Method for producing friction stir welded joint

Abstract

The purpose of the present invention is to provide a method for producing a friction stir welded joint. The present invention is a method for producing a friction stir welded joint, in which a tool tip is inserted into a butt surface of joining workpieces and the tool is moved in the joining direction while being rotated in order to effect stir welding between the joining workpieces. The joining workpieces are steel sheet having a component composition comprising, in mass%, C: 0.01-0.15%, Si: 0.01-0.50%, Mn: 0.05-1.00%, and Ni: 6.5-10.0%. In the friction stir, using A for the tool rotational speed (rpm) and B for the welding speed (mm / min), welding conditions are satisfied at which the value of X, which represents A / B for each pass, is 0 < X ≤ 8.0 and 50 ≤ A ≤ 500.

Description

Manufacturing method for friction stir welded joint

[0001] The present invention relates to a method for manufacturing a friction stir welded joint, and more particularly to a method for manufacturing a friction stir welded joint suitable for joining cryogenic steel materials used in cryogenic environments.

[0002] Steel structures such as liquefied gas storage tanks are used in extremely low temperature environments, so the steel materials used in such structures must not only have the strength and toughness required, but also the toughness at extremely low temperatures (hereinafter sometimes referred to as "cryogenic toughness"), particularly the toughness of the welds in the structures.

[0003] For example, when a hot-rolled steel plate (hot-rolled steel plate) is used in a storage tank for liquefied natural gas, the steel plate needs to ensure toughness at cryogenic temperatures of −164°C or below, which is the boiling point of liquefied natural gas. If the cryogenic toughness of the steel material is poor, there is a risk that the safety of the cryogenic storage structure cannot be maintained. Therefore, there is a strong demand for improved cryogenic toughness of the steel material and its joints used. Conventionally, 7% Ni steel plate or 9% Ni steel plate has been used as a steel material that meets this requirement.

[0004] Examples of 7% Ni steel sheets and 9% Ni steel sheets include those described in Patent Documents 1 and 2.

[0005] Patent Document 1 discloses a steel plate for cryogenic use containing more than 5.0 to less than 10.0 mass% Ni and predetermined amounts of C, Si, Mn, and Al. The steel plate of Patent Document 1 has a plate thickness of 6 to 50 mm and a V-notch Charpy absorbed energy per unit area vE －１９６℃ The average value of 1.25 J / mm 2 It has cryogenic toughness of at least 100%.

[0006] Patent Document 2 discloses a Ni-containing steel for low temperature use containing 7.0 to 10.5 mass% Ni and predetermined amounts of C, Si, Mn, and Al. The steel of Patent Document 2 has a plate thickness of 30 to 60 mm and a Charpy absorbed energy vE －１９６℃ The average value of the cryogenic toughness is 150 J or more.

[0007] When these steel plates for cryogenic use are used in steel structures, welding has traditionally been used to join the steel plates together, and welding materials such as Inconel or Hastelloy containing approximately 70% by mass of Ni are required. The reason for this is as follows: In general, steel materials that are subjected to heat at high temperatures close to their melting points suffer from a deterioration in toughness. Therefore, by using these welding materials that do not cause brittle fracture to weld joints, brittle fracture in steel structures can be prevented.

[0008] However, high-Ni-containing welding materials are very expensive, which increases construction costs. Therefore, the present inventors focused on friction stir welding as a joining method that does not expose steel to high temperatures close to its melting point. Friction stir welding (FSW) is a technique for joining metal materials by pressing a rotating tool into the materials to be joined (e.g., metal materials), generating frictional heat in the metal materials, stirring the softened area with the rotating tool, and causing the metal materials to flow. Since friction stir welding is a solid-state joining method, it is relatively easy to adjust the joining temperature, and the joint characteristics are better than those of general fusion welding. Another advantage is that no filler metal is required for joining.

[0009] As a friction stir welding method for obtaining good joint characteristics, for example, Patent Document 3 can be mentioned. In Patent Document 3, a steel containing 0.20 to 0.45 mass % of C and a predetermined amount of Cr is used as the material to be welded, and the maximum temperature reached during welding in the stir zone is set to the A of the steel. １ The present invention discloses a friction stir welding method having the above points.

