Grain-oriented electrical steel sheet and its manufacturing method

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

[Technical Field]

[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. [Background technology]

[0002] The easy axis of magnetization of iron <001> Grain-oriented electrical steel sheets, which have a crystalline structure with a highly aligned orientation 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 specific 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 describes 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. 2Patent 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 62-53579 [Patent Document 2] International Publication No. 2016 / 171130 [Patent Document 3] Japanese Patent Publication No. 2022-22494 Summary of the Invention [Problem to be solved by the invention]

[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 will be collectively referred to as BF. Reducing 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 perspective of reducing BF, it is desirable to make the coating thickness thinner.

[0008] The present invention has been made in view of the above circumstances, and aims to propose a method for realizing magnetic domain refining by improving the tension on the steel sheet surface in addition to magnetic domain refining by groove formation, without increasing the BF. [Means for solving the problem]

[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 and configuration of the present invention are as follows. [1] A grain-oriented electrical steel sheet having, 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 and arranged at intervals in the rolling direction, and having, on both sides of the steel sheet, a forsterite coating and an insulating coating in this order from the surface side of the steel sheet, A grain-oriented electrical steel sheet, wherein the forsterite coating formed in the center of the groove in a cross section perpendicular to the extension direction of the groove has a surface roughness Ra of 5.00 μm or more, and the insulating coating at the bottom of the groove has an average thickness of 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 grain-oriented electrical steel sheet to obtain a hot-rolled sheet, then subjecting the hot-rolled sheet to hot-rolled sheet annealing as necessary to obtain a hot-rolled annealed sheet, and then cold rolling the hot-rolled sheet or the hot-rolled annealed sheet once or two or more times with intermediate annealing therebetween to obtain a cold-rolled sheet having a final sheet thickness, and then decarburizing annealing the cold-rolled sheet to obtain a decarburization annealed sheet, and then applying an annealing separator mainly composed of MgO to the surface of the decarburization annealed sheet, and then subjecting the decarburization annealed sheet to 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, a plurality of grooves are formed on one or both surfaces 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 arranged at intervals in the rolling direction. (2) applying the annealing separator to at least the surface of the decarburized annealed sheet on which the grooves are formed, scraping off the annealing separator, and then applying the annealing separator to both surfaces of the decarburized annealed sheet; (3) A method for producing a grain-oriented electrical steel sheet, comprising applying the insulating coating to at least the surface of the finish-annealed sheet on which the grooves are formed, scraping off the insulating coating, and then applying insulating coatings to both surfaces of the finish-annealed sheet.

[0016] [4] The method for producing a grain-oriented electrical steel sheet according to [3] above, 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 item [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 arranged on the outer winding side and subjected to final finish annealing. [Effects of the Invention]

[0018] According to the present invention, by increasing the thickness of the insulating coating while roughening the forsterite coating 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, which in turn enables further reduction in iron loss and realization of a low BF. Furthermore, by applying an annealing separator to the grooves present on the surface of a decarburized annealed steel sheet and further applying an insulating coating to the grooves present on the surface of a finish annealed steel sheet according to the manufacturing method of the present invention, magnetic domain refinement is promoted in the same manner as described above, thereby achieving both reduced iron loss and a low BF. Furthermore, according to the preferred embodiment of the present invention, the degree of damage to the forsterite film is also reduced, thereby further promoting magnetic domain refinement, thereby achieving both lower core loss and lower BF. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 3 is a cross-sectional view showing a groove in a steel plate. [Figure 2] 1A and 1B are cross-sectional views showing grooves in a steel plate at two positions spaced apart in the extension direction. DETAILED DESCRIPTION OF THE INVENTION

[0020] Preferred embodiments of the present invention will be described in detail below. Unless otherwise specified, the expression "A to B" for numerical values ​​A and B means "A or more and B or less." In such expressions, 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 that extend linearly in a direction transverse to the rolling direction of the steel sheet (hereinafter sometimes simply referred to as the rolling direction) and are aligned at intervals in the rolling direction, and both sides of the steel sheet are provided with a forsterite coating and an insulating coating, in that order from the steel sheet surface. 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 forsterite coating surface at the center of the grooves in a cross section perpendicular to the extension direction of the grooves is 5.00 μm or more. Furthermore, in a suitable grain-oriented electrical steel sheet, it is desirable that the roughness coefficient of the forsterite coating formed in the grooves be 0.8 or less. The grain-oriented electrical steel sheet according to the present invention can be suitably obtained, for example, by the method for producing a grain-oriented electrical steel sheet according to the present invention described below.

