Mechanism of a CaS Formation in an Al-Killed High-S Containing Steel during a Secondary Refining Process without a Ca-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 Al2O3 + 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.