FIELD OF INVENTION
The present invention relates to an improved process for
continuous casting of stainless steel slabs having excellent
surface and subsurface quality. More particularly, the
invention relates to an improved process wherein the chemistry
of ingredients charged and melted in an electric arc furnace
(EAF) are selected in a manner to keep chemical compositions
of liquid steel in a desirable range for continuous casting of
1.0 - 8.5% nickel stainless steel slabs free from surface
depression and subsurface cracks having excellent quality of
surface and subsurface.
Background AND PRIOR ART
Stainless steel slabs having 1.0 - 8.5% nickel, 15.0 - 18.5%
chromium, 1.5 - 2.0% manganese and 0-2% copper are
traditionally continuously cast by the manufacturers. The cast
slabs can have different dimensions, e.g. 140 - 250 mm
thickness, 500 - 1500 mm width, and 5,000 to 10,000 mm length.
Stainless steels of the above chemistry range are most popular
in the commonly known austenitic series. The standard AISI-304
grade and its cheaper substitutes having 1%, 2% and 4% nickel
belong to this category. The cast slabs of these grades are
inherently prone to having defects, like, surface depression,
appearing like long crater, and subsurface crack. Therefore,
the cast slabs of these steels are susceptible to quality
problems. The surface quality of the final products, i.e. hot
rolled and cold rolled coils or sheets which are processed
from these cast slabs, are consequently inferior.
The incidence of these defects in cast slabs, i.e. surface
depression and subsurface crack, is almost random in nature.
The stainless steel producers face difficulty in controlling

these quality aberrations. The effective yield of good quality
cast slab and processed final product of these grades is
therefore inferior.
The random and uncertain nature of occurrence of this quality
problem owe their genesis to the solidification process of
liquid steel during continuous casting operation. The mode of
solidification, i.e. whether liquid steel solidifies to
'ferrite' or 'austenite' phase, and how the distribution of
these two solid phases change during the solidification
interval, have a profound influence on the surface and
subsurface quality. The specific chemistry of the grades, and
the process parameters for continuous casting have to be
complementary in nature to resist the inherent tendency of
these grades toward formation of depression and crack in cast
slab.
The specified chemical composition ranges of these grades are
large. Such a wide chemistry range makes it difficult to
ensure the desirable solidification mode and the matching
parameters of slab casting. Lack of this understanding on the
crucial solidification behaviour of these grades is
responsible for the incidence of random and uncertain nature
of quality problems.
The cast slabs of these austenitic grade of stainless steels
having 1.0 - 8.5% nickel are inherently susceptible to quality
problems, e.g. surface depression and subsurface crack. These
defects in the parent cast slab are transmitted to the final
product (hot rolled coil), and get aggravated during the
subsequent processing stages, i.e. annealing and cold rolling.
The prime quality, and consequently the yield of product, are
therefore inferior for these grades.

The present invention is distinguishable from this existing
process with respect to the composition of liquid stainless
steel for continuous casting, the process parameters for
continuous casting, specifically, the taper of the mould and
the composition (basicity) and melting range of the casting
powder used in the mould.
The present inventors have found that the chemistry of
ingredients charged and melted in an electric arc furnace
(EAF) being selected in a manner to keep chemical compositions
of liquid steel in a desirable range for continuous casting of
1.0 - 8.5% nickel stainless steel slabs free from surface
depression and subsurface cracks, having excellent quality of
surface and subsurface.
OBJECTS OF INVENTION
Thus the basic object of the present invention is to provide
an improved process for continuous casting slab having
excellent surface and subsurface quality.
Another object of the present invention is to provide a
process avoiding the inherent problem of the generation of
surface depression and subsurface cracks associated with
casting of stainless steel slabs having 1.0 - 8.5% nickel is
thereby avoided in the invented process.
Yet another object of the present invention is to provide a
continuous casting process which removes the uncertainty and
the random nature of the quality aberration in cast slab.
The other object of the present invention is to increase the
yield of the saleable final hot rolled or cold rolled product.

