Fig. 1 Hot stamping procedures and the microstructure evolution of PHS

Fig. 2 Influence of baking on the transition carbide precipitation of 22MnB5 PHS (adapted from [50]). a Time dependence of the height of the P3 peak during internal friction analysis at 127 ℃, which indicates the decrease of solute carbon concentration during aging; b Time dependence of the W value, a measure of fraction of the carbon atoms as interstitial in the microstructure, converted from a; c Bright-field transmission electron microscopy (TEM) of PHS after baked at 127 ℃; d Dark-field TEM in the same region as c, showing needle-shaped carbides

Fig. 3 Hydrogen embrittlement behaviors of Al-Si pre-coated 22MnB5 (adapted from [14]). a Thermal desorption analysis (TDA); b Stress-strain curves. From left to right are before hot stamping, after hot stamping, and after baking, respectively. Here, hydrogen is introduced during austenitization

Fig. 4 Alloying element segregation at grain boundaries in martensitic steels (adapted [62,63]). a A prior austenite grain boundary (PAGB) [62]; b Martensite-martensite (M-M) boundaries, B not detected in the entire probed volume [62]; c Quenched from 1050 ℃ [63]; d Quenched from 900 ℃ [63]. a) and (b are from APT analysis, while c and d are B signals from scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS). M-M boundaries here may be the packet, block, and/or lath boundaries of martensite [62]

Fig. 5 Application of Mo to improve hydrogen embrittlement resistance (adapted from [11]). a 32MnB5 as a reference sample; b 32MnB5 + 0.15 wt% Mo

Fig. 6 APT analysis of a deuterium (D) charged steel sample with a platelet-like VC precipitate [26]. a 3D element maps; b Mass-to-charge spectrum; c Concentration profiles of carbon and vanadium across the platelet of VC

Fig. 7 Influence of NbC on hydrogen embrittlement resistance of high-strength martensitic steels (adapted from [77]). a-c Tensile stress-strain curves with and without hydrogen charging for martensitic steels as-quenched (900-Q’), tempered at 480 ℃ (Q&T-480), and tempered at 560 ℃ (Q&T-560), respectively; d TDA curves; e Schematism how NbC nano-precipitates enhances hydrogen embrittlement resistance

Fig. 8 Microstructure of Cr-containing coating-free PHS (adapted from [41]). a Scanning electron microscopy (SEM) image of coating-free PHS, white arrows pointing to Cr carbides; b TEM coupled with energy-dispersive X-ray spectroscopy (EDS) analysis for a Cr carbide; c, d Prior austenite grain structures of 22MnB5 and the coating-free PHS, respectively

Fig. 9 Influence of inclusions on hydrogen embrittlement. a-c High-resolution tritium autoradiography of FeS, TiN, and MnS, respectively, as hydrogen traps (adapted from [89]); d Fisheye morphology on the fracture surface of PHS1900 after hydrogen charging; e Enlarged image of d; (e1-e5) EDS analysis of the inclusion in e

Fig. 10 Mechanical properties and microstructure of a PHS with 7 wt% Mn (adapted from [10]). a Tensile stress-strain curves, where BP170 refers to the medium Mn PHS after baking; b SEM microstructure; c TEM bright-field microstructure, γ: austenite; d TEM dark-field microstructure, γ: austenite, VC: vanadium carbide

Fig. 11 Hydrogen permeation and TDA results of a martensitic steel (adapted from [101]). a-c Hydrogen permeation curves of martensitic steels as-quenched (AQ), tempered at 300 ℃ (300 T), and tempered at 450 ℃ (450 T), respectively; d-f TDA curves of AQ, 300 T, and 450 T, respectively

Fig. 12 Comparison of surface qualities of PHS after hot stamping. a-b Hat-shaped parts of bare and Al-Si coated 22MnB5, respectively (adapted from [1]); c-d Bare 22MnB5 and coating-free components, respectively

Fig. 13 Influence of decarburization and oxidation on the microstructure and mechanical properties of bare PHS (adapted from [8]). a Austenitized at 900 ℃ for 4 min; b 900 ℃ 6 min; c 900 ℃ 10 min

Fig. 14 Microstructure of Al-Si coating after hot stamping (adapted from [7]). a SEM; b Phase map from electron backscatter diffraction (EBSD); c Al element mapping from EDS; d Si element mapping from EDS

Fig. 15 Influence of Al-Si coating on hydrogen embrittlement. a Al-Si coated PHS without hydrogen charging; b Bare PHS with hydrogen charging; c Al-Si coated PHS with hydrogen charging

Fig. 16 Influence of Zn coating on the hydrogen embrittlement risk of PHS [112,119]. Comparisons of a tensile curves and b TDA curves of aluminized, uncoated, and galvanized PHS immediately after hot stamping [112]; c Evolution of localized necking elongation (elongation from UTS up to fracture) with strain rate for a Zn coated steel in 3% NaCl solution and air conditions [119]

Fig. 17 Influence of trimming on residual stress at PHS surface and hydrogen embrittlement resistance (adapted from [13]). a-b Residual stress and delayed fracture time of cold blanked samples, respectively; c Residual stress of laser trimmed samples. 1 GPa and 1.2 GPa steel sheets are dual phase steels containing both martensite and ferrite, while 1.5 GPa steel sheets are a tempered martensitic steel