Austenitic stainless steel welded joint

Abstract

Provided is an austenitic stainless steel welded joint having excellent polythionate SCC resistance, excellent naphthenic acid corrosion resistance, and excellent creep ductility. An austenitic stainless steel welded joint (1) is provided with a base material (10) and a weld metal (20). The welding metal (20) has a chemical composition of 0.050% or less by mass of C, 0.01-1.00% by mass of Si, 0.01-3.00% by mass of Mn, 0.030% or less by mass of P, 0.015% or less by mass of S, 15.0-25.0% by mass of Cr, 20.0-70.0% by mass of Ni, 1.30-10.00% by mass of Mo, 0.05-3.00% by mass of Nb, 0.150% or less by mass of N, 0.0050% or less by mass of B, and the balance of Fe and impurities at the center of the width of the welding metal (20) and at the center of the thickness of the welding metal (20).

Description

[0001] This application is a divisional application of an application with the application number 201980015417.6, the title of "Austenitic Stainless Steel Welded Joint", and the filing date of 28 February 2019. TECHNICAL FIELD

[0002] The present application relates to a welded joint, and more particularly, to an austenitic stainless steel welded joint. BACKGROUND

[0003] An austenitic stainless steel welded joint is manufactured by welding an austenitic stainless steel material, and contains a base material formed of an austenitic stainless steel and a weld metal. The austenitic stainless steel welded joint can be used for a welded structure of a chemical plant equipment such as a thermal power boiler, a petroleum refining, and a petrochemical equipment. The welded structure of the chemical plant equipment is, for example, a peripheral equipment of a distillation column, a heating furnace tube, a reaction tube, a heat exchanger, a pipe, and the like. Among the components used in these welded structures of the chemical plant equipment, there are components that are used at a high temperature of 600 to 700°C and in an environment containing a corrosive fluid containing sulfides and / or chlorides. In this specification, the high temperature environment of 600 to 700°C and the environment containing the corrosive fluid containing sulfides and / or chlorides are referred to as a "high temperature corrosive environment".

[0004] The welded structure used in the high temperature corrosive environment is stopped from working at the time of periodic inspection of the chemical plant. At the time of work stoppage, the temperature of the welded structure drops to normal temperature. At this time, air, moisture, and sulfide oxide scale react to generate polythionic acid on the surface of the components of the welded structure. This polythionic acid induces stress corrosion cracking at the grain boundaries (hereinafter, referred to as polythionic acid SCC). Therefore, excellent polythionic acid SCC resistance is sought for the components used in the above-described high temperature corrosive environment.

[0005] A steel having improved polythionic acid SCC resistance is proposed in Japanese Patent Application Publication No. 2003-166039 (Patent Literature 1) and International Publication No. 2009 / 044802 (Patent Literature 2). Polythionic acid SCC is generated due to the precipitation of Cr as M 23 C6-type carbide at the grain boundaries, and the formation of a Cr-deficient layer in the vicinity of the grain boundaries. Therefore, in Patent Literature 1 and Patent Literature 2, the amount of C is reduced, and the generation of M 23 C6-type carbide is suppressed, and the polythionic acid SCC resistance is improved.

[0006] Specifically, the austenitic heat-resistant steel disclosed in Patent Literature 1 contains, in mass%, C: 0.005% or more and less than 0.03%, Si: 0.05 to 0.4%, Mn: 0.5 to 2%, P: 0.01 to 0.04%, S: 0.0005 to 0.005%, Cr: 18 to 20%, Ni: 7 to 11%, Nb: 0.2 to 0.5%, V: 0.2 to 0.5%, Cu: 2 to 4%, N: 0.10 to 0.30%, B: 0.0005 to 0.0080%, and the balance of Fe and unavoidable impurities. The total content of Nb and V is 0.6% or more, and the Nb solid solution content in the steel is 0.15% or more. Further, N / 14 is satisfied by Nb / 93 + V / 51, and Cr - 16C - 0.5Nb - V is satisfied by 17.5 or more. In Patent Literature 1, the C content is reduced, and the relationship between Cr and C, Nb, and V is specified, thereby improving the resistance to SCC in conjunction with dithionite acid.

[0007] The austenitic stainless steel disclosed in Patent Literature 2 contains, in mass%, C: less than 0.04%, Si: 1.5% or less, Mn: 2% or less, Cr: 15 to 25%, Ni: 6 to 30%, N: 0.02 to 0.35%, Sol. Al: 0.03% or less, further contains one or two or more kinds selected from the group consisting of Nb: 0.5% or less, Ti: 0.4% or less, V: 0.4% or less, Ta: 0.2% or less, Hf: 0.2% or less, and Zr: 0.2% or less, and the balance of Fe and impurities. Among the impurities, P: 0.04% or less, S: 0.03% or less, Sn: 0.1% or less, As: 0.01% or less, Zn: 0.01% or less, Pb: 0.01% or less, and Sb: 0.01% or less. Further, F1 = S + {(P + Sn) / 2} + {(As + Zn + Pb + Sb) / 5} is satisfied by 0.075 or less, and 0.05 ≤ Nb + Ta + Zr + Hf + 2Ti + (V / 10) ≤ 1.7 - 9xF1 is satisfied. In Patent Literature 2, the C content is made less than 0.05%, thereby improving the resistance to SCC in conjunction with dithionite acid. Further, by reducing the C-fixing elements such as Nb and Ti, the grain boundary embrittlement elements such as P, S, and Sn in the steel are reduced, thereby improving the resistance to embrittlement cracking at the weld heat affected zone (HAZ).

[0008] Prior Art Documents

[0009] Patent Literature

[0010] Patent Literature 1: Japanese Patent Application Publication No. 2003-166039

[0011] Patent Literature 2: International Publication No. 2009 / 044802 SUMMARY

[0012] Problem to be solved by the invention

[0013] As a result, in chemical plant equipment, there are cases where inferior crude oil is used; not only SCC corrosion by multi-sulfuric acid occurs, but also naphthenic acid corrosion occurs. Naphthenic acid is a cyclic saturated hydrocarbon having one or more carboxyl groups. Naphthenic acid does not cause SCC like multi-sulfuric acid, but causes general corrosion. Therefore, in the welded joint for the above-mentioned plant equipment, it is preferable that not only SCC resistance to multi-sulfuric acid be excellent, but also naphthenic acid corrosion resistance be excellent.

[0014] Further, recently, in components used in a high-temperature corrosion environment of 600 to 700°C, high creep ductility is sought. In chemical plant equipment, as described above, there are cases where the equipment is stopped to perform periodic inspection. In the periodic inspection, components that must be replaced in the welded structure of the chemical plant equipment are investigated. At this time, if the creep ductility is high, the degree of deformation of the components can be confirmed at the time of periodic inspection, and can be used as a criterion for judging replacement of the components.

[0015] In Patent Literature 1 and Patent Literature 2, although improvement in SCC resistance to multi-sulfuric acid is aimed at, naphthenic acid corrosion resistance is not researched, and further, improvement in creep ductility is not researched. Furthermore, research on SCC resistance to multi-sulfuric acid and naphthenic acid corrosion resistance of the welded joint including not only the base material but also the welded metal is not performed.

[0016] An object of the present application is to provide an austenitic stainless steel welded joint in which SCC resistance to multi-sulfuric acid and naphthenic acid corrosion resistance are excellent, and in which the creep ductility of the base material is also excellent.

[0017] Solution for solving the problem

[0018] The austenitic stainless steel welded joint according to the present application has a base material and a welded metal,

[0019] The chemical composition of the base material is, in mass%,

[0020] C: 0.030% or less,

[0021] Si: 0.10 to 1.00%,

[0022] Mn: 0.20 to 2.00%,

[0023] P: 0.040% or less,

[0024] S: 0.010% or less,

[0025] Cr: 16.0 to 25.0%,

[0026] Ni: 10.0 to 30.0%,

[0027] Mo: 0.10 to 5.00%,

[0028] Nb: 0.20 to 1.00%,

[0029] N: 0.050 to 0.300%,

[0030] sol. Al: 0.001 to 0.100%,

[0031] B: 0.0010 to 0.0080%,

[0032] Cu: 0 to 5.00%,

[0033] W: 0 to 5.0%,

[0034] Co: 0 to 1.0%,

[0035] V: 0 to 1.00%,

[0036] Ta: 0 to 0.20%,

[0037] Hf: 0 to 0.20%,

[0038] Ca: 0 to 0.010%,

[0039] Mg: 0 to 0.010%,

[0040] rare earth elements: 0 to 0.100%, and

[0041] balance: Fe and impurities, and

[0042] satisfies formula (1);

[0043] in the weld metal,

[0044] the chemical composition at the width center position and the thickness center position of the weld metal is, in mass%,

[0045] C: 0.050% or less,

[0046] Si: 0.01 to 1.00%,

[0047] Mn: 0.01 to 3.00%,

[0048] P: 0.030% or less,

[0049] S: 0.015% or less,

[0050] Cr: 15.0 to 25.0%,

[0051] Ni: 20.0 to 70.0%,

[0052] Mo: 1.30 to 10.00%,

[0053] Nb: 0.05 to 3.00%,

[0054] N: 0.150% or less,

[0055] B: 0.0050% or less,

[0056] sol. Al: 0 to 1.000%,

[0057] Cu: 0 to 2.50%,

[0058] W: 0 to 1.0%,

[0059] Co: 0 to 15.0%,

[0060] V: 0 to 0.10%,

[0061] Ti: 0 to 0.50%,

[0062] Ta: 0 to 0.20%,

[0063] Ca: 0 to 0.010%,

[0064] Mg: 0 to 0.010%,

[0065] rare earth elements: 0 to 0.100%, and

[0066] balance: Fe and impurities.

[0067] B + 0.004 - 0.9C + 0.017Mo 2 ≥ 0 (1)

[0068] wherein the content (mass%) of the corresponding element is substituted for the element symbol in formula (1).

[0069] Effects of the invention

[0070] The austenitic stainless steel welded joint according to the present application is excellent in resistance to SCC in conjunction with multivalent sulfuric acid and in resistance to naphthenic acid corrosion, and is also excellent in creep ductility of the base material. BRIEF DESCRIPTION OF DRAWINGS

[0071] Figure 1 is a plan view showing an example of the austenitic stainless steel welded joint according to the present embodiment.

[0072] Figure 2 is a cross-sectional view showing the austenitic stainless steel welded joint according to Figure 1 cut in the width direction of the weld metal.

[0073] Figure 3 is a cross-sectional view showing the austenitic stainless steel welded joint according to Figure 1A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal.

[0074] Figure 4 A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal. Figure 3

[0075] A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal. Figure 5

[0076] A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal. Figure 6

[0077] A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal. Figure 7 Figure 6 A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal.

[0078] Figure 8 A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal.

[0079] Figure 9 A cross-sectional view of the austenitic stainless steel welded joint in the direction L perpendicular to the extension direction of the weld metal.

[0080] The present inventors have studied a welded joint which is not only excellent in SCC resistance to condensed sulfuric acid, but also excellent in SCC resistance to naphthenic acid, and which is also excellent in creep ductility of the base material.

[0081] If the C content of the base material is reduced to 0.030% or less, the generation of M 23 C6 carbide is suppressed, and a Cr-deficient layer is suppressed from being generated near the grain boundaries. In the base material of the austenitic stainless steel welded joint of the present embodiment, 0.20 to 1.00% of Nb is further contained, thereby fixing C with Nb, and further reducing the solid solution C amount which is a factor of the generation of M 23 C6 carbide. In the base material of the austenitic stainless steel welded joint of the present embodiment, 0.10 to 5.00% of Mo is further contained. Mo suppresses the generation of M 23 C6 carbide. Thus, the generation of a Cr-deficient layer is reduced. According to the above countermeasures, the SCC resistance to condensed sulfuric acid, which is one type of stress corrosion cracking, can be improved.

[0082] ​​Further, the Mo is effective against naphthenic acid corrosion. The Cu is effective against general corrosion such as sulfuric acid corrosion. However, the naphthenic acid is a cyclic saturated hydrocarbon having a carboxyl group as described above, and even if the Cu is contained, the naphthenic acid corrosion resistance is not high. On the other hand, the Mo is extremely effective against the naphthenic acid corrosion. The Mo bonds with S in the high-temperature corrosion environment of the austenitic stainless steel welded joint, and forms a sulfide film on the surface of the austenitic stainless steel welded joint. The sulfide film improves the naphthenic acid corrosion resistance. Therefore, in the chemical composition of the base material, if the C content is set to 0.030% or less, the Nb content is set to 0.20 to 1.00%, and the Mo content is set to 0.10 to 5.00%, the multivalent sulfuric acid SCC resistance is high, and the naphthenic acid corrosion resistance is also high.

