. FUNDAMENTAL PROPERTIES OF CONTINUOUS CASTING POWDERS.

by

A3AY0MI OLUSANYA

A thesis presented to Imperial College, University of London

for the Degree of Doctor of Philosophy.

Department of Metallurgy and Materials Scienc Royal School of Mines.

Imperial College.University of London September, 1983*

ABSTRACT

Mould powders perform many functions in the continuous casting of steel.Their physical properties such as viscosity and various thermal factors,can influence their efficacy in providing lubrication, homogenous heat transfer, etc. which are critical in continuous casting. The formulation and subsequent improvement of these powders has been carried out on a purely empirical basis. It is the purpose of this study to :-

i)Characterise extant casting powders and to determine the crystallographic constitution, melting range and basic physical properties of the slags produced by the powders.this would provide a necessary data-base for continuous casting powders.ii)Determine those physical properties of the powder which strongly affect the functions of the powder in continuous casting.In particular, emphasis will be placed upon the factors which influence the heat transfer characteristics of these powders (thermal conductivity,thickness of slag layer, heat capacities and factors affecting the radiative heat transfer) which in turn are known to influence the amount of cracking present in the cast product.iii)Determination of reliable values for the aforementioned properties which can be of use in the formulation of mathematical models which describe heat transfer in the continuous casting mould.

For this study the physical properties have been characterised by various techniques, including determination of melting range byD.T.A., identification of the glass temperature by D.T.A. andD.S.C. X-Ray, Infra-red and Ultra-violet spectral analysis.Thermal properties studied include heat capacities and enthalpies of fusion which could affect the melting rate of the powder,thermal expansion of the slags to determine the possibility of air gap formation as the steel and slag cool, and radiative properties including emissivity and absorption spectra. Values for these data, coupled with thermal diffusivity values and with simulation experiments also being performed at Imperial College, provide an analysis of the predominant modes of heat transfer operating in continuous casting.

DECLARATION

No part of the work described in this thesis has been submitted in support of any application for any qualification

at this or any other Institute of Education.

A.OLUSANYA. September, 1933.

ACKNOWLEDGEMENT

I duly thank my supervisor Prof.Paul Grieveson,Dr.Ken Mills and all those who have given their assistance during the period of

this research project.

CONTENTS

Abstract.Contents.List of Figures.Li3t of Tables.Literature Survey.The continuous casting of steel.

1010131415

continuous17

1 The use of mould powders in continuous casting.1:1 Protection of the steel meniscus by mould powders.1:2 Absorption of inclusions by mould powders.1:3 Frictional forces and lubrication in the

casting mould.1:1:4 Mould powders and their influence on thermal flux and

cracking. 181:1:5 Fusion behaviour of mould powders. 21

a) Powder particle size. 24b) Carbon type. 25

:6 Crack formation in continuously cast steels. 277 Transverse cracking. 278 Longitudinal cracking. 309 Metallurgical factors affecting longitudinal cracking. 30a) Carbon levels. 30b) Sulphur levels. 33c) Aluminium levels. 33Operational factors affecting longitudinal cracking. 34Thermal transfer in the continuous casting mould. 35Objectives. 42Experimental Methods. 44Differential Thermal Analysis (D.T.A.). 44

1 Heats of fusion of mould powders by D.T.A. 462 Differential Scanning Calorimetry (D.S.C.). 483 Thermal Diffusivity. 44 Thermal Expansion. 505 Emissivity. 506 X-ray Diffraction. 537 Infra-Red and Ultra-Violet Spectral Analysis. 53

Preparation of samples. 571 Preparation of glass samples from mould powders. 57

Results. 59Characterisation of mould powders. 59Phase identification of mould powders. 59

1 Phase identification by U 3e of D.T.A. 612 Determination of the melting range of mould powders. 613 Viscosity of mould powders. 654 Crystallisation temperatures of glasses prepared from

mould powders. 673:1:5 Glass temperatures of glasses prepared from mould

powders. 693:1:6 Deformation temperatures of mould powders. 713:1:7 Solidus temperatures of mould powders. 723:1:8 The effect of carbon on mould powders. 74

a) Melting range of "as received" mould powders. 74b) Determination of the carbon type present in mouldpowders. 76

3:1:9 The water content of mould powders. 773:2 Thermal properties of mould powders. 793:2:1 Heat capacities of mould powders. 79

23456

78

123456

123345678

91

CONTENTS

Estimated heat capacities of mould powders. 86 Heats of fusion of mould powders. 87 Thermal expansion of.mould powders. 88 Emissivity of mould powders. 92 The absorption coefficient of mould powders at room temperature. 96 The calculation of average absorption coefficients. 100 Determination of the ferrous-ferric ratio from the U.V. spectra of mould powders. 103 Discussion. 105 Characterisation of mould powders. 106 Phase analysis. 106 Melting range of mould powders. 114 The effect of carbon on the fusion of mould powders. 116 Viscosity of mould powders. 117 The "structure" of glasses prepared from mould powders. 118 Glass and crystallisation of mould powder glasses. 120 The water content of mould powders. 121 Thermal properties of mould powders. 125 Heat capacities of mould powders. 125 Thermal diffusivity of mould powders. 125 Thermal diffusivity of sintered mould powders. 127 Thermal diffusivity of glass mould powders. 131 Thermal diffusivity of molten flux. 135 Thermal conductivities of mould powders. 137 Thermal radiative properties of mould powders. 140 The ferrous-ferric ratio apertaining to mould powder glasses. 141 An analysis of the flux layer occuring in the continuous casting mould. 143 The absorption coefficient of crystalline mould powders. 145 Emissivity of mould powders. 147 A mathematical analysis of heat transfer in the continuous casting mould. 148 Conclusions. 154 Further work. 158 References. 159

L i s t o f F ig u r e s .

Figure Page1 Schematic representation of the continuous casting 11

mould.

2 Mechanism of inclusion entrapment. 15

3 The influence of the casting speed on the 19incidence of cracking.

4 Fusion systems of mould powders in the continuous 20casting mould.

5 The mode of melting of mould powders during 22unidirectional heating.

6 The effect of viscosity on the formation of facial 23cracks with casting speed.

7 A representation of the carbon skeleton. 25

8 Mechanisms of first shell formation. 27

9 Stress distribution across the broad face of a 28a continuously cast slab.

10 The effect of the steel composition on hot 29ductility.

11 The effect of the carbon content of steel on 30longitudinal surface cracking.

12 The effect of the carbon content of steel on 31mould heat transfer.

13 The effect of the carbon content of steel on 32slab shrinkage.

1H Schematic representation of a D.T.A. head. HU

15 Schematic representation of endotherms recordedfor the enthalpy of transition for inorganic H6salts.

16 The temperature dependence of the conversion factor H7• relating endothermic area and enthalpy.

17 Apparatus used for the measurement of the 51emissivity of mould powders.

18 Cells used in the measurement of the eraissivity of 5219 mould powders.

20 Planck’s distribution for three temperatures. 5H

21 Schematic diagram showing the relationship betweenthe rate of radiant heat transfer and optical 55thickness.

22 Melting range data obtained by D.T.A. 6223 64

24 Crystallisation data obtained by D.T.A. 68

25 A typical thermal expansion curve for a glass mould 70powder.

26 Solidus data obtained by D.T.A. 73

27 A comparision of the D.T.A data obtained for "as 75received" and decarburised mould powders.

28 The amount of carbon in a mould powder as measured 76by D.T.A.

29 A typical -OH I.R absorption band found in glass 78mould powders.

30 7931 The heat capacity of sintered mould powders. 8032

33 8234 The heat capacity of glass mould powders. 8335 8436 85

F ig u r e Page

37 The thermal expansion of three sintered mouldpowders. 89

38 9739 The absorption coefficients of glass mould powders. 9840 99

41 The absorption in mould powders below y^m. 118

42 12343 The absorption due to -OH in glass mould powders. 12344 123

45 12746 Thermal diffusivities of sintered mould powders, 12847 D,A,B and K. 12848 129

49' 13150 Thermal diffusivities of glass mould powders. 13251 133

F ig u r e Page

52 The thermal diffusivity of a molten mould powder. 136

53 Thermal conductivities of sintered mould powders, 137

54 Thermal conductivities of glass mould powders. 139

55 A schematic representation of the layer mould powdersystem occuring between the mould and the steel 143strand.

56 The increase in the absorption coefficient 146with time at 850°C.

57 A mathematical representation of the flux layer system. 149

List of Tables.

Table Page

1 Powder compositions. 58

2 Phases identified by X-ray analysis. 60

3 Liquidus temperatures of mould powders. 63

4 Viscosities of mould powders. 66

5 Crystallisation temperatures of mould powders. 67

6 Glass temperatures of mould powders. 69

7 Deformation temperatures of mould powders. 71

8 Solidus temperatures of mould powders. 72

9 The water content of mould powders. 77

10 Thermal expansion data for glassy mould powders. 90

11 Emissivities of liquid mould powders. 9

12 Emissivities of solid mould powders. 95

13 Average absorption coefficients of glass mould powders. 101

The ferroua/ferric ratio for five mould as determined 10414

from the ultra-violet absorption spectra.

15 X-ray data as obtained by Newcastle University. 108

16 Compari3ion of liquidus temperatures obtained by various investigators.

Average absorption coefficients for powder phases 17 in the layer system occuring in the continuous

-110

115

152

casting mould.

10

Literature Survey.

1 The continuous casting of steel.

The use of continuous casting to produce bulk steel strip, plate and ingots has become of increasing importance in recent years. In fact over 25% of the World’s steel is produced by this technique. Japan produces over 5056 of its bulk steel by this method. During the process, molten steel is poured from the ladle into an oscillating, water-cooled copper mould. Here solidification of the steel surface takes place .Once a skin of sufficient thickness is generated, the resulting strand is pulled from the mould . Upon leaving the mould, the strand of steel is subjected to water jets to produce further cooling. Mould powders are placed on the surface of the steel in the mould and form a very thin layer on the surface of the strand (Figure 1 ). Initially rape seed oil and other mineral oils were used to provide lubrication between the mould and the steel strand, but over the past twenty years this practice has been superceded by the use of mould powders. One of the problems associated with the use of rape seed and other mineral oils was the carburization of the steel surface by carbon, produced by the breakdown of the oils at high temperature. This necessitated an additional heat treatment to remove this surface layer. Formerly, mould powders were simply waste slag products, eg. fly ash, but in recent years a large range of commercial mould powders have been developed. Mould powders are primarily a mixture of various oxides. The two main constituents are SiO and CaO with additions of Al^^,Na20,CaF2. Some transition metal oxides are also present, usually Fe O or MnO . Carbon is also added, to modify the powder’s fusion and melting characteristics.

MO

UL

D

11

F ig u re 1S ch em atic re p re s e n ta t io n o f th e continuous casting mould

12

Despite the widespread adoption of continuous casting by the steel industry, there is little detailed scientific data with regard to the influence of the process variables upon the final product quality. The basis of this study is to:-

1) Characterise the basic physical properties, which are thought to be important in continuous casting.

2) Obtain a fundamental understanding of the factors influencing heat transfer during continuous casting.

3) Obtain reliable values for the relevant physical properties involved in heat transfer.

The results obtained from these studies, are an essential pre-requisite in the development of a mathematical model, representing heat transfer in the continuous casting mould.

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1:1 The use of mould powders in continuous casting.

Mould powders have the following functions

1) Protection of the liquid steel meniscus .

2) Absorption of inclusions (alumina etc.) by the liquid flux, at the boundary with the liquid steel.

3) Provision of a lubricating film at the copper mould/steel strand interface.

4) Provision of a thermal barrier in the solidificationprocess which allows controlled and homogeneous heat transfer between the strand and the mould.

It has been clearly established that in the continuous casting process a thin slag film penetrates the gap between the copper mould and the steel strand. It is this layer which tends to regulate the flow of heat between the steel shell and the mould. This thermal attenuation can differ widely from powder to powder, but the reasons for this variation and its effect on the cast product is not clearly understood.The two major defects which can occur in continuous casting are described below

1) If heat is dissipated too slowly, a very thin steel skinis formed during the period of retention in the mould. On removal from the mould, this skin is unable to withstand the ferrostatic pressure of the molten steel acting outwards, and ruptures. This dangerous release of molten steel is called "breakout”.

14 -

2) If heat is removed too quickly, longitudinal cracks ofvarying depths occur in the final product. If these cracks are severe the cast has to be scrapped.

1:1:1 Protection of the steel meniscus by mould powders.

The liquid steel meniscus must be protected from oxidation by contact with air, the as oxides and scales formed would create surface defects in the strand. These defects retard the formation of a sufficiently thick shell and could consequently lead to breakout.

Meniscus protection can be obtained by any flux which covers the meniscus and hinders oxygen transportation. The iron oxide content in mould powders is kept low, as the transport of oxygen is enhanced by the presence of these oxides. Carbon, which is added in the manufacture of these powders, maintains the reducing conditions present in the flux layer. Breakdown of carbonates in the mould powder produces carbon dioxide. This carbon dioxide, in the presence of carbon is reduced to form carbon monoxide, which also tends to exclude oxygen.

The covering of the surface by mould powders prevents the premature solidification of the steel meniscus by providing a measure of thermal insulation. Early solidification of the steel could entrap inclusions (alumina clusters)(Fig. 2 ) which tend to float to the surface. These solid particles are then drawn down into the solidifying shell resulting in subsurface scum patches^

15

1: 1: 2 Absorption of inclusions by mould powders.

Steel products are produced by rolling, forging, and welding processes and the mechanical properties resulting from these treatments such as, resistance to corrosion, cracking, and fatigue failure are all related to the presence of non-metallic inclusions in the steel. Surface treatments such as enamelling, painting, and coating are all influenced by inclusions in steel. Dendritic clusters of alumina are the main type of inclusion found in continuously cast steels(Fig. 2 ). These are formed from two sources of alumina1) Particles which did not float out of the deoxidised

steel in the tundish.

2) Particles formed by the oxidation of residual aluminium present in the steel with air permeating into the system.

Figure & - Mechanisms of inclusion entrapment in the first shell and of possible anchoring of slag by the inclusion cluster : slag and inclusion will be entrapped together when liquid steel overflows.

- 16 -

Alumina, from whatever source, will be deposited on the sides of thenozzle and can be stripped by the passage of molten steel into themould. The ability of a mould powder to absorb these clusters is

( 2 )dependent upon the composition of the powder . Fastest alumina dissolution is obtained with powders which form homogeneous liquids having low alumina and silica contents and large amounts of sodium fluoride and /or calcium fluoride. Failure of the slag to dissolve these inclusions leads to the formation of an heterogenous slag/metal interface. This interface may become congested with solids, with the consequence that among other effects, lubrication of the mould deteriorates as the infiltration of the slag between the steel shell and

(i)the mould becomes erratic. It has been foundv ' using more basic slags, (Na20,Ca0)/Si02>1 and casting speeds above 1.2 metres per minute, that inclusion dissolution is so rapid that no solid alumina was found in the slag, despite an enrichment of greater than 15% in the alumina content.

