Formation of MgO-based inclusions during AOD and ladle treatment of Al-killed 2205 duplex stainless steel

Pilot trials were performed to study the formation mechanism of MgO-cored inclusions in 2205 stainless steel refined using low-Al high-basicity slag. The composition of melts and slags significantly affected the inclusion types, compositions, and shapes. Experimental results showed characteristic CaO and MgO dual-saturation in the slags. The compositions of the inclusions approached those of the slags, viz. the MgO-rich MgOspinel solid solutions or pure MgO. MgO in the slags provided sufficient Mg to the steel melts that newly generated pure MgO stably approached equilibrium as the Mg-O deoxidization reaction continued. The activity changes of Ca, Mg, and Al during refining were modeled. The activity of Mg exceeded those of Al and Ca, and the Mg-O reaction was always dominant. A kinetics model was established to elucidate the changing mechanisms of inclusions. The transferring kinetics of inclusions was discussed qualitatively to analyze velocity-controlled steps. The concentration of Al was low in molten steel, implying a low potential energy of Al3+ and impeded diffusion into the MgO core to form MA-spinel. Meanwhile, the diffusion of Mg2+ into the MgO core was the rate-limited step.