JPS6235248B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a demagnetization method for removing residual magnetism from steel materials. In general, steel materials such as steel pipes and round steel, especially special steels such as high carbon steel, manganese steel, and chromium molybdenum steel, are used for important structures and high-pressure piping materials, so defects such as surface scratches and internal scratches are removed during the manufacturing process. A commonly used flaw detection device is a magnetic flaw detection device. However, when magnetic flaw detection equipment is used, residual magnetism tends to remain in special steel, causing problems in workability, weldability, etc. When it comes to special steel, the first thing to consider is the situation in which residual magnetism tends to remain.
〜Explained using FIG. 2. FIG. 1a shows the principle of a penetrating flaw detection coil when a steel material 1 is flowing in the x direction at a velocity v. When a DC excitation current _P is passed through the primary coil P with the number of turns N ₁ , it moves in the direction of the arrow.
A magnetomotive force of H ₁ = N ₁ is generated in the steel material 1 Φ = BS =
A magnetic flux of μH ₁ S is generated. When a magnetic flux Φ is generated, the steel material 1 is magnetized as shown in FIG. 1b, with the tip side in the moving direction being magnetized as a N pole and the opposite side being magnetized as an S pole. If there is a defect number 2 inside the steel material 1, an induced electromotive force e is generated between both ends S ₁ and S ₂ of the secondary detection coil S having N ₂ turns of the penetrating flaw detection coil. That is, the magnetic flux Φ inside the steel material 1
When the steel material is moving at a speed v, the induced electric power e is e=N ₂ dΦ/dt=N ₂ dΦ/dx・dx/dt=N ₂・v
・dΦ/dx……(1) is given. dΦ/dx in equation (1) represents the change in the position of magnetic flux, and in a region without defects, dΦ/dx = 0 and e = 0 as shown in Figure 1c, but if there is a defect, dΦ /dx≠0, and induced electromotive force with the velocity v as a proportional number is generated and can be taken out as a defect signal.
When the steel material 1 is transferred while passing through the penetrating flaw detection coil P, a residual magnetism Br is generated according to the hysteresis curve shown in FIG. 1d. Since this residual magnetism Br is large, normally another through-type coil T is provided and an excitation current smaller than _P is passed in the opposite direction to _{the primary excitation current P to} generate a magnetomotive force in the opposite direction to _H1 .
It generates H ₂ and reduces the residual magnetism Br. The steel material 1 that is removed from this reverse magnetic field _H2 has a magnetomotive force H=0, and a residual magnetism of ΔBr remains. Now, the residual magnetism ΔBr generated after penetrating flaw detection is completed.
is a sufficiently small value for ordinary steel, but in the case of special steel, it becomes relatively large, and generally a degaussing coil is used as the first degaussing coil to further reduce ΔBr.
It is installed in a series of lines using the same principle as the tertiary coil in Figure a. However, in the case of a through type, when the outer diameter of the steel material 1 varies, the demagnetizing ability changes depending on the gap between the coil diameter and the steel material diameter, and a large gap requires large power consumption. The following explanation will make clear that special steel has a relatively large residual magnetism ΔBr compared to ordinary steel. In Fig. 2a, the horizontal axis represents the carbon content (%) in the steel material, and the vertical axis represents the holding force H _C , and when the steel materials are blotted for annealing and quenching, they become curves V and U. That is, as the carbon content increases, the holding force H _C increases, and this is remarkable in hardened materials. The hysteresis curve for such special steel with a high carbon content has a demagnetization curve R _S as shown in FIG. 2, which is significantly different from the demagnetization curve R _N of ordinary steel. That is, in ordinary steel, when the coercive force _CN is removed, there is a residual magnetism Br _N , and in special steel, even when the coercive force H _CS is removed, there is a residual magnetic residual Br _S , which is Br _S >Br _N. In order to reduce this Br _S , the magnetomotive force is generally reduced while demagnetizing the hysteresis curve several times. However, as mentioned above, the operating point changes as shown in Fig. 2b due to the gap g (g ₁ < g ₂ < g _{3 .} . .) between the steel material 1 and the through coil, so the reverse magnetic field H _g1 , H for demagnetization is _g29 , H _{g3 . .