A phenomenological model for bake hardening in minimal carbon steels

Abstract

The essence of bake hardening is to exploit the classical strain ageing in a positive way to increase the strength of the formed steel sheets used in outer body panel of a passenger car during the paint-baking operation. A new model that takes into account the strengthening contributions from Cottrell atmosphere and precipitate formation has been developed in the present work. The model predicts the increase in strength as a function of the amount of free solute C (calculated as a function of the annealing temperature), the amount of deformation, ageing temperature and time. The model predictions have been found to agree quite well with the experimental results; the individual contributions of Cottrell atmosphere and precipitation strengthening have been quantified.

Keywords:

• Cottrell atmosphere
• precipitation
• dislocation
• diffusion of C atoms
• bake hardening

Acknowledgements

The authors are grateful to Dr R.N. Ghosh, Steel Chair Professor, IIT-Kharagpur for his helpful discussion on the subject and for reviewing the proof. One of the authors (SD) expresses his deep gratitude to Dr A. Haldar and the management of R&D Division, Tata Steel for support to carry out this research work and to allow for publication.