Electrical steel strip joining and friction stir welding methods, and methods for manufacturing electrical steel strips.

Abstract

Invention details;

Description

Welded joint of electromagnetic steel strip, friction stir welding method, and manufacturing method of electromagnetic steel strip

[0001] The present invention relates to a welded joint of an electromagnetic steel strip, a friction stir welding method, and a method for manufacturing an electromagnetic steel strip.

[0002] In steel sheet production lines, such as those for pickling, cold rolling, annealing, and plating, it is common to perform so-called coil joining before threading steel strips in order to improve productivity and increase yield. Herein, coil joining refers to joining the end (rear end) of a preceding steel strip (hereinafter also referred to as a preceding steel strip) to the end (front end) of a steel strip following the preceding steel strip (hereinafter also referred to as a following steel strip). Hereinafter, the joint formed by coil joining is also referred to as a coil joint. The front end refers to the end of the steel strip in the direction of travel in the production line. The rear end refers to the end of the steel strip opposite the direction of travel in the production line. This coil joining allows the steel strip to be rolled, etc., while being tensioned over its entire length. Furthermore, it also enables the thickness and shape of the steel strip to be controlled with high precision, even at the front and rear ends.

[0003] Conventionally, flash butt welding and other methods have been commonly used for coil joining. However, with advances in laser welding machines, the use of laser welding for coil joining is becoming mainstream, even in production lines for electrical steel sheets, stainless steel sheets, and high-tensile steel sheets.

[0004] For example, Patent Document 1 discloses such a technique as follows: "A laser welding method for high-Si steel, characterized in that when welding high-Si steel, a filler wire containing Ni as the main component is used, or a powder filler containing Ni as the main component is supplied, and welding is performed so that the chemical composition of the weld metal satisfies the following formula (1): X = [%Ni] - [%Si] x 2.5 - ([%Cr] + [%Mo]) x 0.4 ≥ 0 (1) where [%Ni], [%Si], [%Cr], and [%Mo] represent the contents (by weight) of Ni, Si, Cr, and Mo in the weld metal, respectively."

[0005] Patent Document 2 discloses "a laser welding method in which a leading sheet and a trailing sheet are butted together and laser welded using a filler wire, characterized in that the ratio (Gap / DEPO) of the butt gap (Gap) between the leading sheet and the trailing sheet at the beginning of welding to the average width (DEPO) of the weld metal is 0.3 to 0.8."

[0006] Patent Document 3 discloses the following: "A weld formed by laser welding a preceding thin plate and a following thin plate made of special steel transported on a continuous cold rolling line, wherein, when L1 is the minimum thickness of the base material present below an upper extension made of weld metal that has extended to the top surface of the base material due to cold rolling, and L2 is the minimum thickness of the base material sandwiched between the lower extension made of weld metal that has extended to the bottom surface of the base material due to cold rolling and the upper extension, at least one of L1 and L2 is greater than zero."

[0007] Japanese Patent Application Laid-Open No. 5-305466 Japanese Patent Application Laid-Open No. 2004-25284 Japanese Patent Application Laid-Open No. 2011-140026 Japanese Patent Publication No. 07-505090 Japanese Patent No. 3261433 Japanese Patent No. 4838385 Japanese Patent No. 4838388

[0008] Cui, L.; Fujii, H.; Tsuji, N.; Nogi, K. Scripta Mater. 2007, 56, p.637-640.

[0009] However, because laser welding is a fusion welding process, embrittlement can occur due to the segregation of impurities during melting and solidification, as well as due to hydrogen penetration. This can result in deterioration of the mechanical properties of the joint (weld). In particular, because the composition of electrical steel sheets contains a large amount of Si, the mechanical properties of the coil joint can easily deteriorate significantly. Therefore, when laser welding is applied to coil joints of electrical steel strips, as in Patent Documents 1 to 3, there is a problem that fractures can occur at the coil joint in a production line, for example, a continuous cold rolling line, resulting in reduced productivity due to line stoppages and other factors.

[0010] The present invention was developed to solve the above problems, and aims to provide a welded joint of an electromagnetic steel strip that can prevent the occurrence of fractures of the coil joint on a production line due to deterioration of the mechanical properties or shape of the coil joint. Another aim of the present invention is to provide a method for friction stir welding of an electromagnetic steel strip for obtaining the above welded joint of an electromagnetic steel strip. A further aim of the present invention is to provide a method for manufacturing an electromagnetic steel strip using the above method for friction stir welding of an electromagnetic steel strip.

[0011] The inventors have conducted extensive research to achieve the above-mentioned object. First, the inventors investigated and examined the reasons why deterioration in the mechanical properties and shape of the coil joint occurs when laser welding is applied to join coils of electromagnetic steel strips, and have arrived at the following findings.

[0012] (a) As described above, the composition of electrical steel sheets contains a large amount of Si, for example, approximately 2.0 to 5.0 mass %. Si is a ferrite-stabilizing element. Therefore, when conventional laser welding is applied to joining coils of electrical steel strips, the ferrite grains at the fusion zone (the coil joint) and also at the heat-affected zone (the heat-affected zone) become coarse. This significantly deteriorates the mechanical properties of the coil joint, particularly the toughness and bending strength, leading to fractures of the coil joint during production.

[0013] (b) Furthermore, the techniques described in Patent Documents 1 to 3 use a filler metal (filler) primarily composed of Ni, an austenite-stabilizing element. Therefore, the coil joint is primarily formed of an austenite phase. However, the techniques described in Patent Documents 1 to 3 require strict control of the fusion amounts of the filler metal and the steel sheet in the fusion zone (weld metal) by eliminating fluctuations in the butt gap between the preceding and following steel strips, and constantly and appropriately controlling the balance between the Ni equivalent and the Cr equivalent in the fusion zone (weld metal). In other words, if the balance between the Ni equivalent and the Cr equivalent in the fusion zone (weld metal) is not appropriately controlled, a martensite phase, which is a hard and brittle structure, will form in the coil joint. This significantly deteriorates the mechanical properties of the coil joint, particularly its toughness. Furthermore, in the heat-affected zone, the mechanical properties of the coil joint are significantly deteriorated due to the coarsening of ferrite grains. These reasons can lead to fractures in the coil joint during production.

[0014] (c) Furthermore, the above-mentioned fluctuation in the butt gap between the preceding and following steel strips affects the height of the weld. For example, if the weld height becomes too high and the weld becomes excessively convex, stress will be concentrated at the weld toe when a load is applied to the weld. Therefore, the above-mentioned fluctuation in the butt gap between the preceding and following steel strips can also cause breakage of the coil joint on the production line. The weld can be removed by grinding or other methods. However, such an increase in the number of processes significantly reduces productivity.

[0015] Based on the above, the inventors conducted further research and came up with the idea of ​​applying friction stir welding to coil welding of electromagnetic steel strips. Here, friction stir welding is a solid-state welding process that utilizes the frictional heat between a rotating tool and workpieces and the plastic flow of the workpieces. Specifically, the rotating tool frictionally stirs the unwelded portion (the area to be welded) of the workpieces. When the unwelded portion of the workpieces is heated by frictional heat, plastic flow begins. The interface between the plastic flow region and the base material is then significantly elongated. This brings clean, oxide-free interfaces into contact, forming a weld without melting the workpieces. The weld zone is a region that undergoes hot working due to the frictional heat and plastic flow between the rotating tool and workpieces, resulting in a recrystallized structure. It is sometimes referred to as a stir zone. Furthermore, a region adjacent to the weld zone is affected by the frictional heat and hot working due to plastic flow, but the temperature and processing are insufficient to achieve recrystallization. This region is called the thermal processing affected zone. Furthermore, there is a region in the workpiece that is not affected by hot working due to frictional heat and plastic flow. This region is called the base material. Techniques related to friction stir welding are disclosed in, for example, Patent Documents 4 to 7 and Non-Patent Document 1, but none of these are applicable to coil joining of electromagnetic steel strips.

[0016] Based on the above idea, the inventors conducted further research and discovered the following: (d) To effectively solve the problems (a) to (c) above, friction stir welding is used to join coils of electromagnetic steel strips. Furthermore, it is important to make the steel structure of the joint and the thermal processing affected zone a ferrite-based structure, and to simultaneously refine the steel structure of the joint and the thermal processing affected zone and reduce the difference in hardness between the joint and the base material. Specifically, it is important to simultaneously satisfy the relationships of the following equations (1) to (4). This improves the mechanical properties of the coil joint without deteriorating the shape of the coil joint, even when an electromagnetic steel strip is used as the material to be joined, and effectively prevents breakage of the coil joint on the production line. Dsz≦200 μm (1) Dhaz1 ≦Dbm1 (2) Dhaz2 ≦Dbm2 (3) 0.9×(Hbm1+Hbm2) / 2≦Hsz≦1.2×(Hbm1+Hbm2) / 2 (4) where, Dsz is the average value (μm) of the ferrite grain size in the joint, Dhaz1 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the first electromagnetic steel band side, Dhaz2 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the second electromagnetic steel band side, Dbm1 is the average value (μm) of the ferrite grain size in the base material part of the first electromagnetic steel band, Dbm2 is the average value (μm) of the ferrite grain size in the base material part of the second electromagnetic steel band, Hsz is the average value of the hardness of the joint, Hbm1 is the average value of the hardness of the base material portion of the first electromagnetic steel strip, and Hbm2 is the average value of the hardness of the base material portion of the second electromagnetic steel strip.

[0017] (e) Furthermore, it is preferable to satisfy the relationships of the following equations (5) and (6) while simultaneously satisfying the relationships of the above equations (1) to (4). This improves the mechanical properties of the coil joint without deteriorating the shape of the coil joint, even when an electromagnetic steel strip is used as the joined material, and more effectively prevents breakage of the coil joint on the production line. 0.8 × TbmL ≦ TszL (5) TszH ≦ 1.3 × TbmH (6) where, TszL is the minimum thickness of the joint (mm), TszH is the maximum thickness of the joint (mm), TbmL is the thickness of the thinner of the first and second electromagnetic steel strips (mm), and TbmH is the thickness of the thicker of the first and second electromagnetic steel strips (mm). However, when the sheet thickness of the first electromagnetic steel strip and the second electromagnetic steel strip are the same, TbmL = TbmH.

[0018] (f) Furthermore, from the viewpoint of increasing the joining speed while suppressing the occurrence of defects, it is preferable to apply so-called double-sided friction stir welding as the friction stir welding.

[0019] (g) In addition, when the sheet thicknesses of the electromagnetic steel strips to be welded are different from each other, it is preferable to: apply the above-mentioned double-sided friction stir welding as the joining method; and then perform joining using a rotary tool whose tip face has a central portion and an outer peripheral portion disposed adjacent to the periphery of the central portion, the outer peripheral portion having a tapered shape. This makes it possible to increase the joining speed while preventing breakage of the coil joint on the production line due to deterioration in the mechanical properties and shape of the coil joint, even when the sheet thicknesses of the electromagnetic steel strips to be welded are different from each other.

[0020] (h) The diameter D of the center of the rotary tool １ (mm) satisfies the relationship of the following formula (10), and the joining conditions are appropriately controlled, particularly the rotation speed RS (revolutions / min) of the rotary tool, the diameter D of the center of the rotary tool, １ (mm) and welding speed JS (mm / min) １ ３It is preferable that the relationship of the following formula (11) be satisfied for 4×TJ / JS. This makes it possible to increase the joining speed while more advantageously preventing breakage of the coil joint portion on the production line due to deterioration of the mechanical properties and shape of the coil joint portion, even when the sheet thicknesses of the electromagnetic steel strips to be joined are different from each other. 　 ≦ 　 D １　 ≦ 　 10 x TJ ... (10) 200 x TJ 　 ≦ 　 RS×D １ ３ / JS 　 ≦ 　 2000×TJ (11) where TJ is When the unjoined portion is a butt portion, it is the average value (mm) of the sheet thickness of the first electromagnetic steel strip and the sheet thickness of the second electromagnetic steel strip, When the unjoined portion is an overlap portion, it is the thickness (mm) of the overlap portion. The present invention was completed based on the above findings and further investigation.

[0021] That is, the gist of the present invention is as follows: 1. A welded joint of electromagnetic steel bands that joins a first electromagnetic steel band and a second electromagnetic steel band, the welded joint of electromagnetic steel bands having a welded portion and a thermal processing affected zone adjacent to the welded portion, the steel structures of the welded portion and the thermal processing affected zone each being a structure mainly composed of ferrite phase, and satisfying the relationships of the following formulas (1) to (4): Dsz≦200 μm (1) Dhaz1 ≦Dbm1 (2) Dhaz2 ≦Dbm2 (3) 0.9×(Hbm1+Hbm2) / 2≦Hsz≦1.2×(Hbm1+Hbm2) / 2 (4) where, Dsz is the average value (μm) of the ferrite grain size in the joint, Dhaz1 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the first electromagnetic steel band side, Dhaz2 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the second electromagnetic steel band side, Dbm1 is the average value (μm) of the ferrite grain size in the base material part of the first electromagnetic steel band, Dbm2 is the average value (μm) of the ferrite grain size in the base material part of the second electromagnetic steel band, Hsz is the average value of the hardness of the joint, Hbm1 is the average value of the hardness of the base material portion of the first electromagnetic steel strip, and Hbm2 is the average value of the hardness of the base material portion of the second electromagnetic steel strip.

[0022] 2. A welded joint of electromagnetic steel strips according to item 1 above, which satisfies the relationships of the following equations (5) and (6): 0.8 x TbmL ≦ TszL (5) TszH ≦ 1.3 x TbmH (6) where, TszL is the minimum thickness of the weld (mm), TszH is the maximum thickness of the weld (mm), TbmL is the thickness of the thinner of the first and second electromagnetic steel strips (mm), and TbmH is the thickness of the thicker of the first and second electromagnetic steel strips (mm). However, if the thicknesses of the first and second electromagnetic steel strips are the same, then TbmL = TbmH.

