Calcium Modification of Inclusions via Slag/Metal Reactions

This paper examines the consistency between experiments and thermodynamic predictions of the modification of non-metallic inclusions by dissolved Ca in liquid Fe-Al alloys. Current thermodynamic predictions made with FactSage (version 7.2) were found to overestimate the amount of dissolved Ca and the Ca in non-metallic inclusions. This was demonstrated in two ways. First, Al-deoxidized Fe was held in CaO-3 pct ZrO2 crucibles for 100-136 minutes at 1873 K (1600 °C) and then reoxidized to precipitate dissolved Ca as oxide inclusions. The amount of Ca in the inclusions after reoxidation was quantified and considered equal to the dissolved Ca in the liquid Fe prior to reoxidation. Although experimental data were limited, the results suggested that the dissolved Ca was low and that the thermodynamic behavior of Ca could be best described by excluding Ca-O interaction. The assumption of no Ca-O interaction was compared with the associate solution model employed in FactSage by simulating the evolution of inclusion compositions in Fe—2, 1, 0.5, 0.1 wt pct Al alloys exposed to CaO (sat.)-MgO (sat.)-Al2O3 slags. The assumption of no Ca-O interaction led to predictions that were much closer to experimental results. More work is needed to ensure dissolved Ca behavior is accurately described and to ensure the sources for Ca modification of inclusions in industrial samples are properly identified.