FIELD OF THE INVENTION
The present invention relates to grades of steel with improved surface quality, for
continuously cast billet and a method for its production. More particularly, the present
invention relates to a process for continuous casting of high carbon billet with oil
lubrication having carbon in the range of 0.4 - 1.0 wt. % adapted to suit special
applications like forging, agriculture, automobile, wire drawing and the like industries.
Advantageously, the invention is directed to optimizing the composition to have sulphur
content at < 0.3 weight % and Mn/S ratio of > 25 to provide sufficient strength and
ductility to the solid shell and selective use of casting process parameters comprising
casting speed, mould water flow etc. and also augmenting hardwares of the caster to
ensure elimination of surface defects like lap and bleed with irregular oscillation marks.
The lubricating oil temperature is not allowed to exceed its boiling start temperature, by
reducing the peak hot face temperature of mould below 200 CC. for the high carbon steel
cast billets produced following the conventional BOF - LTS/LF - Billet Caster route. The
present invention is thus capable of wide industrial application in continuous casting plants
of steelmaking industries ensuring good quality cast billets free of surface defects,
reducing rejections, improving productivity and significant saving on cost of production.
BACKGROUND ART
It is well known in the art of continuous casting of steel billets having high carbon contents
that the prime reasons for low acceptance and high rejection of cast billets are attributable
to surface lap and bleed associated with irregular oscillation mark. Other quality issues in
terms of longitudinal off-corner crack, midway / reheating crack and central looseness are
taken care of by using the various technological measures practiced in the existing
process.
Lap and Bleed are some common surface defects associated with irregular oscillation
marks in continuous casting of high carbon billets with oil lubrication , following the usual
BOF - LTS/LF - Billet Caster process route. Laps form at meniscus due to overflow of liquid,
whereas bleeds originate just below the meniscus due to tearing of solid shell. However,
mechanism of their formation is same. This is basically related to high hot face
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temperature associated with high peak heat flux in mould for high carbon steel. This is
essentially because the solid shell in the mould is relatively much thinner for steels with
carbon > 0.3 weight % , as compared to that for lower carbon . Excessive metal level
fluctuation associated with high speed, and low high temperature shell strength and
ductility resulting due to high S and low Mn/S ratio further aggravate the situation.
The phenomenon of loss of mould lubrication occurs when peak hot face temperature
exceeds the boiling start temperature of lubricating oil. This leads to sticking of the solid
shell at/near the meniscus. This in turn results in overflow of liquid steel at meniscus and
formation of laps. Moreover, mould sticking below meniscus results in shell tearing causing
bleed formation. High hot face temperature of mould is the consequence of improper
casting process parameters comprising high casting speed, low water velocity in mould
water channel, quality of lubricating oil and very low or high negative strip time of mould
oscillation.
There has therefore been a persistent need for developing a reliable method of production
to meet the stringent requirement of surface quality of continuously cast billet involving oil
lubrication for high ( 0.4 - 1.0 weight %) carbon steels directed to special applications like
forging, agriculture, auto industry, wire drawing and the like. There has also been the
need for identify the cause of surface defects like lap and bleed associated with irregular
oscillation marks and adopt remedial measures for effecting quality improvement of cast
billets. Therefore process for manufacturing such improved surface quality of steel needed
optimizing the chemistry and casting process parameters to achieve certain quality
attributes in cast billet in terms of internal and surface/sub-surface quality for the required
applications, while augmenting the hardwares of the caster to support the process
requirements to achieve reliability of the process in the long run in a simple and cost
effective manner.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to developing cast steel billet
free of surface defects like Lap and Bleed associated with irregular oscillation marks in
continuous cast billets with high carbon contents using oil lubrication during casting,
suitable for applications like forging, agriculture, auto industry, wire drawing etc.
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A further object of the present invention is directed to developing a process for the
manufacture of continuously cast high carbon steel billets with improved surface quality
free of lap and bleed, by reducing the peak hot face temperature of mould preferably
below 200°C.
A still further object of the present invention is directed to developing a process for the
manufacture of continuously cast high carbon steel billets with improved surface quality
free of lap and bleed, by optimizing the chemistry as well as the process parameters to
achieve certain quality attributes in cast billet in terms of internal and surface/sub-surface
quality for the desired applications along with augmenting the hardware implements of the
caster to support the process requirements.
A still further object of the present invention is directed to developing a process for the
manufacture of continuously cast high carbon steel billets with improved surface quality
free of lap and bleed, wherein the reduction in peak hot face temperature of mould is
achieved by controlling the casting parameters comprising casting speed, mould water
flow, Electromagnetic stirring ( EMS ) current intensity, Mould oscillation stroke etc.