[0022] [groove] The grooves are preferably formed on the steel sheet surface at intervals 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 interval is less than 1 mm, the magnetic domain refinement 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 as it increases iron loss and BF. On the other hand, if the interval exceeds 20 mm, the magnetic domain refinement effect decreases, which is undesirable as it insufficiently improves iron loss and BF. The groove interval is preferably 1 to 20 mm, more preferably 2 to 10 mm.

[0023] There are no particular restrictions on the groove formation pattern, but it is preferable to have grooves on both sides of the steel sheet, with the grooves parallel to each other and in the same positions on both sides, rather than having them on only one side of the steel sheet, because this increases the magnetostatic energy in the grooves and strengthens the magnetic domain refinement effect. Furthermore, if the grooves are parallel to each other and positioned alternately on both sides of the steel sheet, the length of the magnetic domain in the rolling direction will be shorter, which will increase the magnetostatic energy and further enhance the magnetic domain refinement effect, which is desirable. Furthermore, if grooves are provided on both surfaces of the steel sheet at positions where they cross on the front and back, the magnetostatic energy in the grooves is large where the grooves cross, and the magnetic domain length in the rolling direction is shorter in the areas where the grooves do not cross than when the grooves are parallel, so the magnetostatic energy is large and the magnetic domain refining effect is strong, which is desirable.

[0024] Figure 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 Figure 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 may, for example, have 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 and preferably 50 µm or less, and more preferably in the range of 10 µm to 50 µm. 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 on an imaginary extension line 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 a sufficient iron loss reduction effect may 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 with the grooves 2 on at least one side, and is provided with a forsterite coating 3 and an insulating coating 4, in that order from the front surface side, on both the front and back 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 methods for grain-oriented electrical steel sheets. Meanwhile, 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] [The surface roughness Ra of the forsterite coating formed at the center of the groove in the cross section perpendicular to the extension direction of the groove is 5.00 μm or more] First, the "center portion of the groove in the cross section perpendicular to the extension direction of the groove" refers to the position (center portion 21) of a perpendicular line drawn through the center of the opening width of the groove 2 to the bottom portion 20, as shown in FIG. 1. The arithmetic mean roughness Ra of the 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 portion of the groove in the cross section perpendicular to the extension direction of the groove." Hereinafter, this 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 of the figure shows a top view of a steel sheet 1 having grooves 2. The left side of the figure shows cross-sectional views of the grooves corresponding to two different positions along the extension direction of the grooves in the top view. Specifically, in FIG. 2, groove cross-sections corresponding to positions spaced a predetermined distance t apart in the extension direction of the grooves 2 are shown as cross-sections AA and BB. 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 is shown as d and d'. The surface irregularities of the forsterite coating 3 on the line connecting 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 must be achieved over a region longer than the width of at least one crystal grain. The definition of the arithmetic mean roughness Ra of the roughness curve shall comply with Japanese Industrial Standard 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 Ra to 5.00 μm or greater, the surface area per unit groove of the forsterite coating increases, which is thought to increase the adhesive area with the insulating coating and improve coating adhesion. Preferably, 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 set to 20.00 μm or less. More preferably, Ra is set to 15.00 μm or less, and even more preferably, Ra is set to 10.00 μm or less.

