Early stages of yielding and strain aging of a vanadium-containing dual-phase steel

Abstract

A study has been made of the early stages of yielding, 10⁻⁵ to 10⁻² plastic strain range, of a vanadium containing dual-phase steel as a function of pct martensite, pct prestrain and aging time and temperature. It is found that the 10⁻⁵ yield stress is independent of the pct martensite, while the 10⁻³ flow stress is proportional to, and increases rapidly with the martensite content. For dual-phase steels of commercial interest (15 to 25 pct martensite content), it is concluded that the yield behavior is controlled by the free dislocations introduced into the ferrite matrix when the austenite regions transform to martensite and their subsequent interaction with the martensite regions. While straining increases the microyield stresses and aging increases both the micro and macroyield stresses, these processes are not additive in a strained plus aged specimen. The activation energy for the production of a yield point plateau was determined to be about 33 kcal/mol. It is proposed that this high value is a result of carbon atoms interacting with vanadium rich clusters which remain in the structure after vanadium carbide precipitates are dissolved during intercritical annealing.