Strain aging behaviour of cold rolled multiphase steel

The bake hardening behaviour of cold rolled multiphase steel, processed to a minimum yield strength of 250 MPa and a minimum tensile strength of 450 MPa, was evaluated within a range of tensile prestrain of up to 20% by means of aging experiments and tensile tests. The influence of room temperature aging time on the steel's mechanical properties and Lüders strain was also examined. It was found that, after reaching a peak value at a prestrain level of between 0·5 and 1·0%, the bake hardening parameter began to decrease upon reaching higher levels of prestrain. This behaviour was explained in terms of changes in the dislocation structure and dislocation interaction with dissolved carbon atoms. Concerning the natural aging behaviour, no significant influence of room temperature aging time on the mechanical properties could be detected. Moreover, the appearance of Lüders strain was not observed during a 6 month evaluation period, indicating that the investigated steel possesses a natural non-aging behaviour.