Microstructural evolution during bainite transformation in a vanadium microalloyed TRIP-assisted steel

The microstructural evolution and subsequent properties of a cold rolled 0.12 wt% V microalloyed transformation induced plasticity assisted steel were studied after intercritical annealing, focusing on isothermal bainite transformation at 460 °C. The heat treated steel was examined by optical microscopy, scanning-transmission electron microscopy and microanalysis, X-ray diffraction analysis and tensile testing. The tensile testing showed considerable improvements in yield strength and total elongation with longer isothermal bainite holding times (i.e. 180 s). According to microscopy observations, these effects were attributed to the increase in the population of film-type retained austenite and newly formed dislocations as a result of bainite transformation. No significant V(C,N) precipitation and growth-coarsening were observed during isothermal bainite transformation, which is consistent with conventional vanadium microalloyed steels. However, the V(C,N) precipitates in dislocation tangles suggested the possible contribution of precipitates on the tensile properties of steel.