Optimizing strength and ductility in 2.8 wt% Mn TRIP steel: unlocking multistage TRIP effects through a co-deformable dual-heterogeneous structure

Achieving an optimal balance between strength and ductility in advanced high-strength steels (AHSS) remains a critical challenge, particularly for medium Mn TRIP steels, where high Mn content (≥5.0 wt%) often leads to severe Mn segregation, poor weldability, and high deformation resistance. Here, we demonstrate a novel co-deformable dual-heterogeneous structure in a low-carbon 2.8 wt% Mn TRIP steel, engineered through a combination of hot-rolling and tailored intercritical annealing strategies. This microstructural design, comprising polygonal ferrite, lath ferrite, and martensite/retained austenite (M/RA) islands with varying morphologies, delivers an exceptional product of strength and elongation (PSE) exceeding 30 GPa%, rivaling conventional high-Mn TRIP steels. A detailed microstructural investigation reveals that the material's superior mechanical performance stems from a multistage transformation-induced plasticity (TRIP) effect, governed by the sequential activation of strain localization across heterogeneous ferritic domains. Quasi in-situ EBSD and microscopic digital image correlation (μ-DIC) analyses uncover a strain partitioning cascade, where deformation first concentrates in polygonal ferrite, thick lath ferrite with high Schmid factors and local areas near the boundaries between lath ferrite blocks with significant crystallographic misorientation, then transitions through fine lath ferrite with high Schmid factors, before culminating in lath ferrite with lower Schmid factors. This orchestrated strain evolution triggers a progressive TRIP effect, effectively delaying strain localization and enhancing work-hardening capacity. Crucially, we show that the key to improved ductility is not merely the martensitic transformation of retained austenite, but rather the synchronized, co-deformable response of the ferritic matrix. These findings establish that strain localization engineering, through hierarchical microstructure design, is a powerful strategy for unlocking the full potential of TRIP steels. By integrating a controllable multistage TRIP effect via dual-heterostructure tuning, this study provides a blueprint for the next generation of low-carbon, low/medium Mn TRIP steels, offering an economically viable and industrially scalable alternative to conventional high-Mn AHSS.