The Effect of Cooling Rate on the Damage Micromechanisms of DP Steels

Abstract

The microstructural failure mechanisms of two DP steel sheets cooled with different cooling rates during their heat treatment are compared in the present study. The as-cold rolled DP steel sheets were annealed at intercritical temperature and cooled down with rates of 45°C/s (quenching) and 2 °C/s (slow cooling). Uniaxial tensile tests were carried out on samples from both sheets and the microstructure of undeformed samples and the broken tensile specimens was evaluated by optical microscopy, scanning electron microscopy and electron back-scatter diffraction technique. Although the grain size did not show significant differences, the amount and size of the constituents, e.g. martensite and bainite, differ between both alloys. Concerning the mechanical properties, the quenched material showed superior strength and ductility besides a less localized deformation at higher strains. The area fraction of voids in the broken specimens was low for both steels. In the slow cooled samples the nucleation of shear bands was on the large voids and cracks were observed along these shears bands. It was concluded that the detrimental effect of void nucleation on both steels is not only attributed to their null-carry capacity but more to the stress concentration close to the voids which gives rise to strain localization in the form of shear bands.