Grain-oriented electrical steel sheet and method for manufacturing same

Abstract

The present invention achieves magnetic domain refinement by means of groove formation and magnetic domain refinement by means of the application of a tension without increasing the building factor. Disclosed is a grain-oriented electrical steel sheet which has a plurality of grooves that are arranged in a predetermined position of a steel sheet, and is provided with a forsterite coating film and an insulating coating film on both surfaces of the steel sheet, wherein: Ra of the forsterite coating film at the center of the grooves is 5.00 μm or more; and the average thickness of the insulating coating film at the bottom of the grooves is 1.50 μm or more.

Description

Grain-oriented electrical steel sheet and its manufacturing method

[0001] The present invention relates to a grain-oriented electrical steel sheet and a manufacturing method thereof, and more particularly to a grain-oriented electrical steel sheet suitable for use as an iron core material for a transformer or the like, and a manufacturing method thereof.

[0002] Grain-oriented electrical steel sheets, which have a crystalline structure in which the <001> orientation, the axis of easy magnetization of iron, is highly aligned in the rolling direction of the steel sheet, are particularly used as core materials for power transformers. Transformers are broadly classified into stacked-core transformers and wound-core transformers based on their core structure. Stacked-core transformers have a core formed by stacking steel sheets cut to a predetermined shape. Wound-core transformers, on the other hand, have a core formed by winding steel sheets. While various characteristics are required for transformer cores, a particularly important one is reduced iron loss. The development of transformer materials with the performance required to reduce iron loss is increasingly required each year.

[0003] From the above perspective, low iron loss is an important characteristic required of grain-oriented electrical steel sheets, which are the material for iron cores. Magnetic domain refining technology is one of the technologies for reducing the iron loss of grain-oriented electrical steel sheets. This magnetic domain refining technology is divided into heat-resistant and non-heat-resistant types. In the case of the wound cores described above, stress relief annealing is performed during the manufacturing process, so heat-resistant magnetic domain refining technology is required for the grain-oriented electrical steel sheets used in these wound cores. A known heat-resistant magnetic domain refining technology is to form grooves on the surface of the steel sheet.

[0004] For example, Patent Document 1 discloses a method for applying a pressure of 882 to 2156 MPa (90 to 220 kgf / mm) to a steel sheet that has been finish-annealed. 2 (2003) describes a technique for forming grooves with a depth of more than 5 μm in a substrate steel portion under a load of 1000 kJ / cm2, followed by heat treatment at a temperature of 750°C or higher, thereby refining magnetic domains. Patent Document 2 proposes forming grooves with an asymmetric shape in the groove width direction with respect to the center of the groove width, and controlling the average depth of the groove, the arithmetic mean height Ra of the roughness curve that defines the outline of the groove bottom region, and the average length RSm of the roughness curve elements that define the outline of the groove bottom region. Patent Document 3 proposes controlling the groove depth, groove width, the roughness Ra value of the groove bottom surface, and the roughness Ra value of the groove side surface within specific ranges.

[0005] Japanese Patent Publication No. 62-53579 International Publication No. 2016 / 171130 Japanese Patent Application Laid-Open No. 2022-22494

[0006] While the application of the above-mentioned conventional techniques achieves a certain degree of iron loss reduction, there is currently a demand for even lower iron loss. Aside from the groove-forming technique described above, a known magnetic domain refining technique involves applying tension to grain-oriented electrical steel sheets, which refines the magnetic domains and reduces iron loss. The application of tension to the steel sheets improves with increasing thickness of the forsterite coating and insulating coating. However, as these two coatings become thicker, there is a problem in that the volume fraction of the steel sheets in the transformer core, known as the space factor, decreases when the steel sheets are assembled into the core.

[0007] Here, the ratio of the iron loss value (transformer iron loss) when grain-oriented electrical steel sheet is used as the iron core of a transformer to the iron loss value of the material (grain-oriented electrical steel sheet) obtained by the Epstein test is generally called the building factor (BF) or distraction factor (DF). Hereinafter, these are collectively referred to as BF. Reduction of this BF is also an item that should be considered as a property required of grain-oriented electrical steel sheet. This BF increases as the space factor mentioned above decreases. Therefore, from the viewpoint of reducing BF, it is desirable to make the coating thickness thin.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to propose a method for realizing magnetic domain refining by improving the tension on the surface of a steel sheet in addition to magnetic domain refining by forming grooves, without increasing the BF.

[0009] The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that by thickening the insulating coating formed in the grooves, particularly at the bottom of the grooves, the tension in the rolling direction generated in the areas where the grooves are present is improved, and the amount of magnetic poles generated on the groove wall surfaces is increased. As a result, magnetic domain refinement is achieved, resulting in low iron loss. They further discovered that this method does not cause a decrease in the space factor, which is a problem when thickening the insulating coating, because the insulating coating is thickened only in the areas of the steel sheet surface where the grooves are present.

[0010] Based on the above findings, the inventors attempted to form a thick insulating coating in the grooves by pouring a coating solution for forming an insulating coating into grooves formed on the surface of a grain-oriented electrical steel sheet and then performing planarization annealing. However, they found that the insulating coating had poor adhesion and easily peeled off. Therefore, the inventors conducted extensive research to solve the above-mentioned problem of coating peeling. As a result, they discovered that by increasing the surface roughness of the forsterite coating formed in predetermined locations in the grooves to a predetermined extent, the adhesion of the insulating coating further formed thereon can be improved, which led to the completion of the present invention.

[0011] Furthermore, the inventors have clarified that damage to the forsterite coating has a significant effect on the magnitude of the tension in the groove in the rolling direction, and have found that suppressing this damage can achieve an even greater and more stable iron loss reduction effect.

[0012] Based on the above further findings, the inventors confirmed that the forsterite coating formed on the surface of grain-oriented electrical steel sheet has many cracks and searched for a method for suppressing these cracks. As a result, they found that damage to the forsterite coating, typified by cracks, is mainly introduced due to the shape correction of the steel sheet during flattening annealing after final finish annealing. They also found that by performing final finish annealing on a coil-shaped steel sheet wound with the grooved surface facing outward, the generation of tensile stress that causes cracks to be introduced during the subsequent shape correction is suppressed, and the frequency of crack occurrence is significantly reduced, leading to the completion of an even more preferable invention.

[0013] The gist of the present invention is as follows: [1] A grain-oriented electrical steel sheet having, on one or both sides of the steel sheet, a plurality of grooves that extend linearly in a direction transverse to the rolling direction of the steel sheet and are aligned at intervals in the rolling direction, and having, on both sides of the steel sheet, a forsterite coating and an insulating coating, in that order from the surface side of the steel sheet, wherein the surface roughness Ra of the forsterite coating formed at the center of the grooves in a cross section perpendicular to the extension direction of the grooves is 5.00 μm or more, and the average thickness of the insulating coating at the bottom of the grooves is 1.50 μm or more.

