阅读说明：本技术 奥氏体系不锈钢焊接接头 (Austenitic stainless steel welded joint ) 是由 净德佳奈 山田健太 冈田浩一 小薄孝裕 于 2019-02-28 设计创作，主要内容包括：本申请提供耐连多硫酸SCC性以及耐环烷酸腐蚀性优异、并且蠕变延性也优异的奥氏体系不锈钢焊接接头。奥氏体系不锈钢焊接接头(1)具备母材(10)和焊接金属(20)。在焊接金属(20)的宽度中央位置且是厚度中央位置的化学组成以质量％计为C：0.050％以下、Si：0.01～1.00％、Mn：0.01～3.00％、P：0.030％以下、S：0.015％以下、Cr：15.0～25.0％、Ni：20.0～70.0％、Mo：1.30～10.00％、Nb：0.05～3.00％、N：0.150％以下、B：0.0050％以下、以及余量：Fe以及杂质。(Provided is an austenitic stainless steel welded joint which is excellent in polythionic acid (SCC) resistance, naphthenic acid corrosion resistance, and creep ductility. An austenitic stainless steel welded joint (1) is provided with a base metal (10) and a weld metal (20). The chemical composition in mass% at the widthwise central position and the thickness central position of the weld metal (20) is C: 0.050% or less, Si: 0.01 to 1.00%, Mn: 0.01-3.00%, P: 0.030% or less, S: 0.015% or less, Cr: 15.0 to 25.0%, Ni: 20.0 to 70.0%, Mo: 1.30-10.00%, Nb: 0.05-3.00%, N: 0.150% or less, B: 0.0050% or less, and the balance: fe and impurities.)

1. An austenitic stainless steel welded joint comprising a base metal and a weld metal,

the chemical composition of the base material is calculated by mass percent

C: less than 0.030%,

Si：0.10～1.00％、

Mn：0.20～2.00％、

P: less than 0.040%,

S: less than 0.010%,

Cr：16.0～25.0％、

Ni：10.0～30.0％、

Mo：0.10～5.00％、

Nb：0.20～1.00％、

N：0.050～0.300％、

sol.Al：0.001～0.100％、

B：0.0010～0.0080％、

Cu：0～5.00％、

W：0～5.0％、

Co：0～1.0％、

V：0～1.00％、

Ta：0～0.20％、

Hf：0～0.20％、

Ca：0～0.010％、

Mg：0～0.010％、

Rare earth elements: 0 to 0.100%, and

and the balance: fe and impurities, and

satisfies formula (1);

in the case of the weld metal,

the chemical composition in mass% at the widthwise central position and the thickness central position of the weld metal

C: less than 0.050%,

Si：0.01～1.00％、

Mn：0.01～3.00％、

P: less than 0.030%,

S: less than 0.015%,

Cr：15.0～25.0％、

Ni：20.0～70.0％、

Mo：1.30～10.00％、

Nb：0.05～3.00％、

N: less than 0.150 percent,

B: less than 0.0050%,

sol.Al：0～1.000％、

Cu：0～2.50％、

W：0～1.0％、

Co：0～15.0％、

V：0～0.10％、

Ti：0～0.50％，

Ta：0～0.20％、

Ca：0～0.010％、

Mg：0～0.010％、

Rare earth elements: 0 to 0.100%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

B+0.004-0.9C+0.017Mo²≥0 (1)

wherein the content of the corresponding element in mass% is substituted at the symbol of the element in the formula (1).

2. The austenitic stainless steel weld joint of claim 1,

the chemical composition of the parent material comprises a chemical composition selected from the group consisting of

Cu：0.10～5.00％、

W：0.1～5.0％、

Co：0.1～1.0％、

V：0.10～1.00％、

Ta：0.01～0.20％、

Hf：0.01～0.20％、

Ca：0.001～0.010％、

Mg: 0.001 to 0.010%, and

rare earth elements: 0.001-0.100% of 1 or more elements in the group.

3. The austenitic stainless steel weld joint according to claim 1 or claim 2,

said chemical composition of said weld metal is selected from the group consisting of

sol.Al：0.001～1.000％、

Cu：0.01～2.50％、

W：0.1～1.0％、

Co：0.1～15.0％、

V：0.01～0.10％、

Ti：0.01～0.50％，

Ta：0.01～0.20％、

Ca：0.001～0.010％、

Mg: 0.001 to 0.010%, and

rare earth elements: 0.001-0.100% of 1 or more elements in the group.

4. The austenitic stainless steel weld joint according to any one of claims 1 to 3,

the chemical composition of the weld metal satisfies formula (2),

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

wherein the content of the corresponding element in mass% is substituted at the symbol of the element in the formula (2).

Technical Field

The present invention relates to a welded joint, and more particularly, to an austenitic stainless steel welded joint.

Background

An austenitic stainless steel welded joint is produced by welding austenitic stainless steel materials, and includes a base material formed of austenitic stainless steel and a weld metal. The austenitic stainless steel welded joint can be used for welded structures of plant equipment such as thermal boilers, oil refining, petrochemical equipment and the like. Examples of the welded structure of the chemical plant equipment include peripheral equipment of a distillation column, a heating furnace tube, a reaction tube, a heat exchanger, and piping. Among the parts used in the welded structure of these chemical plant facilities, there are parts used in an environment containing corrosive fluids including sulfides and/or chlorides at a high temperature of 600 to 700 ℃. In the present specification, an environment of 600 to 700 ℃ high temperature and containing a corrosive fluid containing sulfide and/or chloride is referred to as a "high temperature corrosive environment".

Welded structures used in high temperature corrosive environments stop working during periodic maintenance of chemical plants. When the work stops, the temperature of the welding structure is reduced to normal temperature. At this time, air, moisture, and sulfide scale react to generate polythionic acid on the surface of the member of the welded structure. This polythionic acid induces stress corrosion cracking at grain boundaries (hereinafter referred to as polythionic acid SCC). Therefore, excellent polythionic acid SCC resistance is required for the member used in the high-temperature corrosive environment.

Steels having improved resistance to polythionic acid SCC have been proposed in Japanese patent application laid-open Nos. 2003-166039 (patent document 1) and 2009/044802 (patent document 2). The reason for the polythionic acid SCC is that Cr is expressed as M₂₃C₆The form of the type carbide precipitates at grain boundaries, and forms a Cr-deficient layer in the vicinity of the grain boundaries. Therefore, in patent documents 1 and 2, the amount of C is reduced to suppress M₂₃C₆Formation of a shaped carbide improves the polythionic acid SCC resistance.

