The effect of new ferrite content on the tensile fracture behaviour of dual phase steels

The effect of new ferrite present with different volume fractions and morphologies of martensite on microvoids formation and tensile fracture behaviour in dual phase steels has been studied for a steel containing 0.065% C, 1.58% Mn and 0.5% Ni. Fine and coarse dual phase microstructures were obtained from two different starting conditions. Martensite contents were kept constant at ∼18 and ∼25% and new ferrite content was varied by controlled cooling from intercritical annealing temperature of 740, 750 and 785°C. In both fine and coarse dual phase structures microvoids formed at martensite particles, inclusions and martensite-ferrite interfaces in the necked region. Martensite morphology had an influence in determining martensite cracking. Coarse and interconnected martensite distributed along ferrite grain boundaries cracked easily. Martensite cracking was less frequent and the microvoids were smaller in the fine structure than the coarse ones. Microvoid coalescence was the dominant form of fracture in both structures. The specimens with higher new ferrite contents had higher densities of voids. In these samples, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite