Castability of Aluminum- and Sulfur-bearing Free-cutting Steel

Abstract

Submerged entry nozzle (SEN) blockage is one of the main problems during continuous casting of Al and S-containing steel, which has a negative effect on the smooth production and product quality. The blockage deposits mainly consisted of MgO·Al2O3 spinel, calcium sulfide, or (and) high-melting-point calcium aluminate. To understand completely the formation behavior of non-metallic inclusions and provide an optimization direction for improving castability, thermodynamic discussions on the generation characteristic of non-metallic inclusions before liquid steel flow through SEN were conducted. In addition, an industrial trial comprising of 10 heats made on five casting sequences was performed, including not only Al- and S-containing free-cutting steel, but also a medium-carbon Al-killed steel and a high-carbon Al-deoxidized steel. It was found that inclusions transferred into the MgO·Al2O3 spinel from pure Al2O3 and complex inclusions during vacuum degassing (VD) treatment. Two kinds of MgO·Al2O3 spinel inclusions were observed in the VD-treated steel. One is fine homogeneous inclusion of 1–2 μm in diameter, and the other is 10 μm-sized heterogeneous inclusions, which consisted of an Al2O3 core surrounded by a spinel coating. It was suggested that the former was generated from the mutual combination of fine MgO and Al2O3 particles produced from the erosion of the refractory materials during the VD process, and the latter was considered to be generated from the local reduction of large residual Al2O3 particles by soluble [Mg] provided by the decomposition of MgO in the ladle line during VD treatment. It was also discovered that CaS mainly results from steel-slag desulfurization during soft stirring.