Method for producing grain-oriented electrical steel sheet, method for performing final annealing of coil, and coil for final annealing

Abstract

The present invention reliably prevents the deformation of a coil in a production process for a grain-oriented electrical steel sheet, and improves the magnetic characteristics of the grain-oriented electrical steel sheet by expanding the inner diameter of the coil. This method for producing a grain-oriented electrical steel sheet comprises: subjecting a steel slab having a predetermined component composition to hot rolling to form a hot-rolled steel sheet; subjecting the hot-rolled steel sheet to a single iteration of cold rolling or two or more iterations of cold rolling with intervening intermediate annealing to obtain a cold-rolled steel sheet; subjecting the cold-rolled steel sheet to primary recrystallization annealing; winding the cold-rolled steel sheet after the primary recrystallization annealing to form a coil; and subjecting the coil to final annealing, wherein cold-rolled steel sheets positioned in the inner winding part of the coil are joined at at least one of the following times: immediately before the final annealing and during the final annealing.

Description

Method for manufacturing grain-oriented electrical steel sheet, method for subjecting coil to final annealing, and coil for final annealing

[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.

[0002] Grain-oriented electrical steel sheets are steel materials with improved magnetic properties in the rolling direction by utilizing a metallurgical phenomenon called secondary recrystallization to create a crystal texture in which iron crystal grains are oriented in the {110}<001> direction, known as the Goss orientation. Grain-oriented electrical steel sheets are typically manufactured by hot-rolling, hot-rolled annealing, and cold-rolling a steel slab adjusted to a predetermined composition, coiling the resulting cold-rolled steel sheet into a coil, and then sequentially subjecting the coil to a series of heat treatments consisting of primary recrystallization annealing, decarburization annealing, and final annealing. The final annealing requires heat treatment of the coil at a high temperature of 1000°C or higher. During this process, interdiffusion of metal atoms occurs at the contact surfaces of adjacent cold-rolled steel sheets, potentially resulting in diffusion bonding of the cold-rolled steel sheets. Therefore, in order to prevent diffusion bonding of cold-rolled steel sheets, it is common practice to apply an annealing separator, primarily composed of magnesium oxide powder, to the surface of the cold-rolled steel sheet when it is wound into a coil before final annealing. After final annealing is complete, the coil is set on a rotating shaft called a payoff reel, where it is paid out and returned to its steel sheet state. The steel sheet paid out from the payoff reel is then subjected to flattening annealing to correct any coil curl, resulting in the production of grain-oriented electrical steel sheet.

[0003] The shape of a coil can change during the heat treatment process. Two types of coil deformation are known: kinking and collapse. Kinking occurs when the steel sheet buckles near the center hole of the coil, causing multiple layers of the steel sheet to bend and protrude toward the center of the hole. Kinking is thought to occur because of large compressive stresses in the circumferential direction due to uneven temperature during coil heat treatment. Collapse occurs when the weight of the coil crushes the entire coil in a direction perpendicular to its central axis. The deformed portion of the coil cannot be used in the product, resulting in reduced yield. Furthermore, coil deformation reduces the diameter of the center hole, making it impossible to insert the coil into the payoff reel. Coil deformation is more likely to occur with larger inner diameters.

[0004] Several methods have been proposed to prevent the above-mentioned coil deformation. For example, Patent Document 1 proposes a method of preventing coil collapse by applying greater tension to the steel sheet on the inside of the coil and less tension to the outside of the coil when winding the coil. Patent Document 2 proposes a method of increasing the friction force between the coil layers by adding a coarse particle size powder to magnesium oxide applied to the coil surface as an annealing separator, thereby suppressing slippage between the steel sheets and preventing kinking. As other methods, Patent Document 3 proposes a method of inserting a steel sleeve into a center hole of the coil, and Patent Document 4 proposes a method of winding the steel sheet so that a thick, unrolled portion provided at the tip of the steel sheet is located near the center hole of the coil.

