Friction stir spot joining, method of manufacturing the same, and friction stir spot joining method

Abstract

The present application provides a friction stir spot joining joint having high joint strength even when a high-strength steel sheet, particularly a steel sheet having a tensile strength (TS) of 980 MPa or more, is used as a joining material. A ring-shaped groove portion is formed in an upper surface of an upper sheet of the overlapped steel sheets, and the shape, microstructure, and hardness of the joining portion are appropriately controlled at the same time.

Description

Technical Field

[0001] This invention relates to a friction stirring point joint connector and its manufacturing method, as well as a friction stirring point joining method. Background Technology

[0002] In recent years, due to rising environmental concerns, CO2 emission restrictions have become increasingly stringent. In the automotive sector, lightweighting of vehicle bodies to improve fuel efficiency has become a key issue. Therefore, efforts are underway to promote the application of thinner-walled steel sheets, particularly those with a tensile strength (TS) of 980 MPa or higher, in automotive components.

[0003] In automobile assembly, resistance welding is often used to join stamped parts for cost and efficiency reasons. However, in resistance welding, which involves melting and solidification, the tensile strength of the joint can sometimes decrease due to the embrittlement of the steel structure at the joint.

[0004] Therefore, the application of friction stir point bonding as a non-melting bonding method to high-strength steel sheets is being investigated. Here, friction stir point bonding refers to a point bonding method that utilizes the friction stir phenomenon. Specifically, while rotating a tool having a shoulder and a pin protruding from the shoulder, the pin of the tool is pressed (pushed) into the materials to be bonded, which consist of overlapping metal plates. This softens and plastically flows the metal plates, bonding the overlapping metal plates together.

[0005] As a technology related to such frictional stirring point engagement, for example, the following is disclosed in Patent Document 1:

[0006] "A method for processing a metallic material involves controlling the processing section of steel containing 0.15% by mass or more carbon at 723°C or below, and inserting a rod-shaped rotary tool made of WC into the processing section to process the steel."

[0007] The following content is disclosed in Patent Document 2:

[0008] "A low-temperature bonding method for metallic materials involves placing two metallic materials opposite each other at a bonding portion to form a bonding interface, and pressing a rotating tool rotating at a predetermined speed into the bonding portion, thereby bonding the two metallic materials, characterized in that..."

[0009] By setting the circumferential speed of the outermost periphery of the aforementioned rotating tool to 51 mm / s or less, strong strain is introduced into the joined portion, thereby lowering the recrystallization temperature inherent in the aforementioned metallic material.

[0010] This causes recrystallized grains to form at the bonding interface by lowering the bonding temperature below the original recrystallization temperature of the aforementioned metallic material.

[0011] The following content is disclosed in Patent Document 3:

[0012] "A joint having a joint formed by overlapping one steel plate with another steel plate."

[0013] At the aforementioned joint, there is an interface between one steel plate and the other steel plate. The interface, in the cross-section of the joint, has a parallel interface located at the extension of the boundary between the one steel plate and the other steel plate, and a central interface that bends from the parallel interface toward the one steel plate as it approaches the top center of the joint.

[0014] The average hardness of the region separated by 0.05 mm to 0.5 mm in the direction parallel to the plate surface, viewed from the central interface towards the joint, is 350~450 HV.

[0015] The average hardness along the extension line of the parallel interface, viewed from the boundary point of the parallel interface and the central interface towards the joint on the central side, is 400~550 HV.

[0016] The following content is disclosed in Patent Document 4:

[0017] "A friction stirring point engagement tool, wherein a first component and a second component overlap and are pressed from the side of the first component, thereby engaging the first component and the second component at the friction stirring point, wherein..."

[0018] The first component and the second component mentioned above are both steel plates with a tensile strength of 590 MPa or higher.

[0019] The aforementioned friction stirring point engagement tool comprises: a cylindrical portion capable of rotating about a rotation axis and having a shoulder at its front end; and a cylindrical or frustoconical stirring head protruding from the shoulder in a direction extending from the rotation axis.

[0020] Let the thickness of the second component be t (mm), and the volume of the stirring head inserted into the second component during pressing be the insertion volume V (mm²) of the second component. 3 When V is in relation to t, it satisfies the following equation (1).

[0021] V≥2.75t 2 +8.19t-4.19 (t≥0.5mm)…(1)”.

[0022] Patent Document 1: Japanese Patent No. 5099009

[0023] Patent Document 2: Japanese Patent No. 6579596

[0024] Patent Document 3: Japanese Patent Application Publication No. 2018-111107

[0025] Patent Document 4: International Publication No. 2016 / 006377

[0026] However, when the technology of Patent Document 1 is applied to high-strength steel plates, especially those with a tensile strength (TS) of 980 MPa or higher, the joint strength (hereinafter referred to as joint strength) of the joint cannot be sufficiently obtained. Furthermore, when the technology of Patent Documents 2-4 is applied to high-strength steel plates, the joint strength still cannot be sufficiently obtained. Summary of the Invention

[0027] The present invention was developed in view of the above-mentioned situation, and its purpose is to provide a friction stirring point joint and a method thereof that have high joint strength when high-strength steel plates, especially steel plates with a tensile strength (TS) of 980 MPa or more, are used as the joined materials.

[0028] In addition, the present invention aims to provide a friction stirring point joining method for obtaining the above-mentioned friction stirring point joining joint.

[0029] Here, "high joint strength" means that the cross tensile strength determined by the cross tensile test according to JIS Z 3137 is above 7.0 kN.

[0030] Thus, in order to achieve the above objectives, the inventor conducted repeated and in-depth research. As a result, the inventor arrived at the following insights:

[0031] • An annular groove is formed on the upper surface of the joint, and

[0032] • At the same time, the shape, microstructure and hardness of the joint are appropriately controlled, specifically by making it simultaneously satisfy the following (a) to (d).

[0033] High joint strength can also be obtained when high-strength steel plates, especially those with a tensile strength (TS) of 980 MPa or higher, are used as the joining material.

[0034] (a) The vertical position of the deepest point of the recess of the joint is based on the lower mating surface of the steel plate that is the material to be joined, and satisfies at least one of -0.5 mm or less and -TL×0.50 or less.

[0035] (b) Using the upper mating surface of the steel plate that is being joined as the reference position, the vertical position of the highest point of the joint interface is set to +TU×0.50 or more.

