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.