Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Continuous casting nozzle and production method therefor
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nozzle for continuous casting of molten
metal, and more
particularly to a continuous casting nozzle which comprises a tubular
refractory structure having
an inner bore formed along an axial direction thereof to allow molten metal to
pass therethrough,
wherein a part or an entirety of the tubular refractory structure includes an
inner bore- side laver,
an intermediate layer and an outer periphery-side layer.
As used herein, the term "tubular" means any configuration of a refractory
structure having
an inner bore formed along an axial direction thereof, irrespective of a cross-
sectional shape
thereof in a direction orthogonal to the axial direction. That is. the cross-
sectional shape in the
direction orthogonal to the axial direction is not limited to a circular
shape, but may be any other
shape. such as an oval shape, a rectangular shape or a polygonal shape.
As used herein, the term "inner bore-side layer" collectively means any
refractory layer
located on the side of the inner bore relative to a central region (e.g.,
intermediate layer), in a
horizontal cross-section taken at any position of an overall length of a
continuous casting nozzle
in a molten-metal passing direction (i.e., vertical direction), and covers any
layer structure. For
example, the inner bore-side layer may be made up of a plurality of layers. In
this case, a
thermal expansion coefficient of the inner bore-side layer means a maximum one
of respective
thermal expansion coefficients of the plurality of inner bore-side layers.
As used therein, the term "outer periphery-side layer" collectively means any
refractory
layer located on the side of an outer periphery of a continuous casting nozzle
relative to the
central region (e.g., intermediate layer), in the above horizontal cross-
section. and covers any
layer structure. For example, the outer periphery-side layer may be made up of
a plurality of
layers (e.g., a two-layer structure consisting of an AG (i.e. Aluminum-
Graphite)-based layer and
a ZG(i.e. Zirconia-Graphite)-based layer located outside the AG-based layer).
In this case, a
thermal expansion coefficient of the outer periphery-side layer means a
maximum one of
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respective thermal expansion coefficients of the plurality of Outer periphery-
side layers.
2. Description of the Background Art
A continuous casting nozzle, such as a long nozzle for discharging molten
steel from a ladle
into a tundish, or an immersion nozzle for pouring molten steel from a tundish
into a continuous
casting mold, comprises a tubular refractory structure having an inner bore
formed
approximately along an axial center thereof to allow molten metal, such as
molten steel, to pass
therethrough, wherein the molten steel passing through the inner bore causes a
temperature
gradient betvyeen inner bore-side and outer periphery-side layers of the
continuous casting nozzle.
Particularly, in an initial stage of discharging/passing of the molten steel,
the above phenomenon
becomes prominent due to rapid temperature rise in the inner bore-side layer.
Irrespective of whether a refractory body constituting the refractory
structure is made up of
a single layer or a multi-layer, the temperature gradient gives rise to a
strain due to an internal
stress of the refractory body, and becomes one factor causing breaking, such
as cracking,
particularly in the outer periphery-side layer. Further, as the temperature
gradient becomes
larger, and a thermal expansion coefficient of the inner bore-side layer is
greater than that of the
outer periphery-side layer to a larger degree, a thermal stress will be
increased to cause a higher
risk of breaking in the outer periphery-side layer.
In the continuous casting nozzle, a molten steel flow passes therethrough
while violently
colliding against an inner bore surface thereof. Thus, in particular, a region
of the continuous
casting nozzle adjacent to the inner bore surface is severely damaged due to
abrasion caused by
the molten steel, non-metal inclusions in the molten steel, etc.,
embrittlement of a matrix and
washing (corrosion) due to oxidizing components of the molten steel, etc., and
wear caused by a
reaction with FeO and other component of the molten steel. Moreover, in
connection with a
recent trend of upgrading of steel which involves an increase in amount of non-
metal inclusi ns
in molten steel, such as alumina, deposition of inclusions (mainly, alumina)
onto the inner bore
surface of the continuous casting nozzle, or clogging of the inner bore of the
continuous casting
nozzle due to the inclusions, become one key factor determining a lifetime of
the continuous
casting nozzle.
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In the above circumstances, there has been an increasing need for improving
corrosion
resistance and abrasion/wear resistance of an inner bore surface of a
continuous casting nozzle.
and reducing deposition of non-metal inclusions and others onto the inner bore
surface or
clogging of an inner bore of the continuous casting nozzle due to the non-
metal inclusions and
others, to achieve higher durability and safety (stable casting capability) of
the continuous
casting nozzle.
With a view to meeting the above needs, it has been attempted to extend a
lifetime of a
continuous casting nozzle, for example, by applying a refractory composition
excellent in
thermal shock resistance to a body (i.e., an outer periphery-side layer) of
the nozzle, to form a
skeleton of the nozzle, and applying a refractory composition excellent in
abrasion/wear
resistance and corrosion resistance, or a refractory composition resistant to
deposition of
inclusions such as alumina, to a region of the nozzle on the side of an inner
bore surface thereof
adapted to come into contact with a molten steel flow (i.e., inner bore-side
layer) in such a
manner as to define a part or an entirety of the inner bore surface.
Particularly, with regard to the inner bore-side layer, various functional
enhancement
techniques have been recently developed. For example, with a view to providing
higher
corrosion resistance, there has been developed a technique of incorporating a
corrosion resistant
component, such as A1203, Zr0, or MgO, into a material having a reduced amount
of graphite
and silica which are wear-nonresistant aggregates, or a material devoid of
graphite and silica.
Further, with a view to reducing or preventing deposition of inclusions, such
as AI203, in molten
steel, onto the inner bore surface, or clogging of the inner bore due to the
inclusions, there has
been promoted a practical use of a continuous casting nozzle, such as an
immersion nozzle,
having a refractory layer made of a basic material containing a CaO component
highly reactive
with an A1203 component, and inserted thereinto.
A refractory aggregate including the above components for obtaining such a
highly-functional refractory composition has high thermal expansibility, and
the
highly-functional refractory composition contains the refractory aggregate in
a relatively large
amount. Thus, a thermal expansion amount of the inner bore-side layer is apt
to be increased.
Furthermore, due to an additional factor, such as an increase in thermal
gradient caused by
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lowering in thermal conductivity of the inner bore-side layer relative to the
outer periphery-side
layer, in connection with a reduction in carbon content, a difference between
respective thermal
expansion amounts of the inner bore-side layer and the outer periphery-side
layer, and a resulting
thermal stress, are apt to be more increased, which leads to increasing risk
of breaking of the
continuous casting nozzle, particularly cracking in the outer periphery-side
layer caused by
thermal expansion of the inner bore-side layer (hereinafter referred to as
"expansion cracking").
A typical countermeasure against breaking of a continuous casting nozzle due
to a
temperature gradient (thermal stress) therein includes a technique of reducing
a thermal stress
based on an increase in thermal conductivity, a reduction in thermal expansion
amount, and/or a
lowering in elastic modulus, for example, by incorporating graphite into a
refractory composition
of the continuous casting nozzle in a relatively large amount, or by adding or
quantitatively
increasing fused silica having a relatively small thermal expansion
coefficient. On the other
hand, the increased content of graphite or fused silica has a negative effect
causing deterioration
in durability, such as abrasion/wear resistance and corrosion resistance, due
to deterioration in
oxidation resistance, enhancement in reactivity with other refractory
components and
components in molten steel, etc. Thus, the above technique is not effective as
a realistic
solution due to restrictions in application to the inner bore-side layer.
In the above situation, with a view to avoiding the risk of breaking of a
continuous casting
nozzle, there has been employed a technique of, for example, in a structure
where a shaped
member serving as an inner bore-side layer is installed on the side of an
inner bore of an outer
periphery-side layer, forming a mortar layer therebetween using mud-like
mortar which
comprises a fine powder mainly consisting of a refractory material such as a
conventional oxide,
and a nonorganic binder such as silicate containing a relatively large amount
of solvent, in such a
manner that the mortar layer has a relatively large porosity to reduce a
strength thereof so as to
allow a stress caused by thermal expansion of the inner bore-side layer to be
relieved based on
breakage of the mortar layer itself, i.e., a technique employing mortar
capable of exhibiting a
relatively high porosity although it has a relatively low bonding force, to
avoid cracking of the
nozzle. However, this anti-cracking technique based on mortar has the
following problems.
(1) The mortar layer containing an excess amount of solvent has a properly
that the
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solvent in the mortar layer is absorbed in materials of the remaining lavers
through cnntnct N\ ith
the materials of the remaining layers. Thus, the porosity of the mortar layer
is apt to become
gradually lower or denser toward a boundary surface with each of the remaining
layers, so that.
particularly, when the mortar layer is used in a continuous casting nozzle and
formed to have a
small thickness of several nun, the stress relief function of the mortar layer
itself after
installation will deteriorate or disappear.
(2) It is substantially impossible to control an apparent porosity.
Specifically, a pore
distribution allowing for buckling at a given stress or less cannot be
controlled, and thereby it is
essential to contain an excessive amount of solvent to preclude achievement of
a good balance
with bondability (i.e., bonding capability).
(3) The stress relief function of the mortar layer is based on a mechanism
where an
allowance for thermal deformation of the inner bore-side layer is created to
absorb a stress by an
irreversible breaking of a matrix of the mortar layer defining pores. to
relieve a stress in the inner
bore-side layer. Thus, when the matrix is broken once, the bondability is lost
to increase a risk
of drop-off. Moreover, a gap resulting from breaking of the mortar layer due
to changes in
temperature allows molten metal, such as molten steel and slag, to easily
intrude thereinto, and
the intruding molten steel and slag are highly likely to cause cracking and
corrosion, which leads
to damage of the remaining layers or the continuous casting nozzle.
For example, as another approach to preventing breaking due to a thermal
stress while
seeking higher durability such as corrosion resistance, the following Patent
Document I
discloses a casting nozzle comprising: a carbon-free refractory layer formed
to have high thermal
expansibility and high corrosion resistance and installed only on the side of
an inner bore of the
nozzle; a carbon-containing refractory layer formed to have excellent spalling
resistance and
installed on the side of the remaining part, i.e., outer periphery, of the
nozzle; and a separating
layer allowing at least 80% or more of a contact surface between the two
refractory layers to be
separated from each other, wherein the separating layer is formed by setting a
burnable material.
such as polypropylene or nylon, and then the burnable material is vanished.
However, in the casting nozzle disclosed in the Patent Document 1, less than
20% of the
contact surface between the two refractory layers is bonded together. Even if
a bonded region
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is fairly small, it will be an origin of an expansion cracking phenomenon,
because a stress
causing expansion cracking is transmitted from the carbon-free refractory
layer (i.e., inner
bore-side layer) to the carbon-containing refractory layer (i.e., outer
periphery-side layer)
through the bonded region. If the bonded region is set at zero %, it causes a
basic problem that
the inner bore-side layer cannot be structurally supported. Moreover, molten
steel easily
intrudes into the separating layer to cause problems, such as fissures in the
refractory layers due
to solidification shrinkage of the molten steel occurring when it undergoes
changes in
temperature, and expansion of the solidified steel occurring when it is
heated, and peel-off due to
no bonding between the inner bore-side layer and the outer periphery-side
layer.