[0083] However, the present inventors have investigated, and as a result, it has been clarified that if the C content of the base material is reduced to 0.030% or less, the creep ductility of the base material in the high-temperature corrosion environment of 600 to 700°C is reduced. As a reason therefor, the following matters are considered. The precipitates generated at the grain boundaries increase the grain boundary strength. If the grain boundary strength increases, the creep ductility of the base material increases. However, if the C content is reduced to 0.030% or less, the precipitates (carbides, etc.) generated at the grain boundaries are also reduced. As a result, it is considered that the grain boundary strength is difficult to obtain, and the creep ductility of the base material is reduced.

[0084] Therefore, the present inventors have further investigated an austenitic stainless steel welded joint having excellent multivalent sulfuric acid SCC resistance, excellent naphthenic acid corrosion resistance, and excellent creep ductility of the base material. The present inventors have focused on B (boron) as an element contained in the base material. The present inventors have considered that B (boron) segregates at the grain boundaries in the high-temperature corrosion environment of 600 to 700°C described above, and can increase the grain boundary strength.

[0085] As a result of further studies, the present inventors and others have found that, if the chemical composition of the base metal of the austenitic stainless steel welded joint, in terms of mass%, is one containing C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.10 to 5.00%, Nb: 0.20 to 1.00%, N: 0.050 to 0.300%, sol. Al: 0.001 to 0.100%, B: 0.0010 to 0.0080%, Cu: 0 to 5.00%, W: 0 to 5.0%, Co: 0 to 1.0%, V: 0 to 1.00%, Ta: 0 to 0.20%, Hf: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, and rare earth elements: 0 to 0.100%, with the balance being Fe and impurities, then not only excellent resistance to SCC in conjunction with dithionic acid and excellent resistance to naphthenic acid corrosion can be obtained, but also excellent creep ductility of the base metal can be obtained.

[0086] However, investigating the resistance to SCC in conjunction with dithionic acid, the resistance to naphthenic acid corrosion, and the creep ductility of the base metal of the austenitic stainless steel welded joint having the above chemical composition of the base metal, it was found that, although excellent resistance to SCC in conjunction with dithionic acid and excellent resistance to naphthenic acid corrosion were obtained, there were cases in which excellent creep ductility of the base metal was not obtained. Therefore, the present inventors and others further conducted studies. As a result, it was found that, with respect to the creep ductility of the base metal, the following mechanism was considered.

[0087] As described above, in the present embodiment, in order to improve the resistance to SCC in conjunction with dithionic acid and the resistance to naphthenic acid corrosion, not only is the C content made 0.030% or less, but also 0.20 to 1.00% of Nb is contained, C is fixed to the Nb, and the solid solution C is reduced. Specifically, the Nb, due to the solution treatment or aging under a short time, bonds with the C and precipitates in the form of MX-type carbonitride. However, the austenitic stainless steel welded joint of the present embodiment is used under a high-temperature corrosion environment (a corrosion environment of 600 to 700°C) for a long time (at least 3000 hours or more). In such an environment, the MX-type carbonitride is a metastable phase. Therefore, when the base metal having the above chemical composition is used under a high-temperature corrosion environment of 600 to 700°C for a long time, the MX-type carbonitride of the Nb changes to Z phase (CrNbN) and M 23 C6-type carbide. At this time, the B that is segregated at the grain boundaries is replaced with M 23 C of a part of the C of the C6-type carbide, and is absorbed by M 23 C6-type carbide. Therefore, the amount of B that is segregated at the grain boundaries is reduced, and the grain boundary strength is reduced. As a result, it was found that sufficient creep ductility cannot be obtained.

[0088] Therefore, in the use of the austenite stainless steel welded joint at a high temperature corrosion environment of 600 to 700°C for a long time, a further study was made on the method for suppressing the decrease in the segregation B amount at the grain boundary. As a result, it was found that the following mechanism was considered.

[0089] Mo, as described above, inherently suppresses M 23 C6-type carbide. Mo is further present to replace M 23 C6-type carbide, and M 23 C6-type carbide. In the present specification, M 23 C6-type carbide" in which Mo is solid-solved in M 23 C6-type carbide". Mo is solid-solved in M 23 C6-type carbide. Therefore, in the use of the austenite stainless steel welded joint at a high temperature corrosion environment, even if the MX-type carbonitride containing Nb changes to Z phase and M 23 C6-type carbide, if M 23 C6-type carbide is Mo solid-solved M 23 C6-type carbide, the solid-solution of B into M 23 C6-type carbide can be suppressed, and the decrease in the segregation B amount at the grain boundary can be suppressed. As a result, it was found that excellent resistance to SCC by condensate sulfuric acid, excellent resistance to naphthenic acid corrosion, and excellent creep ductility can be obtained.

[0090] Therefore, in the austenite stainless steel welded joint having the base material having the above chemical composition, a further study was made on the base material having a chemical composition in which, even if the MX-type carbonitride containing Nb changes to Z phase and M 23 C6-type carbide, Mo solid-solved M 23 C6-type carbide is generated, so that the decrease in the segregation B amount at the grain boundary can be suppressed. As a result, it was found that there is a close relationship among B, C, and Mo in the above chemical composition with respect to the decrease in the segregation B amount caused by the generation of Mo solid-solved M 23 C6-type carbide. And, it was found that, in the above chemical composition of the base material, if B, C, and Mo satisfy formula (1), even in the use at a high temperature corrosion environment of 600 to 700°C, excellent resistance to SCC by condensate sulfuric acid, excellent resistance to naphthenic acid corrosion, and excellent creep ductility of the base material can be obtained.

[0091] B + 0.004 - 0.9C + 0.017Mo 2 ≥ 0 (1)

[0092] Here, the content (mass %) of the corresponding element is substituted at each element symbol of formula (1).

[0093] The present inventors further studied, and as a result, it was known that in the case where the base material of the above-described austenite-based stainless steel welded joint contains Cu as an arbitrary element, if the Cu content is 5.00% or less, excellent creep strength is obtained, and the creep ductility can also be maintained, and if the upper limit of the Cu content is set to 1.70% or less, the creep strength can be further improved, and the creep ductility of the base material is further maintained. As a reason therefor, the following matters are considered. Cu precipitates within the grains in use in a high-temperature corrosion environment, and forms a Cu phase. There is a case where the Cu phase exists, and although the creep strength is improved, the creep ductility is reduced. Therefore, in the base material of the welded joint having the above-described chemical composition and satisfying Formula (1), it is more preferable that the Cu content be 1.70% or less. If the Cu content is 1.70% or less, the excellent creep ductility can be more effectively maintained.

[0094] The present inventors further studied, and as a result, it was known that if the Mo content of the chemical composition of the base material of the austenite-based stainless steel welded joint is set to 0.50% or more, the creep ductility of the base material is further improved. The reason therefor is not certain, but the following matters are considered. In the base material of the austenite-based stainless steel welded joint having the above-described chemical composition satisfying Formula (1), in the case where the Mo content is further set to 0.50% or more, in use in a high-temperature corrosion environment of 600 to 700°C, Mo is further segregated at the grain boundaries or forms intermetallic compounds. Due to this grain boundary segregation, intermetallic compounds, the grain boundary strength is further improved. As a result thereof, the creep ductility is further improved. In particular, if the Mo content of the base material is 1.00% or more, the base material can obtain very excellent creep ductility.

[0095] The inventors have also found that, in an austenitic stainless steel welded joint, in the weld metal, if the chemical composition at the position of the center in the width and the center in the thickness of the weld metal is set to C: 0.050% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 3.00%, P: 0.030% or less, S: 0.015% or less, Cr: 15.0 to 25.0%, Ni: 20.0 to 70.0%, Mo: 1.30 to 10.00%, Nb: 0.05 to 3.00%, N: 0.150% or less, B: 0.0050% or less, sol. Al: 0 to 1.000%, Cu: 0 to 2.50%, W: 0 to 1.0%, Co: 0 to 15.0%, V: 0 to 0.10%, Ti: 0 to 0.50%, Ta: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.100%, and the balance being Fe and impurities in mass%, then as the welded joint, excellent resistance to SCC in conjunction with dithionic acid and excellent resistance to naphthenic acid corrosion are obtained, and furthermore excellent weldability is obtained.

[0096] Furthermore, it was found that the above chemical composition of the weld metal is preferably such that, if formula (2) is satisfied, the austenitic stainless steel welded joint has excellent resistance to corrosion in conjunction with dithionic acid and naphthenic acid, and the base metal has excellent creep ductility, and furthermore the toughness of the weld metal after high-temperature aging is high.

[0097] 0.012Cr - 0.005Ni + 0.013Mo + 0.023Nb + 0.02Al - 0.004Co ≤ 0.176 (2)

[0098] wherein the content (mass%) of the corresponding element is substituted for the element symbol in formula (2).

[0099] The gist of the austenitic stainless steel welded joint of the present embodiment, which is completed based on the above insight, is as described below.

[0100] The austenitic stainless steel welded joint of [1] has a base metal and a weld metal,

[0101] The chemical composition of the aforementioned base metal is, in mass%,

[0102] C: 0.030% or less,

[0103] Si: 0.10 to 1.00%,

[0104] Mn: 0.20 to 2.00%,

[0105] P: 0.040% or less,

[0106] S: 0.010% or less,

[0107] Cr: 16.0 to 25.0%,

[0108] Ni: 10.0 to 30.0%,

[0109] Mo: 0.10 to 5.00%,

[0110] Nb: 0.20 to 1.00%,

[0111] N: 0.050 to 0.300%,

[0112] sol. Al: 0.001 to 0.100%,

[0113] B: 0.0010 to 0.0080%,

[0114] Cu: 0 to 5.00%,

[0115] W: 0 to 5.0%,

[0116] Co: 0 to 1.0%,

[0117] V: 0 to 1.00%,

[0118] Ta: 0 to 0.20%,

[0119] Hf: 0 to 0.20%,

[0120] Ca: 0 to 0.010%,

[0121] Mg: 0 to 0.010%,

[0122] rare earth elements: 0 to 0.100%, and

[0123] the balance: Fe and impurities, and

[0124] satisfies formula (1);

[0125] in the weld metal,

[0126] the chemical composition at the width center position and the thickness center position of the weld metal is, in mass%,

[0127] C: 0.050% or less,

[0128] Si: 0.01 to 1.00%,

[0129] Mn: 0.01 to 3.00%,

[0130] P: 0.030% or less,

[0131] S: 0.015% or less,

[0132] Cr: 15.0-25.0%,

[0133] Ni: 20.0-70.0%,

[0134] Mo: 1.30-10.00%,

[0135] Nb: 0.05-3.00%,

[0136] N: 0.150% or less,

[0137] B: 0.0050% or less,

[0138] sol. Al: 0-1.000%,

[0139] Cu: 0-2.50%,

[0140] W: 0-1.0%,

[0141] Co: 0-15.0%,

[0142] V: 0-0.10%,

[0143] Ti: 0-0.50%,

[0144] Ta: 0-0.20%,

[0145] Ca: 0-0.010%,

[0146] Mg: 0-0.010%,

[0147] rare earth elements: 0-0.100%, and

[0148] balance: Fe and impurities.

[0149] B + 0.004-0.9C + 0.017Mo 2 ≥ 0 (1)

[0150] wherein the content (mass%) of the corresponding element is substituted for the element symbol in formula (1).

[0151] Here, the width center position of the weld metal means the central position of the length (width) of the weld metal in the width direction perpendicular to the extending direction of the weld metal. The thickness center position of the weld metal means, in the cross section perpendicular to the extending direction of the weld metal, the position of the weld metal at a depth of t / 2 from the surface of the weld metal when the thickness of the weld metal is defined as tmm.

[0152] For the austenitic stainless steel welded joint of the present embodiment, the content of each element of the chemical composition of the base material is within the above range and satisfies the formula (1), and furthermore, the content of each element of the chemical composition at the central position in the width and the central position in the thickness of the weld metal is within the above range. Therefore, the austenitic stainless steel welded joint of the present embodiment is excellent in the resistance to SCC by condensate sulfuric acid and excellent in the resistance to naphthenic acid corrosion. Furthermore, the base material has excellent creep ductility under a high-temperature corrosion environment at 600 to 700°C.

[0153] The austenitic stainless steel welded joint of [2] is the austenitic stainless steel welded joint described in [1], wherein

[0154] The aforementioned chemical composition of the aforementioned base material contains one element or two or more elements selected from the group consisting of

[0155] Cu: 0.10 to 5.00%,

[0156] W: 0.1 to 5.0%,

[0157] Co: 0.1 to 1.0%,

[0158] V: 0.10 to 1.00%,

[0159] Ta: 0.01 to 0.20%,

[0160] Hf: 0.01 to 0.20%,

[0161] Ca: 0.001 to 0.010%,

[0162] Mg: 0.001 to 0.010%, and

[0163] rare earth element: 0.001 to 0.100%.

