Hot ductility of eco-friendly low carbon resulphurised free cutting steel with bismuth

Abstract

The hot ductility of low carbon resulphurised free cutting steel with different bismuth contents was studied using a Gleeble-1500 thermal–mechanical simulator, and the hot brittleness mechanism was analysed with a combination of SEM and energy dispersive X-ray spectroscopy. The results showed that the steels with bismuth contents of 0·12 and 0·17 wt-% both had a good hot ductility (RA, >70%) at 1100–1200°C, which illustrated that the steels might be suitable for the rolling provided the finish rolling temperature does not go <1100°C. However, the hot ductility of the steels deteriorated with the increase in bismuth content at 950–1100°C because of the presence of the low melting point liquid bismuth films at austenite grain boundaries and the delay of the dynamic recrystallisation by Bi additions. At 850–900°C, the steels all had very poor hot ductility, which was mainly attributed to the presence of the ferrite film and the unrecrystallised austenite as well as the presence of the low melting point phases pure Bi and Fe rich (Fe,Mn)S.