FIELD OF THE INVENTION
The present invention generally relates to a continuous metal casting process.
More particularly, the invention relates to a curved shape multi-strand billet
caster tundish in a continuous metal casting system.
BACKGROUND OF THE INVENTION
Tundishes, located between the ladle delivering liquid steel to the caster floor
and the continuous caster mold, are basically large containers for holding a
reservoir or liquid steel. The liquid steel is transferred from the ladle through a
ladle shroud extending into the tundish, and the liquid steel is fed at a
continuous or semicontinuous flow rate controlled by a stopper rod, or by a slide
gate assembly.
Extensive water flow-model studies have been made throughout the steelmaking
industry to simulate fluid flow patterns of the liquid steel within an actual
tundish. These water flow-models have been beneficial in determining critical
area of tundish design such as depth of bath, well block locations, and placement
of fluid flow control devices within the tundish. As a result of these studies, it is
well-known that the fluid flow generated by an incoming ladle stream is reflected
from the flat tundish floor toward the surface of the liquid steel. This

generated fluid flow causes a turbulent boiling action and extensive wave motion
at the surface of the steel bath. Additionally, where the fluid flow forces are
obstructed by structural barriers such as tundish side and end walls, the ladle
stream fluid flow surges upward, along such barriers, and causes excessive
turbulence at the surface of the liquid steel bath. The excessive turbulence
produced by the upward surge breaks up the tundish flux cover, and produces a
downward surge around the ladle shroud. The broken flux cover allows the liquid
steel to be exposed to the atmosphere which generates conditions conducive to
altering the chemistry of the steel bath. The chemical changes typically involve
loss of aluminum from the bath and/or absorption of oxygen and nitrogen into
the steel. The downward, shear flow of the liquid steel swirling around the ladle
shroud, entraps particles from the broken slag cover within the ladle stream.
Surface requirements, and cleanliness standards for modern high quality steel
products, warrant that impurities and chemical changes are not allowable within
the product. Heretofore, there have been various attempts to reduce or
eliminate surface turbulence within a continuous caster tundish to improve the
quality of the finished steel product.
US 5169591discloses an apparatus for reducing surface turbulence in a molten
metal bath, and more particularly, impact pads for controlling the fluid flow
patterns of an incoming ladle stream for the purpose of reducing surface
turbulence within a continuous caster tundish.

US 2003201588 describes a well block for use in a refractory lining in a
metallurgical vessel for holding molten metal. The well block is comprised of a
body formed of a refractory material. The body has a top surface, a bottom
surface, and inner surface defining a bore that extends through the body from
the top surface to the bottom surface, and a double-curved outer surface having
at least one peak or valley formed thereon.
EP 0376523 discloses a means for purifying molten metal, particularly steel, by
removal of impurities/inclusions during continuous casting. The molten metal is
passed in a tundish through a vertical array of baffles that are spaced apart
transversely across the tundish to provide restricted flow channels. The baffles
may be flat boards or ties and are preferably used in contact with a weir and
dam upstream and downstream respectively of the baffles. The speed and
direction of flow of the molten metal can thereby be controlled and directed to
give more effective dwell time in contact with a surface covering layer of flux.
US 2008210719 discloses a tundish assembly including an element for preventing
or limiting steel reoxidization in the process of continuous casting of molten
steel, which is used in combination with a refractory nozzle. The element has an
orifice engaging the outer surface of the nozzle, a main surface surrounding the
main orifice and having a lowest level than the top outer edge of the nozzle inlet
portion, and a periphery having an upper face surrounding the main surface of
the element. The upper face of the periphery of the element is higher than the

main surface of the element and is higher than the surface of the bottom well of
the tundish. The main surface of the element is arranged so as to contact molten
steel when the tundish is in use.
Fluid Flow phenomenon inside the tundish as described hereinabove, governs the
inclusion flotation characteristics of the tundish. Hence, the metallurgical
performance of any tundish depends upon the flow pattern generated inside it.
The flow phenomenon in the multi-strand tundish is more complex as compared
to single strand tundish. Thus, an innovative configuration of a multistrand
tundish is desired to generate the required flow pattern inside the tundish.
The separation of the inclusion from the molten steel is a way to achieve a
superior quality of steel. Tundish metallurgy is a step in the process of steel
making for removal of inclusion which inter alia improves the quality of steel, as
the inclusion flotation inside the tundish depends upon the establishment of flow
behaviour in the tundish. RTD (Residence time distribution) characteristics in an
established criteria for predicting the inclusion separation in the tundish. Ahuja
and Sahai have postulated certain RTD characteristics for achieving the
maximum inclusion separation ratio. Prior art further describe several processes
to evaluate fluid flow behaviour using physical and mathematical modeling in a
tundish. Accordingly, several configuration of tundish, such as rectangular
tundish, tundish with sloping sides, curved shape tundish and wedge shaped
tundish etc are known.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a curved shape multi-strand
billet caster tundish in a continuous metal casting system, which eliminates the
prior art disadvantages.
Another object of the invention is to propose a curved shape multi-strand billet
caster tundish in a continuous metal casting system which improves removal of
inclusions and enhances quality of the casted metal products.
SUMMARY OF THE INVENTION
According to the invention, there is provided an improved curved shape six
strand billet caster tundish with well block attached to the bottom. The outlet of
the tundish is placed at the bottom of the well block. The well block makes the
flow smooth and reduces the turbulence near the outlet. The well block also acts
as a separate buffer inside the tundish, and enhances the inclusion flotation
characteristics of this multi-strand tundish. The depth of the well block is
selected at about 38 % of the liquid steel bath height along the four outlets
distally placed from the poring position. However, the depth of the well block at
the outlets placed proximal to the poring positions is restricted to 25 % of the
liquid steel bath height. The length of the well block is equal to the length of the