A still further object of the present invention is directed to developing a process for the
manufacture of continuously cast high carbon steel billets with improved surface quality
free of lap and bleed, wherein the selective composition of steel is maintained to ensure
Mn/S ratio ( > 25) to provide sufficient strength and ductility to the solid shell in the
mould at high temperature to avoid shell puncturing during casting.
A still further object of the present invention is directed to developing a process for the
manufacture of continuously cast high carbon steel billets with improved surface quality
free of lap and bleed, wherein synthetic oil with higher boiling start temperature (>200 °C)
is selectively used replacing conventional rapeseed oil which has lower boiling start
temperature, thus avoiding non-lubricating region near the meniscus at casting
temperature , and thereby eliminating occurrence of overflow of liquid steel at meniscus
and formation of laps or mould sticking below meniscus causing shell tearing leading to
bleed formation.
A still further object of the present invention is directed to developing a process for the
manufacture of continuously cast high carbon steel billets with improved surface quality
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free of lap and bleed, wherein mould oscillation stroke has been modified to achieve
adequate stripping action with minimisation of sticking of strand in the mould.
A still further object of the present invention directed to developing a process for the
manufacture of continuously cast high carbon steel billets with improved surface quality in
order to reduce rejection and metal loss, improve yield and throughput, at reduced cost of
production in continuous casting of billets.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to a process for continuous
casting of high carbon billet with oil lubrication comprising:
step of increasing the surface quality of the continuously cast billets comprising reducing
the peak hot face temperature of the mould such as to reduce the chances of boiling of the
lubricating oil , and thereby eliminating the non-lubricating area , and obtaining the said
billets substantially free of any surface lap and bleed associated with irregular oscillation
mark.
Another aspect of the present invention is directed to a process for continuous casting of
high carbon billet with oil lubrication wherein said lubricating oil is selectively synthetic oil
with higher boiling start temperature which is > 200 °C.
A still further aspect of the present invention is directed to a process for continuous
casting of high carbon billet with oil lubrication wherein said peak hot face temperature is
maintained below 200 ° C by reduced casting speed in the range of 2.0 - 2.2 m/min and
increasing the mould water velocity to about 10.5 m/sec.
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According to a further aspect of the present invention directed to a process for continuous
casting of high carbon billet with oil lubrication comprising optimization of electromagnetic
stirring ( EMS ) current intensity wherein current is reduced to about 275 A such as to
control high meniscus level turbulence without hampering the internal quality of cast billet.
A still further aspect of the present invention is directed to a process for continuous
casting of high carbon billet with oil lubrication comprising selectively optimizing the steel
composition in terms of sulphur content at < 0.3 weight % and Mn/S ratio of > 25 to
provide sufficient strength and ductility to the solid shell in the mould at high temperature
to avoid shell puncturing during casting.
A still further aspect of the present invention is directed to a process for continuous
casting of high carbon billet with oil lubrication comprising modifying the mould oscillation
stroke such as to achieve adequate stripping action with minimization of sticking of strand
in the mould.
According to yet another aspect of the present invention is directed to a process for
continuous casting of high carbon billet with oil lubrication wherein the oscillation stroke is
reduced to about 11 mm to keep negative strip time of mould oscillation in the range of
0.12 - 0.15 s.
The present invention and its objects and advantages are described in greater details with
reference to the following non-limiting illustrative figures.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1a & 1b: is the illustration of billet surface showing laps and bleeds associated
with irregular oscillation mark in conventional high carbon continuously cast billets.
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Figure 2: is the illustration of the temperature profile of Mould Copper Tube at different
casting conditions/parameters.
Figure 3: is the illustration of the hot face temperature distribution of the copper mould
tube at various casting conditions/parameters.
Figure 4: is the graphical illustration showing the effect of EMS current on percentage
equiaxed zone.
Figure 5 : is the illustration showing the effect of Mould Water Flow and Casting Speed
on the peak Mould Hot Face Temperature.
Figure 6 : is the illustration of improved billet surface with regular oscillation mark and
absence of lap and bleed after innovation.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
It has already been described that the present invention is directed to developing a
process for eliminating occurrence of common surface defects like lap and bleed associated
with irregular oscillation marks in continuous casting of high carbon steel billets with oil
lubrication. More particularly, the process of the invention is directed to identify the causes
of surface defects like lap and bleed associated with irregular oscillation marks, and
implement remedial measures for quality improvement of continuously cast high carbon
billets. The invention is directed to meet the desired surface quality of billet , and reduce
rate of rejection by optimizing the chemistry of cast billet and also controlling the casting
process parameters, as well as augmenting hardware element of the caster for producing
the desired- grades of steel following the usual process route of BOF - LTS/LF - Billet
Caster. The process of the invention is capable of achieving certain quality attributes in
cast billet in terms of internal and surface/sub-surface quality for the desired application
in forging, agriculture, auto industry, wire drawing etc. in a reliable and cost effective
manner, reducing rejection and metal loss.
