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 cross tensile strength even when a steel sheet having a surface soft layer is used as a material to be joined. A ring groove portion is formed in an upper surface of an upper sheet of the overlapped steel sheets, and the shape 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] When assembling automobiles, considering cost and efficiency, most of the stamped steel materials (parts) are joined by resistance welding.

[0003] In addition, the application of friction stir point bonding in steel materials has been studied in recent years. Here, friction stir point bonding refers to a point bonding method that utilizes the phenomenon of friction stir. Specifically, while rotating a tool with a shoulder and a pin protruding from the shoulder, the pin of the tool is pressed (pushed) into the material to be bonded, which consists of overlapping metal plates. As a result, the metal plates soften and plastically flow, bonding the overlapping metal plates.

[0004] As a technology related to such frictional stirring point engagement, for example, the following is disclosed in Patent Document 1:

[0005] "A friction stirring point bonding method involves overlapping a first component and a second component, both made of metallic material, with their softer portions located at their boundaries. While rotating the pin of a rotating tool, the tool is pressed from the first component into the second component, and the first and second components are stirred, thereby achieving point bonding."

[0006] At least one of the first and second components mentioned above is heated to 800°C and strained at a rate of 10. -3 s -1 It is composed of metallic materials with a tensile strength of over 100 MPa.

[0007] The aforementioned soft portion has a hardness at room temperature that is lower than the hardness of the base material of the aforementioned metallic material with a tensile strength of 100 MPa or more.

[0008] The aforementioned soft portion is a soft layer formed at least on the surface portion of the aforementioned metallic material having a tensile strength of 100 MPa or more.

[0009] The aforementioned metallic material with a tensile strength of 100 MPa or more is carbon steel with a carbon content of 0.3% or more by mass, and the aforementioned soft layer is a decarburized layer with a carbon content of 0.1% or less by mass.

[0010] Patent Document 1: Japanese Patent No. 6160345

[0011] However, if the cross tensile test is performed using the friction-stirring point joint obtained by the technology of Patent Document 1, the following problem exists: the soft part located at the boundary between the first and second parts breaks, and sufficient cross tensile strength cannot be obtained. Summary of the Invention

[0012] The present invention was developed to solve the above-mentioned problems, and its purpose is to provide a friction stirring point joint with high cross tensile strength even when a steel plate with a soft layer (hereinafter also referred to as a soft layer) is used as the bonding material. The manufacturing method thereof is also described.

[0013] In addition, the present invention aims to provide a friction stirring point joining method for obtaining the above-mentioned friction stirring point joining joint.

[0014] Here, "high cross tensile strength" means that the cross tensile strength determined by the cross tensile test according to JIS Z 3137 is above 8.0 kN.

[0015] Thus, in order to achieve the above objectives, the inventors conducted repeated and in-depth research. As a result, they arrived at the following insights:

[0016] • An annular groove is formed on the upper surface of the joint, and,

[0017] • At the same time, the shape and hardness of the joint should be appropriately controlled, specifically, the following (a) to (c) should be satisfied simultaneously.

[0018] Therefore, high joint strength can be obtained when using steel plates with a soft surface layer as the bonding material.

[0019] (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 the positions of -0.5 mm or less and -TL×0.50 or less.

[0020] (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.

[0021] (c) At the vertical position: +TU×0.50 or +TU×0.60, at the horizontal position, the difference between the hardness of the joint interface position and the hardness of the joint interface position +350μm is set to less than 80HV.

[0022] In addition, the inventors conducted further research and came to the following conclusions: In order to obtain the above-mentioned friction stirring point joint, it is important to satisfy the following (d) to (f) at the same time during the joint.

[0023] (d) 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 -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 by the shoulder) when the tool reaches the maximum indentation depth.

[0024] (e) Set the maximum reach temperature to below 850°C.

[0025] (f) Appropriately control the engagement conditions, specifically, the tool rotation speed, tool infeed and pressure should satisfy the following relationship (1).

[0026] [Formula 1]

[0027]

[0028] In equation (1),

[0029] n is the number of times the joining conditions are switched.

[0030] R k+1 It is the tool rotation speed (rpm) after the k-th engagement condition switch.

