ZIRCONIA-MULLITE REFRACTORY RAW MATERIAL AND A PLATE
BRICK
DESCRIPTION
TECHNICAL FIELD
The present invention relates to a zirconia-muUite refractory raw material which is usable as
a refractory raw material for refractory and ceramic products, and, particularly, obtainable
through a fusion process and suitable as a raw material for a refractory product for continuous
casting, such as a plate brick or a nozzle, and a plate brick using the zirconia-mullite refractory
raw material.
BACKGROUND ART
In the fields of iron and steel, nonferrous metals, cement, incinerators, ash melting furnaces,
etc., refractory products are widely employed, and a zirconia-muUite refractory raw material is
commonly used as a raw material for the refractory products. Particularly, it is widely used as a
refractory raw material for a plate brick or a nozzle to be used in a sliding nozzle device for
controlling a flow volume of molten steel during continuous casting of steel.
Generally, a zirconia-mullite refractory raw material is industrially produced through a
fusion process designed to melt a mixture of zircon and alumina or a mixture of zirconia, silica
and alumina, using an electric arc furnace or the like. The zirconia-mullite refractory raw
material comprises a mineral phase consists mainly of crystalline zirconia and mullite, wherein
crystal grains of the crystalline zirconia are dispersed in a microstructure of a refractory product
to prevent the development of a crack. In addition, it is considered to be excellent in thermal
shock resistance because of its low thermal expansion rate as compared with other refractory raw
materials such as alumina.
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In typical zirconia-muUite, crystal grains of crystalline zirconia include a relatively large
crystal (primary zirconia crystal) having a grain size of about 100 µm, which is precipitated as a
primary phase during cooling after melting, and a relatively small crystal (eutectic zirconia
crystal) having a grain size of about 10 µm or less, which is precipitated at a eutectic point in a
last stage of the cooling. As for the primary zirconia crystal to be precipitated as a primary
phase, after a seed crystal is precipitated in an initial stage of the cooling, the crystal will grow
along with the cooling, so that it is formed as a relatively large crystal having a grain size of
about 100 µm. As for the eutectic zirconia crystal to be precipitated at a eutectic point, a liquid
phase is crystallized in the last stage of the cooling at a time, and thereby crystal growth is not
promoted, so that it is formed as a fine crystal having a grain size of about 10 µm or less.
As for the mullite, precipitation is initiated after the primary zirconia crystal is precipitated,
and the precipitated crystal grows up to about 100 µm along with the cooling. A matrix glass
exists to fill each gap between the crystals.
In such a micro structure, the primary zirconia crystals and the mullite exist as relatively
large crystals, so that the matrix glass as a matrix part thereof consisting primarily of SiO2, and
the gap, become larger.
For example, as a refractory product using such a zirconia-mullite refractory raw material, a
refractory product for continuous casting is described in the following Patent Document 1, which
uses a zirconia-muUite refractory raw material having a mineral phase consisting primarily of
mullite and baddeleyite (crystalline zirconia), and comprising, as chemical components, 30 to 80
mass% of Al2O3, 10 to 65 mass% of ZrO2, and 5 to 25 mass% of SiO2. The refractory product
using this zirconia-mullite refractory raw material is described as having a low thermal
expansion rate and excellent corrosion resistance.
Further, a refractory product for continuous casting is described in the following Patent
Document 2, which contains electro-fused alumina-zirconia, wherein an average diameter of
primary alumina crystals is in the range of 5 to 70 µm, and zirconia is contained in an amount of
5 to 43 mass%. It is described therein that, if the average diameter of alumina crystals to be
precipitated as primary phases is set in the range of 5 to 70 µm, the primary alumina crystals and
alumina-zirconia eutectic crystals will be mixed finely and complexly, so that energy required
for cracking an electro-fused alumina-zirconia grain becomes larger, and thereby thermal shock
resistance is improved as compared with a conventional alumina-zirconia raw material.
A fused alumina-zirconia-silica based refractory material is described in the following
Patent Document 3, which has a basic micro structure consisting of corundum crystals,
baddeleyite crystals (crystalline zirconia) and a matrix glass, wherein the refractory material
contains: as chemical components and in mass% on the basis of oxide, ZrO2 in an amount of 25
to 32%; Al2O3 in an amount of 55 to 67%; SiO2 in an amount of 5 to 12%; P2O5 in an amount of
0.05 to 0.5%; B2O3 in an amount of 0.05 to 0.5%; and Na2O and K2O in an individual amount of
0.1 to 0.5% and in a total amount of 0.6% or less. It is described therein that Na2O and K2O as
chemical components are contained in an individual amoimt of 0.1 to 0.5% and in a total amount
of 0.6% or less to achieve an effect of suppressing leaching of a matrix glass consisting primarily
of SiO2 during use under a high temperature of 1400°C or more.
[Patent Document 1] JP 56-96775A
[Patent Document 2] JP 2000-44327A
[Patent Document 3] JP 10-101439A
DISCLOSURE OF THE INVENTION
[PROBLEM TO BE SOLVED BY THE INVENTION]
The refractory product using the zirconia-muUite refractory raw material described in the
Patent Document 1 is formed as a refractory product excellent in thermal shock resistance and
corrosion resistance. However, it is known that, when used with steel of a type having a high

oxygen concentration in steel, or steel of a type added with Ca, the refractory product is
subjected to wear due to a reaction between an SiO2 component of the zirconia-muUite and a
component of the steel, such as FeO or CaO, to cause significant deterioration in durability.
Moreover, if the zirconia-mullite refractory raw material is employed in a refractory product
containing carbon, and used under high-temperature conditions for a long period of time, a
micro structure thereof will be altered and degraded to cause a problem of deterioration in
durability. The cause is assumed that an inside of a microstructure of the refractory product is
placed in a low oxygen concentration and a strong reducing atmosphere, and thereby the matrix
glass consisting primarily of SiO2 is reduced and dissipated as SiO gas, resulting in alteration of
the zirconia-mullite.
