Double tube and welded joint

Abstract

A double tube, which includes a first tube and a second tube. The chemical composition of the first tube, by mass%, is C: greater than 0.060% and 0.400% or less, Si: 0.01 - 1.00%, Mn: 0.01 - 1.20%, P: 0.0350% or less, S: 0.0150% or less, Sn: 0.0005 - 0.0400%, Al: 0.040% or less, N: 0.050% or less, O: 0.030% or less, and the balance: Fe and impurities. The chemical composition of the second tube, by mass%, is C: 0.003 - 0.100%, Si: 0.01 - 1.50%, Mn: 0.01 - 2.20%, P: 0.0400% or less, S: 0.0100% or less, Sn: 0.0005 - 0.0300%, Ni: 7.0 - 52.0%, Cr: 15.0 - 27.0%, Al: 0.001 - 0.600%, N: 0.001 - 0.150%, O: 0.030% or less, and the balance: Fe and impurities. The double tube satisfies [Si ave + 6×P ave + 20×S ave + 2×Sn ave ≤ 1.1000] and [0.0015 ≤ 4×S ave + Sn ave .

Description

Technical Field

[0001] The present invention relates to a double tube and a welded joint. Background Art

[0002] Heater tubes in boilers for thermal power generation, waste incineration power generation boilers, and biomass power generation boilers are corroded by molten salts at high temperatures and abraded by unburned substances, etc., and the outer surface of the tubes is exposed to a harsh environment. On the other hand, regarding the heat exchanger tubes used in the syngas cooler of a coal gasification combined cycle power plant, the inner surface of the tubes is exposed to a high-temperature corrosion environment.

[0003] By selecting appropriate materials for the outer tube and the inner tube, excellent corrosion resistance and abrasion resistance can be imparted to the double tube. Therefore, various proposals have been made for double tubes composed of various materials for the above uses or uses such as energy transportation and storage equipment (for example, Patent Documents 1 to 9).

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Laid-Open No. 4-221034

[0007] Patent Document 2: Japanese Patent Laid-Open No. 4-329852

[0008] Patent Document 3: Japanese Patent Laid-Open No. 5-17841

[0009] Patent Document 4: Japanese Patent Laid-Open No. 6-306518

[0010] Patent Document 5: Japanese Patent Laid-Open No. 7-41911

[0011] Patent Document 6: Japanese Patent Laid-Open No. 7-90496

[0012] Patent Document 7: Japanese Patent Laid-Open No. 7-90540

[0013] Patent Document 8: Japanese Patent Laid-Open No. 8-232031

[0014] Patent Document 9: Japanese Patent Laid-Open No. 2013-159840 Summary of the Invention

[0015] Problems to be Solved by the Invention

[0016] However, when these double tubes are used for structures such as heaters, butt welding assembly is performed. There has been research on technical problems and countermeasures when welding the respective materials of the outer tube or the inner tube constituting the double tube. However, research on the welding of double tubes composed of different materials is insufficient.

[0017] In particular, in recent years, from the perspective of improving construction efficiency, sometimes during multi-pass welding, the welding material is not switched between the inner pipe part and the outer pipe part, and all layers are welded using a welding material for austenitic stainless steel or Ni-based alloy. At this time, cracks sometimes occur in the weld metal near the fusion line near the boundary between the inner pipe and the outer pipe. Therefore, there is a strong demand to prevent such cracks. It should be noted that although clad steel plates are also composed of a combination of different materials, the above technical problems become significant technical problems during circumferential welding of double pipes.

[0018] The present invention has been completed in view of the above situation, and thus aims to provide a double pipe that prevents cracks from occurring in the weld metal during butt welding of pipes and can stably obtain a sound welded joint, and a welded joint using the same.

[0019] Means for Solving the Problem

[0020] The present invention mainly includes the following double pipe and welded joint.

[0021] (1) A double pipe comprising a first pipe and a second pipe,

[0022] The chemical composition of the first pipe is, by mass%,

[0023] C: greater than 0.060% and 0.400% or less,

[0024] Si: 0.01 - 1.00%,

[0025] Mn: 0.01 - 1.20%,

[0026] P: 0.0350% or less,

[0027] S: 0.0150% or less,

[0028] Sn: 0.0005 - 0.0400%,

[0029] Al: 0.040% or less,

[0030] N: 0.050% or less,

[0031] O: 0.030% or less,

[0032] Balance: Fe and impurities,

[0033] The chemical composition of the second pipe is, by mass%,

[0034] C: 0.003 - 0.100%,

[0035] Si: 0.01 - 1.50%,

[0036] Mn: 0.01 to 2.20%,

[0037] P: 0.0400% or less,

[0038] S: 0.0100% or less,

[0039] Sn: 0.0005 to 0.0300%,

[0040] Ni: 7.0 to 52.0%,

[0041] Cr: 15.0 to 27.0%,

[0042] Al: 0.001 to 0.600%,

[0043] N: 0.001 to 0.150%,

[0044] O: 0.030% or less,

[0045] The balance: Fe and impurities,

[0046] The double tube satisfies the following formulas (i) and (ii).

[0047] Si ave + 6×P ave + 20×S ave + 2×Sn ave ≤ 1.1000 ··· (i)

[0048] 0.0015 ≤ 4×S ave + Sn ave ··· (ii)

[0049] Wherein, the meanings of the symbols in the above formulas are as follows.

[0050] Si ave : The average value of the Si content in the first tube and the second tube (mass %).

[0051] P ave : The average value of the P content in the first tube and the second tube (mass %).

[0052] S ave : The average value of the S content in the first tube and the second tube (mass %).

[0053] Sn ave : The average value of the Sn content in the first tube and the second tube (mass %).

