Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Back~round of the Invention
This invention relates to a method of manufacturing long
or immersion type nozzles to be used for continuou~ casting -
hereinafter will be called as "casting no7zles" - having highly
stable quality, and high thermal shock- and corrosion-resistance.
Casting nozzles are very important and effective members
in the continuous casting, for improving the quality of cast
metal products. In response to the recent increasing demand
for high-quality steel material, continuous casting operations
using casting nozzles have been remarkably increased. As the
continuous casting process is locating at an early s~age of
the steel production line, any trouble occuring in this process
may affect the subsequent processes remarkably. Accordingly,
the casting nozzles for use in this process have always been
required to have a long service life and a highly stable quali-
ty. For instance, they are imposed such severe requirements
that the occurrence of troubles in the casting nozzles must
be lessthan once per one or two thousands pieces of nozzles.
Besides that, the use of the continuous casting is ex-
tended to the wide variety of steels, including the kinds of
highly corrosive to the refractories. Also there is an in-
creased use of multl-ladle casting which requires long time
continuous operations. Therefore, there is an increased demand
for the development of casting nozzles having highly stable
2~ quality and high reliability, as well as high thermal shock-
and corrosion-resistance.
According to our investigation of various troubles
occurred in actual operations which seem to have a relation-
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~s, 1
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ship to the quality of casting nozzle, such troubles may be
classified into two major types, i.e.,
1). Occurrence of cracking or breakage which
seems to be often caused at the beginning
of casting operations.
2). Occurrence of pitting or boring on the
inner surface of the nozzles due to local
erosion which may be caused at the inter-
mediate stage of casting operations.
Detailed study of these defective nozzles and other
products of the same lot revealed that in the microscopic
tests, there is a tendency of lack of the uniform distribution
of the refractory constituent in one body of the casting noz-
zle on one hand, while on the other hand in the physical and
chemical test results, such as porosity, specific gravity,
bending strength and modulus of elasticity, considerably wide
fluctuation was observed in the results of measurements. This
may indicate that the cracking or abnormal erosion takes place
at the weak portion caused by the lack of uniform quality in
the body of the casting nozzles.
Generally, two counterplans are considered to avoid such
troubles:
1). Improving physical and chemical properties
such as anit-spalling or anti-corrosion
characteristics of the refractories used for
the casting nozzles.
2). Minimizing the fluctuation of the quality
in one body of the casting nozzle.
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~ oth counterplan~ are intending, after all, to minimize
the occurrence of the weak point described above.
As for the counterplan 1), efforts are being made to
raise the refractoriness, corrosion-resistivity and anti-
spalling characteristics of the ma-terial by controlling its
composition an~ treatments. However, for the counterplan 2),
there has been found no effective procedures to minimize the
fluctuation of the quality, and this may be also fundamentally
important procedure to promo-te counterplan.l).
Next, we have studied the cause of the fluc-tuation of
the quality in connection with the manufacturing process of
the casting nozzles.
In the production of casting nozzles, natural Elake
graphite, alumina and fused silica are generally used as main
refractory materials. As the na-tural flake graphite has low
wetting properties against molten metals and slags as well as
high thermal conductivity, casting nozz:Les containing this
material have low modulus of elasticity and high tharmal shock-
and corrosion-resistivity. As a result, they have a long ser-
vice life.
However, particles of fla]ce graphite are flaky and fle~i-
ble, -these properties are different from those of other kinds
of refractory materials - ,so that it is difficult to obtain
uniform dispersion of particles when the refractory materials
containing such flake graphite are kneaded, and when the mate-
rial is strongly kneaded, par-ticles of flake graphite tend to
be broken to small size by the s-trong shearing force and there-
by cause the increase of modulus of elasticity and increase the
brittleness of the products~
Further, at the time of molding, the flakes of graphite
have the tendency of being arranged in a same direction
perpendicular to the direction of the compression, and also
the flakes are apt to be dis-torted.
These and other peculiar characteristics of the flake
graphite have led us to the conclusion that the special con-
sideration must be paid on the manufacturing method of the
casting nozzles, and particularly on the knead.ing process
of raw material.
