GRAIN-ORIENTED ELECTRICAL STEEL SHEET AND METHOD OF PRODUCING SAME

Abstract

Disclosed is a grain-oriented electrical steel sheet with extremely low iron loss by means of a magnetic domain refining technique. In a grain-oriented electrical steel sheet having a plurality of magnetic domains refined via a local strain introduction portion, when a direct-current external magnetic field is applied to the steel sheet in a rolling direction, for a magnetic flux leaked from the local strain introduction portion at a position 1.0 mm away from a surface of the steel sheet at a side of the local strain introduction portion, a value obtained by dividing an intensity level of a total leakage magnetic flux by an intensity level of a magnetic flux leaked due to causes other than strain is more than 1.2.

Claims

1.-5. (canceled)

6. A grain-oriented electrical steel sheet comprising a plurality of magnetic domains refined via a local strain introduction portion, wherein when a direct-current external magnetic field is applied to the steel sheet in a rolling direction, for a magnetic flux leaked from the local strain introduction portion at a position 1.0 mm away from a surface of the steel sheet at a side of the local strain introduction portion, a value obtained by dividing an intensity level of a total leakage magnetic flux by an intensity level of a magnetic flux leaked due to causes other than strain is more than 1.2.

7. The grain-oriented electrical steel sheet according to claim 6, wherein a magnetic flux density B.sub.8 is 1.94 T or more.

8. A method of producing the grain-oriented electrical steel sheet as recited in claim 6, comprising: subjecting a grain-oriented electrical steel sheet to final annealing; and then applying magnetic domain refining treatment to a surface of the steel sheet by irradiation with an electron beam while adjusting focusing of the electron beam such that a position where a beam diameter of the electron beam is minimized over an entire irradiation width is located inside the surface of the steel sheet.

9. A method of producing the grain-oriented electrical steel sheet as recited in claim 7, comprising: subjecting a grain-oriented electrical steel sheet to final annealing; and then applying magnetic domain refining treatment to a surface of the steel sheet by irradiation with an electron beam while adjusting focusing of the electron beam such that a position where a beam diameter of the electron beam is minimized over an entire irradiation width is located inside the surface of the steel sheet.

10. A method of producing the grain-oriented electrical steel sheet as recited in claim 6, comprising: subjecting a grain-oriented electrical steel sheet to final annealing; and then applying magnetic domain refining treatment to a surface of the steel sheet by irradiation with a laser beam while adjusting focusing of the laser beam such that a position where a beam diameter of the laser beam is minimized over an entire irradiation width is located inside the surface of the steel sheet.

11. A method of producing the grain-oriented electrical steel sheet as recited in claim 7, comprising: subjecting a grain-oriented electrical steel sheet to final annealing; and then applying magnetic domain refining treatment to a surface of the steel sheet by irradiation with a laser beam while adjusting focusing of the laser beam such that a position where a beam diameter of the laser beam is minimized over an entire irradiation width is located inside the surface of the steel sheet.

12. The method of producing the grain-oriented electrical steel sheet according to claim 8, wherein the position where the beam diameter is minimized is set in a region from inside the surface of the steel sheet at the side of the local strain introduction portion to a mid-thickness part.

13. The method of producing the grain-oriented electrical steel sheet according to claim 9, wherein the position where the beam diameter is minimized is set in a region from inside the surface of the steel sheet at the side of the local strain introduction portion to a mid-thickness part.

14. The method of producing the grain-oriented electrical steel sheet according to claim 10, wherein the position where the beam diameter is minimized is set in a region from inside the surface of the steel sheet at the side of the local strain introduction portion to a mid-thickness part.

15. The method of producing the grain-oriented electrical steel sheet according to claim 11, wherein the position where the beam diameter is minimized is set in a region from inside the surface of the steel sheet at the side of the local strain introduction portion to a mid-thickness part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In the accompanying drawings:

[0056] FIG. 1 is a graph illustrating an example of the relationship between the amount of iron loss reduction and the position where the electron beam diameter is minimized;

[0057] FIG. 2A is a graph illustrating an exemplary measurement result of leakage magnetic flux before stress relief annealing;

[0058] FIG. 2B is a graph illustrating an exemplary measurement result of leakage magnetic flux after stress relief annealing;

[0059] FIG. 3 is a graph illustrating an example of the relationship between the external magnetic field and the intensity level ratio of leakage magnetic flux;

