Effect of tramp oil ingress in cold rolling oil bath leading to white spot defect on CRCA at Tinplate mill

Cationic rolling oils for cold rolled steel manufacturing is based on dispersion technology. Cationic surfactants present in rolling oil allows the oil to remain dispersed in water phase thus aiding lubrication. During operational conditions, tramp oil ingress in the form of lube oil, hy-draulic oil and grease often happens and it remain dispersed in roll coolant bath in smaller quantity without creating any surface blemishes on cold rolled steel. During an unprecedented mechanical shutdown in Tin plate cold rolling mill, after mill startup random white spot defect with greyish centre suddenly started appearing on cold rolled sheet leading to quality deviations and mass rejections. Prolonged shutdown disturbed the overall equilibrium condition of the tramp oil dispersion within the rolling oil emulsion. This study highlights the role of tramp oil separation within the rolling oil bath leading to the defect initiation. The deposition of alkaline cleaner during the ECL process further retards the burn off properties of the tramp oil resulting in the defect formation. Techniques like microscopy, SEM-EDS and Raman spectroscopy were employed to understand the nature and morphology of the defect formation whereas TGA and XRF were employed to investigate and identify the probable source.