Grain-oriented electrical steel sheet and its manufacturing method

Abstract

To provide a directional property electrical steel which can achieve further reduction of iron loss in order to further suppress loss of a transformer, in response to recent trends in environmental regulations, energy issues, and affections of carbon neutrality and the like.SOLUTION: A directional property electrical steel includes a prescribed component composition, in which, with respect to all grain boundary neighbor parts in a prescribed range, a ratio of grain boundary neighbor parts in which an area ratio of crystal grains whose crystal orientation is deviated by 10 degrees or more from a Goss orientation is 20% or less (including 0%), is 50% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a grain-oriented electrical steel sheet with low iron loss and a method for manufacturing the same. [Background technology]

[0002] Grain-oriented electrical steel sheets contain 7% or less of Si by mass% and have the easy axis of magnetization of iron. <001> This material has a highly concentrated structure in the rolling direction of the steel sheet, and is mainly used as a transformer core material.

[0003] Low loss is one of the most important characteristics required of a transformer, but the majority of loss in a transformer is iron loss that occurs in the iron core. Therefore, reducing iron loss in the grain-oriented electrical steel sheet that is the iron core material is extremely important for improving the performance of transformers.

[0004] Due to recent energy regulations, there has been a strong demand every year for the development of low-core-loss grain-oriented electrical steel sheets that enable the production of low-loss transformers.

[0005] Therefore, in order to develop grain-oriented electrical steel sheets with low iron loss, efforts have been made to sharpen the orientation of the structure, apply high-tensile coatings, and develop magnetic domain refining technologies by introducing non-uniformity into the steel sheet surface.

[0006] For example, Patent Document 1 describes a technology for creating a highly concentrated texture and achieving low iron loss by controlling the soaking pattern and atmosphere of secondary recrystallization annealing during the production of grain-oriented electrical steel sheets.

[0007] Furthermore, Patent Document 2 describes a technique for achieving high film tension by adjusting the thermal expansion coefficient of the material used to form the film, thereby achieving excellent magnetic properties.

[0008] Furthermore, in Patent Document 3, low iron loss is achieved by locally removing the insulating coating of a grain-oriented electrical steel sheet, performing electrolytic etching, and performing a magnetic domain refining process in which grooves are dug. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2020-7637 [Patent Document 2] Japanese Patent Publication No. 2020-196954 [Patent Document 3] Japanese Patent Application Publication No. 2-50918 [Patent Document 4] Special Publication No. 54-23647 Summary of the Invention [Problem to be solved by the invention]

[0010] As described in the above-mentioned Patent Documents 1 to 3, various techniques have been developed to suppress the iron loss of grain-oriented electrical steel sheets that are used as iron core materials.

[0011] However, in recent years, due to the increasing momentum towards environmental regulations, energy issues, carbon neutrality, etc., there is a demand to further reduce transformer losses.

[0012] The present invention has been made in view of the above circumstances, and has an object to provide a grain-oriented electrical steel sheet that can achieve further reduction in iron loss in order to solve the above problems. [Means for solving the problem]

[0013] There are a wide variety of means for reducing iron loss in grain-oriented electrical steel sheets, but the inventors have focused particularly on means for magnetic domain refinement, and have investigated new methods for magnetic domain refinement.

[0014] Here, magnetic domain refinement in grain-oriented electrical steel sheet refers to the refinement of 180° magnetic domains oriented in the rolling direction in order to reduce the increased magnetostatic energy by increasing the amount of magnetic poles generated on the steel sheet surface.

[0015] Methods that have been developed for this purpose include, for example, forming grooves using a protruding roll and introducing thermal strain onto the steel sheet surface by irradiating the steel sheet with a laser. Both of these technologies involve processing existing product sheets or coils to perform magnetic domain refining treatment, and the base material used is ordinary grain-oriented electrical steel sheet.

[0016] In order to refine the magnetic domains of the grain-oriented electrical steel sheet, which is the base material, the inventors first aimed to artificially reduce the grain size of the secondary recrystallized grains. This is because, by using the base material itself as a magnetic domain refinement material, the grain-oriented electrical steel sheet itself will have low iron loss, and by performing magnetic domain refinement treatments such as groove formation and thermal strain, further reduction in iron loss can be expected. In addition, when the secondary recrystallized grains of the base material are reduced, the number of grain boundaries increases, which increases the number of magnetic poles generated between the grains, and a magnetic domain refinement effect can be expected.