17 -

1:1:3 Frictional forces and lubrication in the continuous

casting mould.

During the development of the continuous casting of steel, rape seed oil and alternative mineral oils were employed to lubricate the interface of the strand and the mould. Their subsequent replacement by mould fluxes indicates that these powders also fulfil this particular function. High frictional forces in the mould are indicative of metal/metal contact zones (ie. poor lubrication) and an increase in the rate of heat transfer at these points of contact ,could lead to crack formation in the strand. The penetration of the liquid flux into the gap between the mould and the strand, (the mechanism of which is discussed fully by Riboud et alv J is dependent upon the slag‘s fluidity (reciprocal viscosity ) and its liquidus temperature. Several workers have measured the frictional forces occurring during casting at plants and by laboratory techniques^ . Forces measured in plant studies were found to be very much greater than laboratory data, indicating that metal/metal contact occurs during casting. Results of this nature prove that the mould powder does not fully fill the gap between the mould and the steel shell. A commercial instrument called the M.L. Tektor (Mould Lubrication Detector), has been developed at Centre de Recherches Metallurgiques (C.R.M.) . This device produces a signal proportionalto the frictional forces present during casting, permitting permanent monitoring of the frictional forces acting on the strand in the mould.

- 18 -

1:1:4 Mould powders and their influence on thermal flux and cracking.

As previously stated, a slag film penetrates the gap between the mould and the steel shell. Gray and Marstonw . have derived a simple relationship between the thickness of the slag, d, in this gap and the speed of casting, V ,equation (1).

where

"Yj = slag viscosity.

Q = gravitational constant.

A p = p(steel) ~ p(slag). wherep is the density.

For a continuous casting mould, with the strand separated from the mould by a thin layer of slag of thicknessi d, the thermal flux,Q, across a mould of area, A, is represented by equation (2).

Q r K A j A d

where K is the mean thermal conductivity of the slag is thetemperature difference across the slag layer.

19 -

Combining (1) and (2) leads to equation (3)

In plant trials, Alberny et al^^ measured the thermal flux in the mouldusing a number of mould powders. For the powders tested, it was foundthat the flux which gave minimum cracking levels, also produced aminimum in the rate of heat transfer. To comply with these findings,either the slag thickness must be high or the conductivity low. Fromequation(3 A p will not vary appreciably with a change in mould powder,therefore for the condition of minimum heat flux, viscosity and casting

(5)speed would need to be large. Gray and Marston report values for two casting powders. Using the theory formulated, it can be seen (Fig. 3 ) that powder 1 follows the relationship described by the equation. However, powder 2 does not follow the same relationship and also shows a minimum in the curve.

Figure 3

In d e x o f

cracking

The influence of th e casting speed cn fbe incidence of cracking.

20

The conclusion drawn from these results is that cracking and its relationship to the casting speed is too complex to be described by such a simple theory . Another basic fault with this equation is that intrinsically,one would expect the heat flux, Q , to increase as the casting speed is increased, not decrease, as is predicted. Nevertheless, one can conclude that specific powders operate best for specified castings speeds.

Figure 4

mould

I

mould

a free flow powderk) sintered layer c molten layer c » c d molten metal L e slag deposit

Slag coating in the mould for two powders according to Leires ei al \l)

21

1:1:5 Fusion behaviour of mould powders.

A simplified representation of the powder layer system is shown in Figure 1. The situation which actually operates during casting is far more complex. Leites et al^ investigated the formation of slags on the meniscus of continuously cast billets using two types of mould powder. Variations in the mechanism of fusion were identified and are shown in Figure 4 . The carbon content is thought to alter the heat transfer characteristics across the various interfaces. The mode of mould powder fusion as shown in Figure 4, I, was for a powder containing 30% carbon. The model shown in Figure 4,11 is for a powder with a carbon level of 10%. A composition gradient occurs in both modes. The molten layer shows an increase in the proportion of alumina, manganese oxide and silica from assimilation of compounds floating upwards from the molten steel. The reduction in the crbon level coupled with the presence of the sintered layer (Fig. 4,11) which increases the depth of the molten pool of slag and decreases the interaction of the molten metal and the carbon in the powder, leads to the elimination of carburization of continuouslycast billets.

22

(o\Emi et alv J related the occurrence and severity of longitudinal cracks to the melting rate and viscosity of the slag. It was found that the melting rate affects the speed at which molten flux is supplied to the gap between the mould and the strand. Rate of melting experiments were performed using three types of powder: granulated,partially pulverized and powder. Their mode of melting is shown in Figure 5 . From the results of these melting rate studies it was concluded that granulated powders offered the best mechanism for control of the melting process.

Figure 5

(T ) Powdered layer (£) Sintered layer Q) Granular layer (s ) Hail-molten layer(5) A;r gap © Molten layer

. — Mode of melting during unidirectional heating of powders(a) . Powder( b ) . Partly pulverized granules iz). Granules

- 23 -

Viscosity is also an important parameter when discussing the incidence( 2 )of longitudinal cracking in continuous casting. Emi et al includes

in his study, results of viscosity measurements on mould powders. The results from these measurements suggest the existence of an optimum value for the viscosity which minimises crack occurrence(Fig. 6 ). This was also suggested by Alberny et al ^ . Summarising the results in Figure 6 ,the incidence of cracking was found to be smaller at:-

a) A lower speed with a viscous slag A.b) At an intermediate speed with a slag of medium viscosity E.c) At high speed with a fluid slag, I.

Powder H covers a wide range of casting speeds despite a low viscosity which could indicate inherent advantages by using a granulated powder in preference to the,more common powder form.

Figure 6

In d exo f

cracking

Y i i m / m T n )

E ffect o f viscosity on the formation

of facial cracks wilh casting speed,

fetter Emi e t a l .)

- 24 -

From this brief discussion two important features are noticeable

1) Lubrication of the mould/steel strand interface is affected by the melting rate of the powder.

2) The formation of the layered system on the steel meniscus is related to the rate of melting of casting powders.

(2 )From experiments carried out by Emi et alv , the two main factors upon which the melting rate depends have been isolated.These are :-

a) The nature, particle size and shape of the constituents of the powder.

b) The amount, nature and size of the carbon particles present.

a)Powder particle size.

(2)Emi et al experimented with granulated, partially pulverized andpowder casting fluxes, with specific reference to their influence upon product surface quality.Granulated powders were deemed to melt with each granule constituting a "melting unit". The sintered layer formed from such a granulated powder tends to have a far more evenly distributed system of pores when compared to either a partially pulverized or powder flux.. Hiromoto

/ o \et a l K deduced that the fusion rate of a powder flux increases in an inver.se square relationship, to the mean particle diameter..

- 25 -

It wa3 found that good protection of the steel meniscus can be obtained by any powder with a low density, using a "dark flux practice”, (low surface powder temperature). Furthermore powder fluxes comprising small particles can be provided with superior properties if their fusion rates are controlled. However problems however occur from the use of ultra-fine powder grades, for example, transference of ultra-fine mould powder to the mould could lead to an increase in dust in the immediate working environment and so lead to a potential hazard. Decreasing particle size means an increase in particle surface area which leads to an increase in the water absorption by the calcium and alkali salts present in the mix. This undesirable feature increases the oxygen and hydrogen levels in the surface layer of the molten steel.

b)Carbon type.

Carbon is added to mould powders to reduce their melting rate. The carbon forms a skeletal type of structure (Fig. 7 ), in which fine carbon particles surround powder particles, inhibiting agglomeration and reducing heat transfer.

Figure 7

Schematic representation of

the carbon skeleton occuring in mould powders

- 26 -

It is found that fine flake graphite decreases the melting rate by agreater amount than coarse flakes and/or cokes. The ability to form thiscarbon-flux skeleton is dependent upon the carbon particle size. Finerparticles lower the amount of carbon required to maintain the meltingrate at a particular value. The reactivity of carbon with air and theslag melt must also be considered. Carbon in the form of coke is themost reactive towards air whilst the coarser flaked graphite tends toremain close to t'he molten slag/sintered layer interface. The presenceof carbon in the mould powders means that possible carburization of the

(0)strand could occur. Evteev et alw/ discussed this factor. It was found for a powder containing 30? carbon, a steel was cast with a carburized layer 3nim thick. Powders which had a carbon content of 6? or less showed no evidence of carburization. Further studies of carburization have been carried out by Takeuchi et al^^. In this study boron nitride was substituted for carbon. The results showed BN particles performed in a similar manner to carbon in their ability to produce a skeleton structure reducing the powder melting rate, producing cast products which are totally free from carburization although an increase of 2-3 ppm in the boron content of the steel was observed. Nonetheless carbon powder is to be prefered for its cheapness and its reducing power.

27 -

1 : 1 : 6 C ra c k fo rm a tio n in c o n t in u o u s ly c a s t s t e e ls .

In c o n t in u o u s ly c a s t s t e e l s , t r a n s v e r s e and lo n g it u d in a l c r a c k in g may

o c c u r . These a re cau sed by d i f f e r e n t m echanism s and w i l l be c o n s id e re d

s e p a r a t e ly .

1 : 1 : 7 T ra n s v e rs e c r a c k in g .

The water cooled copper mould does not remain stationary throughout thecasting procedure but oscillates. This reciprocation reduces the risk ofbreakout by rupture of the steel skin and was introduced at an earlystage in the development of continuous casting. A review of the varioustypes of mould, their design and development is presented by

(11)Samarasekera and Brimacombe

Reciprocation produces ripple marks on the steel shell and there areseveral mechanisms proposed to explain the formation of these marks (2)(.12)(13)(14) (Fig. 8 ).

F ig u re 8

iOverflow

I bisOverflow + remelting

JLSolid meniscus bent backwards

Mechanisms of first shell formation. Visible oscil­lation marks are formed by mechanism I, or possibly I bis when convection movements bring warmer steel.

- 28 -

Transverse cracks are initiated at these marks during the secondarycooling stage, where the strand is either bent or straightened to passthrough the water sprays. Thus transverse cracking appears to be related

\

to thermal stresses occurring during secondary cooling and also to the hot ductility of the particular steel during the bending mode. Figures 9 and 10 ,show the effects of stress and hot ductility on the slab.

Figure 9 , shows that longitudinal tensile stresses o c c u r n e a r the edges of the broad faces of a slab due to the large thermal gradients produced by the cooling water sprays.

F ig u re 9

Stress d istribution across:broad face

29 -

Hot tensile testing during cooling from 1300°C shows that the ductility decreases quickly at 100° above normal straightening temperatures for a particular steel composition (O, in Fig. 10 ). This is attributed to the precipitation of nitrides and carbides of aluminium, vanadium and niobium. The ripple marks which provide the sites for the initiation of transverse cracks may be controlled by an increase in the slag layer thickness. This could be achieved by control of the powder fusion rate ie. by carbon addition, but as stated above, precipitation of carbides may increase with an increase of carbon in casting powders , hence control may be difficult to attain.

F ig u re 1 0

Temperature/T

E ffe c t of steel composition on hoi* ductility

- 30 -

1:1: 8 Longitudinal cracking.

This is the predominant surface defect which occurs in the continuous casting of steel. These cracks occur mainly within the central 50 of the broad face of slabs.

1:1:9 Metallurgical factors affecting longitudinal cracking.

a)Carbon levels.

Longitudinal cracking is highly dependent upon the carbon content of the steel(15)( Fig. 11 ).

F ia u re 11m*

1830 x180nm action

powder casis

o A 1-05 1100x 8 1-06 90

Effect of car ben content on longitudinal

surface cracking

- 31

Extensive laboratory, pilot plant and plant measurement, coupled with computer analysis by Singh and Blazek , revealed that as the carbonlevel in the steel fell to 0.1$ the heat transfer to the mould dropped by 25$ of the value at higher carbon levels ( 0.3$ and above), (Fig 12 ).

Figure 12

Mould heat transfa* pJd ffenhstucV

* Lboricry s hudy

Effect of carbon content on mould heat transfer

I t i s proposed t h a t t h i s minimum i s d i r e c t l y r e la t e d to the U

p hase change o f i r o n w it h i t s a s s o c ia t e d volume changes

toY

solidification .

- 32 -

Gray et calculated linear shrinkage coefficients showing the effect of the carbon level on these values (Fig. 13 )• It is further proposed, that it is this additional shrinkage leads to the wrinkling of the steel skin which, in turn gives rise to a decrease in the contact area between the strand and mould reducing the mould heat transfer.

F ig u re 1 3

Effect of carbon content on shrinkage after solidification

- 33 -

b) Sulphur levels.

The addition of 0.3 sulphur was observed to increase the mould heat/ -i £ \transfer of 0.1 carbon containing steels , but had a neglible effect

on steels with a higher carbon level. Plant trials^ indicate that sulphur levels above 0.02/6 lead to an increase in cracking. This is attributed to the tendency of sulphur to form low melting point compounds which reduce the strength of the steel shell at temperatures below the solidus .

c) Aluminium levels.

The level of aluminium seems to have very little effect on the surface quality, except for the case of very low levels in thin slabs. Situations such as this are rare in normal continuous casting practice.

- 34 -

1: 2 Operational factors affecting longitudinal cracking.

The main factors which affect surface quality are:-

1:2a. The thermal conditions apertaining to the mould and the designand geometry of particular moulds are discussed by a number of

(11) (16)workersk M . In particular, the review by Samarasekera and (11)Briraacombe' discusses the introduction of reciprocation in continuous

casting practice and the use of mould tapering to yield improvements on mould heat transfer. Basic design and the manufacture of moulds is also discussed.

1:2b. The cooling conditions and slab support systems, (which occurafter the strand has left the mould, bending of the strand, rolleralignment and forced cooling by water sprays) are discussed by Irving

(15)and Perkins . The results indicate that the incidence of centre line cracking is strongly influenced by the casting speed and extraction roll pressure, metallurgical factors having a neglible role in this form of defect.

1:2c. Mould powders and their use was discussed by Irving and (15)Perkins' . They state that the type of casting powder used during

casting can affect the severity of cracking in the product. The powderpenetrates the gap formed between the strand and the mould producinggreater uniformity in the transfer of heat during the period in which

(17)the strand is formed. Brimacombe et al have found quantities ofmould powder in longitudinal cracks in samples of commercial steels. This proves that longitudinal cracking is initiated in the mould. However the use of a mould powder can only affect the severity of cracks and cannot alter the composition of the steel at which cracking .is at amaximum.

- 35 -

1:3 Thermal transfer in the continuous casting mould.

To reiterate, it has been clearly established that in continuous casting, a thin slag film penetrates the gap between the copper mould and the steel shell. It is this layer which tends to regulate the flow of heat between the strand and the mould. If heat is removed too quickly, longitudinal cracks can occur in the final product. If heat is removed too slowly, the solidified shell formed is of insufficient thickness to oppose the ferrostatic pressure exerted internally and breakout can occur as the strand exits the mould. A discussion of process parameters (casting speed, mould taper etc.) was given earlier. This section reviews some of the mathematical procedures applied to describe the heat transfer situation applying across the gap between mould wall and steel strand.

