Wound iron core

Abstract

The BF of the wound iron core is further reduced by controlling the residual stress caused by the shape of the groove and the strain of the bending part within a certain range. The present invention provides a wound core comprising a grain-oriented electrical steel sheet having a steel sheet surface in which a groove is formed, the groove extending in a direction intersecting a rolling direction (RD direction), the groove depth direction being the sheet thickness direction (ND direction), and the thickness direction of the groove extending in a direction intersecting the rolling direction (RD direction). The pitch of the grooves in the steel plate rolling direction (RD direction) is 2-10 mm, the depth of the grooves is 10 [mu] m to 40 [mu] m, and the width of the grooves is 10 [mu] m to 200 [mu] m. The angle (sometimes also referred to as "apex angle") formed when the opposing wall surfaces of the groove (where the surface of the groove having a depth of 25-75% with respect to the depth of the groove is defined as the wall surface) extend in the plate thickness direction (ND direction) is 90 DEG or less, and is within 10 [mu] m in the rolling direction (RD direction) from the wall surfaces of the groove positioned in the curved portion of the wound core. A region in which residual stress due to tensile strain in the steel sheet rolling direction (RD direction) of 10 MPa or more is present is 10% by area or more.

Description

Technical Field

[0001] The present invention relates to a wound core, and particularly to a wound core in which the building factor (BF) is suppressed. Background Art

[0002] Directional electromagnetic steel sheets are used as magnetic cores in many electrical devices. A directional electromagnetic steel sheet is a steel sheet containing approximately 1.0% to 5.0% of Si and having a highly concentrated crystal orientation of the product in the {110}<001> orientation. The directional electromagnetic steel sheet has excellent magnetic properties and is used, for example, as a core material for static inductors such as transformers. Therefore, as magnetic properties, a high magnetic flux density represented by the B8 value and a low iron loss represented by W17 / 50 are required.

[0003] Especially in a transformer, which is one of the uses of a directional electromagnetic steel sheet, a low iron loss is required in consideration of energy saving and the surrounding environment. Therefore, in recent years, development has been underway to reduce the iron loss of the directional electromagnetic steel sheet itself.

[0004] The core of a transformer has a laminated core and a wound core. Among them, a wound core (also called a wound wire core) is usually manufactured by overlapping directional electromagnetic steel sheets in layers and winding them into a donut shape (winding shape), and then pressing the wound body to form a substantially square shape (in this specification, the wound core manufactured in this way is sometimes called a box-type core). Through this forming process, mechanical processing strain (plastic deformation strain) enters the entire directional electromagnetic steel sheet, and this processing strain becomes the main cause of a significant deterioration in the iron loss of the directional electromagnetic steel sheet. In order to eliminate the influence of this processing strain, stress relief annealing is performed. However, it is required to further reduce the ratio of the iron loss of the core to the iron loss of the raw material steel sheet, that is, the so-called building factor (BF, also called the "assembly factor").

[0005] Patent Document 1 discloses a directional electromagnetic steel sheet that pays attention to noise characteristics and also considers a reduction in iron loss, and a wound core using this directional electromagnetic steel sheet. Specifically, it is disclosed that excellent noise characteristics are obtained by making the electromagnetic steel sheet have a specific range of warpage amount, and the iron loss can be reduced by introducing thermal strain and forming grooves in the electromagnetic steel sheet.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2020-56080 Summary of the Invention

[0009] Problems to be Solved by the Invention

[0010] For a wound core used in a transformer, further reduction of iron loss is required. For the electromagnetic steel sheet used as a raw material, by forming linear grooves or introducing strain, the 180° magnetic domain width is subdivided, and accordingly, the eddy current loss, which is part of the iron loss, is reduced. However, it is required to further reduce the deterioration of the iron loss when the electromagnetic steel sheet is processed into a wound core, that is, the so-called process factor (BF).

[0011] The present invention has been completed in view of the above problems. In a wound core, particularly in the bent portion of the steel sheet, the deterioration of iron loss is large. Suppressing the deterioration of iron loss in the bent portion (sometimes also referred to as "bending processed portion" or "buckling portion") is important for reducing the BF of the wound core.

[0012] An object thereof is to further reduce the BF of the wound core by controlling the residual stress caused by the shape of the groove and the strain of the bent portion within a certain range.

[0013] Means for Solving the Problem

[0014] The inventors of the present invention have found that in a wound core, the grain-oriented electromagnetic steel sheet used herein has residual stress caused by grooves of a specified shape and the strain of the bent portion, and thus a wound core capable of reducing BF compared with the prior art can be provided.

[0015] The gist of the present invention is a wound core composed of a grain-oriented electromagnetic steel sheet having residual stress caused by strain in the vicinity of the groove, the grain-oriented electromagnetic steel sheet having a steel sheet surface formed with the following grooves, the grooves extending in a direction crossing the rolling direction (RD direction) and the groove depth direction being the plate thickness direction (ND direction), and the grooves satisfying the following conditions.

[0016] a The pitch of the grooves in the steel sheet rolling direction (RD direction) is 2 to 10 mm.

[0017] b The depth of the grooves is 10 μm or more and 40 μm or less, and the width of the grooves is 10 μm or more and 200 μm or less.

[0018] c The angle (sometimes also referred to as "top angle") formed when the opposite wall surfaces of the grooves (the surfaces of the grooves at a depth of 25 to 75% with respect to the depth of the grooves) are extended in the plate thickness direction (ND direction) is 90° or less.

[0019] d In a range within 10 μm from the wall surface of the groove located in the bent portion of the wound core in the steel sheet rolling direction (RD direction), a region having a residual stress caused by a tensile strain in the steel sheet rolling direction (RD direction) of 10 MPa or more exists in an area of 10 area% or more.

