Grain-oriented electrical steel sheet and its manufacturing method

Abstract

This grain-oriented electrical steel sheet has a steel sheet on the surface of which a plurality of grooves extending in a direction intersecting the rolling direction and having a depth in the sheet thickness direction are formed. In a cross-section parallel to the rolling direction and parallel to the sheet thickness direction of the steel sheet, there is at least one protrusion on the side surface or bottom surface of the grooves, the maximum height of said protrusion is 2-50 μm and the maximum width is 2-50 μm, and the difference in orientation between the average crystal orientation in said protrusion and the Goss orientation is 10 degrees or more.

Description

[Technical Field]

[0001] The present invention relates to a grain-oriented electrical steel sheet and a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2022-186168, filed on November 22, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] Grain-oriented electrical steel sheets are soft magnetic materials and are primarily used as iron core materials in transformers. Therefore, grain-oriented electrical steel sheets are required to have magnetic properties, including high magnetization and low iron loss. Iron loss is the power loss consumed as thermal energy when an iron core is excited by an AC magnetic field, and from the perspective of energy conservation, iron loss should be as low as possible. The level of iron loss is affected by factors such as magnetic susceptibility, sheet thickness, coating tension, impurity content, electrical resistivity, grain size, and magnetic domain width. Even now, when various technologies for grain-oriented electrical steel sheets have been developed, research and development continues to reduce iron loss in order to improve energy efficiency. One method proposed for reducing iron loss is laser irradiation, which introduces strain into the surface by irradiating the material with laser light, thereby dividing the 180° magnetic domain width and reducing eddy current loss, which is a part of iron loss.

[0003] For example, Patent Document 1 discloses a method for producing a steel sheet by irradiating a surface of the steel sheet with a focused continuous wave laser beam while scanning the surface in a direction inclined from the rolling direction of the steel sheet, and by shifting the scanning area of ​​the continuous wave laser beam at predetermined intervals, and the method is repeated. The average power of the continuous wave laser beam is represented by P (W), the scanning speed by Vc (mm / s), the predetermined interval by PL (mm), and the input energy Ua is represented by Ua = P / (Vc × PL) (mJ / mm 2 ), 1.0 mm ≦ PL ≦ 3.0 mm, and 0.8 mJ / mm 2 ≦Ua≦2.0mJ / mm 2 The present invention discloses a method for producing a grain-oriented electrical steel sheet in which magnetic domains are controlled by irradiation with laser light, which satisfies the above conditions. Patent Document 1 shows that it is possible to easily reduce iron loss in both the L-direction and C-direction of a grain-oriented electrical steel sheet while ensuring high productivity.

[0004] However, for example, when manufacturing a wound core, stress relief annealing is required because grain-oriented electrical steel sheets are bent and formed. Therefore, in this method, the stress relief annealing releases the strain introduced into the grain-oriented electrical steel sheets, and therefore the effect of controlling the magnetic domains by laser irradiation cannot be obtained. For this reason, it has been proposed to form grooves on the surface of the steel sheet to subdivide the 180° magnetic domain width in the same way as introducing strain, thereby reducing eddy current loss. For example, Patent Document 2 discloses a method for improving the iron loss characteristics of grain-oriented electrical steel sheet to which stress relief annealing can be applied, in which a laser beam is controlled and irradiated to form recesses having a width of 0.5 mm or less in the rolling direction and a depth of 10 μm or more. When grooves are formed by laser irradiation, the molten material generated by the laser irradiation is blown away with an assist gas to form the grooves. However, in this case, although the molten material inside the grooves is generally removed, protrusions may form around the grooves due to solidification of the molten material. If protrusions exist around the grooves, voids are created when the steel sheets are stacked, making it difficult for magnetic flux to flow and increasing iron loss. The protrusions can be removed to some extent by brushing or the like after laser irradiation. However, even if brushing or the like is performed, it is difficult to completely remove the protrusions because it is performed so as not to reduce the plate thickness. Patent Document 2 does not take such protrusions into consideration.

[0005] With regard to solidified portions of a molten material, Patent Document 3 discloses a grain-oriented electrical steel sheet having a surface on which grooves are formed, in which, in regions extending from the ends of the grooves in the groove width direction outward in the groove width direction, surface protrusions protruding from the surface of the steel sheet extend along the longitudinal direction of the grooves, the average protrusion height of the surface protrusions is more than 5 μm and not more than 10 μm, and when the surface protrusions are viewed in a cross section including the longitudinal direction of the groove and the normal direction to the steel sheet surface, the total length in the longitudinal direction of the groove of portions having a height of 50% or more of the height of a peak point appearing on the outline of the surface protrusion is 30% or more of the total length of the surface protrusions in the longitudinal direction of the groove, and an image of the steel sheet surface including the grooves is obtained using an apparatus capable of measuring the three-dimensional shape of the steel sheet surface, and portions in the periphery of the grooves that are higher than a reference plane are identified as the surface protrusions, and the cross section includes the peak points of the heights measured by the apparatus.

[0006] However, in Patent Document 3, although it is possible to prevent an increase in hysteresis loss to some extent by controlling the height of the surface protrusions, it does not reduce eddy current loss, and there is a problem in that it is not possible to obtain a sufficient effect of reducing iron loss.

[0007] Patent Document 4 describes a groove formed on a steel plate to have a first side surface, a second side surface, and a bottom surface facing each other, and a solidification portion formed on the first, second side surface, and bottom surface by solidifying a by-product of the molten steel plate during the groove formation process, in which the solidification portion is formed on the first and second side surfaces of the groove without remaining on the bottom surface, or the solidification portion is formed on the bottom surface and the second side surface, and the solidification portion is formed on the first and second side surfaces of the groove without remaining on the bottom surface. When the solidified portion is formed on the bottom surface and the second side surface, the opening portion is formed on the first side surface, and when the solidified portion is formed on the bottom surface and the second side surface, the solidified portions formed on the first and second side surfaces are formed so that their thickness decreases toward the bottom surface and becomes thicker toward the surface portion of the steel sheet, and the diameter (BW) of the groove in the rolling direction is 10 μm to 70 μm, and the length (BL) of the groove in the width direction of the steel sheet is 10 μm to 150 μm.