[0164] The austenitic stainless steel welded joint of [3] is the austenitic stainless steel welded joint described in [1] or [2], wherein

[0165] The aforementioned chemical composition of the aforementioned weld metal contains one element or two or more elements selected from the group consisting of

[0166] sol. Al: 0.001 to 1.000%,

[0167] Cu: 0.01 to 2.50%,

[0168] W: 0.1 to 1.0%,

[0169] Co: 0.1 to 15.0%,

[0170] V: 0.01 to 0.10%,

[0171] Ti: 0.01 to 0.50%,

[0172] Ta: 0.01 to 0.20%,

[0173] Ca: 0.001 to 0.010%,

[0174] Mg: 0.001 to 0.010%, and

[0175] rare earth element: 1 element or 2 or more elements in the group consisting of 0.001 to 0.100%.

[0176] The austenitic stainless steel welded joint of [4] is the austenitic stainless steel welded joint described in any one of [1] to [3], wherein

[0177] The aforementioned chemical composition of the aforementioned weld metal satisfies Equation (2).

[0178] 0.012Cr - 0.005Ni + 0.013Mo + 0.023Nb + 0.02Al - 0.004Co ≤ 0.176 (2)

[0179] wherein, in Equation (2), the content (mass%) of the corresponding element is substituted at the element symbol.

[0180] At this time, the austenitic stainless steel welded joint is excellent in corrosion resistance to dithionic acid and in corrosion resistance to naphthenic acid, and the base material is excellent in creep ductility, and furthermore, the weld metal is excellent in toughness after high-temperature aging.

[0181] Hereinafter, the austenitic stainless steel welded joint of the present embodiment will be described in detail. In the present specification, "%" of an element means mass% unless otherwise specified.

[0182] [Composition of the austenitic stainless steel welded joint]

[0183] Figure 1 is a plan view showing one example of the austenitic stainless steel welded joint 1 of the present embodiment. In the drawing, the same reference numerals are given to the same components as those in FIG. 1, and the description thereof will not be repeated. Figure 1The austenitic stainless steel welded joint 1 according to the present embodiment includes a base material 10 and a weld metal 20. The end portions of the pair of base materials 10 after being beveled are joined to each other, and then welding is performed to form the weld metal 20. The welding is, for example, Gas Tungsten Arc Welding (GTAW), Shielded Metal Arc Welding (SMAW), Flux Cored Arc Welding (FCAW), Gas Metal Arc Welding (GMAW), or Submerged Arc Welding (SAW).

[0184] In Figure 1 , the direction in which the weld metal 20 extends is defined as a weld metal extension direction L, the direction perpendicular to the weld metal extension direction L in plan view is defined as a weld metal width direction W, and the direction perpendicular to both the weld metal extension direction L and the weld metal width direction W is defined as a weld metal thickness direction T. Figure 2 The austenitic stainless steel welded joint 1 according to the present embodiment is cut in the weld metal width direction W to obtain a cross-sectional view of the Figure 1 . As shown in Figure 1 and Figure 2 , the weld metal 20 is disposed between the pair of base materials 10.

[0185] Figure 3 The austenitic stainless steel welded joint 1 according to the present embodiment is cut in the weld metal extension direction L to obtain a cross-sectional view of the Figure 1 , and Figure 4 The austenitic stainless steel welded joint 1 according to the present embodiment is cut in the weld metal extension direction L to obtain a cross-sectional view of the Figure 3 different from the Figure 3 . As shown in Figure 4 , the shape of the base material 10 can be a plate, or as shown in , the shape of the base material 10 can also be a steel pipe. Although not shown, the shape of the base material 10 can also be a bar steel or a profile steel. Hereinafter, the base material 10 and the weld metal 20 are described.

[0186] [Chemical Composition]

[0187] [Chemical Composition] [Chemical Composition]

[0188] The chemical composition of the base material 10 of the austenitic stainless steel welded joint according to the present embodiment contains the following elements.

[0189] C: 0.030% or less

[0190] It inevitably contains carbon (C). That is, the C content exceeds 0%. In the use of the austenitic stainless steel welded joint of this embodiment in a high-temperature corrosion environment of 600-700°C, C generates M at the grain boundaries in the base metal. 23 C6 type carbides reduce the SCC resistance of the parent material 10. Therefore, the C content is 0.030% or less. The preferred upper limit of the C content is 0.020%, more preferably 0.018%, more preferably 0.016%, and more preferably 0.015%. The C content is preferably as low as possible. However, excessive reduction of the C content will increase manufacturing costs. Therefore, in industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.002%.

[0191] Si: 0.10~1.00%

[0192] Silicon (Si) deoxidizes the steel. Si further improves the oxidation resistance and water vapor oxidation resistance of the base material 10. If the Si content is too low, these effects cannot be achieved. On the other hand, if the Si content is too high, a sigma phase (σ phase) precipitates in the base material 10, reducing the steel's toughness. Therefore, the Si content is 0.10–1.00%. The preferred lower limit of the Si content is 0.15%, more preferably 0.17%, more preferably 0.18%, more preferably 0.20%, and more preferably 0.25%. The preferred upper limit of the Si content is 0.75%, more preferably 0.70%, more preferably 0.50%, and more preferably 0.45%.

[0193] Mn: 0.20~2.00%

[0194] Manganese (Mn) deoxidizes the steel. Mn further stabilizes the austenite, improving the creep strength of the base metal 10. If the Mn content is too low, the above effect cannot be obtained. On the other hand, if the Mn content is too high, the creep strength of the base metal 10 decreases. Therefore, the Mn content is 0.20–2.00%. The preferred lower limit of the Mn content is 0.30%, more preferably 0.40%, more preferably 0.50%, more preferably 0.60%, more preferably 0.70%, and more preferably 0.80%. The preferred upper limit of the Mn content is 1.80%, more preferably 1.70%, more preferably 1.60%, and more preferably 1.50%.

[0195] P: below 0.040%

[0196] Phosphorus (P) is an unavoidable impurity. That is, the P content exceeds 0%. P lowers the hot workability and toughness of the steel. Therefore, the P content is 0.040% or less. The preferable upper limit of the P content is 0.035%, further preferably 0.032%, further preferably 0.028%, further preferably 0.026%. The P content is preferably as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, in industrial production, the preferable lower limit of the P content is 0.001%, further preferably 0.002%.

[0197] S: 0.010% or less

[0198] Sulfur (S) is an unavoidable impurity. That is, the S content exceeds 0%. S lowers the hot workability and creep ductility of the steel. Therefore, the S content is 0.010% or less. The preferable upper limit of the S content is 0.007%, further preferably 0.006%, further preferably 0.005%. The S content is preferably as low as possible. However, excessive reduction of the S content increases the manufacturing cost. Therefore, in industrial production, the preferable lower limit of the S content is 0.001%.

[0199] Cr: 16.0 to 25.0%

[0200] Chromium (Cr) improves the resistance to condensate SCC and the resistance to naphthenic acid corrosion of the base material 10. If the Cr content is too low, the above effects cannot be obtained. On the other hand, if the Cr content is too high, the creep strength and toughness of the base material 10 are reduced. Therefore, the Cr content is 16.0 to 25.0%. The preferable lower limit of the Cr content is 16.5%, further preferably 17.0%, further preferably 17.2%, further preferably 17.4%. The preferable upper limit of the Cr content is 24.0%, further preferably 23.0%, further preferably 22.0%.

[0201] Ni: 10.0 to 30.0%

[0202] Nickel (Ni) stabilizes the austenite and improves the creep strength of the base material 10. Ni further improves the resistance to condensate SCC and the resistance to naphthenic acid corrosion of the base material. If the Ni content is too low, the above effects cannot be obtained. On the other hand, if the Ni content is too high, the above effects are saturated, and further, the manufacturing cost becomes high. Therefore, the Ni content is 10.0 to 30.0%. The preferable lower limit of the Ni content is 11.0%, further preferably 12.0%, further preferably 13.0%, further preferably 13.5%. The preferable upper limit of the Ni content is 27.0%, further preferably 25.0%, further preferably 22.0%, further preferably 20.0%, further preferably 18.0%, further preferably 17.0%.

[0203] Mo: 0.10–5.00%

[0204] In high-temperature corrosion environments (600–700°C), molybdenum (Mo) inhibits the formation of molybdenum (M) at grain boundaries. 23 C6 type carbides. This improves the resistance of the base metal 10 to polythionic acid (SCC). For Mo, further in the use of austenitic stainless steel welded joints 1 in high-temperature corrosive environments, the Mo dissolved in the base metal 10 bonds with S in the operating environment, forming a sulfide coating on the surface of the base metal 10. Through the formation of this sulfide coating, high resistance to naphthenic acid corrosion is achieved. Furthermore, for Mo, in use in high-temperature corrosive environments at 600–700°C, the MX-type carbonitrides of Nb change to M... 23 When C6 type carbides are used, B is suppressed in M. 23 Solid solution in C6-type carbides reduces the amount of boron segregating at grain boundaries under high-temperature corrosion conditions. This results in sufficient creep ductility under high-temperature corrosion. These effects cannot be achieved if the Mo content is too low. On the other hand, excessively high Mo content reduces the stability of austenite. Therefore, the Mo content is 0.10–5.00%. The preferred lower limit of the Mo content is 0.20%, and more preferably 0.30%.

[0205] If the Mo content is 0.50% or higher, Mo will further segregate at grain boundaries or form intermetallic compounds, thereby further improving grain boundary strength. In this case, superior creep ductility can be obtained under high-temperature corrosion conditions. Therefore, the preferred lower limit for the Mo content is 0.50%, more preferably 0.80%, more preferably 1.00%, and more preferably 2.00%. If the Mo content is 1.00% or higher, the base material 10 can obtain particularly excellent creep ductility. The preferred upper limit for the Mo content is 4.50%, more preferably 4.00%.

[0206] Nb: 0.20~1.00%

[0207] Nb forms MX type carbonitride by bonding with C in a high temperature corrosion environment of 600 to 700°C, and reduces the amount of solid solution C in the base material 10. Thus, the base material 10 has high resistance to SCC by condensed sulfuric acid and high resistance to naphthenic acid corrosion. The MX type carbonitride of Nb formed also improves the creep strength of the base material 10. If the content of Nb is too low, the above effects cannot be obtained. On the other hand, if the content of Nb is too high, δ ferrite is formed, and the creep strength, toughness, and weldability of the base material 10 are reduced. Therefore, the content of Nb is 0.20 to 1.00%. The preferable lower limit of the content of Nb is 0.25%, further preferably 0.28%, further preferably 0.30%, further preferably 0.32%. The preferable upper limit of the content of Nb is 0.90%, further preferably 0.80%, further preferably 0.70%, further preferably 0.65%.

[0208] N: 0.050 to 0.300%

[0209] N is solid-solved in the matrix (parent phase), stabilizes austenite, and improves the creep strength of the base material 10. N further forms fine carbonitride in the grains, and improves the creep strength of the base material 10. That is, N contributes to the creep strength of the base material 10 due to both solid-solution strengthening and precipitation strengthening. If the content of N is too low, the above effects cannot be obtained. On the other hand, if the content of N is too high, Cr nitride is formed at the grain boundaries, and the resistance to SCC by condensed sulfuric acid and the resistance to naphthenic acid corrosion in the weld heat-affected zone (HAZ) of the base material 10 are reduced. If the content of N is too high, the workability of the steel is further reduced. Therefore, the content of N is 0.050 to 0.300%. The preferable lower limit of the content of N is 0.060%, further preferably 0.070%, further preferably 0.080%. The preferable upper limit of the content of N is 0.250%, further preferably 0.200%, further preferably 0.190%.

[0210] sol. Al: 0.001 to 0.100%

[0211] Al deoxidizes the steel. If the content of Al is too low, the above effects cannot be obtained. On the other hand, if the content of Al is too high, the cleanliness of the steel is reduced, and the workability and ductility of the steel are reduced. Therefore, the content of Al is 0.001 to 0.100%. The preferable lower limit of the content of Al is 0.002%, further preferably 0.003%. The preferable upper limit of the content of Al is 0.050%, further preferably 0.030%, further preferably 0.025%. In the present embodiment, the content of Al means the content of acid-soluble Al (sol. Al).

[0212] B: 0.0010 to 0.0080%

[0213] B is used in a high-temperature corrosion environment at 600 to 700°C to segregate at grain boundaries and increase the strength of the grain boundaries. As a result, the creep ductility of the base material 10 is improved in a high-temperature corrosion environment at 600 to 700°C. If the B content is too low, the above effects cannot be obtained. On the other hand, if the B content is too high, the weldability and hot workability at high temperatures are reduced. Furthermore, the B content of the weld metal is high at the time of welding, and solidification cracks are generated in the weld metal 20. Therefore, the B content is 0.0010 to 0.0080%. The preferable lower limit of the B content is 0.0015%, further preferably 0.0018%, further preferably 0.0020%, further preferably 0.0022%. The preferable upper limit of the B content is less than 0.0060%, further preferably 0.0050%.