[0017] Here, Patent Document 4 discloses a technique in which a region where grain growth of secondary recrystallization is inhibited is locally introduced on the surface of a steel sheet by plastic processing, thermal processing, or chemical processing of the steel sheet, thereby causing secondary recrystallized grains to grow along the region and reducing the diameter of the crystal grains.

[0018] Furthermore, Patent Document 4 discloses that, with regard to this technology, strain is mechanically introduced into the steel sheet before final annealing, thereby forming portions of abnormal grain growth during final annealing, and these portions block secondary recrystallization, resulting in small secondary recrystallized grains and improving iron loss.

[0019] The inventors actually prepared samples according to the method described in Patent Document 4. That is, they introduced strain into the surface of a steel sheet before final annealing, and then finished the steel sheet into a product sheet by final annealing, and examined the iron loss of the product sheet. As a result, although samples with relatively good iron loss were obtained, some samples with large iron loss were observed.

[0020] Therefore, when the cross sections of these samples along the rolling direction were examined in detail, it was found that many fine grains were formed near the grain boundaries of secondary recrystallized grains introduced directly below the strain-introduced area. Furthermore, samples with relatively good iron loss properties had many secondary recrystallized grain boundaries directly below the strained portion, while samples with poor iron loss properties had many of the fine grains precipitated, although the amount of precipitation had decreased. These fine grains were not secondary recrystallized grains that grow during final annealing, but were crystal grains that had coarsened during the temperature rise in final annealing due to the introduction of strain and had not been eaten away by the secondary recrystallized grains (crystal grains whose crystal orientation was off by 10 degrees or more from the Goss orientation), and since they were not Goss grains, it is presumed that the presence of these crystal grains (fine grains) reduced iron loss.

[0021] Furthermore, the inventors have devised the following experiment [1] based on the method disclosed in Patent Document 4. That is, the following experiment [1] was conducted to explore a method for suppressing the generation of fine grains immediately below the strain-introduced portion after final annealing, thereby introducing artificial grain boundaries of secondary recrystallized grains and reducing the diameter of the secondary recrystallized grains, thereby enhancing the iron loss reduction effect beyond that of the technology described in Patent Document 4.

[0022] Experiment [1] A 0.23 mm thick grain-oriented electrical steel sheet that had been subjected to decarburization annealing was subjected to periodic strain and grooves by varying the load pressure on a protruding roll so that the decarburized steel sheet was pressed down uniformly across the width, perpendicular to the rolling direction. The circumference of the outer periphery of this roll was 2 m, and the protrusions were spaced evenly apart at 10 mm. For comparison, a plain sample with no strain introduced was also prepared.

[0023] Next, an annealing separator containing magnesium oxide as its main component was applied, and then final annealing was carried out at a holding temperature of 1200°C to cause secondary recrystallization and the formation of a forsterite film.

[0024] The product plate thus produced was sheared to a length of 280 mm in the rolling direction and 30 mm in the direction perpendicular to the rolling direction, and then an Epstein test was carried out based on JIS C 2550-1:2011 at an excitation frequency of 50 Hz and an excitation magnetic flux density of 1.7 T to measure the iron loss.

[0025] After iron loss measurement, the steel sheet cross section was observed in the thickness direction in the rolling direction of the sample. As shown in Figure 1, the observation area was a range of ±5 mm along the rolling direction from the grain boundary position, which is the center of the maximum length of the grain boundary of the secondary recrystallized grain in the steel sheet cross section in the rolling direction (referred to as the "neighborhood of the grain boundary" in the present invention). Within this range, the area ratio of crystal grains (fine grains) deviated by 10° or more from the Goss orientation to the observed area was calculated. The grain boundary positions are shown in accordance with the examples of the arrangement of each crystal grain shown in Figures 1, 2, 3, and 4. In the present invention, the strain-introducing location refers to the center, in the rolling direction of the steel sheet, of the portion where strain is applied to the steel sheet surface, and in the case of a protruding roll as in this case, refers to the center, in the rolling direction, of the portion where the protrusions come into contact with the steel sheet.

