Wound iron core and method for manufacturing same

Abstract

A wound core 10 having a wound shape, formed by laminating bent steel sheets, in which, in a side view of the wound shape from a direction along the plane of the steel sheet, the wound core 10 has a plurality of plane parts 4 and a plurality of bent parts 5 adjacent to the plane parts 4, which together form a rectangular shape with a central hollow section 15, and in which, in a cross-section perpendicular to the plane of the steel sheet and along the winding direction, a tensile stress of 10 MPa or more is introduced in the thickness direction of the steel sheet at one or more locations in a predetermined region within 30 mm on the plane part side the boundary b of each of the bent parts and the plane parts of the steel sheet.

Description

FIELD

[0001] The present invention relates to a wound core and a manufacturing method thereof.BACKGROUND

[0002] In a wound core fabricated by performing a bending process on a steel sheet without annealing, it is known in the art to reduce core loss by controlling the shape of its bent parts (for example, PTL 1).

[0003] Furthermore, a low core loss grain-oriented electrical steel sheet is known in the art in which a local stress state inside the steel sheet of the grain-oriented electrical steel sheet is controlled to an appropriate condition (PTL 2).[CITATION LIST][PATENT LITERATURE]

[0004] [PTL 1] Japanese Patent No. 7239089 [PTL 2] Japanese Unexamined Patent Publication (Kokai) No. 2008-127632 SUMMARY[TECHNICAL PROBLEM]

[0005] However, with the technique described in PTL 1, significant obstruction of magnetic flux passage was observed in the bent parts, where a large amount of plastic strain is introduced, and there was room for improvement. Furthermore, the technique described in PTL 2 also did not consider at all the influence of obstacles to magnetic flux passage, such as the bent parts where a large amount of plastic strain is introduced, and there was room for improvement.

[0006] In view of the above-mentioned problems, an object of the present disclosure is to provide a wound core that is capable of reducing core loss by means of a local stress state inside a steel sheet formed in the vicinity of a bent part, and a manufacturing method thereof.[SOLUTION TO PROBLEM]

[0007] A summary of the present disclosure is as follows. (1) A wound core having a wound shape, formed by laminating bent steel sheets, wherein, in a side view of the wound shape from a direction along the plane of the steel sheet, the wound core has a plurality of plane parts and a plurality of bent parts adjacent to the plane parts, which together form a rectangular shape with a central hollow section, wherein, in a cross-section perpendicular to the plane of the steel sheet and along the winding direction, a tensile stress of 10 MPa or more is introduced in the thickness direction of the steel sheet at one or more locations in a predetermined region within 30 mm on the plane part side from the boundary of each of the bent parts and the plane parts of the steel sheet. (2) The wound core having laminated steel sheets of (1) above, wherein a ratio SM / Stot is 2% or more, where SM is the number of the steel sheets in which the tensile stress in the thickness direction is introduced within the predetermined region, and Stot is the total number of the laminated steel sheets. (3) The wound core having laminated steel sheets of (1) or (2), wherein a ratio PM / Ptot is 0.1% or more, where PM is an area of a region in which the tensile stress in the thickness direction is introduced within the predetermined region, and Ptot is a total area of the predetermined region. (4) The wound core of any one of (1) to (3), wherein the tensile stress has a maximum value that is equal to or less than the yield stress of the steel sheet. (5) The wound core of any one of (1) to (4), wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, with respect to the regions in which the tensile stress is introduced, the width of one of the regions in the winding direction is 1.4 mm or less. (6) The wound core of any one of (1) to (5), wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, with respect to the regions in which the tensile stress is introduced, the length of one of the regions in the thickness direction is 0.2 times or more and 1 time or less relative to the thickness of the steel sheet. (7) The wound core of any one of (1) to (6), wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, with respect to the regions in which the tensile stress is introduced, the interval between the regions that are adjacent in the winding direction is 2 mm or more and 10 mm or less. (8) The wound core of any one of (1) to (7), wherein, with respect to the region in which the tensile stress is introduced, the point at which the tensile stress is maximum exists at a depth of more than 10 µm from the surface of the steel sheet as a material. (9) The wound core of any one of (1) to (8), wherein the region in which the tensile stress is introduced is provided so as to extend continuously or at predetermined intervals in a direction that intersects the winding direction of the steel sheet, and to traverse the winding direction. (10) The wound core of any one of (1) to (9), wherein the winding direction coincides with the {110}<001> direction of the steel sheet. (11) The wound core of any one of (1) to (10), wherein the winding direction coincides with the rolling direction of the steel sheet. (12) The wound core of any one of (1) to (11), wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, the tensile stress is introduced only in the predetermined region. (13) A method of manufacturing the wound core of any one of (1) to (12), comprising the steps of: performing a bending process on a steel sheet; cutting the steel sheet; laminating the bent and cut steel sheet in layers and assembling them into a wound shape by butting ends of the steel sheets in the winding direction; and introducing a tensile stress in the thickness direction, either in a vicinity of the bent part or in a vicinity of a position that is to be the bent part before cutting. [ADVANTAGEOUS EFFECTS OF INVENTION]

[0008] According to the present disclosure, a wound core that is capable of reducing core loss by means of a local stress state inside a steel sheet formed in the vicinity of a bent part, and a manufacturing method thereof is provided.BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 is a schematic perspective view illustrating one embodiment of a wound core. FIG. 2 is a side view of the wound core illustrated in FIG. 1 as an embodiment. FIG. 3 is a schematic side view illustrating a wound core in another embodiment. FIG. 4 is a schematic diagram illustrating an example of a bent part (curved portion) of a grain-oriented electrical steel sheet. FIG. 5 is a schematic diagram illustrating an example of a grain-oriented electrical steel sheet corresponding to one layer in the wound core body. FIG. 6 is a schematic diagram illustrating an example of a grain-oriented electrical steel sheet corresponding to one layer in the wound core body. FIG. 7A is a schematic diagram illustrating: a detailed shape and arrangement of bent parts in FIGS. 2 or 3; and a region where a region in which a tensile stress is introduced in the thickness direction is to be provided. FIG. 7B is a diagram for explaining a stress arrangement region M(n). FIG. 8A is a schematic diagram illustrating a cross-section of a steel sheet, and illustrates a state in which regions 12 where a tensile stress is introduced in the thickness direction are periodically provided in the X-axis direction (winding direction). FIG. 8B is a perspective view of a steel sheet, and schematically illustrates a state in which a region where a tensile stress is introduced in the thickness direction is provided in a direction that intersects the winding direction (the Y-axis direction, which is orthogonal to the winding direction). FIG. 9A is a diagram for explaining an effect of providing a region in which a tensile stress is introduced in the thickness direction. FIG. 9B is a diagram for explaining an effect of providing a region in which a tensile stress is introduced in the thickness direction. FIG. 10 is a schematic diagram illustrating a manufacturing apparatus for a wound core in the form of a Unicore. FIG. 11 is a diagram for explaining detailed dimensions of the wound core. DESCRIPTION OF EMBODIMENTS

[0010] Hereinafter, some embodiments according to the present invention will be described with reference to the drawings. However, these descriptions are intended merely to illustrate preferred embodiments of the present invention, not to limit the present invention to such specific embodiments. In the following description, the same reference numbers are given to similar components.

