JP4192333B2

JP4192333B2 - Method for measuring transformation layer thickness of steel

Info

Publication number: JP4192333B2
Application number: JP11167999A
Authority: JP
Inventors: 義徳福田; 章生長棟
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1999-04-20
Filing date: 1999-04-20
Publication date: 2008-12-10
Anticipated expiration: 2019-04-20
Also published as: JP2000304725A

Description

【０００１】
【発明の属する技術分野】
本発明は、鋼材を加熱・冷却したときに表層部に生じる変態層の厚さを計測する方法、特に、磁気的性質を利用した計測方法に関する。
【０００２】
【従来の技術】
鋼材の加熱・冷却したときに起るオーステナイト(γ)相ォフェライト(α)相の変態の様子を監視することは、鋼材の機械的特性や物理的特性を管理する上で極め重要である。
【０００３】
そのため、従来より、鋼材の磁気的性質を利用してオンラインで変態率を計測する方法が種々提案されている。例えば、特公平2-42402号公報、特開平3-123853号公報、特開平8-62181号公報などには、励磁コイルと検出コイルを用い、鋼材の変態に伴なう磁気特性の変化によって生じる磁束の変化から変態率を測定する方法が開示されている。
【０００４】
【発明が解決しようとする課題】
しかしながら、上記公報に記載された方法には、鋼材の板厚方向の平均的な変態率しか測定できず、加熱・冷却時における表面から内部へ向う変態層の厚さの変化を追跡することができない、また、板厚が厚い場合は磁束が表皮効果で表層部にしか侵入できないため正確な変態率の測定ができないといった問題がある。
【０００５】
本発明はこのような問題を解決するためになされたもので、鋼材の板厚が厚くても、表面から内部へ向かう変態層の厚さの変化を追跡できる鋼材の変態層厚さ計測方法を提供することを目的とする。
【０００６】
【課題を解決するための手段】
上記課題は、鋼材に交流磁化を印加し、渦電流によって鋼材表面近傍に発生する誘導磁界を、前記交流磁界の周波数を変化させて測定し、得られた前記誘導磁界の前記周波数を関数とする曲線が大きく屈曲するときの周波数を求め、前記曲線が大きく屈曲するときの周波数が渦電流の表皮効果現象により変態層の厚さにより変化することを利用して計測する鋼材の変態層厚さ計測方法により解決される。
【０００７】
鋼材に交流磁界を印加したとき表層部に発生する渦電流の浸透深さは、交流磁界の周波数f、鋼材の透磁率μ、鋼材の導電率sに依存し、f^1/2、μ^1/2、s^1/2に反比例する。ここで、導電率sは変態の影響を受け難いため、変態挙動を追跡するには周波数fと透磁率μを考えればよい。すなわち、渦電流の浸透深さは交流磁界の周波数が高くなり、鋼材の透磁率μが大きくなると浅くなる。また、渦電流による誘導磁界は周波数が高いほど強くなるという特性がある。
【０００８】
いま、有限の厚さの変態層が存在したときの渦電流による誘導磁界を考えると、渦電流の浸透深さが変態層の厚さに比べて十分に深い場合、すなわち、周波数が低い場合は、誘導磁界そのものが弱いが、渦電流の浸透深さに対する変態層の厚さの比も小さいため、変態層に依存した誘導磁界も弱くなり、その周波数による変化も小さい。一方、渦電流の浸透深さが変態層の厚さに比べて浅くなる高い周波数の場合は、誘導磁界そのものが強くなるが、変態層に依存した誘導磁界も強くなり、その周波数による変化が大きくなる。
【０００９】
したがって、図2に模式的に示すように、誘導磁界による起電力を周波数に対してプロットして得られる曲線には屈曲点が存在し、その周波数fcにおいて渦電流の浸透深さが変態層の厚さに対応することになる。なお、図2の縦軸は印加磁界と誘導磁界の複合磁界により検出コイルに発生する起電力を示しているが、起電力が周波数とともに減少しているのは、周波数とともに渦電流による誘導磁界が増大し印加磁界による起電力を減少させているためである。
【００１０】
具体的には、周波数fcにおいては下記式(1)が成り立つので、表層に生じる変態層の透磁率μが既知の場合は、fcの値から変態層の厚さdtが求まることになる。
【００１１】
dt=α/(fc×μ)^1/2…(1)
ここで、αは定数である。
【００１２】
また、鋼材を磁化し、鋼材表面から漏洩する漏洩磁界φを測定し、下記の式(2)から変態層の厚さdtを求めることもできる。
【００１３】
dt=F₁(μ、φ) …(2)
