Development of an On-line r-value Measurement Technique for Cold Rolled Steel Sheet Using a Resonance Mode Electromagnetic Acoustic Transducer

An on-line nondestructive measurement technique for r-value (plastic strain ratio), which evaluates the press formability of cold-rolled steel sheets, has been developed. This technique uses a thickness resonance mode electromagnetic acoustic transducer (EMAT), which can measure the ratio of three different mode ultrasonic resonance frequencies. The in-plane average Young's modulus is calculated from three frequencies and the elastic stiffnesses of an iron single crystal C011, C012, C044. The in-plane average r-value is obtained from the in-plane average Young's modulus. A good correspondence was obtained between the in-plane average r-value measured by this method and the in-plane average r-value measured by tensile testing. We have proved by experiment that the effects of temperature, tensile stress and moving speed of steel sheets are negligible for practical on-line use.