[0023] 3. The joined joint of electromagnetic steel strips according to 1 or 2 above, wherein the thickness of the first electromagnetic steel strip and the thickness of the second electromagnetic steel strip are different.

[0024] 4. A method for friction stir welding of electromagnetic steel bands, joining a first electromagnetic steel band to a second electromagnetic steel band subsequent to the first electromagnetic steel band, comprising: pressing a rotating tool against the butt joint or overlapping unjoined portion where an end of the first electromagnetic steel band and an end of the second electromagnetic steel band are joined, while rotating the rotating tool; and then moving the rotating tool in the joining direction to join the first electromagnetic steel band to the second electromagnetic steel band; and performing the joining under conditions such that the steel structures of the joint and the thermal processing affected zone formed by joining the first electromagnetic steel band to the second electromagnetic steel band each have a structure mainly composed of ferrite phase, and satisfying the relationships of the following equations (1) to (4). Dsz≦200 μm (1) Dhaz1 ≦Dbm1 (2) Dhaz2 ≦Dbm2 (3) 0.9×(Hbm1+Hbm2) / 2≦Hsz≦1.2×(Hbm1+Hbm2) / 2 (4) where, Dsz is the average value (μm) of the ferrite grain size in the joint, Dhaz1 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the first electromagnetic steel band side, Dhaz2 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the second electromagnetic steel band side, Dbm1 is the average value (μm) of the ferrite grain size in the base material part of the first electromagnetic steel band, Dbm2 is the average value (μm) of the ferrite grain size in the base material part of the second electromagnetic steel band, Hsz is the average value of the hardness of the joint, Hbm1 is the average value of the hardness of the base material portion of the first electromagnetic steel strip, and Hbm2 is the average value of the hardness of the base material portion of the second electromagnetic steel strip.

[0025] 5. The friction stir welding method for electromagnetic steel strips according to 4 above, wherein joining is performed under conditions that satisfy the relationships of the following equations (5) and (6): 0.8 × TbmL ≦ TszL (5) TszH ≦ 1.3 × TbmH (6) where, TszL is the minimum thickness of the weld (mm), TszH is the maximum thickness of the weld (mm), TbmL is the thickness of the thinner of the first and second electromagnetic steel strips (mm), and TbmH is the thickness of the thicker of the first and second electromagnetic steel strips (mm). However, if the thicknesses of the first and second electromagnetic steel strips are the same, then TbmL = TbmH.

[0026] 6. The method for friction stir welding of electromagnetic steel strips according to 4 or 5, wherein the rotating tools are a pair of rotating tools facing each other, and the rotating tools press against both sides of the unjoined portion while rotating in opposite directions, the sheet thickness of the first electromagnetic steel strip and the sheet thickness of the second electromagnetic steel strip are different, the tip surface of the rotating tool has a central portion and an outer periphery disposed adjacent to the periphery of the central portion, and the outer periphery has a tapered shape.

[0027] 7. The method for friction stir welding of electromagnetic steel strips according to 6 above, wherein the taper angle α of the outer circumferential portion is 2 to 20°.

[0028] 8. The method for friction stir welding of an electromagnetic steel strip according to 6 or 7, wherein the circumferential position of the boundary between the central portion and the outer periphery is in the range of 0.15 × D to 0.35 × D. Here, the circumferential position refers to the distance from the rotation axis in the circumferential direction of the tip surface of the rotary tool, and D is the diameter of the tip surface of the rotary tool.

[0029] 9. The method for friction stir welding of electromagnetic steel strips according to any one of 6 to 8, wherein the outer circumferential portion has a spiral-shaped step portion on its surface in the direction of rotation of the rotary tool.

[0030] 10. The method for friction stir welding of an electromagnetic steel strip according to any one of 6 to 9 above, wherein the outer circumferential portion has a spiral-shaped step portion on its surface in the direction opposite to the rotation of the rotary tool.

[0031] 11. The method for friction stir welding of electromagnetic steel strips according to any one of 6 to 10 above, wherein the central portion is a flat surface, a convex curved surface, or a concave curved surface.

[0032] 12. The method for friction stir welding of electromagnetic steel strips according to any one of 6 to 11 above, wherein the central portion has a spiral-shaped step portion on its surface in the direction opposite to the rotation of the rotary tool.

[0033] 13. Diameter D of the center of the rotary tool １ (mm) satisfies the relationship of the following formula (10), and １ (mm) and welding speed JS (mm / min) １ ３13. The method for friction stir welding of an electromagnetic steel strip according to any one of 6 to 12 above, wherein 4×TJ / JS satisfies the relationship of the following formula (11): 　 ≦ 　 D １　 ≦ 　 10 x TJ ... (10) 200 x TJ 　 ≦ 　 RS×D １ ３ / JS 　 ≦ 　 2000×TJ (11) Here, TJ is the average value (mm) of the thickness of the first electromagnetic steel strip and the thickness of the second electromagnetic steel strip when the unjoined portion is a butt joint, and is the thickness (mm) of the overlapping portion when the unjoined portion is an overlapping portion.

[0034] 14. The method for friction stir joining electromagnetic steel strips according to any one of items 6 to 13, wherein a step exists on only one surface at the butt joint between the end of the first electromagnetic steel strip and the end of the second electromagnetic steel strip, the advancing side of the rotating tool placed on the surface with the step is the side of the electromagnetic steel strip with the smaller sheet thickness, of the first electromagnetic steel strip and the second electromagnetic steel strip, and the retreating side of the rotating tool placed on the surface with the step is the side of the electromagnetic steel strip with the larger sheet thickness, of the first electromagnetic steel strip and the second electromagnetic steel strip.

[0035] 15. A method for manufacturing an electromagnetic steel strip, comprising: a step of joining a first electromagnetic steel strip and a second electromagnetic steel strip by the method for friction stir joining an electromagnetic steel strip according to any one of 4 to 14 above to obtain a joined steel strip; and a step of cold rolling the joined steel strip to obtain a cold-rolled steel strip.

[0036] According to the present invention, even when electromagnetic steel strips are used as the joined materials, deterioration of the mechanical properties and shape of the coil joint does not occur, and fractures of the coil joint on the production line are effectively prevented. This makes it possible to further improve the productivity of electromagnetic steel sheets and has great industrial value. Furthermore, even when electromagnetic steel strips of different thicknesses are used as the joined materials, the joining speed can be increased while suppressing the occurrence of defects, which is extremely advantageous in terms of construction efficiency.

[0037] FIG. 2A is a schematic diagram showing an example of a welded joint of an electromagnetic steel strip according to one embodiment of the present invention. FIG. 2B is a schematic diagram illustrating a method for friction stir welding of an electromagnetic steel strip according to one embodiment of the present invention, and is a side perspective view showing an example of a butt joint by the single-sided friction stir welding method. FIG. 2A is a schematic diagram illustrating a method for friction stir welding of an electromagnetic steel strip according to one embodiment of the present invention, and is a side perspective view showing an example of a lap joint by the single-sided friction stir welding method. FIG. 3A is a schematic diagram illustrating a method for friction stir welding of an electromagnetic steel strip according to one embodiment of the present invention, and is a side perspective view showing an example of a butt joint by the double-sided friction stir welding method. FIG. 3B is a schematic diagram illustrating a method for friction stir welding of an electromagnetic steel strip according to one embodiment of the present invention, and is a view showing an example of a butt joint by the double-sided friction stir welding method. FIG. 3C is a schematic diagram illustrating an example of a shape of a rotary tool used in the single-sided friction stir welding method. FIG. 3D is a schematic diagram illustrating an example of a shape of a rotary tool used in the single-sided friction stir welding method. FIG. 3E is a schematic diagram illustrating an example of a shape of a rotary tool used in the single-sided friction stir welding method. FIG. 1 is a schematic diagram showing an example of the shape of a rotary tool used in a double-sided friction stir welding method. FIG. 2 is a schematic diagram showing an example of the shape of a rotary tool used in a double-sided friction stir welding method. FIG. 3 is a schematic diagram showing an example of the shape of a rotary tool used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 4 is a schematic diagram showing an example of the shape of a rotary tool used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 5 is a schematic diagram showing an example of the shape of a rotary tool (flat center rotary tool) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 6 is a schematic diagram showing an example of the shape of a rotary tool (flat center rotary tool) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 7 is a schematic diagram showing an example of the shape of a rotary tool (flat center rotary tool) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention.FIG. 1 is a schematic diagram showing an example of the shape of a rotary tool (centrally convex rotary tool) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of the shape of a rotary tool (centrally convex rotary tool) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 3 is a schematic diagram showing an example of the shape of a rotary tool (centrally concave rotary tool) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 4 is a schematic diagram showing an example of the shape of a rotary tool (centrally concave rotary tool) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 5 is a schematic diagram showing an example of the shape of a rotary tool (centrally flat rotary tool with a recessed step portion provided in the center) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 6 is a schematic diagram showing an example of the shape of a rotary tool (centrally flat rotary tool with recessed step portions provided in the center and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of the shape of a rotary tool (a centrally flat rotary tool provided with recessed step portions at the central and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of the shape of a rotary tool (a centrally convex rotary tool provided with recessed step portions at the central and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 3 is a schematic diagram showing an example of the shape of a rotary tool (a centrally convex rotary tool provided with recessed step portions at the central and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 4 is a schematic diagram showing an example of the shape of a rotary tool (a centrally convex rotary tool provided with recessed step portions at the central and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 5 is a schematic diagram showing an example of the shape of a rotary tool (a centrally concave rotary tool provided with recessed step portions at the central and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention.FIG. 1 is a schematic diagram showing an example of the shape of a rotary tool (a centrally concave rotary tool provided with concave-shaped step portions at the central and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of the shape of a rotary tool (a centrally convex rotary tool provided with a stepped step portion at the central portion) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 3 is a schematic diagram showing an example of the shape of a rotary tool (a centrally convex rotary tool provided with a stepped step portion at the central and outer periphery) used in a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention. FIG. 4 is a schematic diagram for explaining a method for friction stir welding of an electromagnetic steel strip according to an embodiment of the present invention, showing an example of butt welding by a double-sided friction stir welding method.

[0038] The present invention will be described based on the following embodiments. [1] Joint of electromagnetic steel strips First, a joint of electromagnetic steel strips according to one embodiment of the present invention will be described with reference to FIG. 1A. FIG. 1A is a cross-sectional view in the thickness direction of a joint of electromagnetic steel strips, in which the thickness t1 of the first electromagnetic steel strip is equal to the thickness t2 of the second electromagnetic steel strip. In the figure, reference numeral 1 denotes the first electromagnetic steel strip (workpiece to be joined), 2 denotes the second electromagnetic steel strip (workpiece to be joined), 4 denotes the joint, 4-1 denotes the thermal processing affected zone (first electromagnetic steel strip side), and 4-2 denotes the thermal processing affected zone (second electromagnetic steel strip side). In the figure, the vertical direction is the thickness direction. The horizontal direction is the direction perpendicular to the joining direction and perpendicular to the thickness direction (hereinafter also referred to as the joining perpendicular direction). The direction perpendicular to the paper surface is the joining direction. In other words, the plane shown in FIG. 1A (the cross section in the thickness direction) includes the joining perpendicular direction and the thickness direction. 1B is a cross-sectional view in the thickness direction of a joint of electromagnetic steel strips when the thickness t1 of the first electromagnetic steel strip and the thickness t2 of the second electromagnetic steel strip are different. While FIG. 1B shows an example in which the thickness t1 of the first electromagnetic steel strip is smaller than the thickness t2 of the second electromagnetic steel strip, the relationship between t1 and t2 may be t1<t2 or t1>t2. Alternatively, t1=t2 may be true, as in FIG. 1A.

[0039] The term "electrical steel strip" as used herein refers to an intermediate product that serves as a material for producing an electrical steel sheet, and in particular refers to an intermediate product at a stage from the end of hot rolling to before heat treatment for primary recrystallization (e.g., decarburization annealing or primary recrystallization annealing). The electrical steel strip produced by the method for producing an electrical steel strip according to one embodiment of the present invention is obtained by joining a first electrical steel strip and a second electrical steel strip together, followed by cold rolling, as will be described later. Hereinafter, the electrical steel strip in the joined state of the first and second electrical steel strips will be referred to as a "joined steel strip," and the electrical steel strip obtained by cold-rolling the joined steel strip will be referred to as a "cold-rolled steel strip."

[0040] A joined joint of electromagnetic steel bands according to one embodiment of the present invention is a joined joint of electromagnetic steel bands that joins a first electromagnetic steel band and a second electromagnetic steel band, the joined joint of electromagnetic steel bands having a joint and a thermal processing affected zone adjacent to the joint, the steel structures of the joint and the thermal processing affected zone each being a structure mainly composed of ferrite phase, and satisfying the relationships of the following formulas (1) to (4). Dsz≦200 μm (1) Dhaz1 ≦Dbm1 (2) Dhaz2 ≦Dbm2 (3) 0.9×(Hbm1+Hbm2) / 2≦Hsz≦1.2×(Hbm1+Hbm2) / 2 (4) where, Dsz is the average value (μm) of the ferrite grain size in the joint, Dhaz1 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the first electromagnetic steel band side, Dhaz2 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the second electromagnetic steel band side, Dbm1 is the average value (μm) of the ferrite grain size in the base material part of the first electromagnetic steel band, Dbm2 is the average value (μm) of the ferrite grain size in the base material part of the second electromagnetic steel band, Hsz is the average value of the hardness of the joint, Hbm1 is the average value of the hardness of the base material portion of the first electromagnetic steel strip, and Hbm2 is the average value of the hardness of the base material portion of the second electromagnetic steel strip.

[0041] [Workpieces to be joined (first and second electromagnetic steel strips)] The first and second electromagnetic steel strips are electromagnetic steel strips that are the workpieces to be joined. The chemical compositions of the first and second electromagnetic steel strips are not particularly limited as long as they are common for electromagnetic steel strips (electromagnetic steel sheets) in the cold rolling stage.