[0036] (c) The microstructure of a specified region in the joint is such that the volume fraction of polygonal ferrite is 70% to 99% and the volume fraction of hard phase is 1% to 30%, and the average aspect ratio of the polygonal ferrite is 3.0 or less.

[0037] (d) The average hardness of the joint at the same vertical position as the upper mating surface of the steel plate that is the material to be joined is 400 HV or less.

[0038] Furthermore, the inventors conducted repeated research and obtained the following insights: In order to obtain the above-mentioned friction stirring point joint, it is important to simultaneously satisfy the following (e) to (g) during the jointing process.

[0039] (e) Regarding the vertical position of the tool's maximum indentation depth, with the lower mating surface of the steel plate as the reference position, it satisfies at least one of the positions of -0.5 mm or less and -TL×0.50 or less, and is in a state where the shoulder contacts the upper plate of the material being joined (by pressing the material being joined) when the tool reaches the maximum indentation depth.

[0040] (f) Set the maximum reach temperature to above 600°C and below 750°C.

[0041] (g) During engagement, the rotational speed of the tool changes from the initial rotational speed more than once, satisfying the following equation (1).

[0042] Rf / R1≤0.60 …(1)

[0043] here,

[0044] R1 is the initial rotational speed of the tool (rpm).

[0045] Rf is the minimum rotational speed (rpm) of the tool.

[0046] This invention was completed based on further research into the above-mentioned insights.

[0047] That is, the main structure of the present invention is as follows.

[0048] 1. A friction stirring point joint comprising two or more overlapping steel plates, a joint portion of the steel plates, and an annular groove on the upper surface of the joint portion, wherein,

[0049] The aforementioned joint has: a recess; a first flow portion adjacent to the recess; a second flow portion adjacent to the first flow portion; and a joint interface serving as the boundary between the first flow portion and the second flow portion.

[0050] The vertical position of the deepest point of the aforementioned recess, with the lower mating surface of the steel plate as the reference position, satisfies at least one of the following: a position below -0.5mm or below -TL×0.50.

[0051] The vertical position of the highest point of the aforementioned joint interface, with the upper mating surface of the steel plate as the reference position, is +TU×0.50 or greater.

[0052] In the aforementioned joint,

[0053] The vertical position, with the upper mating surface of the aforementioned steel plate as the reference position, is +TU×0.20 to +TU×0.50. The horizontal position, with the center of the aforementioned recess as the reference position, is the region where the microstructure of the area from -400μm to +100μm of the aforementioned joint interface consists of polygonal ferrite with a volume fraction of 70% to 99% and a hard phase volume fraction of 1% to 30%, and the average aspect ratio of the polygonal ferrite is 3.0 or less.

[0054] The average hardness of the joint at the same vertical position as the upper mating surface of the steel plate is below 400 HV.

[0055] Here, the upper plate and the lower plate refer to the topmost and bottommost steel plates among two or more overlapping steel plates, respectively.

[0056] TU is the thickness (mm) of the steel plate used for the top plate.

[0057] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0058] The upper mating surface is the mating surface between the upper plate and the steel plate adjacent to it.

[0059] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0060] In addition, the vertical position is indicated by "+" if it is above the reference position and "-" if it is below the reference position. The horizontal position is the distance from the reference position.

[0061] 2. The friction stirring point joint according to 1 above, wherein,

[0062] The composition of at least one of the above steel plates, expressed as a percentage by mass, is as follows:

[0063] C: Above 0.10%,

[0064] Si and Al: The total content is above 0.5%.

[0065] Mn: more than 1.5%,

[0066] P: below 0.10%,

[0067] S: below 0.050%, and

[0068] N: less than 0.010%, the remainder being Fe and unavoidable impurities.

[0069] 3. The friction stirring point joint according to 2 above, wherein,

[0070] The above-mentioned composition also contains one or more of the following ingredients selected from the following, expressed in % by mass:

[0071] Nb: below 0.050%,

[0072] Ti: below 0.050%,

[0073] B: Below 0.0050%,

[0074] V: Below 0.05%,

[0075] Cr: below 0.50%,

[0076] Mo: 0.50% or less,

[0077] Co: below 0.50%,

[0078] Cu: below 0.50%,

[0079] Ni: below 0.50%,

[0080] Sb: below 0.020%, and

[0081] Ca and REM: The total is below 0.010%.

[0082] 4. The friction stirring point joint according to any one of 1 to 3 above, wherein,

[0083] At least one of the steel plates mentioned above has a tensile strength of 980 MPa or higher.

[0084] 5. The friction stirring point joint according to any one of 1 to 4 above, wherein,

[0085] At least one of the steel plates mentioned above has a coating on its surface.

[0086] 6. The friction stirring point joint according to 5 above, wherein,

[0087] The above coating is a zinc-based coating.

[0088] 7. A friction stirring point joining method, wherein a tool having a shoulder and a pin protruding from the shoulder is rotated while the pin is pressed into two or more overlapping steel plates, which are materials to be joined, thereby joining the steel plates, wherein...

[0089] During engagement, the rotational speed of the aforementioned tool is changed from its initial rotational speed more than once.

[0090] The vertical position of the tool's maximum indentation depth is based on the lower mating surface of the steel plate, satisfying at least one of the following: a position of -0.5mm or less, or -TL×0.50 or less, and is the state in which the shoulder contacts the upper plate when the tool reaches its maximum indentation depth.

[0091] The maximum temperature reached is above 600℃ and below 750℃.

[0092] It satisfies the following equation (1).

[0093] Rf / R1≤0.60 …(1)

[0094] here,

[0095] R1 is the initial rotational speed of the tool (rpm).

[0096] Rf is the minimum rotational speed (rpm) of the tool.

[0097] In addition, the upper plate and the lower plate are respectively the uppermost and lowermost steel plates among two or more overlapping steel plates.

[0098] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0099] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0100] Furthermore, the vertical position is marked as "+" if it is above the reference position and "-" if it is below the reference position.

[0101] 8. A method for manufacturing a friction stirring point joint, wherein,

[0102] Two or more overlapping steel plates are joined using the friction stirring point joining method described in 7 above.

