JP3266128B2 - Leakage magnetic flux inspection method and magnetic flux leakage inspection equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a magnetic flux leaking by detecting a defect by magnetizing a ferromagnetic metal sample and measuring a leak magnetic flux generated due to a defect existing in the sample. And more specifically,
The present invention relates to a magnetic flux leakage inspection method in which a plurality of sensors are arranged for one magnetizer and signals from the sensors are calculated to improve defect detection performance.

[0002]

2. Description of the Related Art As a method of detecting a defect existing inside a ferromagnetic material such as iron, a magnetic flux leakage inspection method is widely used. As one example, FIG. 6 shows a configuration of a magnetic flaw detector using a magnetic sensor, which is incorporated in an iron making inspection line in an iron making plant.

A magnetic flaw detector 14 is provided along a transport path of a thin steel strip 13 which is transported at a substantially constant speed V on a product inspection line by transport rollers 11 and 12. The magnetic flaw detector 14 includes a magnetizer 15 for applying a magnetic field to the running thin steel strip 13, a magnetic sensor 16 disposed at a position facing the magnetizer 15 with the thin steel strip 13 interposed therebetween, and a magnetic sensor 16. 1
And a signal processing device 18 for detecting a defect 17 inside or on the surface of the thin steel strip 13 based on the detection signal from the thin steel strip 13.

If a defect 17 exists in the thin steel strip 13, the line of magnetic force in the thin steel strip 13 is disturbed due to the defect 17, and leaks to the outside of the thin steel strip 13 to become a leakage magnetic flux. The magnetic sensor 16 detects this leakage magnetic flux. Since the intensity of the leakage magnetic flux corresponds to the size of the defect 17, the size of the defect 17 can be evaluated based on the signal level of the detection signal of the magnetic sensor 16.

As described above, in the conventional method of detecting a defect in a ferromagnetic metal object by measuring leakage magnetic flux, the size of the defect is detected by the signal level of a detection signal of a magnetic sensor. I was However, the magnetic signal detected by the magnetic sensor includes not only a leakage magnetic flux signal due to the above-described defect but also a ferromagnetic metal specimen due to a local magnetic property change, unevenness, etc. In some cases, disturbance of the magnetic flux distribution outside the body metal subject and disturbance of the magnetic flux distribution caused by surface roughness are included. The disturbance of the magnetic flux distribution is an unnecessary magnetic flux (noise magnetic flux) from the viewpoint of defect detection.

In order to avoid the influence of the noise magnetic flux, a method of judging a defect by using the fact that the frequency caused by the signal caused by the defect leakage magnetic flux and the frequency caused by the signal caused by the noise magnetic flux is sometimes used. . FIG. 7 is a diagram illustrating an example of the measurement results of the frequency characteristics of the defect signal and the noise magnetic flux. That is, FIG. 7 shows a frequency characteristic of a defect signal when leakage magnetic flux due to a defect is detected by a magnetic sensor and a frequency when noise magnetic flux is detected by a magnetic sensor in a state where a thin steel sheet is run at a constant speed. The characteristics are shown.

[0007] As shown in FIG. 7, a defect signal generally has a higher frequency distribution than a noise flux. Therefore, by incorporating a high-pass filter having a cut-off frequency f into the signal processing device, a defect signal is extracted from the detection signals output from the magnetic sensor to the signal processing device while the defect signal is relatively emphasized as compared with the noise magnetic flux. It is possible.
The method of using a filter having an appropriate constant in order to increase the defect detection capability in the leakage magnetic flux flaw detection method is also disclosed in Japanese Utility Model Laid-Open No. 61-119760.

However, as shown in FIG. 7, since the frequency characteristic of the defect signal and the frequency characteristic of the noise magnetic flux overlap in some parts, the defect to be detected is small and the level of the defect signal is small, or the noise magnetic flux is large. In this case, even if the above-described high-pass filter is provided to discriminate the frequency of the defect signal, it is difficult to remove the noise magnetic flux to a level at which the defect can be detected.