[0031] [Method for measuring the surface roughness Ra of forsterite coating] The Ra can be measured using a laser microscope (a 3D laser microscope using a pinhole confocal optical system). Specifically, 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 . The surface roughness of the forsterite coating on a line connecting the surfaces d and d' of the forsterite coating 3 corresponding to the center of the groove bottom (groove bottom surface) in the rolling direction at each position is measured as the arithmetic mean roughness Ra of the roughness curve. This measurement is performed on 20 random grooves, and the average value of the 20 grooves is taken as the surface roughness Ra of the forsterite coating. Here, when the sample to be used for measurement is one before planarization annealing and coating with an insulating coating liquid, Ra can be measured as is. On the other hand, when the sample to be used for measurement is one after coating with an insulating coating liquid, Ra can be measured after removing the insulating coating with an alkaline solution (e.g., sodium hydroxide solution).

[0032] [The roughness coefficient of the forsterite coating formed in the 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 that the porosity of the forsterite coating is relatively high. 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 to the roughness coefficient of the forsterite coating formed in the grooves, but from the viewpoint of sufficiently covering the grooves with the forsterite coating, it is preferably 0.05 or more.

[0033] [Method for measuring the roughness coefficient of the forsterite coating formed in the grooves] First, the specific surface area of ​​a sample with no grooves and a forsterite film formed is measured by a gas adsorption method using Kr gas. The true surface area, including minute surface irregularities, is then 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 film formed on the steel sheet surface excluding the grooves is derived. Next, the specific surface area of ​​the sample including the grooves on which the forsterite coating is formed is measured by the gas adsorption method using Kr gas in the same manner as above, and the true surface area including minute irregularities on the surface is calculated. After that, the apparent surface area of ​​the portion of the sample including the grooves other than the grooves is calculated, and this is multiplied by the roughness coefficient of the sample not including the grooves calculated earlier, thereby deriving the true surface area of ​​the portion of the sample including the grooves other than the grooves. The difference between this result (the true surface area of ​​the portion other than the grooves in the sample including grooves) and the true surface area derived from the result of the specific surface area of ​​the sample including grooves is taken as the true surface area of ​​the grooves. The true surface area of ​​the grooves is then divided by the apparent surface area of ​​the grooves, and then further divided by Ra to derive the roughness coefficient of the forsterite coating formed in the grooves. This makes it possible to evaluate the amount of microcracks that cannot be evaluated by surface roughness Ra. In summary, the roughness coefficient (unit: μm -1 ) can be calculated using 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 in 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] [The average thickness of the insulating coating at the bottom of the groove is 1.50 μm or more] It is also important that the average thickness of the insulating coating at the bottom of the groove (groove base surface) is 1.50 μm or more. In other words, if the average thickness of the insulating coating at this location is less than 1.50 μm, no further improvement in iron loss will be achieved. The mechanism by which an insulating coating with an average thickness of 1.50 μm or more is effective is not clear, but it is thought that as the insulating coating becomes thicker, tension in the groove area increases locally, improving the magnetic domain refinement effect. The average thickness of the insulating coating at the bottom of the groove is preferably 1.70 μm or more. Although it also depends on the thickness of the forsterite coating formed in the groove, for example, the average thickness of the insulating coating at the bottom of the groove 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-sections shown in Figure 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 sheet thickness. Preferably, cross-sectional images are obtained for 30 or more different grooves. These are analyzed by 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. The average thickness of the insulating coating at the bottom of the groove is then calculated by averaging all of the cross-sectional images obtained. In this measurement, the "bottom" of a 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 described above. The "sidewall" of a groove is defined as the portion of the recess defining the groove excluding the "bottom," i.e., the portions corresponding to 10% of the opening width in the rolling direction from both sides of the steel sheet surface edge defining the groove.

[0036] [Method for evaluating the adhesion of insulating coating] The adhesion of the insulating coating can be evaluated as the degree of peeling when 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 tendency, and therefore the better the adhesion.

[0037] (Method of manufacturing grain-oriented electrical steel sheets) 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 then 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 sides of the hot-rolled sheet, hot-rolled annealed sheet, cold-rolled sheet, or 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 an annealing separator to at least the grooved surface of the decarburized annealed sheet, scraping off the annealing separator, and then applying the annealing separator to both surfaces of the decarburized annealed sheet. (3) It is essential that after applying an insulating coating to at least the grooved surface of the finish-annealed sheet, the insulating coating is scraped off, and then insulating coating is applied to both sides of the finish-annealed sheet. According to the manufacturing method of the present invention, the grain-oriented electrical steel sheet of the present invention described above can be obtained satisfactorily, and therefore the same effects as those of the grain-oriented electrical steel sheet of the present invention can be achieved.

