低炭素鋼の塑性ひずみに誘起された非線形超音波特性と微細組織の変化

Abstract

電磁超音波共鳴法（EMAR）は, 非接触で超音波を送受信できる電磁超音波探触子(EMAT)を共鳴測定に適用した計測法である．この方法は, 高精度な超音波減衰の計測ができるだけでなく，試料と探触子との間に接触に伴う非線形音響効果がない. 本研究では，低炭素鋼S25Cの引張試験中のひずみに誘起された非線形超音波特性（共鳴周波数の移動，3波相互作用法，音響複屈折）と微細組織の変化の関係を調べることを目的とした. さらに，1 つのEMATで3波相互作用法に非線形超音波量を計測するために，3層の平面コイルと永久磁石から構成されるE MATを開発した. 板厚方向に偏向しながら横波を送受信する体積波横波EMATを用いた．3つの非線形超音波パ ラメータと減衰係数は，引張塑性ひずみの増加につれて増加していった. この現象は，転位密度や結晶方位差に よる微細組織の変化に起因していた.それはX線による観察やEBSD（電子後方散乱回折）法による方位差解析の結果から裏付けられた．

Electromagnetic acoustic resonance (EMAR) is a contactless resonant method with an electromagnetic acoustic transducer (EMAT). This method enables not only to measure exact ultrasonic attenuation of measured sample but also to eliminate nonlinear acoustic effect between the sample and transducer. In this study, the EMAR was applied to investigate the relationships between nonlinear acoustic characterizations; resonant frequency shift, three-wave mixing and birefringence acoustoelasticity and microstructural changes induced by tensile plastic strain in a low-carbon steel, JIS-S25C. Furthermore, we developed a single bulk-shear-wave EMAT which was composed of three-layer elongated coils and a pair of permanent magnets to measure in three-wave mixing. The EMAT transmits and receives shear wave propagating in thickness direction of a plate specimen. Three nonlinear acoustic parameters and ultrasonic attenuation increased with increase in tensile plastic strain. This phenomenon is interpreted as resulting from microstructure changes, especially, dislocation density and crystal misorientation. This is supported by X-ray observations for dislocation density and EBSD (electron backscattering diffraction) for the misorientation.