[0005] JP 62-70523 A JP 2012-177148 A JP 60-141828 A JP 61-124529 A

[0006] According to the methods proposed in Patent Documents 1 and 2, the deformation of the coil is prevented by suppressing the sliding between the steel sheets that make up the coil. However, because the frictional force between the layers of the coil is easily affected by the surface condition of the steel sheets, these methods were unable to completely prevent the deformation of the coil. The method using a steel sleeve described in Patent Document 3 had the disadvantage of high costs for the production and maintenance of the sleeve. The method using an unrolled portion described in Patent Document 4 had the problem of a significant decrease in yield.

[0007] From the viewpoint of the magnetic properties of grain-oriented electrical steel sheets, it is preferable that the inner diameter of the coil after final annealing is large. FIG. 1 is a schematic diagram showing the orientation of the <001> orientation of iron crystal grains in a coil 2 that has undergone final annealing. FIG. 1 shows a cross section perpendicular to the central axis of the coil 2. In the coil immediately after final annealing, crystals with Goss orientation preferentially grow, resulting in the <001> orientation indicated by the arrows being aligned in one direction throughout the entire interior of each crystal grain, as shown in FIG. 1( a). When the coil 2 that has undergone final annealing is unwound and plastically deformed into a flat grain-oriented electrical steel sheet 4, the angle between the <001> orientation indicated by the arrows and the rolling direction of the grain-oriented electrical steel sheet 4 varies depending on the position within each crystal grain, as shown in FIG. 1( b). The change in this angle becomes smaller as the inner diameter of the coil 2 after final annealing shown in FIG. 1( a) increases. However, as mentioned above, the larger the inner diameter of the coil, the more likely it is that the coil will deform, so the inner diameter of the coil cannot be made too large in order to prevent deformation.

[0008] The present 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.

[0009] The inventors came up with the idea of ​​directly joining steel sheets together, instead of the indirect methods proposed in Patent Documents 1 and 2, in order to reliably prevent slippage between steel sheets. As specific means for joining cold-rolled steel sheets, they investigated joining by welding and diffusion bonding of the steel sheets together during final annealing without intentionally applying an annealing separator. As a result of their investigation, they found that direct joining of steel sheets together can reliably prevent coil deformation, 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 a grain-oriented electrical steel sheet, comprising: hot rolling a steel slab to obtain a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet once or at least twice with intermediate annealing in between to obtain a cold-rolled steel sheet; subjecting the cold-rolled steel sheet to primary recrystallization annealing; coiling the cold-rolled steel sheet after the primary recrystallization annealing to obtain a coil; and subjecting the coil to final annealing, wherein the cold-rolled steel sheet located in the inner winding portion of the coil is joined at least either before the final annealing or during the final annealing.

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

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

[0013] [4] The method for producing a grain-oriented electrical steel sheet according to the above [1], wherein, during the coiling, an annealing separator is applied only to the surface of the cold-rolled steel sheet located in a portion excluding the inner winding portion to form a coil, and during the final annealing, the joining of the cold-rolled steel sheet located in the inner winding portion is performed by diffusion bonding.

[0014] [5] The method for producing a grain-oriented electrical steel sheet according to the above [4], wherein a first joining of the cold-rolled steel sheets located in the inner winding portion is performed by welding during the coiling before the final annealing, and a second joining of the cold-rolled steel sheets located in the inner winding portion is performed by the diffusion bonding during the final annealing.

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

[0016] [7] A method for performing final annealing on a coil for manufacturing a grain-oriented electrical steel sheet, the method comprising joining a cold-rolled steel sheet located on an inner winding portion of the coil at least either before or during the final annealing.

[0017] [8] A coil for final annealing for producing a grain-oriented electrical steel sheet, characterized in that a cold-rolled steel sheet located on an inner winding portion of the coil is joined.

[0018] According to the present invention, it is possible to reliably prevent deformation of the coil during the manufacturing process of grain-oriented electrical steel sheet by a simple means, thereby realizing a high manufacturing yield. Furthermore, since the inner diameter of the coil can be enlarged without deforming the coil, it is possible to improve the magnetic properties of the grain-oriented electrical steel sheet.