A number of workers^ 19) (20) jjave attempted to develop a mathematicalmodel representing the mode of heat transfer occuring during the

(19)continuous casting operation. Jacobi and Fredriksson andThegerstrom^^ have discussed the formation of the air gap which occursbetween the mould and the steel strand due to the shrinkage of the steel

(2 1 )upon cooling. Fredriksson et al relate the growth rate of the strand shell to the applied cooling rate, equation (4).

hS.

sTg = surface temp, of steel h = : heat transfer coeff.T = liquidus temp, of steelm S thickness of shellTq = temp, of mould K = coeff. of thermal conductivity s J

of the steel shell

- 36 -

The study produced equations in which the thickness of the air gap is described as a function of the strand growth rate and the shell thickness. The effects of radiation are ignored by assuming that for a gap width of less than one millimetre, the mode of heat transfer is primarily by conduction. Thus one can replace h, the heat transfer coefficient withK,the thermal conductivity. The analysis performed by

(in')Jacobiv ^ defines the gap width including the contribution to heat transfer by radiation using various gas atmospheres in the gap. Results from the study show that for the solidification of pure iron in an argon atmosphere, the width of the gap is 0.3nim. Surface active gases,such as

were found to increase the smoothness of the ingot surface when compared to or Ar. These surface active gases have a lower thermal conductivity than or Ar. However heat extraction remainseffective due to an increase in the radiation emissivity due to the

( I Q )formation of oxide and sulphide layers on the ingot surface.Jacobiv concludes his treatise with the statement that the mechanism is not valid when the strand is covered with a slag layer.

Ohmiya et al 20 have devised a method to compare the heat transfer capabilities of various casting powders, using a combination of thermal conductivity and radiation conductivity to describe the overall heat transfer mechanism. The difficulty in describing heat transfer by an amalgamation of two (or more) modes is that a model becomes extremelycomplex. For a full theoretical treatment of thermal radiation and heat

( ? 2 )transfer one must consult specialist . tomes .

- 37 -

Ohmiya et al^ ^ fit their data to three models:-

1) In which heat flux is described using an average"apparent” thermal conductivity (analagous to a case of pure conduction), equation (5).

q = h ( T . - T )^ app s t m

q = heat flux T 4-st = Temp, of steelhapp = heat transfer coeff. Tm = Temp, of mould

- - - ■f R m + R s t ©h K °where 3pp ^app

and R and R . are the contact resistances between the flux and the m stmould and the flux and the steel strand respectively.

By plotting 1/h against d,an average contact resistance was ap presolved (1 /2(Rm + Rst). For the two commercial powders tested,thisresistance was zero. To obtain the apparent thermal conductivity, valuesof (Tst“Tm)/q =^^app were plotted against -T^. This was repeated forvarious thicknesses (d). Apparent thermal conductivities derived by thisprocedure, show an increase with T ana with a reduction in the ironstoxide content of the powder.

- 38 -

2) The diffusion approximation.

An effective thermal conductivity (Keff) can be defined by equation(7).

eff - Kc + Kr ©where K is the radiation conductivity.

Heat flux (q) is given by equation (8)

q = - Keffdjctx ©

Ignoring contact resistances, integration gives, equation (9) .Tsj.

1 = 7T | KeffdT ©T,m

Differentiation with respect toAT = (T ,-T ) yields equation(10).—* st m

dq _ 1 dldSTZ 6 ^ {T ) + Keff(Tst) - Keff (TJ ^

as

W y > » Kef(Tst) - Kef(Tm|V-

dAT

- 39 -

Equation (10) simplifies to:-

d qdAT

1d K e f f ( W m ) ©

Thus K is obtained from the slope of a plot of q versus T.

The accuracy of this method depends upon:-

a) The magnitude of the absorption coefficient.

b) The absorption coefficient remaining constant over the temperature range under study.

c) The refractive index remaining constant over the temperature range studied.

(22)The diffusion approach is not valid near surfaces'1 thus it isincorrect to integrate equation (9) between T , and T . Integrationst mshould be carried out between temperatures in the bulk extrapolated tothe surface. The difference between these extrapolated values andsurface temperatures, decreases with increasing absorption coefficient

(22)and thermal conductivity and decreasing temperature and heat fluxHowever Ohmiya et al^ ^ estimate that for the conditions used in hisstudy the difference is neglible . Comparison of Ke^ obtained by Ohmiya

(20) (23)et al with other workers reveals that values of K obtained in ---- e 11his study are of the same order of magnitude.

- 40 -

3) Combined modes of heat transfer.

If one assumes that the relationship between conduction and radiation isuncoupled (both thermal processes can be regarded as independent). The

(22)resultant fluxes q q can be simply combined equation (12).

q = qc + qr ©

Expansion of the individual terms yields equation (13)»

K( T s t -T J +m

4 0 n2 -(T^-T4)3(Xd +1+ 1 _

£ s t &n

. v st m ' 1

©Where

0n

a

Stefan-Boltzman constant refractive index absorption coefficient emissivity of steel emissivity of mould

Equation(13) is valid only for grey media and temperature independentphysical properties. Ohmiya et al^ ^ derive values for the absorptioncoefficient, an average surface emissivity and a true thermalconductivity. This true thermal conductivity is of the order of 25% the

-1 -1value of the effective thermal conductivity being 0.43Wm K for the(20)two commercial powders. The experiments carried out by Ohmiya et alv

indicate a high contribution to the overall heat transfer by radiation.

- 41

McGraw^^ however, working on the heat transfer mechanisms of glassforming, has calculated that during forming operations with a glassthickness of less than 12mm with rapid cooling, radiation contributesminimally to the temperature distribution within the glass. The overall

(23)net heat transfer being almost 100/6 due to conduction. Fine et alhave developed an equation for the calculation of the effective thermalconductivity of metallurgical slags, combining the effects of thelime-silica ratio,temperature and the ferrous oxide content. Theabsorption coefficients for iron containing glasses were also obtained.Measurement of the thermal diffusivity of various ferrous oxidecontaining slags gave values of 4.6 x 10“' - 2.0 x 10 ms fortemperatures between 1550-1720K comparing favourably with the values

(1 \obtained by Riboud et alv \

Objectives.

The purpose of this study was to determine various physical properties of mould powders which could influence the performance of the continuous casting plant and which could account for the differences between the efficiencies of various.powders.

1) Heat Transfer, could be affected by:-a) Melting range, liquidus temperature, glass temperature.a) Emissivity of the powder.c) Heat capacity , enthalpy of fusion fcbermoA oooAgc'KvM'id) The absorption coefficient.e) Thermal expansion to determine possibility of air gap formation.

2) Melting Rate, dependent upon:-a) Melting range, liquidus temperature.b) Carbon content.c) Type of carbon.d) Powder particle size.e) Enthalpy of fusion.

- U3 -

3) Determination of crystalline constituents.

a) Identification of crystalline species which are present during melting and cooling.

b) Study of their effect on the heat transfer and melting rate, on the constitution of the slag layer.

4) Provision of data for mathematical models.

To provide good values for these physical properties, which will be used to derive a reliable mathematical model for the processes at work in thecontinuous casting mould.

- 44

Experimental Methods.

2 Differential Thermal Analysis (D.T.A.).

The melting range of a casting powder is an important basicparameter.The powders are formulated with regard to their oxidecomponent’s ability to raise or reduce the melting range of a powder. Many methods are avalible for the measurement of the melting rate, the most popular method for workers in this area being the technique of Seeger’s cones. In this method, cones of the material under test are placed in furnaces at the required test temperature for specified time periods. The sample’s melting rate is monitored either by photography or by rapid removal of the sample from the furnace (quenching).This method is considered to be unable to produce as accurate results when compared to Differential Thermal Analysis. In this technique a small sample is heated at a specific rate and can quickly equilibrate with the furnace. A reference material ( calcined alumina) is also present (Fig. 14 ).

F ig u re 14

- 2} 5 -

The temperature difference between the alumina and the sample is monitored by use of a differential thermocouple. Any thermal event ie. crystalline transformation, T melting etc. will cause a change in theOtemperature differential which results in a deviation from a base line .

As D.T.A. is a dynamic method, before U3e it must be calibrated to determine its deviation (temperature lag) from the true temperature. This is performed using materials with well defined transformation and/or melting points

Quartz, 571-574°C;Potassium sulphate, 582-588°C;Barium carbonate, 808-819oC;and Strontium carbonate, 928-938°C,

were all used for calibration purposes. This thermal lag is of the order of 5-10°c for measurements in platinum crucibles. Stanton-Redcroft Differential Analyser models, 6-25 and 675 were used in this study. With this equipment it was possible to work up to ~1600°C and under various gas atmospheres.

Liquidus and other transformations were defined at the point at which the/\T signal returned to a base line position extrapolated from low temperature. This point was thought to give the most reliable value for the temperature at which the sample can be regarded to be totally liquid. The initial deviation from the baseline does not necessarily indicate the onset of melting. D.T.A. records all thermal events, thus the onset of sintering is recorded along with various other transitionsat this time unidentified.

46

2: 1:1 Heats of fusion of mould powders by D.T.A.

Heats of fusion for crystalline materials are usually determined by Drop Calorimetry .Use of this method is precluded by the nature of the materials under test. Drop calorimetry employs a high cooling rate which results in the liquid flux forming a glass,which would have a smaller enthalphy of fusion than that produced by the melting of the corresponding oxide constituents. Therefore a method was devised enabling, quantitative measurements using D.T.A. Calibration of the equipment over a large temperature range was necessary. Calibration was carried out by measurement of the areas of a number endotherms produced by compounds with well documented enthalpy values. Ideally calibration endotherms should be of the form depicted in Figure 15a . Unfortunately many of the calibrants availible for this study produced endotherms of the type depicted in Figure 15b.

S c h e m a tic o f recorded endotherm s

This produces difficulties in the determination of the baseline and therefore the area under the endotherm.However some calibrants produced suitable endotherras and allied with data availible from D.S.C.studies on the calibrants producing type b endotherms,it was possible to determine the conversion factor (KCOnV*)as a function of temperature.The Kc°nv.-Temperature relationship is shown in Figure 16. The accuracy of calculated enthalpies due to scatter is of the order of 555.

Figure It The temperature dependence of the conversion factor, K ,

relating endothermic area and enthalphy.

o -G O CO TRANSITION

con v

17 C

lJr-<I

2:1:2 Differential Scanning Calorimetry (D.S.C.).

Drop Calorimetry is a standard method for the measurement of heat capacities. However it cannot be used for these materials for the reasons stated in the previous section. D.S.C. is a experimental technique in many ways similar to D.T.A., however in this case it is the energy difference between a reference (usually empty) and a sample which is monitored. This energy difference is proportional to the enthalpy. The method and treatment of data is discussed by Mills and

M ORichardson . Values for the heat capacity (C ) are required for the calculation of thermal conductivities of casting powders from thermal diffusivity experiments. The equipment used in this study was a Perkin-Elmer Differential Scanning Calorimeter model . Samples were in the form of pressed, sintered, heat treated powder discs and glassdiscs (ca.lmm x 5mm dia.).

2:1:3 Thermal Diffusivity.

Measurement of thermal diffusivities for this study were carried out by Dr.R.Taylor of U.M.I.S.T. The method used to provide these data was

/ O f7 Nthe laser pulse method .The technique measures the faxhatjcoifanhm>+teJi tKroctjia sample in the beam of a ruby laser. Powder samples were in the form of sintered discs ca. 1mm x 10mm dia. prepared from casting powders in the manner outlined in Section 2:2. Glass samples prepared from casting powders were also used. The technique developed for producing glass samples is detailed in Section 2:2:1. This technique enabled glass discs of varying thicknesses to be prepared. This was important as an unknown proportion or radiative heat transfer may be included in the value for thermal diffusivity. The important physical property governing this effect is the so called optical thickness (CXd) of the sample. This will be discussed in a later section.

- 50 -

Thermal expansion data are required for the calculation of densities for solid slags in order that thermal diffusivity values can be converted into thermal conductivities. However they also provide valuable information concerning the physical changes occurring in the slag layer, furthermore these data can be used in calculating the possibility of air gap formation in the lower part of the mould. Measurements were made under argon. "As received" powders were decarburised by heating at 900°C for two hours,cooled and pressed into rectangular shaped specimens ca. 2cm in length and sintered at ca. 900°c for 14 hours. Samples were also prepared as glass rods using the procedure described in Section 2:2:1.

2: 1:5 Emissivity.

2:1:4 Thermal Expansion.

determined using the apparatus shown in Figures 17 } 18 and 19 • Samples were of the form used for thermal diffusivity measurements ie. glass discs ca. 1mm x 10mm dia. A full description of the apparatus and

The total normal emissivity casting powders was

technique is given by Keene and Quinn(28 )

51

Figure 17 Apparatus for measuring the emissivity and absorption coefficient of casting powders.

b

Figure 18a?be ellS used for the determination of emissivity and absorption oefficient of molten casting powders.

- 52

F ig u re 19

«•

Blackbody

Fe crucible

A cell used in the measurement o f e m is s iv ity

- 53 -

X-ray powder photographs of a number of casting powders were obtained incollaboration with Newcastle University. In the study carried out byNewcastle University, decarburized casting powders were photographedusing an increasing temperature regime. In the study carried out by theauthor, glass samples were prepared as detailed in Section 2:2:1 andcrystallized for various time periods. Hiromoto et al ' carried outX-ray analysis of five mould powders and the results of hisinvestigation are essentially in agreement to those obtained byNewcastle University, despite compositional differences between thepowders used in the two investigations. The effects of the variousphases on the heat transfer characteristics of continuous castingpowders have not been reported in the literature,except the briefstatement that compositions favouring the precipitation of cuspidine

f k )should be avoided . The study of heat transfer and crystalline phases is currently under investigation at Imperial College in a related study^^.

2:1:7 Infra-Red and Ultra-Violet Spectral Analysis.

Steel is transfered into the continuous casting mould at approximately 1600°C. Upon leaving the mould the temperature has been reduced to 400-800°C, the strand then undergoes secondary cooling. Thermalradiation at these temperatures is in the near infra-red region of the spectrum (1 m or greater). The position of the peak of thermal radiation can be easily obtained by application of Wien’s displacement law, equation (14).This describes the shift in the wavelength for which the emitted energy is at a maximum for a given temperature. This maximum shifts towards shorter wavelengths as the temperature increases.

2; 1:6 X-ray Diffraction.

- 54 -

x = - ©' ' ‘ max J w

where T = Temperature in K C = 0.29 x 10"2mK

This equation can be derived from Planck’s distribution. Assuming a steel temperature of 1500°C application of the formula leads us to avalue of 1.63 JJ m max’ n8ar in]ra red region of thespectrum. Planck’s distribution gives the distribution of energy emitted by a black body in relation to wavelength at defined temperatures(Fig. 20 ). The variation in the absorption coefficient of a casting powder with wavelength, becomes very important when used with Planck’s distribution.