[0060] FIG. 4 is a graph illustrating an example of the relationship between the amount of iron loss reduction and the intensity level ratio of leakage magnetic flux for the position where the electron beam diameter is minimized;

[0061] FIG. 5 is a graph illustrating the details of the intensity level ratio of leakage magnetic flux and the amount of iron loss reduction around a position of 0 mm where the electron beam diameter is minimized;

[0062] FIG. 6 is a graph illustrating an example of the relationship between the amount of iron loss reduction and the position where the laser beam diameter is minimized;

[0063] FIG. 7A is a graph illustrating a pattern of focal positions relative to widthwise positions;

[0064] FIG. 7B is a graph illustrating another pattern of focal positions relative to widthwise positions;

[0065] FIG. 7C is a graph illustrating another pattern of focal positions relative to widthwise positions;

[0066] FIG. 7D is a graph illustrating another pattern of focal positions relative to widthwise positions;

[0067] FIG. 7E is a graph illustrating another pattern of focal positions relative to widthwise positions; and

[0068] FIG. 7F is a graph illustrating another pattern of focal positions relative to widthwise positions.

DETAILED DESCRIPTION

[0069] Hereinafter, a grain-oriented electrical steel sheet and a method of producing the same according to the present disclosure will be specifically described.

[0070] [Grain-Oriented Electrical Steel Sheet]

[0071] The grain-oriented electrical steel sheet disclosed herein comprises a plurality of magnetic domains refined via a local strain introduction portion. Here, when a direct-current external magnetic field is applied in the rolling direction of the grain-oriented electrical steel sheet disclosed herein, magnetic flux leaks from the local strain introduction portion. For the leakage magnetic flux, at a position 1.0 mm away from a surface of the steel sheet at a side of the local strain introduction portion, a value obtained by dividing an intensity level of a total leakage magnetic flux by an intensity level of a magnetic flux leaked due to causes other than strain may be more than 1.2. The grain-oriented electrical steel sheet disclosed herein can be obtained, by, for example, a method for producing the grain-oriented electrical steel sheet according to the present disclosure.

[0072] The type of grain-oriented electrical steel sheet to be subjected to magnetic domain refining treatment is not particularly limited. Any of the conventionally known grain-oriented electrical steel sheets can be suitably used, for example, with or without the use of inhibitor components. The steel sheet may be coated with an insulating film or may not be coated therewith without any problem. However, from the viewpoint of iron loss reduction, it is preferable to use a steel sheet having a chemical composition containing Si in a range of 2.0 mass % to 8.0 mass %. In addition, from the viewpoint of sheet passage performance, it is more preferable to use a steel sheet having a chemical composition containing Si in a range of 2.5 mass % to 4.5 mass %. In industrial terms, it is preferable that the thickness of the grain-oriented electrical steel sheet is 0.10 mm or more. It is preferably 0.35 mm or less. It is preferably set in a range of about 0.10 mm to about 0.35 mm.

[0073] In addition, for steel sheets with wider magnetic domains before subjection to the magnetic domain refining treatment, more magnetic poles need to be generated to refine the magnetic domains, and the conventional technology may not be effective enough to improve the iron loss property. Therefore, for example, the effect of further iron loss reduction by applying the method according to the present disclosure is greater when a steel sheet with wider magnetic domains before subjection to the magnetic domain refining treatment is used. A wider magnetic domain before the magnetic domain refining treatment is translated into a higher magnetic flux density, and the method disclosed herein is more suitable to be applied to steel sheets with a magnetic flux density B.sub.8 of 1.94 T or higher.

[0074] [Method of Producing the Grain-Oriented Electrical Steel Sheet]

[0075] The method of producing the grain-oriented electrical steel sheet disclosed herein is a method of producing the above-described grain-oriented electrical steel sheet, which comprises the same features as those described above for the grain-oriented electrical steel sheet disclosed herein. The method of producing the grain-oriented electrical steel sheet comprises: subjecting the grain-oriented electrical steel sheet to final annealing; and then applying magnetic domain refining treatment to a surface thereof by irradiation with an electron beam or laser beam. In the magnetic domain refining treatment, focusing of the beam is adjusted such that a position where a beam diameter is minimized over an entire irradiation width is located inside the surface of the steel sheet.