[0054] (2) The double tube as described in (1) above, wherein the chemical composition of the first tube contains one or more selected from the group consisting of the following elements in mass % to replace a part of Fe,

[0055] Cr: below 9.50%;

[0056] Total of one or more selected from Cu, Ni, and Co: below 1.00%;

[0057] Total of Mo and / or W: below 4.00%;

[0058] Total of one or more selected from V, Nb, Ti, and Ta: below 1.00%;

[0059] B: below 0.0200%;

[0060] Total of Ca and / or Mg: below 0.0100%;

[0061] REM: below 0.0500%.

[0062] (3) The double tube according to (1) or (2) above, wherein the chemical composition of the second tube contains one or more selected from the following elements in mass% to replace a part of Fe,

[0063] Total of Cu and / or Co: below 6.00%;

[0064] Total of Mo and / or W: below 8.00%;

[0065] Total of one or more selected from V, Nb, Ti, and Ta: below 2.00%;

[0066] B: below 0.0200%;

[0067] Total of Ca and / or Mg: below 0.0100%;

[0068] REM: below 0.0500%.

[0069] (4) A welded joint comprising the double tube according to any one of (1) to (3) above.

[0070] Effects of the Invention

[0071] According to the present invention, a double tube can be obtained that prevents cracks from occurring in the weld metal during butt welding of tubes and can stably obtain a sound welded joint. Description of the Drawings

[0072] Figure 1 Schematic cross-sectional view showing the shape of the test material subjected to groove machining in the examples

[0073] Figure 2 Schematic view showing the shape of the restrained welding test piece. Detailed Description

[0074] In order to solve the above-mentioned technical problems and with the aim of improving corrosion resistance, the inventors conducted a detailed investigation of cracks occurring in the weld metal during the butt welding of a double tube formed by combining a low-alloy steel and a high-alloy steel containing 0.0005 to 0.0400% and 0.0005 to 0.0300% of Sn respectively, using a Ni-based alloy welding material. As a result, the following insights were clearly obtained.

[0075] (a) Cracks occurring during welding occur in the weld metal near the fusion line near the boundary between the inner tube and the outer tube, that is, in the region where the inner tube and the outer tube are melted and mixed at approximately the same ratio and are particularly susceptible to the composition of the base metals (inner tube and outer tube). In addition, the crack-occurring region exhibits a single-phase austenite solidification structure.

[0076] (b) Moreover, cracks are likely to occur as the contents of Si, P, S, and Sn in the inner tube and the outer tube increase. In addition, cracks occur at the columnar crystal boundaries of the weld metal near the fusion boundary, and enrichment of Si, P, S, and Sn is confirmed there.

[0077] From these results, it is considered that cracks occur for the following reasons.

[0078] (c) Si, P, S, and Sn are partitioned between the liquid phase and the solid phase (austenite phase) during the solidification of the weld metal and are enriched at the columnar crystal boundaries that are the junctions between the solid phases. It is considered that since these elements all lower the solidus temperature, the liquid phase remains at the columnar crystal boundaries until the end of solidification, and the shrinkage stress during solidification causes openings.

[0079] (d) It has been clarified that in order to stably prevent such cracks, it is necessary to control the average value of the amounts of Si, P, S, and Sn contained in the outer tube and the inner tube to be below a range that satisfies a specified relationship. The lower these elements are, the less likely it is for cracks to occur in the weld metal.

[0080] (e) On the other hand, it has also been clarified that when S and Sn among these elements are extremely reduced, incomplete penetration is likely to occur during multi-pass welding.

[0081] (f) S is a surface-active element and has the effect of enhancing the inward convection in the molten pool during welding. Therefore, the heat from the arc is easily transferred in the depth direction, increasing the penetration depth. In addition, Sn evaporates from the surface of the molten pool during welding, forming a conductive path for the arc and increasing the current density of the arc, also having the effect of increasing the penetration depth.

[0082] (g) Therefore, it is considered that when the contents of S and Sn are extremely low, the above-mentioned effects cannot be obtained sufficiently, and incomplete penetration is likely to occur during multi-pass welding.

[0083] (h) It can be seen that in order to prevent incomplete penetration, it is necessary to control the average values of the contents of S and Sn in the outer tube and the inner tube to be above the range that satisfies the specified relationship.

[0084] The present invention has been completed based on the above insights. The following will explain each feature of the present invention in detail.

[0085] (A) Overall structure

[0086] The double tube refers to a structure in which the outer tube and the inner tube are metallurgically bonded, and is also called a "composite tube". The double tube involved in the present invention includes a first tube and a second tube. In the present invention, according to the use, the first tube can be used for the outer tube and the second tube can be used for the inner tube, or the second tube can be used for the outer tube and the first tube can be used for the inner tube.

[0087] Moreover, the double tube of the present invention is a seamless steel pipe (also called a "seamless steel tube"). In addition, there is no particular limitation on the dimensions of the double tube, but it is preferably an outer diameter of 25.4 to 114.3 mm, a thickness of 2.0 to 15.0 mm, and the proportion of the second tube formed of high alloy steel in the overall thickness of the tube is 0.10 to 0.50 as described below.

[0088] As described below, the first tube is formed of low alloy steel and the second tube is formed of high alloy steel. The chemical compositions of the first tube and the second tube will be described in detail respectively.

[0089] (B) Chemical composition of the first tube

[0090] The reasons for limiting each element are as described below. It should be noted that "%" for the content in the following description refers to "mass %".

