FIELD OF THE INVENTION
The present invention generally relates to a device for minimizing the turbulence
in liquid steel during continuous casing of liquid steel adaptable to curved side
wall Tundishes deployed in steel industry. More particularly, the present
invention relates to a device for minimizing the turbulence in liquid steel to
increase life of working lining in a curved wall tundish deployed in a continuous
billet caster.
BACKGROUND OF THE INVENTION
The existing Flow Modifier like Turbo-stop, Ripple pad, shrouds etc are made of
refractory materials to withstand liquid steel impact and control flow. These Flow
Modifiers are of general purpose and do not give expected long casting durations
in tundish having typical curved side wall configurations and operated under
varying operating conditions.
Drawbacks of existing impact pot shroud assembly practice in tundish
An impact block, which receives the impact of steel stream can be shrouded or
unshrouded. The impact block is suitably positioned in the Tundish such that the
turbulence of flowing liquid steel is kept under control. Turbulence can adversely
impact a casting process due to stream flaring and erosion of working lining of
the tundish. Therefore, positioning of an Impact Block is a critical criteria and
depends on specific tundish geometry including the molten metal impact on the
tundish lining. A higher turbulence causes faster wear of the working lining, due
to shearing strain. Thus, the positioning of the impact block is maintained
normally away from the side walls of the Tundish.
In a Tundish having curved side walls on an out-spout side, the Impact Block
takes a position very close to the curved wall to minimize flickering of the steel
shrouds. Further, during the casting operation when the liquid steel is transferred
into the Tundish from the Ladle through a Shroud, the casting process goes
through two phases namely, (i) the tip of the Shroud is displayed above the
liquid steel pool and (ii) the Shroud is completely submerged in the liquid steel.
In the former case, there is a significant turbulence causing erosion of the
working lining of the tundish in particular, in the event the Laddie change over
time exceeding 2 minutes. In the latter case, though the shear stress is mild, the
working lining of the tundish receives a considerable shear stress and erodes
when the casting duration exceeds 10-15 hours.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a device for minimizing the
turbulence in liquid steel to increase life of working lining in a curved wall tundish
deployed in a continuous billet caster, which reduces the wear rate of working
lining of the tundish in particular at the curved walls, by controlling flow
dynamics of the liquid steel in a continuously casting process of liquid steel to
billets.
Another object of the invention is to propose a device for minimizing the
turbulence in liquid steel to increase life of working lining in a curved wall tundish
deployed in a continuous billet caster, which includes an assembly of Impact
Block and Shroud with shape and dimension geometrically optimized in each
case based on data respecting to fluid flow dynamics of the liquid steel in the
tundish.
A further object of the invention is to propose a device for minimizing the
turbulence in liquid steel to increase life of working lining in a curved wall tundish
deployed in a continuous billet caster, which enables an increase of casting
duration more than 25-hours.
A still further object of the invention is to propose a device for minimizing the
turbulence in liquid steel to increase life of working lining in a curved wall tundish
deployed in a continuous billet caster, which eliminates the possibly of tundish
break-up during casting process including maintenance of safety of the plant and
operators.
SUMMARY OF INVENTION
During a continuous casting process of liquid steel into billets, the liquid steel is
transferred from a Steel Laddie into another refractory lined container called
Tundish having a corresponding geometry and capacity through a refractory tube
known as a Shroud. The Shroud performs the following functions:
1. Prevents the oxidation of steel for metallurgical advantages,
2. Guides the stream of liquid steel into Impact Block of the tundish,
3. As the liquid steel pool starts solidifying, the Shroud is submerged in the
liquid steel pool to reduce the turbulence of the liquid/semi-solid in the
impact block of the tundish.
The impact location of the liquid stream that falls into the Tundish is different in
a curved wall Tundish as compared to a normal Tundish with straight wall. In a
Tundish having curved walls at the spout, the Impact Block is positioned closer
to the out-spout to minimize flickering of the liquid stream of steel at the Mould.
When the Impact Block is located closer to the out-spout, the curved walls are
more prone to turbulence of the liquid steel as compared to that takes place in
tundishes having straight walls.
According to the invention there is provided an improved shroud-impact block
assembly in a continuous billet caster having curved walled tundish at the sprout
side to continuously cast billets. The impact block and the shroud is selected
from various combination of geometrical configurations to operate with minimum
impact on the tundish lining, impact block and shroud from turbulence while
liquid metal powering in the tundish through shroud. The inventive device is
capable of controlling turbulence of liquid metal by maintaining minimum shear
stress on various parts namely, turbo stop, shroud and tundish wall wherein the
impact block of any geometrical shape is constructed with a height of 330 to 530
mm and shroud immersion depth of 400 to 450 mm.
