Effect of Pre-intercritical Annealing Treatments on the Microstructure and Mechanical Properties of 0.33% Carbon Dual-Phase Steel

The present study is aimed at understanding the effect of different prior heat treatments on the microstructure and mechanical properties of 0.33% carbon dual-phase (DP) steel. For this purpose, different dual-phase steels were produced by subjecting the as-received steel to ‘hardening’ (DP-H), ‘hardening + tempering’ (DP-HT), and ‘austempering’ (DP-AT) treatments prior to the intercritical annealing treatment. The study reveals that the prior hardening treatment in DP-H steel results in significant refinement of ferrite grains and formation of fine elongated martensite plates having an aspect ratio = 6.1 ± 3. These fine elongated martensites are responsible for poor ductility in DP-H steel. Although DP steel without any prior treatment (DP-AR) and DP-AT steel exhibit high strength and hardness, their ductility is limited by the presence of very hard martensite islands which act as the failure initiation sites in these steels. On the other hand, prior ‘hardening + tempering’ treatment in DP-HT steel results in the formation of nearly spherical-shaped martensite (aspect ratio = 1.24 ± 0.13) along with coarse martensite laths. In addition, the presence of fine carbides is also found along the lath boundaries. These fine spherical martensites and fine carbides provide higher strain hardening to DP-HT steel. Accordingly, DP-HT steel exhibits adequate ductility as well as strength. Therefore, prior ‘hardening + tempering’ treatment was found to the best prior treatment in the present study.