JP2003344359A - Method and apparatus for magnetic particle inspection - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To quickly detect defects on the entire surface of a material to be inspected and furthermore defects in any direction in a magnetic particle inspection. <P>SOLUTION: The magnetic particle inspection method mainly includes: a process for inserting a current penetration electrode into a hole that is in the thickness direction and is formed in the material to be inspected for energizing a DC current and energizing an AC current to magnetization coils arranged on both the sides of the material to be inspected; a process for spraying a magnetic particle liquid onto the surface of the material to be inspected; and a process for demagnetizing the material to be inspected. The magnetic particle inspection apparatus is provided with the current penetration electrode that is inserted in the shaft hole of an annular body and energizes the DC current; the magnetization coils that are oppositely arranged on both the sides of the annular body and energizes the AC current; and a nozzle for spraying the magnetic particle liquid to the annular body. As a result, the defects in any direction on the entire surface of the material to be inspected can be quickly detected. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic particle flaw detection method and a magnetic particle flaw detector which are applied for quality assurance of a material to be inspected, which is a magnetic material, such as a steel product, and more specifically, a hole penetrating in the thickness direction is formed. Further, the present invention relates to a magnetic particle flaw detection method and a magnetic particle flaw detector which can detect defects on the entire surface of a material to be inspected made of a magnetic material and defects in any direction.

[0002]

2. Description of the Related Art Conventionally, a magnetic particle flaw detection test and a magnetic flux leakage flaw detection test using magnetism have been established as quality assurance technology for steel products, and have been widely adopted for various products.

However, among them, the magnetic particle flaw detection test is a method utilizing the phenomenon that magnetic particles magnetically attracted to the leakage magnetic flux generated at the defect portion are collected, and therefore the magnetic field is leaked unless a magnetic field is applied in a direction orthogonal to the length direction of the defect. No magnetic flux is generated, making it difficult to detect defects. For this reason, when defects may occur in multiple directions, defects in all directions cannot be detected unless the directions of the magnetic field are set in multiple directions.

As an apparatus for detecting defects in multiple directions
No. 60-242363 proposes an apparatus for obtaining a uniform magnetic field by providing a rotating roller on a magnetic pole having an E-shaped cross section and rotating the magnetic roller. Further, in Japanese Patent Laid-Open No. 9-80026, four magnetic poles arranged in a rectangular shape are provided on a magnetic core that creates a closed magnetic field.
A device has been proposed which switches between an operation of energizing two coils provided on two sides of a magnetic core to apply a parallel magnetic field and an operation of energizing two coils provided on an orthogonal magnetic core to apply a orthogonal magnetic field. There is.

In Japanese Patent Laid-Open No. 51-29987, a cylindrical magnetic pole having an E-shaped cross section is used to generate a magnetic field in the circumferential direction on the side surface of an annular product such as a gear or wheel and a magnetic field in the axial direction at the end. A method of adding is proposed.

[0006]

SUMMARY OF THE INVENTION
In the technology proposed in 60-242363 and Japanese Patent Laid-Open No. 9-80026, since the direction of the magnetic field is changed by rotating and switching the magnetic poles, the switching time becomes a problem when the material to be inspected moves in a line work, Multidirectional magnetic fields cannot be applied to the same site. Therefore, quality assurance in the manufacturing process is too time-consuming and difficult to apply. Further, JP-A-51-29987 is a method of simultaneously applying a magnetic field in the circumferential direction of the side surface and a magnetic field in the thickness direction of the end portion, but the problem that vertical cracks on the side surface and cracks in the thickness direction of the end portion cannot be detected. There is.

The present invention has been made to solve the above problems, is suitable for quality assurance in the manufacturing process, and detects defects in any direction on the entire surface of the material to be inspected in a short time. It is an object of the present invention to provide a magnetic particle flaw detection method and a magnetic particle flaw detector capable of performing the magnetic particle flaw detection.

[0008]

In order to achieve the above-mentioned object, a magnetic particle flaw detection method according to the present invention is a direct current by inserting a current through electrode into a hole formed in the material to be inspected in the thickness direction. And the step of applying an alternating current to the magnetizing coils arranged on both sides of the material to be inspected, and the step of spraying the magnetic powder on the surface of the material to be inspected.