The refractory raw material described in the Patent Document 2 contains no SiO2
component, and has a dense microstructure, so that it is excellent in not only corrosion resistance
but also abrasion resistance. However, due to its thermal expansion rate greater than that of a
zirconia-mullite refractory raw material, it is impossible to obtain an effect equivalent to that of
the zirconia-mullite refractory raw material in terms of a thermal shock resistance.
The zirconia-mullite refractory raw material described in the following Patent Document 3
contains a matrix glass in an amount of 15 to 20 mass%. Thus, if the zirconia-mullite refractory
raw material is used in a refractory product for molten steel, it is subjected to wear due to a
reaction with a component of the steel, such as FeO or CaO, to cause significant deterioration in
durability, as mentioned above.
It is therefore an object of the present invention to provide a zirconia-mullite refractory raw
material which is excellent in corrosion resistance against FeO, CaO, etc., and less likely to
undergo alternation and micro structural degradation under high-temperature conditions, while
being low in thermal expansion rate, so as to satisfy required thermal shock resistance and
corrosion resistance, and which is particularly optimal as a refractory raw material for use in a
refractory product for continuous casting, and a plate brick using the zirconia-mullite refractory
raw material.
[MEANS FOR SOLVING THE PROBLEM]
In order to solve the above problems, through various studies with a focus on a
microstructure of a zirconia-muUite refractory raw material, the inventors have accomplished the
present invention.
Specifically, the present invention provides a zirconia-muUite refractory raw material which
is obtained through a fusion process. The zirconia-muUite refractory raw material comprises
crystalline zirconia and muUite as primary components, with the remainder being corundum
and/or a matrix glass, wherein the crystalline zirconia includes a eutectic zirconia crystal having
a grain size of 1.0 µm or less, and the matrix glass is contained in an amount of 5 mass% or less.
In cases where a zirconia-muUite refractory raw material is produced through a fusion
process, a cooling rate during cooling of a molten mixture exerts an influence on a grain size of
the refractory raw material, and a smaller grain size is obtained as the cooling rate becomes
higher. Thus, in a zirconia-muUite refractory raw material which is obtained through a fusion
process to have a small eutectic zirconia crystal, any crystal other than the eutectic zirconia
crystal also becomes small in grain size, which means that it has a dense microstructure.
Further, in a zirconia-muUite refractory raw material where a eutectic zirconia crystal has a grain
size of 1.0 µm or less, preferably, 0.5 µm or less, and a matrix glass is contained in an amount of
5 mass% or less, it has a dense microstructure, i.e., excellent corrosion resistance, and a small
volume change, i.e., excellent thermal shock resistance, as compared with the conventional
zirconia-muUite refractory raw materials. If the grain size of the eutectic zirconia crystal is
greater than 1.0 µm, the microstructure becomes less dense, resulting in insufficient corrosion
resistance and thermal shock resistance.
The dense microstructure of the zirconia-mullite refractory raw material makes it possible
to prevent microstructural alternation and degradation when used under high-temperature
conditions for a long period of time. It is believed that this is because the dense micro structure
can suppress spreading of a strong reducing atmosphere in a microstructure of a refractory
product during use.
As used herein, the term "eutectic zirconia crystal" means a relatively small zirconia crystal
which is precipitated at a eutectic point in a last stage of cooling during production of the
zirconia-muUite refractory raw material through a fusion process. Further, the term "primary
zirconia crystal" means a relatively large zirconia crystal which is precipitated in an initial stage
of the cooling. The two types of crystals can be readily distinguished from each other by a
grain size thereof through microscopic observation. Further, in the microscopic observation,
the eutectic zirconia crystal has a feature that it is observed as an aggregate of fine crystals, and
adjacent ones of the crystals are oriented in the same direction. Differently, in the primary
zirconia crystal, there is almost no directional characteristic between adjacent ones of the
crystals. Further, in terms of a grain size, the eutectic zirconia crystal has a grain size which is
about 1/5 or less of a maximum grain size of the zirconia crystal.
In the present invention, the term "eutectic zirconia crystal having a grain size of 1.0 µm or
less" means that, in a microscopic observation field-of-view, 95%/area or more of eutectic
zirconia crystals have a grain size of 1.0 µm or less. Further, in the present invention, the term
"crystalline zirconia has a maximum grain size of 30 µm or less" means that, when 10 particles
of the zirconia-muUite refractory raw material each having a particle size of 0.5 to 3 mm are
microscopically observed to measure respective grain sizes of 20 crystals in descending order of
grain size, an average value of the measured grain sizes is 30 µm or less, as described in detail
later.
Meanwhile, the term "crystalline zirconia" means a crystalline body containing a ZrO2
component in an amount of 95 mass% or more, wherein a crystalline morphology thereof
comprises one or more selected from the group consisting of a monoclinic system, a cubic
system and a tetragonal system. One reason is that, when the crystalline zirconia is partially
stabilized by incorporating an impurity, such as Y2O3 or TiO2, into ZrO2 as a solid solution, the
stabilized crystal has a tetragonal system or a cubic system. Another reason is that, in a process
of rapidly cooling the molten mixture to produce the zirconia-mullite refractory raw material of
the present invention, zirconia having a tetragonal system in a high-temperature region of
1170°C or more is partially kept in the tetragonal system without a phase transition to a
monoclinic system during the rapid cooling.
In the zirconia-mullite refractory raw material, a smaller amount of the matrix glass
provides enhanced corrosion resistance against FeO or CaO during use as a refractory product.
Thus, it is preferable that no matrix glass is contained. However, as long as a content of the
matrix glass is 5 mass% or less, a negative effect thereof is negligibly small. If the content is
greater than 5 mass%, the corrosion resistance against FeO or CaO becomes deteriorated, and
alternation and microstructural deterioration of the zirconia-mullite refractory raw material due
to dissipation of an SiO2 component during exposure to high-temperature conditions for a long
period of time becomes significant to cause significant deterioration in durability.
