Transformation of cerium-containing inclusions in ultra-low-carbon aluminum-killed steels during solidification and cooling

Laboratory experiments were conducted to study the transformation of cerium-containing inclusions during the solidification and cooling process in ultra-low-carbon aluminum-killed steels with 33 ppm, 83 ppm and 140 ppm. Inclusions were analyzed statistically using an automatic scanning electron microscope in water-cooled and furnace-cooled samples, respectively. Besides, sulfur-containing precipitates were detected in furnace-cooled ingots to validate the transformation of inclusions. During the solidification and cooling process, it was found that homogeneous CeAlO3 inclusions transformed into Al2O3–Ce2S3 dual-phase ones in the steel containing 33 ppm, and Ce2O2S inclusions transformed into CeAlO3–Ce2S3 or Al2O3–Ce2S3 dual-phase ones in the steel containing 83 ppm, while inclusions in the steel containing 140 ppm hardly transformed. The thermodynamic analysis was performed and agreed well with experimental results. A prediction model based on the thermodynamics and the mass balance was established to predict the type and the composition of inclusions after transformation in ingots. When the value of T.Ce/T.S ≥ 2.92 in the steel, all sulfur content was combined with the cerium and precipitates of (Mn, Cu)S hardly formed in the solidified steel. © 2020 The Author(s)