FIELD OF THE INVENTION
The present invention relates to an improved wedge shape single-strand tundish
in a continuous metal casting process.
BACKGROUND OF THE INVENTION
The demand for cleaner steels increases every year. In addition to lowering non-
metallic oxide inclusions and controlling their size distribution, morphology, and
composition, a cleaner steel requires control of sulfur, phosphorus, hydrogen,
nitrogen and even carbon, and minimizing metallic impurity elements such as As,
Sn, Sb, Se, Cu, Zn, Pb, Cd, Te and Bi. These requirements vary with the change
in steel grade, and its end use.
Metallic impurity elements, which are traditionally found only in trace amounts,
are becoming an increasing problem due to their accumulation in the scrap
supply. These elements cause intergranular segregation leading to cracks,
detrimental precipitates and other problems, which are often manifested as
silvers in the final product. These elements can be difficult to remove in
steelmaking and refining, but can be lowered by carefully controlling the scrap
charge, or by charging blast furnace iron, direct-reduced iron, or other relatively
pure iron source.

Inclusions generate many defects in the steel product. For example, LCAK steel
can suffer from cracked flanges due to lack of formability, while axels and
bearings suffer fatigue life problems. Both formability and fatigue life are highly
affected by sulfide and oxide inclusions in the steel. Sliver defects occur as lines
along the steel strip surface parallel to the rolling directions. Slivers plague LCAK
steel sheet for automotive applications, causing both cosmetic surface
imperfections and formability problems. They usually consist of aluminates
originating from deoxidation or from complex non-metallic inclusions from
entrained mold slag.
In addition to the amount of inclusions, steel cleanliness depends greatly on the
size distribution, morphology and composition of non-metallic inclusions in the
steel. The inclusion size distribution is particularly important, because large
macroinclusions are the most harmful to mechanical properties.
It is known that Tundish operations greatly affect steel cleanliness. Depending
on its operation, the tundish may act as a further refining vessel to remove
inclusions to the slag layer or it may contaminate the steel through slag
entrainment, reoxidation, and refractory dissolution. The following important
factors are discussed here:

tundish depth and capacity, casting transitions, tundish lining refractory, tundish
flux, gas stirring, and tundish flow control.
Tundish Depth and Capacity
Deep tundishes with a large capacity increase the residence time of liquid steel
and particles, so encourage inclusion removal. Deep tundishes also discourage
vortex formation, enabling more time for ladle transitions before slag
entrainment becomes a problem. Tundish size for Low carbon Aluminium-killed,
LCAK steel has gradually increased world-wide over the past 20 years, typically
reaching 60-80 tons with over 70 inches depth.
Casting Transitions
Casting transitions occur at the start of a casting sequence, during ladle
exchanges and nozzle changes, and at the end of casting. They are responsible
for most cleanliness defects. Inclusions are often generated during transitions
and may persist for a long time, thus contaminating a lot of steel. The sliver
defect index at the beginning of the first heat was found to be 5 times higher
than that at the middle of the first heat and over 15 times that of successive

heats. During these unsteady casting periods, slag entrainment and air
absorption are more likely, which induce reoxidation problems.
During the first casting heat, the entrainment of air and slag in the tundish pour
box due to the turbulence during ladle open is accompanied by an initial
maximum in total oxygen content in the tundish (including both slag and alumina
inclusions). Open pouring at start cast causes total oxygen in tundish to increase
to twice normal levels for more than an entire heat. Several minutes of filling
are needed before tundish flux can be added. Eventually, during steady casting,
the total oxygen decays to lower levels, consisting mainly of alumina.
One improvement during ladle transitions is to stop the flow of liquid into the
mould until the tundish is filled and to bubble gas through the stopper to
promote inclusion flotation. Another improvement is to open new ladles with
submerged shrouding. With this measure.
Near the end of a ladle, ladle slag may enter the tundish, due in part to the
vortex formed in the liquid steel near the ladle exit. This phenomenon requires
some steel to be kept to the ladle upon closing. In addition, the tundish depth

drops after ladle close, which disrupts normal tundish flow and may produce slag
vortexing, slag entrainment, and increased total oxygen in the mold.
Lining Refractory
Dissolved aluminum in the liquid steel may react with oxygen sources in the
lining refractory. This oxygen may come from carbon monoxide, when carbon in
the refractory reacts with binders and impurities, or from silica refractory
decomposition. Silica-based tundish linings are worse than magnesia-based
sprayed linings.
Three types of materials (high Al2O3, AI2O3-SiC-C, and MgO-C with a wear rate
of 1.0, 0.34, 0.16 mm/heat respectively) when adopted at the slag line, the
refractory tends to be damaged by erosive tundish flux and slag, and the MgO-C
brick shows the highest durability among the three.