[0091] C: greater than 0.060% and 0.400% or less

[0092] C is dissolved in the matrix or precipitates in the form of carbides during use at high temperatures, which helps to ensure the strength at normal and high temperatures. In order to obtain this effect, C is contained in an amount greater than 0.060%. However, when too much is contained, hardening of the heat affected zone will occur during welding, increasing the susceptibility to solidification cracking. Therefore, the C content is set to 0.400% or less. The C content is preferably greater than 0.100%, more preferably 0.110% or more. In addition, the C content is preferably 0.380% or less, more preferably 0.350% or less.

[0093] Si: 0.01 to 1.00%

[0094] Si is an element that is effective in improving corrosion resistance and oxidation resistance at high temperatures while having a deoxidizing effect. To obtain this effect, the Si content is set to 0.01% or more. However, when the content is excessive, it will be mixed into the weld metal during welding, increasing the susceptibility to solidification cracking. Therefore, while the Si content is set to 1.00% or less, it is necessary to satisfy the relationships with the contents of P, S, and Sn described below. The Si content is preferably 0.03% or more, more preferably 0.05% or more. In addition, the Si content is preferably 0.90% or less, more preferably 0.80% or less.

[0095] Mn: 0.01 - 1.20%

[0096] Mn, like Si, has a deoxidizing effect and improves hardenability, contributing to an increase in strength. To obtain this effect, the Mn content is set to 0.01% or more. However, when the content is excessive, embrittlement will occur during use at high temperatures. Therefore, the Mn content is set to 1.20% or less. The Mn content is preferably 0.03% or more, more preferably 0.05% or more. In addition, the Mn content is preferably 1.10% or less, more preferably 1.00% or less.

[0097] P: 0.0350% or less

[0098] P is mixed into the weld metal during welding, increasing the susceptibility to solidification cracking. Therefore, the P content is set to 0.0350% or less. In addition, it is necessary to satisfy the relationships with the contents of Si, S, and Sn described below. The P content is preferably 0.0330% or less, more preferably 0.0300% or less. It should be noted that the lower limit of the P content does not need to be specifically set and can be 0 (zero), but extremely reducing it will increase the steelmaking cost. In addition, P has the effect of significantly increasing strength. When this effect is desired, the P content is preferably set to 0.0015% or more, more preferably 0.0030% or more.

[0099] S: 0.0150% or less

[0100] S, like P, is mixed into the weld metal during welding, significantly increasing the susceptibility to solidification cracking. Therefore, the content of S is set to 0.0150% or less. In addition, it is necessary to satisfy the relationships with the contents of Si, P, and Sn described below. The S content is preferably 0.0130% or less, more preferably 0.0100% or less. It should be noted that the lower limit of the S content does not need to be specifically set and can be 0 (zero), but when it is extremely reduced, the penetration depth during welding becomes smaller and incomplete melting is likely to occur. Therefore, while satisfying the relationship with Sn described below, the S content is preferably set to 0.0001% or more, more preferably 0.0002% or more.

[0101] Sn: 0.0005 - 0.0400%

[0102] Sn is enriched under the scale on the surface of the steel, which has the effect of improving corrosion resistance. In addition, when mixed into the weld metal during welding, it increases the penetration depth and thus suppresses the occurrence of non-fusion. To obtain this effect, the Sn content is set to 0.0005% or more, and at the same time, it is necessary to satisfy the relationship with the S content described below. On the other hand, when the content is excessive, the susceptibility to solidification cracking during welding increases. Therefore, the Sn content is set to 0.0400% or less, and at the same time, it is necessary to satisfy the relationship with the contents of Si, P, and Sn described below. The Sn content is preferably 0.0008% or more, more preferably 0.0010% or more. In addition, the Sn content is preferably 0.0380% or less, more preferably 0.0350% or less.

[0103] Al: 0.040% or less

[0104] Al is contained for deoxidation. However, when the content is excessive, it will cause a decrease in toughness. Therefore, the Al content is set to 0.040% or less. The Al content is preferably 0.035% or less, more preferably 0.030% or less. It should be noted that the lower limit of the Al content does not need to be particularly set and can be 0 (zero), but when it is extremely reduced, the deoxidation effect cannot be fully obtained, the cleanliness of the steel decreases, and at the same time, the production cost increases. Therefore, the Al content is preferably set to 0.001% or more, more preferably 0.002% or more.

[0105] N: 0.050% or less

[0106] When the N content is excessive, it will cause a decrease in toughness. Therefore, the N content is set to 0.050% or less. The N content is preferably 0.045% or less, more preferably 0.040% or less. It should be noted that the lower limit of the N content does not need to be particularly set and can be 0 (zero), but when it is extremely reduced, the steelmaking cost increases. In addition, N forms nitrides and has the effect of significantly increasing the strength. When this effect is desired, the N content is preferably set to 0.001% or more, more preferably 0.003% or more.

[0107] O: 0.030% or less

[0108] When the O content is excessive, it will cause a decrease in workability and ductility. Therefore, the O content is set to 0.030% or less. The O content is preferably 0.025% or less, more preferably 0.020% or less. It should be noted that the lower limit of the O content does not need to be particularly set and can be 0 (zero), but when it is extremely reduced, the steelmaking cost increases. Therefore, the O content is preferably set to 0.001% or more, more preferably 0.003% or more.

[0109] In the chemical composition of the first tube, the balance is Fe and impurities. It should be noted that "impurities" refer to substances mixed in for various reasons during the manufacturing process, typically represented by raw materials such as ores or scrap, in the industrial production of steel materials.

[0110] The chemical composition of the first tube may contain one or more selected from the following groups to replace a part of Fe. The reasons are discussed below.

