Steel Making: Ingot casting defects

cont..

1. 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 top
Figure (d) Narrow end up mold with hot top. Pipe is confined to the
top.

2. MY: 301 Steel Making Processes
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.

3. MY: 301 Steel Making Processes
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.

4. MY: 301 Steel Making Processes
·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.

5. MY: 301 Steel Making Processes
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.

6. MY: 301 Steel Making Processes
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...

7. MY: 301 Steel Making Processes
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.

8. MY: 301 Steel Making Processes
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.

9. MY: 301 Steel Making Processes

10. MY: 301 Steel Making Processes
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.

11. MY: 301 Steel Making Processes
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

12. MY: 301 Steel Making Processes
·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.

13. MY: 301 Steel Making Processes
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.

14. MY: 301 Steel Making Processes
· It can be minimized by prolonged soaking of ingots before
working.