tundish while the width of the well block is substantially equal to the width of a
narrower region of the tundish. The comparison of metallurgical performance of
the inventive tundish with that of a known flat bottom curved shape six strands
tundish is conducted through RTD analysis. The comparative analysis shows a
superior performance of the inventive tundish. However, any further increase in
the well block depth beyond said optimum level was found to be detrimental with
respect to fluid flow characteristics. Thus, the proposed invention is enabled to
improve the flow characteristics in the multi-strand tundish. The improved flow
characteristic inter alia enhances inclusion flotation and results in a better quality
of steel.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 presents a schematic illustration of a continuous casting system.
Figure 2 shows an isometric view of symmetrical half of a six-strand tundish
of the invention.
Figure 3 shows top and sectional views of symmetrical half of six-strand Tundish
according to the invention.
Figure 4 shows top and sectional view of symmetrical half of a known curved
shape six-strand tundish.

Figure 5 shows the flow vectors at vertical sectional XZ plane (above
the outlets) in a prior art flat bottom tundish and that of the inventive
tundish.
Figure 6 shows the flow vectors at vertical sectional YZ plane (above outlet 1)
inside a prior art flat bottom tundish and an inventive tundish.
Figure 7 shows flow vectors at top surface XY plane inside a prior art flat bottom
tundish and an inventive tundish.
Figure 8 shows RTD Curve for a prior art flat bottom tundish and the inventive
tundish.
Table 1 presents the RTD characteristics of the inventive tundish and a prior art
flat bottom tundish.

DETAIL DESCRIPTION OF THE INVENTION
The schematic representation of a continuous casting machine in figure 1 shows
that the incoming molten steel from a ladle goes to a mould through a tundish.
The solidification takes place in the mould . A configuration of the inventive six
strand tundish can be seen from figure 2. The well block attached at the tundish
bottom can be clearly visualized from figure 2. The top view of the inventive
tundish is shown in figure 3. The length of the symmetrical half of the tundish is
typically 3.4 m, while width is 0.78 m at top and 0.46 m at bottom. The bath
height of the tundish is 0.8 m. The well block and its dimension can also be seen
from figure 3. The well block depth showed for section CC in figure 3 is
applicable for the region between the symmetrical plane and section CC
However, the well block depth in rest of the region of the inventive tundish is
equivalent to one shown for section BB' in figure 3. The molten steel is poured in
the tundish through an inlet shown in figure 3. The liquid steel is with drawn
from the tundish through the outlets marked in figure 3. Location of a prior art
flow control device is also shown in figure 3. The well block present in the
inventive tundish allows an increase in the path flow vectors moving towards the
outlet. This further makes the flow smoother through reduction in the turbulence
near the outlet.
Figure 4 presents the top and sectional view of the symmetrical half of a prior art
curved shape flat bottom six- strand tundish. The metallurgical performance of

the inventive tundish was compared with the prior art flat bottom curved shape
tundish, to validate the invention. The turbostop (flow control device) and
operating parameters were kept common for both prior art and inventive
tundishes, during the comparative analysis.
It is known that fluid flow phenomenon in the tundish is responsible for
producing the quality steel from the tundish. The inclusion flotation characteristic
in a tundish is governed by the fluid flow behaviour in the tundish. The RTD
(Residence time distribution) analysis is a well established criterion for judging
the fluid flow behaviour inside the tundish. According to this analysis, the whole
volume of the tundish is divided in three regions; these are plug volume, dead
volume and mixed volume. Plug volume is considered to be the regions where
the flow is smooth and uniform. The maximum inclusion flotation takes place in
the plug volume region. The Dead volume is defined as the region of the tundish
where the flow is either stagnant or circulating in a small cell. The dead region
reduces the effective volume of the tundish and hence results in a reduction of
mean residence time. Thus, inclusion in the tundish gets less time to float out
and hence the inclusion flotation characteristics get hampered due to presence of
the dead region in the tundish. Mixed volume is a region of the tundish where
the flow is mixed and the proportion of the inclusion which removes out in this
region is in between the plug and dead region. Thus, a high ratio of the plug to
dead volumes is always desired for better inclusion flotation characteristics in the
tundish.

