Effect of refining slag composition on the evolution and removal of large inclusions in 50Cr5MoV roll steel

Large Al 2 O 3 and MgO·Al 2 O 3 spinel inclusions tend to remain in 50Cr5MoV roll steel at the end of secondary refining, seriously impairing the quality stability of steel ingots. In this study, an optimised refining slag design method based on the coordinated control of basicity (CaO/SiO 2 ) and the CaO/Al 2 O 3 ratio was proposed. By combining FactSage thermodynamic calculations, industrial trials, microscopic characterisation, and a kinetic model for inclusion separation at the steel-slag interface, a slag composition window of 49–51 wt.% CaO, 7 wt.% MgO, 9–10 wt.% SiO 2 , and 26–27.5 wt.% Al 2 O 3 was determined, corresponding to a CaO/Al 2 O 3 ratio of 1.8–2.0 and a CaO/SiO 2 ratio of 5.0–5.5. After optimisation, the slag entered a high-liquid-fraction region dominated by low-melting-point C12A7, and the effective activity of CaO increased. This improved Ca utilisation and promoted the evolution of inclusions toward CaO-rich and Al 2 O 3 -lean calcium aluminate compositions. Meanwhile, the inclusion–slag interfacial tension and interfacial rebound effect were reduced, facilitating inclusion transfer, absorption, and removal by the slag. After optimisation, the number density of inclusions larger than 5 μm decreased significantly, the maximum inclusion size was reduced from approximately 19 μm to about 10 μm, and the ultrasonic inspection pass rate of the ingots increased from 89.7% to 96.7%.