[0026] As shown in Figure 1, when fine grains precipitate adjacent to the grain boundaries of secondary recrystallized grains that penetrate through to the front and back surfaces of the rolled surface of the steel sheet, the positions where the grain boundaries formed between the secondary recrystallized grains and the fine grains are exposed on the surface are designated as A, B, and C. In this case, point C' is defined as the point obtained by perpendicularly projecting point C on the opposite surface of the steel sheet onto the surfaces on the A and B sides, and the distance between this point C' and the above points A and B, i.e., the lengths AC' and BC', which are the lengths of the grain boundaries of secondary recrystallized grains in the rolling direction, are compared. In the case of Figure 1, AC' > BC', and therefore the combination with the greatest distance, i.e., the maximum length, is A and C'. Therefore, AC, which connects point C and A, which is the base point of C', is adopted, and the center of this AC in the rolling direction is defined as the grain boundary position in the present invention.

[0027] Furthermore, as shown in Figure 2, when there are no fine grains adjacent to the grain boundary of a secondary recrystallized grain, the center in the rolling direction of the grain boundary AB connecting points A and B where the grain boundary of the secondary recrystallized grain is exposed on the steel sheet surface is defined as the grain boundary position.

[0028] Furthermore, when multiple fine grains precipitate so as to be in contact with each other midway through the thickness of the secondary recrystallized grain, as shown in Figure 3, or when fine grains divide the secondary recrystallized grain into left and right halves, as shown in Figure 4, the grain boundaries between the secondary recrystallized grain and the fine grains are exposed on the steel sheet surface at points A, B, C, and D. Next, as shown in Figures 3 and 4, points C' and D' are defined as points obtained when C and D on the opposite surface of the steel sheet are perpendicularly projected onto the surfaces on sides A and B, respectively, and the distances AC', AD', BC', and BD' are considered. Then, the combination with the greatest distance was determined, and the center of the line connecting the point exposed on the steel sheet surface of that combination and the point exposed on the steel sheet surface before projection was defined as the grain boundary position.

[0029] That is, in both Figures 3 and 4, AD' is the longest, so the line AD connecting A and point D before the projection of D' is used, and the center of this AD in the rolling direction is the grain boundary position.

[0030] The area ratio of the fine grains in the vicinity of the grain boundary (within a range of ±5 mm from the grain boundary position along the rolling direction) was calculated. The area ratio was calculated by shearing the Epstein test piece in the center in the sheet width direction in the rolling direction, and the area ratio was taken as the area ratio of the sample. For each condition, the area ratio was calculated for 10 Epstein test pieces, and the average area ratio of the 10 pieces was taken as the area ratio for that condition. Since the strain was applied uniformly across the entire width, this area ratio could be obtained at the center of the sheet width. When the roll contact was poor and strain was not applied across the entire width, the area ratio was calculated at the position in the sheet width direction that could cross the most introduced strain. The periodicity of the grain boundary positions could be obtained by measuring the distance between a given strain and its adjacent strain over a length twice the length per unit of equipment that applied the strain, and examining the repeating unit. For example, in the case of a roll having protrusions as in this example, the length twice the circumference of the outer periphery of the roll was measured and the periodicity was evaluated.

[0031] The crystal orientation for determining fine grains can be measured within the observation area by X-ray diffraction or electron backscatter diffraction (EBSD). Because the reflection angles of X-rays and electron beams from the sample differ depending on the crystal orientation, the crystal orientation intensity can be determined from the reflection intensity of a randomly oriented sample.

[0032] The main magnetic domain width in the magnetostatic state was also measured. Before shearing the 10 Epstein test pieces that had undergone the cross-sectional observation from the center in the width direction for cross-sectional observation, the main magnetic domains generated on the rolled surface were output using a magnetic viewer with magnetic colloid particles, and the main magnetic domain width was calculated by observing them under a microscope.

