Friction stir welding joint manufacturing method

Abstract

The purpose of the present invention is to provide a method for manufacturing a friction stir welding joint. The present invention is a method for manufacturing a friction stir welded joint in which materials to be welded are stir-welded together by inserting the tip of a tool into the abutting surface of the materials to be welded and moving the tool in the welding direction while rotating the tool, the materials to be welded being a steel plate having a component composition containing, in mass%, 0.01-0.15% of C, 0.01-0.50% of Si, 0.05-1.00% of Mn, and 6.5-10.0% of Ni, with the balance being Fe and unavoidable impurities. When the rotation speed (rpm) of the tool is A and the bonding speed (mm / min) is B during the stirring bonding, the bonding conditions that the value of X represented by A / B per pass is 0 < X < = 8.0 and 50 < = A < = 500 are satisfied.

Description

Technical Field

[0001] This 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 ultra-low temperature steels for use in ultra-low temperature environments. Background Technology

[0002] Since steel structures such as liquefied gas storage tanks are used in extremely low-temperature environments, the steel used in these structures is required to not only have strength and toughness, but also toughness at extremely low temperatures (hereinafter sometimes referred to as "low-temperature toughness"), especially the toughness of welded parts in the structure.

[0003] For example, when hot-rolled steel sheets are used in liquefied natural gas (LNG) storage tanks, the steel sheet must ensure toughness at extremely low temperatures below the boiling point of LNG, i.e., -164°C. If the steel's extremely low temperature toughness is poor, it may be unable to maintain the safety of the structure used for extremely low temperature storage. Therefore, there is a high requirement to improve the extremely low temperature toughness of the steel used and its joints. 7% Ni steel sheets or 9% Ni steel sheets have traditionally been used to meet this requirement.

[0004] For example, patent documents 1 and 2 can be cited as examples of 7% Ni steel plates or 9% Ni steel plates.

[0005] Patent Document 1 discloses a thick steel plate for ultra-low temperature applications containing more than 5.0% to less than 10.0% by mass of Ni and specified amounts of C, Si, Mn, and Al. The steel plate of Patent Document 1 has a V-notch Charpy absorption energy vE per unit area in the range of 6 to 50 mm in thickness. －196℃ The average value is 1.25 J / mm. 2 The above-mentioned extremely low temperature toughness.

[0006] Patent Document 2 discloses a low-temperature Ni-containing steel containing 7.0–10.5% by mass of Ni and specified amounts of C, Si, Mn, and Al. The steel in Patent Document 2 exhibits a Charpy absorption energy vE in the range of 30–60 mm plate thickness. －196℃ The average value is above 150J for extremely low temperature toughness.

[0007] When using these ultra-low temperature steel plates in steel structures, welding has traditionally been used to join the plates together. Welding materials, such as Inconel or Hastelloy-based materials containing approximately 70% by mass of Ni, are required. The reasoning is as follows: Generally, steel subjected to thermal effects at temperatures close to its melting point deteriorates in toughness. Therefore, by using these welding materials that do not induce brittle fracture, joints are formed through welding, thus preventing brittle fracture of the steel structure.

[0008] However, welding materials containing high Ni are very expensive, putting pressure on construction costs. Therefore, the inventors have focused on friction stir welding (FSW) as a joining method that does not expose steel to high temperatures near its melting point. Friction stir welding (FSW) is a technique that joins metal materials by pressing a rotating tool into the materials to be joined (e.g., metals) and stirring the softened parts of the metals due to frictional heat, causing the metals to flow and thus achieving a joint. FSW is a solid-state joint, making it easier to adjust the joining temperature, and it produces better joint characteristics compared to conventional fusion welding. Furthermore, it has the advantage of not requiring additional welding materials during the jointing process.

