Grain-oriented electrical steel sheet

Abstract

In order to refine the magnetic domain, the shape of the groove is controlled within a certain range, thereby further reducing the iron loss of the obtained grain-oriented electrical steel sheet. This grain-oriented electrical steel sheet is provided with a steel sheet having a steel sheet surface in which grooves are formed, the grooves extending in a direction intersecting the rolling direction and having a groove depth direction as the sheet thickness direction, and having a groove group configured by arranging a plurality of the grooves with respect to the sheet width direction when the steel sheet surface is viewed from the sheet thickness direction, a plurality of the groove groups are arranged at intervals with respect to the rolling direction, and among 75% or more of the groove groups, the average width of the grooves constituting the groove groups is 42-62 [mu] m, and the average width of the grooves constituting the groove groups is 40-60 [mu] m. The grooves constituting the groove group are arranged so as to overlap with adjacent grooves on a projection plane parallel to the direction of extension of the grooves and the direction of depth of the grooves, the amount of overlap between a second groove and a first groove in the projection plane is 5-30 mm, and the undulation of the edges of the grooves constituting the groove group is 0.5-5.0 [mu] m.

Description

Technical Field

[0001] The present invention relates to a grain-oriented electrical steel sheet, particularly a grain-oriented electrical steel sheet having excellent iron loss characteristics after stress relief annealing. Background Art

[0002] Grain-oriented electrical steel sheets are used as magnetic cores in many electrical devices. A grain-oriented electrical 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 grain-oriented electrical 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] In such grain-oriented electrical steel sheets, various technical developments have been made to improve magnetic properties. In particular, with the recent requirements for energy conservation, further reduction of iron loss is required in grain-oriented electrical steel sheets. For reducing the iron loss of grain-oriented electrical steel sheets, regarding the orientation of the grains of the steel sheet, it is effective to increase the degree of aggregation in the Goss orientation to increase the magnetic flux density and reduce the hysteresis loss.

[0004] For a grain-oriented electrical steel sheet used as a base material for a wound transformer, further reduction of iron loss is particularly required. In an electrical steel sheet, magnetic domain refinement is performed to reduce iron loss, but in a wound transformer, stress relief annealing is performed in the manufacturing process. Therefore, in the case of performing magnetic domain refinement, a heat-resistant magnetic domain refinement technique is required.

[0005] For example, as disclosed in Patent Document 1, it is well known that by using the laser irradiation method, it is relatively easy and stable to form grooves on the steel sheet surface for magnetic domain control.

[0006] Patent Document 2 discloses a grain-oriented electrical steel sheet having grooves. When grooves are formed on the steel sheet surface, the productivity is excellent and the iron loss can be improved.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: International Publication No. 2016 / 1711124

[0010] Patent Document 2: International Publication No. 2016 / 1711129 Summary of the Invention

[0011] Problems to be Solved by the Invention

[0012] For a grain-oriented electrical steel sheet used as a base material for a wound transformer, further reduction of iron loss is required. In heat-resistant magnetic domain refinement, the 180° magnetic domain width is generally refined by forming linear grooves or introducing strain, and along with this, the eddy current loss, which is part of the iron loss, is reduced. However, further reduction of iron loss is needed.

[0013] The present invention has been completed in view of the above problems. The purpose is to further reduce the iron loss of the obtained grain-oriented electrical steel sheet by controlling the shape of the grooves within a certain range for magnetic domain refinement.

[0014] Means for Solving the Problems

[0015] The inventors of the present invention found that the shape of the grooves (especially the groove width and the undulation of the groove edge, which is the boundary line between the steel sheet surface and the groove) and the overlapping amount between adjacent grooves affect the iron loss, and came up with the present invention in which the iron loss can be reduced by restricting these amounts.

[0016] The gist of the present invention is a grain-oriented electrical steel sheet having a steel sheet with a steel sheet surface formed with grooves, the grooves extending in a direction crossing the rolling direction and the groove depth direction being the plate thickness direction.

[0017] When observing the steel sheet surface in the plate thickness direction, there is a groove group formed by arranging a plurality of the grooves with respect to the plate width direction.

[0018] A plurality of the groove groups are arranged at intervals with respect to the rolling direction, wherein

[0019] In 75% or more of the groove groups,

[0020] the average width of the grooves constituting the groove group is 42 to 62 μm.

[0021] The grooves constituting the groove group are arranged so as to overlap with adjacent grooves on a projection plane parallel to the groove extending direction and the groove depth direction.

[0022] In this grain-oriented electrical steel sheet, taking the end portion in the plate width direction of the steel sheet as a reference end portion, and setting adjacent grooves among the plurality of grooves of the groove group as a first groove and a second groove in the order from the closest to the reference end portion.

[0023] Setting the two groove ends in the groove extending direction of each groove constituting the groove group as a first groove end and a second groove end in the order from the closest to the reference end portion.

[0024] In the projection plane, the overlapping amount between the first groove end of the second groove and the second groove end of the first groove is 5 to 30 mm.

[0025] The undulation of the edge of the groove constituting the groove group is 0.5 to 5.0 μm.

[0026] Advantages of the Invention

[0027] According to the present invention, it has been found that the shape of the groove (particularly the groove width and the undulation of the groove edge which is the boundary line between the steel plate surface and the groove) and the overlapping amount of adjacent grooves affect iron loss. By restricting these amounts, pinning during the movement of magnetic domain walls is eliminated, and iron loss is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic view showing the grooves on the steel plate surface of the grain-oriented electrical steel sheet according to one embodiment of the present invention.

