Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02312482 2000-OS-31
A CONTINUOUS CASTING NOZZLE FOR MOLTEN STEEL
FIE n OF E INVENTION
The present invention relates to a continuous casting nozzle, in
particular, a continuous casting nozzle which permits effective prevention of
narrowing and clogging of the inner bore thereof through which molten steel
passes in performing continuous casting of the molten steel containing
aluminum such as aluminum-killed steel.
THE F .A'I .D RT
A continuous casting nozzle for casting molten steel is used for the
following purposes.
A continuous casting nozzle has a function of pouring molten steel
from a tundish to a mold. In continuously casting molten steel, a continuous
casting nozzle is used for such purposes as preventing the molten steel from
being oxidized by contacting with the open air, preventing the molten steel
from splashing when the molten steel is poured from a tundish to a mold,
and rectifying the flow of the poured molten steel so as not entrap non-
metallic inclusion and slag present near or on the mold surface into the cast
steel strand.
A refractory material of a conventional continuous casting nozzle of
molten steel comprises graphite, alumina, silica, silicon carbide or the like,
for example. However, there are following problems when aluminum-
killed steel or the like is cast with the use of the conventional casting
nozzle.
In casting the aluminum-killed steel or the like, aluminum which is
added as a de-oxidizer, reacts with oxygen existing in the molten steel to
produce non-metallic inclusion such as alpha ( a )-alumina or the like. In
addition, when the molten steel flows through the nozzle, the aluminum in
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CA 02312482 2000-OS-31
the molten steel reacts with oxygen in the open air to further produce
alumina.
Therefore, in casting the aluminum-killed steel or the like, the non-
metallic inclusion such as a -alumina adheres and accumulates onto the
surface of the inner bore of the continuous casting nozzle, so that the inner
bore is narrowed or clogged up in the worst case so as to make the stable
casting thereof difficult. Furthermore, the non-metallic inclusion such as
a -alumina adhered or accumulated onto the surface of the inner bore peels
off or falls down, and the non-metallic inclusion thus peeled off or fell down
is entrapped into the cast steel strand, thus degrading the quality of the
cast
steel strand.
In order to prevent the above-mentioned reduction or clogging of the
inner bore of the nozzle caused by the non-metallic inclusion such as a -
alumina, there has widely been used the method in which inert gas is ejected
from the inner surface of the nozzle bore toward the molten steel flowing
through the inner bore so as to prevent the non-metallic inclusion such as a
-alumina existing in the molten steel from adhering or accumulating on the
surface of the inner bore of the nozzle (for example, the method disclosed in
Japanese Patent Publication No. Hei 6-59533/1994).
However, there are problems in the above-mentioned method in
which the inert gas is ejected from the inner surface of the inner bore of the
nozzle, as follows:
When a large amount of inert gas is ejected, bubbles produced by the
inert gas is entrapped into the cast steel strand to cause pinholes in the
cast
steel strand, thus deteriorating the quality of the cast steel. On the other
hand, when a small amount of inert gas is ejected, the sufficient effect of
the
inert gas is not obtained, and non-metallic inclusion such as the a -alumina
is adhered and accumulated onto the surface of the inner bore of the nozzle,
thus causing narrowing or clogging, in the worst case, of the inner bore.
In addition, it is substantially difficult to manufacture the nozzle
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which enable to uniformly eject the inert gas from the surface of the inner
bore of the nozzle toward the molten steel flowing through the inner bore.
Furthermore, when the casting is performed for a long period of time, it
becomes gradually difficult to stably control the amount of ejected inert gas,
since the refractory material of the continuous casting nozzle degrades. As
a result, the non-metallic inclusion such as a -alumina adheres and
accumulates onto the surface of the inner bore of the nozzle in such manner
that the inner bore is narrowed or eventually clogged up.
The clogging of the nozzle by the non-metallic inclusion, particularly
alumina (A12O3) inclusion is deemed to be caused as follows:
(1) Aluminum in the molten steel is oxidized by the entrapped air which
passes through a joint portion of the nozzle refractory and the refractory
structure per se to produce alumina, or silica in the refractory including
carbon is reduced to produce Si0 which supples oxygen to produce alumina.
(2) Alumina inclusion is produced by diffusion and cohesion of the
alumina produced in the above process.
(3) Graphite and carbon on the surface of the inner bore of the nozzle are
taken away in such manner that the feature of the surface of the inner bore
becomes rough, and thus the alumina inclusion is apt to accumulate on the
rough surface of the inner bore.