[0009] As a method for obtaining good joint characteristics, friction stir welding can be cited as an example, Patent Document 3. Patent Document 3 discloses a friction stir welding method that uses steel containing 0.20 to 0.45% by mass of C and a specified amount of Cr as the joining material and makes the highest temperature reached during the joint of the stirring part above the A1 point of the steel.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 2011-219848

[0013] Patent Document 2: Japanese Patent Application Publication No. 2011-214099

[0014] Patent Document 3: Japanese Patent No. 6634616 Summary of the Invention

[0015] As mentioned above, in the manufacture of welded joints for welding thick steel plates used at extremely low temperatures using high-Ni welding wire as the welding material, the high cost due to the high Ni content is a problem. Therefore, if friction stir welding can be applied, the cost of welding materials can be significantly reduced.

[0016] Furthermore, in the technologies described in Patent Documents 1 and 2, the low-temperature toughness was only studied based on the properties of the base material (steel plate). These documents made no mention of the properties of the materials to be joined in friction stir welding using high-Ni steel plates, or the joint formed by joining these steel plates.

[0017] On the other hand, the technology described in Patent Document 3 studies the tensile strength and fracture characteristics of the joint (stirring part) of friction stir welding. However, the technology for ensuring the extremely low temperature toughness of the joint is not studied. In the technology of Patent Document 3, carbon steel with a C content of 0.20% by mass or more is used as the joint material, and high Ni steel plate with a Ni content of 6.5% to 10.0% by mass is not used as the joint material.

[0018] Furthermore, from the perspective of the durability of FSW tools, no technology has yet been proposed for using high-Ni steel plates in friction stir welding of the materials being joined.

[0019] The present invention was made in view of the above circumstances, with the aim of providing a method for manufacturing a friction stir welded joint that can ensure extremely low temperature toughness.

[0020] In order to solve the above-mentioned problems, the inventors conducted an in-depth study on the influencing factors of the characteristics of friction stir welded joints manufactured by using high-Ni steel plates with a Ni content of 6.5 to 10.0% by mass as the bonding material and performing friction stir welding on the bonding material under various bonding conditions. As a result, the following insights were obtained.

[0021] First, when performing friction stir welding on the aforementioned high-Ni steel plates with high toughness and strength, the tool rotation speed (rpm) needs to be 50 to 500.

[0022] Second, in order to reliably ensure the extremely low temperature toughness of the stirring joint (i.e., the joint), it is important to keep the value calculated based on the tool rotation speed and the joint speed (i.e., the value of "tool rotation speed (rpm) / joint speed (mm / min)") below a certain value.

[0023] The present invention was made based on the above insights, and its main points are as follows.

[0024] [1] A method for manufacturing a friction stir welding head, wherein the front end of a tool is inserted into the mating surface of the materials to be joined, and the tool is rotated while moving relative to each other along the joining direction to stir and join the materials to be joined.

[0025] The materials to be joined are steel plates having the following composition, which, in percentage by mass, contains...

[0026] C: 0.01~0.15%

[0027] Si: 0.01~0.50%,

[0028] Mn: 0.05~1.00%, and

[0029] Ni: 6.5%–10.0%

[0030] In the above-mentioned stirring and joining process,

[0031] When the rotational speed (rpm) of the above tool is set to A and the engagement speed (mm / min) is set to B,

[0032] The following connection conditions must be met: the value of X shown in equation (1) for each pass is 0 < X ​​≤ 8.0 and the rotational speed of the above tool is 50 ≤ A ≤ 500.

[0033] X = A / B … (1)

[0034] [2] According to the method for manufacturing a friction stir welded joint as described in [1], wherein the composition of the steel plate further contains, by mass%, a component selected from […].

[0035] Cr: less than 1.00%

[0036] Mo: 0.50% or less,

[0037] P: below 0.03%

[0038] S: Below 0.005%

[0039] N: 0.0010~0.0080%

[0040] Al: below 0.10%

[0041] Cu: less than 0.50%

[0042] Nb: below 0.05%

[0043] V: Below 0.05%

[0044] Ti: below 0.03%, and

[0045] B: Below 0.0030%

[0046] One or more of them.

