Strain aging characterization and physical modelling of over-aging in dual phase steel

This study presents an integrated work of experimental investigations and physical modeling of bake hardening (BH) response in dual-phase steel (DP). A DP steel with a martensite volume percentage of 22% was produced by intercritical annealing followed by quenching in brine. Aging experiments with up to 5% pre-straining were carried out in the temperature range of 100 to 220 degrees C, for 2 to 2.10(4) min at temperature. The DP steel was characterized using light optical, scanning electron and transmission electron microscopy. The pinning effect of the dislocations was revealed by atom probe tomographic analysis. The increase in the yield strength accompanying the aging phenomenon, measured using tensile tests, showed a two-step increase, followed by an over-aging stage. The dependence of the time-interval of each stage on the pre-strain value and aging temperature was analyzed. A physical-based model for interpreting the over-aging stage in DP steel was developed. A new concept for over-aging, correlating it to carbon-diffusion from ferrite to martensite due to the gradient in the chemical potential at the interface between the two phases, was introduced. This diffusion causes a partial dissolution of the already formed Cottrell atmosphere/carbide precipitates. Finally, the time required for the onset of over-aging, calculated using physical simulations, was compared with experimental results showing a good matching between experiment and simulation.