JPH10170481A - Eddy current flaw detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly accurately detect a linear flaw during hot working. SOLUTION: A material 1 to be inspected penetrates inside exciting coils 20, 20 which are spaced in an axial direction. A pair of detection coils 30, 30 connected in a differential manner are arranged between the exciting coils 20 and 20. Axial centers of the detection coils 30, 30 are directed to a radial direction of the exciting coils 20 and a vertical component of a secondary magnetic field is detected. The detection coils 30, 30 are arranged side by side in a circumferential direction of the exciting coils 20 and rotated in the circumferential direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an eddy current flaw detector used for inspecting a surface flaw of a material to be inspected having a circular outer shape such as a wire rod or a steel bar, and more particularly to an eddy current flaw detector suitable for inspecting a surface flaw in a hot working process. Related to the device.

[0002]

2. Description of the Related Art An eddy current flaw detector called a through coil type is used for inspection of surface flaws in a hot rolling process of a steel bar. As shown in FIG. 5A, this eddy current flaw detection apparatus has a penetration type excitation coil 2 through which a material 1 to be inspected penetrates the inside.
And a pair of penetrating detection coils 3 and 3 concentrically combined with the excitation coil 2. The pair of detection coils 3 and 3 are arranged side by side in the axial direction, and the differential output difference between the detection coils 3 and 3 when the inspection target material 1 moves in the axial direction inside the coil is used to detect the detection target material 1. Surface flaws are detected.

[0003] This eddy current flaw detector is based on the principle,
Suitable for detecting large area surface flaws such as scabs 4
It is not suitable for detecting relatively shallow linear flaws 5 that occur continuously in the longitudinal direction of the material 1 to be inspected.

On the other hand, as an eddy current flaw detector capable of detecting the linear flaw 5, there is a so-called probe type. As shown in FIG. 5B, the probe coil 6 facing the surface of the material 1 to be inspected is rotated in the circumferential direction of the material 1 to be inspected. However, this eddy current flaw detection device does not allow so-called lift-off, which is the distance from the surface of the material 1 to be inspected to the coil, so that there is a scale on the surface of the material 1 to be inspected and a hot rolling process that requires wire permeability. Has a restriction that it cannot be applied.

An eddy current flaw detector developed under such circumstances is disclosed in Japanese Patent Application Laid-Open No. 3-115585. As shown in FIG. 6, this eddy current flaw detection apparatus has a penetrating main excitation coil 2a through which a material 1 to be inspected penetrates, and an additional excitation coil 2a disposed outside the main excitation coil 2a with an interval in the axial direction. Coil 2b, 2b, additional coil 2
b, 2b.

[0006] A plurality of detection coils 7 are arranged in the circumferential direction of the exciting coil at two positions in the axial direction of the exciting coil with the axis thereof oriented in the circumferential direction of the exciting coil. Connected differentially.

[0007]

Problems to be Solved by the Invention
In the eddy current flaw detection device described in Japanese Patent Application Publication No. 1 (1993) -1995, the additional coils 2b, 2b are arranged outside the main excitation coil 2a, and the detection coil 7 arranged between the additional coils 2b, 2b.
Is oriented in the circumferential direction of the exciting coil to eliminate the influence of the primary magnetic field (generated mainly in the axial direction of the coil), thereby achieving a relatively large lift-off while maintaining relatively high sensitivity. Secured. Therefore, it is possible to detect a linear flaw in a hot state, and specifically, a linear flaw having a lift-off of 3 mm and a depth of 0.5 mm is detected with a sensitivity of S / N ≧ 3. However, lift-off is 3mm
And a linear flaw having a depth of 0.3 mm cannot be detected with a sensitivity of S / N ≧ 5.

Further, in this eddy current flaw detector, it is necessary to reduce the pitch between the detection coils 7, 7 adjacent in the circumferential direction in order to prevent the inspection from being missed. For this reason, a very large number of detection coils 7 are required, resulting in a complicated structure and high cost.

An object of the present invention is to provide a low-cost eddy current flaw detection apparatus which can detect a linear flaw having a depth of 0.3 mm with high sensitivity of S / N ≧ 5 and has a simple structure. To provide.

[0010]

Means for Solving the Problems In hot flaw detection, it is necessary to increase the lift-off in order to avoid scale existing on the surface of the material to be inspected. I have to raise it. on the other hand,
In order to detect a small and shallow flaw such as a linear flaw, it is necessary to reduce the size of the detection coil, and it is necessary to reduce the lift-off in order to compensate for a decrease in sensitivity due to this. The present inventors have conducted various experiments and analyzes on a coil configuration that can resolve such contradictions and detect a linear flaw with high accuracy even in a state where a relatively large lift-off is secured. As a result, the following facts became clear.

In the eddy current flaw detector described in Japanese Patent Application Laid-Open No. 3-1155851, additional coils 2b, 2b are arranged outside the main excitation coil 2a in order to strengthen the primary magnetic field. The detection coil 7 arranged between 2b is still located outside the main excitation coil 2a. In order to eliminate the influence of the primary magnetic field generated by the excitation coil, the axis of the detection coil 7 is oriented in the circumferential direction of the excitation coil. However, as long as it is located outside the main excitation coil 2a, not less than Affected, noise increases.

