Double-layer pipe

Abstract

A double-layer pipe which comprises an inner pipe that is provided with ribs and is made of ferritic steel, and an outer pipe that is made of austenitic steel, and wherein the chemical composition of the inner pipe contains, in mass%, 0.01-0.30% of C, 0.01-0.80% of Si, 0.01-2.00% of Mn, 0.030% or less of P, 0.0100% or less of S, 0.01-3.50% of Cr, 0.050% or less of Al, and 0.0005-0.0500% of N, with the remainder being Fe and impurities. The outer tube has a chemical composition comprising, in mass%, 0.10% or less of C, 0.01 to 0.80% of Si, 0.01 to 3.50% of Mn, 0.040% or less of P, 0.0100% or less of S, 5.0 to 70.0% of Ni, 15.0 to 35.0% of Cr, 1.500% or less of Al, 0.001 to 0.350% of N, and the balance of Fe and impurities.

Description

Technical Field

[0001] This invention relates to a double-layered tube. Background Technology

[0002] The furnace wall tubes of power generation boilers sometimes use internally threaded tubes, such as those in Patent Document 1, which have multiple grooves (hereinafter also referred to as "ribs") inside the tube. By using internally threaded tubes for the furnace wall tubes, it is possible to create swirling flow of the fluid inside the tube, thereby improving thermal conductivity. As a result, the boiler efficiency is improved.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2006-272392 Summary of the Invention

[0006] The problem the invention aims to solve

[0007] The interior of the furnace wall tubes is not an environment where corrosion is easily observed, as only water flows through it. However, when the outer side of the tube is exposed to environments where chlorides and other pollutants are present, higher corrosion resistance is required. The aforementioned internally threaded tubes are typically made of steel with slightly lower corrosion resistance, known as ferritic steel. Therefore, from a corrosion resistance perspective, further research is warranted.

[0008] In this regard, it is also possible to rib austenitic steel with good corrosion resistance and manufacture it into an internally threaded tube. However, austenitic steel is susceptible to stress corrosion cracking when used in environments in contact with water, and it also suffers from high strength and difficulty in machining. Furthermore, it is more expensive than ferritic steel, thus increasing manufacturing costs. Therefore, it is preferable to manufacture a tightly sealed double-layer tube (hereinafter also referred to as an "internally threaded double-layer tube"), in which only the outer tube of the internally threaded tube is austenitic steel, while the inner tube is made of ferritic steel through rib machining. However, when attempting to manufacture such a double-layer tube, there is a challenge in successfully producing the tube.

[0009] The purpose of this invention is to solve the above-mentioned problems and provide a double-layer tube with ribs on the inner side.

[0010] Solution for solving the problem

[0011] The present invention was made to solve the above-mentioned problems, and its main purpose is the following double-layer tube.

[0012] (1) A double-walled tube comprising: an inner tube with ribs and made of ferritic steel, and an outer tube made of austenitic steel.

[0013] The chemical composition of the aforementioned inner tube, expressed in % by mass, contains

[0014] C: 0.01~0.30%

[0015] Si: 0.01~0.80%

[0016] Mn: 0.01~2.00%

[0017] P: below 0.030%

[0018] S: below 0.0100%

[0019] Cr: 0.01~3.50%

[0020] Al: below 0.050%

[0021] N: 0.0005~0.0500%,

[0022] And the remainder consists of Fe and impurities.

[0023] The chemical composition of the aforementioned outer tube, in mass percent, is as follows:

[0024] C: Below 0.10%

[0025] Si: 0.01~0.80%

[0026] Mn: 0.01~3.50%

[0027] P: below 0.040%

[0028] S: below 0.0100%

[0029] Ni: 5.0–70.0%

[0030] Cr: 15.0–35.0%

[0031] Al: Below 1.500%

[0032] N: 0.001~0.350%

[0033] Balance: Fe and impurities.

[0034] (2) A double-walled tube comprising: an inner tube with ribs and made of ferritic steel, and an outer tube made of austenitic steel.

[0035] The chemical composition of the aforementioned inner tube, expressed in % by mass, contains

[0036] C: 0.01~0.30%

[0037] Si: 0.01~0.80%

[0038] Mn: 0.01~2.00%

[0039] P: below 0.030%

[0040] S: below 0.0100%

[0041] Cr: 0.01~3.50%

[0042] Al: below 0.050%

[0043] N: 0.0005~0.0500%,

[0044] It also contains one or more elements selected from group A below.

[0045] And the balance: Fe and impurities,

[0046] The chemical composition of the aforementioned outer tube, in mass percent, is as follows:

[0047] C: Below 0.10%

[0048] Si: 0.01~0.80%

[0049] Mn: 0.01~3.50%

[0050] P: below 0.040%

[0051] S: below 0.0100%

[0052] Ni: 5.0–70.0%

[0053] Cr: 15.0–35.0%

[0054] Al: Below 1.500%

[0055] N: 0.001~0.350%,

[0056] It also contains one or more elements selected from group B below.

[0057] And the balance: Fe and impurities.

[0058] [Group A] Ni: less than 1.5%, Cu: less than 1.0%, Mo: less than 3.0%, W: less than 3.0%, V: less than 0.50%, Nb: less than 0.10%, Ti: less than 0.30%, B: less than 0.0200%, Co: less than 0.50%, Ca: less than 0.050%, Sb: less than 0.20%.

[0059] [Group B] Cu: less than 5.00%, Nb: less than 5.00%, Ti: less than 1.0%, B: less than 0.010%, W: less than 10.0%, Co: less than 15.0%, Ca: less than 0.010%.

[0060] (3) A double-walled tube comprising: an inner tube with ribs and made of ferritic steel, and an outer tube made of austenitic steel.

[0061] The chemical composition of the aforementioned inner tube, expressed in % by mass, contains

[0062] C: 0.01~0.30%

[0063] Si: 0.01~0.80%

[0064] Mn: 0.01~2.00%

[0065] P: below 0.030%

[0066] S: below 0.0100%

[0067] Cr: 0.01~3.50%

[0068] Al: below 0.050%

[0069] N: 0.0005~0.0500%,

[0070] It also contains one or more elements selected from group A below.

[0071] And the balance: Fe and impurities,

[0072] The chemical composition of the aforementioned outer tube, in mass percent, is as follows:

[0073] C: Below 0.10%

[0074] Si: 0.01~0.80%

[0075] Mn: 0.01~3.50%

[0076] P: below 0.040%

[0077] S: below 0.0100%

[0078] Ni: 5.0–70.0%

[0079] Cr: 15.0–35.0%

[0080] Al: Below 1.500%

[0081] N: 0.001~0.350%,

[0082] It also contains one or more elements selected from group B below.

[0083] And the balance: Fe and impurities.

[0084] [Group A] Ni: less than 1.0%, Cu: less than 1.0%, Mo: less than 3.0%, W: less than 3.0%, V: less than 0.50%, Nb: less than 0.10%, Ti: less than 0.30%, B: less than 0.0200%, Co: less than 0.50%, Ca: less than 0.050%, Sb: less than 0.20%.

[0085] [Group B] Cu: less than 5.00%, Nb: less than 5.00%, Ti: less than 1.0%, B: less than 0.010%, W: less than 10.0%, Co: less than 15.0%, Ca: less than 0.010%.