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

[0020] As described above, in the prior art, when indirect methods such as adjusting the tension applied to the steel sheet during coil winding or adjusting the particle size of the annealing separator applied to the surface of the steel sheet were adopted, there was a certain probability of slippage between the steel sheets, which inevitably resulted in kinking or crushing of the inner winding portion of the coil. Therefore, the inventors came up with the idea of ​​directly joining the steel sheets together instead of the indirect methods, in order to reliably prevent slippage between the steel sheets. Specific means for joining cold-rolled steel sheets include applying an annealing separator to the surface of the cold-rolled steel sheet before final annealing, drying it, and winding it, and then welding the cold-rolled steel sheet to a certain extent while it is wound around a tension reel before final annealing, or diffusion bonding the steel sheets together during final annealing without applying an annealing separator. It was thought that joining and integrating the cold-rolled steel sheets located in the inner diameter portion of the coil using these methods would not only prevent slippage between the steel sheets but also increase the apparent thickness, thereby increasing the strength of the inner winding portion of the coil.

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

[0022] 1. Experiment A cold-rolled steel sheet before final annealing was prepared by the above-described general manufacturing method from a steel slab having a predetermined chemical composition, through the processes up to decarburization annealing. The cold-rolled steel sheet had a thickness of 0.23 mm. Next, the cold-rolled steel sheet was wound onto a tension reel of a winding device to produce a coil. Five coil radii were set: 508 mm, 610 mm, 711 mm, 813 mm, and 914 mm. The inner diameter of the coil was changed using a special tension reel equipped with an attachment with a widening mechanism at the winding section. The tension applied to the cold-rolled steel sheet in the rolling direction during coiling was started at 50% of the tension at the end of winding and increased at a constant rate until the end. The coil winding speed was 20 m per minute at the start of winding, increased after approximately 100 m of winding, and then maintained at a constant speed after reaching the normal winding speed.

[0023] Regarding welding, which was one of the joining methods, an annealing separator was applied to the surface of the cold-rolled steel sheet, which was then dried and wound up 10 times. Laser welding was then performed for one revolution in the circumferential direction using a fixed fiber laser welder. The welding was performed at a position 150 mm from the edge of the coil. The output of the fiber laser welder was 10 kW, and the welding speed was 20 m per minute, the same as the coil winding speed. After one revolution of laser welding, an annealing separator was subsequently applied, and the coil was continued to be wound while drying. When the cross section was observed after joining by laser welding, it was found that the weld was formed across the entire 10 layers of cold-rolled steel sheet located in the inner diameter portion of the coil.

[0024] Among the joining methods, in the case of the method of not applying an annealing separator, the annealing separator was not applied to the cold-rolled steel sheet up to the 10th layer of coiling, and the annealing separator was applied to the surface of the cold-rolled steel sheet after that, dried, and wound into a coil. Coils joined by both the method of not applying an annealing separator and the method of laser welding were also produced. As a comparative material, a coil was also produced in which no joining was performed, but the annealing separator was applied and the coiling was performed while drying. Three coils were produced for each of these four conditions and five levels, and the number of coils that developed kinks when final annealing was performed is shown in Table 1.

[0025]

[0026] According to Table 1, without bonding, kinking occurred when the inner diameter of the coil was 711 mm or more, and the probability increased as the inner diameter increased. In contrast, with laser welding, no kinking occurred, and the shape of the inner wound portion was good even when the inner diameter of the coil was increased. It was also found that the probability of kinking was reduced compared to when no bonding was performed, even when no annealing separator was applied. Furthermore, in coils where kinking occurred, observation of the inner wound portion after final annealing suggested that kinking occurred at a low temperature, and then the steel sheets were diffusion bonded together in the high temperature range.

[0027] Even in the cases where annealing separator was not applied and laser welding was performed, no kinks occurred in any of the coils. Observation of the inner winding after final annealing revealed pressure marks along the weld. Because spatter was generated when welding coils without annealing separator, it was assumed that these pressure marks 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 applied with annealing separator and laser-welded only, spatter generation was suppressed compared to coils without annealing separator. This is likely due to the magnesium oxide, the main component of annealing separator, acting as a flux to prevent oxidation of the molten pool and stabilize the formation of the weld. Initially, it was thought that annealing separators would hinder the welding of cold-rolled steel sheets, but it is believed that not removing the annealing separator is advantageous in terms of suppressing spatter generation.