[0008] However, Patent Document 4 only describes the difference in thickness between the bottom and surface of the solidified portion, and does not consider the increase in surface area due to protrusions inside the groove and the associated reduction in eddy current loss. Therefore, Patent Document 4 has the problem of not being able to achieve a sufficient iron loss reduction effect. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent No. 4669565 [Patent Document 2] Japanese Patent Publication No. 6-57335 [Patent Document 3] Japanese Patent No. 7010311 [Patent Document 4] Japanese Patent No. 6307441 Summary of the Invention [Problem to be solved by the invention]

[0010] As described above, the conventional techniques have not been sufficiently effective in meeting the recent demand for higher iron loss reduction. In view of the above, the present inventors have an object to provide a grain-oriented electrical steel sheet having low iron loss, and a method for manufacturing the same, based on the grain-oriented electrical steel sheet in which grooves are formed by laser irradiation to perform magnetic domain refinement. [Means for solving the problem]

[0011] The present inventors have investigated reducing iron loss by reducing eddy current loss in a method for producing grain-oriented electrical steel sheet in which grooves are formed by laser irradiation to refine magnetic domains. As a result, they found that eddy current loss can be reduced by deliberately forming protrusions of a specific shape inside the grooves, and that the iron loss reduction effect can be further enhanced by controlling the crystal orientation of the protrusions. It was also found that the formation of protrusions inside the grooves was greatly affected by the laser irradiation conditions and the assist gas injection conditions.

[0012] The present invention has been made in light of the above findings. [1] A grain-oriented electrical steel sheet according to one aspect of the present invention has a plurality of grooves formed on the surface thereof, the grooves extending in a direction intersecting the rolling direction and having a depth in the sheet thickness direction. , which is the base steel sheet for grain-oriented electrical steel sheets The groove has a steel plate, and in a cross section of the steel plate parallel to the rolling direction and the plate thickness direction, at least one protrusion is present on a side surface or a bottom surface of the groove, and the maximum height of the protrusion is 5 ~50μm and maximum width 5 The misorientation between the average crystal orientation of the protrusions and the Goss orientation is 50 μm or less, 14 It is more than 100 degrees. [2] In the grain-oriented electrical steel sheet described in [1], the distance from the entrance of a groove to the entrance of the opposite groove in the cross section is defined as the groove width, and the groove width is determined by measuring the groove width and the groove width in the direction opposite to the groove center from the entrance of the groove of the steel sheet in the rolling direction. Same width When the region is defined as a groove edge, the protrusion of the groove edge may have a maximum height of 5 μm or less and a maximum width of 5 μm or less. [3] In the grain-oriented electrical steel sheet according to [1] or [2], a forsterite film may be formed on the surface of the steel sheet. [4] In the grain-oriented electrical steel sheet according to [3], an insulating coating may be formed on the surface of the forsterite coating. [5] In the grain-oriented electrical steel sheet according to [1] or [2], an insulating coating may be formed on the surface of the steel sheet. [6] A method for producing a grain-oriented electrical steel sheet according to another aspect of the present invention is the method for producing a grain-oriented electrical steel sheet according to [1] or [2], comprising: a hot rolling step of heating and hot-rolling a slab to form a hot-rolled sheet; a hot-rolled sheet annealing step of annealing the hot-rolled sheet after the hot rolling step; a cold rolling step of pickling the hot-rolled sheet after the hot-rolled sheet annealing step and cold-rolling it to form a steel sheet; a decarburization annealing step of decarburizing annealing the steel sheet; a finish annealing step of applying an annealing separator to the steel sheet after the decarburization annealing step and finish annealing it; and between the cold rolling step and the finish annealing step, irradiating a surface of the steel sheet with a laser to form grooves on the surface of the steel sheet, and applying a laser beam to the side or bottom of the grooves. and a groove forming step of forming protrusions on the sheet, the groove forming step including a first step of irradiating the sheet with a laser having a laser output of 200 to 3000 W, a focused spot diameter of 10 to 1000 μm, which is a diameter including 86% of the laser output in the rolling direction, and a focused spot diameter of 10 to 1000 μm in the sheet width direction, at a scanning speed of 2 to 50 m / s, and spraying an assist gas with a flow rate of 1 to 500 L / min; and a second step of irradiating the same locations as those irradiated with the laser in the first step with a laser having a laser output of 10 to 150 W, a focused spot diameter of 10 to 1000 μm in the rolling direction, and a focused spot diameter of 10 to 1000 μm in the sheet width direction, at a scanning speed of 2 to 50 m / s. [7] [6] The method for manufacturing a grain-oriented electrical steel sheet described above may further include a grinding step of grinding the surface of the steel sheet after the groove forming step using a brush roll having abrasive grains fixed thereto, and in the grinding step, the brush roll may rotate in a direction opposite to the conveying direction of the steel sheet at the position where it contacts the steel sheet, the conveying speed of the steel sheet may be 5 to 100 mpm, the rotation speed of the brush roll may be 500 to 2000 rpm, the reduction amount of the brush roll may be 2 to 10 mm, the grain size of the abrasive grains may be #40 to #400, and the diameter of the brush roll may be 200 to 500 mm. [Effects of the Invention]

[0013] According to the above aspects of the present invention, it is possible to provide a grain-oriented electrical steel sheet with low iron loss and a method for manufacturing the same. [Brief explanation of the drawings]

[0014] [Figure 1] 3A and 3B are diagrams showing examples of groove formation states of the grain-oriented electrical steel sheet according to the present embodiment. [Figure 2] 2 is a schematic diagram of a groove and a protrusion inside the groove of the grain-oriented electrical steel sheet according to the embodiment, viewed in a cross section perpendicular to the extending direction of the groove. FIG. [Figure 3] 2 is a schematic diagram of a groove and a protrusion at the groove edge of the grain-oriented electrical steel sheet according to the embodiment, viewed in a cross section perpendicular to the extending direction of the groove. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, a grain-oriented electrical steel sheet according to one embodiment of the present invention (grain-oriented electrical steel sheet according to this embodiment) and a method for producing the same will be described.

