Microstructure tailoring for property improvements of DP steel via cyclic intercritical annealing

Multi-step intercritical annealing treatment was applied on the quenched and cold rolled martesnitic microstructures to unravel the effects of thermal cycling on dual phase (DP) steel. During intercritical annealing of the quenched microstructure, hardness decreased with increasing holding time and reached a plateau. However, during intercritical annealing of the cold rolled microstructure, there was a rapid fall of hardness due to recrystallization. Afterwards, hardness increased as a result of austenitization, and then reached a plateau. A rapid grain growth was observed during repetition of intercritical annealing cycle for the DP steel originated from the quenched microstructure with adverse effects on the tensile properties. Conversely, decreased size of martensite islands and formation of chain like morphology of martensite around ferrite grains was observed during thermal cycling of the DP steel originated from the cold rolled microstructure with the consequent enhanced work hardening behavior. Accordingly, the combination of cold rolling and cyclic intercritical annealing was recommended in this work to enhance the work hardening behavior and mechanical properties of DP steels.