[0033] This process was carried out on the entire steel sheet, and the main domain widths of all the samples were averaged to obtain the main domain width of one sample. This was then averaged for 10 samples to obtain the main domain width under the experimental conditions. The results of experiment [1] are shown in Table 1. The abundance ratio in Table 1 is the ratio per unit area (1 m 2 This indicates the ratio of the number of grain boundary vicinity regions, in which the fine grain area ratio is 20% or less (including 0%) relative to all grain boundary vicinity regions, in the grain boundary vicinity regions contained throughout the entire thickness of the steel sheet. The "ratio of the number of grains" here corresponds to the length ratio.

[0034] [Table 1]

[0035] It was found that when the load applied by the rolls with protrusions to the decarburized annealed sheet changes and a specific range of rolling reduction is applied to the steel sheet surface, iron loss is further improved. At this time, the area ratio of fine grains obtained from cross-sectional observation decreases, and iron loss is particularly improved when the ratio of the number of grains near the grain boundaries where the area ratio of fine grains is 20% or less is 50% or more.

[0036] Through the above experiment [1], the inventors discovered that by introducing strain, the magnetic domains of grain-oriented electrical steel sheets with periodic secondary recrystallized grain boundaries can be further subdivided, thereby achieving even lower iron loss.

[0037] The above experiment [1] revealed that iron loss is particularly improved when the ratio of the number of grains near the grain boundaries where the area ratio of fine grains is 20% or less is 50% or more. Therefore, the inventors investigated means for further suppressing fine grains through the following experiment [2].

[0038] Experiment [2] The decarburization-annealed grain-oriented electrical steel sheet, 0.23 mm thick, was sheared to a width of 30 mm and a length of 280 mm, and then strain was introduced in the width direction of the steel sheet using a protruding roll with a protrusion spacing of 10 mm at a load resulting in a rolling reduction of 2.0%. Next, an annealing separator was applied, and final annealing was carried out at 1200°C. During this process, the heating rate up to 800°C, the temperature at which secondary recrystallization begins, was changed to 1°C per hour, 3°C per hour, 5°C per hour, and 10°C per hour. The iron loss of the product sheet after final annealing and the area ratio and abundance ratio of fine grains were determined in the same manner as in the above experiment [1]. The results of the experiment [2] are shown in Table 2.

[0039] [Table 2]

[0040] The results of the experiment [2] shown in Table 2 show that the occurrence of fine grains near the grain boundaries can be suppressed under conditions where the heating rate is relatively fast, at least 5°C per hour. In other words, experiments [2] have shown that in order to suppress the occurrence of fine grains near grain boundaries, it is effective not only to increase the amount of strain but also to adjust the heating rate up to the secondary recrystallization temperature in the final annealing.

[0041] Based on the above findings, the present invention has been completed. That is, the present invention satisfies the following requirements. 1. Grain-oriented electrical steel sheet containing, by mass%, Si: 2.0% to 7.0%, wherein, in a thickness direction cross section along the rolling direction of the steel sheet, the center of the maximum length of the grain boundary of secondary recrystallized grains in the rolling direction is taken as the grain boundary position, and the range from the grain boundary position as the base point to a length of ±5 mm in the rolling direction and across the entire thickness of the steel sheet is taken as the grain boundary vicinity, the grain boundary vicinity is 2 ) A grain-oriented electrical steel sheet in which 50% or more of the grain boundary vicinity area is occupied by crystal grains whose crystal orientation deviates from the Goss orientation by 10 degrees or more, at an area ratio of 20% or less (including 0%), relative to the entire grain boundary vicinity area.

[0042] 2. All or part of the grain boundary positions are located within a unit area (1 m) in the rolling direction. 2 2. The grain-oriented electrical steel sheet according to 1 above, wherein the grains are periodically arranged at intervals that are an integral multiple of the average grain size of secondary recrystallized grains in the grain boundary.