Summary of the Invention
__
In the method of manufacturing casting nozzles of the
present invention, the refractory material containing flake
graphite is kneaded by controlling its kneading force to
prevent serious breakage of the flakes of the graphite.
The method comprises the steps of;~a) kneading refractory
material containing flake graphite by a granulation kneading
and/or a stirring kneading, b) aging said kneaded material
and molding to the nozzle shape; and c) sintering by heating
said molded material to form nozzle body.
Thus the lowering of plastic deformability of the material
may be prevanted by this method, and the products of high ther-
mal shock resistance and highly stable quality can be obtained.
As a result, the service life of the nozzles can be improved
remarkably.
The refractory material used for the present invention
has the following composition, i.e., 20 - 60% by weight of
natural flake graphite, ~0 - 70% by weight of alumina, 5 - 20
by weight of fused s~ilica, and 2 ~ 20~ by weight of bond re-
inforcing material and anti-oxidant with the addition of phenol
resin as a binder. Refractory material ls granulated to
about lQ0 - 1,5Q0 ~m size granules in said kneading step.
The method of the present invention will be ~escribed
in more details with reference to the accompanying drawings.
B ef Description of the Drawings
Fig. 1 illustrates a series of graphs of stress-strain
characteristics of the specimens prepared by three different
methods showing the degree of the fluctuation of test results;
and Fig. 2 illustrates a graph showing AE total count numbers
as a ~unction of the thermal shock application time on the
specimens prepared by three different methods.
Detai'led Description of the Invention
. , . . __ . . . . . . . .
As the natural flake graphite has low wetting properties
against molten metals and slags as well as high thermal con-
2G ducti~ity, casting nozzles containing t'his material have lowmodulus of elasticity, and high thermal shock~ and corrosion-
resistance as described above.
However, the graphite is easily oxidized at high tempera-
ture, so that the amount of -the flake graphite may be limited
to some extent. A preferrably range may be 20 - 60g by weight.
As the particles of this graphite are flaky, coarse grains
thereof may lower the dispersion properties and also may be
easily broken to pieces/ while fine grains thereof may have
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less ability for lowering modulus of elasticity of the mate-
rial and more easily oxidized. Therefore, grain size of the
flake graphite withi~ the range of 1,000 - ~0 ~um, more preferr-
ably within the range of 700 - lOO~um may be used.
Fused alumina and/or sintered alumina may be used for
alumina. As the alumina has high refractoriness and corrosion-
resistivity against the molten metals and slags, casting noz-
zles containing the same have a high thermal shock- a~d
corrosion-resistance. ~0 - 70% by weight of alumina powder
of less than 120~um grain size may be used preferrably.
The use of such fine grain size may improve the dispersion
properties of alumina in the material. The use of less than
40% by weight may cause the lowering of the corrosion resist-
ance and the use of more than 70% by weight may cause the
lowering of the thermal shock resistance because of the decrease
of graphite and fused silica contents.
E'used silica, having a low coefficient of thermal expan-
sion, gives high thermal shock resistance to the casting noz-
zles containing the same. 5 ~ 25% by weight of fused silica
of 500 - lOO~um grai~ size may be used preferrably for the
purpose. Fused silica of more than 500~um grain size may
cause lowering of dispersion properties, and fused silica
of less than 100 ~m grain size may lower the thermal shock
resistance due to the devitrification of silica during the
use in a high temperature. The use of less than 5~ by weight
of fused silica may no~ sufficient for the purpose and the use
of more than 25% by weight may lower the corrosion resistance.
2 - 20% by weight of bond reinforcing material and anti~
oxidant may be added to the main raw material. Mullite,
feldspar, sillimanite, aluminosilicates such as clay minerals,
fine silicon carbide powder, fine alumina powder, fine silica
powder, ~rit and the like materlal may be used as the bond
reinforcing material and anti-oxidant. To the mixture of
these powders, phenol resin may be added as a binder.
Preferrably, powdery and liquid form phenol resins may be
used concurrently.
The above mixture is then kneaded by a granulation
kneading to make granules of about 100 - 1,500~um in size by
the use of, for instance, a controlled roller kneading machine
and/or by a stirring kneading by the use of such as a screw
beater.
