## 1 Introduction  
Steel wire rods are used as raw materials for various mechanical components. This research can be applied to the steel wires, raw material of valve springs and have an influence on keeping smooth cycle motion in a gasoline engine. Under periodic fatigue state, failure of spring valves may occur before expectation of fatigue failure. Therefore, we need a reliable method for steel wires that can evaluate microdefects or early stage damage detection. Up to now, for the research work related with this study not so many articles were published. [PERSON] et al. developed nondestructive evaluation (NDE) techniques using magnetic flux for macro defect [1]. The geometric shape of steel wires before processing of valve springs is cylindrical such as rod and solid shaft. Considering shape characteristics, guided waves have been applied to efficiently inspect long specimens such as wire rods. Research on guided ultrasound has been going on for a long time, but it has not progressed actively due to the complexity of theory and the variety of wave modes. [PERSON] performed basic guided ultrasonic analysis of cylindrical structures [2; 3]. Based on this, the test for guided ultrasound was started by [PERSON] and [PERSON]'s research works [4; 5]. The nonlinear technique has the advantage that it can examine the microstructural changes in the nonlinear elastic section. It is based on information of distance, time and velocity of ultrasonic wave used in linear technique. In addition, the acquired ultrasonic signal is further analyzed in the frequency domain. The acquisition method of this technique is as follows: Information on the defect can be obtained by comparing the change at the second or third harmonic frequency. Through this, the internal microdefect can be diagnosed early. It is also possible to predict the life expectancy based on quantitative results. [PERSON] et al. [6] conducted a study on the change of function of the stress and the second harmonic in which the transition occurs in the material. Subsequently, [PERSON] and colleagues [7; 8; 9] studied the plasticization of materials using nonlinear guided ultrasound. Furthermore, according to using acoustic nonlinear guided wave technique, phase-matching mode is required to generate higher order harmonic amplitude. The basic concept of this research, concerning which factor is related with phase velocity and group velocity, has been improved by [PERSON] [10; 11]. EMAT is a noncontact technique. Research on EMAT has been conducted by [PERSON] and [PERSON] of Osaka University [12; 13; 14]. Quantitative changes of physical nonlinearity should be measured by ultrasonic nonlinear method. Therefore, there is a need to continuously carry out experiments under the same conditions. That is, the same experimental apparatus and conditions are required to minimize the influence of the nonlinearity of the system. At this time depending on the contact conditions of the excitation and receiving transducers the reliability of this experiment may be greatly affected. This may change the nonlinearity. Therefore, to overcome the nonlinearity, a noncontact EMAT was applied. Currently, research on microdefects in the material is proceeding smoothly by combining the contact method and the nonlinear method. However, the contact test method using piezoelectric transducer (PZT) requires a certain size of contact area, but it is impossible to test using PZT in a circular form in the case of the wire rod studied in this study. In addition since the PZT patch requires a contact medium such as an ultrasonic couplant, the signal is irregular depending on the amount or contact strength of the contact medium, so reproducibility is not constant when performing repeated experiments. Therefore, research using noncontact technology is necessary, but the result of the research is insufficient. In particular, there have been no reports on the application of the noncontact technique to the internal microdefects of steel wire rods.