| Summary: | There are many types of non-metallic MgAl<sub>2</sub>O<sub>4</sub> inclusions observed in Al-deoxidized steel coupling with Mg treatment, including single-particle MgAl<sub>2</sub>O<sub>4</sub>, agglomerated MgAl<sub>2</sub>O<sub>4</sub>, and MgAl<sub>2</sub>O<sub>4</sub>-MnS. Thermodynamic calculation shows that MgAl<sub>2</sub>O<sub>4</sub> precipitates in the liquid phase. The phase transformation follows liquid + Al<sub>2</sub>O<sub>3</sub> + MgAl<sub>2</sub>O<sub>4</sub> → liquid + MgAl<sub>2</sub>O<sub>4</sub> → liquid + MgO + MgAl<sub>2</sub>O<sub>4</sub> → liquid + MgO with the Mg content increasing when the Al content is a constant in molten steel, and it is in agreement with the experimental results for the formation of MgAl<sub>2</sub>O<sub>4</sub> in molten steel. The calculation results of various attractive forces between two particles show that the cavity bridge force plays a dominant role in the agglomeration process and results in the agglomerated MgAl<sub>2</sub>O<sub>4</sub>. The lattice mismatch calculation result shows that MgAl<sub>2</sub>O<sub>4</sub> can provide effective sites for MnS nucleating in steel.
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