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Acta Metall Sin  2018, Vol. 54 Issue (7): 1031-1041    DOI: 10.11900/0412.1961.2017.00435
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Hydrogen Embrittlement of Intercritically AnnealedCold-Rolled 0.1C-5Mn Steel
Xiaoli ZHAO1,2, Yongjian ZHANG1, Chengwei SHAO1, Weijun HUI1(), Han DONG2
1 School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China
2 Central Iron and Steel Research Institute, Beijing 100081, China
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Medium-Mn steel typically alloyed with (3%~10%)Mn (mass fraction) has recently regained significant interest as one of the most promising candidates for the third-generation automobile steel due to its excellent combination of ultra-high strength and ductility as well as relatively low material cost and industrial feasibility. Considering the ever increasing strength level as well as the comparatively high amount of reverted austenite (RA) of medium-Mn steel, special attention began to be given to its hydrogen embrittlement (HE) behavior for ensuring the safety service of components made of this kind of steel. However, the effect of RA on HE of medium-Mn steel has not been fully understood. For this purpose, the susceptibility to HE of a cold-rolled medium-Mn steel 0.1C-5Mn intercritically annealed at 650 ℃ for different time to obtain different amounts of RA was investigated by using electrochemical hydrogen charging, thermal desorption spectrometry (TDS), slow strain rate test (SSRT) and SEM. The results show that the annealed samples exhibit a dual-phase microstructure of reverted globular shaped RA and ferrite. The ultimate tensile strength (σb) increases while the yield strength decreases with increasing annealing time, and both the total elongation (δ) and the product of σb to δ (σb×δ) initially increase and then decrease with increasing annealing time. That is to say, an excellent combination of strength and ductility could be obtained when the tested steel was annealed at 650 ℃ for 10 min. However, the results of TDS and SSRT show that both the absorbed diffusible hydrogen concentration and the susceptibility to HE increase with increasing annealing time, and the latter is more significant. SEM analysis of the fracture surfaces of fractured samples revealed that the hydrogen-charged annealed sample was fractured to leave both dimples filled with grains and empty dimples while the uncharged annealed specimen was ductile fractured to leave only empty dimples. The dimples filled with grains were basically a brittle intergranular cracking occurring along the boundaries of RA and/or martensite (formerly RA) grains by the hydrogen-assisted cracking mechanism. It is thus concluded that the HE behavior of intercritically annealed cold-rolled medium-Mn steel is primarily controlled by both the amount and mechanical stability of RA.

Key words:  cold-rolled medium-Mn steel      susceptibility to hydrogen embrittlement      intercritical annealing      microstructure      austenite stability     
Received:  18 October 2017     
ZTFLH:  TG111  
Fund: Supported by Laboratory Program of Beijing Jiaotong University (No.16010211)

Cite this article: 

Xiaoli ZHAO, Yongjian ZHANG, Chengwei SHAO, Weijun HUI, Han DONG. Hydrogen Embrittlement of Intercritically AnnealedCold-Rolled 0.1C-5Mn Steel. Acta Metall Sin, 2018, 54(7): 1031-1041.

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Fig.1  TEM images of 0.1C-5Mn steel as-cold rolled (a) and after annealing at 650 ℃ for 10 min (b), 30 min (c) and 360 min (d) (RA—reverted austenite)
Fig.2  XRD spectra (a) and intercritical annealing time dependences of the austenite volume fraction and carbon content of austenite (b) in 0.1C-5Mn steel
Fig.3  Variations of tensile properties of the cold-rolled 0.1C-5Mn steel with annealing time at 650 ℃(a) ultimate tensile strength σb and yield strength σs(b) total elongation δ and σbδ
Fig.4  Hydrogen desorption rate curves of hydrogen-charging 0.1C-5Mn steel specimens (a) and hydrogen contents in the cold-rolled and intercritically annealed 0.1C-5Mn steel samples (b) before and after hydrogen-charging (Inset in Fig.4a shows the locally enlarged curve)
Fig.5  Engineering stress-strain curves of cold rolled and intercritically annealed 0.1C-5Mn steel samples before and after hydrogen-charging (a) and variations of hydrogen embrittlement index δloss with annealing time (b)
Fig.6  SEM fractographs of hydrogen-charged 0.1C-5Mn steel as-cold rolled (a) and after annealing at 650 ℃ for 10 min (b), 30 min (c) and 360 min (d) (Circles in the map represent brittle fracture zones)
Fig.7  SEM fractographs in crack initiation region of uncharged (a, c, e) and hydrogen-charged (b, d, f) 0.1C-5Mn steel as-cold rolled (a, b) and after annealing at 650 ℃ for 10 min (c, d) and 360 min (e, f) (Arrows in Figs.7d and f represent dimples filled with grain)
Annealing time Average size of filled grain in dimple Average size of reverted austenite
min μm μm
5 0.46±0.08 -
10 0.63±0.12 0.68±0.25
30 0.82±0.11 0.89±0.27
360 1.01±0.19 1.10±0.45
Table 1  Average sizes of filled grains in dimples of fractured specimens and average sizes of reverted austenites
Position in Fig.7f Mass fraction of Mn / % Position in Fig.7f Mass fraction of Mn / %
1 7.39 6 4.81
2 7.83 7 5.55
3 7.26 8 4.68
4 8.24 9 6.07
5 7.29 10 5.32
Mean value 7.60±0.42 Mean value 5.20±0.56
Table 2  EDS results of Mn content in fractographs of the hydrogen-charged annealed for 360 min sample shown in Fig.7f
Fig.8  Low (a) and locally high (b) magnified SEM images taken at the normal direction of the fractured hydrogen-charged 0.1C-5Mn steel annealed at 650 ℃ for 30 min, showing small voids initiated at the interface of reverted austenite and ferrite (Black arrows represent martensites and the white arrows represent ferrites)
Fig.9  Dislocation cells of the as-cold rolled 0.1C-5Mn steel
Fig.10  Variations of the volume fractions of RA after tensile test and transformed RA (a) and the ratio of dimples filled with grains (b) with annealing time
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