In order to strengthen the primary magnetic field and effectively eliminate its influence, it is necessary to completely divide the exciting coil into two parts in the axial direction, and to arrange the detecting coil therebetween.

With respect to the direction of the detection coil, it is necessary to eliminate the influence of the primary magnetic field mainly generated in the axial direction of the exciting coil. For this purpose, the axis is oriented in the circumferential direction or radial direction of the exciting coil. There is a need. In the former case, a secondary magnetic field in the horizontal direction (circumferential direction) parallel to the outer surface of the test material is detected, and in the latter case, a secondary magnetic field in a direction perpendicular to the outer surface of the test material is detected. The former method is adopted in the eddy current flaw detector described in Japanese Patent Application Laid-Open No. 3-115585.

However, the present inventor conducted a detailed analysis on this, and found that the vertical component of the secondary magnetic field was larger than the horizontal component (component in the circumferential direction) near the linear flaw. It has been found that the latter method, that is, the method in which the axis of the detection coil is oriented in the radial direction of the exciting coil and the secondary magnetic field in the vertical direction is detected, is more effective.

The secondary magnetic field generated near the linear flaw is complicated, and the exact reason why the vertical component is larger than the horizontal component (the component in the circumferential direction) is not clear. Thinks as follows. The eddy current generated by the exciting coil flows in the circumferential direction of the test object. Since the secondary magnetic field is generated around this circumferential eddy current, it is divided into an axial component and a vertical component of the test object. If there is a linear flaw, it is considered that these components change. However, since there is almost no horizontal (circumferential) component generated here, Japanese Patent Application Laid-Open Publication No. H10-260, which detects a secondary magnetic field change in the horizontal (circumferential) direction. In the eddy current flaw detector described in Japanese Patent Application Laid-Open No. 3-1155851, the influence of the primary magnetic field generated in the axial direction of the exciting coil (material to be inspected) is eliminated, but the detection power for the linear flaw is weak.

Further, in the eddy current flaw detector described in Japanese Patent Application Laid-Open No. 3-1155851, the detection coils are arranged in the circumferential direction of the excitation coil. However, in this configuration, as described above, the number of coils is increased, and the number of coils is increased. Even if the number is increased, the coil position may not completely correspond to the flaw position, and the peak value of the secondary magnetic field cannot always be detected. To solve such a problem, it is effective to rotate the detection coil in the circumferential direction of the exciting coil. By doing so, the number of coils is reduced, and the peak value of the secondary magnetic field can be reliably detected with a small number of detection coils.

The eddy current flaw detection device of the present invention has been developed based on these findings, and has a pair of penetration type penetrating members in which the material to be inspected penetrates the inside and is spaced apart in the axial direction. An exciting coil, disposed between the pair of exciting coils,
And a pair of differentially connected detection coils, the pair of detection coils having their axes oriented in the radial direction of the exciting coil and being arranged side by side in the circumferential direction of the exciting coil to be driven to rotate in the circumferential direction. It is characterized by being performed.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing a basic configuration of the present invention, and FIG. 2 is a perspective view of an eddy current flaw detector showing an embodiment of the present invention.

In this embodiment, a cylindrical bobbin 10 is concentrically mounted on a rotating body 50 that rotates in the circumferential direction. The material 1 to be inspected moves in a concentric position inside the bobbin 10 in the axial direction. On the outer surface of the bobbin 10, a pair of through-type exciting coils 20, 20 are attached at intervals in the axial direction. The exciting coils 20 and 20 are connected to an AC power supply 40 in the same winding direction and connected in series in order to form a primary magnetic field in the axial direction on the outer surface of the test material 1.

The outer surface of the bobbin 10 also has an exciting coil 2
A pair of detection coils 30, 30 are mounted between 0, 20. The detection coils 30 and 30 have their respective axes perpendicular to the outer surface of the bobbin 10 and the bobbin 1
0 are arranged side by side in the circumferential direction. The detection coils 30, 30 are differentially connected to detect an output difference and cancel noise.

In the flaw detection work, the bobbin 10 is rotated in the circumferential direction while a high-frequency voltage is applied to the exciting coils 20, 20, and the material 1 to be inspected, such as a wire or a steel bar, is passed through the inside thereof. Due to the rotation of the bobbin 10, the exciting coils 20, 20 and the detecting coils 30, 30 rotate around the material 1 to be inspected in the circumferential direction. In addition, by applying a high-frequency voltage to the exciting coils 20, 20, a primary magnetic field is generated in the outer surface of the test object 1 in the axial direction, and an eddy current is generated in the circumferential direction.

