Abstract
Concave and convex defects on steel sheets cause problems in the steel production line. These defects are usually invisible since their height or depth is as small as the normal surface roughness of steel sheets. Therefore, they can only be detected by human inspection after grinding the steel surface with a grindstone.
We found that Magnetic Flux Leakage Testing (MFLT) is effective for detecting these kinds of defects. The MFLT method can detect not only magnetic flux caused by the shape of defects, but also that caused by the strain of defects. This paper describes the results of experiments to verify the principle of the detection method and in-line experiments. As a result of the in-line experiments, it was found that some defects cause MFLT signals which are too small to detect automatically. To solve this problem, we proposed a new periodical method to improve detectability. Based on these results, we developed an in-line concave and convex defect inspection technique.
