Influence of Martensite Mechanical Properties on Failure Mode and Ductility of Dual-Phase Steels
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THE automotive industry is deploying increasingly more complex multiphase (MP) steels in underbody cross members and rails to achieve lightweight vehicle bodies with superior crash-energy management capability. For example, dual-phase (DP) steels are increasingly being used in various vehicle components.[1] Typical DP steels have a composite microstructure of ferrite and martensite phases that exhibits a favorable combination of strength, high work-hardening rate, ductility, and formability over other high-strength low-alloy steels with a similar yield strength.[2,3] These favorable behaviors of DP steels are attributable to the combined behaviors of the hard martensite phase with high strength and the soft ferrite matrix with good ductility. Because the advantages of DP steels were first reported by Rashid,[2] a great deal of research has been carried out by many researchers to explore various aspects of DP steels. It is well known that the macroscopic mechanical behavior of DP steels depends on various factors, such as the grain size of the ferrite and the volume fraction, morphology, and carbon content of the martensite phase.[4–11] The fine grain size, high strength and volume fraction of the martensite phase generally increase the tensile strength of DP steels. On the other hand, a high volume fraction of the martensite phase reduces the ductility of DP steels. For example, K.S. CHOI, Postdoctoral Research Associate, W.N. LIU, Scientist, X. SUN, Chief Scientist, and M.A. KHALEEL, Division Director and Laboratory Fellow, are with the Computational Science and Mathematics Division, Pacific Northwest National Laboratory, Richland, WA 99352. Contact e-mail: [email protected] Manuscript submitted July 10, 2008. Article published online February 10, 2009 796—VOLUME 40A, APRIL 2009
Kim[12] developed an analytical model for use in describing the tensile deformation behavior of DP steels, and found that the strength and ductility of DP steels are strongly influenced by the mechanical properties of the martensite. In his model, the ductility is predicted based on the strain at the ultimate tensile strength (UTS). Recently, the influence of the strain rate and prestrain on the deformation behaviors of DP steels has also been investigated by many researchers, due to the increased use of these steels in various vehicle components.[13–15] A number of previous studies on the strength of martensite suggested that the strength of the martensite depends on its carbon content. For example, Leslie and Sober[16] suggested a simple linear dependency of the strength of the martensite on its carbon content, based on some comprehensive experimental data. Speich and Miller[17] also used a linear relationship in their study of the mechanical properties of ferrite-martensite steels containing 0.06 and 0.29 wt pct carbon. A square-root or cube-root dependency was also suggested by other studies.[18–21] Much effort has been spent on investigating the deformation behavior and failure mechanism of MP steels by analytical, experimental
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