Formation mechanism and control of MgO·Al₂O₃ inclusions in non-oriented silicon steel

On the basis of the practical production of non-oriented silicon steel, the formation of MgO·Al₂O₃ inclusions was analyzed in the process of “basic oxygen furnace (BOF) → RH → compact strip production (CSP)”. The thermodynamic and kinetic conditions of the formation of MgO·Al₂O₃ inclusions were discussed, and the behavior of slag entrapment in molten steel during RH refining was simulated by computational fluid dynamics (CFD) software. The results showed that the MgO/Al₂O₃ mass ratio was in the range from 0.005 to 0.017 and that MgO·Al₂O₃ inclusions were not observed before the RH refining process. In contrast, the MgO/Al₂O₃ mass ratio was in the range from 0.30 to 0.50, and the percentage of MgO·Al₂O₃ spinel inclusions reached 58.4% of the total inclusions after the RH refining process. The compositions of the slag were similar to those of the inclusions; furthermore, the critical velocity of slag entrapment was calculated to be 0.45 m·s⁻¹ at an argon flow rate of 698 L·min⁻¹, as simulated using CFD software. When the test steel was in equilibrium with the slag, [Mg] was 0.00024wt%–0.00028wt% and [Al]_s was 0.31wt%–0.37wt%; these concentrations were theoretically calculated to fall within the MgO·Al₂O₃ formation zone, thereby leading to the formation of MgO·Al₂O₃ inclusions in the steel. Thus, the formation of MgO·Al₂O₃ inclusions would be inhibited by reducing the quantity of slag entrapment, controlling the roughing slag during casting, and controlling the composition of the slag and the MgO content in the ladle refractory.