Investigation of a surface defect and its elimination in automotive grade galvannealed steels

The galvannealed coating have gained importance as a potential solution for auto body panels owing to their superior performance like weldability, paintability and after painting corrosion resistance. Efficient production of galvannealed strip through hot dip galvanising and subsequent annealing root is a challenge as occurrence of surface defects results in material downgrading and rejection. The present work deals with one type of surface defect present at the bottom surface of the galvannealed strip with high defect severity. The defect appearance was like a shining spot by naked eye. It was characterised using Optical Microscope, Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy to analyse shape, size, morphology and composition. The defect was further analysed by Confocal Laser Scanning Microscope (CLSM). The features of CLSM like high resolution, high depth of focus, 3D surface topography were useful in determining the relative depth of the defect at different regions. The results indicated that the defect occurrence was due to mechanical abrasion of coating with deposited dust of Fe–Zn intermetallic phases at the galvannealing tower top roll. The production line parameters were modified to maintain the temperature and cleanliness of the top roll and a reduction in defect severity was achieved.