[0020] Advantages of the Invention

[0021] According to the present invention, in a wound core, the oriented electromagnetic steel sheet used herein has residual stress caused by strain of grooves and bent portions having a specified shape, whereby a wound core capable of reducing BF as compared with the prior art can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 FIG. 6 is a plan view showing an example of a wound core according to an embodiment of the present invention.

[0023] Figure 2 FIG. 10 is a cross-sectional view of a groove portion of an oriented electromagnetic steel sheet according to an embodiment of the present invention (a cross-sectional view perpendicular to the extending direction of the groove).

[0024] Figure 3 FIG. 14 is a view schematically showing an example of a bent portion (curved portion) of an oriented electromagnetic steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, preferred embodiments of the present invention will be described in detail. It should be noted that, unless otherwise specified, with respect to numerical values A and B, the expression "A to B" means "not less than A and not more than B". In the case where a unit is attached only to numerical value B in this expression, the unit also applies to numerical value A.

[0026] [Composition of Steel Sheet]

[0027] First, the wound core of the present embodiment is composed of an oriented electromagnetic steel sheet. The oriented electromagnetic steel sheet is not particularly limited, and an oriented electromagnetic steel sheet composed of a known steel composition can be used. An exemplary description will be given of the oriented electromagnetic steel sheet that can be used for the wound core of the present invention.

[0028] [Composition of Steel Sheet]

[0029] The composition of the steel sheet used in the oriented electromagnetic steel sheet of the invention has a preferred composition in order to control the texture so that crystal orientations are aggregated in the {110}<001> orientation (Goss orientation), and may contain at least Si: 1.0 to 5.0% and Mn: 0.01 to 0.15%.

[0030] (Si: 1.0 to 5.0%)

[0031] (Si: 1.0 to 5.0%)

[0032] The content of Si (silicon) is 1.0 to 5.0%. Si reduces the eddy current loss, which is one of the causes of iron loss, by increasing the resistance of the steel sheet. When the content of Si is less than 1.0%, it is difficult to sufficiently suppress the eddy current loss of the final grain-oriented electrical steel sheet, so it is not preferred. When the content of Si exceeds 5.0%, the workability of the grain-oriented electrical steel sheet decreases, so it is not preferred. Therefore, the content of Si is 1.0 to 5.0%, preferably 2.5 to 4.5%, and more preferably 2.7 to 4.0%.

[0033] (Mn: 0.01 to 0.15%)

[0034] The content of Mn (manganese) is 0.01 to 0.15%. Mn forms MnS, MnSe, etc., which are inhibitors for controlling secondary recrystallization. When the content of Mn is less than 0.01%, the absolute amounts of MnS and MnSe that cause secondary recrystallization are insufficient, so it is not preferred. When the content of Mn exceeds 0.15%, it becomes difficult for Mn to dissolve during slab heating, so it is not preferred. In addition, when the content of Mn exceeds 0.15%, the precipitation sizes of MnS and MnSe as inhibitors tend to coarsen, damaging the optimal size distribution as inhibitors, so it is not preferred. Therefore, the content of Mn is 0.01 to 0.15%, preferably 0.03 to 0.13%.

[0035] Components other than Si and Mn can be components contained in ordinary grain-oriented electrical steel sheets.

[0036] For example, as components other than Si and Mn, by mass%, it may contain C: up to 0.085% or less, acid-soluble Al: up to 0.065% or less, N: up to 0.012% or less, Cr: up to 0.30% or less, Cu: up to 0.40% or less, P: up to 0.50% or less, Sn: up to 0.30% or less, Sb: up to 0.30% or less, Ni: up to 1.000% or less, S: up to 0.015% or less. In addition, as other inhibitor constituent elements, B, Bi, Se, Pb, Sn, Ti, etc. can also be added. The addition amounts can be adjusted appropriately. The upper limit value of the B content can be 0.080%, the upper limit value of the Bi content can be 0.010%, the upper limit value of the Se content can be 0.035%, the upper limit value of the Pb content can be 0.10%, the upper limit value of the Sn content can be 0.10%, and the upper limit value of the Ti content can be 0.015%. These optional addition elements can be contained for known purposes, so there is no need to set a lower limit value for the content of the optional addition elements. For example, the lower limit value can be 0%.

[0037] The remaining part other than the above components of the steel sheet is Fe and impurities. Here, the impurity elements are components contained in the raw materials or components mixed in during the manufacturing process, and are components allowed within a range that does not substantially affect the present embodiment.

[0038] The chemical composition of the steel sheet can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, using an ICPS-8100 etc. (measurement device) manufactured by Shimadzu Corporation, a 35 mm square test piece collected from the steel sheet is measured under the conditions based on a pre-made standard curve, thereby determining the chemical composition. It should be noted that C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0039] [Manufacturing method of grain-oriented electrical steel sheet]

[0040] The manufacturing process of the grain-oriented electrical steel sheet of the present invention will be illustratively described below. It should be noted that this is merely an example of the manufacturing method of the grain-oriented electrical steel sheet of this embodiment, and can be arbitrarily changed within the range that does not impair the effects of this embodiment.

[0041] (Casting process S1)

[0042] In the casting process S1, a slab is prepared. An example of the manufacturing method of the slab is as follows. First, molten steel (smelted) is manufactured. Then, the slab is manufactured using the molten steel. The manufacturing method of the slab is not particularly limited, but for example, the slab can be manufactured by the continuous casting method. It is also possible to manufacture an ingot using the molten steel and perform blooming rolling on the ingot to manufacture the slab. The thickness of the slab is not particularly limited. The thickness of the slab can be, for example, 150 mm to 350 mm. The thickness of the slab is preferably 220 mm to 280 mm. As the slab, so-called thin slabs with a thickness of 10 mm to 70 mm can also be used. In the case of using a thin slab, in the hot rolling process S2, the rough rolling before finish rolling can be omitted.

[0043] The component composition of the slab only needs to be a component composition that generates secondary recrystallization. Regarding the basic components and optional elements of the slab, specifically, it is as follows. It should be noted that the % notation used for the components refers to mass %.

