Note: Descriptions are shown in the official language in which they were submitted.
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IMMERSED POUR TUBE HAVI1VG AN EROSION-RESISTANT SLEEVE AND METHOD OF
MANUFACIVRING THE
SAME
FIELD OF THE INVENTION
This invention relates to metallurgical pour tubes
having at least one end of the tube, typically the
downstream end, immersed in a pool oz molten metal. Pour
tubes conduct molten metal from one metallurgical vessel
into a mold or another vessel. Examples of such tubes
include sub-entry nozzles (SENs) and sub-entry shrouds
(SESs), which find particular utility in the continuous
casting of molten steel.
DESCRIPTION OF THE PRIOR ART
in the continuous casting of steel, a stream of
molten steel is typically transferred via a pour tube
from a first metallurgical vessel into a second
metallurgical vessel or mold. The downstream end of the
pour tube is immersed in a pool of molten steel, and has
sub-surface outlets below the surface level of the molten
steel. Such outlets permit the steel to pass from the
first vessel to the second vessel or mold without
contacting air or slag. This reduces oxidation and
limits contamination by slag.
Pour tubes are typically preheated before use, but
although preheated, the tubes are relatively cold
compared to the molten steel. The molten steel passing
through or around the tube subjects the tube to thermal
shock, which can cause the tube to fracture.
Consequently, pour tubes typically comprise thermal
,30 shock-resistant refractories.
During casting, an immersed pour tube extends
through a layer of slag floating on the molten steel.
Slag may comprise glasses, fluxes, mold powders or
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various impurities. Slag is corrosive, and the pour tube
may erode more quickly where it comes in contact with the
slag, that is, at the slag-line, than the remainder of
the pour tube. The tube may fracture where such erosion
occurs. A fractured tube permits slag to mix with the '
molten steel and also exposes the steel to oxidation.
Additionally, a pour tube immersed in a mold often has
sub-surface outlets designed to affect flow patterns and
crystallization of the molten steel. Loss of the
downstream end having the sub-surface outlets may thereby
compromise steel quality and, in some cases, may permit
breakout in the frozen steel strand issuing from the
mold.
Attempts to prevent erosion of an immersed pour tube
involve the use of collars fitted around the pour tube at
the slag-line. Such collars, or slag-line sleeves,
protect the tube from contact with corrosive slag. The
sleeve may move relative to the outside surface of the
tube, and permit the sleeve to rise and fall with changes
in the.molten steel level. A slag-line sleeve may be
connected to a mechanism capable of raising or lowering
the sleeve in response to melt level. The sleeve~may
even form a type of crucible surrounding the pour tube.
The crucible has at least one opening communicating with
a sub-surface outlet in the pour tube.
Sleeves may also be fixedly attached to the outside
of the pour tube. In practice, sleeves have been
mortared, threaded, or copressed onto the pour tube. A
mortared construction involves cementing an erosion
resistant sleeve onto the exterior of a pour tube.
Alternatively, a threaded, erosion-resistant sleeve may
be screwed onto the outer surface of the tube.
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Copressing involves pressing together two refractory
mixes or one refractory mix and a pre-fired component,
and then firing into a single piece.
Slag-line sleeves often comprise erosion-resistant
refractories, such as zirconia, zirconia-graphite,
silicon nitride, boron nitride, and zirconium diboride.
Additional sleeve compositions include magnesia,
magnesia-graphite, magnesia-alumina spinels and dense
alumina. Unfortunately, such erosion-resistant
refractories often have poor thermal shock-resistance.
This property is especially detrimental with pour tubes
having fixedly attached sleeves. Attempts to improve
thermal shock-resistance by modifying the composition of
the sleeve, for example, by adding graphite, frequently
compromises erosion-resistance.