[0031] A k+1 It is the tool indentation (mm) from the time of the kth engagement condition switch to the time of the (k+1)th engagement condition switch.

[0032] P k+1 It is the applied pressure (N) after the k-th engagement condition switch.

[0033] Where R1 is the initial tool rotation speed (rpm), A1 is the tool indentation under initial engagement conditions (mm), and P1 is the initial applied force (N). Additionally, A... n+1 The tool indentation amount (mm) is the amount of material pressed in from the moment the engagement condition is switched at the nth time until the maximum indentation depth of the tool is reached.

[0034] This invention was completed based on further research into the above-mentioned insights.

[0035] That is, the main structure of the present invention is as follows.

[0036] 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,

[0037] At least one of the aforementioned steel plates has a soft surface layer.

[0038] 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.

[0039] The vertical position of the deepest point of the aforementioned recess, with the lower mating surface of the aforementioned steel plate as the reference position, satisfies at least one of the following: a position of -0.5 mm or less, or a position of -TL×0.50 or less.

[0040] The vertical position of the highest point of the aforementioned joint interface, with the upper mating surface of the aforementioned steel plate as the reference position, is +TU×0.50 or greater.

[0041] At the vertical position: +TU×0.50 or +TU×0.60, and at the horizontal position, the hardness difference between the above-mentioned joint interface position and the hardness at the above-mentioned joint interface position +350μm is less than 80HV.

[0042] Here, the upper plate and the lower plate refer to the topmost and bottommost steel plates among two or more overlapping steel plates, respectively.

[0043] TU is the thickness (mm) of the steel plate used for the top plate.

[0044] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0045] The upper mating surface is the mating surface between the upper plate and the steel plate adjacent to it.

[0046] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0047] 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.

[0048] 2. The friction stirring point joint according to 1 above, wherein the average carbon concentration of the surface soft layer is less than 80% of the carbon concentration of the steel plate having the surface soft layer.

[0049] 3. 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...

[0050] At least one of the steel plates used as the bonding material has a soft surface layer.

[0051] 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.

[0052] The highest temperature reached is below 850℃.

[0053] The following relationship (1) is satisfied.

[0054] [Formula 2]

[0055]

[0056] In equation (1),

[0057] n is the number of times the joining conditions are switched.

[0058] R k+1 It is the tool rotation speed (rpm) after the k-th engagement condition switch.

[0059] A k+1 It is the tool indentation (mm) from the time of the kth engagement condition switch to the time of the (k+1)th engagement condition switch.

[0060] P k+1 It is the applied pressure (N) after the k-th engagement condition switch.

[0061] Where R1 is the initial tool rotation speed (rpm), A1 is the tool indentation under initial engagement conditions (mm), and P1 is the initial applied force (N). Additionally, A... n+1 The tool indentation amount (mm) is the amount of material pressed in from the moment the engagement condition is switched at the nth time until the maximum indentation depth of the tool is reached.

[0062] In addition, the upper plate and the lower plate are respectively the uppermost and lowermost steel plates among two or more overlapping steel plates.

[0063] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0064] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0065] Furthermore, the vertical position is marked as "+" if it is above the reference position and "-" if it is below the reference position.

[0066] 4. The friction stirring point bonding method according to 3 above, wherein the average carbon concentration of the above-mentioned soft surface layer is less than 80% of the carbon concentration of the composition of the steel plate having the above-mentioned soft surface layer.

[0067] 5. A method for manufacturing a friction stirring point joint, wherein two or more overlapping steel plates are joined by the friction stirring point joining method described in 3 or 4 above.

[0068] According to the present invention, a friction stir point joint with high cross tensile strength can be obtained even when a steel plate with a soft surface layer is used as the bonding material. Therefore, friction stir point bonding can be applied to a wider range of steel plates, thus having great industrial application value. Attached Figure Description

[0069] 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.

[0070] 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

[0071] The present invention will be described based on the following embodiments.

[0072] [1] Friction stirring point joint

[0073] 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.

[0074] 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.

[0075] like Figure 1As 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.

[0076] [Joint]

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] Therefore, it is important to control the shape and hardness of the joint as follows.

