Improved Mechanical Properties of Structural Steel via Developing Bimodal Grain Size Distribution and Intercritical Heat Treatment

Intercritical annealing and tempering heat treatment combined with cold rolling were applied for improvement of tensile properties and strain-hardening rate of structural steel. Coarse dual-phase (DP) microstructure was processed via intercritical heat treatment of the ferritic–pearlitic sheet, which resulted in the increase in tensile strength and disappearance of the yield point phenomenon. However, the tensile toughness was not effectively enhanced, which is indicative of unaltered strength–ductility trade-off. A bimodal-sized ferrite structure was obtained by cold rolling of the DP microstructure followed by subcritical annealing (tempering), where its characteristic features are high yield stress and relatively poor work-hardening response. Afterward, the intercritical annealing of the bimodal steel resulted in the development of fine DP steel with excellent work-hardening capacity and pronounced enhancement of tensile toughness. Conclusively, this work demonstrates the possibility of controlling the tensile properties and work-hardening behavior of conventional steels by simple heat treatment routes with the aid of cold rolling.