Encapsulating the sleeve within the body of the pour
tube may minimize thermal shock to the sleeve. The
encapsulated sleeve lies between an inner and outer ring
of thermal shock-resistant material. These rings are
believed to absorb the extreme thermal gradients, which
diffuse to the sleeve only gradually. Reduced thermal
gradients may permit the use of extremely erosion-
resistant materials, such as high-density, sintered
zirconia. The encapsulated sleeve should continue to
protect the pour tube from the slag after the outer ring
of thermal shock-resistant material has eroded away. A
limitation of this design, however, is the high thermal
expansion of erosion-resistant materials. The
encapsulated sleeve will expand more than the body of the
pour tube and could cause the pour tube to fracture from
the inside out.
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An attempt to overcome this deficiency is a pour
tube having an inner and an outer slag-line sleeve. The
inner sleeve, made from a highly erosion-resistant
material, is completely encapsulated between the pour
tube and the outer sleeve. The outer sleeve is made of a
material intermediate between the erosion-resistance and
thermal expansion of the body and the inner sleeve. The
' outer sleeve is expected to decrease thermal stresses
within the pour tube.
A need persists for an integral slag-line sleeve in
an immersed, metallurgical pour tube that possesses
outstanding erosion resistance but resists fracture
itself or fracturing the pour tube when exposed to large
thermal gradients or high temperatures.
SUMMARY OF THE INVENTION
The present invention describes a pour tube and a
method of manufacturing a pour tube both having an
erosion-resistant sleeve. An object of the invention is
to produce a pour tube having an erosion-resistant, slag-
line sleeve, wherein both the body of the pour tube and
the sleeve resist cracking due to thermal shock or
thermal expansion. A further object of the invention is
to include an internal slag-line sleeve within such a
tube.
In a broad aspect, the article describes a pour tube
having a body defining an interior cavity. A sleeve is
located within the cavity. The cavity is larger than the
sleeve so that an accommodation region is defined between
the sleeve and the body. The region is sufficiently
large to permit thermal expansion of the sleeve without ,
fracturing the body of the pour tube.
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One aspect of the article describes the
accommodation region as a gap, or, alternatively, as
containing a compressible material. Another aspect
describes the erosion-resistant sleeve as comprising
zirconia or magnesia. A further aspect describes the
sleeve as copressed with the body of the pour tube.
Still another aspect of the invention describes the
interior cavity as formed by the interface of the body
with a third component.
One method for making the article of the invention
includes coating a sleeve with a spacer material and
pressing the coated sleeve within the body of the pour
tube to form a pressed piece. The pressed piece can be
fired thereby removing at least some of the spacer
material and creating an accommodation region. Vents may
be provided for the elimination of spacer~material. The
spacer material is described as comprising a transient or
compressible material.
Another method of producing the article of the
invention comprises co-filling a mold with erosion-
resistant and thermal shock-resistant particulate
refractories. The erosion-resistant refractory is
segregated to the slag-line by a guide means and a spacer
material is placed adjacent to the erosion-resistant
refractory. The filled mold is pressed and fired to
create a pour tube having a slag-line sleeve separated
from the body by an accommodation region.
An alternative method of producing the article of
the invention describes co-pressing a sleeve of a
'30 transient material inside the pour tube at the slag-line.
The transient material may then be eliminated to form an
interior cavity. A refractory composition is inserted
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into the cavity and subsequently densified. One aspect
of this method describes the refractory composition as an
injectable material comprising, for example, a
particulate refractory and wax. Alternatively, the
refractory composition is described as densifying at
temperatures greater than about 1300°C. In either
embodiment, an accommodation region is produced after
firing.
Still another method of producing the article of the
invention describes mechanically securing an erosion-
resistant, sleeve at the slag-line of a pour tube and
covering the sleeve with a third component. The third
component is described as a refractory piece designed to
fit over the sleeve and create an accommodation region
when positioned around the sleeve. Alternatively, the
third component may be a compressible material, such as a
refractory fiber. An aspect of this method uses a fourth
component to secure the third component in place.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art pour tube 1 having a body 2
with a slag-line sleeve 3 fixedly attached on the
exterior of the body.
FIG. 2 shows a prior art pour tube 1 having a slag-
line sleeve 3 completely encapsulated in the body 2 of
the pour tube.
