The shelf life is defined as the maximum period of storage of finally processed cold rolled annealed sheets at room temperature prior to use, without showing any stretcher strain in the component after forming. Aging kinetics of cold rolled-annealed ultra low carbon bake hardening steel has been studied in the temperature range of 50- 100°C and based on these results, the activation energy for the aging process was determined as 21-24 kCal/mole, which is higher than that of diffusion of carbon or nitrogen in iron. Using this activation energy, Hundy equation was modified. The shelf life predicated using Hundy equation is significantly lower than that of actual test data, whereas the predictions made using modified Hundy equation (from higher activation energy) is close to the actual room temperature aging data. In the present invention a simple method has been developed to determine the shelf life of ultra low carbon steel in an industrial practice.
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