Friction stir welded joint

Abstract

The present invention provides a friction stir welded joint which has excellent joint efficiency and in which the material to be joined is a steel sheet mainly composed of ferrite. In the present invention, the ratio of the average dislocation density of a joined part to the average dislocation density of a base material is appropriately controlled.

Description

Friction stir welded joint

[0001] The present invention relates to a friction stir welded joint.

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

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

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

[0005] However, in conventional friction stir welding methods such as those described in Patent Document 1, when a steel plate having a metal structure mainly composed of ferrite (hereinafter also referred to as a ferrite-based steel plate) is used as the material to be welded, sufficient joint efficiency cannot be obtained. Therefore, there is currently a demand for improvement in this respect.

[0006] The present invention was developed in view of the above-mentioned current situation, and aims to provide a friction stir welded joint in which ferrite-based steel plates are used as welded materials (in other words, the base material has a metallurgical structure mainly composed of ferrite) and which has excellent joint efficiency. Note that in this specification, all numerical ranges expressed using "to" mean ranges that include the numerical values ​​written before and after "to" as the lower and upper limits, respectively.

[0007] The inventors have conducted extensive research to achieve the above-mentioned object. First, the inventors investigated the reason why, in conventional friction stir welding methods, excellent joint efficiency cannot be obtained in friction stir welded joints in which ferritic steel plates are used as welded materials. Specifically, the inventors performed friction stir welding under various conditions using various materials as the welded materials, and analyzed the obtained friction stir welded joints by tensile tests and electron backscatter diffraction measurements (hereinafter also referred to as EBSD measurements).

[0008] As a result, the inventors have discovered the following: Microscopic dislocations introduced into the metal structure of the weld during friction stir welding microscopically reduce the ductility of the weld in a friction stir welded joint. This microscopic reduction in ductility makes the weld more susceptible to fracture, in other words, reduces joint efficiency.

[0009] The inventors consider the reason for the above as follows. That is, in a metal structure mainly composed of ferrite, the hardness distribution usually does not change significantly. However, if a metal structure mainly composed of ferrite contains a microscopic region with low ductility, fracture occurs due to the lack of ductility in that region. This microscopic decrease in ductility is largely due to an increase in microscopic dislocations at the joint introduced by heat treatment, processing, etc. In addition, at a macroscopic level, mismatch between the joint and the base material also has an effect.

[0010] Furthermore, based on the above findings and the fact that the ferrite grains in the welded joint of a friction stir welded joint are fine, the inventors suspected that the amount of microscopic dislocations introduced in the welded joint might be correlated with the average characteristics of the matrix. Based on this idea, the inventors conducted further studies to quantify the amount of microscopic dislocations introduced in the welded joint. As a result, the inventors discovered that when the average dislocation density of the welded joint becomes larger than the average dislocation density of the base metal by a certain amount, the ductility of the welded joint of the friction stir welded joint decreases, leading to a decrease in joint efficiency.

[0011] Based on the above findings, the inventors conducted further studies and obtained the following findings: By appropriately controlling the ratio of the average dislocation density of the weld to the average dislocation density of the base material, a friction stir welded joint with excellent joint efficiency can be obtained even when the welded materials are steel plates mainly composed of ferrite. The present invention was completed based on the above findings and through further studies.

[0012] That is, the gist of the present invention is as follows: A friction stir welded joint having base materials of 1.2 or more and a welded portion between the base materials, wherein the area ratio of ferrite in the base materials and the area ratio of ferrite in the welded portion are both 80% or more, and the friction stir welded joint satisfies the relationship of the following formula (1): Dw / Dm≦10.0 (1) where, Dw: average dislocation density ( / m ２ ) Dm: average dislocation density of base material ( / m ２ )

[0013] 2. The friction stir welded joint according to 1 above, wherein the base material has a chemical composition, in mass%, of C: 0.1% or less, Si: 2.0 to 8.0%, Al: 2.0% or less, and Mn: 1.0% or less.

[0014] 3. The average dislocation density of the junction is 1.0 × 10 １６ / m ２ 3. The friction stir welded joint according to claim 1 or 2, wherein:

[0015] 4. The friction stir welded joint according to any one of 1 to 3 above, which is a double-sided friction stir welded joint.

[0016] According to the present invention, a friction stir welded joint can be obtained that uses ferrite-based steel plates as welded materials and has excellent joint efficiency. The friction stir welded joint of the present invention is also advantageous in terms of manufacturability.

[0017] 1 is an example of a KAM diagram of a base material and a joint obtained by EBSD measurement.

[0018] The present invention will be described based on the following embodiments: First, a friction stir welded joint according to one embodiment of the present invention will be described.

[0019] As described above, a friction stir welded joint according to one embodiment of the present invention has two or more base materials and a joint between the base materials (a joint that joins the base materials together).