[0103] According to the present invention, a friction stirring point joint with high joint strength can be obtained even when high-strength steel plates, particularly steel plates with a tensile strength (TS) of 980 MPa or higher, are used as the bonding materials. This enables improved fuel efficiency based on vehicle body lightweighting, and has significant industrial application value. Attached Figure Description

[0104] Figure 1 This is a schematic diagram of the vertical cross-section of a friction stirring point joint according to an embodiment of the present invention.

[0105] Figure 2 This is a schematic diagram of the vertical cross-section of the friction stirring point joint (friction stirring point joint without an annular groove) of sample number 13. Detailed Implementation

[0106] The present invention will be described based on the following embodiments.

[0107] [1] Friction stirring point joint

[0108] First, using Figure 1 A friction stirring point joint according to an embodiment of the present invention will be described. Furthermore, in Figure 1 In this example, we take two steel plates as the materials being joined. Figure 1 The vertical section (hereinafter also referred to as the vertical section) of the joint is formed by the friction stirring point at the center of the recess of the joint. The positions of each part are all on the vertical section.

[0109] Furthermore, the upper side in the vertical direction (the thickness direction of the steel plate) refers to the side pressed in by the tool pin during friction stirring point joining. The lower side in the vertical direction refers to the side where the shim material is placed during friction stirring point joining (the opposite side to the side pressed in by the tool pin during friction stirring point joining). In addition, in friction stirring point joints, the upper and lower sides in the vertical direction can be determined based on their shape, such as the position of the recess. Hereinafter, the uppermost steel plate (located at the very top) of the steel plates to be joined will be called the upper plate, the lowermost steel plate (located at the very bottom) will be called the lower plate, and the steel plate located between the uppermost and lowermost (excluding the uppermost and lowermost plates) will be called the middle plate. The upper surface is the surface on the upper side in the vertical direction, and the lower surface is the surface on the lower side in the vertical direction.

[0110] like Figure 1 As shown, a friction stirring point joint according to an embodiment of the present invention comprises: overlapping steel plates 1, a joint portion 2 of the steel plates, and an annular groove portion 3 on the upper surface of the joint portion. Furthermore, the gap remaining between the overlapping steel plates (unjoined) is referred to as the unjoined interface 4.

[0111] [Joint]

[0112] like Figure 1 As shown, the joint has: a recess 2-1; a first flow portion 2-2 adjacent to the recess; a second flow portion 2-3 adjacent to the first flow portion; and a joint interface 2-4 serving as the boundary between the first flow portion and the second flow portion.

[0113] Here, the recess is formed by pressing in the pin of the tool during engagement. Furthermore, the recess is approximately circular when viewed from above in the vertical direction, and its diameter is almost the same as the diameter of the pin of the tool. Additionally, the center of the recess is the same as the center of the recess when viewed from above in the vertical direction.

[0114] The first flow section is a flow region formed by the plastic flow of the lower plate during joining, and it is formed adjacent to the periphery of the recess. The second flow section is a flow region formed by the plastic flow of the upper plate, and it is formed adjacent to the first flow section. Furthermore, when there are three or more steel plates being joined, the flow regions formed by the plastic flow of the middle plate are all included in the second flow section.

[0115] Furthermore, the first flow section and the second flow section are divided as follows. Thus, the interface that forms the boundary between the first flow section and the second flow section is determined.

[0116] Specifically, the vertical cross-section of the joint, located at the center of the concave portion of the joint, is ground and etched using a saturated aqueous solution of picric acid. Next, the vertical cross-section is observed using an optical microscope, and a first flow portion and a second flow portion are identified based on the matrix structure and the degree of etching in the flow portion. Furthermore, the boundary between the identified first and second flow portions is used as the joint interface.

[0117] Furthermore, it is important to control the shape, microstructure, and hardness of the joint as follows.

[0118] Here, the upper plate and the lower plate refer to the topmost and bottommost steel plates among two or more overlapping steel plates, respectively.

[0119] TU is the thickness (mm) of the steel plate used for the top plate.

[0120] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0121] The upper mating surface is the mating surface between the upper plate and the steel plate adjacent to it.

[0122] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0123] Additionally, the vertical position is indicated by a plus sign if it is above the reference position, and a minus sign if it is below the reference position. The horizontal position is the distance from the reference position (the reference position is set to 0, not - (not taking a negative value)).

[0124] (a) The vertical position of the deepest point of the concave part (hereinafter also referred to as the grade of the deepest point of the concave part)

[0125] Regarding the grade of the deepest point of the recess, the lower mating surface of the steel plate used as the joining material is set as the reference position, satisfying at least one of the following: -0.5 mm or less, or -TL×0.50 or less. This strengthens the joint interface, resulting in high joint strength even when a high-strength steel plate is used as the joining material. Furthermore, although not specifically limited, the grade of the deepest point of the recess is preferably -TL×0.90 or greater.

[0126] (b) The vertical position of the highest point of the joint interface (hereinafter also referred to as the grade of the highest point of the joint interface).

[0127] The grade of the highest point of the joint interface, based on the upper mating surface of the steel plate being joined, is +TU×0.50 or higher. Preferably, it is +TU×0.60 or higher. This strengthens the joint interface, resulting in high joint strength even when high-strength steel plates are used as the joined materials. Furthermore, a higher grade of the highest point of the joint interface is preferable; for example, the highest point of the joint interface can extend from around the shoulder of the tool to the area where burrs are expelled. A grade of +TU×0.98 or lower is more preferable.

[0128] Furthermore, the joint interface is typically a shape that curves upwards in the vertical direction (a shape that protrudes upwards in the vertical direction) in the vertical section.

[0129] In addition, the grade of the deepest point of the recess and the grade of the highest point of the joint interface can be determined by measuring the vertical cross-section of the joint using calipers or similar means, using images taken from the vertical cross-section of the joint to determine the friction stirring point of the joint interface.

[0130] (c) Microstructure of the region (hereinafter also referred to as the specified region in the joint) with the upper mating surface of the steel plate in the joint as the reference position in the vertical direction: +TU×0.20~+TU×0.50, and with the center of the recess as the reference position in the horizontal direction: -400μm of the joint interface to +100μm of the joint interface.

[0131] The microstructure of a specified region in the joint is such that the volume fraction of polygonal ferrite is 70% to 99%, the volume fraction of the hard phase is 1% to 30%, and the average aspect ratio of the polygonal ferrite is 3.0 or less. Therefore, when a high-strength steel plate is used as the bonding material, good toughness can be obtained in the joint, resulting in high joint strength. On the other hand, if at least one of the volume fraction of polygonal ferrite, the volume fraction of the hard phase, and the average aspect ratio of the polygonal ferrite is outside the above range, good toughness cannot be obtained in the joint when a high-strength steel plate is used as the bonding material. As a result, the joint strength is also reduced.

