Work hardening of dual-phase steels

Abstract

A detailed qualitative and quantitative microstructural examination of four dual-phase steels has been carried out. Dual-phase structures were produced in three of these steels by intercritical annealing; the other was in the as-rolled condition. Similar combinations of tensile properties were obtained with all four steels. In intercritically annealed material, greater austenite hardenability was encountered with the two steels containing strong carbide-forming elements (vanadium or titanium) than with a similar plain carbon steel. It has therefore been proposed that the carbonitride precipitates (which are present during the anneal) inhibit the movement of the austenite/ferrite interface during the post-anneal quench; strong metallographic evidence has been provided to support this view. It has been found that during tensile deformation, martensite islands do not deform until strains well in excess of the maximum uniform strain have been reached; this observation has enabled the successful application of Ashby's work-hardening theory to dual-phase steels. This theory predicts that the work-hardening rate is dependent on √f/d, where f is the volume fraction of second phase and d is the mean secondphase island diameter. Thus, it is demonstrated that optimization of dual-phase properties may be achieved by refining the distribution of second phase, since this leads to increased work-hardening rates and hence increased maximum uniform ductilities, for a given tensile strength; the tensile strength is determined primarily by the volume fraction of second phase.