Tundish Flux
The tundish flux provides several functions. Firstly, it must insulate the molten
steel both thermally (to prevent air entrainment and reoxidation). For example,
by changing tundish flux, nitrogen pickup from ladle to mold decreased from 16
to 5 ppm.
Secondly, in ideal circumstances, the flux should also absorb inclusions to
provide additional steel refining. A common tundish flux is burnt rice hulls, which
is inexpensive, a good insulator, and provides good coverage without crusting.
However, rice hulls are high in silica (around 80% SiO2), which can be reduced
to form a source of inclusions. They also are very dusty and with their high
carbon content (around 10% C), may contaminate ultra low carbon steel.
Basic fluxes (CaO-AI2O3 - SiO2 based) are theoretically much better than rice
hulls at refining LCAK steels, and have been correlated with lower oxygen in the
tundish. For example, the total oxygen decreased from 25-50 ppm to 19-35 ppm
with flux basically increasing from 0.83 to 11. However, it was found that by
using basic tundish flux (40% CaO, 24% Al2O3, 18% MgO, 5% SiO2, 0.5% Fe2O3,

8% C), together with baffles, significantly lowered in total oxygen fluctuation, as
compared to the initial flux (3% CaO, 10-15% Al2O3, 3% MgO, 65-75% SiO2, 2-
3% Fe2O3). The total oxygen decreased from 41 to 21 ppm during ladle
transitions and decreased from 39 to 19 ppm during steady state casting.
Tundish Stirring
Injecting inert gas into the tundish from its bottom improves mixing of the liquid
steel, and promotes the collision and removal of inclusions. The danger of this
technology is that any inclusions-laden bubbles which escape the tundish and
become entrapped in the strand would cause severe defects.
Tundish Flow Control
The tundish flow pattern should be designed to increase the liquid steel
residence time, prevent "short circuiting" and promote inclusion removal.
Tundish flow is controlled by its geometry, level, inlet (shroud) design and flow
control devices such as impact pads, weirs, dams, baffles, and filters. The
tundish impact pad, is an inexpensive flow control device that prevents erosion
of the tundish bottom where the molten steel stream from the ladle impinges.

More importantly, it suppresses turbulence inside the inlet zone, which lessens
slag entrainment. It also diffuses incoming stream momentum and allows the
natural buoyancy of the warm incoming steel to avoid short circuiting,
particularly at startup. If properly aligned, and perhaps together with weir(s) and
dam(s), a pour pad can improve steel cleanliness, especially during ladle
exchanges. Baffles combined with an initial tundish cover lowered the average
total oxygen in the tundish during steady state casting from 39± 8 to 24 ± 5
ppm. Ceramic filters are very effective at removing inclusions. However, their
cost and effective operating time before clogging usually make their use
prohibitive.
Transfer Operations for Clean Steel
Transfer operations from ladle to tundish and from tundish to mold are very
important for steel cleanliness control. One of the most important sources of
oxygen pickup is atmospheric reoxidation of steel during transfer operations. This
generates inclusions which cause production problems such as nozzle clogging,
in addition to defects in the final product. This phenomenon is minimized by
optimizing the use of shrouds, argon injection and submerged entry nozzle (SEN)
operations.

The separation of inclusion from molten steel is a way to achieve the superior
quality of steel.
Nevertheless, a tundish is a steel refining metallurgical vessel acting as a buffer
during continuous casting. The liquid steel flow phenomenon in a tundish
governs the inclusion flotation characteristics. The increase in production results
in an enhanced throughput for tundish. Hence, it leads to loss of residence time
for fluid elements inside the tundish. The bath height of liquid steel in the
tundish can drop below optimum level during ladle change over operation, due
to rise in throughput. The change in throughput can also affect the fluid flow
characteristics of tundish. Thus, metallurgical and refractory performance of
tundish can detoriate with increase in production.
Tundish metallurgy as described hereinabove, is a step in the process of steel
making to remove inclusion. Inclusion flotation inside a tundish depends upon
the establishment of flow behaviour in the tundish. RTD (Residence time
distribution) characteristics is an established criteria for predicting the inclusion
separation in the tundish. Prior publication by Ahuja and Sahai, has postulated
certain RTD characteristics to achieve maximum inclusion separation ratio.