[0111] Cr: 9.50% or less

[0112] Total of one or more selected from Cu, Ni, and Co: 1.00% or less

[0113] Total of Mo and / or W: 4.00% or less

[0114] Total of one or more selected from V, Nb, Ti, and Ta: 1.00% or less

[0115] B: 0.0200% or less

[0116] Total of Ca and / or Mg: 0.0100% or less

[0117] REM: 0.0500% or less

[0118] Cr: 9.50% or less

[0119] Since Cr is effective in improving corrosion resistance and strength at high temperatures, it can be contained as needed. However, when the content is excessive, the toughness will decrease. Therefore, when contained, the Cr content is set to 9.50% or less. The Cr content is preferably 9.40% or less, more preferably 9.20% or less. It should be noted that when reliably obtaining the above effects, the Cr content is preferably 0.01% or more, more preferably 0.02% or more.

[0120] Total of one or more selected from Cu, Ni, and Co: 1.00% or less

[0121] Cu, Ni, and Co all enhance the hardenability and are effective in improving strength. Therefore, they can be contained as needed. However, they are all high-cost elements. Therefore, when contained, the total content of one or more selected from these elements is set to 1.00% or less. The above total content is preferably 0.90% or less, more preferably 0.80% or less. It should be noted that when reliably obtaining the above effects, the above total content is preferably 0.01% or more, more preferably 0.02% or more.

[0122] Total of Mo and / or W: 4.00% or less

[0123] Both Mo and W are dissolved in the matrix, which helps to improve the high-temperature strength, so they can be contained as needed. However, when the content is excessive, coarse intermetallic compounds and / or carbides will be formed during high-temperature use, resulting in a decrease in toughness. Therefore, when contained, the total content of Mo and / or W is set to 4.00% or less. The above total content is preferably 3.80% or less, more preferably 3.50% or less. It should be noted that when the above effects are to be reliably obtained, the above total content is preferably 0.01% or more, more preferably 0.02% or more.

[0124] The sum of one or more selected from V, Nb, Ti, and Ta: 1.00% or less

[0125] V, Nb, Ti, and Ta all form fine carbonitrides during high-temperature use, which helps to improve the high-temperature strength, so they can be contained as needed. However, when the content is excessive, a large amount of coarse carbonitrides will be formed, resulting in a decrease in toughness. Therefore, when contained, the total content of one or more selected from these elements is set to 1.00% or less. The above total content is preferably 0.90% or less, more preferably 0.80% or less. It should be noted that when the above effects are to be reliably obtained, the above total content is preferably 0.01% or more, more preferably 0.02% or more.

[0126] B: 0.0200% or less

[0127] B improves the hardenability and helps to improve the strength, so it can be contained as needed. However, when the content is excessive, it will be mixed into the weld metal during welding, increasing the susceptibility to solidification cracking. Therefore, when contained, the B content is set to 0.0200% or less. The B content is preferably 0.0180% or less, more preferably 0.0150% or less. It should be noted that when the above effects are to be reliably obtained, the B content is preferably 0.0005% or more, more preferably 0.0010% or more.

[0128] The sum of Ca and / or Mg: 0.0100% or less

[0129] Both Ca and Mg improve the hot workability, so they can be contained as needed. However, when the content is excessive, the cleanliness will be significantly reduced, which will instead impair the hot workability. Therefore, when contained, the total content of Ca and / or Mg is set to 0.0100% or less. The above total content is preferably 0.0080% or less, more preferably 0.0060% or less. It should be noted that when the above effects are to be reliably obtained, the above total content is preferably 0.0005% or more, more preferably 0.0010% or more.

[0130] REM: 0.0500% or less

[0131] REM improves hot workability in the same way as Ca and Mg, and thus can be contained as needed. However, when the content is excessive, the cleanliness significantly decreases, which instead impairs the hot workability. Therefore, when contained, the REM content is set to 0.0500% or less. The REM content is preferably 0.0400% or less, and more preferably 0.0300% or less. It should be noted that when reliably obtaining the above effects, the REM content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0132] "REM" is a general term for a total of 17 elements including Sc, Y, and the lanthanide series. The REM content refers to the total content of one or more elements in REM. Additionally, REM is usually contained in a rare earth alloy mixture. Therefore, for example, a rare earth alloy mixture can be added to the alloy to make the REM content within the above range.

[0133] (C) Chemical composition of the second tube

[0134] The reasons for limiting each element are as described below. It should be noted that "%" regarding the content in the following description refers to "mass %".

[0135] C: 0.003 - 0.100%

[0136] C stabilizes the austenite structure and helps ensure high-temperature strength. To obtain this effect, the C content is set to 0.003% or more. However, when the content is excessive, carbides are generated during welding or during use at high temperatures, resulting in a decrease in corrosion resistance. Therefore, the C content is set to 0.100% or less. The C content is preferably 0.005% or more, and more preferably 0.008% or more. Additionally, the C content is preferably 0.090% or less, and more preferably 0.080% or less.

[0137] Si: 0.01 - 1.50%

[0138] Si is an element that is effective in improving corrosion resistance and oxidation resistance at high temperatures while having a deoxidizing effect. To obtain this effect, the Si content is set to 0.01% or more. However, when the content is excessive, it will be mixed into the weld metal during welding, increasing the susceptibility to solidification cracking and impairing the stability of the austenite structure, resulting in a decrease in high-temperature strength. Therefore, while the Si content is set to 1.50% or less, it is necessary to satisfy the relationship with P, S, and Sn described below. The Si content is preferably 0.03% or more, and more preferably 0.05% or more. Additionally, the Si content is preferably 1.30% or less, and more preferably 1.00% or less.