[0014] [2] The grain-oriented electrical steel sheet according to [1] above, wherein the forsterite coating formed in the grooves has a roughness coefficient of 0.8 or less.

[0015] [3] A method for producing a grain-oriented electrical steel sheet, comprising hot rolling a slab for a grain-oriented electrical steel sheet to obtain a hot-rolled sheet, annealing the hot-rolled sheet as necessary to obtain a hot-rolled annealed sheet, cold rolling the hot-rolled sheet or the hot-rolled annealed sheet once or at least twice with intermediate annealing therebetween to obtain a cold-rolled sheet having a final sheet thickness, decarburizing the cold-rolled sheet to obtain a decarburized annealed sheet, applying an annealing separator mainly composed of MgO to the surface of the decarburized annealed sheet, and then performing final annealing to obtain a finish-annealed sheet, and then applying an insulating coating to the finish-annealed sheet, the method comprising: (1) forming a plurality of grooves on one or both surfaces of the hot-rolled sheet, the hot-rolled annealed sheet, the cold-rolled sheet, or the decarburized annealed sheet before applying the annealing separator, the grooves extending linearly in a direction transverse to the rolling direction of the steel sheet and spaced apart in the rolling direction; (2) applying the annealing separator to at least the grooved surface of the decarburization-annealed sheet, then scraping off the annealing separator, and then applying an annealing separator to both surfaces of the decarburization-annealed sheet; (3) applying the insulating coating to at least the grooved surface of the finish-annealed sheet, then scraping off the insulating coating, and then applying an insulating coating to both surfaces of the finish-annealed sheet.

[0016] [4] The method for producing a grain-oriented electrical steel sheet according to the above [3], wherein a doctor blade is used to scrape off the annealing separator and the insulating coating.

[0017] [5] The method for producing a grain-oriented electrical steel sheet according to the above [3] or [4], wherein when the decarburized annealed sheet to which the annealing separator has been applied is subjected to final finish annealing, the decarburized annealed sheet is formed into a coil shape with the surface on which the grooves have been formed being arranged on the outer winding side, and then the final finish annealing is performed.

[0018] According to the present invention, by roughening the forsterite film and thickening the insulating film in the grooves formed on the surface of the grain-oriented electrical steel sheet, stronger tension is generated in the rolling direction of the steel sheet, promoting magnetic domain refinement. As a result, it is possible to achieve a low BF while further reducing iron loss. Furthermore, by applying an annealing separator to the grooves on the surface of a decarburized annealed sheet and further applying an insulating coating to the grooves on the surface of a finish-annealed sheet according to the manufacturing method of the present invention, magnetic domain refinement is promoted in the same manner as above. As a result, it is possible to achieve both reduced iron loss and a low BF. Furthermore, according to a preferred invention, the degree of damage to the forsterite film is also reduced, further promoting magnetic domain refinement. As a result, it is possible to achieve both an even lower iron loss and an even lower BF.

[0019] 1A and 1B are cross-sectional views showing grooves in a steel plate, and FIG. 1C are cross-sectional views showing grooves in a steel plate at two positions spaced apart in an extension direction.

[0020] Preferred embodiments of the present invention will be described in detail below. Unless otherwise specified, the notation "A to B" for numerical values ​​A and B means "A or more and B or less." In such notation, when a unit is assigned only to numerical value B, the unit also applies to numerical value A.

[0021] (Grain-oriented electrical steel sheet) The grain-oriented electrical steel sheet (hereinafter sometimes simply referred to as steel sheet) according to the present invention has, on one or both sides of the steel sheet, a plurality of grooves extending linearly in a direction transverse to the rolling direction of the steel sheet (hereinafter sometimes simply referred to as the rolling direction) and spaced apart in the rolling direction. Both sides of the steel sheet are provided with a forsterite coating and an insulating coating, in that order from the surface side of the steel sheet. Furthermore, it is essential that the average thickness of the insulating coating at the bottom of the grooves is 1.50 μm or more, and that the Ra of the surface of the forsterite coating at the center of the groove in a cross section perpendicular to the extension direction of the grooves is 5.00 μm or more. Furthermore, for a suitable grain-oriented electrical steel sheet, it is desirable that the roughness coefficient of the forsterite coating formed in the grooves is 0.8 or less. The grain-oriented electrical steel sheet according to the present invention can be suitably obtained, for example, by the manufacturing method of the grain-oriented electrical steel sheet according to the present invention described below.

[0022] [Grooves] The grooves are preferably formed on the steel sheet surface with a groove spacing of 1 mm or more in the rolling direction, more preferably 2 mm or more, and preferably 20 mm or less, and even more preferably 10 mm or less. If the spacing is less than 1 mm, the magnetic domain refining effect may saturate, and the eddy current loss reduction effect may be almost completely lost. Furthermore, hysteresis loss increases due to strain, which is undesirable, and iron loss and BF tend to increase. On the other hand, if the spacing exceeds 20 mm, the magnetic domain refining effect decreases, and the iron loss and BF improvement effect is insufficient, which is undesirable. The groove spacing is preferably 1 to 20 mm, more preferably 2 to 10 mm.

[0023] The groove formation pattern is not particularly limited, but it is preferable to have grooves parallel to each other and in the same positions on both sides of the steel sheet, rather than having grooves on only one side of the steel sheet, because this increases the magnetostatic energy in the grooves and strengthens the magnetic domain refinement effect. Also, it is preferable to have grooves parallel to each other and in staggered positions on both sides of the steel sheet, because this shortens the magnetic domain length in the rolling direction, thereby increasing the magnetostatic energy and strengthening the magnetic domain refinement effect. It is also preferable to have grooves crossing each other on both sides of the steel sheet, because this increases the magnetostatic energy at the grooves where the grooves cross, and increases the magnetostatic energy at the grooves where they do not cross, because the magnetic domain length in the rolling direction is shorter than when the grooves are parallel, thereby strengthening the magnetic domain refinement effect.

[0024] FIG. 1 is a cross-sectional view of a groove 2 formed in a grain-oriented electrical steel sheet (steel sheet) 1 according to one embodiment of the present invention, taken perpendicular to the extension direction of the groove. In FIG. 1 , the groove 2 has a shape that is close to a trapezoid in cross section, but the shape of the groove 2 is not particularly limited, and it may have, for example, an arched cross section. In one embodiment of the grain-oriented electrical steel sheet of the present invention, the depth D of the groove 2 is preferably 10 μm or more, preferably 50 μm or less, and more preferably in the range of 10 μm to 50 μm. Here, referring to FIG. 1 , the depth D of the groove 2 is the length of a perpendicular line extending from the center of the opening width of the groove 2 in the rolling direction to the bottom 20. The opening width is defined as the distance L between the opening-side edges of both side walls defining the groove 2, along an imaginary extension of the opening of the groove 2 on the surface of the steel sheet 1.