Kneading by the ordinary type roller kneading machine
may break the flake graphite severely, but in the granulation
kneading, although broken or deformed parts of flake graphite
may increase, a mosaic packing pattern of granules surrounded
by the deformed graphite flakes may be formed.
Stirring type kneading has the advantages that the breakage
~0 of graphite flakes hardly occurs and they are uniformly mixed,
and dispersed with other components of the material.
The kneaded mixture is -then aged by preserving for several
days, after that the mixture is molded to the nozzle shape by
the use of rubber press. The molded refractory material is
then sintered by heating the same under the reducing atmosphere
to form nozzle body. In another way of aging, the kneaded
mixture is fed into the rotary drum, and the hot air is blown
into the drum, while the same is rotated, then a part of
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the volatite components, free water and condensation water
contained in the mixture may be removed and the mixture
may be brought into a condition suitable for the subsequent
molding process in a short period of time.
Advantages of the present invention will be explained
more clearly by the following examples.
Table 1 illustrates the physical test results of the
products prepared by the method in accordance with the present
invention (methods II and III), with those of the products
made by the conventional method (method I) for comparison.
First, the methods I, II and III were compared using the
material composition (a), then the material composi~ions (a),
(b), (c), (d) were compared, in which the same kneading method
III being used.
In the Table, modulus of quasi-elasticit~ E' is defined
by the following equation,
E~ = (P60 P30)L _____--- - (1)
48(~60 -~30)I
20 where P60 and P30 are 60 percent and 30 percent load respect~ve-
ly of maximum load value Pmax in the stress-strain curve obtain-
ed by a three-point precision bending strength test made on a
square pillar specimen having a rectangular section; and ~60
and ~30 are strain values at the load P60 and P30 respectively,
and I is the moment of inertia of area of the specimen, and L
is the distance between the supporting points.
The reason of the use of the modulus of quasi-elasticity
for the evaluation of material quality is that the firebrick
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materi.als containing conside:rable amount of flake graphite
shows relatively large pl.astic deformation and that a large
error would appear in the measurement of elasticity within
a very narrow elastic range. Actually, it was seen that the
evaluation based on the modulus of quasi-elasticity, including
the range of plastic deformation, can explain the quality of
the material more really.
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From the results listed in Table 1, it will be seen that
the products of this invention have physical values nearly
equal to those of conventional products, but they have lower
modulus of ~uasi-elasticity than that of conventional products,
and the thermal shock resistance is improved considerahly.
In comparing the results obtained with the compositions
(a), (b), (c), (d) in the method III, the following has become
clear that composition (b) having a greater amount of graphite
than composition (a) shows slightly lower strength than (a)
but has lower modulus of quasi~elasticity E', so that it shows
an increased plastic deformability. Composition (C) having
a greater amount of silica than composition (a) also shows a
similar trend. Composition (d) having a greater amount of
bond reinforcing material than composition (a) shows a higher
strength than (a), but shows considerabl~ high modulus of
~uasi-elasticity E', so that it shows an increased of brittle-
ness.
Fig. 1 shows stress ~B - strain ~ curves obtained by the
bending test on several specimens prepared by the three diE-
ferent kneading methods Ia, IIa and IIIa. It will be seenthat the methods IIa and IIIa of the present invention cause
less fluctuation of test results than conventional method Ia
and are indicating the stabilization of quality.
Fig. 2 shows the results of a thermal shock resistance
test conducted by an AE (Acoustic Emission) method. When
a piece of brick is heated from one sid~ surface thereof,
remarkably large stress occurs within said brick. In case
this stress exceeds the strength of the brick, cracks may
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occur and are transmitted through the brickO A part of the
energy liberated at this time can be detected by the AE
measuring apparatus as elastic waves. AE data are given in
the form of the AE count number, and as the AE count number
corresponds to a number of occurrence of cracks, the larger
AE count number shows much cracks occurred and transmitted,
and indicates lower thermal shock resistance.
It will be seen from Fig. 2, that according to the
methods IIa and IIIa of ~he present invention, products of
higher thermal shock resistance may be obtained than those
made by the conventional method Ia.