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In order to have a clear understanding of the objective of the present invention, reference
is first invited to the accompanying Figure 1, that illustrates the occurrence of laps and
bleeds associated with irregular oscillation mark on the billet surface obtained in the
conventional continuous casting of the high carbon (with 0.6 weight % carbon) containing
steel billets following the BOF - LTS/LF - Billet Caster route. The reasons behind the
occurrence of such defects are attributable to the higher peak mould face temperature and
consequently reaching the boiling start temperature of lubricating oil. This leads to sticking
of the solid shell at/near the meniscus which further results in overflow of liquid steel at
meniscus and formation of laps. Moreover, mould sticking below meniscus results in shell
tearing causing bleed formation. High hot face temperature of mould has been identified in
the existing process to be the consequence of factors like improper casting process
parameters comprising high casting speed, low water velocity in mould water channel,
quality of lubricating oil and very low or high negative strip time of mould oscillation and
the like.
The present invention aims to solve the occurrences of the surface defects by
implementing the following method steps and optimization of the process parameters.
Calculation of copper mould hot face temperature at different casting conditions:
Reference is next invited to the accompanying Figure 2 that illustrates the temperature
profile of the mould copper tube at different casting conditions. The temperature profile of
the mould tube has been computed through two dimensional heat transfer model by
solving Fourier equation using control volume method. Temperature profile has been
calculated for different casting speed ranging from 2.0 to 2.5 m/min, mould water velocity
ranging from 8.5 (1232 Ipm) to 10.5 (1522 Ipm) m/sec, superheat up to 50 °C and steel
carbon range of 0.50 - 0.65 % . The result of the computations have been plotted
graphically . Temperature profile at different casting conditions has been shown in Figure
2. It can be concluded that lowest mould copper tube temperature distribution could be
achieved at lower casting speed along with higher mould water velocity.
Reference is now invited to the accompanying Figure 3 that illustrates the hot face
temperature of the copper mould tube at various casting condition. It shows hot face
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temperature can be reduced to less than 200 ºC from as high as 275 ºC by reducing
casting speed from 2.5 to 2.0 m/min, and increasing mould water flow from 1232 to 1522
Ipm. In addition , lowering of speed resulted in lower mould level fluctuation which
restricts formation of lap and bleed .
Optimisation of EMS current
Optimization of EMS parameter is important both with respect to centerline quality in
terms of pipe and extent of equiaxed zone ( EAZ ) , as well as surface quality in terms of
lap and bleed . For optimization in case of high carbon billets , EMS current was varied
from 200 to 300 A at 8 Hz frequency. Billet macro sample was taken for each current
variation to observe the extent of EAZ . The result is shown in accompanying Figure 4.
The average equiaxed zone at 300 A is 35 %, which is acceptable for special grade
application. However, at 300 A of EMS current, occasional formation of lap and bleed was
observed on the billet surface. Therefore, the EMS current was further reduced to 275 A ,
where the EAZ is around 30 % along with absence of lap and bleed on the billet surface.
Optimisation of steel chemistry
Higher sulphur and lower Mn/S in steel enhances lap and bleed formation. Sulphur reduces
hign temperature strength and causes embrittlement of the solid shell at high
temperature. Manganese reduces the deleterious effect of sulphur by combing with it .
Sulphur content has been restricted to < 0.03% , and Mn/S ratio has been-kept > 25 to
improve high temperature strength and ductility.
Change in lubricating oil
Synthetic oil with higher boiling start temperature ( > 200 C ) has been introduced in
place of rapeseed oil which has lower boiling start temperature. High hot face temperature
causes boiling of lubricating oil , and thereby generating non-lubricating region near the
meniscus at casting temperature. Since synthetic oil has higher boiling start temperature,
boiling does not take place in association with reduced speed and high mould water
velocity as shown in Figure 5.
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Moification of mould oscillation parameters
Oscillation parameters also have significant influence on caster performance in terms of
surface quality with respect to lap and bleed in solidifying strand. Hot face temperature of
the mould tube depends on oscillation parameters. Higher mould oscillation stroke length
increases shell deformation below the meniscus , leading to sticker formation due to
increased mould strand interaction. The billet casters are normally operated with a mould
stroke of 12 mm at frequency of 150 cycle per minute. The oscillation stroke has been
reduced to 11 mm to keep negative strip time of mould oscillation in the range of 0.12 -
0.15 s.
