Transformation behavior of calcium-modified inclusions in 304 stainless steel during solidification

The behavior of inclusions in 304 stainless steel after calcium modification has been relatively well clarified, yet their evolution during solidification remains insufficiently understood. A systematic investigation was conducted on calcium-modified 304 stainless steel through laboratory experiments and thermodynamic calculations, focusing on the evolution of inclusions throughout solidification. The results indicate that the calcium content in molten steel significantly affects the type and morphology of sulfide precipitation. In low-calcium heats, only MnS precipitates, nucleating either heterogeneously on Al₂O₃-SiO₂-CaO-MgO inclusions or independently from the melt. In high-calcium heats, abundant CaS particles precipitate dispersedly on the surfaces of pre-existing inclusions. After furnace cooling, the Al₂O₃ fraction in inclusions of high-calcium heats increases significantly, accompanied by a decrease in SiO₂ and substantial CaS precipitation. Conversely, in low-calcium heats, theSiO₂ fraction increases, the Al₂O₃ fraction decreases, and MnS predominates. Post-cooling, the inclusion size increases while the number density decreases. Thermodynamic calculations further reveal that with increasing total calcium (T.Ca) content, the precipitation sequence follows: MnS+Ca₂Al₂SiO₇→MnS+Ca₂Al₂SiO₇+Ca₂SiO₄→MnS+Ca₂Al₂SiO₇+Ca₂SiO₄+CaS→Ca₂SiO₄+MnS+CaS. At a fixed T.Ca, an increase in total oxygen (T.O) shifts the sequence to MnS+Ca₂Al₂SiO₇+Ca₂SiO₄+CaS→MnS+Ca₂Al₂SiO₇+Ca₂SiO₄→MnS+Ca₂Al₂SiO₇+CaAl₄O₇+CaAl₂Si₂O₈+Al₂O₃.