Mechanical behavior of 304 austenitic stainless steel with large-scale gradient-bimodal structure fabricated by variable-section cryogenic-rolling and annealing

A low-cost variable-section cryogenic-rolling and annealing process was used to prepare 304 austenitic stainless steel (ASS) with a large-scale gradient-bimodal (LGB) structure, aiming to synergistically optimize its strength and ductility. Microstructural characterization, full-field strain measurement and deformation mechanism analysis were employed to explore the process-microstructure-mechanical behavior correlation. Results showed that after 50%–70% reduction ratio variable-section cryogenic-rolling plus annealing at 650–700 °C or cyclic annealing at 760–800 °C, the steel formed a gradient structure featuring bimodal grain size, non-recrystallization fraction, austenite texture and residual strain-induced α'-martensite volume fraction ( V Rα' ). The strength of LGB steel decreased with increasing austenite grain size ( D γ ) and decreasing V Rα' , while ductility rose, achieving 641–984 MPa yield strength and 25.3%–42.2% total elongation. Its strength-ductility synergy outperformed uniform-structure (US) steel, with yield/tensile strength following the Hall-Petch relationship. Tensile strain first concentrated in larger-grain and transition zones then shifted to smaller-grain regions, alleviating localization. The strain-induced α'-martensite (SIM α' ) kinetics of LGB steel had gradient characteristics, inhibiting strain and phase transformation localization. Tensile deformation involved multi-mechanism synergy: phase transformation in larger grains, twinning in smaller grains and buffering in transition zones, endowing ultrafine-grained LGB steel with better work-hardening ability and strength-ductility synergy than ultrafine-grained US steel. This study provides theoretical and process support for low-cost preparation and strength-ductility control of full-gradient metastable steel.