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Deformation and work hardening behavior of Fe–17Mn–0.02C steel containing ε-martensite within the austenite matrix have been investigated by means of in situ microstructural observations and x-ray diffraction analysis. During deformation, the steel shows the deformation-induced transformation of austenite → ε-martensite → α′-martensite as well as the direct transformation of austenite → α′-martensite. Based on the calculation of changes in the fraction of each constituent phase, we found that the phase transformation of austenite → ε-martensite is more effective in work hardening than that of ε-martensite → α′-martensite. Moreover, reverse transformation of ε-martensite → austenite has also been observed during deformation. It originates from the formation of stacking faults within the deformed ε-martensite, resulting in the formation of 6H-long periodic ordered structure.

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Nb carbides have attracted significant attention to enhance the resistance of tempered martensitic (TM) steel to hydrogen embrittlement (HE). However, previous studies have elucidated the role of Nb carbides in HE resistance without categorizing their types (i.e., undissolved and newly precipitated). This study focuses on the effect of "undissolved" Nb carbides on the tensile and fatigue properties of hydrogen-precharged TM steels. It validated the following two factors for the HE resistance of the TM steels containing undissolved Nb carbides: hydrogen-trapping by the carbides and refinement of prior austenite grain. The former factor rarely affected the HE resistance owing to the interfacial incoherency between the undissolved carbides and ferritic matrix. Such results are distinguished from previous studies focusing on the newly precipitated carbides. In contrast, the latter factor contributed significantly to the HE resistance via the decrease in hydrogen contents per unit surface of prior austenite grain boundaries. Hydrogen embrittlement (HE) indicates the deterioration in mechanical properties owing to hydrogen atoms inside a ferrous alloy 1,2. HE is particularly important for the use of high-strength steels, such as a tempered martensitic (TM) steel. The TM structure can yield a high strength that exceeds 1.2 GPa after a simple heat treatment, rendering this steel important for various industries. Nevertheless, a high density of defects in the TM structure renders it highly vulnerable to HE 3. Consequently, this has resulted in increasing demands for an enhanced HE resistance of TM steels. The addition of Nb is an effective approach to increase the HE resistance of TM steels. It has been reported that HE resistance increases owing to the hydrogen trapping by Nb carbides and the refinement of prior austenite grains (PAGs). Such factors hindered the diffusion and concentration of the hydrogen present inside materials 4. A recent study 5 suggested a different mechanism for achieving an improvement in HE resistance, wherein Nb addition decreased the Σ3 boundary fraction in lath martensites. However, previous studies have primarily investigated Nb carbides, which are newly precipitated during a tempering process. The other type of Nb carbides (i.e., "undissolved" carbides) has attracted significantly less attention despite the fact that their resultant amounts are not negligibly small. Therefore, the effect of undissolved Nb carbides on the HE resistance of TM steels was investigated in this study. Four hydrogen-precharged TM steels were evaluated under uniaxial and cyclic loading conditions by considering the application environment of the material.