[0016] <Grain-oriented electrical steel sheet> 1, the grain-oriented electrical steel sheet 1 according to this embodiment has a steel sheet 11 on the surface of which are formed a plurality of grooves 21 that extend in a direction intersecting the rolling direction RD and have a depth in the sheet thickness direction. The steel sheet 11 is, for example, a cold-rolled sheet that has been subjected to cold rolling. The surface of the steel sheet 11 may be formed with a forsterite coating or an insulating coating. The insulating coating may also be formed on the surface of the forsterite coating. That is, the grain-oriented electrical steel sheet according to this embodiment may not only consist of a steel plate (base steel plate), but may also consist of a base steel plate and a forsterite coating formed on the surface of the base steel plate, or may consist of a base steel plate and an insulating coating formed on the surface of the base steel plate, or may consist of a base steel plate, a forsterite coating formed on the surface of the base steel plate, and an insulating coating formed on the surface of the forsterite coating. The forsterite coating and the insulating coating may be formed on one side or both sides. Each of these will be explained below.

[0017] [Steel plate (base material steel plate)] (groove) As shown in FIG. 1, a steel sheet 11 has a plurality of grooves 21 formed on the surface by laser irradiation, the grooves 21 extending in a direction intersecting the rolling direction RD and having a depth in the sheet thickness direction. Magnetic domain control can be achieved by periodically forming linear grooves in a direction intersecting the rolling direction RD. To achieve this effect, in the grain-oriented electrical steel sheet according to this embodiment, grooves 21 are formed on the surface of the steel sheet 11 (in the case of a grain-oriented electrical steel sheet having a forsterite coating or an insulating coating, this is the so-called base steel sheet excluding these). Here, the direction intersecting the rolling direction RD is a direction at an angle of 60 to 120° with respect to the rolling direction RD. The shape of the grooves 21 is not limited as long as it is within a range that can obtain the effect of magnetic domain control, but it is preferable that the depth is 10 to 50 μm and the width is 10 to 200 μm. Of the multiple grooves, the interval between adjacent grooves in the rolling direction RD is preferably 1 to 20 mm. It is preferable that the multiple grooves 21 are formed at approximately regular intervals (periodically) in the rolling direction RD. The groove interval is the distance from the center of one groove in the width direction to the center of the adjacent groove in the width direction.

[0018] (Protrusion inside the groove) In the grain-oriented electrical steel sheet according to this embodiment, at least one protrusion is present inside the groove (on the side or bottom surface of the groove) in a cross section parallel to the rolling direction and the sheet thickness direction, and the protrusion has a maximum height of 2 to 50 μm and a maximum width of 2 to 50 μm. By forming at least one protrusion on the bottom and side of the groove, with a width of 2 to 50 μm and a height of 2 to 50 μm, the surface area inside the groove is increased. The increased surface area inside the groove increases the amount of magnetic flux leaking from inside the groove when the grain-oriented electrical steel sheet is magnetized, increasing magnetostatic energy. To counteract the increase in magnetostatic energy, the magnetic domains are further subdivided, reducing eddy current loss. If there are no protrusions, or if the height or width of the protrusions is less than 2 μm, the surface area inside the groove does not increase sufficiently, and eddy current loss does not decrease. On the other hand, if the width or height of the protrusions exceeds 50 μm, the protrusions will be larger than the depth or width of the groove, and will protrude from the steel sheet surface. In this case, voids will occur when the steel sheets are stacked, increasing iron loss. If there are multiple protrusions, the evaluation will be based on the maximum height and maximum width. The maximum height and width of the protrusions are preferably 5 to 50 μm, more preferably 10 to 50 μm.

[0019] The crystal orientation of the protrusions inside the grooves is such that the misorientation between the average crystal orientation of the protrusions and the Goss orientation is 10 degrees or more. By making the misorientation between the average crystal orientation of the protrusions and the Goss orientation 10 degrees or more, the magnetic domain refinement effect is further enhanced.

[0020] The determination of whether at least one protrusion is present on the side or bottom surface of the groove and the measurement of the height and width of the protrusion can be carried out by the following method. The cross-sectional shape of a groove is usually a Gaussian function or a shape close to it. This is because the laser intensity distribution usually follows a Gaussian distribution. As shown in Figure 2, the imaginary line when the cross-sectional shape of groove 21 is assumed to be a Gaussian function is taken as reference curve RC, and a region having a width (W in Figure 2) of 1 μm or more and a protrusion height H from reference curve RC (height in the direction perpendicular to the tangent at that position) of 1 μm or more is defined as protrusion 102. The height (protrusion height) H of protrusion 102 is the height from reference curve RC to the tip of the protrusion. The width W of the protrusion is the linear distance from one end of the protrusion to the other end at the position of the bottom surface of the protrusion. The method for measuring the height and width of a protrusion is as follows. First, a sample is taken from the grain-oriented electrical steel sheet to be measured so that a cross section in the sheet thickness direction, perpendicular to the longitudinal direction (extension direction) of the groove, is exposed. The cross section is polished to reveal a cross section including the groove and its surrounding area, as shown in Figure 2. The cross section is then observed with an optical microscope or scanning electron microscope to measure the presence, height, and width of protrusions. Observe 10 or more cross sections, and if there is one or more protrusions (areas that are 1 μm or more wide and protrude 1 μm or more high from the reference curve) on the side or bottom of the groove, it is determined that there is at least one protrusion. If there are multiple protrusions, the maximum height and maximum width of each protrusion are used as the measurement values.

