Steel Making: Ingot casting defects

MY: 301 Steel Making Processes

Ingot defects and Remedies: (Chapter No: 26, Tupkary)

1) Pipe ……….. Cause:

The volumetric contraction resulting on solidification appears in

the form of a cavity known as pipe.

This amounts of about 2.5-3.0% of the total apparent volume of

the ingot.

Rimming and semi-killed steels show tendency for piping which

can be eliminated by careful practice.

Capped steel is particularly free of pipe.

In a WEU mould the pipe is short and wide, while in a NEU

mould the pipe is narrow and long.

Figure (a) Narrow end up mould showing long pipe in killed steel Figure (b) Wide end up mould showing pipe in killed steel Figure (c) Wide end up mould with hot topFigure (d) Narrow end up mold with hot top. Pipe is confined to the top.


Remedies of pipe formation:

By adopting hot top feeder head. It acts as a reservoir to feed

the metal to the main part of the ingot and avoid the formation

of pipe. The volume of the hot top is 10-15% higher than ingot

volume

Use of exothermic materials in the hot top keeps the metal hot

in the top portion and pipe formation can be avoided

Another method = to pour little more metal after partial

solidification, but this is not a very common practice.


2) Columnar Structure:

After the formation of initial chill layer further solidification

results in the formation of dendrites which grow along their

principal axis perpendicular the mold wall.

It is tree like structure…Dendrites initially grow as primary arms

and depending upon the cooling rate, composition and

agitation, secondary arms grow outward from the primary arms.

Likewise, tertiary arms grow outward from the secondary arms.

Figure: Dendritic structure

Their lateral growth is restricted due to the growth of adjoining

dendrites giving rise to the elongated crystal.

If the length of these is appreciable it is known as columnar

structure.


Ingot possessing columnar structure tends to crack during

rolling.

3) Blow Holes:

The entrapment of gas evolved during solidification of

steel produce cavities known as blow holes in all except killed

steels.

These are of two types.

i. Primary blow holes are elongated or like honeycomb

and are located next to the ingot skin.

ii. Secondary blow holes are more spherical and are

located further in.

Remedy: Control of gas evolution during solidification so that blow

hole forms only within the ingot skin of adequate thickness.


4) Segregation:

It is the difference in composition of steel within the ingot than

some average composition. Segregation is due to

a) Difference in solubility of solute elements in liquid and solid steel

i.e. partition coefficient of element in steel. Partition coefficient

of solute (K) is defined as

The value of K ≤ 1.

The solute elements whose K = 1 do not segregate.

All elements whose K < 1 tend to segregate.

b) Rate of solidification: faster rate of solidification avoids the

elements to segregate. The initial chill layer of ingot has

practically the same composition as that of liquid steel.

Decrease in rate of solidification causes elements to

segregate.

c) Larger size ingots: are susceptible to segregation than smaller

size ones. Larger size ingots require more time for solidification.

Remedy: soaking of ingots at high temperature can minimize

segregation.


5) Non-metallic inclusions:

Inclusions are foreign particles that contaminate the metal

surface during rolling or other metal forming processes.

Common inclusion particles include oxides, sulfides or

silicates. Inclusions can be characterized by their shape, size

and distribution.

Non metallic inclusions are inorganic oxides, sulphides and

nitrides formed by reaction between metal like Fe, Ti, Zr, Mn,

Si & Al with non metallic elements like oxygen, nitrogen,

sulphur etc...


Types of non-metallic inclusion s :

Oxides

FeO, Al2O3, SiO2, MnO, Cr2O3 etc.

Al2O3*SiO2, Al2O3*FeO, Cr2O3*FeO, MgO*Al2O3, MnO*SiO2 etc.

Sulfides

FeS, MnS, CaS, MgS, Ce2S3 etc.

Oxysulfides

MnS*MnO, Al2O3*CaS, FeS*FeO etc.

Carbides

Fe3C, WC, Cr3C2, Mn3C, Fe3W3C etc.

Nitrides

TiN, AlN, VN, BN etc.