[0047] [3] The manufacturing method of the friction stir welded joint according to [1] or [2], wherein the above-mentioned joining conditions are as follows:

[0048] When the number of tracks is 2 or more, the value of X shown in equation (1) above for the preceding track is set to X1, and the value of X shown in equation (1) above for the subsequent track following the preceding track is set to X2.

[0049] The relationship between X1 and X2 is X1 / X2≥1.0.

[0050] According to the present invention, a method for manufacturing a friction stir welded joint that ensures extremely low-temperature toughness of the joint obtained by using high-Ni steel plates as the bonding material for friction stir welding is provided. By applying the manufacturing method of the present invention to the manufacture of steel structures used in extremely low-temperature environments, such as liquefied gas storage tanks, the safety of the steel structure can be maintained and the joint can be manufactured at a low cost, thus bringing significant industrial benefits. Detailed Implementation

[0051] The following describes in detail one embodiment of the method for manufacturing the friction stir welded joint of the present invention. It should be noted that the following description represents a preferred embodiment of the present invention, and the present invention is not limited thereto.

[0052] <Jointing Method>

[0053] This invention relates to a method for manufacturing a friction stir welded joint, wherein two steel plates to be joined are used as the materials to be joined. The tip of a tool is inserted between the mating surfaces that bring the ends of the materials to abut together. By rotating the tool while moving it relative to each other along the long side direction (i.e., the joining direction) of each end, the materials to be joined are frictionally joined together. In this invention, the materials to be joined and the joining conditions are specified as follows.

[0054] It should be noted that the friction stir welding of the present invention includes either friction stir welding in which the tool is rotated and moved in the joining direction, or friction stir welding in which the materials to be joined are brought together at the joining site and moved in the joining direction, as well as combinations thereof.

[0055] Furthermore, the friction stir welding method of the present invention can be applied to either single-sided construction where the tool is inserted only into one side (i.e., the surface or the back side) of the materials to be joined for stirring and joining, or double-sided construction where the tool is inserted into both sides (i.e., the surface and the back side) of the materials to be joined for stirring and joining.

[0056] [Materials to be bonded]

[0057] First, the steel plate that can be used as the joining material described above will be explained.

[0058] As mentioned above, from a safety perspective, the steel used in steel structures such as liquefied natural gas storage tanks, which require ensuring toughness in extremely low-temperature environments, has high requirements for improving extremely low-temperature toughness. High-Ni steel plates are used to meet this requirement. Furthermore, for steel structures manufactured by joining multiple steel plates, it is also necessary to ensure the toughness of the joints. Therefore, the steel plates used in this invention are designed with a composition that maintains both the strength and extremely low-temperature toughness of the steel plates, and also ensures the extremely low-temperature toughness of the joints after stir-frying. The composition of the steel plates will be explained below.

[0059] [Composition]

[0060] In this invention, the steel plate supplied as the bonding material has the following specified composition. The elements contained in this composition will be described. It should be noted that, unless otherwise specified, the "%" used in this specification to indicate the content of each element refers to "mass%".

[0061] C: 0.01~0.15%

[0062] Carbon (C) is an element that improves the strength of steel plates. To achieve this effect, the C content is 0.01% or more, preferably 0.03% or more. On the other hand, if the C content exceeds 0.15%, the extremely low temperature toughness of the steel plate decreases. Therefore, the C content is 0.15% or less, preferably 0.12% or less.

[0063] Si: 0.01~0.50%

[0064] Si is an element that helps improve the strength of steel sheets and also acts as a deoxidizer. To achieve these effects, the Si content is 0.01% or more. On the other hand, if the Si content becomes excessively high, the extremely low temperature toughness decreases. Therefore, the Si content is 0.50% or less, preferably 0.30% or less.

[0065] Mn: 0.05~1.00%

[0066] Manganese (Mn) is an effective element for increasing the hardenability of steel and thus enhancing the strength of steel plates. To achieve this effect, the Mn content is 0.05% or more. On the other hand, when the Mn content exceeds 1.00%, temper embrittlement susceptibility increases, and deviations in low-temperature toughness begin to appear. For this reason, the Mn content is limited to 1.00% or less. The Mn content is preferably less than 0.90%, and more preferably less than 0.80%.

