Tailoring banded-equiaxed heterogeneous structure and martensite variants to achieve high toughness and high ductility in a newly designed 2.4 GPa high-strength steel

Ultra-high strength steels with a strength level of 2000 MPa are critical structural materials for some extreme service environments but face problems of low ductility and bad toughness. Current research efforts often focus on improving individual property, such as elongation or toughness. Therefore, it remains a significant challenge to unify both features of high strength, high ductility, and high toughness in one material. Adding precious metals such as Ni and Co and using aging treatment can achieve good strength and ductility in the maraging steels, but the cost is too high. In this study, we report a lean 2.4 GPa ultra-high-strength steel with a uniform elongation of 7.7 % and a V-notched impact toughness of 29 J/cm², which shows a competitive advantage compared with existing aircraft landing gear steels. The alloy composition design of “Mn+ microalloying” and simple possessing route of quenching, deep cryogenic treatment, and low-temperature annealing (Q-D-L) are used to achieve low-cost preparation. The transformation behaviors and mechanisms of strengthening, ductilizing, and toughening are discussed. The developed steel possesses a fine banded-equiaxed heterogeneous original austenite structure, where the CP4 occupies most of the equiaxed austenite, with more high-angle grain boundaries, and the martensitic variant of the banded structure is selectively weakened, resulting in a more uniform deformation, so that the crack nucleation energy and propagation energy can be simultaneously improved. Besides, the low aspect ratio structure originating from the fine parent austenite is beneficial to stimulating out-of-lath plane and in-lath plane multiple slip systems, compared with the coarse martensite with geometric lath constraints, thus increasing the deformation capability of martensite. Consequently, with the yield strength of the sample increased to 1960 MPa, the uniform elongation remained as high as 7.7 %, indicating a notable improvement in both strength and ductility compared to samples with coarse banded austenite structures (1718 MPa & 7.6 %). This study provides new insights into alloy design and processing strategies for the synergistic enhancement of multiple properties in ultra-high-strength steels.