[0021] The misorientation between the average crystal orientation of the protrusions and the Goss orientation can be measured by the following method. First, a sample is taken from the grain-oriented electrical steel sheet to be measured, exposing a cross section perpendicular to the longitudinal direction (extension direction) of the groove. The cross section is polished to reveal a cross section including the groove and its surrounding area, as shown in Figure 2, and then the cross section is observed. The crystal orientation is measured using the electron backscattering diffraction (EBSD) method under the conditions described below. Within the region measured by EBSD, a region that is 1 μm or more wide and protrudes 1 μm or more in height from the reference curve is defined as a protrusion, and the average crystal orientation of the protrusion is determined. The misorientation between the determined average crystal orientation and the Goss orientation is then determined. If multiple protrusions are present, the largest misorientation between the average crystal orientation of each protrusion and the Goss orientation is taken as the misorientation between the average crystal orientation of the protrusion and the Goss orientation. The EBSD measurement conditions are as follows: (a) Measurement equipment: FE-SEM "SU-70" (Hitachi High-Technologies Corporation) EBSD equipment "DigiView" (TSL Solutions) (b) Magnification: 500x (c) Step spacing: 0.25 μm (d) Measurement area: 200 μm in the direction intersecting the groove longitudinal direction × 70 μm in the plate thickness direction

[0022] (Groove edge (flat area around the groove)) As shown in Figure 3, in a cross section parallel to the rolling direction RD and parallel to the sheet thickness direction, when the distance from groove entrance 31 (the position where the side of the groove, which is concave with respect to the reference plane, intersects with an imaginary line extending from the reference plane) to the entrance of the opposite groove is defined as the groove width, and when a region equivalent to the width of groove 21 from groove entrance 31 of steel sheet 11 in the rolling direction RD, in the direction opposite to the center of groove 21, is defined as groove edge 41 (although only the right side of the groove is described in Figure 3, the left side is similar), it is preferable that the maximum height of the protrusion of groove edge 41 is 5 µm or less and the maximum width is 5 µm or less. If protrusions form not inside the grooves but around the grooves (especially around the groove edges where molten material often adheres), gaps will form in the laminations when the grain-oriented electrical steel sheets are stacked, making it difficult for magnetic flux to flow and increasing iron loss. This adverse effect is particularly severe when the maximum height or width of the protrusions exceeds 5 μm. It is preferable to reduce the size of protrusions around the groove edges, and it is even more preferable to remove them.

[0023] The height and width of the protrusions on the groove edges can be determined by the following method. As shown in Figure 3, a region on the steel sheet surface away from the groove entrance 31 in the rolling direction RD in the direction opposite to the center of the groove by at least the width of the groove (i.e., a region farther from the groove than the groove edge) is defined as a reference plane RS, and a region that has a width of 1 µm or more and a height of 1 µm or more protruding from the reference plane is defined as a protrusion 101. The height of the protrusion 101 is the height from the reference plane RS to the tip of the protrusion 101. The width of the protrusion 101 is the linear distance from one end of the protrusion 101 to the other end. To measure the height and width of the protrusion 101, first, a sample is taken from the grain-oriented electrical steel sheet to be measured so that a cross section perpendicular to the longitudinal direction of the groove is exposed. At this time, the cross section should include an area equivalent to the width of the groove, extending from the edge of the groove in the direction opposite to the center of the groove. The cross section is polished to reveal a cross section including the groove 21 and its surrounding area (groove edge 41), as shown in Figure 3, and then the cross section is observed with an optical microscope or scanning electron microscope to measure the height and width of the protrusion 101. Ten or more cross sections are observed, and if multiple protrusions are present, the maximum height and width of each protrusion are taken as the measured values.

[0024] (chemical composition) The chemical composition of the steel sheet (base steel sheet) is not limited, as long as it is the same as that of the base steel sheet of a known grain-oriented electrical steel sheet. For example, the chemical composition may contain, in mass %, Si: 2.5 to 4.5%, Mn: 0.01 to 0.15%, C: 0 to 0.085%, acid-soluble Al: 0 to 0.065%, N: 0 to 0.012%, Cr: 0 to 0.30%, Cu: 0 to 0.40%, P: 0 to 0.50%, Sn: 0 to 0.30%, Sb: 0 to 0.30%, Ni: 0 to 1.000%, S: 0 to 0.015%, Se: 0 to 0.015%, and Bi: 0 to 0.02%. Alternatively, the steel sheet may contain the above elements, with the balance being Fe and impurities. When a grain-oriented electrical steel sheet is made of a base steel sheet (when it does not have a forsterite coating or an insulating coating), the chemical composition of the base steel sheet can be said to be the chemical composition of the grain-oriented electrical steel sheet.

[0025] The chemical composition of a steel sheet (base steel sheet) may be measured using a general steel analysis method. For example, the chemical composition of the base steel sheet may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, for example, a 35 mm square test piece (35 mm in both the rolling direction and the width direction) is obtained from the center of the base steel sheet, and the components are measured using a measuring device such as Shimadzu's ICPS-8100 under conditions based on a pre-established calibration curve. C and S, which are difficult to measure using ICP-AES, may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. When a forsterite coating and an insulating coating (described later) are formed on the grain-oriented electrical steel sheet, the chemical composition of the base steel sheet may be analyzed after removing the forsterite coating and the insulating coating from the grain-oriented electrical steel sheet by a known method such as pickling.

