Improved toughness and ductility in ferrite/acicular ferrite dual-phase steel through intercritical heat treatment

The effect of intercritical heat treatment on mechanical properties of ferrite/acicular ferrite dual-phase steels with and without tempering has been investigated in this study. To obtain different ferrite/acicular ferrite microstructures, HSLA steels were subjected to step normalizing (SN) and intercritical normalizing (IN) to develop polygonal ferrite and lath-type ferrite. The results reveal that IN can improve the toughness and ductility with or without tempering. IN decreases the formation temperature range of acicular ferrite, and can refine the dual-phase microstructure and improve the ductility and toughness. However, SN results in the decrease of toughness and ductility due to large grain sizes and low density dislocations. Tempered IN specimen still retains refined microstructure and has more homogeneously distributing fine M23C6 carbides at grain boundaries. Moreover, tempering leads to the minimum loss in strengths and maximal improvement in toughness and ductility for IN specimen. Refined morphology and homogeneous distribution of M23C6 carbides can be responsible for the improved mechanical properties of tempered IN specimen.