[0086] (4) The double-layer tube according to any one of (1) to (3) above, wherein the ferrite ratio of the inner tube is 40% or more in terms of area ratio.

[0087] The aforementioned inner tube has a hardness of less than 180 HV according to the Vickers hardness tester.

[0088] The hardness of the aforementioned outer tube is below 250 HV according to the Vickers hardness tester.

[0089] (5) The double-layer tube according to any one of (1) to (4) above, wherein the difference between the hardness of the carburized region of the outer tube and the hardness of the inner tube is less than 200 HV in Vickers hardness measurement.

[0090] (6) The double-layer tube according to any one of (1) to (5) above, wherein the shear tensile stress between the inner tube and the outer tube is 200 MPa or more.

[0091] Invention Effects

[0092] According to the present invention, a double-layered tube with ribs on the inner side can be obtained. Attached Figure Description

[0093] Figure 1 Here are schematic diagrams of a double-layered internally threaded pipe: (a) is a schematic diagram of section C (cross-section), and (b) is a schematic diagram of section L.

[0094] Figure 2 This is a schematic diagram of a test piece used to measure shear tensile stress. Detailed Implementation

[0095] The inventors have studied internally threaded double-layered tubes and obtained the following insights (a) to (d).

[0096] (a) When the blank used as the inner tube is too hard, it is difficult to machine the internal threads. Therefore, in order to control the hardness, the chemical composition of the ferritic steel used as the inner tube needs to be controlled within a specified range. In addition, the chemical composition of the austenitic steel used as the outer tube can be appropriately determined according to the required corrosion resistance of the outer tube.

[0097] (b) In the case of manufacturing a double-layer tube, a solid steel billet of ferritic steel serving as the inner tube is inserted into a hollow steel billet of austenitic steel serving as the outer tube to assemble a billet. The billet is then hot-extruded or otherwise manufactured into a tube blank. The resulting tube blank is then internally threaded or otherwise machined to obtain an internally threaded double-layer tube. That is, in the manufacturing of an internally threaded double-layer tube, the final process must be carried out while the outer tube portion (hereinafter referred to as "outer tube blank") and the inner tube portion (hereinafter referred to as "inner tube blank") of the tube blank are already integrated.

[0098] (c) When machining internal threads, heat treatment is sometimes performed to improve machinability. In this case, heat treatment needs to be performed while the outer tube blank and the inner tube blank, which are made of different materials, are integrated. Therefore, to improve machinability and perform internal thread machining efficiently, it is preferable to perform a heat treatment suitable for both the outer and inner tube blanks. Specifically, a three-stage heat treatment consisting of solution treatment, normalizing, and tempering is preferred.

[0099] Solution treatment primarily softens the outer tube blank, with a preferred treatment temperature range of 1000–1200°C. Normalizing controls the microstructure of the inner tube blank and increases the ferrite content, with a preferred treatment temperature range of 850–950°C. Tempering further softens the inner tube blank, with a preferred treatment temperature range of 750–800°C. This results in improved machinability during internal thread machining.

[0100] (d) After the above three-stage heat treatment, further normalizing is also effective. This is because it can further increase the shear stress between the outer and inner tubes, suppressing peeling during internal thread machining. During the above solution treatment, normalizing, and tempering, the carbon contained in the inner tube blank moves towards the outer tube blank side, forming a carbon-rich region at the interface of the outer tube blank side. The hardness of this carbon-rich region increases, thus increasing the hardness difference between the carbon-rich region and the inner tube blank. As a result, the shear tensile stress tends to decrease, and peeling may sometimes occur. Therefore, carbon enrichment can be eliminated by adding normalizing.

[0101] One embodiment of the present invention is based on the above-described understanding. The elements of this embodiment will now be described in detail.

[0102] 1. Structure of a double-layered tube

[0103] Figure 1This diagram schematically illustrates the double-layered tube of this embodiment. Figure 1 (a) is a schematic diagram showing a cross-section (also called "C-section" or "cross section") perpendicular to the long side of the tube. Figure 2 (b) is a schematic diagram showing a cross section (also known as the "L-section") parallel to the long side of the tube.

[0104] like Figure 1 As shown in (a) and (b), the double-layered tube of this embodiment is a double-layered tube having an inner tube 2 and an outer tube 1. Furthermore, the double-layered tube of this embodiment is manufactured as follows: a solid steel billet of ferritic steel serving as the inner tube is inserted into a hollow steel billet of austenitic steel serving as the outer tube to assemble a billet; the billet is then subjected to hot extrusion or the like to manufacture a tube blank; subsequently, the resulting tube blank is precision drawn or the like. Therefore, the inner tube and the outer tube are tightly fitted together. That is, it is a so-called tightly fitted double-layered tube where the outer tube and the inner tube are joined.

[0105] In addition, such as Figure 1 As shown in (a) and (b), the double-layer tube of this embodiment has ribs 3 on the inner side of the inner tube, i.e., the hollow region side. There is no particular limitation on the shape of the ribs; when envisioned for use in furnace wall tubes, for example, multiple spiral ribs are often formed.

[0106] 2. Chemical composition of double-walled tubes

[0107] 2-1. Chemical composition of the steel used for the inner tube

[0108] As described above, the double-layered tube of this embodiment has an inner tube and an outer tube. The inner tube is made of ferritic steel. Here, ferritic steel basically refers to steel containing ferrite structure, and ferritic steel includes, for example, carbon steel, low-alloy steel, and ferritic heat-resistant steel. It should be noted that, for example, STB410 is a carbon steel, and for example, STBA22 is a low-alloy steel. However, it is not limited to the above-mentioned steel types, as long as it is a ferritic steel and meets the range of chemical composition described later.

[0109] The following describes the chemical composition of the inner tube, specifying the range of the content of each element. It should be noted that in the following description, the "%" for content refers to "mass %".

[0110] C: 0.01~0.30%

[0111] Carbon (C) is an effective element for improving hardenability and structural stability. Furthermore, C forms carbides, which contribute to increased strength. Therefore, the C content is set to 0.01% or more. Preferably, the C content is 0.05% or more, more preferably 0.10% or more. However, excessive C content can lead to hardening of the blank, making rib machining difficult. Therefore, the C content is set to 0.30% or less. Preferably, the C content is 0.28% or less, more preferably 0.26% or less.

[0112] Si: 0.01~0.80%

[0113] Silicon (Si) has a deoxidizing effect and is an element that effectively improves corrosion resistance and oxidation resistance at high temperatures. Therefore, the Si content is set to 0.01% or more. Preferably, the Si content is 0.05% or more, and more preferably 0.10% or more. However, excessive Si content can lead to hardening of the blank, making rib processing difficult. Therefore, the Si content is set to 0.80% or less. Preferably, the Si content is 0.60% or less, and more preferably 0.40% or less.

[0114] Mn: 0.01~2.00%

[0115] Like Si, manganese (Mn) is an element with deoxidizing effects, which is effective in improving hardenability and structural stability. Therefore, the Mn content is set to 0.01% or more. Preferably, the Mn content is 0.05% or more, and more preferably 0.10% or more. However, excessive Mn content can lead to hardening of the billet, making rib machining difficult. Furthermore, it can cause creep embrittlement and reduced toughness. Therefore, the Mn content is set to 2.00% or less. Preferably, the Mn content is 1.80% or less, and more preferably 1.60% or less.