[0009]

SUMMARY OF THE INVENTION The present inventors have solved such a problem and invented a method for reliably detecting minute defects of a ferromagnetic specimen.
Patent application No. 9782 (prior application invention). this is,
"A ferromagnetic metal specimen is magnetized under a plurality of different magnetizing conditions, the leakage magnetic flux is measured at the same location under each magnetizing condition, these measurement results are calculated, and a defect is determined based on the calculation results. To perform a magnetic flux leakage inspection method.

[0010] This prior invention is based on the fact that noise which is a hindrance in flaw detection is mainly generated near the surface. Signals from the vicinity of the surface are not greatly affected by the degree of magnetization. Based on the knowledge that detection sensitivity is improved when the magnetization is large and that detection is difficult due to the effect of the magnetic shield when the magnetization is small, the ferromagnetic metal specimen is magnetized under multiple magnetization conditions, By performing arithmetic processing on the measurement signals under the respective magnetization conditions, noise components are eliminated and S /
It is intended to improve the N ratio.

In order to realize a plurality of magnetization levels (change the magnetization level) in a ferromagnetic metal object, a generally conceivable method is to change the shape of the magnetizer. The distance between the magnetizer and the ferromagnetic metal object To change the magnetizing current. For example, the magnetization level can be lowered by making the magnetic pole interval very wide. As another method, the magnetization level can be reduced by moving the magnetizer away from the ferromagnetic metal object. In the method (1), the magnetization level can be increased or decreased according to the increase or decrease of the magnetizing current.

One of the methods for realizing a plurality of magnetization levels using these methods is to provide magnetizers having different shapes and different magnetization currents for each magnetization level condition. However, in this method, the cost is increased due to the use of a plurality of magnetizers, the space required for the apparatus is increased, and the magnetic sensors need to be separated by at least the size of the magnetizers. However, there is a problem that the possibility of a difference is increased.

If it is inappropriate to provide a plurality of magnetizers, a method of changing the magnetization level by changing the condition of one magnetizer is considered. However, this method generally deforms and moves a heavy magnetizer,
This method is not particularly suitable for online flaw detection of steel plates that require high speed. In this method, the magnetization level can be changed by temporally changing the magnetization force with one magnetizer, and only one sensor is required. There is a problem in that it becomes complicated (a circuit for processing a signal for each corresponding magnetization level while changing the magnetization current in synchronization with the change).

Another problem arises when speed is required for measurement. In other words, in order to realize a plurality of magnetization conditions for each measurement point on the ferromagnetic metal object, the magnetization force and the magnetization current must be generated at a period faster than the relative movement speed between the ferromagnetic metal object and the sensing system. It needs to be changed,
If the ferromagnetic metal moves quickly, the frequency of the magnetizing current must be increased, and the leakage current is simply affected by the skin effect and the eddy current component as well as the leakage flux component. A disturbance occurs that is incompatible with the purpose of changing the magnetization level of the magnetic flux.

[0015] The present invention has been made in view of such circumstances, and does not use a plurality of magnetizers and does not add complicated functions. It is an object to provide a flaw detection method and a leakage magnetic flux flaw detection device .

[0016]