In the magnetic particle flaw detector, a current penetrating electrode, which is inserted into the hole of a material to be inspected having a hole and is supplied with a direct current, and an alternating current, which is arranged so as to face each side surface of the material to be inspected. The main structure is a magnetizing coil that is energized.
By doing so, it is possible to catch defects in any direction on the entire surface of the material to be inspected in a short time.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below, taking as an example the inspection of railway wheels (hereinafter simply referred to as wheels) that the inventor has been working on for quality assurance. The inventor of the present invention has been working for many years on quality assurance of a railway wheel which is an annular body having a shaft hole formed in the center thereof. The railway operates day and night, carrying a large number of products and many passengers,
Moreover, because it operates at high speed, we do not pay too much attention to ensuring the safety of the wheels, and many inspections are conducted. The material of the wheel is a steel slab that has passed ultrasonic flaw detection, and the steel slab is subjected to forging, rolling, etc. to form a wheel. The product wheels are subjected to visual inspection along with magnetic particle flaw detection in the final process. Especially the quality of the wheels
It is important from the rim portion to the tread surface, and it is extremely important to improve the defect detection accuracy in this portion.

However, in the inspection of the final process described above, 10,0
There was a case where a minute defect was found on the rim portion by visual inspection in a wheel that passed the magnetic particle flaw detection at a rate of about 1 to 2 with respect to 00 pieces. The inventor decided to investigate the defect detection characteristics of the magnetic particle flaw detection in order to clarify the cause. Table 1 shows the inspection conditions, and Table 2 shows the inspection results. FIG. 2 is a cross-sectional view through the shaft hole of the wheel, showing the upper half thereof, and showing the wheel inspection portion of Table 2, where the A type test piece specified in JIS G0565 shown in Table 1 is shown. Was pasted and observed.

[0012]

[Table 1]

[0013]

[Table 2]

The circles in Table 2 indicate that defects could be detected,
The cross mark indicates that the defect could not be detected, and as a result, the defect detection of the radial defect on the front rim, the tread, and the back rim is bad. The circumferential defect in Table 2 is a hole 11 (axial hole) on the side surface 12 in FIG. 1, which schematically shows the state of the inspected material 1 (wheel) and the defect that has occurred.
Indicates a concentric defect (d) or a concentric defect (c) in the hole 11 at the end, and the radial defect means a defect (a) directed toward the hole 11 on the side surface 12 or a hole 11 at the end. Indicates a defect (a).

As a result, the following two points can be considered as causes and countermeasures. , "Insufficient strength of the outer magnetization" causes "addition of outer circumference magnetized rod" as a countermeasure, and "Causes skin action by alternating current" as a cause, "DC conversion" as a countermeasure.

Therefore, the magnetic field strength analysis for the wheel shown in FIG. 3 was further advanced, and the result is shown in FIG. Figure 3 (a)
Shows the current through electrode 3 provided in the shaft hole 21 of the wheel 2 and the outer peripheral magnetized rod 31 at the outer periphery of the wheel. FIG. 3B shows the same current through electrode 3 as in the above analysis. Of the results of analyzing the currents flowing through the current penetrating pole 3 and the outer peripheral magnetizing rod 31 by alternating current and direct current, FIG. 4 (a) shows a magnetic field analysis by direct current magnetization, and FIG. 4 (b) shows This is the case where magnetic field analysis by alternating-current magnetization was performed.

In FIG. 4, the horizontal axis indicates the distance (m from the axis of the wheel).
m) and the magnetic flux density (T) on the vertical axis.
In the case of (a), the solid symbols indicate the case of FIG. 3 (b). From FIG. 4B, it is understood that as long as an alternating current is applied to the current penetrating pole and the outer peripheral magnetizing rod, only the vicinity of the current penetrating pole and the outer peripheral magnetizing rod are magnetized due to the effect of the skin effect. Further, it can be seen from FIG. 4 (a) that when a direct current is applied to the current penetrating pole and the outer peripheral magnetizing rod, there is no skin effect unlike AC, and the interior of the wheel and its end are magnetized. However, in this case, it is also understood that the effect of the outer peripheral magnetizing bar is almost absent.

Further, FIG. 5 is shown to verify the effect of the outer peripheral magnetizing rod 31. FIG. 5A is a diagram showing the distance between the outer circumference magnetized rod 31 and the wheel 2 by the relationship between the shortest distance L between the outer circumference magnetized rod 31 and the wheel 2 and its angle θ, and FIG. FIG. 6 is a diagram showing the relationship between the value of θ and the magnetic field strength at that position. B ₀ is the _value of (μ ₀ Ι) / (2πL) and indicates the magnetic field strength at the distance L, and B _θ is also the magnetic field strength at the distance at θ. Note that μ ₀ is the magnetic permeability and I is the current.