[0029] Figure 2 is showing Figure 1 the cross-sectional shape of the groove (in the length direction) at the A-A line of

[0030] Figure 3 is showing Figure 1 the cross-sectional shape of the groove (in the short side direction) at the B-B line shown in

[0031] Figure 4 is an explanatory view of the definition of the groove.

[0032] Figure 5 is an explanatory view of the definition of the groove.

[0033] Figure 6 is an explanatory view of the definition of the groove.

[0034] Figure 7 is an explanatory view of the definition of the groove.

[0035] Figure 8 is a view showing the groove length projection lines of adjacent grooves of the grain-oriented electrical steel sheet of this embodiment.

[0036] Figure 9 is a view showing the relationship between the average width of the groove and the undulation of the groove edge.

[0037] Figure 10 is a view showing the relationship between the overlapping amount of adjacent grooves and the undulation of the groove edge. DETAILED DESCRIPTION OF THE INVENTION

[0038] Hereinafter, preferred embodiments of the present invention will be described in detail. It should be noted that, unless otherwise specified, for numerical values A and B, the expression "A to B" means "A or more and B or less". In the case where only the unit is attached to the numerical value B in this expression, the unit also applies to the numerical value A.

[0039] [Composition of the Steel Plate]

[0040] First, the directionality electromagnetic steel sheet of the present embodiment is not particularly limited, and a directionality electromagnetic steel sheet composed of a known steel composition can be used. The steel sheet that can be used as the directionality electromagnetic steel sheet of the present invention will be illustratively described.

[0041] [Composition of the steel sheet]

[0042] The composition of a typical steel sheet will be described. It should be noted that, unless otherwise specified, the expression of "%" is defined to represent "% by mass" hereinafter.

[0043] Regarding the composition of the steel sheet used in the directionality electromagnetic steel sheet of the invention, in order to control the crystal orientation to aggregate in the {110}<001> orientation of the Goss texture, it may preferably contain at least Si: 1.0 to 5.0% and Mn: 0.01 to 0.15%.

[0044] (Si: 1.0 to 5.0%)

[0045] The content of Si (silicon) is 1.0 to 5.0%. Si increases the resistance of the steel sheet and reduces the eddy current loss, which is one of the causes of iron loss. When the content of Si is less than 1.0%, it is difficult to sufficiently suppress the eddy current loss of the final directionality electromagnetic steel sheet, so it is not preferred. When the content of Si exceeds 5.0%, the workability of the directionality electromagnetic steel sheet decreases, so it is not preferred. Therefore, the content of Si is 1.0 to 5.0%, preferably 2.50 to 4.50%, and more preferably 2.70 to 4.00%.

[0046] (Mn: 0.01 to 0.15%)

[0047] 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 amount of MnS and MnSe that cause secondary recrystallization is insufficient, so it is not preferred. When the content of Mn exceeds 0.15%, it is 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 size of MnS and MnSe as inhibitors tends 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%.

[0048] The components other than Si and Mn can be the components contained in a normal directionality electromagnetic steel sheet.

[0049] For example, as components other than Si and Mn, by mass%, it may contain C: ~0.085% or less, acid-soluble Al: ~0.065% or less, N: ~0.012% or less, Cr: ~0.30% or less, Cu: ~0.40% or less, P: ~0.50% or less, Sn: ~0.30% or less, Sb: ~0.30% or less, Ni: ~1.000% or less, S: ~0.0150% or less. Additionally, as other inhibitor constituent elements, B, Bi, Se, Pb, Sn, Ti, etc. may 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 optionally added elements can be contained for known purposes, so there is no need to set a lower limit value for the content of the optionally added elements. For example, the lower limit value can be 0%.

[0050] The remainder 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 refer to components that are allowed within a range that does not substantially affect the present embodiment.

[0051] The chemical composition of the steel sheet can be measured using ICP - AES (Inductively Coupled Plasma - Atomic Emission Spectrometry). Specifically, using an ICPS - 8100 etc. (measurement device) manufactured by Shimadzu Corporation, a test piece with a side length of 35 mm 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.

[0052] [Tank]

[0053] Hereinafter, the tank of the embodiment of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.

[0054] Figure 1 It is a top view of the directionally - oriented electromagnetic steel sheet 1 of the present embodiment. Figure 2 is Figure 1The end view along the A-A line. It should be noted that in the figure, the rolling direction of the grain-oriented electromagnetic steel sheet 1 is defined as X, the sheet width direction of the grain-oriented electromagnetic steel sheet 1 (the direction orthogonal to the rolling direction in the same plane) is defined as Y, and the sheet thickness direction of the grain-oriented electromagnetic steel sheet 1 (the direction orthogonal to the XY plane) is defined as Z. The grain-oriented electromagnetic steel sheet 1 of the present embodiment has grooves 3 for magnetic domain refinement on the steel sheet surface 2a. Figure 1 It is a schematic diagram showing the grooves 3 when observing the grain-oriented electromagnetic steel sheet of the present embodiment from the sheet thickness direction Z (hereinafter, sometimes referred to as "top view").