[0042] An example of the composition of such an electrical steel strip is a composition containing 2.0 to 5.0 mass% Si. Another example is a composition containing 0.005 mass% or less C, 2.0 to 5.0 mass% Si, 3.0 mass% or less Al, 2.00 mass% or less Mn, 0.2 mass% or less P, 0.01 mass% or less S, and 0.01 mass% or less N, with the balance being Fe and unavoidable impurities. The above composition may optionally contain, in mass%, at least one element selected from the group consisting of 0.2 mass% or less Sn, 0.2 mass% or less Sb, 0.01 mass% or less Ca, 0.05 mass% or less REM, and 0.01 mass% or less Mg. Furthermore, the above composition may optionally contain, in mass percent, at least one element selected from the group consisting of Cr: 1% or less, Ni: 1% or less, and Cu: 1% or less. Note that all elements other than Si and Fe may be 0%. The composition of the first and second electrical steel strips may be the same or different.

[0043] Although there are no particular limitations on the thickness t1 of the first electromagnetic steel strip and the thickness t2 of the second electromagnetic steel strip, t1 and t2 are each preferably 5 mm or less, and more preferably 1.2 to 3.2 mm. As mentioned above, t1 and t2 may be the same or different. Furthermore, the thickness ratio between the first and second electromagnetic steel strips (thickness of the larger electromagnetic steel strip / thickness of the smaller electromagnetic steel strip) is preferably 1.6 or less.

[0044] In addition, in the first and second electromagnetic steel strips, which are the materials to be joined, the region that is not affected by hot working due to frictional heat and plastic flow is called the base material portion.

[0045] The base material, as well as the welded portion and thermal processing affected zone described below, are defined as follows. That is, the welded joint of the electromagnetic steel strip is cut in the thickness (vertical) direction so that the plane shown in FIG. 1A (i.e., a plane including the weld perpendicular direction and the plate thickness direction) becomes the cut surface. Next, the cut surface is polished and etched with a saturated aqueous solution of picric acid, nital (a solution of nitric acid and ethanol), or aqua regia (a solution of concentrated hydrochloric acid and concentrated nitric acid mixed in a volume ratio of 3:1). Next, the cut surface is observed with an optical microscope, and the base material, welded portion, and thermal processing affected zone are defined based on the degree of etching, etc.

[0046] [Welded Part] The welded part is a region that undergoes hot working due to frictional heat between the rotating tool and the welded materials and plastic flow, and becomes a recrystallized structure.

[0047] The weld is composed of a steel structure mainly composed of ferrite phase, specifically, ferrite phase with an area ratio of 95% or more. The area ratio of the ferrite phase may be 100%. The area ratio of the remaining structure other than the ferrite phase is 5% or less. Examples of the remaining structure other than the ferrite phase include second phases such as martensite, sulfides, nitrides, and carbides. The area ratio of the remaining structure may be 0%.

[0048] The area ratio of the ferrite phase is measured as follows. That is, a test piece is cut out from a welded joint of an electromagnetic steel strip so that the measurement area of ​​the joint described below is included in the observation surface. The observation surface is the surface shown in FIG. 1A (i.e., the surface including the direction perpendicular to the joint and the sheet thickness direction). Next, the observation surface of the test piece is polished and then etched with 3 vol. % nital, a picric acid saturated aqueous solution, or aqua regia to reveal the structure. Next, a total of 10 fields of view within the measurement area of ​​the joint described below are photographed using an optical microscope at a magnification of 500x. Next, from the obtained structure images, the area of ​​the ferrite phase for each of the 10 fields of view is calculated using Adobe Photoshop from Adobe Systems. Next, the area of ​​the ferrite phase calculated for each field of view is divided by the area of ​​the respective field of view, and the result is multiplied by 100. The arithmetic average of these values ​​is taken as the area ratio of the ferrite phase.

[0049] It is also important to refine the steel structure of the joint, specifically, to reduce the grain size of the ferrite crystal grains (hereinafter also referred to as ferrite grain size) that make up the steel structure of the joint, so as to satisfy the relationship of the following formula (1): This improves the mechanical properties of the coil joint without deteriorating the shape of the coil joint, even when an electromagnetic steel strip is used as the joined material, and effectively prevents breakage of the coil joint on the production line: Dsz≦200 μm (1) where Dsz is the average ferrite grain size (μm) of the joint.

[0050] Here, Dsz is measured in accordance with JIS G 0551. Specifically, it is measured as follows. That is, a welded joint of an electromagnetic steel strip is cut in the thickness (vertical) direction so that the plane shown in FIG. 1A (i.e., a plane including the weld perpendicular direction and the plate thickness direction) becomes the cutting plane. In the cut plane, the weld perpendicular direction is defined as the X axis, and the plate thickness direction is defined as the Y axis. The center position of the weld in the weld perpendicular direction and the center position of the plate thickness of the welded materials in the plate thickness (vertical) direction are defined as the origins of the X axis and the Y axis. The center position of the weld in the weld perpendicular direction is, for example, the center position of the butt gap in the case of a butt joint, and the center position of the lap portion in the case of a lap joint. The thickness center of the welded materials in the thickness (vertical) direction is, for example, the thickness center of the thinner of the first and second electromagnetic steel strips in the case of a butt joint, and the thickness center of the overlapping portion in the case of a lap joint. The measurement area is defined as the region X = -0.2 × t to +0.2 × t and Y = -0.2 × t to +0.2 × t. Here, t is the average thickness (mm) of the first and second electromagnetic steel strips. However, if the measurement area includes areas that are not the welded portion, such as the thermal processing affected zone or the base material, these areas are excluded from the measurement area. The + and - signs on the X and Y axes can be set arbitrarily. Then, at any position within the measurement area, the ferrite grain size of the joint is measured five times in total using a cutting method (evaluated by the number of grains captured per mm of the test line or the number of intersections P) in accordance with JIS G 0551 "Steel - Microscopic test method for grain size," and the average value of these measurements is designated as Dsz. Note that the measurement area of ​​the ferrite grain size of the joint is hereinafter also simply referred to as the measurement area of ​​the joint.

[0051] It is also important to reduce the difference in hardness between the joint and the base material, specifically to satisfy the relationship in the following equation (4). This improves the mechanical properties of the coil joint without deteriorating the shape of the coil joint, even when an electromagnetic steel strip is used as the joined material, and effectively prevents breakage of the coil joint on the production line: 0.9 × (Hbm1 + Hbm2) / 2 ≦ Hsz ≦ 1.2 × (Hbm1 + Hbm2) / 2 (4) where, Hsz is the average hardness of the joint, Hbm1 is the average hardness of the base material of the first electromagnetic steel strip, and Hbm2 is the average hardness of the base material of the second electromagnetic steel strip.

[0052] Here, Hsz, Hbm1, and Hbm2 are measured in accordance with JIS Z 2244. Specifically, they are measured as follows. That is, Vickers hardness (HV) is measured at any five locations within the measurement area of ​​the joint on the cut surface under a test force of 4.9 N. The average value of these values ​​is taken as Hsz. Furthermore, on the cut surface, Vickers hardness (HV) is measured at any five locations within a region (level of the thickness (vertical) direction) of ±0.2 × t1 from the center of the thickness of the base material part of the first electromagnetic steel strip, and within a region (level of the thickness (vertical) direction) of ±0.2 × t2 from the center of the thickness of the base material part of the second electromagnetic steel strip, under a test force of 4.9 N. Note that the positions in the vertical (horizontal) direction of the joint may be selected arbitrarily as long as they are within the base material part. The average values ​​of the Vickers hardness (HV) measured on the base material of the first electromagnetic steel strip and the base material of the second electromagnetic steel strip are defined as Hbm1 and Hbm2, respectively, where t1 and t2 are the sheet thicknesses of the first and second electromagnetic steel strips, respectively.

[0053] Furthermore, although the thickness of the joint is not particularly limited, it is preferable to appropriately control the relationship between the thicknesses of the first and second electromagnetic steel strips, specifically, to satisfy the relationships of the following equations (5) and (6). This improves the mechanical properties of the coil joint without deteriorating the shape of the coil joint, even when electromagnetic steel strips are used as the joined materials, and more effectively prevents breakage of the coil joint on the production line. 0.8×TbmL≦TszL (5) TszH ≦1.3×TbmH (6) where, TszL is the minimum thickness of the joint (mm), TszH is the maximum thickness of the joint (mm), TbmL is the thickness of the thinner of the first and second electromagnetic steel strips (mm), and TbmH is the thickness of the thicker of the first and second electromagnetic steel strips (mm). However, when the sheet thickness of the first electromagnetic steel strip and the second electromagnetic steel strip are the same, TbmL = TbmH.

[0054] TszL and TszH may be measured, for example, as follows: A welded joint of an electromagnetic steel strip is cut in the thickness (vertical) direction so that the cut surface is the plane shown in Figure 1A (i.e., a plane including the weld perpendicular direction and the thickness direction). Then, TszL and TszH are measured on the cut surface using a vernier caliper or the like.

[0055] [Thermal processing affected zone] The thermal processing affected zone is a region adjacent to the weld zone that is affected by hot working due to frictional heat and plastic flow, but where the temperature and working are insufficient to result in a recrystallized structure. The thermal processing affected zone is formed adjacent to the weld zone on both sides of the first and second electromagnetic steel strips.

[0056] Like the welded portion, the thermal processing affected zone is composed of a steel structure mainly composed of ferrite phase, specifically, ferrite phase with an area ratio of 95% or more. The area ratio of the ferrite phase may be 100%. Furthermore, the area ratio of the remaining structure other than the ferrite phase is 5% or less. Examples of the remaining structure other than the ferrite phase include second phases such as martensite, sulfides, nitrides, and carbides. The area ratio of the remaining structure may be 0%. The area ratio of the ferrite phase may be measured in the same manner as the method described above.

[0057] It is also important to refine the steel structure in the thermal processing affected zone, specifically, to make the ferrite grain size in the thermal processing affected zone equal to or smaller than the ferrite grain size in the base material, that is, to satisfy the relationships of the following equations (2) and (3): Dhaz1 ≦ Dbm1 (2) Dhaz2 ≦ Dbm2 (3) where, Dhaz1 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the first electromagnetic steel band side, Dhaz2 is the average value (μm) of the ferrite grain size in the thermal processing affected zone on the second electromagnetic steel band side, Dbm1 is the average value (μm) of the ferrite grain size in the base material of the first electromagnetic steel band, and Dbm2 is the average value (μm) of the ferrite grain size in the base material of the second electromagnetic steel band.

[0058] Here, Dhaz1, Dhaz2, Dbm1, and Dbm2 are measured in the same manner as Dsz, which is the average value of the ferrite grain size in the joint, in accordance with JIS G 0551. The measurement area of ​​the ferrite grain size in the thermal processing affected zone on the first electromagnetic steel strip side (hereinafter also referred to as the measurement area of ​​the thermal processing affected zone on the first electromagnetic steel strip side) is set as follows: The welded joint of the electromagnetic steel strips is cut in the thickness (vertical) direction so that the plane shown in FIG. 1A (i.e., a plane including the weld perpendicular direction and the sheet thickness direction) becomes the cutting plane. In the above-mentioned cutting plane, the weld perpendicular direction is defined as the X-axis, and the sheet thickness direction is defined as the Y-axis. The boundary position between the joint and the thermal processing affected zone on the first electromagnetic steel strip side at the center position (level) of the sheet thickness of the first electromagnetic steel strip is defined as the origin of the X-axis and the Y-axis. The X-axis is defined as the first electromagnetic steel strip side (+) and the welded portion side (-), with the measurement region being X = 0 to +0.4 × t1 and Y = -0.2 × t1 to +0.2 × t1. Here, t1 is the thickness of the first electromagnetic steel strip. Regarding the Y-axis, + and - can be set arbitrarily. However, if the measurement region includes areas that are not thermally processed on the first electromagnetic steel strip side, such as the welded portion or base material, these areas are excluded from the measurement region. As mentioned above, the welded portion refers to the region that undergoes hot working due to frictional heat and plastic flow between the rotating tool and the welded material, resulting in a recrystallized structure. The thermally processed affected zone refers to the region adjacent to the welded portion, which is affected by hot working due to frictional heat and plastic flow, but does not undergo recrystallization due to insufficient temperature or processing. The base material refers to the region that is not affected by hot working due to frictional heat and plastic flow.

[0059] Similarly, the measurement area for the ferrite grain size in the thermal processing affected zone on the second electromagnetic steel strip side (hereinafter also referred to as the measurement area for the thermal processing affected zone on the second electromagnetic steel strip side) is set as follows. Specifically, the welded joint of the electromagnetic steel strips is cut in the thickness (vertical) direction so that the plane shown in FIG. 1A (i.e., a plane including the weld perpendicular direction and the sheet thickness direction) serves as the cutting plane. In the above-mentioned cut plane, the weld perpendicular direction is defined as the X-axis, and the sheet thickness direction is defined as the Y-axis. The boundary position between the weld and the thermal processing affected zone on the second electromagnetic steel strip side at the center position (level) of the sheet thickness of the second electromagnetic steel strip is defined as the origin of the X-axis and the Y-axis. For the X-axis, the second electromagnetic steel strip side is defined as +, and the weld side is defined as -, and the measurement area is the area between X = 0 to +0.4 × t2 and Y = -0.2 × t2 to +0.2 × t2. Here, t2 is the sheet thickness of the second electromagnetic steel strip. For the Y-axis, + and - can be set as desired. However, if the above measurement area includes areas that are not thermal processing affected zones on the second electromagnetic steel strip side, such as joints or base material areas, these areas are to be excluded from the measurement area.