[0020] [Base Material] The base material has a metal structure mainly composed of ferrite, specifically, a metal structure in which the area ratio of ferrite is 80% or more. Each base material is made of a steel plate to be welded.

[0021] The area ratio of ferrite in the base material is 80% or more, preferably 90% or more. The area ratio of ferrite in the base material may be 100%. Furthermore, the area ratio of the remaining structure other than ferrite in the base material is 20% or less, preferably 10% or less, and more preferably 5% or less. Examples of the remaining structure other than ferrite include bainite, martensite, sulfides, nitrides, and carbides. The area ratio of the remaining structure in the base material may be 0%.

[0022] The ferrite area ratio of the base material is measured as follows. That is, a test piece is taken from the base material. Next, the observation surface of the test piece is polished and then etched with 3 vol. % nital (a solution of nitric acid and ethanol) to reveal the structure. Next, an image is taken using an optical microscope at a magnification of 40 to 500 times. The total area of ​​the observation region is preferably 50 μm square (50 μm × 50 μm) or more. Next, the ferrite area is calculated from the obtained structure image using Adobe Photoshop from Adobe Systems. Next, the ferrite area calculated for each field of view is divided by the total area of ​​the observation region, and the result is multiplied by 100 to obtain the ferrite area ratio. The ferrite area ratio can also be determined by EBSD measurement.

[0023] The base metal may have a composition, in mass %, of 0.1% or less C, 2.0 to 8.0% Si, 2.0% or less Al, and 1.0% or less Mn, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include 0.2% or less P, 0.01% or less S, and 0.01% or less N. The above composition may optionally contain, in mass %, at least one element selected from the group consisting of 1% or less Cr, 1% or less Ni, 1% or less Cu, 0.2% or less Sn, 0.2% or less Sb, 0.01% or less Ca, 0.05% or less REM, and 0.01% or less Mg. Note that all elements other than Si and Fe may be 0% by mass. The Si content is more preferably 5.0% by mass or less. The C content is more preferably 0.001% by mass or more. The Mn content and the Al content are each more preferably 0.01 mass % or more. Furthermore, the thickness of the base material 1 is preferably 0.2 to 3.2 mm.

[0024] The number of base materials joined at one joint may be 2 or more. The number of base materials joined at one joint may be, for example, 2, or 3 to 5. Furthermore, the base materials joined at one joint may be the same or different in steel type and thickness as long as they are steel plates mainly composed of ferrite.

[0025] [Joint] The joint is a region that has undergone hot working due to the frictional heat between the rotating tool and the workpieces and plastic flow, resulting in a recrystallized structure. The joint also has a metal structure mainly composed of ferrite, specifically, a metal structure with an area ratio of ferrite of 80% or more.

[0026] The area ratio of ferrite in the joint is 80% or more, preferably 90% or more. The area ratio of ferrite in the joint may be 100%. Furthermore, the area ratio of the remaining structure other than ferrite in the joint is 20% or less, preferably 10% or less, and more preferably 5% or less. Examples of the remaining structure other than ferrite include martensite, sulfides, nitrides, and carbides. The area ratio of the remaining structure in the joint may be 0%.

[0027] The ferrite area ratio of the weld is measured, for example, as follows. That is, the friction stir welded joint is cut in the thickness (vertical) direction so that the cross section of the joint (a plane including the weld normal direction and the thickness direction of the friction stir welded joint, where the weld normal direction is a direction perpendicular to both the weld direction and the thickness direction) serves as the cut surface. Next, a test piece is cut out so that the joint cross section of the weld serves as the observation surface. Next, the observation surface of the test piece is polished and then etched with 3 vol. % nital to reveal the structure. Next, an image is taken using an optical microscope at a magnification of 40 to 500 times. The total area of ​​the observation region is preferably 50 μm square (50 μm × 50 μm) or more. Next, the ferrite area is calculated from the obtained structure image using Adobe Photoshop from Adobe Systems. Next, the area ratio of ferrite is determined by dividing the area of ​​ferrite calculated for each visual field by the total area of ​​the observation region and multiplying the result by 100. The area ratio of ferrite can also be determined by EBSD measurement or the like.

[0028] The base material and the welded joint can be defined, for example, as follows: The friction stir welded joint is cut in the thickness (vertical) direction so that the cross section of the joint becomes the cut surface. The cut surface is then polished and etched with 3 vol. % nital. The cut surface is then observed under an optical microscope, and the base material and the welded joint are defined based on the degree of etching, etc.

