Impact of intercritical annealing temperatures on the stability of retained austenite and hydrogen embrittlement resistance of 34MnB5Nb press-hardened steel

Press-hardened steel (PHS) containing martensite microstructure offers a balance of high strength and formability for automotive structures. However, dislocation-type martensite is susceptible to hydrogen embrittlement (HE). This study evaluates the impact of intercritical annealing temperatures on retained austenite (RA) stability and HE performance in a novel 34MnB5–Nb PHS by thermal desorption spectroscopy, tip-bending constant strain, and slow strain rate tensile test. RA initially increases (4.48 vol%) then decreases (2.13 vol%) due to partial stabilization of C and Mn in austenite during high-temperature annealing. Multiphase microstructure and coherent NbC precipitates at 800 °C annealing results in the lowest elongation loss of 24.3 %, higher bending angle of 130°, and optimal H-trapping of 0.144 ppm. EBSD of fractured surface specify that hydrogen induced cracking initiates from martensite or martensite/ferrite boundary, and proliferate along it, leading intergranular fracture with higher strain localization. Optimizing annealing temperature is key to improving RA stability, HE resistance, and mechanical properties in automotive safety components.