[0025] If the depth D of the grooves 2 is less than 10 μm, the amount of magnetic poles generated from the wall surfaces of the grooves 2 will be small, and there is a risk that a sufficient iron loss reduction effect will not be obtained. On the other hand, if the depth D exceeds 50 μm, the magnetic domains will be subdivided, but the reduction in magnetic flux density due to the formation of the grooves will be so great that there is a risk that a sufficient iron loss reduction effect will not be obtained. The depth D is more preferably 15 μm or more and 30 μm or less, and even more preferably 15 μm to 30 μm.

[0026] As shown in Fig. 1 , the grain-oriented electrical steel sheet of the present invention has a steel sheet 1 having the grooves 2 on at least one side thereof, and is provided with a forsterite coating 3 and an insulating coating 4, in that order from the front surface side, on both surfaces of the steel sheet 1. When the steel sheet 1 has grooves 2 on both sides, the forsterite coating 3 and the insulating coating 4 are formed on the grooves 2 on both sides of the steel sheet 1. When the steel sheet 1 has grooves 2 on one side, the forsterite coating 3 and the insulating coating 4 are formed on the grooves 2 only on that one side of the steel sheet 1. The forsterite coating 3 and the insulating coating 4 can be formed in the steel sheet base metal portion excluding the grooves 2 in accordance with general procedures for grain-oriented electrical steel sheets. On the other hand, in the grooves 2, it is important to specify the surface roughness of the forsterite coating 3 and the average thickness of the insulating coating 4 as follows:

[0027] [Surface roughness Ra of the forsterite coating formed at the center of the groove in a cross section perpendicular to the extension direction of the groove is 5.00 μm or more] First, the "center of the groove in a cross section perpendicular to the extension direction of the groove" refers to the position (center 21) of a perpendicular line drawn through the center of the opening width of the groove 2 to the bottom 20, as shown in FIG. 1 . The arithmetic mean roughness Ra of a roughness curve measured for the surface of the forsterite coating 3 on this perpendicular line over a predetermined distance (e.g., a length of 30 mm) along the extension direction of the groove 2 is the "surface roughness Ra of the forsterite coating formed at the center of the groove in a cross section perpendicular to the extension direction of the groove." Hereinafter, this value will also be referred to simply as the "surface roughness of the forsterite coating."

[0028] The surface roughness of the forsterite coating described above will be specifically described with reference to FIG. 2 . In FIG. 2 , the right side shows a top view of a steel sheet 1 having a groove 2. The left side shows cross-sectional views of the groove corresponding to two different positions along the extension direction of the groove in the top view. That is, in FIG. 2 , groove cross-sections corresponding to positions spaced a predetermined distance t apart in the extension direction of the groove 2 are shown as cross-sections A-A and B-B. In FIG. 2 , the surface of the forsterite coating 3 at the center in the rolling direction at each position spaced a predetermined distance t apart is shown as d and d′. The surface irregularities of the forsterite coating 3 on the line connecting these points d and d′ can be measured as the arithmetic mean roughness Ra of the roughness curve. The predetermined distance t may be any length, but should be approximately 30 to 50 mm. This is because achieving the required conditions locally is meaningless; it is necessary that the conditions be achieved on average across the entire groove of the steel sheet, and that they be achieved in a region longer than the width of at least one crystal grain. The definition of the arithmetic mean roughness Ra of the roughness curve is in accordance with Japanese Industrial Standards JIS B0601 (2013).

[0029] It is essential that the surface roughness Ra of the forsterite coating be 5.00 μm or greater. That is, if the surface roughness Ra of the forsterite coating is less than 5.00 μm, the adhesion of the insulating coating to the forsterite coating decreases, making the insulating coating more likely to peel off. Therefore, by setting the Ra to 5.00 μm or greater, the surface area per unit groove of the forsterite coating increases, which is thought to result in a larger adhesive area with the insulating coating and improved coating adhesion. Preferably, the Ra is 6.00 μm or greater, and more preferably 7.00 μm or greater.

[0030] On the other hand, there is no particular upper limit for Ra, but if Ra exceeds 20.00 μm, the forsterite coating itself may become embrittled. Therefore, Ra is preferably 20.00 μm or less. More preferably, Ra is 15.00 μm or less, and even more preferably, Ra is 10.00 μm or less.

[0031] [Method for Measuring the Surface Roughness Ra of a Forsterite Coating] Ra can be measured using a laser microscope (a 3D laser microscope using a pinhole confocal optical system). That is, one of the grooves formed in the grain-oriented electrical steel sheet is selected, and two groove positions (A) and (B) are determined, for example, 30 mm apart in the extension direction, as shown in FIG. 2 . Then, the surface irregularities of the forsterite coating on a line connecting the surfaces d and d′ of the forsterite coating 3 corresponding to the center in the rolling direction of the groove bottom (groove bottom surface) at each position are measured as the arithmetic mean roughness Ra of the roughness curve. The above measurement is performed on any 20 grooves, and the average value of the 20 grooves is taken as the surface roughness Ra of the forsterite. Here, when using a sample before planarization annealing and coating with an insulating coating liquid for measurement, Ra can be measured as is. On the other hand, when using a sample after coating with an insulating coating liquid for measurement, Ra can be measured after removing the insulating coating with an alkaline solution (e.g., sodium hydroxide solution).

[0032] [Roughness Coefficient of the Forsterite Coating Formed in Grooves is 0.8 or Less] The roughness coefficient of the forsterite coating formed in the grooves is preferably 0.8 or less. A roughness coefficient of the forsterite coating present in the grooves exceeding 0.8 indicates a relatively high porosity of the forsterite coating. As a result, the tension applied by the insulating coating on the forsterite coating is not fully reflected in the steel sheet, which can easily cause variations in properties such as iron loss and BF. Therefore, to further enhance the effect of improving tensile properties by thickening the forsterite coating formed in the grooves, it is desirable to reduce the porosity of the forsterite coating in the grooves. Specifically, the roughness coefficient is preferably 0.8 or less, more preferably 0.6 or less, and even more preferably 0.4 or less. On the other hand, there is no particular lower limit for the roughness coefficient of the forsterite coating formed in the grooves, but from the viewpoint of sufficiently covering the grooves with the forsterite coating, a value of 0.05 or more is preferred.