[0010] JP 2011-219848 A JP 2011-214099 A Japanese Patent No. 6634616 A

[0011] As described above, in the production of welded joints in which steel plates for cryogenic temperatures are welded using high-Ni wire as a welding material, the high Ni content poses a problem of high cost. Therefore, if friction stir welding could be applied, it would be possible to significantly reduce the cost of welding materials.

[0012] Furthermore, in the techniques described in Patent Documents 1 and 2, only the properties of the base material (steel plate) are considered with regard to cryogenic toughness, and these documents make no mention of the application of high Ni steel plate to the workpiece of friction stir welding, or the properties of the joint formed by welding such steel plate.

[0013] On the other hand, the technology described in Patent Document 3 studies the tensile strength and fracture characteristics of a friction stir welded joint (stirred joint). However, no study is made on a technology for ensuring cryogenic toughness of the joint. The technology in Patent Document 3 uses carbon steel with a C content of 0.20 mass% or more as the welded material, and does not use high-Ni steel plate with a Ni content of 6.5 to 10.0 mass% as the welded material.

[0014] Furthermore, from the viewpoint of durability of FSW tools, no technology has been proposed so far regarding friction stir welding using high-Ni steel plates as the workpieces.

[0015] The present invention has been made in view of the above circumstances, and has as its object to provide a method for manufacturing a friction stir welded joint that can ensure cryogenic toughness.

[0016] In order to solve the above problems, the present inventors conducted extensive research into the factors that affect the characteristics of friction stir welded joints produced by friction stir welding high-Ni steel plates with a Ni content of 6.5 to 10.0 mass% under various welding conditions. As a result, the following findings were obtained.

[0017] First, when friction stir welding is performed on the high-Ni steel plate having high toughness and strength, the rotation speed (rpm) of the tool needs to be set to 50 or more and 500 or less.

[0018] Secondly, in order to stably ensure the cryogenic toughness of the stir-welded portion (i.e., the welded portion), it is essential to keep the value calculated from the rotational speed of the tool and the welding speed (i.e., the value of "rotational speed of the tool (rpm) / welding speed (mm / min)") at or below a certain level.

[0019] The present invention was completed based on the above findings, and its gist is as follows: [1] A method for manufacturing a friction stir welded joint by inserting the tip of a tool into the butt surfaces of workpieces and relatively moving the tool in the welding direction while rotating it to stir weld the workpieces together, wherein the workpieces are steel plates having a chemical composition containing, in mass %, C: 0.01 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.05 to 1.00%, and Ni: 6.5 to 10.0%, and the stir welding satisfies the following welding conditions: the value of X expressed by equation (1) for each pass is 0<X≦8.0, and the rotation speed of the tool is 50≦A≦500, where A is the rotation speed (rpm) of the tool and B is the welding speed (mm / min). X=A / B (1) [2] The method for producing a friction stir welded joint according to [1], wherein the chemical composition of the steel plate further contains, in mass%, one or more elements selected from the group consisting of Cr: 1.00% or less, Mo: 0.50% or less, P: 0.03% or less, S: 0.005% or less, N: 0.0010 to 0.0080%, Al: 0.10% or less, Cu: 0.50% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, and B: 0.0030% or less. [3] The method for manufacturing a friction stir welded joint according to [1] or [2], wherein the welding conditions are such that, when the number of passes is two or more, the value of X expressed by the formula (1) of a preceding pass is X1, and the value of X expressed by the formula (1) of a subsequent pass following the preceding pass is X2, the relationship between X1 and X2 is X1 / X2≧1.0.

[0020] According to the present invention, a method for manufacturing a friction stir welded joint can be provided that can ensure cryogenic toughness in a stir welded joint using high-Ni steel plates as the workpieces. By applying the manufacturing method of the present invention to the manufacture of steel structures to be used in cryogenic environments, such as tanks for storing liquefied gas, it becomes possible to manufacture inexpensive joints while maintaining the safety of the steel structures, which brings about significant industrial benefits.

[0021] Hereinafter, one embodiment of the method for manufacturing a friction stir welded joint of the present invention will be specifically described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited thereto.

[0022] <Joining method> The present invention is a method for manufacturing a friction stir welded joint, in which two steel plates to be joined are used as workpieces, and the tip of a tool is inserted between the butt surfaces where the ends of the workpieces are butted together, and the tool is moved relatively while rotating along the longitudinal direction of the ends (i.e., the joining direction), thereby frictionally joining the workpieces together. In the present invention, the workpieces and joining conditions are specified as follows.