[0011] Hereinafter, a wound core according to an embodiment of the present invention will be described in detail in order. However, the present invention is not limited only to the configurations disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention. Note that the following numerical limited ranges include the lower limit value and the upper limit value in those ranges. For numerical values indicated with "more than" or "less than," the values are not included in the numerical ranges. Furthermore, "%" regarding chemical composition means "% by mass" unless otherwise specified.

[0012] Furthermore, as used herein, terms specifying shapes, geometric conditions, and their degrees, such as "parallel," "perpendicular," "identical," and "right-angled," as well as lengths, angles, and other values are to be interpreted as including a range to the extent that a similar function can be expected, without being bound by their strict meaning.

[0013] Furthermore, as used herein, "grain-oriented electrical steel sheet" may be simply described as "steel sheet" or "electrical steel sheet," and "wound core" may be described as "wound core body" or simply "core."

[0014] A wound core according to an embodiment of the present invention is a wound core having a wound shape formed by laminating bent steel sheets, wherein, in a side view of the wound shape from a direction along the plane of the steel sheet, the wound core has a structure having four plane parts and four corner parts adjacent to the plane parts, which together form a rectangular shape with a central hollow section. Here, the plane part refers to a linear portion, excluding the bent parts. An inner surface side radius of curvature r of the bent part in a side view is, for example, 1.0 mm or more and 5.0 mm or less. The grain-oriented electrical steel sheet has a chemical composition, for example, containing 2.0 to 7.0% by mass of Si, with the remainder being Fe and impurities, and has a texture oriented in the Goss orientation. As the grain-oriented electrical steel sheet, for example, a grain-oriented electrical steel strip described in JIS C 2553:2019 can be adopted.

[0015] Next, the shapes of the wound core and the grain-oriented electrical steel sheet according to an embodiment of the present invention will be specifically described. The shapes themselves of the wound core and the grain-oriented electrical steel sheet described here are not particularly novel, and merely conform to the shapes of known wound cores and grain-oriented electrical steel sheets.

[0016] FIG. 1 is a schematic perspective view illustrating one embodiment of a wound core. FIG. 2 is a side view drawing of the wound core illustrated in FIG. 1 as an embodiment, and illustrates a side view of the wound core. Furthermore, FIG. 3 is a schematic side view illustrating another embodiment of a wound core.

[0017] Note that a side view is a case where the wound shape of a wound core is viewed from a direction along the plane of the steel sheet. More specifically, a side view refers to a case where a wound core is viewed from the axial direction of the winding of the wound core (the direction perpendicular to the plane of the paper in FIG. 2). In other words, a side view means viewing in the width direction (Y-axis direction in FIG. 1) of the elongated grain-oriented electrical steel sheet that constitutes the wound core. A side view drawing is a drawing that represents the shape visually recognized by a side view (a view in the Y-axis direction of FIG. 1).

[0018] A wound core 10 according to an embodiment of the present invention is provided with a wound core body having a substantially polygonal shape in a side view. The wound core body 10 has a laminated structure with a substantially rectangular shape in the side view, in which grain-oriented electrical steel sheets 1 are stacked in the thickness direction. The wound core body 10 may be used as a wound core as it is, or may be provided with a known fastener or the like, such as a binding band, in order to integrally fix the plurality of stacked grain-oriented electrical steel sheets as necessary.

[0019] In this embodiment, there is no particular limitation on the core length of the wound core body 10. Even if the core length of the wound core 10 changes, the volume of the bent parts 5 is constant, and therefore if the number of bent parts 5 is the same, the core loss that occurs in the bent parts 5 is constant. The longer the core length, the smaller the volume ratio of the bent parts 5 to the wound core body 10, and therefore the less influence on core loss deterioration. Therefore, it is preferable that the core length of the wound core body 10 is longer. The core length of the wound core body 10 is preferably 1.5 m or more, and more preferably 1.7 m or more. Note that, in the present invention, the core length of the wound core body 10 refers to the circumferential length at a center point in the stacking direction of the wound core body 10 in a side view. Such a wound core can be suitably used for any conventionally known applications.

[0020] The core according to this embodiment is characterized by having a substantially polygonal shape in a side view. In the following description using the drawings, for the sake of simplicity of illustration and explanation, a core with a substantially rectangular shape (a quadrilateral), which is a general shape, will be described. However, cores with various shapes can be manufactured depending on the angle and number of the bent parts 5 and the length of the plane parts. For example, if the angles of all the bent parts 5 are 45° and the lengths of the plane parts 4 are equal, the side view will be an octagon. Furthermore, if six bent parts 5 with an angle of 60° exist, and the lengths of the plane parts 4 are equal, the side view will be a hexagon.

[0021] As illustrated in FIGS. 1 and 2, the wound core body 10 includes a portion where grain-oriented electrical steel sheets 1, in which plane parts 4 and 4a and bent parts 5 are arranged alternately in succession in the longitudinal direction, are stacked in the thickness direction, and has a laminated structure 2 having a substantially rectangular shape and having a hollow section 15 in a side view. A corner part 3 including bent parts 5 has two or more bent parts 5 with a curved shape in a side view, and the sum of the respective bending angles of the bent parts 5 existing in one corner part 3 is, for example, 90°. The corner part 3 has a plane part 4a, which is shorter than the plane part 4, between adjacent bent parts 5. Therefore, the corner part 3 has a form having two or more bent parts 5 and one or more plane parts 4a. Note that in the embodiment illustrated in FIG. 2, the angle of one bent part 5 is 45°. In the embodiment illustrated in FIG. 3, the angle of one bent part 5 is 30°.

[0022] As illustrated in these examples, the wound core of this embodiment can be configured with bent parts having various angles. From the viewpoint of suppressing the generation of strain due to deformation during processing and reducing core loss, the bending angles φ (φ1, φ2, and φ3) of a bent part 5 are preferably 60° or less, and more preferably 45° or less. Bending angles φ of bent parts included in one core can be configured as desired. For example, it is possible to set φ1 = 60° and φ2 = 30°. From the viewpoint of production efficiency, it is preferable that the bending angles are equal. However, if, by reducing the number of locations with deformation beyond a certain level, the core loss of the core to be manufactured can be reduced due to the core loss of the steel sheet to be used, processing using a combination of different angles may be employed. The design can be selected as desired based on the important points in core processing.

