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The aim of this study is to clarify the mechanism of low total oxygen (T.O) contents in high-Al containing steel grades. Steel samples are taken from a ladle during an LF-RH process, and the compositions of both the steels and inclusions are determined. According to thermodynamic considerations, the low T.O contents of high Al steel grades are due to the low insoluble oxygen contents. Due to the high Al contents in a steel melt, thermodynamic driving forces of the Al 2 O 3 modification are lower than those in ordinary Al-killed steels. Both the low thermodynamic driving force of the Al 2 O 3 modification and the inclusion removal from the melts contribute to the low CaO contents in inclusions in high-Al steel melts. The contact angles of inclusions in high Al steel melts are higher than 908 due to the low CaO content in inclusions. Therefore, the removal tendency of inclusions in high Al steel melts is kept high throughout an LF-RH process. Due to this high removal tendency, the T.O contents in high Al steel melts decreases remarkably during an LF refining process. Thereafter, they decrease further during the following RH treatment.

A study is carried out to understand the CaS formation mechanism in an Al-killed high-S containing steel during the secondary refining process without a Ca-treatment. At the initial stage of the LF refining, CaS is formed on existing inclusions by the reaction between Ca and S due to the high S activity before the desulfurization. As the desulfurization progresses, the CaS phase changes into a CaO phase due to the decrease of the CaS stability. Since this composition change takes a long time due to the difficulty of a fast mass diffusion in the solid phase, it cannot be complete during the LF refining. During the following RH treatment, an FeS addition increases the activity of S, which reacts with the CaO in the inclusions to form a CaS phase. At the same time, the CaS phase is formed by the reaction between Ca and S. Consequently, the majority of the inclusions end up as an Al 2 O 3 þ CaS phase. The CaS formation behavior during the secondary refining process without a Ca-treatment obeys the thermodynamic driving forces with respect to two reactions: one is the reaction between CaO in inclusions and S and the other is the reaction between Ca and S.

Continuous casting of high-carbon steel containing 1% carbon tends to be operated with low mold flux consumption, resulting in insufficient lubricity. In this study, a mold flux was developed by increasing the viscosity to form a glassy film easily and improve the lubricity between the mold and the solidified shell. In the casting with the developed mold flux, a film with a thickness of 3 mm was stably formed inside the mold to cover the meniscus, bleeding was prevented, and the frequency of surface crack defects on the bloom was reduced by 80%. In the film of the developed mold flux, a 1.2 mm liquid layer lubricated the initial solidification shell.The increased thermal resistance at the film-mold interface reduced the heat flux in the mold, which contributed to the uniform initial solidification. The formation and the growth mechanisms of the crystalline layer in the film were as follows. Firstly, the mold flux flows into the gap between the mold and the initial solidification shell and forms a glassy film. Subsequently, crystallization of the glassy film starts from the mold plate side. Thereafter, the crystallization progresses in the thickness direction of the film until the position of the solidification temperature of the mold flux. The existence of two phases, a liquid layer and a solid layer, played an important role to achieve high lubricity and uniform initial solidification.