[0023] The friction stir welding of the present invention includes any of the following modes: friction stir welding in which a tool is rotated while being moved in the joining direction; friction stir welding in which the workpieces are butted together at a joining portion and then moved in the joining direction; and modes that combine these modes arbitrarily.

[0024] In addition, the friction stir welding method of the present invention can be applied to both single-sided construction, in which a tool is inserted into only one side of the workpieces (i.e., the front or back side) to perform stir welding, and double-sided construction, in which a tool is inserted into both sides of the workpieces (i.e., the front and back sides) to perform stir welding.

[0025] [Members to be welded] First, a steel plate that can be used as the members to be welded will be described.

[0026] As described above, from the viewpoint of safety, there is a strong demand for improved cryogenic toughness in steels used in steel structures that require toughness in cryogenic environments, such as storage tanks for liquefied natural gas. High-Ni steel plates are used to meet this demand. Furthermore, in steel structures manufactured by joining multiple steel plates, it is also necessary to ensure the toughness of the joints. Therefore, the steel plate used in the present invention has a chemical composition that can maintain the strength and cryogenic toughness of the steel plate while also ensuring the cryogenic toughness of the joint after stir welding. The chemical composition of the steel plate is described below.

[0027] [Composition] The steel sheet used as the material to be joined in the present invention has the following predetermined composition. Each element contained in this composition will be explained. Unless otherwise specified, in this specification, "%" used as the unit of content of each element means "mass%."

[0028] C: 0.01 to 0.15% C is an element that has the effect of improving the strength of steel plate. To achieve this effect, the C content is set to 0.01% or more, and preferably 0.03% or more. On the other hand, if the C content exceeds 0.15%, the cryogenic toughness of the steel plate decreases. Therefore, the C content is set to 0.15% or less, and preferably 0.12% or less.

[0029] Si: 0.01 to 0.50% Si is an element that contributes to improving the strength of steel sheet and also acts as a deoxidizer. To achieve these effects, the Si content is set to 0.01% or more. On the other hand, if the Si content is excessively high, the cryogenic toughness decreases. Therefore, the Si content is set to 0.50% or less, and preferably 0.30% or less.

[0030] Mn: 0.05 to 1.00% Mn is an element that improves the hardenability of steel and is effective in increasing the strength of steel plate. To achieve this effect, the Mn content is set to 0.05% or more. On the other hand, if the Mn content exceeds 1.00%, the temper embrittlement susceptibility increases and the cryogenic toughness begins to vary. For these reasons, the Mn content is limited to 1.00% or less. The Mn content is preferably less than 0.90%, and more preferably 0.80% or less.

[0031] Ni: 6.5 to 10.0% Ni is an element extremely effective in improving the cryogenic temperature toughness of steel plate. If the Ni content is less than 6.5%, cryogenic temperature toughness cannot be stably ensured. Therefore, the Ni content is set to 6.5% or more. On the other hand, since Ni is an expensive element, the cost of the steel plate increases as the Ni content increases. Therefore, in the present invention, the Ni content is set to 10.0% or less. The Ni content is preferably set to 7.0% or more, and preferably 9.5% or less.

[0032] In the present invention, the above-mentioned composition is the basic composition of the steel sheet. In addition to the above-mentioned composition, the present invention may optionally contain one or more elements selected from the group consisting of Cr, Mo, P, S, N, Al, Cu, Nb, V, Ti, and B. Since these elements can be contained as needed, the content of each of Cr, Mo, P, S, Al, Cu, Nb, V, Ti, and B may be 0%.

[0033] Cr: 1.00% or less Cr is an element that can improve the strength of a steel plate without significantly impairing the cryogenic toughness. However, if the Cr content exceeds 1.00%, the cryogenic toughness of the steel plate decreases. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less. The Cr content is preferably 0.90% or less. The Cr content is preferably 0.01% or more.

[0034] Mo: 0.50% or less Like Cr, Mo is an element that can improve the strength of steel plate without significantly impairing cryogenic toughness. However, if the Mo content exceeds 0.50%, the cryogenic toughness decreases. Therefore, when Mo is contained, the Mo content is preferably 0.50% or less. The Mo content is preferably 0.40% or less. The Mo content is preferably 0.01% or more.