The present invention is based on the concept of derivations of device feature
relationship based on study of the important turbulence related issues, as
detailed below, during a process of continuous casting, in a curved tundish. The
inventors through studies and experimentations on mathematical relationship of
the important variables influencing turbulence during a long hour casting
process, have determined the physical device parameters enabling minimization
of turbulence in the liquid steel during the casting process.
The inventors with a view to implement the invention, have identified the
following parameters as the critical ones:
1. the effect of an immersion depth of the shroud, can be varied by:-
a. increasing the length of the shroud, and
b. altering the height of the impact block;
2. the shape of the impact block
3. the diameter of the impact block
4. the geometry of the shroud at the steel exit end.
The constructional features of the impact block and the shroud are decided
based on the quantifiable results obtained through experimentation on the
following aspects:
(a) Position of maximum stress:
Under dynamic casting conditions the maximum stress prone area has been
located. Between the two side walls, i.e. straight back wall and curved spout side
wall of the tundish which are constantly exposed to the liquid stream flashing,
the maximum stress is observed to be developing on the two curved walls close
to the out-spout.
(b) Impact of shroud immersion depth:
With the increase of the shroud immersion depth in liquid steel, the distance of
liquid steel level measured from the tip of an exit end of the shroud, to a highest
level of steel in the tundish, there is found to be an initial reduction of wall shear
stress which, on further increase of the depth, results in an increasing trend of
the stress. This finding indicates that there is an optimum shroud immersion
depth. Since the required increase in the shroud immersion depth can be
achieved by using an increase in the length of the shroud and/or an increase in
the height of the Impact Block, the shearing stresses on the curved wall can be
substantially minimized by selecting a corresponding configuration of the device
consisting of the shroud and the impact block.
(c) Shape of Impact Block
Replacing the shape of the impact block from rectangular to cylindrical does not
bring in any advantage.
A trapezoidal shape of the impact block, has a 50% higher volume compared to
that of a rectangular shaped impact block of the same height (430 mm). The
bottom stress of the trape zooidal shape impact block is seen to be about 26 Pa
and the wall shear stress is about 12-13 Pa. In case of a rectangular impact
block, the bottom stress is 25 Pa and the wall shear is between 12-13 Pa. So by
making the volume of the impact block larger, no significant benefit in terms of
lowering the stress can be achieved.
It is therefore concluded that the prior art practice of operating with an Impact
Block measuring a height of 230 mm and a shroud immersion depth of 130 mm
generate high stress on the wall of the tundish, which leads to break outs in the
tundish.
By increasing the Impact Block height to 530 mm and the shroud immersion
depth to 400 or 450 mm, the maximum stress on the curved wall can
substantially be reduced.
A funnel shaped Shroud although substantially reduces the stress on the Impact
Block bottom wall, but fails to reduce the stress on the curved wall of the
Tundish. Hence, the funnel shaped Shrouds are preferred only to reduce stress
on the bottom wall of the Impact Block.
Adapting a trapezoidal shaped Impact Block having volume increase by 50%
more compared to that of a rectangular Impact Block, also fails to substantially
reduce the stress on the tundish wall. This, a rectangular Impact Block with
higher height, and a longer Shroud combination, can be proposed to extend the
life of a curved-wall Tundish.
In summary, to improve the life of the working lining of a curved wall tundish,
and to prevent break-outs of the tundish during long casting hours, the present
invention provides a device for minimizing the turbulence in liquid steel to
increase life of working lining in a curved wall tundish deployed in a continuous
billet caster, which constitutes an assembly of the Impact Block and a Shroud,
the geometrical configuration being decided based on the depth of alignment of
the Shroud. The invention determines the device parameters based on
mathematical relationship of liquid steel flow data, in a billet caster tundish
having curved out-spout wall which enables to reduce the shear stress on the
curved wall of the tundish. The device allows setting of an immersion depth of
the shroud from the prior art level of 130 mm to 400 to around 450 mm by
deploying a relatively longer shroud, and further adapts an impact block with a
height of approximately 530 mm as opposed to the prior art level of 230 mm.
Further, the circular cross section at the exit end of the Shroud as of the prior art
is reconfigured to a funnel shape.