There is proposed a nozzle as a remedy to solve the above problem,
in which a non-oxide raw material (SiC, Si3N3, BN, ZrB2, Sialon, etc.) that
has a low reactivity with aluminum oxide is added to alumina-graphite
refractory, or a nozzle consisting of the above non-oxide material itself (for
example, refer to Japanese Patent Publication No. Sho 61-38152/1986).
However, it is not practical to add the above non-oxide material to
the widely used alumina-graphite refractory, because the effect of preventing
adhesion is not recognized unless a large amount of the non-oxide material is
added, and furthermore, the corrosion resistance thereof is deteriorated
when a large amount of the non-oxide material is added thereto.
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CA 02312482 2000-OS-31
Also, the nozzle consisting essentially of the non-oxide material is
not suitable for practical use, since the material cost and manufacturing cost
are expensive, while the substantial effect of preventing adhesion may be
expected.
There is further proposed a nozzle, the refractory thereof comprising
graphite-oxide raw material containing CaO, in which an oxide raw material
containing Ca0 (CaO~Zr02, CaO~Si02, 2CaO~Si02, and the like) produces
by a reaction of Ca0 with A1203 a low-melting-point material which is easily
separated from the molten steel (for example, refer to Japanese Patent
Publication No. Sho 62-56101/198'x.
However, since the reactivity of Ca0 with A1203 is apt to be
influenced by a temperature condition of the molten steel in casting, it is
difficult to effectively produce the low-melting-point material. In addition,
a large amount of Ca0 is required to supply when a large amount Of A12O3
inclusion is contained in the steel. However, it is difficult to contain
sufficient amount of Ca0 in the refractory of the nozzle, since spalling
resistance and corrosion resistance thereof are deteriorated. Furthermore,
zirconia (Zr02) is difficult to be separated from the molten steel, since
zirconia in the aggregate flowing into the molten steel from the refractory
has a high specific gravity so that zirconia stays in the molten steel.
SiIMMARY OF E INVENTION
The object of the present invention is to provide a continuous casting
nozzle which may prevents alumina inclusion from adhering and
accumulating on the inner surface of the nozzle, and prevents the inner bore
of the nozzle from being narrowed and clogged so as to enable a stable
casting, by means of forming a glass layer on the surface of the inner bore of
the nozzle when the nozzle is used, thereby preventing air from being
entrapped through refractory structure thus not to produce alumina, and in
addition, smoothing the surface of the inner bore of the nozzle.
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CA 02312482 2000-OS-31
The first embodiment of the present invention is a continuous casting
nozzle for casting molten steel, wherein at least a surface layer of an inner
bore of said continuous casting nozzle contacting with a molten steel is
formed of a refractory comprising:
an aggregate consisting essentially of alumina (A12O3) or an
aggregate comprising alumina (A12O3) as its main ingredient and melting
point thereof being at least 1800 degree centigrade (°C): from 15 to 60
wt. %;
and
roseki as a balance.
The second embodiment of the present invention is a continuous
casting nozzle for casting molten steel, wherein at least a surface layer of
an
inner bore of said continuous casting nozzle contacting with a molten steel is
formed by a process in which binder is added to a refractory material
comprising 15 to 60 wt.% of an aggregate consisting essentially of alumina
(A1203), or an aggregate comprising alumina (A1203) as its main component
and melting point thereof being at least 1800 °C and roseki as a
balance, and
then said refractory material with said binder added is kneaded, formed, and
sintered in an anti-oxidizing atmosphere.
The third embodiment of the present invention is a continuous
casting nozzle for casting molten steel, wherein a mixing weight ratio of
roseki having average grain diameter of up to 250~,m is up to 60 wt.%
relative to a total content of said roseki.
The fourth embodiment of the present invention is a continuous
casting nozzle for casting molten steel, wherein said roseki comprises
pyrophyllite (A12O3'4S1O2'H2O) as its main component.
The fifth embodiment of the present invention is a continuous casting
nozzle for casting molten steel, wherein said roseki comprising roseki which
is calcinated at a temperature of at least 800°C so as to remove
crystal water
therein.
CA 02312482 2000-OS-31
The sixth embodiment of the present invention is a continuous
casting nozzle for casting molten steel, wherein said binder comprises a
thermosetting resin.
FIG. 1 schematically shows a longitudinal cross section of a nozzle
according to the present invention in which the refractory of the invention is
formed at the surface layer of the inner bore of the nozzle contacting with
molten steel.
