The deformation behavior in isothermal compression of 300M ultrahigh-strength steel

The flow stress, the apparent activation energy for deformation and the growth behavior of austenite grains in isothermal compression of 300M steel are investigated at the deformation temperatures ranging from 850 °C to 1250 °C, the strain rates ranging from 0.1 s−1 to 25.0 s−1, and the height reductions ranging from 30% to 70%. Through the analysis of flow curves and microstructure, it is confirmed that dynamic recovery is the dominant softening mechanism at a deformation temperature of 900 °C. At a deformation temperature of 1100 °C, the process of dynamic recrystallization has completed and the austenite grain growth is in progress in isothermal compression of 300M steel. Coarse macro-grains in isothermal compression of 300M steel are observed at the deformation temperatures above 1140 °C. The apparent activation energy for deformation of 300M steel is in the range of 332 ± 61 to 397 ± 31 kJ mol−1. The processing maps of 300M steel at a series of strains are constructed based on the dynamic materials model (DMM). The results show that the unstable region in instability maps decreases with an increase in strain. At a strain of 0.7, the unstable region occurs at the deformation temperatures ranging from 900 °C to 1140 °C and the strain rates ranging from 0.1 s−1 to 2.1 s−1. And the processing maps exhibit the peak efficiency of power dissipation of 0.86 occurring at the deformation temperatures ranging from 1100 °C to 1140 °C and the strain rates ranging from 0.1 s−1 to 0.16 s−1, which is correspondent to the optimal deformation condition of 300M steel.

► DRV is the dominant softening mechanism at a deformation temperature of 900 °C.
► DRX has completed and the austenite grain growth is in progress at 1100 °C.
► Q-values are 332 ± 61 to 397 ± 31 kJ mol−1 which is related to microstructual evolution.
► Coarse macro-grains of 300M steel are observed above 1140 °C.
► The peak efficiency of power dissipation occurs at 1100–1140 °C and 0.1–0.16 s−1.