Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVED REGULATION OF A STREAM OF MOLTEN METAL
Field of the Invention
The present invention relates to refractory articles and methods for use in
the
casting of molten metal, and particularly to such articles and methods
affecting the
flow of a molten metal between metallurgical vessels or molds.
Description of the Related Art
In the continuous casting of steel, refractory articles permit the transfer of
molten steel between various containers, notably between the ladle and the
tundish,
and the tundish and the continuous casting mold. Such articles include stopper
rods,
inner nozzles, metering nozzles, slide gate plates and pour tubes, such as
shrouds and
nozzles, which have at least one surface contacting the stream of molten
steel. The
surface can be an outer surface, such as a nose of a stopper rod, or an inner
surface,
such as a bore through which the molten steel flows.
An important function of refractory articles that contact the flow of molten
steel is to discharge the molten steel in a smooth and steady manner without
interruption or disruption. A smooth, steady discharge facilitates processing,
reduces
costs, and can improve the finished product.
Factors, which can disrupt the steady discharge, include asymmetric flow of
molten steel and clogging of the bore. Asymmetric flow may develop at the
entrance
of or within the bore. During casting, precipitates can accumulate near the
entrance of
a bore and can affect the stream's velocity across the bore. For example, the
stream
may develop higher fluid velocity near the centerline of the bore than along
the sides
of the bore, or lower velocity on one side of the centerline as compared to
the
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opposite side, or higher fluid velocity off the centerline. The disparate
velocities can
cause pulsing and excessive turbulence upon exiting the bore, thereby
complicating
processing and decreasing the quality of the finished product.
Precipitates may also clog or restrict the bore so as to disrupt steady
discharge
of the molten metal. In molten steel, precipitates include high melting point
impurities, such as alumina and titania. An oxygen lance can be used to
dislodge the
precipitate and unclog the bore; however, lancing disrupts the casting
process, reduces
refractory life, and decreases casting efficiency and the quality of the steel
produced.
A total blockage of the bore by precipitates decreases the expected life of
the pour
tube and is very costly and time-consuming to steel producers.
Prior art attempts to improve flow include both chemical and mechanical
means. For example, flow may be improved by reducing precipitates and
subsequent
clogging. Prior art has injected inert gas into the pour tube to shield the
flow from the
pour tube, thereby reducing precipitation and clogging. Unfortunately, gas
injection
requires large volumes of gas, complicated refractory designs, and is not
always an
effective solution. Gas may also dissolve or become entrapped within the metal
causing problems in metal quality including pinhole defects in the steel.
Alternatively
or in combination with gas injection, prior art has lined the bore with
refractory
compositions that are claimed to resist precipitation. Compositions include
lower
melting point refractories, such as Ca0-Mg0-A1z03 eutectics, MgO, calcium
zirconate and calcium silicide, that Slough off as deposits form on surfaces.
These
compositions tend to crack at high temperature, and, during casting, they may
hydrate
and dissipate. For these reasons, their useful life is limited. Other surface
compositions that claim to inhibit deposition include refractories containing
SiAION-
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graphite, metal diborides, boron nitrides, aluminum nitride, and carbon-free
compositions. Such refractories can be expensive, impractical, and
manufacturing
can be both hazardous and time consuming.
Mechanical designs for improving flow include U.S. Pat. No. 5,785,880 to
Heaslip et al., which teaches a pour tube having a diffusing geometry that
smoothly
delivers a stream of molten metal to a mold. Alternative designs include EP 0
765
702, which describes a perforated obstacle inside the bore that deflects the
stream
from a preferred trajectory. Both references attempt to control the
introduction of
molten metal into a mold by mechanically manipulating the stream of molten
metal.
Neither attempts to reduce precipitation or clogging.
Prior art also includes designs that claim to improve flow by reducing
precipitation within the bore. These designs include pour tubes with both
conical and
"stepped" bores. U.S. Pat. No. 4,566,614 to Frykendahl teaches an inert gas-
injection
nozzle having a conical bore intended to reduce "pulsations" in the gas flow.
