Friction stir welding method, friction stir welding device, and method for manufacturing joint member

Abstract

Provided is a friction stir welding method capable of obtaining excellent joint efficiency while preventing the occurrence of a roll flaw, even if a high Si steel sheet is used as the material to be joined. The method includes a joining step, a surface processing step, and a heating step. The maximum thickness reduction amount of the joint member in the surface processing step is set to 5.0% or less of the thickness of the material to be joined, and the maximum arrival temperature in the heating step is set to 400°C-1000°C.

Description

Friction stir welding method, friction stir welding device, and method of manufacturing welded members

[0001] The present invention relates to a friction stir welding method, a friction stir welding apparatus, and a method for manufacturing a welded member.

[0002] In friction stir welding, for example, double-sided friction stir welding, workpieces are joined as follows. That is, a pair of rotary tools is placed on the front and back sides of workpieces having at least two or more metal plates. The rotary tools are then pressed against the front and back sides of the workpieces, respectively, and moved in the joining direction while rotating. This softens the metal plates with frictional heat between the rotary tool and the workpieces, and the softened portions are stirred with the rotary tool. Plastic flow is then generated in the region of the workpieces that will become the joint, and the workpieces are joined. Hereinafter, the region where the workpieces are butted or overlapped and not yet joined is referred to as the "unjoined portion," and the region that has been joined and integrated is referred to as the "joined portion."

[0003] As a technology related to such friction stir welding, for example, Patent Document 1 discloses the following: "A friction stir welding method characterized by performing friction stir welding by butting or overlapping two steel plates together and setting the rotation speed R of a tool that rotates in the friction stir welding to more than 5 rpm and less than 5,000 rpm, setting the leading edge temperature T of the steel plate that contacts the leading edge in the advancing direction of the tool to 25°C or higher, setting the shoulder diameter D of the tool to 8 to 40 mm, and setting the welding speed V associated with the advancement of the tool to 0.1 to 5 m / min."

[0004] Japanese Patent Application Laid-Open No. 2019-166569

[0005] In steel sheet production lines, for example, production lines for pickling, cold rolling, annealing, and plating, it is common to perform so-called coil joining before threading steel sheets (steel strips) in order to improve productivity and increase yield. Here, coil joining refers to joining an end (rear end) of a preceding steel sheet or steel strip (hereinafter also referred to as a preceding steel sheet) and an end (front end) of a following steel sheet (hereinafter also referred to as a following steel sheet) as joined materials in the production line.

[0006] Resistance seam welding and laser welding are generally used for coil joining. Furthermore, in recent years, the application of friction stir welding to coil joining has been considered. However, when coil joining is performed by friction stir welding on steel sheets with a Si content of 2.0 to 12.0 mass %, such as electrical steel sheets (hereinafter also referred to as high-Si steel sheets), equipment defects can occur. Specifically, scratches can occur on the rolls through which the steel sheets pass downstream of the joining process (hereinafter simply referred to as roll scratches or roll scratches). Therefore, improvements in this regard are currently desired.

[0007] The present invention was developed in consideration of the above-mentioned current situation, and aims to provide a friction stir welding method that can prevent the occurrence of roll scratches and achieve excellent joint efficiency, even when high-Si steel plates are used as the welded materials. The present invention also aims to provide a friction stir welding apparatus that can be suitably used for the above-mentioned friction stir welding method. Furthermore, the present invention also aims to provide a method for manufacturing welded members. In this specification, any numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits, respectively.

[0008] The inventors have conducted extensive research to achieve the above object. First, the inventors investigated the cause of roll marks when high-Si steel sheets are used as the workpieces. As a result, the inventors discovered that roll marks are caused by burrs generated at the welded portion during friction stir welding. In particular, when high-Si steel sheets are used as the workpieces, the hardness of the burrs at the welded portion increases, and even if only a small amount of burr remains, this can lead to the occurrence of roll marks.

[0009] Based on the above findings, the inventors attempted to prevent the occurrence of roll scratches by removing burrs at the joint by grinding, etc. However, in this case, the joint efficiency was significantly reduced.

