Method for manufacturing grain-oriented electrical steel sheets, method for performing final annealing on coils, and coil for final annealing.

Abstract

The method for manufacturing grain-oriented electrical steel sheets ensures that deformation of the coil is prevented during the manufacturing process of grain-oriented electrical steel sheets, and improves the magnetic properties of the grain-oriented electrical steel sheets by increasing the inner diameter of the coil. The method involves hot-rolling a steel slab having a predetermined composition to form a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet once or two or more times with an intermediate annealing in between to form a cold-rolled steel sheet, performing primary recrystallization annealing on the cold-rolled steel sheet, winding the cold-rolled steel sheet after primary recrystallization annealing to form a coil, and performing final annealing on the coil, wherein the cold-rolled steel sheet located in the inner winding portion of the coil is joined at least one of the following: immediately before the final annealing and during the final annealing.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet, a method for subjecting a coil to final annealing, and a coil for final annealing.

Background Art

[0002] A grain-oriented electrical steel sheet is a steel material in which the magnetic properties in the rolling direction are enhanced by using a metallurgical phenomenon called secondary recrystallization to form a crystal aggregate structure in which the orientation of iron crystal grains is aligned with the {110}<001> orientation called the Goss orientation. The manufacturing of a grain-oriented electrical steel sheet generally involves manufacturing a coil by winding a cold-rolled steel sheet obtained by subjecting a steel slab adjusted to a predetermined component composition to hot rolling, annealing of the hot-rolled sheet, and cold rolling, and then sequentially performing a series of heat treatments including primary recrystallization annealing, decarburization annealing, and final annealing on the coil. In the final annealing, it is necessary to perform the heat treatment of the coil at a high temperature of 1000°C or higher. At this time, mutual diffusion of metal atoms occurs at the contact surface of adjacent cold-rolled steel sheets, and there is a risk that the cold-rolled steel sheets are diffusion-bonded. Therefore, for the purpose of preventing the diffusion bonding of cold-rolled steel sheets, it is common practice to apply an annealing separating agent mainly composed of magnesium oxide powder to the surface of the cold-rolled steel sheet when manufacturing a coil by winding the cold-rolled steel sheet before the final annealing. The coil after the final annealing is set on a rotating shaft called a payoff reel, paid out, and returned to the state of a steel sheet again. The steel sheet paid out from the payoff reel is corrected for the coil winding distortion by flattening annealing, and the grain-oriented electrical steel sheet is completed.

[0003] The shape of a coil may change during the heat treatment process. Two types of coil deformation are known: kinking and crushing. Kinking occurs when buckling of the steel plate near the central hole causes multiple layers of the steel plate to bend and protrude towards the center of the hole. Kinking is thought to be caused by uneven temperature distribution during heat treatment, resulting in large compressive stress in the circumferential direction. Crushing occurs when the coil's own weight compresses the entire coil in a direction perpendicular to its central axis. The deformed portion of the coil cannot be used in the final product, reducing yield. Furthermore, the deformation reduces the diameter of the central hole, making it impossible to set the coil in the payoff reel. Coil deformation is more likely to occur with larger coil inner diameters.

[0004] Several methods have been proposed to prevent the deformation of the coil described above. For example, Patent Document 1 proposes a method to prevent the coil from collapsing by applying greater tension to the steel plate on the inside of the coil and less tension on the outside when winding the coil. Patent Document 2 proposes a method to prevent kinking by increasing the frictional force between the layers of the coil and suppressing slippage between the steel plates by adding coarse-grained powder to the magnesium oxide applied to the coil surface as an annealing separating agent. Other methods include Patent Document 3, which proposes a method of inserting a steel sleeve into the hole in the center of the coil, and Patent Document 4, which proposes a method of winding the coil so that the thicker unrolled portion at the tip of the steel plate is located near the hole in the center of the coil. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Unexamined Patent Publication No. 62-70523 [Patent Document 2] Japanese Patent Publication No. 2012-177148 [Patent Document 3] Japanese Patent Application Publication No. 60-141828 [Patent Document 4] Japanese Patent Application Laid-open No. 61-124529 [Overview of the project] [Problems that the invention aims to solve]

[0006] According to the methods proposed in Patent Documents 1 and 2, coil deformation is prevented by suppressing the sliding between the steel plates constituting the coil. However, since the frictional force between the layers of the coil is easily affected by the surface condition of the steel plates, these methods could not completely prevent coil deformation. The method using a steel sleeve described in Patent Document 3 had the drawback of being costly in terms of manufacturing and maintenance of the sleeve. The method using unrolled material described in Patent Document 4 had the problem of a significant decrease in yield.