} . change, and the desired demagnetization cannot be achieved without complicated control. The problems with degaussing devices using the through-type coil system can be summarized as follows. (1) Several types of degaussing coils are required for steel materials with varying outer diameters. (2) Since it is non-contact, it is inefficient and requires a large amount of electricity. (3) Simultaneous demagnetization of multiple wires is not possible. (4) Demagnetization is not completed until it passes through the through coil, so the demagnetization time is long. (5) Since demagnetization cannot be performed without moving the steel, separate power is required to move the steel in addition to the electricity used to excite the coil for demagnetization. (6) Since the through-coil exists locally in relation to the length of the steel material, a braking force acts on the moving steel material and an external force is required to pull it out. We have explained in Figure 2 a and b that it is difficult to remove the residual magnetism of special steel, but in addition, Figure 3 a
Supplement by using In general, when the steel material 1 already has an internal magnetomotive force H _{d and} a magnetic field H _ex is applied from the outside, the effective magnetic field H _eff is determined by the local appearance of N and S poles on the surface of the steel material. Since a demagnetizing field H' of the resulting magnetic material is generated, it is expressed as H _eff =H _ex -H'=H _ex -H/μ ₀ J (2). Here, N is the demagnetizing field coefficient and J is the magnetization strength. This demagnetizing field coefficient N changes depending on the length and diameter ratio of the steel material. Therefore, even if an attempt is made to remove residual magnetism using an external magnetic field, if the demagnetizing field is large, it is necessary to apply an external magnetic field commensurate with the large demagnetizing field. When residual magnetism Br is generated at the edge of a steel material, comparing the case of applying an external magnetic field in the axial direction and the case of applying it in the radial direction in order to remove it from the outside, it is found that in terms of the difficulty of applying it, it is better to apply an external magnetic field in the axial direction. It is easy to magnetize, but if you can somehow magnetize it in the radial direction,
It is possible to set Br _R > Br _A , and the residual magnetism can be closed in the radial direction, so that a larger demagnetizing effect can be obtained. The demagnetization method using a through-type coil as shown in FIG. 1 can only demagnetize the steel material in the axial direction, and has the disadvantage that the demagnetization effect is weakened by the influence of the demagnetizing field coefficient mentioned above. An object of the present invention is to provide a demagnetizing method for a demagnetizing device that shortens the demagnetizing time due to residual magnetization of a material to be demagnetized. The demagnetization method of the present invention includes a first cycle in which a forward excitation current and a reverse excitation current having a current value larger than the forward excitation current are passed through the excitation coil, and a polarity opposite to that of the first cycle in the excitation coil. The purpose is to cause an excitation current of 1 to flow in the second cycle. Specific embodiments of the present invention will be described below with reference to FIGS. 4 to 7. In FIG. 4, a U-shaped magnet consisting of a yoke 5, magnetic poles 6 and 7, and an excitation coil 8 is attached to a frame 10 through a non-magnetic material 9 over almost the entire length of the steel material 1. The reason why a plurality of U-shaped magnets are provided is that since a plurality of local demagnetizing fields as explained in FIG. 3 exist at arbitrary points on the steel material, it is necessary to demagnetize each of these demagnetizing fields. If there is a need to demagnetize a plurality of steel materials 1, such as 1A, 1B, 1C, and 1D at the same time, the length l _P of the magnetic poles 6 or 7 may be determined according to the number. The demagnetizing field shown in FIG. 3 is not necessarily in the opposite direction to the demagnetizing field applied from the outside in the length direction of the steel material 1, but its vectors have various directions. Therefore, since it is better for each U-shaped magnet to form an independent magnetic path and for the magnetic flux to have the same direction within the steel material, the non-magnetic material 9
It is better to connect via . FIG. 5 shows the pattern of the excitation current of the excitation coil of the degaussing device shown in FIG. If the material of steel material 1 is ordinary steel, the demagnetization curve becomes R _N as shown in Figure 2b, and - for a positive excitation current of +i, that is, a magnetizing current.