Means for Solving the Problems] To solve the above problems
The first means is that a ferromagnetic metal object is magnetized by a magnetizer, and a leakage magnetic flux generated due to a defect existing in the object is measured by a magnetic sensor arranged on the surface of the object. A magnetic flux leakage detection method for detecting defects, wherein a plurality of magnetic sensors whose installation positions are changed along the magnetization direction of the magnetizer substantially change the magnetization level of the device under test.
A method of measuring a magnetic flux leakage signal corresponding to the same position on the subject under such conditions, calculating the measurement results, and determining a defect based on the calculation result. 1) . The second solution to the above problem
The means of 2 is the first means, wherein the "test object"
The conditions under which the magnetization levels of the
Are different in strength,
The change in the defect signal level in the
Characteristic is that the condition is greater than the rate of change
(Claim 2). To solve the above problems
The third means is the first means or the second means.
Therefore, the above-mentioned "The magnetization level of the test object is substantially different.
Is a condition in which the strength of magnetization is different,
Magnetic flux exists under weak magnetization condition
Characterized by the following conditions (claim 3)
It is. A fourth means for solving the above-mentioned problem is that a strong magnetic
The metallic object is magnetized by the magnetizer and is present in the object.
Magnetic flux generated by the defect
Defect detection by measuring with a placed magnetic sensor
Magnetic flux detector for performing inspection, comprising a magnetizer and a device to be inspected.
Under the condition that the magnetization levels of the magnetizers are substantially different,
Magnetic sensors with different installation positions along the magnetization direction
And the leakage flux signal corresponding to the same position on the subject
Calculate the measurement results and judge the defect based on the calculation result.
Magnetic flux characterized by having a signal processing device for performing
A flaw detection device (claim 4). To solve the above problems
The fifth means is the fourth means, wherein the "test"
"Conditions under which the magnetization level of the specimen is substantially different"
Magnetization strength is different, strong magnetizing condition and weak magnetizing condition
Changes in the defect signal level in the matter is, the noise magnetic flux signal level
Condition that is greater than the rate of change
This is a feature (claim 5). Solving the above problems
The sixth means for performing the above is the fourth means or the fifth means
Wherein the “magnetization level of the device under test is substantially different
Is a condition in which the magnetization intensity is different,
Noise flux in strong magnetization condition exists even in weak magnetization condition
Characterized by conditions that exist (claims
6).

The degree of magnetization in the magnetization direction of the magnetizer is:
The leakage magnetic flux signal corresponding to the same position on the subject is obtained by a plurality of magnetic sensors whose installation positions are changed along the magnetization direction of the magnetizer, the largest being at the center of the magnetizer and decreasing with distance from the center. Is measured under different magnetization levels.
By calculating these signals, the effect of the invention of the prior application can be obtained by using one magnetizer.

The reason why the magnetization level of the ferromagnetic metal sample is changed by shifting the measurement position from the pole center along the magnetization direction will be described below with reference to FIG. FIG. 2 schematically illustrates a part of the magnetic flux supplied from both magnetic poles when the ferromagnetic metal object 1 is magnetized by the magnetizer 5. As shown in FIG. 2, a portion of the magnetic flux emitted from the N pole of the magnetizer 5 passes through the inside of the ferromagnetic metal subject 1 and magnetizes the ferromagnetic metal subject 1. The higher the number of magnetic fluxes passing through the ferromagnetic metal subject 1, that is, the higher the magnetic flux density, the higher the magnetization level. However, the magnetic flux density is maximum on the magnetic pole center, and decreases as the distance from the center increases.

The reason is that, as schematically shown in FIG. 2, the amount of magnetic flux supplied from the magnetizer 5 to the ferromagnetic metal test object 1 increases as it approaches the magnetic pole center. This is because the maximum value is obtained at the magnetic pole center.

FIGS. 3 and 4 show the results of an experiment investigating that the magnetization level can be changed by shifting the magnetic sensor from the magnetic pole center. In FIG. 3, 1 is a steel plate, 2 and 3 are transport rolls, 4 is a magnetic flaw detector, 5 is a magnetizer, 6 is a magnetic sensor, 7 is a signal processing device, and 8 is a defect.

A magnetic flaw detector 4 is provided along a conveying path of the steel sheet 1 which is conveyed at a substantially constant speed V by the conveying rollers 2 and 3 on the product inspection line. This magnetic flaw detector 4
A magnetizer 5 for applying a magnetic field to the running steel sheet 1;
Magnetic sensor 6 disposed at a position facing magnetizer 5 across
And the steel sheet 1 based on the detection signal from the magnetic sensor 6.
And a signal processing device 7 for detecting an internal or surface defect 8.