[0067] Ni: 6.5%–10.0%

[0068] Ni is an extremely effective element for improving the extremely low temperature toughness of steel sheets. If the Ni content is less than 6.5%, extremely low temperature toughness cannot be reliably ensured. Therefore, the Ni content is 6.5% or more. On the other hand, Ni is an expensive element, so the cost of steel sheets increases as its content increases. Therefore, in this invention, the Ni content is 10.0% or less. The Ni content is preferably 7.0% or more, and more preferably 9.5% or less.

[0069] In this invention, the above-described composition is the basic composition of the steel plate. In addition to the above-described composition, this invention may arbitrarily contain one or more elements selected from Cr, Mo, P, S, N, Al, Cu, Nb, V, Ti, and B. It should be noted that, since these elements can be included as needed, the content of each of Cr, Mo, P, S, Al, Cu, Nb, V, Ti, and B can be 0%.

[0070] Cr: less than 1.00%

[0071] Cr is an element that can improve the strength of steel plates without severely compromising their low-temperature toughness. However, if the Cr content exceeds 1.00%, the low-temperature toughness of the steel plate decreases. Therefore, when Cr is present, 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.

[0072] Mo: 0.50% or less

[0073] Like Cr, Mo is an element that can increase the strength of steel plates without severely compromising their low-temperature toughness. However, if the Mo content exceeds 0.50%, the low-temperature toughness decreases. Therefore, when Mo is present, 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.

[0074] P: below 0.03%

[0075] Phosphorus (P) is an unavoidable impurity and a harmful element that negatively affects the extremely low temperature toughness of steel plates. For example, in order to obtain a sound base material and weld joint when welding steel plates into welded structures, it is preferable to minimize the P content as much as possible. Therefore, the P content is preferably suppressed to below 0.03%. It should be noted that from the viewpoint of extremely low temperature toughness, the lower the P content, the better. Therefore, there is no particular limitation on the lower limit of the P content, and it can be 0%, but in this case, it is also permissible as an unavoidable impurity. Excessive reduction of P will lead to increased costs, so from a cost point of view, the lower limit of the P content is preferably 0.001%. That is, the P content is preferably 0.001% or more.

[0076] S: below 0.005%

[0077] Sulfur (S) forms MnS in steel, significantly deteriorating its extremely low-temperature toughness. Therefore, 0.005% is considered the upper limit for S content, and it is preferable to reduce it as much as possible. Thus, the S content is preferably 0.005% or less. More preferably, it is 0.002% or less. It should be noted that the lower the S content, the better. Therefore, the lower limit is not particularly limited and can be 0%, but in this case, its presence as an unavoidable impurity is permissible. That is, the S content is preferably 0.0005% or more.

[0078] N: 0.0010~0.0080%

[0079] Nitrogen (N) forms precipitates in steel, and if its content exceeds 0.0080%, it contributes to a decrease in the extremely low-temperature toughness of the base material. However, N also contributes to the grain refinement of the base material by forming AlN, an effect achieved by keeping the N content at 0.0010% or higher. Therefore, when N is present, the N content is preferably 0.0010 to 0.0080%. More preferably, the N content is 0.0020% or higher, and even more preferably 0.0060% or lower.

[0080] Al: below 0.10%

[0081] Al is an element contained in deoxidizers. Al also contributes to the grain refinement of the base material by forming AlN. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the steel is compromised. Therefore, when Al is present, the Al content is preferably 0.10% or less. More preferably, the Al content is 0.05% or less. The Al content is preferably 0.005% or more.

[0082] Cu: below 0.50%

[0083] Cu is an element that increases the strength of steel plates by improving hardenability. However, if the Cu content exceeds 0.50%, in addition to a decrease in the extremely low temperature toughness of the steel plate, the surface properties of the cast steel material (steel billet) deteriorate. Therefore, when Cu is present, the Cu content is preferably 0.50% or less. More preferably, the Cu content is 0.40% or less, and even more preferably 0.30% or less. On the other hand, there is no particular limitation on the lower limit of the Cu content, but to obtain the aforementioned effects, it is preferable that the Cu content is 0.10% or more.

