FIELD OF THE INVENTION
The present invention generally relates to a technique of using LF out temperature as a
controlling variable to maintain desired super heat during casting process of steel, the
LF out temperature being defined as the temperature at which the heat should be
released from the LF process. The present invention in particular relates to a method for
controlling the superheat of liquid steel within a desired limit in a LF route of steelmaking
and casting.
BACKGROUND OF THE INVENTION
It is known that an optimum control of superheat within the desired limit is required
during casting for producing a good quality steel including minimization of production
losses. An effective temperature control mechanism needs to be adopted for managing
the super heat.
The control of liquid steel temperature during steel making and casting process is an
important factor responsible for productivity of the plant. Ladle is an important
metallurgical vessel used to transfer molten steel from one station to another. The
phenomenon of heat transfer taking place in the ladles needs to be assessed for
prediction of liquid steel temperature. The assessment of thermal profile of liquid steel
from BOF converter to caster can be done through consideration of heat losses at all
points. The current practice of releasing a ladle at high temperature from a ladle furnace
to avoid low superheat problem leads to high super heat cases during the process of
casting. The high superheat compels the operators to reduce the casting speed and
hence leads to production losses.
SUMMARY OF THE INVENTION
The present inventors recognized through experimentations that super heat can be
controlled by accurate prediction of LF out temperature i.e. the temperature at which the
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heat should be released from Ladle Furnace. The prediction of LF out temperature
requires generation of thermal profile of liquid steel in a ladle during its journey from LF
process to casting. The thermal profile of liquid steel is highly dependent upon particular
plant and its practices. There are some temperature prediction system available as a
prior art but these models are applicable for specific plant and their practices. However,
there is no universal model available to fulfil the requirement of all Steel Plants. The
prior art models available in prior art therefore need to be modified and adapted
according to the requirement.
The present invention proposes a simple expression of predicting LF out temperature.
The independent variables of the proposed LF out temperature prediction technique
were identified through extensive CFD investigation. Hence, the proposed invention
generally differs from prior art because of its linear nature, number of independent
parameters, and sensitivity of these parameters.
It was felt that super heat can be controlled by accurate prediction of LF out
temperature i.e the temperature at which the heat should be released from Ladle
Furnace. The prediction of LF out temperature requires the generation of liquid steel
thermal profile from LF process to casting. The thermal profile of liquid steel is highly
dependent upon particular plant and its practices. There are some temperature
prediction system available as a prior art but these models are applicable for specific
plant and their practices. Also, there is no model available to fulfil the requirement of
Tata Steel Plant. The model available in prior art needs to be customized according to
the requirement of Tata Steel Ltd. There is always a scope for improvement as far as
accuracy of such system is concerned. The current invention proposed a simple
expression of predicting LF out temperature. The independent variables of proposed LF
out temperature prediction model were identified through extensive CFD investigation.
Hence, the proposed invention differ with prior art because of its linear nature, number
of independent parameters, and sensitivity of these parameters.
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Accordingly, the present invention provides a method for controlling the superheat of
liquid steel within a desired limit in a LF route of steelmaking and casting. As per the
present invention, an empirical model for prediction of LF out temperature (the
temperature at which heat should be released from Ladle Furnace) is assessed. The
predicted LF out temperature is then applied as a controlling variable to maintain super
heat with in the desired limit during casting. The proposed temperature prediction model
is a linear expression of LF out temperature with parameters identified as contributors
towards heat transfer mechanism. A Computational Fluid Dynamics (CFD) based
investigation is performed for the complete process cycle of a ladle to identify these
parameters. CFD simulations were further performed by varying the identified
parameter to get the sensitivity and trend of the identified parameters. The trial of this
empirical model was done at LD3 tested in an operational steel plant, wherein 97 % of
the predictions show variation in the band of ±10 OC with actual LF out temperature.
