Development of New Mold Flux for Continuous Casting Based

865 © 2014 ISIJ

ISIJ International, Vol. 54 (2014), No. 4, pp. 865–871

Development of New Mold Flux for Continuous Casting Based on Non-Newtonian Fluid Properties

Keiji WATANABE,1)* Koichi TSUTSUMI,2) Makoto SUZUKI,3) Hiroki FUJITA,4) Satoshi HATORI,4)

Takayuki SUZUKI5) and Tomoaki OMOTO6)

1) Steel Research Laboratory, JFE Steel Corp., 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba, 260-0835 Japan.2) Steel Research Laboratory, JFE Steel Corp., 1 Mizushima-Kawasakidori, Kurashiki, Okayama, 712-8511 Japan.3) Formerly Steel Research Laboratory, JFE Steel Corp., Now at JFE Techno-Research Corp., 1-1 Minamiwatarida-cho, Kawasaki-ku,Kawasaki, Kanagawa, 210-0855 Japan. 4) East Japan Works, JFE Steel Corp., 1-1 Ogishima, Kawasaki-ku, Kawasaki,Kanagawa, 210-0868 Japan. 5) Research Center, Shinagawa Refractories Co., Ltd., Bizen, Okayama, 705-8577 Japan.6) Okayama Works, Shinagawa Refractories Co., Ltd., Bizen, Okayama, 701-3202 Japan.

(Received on October 14, 2013; accepted on December 27, 2013)

Due to the importance of the physical and chemical properties of the mold flux used in the productionof high-quality steel, in particular the suppression of surface defects on steel sheets, steelmaking engi-neers have attempted to develop new types of mold flux. This paper presents the results of research onentrapment of mold flux and on heat transfer between the mold and the solidified shell. The authors havebeen developing a mold flux with non-Newtonian fluid properties using nitrogen. That is, the viscosity ofthe molten mold flux is low at a high shear rate to reduce the friction between the mold and the solidifiedshell, but is high at a low shear rate to prevent mold flux entrapment. In order to approximate the proper-ties of mold flux as a non-Newtonian fluid, nitride is added to the conventional flux to adjust the silicatenetwork structure through the reaction between nitrogen and calcium. The contact angle of the non-Newtonian mold flux, which represents the wettability between the mold and the solidified shell, is low incomparison with that of the normal mold flux without nitrogen. It is suggested that the non-Newtonianmold flux increases the heat transfer between the mold and the solidified shell. A casting test was carriedout using this non-Newtonian mold flux, and the results showed that entrapment of mold flux decreasedand heat transfer increased, as assumed.

KEY WORDS: continuous casting; mold flux; viscosity; contact angle; non-Newtonian fluid; wettability.

1. Introduction

Mold flux plays an important role in the continuous cast-ing process for high-quality steel production. It is wellknown that mold flux entrapment causes surface defects insteel sheets, and high viscosity mold flux prevents itsentrapment in the molten steel. Many studies have been car-ried out with the aim of preventing mold flux entrapment inthe continuous casting mold. One approach is to control themolten steel flow in the mold by electromagnetic force,1–4)

for which several technologies have been proposed, includ-ing the use of a static electromagnetic field,1,2) travelingmagnetic field3) and stirring magnetic field.4) These electro-magnetic methods of controlling molten steel flow havebeen applied at many steel plants. Another approach is tooptimize the shape of the submerged entry nozzle5–8) to pre-vent mold flux entrapment. Modifications to the shape at theoutlet and in the interior of the submerged entry nozzle havebeen applied. In another approach, which does not involvecontrol of the molten steel flow, the properties of the moldflux have been newly designed. It is generally considered

important to increase the viscosity and interfacial tensionbetween the molten steel and mold flux to prevent entrap-ment of the mold flux.9,10) The viscosity control techniqueis used in particular.

At the same time lubricating properties are also requiredin order to avoid the operational “break-out” problem,which is caused by insufficient lubricant between the moldand the solidified shell. Thus, a low viscosity flux is desiredfrom the viewpoint of stable operation of the continuouscaster, while, high viscosity is desired to suppress entrap-ment of the flux. Therefore, this paper describes the devel-opment of a mold flux with non-Newtonian fluid propertiesusing nitride.11) Figure 1 shows the concept of such a moldflux in the continuous casting process. Mold flux is easilyentrapped when the surface flow velocity of the molten steelincreases. It is highly possible that this entrapment phenom-enon occurs near the center between the immersion nozzleand the short side of the mold, because the surface velocityis high at this area.12) On the other hand, a lubrication areais a crevice part in which flux flows in between the moldand the solidified shell. Since the two phenomena occur indifferent parts of the mold, it is expected to be possible tosatisfy both the properties of “lubrication” and “hard to

* Corresponding author: E-mail: [email protected]: http://dx.doi.org/10.2355/isijinternational.54.865

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ISIJ International, Vol. 54 (2014), No. 4

entrap” by controlling the viscosity of the flux at theserespective locations.