[0084] Nb: below 0.05%

[0085] Nitrogen (Nb) is an effective element for increasing the strength of steel sheets through precipitation strengthening. However, if the Nb content is excessively high, the extremely low temperature toughness of the steel sheet decreases. Therefore, when Nb is present, the Nb content is preferably 0.05% or less. More preferably, the Nb content is 0.03% or less. On the other hand, there is no particular limitation on the lower limit of the Nb content, but to obtain the aforementioned effect, it is preferable that the Nb content is 0.01% or more.

[0086] V: Below 0.05%

[0087] Like Nb, vanadium (V) is an effective element for increasing the strength of steel sheets through precipitation strengthening. However, if the V content is excessively high, the extremely low temperature toughness of the steel sheet decreases. Therefore, when V is present, the V content is preferably 0.05% or less. More preferably, the V content is 0.04% or less. On the other hand, there is no particular limitation on the lower limit of the V content, but to obtain the aforementioned effects, it is preferable that the V content is 0.01% or more.

[0088] Ti: below 0.03%

[0089] Ti is an element that helps improve low-temperature toughness by refining the microstructure of steel sheets through the formation of precipitates. Therefore, when Ti is present, it can be present in the range of 0.03% or less. More preferably, the Ti content is 0.02% or less, and even more preferably 0.005% or more.

[0090] B: Below 0.0030%

[0091] Boron (B) is an element that improves hardenability when added in trace amounts. To effectively achieve this effect, it can contain 0.0003% or more of B. On the other hand, if the B content exceeds 0.0030%, the extremely low temperature toughness deteriorates. Therefore, when B is present, the B content is preferably 0.0030% or less.

[0092] The steel plate of the present invention contains the elements described above, with the remainder being Fe and unavoidable impurities.

[0093] It should be noted that the steel plate of the present invention preferably contains C, Si, Mn, P, S, Al, Ni and N within the above-mentioned numerical range, with the remainder consisting of Fe and unavoidable impurities.

[0094] [Mechanical properties and thickness of steel plates]

[0095] In this invention, the tensile strength of the steel plate supplied as the bonding material is not particularly limited. The tensile strength of the steel plate having the above-described composition can be 780 MPa or more. More preferably, the tensile strength is 800 MPa or more.

[0096] As mentioned above, from the viewpoint of maintaining the safety of structures used for cryogenic storage, the steel plates used must also possess cryogenic toughness. This allows steel plates with the aforementioned composition to absorb Charpy energy (vE) at -196°C. －196℃ The value is above 120J.

[0097] Furthermore, in this invention, the thickness of the steel plate is not particularly limited. From the perspective that the friction stir welding method of this invention is suitable for manufacturing structures for cryogenic storage, it is preferable that the thickness of the steel plate is, for example, 6 mm or more. This is because steel plates with thicknesses within this range are suitable for use as strength components in structures for cryogenic storage.

[0098] [Connection Conditions]

[0099] Next, the joining conditions in the manufacturing method of the present invention will be described. In the present invention, a material to be joined, consisting of two steel plates having the above-described composition, is prepared. The tip of a tool is inserted into the mating surface of the material to be joined. Under the joining conditions described below, the tool is rotated while moving along the joining direction to join the steel plates together, thereby creating a friction stir welded joint. It should be noted that, as described above, the present invention also includes all methods of single-sided and double-sided welding.

[0100] Specifically, the joining conditions of the stirring joining of the present invention satisfy the following conditions: when the rotation speed (rpm) of the tool is set to A and the moving speed of the tool during joining (hereinafter also referred to as "joining speed") is set to B (mm / min), the value of X shown in equation (1) for each pass is 0 < X ​​≤ 8.0, and the rotation speed of the tool is 50 ≤ A ≤ 500.