F ig u re 2 0

55 -

The overall heat transfer assumes that the casting powder behaves as agrey body ie. a constant absorption coefficient. Fluxes comprising oflarge amounts of transition metal oxides (FeO,Fe O Mn O ) may have high

( 2 1 )and a varying absorption coefficient with wavelength. Fine et al J

have measured absorption coefficients in their study of thermal diffusivity of metallurgical slags containing up to 13% ferrous oxide. Strong absorption due to ferrous ions throughout the visible and infra-red spectrum by iron containing glasses was confirmed. Measurement of the infra-red absorption of glass at high temperatures was carried out by Grove and Jellyman^^, coupled with the calculation of the radiative conductivities of the glasses under test. They found that the absorption coefficient did not change markedly with temperature, whilst the differences observed in the radiation conductivity were due to differing iron concentrations and states of oxidation. Radiation conduction fell with an increase in iron content in accordance with the strong absorbing nature of the ferrous ion. The quantityOC<3> the optical thickness is a governing factor .

Opti ca! t h i c k n e s s (& d ) of s peci men

r e la t io n s h ip b e tw e e n o p t ic a l th ic k n e s s and radiant h e a t t r a n s f e r

T h e

- 56 -

It can be seen (Fig. 21 )that when^d, is small direct radiation could be the predominant mode of heat transfer. An increase in Q. would:-

1) Reduce radiation conduction.2) Increase optical thickness.3) Decrease direct radiation .

Thus a must be determined to verify results of thermal diffusivity measurements as it may contain a contribution from these other modes of heat transfer if samples were not optically thick. The extinction coefficient is composed of two components, equation (15).

£ = a + o s ©

where a = the absorption coefficientQ s = the scattering coefficient

For transparent materials ie. glasses Q is small and a =e • Forscrystalline samples 0 becomes the dominant factor. The scatteringsfactor depends on the size and shape of the crystalites in the matrix. A

(22)full theoretical treatment is given by Siegel and Howell' . Due to experimental limitations this scattering factor was not determined. Samples for spectral analysis were in the form of glass discs of varying thickness with a diameter of ca. 25mm. They were produced by the technique outlined in Section 2:2:1 . The discs were surface ground to produce parallel faces and diamond lapped to eight microns. Crystalline discs were produced by heating glass discs at 800°c overnight,re-grinding and re-polishing.

- 57 -

2:2 Preparation of samples.

Samples of easting powders were heated in alumina vessels at a temperature of ca. 900°C for two hours. They were then cooled, ground using an agate mortar and pestle and stored in a desiccator. This heat treatment minimised effects due to the free carbon in the powder such as the reduction in the rate of melting. Absorption of water during the grinding process was neglible as confirmed by D.T.A. Powders in their "as received" form were also evaluated using D.T.A. This can give a value for the ratio of the types of carbon (amorphous, flake etc.) present in a particular powder.

2:2:1 Preparation of glass samples from mould powders.

An alumina crucible (capacity 30cc ) was filled with the required powder (in the "as received" form) heated to 1400°C and held at this temperature for one hour. The molten flux was poured into a preheated graphite mould (ca.600°C) ,which was held under suction. The glass rods so formed were annealed by heating for thirty minutes at 600°C. The glass rods (ca.30mm x 10mm dia.) were surface ground to a suitable size for thermal diffusivity and D.S.C.measurements.This method was also used to produce samples ca.5mm x 25mm dia. for U.V .and I.R. spectralanalysis.

5 8

T A B L E 1

CODE %

Z i 0 2%

CaO%

A l ^%

MgO%

Na20%

k2o%F

%

^ e 2®3%

MnO%

c( f r e e )

A 3 3 .6 35 2 .7 4 .9 9 .7 0 .5 5 .5 3 .9 0 .1 3 -9B 3 7 .5 37 6 .5 0 . 6 **•5 0 .5 5 .8 0 . 2 0 .1 4 .5C 3 5 .0 34 * .5 0 .6 3 .2 0 .1 3 .6 0 .2 0 5 .5D 2 3 .1 2 8 .9 8 .0 0 . 3 7 -3 0 .1 5 .9 0 .4 8 .9 3 .5E 3 5 .1 3 7 .5 3 .3 4 .4 6 .0 0 .8 6 .3 0 .3 0 2 .6F 3 5 -6 32 1. 1 -5 9 .5 0 .4 4 .5 0 .1 - 3 .9G 3 6 3 2 .7 1 .5 1 .6 8 .1 0 .2 4 .4 0 .6 0 .1 4 .0H 3 3 .6 3 3 .1 5 .4 2 .3 1 2 .4 0 .3 7 .3 0 .3 0 .3 3 .0I 25 32 6 0 6 - 5 -3 0 . 2 7 .5J 3 2 .1 3 1 .6 5 .5 - 5 .8 0 . 4 4 .8 0 .7 - 3 .0K 2 7 .3 2 3 .7 8 .7 0 .9 1 4 .2 0 .7 6 .8 0 .9 0L - • - - - - - - _M 34 34 5 .5 - 4 .0 - 5 .3 0 . 2 0 6 .5N 36 39 0 .2 - 6 .4 - 5 .8 0 .6 - 5 .0G - - - - - - - - - -P 3 5 .5 3 5 -3 6 .0 0 .9 5 .1 0 .1 5 .4 1 .1 0 4 .5k 3 6 .7 3 5 .5 1 .6 1 .1 7 - 5 0 .3 1 .4 0 .6 0 4 .1R 3 2 .0 31.O 8 .5 1 .3 ~ 3 .8 0 .3 4 .0 0 .2 0 .1 5.C'S 33 3 4 .1 6 .2 0 . 8 3 .8 0 .5 5 .4 1 .8 0 3 .9T 28 3^ 5 0 .5 6 1 6 2 5U 3 0 .6 ? 4 .3 8 .1 o . S 2 .8 1 .1 4 .3 0 .7 0V 2 6 .1 3 3 .0 4 .6 0 .6 6 .6 0 .9 5 .7 1 .8W* 3 8 .3 2 8 .7 4 .3 1 .9 5 .5 0 .7 1.8 9 -5 0 .8 2 .7X 4 7 .9 3 7 .8 3 .5 0 . 5 5 .2 0 .7 0 .1 1 .6 0 .1 5 .0Y 2 9 .4 2 3 .it 1 2 .2 1 .0 3 .6 2 .1 4 .1 4 .1 0 .1L 2 6 . A 2 3 .3 1 0 .7 0 .6 1 1 .5 1 .9 6 .3 1 .8 3 .3 4 .9BL 2 9 .0 2 9 .0 1 2 .5 - 4 .5 - 2 .0 4 .8 - 16BLA 2 8 .0 2 3 .5 1 3 .0 - 5 .0 - 5 .0 4 .5 - 1 8 .5CA 2 9 .5 3 0 .8 , 5 .0 - . 4 .0 - 4 .2 0 .3 - 7 .5CB 3 1 .5 3 3 .5 6 .5 - 4 .5 5 .3 0 .5 - 3 .2da 27.5 3 4 .5 5 - 0 - 7-5 - 5.5 2 .5 - 5.5THA 31.3 3 1 .1 5-2 0.4 3.7 0.5 4.8 0.4 0 5.4THB J 25-5 3 2 .3 7.8 0.7 10.7 0.8 6 .0 2 .6 5.7 5.4

* Thin powder contains 5.0% B2O-

- 59 -

Results.

3 Characterisation of mould powders.

As there are a wide range of proprietary powders, (Table 1 ), a series of experiments was used which would allow the characterisation of various properties of mould powders which may influence their performance during continuous casting.

3:1 Phase identification of mould powderr-s.

X-ray analysis was employed to characterise the powder,with attempts to correlate some of the properties with identified phases. A tentative basic identification was carried out by the author on three powders (Table 2 ). A more detailed study was executed at Newcastle Universityand is reported in the Discussion section of this thesis.

- 60 -

TABLE 2

Phase Cuspidine Gehlenite Pectolite Wollastonitec3s2fi c2as nc4s6h CS

PowderI X X -P X X X

Q - X X X

C = CaO S = Si02 N = Na20 A = A1203

FI s CaF2 H = H20ie. 2C = 2Ca0

Samples were prepared by heating a quantity of nas received" powder atca. 1400°C for one hour and casting into a circular carbon mould. Theglass disc so formed was annealed for thirty minutes at ca. 600°C andcooled to room temperature. Samples were crystallised by heating to ca. 800°C and maintained at this temperature overnight.

- 61

In addition to X-ray analysis, D.T.A. was also employed to identify phases formed upon heating a mould powder , correlating peaks obtained (Figs. 22 and 23) with the phases identified by Newcastle University (Table 15), as these data are regarded to have been treated in a similar manner to the samples which have undergone thermal analysis. For the phases identified in Table 2 (Section 3* 1)> no correlation was obtained between these data and those due to D.T.A studies. Further study in this area will be discussed in a later section of this thesis.

3:1:2 Determination of the melting range of mould powders.

The melting range of a mould powder is of prime importance in the selection of a powder to cover specified casting conditions. The melting rate of a powder is dependent upon the melting range of the powder; the melting rate, in turn, affects the rate of flow of flux into the gap between mould and steel strand. As discussed earlier, the melting range is also affected by absorption of inclusions, alumina absorption being of the greatest concern. Alumina, is known to increase the liquidus

( O 1 )temperature of a flux altering the melting range of the powderVJ Theaffect of alumina additions to the physical parameters of castingpowders has not been studied in this research programme, but has been

(i)(2)discussed by a number of workers' .

3:1:1 Pha3e identification by use of D.T.A.

The dta traces (heating cycle) for several casting powders

900

Figure 22

- 63 -

Investigators (2) (5) jia(j previously measured melting ranges and liquidus temperatures of continuous casting powders,by the method of Seegers cones. It was decided that this method could not provide the accuracy that was attainable by the use of Differential Thermal Analysis (D.T.A.). Data obtained are shown in Table 3 and in Figures 22 and 23.

TABLE 3

Powder A C H Q R S U THA THB

T . /oC 1 151 1276 1159 1225 1203 1191 1223 1197 1190liq.

In the trace representing powder H (Fig. 23), it can be seen that the endotherm does not return to a baseline position. This is due to a difference in the nature of the solid and liquid phases of the powder, thus resulting in two different baseline constants.

The powders tested, fused over a large temperature range (200°C). Powder Q however, had a sharply-defined melting point compared to the other powders. This is thought to be due to its composition lying close to a phase field boundary . Additionally various minor peaks and troughs were superimposed on the major endotherm; these are due to the formation and melting of various phases during heating. As mentioned previously it is hoped that these phases can be identified by X-ray analysis .

DTA trace (heating cycle) for several casting powders.

Figure 23

- 65 -

3:1:3 Viscosity of mould powders.

The viscosity of the mould powder is considered by many steel manufacturers to be the most important factor when selecting a mould powder for specific casting conditions. Any change in casting speed would be accompanied by a change to an appropriate powder with a lower or higher viscosity. Certainly viscosity is a factor in determining the thickness of the slag layer formed in the mould, hence influencing the heat transfer between the strand and the mould (Equation 3). Algorithms of the form depicted in equation (16) , have been developed whichallow computation of the viscosity of any powder at elevated temperatures. The results obtained from these models provide reasonable estimates for the viscosity of liquid fluxes at elevated temperatures. Viscosities according to two constant sets developed at C.R.M., Belgium and at the Institut de Recherches de la Siderugie Francaise (I.R.S.I.D.), France, are shown in Table 4 and are thought to lie within +30 of the measured values.

R T

A £ t3 a re constants

= mole fra c tio n silica = mole fraction metal oxide

N = silica equivalence factor fo r m ehal oxide

- 66 -

TABLE 4

TEMPERATURE VISCOSITYK

C.R.M.Powder

1573 1.3462BLA 1673 0.6029

1773 0.29661573 0.9043

C 1673 0.43041773 0.22341573 0.2662

D 1673 0.15121773 0.09181573 0.4075

G 1673 0.21681773 0.12431573 0.3160

H 1673 0.17371773 0.10251573 0.3091

I 1673 0.17101773 0.10151573 0.6640

M 1673 0.33321773 0.18141573 0.5982

P 1673 0.30341773 0.16641573 0.4502

Q 1673 O.233O1773 0.13031573 0.8544

R 1673 0.40911773 0.21361573 0.2392

V 1673 0 .13 7 21773 0.08411573 0.5490

THA 1673 0.28071773 0.15531573 0.2239

THB 1673 0.12791773 0.0781

PaSI.R.S.I.D. (constant sets)

1.21350.52880.25391.08450.52640.2781

0.08350.04930.0310

0.26500.15790.1001

0.10980.06720.04360.1324 0.0765 0.04710.50940.26990.15410.44100.23620.1362

0.47000.26600.1610

0.85350.41390.21850.08940.05450.03540.36670.20030.11750.07090.04270.0274

- 67 -

3:1:4 Crystallisation temperatures of glasses prepared from mould powders.

Values of the crystallisation temperature of mould powders are of use when modelling temperature gradients across the thin slag layer in the mould. Data are obtained from D.T.A. and D.S.C. on glass samples prepared from mould powders and are presented in Table 5 and Figures 24 and 33-36.

TABLE 5

POWDER METHOD TEMPERATURE(°C)

BLA D.S.C. 657C D.S.C. 702D D.S.C. 617V D.S.C. 613D D.T.A. 777G D.T.A. 727V D . T . A . 797

- 6 8 -

F ig u r e 2 4

GLASS D

C r y s ta l l is a t io n d a ta o b ta in e d byD .J.A

- 69 -

3:1:5 Glass temperatures of glasses prepared from mouldpowders.

The temperature,Tg.,marks the temperature region at which dramatic changes in physical properties,such as hardness,are observed.These changes are completely reversible and the transition from a glass to the rubbery state is a function of molecular motion not glass structure. In the rubbery 3tate the molecular chains are in relatively rapid motion, but as the temperature is lowered,movement is progressively slower until eventually the thermal energy availible is insufficient to overcome the rotational energy barriers in the chain.The transition from a glass to a rubber-like state is accompanied by noticeable changes in the specific volume,heat capacity,the refractive index and other physical properties of the glass. The glass transition is not a first order transition in the thermodynamic sense,as no discontinuities are observed when the entropy or volume of the glass is measured as a function of temperature.If the first derivative of this property temperature curve is measured,a change in the region of Tg is found.Thus whilst the change in a physical property can be used to locate Tg,the transition is similar to a relaxation process,and the precise value of Tg can depend on the method used and the rate of the measurement .The method used in this report to determine Tg was Thermal Expansion. A typical thermal expansion curve is shown in Figure 25. An increase of slope in the

C X l E(T) relationship, locates Tg. Results are shown in Table 6 .TABLE 6

Powder C D I K P R T U BLA CB THA

Tg./°C 633 550 620 600 620 595 640 550605 550 620

- 70

temp deg c

Figure 2 5 Thermal expansion of glass t

- 71

Knowledge of thi3 temperature is important as a number of properties

display sharp discontinuities at this temperature, ie. the

Enthalpy-Temperature relationship.Thus the identif ication of the

temperature at which these discontinuities occurs is of use when

developing mathematical models describing heat transfer in continuous (?q)casting' .