[0139] Mn: 0.01 - 2.20%

[0140] While Mn has a deoxidizing effect, it increases the stability of the austenite structure, which helps to ensure high-temperature strength. To achieve this effect, the Mn content is set to 0.01% or more. However, when the content is excessive, embrittlement will occur during use at high temperatures. Therefore, the Mn content is set to 2.20% or less. The Mn content is preferably 0.03% or more, and more preferably 0.05% or more. In addition, the Mn content is preferably 2.00% or less, and more preferably 1.80% or less.

[0141] P: 0.0400% or less

[0142] P is mixed into the weld metal during welding, increasing the susceptibility to solidification cracking. Therefore, the P content is set to 0.0400% or less. In addition, the relationship with the contents of Si, S, and Sn described below needs to be satisfied. The P content is preferably 0.0380% or less, and more preferably 0.0350% or less. It should be noted that the lower limit of the P content does not need to be specifically set and can be 0 (zero), but extremely reducing it will increase the steelmaking cost. In addition, P has the effect of significantly increasing strength. When this effect is desired, the P content is preferably set to 0.0030% or more, and more preferably 0.0050% or more.

[0143] S: 0.0100% or less

[0144] Like P, S is mixed into the weld metal during welding, significantly increasing the susceptibility to solidification cracking. In addition, it increases the susceptibility to liquation cracking in the weld-affected zone. Therefore, the S content is set to 0.0100% or less. In addition, the relationship with the contents of Si, P, and Sn described below needs to be satisfied. The S content is preferably 0.0090% or less, and more preferably 0.0080% or less. It should be noted that the lower limit of the S content does not need to be specifically set and can be 0 (zero), but when it is extremely reduced, the penetration depth during welding becomes smaller and non-fusion is likely to occur. Therefore, while satisfying the relationship with Sn described below, the S content is preferably set to 0.0001% or more, and more preferably 0.0002% or more.

[0145] Sn: 0.0005 - 0.0300%

[0146] Sn has the effect of improving corrosion resistance. In addition, when mixed into the welding metal during welding, it increases the penetration depth and thus suppresses the occurrence of non-fusion. To obtain this effect, the Sn content should be set at 0.0005% or more, and at the same time, it is necessary to satisfy the relationship with the S content described below. On the other hand, when the content is excessive, it increases the susceptibility to solidification cracking during welding and also increases the susceptibility to liquation cracking in the heat-affected zone of welding. Therefore, the Sn content should be set at 0.0300% or less, and at the same time, it is necessary to satisfy the relationship with the contents of Si, P, and Sn described below. The Sn content is preferably 0.0008% or more, more preferably 0.0010% or more. In addition, the Sn content is preferably 0.0280% or less, more preferably 0.0250% or less.

[0147] Ni: 7.0 - 52.0%

[0148] Ni stabilizes the austenite structure and contributes to high-temperature strength. In addition, it improves the corrosion resistance in an environment where chloride ions are present. To obtain this effect, the Ni content is set at 7.0% or more. However, Ni is an element with a high price, so when the content is excessive, it will lead to an increase in cost. Therefore, the Ni content is set at 52.0% or less. The Ni content is preferably 7.2% or more, more preferably 7.5% or more. In addition, the Ni content is preferably 48.0% or less, more preferably 45.0% or less.

[0149] Cr: 15.0 - 27.0%

[0150] Cr contributes to the improvement of oxidation resistance and corrosion resistance at high temperatures. To obtain this effect, the Cr content is set at 15.0% or more. However, when the content is excessive, it will damage the stability of the austenite structure and lead to a decrease in high-temperature strength. Therefore, the Cr content is set at 27.0% or less. The Cr content is preferably 15.2% or more, more preferably 15.5% or more. In addition, the Cr content is preferably 26.8% or less, more preferably 26.5% or less.

[0151] Al: 0.001 - 0.600%

[0152] Al is contained for deoxidation. In addition, during use at high temperatures, it combines with Ni and precipitates in the form of intermetallic compounds, contributing to the improvement of high-temperature strength. To obtain this effect, the Al content is set at 0.001% or more. However, when the content is excessive, it will lead to a decrease in toughness. Therefore, the Al content is set at 0.600% or less. The Al content is preferably 0.002% or more, more preferably 0.003% or more. In addition, the Al content is preferably 0.550% or less, more preferably 0.500% or less.

[0153] N: 0.001 - 0.150%

[0154] N stabilizes the austenite phase and helps improve the high-temperature strength. To achieve this effect, the N content is set to 0.001% or more. However, when the content is excessive, it will cause a decrease in ductility. Therefore, the N content is set to 0.150% or less. The N content is preferably 0.002% or more, more preferably 0.003% or more. In addition, the N content is preferably 0.130% or less, more preferably 0.100% or less.

[0155] O: 0.030% or less

[0156] When the O content is excessive, it will cause a decrease in workability and ductility. Therefore, the O content is set to 0.030% or less. The O content is preferably 0.025% or less, more preferably 0.020% or less. It should be noted that the lower limit of the O content does not need to be specifically set and can be 0 (zero), but extremely reducing it will increase the steelmaking cost. Therefore, the O content is preferably set to 0.001% or more, more preferably 0.003% or more.

[0157] In the chemical composition of the second tube, the balance is Fe and impurities. It should be noted that "impurities" refer to substances that are mixed in due to various reasons in the manufacturing process, represented by raw materials such as ores or scraps, when manufacturing steel materials industrially.

[0158] The chemical composition of the second tube may contain one or more selected from the following groups to replace a part of Fe. The reasons will be discussed below.

