FIELD OF THE INVENTION
The present invention generally relates to continuous casting in steel industries.
More particularly, the invention relates to an improved tundish in a continuous
casting system for receiving molten steel and discharging molten material.
BACKGROUND OF THE INVENTION
The separation of inclusion from the molten steel is a way to achieve the
superior quality of steel. Tundish metallurgy is a step in the process of steel
making for removal of inclusion and thus improving the quality of steel. Inclusion
flotation inside the tundish depends upon the establishment of flow behaviour in
the tundish. RTD (Residence time distribution) characteristics is an establish
criteria for predicting the inclusion separation in the tundish. The tundish is a
metallurgical vessel used as a buffer during continuous casting in steel industry.
The tundish is also used as a steel refining vessel. Ahuja and Sahai have
postulated certain RTD characteristics for achieving the maximum inclusion
separation ratio. The control of fluid flow is a way to achieve the desired
characteristics of RTD in the tundish. The fluid flow in the tundish is controlled
by different types of flow modifiers. The traditional flow modifiers used inside the
tundish are dams and weirs. Dams and weirs have improved the flow
characteristics, but resulted in a reduction of effective volume.
The design of an effective flow modifier is a recent development in tundish
metallurgy. The understanding of flow phenomenon inside the tundish is pre-
requisite for designing of flow modifiers. Experimentations on fluid flow
phenomenon inside the tundish establishes that the configuration of the pouring
region plays a vital role on the flow behaviour in the tundish. The known flow
modifiers are based on suppressing the flow near the pouring region of the
tundish. RD. Morales et al has discussed the role of turbulence inhibitor in
suppressing the turbulence in the incoming stream. Pouring chamber designed
by Foseco constitutes the most relevant prior art in this direction. The pouring
chamber is a device designed to inhibit the turbulence near the pouring region
and thus achieving the required RTD characteristics. The turbulent inhibitors are
mechanical devices and depend on the optimum location of other associated
parameters (such as shroud position, submergence depth and design of tundish)
for giving desired flow characteristics. The replacement of these mechanical
devices with external forces can restrict the dependence on various other
parameters of the tundish.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved tundish with
embedded magnetic field generating device.
Another object of the invention is to propose an improved tundish with
embedded magnetic field generating device, which eliminates the requirement of
a flow modifier including the furniture of the tundish.
A still another object of the invention is to propose an improved tundish with
embedded magnetic field generating device, which is enabled to adapt external
electromagnetic forces to act as a flow-modifier.
Yet another object of the invention is to propose an improved tundish with
embedded magnetic field generating device, which substantially improves the
flow-characteristics in a tundish including quality of steel.
A further object of the invention is to propose an improved tundish with
embedded magnetic field generating device, which ensures magnetic uniformity
of flow of the molten steel in the tundish.
SUMMARY OF THE INVENTION
Accordingly, the invention proposes an improved tundish without any furniture
inside it. An apparatus at the bottom of the tundish is attached for generating a
stationary magnetic field, perpendicular to the principal direction of the flow. This
magnetic field is localized in the vicinity of the inlet pouring region. The magnetic
field can be selected within a range of 0.5 Tesla to 1 Tesla. The height of the
pouring region, where the magnetic field is imposed, is located around 10% of
the bath height of the tundish. Among various devices used to control the flow in
the tundish, the pouring chamber (also named as turbostop) is the latest and
superior device used for controlling the flow. The comparative analysis is
performed for the innovated tundish with the pouring chamber shows an
improved performance. Thus, the invention improves the flow characteristics in
the tundish. The improved flow characteristic enhances inclusion flotation and
results in a better quality of steel. The flow generated in the proposed design is
substantially uniform and minimizes short circulating. The special features of this
invention is to have a control on the flow controlling forces. The inventors
recognized that too much of control of the flow in the tundish leads to undesired
flow characteristics. Thus, the controlling force needs to be optimized for better
inclusion flotation characteristics. The change in the configuration of the tundish
or change in the throughput is enabled to influence the optimized values of the
controlling force. Considering that monitoring and control of the braking forces in
the tundish using conventional mechanical device or pouring chambers,is not
feasible, the invention proposes replacement of the existing mechanical flow
control device with an innovative tundish embedded with magnetic field
generating device. This control according to the invention, can easily be achieved
by varying the magnetic field corresponding to the desired flow controlling force.
