Various reasons for measuring plural distances or thicknesses of multilayer structures exist in steel manufacturing processes. Ultrasonic techniques are widely used in thickness measurement but cannot be applied under conditions in which sound does not travel well, such as layers of gas and powder. In this study, a multi frequency eddy current technique is applied to a thickness measurement of multiple layers in cases where the electrical conductivities of the layers are mutually very different. Eddy current signals contain vector information, viz. amplitudes and phases. When two frequencies are used in a measurement, four kinds of data can be acquired. Therefore, the problem of measuring the thicknesses of three layers can be solved by using this data. In this study, we assume measurement of three layers, layer 1 with very high electrical conductivity, layer 2 with low conductivity and layer 3 with very low conductivity, and use two frequencies. And the measurement of interlayer thickness is focused for an example. In the first step, the coil specifications were designed and the two frequencies that should be used were decided by using a mathematical model. A simple laboratory test was performed. Then an online test in the continuous casting line was carried out and the basic performance was confirmed.

With the cosα method, granular (spotty) diffraction rings (Debye-Scherrer ring) are often measured, which may reduce the measurement accuracy. Three types of countermeasures have been proposed so far for such cases. The first is a method of expanding the X-ray irradiation area (sample plane oscillation method). The second is a method of oscillating the X-ray incident angle (angle oscillation method). The third is a method of averaging the measurement data of multiple adjacent measurement points (In-plane averaging). However, these methods also have problems such as a decrease in position resolution, complexity of the device, and an increase in measurement time. Therefore, in this study, we proposed a diffraction ring intensity distribution adjustment (DRIDA) method as the fourth method, and examined the effectiveness of this DRIDA method through application to SUS304, which is a typical material of austenitic stainless steel.