JP2000304725A - Method for measuring thickness of transformation layer of steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the change in the thickness of a transformation layer able to be tracked even in a thick plate by changing an application alternate magnetic field frequency and measuring the induction magnetic field on the surface of a steel material, and utilizing a frequency where a curve with the induction magnetic field frequency as a function is flexed for a specific expression for operation. SOLUTION: When an AC signal from a transmitter is amplified by a power amplifier 9 and an excitation coil 3 is excited, an eddy current is generated at the surface layer of a steel material 1 that is arranged vertical to a coil axis center close to the excitation coil 3 and has a transformation layer 2 on the surface layer. An induction magnetic field by the eddy current is detected as electromotive force by a detection coil 4 that is coaxial with the excitation coil 3, is converted to a DC signal through a lock-in amplifier 11, and is sent to an arithmetic operation device 13 incorporating an expression dt=α/(fc×μ) (dt: thickness of transformation layer, fc: flex point frequency, μ: permeability, α: constant). By changing the frequency and repeating the similar operation and sending the frequency and electromotive force data to the arithmetic operation device 13, fc is determined. Also, by inputting a known permeability μ to the arithmetic operation device, the thickness dt of a transformation layer 2 can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the thickness of a transformation layer formed on a surface layer when a steel material is heated and cooled, and more particularly to a measurement method utilizing magnetic properties.

[0002]

2. Description of the Related Art Monitoring the transformation of the austenite (γ) phase and the ferrite (α) phase that occurs when heating and cooling steel is important in managing the mechanical and physical properties of steel. Very important.

For this reason, various methods for measuring the transformation rate online using the magnetic properties of steel have been proposed. For example, Japanese Patent Publication No. 2-42402,
No. 23853, JP-A-8-62181, etc., using an excitation coil and a detection coil, disclose a method of measuring a transformation rate from a change in magnetic flux caused by a change in magnetic characteristics accompanying transformation of steel. I have.

[0004]

However, the method described in the above publication can measure only the average transformation rate of the steel material in the thickness direction, and the transformation layer from the surface toward the inside during heating and cooling can be measured. There is a problem that the change in thickness cannot be tracked, and when the plate thickness is large, the magnetic flux can penetrate only into the surface layer due to the skin effect, so that the transformation rate cannot be measured accurately.

The present invention has been made in order to solve such a problem, and even if the thickness of the steel material is large, the change in the thickness of the transformation layer from the surface to the inside can be tracked. It is intended to provide a measurement method.

[0006]

The object of the present invention is to obtain a magnetic recording medium by applying an alternating current magnetization to a steel material and measuring an induced magnetic field generated near the steel material surface by an eddy current by changing the frequency of the alternating magnetic field. Determine the frequency when the curve having the frequency of the induction magnetic field as a function is largely bent, utilizing the fact that the frequency when the curve is largely bent changes with the thickness of the transformation layer due to the skin effect phenomenon of the eddy current. The problem is solved by a method for measuring the thickness of a transformation layer of a steel material to be measured.

[0007] depth of penetration of the eddy current generated in the surface portion upon application of alternating magnetic field in the steel, the AC magnetic field having a frequency of f, the permeability of the steel mu, depending on the conductivity s of steel materials, f ^1/2, μ ^1/2 , s
^It is inversely proportional to ^1/2 . Here, since the conductivity s is hardly affected by the transformation, to track the transformation behavior, the frequency f and the permeability μ
Should be considered. That is, the penetration depth of the eddy current becomes shallower as the frequency of the AC magnetic field increases and the magnetic permeability μ of the steel material increases. Further, there is a characteristic that the induced magnetic field due to the eddy current becomes stronger as the frequency becomes higher.

Considering the induced magnetic field due to the eddy current when a transformation layer having a finite thickness exists, if the penetration depth of the eddy current is sufficiently deeper than the thickness of the transformation layer, that is, if the frequency is When it is low, the induction magnetic field itself is weak, but since the ratio of the thickness of the transformation layer to the penetration depth of the eddy current is small, the induction magnetic field depending on the transformation layer is also weak, and its frequency-dependent change is small. On the other hand, at a high frequency where the penetration depth of the eddy current is 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.

Therefore, as schematically shown in FIG.
A curve obtained by plotting the electromotive force due to the induced magnetic field with respect to the frequency has a bending point, and the penetration depth of the eddy current at that frequency fc corresponds to the thickness of the transformation layer. The vertical axis in FIG. 2 shows the electromotive force generated in the detection coil by the composite magnetic field of the applied magnetic field and the induction magnetic field. This is because the electromotive force due to the applied magnetic field is reduced.

Specifically, at the frequency fc,
Since (1) holds, if the magnetic permeability μ of the transformation layer generated on the surface layer is known, the thickness dt of the transformation layer can be obtained from the value of fc.

Dt = α / (fc × μ) ^1/2 (1) where α is a constant.

Further, the leakage magnetic field φ leaking from the surface of the steel material is measured by magnetizing the steel material, and the thickness d of the transformation layer is calculated from the following equation (2).
You can also find t.

