Achieving ultrahigh strength by tuning the hierarchical structure of low-carbon martensitic steel

The high strength of martensitic steel can be attributed to its fine hierarchical structure and high dislocation density. To further improve the mechanical properties of low-carbon martensitic steel, a nano-lamellar structure in size of 28 nm was first produced by warm rolling on the dual-phase structure. Subsequent annealing at 870 °C for different times was performed to tune the hierarchical structures of the martensitic steels with different prior austenite grain (PAG) sizes. The WRQ-15 steel sample, which underwent a 15-min annealing process, exhibits an outstanding combination of strength and elongation with a yield strength (YS) of 1.55 GPa, ultimate tensile strength (UTS) of 2.0 GPa, and elongation to failure (ETF) of 8.7%. It shows higher strength than the martensitic sample annealed from undeformed steel (UQ-15 steel), which has a YS of 1.31 GPa and UTS of 1.68 GPa. The WRQ-15 sample has a finer PAG size (7.2 μm) than the UQ-15 sample (9.5 μm). The results reveal that the refined PAGs lead to further refinement of packets, blocks, and laths, as well as increased dislocation density, contributing to the superior mechanical properties of the WRQ-15 sample. Moreover, the refined hierarchical structures provide high-density boundaries, impeding the propagation of cleavage cracks to improve the toughness. However, the excessive refinement of the hierarchical structures, increase of dislocation densities, and the undissolved carbides in the WRQ-1 sample are harmful for the strength-toughness balance.