Formation of Plastic Inclusions in U71Mnk High-Speed Heavy-Rail Steel Refined by CaO-SiO₂-Al₂O₃-MgO Slag

To precisely control the characteristics of nonmetallic inclusions with high plasticity in U71Mnk high-speed heavy-rail steel deoxidized by Si and Mn, the chemical composition of CaO-SiO₂-Al₂O₃-MgO slag in the ladle furnace (LF) refining process was optimized by thermodynamic calculations. Relationships among the refining slag, nonmetallic inclusions, and elemental concentrations in the steels were analyzed to obtain the conditions for determining the optimal composition of refining slag. The optimal compositions and the component activities of CaO-SiO₂-Al₂O₃-MgO refining slag were determined and discussed according to calculations using FactSage 7.0. Finally, ten industrial experiments were performed. Variations in the compositions of LF refining slag, inclusions, and steel, as well as the sizes of the inclusions, and metallographic classification assessment of the U71Mnk steel product, were investigated and discussed to verify the feasibility and effectiveness of the optimized LF refining slag. Kinetics of steel–slag and steel-inclusion reactions during LF refining was studied and compared. The results show that to obtain low [O], [Al], and [S] contents and inclusions with high plasticity in molten steel, the optimal composition of the LF refining slag should be CaO: 44.2 to 51.7 wt pct, SiO₂: 41.4 to 47.0 wt pct, Al₂O₃: 0 to 4.7 wt pct and MgO 5.0 to 7.0 wt pct, where the basicity R is above 1.00 and the C/A ratio is above 9.00. After the LF refining process, the contents of total oxygen (T.O.), [Al], and [S] gradually stabilized in the range of 0.0008 to 0.0012 wt pct, 0.002 to 0.0028 wt pct and 0.0026 to 0.0037 wt pct, respectively. The densities of the three kinds of typical inclusions (CaO-SiO₂-Al₂O₃-MgO, MnS, and CaO-SiO₂-Al₂O₃-MgO-MnS) also decreased and reached approximately 0.25/mm², and no inclusion larger than 6.0 μm was found. The composition of the inclusions gradually changed into the optimal range in the CaO-SiO₂-Al₂O₃-MgO slag system during the LF refining process.