[0043] 3. A method for producing grain-oriented electrical steel sheet as described in 1 or 2 above, in which one or both sides of the rolled surface of the steel sheet before final annealing are subjected to rolling reduction in part or the entire sheet width direction at a rolling reduction rate of 0.1% or more, and then the temperature is raised to a temperature range where secondary recrystallization begins at a rate of 5°C per hour or more, and final annealing is carried out at a temperature of 1100°C or more. [Effects of the Invention]

[0044] According to the present invention, a grain-oriented electrical steel sheet with low iron loss can be provided. In addition, by subjecting the grain-oriented electrical steel sheet provided by the present invention to further magnetic domain refinement treatment, a method for producing a grain-oriented electrical steel sheet with extremely low iron loss can be provided. [Brief explanation of the drawings]

[0045] [Figure 1] FIG. 2 is a schematic diagram showing an example of a grain boundary position in a steel sheet cross section in the sheet thickness direction in the rolling direction of a sample. [Figure 2] FIG. 10 is a schematic diagram illustrating another example of the grain boundary position in the steel sheet cross section in the sheet thickness direction in the rolling direction of the sample. [Figure 3]FIG. 10 is a schematic diagram illustrating another example of the grain boundary position in the steel sheet cross section in the sheet thickness direction in the rolling direction of the sample. [Figure 4] FIG. 10 is a schematic diagram illustrating another example of the grain boundary position in the steel sheet cross section in the sheet thickness direction in the rolling direction of the sample. DETAILED DESCRIPTION OF THE INVENTION

[0046] First, the reasons for limiting the constituent elements of the present invention will be described. The present invention relates to grain-oriented electrical steel sheet products. As long as the area ratio of fine grains near the specified grain boundaries satisfies the requirements of the present invention, the composition of the steel sheet other than the Si content is not particularly limited. The remainder may be Fe and unavoidable impurities. Furthermore, by mass%, the steel sheet is permitted to contain 0.005-1.000% Mn and 0.0050% or less C. Furthermore, by mass%, the steel sheet is permitted to contain one or more of the following elements: 0.01-1.50% Ni, 0.01-0.50% Cr, 0.01-0.50% Cu, 0.01-0.50% Bi, 0.01-0.20% Sb, 0.01-0.20% Sn, 0.01-0.20% Mo, 0.01-0.20% P, and 0.001-0.015% Nb. Therefore, the composition of the starting steel slab is not limited as long as it is a composition that allows secondary recrystallization to occur. For example, when an inhibitor is used to grow secondary recrystallized grains, examples include AlN-based and MnSe-based inhibitors, and when using these, it is sufficient to include appropriate amounts of Al, N, Mn, Se, etc.

[0047] The present invention is also applicable to grain-oriented electrical steel sheets that do not use inhibitors. Although the inclusion of Si in steel sheet improves its properties as a soft magnetic material, if the Si content exceeds 7.0% by mass, workability deteriorates significantly, making manufacturing difficult. Therefore, the Si content of the steel sheet used in the present invention is limited to 7.0% by mass or less, and preferably 5.0% by mass or less. On the other hand, the Si content is set to 2.0% by mass or more, which increases the electrical resistance of the steel sheet, reduces eddy current loss, and reduces iron loss.

[0048] In the manufacturing process of grain-oriented electrical steel sheets, the hot-rolled steel sheet is cold-rolled and then subjected to decarburization annealing to create the primary recrystallized structure necessary for the grain growth of secondary recrystallization. To create this primary recrystallized structure, there are no restrictions on the number of cold-rolling steps or additional annealing steps, as long as the final product thickness can be adjusted.

[0049] Furthermore, in order to reduce the diameter of the secondary recrystallized grains, processing strain is introduced before final annealing, but the timing and method are not particularly limited as long as final annealing can be carried out in a state in which grooves or portions where strain has been introduced due to localized rolling remain.

[0050] It is preferable that the processing strain is introduced periodically and has periodicity. Furthermore, the grain boundary position referred to in the present invention is desirably selected from the grain boundary positions of the secondary recrystallized grains that are closest to the strain introduction position. The introduction of such strain periodically means that a certain periodicity is recognized in the strain interval, and the strain is introduced in the rolling direction in a unit area (1 m 2 It is more preferable that the strains are arranged periodically at intervals that are an integral multiple of the average grain size of the secondary recrystallized grains in the grain boundary layer. This is because the periodicity of the strain prevents variations in properties from occurring depending on the location when cutting out iron core material from the manufactured grain-oriented electrical steel sheet, making it possible to maintain industrially consistent quality. Furthermore, the periodicity of the grain boundary positions is preferably 50% or more, more preferably 80% or more, in terms of the number ratio of grain boundary positions in an area having a length of at least two units adjacent to one unit of the equipment that imparts strain.