[0029] In a friction stir welded joint according to one embodiment of the present invention, it is important to appropriately control the relative relationship between the average dislocation density of the weld and the average dislocation density of the base material, specifically, to satisfy the relationship of the following equation (1) for the ratio of the average dislocation density of the weld to the average dislocation density of the base material: Dw / Dm≦10.0 (1) where, Dw: average dislocation density of the weld ( / m ２ ) Dm: average dislocation density of base material ( / m ２ ) In addition, when the average dislocation density differs depending on the base material, Dm is the smallest average dislocation density among the base materials joined by the joining portion.

[0030] As described above, microscopic dislocations introduced into the metal structure of the weld during friction stir welding can be quantified by the average dislocation density. By controlling the relative relationship between the average dislocation density of the weld and the average dislocation density of the base material so as to satisfy the relationship of the above formula (1), excellent joint efficiency can be obtained. Therefore, Dw / Dm≦10.0. Preferably, Dw / Dm≦5.0, and more preferably, Dw / Dm≦3.0. The lower limit of Dw / Dm is not particularly limited, and may be, for example, 1.0≦Dw / Dm.

[0031] Furthermore, Dw is, for example, 1.0×10 １６ / m ２ Preferably, Dw is 1.0×10 or less. １３ / m ２ The above is preferable.

[0032] Here, the average dislocation density is determined by, for example, EBSD measurement using the following formula: D = 2K AM ave ÷ (u × |b|) where D: average dislocation density ( / m ２) KAMave: average KAM value, u: measurement interval (m), b: Burgers vector (m). For example, a friction stir welded joint is cut in the thickness (vertical) direction so that the joint cross section becomes the cut surface, and a test specimen is cut out so that the joint cross section of the base material and the joint become the observation surface. Next, EBSD measurement is performed to obtain local crystal orientation data of the base material and the joint. The measurement interval (step size) is preferably 0.1 to 20 μm. In addition, each measurement area is preferably 50 μm square (50 μm x 50 μm) or larger. Next, from the analysis results of the local crystal orientation data, the KAM value (average value of the orientation difference between the measurement point and all adjacent measurement points) is calculated for each measurement point, and the average value, KAMave, is calculated for the base material and the joint. Then, the average dislocation density of the base material and the joint is calculated. For reference, Figure 1 shows an example of a KAM diagram of the base material and the weld obtained by EBSD measurement for a friction stir welded joint obtained by friction stir welding using a ferrite-based steel plate as the welded material according to a conventional method.

[0033] The friction stir welded joint may be a single-sided friction stir welded joint, but a double-sided friction stir welded joint is preferred. Here, a single-sided friction stir welded joint is a joint in which the workpieces are joined by single-sided friction stir welding, in which a rotary tool is placed on one side of the workpieces. In other words, a single-sided friction stir welded joint has a bead on only one of the weld surfaces in the thickness direction. Furthermore, a double-sided friction stir welded joint is a joint in which the workpieces are joined by double-sided friction stir welding, in which a rotary tool is placed on both the front and back sides of the workpieces. In other words, a double-sided friction stir welded joint has beads on both of the weld surfaces in the thickness direction.

[0034] Furthermore, the friction stir welded joint according to one embodiment of the present invention may be a butt joint or a lap joint.

[0035] Furthermore, in a friction stir welded joint according to one embodiment of the present invention, another base material may be joined to the above-mentioned base material via another welded portion. The type of the another welded portion is not particularly limited.

[0036] Moreover, a friction stir welded joint according to one embodiment of the present invention can be produced, for example, as follows.

[0037] First, two or more ferrite-based steel plates, specifically steel plates having a metallurgical structure with a ferrite area ratio of 80% or more, are prepared as the materials to be joined. Here, the materials to be joined constitute the base material after joining, and the metallurgical structure, chemical composition, thickness, etc. are the same as those of the base material described above. Therefore, a description thereof will be omitted here.

[0038] Next, the workpieces are joined by friction stir welding. The joining conditions are not particularly limited, and may be performed in accordance with conventional methods.

[0039] Examples of joining methods include butt joining and lap joining. In butt joining, the end faces of the materials to be joined are placed opposite each other, and a rotary tool is pressed against the butt joint, including the end faces (butt joints), of the materials to be joined, while rotating. Then, in this state, the rotary tool is moved in the joining direction to join the materials to be joined. In lap joining, at least a portion of the ends of the materials to be joined are overlapped, and the rotary tool is pressed against the overlap joint while rotating. Then, in this state, the rotary tool is moved in the joining direction to join the materials to be joined.

[0040] Next, in order to satisfy the relationship of the above formula (1), the friction stir welded joint obtained as described above is post-heated under the following conditions: Average heating rate in the temperature range of 100 to 700 ° C (hereinafter also referred to as heating rate): 30 to 300 ° C / sec Maximum temperature reached: 710 to 1200 ° C Holding time at maximum temperature reached (hereinafter also referred to as holding time): 0.1 to 10 seconds Average cooling rate in the temperature range of 700 to 100 ° C (hereinafter also referred to as cooling rate): 30 to 300 ° C / sec Note that all temperatures here are based on the surface temperature of the welded portion of the friction stir welded joint. It is also preferable to control the heating rate, holding time, and cooling rate on both sides of the welded portion of the friction stir welded joint within the above ranges.

