Grain-oriented electrical steel sheet

Abstract

This grain-oriented electrical steel sheet has a steel sheet in which a plurality of grooves that extend in a direction intersecting the rolling direction and that have a depth in the sheet thickness direction are formed on the surface thereof. Taking a groove edge part to be the range of 40 µm from a groove entrance in a direction perpendicular to the sheet thickness direction and away from the widthwise center of the groove in a cross-section of the steel sheet parallel to the rolling direction and parallel to the sheet thickness direction: in the groove edge parts in a plurality of said cross-sections, the proportion of the number of protrusions having a height of 2.0 µm to smaller than 6.0 µm is from 10% to lower than 40%; and when viewed from above, brush marks having a width of 4-25 µm and a depth 4 µm or below are present in from 10% to less than 80% of the groove edges in terms of area percentage.

Description

[Technical Field]

[0001] The present invention relates to a grain-oriented electrical steel sheet. This application claims priority based on Japanese Patent Application No. 2022-186167, 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 that are primarily used as iron core materials for transformers, and therefore require magnetic properties such as high magnetization and low core loss. Iron loss is the power loss consumed as heat 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, crystal grain size, and magnetic domain size. Even now, when various technologies have been developed for grain-oriented electrical steel sheets, research and development into reducing iron loss continues 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 laser, narrowing the spacing between the 180° domain walls and reducing eddy current loss, which is part of the 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, when grooves are formed by laser irradiation as described above, the molten material generated by the laser irradiation solidifies at the edges of the grooves, forming protrusions. The presence of such protrusions reduces the magnetic properties.

[0005] To address this issue, it has been proposed to control the height of the protrusions. For example, Patent Document 2 discloses laser-scribed grain-oriented silicon steel that has been stress-relief annealed, in which parallel linear scribe grooves are formed on one or both sides of the grain-oriented silicon steel by laser scribing, the linear scribe grooves are perpendicular to or form a certain angle with the rolling direction of the steel sheet, the maximum height of the protrusions at the edges of the linear scribe grooves is 5 μm or less, the maximum height of the sputtered material in the scribe-free areas between adjacent linear scribe grooves is 5 μm or less, the proportion of the area occupied by the sputtered material per unit area is 5% or less, and the line roughness Ra of the center line at the bottom of the linear scribe grooves is 2.1 μm or less. Patent Document 2 discloses that if the height of the edge protrusion exceeds 5 μm, the space factor will fall to 95% or less, and the manufacturing process requirements for transformer cores will not be met, and therefore the height of the protrusion on the edge of the scribe groove must be controlled to within 5 μm.

[0006] Patent Document 3 discloses a steel plate having a surface on which grooves are formed, the extension direction of which intersects the rolling direction and the depth direction of which is parallel to the plate thickness direction, and molten solidification material is present on both sides of the groove on the surface of the steel plate, and the height at which the height data obtained by measuring the steel plate surface at regular intervals in a specific region including the groove is the maximum frequency in a height distribution, and when the spatial volume of a recess recessed from the virtual plane is V1 and the volume of a protrusion protruding from the virtual plane is V2, the value of V2 / V1 is 0.10. The grain-oriented electrical steel sheet has a R-value greater than 0.80 and less than 0.80, a plurality of protrusions are formed in the specific region, and among the plurality of protrusions, the width of the protrusion closest to the groove is greater than the width of the other protrusions, and when the region having the highest average height in the extension direction in the height distribution is viewed in a groove longitudinal cross section including the extension direction and the sheet thickness direction, the average roughness Ra of the roughness curve forming the surface of the region is 0.30 to 2.00 μm and the average length RSm of the roughness curve elements forming the surface of the region is 10 to 150 μm. Furthermore, Patent Document 3 discloses that if the surface roughness parameters (Ra, RSm) are within the above ranges, film breakdown due to stress concentration on the film is avoided, and the height of the maximum height portion of the protrusions T, where insulation is difficult to ensure, is reduced, thereby improving insulation; furthermore, since fluctuations in the width of the protrusions T are reduced, the maximum protrusion width, which is expected to determine the rate-limiting effect of domain wall movement, is reduced, thereby improving magnetic properties. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent No. 4669565 [Patent Document 2] Japanese Patent No. 6884875 [Patent Document 3] Japanese Patent No. 6569803 Summary of the Invention [Problem to be solved by the invention]