SUMMARY OF INVENTION
Thus according to the main aspect of the present invention
there is provided an improved process for continuous casting
of 1.0 to 8.5% of nickel stainless steel slabs free from
surface depression and subsurface cracks having excellent
quality of surface and subsurface, said process comprising:
(i) charging and melting of material ingredients
comprising chemical composition such that liquid
steel so formed comprises chemical compositions of
Nieq and Creq , wherein said Nieq and Creq being
present in desirable range;
(ii) refining and chemistry adjustment of said liquid
steel;
(iii) adding of casting powder composition at top surface
of said liquid steel such that heat transfer during
solidification in the mould is controlled, said
powder composition having predetermined basicity and
melting point so as to provide requisite lubrication
and heat transferring medium in gap provided between
the mould and solidifying shell of cast; and
(iv) tapering of solidified shell to a predetermined
dimension so as to facilitate exact match between
solidification shrinkage and mould.
DETAILED DESCRIPTION OF INVENTION
In the process of the present invention the chemistry of the
ingredients which are charged and melted in Electric Arc
Furnace (EAF), and refined in Vacuum Oxygen Decarburisation
(VOD) or Argon Oxygen Decarburisation (AOD) is selected, so
that the chemical composition of the liquid steel produced

remains in a identified window within the broad specification.
The said ingredients include ferrochromium, ferronickel or
nickel oxide, ferro-manganese, ferro-silicon, stainless steel
or alloy steel scrap used as input to electric arc furnace.
This ensures that the liquid steel solidifies in desirable
mode i.e. to ferrite phase, during the solidification
interval.
Identification of the close range of composition is one
important factor of the present process. This is done based on
the fundamental knowledge on mode of solidification during the
casting process.
Stainless steel contains a large number of alloying elements
of which Cr, Ni, Mn, Si, Cu, Mo, C, N are the primary ones.
These elements are classified into two broad categories,
depending on whether they stabilize the 'ferrite' or the
'austenite' phase in the solid state. Cr, Si and Mo are
ferrite stabilizers, and in terms of Cr equivalent, depending
upon their relative strength of ferrite affinity

Ni, Mn, Cu, C and N are austenite stabilizers, and in terms of
Ni equivalent depending upon their relative strength of
austenite affinity

The ratio of Nieq and Creq for any stainless steel grade is
taken as its representative chemistry. This composition factor
determines the mode of solidification, i.e the fraction of
ferrite and austenite during solidification interval. The
specification of the 1.0 - 8.5% nickel grades in the invented

process has a smaller window of 0.52 to 0.55 in terms of the
ratio of Nieq and Creq to ensure desirable i.e. ferritic mode of
solidification during casting. The ratio of Nieq to Creq ranges
from 0 . 52 :1 to 0.55 :1.
In the process of the present invention at first starting
material comprising ingredients of identified compositions
which includes ferrochromium, ferronickel or nickel oxide,
ferro-manganese, ferro-silicon, stainless steel or alloy steel
scrap are used as input charge in the electric arc furnace
(EAF) . Then the liquid steel is poured from EAF to ladle,
which is next brought to Vacuum Oxygen Decarborisation (VOD)
or Argon Oxygen Decarborisation (AOD) for refining and
chemistry adjustment, which is accomplished by repeated
chemical analysis of liquid steel and minor addition of
ingredients. Homogenisation of the desired composition and
temperature are ensured at this stage.
Liquid steel is then poured from ladle into tundish placed
above copper mould of a Continuous Casting Machine (CCM). The
temperature of liquid steel in the tundish is maintained at
2 0 - 50°C above its liquidus temperature.
Then the liquid steel from the tundish is transferred to
copper mould via sub-entry nozzle (SEN) of Continuous Casting
Machine (CCM) . The dimension of the mould is adjusted
depending on the final required dimension of the cast slab.
The difference in width between the top and the bottom of the
mould is known as 'taper'. This ensures that the shrinkage of
the solidifying shell matches with the dimension of mould from
top to bottom. The taper varies in closer range of 0.9 to 1.0%
per meter.

