Parameters influencing inclusion compositions in Al-killed steel melts during a secondary refining process

Abstract:      This study was carried out to clarify the factors influencing the evolution of inclusions in Al-killed steel melts during a secondary refining process. First, a case-hardening steel was the subject of study to understand the factors influencing the inclusion compositions in the steel melt. During the LF process, inclusions were transformed from the Al2O3 phase, which was the primary deoxidation product, to both MgO·Al2O3 and CaO-Al2O3-MgOliq phases simultaneously. This composition transition during the LF process occurred by composition evolutions toward thermodynamically stable phases. After the RH process, the inclusion compositions primarily consisted of the Al2O3 and CaO-Al2O3 phases. MgO·Al2O3 inclusions were removed, while the CaO-Al2O3 inclusions remained during the RH process. This behavior can be understood in terms of the interfacial properties of the oxide phases in a steel melt. The detected Al2O3 inclusions were considered to be generated by reoxidation during the RH treatment. Thus, it was confirmed that the equilibrium states, removal, and generation of inclusions determine the inclusion compositions in an Al-killed steel melt during an LF-RH refining process.     Subsequently, the effect of high Al contents in a steel melt on the change in inclusion compositions during the LF-RH process was studied. Due to the high Al content in the steel melt, the thermodynamic driving forces for Al2O3 modification became lower than those in ordinary Al-killed steels. Therefore, the degree of inclusion evolution was restricted. This contributed to the low CaO contents in the inclusions. Due to the low CaO contents, the removability of the inclusions remained high throughout the LF-RH process. According to thermodynamic calculations, the low T.O contents in this steel grade are due to the low insoluble O contents. This can be explained by the fast removal of inclusions. Because the inclusions were removed smoothly, the CaO content in the inclusions was lower than that in the thermodynamically stable phase.     In addition, a study was carried out to understand the formation and behavior of the CaS phase in an Al-killed high-S steel during the LF-RH process without Ca-treatment. In the initial stage of the LF process, a CaS phase was formed on the existing inclusions by a reaction between Ca and S. As the desulfurization of the steel melt progressed, the CaS phase started to be transformed into a CaO phase in the inclusions, which resulted in the formation of CaO-Al2O3-CaS inclusions. After desulfurization of the steel melt, the Al2O3 phase in the inclusions was transformed to the CaO-Al2O3liq phase without being hindered by a CaS phase. During the following RH process, the addition of FeS increased the activity of S, which then reacted with both CaO in the inclusions and with Ca, forming a CaS phase. Consequently, the majority of the inclusions consisted of the Al2O3-CaS phase. Thus, a CaS formation during the LF-RH process without Ca-treatment progresses under the thermodynamic driving forces of the following two reactions: the reaction between CaO in the inclusions and S and the reaction between Ca and S. Due to the formation of a CaS phase during the RH process, inclusions in the high S steel melt were covered by a CaS phase, which is difficult to remove from steel melts. Therefore, the castability of the high S steels can be deteriorated by the CaS inclusions, even without using Ca-treatment.     In summary, it can be concluded that the removal of inclusions, generation, and composition evolution should be considered in order to control the inclusion compositions in Al-killed steel melts. In addition, steel components, such as Al and S, are important to monitor to control the inclusion evolution during secondary refining processes.