[0008] The techniques of Patent Documents 2 and 3 provide certain effects, but they are not sufficient to meet the high demands of recent years. Therefore, an object of the present invention is to provide a grain-oriented electrical steel sheet having lower iron loss than conventional ones, on the premise that the grain-oriented electrical steel sheet has grooves formed by irradiation with a laser or the like to control magnetic domains. [Means for solving the problem]

[0009] The present inventors have investigated methods for reducing iron loss in grain-oriented electrical steel sheets in which grooves are formed by irradiation with a laser or the like to control magnetic domains. As a result, it was discovered that controlling the shape and quantity of protrusions formed on the surface around the grooves can improve the space factor when laminating steel sheets. Furthermore, it was discovered that iron loss can be further reduced by forming uniform irregularities (brush marks) on the surface using a brush. Laser irradiation creates non-uniform sputter marks, and simply controlling Ra, etc., makes it impossible to control uniformity. For this reason, it is believed that there was room for improvement in the iron loss reduction effect of conventional technology.

[0010] 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 is By laser irradiation The steel sheet is a base steel sheet for a grain-oriented electrical steel sheet, and has a surface on which a plurality of grooves are formed that extend in a direction intersecting the rolling direction and that are deep in the plate thickness direction. In a cross section of the steel sheet that is parallel to the rolling direction and the plate thickness direction, when a groove edge is defined as a range of 40 μm from the entrance of the groove in a direction perpendicular to the plate thickness direction and opposite the center of the groove width direction, the number of protrusions in the groove edge portions of the plurality of cross sections that are 2.0 μm or more and less than 6.0 μm accounts for 10% or more and less than 40%, and when viewed from above, brush marks having a width of 4 to 25 μm and a depth of 4 μm or less are present in an area proportion of 10% or more and less than 80% of the groove edge portions. [2] In the grain-oriented electrical steel sheet according to [1], a forsterite coating may be formed on the surface of the steel sheet. [3] In the grain-oriented electrical steel sheet according to [2], an insulating coating may be formed on the surface of the forsterite coating. [4] In the grain-oriented electrical steel sheet according to [1], an insulating coating may be formed on the surface of the steel sheet. [Effects of the Invention]

[0011] According to the above-described aspect of the present invention, it is possible to provide a grain-oriented electrical steel sheet having low iron loss (particularly low iron loss when used as a core). [Brief explanation of the drawings]

[0012] [Figure 1A] 1 is a diagram showing an example of a cross section of a grain-oriented electrical steel sheet according to an embodiment of the present invention, the cross section being parallel to the rolling direction and the sheet thickness direction, including grooves. FIG. [Figure 1B] FIG. 1 is a diagram showing an example of a cross section of a conventional grain-oriented electrical steel sheet, the cross section being parallel to the rolling direction and the sheet thickness direction, including a groove. [Figure 2] FIG. 10 is a diagram showing the relationship between the height of the protrusions and the iron loss of the core when the ratio of protrusions of a predetermined height is set to 25%. [Figure 3] FIG. 10 is a diagram showing the relationship between the percentage of the number of protrusions having a height of 2.0 μm or more and less than 6.0 μm and the iron loss of the core. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, a grain-oriented electrical steel sheet according to one embodiment of the present invention will be described. 1A, the grain-oriented electrical steel sheet according to this embodiment has a steel sheet (base 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 includes not only cases where it is made of a steel plate (base steel plate), but also cases where it is made of a base steel plate and a forsterite coating formed on the surface of the base steel plate, a base steel plate and an insulating coating formed on the surface of the base steel plate, or a base steel plate and 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. 1A , in a cross section of a steel sheet 11 included in a grain-oriented electrical steel sheet, which is parallel to the rolling direction RD and the sheet thickness direction, a range of 40 μm from the groove entrance 31 in a direction perpendicular to the sheet thickness direction and opposite to the center of the width direction of the groove 21 will be described as a groove edge 41. In this embodiment, a convex portion present in the groove edge 41 and having a base length of 10 μm or more and a height of 1.0 μm or more will be referred to as a protrusion 101.

