JPH0821822A - Leakage magnetic flux flaw detecting method and device - Google Patents

Abstract

PURPOSE:To improve fine flaw detection accuracy and reduce an undetected flaw region on an inspected material end section. CONSTITUTION:Through holes 4a, 4b which a steel pipe 5 pass through are provided on opposite sides 3a, 3b of a U-shaped magnetic yoke, and an exciting coil 1 is fitted on a bottom side 3c. A ring-like holder 16 fitted with a circularly distributed magnetism-sensitive element 7 is fixed between the opposite sides 3a, 3b of the yoke coincidentally with the centers of the through holes 4a, 4b. The steel pipe 5 is inserted into the through holes 4a, 4b, the exciting current is fed to the exciting coil 1, and the leakage magnetic flux from the flaw 8 of the steel pipe 5 is detected by the magnetism-sensitive element 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for detecting cracks, pit-like scratches and the like existing on the surface of steel pipes and steel bars and in the vicinity of the surface by leaking magnetic flux.

[0002]

2. Description of the Related Art As a flaw detection method for steel pipes, for example, ultrasonic flaw detection (UT) is described in "Eddy current flaw detection test III (1990)", p. 121, issued by Japan Nondestructive Inspection Association (September 1, 1990). , Various non-destructive inspection methods such as eddy current flaw detection (ET), leaked magnetic particle magnetic flux (MLFT), magnetic particle flaw detection (MT), etc. are described, and one or more kinds of methods are combined and applied depending on the predicted damage. It is suggested to do so.

As such non-destructive inspection method, for example, "JIS G 0568-1982 steel eddy current flaw detection test method" and "JIS G 0583-1978 steel tube eddy current flaw detection test method" of Japanese Industrial Standards.
Describes a method in which an AC current having a frequency of about 0.5 to 500 KHZ is passed through the through-type coil to excite the material to be inspected by self-induction or mutual induction, and the inspection is performed with a drill hole of, for example, 1.0 mmφ as a reference. However, in recent years, microscopic surface scratches of about several hundred μm have become a problem, and microscopic surface scratches cannot be detected by inspection based on a drill hole of about 1.0 mmφ.

As a method for detecting microscopic surface flaws, for example, "Nondestructive Inspection", Vol. 30, No. 7, 468, published by the Japan Nondestructive Inspection Association.
Pp. 477 describes the magnetic flux leakage flaw detection method. Figure 6
[Fig. 4] is a side view of a conventional leakage magnetic flux flaw detector. This device includes two exciting coils 1 in the axial direction of the material 5 to be inspected.
1 is provided, and the magnetic flux 2 in the arrow direction is generated by the exciting coils 1 and 1.
And the leakage magnetic flux 9 due to the scratch 8 is detected by the magnetic sensitive element 7.

[0005]

However, the detection limit of this flaw detection method is, for example, issued by the Japan Iron and Steel Institute (November 1990).
As described in “Leakage magnetic flux flaw detection method for steel products” on page 30 of March 30), it is about 0.15 mm (SN ratio ≧ 3), so minute surface flaws of about several hundred μm cannot be detected. Also,
In this flaw detection method, since two exciting coils 1 and 1 are arranged side by side in the axial direction of the inspected material 5 with the magnetic sensitive element 7 interposed therebetween, a stray magnetic field is large and the inspected material around the exciting coils 1 and 1 is large. There is a large amount of magnetic flux leakage, and as a result, the magnetization efficiency and the SN ratio are low, and minute scratches cannot be reliably detected. Further, since the magnetic flux 2 is disturbed at the end portion of the inspected material, there is a problem that an undetected region at the end portion of the inspected material is 200 to 500 mm.

On the other hand, when the exciting coils 1 and 1 and the magnetic sensitive element 7 are brought close to each other in order to reduce the disturbance of the stray magnetic field and the magnetic flux, the influence of the temperature drift due to the heat generation of the exciting coil 1 makes the semiconductor as the magnetic sensitive element 7 particularly. When an element such as a Hall IC is used, the detection sensitivity is lowered, and it is impossible to detect minute surface scratches.

The present invention, in flaw detection by leakage magnetic flux,
An object of the present invention is to reliably detect minute scratches existing on the surface of a material to be inspected and in the vicinity of the surface and to significantly reduce an undetected flaw region at an end portion of the material to be inspected.

[0008]

According to the magnetic flux leakage flaw detection method of the present invention for solving the above-mentioned problems, a material to be inspected is inserted into a through hole provided at an opposite side of a yoke having a substantially U-shaped cross section, and the yoke is cut. A magnetic flux generated by an exciting coil fitted to the bottom side is applied to the material to be inspected through the yoke, and a magnetic flux leaking from a scratch on the material to be inspected is detected by a magnetic sensitive element provided between the opposing sides. Characterize.