The aforesaid optimum result based on which the device of the invention is
formed are generated by adapting a conductivity mathematical model using the
sequential steps as described hereinbelow:-
- discretizing the entire computational domain to about 5 lakh cells which
constitute the shroud, tubo stop and the exit funnels, the computational
domain representing Navier-Strokes equation and discretized over the control
volume;
- integrating the discretized equation for the control volume to yield an
algebraic equation having velocity and pressure as variables;
- solving the value of the pressure velocity coupling using an algorithm;
- forming algebraic equations representing momentum equation for velocity by
adapting the continuity equation;
- solving the momentum equation for velocity by algebraic multi solver;
- computing the pressure in all the cells by adapting the continuity equation
and through iteration until convergence of both velocity and pressure is
arrived;
- computing by adapting the solved values relating to velocity field the wall
shear stress for different values of shroud immersion depth and turbo stop
height; and
- obtaining a clear optimum value for both the parameters to generate
minimum wall shear stress on the curved wall of the tundish.
Incorporation of changes in height of the Impact Block and alignment depth of
the Shroud through optimizing the length of Shroud to reduce the maximum
shear stress on the curved wall of the tundish by 20% from the existing level.
The device of the invention is applicable to achieve:
(i) casting of all types of steel billets having a pre-selected range of carbon
levels wherein casting temperatures being decided by respective Liquid
temperatures.
(ii) casting speeds up to 5 metres/min.
(iii) casting billet having a varying area of cross sections.
(iv) adaptation of all types of shrouds made by different routes including iso-
static pressing.
(v) adaptation of Impact Blocks having different chemistry and made by
different fabricating routes including casting by using high alumina
castables.
(vi) adaptation of Impact Blocks having different inside and outside cross-
sectional areas.
(vii) adaptation of different Impact Block geometries including cylindrical and
trapezoidal.
(viii) adaptation of different Shroud geometries including bell shaped end and
circular cross sections.
(ix) adaptation of Tundishes having curved walls of different sizes and
capacity.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will be better understood from the following description with
reference to the accompanying drawings in which
Figure 1 shows an assembly of half portion of a symmetrical Tundish
showing position of the shroud and that of the Impact Block
according to prior art.
Figure 2 shows schematic geometry of a funnel shaped Shroud of prior art.
Figure 3 shows a modified configuration of an Impact Block having a height
of 530 mm with shroud immersion depth of 450 mm according to
the invention.
Figure 4 shows shear stress variation graph for variables parameters namely
impact block height and shroud immersion depth according to the
invention.
Figure 5 represents a flow diagram of a device according to the invention
showing the positions of the tundish, shroud and impact block
including shroud immersion depth and impact block height with
molten steel level in the tundish.
Figure 4 depicts the test results obtained through observation of maximum shear
stress (Mss) resulted on the curved wall of the tundish (T) Vs the shroud
immersion depth (SID) in mm for Impact Block (IBH) maintained at four different
heights for example, 230,330, 430 and 530 mm for any shape of the impact
block such as rectangular, cylindrical or trapezoidal.
It can be seen from the graph that the maximum shear stress (Mss) on the
curved wall of the tundish (T) decreases as the height of the Impact Block (IBH)
increases from 230 and 530 mm. The decrease in maximum shear stress (Mss) is
higher for the Impact Block height (IBH) of 230 mm, 330 mm and 430 mm and
then the rate of decrease falls as the height increases to 530 mm. It can be
marked from the graph that there is always an optimum shroud immersion depth
(SID) corresponding to each Impact Block height (IBH) where the maximum
stress (Mss) on the curved wall of the Tundish (T) is the minimum. But other
combinations of the Impact Block height (IBH) of 330, 430, 530, with Shroud
immersion depth (SID) 350, 440, and 450 mm, respectively give similar results.
It is observed from the graph that in all the above cases with the increase of
shroud immersion depths (SID) the stress on the curved wall of the tundish (T)
is .38 to .8 Pa minimum and after a critical limit the stress increases. It is further
observed that for a Impact Block height (IBH) of 530 mm and Shroud immersion
depth (SID) of 400 mm, the stress is minimum.
Maximum shear stress (Mss) as observed in various parts of on the tundish wall
for impact block height (IBH) 430 mm and shroud immersion depth (SID) 330
mm for different shapes of impact blocks are shown in Table 1.
It is observed from Table 1 that maximum stress (Mss) on the tundish wall is
within the range of 0.5 to 0.81, higher is for cylindrical impact block with a
funnel shaped shroud. Maximum stress at bottom of the Impact Block is
minimum for cylindrical impact block. Shear stress on side wall of the impact
block is minimum for cylindrical shape of the impact block and maximum stress
on opposite plane wall of the tundish is minimum for a cylindrical impact block
with a funnel shaped shroud.