[0033] [Method for Measuring the Roughness Coefficient of a Forsterite Coating Formed in Grooves] First, the specific surface area of ​​a sample with no grooves but with a forsterite coating is measured by a gas adsorption method using Kr gas. The true surface area, including minute surface irregularities, is calculated from the results and divided by the apparent surface area if the surface were completely smooth. In this way, the surface roughness coefficient of a grain-oriented electrical steel sheet with a forsterite coating formed on the steel sheet surface other than the grooves is derived. Next, the specific surface area of ​​a sample with grooves and a forsterite coating formed thereon is measured by a gas adsorption method using Kr gas in the same manner as above, and the true surface area, including minute surface irregularities, is calculated. The apparent surface area of ​​the non-groove portion of the sample with grooves is then derived, and this is multiplied by the previously derived roughness coefficient of the sample without grooves to derive the true surface area of ​​the non-groove portion of the sample with grooves. The difference between this result (the true surface area of ​​the non-groove portion of the sample with grooves) and the true surface area derived from the specific surface area of ​​the sample with grooves is defined as the true surface area of ​​the groove portion. The true surface area of ​​the groove is then divided by the apparent surface area of ​​the groove, and then further divided by Ra to derive the roughness coefficient of the forsterite film formed in the groove. This makes it possible to evaluate the amount of microcracks that cannot be evaluated by the surface roughness Ra. In summary, the roughness coefficient (unit: μm －１ ) can be calculated by the following formula (1): true surface area of ​​groove / (apparent surface area of ​​groove×Ra) (1) Here, Ra is the surface roughness (μm) of the forsterite coating formed at the center of the groove in a cross section perpendicular to the extension direction of the groove, and can be measured by the method described above.

[0034] [Average Thickness of Insulating Coating at Groove Bottoms of 1.50 μm or More] It is also important that the average thickness of the insulating coating at the groove bottoms (groove bottom surfaces) be 1.50 μm or more. In other words, if the average thickness of the insulating coating at these locations is less than 1.50 μm, no further improvement in iron loss is achieved. Although the mechanism by which an insulating coating with an average thickness of 1.50 μm or more exhibits its effects is unclear, it is believed that a thicker insulating coating locally increases tension in the grooves, improving the magnetic domain refinement effect. The average thickness of the insulating coating at the groove bottoms is preferably 1.70 μm or more. Furthermore, although it depends on the thickness of the forsterite coating formed in the grooves, for example, the average thickness of the insulating coating at the groove bottoms is preferably 20 μm or less, and more preferably 10 μm or less.

[0035] [Method for Measuring the Average Thickness of the Insulating Coating at the Bottom of the Groove] Using a scanning electron microscope, cross-sectional images along the rolling direction including the groove cross section shown in FIG. 1 are obtained in 30 or more fields of view, with one field being 100 to 200 μm in width in the rolling direction × the entire thickness of the sheet. Preferably, cross-sectional images are obtained for 30 or more different grooves. These are subjected to image analysis to identify the interface position between the forsterite coating and the insulating coating and the surface position of the insulating coating, and the average thickness of the insulating coating at the bottom of the groove is calculated for each image. Furthermore, the average thickness of all the obtained cross-sectional images is determined as the average thickness of the insulating coating at the bottom of the groove. Note that in this measurement, the "bottom" of the groove is determined as the portion of the recess defining the groove that corresponds to the central 80% of the opening width in the rolling direction as described above. The "side wall" of a groove is defined as the portion of the recess defining the groove excluding the "bottom," i.e., the portion corresponding to 10% of the opening width in the rolling direction from both sides of the edge of the steel sheet surface defining the groove.

[0036] [Method for Evaluating Adhesion of Insulation Coating] The adhesion of the insulation coating described above can be evaluated as peeling when the shape of the steel sheet is deformed. Specifically, it can be evaluated by a bending peeling test in which the steel sheet is wrapped around a round bar and the smallest diameter at which the coating does not peel off is determined. The smaller the smallest diameter at which the coating does not peel off, the smaller the peeling, and therefore the better the adhesion.

[0037] (Method for manufacturing grain-oriented electrical steel sheet) In the manufacturing method of the present invention, a slab for grain-oriented electrical steel sheet is hot-rolled to form a hot-rolled sheet, and then the hot-rolled sheet is annealed as necessary to form a hot-rolled annealed sheet, and the hot-rolled sheet or the hot-rolled annealed sheet is cold-rolled once or twice or more times with intermediate annealing in between to form a cold-rolled sheet having a final sheet thickness, and then the cold-rolled sheet is decarburized and annealed to form a decarburized annealed sheet, and then an annealing separator mainly composed of MgO is applied to the surface of the decarburized annealed sheet, and then final annealing is performed to form a finish-annealed sheet, and then an insulating coating is applied to the finish-annealed sheet, (1) before applying the annealing separator, a plurality of grooves are formed on one or both surfaces of the hot-rolled sheet, hot-rolled annealed sheet, cold-rolled sheet or decarburized annealed sheet, the grooves extending linearly in a direction transverse to the rolling direction of the steel sheet and arranged at intervals in the rolling direction, (2) an annealing separator is applied to at least the grooved surface of the decarburization-annealed sheet, the annealing separator is then scraped off, and the annealing separator is then applied to both surfaces of the decarburization-annealed sheet, or (3) an insulating coating is applied to at least the grooved surface of the finish-annealed sheet, the insulating coating is then scraped off, and the insulating coating is then applied to both surfaces of the finish-annealed sheet. According to the manufacturing method of the present invention, the above-mentioned grain-oriented electrical steel sheet of the present invention can be successfully obtained, and therefore the same effects as those of the grain-oriented electrical steel sheet of the present invention can be achieved.

[0038] In addition, a preferred production method includes using a doctor blade to scrape off the annealing separator and insulating coating. In yet another preferred production method, when final annealing the decarburized annealed sheet coated with the annealing separator, the decarburized annealed sheet is formed into a coil shape with the grooved surface positioned on the outer winding side, and then final annealing is performed.

[0039] Next, the method for forming the grooves and the forsterite coating and insulating coating in the grooves will be described in detail. [Groove Formation Method] Linear grooves are formed on one or both sides of a steel sheet before application of an annealing separator (i.e., before final finish annealing), more specifically, on one or both sides of a hot-rolled sheet, a hot-rolled annealed sheet, a cold-rolled sheet, or a decarburization annealed sheet. Among these, it is preferable to form linear grooves on a cold-rolled sheet after final cold rolling and before decarburization annealing. Because the grooves may disappear during cold rolling, there is little need to form grooves before final cold rolling. Furthermore, while fayalite must be formed to form forsterite, it is generally formed during decarburization annealing, so it is preferable to form grooves on a cold-rolled sheet before decarburization annealing. Grooves can be effectively formed by applying and adhering an etching resist ink to the uncoated areas, followed by electrolytic etching. In forming the groove pattern, the non-coated areas are preferably formed by applying a resist ink to the entire surface of one or both sides of the cold-rolled sheet, followed by laser irradiation to remove the ink, or by resist printing. Since it is particularly important that a forsterite film is sufficiently formed at the bottom of the grooves, it is essential that the grooves be formed before the application of an annealing separator that forms a forsterite film and before final annealing.