The leakage magnetic flux flaw detector of the present invention has a yoke having a substantially U-shaped cross section, a through hole for a material to be inspected provided on an opposite side of the yoke, an exciting coil through which a bottom side of the yoke penetrates,
And a magnetic sensitive element that is installed between the opposed sides to detect a leakage magnetic flux of the external material to be inspected.

[0010]

When a material to be inspected for flaw detection is inserted into the through hole and an exciting current is supplied to the exciting coil, the magnetic flux generated by the exciting coil passes through the opposite side of the yoke and concentrates in the through hole.
That is, the magnetic flux generated on the bottom side by the exciting coil flows to one side of the opposite side, mainly enters the material to be inspected from the edge of the through hole, and then flows to the edge of the through hole on the other side of the opposite side to reach the bottom side. Return to. If there is a scratch on the material to be inspected between the opposite sides,
The leakage magnetic flux in that portion is large, and this is detected by the magnetic sensitive element.

The magnetic flux generated by the exciting coil passes through the opposite sides of the yoke and concentrates in the through hole, so that the magnetic flux is focused on the material to be inspected. The magnetic sensitive element is located between the opposite sides of the yoke, and these shield the leakage magnetic flux of the exciting coil. As a result, the disturbance of the stray magnetic field and magnetic flux is remarkably reduced, and the magnetization efficiency and SN ratio are improved, so that minute scratches can be reliably detected, and the undetected region at the end of the inspected material is significantly reduced. You can Further, since the magnetic flux can be focused on the material to be inspected, low current excitation magnetization and miniaturization of the excitation unit are possible. Furthermore, even if the exciting coil and the magnetic sensitive element are installed separately, there is little disturbance of the stray magnetic field and magnetic flux,
Also, by installing them separately, the influence of temperature drift due to heat generation of the exciting coil can be significantly reduced. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0012]

【Example】

-First Embodiment- Fig. 1 shows a first embodiment of the present invention. Figure 1 shows a U-shaped yo
3 shows a vertical cross section, and FIG. 2 shows a cross section taken along the line AA '. The yoke 3 has a substantially U-shaped cross section, and the opposing side 3 of the yoke 3
Through holes 4a and 4b for passing the steel pipe 5 as a material to be inspected are formed in a and 3b, and an exciting coil is wound around the bottom side 3c. In other words, the base 3c penetrates the central opening of the exciting coil. Opposite side 3 of the yoke 3
A cylindrical or ring-shaped holder 1 is provided between a and 3b.
6 is mounted. The holder 16 contains a plurality of magnetic sensitive elements 7 in a ring-shaped array (FIG. 2). The centers of the through holes 4a and 4b, the center of the holder 16 and the center of the array circle of the magnetic sensitive elements 7 are on the same straight line,
In the standard state, the central axis of the steel pipe 5 matches the straight line.

When an exciting current is supplied to the exciting coil 1 from a DC or AC exciting power source (not shown), the magnetic flux 2 generated on the bottom side 3c by the exciting coil 1 passes through the facing side 3a of the yoke 3 and the through hole 4a. And penetrates into the steel pipe 5 through the space from the edge of the through hole 4a, and through the steel pipe 5 and further through the space to reach the edge of the through hole 4b of the opposite side 3b and through the opposite side 3b. To return to the bottom 3c of the center of the exciting coil 1. The exciting current value of the exciting coil 1 is set so that the magnetic flux density of the steel pipe 5 becomes saturated. As a result, a leakage magnetic flux is generated around the outer peripheral surface of the steel pipe 5, but when the steel pipe 5 being conveyed in the direction of the arrow 6 is not damaged, the leakage magnetic flux distribution is substantially uniform, and the magnetic sensing element 7 is extremely large. Does not detect magnetic flux. However, if the steel pipe 5 has a flaw 8 between the opposing sides 3a and 3b, the leakage magnetic flux there becomes extremely large. That is, the magnetic flux is concentrated. In the following, this concentration of magnetic flux is referred to as leakage magnetic flux 9. The leakage magnetic flux 9 is detected by the magnetic sensitive element 7.

