Evolution of Non-metallic Inclusions in Si-Ca-Ba-Killed Heavy Rail Steel During Secondary Refining Process

In order to study the evolution behaviors of inclusions in Si-Ca-Ba-killed heavy rail steel, industrial samples were taken at different stages during the refining process of steel grade U75V, and the composition and size of the inclusions were detected. Laboratory experiments were also carried out to study the formation mechanism of different types of inclusions. The results show that the inclusions transform from SiO₂-MnO(-Al₂O₃)-based inclusions to CaO-SiO₂-Al₂O₃-MgO-based inclusions during the refining process. Some CaO-SiO₂-Al₂O₃-MgO-based inclusions with a high Al₂O₃ content (>30 pct) were observed in the middle stage of LF refining, and vanished at the end of LF refining. The Al₂O₃-containing refractory is supposed to be the main reason for the formation of the high Al₂O₃ inclusions, while the refining slag can lower the content of Al₂O₃ in these high Al₂O₃ inclusions. Due to steel-slag-refractory interaction, CaO-SiO₂-Al₂O₃-MgO-based inclusions with a low Al₂O₃ content are the stable inclusions after refining. Besides, it is found that Si-Ca-Ba alloy mainly acts as a deoxidizer for slag deoxidation, and it plays a very limited role in steel deoxidation. Economic slag deoxidizers and refractory with a low Al₂O₃ content are suggested for heavy rail steel.