[0040] Here, the grooves are formed in a direction transverse to the rolling direction, i.e., in a linear shape extending in a direction at an angle from the rolling direction along the rolled surface. In this specification, "direction transverse to the rolling direction" preferably refers to a deviation of the linear groove from the direction perpendicular to the rolling direction of the grooves within ±30°, and can be 0° (width direction of the steel sheet). In this specification, "linear" includes not only solid lines but also dotted lines and dashed lines. Other characteristics of the grooves can be the same as those described above for grain-oriented electrical steel sheets.

[0041] [Application of annealing separator] After applying an annealing separator (preliminary application) to at least the grooved surface of the decarburized annealed steel sheet, the annealing separator is scraped off, and then an annealing separator is applied (main application) to both sides of the decarburized annealed steel sheet. Therefore, the annealing separator is applied twice to at least the grooved surface. The annealing separator contains MgO as its main component, and the application amount is 8 g / m per side of the steel sheet. ２ or more, 15 g / m ２ or less, 8 to 15 g / m ２ A range of about 75% by mass of MgO is suitable for forming a forsterite coating having a desired thickness. Incidentally, "containing MgO as a main component" means that the annealing separator contains 75% by mass or more of MgO in terms of solid content. The formed solid forsterite coating also has a similar MgO content.

[0042] The procedure for applying the annealing separator involves applying (pre-application) the annealing separator (e.g., an MgO suspension) to at least the grooved surface, scraping off the suspension, and then applying (main application) the MgO suspension to both surfaces. Even if the pre-application of the annealing separator is performed, the suspension in the grooves is not scraped off, and the suspension scraped off from areas other than the grooves flows back into the grooves. This allows for a large amount of suspension to be effectively applied to the grooves. A doctor blade is preferably used as a scraping means. While this application method is preferred, applying a large amount of suspension to the grooves by other methods is also acceptable. Unlike the above-mentioned method, known methods that do not involve scraping (e.g., roll coaters) often have difficulty applying a sufficient amount of suspension to the grooves, resulting in a thin forsterite coating in the grooves. A thin forsterite coating in the grooves reduces the roughness of the coating. Furthermore, it is difficult to increase the tension applied to the steel sheet, making it difficult to reduce iron loss and BF. On the other hand, according to the above-described technique, by thoroughly distributing the annealing separator in the grooves, the Ra of the forsterite coating at a predetermined position in the groove after the subsequent final annealing can be made 5.00 μm or more, which reduces iron loss and makes it easier to achieve a low BF.

[0043] [Final Finish Annealing] The decarburized annealed sheet coated with an annealing separator is subjected to final finish annealing for the purpose of secondary recrystallization and the formation of a forsterite coating. Final finish annealing is often performed on a decarburized annealed sheet wound into a coil. In this case, it is preferable to perform final finish annealing on the decarburized annealed sheet as a coil with the grooved surface on the outer winding side. Since the final finish annealing causes a coil set, shape correction may be performed in a subsequent process. If a forsterite coating is formed with grooves on the outer winding side, compressive stress is applied to the forsterite coating in the grooves when the forsterite coating is flattened by subsequent flattening annealing. Because tensile tension is the cause of damage such as cracks that increase the porosity of the forsterite coating, damage to the forsterite coating can be effectively avoided in this case. On the other hand, if a forsterite coating is formed with grooves on the inner winding side, tensile stress is applied to the forsterite coating in the grooves when the sheet is flattened, introducing damage such as cracks into the forsterite coating and increasing the porosity. Furthermore, even if the finish-annealed sheet is bent in the opposite direction to the curl during the sheet threading process after the forsterite coating formation, compressive stress is applied, making the forsterite coating less susceptible to damage. Conversely, if the finish-annealed sheet is bent in the same direction as the curl, tensile stress is applied, but the application of tensile stress is reduced by the amount of the curl, thereby suppressing damage to the forsterite coating. Thus, by controlling the shape of the decarburized-annealed sheet during final annealing as described above, it is possible to more effectively and stably reduce the iron loss of grain-oriented electrical steel sheets and more effectively prevent an increase in BF.

[0044] [Insulating Coating and Planarizing Annealing] An insulating coating is applied (pre-application) to at least the grooved surface of the finish-annealed sheet obtained in this manner, and then the insulating coating is scraped off. Further insulating coatings are applied to both sides of the finish-annealed sheet (main application). Therefore, an insulating coating is applied twice, at least to the grooved surface. By insulating coating, we mean a coating that applies tension to the steel sheet to reduce iron loss. After final finish annealing, flattening annealing to correct the shape is effective in improving the space factor when the grain-oriented electrical steel sheet is processed into an iron core. The flattening annealing temperature is preferably 750°C or higher and 950°C or lower, approximately in the range of 750 to 950°C. The flattening annealing time is preferably 10 seconds or higher and 200 seconds or lower, approximately in the range of 10 to 200 seconds (seconds). The insulating coating is preferably formed before or after flattening annealing.

[0045] In one example of an insulating coating formation process, an insulating coating liquid containing, for example, colloidal silica and phosphate is applied over the forsterite coating to the surface of the finish-annealed sheet on which at least the grooves are formed. A suitable application procedure involves applying the insulating coating liquid to at least the grooved surface (pre-application), scraping off the insulating coating liquid, and then applying the insulating coating liquid again to both surfaces, including the grooved surface (main application). Even if the pre-application insulating coating liquid is scraped off, the insulating coating liquid in the grooves is not scraped off, and the insulating coating liquid scraped off from areas other than the grooves flows back into the grooves. This allows for an effective application of a large amount of insulating coating liquid to the grooves. A doctor blade is preferably used as a scraping means. The above application method is merely one suitable example, and applying a large amount of coating liquid to the grooves using other methods is not problematic. Unlike the above method, known methods that do not involve scraping (e.g., roll coaters) have difficulty applying a sufficient amount of insulating coating liquid to the grooves, which tends to result in a thin insulating coating in the grooves. If the insulating coating in the grooves becomes thin, it becomes difficult to increase the tension applied to the steel sheet and reduce iron loss and BF. On the other hand, the above-mentioned technique allows the insulating coating liquid to be sufficiently distributed throughout the grooves, making it possible to achieve an average thickness of 1.50 μm or more in the grooves of the final grain-oriented electrical steel sheet. This also facilitates reducing iron loss and achieving a low BF. Normally, forming an insulating coating as thick as 1.50 μm poses problems with adhesion (peelability). However, in the present invention, by controlling the surface roughness of the forsterite coating to be even coarser than before, as described above, the peelability of the insulating coating is eliminated. Subsequently, heat treatment can be performed at any suitable temperature (e.g., 840 to 920°C). Finally, a grain-oriented electrical steel sheet having the desired forsterite coating and insulating coating inside the grooves is obtained.

