Static strain aging phenomena in cold-rolled dual-phase steels
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I. INTRODUCTION
IN recent years, there has been a growing interest in the automotive industry in the application of high-strength steels such as dual-phase (DP) ferrite-martensite steels, bainitic steels, ferrite-bainite steels, and transformation induced plasticity–aided steels, consisting of ferrite, bainite, and retained austenite, for use in car structural parts. Dual-phase steel is characterized by an interesting combination of high strength, good ductility, continuous yielding, high initial work-hardening rates (n values), and a low yield stress to tensile strength ratio (YS/TS). The increase in the yield stress by means of bake hardening (BH) will be an important contribution to the additional in-service strength, e.g., with respect to the dent resistance of the components made out of this steel, when they are applied for outer body parts. Earlier studies of aging phenomena in DP steel focused mainly on the determination of BH0 and BH2 values and the room-temperature aging sensitivity of the steels. BH0 is defined as the yield stress increase due to an aging treatment of 20 minutes at 170 °C. BH2 is determined as the yield stress increase of a 2 pct prestrained sample due to an aging treatment of 20 minutes at 170 °C. Tanaka et al.[1] reported BH2 values between 45 and 80 MPa, as well as very low BH0 values (0 to 20 MPa). More recently, Faral and Hourman[2] reported BH2 values for different DP grades between about 50 and 100 MPa. No specific research has yet been reported on the fundamental mechanisms of the bake-hardening behavior of DP steel. Unlike the extra low carbon (ELC, C ⬃ 200 ppm) and ultralow carbon (ULC, C ⬍ 50 ppm) steels, where aging T. WATERSCHOOT, Graduate Student, and B.C. De COOMAN, Professor, are with the Laboratory of Iron and Steelmaking, Ghent University (RUG), B-9052 Zwijnaarde (Ghent), Belgium. Contact e-mail: bruno. [email protected] Dr. A.K. DE, Research Associate, is with the Department of Metallurgy and Materials Engineering, Colorado School of Mines, Golden, CO 80401. S. VANDEPUTTE, Technology Manager, is with the Research Centre, Arcelor Group, OCAS NV, B-9060 Zelzate, Belgium. Manuscript submitted May 5, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
phenomena are principally guided by the amount of interstitial carbon atoms and the dislocation density, the bake hardening in DP steel is expected to be more complex in nature, as a result of its complex two-phase microstructure and strain partitioning. It can be expected that the bakehardening mechanism in the ferritic phase is influenced by the interstitial carbon, the carbon in the grain boundaries, the specific distribution of the dislocations and residual stresses, and possibly by carbon released from the martensite as a result of tempering. Furthermore, the volume decrease of about 0.5 vol pct due to martensite tempering in the 80 °C to 200 °C region,[3] can have a crucial influence on the yielding behavior. In DP steels, therefore, several stages must be considered in both the ferrite (static strain ageing processes) an
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