TITLE OF THE INVENTION
Refractories containing thick flake graphite
TECHNICAL FIELD
[0001]
The present invention relates to refractories containing graphite.
BACKGROUND ART
[0002]
As a refractory lining for a converter, a ladle, a secondary refining furnace, a blast furnace,
a torpedo car and others in an steel-making process, and a continuous casting nozzle, an injection
refractories, a stamping material, etc., graphite containing refractories is widely used which
contains a combination of a refractory raw material such as magnesia, alumina, zirconia or spinel.
In view of a potential to provide a hardly wettable property with slag, or characteristics such as a
reduction in elastic modulus and thermal expansion, and an increase in thermal conductivity,
utilization of graphite is significantly effective in improving corrosion resistance and spalling
resistance of refractories.
[0003]
Heretofore, regarding a shape of graphite to be added to refractories, several findings have
been obtained. The following Patent Document 1 discloses a graphite-containing refractories
containing a thin flake graphite in which a breadth size thereof in a layer plane is 0.105 mm or
more, and a flake thickness is 20 µm or less. The following Patent Document 2 discloses a
magnesia-carbon based unburned brick using thin expanded graphite having a thickness of 12
µm or less. The following Patent Document 3 discloses a method in which a aggregates and
graphite are subjected to dry-mixing to reduce a thickness of the graphite. The Non-Patent
Document 1 mentions that, as compared to commonly-used graphite having a thickness of 36 µm
or 25 µm, thin graphite having a thickness of 9 µm or 10 µm can improve the properties. As
a result of the reduction in thickness, the graphite disclosed in each of the above documents has a
large aspect ratio of about 15 to 40, wherein the aspect ratio is defined as particle size / thickness.
[0004]
However, due to such a large aspect ratio, the thin graphite has great difficulty in being
uniformly dispersed during kneading. Thus, obtained graphite-containing refractories are liable
to have a large fluctuation in strength, which causes breakage in nozzle-shaped refractories,
cracking in refractories lining of steel-making vessel, etc and durability of the refractories
becomes unstable.
[0005]
On the other hand, the following Patent Document 4 discloses refractories which contain
granular graphite having a particle size of 50 µm or more and an aspect ratio of 0.5 to less than 5.
The following Patent Document 5 discloses refractories which contains massive graphite having
cross-sectional dimensions of 0.1 to 1 mm in a longitudinal direction thereof and 0.1 mm or
more in a direction perpendicular to the longitudinal direction. Both of the refractories
disclosed in the Patent Documents 4 and 5 are characterized by using graphite having a shape
close to a spherical shape, i.e., having a small aspect ratio.
[0006]
Differently from commonly-used refractory aggregates, graphite has a peculiar shape, i.e., a
flaky shape, and exhibits a property of easily come off from interlayer, so that it has effects of
causing crack which would otherwise continuously grow to branch or detour, and absorbing
energy which would otherwise cause crack growth, based on delamination of the graphite.
However, in cases where graphite having a small aspect ratio is dispersed in refractories, crack is
more likely to be propagated in a refractory aggregate other than the graphite, so that the crack
growth-causing energy absorbing effect based on the graphite addition will be deteriorated.
Particularly, in graphite having a large thickness of 100 µm or more, as disclosed in the Patent
Document 5, the number of graphite particles per unit mass decreases, and a probability of
contact between crack and graphite becomes lower, so that further deterioration in the above
improvement effect will occur.
LIST OF PRIOR ART DOCUMENTS
[PATENT DOCUMENTS]
[0007]
Patent Document 1: JP 02-043698B
Patent Document 2: JP 2000-319063A
Patent Document 3: JP 3077942
Patent Document 4: JP 3327884
Patent Document 5: JP 3327883
[NON-PATENT DOCUMENTS]
[0008]
Non-Patent Document: Refractories, Vol. 37 (1985) P25
SUMMARY OF THE INVENTION
[PROBLEM TO BE SOLVED BY THE INVENTION]
[0009]
It is an object of the present invention to provide a graphite-containing refractories having a
property that a fluctuation in strength is small and crack growth is less likely to occur, and
thereby exhibiting stable durability.