FIG. 3 shows a prior art pour tube 1 having two
slag-line sleeves, a first sleeve 3 comprising a highly
erosion-resistant material and a second sleeve 4
comprised of a less erosion-resistant material, arranged
so that the first sleeve 3 is sandwiched between the body
2 of the pour tube 1 and the second sleeve 4.
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FIG. 4 shows a pour tube 1 of the current invention
having a body 2 with a slag-line sleeve 4 disposed within
an interior cavity 3. An accommodation region 5, shown
as a gap 6, exists in the region between the sleeve 4 and
the body 2.
FIG. 5 shows a pour tube 1 of the current invention
having an accommodation region 5 and vents 7 for the
elimination of transient material.
FIG. 6 shows a pour tube 1 of the current invention
where the slag-line sleeve 3 is covered by a third
component 8 which is secured to the pour tube 1 by a
fourth component 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An article of the present invention is shown in
FIG. 4 and comprises a pour tube 1 having a body 2 with
an interior cavity 3. A sleeve 4 is enclosed within the
interior cavity 3. An accommodation region 5 exists in
the interior cavity 3 between the sleeve 4 and the body
2. In this embodiment,~the accommodation region 5 is
shown as a gap 6.
In operation, the pour tube is subjected to extreme
thermal gradients. The body of the pour tube insulates
the annular sleeve from the resulting thermal shock and
allows the sleeve's temperature to change only slowly,
thereby reducing the likelihood that the sleeve will
fracture'. The accommodation region permits the sleeve to
expand without fracturing the body.
The body comprises a material possessing good
thermal shock-resistance, and includes, for example,
alumina-graphite and fused silica refractories. Most
commonly, the tube will be an alumina-graphite
composition, ranging from about 45 to about 80 weight
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percent alumina with the balance comprising graphite.
Preferably, the composition is about 62-67 wt.% alumina,
about 20-25 wt.% graphite, with the balance comprising
silica, zirconia, silicon, and other oxides. A suitable
refractory for the body portion will generally have a '
coefficient of thermal expansion below about 6 x 10-6/°C,
and preferably about 4 x 10-6/°C .
The sleeve is within the interior cavity of the
pour tube, preferably at the slag-line. The shape of the
sleeve will depend on several variables, such as the
shape of the pour tube, the depth of immersion, and the
depth of the slag. A sleeve will most commonly be
cylindrical; however, it is anticipated that other shapes
may be used, such as flat plates or asymmetric shapes.
Reference to a sleeve will assume various shapes and
should not be construed as limiting the sleeve to a
cylindrical tube.
The sleeve must resist erosion caused by slag.
Slag may comprise glasses, fluxes, oxides, mold powders,
insulating powders or various impurities that float on
the surface of molten steel during casting. The sleeve
may comprise various erosion-resistant compositions
including, for example, zirconia, titanates, nitrides,
magnesia, dense alumina, and spinels of magnesia, alumina
and graphite. Such compositions may be sintered or
carbon-bonded,. For example, carbon-bonded zirconia will
comprise about 80-99.5 wt.% zirconia and about 0.5-20
wt.% carbon. A typical carbon-bonded composition
contains 88 wt.% zirconia and 6 wt.% graphite. In
contrast, sintered zirconia may be nearly pure zirconia ,
with little or no graphite.
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Erosion-resistant compositions used as slag-line
sleeves typically have thermal expansion coefficients
greater than 6 x 10-6/°C. The difference in thermal
expansion coefficients between the body and the sleeve
causes the sleeve to expand with temperature more than
the body. In practice, the sleeve often expands more
than twice as much as the body. In prior art pour tubes,
as shown in FIGS. 1, 2 and 3, thermal shock or thermal
expansion may fracture the pour tube or the sleeve.
The present invention has an accommodation region
between the sleeve and the body. This region permits
expansion of the sleeve without fracturing the body or
the sleeve. The region is defined as large enough that
stresses caused by thermal expansion will not fracture
the body or the sleeve. The region may be made large
enough to accommodate the entire expansion of the sleeve.