The following Patent Document 2 discloses a technique intended to suppress
deposition of
inclusions, wherein a CaO nozzle member containing 70 wt% or more of CaO and
having an
apparent porosity of 50% or less is inserted into a nozzle body of an
immersion nozzle, in such a
manner that a gap depending on a thermal expansion amount of the CaO nozzle
member is
provided between the CaO nozzle member and the nozzle body. The Patent
Document 2 also
discloses a technique of packing thin ceramic fibers or a small amount of
mortar between an end
of the CaO nozzle member and the nozzle body to fix the CaO nozzle member to
the nozzle body.
according to need.
In the above structure where a gap equivalent to a thermal expansion amount of
the CaO
nozzle member on the side of an inner bore of the immersion nozzle (i.e.,
inner bore-side layer)
is provided between the nozzle body on the side of an outer periphery of the
immersion nozzle
(i.e., outer periphery-side layer) and the CaO nozzle member, the expansion
cracking
phenomenon of the outer periphery-side nozzle body caused by the highly-
expandable CaO
nozzle member can be suppressed. However, in view of the description "the gap
is preferably
set to be 3% or more of an outer diameter of the CaO nozzle member, during
preheating" in the
paragraph [0022] of the Patent Document 2, it is assumed that the inner bore-
side CaO nozzle
member is not in close contact with the outer periphery-side nozzle body in a
high-temperature
state (a thermal expansion coefficient of a CaO-based material is about 2% or
less at about
1500 C, even in a material consisting substantially only of CaO and having a
maximum level of
thermal expansion coefficient). If the CaO nozzle member is not in close
contact with the
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nozzle body in the high-temperature state, i.e.. during use of the immersion
nozzle the CaO
nozzle member is likely to have displacement or drop off due to a compression
stress receiving
during use, Moreover, molten steel easily intrudes into the gap between the
CaO nozzle
member and the nozzle body. This involves a risk of damage of the CaO nozzle
member and
the outer periphery-side nozzle body due to solidification shrinkage of the
molten steel and
thermal expansion of the solidified steel. Furthermore, a material, such as
CaO, capable of
reacting with deoxidation products in molten steel to produce a low-melting
point compound is
fundamentally premised on wearing out. Thus, the CaO nozzle member involves a
risk of
drop-off or breaking due to reduction in thickness caused by wear, and the
structure having no
support base therebehind.
As above, if a joint portion between the inner bore-side layer and the outer
periphery-side
layer, such as the separating layer in the Patent Document I or the gap in the
Patent Document 2,
is set to be excessively broad, a resulting intrusion of molten steel is
likely to cause peel-off and
damage of the inner bore-side layer, and damage of the outer periphery-side
layer. Further, if
the joint portion is set to be excessively narrow, a tensile stress acting on
the outer periphery-side
layer in a circumferential direction thereof due to thermal expansion of the
inner bore-side layer
is likely to cause occurrence of longitudinal crack along an axial direction
of the tubular
refractory structure, or transverse crack (crack along a direction having an
angle relative to the
axial direction; so-called "fracture", etc.)
Thus, in a continuous casting nozzle having a highly-expandable inner bore-
side layer
installed therein, it would be critical to form a matrix structure capable of
preventing intrusion or
passing of molten metal, and have a function of allowing the inner bore-side
layer to be bonded
to an outer periphery-side layer, in addition to a function of reducing an
influence of a stress
from the inner bore-side layer. However, heretofore, a solution for giving the
above three
functions or structures has seldom been discussed.
Further, as disclosed in the Patent Documents I and 2, a conventional
installation process
essentially includes a step of preparing the outer periphery-side layer as a
nozzle body of the
continuous casting nozzle, and the inner bore-side layer, separately from each
other, and a step of
assembling the two layers together in a final stage by use of mortar or the
like. This causes
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deterioration in productivity and an increase in production cost. Moreover. in
the assembling of
the refractory layers prepared as separate components, the layers are brought
into contact vvith
each other through respective flat and smooth surfaces thereof. Thus, it is
difficult to obtain a
bonding strength and a fixing force therebetween sufficient to solve the above
problems, which
leads to a need for additional means to enhance the bonding strength, based on
an adhesive or the
like.
[Patent Document 1] JP 60-152362A
[Patent Document 2] JP 07-232249A
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a continuous casting
nozzle comprising a
refractory layer formed to have high functions, such as high corrosion
resistance and high anti-
deposition capability, and disposed on the side of an inner bore thereof to
serve as an inner
bore-side layer, so as to enhance durability, wherein the continuous casting
nozzle is capable of
preventing expansion cracking of an outer periphery-side layer serving as a
nozzle body thereof,
due to a difference in thermal expansion between respective compositions of
the inner bore-side
layer and the outer periphery-side layer, while preventing displacement and
peel-off of the inner
bore-side layer during casting. It is another object of the present invention
to provide a method
of producing the continuous casting nozzle in a stable and easy manner.
More specifically, in a continuous casting nozzle comprising a tubular
refractory structure
which has an inner bore formed along an axial direction thereof to allow
molten metal to pass
therethrough, and at least partly includes an inner bore-side layer disposed
on the side of the
inner bore, and an outer periphery-side layer disposed on a radially outward
side relative to the
inner bore-side layer, wherein the inner bore-side layer has the thermal
expansion greater than
that of the outer periphery-side layer, it is an object of the present
invention to (1) prevent
breaking of the outer periphery-side layer, and (2) enhance stability of the
inner bore-side layer
during casting, while (3) preventing intrusion of molten steel and others
between respective ones
of a plurality of layers including an intermediate layer. In other words, it
is an object of the
present invention to provide a continuous casting nozzle having a structure
capable of satisfying
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these functions. It is another object of the present invention to provide a
production method
capable of stably obtaining the continuous casting nozzle in an optimized and
laborsaving
manner.
In order to achieve the above objects, according to one aspect of the present
invention, there
is provided:
(1) a continuous casting nozzle comprising a tubular refractory structure v
hich has an
inner bore formed along an axial direction thereof to allow molten metal to
pass therethrough.
and at least partly includes an inner bore-side layer disposed on the side of
the inner bore, and an
outer periphery-side layer disposed on a radially outward side relative to the
inner bore-side layer.
wherein the inner bore-side layer has the thermal expansion greater than that
of the outer
periphery-side layer. The continuous casting nozzle is characterized in that
the tubular
refractory structure includes an intermediate layer having compressability and
lying between the
inner bore-side layer and the outer periphery-side layer, wherein: the inner
bore-side layer. the
intermediate layer and the outer periphery-side layer are simultaneously
integrated together
during a forming process to form a multi-layer structure; a bonding strength
between the
intermediate layer and each of the inner bore-side layer and the outer
periphery-side layer
adjacent to the intermediate layer is in the range of 0.01 to 1.5 MPa, as
measured in a
non-oxidation atmosphere at 1000 C; and the intermediate layer has a
compressive rate K (%)
satisfies the following Formula I as measured in a non-oxidation atmosphere at
1000 C under a
pressure of 2.5 MPa,
K > [(Di x (i - Do x ao) / (2 x Tin)] --- Formula I
wherein: Di is an outer diameter (min) of the inner bore-side layer;
Do is an inner diameter (mm) of the outer periphery-side layer;
Tin is an initial thickness (min) of the intermediate layer at room
temperature;
ai is a maximum thermal expansion coefficient (%) of the refractory
composition of the inner bore-side layer in a temperature range of room
temperature to I 500`C;
and
ao is a thermal expansion coefficient (%) of the refractory composition of
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the outer periphery-side layer at a temperature at start of discharge or
pouring of molten
metal through the continuous casting nozzle (i.e., passing of molten steel).
(2) Preferably, in the continuous casting nozzle, the intermediate layer in a
state
after being subjected to a heat treatment in a non-oxidation atmosphere at 600
C or more
contains expanded expansive graphite particles (hereinafter referred to as
"expanded graphite
particles").
(3) Preferably, in the continuous casting nozzle, the intermediate layer in a
state
after being subjected to a heat treatment in a non-oxidation atmosphere at
1000 C contains a
carbon component (except any carbon compound with the remaining components) in
a total
amount of 16 mass% or more (including 100 mass%).
(4) Preferably, in the continuous casting nozzle, the intermediate layer in a
state
after being subjected to a heat treatment in a non-oxidation atmosphere at
1000 C contains a
carbon component (except any carbon compound with the remaining components) in
a total
amount of 16 mass% or more, with the remainder other than the carbon component
being a
refractory material comprising one or more selected from the group consisting
of oxide,
carbide, nitride and metal.
According to another aspect of the present invention, there is provided:
(5) a method of producing a continuous casting nozzle comprising a tubular
refractory structure which has an inner bore formed along an axial direction
thereof to allow
molten metal to pass therethrough, and at least partly includes an inner bore-
side layer, an
intermediate layer and an outer periphery-side layer which are arranged in
this order in a
radially outward direction with respect to the inner bore. The method
comprises the steps
of: preparing a mixture (ingredients) for the intermediate layer, which
contains un-expanded
expansive graphite particles in an amount ranging from 5 to 45 mass%, and
burnable
particles in an amount ranging from 55 to 95 mass%, and further contains an
organic binder
in a given mass% with respect to a total mass% of the un-expanded expansive
graphite
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particles and the burnable particles, and in addition to the total mass%,
wherein the given
mass% of the organic binder is set to allow a ratio of a carbon component only
of the organic
binder (except any carbon compound with the remaining components) to an entire
refractory
composition of the intermediate layer, in a state after the refractory
composition of the
intermediate layer is subjected to a heat treatment in a non-oxidation
atmosphere at 1000 C,
to fall within the range of 2.5 to 15 mass%; subjecting the mixture
(ingredients) for the
intermediate layer to a pressure forming using a cold isostatic press (CIP)
machine,
simultaneously and integrally together with a mixture (ingredients) for the
inner bore-side
layer and a mixture (ingredients) for outer periphery-side layer, to obtain a
single shaped
body; and subjecting the shaped body to a heat treatment at a temperature of
600 to 1300 C
to allow the burnable particles contained in the mixture (ingredients, i.e.,
green body after
pressing) for the intermediate layer in the shaped body to be vanished so as
to form voids,
and then expand the un-expanded expansive graphite particles contained in the
mixture
(ingredients, i.e., green body after pressing) for the intermediate layer in
the shaped body so
as to allow the voids to be filled with the expanded graphite particles.
