Characteristics, Formation Mechanism, and Removal of Large-Sized CaO–Al2O3–SiO2 Inclusions in D2 High-Speed Railway Wheel Steel

The characteristics, formation mechanism, and removal of large-sized CaO–Al₂O₃–SiO₂ inclusions in D2 high-speed railway wheel steel are investigated. Large-sized CaO–Al₂O₃–SiO₂ inclusions are present in the continuous casting billet, with SiO₂ and CaO content ranging from 5 to 15 and 30 to 65 wt%, respectively. The size mainly exceeds 10 μm. These inclusions originate from the calcium-treatment stage of the refining process, during which liquid CaO–Al₂O₃–SiO₂ inclusions are formed in the liquid steel. The contact angle between these inclusions and the liquid steel is below 40°, which results in excellent wettability. Consequently, the inclusions are difficult to remove from the liquid steel and are thus inherited into the billet. By reducing the SiO₂ content and controlling CaO content between 8 and 30 wt%, small-sized inclusions are formed. This requires reasonable control of the Al and Ca content in the liquid steel. When the Al and Ca content in liquid steel is maintained at 0.012 wt% and 8 ppm, respectively, the inclusions in the billet are mainly CaO·6Al₂O₃ (CA₆) and CaO·2Al₂O₃ (CA₂), both under 5 μm in size. These inclusions represent the suitable inclusions in steel.