The effects of addition of deoxidation elements on the morphology of (Mn,Cr)S inclusions in stainless steel

The effects of additions of C, Si, Ti, and Al on the morphology of (Mn,Cr)S inclusions in Fe-15Cr-1Mn-0.3S (mass%) and Fe-15Cr-0.5Mn-0.3S base alloys have been investigated using optical and scanning electron microscopy. Three types of (Mn,Cr)S inclusions morphology have been observed (1) globular-shaped sulfide, (2) rodlike (Mn,Cr)S, and (3) angular (Mn,Cr)S, depending on the Mn content and the presence of additional elements. The formation mechanisms of (Mn,Cr)S inclusions are discussed on the basis of phase diagram information. It is shown that (a) the globular (Mn,Cr)S inclusions observed in 1% Mn alloys are formed through the metastable monotectic reaction, (b) the rodlike (Mn,Cr)S observed on the addition of Ti and Al are the products of a stable eutectic reaction that proceeds by nucleation of (Mn,Cr)S crystals by the high-melting-point particles such as TiN and Al₂O₃, (c) the globular (Mn,Cr)S morphology observed in 0.5% Mn base alloys is the result of a stable monotectic reaction that proceeds through the formation of the low-melting globular (Mn,Cr)S inclusions, and (d) the addition of C and Si promote the stable eutectic reaction by lowering the melting point of Fe-rich solid solution.