Since the detection signal of the leakage magnetic flux 9 generated by the magnetic sensitive element 7 is weak when viewed from a general electric signal, it is amplified by the amplifier 10, and the noise signal is removed by the filter 11 from the amplified signal, Further, after removing an unnecessary signal from the signal from which the noise signal has been removed by the waveform shaper 12, the signal is recorded by the recorder 13A. Further, the vibration component (before and after the upper and lower peaks) of the waveform-shaped signal is extracted by the comparator 14, and the vibration component is judged by the judging device 15 as to whether it is caused by a scratch. Record.
A synchronizing signal generator (not shown) generates a velocity synchronizing pulse of one pulse for each predetermined short distance movement of the steel pipe 5, which is supplied to the recorders 13A and 13B.
3A and 13B count the speed synchronization pulse and the steel pipe 5
The position in the longitudinal direction of is calculated, and recording is performed in correspondence with the position. Note that, as shown in FIG. 2, in this embodiment, eight magnetic sensitive elements 7 are arranged at equal pitches on the same circumference, so the above-described recording corresponds to each magnetic sensitive element (steel pipe 5).
(Corresponding to each of the seven divided areas on the outer circumference).

The magnetic flux 2 generated by the exciting coil 1 is
The magnetic flux 2 can be focused on the steel pipe 5 because it is concentrated on the through holes 4a and 4b through the opposing sides 3a of the yoke. As a result, the disturbance of the stray magnetic field and the magnetic flux is significantly reduced, and the magnetization efficiency and the SN ratio (ratio of the scratch signal level to the noise signal level) are improved, so that minute scratches can be reliably detected, and FIG. When the steel pipe 5 is transferred to the right as shown in FIG. 2, while the front end of the steel pipe 5 is in the thickness range of the facing side 3a, the rear end of the steel pipe 5 is in the thickness range of the facing side 3b. Flaw detection is possible, and the non-flaw detection area at the front and rear ends of the steel pipe 5 is extremely short. That is, the undetected region at the end is significantly reduced. Further, since the magnetic flux 2 can be concentrated on the steel pipe 5, it becomes possible to lower the exciting current of the exciting current and downsize the exciting portion. Further, even if the exciting coil 1 and the magnetic sensitive element 7 are installed separately, the stray magnetic field and the magnetic flux 2 are less disturbed, and by installing them separately, the influence of temperature drift due to heat generation of the exciting coil 1 can be significantly reduced. . Next, a specific numerical example according to this embodiment will be described.

Stretch reducer (drawing and rolling machine) of carbon steel ERW pipe with outer diameter 89.1 mmφ, wall thickness 2.8 mm and length 100 m
After being hot-rolled by, the outer diameter was 21.7 mmφ and the wall thickness was 2.3 mm, and then cut in units of 8 m in length. Steel pipe (measurement target material) including various surface scratches (natural scratches) detected by visual inspection, and a sound steel pipe with a 0.1 mm deep notch and a 2.0 mmφ drill with a 0.5 mm deep hole from the pipe end. What was processed at a pitch of 10 mm (comparative material) was used as the material to be inspected.

The exciting coil 1 was made by winding a 2.0 mmφ copper wire for 600 turns, and a direct current of 2 A was passed through the exciting coil 1. The yoke thickness t shown in FIG. 1 is 20 mm, the through holes 4a and 4b.
Has a diameter d of 32 mm, a yoke spacing p of 30 mm, and a yoke height h of
The width Z of the yoke is 200 mm and the width Z of the yoke is 90 mm. As the magnetic sensitive element 7, an SMD sensor (SONY Magnet Diode, a trademark of Sony Corporation) having a tube circumferential width w of 3.5 mm and a tube axial length L of 0.6 mm is used, and a sensor interval r is 1.0 mm, The lift-off (distance between the steel pipe 5 and the SMD sensor) was set to 2.0 mm, and the holder 16 was attached. In order to stably convey the steel pipe 5, pinch rolls (not shown) are installed in front of and behind the yokes 3a and 3b, flaw detection is performed at a conveyance speed of 120 m / min, and flaw detection signals are recorded in the recorder 13A to detect flaws. Information was recorded on recorder 13B.

The detectability of natural scratches was evaluated by the SN ratio, and further, it was 0.
The signal level from the notch having a depth of 1 mm was used as the reference value (threshold judgment threshold value) of the judging device 15. Confirmation of the undetected flaw region at the pipe end was performed using a drill hole having a hole diameter of 2.0 mmφ and a depth of 0.5 mm formed at the pipe end. As a conventional method (comparative example),
A 600 mm winding of a 2.0 mmφ copper wire is wound, and an exciting coil 1 having a coil length CL of 80 mm and an inner diameter D of 40 mmφ shown in FIG. It was set up and a direct current of 2A was passed. After the flaw detection in the same manner as in the present invention, the cut portion was subjected to a cutting test to measure the depth of the natural flaw.

FIG. 3 shows a depth of 0.
The detection characteristics of various natural scratches of 10 mm or more are shown. In FIG. 3, the SN ratio of a scratch having a depth of 0.10 to 0.15 mm is 3 or less in the conventional method (broken line), whereas the SN ratio of the present invention method (dashed-dot method) is 3 or more and can be detected. there were. As a result, scratches of 0.10 to 0.15 mm, which were difficult to detect with the conventional method,
It could be detected by the method of the present invention.