ここで、F₁は鋼材の強磁性体層と磁化装置の間に形成される磁気回路に依存する関数、μは変態層の透磁率である。
【００１４】
例えば、表層に変態層を有する鋼材をU字形の磁化器により交流磁化する磁気回路を考えた場合、U字形のヨーク間で磁化側に変態層と平行に漏洩する磁界は変態層の磁気抵抗の増加に応じて増加する。すなわち、変態層の磁気抵抗が増すと磁束は変態層内を流れ難くなり漏洩磁界は増加する。一方、変態層の磁気抵抗が減ると磁束は変態層内を流れ易くなり漏洩磁界は減少する。この磁気回路においては、磁気抵抗は変態層の透磁率μと厚さdtの積に反比例するので、漏洩磁界φは下記の式(4)で表せる。
【００１５】
φ=φ₀/(1+β・μ・dt）…(4)
ここで、φ₀は変態層がないときの磁界、βは定数
【００１６】
したがって、変態層の厚さdtは上記式(2)のように漏洩磁界φと透磁率μの関数で表せるので、表層に生じる変態層の透磁率μが既知の場合は、漏洩磁界φを測定することにより変態層の厚さdtを求めることができる。
【００１７】
なお、式(2)の関数形は、式(4)で一義的に表せなく、装置構成により変わるのでその都度決定する必要がある。
【００１８】
一般に、鋼材の透磁率は成分や温度により大きく変化するので、上述したような変態層の透磁率が既知の場合はむしろ希である。しかし、変態層の透磁率が未知の場合でも、上記した周波数fcと漏洩磁界φを求め、上記式(1)と式(2)から透磁率μを消去した下記の式(3)から変態層の厚さdtを求めることができる。
【００１９】
dt=F₂(fc、φ) …(3)
式(2)の関数形として式(4)を用いる場合は、下記の式(5)から変態層の厚さdtを求めることができる。
【００２０】
dt=γ/fc/(φ₀/φ-1) …(5)
ここで、γは定数である。
【００２１】
ところで、式(1)から周波数fcの変態層の厚さdtに対する変化はdtの3乗のオーダーで小さくなり、また、式(4)から漏洩磁界φの変態層の厚さdtに対する変化はdtの2乗のオーダーで小さくなることがわかる。これは、計測の観点からは、変態層の厚さdtが厚くなるにしたがい周波数fcや漏洩磁界φの測定精度が低下することを意味する。そこで、変態層の厚さdtが厚い場合でも周波数fcや漏洩磁界φの変態層の厚さdtに対する変化を大きして測定精度を上げるための検討を行ったところ、変態層の透磁率μ自体を小さくすればよく、そのためには、上記の方法で印加する交流磁界に直流磁界を重畳すればよいことを見出した。特に、式(3)を用いて変態層の厚さdtを測定する場合は、透磁率μによらず測定できるので非常に有効である。なお、変態層の厚さdtが薄い場合には透磁率μが大きい方が測定精度が上がるので、変態層の厚さdtに応じて直流磁界の強さを適宜選択することが好ましい。
【００２２】
【発明の実施の形態】
図1に、本発明法の1実施の形態を示す。
【００２３】
発信器(図示されてない)から送られてくる交流信号を電力増幅器9で増幅し、感度向上のためにフェライトヨーク5を有する励磁コイル3を励磁すると、この励磁コイル3に近接し、コイル軸芯に垂直に配置され、表層に変態層2を有する鋼材1の表層には渦電流が発生する。この渦電流により誘導磁界が発生するが、その磁界はフェライトヨーク5を有し、励磁コイル3と同一の軸上にある検出コイル4に発生する起電力として検出される。この起電力はロックインアンプ11へ送られ、発信器から送られてくる交流信号に基づき同期検波され直流信号に変換されて、上記式(1)の組み込まれた演算処理装置13に送られる。交流信号の周波数を変えて同様な操作を繰り返し、周波数と起電力のデータを演算処理装置13に送ればfcが求まり、変態層2の透磁率μが既知の場合は、μを演算処理装置13に入力することにより変態層2の厚さdtが求まる。
【００２４】
また、発信器から送られてくる交流信号を電力増幅器10で増幅し、鋼材1に近接させたU字形ヨーク７を有する励磁コイル６を励磁すると、鋼材1の表面に平行に磁界が漏洩するが、この漏洩磁界φをたとえばホール素子のような磁気センサー8で起電力として検出しロックインアンプ12へ送り、変態層2の透磁率μが既知の場合は、μを例えば上記式(4)の組み込まれた演算処理装置13に入力することにより変態層2の厚さdtを求めることもできる。
【００２５】
変態層2の透磁率μが未知の場合は、上記両方の測定を行い、周波数と起電力のデータと漏洩磁界のデータを例えば上記式(5)の組み込まれた演算処理装置13に入力することにより変態層2の厚さdtを求めることができる。
【００２６】
発信器から送られてくる交流信号に直流成分を重畳すると変態層の透磁率μ自体を小さくできるので、変態層の厚さdtが厚い場合でも、周波数fcや漏洩磁界φを精度よく測定できる。
【００２７】
【実施例】