[0038] A suitable manufacturing method includes using a doctor blade to scrape off the annealing separator and insulating coating. In yet another preferred manufacturing method, when a decarburized annealed sheet coated with an annealing separator is subjected to final finish annealing, the decarburized annealed sheet is formed into a coil shape with the surface on which the grooves are formed being arranged on the outer winding side, and then the final finish 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] The linear grooves are formed on one or both sides of the steel sheet before application of the annealing separator (i.e., before final finish annealing), more specifically, on one or both sides of any of the hot-rolled sheet, hot-rolled annealed sheet, cold-rolled sheet, and decarburization annealed sheet. Among these, it is preferable to form linear grooves on the cold-rolled sheet after final cold rolling and before decarburization annealing. Since the grooves may disappear during cold rolling, there is no strong need to form grooves before final cold rolling. Furthermore, while fayalite must be formed in order to form forsterite, fayalite is usually formed during decarburization annealing, so it is preferable to form grooves on the cold-rolled sheet before decarburization annealing. The grooves can be effectively formed by applying and depositing an etching resist ink, followed by electrolytic etching of the non-coated areas. When forming a groove pattern, the non-coated areas can be formed by applying the resist ink to, for example, the entire surface of one or both sides of a cold-rolled sheet, and then removing the ink by laser irradiation, or by resist printing. Since it is particularly important that a sufficient forsterite film is formed at the bottom of the grooves, it is essential that the grooves be formed before applying 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 addition, in this specification, "linear" includes not only solid lines but also dotted lines and dashed lines. Other features of the grooves may be the same as those described above for the grain-oriented electrical steel sheet.

[0041] [Application of annealing separator] After applying an annealing separator to at least the grooved surface of the decarburized annealed steel sheet (preliminary application), the annealing separator is scraped off, and then the annealing separator is applied to both sides of the decarburized annealed steel sheet (main application). 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 / m2 per side of the steel sheet. 2 More than 15g / m 2 or less, 8 to 15 g / m 2 A range of about 1000 to 15000 is suitable for forming a forsterite coating having a desired thickness. Incidentally, "containing MgO as the main component" means that the annealing separator contains 75 mass % or more of MgO in terms of solid content. The formed solid forsterite coating also contains the same MgO content.

[0042] The procedure for applying the annealing separator involves applying (pre-application) the annealing separator (e.g., MgO suspension) to at least the grooved surface, scraping off the suspension, and then applying the MgO suspension to both surfaces again (main application). 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, by using the above-described method, the annealing separator is sufficiently spread throughout the grooves, and 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. This also reduces iron loss and makes it easier to achieve a low BF.

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

[0044] [Insulation coating and flattening annealing] An insulating coating is applied (pre-application) to at least the grooved surface of the finish-annealed sheet obtained in this way, and then the insulating coating is scraped off, and an insulating coating is applied to both sides of the finish-annealed sheet (main application). Therefore, an insulating coating is applied twice to at least the grooved surface. The insulating coating means a coating that imparts tension to the steel sheet in order to reduce iron loss. After the final annealing, flattening annealing is performed to correct the shape, which is effective in improving the space factor when the grain-oriented electrical steel sheet is processed into an iron core. The flattening annealing is performed at an annealing temperature of 750°C or higher and 950°C or lower, preferably in the range of about 750 to 950°C. The flattening annealing is also preferably performed for an annealing time of 10 seconds or higher and 200 seconds or lower, preferably in the range of about 10 to 200 seconds. An insulating coating is preferably formed before or after the 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. Furthermore, 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 also acceptable. 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 can 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, making it difficult to reduce iron loss and BF. On the other hand, the above-mentioned method allows the insulating coating liquid to be sufficiently distributed in 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 makes it easier to reduce iron loss and achieve a low BF. Normally, forming an insulating coating as thick as 1.50 μm poses a problem of adhesion (peelability). However, in the present invention, by controlling the surface roughness of the forsterite coating to be even greater than before, as described above, peelability of the insulating coating is no longer an issue. Subsequently, heat treatment can be optionally performed under suitable temperature conditions (e.g., 840 to 920°C). Finally, a grain-oriented electrical steel sheet is obtained that has the desired forsterite coating and insulating coating inside the grooves.

