Abstract: The inelastic recovery behavior of DP780 steel was studied in continuous cyclic loading-unloading tensile tests. The composition of springback and the percentage of inelastic strain recovery of the steel were determined. Then, a mathematical model of unloading modulus and plastic strain was proposed. The model was verified by numerical simulation and experiment. It is shown that the plane anisotropy of DP780 steel is not obvious. The unloading and loading paths constitute a closed loop, which is formed due to the nonlinearity of the elastic modulus in the unloading and reloading cycles. With the increase of the pre-strain, the elastic modulus decreases rapidly, which then increases slowly and gradually tends to be stable after the pre-strain reaches 6%, the unloading elastic modulus decreases by 17%. The total unloading strain recovery is composed of elastic strain recovery and inelastic strain recovery, and the inelastic strain recovery can reach as high as 11% of the total strain recovery. The microstructures of DP780 steel were detected by SEM and TEM, and the microscopic mechanism of the inelastic recovery was revealed. It is shown that the plastic deformation and austenitic phase transformation cause the increase of dislocation density; the martensite dislocation moving obstacles lead to a large amount of dislocation pile-up and the movable dislocation bending, increasing the length of the dislocation line and additional elastic deformation. This results in significant inelastic recovery behavior. Integrating the model to the finite element simulation improves the accuracy of simulated springback and the mold springback compensation is more accurate.

Key words: materials synthesis and processing technology, DP780 steel, elastic modulus, closed loop, inelastic recovery, dislocation pile-up