[0060] Furthermore, the measurement regions of the ferrite grain size in the base metal portions of the first and second electromagnetic steel strips (hereinafter also referred to as the measurement regions of the base metal portions of the first and second electromagnetic steel strips) may be the region (level in the thickness (vertical) direction) of the thickness center position of the base metal portion of the first electromagnetic steel strip on the above-mentioned cut surface ±0.2 × t1 and the region (level in the thickness (vertical) direction) of the thickness center position of the base metal portion of the second electromagnetic steel strip, respectively. Note that the position in the joining vertical (horizontal) direction may be selected arbitrarily as long as it is in the base metal portion. Here, t1 and t2 are the thicknesses of the first and second electromagnetic steel strips, respectively.

[0061] Examples of the joint type include a butt joint and a lap joint.

[0062] [2] Friction Stir Welding Method for Electromagnetic Steel Strip Next, a description will be given of a friction stir welding method for electromagnetic steel strip according to one embodiment of the present invention. As described above, the friction stir welding method for electromagnetic steel strip according to one embodiment of the present invention is characterized in that, after applying friction stir welding to join the coils of the electromagnetic steel strip, the steel structure of the welded portion and the thermal processing affected zone is made to be a ferrite-based structure, and the steel structure of the welded portion and the thermal processing affected zone is refined, thereby reducing the difference in hardness between the welded portion and the base material. More specifically, a friction stir welding method for joining a first electromagnetic steel strip to a second electromagnetic steel strip following the first electromagnetic steel strip includes: pressing a rotating tool against an unjoined portion, which is an abutment portion or an overlap portion, of an end of the first electromagnetic steel strip and an end of the second electromagnetic steel strip; and then moving the rotating tool in the joining direction to join the first electromagnetic steel strip to the second electromagnetic steel strip; and performing the joining under conditions such that the steel structures of the joint and the thermally affected zone formed by joining the first electromagnetic steel strip to the second electromagnetic steel strip are each primarily composed of ferrite phase and satisfy the relationships of the above formulas (1) to (4). Note that the friction stir welding method for joining an electromagnetic steel strip according to an embodiment of the present invention is preferably performed, for example, in an electromagnetic steel strip production line, particularly a continuous cold rolling line. Here, the continuous cold rolling line is a production line in which a steel strip is continuously cold rolled by a cold rolling mill. The continuous cold rolling line includes, for example, a steel strip conveying device and a cold rolling mill. The continuous cold rolling line may optionally further include a pickling device, an annealing furnace, a coating device, etc.

[0063] Suitable examples of joining types include butt joining and lap joining. In butt joining, as shown in FIGS. 2A and 2B , the end faces of the first and second electromagnetic steel strips are opposed to each other, and a rotating tool is pressed against the butt joint including the end faces (butt joints) of the first and second electromagnetic steel strips while rotating. Then, in this state, the rotating tool is moved in the joining direction to join the first and second electromagnetic steel strips. FIG. 2A is a side perspective view, and FIG. 2B is a view taken along the arrows A-A in FIG. 2A . In lap joining, as shown in FIG. 2C , at least a portion of the ends of the first and second electromagnetic steel strips are overlapped, and the rotating tool is pressed against the overlap joint while rotating. Then, in this state, the rotating tool is moved in the joining direction to join the first and second electromagnetic steel strips. In the figures, reference numeral 1 denotes a first electromagnetic steel strip (workpiece), 2 denotes a second electromagnetic steel strip (workpiece), 3 denotes a rotary tool, 4 denotes a joining portion, 5 denotes a shoulder portion, 6 denotes a probe (pin), 7 denotes a gripping device, 8 denotes a surface plate (backing), and 9 denotes a tip portion. The gripping devices are not shown in Figures 2A and 2C. In Figure 2B, the vertical direction is the thickness direction, the horizontal direction is the joining vertical direction (the direction of travel of the electromagnetic steel strip), and the direction toward the front of the page is the joining direction. Here, the first electromagnetic steel strip is a preceding steel strip in a continuous cold rolling line, and the second electromagnetic steel strip is a following steel strip in a continuous cold rolling line (the electromagnetic steel strip following the first electromagnetic steel strip).

[0064] Butt joining and lap joining differ only in the shape of the unjoined portion, and the other configurations of the equipment are basically the same. Therefore, here, we will explain an example of butt joining by so-called one-sided friction stir welding, as shown in Figures 2A and 2B.

[0065] In single-sided friction stir welding, for example, a friction stir welding apparatus is used that includes a rotary tool, a gripping device, and a control device (not shown) that controls the operation of the rotary tool. The control device controls, for example, the tilt angle θ of the rotary tool, the position of the tip of the rotary tool, the welding speed, the plunge load, the rotation speed and rotation torque of the rotary tool, etc.

[0066] In single-sided friction stir welding, a rotating tool is placed on one side of the first and second electromagnetic steel strips, which are the workpieces. The first and second electromagnetic steel strips are placed on a surface plate (backing) so that they are parallel to the welding center line shown in the figure, and are each gripped by a gripping device. The rotating tool is then rotated and pressed against the unwelded portion (the area to be welded) located on the welding center line, i.e., the butt joint between the end (rear end) of the first electromagnetic steel strip and the end (front end) of the second electromagnetic steel strip. Next, in this state, the rotating tool is moved in the welding direction. This softens the workpieces due to frictional heat between the rotating tool and the first and second electromagnetic steel strips. The softened portion is then stirred with the rotating tool, causing plastic flow and joining the first and second electromagnetic steel strips. A weld is formed where the welding is completed. Additionally, a thermomechanically affected zone is formed adjacent to the weld.

[0067] Furthermore, instead of single-sided friction stir welding, a double-sided friction stir welding method may be performed. The double-sided friction stir welding method is a friction stir welding method in which a first electromagnetic steel strip and a second electromagnetic steel strip are joined using a pair of opposing rotary tools. That is, the pair of opposing rotary tools are pressed against both sides of the unjoined portion while rotating in opposite directions, and in this state, the rotary tools are moved in the joining direction to join the first electromagnetic steel strip and the second electromagnetic steel strip. Figures 3A and 3B illustrate an example of butt joining by double-sided friction stir welding. Figure 3A is a side perspective view, and Figure 3B is a view taken along the arrows A-A in Figure 3A. In the figure, reference numeral 1 denotes a first electromagnetic steel strip (workpiece), 2 denotes a second electromagnetic steel strip (workpiece), 3-1 denotes a rotary tool (front-side rotary tool), 3-2 denotes a rotary tool (rear-side rotary tool), 4 denotes a joint, 5-1 and 5-2 denote shoulders, 6-1 and 6-2 denote probes (pins), 7 denotes a gripping device, and 9-1 and 9-2 denote tip portions. The gripping devices are not shown in Figure 3A. In Figure 3B, the vertical direction is the thickness direction, the horizontal direction is the vertical direction of the joint (the direction of travel of the electromagnetic steel strip), and the direction toward the front of the page is the joint direction.

[0068] In double-sided friction stir welding, for example, a double-sided friction stir welding apparatus is used that includes a pair of opposing rotary tools, a gripping device, and a control device (not shown) that controls the operation of the rotary tools. The control device controls, for example, the tilt angle θ of the rotary tools, the positions of the tips of the rotary tools and the distance between the tips (probes) (hereinafter also referred to as the gap between the probes), the gap between the shoulders of the rotary tools (i.e., the distance between the shoulder of the front-side rotary tool and the shoulder of the back-side rotary tool in the plate thickness direction), the welding speed, the plunge load, the rotation speed of the rotary tools, the rotation torque, etc.

[0069] In double-sided friction stir welding, the rotating tools of the friction stir welding apparatus are placed on both sides of the first and second electromagnetic steel strips, which are the materials to be welded. The rotating tools placed on the front side (vertically upper side) of the first and second electromagnetic steel strips are sometimes referred to as the front-side rotating tools, and the rotating tools placed on the back side (vertically lower side) of the first and second electromagnetic steel strips are sometimes referred to as the back-side rotating tools. The first and second electromagnetic steel strips are placed parallel to the weld center line shown in the figure and are each gripped by a gripping device. Then, the rotating tools are rotated and pressed against both sides of the unwelded portion (region to be welded) located on the weld center line, i.e., the butt joint between the end (rear end) of the first electromagnetic steel strip and the end (front end) of the second electromagnetic steel strip. Then, in this state, the rotating tools are moved in the welding direction. This softens the workpieces due to frictional heat between the rotating tool and the first and second electromagnetic steel bands. The softened area is then stirred with the rotating tool, causing plastic flow and joining the first and second electromagnetic steel bands. A weld is formed where the joining is complete. A thermally affected zone is also formed adjacent to the weld.

[0070] In the friction stir welding method for electromagnetic steel strips according to one embodiment of the present invention, it is important to perform the joining under conditions in which the steel structures of the weld and the thermal processing affected zone formed by the joining are each primarily composed of ferrite phase and satisfy the relationships of the above formulas (1) to (4). This improves the mechanical properties of the coil joint without deteriorating the shape of the coil joint, even when an electromagnetic steel strip is used as the joined material, and effectively prevents breakage of the coil joint on the production line.

[0071] To ensure that the steel structures of the weld and the thermal processing affected zone formed by welding are each mainly composed of ferrite phase and satisfy the relationships of the above formulas (1) to (4), it is preferable to do the following, for example: When performing one-sided friction stir welding, the diameter D (mm) of the shoulder of the rotary tool must satisfy the relationship of the following formula (7), and RS x D, which is expressed by the rotation speed RS (revolutions / min) of the rotary tool, the diameter D (mm) of the shoulder of the rotary tool, and the welding speed JS (mm / min), must be ３ / JS satisfies the relationship of the following equation (8): 4×TJ 　 ≦ 　 D 　 ≦ 　 10 x TJ (7) 1000 x TJ 　 ≦ 　 RS×D ３ / JS 　 ≦ 　 9000×TJ (8) Here, TJ in equations (7) and (8) is, when the unjoined portion is a butt portion, the average value (mm) of the thickness of the first electromagnetic steel strip and the thickness of the second electromagnetic steel strip, and when the unjoined portion is an overlap portion, it is the thickness (mm) of the overlap portion (i.e., the sum of the thickness (mm) of the first electromagnetic steel strip and the thickness (mm) of the second electromagnetic steel strip).

[0072] That is, the diameter D of the shoulder of the rotary tool (hereinafter simply referred to as shoulder diameter D) is appropriately controlled according to the thickness of the unwelded portion. This allows the temperature rise due to frictional heat generated between the rotary tool and the first and second electromagnetic steel strips, which are the workpieces, and the shear stress due to the frictional force to be effectively applied to the workpieces. Here, if the shoulder diameter D is less than 4 × TJ (mm), sufficient plastic flow may not be obtained, making it difficult to satisfy the specified relationship. On the other hand, if the shoulder diameter D exceeds 10 × TJ (mm), the region where plastic flow occurs unnecessarily expands, and excessive heat is input to the weld. This may result in coarsening of the recrystallized structure of the weld, making it difficult to satisfy the specified relationship. Therefore, to satisfy the specified relationship, it is preferable that the shoulder diameter D satisfy the relationship of the above formula (7). It is more preferable that it is 5 × TJ (mm) or more. It is also more preferable that it is 9 × TJ (mm) or less.

[0073] Also, RS×D ３ / JS is a parameter that correlates with the amount of heat generated per unit joint length. ３ By setting the range of RS / JS to 1000×TJ to 9000×TJ, it is possible to effectively apply to the workpieces a temperature rise due to frictional heat generated between the rotating tool and the first and second electromagnetic steel strips, which are the workpieces to be welded, and a shear stress due to the frictional force. ３ If / JS is less than 1000×TJ, the amount of heat generated may be insufficient. As a result, it may be difficult to form a metallurgically joined joint interface between the first and second electromagnetic steel strips, making it difficult to satisfy the predetermined relationship. ３ When / JS exceeds 9000 × TJ, the amount of heat generated by friction stirring becomes excessive, and an excessive amount of heat is input to the joint. This causes the peak temperature (maximum temperature reached) of the joint to rise, the cooling rate to decrease, and the recrystallized structure of the joint to become coarse. As a result, it may be difficult to satisfy the specified relationship. Therefore, from the viewpoint of satisfying the condition for the specified relationship, RS × D ３ It is preferable that / JS satisfies the relationship of the above formula (8).３ / JS is more preferably 1400×TJ or more. ３ / JS is more preferably 7000×TJ or less.

[0074] When performing double-sided friction stir welding, the diameter D (mm) of the shoulder of the rotary tool must satisfy the relationship of the above formula (7), and RS × D, which is expressed by the rotation speed RS (rpm), the diameter D (mm) of the shoulder of the rotary tool, and the welding speed JS (mm / min), must be ３ / JS satisfies the relationship of the following equation (9): 200×TJ 　 ≦ 　 RS×D ３ / JS 　 ≦ 　 2000×TJ (9) Here, TJ in equation (9) is, when the unjoined portion is a butt portion, the average value (mm) of the thickness of the first electromagnetic steel strip and the thickness of the second electromagnetic steel strip, and when the unjoined portion is an overlap portion, it is the thickness (mm) of the overlap portion (i.e., the sum of the thickness (mm) of the first electromagnetic steel strip and the thickness (mm) of the second electromagnetic steel strip).

[0075] As mentioned above, RS×D ３ / JS is a parameter that correlates with the amount of heat generated per unit joint length. ３ By setting the range of RS / JS to 200×TJ to 2000×TJ, it is possible to effectively apply to the workpieces a temperature rise due to frictional heat generated between the rotating tool and the first and second electromagnetic steel strips, which are the workpieces to be welded, and a shear stress due to the frictional force. ３ If / JS is less than 200×TJ, the amount of heat generated may be insufficient. As a result, it may be difficult to form a metallurgically joined joint interface between the first and second electromagnetic steel strips, making it difficult to satisfy the predetermined relationship. ３When / JS exceeds 2000 × TJ, the amount of heat generated by friction stirring becomes excessive, and an excessive amount of heat is input to the joint. This causes the peak temperature (maximum temperature reached) of the joint to rise, the cooling rate to decrease, and the recrystallized structure of the joint to become coarse. As a result, it may be difficult to satisfy the specified relationship. Therefore, from the viewpoint of satisfying the condition for the specified relationship, RS × D ３ It is preferable that / JS satisfies the relationship of the above formula (9). ３ / JS is more preferably 280×TJ or more. ３ / JS is more preferably 1600×TJ or less.