[0046] [Other processes] 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 and suitable chemical compositions and other production conditions are described below.

[0047] [Component composition] The chemical composition of grain-oriented electrical steel slabs is preferably such that secondary recrystallization occurs. When an inhibitor is used, for example, an AlN-based inhibitor contains appropriate amounts of Al and N, while an MnS·MnSe-based inhibitor contains appropriate amounts of Mn and S and / or Se. Naturally, both inhibitors may be used in combination. In this case, the preferred contents of Al, N, S, and / or Se are 0.01-0.04 mass% Al, 0.005-0.02 mass% N, and 0.005-0.03 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 100 ppm by mass or less for Al, 50 ppm by mass or less for N, 50 ppm by mass or less for S, and 50 ppm by mass or less for Se, respectively.

[0049] The basic components and optional additional components other than those mentioned above are as follows: C: 0.08% by 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, and the content is reduced to the same level as an unavoidable impurity.

[0050] Si:2.0~8.0% by mass Silicon is an element that effectively increases the electrical resistance of steel and improves iron loss. However, if the content is less than 2.0% by mass, sufficient iron loss reduction effects cannot be expected. On the other hand, if the content exceeds 8.0% by mass, workability is significantly reduced and magnetic flux density is also reduced. 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:0.005~1.0% 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 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 above basic components, the following elements may be suitably contained as components for improving magnetic properties. At least one selected from Ni: 0.03 to 1.50 mass%, Sn: 0.01 to 1.50 mass%, Sb: 0.005 to 1.50 mass%, Cu: 0.03 to 3.0 mass%, P: 0.03 to 0.50 mass%, Mo: 0.005 to 0.10 mass%, and Cr: 0.03 to 1.50 mass%. Ni is a useful element for improving the hot-rolled sheet structure and magnetic properties. However, if the Ni content is less than 0.03 mass%, the effect of improving the magnetic properties is small, while if the Ni content exceeds 1.50 mass%, secondary recrystallization becomes unstable and the magnetic properties deteriorate. Therefore, the Ni content is preferably 0.03 mass% or more and 1.50 mass% or less, and more preferably in the range of 0.03 to 1.50 mass%.

[0053] Furthermore, Sn, Sb, Cu, P, Mo, and Cr are each elements useful 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 the grain-oriented electrical steel sheet will be described. [Slab heating] The slab for grain-oriented electrical steel sheet having the above-mentioned composition can be heated by a conventional method. The heating temperature is preferably 1150°C or higher and 1450°C or lower, more preferably in the range of 1150 to 1450°C.

[0055] [Hot rolling] After the slab is heated, it can be hot-rolled to form 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 that the rolling temperature for the final pass of rough rolling is 900°C or higher, and the rolling temperature for the final pass of finish rolling is 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 develop a Goss structure to a high degree in the product sheet (grain-oriented electrical steel sheet), the annealing temperature of the hot-rolled sheet is preferably 800°C or higher, preferably 1100°C or lower, and more preferably in the range of 800 to 1100°C. If the annealing temperature of the hot-rolled sheet is lower 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 annealing temperature of the hot-rolled sheet exceeds 1100°C, the grain size after annealing of the hot-rolled sheet 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 cold-rolled 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 shorter, and is 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 900°C or lower, and more preferably about 750 to 900°C. The atmospheric oxidation (oxidation degree) PH2O / PH2 is preferably 0.25 or higher and 0.60 or lower, and more preferably about 0.25 to 0.60. Furthermore, the annealing time is preferably 50 seconds or higher and 300 seconds or lower, and more preferably about 50 to 300 seconds.

[0059] [Magnetic domain refinement processing] The magnetic domain refining process can be carried out in accordance with the groove forming 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-mentioned steps before the application of the annealing separator. Linear grooves can be formed by localized etching, scribing with a blade, rolling with a protruding roll, and other methods. 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 is effective for magnetic domain refinement, the grooves are formed before the application of the annealing separator, which forms the forsterite film, and before the final annealing.