[0010] The inventors have therefore thoroughly investigated the above-mentioned causes and further studied to achieve the desired objective. As a result, they have made the following findings: (1) During the burr removal process, a deformed structure is formed at the joint. In particular, if a hardened phase, i.e., a structure with reduced ductility compared to the base material, is present in the region of the joint between the surface and the base material (hereinafter referred to as the surface region of the joint), defects will occur in this structure. These defects will then initiate cracks, reducing joint efficiency. (2) To prevent roll damage while achieving excellent joint efficiency, it is important to simultaneously satisfy the following points: - In the surface processing process, which is the burr removal process, the maximum thickness reduction of the joint members is limited to 5.0% or less of the thickness of the joined materials. - After the surface processing process, the joint members are heated, and the heating conditions are appropriately controlled to reduce deformation strain in the surface region of the joint. The present invention was completed based on the above findings and further study.

[0011] That is, the gist of the present invention is as follows: 1. A friction stir welding method comprising: a joining step of pressing a rotary tool against workpieces and moving the rotary tool in a joining direction while rotating, thereby joining the workpieces to obtain a joined member; a surface processing step of performing a surface processing on the joined member; and a heating step of heating the joined member, wherein the workpieces are steel plates with a Si content of 2.0 to 12.0 mass %, the maximum thickness reduction of the joined member in the surface processing step is 5.0% or less of the thickness of the workpieces, and the maximum temperature reached in the heating step is 400°C or higher and 1000°C or lower.

[0012] 2. The friction stir welding method according to 1 above, wherein the surface processing is carried out by grinding or pressing.

[0013] 3. The friction stir welding method according to 1 or 2 above, wherein the heating is performed by induction heating or resistance heating.

[0014] 4. The friction stir welding method according to any one of 1 to 3, wherein after the heating step, the hardness of a surface layer region of the welded portion of the welded members is in the range of 95 to 115% of the hardness of the base material of the welded members, and the surface layer region of the welded portion of the welded members is a region from the surface of the welded portion of the welded members to a position 1 / 10 of the thickness of the welded portion.

[0015] 5. The friction stir welding method according to any one of 1 to 4 above, further comprising a preliminary surface processing step of removing at least a region from the surface of the workpieces to a depth of 5 μm before the joining step.

[0016] 6. A friction stir welding apparatus for use in the friction stir welding method according to any one of 1 to 4 above, comprising, in the welding direction, a joining device that joins workpieces to obtain a joined member, a surface processing device that performs surface processing on the joined member, and a heating device that heats the joined member.

[0017] 7. A friction stir welding apparatus for use in the friction stir welding method described in 5 above, comprising, in the above order in the welding direction, a preliminary surface processing device that performs preliminary surface processing on materials to be welded, a joining device that joins the materials to be welded to obtain a welded member, a surface processing device that performs surface processing on the welded member, and a heating device that heats the welded member.

[0018] 8. A method for manufacturing a welded member, comprising joining workpieces by the friction stir welding method according to any one of 1 to 5 above to obtain a welded member.

[0019] According to the present invention, even when high-Si steel plates are used as the materials to be joined, it is possible to prevent the occurrence of roll scratches and obtain excellent joint efficiency.

[0020] 1A and 1B are schematic diagrams showing an example of a friction stir welding method according to an embodiment of the present invention;

[0021] The present invention will be described based on the following embodiments. [1] Friction Stir Welding Method First, a friction stir welding method according to one embodiment of the present invention will be described. As shown in Fig. 1, the friction stir welding method according to one embodiment of the present invention includes a joining step in which a rotary tool is pressed against workpieces and moved in a joining direction while rotating the rotary tool to join the workpieces and obtain a joined member; a surface processing step in which the joined member is surface-processed; and a heating step in which the joined member is heated. In Fig. 1, reference numeral 1 denotes the rotary tool (front surface-side rotary tool), 3 denotes the workpieces, 4 denotes a joint, 5 denotes a grinding device, and 6 denotes a heating device. Each of the above steps will be described below.

[0022] (Joining Step) In the joining step, the rotary tool is pressed against the workpieces to be joined, and the rotary tool is moved in the joining direction while being rotated, thereby joining the workpieces to obtain a joined member.

[0023] The bonding conditions are not particularly limited, and may be in accordance with conventional methods.

[0024] The joining method may be single-sided friction stir welding, in which a rotary tool is placed on one side of the workpieces to be joined, or double-sided friction stir welding, in which a rotary tool is placed on both the front and back surfaces of the workpieces to be joined.

