Investigation of the relationship between the microstructural evolution mechanism and mechanical properties in hot rolled Fe-0.2C–6Mn–3Al steel

The objective of the present research is to investigate the influence of the microstructural evolution mechanism on mechanical properties during deformation. For this purpose, hot rolled Fe-0.22C-6.12Mn-3.08Al steel is subjected to intercritical annealing at 740 °C for 10 min. The annealed specimen contains a mixture of lath-like α-ferrite and reverted γ, whose volume fraction is approximately 38.7%. During deformation, once the critical stress caused by the severe dislocation pile-ups is reached, the strain/stress-induced phase transformation occurs. First, grain boundaries of γ/α serve as the preferential nucleation sites for fresh α′-martensite. Subsequently, fresh α′-martensite nucleates inside reverted γ. In the meantime, the interfacial α′-martensite grows continuously into the reverted γ interior. During deformation (prestrain at 0.0%, 5.1% and 7.8%), the misorientation (MO) in α-ferrite+α′-martensite continuously rises from 0.63° to 0.67°; nevertheless, the MO in reverted γ increases from 0.65° to 0.73° and then rapidly decreases to 0.69°. Hence, the phase transformation of reverted γ is accompanied by obvious stress softening; thus, effective work hardening occurs in the subsequent deformation region due to α-ferrite and fresh α′-martensite. This phenomenon greatly enhances the strength and delays necking, e.g., the annealed specimen possesses outstanding mechanical properties (992 MPa, 47.6% and 47.2 GPa × %). Once necking occurs, the residual reverted γ is rapidly consumed in the necking region. Hence, the hardness in the necking region (5.31 ± 0.20 GPa) is much higher than that in the uniform deformation region (4.53 ± 0.17 GPa) due to the abundant hard fresh α′-martensite.