Carbonitrides

Titanium carbonitrides, vanadium carbonitrides, niobium carbonitrides etc.

Phosphides

Fe3P, Fe2P, Mn5P2

Depending on the source, from which non-metallic inclusion are

derived, they are subdivided into two groups: indigenous and

exogenous inclusions.


1. Indigenous inclusions are formed in liquid, solidified or solid steel

as a result of chemical reactions (deoxidation, desulfurization)

between the elements dissolved in steel.

2. Exogenous inclusions are derived from external sources such as

furnace refractories, ladle lining, mold materials etc. Amount of

exogenous inclusions and their influence on the steel properties are

insufficient.

Distribution of non-metallic inclusion s :

Besides of the shape of non-metallic inclusions their distribution throughout

the steel grain structure is very important factor determining mechanical

properties of the steel.

1. Homogeneous distribution of small inclusions is the most desirable type of distribution. In some steels microscopic carbides or nitrides

homogeneously distributed in the steel are created by purpose in order to

increase the steel strength.

2. Location of inclusions along the grain boundaries is undesirable

since this type of distribution weakens the metal.

3. Clusters of inclusions are also unfavorable since they may result in

local drop of mechanical properties such as toughness and fatigue

strength.

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Solidification of Ingots: (Chapter No. 25, Tupkary)

Types of steels

Molten steel contains dissolved gases. During cooling of the

steel the solubility of dissolved gases is decreases and the

excess come out of solution. (e.g., in liquid steel solubility of oxygen is 0.16% but in solid steel is only 0.003%)

The amount of oxygen in solution and the amount that is

expelled as CO is decided by its carbon content, the type and

amount of deoxidizer added to steel prior to solidification.

Steel that is fully oxidized by a strong deoxidizer is called Killed Steel .

If the evolution of the gas is appreciable, in other words

deoxidation is not fully carried out, it gives appearance of boiling

to liquid steel in the mould. This boiling action is termed as

Rimming and the steel known as Rimming Steel .

In between violently rimming and killed steel lies the Semi Killed Steel , which is only partially deoxidized such that some gas

evolution takes place during later stages of solidification.

The capped steel is only a special variety of rimming steels in

which the rimming action is less violent.


Mechanism of Solidification:

Killed steel solidifies in three zones in an ingot.

The metal next to the mould walls and bottom is chilled by the

cold mould surfaces. This is a thin layer and is known as chill, shell or skin of an ingot and has a fine equiaxed grains.

The rate of solidification is very high in forming the skin, however

the rate of solidification soon slow down.

The mould expands on heating and the skin contracts on

solidification; it reduces the rate of heat flow and thereby slows

down the cooling of an ingot.

The solidification front moves inwards perpendicular to the

mould faces resulting in columnar grains next to the chill. OR

After the formation of initial chill layer further solidification results

in the formation of dendrities which row along their principal axis

perpendicular to the mould walls.

Their lateral growth is restricted due to the growth of adjoining

dendrities giving rise to elongated crystal. If the length of these is

appreciable is known as columnar structure


In general columnar structure does not extend to the centre of

the ingot. The central portion solidifies as equiaxed grains of bigger sizes than those in the chill due to slow cooling.

One zone blends into the next gradually. The extent of each

zone varies with composition and temperature of liquid steel,

mould design and its temperature at the time of teeming.


Segregation:

Segregation means departure from the average

composition.

Segregation is the result of the differential solidification

characteristic of all liquid solution.

In case of Steel, is an alloy (liquid solution) of S, Si, C, P,

Mn etc. in iron and hence is prone to segregate during

solidification.

The initial chill layer of the ingot has practically the same

composition as that of the steel poured in the mould, i.e. there

is no segregation in the chill layer because of vary rapid rate of

solidification.

The progressive solidification there after results in

solidification of purer phase (rich in iron) while the remaining

liquid gets richer in impurity contents.

If the concentration > the average it is called positive

segregation.

If the concentration < the average it is called positive

segregation.


It can be minimized by prolonged soaking of ingots before

working.

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Steel Making: Ingot casting defects