[0025] To explain double-sided friction stir welding as an example, as shown in FIG. 2, a pair of rotary tools are placed on the front and back sides of workpieces having at least two metal plates. Hereinafter, the rotary tool placed on the front side of the workpieces will be referred to as the front-side rotary tool. Also, the rotary tool placed on the back side of the workpieces will be referred to as the back-side rotary tool. In FIG. 2, reference numerals 1 and 2 denote the rotary tool (front-side rotary tool), 2 (back-side rotary tool), 3 and 4, respectively. Then, the pair of opposing rotary tools are pressed against the front and back sides of the workpieces, respectively, and the rotary tools are rotated and moved in the joining direction. That is, the front-side rotary tool is pressed against the front side of the workpieces, and the back-side rotary tool is pressed against the back side of the workpieces, respectively, and these rotary tools are rotated and moved in the joining direction. This softens the metal plates with frictional heat between the rotary tool and the workpieces, and the softened portion is stirred by the rotary tool. Then, plastic flow is generated in the region of the workpieces that will become the joining portion, and the workpieces are joined.

[0026] Examples of joining methods include butt joining and lap joining. In butt joining, the end faces of the workpieces are placed opposite each other, and a rotary tool is pressed against the butt joint, including the end faces (butt joints), of the workpieces while rotating. Then, in this state, the rotary tool is moved in the joining direction to join the workpieces. In lap joining, at least a portion of the end faces of the workpieces are overlapped, and the rotary tool is pressed against the overlapping portion while rotating. Then, in this state, the rotary tool is moved in the joining direction to join the workpieces.

[0027] (Surface Processing Step) In the surface processing step, burrs on the surfaces of the joined members that were generated in the joining step are removed. Note that the equipment on the back side is not shown in Figure 1. In this step, it is important to limit the amount of joined member that is removed along with the burrs, and in particular, to limit the maximum thickness reduction of the joined member to 5.0% or less of the thickness of the joined materials.

[0028] Maximum thickness reduction of the joined members: 5.0% or less of the thickness of the joined materials. When high-Si steel plates are used as the joined materials and burrs are removed from the weld, a structure with reduced ductility compared to the base material is formed in the surface region of the weld, resulting in defects. These defects then initiate cracks, resulting in reduced joint efficiency. As described above, to suppress deformation strain in this region, it is effective to heat the joined members in the heating process described below and appropriately control the heating conditions. However, when the maximum thickness reduction of the joined members exceeds 5.0% of the thickness of the joined materials, i.e., when the ratio of the maximum thickness reduction of the joined members to the thickness of the joined materials (= [maximum thickness reduction of the joined members (mm)] / [thickness of the joined materials (mm)] × 100, hereinafter also referred to as the thickness reduction ratio of the joined members) exceeds 5.0%, the joint efficiency decreases due to metal thinning. Furthermore, even if the joined members are heated after the surface processing process, the deformation strain in this region cannot be sufficiently reduced, and excellent joint efficiency cannot be achieved. Therefore, the thickness reduction ratio of the joined members is set to 5.0% or less. The thickness reduction ratio of the joined members is preferably 3.0% or less. Note that, as long as burrs generated in the joining process can be removed, the thickness reduction ratio of the joined members is not particularly limited and may be 0%. From the viewpoint of sufficiently removing burrs generated in the joining process and more reliably preventing the occurrence of roll scratches, the thickness reduction ratio of the joined members is preferably 1.0% or more.

[0029] Here, the maximum thickness reduction of the joined members is the maximum value of the thickness reduction in each region of the joined members that have been surface-treated. In the case of butt joining, the thickness of the joined materials is the thickness of each steel plate that will be the joined material. However, if the thicknesses of the steel plates that will be the joined materials are different, the thickness of the joined material is the average thickness of the steel plates that will be the joined material. In the case of lap joining, the thickness is the thickness of the overlapping portion of the joined materials (i.e., the total thickness of the steel plates that overlap at the overlapping portion).

[0030] The surface processing method is not particularly limited. From the viewpoint of processing efficiency, grinding or rolling is preferable as the surface processing method. In order to control the maximum thickness reduction of the joined members, it is preferable to check the surface condition of the joined members (such as the degree of burrs) before surface processing and then adjust the position and height of the surface processing.