[0028] Next, an embodiment of the present invention will be described in detail.

[0029] 2. Manufacturing Method of Grain-Oriented Electrical Steel Sheet In one embodiment, the present invention is a manufacturing method of a grain-orientated electrical steel sheet, which comprises hot rolling a steel slab to form a hot-rolled steel sheet, cold rolling the hot-rolled steel sheet once or two or more times with intermediate annealing in between to form a cold-rolled steel sheet, subjecting the cold-rolled steel sheet to primary recrystallization annealing, coiling the cold-rolled steel sheet after the primary recrystallization annealing to form a coil, and subjecting the coil to final annealing, wherein joining of the cold-rolled steel sheet located at an inner winding portion of the coil is performed at least either before or during the final annealing.

[0030] The grain-oriented electrical steel sheet of the present invention is not particularly limited to any particular steel type, as long as it has a coating mainly composed of forsterite on the surface. Typically, such grain-oriented electrical steel sheets are produced by hot-rolling a silicon-containing steel slab produced in a blast furnace or electric furnace by a known method to obtain a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet once or twice or more times with intermediate annealing therebetween to obtain a cold-rolled steel sheet finished to a final thickness, subjecting the cold-rolled steel sheet to primary recrystallization annealing, coiling the cold-rolled steel sheet after primary recrystallization annealing to obtain a coil, and subjecting the coil to final annealing. The primary recrystallization annealing may also be annealing combined with decarburization annealing. The final annealing is annealing including secondary recrystallization annealing. Planarization annealing may be performed after the final annealing.

[0031] In the present invention, the cold-rolled steel sheets located in the inner winding portion of the coil are joined at least either before or during the final annealing in the above series of steps. The inner winding portion of the coil refers to the inner peripheral portion of the coil where the final annealing is performed. The scope of the inner winding portion of the coil is not particularly limited, but it preferably includes the innermost cold-rolled steel sheet of the coil and preferably includes at least two layers of the inner winding portion from there toward the outer periphery. In the present invention, "joining of 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 to each other and do not slip. This makes the inner winding portion of the coil less susceptible to buckling and prevents the occurrence of kinks. Furthermore, the joining integrates the cold-rolled steel sheets located in the inner winding portion, increasing the apparent thickness, thereby improving the strength of the inner winding portion of the coil. This prevents the occurrence of coil collapse.

[0032] In a preferred embodiment of the present invention, three or more layers of cold-rolled steel sheets located in the inner winding portion are joined. Joining three or more layers of cold-rolled steel sheets further improves the interlayer bonding strength, making it possible to increase the inner diameter of the coil and more reliably suppress the occurrence of kinking. The cold-rolled steel sheets do not need to be joined over the entire surface as long as they are joined so that the steel sheets do not slip, and the joint may be strip-shaped or linear. Furthermore, the joint does not need to be continuous along the circumferential or width direction of the coil, and the joint may have interruptions along the way. It is preferable that the number of layers of cold-rolled steel sheets to be joined is 20 or less, as this reduces the amount of waste in the inner winding portion and improves product yield. A more preferred number of layers is 10 or less.

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

[0034] Welding offers greater freedom in designing the joining position than diffusion bonding, which will be described later. When a cold-rolled steel sheet is wound around a tension reel to form a coil, welding can be performed at a position close to the end of the coil to more reliably prevent the steel sheets from slipping apart. The welding position may be one position in the width direction of the coil, but two or more positions are preferred because the joining is more stable. Instead of welding one circumference along the circumferential direction of the coil, welding may be performed three times, for example, by changing the position, for each third of the circumference. The thickness of the weld formed by welding may be the thickness including the entire cold-rolled steel sheet to be welded, or may be a portion thereof. When welding a portion of the cold-rolled steel sheet in the thickness direction, the entire cold-rolled steel sheet may be welded at multiple positions while gradually changing the welding position.