According to another aspect of the present invention, there is provided:
(6) a method of producing a continuous casting nozzle comprising a tubular
refractory structure which has an inner bore formed along an axial direction
thereof to allow
molten metal to pass therethrough, and at least partly includes an inner bore-
side layer, an
intermediate layer and an outer periphery-side layer which are arranged in
this order in a
radially outward direction with respect to the inner bore. The method
comprises the steps
of. preparing a mixture (ingredient) for the intermediate layer, which
contains un-expanded
expansive graphite particles in an amount ranging from 5 to 45 mass%, and
burnable
particles in an amount ranging from 55 to 95 mass%, a refractory material
which is one or
more selected from the group consisting of oxide, carbide, nitride and metal,
in a total
amount of 40 mass% or less, and further contains an organic binder in a given
mass% with
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respect to a total mass% of the un-expanded expansive graphite particles, the
burnable
particles and the refractory material which is one or more selected from the
group consisting
of oxide, carbide, nitride and metal, and in addition to the total mass%,
wherein the given
mass% of the organic binder is set to allow a ratio of a carbon component only
of the organic
binder (except any carbon compound with the remaining components) to an entire
refractory
composition of the intermediate layer, in a state after the refractory
composition of the
intermediate layer is subjected to a heat treatment in a non-oxidation
atmosphere at 1000 C,
to fall within the range of 2.5 to 15 mass%; subjecting the mixture
(ingredients) for the
intermediate layer to a pressure forming using a cold isostatic press (CIP)
machine,
simultaneously and integrally together with a mixture (ingredients) for the
inner bore-side
layer and a mixture (ingredients) for outer periphery-side layer, to obtain a
single shaped
body; and subjecting the shaped body to a heat treatment at a temperature of
600 to 1300 C
to allow the burnable particles contained in the mixture (ingredients, i.e.,
green body after
pressing) for the intermediate layer in the shaped body to be vanished so as
to form voids,
and then expand the un-expanded expansive graphite particles contained in the
mixture
(ingredients, i.e., green body after pressing) for the internediate layer in
the shaped body so
as to allow the voids to be filled with the expanded graphite particles.
Specifically, in order to achieve the above objects, a continuous casting
nozzle of the
present invention is intended to meet the following fundamental requirements:
(1) to install an intermediate layer having a stress relief function, between
the inner
bore-side layer and the outer periphery-side layer;
(2) to maintain a layer configuration of the intermediate layer so as to
prevent
breaking and other adverse effect due to layer destruction, i.e., to enhance
layer stability; and
(3) to simultaneously integrate the intermediate layer, the inner bore-side
layer and
the outer periphery-side layer together during a forming process to form a
multi-layer
structure so as to fixedly bond between the intermediate layer and each of the
inner bore-side
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layer and the outer periphery-side layer.
(The above requirements (1), (2) and (3) will hereinafter be referred to as
respectively
as "compressability requirement", "stability requirement" and "bondability
requirement".)
Each of the above requirements will be specifically described below.
(1) COMPRESSABILITY REQUIREMENT
As mentioned above, with a view to enhancement in corrosion resistance and
abrasion/wear resistance, suppression of elution of a carbon component from a
refractory
composition into molten steel, and prevention of deposition of inclusions,
mainly nonmetal
inclusions such as alumina, onto an inner bore surface or nozzle clogging due
to the
inclusions, the inner bore-side layer tends to be made of a refractory
composition having an
increased amount of A1203, MgO,
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Z_rO, and/or CaO and exhibiting excellent corrosion resistance and
abrasion/wear resistance.
In many cases, the outer periphery-side layer (including an outer periphery-
side layer as a
part of a nozzle body) to be designed while placing great important on thermal
shock resistance
has a smaller content of A1203, MgO, ZrO2 and/or CaO as compared with the
inner bore-side
layer. Thus, a thermal expansion coefficient of the inner bore-side layer
inevitably becomes
greater than that of the outer periphery side layer.
When a refractory composition having a larger thermal expansion coefficient
than that of
the outer periphery-side layer is used for the inner bore-side layer, breaking
of a continuous
casting nozzle due to fissures and expansion cracking in the outer periphery-
side layer caused by
the inner bore-side layer will more frequently occur. Even if the inner bore-
side layer and the
outer periphery-side layer are made of the same refractory composition or,
made of respectively,
of refractory compositions having the same level of thermal expansion
characteristic, the
breaking occurs when the inner bore-side layer is heated up to a temperature
greater than that of
the outer periphery-side layer due to preheating or rapid heating from the
side of the inner bore.
passing of molten steel or the like, to create a large temperature gradient
between the inner
bore-side layer and the outer periphery-side layer.
That is, in the present invention, the requirement that "the inner bore-side
layer has the
thermal expansion greater than that of the outer periphery-side layer" means
not only a condition
that a maximum thermal expansion coefficient of the refractory composition of
the inner
bore-side layer at 1500 C (substantially close to a casting temperature
region) or less is greater
than that of the refractory composition of the outer periphery-side layer at
1500 C or less, but
also a condition that a level of thermal expansion of the inner bore-side
layer becomes greater
than that of the outer periphery-side layer, due to a temperature difference
between the inner
bore-side layer and the outer periphery-side layer occurring during heating,
such as receiving of
molten steel or preheating from the side of the inner bore, even though each
of the inner
bore-side layer and the outer periphery-side layer has the same maximum
thermal expansion
coefficient or exhibits the same thermal expansion behavior (e.g., a material
having the same
composition and structure).
In cases where there is no stress relief function or only an extremely low
stress relief
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CA 02701848 2010-04-06
function, between an inner bore-side laver and an outer periphery-side laver a
stress of the inner
bore-side layer is applied to the outer periphery-side layer as a compression
stress oriented in a
radial direction on a horizontal section of the nozzle. Further, if the outer
periphery-side layer is
designed to extend to cover opposite longitudinally or axially outward ends of
the inner
bore-side layer, the stress of the inner bore-side layer is also applied to
the outer periphery-side
laver as a compression stress oriented in the axial direction. Then, the
radial compression stress
is converted to a tensile stress oriented in a circumferential direction, and
the axial compression
stress is converted to a tensile stress in the axial direction. Subsequently,
when these tensile
stresses become greater than a tensile strength limit of the refractory
composition of the outer
periphery-side layer, the circumferential tensile stress causes an axial
(vertical) crack, and the
axial tensile stress causes a horizontal (transverse) crack to damage the
outer periphery-side
layer.
In the present invention, as means to provide the stress relief function
between the inner
bore-side layer and the outer periphery-side layer having the above
relationship, the intermediate
layer having compressability and bondability during a nozzle preheating
operation and during
hearing up to 1500 C (substantially close to a casting temperature region) is
installed.
This allows a stress due to thermal expansion of the outer periphery-side
layer to be applied
to the installed intermediate layer as a compression stress without being
directly applied to the
outer periphery-side layer. During this process, in response to the
compression stress, a
thickness of the intermediate layer itself is reduced in the radial direction,
and in the axial
direction at the axial end. In other words, a stress due to thermal expansion
the intermediate
layer can be relieved by reducing a volume of the intermediate layer. In the
present invention.
such a capability to be reduced in thickness and volume is referred to as
"compressability".
Generally, in a tubular refractory structure comprising an A12O3-C based
material which is a
typical material of an outer periphery-side layer of a conventional immersion
nozzle, the outer
periphery-side layer is broken by a pressure of about 2.5 MPa applied to an
inner wall surface
thereof. For example, in an AhO3-graphite based refractory structure
comprising an outer
periphery-side layer which has practically minimum radial dimensions (inner
diameter ~ = 80
mm, outer diameter = 135 mm) and a maximum tensile strength of 6 MPa, when a
pressure
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CA 02701848 2010-04-06
load is applied from the side of an inner wall surface of the outer periphery-
side layer, the outer
periphery-side layer reaches breaking when the pressure load is applied to the
inner wall surface
at about 2.5 MPa, according to calculation using a formula for a thick-walled
cylinder.
In a continuous casting nozzle where an intermediate layer and an inner bore-
side layer are
disposed on the side of an inner bore relative to an outer periphery-side
layer, the intermediate
layer itself is required to exhibit a deformation behavior in order to relieve
a stress due to thermal
expansion of the inner bore-side layer, which is oriented toward the outer
periphery-side layer.
That is, the stress oriented toward the outer periphery-side layer has to be
reduced to 2.5 MPa or
less by deformation (contraction) of the intermediate layer.
Thus, during heating of the inner bore-side layer or during passing of molten
steel, a tensile
stress to be generated in the outer periphery-side layer is preferably reduced
to 2.5 MPa or less,
more preferably further reduced as low as possible to provide enhanced safety,
and the
intermediate layer itself is required to exhibit a deformation behavior
capable of reducing a
compression stress to a value corresponding to such a tensile stress value.
Compressability required for the intermediate layer under a pressing force of
2.5 MPa or
more can be expressed as a compressive rate K (%) in the following Formula 1:
K > [(Di x ai - Do x ao) / (2 x Tin)] --- Formula I
wherein: Di is an outer diameter (mnnm) of the inner bore-side layer;
Do is an inner diameter (mm) of the outer periphery-side layer;
Tm is an initial thickness (mm) of the intermediate layer at room
temperature;
ai is a maximum thermal expansion coefficient (%) of the refractory
composition of the inner bore-side layer in a temperature range of room
temperature to 1500`C;
and
ao is a thermal expansion coefficient (%) of the refractory composition of
the outer periphery-side layer at a temperature at start of passing of molten
metal.
Di and Do are, respectively, a diameter measured on an outer periphery-side
surface of the
inner bore-side layer and a diameter measured on an inner bore-side surface of
the outer
periphery-side layer, in respective horizontal cross-sections (i.e., cross-
sections taken along a
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CA 02701848 2010-04-06
direction perpendicular to the axial direction) of the inner bore-side layer
and the outer
periphery-side layer. When a horizontal cross-sectional shape of each of the
inner bore-side
layer and the outer periphery-side layer is not circle, Di may be defined as a
distance between
two positions where a straight line extending radially from a center of the
horizontal
cross-sectional shape of the inner bore-side layer intersects with the outer
periphery-side surface
of the inner bore-side layer, and Do may be defined as a distance between two
positions where
the above straight line intersects with the inner bore-side surface of the
outer periphery-side layer
in the cross-section. Then, the entire dimensions may be determined to satisfy
the Formula 1.
With regard to compressability in an axial end of the nozzle. Di may be
replaced vv ith an
axial distance between respective opposite axially outward end surfaces of the
inner bore-side
layer, and Do may be replaced with an axial distance between respective
opposite axially inward
surfaces of the outer periphery-side layer each facing a corresponding one of
the axially outward
end surfaces of the inner bore-side layer, in respective vertical cross-
sections of the inner
bore-side layer and the outer periphery-side layer, taken along a longitudinal
(vertical) axis of the
nozzle.
In the Formula 1, ai is a maximum thermal expansion coefficient (%) of the
refractory
composition of the inner bore-side layer in a temperature range of room
temperature to 1500 C,
which means that ai is a maximum thermal expansion coefficient of the
refractory composition
of the inner bore-side layer in a temperature range of room temperature to
substantially a molten
steel temperature. Further, ao is a thermal expansion coefficient (%) of the
refractory
composition of the outer periphery-side layer at a temperature at start of
passing of molten metal.
and the temperature to which the outer periphery-side layer is exposed at
start of passing of
molten metal, varies depending on operation conditions, such as a preheating
condition. Thus.
ao is determined for each job site on a case-by-case basis.