3:1:6 Deformation temperatures of mould powders.

As can be seen from Figure 25» at temperatures 20-50°C above Tg. a

maximum occurs in the curve. This is purely a consequence of the failure

in the experimental technique when linear thermal expansion is measured by conventional methods. Due to the viscosity of the sample falling with

an increase in temperature,a point is reached where the glass is so soft that it no longer behaves as a rigid specimen. The sample thu3 deforms under the small force applied to maintain the measuring device in

contact with the specimen. This temperature is conventionally referred

to as the softening temperature or the deformation temperature ,T ,

and is reported in Table 7TABLE 7

Powder C D I K P R T U BLA CB THA THB

T /°C 678 590 660 6*40 600 660 620det. 680 630 675 580 795

- 72 -

Determination of the solidus temperature, (T ),is theoretically possible from D.T.A via the initial deviation of the baseline cf.the data shown in Figures 22 and 23. In practice, the value so obtained is seriously affected by the effects due to sintering. It is for this reason that T was determined from an appropriate point in the data obtained for the cooling cycle. Values of T are presented in Figure 26 and Table 8 .

TABLE 8

Powder A B E F G J Q U Y Z T H A TKB

T _ /°C 905 855 960 935 965 1000 1000 1210 1095 -930 988 912sol.

A feature observed during cooling of powder Q, is the appearance of three definite peaks. This indicates solidification of three distinct phases in the sample. It was hoped that the use of X-ray photography would identify these phases but as reported thus far no correlation has been made. (A more detailed X-ray study will be discussed later.)

3: 1: 7 Solidus temperatures of mould powders.

- 7

3

-

- 74 -

As discussed previously, carbon is added to mould powders to reduce the melting rate. Initial studies were carried out on mould powders with the carbon being removed as detailed in Section 2:2 , as the slag layer adjacent to the mould contains very little carbon. However a number of measurements were performed on the "as received" powders to determine the effect of carbon on various parameters.

a)Melting range of "as received1* mould powders.

Differential thermal analysis measurements were carried out using the "as received" powders and the results obtaining are essentially similar (Fig. 27) to those presented in Figures 22 and 23.

3:1:8 The effect of carbon on mould powders.

Figure 27- 75 -

- 76 -

b)Determination of the carbon type present in mould powders.

As can be seen from Figure 27 ,the exotherms at temperatures up to ca. 800°C are due to the breakdown of carbonates and the oxidation of free carbon in the sample. The exotherm occurring at ca. 500°C is due to an amorphous form of carbon, whilst the exotherm at the higher temperature can be attributed to cokes and/or flake carbon. Values calculated for the amounts of carbon in the powder are 53*7$ and 47.3 respectively. Comparing Figure 28a with Figure 28b (which represents a sample decarburized at 900&C ; yielding an area which represents the total amount of carbon present in the powder), produces areas which do not correlate precisely. This is due to sintering which contributes to the greater area of the exotherm in Figure 28b.

Figure 28

Low Temperature Range

b

- 77 -

The identification of a hydrated phase in crystalline mould powders by X-ray analysis provides evidence that water is present in the powders at

elevated temperatures. The ability to identify the X-ray pattern of

pectolite, indicates that it is present in a substantial amount >_10%,/ Q \cf. Hiromoto et alv . Quantitative measurement of water in flux

mixtures is however extremely difficult. Attempts were made using D.T.A.in a quantitative mode, but due to the small sample size and restrictedsensitivity of the instrument,initial measurements provided no usefulinformation. Evacuation of the D.T.A. furnace chamber coupled withlengthy analysis periods was tried, but these gave erroneous results.However from the infra-red absorption spectra of glasses prepared frommould powders, an absorption band due to the -0-H stretching mode wasobserved (Figure 29). By use of the extinction coefficient reported for

(33)this band , the quantity of water in the sample was calculated. Values are shown in Table 9 •

3:1:9 The water content of mould powders.

TABLE 9

Powder I M P R CB THA THB

H20/ppm 775 685 378 937 684 901 973

- 78 -

The values shown are calculated by use of equation (17) using the band-1 -1at ca. 2.73j jm with an extinction coefficient of 77.00 1 mole cm ,

0 * 7 3 = £ C ©

wherea - absorption coeff.P = Extinction coeff.

_ 1C = concentration (mole 1 ).

This band however is not normally used for such calculations. Usually overtone bands due to water in silica glass, appear at 2.2 and 1.4|jjn, but owing to the large background absorption due to Fe2+ in these samples these bands were not observed.

Figure 29

A typical -OH IR absorption band

- 79 -

This section deals with results for properties which affect thermal transfer in the continuous casting mould.

3:2:1 Heat capacities of mould powders.

Heat capacity data when coupled with the density values of the powders enables calculation of thermal conductivities from reported measurements of thermal diffusivity. This will be discussed in a later section.

Heat capacities for a number of powders were determined for temperatures between 50 and 730°C. Powders were in the form of pressed sintered, and glass discs,ca. 1 mm thick x 0.5 mm diameter Results for the sintered samples are shown in Figures 30,31 and 32. It can be seen that within an experimental uncertainty of +2% heat capacities for slags F, Q and S (Fig.32) can be represented by a single Cp-(T) relationship. Measured heat capacities for powder E are some 2—3% lower compared to the samples of similar form, there are also some inconsistencies at higher temperatures necessitating redetermination.

3:2 Thermal properties of mould powders.

capacity of three casting powders; — _ — , slag 3..............., slag D; _ — ------------ , slag K.

Heat

80

Figure 31

Heat capacity of three casting powders; — .................... , slag Y; ------------- -

— -— , slag X;slag 2.

Figure 32

- 81

Results for the glassy samples are reproduced in Figures 33-36. It canbe seen that the main difference between glass and the sintered samplesis the rapid increase in heat capacity above 800K for the glass samples.This is attributed to crystallisation of the sample. As the sampletemperature is increased, the motion of the atoms increases. At thecrystallisation temperature, nucleation occurs and crystal growth isinitiated. Once crystallisation starts the motion of the atoms isrestricted and the heat capacity falls. Determination of the glas3temperatures from the results of Cp measurement was thought not to be

(34)feasible by the usual method of calculating and plotting the ^ ” 298 vs T relationship from which a transition can be noted by a change in slope, as the data required for H at temperatures above T .can only be obtained by cooling. Thus use of this method is unlikely to produceaccurate results.

82

Figure

33

The heat

capacityof glass B LA.

1030

- 83 -

Fig

ure

34

The

heat

cap

acity

of

gla

ss C

84

Figure

35

T/K

- 85

-

Fig

ure

36

ODTD

The

heat

cap

acity

ot

gla

ss V

i i

U8‘L

- 86 -

A model has been produced using heat capacity data published for oxides and fluorides. The method consists essentially of determining the partial molar heat capacities and the mole fractions for the various constituents of the slag (Cp=x1Cp1 + x2Cp2 + x Cp +***«)» with heat capacities of the pure components being used for Cp etc.. These routine calculations were carried out by computer. Data calculated by this method are plotted with the heat capacities of the glasses produced by powders C, D, V and BLA (Figures 33-36).One can see that the calculated values are in close agreement up to 800K. Comparison of computed values with the experimental data obtained for sintered samples show close agreement at all temperatures.Thus values for the heat capacity of sintered casting powders can be readily obtained.

To obtain an improved correlation between the heat capacity of glassy powders and the computed values,a modification of the heat capacity program will have to be undertaken,allowing for the sharp rise in heat capacity above 800K.

3: 2: 2 Estimated heat capacities of mould powders.

- 37 -

Numerous models have been proposed for the calculation of the melting rates of powders.However these models ignore differences in the enthalpy of fusion of the various powders. Enthalpies of fusion were thus determined to see if any large variations occurred which could lead- to differences in powder melting rate. The heat of fusion for powder C (decarburized),was measured using a quantitative D.T.A. technique.Difficulties were encountered in the experiment and only one measurement was obtained due to attack of the alumina D.T.A. head assembly by molten casting flux.Due to this attack, weighing of the sample after analysis to determine the weight/area endotherm ratio could not be performed.However using data supplied • by the manufacturers weight loss of approximately 19$ is to be expected on decarburization and heating of the powder.Using this figure,values of M28 J (g.powder)~ '

_ iand 528 J (g slag) were calculated for the casting powder ("asreceived") and the slag formed. This measured enthalpy of fusion of the

(34)slag,is of the same order as that reported recently for thecrystalline phases,CaO.7Al202.CaF2 and 3CaO.3A120^.CaF2.Rapid cooling,to produce a glass yields a much lower value for the enthalpy of fusion ca.- 200 Jg“1.

3:2:3 Heats of fusion of mould powders.

Measurements were made on two different forms of the mould powder:-i) pressed and sintered mould powders,ii) glass rods.

Thermal expansion values are needed to calculate the possibility of air gap formation between the copper mould and the slag.However the data are also useful for the calculation of densities at elevated temperatures thereby allowing conversion of thermal diffusivity data into thermal conductivity values.They also provide valuable information concerning the physical changes which occur in the slag.Thermal expansion results are shown in Figure 37 for sintered samples.Hysteresis was observed in all the samples,but was least marked for sample D.This hysteresis is attributed to the healing of voids and microcracks present in the sample

- 88 -

3:2:4 Thermal expansion of mould powders.

and the formation of these imperfections during cooling.

Ther

mal

.E

xp

ansi

on

- 39 -

Figure 37

T/ ° C

if

i

Thermal expansion of three casting powders; ^ ............... , s l a g D ; ,------------------------------------- ;1&K l<

slag B.;

- 90 -

The results for the glass samples are summarized in Table 10.

TABLE 10

Powder C D I K R T BLA CB THA THB%

(xltiS1 heat 97 117 105 105 96 110 93 86 118 1261 cool 93 81 91 99 96 87 91 90 99 130average 95 99 98 102 96 99 92 88 109 128

2 heat 94 99 97 102 96 96 93 93 105 1302 cool 92 102 91 92 95 80 93 89 89 130average 93 100 94 97 96 88 93 91 97 130

A typical thermal expansion-temperature plot {6K (T)} is shown inLt>Figure 25. The relationship is smooth and not quite linear up to the sample’s glass temperature,(Tg.),at this temperature there is a discontinuity in the slope.

Upon cooling from the deformation temperature T ,the slope of<£><T„-T relationship is slightly different to that shown during the heatingcycle.

There arc a number of reasons for this difference

a) devitrification may have been initiated in thesample,therefore the structure of the glass on cooling i3 different to that v/hich underwent heating.

b) measurements on glass materials are time-dependent depending also upon the thermal history of the glass,for example annealing temperature and annealing time. Therefore values are hard to duplicate and slight changes in the parameters of the glass are to beexpected.

- 92 -

Casting powders at elevated temperatures belong to a class of materials known as semi-transparent or diathermanous.For these materials heat conduction can occur by a process called radiation conduction or internal radiative transfer.During this process,radiant heat incident on the surface of the material is transferred through thin layers by radiation as opposed to direct radiation.The effect is dependent upon the product of the thickness (d) and the absorption coefficient (&<) of the material viz.the optical thicknesstO C d.

As mentioned previously the apparent thermal conductivity will increase until the condition C><d>3 is attained at which point the sample is considered to be optically thick.For the slag layers formed in the mould and for the samples used in the determination of thermal diffusivity the condition of optical thickness may not be fulfilled and therefore an unknown proportion of total thermal conductivity could occur by radiation conduction.Therefore a knowledge of the magnitude of C X ,the absorption coefficient is important in attaining an understanding of the heat transfer in the mould and the meaningfulness of experimental thermal conductivity values. For an opaque medium the emissivity is given by the ratio (E^/Eg),where E is the rate of energy radiated per unit area and the subscripts denote the surface in question and a black body respectively.For a diathermanous material, energy can be radiated by reflection from the base of the container in addition to that which is radiated from the surface and the bulk of the medium.The general relationship which describes the radiation emitted normally from the surface of a diathermanous material is given by equation (18).

3: 2:5 Emissivity of mould powders.

- 93

Es B(l - r b exp -2gd) I1-rs rbexp-2ad)

cx

d

= total normal emissivity = reflectivity of the medium = reflectivity of the base= relative absorption coefficient of the medium = depth of medium

If r*b = 0, equation (18), can be simplified to:-

This condition is achieved by studying the radiation emitted from the surface,above a submerged blackbody. From equation (19),it can be 3een that the introduction of polished inserts of known reflectivity at varying, depths below the medium surface would alter the emitted radiation if the medium was semi-transparent to I.R. radiation.By solving simultaneous equations, values of£X can be obtained.

Emissivities of liquid and solid fluxes of varying thickness were measured with the apparatus shown in Figures (17),(18) and (19).

However the iron crucibles melted and oxidised during measurement using

an argon atmosphere. Trials were repeated using a Hylite (99*ArI H ) atmosphere, and under vacuo with no success.Breakdown of the iron crucibles occurred at temperatures ca. 1100°C.

Emissivities were measured by use of the cells shown in Figures (18) and (19) enabling the emissivity of liquid and solid fluxes to be obtained.Results are shown in Tables 11 and 12.

TABLE 11

Powder Temperature Erai3sivity

(°C) (f )

P 1154 0.911154 0.99

Measurements carried out using a carbon cruciblea Hylite atmosphere.

1090 0.921190 O'. 92

C 1270 0.921400 0.89 •1445 0.88

Measurements carried out using a molybdenum crucible in a Hylite atmosphere.

- 95 -

TABLE 12

Powder Temperature Emissivity(°C) (<£Trj)

759 0.95860 0.94

P 910 . 0.96966 0.92

Measurements carried out using a carbon plaque under vacuum

858 0.90893 0.89

U 930 0.91977 0.99

THA 792 0.99

Measurements carried out using a polished stainless steel plaque under vacuum.

As can be seen from Tables 11 and 12,the measured emissivities for all the powders tested were large.The inability to view the blackbody beneath the sample,precluded the use of equation (18) to calculate the absorption coefficient,therefore an alternative method was devised to measure the absorption coefficient.

- 96 -

3:2:6 The absorption coefficient of mould powders at room temperature.

In view of the shortcomings of the previous method for determining the absorptance of mould powders,a method was devised in which thin sections of glass could be produced to determine their absorption characteristics.From information in extant literature ^^\it wasconcluded that the spectra at ca.1300°C was similar to that at ambient temperatures.Samples were prepared as detailed in Section 2:2:1.Values of the absorption coefficient at room temperature were determined for the wavelength region 0.3-5/x/m by use of equation (20).

d

= the spectral absorption coefficient= the spectral transmittance = thickness of sample

Results are shown in Figures 38,39 and 40.

- 97

Figure 38

A

3a

cm •*1

2

1

1 2 3 AA/[J.m

Absorption spectrum for casting powders at 25°C; , THA;CB. .

- 98 -

Figure 39

acm

bsorption spsctrum for* cssting pov»dsrs st 25 C }

- 99 -

Figure 40

T

acm -1

1 -

1 2 3 4

\ i [im

Absorption spectrum for casting powders at 25°C; R;-- , M.