[0026] (plate thickness) The thickness of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment is not limited, but from the viewpoint that a thinner sheet thickness reduces eddy current loss, it is preferably 0.15 to 0.35 mm, for example.

[0027] [Forsterite coating] In the grain-oriented electrical steel sheet according to this embodiment, a forsterite film may be formed on the surface of the base steel sheet. The forsterite coating is an inorganic coating primarily composed of magnesium silicate. The forsterite coating is formed during finish annealing by a reaction between an annealing separator containing magnesia (MgO) applied to the surface of the base steel sheet and the components of the surface of the base steel sheet, and has a composition derived from the components of the annealing separator and the base steel sheet (more specifically, a composition primarily composed of Mg2SiO4). On the other hand, when an annealing separator mainly containing Al2O3 is used in the finish annealing, the forsterite film may not be formed.

[0028] [Insulating coating] In the grain-oriented electrical steel sheet according to this embodiment, an insulating coating may be formed on the surface of the base steel sheet or on the surface of the forsterite coating. The insulating coating imparts electrical insulation to the grain-oriented electrical steel sheet, thereby reducing eddy current loss and thereby reducing iron loss in the grain-oriented electrical steel sheet. The insulating coating also has the function of applying tension to the grain-oriented electrical steel sheet. Applying tension to the grain-oriented electrical steel sheet facilitates domain wall movement in the grain-oriented electrical steel sheet, thereby reducing iron loss in the grain-oriented electrical steel sheet. In addition to the electrical insulation properties described above, the insulating coating also provides various other properties such as corrosion resistance, heat resistance, and slipperiness. In the grain-oriented electrical steel sheet according to this embodiment, the insulating coating may be a known coating formed, for example, by applying a coating solution containing phosphate and colloidal silica as main components to the surface of the forsterite coating and baking the coating.

[0029] <Manufacturing method> The grain-oriented electrical steel sheet according to this embodiment can achieve the above-described effects as long as it has the characteristics regardless of the manufacturing method, but a manufacturing method including the following steps is preferred because it can be manufactured stably. (I) a hot rolling step in which the slab is heated and hot-rolled to form a hot-rolled sheet; (II) a hot-rolled sheet annealing step of annealing the hot-rolled sheet after the hot-rolling step; (III) a cold rolling step in which the hot-rolled sheet after the hot-rolled sheet annealing step is pickled and cold-rolled to form a steel sheet (cold-rolled sheet); (IV) a decarburization annealing step of performing decarburization annealing on the steel sheet; (V) a finish annealing step of applying an annealing separator to the steel sheet after the decarburization annealing step and finish annealing the steel sheet; (VI) A groove forming step between the cold rolling step and the finish annealing step, in which a groove is formed on the surface of the steel sheet by irradiating the surface of the steel sheet with a laser. Furthermore, the grain-oriented electrical steel sheet according to this embodiment may further include one or more of the following steps. (VII) a nitriding treatment step for increasing the nitrogen content of the steel sheet after the decarburization annealing step; (VIII) a grinding step of grinding the surface of the steel plate after the groove forming step; (IX) An insulating coating forming step of forming an insulating coating on the surface of the steel sheet (or on the surface of the forsterite coating formed on the surface of the steel sheet) after the finish annealing step.

[0030] [Hot rolling process] In the hot rolling step, a slab having a predetermined chemical composition (a chemical composition corresponding to the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment) is heated and hot-rolled to form a hot-rolled sheet. The conditions are not limited, but the heating temperature is, for example, 1050 to 1400°C.

[0031] The chemical composition of the slab to be subjected to hot rolling may be determined in accordance with the desired chemical composition of the grain-oriented electrical steel sheet, taking into consideration changes in the chemical composition in each process. For example, when obtaining the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to the preferred embodiment described above, it is preferable to use a slab having the following chemical composition. That is, the slab may contain, in mass %, as a chemical composition, 2.5-4.5% Si, 0.01-0.15% Mn, 0.02-0.10% C, 0-0.065% acid-soluble Al, 0.002-0.030% N, 0-0.30% Cr, 0-0.40% Cu, 0-0.50% P, 0-0.30% Sn, 0-0.30% Sb, 0-1.000% Ni, 0.001-0.050% S, 0-0.050% Se, and 0-0.02% Bi. Alternatively, the slab may contain the above, with the remainder being Fe and impurities.

[0032] The method for obtaining a slab is not limited. For example, molten steel having a predetermined chemical composition may be melted and used to produce a slab. A slab may be produced by continuous casting, or an ingot may be produced from the molten steel and then bloomed to produce a slab. Alternatively, a slab may be produced by other methods. The thickness of the slab is not particularly limited, but is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. A so-called thin slab having a thickness of 10 to 70 mm may also be used.

[0033] [Hot-rolled sheet annealing process] In the hot-rolled sheet annealing step, the hot-rolled sheet after the hot rolling step is annealed. By carrying out such annealing treatment, recrystallization occurs in the steel sheet structure, and it becomes possible to realize good magnetic properties. In the hot-rolled sheet annealing step of this embodiment, the hot-rolled sheet manufactured through the hot rolling step may be annealed according to a known method. The means for heating the hot-rolled sheet during annealing is not particularly limited, and known heating methods can be adopted. The annealing conditions are also not particularly limited, but for example, the hot-rolled sheet may be annealed in a temperature range of 900 to 1200°C for 10 seconds to 5 minutes.

