Investigations of damaging mechanisms of coil-coated steel sheet durlng forming in order to enhance the performance predictability of finished parts

The report presents results on the formability and the cutting process of polymer-coated galvanised steel. Finite element modelling (FEM) analysis of the processes of forming and cutting are directly linked to the investigation of resulting defects such as pores in the coating, polymer/metal interfacial de-adhesion or the formation and structure of cutedges. A two-Iayer coil coating system in combination with different metal pre-treatments and thin tailored plasma polymer layers were chosen as model systems. The investigation of the defects has been carried out at different levels, from macroscopic, over microscopie to nanoscopic aspects. Observed defects were correlated with the corrosion resistance and the electrochemical behaviour of the coatings. Based on these results, defect formation and a reduced corrosion resistance of coil-coated material can be foreseen more easily on a profound knowledge basis. Applied microscopic techniques were scanning electron microscopy, Raman microscopy, and atomic force microscopy. Electrochemical investigations were based on local electrochemical impedance spectroscopy and the scanning Kevin probe. Moreover, the nanoscopic investigations allow the design of new interfacial layer systems based on plasma polymer films for highly corrosion-resistant coil-coated steel sheet and have opened the field of a better understanding of the forming behaviour of zinc coatIngs, which has not been addressed to date.