Tensile flow stress of an ultra low carbon steel was investigated at temperatures between 77K and 293K with strain rates from 2×10-2/s to 2×103/s. Stress-strain curves were described by using the Kocks-Mecking model in which several parameters were determined from the experimental data obtained by conventional tensile tests with lower strain rates less than 2×100/s. It is found that the flow stress at 2×103/s estimated by the Kocks-Mecking model agrees well with the stress measured by Hopkinson-bar method. The decrease in work-hardening rate at 2×103/s was discussed from the viewpoints of temperature rise and microstructural evolution during high speed tensile deformation, leading to a conclusion that the former was a main reason.