A non-Newtonian fluid property means that the viscosityof the molten mold flux is low at a high shear rate, whichis advantageous for reducing the friction between the moldand the solidified shell, thereby satisfying “lubrication,” butis high at a low shear rate, which prevents mold flux entrap-ment and satisfies the “hard-to-entrap” property.

2. Concept of Non-Newtonian Fluid Mold Flux

According to the theory of hydrodynamics, a Newtonianfluid is defined as:

.................................. (1)

In this equation, τ , μ , v and z denote shear stress, viscos-ity, velocity and displacement, respectively. When this rela-tionship does not hold, the fluid has non-Newtonian properties.

Dilatant flow, Bingham flow, etc. are typical non-Newtonianbehaviors. In this study, a pseudoplastic flow, which decreasesthe viscosity in the high shear rate region, is desired.

Research on the non-Newtonian flow of liquid slag wascarried out by Shiraishi,13) who proposed two mecha-nisms.14,15) One is the dispersion of high melting point par-ticles, for instance Cr2O3, in the slag, which then behaves asa suspension, resulting in non-Newtonian properties. Theother is changing the silicate network structure to a heteronetwork. In the latter case, nitrogen exists with a hetero

structure such as the bonding of

rather than existing in a uniform network as SiO42–. This

causes the slag to behave as a non-Newtonian fluid.

3. Experimental Method and Conditions

3.1 Preparation of Mold Flux Containing NitrogenIn general, various methods of charging nitrogen into slag

are possible, for example, by blowing nitrogen gas, by add-ing Si3N4 powder, and so on. In the present study, a methodof adding Si3N4 powder to the mold flux was employed.Figure 2 shows the flow of the procedure for preparing thenon-Newtonian flux.

The melting process is required in order to dissolvenitride in slag. In actual processes, mold flux is used in apowder or granule form, so a water granulation and pulver-ization process is suitable from the viewpoint of industrialapplication. It is known that blast furnace slag is foamed bygeneration of N2 and H2 gases when the slag comes intocontact with steam in the water granulation process.16)

Because the composition of this mold flux is similar to thatof blast furnace slag, which mainly contains CaO and SiO2,it is important to confirm the yield of nitrogen when consid-ering an industrial process.

Fig. 1. Concept of mold flux of non-Newtonian fluid in a continuous casting process.

τ μ= dv

dz

≡ = = ≡ ≡Si N Si or Si N Si– – –|

Fig. 2. Method of preparing the non-Newtonian mold flux.


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Therefore, 3.8 kg of silicon nitride was added to 252.5 kgof the main material of the mold flux (CaO: 48%, SiO2:43%, Al2O3: 3%). The mixture was heated, melted andwater granulated under the conditions shown in Table 1, andthe yield of nitrogen was estimated by the amount of nitro-gen in the flux product. The Kjeldahl method was adoptedfor the analysis of nitrogen.17) Since there was no largereduction in yield in the granulation processing, as describedlater, the investigation of properties and the casting test werecarried out after the flux was prepared by the procedureshown in Fig. 2.

Table 2 shows the chemical composition of a convention-al industrial mold flux and the non-Newtonian fluid moldflux used in the present work. The conventional mold fluxis used for high-speed casting of ultra-low carbon steel.

In the next step, the viscosity and contact angles of thefluxes were measured as essential physical properties ofnon-Newtonian fluid mold flux. Viscosity was measured

with the rotational viscometer shown in Fig. 3. A graphitecrucible with a radius of 21 mm was filled with 200 g ofmold flux. The specimen was heated in an electric furnaceto 1 573 K under a purified Ar flow. Viscosity was measuredby changing the rotational speed (5–300 rpm). Due to con-cerns about a rise in torque in the low shear velocity region,before measuring the viscosity of the mold flux, the correct-ed value of viscosity was measured using commercial stan-dard silicon oils (JS-100, 200, 500, manufactured by NipponGrease Co., Ltd.).