According to the invention, external forces are used in controlling the flow in the
tundish which interalia enables configuration an improved flow modifier. The
electro-magnetic forces in this case, is used to direct the flow without delimiting
within the mould itself. Thus, the invention proposes an alternative methodology
of controlling the flow through electro-magnetic forces by designing a tundish
embedded with a magnetic field generating device.
The present invention enables a substantial improvement in the flow
characteristic in a tundish. Thus, the invention adapt external electromagnetic
forces to act as a flow modifier in the tundish. The invention proposes a new
configuration of tundish and its furniture. This new configuration eliminates all
the existing furniture of the tundish and incorporates a device for generating a
magnetic field in the tundish. The magnetic field generates electromagnetic force
after coming in contact with the liquid steel. The electromagnetic force
generated in the tundish acts as a flow modifying force. The flow modification
allows improvement in the characteristic of inclusion flotation in the tundish.
Thus, a substantial improvement in the quality of steel is achieved through the
innovative tundish. The removal of tundish furniture according to the invention
enables an increase in the effective volume of the tundish. The innovative
approach of using a tundish embedded with a magnetic field generation device,
as a replacement for tundish furniture allows to incorporate a flow control
technology for the tundish. The invention ensures improvement in the quality of
the steel with maximum utilization of the tundish volume.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig.l presents a schematic illustration of a generic continuous casting system.
Fig.2 shows a vertical sectional view of a prior art tundish at symmetrical plane,
indicating the section view and location of the best known pouring chamber used
in prior art.
Fig.3 presents a top view of a symmetrical half of the tundish (embedded with
magnetic field generating system) according to the invention.
Figure 4 shows a vertical sectional view of the tundish of the present invention.
Fig.5 shows a graphical comparison of RTD curve for different configurations of
prior art tundish vis-a-vis the inventive tundish.
Fig.6 is a velocity magnitude profile at the meniscus according to the prior art.
Fig.7 is a velocity magnitude profile for the tundish of the present invention.
Table 1 presents a comparison of RTD characteristics of the inventive tundish
vis-a-vis that of prior art.
DETAILED DESCRIPTION OF THE INVENTION
The schematic representation of a continuous casting machine in fig.l shows
that the incoming molten steel from a ladle (1) goes to a mould (3) through
tundish (2). The solidification takes place in the mould (3). The vertical section
view of the tundish (2) used for the invention is shown in fig.2. The length (L) of
the tundish (2) is typically 4 m and width (w) is around 0.67 m. The bath height
(R) of the tundish (2) is 1.1 m. The molten steel is poured in the tundish (2)
through an inlet (4). The liquid steel is withdrawn from the tundish (2) through
the outlet (5). A flow control device (6) used in prior art and its location is shown
in the fig.2.
The invention proposes placement of a magnetic field generating device (7) at
the tundish bottom as marked in fig.3. Fig.3 presents the top view of the
symmetrical half (2a) of proposed inventive tundish (2). The electro-magnetic (7)
placed at the tundish bottom (in the region marked in fig.3) generates a
magnetic field in the range of 0.5 to 1 tesla. The region of the imposed magnetic
field can be seen from the vertical sectional view of the inventive tundish (2) in
fig.4. The height of the magnetic field region (8) is 10% of the bath height (h) of
the tundish (2). The magnetic field direction (9) in the magnetic field zone
marked in fig.4 is substantially perpendicular to the principal direction (10) of the
molten steel flow from the inlet (4) to outlet portion (5). The direction of the
imposed magnetic field is marked in fig.4. The molten metal flows through the
magnetic field region marked in the fig.4 and hence, the current will be induced
in the liquid steel. The currents generate their own magnetic field, which
interact with the externally applied field to result in a braking force opposing the
molten steel. Thus, the application of the magnetic field slow down the tundish
(2) near the inlet (4). The high turbulence near the inlet pouring region (4) is the
logic behind imposing the magnetic field near the inlet region (4). The imposed
magnetic field will capture the incoming stream near the pouring region (6) and
will evenly distribute the molten steel in the tundish (2).
