The performance and behavior of submerged nozzle directly affect the continuous casting efficiency and slab quality. It is required that the submerged nozzle shall be resistant to the corrosion of molten steel and mold flux, and shall not react with substances in molten steel to form blockage. The original submerged nozzle was made of fused silica, but its corrosion resistance was poor, which could not meet the needs of multi furnace continuous casting and clean steel production. At present, the composite submerged nozzle whose body is Al2O3-C and slag line composite zro2-c is mainly used. On the premise of meeting the thermal shock resistance, the corrosion resistance of slag line material to mold flux and the resistance to Al2O3 nodulation inside the nozzle are the key factors determining the service life of the submerged nozzle.
- Alumina blockage and its preventive measures
The Al2O3-C or Al2O3-ZrO2-C submerged nozzle used in casting some special steels and Al or Al Si killed steels often produces Al2O3 nodulation, resulting in instability of liquid steel flow, even blockage of nozzle, destroying normal casting flow and affecting billet quality. It has become the main obstacle to the realization of multi furnace continuous casting and improving continuous casting efficiency of Al2O3-C or Al2O3-ZrO2-C submerged nozzle. The possible reasons for the nodulation of alumina on the inner wall of the nozzle are as follows:
One is deoxidation of molten steel to produce Al2O3;
Second, SiO2 and carbon contained in the refractory promote the formation of Al2O3, and its reaction is as follows:
SiO2(s)+C(s)=SiO(g)+CO(g)
3SiO(g)+2Al(l)=Al2O3(s)+3Si(l)
3CO(g)+2Al(l)=Al2O3(s)+3C(s)
Thirdly, Al2O3 particles contact and adhere to the inner wall of submerged nozzle under the action of surface tension of alumina and molten steel. In order to reduce Al2O3 nodulation, the following measures can be taken from the perspective of mechanism: making the deoxidized product Al2O3 low melting point, for example, treating molten steel with CA to change Al2O3 into Al2O3 Cao system low melting point material; Degassing technology is used to purify molten steel; Control the temperature drop of molten steel; Change the lining material of the nozzle so that one of its components acts with Al2O3 at high temperature, forming a low melting point phase at the interface between the molten steel and the refractory, which is taken away by the molten steel and enters the slag to eliminate the deposition of Al2O3. For example, the inner wall of the nozzle is compounded with Cao containing substances; Silicon free and carbon free nozzle lining material is adopted to fundamentally reduce the formation of Al2O3.
At present, the research on preventing Al2O3 clogging submerged nozzle mainly focuses on the lining material of nozzle. Materials that have been researched and developed include: sialon-zro2, cao-mgo-al2o3, zro2-zrb2-c, bn-aln-c, zro2-cao-c, etc. Recently, silicon free and carbon free lining materials have been developed. According to the principle that alumina reacts with a component of refractory to form a low melting point material, the developed zro2-cao-c material has been successfully applied in practice. However, according to specific steel grades and casting conditions, it is difficult to determine the CaO content in the lining material that achieves the best anti Al2O3 plugging effect. Moreover, with the casting process, graphite oxidation often causes the rough working surface of the inner wall, so it is difficult to obtain a better anti alumina plugging effect.
Japan has recently developed and tried the silicon free and carbon free submerged nozzle lining material. Through the internal wall composite spinel material, the real machine casting test shows that the Al2O3 nodulation is significantly reduced, the internal wall working surface is smooth after casting, and the material has good thermal shock resistance. Because this kind of material fundamentally reduces the source of alumina, it is a very promising lining material to prevent alumina from blocking. This kind of lining material is also more suitable for smelting clean steel and ultra-low carbon steel. At present, China has successfully developed zro2-cao-c materials, which have been successfully applied in Wuhan Iron and Steel Group, Tianjin Seamless and other manufacturers. In order to meet the needs of high-purity steel smelting in China, the development of silicon-free and carbon-free submerged nozzle lining material is of great significance.
- Erosion resistance of slag line material
Corrosion resistance of slag line of submerged nozzle is another important factor affecting its service life. At present, zro2-c material is widely used at the slag line. Compared with the previous Al2O3-C material, the corrosion resistance has been significantly improved. Generally speaking, zro2-c material with 15% carbon content has better comprehensive properties.
The erosion mechanism of zro2-c material includes: oxidation and dissolution of carbon caused by molten steel; Cao, the stabilizer of ZrO2, reacts with corrosive components in the slag such as SiO2 and Na2O to desolvate, resulting in the bursting of some stable zirconia into fine monoclinic zirconia. The former results in the embrittlement of the microstructure of zro2-c material, while the latter results in the generation of m-ZrO2 and glass phase, and the zro2-c material is washed away into the slag with the oscillation of the cleaner. The corrosion resistance of zro2-c material faces a more severe test when casting some special steels or under some specific casting processes, such as casting high oxygen steel, limiting the casting of tundish ladle and thin slab continuous casting. It is impossible to achieve high-efficiency multi furnace continuous casting.
In order to further improve the corrosion resistance of slag line material, the nozzle structure and material composition can be considered. According to the corrosion mechanism of zro2-c material, reducing the carbon content in zro2-c material may improve its corrosion resistance, but the reduction of carbon content will sacrifice the thermal shock resistance. Therefore, Didier has developed a submerged nozzle composed of three layers at the slag line.