Effect of Mn/S Ratio on the Hot Ductility of Eco-friendly Bi-S based Free Cutting Steel

The hot ductility of eco-friendly Bi-S based free cutting steels with different Mn/S ratios was studied using a Gleeble-1500 thermal–mechanical simulator. The hot ductility of the steel was found to depend on the Mn/S ratio, and the Mn/S ratio of the steel should be greater than 3.5 for hot rolling of billets without crack development. The low Mn/S ratio would inhibit the occurrence of the dynamic recrystallization and cause the formation of the low melting point sulfide Fe-rich (Fe,Mn)S as secondary phases, which could obviously reduce the strength of the grain boundary and resulted in the formation of cracks along the grain boundary. The higher the Mn/S ratio in the steel, the lower the Fe content in the Fe-rich (Fe,Mn)S phases. When the Mn/S ratio in the steel was high enough, the sulfide phases in the steel were mainly MnS as primary inclusions and the low melting point sulfide phases could be effectively avoided forming. While the Mn/S ratio could influence the hot ductility of the steel over the whole temperature range of 900–1200 °C, the segregation of bismuth along grain boundary could be harmful to the hot ductility in addition to the lower Mn/S ratio for the temperature was no more than 1050 °C