The present invention thus is directed to achieving the following major objectives in order
to eliminate the surface defects, in particular the defects like the lap and the bleed, by way
of attaining the end results in the optimized parameter values as follows:
i) Reduction of peak hot face temperature of mould by reducing casting speed to
2.0 - 2.2 m/min from a level of 2.5 m/min to reduce the chances of boiling of
lubricating oil , thereby eliminating the non-lubricating area ;
ii) Increase of mould water velocity to 10.5 m/sec by increasing the water flow
rate in the mould water channel to reduce peak hot face temperature below 200
C which is the boiling point of lubricating oil ;
iii) Optimization of Electromagnetic stirring ( EMS ) current intensity : current
reduced to 275 A from 350 A to control high meniscus level turbulence without
hampering the internal quality of cast billet ;
iv) Steel chemistry optimization in terms of sulphur content ( < 0.3 weight % ) ,
and Mn/S ratio ( > 25) to provide sufficient strength and ductility to the thin
solid shell in the mould at high temperature to avoid shell puncturing during
casting;
v) Synthetic oil has been introduced in place of rapeseed oil as a lubricating oil to
improve the boiling characteristics ( boiling start temperature > 200 C ) ;
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vi) Mould oscillation stroke ( 11 mm ) has been modified to achieve adequate
stripping action with minimization of sticking of strand in the mould ;
The improved casting process has led to complete elimination of lap and bleed in high
carbon special quality steels. A typical billet surface photograph with modified casting
process is shown in Figure 6. The billet surface clearly shows regular oscillation mark and
elimination of lap and bleed .
It is thus possible by way of the present invention to produce high carbon grades of
continuous cast steel billet free of surface defects like the Lap and the Bleed by optimizing
the chemical composition and controlling the process parameters for the optimum result
directed to limiting the peak mould surface temperature to below 200°C with
advantageous use of synthetic lubricant oil having higher boiling start temperature and to
achieve adequate stripping action with minimization of sticking of strand in the mould. The
quality of such continuous cast billet thus would favour less rejection or in-process metal
loss leading to improved yield and acceptance, and resultant reduction in cost of
manufacturing of continuously cast high carbon billets in still mills, for special industrial
applications comprising forging, agri/auto industry, wire drawing and the like, ensuring
reliable performance.
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WE CLAIM :
1. A process for continuous casting of high carbon billet with oil lubrication comprising:
step of increasing the surface quality of the continuously cast billets comprising reducing
the peak hot face temperature of the mould such as to reduce the chances of boiling of the
lubricating oil and thereby eliminating the non-lubricating area , and obtaining the said
billets substantially free of any surface lap and bleed associated with irregular oscillation
mark.
2. A process for continuous casting of high carbon billet with oil lubrication as claimed in
claim 1 wherein said lubricating oil is selectively synthetic oil with higher boiling start
temperature which is > 200 C.
3. A process for continuous casting of high carbon billet with oil lubrication as claimed in
anyone of claims 1 or 2 wherein said peak hot face temperature is maintained below 200°C
by reduced casting speed in the range of 2.0 - 2.2 m/min and increasing the mould water
velocity to about 10.5 m/sec.
4. A process for continuous casting of high carbon billet with oil lubrication as claimed in
anyone of claims 1 to 3 comprising optimization of electromagnetic stirring ( EMS ) current
intensity wherein current is reduced to about 275 A such as to control high meniscus
level turbulence without hampering the internal quality of cast billet.
5. A process for continuous casting of high carbon billet with oil lubrication as claimed in
anyone of claims 1 to 4 comprising selectively optimizing the steel composition in terms of
sulphur content at < 0.3 weight % and Mn/S ratio of > 25 to provide sufficient
12

strength and ductility to the solid shell in the mould at high temperature to avoid shell
puncturing during casting.
6. A process for continuous casting of high carbon billet with oil lubrication as claimed in
anyone of claims 1 to 5 comprising modifying the mould oscillation stroke such as to
achieve adequate stripping action with minimization of sticking of strand in the mould.
7. A process for continuous casting of high carbon billet with oil lubrication as claimed in
anyone of claims 1 to 6 wherein the oscillation stroke is reduced to about 11 mm to keep
negative strip time of mould oscillation in the range of 0.12 - 0.15 s.
8. A process for continuous casting of high carbon billet with oil lubrication substantially as
hereindescribed and illustrated with reference to the accompanying figures.

A process for continuous casting of high carbon steel billet with improved surface quality.
More particularly, the invention relates to continuous casting of high carbon billet with oil
lubrication having carbon in the range of 0.4 - 1.0 wt. % for applications like forging,
agriculture, automobile, wire drawing etc. Advantageously, the invention is directed to
optimizing the composition to have sulphur content at ≤ 0.3 weight % and Mn/S ratio of
> 25 to provide sufficient strength and ductility to the solid shell . and selective use of casting process parameters comprising casting speed, mould water flow etc. and also augmenting hardwares of the caster to ensure elimination of surface defects like lap and bleed with irregular oscillation marks. The lubricating oil temperature is kept below its boiling start temperature by reducing peak hot face temperature of mould below 200 ºC. The present invention is thus capable of wide industrial application in continuous casting plants of steel producers ensuring good quality cast billets free of surface defects.