Formation and Evolution of Inclusions with Different Adding Order of Magnesium and Sulfur in Al-Killed Free-Cutting Steel

To reveal the effect of adding order of magnesium and sulfur on the evolution of inclusions in Al-killed free-cutting steel, both deoxidized experiments and thermodynamic calculations were carried out in this paper. The samples, which were extracted from the liquid steel at different time after magnesium and sulfur addition, were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The results showed that the adding order had a significant influence on the formation and evolution of inclusions. In the case of magnesium added before sulfur, MgO formed immediately after magnesium addition and then transformed to spinel accompanied by MeS (Mg0.9Mn0.1S) after sulfur addition. In the case of sulfur added prior to magnesium, MgO and MeS precipitated simultaneously after magnesium addition. During the equilibrium solidification, the transformation of MgO to spinel was calculated to take place before MnS precipitated from the saturated liquid steel.