Dynamic recrystallization behavior during hot deformation of as-cast 4Cr5MoSiV1 steel

The dynamic recrystallization behavior of as-cast 4Cr5MoSiV1 steel was studied by hot compression tests conducted at various temperatures (900-1150 degrees C) and strain rates (0.01-10 s(-1)). Flow stress curves and microstructural observation were employed to experimentally identify the various flow mechanisms during deformation. A revised Sellars' constitutive equation was adopted to construct the thermal activation energy map, which considered the effects of deformation temperature and strain rate on the material variables. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) type equation X-D = 1-exp[-k(epsilon-epsilon(c)/epsilon(p))(m)], (epsilon >=epsilon(c)) was applied to characterize the evolution of dynamic recrystallization (DRX) volume fraction. The nucleation of DRX was performed by the bulging, sub-grain swallowing. The presence of dendrite segregation would affect the DRX to a large extent that the segregated alloying elements and carbide precipitates inhibited the migration of boundaries. The thermal activation energy varied from 4310 to 470 kJ/mol and the thermal activation energy increased sharply at temperature below 1000 degrees C due to the dendrite segregation. By further analysis of the true stress-strain curves, the material constant m in JMAK type equation was determined to be 1.29366, indicating the DRX was difficult.