If a defect 8 exists in the steel sheet 1, the defect 8
The magnetic field lines in the steel plate 1 are disturbed due to this, and leak to the outside of the steel plate 1 to become a leakage magnetic flux. The magnetic sensor 6 detects the leakage magnetic flux. Since the strength of the leakage magnetic flux corresponds to the size of the defect 8, the signal level of the detection signal of the magnetic sensor 6 indicates the defect 8.
The size of can be evaluated.

As shown in FIG. 3, the distance S from the pole center is
The magnetic flux was measured by the magnetic sensor 6 at a position shifted only along the magnetization direction, and the effect was examined. Here, a thin steel plate having a thickness of 1 [mm] was used as the steel plate 1 as the ferromagnetic metal specimen. The distance between the steel sheet 1 and the magnetic sensor 6, that is, the lift-off L is 1 [mm], the steel sheet speed V is 300 [m / min], and the magnetic pole interval of the magnetizer 5 is 12 [m].
m], the distance between the magnetizer 5 and the steel plate 1 is 4 [mm], and the magnetizing force is 3000
[AT].

When the signal level from a certain defect was examined, it was found that as the position deviated from the magnetic pole center, the leakage magnetic flux signal tended to decrease in accordance with the above concept (not shown). In order to investigate in detail whether this phenomenon is caused by a change in the magnetization level, or by chance, the defect output has changed due to another factor, a method conventionally used for changing the magnetization level, that is, on the pole center, is used. FIG. 4 shows the result of comparison between the case where the magnetic sensor 6 is fixed and the magnetizing force alone is changed (magnetizing force M) and the case where the sensor is shifted from the magnetic pole center (shift amount S).

FIG. 4 is a graph in which two types of defects and one type of material-induced noise are used as sources of leakage magnetic flux, and a set of S and M at which the output levels are the same in both methods is plotted.
Since this plot is almost at the same position regardless of the source of the leakage magnetic flux, the magnetization level of the ferromagnetic metal object changes by measuring the magnetic sensor shifted from the magnetic pole center in the magnetization direction. It can be said that.

Here, for the sake of simplicity, results are shown for only three types of leakage magnetic flux sources, but the same applies to other defects and noises caused by materials.
As described above, the measurement can be performed in a state where the magnetization level of the ferromagnetic metal object is different by measuring the magnetic sensor while displacing the magnetic sensor from the magnetic pole center along the magnetization direction. It is understood that the leakage magnetic flux measurement can be realized with a simple configuration without using a plurality of magnetizers, and can be applied to a case where high-speed measurement is required.

Although the case where the measurement is performed at two kinds of magnetization levels has been described here, it goes without saying that the same method can be applied to the case where the measurement is performed at three or more kinds of magnetization levels. . Further, in this experiment, the magnetic sensor 6 and the magnetizer 5 were arranged on opposite sides with the subject interposed therebetween, but the same result can be obtained even on the same side. Also, in FIG. 3, the direction in which the sensor position is shifted to lower the magnetization level is the direction in which the steel strip is moved.
Similar results can be obtained by shifting to the opposite side.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a magnetic flux leakage inspection apparatus used for carrying out the present invention. 1, the same components as those shown in FIG. 3 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 1, reference numerals 6a and 6b denote magnetic sensors, and 6a is located near the center of the magnetizer 5 in the magnetization direction.
b was provided at a position 4 [mm] away from it. In this embodiment, the distance between the magnetic sensors 6a and 6b and the steel plate 1, that is, the lift-off L is the same for both magnetic sensors (0.
7 [mm]), but this is not necessarily the same.