[0159] The total of Cu and / or Co: 6.00% or less

[0160] The total of Mo and / or W: 8.00% or less

[0161] The total of one or more selected from V, Nb, Ti, and Ta: 2.00% or less

[0162] B: 0.0200% or less

[0163] The total of Ca and / or Mg: 0.0100% or less

[0164] REM: 0.0500% or less

[0165] The total of Cu and / or Co: 6.00% or less

[0166] Both Cu and Co increase the stability of the austenitic structure and are effective in increasing the high-temperature strength, so they can be contained as needed. However, they are both elements with high prices, and when contained in excessive amounts, they will cause a decrease in ductility. Therefore, when contained, the total content of Cu and / or Co is set to 6.00% or less. The above total content is preferably 5.50% or less, more preferably 5.00% or less. It should be noted that when it is desired to reliably obtain the above effects, the above total content is preferably 0.01% or more, more preferably 0.02% or more.

[0167] Total of Mo and / or W: 8.00% or less

[0168] Both Mo and W dissolve in the matrix and contribute to the increase in high-temperature strength, so they can be contained as needed. However, when contained in excessive amounts, it will damage the stability of the austenitic structure, and at the same time, coarse intermetallic compounds and / or carbides will be formed during use at high temperatures, resulting in a decrease in toughness. Therefore, when contained, the total content of Mo and / or W is set to 8.00% or less. The above total content is preferably 7.50% or less, more preferably 7.00% or less. It should be noted that when it is desired to reliably obtain the above effects, the above total content is preferably 0.01% or more, more preferably 0.02% or more.

[0169] Total of one or more selected from V, Nb, Ti, and Ta: 2.00% or less

[0170] V, Nb, Ti, and Ta all form fine carbonitrides during use at high temperatures and contribute to the increase in high-temperature strength, so they can be contained as needed. However, when contained in excessive amounts, a large amount of coarse carbonitrides will be generated, resulting in a decrease in toughness. Therefore, when contained, the total content of one or more selected from these elements is set to 2.00% or less. The above total content is preferably 1.90% or less, more preferably 1.80% or less. It should be noted that when it is desired to reliably obtain the above effects, the above total content is preferably 0.01% or more, more preferably 0.02% or more.

[0171] B: 0.0200% or less

[0172] B dissolves and is finely dispersed in carbides during use at high temperatures and contributes to the increase in high-temperature strength, so it can be contained as needed. However, when contained in excessive amounts, it will be mixed into the weld metal during welding, increasing the susceptibility to solidification cracking. Therefore, when contained, the B content is set to 0.0200% or less. The B content is preferably 0.0180% or less, more preferably 0.0150% or less. It should be noted that when it is desired to reliably obtain the above effects, the B content is preferably 0.0005% or more, more preferably 0.0010% or more.

[0173] Total of Ca and / or Mg: 0.0100% or less

[0174] Both Ca and Mg improve hot workability and can thus be contained as needed. However, when the content is excessive, the cleanliness significantly decreases, which instead impairs hot workability. Therefore, when contained, the total content of Ca and / or Mg is set to 0.0100% or less. The above total content is preferably 0.0080% or less, more preferably 0.0060% or less. It should be noted that when reliably obtaining the above effects, the above total content is preferably 0.0005% or more, more preferably 0.0010% or more.

[0175] REM: 0.0500% or less

[0176] REM improves hot workability in the same way as Ca and Mg and can thus be contained as needed. However, when the content is excessive, the cleanliness significantly decreases, which instead impairs hot workability. Therefore, when contained, the REM content is set to 0.0500% or less. The REM content is preferably 0.0400% or less, more preferably 0.0300% or less. It should be noted that when reliably obtaining the above effects, the REM content is preferably 0.0005% or more, more preferably 0.0010% or more.

[0177] "REM" is the general term for a total of 17 elements including Sc, Y, and the lanthanide series. The REM content refers to the total content of one or more elements in REM. Additionally, REM is usually contained in mischmetal alloys. Therefore, for example, a mischmetal alloy can be added to the alloy so that the REM content is within the above range.

[0178] (D) Average chemical composition of the first tube and the second tube

[0179] In addition to having the above chemical compositions respectively, the average chemical composition of the first tube and the second tube related to the present invention also needs to satisfy a specified relational expression. The reasons are as described below.

[0180] When welding the duplex tube related to the present invention using an austenitic stainless steel or a welding material made of a Ni alloy, the duplex tube (base material) melts, and the contained Si, P, S, and Sn mix into the weld metal. These elements all lower the solidus temperature and increase the susceptibility of the weld metal to solidification cracking. In particular, in the weld metal near the melting boundary, the mixing during the welding process is insufficient, and the chemical composition of the weld metal is dominantly affected by the base material.

[0181] In addition, the weld metal becomes an austenite single-phase solidification structure near the boundary between the inner tube and the outer tube, and thus is affected by these elements and is prone to solidification cracking. In order to stably prevent solidification cracking occurring in this region, for the average values of the contents of Si, P, S, and Sn in the first tube and the second tube, it is necessary to control the relational expression considering the influence degree of each element to be below a specified range.

[0182] Specifically, the following formula (i) needs to be satisfied. The left-end value of the following formula (i) is preferably 1.0500 or less, more preferably 1.0000 or less.

[0183] Si ave +6×P ave +20×S ave +2×Sn ave ≤1.1000 · · · (i)

[0184] Among them, the meanings of the symbols in the above formula are as follows.

[0185] Si ave : The average value of the Si content of the first tube and the second tube (mass %)

[0186] P ave : The average value of the P content of the first tube and the second tube (mass %)

[0187] S ave : The average value of the S content of the first tube and the second tube (mass %)

[0188] Sn ave : The average value of the Sn content of the first tube and the second tube (mass %)

[0189] As described above, during welding, the double tube (base material) melts, and the contained S and Sn are mixed into the welding metal. These elements increase the susceptibility to welding cracks. On the other hand, when they are too low, the penetration depth during welding becomes smaller, and incomplete penetration is likely to occur. In order to stably prevent incomplete penetration, regarding the average amounts of S and Sn in the first tube and the second tube, the relational expression considering the influence degree of each element needs to be controlled above a specified range.