[0116] P: below 0.030%

[0117] Phosphorus (P) is contained in steel as an impurity and is an element that reduces mechanical properties. Therefore, the P content is set to 0.030% or less. The P content is preferably set to 0.028% or less, more preferably 0.025% or less. It is preferable to reduce the P content as much as possible, but excessive reduction will increase steelmaking costs. Therefore, the P content is preferably set to 0.0005% or more.

[0118] S: below 0.0100%

[0119] Like phosphorus (P), sulfur (S) is present in steel as an impurity and reduces its mechanical properties. Therefore, the S content is set to 0.0100% or less. Preferably, the S content is set to 0.0080% or less, more preferably 0.0050% or less. While it is preferable to reduce the S content as much as possible, excessive reduction will increase steelmaking costs. Therefore, the S content is preferably set to 0.0001% or more.

[0120] Cr: 0.01~3.50%

[0121] Chromium (Cr) ensures resistance to oxidation and high-temperature corrosion, and stabilizes the microstructure. Therefore, the Cr content is set to 0.01% or more. Preferably, the Cr content is 0.10% or more, and more preferably 0.50% or more. However, excessive Cr content leads to hardening of the billet, making rib machining difficult. Furthermore, it reduces the stability of carbides and deteriorates toughness. Therefore, the Cr content is set to 3.50% or less. Preferably, the Cr content is 3.20% or less, and more preferably 3.00% or less.

[0122] Al: below 0.050%

[0123] Al (aluminum) is an element with deoxidizing effects, but excessive content can lead to hardening of the billet, making rib processing difficult. It also reduces toughness. Therefore, the Al content is set to 0.050% or less. Preferably, the Al content is set to 0.045% or less, more preferably 0.040% or less. On the other hand, to obtain the above-mentioned effects, the Al content is preferably set to 0.0005% or more.

[0124] N: 0.0005~0.0500%

[0125] Nitrogen (N) is an element that effectively increases strength by combining with V, Nb, and Ti to form fine nitrides. Therefore, the N content is set to 0.0005% or more. Preferably, the N content is set to 0.0008% or more, and more preferably 0.0010% or more. However, excessive N content can lead to hardening of the billet, making rib processing difficult. Furthermore, it can cause excessive precipitation of nitrides, reducing toughness. Therefore, the N content is set to 0.0500% or less. Preferably, the N content is set to 0.0450% or less, more preferably 0.0400% or less, and even more preferably 0.0300%, 0.0200%, 0.0150%, or 0.0100% or less.

[0126] In addition to the elements mentioned above, it may also contain one or more elements selected from group A below (Ni, Cu, Mo, W, V, Nb, Ti, B, Co, Ca, and Sb). That is, the lower limit for these elements is 0%. The reasons for the limitation of each element are explained below.

[0127] Ni: below 1.5%

[0128] Ni (nickel) contributes to increased strength. Therefore, it can be included as needed. However, excessive Ni content can lead to hardening of the billet, making rib processing difficult. Therefore, the Ni content is set to 1.5% or less. Preferably, the Ni content is set to 1.4% or less, more preferably 1.3% or less, and even more preferably 1.0%, 0.8%, or 0.6% or less. On the other hand, to obtain the above-mentioned effects, the Ni content is preferably set to 0.01% or more.

[0129] Cu: below 1.0%

[0130] Cu (copper) contributes to increased strength. Therefore, it can be included as needed. However, excessive Cu content can lead to hardening of the billet, making rib processing difficult. Therefore, the Cu content is set to 1.0% or less. The Cu content is preferably set to 0.90% or less, more preferably 0.80% or less. On the other hand, to obtain the above-mentioned effects, the Cu content is preferably set to 0.01% or more.

[0131] When the sample contains one or more of the above-mentioned Ni and Cu, it is preferable to satisfy the following formula (i).

[0132] 0.01≤Ni+Cu≤2.5···(i)

[0133] In the above formula, the symbols of each element represent the content (mass%) of each element contained in the steel, and are denoted as 0 when no element is contained.

[0134] Because the strength is easily improved by ensuring that the total content of Ni and Cu, i.e., the value in the middle of formula (i), is 0.01% or more. Therefore, the value in the middle of formula (i) is preferably set to 0.01% or more. More preferably, the value in the middle of formula (i) is set to 0.02% or more, and even more preferably, 0.03% or more.

[0135] On the other hand, when the value of the middle part of formula (i) exceeds 1.0%, it may sometimes lead to hardening of the blank, making it difficult to perform rib processing. Therefore, the value of the middle part of formula (i) is preferably set to 2.5% or less. More preferably, the value of the middle part of formula (i) is set to 2.3% or less, and even more preferably, 2.0% or less, 1.5% or less, 1.0% or less, 0.9% or less, or 0.8% or less.

[0136] Mo: 3.0% or less

[0137] Mo (molybdenum) has the effect of solid solution strengthening of the matrix and improving its strength. Therefore, it can be included as needed. However, excessive Mo content can lead to hardening of the billet, making rib processing difficult. Therefore, the Mo content is set to 3.0% or less. The Mo content is preferably set to 2.8% or less, more preferably 2.5% or less, 2.0% or less, 1.5% or less, 1.3% or less, or 1.2% or less. On the other hand, in order to obtain the above-mentioned effects, the Mo content is preferably set to 0.01% or more.

[0138] W: Below 3.0%

[0139] Tungsten (W) has the effect of solid solution strengthening of the matrix and improving its strength. Therefore, it can be included as needed. However, excessive W content can lead to hardening of the billet, making rib processing difficult. Therefore, the W content is set to 3.0% or less. The W content is preferably set to 2.8% or less; more preferably to 2.5% or less, 2.0% or less, 1.5% or less, 1.0% or less, 0.8% or less, or 0.5% or less. On the other hand, in order to obtain the above-mentioned effects, the W content is preferably set to 0.01% or more.

[0140] When the material contains one or more of the above-mentioned Mo and W, it is preferable to satisfy the following formula (ii).

[0141] 0.01≤Mo+W≤3.0···(ii)

[0142] In the above formula, the symbols of each element represent the content (mass%) of each element contained in the steel, and are denoted as 0 when no element is contained.

[0143] By ensuring that the total content of Mo and W, i.e., the value in the middle of formula (ii), is 0.01% or more, the matrix is ​​strengthened by solid solution, which easily improves the strength. Therefore, the value in the middle of formula (ii) is preferably 0.01% or more. More preferably, the value in the middle of formula (ii) is 0.03% or more, and even more preferably 0.05% or more.

[0144] On the other hand, when the value of the middle part of formula (ii) exceeds 3.0%, it can sometimes lead to hardening of the billet, making rib processing difficult. Therefore, the value of the middle part of formula (ii) is preferably set to 3.0% or less. More preferably, the value of the middle part of formula (ii) is set to 2.8% or less, and even more preferably, 2.5% or less, 2.0% or less, 1.5% or less, or 1.3% or less.