As shown in FIG. 5 (b), the magnetic field strength greatly decreases with a slight change in the distance between the outer circumference magnetized rod and the wheel (for example, the magnetic field strength is about 70% (-3 dB) at a position where the angle θ is 45 °).
Is. ) I understand. Therefore, it cannot be said that it is a good idea to adopt the outer peripheral magnetizing rod according to the shape of the wheel in order to stabilize the magnetic field strength, and it cannot be said that the use of the outer peripheral magnetizing rod is effective from FIG. 4 (a). I understand.

Therefore, the effect of the difference between the direct current and the alternating current was confirmed again using only the current through electrode. The results are shown in Table 3. As shown in Table 3, it was confirmed that even with the same current value, direct current has higher detectability, and by further increasing the current value, flaw detection up to the tread surface is possible.

[0021]

[Table 3]

The magnetic particle flaw detection method according to the first aspect of the present invention has been made based on the above findings, and a current penetrating electrode is provided in a hole of a material to be inspected made of a magnetic material having a hole penetrating in the thickness direction. Inserting and applying a direct current to the current through electrode, and applying an alternating current to a magnetizing coil arranged to face both sides of the inspected material to magnetize the inspected material,
A magnetic particle flaw detection method comprising: a step of spraying a magnetic powder solution on the surface of the inspection material; a step of observing the presence or absence of magnetic powder adhering to the inspection material; and a step of demagnetizing the inspection material.

In the present invention, the current penetrating electrode is inserted into the hole of the material to be inspected and a direct current is passed through the current penetrating electrode, because the magnetic field penetrates into the material to be inspected and its end portions.
This is because it is excellent in detecting radial defects with the hole as the axis. Further, to magnetize the material to be inspected by passing an alternating current through a magnetizing coil arranged on both sides of the material to be inspected, a magnetic field permeates all over both sides of the material to be inspected by a skin effect, This is because it is particularly excellent in detecting a circumferential defect centered on the hole. As described above, by using the current penetrating pole and the magnetizing coil together and supplying a direct current to the current penetrating pole and an alternating current to the magnetizing coil, a defect at any position in any direction can be detected.

The function of the current through electrode is to allow a direct current to pass through the hole of the material to be inspected. Therefore, a method of penetrating one current through electrode into the hole of the material to be inspected, Alternatively, two current penetrating electrodes may be inserted from both side surfaces of the material to be inspected, the ends of the current penetrating electrodes may be butt-connected to each other, and electricity may be applied. In the latter case, since the length is half that of a single current through electrode, the operating time of the flaw detection device can be shortened, and the amount of protrusion of the current through electrode can be reduced, which reduces the size of the device itself. is there. Therefore, inserting the current through electrode into the hole of the material to be inspected does not necessarily mean that one current through electrode is inserted into the hole of the material to be inspected.

Further, the demagnetization treatment of the material to be inspected before and after the presence / absence of the adhered magnetic powder is carried out by an electric treatment in the vicinity of the material to be inspected in the state where the strong magnetism due to the magnetic field at the time of flaw detection of the magnetic powder remains. This is because, for example, there are various concerns such as a defect such as arc welding, a possibility of malfunction of a solenoid valve, a defect due to suction of magnetic powder to peripheral equipment, and an electric corrosion of a material to be inspected.

Regarding the demagnetization method, in the present invention, since a direct current is adopted as the current through electrode in order to permeate the magnetic field into the inside and the end of the material to be inspected, the magnetic flux remains as compared with the alternating magnetization. It is easy and the residual magnetic flux is hard to appear on the surface,
Any structure may be used as long as it can be effectively demagnetized.

According to the experiment conducted by the present inventor, even after a demagnetizing machine is separately installed and subjected to demagnetizing treatment, a small amount of magnetism remains and the magnetic powder attached to the defect remains. It turns out. Therefore, even if demagnetization is performed by providing a direct current reversal current to the current through pole as shown in the current waveform in FIG. We have confirmed equivalent effects. In this case, since it is a reversing direct current, the demagnetizing action works to the inside and the end of the material to be inspected, and since only the switching of the energizing current is performed, the cycle time of magnetization and demagnetization becomes extremely short, Compared to the case of installing a new demagnetizer separately, the installation place and installation cost are unnecessary.