[0007] Incidentally, from the viewpoint of the magnetic properties of grain-oriented electrical steel sheets, it is preferable for the inner diameter of the coil to be large in the final annealing stage. Figure 1 shows the iron crystal grains in coil 2 that has undergone final annealing. <001> This is a schematic diagram showing the orientation. Figure 1 shows a cross-section perpendicular to the central axis of coil 2. In the coil immediately after the final annealing, crystals with the Goss orientation preferentially grow, resulting in the arrows shown in Figure 1(a). <001> The orientation is aligned in one direction throughout the entire interior of a single crystal grain. When the coil 2, which has completed the final annealing, is discharged and plastically deformed into a flat grain-oriented electrical steel sheet 4, it becomes as shown by the arrows in Figure 1(b). <001> The angle between the grain orientation and the rolling direction of the grain-oriented electrical steel sheet 4 changes depending on the position within a single grain. This change in angle becomes smaller as the inner diameter of the coil 2 during the final annealing stage, as shown in Figure 1(a), increases. However, as mentioned above, coil deformation is more likely to occur as the inner diameter of the coil increases, so the inner diameter of the coil could not be made too large in order to prevent deformation.

[0008] This invention has been made in view of the above-mentioned problems, and aims to reliably prevent deformation of coils in the manufacturing process of grain-oriented electrical steel sheets, and to improve the magnetic properties of grain-oriented electrical steel sheets by increasing the inner diameter of the coils. [Means for solving the problem]

[0009] The inventors conceived of directly joining steel plates together, instead of the indirect methods proposed in Patent Documents 1 and 2, in order to reliably prevent slippage between steel plates. As specific means of joining cold-rolled steel plates, they considered joining by welding and a method of diffusion joining the steel plates together during the final annealing process without applying an annealing release agent. As a result of their investigations, they found that deformation of the coil could be reliably prevented by directly joining the steel plates together, and thus completed the present invention. The gist of the present invention completed by the inventors is as follows.

[0010] [1] A method for manufacturing grain-oriented electromagnetic steel sheets, comprising: hot rolling a steel slab to obtain a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet once or twice with an intermediate annealing in between to obtain a cold-rolled steel sheet; primary recrystallization annealing of the cold-rolled steel sheet; winding the cold-rolled steel sheet after primary recrystallization annealing to obtain a coil; and final annealing of the coil, A method for manufacturing grain-oriented electrical steel sheets, characterized by joining the cold-rolled steel sheets located in the inner winding portion of the coil at at least one of the following: before the final annealing and during the final annealing.

[0011] [2] The method for manufacturing grain-oriented electrical steel sheets according to [1] above, wherein in the winding before the final annealing, the joining of the cold-rolled steel sheets located in the inner winding portion is performed by welding.

[0012] [3] The method for manufacturing grain-oriented electrical steel sheets according to [2] above, wherein the welding is laser welding.

[0013] [4] In the winding process, an annealing release agent is applied only to the surface of the cold-rolled steel sheet located in the portion excluding the inner winding portion to form a coil. A method for manufacturing grain-oriented electrical steel sheets according to [1] above, wherein the joining of the cold-rolled steel sheets located in the inner coil portion is performed by diffusion joining during the final annealing process.

[0014] [5] The initial joining of the cold-rolled steel sheet located in the inner coil portion is performed by welding during the coiling process prior to the final annealing. A method for manufacturing grain-oriented electrical steel sheets according to [4] above, wherein the second joining of the cold-rolled steel sheet located in the inner coil portion is performed by diffusion joining during the final annealing process.

[0015] [6] A method for manufacturing grain-oriented electrical steel sheets according to any one of [1] to [5] above, wherein the joining is performed on three or more layers of cold-rolled steel sheets located in the inner coil portion.

