Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-08-24
2002-10-08
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S100000, C501S101000, C501S104000, C501S105000, C501S108000, C501S119000, C501S120000, C501S121000, C501S125000, C501S127000
Reexamination Certificate
active
06461991
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an alumina-magnesia-graphite refractory. In particular, the present invention relates to a reusable alumina-magnesia-graphite refractory which is suitably applied to refractory products for continuous casting, such as a long nozzle which is used for pouring a molten metal from a ladle into a tundish, a submerged nozzle which is used for pouring the molten metal from the tundish into a mold, and a long stopper which controls a flow rate of the molten steel.
BACKGROUND ART
In recent years, refractory products for continuous casting require to be a long life material excellent in corrosion resistance in order to reduce unit consumption and unit cost. Alumina-graphite refractories have been used as desirable refractories which fulfill these requirements.
For example, in practical applications of such a refractory product for continuous casting, it is gradually accepted as a common way that a long nozzle or a long stopper is saved after completing a casting operation and then reused for another casting operations, even though these nozzle and stopper were heretofore replaced with new one on each casting operation.
However, such refractory product after worked under thermal load from molten steel causes deterioration in physical properties, specifically in thermal shock resistance, compared to that in its initial use.
Japanese Patent Publication No. Sho 47-49409 discloses an alumina-graphite refractory as one refractory having an improved thermal shock resistance for use in a long nozzle and a long stopper for continuous casting, wherein a fused silica having a small thermal expansion properties is added, and this refractory has been commonly used. However, when an amount of the added fused silica is increased in order to prevent a deterioration of thermal shock resistance for reuse, a problem of a deterioration of corrosion resistance is brought out, because the fused silica is easily corroded by a slag.
An alumina-graphite refractory being free of fused silica shows excellent corrosion resistance, while its elastic modulus is increased due to mutually sintered alumina particles caused by thermal load from received molten steel, resulting in a deteriorated thermal shock resistance and its thermal shock resistance is essentially not so high due to its large thermal expansion coefficient. Thus, on reuse of this type of refractory, a problem causing cracks or shakes is more easily coming out than the case of initial use of a new refractory product.
In this specification, the word of “reuse” does not include usually a case of storing temporarily a long nozzle or long stopper under a condition of heat insulation after a casting operation until a next casting operation before complete cooling. This is referred as “Intermittent Use” which should be in distinction from “reuse”, strictly.
Even under the intermittent use, there is, however, caused a problem similar to that in reuse because the long nozzle or long stopper which worked under a severe thermal load through a casting operation causing large thermal shock during an initial stage of a following casting operation.
The reuse or intermittent use of long nozzles and long stoppers is not common in submerged nozzles. However, in some cases where different kinds of steels are continuously cast, at a time when several minutes passes after a submerged nozzle has been pulled up out of a mold, the submerged nozzle is used again. In this case, the submerged nozzle is left standing to cool during the interrupted operation. Thus, when the submerged nozzle receives molten steel again, the submerged nozzle would be used under the substantially same condition as the intermittent use condition of a long nozzle.
Among various refractory materials, magnesia particularly has a high melting point and is thereby excellent in corrosion resistance. In addition, magnesia is a relatively inexpensive material and thereby economically useful. However, magnesia has an extremely large thermal expansion coefficient compared to alumina. Thus, when the amount of magnesia added to a refractory is increased, thermal expansion coefficient of the refractory is increased, resulting in a deteriorated thermal shock resistance of the refractory. Consequently, magnesia-graphite refractories are applied only to specific regions, such as a protection sleeve of a temperature measuring probe having a small sleeve shape which is advantageous to restraining occurrence of thermal shock as disclosed in “Refractories” 48 [11] 606 (1996), or fitting portions of stopper heads and fitting portions of nozzles wherein slim thermal shock is occurred as disclosed in “Refractories” 48 [11]608 (1996).
An alumina-graphite material including added magnesia has been known. For example, Japanese Patent Laid-Open Publication No. Sho 58-120569 discloses an alumina-graphite material wherein magnesia is added by 35 to 70% in order to prevent a non-metallic inclusion, such as alumina, from being attached to a refractory product. Japanese Patent Laid-Open Publication No. Sho 61-232266 and Japanese Patent Laid-Open Publication No. Sho 61-215251 also disclose an alumina-graphite material wherein magnesia is added by 0.1 to 5.0%, and Japanese Patent Laid-Open Publication No. Sho 59-3069 discloses an alumina-silicon carbide-carbon material wherein magnesia is added by 0.5 to 4.0%.
However, in these known alumina-graphite materials, magnesia is added to act as a sintering aid of alumina and thereby leads to increase elastic modulus. Accordingly, these materials are fundamentally inappropriate to reuse.
Thus, the conventional alumina-graphite materials including added magnesia cannot avoid a deterioration of corrosion resistance and thermal shock resistance in reuse and intermittent use.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide an alumina-magnesia-graphite refractory which has a low deterioration of corrosion resistance and thermal shock resistance under a reuse condition and thereby allows reuse or intermittent use.
As the result of studying an effect of adding magnesia to an alumina-graphite refractory, it has been found that the refractory was improved in corrosion resistance by using a particular grain size of magnesia. It has been also found that a void was also created around the magnesia through thermal load by defining the added amount within a particular range, wherein the void made a contribution to an improvement of thermal shock resistance so that a deterioration level in reuse of the refractory could be minimized.
When suffering thermal load from received molten steel, conventional alumina-graphite refractories are generally deteriorated in thermal shock resistance to a large degree due to significantly increased elastic modulus caused by mutually sintered alumina particles. In contrast, when the alumina-graphite refractory including the added magnesia receives thermal load during a casting operation, the added magnesia is deoxidized by a surrounding carbon and a resulting gaseous metallic magnesium then creates a spinel by reacting with a surrounding alumina. During this reaction, the void is created around the magnesia grains. This void manifests a buffering function to stress so that the elastic modulus of the refractory is restrained to increase. In addition, the spinel created around the void prevents that strength of the refractory is reduced in connection with the created void so that a ratio of strength to elastic modulus may become larger and thermal shock resistance may resultingly be improved.
For improving durability of a long nozzle and a long stopper, corrosion resistance of the long nozzle or long stopper portion corresponding to a slag level should be enhanced. Since an inner wall of a tundish is generally provided with a coating layer including magnesia, some magnesia is inevitably contained in a slag. In this background, it has been found that the addition of magnesia was advantageous to improving corrosion resistance of the long nozzle/long stopper because a difference in
Iitsuka Shoji
Ogata Koji
Jordan and Hamburg LLP
Krosaki Corporation
Marcantoni Paul
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