Obviously, the size of the region depends on a number of
factors, including, but not limited to, the thermal
expansions and geometries of the body and the sleeve, and
the casting temperature of the steel.
The accommodation region may be a gap. The gap
should be large enough to permit the sleeve to expand
without placing unacceptable stress on the body of the
pour tube. Conveniently, the gap is made large enough to
accommodate thermal expansion of the sleeve at the
temperature of casting. The accommodation region may
also be a compressible material, instead of or in
conjunction with a gap. As the sleeve expands, the
compressible material compacts thereby minimizing
stresses transmitted to the body. The compressible
material should have a refractory composition, and most
commonly will be a refractory fiber, for example, a
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ceramic fiber, such as silica or alumina. The
compressible material may also advantageously secure the
slag-line sleeve within the interior cavity.
The article of the present invention may be made by
several methods. These methods may make use of a spacer '
material comprising a transient or compressible material.
A transient material is any composition that can be
eliminated from around a sleeve after pressing.
Elimination of the transient material creates a gap
between the body of the pour tube and the sleeve where
the transient material had been. Transient materials may
be eliminated by, for example, melting, volatilizing,
combusting, degrading, or shrinking. Heat from the
firing or actual use of the article may be used to effect
these transitions. Transient materials may comprise
metals, ceramics and organic compounds. Metals will
typically be low melting point metals or alloys, such as
lead. A ceramic may leave a gap between the sleeve and
the body by, for example; shrinking as a result of
sintering or degradation. Preferably, the transient
material will be an organic material, such as wax, which
can both melt and volatilize at elevated temperatures.
In a preferred embodiment, as shown in FIG. 5, the body 2
of the pour tube 1 will have one or more vents 7, which
permit elimination of the transient material or its
degradation products.
A compressible material may be used in conjunction
with or independent of the transient material. The
compressible material may expand to occupy the gap
created by elimination of the transient material. Use of
a compressible material may reduce or eliminate the need
for vents. The compressible material should be a
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refractory fiber, such as a ceramic fiber, or an expanded
refractory material.
The amount of spacer material required depends upon
the disparity in thermal expansion and processing
shrinkage between the body of the pour tube and the
sleeve. A larger disparity suggests the use of a greater
amount of spacer material. The spacer material should be
present at least in sufficient amount to prevent fracture
of the body by thermal expansion of the sleeve.
Preferably, the amount of spacer material will fully
compensate for the disparity. In other words, at casting
temperatures, the sleeve will expand to completely fill
the region between the body and the sleeve.
One method of making the article of the present
invention involves placing a pre-shaped sleeve inside a
thermal shock-resistant, particulate, refractory body and
subsequently pressing the sleeve within the body.
Particulate means any type of material whether powdered,
granular, fibrous, chunkeel; or any shape or combination
of shapes, and of whatever size, which is amenable to
being pressed into a form. The sleeve comprises an
erosion-resistant refractory and may be pre-fired. The
sleeve is coated with a spacer material before pressing
within the body. The sleeve and body are pressed to form
a piece, so that the refractory body is compacted around
the sleeve. Preferably, the piece is isopressed, and
most preferably the piece is isopressed on the inside and
outside of the piece. The piece is then fired, and an
interior cavity forms that is slightly larger than the
,30 sleeve so that a region is created between the body and
the sleeve. The region may include a gap when the spacer
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material used to coat the sleeve comprises a transient
material.
The article of the present invention may also be
made by co-filling a mold with an erosion-resistant
particulate refractory and a thermal shock-resistant
particulate refractory. A guide means directs the
erosion-resistant refractory to its proper place in the
mold, that is, where the slag-line sleeve will be. The
guide means is often a funnel, tube or annular form, but
may be anything capable of directing a particulate into a
mold. Often, a plurality of guide means are used. A
spacer material is then introduced adjacent to the
erosion-resistant ref ractory. Conveniently, the guide
means may comprise the spacer material, such as, for
example, wax slips. The filled mold is then pressed to
form a piece and the piece is fired to produce the
article. Pressing is most commonly done by isopressing.