[0083] Here, the upper plate and the lower plate refer to the topmost and bottommost steel plates among two or more overlapping steel plates, respectively.

[0084] TU is the thickness (mm) of the steel plate used for the top plate.

[0085] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0086] The upper mating surface is the mating surface between the upper plate and the steel plate adjacent to it.

[0087] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0088] 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)).

[0089] (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)

[0090] Regarding the grade of the deepest point of the recess, the lower mating surface of the steel plate used as the bonding material is set as the reference position, satisfying at least one of -0.5 mm or less and -TL×0.50 or less. This strengthens the bonding interface, and even when a steel plate with a soft surface layer is used as the bonding material, high cross tensile strength can be obtained. Furthermore, although not particularly limited, the grade of the deepest point of the recess is preferably -TL×0.90 or greater.

[0091] (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).

[0092] 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 cross tensile strength even when a steel plate with a soft surface layer is used as the joined material. 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.

[0093] 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.

[0094] 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.

[0095] (c) Vertical position: The difference between the hardness of the joint interface at the horizontal position under +TU×0.50 or +TU×0.60 and the hardness of the joint interface at +350μm (hereinafter also referred to as the hardness difference of the joint under the specified grade).

[0096] The inventors investigated the cause of insufficient cross tensile strength in cross tensile tests when using the friction-stirring point joint obtained by the technology described in Patent Document 1, where fracture occurred along the soft portion at the boundary between the first and second components. As a result, the inventors discovered that if a steel plate with a soft surface layer is used as the bonding material, the soft surface layer causes a decrease in hardness at the bonding interface, which is the cause of fracture along the soft portion at the boundary between the first and second components during cross tensile tests. Furthermore, based on this insight, the inventors conducted repeated research and discovered that by forming an annular groove on the upper surface of the joint and satisfying the conditions (a) and (b) above, the hardness difference at the specified level of the joint—that is, the hardness difference between the bonding interface and the second flow portion at the upper plate setting level—is reduced, thereby preventing fracture along the aforementioned soft portion.

[0097] In particular, if the hardness difference at the specified grade of the joint is 80 HV or less, fracture along the soft portion is effectively prevented, and the cross tensile strength is improved. On the other hand, if the hardness difference at the specified grade of the joint exceeds 80 HV, fracture along the soft portion cannot be adequately prevented, and the cross tensile strength is reduced. Therefore, the hardness difference at the specified grade of the joint is 80 HV or less. Preferably, the hardness difference at the specified grade of the joint is 70 HV or less, more preferably 60 HV or less. The lower limit of the hardness difference at the specified grade of the joint is not particularly limited, and may also be 0.

[0098] Furthermore, the hardness difference of the joint under the specified grade is generally measured at the vertical position: +TU×0.60. However, if the grade at the highest point of the joint interface is less than +TU×0.60, the hardness difference of the joint under the specified grade is measured at the vertical position: +TU×0.50.

[0099] In addition, the reason for measuring the hardness difference of the joint at the specified grade in the vertical direction: +TU×0.60 (hereinafter also referred to as +TU×0.60) is as follows.

[0100] In other words, in the case of a well-established friction-stirring joint, +TU×0.60 is a position where a sufficiently strong bond can be achieved at the joint interface. Therefore, if no soft portion is formed near the joint interface, fracture along the joint interface will not occur at this position. On the other hand, if a soft portion is formed near the joint interface, the soft tissue at this position will fracture along the joint interface. That is, +TU×0.60 is a position where the difference in hardness between the joint interface and the position at +350 μm of the joint interface has a significant impact on the fracture path. Therefore, +TU×0.60 is a suitable position for measuring the hardness difference (in other words, confirming the presence or absence of a soft layer near the joint interface). Additionally, the vertical position: +TU×0.50 is also suitable for measuring the hardness difference for the same reason; however, at the vertical position: +TU×0.50, the measured hardness value may be affected by segregation occurring in the central part of the steel plate thickness. Therefore, where possible, it is preferable to perform the measurement at +TU×0.60.

[0101] Furthermore, the hardness at each location was measured according to JIS Z 2244. Specifically, it was performed as follows.