[0031] If double-sided friction stir welding is performed in the joining process, the above surface treatment is performed on both sides of the joining members. If single-sided friction stir welding is performed in the joining process, the above surface treatment may be performed only on one side of the joining members (the side on which the rotary tool is placed (the side on which the bead is formed)), or on both sides of the joining members. The former is advantageous in terms of efficiency. If the above surface treatment is performed on both sides of the joining members, the thickness reduction ratio of the joining members on both sides is controlled to be within the above range. In the heating process described below, similarly to the above, heating may be performed on one or both sides of the joining members depending on the joining method.

[0032] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0033] (Heating Step) In the heating step, the joining members are heated. As described above, when friction stir welding is performed using high-Si steel plates as the joining materials and then burrs formed at the joining portion are removed, a structure with reduced ductility compared to the base material is formed in the surface layer region of the joining portion, and defects occur in the structure. Then, cracks are generated from the defects, and the joint efficiency is reduced. In order to reduce deformation strain in the surface layer region of the joining portion, it is important to heat the joining members after removing the burrs and control the heating conditions as follows.

[0034] Maximum temperature: 400°C or higher and 1000°C or lower By setting the maximum temperature to 400°C or higher, the deformation strain in the surface layer region of the joint as described above is reduced, resulting in excellent joint efficiency. However, if the maximum temperature exceeds 1000°C, coarsening of the crystal grains occurs. Therefore, the maximum temperature is set to 400°C or higher and 1000°C or lower. The maximum temperature is preferably 700°C or higher. Furthermore, the maximum temperature is preferably 900°C or lower.

[0035] Furthermore, it is preferable that the heating step satisfy the following conditions. This more advantageously reduces the deformation strain in the surface layer region of the joint, as described above, and provides excellent joint efficiency. Average heating rate in the temperature range of 100 to 400°C (hereinafter also referred to as heating rate): 10 to 300°C / sec. Holding time at maximum temperature (hereinafter also referred to as holding time): 0.1 to 10 seconds. Average cooling rate in the temperature range of 400 to 100°C (hereinafter also referred to as cooling rate): 10 to 300°C / sec.

[0036] All temperatures referred to here are based on the surface temperature of the welded portions of the welded members. When single-sided friction stir welding is performed and the above surface processing is performed only on one side of the welded members (the side on which the rotary tool is placed (the side on which the bead is formed)), the maximum temperature is controlled within the above ranges on at least that side of the welded portions of the welded members. In this case, it is preferable to control the heating rate, holding time, and cooling rate within the above ranges on that side of the welded portions of the welded members. When double-sided friction stir welding is performed and the above surface processing is performed on both sides of the welded members, the maximum temperature is controlled within the above ranges on both sides of the welded portions of the welded members. In this case, it is preferable to control the heating rate, holding time, and cooling rate within the above ranges on both sides of the welded portions of the welded members.

[0037] The heating method is not particularly limited, but from the viewpoint of heating efficiency, high-frequency induction heating or resistance heating (where electricity is passed between electrodes) is preferred.

[0038] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0039] (Preliminary Surface Treatment Step) In friction stir welding, if an oxide film or contaminants remain on the surface of the workpieces, the joint strength may be reduced. To avoid this, a friction stir welding method according to one embodiment of the present invention may optionally further include a preliminary surface treatment step of removing at least a region of the workpieces up to a depth of 5 μm from the surface prior to the joining step. It is preferable that the removed region of the workpieces in the preliminary surface treatment step be a region up to a depth of 50 μm from the surface of the workpieces at most. Furthermore, the preliminary surface treatment may be performed on only one side of the workpieces or on both sides of the workpieces, depending on the surface condition of the workpieces.

[0040] The surface processing method is not particularly limited, but from the viewpoint of processing efficiency, grinding or polishing is preferred.

[0041] When preliminary surface treatment is performed, the thickness of the joined materials, which is the denominator of the thickness reduction ratio of the joined members, is the thickness of the joined materials after the preliminary surface treatment. In other words, the thickness of the joined materials, which is the denominator of the thickness reduction ratio of the joined members, is the thickness of the joined materials provided for the joining process.