Dt = F ₁ (μ, φ) (2) where F ₁ is a function dependent on a magnetic circuit formed between the ferromagnetic layer made of steel and the magnetizing device, and μ is the permeability of the transformation layer. The magnetic susceptibility.

For example, when considering a magnetic circuit in which a steel material having a transformation layer on its surface is AC-magnetized by a U-shaped magnetizer,
The magnetic field leaking between the U-shaped yokes on the magnetization side in parallel with the transformation layer increases as the magnetoresistance of the transformation layer increases. That is, when the magnetic resistance 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 magnetic resistance 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 magnetic permeability μ of the transformation layer and the thickness dt, the leakage magnetic field φ can be expressed by the following equation (4).

Φ = φ ₀ / (1 + β · μ · dt) (4) where φ ₀ is a magnetic field when there is no transformation layer, and β is a constant

Therefore, the thickness dt of the transformation layer is given by the above equation (2)
Can be expressed as a function of the leakage magnetic field φ and the magnetic permeability μ, so if the permeability μ of the transformation layer generated on the surface layer is known, the thickness dt of the transformation layer can be obtained by measuring the leakage magnetic field φ .

The function form of the equation (2) cannot be uniquely expressed by the equation (4) and varies depending on the device configuration, so it must be determined each time.

In general, the magnetic permeability of a steel material greatly changes depending on components and temperature, and it is rather rare that the magnetic permeability of the above-mentioned transformation layer is known. However, even when the magnetic permeability of the transformation layer is unknown, the following equation is obtained in which the above-mentioned frequency fc and the leakage magnetic field φ are obtained, and the magnetic permeability μ is eliminated from the above equations (1) and (2).
From (3), the thickness dt of the transformation layer can be obtained.

Dt = F ₂ (fc, φ) (3) When the equation (4) is used as a function form of the equation (2), the following equation is used.
From (5), the thickness dt of the transformation layer can be obtained.

Dt = γ / fc / (φ ₀ / φ−1) (5) where γ is a constant.

From the equation (1), the change of the frequency fc with respect to the thickness dt of the transformation layer becomes smaller on the order of the third power of dt, and from the equation (4), the change of the leakage magnetic field φ with respect to the thickness dt of the transformation layer is obtained. It can be seen that the change becomes smaller in the order of the square of dt. This means that from the viewpoint of measurement, as the thickness dt of the transformation layer increases, the measurement accuracy of the frequency fc and the leakage magnetic field φ decreases. Therefore, even when the thickness dt of the transformation layer was large, a study was conducted to increase the change in the frequency fc and the leakage magnetic field φ with respect to the thickness dt of the transformation layer to improve the measurement accuracy. Has been found to be small, and for this purpose, a DC magnetic field may be superimposed on the AC magnetic field applied by the above method. In particular, when the thickness dt of the transformation layer is measured by using the equation (3), it can be measured regardless of the magnetic permeability μ, which is very effective. The thickness dt of the transformation layer
When the thickness is small, the higher the permeability μ, the higher the measurement accuracy. Therefore, it is preferable to appropriately select the intensity of the DC magnetic field according to the thickness dt of the transformation layer.

[0022]

FIG. 1 shows an embodiment of the method of the present invention.

When an AC signal sent from a transmitter (not shown) is amplified by a power amplifier 9 and an exciting coil 3 having a ferrite yoke 5 is excited to improve the sensitivity, the AC coil 3 becomes close to the exciting coil 3. An eddy current is generated on the surface of the steel material 1 which is arranged perpendicular to the coil axis and has the transformation layer 2 on the surface. An induced magnetic field is generated by the eddy current, and the magnetic field is detected as an electromotive force generated in the detection coil 4 having the ferrite yoke 5 and on the same axis as the excitation coil 3. This electromotive force is sent to the lock-in amplifier 11, synchronously detected based on the AC signal sent from the transmitter, converted into a DC signal, and sent to the arithmetic processing unit 13 in which the above equation (1) is incorporated. The same operation is repeated by changing the frequency of the AC signal, and the frequency and electromotive force data are processed by the arithmetic processing unit.
When fc is sent to the transformation layer 13 and the magnetic permeability μ of the transformation layer 2 is known, the transformation coefficient
The thickness dt of 2 is obtained.

When the AC signal sent from the transmitter is amplified by the power amplifier 10 and the exciting coil 6 having the U-shaped yoke 7 close to the steel material 1 is excited, a magnetic field is generated in parallel to the surface of the steel material 1. Although leakage occurs, this 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 magnetic permeability μ of the transformation layer 2 is known, μ is expressed by, for example, the above equation ( The thickness dt of the transformation layer 2 can also be obtained by inputting to the arithmetic processing unit 13 in which 4) is incorporated.

When the magnetic permeability μ of the transformation layer 2 is unknown, both of the above measurements are performed, and the data of the frequency and the electromotive force and the data of the leakage magnetic field are sent to, for example, the arithmetic processing unit 13 in which the above equation (5) is incorporated. By inputting, the thickness dt of the transformation layer 2 can be obtained.