[0060] [Application of annealing separator] The application of the annealing separator is as described above. 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 the application of the annealing separator, the steel sheet is subjected to final annealing for the purpose of secondary recrystallization and the formation of a forsterite film, to obtain a finish-annealed steel sheet. The annealing temperature for the final annealing is preferably 1100°C or higher, and the annealing time is preferably 30 minutes or longer. As described above, it is preferable to perform final annealing on the decarburized annealed sheet in the form of a coil with the grooved surface disposed on the outer winding side.

[0062] [Insulation coating and flattening annealing] The insulating coating and planarizing annealing are as described above. After the final annealing, the insulating coating is applied by the above-mentioned 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 the insulating coating, or the application of the insulating coating may be performed after the planarizing annealing. [Example]

[0063] A steel slab (slab for grain-oriented electrical steel sheet) containing 0.07% by mass of C, 3.4% by mass of Si, 0.1% by mass of Mn, 0.2% by mass of Ni, 240 ppm by mass of Al, 20 ppm by mass of S, 90 ppm by mass of N, and 180 ppm by mass of Se, with the remainder being Fe and unavoidable impurities, was produced by continuous casting. The slab was heated to 1430°C and hot-rolled to a thickness of 2.2 mm. The hot-rolled sheet was subjected to hot-rolled annealing at 1100°C for 20 seconds. Next, the hot-rolled annealed sheet after the hot-rolled annealing was cold-rolled to an intermediate thickness of 0.40 mm, and then subjected to intermediate annealing under the following conditions: oxidation degree P(H2O) / P(H2) = 0.40, temperature: 1000°C, time: 70 seconds. Next, the hot-rolled annealed sheet after 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 the laser was scanned linearly across the coated surface, repeatedly at 3 mm intervals in the rolling direction, to peel and remove the resist ink at 3 mm intervals in the rolling direction. The laser irradiation was performed using a single-mode fiber laser with a galvanometer scanner, 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: one-sided only (offset from the direction perpendicular to the rolling direction: 1 to 30°, indicated as "one-sided" in the table); parallel and staggered on both sides (offset: 1 to 30°, indicated as "both parallel and staggered" in the table); parallel and parallel on both sides with the same positions on both sides (offset: 1 to 30°, indicated as "both same positions" in the table); and crossed on both sides with the same positions on both sides (offset: 1 to 30°, symmetrical to the rolling direction, indicated as "both 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 steel was subjected to decarburization annealing at an oxidation degree P(H2O) / P(H2) = 0.44 and a soaking temperature of 820°C for 300 seconds. After that, an annealing separator mainly composed of MgO was applied to the surface of the decarburized annealed steel sheet, and final annealing was carried out at 1160°C for 10 hours for the purposes of secondary recrystallization, forsterite film formation, and purification. The annealing separator was applied in two ways: by conventional application using only a roll coater, and by the application method according to the present invention (pre-application: injecting the annealing separator into the grooves using a roll coater + scraping off the annealing separator + main application: applying the entire surface using a roll coater). In the table, the former is shown as "without pre-application and scraping," and the latter is shown as "with pre-application and scraping." In addition, to promote inflow into the grooves, the viscosity of the annealing separator (suspension) was changed to 1000, 100, and 10 Pa·s. After pre-application of the annealing separator, the solution on the surface was scraped off using a doctor blade.

[0066] Furthermore, when the grooves were formed on only one side of the decarburized annealed steel sheet during the final annealing, 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. In the table, the "groove position in the coil" is indicated as "outside" for the former and "inside" for the latter.

