Verma, N., P.C. Pistorius, R.J. Fruehan, M.S. Potter, H.G. Oltmann and E.B. Pretorius, “Calcium Modification of Spinel Inclusions in Aluminum-Killed Steel: Reaction Steps”, Metallurgical and Materials Transactions B, Vol. 43(4), pp. 830-840, 2012. doi:10.1007/s11663-012-9660-4.
Calcium treatment is a well-established way to modify solid alumina inclusions to liquid or partially liquid calcium aluminates. Spinels (Al2O3·xMgO) can also form in liquid steel after aluminum deoxidation. Like alumina, the spinels can be modified readily to liquid inclusions by a calcium treatment. The modification of spinels was studied by observing the transient evolution of inclusions, in laboratory and industrial heats. Spinel modification involves the preferential reduction of MgO from the spinel, with Mg dissolving in the steel, and it proceeds through transient calcium sulfide formation, just like in the case of alumina inclusions. Because magnesium dissolves in steel after the calcium treatment of spinels, the reoxidation of the melt will produce new spinels.