[0076] In addition, when performing double-sided friction stir welding, if the rotation speed RS and / or shoulder diameter D of the front-side rotating tool and the back-side rotating tool are different, it is preferable that the front-side rotating tool and the back-side rotating tool each satisfy the relationships of the above-mentioned formulas (7) and (9).

[0077] Furthermore, the joining conditions, such as the tilt angle θ of the rotary tool, the rotation speed of the rotary tool, the welding speed, the position of the tip of the rotary tool, the plunge load, and the rotation torque, are not particularly limited, and may be selected in accordance with ordinary methods, as long as the conditions are such that the steel structures of the weld and the thermal processing affected zone formed by welding are each mainly composed of ferrite phase, and the relationships of the above formulas (1) to (4) are satisfied.

[0078] For example, in the case of one-sided friction stir welding, the inclination angle θ of the rotary tool is preferably 0°≦θ≦4°, more preferably 0°<θ≦3°. Here, θ is the inclination angle of the rotation axis of the rotary tool (hereinafter also referred to as the tool rotation axis) from the thickness direction (the direction perpendicular to the surface of the workpieces) in a plane including the welding direction and the thickness direction (the direction perpendicular to the surface of the workpieces). The direction (angle) in which the tip of the rotary tool leads the welding direction is defined as +. The rotation speed of the rotary tool is preferably 100 to 2000 r / min. Setting the rotation speed of the rotary tool within this range is advantageous because it can maintain a good surface shape while suppressing deterioration of mechanical properties due to excessive heat input. The rotation speed of the rotary tool is more preferably 150 r / min or more. Furthermore, the rotation speed of the rotary tool is more preferably 1500 r / min or less. The welding speed is preferably 80 to 600 mm / min. The welding speed is more preferably 100 mm / min or more. The welding speed is more preferably 500 mm / min or less. The position of the tip of the rotary tool, the plunge load, the rotational torque, etc. may be set appropriately in accordance with common methods.

[0079] Furthermore, in the case of double-sided friction stir welding, the inclination angle θ of the rotary tool is preferably 0°≦θ≦3°, more preferably 0°<θ≦2°, for both the front-side rotary tool and the back-side rotary tool. The rotation speed of the rotary tool is preferably 100 to 9000 r / min. Setting the rotation speed of the rotary tool within this range is advantageous because it is possible to maintain a good surface shape while suppressing deterioration of mechanical properties due to excessive heat input. The rotation speed of the rotary tool is more preferably 300 r / min or more, even more preferably 400 r / min or more. The rotation speed of the rotary tool is more preferably 8000 r / min or less. Note that when t1 and t2 are different, the rotation speed of the rotary tool is more preferably 100 to 5000 r / min. In this case, the rotation speed of the rotary tool is more preferably 500 r / min or more. In this case, the rotation speed of the rotary tool is more preferably 3000 r / min or less. The welding speed is preferably 800 to 5000 mm / min. The welding speed is more preferably 1000 mm / min or more. The welding speed is more preferably 2000 mm / min or more, and further preferably 4000 mm / min or less. The position of the tip of the rotary tool, the plunge load, the rotational torque, the gap between the probes, the gap between the shoulder portions of the rotary tool, and the like may be set appropriately in accordance with conventional methods.

[0080] 3A, when performing double-sided friction stir welding, it is preferable that the rotation directions of the front-side rotary tool and the back-side rotary tool are opposite when viewed from the front side (or back side) of the workpieces, and that the rotation speeds are the same. This allows the rotational torques applied to the workpieces from the front-side rotary tool and the back-side rotary tool to cancel each other out. As a result, compared to single-sided friction stir welding, which joins the unjoined parts by pressing them from one side, it is possible to simplify the structure of the jig that restrains the workpieces.

[0081] Furthermore, if the rotation directions of the front-side rotary tool and the back-side rotary tool are the same as viewed from the front side (or back side) of the workpieces, the relative speed of one rotary tool with respect to the other rotary tool approaches zero. As a result, the plastic flow of the workpieces approaches a homogeneous state, and plastic deformation also decreases. Therefore, heat generation due to plastic deformation of the material is no longer obtained, making it difficult to achieve a good joining state. Therefore, from the viewpoint of obtaining a temperature rise and shear stress sufficient to achieve a good joining state homogeneously in the thickness direction of the workpieces, it is preferable to rotate the front-side rotary tool and the back-side rotary tool in opposite directions as viewed from the front side (or back side) of the workpieces.

[0082] Furthermore, the rotating tool used in friction stir welding is not particularly limited as long as the steel microstructures of the weld and the thermal processing affected zone formed by the welding are each primarily composed of ferrite phase and satisfy the above-mentioned formulas (1) to (4). For example, the tip of the rotating tool contacts the first and second electromagnetic steel bands, which are the workpieces, during welding. Therefore, the tip of the rotating tool is formed of a material harder than the first and second electromagnetic steel bands at the high temperatures to which it is exposed during welding. This allows the rotating tool to deform the first and second electromagnetic steel bands while maintaining the shape of the tip during welding. As a result, high stirring ability can be sustained, enabling proper welding. The hardness of the tip of the rotating tool and the first and second electromagnetic steel bands can be measured and compared using a high-temperature Vickers hardness test method. Alternatively, only the tip of the rotating tool may be formed of a material harder than the first and second electromagnetic steel bands. The entire rotary tool may be made of a material harder than the first and second electromagnetic steel strips.

[0083] 4A to 4C show examples of rotary tools used in single-sided friction stir welding. As shown in FIGS. 4A to 4C, the rotary tool used in single-sided friction stir welding has a tip end including a shoulder (the area indicated by the shoulder diameter in the figure) and a probe (the area indicated by the pin diameter in the figure) that is positioned on the shoulder and shares a rotation axis with the shoulder. In the example of the rotary tool shown in FIG. 4A, the rotary tool has a shoulder diameter D of 13 mm, a probe diameter (hereinafter also referred to as the pin diameter) of 4 mm, a probe length (hereinafter also referred to as the pin length) of 1.8 mm, and a concave depth (not shown) of 0.3 mm. In the example of the rotary tool shown in FIG. 4B, the rotary tool has a shoulder diameter D of 18 mm, a pin diameter of 5.5 mm, a pin length of 2.1 mm, and a concave depth (not shown) of 0.3 mm. In the example of the rotary tool shown in FIG. 4C, the shape of the rotary tool is as follows: shoulder diameter D: 21 mm, pin diameter: 6.7 mm, pin length: 2.4 mm, and concave depth (not shown): 0.3 mm.

[0084] FIGS. 5A to 5C show examples of rotary tools used in double-sided friction stir welding. As shown in FIGS. 5A to 5C, the tip of the rotary tool used in double-sided friction stir welding also has a shoulder (the area indicated by the shoulder diameter in the figure) and a probe (the area indicated by the pin diameter in the figure) that is positioned on the shoulder and shares a rotation axis with the shoulder. In the example of the rotary tool shown in FIG. 5A, the rotary tool has a shoulder diameter D of 13 mm, a pin diameter of 4 mm, a pin length of 0.6 mm, and a concave depth (not shown) of 0.6 mm. In the example of the rotary tool shown in FIG. 5B, the rotary tool has a shoulder diameter D of 18 mm, a pin diameter of 5.5 mm, a pin length of 0.8 mm, and a concave depth (not shown) of 0.3 mm. In the example of the rotary tool shown in FIG. 5C, the rotary tool has a shoulder diameter D of 21 mm, a pin diameter of 6.7 mm, a pin length of 0.9 mm, and a concave depth (not shown) of 0.3 mm.

[0085] As shown in Figures 4A-C and 5A-C, the tip of a typical rotary tool consists of a shoulder (the area indicated by the shoulder diameter in the figure) and a probe (the area indicated by the pin diameter in the figure). The shoulder has a flat shape formed by a substantially flat or gently curved surface. The shoulder contacts the first and second electromagnetic steel strips while rotating during welding, generating frictional heat. The shoulder also presses the heat-softened portions to prevent material dispersion and promote plastic flow in the direction of rotation. The probe is discontinuous with the shoulder and protrudes approximately perpendicularly toward the workpieces (not shown). The probe penetrates toward the center of the thickness of the softened portions of the first and second electromagnetic steel strips during welding, thereby improving the stirring ability near the center of the thickness. The probe is also typically located at the center of the shoulder.

[0086] The shoulder diameter D (mm) is not particularly limited, but as described above, it is preferable that the shoulder diameter D (mm) satisfies the relationships of the above formulas (7) to (9) from the viewpoint that the steel structures of the welded portion and the thermal processing affected zone formed by joining each have a structure mainly composed of ferrite phase and satisfy the relationships of the above formulas (1) to (4).

[0087] Furthermore, the pin diameter and pin length of the rotary tool are not particularly limited and may be set appropriately in accordance with conventional methods. For example, when butt-joining a first electromagnetic steel strip and a second electromagnetic steel strip having different thicknesses, the pin diameter and pin length of the rotary tool may be set in accordance with conventional methods, taking into account the average thickness of the first electromagnetic steel strip and the second electromagnetic steel strip. Furthermore, when lap-joining a first electromagnetic steel strip and a second electromagnetic steel strip, the pin diameter and pin length of the rotary tool may be set in accordance with conventional methods, taking into account the total thickness of the first electromagnetic steel strip and the second electromagnetic steel strip.

[0088] 3C , when the sheet thicknesses of the electromagnetic steel strips to be welded are different (t1 and t2 are different), it is preferable to: apply double-sided friction stir welding to coil joining of the electromagnetic steel strips, and then perform joining using a rotary tool whose tip face has a central portion and an outer peripheral portion disposed adjacent to the periphery of the central portion, the outer peripheral portion having a tapered shape. This makes it possible to increase the joining speed while preventing breakage of the coil joint on the production line due to deterioration in the mechanical properties and shape of the coil joint, even when the sheet thicknesses of the electromagnetic steel strips to be welded are different.

[0089] In FIG. 3C , reference numeral 1 denotes a first electromagnetic steel strip (workpiece), 2 denotes a second electromagnetic steel strip (workpiece), 3-1 denotes a rotary tool (front surface side rotary tool), 3-2 denotes a rotary tool (rear surface side rotary tool), 4 denotes a joining portion, 5-3 denotes a tip surface (front surface side rotary tool), 5-4 denotes a tip surface (rear surface side rotary tool), 5-5 denotes a center portion (front surface side rotary tool), 5-6 denotes a center portion (rear surface side rotary tool), 5-7 denotes an outer periphery portion (front surface side rotary tool), 5-8 denotes an outer periphery portion (rear surface side rotary tool), 6-3 denotes a probe (pin) (front surface side rotary tool), 6-4 denotes a probe (pin) (rear surface side rotary tool), 5-9 denotes a shoulder end portion (circumferential end portion) (front surface side rotary tool), 5-10 denotes a shoulder end portion (circumferential end portion) (rear surface side rotary tool), 5-11 denotes a boundary between the center portion and the outer periphery portion (front surface side rotary tool), and 5-12 denotes a boundary between the center portion and the outer periphery portion (rear surface side rotary tool). In addition, in the view of arrow AA in Fig. 3C, the vertical direction is the plate thickness direction, the horizontal direction is the joint vertical direction, and the direction toward the back of the paper is the joint direction. In other words, the plane shown in the view of arrow AA in Fig. 3C includes the joint vertical direction and the plate thickness direction.

[0090] The first and second electromagnetic steel strips may be arranged so that a step is formed on only one side, as shown in FIG. 3C . In this case, a rotary tool with a tapered outer periphery, as described below, may be used as the front-side rotary tool arranged at least on the front side (vertically upper side) where the step is present. Rotary tools with the same shape may be used for both the front-side rotary tool and the back-side rotary tool. Alternatively, as shown in FIG. 15 , the first and second electromagnetic steel strips may be arranged so that a step is formed on both sides. In the example of FIG. 15 , a step is provided so that the second electromagnetic steel strip protrudes on the vertically upper side, and a step is provided so that the first electromagnetic steel strip protrudes on the vertically lower side. In this case, it is preferable to use rotary tools with a tapered outer periphery, as described below, for both the front-side rotary tool and the back-side rotary tool.

[0091] Rotary tools suitable for use when t1 and t2 are different will be described below with reference to Figures 6 to 14. In each figure, the upper figure is a cross-sectional view of the rotary tool as viewed from the side (a cross-section that includes and is parallel to the rotation axis), and the lower figure is a plan view of the rotary tool as viewed from the top (as viewed from the tip surface side). Note that, because the shapes of the front-side rotary tool and the back-side rotary tool are basically the same or point-symmetric, the following description will be given using the front-side rotary tool as an example.

[0092] Figure 6 shows the same embodiment of the rotary tool used in Figure 3C, but the tip surface has a central portion and an outer periphery disposed adjacent to the periphery of the central portion. It is important that this outer periphery is tapered. If the outer periphery is tapered, the tip of the rotary tool, excluding the probe, will have a generally truncated cone shape, as shown in Figures 6 to 14.

[0093] 3C, in friction stir welding, the rotary tool penetrates into the softened portion of the workpieces to near the center in the thickness direction. When workpieces of different thickness are butt-joined, a step occurs on at least one surface of the workpieces.

[0094] In order to make the steel structure of the welded joint and the thermal processing affected zone formed by joining each be a structure mainly composed of ferrite phase and to satisfy the relationships of the above formulas (1) to (4), this step is smoothed when the rotary tool passes through, thereby forming an appropriate bead.