The corundum in the zirconia-mullite refractory raw material can improve corrosion
resistance against molten metal. Thus, in cases where the zirconia-mullite refractory raw
material is used for a refractory product necessary to give priority to corrosion resistance, a
certain amount of corundum may be contained. Otherwise, if it is used for a refractory product
necessary to give priority to thermal shock resistance, it is preferable to minimize a content ratio
of corundum in the zirconia-mullite refractory raw material, or contain no corundum. For
example, in cases where it is used as a refractory raw material for a refractory product for
continuous casting, it is preferable to minimize a content ratio of corundum.
The present invention further provides a zirconia-mullite refractory raw material which
consists of crystalline zirconia and mullite, wherein the crystalline zirconia includes a eutectic
zirconia crystal having a grain size of 1.0 µm or less.
Preferably, in the zirconia-mullite refractory raw material of the present invention, the
crystalline zirconia has a maximum grain size of 30 µm or less. In this case, a microstructure
thereof becomes denser to provide enhanced corrosion resistance.
Preferably, a chemical composition of the zirconia-mullite refractory raw material of the
present invention satisfies the following requirements: it comprises 30 to 55 mass% of ZrO2, 30
to 55 mass% of Al2O3 and 10 to 25 mass% of SiOa, and each of the chemical components falls
within a primary phase region of ZrO2 in an Al2O3-ZrO2-SiO2 system phase diagram.
The zirconia-mullite refractory raw material satisfying the above requirements has a smaller
zirconia grain size and a denser microstructure, which provides higher thermal shock resistance
and corrosion resistance. It is assumed that this is because, in the zirconia-mullite refractory
raw material satisfying the above requirements, in the course of cooling of a molten mixture
during production, primary zirconia crystals are firstly precipitated, and then mullite and
corundum are sequentially precipitated in this order, which suppresses excessive formation of
corundum and glass.
The primary phase region 1 of ZrO2 is shown in the Al2O3-ZrO2-SiO2 system phase diagram
of FIG. 1. In FIG. 1, the line A indicates that a content ratio of ZrO2 is 30 mass%, and the line
B indicates that the content ratio of ZrO2 is 55 mass%. The line C indicates that a content ratio
of Al2O3 is 30 mass%, and the line D indicates that the content ratio of Al2O3 is 55 mass%. The
line E indicates that a content ratio of SiO2 is 10 mass%, and the line F indicates that content
ratio of SiO2 is 25 mass%.
If an amount of ZrO2 in the zirconia-mullite refractory raw material is less than 30 mass%,
formation of microcracks in a matrix during a phase transition of the crystalline zirconia
becomes deteriorated to cause deterioration in an effect of reducing an elastic modulus of a
refractory product. If the amount is greater than 55 mass%, an influence of the phase transition
of the crystalline zirconia on volume change characteristics becomes larger to cause destruction
of a matrix structure of the refractory product and deterioration in strength and thermal shock
resistance of a refractory product.
The Al2O3 component in the zirconia-mullite refractory raw material reacts with the SiO2
component to form mullite. Thus, if an amount of Al2O3 is less than 30 mass%, an amount of
mullite to be formed in the refractory raw material is reduced to cause deterioration in thermal
shock resistance. Moreover, an amount of the SiO2 component and/or an amount of the ZrO2
component are relatively increased. If the ZrO2 component is excessively increased, the
influence of a phase transition of the crystalline zirconia on volume change characteristics
becomes large to cause not only deterioration in strength and thermal shock resistance of a
refractory product due to destruction of a matrix structure of the refractory product, but also
deterioration in corrosion resistance due to accelerated formation of a low-melting point
substance from a reaction with FeO. If the SiO2 component is excessively increased, it forms a
low-melting point substance from a reaction with FeO or CaO to cause deterioration in corrosion
resistance. If the amount of Al2O3 is greater than 55 mass%, the amount of SiO2 and/or the
amount of ZrO2 are relatively reduced to cause deterioration in the effect of reducing a thermal
expansion rate and an elastic modulus.
If an amount of SiO2 in the zirconia-mullite refractory raw material is less than 10 mass%,
an amount of mullite becomes insufficient to cause an increase in thermal expansion rate. If the
amount is greater than 25 mass%, it is more likely to form a low-melting point substance from a
reaction with FeO or CaO to cause deterioration in corrosion resistance.
An apparent porosity of the zirconia-mullite refractory raw material of the present invention
is preferably 3.0% or less, more preferably 2.0% or less. If the apparent porosity is greater than
3.0%, a level of denseness of the zirconia-mullite refractory raw material becomes insufficient,
so that SiO gas is easily dissipated through gaps existing in the micro structure of the
zirconia-mullite refractory raw material, which is liable to accelerate alternation and
micro structural degradation of the zirconia-mullite refractory raw material. Moreover, a
specific surface area of the zirconia-mullite refractory raw material becomes larger, which is
liable to accelerate wear due to a reaction with FeO or CaO
Preferably, the zirconia-mullite refractory raw material of the present invention contains
Na2O, K2O, CaO, MgO, P2O5, B2O3, Fe2O3 and MnO2 in a total amount of 1.0 mass% or less.
If the total amount of these components is greater than 1.0 mass%, a melting point of the matrix
glass becomes lower to cause deterioration in corrosion resistance of the zirconia-mullite
refractory raw material, and further cause deterioration in thermal shock resistance due to
accelerated microstructural degradation in the zirconia-mullite refractory raw material itself
during heat receiving at high temperatures.
As used herein, the term "matrix glass" means an amorphous glass phase which does not
have a specific crystal structure consisting primarily of SiO2. In the zirconia-mullite refractory
raw material, the matrix glass exists to fill each gap between crystals, such as the crystalline
zirconia and the mullite. An amount of the matrix glass can be quantitatively determined from
chemical components and quantitative determination of a mineral phase by an X-ray
diffractometry-based internal reference method.
