Effect of ausforming on the bainitic transformation and microstructure in medium carbon V–N micro-alloyed steel

Ausforming at 900 °C with 30% reduction in V and N micro-alloyed steel could induce VN precipitation, which has low lattice misfit (1.8%) with bainitic ferrite in a particular crystal orientation. The incubation period of bainitic transformation was shortened by introducing VN. A further ausforming at 300 °C with different strains prior to isothermal transformation was carried out, which provided more nucleation points and more distortion-energy storage for bainitic transformation. The incubation period was dramatically shortened and bainitic transformation was notably accelerated. There existed a competition between accelerating nucleation and suppressing growth after low temperature ausforming with different strains, and a small strain resulted in a more significant acceleration and a higher degree of bainitic transformation. In addition, thinner bainitic ferrite and finer blocky retained austenite were obtained after low temperature ausforming. The orientation relationship between bainitic ferrite and austenite in the ausformed steel were K–S and N–W relationships.