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

The austenitic stainless steel disclosed in patent document 2 contains, in mass%, C: less than 0.04%, Si: 1.5% or less, Mn: 2% or less, Cr: 15-25%, Ni: 6-30%, N: 0.02-0.35%, Sol.Al: 0.03% or less, further containing 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 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: less than 0.01 percent. Further, F1 { (P + Sn)/2} + { (As + Zn + Pb + Sb)/5} ≦ 0.075, and 0.05 ≦ Nb + Ta + Zr + Hf +2Ti + (V/10) ≦ 1.7 to 9 XF 1. In patent document 2, the resistance to polythionic acid SCC is improved by making the C content less than 0.05%. Further, by reducing C-fixing elements such as Nb and Ti, grain boundary embrittlement elements such as P, S, Sn in steel are reduced, and embrittlement cracking resistance at the weld Heat Affected Zone (HAZ) is improved.

Disclosure of Invention

Problems to be solved by the invention

As a result, in chemical plant equipment, inferior crude oil may be used; not only the corrosion of polythionic acid SCC but also the corrosion of naphthenic acid. Naphthenic acids are cyclic saturated hydrocarbons having 1 or more carboxyl groups. Naphthenic acids do not cause SCC, as polythionic acid does, but instead cause general corrosion. Therefore, in the welded joint used in the plant equipment, it is preferable that not only the resistance to SCC, polythionic acid, but also the resistance to naphthenic acid corrosion be excellent.

In addition, in recent years, high creep ductility has been required for parts used in a high-temperature corrosive environment of 600 to 700 ℃. In chemical plant facilities, as described above, the facilities may be stopped and regular maintenance may be performed. In the periodic maintenance, parts that must be replaced in a welded structure of chemical plant equipment are investigated. In this case, if the creep ductility is high, the degree of deformation of the component can be checked at the time of regular maintenance, and this can be used as a criterion for determining replacement of the component.

Although patent documents 1 and 2 aim to improve the resistance to SCC of polythionic acid, they have studied naphthenic acid corrosion resistance and have not studied to improve creep ductility. Further, no studies have been made on the polythionic acid SCC resistance and naphthenic acid corrosion resistance of a weld joint including not only a base material but also a weld metal.

The purpose of the present application is to provide an austenitic stainless steel welded joint that is excellent in the resistance to SCC of polythionic acid and naphthenic acid corrosion, and also excellent in the creep ductility of the base metal.

Means for solving the problems

The austenitic stainless steel welded joint according to the present application includes a base metal and a weld metal,

the chemical composition of the base material is calculated by mass percent

C: less than 0.030%,

Si：0.10～1.00％、

Mn：0.20～2.00％、

P: less than 0.040%,

S: less than 0.010%,

Cr：16.0～25.0％、

Ni：10.0～30.0％、

Mo：0.10～5.00％、

Nb：0.20～1.00％、

N：0.050～0.300％、

sol.Al：0.001～0.100％、

B：0.0010～0.0080％、

Cu：0～5.00％、

W：0～5.0％、

Co：0～1.0％、

V：0～1.00％、

Ta：0～0.20％、

Hf：0～0.20％、

Ca：0～0.010％、

Mg：0～0.010％、

Rare earth elements: 0 to 0.100%, and

and the balance: fe and impurities, and

satisfies formula (1);

in the case of the weld metal,

the chemical composition in mass% at the widthwise central position and the thickness central position of the weld metal

C: less than 0.050%,

Si：0.01～1.00％、

Mn：0.01～3.00％、

P: less than 0.030%,

S: less than 0.015%,

Cr：15.0～25.0％、

Ni：20.0～70.0％、

Mo：1.30～10.00％、

Nb：0.05～3.00％、

N: less than 0.150 percent,

B: less than 0.0050%,

sol.Al：0～1.000％、

Cu：0～2.50％、

W：0～1.0％、

Co：0～15.0％、

V：0～0.10％、

Ti：0～0.50％，

Ta：0～0.20％、

Ca：0～0.010％、

Mg：0～0.010％、

Rare earth elements: 0 to 0.100%, and

and the balance: fe and impurities.

B+0.004-0.9C+0.017Mo²≥0 (1)

Wherein the content (mass%) of the corresponding element is substituted at the symbol of the element in the formula (1).

ADVANTAGEOUS EFFECTS OF INVENTION

The austenitic stainless steel welded joint according to the present application is excellent in polythionic acid SCC resistance and naphthenic acid corrosion resistance, and also excellent in creep ductility of the base metal.

Drawings

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

Fig. 2 is a cross-sectional view of the austenitic stainless steel weld joint of fig. 1, taken in the width direction of the weld metal.

Fig. 3 is a sectional view of the austenitic stainless steel weld joint of fig. 1, taken in the weld metal extending direction L.

Fig. 4 is a cross-sectional view of an austenitic stainless steel weld joint different from that of fig. 3, taken along the weld metal extending direction L.

Fig. 5 is a cross-sectional view perpendicular to the weld metal extending direction L in the austenitic stainless steel welded joint according to the present embodiment.

FIG. 6 is a schematic view for explaining the groove shape of the parent material in the example.

Fig. 7 is a schematic view of a welded joint using the base material of fig. 6.

Fig. 8 is a schematic diagram showing the sampling position of the plate-like test piece used in the examples.

FIG. 9 is a schematic view showing the assumed position of the V-notch test piece used in the examples.

Detailed Description

The present inventors have studied a welded joint having excellent SCC resistance to polythionic acid, excellent naphthenic acid corrosion resistance, and excellent creep ductility of a base material.

When the C content of the base material is reduced to 0.030% or less, M is suppressed in use under a high-temperature corrosive environment₂₃C₆The formation of the carbide suppresses the formation of a Cr-deficient layer in the vicinity of the grain boundary. The base material of the austenitic stainless steel welded joint of the present embodiment further contains 0.20 to 1.00% of Nb, so that C is fixed by Nb, and M is further reduced₂₃C₆The amount of solid solution C as a formation element of the form carbide. The base material of the austenitic stainless steel welded joint according to the present embodiment further contains 0.10 to 5.00% of Mo. Mo inhibition of M₂₃C₆Formation of type carbides. Therefore, the generation of the Cr-deficient layer is reduced. According to the above measures, the resistance to SCC, which is one of stress corrosion cracking, to polythionic acid can be improved.