[0016] [7] A method for performing final annealing on a coil for manufacturing grain-oriented electrical steel sheets, A method characterized by joining cold-rolled steel sheets located in the inner winding portion of the coil before the final annealing and at least one of the following during the final annealing.

[0017] [8] A coil for final annealing for manufacturing grain-oriented electrical steel sheets, A coil characterized in that a cold-rolled steel sheet located in the inner winding portion of the coil is joined to it. [Effects of the Invention]

[0018] According to the present invention, deformation of coils in the manufacturing process of grain-oriented electrical steel sheets can be reliably prevented by simple means, thereby achieving a high manufacturing yield. Furthermore, since the inner diameter of the coil can be increased without deforming the coil, the magnetic properties of the grain-oriented electrical steel sheet can be improved. [Brief explanation of the drawing]

[0019] [Figure 1]It is a schematic diagram showing the orientation of the <001> plane of the iron crystal grains in the coil after final annealing. (a) shows the state of the coil immediately after final annealing, and (b) shows the state of the coil after paying out the coil after final annealing. [Figure 2] It is a schematic diagram showing an example of a method for joining cold-rolled steel sheets by laser welding.

Embodiments for Carrying out the Invention

[0020] As described above, in the prior art, when adopting indirect methods such as a method for adjusting the tension applied to the steel sheet when winding the coil or a method for adjusting the particle size of the annealing release agent applied to the surface of the steel sheet, slipping between the steel sheets occurs with a certain probability, and it was inevitable that kinks and collapses occurred in the inner wound portion of the coil. Therefore, the inventors conceived of directly joining the steel sheets instead of the above indirect methods for the purpose of surely preventing the occurrence of slipping between the steel sheets. Specific means for joining cold-rolled steel sheets include, after applying an annealing release agent to the surface of the cold-rolled steel sheet before final annealing and drying and winding it up, joining by welding before final annealing in a state where the cold-rolled steel sheet is wound around the tension reel to a certain extent, or means for diffusion bonding the steel sheets to each other during final annealing without applying the annealing release agent deliberately. By joining and integrating the cold-rolled steel sheets located in the inner diameter portion of the coil using these means, not only can the slipping between the steel sheets be suppressed, but also the apparent plate thickness increases, so it was thought that the strength of the inner wound portion of the coil could be increased.

[0021] First, an experiment that actually verified the above concept and led to the present invention will be described.

[0022] 1. Experiment Cold-rolled steel sheets were prepared from steel slabs having a predetermined composition, following the general manufacturing method described above, through the process up to decarburization and annealing, before final annealing. The thickness of the cold-rolled steel sheets was 0.23 mm. Next, the cold-rolled steel sheets were wound onto a tension reel of a winding device to produce coils. The coil radii were set to five levels: 508 mm, 610 mm, 711 mm, 813 mm, and 914 mm. The inner diameter of the coils was changed using a special tension reel equipped with an attachment having a widening mechanism in the winding section. The tension applied to the cold-rolled steel sheets in the rolling direction during coil winding was set so that winding started at 50% of the final tension and increased at a constant rate until completion. The winding speed was initially 20 m per minute, but increased after winding about 100 m, and then maintained at a constant speed after reaching the normal winding speed.

[0023] Regarding the welding method of joining, an annealing release agent was applied to the surface of the cold-rolled steel sheet, dried, and then wound 10 times. After that, laser welding was performed around the circumference using a fixed fiber laser welding machine. The welding was performed at a position 150 mm from the edge of the coil. The output of the fiber laser welding machine was 10 kW, and the welding speed was 20 m / min, the same as the winding speed of the coil. After laser welding one full turn, the annealing release agent was applied again, and the coil winding continued while drying. When the cross-section was observed after joining by laser welding, the weld was formed over the entire 10 layers of cold-rolled steel sheet located in the inner diameter portion of the coil.

[0024] For the joining method that does not involve applying an annealing release agent, the cold-rolled steel sheet was not coated with the annealing release agent up to 10 layers of winding. The annealing release agent was then applied to the surface of the cold-rolled steel sheet from that point onward, dried, and wound into a coil. Coils joined using both the method without applying the annealing release agent and laser welding were also prepared. As a comparison, a coil was prepared without joining, with the annealing release agent applied and wound while drying. Three coils were prepared for each of these four conditions and five levels, and Table 1 shows the number of coils that developed kinks after final annealing.

