Effect of Al and S on the Evolution of Inclusion and Formation of Acicular Ferrite in the Mg-RE-Ti-Treated Steel

The composition, number density and size distribution of inclusions in Mg-RE-Ti-treated steels with different Al and S content were investigated systematically, while the microstructure and the probability of AF nucleation on inclusions were analyzed. The results show that the dominate oxide was composed of two layered structures with Mg-bearing core and La-bearing shell. As the S content enhanced, the formation of MnS was promoted, while the composition of La-bearing inclusion did not change much. With the increase in Al content, the inclusions were easy to form cluster, and the La-bearing inclusions evolved from La2O3 to LaAlO3. With MnS precipitation on the inclusions, the variation of the La-bearing inclusions composition did not influence the AF nucleation significantly. The AF fraction decreased with Al addition mainly because the inclusions had higher tendency to aggregate in high Al samples in this study.