Effect of multi-pass DRECE processing on microstructure and mechanical behaviour of TRIP steel

In this study, TRIP steel was subjected to one, two, and three passes of Dual Rolls Equal Channel Extrusion (DRECE) at room temperature to investigate the effect of successive deformation on the microstructure, retained-austenite stability, and mechanical properties. Microstructural changes were analysed using SEM, TEM, EBSD, and X-ray diffraction, while mechanical properties were determined by tensile and hardness tests. The initial ferritic–bainitic microstructure contained retained austenite and martensite/austenite (M/A) islands. DRECE processing was accompanied by a progressive decrease in retained-austenite fraction from 10% in the initial state to 4.8%, 4.2%, and 1.7% after successive passes, consistent with strain-induced martensitic transformation. The decrease in retained-austenite fraction was non-linear and was accompanied by dislocation accumulation, subgrain formation, and partial recovery. The mechanical response was clearly non-monotonic: DRECE-1 produced strong strengthening, DRECE-2 was associated with a partial recovery of ductility, whereas DRECE-3 resulted in renewed strengthening and reduced remaining strain-hardening capacity. Hollomon analysis revealed two approximately linear strain-hardening regions in all investigated conditions, with markedly lower strain-hardening exponents after DRECE processing. Fractographic observations showed predominantly ductile fracture controlled by microvoid nucleation, growth, and coalescence, despite the reduction in macroscopic elongation after deformation. The results demonstrate that the mechanical response of DRECE-processed TRIP steel is governed by the interaction between SPD-induced defect evolution and strain-induced martensitic transformation.