[0025] [Table 1]

[0026] According to Table 1, in the case of no joining, kinking occurred when the inner diameter of the coil was 711 mm or larger, and the probability of kinking increased with increasing inner diameter. In contrast, with laser welding, no kinking occurred, and the shape of the inner winding remained good even when the inner diameter of the coil was increased. It was also found that the probability of kinking was suppressed compared to the case of no joining when no annealing release agent was applied. Furthermore, in coils where kinking occurred, observation of the inner winding after final annealing suggested that kinking occurred at low temperatures, followed by diffusion bonding of the steel plates at high temperatures.

[0027] Even in cases where the annealing release agent was not applied and laser welding was performed, no kinking occurred in any of the coils. Upon observation of the inner coil section after final annealing, indentations were found along the weld. Since spatter was generated when welding coils without the annealing release agent, it was presumed that these indentations were caused by spatter generated during welding adhering to the surface of the cold-rolled steel sheet. On the other hand, in coils that were laser-welded with the annealing release agent applied, spatter generation was suppressed compared to the case without the annealing release agent. This is presumed to be because magnesium oxide, the main component of the annealing release agent, acted like a flux, preventing oxidation of the molten pool and stabilizing the formation of the weld. Initially, it was thought that the annealing release agent would inhibit the welding of cold-rolled steel sheets, but it is now considered advantageous not to remove the annealing release agent in order to suppress spatter generation.

[0028] Next, embodiments for carrying out the present invention will be described in detail.

[0029] 2. Manufacturing method of grain-oriented electrical steel sheets In one embodiment, the present invention relates to a method for manufacturing grain-oriented electrical steel sheets, comprising: hot rolling a steel slab to obtain a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet once or two or more times with intermediate annealing in between to obtain a cold-rolled steel sheet; primary recrystallization annealing of the cold-rolled steel sheet; winding the cold-rolled steel sheet after primary recrystallization annealing to form a coil; and final annealing of the coil, characterized in that, at least one of the following—before the final annealing and during the final annealing—the cold-rolled steel sheets located in the inner winding portion of the coil are joined.

[0030] The grain-oriented electrical steel sheets covered by this invention are of any type of steel, as long as they have a forsterite-based coating on their surface. Typically, such grain-oriented electrical steel sheets are produced by hot-rolling a silicon-containing steel slab, melted in a blast furnace or electric furnace, using a known method; cold-rolling the hot-rolled steel sheet once or two or more times with an intermediate annealing in between to obtain a cold-rolled steel sheet to the final thickness; primary recrystallization annealing of the cold-rolled steel sheet; winding the cold-rolled steel sheet after primary recrystallization annealing into a coil; and final annealing of the coil. Primary recrystallization annealing may also be combined with decarburization annealing. Final annealing is annealing that includes secondary recrystallization annealing. Planarization annealing may be performed after final annealing.

[0031] In the present invention, among the series of steps described above, the cold-rolled steel sheets located in the inner winding portion of the coil are joined before the final annealing and during the final annealing process. The inner winding portion of the coil refers to the inner circumference portion of the coil where the final annealing is performed. The range of the inner winding portion of the coil is not particularly limited, but it is preferable to include the innermost cold-rolled steel sheet of the coil and preferably include at least two layers extending outward from there. In the present invention, "joining cold-rolled steel sheets" refers to joining adjacent cold-rolled steel sheets together. By joining the cold-rolled steel sheets located in the inner winding portion of the coil, the steel sheets located in the inner winding portion are constrained by each other, preventing slippage. This makes buckling in the inner winding portion of the coil less likely to occur, preventing the formation of kinks. In addition, the cold-rolled steel sheets located in the inner winding portion are integrated by the joining, increasing the apparent thickness of the sheet, thus improving the strength of the inner winding portion of the coil. This prevents the coil from collapsing.