[0034] [Cold rolling process] In the cold rolling process, the hot-rolled sheet after the hot-rolled sheet annealing process is pickled and cold-rolled to produce a steel sheet (cold-rolled sheet). The cold rolling may be a single cold rolling (a series of cold rolling without intermediate annealing), or may be multiple cold rolling passes with intermediate annealing between them, with the cold rolling being interrupted and at least one or two or more intermediate annealing passes being performed before the final pass of the cold rolling process. The cold rolling conditions may be those of a known method, for example, the final rolling reduction may be in the range of 80% to 95%. The final rolling reduction is the cumulative rolling reduction of cold rolling, and in the case where intermediate annealing is performed, it is the cumulative rolling reduction of cold rolling after final intermediate annealing. When intermediate annealing is performed, the steel sheet is held at a temperature of 1000 to 1200° C. for 5 to 180 seconds, for example. The annealing atmosphere is not particularly limited. In consideration of production costs, the number of times intermediate annealing is performed is preferably three or less. Furthermore, pickling may also be carried out under known conditions.

[0035] [Decarburization annealing process] In the decarburization annealing process, the steel sheet (cold-rolled sheet) is subjected to decarburization annealing, which removes carbon from the steel sheet, which has a negative effect on the magnetic properties, and also causes primary recrystallization of the steel sheet. The decarburization annealing conditions are not limited, but may be, for example, conditions of heating at 700 to 900° C. for 1 to 3 minutes.

[0036] [Nitriding process] After the decarburization annealing step and before the finish annealing step, a nitriding treatment may be performed to increase the nitrogen content of the grain-oriented electrical steel sheet. The nitriding treatment may be performed by a known method, for example, by annealing in an atmosphere containing a nitriding gas such as ammonia, thereby allowing nitrogen to penetrate into the steel. This allows the formation of a nitride that acts as an inhibitor of secondary recrystallization.

[0037] [Groove formation process] In the groove formation process, grooves are formed on the surface of the steel sheet (cold-rolled sheet) by irradiating the surface with a laser. At the same time, protrusions are formed inside the grooves (on the side or bottom surfaces). The crystal orientation of the protrusions is also controlled. The groove forming step and the subsequent grinding step, which are carried out as necessary, are carried out between the cold rolling step and the finish annealing step, i.e., between the cold rolling step and the decarburization annealing step, or between the decarburization annealing step and the finish annealing step, or between the nitriding treatment step and the finish annealing step, etc. In the groove forming step, laser irradiation is performed in two stages, a first stage and a second stage, to form the grooves and the protrusions at the same time. In this case, in the first stage, it is necessary to control the laser irradiation conditions and the injection conditions of the assist gas for removing the molten material generated by the laser irradiation from the surface with the assist gas, and in the second stage, it is necessary to control the laser irradiation conditions. Specifically, in the first stage, the laser is irradiated at a scanning speed of 2 to 50 m / s with a laser having a laser output of 200 to 3000 W, a focused spot diameter in the rolling direction (i.e., the diameter containing 86% of the laser output) of 10 to 1000 μm, and a focused spot diameter in the sheet width direction (i.e., the diameter containing 86% of the laser output), of 10 to 1000 μm, while simultaneously spraying assist gas at a flow rate of 1 to 500 L / min. By performing laser irradiation and assist gas spraying under the above conditions, grooves are formed on the surface of the steel sheet, and protrusions of a predetermined size are formed within the grooves (on the side or bottom). In the second step, the same locations as those irradiated with the laser in the first step are irradiated with a laser having a laser output of 10 to 150 W, a focused spot diameter of 10 to 1000 μm in the rolling direction, and a focused spot diameter of 10 to 1000 μm in the sheet width direction, at a scanning speed of 2 to 50 m / s. By performing laser irradiation under the above conditions following the first stage, the misorientation between the average crystal orientation of the protrusions and the Goss orientation can be made 10 degrees or more.

[0038] In the first stage, if the laser power is less than 200 W, the generation of the molten material is insufficient, resulting in the formation of protrusions smaller than the desired shape inside the groove, or no protrusions at all.On the other hand, if the laser power is more than 3000 W, the laser power is too strong, resulting in the generation of excessive molten material, resulting in the formation of protrusions larger than the desired shape inside the groove. Furthermore, if the laser focused spot diameter in the rolling direction and width direction is less than 10 μm, grooves of a desirable shape are not formed, and the protrusions formed inside the grooves are smaller than desirable, so that the iron loss reduction effect cannot be obtained. On the other hand, if it exceeds 1000 μm, grooves of a desirable shape are not formed, and the protrusions formed inside the grooves are larger than desirable, so that the iron loss reduction effect cannot be obtained. Furthermore, if the scanning speed is less than 2 m / s, an excessive amount of molten material is generated, resulting in larger than desired protrusions being formed inside the grooves, whereas if the scanning speed is more than 50 m / s, the amount of molten material generated is insufficient, resulting in smaller than desired protrusions or no protrusions being formed inside the grooves. If the assist gas flow rate is less than 1 L / min, the removal of the molten material is insufficient, resulting in the formation of protrusions inside the grooves that are larger than the desired shape, whereas if the assist gas flow rate is more than 500 L / min, the removal of the molten material is excessive, resulting in the formation of protrusions inside the grooves that are smaller than the desired shape, or no protrusions at all. Preferably, the laser output is 500 to 3000 W, the laser focused spot diameter in the rolling direction (i.e., the diameter containing 86% of the laser output) is 10 to 500 μm, the laser focused spot diameter in the sheet width direction (i.e., the diameter containing 86% of the laser output) is 10 to 500 μm, the laser scanning speed is 5 to 50 m / s, and the flow rate of the assist gas is 1 to 200 L / min.

