The dynamic tensile behavior and deformation mechanism of ultra-high-strength dual-phase (UHS-DP1000) steel were investigated over a wide range of strain rates from 10−4 to 103 s−1. As the strain rate increases, the transition strain decreases from 2.73 to 1.92, and the martensite plastic deformation starts earlier. At strain rate of 10−4–0.5 s−1, the inhomogeneous plastic deformation ability increases because the dislocation density in the ferrite matrix increases. This leads to a decrease in uniform elongation and an increase in fracture elongation. When the strain rate increases from 0.5 to 500 s−1, the amount of mobile dislocation increases, which is the main reason for the enhancing uniform elongation and fracture elongation. Meanwhile, because the dislocation motion resistance rapidly increases, the yield strength and ultimate tensile strength also increase. When the strain rate is higher than 500 s−1, the hardening behavior caused by the dislocation motion resistance has not been offset by softening due to the mobile dislocation and adiabatic heating. The voids at the early stage of deformation could not uniformly form and grow, and thus the homogeneous plastic deformation ability decreases.
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The authors would like to acknowledge the financial support of the National Key R&D Program of China (Grant No. 2017YFB0304404) and Shandong Provincial Natural Science Foundation of China (Grant No. ZR2018MEM007).