The zirconia-mullite refractory raw material of the present invention may be contained in a
plate brick in an amount of 5 to 40 mass% to provide a plate brick excellent in corrosion
resistance and thermal shock resistance.
In the plate brick containing the zirconia-mullite refractory raw material of the present
invention, the small thermal expansion rate of the zirconia-mullite refractory raw material
prevents an excessive formation of microspaces between the zirconia-mullite particles and a
matrix during heat receiving, to provide adequate strength and elastic modulus. In addition, a
thermal expansion rate around 1000°C is reduced, so that thermal shock resistance is improved.
More specifically, during actual use of the plate brick, heating and cooling are repeated.
Thus, a plate brick containing the conventional zirconia-mullite refractory raw material has large
residual expansion, so that a microstructure thereof becomes more loosened along with an
increase in the number of usages to cause degradation in the microstructure. In contrast, the
plate brick containing the zirconia-mullite refractory raw material of the present invention has
almost no residual expansion, so that micro structural degradation during repetitive use is
suppressed to allow the number of usable cycles to be increased. Further, a transition
temperature becomes lower as the crystalline zirconia of the zirconia-mullite refractory raw
material has a smaller grain size. This makes it possible to form an adequate microspace at a
relatively low temperature so as to reduce an elastic modulus to effectively provide thermal
shock resistance to the plate brick.
If a content of the zirconia-mullite refractory raw material in the plate brick is less than 5
mass %, the effect of reducing a thermal expansion rate to improve spaUing resistance becomes
deteriorated. If the content is greater than 40 mass%, corrosion resistance becomes
deteriorated.
[EFFECT OF THE INVENTION]
The zirconia-mullite refractory raw material of the present invention is significantly low in
alternation and micro structural degradation under high-temperature conditions, and excellent in
thermal shock resistance and corrosion resistance, as compared with the conventional
zirconia-mullite refractory raw materials. Thus, it can be used in place of the conventional
zirconia-mullite refractory raw materials to improve durability of refractory and ceramic
products. Further, it is usable in a plate brick to improve durability thereof
BEST MODE FOR CARRYING OUT THE INVENTION
The zirconia-mullite refractory raw material of the present invention is produced or
obtained by: preparing a mixture of starting materials consisting of zircon and alumina or of a
zirconia, silica and alumina; subjecting the starting-material mixture to a fusion process designed
to melt the starting-material mixture using an electric arc furnace or the like; and, after melting
the starting-material mixture, rapidly cooling the molten mixture. The zircon may be zircon
sand. The alumina may be calcinated alumina or sintered alumina, and the zirconia may be
naturally-occurring baddeleyite or desiliconized baddeleyite. The silica may be a naturally-
occurring silica material, such as silica stone, or a synthetic silica material, such as microsilica or
silica flour.
The rapid cooling may include a technique of allowing the molten mixture to flow along a
water-cooled iron plate, a technique of pouring the molten mixture into a frame formed by
assembling iron plates together, and a technique of pouring the molten mixture into a vessel
bedded with iron balls.
The zirconia-mullite refractory raw material obtained in the above manner has a dense
micro structure with fine crystals. Specifically, a dense micro structure with fine crystals can be
obtained by setting a cooling rate of the starting-material mixture in a molten state to a high
value to suppress crystal growth of crystalline zirconia and mullite. Further, a grain size of
primary and eutectic zirconia crystals can be controlled by controlling rapid-cooling conditions,
i.e., a cooling amount. Consequently, a porosity of the zirconia-mullite refractory raw material
can also be controlled.
A chemical composition of the starting-material mixture, i.e., a mixture of the starting
materials to be subjected to a fusion process, is adjusted to have comprising 30 to 55 mass% of
ZrO2, 30 to 55 mass% of Al2O3 and 10 to 25 mass% of SiO2, wherein each of the chemical
components falls within a primary phase region of ZrO2 in an Al2O3-ZrO2-SiO2 system phase
diagram. In this case, it becomes possible to more easily obtain a zirconia-mullite refractory
raw material in which crystalline zirconia includes a eutectic zirconia crystal having a grain size
of 1.0 µm or less, and has a maximum grain size of 30 µm or less, and a matrix glass is contained
in an amount of 5 mass% or less.
In a situation where each of the chemical components is out of the primary phase region of
ZrO2, i.e., falls within a composition range 2 in FIG. 1 which is a primary phase region of
corundum, during rapid cooling of the melting raw materials in a molten state, the Al2O3
component in the melting raw materials causes rapid precipitation of corundum crystals, so that
it becomes difficult to form the SiO2 component and the mullite. As a result, the content of
matrix glass phase is liable to be 10 mass% or more.
In another situation where each of the chemical components falls within a composition
range 3 in FIG. 1 which is a primary phase region of mullite, although a matrix glass phase is not
significantly formed, the content of ZrO2 is relatively reduced to less than about 30 mass%, so
that formation of microcracks, i.e., an effect of reducing an elastic modulus of a refractory
product, is liable to becomes deteriorated.
Further, a total amount of Na2O, K2O, CaO, MgO, P2O5, B2O3, Fe2O3 and MnO2 in the
zirconia-mullite refractory raw material is set to 1.0 mass% or less. For this purpose, a content
of theses components in the starting materials may be preliminarily controlled. A level of
formation of the matrix glass can be reduced by reducing the content of the components.
A micro structure of the zirconia-mullite refractory raw material of the present invention
comprises crystalline zirconia and mullite as primary components, with the remainder being
corundum and/or a matrix glass. For example, the micro structure may consist only of
crystalline zirconia and mullite.
