Effect of MgO on solidification crystallisation behaviour of high SiO 2 content MnO–SiO 2 –Al 2 O 3 -based inclusions in Si–Mn-killed steel

Abstract

Aiming at avoiding the formation of high-hardness crystalline phase in inclusions, the effect of MgO on the solidification crystallisation behaviour of high SiO₂ content MnO–SiO₂–Al₂O₃-based inclusions was investigated. The results showed that the high SiO₂ content MnO–SiO₂–Al₂O₃ inclusions precipitated the pure SiO₂ phase during the solidification process. With the increase of MgO content in inclusions, the content of the SiO₂ phase decreased continuously, while the MgSiO₃ phase began to precipitate and the content increased gradually. Besides, 20 wt.% MgO content inclusions first precipitated the Mg₂SiO₄ phase, and the MgSiO₃ phase was formed in the subsequent solidification process. Thermodynamic calculation results indicated that the beginning of crystallisation temperature first decreased and then increased with the increase of MgO content in inclusions. However, the calculation results were partially different from the experiment results, because inclusions became the undercooling system during solidification and led to crystallisation hysteresis. The increase of MgO content in inclusions decreased the content of bridging oxygen and resulted in the depolymerisation of complex network structures, which promoted the crystallisation of inclusions. Moreover, the crystalline phase formed in inclusions transformed along the route of “SiO₂ phase–MgSiO₃ phase–Mg₂SiO₄ phase” with the increase of MgO content in inclusions.