[0145] V: Below 0.50%

[0146] Vanadium (V) forms fine carbonitrides, which contributes to increased strength. Therefore, it can be included as needed. However, excessive V content leads to hardening of the billet, making rib processing difficult. Furthermore, the large-scale precipitation of V as carbides results in decreased toughness. Therefore, the V content is set to 0.50% or less. Preferably, the V content is set to 0.45% or less, more preferably 0.40%, 0.30%, or 0.20% or less. On the other hand, to obtain the above-mentioned effects, the V content is preferably set to 0.01% or more.

[0147] Nb: below 0.10%

[0148] Like V, niobium (Nb) forms fine carbonitrides, which contribute to increased strength. Therefore, it can be included as needed. However, excessive Nb content leads to hardening of the billet, making rib processing difficult. Furthermore, the large-scale precipitation of Nb as carbonitrides reduces toughness. Therefore, the Nb content is set to 0.10% or less. Preferably, the Nb content is set to 0.09% or less, more preferably 0.08% or less, or 0.05% or less. On the other hand, to obtain the above-mentioned effects, the Nb content is preferably set to 0.005% or more.

[0149] Ti: below 0.30%

[0150] Titanium (Ti) precipitates within the grains as fine carbonitrides, contributing to increased strength. Furthermore, Ti suppresses grain coarsening in the heat-affected zone of welds, indirectly preventing low-temperature cracking. Therefore, it can be included as needed. However, excessive Ti content leads to hardening of the billet, making rib machining difficult. Additionally, like V or Nb, it precipitates in large quantities as carbonitrides, resulting in reduced toughness. Therefore, the Ti content is set to 0.30% or less. Preferably, the Ti content is 0.28% or less, more preferably 0.25%, 0.20%, 0.15%, 0.10%, 0.08%, or 0.05% or less. On the other hand, to obtain the above effects, the Ti content is preferably 0.0005% or more.

[0151] B: Below 0.0200%

[0152] Boron (B) has the effect of reducing the susceptibility to low-temperature cracking. Furthermore, even trace amounts of B improve the hardenability of steel and stabilize its microstructure. Therefore, it can be included as needed. However, excessive B content leads to hardening of the billet, making rib machining difficult. Additionally, in the case of welding, segregation occurs at grain boundaries in the weld heat-affected zone, making liquefaction cracks more likely. Therefore, the B content is set to 0.0200% or less. Preferably, the B content is set to 0.0180% or less, more preferably 0.0150%, 0.0100%, 0.0080%, or 0.0050% or less. On the other hand, to obtain the above-mentioned effects, the B content is preferably set to 0.0001% or more.

[0153] Co: less than 0.50%

[0154] Co (cobalt) has the effect of improving high-temperature strength. Therefore, it can be included as needed. However, excessive Co content reduces processability and increases manufacturing costs. Therefore, the Co content is set to 0.50% or less. Preferably, the Co content is set to 0.40% or less, more preferably 0.30% or less. On the other hand, to obtain the above-mentioned effects, the Co content is preferably set to 0.0001% or more.

[0155] Ca: below 0.050%

[0156] Ca (calcium) has the effect of improving ductility at high temperatures. Therefore, it can be included as needed. However, excessive Ca content will reduce the cleanliness of the steel. Therefore, the Ca content is set to 0.050% or less. The Ca content is preferably set to 0.040% or less, more preferably 0.030% or less. On the other hand, in order to obtain the above-mentioned effect, the Ca content is preferably set to 0.0001% or more.

[0157] Sb: below 0.20%

[0158] Antimony (Sb) has the effect of improving high-temperature strength. Therefore, it can be included as needed. However, excessive Sb content reduces weldability and toughness. Therefore, the Sb content is set to 0.20% or less. The Sb content is preferably set to 0.19% or less, more preferably 0.18% or less. On the other hand, in order to obtain the above-mentioned effect, the Sb content is preferably set to 0.0001% or more.

[0159] The chemical composition of the inner tube consists of Fe and impurities. Here, "impurities" refers to components introduced during the industrial production of ferritic steel due to various factors related to raw materials such as ore and waste, and the manufacturing process, and is permissible within a range that does not adversely affect this embodiment. It should be noted that impurities may include, for example, O (oxygen), but the O content is preferably set to 0.01% or less.

[0160] 2-2. Chemical composition of the steel used for the outer tube

[0161] The outer tube is made of austenitic steel. Here, austenitic steel basically refers to steel containing an austenitic structure, including, for example, austenitic heat-resistant steels. Examples of austenitic heat-resistant steels include SUS304LTB and (Japanese Fire Technology Explanation) 310J1TB. However, it is not particularly limited to these specific steel types, as long as it meets the range of chemical composition described later. Austenitic steel, for example, by having the chemical composition described later, can ensure corrosion resistance in environments where chlorides and other pollutants are present.

[0162] The following describes the chemical composition of the outer tube, including the specific content range of each element. It should be noted that, as with the inner tube, the "%" for content refers to "mass %".

[0163] C: Below 0.10%

[0164] Carbon (C) improves strength and high-temperature strength, but excessive amounts can reduce mechanical properties such as ductility and toughness, as well as corrosion resistance. Therefore, the C content is set to 0.10% or less. Preferably, the C content is 0.06% or less, and more preferably 0.03% or less. On the other hand, to achieve the aforementioned effects, the C content is preferably 0.003% or more.

[0165] Si: 0.01~0.80%

[0166] Silicon (Si) is an element with deoxidizing effects. It is also effective in improving oxidation resistance and resistance to water vapor oxidation. Therefore, the Si content is set to 0.01% or more. Preferably, the Si content is 0.05% or more, more preferably 0.1% or more. However, when Si is present in excess, the precipitation of intermetallic compound phases such as the σ phase is promoted, the microstructure stability at high temperatures decreases, and corrosion resistance also decreases. Furthermore, weldability also decreases. Therefore, the Si content is set to 0.80% or less. Preferably, the Si content is 0.75% or less, more preferably 0.70% or less.

[0167] Mn: 0.01~3.50%

[0168] Manganese (Mn) has the effect of improving strength. Therefore, the Mn content is set to 0.01% or more. Preferably, the Mn content is 0.05% or more, more preferably 0.10% or more. However, when Mn is present in excess, the precipitation of intermetallic compound phases such as the σ phase is promoted, resulting in decreased microstructure stability, high-temperature strength, and mechanical properties. Therefore, the Mn content is set to 3.50% or less. Preferably, the Mn content is 3.40% or less, more preferably 3.3% or less.

[0169] P: below 0.040%

[0170] Phosphorus (P) is contained in steel as an impurity and is an element that reduces mechanical properties. Therefore, the P content is set to 0.040% or less. The P content is preferably set to 0.035% or less, more preferably 0.030% or less. It is preferable to reduce the P content as much as possible, but excessive reduction will increase steelmaking costs. Therefore, the P content is preferably set to 0.0005% or more.

[0171] S: below 0.0100%

[0172] Like phosphorus (P), sulfur (S) is present in steel as an impurity and reduces its mechanical properties. Therefore, the S content is set to 0.0100% or less. Preferably, the S content is set to 0.0080% or less, more preferably 0.0050% or less. While it is preferable to reduce the S content as much as possible, excessive reduction will increase steelmaking costs. Therefore, the S content is preferably set to 0.0001% or more.

[0173] Ni: 5.0–70.0%

[0174] Nickel (Ni) is an element that stabilizes the microstructure of austenitic steel and improves its corrosion resistance. It also increases strength. Therefore, the Ni content is set at 5.0% or more. Preferably, the Ni content is 6.0% or more, more preferably 7.0% or more. However, excessive Ni content impairs weldability and economy. Therefore, the Ni content is set at 70.0% or less. Preferably, the Ni content is 69.0% or less, more preferably 68.0% or less, and even more preferably 65.0% or less.

[0175] Cr: 15.0–35.0%

[0176] Chromium (Cr) has the effect of improving oxidation resistance and corrosion resistance. Therefore, the Cr content is set to 15.0% or more. Preferably, the Cr content is 15.5% or more, and more preferably 16.0% or more. However, excessive Cr content can lead to instability in the microstructure. Furthermore, weldability will also decrease. Therefore, the Cr content is set to 35.0% or less. Preferably, the Cr content is 34.0% or less, and more preferably 33.0% or less.

[0177] Al: Below 1.500%

[0178] Al (aluminum) is an element with deoxidizing effects, but when present in excess, it causes a large amount of non-metallic inclusions to precipitate, reducing mechanical properties. Therefore, the Al content is set to 1.500% or less. Preferably, the Al content is set to 1.400% or less, more preferably 1.300% or less. On the other hand, to obtain the aforementioned effects, the Al content is preferably set to 0.0005% or more.

[0179] N: 0.001~0.350%

[0180] Nitrogen (N) has the effect of forming nitrides with V, Nb, etc., thereby improving strength and high-temperature strength. In addition, it also has the effect of stabilizing the austenitic structure. Therefore, the N content is set to 0.001% or more. The N content is preferably set to 0.002% or more, and more preferably 0.003% or more. However, when N is present in excess, excessive nitride formation occurs, thus reducing mechanical properties. Therefore, the N content is set to 0.350% or less. The N content is preferably set to 0.340% or less, and more preferably 0.330% or less.

[0181] In addition to the elements mentioned above, it may contain one or more elements selected from group B below (Cu, Nb, Ti, B, W, Co, W, and Ca). That is, the lower limit for these elements is 0%. The reasons for the restrictions on each element are explained below.

[0182] Cu: below 5.00%

[0183] Copper (Cu) improves high-temperature strength and corrosion resistance. Therefore, it can be included as needed. However, excessive Cu content reduces processability. Therefore, the Cu content is set to 5.00% or less. Preferably, the Cu content is 4.00% or less, more preferably 3.50% or less. On the other hand, to obtain the above-mentioned effects, the Cu content is preferably 0.0001% or more.

[0184] Nb: below 5.00%

[0185] Niobium (Nb) has the effect of improving high-temperature strength. Therefore, it can be included as needed. However, excessive Nb content reduces processability. Therefore, the Nb content is set to 5.00% or less. The Nb content is preferably set to 4.80% or less, more preferably 4.50% or less. On the other hand, in order to obtain the above-mentioned effect, the Nb content is preferably set to 0.0001% or more.

[0186] Ti: below 1.0%

[0187] Titanium (Ti) improves corrosion resistance. Therefore, it can be included as needed. However, excessive Ti content increases manufacturing costs. Therefore, the Ti content is set to 1.0% or less. Preferably, the Ti content is 0.90% or less, more preferably 0.80% or less. On the other hand, to obtain the above-mentioned effect, the Ti content is preferably 0.0001% or more.

[0188] B: Below 0.010%

[0189] Boron (B) improves corrosion resistance. Therefore, it can be included as needed. However, excessive B content reduces hot workability. Therefore, the B content is set to 0.010% or less. Preferably, the B content is set to 0.009% or less, more preferably 0.008% or less. On the other hand, to obtain the above-mentioned effect, the B content is preferably set to 0.0001% or more.

[0190] W: Below 10.0%

[0191] Tungsten (W) has the effect of improving high-temperature strength. Therefore, it can be included as needed. However, excessive W content will reduce hot workability. Therefore, the W content is set to 10.0% or less. The W content is preferably set to 9.5% or less, more preferably 9.0% or less. On the other hand, in order to obtain the above-mentioned effect, the W content is preferably set to 0.0001% or more.

[0192] Co: below 15.0%

[0193] Co (cobalt) has the effect of improving high-temperature strength. Therefore, it can be included as needed. However, excessive Co content reduces processability and increases manufacturing costs. Therefore, the Co content is set to 15.0% or less. Preferably, the Co content is set to 14.0% or less, more preferably 13.0% or less. On the other hand, to obtain the above-mentioned effects, the Co content is preferably set to 0.0001% or more.

[0194] Ca: below 0.010%

[0195] Ca (calcium) has the effect of improving ductility at high temperatures. Therefore, it can be included as needed. However, excessive Ca content will reduce the cleanliness of the steel. Therefore, the Ca content is set to 0.010% or less. On the other hand, to obtain the above-mentioned effect, the Ca content is preferably set to 0.0001% or more.

[0196] The chemical composition of the outer tube consists of Fe and impurities. Here, "impurities" refers to components introduced during the industrial manufacturing of austenitic steel due to various factors related to raw materials such as ore and waste, and the manufacturing process, and is permissible within a range that does not adversely affect this embodiment. It should be noted that impurities may include, for example, O (oxygen), but the O content is preferably set to 0.01% or less.

[0197] 3. Metallographic structure of the inner tube

[0198] As described above, the inner tube is made of ferritic steel. Therefore, the metallographic structure of the ferritic steel is not particularly limited. However, in the metallographic structure, the ferrite content, in terms of area ratio, is preferably 40% or more. Here, ferrite content refers to the proportion of ferrite in the metallographic structure. When the ferrite content, in terms of area ratio, is 40% or more, the processing characteristics are better, and it is easier to perform rib machining. More preferably, the ferrite content, in terms of area ratio, is 45% or more, and even more preferably, it is 50% or more. There is no specific upper limit set for the ferrite content. The higher the ferrite content, the better.

[0199] It should be noted that the ferrite ratio mentioned above can be calculated through microscopic observation. In microscopic observation, the sample is collected with the cross-section of the inner tube (the section perpendicular to the long side) as the observation surface. Then, this observation surface is mirror-polished and etched with nitric acid and ethanol. Next, an optical microscope is used to take photographs at 100x magnification, with the center of the wall thickness located at the center of the observation field. In the photographs, only white areas are identified as ferrite. Then, the proportion of white areas in the entire observation field is calculated and used as the ferrite ratio.

[0200] 4. Hardness

[0201] 4-1. Hardness of the inner tube

[0202] The hardness of the inner tube, measured using a Vickers hardness tester, is preferably 180 HV or less. When the hardness of the inner tube is 180 HV or less, its machinability is better, and it is easier to perform the precision drawing described later. As a result, the yield of double-layer tubes is improved. Therefore, the hardness of the inner tube, measured using a Vickers hardness tester, is preferably 180 HV or less, more preferably 175 HV or less, and even more preferably 170 HV or less.

[0203] The hardness of the inner tube is determined by the following steps. Specifically, a test piece is collected from the cross-section of the inner tube. In the collected test piece, a load of 10 kgf is applied at the center of the inner tube wall thickness, and hardness tests are performed at three locations. The average value of these tests is taken as the hardness of the inner tube. It should be noted that a Vickers hardness tester can be used for the hardness test. Furthermore, appropriate intervals should be allowed during the hardness measurement to avoid influence from different measurement points. In addition, specific conditions should be followed according to JIS Z 2244-1:2020.

[0204] 4-2. Hardness of the outer tube

[0205] The hardness of the outer tube, measured using a Vickers hardness tester, is preferably 250 HV or less. For the double-layered tube of this embodiment, as described later, the hardness of the inner tube can reach 180 HV or less through three-stage heat treatment. However, in this case, the hardness of the outer tube, which is also heat-treated, will reach 250 HV or less. Therefore, the hardness of the outer tube, measured using a Vickers hardness tester, is preferably 250 HV or less, more preferably 240 HV or less, and even more preferably 230 HV or less.

[0206] The hardness of the outer tube was determined according to the following steps. Specifically, a test piece was collected from the cross-section of the outer tube. In the collected test piece, a load of 10 kgf was applied at the center of the outer tube wall thickness, and hardness tests were performed at three locations. The average value of these tests was taken as the hardness of the outer tube. That is, the same method as for the inner tube was used for the determination.

[0207] 4-3. The difference in hardness between the carburized area of ​​the outer tube and the inner tube.

[0208] As described above, during the three-stage heat treatment process of solution treatment, normalizing, and tempering, carbon contained in the inner tube blank migrates towards the outer tube side, forming a carbon-rich region at the interface of the outer tube blank. This region is called the carburized region. In the double-layer tube of this embodiment, the carburized region refers to the area extending 80 μm outward from the interface between the outer and inner tubes when observing the cross-section of the double-layer tube. This is because carbon tends to accumulate in this region extending 80 μm from the interface between the outer and inner tubes. When carbon accumulates in this carburized region, the hardness locally increases, the hardness difference between the inner and outer tubes increases, and the shear tensile stress decreases to below 200 MPa. As a result, peeling can sometimes occur easily. In addition, the machinability of fine drawing may also decrease.

[0209] Therefore, the difference between the hardness of the carburized region of the outer tube and the hardness of the inner tube (hereinafter referred to as "ΔHV") is preferably set to 200 HV or less using a Vickers hardness tester. ΔHV is more preferably 150 HV or less using a Vickers hardness tester. It should be noted that it is preferable to minimize the hardness difference as much as possible.

[0210] It should be noted that, regarding the hardness of the carburized region of the outer tube, hardness tests were conducted at three locations at the center of the carburized region, i.e., 40 μm outward from the interface of the double-layer tube cross-section. The average value of these tests was taken as the hardness of the carburized region of the outer tube. It should also be noted that the load was set to 100 gf, and the measurement conditions were set to be the same as those used when measuring the hardness of the inner tube.

[0211] 5. Shear tensile stress

[0212] In double-walled tubes used for furnace wall tubes, water flows inside the inner tube, thus generating shear tensile stress. Therefore, in double-walled tubes used for the above-mentioned applications, a higher shear tensile stress is preferable. Furthermore, a higher shear tensile stress also improves the machinability during the manufacturing process. Therefore, the shear tensile stress between the inner and outer tubes is preferably 200 MPa or more. More preferably, it is 300 MPa or more, and even more preferably 400 MPa or more.

[0213] It should be noted that the shear tensile stress can be measured according to the following steps. Data is collected with the long side of the double-layer tube aligned with the long side of the test piece. Figure 2 Such a test piece. It should be noted that the width of the parallel portion of the test piece is set to 10 mm, and the length of the sealing portion between the outer and inner tubes along the long side is set to 2 mm. Tensile tests are performed using this type of test piece to determine the shear tensile stress.

[0214] 6. Manufacturing method

[0215] A preferred manufacturing method for the double-layered tube of this embodiment will be described. The double-layered tube of this embodiment can be stably manufactured using the following manufacturing method.

[0216] First, the billets are prepared as follows: a solid steel billet of ferritic steel, serving as the inner tube, is inserted into a hollow austenitic steel billet, which serves as the outer tube, and then assembled. The austenitic steel serving as the outer tube preferably satisfies the chemical composition described above. The ferritic steel serving as the inner tube only needs to satisfy the chemical composition described above.

[0217] 6-1. Heat-pressed tube

[0218] Next, the above-mentioned billet is hot-formed into a tube blank. Hot forming involves hot extrusion and intermediate drawing. There are no particular limitations on the conditions for hot forming; conventional methods are sufficient. Through the aforementioned intermediate drawing, a tube blank with an inner tube tightly sealed inside an outer tube is formed.

[0219] 6-2. Heat Treatment

[0220] Next, the tube blank formed by hot forming is preferably subjected to heat treatment. The heat treatment preferably involves solution treatment first, followed by normalizing, and then tempering. Furthermore, it is more preferable to perform further normalizing after tempering (hereinafter, normalizing after tempering will also be referred to as "additional normalizing"). Each heat treatment will be described in detail below.

[0221] 6-2-1. Solution treatment

[0222] When heat-treating the tube blank, solution treatment is preferably performed first. This is because solution treatment softens the outer tube blank more easily. As a result, manufacturability is improved, and the hardness of the final product's outer tube reaches 250 HV or less. The heat treatment temperature for solution treatment is preferably set in the range of 1000–1200 °C. If the heat treatment temperature for solution treatment is below 1000 °C, the outer tube blank is less likely to soften, and the hardness of the outer tube easily exceeds 250 HV. As a result, precision drawing is difficult. Therefore, the heat treatment temperature for solution treatment is preferably set at 1000 °C or higher, more preferably at 1050 °C or higher.

[0223] On the other hand, when the heat treatment temperature for solution treatment exceeds 1200°C, a coarse metallographic structure is formed, making it impossible to obtain the desired properties. Therefore, the heat treatment temperature for solution treatment is preferably set to 1200°C or below, and more preferably to 1190°C or below.

[0224] It should be noted that there is no particular limit to the heat treatment time for solution treatment, which is usually around 1 to 10 minutes.

[0225] 6-2-2. Positive Fire

[0226] Next, the solution-treated tube blank is preferably normalized. Normalizing makes it easier to control the microstructure of the inner tube blank. The normalizing heat treatment temperature is preferably set in the range of 850–950°C. If the normalizing heat treatment temperature is below 850°C, the desired microstructure cannot be formed. Therefore, the normalizing heat treatment temperature is preferably set at 850°C or higher, more preferably at 900°C or higher.

[0227] On the other hand, when the normalizing heat treatment temperature exceeds 950°C, the ferrite content of the inner tube of the final product will be less than 40%. Consequently, the hardness of the inner tube easily exceeds 180 HV on a Vickers hardness tester. That is, it is difficult to perform rib machining, and the machinability is sometimes reduced. Therefore, the normalizing heat treatment temperature is preferably set to 950°C or below, more preferably 940°C or below.

