Interactions of nanoprecipitates, metastable austenite, and Lüders banding in an ultra-low carbon medium Mn steel with abnormal strength dependence of hydrogen embrittlement

A hierarchical microstructure, containing an ultrafine-grained matrix with mixed lamellar and equiaxed morphology, metastable austenite, and intermetallic nanoprecipitate, was designed to improve the hydrogen embrittlement (HE) resistance of ultra-low carbon medium Mn steels. Compared with the warm-rolled (WR) state, the warm-rolled-tempering (WRT) sample shows a simultaneous enhancement of yield strength (YS) and tensile ductility, but lower total elongation loss (∼3%) after hydrogen charging, thus exhibiting an abnormal strength dependence of HE. The microstructural analysis with numerical calculation proves that an additional tempering not only introduces a heterogeneous distribution of Ni(Al, Mn) precipitations, but also results in a reverse γ→α transformation with Mn partitioning, thus contributing to a self-stabilization of austenite with refined grain size and enriched element stabilizer. On one hand, the introduction of intermetallic Ni(Al, Mn) precipitations could be beneficial for the intrinsic hydrogen capture and storage, although its limited number density only contributes little to impeding hydrogen diffusion. Moreover, the introduction of nanoprecipitate can improve the strain compatibility and suppress the strain concentration in ferrite phases, which is consistent with a lower kernel average misorientation (KAM) value of the WRT sample after deformation. Meanwhile, the increased austenite stability leads to a shorter yield point elongation (YPE) and an effective transformation-induced plasticity (TRIP) effect at large strains, and the transformation product of nano-lamellar α’-martensite from self-stabilized austenite contributes little to the hydrogen-accelerated strain localization in weak α/γ interfaces of the WRT sample. In contrast, the low HE resistance of the WR sample is mainly attributed to the enhanced martensitic transformation process during the propagation of Lüders banding, which leads to a strong localized stress concentration in α/α’ interfaces, and the premature failure due to hydrogen-enhanced decohesion effect (HEDE). The interactions of nanoprecipitate, metastable austenite, and Lüders banding provide new insight into the design of high-strength steel with excellent HE resistance.