The model further shows that 82 % of the predictions were in the band of + 5 OC.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
Fig. 1 shows the Process route of ladle cycle through LF route.
Fig. 2 shows schematic representations of ladle operations happening in the process
route of ladle.
Fig. 3 shows liquid steel thermal profile predicted by CFD model.
Fig. 4 shows liquid steel thermal profile for different slag layer thicknesses.
Fig. 5 shows variation of liquid steel temperature with time for different ladle life.
Fig. 6 shows plant trial results of LF out temperature prediction model.
Table 1 shows chill Factor of Alloys.
Table 2 shows coefficients and constants of equation 1 for LF out temperature
DETAILED DESCRIPTION OF THE INVENTION
A series of ladles completes the process cycle through three different routes named as
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Direct, LF and RH. CFD investigations were performed for all the processes in LF route
to develop comprehensive understanding of ladle operations in this route. Fig 1 shows
the process cycle of ladle through LF route at LD3. The schematic description of the
phenomenon taking place in the ladle through this route can be seen from Fig.2. The
brief description of these processes are given below:
Green Ladle pre-heating: Green ladle is a ladle, which is included in the ladle process
cycle for the first time. This ladle is pre-heated up to hot face temperature of 1000oC,
before being sent for tapping. The phenomenon of pre-heating takes place for
approximately 10 hrs. Thus, this is one of the important steps in ladle cycle.
Tapping: The next step in ladle cycle is tapping. The ladle after attaining required hot
face temperature was sent for tapping. In this process, the molten steel from BOF
vessel is poured to heated ladle and ferroalloy additions are made in the ladle.
OLP: The online purging is next process in the cycle of ladle. The molten steel in the
ladle is purged to attain uniform temperature and chemistry at this process. Ferroalloy
additions are also made during purging to attain required composition of steel.
LF: After online purging ladle goes to LF for further treatment. The ladle additions with
purging is being performed at LF. LF process also have a facility of arcing. Arcing is
done to increase the temperature.
Holding Period: This is the time period between end of Online purging and start of
teeming. During this period ladle is kept on hold. The ladle cover is placed on the ladle
during this period. However, ladle can be without the cover for some time especially
during transportation.
Teeming: Teeming is defined as the phenomenon during which the molten steel is
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poured from the ladle to mould through tundish in the continuous casting process. In
this process, the molten steel is flowed in the tundish through the ladle nozzle.
TAT: The emptied ladle is cleaned and repaired to prepare for next tapping. The time
spent during these processes is defined as turn around time (TAT). The continuous heat
losses from refractory walls take place during this period.
The CFD model was first developed by formulation of governing equations for all the
processes described above. The equations were then solved for the computational
domain of the ladle. The drop in temperature due to ferroalloy additions was calculated
based on the chill factor as mentioned in Table 1. CFD simulations were further
performed by the varying different parameters to study their sensitivity w.r.t liquid steel
temperature drop. The CFD analysis of ladle operations in LF route is mentioned below:
Prediction of Liquid steel thermal profile in ladle through LF route: The thermal profile for
molten steel during tapping to OLP and then from LF to the end of holding period was
generated through CFD simulation. The temperature drop of molten steel ferroalloy
addition was also considered in the simulation. Fig. 3 shows the plot of steel
temperature with time for various stages of the ladle. The linear variation of the
temperature with time can be observed till OLP after that there is an increase in
temperature at LF due to arcing. Fig. 3 shows the matching between predicted and
measured thermal profile at plant.
Effect of slag thickness on liquid steel temperature drop: The thickness of slag layer
was considered to be one of the important parameters for reduction of the heat loss at
the top surface. Hence, simulations were performed for three different slag layer
thicknesses. Fig. 4 shows the variation of steel temperature with time for three different
slag layer thicknesses. Though, the slag layer has an effective role in control of heat
losses, but the significance of its thickness is doubtful and needs an extensive
investigation.