In cases where the continuous casting nozzle is used without preheating, a
temperature of
the outer periphery-side layer is equal to room temperature (ambient
temperature). In this case,
ao may be considered as a thermal expansion coefficient at room temperature
which is a
reference point of a measurement of thermal expansion coefficient, i.e.,
"zero", and therefore the
Formula I can be expressed as the following Formula 2:
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CA 02701848 2010-04-06
K > [(Di x ai) / (2 x Tm)1 --- Formula 2
The compressive rate K satisfying the Formula 2 is a value in consideration of
the most
severe condition, i.e., a condition that a difference in thermal expansion
between the inner
bore-side layer and the outer periphery-side layer is maximized. Thus, if the
compressive rate
K is determined at a value satisfying the Formula 2, the outer periphery-side
layer will never be
broken. Preferably, the compressive rate K is set at a value satisfying the
Formula 2 in all the
operation conditions.
The compressive rate K is a value determined under a condition that a target
refractory
composition (sample) is not oxidized, for example, in a non-oxidation
atmosphere, such as a
reducing gas atmosphere or an inert gas atmosphere, or in an oxidizing gas
atmosphere, such as
an air atmosphere, under a condition that an antioxidant is applied on a
surface of the target
refractory (sample). During an actual use of the continuous casting nozzle,
the intermediate
layer is placed in a non-oxidation atmosphere. If a target sample is oxidized
during a
measurement of the compressive rate K. properties of the sample cannot be
accurately figured
out.
Preferably, in the present invention, the compressive rate K of the
intermediate layer is
fundamentally set in the range of 10 to 80%.
A thickness of the intermediate layer can be adjusted depending on the
compressive rate K
of the intermediate layer to absorb expanded dimensions of the inner bore-side
layer. If the
compressive rate K is less than 10%, the thickness of the intermediate layer
will be increased
depending on a difference in thermal expansion coefficient between the inner
bore-side layer and
the outer periphery-side layer. Thus, due to restrictions on an overall wall
thickness of the
continuous casting nozzle, a wall thickness of the nozzle body is inevitably
reduced to cause a
problem about deterioration in structural strength. If the compressive rate K
is greater than
80%, an excessively reduced thickness of the intermediate layer is likely to
cause a problem;
about production difficulty in forming such a thin intermediate layer, and a
problem about
deterioration in bonding strength between the inner bore-side layer and the
outer periphery-side
layer, although the thickness of the intermediate layer can be sufficiently
reduced to prevent
occurrence of the above problem about deterioration in structural strength.
For example, on an
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CA 02701848 2010-04-06
assumption that the inner diameter of the outer periphery-side laver, the
thermal expansion
coefficient of the inner bore-side layer, and the thermal expansion
coefficient of the outer
periphery-side layer, are set, respectively, at about 80 mm ~, 2.0% and 0.8%,
which are close to
the smallest size in conventional continuous casting nozzles, the thickness of
the intermediate
layer is about 4 min, and the compressive rate necessary for the refractory
composition of the
intermediate layer is 10%. Further. on an assumption that the inner diameter
of the outer
periphery-side layer, the thermal expansion coefficient of the inner bore-side
layer. and the
thermal expansion coefficient of the outer periphery-side layer, are set,
respectively, at about 150
mm ~. 2.0% and 0.8%, which are close to the largest size in conventional
continuous casting
nozzles, the thickness of the intermediate layer is about 1.2 mm, and the
compressive rate
necessary for the refractory composition of the intermediate layer is 78%.
The above compressive rate may be measured by the following method, and a
resulting
measured value can be regarded as the compressive rate.
A columnar refractory body (20 min ~ x 5 min t) made of a Mixture having a
characteristic
of exhibiting compressability after being formed under a pressure equal to a
shaping pressure
and subjected to a heat treatment is put in a holding cavity of a carbon-based
member having the
same shape as that of the columnar refractory body, and subjected to a heat
treatment under a
non-oxidation atmosphere in a given temperature rise pattern to allow a
burnable component to
be vanished so as to obtain a columnar sample (about 20 mm ~ x about 5 mm t).
This
heat-treated columnar sample is disposed between respective end surfaces of
two refractory jigs
each having a size of 20 mm ~ x 40 mm L. Further, a cylindrical sample guide
made of a
refractory composition and formed to have an inner diameter 20 mnm 4, an outer
diameter of 50
mm 4 and a height of 78 mnm is fitted on the sample in order to prevent the
sample from dropping
off laterally during an operation of longitudinally pressing the columnar
sample clamped
between the jigs, to obtain a measurement sample.
The measurement sample is set inside a furnace of a material testing machine
adapted to
controllably adjust a temperature, an atmosphere and a pressing speed. Then,
after increasing a
furnace temperature up to a given value under a non-oxidation atmosphere, and
holding the
temperature until it is uniformed, a pressing operation is initiated to carry
out the measurement.
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CA 02701848 2010-04-06
Specifically, an initial thickness to (min) of the cylindrical measurement
sample is firstly
measured under a pressureless condition. After holding a temperature of the
measurement
sample at a given value, the measurement sample is compressed from upward and
downward
directions while setting a crosshead moving speed in the range of 0.001 to
0.01 mm/sec, in such
a manner as to increase a pressing force up to 2.5 MPa, and then a
displacement (i.e.,
deformation amount) h 1 (mm) of the measurement sample is measured.
Furthermore, in order
to measure a blank value (i.e., a value in a measurement sample devoid of the
intermediate layer),
under the same temperature and the same load of the refractory jigs for
clamping the
measurement sample, the measurement sample is pressed under the same
conditions, except that
it is clamped, to measure a displacement h2. These measured values can be
assigned to the
following Formula 3 to calculate the compressive rate K (%) at each
temperature:
K = (h 1 - h2) / tO x 100 (%) ----- Formula 3
The compressive rate K can also be measured from an actual casting nozzle
having a
structure where the inner bore-side layer is continuously integrated with the
outer periphery-side
layer through the intermediate layer during the forming process. Specifically,
the actual casting
nozzle is subjected to core boring of 20 mm ~ from the outer periphery-side
layer toward an axis
of a refractory body of the nozzle in a direction perpendicular to the axis,
to obtain a core sample
integrally including respective portions of the inner bore-side layer, the
intermediate layer and
the outer periphery-side layer, and having a diameter of about 20 nmm ~ and
opposite curved ends
consisting of respective portions of an inner bore surface and an outer
peripheral surface of the
nozzle. In order to uniformly measure the compressive rate of the intermediate
layer, the core
sample is bonded to two refractory jigs after flatly machining each of top and
bottom curved
surfaces of the ends, or bonded to refractory jigs each having the same
curvature as that of a
corresponding one of the ends of the core sample, to obtain a measurement
sample including the
inner bore-side layer, the intermediate layer and the outer periphery-side
layer and having a size
of 20 min ~ x 80 to 100 mmrm L (when the measurement sample is smaller than
this size.
conditions, the measurement may be performed under a condition that parameter
values, such as
a unit area and a unit length, are set at the same level as those in the above
measurement sample
on a calculation basis, and then a measured value may be subjected to
conversion). Then, in the
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CA 02701848 2010-04-06
same manner as that in the above method, the initial thickness t0 (mnl) of the
intermediate layer
is accurately measured under a pressureless condition. Further, the
displacement hl of the
intermediate layer is measured in a non-oxidation atmosphere, and the
displacement h2 as a
blank value in a state devoid of the intermediate layer is measured, so as to
calculate the
compressive rate K. The measurement sample obtained from the actual nozzle
makes it
possible to accurately measure compressability of the intermediate layer.
(2) STABILITY REQUIREMENT AND BONDABILITY REQUIREMENT
The intermediate layer is required to satisfy the above compressability
requirement, and
maintain sufficient stability and bondability relative to the inner bore-side
layer and the outer
periphery-side layer after production and during use of the continuous casting
nozzle.
As mentioned in the section "Background Art", the conventional technique of
obtaining
compressability based on buckling or destruction of a large amount of low-
strength mortar or a
matrix having a high porosity, the matrix after buckling or destruction is
simply formed as
powder, and thereby neither the interlayer bonding force nor the intermediate
layer itself can be
maintained. If a gap exists between the inner bore-side layer and the outer
periphery-side layer
due to the powdered intermediate layer, i.e., the layers are in a peeled state
without being fixed to
each other, the intermediate layer itself is locally destroyed due to a stress
unevenly applied
thereto to further increase a volume of gap. If the intermediate layer is
vanished, the inner
bore-side layer is likely to become movable and cause drop-off or destruction,
and the inner
bore-side layer is likely to be locally brought into contact with the outer
periphery-side layer and
cause stress concentration and destruction. Moreover, such a gap allows
intrusion of molten
metal, such as molten steel, which is likely to accelerate layer destruction
and interlayer peeling.
Thus, the intermediate layer having compressability is required to maintain
healthy matrix
and given bondability (as will be described later) relative to the inner bore-
side layer and the
outer periphery-side layer, even after being compressed.
It is understood that the given bondability of the intermediate layer relative
to the inner
bore-side layer and the outer periphery-side layer is based on an assumption
that the refractory
composition itself of the intermediate layer has a certain level or more of
layer strength
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CA 02701848 2010-04-06
necessary for exhibiting the given bondability, to allow a healthy layer
condition to be
maintained.
In order to enhance stability of the intermediate layer and fixability of the
intermediate layer
relative to the inner bore-side layer and the outer periphery-side layer, the
present invention is
intended to enhance bondability of the intermediate layer relative to the
inner bore-side layer and
the outer periphery-side layer.
Bondability, i.e., joinability or fixability. can be evaluated as bonding
strength (as
mentioned above, on the assumption that stability enough to exhibit the
bonding strength is
ensured, the bondability is evaluated). The inventers have found that an
optimal value of the
bonding strength is in the range of 0.01 to 1.5 MPa, as measured in a non-
oxidation atmosphere
at 1000 C.
The minimum value "0.01 MPa" of the bonding strength is determined based on a
result of
various experimental tests, and a value which allows a frictional resistance
between the layers to
be obtained in a minimum level for maintaining each of the inner bore-side
layer and the outer
periphery-side layer in its installation position. If the bonding strength is
less than 0.01 MPa,
a retention capability for the inner bore-side layer is low although it might
prevent drop-off of
the inner bore-side layer before start of passing of molten steel, and thereby
the inner bore-side
layer is likely to be peeled off, due to shock at start of passing of molten
steel, vibration caused
by changes in flow speed of molten steel, or occurrence of local wear in the
inner bore-side layer.
Moreover, in various stages, such as transportation of the continuous casting
nozzle, installation
of the continuous casting nozzle to a continuous casting apparatus, preheating
and passing of
molten steel, external force in each of the stages is likely to cause
displacement from a
predetermined position, peel-off or drop-off of the inner bore-side layer.