Further, the defect detection limit at the pipe end was investigated by using drill holes formed at various distances from the pipe end.
As a result, in the conventional method, the drill hole at a position of 100 mm from the pipe end was the detection limit, whereas in the method of the present invention,
It was revealed that the drill hole at the position of 20 mm could be detected with certainty, and the undetected area was significantly reduced compared to the conventional coil method (Fig. 6). Further, according to the method of the present invention, the exciting force is approximately tripled by concentrating the magnetic flux on the material to be inspected as compared with the conventional method, so that a small-sized and high-performance flaw detector can be obtained.

Second Embodiment FIGS. 4 and 5 show a second embodiment of the present invention. This second
In the embodiment, ring-shaped auxiliary yokes 17a and 17b are added to the first embodiment shown in FIGS. These auxiliary yokes 17a and 17b are opposite sides 3a,
Since the magnetic sensitive element 7 projects between the auxiliary yokes 17a and 17b and protrudes into the space between 3b, the leakage magnetic flux from the scratch 8 has a strong component perpendicular to the tube axis of the steel pipe 5, and the magnetic sensitive element 7 The scratch detection signal level becomes high. Also, the opposite sides 3a, 3b
Since the stray magnetic field between the through holes 4a and 4b and the disturbance of the magnetic flux are further reduced, the magnetization efficiency and the SN ratio are improved, and the undetected region at the end of the inspected material is further reduced.

In the first and second embodiments described above, a magnetic sensitive diode, which is one of the semiconductor elements, is used as the magnetic sensitive element 7, but a Hall element, a search coil or the like may be used. Further, the substantially U-shaped yoke 3 is installed upright, but it may be installed upside down or laid down so that the installation direction is arbitrary.

[0023]

According to the present invention, since the magnetic flux generated from the exciting coil 1 can be concentrated on the material 5 to be inspected through the substantially U-shaped yoke 3, the stray magnetic field and the magnetic flux are significantly disturbed. Less. Further, since the exciting coil 1 and the magnetic sensitive element 7 can be installed separately from each other, the influence of the temperature drift of the magnetic sensitive element 7 due to the heat generation of the exciting coil 1 can be significantly reduced.
As a result, the magnetization efficiency and the S / N ratio are improved, and the undetected region at the end of the material to be inspected is significantly reduced, so that minute scratches can be reliably detected. Further, since the magnetic flux can be concentrated on the material to be inspected, the device can be downsized.

[Brief description of drawings]

FIG. 1 shows a first embodiment of the present invention, in which a detection end portion shows a vertical section.

FIG. 2 is a cross-sectional view taken along the line A-A ′ of FIG.

FIG. 3 is a graph showing the flaw detection result of the present invention according to the first embodiment and the flaw detection result of the conventional method (FIG. 6).

FIG. 4 shows a second embodiment of the present invention, in which the detection end shows a vertical section.

5 is a cross-sectional view taken along the line A-A ′ of FIG.

FIG. 6 is a side view showing one conventional leakage magnetic flux flaw detector, and a detection end shows a cross section.

[Explanation of symbols]

1: Excitation coil 2: Magnetic flux 3: Yoke 3a: Opposite side of yoke 3b: Opposite side of yoke 3c: Bottom side of yoke 4a: Through hole of yoke 4b: Through hole of yoke 5: Steel pipe (inspected material) 6: Transport Direction 7: Magnetosensitive element 8: Scratch 9: Leakage magnetic flux 10: Amplifier 11: Filter 12: Waveform shaper 13A, 13B: Recorder 14: Comparator 15: Judgment device 16: Holder 17a: Auxiliary yoke 17b: Auxiliary yoke:

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Fujii Akira 5 Tokuyama, Tokuyama City, Yamaguchi Prefecture 6 System High Tech Co., Ltd.

Claims (2)

[Claims] 1. A material to be inspected is inserted into a through hole provided on an opposing side of a yoke having a substantially U-shaped cross section, and a magnetic flux generated by an exciting coil fitted on the bottom side of the yoke is covered via the yoke. A leakage magnetic flux flaw detection method, which comprises applying a magnetic flux to an inspection material and detecting a leakage magnetic flux from a flaw of the inspection material by a magnetic sensitive element provided between the facing sides. 2. A yoke having a substantially U-shaped cross section, a through hole of a material to be inspected provided on an opposite side of the yoke, an exciting coil penetrated by a bottom side of the yoke, and the yoke are provided between the opposite sides. A leak magnetic flux flaw detector comprising a magnetic sensitive element for detecting a leak magnetic flux of an external material to be inspected.