When the DC component is superimposed on the AC signal sent from the transmitter, the magnetic permeability μ itself of the transformation layer can be reduced. Therefore, even when the thickness dt of the transformation layer is large, the frequency fc and the leakage magnetic field φ can be accurately determined. Can be measured.

[0027]

EXAMPLE A ferromagnetic steel plate having a thickness of 2.0 to 8.0 mm and a magnetic permeability μ of 25 to 150 is laminated on bronze, a nonmagnetic material having substantially the same conductivity as steel, and the steel material is austenitic. Phase →
A sample was prepared which simulated the transition of the thickness of the transformed layer in the surface layer during the transformation to the ferrite phase. Then, using the apparatus having the configuration shown in FIG. 1, the frequency fc, the leakage magnetic field φ (electromotive force), and the thickness of the steel sheet (transformation layer thickness) were measured. Note that a 0.2 A, 10 Hz AC signal was used for the measurement of the leakage magnetic field φ.
Are superimposed.

FIG. 3 shows the relationship between the induced electromotive force ratio and the frequency when the thickness of the steel sheet is changed. Here, the induced electromotive force ratio is a ratio of the electromotive force to the electromotive force without a steel plate. The results are obtained when a steel plate having a constant magnetic permeability μ is used.

At each steel sheet thickness, there is a bending point in the relationship between the induced electromotive force and the frequency, and the frequency at the bending point decreases as the steel sheet thickness increases, ie, the thickness of the transformation layer. It can be seen that fc decreases as the thickness increases. When plotting the relationship between frequency fc and steel plate thickness,
As shown in FIG. 4, it can be seen that there is a linear relationship between the thickness of the steel sheet, that is, the thickness of the transformation layer and fc ^-1/2, and the thickness of the transformation layer can be calculated by the above equation (1).

FIG. 5 shows the relationship between the leakage magnetic field φ and the thickness of the steel sheet. Here, the leakage magnetic field is measured as an electromotive force. The results are obtained when a steel plate having a constant magnetic permeability μ is used.

The leakage magnetic field φ decreases linearly as the thickness of the steel sheet increases, and it can be seen that the thickness of the transformation layer can be calculated by the above equation (4) in the case of the apparatus configuration of FIG.

Table 1 shows the results of calculating the thickness of the steel sheet from the frequency fc and the leakage magnetic field φ measured by variously changing the magnetic permeability μ of the steel sheet using the above equation (5).

As described above, the thickness calculated using the equation (5) matches the actual thickness within 0.3 mm, and even when the magnetic permeability μ is unknown, the thickness of the transformation layer can be reduced by the method of the present invention. It can be obtained with high accuracy.

[0034]

[Table 1]

[0035]

Since the present invention is configured as described above, even if the thickness of the steel material is large, the change in the thickness of the transformation layer from the surface to the inside can be tracked, and the thickness of the transformation layer of the steel material can be measured. We can provide a method.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of the method of the present invention.

FIG. 2 is a diagram schematically illustrating a relationship between an induced electromotive force and a frequency.

FIG. 3 is a diagram illustrating a relationship between an induced electromotive force ratio and a frequency when a thickness of a steel sheet is changed.

FIG. 4 is a diagram illustrating a relationship between a frequency fc and a steel sheet thickness.

FIG. 5 is a diagram illustrating a relationship between a leakage magnetic field φ and a steel sheet thickness.

Claims (5)

[Claims] An AC magnetic field is applied to a steel material, and an induced magnetic field generated near the steel material surface due to an eddy current is measured by changing a frequency of the AC magnetic field, and the obtained frequency of the induced magnetic field is defined as a function. Determine the frequency at which the curve to be bent greatly bends, the thickness of the transformation layer of the steel material measured by utilizing the fact that the frequency at which the curve is greatly bent changes with the thickness of the transformation layer due to the skin effect phenomenon of eddy current Measurement method. 2. When the magnetic permeability of the transformation layer is μ and the frequency at which the curve having the function of the frequency of the induced magnetic field is greatly bent is fc, the thickness dt of the transformation layer is obtained from the following equation (2). 2. The method for measuring the thickness of a transformation layer of a steel material according to claim 1. dt = a / (fc × μ) ¹ / ² … (2) where a is a constant 3. A steel material is magnetized, a leakage magnetic field φ leaking from the steel material surface is measured, and the thickness d of the transformation layer is calculated from the following equation (1).
A method for measuring the thickness of the transformed layer of a steel material for which t is to be determined. dt = F ₁ (μ, φ)… (1) where F ₁ is a function that depends on the magnetic circuit formed between the ferromagnetic layer of steel and the magnetizing device, and μ is the magnetic permeability of the transformation layer 4. A method for measuring the thickness of a transformation layer of a steel material, wherein the frequency fc and the leakage magnetic field φ are determined, and the thickness dt of the transformation layer is determined from the following equation (3). dt = F ₂ (fc, φ) (3) where F ₂ is a function obtained by eliminating the magnetic permeability μ from the above equations (1) and (2). 5. The method according to claim 2, wherein a DC magnetic field is superimposed on an AC magnetic field.