[0014] [Steel plate (base material steel plate)] (groove) The steel plate has grooves formed on the surface by laser irradiation, the grooves extending in a direction intersecting the rolling direction and having a depth in the plate thickness direction. By forming linear grooves in a direction intersecting the rolling direction, magnetic domain control can be achieved. In the grain-oriented electrical steel sheet according to this embodiment, grooves are formed on the surface of the steel sheet (base steel sheet) to achieve this effect. In this embodiment, the direction intersecting the rolling direction is a direction at an angle of 75 to 105° to the rolling direction. The shape of the grooves is not limited as long as it is within a range that can obtain the effect of magnetic domain control, but for example, the depth is 4.0 to 40.0 μm and the groove width is 20.0 to 400.0 μm. Of the multiple grooves, the spacing between adjacent grooves in the rolling direction is preferably 2 to 20 mm. The multiple grooves are preferably formed at approximately regular intervals (periodically) in the rolling direction. The groove spacing is the distance from the center of one groove in the width direction to the center of the adjacent groove in the width direction.

[0015] (groove edge) In the grain-oriented electrical steel sheet according to this embodiment, when multiple cross sections parallel to the rolling direction and the thickness direction of the steel sheet are observed and multiple groove edge portions are observed, protrusions having a height of 2.0 μm or more and less than 6.0 μm account for 10% or more and less than 40% of the observed protrusions. Here, in this embodiment, a protrusion is defined as a convex portion having a base length of 10 μm or more and a height of 1.0 μm or more. The reason for controlling the proportion of protrusions with a height of 2.0 μm or more but less than 6.0 μm (base length of 10 μm or more) is that when protrusions with a height of 2.0 μm or more are present, the stress induced by the protrusions subdivides the magnetic domains (reducing the magnetic domain width), which has the effect of reducing iron loss.However, if the protrusion height is 6.0 μm or more, there is concern that the space factor will decrease when the core is manufactured, causing an increase in the core's iron loss (deteriorating iron loss characteristics). Furthermore, even if protrusions having a height of 2.0 μm or more and less than 6.0 μm are present, if the proportion of protrusions having a height of 2.0 μm or more and less than 6.0 μm among the certain number of protrusions present is less than 10%, the effect cannot be sufficiently obtained. On the other hand, if the proportion of the number of protrusions having a height of 2.0 μm or more and less than 6.0 μm is 40% or more, the space factor decreases, causing an increase in core iron loss. 2 and 3 show the iron loss (W17 / 50) values ​​of cores manufactured using grain-oriented electrical steel sheets with different protrusion heights and different ratios of protrusions with heights of 2.0 μm or more and less than 6.0 μm. It can be seen that by setting the protrusion height and the proportion of the specified protrusions within a specified range, the iron loss of the core can be reduced. The maximum height of the protrusions observed on the groove edges is preferably less than 6.0 μm. That is, the proportion of protrusions having a height of 2.0 μm or more but less than 6.0 μm is preferably 10 to 40%, and the height of the other protrusions is preferably less than 2.0 μm.