Moreover, the representative values of the shear velocityof the entrapment area in this study were calculated by theflow velocity of molten steel and the molten pool thickness,whose values are assumed to be 20 cm/s12) and 10 mm,18)

respectively.

Therefore, in this research, a shear rate of 20 1/s at theentrapment area was used as a representative value.

The shear rate of the lubrication area is calculated by thefollowing equation from the thickness of the film (0.75mm)19) and the casting conditions (casting speed Vc: 1.6m/min = 26.7 mm/s, frequency f: 160 cpm = 2.7 Hz, andamplitude A: ±4 mm; oscillating system: sin).

Vm : mold speed (mm/s)Vc : casting speed (mm/s)f : frequency (Hz)df : thickness of film (mm)

Thus, the shear velocity changes between 0–125 1/s in oneoscillation period.

Various lubrication mechanisms at the boundary of themold and the strand have been proposed.20,21) However, it isconsidered that flux flows in the positive strip period ofoscillation, and during this period, the shear rate is maxi-mum; therefore, in this research, a shear rate of 125 1/s atthe lubrication area was used as a representative value.

The contact angle between a pure iron sheet and the mold

Table 1. Test conditions for melting Si3N4 powder.

The amount of melting (kg) 256

Melting time(min) 30–138

Temperature(K) 1 873

Water/flux ratio 20–40

Table 2. Chemical composition of mold fluxes.

Conventional Non-Newtonian

SiO2

(mass%)

39.4 37.8

Al2O3 5.7 5.2

CaO 34.5 31.7

F 3.7 3.4

Na2O 4.2 3.9

MgO 6.5 6.0

Li2O 1.3 1.3

N 0.0 0.2

C/S (–) 0.9 0.8

dv

dz= =0 2

1020 1

. //

m s

mms

V V

d

ftm c

f

+=

+21 3 2 26 7

0 75

. cos( ) .

.

π π

Fig. 3. High-temperature viscosity meter using concentric cylinder method.

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flux was also measured using the sessile drop technique tostudy wettability as an important factor for lubrication. Asit is known that oxygen in steel strongly affects interfacialtension,10,22) the oxygen content of the pure iron sheet (10mm2) was under 10 ppm. The specimen was rapidly cooledby pouring molten mold flux on a water-cooled copper plateand forming 5-mm cubes by polishing the solidified moldflux. These cubes were set in a high-temperature microscopeand heated to the target temperatures (1 373, 1 473, 1 573,1 673 K) at a rate of 5 K/min, and were kept at the targettemperature for 10 min, then photographed through thequartz window of the high-temperature microscope. Fromthe profiles of mold flux droplets, the contact angle of eachflux against the steel sheet was measured.

3.2. Laboratory Continuous Caster TestMolten steel (250 kg) was cast using a small laboratory

continuous caster (mold size: 100 mm in diameter and 310mm in length) with conventional mold flux and the non-Newtonian mold flux. The cast billet length was about 0.8m, and the carbon content of the molten steel was selectedto be about 0.2%. The oscillation stroke was 8 mm, and thecasting velocity was from 1 to 2 m/min. After the castingexperiment, the mold flux film was collected from the sur-face of the mold and cast billet. Flux consumption was cal-culated from the weight and thickness of the mold flux film.

4. Experimental Results and Discussion

4.1. Nitrogen Yield of Mold FluxFigure 4 shows the results for the nitrogen yield.

Although the nitrogen content decreased gradually with pro-cessing time, the content was maintained at about 80% ofthe initial value until 138 min. Part of the dissolved nitrogengenerated fine gas bubbles after heat treatment at 1 873 Kand water granulation, and as a result, the bulk density ofthe flux grains was reduced by 40% or more compared withnon-N additives. However, no large decrease in yield wasobserved.

According to Fuwa et al., the nitrogen content of slag thatcontains about 800–1 000 ppm of nitrogen is reduced byapproximately one-half during water granulation.16) Nitrogenevolves as shown in Eq. (2)23) when the slag is placed incontact with steam. However, because the water/slag ratio inthis research was 20–40 in mass, which is large in compar-ison with water granulation of blast furnace slag, the coolingrate was fast enough to avoid nitrogen evolution. Thus, the

contact time between the water and slag was short, and as aresult, nitrogen yield was high.

