Microstructural design for hydrogen-resistant medium Mn steels

Abstract

To address the industrial demand for energy saving, carbon reduction, and cost reduction, the development of advanced high-strength steels has become imperative, among which medium manganese steels (MMS) have aroused significant interest and research within the academic community. However, as the strength of steel continues to increase, the issue of hydrogen embrittlement is bound to become the "Achilles' heel" for advanced high-strength steels. Therefore, designing high-strength steels that resist hydrogen embrittlement is of primary significance. The objective of this thesis is to investigate the hydrogen embrittlement behavior of MMS and to exploit effective microstructural design strategies that enhance their resistance to hydrogen embrittlement. First, to rationalize the study of hydrogen embrittlement under slow strain rates, a preliminary study on the strain rate-dependent deformation mechanisms of a fully austenitic MMS was conducted. Surprisingly, a rate-dependent ductile-brittle transition was observed. This transition was proven to be the result of the rate-dependent deformability of the fresh martensite, and the critical role of carbon clusters in dragging dislocation movements in martensite was proposed to be the ultimate reason. Finally, a slow strain rate was always found to cause a more serious hydrogen embrittlement in different MMS, which should be attributed to the rate-dependent deformability of fresh martensite to some extent. Second, the role of intercritical annealing (IA) duration on hydrogen embrittlement was revisited. It demonstrated an initially alleviated, but then deteriorated, hydrogen embrittlement with increasing duration. The decreased residual stress and corresponding flow stress were proposed to contribute to the initial improvement, whilst the later exacerbation was attributed to reduced mechanical stability of austenite. Third, to maximize the transformation-induced plasticity (TRIP) effect while alleviating hydrogen embrittlement, a morphology optimization was conducted in a MMS through warm rolling (WR). An elongated banded morphology was obtained and proven to enhance hydrogen resistance, compared with equiaxed morphology by IA. The stable filmy austenite and band boundaries provide significant blunting and deflections to the main crack. Finally, combined with adequate residual stress and elongated banded morphology made by short-duration IA and WR, a MMS achieved a high fracture strength exceeding 1 GPa regardless of hydrogen content. Particularly, its uniform elongation can be fully preserved at a hydrogen content of ~10 wppm. Both constituent phases are proposed to make contributions to the enhanced hydrogen resistance. The yield drop and nonhardenable behavior of the ferrite requires more hydrogen content to embrittle ferrite boundaries. The austenite plays a threefold role that provides plastic deformation, acts as an effective hydrogen reservoir, and blunts crack propagation, due to its high-volume fraction and high-mechanical stability before necking. Overall, the thesis determines the cornerstone of the issue of hydrogen embrittlement and provides microstructural strategies for designing hydrogen-resistant MMS with significant breakthroughs.