[0076] [Local Strain Introduction]

[0077] Local strain introduction can be applied by a method using an electron beam or laser beam. However, it is more preferable to use an electron beam that had a higher effect on iron loss reduction, etc., as demonstrated in the experiments conducted by the inventors described above. In forming a local strain introduction portion, it is important to set the position (focal position) where the beam diameter is minimized over the entire irradiation width inside the surface of the steel sheet. More preferably, this focal position is adjusted to a position from inside the surface (irradiation surface) of the steel sheet at the side of the local strain introduction portion to a mid-thickness part. There is no particular limitation on the way of focal position adjustment, yet in the case of electron beam irradiation, it is preferable to apply dynamic focus control and adjust the focusing current. In the case of laser irradiation, it is preferable to adjust the height of the laser condenser lens (i.e., the distance from the surface of the steel sheet). Although the reason why setting the focal position inside the surface of the steel sheet improves the iron loss reducing effect is uncertain, the inventors believe that it may be because the strain distribution inside the steel sheet in the local strain introduction portion changes even if the volume of closure domains (the volume of the local strain introduction portion) is the same, and as a result, the ratio of magnetic poles generated increases. Although conditions other than the above for magnetic domain refining treatment are not particularly limited, the irradiation direction is preferably a direction that crosses the rolling direction of the steel sheet, more preferably a direction of 60° to 90° relative to the rolling direction, and even more preferably a direction of 90° relative to the rolling direction (i.e., the transverse direction). The irradiation interval is preferably 3 mm or more in the rolling direction. It is preferably 15 mm or less in the rolling direction. It is more preferably in a range of about 3 mm to about 15 mm. When an electron beam is used, the accelerating voltage is preferably 10 kV or higher. It is preferably 200 kV or lower. It is more preferably in a range of 10 kV to 200 kV. The beam current is preferably 0.1 mA or higher. It is preferably 100 mA or lower. It is more preferably in a range of 0.1 mA to 100 mA. The beam diameter is preferably 0.01 mm or more. It is preferably 0.3 mm or less. It is more preferably in a range of 0.01 mm to 0.3 mm. When a laser beam is used, the amount of heat per unit length is preferably 5 J/m or more. It is preferably 100 J/m or less. It is more preferably in a range of about 5 J/m to about 100 J/m. The spot diameter is preferably 0.01 mm or more. It is preferably 0.3 mm or less. It is more preferably in a range of about 0.01 mm to about 0.3 mm.

[0078] Controlling the focal position to a predetermined position, which is a feature of the production method disclosed herein, means that the focal position is defocused from the surface of the steel sheet. For defocusing, several techniques have been reported in, for example, JPS62-49322B (PTL 6), WO2013/0099160 (PTL 7), JP2015-4090A (PTL 8), and JPH5-43944A (PTL 9). Now, the difference between these techniques and the present disclosure is described.

[0079] First, PTL 9 describes a magnetic domain refining technique using an electron beam, in which the focal position is set farther away from the surface of the steel sheet without applying dynamic focusing technology. In some of the examples in PTL 9, the focal position is set outside, rather than inside, the steel sheet, which is clearly different from the present disclosure.

[0080] Also, PTL 6 describes a laser-based magnetic domain refining technique that suppresses peeling of coating through defocusing. Defocusing on the under-focus side is important in the present disclosure, but PTL 6 does not distinguish between upper focus and under focus, and does not suggest that there is a small region on the under-focus side where iron loss could be further reduced. In addition, the technology in PTL 6 is aimed at reducing the amount of strain introduced to minimize the degradation of iron loss properties while reducing the damage to the coating, but is not intended to further reduce iron loss.

[0081] Furthermore, the technologies described in PTLs 7 and 8 are aimed at improving the noise properties and building factors of transformers, and do not focus on further reducing the material iron loss, which is an object of the present disclosure. The examples of PTLs 7 and 8 do not distinguish between upper focus and under focus, and there is no specific statement regarding the degree of defocus.

