Abstract: In view of the distribution characteristics of inclusions in aluminum-killed 25Mn steel round billets, the two-dimensional morphology, compositions and quantity density of inclusions in steel at different radial positions were analyzed with SEM-EDS equipped with an automatic inclusion analyzer. The inclusions in the steel were extracted by non-aqueous electrolysis and their three-dimensional morphologies and compositions were observed. The calculation with FactSage8.0 software was also conducted. The results show that with the decrease of temperature in the solidification and cooling process, the Al₂O₃ content in inclusions increases gradually, the CaS content increases first and then decreases, and the CaO and SiO₂ contents decrease gradually. With the increase of [O] content in steel, the stable region of the single liquid steel phase decreases gradually, and the stable region of liquid oxide phase increases. The inclusions in steel can be mainly divided into type S containing SiO₂, type A containing Al₂O₃, type M containing MgO and type C containing CaO or CaS. With closing to the center of the round billet, the total number density of inclusions increases slightly, the number densities of types S, A and M inclusions increase slightly, and the number density of type C inclusions decreases slightly. With closing to the surface of the round billet, the compositions of CaO-Al₂O₃-MgO inclusions are located near the liquid phase area. With approaching the center of the round billet, the compositions of CaO-Al₂O₃-MgO inclusions are gradually away from the liquid phase area. The average compositions of these inclusions are gradually transferred from the liquid phase area to the solid phase area, and the MgO content in the inclusions increases. The types of inclusions extracted with non-aqueous electrolysis are basically consistent with those observed with inclusion automatic analysis apparatus.

Key words: inclusion equilibrium phase diagram, inclusion characteristics, non aqueous electrolysis, round billet, 25Mn steel