Modification of sulphide inclusions in Q355C steel by rare earth Ce and industrial tests

This paper systematically investigates the modification effect of rare earth Ce on sulphide inclusions in Q355C steel. Thermodynamic calculations and experiments revealed that after Ce addition, MnS inclusions precipitate using rare earth inclusions as nucleation cores, forming spherical or ellipsoidal rare earth-containing composite inclusions. With increasing Ce content, the nucleation core for MnS showed a regular evolution: Al₂O₃ → CeAl₁₁O₁₈–Al₂O₃ → CeAl₁₁O₁₈ → CeAl₁₁O₁₈–CeAlO₃ → CeAlO₃ → Ce₂O₂S–CeAlO₃. Industrial experiments on rare earth alloying of Q355C steel show that in the absence of rare earth elements, the main inclusions in steel are Al₂O₃, MnS, (Ca,Mn)S and composite Al₂O₃-(Ca,Mn)S inclusions. After rare earth Ce is added in the form of Ce-Fe alloy, Al₂O₃ and sulphide inclusions in steel are modified into CeAlO₃-Al₂O₃-(Ca,Mn)S composite inclusions. The average size of sulphur-containing inclusions decreases from 2.51 μm to 2.07 μm. Further laboratory investigations reveal that the addition of rare earth Ce can remarkably refine the size of MnS inclusions. When rare earth Ce with a content of 60 ppm is added via Al-Ce alloy, the average size of sulphide inclusions is reduced to 1.69 μm, presenting the optimal modification effect.