Plasticity enhancement mechanism of DP600 steel sheets during uniaxial quasi-static/dynamic forming

The main goals of this work are to explore the macro- and micro- mechanisms of plasticity enhancement of DP600 steel sheets during the hybrid uniaxial tension consisting of quasi-static forming and then dynamic electrohydraulic forming (EHF). The experimental results showed that the room-temperature plasticity of DP600 sheets during the hybrid forming process was improved significantly: the limit strain was improved by 35 %, and the total percentage elongation was improved by 25 % in contrast to that during the quasi-static forming process. Then, the mechanical behavior, fractography and microstructure evolution of the specimens were analyzed. The quasi-static pre-deformation led to the change of the material constitutive of DP600 steel at high strain rate, improved the formability in the subsequent dynamic step and resulted in an excellent final plasticity. The inertial effect in the dynamic EHF process and the additional stress increment Δσ_θ caused by quasi-static pre-deformation reduced the velocity gradient of the mass particles along the tensile direction after starting plastic instability, thereby coordinating the subsequent deformation and enhancing the final plasticity of the hybrid uniaxial tension specimens. Under high strain rate, the nucleation from the existing dislocations caused higher dislocation density in the ferrite phase, which offset the easy stacking, winding, and uneven distribution of the dislocations caused by quasi-static pre-deformation. Besides, typical [0 1−1]_MT deformation twins presenting in the martensite phase, enhanced the plastic compatibility of the ferrite and martensite phases and contributed to the plasticity improvement.