Evolution Mechanism of Inclusions in Al-killed, Ti-bearing 11Cr Stainless Steel with Ca Treatment

Abstract

The evolution mechanism of non-metallic inclusions in Al-killed, Ti-bearing 11Cr stainless steel with Ca treatment was investigated by industrial trials and thermodynamic calculation. The morphology, composition, and size distribution of inclusions in steel specimens were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The alloy addition and calcium contents in steel significantly influenced the characteristics of inclusions according to the present study. After the addition of Al, there were mainly alumina-rich inclusions in steel. With the titanium addition after calcium treatment, three types of inclusions were formed: irregular MgO–Al₂O₃–TiO_x inclusion, spherical CaO–MgO–Al₂O₃–TiO_x inclusion, and irregular dual phase Ti-containing inclusion. At the end of LF refining process, solid calcium titanates inclusions were formed due to the high calcium content in steel. The evolution of these inclusions was consistent with thermodynamic calculation, which indicated that the compositions of inclusions in steel specimens after the addition of titanium were mostly located in Al₂O₃–TiO_x stable phase. Based on the characteristics of inclusions in steel and thermodynamic calculation, calcium could reduce the stability of spinel and strongly modify solid alumina and Al₂O₃–TiO_x inclusions to form liquid oxides or solid calcium titanates. At the same time, the effects of calcium content in Al-killed, Ti-bearing 11Cr molten steel on the formation of inclusions were discussed through the coupling of thermodynamics calculation and experimental results.