The casting process is initiated by pulling down dummy bar at
the bottom of mould, at a rate depending upon the dimension of
slab or mould. This rate is known as casting speed.
The heat transfer during solidification in the mould is
controlled by casting powder composition added at the top of
the liquid steel. The powder composition is such that its
basicity i.e. ratio of CaO to SiO2 ratio is 1.1 - 1.2, and its
melting point is in the range of 1120-1140°C. The said powder
melts and penetrates into the gap between the mould wall and
the solidifying shell of the cast providing requisite
lubrication. The melted powder behaves as heat transferring
medium between the mould wall and the solidifying shell during
the casting process thus facilitating relatively higher
fraction of solid slag layer with thick crystalline component.
The heat transfer in the mould is thus maintained at a
relatively lower level for these grades to get good surface
and subsurface quality of the cast slab. Water is used as
cooling agent for the solid strands coming out of the mould.
The amount of water used to cool the solidifying strand coming
out of the mould, known as secondary cooling intensity, is
maintained in the range of 0.6 - 0.7 1/kg of steel. The
solidified material is finally cut into the desirable lengths
of slabs.
The improved process of the present invention for the cast
slabs of stainless steels with chemical composition and the
casting parameters in the range as mentioned above, are free
from surface depression and subsurface crack. The prime
quality rating of the hot rolled and cold rolled products
processed from these cast slabs have improved to 80% level, as
compared to 60% in case of the conventional prior art.

WE CLAIM
1. An improved process for continuous casting of 1.0 to 8.5%
of nickel stainless steel slabs free from surface
depression and subsurface cracks having excellent quality
of surface and subsurface, said process comprising:
(i) charging and melting of material ingredients
comprising chemical composition such that liquid
steel so formed comprises chemical compositions of
Nieq and Creq , wherein said Nieq and Creq being
present in desirable range;
(ii) refining and chemistry adjustment of said liquid
steel;
(iii) adding of casting powder composition at top surface
of said liquid steel such that heat transfer during
solidification in the mould is controlled, said
powder composition having predetermined basicity and
melting point so as to provide requisite lubrication
and heat transferring medium in gap provided between
the mould and solidifying shell of cast; and
(iv) tapering of solidified shell to a predetermined
dimension so as to facilitate exact match between
solidification shrinkage and mould.
2. A process as claimed in claim 1, wherein the ratio of Nieq
to Creq ranges from 0.52:1 to 0.55:1.
3. A process as claimed in claim 1, wherein the casting
powder composition comprises CaO, SiO2 and mixtures
thereof.

4. A process as claimed in claims 1 to 3, wherein basicity
of the casting powder composition ranges from 1.1 to 1.2.
5. A process as claimed in claims 1 to 4, wherein the
melting point of casting powder composition ranges from
1120°C to 1140°C.
6. A process as claimed in claims 1 to 5, wherein tapering
is maintained in the range of 0.9 to 1.0% per meter of
the mould height.

An improved process for continuous casting of 1.0 to 8.5% of nickel stainless steel slabs free from surface depression and subsurface cracks having excellent quality of surface and
subsurface. The said process comprises charging and melting of material ingredients comprising chemical composition such that liquid steel so formed comprises chemical compositions of Nieq
and Creq in desirable range, refining and chemistry adjustment of said liquid steel, adding of casting powder composition at
top surface of said liquid steel such that heat transfer during solidification in the mould is controlled. The casting powder composition has predetermined basicity and melting point to provide requisite lubrication and heat transferring medium in gap provided between the mould and solidifying shell of cast. The process comprises tapering of solidified shell to
a predetermined dimension so as to facilitate exact match between solidification shrinkage and mould.