Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels

A series of dual-phase (DP) steels containing finely dispersed martensite w ith different volume fractions of martensite (V-m) were produced by interme diate quenching of a boron- and vanadium-containing microalloyed steel. The volume fraction of martensite was varied from 0.3 to 0.8 by changing the i ntercritical annealing temperature. The tensile and impact properties of th ese steels were studied and compared to those of step-quenched steels, whic h showed banded microstructures. The experimental results show that DP stee ls with finely dispersed microstructures have excellent mechanical properti es, including high impact toughness values, with an optimum in properties o btained at similar to 0.55 V-m. A further increase in V-m was found to decr ease the yield and tensile strengths as well as the impact properties. It w as shown that models developed on the basis of a rule of mixtures are inade quate in capturing the tensile properties of DP steels with V-m > 0.55. Jao ul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation .