Ni-based alloy seamless pipes and welded joints

Abstract

To provide an Ni-based alloy tube that allows stable formation of an internal bead having good usability of its weldment, and a welded joint.SOLUTION: An Ni-based alloy tube includes a chemical composition containing, in mass%, C: 0.005 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P: 0.015% or less, S: 0.0001 to 0.0030%, Cr: 20.0 to 23.5%, Mo: 8.0 to 10.5%, Ti: 0.01 to 0.40%, N: 0.0010 to 0.0400%, Al: 0.01 to 0.40%, O: 0.0004 to 0.0100%, one or more elements selected from Nb and Ta, and Sn: 0 to 0.010%, with the balance: Ni and impurities, and satisfying [0.0010≤S+2O+0.2Sn≤0.0180] and [2.50≤Nb+Ta≤4.60].SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a Ni-based alloy pipe and a welded joint. [Background technology]

[0002] Chemical plants and power plants are equipped with a variety of plant equipment, including flue gas treatment equipment and seawater treatment equipment. Plant interiors are severely corrosive environments, with many substances that accelerate corrosion, such as chlorides and hydrogen sulfide. Therefore, materials used in plant equipment are required to have not only strength but also corrosion resistance. Therefore, as disclosed in Patent Documents 1 to 8, Ni-based alloys with enhanced corrosion resistance have been developed for use in plant equipment. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 54-110918 [Patent Document 2] Japanese Patent Application Publication No. 63-89637 [Patent Document 3] Japanese Patent Application Publication No. 2-156034 [Patent Document 4] Japanese Patent Application Publication No. 3-173732 [Patent Document 5] Japanese Patent Application Publication No. 5-271832 [Patent Document 6] Japanese Patent Application Publication No. 9-87786 [Patent Document 7] Japanese Patent Application Publication No. 10-30140 [Patent Document 8] Japanese Patent Application Laid-Open No. 2012-72446 Summary of the Invention [Problem to be solved by the invention]

[0004] Some plant equipment is manufactured by assembling components together through welding. In such equipment, the state of weld bead formation can affect the progression of corrosion.

[0005] For example, some heat exchangers are manufactured by butt-welding multiple Ni-based alloy pipes that serve as flow paths for refrigerants and the like. When used as a heat exchanger, various corrosive fluids flow inside the pipes. In this case, if the height of the bead formed inside the pipe by welding, i.e., the weld height, is too high, the corrosive fluids will accumulate and concentrate at the toe where the bead surface intersects with the surface of the base material. As a result, there is a problem in that corrosion is more likely to progress at the toe.

[0006] On the other hand, if the amount of heat input during welding is reduced by trying to reduce the reinforcement height too much, the butt surfaces of the pipes do not melt completely, making it difficult to form a stable bead. As a result, there are problems such as the occurrence of weld defects and the retention and concentration of corrosive fluid in the weld defects, facilitating the progression of corrosion. However, Patent Documents 1 to 8 do not consider these problems at all.

[0007] Therefore, even if a pipe is manufactured using a highly corrosion-resistant Ni-based alloy, it is difficult to form a bead of an appropriate shape on the pipe during butt welding, which is resistant to the progression of corrosion. In other words, there is a problem in that it is difficult to obtain a Ni-based alloy pipe in which a bead is stably formed on the inner surface during welding and the reinforcement height is not too high.

[0008] In light of the above, an object of the present invention is to solve the above problems and to provide a Ni-based alloy pipe and welded joint that can stably form an inner bead that has good usability at the weld. [Means for solving the problem]

[0009] The present invention has been made to solve the above problems, and is summarized as the following Ni-based alloy pipe and welded joint.

[0010] (1) Chemical composition, in mass%, C: 0.005~0.080%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P: 0.015% or less, S: 0.0001 to 0.0030%, Cr: 20.0~23.5%, Mo: 8.0-10.5% Ti: 0.01 to 0.40% N: 0.0010~0.0400%, Al: 0.01 to 0.40%, O: 0.0004 to 0.0100% One or more selected from Nb and Ta, Sn: 0 to 0.010% The balance is Ni and impurities, A Ni-based alloy tube that satisfies the following formulas (i) and (ii): 0.0010≦S+2O+0.2Sn≦0.0180 (i) 2.50≦Nb+Ta≦4.60 (ii) However, the element symbols in the above formula represent the content (mass %) of each element contained in the Ni-based alloy, and if no element is contained, it is set to zero.

[0011] (2) The Ni-based alloy pipe according to (1) above, wherein the arithmetic mean roughness Ra of the inner surface of the Ni-based alloy pipe in the longitudinal direction of the pipe is 7.0 μm or less.

[0012] (3) The chemical composition contains, in mass %, a part of Ni replaced by The Ni-based alloy pipe according to (1) or (2) above, containing Fe: 5.50% or less.

[0013] (4) A Ni-based alloy pipe according to any one of (1) to (3) above, wherein the chemical composition contains at least one element selected from Cu and Co in place of a portion of the Ni, and satisfies the following formula (iii): 0.01≦Cu+Co≦1.50 (iii) However, the element symbols in the above formula represent the content (mass %) of each element contained in the Ni-based alloy, and if no element is contained, it is set to zero.

[0014] (5) The chemical composition contains, in mass %, a part of Ni replaced by W: 1.00% or less, V: 0.40% or less, Ca: 0.0030% or less, Mg: 0.0030% or less, B: 0.0100% or less, and REM: 0.0100% or less, The Ni-based alloy pipe according to any one of (1) to (4) above, which contains one or more selected from the following:

[0015] (6) A welded joint using the Ni-based alloy pipe according to any one of (1) to (5) above. [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain a Ni-based alloy pipe capable of stably forming an inner bead having good usability at the welded portion. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing the groove shape in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present inventors have conducted research into beads in Ni-based alloy pipes and have obtained the following findings (a) to (d).

[0019] (a) The shape of the inner bead formed during butt welding of a pipe is affected by the contents of S and O contained in the Ni-based alloy pipe. The inventors have clarified that when the contents of S and O are low, the inner bead is not stably formed, and part of the butt surface remains unmelted.

[0020] On the other hand, if the S and O contents are excessive, the bead will be formed stably, but the bead height will be excessively high. Therefore, when used as an alloy pipe, corrosive fluid will remain near the bead, making corrosion more likely to progress. Therefore, in order to form a stable bead and prevent the bead height from becoming excessively high, it is necessary to adjust the S and O contents to within a predetermined range.

[0021] (b) The reasons why S and O affect the formation of the bead are thought to be as follows: S and O are surface active elements, which strengthen the inward convection in the weld pool during welding. As a result, the welding heat is more easily transmitted in the depth direction, allowing the bead to be formed stably. On the other hand, if S and O are contained in excess, the surface tension of the molten metal decreases excessively, making the molten metal more likely to sag. As a result, the shape of the bead becomes excessively raised, or convex (hereinafter simply referred to as "convex shape"), and the height of the excess weld increases.

[0022] (c) In addition, the shape of the bead on the inner surface of the pipe is affected by the surface roughness of the pipe inner surface in the longitudinal direction. The inventors have found that when the surface roughness is large, the reinforcement height increases and tends to become convex. For this reason, it is desirable to control the surface roughness within a predetermined range. In particular, when the surface roughness of the pipe inner surface in the longitudinal direction is large, the spread of the molten metal in the width direction is suppressed, and the bead shape tends to become convex and the reinforcement height tends to increase.

[0023] (d) Furthermore, the inventors have also found that the shape of the inner bead is also affected by the Sn content. When Sn is added, the penetration depth increases, making it easier to form a stable inner bead. On the other hand, when an excessive amount of Sn is added, the penetration becomes excessive, making the inner bead more likely to have a convex shape. This is thought to be because Sn evaporates from the surface of the molten pool during welding, increasing the concentration of the arc. Therefore, when Sn is added, in order to obtain an inner bead with an appropriate shape, it is necessary to control the Sn content within a specified range, and also to ensure that the relationship between the S content, the O content, and the Sn content satisfies specified ranges.

[0024] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0025] 1. Chemical composition of alloy tubes The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0026] C: 0.005 to 0.080% C has the effect of stabilizing the structure. Therefore, the C content is set to 0.005% or more. The C content is preferably set to 0.008% or more, more preferably 0.010% or more, and even more preferably 0.012% or more. However, if C is added in excess, it bonds with Cr during the welding heat cycle to form carbides at the grain boundaries in the weld heat affected zone. As a result, Cr-depleted zones are formed near the grain boundaries, reducing corrosion resistance. Therefore, the C content is set to 0.080% or less. The C content is preferably set to 0.050% or less, more preferably 0.030% or less, and even more preferably 0.025% or less.

[0027] Si: 0.01 to 0.50% Si has a deoxidizing effect. Therefore, the Si content is set to 0.01% or more. The Si content is preferably set to 0.02% or more, and more preferably set to 0.03% or more. The Si content is further preferably set to 0.05% or more. However, excessive Si content reduces the structural stability of the alloy and increases the susceptibility to weld cracking. It may also make it difficult to stably form a bead on the inner surface. Therefore, the Si content is set to 0.50% or less. The Si content is preferably set to 0.48% or less, and more preferably set to 0.45% or less. The Si content is further preferably set to 0.43% or less.

[0028] Mn: 0.01 to 0.50% Mn, like Si, has a deoxidizing effect. It also has the effect of enhancing structural stability and contributes significantly to the stable formation of a bead on the inner surface. Therefore, the Mn content is set to 0.01% or more. The Mn content is preferably set to 0.03% or more, and more preferably set to 0.05% or more. The Mn content is further preferably set to 0.08% or more. However, excessive Mn content reduces hot workability. Therefore, the Mn content is set to 0.50% or less. The Mn content is preferably set to 0.48% or less, and more preferably set to 0.45% or less. The Mn content is further preferably set to 0.40% or less.

[0029] P:0.015% or less P is contained in Ni-based alloys as an impurity and significantly increases weld crack susceptibility. For this reason, the P content is set to 0.015% or less. The P content is preferably set to 0.013% or less, and more preferably set to 0.012% or less. It is preferable to reduce the P content as much as possible, but excessive reduction increases manufacturing costs. For this reason, the P content is preferably set to 0.001% or more, and more preferably set to 0.002% or more.

[0030] S: 0.0001 to 0.0030% S is generally contained as an impurity in Ni-based alloys, but in the alloy pipe of the present invention, together with O, it has the effect of improving the ability to form an inner bead during welding. For this reason, the S content is set to 0.0001% or more. The S content is preferably set to 0.0002% or more, and more preferably 0.0003% or more. However, if S is contained in excess, the bead on the inner surface of the pipe will become convex and the susceptibility to weld cracking will increase. For this reason, the S content is set to 0.0030% or less. The S content is preferably set to 0.0025% or less, and more preferably 0.0020% or less. Note that S, together with O and Sn, must satisfy the following formula (i).

[0031] Cr: 20.0~23.5% Cr is an essential element for ensuring corrosion resistance. Cr forms a passive film on the surface, particularly improving corrosion resistance in an oxidizing acid environment. For this reason, the Cr content is set to 20.0% or more. The Cr content is preferably set to 20.5% or more, more preferably set to 21.0% or more, and even more preferably set to 21.2% or more. However, excessive Cr content reduces structural stability. For this reason, the Cr content is set to 23.5% or less. The Cr content is preferably set to 23.3% or less, more preferably set to 23.0% or less, and even more preferably set to 22.8% or less.

[0032] Mo: 8.0-10.5% Mo improves corrosion resistance in environments where non-oxidizing acids and chlorides are present. For this reason, the Mo content is set to 8.0% or more. The Mo content is preferably set to 8.2% or more, more preferably set to 8.5% or more, and even more preferably set to 8.7% or more. However, excessive Mo content reduces structural stability. Furthermore, Mo is an expensive element, which increases manufacturing costs. For this reason, the Mo content is set to 10.5% or less. The Mo content is preferably set to 10.3% or less, more preferably set to 10.0% or less, and even more preferably set to 9.8% or less.

[0033] Ti: 0.01 to 0.40% Ti forms carbides, contributing to strengthening, and also suppresses the formation of Cr carbides, thereby reducing the deterioration of corrosion resistance at grain boundaries. Therefore, the Ti content is set to 0.01% or more. The Ti content is preferably set to 0.05% or more, more preferably 0.08% or more, and even more preferably 0.10% or more. However, excessive Ti content causes large amounts of Ti carbides and carbonitrides to precipitate, reducing ductility. Therefore, the Ti content is set to 0.40% or less. The Ti content is preferably set to 0.38% or less, more preferably 0.35% or less, and even more preferably 0.32% or less.

