Note: Descriptions are shown in the official language in which they were submitted.
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TUNDISH STOPPER ROD FOR CONTINUOUS MOLTEN METAL CASTING
FIELD OF THE INVENTION
The present invention relates generally to the casting of molten metal,
such as steel, copper, aluminium and alloys thereof. More specifically, the
present invention relates to stopper rods for regulating the flow rate of
molten
metal discharged from a tundish during a continuous casting operation.
BACKGROUND AND SUMMARY OF THE INVENTION
The continuous casting of molten metal involves providing an available
source of molten metal in a suitable vessel, for example, a tundish or ladle,
which is located physically above a mould in the continuous casting apparatus.
When using a tundish as the vessel for holding the molten metal, a flow of
molten metal is discharged therefrom into the mould via a tundish discharge
nozzle at a flow rate which is suitable for the casting conditions. The flow
rate
of molten metal being discharged from the tundish nozzle is controllably
regulated by a stopper rod. More specifically, the stopper rod is moveable
relative to the tundish nozzle between seated and unseated conditions. Thus,
movement of the stopper rod relative to the tundish nozzle will selective
adjust
the annular orifice area defined between the stopper rod tip and the tundish
nozzle through which molten metal is allowed to flow. Adjustably varying the
effective annular orifice area will thereby in turn adjustably control over
the
flow rate of the molten metal being discharged from the tundish.
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SUBSTITUTE SHEET (RULE 26)
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One problem associated with controlling molten metal flow is that so-
called inclusions (e.g., contaminants in the molten metal such as oxidized
particles of the metal being cast) are typically present in the molten metal
as it is
discharged from the tundish through the nozzle. The particular geometry of the
stopper rod tip which is capable of seating with the tundish nozzle may create
flow profiles which encourage the inclusions to deposit on the surface of the
stopper rod and/or tundish nozzle. Over time, therefore, the geometry of the
orifice are defined between the tundish nozzle and the stopper rod tip may
change due to continual deposit of inclusions thereby eventually detrimentally
affecting the stopper rod's flow control characteristics.
A boundary layer is developed when a fluid flows over solid surfaces
which may be flat, curved or three-dimensional (3-D) curves along 3-D
surfaces. It is characterized mainly because there is a velocity gradient at
the
surface and far away from it, out of the so called boundary layer; the flow
behaves as an inviscid flow. Then, inside the boundary layer viscous forces
have preponderance over inertial or convective forces.
Velocity profiles inside boundary layers are very dependent on the radius
of curvature of the surfaces. This radius can be so determinant that flow
separation phenomena may arise depending on the specific flow conditions.
Separation phenomena mean that the fluid instead of continuing flowing
downstream along a surface driven by a negative pressure gradient, finds
itself
in front of a positive pressure gradient. A flow condition like this brings
about
downstream flow instability causing an incomplete useage of a port area in a
SEN. In other words, separation phenomena may bring about instability in the
internal walls of a SEN and its ports promoting flow fluctuations, meniscus
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oscillations of bath in the mould and out of control turbulent spikes of
velocity.
All those phenomena induce severe slab defects.
The main source of flow control in a tundish using stopper rods is the
shape of the stopper tip. Most of the stoppers in the continuous casting
industry
have rounded tips with different radiuses. When a rounded tip is used steel
forms a very thick boundary layer where fluid velocities are very small. Then
just at the tip a stagnant zone is formed and any inclusion that touches that
area
loses momentum and can be easily trapped on the ceramic surface of the tip
becoming, with time, into a clogging problem. Moreover, rounded tips enhance
boundary layer thickness in zones upstream the tip and these are prone for
inclusions trapping increasing danger of nozzle clogging.
It would therefore be desirable if a stopper rod for continuous molten
metal casting could be provided which minimizes (if not eliminates entirely)
deposition of inclusions onto the stopper rod tip thereby allowing the stopper
rod
to be maintained in service for prolonged time periods and/or improving steel
cleanliness. It is towards fulfilling such a need that the present invention
is
directed.