Smoother gas flow into the bore is said to reduce clogging. "Stepped" designs
include pour tubes that have discontinuous changes in bore diameter. Stepped
designs
also include pour tubes having a spiral bore. JP Kokai 61-72361 is
illustrative of
stepped pour tubes, and describes a pour tube having a bore with at least one
convex
or concave section that generates turbulent flow in the molten metal.
Turbulent flow,
as contrasted with laminar flow, is described as reducing alumina clogging. U.
S. Pat.
No. 5,328,064 to Nanbo et al. teaches a bore having a plurality of concave
sections
separated by steps having a constant diameter, d. Each section has a diameter
greater
than d, and preferably the diameters of the sections decrease along the
direction of
flow. The steps are described as generating turbulence that reduces alumina
clogging.
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PCT/LTS00/23601 to Heaslip et al. comprises a pour tube having a bore
comprising a
series of fluidly connected sections each of which converges and diverges to
continuously alter and diffuse the contained stream.
Prior art designs are directed at reducing precipitation and clogging by
altering
the bulk flow of molten steel within a bore. A need persists for a refractory
article
that inhibits precipitation and clogging at the inlet and outlet of a bore
while
maintaining flow patterns consistent with a smooth and steady discharge of
molten
steel from the article. Such an article should also reduce turbulence in a
mold and
extend the life of the refractory article. Ideally, the article could be
adapted for use in
combination with inert gas.
Summary of the Invention
The objective of the present invention is a refractory article and a method of
making the article that is used in the transfer of molten steel from a first
metallurgical
vessel to a second metallurgical vessel or mold. The article includes a
surface
contacting a stream of molten steel and at least one perturbation at the
surface
sufficient to interrupt laminar flow of the molten steel in a boundary layer
at the
surface.
The article comprises a stopper rod, inner nozzle, metering nozzle, slide gate
plate, pour tube, such as a submerged-entry shroud or submerged-entry nozzle,
and
combinations thereof. Interruption of laminar flow in the boundary layer is
described
as reducing diffusion of corrosive and erosive agents towards the surface.
One aspect of the invention shows a refractory piece having a perturbation at
or near an inlet to or outlet of a bore of a refractory article. The
perturbation is
described as reducing precipitation at the inlet and outlet and promoting
smoother
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flow of the stream of molten metal through the bore. Such a perturbation can
improve
refractory life and casting efficiency. Another aspect of the invention
includes a
plurality of perhirbations along the surfaces exposed to the stream of molten
metal.
In another aspect of the invention, the perturbation is described as a
discontinuity and comprises a protrusion or indentation in the surface that is
taller or
deeper, respectively, than the boundary layer is thick. For example, the
discontinuity
may be a step or groove. Alternatively or in combination with a step or
groove, the
discontinuity may comprise the intersection of two surfaces at a non-zero
angle. The
angle can vary depending on its location along the flow path. For example, an
angle
along the bore of a nozzle will typically be from four to twenty degrees,
while at the
entrance to a nozzle, the angle can be greater than sixty degrees. Similarly,
a stopper
rod can have an angle of ninety degrees.
In one embodiment, the perturbation is described as affecting the boundary
layer over a working region. The working region has a length that is between
four
and twenty times the size of the perturbation. Depending on the length of the
region,
one or more perturbations may be necessary. The working region preferably
includes
those areas known to accumulate deposits that could interfere with casting.
In still another embodiment, the article is adapted to receive an inert gas
that
protects the molten steel from oxygen infiltration. The perturbation affects
the
boundary layer of steel in such a way that inert gas leaking into the molten
steel and
flows along the surface adjacent to the molten steel instead diffusing into
the steel.
The method of the present invention includes disrupting laminar flow in a
boundary layer adjacent to the surface without significantly affecting flow in
a
remainder of the stream. Non-laminar flow in the boundary layer is believed to
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reduce diffusion of precipitating compounds to the surface and to improve the
effect
of inert gas injection.
Brief Description of the Drawings
Figure 1 shows a longitudinal cross-section of a refractory article of the
present invention, specifically a collector nozzle.
Figure 2 shows a longitudinal cross-section of a variation of the collector
nozzle.