[0067] Furthermore, an insulating coating consisting of 60 mass% 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 flattening annealing. The insulating coating liquid 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: applying the insulating coating to 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 shown as "without preliminary application and scraping," and the latter is shown as "with preliminary application and scraping." In addition, to promote flow into the grooves, the viscosity of the insulating coating liquid was changed to 1000, 100, and 10 Pa·s. After preliminary application of the insulating coating liquid, 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 groove of the grain-oriented electrical steel sheet thus obtained, the roughness coefficient of the forsterite coating formed in the groove, and the average thickness of the insulating coating at the bottom of the groove were measured according to the methods described above and below. 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 solution. For these samples, the unevenness was measured at 20 random locations (for 20 different grooves) at a distance t of 30 mm according to the surface roughness Ra measurement method described above, and the Ra value was calculated. The average value of these 20 locations was then used as the surface roughness Ra of the forsterite coating formed at the center of the groove. The annealing temperature during the final annealing to form the forsterite coating is higher than the annealing temperature to form the insulating coating, and 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, as in 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) after the insulating coating, which also served as the flattening annealing, was applied. 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 (to eliminate the effects of shearing) in an argon atmosphere at 800°C for 3 hours. Magnetic properties of the test pieces were then measured 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] [Table 1] TIFF0007792057000002.tif188170TIFF0007792057000003.tif192170TIFF00077920570 00004.tif193170TIFF0007792057000005.tif192170TIFF0007792057000006.tif192170

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

[0076] In particular, when the roughness coefficient of the forsterite coating further satisfies 0.8 or less, even better magnetic properties are obtained. Furthermore, for grain-oriented electrical steel sheets in which grooves were formed on only one side, the roughness coefficient was smaller and iron loss was further reduced when final annealing was performed with the grooves on the outer winding side than when final annealing was performed with the grooves 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 refinement effect of the grooves on both sides. However, because the forsterite coating formed on one side was susceptible to damage, the roughness coefficient tended to be larger than when grooves were formed on only one side and final annealed with the grooves on the outer winding side. As a result, due to the balance between the improved magnetic domain refinement effect from both sides and the increased roughness coefficient, the iron loss properties were almost the same as when grooves were formed on only one side and final annealed with the grooves on the outer winding side. [Explanation of symbols]

[0077] 1 Grain-oriented electrical steel sheet (steel sheet) 2 grooves 20 bottom 21 Center 3 Forsterite coating 4. Insulation coating

Claims

1. A grain-oriented electrical steel sheet having, on one or both surfaces of the steel sheet, a plurality of grooves extending linearly in a direction transverse to the rolling direction of the steel sheet and arranged at intervals in the rolling direction, and having, on both surfaces of the steel sheet, a forsterite coating and an insulating coating in this order from the surface side of the steel sheet, a grain-oriented electrical steel sheet, wherein the forsterite coating formed in the center of the groove in a cross section perpendicular to the extension direction of the groove has a surface roughness Ra of 5.00 μm or more, and the insulating coating at the bottom of the groove has an average thickness of 1.50 μm or more.

2. 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 a grain-oriented electrical steel sheet, comprising: hot rolling a slab for grain-oriented electrical steel sheet to form a hot-rolled sheet; annealing the hot-rolled sheet as necessary to form 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 form a cold-rolled sheet having a final thickness; decarburization annealing the cold-rolled sheet to form a decarburization annealed sheet; applying an annealing separator mainly composed of MgO to the surface of the decarburization annealed sheet; and then final annealing to form a finish-annealed sheet; and then applying an insulating coating 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, 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 arranged at intervals in the rolling direction. (2) After applying the annealing separator to at least the surface of the decarburization-annealed sheet on which the grooves are formed, the annealing separator is scraped off, and then the annealing separator is applied to both surfaces of the decarburization-annealed sheet. (3) After applying the insulating coating to at least the surface of the finish-annealed sheet where the grooves are formed, the insulating coating is scraped off, and then insulating coatings are applied to both surfaces of the finish-annealed sheet, A method for producing the grain-oriented electrical steel sheet according to claim 1.

4. The method for producing a 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. When the decarburized annealed sheet to which the annealing separator is applied is subjected to final finish annealing, the decarburized annealed sheet is formed into a coil shape with the surface on which the grooves are formed being disposed on the outer winding side, and then the final finish annealing is performed. A method for producing the grain-oriented electrical steel sheet according to claim 3 or 4, which produces the grain-oriented electrical steel sheet according to claim 2.

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