Effect of the flow behavior and dynamic recrystallization of EH420 marine steel on the morphology evolution of martensite substructures

The hot deformation behavior of EH420 marine steel was investigated by isothermal compression tests in the temperature range of 850-1050 degrees C and the strain rate range of 0.01-10 s- 1. A high-temperature constitutive model and a hot processing map were developed. The results indicated that the microstructure of EH420 marine steel after hot deformation was mainly bainite and lath martensite. The martensite blocks gradually coarsened with the increase in temperature within the range of 950-1050 degrees C/0.01 s- 1, and the average width increased from about 0.7 mu m to 2.3 mu m. At high strain rates, martensite blocks became coarse and disordered due to flow instability. The flow stress curve showed a single peak when the strain rate was low. There were more dynamic recrystallization (DRX) structures, resulting in fine and ordered martensite blocks. Under the deformation conditions of 0.1-10 s- 1/1000 degrees C, the proportion of high-angle grain boundaries (HAGBs) decreased from 53.5 % to 40.7 % as the strain rate increased. Meanwhile, the hot deformation mechanism shifted from discontinuous dynamic recrystallization (DDRX) to the combined effect of DDRX and dynamic recovery (DRV) and finally transformed into DRV. The flow stress derived from the constitutive model was consistent with the experimental results. The activation energy of EH420 marine steel was 3.16x105 J/mol. The optimal hot processing parameters were 950-1050 degrees C and 0.01-0.1 s- 1.