[0016] Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, when the steel sheet is viewed from above (observed from a direction perpendicular to the surface of the steel sheet), brush marks with a width of 4 to 25 μm and a depth of 4 μm or less are present over an area ratio of 10% or more but less than 80% of the groove edges. When grooves are formed by laser irradiation, uneven sputter marks are formed at the edges of the grooves. In this embodiment, the surface of the steel sheet is ground with a brush (brush roll), and the uneven distribution of the sputter marks and other irregularities is made uniform by the brush. This grinding also controls the height of the protrusions. As will be described later, the brush roll is generally arranged so as to rotate in a direction opposite to the conveyance direction of the steel sheet, and therefore the brush marks are provided so as to extend in the rolling direction. If the proportion of brush marks is less than 10%, sufficient effect cannot be obtained. On the other hand, if the proportion of brush marks is 80% or more, the surface roughness increases excessively, hindering the movement of domain walls that affect magnetic properties, resulting in degradation of magnetic properties. Furthermore, if the width of the brush marks is less than 4 μm, they hinder the movement of the 180° domain walls, increasing iron loss.On the other hand, if the width is more than 25 μm, the magnetostatic energy on the surface does not increase, the spacing between the 180° domain walls does not narrow, and the effect of reducing iron loss cannot be obtained. Furthermore, if the brush mark depth exceeds 4 μm, the domain wall motion is further hindered, resulting in a large increase in iron loss. Unlike protrusions, brush marks do not leave deposits in the thickness direction of the plate, so they do not affect each other even when oriented electromagnetic steel plates are stacked to form a core.

[0017] The proportion of protrusions and the proportion of brush marks can be measured using an optical microscope by the following method. The grain-oriented electrical steel sheet is cut so that a cross section in the thickness direction perpendicular to the grooves can be observed, and an image of a 2.6 mm range in the rolling direction is taken at 300x magnification using an optical microscope to determine the presence or absence of protrusions with a base length (the length on an extension of the surface of the groove edge) of 10 μm or more. If present, their height (the distance from the base to the longest point in the direction perpendicular to the base) is measured. If the measurement results in a height of 1.0 μm or more, they are determined to be protrusions. This observation is carried out at five or more locations, and the number of protrusions in the observation field and the number of protrusions with a height of 2.0 μm or more but less than 6.0 μm are counted, and the proportion of protrusions with a height of 2.0 μm or more but less than 6.0 μm is calculated from these. At this time, the width and depth of the groove can also be measured. In addition, the grain-oriented electrical steel sheet is cut at the groove edge near the groove so that a cross section in the sheet thickness direction parallel to the groove can be observed, and an image of an area of ​​2.6 mm in the rolling direction is taken at 300x magnification using an optical microscope to determine whether the brush marks are 4 to 25 μm in width and 4 μm or less in depth. Since the brush marks are formed approximately uniformly, the grain-oriented electrical steel sheet is viewed from above and an area of ​​2.0 mm in the width direction and 2.6 mm in the rolling direction from the center of the groove is observed, and the area of ​​the groove edge where irregularities similar to the irregularities determined above to be brush marks are formed is measured, and the ratio to the area of ​​the groove edge is calculated. When measuring the area ratio of brush marks, if a forsterite film or insulating film is formed on the surface, these should be removed before measurement.

[0018] (chemical composition) The chemical composition of the base steel sheet is not particularly limited, as long as it is the same as that of the base steel sheet of a known grain-oriented electrical steel sheet. The chemical composition of the base steel sheet preferably contains, for example, in mass %, Si: 0.80% to 7.00%, C: greater than 0% and not more than 0.085%, acid-soluble Al: 0% to 0.065%, N: 0% to 0.0120%, Mn: 0% to 1.00%, Cr: 0% to 0.30%, Cu: 0% to 0.4%, P: 0% to 0.5%, Sn: 0% to 0.3%, Sb: 0% to 0.3%, Ni: 0% to 1%, S: 0% to 0.015%, Se: 0% to 0.015%, with the balance being Fe and impurities. When the grain-oriented electrical steel sheet according to this embodiment is made of only a steel sheet (base steel sheet), the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet can also be said to be the chemical composition of the grain-oriented electrical steel sheet.

