Static strain aging phenomena in cold-rolled dual-phase steels

The effect of low-temperature aging, with aging temperatures up to 170 °C, on a cold-rolled CMn-CrMo dual-phase (DP) ferrite-martensite steel was investigated. This material was processed using three different intercritical annealing treatments, leading to DP structures with different microstructures and properties. It has been found that both the aging in the ferrite phase and the tempering in the martensite play an important role in the mechanical behavior of the material with regard to the strain aging phenomena. The yield stress increase accompanying the aging phenomenon revealed three separate aging stages. In the present study, those stages were determined to be the result of the pinning of dislocations in the ferrite, the C-cluster formation, or low-temperature carbide precipitation in the ferrite and the volume contraction of the martensite due to formation of low-temperature carbides, leading to the relief of residual stresses in the ferrite. In the absence of a clear yield point, a new method is proposed to measure the increase in yield stress due to aging only.