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

Abstract

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 Fe3C (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 Fe3C particles were formed in pearlite colonies. The produced steel with BGSD and fine spherical Fe3C 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/cm3). 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.

Keywords

• Bimodal grain size distribution (BGSD)
• Rolling strain
• Mechanical properties