[0023] The bent part 5 will be described in further detail with reference to FIG. 4. FIG. 4 is a schematic diagram illustrating an example of a bent part (curved portion) 5 of a grain-oriented electrical steel sheet 1. A bending angle of a bent part 5 means an angular difference generated between a plane part on a rear side and a plane part on a front side in the bending direction in a bent part of a grain-oriented electrical steel sheet. The bending angle is represented as an angle φ, which is the supplementary angle of the angle formed by two virtual lines, Lb-elongation 1 and Lb-elongation 2, obtained by extending the linear portions that are the surfaces of the plane parts 4 and 4a on both sides sandwiching the bent part 5, on the outer surface of the grain-oriented electrical steel sheet 1. Here, the points at which the extending straight lines depart from the steel sheet surface are the boundaries between the plane part 4 and the bent part 5 on the outer surface side of the steel sheet, and are points F and G in FIG. 4.

[0024] Furthermore, from each of points F and G, straight lines perpendicular to the outer surface of the steel sheet are extended, and the intersections with the surfaces in the inner surface side of the steel sheet are designated as points E and D, respectively. These points E and D are the boundaries between the plane part 4 and the bent part 5 on the surface in the inner surface side of the steel sheet.

[0025] Then, in the present invention, the bent part 5 is a portion of the grain-oriented electrical steel sheet 1, surrounded by the above-mentioned points D, E, F, and G in a side view of the grain-oriented electrical steel sheet 1. In FIG. 4, the steel sheet surface between point D and point E, that is, the inner surface of the bent part 5, is designated as La, while the steel sheet surface between point F and point G, that is, the outer surface of the bent part 5, is designated as Lb.

[0026] Furthermore, this drawing illustrates an inner radius of curvature r of the bent part 5 in a side view. The inner radius of curvature r of the bent part 5 is obtained by approximating La with a circular arc that passes through points E and D. The smaller the inner radius of curvature r, the sharper the curve of the curved portion of the bent part 5 is, while the larger the inner radius of curvature r, the gentler the curve of the curved portion of the bent part 5 is.

[0027] In the wound core of the present invention, the radius of curvature r in each bent part 5 of each grain-oriented electrical steel sheet 1 stacked in the thickness direction may vary to some extent. This variation may be caused by forming accuracy, or an unintended variation may occur due to handling during lamination or the like. Such unintended errors can be suppressed to about 0.2 mm or less in current normal industrial manufacturing. When such variation is large, a representative value can be obtained by measuring the radius of curvature for a sufficiently large number of steel sheets and averaging them. Furthermore, it is also conceivable to intentionally change the radius of curvature for some reason, but the present invention does not exclude such embodiments. It is preferable that r, the radius of curvature of the bent part 5 (the inner radius of curvature in a side view of the bent part 5), is 1 mm or more and 5 mm or less. By setting the radius of curvature r to 1 mm or more and 5 mm or less, the building factor (BF) can be further reduced.

[0028] Note that there is no particular limitation on the method for measuring the radius of curvature r of the bent part 5, but it can be measured, for example, by observation at 200× magnification using a commercially available microscope (Nikon ECLIPSE LV150). Specifically, the center of curvature, point A, is determined from the observation result. As a method for determining point A, for example, line segments EF and DG are extended to the inner side opposite to point B, and their intersection is defined as A. Then, the magnitude of the radius of curvature r corresponds to the length of line segment AC. Here, point A and point B are drawn by a straight line, and the intersection with the inner circular arc DE of the bent part of the steel sheet is defined as C.

[0029] FIGS. 5 and 6 are schematic diagrams each illustrating an example of a grain-oriented electrical steel sheet 1 corresponding to one layer in the wound core body. The grain-oriented electrical steel sheet 1 used in the examples in FIGS. 5 and 6 has been subjected to a bending process in order to realize a wound core in the form of a Unicore, has two or more bent parts 5 and two or more plane parts 4, and forms a substantially polygonal ring in a side view via one or more joint parts 6 (gaps), which are the end faces in the longitudinal direction of the grain-oriented electrical steel sheet 1.

[0030] In this embodiment, it is sufficient that the wound core body 10 has a laminated structure in a substantially polygonal shape as a whole in a side view. As illustrated in the example of FIG. 5, one grain-oriented electrical steel sheet may constitute one layer of the wound core body via one joint part 6 (one grain-oriented electrical steel sheet is connected via one joint part 6 per turn). Alternatively, as illustrated in the example of FIG. 6, one grain-oriented electrical steel sheet 1 may constitute about half a turn of the wound core, and two grain-oriented electrical steel sheets 1 may constitute one layer of the wound core body via two joint parts 6 (two grain-oriented electrical steel sheets 1 are connected to each other via two joint parts 6 per turn).

[0031] The sheet thickness of the grain-oriented electrical steel sheet 1 used in this embodiment is not particularly limited and may be appropriately selected according to the application or the like, but it is usually in the range of 0.15 mm to 0.35 mm, and preferably in the range of 0.18 mm to 0.27 mm.

[0032] In the case of the example in FIG. 5, one joint part 6 of the one grain-oriented electrical steel sheet constituting one layer of the wound core body is located in a region A1 illustrated in FIGS. 2 and 3. Furthermore, in the case of the example in FIG. 6, the two joint parts 6 of the two grain-oriented electrical steel sheets constituting one layer of the wound core body are located in regions A1 and A2 illustrated in FIGS. 2 and 3. Note that in FIGS. 2 and 3, the detailed shape and arrangement of the joint part 6 in the regions A1 or A2 are omitted from the illustration.

[0033] FIG. 7A is a schematic diagram for explaining: a detailed shape and arrangement of bent parts 5 in FIGS. 2 or 3; and a region where a region 12 in which a tensile stress is introduced in the thickness direction is to be provided. FIG. 7A schematically illustrates a cross-section perpendicular to the plane of the steel sheet and along the winding direction.

[0034] The present inventors intensively studied the influence of the stress state inside the steel sheet in the vicinity of bent parts 5 on the core characteristics, and found that controlling the strain in the vicinity of bent parts is particularly effective for reducing the core loss of a wound core having bent parts 5 with relatively small radius of curvature. Based on this finding, in this embodiment, the tensile stress state (strain distribution) in the thickness direction inside the steel sheet is controlled in the vicinity of bent parts 5 of the wound core.