[0042] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0043] (Metal to be welded) The metal to be welded is a steel sheet having a Si content of 2.0 to 12.0% by mass. The content of elements other than Si is not particularly limited. An example of the composition of such a steel sheet is, by mass, C: 0.1% or less, Si: 2.0 to 12.0%, Al: 2.0% or less, and Mn: 1.0% or less, with the balance being Fe and unavoidable impurities. Examples of the unavoidable impurities include P: 0.2% or less, S: 0.01% or less, and N: 0.01% or less. The above metal composition may optionally contain, by mass, at least one element selected from the group consisting of Cr: 1% or less, Ni: 1% or less, Cu: 1% or less, Sn: 0.2% or less, Sb: 0.2% or less, Ca: 0.01% or less, REM: 0.05% or less, and Mg: 0.01% or less. Note that all elements other than Si and Fe may be 0% by mass. The Si content is preferably 3.0% by mass or more. The Si content is preferably 8.0% by mass or less, more preferably 5.0% by mass or less. The C content is more preferably 0.001% by mass or more. The Mn content and the Al content are each more preferably 0.01% by mass or more. The steel plates used for the workpieces may be of the same steel type or different steel types. Furthermore, the thickness of each steel plate used for the workpieces is preferably 0.2 to 3.2 mm.

[0044] (Joined Member) A joined member obtained by a friction stir welding method according to one embodiment of the present invention has two or more base materials (joined materials) and a joint between the base materials (joining the base materials). Here, each base material is composed of a steel plate that serves as the joined material. The joint is a region that has undergone hot working due to frictional heat and plastic flow between the rotary tool and the joined materials, resulting in a recrystallized structure. Furthermore, in the joined member, the hardness of the surface layer region of the joint is preferably in the range of 95 to 115% of the hardness of the base materials, and more preferably in the range of 100 to 110% of the hardness of the base materials. In other words, the conditions of each of the above steps, particularly the heating step, are controlled so that the hardness of the surface layer region of the joint of the finally obtained joined member is preferably in the range of 95 to 115% of the hardness of the base materials, and more preferably in the range of 100 to 110% of the hardness of the base materials.

[0045] Here, the surface region refers to the region from the surface to 1 / 10 of the thickness at the joint of the joined members (depth (in the thickness direction) from the surface: region up to 1 / 10 of the thickness of the part). When performing single-sided friction stir welding and performing the above-mentioned surface processing and heating only on one side of the joined members (the side on which the rotary tool is placed (the side on which the bead is formed)), the hardness of the surface region of the joint on at least that side of the joined members is preferably in the range of 95 to 115% of the hardness of the base material, and more preferably in the range of 100 to 110% of the hardness of the base material. When performing double-sided friction stir welding and performing the above-mentioned surface processing and heating on both sides of the joined members, the hardness of the surface region of the joint on both sides of the joined members is preferably in the range of 95 to 115% of the hardness of the base material, and more preferably in the range of 100 to 110% of the hardness of the base material.

[0046] The hardness of the surface layer region of the joint and the hardness of the base material are measured by a Vickers hardness test in accordance with JIS Z 2244 (2009). The test force is 100 gf.

[0047] Specifically, the joined members are cut in the thickness (vertical) direction so that the cut surface is the joint cross section (a plane including the joint-normal direction and the thickness direction of the joined members, where the joint-normal direction is perpendicular to both the joint direction and the thickness direction). Next, in the surface region of the joint at the cut surface, the Vickers hardness is measured at intervals of 50 μm in the thickness direction and 50 μm in the joint-normal direction, starting from a position 50 μm deep from the surface of the joint and 50 μm away from one boundary between the base material and the joint in the joint-normal direction (toward the joint), in a grid pattern. The Vickers hardness is also measured at the center of the thickness of the base material at the cut surface under the above conditions. If the Vickers hardnesses measured at the surface region of the joint at the cut surface are all within the range of 95 to 115% of the Vickers hardness measured at the center of the thickness of the base material at the cut surface, the hardness of the surface region of the joint is determined to be within the range of 95 to 115% of the hardness of the base material. In addition, when the Vickers hardness differs depending on the base material joined via the joint, the above judgment is made based on the Vickers hardness of each base material joined via the joint. For example, in a joined member in which a first base material and a second base material are joined via a joint, if the Vickers hardness measured in the surface region of the joint at the cut surface is within the range of 95 to 115% of the Vickers hardness measured at the center of the thickness of the first base material at the cut surface and is also within the range of 95 to 115% of the Vickers hardness measured at the center of the thickness of the second base material at the cut surface, the hardness of the surface region of the joint is judged to be within the range of 95 to 115% of the hardness of the base materials.