................ (2)

4.2. Measurement of Viscosity by Rotational ViscometerFigure 5 shows the results of viscosity measurement

using the concentric cylinder method. The measured data forthe viscosity of the conventional mold flux was constant(0.48 Pa·s), while the data for the non-Newtonian mold fluxchanged from 0.5 to 0.7 Pa·s in proportion to the shear rate.In particular, the low shear rate (20 1/s) was assumed to bethe area of entrapment at the meniscus, and the high shearrate (125 1/s) was assumed to be the area of lubricationbetween the mold and the solidified shell. The viscosity ofthe non-Newtonian mold flux at the low shear rate was high-er than that of the conventional mold flux, and its viscosityat the high shear rate was the same as that of the conven-tional mold flux. Therefore, there should be no possibilityof break-out between the mold and the solidified shell, andthe entrapment of mold flux in the molten steel is reducedbecause the viscosity of the flux is high at the low shear rate.

4.3. Observation of Flowing Flux with Small Labora-tory Continuous Caster and Measurement of FluxConsumption

Flux consumption was measured with a small laboratorycontinuous caster using another non-Newtonian fluid moldflux for which the viscosity in the lubricant region was 0.35Pa·s. Figure 6 shows the flux consumption by the laboratorycaster.

At a 1.6 m/min casting velocity, the observed flux con-sumption of the non-Newtonian fluid mold flux was 0.50kg/m2 and that of the conventional mold flux was 0.38 kg/m2.That is, the flux consumption of the non-Newtonian fluidmold flux increased by more than 20% in comparison withthat of the conventional mold flux. Figure 7 shows a viewof the lubricant as photographed from under the mold.Based on observation through the video camera under themold of the laboratory continuous caster, the wettability ofthe non-Newtonian fluid mold flux increased.

Figure 8 shows the measurement results for the contact

Fig. 4. Relationship between melting time and nitrogen content. Fig. 5. Shear rate dependency of viscosity of mold fluxes at 1 573 K.

2 3 332 2 2

2N H O H N O− −+ = + +

0.2

0.3

0.4

0.6

0.7

0.8

0 50 100 150 200 250 Shear velocity[1/s]

Engulfment area Lubrication area

Shear rate (1/s)

Visc

osity

(Pa

s)

0.5

Conventional flux Non-Newtonian flux


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angle between the pure iron sheet and the mold flux in thehigh-temperature microscope. The contact angle of the non-Newtonian fluid mold flux was smaller than that of the con-ventional mold flux at high temperature. These resultsshowed that the wettability between the molten mold flux andthe pure iron sheet was improved with the non-Newtonianfluid mold flux.

In explaining this phenomenon, since the flux containednitride, it is presumed that part of the nitrogen in the fluxwas absorbed to the steel surface, and this interaction influ-enced the wettability.24) In this manner, consumption of thenon-Newtonian mold flux in the small casting test increasedby about 20% in comparison with the conventional flux witha similar viscosity in the lubrication region. This fact indi-cated the excellent wettability of the non-Newtonian fluidflux.

5. Actual Casting Test

An actual casting test was carried out in order to confirmthe effect of the non-Newtonian fluid mold flux using twostrands. In one strand, the trial mold flux (hereafter, “con-ventional flux”) was used until casting was completed. Inthe other strand, the conventional flux was changed to non-Newtonian fluid flux at the casting length position of 700 m.

The material being cast was steel for oil well tubulars.

The strand size and casting speed were 210 mmϕ and 1.4–1.6 m/min, respectively.

Flux consumption was evaluated as an index of lubrica-tion. To measure entrapment of mold flux, four specimenswith dimensions of 20 mm × 40 mm × 40 mm were cut andtaken from the strand at the casting length positions of 700m and 800 m. In the test strand, the 700 m position wasbefore the flux change, i.e., while using the conventionalflux, and the 800 m position was after changing to the non-Newtonian fluid flux.

Flux inclusions were investigated by sampling from thestrands at each length position (Fig. 9).25) The viscosity ofthe non-Newtonian fluid flux in the low shear zone showsa difference of 0.1 Pa·s or more, even though the viscosity

Fig. 6. Results for flux consumption by laboratory caster.

Fig. 8. Temperature dependency of contact angle of mold fluxes.

Fig. 9. Schematic of flux analysis position at the sampling strand.

Fig. 7. Observation of lubricant (photograph taken under mold of laboratory continuous caster).

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of both the conventional flux and the non-Newtonian fluidflux in the lubrication region is at the same level, 0.81–0.82Pa·s.

Figure 10 shows the consumption of each flux: the con-ventional flux was 0.13 kg/m2 and the non-Newtonian fluidflux was 0.14 kg/m2. Although no clear difference like thatin the laboratory casting test was observed in the actual cast-ing test, the tendency of flux use was almost the same as inthe laboratory caster test.

