Study on the mechanism of AF nucleation induced by complex oxide inclusions after LF refining in oxide metallurgical steel

The ladle furnace refining process (before LF, add Ti, add Al, add Mg, add Ca) of oxide metallurgical steel was sampled, and the effect of complex oxide inclusions generated after LF refining on acicular ferrite (AF) nucleation were studied by SEM, TEM, TKD and EBSD. The results show that the complex oxide inclusions after LF refining are composed of MgAl2O4, CaTiO3, TiO2, TiN, and MnS. Among them, MgAl2O4, CaTiO3, TiO2 and TiN have NW orientation relationship with adjacent AF laths. There are Mn depletion zone(MDZ) with a width of about 200 nm outside MgAl2O4, CaTiO3, TiO2 and MnS. Some AF laths can be nucleated by different inclusions under the combined action of MDZ mechanism and lattice mismatch mechanism. The first principle calculations show that MgAl2O4, CaTiO3 and TiO2 all have cation vacancies, and the diffusion of Mn ions into cation vacancies has thermodynamic advantages. The formation of MDZs is the combined effect of MnS precipitation on oxidized inclusions and Mn ions occupying cation vacancies in inclusions. •The oxide inclusions and TiN have N-W orientation relationship with adjacent AF laths. The outside of oxide inclusions and TiN have 200nm wide MDZs.•The first-principles calculation results show that MgAl2O4, CaTiO3 and TiO2 have the thermodynamic advantage of Mn ion entering cation vacancies.•The precipitation of MnS on oxide inclusions and the entry of Mn ions into cation vacancies of inclusions are two reasons for the formation of MDZs.