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Effect of ladle life: Ladle life is one of the important parameters to be considered for
investigation of temperature drop of molten steel. The refractory of the ladle gets eroded
with increase in the ladle life. The reducing thickness enhances the heat transfer from
hot faces to steel shell. The simulation was performed for five different life of ladle.
These are 15, 30, 45, 60 & 75 Heats. Fig. 5 shows the variation of steel temperature
with the time (from tapping start to end of holding period) for different life of ladles.
Though, it was expected that temperature drop will increase with the ladle life, but no
sign of increase in temperature drop with ladle life was observed. The comparison of
these cases with green ladle can also be seen from Fig. 5. The trend of the curve in Fig.
5 is similar for all the cases. However, the presence of comparatively high initial
refractory temperature leads to low temperature drop inside the green ladle. Thus, it can
be derived from these plots that the role of ladle life w.r.t molten steel temperature is
minor.
Empirical expression for prediction of LF out temperature: CFD analysis of ladle
operations resulted in comprehensive information about the liquid steel thermal profile.
The information from CFD simulation needs to be translated into an empirical model of
LF out temperature for developing effective super heat control mechanism. The linear
expression for LF out temperature was developed through regression analysis of CFD
data and incorporating the chill factor given in Table 2. Eqn. 1 presents this expression
of LF out temperature.
Predicted LF out Temperature = Liquidus temperature + a1* (Turn Around time) + a2 *
(Holding Time) + a3*(casting duration)+ a0 + S. ----------- (1)
Here,
Predicted LF out Temperature (in OC)
Liquidus temperature (in OC)
Holding Time: Estimated time (in minutes) between LF end to casting start.
Turn Around Time: Time (in minutes) spends in repairing and slag dumping in previous
heat of ladle.
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Casting Duration: Estimated Casting duration (in minutes) of current heat.
S: Desired super heat at caster for current heat (in OC).
The value of coefficient a0 to a3 of eq. 1 can be seen from Table 2.
The plant trail of eq. 1 can be seen from Fig. 6. The trial was done for 99 heats of LF
route at LD3 and 97 % of the predictions show variation in the range of ±10OC with
actual LF out temperature. It was further noticed that 82 % of the predictions shows
variation of + 5 OC with actual LF out temperature.

WE CLAIM
1. A method for controlling the superheat of liquid steel within a desired limit in a LF
route of steelmaking and casting, the process of ladle operation in the LF route
comprising:-
- pre-heading a green ladle in the process of ladle operation;
- pouring the molten steel from the BOF vessel to the pre-heated vessel
including addition of ferro-alloys;
- on-line purging of the molten steel in the ladle including addition of ferroalloys
to attain uniform temperature and chemistry;
- arcing to increase the temperature of the ladle including addition of ladles in
the LF and process route;
- maintaining a holding period in which the ladle cover is placed over the ladle
after undergoing a transportation activity of the ladle when the cover remains
open;
- performing a teeming operation during which the molten steel is injected
through nozzles from the ladle to a tundish;
the method comprising the steps of:-
- predicting a LF-out temperature representation a temperature at which the
ladle being released from the LF process by forming a parametric relationship
in the form of an empirical model; and
- controlling super-heat temperature of the liquid steel with in an optimum
range during the casting process to improve productivity and quality of the
produce,
wherein the empirical model representing a relationship between the parameters
of liquids temperature, holding time, turn-around time, casting duration, and a
desired superheat at the caster for the current heat surcharged with a co-efficient
value in case of all said parameters, and wherein the co-efficient and constant
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used to determine the empirical model have a value-range relationship as herein
described in Table-2.
2. The method as claimed in claim 1, wherein the empirical model for LF out
temperature exhibiting a relationship of linear nature and depends upon three
variables and one constant.
3. The method as claimed in claim 1, wherein the coefficients values are applicable
for 170 – 300 tonne ladle.
4. The method as claimed in claim 1, wherein the turnaround times is in range of 40
to 120 minutes.