[0032] In a preferred embodiment of the present invention, three or more layers of cold-rolled steel sheets located in the inner coil section are joined. By joining three or more layers of cold-rolled steel sheets, the inter-layer joint strength is further improved, making it possible to enlarge the inner diameter of the coil and more reliably suppress the occurrence of kinks. The joining of cold-rolled steel sheets does not need to cover the entire surface, as long as the steel sheets are joined in a way that prevents them from slipping, and the joint can be in the shape of a strip or a line. Furthermore, the joint does not need to be continuous along the circumferential or width direction of the coil, and there may be gaps in the joint. The number of layers of cold-rolled steel sheets to be joined is preferably 20 layers or less, as this reduces the amount of waste in the inner coil section and improves the product yield. A more preferable number of layers is 10 layers or less.

[0033] [Joining by welding] In a preferred embodiment of the present invention, the cold-rolled steel sheets located in the inner winding portion are joined by welding during winding before the final annealing. Welding is a simple method in that it allows cold-rolled steel sheets to be joined at room temperature in a short time. Specifically, it is reasonable to fix the welding machine near the winding device and, after winding three or more layers of cold-rolled steel sheets onto the tension reel, perform welding along the circumference of the coil for about one full turn. In this case, the welding speed is the same as the conveying speed of the cold-rolled steel sheets, preferably, for example, 10 m / min or more and 30 m / min or less.

[0034] Welding offers greater design flexibility in terms of the joint location compared to diffusion bonding, which will be discussed later. When cold-rolled steel sheets are wound onto a tension reel to form a coil, welding near the end of the coil can more reliably prevent misalignment between the steel sheets. While welding may be performed in only one location along the width of the coil, two or more locations are preferable for more stable joints. Instead of welding the entire circumference of the coil, for example, the welding can be done in three separate passes, each covering one-third of the circumference, with the welding location varying each time. The thickness of the welded joint may include the entire thickness of the cold-rolled steel sheet to be welded, or it may only be a portion of it. When welding only a portion of the thickness of the cold-rolled steel sheet, the entire sheet can be welded at multiple locations, gradually changing the welding points.

[0035] As a welding method, arc welding such as TIG welding can be used, but laser welding is more preferable. Laser welding is preferable because, unlike arc welding, it is not necessary to bring the welding rod close to the workpiece and it is less affected by disturbances. Figure 2 is a schematic diagram showing an example of a method for joining cold-rolled steel sheets by laser welding. In the winding of the cold-rolled steel sheet 1, the cold-rolled steel sheet is welded by irradiating the surface of the wound coil 2 with a laser beam 3, and a welded portion 2a is formed along the circumferential direction of the coil 2. The output of the laser when performing laser welding is not particularly limited, but for example, in the case of grain-oriented electrical steel sheet with a thickness of 0.23 mm and coated with an annealing release agent, the output is preferably 2 kW or more and 20 kW or less. If spatter is generated during welding, it will cause depressions on the surface of the cold-rolled steel sheet during the final annealing, so it is preferable to perform welding under conditions that minimize spatter generation. As mentioned above, the application of an annealing release agent is effective in suppressing the generation of spatter. Furthermore, in the case of fiber laser welding, spatter generation can be suppressed by applying a condition called ring mode, where the energy around the laser beam is high.

[0036] [Joining by diffusion bonding] In a preferred embodiment of the present invention, during winding, the annealing release agent is applied only to the surface of the cold-rolled steel sheet located in the portion excluding the inner winding portion to form a coil, and during the final annealing, the cold-rolled steel sheet located in the inner winding portion is joined by diffusion bonding. The annealing release agent is applied to the surface of the cold-rolled steel sheet to prevent the cold-rolled steel sheets from being diffusely bonded to each other during the final annealing of the coil. By not applying the annealing release agent to the inner winding portion of the coil, but to the portion other than the inner winding portion, the cold-rolled steel sheet located in the inner winding portion to which the annealing release agent has not been applied can be joined by diffusion bonding or fusion bonding, while the joining of the cold-rolled steel sheet located in the portion other than the inner winding portion can be prevented by the action of the annealing release agent. Diffusion bonding does not require a welding machine, which is necessary for welding, and can be easily carried out with existing equipment.

