Optical Approach for Suppressing Over-detection in Surface Inspection for High Strength Steel Sheets

In a process of manufacturing thin steel sheets such as for automobiles, it is common to install and use optical surface inspection system using a light source and cameras that receives regular reflection and non-regular reflection with properly settled incident and observation angles. However, surface inspection is getting more difficult for automobile steel sheets which are being promoted to high tensile strength steel sheets for safety. For example, harmless glittering glossy parts existing on the surface of the steel sheet are detected and mistakenly recognized as a harmful defect, which is called over-detection, or a large amount of over-detection interferes with proper inspection. That is the reason why conventional technologies could not be applied to high tensile strength steel sheets with harmless glittering glossy parts. This paper describes the cause of harmless glittering glossy parts and essential measures to completely prevent the over-detections by optical means.