[0034] N: 0.0010~0.0400% N contributes to structural stability and has the effect of enhancing pitting corrosion resistance. Therefore, the N content is set to 0.0010% or more. The N content is preferably set to 0.0020% or more, more preferably set to 0.0030% or more, and even more preferably set to 0.0040% or more. However, excessive N content causes nitrides to precipitate, reducing ductility. Therefore, the N content is set to 0.0400% or less. The N content is preferably set to 0.0350% or less, more preferably set to 0.0300% or less. The N content is even more preferably set to 0.0250% or less.

[0035] Al: 0.01 to 0.40% Al has a deoxidizing effect and also contributes to improving oxidation resistance at high temperatures. For this reason, the Al content is set to 0.01% or more. The Al content is preferably set to 0.02% or more, more preferably 0.03% or more. The Al content is further preferably set to 0.05% or more. However, excessive Al content generates brittle compounds with Ni, reducing hot workability. It may also make it difficult to stably form a bead on the inner surface. For this reason, the Al content is set to 0.40% or less. The Al content is preferably set to 0.35% or less, more preferably 0.30% or less, and even more preferably 0.28% or less.

[0036] O: 0.0004 to 0.0100% O is generally contained as an impurity in Ni-based alloys, but in the alloy pipe of the present invention, together with S, it has the effect of improving the ability to form a bead on the inner surface of the pipe during welding. Therefore, the O content is set to 0.0004% or more. The O content is preferably set to 0.0006% or more, and more preferably set to 0.0008% or more. However, if O is contained in excess, the bead on the inner surface of the pipe will become convex and the hot workability will deteriorate. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0080% or less, and more preferably set to 0.0060% or less. Note that O, together with S and Sn, must satisfy the following formula (i).

[0037] One or more selected from Nb and Ta, 2.50% or more and 4.60% or less in total Like Ti, Nb and Ta combine with carbon to form carbides, contributing to strengthening, while also suppressing the formation of Cr carbides and reducing deterioration of corrosion resistance at grain boundaries. For this reason, it is necessary for the steel to contain at least one element selected from Nb and Ta, and for the total content of these elements to satisfy the following formula (ii):

[0038] 2.50≦Nb+Ta≦4.60 (ii) However, the element symbols in the above formula represent the content (mass %) of each element contained in the Ni-based alloy, and if no element is contained, it is set to zero.

[0039] If the value in formula (ii), which is the total content of Nb and Ta, is less than 2.50%, the effects of improving strength and reducing deterioration of intergranular corrosion resistance described above cannot be obtained. Therefore, the value in formula (ii) is set to 2.50% or more. The value in formula (ii) is preferably set to 2.70% or more, and more preferably set to 3.00% or more.

[0040] On the other hand, if the value in equation (ii) exceeds 4.60%, large amounts of Nb and Ta carbides and carbonitrides precipitate, reducing ductility. Furthermore, the susceptibility to weld cracking also increases. Therefore, the value in equation (ii) should be set to 4.60% or less, preferably 4.40% or less, and more preferably 4.20% or less.

[0041] In addition to the above elements, the chemical composition may further contain Sn within the range shown below.

[0042] Sn: 0 to 0.010% Sn has the effect of increasing the penetration depth during welding and improving the ability to form a bead on the inner surface of the pipe. Therefore, it may be added as needed. However, excessive Sn content reduces hot workability and increases susceptibility to weld cracking. In addition, it makes the inner surface bead more likely to have a convex shape. For this reason, the Sn content is set to 0.010% or less. The Sn content is preferably set to 0.009% or less, and more preferably set to 0.008% or less. On the other hand, to obtain the above effect, the Sn content is preferably set to 0.001% or more, more preferably set to 0.002% or more, and even more preferably set to 0.003% or more. Note that Sn, together with S and O, must satisfy the following formula (i).

[0043] As described above, S, O, and Sn effectively contribute to the formation of beads on the inner surface of the tube, and therefore, in the Ni-based alloy tube according to the present invention, it is necessary to satisfy the following formula (i), which is a relational expression between the S content, the O content, and the Sn content:

[0044] 0.0010≦S+2O+0.2Sn≦0.0180 (i) In the above formula, the element symbols represent the content (mass%) of each element contained in the Ni-based alloy, and if the element is not contained, it is set to 0. Also, in the above formula, if the Sn content is less than 0.001%, Sn = 0.

