A new ultrasonic immersion testing method for the detection of internal flaws in a running steel strip has been developed. It consists of a transmitting probe array and a receiving probe array arranged face-to-face on opposite sides of the strip in water, and two flaw echoes given as follows are received by the receiving probe array.
(1) Flaw echo reflected first at a internal flaw and next at the surface wall of the strip.
(2) Flaw echo reflected first at the back wall of the strip and next at a internal flaw.
A linear area in the strip can be tested within a cycle of ultrasonic pulse repetition by integrating transmission of line-focused ultrasonic beam (25 MHz in frequency) using all elements in the transmitting probe array at once with parallel processing of signals received by the receiving probe array.
In succession, an ultrasonic detecting system has been installed in No. 6 Pickling Line at JFE Steel Chiba District. Steel strip being tested is immersed in water by using 6 additional deflector rolls. The transmitting probe array and the receiving probe array cover whole width of the strip so that testing of whole volume of the strip is realized. It is confirmed that an inclusion with 5×10-5mm3 in minimum volume can be detected with signal-to-noise ratio in the range of 9-10 dB. The system is being used for the evaluation of commercial products without trouble, and very useful for internal quality control and assurance in the production line.