[0102] That is, with Figure 1 The vertical section of the friction stirring point joint shown (the vertical section of the friction stirring point joint through the center of the concave part of the joint) is used as the measurement surface. Vickers hardness (HV) is measured at each position under a test force of 2.942 N.

[0103] [Annular groove]

[0104] like Figure 1 As shown, an annular groove is formed on the upper surface of the joint. Furthermore, the annular groove is positioned on the upper surface of the joint to surround the recess. That is, the annular groove is formed during joining by contact between the shoulder of the tool and the upper plate. By forming the annular groove, the joint interface is strengthened. Therefore, even when using a steel plate with a soft surface layer as the joined material, high joint strength can be obtained.

[0105] 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.

[0106] [Steel plate]

[0107] At least one of the steel plates used as bonding materials has a soft surface layer. Furthermore, there is no particular limitation on the number of steel plates with a soft surface layer, and all steel plates used as bonding materials may have a soft surface layer.

[0108] Here, the surface soft layer refers to a region where the ferrite fraction at the 1 / 3 position of the plate thickness is more than twice that of the region at which the ferrite fraction is located. Furthermore, if a surface soft layer with a thickness of 1 μm or more is present on at least one side of the steel plate, it is determined that a surface soft layer is present.

[0109] That is, "having a surface soft layer" means that on at least one side of the steel plate, there is a region where the ferrite fraction is more than twice that at the position of 1 / 3 of the plate thickness, and the thickness of the region is more than 1 μm.

[0110] In addition, the thickness of the surface soft layer is measured, for example, as follows.

[0111] Specifically, test pieces were cut from the steel plate using an arbitrary cross-section perpendicular to the steel plate surface as the observation plane. The observation plane was then ground and etched with 3 vol.% nitric acid ethanol to reveal the microstructure. Next, microstructures at various thicknesses from the top to the bottom of the steel plate were imaged at 100 μm intervals using a SEM (Scanning Electron Microscope) at 2000x magnification. Based on the obtained microstructure images at various thicknesses, the area of ​​ferrite at each thickness was calculated using Adobe Photoshop from Adobe Systems. Then, the area of ​​ferrite at each thickness was divided by the area of ​​its respective field of view, and these values ​​were used as the ferrite fraction at each thickness. Here, the black phase observed on relatively smooth surfaces was considered as ferrite. Next, the deepest point (hereinafter referred to as the deepest point) of the thickness at which the ferrite fraction at the 1 / 3 thickness position is more than twice that at the 1 / 3 thickness position of the steel plate is determined on both sides (upper and lower surfaces). Based on these thickness positions, the thickness of the surface soft layer on both sides of the steel plate (distance from the upper surface to the deepest point on the upper surface side, and distance from the lower surface to the deepest point on the lower surface side) is determined. Furthermore, when the thickness of the surface soft layer is less than 10 μm, 1 to 3 micrographs of the microstructure at various thickness positions near the upper and lower surfaces of the steel plate are taken using SEM at a magnification of 10000x and a spacing (thickness direction interval) of 5 μm. Similarly, Adobe Photoshop from Adobe Systems is used to calculate the ferrite area and ferrite fraction at each thickness position. Thus, the thickness of the surface soft layer is determined. In addition, the starting point at the 1 / 3 thickness position (thickness 0 position) can be any surface on either the upper or lower surface. However, when the ferrite fraction at the 1 / 3 position of the plate thickness is different, whether the starting surface is the above surface or the starting surface is below surface, the value with the larger ferrite fraction is preferred.

[0112] Furthermore, the average carbon concentration of the surface soft layer is preferably 80% or less of the carbon concentration of the composition of the steel plate having the surface soft layer. Here, the average carbon concentration of the surface soft layer is measured, for example, by FE-EPMA.

[0113] Specifically, test pieces are cut from a steel plate with an arbitrary cross-section perpendicular to the plate surface as the observation plane. Then, FE-EPMA is used to measure the carbon concentration at various depths of the test pieces to determine the carbon concentration distribution along the depth direction of the steel plate. Furthermore, the average carbon concentration at depths corresponding to the surface soft layer is calculated and used as the average carbon concentration of the surface soft layer.

[0114] Furthermore, 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 they can be steel plates of different steel grades.