Furthermore, in
connection with the above phenomena, a portion of the intermediate layer
becomes impossible to
satisfy the above compressability requirement to cause a higher risk of
breaking of the inner
bore-side layer or the outer periphery-side layer.
If the bonding strength is increased to a value greater than 1.5 MPa, the
strength of the
refractory composition itself of the intermediate layer will be increased
accordingly. That is, a
strength of an internal matrix of the intermediate layer is also largely
increased at the same level
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CA 02701848 2010-04-06
as that of the bonding strength, to spoil the compressability of the
intermediate layer. This
makes it easy to transmit a stress due to thermal expansion of the inner bore-
side layer to the
outer periphery-side layer without being relived, to increase a risk of
occurrence of breaking of
the continuous casting nozzle.
As shown in FIG. 3, the bonding strength may be measured by cutting a portion
of a
continuous casting nozzle having an inner bore-side layer 2, an intermediate
layer 4 and an outer
periphery-side layer 3, along two planes parallel to a horizontal cross-
section of the nozzle (taken
along a direction perpendicular to a longitudinal or axial direction of the
nozzle), to obtain a
cylindrical slice-shaped sample 10 having a thickness of about 100 mm. pushing
only the inner
bore-side layer 2 dowmwardly using a pushing member II (a columnar pusher made
of a
refractory composition and formed to have a flat end) having an outer diameter
approximately
equal to that of the inner bore-side layer, wherein a full weight overall load
is divided by a
bonding area to derive the bonding strength. The measurement is performed in a
non-oxidation
atmosphere at a temperature of 1000 C.
The reason for performing the measurements of the compressability and the
bonding
strength in a non-oxidation atmosphere at a temperature of 1000 C, and
defining components of
the refractory composition of the intermediate layer in a state after the
intermediate layer is
subjected to a heat treatment in a non-oxidation atmosphere at a temperature
of 1000 C. is that
1000`C corresponds to a temperature at which a volatile component in the
organic hinder is
sufficiently volatilized to establish a carbon-based joint matrix so as to
provide stable
compressability and bonded state.
In the present invention, the refractory structure at least partly has an
integral multi-layer
structure obtained by forming the inner bore-side layer simultaneously
together with the
intermediate layer and the outer periphery-side layer during a forming
process. The integral
multi-layer structure means that respective matrix portions of the three
layers around boundaries
therebetween are joined to each other in a direct contact manner without any
gap therebetwecn.
Specifically, in the present invention, respective mixtures (ingredients) for
the inner
bore-side layer, the intermediate layer and the outer periphery-side layer are
simultaneously
subjected to a cold isostatic pressing (CIP) process to form an integral
continuous structure, as
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CA 02701848 2010-04-06
will be described later.
A conventional technique based on installation of adhesive or mortar, for
example, a
technique of forming each of the inner bore-side layer, the intermediate layer
and the outer
periphery-side layer into a shaped body separately, and then bonding the
shaped bodies together
through a silicate-based adhesive or the like (the "adhesive or mortar" will
hereinafter be referred
to collectively as "mortal/adhesive") involves the following problems.
(1) During an operation of charging mortar etc., into a narrow gap with, an un-
bonded
zone is likely to be created due to occurrence of an unfilled zone and/or
mixing of large air
bubbles, to cause instability in quality of the mortar/adhesive region.
(2) A large amount of liquid is contained to ensure operation efficiency.
Thus, the
liquid is liable to be absorbed in a target layer to cause shrinkage of the
mortars, which leads to
shrinkage crack in the mortal/adhesive itself or formation of a gap (peel-off)
on a bonding
surface.
(3) A boundary region between the respective layers has a sudden change in
matrix
phase. Thus, stress concentration is likely to occur in the boundary region to
cause destruction
or peel-off of a bonded portion.
(4) In a high-temperature region, e.g., during passing of molten steel,
components of
mortal/adhesive react with components of the layers. Thus, the mortal/adhesive
is softened or
molten to cause deterioration in bonding strength, and the layers themselves
shrink and deform
to cause deterioration in fixing force of the layers, which increases a risk
of occurrence of
drop-off or peel-off of the inner bore-side layer, or breaking of the outer
periphery-side layer.
Differently from the above conventional technique, the integral structure in
the present
invention employs a bonding mechanism based on a mechanical entanglement
between
respective components of the layers. This provides the following effects.
(1) During preparation of a mixture (ingredients). a given amount of burnable
particles
can be uniformly dispersed over the mixture (ingredients), so that voids
resulting from
vanishment of the burnable particles caused by a heat treatment are
substituted and filled with
expansive graphite particles having micro-voids. This makes it possible to
control a
compression amount uniformly and stably.
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CA 02701848 2010-04-06
(2) A bonding through a binder component, such as a carbonaceous material, in
the
intermediate layer, is also developed during the heat treatment, to provide
enhanced uniformity
and stability.
(3) The micro-voids of the expansive graphite particles are finely dispersed
over the
entire matrix during a heat treatment after the forming process. The expanded
graphite particle
region having micro-void layers in the intermediate layer stretches and
contracts in an accordion
manner, so that a bonding force can be maintained without significant
destruction of the matrix
of the intermediate layer, to uniformly absorb a thermal expansion of the
inner bore-side layer.
The above effects make it possible to obtain robust fixability between the
respective layers
or stability in each of the three layers while reducing the risk of occurrence
of drop-off or
peel-off of the inner bore-side layer or breaking of the outer periphery-side
layer.
Further, as compared with the conventional technique based on installation of
mortal/adhesive, the simultaneous integral forming process makes it possible
to obtain the
continuous casting nozzle having the above features, with a high degree of
accuracy, i.e., Mille
suppressing variation in matrix structure and dimensions such as thickness
(the Lill iformization is
promoted because, during a course of expansion of the expansive graphite
particles, the
expansion generates a pressure inside the intermediate layer, to allow
components. mainly the
expansive graphite particles in the intermediate layer, to be automatically
charged and dispersed
in the voids resulting from vanishment of the burnable particles), and to
achieve simplification
and laborsaving in a production process, and reductions in required production
lead time and
cost.
In order to ensure the compressability, the bondability and the stability, the
intermediate
layer in the present invention may have a structure where a plurality of
carbonaceous lamellar
unit layers each having a thickness of 1 m or less are arranged in a layered
pattern while
interposing a space therebetween (this arrangement will hereinafter be
referred to simply as
"laminar configuration". This structure mainly comprises graphite particles
having
expansibility, wherein the graphite particles are in an expanded state (in the
present invention,
the graphite particles before expansion will be referred to as "expansive
graphite particles", and
the graphite particles after expansion will be referred to as "expanded
graphite particles").
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CA 02701848 2010-04-06
In the present invention, the compressability of the intermediate layer is
provided primarily'
based on a phenomenon that the expanded graphite particles arranged in the
laminar
configuration compress the spaces between the laminar layers, in response to
an external force,
and each of the laminar unit layers is flexibly deformed. When each of the
carbonadoes
laminar unit layers has a thickness of I n1 or less, a property of being
flexibly deformed by an
external force while maintaining the original configuration of the carbonadoes
laminar unit layer
is enhanced, and the spaces each formed between adjacent ones of the
carbonadoes laminar unit
layers at a distance of about 10 to 200 tun serve as a space necessary for
deformation and
displacement of the carbonadoes laminar unit layers. The carbonadoes laminar
unit layers and
the spaces exist in such a manner as to be entangled with each other 3-
dinientionally and
complicatedly, so that a stress can be dispersed in all directions to enhance
the conlpressability.
i.e., the stress relieve function.
In the present invention, a mixture (ingredients) for the intermediate layer
is used in such a
manner that it contains un-expanded expansive graphite particles in an amount
ranging from 5 to
45 mass%. and burnable particles in an amount ranging from 55 to 95 mass%. and
further
contains an organic binder in a given mass%, with respect to a total mass% of
the un-expanded
expansive graphite particles and the burnable particles, and in addition to
the total mass%.
wherein the given mass% of the organic binder is set to allow a ratio of a
carbon component of
the organic binder to an entire refractory composition of the intermediate
layer. to fall within the
range of 2.5 to 15 mass%, as a corresponding value in a state after the
refractory composition of
the intermediate layer is subjected to a heat treatment in a non-oxidation
atmosphere at 1000`C.
This makes it possible to form the intermediate layer having both desired
compressive rate and
bondability without crack and peeling, through a heat treatment in a non-
oxidation atmosphere at
600 to 1300 C.
Alternatively. in the present invention, a mixture (ingredients) for the
internlediatc layer is
used in such a manner that it contains un-expanded expansive graphite
particles in an amount
ranging from 5 to 45 mass%. and burnable particles in an amount ranging from
55 to 95 mass%,
with the remainder being 40 mass% or less of refractory material which is one
or more selected
from the group consisting of oxide, carbide, nitride and metal, and further
contains an organic
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CA 02701848 2010-04-06
binder in a given mass%.[ with respect to a total mass% of the un-expanded
expansive graphite
particles, the burnable particles and the refractory material, and in addition
to the total mass%,
wherein the given mass% of the organic binder is set to allow a ratio of a
carbon component of
the organic binder to an entire refractory composition of the intermediate
layer, to fall within the
range of 2.5 to 15 mass% as a corresponding value in a state after the
refractory composition of
the intermediate layer is subjected to a heat treatment in a non-oxidation
atmosphere at 1000 C.
This makes it possible to form the intermediate layer having both desired
compressive rate and
bondability without crack and peeling, through a heat treatment in a non-
oxidation atmosphere at
600 to 1300 C.
In the present invention, the compressability of the intermediate layer is
provided based on
a phenomenon that, during a course of allowing a component of the burnable
particles and the
organic binder, which does not form a carbon bond, to be vanished, a part or
entirety of voids
resulting from the vanishment is substitute with the expanded graphite
particles. Preferably, the
expansive graphite particle is made of a material which initiates expansion at
approximately the
same temperature as that at which the organic binder initiates burning and
vanishment. The
material is appropriately selected from expansive graphite particles different
in expansion
initiation temperature, in conformity to the vanishment initiation temperature
of the burnable
particles. Typically, the material is appropriately selected from expansive
graphite particles
having an expansion initiation temperature of 130 to 350 C. A particle size of
the expansive
graphite particle is preferably set in the range of 50 to 800 pun, more
preferably in the range of
100 to 600 m. If the particle size is less than 50 pn, the expansibility
during heating
deteriorates to cause difficulty in obtaining desired compressability,
although a capability to fill
the micro-voids is enhanced. If the particle size is greater than 800 m, the
3-dimensional
entanglement of the expanded graphite particles is reduced to cause
deterioration in bonding
strength, although the expansibility is improved to provide enhanced
compressability.
In the components of the refractory composition of the intermediate layer
after the heat
treatment in a non-oxidation atmosphere at 1000 C, the remainder other than
the expanded
graphite particles may include a refractory material comprising one or more
selected from the
group consisting of oxide, carbide, nitride and metal.