[0214] The balance of the chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment is Fe and impurities. Here, the impurities refer to substances mixed from ores, waste materials, or manufacturing environments, etc. as raw materials when the above base material 10 is industrially manufactured, and mean substances allowed within a range that does not adversely affect the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment.

[0215] [For arbitrary elements]

[0216] [1st group arbitrary elements]

[0217] The chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment can also contain one element or two or more elements selected from the group consisting of Cu, W, and Co instead of part of Fe. These elements all increase the creep strength of the base material 10.

[0218] Cu: 0 to 5.00%

[0219] Copper (Cu) is an optional element and can be contained or not contained. That is, Cu can be 0%. In the case of being contained, Cu is precipitated in the form of Cu phase in the grain in use at a high temperature corrosion environment of 600 to 700°C, and the creep strength of the base material 10 is improved due to precipitation strengthening. However, if the Cu content is excessively high, the hot workability and weldability of the steel are reduced. Therefore, the Cu content is 0 to 5.00%. In order to more effectively improve the creep strength of the base material 10 of the austenitic stainless steel welded joint 1, the preferable lower limit of the Cu content exceeds 0%, and is further preferably 0.10%, further preferably 0.11%, further preferably 0.12%, further preferably 2.00%, further preferably 2.50%. The preferable upper limit of the Cu content is 4.50%, further preferably 4.00%, further preferably 3.80%, further preferably 3.70%, further preferably 3.60%, further preferably 3.50%, further preferably 1.90%. In particular, in order to maintain more excellent creep ductility at a high temperature corrosion environment of 600 to 700°C, the preferable Cu content is 0 to 1.70%, and the upper limit of the Cu content is further preferably 1.60%.

[0220] W: 0 to 5.0%

[0221] Tungsten (W) is an optional element and can be contained or not contained. That is, W can be 0%. In the case of being contained, W is solid-solved in the matrix (parent phase), and the creep strength of the base material 10 of the austenitic stainless steel welded joint 1 is improved. However, if the W content is excessively high, the stability of the austenite is reduced, and the creep strength and toughness of the base material 10 are reduced. Therefore, the W content is 0 to 5.0%. The preferable lower limit of the W content exceeds 0%, and is further preferably 0.1%, further preferably 0.2%, further preferably 0.5%. The preferable upper limit of the W content is 4.5%, further preferably 4.0%, further preferably 3.5%.

[0222] Co: 0 to 1.0%

[0223] Cobalt (Co) is an optional element and can be contained or not contained. That is, the Co content can be 0%. In the case of being contained, Co stabilizes the austenite, and the creep strength of the base material 10 of the austenitic stainless steel welded joint 1 is improved. However, if the Co content is excessively high, the raw material cost is high. Therefore, the Co content is 0 to 1.0%. The preferable lower limit of the Co content exceeds 0%, and is further preferably 0.1%, further preferably 0.2%, further preferably 0.3%. The preferable upper limit of the Co content is 0.9%, further preferably 0.8%.

[0224] [2nd Group Optional Element]

[0225] The chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment can also contain, instead of a part of Fe, one or two or more elements selected from the group consisting of V, Ta, and Hf. These elements each improve the resistance to SCC of the base material 10 in conjunction with condensed sulfuric acid and the creep strength.

[0226] V: 0 to 1.00%

[0227] Vanadium (V) is an optional element and can not be contained. That is, the V content can be 0%. In the case of being contained, V forms a carbonitride by bonding with C in use in a high-temperature corrosion environment of 600 to 700°C, reduces the solid-solution C, and improves the resistance to SCC of the base material 10 in conjunction with condensed sulfuric acid. The V carbonitride formed also improves the creep strength of the base material 10. However, if the V content is too high, δ-ferrite is formed, and the creep strength, toughness, and weldability of the base material 10 are reduced. Therefore, the V content is 0 to 1.00%. The preferable lower limit of the V content for more effectively improving the resistance to SCC in conjunction with condensed sulfuric acid and the creep strength is more than 0%, and further preferably 0.10%. The preferable upper limit of the V content is 0.90%, and further preferably 0.80%.

[0228] Ta: 0 to 0.20%

[0229] Tantalum (Ta) is an optional element and can not be contained. That is, the Ta content can be 0%. In the case of being contained, Ta forms a carbonitride by bonding with C in use in a high-temperature corrosion environment of 600 to 700°C, reduces the solid-solution C, and improves the resistance to SCC of the base material 10 in conjunction with condensed sulfuric acid. The Ta carbonitride formed also improves the creep strength. However, if the Ta content is too high, δ-ferrite is formed, and the creep strength, toughness, and weldability of the base material 10 are reduced. Therefore, the Ta content is 0 to 0.20%. The preferable lower limit of the Ta content for more effectively improving the resistance to SCC in conjunction with condensed sulfuric acid and the creep strength is more than 0%, and further preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the Ta content is 0.18%, and further preferably 0.16%.

[0230] Hf: 0 to 0.20%

[0231] Hafnium (Hf) is an optional element, and can not be contained. That is, the Hf content can be 0%. In the case of being contained, Hf forms a carbonitride by bonding with C in a high-temperature corrosion environment of 600 to 700°C, reduces solid solution C, and improves the SC resistance of the base material 10 to condensed sulfuric acid. The Hf carbonitride formed also improves the creep strength of the base material 10. However, if the Hf content is too high, δ-ferrite is generated, and the creep strength, toughness, and weldability of the base material 10 are reduced. Therefore, the Hf content is 0 to 0.20%. The preferable lower limit of the Hf content is more than 0%, and further preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the Hf content is 0.18%, and further preferably 0.16%.

[0232] [Group 3 Optional Element]

[0233] The chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment can also contain one or two or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM) instead of a part of Fe. These elements all improve the hot workability and creep ductility of the base material.

[0234] Ca: 0 to 0.010%

[0235] Calcium (Ca) is an optional element, and can not be contained. That is, the Ca content can be 0%. In the case of being contained, Ca fixes O (oxygen) and S (sulfur) in the form of an inclusion, and improves the hot workability and creep ductility of the base material 10. However, if the Ca content is too high, the hot workability and creep ductility of the base material 10 are reduced. Therefore, the Ca content is 0 to 0.010%. The preferable lower limit of the Ca content is more than 0%, and further preferably 0.001%, and further preferably 0.002%. The preferable upper limit of the Ca content is 0.008%, and further preferably 0.006%.

[0236] Mg: 0 to 0.010%

[0237] Magnesium (Mg) is an optional element, and can not be contained. That is, the Mg content can be 0%. In the case of being contained, Mg fixes O (oxygen) and S (sulfur) in the form of an inclusion, and improves the hot workability and creep ductility of the base material 10. However, if the Mg content is too high, the hot workability and creep ductility of the base material 10 are reduced. Therefore, the Mg content is 0 to 0.010%. The preferable lower limit of the Mg content is more than 0%, and further preferably 0.001%, and further preferably 0.002%. The preferable upper limit of the Mg content is 0.008%, and further preferably 0.006%.

[0238] Rare earth element: 0 to 0.100%

[0239] The rare earth element (REM) is any element, and can not be contained. That is, the REM content can be 0%. In the case of being contained, the REM fixes O (oxygen) and S (sulfur) in the form of an inclusion, and improves the hot workability and creep ductility of the base material. However, if the REM content is too high, the hot workability and creep ductility of the base material decrease. Therefore, the REM content is 0 to 0.10%. The preferable lower limit of the REM content is more than 0%, and further preferably 0.001%, and further preferably 0.002%. The preferable upper limit of the REM content is 0.080%, and further preferably 0.060%.

[0240] The REM in the present specification contains at least one or more of Sc, Y, and lanthanoid elements (La of atomic number 57 to Lu of atomic number 71), and the REM content means the total content of these elements.

[0241] [For Formula (1)]

[0242] The chemical composition of the above base material 10 also satisfies Formula (1).

[0243] B + 0.004 - 0.9C + 0.017Mo 2 ≥ 0 (1)

[0244] The content (mass%) of the corresponding element is substituted at each element symbol in Formula (1).

[0245] As described above, in the present embodiment, in order to improve the endurance to condensated sulfuric acid SCC and the endurance to naphthenic acid corrosion, not only is the C content set to 0.030% or less, but also 0.20 to 1.00% of Nb is contained. Thereby, in use in a high temperature corrosion environment of 600 to 700°C, MX type carbonitride of Nb is generated, and the solid solution C amount decreases. However, the MX type carbonitride of Nb is a metastable phase, and therefore, in long time use in the above high temperature corrosion environment, the MX type carbonitride of Nb changes to Z phase and M 23 C6 type carbide. At this time, B segregated at the grain boundary is solid-solved in the M 23 C6 type carbide, and the B segregation amount at the grain boundary decreases. As a result, the creep ductility of the base material 10 decreases.

[0246] However, if Mo is solid-solved in the M 23 C6 type carbide, "Mo solid-solution M 23 C6 type carbide" is generated. In the Mo solid-solution M 23 C6 type carbide, B is not easily solid-solved. Therefore, the B segregation amount at the grain boundary is maintained, and not only excellent endurance to condensated sulfuric acid SCC and endurance to naphthenic acid corrosion are obtained, but also excellent creep ductility of the base material 10 is obtained.

[0247] F1 = B + 0.004 - 0.9C + 0.017Mo is defined.2 F1 is an index indicating the amount of M 23 Mo solid solution M 23 F1 is an index indicating the proportion of C6 type carbide. If F1 is 0 or more, even if a plurality of M 23 Mo solid solution M 23 C6 type carbide, Mo solid solution M 23 C6 type carbide, the amount of segregation B at the grain boundary is maintained. Therefore, the base material 10 can have excellent resistance to SCC of condensed sulfuric acid, excellent resistance to naphthenic acid corrosion, and excellent creep ductility. Therefore, F1 is 0 (0.00000) or more. It is preferable that F1 be 0.00100 or more, further preferably 0.00200 or more, further preferably 0.00400 or more, further preferably 0.00500 or more, further preferably 0.00800 or more, further preferably 0.01000 or more, further preferably 0.02000 or more.

[0248] It is preferable that the chemical composition of the above-described base material contain Cu. In this case, as described above, the upper limit of the Cu content is 1.70% or less. That is, if the creep strength is considered to be improved and excellent creep ductility is further obtained, it is preferable that the Cu content be greater than 0% and 1.70% or less. If the Cu content is 1.70% or less, excellent creep strength can be obtained due to the precipitation strengthening of Cu phase, and excellent creep ductility of the base material is maintained.

[0249] In the chemical composition of the above-described base material, the lower limit of the Mo content is preferably 0.50%. In this case, in the use under a high-temperature corrosion environment of 600 to 700°C, Mo is also segregated at the grain boundary or forms an intermetallic compound. Due to this grain boundary segregation, intermetallic compound, the grain boundary strength is further improved. As a result, the creep ductility of the base material is further improved. Therefore, it is preferable that the lower limit of the Mo content be 0.50%. Note that, in the case where the lower limit of the Mo content is 1.00% or more, the creep ductility of the base material is significantly improved. Note that, in the case where the Mo content is 0.50% or more, the preferable F1 value is 0.00500 or more, further preferably 0.00800 or more, further preferably 0.01000 or more, further preferably 0.02000 or more.

[0250] [For the Weld Metal 20]

[0251] [Chemical Composition]

[0252] Figure 5 is a cross-sectional view of the austenitic stainless steel welded joint 1 of the present embodiment perpendicular to the extension direction L of the weld metal. Referring toFigure 5 In a cross-section of the weld metal 20 perpendicular to the extension direction L of the weld metal, the width of the outermost surface of the weld metal 20 is defined as W (mm). The thickness of the weld metal 20 at the central position of the width W is defined as t (mm). The chemical composition of region P, which is the central position of both the width and thickness of the weld metal 20 (i.e., the position at a depth t / 2 from the outermost surface of the weld metal 20), contains the following elements.

[0253] C: Below 0.050%

[0254] It inevitably contains carbon (C). That is, the C content exceeds 0%. In the use of the austenitic stainless steel welded joint 1 of this embodiment in a high-temperature corrosion environment of 600-700°C, C forms M at the grain boundaries in the weld metal 20. 23 C6 type carbides reduce the resistance of weld metal 20 to polysulfides (SCC) and naphthenic acids. Therefore, the C content is 0.050% or less. The preferred upper limit of the C content is 0.040%, more preferably 0.030%, more preferably 0.025%, and even more preferably 0.020%. The C content is preferably as low as possible. However, excessive reduction of the C content will increase manufacturing costs. Therefore, in industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.005%.