[0044] Si is an important element in terms of increasing resistance and reducing iron loss. If the content rate exceeds 5.0%, the material becomes prone to cracking during cold rolling and cannot be rolled. On the other hand, if the Si amount is reduced, the α→γ phase transformation occurs during the final annealing, and the crystal orientation is damaged. Therefore, 1.0% that does not affect the crystal orientation during the final annealing can also be used as the lower limit. Therefore, the Si content can be 1.0 to 5.0%.

[0045] Mn and S precipitate as MnS and act as inhibitors. When the Mn content is less than 0.01% and the S content is less than 0.005% additionally, it may not be possible to ensure a specified amount of effective MnS inhibitor. Further, if the Mn content is more than 0.15% and the S content is more than 0.015%, solid solution during slab heating becomes insufficient, and it may not be possible to stably perform secondary recrystallization. Therefore, the Mn content can be 0.01 - 0.15%, and the S content can be 0.005 - 0.015%.

[0046] C is an element effective for controlling the primary recrystallization structure in the manufacturing process, but if the content in the final product is excessive, it may have an adverse effect on magnetic properties. Therefore, the C content can also be set to 0.085% or less. The preferred upper limit of the C content is 0.080%. C can be purified in the decarburizing annealing process S5 and the final annealing process S8 described later, and can be 0.005% or less after the final annealing process S8. When the slab contains C, considering productivity in industrial production, the lower limit of the C content can exceed 0%, or can be 0.001%.

[0047] Acid-soluble Al functions as an inhibitor in a state where it combines with N to form AlN or (Al, Si)N. The content of acid-soluble Al can be set to 0.012% - 0.065% where the magnetic flux density becomes higher.

[0048] When 0.012% or more of N is added during steelmaking, voids in the steel sheet called blister are generated. Therefore, the upper limit of the N content can be 0.012%. N can be contained by nitriding during the manufacturing process, so the lower limit is not particularly limited and can be 0%. However, the detection limit of N is 0.0001%, so the practical lower limit is 0.0001%.

[0049] It is also possible to add B, Bi, Se, Pb, Sn, Ti, etc. to the slab as other inhibitor constituent elements. The addition amount can be adjusted appropriately. The upper limit value of the B content can be 0.080%, the upper limit value of the Bi content can be 0.010%, the upper limit value of the Se content can be 0.035%, the upper limit value of the Pb content can be 0.10%, the upper limit value of the Sn content can be 0.10%, and the upper limit value of the Ti content can be 0.015%. These optional addition elements can be contained in the slab for known purposes, so there is no need to set a lower limit value for the content of the optional addition elements. For example, the lower limit can be 0%.

[0050] The remaining part of the chemical composition of the slab consists of Fe and impurities. It should be noted that the "impurities" mentioned here refer to the components mixed into the slab due to various reasons of raw materials such as ore and waste and manufacturing processes during the industrial production of the slab, and are substances that are allowed within a range that does not substantially affect the directionality electromagnetic steel sheet of the present embodiment.

[0051] In the slab, considering the enhancement of the inhibitor function brought about by compound formation and the influence on magnetic properties, known arbitrary elements can also be contained (added) in place of a part of Fe. Examples of the arbitrary elements contained in the slab in place of a part of Fe include Cu, P, Sb, Sn, Cr, Ni, etc. Any one or two or more of these can be added to the slab. The upper limit value of the Cu content can be 0.30%, the upper limit value of the P content can be 0.50%, the upper limit value of the Sb content can be 0.30%, the upper limit value of the Sn content can be 0.30%, the upper limit value of the Cr content can be 0.30%, and the upper limit value of the Ni content can be 1.000%. These arbitrary additive elements can be contained in the slab for known purposes, so there is no need to set a lower limit value for the content of the arbitrary additive elements, and the lower limit value can be 0%.

[0052] The chemical composition of the slab can be measured using ICP - AES (Inductively Coupled Plasma - Atomic Emission Spectrometry). Specifically, using an ICPS - 8100 manufactured by Shimadzu Corporation, etc. (measurement device), a test piece with a side length of 35 mm collected from the slab is measured under the conditions based on a pre - made standard curve, thereby determining the chemical composition. It should be noted that C and S can be measured using the combustion - infrared absorption method, and N can be measured using the inert gas fusion - thermal conductivity method.

[0053] (Hot rolling process S2)

[0054] The hot rolling process S2 is a process of hot rolling a slab heated to a specified heating temperature (for example, 1100°C to 1400°C) to obtain a hot - rolled steel sheet. From the perspective of ensuring the temperature during hot rolling, the heating temperature during hot rolling can be, for example, 1100°C or higher. Furthermore, from the perspective of not completely solid - solving AlN, which is an inhibitor component, the heating temperature during hot rolling can be 1280°C or lower. It should be noted that in the case where AlN and MnS are the main inhibitors, the heating temperature during hot rolling can be set to 1300°C or higher to completely solid - solve these inhibitor components.

[0055] (Annealing process of hot - rolled steel sheet S3)

[0056] The annealing process S3 of the hot-rolled steel sheet is a process of immediately or shortly annealing the hot-rolled steel sheet obtained in the hot-rolling process S2 to obtain an annealed steel sheet. The annealing can be carried out in a temperature range of 750 °C to 1200 °C for 30 seconds to 30 minutes. This annealing is effective for improving the magnetic properties of the product.

[0057] (Cold rolling process S4)

[0058] The cold rolling process S4 is a process of obtaining a cold-rolled steel sheet by subjecting the annealed steel sheet obtained in the annealing process S3 of the hot-rolled steel sheet to one-time cold rolling or multiple (two or more times) cold rolling with intermediate annealing (intermediate annealing) (for example, the total cold rolling rate is 80% to 95%). The thickness of the cold-rolled steel sheet can be, for example, 0.10 mm to 0.50 mm.

