Since various kinds of steel contain chemical elements selected in accordance with the use of the steel, refratory parts used for casting the steel, such as a nozzle, can be improved in such a manner that they have a higher chemical stability with respect to the various elements.
A sintered body with a chemical stability which is improved over that of a conventional sintered body must also be so flexible that it can absorb in itself any radial expansion force occurring in the sintered body due to the intense heat applied thereto when it is used, to prevent the sintered body from breaking. In addition, it is necessary that such a sintered body has a sufficient heat-insulating capacity to prevent the molten steel or non-metallic inclusions therein from being deposited on the surface thereof.
Providing a nozzle for steel casting with this chemical stability is very important in improving its corrosion resistance. Improving the chemical stability of a nozzle with a texture which can withstand the high frictional force generated by the molten steel of a high specific gravity, causes the toughness thereof to decrease, so a nozzle with an improved chemical stability is not suitable for practical use. The chemical stability of a nozzle, which prevents melting losses which could be caused by molten steel, includes the capacity of preventing the deposition of molten steel onto the superficial layer of the nozzle to increase the thickness thereof and thereby block the nozzle. In any case,the nozzle must have a chemical stability that prevents the occurrence of reactions between the nozzle and the components of the molten steel, inasmuch as these reactions would result in degeneration of the nozzle.
U.S. Pat. No. 4,069,060 is directed to an improvement in conventional Al.sub.2 O.sub.3 -SiC refractory material. The improved refractory material is obtained by adding pulverised Al.sub.2 O.sub.3 to Al.sub.2 O.sub.3 SiC, which is known as refractory clay, and mixing them together, molding the mixture thus obtained, drying the resultant molded product, and finally calcining the dried product in an atmosphere of CO and CO.sub.2. The final product contains 2-35% by weight of corundum and SiC.
U.S. Pat. No. 4,104,075 concerns Al.sub.2 O.sub.3 -SiC refractory material containing TiN (titanium nitride). This refractory material necessarily has a high thermal resistance and a high corrosion resistance, which are properties peculiar to TiN. These properties of TiN are identical with the characteristic properties of SiC. However, SiC oxidizes readily, and the properties thereof are easily lost. Therefore TiN is utilized as a component to compensate for the loss of the properties of SiC.
U.S. Pat. No. 4,210,264 concerns a nozzle of a refractory material for continuous casting, which has a second layer of a refractory material containing 70-90% by weight ZrO.sub.2 formed on the outer surface of a refractory nozzle body, to increase the life of the nozzle.