[0035] Although this mechanism itself is not completely clear at the present time, it can be presumed from the results of intensive studies by the present inventors that this is because, in bent parts 5 in which the plastic strain is very large and through which it would normally be difficult for magnetic flux to pass, relatively small strains arranged in the vicinity of the bent parts 5 makes it easier for the magnetic flux to pass through the inside of the steel sheet (inside the bent parts 5). At the boundary of a bent part 5 of the wound core 10, the magnetic flux needs to enter the bent part 5, where a large amount of plastic strain has been introduced and the crystallinity has been largely lost, from the plane part 4, which is a nearly perfect crystal with almost no strain. It is considered that when the magnetic flux enters a region where the crystalline states (that is, the magnetic permeability of the steel sheet) differs greatly in this way, smooth entry of the magnetic flux is greatly obstructed. As a result, along the innermost and outermost peripheries of the laminated steel sheets 1 of the wound core 10, the magnetic flux is more likely to leak out of the steel sheet. An adverse effect also occurs in the central portion in the stacking direction of the laminated steel sheets 1 due to the obstruction of the smooth entry of the magnetic flux into bent parts 5. In this case, if a relatively small strain exists in the vicinity of a bent part 5, the steepness of the change in crystallinity (magnetic permeability) is mitigated, and the strain acts to make the entry of the magnetic flux relatively smooth from the plane part 4, which is a perfect crystal and has a very high magnetic permeability, to the bent part 5, where the crystallinity has been largely lost and the magnetic permeability is very low. This results in reduction of the adverse effect due to the entry of magnetic flux into the bent part 5 in the central portion of the laminated steel sheets 1, as well as reduction of the leakage of magnetic flux out of the steel sheet in the innermost and outermost peripheries of the laminated steel sheets. Therefore, the relatively small strain arranged in the vicinity of bent parts 5 can be represented by a tensile stress in the thickness direction as an index. Accordingly, it is considered that these make it possible to suppress the generation of core loss in bent parts 5 of the wound core 10. In the following description, the mechanism of action of the tensile stress in the thickness direction in the vicinity of the bent part 5 in the present invention will be explained on the premise of the above-mentioned phenomenon (smooth entry of magnetic flux to the bent part 5), but it is noted that this is merely one current hypothesis to theoretically support the empirical rules found by the present inventors as a result of intensive studies. It is hoped that the academically correct mechanism will be elucidated in the future.

[0036] In this embodiment, as illustrated in FIG. 7A, the grain-oriented electrical steel sheet 1 is provided with a continuous region 12 (hereinafter also referred to as a stress region 12) in which a tensile stress of 10 MPa or more is introduced in the thickness direction. Regarding the stress value inside the sheet thickness, the strains of the crystal lattice in three directions were measured by an X-ray diffraction method, and the stress values in the rolling direction, the thickness direction, and the sheet width direction were determined based on the material property values such as the elastic modulus. These crystal strains in three directions can be determined by irradiating the steel sheet with X-rays and detecting and evaluating the reflected X-rays and the transmitted X-rays. Specifically, the strain in the thickness direction is determined by moving the steel sheet (test piece) during scanning, and evaluating the shift in the diffraction angle (2θ) of the diffraction peak of the reflected X-ray by a reflection method. Furthermore, the strains in the rolling direction and the sheet width direction are determined by evaluating the shift in the diffraction angle of the diffraction peak of the transmitted X-ray by a transmission method. Then, the conversion from strain to a stress value can be performed with a relational expression between the stress and strain in three directions according to the material property values (Young's modulus, Poisson's ratio). These methods are known in the art, and it is possible to determine the stress value inside the sheet thickness as described above by using, for example, the residual stress distribution measurement by a strain scanning method using high-energy synchrotron radiation - a method described in (Reference: Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 711, 2005, pp. 1530).

[0037] A stress region 12 is basically provided in the vicinity of the bent part 5 of each steel sheet. When focusing on any one bent part 5, it is preferable to determine the region where a stress region 12 should be placed. Hereinafter, the region where a stress region 12 should be placed (hereinafter also referred to as the stress arrangement region) will be described in detail.

[0038] First, the stress arrangement region is set in a region inward for a distance two or more times the steel sheet thickness from the ends in the in-plane direction (the direction along the surface of the steel sheet) of the steel sheet member constituting the wound core. The ends in the in-plane direction of the steel sheet member are generally formed by a shearing process and are thus largely plastically deformed, and therefore are highly likely to be in a stress state that is not preferable for the stress region intended by the present application. For this reason, the stress arrangement region shall exclude such an end region. The end region, in the wound core illustrated in FIG. 1, is a region extending from the surface in the Y-axis direction, that is, the surface when the wound core is viewed in a side view, inward into the steel sheet (the Y-axis direction), for the distance of two times the sheet thickness.

[0039] Hereinafter, the stress arrangement region provided in the vicinity of the bent part 5 will be described in detail. FIG. 7A illustrates a cross-section along the XZ plane with respect to one corner part 3 of the wound core 10, separated by a distance of two times or more the sheet thickness inward into the steel material from the Y-axis end of the wound core 6. Here, the phrase "a cross-section separated by a distance of two times or more the sheet thickness inward into the steel material" is intended to avoid the above-mentioned end region. As illustrated in FIG. 7A, a corner part 3 has two bent parts 5, stress regions 12 are provided in plane parts 4 in the vicinity of bent parts 5 of steel sheets.

[0040] FIG. 7B is a diagram for explaining the stress arrangement region M(n), and illustrates, in an enlarged view of FIG. 7A, only the n-th steel sheet 1 from the innermost side in the direction from the inside to the outside of the wound core body 10 (the stacking direction of the steel sheets). Here, the third steel sheet from the inside in FIG. 7A is illustrated as the n-th steel sheet. As illustrated in FIG. 7B, in the n-th steel sheet from the innermost side, the stress arrangement region M(n) is a region from the boundary b between the plane part 4 and the bent part 5 toward the plane part 4 for a predetermined distance L (L0 = 30 mm). In FIG. 7B, the stress arrangement region M(n) is illustrated with hatching.

[0041] Note that when one steel sheet, for example, the n-th steel sheet has a plurality of bent parts 5, and stress arrangement regions M(n) are set for one bent part 5 as described above, it is conceivable that if the distance between the adjacent bent parts 5 is short, the stress arrangement region may reach the adjacent, different bent part 5. In such a case, the stress arrangement region shall not deviate from the plane part 4 and shall not be a region that includes the adjacent, different bent part 5. Furthermore, it is conceivable that if the distance between a bent part 5 and a joint part 6 is short, the stress arrangement region may reach the joint part 6. In such a case, the stress arrangement region shall not be a region that includes the joint part 6 and the above-mentioned end region formed in association with the joint part 6. Thus, in such situations, the length of the stress arrangement region in that direction is less than 30 mm.