Therefore, in the first aspect of the present invention, it is possible to observe the presence or absence of the adhered magnetic powder before the demagnetization step or after the demagnetization step. Further, the dispersion of the magnetic powder liquid may be started before the magnetization is started or after the magnetization is started. However, the spreading must be finished before the magnetization is finished. This is because the sprinkled magnetic powder liquid continues to be magnetized while flowing on the surface of the material to be inspected, and the magnetic sticking at the defective portion is prevented from flowing off by the flow of the magnetic powder liquid.
Further, the observation of the presence or absence of magnetic particles attached is usually performed before the demagnetization step. However, according to the experiments of the present inventor, the magnetization remains to some extent even after demagnetization, and the magnetic powder attached to the defect remains as it is, so it may be observed after demagnetization.

Further, in the present invention, when the material to be inspected is an annular body having a shaft hole formed in the center thereof, it is less likely to sprinkle the magnetic powder liquid on the surface while rotating the shaft hole around the shaft center. It can be sprayed evenly with a nozzle in a short time. In this case, the effect is exhibited even if the rotation speed is 1 rotation or more, for example, 1.5 rotations. Further, since the shape is symmetrical with respect to the shaft hole, the magnetic field strength is constant at any position on the outer peripheral portion, which is desirable because there is no unevenness in defect detection. This is the second,
3 of the present invention.

In the magnetic particle flaw detector according to the present invention, a current penetrating electrode which is inserted into a shaft hole of a material to be inspected having a shaft hole and through which a direct current is applied, and both side surfaces of the material to be inspected are arranged to face each other. And a magnetizing coil to which an alternating current is applied, and a nozzle for spraying a magnetic powder liquid on the material to be inspected. Desirably, it is provided with a rotation mechanism that rotates the shaft hole of the material to be inspected around the shaft center. With such a configuration, the magnetic particle flaw detection method of the present invention can be implemented. In particular, when the wheel is inspected by the magnetic particle flaw detector of the present invention, all the defects such as the rim portion and the tread portion of the wheel are completely detected, and the reliability of the quality after the inspection becomes very high. This is the fourth and fifth aspects of the present invention.

[0031]

Embodiments (Embodiment 1) The magnetic particle flaw detection method according to the present invention will be described below with reference to embodiments. A test piece of type A specified by JIS G0565 is attached to the positions of ~ shown in Fig. 2 of a wheel with an outer diameter of 910 mm, and a through current is passed from the current through electrode to the shaft hole of the wheel, and at the same time, it faces both sides of the wheel. The magnetizing coil placed was also energized and evaluated by visual observation.

As shown in FIG. 7, the procedure was as follows: the magnetic powder was sprayed for 15 seconds while the wheel was rotated, the magnetization was started before the end of the magnetic powder spraying, and the magnetization was performed for 10 seconds at the current values shown in Table 4. In Table 4, the mark ◯ indicates that the attracted magnetic powder pattern is clearly visible, the mark Δ indicates that it is faint, and the mark x indicates that it cannot be seen.

[0033]

[Table 4]

From Table 4, in the comparative example, the rim portion and the tread surface could not be detected at all. On the other hand, in the method of the present invention, good detection ability was obtained for both the boss portion and the rim portion, and furthermore, good detection was possible on the tread surface without any problems.

Next, Table 5 shows the results of the demagnetization treatment of the wheels after the magnetic particle flaw detection test shown in Table 4 of the present invention. The demagnetization treatment was performed for 15 seconds as shown in FIG. 7 by the direct current inversion demagnetization shown in FIG. In the DC reversal demagnetization of FIG. 6, the wavelength and the cycle were gradually reduced and finally set to 0. In Table 5, the residual magnetic flux density (mT) was measured including the edge portion where it is easy to appear, and the detected value is shown as the maximum value. The machining holes in Table 5 are, for example, the oil supply holes in the final machining of the wheel, and the chips are the chips generated during the finishing of the shaft holes and the oil supply holes.

[0036]

[Table 5]

According to Table 5, the demagnetizing effect is not exhibited in the portion having a small residual magnetic flux, but the portion having a large residual magnetic flux, for example, inside the machined hole or the machined hole edge, should be reduced to 1/2 or less. I understand.