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CA 02701848 2010-04-06
Among them, particularly the refractory material particles constituting the
remainder other
than the carbon component take on a function of ensuring corrosion resistance
of the
intermediate layer. Specifically, the particles have a function of suppressing
or preventing
molten metal. such as molten steel, from coming into direct contact with the
outer periphery-side
layer having relatively low corrosion resistance, when the intermediate layer
is damaged, and a
function of ensuring corrosion resistance and abrasion/wear resistance of the
intermediate layer
itself. The particles also have a skeletal function for maintaining the
strength of the
intermediate layer as a refractory body.
In addition to a damageable portion of the inner bore-side layer itself, a
boundary portion
between the inner bore-side layer and the nozzle body, a locally damageable
portion having
brittleness, such as a gas-injecting gas spool portion or a joint portion
between the respective
layers, the continuous casting nozzle has a portion to be directly exposed to
molten steel all the
time as a product of a continuous casting nozzle, such as an outlet portion of
an immersion
nozzle. For example, if the portion to be directly exposed to molten steel has
poor collusion
resistance and abrasion/wear resistance, a selective loss of the portion
causes a breaking of the
continuous casting nozzle which is a fatal problem in operation of continuous
casting, due to
intrusion of molten steel between the inner bore-side layer and the outer
periphery-side layer.
As a refractory material for use in a refractory portion of the intermediate
layer to be
directly exposed to molten steel, a refractory aggregate comprising one or
more selected from the
group consisting of A1203, SiO2, MgO, CaO and ZrO2, specifically, alumina-
silica based
aggregate (e.g., corundum, mullite, sillimanite, kyanite and kaolinite; in
view of obtain corrosion
resistance to molten steel, it is preferable to select one of them in the
above order);
alumina-magnesia based spinet; zirconia; zircon; and/or alkaline earth metal
oxide, may be
selectively used depending on a level of corrosion resistance required under
individual casting
operation conditions, or other factor. It is recommended to avoid the use of a
refractor'
material consisting only of silica, and a glassy refractory material including
an alkali metal
component, because they have a problem about oxidation of a carbon component,
and formation
of a low-melting point substance resulting from a reaction with other
refractory material.
Further, with a view to suppressing oxidation of the refractory composition of
the
-27-
CA 02701848 2010-04-06
intermediate laver, carbide such as silicon carbide or titanium carbide, and
nitride such as RN or
silicon nitride, may be contained in the refractory composition of the
intermediate layer.
The refractory material for the above mentioned remainder is not an essential
component.
Thus, if there is not a need for relying on the refractory composition as the
remainder, to ensure
corrosion resistance and/or abrasion/wear resistance in view of individual
casting operation
conditions, a situation of damage of the continuous casting nozzle, and other
factor. the
refractory material for the remainder is not necessarily contained.
In the present invention, the refractory composition of the intermediate layer
primarily
comprises carbon having high stability even in a high-temperature range.
specifically of about
1000`C or more at which sintering of oxide or the like, and a reaction for a
low-melting point
substance are initiated or accelerated, so that the intermediate layer itself
definitely has high
stability. In addition, the expanded graphite particles are dispersed in such
a manner as to cover
the remaining refractory material particles. This makes it possible to almost
prevent sintering.
shrinkage and formation of a low-melting point substance, due to a reaction
between respective
remaining refractory aggregates, such as oxides, while preventing generation
of voids caused by
sintering and softening of the refractory composition of the intermediate
layer. This is also an
advantage of the present invention using the expanded graphite particles as a
primary component
of the intermediate layer.
The refractory material particles constituting the refractory composition of
the intermediate
layer are hound to each other through the binder. In view of maintaining the
compressability of
the refractory composition of the intermediate layer, and preventing softening
or melting of the
refractory composition of the intermediate layer so as to maintain a bonding
function even at
high temperatures, the binder is preferably made of thermosetting resin, tar
or pitch, as a starting
material, wherein it has a carbonaceous bondable matrix in a state after a
heat treatment at 600 C
or more.
Thus, in the present invention, the organic binder is added to allow a ratio
of a content of a
carbon component of the binder as a corresponding value in a state after being
subjected to a heat
treatment in a non-oxidation atmosphere at 1000 C, to a component (including a
carbon
component other than the carbon component of the binder) of the entire
refractory composition
-28-
CA 02701848 2010-04-06
of the intermediate layer in a state after being subjected to a heat treatment
in a non-oxidation
atmosphere at 1000 C, to fall within the range of 2.5 to 15 mass%. If the
ratio is less than 2.5
mass%. a sufficient bonding strength between the inner bore-side layer and the
outer
periphery-side layer cannot be obtained, although it is favorable to expansion
of the expansive
graphite particles and compressability of the intermediate layer. If the ratio
is greater than 15
mass%, the expansion of the expansive graphite particles during the heat
treatment is hindered to
cause difficulty in ensuring required compressability of the intermediate
layer, although it is
favorable to the bonding strength.
Further, the expanded graphite particles as the carbon component are required
to contain
13.5 mass% or more in the intermediate layer, in order to ensure required
compressability and a
healthy matrix. If the content is less than 13.5 mass%, the matrix of the
intermediate layer is
more likely to have a brittle portion to increase a risk of occurrence of
shrinkage crack and
deterioration in the bondability relative to the outer periphery-side layer.
Thus, in the present invention, the intermediate layer after being subjected
to the heat
treatment in a non-oxidation atmosphere at 1000 C includes expanded graphite
particles and an
organic binder component, and contains respective carbon components (except
any compound.
such as SiC, B4C and AIC, with the remaining components) of the expanded
graphite particles
and the organic binder component, in a total amount of 16 mass% or more
(including 100
mass%). Specifically, 16 mass% as a lower limit of the total of the carbon
components is a sum
of 13.5 mass% as a minimum content of the expanded graphite particles, and 2.5
mass% as a
minimum content of the organic binder component. A portion exceeding 16 mass%
may
consist only of the organic binder component (maximum content: 15 mass%) and
the expanded
graphite particles, or may comprise a carbonaceous component, such as flaky
graphite or carbon
black, other than the expanded graphite particles and the organic binder
component.
When the total of the carbon contents is in the range of 16 to less than 100
mass%. a
refractory material which is one or more selected from the group consisting of
oxide, carbide,
nitride and metal, may be contained in the remainder, in a total amount of 84
mass% or less.
The bonding strength is changed depending on a balance between a content of
the binder
and a content of the burnable particles in the mixture (ingredients).
-29-
CA 02701848 2010-04-06
It is recommended to avoid the use of a binder containing a large amount of
alkali metal,
because it is likely to cause softening or melting of oxides, or vaporization
of the carbon
components, which leads to degradation in matrix of the intermediate layer or
the adjacent layers.
Particularly, in view of maintaining a matrix strength in a low-temperature
region (e.g., 600 C or
less), an organic resin which does not remain in the form of a carbon
component may be used.
A production method for obtaining the refractory composition of the
intermediate layer in
the present invention, and a continuous casting nozzle comprising the
intermediate layer having
the refractory composition, will be described below.
A continuous casting nozzle having the compressable intermediate layer can be
obtained by
a production method which comprising the steps of, preparing three types of
mixtures
(ingredients) for the inner bore-side layer, the intermediate layer and the
outer periphery-side
layer, on a layer-by-layer basis; providing to a forming mold a plurality of
cavities for forming
the inner bore-side layer, the intermediate layer and the outer periphery-side
layer. vyherein the
cavities are separated from each other to allow each of the cavities to have a
given size and
configuration; filling each of the cavities of the forming mold with a
corresponding one of the
mixtures (ingredients), and allowing adjacent ones of the mixtures
(ingredients) to be brought
into direct contact with each other, for example, by removing partitions:
subjecting the mixtures
(ingredients) in direct contact with each other, to a pressure forming process
using a CIP machine
to form a shaped body; and subjecting the shaped body to a heat treatment at a
temperature of
600 to 1300 C, in a non-oxidation atmosphere, or an oxidation atmosphere after
subjecting a
surface of the shaped body to an anti-oxidation treatment. The production
method may further
includes, in advance of the above mentioned heat treatment step, an
independent step of
subjecting the shaped body to a heat treatment at a temperature less than the
above temperature.
to remove a volatile component and cure a resin therein.
While a fundamental handling/operation and a required apparatus in each of the
steps are
the same as that in a conventional continuous casting nozzle production
method, the continuous
casting nozzle production method of the present invention has the following
first to third
features.
The first feature is a composition of the refractory mixture (ingredients) for
the intermediate
-30-
CA 02701848 2010-04-06
layer (intermediate-layer mixture (ingredients)). The intermediate-layer
mixture (ingredients)
contains, as a powder component except a volatile component, (1) un-expanded
expansive
graphite particles in an amount ranging from 5 to 45 mass%, (2) burnable
particles in an amount
ranging from 55 to 95 mass%, and (3), as needed, a refractory material which
is one or more
selected from the group consisting of oxide. carbide, nitride and metal, in a
total amount of 40
mass% or less (including zero), as the remainder, and further contains an
organic binder in a
given mass%, with respect to a total mass% of the powder component, and in
addition to the
total mass%, wherein the given mass% of the organic binder is set to allow a
ratio of a carbon
component only of the organic binder (except any carbon compound with the
remaining
components) to an entire refractory composition of the intermediate layer. in
a state after the
refractory composition of the intermediate layer is subjected to a heat
treatment in a
non-oxidation atmosphere at 1000 C, to fall within the range of 2.5 to 15
mass%;
The second feature is to simultaneously integrate the three layers together by
a CIP process,
without any joint at boundary regions therebetween.
The third feature is to remove the burnable particles in the shaped body of
the
intermediate-layer mixture (ingredients) to form voids therein, and then
expand the un-expanded
expansive graphite particles, in the step of subjecting the integrated shaped
body to a heat
treatment at 600 to 1300 C:.
The above features will be more specifically described below.
It is practically difficult to obtain the refractory composition of the
intermediate layer in the
present invention which contains the expanded graphite particles as a primary
component, by
using a refractory composition or a mixture (ingredients) for the intermediate
layer which
already has compressive rate approximately equal to the aforementioned
compressability
required in a state after a continuous casting nozzle is formed as a product
(this compressability
has the same meaning as the compressive rate satisfying the Formula 1;
thereinafter referred to
as "product compressability"), and installing the refractory composition or
mixture (ingredients)
into an integral structure simultaneously together with other layers during a
step of forming the
continuous casting nozzle. Specifically, in a CIP process (typically, a
forming pressure is far
greater than 2.5 MPa) designed to produce a continuous casting nozzle based on
an apparatus
-31-
CA 02701848 2010-04-06
and a fundamental handling/operation according to a conventional continuous
casting nozzle
production method. the refractory composition having compressability is
compressed during the
forming step, and thereby the refractory composition after the forming step
Nvill lose the
compressability. Therefore, it is difficult to achieve a balance between the
purpose of
producing a continuous casting nozzle using a refractory composition or a
mixture (ingredients)
which already has compressability approximately equal to product
compressability, and the
purpose of obtaining a continuous casting nozzle having an integral structure
with the product
compressability.
