Effect of microstructural refinement and intercritical annealing time on mechanical properties of high-formability dual phase steel

The impact of the intercritical annealing time and refinement of microstructure upon the tensile properties and work-hardening capacity of a dual phase (DP) steel were evaluated. For the undeformed pre-intercritical microstructures, the sensitivity of the obtained DP microstructures on the time of annealing was low, and in all cases, a relatively coarse microstructure was obtained. It was shown that a fine-grained DP steel with chain-network martensite morphology can be readily obtained by carefully controlled intercritical annealing of a cold deformed martensitic microstructure. By continued intercritical annealing beyond the optimum value, the growth of ferrite grains and fading of the chain-network martensite morphology were found to be responsible for the lowering of work-hardening rate and obtained tensile properties. By optimization of the intercritical annealing condition, a DP300/600 steel with high work-hardening rate, low yield ratio, high tensile toughness, and good ductility was obtained, which exhibited significant enhancements compared with the conventional DP350/600 grades.