[0019] The chemical composition of the base steel plate is determined by a known elemental analysis method. Specifically, chips are generated from the base steel plate using a drill, the chips are collected, and the collected chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES to perform elemental analysis of the chemical composition. However, since Si is difficult to measure using ICP-AES, it is determined using the method (silicon determination method) specified in JIS G 1212 (1997). Specifically, when the above-mentioned chips are dissolved in acid, silicon oxide is precipitated, and this precipitate (silicon oxide) is filtered out with filter paper and its mass is measured to determine the Si content. The carbon and sulfur contents are determined by the well-known high-frequency combustion method (combustion-infrared absorption method). Specifically, the solution is combusted in an oxygen stream by high-frequency heating, and the carbon dioxide and sulfur dioxide generated are detected to determine the carbon and sulfur contents. The N content is determined using the well-known inert gas fusion-thermal conductivity method.

[0020] If a forsterite film and / or an insulating film has formed on the surface, these can be removed before measurement. Specifically, if an insulating coating is formed, the insulating coating is removed by immersing the grain-oriented electrical steel sheet with the insulating coating in an aqueous sodium hydroxide solution containing 30 to 50 mass% NaOH and 50 to 70 mass% HO at 80 to 90°C for 7 to 10 minutes. The grain-oriented electrical steel sheet from which the insulating coating has been removed is then rinsed with water, and then dried with a hot air blower for just under 1 minute. If a forsterite film has formed, the forsterite film is removed by immersing the grain-oriented electrical steel sheet having the forsterite film for 1 to 10 minutes in an aqueous hydrochloric acid solution containing 30 to 40 mass% HCl at 80 to 90°C. After immersion, the base steel sheet is rinsed with water and then dried with a hot air blower for just under 1 minute. Through the above steps, a base steel sheet can be taken out from the grain-oriented electrical steel sheet on which the forsterite coating and / or insulating coating has been formed.

[0021] (plate thickness) There are no limitations on the thickness of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, but when iron loss is taken into consideration, it is preferably 0.15 to 0.40 mm.

[0022] [Forsterite coating] In the grain-oriented electrical steel sheet according to this embodiment, a forsterite coating may be formed on the surface of the steel sheet that serves as 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.

[0023] [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 improving the iron loss characteristics of 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 improving the iron loss characteristics of 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 by, for example, applying a coating liquid containing phosphate and colloidal silica as main components to the surface of the forsterite coating and baking it.

[0024] <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 a slab having a predetermined chemical composition 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 groove forming step of irradiating a laser onto the surface of the steel sheet after the cold rolling step, and removing a molten material generated by the laser irradiation from the surface with an assist gas, thereby forming grooves on the surface of the steel sheet; (V) a grinding step of grinding the surface of the steel plate after the groove forming step using a brush roll; (VI) a decarburization annealing step of performing decarburization annealing on the steel sheet after the grinding step; (VII) A finish annealing step of applying an annealing separator to the steel sheet after the decarburization annealing step and finish annealing the steel sheet. The method for manufacturing a grain-oriented electrical steel sheet according to this embodiment may further include the following steps. (VIII) 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.

[0025] [Hot rolling process] In the hot rolling process, a slab having a predetermined chemical composition (a chemical composition corresponding to the chemical composition of the steel sheet (base steel sheet) contained in 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 and may be set according to the required thickness and strength.

[0026] 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 step. 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: For example, it preferably contains, in mass %, Si: 0.80% to 7.10%, C: greater than 0% and not more than 0.090%, acid-soluble Al: 0% to 0.070%, N: 0% to 0.0200%, Mn: 0% to 1.50%, Cr: 0% to 0.35%, Cu: 0% to 0.45%, P: 0% to 0.55%, Sn: 0% to 0.4%, Sb: 0% to 0.3%, Ni: 0% to 1%, S: 0% to 0.020%, Se: 0% to 0.015%, and the remainder being Fe and impurities.

[0027] 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.

[0028] [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, making it possible to achieve 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.

[0029] [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 in accordance with known methods. 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.