[0255] Si: 0.01~1.00%

[0256] Silicon (Si) deoxidizes the weld metal 20 during welding. If the Si content is too low, this effect cannot be achieved. On the other hand, if the Si content is too high, the toughness of the weld metal 20 decreases. Therefore, the Si content is 0.01 to 1.00%. The preferred lower limit of the Si content is 0.02%, more preferably 0.03%, and even more preferably 0.10%. The preferred upper limit of the Si content is 0.80%, more preferably 0.65%, even more preferably 0.40%, and even more preferably 0.35%.

[0257] Mn: 0.01~3.00%

[0258] Manganese (Mn) deoxidizes the weld metal 20 during welding. If the Mn content is too low, this effect cannot be achieved. On the other hand, if the Mn content is too high, the creep strength of the weld metal 20 decreases. Therefore, the Mn content is 0.01% to 3.00%. The preferred lower limit of the Mn content is 0.05%, more preferably 0.08%, more preferably 0.10%, more preferably 0.14%, and more preferably 0.16%. The preferred upper limit of the Mn content is 2.70%, more preferably 2.50%, and more preferably 2.30%.

[0259] P: 0.030% or less

[0260] Phosphorus (P) is an impurity that is inevitably contained. That is, the P content exceeds 0%. P decreases the toughness of the weld metal. P further increases the high-temperature crack sensitivity of the weld metal 20. Therefore, the P content is 0.030% or less. The preferable upper limit of the P content is 0.025%, further preferably 0.020%. The P content is preferably as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, in industrial production, the preferable lower limit of the P content is 0.001%, further preferably 0.002%.

[0261] S: 0.015% or less

[0262] Sulfur (S) is an impurity that is inevitably contained. That is, the S content exceeds 0%. S decreases the ductility of the weld metal and increases the high-temperature crack sensitivity of the weld metal 20. Therefore, the S content is 0.015% or less. The preferable upper limit of the S content is 0.010%, further preferably 0.007%. The S content is preferably as low as possible. However, excessive reduction of the S content increases the manufacturing cost. Therefore, in industrial production, the preferable lower limit of the S content is 0.001%, further preferably 0.002%.

[0263] Cr: 15.0 to 25.0%

[0264] Chromium (Cr) improves the resistance to condensate SCC and the resistance to naphthenic acid corrosion of the weld metal 20. Cr further improves the resistance to oxidation, the resistance to water vapor oxidation, the resistance to high-temperature corrosion, and the like. If the Cr content is too low, the above effects cannot be obtained. On the other hand, if the Cr content is too high, the creep strength and the toughness of the weld metal 20 decrease. Therefore, the Cr content is 15.0 to 25.0%. The preferable lower limit of the Cr content is 16.5%, further preferably 17.0%. The preferable upper limit of the Cr content is 24.0%, further preferably 23.0%.

[0265] Ni: 20.0 to 70.0%

[0266] Nickel (Ni) stabilizes austenite and improves the creep strength of the weld metal 20. Ni further improves the resistance to condensate SCC and the resistance to naphthenic acid corrosion of the weld metal 20. If the Ni content is too low, the above effects cannot be obtained. On the other hand, if the Ni content is too high, the above effects are saturated, and further, the manufacturing cost becomes high. Therefore, the Ni content is 20.0 to 70.0%. The preferable lower limit of the Ni content is 21.0%, further preferably 23.0%, further preferably 25.0%, further preferably 27.0%. The preferable upper limit of the Ni content is 60.0%, further preferably 55.0%, further preferably 50.0%.

[0267] Mo: 1.30~10.00%

[0268] In high-temperature corrosive environments of 600–700°C, molybdenum (Mo) suppresses grain boundary formation in weld metal 20. 23 C6 type carbides. This improves the resistance of weld metal 20 to polythionic acid (SCC). Furthermore, in the use of welded joints in high-temperature corrosive environments, the Mo dissolved in weld metal 20 bonds with S in the environment, forming a sulfide coating on the surface of weld metal 20. Through the formation of this sulfide coating, the resistance of weld metal 20 to naphthenic acid corrosion is improved. For Mo, further in high-temperature corrosive environments at 600–700°C, the MX-type carbonitrides of Nb change to M... 23 When C6 type carbides are used, B is suppressed in M. 23 Solid solution in C6-type carbides reduces the amount of boron segregating at grain boundaries under high-temperature corrosion conditions. This results in sufficient creep ductility under high-temperature corrosion. These effects cannot be achieved if the Mo content is too low. On the other hand, if the Mo content is too high, the stability of austenite decreases. Therefore, the Mo content is 1.30–10.00%. The preferred lower limit of the Mo content is 1.40%, more preferably 1.50%, more preferably 2.00%, more preferably 3.00%, more preferably 4.00%, and more preferably more than 5.00%. The preferred upper limit of the Mo content is 9.00%, more preferably 8.50%.

[0269] Nb: 0.05~3.00%

[0270] When used in high-temperature corrosive environments of 600–700°C, niobium (Nb) bonds with carbon to form MX-type carbonitrides, reducing the amount of dissolved carbon in the weld metal 20. This improves the resistance of the weld metal 20 to polythionite solids (SCC). The formed MX-type carbonitrides of Nb also improve creep strength. However, if the Nb content is too high, δ-ferrite forms, reducing the long-term creep strength, toughness, and weldability of the weld metal 20. Therefore, the Nb content is 0.05–3.00%. The preferred lower limit of the Nb content is 0.06%, more preferably 0.07%, more preferably 0.10%, more preferably 0.15%, more preferably 0.18%, and more preferably 0.20%. The preferred upper limit of the Nb content is 2.90%, more preferably 2.50%, and more preferably 2.00%.

[0271] N: below 0.150%

[0272] N is inevitably contained. That is, the N content exceeds 0%. N is solid-solved in the matrix (parent phase), stabilizes the austenite, and increases the creep strength of the weld metal 20. N also forms fine carbonitrides in the grains, and increases the creep strength of the weld metal 20. That is, N contributes to the creep strength of the weld metal due to both solid-solution strengthening and precipitation strengthening. However, if the N content is too high, Cr nitrides are formed at the grain boundaries, and the resistance to SC C of condensated sulfuric acid and the resistance to naphthenic acid corrosion of the weld metal 20 are reduced. If the N content is too high, the ductility of the weld metal 20 is further reduced. Therefore, the N content is 0.150% or less. The preferable lower limit of the N content is 0.010%, further preferably 0.050%, further preferably 0.080%, further preferably 0.100%. The preferable upper limit of the N content is 0.140%, further preferably 0.130%.

[0273] B: 0.0050% or less

[0274] B is inevitably contained. That is, the B content exceeds 0%. B segregates at the grain boundaries in use in a high-temperature corrosion environment of 600 to 700°C, and increases the grain boundary strength. As a result, the creep ductility of the weld metal 20 is increased in a high-temperature corrosion environment of 600 to 700°C. B further increases the creep strength of the weld metal 20 in a high-temperature corrosion environment of 600 to 700°C. However, if the B content is too high, solidification cracks are generated in the weld metal 20 at the time of welding. Therefore, the B content in the chemical composition of the weld metal 20 is 0.0050% or less. The preferable lower limit of the B content is 0.0001%, further preferably 0.0005%, further preferably 0.0010%, further preferably 0.0015%, further preferably 0.0020%, further preferably 0.0030%. If the B content in the weld metal 20 is 0.0030% or more, the weld joint 1 is particularly excellent in the creep strength. The preferable upper limit of the B content is 0.0045%, further preferably 0.0040%.

[0275] The balance of the chemical composition of the weld metal 20 of the austenitic stainless steel weld joint 1 according to the present embodiment is Fe and impurities. Here, the impurities refer to substances mixed from the welding material as a raw material and the environment at the time of welding at the time of forming the above-described weld metal 20, and are allowed within a range that does not adversely affect the above-described weld metal 20.

[0276] [For arbitrary elements]

[0277] [1st group arbitrary elements]

[0278] The chemical composition of the weld metal 20 of the austenitic stainless steel weld joint 1 according to the present embodiment further contains Al instead of a part of Fe.

[0279] sol. Al: 0 to 1.000%

[0280] Aluminum (Al) is an optional element, and can not be contained. That is, the Al content can be 0%. In the case of being contained, Al deoxidizes the weld metal 20 at the time of welding. However, if the Al content is too high, the ductility of the weld metal 20 decreases. Therefore, the Al content is 0 to 1.000%. The preferable lower limit of the Al content is more than 0%, further preferably 0.001%, further preferably 0.002%, further preferably 0.010%, further preferably 0.050%. The preferable upper limit of the Al content is 0.850%, further preferably 0.800%. In the present embodiment, the Al content means the content of acid-soluble Al (sol. Al).

[0281] [Group 2 Optional Element]

[0282] The chemical composition of the above-described weld metal 20 based on the present embodiment can also contain 1 element or 2 or more elements selected from the group consisting of Cu, W, and Co instead of a part of Fe. Each of these elements increases the creep strength of the weld metal 20.

[0283] Cu: 0 to 2.50%

[0284] Copper (Cu) is an optional element, and can not be contained. That is, the Cu content can be 0%. In the case of being contained, Cu precipitates in the form of a Cu phase within the crystal grains in the use at a high temperature corrosion environment of 600 to 700°C, and increases the creep strength of the weld metal 20 through precipitation strengthening. However, if the Cu content is too high, the weldability of the weld metal 20 decreases, and there is a case where a crack is generated at the time of welding. Therefore, the Cu content is 0 to 2.50%. In order to further effectively increase the creep strength of the weld metal 20, the preferable lower limit of the Cu content is more than 0%, further preferably 0.01%, further preferably 0.05%, further preferably 0.10%, further preferably 0.50%, further preferably 1.00%, further preferably 1.20%. The preferable upper limit of the Cu content is 2.30%, further preferably 2.10%, further preferably 1.90%, further preferably 1.80%, further preferably 1.70%.

[0285] W: 0 to 1.0%

[0286] Tungsten (W) is an optional element and can not be contained. That is, the W content can be 0%. In the case of being contained, W is solid-solved in the weld metal 20 and, in use at a high temperature corrosion environment of 600 to 700°C, improves the creep strength of the weld metal 20. However, if the W content is too high, the stability of austenite decreases and the creep strength and toughness of the weld metal 20 decrease. Therefore, the W content is 0 to 1.0%. The preferable lower limit of the W content is more than 0%, further preferably 0.1%, and further preferably 0.2%. The preferable upper limit of the W content is 0.9%, further preferably 0.7%, and further preferably 0.5%.

[0287] Co: 0 to 15.0%

[0288] Cobalt (Co) is an optional element and can not be contained. That is, the Co content can be 0%. In the case of being contained, Co stabilizes austenite and improves the creep strength of the weld metal 20. However, if the Co content is too high, the raw material cost is high. Therefore, the Co content is 0 to 15.0%. The preferable lower limit of the Co content is more than 0%, further preferably 0.1%, further preferably 1.0%, further preferably 2.0%, and further preferably 2.5%. The preferable upper limit of the Co content is 12.0%, further preferably 11.0%.

[0289] [Group 3 Optional Element]

[0290] The chemical composition of the weld metal 20 of the austenitic stainless steel welded joint 1 according to the present embodiment can further contain one or two or more elements selected from the group consisting of V, Ti, and Ta instead of a part of Fe. These elements all improve the resistance to SCC of the steel to condensate sulfuric acid and the creep strength.

[0291] V: 0 to 0.10%

[0292] Vanadium (V) is an optional element and can not be contained. That is, the V content can be 0%. In the case of being contained, V, in use at a high temperature corrosion environment of 600 to 700°C, bonds with C to generate a carbonitride and reduces solid-solved C, thereby improving the resistance to SCC of the weld metal 20 to condensate sulfuric acid. The generated V carbonitride also improves the creep strength of the weld metal 20. However, if the V content is too high, the toughness and weldability of the weld metal 20 decrease. Therefore, the V content is 0 to 0.10%. The preferable lower limit of the V content is more than 0%, further preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the V content is 0.09%, further preferably 0.08%.

[0293] Ti: 0 to 0.50%

[0294] Titanium (Ti) is an optional element and can not be contained. That is, the Ti content can be 0%. In the case of being contained, Ti forms carbide by bonding with C in use at a high temperature corrosion environment of 600 to 700°C, reduces solid solution C, and improves the multivalent sulfuric acid SCC resistance of the weld metal 20. The Ti carbide formed also improves the creep strength of the weld metal 20. However, if the Ti content is too high, the toughness and weldability of the weld metal 20 are reduced. Therefore, the Ti content is 0 to 0.50%. The preferable lower limit of the Ti content is more than 0%, further preferably 0.01%, further preferably 0.02%, further preferably 0.10%, further preferably 0.15%. The preferable upper limit of the Ti content is 0.45%, further preferably 0.40%.