[0042] FIG. 8A is a schematic diagram illustrating a cross-section of a steel sheet, and illustrates a state in which a plurality of stress regions 12 are provided in the X-axis direction (winding direction). The stress regions 12 may be provided at predetermined intervals in the X-axis direction in the cross-section of the steel sheet. In other words, the stress regions 12 may be provided periodically. Therefore, the stress regions 12 are arranged in the stress arrangement region, but may be arranged in the entire area of the steel sheet other than the stress arrangement region.

[0043] In FIG. 8A, it is preferable that the width D in the X-axis direction (the width in the winding direction) of a stress region 12 is 1.4 mm or less. The width D is preferably 0.3 to 1.1 mm, and more preferably 0.7 to 1.1 mm. It is preferable that the length L in the Z-axis direction (the width in the thickness direction) is 0.2*t ≤ L < t. The length L is more preferably 0.4*t ≤ L < 0.8*t.

[0044] The maximum stress of the stress region 12 desirably exists at a depth of more than 10 µm from the steel sheet surface. If the maximum stress of the stress region 12 exists in a region shallower than 10 µm, the bending of the magnetic flux during its transfer is steep, and as a result, eddy current loss increases, and the effect of reducing the core loss is reduced. Therefore, the maximum stress of the stress region 12 exists at a depth of more than 10 µm from the steel sheet surface, and preferably exists at a depth between 30 and 80 µm, and more preferably at a depth between 40 and 70 µm. When the outer side of the bent part formed by the bending process is taken as the surface, it is desirable that the stress region 12 exists at the above-described depth. However, even when the inner side of the bent part is taken as the surface, the effect does not change as long as the stress region 12 exists at the above-described depth.

[0045] When the stress regions 12 are periodically provided at predetermined intervals in the X-axis direction, the interval in the X-axis direction of the stress regions 12 affects magnetic domain refining due to the interaction between the respective adjacent stress regions 12. Therefore, if the interval is too large, the effect of reducing the core loss decreases. In order to sufficiently reduce the core loss, the interval of the stress regions 12 is preferably 10.0 mm or less, and more preferably 7.0 mm or less. The interval of the stress regions 12 is desirably about 5 mm. However, on the other hand, since the stress regions 12 can also be an obstacle to the magnetic flux in the in-plane direction of the steel sheet (magnetization direction), and too narrow interval would cause increase in the core loss, it is preferable that the interval in the X-axis direction of the stress regions 12 is 2 mm or more.

[0046] FIG. 8B is a perspective view of a steel sheet, and schematically illustrates a state in which the stress region 12 is provided in a direction that intersects the winding direction (in FIG. 8B, the Y-axis direction that is orthogonal to the winding direction). As illustrated in FIG. 8B, the stress region 12 may extend in a direction that intersects the winding direction, and be provided so as to traverse the winding direction. Furthermore, the stress region 12 may be provided in a direction that intersects the winding direction continuously, or periodically at predetermined intervals. In order to make the transfer of magnetic flux in the vicinity of the bent part smoother and to further suppress the reduction in the core loss, it is preferable that the stress region 12 extends in a direction that forms an angle of 60 to 120° with respect to the X-axis direction, in a field of view along the Z-axis. More preferably, the stress region 12 is extended in a direction that forms an angle of 80 to 110°.

[0047] In the present invention, the stress region 12 is defined as a continuous region having a tensile stress in the thickness direction of 10 MPa or more. This is because if the tensile stress in the thickness direction is less than 10 MPa, the influence on the magnetic permeability is small, and therefore the effect (the effect of assisting the smooth entry of magnetic flux from the plane part 4 having a high magnetic permeability to the bent part 5 having a low magnetic permeability) of providing a region in which a tensile stress is introduced cannot be sufficiently obtained.

[0048] Furthermore, in the stress region 12, it is preferable that the maximum value of the tensile stress in the thickness direction is equal to or less than the yield stress of the steel sheet as a material. A larger maximum value of the tensile stress would lead to an increase in the plastic region, and a significant decrease in the magnetic permeability due to plastic strain, resulting in the deterioration of the magnetic properties of the plane part 4 itself, and the worsening of the core loss of the core. In general, the point at which the stress state changes greatly from an elastic region to a plastic region can be defined by the yield stress of the material.

[0049] For the reasons above, in this embodiment, it is preferable that the maximum value of the tensile stress in the thickness direction in the stress region 12 is equal to or less than the yield stress value of the material. More preferably, the maximum value of the tensile stress is 80 to 200 MPa.

[0050] FIGS. 9A and 9B are diagrams for explaining the effect of providing stress regions 12. As described above, it is noted that the description of FIGS. 9A and 9B is one possibility for the mechanism of action of the present invention. In FIG. 9A, the flows of magnetic flux in a case where stress regions 12 are provided are represented as arrows.

[0051] On the other hand, in FIG. 9B, the flows of magnetic flux in a case where stress regions 12 are not provided are represented as arrows. When the stress region 12 is not provided, compared to the case where the stress region 12 is provided, the leakage of magnetic flux out of the steel sheet is larger at the innermost and outermost peripheries of the laminated steel sheets 1. In addition, in the central portion of the laminated steel sheets 1, the smooth entry of magnetic flux from the plane part 4 to the bent part 5 is obstructed. As a result, the core loss of the core increases.

[0052] In the present invention, the stress region 12 and the stress arrangement region M are defined as described above. In the present invention, the effect of the invention is exhibited by the presence of the stress region 12 within the stress arrangement region M. In this case, it is not necessary for the entire region of one stress region 12 to exist within the stress arrangement region M, and the effect of the invention is exhibited if even a part of one stress region 12 overlaps with the stress arrangement region M. Furthermore, the effect of the invention is exhibited as long as the stress region 12 and the stress arrangement region M overlap even if only in a part of steel sheets in the laminated steel sheets. With respect to the degree of the above-mentioned overlap, when observing a region covering 1 / 2 of the lamination thickness in the Z-axis direction and 1 / 2 of the lamination thickness in the X-axis direction, it is preferable that PM / Ptot is 0.1% or more, where Ptot is the total area of the stress arrangement region M in the observation region, and PM is the total area of the stress region 12 within the stress arrangement region M. If PM / Ptot is less than 0.1%, a sufficient effect of the invention cannot be obtained. PM / Ptot is preferably 2% or more, and more preferably 5% or more. However, since the stress region 12 can also be an obstacle to the magnetic flux in the in-plane direction of the steel sheet (magnetization direction), the existence of excessive stress regions 12 within the stress arrangement region M would also cause deterioration of the core loss. Therefore, in a preferred embodiment, PM / Ptot is 40% or less, and more preferably 30% or less.