[0132] Here, the volume fraction of the polygonal ferrite is preferably 75% or more. The volume fraction of the polygonal ferrite is preferably 97% or less. The volume fraction of the hard phase is preferably 3% or more. The volume fraction of the hard phase is preferably 25% or less.

[0133] Furthermore, the volume fraction of the remaining microstructure other than polygonal ferrite and hard phase is preferably 20% or less. Examples of the remaining microstructure include bainite, retained austenite, and pearlite, which are well-known microstructures in other steel plates. The remaining microstructure can be identified, for example, by observation based on SEM (Scanning Electron Microscope).

[0134] In addition, the volume fraction of the remaining tissue is calculated using the following formula.

[0135] [Volume fraction of remaining tissue (%)] = 100 - [Volume fraction of polygonal ferrite (%)] - [Volume fraction of hard phase (%)]

[0136] In addition, the volume fraction of polygonal ferrite and hard phase is determined, for example, as follows.

[0137] That is, with Figure 1 The test piece was cut from the friction-stirring point joint using a designated area within the joint as the observation surface, based on the vertical cross-section of the joint (the vertical cross-section of the friction-stirring point joint through the center of the concave part of the joint). Next, after grinding the observation surface of the test piece, it was etched with 3 vol.% nitric acid ethanol to reveal the microstructure. Then, for the designated area within the joint, six fields of view (SEM) were captured at 10000x magnification: four fields of view from the region closer to the concave part of the joint interface (first flow section) and two fields of view from the region opposite to the concave part of the joint interface (second flow section) (side away from the concave part). Based on the obtained microstructure images, the area of ​​polygonal ferrite and hard phase in each of the six fields of view was calculated using Adobe Photoshop (Adobe Systems). The area of ​​polygonal ferrite and hard phase calculated for each field of view was then divided by the area of ​​the respective field of view. The arithmetic mean of these values ​​was then used as the volume fraction of polygonal ferrite and hard phase, respectively.

[0138] Furthermore, in the aforementioned microstructure images, the polygonal ferrite appears as a black microstructure, while the hard phase appears as a gray microstructure. This allows for differentiation between the two. Additionally, the hard phase is composed of carbides such as martensite and cementite.

[0139] Furthermore, the average aspect ratio of the polygonal ferrite is preferably 2.5 or less. While there is no particular limitation on the lower limit of the average aspect ratio of the polygonal ferrite, it is preferably 1.2 or more.

[0140] Here, the average aspect ratio of the polygonal ferrite is determined, for example, as follows.

[0141] That is, in the microstructure image obtained by measuring the volume fraction of polygonal ferrite and the hard phase, five grains observed in the region defined as polygonal ferrite are randomly selected for each field of view. For the five selected grains, the major and minor axes are calculated, and the major axis is divided by the minor axis. Then, the average of these values ​​is taken as the average aspect ratio of the polygonal ferrite.

[0142] Furthermore, the major axis refers to the diameter of the largest grain on a straight line passing through the centroid of the grain. The minor axis refers to the diameter of the smallest grain on a straight line passing through the centroid of the grain.

[0143] In addition, the region in the joint where the microstructure is determined is set as follows: the vertical position with the upper mating surface of the steel plate as the reference position: +TU×0.20~+TU×0.50, and the horizontal position with the center of the recess as the reference position: the region from the position of the joint interface to the position of the joint interface +100μm.

[0144] That is, in cross tensile tests, fracture sometimes occurs around the joint interface. In this case, the fracture initiation point corresponds to the vertical direction position near the joint interface and with the upper mating surface as the reference: the area of ​​+TU×0.20 to +TU×0.50. In particular, the horizontal direction position with the center of the recess as the reference position: the area of ​​the joint interface position -400 μm to the joint interface position +100 μm is often used as the fracture path. Therefore, the area in the joint where the microstructure is determined is the area described above.

[0145] Furthermore, the position of the joint interface (horizontal position) varies depending on the vertical position (grade), so the position of the joint interface at -400μm and the position of the joint interface at +100μm also vary depending on the vertical position (grade).

[0146] For example, the horizontal position of the joint interface under vertical position: +TU×0.20 may not be the same as the horizontal position of the joint interface under vertical position: +TU×0.50. Therefore, the horizontal position of the joint interface at -400μm under vertical position: +TU×0.20 may also not be the same as the horizontal position of the joint interface at -400μm under vertical position: +TU×0.50.

[0147] (d) The average hardness of the joint at the same vertical position as the upper mating surface of the steel plate (hereinafter also referred to as the average hardness of the joint).

[0148] The average hardness of the joint is kept below 400 HV. This ensures that, when high-strength steel plates are used as the joined materials, good toughness is achieved at the joint, resulting in high joint strength. Conversely, if the average hardness of the joint exceeds 400 HV, good toughness is not achieved at the joint when high-strength steel plates are used as the joined materials. Consequently, the joint strength also decreases. Furthermore, the average hardness of the joint is preferably below 380 HV. Additionally, the average hardness of the joint is preferably above 250 HV.

[0149] Here, the average hardness of the joint is measured according to JIS Z 2244. Specifically, it is performed as follows.

[0150] That is, to Figure 1 The vertical section of the friction stirring point joint shown (the vertical section of the friction stirring point joint at the center of the recess of the joint) is used as the measurement surface. Furthermore, taking the position of the joint interface at the same vertical direction (grade) as the upper mating surface of the steel plate as the starting point on this measurement surface, and moving horizontally towards the recess, the Vickers hardness (HV) is measured under the conditions of a test force of 2.942 N and a measurement interval of 0.2 mm (until reaching the recess). The arithmetic mean of these measured values ​​is then taken as the average hardness of the joint.

[0151] [Annular groove]

[0152] like Figure 1 As shown, an annular groove is formed on the upper surface of the joint. Furthermore, the annular groove is configured to surround a recess on the upper surface of the joint. That is, the annular groove is formed during jointing by contact between the shoulder of the tool and the upper plate. By forming the annular groove, the joint interface is strengthened, and high joint strength can be obtained even when high-strength steel plates are used as the joined materials.