The steel sheet 1 transported on the product inspection line
Was 1 [mm]. The steel sheet 1 was transported by the transport rollers 2 and 3 at a substantially constant speed V = [100 m / mim]. Although not shown, a plurality of magnetic sensors 6a and 6b are linearly arranged at a pitch of 5 mm in the plate width direction, and 200 sets of 400 magnetic sensors 6a and 6b are used in the plate width direction. Covers 1m. The interval between the magnetic poles of the magnetizer 5 is 12 [mm]
It is. As a stronger magnetization level condition (strong magnetization condition), the magnetic sensor 6a is disposed on the magnetic pole center as described above, and as a magnetization force, a saturation magnetization level near which both the defect signal and the noise magnetic flux are both large is detected. (3000AT). At this time, in order to make the noise magnetic flux signal under the strong magnetization condition exist also under the weaker magnetization condition, the condition was set that the strong magnetization level was not unnecessarily increased in order to approach the weak magnetization condition.

As a weaker magnetization level condition (weak magnetization condition), the distance S of the magnetic sensor 6b from the center of the magnetization direction of the magnetizer 5 was selected so as to satisfy the following condition.
That is, as the magnetization condition, the change in the defect signal level due to the lowering of the magnetization is in a range as large as possible than the rate of the change in the noise magnetic flux signal level, and the noise magnetic flux in the strong magnetization condition exists even in the weak magnetization condition. The level was set so as not to be too small. As a result, the distance S was 4 [mm] as described above.

The signal processing device 7 includes the magnetic sensors 6a, 6
The analog-to-digital conversion of the detection signals Va (t) and Vb (t) is performed at a sampling frequency of 20 kHz. In addition, a time difference Δt corresponding to the same steel sheet position is obtained by dividing the positional deviation S between the magnetic sensor 6a and the magnetic sensor 6b in the moving direction of the steel sheet 2 by the steel sheet speed V that is sequentially measured, and a delay processing circuit (FIG. (Not shown), the signal Va (t) of 6a
Is relatively delayed with respect to the signal Vb (t) of the magnetic sensor 6b so that Va (t−Δt) and Vb (t) correspond to each other. Further, the detection signal Va (t−Δt) and V
b (t) is subjected to a band-pass filter in order to reduce a direct current component and a formation noise component having a low frequency, and to cut electric noise higher than a defect signal frequency. The pass band is the same under both magnetization conditions, and is 1 kHz to 2 kHz.

FIG. 5 shows the effect of improving detection performance in this embodiment. Under the strong magnetizing condition (signal of the magnetic sensor 6a), noise caused by the material is large and the S / N is 1.3. Under the weak magnetization condition (the signal of the magnetic sensor 6b), it can be seen that the noise magnetic flux generated under the strong magnetization condition similarly appears. The result obtained by multiplying the weak magnetization signal by 2.5 and subtracting it from the strong magnetization signal at the corresponding position is the difference processing result. It can be seen that the noise magnetic flux is drastically reduced, the defect signal is relatively emphasized, and the S / N is improved. Processing such as subtraction, delay processing, and filtering of the measurement value under the strong magnetization condition and the measurement value under the weak magnetization condition may be performed using an analog signal, or may be performed after converting the analog signal into a digital signal. Good. Further, even when the conversion is performed after converting into a digital signal, the conversion may be performed by hardware or software.

[0033]

As described above, in the method for detecting magnetic flux leakage according to the present invention, in order to perform flaw detection of a ferromagnetic metal object by magnetic flux leakage at a plurality of magnetization levels, the magnetic flux from the magnetic pole center of the magnetizer is required. A plurality of sensors are arranged at positions where the amounts of displacement in the directions are different from each other. Then, the magnetization level of the test object is
Under such conditions that the sensors are substantially different from each other, an internal defect is detected by mutually calculating the measured outputs. Since the magnitude of the signal level change when the magnetization conditions are changed differs between the noise and defect signal caused by the material, the measurement results for the strong magnetization and the measurement results for the weak magnetization are appropriately calculated by appropriate calculation. , Noise can be reduced and the defective signal can be relatively emphasized. This allows
With one magnetizer, without adding particularly complicated functions,
With the simple system configuration as in the past, it is possible to perform the leakage magnetic flux flaw detection with a high S / N ratio.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a magnetic flux leakage inspection device used for carrying out the present invention.