Demagnetization can be easily achieved by passing one cycle of reverse excitation current, that is, demagnetization current of i. For ordinary steel, the demagnetizing current may be 50 to 100% of the magnetizing current. When steel material 1 becomes special steel, the demagnetization curve becomes R _S as shown in Figure 2b, so relatively good demagnetization can be achieved by flowing one cycle of -i' demagnetizing current for +i magnetizing current. Ru.
The demagnetizing device proposed in Fig. 5 is in contact with or close to contact with the steel material 1 and the air gap is small, so the holding force is as shown in Fig. 2b-H _g1 , for example, so it is relatively easy to demagnetize even though it is made of special steel. obtained, −
The appropriate demagnetizing current for i' is 100 to 300% of the magnetizing current. Even among special steels, when it comes to chromium-molybdenum steel or high manganese steel, the demagnetizing field explained in FIG. 3a becomes large and has a complicated distribution, so two-cycle demagnetization, in which the polarity is reversed and demagnetized as shown in FIG. 5, is effective. In other words, the direction of the magnetic flux within the steel material 1 of each U-shaped magnet in Fig. 4 is directed in one direction during magnetization in the first 1/2 cycle, and is directed in the opposite direction during demagnetization in the next 1/2 cycle. Turn in one direction. For the next second cycle, an effective method is to magnetize for 1/2 cycle in the demagnetization direction of the previous one cycle and demagnetize the remaining 1/2 cycle in the magnetization direction of the previous one cycle. The details will be explained using FIG. 6. First, a magnetizing current of +i is applied to magnetize from 0 ₀ to 0 ₁ . Next, this current is cut off and the flow goes from 0 ₁ to 0 ₂ , and a demagnetizing current of -i' is caused to flow until it reaches 0 ₂ → 0 _3. This current is cut off and the flow goes to 0 ₃ → 0 ₄ , completing one cycle. Next, a magnetizing current of -i is passed in the opposite direction to the first cycle, that is, in the demagnetizing direction of the first cycle, magnetizing the magnet from 0 ₄ to 0 ₅ , and then this current is cut to reach the magnetization from 0 ₅ to 0 ₆ . Next, a current of +i' is applied to demagnetize from 0 ₆ to 0 ₇ in the first cycle magnetization direction. The method is to cut off this current at the end so that it reaches 0 ₇ → 0 ₀ and approaches the origin. If you organize it repeatedly

[Table] This is a demagnetization method that creates a demagnetization pattern. FIG. 7 and Table 1 show the results of experiments conducted by the authors using various special steels with the degaussing device shown in FIGS. 4 to 6 above. It can be seen that the demagnetization is approximately 1/2 to 1/5 in the first cycle and approximately 1/5 to 1/10 in the second cycle of polarity reversal demagnetization compared to before demagnetization. In the experimental example, demagnetization was completed in an unprecedentedly short time of 2 seconds for 1 cycle and 4 seconds for 2 cycles.

[Table] As described above, according to the present invention, it is possible to construct a plurality of simple magnets and shorten the demagnetization time for a plurality of magnets simultaneously by one or two demagnetization cycles of forward/reverse switching, thereby significantly improving the efficiency of demagnetization work.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining magnetic flaw detection and residual magnetism, Figure 2 is a diagram explaining the magnitude of residual magnetism, Figure 3 is a diagram explaining other problems with demagnetization, Figures 4 to 5 FIG. 6 is a diagram showing an embodiment of the present invention, and FIGS. 6 and 7 are diagrams explaining the effects of the present invention. 5... Yoke, 6, 7... Magnetic pole, 8... Coil, 1A
~1D...Material to be demagnetized.

Claims (1)

[Claims] 1 A material to be demagnetized having a residual magnetic flux and a material to be demagnetized are arranged in correspondence with a plurality of yokes, an excitation coil is wound around each yoke to form a magnetic path, and the positive direction current flowing through the excitation coil is switched. In a device that demagnetizes the residual magnetic flux of a material to be demagnetized by passing a reverse excitation current, the forward excitation current applied to the excitation coil is switched to allow a reverse excitation current having a larger current value than the forward excitation current to flow. A degaussing method for a degaussing device, characterized in that in a first cycle, and in a second cycle, an excitation current having a polarity opposite to that in the first cycle is caused to flow through the excitation coil.