[0048] The base material and the joint can be defined, for example, as follows: The joining members are cut in the thickness (vertical) direction so that the joint cross section is the cut surface. The cut surface is then polished and etched with 3 vol. % nital (a solution of nitric acid and ethanol), a saturated aqueous solution of picric acid, or aqua regia (a solution of hydrochloric acid and nitric acid mixed in a 3:1 volume ratio). The cut surface is then observed under an optical microscope, and the base material and the joint are defined based on the degree of etching, etc.

[0049] The welded member may be a single-sided friction stir welded joint or a double-sided friction stir welded joint. Here, a single-sided friction stir welded joint is a welded member in which workpieces are welded by single-sided friction stir welding, in which a rotary tool is placed on one side of the workpieces. That is, a single-sided friction stir welded joint has a bead on only one surface of the weld. In a single-sided friction stir welded joint, it is preferable to control the conditions of each of the above processes, particularly the conditions of the heating process, so that the above-mentioned surface layer region is obtained on at least one surface side of the weld. Furthermore, a double-sided friction stir welded joint is a welded member in which workpieces are welded by double-sided friction stir welding, in which a rotary tool is placed on both the front and back surfaces of the workpieces. That is, a double-sided friction stir welded joint has beads on both surfaces of the weld. When the welded member is a double-sided friction stir welded joint, it is preferable to control the conditions of each of the above processes, particularly the conditions of the heating process, so that the above-mentioned surface layer region is obtained near both surfaces of the weld.

[0050] [2] Friction stir welding apparatus Next, a friction stir welding apparatus according to one embodiment of the present invention will be described. The friction stir welding apparatus according to one embodiment of the present invention is a friction stir welding apparatus that can be suitably used for the friction stir welding method of [1] above, characterized in that it comprises, in the above order in the joining direction, an optional preliminary surface treatment device that performs preliminary surface treatment on the workpieces to be joined, a joining device that joins the workpieces to obtain a joined member, a surface treatment device that performs surface treatment on the joined member, and a heating device that heats the joined member. Note that, if necessary, a surface shape measuring device may be provided between the joining device and the surface treatment device.

[0051] Here, the type of the preliminary surface processing device is not particularly limited as long as it can process the surfaces of the workpieces and partially remove the workpieces. Examples of the preliminary surface processing device include a grinding device such as a grinder, or a polishing device.

[0052] The welding apparatus is not particularly limited as long as it is an apparatus that performs friction stir welding. The welding apparatus includes, for example, a rotary tool and a drive device for the rotary tool.

[0053] The type of the surface profile measuring device is not particularly limited as long as it can check the degree of burrs on the surface of the joint of the joined members. Examples of the surface profile measuring device include a contact type (which traces the surface of the sample with a stylus) and a non-contact type (such as a laser) surface profile measuring device.

[0054] The type of the surface processing device is not particularly limited as long as it can process the surfaces of the joined members and remove burrs from the surfaces of the joints of the joined members. Examples of the surface processing device include a grinding device such as a grinder, or a screw-down device.

[0055] The heating device is not particularly limited. Examples of the heating device include an atmosphere furnace (a furnace that heats the entire joint by heating it in an atmosphere while covering it), an induction heating device that heats locally without contact, and a resistance heating device. In addition, the heating device preferably has a device for measuring the surface temperature of the joint members, such as a thermograph.

[0056] Furthermore, the friction stir welding apparatus according to one embodiment of the present invention preferably has a drive device that moves the optional preliminary surface processing device, the welding device, the optional surface shape measuring device, the surface processing device, and the heating device together in the welding direction in accordance with the welding speed. This allows a series of steps to be performed continuously, which is extremely advantageous in terms of efficiency.

[0057] Other than the above, there are no particular limitations, and the device configuration may be the same as that of a conventionally known device.

[0058] [3] Manufacturing Method of Welded Member Next, a manufacturing method of a welded member according to one embodiment of the present invention will be described. The manufacturing method of a welded member according to one embodiment of the present invention involves joining workpieces by the friction stir welding method described in [1] above to obtain a welded member. The welded member obtained is as described in [1] above. A specific example of the welded member obtained is a coil having a welded portion that joins a preceding steel sheet and a following steel sheet. Conditions other than those described above are not particularly limited, and may be performed in accordance with conventional methods.