[MEANS FOR SOLVING THE PROBLEM]
[0010]
On the assumption that a technique of allowing a graphite-containing refractories to have a
property that a fluctuation in strength is small and crack growth is less likely to occur,
contributes significantly to stability in durability of the graphite-containing refractories, the
inventors diligently conducted researches on a shape of graphite to be used. As described in the
Background Art, there have heretofore been known graphite having a large aspect ratio of about
15 to 40 and graphite having a small aspect ratio of less than 5, wherein the aspect ratio is
defined as particle size / thickness. However, it was proven that they have the aforementioned
problems when actually used as a raw material for refractories. As a result of checking an
influence of a shape of graphite on the above properties, the inventors found that the above
problems can be solved by using graphite having an aspect ratio intermediate between the
conventional ratios and a specific thickness, and have finally accomplished the present invention.
[0011]
Specifically, the present invention provides refractories comprising a refractory raw
material and graphite. The refractories is characterized in that the graphite in the refractories
contains 20 rnass% or more of thick flake graphite having a thickness of 50 µm to less than 100
µm and an aspect ratio of 5 to 12, wherein the aspect ratio is defined as particle size / thickness.
[0012]
A shape of the graphite in the present invention is clarified by subjecting graphite by itself
or a kneaded mixture of graphite and a refractory aggregate to uniaxial pressing to obtain a
shaped body, embedding the shaped body into resin to form a specimen, polishing a surface of
the specimen parallel to a direction of the pressing, and observing a cross-section of the graphite
by a reflecting microscope. As used here, the term "particle size" of the graphite means a size
of the graphite in a longitudinal direction thereof, and the term "thickness" of the graphite means
a size of the graphite in a direction perpendicular to the longitudinal direction. In each type of
graphite, graphite flakes are not exactly constant in particle size and thickness. Thus, according
to need, each of the particle size and thickness is determined by performing measurement at
several arbitrary points, and averaging the measured values. The aspect ratio is defined as the
particle size / thickness measured through the observation.
[0013]
FIGS. 1 (a) and 1(b) show microscope photographs of two types of graphites each
embedded in resin in the above manner. It is evident that the thick flake graphite according to
the present invention has a thickness greater than that of natural flake graphite used for
conventional graphite-containing refractories.
[0014]
In regard to graphite for use in a graphite-containing refractories, when the graphite has a
large aspect ratio of about 15 to 40, it becomes difficult to uniformly disperse, resulting in a large
fluctuation in strength between obtained graphite-containing refractories. On the other hand,
when the graphite has a small aspect ratio, for example, of less than 5, the crack growth-causing
energy absorbing effect becomes poor. In contrast, graphite having an aspect ratio of 5 to 12
intermediate between the large and small ratios can solve the above problems. However, this
improvement effect is significantly exerted only in graphite having a thickness falling within a
specific range. Specifically, the thickness is in the range of 50 µm to less than 100 µm. If the
thickness is less than 50 µm, the number of graphite flakes per unit mass increases, which causes
deterioration in dispersibility and promotes a fluctuation in strength. Thus, the commonly-used,
36 or 25 µm thickness, natural flake graphite as disclosed in the Non-Patent Document 1 cannot
obtain a significant improvement effect in regard to a reduction of the fluctuation in strength,
even if it has an aspect ratio of 5 to 12. On the other hand, if the thickness is 100 µm or more, a
probability of contact between crack and graphite becomes lower, so that the crack
growth-causing energy absorbing effect based on graphite will be deteriorated.
