The Effects of Room Temperature Aging on Subsequent Bake-Hardening of Automotive Sheet Steels

Bake-hardening steels used for exposed auto-body panels provide low yield strengths before forming, and increased strength and dent resistance after the forming and paint-baking processes. Room temperature aging can alter the sheet properties before forming, after forming, or after baking. Knowledge of the evolution of mechanical properties is important, and the effects of room temperature and simulated room temperature aging (at 50°C and 100°C) on the yielding behavior and the bake hardening response of two different bake-hardening steel grades were studied. The steels included a low strength ultra-low carbon steel and a dual-phase steel with higher strength and greater bake-hardening index. Neither steel exhibited a substantial response to aging prior to tensile pre-straining, although both steels exhibited strength increases after either aging or baking following straining. The total strengthening increment was essentially independent of aging history for both steels, and followed t^2/3 kinetics, with an apparent activation energy somewhat greater than the activation energy for carbon diffusion in ferrite.