Strengthening and strain hardening mechanisms of a plain medium carbon steel by multiscale lamellar structures

Abstract

Plain medium carbon steel with the multiscale lamellar structure was produced through quenching and subsequent warm rolling. The microstructure characteristic was conducted based on statistical parameters and physical metallurgy by electron backscattered diffraction (EBSD), the mechanical properties were investigated using uniaxial tensile tests at room temperature. The heterogeneous structure is characterized with multiscale lamellar distribution with three-level grain size spanning the nano-to micro-meter range, which is defined as ultrafine-grained (UFG), fine-grained (FG) and coarse-grained (CG). The results of kernel average misorientation (KAM) and Taylor factor (TF) revealed that the existence of interface effects. The nucleation and accumulation of geometrically necessary dislocation (GND) were activated by multiscale lamellae, to cause the formation of sharp up-turn in strain hardening curves. The results of multiple model fits showed that the Swift model was the most appropriate for the studied steel, and the instantaneous strain hardening values were obtained on its basis. The existence of an extra strengthening mechanism (hetero-deformation induced strengthening) was confirmed by calculating. The results showed WR84 had outstanding mechanical properties and strain hardening ability. The improved toughness of WR84 is mainly attributed to ultrafine grain strengthening, and the reasonable ductility is owing to hierarchical hetero-structured of the lamellae, particle dispersion distribution and the three-level coordinated control of grain size as they can increase the strain hardening ability by the interaction constraint.