The increase in throughput at the casters is desired to meet the high production
target. The rise in throughput will reduce the theoretical residence time and can
also detoriate the RTD characteristics of tundish. Thus, the increase in volume
of tundish is required to compensate the loss in residence time. The volume
increase can either be incorporated through change in shape of tundish or
change in dimensions of existing wedge shape tundish. The presence of well
block near the outlet is an important feature of existing wedge shape tundish.
The present invention confirms the superior performance of existing wedge
shape tunidsh (having well block) over conventional tundish. Hence, the change
in dimensions of existing tundish was required to increase the theoretical
residence time. The change in dimensions will alter the RTD characteristics of
tundish. Thus, the investigation was performed to propose an improved design
of existing wedge shape tundish.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved wedge shape
single-strand tundish in a continuous metal casting process, which eliminates the
disadvantages of prior art.

Another object of the invention is to propose an improved wedge shape single-
strand tundish in a continuous metal casting process, which is enabled to
maintain the desired flow characteristics at high casting speed.
SUMMARY OF THE INVENTION
According to the invention, there is provided an improved wedge shape tundish
of 35 ton capacity, which is applicable for maximum throughput of 3.5 ton/min.
According to the invention, the bath height of an existing tundish is increased by
12%. The rise in height increases the capacity of the tundish by 5 ton. The
increase in capacity compensate the loss in residence time for the increased
throughput. Wedge shape tundish (without well block) is the most popular
tundish used at high throughput in steel industry. The comparative analysis
performed for this existing tundish with the proposed tundish shows a better
performance of the later. However, the increase in height beyond 12% is found
to be detrimental in respect of fluid flow characteristics. Nevertheless, the
inventive tundish is enabled to improve the flow characteristics in the tundish at
high range of throughput. The improved flow characteristic enhances flotation
and results in a better quality of steel. The current invention will also recover the

loss in bath height at higher throughput during ladle change over operation.
Hence, the throughput at the caster can be increased to 3.5 ton/min with the
inventive tundish.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS AND TABLES
Fig. 1 shows a schematic illustration of a continuous casting system.
Fig. 2 shows the vertical sectional view of the wedge shape tundish according to
the invention.
Fig. 3 shows the variation of theoretical residence time with throughput for a
prior art wedge shape tundish.
Fig. 4a shows the RTD curve of a prior art tundish at throughput of 2 ton/min.
Fig. 4b shows the RTD curve of the inventive tundish at throughput of 3.5
ton/min.
Fig. 5a shows the velocity vectors at meniscus of prior art tundish for throughput
of 2 ton/min.
Fig. 5b shows the velocity vectors at meniscus of the inventive tundish for
throughput of 3.5 ton/min.

Table 1 presents the RTD characteristics of various configuration of wedge shape
tundish (with well block).
Table 2 shows the bath height retained after ladle change over operation for
different cases of wedge shape tundish (with well block).
DETAILED DESCRIPTION OF THE INVENTION
The schematic representation of a continuous casting machine of Fg.l shows
that the incoming molten steel from the ladle goes to the mould through a
wedge shape tundish. The solidification takes place in the mould. The vertical
sectional view of the wedge shape tundish having well block at the bottom with a
bath height as shown in Fig.2. The length of the tundish is typically 4 m and
width is around 0.67 m. The bath height of prior art tundish is 1.1 m. The molten
steel is poured in the tundish through an inlet shown in Fig. 2. It is with drawn
from the tundish through the outlet marked in Fig.2. The flow control device
known as pouring chamber used in such type of tundish is also shown in Fig. 2.
Its location can easily be identified from Fig. 2.

The invention proposes an increases in bath height of prior art wedge shape
tundish by 130 mm. The wedge shape tundish with bath height of 1.23 m, as
proposed by the invention can be seen from Fig.2. The rest of dimensions of
prior art tundish is allowed to remain unaltered according to the invention. The
increase in bath height results in increased residence time of the tundish. Thus,
the invention enhances the inclusion flotation at high range of throughput. Fig. 3
shows a comparison of residence time of prior art and the inventive tundish for
different throughputs. The rise in residence time at high range of throughput for
the inventive tundish can be seen from Fig. 3. The performance of the new
tundish cannot be judged without evaluation of fluid flow characteristics. It is
observed that fluid flow characteristics of prior art tundish at 2 ton/min does not
detoriate through increase in bath height proposed through other invention.
Hence, RTD analysis of the new tundish was performed.
The fluid flow behavior governs the inclusion flotation characteristic of a tundish.
The RTD (Residence time distribution) analysis is a well established criterion to
judge the fluid flow behavior inside the tundish. According to this analysis, the
whole volume of the tundish is divided into three regions for example, plug

volume, dead volume and mixed volume. Plug volume is considered to be the
region with smooth and uniform flow. The maximum inclusion flotation takes
place in this region. Dead volume is defined as that 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, inclusions in the tundish get less time to float out. Mixed
volume is considered to be a mixed flow region of the tundish. The proportion of
inclusion removal in this region is in between the plug and dead region. Thus, a
high ratio of plug to dead volume is desired for better inclusion flotation
characteristics inside the tundish.
The RTD analysis is performed by injecting a tracer in the inlet stream and then
its concentration is measured 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 through
known process based on exit concentration and time variation of the tracer.

