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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.

Punch-stretch tests to determine formability of type 304 stainless steel sheet were conducted using a hemispherical dome test. Sheets of 19.1 mm width and 177.8 mm width were stretched on a 101.6 mm diameter punch at punch rates between 0.042 to 2.12 mm/sec with three lubricant systems: a mineral seal oil, thin polytetrafluoroethelyne sheet with mineral seal oil, and silicone rubber with mineral seal oil. The resulting strain distributions were measured and the amount of martensite was determined by magnetic means. Increasing lubricity resulted in more uniform strain distributions while increased punch rates tended to decrease both strain and transformation distributions. High forming limit values were related to the formation of high and uniformly distributed martensite volume fractions during deformation. The results of this study are interpreted with an analysis of the effects of strain and temperature on strain induced martensite formation in metastable austenitic stainless steels.