[0059] (Decarburizing annealing process S5)

[0060] The decarburizing annealing process S5 is a process of subjecting the cold-rolled steel sheet obtained in the cold rolling process S4 to decarburizing annealing to obtain a decarburized annealed steel sheet (cold-rolled steel sheet subjected to the decarburizing annealing process) in which primary recrystallization has occurred. The decarburizing annealing can be carried out, for example, at 700 °C to 900 °C for 1 minute to 3 minutes.

[0061] By subjecting the cold-rolled steel sheet to decarburizing annealing, the C component contained in the cold-rolled steel sheet is removed. In order to remove the C component contained in the cold-rolled steel sheet, the decarburizing annealing is preferably carried out in a moist atmosphere.

[0062] (Nitriding treatment process S6)

[0063] The nitriding treatment process S6 is a process that is carried out as needed to adjust the strength of the inhibitor in secondary recrystallization. The nitriding treatment is a treatment that increases the nitrogen content of the cold-rolled steel sheet by about 40 ppm to 200 ppm during the period from the start of the decarburizing annealing process to the start of secondary recrystallization in the final annealing process. As the nitriding treatment, for example, a treatment of annealing the decarburized annealed steel sheet in an atmosphere containing a gas having nitriding ability such as ammonia, a treatment of applying an annealing release agent containing a powder having nitriding ability such as MnN to the decarburized annealed steel sheet in the subsequent annealing release agent coating process S7, etc. can be cited.

[0064] (Annealing release agent coating process S7)

[0065] The annealing release agent coating process S7 is a process of coating an annealing release agent on the decarburized annealed steel sheet. As the annealing release agent, for example, an annealing release agent mainly composed of alumina (Al2O3) can be used. The decarburized annealed steel sheet after coating the annealing release agent is subjected to final annealing in the subsequent final annealing process S8 in a state of being wound into a coil.

[0066] In addition, in the case of forming a glass film containing Mg2SiO4, an annealing parting agent mainly composed of magnesium oxide (MgO) is used.

[0067] (Final annealing process S8)

[0068] The final annealing process S8 is a process of performing final annealing on a decarburized annealed steel sheet coated with an annealing parting agent to cause secondary recrystallization. In the final annealing process S8 accompanied by this secondary recrystallization, by performing secondary recrystallization in a state where the growth of primary recrystallized grains is suppressed by an inhibitor, {100}<001> oriented grains preferentially grow, and the magnetic flux density is dramatically increased.

[0069] In addition, in the case of coating magnesium oxide (MgO) in the above-described annealing parting agent coating process S7, a glass film containing Mg2SiO4 is formed through this final annealing process S8. It should be noted that in this embodiment, such a glass film is also included in the base material steel sheet (the final annealed steel sheet described later). Therefore, for example, in the case of forming a glass film on the final annealed steel sheet, the "surface of the final annealed steel sheet" refers to the surface of the glass film. By forming the glass film, it is expected that the characteristics of the finally obtained grain-oriented electrical steel sheet will be further improved.

[0070] (Groove forming process S9)

[0071] The groove forming process S9 is a process of forming grooves on the steel sheet for the purpose of magnetic domain control (magnetic domain refinement). The grooves can be formed by known methods such as laser, electron beam, plasma, mechanical method, and etching.

[0072] In the process described above, the groove forming process S9 is performed after the final annealing process S8. However, the groove forming process S9 can also be performed on the steel sheet (i.e., the cold-rolled steel sheet) that has undergone the cold rolling process S4. In this case, it is also possible to maintain the ideal cross-sectional shape of the linear groove G in the magnetic domain refinement. Therefore, the timing of performing the groove forming process S9 can be before or after the final annealing process S8. However, in the case of performing the tension film application process S10 described later, it is necessary to perform the groove forming process 9 in advance before this process S10. Details regarding the shape of the grooves, the forming method, etc. will be described later.

[0073] (Tension film application process S10)

[0074] The tension film application process S10 is a process of forming an insulating film (tension film) on the groove forming surface by coating a coating solution on the groove forming surface of the final annealed steel sheet and baking it. By forming the insulating coating film (tension coating film), it is expected that the characteristics of the finally obtained grain-oriented electrical steel sheet will be further improved.

[0075] Here, the coating solution contains, for example, compounds of phosphoric acid, phosphates, chromic anhydride, chromates, alumina, or silica. Baking can be performed, for example, under the conditions of 350°C to 1150°C and 5 seconds to 300 seconds.

[0076] [Wound core]

[0077] Next, with reference to Figure 1 a wound core according to an embodiment of the present invention will be described. Figure 1 is a top view showing an example of a wound core according to an embodiment of the present invention. In addition, hereinafter, the wound core transformer may sometimes be simply referred to as a transformer.

[0078] The wound core 10 is formed by winding a directionally electromagnetic steel sheet 100. The shape of the wound core 10 is not limited to the illustrated rounded square shape, and can be, for example, an elliptical shape, an oblong shape, or a rounded square shape.

[0079] By providing a primary winding 20A and a secondary winding 20B wound around the wound core 10, a transformer 1 can also be manufactured.

[0080] The primary winding 20A and the secondary winding 20B are wound around the wound core 10 at opposite positions of the wound core 10. In the primary winding 20A and the secondary winding 20B, existing wires can be used. For example, wires obtained by covering a highly conductive metal wire with an insulator can be used. As the metal wire, for example, a copper wire covered with enamel coated with copper, a copper alloy, aluminum, and an insulating material and sintered can be used. As the insulator covering the surface of the metal wire, for example, polyvinyl chloride, polyethylene, fluororesin, or polyester can be used.

[0081] The number of turns of the primary winding 20A and the number of turns of the secondary winding 20B are not particularly limited. For example, according to the specifications of the wound core transformer 1, the number of turns of the primary winding 20A and the number of turns of the secondary winding 20B can be determined.

