Online magnetic flux leakage detection of inclusions and inhomogeneities in cold rolled steel plate

Abstract

Abstract Cold rolled steel plate is found in different applications. In manufacturing processes with high stretch ratios like bending or deep drawing, non-metallic inclusions, or segregations, which remain in the material during slab casting, can lead to ruptures of the plate. While not being visible optically, those inclusions are detectable by magnetic flux leakage inspection. Therefore, an online inspection system using giant magnetoresistance gradiometers was developed. The individual components were optimized by researching the magnetic properties of the cold rolled plate and natural as well as artificial defects on a laboratory setup for moving material simulation. Correlations between defect properties, inspection parameters and the resulting signals were determined. By optimizing components and configuration accordingly, a sensor module containing 48 sensors, signal conditioning, digitization, and an adjustable magnet was arrived at. Due to the necessarily small distance to the moving material, attention was paid to mechanical repeatability and ruggedness. An inclusion detection system, fully covering 1344 mm strip width, was installed in a coating line. Operating in production for over a year, the inspection system has shown high sensitivity and is, in addition to internal defects, able to detect various strip damages like roll marks and scratches.