In view of enhancing thermal shock resistance, it is preferable that a total content ratio of
the crystalline zirconia and the mullite in the entire micro structure is 85 mass% or more. If the
total content ratio is less than 85 mass%, the thermal shock resistance tends to largely
deteriorate. Further, it is preferable that a content ratio of the crystalline zirconia is in the range
of 30 to 55 mass%. If the content ratio is less than 30 mass%, formation of microcracks in a
matrix during a phase transition of the crystalline zirconia becomes deteriorated to cause
deterioration in the effect of reducing an elastic modulus of a refractory product. If the content
ratio is greater than 55 mass%, an influence of the phase transition of the crystalline zirconia on
volume change characteristics becomes larger, which is liable to cause destruction of a matrix
structure of the refractory product and deterioration in strength and thermal shock resistance of a
refractory product.
In the corundum of the zirconia-mullite refractory raw material, a cleavage crack is likely to
grow in a crystal thereof Thus, if a corundum crystal is formed in the zirconia-mullite
refractory raw material, a cleavage crack in the corundum crystal is liable to become a defect,
which could be a factor spoiling denseness of the zirconia-mullite refractory raw material. For
this reason, it is preferable to minimize a content ratio of corundum or contain no corundum.
As long as the content ratio is 20 mass% or less, the adverse effect is negligibly small, and a
corrosion resistance-enhancing effect can be obtained. If the content ratio is greater than 20
mass%, a thermal expansion rate becomes large to cause deterioration in thermal shock
resistance.
Each of the content ratios of the above mineral phases can be quantitatively determined
based on an X-ray intensity ratio. More specifically, it can be quantitatively determined by
chemical component analysis, such as fluorescent X-ray analysis or electron probe X-ray
microanalysis (EPMA), or by a commonly-used X-ray diffractometry-based internal reference
method using standard samples of corundum, crystalline zirconia, mullite, etc.
The content ratio of the matrix glass consisting primarily of SiO2 can be obtained by
quantitatively determining a content of SiO2 through chemical component analysis, such as
fluorescent X-ray analysis or EPMA, and subtracting, from the SiO2 content, an amount of SiO2
contained in the mullite phase, which is quantitatively determined through the X-ray
diffractometry-based internal reference method.
In the present invention, the crystalline zirconia includes a eutectic zirconia crystal having a
grain size of 1.0 µm or less, and has a maximum grain size of 30 µm or less. In view of
obtaining a denser micro structure, it is preferable to minimize a maximum grain size of the
mullite. Specifically, the maximum grain size of the mullite is preferably 50 µm or less, more
preferably 30 µm or less. As with the crystalline zirconia, the maximum grain size of the
mullite can be controlled by controlling the cooling rate of the molten mixture.
As used herein, the term "maximum grain size" means a value obtained by microscopically
observing 10 particles of the zirconia-mullite refractory raw material each having a particle size
of 0.5 to 3 mm are microscopically observed to measure respective grain sizes of 20 crystals in
descending order of grain size, and averaging the measured grain sizes. Generally, each of
crystalline zirconia and crystalline mullite includes an elongate columnar crystal, a block-like
crystal, or a dendritic crystal. In the present invention, the term "grain size" means a maximum
outer diameter of such a crystal in a direction perpendicular to a longitudinal axis of the crystal.
Further, in an actual microscopic observation, the crystal is observed in the form of a cross
section cut at an arbitrary position. Therefore, a length of the outer diameter of the crystal in
the direction perpendicular to the longitudinal axis of the crystal is measured in consideration of
an orientation of the crystal in each cross section.
The zirconia-mullite refractory raw material of the present invention can be used in place of
the conventional zirconia-mullite refractory raw material to improve thermal shock resistance
and corrosion resistance without any adverse effect. In particular, when it is used in a
refractory product for iron/steel making, the advantage becomes prominent. Specifically, it can
be used in a plate brick so as to improve durability thereof
[EXAMPLE]
Tables 1 and 2 show Inventive Examples and Comparative Examples.
The zirconia-mullite refractory raw material was experimentally produced by; melting
about 1 ton of each mixture of starting materials in Tables 1 and 2 at about 2000°C using an
electric fusion furnace; and, after uniformly melting the mixture, subjecting the molten mixture
to a rapid-cooling process designed to pour the molten mixture kept at the temperature into a
grid-shaped iron frame to rapidly cool the molten mixture, or a slow-cooling process designed to
cooling the molten mixture while being left in a molten material vessel. Control of a grain size
of the crystalline zirconia was performed by changing a size of the iron frame (cooling amount).
As for the chemical composition or chemical components, a quantitative analysis was
performed by fluorescent X-ray analysis based on JIS R2216. As for the mineral phases,
quantitative determination for each mineral phase was performed by an X-ray diffractometry-
based internal reference method. As for the matrix glass, an amount of the matrix glass was
determined by subtracting, from a quantitative analysis result on the SiO2 component based on
the fluorescent X-ray analysis, an amount of SiO2 contained in the mullite phase, which is
obtained by the X-ray diffractometry.
Each of the experimentally-produced refractory raw materials was embedded in resin, and,
after curing the resin, an obtained specimen was polished. Then, a microstructure of the
refractory raw material was observed by a reflecting microscope to check a maximum grain size
of the crystalline zirconia.
As for a thermal expansion rate, each of the experimentally-produced refractory raw
materials was pulverized to a particle size of 0.044 mm or less, and a columnar-shaped sample
was formed, and subjected to a heat treatment. Then, thermal expansion characteristics were
checked by thermo-mechanical analysis (TMA).
As for an apparent porosity, after each of the experimentally-produced refractory raw
materials was pulverized, obtained particles were sieved to select particles each having a size of
3.35 to 2.0 mm, and the selected particles were checked based on JIS R2205.
Further, after measuring an apparent porosity, each of the experimentally-produced
refractory raw materials was put in an alumina crucible, and the alumina crucible was put in a
vessel made of a silicon carbide-based refractory material. Then, the vessel was filled with
coke particles, and closed by a cover. In this state, each of the experimentally-produced
refractory raw materials was continuously subjected to a heat treatment in an electric furnace at a
temperature of 1500°C for 10 hours, and then an apparent porosity was measured again to
evaluate micro structural degradation due to alternation and dissipation of the matrix glass under
high temperatures, from the post-heat treatment apparent porosity.