[0082] The primary winding 20A is connected to the circuit on the power supply side during use, and an AC voltage is applied from the power supply. By applying an AC voltage to the primary winding 20A, a magnetic flux is generated in the wound core 10, and according to the change in the generated magnetic flux, a voltage corresponding to the number of turns of the primary winding 20A and the number of turns of the secondary winding 20B is generated in the secondary winding 20B connected to the circuit on the load side.

[0083] [Grooves of the directionally electromagnetic steel sheet]

[0084] The directionally electromagnetic steel sheet that constitutes the wound core is made to have low iron loss by refining magnetic domains, and grooves are formed on the surface of the steel sheet that extend in a direction crossing the rolling direction (RD direction) and whose groove depth direction is the plate thickness direction (ND direction). Additionally, the grooves only need to be arranged in a way that crosses the rolling direction, and it is not necessary for the groove extension direction to be orthogonal to the rolling direction. However, they can also be arranged in a direction forming an angle of 0 to 30° with the direction perpendicular to the rolling. Also, when viewed from the plate thickness direction (when looking down at the grooves), the grooves do not necessarily have to have a straight shape and can also have an arcuate shape. The measurement of the following groove shape is carried out after at least removing the glass coating and insulation coating inside the grooves from the final product by pickling or the like.

[0085] The above-mentioned grooves are formed on the surface of the steel sheet at intervals (interval widths) of 2 to 20 mm in the steel sheet rolling direction (RD direction). If the groove interval is less than 2 mm, the magnetic domain refinement effect saturates, and almost no reduction effect of eddy current loss can be obtained, which is not preferred. When it exceeds 20 mm, the magnetic domain refinement effect decreases, so the iron loss improvement effect is insufficient, which is not preferred. The preferred groove interval is 2 to 10 mm.

[0086] It should be noted that the interval in the rolling direction can be measured by the following method. That is, focus on any two adjacent groove groups when looking down. Then, measure the distance in the rolling direction between the center points in the width direction of these grooves at multiple locations, and take their average value as the rolling direction interval of this groove group.

[0087] Figure 2 It is a cross-sectional view of the groove part (a cross-sectional view perpendicular to the groove extension direction) of an embodiment of the present invention. In this figure, the groove part has a shape close to a trapezoid, but the groove shape can also be arcuate. In an embodiment of the electromagnetic steel sheet of the present invention, the depth D of the groove part is in the range of 10 μm to 40 μm in the plate thickness direction (ND direction). When the depth D is less than 10 μm, the amount of generated magnetic poles from the groove wall surface becomes small, magnetic domains are not refined, and sufficient iron loss reduction effect cannot be obtained. When the depth D exceeds 40 μm, the magnetic domains are refined, but the reduction of the magnetic flux density caused by the formation of the grooves becomes large, and sufficient iron loss reduction effect cannot be obtained. The preferred depth is 15 μm to 30 μm.

[0088] (Measurement of the depth D of the groove)

[0089] The measurement method of the "depth D" of the present invention is as described below.

[0090] Select any slot, and measure the maximum depth on the cross-section (the plane perpendicular to the extending direction of the slot) of any point of the slot as the depth D of the slot, using a laser microscope (a 3D laser microscope using a pinhole-based confocal optical system). Set the height of the steel plate surface adjacent to the edge of the slot to 0, and measure the depth in the normal direction of the steel plate surface (set the inside direction of the steel plate as positive).

[0091] The "width W of the slot portion" as mentioned in the present invention refers to the width of the slot at a depth of half of the slot depth D (D / 2). The width W of the slot portion ranges from 10 μm to 200 μm. When the width W is less than 10 μm, the magnetic flux leaking from the wall surface of the slot enters the wall surface of the opposite slot, but the amount of magnetic poles generated becomes less, no magnetic domain refinement is performed, and sufficient iron loss reduction effect cannot be obtained. When the width W exceeds 200 μm, the iron loss reduction effect saturates, the laser power required to form the slot becomes larger, and the manufacturing cost increases. The preferred width W is 30 to 100 mm.

[0092] (Measurement of the width W of the slot portion)

[0093] The measurement method of the "slot width W" of the present invention is as described below.

[0094] Select any slot, and set the wider one of the widths of the slot at a depth of half of the slot depth D (D / 2) on the cross-sections (the planes perpendicular to the extending direction of the slot) of any point (A) of the slot and a point (B) that is 3 mm away from this point in the length direction of the slot as w, and set the narrower one as w'. Measure using a laser microscope (a 3D laser microscope using a pinhole-based confocal optical system). The slot portion width W is the average value of these values.

[0095] As Figure 2As shown, in a cross-section perpendicular to the extending direction of the groove of the present invention, an angle (sometimes referred to as the "top angle") formed when the opposite wall surfaces of the groove are extended in the plate thickness direction (ND direction) can be defined. Here, the wall surface of the groove refers to the surface of the groove at a depth of 25 to 75% of the depth of the groove. To determine the top angle, a line is connected from the point at 25% depth of the wall surface to the point at 75% depth and extended in the direction into the plate thickness. This top angle is in the range of 90° or less. When the opposite wall surfaces are parallel, the top angle cannot be formed, so the top angle is greater than 0°. When the top angle exceeds 90°, the wall surface of the groove forms a gentle inclined plane, and the leakage of magnetic flux is reduced, and a sufficient iron loss reduction effect cannot be obtained. Regarding the reason, it is not desired to be restricted to a specific theory, but the following is considered. In a grain-oriented electrical steel sheet formed with grooves, the magnetic flux reaching one wall surface of the groove in the steel sheet leaks from the groove wall (i.e., due to the leakage of magnetic flux), thereby increasing the magnetostatic energy. Refining the main magnetic domains to reduce this magnetostatic energy is the reason for generating the magnetic domain control effect. When the wall surface is a gentle inclined plane, the magnetic flux reaching one wall surface of the groove in the steel sheet flows along the gentle inclined plane, that is, the wall surface, and the leakage from the wall surface of the groove is suppressed. As a result, the magnetostatic energy does not increase, and in order to reduce this magnetostatic energy, the main magnetic domains are not subdivided, and a sufficient magnetic domain control effect cannot be obtained.