[0035] Arc welding such as TIG welding can be used as a welding method, but laser welding is more preferred. Laser welding is preferable because it does not require the welding rod to be close to the workpiece as in arc welding and is less susceptible to external disturbances. FIG. 2 is a schematic diagram showing an example of a method for joining cold-rolled steel sheets by laser welding. During winding of a cold-rolled steel sheet 1, a laser beam 3 is irradiated onto the surface of the wound coil 2 to weld the cold-rolled steel sheet, forming a weld 2a along the circumferential direction of the coil 2. The laser output when performing laser welding is not particularly limited. For example, in the case of a grain-oriented electrical steel sheet having a thickness of 0.23 mm and coated with an annealing separator, the output is preferably 2 kW or more and 20 kW or less. Spatter generated during welding can cause pressure scratches on the surface of the cold-rolled steel sheet during final annealing, so welding is preferably performed under conditions that minimize spatter generation. As described above, applying an annealing separator is effective in suppressing spatter generation. In the case of fiber laser welding, the occurrence of spatter can be suppressed by applying a condition called ring mode, in which the peripheral energy of the laser beam is high.

[0036] [Joining by Diffusion Bonding] In a preferred embodiment of the present invention, an annealing separator is applied only to the surface of the cold-rolled steel sheet located in the portion excluding the inner winding portion during coiling to form a coil, and the cold-rolled steel sheet located in the inner winding portion is joined by diffusion bonding during final annealing. The annealing separator is applied to the surface of the cold-rolled steel sheet for the purpose of preventing diffusion bonding of the cold-rolled steel sheets to each other during final annealing of the coil. By not applying the annealing separator to the inner winding portion of the coil but to the portion other than the inner winding portion, the cold-rolled steel sheets located in the inner winding portion where the annealing separator is not applied can be joined by diffusion bonding or fusion, and the action of the annealing separator can prevent joining of the cold-rolled steel sheets located in the portion other than the inner winding portion. Bonding by diffusion bonding does not require a welding machine required for welding, and can be easily performed using existing equipment.

[0037] The annealing separator used in the present invention preferably contains magnesium oxide as a main component. That is, the annealing separator contains 50 mass% or more of magnesium oxide. If the magnesium oxide content is less than 50 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 separator becomes insufficient. The proportion of magnesium oxide contained in the annealing separator is preferably 60 mass% or more, more preferably 80 mass% or more. The annealing separator may contain reaction aids such as titanium oxide and strontium hydroxide as components other than magnesium oxide, and may be mixed with other components to form a liquid or slurry annealing separator, which can be applied to a coil. The liquid or slurry annealing separator applied to the surface of a cold-rolled steel sheet is dried, and the cold-rolled steel sheet is then wound into a coil, thereby realizing a state in which the annealing separator is present between the layers of the coil.

[0038] It is believed that diffusion bonding progresses when the coil is heated to 900°C or higher during final annealing. For this reason, diffusion bonding is only marginally effective 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 diffusion bonding with a conventional technique such as adjusting the tension when winding the coil.

[0039] In the present invention, joining by diffusion bonding may be performed in combination 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 winding portion is performed by welding during coiling before final annealing, and the second joining of the cold-rolled steel sheets located in the inner winding portion is performed by diffusion bonding during final annealing. By combining the two types of joining, it is possible to more reliably prevent coil deformation.

[0040] 3. Method for subjecting a coil to final annealing In another embodiment, the present invention is a method for subjecting a coil for producing a grain-oriented electrical steel sheet to final annealing, characterized in that a cold-rolled steel sheet located on an inner winding portion of the coil is joined at least either before or during the final annealing. By subjecting a coil for producing a grain-oriented electrical steel sheet to final annealing 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 for producing grain-oriented electrical steel sheet, characterized in that the cold-rolled steel sheet located on the inner winding portion of the coil is joined. In the coil according to the present invention, the joining of the cold-rolled steel sheet may be performed before the final annealing, during the final annealing, or both. By subjecting the coil according to the present invention to final annealing, deformation of the coil is effectively prevented.