[0045] S and O are surface active elements that strengthen inward convection within the molten pool during welding. Sn also contributes to the formation of an arc current path, increasing the arc concentration. It also transports welding heat toward the depth of the center of the molten pool. As a result, these elements have the effect of stably forming an inner bead, but this effect cannot be achieved if the value in equation (i) is less than 0.0010%. Therefore, the value in equation (i) must be 0.0010% or greater. The value in equation (i) is preferably 0.0012% or greater, and more preferably 0.0015% or greater.

[0046] On the other hand, if the value in equation (i) exceeds 0.0180%, the surface tension of the molten metal decreases, or melting in the center of the molten pool is promoted, causing sagging. As a result, the bead becomes convex, and a stable bead cannot be formed on the inner surface of the pipe. For this reason, the value in equation (i) should be 0.0180% or less. The value in equation (i) is preferably 0.0175% or less, and more preferably 0.0170% or less.

[0047] In addition to the above elements, the chemical composition may further contain Fe within the range shown below.

[0048] Fe:5.50% or less Fe is effective in improving hot workability. Furthermore, it also contributes to reducing alloy costs. Therefore, it may be contained as needed. However, excessive Fe content reduces structural stability. Therefore, the Fe content is set to 5.50% or less. The Fe content is preferably set to 5.30% or less, and more preferably set to 5.00% or less. On the other hand, to obtain the above effects, the Fe content is preferably set to 0.01% or more, more preferably set to 0.50% or more, and even more preferably set to 1.50% or more.

[0049] In addition to the above elements, the chemical composition may further contain Cu and Co within the ranges shown below.

[0050] One or more selected from Cu and Co, total of 1.50% or less Cu and Co have the effect of increasing structural stability and improving corrosion resistance in non-oxidizing acid and chloride environments. Therefore, one or more elements selected from Cu and Co may be contained as needed. Furthermore, when Cu and Co are contained, it is preferable that the chemical composition satisfies the following formula (iii):

[0051] 0.01≦Cu+Co≦1.50 (iii) However, the element symbols in the above formula represent the content (mass %) of each element contained in the Ni-based alloy, and if no element is contained, it is set to zero.

[0052] If the value in formula (iii), which is the total content of Cu and Co, is less than 0.01%, it becomes difficult to obtain the above-mentioned effect. Therefore, the value in formula (iii) is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more. However, if the value in formula (iii) exceeds 1.50%, the hot workability deteriorates and the manufacturing cost increases. Therefore, the value in formula (iii) is preferably 1.50% or less, more preferably 1.30% or less. The value in formula (iii) is even more preferably 1.00% or less.

[0053] In addition to the above elements, the chemical composition may contain one or more elements selected from W, V, Ca, Mg, B, and REM within the ranges shown below. The reasons for limiting each element will be explained below.

[0054] W: 1.00% or less W improves corrosion resistance in environments where non-oxidizing acids and chlorides are present. Therefore, it may be added as needed. However, excessive W content reduces structural stability. Furthermore, W is an expensive element, which increases manufacturing costs. Therefore, the W content is set to 1.00% or less. The W content is preferably set to 0.90% or less, and more preferably set to 0.80% or less. On the other hand, to obtain the above effects, the W content is preferably set to 0.01% or more, and more preferably set to 0.02% or more.

[0055] V:0.40% or less V combines with carbon to form carbides and suppresses the formation of Cr carbides, thereby reducing the deterioration of corrosion resistance at grain boundaries. Therefore, V may be added as needed. However, excessive V content causes large amounts of V carbides and carbonitrides to precipitate, reducing ductility. Therefore, the V content is set to 0.40% or less. The V content is preferably set to 0.35% or less, and more preferably set to 0.30% or less. On the other hand, to obtain the above effects, the V content is preferably set to 0.01% or more, and more preferably set to 0.02% or more.

[0056] Ca:0.0030% or less Ca has the effect of improving hot workability. Therefore, it may be added as necessary. However, if an excessive amount of Ca is added, it combines with oxygen and significantly reduces cleanliness. As a result, hot workability is actually reduced. Therefore, the Ca content is set to 0.0030% or less. The Ca content is preferably set to 0.0020% or less, and more preferably set to 0.0010% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more.