[0059] Two steel plates with the Si content and thickness listed in Table 1 were butted together and subjected to double-sided friction stir welding to obtain a joined member (joining process). In both cases, the rotation speed was 1000 rpm and the joining speed was 2 m / min on both sides. Ceramic rotary tools (front-side rotary tool and back-side rotary tool) were used, each with a shoulder diameter (shoulder diameter) of 16 mm, a probe diameter (probe diameter) of 8 mm, and a probe length of 1 mm. Next, both sides of the resulting joined member were subjected to surface grinding using a 20 mm wide grinder under the conditions listed in Table 1 to remove burrs from the joints of the joined member (surface processing process). The maximum thickness reduction of the joined member was measured using a laser displacement meter. The joined member was then heated under the conditions listed in Table 1 (heating process). Heating was performed using an atmospheric furnace. The surfaces (both sides) of the joined member were measured using thermography to determine the maximum temperature. Conditions not specified were as described above or in accordance with standard practice. In addition, in No. 15, the surface treatment process was not performed. In No. 16, the heating process was not performed. In both cases, the conditions of the surface treatment process and the heating process, as well as the hardness of the surface layer region of the joint / hardness of the base material x 100, which will be described later, were almost the same on both sides, so only one side is shown as a representative in Table 1. In addition, the prevention of roll scratches, which will be described later, was confirmed on both sides of the joint of the joined members.

[0060] For the bonded members thus obtained, the hardness of the surface layer region of the bonded portion and the hardness of the base material were measured in the same manner as described above, and it was determined whether the hardness of the surface layer region of the bonded portion was within the range of 95 to 115% of the hardness of the base material. The hardness of the surface layer region of the bonded portion / hardness of the base material × 100 is also shown in Table 1.

[0061] The resulting joined members were also evaluated for joint efficiency. In the evaluation of joint efficiency, a joint efficiency (= fracture strength of joint / fracture strength of base material × 100) of over 95% was evaluated as "excellent," a joint efficiency of 95% or less but over 90% was evaluated as "good," and a joint efficiency of 90% or less was evaluated as "poor." The results are also shown in Table 1.

[0062] Here, the fracture strength of the joint was measured as follows. That is, a test piece having the same shape as the No. 1 test piece specified in JIS Z 3121 (2013) was taken from the obtained friction stir welded joint so that the joining direction and thickness direction were perpendicular to the longitudinal direction of the test piece and the joint was located at the center of the parallel part. Next, a tensile test was performed in accordance with JIS Z 3121 (2013) using the taken test piece, and the maximum test force (N) was determined. The determined maximum test force (N) was then multiplied by the cross-sectional area (mm ) of the parallel part of the test piece. ２ The value obtained by dividing the test piece by the test piece strength was taken as the breaking strength of the joint.

[0063] The breaking strength of the base material can also be referred to as the tensile strength (TS) of the base material (the material to be joined). That is, the breaking strength of the base material can be measured by conducting a tensile test in accordance with JIS Z 2241 (2022). For example, a JIS No. 5 test piece is taken from the base material or the same material as the base material. Next, a tensile test is conducted using the taken test piece at a crosshead speed of 10 mm / min to measure the tensile strength (TS). Note that when the breaking strength differs depending on the base material, the maximum breaking strength of each base material is used in evaluating the joint efficiency.

[0064] To confirm the roll scratch prevention, the bonded portions of the bonding members were pressed against a surface-polished high-carbon chromium bearing steel (SUJ-2, hereinafter also referred to as bearing steel) (simulating a roll) and slid 100 times. At this time, the pressing pressure (the pressure applied when the bonded portions of the bonding members were pressed against the bearing steel) was 400 MPa. Next, the presence or absence of scratches on the bearing steel was confirmed visually. If scratches were found, the depth of the scratches was measured by cross-sectional observation using an optical microscope. The roll scratch prevention was then evaluated according to the following criteria. The results are also shown in Table 1. Good: No scratches of a depth of 50 μm or more were found. Poor: Scratches of a depth of 50 μm or more were found.

[0065]

[0066] As shown in Table 1, in all of the inventive examples, excellent joint efficiency was obtained while preventing the occurrence of roll marks. On the other hand, in the comparative examples, roll marks occurred or sufficient joint efficiency was not obtained.