[0030] [Groove formation process] In the groove forming step, a laser is irradiated onto the surface of the steel sheet, and a molten material generated by the laser irradiation is removed from the surface with an assist gas, thereby forming grooves on the surface of the steel sheet. The laser irradiation conditions and the assist gas injection conditions are not particularly limited as long as they are conditions under which grooves can be formed.

[0031] [Grinding process] In the grinding process, the surface of the steel sheet after the groove forming process is ground using a brush roll. The purpose of this grinding process is to control the height of the protrusions and to form uniformly uneven brush marks, rather than simply polishing the surface of the steel sheet to make it smooth, as seen in conventional techniques. The grinding range should be at least the groove edges, but if the rotation direction of the brush roll is parallel to the longitudinal direction of the sheet and the steel sheet is ground while being transported, the entire steel sheet may be ground in consideration of productivity. During grinding, in order to obtain predetermined brush marks, the grain size of the grindstone at the tips of the bristles constituting the brush, the wire diameter of the brush bristles, the number of rotations of the brush, the amount of pressing down of the brush, and the like are controlled. Preferred conditions are, for example, a brush bristle diameter φ of more than 1.0 mm and less than 1.4 mm, and a grit size of #80 (0.20 mm) to #200 (0.08 mm). The grit size of the abrasive grains used here is primarily intended for the removal of protrusions, and is different from the fine abrasive grains used in conventional smoothing. The brush rotation speed is 700 to 2000 rpm, and the brush reduction is 2 to 8 mm. The rotation speed and reduction are also slightly higher and larger than conventional methods, as the primary focus is on removing protrusions.

[0032] [Decarburization annealing process] In the decarburization annealing process, the steel sheet after the grinding process is subjected to decarburization annealing. In this decarburization annealing, C, which has a negative effect on magnetic properties, is removed from the steel sheet (decarburization), and the steel sheet undergoes primary recrystallization. The decarburization annealing conditions are not limited, but are preferably, for example, in a wet hydrogen and nitrogen atmosphere, at an annealing temperature of 750 to 900° C., and held for 10 to 600 seconds.

[0033] [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 is used, a forsterite (Mg2SiO4) coating layer is formed. If an annealing separator mainly composed of Al2O3 is used, the forsterite (Mg2SiO4) coating may not be formed. In the final annealing process, the steel sheet is heated to the annealing temperature, causing secondary recrystallization of the primary recrystallized grains obtained in the decarburization annealing process, and the rolling direction of the steel sheet is <100> The steel sheet surface is annealed to obtain crystal grains aligned in the GOSS orientation, which indicate the {110} plane, and by holding it at the annealing temperature for a specified time, inhibitors and other substances that adversely affect the magnetic properties are removed (purified). The conditions for the finish annealing are not limited, but for example, the temperature is raised to 1150 to 1250° C. in an atmospheric gas containing hydrogen and nitrogen, and held at that temperature range for 10 to 60 hours.

[0034] [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 which includes a base steel sheet (cold-rolled sheet) and, if necessary, a forsterite coating and / or an insulating coating. [Example]

[0035] A slab containing, by mass%, C: 0.055%, Si: 0.86 to 3.15%, Mn: 0.14%, S: 0.007%, acid-soluble Al: 0.027%, Cr: 0.12%, N: 0.0075%, and the remainder being Fe and impurities was prepared by continuous casting. The slab was heated to 1100 to 1450°C and hot rolled to obtain a hot rolled sheet having a thickness of 2.3 to 3.0 mm. This hot-rolled sheet was subjected to hot-rolled sheet annealing by holding at 1120°C for 180 seconds. The hot-rolled sheet after annealing was pickled and then cold-rolled under conditions where the final reduction was 90% to obtain a steel sheet (cold-rolled sheet) with a thickness of 0.23 mm. This steel sheet was irradiated with a laser to form a plurality of grooves extending in a direction at an angle of 80° to the rolling direction, periodically at intervals of 4 mm in the rolling direction. The steel plate after groove formation was ground using the brush roll shown in Table 1 under the conditions shown in Table 1. During grinding, the steel plate was conveyed while being ground so that the rotation direction of the brush roll was parallel to the longitudinal direction of the plate and opposed to the conveying direction at the portion in contact with the steel plate. After the grinding process, the steel sheet was heated to 840°C in a wet hydrogen and nitrogen atmosphere and held for 150 seconds to perform decarburization annealing. After the decarburization annealing process, the steel sheets were coated with an annealing separator mainly composed of MgO and TiO2, and then subjected to finish annealing at 1200°C for 20 hours. After the finish annealing, a forsterite film was formed on the surface of the steel sheet. After finish annealing, a coating liquid containing chromic anhydride and aluminum phosphate as main components was applied to the surface of this steel sheet (a steel sheet having a forsterite coating formed on the surface of the base steel sheet), and baked by heating to 700 to 900°C to form an insulating coating, thereby obtaining a grain-oriented electrical steel sheet.

