Requirements for refractory materials and "three major parts" in stainless steel continuous casting

The main technical requirements for continuous casting refractory materials are: resistance to high temperatures above 1600~1650℃; good thermal shock resistance; high high-temperature flexural strength and compressive strength; small changes in thermal expansion and contraction; good resistance to slag corrosion; high resistance to molten steel impact; convenient construction and low cost. For stainless steel, since this type of steel requires the carbon content to be as low as possible, the refractory material is required not to easily cause carbon increase in molten steel, nor to bring inclusions to the molten steel.

In order to meet the requirements of refractory materials for special steel continuous casting, after several years of efforts, my country has initially established a refractory material system for special steel continuous casting. Their configuration is: for steel flow protection between ladle and tundish, a baked A12O3-C long nozzle (protection tube) containing about 40% A12O3 is used; the tundish uses an A12O3-C integral stopper containing about 50% A12O3; for molten steel protection between the tundish and the crystallizer, a combined and integral A12O3-C immersion nozzle containing 40% to 50% A12O3 is used; the tundish lining is a magnesium insulation board; fused quartz products are also used.

At present, there are problems of nozzle blockage, stopper rod argon blowing control, and refractory corrosion resistance and pollution of molten steel in stainless steel continuous casting. The "three major parts" of continuous casting refer to the long nozzle (also known as ladle protection sleeve) between the large ladle and the medium ladle, the stopper rod, and the immersion nozzle between the medium ladle and the crystallizer. The functions of the long nozzle and the submerged nozzle are to guide the flow, prevent the splashing and oxidation of molten steel, and adjust the flow state of molten steel. The bowl of the long nozzle is connected to the lower nozzle of the ladle sliding nozzle. Due to the rapid flow of molten steel, negative pressure is generated at this place and air is sucked in, causing the molten steel to be secondary oxidized to form mixed and contaminated molten steel.

In order to prevent the secondary oxidation of molten steel between the ladle and the intermediate tank, argon sealing measures must be taken, including argon sealing of the ladle sliding nozzle system and argon sealing of the long nozzle from the ladle to the intermediate tank. At present, aluminum carbon long nozzles are used, in which A12O3 is 50%~55% and C content is 28%~31%. The submerged nozzle guides the molten steel in the tundish into the crystallizer. Due to contact with molten steel and slag, the refractory containing oxides and graphite is initially covered with a slag film, which wets the oxides, melts them in a favorable condition for graphite, and produces a graphite-rich layer on the surface.

The molten steel wets and dissolves the graphite in a condition that is favorable to the oxide, resulting in the disappearance of the graphite layer, and the surface of the refractory material is covered with a slag film, and this process is repeated. In other words, the oxide is eroded by the slag, and the graphite is eroded by the molten steel. For stainless steel continuous casting, the refractory materials of the long nozzle and the submerged nozzle are required to have excellent thermal shock resistance and slag erosion resistance, high resistance to molten steel erosion, and high high temperature strength.