[0055] As Figure 1 shown, when observing the grooves 3 from the sheet thickness direction Z (when looking down at the grooves 3), the extending direction of the grooves 3 ( Figure 1 the arrow L shown) is called the groove length direction L. When looking down at the grooves 3, the direction orthogonal to the groove length direction L of the grooves 3 ( Figure 1 the arrow Q shown) is called the groove width direction Q. The steel sheet surface 2a of the actual grain-oriented electromagnetic steel sheet and the surface of the grooves 3 are not uniformly formed, but for explaining the features of the invention, at least a part is schematically shown in Figures 1 to 3 , Figures 5 to 8 . In addition, the grooves 3 may have an arcuate shape when observed from the sheet thickness direction Z (when looking down at the grooves 3). However, in the present embodiment, for the sake of convenience in explanation, the grooves 3 having a straight shape are exemplified.

[0056] The grain-oriented electromagnetic steel sheet 1 includes a steel sheet (base metal) 2 whose crystal orientation is controlled by a combination of cold rolling treatment and annealing treatment so that the easy magnetization axis of the crystal grains coincides with the rolling direction X, and has grooves 3 on the surface of the steel sheet (steel sheet surface 2a).

[0057] As Figure 1 shown, the grooves 3 are formed to extend in a direction crossing the rolling direction X and the depth direction is the sheet thickness direction Z. The grain-oriented electromagnetic steel sheet 1, when observing the steel sheet surface 2a from the sheet thickness direction Z, has a groove group 30 composed of a plurality of grooves 3 arranged with respect to the sheet width direction Y, and the plurality of groove groups are arranged at intervals with respect to the rolling direction. The extending direction of the grooves 3 is not particularly limited, but when the sheet width direction Y is set to 0°, the angle formed by the extending direction of the sheet width grooves 3 and the sheet width direction Y may also be in the range of 0° to ±60°. The grooves 3 constituting the groove group 30 are substantially parallel to each other. The so-called substantially parallel may also have an angular difference of several degrees, for example, an angular difference within ±5° or within ±3° is allowed.

[0058] In more than 75% of all the slot groups, by setting the average width of the slots constituting the slot group, the overlapping amount between adjacent slots, and the undulation of the slot edges within a specific range, the iron loss can be effectively improved. It should be noted that the measurement of the slot shape is carried out after at least removing the glass coating film and the insulating coating film inside the slot from the final product by pickling or the like.

[0059] Define the terms in the following description.

[0060] (Average slot depth D)

[0061] As Figure 2 shown in the example, the depth of slot 3 refers to the length in the plate thickness direction Z from the height of the surface of steel plate 2a to the surface (bottom 4) of slot 3. The average slot depth D can be measured as follows. When observing slot 3 in the plate thickness direction Z (looking down on slot 3), set the observation range as a part of slot 3. It is desirable to set the observation range as the area other than the ends in the slot length direction L of slot 3 (i.e., the area where the shape of the slot bottom is stable). For example, the observation range can be set as the observation area at the approximate center in the slot length direction L with a length of about 30 μm to 300 μm in the slot length direction L. Then, use a laser microscope to obtain the height distribution (slot depth distribution) within the observation range, and find the maximum slot depth within this observation range. Change the observation range and perform the same measurement in at least 3 or more areas, more preferably 10 areas. Then, calculate the average value of the maximum slot depths in each observation area and define it as the average slot depth D. In order to preferably obtain the effect of magnetic domain refinement, the average slot depth D of slot 3 in this embodiment is, for example, preferably 5 μm or more and 100 μm or less, and more preferably more than 10 μm and 40 μm or less.

[0062] It should be noted that in order to measure the distance between the surface of steel plate 2a and the surface of slot 3, it is necessary to pre-measure the position (height) of the surface of steel plate 2a in the plate thickness direction Z. For example, the position (height) in the plate thickness direction Z of multiple parts of the surface of steel plate 2a within each observation range can be measured separately using a laser microscope, and the average value of these measurement results can be used as the height of the surface of steel plate 2a. In addition, in this embodiment, as described later, a slot short side cross-section is used when measuring the average slot width W, so the surface of steel plate 2a can also be measured from this slot short side cross-section. It should be noted that when observing a steel plate sample with a laser microscope, it is preferable that the two plate surfaces (the observation surface and its back surface) of the steel plate sample are substantially parallel.

[0063] (Average slot width W)

[0064] As Figure 3As shown in the example, the width of the groove 3 refers to the length of the groove opening in the groove short side direction Q when observing the groove 3 in a cross section orthogonal to the groove length direction L (groove width direction cross section or groove short side cross section). The average groove width W can be measured as follows. Similar to the average groove depth D, when observing the groove 3 from the plate thickness direction Z (when looking down at the groove 3), the observation range is set to a part of the groove 3. It is desirable to set the observation range to the area other than the ends in the groove length direction L of the groove 3 (i.e., the area where the shape of the groove bottom is stable).

[0065] For example, the observation range can be set to an observation area at approximately the center in the groove length direction L and with a length of about 30 μm to 300 μm in the groove length direction L. Then, using a laser microscope, at any position within the observation range (for example, the position of the maximum groove depth in the observation area), a groove short side cross section orthogonal to the groove length direction L is obtained. The length of the groove opening is obtained based on the steel plate surface 2a and the contour curve of the groove 3 that appear in this groove short side cross section.