[0096] When using a laser to form the groove, through the following example of laser irradiation conditions, an electrical steel sheet capable of constituting the wound core of the present embodiment, that is, an electrical steel sheet having a specified groove, can be obtained.

[0097] In the laser irradiation process, the surface of the steel sheet (only one side) is irradiated with a laser, and on the surface of the steel sheet, a plurality of grooves extending in a direction intersecting the rolling direction are formed at a desired pitch interval in the range of 2 to 20 mm along the rolling direction.

[0098] In the laser irradiation process, the laser irradiation device can also irradiate the surface of the steel sheet by the rotation drive of a polygon mirror, and scan the laser in a direction at an angle of 0 to 30° with respect to the direction perpendicular to the rolling.

[0099] It is also possible to blow auxiliary gases such as air or inert gas to the part of the steel sheet irradiated with the laser while irradiating the laser. The inert gas is, for example, nitrogen or argon. The auxiliary gas serves to remove the components melted or evaporated from the steel sheet due to the laser irradiation. By blowing the auxiliary gas, the laser can reach the steel sheet without being blocked by the above-mentioned melted or evaporated components, so that the groove and the strain introduction part in its vicinity are stably formed.

[0100] As the laser light source, for example, a fiber laser, a YAG laser, a semiconductor laser, a CO2 laser, or the like, which are high-output lasers generally used in industry, can be used. In addition, as long as the groove and the strain introduction portion in the vicinity thereof can be stably formed, a pulsed laser or a continuous wave laser can also be used as the laser light source. From the viewpoint of obtaining a groove having a vertex angle of 90° or less, that is, a steep wall surface, in the present embodiment, a single-mode laser having high condensing property for forming the groove is preferably used.

[0101] As an example of the irradiation conditions of the laser, for example, by setting the laser output to 200 W to 3000 W and forming a groove using a top hat type laser having a high power at the center of the laser, the groove shape of the present embodiment can be formed.

[0102] The condensing spot diameter of the portion having the largest beam diameter may be set to 10 μm to 200 μm, and the laser scanning speed may be set to 5 m / s to 50 m / s. These laser irradiation conditions are appropriately adjusted to obtain a desired groove and the strain introduction portion in the vicinity thereof.

[0103] [Residual stress caused by tensile strain at the bent portion of the wound core]

[0104] According to the present embodiment, grooves are formed on the steel plate surface for magnetic domain refinement. It is considered that the magnetic flux leaks from the wall of the groove to increase the magnetostatic energy, and in order to reduce this magnetostatic energy, the main magnetic domains are subdivided. However, in the wound core, in the bent portion, usually the magnetic domain structure changes to suppress the leakage magnetic flux. As a result, the magnetostatic energy does not increase, and in order to reduce this magnetostatic energy, the main magnetic domains are not subdivided, and a sufficient magnetic domain control effect cannot be obtained. This becomes the main cause of the deterioration of the process coefficient of the wound core. Therefore, in the present embodiment, in the bent portion of the wound core, particularly by forming a groove having a steep shape with a vertex angle of 90° or less, a tensile strain in the rolling direction of the steel plate is introduced in the vicinity of the groove, and a prescribed residual stress caused by this strain exists. This residual stress produces an effect of inducing magnetic anisotropy in the bent portion, particularly in the vicinity of the groove, the magnetic flux leakage of the groove portion increases, and the magnetic domains are subdivided.

[0105] Here, the bent portion of the wound core will be described. In Figure 1 the example of the wound core 10 shown, it is substantially rectangular (quadrilateral), having four bent portions 5 (vertices) and four flat portions 4 (sides) sandwiching the bent portions. Referring to Figure 3 the bent portion 5 will be described in more detail. Figure 3This is a diagram schematically showing an example of the bent portion (curved portion) 5 of the oriented electromagnetic steel sheet 100. The bending angle of the bent portion 5 means the angle difference generated between the straight portion on the rear side in the bending direction and the straight portion on the front side in the bent portion 5 of the oriented electromagnetic steel sheet 100, and is represented in the form of the supplementary angle φ of the angle formed by two imaginary lines Lb-elongation1 and Lb-elongation2 obtained by extending the surfaces of the flat portions 4 and 4a on both sides sandwiching the bent portion 5 on the outer surface of the oriented electromagnetic steel sheet 100. At this time, the point where the extended straight line separates from the steel sheet surface is the boundary between the flat portion 4 on the outer surface side of the steel sheet and the bent portion 5, and in Figure 3 it is point F and point G.

[0106] Furthermore, straight lines perpendicular to the outer surface of the steel sheet are extended from point F and point G respectively, and the intersection points with the inner surface side of the steel sheet are set as point E and point D respectively. These point E and point D are the boundaries between the flat portion 4 on the inner surface side of the steel sheet and the bent portion 5.

[0107] Moreover, in the present invention, the bent portion 5 is the portion of the oriented electromagnetic steel sheet 100 surrounded by the above-mentioned points D, E, F, and G in the side view of the oriented electromagnetic steel sheet 100. In Figure 3 it, the steel sheet surface between point D and point E, that is, the inner surface of the bent portion 5 is represented as La, and the steel sheet surface between point F and point G, that is, the outer surface of the bent portion 5 is represented as Lb.

[0108] In addition, in this figure, the inner surface side curvature radius r of the bent portion 5 when viewed from the side is shown. By approximating the arc passing through point E and point D to the above-mentioned La, the curvature radius r of the bent portion 5 is obtained. The smaller the curvature radius r, the steeper the curve of the curved portion 5, and the larger the curvature radius r, the gentler the curve of the curved portion 5.