[0190] Specifically, the following formula (ii) needs to be satisfied. The right-end value of the following formula (ii) is preferably 0.0020 or more, more preferably 0.0025 or more.

[0191] 0.0015 ≤ 4×S ave +Sn ave · · · (ii)

[0192] Among them, the meanings of the symbols in the above formula are as follows.

[0193] S ave : The average value of the S content of the first tube and the second tube (mass %)

[0194] Sn ave : The average value of the Sn content of the first tube and the second tube (mass %)

[0195] (E) Welded joint

[0196] The welded joint involved in the present invention has the above double tubes. That is, a plurality of double tubes are welded and joined. When butt-welding and assembling the double tubes, a suitable welding material can be selected and determined according to its intended use.

[0197] For example, when welding a double tube with the outer tube being the first tube (low alloy steel) and the inner tube being the second tube (high alloy steel), as is usually done, after welding the high alloy steel part with a welding material for austenitic stainless steel or Ni-based alloy, welding is performed near the boundary part with a pure Ni welding material, and the remaining low alloy steel part is welded with a welding material for carbon steel, and a welded joint with the required performance can be obtained. In addition, when the materials constituting the outer tube and the inner tube are reversed, the reverse method can be used for welding construction.

[0198] In addition, in the double tube involved in the present embodiment, as described above, without using a variety of welding materials, only using a welding material for austenitic stainless steel or Ni-based alloy with austenitic solidification can also obtain a welded joint with the required performance. Therefore, the welded joint involved in an embodiment of the present invention is preferably welded with any one of the welding materials for austenitic stainless steel or Ni-based alloy. It should be noted that in this case, the welding material used and the formed weld metal preferably have the following chemical compositions.

[0199] That is, the chemical compositions of the welding material and the weld metal are in mass%

[0200] C: 0.003 to 0.100%,

[0201] Si: 0.01 to 1.50%,

[0202] Mn: 0.01 to 2.50%,

[0203] P: 0.0400% or less,

[0204] S: 0.0100% or less,

[0205] Sn: 0.0300% or less,

[0206] The total of Cu and / or Co: 0 to 15.00%,

[0207] Ni: 12.0 to 75.0%,

[0208] Cr: 18.0% to 27.0%,

[0209] The total of Mo and / or W: 0 to 10.00%,

[0210] Total of one or more selected from V, Nb, Ti, and Ta: 0 to 4.00%, B: 0 to 0.0200%,

[0211] Total of Ca and / or Mg: 0 to 0.0100%,

[0212] Al: 0.001 to 1.500%,

[0213] N: 0.001 to 0.150%,

[0214] O: 0.030% or less,

[0215] Balance: Fe and impurities,

[0216] Preferably, the following formulas (iii) and (iv) are satisfied.

[0217] Si w + 6×P w + 20×S w + 2×Sn w ≤ 1.1000% ··· (iii)

[0218] 0.0015% ≤ 4×S w + Sn w ···(iv)

[0219] Among them, the meanings of the symbols in the above formulas are as follows.

[0220] Si w : Si content (mass%) of the welding material or welding metal

[0221] P w : P content (mass%) of the welding material or welding metal

[0222] S w : S content (mass%) of the welding material or welding metal

[0223] Sn w : Sn content (mass%) of the welding material or welding metal

[0224] (F) Manufacturing method

[0225] There is no particular limitation on the manufacturing method of the double tube. For example, for the raw material assembled by inserting a solid blank of high alloy steel or low alloy steel constituting the inner tube into a hollow blank of low alloy steel or high alloy steel constituting the outer tube, so-called "hot working pipe making" such as hot extrusion and roll rolling is performed, and the outer tube and the inner tube are integrated to make a pipe, thereby enabling the manufacture of a double tube. Thus, a double tube with the outer tube as the first tube and the inner tube as the second tube, or a double tube with the outer tube as the second tube and the inner tube as the first tube can be obtained.

[0226] It should be noted that, generally, in order to ensure the cleanliness of the joint surface, the assembly of the above-mentioned blank is carried out in a vacuum or an inert gas atmosphere. Then, cold working such as rolling or drawing can be performed on the double tube of the above-mentioned hot-worked pipe, and heat treatment is also carried out to form a double tube of the required shape.

[0227] Hereinafter, the present invention will be described in more detail by way of examples. It should be noted that the present invention is not limited by these examples.

[0228] Example

[0229] Low alloy steels L1 to L7 and high alloy steels H1 to H7 having the chemical compositions shown in Table 1 were combined, and a double tube having a first tube formed of a low alloy steel and a second tube formed of a high alloy steel, a thickness of 6.5 mm, and an outer diameter of 63 mm was produced as a test tube by a hot-worked pipe manufacturing method. It should be noted that when the outer tube is the first tube and the inner tube is the second tube, the thickness of the outer tube is set to 4.2 mm, and the thickness of the inner tube is set to 2.3 mm, that is, the proportion of the second tube in the overall thickness of the tube is set to 0.35. On the other hand, when the outer tube is the second tube and the inner tube is the first tube, the thickness of the outer tube is set to 1.6 mm, and the thickness of the inner tube is set to 4.9 mm, that is, the proportion of the second tube in the overall thickness of the tube is set to 0.25.

[0230] [Table 1].

