Effect of Thermo-Mechanical Process on Phase Transformation Behaviors of VTiN Microalloyed Steel

Thermo-mechanical simulation tests were performed on VTiN microalloyed steel under three hot working conditions by using Gleeble-3800 thermo-mechanical simulator to study the effects of hot deformation and post-deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post-deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post-deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post-deformation holding process reduces the effect of hot deformation on the bainite start temperature.