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

[0043] During coil winding before the final annealing, a fiber laser welder was installed above the tension reel, and 10 layers of cold-rolled steel sheet coated with an annealing separator were wound. After that, laser welding was performed around the coil in the circumferential direction 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 continued to be wound without laser irradiation. As a comparative example, a condition in which no welding was performed was also performed. The coil was wound under a condition in which the winding tension was constant over the entire length of the coil. The shape of the inner winding portion of the obtained coil was evaluated at each stage of secondary recrystallization by final annealing and subsequent flattening annealing, which also served to form an insulating coating. Furthermore, samples for measuring magnetic flux density were taken from the inner winding portion and the outer winding portion of the coil after flattening annealing, and the magnetic flux density B ８ The results are summarized in Table 2.

[0044]

[0045] In Comparative Examples No. 1, 3, 5, and 7 shown in Table 2, the shape of the inner winding portion of the coil was deformed. In Comparative Example No. 5, in which the inner diameter of the coil was 711 mm, a kink occurred after final annealing, and the kink got caught on the mandrel of the payoff reel during flattening annealing, making it impossible to insert. For this reason, flattening annealing was omitted, and samples were taken from the coil after final annealing to evaluate the magnetic flux density, so the measured magnetic flux density values ​​are shown in parentheses as reference values. In addition, in Comparative Example No. 7, in which the inner diameter of the coil was 813 mm, a large kink occurred during winding after primary recrystallization annealing, so the entire coil was discarded without undergoing final annealing.

[0046] On the other hand, in Examples Nos. 2, 4, 6, and 8, in which ten layers of cold-rolled steel sheets located in the inner winding portion were joined by laser welding, the shape of the inner winding portion was good in all cases, and no manufacturing problems due to poor shape occurred. Although approximately 30 m of the welded ten layers was discarded, considering the cost of sleeve maintenance required when a steel sleeve is used to prevent coil deformation and the cost of disposal after use, and the fact that the amount of discarded material was smaller than the amount of discarded material in Comparative Examples Nos. 1 and 3, the present invention can be said to be superior in terms of cost to the prior art. Note that, although ten layers of cold-rolled steel sheets were welded in these Examples, it is possible to reduce the number of layers to be welded and the amount of discarded material by combining this with the prior art.

[0047] Also, according to Table 2, the magnetic flux density B ８ It was confirmed that the magnetic flux density at the inner diameter increases as the coil diameter increases, and the difference with the magnetic flux density at the outer diameter decreases. From these results, it can be seen that the magnetic properties can be improved by enlarging the inner diameter of the coil while reliably preventing deformation of the coil.

[0048] 1 Cold-rolled steel sheet 2 Coil 2a Welded portion 3 Laser beam 4 Grain-oriented electrical steel sheet

Claims

1. A method for manufacturing a grain-oriented electrical steel sheet, which comprises hot rolling a steel slab to form a hot-rolled steel sheet, cold rolling the hot-rolled steel sheet once or two or more times with intermediate annealing between them to form a cold-rolled steel sheet, subjecting the cold-rolled steel sheet to primary recrystallization annealing, coiling the cold-rolled steel sheet after the primary recrystallization annealing to form a coil, and subjecting the coil to final annealing, characterized in that the cold-rolled steel sheet located on the inner winding portion of the coil is joined at least either before the final annealing or during the final annealing.

2. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein the joining of the cold-rolled steel sheet located in the inner winding portion is performed by welding during the coiling before the final annealing.

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

4. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein, during the coiling, an annealing separator 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 joining of the cold-rolled steel sheet located in the inner winding portion is performed by diffusion bonding.

5. The method for producing grain-oriented electrical steel sheet according to claim 4, wherein the cold-rolled steel sheet located in the inner winding portion is first joined by welding during the coiling before the final annealing, and the cold-rolled steel sheet located in the inner winding portion is second joined by diffusion bonding during the final annealing.

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

7. A method for subjecting a coil to final annealing for manufacturing grain-oriented electrical steel sheet, characterized in that the cold-rolled steel sheet located on the inner winding portion of the coil is joined at least either before or during the final annealing.

8. A coil for final annealing in the manufacture of grain-oriented electrical steel sheet, characterized in that a cold-rolled steel sheet located on the inner winding portion of the coil is joined.

Images