2004 Volume 90 Issue 11 Pages 870-876
It is necessary to detect certain types of "shallow defects", which can be harmful in the press processes of steel sheets customers. Since it is difficult to detect them, the thresholds of automatic inspection systems should be lowered, which may cause enormous excessive detection of harmless patterns, such as "shallow" oil patterns.
In this paper, an automatic inspection technique to discriminate harmful defects from harmless patterns clearly in steel strips is studied.
By investigating the microscopic surface structure of defects and harmless patterns, we have found that micro flat elements of metal are dominant in the defect surface, while the harmless patterns have coatings of dielectric materials in the surface layer. Both surfaces show similar mirror reflection characteristic due to their surface structures. This fact accounts for the difficulty of the discrimination between defects and harmless patterns.
Considering above facts, we have adopted ellipsometry to obtain polarized images of steel sheet samples for quantitative analysis. By utilizing linearly polarized light with an azimuth angle of 45 degrees as the incident light, the following results are obtained.
(1) Alternation to elliptical polarization of reflected light occurs to more degree in the case of the defect than in that of the harmless pattern.
(2) Reflected light from the harmless pattern keeps linearity of the polarized incident light.
(3) Defects and harmless patterns can be discriminated by ellipsometric parameters Δ and Ψ.
Based on these results, we have designed a practical instrument for in-line automatic inspection with three polarization cameras for three different azimuth angles. This inspection system is successfully under operation in West Japan Works of JFE Steel Corporation.