[0015]
In the present invention, it is essential that the graphite in the refractories contains 20
mass% or more of the thick flake graphite which has a thickness of 50 µm to less than 100 µm
and an aspect ratio of 5 to 12. If the content is less than 20 mass%, it becomes impossible to
obtain the significant improvement effect of the present invention. It is desirable to increase the
content of the thick flake graphite as much as possible, preferably up to 40 mass% or more, more
preferably up to 60 mass% or more. The graphite may entirely consist of the thick flake
graphite. The remaining graphite other than the thick flake graphite is not particularly limited.
However, in view of cost, it is most preferable to use natural flake graphite.
[0016]
As to a production method for the thick flake graphite according to the present invention,
the thick flake graphite may be sorted from natural flake graphite. However, considering that
graphite falling under the thick flake graphite is naturally produced in a small amount, it is
preferable to produce the thick flake graphite by artificially processing commonly-used natural
flake graphite. For example, a binder such as phenolic resin is added to flake graphite, and they
are mixed using a mixer, so that graphite flakes are stacked in layers to form graphite having a
larger thickness. The resulting graphite is pulverized into pieces having a desired shape, and
the obtained graphite pieces are subjected to classification. In this manner, the thick flake
graphite can be obtained. Alternatively, the thick flake graphite may be produced by applying a
compression force to natural flake graphite to cause graphite flakes to be stacked in layers in a
thickness direction.
[0017]
The present invention is characterized in that the thick flake graphite is contained in an
amount of 20 mass% or more with respect to the entire graphite. As to a raw material other
than the graphite, any suitable conventional raw material may be used in combination. For
example, the refractory aggregate includes magnesia, alumina, silica, mullite, zirconia, zircon,
spinel, agalmatolite, and titania. Other additives may include, for example: metal such as
aluminum, silicon or magnesium, and their alloys; a boron compound such as boron carbide; a
carbon raw material such as carbon black, pitch or anthracite; carbide such as silicon carbide;
and a binder such as phenolic resin, furan resin, pitch or tar. These raw materials may be
formed as a shaped refractories through processes, such as kneading, shaping, drying and
burning, or may be formed as a castable or unshaped refractories through processes, such as
kneading, construction and drying.
[0018]
A specific example of the graphite-containing refractories may include, as a shaped
refractories: a magnesia-graphite based refractories for use in a converter and secondary refining
vessel, ladle or an electric furnace; an alumina-graphite based refractories or a zirconia-graphite
based refractories for use in a continuous casting nozzle; an alumina-silicon carbide-graphite
based refractories for use in a torpedo car; a carbon-based refractories or an alumina-graphite
based refractories for a blast furnace; and an alumina-agalmatolite-silicon carbide-graphite based
refractories or an agalmatolite-silicon carbide-graphite based refractories for a hot-metal ladle,
and includes, as an unshaped refractories: an agalmatolite-silicon carbide-carbon based
refractories for use as a filling material for a taphole of a blast furnace; an alumina-graphite
based refractories for use as a castable material for a gutter of a blast furnace; a carbon-based
injection refractories for a blast furnace; a magnesia-graphite based stamping material for a
hearth of an electric furnace; and a castable material for a slag line of a hot-metal ladle, a torpedo
car, a ladle or the like.
[EFFECT OF THE INVENTION]
[0019]
The graphite-containing refractories of the present invention has a small fluctuation in
strength and an excellent capability to prevent crack growth, and thereby exhibits stable
durability, so that it is usable as various refractories for an steel-making process to contribute to
improvement in productivity and stabilization in operation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
FIGS. 1(a) and 1(b) are microscope photographs illustrating two types of graphites each
embedded in resin, wherein FIG. 1(a) illustrates thick flake graphite according to the present
invention, and FIG. 1 (b) shows natural flake graphite used for conventional graphite-containing
refractories.
FIG. 2 is a graph shows an average thickness and an average particle size of each type of
graphite, together with a range of thick flake graphite defined in the present invention.