[0095] When using a general rotary tool, in order to smooth out this step as the rotary tool passes, it is necessary to take measures such as tilting the rotation axis of the rotary tool in the direction perpendicular to the joining (especially toward the thinner workpiece). However, when the workpiece is an electromagnetic steel strip, particularly a high-melting-point or high-hardness material, tilting the rotary tool in the direction perpendicular to the joining generates a large load, which can make it difficult to form an appropriate bead. In addition, there are problems such as damage to the rotary tool and the need to extremely increase the rigidity of the device in which the rotary tool is installed.

[0096] In this regard, by tapering the outer periphery of the tip face of the rotary tool, steps in the workpieces are smoothed as the rotary tool passes, and an appropriate bead is formed, even without tilting the rotation axis of the rotary tool perpendicular to the joining direction. As a result, even if the thicknesses of the electromagnetic steel strips to be joined are different, it is possible to increase the joining speed while preventing breakage of the coil joint on the production line due to deterioration of the mechanical properties and shape of the coil joint.

[0097] Here, the outer periphery is defined as the region from the shoulder end (circumferential end) of the tip surface of the rotary tool to the boundary between the center and the outer periphery (donut-shaped region). The center is defined as the region of the tip surface of the rotary tool other than the outer periphery. Furthermore, the boundary between the center and the outer periphery (hereinafter also referred to as the center-outer periphery boundary) is defined as the position closest to the shoulder end where the shape of the tip surface becomes discontinuous from the shoulder end toward the center (rotation axis) in a cross section of the rotary tool viewed from the side (a cross section including the rotation axis and parallel to the rotation axis). Note that, when a step portion, as described below, is provided, the positions where discontinuity occurs due to the step portion (rising portions of the steps, recessed portions (grooves), and convex portions) are ignored.

[0098] Furthermore, from the viewpoint of achieving the above-mentioned effect, the taper angle α of the outer periphery is preferably set to 2 to 20°. Here, if the taper angle α of the outer periphery is smaller than 2°, there is a risk that the ability to smooth out steps on the workpieces as the rotary tool passes through may be reduced. Furthermore, a large stress may be applied to the outer periphery of the rotary tool, increasing the risk of the rotary tool being damaged. On the other hand, if the taper angle α of the outer periphery is larger than 20°, the contact area between the outer periphery and the workpieces may be reduced. This may result in a reduction in welding ability. Therefore, the taper angle α of the outer periphery is preferably set to 2 to 20°. The taper angle α of the outer periphery is more preferably 8° or more. Furthermore, the taper angle α of the outer periphery is more preferably 15° or less.

[0099] Here, the taper angle α of the outer periphery is the angle formed by a line connecting a shoulder end (circumferential end) of the tip surface of the rotary tool to the boundary between the center and the outer periphery, and a line perpendicular to the rotation axis, in a cross section of the rotary tool viewed from the side (a cross section including the rotation axis and parallel to the rotation axis). Note that the shape of the outer periphery in the cross section of the tip surface (a cross section including the rotation axis and parallel to the rotation axis) may be a straight line (line segment) or a continuous curve, but is preferably a straight line (line segment).

[0100] Furthermore, from the viewpoint of obtaining the above-mentioned effects, it is preferable that the circumferential position of the boundary between the central portion and the outer periphery be in the range of 0.15×D to 0.35×D (in other words, the diameter D1 of the central portion of the rotary tool is in the range of 0.3×D to 0.7×D). Here, the circumferential position refers to the distance from the rotation axis in the circumferential direction (direction perpendicular to the rotation axis) of the tip face of the rotary tool. Furthermore, D is the diameter of the tip face of the rotary tool (hereinafter also referred to as tip face diameter D).

[0101] In addition, from the viewpoint of achieving the above-mentioned effects, it is preferable that the target position (position of the rotation axis) of the rotary tool in the direction perpendicular to the joining (width direction of the plate) is within ±2 mm of the butt joint (joining center line) of the first electromagnetic steel strip and the second electromagnetic steel strip in the case of butt joining, and within ±2 mm of the center position (in the direction perpendicular to the joining) of the overlapping portion of the first electromagnetic steel strip and the second electromagnetic steel strip in the case of lap joining.

[0102] Even if the tool target position is shifted in the normal direction to the surface of the workpiece, the tapered outer periphery can press the workpiece, so an appropriate bead is formed. Also, there is no particular problem with the shape (dimension) error of the tip surface as long as it is within ±0.005 mm of the specified dimension, as determined by tool measurement using a displacement meter.

[0103] As shown in Figures 7A and 7B, it is also preferable to provide a spiral-shaped (helical, helical) step portion on the outer periphery. The spiral-shaped step portion is defined, for example, by a radial curve (vortex) that starts from the periphery of the center (the boundary between the center and the outer periphery) and extends to the outer periphery of the tip surface of the rotary tool (the shoulder end of the tip surface of the rotary tool). The direction of the vortex may be the same direction (same direction) as the rotation direction of the rotary tool as shown in Figure 7A, or may be the opposite direction (opposite direction) as shown in Figure 7B. Note that in both Figures 7A and 7B, the step portion has a recessed (groove) shape and there are four vortices.

[0104] That is, in friction stir welding, frictional heat is generated when the rotating tool presses and stirs the workpieces, softening and flowing the metal material to be welded. In this regard, as shown in FIG. 7A, by providing a spiral-shaped step portion on the outer periphery in the same direction as the rotation direction of the rotating tool, frictional heat is generated from the inside to the outside of the rotating tool. This allows excess metal material generated by differences in thickness of the workpieces to flow out of the pressing portion pressed by the rotating tool. As a result, it is possible to reduce the load on the rotating tool.

[0105] On the other hand, as shown in Figure 7B, by providing a spiral-shaped step portion on the outer periphery in the opposite direction to the rotation direction of the rotary tool, frictional heat is generated from the outside to the inside of the rotary tool. This prevents the metal material from flowing out of the pressed portion by the rotary tool. As a result, plastic flow in the pressed portion can be promoted. In addition, it is possible to prevent the thickness of the joint from decreasing relative to the thickness of the base material. Furthermore, it is possible to form a joint surface with fewer burrs.

[0106] The number of vortices defining the step portion may be one or more. However, if the number of vortices defining the step portion exceeds six, not only will the effect of promoting material flow be reduced, but the shape may become more complex and more susceptible to breakage. Therefore, it is preferable that the number of vortices defining the step portion be six or less.

[0107] Furthermore, from the viewpoint of preventing damage to the tip of the rotary tool while improving material flow, it is preferable to adjust the number of vortices that define the step portion according to the tip surface diameter D. For example, it is preferable to increase the number of vortices that define the step portion as the tip surface diameter D increases, and decrease the number of vortices that define the step portion as the tip surface diameter D decreases. Specifically, when the tip surface diameter D is less than 6 mm, it is preferable to set the number of vortices that define the step portion to two or less. On the other hand, when the tip surface diameter D is 6 mm or more, it is preferable to set the number of vortices that define the step portion to three to six.

[0108] In addition, the length of each vortex is preferably 0.5 to 2 times the circumferential length of the tip face. The length of the vortex is also preferably adjusted according to the tip face diameter D. For example, it is preferable to make the vortex length longer as the tip face diameter D increases, and shorter as the tip face diameter D decreases.

[0109] In one example, the step portion changes its height position in a stepped manner for each region between vortices. For example, as shown in FIGS. 14B and 14C , the step portion is configured by gradually lowering from the periphery of the center (the boundary between the center and the outer periphery) toward the outer periphery of the tip surface of the rotary tool (the shoulder end of the tip surface of the rotary tool). Hereinafter, this type of step portion configuration will also be referred to as a stepped configuration. Furthermore, in a cross section that includes the rotation axis and is parallel to the rotation axis, each step portion may be, for example, approximately horizontal.

[0110] In another example, the step portion is formed by providing a recess (groove) or a protrusion at the position of the spiral. Hereinafter, such a step portion shape will also be referred to as a recess-shaped or a protrusion-shaped. Examples of the cross-sectional shape of the recess (groove) include a U-shape (including a square shape and a semicircular shape), a V-shape, and a V-shape, but any shape that can achieve the above-mentioned effect is acceptable.

[0111] The center of the rotary tool is preferably a flat surface, a convex curved surface, or a concave curved surface. Furthermore, the shape of the center in a plane perpendicular to the tool rotation axis (the projected area when the center of the rotary tool is projected in a direction parallel to the rotation axis) is basically circular. Figures 8A to 8C show an example of a rotary tool with a flat center (hereinafter also referred to as a centrally flat rotary tool). Figures 9A to 9C show an example of a rotary tool with a centrally convex curved surface (hereinafter also referred to as a centrally convex rotary tool). Figures 10A to 10C show an example of a rotary tool with a centrally concave curved surface (hereinafter also referred to as a centrally concave rotary tool).

[0112] In the case of a center-plane rotary tool as shown in Figures 8A to 8C, for example, the center that comes into contact with the workpiece is made up of a single plane perpendicular to the rotation axis of the tool. Figure 8A is an example of a center-plane rotary tool without a step. Figure 8B is an example of a center-plane rotary tool with a spiral-shaped step on the outer periphery that faces in the same direction as the rotation direction. Figure 8C is an example of a center-plane rotary tool with a spiral-shaped step on the outer periphery that faces in the opposite direction to the rotation direction.

[0113] In a center-convex rotary tool such as those shown in Figures 9A to 9C, for example, the center that contacts the workpiece has a continuous shape and forms a substantially uniformly inclined surface. More specifically, the tip forms a single curved surface (paraboloid, prolate spheroid, or sphere) that protrudes from the center toward the center. Furthermore, as shown in Figures 9A to 9C, the cross section of the tip (a cross section that includes the rotation axis and is parallel to the rotation axis) forms a curve with a substantially uniform radius of curvature. Figure 9A is an example of a center-convex rotary tool without a step. Figure 9B is an example of a center-convex rotary tool in which a spiral-shaped step is provided on the outer periphery in the same direction as the rotation direction. Figure 9C is an example of a center-convex rotary tool in which a spiral-shaped step is provided on the outer periphery in the opposite direction to the rotation direction.

[0114] In a central concave rotary tool such as those shown in Figures 10A to 10C, the center that comes into contact with the workpiece has a continuous shape and forms a substantially uniformly inclined surface. More specifically, the tip forms a single curved surface (paraboloid, prolate spheroid, or sphere) that is recessed from the periphery toward the center. Furthermore, as shown in Figures 10A to 10C, the cross section of the tip (a cross section that includes the rotation axis and is parallel to the rotation axis) forms a curve with a substantially uniform radius of curvature. Figure 10A is an example of a central concave rotary tool without a step. Figure 10B is an example of a central concave rotary tool in which a spiral-shaped step is provided on the outer periphery in the same direction as the rotation direction. Figure 10C is an example of a central concave rotary tool in which a spiral-shaped step is provided on the outer periphery in the opposite direction to the rotation direction.

[0115] In addition, with a central convex rotary tool, the higher the curved surface height, the greater the ability to cause plastic flow during joining (hereinafter also referred to as plastic flow capacity). With a central concave rotary tool, the deeper the curved surface, the smaller the plastic flow capacity. With a central flat rotary tool, the plastic flow capacity is intermediate between that of a central convex rotary tool and a central concave rotary tool. In this way, the plastic flow capacity can be adjusted by adjusting the shape of the central part of the rotary tool, so it is preferable to select the shape and size of the central part appropriately depending on the strength of the materials to be joined and the required properties. For example, the diameter D of the central part of the rotary tool is １ When the diameter is 5.0 to 15.0 mm, it is preferable that the curved surface height is 0.2 to 1.4 mm for the central convex rotary tool and the curved surface depth is 0.2 to 1.4 mm for the central concave rotary tool.

[0116] 11 to 14, a spiral-shaped step portion may be provided in the center of the rotary tool in the opposite direction (opposite direction) to the rotation direction. The shape of the step portion, other than the direction of the spiral, is the same as the shape of the step portion provided in the outer periphery described above. For example, the spiral-shaped step portion is defined by a radial curve (swirl) that starts from the center of the tip face of the rotary tool or the periphery of the central circle of the tip face of the rotary tool and extends to the periphery of the center (the center-periphery boundary). The central circle of the tip face of the rotary tool is a circle whose center is the center of the tip face of the rotary tool (the position of the rotation axis) and whose diameter is less than the center diameter. The number of vortices defining the step portion may be one or more. However, if the number of vortices defining the step portion exceeds six, the effect of promoting material flow is reduced. Furthermore, the complex shape may be prone to breakage. Therefore, it is preferable that the number of vortices defining the step portion be six or less.

[0117] Figure 11A shows an example of a central plane rotary tool with a step only in the center, Figure 11B shows an example of a central plane rotary tool with a step in the center and a spiral-shaped step in the same direction as the rotation direction on the outer periphery, and Figure 11C shows an example of a central plane rotary tool with a step in the center and a spiral-shaped step in the opposite direction to the rotation direction on the outer periphery.

[0118] Fig. 12A shows an example of a centrally convex rotary tool in which a step portion is provided only in the central portion. Fig. 12B shows an example of a centrally convex rotary tool in which a step portion is provided in the central portion and a spiral-shaped step portion is also provided on the outer periphery in the same direction as the rotation direction. Fig. 12C shows an example of a centrally convex rotary tool in which a step portion is provided in the central portion and a spiral-shaped step portion is also provided on the outer periphery in the opposite direction to the rotation direction.

[0119] Fig. 13A shows an example of a central concave rotary tool in which a step portion is provided only in the center. Fig. 13B shows an example of a central concave rotary tool in which a step portion is provided in the center and a spiral-shaped step portion is also provided on the outer periphery in the same direction as the rotation direction. Fig. 13C shows an example of a central concave rotary tool in which a step portion is provided in the center and a spiral-shaped step portion is also provided on the outer periphery in the opposite direction to the rotation direction.