[0228] It should be noted that there is no particular limit to the normalizing heat treatment time, which is usually around 1 to 30 minutes.

[0229] 6-2-3. Tempering

[0230] Next, the normalized tube blank is preferably tempered. This is because tempering softens the inner tube blank more easily. The tempering heat treatment temperature is preferably set in the range of 750–800°C. This is because when the tempering heat treatment temperature is below 750°C, the inner tube blank is difficult to soften, and sometimes it is difficult to perform rib machining. In addition, the hardness of the inner tube of the final product will exceed 180 HV. That is, the machinability of fine drawing may be reduced. Therefore, the tempering heat treatment temperature is preferably set to 750°C or higher.

[0231] On the other hand, if the tempering heat treatment temperature exceeds 800°C, a phase transformation will occur, and the desired characteristics cannot be obtained. Therefore, the tempering heat treatment temperature is preferably set to below 800°C.

[0232] It should be noted that there is no particular limit to the heat treatment time for tempering, which is usually around 1 to 60 minutes.

[0233] 6-2-4. Add positive heat

[0234] Alternatively, the tempered billet can be further normalized as needed. To soften both the inner and outer tube blanks together and to perform rib machining on the inner tube blank, the aforementioned solution treatment, normalizing, and tempering heat treatment are preferred. However, due to these heat treatments, carbon (C) from the inner tube blank accumulates in the carburized region of the outer tube, leading to a localized increase in hardness. As a result, the hardness difference between the inner and outer tube blanks becomes larger; that is, ΔHV sometimes exceeds 200. Furthermore, shear tensile stress tends to decrease, and the machinability of fine drawing may sometimes decrease. Here, by performing additional normalizing, the aforementioned C enrichment can be reduced, and the shear tensile stress can be improved.

[0235] The heat treatment temperature for the additional normalizing is preferably set in the range of 850–950°C. This is because if the heat treatment temperature for the additional normalizing is below 850°C, the aforementioned carbon enrichment cannot be sufficiently eliminated. As a result, the hardness difference between the inner and outer tubes of the final product becomes larger, and the shear tensile stress is less than 200 MPa. Therefore, the heat treatment temperature for the additional normalizing is preferably set to 850°C or higher, and more preferably 900°C or higher.

[0236] On the other hand, when the heat treatment temperature for the additional normalizing exceeds 950°C, quenching and hardening occur, sometimes making fine drawing (described later) difficult. Additionally, the inner tube may harden, reducing the ferrite content. Therefore, the heat treatment temperature for the additional normalizing is preferably set to 950°C or below, more preferably 940°C or below.

[0237] It should be noted that there is no particular limit to the heat treatment time for the additional normalizing, which is usually around 1 to 60 minutes.

[0238] 6-3. Exceptional Selection

[0239] The tube blank that has undergone process 6-1 above, or the tube blank that has undergone process 6-2 after process 6-1 as needed, is precision drawn to perform internal threading on the inner tube blank. During internal threading, with the drawing tool (hereinafter also referred to as the "mandrel") inserted into the inner tube blank, the tube blank is pressed using a die while being drawn. The shape of the mandrel and die is not particularly limited; they can be appropriately selected according to the desired rib shape.

[0240] It should be noted that, before machining the internal threads, a portion of both the inner and outer tube blanks may be coated with lubricating material or degreased as needed. Furthermore, after precision drawing, heat treatment or shot peening may be performed as required. Through these processes, the double-layered tube of this embodiment can be obtained.

[0241] The double-layer tube of the present invention will be described in more detail below through embodiments, but the embodiments are not limited to these embodiments.

[0242] Example

[0243] The billet is prepared as follows: a solid steel billet of ferritic steel with the chemical composition of Table 1 is inserted into a hollow steel billet of austenitic steel having the chemical composition described in Table 2, and then assembled. Next, the billet is subjected to hot extrusion, intermediate drawing, etc., to produce a tube blank.

[0244] [Table 1]

[0245]

[0246] [Table 2]

[0247]

[0248] For the obtained tube blank, solution treatment, normalizing, tempering, and additional normalizing are performed as needed under the conditions shown in Table 3. Then, fine drawing is performed to machine the internal threads of the inner tube blank, resulting in a double-layered tube with ribs, where the inner tube is made of ferritic steel and the outer tube is made of austenitic steel. It should be noted that in Table 3, ○ in the fine drawing items indicates that fine drawing can be performed without problems, △ indicates that fine drawing can be performed even with uneven yields, and × indicates that fine drawing cannot be performed.

[0249] For the obtained internally threaded double-layer tube, the hardness, ferrite ratio of the inner tube, and shear tensile stress are determined according to the following steps.

[0250] (Ferrite content of the inner tube)

[0251] The ferrite ratio can be calculated through microscopic observation. In microscopic observation, the sample is collected with the cross-section of the inner tube (perpendicular to the long side) as the observation surface. This observation surface is then mirror-polished and etched with nitric acid and ethanol. An optical microscope is then used to take photographs at 100x magnification, with the center of the wall thickness at the center of the observation field. In the photographs, only white areas are identified as ferrite. The proportion of these white areas in the entire observation field is then calculated and used as the ferrite ratio.

[0252] (Hardness of the inner tube)

[0253] The hardness of the inner tube was determined according to the following steps. A test piece was collected from the cross-section of the inner tube. In the collected test piece, a load of 10 kgf was applied at the center of the inner tube wall thickness, and hardness tests were performed at three points. The average value of these tests was taken as the hardness of the inner tube. It should be noted that a Vickers hardness tester was used for the hardness test. Furthermore, appropriate intervals were allowed during the hardness measurement to avoid influence from different measurement points. In addition, specific conditions were performed according to JIS Z 2244-1:2020.

[0254] (Hardness of the outer tube)

[0255] The hardness of the outer tube was determined according to the following steps. A test piece was collected from the cross-section of the outer tube. In the collected test piece, a load of 10 kgf was applied at the center of the outer tube wall thickness, and hardness tests were performed at three locations. The average value of these tests was taken as the hardness of the outer tube. Furthermore, the testing conditions were set to be the same as those used when determining the hardness of the inner tube.

[0256] (Hardness of the carburized area of ​​the outer tube)

[0257] The hardness of the carburized region of the outer tube was determined according to the following steps. Hardness tests were performed at three locations at the center of the carburized region, i.e., 40 μm outward from the interface of the double-layer tube cross-section. The average value of these tests was taken as the hardness of the carburized region of the outer tube. It should be noted that the load was set to 100 gf, and the measurement conditions were set to be the same as those used when measuring the hardness of the inner tube.

[0258] (Shear tensile stress)

[0259] Shear tensile stress can be determined by following these steps: Data is collected with the long side of the double-layered tube aligned with the long side of the test piece. Figure 2 Such a test piece. It should be noted that the width of the parallel portion of the test piece is set to 10 mm, and the length of the sealing portion between the outer and inner tubes along the long side is set to 2 mm. Tensile tests are performed using this type of test piece to determine the shear tensile stress.