[0153] Furthermore, the presence or absence of the annular groove can be confirmed by visually observing the appearance of the upper surface of the friction stirring point joint and the vertical cross-section of the friction stirring point joint.

[0154] [Steel plate]

[0155] The number of steel plates used as joining materials only needs to be two or more. There is no particular upper limit to the number of steel plates used as joining materials, but it is preferable to use five or fewer. The steel plates used as joining materials can be all of the same steel grade or different steel grades.

[0156] Furthermore, the thickness of the steel plates used as the joining material is preferably 0.4 to 3.2 mm. Additionally, steel plates of the same thickness can be used as the joining material, or steel plates of different thicknesses can be used.

[0157] In addition, as at least one of the steel plates that are to be joined, the high-strength steel plate is, for example, expressed as a percentage by mass:

[0158] C: Above 0.10%,

[0159] Si and Al: The total content is above 0.5%.

[0160] Mn: more than 1.5%,

[0161] P: below 0.10%,

[0162] S: below 0.050%, and

[0163] N: below 0.010%,

[0164] It is preferable to use steel plates whose remaining portion consists of Fe and unavoidable impurities. Furthermore, all composition units are "mass%" and will be expressed as "%" unless otherwise specified.

[0165] C: 0.10% or more

[0166] C is an effective element for increasing the strength of steel plates. Furthermore, C effectively contributes to the formation of carbides in the microstructure (corresponding to the hard phase) of a specified region within the joint. Therefore, the C content is preferably 0.10% or more. The C content is more preferably 0.15% or more. In addition, although not particularly limited, from the viewpoint of preventing a decrease in resistance to delayed failure, the C content is preferably 1.0% or less.

[0167] Si and Al: Total 0.5% or more

[0168] Si and Al are elements that suppress the excessive formation of carbides in the joint and prevent a decrease in the toughness of the joint. Therefore, the total content of Si and Al is preferably 0.5% or more. Furthermore, although not particularly limited, from the viewpoint of preventing a decrease in plating performance, the total content of Si and Al is preferably 4.0% or less.

[0169] Mn: 1.5% or more

[0170] Mn is an element that contributes to high strength by generating solid solution strengthening and a hard phase (as a second phase). Furthermore, Mn is an element that stabilizes austenite and is effective in controlling the proportion of the hard phase. Therefore, the Mn content is preferably 1.5% or more. In addition, although not particularly limited, from the viewpoint of preventing a decrease in plating properties, the Mn content is preferably 10% or less.

[0171] P: below 0.10%

[0172] In cases where phosphorus (P) is present in excess, its segregation towards grain boundaries becomes significant, raising concerns about grain boundary embrittlement. Therefore, the P content is preferably 0.10% or less. Furthermore, although not particularly limited, extremely low P content leads to increased steelmaking costs, so the P content is preferably 0.005% or more.

[0173] S: below 0.050%

[0174] When the sulfur content is high, many sulfides such as MnS are formed, raising concerns about reduced joint strength. Therefore, the sulfur content is preferably 0.050% or less. Furthermore, although not specifically limited, extremely low sulfur content leads to increased steelmaking costs; therefore, the sulfur content is preferably 0.0002% or more.

[0175] N: below 0.010%

[0176] Nitrogen (N) can form coarse nitrides, which may reduce joint strength. Therefore, it is preferable to reduce the N content, preferably to 0.010% or less. Furthermore, while not particularly limited, extremely low N content leads to increased steelmaking costs; therefore, the N content is preferably 0.0005% or more.

[0177] In addition to the basic components mentioned above, it may also contain one or more of the following:

[0178] Nb: below 0.050%,

[0179] Ti: below 0.050%,

[0180] B: Below 0.0050%,

[0181] V: Below 0.05%,

[0182] Cr: below 0.50%,

[0183] Mo: 0.50% or less,

[0184] Co: below 0.50%,

[0185] Cu: below 0.50%,

[0186] Ni: below 0.50%,

[0187] Sb: below 0.020%, and

[0188] Ca and REM: The total is below 0.010%.

[0189] Nb: below 0.050%

[0190] Nb forms fine carbonitride compounds, reducing the grain size of the old austenite. This helps to improve joint strength. From the viewpoint of achieving this effect, the Nb content is preferably 0.005% or more. On the other hand, if the Nb content is excessive, it leads to a decrease in elongation. In addition, it is prone to causing slab cracks after continuous casting. Therefore, when Nb is present, its content is preferably 0.050% or less. The Nb content is more preferably 0.045% or less, and even more preferably 0.040% or less.

[0191] Ti: below 0.050%

[0192] Like Nb, Ti forms fine carbonitride compounds, reducing the particle size of the old austenite. This helps to improve joint strength. From the viewpoint of achieving this effect, the Ti content is preferably 0.005% or more. The Ti content is more preferably 0.008% or more. On the other hand, if Ti is present in excess, it leads to a decrease in elongation. Therefore, when Ti is present, its content is preferably 0.050% or less. The Ti content is more preferably 0.045% or less.

[0193] B: Below 0.0050%

[0194] Boron (B) improves hardenability and inhibits the formation of ferrite and pearlite. This contributes to increasing the strength of the base material and the joint. From the viewpoint of achieving this effect, the B content is preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0050%, the effect becomes saturated. Therefore, when B is present, its content is preferably 0.0050% or less.

[0195] V: Below 0.05%

[0196] V is an element that contributes to increased strength by forming fine carbonitridium compounds. From the viewpoint of achieving this effect, the content of V is preferably 0.005% or more. On the other hand, if the content of V exceeds 0.05%, the strength-increasing effect becomes smaller. In addition, it leads to an increase in cost. Therefore, when V is present, its content is preferably 0.05% or less.

[0197] Cr: less than 0.50%

[0198] Cr is an element that contributes to high strength by forming a hard phase. From the viewpoint of achieving this effect, the Cr content is preferably 0.05% or more. On the other hand, if the Cr content exceeds 0.50%, excessive martensite formation can easily lead to surface defects. Therefore, when Cr is present, its content is preferably 0.50% or less. The Cr content is more preferably 0.45% or less.

