Abstract: The effects of compositional changes on the precipitation of sulfides in non-quenched and tempered steel C38N2 were calculated by Thermo-Calc/DICTRA thermodynamics and kinetics software. It is calculated that the increase of C, Si and Al content can reduce the initial precipitation temperature of MnS in non-quenched and tempered steel. Among them, the change of C content has the most obvious influence on the precipitation temperature, the change of Al content has little effect on the initial precipitation temperature of MnS, and the effect of Si content change on the initial precipitation temperature of MnS is between C and Al. The change of Mn content has a great influence on the equilibrium precipitation of MnS phase. With the increase of Mn content, the precipitation temperature of MnS also increases, and the precipitation of MnS in high temperature zone can be promoted. The change of N content has little effect on the precipitation temperature of MnS, but with the increase of N content, it can promote the precipitation of MnS in high temperature zone. However, when the N mass fraction is increased to a certain extent (0.015%), the change in the N content does not affect the precipitation behavior of MnS. The change in Nb and V content has little effect on the thermodynamics of MnS precipitation in non-quenched and tempered steel, and the initial precipitation temperature of the second phase is lower than the initial precipitation temperature of MnS. However, Ti has a great influence on the precipitation of MnS in non-quenched and tempered steel. When the Ti mass fraction reaches 0.01%, the initial precipitation temperature of TiN is higher than the initial precipitation temperature of MnS, so that TiN is precipitated earlier than MnS. When the Ti mass fraction is more than 0.02%, Ti will suppress the precipitation of MnS in the high temperature region. And this phenomenon also explains the presence of MnS inclusions with (Nb, Ti)N as the core in non-quenched and tempered steel.

Key words: non-quenched and tempered steel, sulfide, thermodynamics, Thermo-Calc/DICTRA