(Embodiment 2) Next, a magnetic particle flaw detector of the present invention will be described based on an embodiment shown in FIGS. Figure 8
Shows a front view of the whole magnetic particle flaw detector of the present invention, FIG. 9 is a front view of a dolly to which a current through pole and a magnetizing coil are attached, FIG. 10 is a plan view of FIG. 9, and FIG. 11 is a current through pole and a magnetizing coil. FIG. 12 is an enlarged view of the inside of the elevating mechanism of FIG.

In FIG. 8, the wheel 2 which is the material to be inspected with the shaft hole 21 horizontal in the center and the thickness direction facing the front is placed on the roller 61 constituting the rotating mechanism 6, and the wheel is Two
On the left and right sides of the current-carrying pole 3, a current through pole 3 and a magnetizing coil 4 mounted on a carriage 7 are arranged so as to face each other. A motor 62 with a speed reducer for rotating the roller 61 is attached to the frame 9 located below the carriage 7, and a belt is interposed between the motor 62 and the roller 61 to rotate the roller 61. There is. The two rollers 61 are paired and kept at a predetermined distance, and the wheels 2 are placed between the rollers 61 in a standing manner.
It is carried out by a separately provided conveyor type and by lowering one roller 61.

A pump 81 for supplying magnetic powder liquid is connected to the motor 82 on the opposite side of the motor 62, and the magnetic powder liquid pumped up from the pump 81 is supplied to a large number of nozzles 84 via a supply pipe 83. ing. Since the magnetic powder from the nozzle 84 needs to be sufficiently dispersed to the wheel tread, which is the upper end of the wheel 2 in the view of the drawing, one of the nozzles 84 arranged in large numbers should be located above the wheel tread. There is. Although not shown in this drawing, the supply pipe 8
3 has a part thereof made flexible, and is fixed to the carriage 7 together with the supply pipe 83 and the nozzle 84.

The dolly 7 is constructed so as to move toward and away from the wheel 2 when the rod of the air cylinder 71 fixed to the frame 9 moves in and out. When magnetizing the wheel 2, the bogies 7 on both sides of the wheel 2 move forward equidistantly from each other, approach the front and back sides of the wheel 2, and the current penetrating electrode 3
Are inserted into the shaft hole 21, and the current through electrodes 3 are butt-connected to each other in the shaft hole 21 so that a direct current can flow.

As for the positional relationship between the wheel 2 which is the material to be inspected and the carriage 7, the mounting position of the wheel 2 on the roller 61 is fixed,
By setting the amount of forward movement of the two carts 7 to be constant at all times, relative variations between the wheel 2 as the material to be inspected, the current through pole 3 and the magnetizing coil 4 are eliminated, and the current through pole 3 is eliminated.
If a specified current is applied to the magnetizing coil 4, the wheel 2
The specified magnetic field is applied to. In addition,
The thickness variation of the wheel 2 due to the size variation of the wheel 2 is small,
It has almost no effect on magnetic particle flaw detection.

The positional relationship between the current through pole 3 and the magnetizing coil 4 mounted on the carriage 7 is fixed to the mounting base 72 to which each of them is mounted. However, this mounting base 7
2 can be moved in the vertical direction so that the position of the wheel 2 can be adjusted according to the size of the wheel 2 (change of wheel diameter). Since the size of the wheel 2 is a lot production that is integrated in the manufacturing process and does not change frequently, in the present embodiment,
A handle 73 is provided for manually adjusting the mounting base 72 in the vertical direction. By rotating the handle 73 in the forward and reverse directions, the current through pole 3 and the magnetizing coil 4 are
Can be moved up and down with respect to the bogie frame 74.

FIG. 11 is an enlarged view of the lifting mechanism. When the handle 73 is turned, the shaft 73 to which the handle 73 is attached is attached.
a, a transmission shaft 73d through a worm 73b attached to the shaft 73a and a worm wheel 73c meshing with the worm 73b.
The second worm 73e and the worm wheel 73f meshed with the second worm 73e are rotated by the rotation of the transmission shaft 73d, and the worm wheel 73f moves the rack 73g up and down. Since the rack 73g and the mounting base 72 are fixed with screws, the rack 73g
The vertical movement of g is the vertical movement of the mounting base 72. Therefore, the current through pole 3 and the magnetizing coil 4 fixed to the mounting base 72 are integrated with the vertical movement of the rack 73g, and are smoothly moved up and down by being guided by the two guide rods 73h. The reference numeral 75 indicates the wheels of the carriage 7. Also,
The power supply to the current through pole 3 and the magnetizing coil 4 is separately supplied from a power source.