FIG. 2 is a diagram schematically illustrating a part of a magnetic flux when a ferromagnetic metal object is magnetized by a magnetizer.

FIG. 3 is a schematic configuration diagram of a magnetic flux leakage inspection apparatus.

FIG. 4 shows the amount of deviation [mm] from the magnetic pole center of the sensor, in which the same magnitude signal can be obtained from the same leakage magnetic flux generation source.
FIG. 6 is a diagram showing a correspondence relationship between a magnetic field and a magnetization force [AT].

FIG. 5 is a diagram showing the effect of improving detection performance in an example of the present invention.

FIG. 6 is a diagram showing a schematic configuration of a conventional magnetic flux leakage inspection apparatus.

FIG. 7 is a diagram illustrating an example of measurement results of frequency characteristics of a defect signal and a noise magnetic flux.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Steel plate 2,3 ... Conveying roll 4 ... Magnetic flaw detector 5 ... Magnetizer 6,6a, 6b ... Magnetic sensor 7 ... Signal processing device 8 ... Defect

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-274016 (JP, A) JP-A-58-2649 (JP, A) JP-A-63-133054 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G01N 27/72-27/90

Claims (6)

(57) [Claims] A ferromagnetic metal object is magnetized by a magnetizer, and leakage magnetic flux generated due to a defect existing in the object is measured by a magnetic sensor arranged on the surface of the object, thereby detecting the defect. A magnetic flux leakage detection method for detecting
Due to multiple magnetic sensors whose positions are changed along the magnetization direction of the magnetizer, the magnetization level of the device under test is substantially different
Under such conditions, a magnetic flux leakage signal corresponding to the same position on the subject is measured, the measurement results are calculated, and a defect is determined based on the calculation result. 2. The leakage magnetic flux inspection method according to claim 1, wherein
Therefore, the above-mentioned "The magnetization level of the test object is substantially different.
Is a condition in which the strength of magnetization is different,
The change in defect signal level under the magnetization condition and the weak magnetization condition
Noise flux level greater than the rate of change
A magnetic flux leakage detection method characterized by the following conditions. 3. The leakage magnetic flux according to claim 1 or 2.
The flaw detection method, wherein the "magnetization level of the test object is substantially
"Different conditions" means that the magnetization intensity is different
Noise flux under strong magnetization conditions
Leaks characterized by conditions that exist even if they exist
Magnetic flux inspection method. 4. A ferromagnetic metal object is magnetized by a magnetizer.
And the leakage magnetic flux generated due to the defect existing in the subject
Is measured with a magnetic sensor placed on the surface of the subject
With this, it is a leakage magnetic flux flaw detection device that detects a defect,
The magnetizer and the test object have substantially different magnetization levels.
Under different conditions along the magnetizing direction of the magnetizer
Multiple magnetic sensors and the same position on the subject
Measure the leakage flux signal, calculate the measurement results,
It is necessary to have a signal processing device that performs defect judgment from the
Features a magnetic flux leakage inspection device. 5. The leakage magnetic flux detection device according to claim 4,
Therefore, the above-mentioned “the magnetization level of the test object is substantially different.
Is a condition in which the strength of magnetization is different,
The change in defect signal level under the magnetization condition and the weak magnetization condition
Noise flux level greater than the rate of change
A magnetic flux leakage inspection device characterized by the following conditions. 6. The leakage magnetic flux according to claim 4 or claim 5.
The flaw detection apparatus, wherein the "magnetization level of the test object is substantially
"Conditions that are different from each other" means that the magnetization intensity is different.
Noise flux under strong magnetization conditions
Leaks characterized by conditions that exist even if they exist
Magnetic flux testing equipment.