FIELD OF THE INVENTION
The present invention is an improved tundish with a recessed bottom at outlet to
reduce skull volume and to provide more steel height to avoid vortexing. The
invention can find application in continuous casting tundish of steel plants which
utilize a dry vibratable refractory as the working lining of the tundish.
BACKGROUND OF THE INVENTION
Continuous casting of steel is currently one of the most popular and important steps
in converting the refined molten steel to semi-finished long or flat product. The
process essentially consists of an open ended water cooled cooper mould through
which the molten steel is passed. Heat transfer to the mould quickly freezes a thin
shell of steel around the liquid, the strength of which is enough to contain the liquid
when it emerges out of the mould. The remainder of the liquid steel is frozen outside
of the mould by water sprays. In this way casting of steel is a continuous process
where molten steel is poured into the mould from the top and partially solidified steel
is withdrawn from the bottom.
Steel making on the contrary is a batch process where a fixed weight of steel can be
refined together, which must be sent to the casting machine at one go. The caster
would then have to wait for a certain amount of time for the next batch of steel to
arrive. In order to bridge a batch process to a continuous operation, a tundish (2) is
used in-between the steel carrying ladle and the mould. The molten steel from the
ladle is poured into the tundish from the top (4) and one or more outlets at the
bottom (1) transfers the steel to the mould. When the liquid steel at the ladle runs
low and a new batch is awaited, the remainder of the steel contained in the tundish is
enough to keep feeding the mould. Thus the volume of the liquid steel in the tundish
acts as a buffer.

Depending on operational practices, a continuous casting machine can operate non-
stop for upwards of 72 hours. However at the end of its casting sequence, the
remainder of the steel in the tundish is drained as much as possible to be converted
into the solidified product. Any steel remaining in the tundish is essentially wasted.
This loss of metal in the tundish is called as skull and accounts for reduced yield of
the caster calculated as the percentage difference in weight of liquid steel accepted by
the caster and the weight of steel cast. Thus in the interest of higher productivity, it is
necessary to drain the tundish fully. However in order to cut down on heat loss and
oxidation on the exposed metal surface, a layer of slag is floated on top of the molten
steel in the tundish. When the level of steel in the tundish runs low, there arises a
possibility of vortex formation near the tundish outlets. These vortices entrain surface
slag and carry them into the mould. Since the thermal and mechanical properties of
these slag is very different from the steel, they severely weaken the solidified steel
shell which can no longer hold the pressure of liquid steel. This leads to a breakout
which can lead to significant downtime of the casting machine. Even without the slag
layer on top, most vortices may also entrain air with it, leading to a similar result.
Thus operationally an amount of steel is wasted as skull in the tundish to safeguard
the caster.
Depending on the range of flow rates of liquid steel from the ladle to the tundish and
from the tundish to the mould, there may exist one or more outlets on the tundish
bottom, each feeding its own mould, and in some cases more than one ladle may
simultaneously feed a single tundish. Each of the tundish outlets are throttled by a
flow control device e.g. a slide gate or a stopper rod to control the flow rate. The
tundish structure consists of a steel open top vessel lined internally with suitable
refractory, with its exact shape depending on a number of factors including but not
limited to number of outlets, number of ladles that may be used simultaneously, and
the exact layout of the plant. While the tundish refractory can consist of several
different layers, use of dry vibratable type of refractory has gained prominence
especially for the innermost layers in contact with the liquid steel.

A metallic mould (Performer) whose outer surface confirms to the required shape of
the tundish working lining is used. The space between the outer walls of the mould
and the tundish walls are packed with a powdered refractory. The mould is then
heated to set the powdered refractory into a solid mass. After the required heating
period, the mould is withdrawn revealing the finished tundish.
Patent documents US 4770395, US 20010045692A1 divulges information about similar
modifications, but remains limited to a specific tundish design without adequate
information on linear dimensions and methodology about how to arrive at the
optimum tundish design.
OBJECTS OF THE INVENTION
The object of the present invention is to develop an improved tundish with a recessed
bottom at outlet that reduces the volume of skull that needs to be left in tundish at
the end of casting thus ensuring cost saving.
Another object of the invention is to develop an improved tundish with a recess
bottom to create comparatively more steel height as steel drains out of tundish to
avoid vortexing.
A further object of the invention is to develop an improved tundish with a recessed
bottom that ensures the case with which solidified skull to be dislodged during
deskulling.
A still further object of the invention is to develop an improved tundish where the
modified recess can be applied for tundishes of varying shape and size with both
single and multiple outlets.

SUMMARY OF THE INVENTION
The present invention proposes the use of a modified tundish bottom refractory layout
which alone would provide substantial benefit by reducing the volume of skull that
needs to be left in the tundish at cast end. More importantly there needs to be no
costly and time consuming change in the external steel structure of the tundish.
Modification of shape of the performer alone would be necessary and sufficient. The
objective of the modification is to generate a recess in the shape of inverted frustum
located such that one of the tundish outlets is placed in an asymmetrical position. The
vertical height of the frustum and the altitude of the origin pyramid requiring to be
calculated depending on other operational and design parameters.
BRIEF DESCRIPTION OF DRAWING
Figure 1 – Existing tundish prior to modification.
Figure 2 – View of a generic tundish outlet before and after incorporation of a
recessed region at the outlet.
Figure 3 – Incorporation of proposed modification on a four strand delta shaped
tundish.
Figure 4 – Incorporation of proposed modification on a four strand delta shaped
tundish.
DETAILED DESCRIPTION OF THE INVENTION
The objective of making the proposed tundish modification is to reduce the possibility
and / or delay the onset of vortices during tundish draining. The change of vortex
formation however depends almost exclusively on the vertical height of liquid above
the tundish outlet and not on the total amount of liquid steel in the tundish. This
knowledge about vortex formation can be exploited by varying the depth of the liquid