Further, it is effective to contain the above-mentioned Mo against naphthenic acid corrosion. It is effective to contain Cu in the base material against general corrosion such as sulfuric acid corrosion. However, naphthenic acids, as described above, are cyclic saturated hydrocarbons having a carboxyl group, and even if Cu is contained, naphthenic acid corrosion resistance is not high. On the other hand, Mo is extremely effective for naphthenic acid corrosion. Mo is bonded with S in the high-temperature corrosion environment when used in the high-temperature corrosion environment of the austenitic stainless steel welding joint, and a sulfide coating is formed on the surface of the austenitic stainless steel welding joint. The sulfide film improves naphthenic acid corrosion resistance. Therefore, when the C content is 0.030% or less, the Nb content is 0.20 to 1.00%, and the Mo content is 0.10 to 5.00% in the chemical composition of the base material, the resistance to SCC of polythionic acid is high, and the resistance to naphthenic acid corrosion is also high.

However, the present inventors have investigated and found that when the C content of the base material is reduced to 0.030% or less, the creep ductility of the base material in a high-temperature corrosive environment of 600 to 700 ℃ is reduced. The reason for this is considered as follows. Precipitates formed at grain boundaries improve the grain boundary strength. When the grain boundary strength is increased, the creep ductility of the base material is increased. However, when the C content is reduced to 0.030% or less, precipitates (carbides and the like) formed in grain boundaries are also reduced. As a result, it is considered that it is difficult to obtain grain boundary strength, and creep ductility of the base material is reduced.

Therefore, the present inventors have further studied an austenitic stainless steel welded joint having excellent polythionic acid SCC resistance, excellent naphthenic acid corrosion resistance, and excellent creep ductility of the base material. The present inventors focused on B (boron) as an element contained in the base material. The present inventors have considered that B (boron) segregates in grain boundaries under the high temperature corrosive environment of 600 to 700 ℃ described above, and thus the grain boundary strength can be improved.

As a result of further investigation, the present inventors considered that the chemical composition of the base material of the austenitic stainless steel welded joint is, in mass%, a composition containing C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20-2.00%, 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: 0.20-1.00%, N: 0.050 to 0.300%, sol.Al: 0.001-0.100%, B: 0.0010-0.0080%, Cu: 0-5.00%, W: 0-5.0%, Co: 0-1.0%, V: 0-1.00%, Ta: 0-0.20%, Hf: 0-0.20%, Ca: 0-0.010%, Mg: 0 to 0.010%, and a rare earth element: 0 to 0.100% and the balance of Fe and impurities, not only can excellent polythionic acid SCC resistance and naphthenic acid corrosion resistance be obtained, but also excellent creep ductility can be obtained.

However, it was found that although the resistance to polythionic acid SCC, the resistance to naphthenic acid corrosion, and the creep ductility of the base material of the austenitic stainless steel welded joint having the above chemical composition as the base material were investigated, excellent polythionic acid SCC resistance and excellent naphthenic acid corrosion resistance were obtained, but the excellent creep ductility of the base material was not necessarily obtained in some cases. Therefore, the present inventors have conducted further studies. As a result, the creep ductility of the base material is considered to be the following mechanism.

As described above, in the present embodiment, in order to improve the polythionic acid SCC resistance and the naphthenic acid corrosion resistance, the content of C is not more than 0.030%, and Nb is contained in an amount of 0.20 to 1.00%, and C is fixed to Nb to reduce solid solution C. Specifically, Nb is bonded to C by solution treatment or aging in a short time, and precipitates as MX-type carbonitride. However, the austenitic stainless steel welded joint according to the present embodiment is used for a long time (at least 3000 hours or more) in a high-temperature corrosive environment (a corrosive environment of 600 to 700 ℃). In such an environment, MX type carbonitrides are metastable phases. Therefore, when the base material having the above chemical composition is used for a long time in a high-temperature corrosive environment of 600 to 700 ℃, MX-type carbonitride of Nb changes into Z-phase (CrNbN) and M as steady-state phases₂₃C₆A type carbide. At this time, B and M segregated in the grain boundary₂₃C₆A part of C in the type carbide is replaced by M₂₃C₆And absorbed by the carbide. Therefore, the amount of B segregated in the grain boundaries decreases, and the grain boundary strength decreases. As a result, it is considered that sufficient creep ductility cannot be obtained.

Therefore, a method for suppressing the reduction of the amount of segregated B at the grain boundary in the use of an austenitic stainless steel welded joint under a high-temperature corrosive environment of 600 to 700 ℃ for a long time is further studied. As a result, the following mechanism is considered.

Mo is in the above-mentioned range, and M is suppressed₂₃C₆Formation of type carbides. Mo is further present with M₂₃C₆A part of M in the type carbide is substituted at M₂₃C₆The case of solid solution in the form of carbide. In this specification, M having Mo dissolved therein will be described₂₃C₆Type carbide is defined as "Mo solid solution M₂₃C₆Type carbide ". Mo solid solution M₂₃C₆The type carbide is not easy to dissolve B in solid. Therefore, even if MX-type carbonitride containing Nb is changed to Z phase and M phase in the use of welded austenitic stainless steel joints in high-temperature corrosive environments₂₃C₆In the case of type carbide, if M₂₃C₆The carbide type is Mo solid solution M₂₃C₆Type carbide, B can be inhibited from going to M₂₃C₆The solid solution of the form carbide suppresses the reduction of the amount of B segregated at the grain boundary. As a result, it is considered that excellent polythionic acid SCC resistance, excellent naphthenic acid corrosion resistance, and excellent creep ductility can be obtained.

Therefore, in the austenitic stainless steel welded joint having the base material having the above chemical composition, further studies have been made on the transformation of MX type carbonitride containing Nb into Z phase and M phase in the use thereof in a high temperature corrosive environment at 600 to 700 ℃₂₃C₆In the case of the type carbide, Mo solid solution M is also formed₂₃C₆And a type carbide, whereby the chemical composition of the base material in which the amount of segregated B at the grain boundaries is reduced can be suppressed. As a result, it was found that M is dissolved in Mo as a solid solution₂₃C₆The segregation B amount is suppressed by the formation of the type carbide, and B, C and Mo in the chemical composition are closely related to each other. Further, it is known that B, C and Mo in the chemical composition of the base material satisfy formula (1), and thus the base material can have excellent polythionic acid SCC resistance, excellent naphthenic acid corrosion resistance, and excellent creep ductility of the base material even when used in a high-temperature corrosion environment of 600 to 700 ℃.

