Abstract: The formation mechanism and evolution of inclusions in SAE8620RH gear steel produced by EAF→LF→VD→CC process in a domestic steel mill were studied by SEM-EDS and thermodynamic calculation. The results show that the oxygen-sulfur composite inclusions in steel mainly exist in the composite form of  MgO·Al₂O₃ as the core and CaS wrapped outside. The experimental results show that the inclusions in liquid steel at the early stage of LF refining are mainly MgO·Al₂O₃, with a small amount of CaS wrapped around the outside. After Ca treatment, part of MgO·Al₂O₃ is modified into liquid calcium aluminate. After VD vacuum treatment, the CaS proportion of MgO·Al₂O₃ coating increased obviously. MgO·Al₂O₃ in the billet precipitates MnS again, forms a composite form of MgO·Al₂O_3-(Ca,Mn)S. Thermodynamic calculation results show that MgO·Al₂O₃ can be generated when the mass fraction of Al in liquid steel is 0.03 % and the mass fraction of Mg is 1.85×10^-6. In the early stage of LF refining, CaS is mainly generated by the direct reaction of [S] and [Ca], while in VD vacuum process, [S] and [Ca] in liquid steel will further reacted to form CaS, and calcium aluminate outside MgO·Al₂O₃ will also reacted with [S] to form CaS.

Key words: SAE8620RH gear steel, inclusions, formation mechanism, evolution mechanism