The effect of microstructure and strain rate on the stage III strain hardening and ductility of dual-phase steels

Abstract

The effects of microstructure and strain rate on stage III strain hardening (i.e. the strain hardening which occurs just prior to instability) and ductility in dual-phase steels were studied by tensile testing specimens produced by intercritically annealing 0.08C-1.45Mn-0.21Si steel. The steel was cold-rolled 50% and heat treated to obtain either normalized, martensitic or tempered martensitic microstructures prior to intercritical annealing at 717°C for 5 min and ice-water quenching. Some specimens were intercritically annealed directly after cold rolling. Stage III strain hardening was found to be a function of microstructure, with specimens produced from cold-rolled and normalized initial microstructures exhibiting higher strain hardening rates than those annealed with initially martensitic or tempered martensitic microstructures. The difference in strain hardening behavior was attributed to the effects of microstructure on the rate of dislocation substructure generation and consolidation. The strain dependence of strain hardening rates at high strain was shown to be an important factor in controlling the uniform strain. Three strain rates covering two orders of magnitude were used in strain rate jump tests to determine values of strain rate sensitivity. The strain rate sensitivity was 0.002 for all microstructures when tested at the lowest strain rates and 0.005 at the two highest rates. The strain rate dependence of the strain rate sensitivity was attributed to dynamic strain aging effects. Both uniform and post-uniform elongation increased with increasing strain rates. Low uniform elongations at low strain rates arose from lower strain hardening rates observed near instability. Post-uniform elongation increased because strain rate sensitivity increased with increasing strain rate.