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.

Automated summary

The improvement of strength in low-carbon steel usually leads to a reduction in ductility due to the low strain-hardening rate of ultrafine-grained materials. The heterostructured strategy, achieved through phase transformation, offers a new way to enhance the strain-hardening rate. In this study, intercritical annealing was used to produce a dual-phase heterostructure in low-carbon steel, leading to improved strength and ductility. The presence of ferrite and martensite in the heterostructured steel resulted in a higher density of low-angle grain boundaries and geometrically necessary dislocations, promoting strain partitioning and ultimately improving the combination of strength and ductility.