Hot Deformation Behavior and Constitutive Modelling of a Medium-Carbon Structural Steel

The purpose of this study was to investigate the effects of temperature and strain rate on the hot deformation behavior of a medium-carbon structural steel. This steel is known as 30X Gamma CA steel in GOST standard and contains 0.3 wt % carbon. To investigate the hot deformation behavior of this steel, hot compression tests were performed on cylindrical samples in temperature ranges from 800 to 1000 degrees C and strain ranges from 0.001 to 0.1 s(-1). Microstructure of samples was characterized using optical microscopy (OM) and scanning electron microscopy (SEM). The true stress-strain curves of 30X Gamma CA steel obtained at various deformation conditions showed that the softening phenomenon based on recovery and dynamic recrystallization (DRX) occurred during hot deformation tests. The results indicated that the peak stress decreased with increasing temperature and decreasing strain rate. With increasing temperature from 800 to 1000 degrees C, the peak stress decreased by 52, 60, and 38% at strain rates of 0.001, 0.01 and 0.1 s(-1), respectively. According to the results, it can be claimed that the DRX was a dominate mechanism of softening in all deformation conditions. The changes of work hardening rate for 30X Gamma CA steel during hot deformation were analyzed to confirm the results. Accordingly, it was observed that critical stress for the initiation of DRX decreased with increasing temperature and decreasing strain rate. Furthermore, constitutive equations and activation energy of deformation for 30X Gamma CA steel were successfully determined. Based on the hyperbolic sine equation, the activation energy of hot deformation of 30X Gamma CA steel was 256.11 kJ/mol.