[0295] Ta: 0 to 0.20%

[0296] Tantalum (Ta) is an optional element and can not be contained. That is, the Ta content can be 0%. In the case of being contained, Ta forms carbonitride by bonding with C in use at a high temperature corrosion environment of 600 to 700°C, reduces solid solution C, and improves the multivalent sulfuric acid SCC resistance of the weld metal 20. The Ta carbonitride formed also improves the creep strength. However, if the Ta content is too high, the creep strength, toughness, and weldability of the weld metal 20 are reduced. Therefore, the Ta content is 0 to 0.20%. In order to further effectively improve the multivalent sulfuric acid SCC resistance and the creep strength, the preferable lower limit of the Ta content is more than 0%, further preferably 0.01%, further preferably 0.02%. The preferable upper limit of the Ta content is 0.18%, further preferably 0.16%.

[0297] [Group 4 Optional Element]

[0298] The chemical composition of the weld metal 20 of the austenitic stainless steel welded joint 1 according to the present embodiment can also contain one or two or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM) instead of a part of Fe. These elements all improve the deformability of the weld metal 20 at high temperatures (hot conditions).

[0299] Ca: 0 to 0.010%

[0300] Calcium (Ca) is an optional element and can not be contained. That is, the Ca content can be 0%. In the case of being contained, Ca fixes O (oxygen) and S (sulfur) in the form of inclusions, and improves the reheat crack resistance of the weld metal 20. However, if the Ca content is too high, the solidification crack resistance of the weld metal 20 is rather reduced. Therefore, the Ca content is 0 to 0.010%. The preferable lower limit of the Ca content is more than 0%, further preferably 0.001%, further preferably 0.002%. The preferable upper limit of the Ca content is 0.008%, further preferably 0.006%.

[0301] Mg: 0 to 0.010%

[0302] Mg is an optional element, and can not be contained. That is, the content of Mg can be 0%. In the case of being contained, Mg fixes O (oxygen) and S (sulfur) in the form of inclusions, and improves the reheat crack resistance of the weld metal 20. However, if the content of Mg is too high, the solidification crack resistance of the weld metal 20 under a hot condition decreases. Therefore, the content of Mg is 0 to 0.010%. The preferable lower limit of the content of Mg is more than 0%, and further preferably 0.001%. The preferable upper limit of the content of Mg is 0.008%, and further preferably 0.006%.

[0303] Rare earth element: 0 to 0.100%

[0304] REM is an optional element, and can not be contained. That is, the content of REM can be 0%. In the case of being contained, REM fixes O (oxygen) and S (sulfur) in the form of inclusions, and improves the reheat crack resistance of the weld metal 20. However, if the content of REM is too high, the solidification crack resistance of the weld metal 20 decreases. Therefore, the content of REM is 0 to 0.100%. The preferable lower limit of the content of REM is more than 0%, and further preferably 0.001%, and further preferably 0.002%. The preferable upper limit of the content of REM is 0.080%, and further preferably 0.060%.

[0305] The austenitic stainless steel welded joint 1 based on the present embodiment has the base material 10 having the above-described chemical composition and the weld metal 20 having the above-described chemical composition. Therefore, the resistance to SCC of the condensed sulfuric acid is excellent, and the resistance to naphthenic acid corrosion is excellent. Furthermore, the base material 10 has excellent creep ductility under a high-temperature corrosion environment at 600 to 700°C.

[0306] [For Equation (2)]

[0307] It is preferable that the chemical composition of the region P at the center of the width and at the center of the thickness of the weld metal 20 further satisfies the following Equation (2).

[0308] 0.012Cr - 0.005Ni + 0.013Mo + 0.023Nb + 0.02Al - 0.004Co ≤ 0.176 (2)

[0309] In Equation (2), the content (mass%) of the corresponding element is substituted at the element symbol.

[0310] F2 = 0.012Cr - 0.005Ni + 0.013Mo + 0.023Nb + 0.02Al - 0.004Co. As described above, in the case where the austenitic stainless steel welded joint 1 of the present embodiment is used as a member of a welded structure of a chemical plant equipment, it is used in a high temperature corrosion environment at 600 to 700°C during operation as described above. Therefore, the weld metal 20 of the austenitic stainless steel welded joint 1 is subjected to high temperature aging. On the other hand, during the stop of the equipment, the temperature of the austenitic stainless steel welded joint 1 drops to room temperature. The weld metal 20 is significantly affected by high temperature aging at 600 to 700°C compared to the base metal 10 due to re-heating during welding and solidification segregation. Therefore, in the austenitic stainless steel welded joint 1 used in the use where corrosion resistance to dithionic acid and corrosion resistance to naphthenic acid are sought, it is preferable that the weld metal 20 has excellent toughness even after high temperature aging.

[0311] F2 is an index of toughness of the weld metal 20 after high temperature aging. As for Ni and Co in F2, in the case where the elements in the chemical composition of the weld metal 20 are within the range of the present embodiment, the toughness of the weld metal 20 is improved. That is, Ni and Co are elements that improve the toughness of the weld metal in the use environment of the austenitic stainless steel welded joint 1 of the present embodiment. On the other hand, Cr, Mo, Nb, and Al all generate precipitates due to high temperature aging at 600 to 700°C. The generation of precipitates reduces the toughness due to an increase in the hardness of the weld metal 20. That is, Cr, Mo, Nb, and Al are elements that reduce the toughness of the weld metal in the use environment of the austenitic stainless steel welded joint 1 of the present embodiment.

[0312] In F2, Ni and Co, which are elements that improve the toughness of the weld metal, are made negative, and Cr, Mo, Nb, and Al, which are elements that reduce the toughness of the weld metal, are made positive. If F2 is greater than 0.176, the proportion of elements that reduce the toughness of the weld metal increases relative to elements that improve the toughness of the weld metal. At this time, the corrosion resistance to dithionic acid and the corrosion resistance to naphthenic acid of the austenitic stainless steel welded joint 1 are excellent, and the creep ductility of the base metal 10 is also excellent, and the toughness of the weld metal 20 after high temperature aging is low. Specifically, the Charpy impact value at room temperature (25°C ± 15°C) of the weld metal 20 of the austenitic stainless steel welded joint 1 of the present embodiment after 3000 hours of aging treatment at 700°C is less than 20 J / cm 2 .

[0313] If F2 is 0.176 or less, the chemical composition of the base material 10 of the austenite stainless steel welded joint 1 is within the range of the present embodiment and satisfies the formula (1), and the chemical composition of the weld metal 20 at the width center position and the thickness center position is within the range of the present embodiment, the corrosion resistance to condensate sulfuric acid and the corrosion resistance to naphthenic acid of the austenite stainless steel welded joint 1 are excellent, the creep ductility of the base material 10 is excellent, and the toughness of the weld metal 20 after high-temperature aging is high. Specifically, the Charpy impact value of the weld metal 20 of the austenite stainless steel welded joint 1 of the present embodiment after aging treatment at 700°C for 3000 hours is 20 J / cm2or more at normal temperature (25°C ± 15°C). 2 The above.

[0314] The preferable upper limit of the F2 value is 0.174, further preferably 0.172, further preferably 0.170, and further preferably 0.165.

[0315] As described above, in the austenite stainless steel welded joint 1 based on the present embodiment, the chemical composition of the base material 10 is within the above range and satisfies the formula (1), and further the chemical composition of the weld metal 20 at the width center position and the thickness center position is within the above range. Therefore, for the austenite stainless steel welded joint 1 of the present embodiment, the corrosion resistance to condensate sulfuric acid SCC of the base material 10 and the weld metal 20 is excellent, and the corrosion resistance to naphthenic acid is excellent. Further, under a high-temperature corrosion environment at 600 to 700°C, the base material 10 has excellent creep ductility. It is preferable that the chemical composition at the width center position and the thickness center position of the weld metal 20 further satisfy the formula (2). At this time, the corrosion resistance to condensate sulfuric acid and the corrosion resistance to naphthenic acid of the austenite stainless steel welded joint 1 are excellent, the creep ductility of the base material 10 is excellent, and the toughness of the weld metal 20 after high-temperature aging is excellent.

[0316] [Manufacturing method]

[0317] An example of the manufacturing method of the austenite stainless steel welded joint 1 of the present embodiment is described. One example of the manufacturing method includes a step of preparing the base material 10 (base material preparation step) and a step of forming the austenite stainless steel welded joint 1 by welding the base material 10 (welding step). Hereinafter, each step is described in detail.

[0318] [Base material preparation step]

[0319] In the mother material preparation step, a mother material 10 having the above chemical composition satisfying formula (1) is prepared. The mother material 10 can be a steel sheet as described above, or can also be a steel pipe. For the mother material 10, a material for implementing the welding step described later can be obtained as an article, or a material for implementing the welding step can also be manufactured.

[0320] In manufacturing the mother material 10, the mother material preparation step includes: a preparation step of preparing a blank; a hot working step of implementing hot working on the blank to manufacture the mother material 10; a cold working step of, as necessary, performing cold working on the mother material 10 after the hot working step; and a solution treatment step of, as necessary, implementing solution treatment on the mother material 10. Hereinafter, the mother material preparation step in the case of manufacturing the mother material 10 will be described.

[0321] [Preparation Step]

[0322] A molten steel having the above chemical composition and satisfying formula (1) is manufactured. For example, an electric furnace, an AOD (Argon Oxygen Decarburization) furnace, or a VOD (Vacuum Oxygen Decarburization) furnace is used to manufacture the above molten steel. For the manufactured molten steel, as necessary, a publicly known degassing treatment is implemented. A blank is manufactured from the molten steel on which the degassing treatment has been implemented. The method of manufacturing the blank is, for example, continuous casting. By the continuous casting, a continuously cast material (blank) is manufactured. The continuously cast material is, for example, a slab, a bloom, and a billet, or the like. The molten steel can be manufactured into an ingot by an ingot casting method.

[0323] [Hot Working Step]

[0324] The prepared blank (continuously cast material or ingot) is subjected to hot working to manufacture the mother material. For example, hot rolling is implemented on the blank to manufacture a steel sheet as the mother material 10. In addition, hot extrusion, hot piercing, or the like is implemented on the blank to manufacture a steel pipe as the mother material 10. The specific method of the hot working is not particularly limited, and hot working according to the shape of the final article can be implemented. The processing termination temperature of the hot working is, for example, 1000°C or higher, and further preferably 1050°C or higher. The processing termination temperature herein means the temperature of the mother material 10 immediately after the final hot working is completed.

[0325] [Cold Working Step]

[0326] Cold working can also be implemented on the mother material after the hot working step, as necessary. In the case where the mother material 10 is a steel pipe, the cold working is, for example, cold drawing or cold rolling. In the case where the mother material 10 is a steel sheet, the cold working is, for example, cold rolling or the like.

[0327] [Solution Treatment Step]

[0328] After the hot working step, or after the cold working step, as necessary, the base material 10 can be subjected to a solution treatment. In the solution treatment step, the microstructure is homogenized, and the carbonitride is dissolved. The solution treatment temperature is preferably as described below.

[0329] Solution treatment temperature: 1000 to 1250°C

[0330] If the solution treatment temperature is 1000°C or higher, the carbonitride of Nb is sufficiently dissolved, and the creep strength is further improved when used in a high-temperature corrosion environment. If the heat treatment temperature is 1250°C or lower, the excessive solution of C can be suppressed, and the resistance to condensate SCC is further improved.

[0331] The holding time at the above solution treatment temperature during the solution treatment is not particularly limited, and is, for example, 2 minutes to 60 minutes.

[0332] Note that, for the base material 10 manufactured by the hot working step, quenching can be performed immediately after the hot working instead of the above solution treatment. In this case, the finish temperature of the hot working is preferably 1000°C or higher. If the finish temperature of the hot working is 1000°C or higher, the carbonitride of Nb is sufficiently dissolved, and excellent resistance to condensate SCC and excellent creep ductility are obtained when used in a high-temperature corrosion environment of 600 to 700°C. Furthermore, sufficient creep strength is also obtained by the generation of the carbonitride of Nb when used in a high-temperature corrosion environment.

[0333] [Step of welding]

[0334] Welding is performed on the prepared base material 10, and the austenitic stainless steel welded joint 1 is manufactured. A groove is formed at the end of the base material 10. Two base materials 10 in which a groove is formed are prepared. The grooves of the prepared base materials 10 are butted against each other. Then, welding is performed on the butted pair of groove portions using a welding material, and the welding metal 20 having the above chemical composition is formed. The chemical composition of the welding material used in the welding, and the chemical composition of the welding metal 20 formed are not particularly limited as long as they are within the above ranges. The welding method is, for example, gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), flux cored arc welding (FCAW), gas metal arc welding (GMAW), or submerged arc welding (SAW).