The firing temperature should be sufficiently high to
densify the erosion-resistant refractory. Such a
temperature is typically above 1300°C.
An alternative method for producing the article
involves first creating an annular cavity within the
thermal shock-resistant body of the pour tube. This may
be done by forming an annular piece comprising a spacer
material, typically an incompressible transient material
such as wax or a low melting point metal. The annular
piece is copressed with the thermal shock-resistant body.
The spacer material is then substantially eliminated from
the cavity, for example, by melting. The spacer material
may also sublime, volatilize or otherwise be removed from .
the cavity. A refractory material having good erosion-
resistance may then be inserted into the cavity. A
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representative composition includes zirconia or zirconia-
graphite. Insertion is preferably accomplished using an
injectable refractory. Injectable refractories comprise
a particulate refractory with a transient flow agent,
such as wax. Firing the resulting pour tube at elevated
temperatures removes the transient flow agent and causes
the refractory to shrink as carbon-bonding or sintering
takes place. A suitable temperature for this process
will be greater than about 1300°C. A gap is thereby
formed between the injected erosion-resistant sleeve and
the body of the pour tube. Care must be taken to achieve
at least a minimum densification of the refractory for
good erosion-resistance. It should be appreciated that
injecting a refractory into a cavity of the pour tube may
be used in other applications besides slag-line sleeves,
for example, porous gas inserts.
Still another method of making the present ,
invention, as illustrated by the article of FIG. 6,
comprises securing a sleeve 4 onto a body 2 and encasing
the sleeve 4 between the body 2 and a third component 8.
The sleeve may be fixedly secured to the body with mortar
or may simply engage the body until the third component
secures the sleeve in place. The third component may be
a refractory piece designed to fit around the sleeve and
the body while leaving a gap between the two.
Alternatively, the third component may be a compressible
material, such as refractory fiber. Both embodiments
enable the sleeve to expand without creating destructive
stresses in the body. Frequently, a fourth component 9
'30 may be used to lock the third component 8 and the sleeve
4 in place. A fourth component is especially useful
where the third component is a refractory fiber or would
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otherwise be difficult to mortar in place. Both the
third and fourth components often comprise a plurality of
pieces so as fit around the body.
Example 1
An erosion-resistant composition consisting
essentially of zirconia is fired to form a cylindrical
sleeve. The sleeve is then coated with wax to a
thickness approximating the size of the sleeve at the
casting temperature of steel. The coated sleeve is
placed in a pour tube mold so that the sleeve encircles
the flow passage and will be at the slag-line when the
resultant pour tube is in operation. The sleeve is
surrounded by a particulate alumina-graphite. The filled
mold is pressed at 5000 psi, with pressure being applied
on the inside and outside of the mold. The resultant
piece is fired at greater than 800°C for greater than 2
hours. During firing the wax is eliminated and a gap is
created between the sleeve and the body.
Example 2
Wax is formed into a cylindrical shape and placed in
a pour tube mold around the flow passage and at the slag-
line. The shape is surrounded by alumina-graphite. The
filled mold is pressed at 5000 psi. A vent is created
between the wax and the exterior surface of the pressed
piece. The wax is melted out of the piece through the
vent, thereby creating an interior cavity. A material
comprising 80 wt.% zirconia and 20 wt.% wax is injected
through the vent into the interior cavity. The piece is
then fired at greater than 1300°C for greater than 5
hours. During firing the wax is eliminated, the zirconia .
densifies to form an erosion-resistant material, and a
gap is created between the zirconia and the body.
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Example 3
A pour tube mold is co-filled with a particulate
zirconia and an alumina-graphite refractory mix. The
zirconia is directed into a pour tube mold at the slag-
line using concentric funnels. An annular wax sleeve is
placed inside of the zirconia around the flow passage.
The zirconia, alumina-graphite and wax sleeve are
copressed at 5000 psi and fired at greater than 1300°C for
greater than 5 hours. During firing the wax is
eliminated, the zirconia densifies to form an erosion-
resistant material, and a gap is created between the
zirconia and the body.
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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