[0035] P: 0.03% or less P is an inevitable impurity and a harmful element that adversely affects the cryogenic toughness of steel sheets. For example, to obtain a sound base material and welded joint when steel sheets are welded to form a welded structure, it is preferable to reduce the P content as much as possible. Therefore, it is preferable to suppress the P content to 0.03% or less. From the viewpoint of cryogenic toughness, the lower the P content, the better. Therefore, the lower limit of the P content is not particularly limited and may be 0%, but even in this case, it is acceptable for P to be contained as an inevitable impurity. Since excessive reduction of P causes an increase in cost, from the viewpoint of cost, it is preferable that the lower limit of the P content be 0.001%. That is, it is preferable that the P content be 0.001% or more.

[0036] S: 0.005% or less S forms MnS in steel and significantly deteriorates cryogenic toughness, so 0.005% is the upper limit of the S content, and it is desirable to reduce it as much as possible. Therefore, the S content is preferably 0.005% or less. The S content is more preferably 0.002% or less. The lower the S content, the better. Therefore, the lower limit is not particularly limited, and it may be 0%, but even in that case, it is acceptable for S to be contained as an unavoidable impurity. In other words, the S content is preferably 0.0005% or more.

[0037] N: 0.0010 to 0.0080% N forms precipitates in steel, and if the N content exceeds 0.0080%, it causes a decrease in the cryogenic toughness of the base material. However, N is also an element that contributes to the refinement of the base material's grains by forming AlN, and this effect can be achieved by setting the N content to 0.0010% or more. Therefore, if N is contained, the N content is preferably 0.0010 to 0.0080%. The N content is more preferably 0.0020% or more, and more preferably 0.0060% or less.

[0038] Al: 0.10% or less Al is an element contained in a deoxidizer. Al also contributes to the refinement of the base material grains by forming AlN. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the steel is impaired. Therefore, when Al is contained, the Al content is preferably 0.10% or less. The Al content is more preferably 0.05% or less. The Al content is preferably 0.005% or more.

[0039] Cu: 0.50% or less Cu is an element that has the effect of increasing the strength of steel sheet by improving hardenability. However, if the Cu content exceeds 0.50%, the cryogenic toughness of the steel sheet decreases, and the surface properties of the steel material (slab) after casting deteriorate. Therefore, when Cu is contained, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.40% or less, and even more preferably 0.30% or less. On the other hand, although there is no particular limitation on the lower limit of the Cu content, in order to obtain the above effect, the Cu content is preferably 0.10% or more.

[0040] Nb: 0.05% or less Nb is an effective element for increasing the strength of steel sheet through precipitation strengthening. However, if the Nb content is excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when Nb is contained, the Nb content is preferably 0.05% or less. The Nb content is more preferably 0.03% or less. On the other hand, although there is no particular lower limit for the Nb content, in order to obtain the above-mentioned effect, the Nb content is preferably 0.01% or more.

[0041] V: 0.05% or less Like Nb, V is an effective element for increasing the strength of steel sheet by precipitation strengthening. However, if the V content is excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when V is contained, the V content is preferably 0.05% or less. The V content is more preferably 0.04% or less. On the other hand, although there is no particular lower limit for the V content, in order to obtain the above effect, the V content is preferably 0.01% or more.

[0042] Ti: 0.03% or less Ti is an element that contributes to improving cryogenic toughness by forming precipitates and refining the steel sheet structure. Therefore, when Ti is contained, it can be contained in a range of 0.03% or less. The Ti content is more preferably 0.02% or less, and preferably 0.005% or more.

[0043] B: 0.0030% or less B is an element that improves hardenability when added in small amounts. To effectively exert this effect, B can be contained in an amount of 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, the cryogenic toughness deteriorates. Therefore, when B is contained, the B content is preferably 0.0030% or less.

[0044] The steel sheet of the present invention may have a chemical composition containing the above-mentioned elements with the balance being Fe and inevitable impurities. Preferably, the steel sheet of the present invention has a chemical composition containing C, Si, Mn, P, S, Al, Ni, and N within the above-mentioned numerical ranges with the balance being Fe and inevitable impurities.