[0057] Mg: 0.0030% or less Like Ca, Mg has the effect of improving hot workability. Therefore, it may be added as needed. However, if an excessive amount of Mg is added, it combines with oxygen and significantly reduces cleanliness. As a result, hot workability is actually reduced. Therefore, the Mg content is set to 0.0030% or less. The Mg content is preferably set to 0.0020% or less, and more preferably set to 0.0010% or less. On the other hand, to obtain the above effect, the Mg content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more.

[0058] B: 0.0100% or less B segregates at grain boundaries at high temperatures, strengthening the grain boundaries and improving hot workability. Therefore, it may be added as needed. However, excessive B content increases weld crack susceptibility. Therefore, the B content is set to 0.0100% or less. The B content is preferably set to 0.0080% or less, and more preferably set to 0.0060% or less. On the other hand, to obtain the above effects, the B content is preferably set to 0.0002% or more, and more preferably set to 0.0005% or more.

[0059] REM: 0.0100% or less Like Ca and Mg, REM has the effect of improving hot workability during manufacturing. Therefore, it may be added as needed. However, if REM is added in excess, it combines with oxygen and significantly reduces cleanliness. As a result, hot workability is actually reduced. Therefore, the REM content is set to 0.0100% or less. The REM content is preferably set to 0.0050% or less, and more preferably 0.0030% or less. On the other hand, to obtain the above effect, the REM content is preferably set to 0.0001% or more, and more preferably 0.0003% or more. Here, REM refers to Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements.

[0060] In the chemical composition of the Ni-based alloy according to the present invention, the balance is Ni and impurities. Here, "impurities" refer to components that are not intentionally added but are mixed in during industrial production of the Ni-based alloy due to various factors such as raw materials and production processes, and are acceptable within a range that does not adversely affect the present invention.

[0061] 2. Surface roughness of alloy pipe The bead is formed when welding the end of the alloy pipe. To form a good bead, it is preferable to control the arithmetic mean roughness Ra in the longitudinal direction on the inner surface of the alloy pipe. Here, the surface roughness of the alloy pipe refers to the surface roughness after the final process in the manufacturing process. In other words, the surface roughness of the alloy pipe changes during the manufacturing process, but to obtain the effects of the present invention, it is sufficient that the surface roughness in the longitudinal direction of the pipe after the final process satisfies the range specified in the present invention, regardless of the surface roughness during manufacturing.

[0062] If the arithmetic mean roughness Ra of the inner surface of a Ni-based alloy pipe in the longitudinal direction of the pipe exceeds 7.0 μm, wetting of the weld metal on the inner surface of the pipe is hindered, making it difficult for the weld metal to spread in the width direction, i.e., along the circumference of the pipe. As a result, the bead tends to have a convex shape, and the weld height tends to be large. For this reason, the arithmetic mean roughness Ra of the inner surface of the Ni-based alloy pipe in the longitudinal direction of the pipe is preferably 7.0 μm or less. The arithmetic mean roughness Ra is preferably 5.0 μm or less, and more preferably 3.0 μm or less. There is no particular lower limit for the arithmetic mean roughness Ra, but when using the manufacturing method described below, it is usually 0.1 to 1.0 μm or more.

[0063] Here, the arithmetic mean roughness Ra is defined in JIS B 0601:2001, and can be measured using a contact-type surface roughness measuring device.

[0064] 3. Welded joints A welded joint of Ni-based alloy pipe can be obtained by butt-welding the ends of the above-mentioned Ni-based alloy pipes together under specified conditions. A welded joint of Ni-based alloy pipes has a weld metal, which is formed when the molten metal solidifies and becomes a joint, and a base material. The base material includes a weld heat-affected zone, which is affected by the heat input from welding. The base material, excluding the weld heat-affected zone, inherits the chemical composition, surface roughness, and other properties of the Ni-based alloy pipe described in items 1 and 2 above. The weld refers to the weld metal and the weld heat-affected zone.

[0065] 4. Manufacturing method A preferred method for manufacturing the Ni-based alloy pipe according to the present invention will now be described. The Ni-based alloy pipe according to the present invention can obtain the effects described above regardless of the manufacturing method, but can be stably manufactured, for example, by the following manufacturing method.

