The formation of CaO-TiO_2-MgO-Al_2O_3 dual phase inclusion in 321 stainless steel was investigated in the laboratory. The result indicated that the condition for the formation of CaOTiO_2-MgOAl_2O_3 in 321 steel is [Ca]＞0.001wt％, [Ti]＞0.1wt％, and [Al]＞0.01wt％. The mechanism is the following: Al_2O_3 inclusion turns into CaO-Al_2O_3 after Ca-Si wire is fed into the molten steel; [Mg] is then obtained by reducing MgO in slag or crucible wall by [Al] and [Ti]; finally CaO-Al_2O_3 inclusion is changed into CaO-TiO_2-MgO-Al_2O_3 by the reaction with [Mg], [Ti], and [O] in the molten steel simultaneously.
