Thermo-viscoplastic behavior of DP800 steel at quasi-static, intermediate, high and ultra-high strain rates

Using the shear-compression specimen (SCS), thermo-viscoplastic behavior of DP800 steel has been investigated systematically over a wide range of strain rates, including quasi-static (0.001–1 s⁻¹), intermediate (10–100 s⁻¹), high (2000–8000 s⁻¹) and ultra-high (16,000–50,000 s⁻¹) regions. Temperatures between 293 and 1073 K were considered. Experimental results show: (1) DP800 steel exhibits non-linear strain rate sensitivity. For tests at strain rates lower than 4000 s⁻¹, the flow stress increases slightly. However, for tests at strain rates above 8000 s⁻¹, a significant flow stress upturn exists; (2) dynamic strain aging (DSA) occurs within the temperature range of 473–873 K and strain rates below 100 s⁻¹; (3) both strain rate sensitivity and temperature sensitivity of DP800 steel are influenced by DSA, and bell-shaped sensitivity curves form; (4) with increasing strain rates, peaks of the bell-shaped temperature sensitivity curves move to higher temperature regions, and this law is equally valid for the effect of temperature on the strain rate sensitivity curves; (5) strain hardening behavior of DP800 steel is dominated by a competition between strain rate hardening, DSA hardening and thermal softening. With increasing strain rate, single peak or double peaks are observed in average strain hardening rate-strain rate curves. According to experiments, a semi-physical constitutive relation has been established, taking into account both the viscous drag effect and the DSA phenomenon. The established constitutive model can predict flow stress curves of DP800 steel accurately with an average error of 7.12%. Both the enhanced strain rate sensitivity due to viscous drag and the bell-shaped flow stress curves caused by DSA are captured correctly.