[0036] The chemical composition of the obtained base steel sheet for the grain-oriented electrical steel sheet was measured using the method described above. The results were, in mass%, C content of 0.014-0.055%, S content of 0.001-0.007%, acid-soluble Al content of 0.011-0.027%, and N content of 0.0049-0.0075%. The Si content, Mn content, and Cr content did not show any significant changes compared to the slab stage.

[0037] The grain-oriented electrical steel sheets were also measured for the number of protrusions at the groove edges that were 2.0 μm or more and less than 6.0 μm in height, and the area ratio of brush marks at the groove edges, using the methods described above. Additionally, the width and depth of the groove were measured. The results are shown in Tables 1 and 2.

[0038] Furthermore, a wound core with a capacity of 25 kVA was manufactured by a known method using the obtained grain-oriented electrical steel sheet.

[0039] The primary and secondary windings were wound around the obtained core, an excitation current was passed through the primary winding, and the iron loss W17 / 50 was measured using a wattmeter from the current and the secondary voltage of the secondary winding. If the iron loss was 0.75 (W / kg) or less, it was determined that the core iron loss was low. The results are shown in Table 2.

[0040] [Table 1]

[0041] [Table 2]

[0042] As can be seen from Tables 1 and 2, in examples where the ratio of the number of protrusions with a height of 2.0 μm or more and less than 6.0 μm and the ratio of the area of ​​brush marks on the groove edges were within the range of the present invention, the core iron loss was low. In contrast, in examples where the grinding conditions were outside the preferred range, the ratio of the number of protrusions with a height of 2.0 μm or more and less than 6.0 μm and the ratio of the area of ​​brush marks on the groove edges were outside the range of the present invention, and as a result, the core iron loss was also high. [Industrial Applicability]

[0043] According to the present invention, it is possible to provide a grain-oriented electrical steel sheet with low iron loss (particularly low iron loss when used as a core), which has high industrial applicability. [Explanation of symbols]

[0044] 11 Steel plate (base material steel plate) 21 Groove 31 Groove entrance 41 Groove edge 101 Protrusion RD rolling direction TD Sheet width direction

Claims

1. A method for manufacturing a steel sheet, which is a base steel sheet for a grain-oriented electrical steel sheet, having a plurality of grooves formed on its surface by laser irradiation, the grooves extending in a direction intersecting the rolling direction and having a depth in the plate thickness direction, In a cross section of the steel plate parallel to the rolling direction and parallel to the plate thickness direction, when a range of 40 μm from the entrance of the groove in a direction perpendicular to the plate thickness direction and opposite to the center of the width direction of the groove is defined as a groove edge portion, the ratio of the number of protrusions having a height of 2.0 μm or more and less than 6.0 μm to the number of protrusions in the groove edge portion of the plurality of cross sections is 10% or more and less than 40%; When viewed from above, brush marks having a width of 4 to 25 μm and a depth of 4 μm or less are present in an area ratio of 10% or more but less than 80% of the groove edge portion. A directional electrical steel sheet characterized by:

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

3. an insulating coating is formed on the surface of the forsterite coating; The grain-oriented electrical steel sheet according to claim 2 ,

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

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