EXAMPLE
The present invention has been applied in a Billet Caster Tundish having six
Strand casting facility and curved walls at the out-spout and maintained with
liquid steel depth up to 900 mm for minimizing shear stress on the tundish wall,
impact block and shroud with enhanced casting duration through the following
characterized features:-
(i) The shape of Flow Modifier or Impact Block has little impact on turbulence
control.
(ii) Impact Block deployed can have any shape including circular, rectangular,
trapezoidal etc.
(iii) Flow Modifier or Impact Block height (IBH) and Shroud depth of
immersion (SID) plays vital role in turbulence control.
(iv) Maintenance of (a) the maximum Shroud immersion depth up to 450 mm
and (b) Impact Block height up to 530 mm, and (c) any combination
within the specified limits.
(v) Use of typical Shroud having bell shaped, diverging geometry, towards
liquid steel exit cross-section is also included.
The invention as herein described and illustrated with an embodiment, should
not be read in a restrictive manner as various adaptations, alterations,
modifications are possible within the scope and limit of the invention as defined
in the appended claims.
WE CLAIM
1. A device for minimizing turbulence in liquid steel to increase the life of the
working lining in a tundish deployed in a continuous billet caster, the device
being adaptable to a curve walled tundish (T), and comprising:
an impact block (IB) having a geometrical configuration to operate with
minimum impact on the lining of the tundish (T) and a shroud (SH) having a
length disposable within the liquid metal inside the tundish (T) with a
corresponding immersion depth, characterized in that the relationship
between the height of the Impact Block (IBH) and the shroud immersion
depth (SID) is maintained at a ratio of 1.178:1.
2. The device as claimed in claim 1, wherein the height of the impact block
(IBH) is 330 mm, prefereably 430 mm, and more preferably 530 mm.
3. The device as claimed in claim 1 or 2, wherein the shroud immersion depth
(SID) is 350 mm, preferably 440 mm, and more preferably 450 mm.
4. The device as claimed in claim 1 to 3, wherein the casting speed is 5
meters/minute.
5. The device as claimed in the preceding claims wherein the impact block (IB)
is configured with one of a geometrical shape of rectangular, cylindrical or
trapezoidal with different inside and outside cross sections.
6. The device as claimed in claim 1, wherein the shroud (SH) comprises a bell
shaped end with one of a circular cross section and funnel shaped
configuration.
7. The device as claimed in one of the preceding claims, wherein the casting
duration length exceeds 25 hours.
8. The device as claimed in one of the preceding claims, wherein by maintaining
Impact Block height (IBH) of 530 mm and shroud immersion depth (SID) of
400-450 mm, the stress on the curved wall of the tundish (T) is kept at
minimum of 0.38 to 0.8 Pa.
9. A method of configurating a device for minimizing turbulence in liquid steel to
increase the life of working lining in a curved wall tundish deployed in a
continuous billet caster as claimed in claim 1, the method comprising the
steps of :-
- discretizing the entire computational domain to about 5 lakh cells which
constitute the shroud, tubo stop and the exit funnels, the computational
domain representing Navier-Strokes equation and discretized over the control
volume;
- integrating the discretized equation for the control volume to yield an
algebraic equation having velocity and pressure as variables;
- solving the value of the pressure velocity coupling using an algorithm;
- forming algebraic equations representing momentum equation for velocity by
adapting the continuity equation;
- solving the momentum equation for velocity by algebraic multi solver;
- computing the pressure in all the cells by adaptihg the continuity equation
and through iteration until convergence of both velocity and pressure is
arrived;
- computing by adapting the solved values relating to velocity field the wall
shear stress for different values of shroud immersion depth and turbo stop
height; and
- obtaining a clear optimum value for both the parameters to generate
minimum wall shear stress on the curved wall of the tundish.

The invention relates to a device for minimizing turbulence in liquid steel to
increase the life of the working lining in a tundish deployed in a continuous billet
caster, the device being adaptable to a curve walled tundish (T), and comprising
an impact block (IB) having a geometrical configuration to operate with
minimum impact on the lining of the tundish (T) and a shroud (SH) having a
length disposable within the liquid metal inside the tundish (T) with a
corresponding immersion depth, the relationship between the height of the
Impact Block (IBH) and the shroud immersion depth (SID) is maintained at a
ratio of 1.178:1.