[0199] Mo: 0.50% or less

[0200] Like Cr, Mo is an element that contributes to increased strength by forming a hard phase. Furthermore, Mo also contributes to increased strength by forming some carbides. From the viewpoint of achieving such effects, the Mo content is preferably 0.05% or more. On the other hand, if the Mo content exceeds 0.50%, the effect saturates, leading to increased costs. Therefore, when Mo is present, its content is preferably 0.50% or less. More preferably, the Mo content is 0.32% or less.

[0201] Co: below 0.50%

[0202] Co is an element that helps improve resistance to delayed degradation by increasing hydrogen overvoltage. From the viewpoint of achieving this effect, the Co content is preferably 0.05% or more. The Co content is more preferably 0.10% or more. On the other hand, if the Co content exceeds 0.50%, the effect saturates, leading to an increase in cost. Therefore, when Co is present, its content is preferably 0.50% or less. The Co content is more preferably 0.32% or less.

[0203] Cu: below 0.50%

[0204] Cu is an element that contributes to increased strength through solid solution strengthening. Furthermore, Cu also contributes to increased strength by forming a hard phase. From the viewpoint of achieving these effects, the Cu content is preferably 0.005% or more. On the other hand, if the Cu content exceeds 0.50%, the effect saturates, leading to increased costs. Additionally, surface defects caused by Cu are more likely to occur. Therefore, when Cu is present, its content is preferably 0.50% or less.

[0205] Ni: below 0.50%

[0206] Like Cu, Ni is an element that contributes to increased strength through solid solution strengthening. Furthermore, Ni also contributes to increased strength by forming a hard phase. Additionally, when Ni is present alongside Cu, it has the effect of suppressing surface defects caused by Cu. Therefore, when Cu is present, the simultaneous presence of Ni is effective. From the viewpoint of achieving this effect, the Ni content is preferably 0.005% or more. On the other hand, if the Ni content exceeds 0.50%, the effect saturates. Therefore, when Ni is present, its content is preferably 0.50% or less.

[0207] Sb: below 0.020%

[0208] Sb contributes to increased strength by suppressing the formation of a decarburized layer on the surface of the steel sheet. From the viewpoint of achieving this effect, the Sb content is preferably 0.001% or more. On the other hand, if the Sb content exceeds 0.020%, the rolling load increases, and productivity decreases. Therefore, in the case of Sb, its content is preferably 0.020% or less.

[0209] Ca and REM: The total is below 0.010%.

[0210] Ca and REM are elements that contribute to increased strength by spherizing sulfides. From the viewpoint of achieving this effect, the combined content of Ca and REM is preferably 0.0005% or more. On the other hand, if the combined content of Ca and REM exceeds 0.010%, the effect saturates. Therefore, when Ca and / or REM are present, their combined content is preferably 0.010% or less. Furthermore, the combined content of Ca and REM is more preferably 0.0050% or less.

[0211] The remaining portion, excluding the above, consists of Fe and unavoidable impurities. Examples of unavoidable impurities include Sn and Zn, which are permissible as long as Sn is 0.01% or less and Zn is 0.01% or less. Furthermore, even within the typical steel composition range, specifically containing 0.01% or less of Ta, Mg, and Zr respectively, will not diminish its effectiveness.

[0212] Furthermore, regarding the aforementioned arbitrary added element, if its content is below the (preferred) lower limit, the presence of the arbitrary added element is an unavoidable impurity.

[0213] Furthermore, at least one of the steel plates used as the joining materials is preferably a steel plate with a tensile strength of 980 MPa or higher. The tensile strength is determined according to JIS Z 2241 (1998).

[0214] In addition, at least one of the steel plates used as joining materials may also have a coating on its surface. The coating may be applied to both sides of the steel plate or to only one side. Examples of coatings include zinc-based coatings. Furthermore, zinc-based coatings refer to coatings with zinc as the main component (coatings with a zinc content of 50% by mass or more), such as hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electro-galvanized layers.

[0215] Furthermore, the aforementioned steel plates can be manufactured using conventional methods.

[0216] [2] Friction stirring point joining method and manufacturing method of friction stirring point joining joint

[0217] Next, a friction stirring point joining method according to an embodiment of the present invention and a friction stirring point joining joint manufacturing method according to an embodiment of the present invention will be described.

[0218] According to one embodiment of the present invention, a friction stirring point joining method involves rotating a tool having a shoulder and a pin protruding from the shoulder while pressing the pin into two or more overlapping steel plates that are being joined, thereby joining the steel plates.

[0219] During engagement, the rotational speed of the aforementioned tool is changed from its initial rotational speed more than once.

[0220] The vertical position of the tool's maximum indentation depth is based on the lower mating surface of the steel plate, satisfying at least one of the following: a position of -0.5mm or less, or -TL×0.50 or less, and is the state in which the shoulder contacts the upper plate when the tool reaches its maximum indentation depth.

[0221] The maximum temperature reached is above 600℃ and below 750℃.

[0222] It satisfies the following equation (1).

[0223] Rf / R1≤0.60 …(1)

[0224] here,

[0225] R1 is the initial rotational speed of the tool (rpm).

[0226] Rf is the minimum rotational speed (rpm) of the tool.

[0227] In addition, the upper plate and the lower plate are respectively the uppermost and lowermost steel plates among two or more overlapping steel plates.

[0228] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0229] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0230] Furthermore, the vertical position is marked as "+" if it is above the reference position and "-" if it is below the reference position.

[0231] Furthermore, the friction stirring point joining method according to an embodiment of the present invention is a method for obtaining a friction stirring point joining joint according to the above-described embodiment of the present invention.

[0232] Furthermore, the method for manufacturing a friction stirring point joint according to an embodiment of the present invention involves joining two or more overlapping steel plates using the friction stirring point joining method according to the above-described embodiment of the present invention.

[0233] Furthermore, the method for manufacturing a friction stirring point joint according to an embodiment of the present invention is a method for manufacturing a friction stirring point joint according to the above-described embodiment of the present invention.

[0234] Two or more overlapping steel plates, which are to be joined, are joined using a tool having a shoulder and a pin protruding from the shoulder. Specifically, the pin of the tool is pressed into the materials to be joined while the tool is rotated, thus joining the steel plates. Here, the tool is not particularly limited, and a common tool can be used. In addition, the shape of the shoulder and pin, such as the diameter of the shoulder (hereinafter also referred to as shoulder diameter), the diameter of the pin (hereinafter also referred to as pin diameter), the length of the pin (hereinafter also referred to as pin length), and the inclination angle of the pin side are not particularly limited, as long as they are determined according to the steel grade and thickness of the steel plates to be joined. For example, when two steel plates with a tensile strength of 980 MPa or more and a thickness of 1.6 mm are used as the materials to be joined, it is preferable that the shoulder diameter is 6.0 to 16.0 mm, the pin diameter is 3.0 to 8.0 mm, the pin length is 2.0 to 3.5 mm, and the inclination angle of the pin side is 3 to 30°.

