Effect of Annealing Cycles on the Properties and Microstructures of Several Dual Phase Steels

A study was undertaken to describe the effects of various annealing treatments, both intercritical and supercritical, on the mechanical properties and microstructures of six dual-phase, Mn-Si, Mn-Si-Cr, Mn-Si-Mo and Mn-Si-V steels having nearly equal hardenability. It has been found that the combination of tensile strength and total elongation after intercritical treatments may be slightly better than that after supercritical treatments. Although hardenabilities of these six alloys were more or less equal, it appears that the strength-ductility relationship varies a little (though not very significantly) with chemistry. The strength-ductility behavior of Mn-Si-Cr steels compares quite favorably with that of commercial Mn-Si-V steels with optimum processing conditions for both. It has been confirmed that the tensile strength of dual-phase steels is primarily a function of the hard-phase (mainly martensite and bainite, if any) content at a given ferrite grain size. The full curve n values of these six alloys are higher than the uniform strain “n” (n = ε_u) values. The observed variation in the work hardening behaviors of these six alloys, expressed in terms of uniform true strain “n”, with the volume fraction hard-phase content, can be explained in terms of the Araki et.al., theory. On the other hand, full curve values follow those predicted by the Davies-Mileiko theory. The minimum amount of hard-phase content needed for the elimination of yield point elongation in these alloys varied from one alloy to the other.