鋼とほぼ同じ導電率を有する非磁性体である青銅の上に、厚さが2.0〜8.0mm、透磁率μが25〜150の強磁性体である鋼板を重ね、鋼材がオーステナイト相→フェライト相に変態しているときの表層における変態層厚さの推移をシミュレートした試料を作製した。そして、図1に示した構成の装置を用い、周波数fc、漏洩磁界φ(起電力)、鋼板厚さ(変態層厚さ)を測定した。なお、漏洩磁界φの測定には0.2A、10Hzの交流信号を用い、また、いずれの測定においても、交流信号に2.08Aの直流成分を重畳した。
【００２８】
図3に、鋼板厚さを変えた場合の誘導起電力比と周波数との関係を示す。ここで、誘導起電力比とは、起電力を鋼板のないときの起電力に対する比で表したものである。また、透磁率μが一定の鋼板を用いたときの結果である。
【００２９】
各鋼板厚さにおいても、誘導起電力と周波数の関係には屈曲点が存在しており、鋼板厚さが厚くなるにしたがい屈曲点の周波数が低下する、すなわち、変態層の厚さが厚くなるにしたがいfcが小さくなることがわかる。周波数fcと鋼板厚さの関係をプロットすると、図4に示すように、鋼板の厚さ、すなわち、変態層の厚さとfc^-1/2とには直線関係があり、上記式(1)により変態層の厚さが計算できることがわかる。
【００３０】
図5に、漏洩磁界φと鋼板厚さとの関係を示す。ここで、漏洩磁界は起電力として測定されている。また、透磁率μが一定の鋼板を用いたときの結果である。
【００３１】
漏洩磁界φは鋼板厚さが厚くなるにしたがい直線的に低下しており、図1の装置構成の場合の上記式(4)により変態層の厚さが計算できることがわかる。
【００３２】
鋼板の透磁率μを種々変化させて測定した周波数fcと漏洩磁界φから、上記式(5)を用いて鋼板厚さを計算した結果を表1に示す。
【００３３】
このように、式(5)を用いて計算した厚さは実際の厚さと0.3mm以内で一致しており、透磁率μが未知の場合でも本発明法により変態層の厚さを精度よく求めることができる。
【００３４】
【表１】

【００３５】
【発明の効果】
本発明は以上説明したように構成されているので、鋼材の板厚が厚くても、表面から内部へ向かう変態層の厚さの変化を追跡できる鋼材の変態層厚さ計測方法を提供できる。
【図面の簡単な説明】
【図１】本発明法の1実施の形態を示す図である。
【図２】誘導起電力と周波数との関係を模式的に示す図である。
【図３】鋼板厚さを変えた場合の誘導起電力比と周波数との関係を示す図である。
【図４】周波数fcと鋼板厚さとの関係を示す図である。
【図５】漏洩磁界φと鋼板厚さとの関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for measuring the thickness of a transformation layer generated in a surface layer portion when a steel material is heated and cooled, and more particularly to a measurement method using magnetic properties.
[0002]
[Prior art]
Monitoring the transformation of the austenite (γ) phase and ferrite (α) phase that occurs when steel materials are heated and cooled is extremely important in managing the mechanical and physical properties of the steel materials.
[0003]
For this reason, various methods for measuring the transformation rate online using the magnetic properties of steel have been proposed. For example, in Japanese Patent Publication No. 2-42402, Japanese Patent Application Laid-Open No. 3-13853, Japanese Patent Application Laid-Open No. 8-62181, etc., an excitation coil and a detection coil are used, which are caused by changes in magnetic properties accompanying the transformation of steel. A method for measuring a transformation rate from a change in magnetic flux is disclosed.