Broadly the present invention is embodied in a stopper rod for continuous
molten metal casting which creates flow profiles of the molten metal as it is
being discharged from a vessel holding the molten metal through a nozzle so as
to enhance metal cleanliness. That is, the stopper rods according to the
present
invention discourage the deposition of inclusions (e.g., undesirable
particulates,
such as metal oxides) onto the stopper rod tip. According to one especially
preferred aspect of the present invention, a stopper rod for continuous molten
metal casting is provided which has a geometric profile so as to increase the
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velocity of the flowing molten metal sufficient to reduce the boundary layer
thickness of such flowing molten metal adjacent the nozzle and stopper rod tip
surfaces so as to minimize the deposition of inclusions thereon.
Advantageously, the geometric profile of the stopper rod tip according to
the present invention also does not detrimentally affect the lifting force
sensitivities of the stopper rod. That is, the geometric profile of the
stopper rod
tip does not create molten metal flow profiles which would make it difficult
to
exercise physical displacement of the stopper rod tip relative to the tundish
nozzle. As a result, the stopper rods of the present invention exhibit a
relatively
wide range of flow rate control of the molten metal flow discharged from the
tundish.
According to one aspect of the present invention, a tundish stopper rod
for continuous casting of molten metal comprises a stopper rod body, and a
stopper rod tip at a lower end of the stopper rod body, said stopper rod tip
having a frustoconically shaped exterior surface which terminates in a
recessed
nose.
Advantageously the recessed nose is a recessed curvilinear surface (e.g.,
a spherical segment, but non-curvilinear surfaces (e.g. prismatic, pyramidal,
triangular, and quadrangular surfaces) may also be provided.
In especially preferred embodiments, the frustoconically shaped surface
forms an angle 0 with a horizontal surface which is between about 55 to
about
85 , and more preferably, greater than 70 . A central gas flow channel and
tip
gas flow channel are advantageously formed in the stopper rod body and stopper
rod tip, respectively, to allow inert gas to be supplied to the recessed nose.
The
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junction between the frustoconical exterior surface and the exterior surface
of
the body is desirably smooth, i.e. without a sharp change of direction.
According to another aspect of the invention, there is a provided a system
for continuous casting of molten metal comprising a vessel for holding a
supply
of molten metal to be cast, a discharge nozzle for discharging molten metal
from
the vessel, and a stopper rod mounted for reciprocal rectilinear movements
towards and away from the discharge nozzle to allow flow rate regulation of
the
molten metal discharged from the vessel through the discharge nozzle, wherein
the stopper rod includes a stopper rod body, and a stopper rod tip at a lower
end
of the stopper rod body, the stopper rod tip having a frustoconically shaped
exterior surface which terminates in a recessed nose. Advantageously, the
vessel is a tundish or a ladle.
According to a third aspect of the invention, there is provided a method
of regulating the flow rate of molten metal being discharged from a vessel
through a discharge nozzle during a continuous casting operation, the method
comprising providing a stopper rod comprising a stopper rod body, and a
stopper rod tip at a lower end of the stopper rod body, the stopper rod tip
having
a frustoconically shaped exterior surface which terminates in a recessed nose;
and positioning the stopper rod within the vessel so that the stopper rod tip
operatively cooperates with the discharge nozzle; and controllably displacing
the
stopper rod tip relative to the discharge nozzle so as to regulate the flow
rate of
molten metal being discharged from the vessel through the vessel discharge
nozzle.
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These and other aspects and advantages will become more apparent after
careful consideration is given to the following detailed description of the
preferred exemplary embodiments thereof.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Reference will hereinafter be made to the accompanying drawings,
wherein like reference numerals throughout the various FIGURES denote like
structural elements, and wherein;
FIGURE 1 is a schematic cross-sectional elevational view of a tundish
having a stopper rod for continuous molten metal casting according to an
especially preferred embodiment of the present invention;
FIGURE 2 is a cross-sectional elevational view of the stopper rod
depicted in FIGURE 1;
FIGURE 3 is an enlarged detailed cross-sectional view of the stopper rod
tip; and
FIGURE 4 is a bar graph of the percent of trapped inclusions in the
molten metal that can expected to be trapped on the refractory surfaces of a
stopper rod in accordance with the present invention in comparison to stopper
rods outside the scope of the present invention,
FIGURE 5 is a view like FIGURE 2 of a further example of a stopper rod
of the invention, and
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FIGURE 6 is a view like FIGURES 2 and 5 of a still further example of a
stopper rod of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Accompanying FIGURE 1 depicts a continuous casting tundish system 10
which includes a tundish 12 for containing a supply of molten metal 14
therein.