[0260] It should be noted that, considering the differences in the state, texture, hardness, and stiffness of the finished product, in the characteristic evaluation, those whose overall characteristics do not meet the expected benchmark are marked with ×, and those whose characteristic evaluation is slightly better than × are marked with △. Furthermore, those slightly better than △ are marked with ○, and those slightly better than ○ are marked with ◎. The results are summarized in Table 3 below.

[0261] [Table 3]

[0262]

[0263] Tests No. 1 to 15 that meet the conditions of this embodiment have a characteristic evaluation of △ or higher. On the other hand, Test No. 16, which does not meet the conditions of this embodiment, cannot be finely drawn, and a double-layered internally threaded tube cannot be obtained.

[0264] (Postscript)

[0265] A double-walled tube comprising: an inner tube with ribs and made of ferritic steel, and an outer tube made of austenitic steel.

[0266] The chemical composition of the aforementioned inner tube, in mass percent, is as follows:

[0267] C: 0.01~0.30%

[0268] Si: 0.01~0.80%

[0269] Mn: 0.01~2.00%

[0270] P: below 0.030%

[0271] S: below 0.0100%

[0272] Cr: 0.01~3.50%

[0273] Al: below 0.050%

[0274] N: 0.0005~0.0500%

[0275] Ni: 0-1.5%

[0276] Cu: 0–1.0%

[0277] Mo: 0–3.0%

[0278] W: 0–3.0%

[0279] V: 0~0.50%

[0280] Nb: 0–0.10%

[0281] Ti: 0-0.30%

[0282] B: 0~0.0200%

[0283] Co: 0-0.50%

[0284] Ca: 0–0.050%

[0285] Sb: 0~0.20%

[0286] Balance: Fe and impurities,

[0287] The chemical composition of the aforementioned outer tube, in mass percent, is as follows: C: less than 0.10%, Si: 0.01–0.80%, Mn: 0.01–3.50%, P: less than 0.040%, S: less than 0.0100%, Ni: 5.0–70.0%, Cr: 15.0–35.0%, Al: less than 1.500%, N: 0.001–0.350%, Cu: 0–5.00%, Nb: 0–5.00%, Ti: 0–1.0%, B: 0–0.010%, W: 0–10.0%, Co: 0–15.0%, Ca: 0–0.010%.

[0288] Balance: Fe and impurities.

[0289] Explanation of reference numerals in the attached figures

[0290] 1. Outer pipe

[0291] 2. Inner tube

[0292] 3. Ribs

Claims

1. A double-walled tube comprising: an inner tube with ribs and made of ferritic steel, and an outer tube made of austenitic steel. The chemical composition of the inner tube, expressed in % by mass, contains C：0.01～0.30％、 Si: 0.01~0.80% Mn: 0.01~2.00% P: below 0.030% S: below 0.0100% Cr：0.01～3.50％、 Al: below 0.050% N：0.0005～0.0500％， And the remainder consists of Fe and impurities. The chemical composition of the outer tube, expressed as mass%, is C: less than 0.10%. Si: 0.01~0.80% Mn: 0.01~3.50% P: below 0.040% S: below 0.0100% Ni: 5.0–70.0% Cr：15.0～35.0％、 Al: Below 1.500% N：0.001～0.350％、 Balance: Fe and impurities.

2. A double-walled pipe comprising: an inner tube with ribs and made of ferritic steel, and an outer tube made of austenitic steel. The chemical composition of the inner tube, expressed in % by mass, contains C：0.01～0.30％、 Si: 0.01~0.80% Mn: 0.01~2.00% P: below 0.030% S: below 0.0100% Cr：0.01～3.50％、 Al: below 0.050% N：0.0005～0.0500％， It also contains one or more elements selected from group A below. And the balance: Fe and impurities, The chemical composition of the outer tube, expressed as mass%, is C: less than 0.10%. Si: 0.01~0.80% Mn: 0.01~3.50% P: below 0.040% S: below 0.0100% Ni: 5.0–70.0% Cr：15.0～35.0％、 Al: Below 1.500% N：0.001～0.350％， It also contains one or more elements selected from group B below. And the balance: Fe and impurities, [Group A] Ni: less than 1.5%, Cu: less than 1.0%, Mo: less than 3.0%, W: less than 3.0%, V: less than 0.50%, Nb: less than 0.10%, Ti: less than 0.30%, B: less than 0.0200%, Co: less than 0.50%, Ca: less than 0.050%, Sb: less than 0.20%. [Group B] Cu: less than 5.00%, Nb: less than 5.00%, Ti: less than 1.0%, B: less than 0.010%, W: less than 10.0%, Co: less than 15.0%, Ca: less than 0.010%.

3. A double-walled tube comprising: an inner tube with ribs and made of ferritic steel, and an outer tube made of austenitic steel. The chemical composition of the inner tube, expressed in % by mass, contains C：0.01～0.30％、 Si: 0.01~0.80% Mn: 0.01~2.00% P: below 0.030% S: below 0.0100% Cr：0.01～3.50％、 Al: below 0.050% N：0.0005～0.0500％， It also contains one or more elements selected from group A below. And the balance: Fe and impurities, The chemical composition of the outer tube, expressed as mass%, is C: less than 0.10%. Si: 0.01~0.80% Mn: 0.01~3.50% P: below 0.040% S: below 0.0100% Ni: 5.0–70.0% Cr：15.0～35.0％、 Al: Below 1.500% N：0.001～0.350％， It also contains one or more elements selected from group B below. And the balance: Fe and impurities, [Group A] Ni: less than 1.0%, Cu: less than 1.0%, Mo: less than 3.0%, W: less than 3.0%, V: less than 0.50%, Nb: less than 0.10%, Ti: less than 0.30%, B: less than 0.0200%, Co: less than 0.50%, Ca: less than 0.050%, Sb: less than 0.20%. [Group B] Cu: less than 5.00%, Nb: less than 5.00%, Ti: less than 1.0%, B: less than 0.010%, W: less than 10.0%, Co: less than 15.0%, Ca: less than 0.010%.

4. The double-layered tube according to claim 1, wherein, The ferrite content of the inner tube, in terms of area ratio, is 40% or more. The hardness of the inner tube, measured by a Vickers hardness tester, is below 180 HV. The hardness of the outer tube is below 250 HV according to the Vickers hardness tester.

5. The double-layered tube according to claim 2, wherein, The ferrite content of the inner tube, in terms of area ratio, is 40% or more. The hardness of the inner tube, measured by a Vickers hardness tester, is below 180 HV. The hardness of the outer tube is below 250 HV according to the Vickers hardness tester.

6. The double-layered tube according to claim 3, wherein, The ferrite content of the inner tube, in terms of area ratio, is 40% or more. The hardness of the inner tube, measured by a Vickers hardness tester, is below 180 HV. The hardness of the outer tube is below 250 HV according to the Vickers hardness tester.

7. The double-walled tube according to any one of claims 1 to 6, wherein, The difference between the hardness of the carburized area of ​​the outer tube and the hardness of the inner tube is less than 200 HV on a Vickers hardness scale.

8. The double-walled tube according to any one of claims 1 to 6, wherein, The shear tensile stress between the inner tube and the outer tube is above 200 MPa.

9. The double-layered tube according to claim 7, wherein, The shear tensile stress between the inner tube and the outer tube is above 200 MPa.

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