Effects of Mg and La on the evolution of inclusions and microstructure in Ca-Ti treated steel

The evolution of inclusions and the formation of acicular ferrite (AF) in Ca-Ti treated steel was systematically investigated after Mg and La addition. The inclusions in the molten steel were Ca-Al-O, Ca-Al-Mg-O, and La-Mg-Ca-Al-O after Ca, Mg, and La addition, respectively. The type of oxide inclusion in the final quenched samples was the same as that in the molten steel. However, unlike those in molten steel, the inclusions were Ca-Al-Ti-O + MnS, Ca-Mg-Al-Ti-O + MnS, and La-Ca-Mg-Al-Ti-O + MnS in Mg-free, Mg-containing, and La-containing samples, respectively. The inclusions distributed dispersedly in the La-containing sample. In addition, the average size of the inclusions in the La-containing sample was the smallest, while the number density of inclusions was the highest. The size of effective inclusions (nucleus of AF formation) was mainly in the range of 1-3 mu m. In addition, the content of ferrite side plates (FSP) decreased, while the percentage of AF increased by 16.2% due to the increase in the number of effective inclusions in the La-containing sample in this study.