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. IntroductionBake hardening (BH) steels are low carbon ferritic steels that are processed in such a way that the ferrite contains an extremely small amount (approximately 15-20 ppm by weight) of carbon in solution in the final strip material [1]. The main use of this grade of steel is in the automobile sector where the autobody parts made of this steel are painted to give them an aesthetic look. When the paint is baked at an elevated temperature (typically 150-200°C), the dissolved free C atoms migrate to pin the dislocations introduced during the forming operations and the resultant increase in the strength of the material is known as the bake hardening response, Δσ BH . The BH response is measured by first prestraining the sample to 2% strain in tension at room temperature. The samples are then unloaded, aged at the required temperature (usually at 170°C) for a fixed time (usually 20 min) and then tensile tested again at room temperature. The difference between the lower yield stress (YS) after aging and the flow stress at unloading is known as the BH response [1] or Δσ BH . The test procedure is schematically shown in Figure 1; the prestrain represents the forming operation during the actual production of the component, the ageing treatment represents the paint-baking operation and the