FIG. 2 schematically shows a longitudinal cross section of a nozzle
according to the present invention in which the refractory of the invention is
formed both at the surface layer of the inner bore of the nozzle and the lower
part (a part immersed in the molten steel) of the nozzle.
The most important feature of the casting nozzle of the present
invention resides in that roseki is used as its main ingredient of the
refractory
material of the nozzle and graphite which is often used in the conventional
nozzle is not contained.
Graphite contained in the conventional nozzle reacts with silica
contained in the nozzle, when the nozzle is used in operation, as follows:
sio2(s) + c(s) = sio(g) + co(g)
3Si0(g) + 2A1= A12o3(s) + 3Si
3C0(g) + 2A1= A12O3 (S) + 3C
According to the above reactions, the silica in the nozzle is
decomposed to produce Si0(g) and CO(g), which become an origin to
supply oxygen to the molten steel, and thus supplied oxygen reacts with
aluminum in the molten steel to produce A12O3.
However, the particles of roseki does not decompose even if carbon
coexists in the molten steel. In particular, the Si02 contained in
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CA 02312482 2000-OS-31
pyrophyllite (A12O3'4SIO2'H2O) or the like which is the main mineral of the
roseki is stable. The above-mentioned fact is acknowledged in the
following manner: a briquette comprising the roseki, resin powders and
carbon powders was formed and buried in a breeze, and heat-treated at a
temperature of 1500°C for 24 hours, and then the thus treated briquett
was
investigated with a microscope to find that the particles did not decay and
bubbles were not produced.
The conventional refractory material with 10 wt.% of graphite added
has a thermal conductivity of 9.8 (kcal/m/hr/°C), whereas the
refractory
material of the invention which does not contain graphite has such a low
thermal conductivity as 2.4 (kcal/m/hr/°C), and excellent heat
resistance.
As a result, the refractory material of the invention shows effective
prevention of metal from being adhered or non-metallic inclusion such as a
-alumina (A12O3) from being precipitated.
In addition, in the conventional nozzle containing graphite, when the
graphite is oxidized, the smoothness of the surface of the inner bore is
lowered. Furthermore, the molten steel flowing through the inner bore of
the nozzle produces turbulence so as to cause the non-metallic inclusion
such as CX -alumina to accumulate on the inner surface of the nozzle.
However, since the surface layer of the inner bore of the nozzle of the
invention does not contain graphite, the smoothness of the surface of the
inner bore is not lowered. In other words, concave and convex portions are
not formed on the inner surface of the nozzle of the invention, thus the non-
metallic inclusion such as Cr -alumina is not accumulated on the inner
surface of the nozzle of the invention.
The half melting temperature of the roseki is around 1500°C, so
that
it melts at the surface of the inner bore of the nozzle contacting with the
molten steel to form a glass coat in such manner that the structure of the
surface of the inner bore becomes to be smooth and air is prevented from
being entrapped through a refractory structure.
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The above-mentioned fact was acknowledged in the following
manner: the refractory comprising alumina-roseki with graphite added is
heat-treated at a temperature of 1500°C for 1 hour in the oxidizing
atmosphere, and the permeability thereof was investigated to find out to be
about 6.5x10-4 darcy, whereas the refractory comprising alumina-roseki
without graphite was heat-treated at a temperature of 1500°C for 1 hour
in
the oxidizing atmosphere, and the permeability thereof was investigated to
find out to be about l.OxlO~ darcy, thus the permeability is lowered.
The roseki is contained in the refractory of the surface layer of the
inner bore of the nozzle of the invention as a balance, i.e., the remaining
ingredient of the refractory. To actively form the glass coat on the surface
of the inner bore when used as a continuous casting nozzle, a mixing weight
ratio of the roseki in the surface layer of the inner bore of the nozzle is
preferably at least 40 wt%. Also, it is preferable that the mixing weight
ratio of the roseki in the surface layer is up to 85 wt%, because with the
mixing weight ratio of the roseki over 85 wt%, degree of softening
deformation is large, and corrosion resistance against molten steel is
deteriorated.
In the continuous casting nozzle of the present invention, the
refractory of the surface layer of the bore of the nozzle comprises roseki and
15 to 60 wt.% of an aggregate consisting essentially Of A12O3 or an
aggregate comprising A1203 as its main ingredient and melting point thereof
being at least 1800 degree centigrade. As the aggregate comprising A1203
as its main ingredient, spinet (MgO. A1203) is used which has a function to
provide the surface layer of the inner bore of the nozzle with strength and
corrosion resistance.