, THB,

100

There is a wide variation in the value of the absorption coefficient with wavelength.Simple mathematical relationships which describe combined modes of heat transfer ie.equation (13),require an average absorption coefficient which can adequately describe the absorption over the wavelength region in question. The calculation of an average absorption coefficient for this study is discussed in the following section.

3:2*.7 The calculation of average absorption coefficients.

As already described in a previous section,there is a specific wavelength range over which radiation in the mould is important (if at all).This range is dependent upon temperature and is much broader for lower temperature ranges involved;for 1500°C the range was determined to be I.S- .b tm (Section 2:1:7).For the purpose of calculating an average absorption coefficient a method of weighting has to be employed. The method chosen involved the determination of the energy for small wavelength increments (0.5jllw) from Figure 20 and multiplying this by an average absorption coefficient over the corresponding wavelength interval.The product is summed and the total is divided by the total energy between 1.5 and k.5n,m.

101

Mathematically the process is detailed in equation (21).

a

Results are shown in Table 13.

TABLE 13

Powder I M P R CB THA

£?< /cm”^ 2.11 1.78 0.71 3.48 1.93 2.66

— 1cf.silica glass = 0.2cm

THB

4.28

102

The prime absorbing agent in glasses prepared from casting powders is the ferrous ion.To evaluate the effect of furnace conditions on the ferrous/ ferric equilibrium,a glass from powder P was prepared under various conditions

i)by fusing an "as received powder" in a carbon crucible with a carbon lid.This created a reducing atmosphere as the crucible is slowly oxidised during heating.The crucible was then transfered to another furnace ca.600°C and poured into a carbon mould held in the furnace.The sample was held at ca. 600°c to anneal for thirty minutes, and furnace cooled.

ii)conditions which were regarded as oxidising were produced by fusing a decarburised sample in an alumina pot in air.

iii)conditions which were regarded as a compromise between those outlined in i) and ii) are detailed in Section 2:2:1.

Comparing the transmission spectra for the three samples,revealed no major differences (+2%) in the transmission spectra.

2+ *5 j-From these observations one deduces that either the Fe /Fe is unimportant or that the experimental conditions affected the total iron concentration in some way.It was found in some experiments that liquid iron had been formed from the reduction of the iron oxide contained in the powder when heated in a carbon crucible. This iron formation would explain the similarity in the absorption coefficients for glasses produced under different oxygen potentials.

103 -

3:2:8 Determination of the ferrous-ferric ratio from the U.V. spectra of mould powders.

As can be seen from Table 13,the average absorption coefficient of glassR is relatively high,taking into account the iron content of the powder(Table 1).This increase in absorption is related to the ferrous/ ferricequilibrium.From a knowledge of the mass of the glass subjected to I.R,and thus by application of the molar extinction coefficient for Fe +which according to Steel and Douglas ' has a value of 28.90

-1 -1lmole cm , one can calculate the amount of ferrous iron present in the sample, a3 the Beer-Lambert law is obeyed for small concentrations of iron (less than 5/5 total iron). By reference to Table 1,a value for the total amount of iron (Fe +) present initially can be calculated,thus data, relating to the redox behavior of the casting powder can be obtained. Initial impressions seem to suggest that the composition of powder R tends to favour formation of Fe^ions over Fe'i+ ions.This is confirmed by reference to Table 14. As it was not possible to produce a representative base glass which could yield data referring to 0% iron,the base absorption due to a fused silica glass,Spectrosil B,was used in order to calculate the true transmission through the sample.As this transmission was of the order of 2-355,it was deemed to have a neglible effect on the overall data.

104 -

TABLE 14

Glass cx

ojt

TP 2 +Fe Fe3+ Fe3+ rre

/depth mass 0.875/fm measured equiv­alent

initial converted%

cm g cm"* { gx106 } x103

THA(0.196)

2.805 1.930 1.184 1.303 6086 21

M(0.233)

3.247 5.619 4.099 4.509' 4545 99

CB(0.398)

5.641 3.110 3.876 4.264 19740 22

P(0.520)

7.431 1.579 2.571 2.828 57220 5

R 3-627 7.030 5.550 ' 6.105 5077 120(0.252)

All samples had a diameter of 25mm.

105

D is c u s s io n .

The o b je c t iv e s o f t h i s s tu d y w ere to

i C h a r a c t e r i s e p r o p r ie t o r y c a s t in g powders to o b ta in

an u n d e rs t a n d in g o f t h e ir p r o p e r t ie s and to p ro v id e a

d a ta base f o r f u r t h e r s tu d y .

i i ) t o i s o l a t e th o s e fundam ental p r o p e r t ie s o f c a s t in g

powders w hich p la y an im p o rta n t r o le in d e te rm in in g the

mode o f h e a t t r a n s f e r o c c u r in g in the m ould.

And th u s

i i i ) t o o b t a in v a lu e s f o r tho se p r o p e r t ie s w hich

in f lu e n c e h e a t t r a n s f e r in th e co n tin u o u s c a s t in g mould

w hich c o u ld be u se d in subseq uent m a th e m a tica l m odels

d e s c r ib in g th e h e a t t r a n s f e r i n the co n tin u o u s c a s t in g

m ou ld .

106 -

4 Characterisation of mould powders.

4:1 Phase analysis.

The difficulties of X-ray analysis of casting powders was mentioned in brief in the results section of this thesis.This section is to present a more rigorous interpretation of the various investigations into this method of analysis for samples of this type. A basic problem encountered in determining the phases present in these powders was the difficulty in preparing cuspidine,gehlenite and the other phases which could be present,in a pure form from Analar reagents. These efforts were abandoned after considerable time and effort and samples of mould powders were dispatched to Newcastle University where greater expertisecould be drawn upon.

107 -

To produce an e f f e c t i v e c o r r e l a t i o n between the r e s u l t s o b ta in e d by

X - r a y d i f f r a c t i o n and r e s u l t s o b ta in e d i n therm al s t u d ie s the

p r e p a r a t iv e method employed i n p ro d u c in g sam ples f o r X - r a y s tu d y i s

e x t r e m e ly im p o r t a n t .

Newcastle University produced data from samples which were heated at the required temperature for one hour,rapidly cooled, photographed and then reheated at a higher test temperature (intervals of approx. 100°C) up to fusion.,rAs-received” refers to powders supplied as prepared in Section 2: 2 .

/ Q \Hiromoto et alv ' prepared their samples in the following manner.Ten grams of mould flux held in a crucible was heated for ten minutes at the required test temperature.The sample was cooled at room temperature, ground and analysed.Bearing in mind compositional differences between the powders used in these studies,phases which can be identified in the latter study are of a transient nature as compared to the data presented In Table 15 from Newcastle University.The holding period employed in their investigation allows a greater approach to equilibrium conditions.

S a m p le Temp.°C

C usp id iqa C a rn e g e ife -W o llaston ifeC ^ i f i n a s 2 c s

K as rec 709797910

1000*HOO*

X XX X - AX X X XX x -

P asrec.709797

. 910

X - X X ■ - - X X - X X - X

10001100

X ~ X ? - ?

Q as rec. ■ 709

797 910

10001100

XX

~ - X - - X

_ xX

■o X

G e h la n ife Pecfolite N C S 3 NC^S^ NaF UnkncwnPhase C2 AS N Q ^ H

x 1x i

- . - - x 1 -

xX

XXX

XXXXX

??7XXX

XXXXX

TAB

LE

15

TAB

LE

15Sample Temp. Cuspldlr)Q Carnegeif'e Wollastonife Gehlenihe Pecfolite n c s 3 NGjS, NaF Unknown Phase

°C C ^ F l ' n a s 2 CS c2a s NQ*%H

A as rec. X X _ . . . X X _

709 X X X X809 Y y Y V698

AX X — — A

XAX —- — —

1005 X X __ X X ,n-‘, — __ — -1109 X — — '— — — — ■ — —

~C as rec. -X — X — X __ 4____ _709 X — X — — X ■ — — —

809 X ___ X _ . — X __ __ __

898 X __ X X __ ___ __1005 X — X — X — — —

1109 X — X X X' — — — —

i C8 as rec X — X ___ X ? ___ - __

709 X — X — X X — — —809 X ___ X __ X x _ _ _898 X — X ___ X X . n - , ■1005 X ___ X _ X X1109 X — X ? X — — —

Sample

81

8 LA

V

Temp. Cu s p id in e , Carnege°C NAS-

as rec. X X7 0 9 X X809 X X898 X X10C5 X ______

1109 — .

asrac. X _

709 X —

809 X —

698 x ____

1005 X — -

1109 ' —.A

as rsc. 7 0 9

X -

777 X710 X1000 XMOD X

Qehlenihe Pedolife NCS S2 NaF Unknown Phasec2 a s nq^ h

XXXXX‘X

4/,-r4L

— I>CDI— rr\

cn

x

01

?7

?•*>

3x? x

The a fo rem e n tio n e d method o f p r e p a r a t io n o f sam p les i s d i f f e r e n t to the

(1 2)sample p r e p a r a t io n c a r r i e d out by L a n y i and Rosa , Bagha and

G r i e v e s o n ^ ^ and t h i s i n v e s t i g a t o r ( T a b le 2 ) . The p r e p a r a t iv e method

(2 9 )used by Bagha and Grieveson and this investigator was to crystallise samples from the melt. This method produces phases which can be regarded as being preferentially precipitated from the melt,thus this situation can be regarded as representing the situation existing between the mould wall and the steel 3trand,whereas the techniques used by Hiromoto

/ o \et al and Newcastle University can be regarded as relating to the powder and sintered layers on the surface of the steel protecting the surface from the atmosphere.

(3 2 )The technique used by Lanyi and Rosa was a "quick-chill" method inwhich molten flux was poured into a steel container and the cooledmaterial was subjected to X-ray analysis.This method is somewhat similar

(29 )to to the method used by Bagha et al (32)Lanyi and RosaVJ identify four phases in their investigation; calcium

fluoride,sodium fluoride,cuspidine Ca4F2Si2°7^, and xenolite(CagSi^0^(0H)2) .They found no evidence of any alumina phases.These conflicting results suggest that alumina containing phases are formed after the initial cooling process.

The data provided by Newcastle University (Table 15) suggest that:-i) powders with a fluoride content of greater than 1.4$,have cuspidine as an integral part of their matrix.

ii) pectolite can be present at elevated temperatures ca.1109°C.

- 111 -

University, it was thought that a correlation may be achieved betweenthe data due to X-ray studies and those determined by thermal analysis.However,due to the rate at which the D.T.A equipment was operated (10°C

-1min. ),it means that the phases formed during the heating cycle of a D.T.A run were of a transitory nature as compared to the treatment of the samples used in X-ray analyses.The extended heating period thus allows a greater approach to the equilibrium state.Therefore any correlation made between these two sets of data is open to critisism.

Due to the manner in which the samples were treated by Newcastle

113 -

The determination of crystallisation temperatures of mould fluxes by the use of D.T.A needs the use of glass forms of mould powders. If any endotherms were obtained during a run these could be attributed to the formation of the various crystalline phases in a similar manner to the way the phases are created in the investigations of Bagha and Grieveson

and for those phases presented in Section 3:1. (Table 2).However if any endotherms were present these were masked by the deviations due to the glass and crystallisation temperatures.Despite the difficulty in the use of X-ray techniques to identify the various phases present in these powders,it has proved crucial along with infra-red spectroscopy to verify the presence of water (OH) in these powders at high temperatures (>1400°C).Pectolite (NC S H) is presumed to decompose at temperatures greater than 600°C (Eitel^^) .This decomposition leads to the formation of silica,wollastonite and 2Ca0.Na20.SSiO .However,the reaction product ’’CaO.Na O. SSiO " which wa3 found in some powders,did not seem to be formed from the breakdown of pectolite (cf. Powders A, U and C in Table15 ).

- 114 -

The melting range of a powder is of importance as this is a property which needs to be considered when selecting a particular powder for a specific casting operation.D.T.A. as explained earlier has been adopted as the method for the determination of a powder’s melting range.Results from these studies, show a good correlation with other workers and the data supplied by the manufacturers (Table 16),in the determination of the liquidus temperatures of the powders. As discussed previously the other methods which have been used to determine the melting points of these powders have been based on the method of Seeger’s cones.This method has limitations especially if the powders contain large quantities of carbon,which may not have been totally removed in the pretreatment stage.

4:1:1 Melting range of mould powders.

Powder

BCDI

JKMP

QVUBL

BLACACBDA

- 1 1 5 -

TABLE 16

Liquidus Point /°CThis study I.R.S.I.D. B.S.C. Manufacturers+ 10 C

1212 1215 — 11601276 1260 1275 12601192 — — 11651212 — — 1182

1215 — 1250 —

1133 1125 — —

1212 — — 11651187 — — 1165

1125 — ------------ . 12701216 — — .11501223 1230 — —

1222 — — 1250

1118 — — 12601244 — — 12101228 — — 11901202 — — — — 1145

- 116 -

The role of carbon in mould powders has been discussed in the literature survey of this report.The belief is widely held that is the formation of the so-called carbon skeleton upon heating of a casting powder which affects the overall melting rate of the powder. However from research carried out in support of this investigation an alternative view of the effect of carbon is proposed.

It was found during preliminary experiments,to produce melts for the casting of glasses,that an ,,as-received,, powder contained in a carbon crucible with a carbon lid at temperatures far in excess of its fusion point,could be poured from the crucible in an apparently unchanged form ie. spherical carbon coated particles.This of course cannot be true,the powder must have fused and been retained by the carbon coating,which owing to the experimental conditions applicable in the furnace cannot oxidise. All casting powders exhibit a melting range which is quite extensive in several cases.Under conditions of a temperature gradient,the first melting liquid will drain from the solid causing variations in the liquid composition. Thus far from producing a carbon skeleton the role of carbon is to inhibit the agglomeration of the individual liquid flux until complete melting is achieved.Therefore reviewing the situation which can occur in the mould,it seems feasible that this carbon contained in the powder creates an environment in which the liquid formed from a powder produces a homogenous liquid layer.

4:1:2 The effect of carbon on the fusion of mould powders.

H : 1 : 3 V i s c o s i t y o f mould powders.

The viscosity of a powder is thought by workers on plant to be crucial, as it thought that it is this property (actually fluidity,reciprocal viscosity) which determines the speed at which flux is supplied to the gap between the mould and the steel strand.The estimated viscosities presented in the Results section have been calculated using constant sets reported by C.R.M. and I.R.S.I.D. Both are based around a form of the Weymann equation used by Lanyi andRosa^^equation (22).

7] - 1-8x10 ^exp 12-2Xc.„ + NXV

SIO A l2°B

where RT

T ) = viscosityX _= mole fraction of silicaa S 0 2X ai n = mo e fraction of aluminaa 7uBN = silica equivalence factor for alumina

'Equivalence factors for a number of oxides have been calculated by C.R.M. and I.R.S.I.D. and by use of a computer program viscosities for a wide range of compositions at various temperatures can be rapidly calculated.The viscosities thus calculated are estimated to be within +30% of their actual values.A rotating bob viscometer' is currently under construction at N.P.L. which will be used to measure the viscosities of mould powders at temperatures up to 1600°C which will allow comparison between these estimates and experimental data.