[0115] Furthermore, the thickness of the steel plates used as the joining material is preferably 0.4 to 3.2 mm. Additionally, the steel plates used as the joining material can be all of the same thickness, or steel plates of different thicknesses can be used.

[0116] In addition, the composition of the steel plates that become the joined materials, for example, in terms of mass percentage,

[0117] The composition is as follows: C: 0.05~0.6%, Si: 0.2~2.5%, Mn: 1.0~4.0%, P: less than 0.10%, S: less than 0.050%, Al: 0.01~0.80%, and N: less than 0.010%.

[0118] Arbitrarily, examples can be given of a composition having one or more elements selected from Nb: 0.050% or less, Ti: 0.050% or less, B: 0.0050% or less, V: 0.05% or less, Cr: 0.50% or less, Mo: 0.50% or less, Co: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sb: 0.020% or less, and Ca and REM: a total of 0.010% or less, with the remainder being Fe and unavoidable impurities.

[0119] Furthermore, the tensile strength of the steel plate used as the bonding material is preferably 980 MPa or more, and more preferably 1470 MPa or more. The tensile strength is measured according to JIS Z 2241 (1998).

[0120] In addition, the steel plates used as the bonding materials may also have a coating on their 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.

[0121] Furthermore, the aforementioned steel plates can be manufactured using conventional methods.

[0122] [2] Friction stirring point joining method and manufacturing method of friction stirring point joining joint

[0123] 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.

[0124] 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.

[0125] At least one of the steel plates used as the bonding material has a soft surface layer.

[0126] The vertical position of the maximum indentation depth of the aforementioned tool is based on the lower mating surface of the aforementioned steel plate, satisfying at least one of -0.5mm or less and -TL×0.50 or less, and is the state in which the shoulder contacts the upper plate when the aforementioned tool reaches the maximum indentation depth.

[0127] The highest temperature reached is below 850℃.

[0128] The following relationship (1) is satisfied.

[0129] [Formula 3]

[0130]

[0131] In equation (1),

[0132] n is the number of times the joining conditions are switched.

[0133] R k+1 It is the tool rotation speed (rpm) after the k-th engagement condition switch.

[0134] A k+1 It is the tool indentation (mm) from the time of the kth engagement condition switch to the time of the (k+1)th engagement condition switch.

[0135] Pk+1 It is the applied pressure (N) after the k-th engagement condition switch.

[0136] Where R1 is the initial tool rotation speed (rpm), A1 is the tool indentation under initial engagement conditions (mm), and P1 is the initial applied force (N). Additionally, A... n+1 The tool indentation amount (mm) is the amount of material pressed in from the moment the engagement condition is switched at the nth time until the maximum indentation depth of the tool is reached.

[0137] In addition, the upper plate and the lower plate are respectively the uppermost and lowermost steel plates among two or more overlapping steel plates.

[0138] TL is the thickness (mm) of the steel plate used for the bottom plate.

[0139] The lower mating surface is the mating surface between the lower plate and the steel plate adjacent to the lower plate.

[0140] Furthermore, the vertical position is marked as "+" if it is above the reference position and "-" if it is below the reference position.

[0141] Furthermore, the friction stirring point joining method according to an embodiment of the present invention is a method for obtaining (manufacturing) a friction stirring point joining joint according to the above-described embodiment of the present invention.

[0142] 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.

[0143] 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.

[0144] Two or more overlapping steel plates, which are the materials 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 using two steel plates with a surface soft layer, a tensile strength of 980 MPa or more, and a thickness of 1.6 mm 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°.

[0145] 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).

[0146] Furthermore, at least one of the steel plates used as the joining materials is a steel plate having a surface soft layer. The definition of the surface soft layer and the method for measuring its thickness are as described above. Additionally, if the steel plates used as the joining materials have a surface soft layer before joining, it can generally be determined that they will also have a surface soft layer after joining. The description of the steel plates used as the joining materials is as described above.

[0147] Furthermore, in the friction stirring point bonding method according to an embodiment of the present invention, it is important to satisfy the following conditions.

[0148] (d) Vertical position of the tool's maximum indentation depth

[0149] 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 satisfies at least one 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 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 more.