The molten metal 14 is introduced into the tundish 10 by a tundish inlet
nozzle
16, and is discharged from the tundish through the tundish discharge nozzle
18.
Typically, the tundish discharge nozzle is connected to an immersion nozzle 20
associated with the continuous casting mould (not shown) so as to transfer the
molten metal to the mould. Although a tundish 12 is shown and will be
referenced specifically hereinafter, a ladle may also be employed as the
vessel
for holding the molten metal 14. However, for ease of discussion, a tundish 12
will be described specifically below as it represents an especially preferred
vessel for use in the present invention.
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A stopper rod 22 in accordance with the present invention is mounted for
reciprocal rectilinear movements relative to the tundish discharge nozzle 18
so
that the stopper rod tip 22-1 is capable of substantial vertically
displacements
towards and away from the nozzle 18 (arrow Al in FIGURE 1). Displacement of
the stopper rod tip 22-1 thereby varies the orifice area defined with the
nozzle 18
which in turn controls the flow rate of the molten metal being discharged
therethrough. In this regard, the stopper rod 22 includes a cross-pin 22-2
which
is coupled to an arm 24 associated with a lifting mechanism, for example, a
hydraulic actuator (not shown).
The stopper rod 22 according to the present invention is perhaps more
clearly depicted in accompanying FIGURES 2 and 3. As shown therein, the
stopper rod 22 is an elongate generaily cyiindrical stopper rod body member 22-
3
which is tapered somewhat between its upper end 22-4 and its lower end 22-5.
The stopper rod body 22-3 is most preferably formed of a high temperature
ceramic material adapted to being immersed in molten metal.
The stopper rod tip 22-1 is joined to the lower end of the stopper rod body
22-3 at the lower end 22-5 thereof and is also most preferably formed of a
high
temperature ceramic material adapted to being immersed into molten metal. The
stopper rod tip 22-1 may be formed of the same or different ceramic material
as
compared to the stopper rod body 22-3. In addition, although the stopper rod
body 22-3 and tip 22-1 are depicted as separate structural component, they
could for example be formed as a unitary structure is desired.
A tip vent channel 22-6 is centrally formed in the stopper rod tip 22-1 and
communicates with the central vent channel 22-7 formed in the
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stopper rod body 22-3. The tip vent channel 22-6 is most preferably a smaller
diameter as compared to the central vent channel 22-7. Collectively, the vent
channels 22-6 and 22-7 allow off-gas that may be generated by the ceramic
materials forming the stopper rod body 22-3 and/or tip 22-1 to be vented out
of
the tundish 12 so it does not contaminate the molten metal 14 therein.
Although
vent channels 22-6 and 22-7 are shown in the accompanying FIGURES 1-3, the
stopper rod 22 in accordance with the present invention could be provided
without any such vent channels. In addition, as shown in FIGURE 5, a channel
22-9 through the tip 22-1 may be provided and is especially useful if an inert
purge gas is desired to be introduced therethrough. Argon is typically
supplied
through channels 22-7 and 22-9 to the tip.
An aperture 22-8 is formed through the stopper rod body 22-3 so as to
receive the cross-pin 22-2. Alternative means of lifting the stopper rod 22
may
also be provided. For example, a threaded nut may be physically embedded or
affixed to the upper end of the stopper rod body 22-3 so that the stopper rod
22
may be threadably attached to a lifting rod. Alternatively or additionally, an
external pressure collar may be attached to the upper end of the stopper rod
body 22-3 for such purpose.
Important to the present invention, the stopper rod tip 22-1 has a
frustoconically shaped exterior surface 22-la which includes a recessed nose
22-
lb. Most preferably, the recessed nose 22-lb is a smoothly arcuate concavity
such as a spherical segment. However, virtually any concavity may be
employed in the practice of the present invention. Thus, the recessed nose 22-
lb may be embodied in regular and irregular curvilinear surfaces.
Alternatively, non-curvilinear surfaces (e.g., prismatic, pyramidal,
triangular,
quadrangular and the like) may be employed to form the recessed nose 22-lb.