118 -

4:1:4 The "structure” of glasses prepared from mould powders.

Furtherto the. measurement of the absorption coefficient and thedetermination of the water content of mould powders,structuralinformation can be gleaned from the spectra of the glasses (Fig.4l).

2+ 34-Bands due to the absorption of Fe and Fe ions are observed for certain powders (THA.M.etc.).These bands are well documented,however due to the presence of modifying ions in the glass matrix,these bands do not

/ o o \occur at the precise wavelengths quoted in the literature0 .

Figure 41

Thus the band at ca. 0.

3 r5

co-ordination assumed by an ion,the radius of the ion,the radius and thepolarisability of the ligands,the radius ratio and ligand fieldstabilisation. Ligand field stabilisation and the radius ratio rulespredict that for ferrous ions surrounded by oxygen ligands,octahedralco-ordination is prefered.The application of ligand field theory

24-predicts a single absorption band at ca.I^m in the spectrum for Fe in octahedral co-ordination.

transition r 5 -

9 jjum has been assigned to the ferrous ion .Various factors contribute to the

Difficulties arise in assigning a co-ordination to the ferric ion asthis ion can exist in tetrahedral and octahedral co-ordination. The bands which are indicative of tetrahedral ferric co-ordination are predicted by ligand field theory to occur at 0.500,0.446,0. H27 and 0.38Cy m.The ultra-violet cutoff due to the Fe + and Fe + ions in these glasses makes the identification of absorption bands due to iron very difficult to isolate below 0.450 , m.As can be seen from Figure 41,only two bands are observed ca.0.440 and 0.480 , m,therefore a co-ordination for the ferric ion cannot be assigned with such authority as wa3 the co-ordination for the ferrous ion.Nevertheless as suggested by extant literature^^\ the ferric ion is probably in tetrahedral co-ordination. However in spite of the extensive studies made with regard to the 3tate of dilute iron in glasses,there are still some disagreements in assigning the co-ordination number and the predominant oxidation state in the different glass systems.

120

The crystallisation temperatures of a number of mould powders were determined by D.T.A and D.S.C.Values obtained are in good agreement within the experimental limits.These data are of use when proposing mathematical models describing thermal transfer in continuous casting. Glass temperatures are also theoretically available from both experimental methods,however the glass transition is of a much lower intensity than other transitions and thus cannot be determined from the data collected for calculation of heat capacities.By increasing the sensitivity of the D.T.A equipment a glass temperature for powder V was obtained ca.700°C.This is in the same temperature region as the glass temperatures obtained by thermal expansion measurements, (Figure 24 and Table 6). .

4;1;5 Glass and crystallisation of mould powder glasses.

121

This factor is becoming of increasing importance in relation to cracking(S9)in continuous casting.Gray states that there is a relationship

between the moisture level and the incidence of cracking for plate grade steels.Investigation of this feature by B.S.C is at a preliminary stage. The aspect of water contained in these powders was given little attention, as it was regarded that any moisture present in the powders would simply evaporate due to the high temperatures in the mould.Water was found to be present in samples treated in a similar manner to the systems operating in the mould ie.the X-ray studies reported by Newcastle University and those presented in Table 2.This water which is availible at high temperatures can react with the carbon present in these powders producing hydrogen by the following reactions

h2o + C = h2 + CO

h2o + CO = co2 + h2

This hydrogen could ultimately cause hydrogen embrittlement in the steel, which is deleterious to the final product.

l | : 1 :6 The w ater content, o f mould powders.

122

standard sample from Analar materials which would enable the background(33)transmittance to be calculated .A standard silica glass was

obtained,but this was found to have a water level which was too high for the instrument to record (Fig.42). Nevertheless the base transmission of the silica glass was used in order to calculate the true transmission due to the sample.As this base transmission was of the order of 2-3% it was ignored from the the calculations a3 it was thought that it would not greatly influence the value calculated for the moisture content of the glasses These calculated values are thought to be within +10$. Despite the standard silica sample having a moisture level too high to determine,it locates the wavelength at which the absorption due to "free hydroxyl" groups (ie. water which is not chemically bonded) in a standard glass occurs.For the case of mould powders,this absorption band occurs at a higher wavelength,ca.2.90 Lm as compared to 2 . 7 3 in the standard glass.Additionally the band covers a wider range.Florence et al^ ^ suggest that this shift in the absorption peak gives an indication of the strength of the bonds which maintain the water (OH) in the matrix.lt was found that the further the peak is displaced towards the higher wavelengths, the greater is the association of the OH molecules in the glass. As can be seen from Figures 29 and 42-44,it would seem that the -OH is tightly bonded into the glass matrix.

Due to the wide range of powders,it was not possible to produce a

123 -

Figure 4 2

Figure 4 3

W ength/ p_mF ig u re 44-

124

Attempts to measure values for the water content by a quantitative mode of D.T.A proved unsuccessful,the high sensitivity settings required and the difficulties in obtaining calibrants for the equipment do not favour the use of this method. An experiment was devised in which a casting powder (P) was fused using a vacuum furnace. The sample (contained in a 30cc alumina crucible) was heated to ca.l400°C under vacuum,and held for a period of two hours.The sample was then rapidly cooled to ca.600°C,held at this temperature for thirty minutes and furnace cooled to ambient temperature.The sample was sectioned,producing a circular sample.This sample was ground and polished in a similar manner to that described in Section 2:2:1 and subjected to infra-red analysis.The region under investigation (2.7- 3-0j m) was devoid of any absorption due to OH.However the sample was subject to a large number of cracks which reduce the accuracy of this experimental method.Despite this inaccuracy,the level of water in the sample was reduced by this method of treatment,but calculation of the water remaining in the sample could not be undertaken due to the unavailibility of a sample which containedno water which could be used as a reference.

125 -

Jf: 2 Thermal properbi.es of mould powders.

These data are directly of use in mathematically representing the heat transfer mechanisms operating in the mould.A balance can thus be constructed of the relative importance of the three major heat transfer mechanisms which can occur in the continuous casting mould,i) thermal conductivity; ii) direct radiation and iii) radiation conduction.

4:2:1 Heat capacities of mould powders.

As can be seen from Figures 3 ,35 and 36,values for the heat capacities of mould powders can now be estimated by U3e of data contained on a computer program.This program calculates values in good agreement with those measured for sintered samples and for glass samples up to their glass transition temperature.To account for the sharp rise in the heat capacity of glass samples above their glass transition the model will have to be modified.

H:2.*2 Thermal diffusivity of mould powders.

In combination with the heat capacity data,values for the thermal conductivity,(K),can be calculated by application of equation (23).

wherea = thermal diffusivityCp = heat capacity

= density

- 126 -

Values for the thermal diffusivity-temperature relationship have been supplied by Dr.R.Taylor of U.M.I.S.T.The technique by which these values were obtained,has been described in an earlier section. Values have been measured for sintered and glass samples,the data obtained will bediscussed separately.

(41)Extant literature indicates that a reduction in diffusivity can occur as a result of incomplete sintering and microcrack formation. Thus the degree of sintering is an important factor in determining the final value measured for the thermal diffusivity.However,partial compensation of this effect occurs as a consequence of gaseous conduction across gas filled pores and cracks.As a result of these effects,values for the thermal diffusivity determined in a gas atmosphere will be higher than those measured in vacuo (Figures 45 -48).These features can be seen in the thermal diffusivity- temperature relationship of sintered powder D,depicted in Figure 45.

Fjgure 45

-127-

4:2:3 Thermal diffusivity of sintered mould powders.

107a /

rris-'

l07.a

/ m2

- 128 -

F ig u re 46T/°C

Therm al d i f f u s i v i t y o f s l a g A; sp ecim en s i n t e r e d 26 h ou rs a t 775°C , i n v a c u o , l-H -l- ; i n h e liu m , ; sp ecim ens i n t e r e d f o r a f u r th e r 16 h ou rs a t 850°C j i n v a cu o , **** ; i n h e liu m , —— ; > , h e a t in g c y c le ; , c o o l in g c y c le *

i

F ig u re 47— -

- 129

Figure A#

Therm al d i f f u s i v i t y o f s l a g K; a l l sp ecim en s s i n t e r e d 2.6 hours a t 900°C ; i n v a cu o ; ru n 1 , **** ; run 2 , “ ** “ ;i n h e liu m ; run 1 , i H - ; ru n 2 , ------- - ; > , h e a t in g c y c le ;4 . , c o o l in g c y c l e .

Various features are seen in the results presented for powder D (Fig.45) which has been sintered for 26 hours at 900°C:-

i) hysteresis.ii) higher thermal diffusivity values in helium than those obtained in vacuo.iii) further sintering (ninety minutes at ca.900°C) results in a higher value for the thermal diffusivity.

- 130 -

Results of this nature indicate that the sintering of the sample was incomplete.To allow for this,a batch of specimens were sintered for a further three hours at ca.950°C.This produced the following1) thermal diffusivity increased by 205L

2) identical values for the thermal diffusivity on the heating and cooling cycle in helium.3) identical values for the thermal diffusivity for the cooling cycles carried out in vacuo and in helium.

These results indicate that:-a) sintering is complete.

b) microcrack formation does not occur above 30ti°C in powder D.

As can be seen from the data presented in Figures 45-48, incomplete sintering and microcracks are all features in the values obtained for the thermal diffusivity of powders,A,B and K.The thermal diffusivity-temperature relationship shown in Figure 46,for powder A, show maxima at 330 and 430°C and a minimum at 380°C.D.T.A measurements for this powder did not indicate the presence of any transitions occuring at these temperatures.This suggests that these peaks are due tomicrocrack formation.

131

The thermal diffusivitie3 of a number of casting powders in the glass phase have been determined and are shown in Figures *19, 50 and 51.

Figure 49

Jt: 2: 3 Thermal diffusivity of glass mould powders.

c | ig i

----- -A— A -A . . 4- - .

Ap-Z* m r t r TT^u

5

4

3

4- 4* i,,4^rAA ----Z&— ■■■ -A- \— — 4 -4 r

THB

400 600 800

Temperature/°C

UJ/'D .01

132 -

Figure 50

iinCM

Temperature/°C

• 107 Q.

/m

133 -

Figure 51

iinIN

A b -4.—-♦A— f—BLA

•y'Art

figt400 600 800'

Temperature/°C

- 134 -

The following observations were made

i)the thermal diffusivity for all the powders tested were similar.—1 2 -1Values varying between 4-5 x10 m s

ii) for the glasses,K,M,U,CB and THA,a sharp decrease in the thermal diffusivity was observed in the region of the glass transition temperature.

iii) for the glasses K,M,U and possibly CB,the decrease mentioned above was accompanied by an enhanced thermal diffusivity during the cooling cycle.This feature was also observed in the data for THB which exhibited no irregularities in the a(T) relationship during the heating cycle. These enhanced values which are observed during, the cooling cycle can be explained by two theories

1) microcracks which occur in the sample are cured during the heating cycle and thus an increase in thermal diffusivity occurs upon cooling.

2) the thermal diffusivity of the crystalline modifications of the powder is higher than the value associated with the glassy phase. Therefore the nature of the sample which undergoes the cooling cycle contains different phases as compared to the original sample which undergoes heating.

135 -

T h is l a t t e r th e o ry i s re g a rd e d a s g i v i n g the b e st e x p la n a t io n o f the

o b serve d f a c t s .

Thus the factors investigated in this study which were deemed instrumental in determining the magnitude of the thermal conductivity of a powder, were found to be of the same order,therefore the differences which are noted between powders in terms of their ability to prevent longitudinal cracking and/or breakout may be explained by a powder’s ability to (or not to) transfer heat by radiative means.

4;2j4 Thermal diffusivity of molten flux.

Preliminary values have been obtained for the thermal diffusivity of themolten powder M,using an adaptation of the laser pulse technique (27).The results are shown in Figure 52.The thermal diffusivity

-7 ? -1increases from 5 to 9 x10 m's as the powder becomes molten. Thisincrease is equivalent to a rise in thermal conductivity of ca.1.5

-1 -1Wm K .This increase can be attributed to either an increase in thephonon conduction (K ) or to an increase in the radiation conductionc(K ).The relative proportions of thermal conductivity and radiation conduction will be discussed in a later section.

- 136 -

F ig u r e 5 2

The thermal diffusivity of molten powder M

137 -

4:2:5 Thermal conductivities of mould powders.

Thermal diffusivity data can be converted into thermal conductivities by application of the relationship described by equation (23).

K = S C p O ©

This calculation has been performed for the sintered samples B,D,and K. The densities ( p ) of the three samples being 2.00,1.99,and 1.98 g m“ at 25°C respectively.The values calculated for B and K show an increase with temperature as one would expect for these types of material (Fig.53).

1

TE

5

However the k(T) relationship for D exhibits the reverse trend.The plotalso shows two discontinuities ca.500 and 700°C.These data do notcorrespond with the theories currently proposed to describe thermal

(42)conductivity in these and similar materials .Some re-evaluation ofthese results will be necessary at a later date.

- 138 -

Thermal conductivities for glass samples have also been calculated by use of the thermal diffusivity and heat capacity data presented in this thesis.The k(T) relationship for the three glasses shown in Figure 54,BLA,C and V display a similar trend to the data presented in Figure 53,an increase of thermal conductivity with temperature. However for this form of sample,a maximum occurs in the K(T) relationship corresponding to the crystallisation temperature of each glass.

Data presented in this thesis for thermal conductivities and thermaldiffusivities obtained by Dr.R.Taylor (U.M.I.S.T) compare favourably

(23) (43)with those reported by Fine et alv ,Gieger and Poirierv and Riboud(1) (44)et al .These data indicate that differences between powders are

not due to any major variation these powder’s experimentally determined thermal conductivity. Thus other properties seem to be a factor in producing the variations noted between powders.

THEfm CONDUCTIVITY OF POWDERS^ BLA.CANDV.

ioov_0

I

Figure 54

- 140 -

4:2:6 Thermal radiative properties of mould powders.

The previous section has shown that the differences in thermalconductivity of these powders cannot explain the wide variation inobserved' properties.Therefore it is possible that these differencescould be due to the amount of heat that may be transfered by radiativemechanisms. As is shown in the Results Section,the composition of apowder affects the position of the ferrous/ferric equilibrium.lt is well

2+ R+reported that it is this Fe ' —-y FeJ transition which has the greatest influence upon the value of the overall absorption coefficient displayed by a glass,and this influence is also apparent in these powders. An increase in the amount of ferrous ion reduces:-

1) direct radiation.2) radiation conductivity.

Thus affecting the overall heat transfer coefficient of .the powder.

Therefore powders which contain large amounts of iron oxide >5$,can be regarded as opaque to radiative heat processes and thus the can be viewed as transfering heat solely by thermal conductivity.

im

l \ : 2 : rf The ferrous-ferric ratio apertaining to mould powder

glasses.