[0335] The welding material can be melted, for example, in the same manner as the base material 10 described above. At this time, the molten welding material is cast to produce an ingot. The ingot is subjected to hot working to produce the welding material. The welding material after the hot working can be further subjected to cold working. In addition, the welding material can be subjected to a known heat treatment. The heat treatment is, for example, the same solid solution treatment as the base material 10. The heat treatment can not be performed. The welding material can be in a bar shape or in a small piece shape.

[0336] In the welding step, in the case where the base material 10 is a steel plate, a groove is formed, for example, in an end surface or a side surface of the steel plate. In the case where the base material 10 is a steel pipe, a groove is formed in a pipe end portion in the axial direction of the steel pipe. In the case where the base material 10 is a steel pipe, an austenitic stainless steel welded joint 1 is formed, for example, by performing a circumferential welding. Note that in the welding step, by adjusting the chemical composition of the welding material and the dilution of the base material 10, a welding metal 20 having the above-described chemical composition, preferably satisfying the formula (2), can be produced.

[0337] By the above-described manufacturing process, the austenitic stainless steel welded joint 1 according to the present embodiment can be manufactured. Note that the manufacturing method of the austenitic stainless steel welded joint 1 according to the present embodiment is not limited to the above-described manufacturing method. As long as the above-described austenitic stainless steel welded joint 1 having the above-described chemical composition is manufactured by performing welding using the base material 10 having the above-described chemical composition satisfying the formula (1) to produce the welding metal 20 having the above-described chemical composition, it can be manufactured by other methods.

[0338] Example

[0339] [Manufacture of Austenitic Stainless Steel Welded Joint]

[0340] [Manufacture of Base Material]

[0341] A molten steel having the chemical composition of Table 1 was produced to manufacture a base material.

[0342] [Table 1]

[0343]

[0344] In the column of "F1" in Table 1, the F1 value of each steel grade was substituted. In addition, the element symbol and the numerical value attached to the element symbol in the column of "Others" in the column of "Chemical Composition" mean an arbitrary element and the content (mass %) thereof contained. For example, in the steel grade A, 0.10% of V was shown as an other element. In the steel grade K, 0.15% of V and further 0.002% of Ca were shown as other elements. In the chemical composition of each steel grade, the balance other than the elements described in Table 1 was Fe and impurities. Note that "-" in Table 1 means that the corresponding element is not contained (less than the detection limit).

[0345] Molten steel is used to manufacture ingots with an outer diameter of 120 mm and a weight of 30 kg. These ingots are then hot-forged to produce steel plates with a thickness of 40 mm. Next, they are hot-rolled to produce steel plates with a thickness of 15 mm. The final processing temperature during hot rolling is always above 1050℃. The hot-rolled steel plates are then subjected to solution treatment. In all steel plates, the solution treatment temperature is 1150℃, and the solution treatment time is 10 minutes. The solution-treated base material is then water-cooled. Through these manufacturing processes, steel plates (base materials) with a thickness of 15 mm, a width of 50 mm, and a length of 100 mm are produced.

[0346] [Manufacturing of welding materials]

[0347] Molten steel with the chemical composition of steel grades V to Z is manufactured, wherein steel grades V to Z have the chemical compositions shown in Table 2. In the chemical composition of each steel grade, the balance other than the elements listed in Table 2 is Fe and impurities. It should be noted that "-" in Table 2 means that the corresponding element is not present (below the detection limit).

[0348] [Table 2]

[0349]

[0350] Molten steel is used to manufacture ingots with an outer diameter of 120 mm and a weight of 30 kg. The ingots are then hot-forged, hot-rolled, cold-rolled, and heat-treated using known methods to produce welding wire (welding material) with an outer diameter of 1.2 mm.

[0351] [Manufacturing of welded joints in austenitic stainless steel systems]

[0352] Two pieces were manufactured from the base materials of each steel grade listed in Table 1 through machining. Figure 6 The sheet material shown in the image. Figure 6 In this text, the numerical value with "mm" indicates the dimensions (in mm) of the steel plate used as the base material. The steel plate has a beveled surface on its side along its length. The beveled surface is a V-bevel with a bevel angle of 30° and a root thickness of 1 mm.

[0353] like Figure 7 As shown, a constraint plate 30 is prepared. The constraint plate 30 has a thickness of 25 mm, a width of 200 mm, and a length of 200 mm, and has a chemical composition equivalent to "SM400C" as described in JIS G 3106 (2008).

[0354] Two base materials (plates) 10 are placed on the constraint plate 30. The bevel surfaces of the two base materials 10 are then butt-jointed. After placing the two base materials 10, a shielded arc welding rod is used to perform constraint welding around the base materials 10. The shielded arc welding rod has a chemical composition equivalent to "ENiCrMo-3" as specified in JIS Z 3224 (2010).

[0355] After the restraint welding of the periphery of the base material 10, multi-layer welding was performed using a welding wire having the chemical composition shown in Table 2. Specifically, gas tungsten arc welding (GTAW) was performed. The heat input amount in each welding was adjusted to 6 to 18 kJ / cm. At the time of the gas tungsten arc welding (GTAW), 100% Ar gas was used as the shielding gas.

[0356] By the above welding, an austenitic stainless steel welded joint having the base material 10 and the weld metal 20 was manufactured. The chemical composition at the position of the center of the width and the center of the thickness of the weld metal 20 of the welded joint was analyzed. The chemical composition at the position of the center of the width and the center of the thickness of the weld metal 20 is shown in Table 3.

[0357] [Table 3]

[0358]

[0359] The F2 value of the weld metal of each test number is written in the column of "F2" in Table 3. Further, the element symbol of the column of "Others" in the column of "Chemical composition" and the numerical value attached to the front of the element symbol mean an arbitrary element contained and the content amount thereof (mass %). For example, in the test number 1, it is indicated that 0.04% of V is contained as the other element. In the test number 15, it is indicated that 0.6% of W and further 0.06% of Ta are contained as the other elements. Of the chemical composition of each steel, the balance other than the elements described in Table 3 is Fe and impurities. Note that "-" in Table 3 means that the corresponding element is not contained (less than the detection limit).

[0360] [Evaluation Test]

[0361] The following evaluation test was performed on the austenitic stainless steel welded joint of each test number.

[0362] [Flame Resistance Evaluation Test]

[0363] From the portion of the weld metal of the welded joint of each test number shown in Table 3, 10 test pieces for cross-sectional microstructure observation orthogonal to the welding line were taken out each. The surface of the taken-out test piece was mirror-polished and etched. The surface of the etched test piece was observed using an optical microscope at 200 times. Further, it was visually judged whether or not a high-temperature crack was generated in the weld metal in the cross section. The test result is shown in the column of "Weldability" in Table 4. In the column of "Weldability" in Table 4, "O" indicates that no crack was generated in the weld metal of any one of the 10 test pieces. "X" indicates that a crack was generated in the weld metal of one or more of the 10 test pieces.

[0364] [Table 4]

[0365]

[0366] [Evaluation Test of Resistance of Weld Metals to SCC (Symplocity Crush)]

[0367] For welded joints that did not produce cracks in the aforementioned weldability evaluation tests, a hypothetical high-temperature environment was assumed, involving an aging treatment at 600°C for 5000 hours. In the welded joints after aging treatment, if... Figure 8 As shown, a plate-shaped test piece 40 containing region P, with a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was prepared. The thickness of the test piece 40 (2 mm) corresponds to the length T in the thickness direction of the weld metal, the length of the test piece 40 (75 mm) corresponds to the width direction W of the weld metal, and the width of the test piece 40 (10 mm) corresponds to the extension direction L of the weld metal. The plate-shaped test piece was prepared so that region P of the weld metal 20 was located at the center of the 75 mm length of the plate-shaped test piece. Based on JIS G 0576 (2001) "Stress Corrosion Crack Test Method for Stainless Steel", a test to evaluate the resistance to polythionite (SCC) was conducted. Specifically, the test piece was bent into a U-shape around a punch with an inner radius of 5 mm (the bent part corresponds to the weld metal). The U-shaped bent test piece was immersed in Wackenroder solution (a solution obtained by blowing a large amount of H2S gas into a saturated aqueous solution of H2SO3 prepared by blowing SO2 gas into distilled water) at room temperature for 100 hours. After impregnation, the welded metal parts of the test pieces are examined under a microscope at a magnification of 500x to check for cracks.

[0368] If no crack is found, the weld metal is judged to have excellent SCC resistance to polysulfide (marked as "○" in the "Polysulfide SCC Resistance" column of the "Weld Metal" section in Table 4). Even if only one crack is found in the weld metal, the weld metal is judged to have low SCC resistance to polysulfide (marked as "×" in the "Polysulfide SCC Resistance" column of the "Weld Metal" section in Table 4).

[0369] [Evaluation Test of Resistance to Naphthenic Acid Corrosion of Welded Metals]

[0370] For austenitic stainless steel welded joints that did not develop cracks in the aforementioned weldability evaluation tests, a weld metal sample including region P was taken. The base metal was not included in the weld metal sample.

[0371] The welded metal sample was immersed in a poor-quality crude oil at 135 MPa, 350°C for 48 hours under a nitrogen atmosphere using an autoclave. The poor-quality crude oil corresponds to a total acid number of 6 as defined in ASTM D664-11a. After 48 hours, the welded metal sample was taken out. Note that as the corrosion test proceeds, the acid in the poor-quality crude oil is consumed, and the TAN value (total acid number) decreases, so the poor-quality crude oil was completely replaced after 24 hours of immersion of the corrosion test piece, and the immersion was performed for a total of 48 hours using the water inlet and outlet of the autoclave.

[0372] After 48 hours, black soot was firmly attached to the welded metal sample taken out of the autoclave. Therefore, after performing a 5-second alumina blast treatment on the welded metal sample, the plate-shaped test piece was subjected to pickling in an ammonium citrate solution at 100°C for 60 minutes. Thereafter, an ultrasonic cleaning was performed for 3 minutes using acetone.

[0373] The difference between the mass of the welded metal sample before the test and the mass of the welded metal sample after the ultrasonic cleaning was calculated as the corrosion loss. Further, the corrosion rate (mm / y) was calculated from the surface area of the plate-shaped test piece, the specific gravity, and the test time.

[0374] In the case where the corrosion rate was 1.50 mm / y or less, it was judged that the naphthenic acid corrosion resistance of the welded metal was excellent (in the "naphthenic acid corrosion resistance" column of the "welded metal" column in Table 4, "O"). On the other hand, in the case where the corrosion rate exceeded 1.50 mm / y, it was judged that the naphthenic acid corrosion resistance of the welded metal was low (in the "naphthenic acid corrosion resistance" column of the "welded metal" column in Table 4, "X").

[0375] [Creep strength test of welded joint]

[0376] For the welded joint in which no cracks were generated in the weldability evaluation test described above, a creep rupture test piece based on JIS Z2271 (2010) was prepared. The cross section perpendicular to the axial direction of the creep rupture test piece was circular, the outer diameter of the creep rupture test piece was 6 mm, and the parallel portion was 30 mm. The axial direction of the parallel portion of the creep rupture test piece was the direction in which the welded metal 20 of the welded joint was oriented. Figure 8 The welded metal 20 of the welded joint in the direction of the width of the welded metal, which included the region P, was located at the central position of the parallel portion.

[0377] Using the prepared creep rupture test piece, a creep rupture test based on JIS Z2271 (2010) was performed. Specifically, after heating the creep rupture test piece at 750°C, the creep rupture test was performed. The test stress was set to 45 MPa, and the creep rupture time (hours) was calculated.

[0378] With respect to the creep strength, in the case where the creep rupture time is 10,000 hours or more, the creep strength of the welded joint was judged to be significantly excellent ("◎" in the "Creep strength" column of the "Welded joint" column in Table 4). In the case where the creep rupture time is 3,000 hours or more and less than 10,000 hours, the creep strength of the welded joint was judged to be excellent ("O" in the "Creep strength" column of the "Welded joint" column in Table 4). In the case where the creep rupture time is less than 3,000 hours, the creep strength of the welded joint was judged to be low ("X" in the "Creep strength" column of the "Welded joint" column in Table 4). In the case where the creep strength is "O" or "◎", the welded joint was judged to have sufficient creep strength.

[0379] [SCC resistance evaluation test of base material]

[0380] For the base material (steel plate) of each test number, use under a high temperature environment was assumed, and aging treatment was performed at 600°C for 5,000 hours. From the aged material, a plate-shaped test piece having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was taken. An SCC resistance evaluation test was performed based on JIS G 0576 (2001) "Stress Corrosion Cracking Test Method for Stainless Steels". Specifically, the test piece was bent around a punch having an inner radius of 5 mm to be in a U-bend shape. The test piece in the U-bend shape was immersed in Wackenroder solution (a solution obtained by bubbling a large amount of H2S gas into H2SO3-saturated aqueous solution prepared by bubbling SO2 gas into distilled water) at room temperature for 100 hours. For the test piece after the immersion, microscopic observation was performed at a magnification of 500 times to confirm the presence or absence of cracks.

