Magnetic Leakage Testing Using Linearly Integrated Hall and GMR Sensor Arrays to Inspect Inclusions in Cold-Rolled Strips

Inclusions in steel are entrapped foreign materials, which may be metallic or non-metallic; they may be distributed throughout the volume of steel strips during the rolling process and may propagate as narrow cracks in steel strips. In this paper, for inspecting inclusions in steel strips, we propose the use of magnetic flux leakage testing involving a magnetizer and magnetic field sensor arrays, such as linearly integrated Hall sensors (LIHaS) in differential mode and linearly integrated giant magnetoresistance sensors (LIGiS). In the proposed method, images of the distribution of Hall voltage (V_H) and giant magnetoresistance voltage (V_G) due to the magnetic leakage field around an inclusion are obtained by means of LIHaS and LIGiS, respectively. The magnetic leakage field images are obtained at different magnetization directions relative to the rolling direction by rotating the magnetizer. Different data processing techniques, such as differentiation, noise reduction, and fusion of image data of the inclusion obtained by LIHaS and LIGiS, are applied to enhance the detectability of the inclusion. The effectiveness of the proposed technique was experimentally verified by detecting actual inclusions situated in the rolling direction in a cold-rolled steel strip.