Significant Hetero-Deformation Induced Strain Hardening in a Dual-Phase Low-Carbon Steel

The strength improvement of low-carbon steel usually occurs at the expense of its ductility because the strain-hardening rate of ultrafine-grained material is relatively low. The heterostructured strategy is a new way to improve the strain-hardening rate in carbon steels, and is easy to achieve by phase transformation. In this work, intercritical annealing was performed on a low-carbon steel to produce a dual-phase heterostructure, which had higher strength and ductility than that of a uniform ferrite microstructure. An outstanding UTS of 960 MPa and a high uniform elongation of 16.5% were obtained in the heterostructured sample. In situ electron backscattered diffraction was performed to investigate the underlying deformation mechanism. Due to the mechanical incompatibility between ferrite and martensite, a much higher density of low-angle grain boundaries was found in the dual-phase heterostructured steel. Significant strain partitioning during deformation leads to a high density of geometrically necessary dislocations (GNDs) near zone boundaries. These GNDs provide hetero-deformation-induced strain hardening, eventually improving the combination of strength and ductility.