Figure 3A shows flow contours of a boundary layer near a surface absent a
perturbation.
Figure 3B shows flow contours of a boundary layer near a surface with a
perturbation.
Figure 4 shows a longitudinal cross-section of a stopper rod in proximity to a
collector nozzle.
Figure 5 shows a longitudinal cross-section of an outlet of a collector nozzle
having a diverging section.
Detailed Description of the Preferred Embodiments
The present invention comprises a refractory article for use in casting a
stream
of molten steel through a bore from a first metallurgical vessel to a second
metallurgical vessel or mold. The article includes a surface adjacent to the
stream and
at least one perturbation on the surface. The perturbation is located
sufficiently near
the inlet or outlet of the bore to affect precipitation on the surface at
these locations.
The article may include a stopper rod, inner nozzle, metering nozzle, slide
gate plate,
pour tube, such as a submerged-entry shroud or submerged-entry nozzle, and
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combinations thereof, which have at least one surface contacting the stream of
molten
steel.
A stream of molten steel can be described as a combination of flow regions
having laminar, turbulent and transitional flow regimes. Flow regimes are
often
identified based on their Reynolds numbers, which is a dimensionless number
that
relates inertial and viscous effects in a fluid. Reynolds number, Re, equals:
p*V*D/r~,
where p is the density of the fluid, V is the velocity, D is the diameter of
the bore, and
r1 is the viscosity of the fluid. When Re is less than about 2100, flow is
laminar. A
Re above about 3000 identifies turbulent flow. Vaules of Re between 2100 and
3000
correspond to transitional flow, where the stream exhibits both laminar and
turbulent
flow patterns.
Laminar flow means a state of fluid flow where the flow moves along parallel,
ordered paths. Laminar flow results in lower friction on adjacent surfaces,
but has
problems following retreating or protruding surfaces. Fluid flow that is
immediately
adjacent to a surface forms a boundary layer that is commonly laminar. The
size of
the boundary layer depends on physical properties of bore and the Reynolds
number.
In contrast to laminar flow, turbulent flow is a state where the particles
move in
irregular, wavy paths. Turbulent flow causes more friction on adjacent
surfaces, but
can more easily follow retreating or protruding surfaces.
Precipitation can occur, if at all, on surfaces adjacent to the stream of
molten
metal. Along these surfaces, boundary layers exist that usually present with
laminar
flow. Laminar flow is believed to promote precipitation. In the present
invention, a
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perturbation disrupts laminar flow in the boundary layer, thereby reducing
precipitation on adjacent surfaces. Additionally, turbulent or transitional
flow tends
to decrease pressure in the fluid. This feature is relevant to gas-injection
as will be
explained in greater detail later.
A perturbation comprises an alteration in a surface adjacent to the stream of
molten metal. The perturbation can be any alteration in the surface and should
be
sufficiently large to disturb laminar flow of the boundary layer but not so
large as to
affect significantly the flow regimes of the bulk fluid, such as the creation
of "dead"
zones. A "dead" zone is a region downstream of a discontinuity in which the
pressure
is substantially reduced relative to the bulk of the stream and small vortexes
appear.
Alterations in the surface include any discontinuity that affects the boundary
layer,
and can include indentations, grooves, protrusions, such as bumps or ridges,
or the
intersection formed by two surfaces. The latter includes, for example, the
transition
from a straight bore to a diverging bore. During casting, a boundary layer of
steel
commonly extends about 1-2 mm perpendicularly from the surface. A perturbation
slightly larger than the boundary layer, that is, from 3-10 mm, can disrupt
laminar
flow in the boundary layer. Such a perturbation is typically smaller than
"steps"
found in prior art.
Figure 1 shows a nozzle 1 of the present invention. The nozzle 1 has an inner
surface 2 defining a bore 3 with an inlet 4 and an outlet 5. In the embodiment
shown,
the inlet 4 includes a first perturbation 6 in the form of a step and a second
perturbation 7 comprising an intersection of two surfaces. The perturbations 6
are
exaggerated for clarity. An alternative embodiment in Figure 2 shows the first
perturbation 6 as a groove near the inlet 4 of the nozzle 1.
