Hot-deformation characteristics of Al-alloyed δ-ferritic and martensitic dual-phase steel

Abstract

The hot working behavior of Al-alloyed δ-ferritic/martensitic dual-phase steel was investigated using a Gleeble-3800 thermomechanical simulator and compression tests at deformation temperatures and strain rates ranging from 800 to 1100 °C and 0.1 to 3 s−1, respectively. The shape of the flow stress curve of the experimental steel resembled that of traditional dynamic recovery (DRV) flow stress curves produced by work hardening, ferritic DRV, ferritic dynamic recrystallization (DRX), and austenitic dynamic phase transformation (DPT). Additionally, constitutive equations were established to predict the steady-state stress. The apparent deformation activation energy of the steel was approximately 264.7 kJ/mol. Moreover, the microstructure following deformation was investigated using electron backscatter diffraction. The primary dynamic restoration mechanisms of δ-ferrite comprised DRV and DRX at temperatures below and above 900 °C, respectively. However, with increasing deformation temperature and strain rate, the primary dynamic restoration mechanism of δ-ferrite shifted from continuous DRX (CDRX) to discontinuous DRX (DDRX). Additionally, the processing maps of the steel were established using the dynamic materials model. Further, based on the microstructure results, it was found that the power dissipation efficiency of ferritic CDRX, ferritic DDRX, ferritic DRV, and austenitic DPT gradually decreased. Additionally, only one domain of flow instability was identified, and it was located at 1025–1075 °C/0.6–3 s−1. Thus, to avoid unstable deformation regions, a two-stage design of the optimal process parameters for industrial processing is highly essential.