The amount of ferrous iron calculated from the U. V spectra of these glasses does not represent the equilibrium situation.The presence of carbon produces an additional time dependent reduction effect.Here now follows a rigid definition of the conditions apertaining to the results presented in thi3 thesis.

30cc.of the "as-received” powder contained in an alumina crucible w (\S

heated in a furnace from 500 to 1*l00$.over a period of two hours. The powder was maintained at this temperature for one hour and then cast in air into a circular carbon mould (ca.25mm dia.).The cast glas3 was annealed by placing in a furnace held at 600°C for thirty minutes and furnace cooled. The glass was surface ground producing parallel faces and samples of the required thickness.The faces of the sample were polished to eight microns using a diamond lap.In addition to this procedure all samples . were prepared singularly,thus the carbon which influences the furnace atmosphere is solely due to the amount present in the initial sample.The overall influence this carbon has on the ferrous/ ferric ratio is obviously dependent upon the length of time it takes the carbon to burn off.For example it may take two hours under these experimental conditions for the carbon in powder I to be removed and therefore the ferrous/ferric ratio would be expected to be different when compared to a powder whose carbon is removed in a shorter time.

Visual examination of the specimens in terms of their colour,with reference to their associated spectra leads to an indication of thepredominant valence state of the iron.

- 142 -

It is well known that the colour attributed to the Fe state is blue or3+green and to the FeJ state brown or yellow.Therefore glasses which

contain a higher proportion of either ion will tend to give rise to its associated colour.Visual examination of the five glasses thus far analysed spectrally for their iron content revealed the following correlation between their

2+

spectra and colour.

GLASS COLOUR ASSUMPTIONS

R GREEN Fe2+»Fe3+

M ' pale GREEN Fe2+>Fe3+

THA CLEAR Fe2+=Fe3+

P pale YELLOW Fe2+<Fe3+

CB deeper pale YELLOW Fe2+<Fe3+

In addition to these assumptions a basic correlation between the background spectrum and the overall depth of colour could be made.It was found that the highest background transmission levels (bearing in mind the thickness of the samples) were found for glasses R and CB,this background transmission was found to decrease for glasses M and P,whilstTHA had the lowest relative background transmission, corresponding to the weakest colouration.Quantitative measurements of the total iron content of these glasses are being performed at the time of reporting.

H: 2:8 An analysis of the flux layer occuring in the continuouscasting mould.

Ecfore a detailed discussion of radiative transfer a knowledge of the material through which the heat is transfered is required. Detailed analysis of the situation occuring between the strand and the mould has lead to a complex model being formulated, Figure 55. Due to thetemperature gradient across the flux,various boundaries are formed eg.glass/crystalline/liquid.Obviously there are no definite cutoffs but agradual merging of the layers.This situation has been verified by

(45)Riboud and also by research carried out for a similar situation in the glass industry^*^.

Copper toould

Glassy slag

Molten steel

Crystalline layer of slag

Steel shell

Liquid slag

Figure 55

144 -

As a result, powder consumption studies have had to be reviewed.lt hasbeen found that for this pseudo "three" layer system,the liquid layerprovides lubrication between the strand and the crystalline and glasslayers adhere to and move with the reciprocating mould.Thus powderconsumption studies which were initially interpreted to yield a fluxthickness of one millimeter,refer (once the steady state has beenachieved) only to the liquid layer .Therefore a much thicker flux layeroccurs between the mould and the strand.This system has been verified by

(45)Riboud from analysis of fragments found after breakout has occured.

With this additional information,the modelling of the heat transfer characteristics of a powder becomes very complex indeed as each phase will contribute separately to the overall heat transfer coefficient observed for the process.Thus measurements have to be made for each powder modification which can occur in the process to obtain a value for an overall heat transfer coefficient for a powder.

For the case of absorption coefficient measurements,it is proposed that the liquid and glass phases have similar coefficients. No data is however availible for crystalline mould powders. Values have been determined for this study and are presented in the following section along with their dependence upon time.

- 145 -

4:2:9 The absorption coefficient of crystalline mould powders.

Crystalline mould powders were prepared by heating glass discs of the

samples scattering becomes very important. Thus the assumption that the extinction coefficient is equivalent to the absorption coefficient is no longer valid.The extinction coefficient for these samples is dependent upon the magnitude of the scattering coefficient ).This scatteringcoefficient is dependent upon the size of the crystallites found in the sample.However it was not possible to measure the scattering coefficient during this study. Values obtained for the absorption coefficients were found to be very much greater than the values measured for the glass samples. The values obtained for crystalline samples indicate an absorption coefficient greater than 140cm~ .This value was the limit to which the samples could be ground for safe handling. The minimum thickness which could safely be handled was 0.3mm.

These values obtained are not necessarily values for the absorption coefficient (equation 15).

desired powder at ca.800°C overnight. For the case of crystalline

- 146 -

The high values obtained for the absorption coefficients of crystallinemould powders indicate that heat transfer by direct radiation andradiation conduction would be very low. Thus this crystalline layerwould appear to act as a barrier to radiative heat transfer.The rate atwhich this barrier is formed is dependent upon the rate ofcrystallisation.A basic measurement of this rate was performed bymeasuring the increase in the absorption coefficient with time atca.800°C for powder THA.The results obtained are shown in Figure56. Initially the rate of increase in the absorption coefficient isrelatively slow but increases rapidly after two hours.This experimentwas repeated for powders I and R These we re found to have absorption

-1coefficients of greater than 100cm after five minutes.Further study will have to be undertaken to determine the rates for other powders.

- 1H7 -

H:3 Fmissivity of mould powders.

The use of emissivity measurements to obtain the absorption coefficientsof mould powders was precluded by the inability to view the blackbodywhich wa3 below the sample (Fig.18).This feature indicates that thecasting powders tested were not of a semi-transparent nature, henceradiation conduction cannot occur in these samples. The emissivities ofthe fluxes measured are similar to those obtained by Keene and

(47)Mills for fluxes used in the electro-slag remelting of steel.

148 -

4: 3: 1 A mathematical analysis of heat transfer in the continuous casting mould.

This section gives a brief mathematical treatment of the heat transfer across the flux layer,making use of the results and theories put forward in this study.

One of the most recent studies of the heat transfer characteristics of the continuous casting mould,has been performed by Omhiya et al^^. The results of his study indicate that for flux thicknesses in the gap between the mould and the strand of up to 3mm,radiation contributes almost 75% to the overall heat transfer.

Now, for a flux of thickness 3mm.in the gap between the mould and the strand, which are at 1500 and 300°C respectively.Assuming the flux has '’average’* physical properties as detailed:-

T, . = 1150°C CK = 150cm"*1liq.

Tg. = 600°C CX = 3cuf1

Tcrys. * 825°C = 70°ra"1

Assuming a linear thermal gradient across the flux layers, the following picture emerges,Figure 57.

149 -

F i g u r e 57

One can see that the liquid layer complies with the data availible from powder consumption studies.Using the data obtained in this study, leads to further points of reference.One notes that the glass thickness would be of the order of 1.5mm.,however not all of this layer allows the passage of I.R radiation.The silica cutoff which by use of Wien’s law, equation (14) to yield a temperature of~400°C.It is clear that only ca. 1mra. of the glass is thus able to transmit I.R radiation. Note the temperature of this layer 400°C,at this temperature the passage of heat by radiative methods becomes neglible.

150 -

In addition to the glass layer,the two other regions which could transmit I.R radiation in significant amounts are the crystalline and and the semi-crystalline layers.These by virtue of their large extinction coefficients will absorb all the radiation which is passed through the very thin liquid layer. From the data available,an average absorption coefficient can be calculated.This figure is valid only to the boundary of glass”A”with glass”B” (the mould thus appears to see only heat transfered by phonon (or lattice) thermal conductivity).An estimate of the amount of energy transfered by radiation can be calculated by use of equation (2H).

Where andrespectively.

^ are the emissivities of the strand and the mould

Using the parameters quoted,one calculates a figure of ca.8 x10 Wmfor the radiation heat flux

151

Calculation of the heat transfer by thermal conductivity can be performed by use of equation (25).

qc = K /d ( T2- I,)

Where d is the total thickness of the flux, in this case 3mm.

Assuming an average value for the thermal conductivity of a mould powder - 1 - 1 M ) ( 2 9 ) ( 4 1 )of 1.5Wra K and for the temperatures quoted in Figure

4 -2571yields a value of ca.65 x10 Wm for the heat transfered by thermal conductivity.Thus one can see that radiation contributes of the order of 10/6 to the overall heat transfer.

-2The total heat flux (0.68MWm ) calculated compares favourably with the(29)results obtained by Bagha and Grieveson in a related study.The U3 e

-2of an average heat flux (1 MWra ) from the data provided by Bagha andGrievesonenables an effective thermal conductivity to be calculated

_2by use of equation (25),for q = 0.08MWm — -1 -1.The value of K thus calculated,0.5Wm K agrees with the value obtained

by Ohmiya et al 2^ of, 0.43Wm~^K~\ One can see that by use of the appropriate equations values for the physical properties of casting powders can be obtained.However whether these purely calculated results lead to reliable values for the actual physical properties of mould powders is open to debate.

- 152 -

Ths experimental technique used by Omhiya et al(20) to evaluate castingpowders used a decreasing temperature programme when measuring the heat flux passing through a casting powder. The use of such an experimental procedure would create at the crystallisation point discontinuities in:-

a) the absorption coefficient and therefore a change in the radiation contribution to the overall heat flux.

b) the heat capacity cf. Figures 33-36,leading to an increase in the thermal conductivity.

These effects may be masked by errors inherent in the experimental procedure adopted.

Data obtained for this study for example gives individual values for three phases of a mould which occur in continuous casting,Table 17 .

TABLE 17.

Phase C X /cm”^

Glass 3

Crystalline > 100

Liquid 3

- 153 -

The average absorption coefficient of the flux layer in the mould wouldbe governed by the high value due to the crystalline portion of thelayer 3y3tem. Thus it is proposed that an overall absorption coefficient

-1 ( 2 0 )would have a value of the order of 50cm cf.Ohrniya et al 10 -20em” ^ .

It is the opinion of many workers ^9)(21)(23)(29)(il8)that heat transfer

by radiation is neglible,and that though one can calculate contributions due to this mode they have no real bearing upon the situation occuring in the mould.lt is therefore possible that the factors which determine the degree of contact and adhesion between the flux and the two metal surfaces namely:- the thickness and the type of glass,the type of phase initially deposited on the copper mould, the degree of iron transfer between the glass in contact with the strand surface,the shrinkage of the flux layers are the properties which have the largest influence upon the heat transfer situation prevalent in the continuous casting mould.

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Conclusions.

As stated,this project was designed to produce basic physical parameters for mould powders,which would be of use in gaining a fundamental understanding of heat transfer in the continuous casting of steel. Reliable conclusions for investigations on glassy materials can only be drawn when specific chemical and physical properties are defined.Thus these results may not concur with other studies whose samples have a different thermal history.

Characterisation by X-ray and D.T.A gave a large amount of data concerning the phases present upon crystallisation of the glass of which small changes in the composition of the powder form additional and/or alternative phases.However attempts to correlate peaks observed during D.T.A with the phases identified by X-ray analysis met with little success.

X-ray analysis did produce evidence of hydrated phases which were present in the powder at elevated temperatures.This phenomenora is now

( OQ )under investigation at B.S.C J .

Quantitative use of the I.R spectra of mould powder glasses yielded values of the amount of water present in powders at working temperatures. It must be noted that these values are not corrected for the absorption due to the base glass due to the difficulty in designing a synthetic powder with "average" properties. Also undetermined at this present moment is the transfer rate of the water in the powder to the strand (at these temperatures and in the presence of carbon the water would produce hydrogen).

155

D.T.A allowed the identification of the carbon type and the quantity present in the powder.Overall in the context of this study D.T.A was an invaluable analytical tool.

Divers thermal properties of continuous casting mould powders were investigated.Heat capacities were measured for sintered and glassy states,whilst the use a computer program allowed data to be calculated.The calculated data showing close agreement to the values measured for sintered mould powders.

In general the heat capacity was found to be greater for the crystalline materials as compared to the glass or sintered forms. Data for the heat capacity above 1000K is not available due to lack of suitable equipment.

Using data supplied by Dr. R. Taylor of U.M.I.S.T for the thermal diffusivities of mould powders,allied with their densities and their heat capacities which were measured in . this study allowed the calculation of thermal conductivities.The underlying trend was that the thermal conductivity for all forms of the powder increased with an increase in temperature.

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Measurement of the powder's emissivity revealed that the powders were not semi-transparent,therefore heat transfer by radiation conduction did not occur.The measured emissivities of glass,liquid and crystalline forms of the powders were between 0.9 and 1.Prior to this study the effects of radiation on the heat transfer characteristics of continuous casting mould powders were unqualified. Measurement of their absorption coefficients indicates that the composition of a powder has an effect upon the ferrous/ferric equilibrium which is instrumental in determining the magnitude of the absorption coefficient at working temperatures.The use of Planck’s distribution allowed the calculation of an average (wavelength independent) absorption coefficient for the glass form. However,due to the complex layer system prevailing in the mould, the high absorption coefficients of the crystalline mould powder and the very strong absorption of the layer of glass in contact with the copper mould (due to the silica cutoff) ,the overall situation is of heat transfer through a mould powder being predominately by thermalconductivity.

157

Powder consumption studies lead to the conclusion that the flux layer thickness was of the order of 0.1-1mm.The verification of this multiple layer system with a thickness of up to ca.3mm. required reappraisal of these data.It was found that once the steady state was achieved,that these powder consumption data related primarily to the liquid layer. From the discrepancies of the data of Ohmiya et al 20 with this study,the factors which contribute to the overall heat transfer coefficient in the continuous casting mould require careful appraisal to verify their magnitude and influence.

Finally,the present investigation like most interesting doctorate theses has has raised as many questions as have been answered.However a considerable body of information is made availible which it is hoped will make some contribution to the elucidation of this complex problem.

Further work.

In relation to this study,further investigation should be undertaken to try and correlate the transitions observed in the data availible from D.T.A studies and X-ray analysis. Results from such an investigation could yield the possibility of tailoring the phases formed to create the required heat transfer situation in the mould.As it seems that the primary mode of heat transfer in the continuous casting mould is by thermal conductivity,it would be useful to have an average value for the multi-layer system which is found in the mould.This could be calculated in a similar manner to the average absorption coefficient presented in thi3 thesis once the individual components were measured.Thermal diffusivities will continue to be measured by Dr.R.Taylor atU.M.I.S.T.Simulation experiments relating to the heat transfer in the continuous

(29)mould have been performed at Imperial College,London and arecontinuing at this present time.As previously mentioned,the moisture level of mould powders at elevated

(39)temperatures is currently under investigation at B.S.C Viscosities which at this present time are estimated by use of computer programs,are hoped to be measured by a high temperature rotating bob viscometer,which is currently under construction at N.P.L.At this present time there is a great interest in continuous casting and a number of research projects are currently in progress in a number ofcountries.

159 -

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