B+0.004-0.9C+0.017Mo²≥0 (1)

Here, the content (mass%) of the corresponding element is substituted into each element symbol of formula (1).

As a result of further studies, the present inventors have found that when Cu is contained in the base material of the austenitic stainless steel welded joint as an arbitrary element, excellent creep strength is obtained and creep ductility can be maintained even if Cu is contained at 5.00% or less, and that when the upper limit of the Cu content is 1.70% or less, creep strength can be further improved and high creep ductility of the base material can be further maintained. The reason for this is considered as follows. When used in a high-temperature corrosive environment, Cu precipitates in the grains to form a Cu phase. The Cu phase may increase creep strength but decrease creep ductility. Therefore, the Cu content in the base material of the welded joint having the chemical composition and satisfying the formula (1) is more preferably 1.70% or less. When the Cu content is 1.70% or less, excellent creep ductility can be maintained more effectively.

As a result of further studies, the present inventors have found that the creep ductility of the base material is further improved when the Mo content in the chemical composition of the base material of the austenitic stainless steel welded joint is 0.50% or more. The reason is not clear, but the following is considered. When the content of Mo in the base material of the austenitic stainless steel welded joint having the chemical composition satisfying the formula (1) is further 0.50% or more, Mo is further segregated in grain boundaries or generates intermetallic compounds during use in a high-temperature corrosive environment at 600 to 700 ℃. The grain boundary strength is further improved by the segregation and intermetallic compound of the grain boundary. As a result, the creep ductility is further improved. In particular, when the Mo content of the base material is 1.00% or more, the base material can obtain very excellent creep ductility.

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

Further, it has been found that when the above chemical composition of the weld metal satisfies formula (2), the austenitic stainless steel weld joint is excellent in polythionic acid corrosion resistance and naphthenic acid corrosion resistance, and the base metal has excellent creep ductility, and the weld metal has high toughness after high-temperature aging.

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

Wherein the content (mass%) of the corresponding element is substituted into the symbol of the element in the formula (2).

The gist of the austenitic stainless steel welded joint according to the present embodiment completed based on the above findings is as follows.

[1] The austenitic stainless steel welded joint of (1) comprises a base metal and a weld metal,

the chemical composition of the base material is calculated by mass%

C: less than 0.030%,

Si：0.10～1.00％、

Mn：0.20～2.00％、

P: less than 0.040%,

S: less than 0.010%,

Cr：16.0～25.0％、

Ni：10.0～30.0％、

Mo：0.10～5.00％、

Nb：0.20～1.00％、

N：0.050～0.300％、

sol.Al：0.001～0.100％、

B：0.0010～0.0080％、

Cu：0～5.00％、

W：0～5.0％、

Co：0～1.0％、

V：0～1.00％、

Ta：0～0.20％、

Hf：0～0.20％、

Ca：0～0.010％、

Mg：0～0.010％、

Rare earth elements: 0 to 0.100%, and

and the balance: fe and impurities, and

satisfies formula (1);

in the case of the weld metal,

the chemical composition in mass% at the widthwise central position and the thickness central position of the weld metal

C: less than 0.050%,

Si：0.01～1.00％、

Mn：0.01～3.00％、

P: less than 0.030%,

S: less than 0.015%,

Cr：15.0～25.0％、

Ni：20.0～70.0％、

Mo：1.30～10.00％、

Nb：0.05～3.00％、

N: less than 0.150 percent,

B: less than 0.0050%,

sol.Al：0～1.000％、

Cu：0～2.50％、

W：0～1.0％、

Co：0～15.0％、

V：0～0.10％、

Ti：0～0.50％，

Ta：0～0.20％、

Ca：0～0.010％、

Mg：0～0.010％、

Rare earth elements: 0 to 0.100%, and

and the balance: fe and impurities.

B+0.004-0.9C+0.017Mo²≥0 (1)

Wherein the content (mass%) of the corresponding element is substituted at the symbol of the element in the formula (1).

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

In the austenitic stainless steel welded joint according to the present embodiment, the chemical composition of the base material contains the elements in the above-described ranges and satisfies the formula (1), and further, the chemical composition of the welded metal at the center of the width and the center of the thickness contains the elements in the above-described ranges. Therefore, the austenitic stainless steel weld joint of the present embodiment is excellent in polythionic acid SCC resistance and naphthenic acid corrosion resistance. Furthermore, the base material has excellent creep ductility under a high-temperature corrosion environment at 600-700 ℃.

[2] The austenitic stainless steel welded joint of [1], wherein,

the chemical composition of the base material is selected from the group consisting of

Cu：0.10～5.00％、

W：0.1～5.0％、

Co：0.1～1.0％、

V：0.10～1.00％、

Ta：0.01～0.20％、

Hf：0.01～0.20％、

Ca：0.001～0.010％、

Mg: 0.001 to 0.010%, and

rare earth elements: 0.001-0.100% of 1 or more elements in the group.

[3] The austenitic stainless steel welded joint of [1] or [2], wherein,

the aforementioned chemical composition of the aforementioned weld metal comprises a composition selected from the group consisting of

sol.Al：0.001～1.000％、

Cu：0.01～2.50％、

W：0.1～1.0％、

Co：0.1～15.0％、

V：0.01～0.10％、

Ti：0.01～0.50％，

Ta：0.01～0.20％、

Ca：0.001～0.010％、

Mg: 0.001 to 0.010%, and

rare earth elements: 0.001-0.100% of 1 or more elements in the group.

[4] The austenitic stainless steel welded joint of (1) to (3), wherein,

the chemical composition of the weld metal satisfies formula (2).

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

Wherein the content (mass%) of the corresponding element is substituted into the symbol of the element in the formula (2).

In this case, the austenitic stainless steel welded joint is excellent in polythionic acid corrosion resistance and naphthenic acid corrosion resistance, and also excellent in creep ductility of the base metal, and further excellent in toughness after high-temperature aging of the weld metal.

The austenitic stainless steel welded joint according to the present embodiment will be described in detail below. In the present specification, "%" of an element means mass% unless otherwise specified.

