Metallurgical Analyses of Surface Defects in Cold-Rolled Steel Sheets

Six cold-rolled steel sheet samples having surface defects like slivers and dents have been metallurgically analyzed to identify the reasons for generation of such defects. In all the cases, large inclusions/entrapments were located at the defect site. Energy-dispersive spectroscopy (EDS) of the entrapments in samples 1 and 2 having dents revealed incidences of O, Mg and Ca, confirming that these entrapments are essentially complex oxides of probable slag genesis. The dents in samples 1 and 2 must have occurred due to heavy cold rolling load on the preexisting cracks in the coil. Apart from dents, sliver defects were also observed in some of the samples. Slivers are, however, of various types and appearances, and there could be many reasons for occurrences of these slivers in the cold-rolled sheets. In the present study, the SEM-EDS analyses of the subsurface entrapment/deposit observed in sample 3 revealed complex oxides of CaO-MgO of possible slag genesis and could be the reason for incidences of slivers in this sample. On the other hand, the wavelength-dispersive spectroscopy of samples 4 and 5 having sliver defects carried out in an electron probe microanalyzer (EPMA) revealed subsurface chains of dark and angular brittle inclusions of either alumina or silicate of possible deoxidation product genesis. EPMA analysis of sample 6 having pencil line sliver defects, however, revealed entrapments of possible casting powder origin.