Diffusion and precipitation of MnS at interface between Mn–Al–Si oxides and steel during solidification

Sulfide inclusions in steel occur mainly as pure manganese sulfide (MnS) and manganese oxysulfide. The high-temperature precipitation of MnS at the oxide–steel interface was studied by inserting Mn–Al–Si oxide rods with or without MnS into different molten metals and solidifying under different cooling conditions. No MnS precipitated at the oxide–metal interface if the oxide contained no MnS, even if the steel contained Mn and S. At very low cooling rate, MnS in steel cannot diffuse to the oxide–steel interface, irrespective of the sulfide capacity of the oxide. MnS precipitated at the oxide–steel interface originated mainly from the oxide; thus, MnS in oxysulfide inclusions originates from diffusion from the oxide phase, not from the steel. Dispersed pure MnS inclusions precipitated from steel during solidification. A Mn-depletion zone existed on the oxide side with width larger than most inclusion radii in cast steel. This demonstrates that most oxysulfide inclusions are not large enough to present as residual MnS in the oxide phase.