[0082] [Evaluation Parameters for the Local Strain Introduction Portion]

[0083] The evaluation of the depth and width of closure domains used in conventional strain evaluation cannot evaluate the intended strain distribution state of the grain-oriented electrical steel sheet disclosed herein. To identify the strain state in the grain-oriented electrical steel sheet disclosed herein, the evaluation method using the leakage magnetic flux described above is effective. Specifically, this method uses a magnetic sensor to measure the magnetic flux that is caused to leak above the surface of the steel sheet due to the fact that the magnetic flux that has been passed inside the steel sheet using a magnetizer is difficult to pass through the steel sheet due to strain. The measurement data was subjected to FFT in the direction of the easy magnetization axis, and the absolute value of the complex number of the FFT result was used as the signal intensity level of the leakage magnetic flux (intensity level of the total leakage magnetic flux). This signal intensity level includes not only the leakage magnetic flux due to strain, but also the leakage magnetic flux due to other factors. Therefore, for the strain evaluation, the signal intensity ratio (the ratio of the intensity level of the total leakage magnetic flux to the intensity level of the magnetic flux leaked due to causes other than strain) is used, rather than the above signal intensity level itself. As mentioned above, very good iron loss properties can be obtained when the obtained signal intensity ratio (ratio of the intensity levels of the leakage magnetic flux) is more than 1.2. Preferably, the signal intensity ratio is 2.5 times or more, 3.0 times or more, or 4.0 times or more.

EXAMPLES

Example 1

[0084] The present disclosure will be described in more detail below. The following examples merely represent preferred examples, and the present disclosure is not limited to these examples. Embodiments of the present disclosure may be changed appropriately within the range conforming to the purpose of the disclosure, all of such changes being included within the technical scope of the disclosure.

[0085] Steel slabs (Steel Nos. A and B) containing the components listed in Table 1 with the balance being Fe and inevitable impurities were prepared by continuous casting, heated to 1400° C., and hot rolled to obtain a hot-rolled sheet having a thickness of 2.6 mm, and then subjected to hot-rolled sheet annealing at 950° C. for 10 seconds. Then, each resulting sheet was cold rolled to an intermediate thickness of 0.80 mm, and intermediate annealing was carried out under a set of conditions including an oxidation degree of PH.sub.2O/PH.sub.2=0.35, a temperature of 1070° C., and a duration of 200 seconds. Thereafter, the subscale on the surface was removed by pickling with hydrochloric acid, and then cold rolling was performed again to produce a cold-rolled sheet having a thickness of 0.22 mm.

[0086] Each cold-rolled sheet was then subjected to decarburization annealing in which it was held at a soaking temperature of 860° C. for 30 seconds, then applied with an annealing separator mainly composed of MgO, and then subjected to final annealing at 1220° C. for 20 hours intended for secondary recrystallization, forsterite film formation, and purification. After removing any unreacted annealing separator, a coating liquid containing 50% of colloidal silica and aluminum phosphate was applied, and tension coating baking treatment (baking temperature: 850° C.) also serving as flattening annealing was performed. Then, magnetic domain refining treatment was performed on one surface of each steel sheet, where the surface was irradiated with an electron beam or laser beam in a direction perpendicular to the rolling direction. The irradiation conditions of the electron beam and laser beam were adjusted as in Table 2, and the position where the beam diameter was minimized over the entire irradiation width was also adjusted as in Table 2.

[0087] The evaluation results for iron loss, magnetic flux density, and signal intensity ratio (the value obtained by dividing the intensity level of the total leakage magnetic flux by the intensity level of the magnetic flux leaked due to causes other than strain for the magnetic flux leaked from the local strain introduction portion) are listed in Table 2. With reference to Table 2, comparing between Condition Nos. 4 to 8 with Nos. 14 to 18 and between Condition Nos. 24 to 28 with Nos. 34 to 38, it can be seen that the improvement in the iron loss property at the same focal position was much larger than that at the focal position of 0 mm when grain-oriented electrical steel sheets with a higher magnetic flux density were used, regardless of the strain introduction method.

[0088] Comparing between Condition Nos. 4, 5, 6, 7 (Steel No. A) and Nos. 14, 15, 16, 17 (Steel No. B) where electron beam irradiation was performed with Condition Nos. 24, 25, 26, 27 (Steel No. A) and Nos. 34, 35, 36, 37 (Steel No. B) where laser beam irradiation was performed for each steel grade, it can be seen that for the same steel grade, the signal intensity ratio was larger in the samples with electron beam irradiation and the iron loss reducing effect was also larger in these samples, although both of the electron beam and laser beam conditions are within the scope of the present disclosure. In contrast, for the comparative examples outside the scope of the present disclosure in which the focal position was set on the irradiation surface (i.e., the focal position was set at 0 mm), it can be seen that the iron loss was larger than our examples.