[0045] [Mechanical Properties and Thickness of Steel Sheet] In the present invention, the tensile strength of the steel sheet used as the joined material is not particularly limited. The steel sheet having the above-mentioned composition can have a tensile strength of 780 MPa or more. The tensile strength is more preferably 800 MPa or more.

[0046] As mentioned above, from the viewpoint of maintaining safety as a cryogenic storage structure, the steel plate used is also required to have cryogenic temperature toughness. －１９６℃ ) can be 120 J or more.

[0047] In the present invention, the thickness of the steel plate is not particularly limited. Since the friction stir welding method of the present invention is suitable for manufacturing a cryogenic storage structure, it is preferable that the thickness of the steel plate is, for example, 6 mm or more. This is because steel plates having a thickness within this range can be suitably used as strength members for use in a cryogenic storage structure.

[0048] [Joining conditions] Next, the joining conditions in the manufacturing method of the present invention will be explained. In the present invention, workpieces made of two steel plates having the above-mentioned component compositions are prepared, the tip of a tool is inserted into the butt surfaces of the workpieces, and the steel plates are joined together by moving the tool in the joining direction while rotating it under the joining conditions described below, thereby manufacturing a friction stir welded joint. As mentioned above, the present invention includes both single-sided and double-sided joining modes.

[0049] Specifically, the joining conditions for stir welding of the present invention are such that, when the rotation speed (rpm) of the tool is A and the moving speed of the tool during joining (hereinafter also referred to as "joining speed") is B (mm / min), the value of X expressed by equation (1) for each pass is 0<X≦8.0, and the rotation speed of the tool is 50≦A≦500. X=A / B (1)

[0050] In the present invention, we focus on the tool rotation speed (A) and welding speed (B) per pass. By appropriately controlling these two factors, it is possible to control the heat generated during friction stir welding of the workpieces and the accompanying changes in their structure. As a result, the cryogenic temperature toughness of the stir-fry welded portion (i.e., the weld) can be stably ensured. If the value obtained by dividing A by B in equation (1), i.e., the value of "A / B" (hereinafter sometimes referred to as "X"), exceeds 8.0, the microstructure becomes coarse due to the heat generated during friction stir welding, resulting in a decrease in the cryogenic temperature toughness of the weld. Therefore, the value of X is set to 8.0 or less. The value of X is preferably set to 7.5 or less.

[0051] The lower limit of the X value is set to be more than 0. If the X value is less than 2.0, the load on the welding tool increases, resulting in a shortened tool life and increased costs. Therefore, the X value is preferably set to be 2.0 or more, more preferably 2.5 or more, and even more preferably 3.0 or more.

[0052] In addition to this condition, the tool rotation speed must be 50 rpm or more and 500 rpm or less. As described above, the steel plate used as the workpiece in the present invention is a Ni steel plate having high toughness and strength. When friction stir welding this steel plate, by appropriately controlling the tool rotation speed, the tool's useful life can be extended and construction costs can be reduced. If the tool rotation speed is less than 50 rpm, the load on the tool is high and the tool's life is shortened. On the other hand, the same applies when the tool rotation speed exceeds 500 rpm. The tool rotation speed is preferably 75 rpm or more and preferably 450 rpm or less. The tool rotation speed is more preferably 100 rpm or more and more preferably 400 rpm or less.

[0053] As explained above, by manufacturing a joint by friction stir welding that satisfies these two conditions in both single-sided and double-sided construction, stable cryogenic toughness can be achieved in the joint even when the above-mentioned high Ni steel plate is used as the welded material.

[0054] The joining method of the present invention is also applicable to cases where the thickness of the workpieces to be joined is greater than the height of the tool.

[0055] For example, in stir welding, where the thickness of the workpieces is greater than the tool height, double-sided welding (i.e., welding is performed in one pass on both sides) can be considered. However, even if each pass is controlled to satisfy the above two conditions, the toughness of the area welded in the first pass may be reduced due to the influence of the second pass. Therefore, in order to more effectively avoid this reduction in toughness during double-sided welding, in addition to the above two conditions, it is effective to control the ratio of the values ​​calculated from the tool rotation speed and welding speed in the first and second passes to a certain value or higher.

[0056] Specifically, when the number of passes in stir welding is two or more, it is preferable that each pass satisfies the above two conditions, and when the value of X expressed by equation (1) of a preceding pass is X1 and the value of X expressed by equation (1) of a subsequent pass following the preceding pass is X2, the relationship between the preceding pass and the subsequent pass satisfies the condition X1 / X2≧1.0.

