Flow stress analysis using the Kocks-Mecking model for ferrite-cementite steels with various ferrite grain sizes

True stress (sigma)-true strain (epsilon) curves were calculated by using the Kocks-Mecking (KM) model for the ferrite-cementite steels with various ferrite grain sizes between 0.47 and 13.6 mu m. In the KM model, the effect of ferrite grain size on flow stress is described by the athermal stress component that follows the Hall-Fetch equation. The effects of temperature and strain rate on flow stress, which are correlated with the thermal stress component, are independent of the ferrite grain size. The calculated sigma-epsilon curves by using the KM model agree with the measured ones at various temperatures and strain rates including the high-speed tensile test with a strain rate of 10(3)s(-1). From the calculations based on a micromechanic model, it is found that the volume fraction of second phase affects the grain size dependence in multi-phase steels. The m-value showing strain rate sensitivity for the external stress was decreased with a decrease in grain size and that for the thermal stress was independent of grain size.