[0004]
[Problems to be solved by the invention]
However, the method described in the above publication can only measure the average transformation rate in the thickness direction of the steel material, and can track the change in the thickness of the transformation layer from the surface to the inside during heating and cooling. In addition, when the plate thickness is large, the magnetic flux can only enter the surface layer due to the skin effect, so that there is a problem that an accurate transformation rate cannot be measured.
[0005]
The present invention has been made to solve such problems, and provides a method for measuring the thickness of a transformation layer of a steel material that can track the change in thickness of the transformation layer from the surface toward the inside even if the thickness of the steel material is large. The purpose is to provide.
[0006]
[Means for Solving the Problems]
The above-mentioned problem is that an AC magnetic field is applied to a steel material, an induced magnetic field generated near the steel material surface by an eddy current is measured by changing the frequency of the AC magnetic field, and the obtained frequency of the induced magnetic field is used as a function. Obtain the frequency when the curve bends greatly, and measure the thickness of the transformation layer of steel by measuring the frequency when the curve bends greatly depending on the thickness of the transformation layer due to the skin effect phenomenon of eddy current Solved by the method.
[0007]
The penetration depth of the eddy current generated in the surface layer when an AC magnetic field is applied to the steel depends on the frequency f of the AC magnetic field, the permeability μ of the steel, the conductivity s of the steel, f ^1/2 , μ ^{1 / 2} is inversely proportional to s ^1/2 . Here, since the conductivity s is hardly affected by the transformation, the frequency f and the magnetic permeability μ can be considered in order to trace the transformation behavior. That is, the penetration depth of the eddy current becomes shallow as the frequency of the alternating magnetic field increases and the permeability μ of the steel material increases. In addition, the induced magnetic field due to eddy current has a characteristic that it becomes stronger as the frequency is higher.
[0008]
Considering the induced magnetic field due to the eddy current when a transformation layer with a finite thickness exists, if the penetration depth of the eddy current is sufficiently deep compared to the thickness of the transformation layer, that is, if the frequency is low Although the induced magnetic field itself is weak, since the ratio of the thickness of the transformation layer to the penetration depth of the eddy current is small, the induced magnetic field depending on the transformation layer is also weak, and the change due to the frequency is also small. On the other hand, in the case of a high frequency at which the penetration depth of eddy current becomes shallower than the thickness of the transformation layer, the induced magnetic field itself becomes stronger, but the induced magnetic field depending on the transformation layer also becomes stronger, and the change due to the frequency is large. Become.
[0009]
Therefore, as schematically shown in FIG. 2, there is an inflection point in the curve obtained by plotting the electromotive force due to the induced magnetic field against the frequency, and the penetration depth of the eddy current at the frequency fc is that of the transformation layer. It will correspond to the thickness. The vertical axis in FIG. 2 indicates the electromotive force generated in the detection coil by the combined magnetic field of the applied magnetic field and the induced magnetic field. The electromotive force decreases with the frequency because the induced magnetic field due to the eddy current increases with the frequency. This is because the electromotive force due to the increase and the applied magnetic field is reduced.
[0010]
Specifically, since the following formula (1) is established at the frequency fc, if the permeability μ of the transformation layer generated on the surface layer is known, the thickness dt of the transformation layer is obtained from the value of fc.
[0011]
dt = α / (fc × μ) ¹ / ² … (1)
Here, α is a constant.
[0012]
Further, the leakage magnetic field φ leaked from the steel material surface can be measured by magnetizing the steel material, and the thickness dt of the transformation layer can be obtained from the following equation (2).
[0013]
dt = F ₁ (μ, φ)… (2)
Here, F ₁ is a function depending on the magnetic circuit formed between the ferromagnetic layer of the steel material and the magnetizing device, and μ is the permeability of the transformation layer.
[0014]
For example, when considering a magnetic circuit in which a steel material having a transformation layer on the surface is AC magnetized by a U-shaped magnetizer, the magnetic field leaking parallel to the transformation layer on the magnetization side between the U-shaped yokes is the magnetoresistance of the transformation layer. Increases with increase. That is, when the magnetoresistance of the transformation layer increases, the magnetic flux hardly flows in the transformation layer and the leakage magnetic field increases. On the other hand, when the magnetoresistance of the transformation layer decreases, the magnetic flux easily flows in the transformation layer and the leakage magnetic field decreases. In this magnetic circuit, since the magnetic resistance is inversely proportional to the product of the permeability μ and the thickness dt of the transformation layer, the leakage magnetic field φ can be expressed by the following equation (4).