[0066] Specifically, after applying a low-pass filter (cutoff value λs) to the measurement cross section curve MCL formed by the steel plate surface 2a and the contour of the groove 3 that appear in the above-mentioned groove short side cross section to obtain a cross section curve, and then applying a band-pass filter (cutoff values λf, λc) to this cross section curve, if the long wavelength components and short wavelength components are removed from the cross section curve, as Figure 3 shown, the undulation curve WWC that constitutes the contour of the groove 3 in the groove short side cross section is obtained. The undulation curve is a type of contour curve suitable for simplifying the shape of the contour itself with a smooth line.

[0067] As Figure 3 shown, on the undulation curve WWC of the groove 3 in the groove short side cross section, the length (groove opening) W of the line segment connecting two points (the third point 33 and the fourth point 34) on the steel plate surface 2a separated by the groove 3 is obtained n .

[0068] Change the observation range and perform the same measurement in at least 3 or more areas, more preferably 10 areas. Then, calculate the average value of the groove openings in each observation area and define it as the average groove width W. In order to preferably obtain the effect of magnetic domain refinement, the average groove width W of the groove 3 in this embodiment is, for example, 42 μm or more and 62 μm or less.

[0069] It should be noted that in order to measure two points (the third point 33 and the fourth point 34) on the steel plate surface 2a separated by the groove 3, it is necessary to previously measure the position (height) of the steel plate surface 2a in the plate thickness direction Z. For example, the position (height) of the plate thickness direction Z can also be measured for multiple parts of the steel plate surface 2a on the undulation curve within each groove short side cross section, and the average value of these measurement results can be used as the height of the steel plate surface 2a.

[0070] (Overlap of slots)

[0071] On the projection plane ( Figure 1 When observed from the cross section indicated by the dotted line 11a, the grooves 3 constituting the groove group 30 are arranged in a manner such that adjacent grooves overlap each other.

[0072] According to this configuration, when the grain-oriented electromagnetic steel sheet 1 has a plurality of grooves 3 formed in the sheet width direction Y, the grooves 3 are ensured to be formed in the sheet width direction Y, thereby improving iron loss.

[0073] When one end of the steel plate in the plate width direction Y is regarded as the reference end 21a, the plurality of grooves 3 constituting the groove group 30 are formed in the order from near to far from the reference end 21a, namely, the first groove 31, the second groove 32, and the nth groove 3n. Figure 1 As shown, the first groove 31, the second groove 32, and the nth groove 3n are arranged in a manner such that the ends of adjacent grooves 3 overlap (coincide) on a projection plane parallel to the groove extension direction (or groove length direction L) and the groove depth direction (or plate thickness direction Z).

[0074] In addition, if Figure 1 As shown, the groove group 30 is arranged in a manner separated from other groove groups 30 in the rolling direction X.

[0075] If the slots 3 are arranged so that the ends of adjacent slots 3 overlap (superimpose) on a projection plane parallel to the slot extending direction (or slot length direction L) and the slot depth direction (or plate thickness direction Z), iron loss can be effectively improved.

[0076] When a plane parallel to the groove length direction L and the plate thickness direction Z is set as a projection plane and the outline of the groove 3 in the groove length direction L is projected onto the projection plane, the outline of the groove in the groove length direction L projected onto the projection plane is defined as the groove length projection line LWP. The groove length projection line LWP can be measured as follows. When the groove 3 is viewed from the plate thickness direction Z (refer to Figure 6 ), as the observation range, set an area including the entirety of the groove 3, or an area including the end of the groove 3 (that is, an area including the beginning of the groove 3 in the groove length direction L to the area where the shape of the groove bottom is stable). Within this observation range, multiple imaginary lines along the groove length direction L are hypothetically set. The imaginary lines L1~Ln can be set to any height in the plate thickness direction Z. Then, select an imaginary line that satisfies the conditions that the average depth of the groove is the maximum among the above-mentioned imaginary lines L1~Ln as the groove reference line BL. For example, Figure 6As shown, when the groove depth D2 among the groove depths D1 to Dn respectively obtained for the imaginary lines L1 to Ln is the maximum, the imaginary line L2 is defined as the groove reference line BL. A curve obtained by projecting the groove depth distribution along the selected imaginary line onto the above-mentioned projection plane as the overall profile (undulation curve) in the groove length direction L of the groove 3 is defined as the groove length projection line LWP. In addition, as the above-mentioned observation range, it is preferable to set a region including the whole of two adjacent grooves, or a region including the overlapping end portions of two adjacent grooves (that is, a region including a region where the shape of the bottom of one groove is stable, a region where the ends of two adjacent grooves overlap, and a region where the shape of the bottom of the other groove is stable). The two groove ends in the groove length direction L of each groove constituting the groove group 30 are set as the first groove end and the second groove end in the order from the nearer to the farther from the reference end portion 21a. Figure 8 Schematically shown are the first groove end 31a and the second groove end 31b of the first groove length projection line LWP1 of the first groove 31, and the first groove end 32a and the second groove end 32b of the second groove length projection line LWP2 of the second groove 32. In addition, in order to illustrate the positional relationship in the groove length direction L between adjacent grooves, in Figure 8 only two adjacent grooves 31 and 32 among the plurality of grooves 3 of the directionality electromagnetic steel sheet 1 of the present embodiment in the groove length direction L are extracted and described.