[0053] In the present invention, the degree of this overlap is defined, for the laminated steel sheets, by the ratio of the number of the steel sheets SM, in which a stress region 12 exists within a stress arrangement region M, to the total number of the laminated sheets Stot. A steel sheet in which a stress region 12 exists within a stress arrangement region M is a steel sheet in which at least one stress region 12 exists within at least one stress arrangement region M. In the present invention, SM / Stot is preferably 2% or more. SM / Stot is preferably 5% or more, and more preferably 10% or more. Of course, a preferable embodiment is where stress regions 12 exist within stress arrangement regions M in all the steel sheets, and SM / Stot is 100%.

[0054] Since the stress region 12 can be an obstacle to the magnetic flux in the in-plane direction of the steel sheet and can be a cause of core loss deterioration, the stress region 12 may be provided only within the stress arrangement region M, and not outside the stress arrangement region M. That is, the tensile stress may be introduced only within the stress arrangement region M.

[0055] Note that the winding direction of the wound core may be considered as the rolling direction of the steel sheet. In addition, the winding direction of the wound core may be considered as the {110}<001> direction of the steel sheet. Furthermore, in the descriptions of FIGS. 7A, 7B, 8A, 8B, 9A, and 9B, the X-axis direction may be considered as the rolling direction of the steel sheet, and may be considered as the {110}<001> direction of the steel sheet.

[0056] The method for manufacturing the grain-oriented electrical steel sheet is not particularly limited, and a conventionally known method for manufacturing a grain-oriented electrical steel sheet can be appropriately selected. A preferred, specific example of the manufacturing method includes: heating a slab having a chemical composition of 0.04 to 0.1% by mass of C, the balance being as described above for the grain-oriented electrical steel sheet, to 1000°C or higher and performing hot rolling; performing hot-rolled sheet annealing as necessary; then performing cold rolling once or two or more times with an intermediate annealing to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet to 700 to 900°C in, for example, a wet hydrogen-inert gas atmosphere for decarburization annealing; further performing nitriding annealing as necessary; applying an annealing separator; performing finish annealing at about 1000°C; and forming an insulating coating at about 900°C.

[0057] In this embodiment, the wound core, which is constituted by the grain-oriented electrical steel sheet 1 having the form as described above, is formed by laminating in layers the grain-oriented electrical steel sheets 1 that have been individually bent and assembling them into a wound shape. A plurality of the grain-oriented electrical steel sheets 1 are connected to each other via at least one joint part 6 per turn.

[0058] In this embodiment, the method for introducing a tensile stress in the thickness direction into the interior of the grain-oriented electrical steel sheet is, for example, a method of irradiating with a laser, an electron beam, or the like, and by irradiating in a medium such that the viscosity (mPa·s) of the aqueous solution is 2 to 8, the vibration of the steel sheet due to the irradiation of the laser or electron beam is reduced, and the intended stress distribution is more easily obtained. If the viscosity exceeds 8, it becomes difficult to introduce the stress. A method performed in a situation where the viscosity is controlled by the concentration and temperature of the aqueous solution is desirable because the conditions can be appropriately adjusted. For example, a method using an aqueous ethanol solution with a concentration of 40 to 60% at a temperature of -10°C to 20°C is a preferable example.

[0059] Furthermore, regarding the timing for introducing the tensile stress to the steel sheet, the tensile stress may be introduced at the stage of the coil before the steel sheet is cut in the rolling direction, or at the stage of the bending process.

[0060] An apparatus that enables the manufacturing of a wound core involving the bending of a steel sheet as described above is schematically illustrated in a block diagram in FIG. 10. FIG. 10 schematically illustrates a manufacturing apparatus 70 for a wound core in the form of a Unicore. The manufacturing apparatus 70 comprises a bending processing section 71 that individually performs a bending process on the grain-oriented electrical steel sheets 1. The manufacturing apparatus 70 may also comprise an assembly section 72 where the bent grain-oriented electrical steel sheets 1 are laminated in layers and assembled into a wound shape, thereby forming a wound core having a wound shape comprising a portion where the grain-oriented electrical steel sheets 1 with plane parts 4 and bent parts 5 being alternately continuous in the longitudinal direction are stacked in the thickness direction.

[0061] The grain-oriented electrical steel sheet 1 is supplied to the bending processing section 71 by being drawn out at a predetermined transport speed from a steel sheet supply section 75 that holds a hoop material formed by winding the grain-oriented electrical steel sheet 1 in a roll shape. The grain-oriented electrical steel sheet 1 supplied in this manner is subjected to a bending process in the bending processing section 71, in which the grain-oriented electrical steel sheet 1 is cut into appropriate sizes as needed and is bent individually in small batches, such as one sheet at a time.

[0062] The bending processing section 71 comprises a tensile stress introduction section 71a. The tensile stress introduction section 71a introduces a tensile stress into the grain-oriented electrical steel sheet 1 by the above-mentioned method, either before cutting the grain-oriented electrical steel sheet 1 or during the bending process after cutting the grain-oriented electrical steel sheet 1.

[0063] When introducing the tensile stress only in the stress arrangement region M, the stress arrangement region M in each individual grain-oriented electrical steel sheet 1 is determined on the basis of the design, based on the planned position of each individual grain-oriented electrical steel sheet 1 when the grain-oriented electrical steel sheets 1, in which the plane parts 4 and the bent parts 5 are alternately continuous, are stacked in the thickness direction. Thus, the tensile stress can be introduced only into the stress arrangement region M by irradiating, in each individual grain-oriented electrical steel sheet 1, an assumed position that will be the stress arrangement region M on the basis of the design, with a laser, an electron beam, or the like.

[0064] In the grain-oriented electrical steel sheet 1 thus obtained, the radius of curvature of the bent part 5 generated in the bending process is extremely small, and thus the processing strain imparted to the grain-oriented electrical steel sheet 1 by the bending process is extremely small. In this way, while it is assumed that the density of the processing strain will increase, if the volume affected by the processing strain can be made smaller, the annealing process can be omitted.

[0065] The present disclosure is not limited to the above-described embodiments. The above-described embodiments are illustrative, and any embodiment that has substantially the same configuration as the technical idea described in the claims of the present disclosure and exhibits similar functions and effects is included in the technical scope of the present disclosure.EXAMPLES

[0066] Hereinafter, the present disclosure will be specifically described by illustrating Examples. Note that the conditions in Examples are an example adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to the conditions in Examples. The present disclosure can adopt various conditions as long as they do not depart from the spirit of the disclosure and achieve the object of the disclosure.

