Mechanistic Study on Nb Microalloying Synergistically Enhancing the Cryogenic Strength–Toughness of Austenitic Stainless Steel

Overcoming the strength–ductility trade-off at cryogenic temperatures is a key bottleneck for austenitic stainless steels. This study investigated the effects of Nb additions (0, 0.03, and 0.06 wt.%) on the mechanical properties and fracture behavior of 304 steel from room temperature (RT) to −196 °C. Within the investigated range, the 0.06 wt.% Nb steel produced the largest measured response. Compared with the Nb-free steel, the RT yield strength and elongation after fracture increased by ~5% and 2.4%, respectively; at −196 °C, they increased by 11.1% and 4.1%, and impact toughness by 31.8%, mitigating the low-temperature strength–ductility trade-off. Fractography revealed a transition from brittle cleavage/mixed fracture to dimple-dominated ductile fracture. The improvement in mechanical properties is consistent with the refined microstructure and the modified inclusion distribution, with the underlying mechanisms likely involving grain refinement and the solid-solution strengthening of the austenite matrix by Nb. It should be noted that, owing to experimental limitations, the deformation-induced α′-martensite was not quantitatively characterized in the present study, and no rigorous quantitative correlation has yet been established between inclusion characteristics and low-temperature impact toughness. Consequently, the relative contribution of each contributing factor to the observed performance enhancement remains unresolved. Nevertheless, this work provides experimental support for the microalloying design of ultra-low-temperature austenitic stainless steels.