[0046] [Other Steps] The method for producing a grain-oriented electrical steel sheet of the present invention is not particularly limited with respect to steps not directly related to the magnetic domain refinement treatment. Recommended suitable chemical compositions and other production conditions are described below.

[0047] [Composition] The composition of grain-oriented electrical steel slabs is preferably one that allows secondary recrystallization to occur. When using an inhibitor, for example, an AlN-based inhibitor may contain appropriate amounts of Al and N, while an MnS / MnSe-based inhibitor may contain appropriate amounts of Mn and S and / or Se. Of course, both inhibitors may be used in combination. In this case, the preferred contents of Al, N, S, and / or Se are 0.01 to 0.04 mass% Al, 0.005 to 0.02 mass% N, and 0.005 to 0.03 mass% S and / or Se in total, respectively. During final annealing, Al, N, S, and Se are purified and their contents are reduced to the level of unavoidable impurities.

[0048] Furthermore, the present invention can also be applied to grain-oriented electrical steel sheets in which the contents of Al, N, S, and Se are limited and no inhibitor is used. In this case, the amounts of Al, N, S, and Se are preferably limited to Al: 100 ppm by mass or less, N: 50 ppm by mass or less, S: 50 ppm by mass or less, and Se: 50 ppm by mass or less, respectively.

[0049] The other basic components and optional added components are as follows: C: 0.08 mass% or less If the C content exceeds 0.08 mass%, it becomes difficult to reduce the C content to 50 mass ppm or less, at which point magnetic aging does not occur, during the manufacturing process, so it is preferable to set it to 0.08 mass% or less. On the other hand, since secondary recrystallization is possible even in materials that do not contain C, there is no need to set a lower limit, and it can be 0 mass%. Note that C is removed from the steel during decarburization annealing, reducing the content to the level of an unavoidable impurity.

[0050] Si: 2.0 to 8.0% by mass: Si is an element that is effective in increasing the electrical resistance of steel and improving iron loss. However, if the content is less than 2.0% by mass, sufficient iron loss reduction effect cannot be expected. On the other hand, if the content exceeds 8.0% by mass, workability is significantly reduced and magnetic flux density also decreases. Therefore, the Si content is preferably 2.0% by mass or more and 8.0% by mass or less, and more preferably in the range of 2.0 to 8.0% by mass.

[0051] Mn: 0.005 to 1.0% by mass. Mn is an element necessary for improving hot workability, but if the content is less than 0.005% by mass, the effect of adding it is poor. On the other hand, if the Mn content exceeds 1.0% by mass, the magnetic flux density of the finished sheet decreases. Therefore, the Mn content is preferably 0.005% by mass or more, and preferably 1.0% by mass or less, and more preferably in the range of 0.005 to 1.0% by mass.

[0052] In addition to the basic components described above, the following elements may be appropriately added to improve magnetic properties: at least one selected from the group consisting of Ni: 0.03 to 1.50% by mass, Sn: 0.01 to 1.50% by mass, Sb: 0.005 to 1.50% by mass, Cu: 0.03 to 3.0% by mass, P: 0.03 to 0.50% by mass, Mo: 0.005 to 0.10% by mass, and Cr: 0.03 to 1.50% by mass. Ni is a useful element for improving the hot-rolled sheet structure and magnetic properties. However, a Ni content of less than 0.03% by mass is ineffective in improving magnetic properties, while a Ni content of more than 1.50% by mass results in unstable secondary recrystallization and degraded magnetic properties. Therefore, the Ni content is preferably 0.03% by mass or more, but preferably 1.50% by mass or less, and more preferably in the range of 0.03 to 1.50% by mass.

[0053] Furthermore, Sn, Sb, Cu, P, Mo, and Cr are each useful elements for improving magnetic properties, but if the amount of each element is less than the lower limit of the respective element, the effect of improving magnetic properties is small. On the other hand, if the amount of each element exceeds the upper limit, the development of secondary recrystallized grains is inhibited. Therefore, it is preferable to contain each element within the above-mentioned range. The remainder other than the above elements can be unavoidable impurities mixed in during the manufacturing process and Fe.

[0054] Next, other manufacturing conditions in the manufacturing method of grain-oriented electrical steel sheet will be described. [Slab heating] A slab for grain-oriented electrical steel sheet having the above-mentioned composition can be heated in a conventional manner. The heating temperature is preferably 1150°C or higher and 1450°C or lower, and more preferably in the range of 1150 to 1450°C.

[0055] [Hot Rolling] After the slab heating, hot rolling can be performed to produce a hot-rolled sheet. Hot rolling may be performed immediately after casting without heating. In the case of a thin cast slab, hot rolling may be performed immediately, or hot rolling may be omitted. When hot rolling is performed, it is preferable to set the rolling temperature for the final pass of rough rolling to 900°C or higher and the rolling temperature for the final pass of finish rolling to 700°C or higher.

[0056] [Hot-rolled sheet annealing] After hot rolling, the hot-rolled sheet can be annealed as needed to produce a hot-rolled annealed sheet. In this case, in order to highly develop the Goss structure in the product sheet (grain-oriented electrical steel sheet), the hot-rolled sheet annealing temperature is preferably 800°C or higher, preferably 1100°C or lower, and more preferably in the range of 800 to 1100°C. If the hot-rolled sheet annealing temperature is less than 800°C, the band structure from hot rolling remains, making it difficult to achieve a uniformly sized primary recrystallized structure and inhibiting the development of secondary recrystallization. On the other hand, if the hot-rolled sheet annealing temperature exceeds 1100°C, the grain size after hot-rolled sheet annealing becomes too coarse, making it difficult to achieve a uniformly sized primary recrystallized structure.

[0057] [Cold Rolling] The hot-rolled sheet after hot rolling or the hot-rolled and annealed sheet after hot-rolled sheet annealing is subjected to cold rolling once or twice or more times with intermediate annealing in between to obtain a cold-rolled sheet having the final thickness. The intermediate annealing temperature is preferably 800°C or higher and 1150°C or lower, and more preferably 800°C or higher and 1150°C or lower. The intermediate annealing time is preferably 10 seconds or longer and 100 seconds or lower, and preferably about 10 to 100 seconds. As described above, the above-mentioned grooves are preferably formed in this cold-rolled sheet.

[0058] [Decarburization annealing] The cold-rolled sheet after cold rolling is subjected to decarburization annealing to obtain a decarburization annealed sheet. In the decarburization annealing, the annealing temperature is preferably 750°C or higher, and preferably 900°C or lower, more preferably about 750 to 900°C. In addition, the oxidizing atmosphere (oxidation degree) PH ２ O / PH ２The annealing time is preferably 50 seconds or more, and more preferably 300 seconds or less, and more preferably 50 to 300 seconds.