In the magnetic particle flaw detector of the present invention having the above-described structure, if the operating procedure is stored in a separately provided control device, the method of the present invention is executed according to the instruction. In particular, efficient processing such as simultaneous operation of spraying the magnetic powder liquid, rotation of the material to be inspected and forward movement of the carriage 7, simultaneous operation of spraying the magnetic powder liquid and magnetization, continuous switching of magnetization and demagnetization is possible. . Of course, it is possible to perform magnetic particle flaw detection without providing a control device.

[0046]

As described above, according to the present invention,
Not only the wheel but also defects on the entire surface of the material to be inspected made of a magnetic material in which a hole penetrating in the thickness direction is formed, and also defects in any direction can be detected, and in a shorter time than the conventional technology Magnetic particle flaw detection can be performed. In particular, when the material to be inspected is a circular railway wheel, it is possible to perform a complete inspection up to the end surface, which is the tread surface.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a state of a material to be inspected and defects generated therein.

FIG. 2 is a diagram showing an inspection portion of a wheel.

FIG. 3 is a diagram showing a magnetic field analysis on wheels, where (a) is
FIG. 7B is a diagram in which a current penetrating pole is provided on the wheel shaft center and an outer magnetizing rod is provided on the wheel outer peripheral portion, and FIG.

FIG. 4 is a diagram showing the results when the current passing through the current penetrating pole and the outer magnetizing rod is selectively used for alternating current and direct current in the arrangement shown in FIG. 3, and FIG. 3) when a direct current is applied to any of
It is a figure at the time of supplying an alternating current to both of (b).

FIG. 5 (a) is a diagram showing the relationship between the distance between the outer magnetized rod and the wheel, and FIG. 5 (b) is a diagram showing the relationship of variation of the magnetic field strength with a change in the distance.

FIG. 6 is a diagram showing a current waveform during demagnetization conduction.

FIG. 7 is a diagram showing a procedure for carrying out the method of the present invention in Examples.

FIG. 8 is a front view showing the entire magnetic particle flaw detector of the present invention.

FIG. 9 is a front view of a dolly to which a current through electrode and a magnetizing coil are attached.

FIG. 10 is a plan view of FIG.

FIG. 11 is an enlarged view showing the inside of a lifting mechanism for a current through pole and a magnetizing coil.

FIG. 12 is a side view showing an attachment portion of a current through electrode and a magnetizing coil.

[Explanation of symbols]

1 Inspected material Two wheels 21 shaft hole 3 Current penetration pole 31 Peripheral magnetized rod 4 magnetizing coil 6 rotation mechanism 61 Laura 62 motor 7 dolly 71 Air cylinder 72 Mounting base 73 handle 74 bogie frame 75 dolly wheels 81 pumps 82 motor 83 Supply piping 84 nozzles 9 frame

Claims (5)

[Claims] 1. A current penetrating electrode is inserted into a hole of a material to be inspected, which is made of a magnetic material and has a hole penetrating in the thickness direction, and a direct current is applied to the current penetrating pole, and both side surfaces of the material to be inspected are provided. A step of magnetizing the material to be inspected by applying an alternating current to a magnetizing coil arranged to face, a step of spraying a magnetic powder liquid on the surface of the material to be inspected, and a step of applying magnetic powder to the material to be inspected. A magnetic particle flaw detection method comprising a step of observing the presence or absence and a step of demagnetizing the material to be inspected. 2. The magnetic particle flaw detection method according to claim 1, wherein the material to be inspected is an annular body having a shaft hole. 3. The magnetic particle flaw detection method according to claim 1, wherein the magnetic powder liquid is sprayed on the surface of the material to be inspected while the material to be inspected is rotated about the hole. 4. An apparatus for performing the magnetic particle flaw detection method according to claim 1 or 2, wherein a current through electrode inserted into a hole of a material to be inspected and supplied with a direct current, and both side surfaces of the material to be inspected, respectively. A magnetic particle flaw detector, comprising: a magnetizing coil, which is disposed so as to face the magnet, and which is supplied with an alternating current; and a nozzle for spraying a magnetic particle liquid onto the material to be inspected. 5. The magnetic particle flaw detector according to claim 4, further comprising a rotating mechanism for rotating the material to be inspected around a hole as an axis.