Thus, at least in the step of forming the mixtures (ingredients)
simultaneously and integrally
under a high pressure, the mixture (ingredients) to be formed must not have
compressability
approximately equal to product compressability, except compressability
equivalent to volume
shrinkage occurring in a filling step during a CIP process for a powder of a
refractory
composition for a conventional continuous casting nozzle (the volume shrinkage
Nvill hereinafter
referred to as "compaction" in order to discriminate from the "compressabilit)
").
The production method of the present invention is intended to obtain a
continuous casting
nozzle having an integral structure, wherein an intermediate layer has a
refractory composition
exhibiting product compressability.
In the mixture (ingredients) having the composition as described in the first
feature, the
expanded expansive graphite particles as a primary component taking a major
roll in
compressability in a product state has almost no significant compressability
in its un-expanded
state. Thus, even if the mixture (ingredients) is exposed to a high pressure
during a CIP process.
a volume of the composition is not reduced, and this volume shrinkage can be
limited to about
the compaction. This makes it possible to ensure a given volume, such as a
wall thickness and
an axial dimension, of the intermediate layer.
The mixture (in(,,redients) resistive to a CIP process allows the intermediate-
layer mixture
(ingredients) to be formed simultaneously and integrally together with other
refractory mixtures
(ingredients) for the inner bore-side layer and the outer periphery-side
layer.
The un-expanded expansive graphite particles in the intermediate layer is
preferably
contained in the range of 5 to 45 mass%. If the content is less than 5 mass%,
compressability
-32-
CA 02701848 2010-04-06
of the refractory composition of the intermediate laver after the heat
treatment is excessively
lowered. Thus, it is necessary to excessively increase a thickness of the
intermediate layer to
satisfy the Formula I about compressability, which leads to an undesirable
situation where a
thickness of a continuous casting nozzle is restricted in design, and a
variation in compressability
is likely to occur in each region of the intermediate layer. If the content of
the un-expanded
expansive graphite particles is greater than 45 mass%, the expansive graphite
particles are
excessively expanded beyond a volume of voids generated by banishment of the
burnable
particles, and a pressure in the refractory composition of the intermediate
layer is excessively
increase due to an expansion of the intermediate layer. to cause a problem
about deterioration in
production yield.
The burnable particles in the intermediate-layer mixture (ingredients) is
preferably
contained in the range of 55 to 95 mass%. If the content is less than 55
mass%, a volume of
voids vanished by heating of the burnable particles during the heat treatment
in the production
process is excessively reduced to cause difficulty in sufficiently ensuring a
space between
respective laminar lavers of the expanded graphite particles after the voids
are filled according to
expansion of the expansive graphite particles, which increases a risk of
deterioration in
compressability. If the content is greater than 95 mass%, a volume of voids
after vanishment of
the burnable particles is excessively increased, and thereby a space including
a space between
respective laminar layers of the expanded graphite particles is excessively
created after the voids
are filled according to expansion of the expansive graphite particles, to
increase a risk of
deterioration in strength and bondability of the intermediate layer itself.
The amount of the
burnable particles is preferably set at a value equal to greater than the
amount of the expansive
graphite particles. As the burnable particle, a polyethylene particle, a
polyester powder or a
grain powder may be used. Preferably, the burnable particle has a minimum
particle size, such
as about 45 ttm or less, to evenly form voids and obtain an even distribution
of the expanded
graphite particles.
The respective amounts of the un-expanded expansive graphite particles and the
burnable
particles in the intermediate-layer mixture (ingredients) may be determined by
relatively
adjusting them to satisfy the Formula I about compressability.
- 33 -
CA 02701848 2010-04-06
The remainder contained the intermediate-layer mixture (ingredients) in an
amount of 40
mass% or less (a minimum amount of the un-expanded expansive graphite
particles is 5 mass%.
and a minimum amount of the burnable particles is 55 mass%. Thus, a maximum
value of the
remainder is 100 - 5 - 55 = 40, and a minimum thereof amount is zero) may be a
refractory
material comprising one or more selected from the group consisting of oxide,
carbide, nitride and
metal. This component is not essential, i.e., may not be contained. The
refractory material,
such as atype, a combination and amount thereof, may be determined depending
on conditions
of individual casting operation and a structure and configuration of
equipment, i.e., a level of
corrosion resistance required for the intermediate layer, and depending on an
amount of the
expansive graphite particles and the burnable particles in conformity to a
level of compressability,
and in consideration of control of a reaction with respective materials of the
inner bore-side layer
and the outer periphery-side layer.
Thus, in the continuous casting nozzle of the present invention, a maximum
amount of
carbon component in the refractory composition of the intermediate layer in a
state after being;
subjected to a heat treatment at 600 C or more may be set at 100 mass%, with
respect to the
entire refractory composition except the binder, or the entire refractory
composition including the
binder when the binder consists only of a carbonaceous material.
When the intermediate-layer mixture (ingredients) contains the un-expanded
expansive
graphite particles, the burnable particles, and the additional refractory
material particles as the
remainder, these refractory material particles (hereinafter referred to
collectively as "raw material
powder") are uniformly mixed. Then, a binder is added (a solvent may be used.
In this case, a
solvent is also added) to the uniformly mixed raw material powder while
uniformly kneading the
mixture, to give fluidity, wettability, shape-retainability and bonding
function to the raw material
powder.
The mixture (ingredients) is required to ensure shape-retainability during the
forming step
and a subsequent step, and strength of the refractory composition after the
heat treatment. Thus,
in the present invention, an organic binder. such as one or more selected from
the various types
of tars, pitches, phenol resins and furan resins, is added in such a manner as
to allow a ratio of
the organic binder to an entire refractory composition of the intermediate
layer, to fall within the
34
CA 02701848 2010-04-06
ranee of 2.5 to 15 mass% in terms of a carbon component of the organic binder
in a state after
being subjected to a heat treatment in a non-oxidation atmosphere at 1000 C.
Preferably. the
organic binder is added in such a manner as to allow a total mass of a solid
of the organic hinder
(a sum of an amount of carbon component after a heat treatment in a non-
oxidation atmosphere
at 1000 C, and an amount of additional organic binder which does not remain in
the form of a
carbon component in the state after the heat treatment in a non-oxidation
atmosphere at 1000 C,
in terms of a solid except a solvent, at room temperature) to fall within the
range of 5 to 30
mass%, with respect to total 100 mass% of a power component consisting of the
expansive
graphite particles, the burnable particles and the refractory material as the
remainder, and in
addition to the power component. The reason is that the amount of the solid of
the organic
binder is less than 5 mass% or greater than 30 mass%, the mixture
(ingredients) deteriorates in
fluidity and compressability during the forming step, and the refractory
composition deteriorates
in strength after the forming step.
With a view to ensuring strength in a low temperature range of room
temperature to about
300 C, primarily in order to ensure shape-retainability after the forming
step, an organic binder
which does not remain in the form of a carbon component (carbon bond) at about
600 C or more
(hereinafter referred to as "second organic binder") may be used in
combination with the
aforementioned organic binder (hereinafter referred to as "first organic
binder").
As the additional, second, organic binder which does not remain in the form of
a carbon
component (carbon bond), an organic adhesive material/resin, such as an
acrylic resin, a vinyl
acetate resin, a polyester resin or a polyacrylonitrile resin, may be used.
When the second organic binder is used in combination with the first organic
binder, the
second organic binder may be added in such a manner as to allow a sum of an
amount of solid of
the second organic binders, except a solvent (at room temperature), and an
amount of carbon
component of the first organic binder in a state after a heat treatment in a
non-oxidation
atmosphere at 1000 C, to fall within the range of 5 to 30 mass%, in addition
to the total mass%
of the powder component.
Preferably, with a view to enhancing strength after the heat treatment in the
continuous
casting nozzle production method of the present invention, a ratio of pitches
to be used may be
- 35 -
CA 02701848 2010-04-06
increased. This strength is set on the assumption that it is equal to or
grater than the bonding
strength, and the above binder bondable to carbon contributes to enhancement
in strength of the
intermediate layer itself.
In an operation of bonding a commonly used oxide, for example, using
mortar/adhesive
containing an inorganic binder, such as silicate (or forming a layer
corresponding to the
intermediate layer), in a high-temperature range of 1000 to 1500 C, for
example, an oxide
component and an alkali metal oxide are softened due to a reaction
therebetween. and a bonding
strength will be gradually lowered. Further, at a temperature of 1200 C or
more, due to
occurrence of melting, the bonding strength is significantly lowered to cause
shrinkage or
thermal decay, and gap between the layers, which spoils a healthy structure of
a continuous
casting nozzle, in many cases.
The joint structure in the present invention can solve the above conventional
problem.
because it has a structure primarily based on carbon bond, which is almost
free of a component
causing acceleration of sintering or formation of a low-melting point
substance, and
high-temperature degradation.
Separately from the intermediate-la)er mixture (ingredients), the respective
mixtures
(ingredients) for the inner bore-side layer and the outer periphery-side layer
are prepared.
Respective compositions of the mixtures (ingredients) for the inner bore-side
layer and the
outer periphery-side layer may be determined in conformity t o conditions and
an intended
purpose of individual continuous casting, and based on a premise that they has
characteristics.
such as fillability, shape-retainability and strength performance, which allow
for formation
simultaneously with the intermediate layer by a CIP process.
Then, an inner space of a forming mold for a CIP process is divided into a
plurality of
cavities each having a given size and configuration to form the inner bore-
side la} er, the
intermediate layer and the outer periphery-side layer and each of the cavities
is with a
corresponding one of the mixtures (ingredients).
Subsequently, adjacent ones of the mixtures (ingredients) are brought into
direct contact
with each other without being spaced apart from each other. This step may
comprise providing
a plurality of cavities divided by partition plates to allow each of the
cavities to be filled with a
- 36 -
CA 02701848 2010-04-06
corresponding one of the mixtures (ingredients), tilling each of the cavities
with a corresponding
one of the mixtures (ingredients), and removing the partition plates to allow
the
intermediate-layer mixture (ingredients) to be brought into direct contact
with adjacent ones the
mixtures (ingredients) of the inner bore-side layer and the outer periphery-
side layer in a
borderless manner. Alternatively, the step may comprise temporarily forming
the mixture
(ingredients) for one or two of the inner bore-side layer, intermediate layer
and the outer
periphery-side layer into a given shape (forming a temporary shaped body),
setting the
temporary shaped body in a forming mold for a CIP process, and filling given
cavities with the
mixtures (ingredients) for the layers adjacent to the temporary shaped body.
Further, the
mixtures (ingredients) may be supplied in the same mold to fill the respective
cavities in a
stepwise manner, and compressed plural times every filling operation, and
finally simultaneously
pressed to integrate them.
Then, the mixtures (ingredients) are subjected to a press forming process
using a CIP
machine. Forming conditions, such as a pressure and a compression time, may be
the same as
those for a conventional process for a continuous casting nozzle (e.g., about
150 MPa).
Through the above steps, an integral shaped body having the respective
refractory
compositions of the layers formed as a multi-layer structure can be obtained.
