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.