Evolution of sulphide inclusion in Mg–Ca treating gear steel

Abstract

ABSTRACT The behaviour of sulphide inclusion in Ti-S-containing gear steel is studied, when Mg–Ca treatment is applied to control the sulphide shape. The results show that MgO + CaS will appear in molten steel after Mg–Ca treatment is carried out and act as the heterogeneous nucleus of TiN and MnS during subsequent solidification process. CaS in molten steel has the tendency to transform into (Ca,Mn)S with the development of solidification. The transformation degree is influenced by cooling rate and even stopped when CaS is totally covered by a TiN shell, resulting in the formation of three types of sulphides with evidently different chemical compositional characteristics in casted billet. Based on the study in the paper, the improvement of sulphide morphology under heterogeneous nucleation effect by oxide core and modification effect by calcium will be prevented by the titanium nitride which precipitates first in Ti-containing gear steel.