The RTD analysis is performed by injecting a tracer in the inlet stream and then
measuring the tracer concentration at the exit stream. The RTD curve is then
plotted, which represents the variation of exit concentration of the tracer with
time. Finally, the percentage of the plug, dead and mixed volume is calculated
based on well established formula on exit concentration of trace and time
variation of the tracer.
The fluid flow pattern inside the inventive tundish was generated through CFD
simulation. The flow patterns were then compared with a prior art flat bottom
tundish. Figure 5 shows the flow vectors at vertical XZ plane for both the
tundishes. The flow reversal towards the pouring region of a prior art flat bottom
tundish can be clearly seen from Figure 5. However, the flow vectors in the
invented tundish move towards the outlet 3 from the pouring region. The intense
flow with high magnitude of velocity can be seen for the prior art flat bottom
tundish as compared to the invented tundish. The flow patterns at vertical YZ
plane above outlet 1 for both the prior art and the inventive tundishes can be
seen from figure 6. An increase in path of the flow vectors moving towards the
outlet can be observed for the invented curved shape tundish as compared to
the prior art flat bottom tundish. The reduction in velocity magnitude near the
outlet for proposed tundish can also be observed from figure 6. The flow field at
the top surface of both the prior art and the inventive tundishes can be seen
from figure 7. The high velocity region appears to be larger for the prior art flat
bottom tundish as compared to the inventive tundish. The high surface velocity

leads to a surface fluctuation. The diversion of flow vectors towards the curved
wall for the inventive tundish can be seen from figure 7. This type of diversion
leads to a rotatory flow in the pouring region and thus the path of the fluid
elements is increased.
The RTD analysis is performed for the present invention and the results obtained
were compared with the prior art curved shape flat bottom tundish. Figure 8
shows the RTD curve of both the prior art and inventive curved shape tundishes.
The high peak concentration for the prior art flat bottom tundish can be seen
from figure 8. The low value of tmin and tpeak for the inventive tundish can be
clearly observed for the inventive tundish. The above mentioned findings
sufficiently indicate the rise in plug volume and fall in dead volume for the
invented curved shape tundish as compared to a prior art flat bottom tundish.
The Comparison of the RTD characteristics derived from the above mentioned
RTD analysis can be seen from table 1. The parameter named as 'dispersed plug
volume' is considered by the present inventors as the influencing factor to judge
the fluid flow characteristics inside the tundish.
Dispersed plug volume is the ratio of average of tminand tpeak of the residence
time of the tundish. A high ratio of the dispersed plug to dead volume for the
inventive tundish as compared to the prior art fiat bottom tundish indicates a
superior metallurgical performance of the inventive tundish.

WE CLAIM:
1. A multi-strand tundish in a continuous metal casting process, the tundish
constituting a buffer between a ladle and a mold for receiving molten
metal in an inlet and discharging the molten metal at one or more outlet,
the tundish provided with a well block at the bottom of a curved shape
wall of the tundish, being equal to smallest width of the tundish in the
narrower region, wherein the well block depth is between 20 % to 30 %
of the liquid steel bath height near the pouring region and 30 % to 40 %
of the bath height along the rest of the bath, wherein the width of the
well block is between 0.4m to 0.5 m with the length equaling that of the
tundish, and wherein the shroud submergence depth of the tundish is
between 30% to 50 % of the bath height.
2. The tundish as claimed in claim 1, wherein the pouring chamber is
enabled to act as a flow control device.
3. The tundish as claimed in claim 1, wherein the curved wall of the tundish
is disposed near the pouring point.
4. The tundish as claimed in claim 1, wherein the tundish has six strands.

5. The tundish as claimed in claim 1, wherein the metal is steel.

The invention relates to a multi-strand tundish in a continuous metal casting
process, the tundish constituting a buffer between a ladle and mold for
receiving molten metal in an inlet and discharging the molten metal atone or
more outlet, the tundish provided with a well block at the bottom of a curved
shape wall of the tundish, being equal to smallest width of the tundish in the
narrower region, wherein the well block depth is 25 % of the liquid steel bath
height near the pouring region and 38 % of the bath height along the rest of
the bath, wherein the width of the well block is 0.46 m with the length
equaling that of the tundish, and wherein the shroud submergence depth of
the tundish is 50 % of the bath height.