Enhancement of mechanical properties of low carbon steel based on heat treatment and thermo-mechanical processing routes

Thermal treatments and thermo-mechanical processing routes were applied on a conventional structural steel (st37 steel: 0.12C-1.11Mn-0.16Si) for improvement of tensile properties and enhancement of work-hardening behavior. Full annealing resulted in a sheet with coarse ferrite grains and pearlite colonies arranged alternatively in distinct bands, which showed high ductility, low strength, and the presence of the yield point elongation at the beginning of the plastic flow. The cold-rolled sheet, however, showed poor ductility but much higher strength level. The dual phase (DP) sheet, resulted from intercritical annealing in the austenite plus ferrite region, showed a remarkable strength-ductility balance. The latter was related to the excellent work-hardening behavior as a result of the glide and interaction of the quench-induced unpinned dislocations. A bimodal-sized ferritic structure with the appearance of a poor strain hardening regime after experiencing a high yield stress was obtained from the subcritically annealed cold-rolled DP microstructure. The ultrafine-grained sheet was processed by applying the abovementioned route on a martensitic microstructure, which resulted in low ductility but high strength at ambient temperature. These results demonstrated the ability to control the properties of conventional steels by simple thermal and thermo-mechanical treatments. Low carbon steel, Grain refinement, Mechanical properties, Strain hardening rate