[0051] Here, the term "periodic grain boundary positions" as used herein means that three or more strains are repeatedly introduced in the rolling direction of the steel sheet with an error range of ±0.5 mm or less. The value of the interval is not particularly limited, but is preferably in the range of about 5 to 200 mm from an industrial perspective.

[0052] The grain boundaries of secondary recrystallized grains do not extend perpendicular to the rolling direction from directly below the point where strain is introduced, but may extend diagonally or in a semicircular shape depending on the speed at which the secondary recrystallized grains grow before and after strain is introduced and the non-uniformity of strain in the thickness direction of the steel sheet. The above-mentioned tolerance ranges were set in consideration of such influences. Note that the width and length of the strain-introduced location are the width and length of the part where stress and load are applied to the decarburized annealed sheet when strain is introduced.

[0053] In the present invention, the length of the strain in the sheet width direction does not need to be the entire width of the steel sheet, and as long as it is 50% or more of the width of the steel sheet, the grain size of the secondary recrystallized grains is effectively reduced, and a steel sheet with low iron loss can be obtained. In order to reduce iron loss, it is preferable to increase the length of the portion where strain is introduced in the width direction, and the most preferable condition is to introduce strain throughout the entire sheet width direction.

[0054] Furthermore, as described above, by imparting the aforementioned periodicity to at least 50% of the introduced strain in terms of number ratio, it is possible to effectively reduce the variation in grain size of secondary recrystallized grains when the steel sheet is cut out.

[0055] That is, when grain-oriented electrical steel sheets are processed into transformers, except for those called non-cut cores, they are sheared to a certain length and then assembled into the transformer core. However, if the grain size of secondary recrystallized grains differs for each steel sheet after shearing, the iron loss will differ for each steel sheet, and steel sheets with large iron loss will become a bottleneck, preventing the iron loss improvement effect of the present invention from being fully exerted in the entire transformer core and causing a deterioration in the characteristics when the core is magnetized.

[0056] Therefore, in the present invention, in order to prevent an unnecessary increase in iron loss when a transformer is manufactured using the obtained grain-oriented electrical steel sheet, it is preferable to form a crystal structure having periodic grain boundaries of secondary recrystallized grains in the above-mentioned number ratio.

[0057] Furthermore, the primary recrystallized structure of grain-oriented electrical steel sheets is easily encroached upon by Goss grains and is controlled to a structure that promotes the growth of Goss grains. However, if strain is locally introduced into the steel sheet before final annealing, continuous recrystallization occurs during the temperature rise of the final annealing, driven by the strain, and new recrystallized grains are formed. However, the recrystallized grains formed here have significantly different orientations and are less susceptible to encroachment. Therefore, the rolling section is not encroached upon, and different secondary recrystallized grains grow across this boundary.

[0058] If the annealing temperature is further increased, the grown secondary recrystallized grains will eat away at the recrystallized grains in between, resulting in the recrystallized grains in the distorted areas and the creation of only secondary recrystallized grains with a specific grain size. If the amount of strain introduced is inappropriate, the primary recrystallized grains will not have enough driving force to cause continuous recrystallization, resulting in discontinuous recrystallization, in which abnormal grain growth occurs in the same orientation as the grains around them, with the grains being significantly misoriented relative to the surrounding grains.

[0059] These discontinuous recrystallizations are larger than the primary recrystallized grains and are not encroached upon by the grown Goss grains. Therefore, these discontinuous recrystallizations remain as finer crystal grains than the secondary recrystallized grains even after final annealing. Furthermore, since these discontinuous recrystallizations are not secondary recrystallized, their crystal orientation is significantly different from that of the Goss grains.