[0041] The post-heating method is not particularly limited, and examples thereof include high-frequency induction heating, laser irradiation, and gas heating. Furnace heating is also possible.

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

[0043] Two steel plates with a thickness of 2.0 mm having the above-mentioned preferred composition (in mass%, C: 0.1% or less, Si: 2.0 to 8.0%, Al: 2.0% or less, and Mn: 1.0% or less, and a portion further containing at least one element selected from the group consisting of Cr: 1% or less, Ni: 0.5% or less, Cu: 0.5% or less, Sn: 0.2% or less, Sb: 0.2% or less, Ca: 0.01% or less, REM: 0.05% or less, and Mg: 0.01% or less, with the balance being Fe and unavoidable impurities) were butted together and subjected to double-sided friction stir welding under the conditions shown in Table 1 to obtain a friction stir welded joint. Next, the friction stir welded joint was post-heated under the conditions shown in Table 1. Note that conditions not specified were as described above or in accordance with conventional methods. In either case, the bonding conditions and post-heating conditions were almost the same for both sides, so Table 1 shows only one side as a representative.

[0044] For the friction stir welded joints thus obtained, the ferrite area fractions of the base metals and the welded joints, as well as Dw / Dm, were determined in the same manner as described above. The results are shown in Table 1. In Table 1, when the ferrite area fractions of the base metals joined at one welded joint differ, the value for the smaller ferrite area fraction is shown as a representative.

[0045] The resulting friction stir welded joints were also evaluated for joint efficiency. In the evaluation of joint efficiency, joint efficiency (= fracture strength of welded joint / fracture strength of base material × 100) of over 90% was evaluated as "excellent," and joint efficiency of 90% or less was evaluated as "poor." The results are also shown in Table 1.

[0046] Here, the fracture strength of the joint was measured as follows. That is, a test piece having the same shape as the No. 1 test piece specified in JIS Z 3121 (2013) was taken from the obtained friction stir welded joint so that the joining direction and thickness direction were perpendicular to the longitudinal direction of the test piece and the joint was located at the center of the parallel part. Next, a tensile test was performed in accordance with JIS Z 3121 (2013) using the taken test piece, and the maximum test force (N) was determined. The determined maximum test force (N) was then multiplied by the cross-sectional area (mm ２ The value obtained by dividing the test piece by the test piece strength was taken as the breaking strength of the joint.

[0047] The breaking strength of the base material can also be referred to as the tensile strength (TS) of the base material (joined material). That is, the breaking strength of the base material can be measured by conducting a tensile test in accordance with JIS Z 2241 (2022). For example, a JIS No. 5 test piece is taken from the base material or the same material as the base material. Next, a tensile test is conducted using the taken test piece at a crosshead speed of 10 mm / min to measure the tensile strength (TS). Note that when the breaking strength differs depending on the base material, the maximum breaking strength of each base material is used in evaluating the joint efficiency.

[0048]

[0049] As shown in Table 1, in the inventive examples, the steel plates mainly composed of ferrite were used as the welded materials, and friction stir welded joints with excellent joint efficiency were obtained. On the other hand, in the comparative examples, sufficient joint efficiency was not obtained.

[0050] Furthermore, in friction stir welded butt joints and lap joints (i.e., the area ratio of ferrite in the base material and the area ratio of ferrite in the weld joint were both 80% or more) using various separately prepared steel plates mainly composed of ferrite (various steel plates having the above-mentioned preferred composition and thicknesses of 0.2 to 3.2 mm) as the welded materials, excellent joint efficiency was obtained similarly to the above when Dw / Dm satisfied the above formula (1).

Claims

1. A friction stir welded joint having two or more base materials and a welded portion between the base materials, wherein the area ratio of ferrite in the base materials and the area ratio of ferrite in the welded portion are both 80% or more, and the following formula (1) is satisfied: Dw / Dm≦10.0 (1), where Dw is the average dislocation density ( / m ２ ) Dm: Average dislocation density of the base material ( / m ２ ) 2. A friction stir welded joint as described in claim 1, wherein the composition of the base material is, in mass%, C: 0.1% or less, Si: 2.0 to 8.0%, Al: 2.0% or less, and Mn: 1.0% or less.

3. The average dislocation density of the joint is 1.0 x 10 １６ / m ２ 3. The friction stir welded joint according to claim 1 or 2, wherein:

4. The friction stir welded joint according to claim 1 or 2, which is a double-sided friction stir welded joint.

5. The friction stir welded joint according to claim 3, which is a double-sided friction stir welded joint.

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