[0077] As Figure 1 shown, the directionality electromagnetic steel sheet 1 of the present embodiment is arranged such that the second groove end 31b of the first groove 31 adjacent in the groove length direction L overlaps the first groove end 32a of the second groove 32 in the groove length direction L. In Figure 1 an example is shown in which when viewed from the plate thickness direction Z, the end portions of the first groove 31 and the second groove 32 adjacent in the groove length direction L do not overlap. However, when viewed from the plate thickness direction Z, the end portions of the first groove 31 and the second groove 32 may also overlap. For example, when viewed from the plate thickness direction Z, if the end portions of the first groove 31 and the second groove 32 completely overlap, they can be regarded as one groove.

[0078] Adjacent grooves overlap each other in the groove length direction L such that the position in the groove length direction L of the first groove end 32a of the second groove 32 in the second groove length projection line LWP2 is located closer to the reference end portion 21a side than the position in the groove length direction L of the second groove end 31b of the first groove 31 in the first groove length projection line LWP1. As Figure 8 shown, between the second groove end 31b of the first groove 31 and the first groove end 32a of the second groove 32 is a region R where the first groove 31 and the second groove 32 overlap in the groove length direction L.

[0079] In the overlapping region R, the distance in the longitudinal direction L between the second slot end 31b of the first slot 31 and the first slot end 32a of the second slot 32 is referred to as the "overlap amount". The "overlap amount" of the directional electromagnetic steel sheet 1 of the present embodiment is 5 to 30 mm. In the directional electromagnetic steel sheet 1, a plurality of slots are formed in the slot longitudinal direction L, and adjacent slots 31 and 32 overlap each other. With the "overlap amount" being 5 to 30 mm, the iron loss can be suppressed to a low level. If the overlap amount is less than 5 mm, as approaching the end of the slot, the effect of magnetic domain refinement becomes smaller, and it is difficult to refine the magnetic domains. Therefore, the magnetic domains in the overlapping amount part will not be refined, and the effect of reducing iron loss cannot be expected. On the other hand, if the overlap amount exceeds 30 mm, the portion where the slot interval is narrow relatively increases. If the slot interval is narrow, the magnetic domains are refined, but the magnetic domain walls are fixed in the slot part, and it is difficult for the magnetic domain walls to move according to the change of the external magnetic field. As a result, the hysteresis loss that constitutes the iron loss becomes large, offsetting the effect brought by the magnetic domain refinement. Therefore, the iron loss increases, and the improvement of magnetic properties cannot be expected. That is, by arranging a plurality of slots 3 in the slot longitudinal direction L and overlapping the end portions of adjacent slots appropriately in the slot longitudinal direction L, the iron loss can be improved in the same way as in the case of forming a slot with a uniform depth in the slot longitudinal direction L.

[0080] The separation distance in the rolling direction X between the first slot 31 and the second slot 32 adjacent in the plate width direction Y ( Figure 1 the distance F1 shown) can also be set to be smaller than the separation distance in the rolling direction X between adjacent slot groups 30 in the rolling direction X ( Figure 1 the distance F2 shown). When the average depth of the slot group of a plurality of slots 31, 32, …, 3n arranged along the plate width direction Y is set to DA, the first slot 31 and the second slot 32 can also be arranged such that the distance between the first slot end 32a of the second slot 32 and the reference end 21a of the steel sheet 2 is shorter than the distance between the second slot end 31b of the first slot 31 and the reference end 21a of the steel sheet 2. Thus, a plurality of slots 3 can be arranged in the plate width direction Y, and the end portions of adjacent slots 31 and 32 can be overlapped in the plate width direction Y, and the iron loss can be improved in the same way as in the case of forming a slot with a uniform depth in the plate width direction Y.

[0081] (The undulation of the edge of the slot)

[0082] The edge of the slot 3, that is, the ridge line (boundary line) between the slot 3 and the steel sheet 2 is not necessarily Figure 6 linear as shown, and sometimes becomes a curved shape with undulation. Therefore, it is necessary to clarify the undulation of the edge of the slot when observing the slot 3 from the plate thickness direction Z (looking down on the slot 3). Hereinafter, the method for determining the undulation of the edge of the slot when looking down on the slot 3 will be described.

[0083] When observing the groove 3 in the direction of the plate thickness Z (when looking down on the groove 3), when measuring the edge in the groove extension direction of the groove 3 using a laser microscope or the like, the measured edge curve MEL constituting the edge of the groove 3 as shown in Figure 7 is obtained. Here, the point where the position (height) of the steel plate surface 2 starts to decrease toward the bottom of the groove is defined as the boundary between the groove 3 and the steel plate 2. The ridge line (boundary line) of the groove 3 and the steel plate 2 that connects this boundary delimits the edge of the groove. In order to measure the edge of the groove 3, that is, the ridge line (boundary line) between the groove 3 and the steel plate 2, it is necessary to previously measure the position (height) of the steel plate surface 2 in the Z direction. For example, the position (height) in the Z direction can be measured for a plurality of parts of the steel plate surface 2 within the observation range 50 using a laser surface roughness measuring instrument respectively, and the average value of these measurement results can be used as the height of the steel plate surface 2. When the groove 3 is longer than the observation field of view of a laser microscope or the like in the groove extension direction, it is only necessary to divide one end 31a to the other end 31b of the groove 3 into several observation ranges 50 for observation. For example, the observation range 50 can be set as an observation area with a length of about 300 μm in the groove extension direction.