[0067] In Examples, as a method for introducing a tensile stress into a steel sheet, a method of irradiating with a laser is used. First, a laser was irradiated onto a steel sheet in ethanol at a temperature of about 3°C to 15°C to introduce a tensile stress into the steel sheet. A laser was also irradiated in air, in water at 24°C, or in ethanol at 24°C to introduce a tensile stress into a steel sheet.

[0068] At this time, the beam diameter was set to 150 µm under all conditions, the output was appropriately adjusted, and a laser was irradiated in advance onto the location of the steel sheet corresponding to the bent part 6.

[0069] The steel sheets A, B, C, D, D-2, E, F, G, H, I, J, K, L, M, M-2, M-3, M-4, N, O, P, Q, R, S, T, U, V, W, Z, AA, BB, CC, CC-2, CC-3, CC-4, CC-5, CC-6, DD, EE, FF, and GG thus obtained are shown in the following Table 1. [Table 1]Tensile stress introduction conditionsSteel sheet thickness (mm)Maximum tensile stress in the thickness direction (MPa)Width in winding direction of one region where tensile stress is introduced (mm)Ratio of sheet thickness to length in the direction of plate thicknessInterval between adjacent tensile stress introduction regions in the winding direction (mm)Depth of maximum tensile stress from the surface layer (µm)Irradiation angle (°)Remarks: Tensile stress introduction methodA0.2330.70.64550in a 60% ethanol solution at 40°CB0.23100.50.2101060in a 60% ethanol solution at 20°CC0.23101.10.6430110in a 60% ethanol solution at 20°CD0.23800.30.41080120in a 60% ethanol solution at 20°CD-20.23800.30.4105120in a 60% ethanol solution at 20°CE0.23800.70.8201060in a 60% ethanol solution at 20°CF0.23801.30.6120120in a 60% ethanol solution at -10°CG0.23801.70.8230100in a 60% ethanol solution at -10°CH0.23800.30.42040110in a 60% ethanol solution at -10°CI0.23801.10.61150120in a 60% ethanol solution at -10°CJ0.23801.40.15160120in a 60% ethanol solution at -10°CK0.231100.30.81180120in a 60% ethanol solution at 0°CL0.231100.70.484080in a 60% ethanol solution at 0°CM0.231101.10.6460110in a 60% ethanol solution at 0°CM-20.231101.10.646030in a 60% ethanol solution at 0°CM-30.231101.10.6460120in a 60% ethanol solution at 0°CM-40.231101.10.6460150in a 60% ethanol solution at 0°CN0.231101.40.2270110in a 60% ethanol solution at 0°CO0.231100.030.15110120in a 60% ethanol solution at 0°CP0.231102121060in a 60% ethanol solution at 10°CQ0.232001.4143060in a 60% ethanol solution at 10°CR0.232000.30.454070in a 60% ethanol solution at 10°CS0.232000.70.6870100in a 60% ethanol solution at 10°CT0.232001.10.81080110in a 60% ethanol solution at 10°CU0.232001.40.21410120in a 60% ethanol solution at 10°CV0.232001.70.1111090in a 60% ethanol solution at 10°CW0.234500.50.843060in a 60% ethanol solution at 10°CZ0.236001.10.543060in a 60% ethanol solution at 10°CAA0.2800.50.643060in a 60% ethanol solution at 10°CBB0.21100.50.644080in a 60% ethanol solution at 10°CCC0.22000.50.6470110in a 60% ethanol solution at 10°CCC-20.22000.50.645110in a 60% ethanol solution at 10°CCC-30.22000.50.647020in a 60% ethanol solution at 10°CCC-40.22000.50.647060in a 60% ethanol solution at 10°CCC-50.22000.50.6470120in a 60% ethanol solution at 10°CCC-60.22000.50.6470160in a 60% ethanol solution at 10°CDD0.26000.50.6480120in a 60% ethanol solution at 10°CEE0.18800.50.643060in a 60% ethanol solution at 10°CFF0.182000.50.6470110in a 60% ethanol solution at 10°CGG0.186000.50.6480120in a 60% ethanol solution at 10°C

[0070] Then, wound cores were fabricated from the steel sheets shown in Table 1. The detailed dimensions of the wound cores are illustrated in FIG. 11 and Table 2. Note that the lap length in Table 2 indicates the distance between two adjacent joint parts 6 in the X-axis direction. [Table 2]Core No.Core ShapeL1L2L3L4L5ϕNumber of bent parts per cornerNumber of joint partsNumber of stepsLap lengthmmmmmmmmmm°mma1976647152.454521910

