Influence of hot deformation on dynamic recrystallization behavior of 300M steel: Rules and modeling

Abstract

Dynamic recrystallization (DRX) is a major softening mechanism that impacts the microstructural properties of 300M steel during the forging of large parts. It refines the grains and eliminates defects from the material, and an in-depth analysis of the process is thus important. In the present study, the hot deformation behaviour of 300M steel was investigated by hot compression tests conducted at temperatures of 900–1150 °C and strain rates of 0.01–10 s−1. The results were used to plot the true stress–strain and work hardening rate curves of the material, and the latter curves were subsequently used to determine the critical deformation conditions for the occurrence of DRX. The DRX nucleation mechanism was determined to mainly involve bulging of the initial grain boundaries. Further, based on a dynamic material model, the hot processing map of 300M steel was developed, from which the regions other than the instability region were observed to have higher DRX volume fractions. The relationships between the efficiency of power dissipation (η), the microstructure, and the softening mechanism were analysed. It is found that there may be some association between the change rules of η and DRX. When peak η (ηp) < 0.3 and η gradually decreases with increasing true strain, the softening is mainly by dynamic recovery (DRV). When ηp > 0.3 and η increases with increasing true strain, the softening is dominated by DRX. In addition, η gradually decreased after the peak value in the late stage of the deformation, indicating coarsening of the DRX grains. Finally, the DRX volume fraction was calculated for different deformation conditions and the DRX volume fraction model was corrected. The effectiveness of the original and corrected models were statistically evaluated and the latter was confirmed to more accurately depict the DRX volume fraction of 300M steel.