Nonmetallic inclusions are harmful to the quality of 42CrMo4 steel. Therefore, the formation and removal mechanism of inclusions in 42CrMo4 steel during the steelmaking process is investigated by industrial trials. The characteristics of inclusions in specimens were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The main type of inclusions in molten steel in the early stage of ladle furnace (LF) refining is MgO-Al₂O₃ inclusions of irregular shape. CaO begins to appear in MgO-Al₂O₃ inclusions in the middle and late stages of LF. In the vacuum degassing (VD) refining stage, the inclusions in molten steel completely change into low-melting-point CaO-MgO-Al₂O₃ inclusions. The existence of [Mg] in molten steel is the fundamental reason for the formation of a large number of MgO-Al₂O₃ inclusions. Thermodynamic calculation shows that the refractory mainly transfers [Mg] to the liquid steel in the LF refining stage, whereas the slag mainly transfers [Mg] to the liquid steel in the VD refining stage. Kinetic calculation indicates that MgO-Al₂O₃ inclusions could be removed from molten steel faster than low-melting-point CaO-MgO-Al₂O₃ inclusions. The fundamental reason for the different removal behavior of the two types of inclusions is that the interfacial tension between the low-melting-point CaO-MgO-Al₂O₃ inclusions and the liquid steel is 50% lower than that of the MgO-Al₂O₃ inclusions.

Few reports exist on the effect of the basicity of refining slag on inclusions in 15-5PH stainless steel and its removal efficiency. In this study, the effects of various basicities on the formation and removal efficiency of inclusions in molten steel were investigated. To investigate the effect of the chemical makeup of slag on the non-metallic inclusions in liquid steel, laboratory experiments and thermodynamic calculations were conducted on CaO-MgO-SiO₂-Al₂O₃-CaF₂ slag with various slag basicities and 15-5PH stainless steel. In the steel samples that had reacted with high-basicity slag samples, the magnesium content and aluminum yield were higher. Thermodynamic findings according to the ion and molecule coexistence theory showed that log ($a_{{\mathrm{SiO}}_2}^3/a_{{\mathrm{Al}}_2{\mathrm{O}}_3}^2$) decreases as slag basicity increases. This increases the Al concentration in liquid steel while decreasing the Si content. Log ($a_{\mathrm{MgO}}^3/a_{{\mathrm{Al}}_2{\mathrm{O}}_3}$) also increases, increasing the Mg content of the molten steel. With this, the transformation order of oxide inclusions is Al₂O₃ → MgAl₂O₄ → MgO. High-basicity slag increases the attachment of slag to inclusions and generates MgAl₂O₄ inclusions that are more easily adsorbed by inclusions in molten steel, thereby improving the cleanliness of molten steel.