[0066] 4-1.Ni-based alloy tube First, a Ni-based alloy ingot is produced as a raw material for the Ni-based alloy pipe. The Ni-based alloy ingot is preferably produced by melting an alloy having the above-mentioned chemical composition in an electric furnace or the like, removing impurities by refining, and then casting the melt. Next, the obtained ingot is preferably hot forged into a cylindrical billet. The obtained billet is then processed to form a pipe.

[0067] Specifically, it is preferable to hot extrude the billet, followed by cold rolling or cold drawing. During the processing, softening heat treatment and intermediate pickling may be performed in between, if necessary. Thereafter, it is preferable to subject the alloy pipe to a solution treatment as a heat treatment. After the solution treatment, pickling or processing may be performed, if necessary.

[0068] To achieve an arithmetic mean roughness Ra of 7.0 μm or less in the longitudinal direction of the pipe, the following steps are preferably carried out. Specifically, solution treatment is preferably carried out by heating in the temperature range of 950°C to 1230°C for 1 to 15 minutes, followed by water cooling. It is also preferable to subject the inner surface of the pipe to machining such as grinding or grinding, or shot blasting or shot peening.

[0069] Although the arithmetic mean roughness Ra changes during the manufacturing process, the effect of the present invention is affected only by the surface roughness in the longitudinal direction of the pipe after the final process, regardless of the surface roughness during the intermediate processes.

[0070] 4-2. Welded joints of Ni-based alloy pipes A welded joint can be obtained by welding the ends of the Ni-based alloy pipe according to the present invention. The welding method is not particularly limited, but arc welding may be used, for example. The arc welding conditions are preferably, for example, a heat input in the range of 4 to 20 kJ / cm. During welding, Ar gas is preferably used as a shielding gas and a back-shielding gas. The flow rate of the gas flowing through the welded area is preferably adjusted as appropriate.

[0071] The chemical composition of the welding material (filler metal) used is not particularly limited, but preferably has the following composition: In mass %, it is preferable that the composition contains C: 0.150% or less, Si: 1.00% or less, Mn: 3.50% or less, P: 0.030% or less, S: 0.0001 to 0.0100%, Fe: 38.0% or less, Cu: 3.00% or less, Co: 15.0% or less, Cr: 14.0 to 26.0%, Mo: 17.0% or less, W: 4.5% or less, at least one of Nb and Ta in total 4.20% or less, Ti: 1.50% or less, V: 0.35% or less, N: 0.0500% or less, Al: 1.50% or less, O: 0.0004 to 0.0100%, with the balance being Ni and impurities, and the relationship between the S and O contents satisfies the following formula (a):

[0072] 0.0010≦S+2O≦0.0180 (a) However, the element symbols in the above formula represent the content (mass%) of each element contained in the welding material, and if no element is contained, it is set to zero.

[0073] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0074] Alloys having the chemical compositions shown in Table 1 were melted and produced into ingots. They were then hot forged and hot rolled to a thickness of 10 mm. The scale formed on the surface was then removed by pickling. The arithmetic mean roughness Ra was approximately 10 μm for all steel types. Subsequently, in anticipation of the alloy pipe manufacturing process, 3 mm alloy plates were obtained by cold rolling, with intermediate softening heat treatment and intermediate pickling performed.

[0075] Next, the alloy plate was subjected to solution treatment in a hydrogen furnace at 1150°C for 10 minutes, followed by water cooling. Plates measuring 50 mm in width and 100 mm in length were then cut out of the alloy plate. Some of the cut-out plates were shot peened on only one side, simulating the inner surface of an alloy pipe, as shown in Table 2. Plates that were not shot peened were ground on one side or polished 1 to 5 times using a grinding stone with a grit size of 40 or 60. In Table 2, for example, "Polished (#40 x 1 time)" indicates that the plate was polished once with a grinding stone with a grit size of 40.

[0076] [Table 1]

[0077] [Table 2]

[0078] The arithmetic mean roughness of each plate was measured using a contact roughness meter. Two plates of each alloy type were prepared, and the end faces in the rolling direction were grooved as shown in Figure 1. The grooved end faces of these plates were butted together, and a first-pass weld was performed using a filler metal with an outer diameter of 1.0 mm and the chemical composition shown in Table 3 to obtain a welded joint. The heat input during welding was approximately 5 kJ / cm, and Ar gas was used as the shielding gas and back-shielding gas, flowing through the weld at a flow rate of 10 L / min.