[0101] X = A / B … (1)

[0102] In this invention, the focus is on the tool rotation speed (A) and the joining speed (B) for each pass. By appropriately controlling these two factors, the heat generated during the processing of the materials being joined and the resulting microstructural changes can be controlled during friction stir welding. As a result, the extremely low temperature toughness of the stir-jointed portion (i.e., the joint) can be stably ensured. If the value of A divided by B as shown in equation (1), i.e., the value of "A / B" (hereinafter sometimes referred to as "X"), exceeds 8.0, the microstructure becomes coarser under the influence of the processing heat generated by friction stir welding, resulting in a decrease in the extremely low temperature toughness of the joint. Therefore, the value of X is 8.0 or less. The value of X is preferably 7.5 or less.

[0103] The lower limit of the X value is greater than 0. It should be noted that if the X value is less than 2.0, the load on the engagement tool increases, resulting in a reduced tool life and putting pressure on costs. Therefore, the X value is preferably 2.0 or higher, more preferably 2.5 or higher, and even more preferably 3.0 or higher.

[0104] In addition to this condition, the tool rotation speed needs to be between 50 rpm and 500 rpm. As described above, the steel plate used for the joined materials in this invention is a Ni steel plate with high toughness and strength. Because when performing friction stir welding on this steel plate, by properly controlling the tool rotation speed, the tool's service life can be extended and construction costs reduced. If the tool rotation speed is less than 50 rpm, the load applied to the tool is high and its service life is reduced. 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 more preferably 450 rpm or less. The tool rotation speed is more preferably 100 rpm or more, and more preferably 400 rpm or less.

[0105] As explained above, in both single-sided and double-sided construction, joints are manufactured by performing friction stir welding that meets these two conditions. Thus, even when the aforementioned high-Ni steel plate is used as the material to be joined, stable low-temperature toughness of the joint can be achieved.

[0106] It should be noted that the joining method of the present invention can also be applied when the thickness of the materials to be joined is greater than the height of the tool.

[0107] For example, in stir bonding where the thickness of the materials being joined is greater than the tool height, a case of double-sided application (i.e., double-sided single-pass application) can be cited. However, even if each pass is controlled in a way that satisfies the two conditions mentioned above, the toughness of the first pass may sometimes decrease due to the influence of the second pass. Therefore, in order to more effectively avoid this reduction in toughness during double-sided application, in addition to the two conditions mentioned above, it is effective to control the process by ensuring that the ratio of the values ​​calculated from the tool rotation speed and the bonding speed in the first pass to those in the second pass is a certain value or higher.

[0108] Specifically, when there are two or more passes in the mixing process, each pass satisfies the two conditions mentioned above, and when the value of X shown in equation (1) of the preceding pass is set as X1 and the value of X shown in equation (1) of the subsequent pass after the preceding pass is set as X2, the relationship between the preceding pass and the subsequent pass preferably satisfies the condition X1 / X2≥1.0.

[0109] For example, in the case of double-sided single-pass construction, the "preceding pass" mentioned above refers to the first pass in the double-sided construction, and the "subsequent pass" mentioned above refers to the second pass in the double-sided construction. That is, it is the first pass performed from the surface side of the joined materials (joining area) and the second pass performed from the back side of the joined materials, which is the opposite side of the surface side of the joined materials.

[0110] The reasons are as follows. During double-sided welding, if the value of X exceeds 8.0 on either side or in any pass, the microstructure becomes coarser due to the heat generated during friction stir welding, resulting in reduced low-temperature toughness of the joint. Therefore, the value of X for the first and second passes is kept below 8.0. For the same reasons, the value of X for each pass is preferably below 7.5, and more preferably below 7.0. It should be noted that in the case of two or more passes, the value of X should also be kept below 8.0 in all passes for the same reasoning.

