Thermal-mechanical and springback behavior of dual-phase steel at warm temperatures

for non-quenchable dual-phase (DP) steel sheet, the warm forming process can effectively reduce the amount of springback, and the mechanical parameters that influence its elastic and inelastic recovery to decrease exhibit a strong temperature dependence, especially under cyclic loading conditions. In this paper, the monotonic and cyclic loading tests of DP980 steel sheets are conducted at the temperatures ranging from 25 degrees C to 500 degrees C. The temperature-dependent flow stress, nonlinear elastic recovery, and Bauschinger effect are investigated. The results demonstrate that both the elastic modulus and Bauschinger effect show an exponential law with pre-strain, and decrease with the increase of forming temperature, while there will be an abnormal phenomenon of rebound due to the influence of dynamic strain aging effect. Meanwhile, a linear relationship between the Bauschinger effect and inelastic strain is observed at various temperatures, and the weight of the Bauschinger effect in the total strain reduces with temperature increasing, which indicates that the springback is dominated by linear elastic recovery. Furthermore, the U-draw bending tests are carried out to clarify the influence of Vickers hardness distribution and martensite size effect on the springback behavior.