[0067] Furthermore, even when joining members were manufactured in the same manner as described above except for carrying out a preliminary surface treatment process, in all cases where the thickness reduction ratio of the joining members in the surface treatment process was 5.0% or less and the maximum temperature reached in the heating process was 400°C or higher and 1000°C or lower, excellent joint efficiency was obtained while preventing the occurrence of roll scratches.

[0068] Furthermore, even when various steel plates having the above-mentioned preferred component composition with a Si content of 2.0 to 12.0 mass % and a thickness of 0.2 to 3.2 mm were used as the materials to be joined, and joined members were produced by butt joining or lap joining under various joining conditions, as long as the thickness reduction ratio of the joined members was 5.0% or less and the maximum temperature reached in the heating step was 400°C or higher and 1000°C or lower, excellent joint efficiency was obtained while preventing the occurrence of roll scratches.

[0069] REFERENCE SIGNS LIST 1 Rotary tool (front surface side rotary tool) 2 Rotary tool (rear surface side rotary tool) 3 Workpiece 4 Joint 5 Grinding device 6 Heating device

Claims

1. A friction stir welding method comprising: a joining step of pressing a rotating tool against materials to be joined and moving the rotating tool in a joining direction while rotating, thereby joining the materials to obtain a joined member; a surface processing step of performing a surface processing on the joined member; and a heating step of heating the joined member; wherein the materials to be joined are steel plates with a Si content of 2.0 to 12.0 mass%, the maximum thickness reduction of the joined member in the surface processing step is 5.0% or less of the thickness of the materials to be joined, and the maximum temperature reached in the heating step is 400°C or higher and 1000°C or lower.

2. The friction stir welding method according to claim 1, wherein the surface processing is carried out by grinding or pressing.

3. The friction stir welding method according to claim 1, wherein the heating is performed by induction heating or resistance heating.

4. The friction stir welding method according to claim 2, wherein the heating is performed by induction heating or resistance heating.

5. The friction stir welding method according to claim 1, wherein after the heating step, the hardness of the surface region of the joint of the joined members is in the range of 95 to 115% of the hardness of the base material of the joined members, and the surface region of the joint of the joined members is a region extending from the surface to a position 1 / 10 of the thickness of the joint of the joined members.

6. The friction stir welding method according to claim 2, wherein after the heating step, the hardness of the surface region of the joint of the joined members is in the range of 95 to 115% of the hardness of the base material of the joined members, and the surface region of the joint of the joined members is a region extending from the surface to a position 1 / 10 of the thickness of the joint of the joined members.

7. The friction stir welding method according to claim 3, wherein after the heating step, the hardness of the surface region of the joint of the joined members is in the range of 95 to 115% of the hardness of the base material of the joined members, and the surface region of the joint of the joined members is a region extending from the surface to a position 1 / 10 of the thickness of the joint of the joined members.

8. The friction stir welding method according to claim 4, wherein after the heating step, the hardness of the surface region of the joint of the joined members is in the range of 95 to 115% of the hardness of the base material of the joined members, and the surface region of the joint of the joined members is a region from the surface to a position 1 / 10 of the thickness of the joint of the joined members.

9. A friction stir welding method according to any one of claims 1 to 8, further comprising a preliminary surface treatment step of removing at least a region from the surface of the workpieces to a depth of 5 μm prior to the joining step.

10. A friction stir welding apparatus for use in the friction stir welding method according to any one of claims 1 to 8, comprising: a joining device for joining materials to be joined to obtain a joined member; a surface processing device for performing surface processing on the joined member; and a heating device for heating the joined member, in the above order in the joining direction.

11. A friction stir welding apparatus for use in the friction stir welding method according to claim 9, comprising: a preliminary surface treatment device for performing preliminary surface treatment on materials to be joined; a joining device for joining the materials to be joined to obtain a joined member; a surface treatment device for performing surface treatment on the joined member; and a heating device for heating the joined member, in the above order in the joining direction.

12. A method for producing a welded member, comprising joining workpieces by the friction stir welding method according to any one of claims 1 to 8 to obtain a welded member.

13. A method for producing a welded member, comprising the steps of: welding materials by the friction stir welding method according to claim 9 to obtain a welded member.

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