[0060] That is, as shown in the above experiment [1], iron loss is further improved if there are fewer discontinuously recrystallized grains. Therefore, in the present invention, when the grain boundary vicinity area is defined as a range of ±5 mm in the rolling direction from the grain boundary position as the base point and across the entire thickness of the steel plate, the grain boundary vicinity area is defined as a range of ±5 mm in the rolling direction from the grain boundary position as the base point and across the entire thickness of the steel plate. 2 By controlling the abundance ratio of the grain boundary vicinity in the steel sheet (i.e., the area ratio of crystal grains (fine grains) whose crystal orientation deviates from the Goss orientation by 10 degrees or more to the entire grain boundary vicinity, which is 20% or less (including 0%), to 50% or more, a grain-oriented electrical steel sheet with extremely low iron loss can be obtained. The abundance ratio is preferably 80% or more.

[0061] In the present invention, it is essential that strain is introduced into the surface of the steel sheet that serves as the base material, and that when the steel sheet is heated, the crystal grains recover and recrystallize to release the strain within the steel sheet. The strain introduced is achieved by rolling one or both sides of the rolled surface of the steel sheet before final annealing at a reduction rate of 0.1% or more across part or the entire width of the sheet. The upper limit of the reduction rate is not particularly limited as long as it does not result in grooves with a depth of 50 μm or more, but 20 μm or less is preferred.

[0062] Furthermore, if the heating rate during such heating is slow, i.e., less than 5°C per hour, the strain is partially released by recovery before the continuous recrystallization described above occurs, and there is not enough strain left to cause continuous recrystallization, causing discontinuous recrystallization and the growth of many fine grains. Therefore, by increasing the temperature rise rate to 5° C. or more per hour, the effect of reducing iron loss due to the reduction in the grain size of secondary recrystallized grains caused by strain can be further increased.

[0063] Furthermore, final annealing must be performed at a temperature of 1100°C or higher. This is because it is necessary to purify impurities in the steel sheet. It is preferable to perform the final annealing at a holding temperature of 1200°C or higher, which is a temperature more suitable for the above-mentioned purification. On the other hand, although there is no particular upper limit on the holding temperature for final annealing, a temperature of around 1200°C is preferable in consideration of the capacity of the equipment and the cost required for heating.

[0064] In the present invention, the manufacturing conditions of the decarburized annealed steel sheet, which is the base material when strain is introduced, are not particularly limited as long as the process allows secondary recrystallization to occur during final annealing. Therefore, the present invention can also be applied to materials processed by subjecting the steel sheet surface to laser irradiation or electrolytic etching, and further reduction in iron loss can be achieved by reducing the diameter of the secondary recrystallized grains.

[0065] Furthermore, the processes after the final annealing are not particularly limited, and flattening annealing, application of an insulating coating, baking, etc. Furthermore, it is preferable to carry out a magnetic domain refining process by introducing thermal strain, typically by laser irradiation, or by forming additional grooves, as this leads to further reduction in iron loss. [Example]

[0066] Next, the present invention will be specifically described based on examples. The following examples are representative examples of the present invention, and the present invention is not limited by these examples. The embodiments of the present invention can be appropriately modified within the scope of the invention, and all such modifications are included in the technical scope of the present invention.

[0067] A steel slab containing the components shown in Table 3, with the remainder being Fe and unavoidable impurities, was produced by continuous casting, heated to 1420°C, and then hot-rolled to a 2.0 mm thick hot-rolled sheet, which was then annealed at 900°C for 10 seconds. This was then cold-rolled to an intermediate thickness of 1.1 mm, and intermediate annealed at 1070°C for 30 seconds with an oxidation degree of PH2O / PH2 = 0.32. This was then cold-rolled again to a 0.23 mm thick cold-rolled steel sheet. The cold-rolled steel sheet with the final thickness was then decarburized in wet hydrogen at 850°C for 150 seconds.

[0068] [Table 3]

[0069] The steel sheet was then rolled using a 2 m long protrusion roll on the outer periphery with 50 μm wide protrusions formed randomly at intervals of 3 to 10 mm under five conditions of reduction ratio: 0.0%, 0.1%, 2.0%, 5.0%, and 10.0%. Next, an annealing separator containing MgO as the main component was applied, and final annealing was carried out for the purpose of secondary recrystallization and the formation of a forsterite film. The rolling reduction of 0.0% means that the steel sheet was passed through the protruding rolls with a load that did not create grooves on the steel sheet surface.