As for corrosion resistance, each of the experimentally-produced refractory raw materials
having a given particle size by the particle size selection was mixed with a phenol resin. Then,
the mixture was formed in a square column shape having a size of 30 x 30 x 150, and fired
within Coke Breeze at 1000°C for 3 hours to obtain a sample. The sample was evaluated by
using an iron oxide powder as a corrosive material, in a high-frequency induction furnace. In
Tables 1 and 2, the corrosion resistance is indicated by a wear index, wherein a larger value
means a higher level of wear, i.e., that the corrosion resistance becomes more inadequate. The
wear index is determined on an assumption that a wear amount in Comparative Example 7 is
100.
In the "chemical components" in Tables 1 and 2, the "others 1" shows a total content ratio
of Na2O, K2O, CaO, MgO, P2O5, B2O3, Fe2O3 and MnO2, and the "others 2" shows a total
content ratio of Al2O3, ZrO2, SiO2, and other chemical components except the "others 1".
In Table 1, each of Inventive Examples 1 to 4 falls within the scope of the present
invention, i.e., the crystalline zirconia includes a eutectic zirconia crystal having a grain size of
1.0 µm or less, and has a maximum grain size of 30 µm or less, and the matrix glass is contained
in an amount of 5 mass% or less. As seen in Table 1, each of Inventive Examples 1 to 4 is less
likely to undergo alternation and microstructural degradation under high-temperature conditions,
because its apparent porosity (pre-heat treatment apparent porosity) and post-heat treatment
apparent porosity are smaller than those of Comparative Example 7 which is a conventional
zirconia-mullite refractory raw material, and excellent in corrosion resistance against FeO,
because its wear index is smaller than that of Comparative Example 7.
Each of Comperative Examples 1 to 6 is out of the scope of the present invention, because it
does not contain mullite as a mineral phase. Each of Comperative Examples 1 to 6 is
insufficient in thermal shock resistance due to its high thermal expansion rate. Moreover, it
contains a relatively small amount of SiO2 and ZrO2 components, and a relatively large amount
of Al2O3, so that corundum precipitated in an initial stage of the cooling is excessively
crystallized to preclude formation of mullite. Thus, the SiO2 component entirely exists as a
matrix glass, and thereby the post-heat treatment apparent porosity is increased up to 3.0% or
more, resulting in significant microstructural degradation.
Each of Comperative Examples 7 to 9 was prepared using the same starting materials as
those of Inventive Example 3 and at a cooling rate reduced by changing the size (e.g., thickness)
of the iron frame for use in the experimental production, so that a eutectic zirconia crystal had a
grain size of greater than 1.0 µm which is out of the scope of the present invention.
Consequently, the pre-heat treatment apparent porosity was increased to 4.8 to 6.2%, and thereby
an intended dense microstructure was not obtained. Moreover, this caused an increase in the
post-heat treatment apparent porosity, which led to deterioration in corrosion resistance due to
significant microstructural degradation after heat receiving and degradation in the denseness.
Comporting between pre-heat treatment apparent porosities of Inventive Example 3 and
Comparative Example 7, there is a large difference therebetween. Specifically, the pre-heat
treatment apparent porosity in Inventive Example 3 is 0.7%, whereas the pre-heat treatment
apparent porosity in Comparative Example 7 is 4.8%. Further, as for the gain size of the
eutectic zirconia crystal and the maximum grain size of the crystalline zirconia, there is also a
large difference therebetween. Specifically, the gain size of the eutectic zirconia crystal in
Inventive Example 3 is 0.2 µm, whereas the grain size of the eutectic zirconia crystal in
Comparative Example 7 is 1.2 µL, and the maximum grain size of the crystalline zirconia in
Inventive Example 3 is 21 µm, whereas the maximum grain size of the crystalline zirconia in
Comparative Example 7 is 52 µm Consequently, Inventive Example 3 had a significantly
superior result in terms of corrosion resistance. This means that, if the grain size of the zirconia
is reduced, a microstructure becomes dense, and thereby corrosion resistance against FeO is
enhanced. It can be also said that corrosion resistance against CaO is enhanced.
Each of Comparative Examples 10 and 11 contains an excessive amount of SiO2
component, and thereby a large amount (8.0% or 18.7%) of matrix glass, so that post-heat
treatment apparent porosity is increased to 4.7% or 5.2%, which caused significant
microstructural degradation. Moreover, an amount of silica glass is also increased, which
caused significant deterioration in corrosion resistance.
FIG. 2A shows a microscope (reflecting microscope) photograph of a zirconia-mullite
refractory raw material obtained in Inventive Example 3, and FIG. 2B shows a microscope
(electron microscope) closeup-photograph of a eutectic zirconia crystal thereof FIG. 3A shows
a microscope (reflecting microscope) photograph of a zirconia-mullite refractory raw material
obtained in Comparative Example 7, and FIG. 3B shows a microscope (electron microscope)
closeup-photograph of a eutectic zirconia crystal thereof In each figure, a whitish portion is
crystalline zirconia.
In FIG. 2A, a while crystal having a clearly recognizable shape is a primary zirconia crystal,
whereas a eutectic zirconia crystal is unrecognizable due to its excessively small size. Thus, the
primary zirconia crystal and the eutectic zirconia crystal are different in size, and can be clearly
distinguished from each other. In FIG. 2A, the maximum grain size of the crystalline zirconia
is 30 µm or less. In FIG. 2B which is a closeup-photograph of the eutectic zirconia crystal, the
grain size thereof is 1.0 µm or less.
In FIG. 3A, a white and large crystal is a primary zirconia crystal. The primary zirconia
crystal comprises a rectangular crystal and an oval crystal each including long and short types.