[0057] For example, when performing construction in one pass on both sides, the above-mentioned "preceding pass" refers to the first pass in the construction on both sides, and the above-mentioned "subsequent pass" refers to the second pass in the construction on both sides. That is, the first pass is from the front side of the butt surfaces (joining portion) of the workpieces to be joined, and the second pass is from the back side of the workpieces to be joined, which is the surface opposite to the front side.

[0058] The reason is as follows: In double-sided welding, if the value of X exceeds 8.0 in both or either of the passes, the microstructure will coarsen due to the influence of processing heat generated by friction stirring, resulting in a decrease in the cryogenic toughness of the joint. Therefore, the values ​​of X in the first and second passes are set to 8.0 or less. As with the above-mentioned reasons, the value of X in each pass is preferably 7.5 or less, and more preferably 7.0 or less. Note that, even in the case of two or more passes, for the same reason, it is sufficient to set the value of X to 8.0 or less in all passes.

[0059] Furthermore, if the thermal effect of the second pass causes reverse transformation over a wide range in the structure after the first pass, the reverse transformation will result in a coarse, low-toughness structure, resulting in a decrease in cryogenic toughness. To prevent this, the thermal effect of the subsequent passes should be less than that of the preceding passes. Therefore, X1 / X2, which is the ratio of the tool rotation speed in the first and second passes divided by the joining speed, is controlled to be 1.0 or greater. For the same reason, even when two or more passes are used, the relationship between the preceding and subsequent passes in all passes should be controlled to satisfy the above ratio. There is no particular upper limit to the ratio. From the viewpoint of suppressing deformation of the joint, it is more preferable that X1 / X2 be 5.0 or less.

[0060] In the present invention, in addition to the above conditions, the tool lead angle may be specified. The tool lead angle refers to the inclination from a line perpendicular to the surface of the workpieces to the direction in which the tip of the tool leads the welding direction. The tool lead angle is preferably set to 0 to 3°. This is because, when the tool lead angle is within this range, the load applied to the tool can be distributed and tool wear can be suppressed.

[0061] As described above, the manufacturing method of the present invention can stably ensure excellent cryogenic toughness in joints manufactured by friction stirring steel plates for cryogenic use. The manufacturing method of the present invention can be suitably used for steel for structures used in cryogenic environments, such as liquefied gas storage tanks for ships and on land.

[0062] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0063] Example 1 Example 1 shows the results of evaluating the cryogenic toughness of a joint manufactured by one-side, one-pass welding using the friction stir welding method of the present invention.

[0064] Steel plates (test steels) manufactured by the procedure described below were used as the materials to be joined. First, molten steel having the chemical composition shown in Table 1 was melted in a converter and made into a steel material by continuous casting. The obtained steel material (slab) was heated and hot-rolled to make a steel plate with a thickness of 6 mm. Next, the hot-rolled steel plate was subjected to accelerated cooling and heat treatment to make the test steel. The symbol "-" in Table 1 indicates that no element is intentionally added, and includes not only cases where no element is contained (i.e., the content is 0%), but also cases where an element is unavoidably contained. The cryogenic toughness of the test steels having the chemical composition of the present invention described above was measured by the Charpy absorbed energy (vE －１９６℃ ) was 120J or more.

[0065] The obtained steel plates were then stir-joined to produce joints. Here, joints were produced using each of the steel plates shown in Table 2, with the tool rotation speed and welding speed varied as shown in Table 2. An I-groove was used. The tool used was a cemented carbide tool (note that the tool did not have a thread on the probe) with a shoulder diameter of 12 mm, a probe diameter of 2 mm, a probe length of 6 mm, and a tool height of 10 mm. Using the obtained joints, the cryogenic toughness was evaluated by the following method.

[0066] [Evaluation of cryogenic toughness] A V-notch test piece was taken from the center of the plate width (center of the joint) and the center of the plate thickness of the obtained joint in accordance with the provisions of JIS Z 3128, and a Charpy impact test was carried out. －１９６℃ The Charpy impact test was performed three times. The obtained Charpy absorbed energy (vE －１９６℃ ) are shown in Table 2.

