Inclusion characteristics and acicular ferrite formation

The effect of adding MgO nanoparticles (0%, 0.01%, 0.05%, and 0.10%) on the inclusion and acicular ferrite (AF) formation in 1.8%Ni low carbon Al-killed (LCAK) steel was explored. The inclusion analysis reveals that MgO nanoparticles react with dissolved Al or Al2O3 inclusions and finally change to fine MgAl2O4 wrapped by MnS in the case of adding 0.05% MgO nanoparticles. In contrast, MgO nanoparticles are agglomerated and form Mg-bearing inclusion clusters in the case of 0.10% or reduced to high Al2O3 inclusions by dissolved Al in the case of 0.10%. The microstructure observations show that the AF formation was promoted in the case with addition of 0.05% MgO nanoparticles. The process of inclusion-inducing AF nucleation is essentially characterized via the high-temperature confocal scanning laser microscopy experiment. delta min (delta denotes disregistry) of Al2O3/alpha-Fe, MgAl2O4/alpha-Fe, and MnS/alpha-Fe is evaluated to be 11.5%, 6.5%, and 7.4%, respectively. The high AF fraction in the case of 0.05% MgO addition can be attributed to the low disregistry of MnS-MgAl2O4/ferrite and Mn-depleted zone adjacent to the MnS layer. Based on the results, 0.05% was recommended as the proper addition amount of MgO nanoparticles in 1.8%Ni LCAK steel.