Identification of the “rolled-in scale” type sheet surface defect

Abstract

Rolled-in scale” defect is put down to one of main surface defects of sheet rolled products. It deteriorates salable condition of cold-rolled sheet, adhesion of surface coatings to metal base during galvanizing, chrome plating, applying of polymer materials, stipulates increased corrosion of metal rolled products etc. External view of “rolled-in scale” defect is very similar to the whole number of sheet surface failures, as scratch, rolled impurities, slag inclusions, rolled undersurface gas bubble etc. Exact identification of the abovementioned defect is important for elaboration of measures for its elimination and is possible only by application of metallographic methods. Results of metallographic study of cold-rolled sheet defect, identified as “rolled-in scale” presented. Technology of hot-rolled sheet production usually is the reason for this defect, when formed on the billet surface furnace and/or air scale is partially rolled into the rolled product surface. Such a defect on the cold-rolled sheet surface, inherited from the hot-rolled sheet surface is considered as a rejected feature. Studies by optical and raster electron microscopes using X-ray spectral microscopy analysis results in determining real reason of the presumable “rolled-in scale” defect. In this case the rolling-in of residuals of slag surface layer and slag-making mixtures into the hot-rolled sheet surface is the reason. The residuals are inherited from metallurgical stage, mainly continuous casting, as well as pickling process and cold-rolled sheet alkaline cleaning.