<p>A series of laboratory-scale experiments were conducted to investigate the effects of MgO and SiO₂ contents in slag on the control of inclusions in steel and the erosion of MgO-based refractory. The results demonstrated that both CaO–Al₂O₃ slag in Experiments A and CaO–Al₂O₃–MgO slag in Experiments B achieved high cleanliness of steel, with T[O] and [S] contents below 0.0007 and 0.0006&#xa0;pct, respectively, after 60 min of refining. However, as SiO₂ content in slag increased from 0 to 30 pct, the desulfurization rates of steel in Experiments A and B decreased from 95.71 and 96.43 pct to 34.29 and 29.29 pct, respectively. Moreover, when SiO₂ exceeded 10 pct, the T[O] content began to increase. Regarding inclusions, when no SiO₂ was added to slag, the inclusions in both Experiments A and B were mainly MgO-based, MgO–Al₂O₃, CaO–MgO–Al₂O₃, and a few Al₂O₃-based, with number density of 9.45/mm² and 11.58 to 12.68/mm², respectively. As SiO₂ content increased, the proportion of MgO-based decreased in both experiments, while the proportion of MgO–Al₂O₃ first increased and then decreased. When SiO₂ content reached 30 pct, the inclusions were primarily MgO–Al₂O₃ and MnO–SiO₂–Al₂O₃. The number density reached its minimum at 15&#xa0;pct SiO₂ and then gradually increased, although the average size remained below 3&#xa0;μm. Additionally, the refining slag eroded the MgO crucible, with penetration occurring along the gaps among MgO grains. The extent of erosion first increased linearly with rising SiO₂ content in slag and then followed a parabolic trend. The addition of an appropriate amount of MgO effectively suppressed the erosion. Thermodynamic analysis revealed that although the initial slag compositions in both experiments did not meet the conditions for MgO–Al₂O₃ formation, the gradual addition of SiO₂ coupled with MgO dissolution from the crucible led to the formation of MgO–Al₂O₃ in slag. This phase precipitated extensively during cooling, resulting in a significant increase in its size and number.</p>