[0235] Additionally, during engagement, a liner is placed on the lower surface of the material being joined (the side opposite to the pin of the press-in tool).

[0236] Furthermore, in the friction stirring point bonding method according to an embodiment of the present invention, it is important to satisfy the following conditions.

[0237] (e) Vertical position of the tool's maximum indentation depth

[0238] Regarding the vertical position of the tool's maximum indentation depth, taking the lower mating surface of the steel plate as a reference position, it must satisfy at least one of the following: -0.5 mm or less, or -TL×0.50 or less, and must be in a state where the shoulder contacts the upper plate of the material being joined (by pressing the material being joined) when the tool reaches its maximum indentation depth. By appropriately controlling the vertical position of the tool's indentation depth, material flow can be promoted, resulting in a higher highest point of the joint interface. Furthermore, by having the shoulder contact the upper plate of the material being joined, the overlapping steel plates are brought closer together, strengthening the joint interface. Although not specifically limited, the vertical position of the tool's indentation depth is preferably -TL×0.90 or greater.

[0239] (f) Maximum temperature reached

[0240] The maximum temperature reached at the joint is set to 600°C or higher and 750°C or lower. This allows the microstructure of a defined region within the joint to have a volume fraction of polygonal ferrite of 70% to 99% and a volume fraction of hard phase of 1% to 30%. The maximum temperature reached is preferably 640°C or higher. Furthermore, the maximum temperature reached is preferably 720°C or lower.

[0241] Here, the highest temperature reached is measured, for example, as follows.

[0242] That is, a thermocouple is installed inside the tool, specifically near the tip of the pin of the tool that contacts the joint. Then, the temperature during the joint (the temperature rising at the joint) is continuously measured using the thermocouple inside the tool. The highest temperature measured during the joint is then set as the highest reached temperature.

[0243] (g) Rotational speed of the tool

[0244] During the joining process, the rotational speed of the tool is changed from the initial rotational speed more than once, satisfying the following formula (1). As a result, in the microstructure of a specified area in the joint, the average aspect ratio of the polygonal ferrite is 3.0 or less.

[0245] Rf / R1≤0.60 …(1)

[0246] here,

[0247] R1 is the initial rotational speed of the tool (rpm).

[0248] Rf is the minimum rotational speed (rpm) of the tool.

[0249] Rf / R1 is preferably 0.50 or less. Alternatively, Rf / R1 is preferably 0.10 or more.

[0250] Furthermore, the initial rotational speed of the tool is not particularly limited as long as it satisfies the above formula (1), and can be determined according to the steel grade and thickness of the steel plates used as the joining materials. For example, when two steel plates with a tensile strength of 980 MPa or more and a thickness of 1.6 mm are used as the joining materials, the initial rotational speed of the tool is preferably 300 to 1000 rpm.

[0251] In addition, the tool's rotation speed only needs to be switched more than once. Although there is no specific limitation, it is preferable that the tool's rotation speed is switched no more than 5 times.

[0252] There is no particular limitation on when to switch the rotation speed of the tool, but for example, when two steel plates with a tensile strength of 980 MPa or more and a thickness of 1.6 mm are used as the materials to be joined, it is preferable when the tool (pin) is pressed into the upper surface of the upper plate to a depth of 0.8 to 2.4 mm.

[0253] Furthermore, there is no particular limitation on the applied pressure, but it is preferable to switch the applied pressure in conjunction with the switching of the tool's rotation speed. For example, when using two steel plates with a tensile strength of 980 MPa or more and a thickness of 1.6 mm as the materials to be joined, it is preferable to set the applied pressure to 10~60 kN at the tool's rotation speed = R1 (initial rotation speed) and to set the applied pressure to 20~70 kN at the tool's rotation speed = Rf (minimum rotation speed).

[0254] Furthermore, the engagement time is preferably 3 to 60 seconds.

[0255] There are no special restrictions on conditions other than those mentioned above; just follow the usual methods.

[0256] Example

[0257] The embodiments of the present invention will be described in more detail below. However, the present invention is not limited to the following embodiments, and modifications can be appropriately made to implement it within the scope that conforms to the spirit of the present invention. All such modifications are included within the technical scope of the present invention.

[0258] Steel with the composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was smelted and continuously cast into slabs. Next, the slabs were hot-rolled to obtain hot-rolled steel sheets. Then, the hot-rolled steel sheets were pickled. Next, the hot-rolled steel sheets were cold-rolled and annealed to obtain cold-rolled steel sheets (thickness: 1.2~1.6 mm). Then, for steel grade C in Table 1, the cold-rolled steel sheets were further coated to obtain steel sheets with a coating (hot-dip galvanized layer) on both sides (hot-dip galvanized steel sheets). Next, JIS No. 5 tensile test specimens were collected from the obtained steel sheets with the rolling right angle as the length direction (tensile direction), and tensile tests were performed according to JIS Z 2241 (1998) to determine the tensile strength (TS). The results are recorded in Table 1.

[0259] 50mm × 150mm test pieces were cut from the steel plates obtained in this way, and overlapped into a cross shape according to the combinations shown in Table 2. Furthermore, in the case of three overlapping pieces, the upper plate and middle plate were overlapped in a cross shape, and the middle plate and lower plate were overlapped parallel to each other (i.e., the cross tensile strength between the upper plate and middle plate was measured in the cross tensile strength determination described later). Next, friction stirring point bonding was performed on the center of the cross overlap portion of the test pieces under the conditions shown in Table 3, resulting in a friction stirring point bonded joint.

[0260] Furthermore, the tool has the following dimensions: shoulder diameter: 10 mm, pin diameter: 4.8 mm, pin side inclination angle: 10°, and pin length: 1.8 mm to 4.6 mm depending on the engagement conditions. In addition, rotational speeds were switched for all specimens except specimen 16 and 18. For specimens with switched rotational speeds, except for specimen 24, the speed switch was performed once. For specimen 24, the speed switch was performed twice. In all cases, the timing of the speed switch was based on the tool's indentation depth from the upper surface of the upper plate (the "tool indentation depth at switching" listed in Table 3 refers to the tool's indentation depth from the upper surface of the upper plate). The applied pressure was also switched at the time of the speed switch.