Fluid flow phenomenon in the tundish (2) is responsible for producing the quality
steel from the tundish (2). The inclusion flotation characteristic in the tundish (2)
is governed by the fluid flow behaviour in the tundish (2). The RTD (Residence
time distribution) analysis is a well established criterion for judging the fluid flow
behaviour inside the tundish (2). According to this analysis, the whole volume of
the tundish (2) is divided into 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 this type of
region. Dead volume is defined as the region of the tundish (2) where the flow is
either stagnant or circulating in a small cell. The dead region reduces the
effective volume of the tundish (2) and hence results in a reduction of mean
residence time. Thus, inclusion in the tundish (2) gets less time to float out and
hence the inclusion flotation characteristics get hampered due to the presence of
dead region in the tundish (2). 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 plug and dead region. Thus, the high ratio of the plug to
dead volumes is desired for better inclusion flotation characteristics in the
tundish (2).
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 plug, dead and mixed volume is calculated the
well established formula based on exit concentration of trace and time variation
of tracer.
The RTD analysis is performed for the present invention and the results obtained
are compared with the tundish of prior art. The results are also compared with
the tundish without any flow control device. Fig. 5 shows the comparison of the
RTD curve of all the three cases. The curve representing the magnetic flow
modifier in fig.5 represents results of the invention. This RTD curve is for the
magnetic field strength of 0.5 tesla. It can be noticed from fig.5 that the peak in
tracer concentration of the curve for inventive tundish is significantly shifted in
time compared to other curves representing other flow control device. Thus, the
efficiency of magnetic uniformity of the flow can be easily illustrated.
Fig.6 and fig.7 shows the contours of the magnitude of velocity at the meniscus
for a prior art tundish and the inventive tundish, respectively. The significantly
variation of centerline velocity from inlet to exit can be noticed in fig.6 as
compared to the velocity magnitude in fig.7. The variation of velocity magnitude
through the width of the tundish (i.e. through z-axis in fig.6 and fig.7) also
shows more uniformity in the flow for the inventive tundish. Thus, the imposition
of the magnetic field strength of 0.5 tesla results in the uniformity of the
centerline velocity.
The comparison of the RTD characteristics derived from the above mentioned
RTD analysis can be seen from table 1. The high ratio of plug to dead volume for
the inventive tundish with magnetic field strength of 0.5 tesla as compared to
the one calculated for the best known flow control device can be notice from
table 1.
WE CLAIM:
1. An improved tundish in a continuous casting system for receiving molten
steel at an inlet and discharging the molten metal at the one or more outlet,
comprising a magnetic field generating device disposed art the bottom of the
tundish, the magnetic field generating in a confined region near the inlet pouring
region, a height of the magnetic field region is selected at a location about 10%
of the bath height of the tundish, the imposed magnetic field being substantially
perpendicular to the principal direction of the flow of liquid steel in the tundish,
characterized in that the magnetic field generating device is embedded in the
tundish.
2. The tundish as claimed in claim 1 wherein the magnetic field strength is in
the range of 0.5 to 1 tesla.
3. The tundish as claimed in claim 1, wherein the optimized magnetic field
strength of 0.5 tesla.
4. The tundish as claimed in claim 1, wherein the tundish is enabled to act as
a flow-modifier.
5. The tundish as claimed in claim 1 wherein the metal is steel.
6. The tundish as claimed in claim 3, wherein the tundish has a length of 4
cm, depth of 1.1 m and with of 0.67 m
7. The tundish as claimed in claim 1 wherein the magnetic field encompasses
an area near the inlet region of the tundish where the incoming stream is falling.
8. The tundish as claimed in claim 1 wherein the magnetic field generating
device is placed at the tundish bottom encompassing the inlet pouring.
9. The tundish as claimed in claim 1 wherein the height of the magnetic field
region is about 10% of the bath height of the tundish.

An improved tundish in a continuous casting system for receiving molten steel at
an inlet and discharging the molten metal at the one or more outlet, comprising
a magnetic field generating device disposed art the bottom of the tundish, the
magnetic field generating in a confined region near the inlet pouring region, a
height of the magnetic field region is selected at a location about 10% of the
bath height of the tundish, the imposed magnetic field being substantially
perpendicular to the principal direction of the flow of liquid steel in the tundish,
characterized in that the magnetic field generating device is embedded in the
tundish.