[0039] In the second stage, if the laser output is less than 10 W, the projections are not heated sufficiently and no effect can be obtained, whereas if it exceeds 150 W, the projections will melt. If the laser focused spot diameter in the rolling direction and width direction is less than 10 μm, the laser output per unit area will be too high, causing the protrusions to melt. On the other hand, if it exceeds 1000 μm, areas other than the protrusions will also be heated, which is undesirable. The scanning speed is preferably the same as that in the first step in terms of production efficiency, but may be changed as appropriate within the range of 2 to 50 m / s, as this is sufficient for heating the protrusions. The reason why the crystal orientation of the protrusions can be controlled by laser irradiation is presumed to be as follows. Laser irradiation causes recrystallization and grain growth near the protrusions. Alternatively, thermal strain is applied, which acts as a driving force for recrystallization and grain growth prior to the subsequent decarburization annealing and finish annealing (before secondary recrystallization begins), resulting in coarsening compared to the matrix grains prior to secondary recrystallization (grains consumed by secondary recrystallized grains). These coarse grains are less likely to be consumed by secondary recrystallized grains, and because their orientation is random, there is a large misorientation with the Goss orientation. Therefore, the misorientation between the average crystal orientation of the protrusions and the Goss orientation can be made 10 degrees or more.

[0040] [Grinding process] In the grinding process, the surface of the steel sheet (cold-rolled sheet) after the groove forming process is ground using a brush roll with fixed abrasive grains. This makes it possible to reduce the height of protrusions on the flat areas around the groove edges or to remove the protrusions. When grinding, the brush roll is controlled so that it rotates in a direction opposite to the conveyance direction of the steel plate at the position where it comes into contact with the steel plate, and grinding is performed under the following conditions: the conveyance speed (line speed) of the steel plate is 5 to 100 mpm (meter per minute), the brush roll rotation speed is 500 to 2000 rpm, the brush roll reduction is 2 to 10 mm, the abrasive grain size is #40 to #400, and the brush roll diameter is 200 to 500 mm. Outside the above range, sufficient effects cannot be obtained. The grinding conditions are preferably a steel plate conveying speed of 5 to 100 mpm, a brush roll rotation speed of 1000 to 2000 rpm, a brush roll reduction of 4 to 10 mm, an abrasive grain size of #40 to #100, and a brush roll diameter of 200 to 500 mm. In this case, the effect of removing protrusions from groove edges is enhanced.

[0041] [Finishing annealing process] In the final annealing step, an annealing separator is applied to the steel sheet after the decarburization annealing step, and the steel sheet is then final annealed. Finish annealing is performed after the steel sheet has been wound into a coil, and an annealing separator is applied to prevent the steel sheet from seizing during finish annealing. Annealing separators generally consist mainly of MgO or Al2O3. Finish annealing is performed after applying such an annealing separator. For example, if an annealing separator containing MgO as the main component is used, a forsterite (Mg2SiO4) coating layer is formed. If an annealing separator containing Al2O3 as the main component is used, a forsterite coating layer may not be formed. In the final annealing process, the material is heated to the annealing temperature, which causes secondary recrystallization of the primary recrystallized grains obtained in the decarburization annealing process, resulting in crystal grains aligned in the Goss orientation. Furthermore, by holding the material at the annealing temperature for a predetermined time, impurities (such as N and S) that adversely affect the magnetic properties are removed (purified). The conditions for the finish annealing are not limited, but for example, the material is heated to 1100 to 1300° C. and held for 20 to 24 hours.

[0042] [Insulating film formation process] In the insulating coating forming step, an insulating coating is formed on the steel sheet after the finish annealing step (on the surface of the forsterite coating if a forsterite coating has been formed on the surface of the steel sheet by finish annealing). For example, the insulating coating can be formed by applying a coating solution containing phosphoric acid or a phosphate, colloidal silica, and chromic anhydride or a chromate to a steel sheet (including those having a forsterite coating) after finish annealing, and baking and drying at 300 to 950°C for 10 seconds or more. Through these steps, a grain-oriented electrical steel sheet can be obtained that includes a base steel sheet and, if necessary, a forsterite coating and / or an insulating coating. [Example]

[0043] A slab having a chemical composition containing, by mass fraction, Si: 3.3%, C: 0.060%, acid-soluble Al: 0.028%, N: 0.008%, Mn: 0.12%, Cr: 0.05%, Cu: 0.04%, P: 0.01%, Sn: 0.02%, Ni: 0.005%, S: 0.007%, with the remainder being Fe and impurities, was hot-rolled to form a hot-rolled sheet with a thickness of 2.6 mm. This hot-rolled sheet was annealed by heating to 1000°C and holding for 1 minute. The hot-rolled sheet after annealing was pickled and cold-rolled to obtain a steel sheet (cold-rolled sheet) having a thickness of 0.23 mm. This steel sheet was irradiated with a laser under the conditions shown in Tables 1A and 1B to form grooves on the surface extending in a direction at an angle of 90° to the rolling direction. A plurality of these grooves were formed in parallel to each other at intervals of 5 mm in the rolling direction. The surface of the steel plate on which the grooves were formed was ground under the conditions shown in Table 2 using a brush roll rotating in the direction opposite to the conveyance direction of the steel plate. After grinding, the specimen was heated to 800°C and held there for 2 minutes for decarburization annealing. After decarburization annealing, the steel sheet was coated with an annealing separator mainly composed of magnesia (MgO), heated to 1200°C, and subjected to finish annealing by holding for 20 hours. As a result, a grain-oriented electrical steel sheet was obtained in which a forsterite film was formed on the surface of the steel sheet (base steel sheet). A coating solution containing colloidal silica and phosphate was applied to the obtained grain-oriented electrical steel sheet, and heat treatment was carried out by heating to 850°C and holding for 1 minute to form an insulating coating.

[0044] The chemical composition of the base steel sheet of the obtained grain-oriented electrical steel sheet was determined to contain, by mass fraction, Si: 3.3%, C: 0.001% or less, acid-soluble Al: 0.004% or less, N: 0.001% or less, Mn: 0.12%, Cr: 0.05%, Cu: 0.04%, P: 0.01%, Sn: 0.02%, Ni: 0.005%, S: 0.001% or less, with the remainder being Fe and impurities.