TABLE-US-00001 TABLE 1 Steel sample Chemical composition (mass %) No. C Si Mn Ni Cr P Mo Sb Sn Al N Se S O A 0.03 2.8 0.010 0.01 0.01 0.01 0.001 0.020 0.001 0.025 0.0050 0.001 0.004 0.0010 B 0.07 3.4 0.030 0.07 0.05 0.05 0.010 0.001 0.04 0.028 0.0038 0.012 0.001 0.0013

TABLE-US-00002 TABLE 2 Focal position (mm) with Irradiation reference to Properties of grain-oriented interval in Beam irradiation electrical steel sheet Steel Strain rolling Beam scanning surface Iron loss Magnetic Signal Condition sample introduction direction output rate (−: upper focus, W.sub.17/50 flux density intensity No. No. means (mm) (W) (m/s) +: under focus) (W/kg) B.sub.8 (T) ratio Remarks 1 A Electron beam 4 480 32 1.00 0.78 1.93 1.05 Comparative example 2 Electron beam 4 480 32 0.50 0.75 1.93 1.1 Comparative example 3 Electron beam 4 480 32 0.22 0.74 1.93 1.18 Comparative example 4 Electron beam 4 480 32 0.21 0.71 1.93 2.1 Example 5 Electron beam 4 480 32 0.15 0.69 1.93 2.8 Example 6 Electron beam 4 480 32 0.10 0.68 1.93 3.1 Example 7 Electron beam 4 480 32 0.05 0.68 1.93 3.5 Example 8 Electron beam 4 480 32 0.00 0.73 1.93 1.12 Comparative example 9 Electron beam 4 480 32 −0.05 0.76 1.93 1.08 Comparative example 10 Electron beam 4 480 32 −0.15 0.80 1.93 1.03 Comparative example 11 B Electron beam 4 480 32 1.00 0.80 1.96 1.02 Comparative example 12 Electron beam 4 480 32 0.50 0.76 1.96 1.03 Comparative example 13 Electron beam 4 480 32 0.22 0.74 1.96 1.1 Comparative example 14 Electron beam 4 480 32 0.21 0.65 1.96 3.1 Example 15 Electron beam 4 480 32 0.15 0.62 1.96 3.6 Example 16 Electron beam 4 480 32 0.10 0.61 1.96 4.2 Example 17 Electron beam 4 480 32 0.05 0.61 1.96 4.8 Example 18 Electron beam 4 480 32 0.00 0.68 1.96 1.18 Comparative example 19 Electron beam 4 480 32 −0.05 0.80 1.96 1.05 Comparative example 20 Electron beam 4 480 32 −0.15 0.83 1.96 1.03 Comparative example 21 A Laser beam 8 2000 150 1.00 0.82 1.92 1.04 Comparative example 22 Laser beam 8 2000 150 0.50 0.78 1.92 1.1 Comparative example 23 Laser beam 8 2000 150 0.22 0.77 1.92 1.19 Comparative example 24 Laser beam 8 2000 150 0.21 0.74 1.92 1.8 Example 25 Laser beam 8 2000 150 0.15 0.73 1.92 2.5 Example 26 Laser beam 8 2000 150 0.10 0.72 1.92 2.7 Example 27 Laser beam 8 2000 150 0.05 0.72 1.92 2.8 Example 28 Laser beam 8 2000 150 0.00 0.75 1.92 1.2 Comparative example 29 Laser beam 8 2000 150 −0.05 0.79 1.92 1.1 Comparative example 30 Laser beam 8 2000 150 −0.15 0.81 1.92 1.06 Comparative example 31 B Laser beam 8 2000 150 1.00 0.78 1.94 1.03 Comparative example 32 Laser beam 8 2000 150 0.50 0.71 1.94 1.17 Comparative example 33 Laser beam 8 2000 150 0.22 0.70 1.94 1.19 Comparative example 34 Laser beam 8 2000 150 0.21 0.66 1.94 2.8 Example 35 Laser beam 8 2000 150 0.15 0.65 1.94 3.5 Example 36 Laser beam 8 2000 150 0.10 0.64 1.94 4.0 Example 37 Laser beam 8 2000 150 0.05 0.64 1.94 4.4 Example 38 Laser beam 8 2000 150 0.00 0.68 1.94 1.19 Comparative example 39 Laser beam 8 2000 150 −0.05 0.79 1.94 1.05 Comparative example 40 Laser beam 8 2000 150 −0.15 0.82 1.94 1.03 Comparative example “Signal intensity ratio” represents the value obtained by dividing the intensity level of the total leakage magnetic flux by the intensity level of the magnetic flux leaked due to causes other than strain for the magnetic flux leaked from the local strain introduction portion. “Focal point” represents the position where the beam diameter is minimized over the entire irradiation width.