pool at various locations of the tundish, keeping the maximum near the outlets. Since
the external steel structure of the tundish is not being changed, any attempt at
deepening the tundish near its outlet(s) would reduce the refractory thickness at that
location; this would not be acceptable in most cases. Increasing the refractory
thickness at locations other than the outlets provides a convenient means of achieving
the above mentioned result. Essentially a thicker layer of bottom refractory would
replace an equal volume of steel, for a given height of liquid near the outlet. The
region of higher liquid depth near the outlet would be shaped in the form of a
recessed depression on the tundish bottom wall. The sidewalls of this recessed region
has been given a gentle slope outwards. The more gentle the slope, the easier it
would be for the solidified skull to be dislodged during deskulling. A solidified skull will
tend to become wedged into a steeper slope and would be hard to remove. The
invention thus proposes a general idea of introducing an inverted frustum shaped well
/ depression above each tundish outlet to provide the benefit of reduced skull losses.
The depressions would be placed such that the particular outlet is placed at the 1/3rd
position along the bottom face diagonal, any centre line or any other asymmetrical
position in decreasing order of preference. This is because an outlet is more likely to
experience a vortex if placed at a symmetrical location in a fluid field.
Designing of the recess would be initiated by initially calculating the minimum liquid
level that can safely be reached without vortex formation. This limiting fluid level is
called as critical vortexing height and can be calculated by the equation.

where hcr = critical vortexing height
d = diameter of intake pipe
Fr = Froude number

The Froude number can be calculated by the following equation.

where v = resultant velocity of fluid at the tundish outlet
g = acceleration due to gravity (9.8 m / s2) and t = Khcr
Additionally a factor of safety ‘K’ with a value between 1.5 to 2 should be considered
over and above the calculated minimum safe fluid level to account for the fact that
equation 1 is of and empirical nature, and a certain amount of uncertainty is inherent.
This measure denoted by ‘t’ equals the depth of the recess.
The upper surface of the frustum when possible would be in the shape of a square
with each length being defined as ‘a’. When permitted by refractory design, the length
‘a’ should be equal to the smallest distance of approach between two opposite tundish
walls. However the measure of ‘a’ can be limited to 10 times the outlet diameter ‘d’ in
case the smallest distance of approach between two opposite walls is larger or
otherwise unavailable. For multi outlet tundishes, the shape of the recess would be
kept identical across each outlet to simplify design of performer and keep the nature
of steel flow across all the outlets identical.
As mentioned previously, the slope of the recess side walls has an important effect on
the ease of deskulling. However since ‘ease of deskulling’ cannot be quantified, it is
only worthwhile to mention that the more gentle the slop, the easier it is to dislodge
the solidified mass. The exact measure of the slope angle ‘θ’ would depend on both
tundish size and design. With a view to maintain generality across varying tundish
designs with single as well as multiple outlets, the following consideration is proposed
to set the magnitude of the slope. The recess which is in the shape of a frustum is a
transverse section of a square based pyramid or a triangular prism (with side walls
being non-parallel). In this case the altitude of the original pyramid / prism directly

relates to the slope of the side walls. For the purpose of designing the recess, the
altitude h of the originating solid should be between 0.5 to 1.5 times of ‘a’.
Although particular embodiments of the invention have been shown and described in
full here, there is no intention to thereby limit the invention to the details of such
embodiments. On the contrary, the intention is to cover all modifications, alternatives,
embodiments, usages and equivalents as fall within the spirit and scope of the present
invention, specification and appended claims.

We claim
1. An improved tundish with a recessed bottom at outlet and to provide more steel
height to avoid vortexing, the said tundish comprising:
a modified tundish (2) bottom refractory layout for providing reduction in
volume of skull, the said modification being created by generating a recess in
the shape of an inverted frustum with depression at tundish outlet (1), the said
depression in the outlet (1) is placed at 1/3rd position along the bottom face
diagonal in an asymmetrical position for avoiding vortex, wherein the recess is
formed initially by calculating the minimum liquid level or critical vortexing
height based on the equation

where hcr = critical vortexing height
d = diameter of intake pipe
Fr = Froude number which can be calculated by the equation.

where v = resultant velocity of fluid at the tundish outlet
g = acceleration due to gravity (9.8 m / s2)
and t = Khcr where t = depth of recess
and k = factor of safety
when with a factor of safety ‘K’ of 1.5 to 2 to be considered over and above the
calculated minimum fluid level to account for the fact that the equation is of an
empirical nature and a certain amount of uncertainty is inherent.

2. The improved tundish as claimed in claim 1, wherein the upper surface of the
frustum is in the shape of a square with each length defined as 'a' is equal to
the smallest distance of approach between two opposite tundish walls.
3. The improved tundish as claimed in claim 1, wherein the tundish is provided
with gentle slope 'θ' for deskulling.