[0015]
φ = φ ₀ / (1 + β ・ μ ・ dt) (4)
Here, φ ₀ is a magnetic field when there is no transformation layer, β is a constant
Therefore, the thickness dt of the transformation layer can be expressed as a function of the leakage magnetic field φ and the magnetic permeability μ as in the above equation (2). Therefore, when the magnetic permeability μ of the transformation layer generated in the surface layer is known, the leakage magnetic field φ is measured. By doing so, the thickness dt of the transformation layer can be obtained.
[0017]
Note that the function form of Equation (2) cannot be uniquely expressed by Equation (4), and changes depending on the device configuration, so it must be determined each time.
[0018]
In general, the permeability of a steel material varies greatly depending on the component and temperature, so it is rather rare when the permeability of the transformation layer as described above is known. However, even when the permeability of the transformation layer is unknown, the frequency fc and the leakage magnetic field φ described above are obtained, and the transformation layer is obtained from the following formula (3) in which the permeability μ is eliminated from the formulas (1) and (2). Thickness dt can be obtained.
[0019]
dt = F ₂ (fc, φ) (3)
When equation (4) is used as a function form of equation (2), the thickness dt of the transformation layer can be obtained from equation (5) below.
[0020]
dt = γ / fc / (φ ₀ / φ-1) (5)
Here, γ is a constant.
[0021]
By the way, the change of the frequency fc with respect to the thickness dt of the transformation layer becomes smaller in the order of the third power of dt from the equation (1), and the change of the leakage magnetic field φ with respect to the thickness dt of the transformation layer becomes dt from the equation (4). It turns out that it becomes small in the order of the square of. This means that, from the viewpoint of measurement, the measurement accuracy of the frequency fc and the leakage magnetic field φ decreases as the thickness dt of the transformation layer increases. Therefore, even when the thickness dt of the transformation layer is large, studies were conducted to increase the measurement accuracy by increasing the change of the frequency fc and the leakage magnetic field φ with respect to the thickness dt of the transformation layer. In order to achieve this, it has been found that a DC magnetic field may be superimposed on an AC magnetic field applied by the above method. In particular, when the thickness dt of the transformation layer is measured using the equation (3), it is very effective because it can be measured regardless of the magnetic permeability μ. Note that when the thickness dt of the transformation layer is thin, the measurement accuracy increases as the permeability μ is larger. Therefore, it is preferable to appropriately select the strength of the DC magnetic field according to the thickness dt of the transformation layer.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment of the method of the present invention.
[0023]
When an AC signal sent from a transmitter (not shown) is amplified by a power amplifier 9 and an excitation coil 3 having a ferrite yoke 5 is excited to improve sensitivity, the excitation coil 3 is close to the coil axis, Eddy current is generated in the surface layer of the steel material 1 that is arranged perpendicular to the core and has the transformation layer 2 in the surface layer. An induced magnetic field is generated by this eddy current, and the magnetic field has a ferrite yoke 5 and is detected as an electromotive force generated in the detection coil 4 on the same axis as the exciting coil 3. This electromotive force is sent to the lock-in amplifier 11, synchronously detected and converted into a DC signal based on the AC signal sent from the transmitter, and sent to the arithmetic processing unit 13 incorporating the above equation (1). The same operation is repeated by changing the frequency of the AC signal, and if the frequency and electromotive force data are sent to the arithmetic processing unit 13, fc is obtained.If the permeability μ of the transformation layer 2 is known, μ is calculated as the arithmetic processing unit 13 , The thickness dt of the transformation layer 2 is obtained.
[0024]
In addition, when the AC signal sent from the transmitter is amplified by the power amplifier 10 and the excitation coil 6 having the U-shaped yoke 7 close to the steel material 1 is excited, a magnetic field leaks in parallel to the surface of the steel material 1. The leakage magnetic field φ is detected as an electromotive force by a magnetic sensor 8 such as a Hall element and sent to the lock-in amplifier 12, and when the permeability μ of the transformation layer 2 is known, μ is expressed by, for example, the above formula (4) The thickness dt of the transformation layer 2 can also be obtained by inputting to the incorporated arithmetic processing unit 13.
[0025]
If the permeability μ of the transformation layer 2 is unknown, perform both of the above measurements, and input the frequency, electromotive force data, and leakage magnetic field data, for example, to the arithmetic processing unit 13 incorporating the above equation (5). Thus, the thickness dt of the transformation layer 2 can be obtained.
[0026]
When the DC component is superimposed on the AC signal sent from the transmitter, the permeability μ itself of the transformation layer can be reduced, so that the frequency fc and the leakage magnetic field φ can be accurately measured even when the transformation layer has a large thickness dt.
