Evolution of inclusions in Mg-RE-Ti treated steels with different Al contents and their influence on acicular ferrite

Abstract

The size distribution and composition of inclusions in Mg-RE-Ti treated steels with different Al contents were investigated systematically, while the microstructure and the nucleation potency of inclusions for AF were analyzed. The results show that the inclusions were refined with Mg treatment, and then coarsen following La addition in the samples. The particles in high Al sample were more likely to collide, aggregate and form clusters due to the relatively high amount of inclusions in melt steel. The dominate inclusions were composed of two layer structures with Mg-bearing inclusions cores and La-bearing inclusions shells. With the increase of Al content, the cores evolved from MgO to MgO•Al2O3 and the shells evolved from La2O2S and La2O3 to LaAlO3. The nucleation potency of inclusions for AF decreased due to relatively high lattice misfit between inclusions and α-Fe in high Al sample. The AF fraction decreased with the increase of Al content because of the decrease of the amount of effective inclusions.