The shape of the primary zirconia crystal varies depending on a cross section of the crystal
appearing on an observation surface. In FIG. 3A, a large crystalline zirconia has a maximum
grain size of greater than 30 µm, and a portion recognizable as a white point or line with a size of
10 |xm or less is a eutectic zirconia crystal but not a primary zirconia crystal. The eutectic
zirconia crystal has a feature that it is observed as a group of a plurality of crystals oriented in the
same direction, or an aggregate of points or lines. In FIG. 3B which is a closeup-photograph of
the eutectic zirconia crystal, a cross section of the crystal in a direction approximately
perpendicular and slightly oblique to a longitudinal axis thereof is observed. As seen in FIG.
3B, a contour (grain size) of the crystal in this cross section is greater than 1.0 µm. As in this
photogram, a eutectic zirconia crystal has a columnar shape, in many cases.
In Table 2, each of Inventive Examples 5 to 7 contains a small amount of impurity
consisting of the "others 1", so that it contains a matrix glass in a small amount of 5 mass% or
less, and thereby maintains denseness in a micro structure even after the heat treatment and
excellent corrosion resistance. In addition, it has a low thermal expansion rate of 0.6 to 0.63
and sufficiently high thermal shock resistance.
In contrast, each of Comparative Examples 12 and 13 contains a matrix glass in an amount
out of the scope of the present invention, so that the post-heat treatment apparent porosity is
increased to 3.1% or 3.6%, which caused significant micro structural degradation after heat
receiving and significant deterioration in corrosion resistance.
In Inventive Examples 8 and 9, baddeleyite, calcinated alumina and silica flour were used
as starting materials, and a mixing ratio thereof was changed to have 29.0 to 54.0 mass% of
Al2O3, 55.0 to 30.0 mass% of ZrO2 and 15.0 mass% of SiO2. After melting a mixture of the
starting materials, the molten mixture was rapidly cooled to obtain a zirconia-mullite refractory
raw material.
Each of Inventive Examples 8 and 9 has a low thermal expansion rate of 0.62% or 0.66%,
and sufficient thermal shock resistance. In addition, it contains a matrix glass in a small
amount of 3.6 mass% or 0.8 mass%, and thereby maintains denseness in a micro structure even
after the heat treatment. Further, it has excellent corrosion resistance.
FIG. 4A shows a measurement result on a thermal expansion rate of the zirconia-mullite
refractory raw material in Inventive Example 3 under repetitive thermal loading, and FIG. 4B
shows a measurement result on a thermal expansion rate of the zirconia-mullite refractory raw
material in Comparative Example 7 under repetitive thermal loading.
In order to check thermal expansion characteristics of a zirconia-mullite refractory raw
material subjected to repetitive thermal loading, a columnar test piece cut out from a
post-cooling fused sample to have a height diminution of 20 mm and a diameter of 10 mm was
used as a measurement sample. A heating process from room temperature to 1300°C and a
cooling process were repeated 3 times to measure the thermal expansion characteristics based on
thermo-mechanical analysis (TMA).
As seen in FIG. 4A, the zirconia-mullite refractory raw material in Inventive Example 3 has
a small thermal expansion rate at 1300°C, and a small volume change during a phase transition.
In addition, it has a small residual expansion in the first cycle of thermal loading, and has almost
no residual expansion in the second and third cycles of thermal loading. In contrast, as seen in
FIG. 4B, the zirconia-mullite refractory raw material in Comparative Example 7 has a high
thermal expansion rate at about 1100°C, and largely shrinks at a temperature higher than about
1100°C due to a phase transition. Moreover, a residual expansion of about 0.2% occurs when
cooled to room temperature in the first cycle of thermal loading. Then, a residual expansion is
increased by repeating the heating in the second and third cycles of thermal loading.
In Table 1, the maximum grain size of the crystalline zirconia is 21 µm in Inventive
Example 3 and 52 µm in Comparative Example 7, and the grain size of the eutectic zirconia
crystal is 0.2 µm in Inventive Example 3 and 1.2 µm in Comparative Example 7. Thus, it is
proven that the above difference in the thermal expansion characteristics is caused by a
difference in the grain size of the crystalline zirconia. As above, the zirconia-mullite refractory
raw material of the present invention is small in thermal expansion rate and residual expansion,
and small in volume change during a phase transition, so that it serves as a refractory raw
material having significantly excellent thermal shock resistance. In particular, it can drastically
improve a life of a refractory or ceramic product to be used under conditions subjected to
repetitive thermal loading. For example, it can be used as a refractory material for a plate brick
so as to drastically increase the number of usable cycles of the plate brick.
A plate brick as a carbon-containing refractory product was prepared using the
zirconia-mullite refractory raw material obtained in Inventive Example 3, and an effect thereof
was compared with that of a plate brick prepared using the zirconia-mullite refractory raw
material in Comparative Example 7 as an example of a conventional zirconia-mullite refractory
raw material. A particle size of the zirconia-mullite refractory raw material was set in the range
of 1 mm to less than 2 mm. This plate brick was obtained by embedding the zirconia-mullite
refractory raw material in a refractory vessel filled with coke particles and firing it at 1000°C.
Table 3 shows a result of a comparative evaluation using a so-called fired plate brick
obtained by firing a shaped body of a mixture prepared using each of the zirconia-mullite
refractory raw materials, under the above conditions.
A conventional zirconia-mullite refractory raw material (Comparative Example 7) and an
alumina-zirconia refractory raw material were added, respectively, to Comparative Example 14
and Comparative Example 15, each in an amount of 20 mass%.
As compared with conventional plate bricks. Inventive Example 10 produced using
zirconia-mullite refractory raw material obtained in inventive Example 3 is superior in both
thermal shock resistance and corrosion resistance. Further, Inventive Example 10 was
experimentally used in an A steel plant as a plate brick of a sliding nozzle device for a ladle.
As a result, Inventive Example 10 could be used for 7 operation cycles with less wear in a nozzle
hole, and durability was improved by about 30% as compared with the conventional
zirconia-mullite refractory raw material.
