Thermodynamics on the formation of spinel nonmetallic inclusion in liquid steel

Abstract

The nonmetallic spinel inclusion, MgOzAl2O3, has a high melting point and produces an undeformed C-type inclusion in steel products. Therefore, it is very harmful in the production of high grade wire, spring, and bearing steels. Thus, it is necessary to develop a methodology by which we can predict the occurrence of such defects. The thermodynamics of the formation of spinel nonmetallic inclusions during co-deoxidation with aluminum and magnesium is discussed in the present work. Previous literature thermodynamic values for strong deoxidizers such as aluminum, magnesium, and calcium are not in good agreement with the observed results and measurements. This discrepancy between predicted and measured results is due to an incomplete expression of the activities of dissolved oxygen and deoxidizers in current literature. Namely, only first-order interaction parameters are available despite the very strong interactions among the dissolved elements. In the present work, the deoxidation equilibrium with magnesium in liquid iron was studied in order to evaluate the activities of oxygen and dissolved magnesium and, include in the thermodynamic description, the firstand second-order interaction parameters, including the cross-product terms. A dolomite crucible was used to enhance the magnesium content in liquid iron in the present experiment on thermodynamics of spinel formation with reference to the study on desulfurization with a dolomite crucible undertaken by Ototani et al.[1] The assessment of aluminum-oxygen equilibrium in liquid iron reported by other investigators was also reviewed to develop values for the second-order interaction parameters. Finally, thermodynamic data on calcium deoxidation equilibrium were obtained. The iron sample was melted by a high frequency induction furnace. Iron of about 90 grams was charged into a crucible. The added weight of a deoxidizer such as magnesium and calcium was 1 mass pct of the weight of iron. The experimental procedure is briefly shown as follows. After the iron melted under a gas flow of a mixture of Ar-50 vol pct H2, hydrogen was impinged onto the melt surface to reduce dissolved oxygen content until it was less