Formation and Modification of MgO·Al2O3-Based Inclusions in Alloy Steels

The current study performed thermadynamic calculation, laboratory experiments, and industrial trials for the formation and modification of MgO-Al2O3 spinel inclusions in alloy steels. The stability Mg-Al-O diagram was obtained using the thermodymanic study. The resulting MgO-Al2O3-CaO inclusions from MgO-Al2O3 spinel inclusions after the calcium treatment were spherical, and > 5 μm MgO-Al2O3-CaO inclusions have a two-layer structure: an outside CaO-Al2O3 layer and a MgO-Al2O3 core. The modification of > 5 μm MgO·Al2O3 spinel inclusions by calcium treatment includes two steps: (1) reducing MgO in the inclusion into the dissolved magnesium by the dissolved calcium in the steel and (2) generating a liquid xCaO·yAl2O3 layer at the outside of the spinel inclusion. For <2 μm MgO·Al2O3 spinel inclusions, they can possibly be modified into a xCaO·yAl2O3 inclusion by reducing all MgO component in the spinel inclusions with the added calcium.