[0084] After applying a low-pass filter (cutoff value λs) to the measured edge curve MEL obtained as described above to obtain an edge curve, and then applying a band-pass filter (cutoff values λf, λc) to this edge curve, if the long-wavelength components and short-wavelength components are removed from the measured edge curve, as shown in Figure 7 , the undulation curve EWC constituting the undulation of the edge in the groove extension direction of the groove 3 is obtained. The undulation curve is a contour curve suitable for simplifying the shape of the contour itself with a smooth line. On the other hand, a hypothetical line that satisfies the condition of being along the groove length direction L among the hypothetical lines L1 to Ln and having the smallest difference from the measured edge curve MEL of the groove is selected as the edge reference line EBL. For example, as shown in Figure 6 , regarding the hypothetical lines L1 to Ln, the hypothetical line Ln with the smallest displacement difference from the measured edge curve MEL calculated by the least squares method is defined as the edge reference line EBL. As shown in Figure 7 , if the difference between the edge reference line EBL and the undulation curve EWC is used, the undulation of the edge of the groove can be evaluated with high precision.

[0085] (Undulation Rag of the edge of the groove)

[0086] In the present embodiment, the undulation of the edge of the groove is 0.5 to 5.0 μm. If the undulation Rag of the edge of the groove is less than 0.5 μm, it has a small effect as a starting point for the generation of magnetic domain walls, and sometimes magnetic domain refinement cannot be sufficiently performed. In addition, if the undulation of the edge of the groove exceeds 5.0 μm, pinning that hinders the movement of magnetic domain walls during the magnetization process of the electromagnetic steel plate will occur, and it becomes difficult to reduce iron loss.

[0087] The undulation of the edge of the groove can be defined by the following formula (1).

[0088] [Number 1]

[0089]

[0090] Among them, Rag is the undulation (μm) of the edge of the groove, l is the length in the groove length direction, x is an arbitrary position in the groove length direction, and f(x) is the difference between the edge baseline EBL of the groove and the undulation curve EWC of the contour forming the edge.

[0091] [Manufacturing method of grain-oriented electrical steel sheet]

[0092] 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 only an example of the manufacturing method of the grain-oriented electrical steel sheet of this embodiment, and it can be arbitrarily changed within the range that does not damage the effects of this embodiment.

[0093] (Casting process S1)

[0094] In the casting process S1, a slab is prepared. An example of the manufacturing method of the slab is as follows. First, molten steel is manufactured (smelted). 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 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, a so-called thin slab 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.

[0095] 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 as follows. It should be noted that the % notation used for the components refers to mass %.

[0096] 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 content 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%.

[0097] 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% in addition, it may not be possible to ensure a specified amount of effective MnS inhibitor. In addition, if the Mn content is more than 0.15% and the S content is more than 0.150%, 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 - 0.150%.

[0098] C is an element effective for controlling the primary recrystallized 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 is purified in the decarburizing annealing process S5 and the final annealing process S8 described later and is 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%.

[0099] 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% at which the magnetic flux density becomes higher.

[0100] When 0.012% or more of N is added during steelmaking, voids in the steel sheet called blisters 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%.

[0101] 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%.

[0102] The remainder 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 ores and scraps and manufacturing processes during the industrial production of the slab, and are substances tolerated within the range that does not substantially affect the directionality electromagnetic steel sheet of the present embodiment.

[0103] In the slab, in consideration of the enhancement of the inhibitor function brought about by compound formation and the influence on magnetic properties, any known element can also be contained (added) instead of a part of Fe. Examples of the arbitrary element contained in the slab instead 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 0.3%, the upper limit value of the P content can be 0.5%, the upper limit value of the Sb content can be 0.3%, the upper limit value of the Sn content can be 0.3%, the upper limit value of the Cr content can be 0.30%, and the upper limit value of the Ni content can be 1.0%. 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%.

[0104] The chemical composition of the slab can be measured by ICP - AES (Inductively Coupled Plasma - Atomic Emission Spectrometry). Specifically, using an ICPS - 8100 etc. (measurement device) manufactured by Shimadzu Corporation, 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 by combustion - infrared absorption method, and N can be measured by inert gas fusion - thermal conductivity method.

[0105] (Hot rolling process S2)

[0106] 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 as an inhibitor component, the heating temperature during hot rolling can be 1280°C or lower. It should be noted that in the case of using AlN and MnS as main inhibitors, the heating temperature during hot rolling can be set to 1300°C or higher to completely solid - solve these inhibitor components.

[0107] (Annealing process of hot - rolled steel sheet S3)

[0108] 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.

[0109] (Cold rolling process S4)

[0110] 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.

[0111] (Decarburizing annealing process S5)

[0112] 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 with primary recrystallization (cold-rolled steel sheet subjected to the decarburizing annealing process). The decarburizing annealing can be carried out, for example, at 700 °C to 900 °C for 1 minute to 3 minutes.

[0113] 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 humid atmosphere.

[0114] (Nitriding treatment process S6)

[0115] 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 separating agent containing a powder having nitriding ability such as MnN to the decarburized annealed steel sheet in the annealing separating agent coating process S7 described below, etc. can be cited.