DESCRIPTION OF EMBODIMENTS
[0021]
An embodiment of the present invention will now be described based on examples.
[EXAMPLES]
[0022]
As shown in Table 1, twelve types of graphites different in thickness and particle size were
prepared. Specifically, the thick flake graphite, the coarse graphite, the granular graphite and
the massive graphite were prepared by adding a liquid phenolic resin to natural flake graphite,
agglomerating graphite flakes using a Henschel mixer, and subjecting the obtained agglomerate
to pulverization and classification. The thickness and particle size were set to desired values by
adjusting an amount of resin to be added, a period of time for the agglomeration, a method for
the pulverization, etc. For example, graphite having a large thickness can be obtained by
'ncreasing the amount of resin to be added, and graphite having a small particle size can be
obtained by lengthening the period of time for the agglomeration. The thin-walled graphite was
selected from commercialized products.
[0023]
TABLE 1
[0024]
Each type of graphite was embedded in resin and a cross-section of the graphite was
observed by a microscope, as described above, to measure a thickness and a particle size thereof
and calculate an aspect ratio from the particle size / thickness. Each type of graphite has
approximately the same purity of about 98 mass%.
[0025]
The thick flake graphite A is the entirety of which falls within the scope of the present
invention, and each of the thick flake graphites B to F is graphite 50 mass% of which falls within
the scope of the present invention.
[0026]
FIG 2 illustrates an average thickness and an average particle size of each type of graphite,
together with a range of the thick flake graphite having a thickness of 50 µm to less than 100 µm
and an aspect ratio of 5 to 12, which falls within the scope of the present invention.
[0027]
It was checked how the fluctuation in strength and breaking energy indicative of the crack
growth-suppressing capability are affected by a ratio of the thick flake graphite to the entire
graphite in the refractories. As illustrated in Table 2, an appropriate amount of phenolic resin
was added to 10 mass% of graphite, 89 mass% of magnesia clinker having a maximµm particle
size of 5 mm, and 1 mass% of aluminµm having a maximµm particle size of 0.1 mm, and they
were uniformly kneaded to obtain a mixture. Then, the mixture was formed into a standard
shape by uniaxial pressing under a pressure of 1500 kg/cm2, and the shaped body was subjected
to a heat treatment at 250°C for 5 hours to prepare a sample refractories
[0028]
TABLE 2
[0029]
Then, as illustrated in Table 2, a ratio of the thick flake graphite falling within the scope of
the present invention to the entire graphite was changed by combining the thick flake graphite A
with the natural flake graphite. The thick flake graphite A consists entirely of the graphite
falling within the scope of the present invention, and the natural flake graphite does not contain
the graphite falling within the scope of the present invention. Thus, the ratio of the graphite
falling within the scope of the present invention is equal to a ratio of the thick flake graphite A to
the entire graphite.
[0030]
In evaluation of each sample refractories, ten rectangular parallelepiped-shaped
measurement samples each having a size of 160 mm length x 25 mm width x 16 mm thickness
were cut from the sample refractories, and subjected to a three-point bending test under a span of
140 mm, and a cross head speed of 0.2 mm/min, to obtain a stress-strain curve in a period from
start of loading through until breaking, and the breaking energy was calculated from bending
strength calculated from a maximµm stress and an integrated value in the curve. The
fluctuation in strength was evaluated by calculating a difference between a maximµm value and a
minimµm value among the calculated bending strength values (n = 10), and the breaking energy
was evaluated based on an average of the calculated breaking energy values (n =10). A result
of the evaluations each represented by an index based on the assumption that a value of
Inventive Sample 1 is 100, is illustrated in Table 2. In regard to the index of the fluctuation in
strength, a larger value indicates that the fluctuation becomes larger, i.e., durability becomes
worse. In regard to the index of the breaking energy, a larger value means that breaking is less
likely to occur, i.e., durability becomes better. In an actual refractories, it is a key point to
ensure a balance between the two factors. Thus, the comprehensive evaluation index defined as
"index of the fluctuation in strength / index of the breaking energy x 100" was calculated.