[0027]
【Example】
A steel sheet made of ferromagnetic material with a thickness of 2.0 to 8.0 mm and a magnetic permeability μ of 25 to 150 is layered on bronze, which is a non-magnetic material having substantially the same conductivity as steel, and the steel material becomes an austenite phase → ferrite phase. A sample was prepared to simulate the transition of the thickness of the transformation layer in the surface layer during transformation. The frequency fc, leakage magnetic field φ (electromotive force), and steel plate thickness (transformation layer thickness) were measured using the apparatus having the configuration shown in FIG. Note that a 0.2 A, 10 Hz AC signal was used for measurement of the leakage magnetic field φ, and in each measurement, a 2.08 A DC component was superimposed on the AC signal.
[0028]
FIG. 3 shows the relationship between the induced electromotive force ratio and the frequency when the steel plate thickness is changed. Here, the induced electromotive force ratio is the ratio of the electromotive force to the electromotive force when there is no steel plate. Moreover, it is a result when the steel plate with constant permeability μ is used.
[0029]
Even at each steel plate thickness, there exists a bending point in the relationship between the induced electromotive force and the frequency, and as the steel plate thickness increases, the frequency at the bending point decreases, that is, the thickness of the transformation layer increases. It turns out that fc becomes small according to. When the relationship between the frequency fc and the steel plate thickness is plotted, as shown in FIG. 4, the thickness of the steel plate, that is, the thickness of the transformation layer and fc ^-1/2 has a linear relationship. It can be seen that the thickness of the transformation layer can be calculated.
[0030]
FIG. 5 shows the relationship between the leakage magnetic field φ and the steel plate thickness. Here, the leakage magnetic field is measured as an electromotive force. Moreover, it is a result when the steel plate with constant permeability μ is used.
[0031]
The leakage magnetic field φ decreases linearly as the thickness of the steel plate increases, and it can be seen that the thickness of the transformation layer can be calculated from the above equation (4) in the case of the apparatus configuration of FIG.
[0032]
Table 1 shows the results of calculating the steel plate thickness from the frequency fc and the leakage magnetic field φ measured by variously changing the magnetic permeability μ of the steel plate using the above formula (5).
[0033]
Thus, the thickness calculated using Equation (5) agrees with the actual thickness within 0.3 mm, and the thickness of the transformation layer is accurately obtained by the method of the present invention even when the magnetic permeability μ is unknown. be able to.
[0034]
[Table 1]

[0035]
【The invention's effect】
Since the present invention is configured as described above, it is possible to provide a method for measuring the thickness of a transformation layer of a steel material that can track the change in thickness of the transformation layer from the surface toward the inside even if the thickness of the steel material is large.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of the method of the present invention.
FIG. 2 is a diagram schematically showing a relationship between induced electromotive force and frequency.
FIG. 3 is a diagram showing a relationship between an induced electromotive force ratio and a frequency when a steel plate thickness is changed.
FIG. 4 is a diagram showing a relationship between a frequency fc and a steel plate thickness.
FIG. 5 is a diagram showing a relationship between a leakage magnetic field φ and a steel plate thickness.

Claims

The AC magnetic field is applied to the steel material, and the induction magnetic field generated near the steel surface due to the eddy current is measured by changing the frequency of the AC magnetic field, and the curve obtained as a function of the frequency of the induction magnetic field is greatly bent. And the following relational expression (1) of the frequency fc, the permeability μ of the transformation layer, and the thickness dt of the transformation layer:
dt = a / (fc × μ) ^1/2 (1) (where a is a constant)
From the following relational expression (2) between the leakage magnetic field φ magnetizing the steel material and leaking from the steel material surface, the permeability μ of the transformation layer, and the thickness dt of the transformation layer:
dt = F ₁ (μ, φ) (2) (where F ₁ is a function depending on the magnetic circuit formed between the ferromagnetic layer of steel and the magnetizing device)
Using the following relational expression (3) obtained by erasing the magnetic permeability μ of the transformation layer,
dt = F ₂ (fc, φ) (3) (where F ₂ is a new value of the frequency fc and the leakage magnetic field φ obtained by erasing μ of the transformation layer from the equations (1) and (2). function)
A method for measuring the thickness of a transformation layer of a steel material to obtain the thickness dt of the transformation layer.

The method for measuring a transformation layer thickness of a steel material according to claim 1, wherein a DC magnetic field is superimposed on an AC magnetic field.