The effects of ferritic transformation on hot ductility of medium carbon steel

The hot ductilities of Nb-bearing and Nb-free medium carbon steel have been investigated by the hot tensile tests. A high carbon steel (0.4%) was also tested as a reference to determine the relationship between transformation temperature and hot ductility. C and Nb are very effective in delaying the ferrite transformation so that the ductility trough extended to a lower temperature compared to Nb-free medium carbon steel. Decreasing the strain rate promotes the formation of grain boundary ferrite films at higher temperatures and, on the low temperature side, the intergranular failure could still occur even when the amount of ferrite reached 40% or more. The short time temperature oscillation accelerated the precipitation of grain boundary films at higher temperatures, extending the ductility trough to higher temperatures. The samples tested on different machines (direct heating and induction heating) exhibited different hot ductility behavior.