2011 Volume 52 Issue 3 Pages 531-538
A new ultrasonic immersion test method for detecting internal flaws in a running steel strip has been developed. A transmitting probe array and a receiving probe array are arranged face-to-face on opposite sides of the strip in water, and two flaw echoes are received by the receiving probe array: (1) Flaw echo reflected first at an internal flaw and next at the surface wall of the strip; and (2) Flaw echo reflected first at the back wall of the strip and next at an internal flaw. A linear area in the strip can be tested within a cycle of ultrasonic pulse repetition by integrating the transmission of a line-focused ultrasonic beam (25 MHz in frequency) using all elements in the transmitting probe array at once with the parallel processing of signals received by the receiving probe array. An ultrasonic detection system based on this method was subsequently installed in the No. 6 pickling line at JFE Steel, Chiba District. The steel strip being tested is immersed in water by using six additional deflector rolls. The transmitting probe array and receiving probe array cover the width of the strip so that testing of the entire volume of the strip is realized. It is confirmed that inclusions with a minimum volume of 5×10−5 mm3 can be detected at a signal-to-noise ratio in the range of 9–10 dB. The detection results are rapidly fed back to the steelmaking process to improve steel cleanliness. As a result, steel coils have maintained a high level of internal cleanliness with a dramatic reduction in defects found during the users’ manufacturing process.