Characteristics and Formation Mechanism of Oxide-Sulfide Complex Inclusions in High-Speed Railway Wheel Steel

The deleterious effects of brittle and rigid oxides on the quality of high-speed railway wheel steel can be mitigated by wrapping the oxides with sulfides. The investigation reveals the presence of three types of oxide-sulfide complex inclusions in the continuous casting billet. Type 1 possesses a core oxide with high CaO content, and its shell sulfide predominantly comprises CaS. Type 2 presents a core oxide with low CaO content, accompanied by a (Mn,Ca)S shell. The core oxide in Type 3 contains almost no CaO, and the shell sulfide is primarily MnS. The area ratio, defined as the ratio of sulfide area to oxide area, increases successively among the three types of complex inclusions. The formation mechanisms of the three types of complex inclusions vary. In Type 1, the core oxide originates from the refining process. The CaS in the shell sulfide is a product of the reaction between Al and S in the liquid steel and CaO in the oxide. In Type 2, the core oxide is formed during the cooling and solidification process. As the liquid steel solidifies, the segregation of Ca, Mn, and S leads to the formation of (Mn,Ca)S shell around the oxide. The formation temperature of the core oxide in Type 3 is lower than that of Type 1 and Type 2, occurring in the late stages of solidification. A substantial amount of MnS is formed around the oxide near the end of solidification. In the finished wheel, Type 2, characterized by its (Mn,Ca)S shell, exhibits appropriate hardness and resistance to deformation, rendering it the most optimal among three types of complex inclusions.