Effect of heat treatment path on the cold formability of drawn dual-phase steels

Dual-phase steel has received much attention as a candidate for non-heat treated steel due to its high strain hardening capability. Present study was performed to examine the optimum microstructure and corresponding heat treatment path revealing excellent cold formability of drawn dual-phase steel. Microstructures were varied by three different heat treatments (or quenching paths), namely intercritical quenching (IcQ), intermediate quenching (ImQ), and step quenching. Resulting microstructures consisted of ferrite and martensite phases, but the volume fraction and morphology were largely varied by the quenching paths. Tensile and compression tests for all three microstructures showed continuous yielding curves with high strain hardening exponent. However, after imposing some amounts of drawing strain, all the specimens were significantly strengthened losing strain hardening capability. Cold formability was studied by estimating both the forming limit and the deformation resistance. The IcQ microstructure showed excellent forming limit and deformation resistance as compared to other two microstructures. It was attributed to the beneficial morphology of the IcQ microstructure, which effectively suppressed crack propagation.