Fig. 1 Sketch of the processing routes of one-step and two-step ART sample

Fig. 2 a, b Secondary electron (SE) images and EBSD phase-image quality (Phase-IQ) maps of the one-step a and two-step b ART sample; c EBSD inverse pole figure (IPF) map, Phase-IQ map and the corresponding SE image taken at the white box in the Phase-IQ map of b, showing the detailed morphology, phase condition and crystallographic orientation of the core-shell austenite in the two-step ART sample. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3 Mn distribution in austenite of one-step a, c, e, g and two-step b, d, f, h ART sample measured by AES-EBSD a-d and STEM-EDX e-h. a, b EBSD Phase-IQ maps of the one-step a and two-step b ART sample; c, d profiles of Mn intensity measured along the AES scanning line in a, b, and the insert shows the SEM morphology of the measured austenite grain; e, f STEM bright field (BF) images and the corresponding EDX maps of Mn of one-step e and two-step f ART sample. The white solid and dash closed curves show the core-shell austenite with distinct Mn enrichment in the shell region; g, h profiles of Mn content taken along the arrow in the EDX map

Fig. 4 Electrochemical H permeation curves of the one-step and two-step ART sample

Fig. 5 a, b Engineering stress–strain curves of the uncharged and H-charged (at current densities of 5 mA/cm2 and 60 mA/cm2) one-step a and two-step b ART sample; c the work hardening rate of the uncharged samples; d HE resistance index (HERI) as a function of current density of H charging, where ${\text{TEL}}_{{\text{H}}}$ and ${\text{TEL}}_{0}$ are the total elongation for the H-charged and uncharged specimen, respectively

Fig. 6 SEM images of the fracture surface of one-step a-c and two-step d-f ART samples. a, d Uncharged; b, e near-edge region of H-charged samples; c, f region located at ~ 100 μm away from the edge of H-charged samples. Arrowheads in b, c and e indicate intergranular facets. (The current density of H-charging is 60 mA/cm2.)

Fig. 7 Ex situ EBSD results of one-step ART sample (SEM images, EBSD Phase-IQ maps and IPF ($\gamma$) maps), showing two types of martensite transformation of individual austenite grains: a fully transformed at low strain (~ 12%); b transformation persisted at higher strain

Fig. 8 Ex situ EBSD results of martensite transformation of core–shell austenite grains in the two-step ART sample (SEM images, EBSD Phase-IQ maps and IPF ($\gamma$) maps): a complete transformation of both core and shell region into martensite at low strain (~ 12%); b stepwise transformation with the shell region transformed first; c stepwise transformation with the core region transformed first followed by the shell region; d almost no transformation occurred at analyzed strain levels

Fig. 9 Proportion of different types of martensitic transformation behavior in the one-step and two-step ART sample

Fig. 10 a, b SEM images of the deformed region of H-charged (at current density of 60 mA/cm2) one-step a and two-step b ART sample stretched to a macroscopic strain approximately equal to one fifth of the YPE; c, d SEM images, EBSD Phase-IQ maps and IPF maps of the magnified regions near the main cracks in a, b, showing the formation of micro-cracks in H-charged one-step c and two-step d ART sample. The inset in d2 is the profile of Mn intensity measured along the AES scanning line, showing the Mn enrichment in the austenite shell. $\alpha$ represents ferrite and $\alpha^{\prime}$ stands for $\alpha^{\prime}$-martensite, which is distinguished based on the combination of SEM morphology and EBSD image quality map. PAGB represents prior-austenite grain boundaries

Fig. 11 a Phase-IQ map and the STEM-EDX measurements of the sample subjected to the first ART at 700 °C. The Mn profile is taken along the arrow in the EDX map of the corresponding STEM BF image. The inset in the Phase-IQ map is the setup of the DICTRA simulation; b length change during the first and second ART (note: the cooling stage after the first ART and the reheating stage of the second ART are not simulated here and they should not influence the dilatometry results as no phase transformation is expected during these two stages); c the simulated evolution of Mn profiles during the second ART; d magnified profiles taken at the black-dash rectangular frame in c; e EDX mapping of the two-step ART sample showing the Mn-depleted region in the ${\alpha }$ side near the phase boundary. The Mn profile is taken along the arrow in the EDX map, showing the Mn gradient inside austenite

Fig. 12 Schematic illustration showing the H-induced cracking behavior in one-step ART sample a, sample with randomly-distributed Mn-rich zone inside austenite [26] b and two-step ART sample with core–shell austenite c. With regard to intergranular cracking, only cracking along $\alpha$/$\alpha^{^{\prime}}$-martensite interface is presented for better comparison, as it is the most prevalent cracking behavior in H-charged MMS