Dual-phase hetero-structured strategy to improve ductility of a low carbon martensitic steel
Jie Huang, Ying Liu, Tao Xu, X.F. Chen, Qingquan Lai, Lirong Xiao, Zhiyi Pan, Bo Gao, Hao Zhou, Yuntian Zhu
Abstract
Martensitic transformation significantly increases the strength of low-carbon steels, while it is usually at expense of the formability and ductility. In order to further improve the mechanical properties of low carbon martensitic steel, the strategy of dual-phase heterostructure was proposed. The steel with nano-lamellar structure in size of 83 nm was produced by cyclic annealing & cold rolling (AnnCR) on the martensitic structure. Then, the ultrafine-grained heterostructured dual-phase (UFG-HSDP) steels with outstanding combination of strength and ductility were achieved by subsequent short-time intercritical annealing. A promising heterostructure of soft ferrite grains completely embedded in hard martensite grains was formed in the sample annealed at 820 °C. A high strength of ∼1.1 GPa, close to the as-quenched full martensite steel, was retained in the HSDP steel. While, the uniform elongation was significantly improved to 6% by tailoring the dual-phase distribution. Hetero-deformation induced (HDI) stress, derived from the mechanical incompatibility of the dual-phase, is proposed to provide an extra strain hardening in the HSDP steels. Detailed microstructure analysis indicates that geometrically necessary dislocations piled-up near the zone interfaces produce a long-range back stress in the ferrite zones as well as a corresponding forward stress in the martensite zones, collectively resulting in the hetero-deformation induced (HDI) stress.