[ constitution of Austenitic stainless Steel welded Joint ]

Fig. 1 is a plan view showing an example of an austenitic stainless steel welded joint 1 according to the present embodiment. Referring to fig. 1, an austenitic stainless steel welded joint 1 according to the present embodiment includes a base material 10 and a weld metal 20. After the end portions of the pair of base materials 10 whose end portions have been beveled are joined to each other, welding is performed to form the weld metal 20. Examples of Welding are Gas Tungsten Arc Welding (GTAW), Shielded Metal Arc Welding (SMAW), Flux Cored Arc Welding (FCAW), Gas Metal Arc Welding (GMAW), and Submerged Arc Welding (SAW).

In fig. 1, a direction in which the weld metal 20 extends is defined as a weld metal extending direction L, a direction perpendicular to the weld metal extending direction L in a plan view is defined as a weld metal width direction W, and a direction perpendicular to the weld metal extending direction L and the weld metal width direction W is defined as a weld metal thickness direction T. Fig. 2 is a cross-sectional view of the austenitic stainless steel welded joint 1 of fig. 1 cut in the weld metal width direction W. As shown in fig. 1 and 2, the weld metal 20 is disposed between the pair of base materials 10.

Fig. 3 is a sectional view of the austenitic stainless steel welded joint 1 shown in fig. 1 cut in the weld metal extending direction L, and fig. 4 is a sectional view of the austenitic stainless steel welded joint 1 different from fig. 3 cut in the weld metal extending direction L. As shown in fig. 3, the base material 10 may be a plate material, or as shown in fig. 4, the base material 10 may be a steel pipe. Although not shown, the base material 10 may be a bar steel or a section steel. The base material 10 and the weld metal 20 will be described below.

[ base Material 10]

[ chemical composition ]

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

C: less than 0.030%

Carbon (C) is inevitably contained. That is, the C content exceeds 0%. C in the use of the austenitic stainless steel welded joint of the present embodiment in a high temperature corrosive environment of 600 to 700 ℃, M is generated at the grain boundary in the base material₂₃C₆The carbonitride reduces the polythioacid SCC resistance of the base material 10. Therefore, the C content is 0.030% or less. The upper limit of the C content is preferably 0.020%, more preferably 0.018%, still more preferably 0.016%, and yet more preferably 0.015%. The C content is preferably as low as possible. However, an excessive reduction in the C content increases the manufacturing cost. Therefore, the lower limit of the C content is preferably 0.001%, more preferably 0.002% in industrial production.

Si：0.10～1.00％

Silicon (Si) deoxidizes steel. Si further improves the oxidation resistance and steam oxidation resistance of the base material 10. If the Si content is too low, the above-mentioned effects cannot be obtained. On the other hand, if the Si content is too high, a sigma phase (σ phase) precipitates in the base material 10, and the toughness of the steel decreases. Therefore, the Si content is 0.10 to 1.00%. The lower limit of the Si content is preferably 0.15%, more preferably 0.17%, even more preferably 0.18%, even more preferably 0.20%, and even more preferably 0.25%. The upper limit of the Si content is preferably 0.75%, more preferably 0.70%, even more preferably 0.50%, and even more preferably 0.45%.

Mn：0.20～2.00％

Manganese (Mn) deoxidizes steel. Mn further stabilizes austenite, and improves creep strength of the base material 10. If the Mn content is too low, the above-described effects cannot be obtained. On the other hand, if the Mn content is too high, the creep strength of the base material 10 is rather lowered. Therefore, the Mn content is 0.20 to 2.00%. The lower limit of the Mn content is preferably 0.30%, more preferably 0.40%, further preferably 0.50%, further preferably 0.60%, further preferably 0.70%, further preferably 0.80%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.70%, still more preferably 1.60%, and still more preferably 1.50%.

P: less than 0.040%

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

S: 0.010% or less

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

Cr：16.0～25.0％

Chromium (Cr) improves the polythioacid SCC resistance and naphthenic acid corrosion resistance of the base material 10. If the Cr content is too low, the above-described 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 lower limit of the Cr content is preferably 16.5%, more preferably 17.0%, even more preferably 17.2%, and even more preferably 17.4%. The upper limit of the Cr content is preferably 24.0%, more preferably 23.0%, and still more preferably 22.0%.

Ni：10.0～30.0％

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

Mo：0.10～5.00％

When molybdenum (Mo) is used in a high-temperature corrosion environment at 600-700 ℃, M is inhibited from being generated in grain boundaries₂₃C₆A type carbide. This improves the resistance of base material 10 to polythionic acid SCC. In addition, when Mo is used in the austenitic stainless steel welded joint 1 in a high-temperature corrosive environment, the solid-solution Mo in the base material 10 bonds with S in the use environment, and a sulfide film is formed on the surface of the base material 10. The sulfide film is formed, thereby providing high naphthenic acid corrosion resistance. For Mo, and further in the use of the Mo in a high-temperature corrosion environment at 600-700 ℃, MX-type carbo-nitriding of NbChange of substance to M₂₃C₆Type carbide, B is inhibited at M₂₃C₆The form carbide is dissolved in a solid solution, and the amount of B segregation in the grain boundary under a high-temperature corrosive environment is suppressed to decrease. Thereby, sufficient creep ductility can be obtained in a high-temperature corrosive environment. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the austenite steady-state property is lowered. Therefore, the Mo content is 0.10 to 5.00%. The lower limit of the Mo content is preferably 0.20%, and more preferably 0.30%.

When the Mo content is 0.50% or more, Mo is further segregated in the grain boundary or an intermetallic compound is generated, and the grain boundary strength is further improved. In this case, even more excellent creep ductility can be obtained in a high-temperature corrosive environment. Therefore, the lower limit of the Mo content is more preferably 0.50%, still more preferably 0.80%, still more preferably 1.00%, and still more preferably 2.00%. When the Mo content is 1.00% or more, the base material 10 can obtain particularly excellent creep ductility. The upper limit of the Mo content is preferably 4.50%, and more preferably 4.00%.

Nb：0.20～1.00％

When used in a high-temperature corrosive environment of 600-700 ℃, niobium (Nb) bonds with C to form MX-type carbonitride, thereby reducing the amount of solid solution C in the parent material 10. This provides base material 10 with high resistance to SCC polythionate and naphthenic acid corrosion. The resulting MX-type carbonitride of Nb also improves the creep strength of the base material 10. If the Nb content is too low, the above-described effects cannot be obtained. On the other hand, if the Nb content is too high, ferrite is generated, and the creep strength, toughness, and weldability of the base material 10 are reduced. Therefore, the Nb content is 0.20 to 1.00%. The lower limit of the Nb content is preferably 0.25%, more preferably 0.28%, even more preferably 0.30%, and even more preferably 0.32%. The upper limit of the Nb content is preferably 0.90%, more preferably 0.80%, still more preferably 0.70%, and still more preferably 0.65%.

