Factors influencing the resistance of dual-phase steels to edge cracking

The key microstructural factors affecting the resistance to edge cracking of two cold-rolled dual-phase (DP) steels, a DP600 and a DP800 DH, were investigated. The resistance to edge cracking was characterised by the hole-expansion ratio, λ, which was measured according to ISO 16630:2017. It was shown that for both steels λ could be greatly improved by adjusting the temperatures in the continuous annealing line. However, the adjustments also resulted in changes in the mechanical properties of the materials, in particular the yield strength, Rp0.2. For both materials the increase in λ correlated with a decrease in the proportion of martensite at the stamped edge, which was approximated by the total proportion of martensite and retained austenite in the bulk microstructure, and an increase in the proportion of bainite. There was no correlation between λ and the hardness of the ferrite and martensite in either steel. For the DP800 DH an increase in λ correlated with (i) a decrease in the intensity of the crystallographic texture, characterised by maximum orientation distribution function, f(g)max, and the proportions of the microstructure corresponding to the α and γ fibres, and (ii) a decrease in grain size resulting from a more complete austenitisation of the microstructure. For the DP600 there was no correlation between the crystallographic texture and λ.