[0150] (e) Maximum reached temperature

[0151] The maximum temperature reached at the joint during the bonding process is set to 850°C or below. This ensures that the hardness difference at the specified grade of the joint is 80 HV or below. The maximum temperature reached is preferably 830°C or below, more preferably 810°C or below. Furthermore, the maximum temperature reached is preferably 650°C or above.

[0152] Here, the highest temperature reached is measured, for example, as follows.

[0153] 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.

[0154] (f) Control of bonding conditions

[0155] Properly controlling the engagement conditions, specifically regarding the tool rotation speed, tool infeed, and applied force, is important to satisfy the relationship of Equation (1). Furthermore, the left side of Equation (1) will also be referred to as E below.

[0156] [Formula 4]

[0157]

[0158] In equation (1),

[0159] N is the number of times the engagement condition is switched.

[0160] R k+1 It is the tool rotation speed (rpm) after the k-th engagement condition switch.

[0161] A k+1 It is the tool indentation (mm) from the time of the kth engagement condition switch to the time of the (k+1)th engagement condition switch.

[0162] P k+1 It is the applied pressure (N) after the k-th engagement condition switch.

[0163] Where R1 is the initial tool rotation speed (rpm), A1 is the tool indentation (mm) under the initial engagement conditions (in other words, the tool indentation (mm) from the start of engagement to the first change in engagement conditions), and P1 is the initial applied force (N). Additionally, A... n+1 The tool indentation amount (mm) is the amount of material pressed in from the moment the engagement condition is switched at the nth time until the maximum indentation depth of the tool is reached.

[0164] Furthermore, the engagement condition for switching is at least one of the tool rotation speed and the applied pressure.

[0165] By satisfying the relationship in equation (1) above, that is, by making E 41.0 or less, the hardness difference of the joint at the specified grade can be made to be 80 HV or less. Furthermore, E is preferably 40 or less. In addition, the lower limit of E is not particularly limited, but E is preferably 10 or more.

[0166] Here, n (the number of times the engagement conditions are switched) can be any integer greater than or equal to 0. That is, the switching of engagement conditions (tool rotation speed and pressure) is arbitrary and can be performed or not. Furthermore, when n is 0, the tool indentation under the initial engagement conditions, i.e., A1, becomes the tool indentation (mm) from the start of engagement until the maximum tool indentation depth is reached. In addition, although not specifically limited, n is preferably 5 times or less.

[0167] Furthermore, the initial tool rotation speed and initial applied pressure, which are the initial joining conditions—that is, the joining conditions before the first joining condition switch—are not particularly limited as long as the maximum temperature reached is below 850°C and the relationship of the above formula (1) is satisfied. They can be determined according to the steel grade and thickness of the steel plates used as joining materials. For example, when using two steel plates with a surface soft layer, a tensile strength of 980 MPa or more, and a thickness of 1.6 mm as joining materials, the initial tool rotation speed (R1) is preferably 200 to 2000 rpm. In addition, the initial applied pressure (P1) is preferably 10 to 60 kN.

[0168] In addition, when switching engagement conditions, it is preferable to make the rotational speed of the tool after switching lower than the rotational speed of the initial tool, and on the other hand, to make the applied pressure after switching greater than the applied pressure at the beginning.

[0169] For example, when using two steel plates with a surface soft layer, a tensile strength of 980 MPa or more, and a thickness of 1.6 mm as the bonding material, and setting n (the number of times the bonding condition is switched) to 1, the tool rotation speed (R2) after switching is preferably 40 to 400 rpm. Furthermore, the applied pressure (P2) after switching is preferably 20 to 70 kN.

[0170] Furthermore, when switching the tool rotation speed and applying pressure, for example, when using two steel plates with a surface soft layer, a tensile strength of 980 MPa or more, and a plate thickness of 1.6 mm as the materials to be joined, and when n (the number of times the joining conditions are switched) is set to 1, it is preferable that the tool (pin) reaches a pressing depth (A1) of 0.8 to 2.4 mm from the upper surface of the upper plate.

[0171] Furthermore, the engagement time is preferably 3 to 60 seconds.