[0261] Furthermore, when the tool reached its maximum indentation depth, the contact state between the outer periphery of the shoulder and the upper plate was visually confirmed. In specimen number 13, the outer periphery of the shoulder did not contact the upper plate. On the other hand, in all specimens except specimen number 13, the outer periphery of the shoulder contacted the upper plate when the tool reached its maximum indentation depth. For reference, in Figure 2 The diagram shows a schematic of the vertical cross-section of the friction stirring point joint (friction stirring point joint without an annular groove) of specimen number 13.

[0262] For the friction stirring point joint obtained in this way, the following methods were used to determine the grade of the deepest point of the recess, the grade of the highest point of the joint interface, the identification of the microstructure of a specified area in the joint, the determination of the average hardness of the joint, and the confirmation of the presence or absence of the annular groove. The results are shown in Table 4.

[0263] Furthermore, the same friction stirring joint was fabricated under the same conditions, and a cross tensile test was performed using it according to JIS Z 3137 to determine the cross tensile strength. The results are shown in Table 4. Furthermore, a cross tensile strength of 7.0 kN or higher was considered acceptable.

[0264] [Table 1]

[0265]

[0266] [Table 2]

[0267]

[0268] [Table 3]

[0269]

[0270] [Table 4]

[0271]

[0272] As shown in Table 4, in the invention examples, the cross tensile strength is above 7.0kN, resulting in high joint strength.

[0273] On the other hand, in the comparative example, sufficient joint strength was not obtained.

[0274] Explanation of reference numerals in the attached figures

[0275] 1…steel plate; 2…joint; 3…annular groove; 4…unjoined interface; 2-1…recess; 2-2…first flow section; 2-3…second flow section; 2-4…joint interface.

Claims

1. A friction stirring point joint comprising two or more overlapping steel plates, a joint portion of the steel plates, and an annular groove on the upper surface of the joint portion, characterized in that, The joint has: a recess; a first flow portion adjacent to the recess; a second flow portion adjacent to the first flow portion; and a joint interface serving as the boundary between the first flow portion and the second flow portion. The vertical position of the deepest point of the recess, with the lower mating surface of the steel plate as the reference position, satisfies at least one of the following: a position below -0.5mm or below -TL×0.

50. The vertical position of the highest point of the joint interface is based on the upper mating surface of the steel plate, and is greater than +TU×0.

50. In the joint, The vertical position with the upper mating surface of the steel plate as the reference position is +TU×0.20 to +TU×0.

50. The horizontal position with the center of the recess as the reference position is: the microstructure of the region in the horizontal direction from the joint interface offset 400 μm towards the recess to the position offset 100 μm from the joint interface to the opposite side of the recess is composed of polygonal ferrite with a volume fraction of 70% to 99% and a hard phase with a volume fraction of 1% to 30%, and the average aspect ratio of the polygonal ferrite is 3.0 or less. The average hardness of the joint at the same vertical position as the upper mating surface of the steel plate is below 400 HV. Here, the upper plate and the lower plate refer to the topmost and bottommost steel plates among two or more overlapping steel plates, respectively. TU is the thickness of the steel plate used for the top plate, and its unit is mm. TL represents the thickness of the steel plate used for the bottom plate, and its unit is mm. The upper mating surface is the mating surface between the upper plate and the steel plate adjacent to it. The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to it. In addition, the vertical position is marked as "+" if it is higher than the reference position and "-" if it is lower than the reference position, and the horizontal position is the distance from the reference position.

2. The friction stirring point joint according to claim 1, characterized in that, The composition of at least one of the steel plates, expressed as a percentage by mass, is as follows: C: Above 0.10%, Si and Al: The total content is above 0.5%. Mn: more than 1.5%, P: below 0.10%, S: below 0.050%, and N: below 0.010%, The remainder consists of Fe and unavoidable impurities.

3. The friction stirring point joint according to claim 2, characterized in that, The composition further comprises one or more of the following components, expressed in mass percent: Nb: below 0.050%, Ti: below 0.050%, B: Below 0.0050%, V: Below 0.05%, Cr: below 0.50%, Mo: 0.50% or less, Co: below 0.50%, Cu: below 0.50%, Ni: below 0.50%, Sb: below 0.020%, and Ca and REM: The total is below 0.010%.

4. The friction stirring point joint according to any one of claims 1 to 3, characterized in that, At least one of the steel plates has a tensile strength of 980 MPa or higher.

5. The friction stirring point joint according to any one of claims 1 to 3, characterized in that, At least one of the steel plates has a coating on its surface.

6. The friction stirring point joint according to claim 4, characterized in that, At least one of the steel plates has a coating on its surface.

7. The friction stirring point joint according to claim 5, characterized in that, The coating is a zinc-based coating.

8. The friction stirring point joint according to claim 6, characterized in that, The coating is a zinc-based coating.

9. A friction stirring point joining method, wherein a tool having a shoulder and a pin protruding from the shoulder is rotated while the pin is pressed into two or more overlapping steel plates, which are materials to be joined, thereby joining the steel plates, characterized in that, During engagement, the rotational speed of the tool is changed from its initial rotational speed more than once. The vertical position of the tool at its maximum indentation depth is based on the lower mating surface of the steel plate, and satisfies at least one of the following: below -0.5mm or below -TL×0.

50. Furthermore, the tool is in contact with the upper plate when it reaches its maximum indentation depth. The maximum temperature reached is above 600℃ and below 750℃. It satisfies the following equation (1). Rf / R1≤0.60 …(1) here, R1 is the initial rotational speed of the tool, and its unit is rpm. Rf is the minimum rotational speed of the tool, and its unit is rpm. In addition, the upper plate and the lower plate are respectively the uppermost and lowermost steel plates among two or more overlapping steel plates. TL represents the thickness of the steel plate used for the bottom plate, and its unit is mm. The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to it. Furthermore, the vertical position is marked as "+" if it is above the reference position and "-" if it is below the reference position.

10. A method for manufacturing a friction stirring point joint, characterized in that, Two or more overlapping steel plates are joined by the friction stirring point joining method as described in claim 9.

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