A novel process to obtain lamella structured low-carbon steel with bimodal grain size distribution for potentially improving mechanical property

A lamella structured low-carbon steel plate with bimodal grain size distribution (LSBG steel) was produced by a two-step warm rolling and subsequently annealing, and its mechanical properties, strengthening and toughening mechanisms were studied. The heterogeneous lamellar structure is characterized with ultrafine-grained (UFG) lamellae (with average grain diameter about 1 μm) embedded in coarse-grained (CG) lamellae matrix. The LSBG steel shows an improved combination of strength and toughness when compared with corresponding CG specimens, and also evades strength-ductility trade-off compared with UFG ones. When comparing with initial CG steel, the yield strength and tensile strength are increased by 87.4% and 35% respectively, but the ductility is only with a small sacrifice, and the ductile-to-brittle transition temperature is significantly decreased from about −70 °C to −110 °C. The improved strength is mainly attributed to ultrafine grain strengthening, and the reasonable ductility can be attributed to both the bimodal grain size and the lamellar structure as they can increase the work hardening rate by the accumulation of geometrically necessary dislocations in their vicinity. And the improved toughness of the LSBG steel is thought to be mainly attributed to grain refinement and the lamellar structure.