Synergistically achieving high strength and ductility in low-carbon steel sheet with bimodal microstructure

• The PSN mechanism was activated by the fragmented GBC particles. • The steel showed BGSD including fine grains (1–5 µm) and coarse grains (>5 µm). • The steel with BGSD revealed highest strength, and good ductility and toughness. • The amount of pre-strain has a significant effect in achieving BGSD. • Fracture surface of the steel with BGSD revealed both fine and coarse dimples. In the present research, simple thermomechanical processing (cold rolling and short-term annealing at 550 °C for 45 min in an air atmosphere) was used to achieve bimodal grain size distribution (BGSD) in the AISI 1016 low-carbon steel sheet. There were both eutectoid Fe 3 C (with lamellar morphology) and grain boundary cementite (GBC) in the as-received steel. Not performing the homogenization treatment in the present work led to the activation of the particle-stimulated nucleation (PSN) mechanism by the fragmented GBC particles. The grain size distribution of the produced steel showed BGSD including fine grains (1–5 µm) and coarse grains (>5 µm). Also, a large number of fine spherical Fe 3 C particles were formed in pearlite colonies. The produced steel with BGSD and fine spherical Fe 3 C particles revealed the highest hardness (223.5 HV), yield strength (733.6 MPa) and ultimate tensile strength (748.2 MPa), good ductility (12.9 %) and toughness (104.4 J /cm 3 ). The amount of pre-strain had a significant effect in achieving BGSD and on the final mechanical properties. On the fracture surface of the produced steel with BGSD, there were both fine and coarse dimples.