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 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.