Effect of strain rate on true stress-true strain relationships of ultrafine-grained ferrite-cementite steels up to plastic deformation limit

The true stress (σ)-true strain (ε) relationships up to the plastic deformation limit of low-carbon ferrite-cementite (FC) steels with ferrite grain sizes between 0.5 and 34 μm were estimated at strain rates between 3.3×10−1 and 5.0×10−4 s−1 to investigate the effect of the strain rate on the σ-ε relationship. Both σ and ε increased with increasing strain rate for each of the FC steels considered, and larger work-hardening rates were observed with increasing strain rate in the ultrafine-grained FC specimens with average ferrite grain sizes less than 1 μm. Increasing the strain rate was found to be effective in improving σ and ε up to the plastic deformation limit. The important factors in obtaining good σ-ε relationships in FC steels are the loads and the radius of curvature of the neck profile after the maximum load point. On the basis of experimental results that both σ and ε can improve under high-speed deformation such as at 10−1 s−1 (as in static tensile tests) and that the decrease in ductility with increasing strain rate was small, grain refinement of up to 0.8 μm was concluded to be effective in improving the tensile deformation behavior of the low-carbon FC steels considered in this study.