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A perturbation affects a boundary layer of a stream of molten steel near a
surface 2. Absent a perturbation, the boundary layer would normally exhibit
laminar
flow as shown in Figure 3A, in which flow contours 8 appear parallel to the
surface 1.
A perturbation 6, as shown in Figure 3B, interrupts the boundary layer and
changes
the flow from laminar to transitional or turbulent. Flow contours 8 near the
surface
are no longer parallel and can develop eddies or vortexes that promote mixing
of the
fluid. Such non-laminar flow can reduce precipitation and decrease fluid
pressure
near the perturbation 6.
A perturbation typically affects flow patterns over a working surface that is
as
long along the bore as four to twenty times the size of the perturbation. In
other
words, a perturbation of 3 mm can affect the boundary layer for 12-60 mm along
the
length of the bore. The geometry of the refractory article, the type of steel
being cast,
and casting conditions will determine the placement, size and number of
perturbations
in a refractory article. One skilled in the art would identify such parameters
after
identifying the likely locations where precipitation can occur. Frequently,
more than
one perturbation may be necessary. Conveniently, these perturbations may be
arranged in series along at least a portion of the bore, where precipitates
are more
likely to form.
Advantageously, a perturbation can be placed at an inlet or outlet of a bore.
Figure 4 shows a stopper rod 9 in combination with a nozzle 1. The angle 15 of
the
stopper nose can be from about twenty to seventy degrees, and is typically
about forty
to sixty degrees. Precipitation on the nose 10 of the stopper rod 9 can
seriously affect
the metering of the stream of molten metal. When the nose 10 of the stopper
rod 9 is
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near the entrance 4 of the nozzle l, a perturbation 6 on the surface of the
nose 10 can
alter the boundary layer to resist precipitation.
Similarly, an outlet of a collector nozzle includes a bore 3 having a
perturbation 6 near the outlet 5 of the nozzle 1. The perturbation 6 includes
a
discontinuity 11 formed by the intersection of two portions of the inner
surface 2 that
define the bore 3. The size of the perturbation 6 is measured as the
orthogonal depth
12 of the perturbation relative to the bore 3. At the outlet S, a collector
nozzle may
also include a diverging section 13 having a diverging angle 14 defined by the
inner
surface of the diverging section and the direction of the stream. The
diverging angle
is between about four to twenty degrees. Diverging sections have been
described as
reducing the formation of metal droplets by reducing the pressure drop, and
resultant
shear stresses, from within the bore to outside the bore. Conveniently, the
diverging
section 13 will be about four times the size of the perturbation.
A perturbation affects laminar flow in a boundary layer and changes the
velocity of the stream of molten metal at the boundary layer. Velocity and
pressure
are inversely related as explained by Bernoulli's law. As velocity increases
around a
perturbation, pressure decreases. Such a decrease can be exploited for the
injection of
an inert gas, such as, for example, argon. Inert gas is frequently used to
protect the
stream of molten steel from contact with oxygen and the resultant oxidation
and
precipitation.
Ideally, inert gas diffuses into the bore and covers the inner surface,
thereby
enshrouding the stream. In prior art, the inert gas is frequently injected at
higher than
desired pressure to overcome the resistance from pressure in the boundary
layer.
High-pressure, injected gas can escape from the surface and dissolve in the
molten
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steel stream. This limits the amount of inert gas actually enshrouding the
stream and
causes defects in the final product because of dissolved gas bubbles.
A perturbation induces non-laminar flow in the boundary layer and, therefore,
lowers pressure in the boundary layer and also at the inner surface. The
pressure
needed to inject inert gas decreases. The injected gas can be at a low enough
pressure
that it remains on the surface and, because of the relatively higher pressure
away from
the boundary layer, the inert gas does not easily diffuse away from surface
into the
molten steel. The inert gas remains along the inner surface where the gas more
effectively can enshroud the steel from oxygen.
Obviously, numerous modifications and variations of the present invention are
possible. It is, therefore, to be understood that within the scope of the
following
claims, the invention may be practiced otherwise than as specifically
described.
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