Unprecedented high strength and ductility in plain low-carbon steel via trimodal grain size distribution

In this work, unprecedented high strength and ductility in plain low-carbon steel (PLCS) was achieved via the formation of trimodal grain size distribution (TGSD). After intercritical annealing, 75% cold deformation, and post-annealing, a TGSD includes three groups of α grains with sizes smaller than 1 μm (ultrafine grains), between 1 and 5 μm (fine grains), and larger than 5 μm (coarse grains) were formed. Both yield strength and tensile strength of annealed samples were higher than that of the as-received sheet owing to the formation of TGSD and fine θ particles. By increasing the intercritical annealing temperature, the strength of the post-annealed samples was enhanced. The formation of TGSD had a great effect on improving the mechanical properties of PLCS and could produce much higher yield strength (39% higher than the steel with bimodal microstructure), larger tensile strength (28% higher than the steel with bimodal microstructure), and better ductility (49% higher than the steel with bimodal microstructure). The mechanical properties of the produced ferritic-pearlitic steel were in the range of dual-phase (DP) and transformation-induced plasticity (TRIP) steels. The number, size, and depth of the dimples were increased after the final annealing and the annealed sheets indicated a ductile fracture.