Modelling of constitutive relationship, dynamic recrystallization and grain size of 40Cr steel during hot deformation process

The hot deformation and dynamic recrystallization (DRX) behaviors of 40Cr steel were studied by hot compression tests over a wide range of deformation temperature (1023 K-1323 K) and strain rate (0.01 s(-1)-10 s(-1)). The constitutive relationship, DRX kinetics and DRXed grain size were modelled, and numerical simulation was performed to verify their predicting accuracy. Moreover, the effects of deformation parameters on martensite structure after hot compression and quenching were analyzed. The results show that the strain compensated Arrhenius can accurately predict the flow stress with the average absolute relative error of 3.89% and correlation coefficient of 0.992. The fraction and grain size of DRX are strongly related to deformation conditions, and low strain rate and high temperature are beneficial for the occurrence of DRX. Moreover, the equation of DRX fraction was developed based on Avrami model, and the relationship between DRXed grain size and Zener-Holloman parameter was also established. The simulated results of true strain, DRX fraction and grain size agree well with experimental findings. Finally, it was found that the size of martensite packet became larger under high strain rate, while the aspect ratio of martensite lath increases at higher deformation temperature.