[0116] (Annealing separating agent coating process S7)

[0117] The annealing separating agent coating process S7 is a process of applying an annealing separating agent to the decarburized annealed steel sheet. As the annealing separating agent, for example, an annealing separating agent mainly composed of alumina (Al2O3) can be used. The decarburized annealed steel sheet after applying the annealing separating agent is subjected to final annealing in the next final annealing process S8 in a coiled state.

[0118] In addition, when forming a glass film containing Mg2SiO4, an annealing release agent mainly composed of magnesium oxide (MgO) is used.

[0119] (Final annealing process S8)

[0120] The final annealing process S8 is a process of performing final annealing on a decarburized annealed steel sheet coated with an annealing release agent to generate 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.

[0121] In addition, when magnesium oxide (MgO) is coated in the above annealing release 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 steel sheet (the final annealed steel sheet described later). Therefore, for example, when 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 to further improve the characteristics of the finally obtained grain-oriented electrical steel sheet.

[0122] (Groove forming process S9)

[0123] 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.

[0124] In the process flow 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., cold-rolled steel sheet) that has undergone the cold rolling process S4. In this case, it is also possible to maintain an ideal cross-sectional shape of the linear groove G in magnetic domain refinement. Therefore, the timing of performing the groove forming process S9 can be before or after the final annealing process S8. However, when 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.

[0125] When using a laser to form grooves, the electrical steel sheet of the present embodiment can be obtained through the following example of laser irradiation conditions.

[0126] In the laser irradiation process, the laser can also be irradiated on the surface (only one side) of the steel sheet, and a plurality of grooves extending in a direction intersecting the rolling direction are formed at desired intervals in a range of 2 to 20 mm along the rolling direction on the surface of the steel sheet.

[0127] In the laser irradiation process, the laser irradiation device can also irradiate the surface of the steel plate with laser light by rotating and driving a polygonal mirror, and scan the laser light in a direction at an angle of 0 to 30° with respect to the direction perpendicular to rolling.

[0128] It is also possible to spray a water jet onto the part of the steel plate irradiated with the laser while irradiating the laser. In the water jet, by causing a super-high-speed water flow formed by ejecting water pressurized by a high-pressure water pump from a nozzle to collide with the target part, effects such as cleaning, peeling, and cutting of the target part can be generated. The pressurization can be up to 50 to 350 MPa, the nozzle can be about 0.1 mm to 1 mm in size, and abrasive materials (such as abrasive garnet) can also be added to the solvent (water). The material, particle size (#), etc. of the abrasive material can be appropriately selected. It can also be in the range of particle size (#) 10 to 1000. The water jet plays a role in removing the components melted or evaporated from the steel plate due to laser irradiation. By spraying the water jet, the above-mentioned melted or evaporated components can be prevented from remaining in the tank, and the undulations of the edge of the tank can be stably formed within the desired range.

[0129] It is also possible to blow auxiliary gases such as air or inert gas onto the part of the steel plate irradiated with the laser while irradiating the laser. The inert gas is, for example, nitrogen or argon. The auxiliary gas plays a role in removing the components melted or evaporated from the steel plate due to laser irradiation. By blowing the auxiliary gas, the laser can reach the steel plate without being blocked by the above-mentioned melted or evaporated components, so the tank is stably formed.

[0130] As the laser light source, for example, a fiber laser, a YAG laser, a semiconductor laser, or a CO2 laser, etc., which are commonly used high-output power lasers for industrial use, can be used. In addition, as long as the tank can be stably formed, a pulsed laser or a continuous wave laser can also be used as the laser light source. As the laser, a single-mode laser with high beam condensing property is usually used to form the tank, but a multi-mode laser with an appropriately distributed power peak can also be used.

[0131] As an example of the irradiation conditions of the laser, for example, the laser output power is set to 200 W to 3000 W, and the beam shape of the laser during tank formation is changed for implementation.

[0132] It is also possible to set the condensing spot diameter of the laser (that is, the diameter including 86% of the laser output, hereinafter sometimes also abbreviated as 86% diameter) to 10 μm to 1000 μm, set the laser scanning speed to 1 m / s to 100 m / s, and set the laser scanning pitch (interval PL) to 2 mm to 10 mm. In order to obtain a desired tank, these laser irradiation conditions are appropriately adjusted.

[0133] (Tension film application process S10)

[0134] The tension film application step S10 is a step of forming an insulating film (tension film) on the groove forming surface by coating a coating solution on the groove forming surface of the finally annealed steel sheet and baking it. By forming the insulating coating film (tension coating film), it is expected to further improve the characteristics of the finally obtained grain-oriented electrical steel sheet.

[0135] Here, the coating solution contains, for example, compounds of phosphoric acid, phosphate, chromic anhydride, chromate, alumina or silica. The baking can be carried out, for example, under the conditions of 350°C to 1150°C and 5 seconds to 300 seconds.

[0136] Examples

[0137] 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.

[0138] A slab prepared in such a way that the grain-oriented electrical steel sheet has a chemical composition containing 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 being composed of Fe and impurities is subjected to hot rolling to obtain a hot-rolled steel sheet with a thickness of 2.3 mm.

[0139] Next, the above hot-rolled steel sheet is subjected to annealing treatment under the temperature condition of heating at 1000°C for 1 minute.

