Structure–property relationships in dual-phase steels

Abstract

A study has been made of the formation of the dual-phase structure in intercritically annealed steels. The effects of annealing temperature, time, and cooling rate on the formation of the M–A (martensite and/or retained austenite) constituent have been described, and a discussion given of the factors affecting its development. It has been shown that nitrogen refines the M–A constituent particle size, and possible reasons for this have been discussed. The flow stresses and work-hardening rates have been analysed in terms of established dispersion-strengthening models, with excellent agreement between the theoretical predictions and the experimental results. Both the flow stress and work-hardening rate increase with increasing volume fraction and decreasing particle size of the M–A constituent, but particle size has more effect on the work-hardening rate than on the flow stress. Hence a fine particle size is required to optimize the formability. On the basis of the results obtained, the optimum microstructural profile for a dual-phase steel is suggested, and the processing by which it may be achieved has been discussed.