Three kinds of roseki may be used as the above-mentioned roseki,
that is pyrophyllite roseki, kaolin roseki, and sericite roseki. The
pyrophyllite roseki with refractoriness from SK29 to SK32 (SK is a
Japanese Standard for refractoriness ) is the most suitable, because the
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roseki is half molten when the surface layer of the inner bore contacts with
the molten steel to form a glass layer and the erosion resistance thereof
against the molten steel is excellent. On the other hand, the kaolin roseki
has a greater refractoriness from SK33 to SK36, and the sericite roseki has a
smaller refractoriness from SK26 to SK29, both of which are not preferable.
It is preferable to use the roseki calcinated at a temperature at least
800°C to vanish (remove) crystal water. The reason for using the above
roseki is that when the nozzle containing not calcinated roseki is formed and
sintered, the crystal water is released from the roseki at a temperature
within
a range of from 500 to 800°C in sintering thereof, and then, the formed
body
may cracks by virtue of an unusually large thermal expansion coefficient.
It is preferable that a mixing weight ratio of roseki having average
grain diameter of up to 250~m should be up to 60 wt.% relative to the total
content of the roseki, because when a mixing weight ratio of roseki having
average grain diameter of up to 250p,m is over 60 wt.%, structural defects
such as lamination are inclined to be produced in molding and softening
deformation of roseki particles is inclined to happen when used in operation
as a continuous casting nozzle.
Roseki comprising pyrophyllite (A12O3'4S1O2'H2O) as its main
component may be more preferably contained in the refractory within a
range from 65 to 85 wt.%. In the refractory comprising an aggregate
consisting essentially of alumina (A1203), or an aggregate comprising
alumina (A1203) as its main ingredient and the melting point thereof being at
least 1800 degree centigrade, for example, spinet within a range of from 15
to 60 wt.%, and roseki as the balance, roseki particles are not discomposed,
so that oxygen is not supplied into the molten steel, contrary to Si02.
The half melting temperature of the roseki is about 1500°C which
is
almost the same temperature as the casting temperature of molten steel.
Accordingly, the roseki melts at the surface of the inner bore of the nozzle
contacting with the molten steel to form a glass coat in such manner that the
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structure of the surface of the inner bore is smoothed and air is prevented
from being entrapped through a refractory structure, thus preventing alumina
(A12O3) and metal from adhering thereon.
Thermosetting resin, for example, phenol resin, furan resin or the like
is added as a binder within a range of from 5 to 15 wt.% to the above-
mentioned material comprising the roseki and the aggregate, and then, a
formed body of a nozzle is prepared and sintered. It is preferable that the
above-mentioned formed body is prepared by the CIP (Cold Isostatic
Pressing) process, considering that the formed body is uniformly
compressed through the CIP process. The sintering temperature is
preferably within a range of from 1000 to 1300 °C. Reduction
atmosphere,
namely the anti-oxidizing atmosphere is preferable than the oxidizing
atmosphere as the sintering atmosphere because the added thermosetting
resin is not oxidized in the reduction atmosphere.
The continuous casting nozzle for molten steel according to the
present invention is described in detail with reference to the drawings.
Fig. 1 schematically shows a vertical sectional view of the
immersion nozzle for continuous casting according to the present invention.
The continuous casting nozzle 10 is placed between a tundish and a mold,
and used in operation as an immersion nozzle for pouring the molten steel
from the tundish to the mold.
As shown in Fig. 1, a surface layer 2 of the inner bore 1, through
which the molten steel flows, of the immersion nozzle 10 is formed by a
refractory having the chemical composition as described above. The
remaining part of the nozzle 3 is formed by a conventional alumina-gaphite
refractory.
The dimensions of the nozzle are about 1m in total length, about
60mm in diameter of the inner bore, 160mm in outer diameter of the nozzle,
and about SOmm in thickness. The thickness of the surface layer of the
inner bore made of the refractory in the present invention is from about 2
CA 02312482 2000-OS-31
mm to about l5mm. The above-mentioned dimensions are shown as the
example, and the nozzle of the present invention is not limited to the above
dimensions. More specifically, the dimensions vary in accordance with the
composition of the molten steel to be cast, and the size of the cast strand.
FIG. 2 schematically shows a longitudinal cross section of a nozzle
in which the surface layer of the bore of the nozzle and the lower part (a
part
immersed in the molten steel) of the nozzle is made of the refractory
according to the present invention. In either case, alumina which clogs the
inner bore of the nozzle is collected at the inner bore in the lower part of
the
nozzle. According to the immersion nozzle of the present invention, the
non-metallic inclusion such as alumina or the like is prevented from adhering
and accumulating on the surface layer portion 2 of the inner bore 1. The
present invention is described by the examples.