[0109] It should be noted that the method for measuring the curvature radius r of the bent portion 5 is not particularly limited. For example, it can be measured by observing at 200 times using a commercially available microscope (Nikon ECLIPSE LV150). Specifically, according to the observation results, the curvature center point A is obtained. As this obtaining method, for example, if the intersection point on the inner side of the opposite side of point B when the line segments EF and DG are extended is defined as A, the magnitude of the curvature radius r is equivalent to the length of the line segment AC. Here, when connecting point A and point B with a straight line, the intersection point on the inner arc DE of the bent portion of the steel sheet is set as C.

[0110] The bent portion 5 can be obtained by bending the directionality electromagnetic steel sheet 100. By the bending process, tensile strain is generated in and around the bent portion 5, and residual stress caused by the tensile strain is generated. If the processing speed of the bending process is too slow, the residual stress will not increase. If it is too fast, fracture or plastic deformation may occur. Therefore, for example, by appropriately adjusting the conditions of the bending process such as bending at a strain rate of 5 mm / second or more and 100 mm / second or less, tensile strain can be obtained at a desired position and size. In other words, residual stress can be obtained at a desired position and size.

[0111] According to the present embodiment, the position where there is residual stress caused by tensile strain is within a range of 10 μm or less in the rolling direction (RD direction) from the wall surface of the groove located in the bent portion of the wound core. This range within 10 μm is defined in a cross section perpendicular to the extending direction of the groove. In the range exceeding 10 μm from the wall surface in the rolling direction, the influence of the residual stress caused by the strain on the groove is small, and the effect of magnetic domain refinement brought by the groove cannot be sufficiently improved. Here, the wall surface of the groove refers to the surface of the groove at a depth of 25 to 75% of the depth of the groove.

[0112] The residual stress caused by tensile strain is a residual stress of 10 MPa or more in the rolling direction (RD direction). If the residual stress is less than 10 MPa, the residual stress caused by the strain is small, and the effect of magnetic domain refinement brought by the groove cannot be sufficiently improved. The upper limit of the residual stress is not particularly limited, and the substantial upper limit is the yield strength of the material. However, if it is too large, the magnetic properties including iron loss will deteriorate instead. Therefore, it can be generally 30 MPa or less. In addition, excessive residual stress may affect the magnetic properties, so the upper limit of the residual stress can be restricted, and it can usually be 10 MPa or less.

[0113] The area where there is residual stress caused by compressive strain of 10 MPa or more in the rolling direction (RD direction) exists in an area of 10 area% or more within a range of 10 μm or less in the rolling direction (RD direction) from the wall surface of the groove located in the bent portion of the wound core. If this area is less than 10 area%, the effect of magnetic domain refinement cannot be sufficiently obtained. The upper limit of the area% of this area is not particularly limited and can be 100 area%. However, the effect of magnetic domain refinement becomes somewhat saturated. In addition, if the residual stress is large and the area% is large, the effect of reducing iron loss brought by the magnetic domain refinement effect and the effect of deteriorating iron loss caused by stress as described above are offset, and sometimes the magnetic properties including iron loss deteriorate. The upper limit of the area% of this area can be adjusted, and it can generally be 50 area% or less. In addition, the upper limit of the area% of this area can be adjusted according to the desired magnetic properties, and it can be 30 area% or less, or even 20 area% or less.

[0114] (Measurement of Residual Stress Caused by Strain)

[0115] The residual stress caused by strain is measured by EBSD.

[0116] For the cross-section in the plate thickness direction perpendicular to the scanning direction of the energy ray (in the case of a directional electromagnetic steel sheet, the energy ray is usually irradiated while scanning in the plate width direction, and in this case, the cross-section in the plate thickness direction perpendicular to the plate width direction), the total thickness in the plate thickness direction and the width (rolling direction) that is twice the irradiation width of the energy ray centered on the irradiation position of the energy ray is taken as the measurement area, and area distribution measurement is carried out by electron backscatter diffraction (EBSD). During the area distribution measurement, for a specimen with the observation surface (steel sheet surface) tilted 70 degrees relative to the irradiation direction of the electron beam (the tilt angle is set to 0 degrees when the incident direction of the electron beam is perpendicular to the steel sheet cross-section), the electron beam is irradiated at a step size of 2 μm or less to obtain an EBSD image. The obtained image is saved with 956×956 pixels, and the strain is calculated using CrossCourt4 of BLG Vantage Co., Ltd. to calculate the intensity of the strain.

[0117] As the derivation method of area%, the following is adopted

[0118] (Area of the region where the compressive strain is 10 MPa or more) / (groove depth (depth of 25 - 75%) × 10 μm) × 100 Figure 2 Schematically shows the range within 10 μm from the groove wall surface in the cross-section perpendicular to the groove extension direction.

[0119] Regarding the calculation of area%, the surface for area distribution measurement is set as the surface including the midpoint between the point where the groove formation starts and the point where it ends by laser.

[0120] (Measurement of process coefficient (BF))

[0121] The process coefficient (BF) is measured based on the iron loss (W / kg) of the iron core and the iron loss (W / kg) of the steel sheet.

[0122] The process coefficient is measured by the following method. Regarding the wound core, the measurement using the exciting current method described in JIS C 2550-1:2011 is carried out under the conditions of a frequency of 50 Hz and a magnetic flux density of 1.7 T, and the iron loss value (core iron loss) WA of the wound core is measured. In addition, a specimen with a width of 60 mm × a length of 300 mm is collected from the grain-oriented electrical steel sheet (sheet width: 152.4 mm) used in the core. For this specimen, the magnetic property test of a single electrical steel sheet using the H-coil method described in JIS C 2556:2015 is carried out under the conditions of a frequency of 50 Hz and a magnetic flux density of 1.7 T, and the iron loss value (iron loss of the steel sheet) WB of the single raw material steel sheet is measured. The process coefficient (BF) is obtained by dividing the obtained iron loss value WA by the iron loss value WB.