[0231]

[0232] After cutting out a 100 mm long test material from the test tube, the end on one side of the test material was machined into Figure 1 the groove shown. The grooves were butted, and a solid bar was inserted into the steel pipe. The solid bar was made of a commercially available steel plate equivalent to SM400B specified in JIS G 3106 (2008) by machining, and had an outer diameter of 48 mm and a length of 250 mm. Then, using a coated electrode specified in AWS A5.11-2005 ENiCrMo-3, welding was performed on both ends, and two restraint welded test specimens as shown in Figure 2 each test number were fabricated.

[0233] In the groove of the restraint welded test specimen, a filler wire specified in AWS A.5.14-2009 ERNiFeCr-1 having the chemical composition shown in Table 2 was used, and multi-pass welding was performed by TIG welding with a heat input of 8 to 12 J / cm.

[0234] [Table 2]

[0235] Table 2

[0236]

[0237] Further, four test pieces each were cut out from the restraint welding test pieces after multi-pass welding with the cross-section of the welded joint as the observation surface, and mirror polishing was performed.

[0238] Then, after the test pieces were corroded, the cross-sections of the welded parts of a total of eight cross-sections (2 test pieces × 4 cross-sections) were investigated for defects by an optical microscope for each test number. And, the test pieces in which no solidification cracks or incomplete penetration were observed in all cross-sections were regarded as "A", the test pieces in which they were observed only in one cross-section were regarded as "B", and both of these were judged to be qualified, and the test pieces in which they were observed in two or more cross-sections were regarded as "F" and judged to be unqualified.

[0239] The evaluation results are shown in Table 3.

[0240] [Table 3]

[0241] Table 3

[0242]

[0243] Si ave +6×P ave +20×S ave +2×Sn ave ≤1.1000…(i)

[0244] 0.0015≤4×S ave +Sn ave …(ii)

[0245] As can be seen from Table 3, no defects such as solidification cracks or incomplete penetration occurred in the weld metal of the double pipes that satisfied all the regulations of the present invention, and a sound welded joint could be obtained. It should be noted that it was confirmed that the chemical composition of the obtained weld metal satisfied the aforementioned preferred chemical composition.

[0246] On the contrary, in test numbers T2-2, T3-2, T3-9, T5-2, and T5-5, since the formula (i) was not satisfied, solidification cracks occurred in the weld metal near the fusion line near the boundary between the inner pipe and the outer pipe. In addition, in test numbers T3-3, T3-10, T5-3, and T5-6, since the formula (ii) was not satisfied, the groove face was not sufficiently melted, and so-called incomplete penetration occurred.

[0247] Although the chemical composition of the weld metal of these welded joints that did not satisfy the present invention satisfied the aforementioned preferred chemical composition range on average, since the formula (i) or (ii) was not satisfied in the local region of the weld metal at the fusion boundary near the boundary between the outer pipe and the inner pipe, it was judged that solidification cracks or incomplete penetration occurred.

[0248] As described above, a sound welded joint can be obtained only when a double tube that meets all the requirements of the present invention is used.

[0249] Industrial Applicability

[0250] According to the present invention, a double tube that can prevent cracks in the weld metal during butt welding of tubes and can stably obtain a sound welded joint can be obtained.

Claims

1. A double tube, which includes a first tube and a second tube, The double tube has a structure in which an outer tube and an inner tube are metallurgically bonded, The outer tube is the first tube and the inner tube is the second tube, or the outer tube is the second tube and the inner tube is the first tube, The chemical composition of the first tube is by mass% C: greater than 0.060% and 0.400% or less, Si: 0.01 - 1.00%, Mn: 0.01 - 1.20%, P: 0.0350% or less, S: 0.0150% or less, Sn: 0.0005 - 0.0400%, Al: 0.040% or less, N: 0.050% or less, O: 0.030% or less, Balance: Fe and impurities, The chemical composition of the second tube is by mass% C: 0.003 - 0.100%, Si: 0.01 - 1.50%, Mn: 0.01 - 2.20%, P: 0.0400% or less, S: 0.0100% or less, Sn: 0.0005 - 0.0300%, Ni: 7.0 - 52.0%, Cr：15.0～27.0％、 Al：0.001～0.600％、 N：0.001～0.150％、 O: 0.030% or less, Balance: Fe and impurities, The double tube satisfies the following (i) and (ii) formulas, Si ave + 6 × P ave + 20 × S ave + 2 × Sn ave ≤ 1.1000···(i) 0.0015 ≤ 4×S ave + Sn ave ···(ii) Among them, The meanings of the symbols in the above formulas are as follows, Si ave : The average value of the Si content in the first tube and the second tube in mass percentage, P ave : The average value of the P content in the 1st tube and the 2nd tube in mass%, S ave : The average value of the S content in the first tube and the second tube in mass percentage, Sn ave : The average value of the Sn content in the 1st tube and the 2nd tube in mass%.

2. The double tube according to claim 1, wherein, The chemical composition of the first tube contains one or more selected from the group consisting of the following elements by mass% to replace a part of Fe, Cr: 9.50% or less; Total of one or more selected from Cu, Ni and Co: 1.00% or less; Total of Mo and / or W: 4.00% or less; Total of one or more selected from V, Nb, Ti and Ta: 1.00% or less; B: 0.0200% or less; Total of Ca and / or Mg: 0.0100% or less; REM: 0.0500% or less.

3. The double tube according to claim 1 or claim 2, wherein, The chemical composition of the second tube contains one or more selected from the following elements by mass% to replace a part of Fe, Total of Cu and / or Co: 6.00% or less; Total of Mo and / or W: 8.00% or less; Total of one or more selected from V, Nb, Ti and Ta: 2.00% or less; B: 0.0200% or less; Total of Ca and / or Mg: 0.0100% or less; REM: 0.0500% or less.

4. A welded joint, which includes the double tube according to any one of claims 1 to 3.

Images