N：0.050～0.300％

Nitrogen (N) is dissolved in the matrix (parent phase) to stabilize austenite, thereby improving the creep strength of the base material 10. N further forms fine carbonitrides 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 N content is too low, the above-mentioned effects cannot be obtained. On the other hand, if the N content is too high, Cr nitrides are formed at the grain boundaries, and the polythionic acid SCC resistance and the naphthenic acid corrosion resistance in the welding Heat Affected Zone (HAZ) of base material 10 are reduced. If the N content is too high, the workability of the steel is further lowered. Therefore, the N content is 0.050 to 0.300%. The lower limit of the N content is preferably 0.060%, more preferably 0.070%, and still more preferably 0.080%. The upper limit of the N content is preferably 0.250%, more preferably 0.200%, and still more preferably 0.190%.

sol.Al：0.001～0.100％

Aluminum (Al) deoxidizes steel. If the Al content is too low, the above-mentioned effects cannot be obtained. On the other hand, if the Al content is too high, the cleanliness of the steel decreases, and the workability and ductility of the steel decrease. Therefore, the Al content is 0.001 to 0.100%. The lower limit of the Al content is preferably 0.002%, and more preferably 0.003%. The upper limit of the Al content is preferably 0.050%, more preferably 0.030%, and still more preferably 0.025%. In the present embodiment, the Al content means the content of acid-soluble Al (sol.al).

B：0.0010～0.0080％

Boron (B) is segregated in the grain boundary and improves the grain boundary strength when used in a high-temperature corrosion environment at 600-700 ℃. As a result, the creep ductility of the base material 10 is improved in a high-temperature corrosive environment at 600 to 700 ℃. If the content of B is too low, the above-mentioned effects cannot be obtained. On the other hand, if the B content is too high, weldability and hot workability at high temperature are degraded. Further, the content of B in the weld metal is high during welding, and solidification cracks occur in the weld metal 20. Therefore, the content of B is 0.0010-0.0080%. The lower limit of the B content is preferably 0.0015%, more preferably 0.0018%, even more preferably 0.0020%, and even more preferably 0.0022%. The upper limit of the B content is preferably less than 0.0060%, and more preferably 0.0050%.

The remainder 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 mean substances mixed from ores, scraps, manufacturing environments, and the like as raw materials in the industrial production of the base material 10, and mean substances that are acceptable within a range that does not adversely affect the base material 10 of the austenitic stainless steel welded joint 1 of the present embodiment.

[ for any element ]

[ any element of group 1]

The chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of Cu, W, and Co in place of a part of Fe. These elements all improve the creep strength of the base material 10.

Cu：0～5.00％

Copper (Cu) is an arbitrary element, and may not be contained. That is, Cu may be 0%. When Cu is contained, Cu precipitates as a Cu phase in grains when used in a high-temperature corrosive environment of 600 to 700 ℃, and the creep strength of the base material 10 is improved by precipitation strengthening. However, if the Cu content is too high, hot workability and weldability of the steel decrease. Therefore, the Cu content is 0 to 5.00%. The lower limit of the Cu content is preferably more than 0%, more preferably 0.10%, even more preferably 0.11%, even more preferably 0.12%, even more preferably 2.00%, and even more preferably 2.50% in order to more effectively improve the creep strength of the base material 10 of the austenitic stainless steel welded joint 1. The upper limit of the Cu content is preferably 4.50%, more preferably 4.00%, further preferably 3.80%, further preferably 3.70%, further preferably 3.60%, further preferably 3.50%, further preferably 1.90%. Particularly, in a high-temperature corrosive environment of 600 to 700 ℃, the Cu content is preferably 0 to 1.70%, and the upper limit of the Cu content is more preferably 1.60% in order to maintain more excellent creep ductility.

W：0～5.0％

Tungsten (W) is an arbitrary element, and may not be contained. That is, W may be 0%. If contained, W is dissolved in the matrix (parent phase) to improve the creep strength of the base material 10 of the austenitic stainless steel welded joint 1. However, if the W content is too high, the austenite steady state property is lowered, and the creep strength and toughness of the base material 10 are lowered. Therefore, the W content is 0 to 5.0%. The lower limit of the W content is preferably more than 0%, more preferably 0.1%, still more preferably 0.2%, and still more preferably 0.5%. The upper limit of the W content is preferably 4.5%, more preferably 4.0%, and still more preferably 3.5%.

Co：0～1.0％

Cobalt (Co) is an arbitrary element, and may or may not be contained. That is, the Co content may be 0%. When contained, Co stabilizes austenite and improves creep strength of the base material 10 of the austenitic stainless steel welded joint 1. However, if the Co content is too high, the raw material cost is high. Therefore, the Co content is 0 to 1.0%. The lower limit of the Co content is preferably more than 0%, more preferably 0.1%, even more preferably 0.2%, and even more preferably 0.3%. The upper limit of the Co content is preferably 0.9%, and more preferably 0.8%.

[ any element of group 2]

The chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of V, Ta and Hf in place of a part of Fe. These elements all improve the resistance of the base material to polythionic acid SCC and the creep strength.

V：0～1.00％

Vanadium (V) is an arbitrary element, and may or may not be contained. That is, the V content may be 0%. When V is contained, V bonds with C to form carbonitride, reduces solid solution C, and improves the resistance of the base material 10 to polythionic acid SCC when V is used in a high-temperature corrosive environment of 600 to 700 ℃. The generated V carbonitride also improves the creep strength of the base material 10. However, if the V content is too high, ferrite is generated, and the creep strength, toughness, and weldability of the base material 10 are lowered. Therefore, the V content is 0 to 1.00%. The lower limit of the V content is preferably more than 0%, more preferably 0.10% in order to more effectively improve the polythionic acid SCC resistance and the creep strength. The upper limit of the V content is preferably 0.90%, and more preferably 0.80%.

