Study of White Patch Defect in Automotive Grade Interstitial Free Steel

Surface of cold-rolled steel intended for automotive body panel applications is expected to be defect free. The presence of surface defects hampers the aesthetics and average surface roughness and thereby deteriorates the paintability of the steel sheet. This study aims to investigate the source of white patch defect in cold-rolled and skin-passed interstitial free steel sheets. Macroscopic observations of as-received white patch sample revealed the presence of blackish deposit having cracks inside the deposits, suggesting that these deposits were heated up possibly during annealing process. Further during macroscopic observation, it was revealed that the black deposits were of near round shape. If deposit is present before rolling, then it is expected that it will elongate during rolling and will not retain round shape. Round shape of the deposits under investigation suggested that it is not before rolling. Electrolytic cleaning line (ECL) is the only in-between process. Surface characterization techniques such as SEM–EDS, Raman spectroscopy, Fourier transform infrared spectroscopy were used to analyze the defect and good region of the steel sheet. Overall analysis indicated grease contamination on cold-rolled steel sheet at the entry of ECL caused generation of white patch defect. Regular cleaning and preventive maintenance of plant machinery were recommended to avoid such leakages to arrest the defect generation in the future.