1. Key Laboratory of the Ministry of Education for Modern Metallurgy Technology, North China University of Science and Technology, Tangshan 063210, China 2. College of Electrical Engineering, North China University of Science and Technology, Tangshan 063210, China 3. The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
Cite this article:
Futao DONG,Fei XUE,Yaqiang TIAN,Liansheng CHEN,Linxiu DU,Xianghua LIU. Effect of Annealing Temperature on Microstructure, Properties and Hydrogen Embrittlement of TWIP Steel. Acta Metall Sin, 2019, 55(6): 792-800.
TWIP steel as the representative of advanced high strength steel (AHSS) has a bright future in market and application owing to its excellent strength and ductility. Hydrogen embrittlement (HE) as a difficult problem of TWIP steel, researches on solving it mainly focus on alloying, few effort has been made on the mechanism of improving HE resistance by process adjustment and microstructure optimization. In this work, electrochemically combined with slow strain rate tensile test (SSRT) and OM, SEM have been used to study the effect of annealing temperature on mechanical property and deformation behavior of a Fe-18Mn-0.6C (mass fraction, %) twinning-induced plasticity (TWIP) steel, and also the influence of various microstructures on HE were discussed. The results showed that the grain size of TWIP steel increased with the increasing annealing temperature. In 700 ℃ annealed sheet, grain boundary (Fe, Mn)3C cementite was obvious. TWIP steel with uniform medium-sized grains by 900 ℃ annealing had the highest strength-ductility balance. After SSRT under ongoing hydrogen charging, strength and plasticity reduced significantly. The strength-ductility balance loss rate (R) showed a tendency of increasing with the increasing annealing temperature. Deformation twins were more likely to be produced in large-sized grains by high temperature annealing. The junctions of twin/twin and twin/grain boundary were the main sources of hydrogen induced cracks. Although relatively low temperature annealing resulted in fine grains and the change of stacking fault energy (SFE) along grain boundary, deformation twins were not easily formed. But it was very vulnerable to generate vacancies where the stress concentrated between the local coarse carbides and deformation twins, then evolved into crack sources. As a result, the plasticity of TWIP steel itself was not high when annealed at a relatively low temperature. It had no apparent effect on improving HE resistance for TWIP steel annealed below 800 ℃.
Fund: National Natural Science Foundation of China(No.51501056);Natural Science Foundation of Hebei Province(No.E2016209341);Educational Commission of Hebei Province(No.BJ2014031);Foundation of North China University of Science and Technology(No.JP201510)
Fig.1 OM images of Fe-18Mn-0.6C twinning induced plasticity (TWIP) steel annealed sheets with annealing temperatures of 700 ℃ (a), 800 ℃ (b), 900 ℃ (c) and 1000 ℃ (d)
Fig.2 Austenite grain size distributions of Fe-18Mn-0.6C TWIP steel annealed sheets with annealing temperatures of 700 ℃ (a), 800 ℃ (b), 900 ℃ (c) and 1000 ℃ (d)
Fig.3 Engineering stress-engineering strain curves of Fe-18Mn-0.6C TWIP steel annealed sheets without hydrogen charging
Fig.4 Curves of strain hardening rate (a) and strain hardening exponent (n) (b) vs true strain of Fe-18Mn-0.6C TWIP steel annealed sheets without hydrogen charging
Fig.5 Engineering stress-engineering strain curves of Fe-18Mn-0.6C TWIP steel annealed sheets from slow strain rate tensile test conducted under ongoing electrochemical hydrogen charging
Fig.6 Strength-ductility balances of Fe-18Mn-0.6C TWIP steel annealed sheets without hydrogen charging, from low strain rate tensile test conducted under ongoing electrochemical hydrogen charging and strength-ductility balance loss rate (R) influenced by hydrogen (σb,σb'—tensile strengths without and with hydrogen charging, respectively;δ,δ'—elongation after breaking without and with hydrogen charging, respectively)
Fig.7 Magnification of engineering stress-engineering strain curves of Fe-18Mn-0.6C TWIP steel annealed sheets (Arrows indicate the start points of serration)
Fig.8 SEM images of Fe-18Mn-0.6C TWIP steel annealed sheets with annealing temperatures of 700 ℃ (a~c) and 900 ℃ (d~f), and strains of ε=0.10 (a, d), ε=0.25 (b, e) and ε=0.40 (c, f)
Fig.9 SEM images of carbides in Fe-18Mn-0.6C TWIP steel annealed sheets with annealing temperatures of 700 ℃ (a) and EDS analysis (b), 800 ℃ (c) and 900 ℃ (d)
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