[0079] [Table 3]

[0080] For the obtained welded joints, those in which a back bead was formed along the entire length of the weld line were judged to have no problems with the ability to form a bead on the inner surface of the alloy pipe and were rated "pass." Among them, those in which the width of the back bead was 2 mm or more along the entire length of the weld line were rated "excellent," and those in which a back bead of 1 mm or more was formed but with a width less than 2 mm were rated "fair." In this example, the back bead corresponds to the inner bead formed when welding is performed from the outside of the alloy pipe.

[0081] Three cross sections were then taken from the welded joint, and if the bead height on the back side in all cross sections was 1.0 mm or less, it was judged that the shape of the bead on the inner side of the alloy pipe was good and was rated "pass." Among these, if the bead height on the back side in all cross sections was 0.8 mm or less, it was rated "excellent," and all other cases were rated "fair." The results are summarized in Table 4.

[0082] [Table 4]

[0083] All of the test specimens using alloys A to H and L to N satisfied the requirements of the present invention, and the formability and shape of the bead on the back side were good. Of these, test specimen N1 using alloy N satisfied the range specified by formula (i), and therefore satisfied both the formability and height of the bead on the back side.

[0084] On the other hand, specimens I1 and K1, which used alloys I and K, had S, O, and Sn contents that did not satisfy formula (i) and were higher than the specified range. As a result, the molten metal sagged significantly, and the height of the bead on the back side did not meet the target. Specimen J1, which used alloy J, had a relationship between S and O content that did not satisfy formula (i) and was lower than the specified range. As a result, melting in the thickness direction was insufficient, and the desired ability to form a bead on the back side was not achieved.

[0085] Specimens O1 and P1, which were made using alloys O and P, respectively, had Sn contents exceeding the specified range, or S, O, and Sn contents higher than the range specified in formula (i). As a result, the molten metal sagged significantly, and the target bead height on the back side was not achieved. Furthermore, specimen Q1, which was made using alloy Q, had S, O, and Sn contents that did not satisfy formula (i). As a result, melting in the thickness direction was insufficient, and the formation of the bead on the back side did not meet the target. [Industrial Applicability]

[0086] According to the present invention, it is possible to obtain a Ni-based alloy pipe in which an inner bead is stably formed during butt welding and the bead height is not excessive.

Claims

1. The chemical composition, in mass%, is C: 0.005-0.080%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P: 0.015% or less, S: 0.0001-0.0030%, Cr: 20.0-23.5%, Mo: 8.0 to 10.5%, Ti: 0.01-0.40%, N: 0.0010-0.0400%, Al: 0.01-0.40%, O: 0.0004 to 0.0100%; One or more selected from Nb and Ta, Sn: 0 to 0.010%, The balance is Ni and impurities, A Ni-based alloy seamless pipe that satisfies the following formulas (i) and (ii): The Ni-based alloy seamless pipe has an arithmetic mean roughness Ra of 7.0 μm or less in the longitudinal direction of the pipe on the inner surface side of the Ni-based alloy seamless pipe. 0.0010≦S+2O+0.2Sn≦0.0180...(i) 2.50≦Nb+Ta≦4.60...(ii) However, the element symbols in the above formula represent the content (mass %) of each element contained in the Ni-based alloy, and if the element is not contained, it is set to zero.

2. The chemical composition is, in mass%, replacing a part of the Ni, The Ni-based alloy seamless pipe according to claim 1, containing Fe: 5.50% or less.

3. The chemical composition contains one or more selected from Cu and Co in place of a part of the Ni. The Ni-based alloy seamless pipe according to claim 1 or 2, which contains Zn and satisfies the following formula (iii): 0.01≦Cu+Co≦1.50...(iii) However, the element symbols in the above formula represent the content (mass%) of each element contained in the Ni-based alloy. If not contained, it is set to zero.

4. The chemical composition is, in mass%, replacing a part of the Ni, W: 1.00% or less, V: 0.40% or less, Ca: 0.0030% or less, Mg: 0.0030% or less, B: 0.0100% or less, and REM: 0.0100% or less, The Ni-based alloy seamless pipe according to any one of claims 1 to 3, comprising one or more selected from the following:

5. A welded joint using the Ni-based alloy seamless pipe according to any one of claims 1 to 4.

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