[0120] Although Figures 11 to 13 all illustrate examples in which the step portion in the center is recessed (groove-shaped), the step portion in the center may also be stepped, as shown in Figures 14A to 14C. Figure 14A is an example of a centrally convex rotary tool in which a stepped step portion is provided only in the center. Figure 14B is an example of a centrally convex rotary tool in which a stepped step portion is provided in the center and a spiral-shaped step portion is also provided on the outer periphery in the same direction as the rotation direction. Figure 14C is an example of a centrally convex rotary tool in which a stepped step portion is provided in the center and a spiral-shaped step portion is also provided on the outer periphery in the opposite direction to the rotation direction.

[0121] Furthermore, as with the outer periphery, the plastic flow capacity can also be adjusted by the form and shape of the step portion provided in the center, so it is preferable to appropriately select the form and shape of the step portions provided in the outer periphery and center depending on the strength and required characteristics of the materials to be joined.

[0122] 6, a probe that shares a rotation axis with the rotary tool may be provided at the center of the rotary tool. The probe has a shape that is discontinuous with the center of the rotary tool and projects substantially perpendicularly toward the workpiece (not shown). The dimensions of the probe are not particularly limited and may be set appropriately depending on the shape and dimensions of the center. For example, if the diameter D of the center of the rotary tool is １ When the diameter is 5.0 to 15.0 mm, it is preferable that the probe diameter is 2.0 to 5.0 mm and the probe length is 0.1 to 1.5 mm.

[0123] Furthermore, when t1 and t2 are different, in order to ensure that the steel structures of the welded portion and the thermal processing affected zone formed by joining are each mainly composed of ferrite phase and satisfy the relationships of the above formulas (1) to (4), it is preferable to satisfy the relationships of the following formulas (10) and (11) instead of the above formulas (7) and (9). １ (mm) satisfies the relationship of the following formula (10), and the rotation speed RS (rpm) of the rotary tool and the diameter D of the center of the rotary tool １ (mm) and welding speed JS (mm / min) １３ It is preferable that / JS satisfies the relationship of the following equation (11): 4×TJ 　 ≦ 　 D １　 ≦ 　 10 x TJ ... (10) 200 x TJ 　 ≦ 　 RS×D １ ３ / JS 　 ≦ 　 2000×TJ (11) Here, TJ is the average value (mm) of the thickness of the first electromagnetic steel strip and the thickness of the second electromagnetic steel strip when the unjoined portion is a butt joint, and is the thickness (mm) of the overlapping portion when the unjoined portion is an overlapping portion.

[0124] That is, the diameter D of the center of the rotary tool １ (Hereinafter, simply referred to as the central diameter D １ It is important to appropriately control the center diameter D in accordance with the thickness of the unjoined portion so as to satisfy the relationship of the above formula (10). This allows the temperature rise due to frictional heat generated between the rotary tool and the first and second electromagnetic steel strips, which are the workpieces to be welded, and the shear stress due to the frictional force to be effectively applied to the workpieces. Here, the center diameter D １ If the center diameter D is less than 4×TJ (mm), sufficient plastic flow cannot be obtained, and it may be difficult to satisfy the predetermined relationship. １ If the center diameter D exceeds 10 × TJ (mm), the area where plastic flow occurs will unnecessarily widen, and excessive heat will be applied to the weld. This will cause the recrystallized structure of the weld to become coarse, making it difficult to satisfy the specified relationship. Therefore, in order to satisfy the specified relationship, the center diameter D １ It is preferable that the relationship of the above formula (10) is satisfied for the center diameter D １ is more preferably 5.5×TJ (mm) or more. １ , and more preferably 8×TJ (mm) or less.

[0125] Also, RS×D １ ３ / JS is a parameter that correlates with the amount of heat generated per unit joint length.１ ３ By setting the range of RS / JS to 200×TJ to 2000×TJ, it is possible to effectively apply to the workpieces a temperature rise due to frictional heat generated between the rotating tool and the first and second electromagnetic steel strips, which are the workpieces to be welded, and a shear stress due to the frictional force. １ ３ If / JS is less than 200×TJ, the amount of heat generated may be insufficient, which may make it difficult to form a metallurgically bonded joint interface between the mating surfaces of the first and second electromagnetic steel strips. １ ３ If / JS exceeds 2000 × TJ, the amount of heat generated by friction stirring becomes excessive, and there is a risk that excessive heat will be input to the joint. This will increase the peak temperature (maximum temperature reached) of the joint and reduce the cooling rate, leading to coarsening of the recrystallized structure of the joint. Therefore, RS × D １ ３ It is preferable that / JS satisfies the relationship of the above formula (11). １ ３ / JS is more preferably 280×TJ or more. １ ３ / JS is more preferably 1600×TJ or less.

[0126] The rotation speed RS and / or the center diameter D of the rotary tool on the front side and the rear side are １ When the rotational tool and the rotational tool are different, it is preferable that the front side rotational tool and the back side rotational tool satisfy the relationships of the above formulas (10) and (11), respectively.

[0127] In addition, when a step is provided on only one side at the butt joint between the end of the first electromagnetic steel strip and the end of the second electromagnetic steel strip, it is preferable that the advancing side of the rotating tool arranged on the side where the step is located is the side of the first electromagnetic steel strip or the second electromagnetic steel strip that has the smaller thickness, and that the retreating side of the rotating tool arranged on the side where the step is located is the side of the first electromagnetic steel strip or the second electromagnetic steel strip that has the larger thickness.

[0128] Specifically, as shown in Figure 3C, when the thickness of the first electromagnetic steel strip is less than the thickness of the second electromagnetic steel strip and a step is provided only on the upper vertical side (surface side) at the butt joint between the end of the first electromagnetic steel strip and the end of the second electromagnetic steel strip, it is preferable that the advancing side of the surface-side rotating tool is on the first electromagnetic steel strip side and the retreating side of the surface-side rotating tool is on the second electromagnetic steel strip side.

[0129] The advancing side is the side on which the rotation direction of the rotary tool and the joining direction are the same, and the retreating side is the side on which the rotation direction of the rotary tool and the joining direction are opposite.

[0130] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0131] [3] Manufacturing Method of an Electrical Steel Strip Next, a manufacturing method of an electrical steel strip according to one embodiment of the present invention will be described. The manufacturing method of an electrical steel strip according to one embodiment of the present invention includes the steps of joining a first electrical steel strip and a second electrical steel strip by the friction stir joining method of an electrical steel strip according to one embodiment of the present invention to obtain a joined steel strip, and cold rolling the joined steel strip to obtain a cold-rolled steel strip. Here, the joined steel strip preferably includes a first electrical steel strip, a second electrical steel strip, and a joint of the electrical steel strips according to one embodiment of the present invention, and the first and second electrical steel strips are joined via the joint of the electrical steel strips according to one embodiment of the present invention. The cold rolling conditions are not particularly limited and may be conventional. Optionally, pickling may be performed before cold rolling.

[0132] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0133] Example 1: The materials to be joined (first and second electromagnetic steel strips) were electromagnetic steel strips having the chemical compositions shown in Table 1 (the remainder being Fe and unavoidable impurities). The first electromagnetic steel strip (preceding steel strip) and the second electromagnetic steel strip (following process) were joined by friction stir welding under the conditions shown in Tables 2 and 3, simulating a continuous cold rolling line, to produce a welded joint of the electromagnetic steel strips. In the case of butt joining, the grooves were formed into a so-called I-groove, with no groove angle on the end faces of the two electromagnetic steel strips to be joined, and the two electromagnetic steel strips were butt-joined with their surfaces in a milled state. The same applies to Table 4, which will be described later. Table 1 also lists the average ferrite grain size, average hardness, and Erichsen value of the base material of the electromagnetic steel strip. The average ferrite grain size and average hardness of the base material of the electromagnetic steel strip were determined by the above-mentioned method. The Erichsen value is a value measured in accordance with the Erichsen test method specified in JIS Z 2247. Conditions not specified were set in accordance with standard methods.

[0134] In single-sided friction stir welding, in the case of butt welding, as shown in FIG. 2A, the rotary tool was rotated clockwise when viewed from above in the vertical direction (counterclockwise when the tip of the rotary tool is viewed from the front). Rotary tools with one of the three types of cross-sectional dimensions and shapes shown in FIGS. 4A to 4C were used. All of these rotary tools were made of tungsten carbide (WC) with a Vickers hardness of HV 1090, which is harder than the materials to be welded. When the thicknesses of the first and second electromagnetic steel strips were different, the back surface (the surface facing the surface plate) of the butt joint between the first and second electromagnetic steel strips was flat, while the front surface (the surface where the rotary tool was placed) had a step. In the case of lap welding, as shown in FIG. 2C, the first electromagnetic steel strip (leading steel strip) was joined on the upper side of the lap, and the second electromagnetic steel strip (following steel strip) was joined on the lower side of the lap. The rotation direction and shape of the rotary tool are the same as those in the case of butt welding.

[0135] In double-sided friction stir welding, in the case of butt welding, as shown in Figure 3A, the rotation direction of the front-side rotary tool located on the upper vertical side was clockwise as viewed from above in the vertical direction, and the back-side rotary tool located on the lower vertical side was rotated counterclockwise as viewed from above in the vertical direction. In other words, when viewed from the front, both rotary tools rotated counterclockwise. Furthermore, one of the three types of rotary tools with cross-sectional dimensions and shapes shown in Figures 5A to 5C was used. Furthermore, the front-side rotary tool and the back-side rotary tool used had the same cross-sectional dimensions and shape. Note that all of these rotary tools were made of tungsten carbide (WC) with a Vickers hardness of HV 1090, which is harder than the materials to be welded. Furthermore, when the sheet thicknesses of the first and second electromagnetic steel strips are different, the butt joint between the first and second electromagnetic steel strips has a flat back surface (the surface on which the back-side rotating tool is placed) and a stepped front surface (the surface on which the front-side rotating tool is placed). In the case of lap joining, the first electromagnetic steel strip (leading steel strip) is joined on the upper side of the lap, and the second electromagnetic steel strip (following steel strip) is joined on the lower side of the lap. The rotation direction and shape of the rotating tool are the same as in the case of butt joining.

[0136] In addition, for comparison, the materials to be joined (first electromagnetic steel strip and second electromagnetic steel strip) shown in Table 4 were laser welded together under the conditions shown in Table 4 to produce a joined joint of the electromagnetic steel strips.

[0137] In laser welding, the maximum output is 5.5 kW. ２ A laser oscillator was used. Helium was used as the shielding gas, and the shielding gas flow rate was 40 liters / minute. In Table 4, the "No filler wire added" column indicates that welding was performed without using a filler wire. In addition, the "Yes filler wire added" column indicates that welding was performed using a MIG wire (0.9 mm diameter) with the chemical composition shown in Table 5 (the balance being Fe and unavoidable impurities) as the filler wire.

[0138] All of the welded joints of the electromagnetic steel strips obtained as described above were in a sound condition with no defects both in appearance and inner surface.

[0139] For the welded joint of the electromagnetic steel bands thus obtained, the welded portion, the thermal processing affected zone, and the base material portion were defined in the manner described above. Also, in the manner described above, the following were measured: Dsz: average value of ferrite grain size (μm) of the welded portion, Dhaz1: average value of ferrite grain size (μm) of the thermal processing affected zone on the first electromagnetic steel band side, Dhaz2: average value of ferrite grain size (μm) of the thermal processing affected zone on the second electromagnetic steel band side, Dbm1: average value of ferrite grain size (μm) of the base material portion of the first electromagnetic steel band, Dbm2: average value of ferrite grain size (μm) of the base material portion of the second electromagnetic steel band, Hsz: average value of hardness of the welded portion, Hbm1: average value of hardness of the base material portion of the first electromagnetic steel band, Hbm2: average value of hardness of the base material portion of the second electromagnetic steel band. Furthermore, TszL: minimum thickness of the weld (mm) and TszH: maximum thickness of the weld (mm) were measured on the vertical cut surface of the obtained welded joint of the electromagnetic steel strip (the surface shown in Figure 1A (i.e., the surface including the direction perpendicular to the weld and the sheet thickness direction)). Note that for welded joints of electromagnetic steel strips obtained by laser welding, the above measurements were performed by regarding the welded portion as the weld and the heat-affected zone as the thermal processing affected zone. The measurement procedures were the same as for welded joints of electromagnetic steel strips obtained by friction stir welding. The results are shown in Tables 6 and 7.

[0140] Furthermore, the obtained welded joints of the electromagnetic steel strips were evaluated for their effect of preventing breakage of the coil joints on the production line (hereinafter also referred to as breakage prevention effect) in the following manner.

[0141] That is, test pieces were taken from the obtained welded joints of the electromagnetic steel strips so as to include the welded joint, the thermally processed affected zone and base material on the first electromagnetic steel strip side, and the thermally processed affected zone and base material on the second electromagnetic steel strip side. Next, using the taken test pieces, the Erichsen values ​​of the welded joints were measured in accordance with the Erichsen test method specified in JIS Z 2247. The fracture suppression effect was evaluated according to the following criteria, based on the ratio of the Erichsen value of the welded joint to the Erichsen value of the base material (hereinafter also referred to as the Erichsen value ratio). The results are shown in Table 8. [Erichsen value ratio (%)] = [Erichsen value of joined joint] / [Erichsen value of base material] x 100 〇 (Pass): Erichsen value ratio is 80% or more × (Fail): Erichsen value ratio is less than 80% In addition, if the Erichsen value of the base material part of the first electromagnetic steel strip differs from the Erichsen value of the base material part of the second electromagnetic steel strip, the Erichsen value of the base material part shall be the smaller of the Erichsen value of the base material part of the first electromagnetic steel strip and the Erichsen value of the base material part of the second electromagnetic steel strip.

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] From Table 8, it can be seen that in all of the inventive examples in which the steel structure of the joint and the thermal processing affected zone was mainly composed of ferrite phase and satisfied the relationships of the above formulas (1) to (4), an excellent fracture prevention effect was obtained. On the other hand, in the comparative examples in which the steel structure of the joint and the thermal processing affected zone was not mainly composed of ferrite phase and / or did not satisfy at least one of the relationships of the above formulas (1) to (4), a sufficient fracture prevention effect was not obtained.