[0037] The annealing separating agent used in the present invention preferably has magnesium oxide as its main component. That is, the annealing separating agent contains 50% by mass or more of magnesium oxide. If the magnesium oxide content is less than 50% by mass, the amount of forsterite film formed by the reaction between the internal silica oxide formed during decarburization annealing and the magnesium oxide contained in the annealing separating agent will be insufficient. The proportion of magnesium oxide contained in the annealing separating agent is preferably 60% by mass or more, more preferably 80% by mass or more. The annealing separating agent may contain reaction aids such as titanium oxide and strontium hydroxide as other components besides magnesium oxide, and can be mixed with other components to form a liquid or slurry-like annealing separating agent which can then be applied to a coil. After drying the liquid or slurry-like annealing separating agent applied to the surface of the cold-rolled steel sheet, the cold-rolled steel sheet can be wound into a coil, thereby achieving a state in which the annealing separating agent exists between the layers of the coil.

[0038] Diffusion bonding is thought to proceed when the coil is heated to over 900°C during the final annealing process. Therefore, diffusion bonding has little effect in preventing kinking that occurs before the coil temperature reaches 900°C. In this case, it is preferable to prevent kinking in the low-temperature range by combining it with conventional techniques such as adjusting the tension when winding the coil.

[0039] In the present invention, joining by diffusion bonding may be combined with joining by welding. In a preferred embodiment of the present invention, the first joining of the cold-rolled steel sheets located in the inner coil portion is performed by welding during winding before the final annealing, and the second joining of the cold-rolled steel sheets located in the inner coil portion is performed by diffusion bonding during the final annealing. By combining the two types of joining, deformation of the coil can be prevented more reliably.

[0040] 3. Method for performing final annealing on the coil In another embodiment, the present invention relates to a method for performing final annealing on a coil for manufacturing grain-oriented electrical steel sheets, characterized in that cold-rolled steel sheets located in the inner winding portion of the coil are joined before and during the final annealing. By performing final annealing on a coil for manufacturing grain-oriented electrical steel sheets using the method according to the present invention, deformation of the coil is effectively prevented.

[0041] 4. Coil for final annealing In another embodiment, the present invention relates to a coil for final annealing of grain-oriented electrical steel sheets, characterized in that cold-rolled steel sheets located in the inner winding portion of the coil are joined. In the coil according to the present invention, the joining of the cold-rolled steel sheets may be performed before the final annealing, during the final annealing, or both. By performing final annealing on the coil according to the present invention, deformation of the coil is effectively prevented. [Examples]

[0042] A steel slab having a composition of C:0.07%, Si:3.4%, Mn:0.07%, Al:240ppm, S:60ppm, Se:150ppm, N:90ppm, Cu:0.1%, Sb:0.05%, Mo:0.01%, with the remainder being Fe and unavoidable impurities, was heated to 1400°C, then hot-rolled to a thickness of 2.4 mm, and then hot-rolled to a hot-rolled steel sheet by hot-rolling to annealing at 1030°C. The hot-rolled steel sheet was subjected to two cold-rolling processes with an intermediate annealing at 1070°C in between to obtain a cold-rolled steel sheet with a final thickness of 0.23 mm. Next, the cold-rolled steel sheet was heated in a humid hydrogen atmosphere at 840°C for 2 minutes to perform primary recrystallization annealing, which also served as decarburization annealing. After primary recrystallization annealing, a cold-rolled steel sheet was coated with an annealing release agent mainly composed of magnesium oxide, dried, and then wound onto a tension reel of a winding machine to form a coil. The inner diameter of the coil was changed to four levels—508 mm, 610 mm, 711 mm, and 813 mm—using a special tension reel equipped with an attachment that has a widening mechanism in the winding section.

[0043] In the coil winding process prior to the final annealing described above, a fiber laser welding machine was installed on top of the tension reel. After winding 10 layers of cold-rolled steel sheet coated with an annealing release agent, laser welding was performed around the circumference of the coil at a position 100 mm from the edge of the coil. The laser output was 8 kW, and the welding speed was 15 m / min, the same as the winding speed. After laser welding, the coil winding was continued without further laser irradiation. As a comparative example, a test without welding was also performed. The coil winding was carried out under the condition that the winding tension was kept constant throughout the entire length of the coil. For the obtained coils, the shape of the inner winding portion of the coil was evaluated at each stage during secondary recrystallization by final annealing and subsequent planarization annealing which also formed an insulating coating. In addition, samples for magnetic flux density measurement were taken from the inner and outer winding portions of the coil after planarization annealing, and the magnetic flux density B8 was measured. The obtained results are summarized in Table 2.

