Effect of MgO on solidification crystallisation behaviour of high SiO₂ content MnO–SiO₂–Al₂O₃-based inclusions in Si–Mn-killed steel

Aiming at avoiding the formation of high-hardness crystalline phase in inclusions, the effect of MgO on the solidification crystallisation behaviour of high SiO 2 content MnO–SiO 2 –Al 2 O 3 -based inclusions was investigated. The results showed that the high SiO 2 content MnO–SiO 2 –Al 2 O 3 inclusions precipitated the pure SiO 2 phase during the solidification process. With the increase of MgO content in inclusions, the content of the SiO 2 phase decreased continuously, while the MgSiO 3 phase began to precipitate and the content increased gradually. Besides, 20 wt.% MgO content inclusions first precipitated the Mg 2 SiO 4 phase, and the MgSiO 3 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 “SiO 2 phase–MgSiO 3 phase–Mg 2 SiO 4 phase” with the increase of MgO content in inclusions.