Mechanisms and Modeling of Bake-Hardening Steels: Part I. Uniaxial Tension
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RODUCTION
REQUIREMENTS of increased dent resistance of steels in the automotive industry have largely contributed to the development of new bake-hardening (BH) ultra-low-carbon steels (ULC). These low-strength deep drawing steels initially offer low yield strength before forming, but they exhibit an increased yield strength after forming and paint baking in automotive manufacturing. This strengthening is due to work hardening during sheet forming processes and to BH during paintbaking processes. The latter phenomenon corresponds to strain aging, which is the interaction between solute carbon and dislocations.[1] Recent BH steels initially present a relatively low dislocation density and a controlled level of carbon in solution, so that the strain aging phenomenon only occurs during the paint-baking process. Diffusion of carbon leads to formation of Cottrell atmospheres[1] and pinning of dislocations. If subsequent strain occurs after stamping and baking, the material exhibits higher yield strength but also the return of a sharp yield point during tensile tests. In that case, plastic instabilities called Piobert–Lu¨ders bands propagate along the testpiece. This corresponds to a localization of plastic strain, which leads to a plateau on the macroscopic stress-strain response.
V. BALLARIN, Research Engineer, is with the Automotive Applications Research Center, Arcelor-Mittal Montataire, 60761 Montataire, France. M. SOLER, A. PERLADE, and X. LEMOINE, Research Engineers, are with the Automotive Products Research Center, Arcelor-Mittal Maizie`res, Voie Romaine, 57283 Maizie`res-le`sMetz, France. Contact e-mail: [email protected] S. FOREST, CNRS Research Director, is with Mines ParisTech, Centre des Mate´riaux, UMR CNRS 7633, 91003 Evry Cedex, France. Manuscript submitted October 16, 2007. Article published online April 15, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
Descriptions of bake hardenability are usually based on uniaxial tensile tests. First, a specimen is loaded to 2 pct strain, which is a typical amount of strain received by outer panels during stamping. Then, the specimen is baked at 170 °C for 20 minutes to simulate the industrial paint-baking process. Finally, the prestrained and baked specimen is uniaxially tested in the same direction as the prestrain. The BH is defined as the difference between the lower yield stress after baking and the final flow stress after prestraining, as displayed in Figure 1. This phenomenon has been widely studied in the literature. From a metallurgical point of view, strain aging, mainly due to Cottrell atmosphere formation, is influenced by numerous and often coupled parameters,[2,3] mainly the chemical composition and especially the solid solution carbon content, the grain size, and the aging conditions (temperature and time). For instance, grain size plays a particular role on solute carbon content, but the resulting effect on BH is not very well understood.[2] The Piobert–Lu¨ders phenomenon has also been investigated from a mechanical point of view. Models have
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