It is considered that this is because the zirconia-mullite refractory raw material in Inventive
Example 3 has a dense micro structure based on crystalline zirconia with a small grain size,
which functions to suppress penetration of FeO into the zirconia-mullite so as to prevent a
reaction with the SiO2 component. It is also assumed that this is because the zirconia-mullite
refractory raw material in Inventive Example 3 has a small residual expansion, which functions
to suppress microstructural degradation even in repetitive usage. As above, the zirconia-mullite
refractory raw material of the present invention can be used in a plate brick so as to extend the
life of the plate brick.
Table 4 shows a result of a comparative evaluation using a so-called unfired plate brick
obtained by subjecting the shaped body to a heat treatment at 300°C. A particle size of the
zirconia-mullite refractory raw material was set in the range of 1 mm to less than 2 mm.
A conventional zirconia-mullite refractory raw material (Comparative Example 7) and an
alumina-zirconia refractory raw material were added, respectively, to Comparative Example 16
and Comparative Example 17, each in an amount of 20 mass%.
As compared with conventional plate bricks, Inventive Example 11 produced using
zirconia-mullite refractory raw material obtained in inventive Example 3 is superior in both
thermal shock resistance and corrosion resistance. Further, Inventive Example 11 was
experimentally used in a B steel plant as a plate brick for a ladle. As a result. Inventive
durability was improved by about 30%. As above, it was verified that durability is improved
even in the unfired plate brick as with the fired plate brick in Table 3.

brief description of the drawings
FIG. 1 is an Al2O3-ZrO2-SiO2 system phase diagram.
FIG. 2A is a microscope photograph of a zirconia-mullite refractory raw material obtained
in Inventive Example 3 (Table 1).
FIG. 2B is a microscope closeup-photograph of a eutectic zirconia crystal of the
zirconia-mullite refractory raw material obtained in Inventive Example 3 (Table 1).
FIG. 3A is a microscope photograph of a zirconia-mullite refractory raw material obtained
in Comparative Example 7 (Table 1).
FIG. 3B is a microscope closeup-photograph of a eutectic zirconia crystal of the
zirconia-mullite refractory raw material obtained in Comparative Example 7 (Table 1).
FIG. 4A is a graph showing a measurement result on a thermal expansion rate of the
zirconia-mullite refractory raw material obtained in Inventive Example 3 (Table 1), under
repetitive thermal loading.
FIG. 4B is a graph showing a measurement result on a thermal expansion rate of the
zirconia-mullite refractory raw material obtained in Comparative Example 7 (Table 1), under
repetitive thermal loading.
*1: An additional mess part with respect to 100 mass parts which is a total amount of other mixed materials
*2: A level of crack was evaluated after heating a sample at 1500°C for 5 hours and then immersing it in hot
metal at 1600°C, for 3 minutes.
*3: A sample was evaluated using molten FeO as a corrosive material, in an induction furnace.
A smaller value indicates better corrosion resistance.
*4: A test result obtained by using a sample as an SN plate in an A steel plant. The value indicates the
number of usable cycles.
*1: An additional mess part with respect to 100 mass parts which is a total amount of other mixed materials
*2: A level of crack was evaluated after heating a sample at 1500°C for 5 hours and then immersing it in hot
metal at 1600°C, for 3 minutes.
*3: A sample was evaluated using molten FeO as a corrosive material, in an induction furnace.
A smaller value indicates better corrosion resistance.
*4: A test result obtained by using a sample as an SN plate in an B steel plant. The value indicates the
number of usable cycles.
we claim:
1. A zirconia-mullite refractory raw material which is obtained through a fusion process, the
zirconia-mullite refractory raw material comprising crystalline zirconia and mullite as primary
components, with the remainder being corundum and/or a matrix glass, wherein the crystalline
zirconia includes a eutectic zirconia crystal having a grain size of 1.0 µm or less, and the matrix
glass is contained in an amount of 5 mass% or less.
2. A zirconia-mullite refractory raw material which is obtained through a fusion process, the
zirconia-mullite refractory raw material consisting of crystalline zirconia and mullite, wherein
the crystalline zirconia includes a eutectic zirconia crystal having a grain size of 1.0 µm or less.
3. The zirconia-mullite refractory raw material as defined in claim 1 or 2, wherein the
crystalline zirconia has a maximum grain size of 30 µm or less.
4. The zirconia-mullite refractory raw material as defined in any one of claims 1 to 3, which
has a chemical composition comprising 30 to 55 mass% of ZrO2, 30 to 55 mass% of Al2O3 and
10 to 25 mass% of SiO2, wherein each of the chemical components falls within a primary phase
region of ZrO2 in an Al2O3-ZrO2-SiO2 system phase diagram
5. The zirconia-mullite refractory raw material as defined in any one of claims 1 to 4, which
has an apparent porosity of 3.0% or less.
6. The zirconia-mullite refractory raw material as defined in any one of claims 1 to 5, which
contains Na2O, K2O, CaO, MgO, P2O5, B2O3, Fe2O3 and MnO2 in a total amount of 1.0 mass%
or less.
7. A plate brick which contains the zirconia-mullite refractory raw material as defined in any
one of claims 1 to 6, in an amount of 5 to 40 mass%.

Provided is a zirconia-mullite refractory raw material which is less likely to undergo
alternation and micro structural degradation under high-temperature conditions, and low in
thermal expansion rate, so as to have thermal shock resistance and corrosion resistance. The
zirconia-mullite refractory raw material comprises crystalline zirconia and mullite as primary
components, with the remainder being corundum and/or a matrix glass, wherein the crystalline
zirconia includes a eutectic zirconia crystal having a grain size of 1.0 µm or less, and has a
maximum grain size of 30 µm or less, and the matrix glass is contained in an amount of 5 mass%
or less. The zirconia-mullite refractory raw material has a chemical composition comprising 30
to 55 mass% of ZrO2, 30 to 55 mass% of Al2O3 and 10 to 25 mass% of SiO2, wherein each of the
chemical components falls within a primary phase region of ZrO2 in an Al2O3-ZrO2-SiO2 system
phase diagram.