Factors to Determine Inclusion Compositions in Molten Steel during the Secondary Refining Process of Case-Hardening Steel

A study was carried out to observe inclusions during the secondary refining process of case-hardening steel to understand the factors to determine inclusion compositions. During the LF refining, inclusions changed from the primary deoxidation product of Al2O3 to MgO center dot Al2O3 and the CaO-Al2O3 system. At the end of LF refining, the compositions were placed on the tie-line connecting the areas of MgO.Al2O3 and CaO-Al2O3-MgOhc, in the CaO-Al2O3-MgO diagram. This change took place by the composition evolution targeting the thermodynamic equilibrium states. After the RH treatment, the inclusion compositions changed mainly to Al2O3 and the CaO-Al2O3 system because MgO.Al2O3 inclusions were removed, while the CaO-Al2O3 system inclusions, the most stable oxide, were remained. This behavior could be described by the interfacial properties between the oxides and the molten steel. Al2O3 inclusions were considered to be newly generated during the RH treatment. It was confirmed that three factors of (1) equilibrium state (2) removal and (3) generation of inclusions dominated to determine the inclusion compositions.