[0044] [Table 2]

[0045] In the comparative examples of No. 1, 3, 5, and 7 shown in Table 2, the shape of the inner winding portion of the coil was deformed in all cases. In comparative example No. 5, which had an inner diameter of 711 mm, a kink occurred after final annealing, and the kink caught on the mandrel of the payoff reel during planarization annealing, preventing insertion. Therefore, planarization annealing was omitted, and a sample was taken from the coil after final annealing to evaluate the magnetic flux density; the measured magnetic flux density is shown in parentheses as a reference value. In comparative example No. 7, which had an inner diameter of 813 mm, a large kink occurred during winding after primary recrystallization annealing, so the entire coil was discarded without performing final annealing.

[0046] On the other hand, in the invention examples No. 2, 4, 6, and 8, in which 10 layers of cold-rolled steel sheets located in the inner coil section were joined by laser welding, the shape of the inner coil section was good in all cases, and no manufacturing problems due to shape defects occurred. Although approximately 30m of the 10 welded layers were discarded, considering the cost of maintenance and disposal of the sleeves required when using steel sleeves to prevent coil deformation, and the amount of waste compared to the comparative examples No. 1 and 3, the present invention can be said to be superior to the conventional technology in terms of cost. In this embodiment, 10 layers of cold-rolled steel sheets were welded, but by combining it with the conventional technology, it is possible to reduce the number of layers to be welded and the amount of waste.

[0047] Furthermore, as shown in Table 2, it was confirmed that the magnetic flux density B8 in the inner diameter portion of the inventive example increased as the coil diameter increased, and the difference with the magnetic flux density in the outer diameter portion decreased. From these results, it can be seen that, according to the present invention, the magnetic properties can be improved by enlarging the inner diameter of the coil while reliably preventing deformation of the coil. [Explanation of symbols]

[0048] 1 Cold rolled steel plate 2 coils 2a Weld 3 laser beams 4 Grain-oriented electrical steel sheet

Claims

1. A method for manufacturing grain-oriented electrical steel sheets, comprising: hot rolling a steel slab to obtain a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet once or two or more times with an intermediate annealing in between to obtain a cold-rolled steel sheet; primary recrystallization annealing of the cold-rolled steel sheet; winding the cold-rolled steel sheet after primary recrystallization annealing to obtain a coil; and final annealing of the coil, A method for manufacturing grain-oriented electrical steel sheets, characterized by joining adjacent cold-rolled steel sheets located in the inner winding portion of the coil, at least one of the following: before the final annealing and during the final annealing.

2. A method for manufacturing grain-oriented electrical steel sheets according to claim 1, wherein, in the winding before the final annealing, the joining of adjacent cold-rolled steel sheets located in the inner winding portion is performed by welding.

3. The method for manufacturing grain-oriented electrical steel sheets according to claim 2, wherein the welding is laser welding.

4. In the winding process described above, an annealing release agent is applied only to the surface of the cold-rolled steel sheet located in the portion excluding the inner winding portion to form a coil. A method for manufacturing grain-oriented electrical steel sheets according to claim 1, wherein, during the final annealing, the joining of adjacent cold-rolled steel sheets located in the inner coil portion is performed by diffusion bonding.

5. The initial joining of adjacent cold-rolled steel sheets located in the inner winding portion is performed by welding during the winding process prior to the final annealing. A method for manufacturing grain-oriented electrical steel sheets according to claim 4, wherein the second joining of adjacent cold-rolled steel sheets located in the inner coil portion is performed by diffusion joining during the final annealing process.

6. A method for manufacturing grain-oriented electrical steel sheets according to any one of claims 1 to 5, wherein the joining is performed on three or more adjacent layers of cold-rolled steel sheets located in the inner winding portion.

7. A method for performing final annealing on a coil for manufacturing grain-oriented electrical steel sheets, A method characterized by joining adjacent cold-rolled steel sheets located in the inner winding portion of the coil, either before the final annealing or during the final annealing.

8. A coil before the final annealing process for manufacturing grain-oriented electrical steel sheets, A coil characterized in that adjacent cold-rolled steel sheets located in the inner winding portion of the coil are joined together.

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