[0059] [Magnetic Domain Refinement Treatment] The magnetic domain refinement treatment can be performed in accordance with the groove formation method described above. In the magnetic domain refinement treatment, which is one of the features of the present invention, linear grooves are formed between any of the above-described steps before the application of the annealing separator. The linear grooves can be formed by local etching, scribing with a blade, rolling with a protruding roll, or the like. Among these, a preferred method is to apply an etching resist to the cold-rolled sheet after final cold rolling by printing or the like, and then form linear grooves in the non-adhered areas by electrolytic etching. Furthermore, since the formation of a predetermined forsterite film at the bottom of the grooves, which forms a forsterite film, is effective for magnetic domain refinement, the grooves are formed before the application of the annealing separator and the final annealing.

[0060] [Application of Annealing Separator] After decarburization annealing, the annealing separator is applied by the above-mentioned predetermined method, and then final annealing is performed to form a forsterite coating.

[0061] [Final annealing] The final annealing is as described above. After application of the annealing separator, final annealing is performed for the purpose of secondary recrystallization and formation of a forsterite film to obtain a finish-annealed sheet. The annealing temperature for such final annealing is preferably 1100°C or higher, and the annealing time is preferably 30 minutes or longer. As described above, it is also preferable to perform final annealing on the decarburized annealed sheet in a coil shape with the grooved surface disposed on the outer winding side.

[0062] [Insulating Coating and Planarizing Annealing] The insulating coating and planarizing annealing are as described above. After the final annealing, an insulating coating is applied by the above-described predetermined method to form an insulating film. It is also preferable to perform planarizing annealing to correct the shape. Planarizing annealing may be performed after insulating coating, or the application of the insulating coating may be performed after planarizing annealing.

[0063] A steel slab (slab for grain-oriented electrical steel sheet) containing 0.07 mass% C, 3.4 mass% Si, 0.1 mass% Mn, 0.2 mass% Ni, 240 mass ppm Al, 20 mass ppm S, 90 mass ppm N, and 180 mass ppm Se, with the remainder being Fe and unavoidable impurities, was produced by continuous casting. The slab was heated to 1430°C and then hot-rolled to a hot-rolled sheet having a thickness of 2.2 mm. The hot-rolled sheet was subjected to hot-rolled sheet annealing at 1100°C for 20 seconds. Next, the hot-rolled annealed sheet after the hot-rolled sheet annealing was cold-rolled to an intermediate thickness of 0.40 mm, and the oxidation degree P (H ２ O) / P(H ２ ) = 0.40, temperature: 1000°C, and time: 70 seconds. Next, the hot-rolled annealed sheet after the intermediate annealing was pickled with hydrochloric acid to remove subscale from the surface, and then cold-rolled again to obtain a cold-rolled sheet with a final thickness of 0.23 mm.

[0064] After that, resist ink was applied to one or both sides of the cold-rolled sheet, and then a laser was scanned linearly across the coated surface in a direction transverse to the rolling direction at intervals of 3 mm in the rolling direction, to peel and remove the resist ink at intervals of 3 mm in the rolling direction. The laser irradiation was performed using a single-mode fiber laser with a galvanometer scanner system, and the resist ink was completely peeled off continuously from one end to the other in the width direction of the cold-rolled sheet. As shown in Table 1, the laser pattern, i.e., the groove formation pattern, was performed in four ways: on one side only (offset from the direction perpendicular to the rolling direction: 1 to 30°, shown as "one side" in the table); on both sides, parallel and staggered (offset: 1 to 30°, shown as "both sides, parallel, staggered" in the table); on both sides, parallel and in the same position on both sides (offset: 1 to 30°, shown as "both sides, same location" in the table); and on both sides, crossed on both sides (offset: 1 to 30° symmetrically with the rolling direction, shown as "both sides, crossed" in the table). Subsequently, electrolytic etching was performed to form grooves with a depth D of 25 μm and a roughly trapezoidal cross section. After electrolytic etching, the resist ink remaining on the steel sheet was removed.

[0065] Next, the oxidation degree P (H ２ O) / P(H ２ After decarburization annealing at a soaking temperature of 820°C for 300 seconds, an annealing separator primarily composed of MgO was applied to the surface of the decarburized annealed steel sheet. Final annealing for secondary recrystallization, forsterite film formation, and purification was performed at 1160°C for 10 hours. The annealing separator was applied in two ways: by conventional roll coater alone, and by the application method according to the present invention (pre-application: inflow of the annealing separator into the grooves using a roll coater + scraping of the annealing separator + main application: full-surface application using a roll coater). In the table, the former is indicated as "without pre-application and scraping," and the latter is indicated as "with pre-application and scraping." To promote inflow into the grooves, the viscosity of the annealing separator (suspension) was varied to 1000, 100, and 10 Pa·s, respectively. After pre-application of the annealing separator, the solution on the surface was scraped off using a doctor blade.

[0066] Furthermore, in the final annealing, when grooves were formed on only one side of the decarburized annealed steel sheet, two conditions were used: one in which the grooves were located on the outer winding side of the coil, and the other in which the grooves were located on the inner winding side of the coil. In the table, the former is indicated as "outside" and the latter as "inside" for "groove position in the coil."

[0067] Furthermore, an insulating coating consisting of 60% by mass of colloidal silica and aluminum phosphate was applied and baked at 850°C to form an insulating film, and each test specimen was prepared. This baking treatment of the insulating coating also served as planarization annealing. The insulating coating solution was applied in two ways: by conventional application using only a roll coater, and by the application method according to the present invention (preliminary application: injecting the insulating coating into the grooves using a roll coater + scraping off the insulating coating + final application: applying the entire surface using a roll coater). In the table, the former is indicated as "without preliminary application and scraping," and the latter is indicated as "with preliminary application and scraping." To promote inflow into the grooves, the viscosity of the insulating coating solution was varied to 1000, 100, and 10 Pa·s. After preliminary application of the insulating coating solution, the solution on the surface was scraped off using a doctor blade.

[0068] The surface roughness Ra of the forsterite coating at the center of the grooves of the grain-oriented electrical steel sheet thus obtained, the roughness coefficient of the forsterite coating formed in the grooves, and the average thickness of the insulating coating at the bottom of the grooves were measured according to the above and following methods. First, the surface roughness Ra of the forsterite coating was measured using samples (finish-annealed sheets) before planarization annealing and application of the insulating coating liquid. For these samples, the unevenness was measured at 20 random locations (for 20 different grooves) on the sample at a distance t of 30 mm according to the above-mentioned method for measuring surface roughness Ra, and the Ra value was determined. The average value of the 20 locations was then used as the surface roughness Ra of the forsterite coating formed at the center of the grooves. Note that the annealing temperature during the final finish annealing at which the forsterite coating is formed is higher than the annealing temperature at which the insulating coating is formed. Therefore, the surface roughness of the forsterite coating does not change before and after the formation of the insulating coating.