Ta：0～0.20％

Tantalum (Ta) is an arbitrary element, and may not be contained. That is, the Ta content may be 0%. When Ta is contained, Ta bonds with C to form carbonitride when used in a high-temperature corrosive environment of 600 to 700 ℃, reduces solid solution C, and improves the polythionic acid SCC resistance of the base material 10. The resulting Ta carbonitride also improves creep strength. However, if the Ta content is too high, ferrite is generated, and the creep strength, toughness, and weldability of the base material 10 are lowered. Therefore, the Ta content is 0 to 0.20%. The lower limit of the Ta content is preferably more than 0%, more preferably 0.01%, and still more preferably 0.02% in order to further effectively improve the polythionic acid SCC resistance and the creep strength. The upper limit of the Ta content is preferably 0.18%, and more preferably 0.16%.

Hf：0～0.20％

Hafnium (Hf) is an arbitrary element, and may not be contained. That is, the Hf content may be 0%. When contained, Hf bonds with C to form carbonitride in use in a high-temperature corrosive environment of 600 to 700 ℃, thereby reducing solid solution C and improving the polythionic acid SCC resistance of the base material 10. The generated Hf carbonitride also improves the creep strength of the base material 10. However, if the Hf content is too high, ferrite is generated, and the creep strength, toughness, and weldability of the base material 10 are lowered. Therefore, the Hf content is 0 to 0.20%. The lower limit of the Hf content is preferably more than 0%, more preferably 0.01%, and still more preferably 0.02%. The preferable upper limit of the Hf content is 0.18%, and more preferably 0.16%.

[ any element of group 3]

The chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM) in place of a part of Fe. These elements all improve hot workability and creep ductility of the base material.

Ca：0～0.010％

Calcium (Ca) is an arbitrary element, and may or may not be contained. That is, the Ca content may be 0%. If it is contained, Ca fixes O (oxygen) and S (sulfur) as inclusions, and improves hot workability and creep ductility of the base material 10. However, if the Ca content is too high, hot workability and creep ductility of the base material 10 are reduced. Therefore, the Ca content is 0 to 0.010%. The lower limit of the Ca content is preferably more than 0%, more preferably 0.001%, and still more preferably 0.002%. The upper limit of the Ca content is preferably 0.008%, and more preferably 0.006%.

Mg：0～0.010％

Magnesium (Mg) is an arbitrary element, and may or may not be contained. That is, the Mg content may be 0%. If contained, Mg fixes O (oxygen) and S (sulfur) as inclusions, and improves hot workability and creep ductility of the base material 10. However, if the Mg content is too high, hot workability and creep ductility of the base material 10 are reduced. Therefore, the Mg content is 0 to 0.010%. The lower limit of the Mg content is preferably more than 0%, more preferably 0.001%, and still more preferably 0.002%. The upper limit of the Mg content is preferably 0.008%, and more preferably 0.006%.

Rare earth elements: 0 to 0.100 percent

The rare earth element (REM) is an arbitrary element and may or may not be contained. That is, the REM content may be 0%. When contained, REM fixes O (oxygen) and S (sulfur) as inclusions, and improves hot workability and creep ductility of the base material. However, if the REM content is too high, hot workability and creep ductility of the base material decrease. Therefore, the REM content is 0 to 0.10%. The lower limit of the REM content is preferably more than 0%, more preferably 0.001%, and still more preferably 0.002%. The upper limit of the REM content is preferably 0.080%, and more preferably 0.060%.

REM in the present specification contains at least 1 or more of Sc, Y and lanthanoid (La of atomic number 57-71 Lu), and the REM content means the total content of these elements.

[ for formula (1) ]

The chemical composition of the base material 10 also satisfies formula (1).

B+0.004-0.9C+0.017Mo²≥0 (1)

The content (mass%) of the corresponding element is substituted into each element symbol in the formula (1).

As described above, in the present embodiment, in order to improve the polythionic acid SCC resistance and the naphthenic acid corrosion resistance, the C content is 0.030% or less and Nb is contained in an amount of 0.20 to 1.00%. Therefore, the high-temperature corrosion environment at 600-700 ℃ is realizedIn the following use, MX-type carbonitride of Nb is formed, and the amount of solid-solution C is reduced. However, MX-type carbonitride of Nb is a metastable phase, and therefore, during long-term use under the above-described high-temperature corrosive environment, MX-type carbonitride of Nb changes into a Z-phase and M-phase₂₃C₆A type carbide. In this case, B segregated in the grain boundary is in M₂₃C₆The form carbide is dissolved in the matrix carbide, and the amount of B segregation in the grain boundary is reduced. As a result, the creep ductility of the base material 10 is reduced.

However, if Mo is in M₂₃C₆Form carbide is dissolved in solid solution to form' Mo solid solution M₂₃C₆Type carbide ", M is dissolved in Mo as a solid₂₃C₆B in the carbide is not easily dissolved in a solid. Therefore, the B segregation amount at the grain boundary is maintained, and not only excellent polythionic acid SCC resistance and naphthenic acid corrosion resistance are obtained, but also excellent creep ductility is obtained in the base material 10.

Definition of F1 ═ B +0.004-0.9C +0.017Mo². F1 represents a plurality of M species generated in the base material 10 in use in a high-temperature corrosive environment₂₃C₆Mo in the form of carbide solid solution M₂₃C₆An indication of the proportion of type carbide. When F1 is 0 or more, even if a plurality of kinds of M are formed in the steel in use under a high-temperature corrosive environment₂₃C₆Form carbide, Mo solid solution M₂₃C₆The proportion of type carbides is also high. Therefore, B segregated in the grain boundary is not easily dissolved in M₂₃C₆The carbide is maintained in the amount of B segregated in the grain boundary. Therefore, the base material 10 can have excellent polythionic acid SCC resistance, excellent naphthenic acid corrosion resistance, and excellent creep ductility. Therefore, F1 is 0(0.00000) or more. F1 is preferably 0.00100 or more, more 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, and further preferably 0.02000 or more.

When the chemical composition of the base material contains Cu, the upper limit of the Cu content is preferably 1.70% or less, as described above. That is, in view of improving creep strength and further obtaining excellent creep ductility, the Cu content is preferably more than 0% and 1.70% or less. When the Cu content is 1.70% or less, excellent creep strength can be obtained due to precipitation strengthening of the Cu phase, and excellent creep ductility of the base material can be maintained.