[0045] Samples were taken from the obtained grain-oriented electrical steel sheets in the manner described above, and the presence or absence of protrusions in the grooves and at the groove edges, the maximum height and maximum width of the protrusions, and the misorientation between the average crystal orientation of the protrusions in the grooves and the GOSS orientation were measured. The results are shown in Table 3.

[0046] The obtained grain-oriented electrical steel sheets were sheared to a width of 30 mm and a length of 320 mm to obtain test specimens. The length of the test specimens was parallel to the rolling direction. The obtained test specimens were subjected to an Epstein test in accordance with JIS C2550-1 (2011), and the iron loss W17 / 50 was measured at a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T. It was determined that low iron loss was obtained if W17 / 50 was 0.750 W / kg or less. The results are shown in Table 3.

[0047] [Table 1A]

[0048] [Table 1B]

[0049] [Table 2]

[0050] [Table 3]

[0051] As shown in Tables 1A to 3, when the laser irradiation conditions or assist gas injection conditions were outside the ranges specified in the present invention, protrusions of a desirable shape were not formed in the grooves. Alternatively, the average crystal orientation of the protrusions was outside the ranges specified in the present invention. These grain-oriented electrical steel sheets had high iron loss (Comparative Examples 1 to 20). On the other hand, when the laser irradiation conditions and assist gas injection conditions were within the ranges specified in the present invention, protrusions as specified in the present invention were formed in the grooves. Furthermore, the misorientation between the average crystal orientation of the protrusions and the Goss orientation was 10 degrees or more. These grain-oriented electrical steel sheets had low iron loss (Invention Examples 1 to 21). Furthermore, when the grinding conditions using the brush roll were specified, the protrusions at the groove edges became smaller, and iron loss was further reduced (Invention Examples 1 to 15). [Industrial Applicability]

[0052] According to the present invention, a grain-oriented electrical steel sheet with low iron loss and a method for manufacturing the same can be provided, and therefore, has high industrial applicability. [Explanation of symbols]

[0053] 1 Grain-oriented electrical steel sheet 11 Steel plate (base material steel plate) 21 Groove 31 Groove entrance 41 Groove edge 102 Protrusion (inside groove) H Protrusion height (protrusion height) W width of protrusion RD rolling direction TD Sheet width direction RC reference curve 101 Protrusion (groove edge) RS reference plane

Claims

1. The method includes the step of forming a steel sheet that is a base steel sheet of a grain-oriented electrical steel sheet, the base steel sheet having a plurality of grooves formed on the surface thereof, the grooves extending in a direction intersecting the rolling direction and having a depth in the sheet thickness direction, In a cross section of the steel plate parallel to the rolling direction and parallel to the plate thickness direction, At least one protrusion is present on the side or bottom surface of the groove, The protrusions have a maximum height of 5 to 50 μm and a maximum width of 5 to 50 μm, the misorientation between the average crystal orientation of the protrusions and the Goss orientation is 14 degrees or more; A directional electrical steel sheet characterized by:

2. In the cross section, when the distance from the entrance of a groove to the entrance of the opposite groove is defined as the groove width, and a region having the same width as the width of the groove from the entrance of the groove of the steel plate in the rolling direction in the direction opposite to the center of the groove is defined as a groove edge, the maximum height of the protrusion of the groove edge is 5 μm or less and the maximum width is 5 μm or less. The grain-oriented electrical steel sheet according to claim 1 .

3. a forsterite coating is formed on the surface of the steel plate; The grain-oriented electrical steel sheet according to claim 1 or 2,

4. an insulating coating is formed on the surface of the forsterite coating; The grain-oriented electrical steel sheet according to claim 3 ,

5. An insulating coating is formed on the surface of the steel plate. The grain-oriented electrical steel sheet according to claim 1 or 2,

6. The method for producing the grain-oriented electrical steel sheet according to claim 1 or 2, a hot rolling step in which the slab is heated and hot-rolled to form a hot-rolled sheet; a hot-rolled sheet annealing step of annealing the hot-rolled sheet after the hot-rolling step; A cold rolling process in which the hot-rolled sheet after the hot-rolled sheet annealing process is pickled and cold-rolled to obtain a steel sheet; a decarburization annealing step of performing decarburization annealing on the steel sheet; a finish annealing step of applying an annealing separator to the steel sheet after the decarburization annealing step and finish annealing the steel sheet; Between the cold rolling step and the finish annealing step, a groove forming step is performed in which a groove is formed on the surface of the steel sheet by irradiating the surface of the steel sheet with a laser, and a protrusion is formed on a side surface or a bottom surface of the groove. Including, In the groove forming step, a first step of irradiating a laser having a laser output of 200 to 3000 W, a focused spot diameter of 10 to 1000 μm, which is a diameter including 86% of the laser output in the rolling direction, and a focused spot diameter of 10 to 1000 μm in the plate width direction, at a scanning speed of 2 to 50 m / s, and blowing an assist gas at a flow rate of 1 to 500 L / min; A second step of irradiating the same location as the location irradiated with the laser in the first step with a laser having a laser output of 10 to 150 W, a focused spot diameter of 10 to 1000 μm in the rolling direction, and a focused spot diameter of 10 to 1000 μm in the sheet width direction at a scanning speed of 2 to 50 m / s. A method for producing a grain-oriented electrical steel sheet.

7. Further, the groove forming step includes a grinding step of grinding the surface of the steel plate using a brush roll to which abrasive grains are fixed, In the grinding step, the brush roll rotates in a direction opposite to the conveyance direction of the steel plate at a position where the brush roll contacts the steel plate, The conveying speed of the steel plate is 5 to 100 mpm, The rotation speed of the brush roll is 500 to 2000 rpm, The brush roll has a pressing amount of 2 to 10 mm, The grain size of the abrasive grains is #40 to #400, The diameter of the brush roll is 200 to 500 mm. The method for producing a grain-oriented electrical steel sheet according to claim 6 .

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