If there is a linear flaw on the surface of the material 1 to be inspected, the detection coils 30, 30 sequentially cross it at right angles. In addition, disturbance occurs in the secondary magnetic field generated by the eddy current. It has been confirmed that the vertical component of the secondary magnetic field in the vicinity of the linear flaw is larger than the horizontal component (component in the circumferential direction). Detection coil 3
Reference numerals 0 and 30 detect the vertical component of the secondary magnetic field. When one detection coil 30 is located on a linear flaw, the other detection coil 30 is located off the linear flaw. By detecting the difference, the linear flaw is detected with high accuracy.

For the purpose of flaw detection in hot rolling, a lift-off of at least 3 mm is required. When the lift-off is 3 mm, in the eddy current flaw detector described in JP-A-3-115585, a linear flaw having a depth of 0.5 mm has a S / N ≧
Detected with a sensitivity of 3. However, the depth is 0.3mm
Cannot be detected with a sensitivity of S / N ≧ 5. In contrast, the eddy current flaw detector of the present invention has a depth of 0.3 m.
m linear flaws can be detected with a sensitivity of S / N ≧ 5.

This is because the influence of the primary magnetic field generated by the exciting coils 20, 20 is effectively eliminated by separating the exciting coils 20, 20 in the axial direction and disposing the detecting coils 30, 30 therebetween. By directing the axes of the detection coils 30 and 30 in the radial direction of the inspection target material 1, it is possible to detect the vertical component of the secondary magnetic field, which frequently occurs near the linear flaw, and This is because, by rotating the inspection material 1 in the circumferential direction, the peak value of the vertical component is reliably detected.

The size of the gap between the exciting coils 20 is
In order to increase the primary magnetic field generated in the gap, it is preferable that the primary magnetic field be as small as possible within a range where the detection coils 30, 30 can be arranged. The diameter of the detection coils 30 is 3 to 9
mm is preferred. Detection sensitivity of coil itself, excitation coil 2
The point of reducing the gap between 0, 20 and the detection coil 30,
The smaller the distance, the better the diameter is, but the extremely small the diameter makes it difficult to manufacture the detection coils 30. The distance between the detection coils 30, 30 is preferably 3 mm or less because it is better to pick up the signal as close as possible and take the difference to increase the detection sensitivity.

FIG. 3 is a graph showing the result of analyzing the effect of the direction of the detection coil on the detection sensitivity. A is FIG.
When the axis of the detection coil is oriented in the radial direction of the excitation coil as shown in FIG. 4A, B is obtained when the axis of the detection coil is oriented in the axis direction of the excitation coil as shown in FIG. FIG. 4C shows a case where the axis of the detection coil is oriented in the circumferential direction of the exciting coil as shown in FIG. Common conditions include a linear flaw depth of 0.3 mm, a frequency of 1 MHz, and a detection coil diameter of 4.
5 mm and a lift-off of 3.0 mm were employed.

As can be seen from FIG. 3, the secondary magnetic field generated near the linear flaw has a vertical component larger than the horizontal component (a component in the circumferential direction), and the secondary magnetic field is generated by the detection coil whose axis is directed in the radial direction of the exciting coil. The highest sensitivity is obtained by detecting the vertical component. Also, it can be seen that the horizontal component (component in the circumferential direction) is unexpectedly small. This is because, as described above, the secondary magnetic field generated by the exciting coil is mainly composed of the axial component and the vertical component of the material to be detected, and hardly generates a horizontal (circumferential) component.

In this embodiment, the exciting coils 20 and 2
Although both 0 and the detection coils 30 and 30 are rotated, this rotation may be performed only for the detection coils 30 and 30.

[0029]

As is apparent from the above description, in the eddy current flaw detector of the present invention, the axial center is directed in the axial direction of the exciting coil between a pair of exciting coils spaced apart in the axial direction. By rotating the detection coil in the circumferential direction of the excitation coil, it is possible to ensure high sensitivity to linear flaws in a state where a large lift-off is ensured. Therefore, the detection of shallow linear flaws in the hot step, which has been difficult so far, can be performed with high accuracy. Moreover, since a large number of detection coils are not required, the structure is simple and inexpensive.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a basic configuration of the present invention.

FIG. 2 is a perspective view of an eddy current flaw detector showing an embodiment of the present invention.

FIG. 3 is a graph showing the result of analyzing the effect of the direction of a detection coil on detection sensitivity.

FIG. 4 is a schematic diagram showing an arrangement of detection coils in an analysis test.

FIG. 5 is a schematic diagram showing a configuration of a conventional eddy current flaw detection device.

FIG. 6 is a schematic diagram showing a configuration of another conventional eddy current flaw detection device.

[Explanation of symbols]

 Reference Signs List 10 bobbin 20 excitation coil 30 detection coil 40 high frequency power supply 50 rotating body

Claims (1)

[Claims] 1. A pair of through-type exciting coils, through which a material to be inspected penetrates the inside and spaced apart in the axial direction,
A pair of detection coils disposed between the pair of excitation coils and connected differentially, the pair of detection coils having their axes oriented in the radial direction of the excitation coil and arranged in the circumferential direction of the excitation coil. The eddy current flaw detection device is arranged and rotated in the circumferential direction.