Dual-phase hetero-structured strategy to improve ductility of a low carbon martensitic steel

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.