[0140] After the annealing treatment, cold rolling is carried out to obtain a cold-rolled steel sheet with a thickness of 0.23 mm. Next, after subjecting the cold-rolled steel sheet to decarburization annealing treatment under the temperature condition of heating at 800°C for 2 minutes, an annealing parting agent containing magnesium oxide (MgO) as the main component is coated on the surface of the cold-rolled steel sheet.

[0141] Next, for the cold-rolled steel sheet coated with the annealing parting agent, final annealing treatment is carried out under the temperature condition of heating at 1200°C for 20 hours. As a result, a steel sheet having the above chemical composition, in which the easy magnetization axis of the crystal grains is controlled to be consistent with the rolling direction and a glass film is formed on the surface, is obtained.

[0142] Next, while irradiating a laser on one side of the surface of the steel sheet, a water jet was sprayed under the conditions shown in Table 1 to impart grooves to the surface of the steel sheet. In the water jet, high-hardness garnet Type-2 (800 - 300 μm) manufactured by Nitto Co., Ltd. was used as the abrasive material.

[0143] The laser irradiation device used a fiber laser manufactured by IPG. The laser irradiation conditions were adjusted such that the laser output power was 300 W, the laser scanning speed was 50 m / s, and the laser scanning pitch (interval PL) was 3 mm. The laser was adjusted such that the focused spot diameter was 50 μm. Linear grooves with a depth D of approximately 20 μm were formed at intervals of 3 mm in a direction perpendicular to the rolling direction. A grain-oriented electrical steel sheet was manufactured under the conditions shown in Table 1 such that the width of the grooves, the overlapping amount between adjacent grooves, and the groove group in which the undulations of the groove edges were adjusted accounted for 75% of all the groove groups. It should be noted that the process of forming the grooves was carried out in either the final annealing process or the cold rolling process.

[0144] A bobbin with a capacity of 25 kVA was manufactured using the obtained electrical steel sheet. After stress relief annealing (holding at a soaking temperature of 750 °C for 4 hours), the iron loss W17 / 50 was measured. The results are shown in Table 1.

[0145] [Magnetic property evaluation]

[0146] Regarding the bobbin, under the conditions of a frequency of 50 Hz and a magnetic flux density of 1.7 T, the iron loss value (sometimes also referred to as core iron loss) WA of the wound core was measured using the exciting current method described in JIS C2550 - 1:2011.

[0147] Table 1

[0148]

[0149] From these results, it can be seen that the magnetic properties of the bobbins trial-produced using the electrical steel sheets within the scope of the present invention are good, and an increase in core iron loss can be suppressed. Specifically, when a box-type core was made using a material with a raw material iron loss of 0.74 W / kg, the measured core iron loss deteriorated. Regarding the core iron loss, a judgment criterion of not exceeding 0.80 W / kg was set, and as a result, the conditions of No. 2 and No. 5 satisfied this criterion.

[0150] In addition, in No. 10, as an example of forming grooves using a laser without spraying a water jet, the laser output power was set to 3100 W, the focused spot diameter was set to 8 μm, the laser scanning speed was set to 1 m / s, and the laser scanning pitch (interval PL) was set to 2 mm to form grooves. At this time, the undulation of the groove edge was 15 μm, and the iron loss increased to 0.89 W / kg.

[0151] In addition, the case where the overlapping amount between adjacent slots is fixed at 25 mm and the width of the slots and the undulations of the slot edges are changed was investigated. The results are shown in Figure 9 . In Figure 9 , it was confirmed that when the width of the slots and the undulations of the slot edges are within the scope of the present invention, the magnetic properties are good. In this case, the core iron loss was also measured using a material with a raw material of 0.74 W / kg, and as a result, good magnetic properties not exceeding 0.80 W / kg were obtained.

[0152] In addition, the case where the width of the slots is fixed at 50 μm and the overlapping amount between adjacent slots and the undulations of the slot edges are changed was investigated. The results are shown in Figure 10 . In Figure 10 , it was confirmed that when the overlapping amount between adjacent slots and the undulations of the slot edges are within the scope of the present invention, the magnetic properties are good. In this case, the core iron loss was also measured using a material with a raw material of 0.74 W / kg, and as a result, good magnetic properties not exceeding 0.80 W / kg were obtained.

Claims

1. A directional electromagnetic steel sheet, comprising a steel sheet having a steel sheet surface formed with grooves, the grooves extending in a direction crossing the rolling direction and the groove depth direction being the plate thickness direction, when observing the steel sheet surface in the plate thickness direction, there are groove groups formed by arranging a plurality of the grooves with respect to the plate width direction, and a plurality of the groove groups are arranged at intervals with respect to the rolling direction, wherein, in more than 75% of the groove groups, the average width of the grooves constituting the groove group is 42 to 62 μm, the grooves constituting the groove group are arranged so as to overlap with adjacent grooves on a projection plane parallel to the groove extending direction and the groove depth direction, in this directional electromagnetic steel sheet, taking the end portion in the plate width direction of the steel sheet as a reference end portion, and setting adjacent grooves among the plurality of grooves of the groove group as a first groove and a second groove in the order from the closest to the reference end portion, setting the two groove ends in the groove extending direction of each groove constituting the groove group as a first groove end and a second groove end in the order from the closest to the reference end portion, in the projection plane, the overlapping amount between the first groove end of the second groove and the second groove end of the first groove is 5 to 30 mm, the undulation of the edge of the groove constituting the groove group is 0.5 to 5.0 μm.

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