[0069] The roughness coefficient of the forsterite coating formed in the grooves was also measured using finish-annealed sheets, similar to the measurement of Ra. Following the roughness coefficient measurement method described above, 30 g of samples without grooves and 30 g of samples with grooves were used per analysis, and measurements were performed on 20 samples of each type, with the average value being used as the roughness coefficient result. A TriStar (registered trademark) II series automatic specific surface area and pore distribution analyzer was used for the measurements. For the same reason as in the case of Ra, the roughness coefficient of the forsterite coating does not change before and after the formation of the insulating coating.

[0070] Next, the average thickness of the insulating coating at the bottom of the groove was measured using test pieces (grain-oriented electrical steel sheets) that had been insulating-coated and had also been subjected to flattening annealing. The average thickness of the insulating coating was obtained for these test pieces according to the average thickness measurement method described above.

[0071] The test pieces were sheared to a size of 30 mm x 280 mm, and then subjected to stress relief annealing (for the purpose of eliminating the influence of shearing) in an argon atmosphere at 800°C for 3 hours. Thereafter, the test pieces were subjected to magnetic measurements using the Epstein test method.

[0072] The adhesion (peelability) of the insulating coating was evaluated by a bending peeling test, in which a grain-oriented electrical steel sheet with the coating was wrapped around a round bar and the smallest diameter (mm) at which the insulating coating did not peel off was determined, as described above. This test evaluates the adhesion of the insulating coating in the grooves, and the insulating coating is more damaged in tension than in compression. For this reason, the steel sheet was wrapped with the grooves facing outward during the test.

[0073] Table 1 shows the surface roughness Ra (average value) of the forsterite coating at the center of the groove, the thickness (average value) of the insulating coating at the bottom of the groove, the roughness and roughness coefficient of the forsterite coating in the groove, as well as the evaluation results of the adhesion (peelability) of the insulating coating and the magnetic properties of the grain-oriented electrical steel sheet.

[0074]

[0075] As shown in Table 1, when a forsterite coating satisfying a predetermined surface roughness is formed in the grooves according to the present invention, the insulating coating on the forsterite coating can be made thick to a predetermined extent, the adhesion (peelability) of the insulating coating is improved, and a grain-oriented electrical steel sheet with excellent magnetic properties can be obtained. Furthermore, the method according to the present invention provides grain-oriented electrical steel sheets that exhibit consistently excellent magnetic properties regardless of the viscosity of the annealing separator (suspension) and the insulating coating liquid. Furthermore, grain-oriented electrical steel sheets that exhibit consistently excellent magnetic properties regardless of the groove pattern formed can be obtained. Thus, by using the grain-oriented electrical steel sheet of the present invention, which exhibits excellent adhesion and low iron loss despite the thick insulating coating in the grooves, it is believed that the BF of the transformer can also be reduced.

[0076] In particular, even better magnetic properties were obtained when the roughness coefficient of the forsterite coating further satisfied the requirement of 0.8 or less. Furthermore, for grain-oriented electrical steel sheets with grooves formed on only one side, the roughness coefficient was smaller and iron loss was further reduced when the grooves were final annealed on the outer winding side than when the grooves were final annealed on the inner winding side. Furthermore, for grain-oriented electrical steel sheets with grooves formed on both sides, the magnetic properties were better than those of the comparative example due to the magnetic domain refining effect of the grooves on the front and back sides. However, because the forsterite coating formed on either side was susceptible to damage, the roughness coefficient tended to be larger than when the grooves were formed on only one side and final annealed on the outer winding side. As a result, due to the balance between the improved magnetic domain refining effect from both sides and the increased roughness coefficient, the iron loss properties were approximately the same as when the grooves were formed on only one side and final annealed on the outer winding side.

[0077] REFERENCE SIGNS LIST 1 Grain-oriented electrical steel sheet (steel sheet) 2 Groove 20 Bottom 21 Center 3 Forsterite coating 4 Insulating coating

Claims

1. A grain-oriented electrical steel sheet having on one or both sides thereof a plurality of grooves which extend linearly in a direction transverse to the rolling direction of the steel sheet and are spaced apart in the rolling direction, and on both sides of the steel sheet, a forsterite coating and an insulating coating, in that order from the surface side of the steel sheet, wherein the surface roughness Ra of the forsterite coating formed in the center of the grooves in a cross section perpendicular to the extension direction of the grooves is 5.00 μm or more, and the average thickness of the insulating coating at the bottom of the grooves is 1.50 μm or more.

2. The grain-oriented electrical steel sheet according to claim 1, wherein the forsterite coating formed in the grooves has a roughness coefficient of 0.8 or less.

3. A method for producing grain-oriented electrical steel sheet, comprising hot rolling a slab for grain-oriented electrical steel sheet to obtain a hot-rolled sheet, annealing the hot-rolled sheet as necessary to obtain a hot-rolled annealed sheet, cold rolling the hot-rolled sheet or the hot-rolled annealed sheet once or at least twice with intermediate annealing to obtain a cold-rolled sheet having a final sheet thickness, decarburization annealing the cold-rolled sheet to obtain a decarburization annealed sheet, applying an annealing separator mainly composed of MgO to the surface of the decarburization annealed sheet, performing final annealing to obtain a finish annealed sheet, and then applying an insulating coating to the finish annealed sheet, (1) before applying the annealing separator, forming a plurality of grooves on one or both sides of the hot-rolled sheet, the hot-rolled annealed sheet, the cold-rolled sheet or the decarburization annealed sheet, the grooves extending linearly in a direction transverse to the rolling direction of the steel sheet and spaced apart in the rolling direction, (2) applying the annealing separator to at least the surface of the decarburization-annealed sheet where the grooves are formed, then scraping off the annealing separator, and then applying the annealing separator to both surfaces of the decarburization-annealed sheet; (3) applying the insulating coating to at least the surface of the finish-annealed sheet where the grooves are formed, then scraping off the insulating coating, and then applying an insulating coating to both surfaces of the finish-annealed sheet.

4. The method for producing grain-oriented electrical steel sheet according to claim 3, wherein a doctor blade is used to scrape off the annealing separator and the insulating coating.

5. A method for producing grain-oriented electrical steel sheet according to claim 3 or 4, wherein when the decarburized annealed sheet to which the annealing separator has been applied is subjected to final finish annealing, the decarburized annealed sheet is formed into a coil shape with the surface having the grooves formed thereon positioned on the outer winding side and then subjected to final finish annealing.