Strain rate-temperature effects and constitutive models for Q690D QT steel

This paper investigates the mechanical properties of the high-strength structural steel Q690D QT (Quenched and Tempered) over a wide temperature range and a wide strain rate range. Firstly, a series of quasi-static and dynamic tests adopting the universal testing machine and Split Hopkinson Pressure Bar were carried out to investigate the mechanical properties at strain rates from 0.00025 s-1 to 5000 s-1 and temperatures from room temperature (20 degrees C) to 700 degrees C. Stress-strain curves and dynamic yield strength increase factors were extracted and investigated. Test results show that Q690D QT exhibits different strain rate effects at different temperatures, more significant at the temperatures of 600 and 700 degrees C, as well as the high-temperature softening effect. Dynamic strain aging effect is identified at temperatures between 300 - 600 degrees C for plastic strain rates of 800 s-1 and 2300 s-1, nevertheless, this effect is observed at a higher temperature range of 500-700 degrees C for higher plastic strain rates of 4000 s-1 and 6400 s-1. From the analysis, the adiabatic thermosoftening effect cannot be ignored when the plastic strain rates reach 4000 s-1. Finally, an improved form of the Johnson-Cook model is proposed for capturing the adiabatic temperature effects at different temperatures and validated by the test data. A parameter dynamic temperature factor is proposed to describe the comprehensive effect of strain rate, test temperature, and adiabatic thermosoftening.