When the comprehensive evaluation index is: 85 or less; in the range of 86 to 105; in the range
of 106 to 120; and 121 or more, the comprehensive evaluation is made as: ?; o; ?; and x,
respectively. The marks ?, o, ?, and x indicate better evaluation, in this order.
[0031]
As is clear from the comprehensive evaluation result in Table 2, it is proven that Inventive
Samples 1 to 5 in which the ratio of the thick flake graphite falling within the scope of the
present invention to the entire graphite is 20 mass% or more, are better than Comparative
Samples 1 and 2. It is also proven that the ratio of the thick flake graphite falling within the
scope of the present invention is preferably 40 mass% or more, more preferably 60 mass% or
more. If the ratio of the thick flake graphite falling within the scope of the present invention is
less than 20 mass%, the fluctuation in strength becomes significantly large, which is practically
unsuitable.
[0032]
As illustrated in Table 3, the evaluations were performed using the graphites illustrated in
Table 1 having various shapes. Each of the thick flake graphites B to F is graphite 50 mass% of
which falls within the scope of the present invention. Thus, when each of the thick flake
graphites B to F is added in an amount of 4 mass%, the ratio of the thick flake graphite falling
within the scope of the present invention to the entire graphite is 20 mass%. The preparation of
sample refractories and the evaluation of the sample refractories were performed in the same
manner as described above.
[0033]
TABLE 3
[0034]
From a result of comprehensive evaluation in Table 3, it is evident that an excellently
balanced refractories can be obtained by allowing thick flake graphite having a thickness of 50
µm to less than 100 µm and an aspect ratio of 5 to 12 to be contained in an amount of 20 mass%
or more with respect to the entire graphite. The refractories containing the coarse graphite or
hunned graphite having a large aspect ratio is practically unsuitable, because the fluctuation in
trength is significantly large, and the refractories containing the granular graphite A or granular
graphite B having a small aspect ratio is also practically unsuitable because the breaking energy
is significantly low.
[0035]
In order to check an influence of a total amount of graphite to be added, and a type of
refractory raw material other than the graphite, as illustrated in Table 4, a total amount of
graphite was set to 30 mass%, and 50 mass% of alumina clinker having a maximµm particle size
of 0.5 mm and 20 mass% of fused silica having a maximµm particle size of 0.3 mm were added.
Then, an appropriate amount of phenolic resin was added thereto, and they were uniformly
kneaded to obtain a mixture. This mixture was formed into a cylindrical shape having an outer
diameter of 120 mm, an inner diameter of 50 mm and a height of 400 mm by a CIP process
under 1000 kg/cm2, and the shaped body was subjected to a hear treatment at 250°C for 5 hours.
Then, the heat-treated body was subjected to reduction burning in coke breeze at 1000°C for 8
hours to obtain sample refractories. The evaluation of the sample refractories were performed
in the same manner as described above.
[0036]
TABLE 4
[0037]
From a result of comprehensive evaluation in Table 4, it is evident that excellently balanced
refractories can be obtained by using the thick flake graphite according to the present invention,
even if the total amount of graphite is increased, or the type of refractory raw material other than
the graphite is changed
[0038]
As mentioned above, the use of the thick flake graphite according to the present invention
makes it possible to obtain refractories in which crack growth is less likely to occur. This
means that flexibility in material design is enhanced, and corrosion resistance, abrasion
resistance and oxidation resistance are significantly improved.
We Claim:
1. Refractories comprising a refractory raw material and graphite, characterized in that the
graphite in the refractories contains 20 mass% or more of thick flake graphite having a thickness
of 50 µm to less than 100 µm and an aspect ratio of 5 to 12, wherein the aspect ratio is defined as
particle size / thickness.