The dependence of some tensile and fatigue properties of a dual-phase steel on its microstructure

Abstract

Five different dual-phase microstructures have been produced in a steel containing 0.11 C, 1.60 Mn, 0.73 Si. By careful design of the heat treatment schedules, it was arranged that all the specimens had the same fine austenite grain size before intercritical annealing and that they all contained close to 30 vol pct martensite and a negligible fraction of retained austenite after annealing. The microstructures were characterized by measurement of the mean ferrite path and the density and distribution of the extrinsic transformation accommodation dislocations in the substructure of the ferrite. Specimens representative of each of the microstructures were tested in tension at room temperature. The strength, work-hardening, and fracture of three of them were examined in detail and correlated with the microstructural parameters. The microstructural features which most influence the tensile properties are identified. Two specimens, representative of the finest and coarsest microstructures, were the subject of a detailed exploration of the initiation of persistent slip and microcracks and of crack propagation in bending fatigue. The fatigue endurance limit was also measured. It is shown that the initiation of persistent slip is primarily influenced by the heterogeneity in the density of the dislocations in the ferrite. The microstructural features of greatest importance in controlling the formation and propagation of microcracks are the local dislocation density and the mean ferrite path.