[0172] Furthermore, the maximum temperature can be adjusted using an external heat source. As for conditions other than those mentioned above, there are no particular limitations; any standard methods will suffice.

[0173] Example

[0174] 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.

[0175] 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). 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). Next, tensile tests were performed on the collected test specimens according to JIS Z 2241 (1998), and the tensile strength (TS) was determined. In addition, the presence or absence of a surface soft layer and the average carbon concentration of the surface soft layer were determined using the above methods. The results are recorded in Table 1.

[0176] In addition, 50mm × 150mm test pieces were cut from the obtained steel plates and overlapped into a cross shape according to the combination shown in Table 2. Furthermore, with three plates overlapping, the upper plate and the middle plate were overlapped in a cross shape, and the middle plate and the lower plate were overlapped parallel to each other (that is, in the cross tensile strength determination described later, the cross tensile strength between the upper plate and the middle plate was measured). Next, friction stirring point bonding was performed on the center of the cross overlap of the test pieces under the conditions shown in Table 2 to obtain a friction stirring point bonded joint.

[0177] Furthermore, the tool has the following characteristics: 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 joining conditions. In specimens 1-5, 9-14, and 16-21, the tool rotation speed and applied pressure were switched simultaneously during joining. In specimen 6, only one switch in applied pressure was performed during joining. Here, the timing of switching the tool rotation speed and applied pressure was based on the tool's indentation depth from the upper surface of the upper plate (the "tool indentation depth at switching" recorded in Table 2 refers to the tool's indentation depth from the upper surface of the upper plate).

[0178] 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.

[0179] For the friction stirring point joint obtained in this way, the grade of the deepest point of the recess, the grade of the highest point of the joint interface, the hardness of the joint at the specified grade, and the presence or absence of the annular groove were determined using the methods described above. The results are shown in Table 3. Here, for the hardness of the joint at the specified grade listed in Table 3, the hardness measured at the vertical position of +TU×0.60 for Nos. 1 to 9 and 13 to 21, and at the vertical position of +TU×0.50 for Nos. 10 and 12 are recorded. In addition, for Nos. 1 to 4, 9 to 10, and 12 to 21, the hardness difference measured at the vertical position of +TU×0.50 or +TU×0.60 is 80 HV or less.

[0180] 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 3. Furthermore, a cross tensile strength of 8.0 kN or higher was considered acceptable.

[0181] [Table 1]

[0182]

[0183] [Table 2]

[0184]

[0185] [Table 3]

[0186]

[0187] As shown in Table 3, in the invention examples, the cross tensile strength is above 8.0 kN, resulting in high cross tensile strength.

[0188] On the other hand, in the comparative example, sufficient joint strength was not obtained.

[0189] Explanation of reference numerals in the attached figures

[0190] 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, At least one of the steel plates has a soft surface layer. 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. At a vertical position of +TU×0.50 or +TU×0.60, and in the horizontal position, the hardness difference between the location of the joint interface and the location at +350μm of the joint interface is less than 80HV. 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 average carbon concentration of the surface soft layer is less than 80% of the carbon concentration of the composition of the steel plate having the surface soft layer.

3. 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, At least one of the steel plates used as the bonding material has a soft surface layer. 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 conditions: 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 highest temperature reached is below 850℃. The following relationship (1) must be satisfied. [Formula 1] In equation (1), n is the number of times the joining conditions are switched. R k+1 It is the tool rotation speed after the k-th engagement condition switch, and its unit is rpm. A k+1 It represents the tool indentation amount from the time of the k-th engagement condition switch to the time of the (k+1)-th engagement condition switch, and its unit is mm. P k+1 It is the pressure applied after the k-th engagement condition switch, and its unit is N. Where R1 is the initial tool rotation speed in rpm, A1 is the tool indentation under initial engagement conditions in mm, P1 is the initial applied force in N, and A... n+1 The tool indentation amount, expressed in mm, is the amount of material inserted from the moment the engagement condition changes (n-th time) until the maximum tool indentation depth is reached. 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.

4. The friction stirring point joining method according to claim 3, characterized in that, The average carbon concentration of the surface soft layer is less than 80% of the carbon concentration of the composition of the steel plate having the surface soft layer.

5. 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 3 or 4.

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