[0070] As a comparative example, a sample was prepared in which, after decarburization annealing, no rolling with projecting rolls was performed, an annealing separator was applied, and final annealing was performed.

[0071] The final annealing was performed using a thermal pattern in which the temperature was increased at a rate of 1°C per hour, 2°C per hour, 5°C per hour, or 10°C per hour up to 800°C, where secondary recrystallization begins, and then increased at a rate of 5°C per hour up to 1200°C.

[0072] Next, after removing the unreacted annealing separator, a coating liquid consisting of 50% by mass of colloidal silica and 50% by mass of aluminum phosphate was applied, and a tension coating baking treatment, which also served as flattening annealing, was carried out at a baking temperature of 850°C.

[0073] The grain-oriented electrical steel sheets thus obtained were subjected to Epstein tests and magnetic domain observations to determine the iron loss and magnetic domain width. The Epstein tests were performed on samples cut out of 36 samples per condition, each 280 mm in the rolling direction and 30 mm in the sheet width direction, under the excitation conditions of 1.7 T and 50 Hz. In this example, the average of the iron loss values ​​of the 36 samples was taken as the iron loss under the conditions. The magnetic domain width was observed under a microscope using a magnet viewer to display the main magnetic domain width in a static state. After observing the main magnetic domain width, the cross section of the steel sheet was observed to determine the number ratio of periodically introduced grain boundary positions, the area ratio of fine grains in the vicinity of grain boundaries, and the abundance ratio of areas in the vicinity of grain boundaries where the area ratio of such fine grains was 20% or less.

[0074] The cross section was observed and the area ratio of fine particles was measured by the methods described above. The area ratio of fine particles is the average value of measurement data from 10 sheets per condition. The number ratio was calculated from the formula: (number of periodic grain boundary positions / number of grain boundaries within a range twice the circumferential length of the outer periphery of the roll). Furthermore, the above-mentioned ratio is calculated by dividing the number of grain boundaries where the area ratio of fine grains is 20% or less by the unit area of ​​the steel sheet surface (1 m 2 The number of grain boundaries identified in the The results of such measurements are shown in Table 4.

[0075] [Table 4]

[0076] As shown in Table 4, it is clear that by satisfying the requirements of the present invention, grain-oriented electrical steel sheets with superior properties can be obtained. In particular, it is understood that when the above number ratio is in the range of 93% or more and the above existence ratio is in the range of 88% or more, grain-oriented electrical steel sheets with better properties can be obtained.

Claims

1. A grain-oriented electrical steel sheet containing, by mass%, Si: 2.0% or more and 7.0% or less, In a thickness direction cross section along the rolling direction of a steel sheet, when the center of the maximum length of the grain boundary of a secondary recrystallized grain in the rolling direction is defined as the grain boundary position, and the range from the grain boundary position as a base point to a length of ±5 mm in the rolling direction and across the entire thickness of the steel sheet is defined as the grain boundary vicinity, The unit area (1 m) of the steel plate surface across the entire thickness of the steel plate ２ ) In the grain boundary vicinity region, crystal grains whose crystal orientation deviates from the Goss orientation by 10 degrees or more account for 50% or more of the entire grain boundary vicinity region, and the crystal grains have an area ratio of 20% or less (including 0%), Furthermore, the grain-oriented electrical steel sheet has all or some of the grain boundary positions arranged periodically in the rolling direction at intervals that are an integral multiple of the average grain size of secondary recrystallized grains per unit area (1 m 2 ).

2. A method for producing the grain-oriented electrical steel sheet according to claim 1, A method for producing a grain-oriented electrical steel sheet, in which one or both sides of the rolled surface of a steel sheet before final annealing are reduced using rolls having protrusions provided on part or the entire sheet width direction at a reduction rate of 0.1% or more, and then the temperature is increased at a rate of 5°C per hour or more to a temperature range at which secondary recrystallization begins, and final annealing is then performed at a temperature of 1100°C or higher.

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