Correlation of Optical Microstructure and Macroscopic Intensity of Defects on Steel Strip Surface

Abstract

Low contrast defects which originate from inclusions under the surface layer are often observed as patterns of shallow bright streaks with the exception of exposed inclusions on the surface of steel strips. These defects also appear brighter than the normal part under most optical conditions. In this research, the relationship between the macroscopic intensity of the defective parts and the defect microstructure was studied in detail with galvannealed (iron–zinc alloy) steel strips. The following points were observed: 1) Defective parts contain a number of microscopic flat portions which have mirror reflection parallel to the surface, and the reflected intensity of these micro-flat portions is dominant in specular reflection. 2) A clear correlation exists between the specular reflected intensity in the macroscopic images of the defective part and the unit area ratio of the microscopic flat portions. 3) When the intensity from macroscopic observations can be estimated while considering the polarized reflection characteristics of the defect and treating the flat portion as a mirror facet, it agrees with the measured intensity.The results of this study led to a better understanding of the reason why defects originating from inclusions under the surface layer appear slightly brighter than the normal area on steel strips.