Figure 11 shows the entrapment amount for each flux.The conventional flux was used at “strand A” and the non-Newtonian flux was used at “strand B (after 700 m)”,respectively. When elements (Si, Ca, Al, and Na) that areconstituents of the flux were detected in the residue aftergalvanostatic electrolysis, the residue was assumed to be aflux inclusion.

At the 700 m position, that is, where both strands wereusing the conventional flux, the entrapment in strand B was13% lower than in strand A. This difference is presumablydue to the different characteristics of strand A and B, suchas the difference of inclusion adhesion on the respectiveimmersion nozzles. However, at the 800 m position, wherestrand B was using the non-Newtonian flux, the entrapmentin strand B decreased to less than one-half of that in strandA. Because the reduction of entrapment in strand B (>50%)

is substantially larger than the initial difference between thestrands (13%), the non-Newtonian flux was considered todisplay a “hard-to-entrap” property.

Figure 12 shows the heat flux in the mold calculatedfrom the circulating water temperature and the flow veloci-ty. The heat flux of the non-Newtonian type was 1.2 timesthat of the conventional flux.

A phenomenon in which mold flux adheres to the caststrand, as observed in the laboratory caster, causes the heatflux to increase. It is well known that the interfacial thermalresistance between a flux film and a mold has an influenceon the heat flux.26) In the actual casting test using the non-Newtonian fluid flux, it is considered that the wettabilitywith steel improved, as in the laboratory caster test, enablinguniform flux flow between the mold and the strand. Thiscaused the air gap to decrease, and as a result, the heat fluxincreased.

6. Mechanism of Non-Newtonian Fluid Mold Flux

Finally, the mechanism of the non-Newtonian fluid moldflux was considered based on the results of these laboratoryexperiments. First, the possibility of the effect of a suspen-sion of high melting point particles, which was the first pro-posed mechanism mentioned in Chapter 2,15) was examinedby an SEM investigation of the flux film sampled in the lab-oratory caster, as described in section 4.3. Figure 13 showsa photograph of the flux film, which has a glassy structure,as in the case of the conventional flux film, and also con-tains no Si3N4 particles. Figure 14 shows its X-ray-diffrac-tion pattern. As shown in the XRD chart, no peak of thecrystal of Si3N4 was observed. These facts show that nitridewas melted into the flux. From this, it was presumed that thenon-Newton property observed in this study was influencednot by suspension of Si3N4 particles, but rather, by dissolvednitrogen.

Next, the mechanism of the change in flux viscosity withshear rate when the mold flux contained dissolved nitridewas assumed to be as follows.13,15) The viscosity of the non-Newtonian mold flux remains high because there are weakbonds between the nitrogen and calcium in the molten non-Newtonian fluid mold flux at the low shear rate. However,at the high shear rate, these weak bonds between the nitro-gen and calcium in the molten non-Newtonian fluid moldflux are broken, and as a result, its viscosity decreases.

Fig. 12. Heat transfer in the billet mold.

Fig. 10. Flux consumption (Vc = 1.6 m/min) in test casting.

Fig. 11. Mold flux entrapment in test casting.


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7. Conclusion

A non-Newtonian fluid mold flux containing nitrogenwas developed for the continuous casting process of high-quality steel. The basic properties of this flux such as vis-cosity and wettability were measured, and casting tests wereperformed in the laboratory and with an actual continuouscaster. The following conclusions were obtained.

(1) Viscosity of the mold flux containing nitrogen ishigh at a low shear rate, and low at a high shear rate.

(2) Wettability between the molten mold flux and steelis improved with the non-Newtonian mold flux, resulting inimproved lubrication.

(3) The actual casting test using the non-Newtonianflux clarified the fact that inclusions of the non-Newtonianflux in the strand decrease, even though the lubricant prop-

erties of the two fluxes are on the same level. This differ-ence in inclusions is attributed to the higher viscosity of thenon-Newtonian flux under the low shear rate condition inthe entrapment region.

Therefore, this flux enables the production of high-qualityslabs by preventing entrapment of mold flux during high-speed casting, while its high lubrication property in the highshear rate region reduces the frequency of the operationalproblem known as break-out, which is caused by inadequatelubrication.

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Fig. 13. Photograph of mold flux film.

Fig. 14. XRD results of the flux film in laboratory caster.

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Development of New Mold Flux for Continuous Casting Based