Formation and Development of Shear Bands Preceding the Cracking in Sheet Bending of a Dual-phase Steel

The microstructures developed by inhomogeneous plastic flow are known to play a crucial role in the cracking in sheet metal bending. Observations on the cross-section perpendicular to the bending axis show that cracks propagate along banded microstructures, known as shear bands, which have already developed almost diagonally to the depth direction. However, the relationship between the shear-band formation and the initial microstructures has not been clarified for dual-phase (DP) steel with hard martensite and soft ferrite. In this study, the evolution of shear bands under three-point bending was investigated for a coarse-grained DP steel. It was shown that shear bands initially develop in ferrite at the boundaries with martensite. They are originally planar and tend to stretch diagonally to the depth direction. When further extension of a shear band is obstructed by hard martensite, it changes direction to circumvent it. Furthermore, these shear bands already develop at the early stages of bending, whereas the formation of surface grooves and voids requires larger bending angles, and their shapes and positions are strongly affected by the shear bands. It is, thus, suggested that the shear-band development is essential to the formation of grooves and voids, which eventually leads to cracking.