Abstract: To investigate the controlling effect of magnesia inclusions on the sulfide morphology in sulfur-containing gear steel, this study performed magnesium treatment on 40Cr steel bearing sulfur. Combining thermodynamic analysis, the evolution behavior of inclusions during smelting and rolling processes was analyzed. The research findings revealed that after introducing metallic magnesium into the molten steel, the Al₂O₃ inclusions in the steel could be transformed into dispersed MgAl₂O₄ inclusions. During the solidification process, the transformed MgAl₂O₄ inclusions could serve as the precipitate core of sulfide manganese, forming multiphase inclusions. During the rolling process, the core MgAl₂O₄ further played a skeletal role, inhibiting the extension of surrounding sulfide manganese. This effectively reduced the aspect ratio of sulfide manganese and is expected to improve the cutting and mechanical performance of gear steel. The research results also indicated that to fully utilize the skeletal role of MgAl₂O₄, it was necessary to ensure a sufficiently high dissolved magnesium content in the steel for the full transformation of Al₂O₃, and to promote suitable particle size and quantity of MgAl₂O₄ to serve as an effective core for controlling sulfide manganese precipitation and deformation.

Key words: gear steel, magnesium treatment, manganese sulfide, morphological control