Abstract

The clogging of a submerged entry nozzle (SEN), caused by the alumina inclusions that precipitate from steel, is a serious problem for the continuous casting operation and steel quality. It is known that the addition of calcium (Ca) to the steel is effective for preventing the clogging because Ca reacts with alumina inclusions, forming calcium aluminates, which have low melting points and do not form precipitated layers. However, it was observed that the corrosion of Al2O3-ZrO2-C (AZC) slide gate plates increased when they are used for Ca-added steel casting. Therefore, it became necessary to improve the corrosion resistance of the AZC slide gate plates.

Based on the microscopic observation of used AZC slide gate plates, it became clear that when these slide gate plates were used for Ca-added steel casting, they developed many more pores than the original material, and calcium aluminates penetrated into the slide gate plate structure, along the pores. It was considered that because more pores developed in the structure, the corrosion of the AZC slide gate plates was accelerated.

To clarify the porosity increase mechanism of the Al2O3-ZrO2-C slide gate plate structure, the gas generation from the refractory materials was thermodynamically investigated. The thermodynamic calculation was conducted for the condition where solid state Al2O3, ZrO2, mullite and C, which are components of the refractory materials, coexisted with pure Fe or Fe-Ca in the liquid condition at 1550C (1823K). The analysis indicated that the partial pressure of SiO gas generated in the slide gate plate material, when used with Ca-added steel, was much higher than the cases of the other steels. Therefore, it was suggested that it is necessary to suppress the gas generation from the slide gate plate materials, and maintain the dense structure, to improve the corrosion resistance of the Al2O3-ZrO2-C slide gate plates.

So, the sample refractory material which consisted of Calcium Hexaaluminate(CA6) aggregates used for suppressing the SiO gas generation because of its stableness as the solid state was tested by the corrosion test using the Ca-added steel at 1550C (1823K). Then, it was revealed that the corrosion resistance of the sample material increased by 20% compared with the conventional AZC slide gate material.