The obtained shaped body may be subjected to a drying process at about several
hundred C
or less. Then, the shaped body is subjected to a heat treatment in a non-
oxidation atmosphere.
or in an oxidation atmosphere after subjecting a surface of the shaped body to
an anti-oxidation
treatment, at 600 to 1300 C. In this heat operation step. burnable material
(burnable particles,
solvent, etc.) in the shaped body of the intermediate-layer mixture
(ingredients) are vanished to
form voids therein, and then the un-expanded expansive graphite particles are
expanded to fill
the voids formed by vanishment of the burnable materials with the expanded
graphite particles).
Specifically. a volume occupied by the burnable particles in the mixture
(ingredients) is
substituted with particles constituting a laminar structure consisting of a
plurality of
carbonaceous layers as the result of expansion of the expansive graphite
particles. This makes
it possible to obtain a refractory layer exhibiting compressability based on
evenly distributed
small spaces.
- 37 -
CA 02701848 2010-04-06
The vanishment of the burnable material and the expansion phenomenoil of the
un-expanded expansive graphite particles are promoted at a temperature of
about several
hundred T. Preferably, the intermediate-layer mixture (ingredients) is treated
at a temperature
of 600 C or more to reliably complete the above phenomena. If the heat
treatment temperature
is greater than I300 C, properties, such as thermal shock resistance, of the
refractory
compositions of a portion of the continuous casting nozzle other than the
intermediate layer in
the present invention, such as a refractory composition of the nozzle body, is
more likely to
undesirably change. Thus. a maximum heat treatment temperature is set at 1300
C.
Subsequently, the heat-treated body may be subjected to machining /treatment.
such as
cutting, grinding and anti-oxidation treatment. Through the above steps, the
continuous casting
nozzle of the present invention can be obtained.
The production method of the present invention having the above features makes
it possible
to obtain a continuous casting nozzle excellent in compressability and
bondability. In addition,
as compared with the conventional continuous casting nozzle, i.e., a
production method
comprising the steps of preparing separate members for respective layers,
assembling and joining
them together using mortar/adhesive, and subjecting the joined body to a
drying process. the
production method of the present invention makes it possible to achieve
significant reduction in
the number of production processes and cost, and provide enhanced productivity
and enhanced
accuracy, such as dimensional accuracy, of the continuous casting nozzle.
As mentioned above, the present invention can prevent expansion cracking of
the outer
periphery-side layer due to a difference in thermal expansion between the
inner bore-side layer
and the outer periphery-side layer, in a continuous casting nozzle where a
layer having a high
function, such as a capability to prevent deposition of inclusions, is
disposed on the side of an
inner bore to enhance durability, i.e., the inner bore-side layer has a
thermal expansion
coefficient greater than that of the outer p eriphery Side lager, or in a
continuous casting nOZZIC
having a large thermal gradient due to rapid heating even though the inner
bore-side layer has a
thermal expansion coefficient approximately equal to that of the outer
periphery-side layer.
The continuous casting nozzle of the present invention has a structure Ns here
the three layers
are integrated together. Thus, enhanced bonding force and fixing force between
the respective
-38-
CA 02701848 2010-04-06
layers can be obtained as compared with the joining technique based on
adhesive. mortar or the
like, without a need for a particular bonding material.
These make it possible to significantly enhance durability of the continuous
casting nozzle,
such as thermal shock resistance and stability, and promote achievement of
higher function and
high durability in the continuous casting nozzle. based on the multi-layer
structure,
The production method of the present invention makes it possible to achieve a
simultaneous
internal forming process, to obtain the continues casting nozzle having the
above excellent
features, stably with high accuracy and high quality, and achieve
simplification and laborsaving
in a production process, and reductions in required production lead time and
cost.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. I is a sectional view showing a long nozzle according to one embodiment
of the
present invention.
FIG. 2 is a sectional view showing a continuous casting nozzle according to
another
embodiment of the present invention.
FIG. 3 is an explanatory diagram showing a measurement method of a hot bonding
strength
of an intermediate layer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be more specifically described based on an
embodiment thereof
and an example.
The present invention was applied to a tubular refractory structure called
"long nozzle" for
use in transferring molten steel between a ladle and a tundish in a continuous
casting process.
As shown in FIG. 1, in a refractory structure applied to a long nozzle I
(diameter of an inner
bore: 140 mm diameter of a straight body: 226 mm 6. length: 1500 min), an MgO-
C hased
material (MgO = 77 mass%, C = 19 mass%) having a maximum thermal expansion
coefficient at
temperature of room temperature to 1500 C of 1.8 % was used for an inner bore-
side layer 2, and
applied to the entire inner bore surface at a thickness of 10 mm, and an AI2O3-
SiO2-C material
(AI-203 = 50 inass%, Si02 25 mass%. C = 25 mass%) having a maximum thermal
expansion
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coefficient at temperature of room temperature to 1500 C of 0.5 %, was used
for an outer
periphery-side layer 3, and applied to the side of a region which is not
immersed in molten steel
bath (non-immersed portion). Further, a thickness of an intermediate layer 4
for reliving
thermal expansion of the inner bore-side layer 2 was set at 3.0 mm.
A mixture (ingredients) for the intermediate layer 4 was prepared by; mixing
un-expanded
expansive graphite particles as the expansive material, polyethylene particles
as the burnable
particles, and alumina and magnesia as the refractory aggregate, together, and
adding a pitch
powder and an acrylic resin as the organic binder into the mixture; subjecting
the mixture to a
granulation process in a high-speed mixer; adjusting an amount of residual
volatile component in
a fluidized drying furnace to adjust plasticity during forming; and regulating
the granulated
mixture obtained through the drying, to have a particle size of 1 mm or less.
The derails are shown in Table 1. A compressive rate and a hot bonding
strength
(compressive shear strength) in Table I were measured by the aforementioned
method. In the
long nozzle I in this example. the compressive rate required for the
intermediate layer, according
to the Formula I is 34 % or more.
For comparison, the inner bore-side layer 2 was inserted into the outer
periphery-side layer
3 using a conventional mud-like mortar to prepare Comparative Example 1. In
Comparative
Example 1, no compressability of the intermediate layer was observed in a
measurement of a
solid shape of the nozzle, and crack and peel-off occurred, respectively, in
the outer
periphery-side layer and the inner bore-side layer in a first cycle of pouring
test.
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A sample in Comparative Example 2 was prepared in such a manner as to contain
50
mass% of expansive graphite particles, with the remainder being 50 mass% of
burnable particles.
and further contain 5 mass part of pitch in addition to a total mass% of the
expansive graphite
particles and the burnable particles. In Comparative Example 2, expansion
cracking occurred
in the outer periphery-side layer during a heat treatment in a production
process. The reason is
that the expansive graphite particles is contained in an amount of 50 mmass%
which exceeds 45
mass% as the upper limit value, and thereby the outer periphery-side layer is
pressed radially
outwardly and broken by an expansion force of the expansive graphite particles
during the heat
treatment.
Inventive Examples I to 3 and Comparative Examples 3 and 4 are results of
evaluation
performed by setting an amount of the expansive graphite particles at a
constant value of 45
mass%, and using a pitch powder as an organic binder, and changing. in the
range of 2.0 to 16
mass%, an amount of carbon content of the pitch powder (hereinafter referred
to as "pitch carbon
component") in terms of an amount of carbon content in a non-oxidation
atmosphere at 1000 C.
As increase in bonding strength along with an increase in the pitch carbon
component is
observed.
In Inventive Examples I to 3 and Comparative Example 3, the expansive graphite
particles
were sufficiently expanded during the heat treatment, and voids which have
been occupied by
the polyethylene particles were fully filled with the expanded graphite
particles, so that a large
compressive ratio could be obtained. Ilowever, in Comparative Example 3 where
the pitch
carbon component is reduced to 2 mass%, the bonding strength could not be
sufficiently
obtained to cause peel-off of the inner bore-side layer (inner bore-side
member). Further, in
Comparative Example 4 where the pitch carbon component is increased up to 16
mass%. the
bonding strength was excessively increased, and the compensability was not
sufficiently
obtained. Consequently, crack occurred in the first cycle of the pouring test.
In Inventive Examples I to 3, excellent results could be obtained in "yield of
solid shape"
and "pouring cycle repetition test" without any problem.
In Comparative Example 5 where no expansive graphite particle is used, both
the
compressability and the bondability could not be obtained at a required level,
and a peel-off
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phenomenon occurred in the pouring test.
In Inventive Examples 4 to 6 where an amount of the burnable particles is
further increased,
both the compressability and the bonding strength could be sufficiently
obtained.
In Comparative Example 6 where the amount of the burnable particles is further
increased.
the bonding strength could not be sufficiently obtained, and a peel-off
phenomenon occurred in
the pouring test, although the compressability could be sufficiently obtained.
In Inventive Examples 7 and 8 where a part of the burnable particles is
substituted with
refractory particles, the compressability and the bonding strength could be
sufficiently obtained,
and excellent results could be obtained. In Comparative Example 7 where the
amount of the
refractory particles is further increased, the compressability could not be
sufficiently obtained.
Consequently, shrink crack was observed in the intermediate layer, and the
bonding strength
could not be sufficiently obtained. Moreover, a peel-off phenomenon occurred
in the inner
bore-side layer in a second cycle of poring test.
Although, in the above examples, the present invention is applied to the long
nozzle
illustrated in FIG. 1, a nozzle type to be covered by the present invention is
not limited the lone
nozzle. For example, the present invention may be applied to a tubular
refractory structure as
shown in FIG. 2.
FIGS. 2(a) and (b) show two types of long nozzle utilizing the present
invention as with
the embodiment illustrated in FIG. I. In the nozzle illustrated in FIG. 2(a),
the outer
periphery-side layer 3 is provided to extend up to a bottom end of the long
nozzle 1. and the
intermediate layer 4 is provided between a lower end of the inner bore-side
layer 2 and the outer
periphery-side layer 3. In FIG. 2(b), the refractory composition of the outer
periphery-side
layer 3 is provided to extend up to both top and bottom ends of the long
nozzle 1. and the
intermediate layer 4 is disposed between an upper end of the inner bore-side
layer 2 and the outer
periphery-side layer 3 and between the lower end of the inner bore-side layer
2 and the, outer
periphery-side layer 3.
FIG. 2(c) shows an example where the present invention is applied to an
immersion nozzle.
An immersion nozzle I' illustrated in FIG. 2(c) comprises an outer periphery-
side layer 3
consisting of an AG composition 3a and a ZG composition 3b, wherein the AG
composition 3a is
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has a bottom wall, and a lateral wall formed with an outlet ports 5. An inner
bore-side layer 2
also has a bottom wall, and an intermediate layer 4 is provided between the
inner bore-side layer
2 and the outer periphery-side layer 3 to extend over substantially the entire
region thereof.
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