Example 2

[0089] Steel slabs containing the components of Steel sample No. A listed in Table 1 with the balance being Fe and inevitable impurities were prepared by continuous casting, heated to 1400° C., and hot rolled to obtain a hot-rolled sheet having a thickness of 2.4 mm, and then subjected to hot-rolled sheet annealing at 1000° C. for 30 seconds. Then, each resulting sheet was cold rolled to an intermediate thickness of 1.0 mm, and intermediate annealing was carried out under a set of conditions including an oxidation degree of PH.sub.2O/PH.sub.2=0.30, a temperature of 1050° C., and a duration of 30 seconds. Thereafter, the subscale on the surface was removed by pickling with hydrochloric acid, and then cold rolling was performed again to produce a cold-rolled sheet having a thickness of 0.27 mm.

[0090] Each cold-rolled sheet was then subjected to decarburization annealing in which it was held at a soaking temperature of 820° C. for 120 seconds, then applied with an annealing separator mainly composed of MgO, and then subjected to final annealing at 1180° C. for 50 hours intended for secondary recrystallization, forsterite film formation, and purification. After removing any unreacted annealing separator, a coating liquid containing 50% of colloidal silica and aluminum phosphate was applied, and tension coating baking treatment (baking temperature: 880° C.) also serving as flattening annealing was performed. Then, magnetic domain refining treatment was performed on one surface of each steel sheet, where the surface was irradiated with an electron beam in a direction perpendicular to the rolling direction. The focal position was varied in the widthwise direction of the steel sheet by continuously changing the focus coil. FIGS. 7A to 7F illustrate focal position Patterns 1 to 6 relative to widthwise positions. Other electron beam irradiation conditions are as listed in Table 3. The evaluation samples were taken from the entire irradiation width.

[0091] The obtained evaluation results (iron loss, magnetic flux density, and signal intensity ratio) are listed in Table 3. It can be seen that good iron loss properties were obtained in Pattern Nos. 2 and 5, which are within the range of the present disclosure, where the focal positions were above 0 across the entire width of the steel sheets and the signal intensity ratios were above 1.2. In contrast, the iron loss was larger in Pattern Nos. 1, 3, 4, and 6, which are outside the scope of the present disclosure, where the focal positions were 0 or less or the signal intensity ratios were 1.2 or lower, even partially in the width direction of the steel sheets.

TABLE-US-00003 TABLE 3 Irradiation Properties of grain-oriented interval in Beam electrical steel sheet Steel Strain rolling Beam scanning Focal Iron loss Magnetic Signal Condition sample introduction direction output rate position W.sub.17/50 flux density intensity No. No. means (mm) (W) (m/s) pattern (W/kg) B.sub.8 (T) ratio Remarks 1 A Electron beam 5 1500 90 1 (FIG. 7A) 0.82 1.95 1.12 Comparative example 2 Electron beam 5 1500 90 2 (FIG. 7B) 0.76 1.95 4.2 Example 3 Electron beam 5 1500 90 3 (FIG. 7C) 0.84 1.95 1.02 Comparative example 4 Electron beam 5 1500 90 4 (FIG. 7D) 0.82 1.95 1.1 Comparative example 5 Electron beam 5 1500 90 5 (FIG. 7E) 0.79 1.95 2.4 Example 6 Electron beam 5 1500 90 6 (FIG. 7F) 0.81 1.95 1.15 Comparative example “Signal intensity ratio” represents the value obtained by dividing the intensity level of the total leakage magnetic flux by the intensity level of the magnetic flux leaked due to causes other than strain for the magnetic flux leaked from the local strain introduction portion. “Focal point” represents the position where the beam diameter is minimized over the entire irradiation width.