The paper examines the roughening along a metal-polymer interface, to find out whether the relevant length scale is on a sub-grain level or on the grain-size level. This is relevant for understanding the possible delamination of a polymer coating on a metallic substrate. Therefore we have investigated the local lattice orientation in heavily strained ferritic steel using electron back-scatter diffraction. From that data we have calculated the components of the local orientation gradient tensor as well as the local Schmid factor for deformation along [100] and [001] on {101} and {112} slip systems. The curvature of the draw-and-redraw steel- polyethylene terephthalate (PET) laminate interface as well as the curvature of the underlying steel lattice was examined in detail. It is concluded that roughening at a sub-grain length scale along the interface is due to plasticity in the interior of the grains.