[0111] Furthermore, if the microstructure after the first pass undergoes a wide-ranging reverse phase transformation due to the heat effect of the second pass, the portion where this reverse phase transformation occurs becomes a coarse, low-toughness microstructure, thus reducing low-temperature toughness. To prevent this, the heat effect of subsequent passes should be less than that of preceding passes. Therefore, control is implemented such that the ratio of the tool rotation speed divided by the engagement speed in the first and second passes, i.e., X1 / X2, is 1.0 or higher. It should be noted that for two or more passes, control is also implemented for the same reason, ensuring that the relationship between preceding and subsequent passes in all passes satisfies the above ratio.

[0112] It should be noted that no upper limit is specifically specified for the above ratio. From the viewpoint of suppressing various deformations of the joint, X1 / X2 is more preferably 5.0 or less.

[0113] In this invention, in addition to the conditions described above, the tool's tilt angle can be specified. The tool's tilt angle refers to the inclination of the tool's tip from a line perpendicular to the surface of the materials being joined towards a direction forward relative to the joining direction. The tool's tilt angle is preferably 0 to 3°. Because within this range, the load applied to the tool can be distributed, suppressing tool wear.

[0114] As explained above, the manufacturing method according to the present invention ensures stable and excellent cryogenic toughness in joints made from steel plates used in cryogenic environments via friction stir welding. The manufacturing method of the present invention is suitable, for example, for structural steel used in cryogenic environments, such as liquefied gas storage tanks for marine and land-based applications.

[0115] Example

[0116] The following examples illustrate the function and effects of the present invention. It should be noted that the present invention is not limited to the following examples.

[0117] [Example 1]

[0118] Example 1 shows the evaluation results of the cryogenic toughness of a joint manufactured using the friction stir welding method of the present invention in a single-sided, single-pass manner.

[0119] The steel plate (test steel) manufactured according to the steps described below is used for the materials to be joined.

[0120] First, molten steel with the composition shown in Table 1 is smelted in a converter and produced as steel raw material by continuous casting. The obtained steel raw material (steel billet) is heated and hot-rolled to produce a steel plate with a thickness of 6 mm. Next, the hot-rolled steel plate is subjected to accelerated cooling and heat treatment to produce test steel. In Table 1, "-" indicates that the element was not intentionally added, including not only cases where the element is not present (i.e., the content is 0%), but also cases where the element is unavoidably present. It should be noted that the extremely low temperature toughness of the test steel with the composition of the present invention described above is measured by the Charpy absorption energy (vE) at -196°C. －196℃ The value is above 120J.

[0121] Next, the obtained steel plates were stirred and bonded to create a joint. Here, the steel plates shown in Table 2 were used, and the tool rotation speed and bonding speed were varied as shown in Table 2 to create the joint. The bevel was an I-bevel. The tool used was a superhard alloy tool with a shoulder diameter of 12 mm, a stirring head diameter of 2 mm, a stirring head length of 6 mm, and a tool height of 10 mm (it should be noted that this tool does not have threads at the stirring head). The resulting joint was then used to evaluate its cryogenic toughness using the following method.

[0122] [Evaluation of Low Temperature Toughness]

[0123] According to JIS Z 3128, a V-notch test piece is taken from the center of the width (center of the joint) and the center of the thickness of the obtained joint, and a Charpy impact test is performed to determine the Charpy absorption energy (vE) at -196℃. －196℃ Three Charpy impact tests were conducted. The Charpy absorbed energy (vE) obtained was... －196℃ The values ​​of ) are shown in Table 2.

[0124] It should be noted that Charpy absorption energy can be considered as an indicator of the extremely low temperature toughness of the steel plate. Here, in all three test results, the Charpy absorption energy (vE) is the highest. －196℃ When the value is above 60 J, it is evaluated as having "excellent low-temperature toughness". It should be noted that in Example 1, a small-sized V-notch test piece was used.

[0125]

[0126]

[0127] As shown in Tables 1 and 2, the joint manufactured according to the method of the present invention exhibits a high absorption energy of over 60 J when using a small size at -196°C, confirming excellent low-temperature toughness. On the other hand, in comparative examples that deviate from the scope of the present invention, the Charpy absorption energy is low, less than 60 J. In other words, good low-temperature toughness was not obtained in the comparative examples.