[0123] In the present embodiment, the core length of the wound core body 10 is not particularly limited. If the number of the bent portions 5 is the same, even if the core length varies in the wound core body 10, since the volume of the bent portion 5 is constant, the iron loss generated in the bent portion 5 is constant. For a longer core length, the volume ratio of the bent portion 5 to the wound core body 10 decreases, and thus the influence on the deterioration of the iron loss is also small. Therefore, it is preferable that the core length of the wound core body 10 is long. The core length of the wound core body 10 may be 1.0 m or more, preferably 1.5 m or more, and more preferably 1.7 m or more. In addition, in the present invention, the core length of the wound core body 10 refers to the perimeter of the grain-oriented electrical steel sheet 100 at the center point in the stacking direction of the wound core body 10 when viewed from the side.

[0124] Examples

[0125] Hereinafter, while showing examples, a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described in more detail. It should be noted that the examples shown below are merely examples of the grain-oriented electrical steel sheet of the present embodiment, and the grain-oriented electrical steel sheet of the present embodiment is not limited to the examples shown below.

[0126] A slab prepared in such a chemical composition that the grain-oriented electrical steel sheet contains Si: 3.0%, C: 0.080%, acid-soluble Al: 0.028%, N: 0.010%, Mn: 0.12%, Cr: 0.05%, Cu: 0.04%, P: 0.01%, Sn: 0.02%, Sb: 0.01%, Ni: 0.005%, S: 0.007%, Se: 0.001% by mass fraction, and the balance is composed of Fe and impurities is subjected to hot rolling to obtain a hot-rolled steel sheet with a thickness of 2.3 mm.

[0127] Next, the above hot-rolled steel sheet is subjected to an annealing treatment under the temperature condition of heating at 1000 °C for 1 minute.

[0128] After annealing treatment, cold rolling was carried out to obtain a cold-rolled steel sheet with a thickness of 0.23 mm. Then, after decarburization annealing treatment was carried out under the temperature condition of heating the cold-rolled steel sheet at 800 °C for 2 minutes, an annealing separating agent containing magnesium oxide (MgO) as the main component was coated on the surface of the cold-rolled steel sheet.

[0129] Next, for the cold-rolled steel sheet coated with the annealing separating agent, final annealing treatment was carried out under the temperature condition of heating at 1200 °C for 20 hours. As a result, a steel sheet with the above chemical composition, controlled crystal orientation in such a way that the easy magnetization axis of the crystal grains is consistent with the rolling direction, and a glass film formed on the surface was obtained.

[0130] Next, a laser was irradiated on the surface of the steel sheet to impart at least one of a groove or strain to the surface of the steel sheet.

[0131] The laser irradiation device uses a fiber laser manufactured by IPG Corporation. The irradiation conditions of the laser are adjusted such that the laser output is 1800 W, the laser scanning speed is 50 m / s, and the laser scanning pitch (interval PL) is 3 mm. The condensing spot diameter of the part with the largest beam diameter in the rolling direction of the laser on the steel sheet is 10 μm to 200 μm, and the groove with the apex angle shown in Table 1 is adjusted by a flat-top laser. Linear grooves with a width w of about 50 μm and a depth D of about 20 μm are formed at intervals of 3 mm in the direction perpendicular to the rolling direction.

[0132] Using each steel sheet as a raw material, a wound core was manufactured. The core length was set to 1.0 m, and the number of bent parts was 4. In the bending process, tensile strain was generated in the bent parts of the wound core, resulting in the residual stress shown in Table 1. This residual stress was measured by the above measurement method and is the residual stress at a position within 10 μm in the rolling direction (RD direction) from the wall surface of the groove located in the bent part of the wound core.

[0133] [Magnetic property evaluation]

[0134] Samples with a width of 60 mm × a length of 300 mm were collected at the center of the plate width of the directional electromagnetic steel sheets including each test number. The length of the sample is parallel to the rolling direction.

[0135] Using this sample, according to JIS C2256 (2011), the magnetic flux density (T) was obtained through a single-plate magnetic property test (SST test). Specifically, a magnetic field of 800 A / m was applied to the sample, and the magnetic flux density (T) was obtained.

[0136] Furthermore, using the above sample, according to JIS C2256 (2011), the iron loss W 17 / 50 (W / kg) was measured when the frequency was set to 50 Hz and the maximum magnetic flux density was set to 1.7 T.

[0137] In addition, the process coefficient (BF) was also measured by the above measurement method. The measurement results are shown in Table 1.

[0138] Table 1

[0139]

[0140] From this result, it can be seen that for the electromagnetic steel sheet with residual stress caused by strain in the grooves and bent portions where the embodiments of the present invention exist, the process coefficient (BF) is further reduced compared to the comparative example.

[0141] Description of reference numerals

[0142] 1 Transformer

[0143] 10 Winding core

[0144] 20A Primary winding

[0145] 20B Secondary winding

[0146] 100 Directional electromagnetic steel sheet

Claims

1. A wound iron core, characterized in that, It is a wound core made of a grain-oriented electromagnetic steel sheet, wherein the grain-oriented electromagnetic steel sheet has a steel sheet surface formed with the following grooves, the grooves extending in a direction crossing the rolling direction and the groove depth direction being the plate thickness direction, the pitch of the grooves in the steel sheet rolling direction is 2 to 10 mm, the depth of the grooves is 10 μm or more and 40 μm or less, and the width of the grooves is 10 μm or more and 200 μm or less, the angle formed when the opposite wall surfaces of the grooves are extended in the plate thickness direction is 90° or less, where the surface of the groove at a depth of 25 to 75% relative to the depth of the groove is set as the wall surface, in a range within 10 μm in the rolling direction from the wall surface of the groove located at the bent portion of the wound core, there is an area with a residual stress caused by a tensile strain in the steel sheet rolling direction of 10 MPa or more accounting for 10 area% or more.

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