[0067] The Charpy absorbed energy can be regarded as an index of the cryogenic toughness of the steel plate. In all three test results, the Charpy absorbed energy (vE －１９６℃ When the value of σ σ ) was 60 J or more, the specimen was evaluated as having "excellent cryogenic toughness." In Example 1, a sub-size V-notch test piece was used.

[0068]

[0069]

[0070] As can be seen from Tables 1 and 2, the joints manufactured according to the manufacturing method of the present invention exhibited a high absorbed energy of 60 J or more at -196°C for the sub-size joints, confirming that they had excellent cryogenic temperature toughness. On the other hand, the comparative examples outside the range of the present invention had low Charpy absorbed energies of less than 60 J. In other words, the comparative examples did not achieve good cryogenic temperature toughness.

[0071] Example 2 Example 2 shows the results of evaluating the cryogenic toughness of a joint manufactured by a double-sided single-pass method using the friction stir welding method of the present invention.

[0072] Steel plates (test steels) manufactured by the procedure described below were used as the materials to be joined. First, molten steel having the chemical composition shown in Table 1 was melted in a converter and made into a steel material by continuous casting. The obtained steel material (slab) was heated and hot-rolled to make a steel plate with a thickness of 12 mm. Next, the hot-rolled steel plate was subjected to accelerated cooling and heat treatment to obtain the test steel. "-" in Table 1 has the same meaning as above. The cryogenic toughness of the test steel having the chemical composition of the present invention described above was measured by the Charpy absorbed energy (vE －１９６℃ ) was 120J or more.

[0073] Next, the obtained steel plates were friction stir welded to produce joints. Here, joints were produced using the steel plates shown in Table 3, with the tool rotation speed and welding speed varied as shown in Table 3. An I-groove was used. The tool shape was the same as in Example 1. The "first pass" refers to friction stir welding performed on the front surface, and the "second pass" refers to friction stir welding performed on the back surface.

[0074] Using the obtained joint, the cryogenic temperature toughness was evaluated in the same manner as in Example 1. The obtained Charpy absorbed energy (vE －１９６℃ The values ​​of Charpy absorbed energy (vE) are shown in Table 3. In Example 2, a full-size V-notch test piece was used. In Example 2, the Charpy absorbed energy (vE) was measured in all three test results. －１９６℃ ) was 120 J or more, the steel was evaluated as having "excellent cryogenic toughness."

[0075]

[0076] As can be seen from Tables 1 and 3, the joints manufactured according to the manufacturing method of the present invention exhibited a high absorbed energy of 120 J or more at -196°C in full size, confirming that they ensured excellent cryogenic toughness. On the other hand, the comparative examples outside the range of the present invention had low Charpy absorbed energies of less than 120 J. In other words, the comparative examples did not achieve good cryogenic toughness.

[0077] According to the present invention, a joint having excellent cryogenic toughness can be manufactured inexpensively using a high-Ni steel plate having cryogenic toughness.

Claims

1. A manufacturing method for a friction stir welded joint in which the tip of a tool is inserted into the butt surfaces of the workpieces and the tool is moved relatively in the welding direction while rotating to stir weld the workpieces together, the workpieces being steel plates having a composition containing, in mass%, C: 0.01-0.15%, Si: 0.01-0.50%, Mn: 0.05-1.00%, and Ni: 6.5-10.0%, and the stir welding satisfies the following welding conditions: the value of X expressed by formula (1) for each pass is 0<X≦8.0, and the rotation speed of the tool is 50≦A≦500, where A is the rotation speed (rpm) of the tool and B is the welding speed. X=A / B...(1) 2. A method for manufacturing a friction stir welded joint as described in claim 1, wherein the chemical composition of the steel plate further contains, by mass%, one or more elements selected from the group consisting of Cr: 1.00% or less, Mo: 0.50% or less, P: 0.03% or less, S: 0.005% or less, N: 0.0010 to 0.0080%, Al: 0.10% or less, Cu: 0.50% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, and B: 0.0030% or less.

3. A method for producing a friction stir welded joint according to claim 1 or 2, wherein the welding conditions are, when the number of passes is two or more, where the value of X expressed by said formula (1) in a preceding pass is X1 and the value of X expressed by said formula (1) in a subsequent pass following the preceding pass is X2, the relationship between X1 and X2 is X1 / X2≧1.0.

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