[0128] [Example 2]

[0129] Example 2 shows the evaluation results of the extremely low temperature toughness of a joint manufactured using the friction stir welding method of the present invention in a double-sided single-pass manner.

[0130] The steel plate (test steel) manufactured using the steps described below will be used for the materials to be joined.

[0131] First, molten steel with the composition shown in Table 1 is smelted in a converter and produced as steel raw material by continuous casting. The obtained steel raw material (slab) is heated and hot-rolled to produce a steel plate with a thickness of 12 mm. Next, the hot-rolled steel plate is subjected to accelerated cooling and heat treatment to produce test steel. The "-" in Table 1 is the same as described above. It should be noted that the extremely low temperature toughness of the test steel with the composition of the present invention described above is measured by the Charpy absorption energy (vE) at -196°C. －196℃ The value is above 120J.

[0132] Next, the obtained steel plates are stir-welded to create a joint. Here, the steel plates shown in Table 3 are used, and the tool rotation speed and joining speed are varied as shown in Table 3 to create the joint. The bevel is an I bevel. The tool shape is the same as in Example 1. It should be noted that "first pass" refers to friction stir welding performed from the surface, and "second pass" refers to friction stir welding performed from the back side.

[0133] The obtained joint was used to evaluate its cryogenic toughness using the same method as in Example 1. The obtained Charpy absorption energy (vE) was then used to evaluate the low-temperature toughness. －196℃ The values ​​of ) are shown in Table 3. In Example 2, a full-size V-notch test piece was used. It should be noted that in Example 2, in all three test results, the Charpy absorption energy (vE) was the highest. －196℃ When the value is above 120J, it is evaluated as having "excellent low-temperature toughness".

[0134]

[0135] As shown in Tables 1 and 3, the joint manufactured according to the method of the present invention exhibits a high absorption energy of over 120 J when used at full size at -196°C, confirming excellent cryogenic toughness. On the other hand, in comparative examples that deviate from the scope of the present invention, the Charpy absorption energy is low, less than 120 J. In other words, good cryogenic toughness was not obtained in the comparative examples.

[0136] Industrial availability

[0137] According to the present invention, it is possible to manufacture inexpensive joints with excellent low-temperature toughness using high-Ni steel plates with extremely low-temperature toughness.

Claims

1. A method for manufacturing a friction stir welding head, comprising inserting the tip of a tool into the mating surfaces of the materials to be joined, and rotating the tool while moving it relative to each other along the joining direction to stir and join the materials together. The material to be joined is a steel plate having the following composition, which, in mass percent, contains C：0.01～0.15%、 Si: 0.01~0.50%, Mn: 0.05~1.00%, and Ni: 6.5%–10.0% In the stirring engagement, When the rotational speed (rpm) of the tool is set to A and the engagement speed (mm / min) is set to B, The following engagement conditions are met: the value of X shown in equation (1) for each pass is 0 < X ​​≤ 8.0 and the rotational speed of the tool is 50 ≤ A ≤ 500, where, The unit for rotational speed is rpm, and the unit for engagement speed is mm / min. X = A / B … (1).

2. The method for manufacturing a friction stir welded joint according to claim 1, wherein, The composition of the steel plate, by mass%, further contains ingredients selected from... Cr: less than 1.00% Mo: 0.50% or less, P: below 0.03% S: Below 0.005% N：0.0010～0.0080%、 Al: below 0.10% Cu: less than 0.50% Nb: below 0.05% V： 0.05% or less, Ti: below 0.03%, and B: Below 0.0030% One or more of them.

3. The method for manufacturing a friction stir welded joint according to claim 1 or 2, wherein, The bonding conditions are as follows: When the number of tracks is 2 or more, if the value of X shown in equation (1) for the preceding track is set to X1, and the value of X shown in equation (1) for the subsequent track following the preceding track is set to X2, The relationship between X1 and X2 is X1 / X2≥1.0.

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