Investigation on Hydrogen-Induced Delayed Fracture of Cold-Rolled DP980 Steels
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JMEPEG DOI: 10.1007/s11665-017-2636-4
Investigation on Hydrogen-Induced Delayed Fracture of Cold-Rolled DP980 Steels Yun Han, Liang Chen, Shuang Kuang, and Chunqian Xie (Submitted August 11, 2016; in revised form November 16, 2016)
In this study, the phenomenon of hydrogen-induced delayed fracture of two cold-rolled DP980 steels with different chemical compositions was studied. The results show that the microstructure of both steels is composed of ferrite matrix, martensite-austenite islands and small amount of bainite. DP980-1 having higher contents of C and Si exhibits higher tensile strength, lower yield strength and higher elongation in comparison with DP980-2 having lower contents of C and Si. According to the results of slow strain rate tensile tests, the tensile strength of DP980-1 after hydrogen charging is reduced by 20.8%, while it is just 5.4% for DP980-2. Moreover, very fine dimples can still be observed in the fracture surface of DP980-2 after hydrogen charging, which indicates a good ductile. The main reasons leading to the better delayed fracture resistance of DP980-2 are the lower volume fraction of martensite-austenite islands, lower content of diffusible hydrogen and the grain refinement effects. Keywords
DP steel, hydrogen embrittlement, hydrogen charging, slow strain rate tensile test, diffusible hydrogen
1. Introduction With the rapid development of automobile industry, more and more advanced high-strength steels have been used in producing ‘‘body-in-white’’ components. The application of high-strength steel has the advantages of reducing the weight of automobile body, saving the fuel consumption and increasing the safety of passengers, which meet the expected energy efficiency and safety standards (Ref 1-4). Among variety of high-strength steels, dual-phase (DP) steels are the most widely used one due to its high tensile strength, low yield strength, high work hardening rate and superior formability (Ref 5-7). The microstructure of DP steels is mainly composed of ferrite matrix and dispersed martensite islands, and the fraction of martensite islands varies depending on the designed mechanical properties. The balance between soft ferrite and hard martensite confers the combination of excellent mechanical properties and plastic formability of DP steels (Ref 8). However, as the strength of steel increases, the problem of hydrogen embrittlement usually becomes more serious (Ref 9). Martensite has high sensitivity on hydrogen embrittlement, and the delayed fracture easily occurs during the service of DP steel components. Thus, high attention should be paid on the hydrogeninduced delayed fracture (HDF) of DP steels, which is an important issue considering the safety of passengers.
Yun Han, Shuang Kuang, and Chunqian Xie, Shougang Research Institute of Technology, Beijing 100043, China; and Liang Chen, Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China. Contact e-mail: [email protected].
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