[0151] Example 2: The materials to be joined (first and second electromagnetic steel strips) were electromagnetic steel strips having the same composition as in Example 1 shown in Table 1 (the remainder being Fe and unavoidable impurities). Then, the first electromagnetic steel strip (preceding steel strip) and the second electromagnetic steel strip (following process) were joined by double-sided friction stir welding under the conditions shown in Table 9, simulating a continuous cold rolling line, to produce a welded joint of the electromagnetic steel strips. Here, in the case of butt joining, the grooves were made into a so-called I-groove, with no groove angle on the end faces of the two electromagnetic steel strips to be joined, and the two electromagnetic steel strips were butt-joined with their surfaces in a milled condition. Conditions not specified were set according to standard methods.

[0152] In all of the above-described double-sided friction stir welding, the rotation direction of the front-side rotary tool located on the upper vertical side was counterclockwise as viewed from above, as shown in FIG. 3C , and the back-side rotary tool located on the lower vertical side was clockwise as viewed from above. Furthermore, the butt joint between the first and second electromagnetic steel strips had a flat back surface (the surface on which the back-side rotary tool was located) and a stepped front surface (the surface on which the front-side rotary tool was located). Furthermore, the electromagnetic steel strip with the smaller sheet thickness (the first electromagnetic steel strip in the example of FIG. 3C ) was placed on the advancing side of the front-side rotary tool, and the electromagnetic steel strip with the larger sheet thickness (the second electromagnetic steel strip in the example of FIG. 3C ) was placed on the retreating side of the front-side rotary tool. Additionally, the rotary tools were not inclined vertically in the joining direction.

[0153] Here, the rotary tools used had the shapes shown in Table 9. The dimensions of each part are as shown in Table 9 and below. Fig. 6: Probe diameter: 3.5 mm, probe length: 0.5 mm Fig. 7B: Probe diameter: 3.5 mm, probe length: 0.5 mm, groove depth: 0.5 mm, groove width: 0.5 mm Fig. 9A: Curved surface height: 0.5 mm Fig. 9C: Curved surface height: 0.5 mm, groove depth (outer periphery): 0.5 mm, groove width (outer periphery): 0.5 mm Fig. 10A: Curved surface depth: 0.5 mm Fig. 14C: Curved surface height: 0.5 mm, staircase height (center and outer periphery): 0.125 mm, staircase width (center and outer periphery): 1.25 mm

[0154] The front-side rotary tool and the back-side rotary tool used had the same cross-sectional dimensions and shape, and were both made of tungsten carbide (WC) with a Vickers hardness of HV 1090, which is harder than the workpieces.

[0155] For comparison, the materials to be joined (first electromagnetic steel strip and second electromagnetic steel strip) listed in Table 1 were laser welded together under the conditions listed in Table 10 to produce a joined joint of the electromagnetic steel strips.

[0156] In laser welding, the maximum output is 5.5 kW. ２ A laser oscillator was used. Helium was used as the shielding gas, and the shielding gas flow rate was 40 liters / minute. In Table 10, the "No filler wire added" column indicates that welding was performed without using a filler wire. In addition, in Table 10, the "Yes filler wire added" column indicates that welding was performed using a MIG wire (0.9 mm diameter) with the same composition as in Example 1, shown in Table 5 (the balance being Fe and unavoidable impurities).

[0157] All of the welded joints of the electromagnetic steel strips obtained as described above were in a sound condition with no defects both in appearance and inner surface.

[0158] For the welded joint of the electromagnetic steel bands thus obtained, the welded portion, the thermal processing affected zone, and the base material portion were defined in the manner described above. Also, in the manner described above, the following were measured: Dsz: average value of ferrite grain size (μm) of the welded portion, Dhaz1: average value of ferrite grain size (μm) of the thermal processing affected zone on the first electromagnetic steel band side, Dhaz2: average value of ferrite grain size (μm) of the thermal processing affected zone on the second electromagnetic steel band side, Dbm1: average value of ferrite grain size (μm) of the base material portion of the first electromagnetic steel band, Dbm2: average value of ferrite grain size (μm) of the base material portion of the second electromagnetic steel band, Hsz: average value of hardness of the welded portion, Hbm1: average value of hardness of the base material portion of the first electromagnetic steel band, Hbm2: average value of hardness of the base material portion of the second electromagnetic steel band. Furthermore, TszL: minimum thickness of the weld (mm) and TszH: maximum thickness of the weld (mm) were measured on the vertical cut surface of the obtained welded joint of the electromagnetic steel strip (the surface shown in Figure 1B (i.e., the surface including the direction perpendicular to the weld and the sheet thickness direction)). Note that for welded joints of electromagnetic steel strips obtained by laser welding, the above measurements were performed by regarding the welded portion as the weld and the heat-affected zone as the thermal processing affected zone. The measurement procedures were the same as for welded joints of electromagnetic steel strips obtained by friction stir welding. The results are shown in Tables 11 and 12.

[0159] The obtained welded joints of the electromagnetic steel strips were also evaluated for their effect of preventing breakage of the coil joints on the production line (hereinafter also referred to as breakage prevention effect) in the same manner as in Example 1. The evaluation results are shown in Table 13.

[0160]

[0161]

[0162]

[0163]

[0164]

[0165] As can be seen from Table 13, in all of the inventive examples, even though the sheet thickness of the first electromagnetic steel strip and the sheet thickness of the second electromagnetic steel strip were different from each other, joining was performed at high speed, and a defect-free joined joint of the electromagnetic steel strips having an excellent fracture prevention effect was obtained. On the other hand, in the comparative examples, a sufficient fracture prevention effect was not obtained.

[0166] 1 First electromagnetic steel strip (workpiece) 2 Second electromagnetic steel strip (workpiece) 3 Rotating tool 3-1 Rotating tool (surface-side rotating tool) 3-2 Rotating tool (rear-side rotating tool) 4 Joint 4-1 Thermal processing affected zone (first electromagnetic steel strip side) 4-2 Thermal processing affected zone (second electromagnetic steel strip side) 5, 5-1, 5-2 Shoulder 5-3 Tip surface (surface-side rotating tool) 5-4 Tip surface (rear-side rotating tool) 5-5 Center portion (surface-side rotating tool) 5-6 Center portion (rear-side rotating tool) 5-7 Outer periphery portion (surface-side rotating tool) 5-8 Outer periphery portion (rear-side rotating tool) 5-9 Shoulder end portion (circumferential end portion) (surface-side rotating tool) 5-10 Shoulder end portion (circumferential end portion) (rear-side rotating tool) 5-11 Center portion-outer periphery boundary (surface-side rotating tool) 5-12 Center-periphery boundary (rear side rotating tool) 6, 6-1, 6-2 Probe (pin) 6-3 Probe (pin) (front side rotating tool) 6-4 Probe (pin) (rear side rotating tool) 7 Grip device 8 Surface plate (backing) 9, 9-1, 9-2 Tip

Claims

DEPCT671. Electrical steel strip joint, First electrical steel strip joint and Second electrical steel strip joint, Electrical steel strip joint which includes the joint and the mechanically-thermal affected area near the joint, where the steel microstructure of the joint and the mechanically-thermal affected area is primarily ferritic, and satisfies the following expressions (1) to (4), Dsz less than or equal to 200 μm...(1)Dhaz1 less than or equal to Dbm1...(2)Dhaz2 less than or equal to Dbm2...(3)0.9x(Hbm1+Hbm2) / 2 less than or equal to Hsz less than or equal to 1.2x(Hbm1+Hbm2) / 2...(4)Where Dsz is the average (µm) of the ferrite grain size of the welded section, Dhaz1 is the average (µm) of the ferrite grain size of the thermally mechanically affected zone on the first side of the electrical steel strip piece, Dhaz2 is the average (µm) of the ferrite grain size of the thermally mechanically affected zone on the second side of the electrical steel strip piece, Dbm1 is the average (µm) of the ferrite grain size of the base metal section of the first side of the electrical steel strip piece, Dbm2 is the average (µm) of the ferrite grain size of the base metal section of the first side of the electrical steel strip piece. The ferrite core of the base metal of the second electrical steel strip, Hsz is the average of the hardness of the welded section, Hbm1 is the average of the hardness of the base metal of the first electrical steel strip, and Hbm2 is the average of the hardness of the base metal of the second electrical steel strip.

2. The welded section of the electrical steel strip according to claim 1, satisfies the relationship of the following expressions (5) and (6), 0.8xTbmL is less than or equal to TszL....(5)TszH is less than or equal to 1.3xTbmH....(6) where TszL is the minimum (mm) value of the thickness of the welded section, TszH is the maximum (mm) value of the thickness of the welded section, TbmL is the thickness (mm) of the thinner section of the first and second electrical steel strip sections, TbmH is the thickness (mm) of the thicker section of the first and second electrical steel strip sections, and when the thickness of the first and second electrical steel strip sections is the same, TbmL=TbmH3. Electrical steel strip welded section according to claim 1 or 2, where the thickness of the first electrical steel strip section is different from the thickness of the second electrical steel strip section4.The friction stir welding method for electrical steel strips for joining the first and second electrical steel strips (following the first) consists of: pressing the rotary tool into the unjoined section, which is the butt or overlapping part of the end of the first and the end of the second electrical steel strip, while rotating the tool; and joining the first and second electrical steel strips by moving the rotary tool. In the connection direction, where the connection is performed under conditions in which the steel microstructure of the welded section and the heat-mechanical affected zone created by the connection of the first electrical steel strip and the second electrical steel strip become primarily ferritic phases and conform to the following expressions (1) to (4): Dsz less than or equal to 200 μm...(1)Dhaz1 less than or equal to Dbm1...(2)Dhaz2 less than or equal to Dbm2...(3)0.9x(Hbm1+Hbm2) / 2 less than or equal to Hsz less than or equal to 1.2x(Hbm1+Hbm2) / 2..(4) where Dsz is the average (µm) of the ferrite grain size of the welded section, Dhaz1 is the average (µm) of the ferrite grain size of the thermally mechanically affected zone on the first side of the electrical steel strip piece, Dhaz2 is the average (µm) of the ferrite grain size of the thermally mechanically affected zone on the second side of the electrical steel strip piece, Dbm1 is the average (µm) of the ferrite grain size of the base metal section of the first electrical steel strip piece, Dbm2 is the average (µm) of the ferrite grain size of the base metal section of the second electrical steel strip piece.

5. Friction stir welding method for electrical steel strips according to claim 4, where the welding is performed under conditions which satisfy the relationship of the following expressions (5) and (6), 0.8xTbmL less than or equal to TszL....(5)TszH less than or equal to 1.3xTbmH....(6) Where TszL is the minimum (mm) value of the thickness of the welded section, TszH is the maximum (mm) value of the thickness of the welded section, TbmL is the thickness (mm) of the thinner section of the first and second electrical steel strip, TbmH is the thickness (mm) of the thicker section of the first and second electrical steel strip, and when the thickness of the first and second electrical steel strips are equal, TbmL=TbmH6. Friction stir welding method for electrical steel strips according to claim 4 or 5, where the rotating tool is a pair of rotating tools facing each other, and the rotating tools are pressed from both sides of the unwelded section while rotating in opposite directions to each other, the thickness of the first electrical steel strip is different from the thickness of the second electrical steel strip, the leading end of the rotating tool at each section includes the central and outer circumferential sections arranged close to the circumferential section of this central section, and the outer circumferential section has a tapered shape7.

7. Friction stir welding method for electrical steel strips under claim 6, where the alpha bending angle of the outer circumferential section is from 2 degrees to 20 degrees.

8. Friction stir welding method for electrical steel strips under claim 6 or 7, where the radial position of the boundary between the center and the outer circumferential section is in the range from 0.15xD to 0.35xD, where the radial position is the distance from the axis of rotation in the radial direction of the leading end of the rotating tool, and D is the diameter of the leading end of the rotating tool.

9. Friction stir welding method for electrical steel strips under any one of claim 6 to 8, where the surface of the outer circumferential section has a helical step that is twisted in the direction of rotation of the rotating tool.

10. Friction stir welding method for electrical steel strips under any one of claim 6 to 9, where the surface of the outer circumferential section has a helical step that is twisted in the opposite direction of rotation of the rotating tool. 11.

12. Friction stir welding method for electrical steel strips under any of the claims 6 to 10, where the center surface has a flat, convex, or concave surface.

13. Friction stir welding method for electrical steel strips under any of the claims 6 to 11, where the center surface has a helical step that is twisted in the opposite direction to the rotation of the rotating tool. The diameter D1 (mm) of the center of the rotating tool satisfies the relationship of the following expression (10), and the rotation speed RS (rpm) of the rotating tool, the diameter D1 (mm) of the center of the rotating tool, and the connection speed JS (mm / min), which is expressed as RSxD13 / JS, satisfies the relationship of the following expression (11), 4xTJ less than or equal to D1 less than or equal to 10xTJ...(10)200xTJ less than or equal to RSxD13 / JS less than or equal to 2000xTJ...(11) Where TJ is defined in such a way that, when the unwelded section is a butt joint, TJ is the average (mm) of the thickness of the first electrical steel strip and the thickness of the second electrical steel strip, and when the unwelded section is an overlap, TJ is the thickness (mm) of the overlap.

14. Any one of the claims 6 to 13 of the friction stir welding method for electrical steel strips, where at the butt joint of the end of the first electrical steel strip and the end of the second electrical steel strip, there is only one upper step, the forward side of the rotating tool which is placed on the side with the step on the side of the electrical steel strip that is thicker than the first and second electrical steel strips, and the backward side of the rotating tool which is placed on the side with the step on the side of the electrical steel strip that is thicker than the first and second electrical steel strips.15.The method of producing electrical steel strips, which includes: joining the first and second electrical steel strips by friction stir welding for electrical steel strips according to any one of claims 4 to 14 to obtain a welded steel strip; and cold rolling the welded steel strip to obtain a cold-rolled steel strip.

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