[0381] In the case where no cracks were confirmed, the SCC resistance of the base material was judged to be excellent ("O" in the "SCC resistance" column of the "Base material" column in Table 4). Even in the case where one crack was confirmed, the SCC resistance of the base material was judged to be low ("X" in the "SCC resistance" column of the "Base material" column in Table 4).

[0382] [Corrosion resistance evaluation test of base material]

[0383] For the base material (steel plate) of each test number, a base material sample having a thickness of 3 mm, a length of 25 m, and a width of 25 mm was taken. The base material sample did not include weld metal.

[0384] The base material sample was immersed in a poor-quality crude oil at 135 MPa, 350°C for 48 hours under a nitrogen atmosphere using an autoclave. The poor-quality crude oil corresponds to a total acid number of 6 as defined in ASTM D664-11a. After 48 hours, the base material sample was removed. Note that as the corrosion test proceeds, the acid in the poor-quality crude oil is consumed, and the TAN value (total acid number) decreases, so the poor-quality crude oil was completely replaced after 24 hours of immersion of the corrosion test piece, and the immersion was performed for a total of 48 hours using the drain and the water inlet of the autoclave.

[0385] After 48 hours, black soot was firmly attached to the base material sample removed from the autoclave. Therefore, after performing a 5-second blast treatment using alumina on the base material sample, the base material sample was subjected to pickling in an ammonium citrate solution at 100°C for 60 minutes. Thereafter, an ultrasonic cleaning was performed using acetone for 3 minutes.

[0386] The difference between the mass of the base material sample before the test and the mass of the base material sample after the ultrasonic cleaning was calculated as the corrosion loss. Furthermore, the corrosion rate (mm / y) was calculated from the surface area of the base material sample, the specific gravity, and the test time.

[0387] In the case where the corrosion rate was 1.50 mm / y or less, it was judged that the naphthenic acid corrosion resistance of the base material was excellent (in the "naphthenic acid corrosion resistance" column of the "base material" column in Table 4, "O"). On the other hand, in the case where the corrosion rate exceeded 1.50 mm / y, it was judged that the naphthenic acid corrosion resistance of the base material was low (in the "naphthenic acid corrosion resistance" column of the "base material" column in Table 4, "X").

[0388] [Creep ductility test of base material]

[0389] For the base material of the welded joint of each test number (steel types A to U5 shown in Table 1), a creep rupture test piece based on JIS Z2271 (2010) was prepared. The cross section perpendicular to the axial direction of the creep rupture test piece was circular, and the outer diameter of the creep rupture test piece was 6 mm, and the parallel portion was 30 mm.

[0390] Using the prepared creep rupture test piece, a creep rupture test based on JIS Z2271 (2010) was performed. Specifically, after heating the creep rupture test piece at 750°C, the creep rupture test was performed. The test stress was set to 45 MPa, and the creep rupture elongation (%) was calculated.

[0391] When the creep elongation at break is 20.0% or more but less than 30.0%, the creep ductility of the base material is considered good (marked as "P" in the "Creep Ductility" column of the "Base Material" section in Table 4). When the creep elongation at break is greater than 30.0% but less than 50.0%, the creep ductility of the base material is considered excellent (marked as "○" in the "Creep Ductility" column of the "Base Material" section in Table 4). Furthermore, when the creep elongation at break exceeds 50.0%, the creep ductility of the base material is considered significantly excellent (marked as "◎" (Excellent) in the "Creep Ductility" column of the "Base Material" section in Table 4). When the creep elongation at break is less than 20.0%, the creep ductility of the base material is considered low (marked as "×" in the "Creep Ductility" column of the "Base Material" section in Table 4). When the creep elongation at break is P, ○, or ◎, sufficient creep ductility of the base material is considered to be obtained.

[0392] [Toughness evaluation test after high temperature aging]

[0393] from Figure 9 A V-cut test piece 50 containing region P was fabricated at the location shown in the figure. The width of the test piece 50 was set to 10 mm, the thickness to 10 mm, and the length to 55 mm. The test piece 50 was fabricated with region P of the weld metal 20 located at the center of its length. A V-cut was formed at the center of the length of the test piece 50. The V-cut angle was set to 45°, the cut depth to 2 mm, and the cut bottom radius to 0.25 mm. Three V-cut test pieces 50 were fabricated for each test number. For the test piece 50, an aging treatment was performed at 700°C for 3000 hours. After 3000 hours, the test piece was allowed to cool. For the aged test piece, a Charpy impact test based on JIS Z 2242 (2005) was performed at room temperature and in the atmosphere. The three impact values ​​(J / cm) obtained from the test were compared. 2 The arithmetic mean of the values ​​is defined as the impact values ​​(J / cm²) after high-temperature aging treatment for that test number. 2 ).

[0394] If the obtained impact value is 20 J / cm 2 Based on the above, it can be determined that the toughness of the weld metal remains high even after high-temperature aging (marked as "○" in the "Toughness after High-Temperature Aging" column of the "Weld Metal" section in Table 4). On the other hand, if the impact value is less than 20 J / cm... 2 If the toughness of the weld metal after high-temperature aging is low, it will be marked as "×" in the "Toughness after High-Temperature Aging" column of the "Weld Metal" column in Table 4.

[0395] [Experimental Results]

[0396] The test results are shown in Table 4. Note that "-" in Table 4 means that the test was not performed. In Test Nos. 1 to 24, the chemical composition of the base metal was appropriate, satisfying Formula (1). Further, the chemical composition of the weld metal was appropriate. Therefore, in the weldability evaluation test, no cracks were confirmed in the weld metal, showing excellent weldability. Further, in the weld metal, the resistance to condensate SCC and the resistance to naphthenic acid corrosion were excellent. Further, the creep strength of the welded joint was also high. Further, in the base metal of these test numbers, the resistance to condensate SCC and the resistance to naphthenic acid corrosion were excellent. Further, the creep ductility of the base metal was also good.

[0397] In particular, in Test Nos. 1, 6, 9 to 19, and 22, the B content in the weld metal was 0.0030% or more. Therefore, the welded joint obtained particularly excellent creep strength ("◎" in the "Creep Strength" column of the "Welded Joint" column of Table 4).

[0398] Further, in Test Nos. 4, 5, 11, 12, 15, 17, and 19 to 24, the Mo content of the chemical composition of the base metal was 1.00% or more and the Cu content was 1.7% or less. Therefore, the base metal obtained particularly excellent creep ductility ("◎" in the "Creep Ductility" column of the "Base Metal" column of Table 4).

[0399] Further, in Test Nos. 1 to 21, F2 in the chemical composition of the weld metal satisfied Formula (2). Therefore, in Test Nos. 1 to 21, the toughness after high-temperature aging was more excellent compared to Test Nos. 22 to 24 in which F2 in the chemical composition of the weld metal did not satisfy Formula (2).

[0400] On the other hand, in Test Nos. 25, 28, and 30, the Ni content of the welding material Z shown in Table 2 was low and Nb was not contained, so the Ni content and the Nb content of the weld metal were too low. Therefore, the resistance to condensate SCC of the weld metal was low.

[0401] In Test No. 26, the B content in the chemical composition of the weld metal was too high. Therefore, in the weldability evaluation test, cracks were confirmed in the weld metal, and the weldability of the weld metal was low.

[0402] In Test Nos. 27 and 29, the Mo content in the chemical composition of the weld metal was too low. Therefore, the resistance to naphthenic acid corrosion of the weld metal was low.

[0403] In Test Nos. 31 and 32, F1 in the chemical composition of the base metal did not satisfy Formula (1). As a result, the creep ductility of the base metal was low.

[0404] The C content of the weld metal of Test No. 33 is too high. Therefore, the weld metal has low resistance to SCC by condensate sulfuric acid. Further, the C content of the base metal is high, and therefore the base metal has low resistance to SCC by condensate sulfuric acid. Further, the F1 of the base metal does not satisfy the formula (1). Therefore, the base metal has low creep ductility.

[0405] In Test No. 34, the Mo content of the weld metal is too low. Therefore, the weld metal has low resistance to naphthenic acid corrosion. Further, in the chemical composition of the base metal, the F1 does not satisfy the formula (1). As a result, the base metal has low creep ductility.

[0406] In Test No. 35, the B content of the base metal is too low. Therefore, the base metal has too low creep ductility.

[0407] In Test No. 36, the B content of the base metal is too high, and as a result, the B content of the weld metal is too high. Therefore, in the weldability evaluation test, a crack is confirmed in the weld metal, and the weldability is low.

[0408] In Test No. 37, the Nb content of the base metal is too low. As a result, the base metal has low resistance to SCC by condensate sulfuric acid.

[0409] In Test No. 38, the Mo content of the base metal is too low. As a result, the base metal has low resistance to naphthenic acid corrosion.

[0410] The above describes the embodiments of the present application. However, the above-described embodiments are only examples for implementing the present application. Therefore, the present application is not limited to the above-described embodiments, and the above-described embodiments can be appropriately changed and implemented within the scope of the gist of the present application.

[0411] Reference Signs List

[0412] 1 Austenitic stainless steel welded joint

[0413] 10 Base metal

[0414] 20 Weld metal

[0415] 30 Restraint plate.

Claims

1. An austenitic stainless steel welded joint having a base material and a weld metal, the chemical composition of the base material consisting of, in mass %, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr：16.0～25.0%、 Ni: 10.0 to 30.0%, Mo: 0.10 to 5.00%, Nb: 0.20 to 1.00%, N：0.050～0.300%、 sol. Al: 0.001 to 0.100%, B：0.0010～0.0080%、 Cu: 0 to 5.00%, W：0～5.0%、 Co: 0 to 1.0%, V：0～1.00%、 Ta: 0 to 0.20%, Hf: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.100%, and the balance: Fe and impurities, and satisfying formula (1); in the weld metal, the chemical composition at the width center position and the thickness center position of the weld metal consisting of, in mass %, C: 0.050% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 3.00%, P: 0.030% or less, S: 0.015% or less, Cr：15.0～25.0%、 Ni: 21.0 to 70.0%, Mo: 1.30 to 10.00%, Nb: 0.05 to 3.00%, N: 0.150% or less, B: 0.0050% or less, sol. Al: 0 to 1.000%, Cu: 0 to 2.50%, W：0～1.0%、 Co: 0 to 15.0%, V：0～0.10%、 Ti: 0 to 0.50%, Ta: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.100%, and the balance: Fe and impurities, the chemical composition of the weld metal satisfying formula (2), B + 0.004 - 0.9 C + 0.017 Mo 2 ≥ 0.00400 (1) 0.012Cr - 0.005Ni + 0.013Mo + 0.023Nb + 0.02Al - 0.004Co ≤ 0.174 (2) wherein, the mass % content of the corresponding element is substituted at the element symbol in formula (1), and the mass % content of the corresponding element is substituted at the element symbol in formula (2).

2. The austenitic stainless steel welded joint according to claim 1, wherein the chemical composition of the base material contains one element or two or more elements selected from the group consisting of Cu: 0.10 to 5.00%, W：0.1～5.0%、 Co: 0.1 to 1.0%, V：0.10～1.00%、 Ta: 0.01 to 0.20%, Hf: 0.01 to 0.20%, Ca: 0.001 to 0.010%, Mg: 0.001 to 0.010%, and rare earth elements: 0.001 to 0.100%.

3. The austenitic stainless steel welded joint according to claim 1, wherein the chemical composition of the weld metal contains one element or two or more elements selected from the group consisting of sol. Al: 0.001 to 1.000%, Cu: 0.01 to 2.50%, W：0.1～1.0%、 Co: 0.1 to 15.0%, V：0.01～0.10%、 Ti: 0.01 to 0.50%, Ta: 0.01 to 0.20%, Ca: 0.001 to 0.010%, Mg: 0.001 to 0.010%, and Rare earth element: 1 element or 2 or more elements in the group consisting of 0.001 to 0.100%.

4. The austenitic stainless steel welded joint according to claim 2, wherein the chemical composition of the weld metal contains one or more elements selected from the group consisting of sol. Al: 0.001 to 1.000%, Cu: 0.01 to 2.50%, W：0.1～1.0%、 Co: 0.1 to 15.0%, V：0.01～0.10%、 Ti: 0.01 to 0.50%, Ta: 0.01 to 0.20%, Ca: 0.001 to 0.010%, Mg: 0.001 to 0.010%, and Rare earth element: 1 element or 2 or more elements in the group consisting of 0.001 to 0.100%.

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