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The frustoconically shaped exterior surface 22-la of the stopper rod tip
22-1 most preferably forms an angle 0 with respect to a horizontal plane which
is sufficiently great so as to increase the velocity of the flowing molten
metal to
reduce the boundary layer thickness thereof adjacent the nozzle and stopper
rod
tip surfaces so as to minimize the deposition of inclusions thereon. In
addition,
the angle 0 formed by the frustoconically shaped exterior surface 22-la is not
so
great as to detrimentally affect the lifting force sensitivities of the
stopper rod
22. Advantageously, the frustoconically shaped exterior surface 22-1 a of the
stopper rod tip 22-1 forms an angle 0 with a horizontal surface which is
between
about 55 to about 85 . According to a particularly preferred embodiment of
the
invention, the angle 0 formed by the frustoconically shaped exterior surface
22-
la is greater than about 70 , for example between 70 and 76 .
The use of a recessed or dimpled nose 22-lb causes, in use, a sudden
change of pressure increasing cast metal (steel) velocities and then
decreasing
the size of the stagnant metal. The dimple depth should be approximately 5mm.
When argon gas is injected through the stopper rod, it reaches higher
velocities at the recess or dimple, the gas bubbles stripping away clogging
material at the nose. Fluid flow patterns through the stopper-nozzle are
improved by the supply of argon, because the thickness of the boundary layers
along surfaces of the stopper-tip cone, the tundish bottom and the region
above
the junction between the stopper nose and the cylindrical body are streamlined
and thinner than those observed with single-phase flow. The floating rate of
inclusions in the tundish is increased. In addition to reducing the boundary
layer
on the surface of the recess or dimple, the injection of inert gas reduces the
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density of the steel-gas mixture and the result is that buoyancy forces in the
nozzle-dimple throat are higher. This facilitates the floating out of
inclusions in
the proximity. Inert gas (i.e. argon) injection stabilises the flow pattern in
the
tundish. The flow pattern in the entire liquid will be consistent, independent
of
the inert gas flow rate. Large bore sizes for injecting the inert gas exlhance
the
above advantages, although some chilling effect is expected. This is the only
reason why small bore sizes, like 2mm, may be preferred over, say, a 5rnm
bore.
To reduce further the boundary layer at the (sharp) junction between the
frustoconical surface of the nose and the cylindrical body of the stopper rod
body, this junction can be rounded, as shown at 22-1c in FIGURE 5.
A rounding of this annular junction produces several benefits, namely:
i) It decreases the velocity of steel flowing along the
cylindrical stopper rod body and the frustoconical tip.
ii) It reduces the size of the high velocity field concentrating
high velocities very close to the stopper rod nozzle throat.
iii) Inclusions 'swimming' close to the stopper rod throat will
find themselves in a field of smaller velocities, with the
result that buoyancy forces may drive the inclusion upwards
instead of being dragged toward the nozzle throat by inertial
forces.
iv) It will enhance adherence of inclusions on the body of the
stopper rod instead of at regions located at the tip or in the
nozzle throat.
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The present invention will be further understood by reference to the
following non-limiting Examples.
EXAMPLES
Computer simulations were conducted based on a mathematical fluid
turbulence model coupled with a Lagrange model for trajectory of particles.
The selected model for momentum transfer was the turbulent regime known as
k-F-. A constant casting rate of 4.0 tons/minute was simulated for each of the
stopper rods being examined.
Comparative Stopper Rod No. 1 (CSR1) was formed with a stopper rod
tip configured to have an upper cylindrical surface portion, an intermediate
frustoconical surface portion, and a terminal smoothly convex nose. The
Comparative Stopper Rod No. 2 (CSR2) was formed with a stopper rod tip
having a hemi-ellipsoidal surface configuration. The stopper rod according to
the present invention was configured as shown in FIGURES 1-3 above. A bar
graph plot of percentage of trapped inclusions to form a clog for each of the
stopper rods is shown in FIGURE 4. It is evident that the stopper rod
according
to the present invention results in substantially less percentage of trapped
inclusions as compared to the stopper rods CSR1 and CSR 2 outside the scope of
this invention.
While the invention has been described in connection with what is
presently considered to be the most practical and preferred embodiment, it is
to
be understood that the invention is not to be limited to the disclosed
embodiment, but on the contrary, is intended to cover various modifications
and
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equivalent arrangements included within the spirit and scope of the appended
claims.
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