[0071] Finally, the core loss of the prepared wound core was evaluated. The results are shown in Table 3 below. [Table 3]Test No.Steel Sheet No.Core No.Distance at which tensile stress is applied, L (mm)Total number of laminated sheets, S tot Number of steel sheets with tensile stress in region M, S M Ratio of Region M with tensile stress in all Regions M, % (S M / S tot ×100)Ratio of Region (PM) with tensile stress in all Regions M (Ptot), % (PM / Ptot×100)Material core loss (W / kg)Wound core loss (W / kg)Core loss ratio (= Wound core loss / Material core loss)Evaluation1Non-irradiated materiala0204000.000.720.8281.15Comparative Example2Aa1002042041000.090.720.8281.15Comparative Example3Aa102042041000.270.720.8281.15Comparative Example4Aa52042041001.220.720.8281.15Comparative Example5Ba1002042041002.100.720.8281.15Comparative Example6Ba402042041004.510.720.8281.15Comparative Example7Ba3020420410018.450.720.7631.06Inventive Example8Ba2020420410042.560.720.7491.04Inventive Example9Ba102042041000.090.720.7631.06Inventive Example10Ba52042041000.210.720.7421.03Inventive Example11Ba40204421.180.720.8281.15Comparative Example12Ba40204421.240.720.8281.15Comparative Example13Ba30204422.050.720.7991.11Inventive Example14Ba30204422.080.7Z0.7991.11Inventive Example15Ba20204424.530.720.7851.09Inventive Example16Ba102041025020.500.720.7341.02Inventive Example17Ba520420410042.400.720.7341.02Inventive Example18Ba402041782.110.720.8281.15Comparative Example19Ba302041782.110.120.7991.11Inventive Example20Ba2020420104.530.720.7781.08Inventive Example21Ba102041025020.500.720.7421.03Inventive Example22Ba520420410042.400.720.7271.01Inventive Example23Ba402041782.110.720.8281.15Comparative Example24Ba302041782.110.720.7991.11Inventive Example25Ba2020420104.530.720.7781.08Inventive Example26Ba402041782.110.720.8281.15Comparative Example27Ba302041782.110.720.7991.11Inventive Example28Ba2020420104.530.720.7631.06Inventive Example29Ba2020420104.530.720.7631.06Inventive Example30Ca40204210.960.720.8281.15Comparative Example31Ca30204632.360.720.8141.13Inventive Example32Ca1020420410060.300.720.7271.01Inventive Example33Da30204632.030JZ0.8141.13Inventive Example34Ea1020420410044.200.720.7341.02Inventive Example35Fa5204637.400.720.8141.13Inventive Example36Ga1520417811.600.720.8061.12Inventive Example37Ha40204630.100.720.8281.15Comparative Example38Ia3020420410028.400.720.7421.03Inventive Example39Ja2020420410098.000.720.7271.01Inventive Example40Ka5204630.100.720.8141.13Inventive Example41La10204630.130.720.8061.12Inventive Example42Ma102041004923.000.720.7491.04Inventive Example43Na4020420410051.100.720.8281.15Comparative Example44Oa30204631.230.720.8211.14Inventive Example45Oa1020420410047.800.720.7631.06Inventive Example46Pa1020420410097.430.720.7271.01Inventive Example47Qa2020420410086.230.720.7491.04Inventive Example48Ra3020420410083.230.720.7561.05Inventive Example49Sa3020420410085.320.720.7561.05Inventive Example50Ta1020420410081.020.720.7491.04Inventive Example51Ua10204630.100.720.8141.13Inventive Example52Va30204630.120.720.8211.14Inventive Example53Wa3020420410098.400.720.7561.05Inventive Example54Za1020420410099.200.720.7271.01Inventive Example55AAa4023523510049.400.680.7821.15Comparative Example56BBa1002357372.300.680.7821.15Comparative Example57CCa2023523510055.600.680.6941.02Inventive Example58DDa2023523510052.700.680.6941.02Inventive Example59EEa10261730.100.580.6381.10Inventive Example60FFa1026126110076.200.580.5921.02Inventive Example61GGa1026126110083.500.580.5801.00Inventive Example62D-2a10204630.100.720.8211.14Inventive Example63M-2a302041004967.300.720.7781.08Inventive Example64M-3a402041004952.400.720.8281.15Comparative Example65M-4a702041004952.600.720.8281.15Comparative Example66CC-2a3023523510095.600.580.6151.06Inventive Example67CC-3a3023523510097.400.580.6091.05Inventive Example68CC-4a3023523510098.100.580.6031.04Inventive Example69CC-5a3023523510095.300.580.6031.04Inventive Example70CC-6a4023523510099.800.580.6671.15Comparative Example

[0072] In the evaluation in Table 3, the core loss at 50 Hz and a magnetic flux density of 1.7 T was measured by the magnetizing current method. The obtained results showed that the core loss ratio was low for the Test Nos., where L is 30 mm or less and a tensile stress of 10 MPa or more was introduced, and the core loss ratio was high for the other Test Nos. Note that Test No. 1 is a conventional, normal wound core in which no stress region 12 is provided. As can be seen from the results in Table 3, when the conditions are satisfied in which a tensile stress of 10 MPa or more is introduced in the thickness direction and L is 30 mm or less, the core loss ratio is 1.14 or less. It can also be seen that when the conditions are satisfied in which PM / Ptot is 0.1% or more, or SM / Stot is 2% or more, the core loss ratio is further improved.REFERENCE SIGNS LIST

[0073] 1grain-oriented electrical steel sheet 2laminated structure 3corner part 4,4aplane part 5bent part 6joint part 10wound core 12region 15hollow section 70manufacturing apparatus 71processing section 71atensile stress introduction section 72assembly section 75steel sheet supply section

Claims

1. A wound core having a wound shape, formed by laminating bent steel sheets, wherein, in a side view of the wound shape from a direction along the plane of the steel sheet, the wound core has a plurality of plane parts and a plurality of bent parts adjacent to the plane parts, which together form a rectangular shape with a central hollow section, and wherein, in a cross-section perpendicular to the plane of the steel sheet and along the winding direction, a tensile stress of 10 MPa or more is introduced in the thickness direction of the steel sheet at one or more locations in a predetermined region within 30 mm on the plane part side from the boundary of each of the bent parts and the plane parts of the steel sheet.

2. The wound core having laminated steel sheets of claim 1, wherein a ratio SM / Stot is 2% or more, where SM is the number of the steel sheets in which the tensile stress in the thickness direction is introduced within the predetermined region, and Stot is the total number of the laminated steel sheets.

3. The wound core having laminated steel sheets of claim 1 or 2, wherein a ratio PM / Ptot is 0.1% or more, where PM is an area of a region in which the tensile stress in the thickness direction is introduced within the predetermined region, and Ptot is a total area of the predetermined region.

4. The wound core of claim 1 or 2, wherein the tensile stress has a maximum value that is equal to or less than the yield stress of the steel sheet.

5. The wound core of claim 1 or 2, wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, with respect to the regions in which the tensile stress is introduced, the width of one of the regions in the winding direction is 1.4 mm or less.

6. The wound core of claim 1 or 2, wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, with respect to the regions in which the tensile stress is introduced, the length of one of the regions in the thickness direction is 0.2 times or more and 1 time or less relative to the thickness of the steel sheet.

7. The wound core of claim 1 or 2, wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, with respect to the regions in which the tensile stress is introduced, the interval between the regions that are adjacent in the winding direction is 2 mm or more and 10 mm or less.

8. The wound core of claim 1 or 2, wherein, with respect to the region in which the tensile stress is introduced, the point at which the tensile stress is maximum exists at a depth of more than 10 µm from the surface of the steel sheet as a material.

9. The wound core of claim 1 or 2, wherein the region in which the tensile stress is introduced is provided so as to extend continuously or at predetermined intervals in a direction that intersects the winding direction of the steel sheet, and to traverse the winding direction.

10. The wound core of claim 1 or 2, wherein the winding direction coincides with the {110}<001> direction of the steel sheet.

11. The wound core of claim 1 or 2, wherein the winding direction coincides with the rolling direction of the steel sheet.

12. The wound core of claim 1 or 2, wherein, in the cross-section perpendicular to the plane of the steel sheet and along the winding direction, the tensile stress is introduced only in the predetermined region.

13. The method of manufacturing the wound core of claim 1 or 2, comprising the steps of: performing a bending process on a steel sheet; cutting the steel sheet; laminating the bent and cut steel sheet in layers and assembling them into a wound shape by butting ends of the steel sheets in the winding direction; and introducing a tensile stress in the thickness direction, either in a vicinity of the bent part or in a vicinity of a position that is to be the bent part before cutting.

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