EXAMPLES
Phenol resin in the state of powder and liquid was added in an
amount within a range of from 5 to 10 wt.% to each of nine pieces of mixed
materials having a different chemical composition, and kneaded. From the
thus kneaded materials, a first formed body (hereinafter referred to as the
"formed body 1 ") with dimensions of 30mm x 30mm x 230mm for
investigating an amount of adhesion of non-metallic inclusion such as
alumina and corrosion resistance against the molten steel, a second formed
body (hereinafter referred to as the "formed body 2") with dimensions of
SOmm ~S x 20mm for investigating permeability, and a third formed body
(hereinafter referred to as the "formed body 3") with dimensions of 100mm
in outer diameter, 60mm in inner diameter and 250mm in length for
investigating spalling resistance, were respectively prepared, and then the
respective bodies were sintered in reducing atmosphere at a temperature
within a range from 1000 to 1200°C to prepare samples No. 1 to 9.
The samples No. 1 to 5 (hereinafter referred to as the "sample of the
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present invention") have the chemical compositions within the scope of the
present invention, and the samples No. 6 to 9 (hereinafter referred to as
"sample for comparison") have the chemical compositions out of the scope
of the present invention.
Physical properties (porosity and bulk density) for each of the above-
mentioned samples of the present invention Nos. 1 to 5 and the samples for
comparison Nos. 6 to 9 are shown in Table 1. The spalling resistance of
the samples of the present invention Nos. 1 to 5 and the samples for
comparison Nos. 6 to 9 prepared by the formed body 3 were investigated
after being heated at a temperature of 1500°C for 30 minutes in an
electric
furnace and then rapidly cooled by water. The results thereof are shown in
Table 1.
An erosion ratio (%) and an amount of adhesion of non-metallic
inclusion such as alumina of the samples of the present invention Nos. 1 to S
and the samples for comparison Nos. 6 to 9 prepared by the formed body 1
were investigated after being immersed in molten steel having a temperature
of 1520°C for 180 minutes, which contains aluminum within a range from
0.02 to 0.05 wt%. The results thereof are also shown in Table 1.
The permeability of the samples of the present invention Nos. 1 to 5
and the samples for comparison Nos. 6 to 9 prepared by the formed body 2
were investigated after being heated at a temperature of 1500°C for 60
minutes in an electric furnace and then cooled. The results thereof are also
shown in Table 1.
As is clear from Table 1, all of the samples of the present invention
show excellent spalling resistance, and the non-metallic inclusion such as
alumina are not adhered in spite of the low erosion rate, thereby effectively
preventing narrowing or clogging of the continuous casting nozzle for
molten steel.
Also, it is possible for the samples of the present invention to prevent
air from being entrapped through the refractory in practical use because of
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small permeability.
On the other hand, it is obvious that the sample for comparison No. 6
is remarkably inferior in the corrosion resistance against the molten steel,
although a small amount of alumina adheres due to a large amount of roseki
content.
As for the sample for comparison No. 7, the amount of adhesion of
alumina is remarkably large, because it contains simple alumina (AI2O3) and
simple silica (Si02), in which Si02 decomposes to supply oxygen into the
steel. The sample for comparison No. 8 is remarkably inferior in spalling
resistance, has a high permeability and shows adhesion of large amount of
non-metallic inclusion such as alumina, due to the small amount of roseki
content and the large amount of alumina (A1203), inspite of the mineral
supplying oxygen into the molten steel is removed.
As for the sample for comparison No. 9 which comprises graphite,
roseki and alumina (A1203), the amount of alumina adhesion is slightly large
and the amount of metal adhesion is large when the temperature of the
molten steel is as low as 1520~ 10°C due to the containing of graphite.
Therefore, with the use of the continuous casting nozzle according to
the present invention, it is possible to perform stable casting without
deterioration of the refractory structure, while preventing narrowing or
clogging of the bore caused by the non-metallic inclusion such as alumina.
According to the present invention, when 300 ton per charge of a
low carbon aluminum killed steel was continuously cast by 2 strand slab
caster, 5 to 7 charges of the steel were cast with the use of single nozzle
without clogging.
Meanwhile, according to the conventional nozzle, when 2 to 4
charges of the steel were cast, the nozzle was clogged up and the casting
had to be interrupted. As mentioned above, the effect of this invention was
remarkable.
13
CA 02312482 2004-09-15
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