Effect of Austenite Grain Size on Phase Transformation Structure of Low‐Carbon Microalloyed Steel

The effect of austenite grain on the phase transformation of low‐carbon microalloyed steel during continuous cooling is investigated by using Gleeble thermal simulator. The austenite grain size is changed by altering the holding period at the peak temperature. The findings demonstrate that for a given cooling rate, the longer the holding time, the microstructure becomes finer, and bainite begins to show up in the microstructure. The critical temperature of phase transformation and the diagram of phase transformation reaction rate during continuous cooling are determined by dilatometry. The Johnson–Mehl–Avrami–Kolmogorov equation is used to fit the transformation volume fraction, and the variation of kinetic parameters k and n with prior austenite grain size under nonisothermal condition is determined. The n value is trending downward as the prior austenite grain size increases, while the kinetic parameter k barely changes at all. The activation energy of phase transition is calculated by Kissinger method. The activation energy of fine austenite grain increases from 248.6 to 286.3 kJ mol −1 with the increase of austenite grain size, while the activation energy of coarse austenite grain decreases to 176.5 kJ mol −1 due to the competitive nucleation mechanism.