The RTD analysis was performed for the inventive tundish at throughput of 3.5
ton/min. The results obtained were compared with that of RTD analysis of a prior
art tundish at throughput of 2 ton/min. Fig. 4a and Fig. 4b show the RTD curve
of a prior art tundish and the inventive tundish respectively. The features of RTD
curve in Fig. 4a and Fig. 4b look similar. The type of similarity suggests that a
fluid flow characteristic of a prior art tundish, which is operated at 2 ton/min,
could be maintained in the inventive tundish too. However, the throughput was
increased to 3.5 ton/min in the invention.
Fig. 5a and Fig. 5b shows the velocity vectors with their magnitude at the
meniscus for a prior art tundish and the inventive tundish, respectively. It may
be noticed that velocity vectors in Fig. 5a was obtained for the tundish operated
at 2 ton/min, while Fig. 5b presents flow vectors at throughput of 3.5 ton/min.
The similarity in the flow pattern can be seen from these figures. However, the
differences in the size above the outlet can also be noticed from Fig. 5a and Fig.
5b. The variation in the magnitude of flow vectors, as observed between these
two figures is due to difference in throughput for the two cases. The flow pattern
can be further diagnosed by comparison of flow vectors at vertical symmetrical

plan of the tundish. Fig. 6a and Fig. 6b shows the flow vectors at vertical
symmetrical plane for prior art and new tundishes respectively. The flow pattern
in Fig. 6a and Fig. 6b again looks similar, through some differences in contour
may be noticed towards the bottom of the tundish. Thus, the recognition that
the tundish of the invention retains the fluid flow characteristics of prior art
tundish is established.
The comparison of the RTD characteristics derived from the RTD analysis can be
seen from Table 1. The marginal high ratio of the plug to dead volume for the
inventive tundish as compared to a prior art tundish can be noticed from Table 1.
The degradation of RTD characteristics with further increase in bath height can
also be noticed from Table 1. Hence, the bath height of 1.23 m has been
recommended in the proposed invention. Any further increase in bath is likely to
alter the fluid flow characteristics.
The ladle changeover operation is one of the phenomenon needs to be
considered to design a tundish. This operation generally takes 5 minutes to
complete. It is always desired that the bath height should not be reduced below

an optimum level. The increase in throughput will reduce the bath height for a
prior art tundish, during this operation. The inventive tundish is enabled to
recover the loss of bath height at high range of throughput. Table 2 shows the
bath height retained after 5 minutes for various tundishes, which are operated at
their respective throughputs.
The current invention further allows increase in metallurgical performances of
the tundish through enhanced residence time. The increased refractory
performance is also achieved through the inventive tundish. The productivity of a
plant increases through implementation of the disclosed invention.

WE CLAIM:
1. A single strand wedge shape tundish to receive molten metal from a ladle
at an inlet and discharge to a mould through an outlet for solidification in a
continuous casting process, the tundish having a well block at the bottom near
the outlet, and a flow-control device, characterized in that the bath height of the
tundish is configured at a value not more than 1.23 meters for casting at a
maximum throughput of 3.5 ton/minute, and in that the length and width is
maintained at 4.0 meters and 0.67 meters respectively.
2. The tundish as claimed in claim 1, wherein the tundish is configured to
provide a desired fluid flow characteristics at high casting speed.
3. The tundish as claimed in claim 1 wherein the flow control device is a
pouring chamber.
4. The tundish as claimed in claim 1 wherein the metal is steel.

ABSTRACT

A single strand wedge shape tundish to receive molten metal from a ladle at an
inlet and discharge to a mould through an outlet for solidification in a continuous
casting process, the tundish having a well block at the bottom near the outlet,
and a flow-control device, characterized in that the bath height of the tundish is
configured at a value not more than 1.23 meters for casting at a maximum
throughput of 3.5 ton/minute, and in that the length and width is maintained at
4.0 meters and 0.67 meters respectively.