Effect of prestrain on aging and bake hardening of cold-rolled, continuously annealed steel sheets

To understand the bake hardening (BH) behavior in an actual automotive part, 40 tensile specimens were machined from the actual press-formed outer-door panel of a compact car and both bake and work hardenability distribution data were determined. Strain applied by actual press forming was estimated from the work hardening data. Finally, the effects of prestraining mode and amount on ambient aging and BH response were also investigated. The BH widely ranged from 10-54 MPa and the work hardenability was between 17-82 MPa, depending on panel location. Bake hardening in the outer-door panel decreased as the work hardening increased, indicating that the BH steel must be applied to the shallow drawn parts in order to maximize the BH effect in dent resistance. In order to establish the effects of prestrain and ambient aging time on the age and subsequent BH, the specimens were prestrained and aged at ambient temperature for various time intervals, and the baked at 170 deg C for 20 min. In the as-temper-rolled and press-formed condition, the steels were extremely resistant to ambient aging. However, it was found that a 0.3% tensile prestrain was sufficient to initiate ambient aging within 1 day, and the effect was accelerated with greater prestrain. With 8 days of ambient aging, all prestrained steels exhibited 20-25 MPa of age hardening. Irrespective of prestrain amount in the range of 0.5-5.0%, the BH decreased as the aging time increased.