In the chemical composition of the base material, the lower limit of the Mo content is preferably 0.50%. In this case, Mo is segregated in grain boundaries or forms intermetallic compounds in the use under a high-temperature corrosive environment of 600 to 700 ℃. The grain boundary strength is further improved by the segregation and intermetallic compound of the grain boundary. As a result, the creep ductility of the base material is further improved. Therefore, the lower limit of the Mo content is preferably 0.50%. When the lower limit of the Mo content is 1.00% or more, the creep ductility of the base material is significantly improved. When the Mo content is 0.50% or more, the F1 value is preferably 0.00500 or more, more preferably 0.00800 or more, more preferably 0.01000 or more, and more preferably 0.02000 or more.

[ for weld metal 20]

[ chemical composition ]

Fig. 5 is a cross-sectional view perpendicular to the weld metal extending direction L in the austenitic stainless steel welded joint 1 according to the present embodiment. Referring to fig. 5, in a cross section of the weld metal 20 perpendicular to the weld metal extending direction L, a width at the outermost surface of the weld metal 20 is defined as w (mm). The thickness of the weld metal 20 at the center of the width W is defined as t (mm). At this time, the chemical composition of the region P, which is the width center position and the thickness center position of the weld metal 20 (i.e., t/2 depth position from the outermost surface of the weld metal 20), contains the following elements.

C: 0.050% or less

Carbon (C) is inevitably contained. Namely, the C content exceeds 0%. C in the use of the austenitic stainless steel welded joint 1 of the present embodiment in a high-temperature corrosive environment of 600 to 700 ℃, M is generated in the grain boundaries in the weld metal 20₂₃C₆The carbide type reduces the resistance of the weld metal 20 to polythionic acid SCC and naphthenic acid corrosion. Therefore, the C content is 0.050% or less. The upper limit of the C content is preferably 0.040%, more preferably 0.030%, even more preferably 0.025%, and even more preferablyThe step is preferably 0.020%. The C content is preferably as low as possible. However, an excessive reduction in the C content increases the manufacturing cost. Therefore, the lower limit of the C content is preferably 0.001%, more preferably 0.005%, in terms of industrial production.

Si：0.01～1.00％

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

Mn：0.01～3.00％

Manganese (Mn) deoxidizes the weld metal 20 at the time of welding. If the Mn content is too low, the effect cannot be obtained. 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 lower limit of the Mn content is preferably 0.05%, more preferably 0.08%, even more preferably 0.10%, even more preferably 0.14%, and even more preferably 0.16%. The upper limit of the Mn content is preferably 2.70%, more preferably 2.50%, and still more preferably 2.30%.

P: less than 0.030%

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

S: less than 0.015%

Sulfur (S) is an impurity inevitably contained. That is, the S content exceeds 0%. S reduces 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 upper limit of the S content is preferably 0.010%, and more preferably 0.007%. The S content is preferably as low as possible. However, an excessive reduction in the S content increases the manufacturing cost. Therefore, the preferable lower limit of the S content is 0.001%, and more preferably 0.002% in industrial production.

Cr：15.0～25.0％

Chromium (Cr) improves the polythionic acid SCC resistance and naphthenic acid corrosion resistance of the weld metal 20. Cr further improves oxidation resistance, steam oxidation resistance, high-temperature corrosion resistance, and the like. If the Cr content is too low, the above-described effects cannot be obtained. On the other hand, if the Cr content is too high, the creep strength and toughness of the weld metal 20 are reduced. Therefore, the Cr content is 15.0 to 25.0%. The lower limit of the Cr content is preferably 16.5%, and more preferably 17.0%. The upper limit of the Cr content is preferably 24.0%, and more preferably 23.0%.

Ni：20.0～70.0％

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

Mo：1.30～10.00％

Molybdenum (Mo) inhibits the grain boundary formation M in the weld metal 20 when used in a high-temperature corrosion environment of 600-700 DEG C₂₃C₆A type carbide. This improves the resistance of the weld metal 20 to polythionic acid SCC. Further, when Mo is used in the welded joint in the high-temperature corrosive environment, the solid-solution Mo in the weld metal 20 bonds with S in the use environment, and a sulfide film is formed on the surface of the weld metal 20. The formation of the sulfide film improves the naphthenic acid corrosion resistance of the weld metal 20. For Mo, and further in use in a high-temperature corrosion environment at 600-700 ℃, MX-type carbon nitrogen of NbChange of chemical compound to M₂₃C₆Type carbide, B is inhibited at M₂₃C₆The form carbide is dissolved in a solid solution, and the amount of B segregation in the grain boundary under a high-temperature corrosive environment is suppressed to decrease. Thereby, sufficient creep ductility can be obtained in a high-temperature corrosive environment. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the austenite steady-state property is lowered. Therefore, the Mo content is 1.30 to 10.00%. The lower limit of the Mo content is preferably 1.40%, more preferably 1.50%, further preferably 2.00%, further preferably 3.00%, further preferably 4.00%, further preferably more than 5.00%. The upper limit of the Mo content is preferably 9.00%, and more preferably 8.50%.

Nb：0.05～3.00％

When the niobium (Nb) is used in a high-temperature corrosion environment at 600-700 ℃, the Nb and C are bonded to generate MX-type carbonitride, and the amount of solid solution C in the weld metal 20 is reduced. This improves the resistance of the weld metal 20 to polythionic acid SCC. The resulting MX-type carbonitride of Nb also improves creep strength. However, if the Nb content is too high, ferrite is generated, and the long-term creep strength, toughness, and weldability of the weld metal 20 are lowered. Therefore, the Nb content is 0.05 to 3.00%. The lower limit of the Nb content is preferably 0.06%, more preferably 0.07%, further preferably 0.10%, further preferably 0.15%, further preferably 0.18%, further preferably 0.20%. The upper limit of the Nb content is preferably 2.90%, more preferably 2.50%, and still more preferably 2.00%.

N: less than 0.150%

Nitrogen (N) is inevitably contained. That is, the N content exceeds 0%. N is dissolved in the matrix (parent phase) to stabilize austenite, thereby improving the creep strength of the weld metal 20. N also forms fine carbonitrides within the grains, improving the creep strength of the weld metal 20. That is, N contributes to 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 grain boundaries, and the resistance of the weld metal 20 to polythionic acid SCC and naphthenic acid corrosion is reduced. If the N content is too high, the ductility of the weld metal 20 further decreases. Therefore, the N content is 0.150% or less. The lower limit of the N content is preferably 0.010%, more preferably 0.050%, further preferably 0.080%, and further preferably 0.100%. The upper limit of the N content is preferably 0.140%, and more preferably 0.130%.

B: 0.0050% or less

Boron (B) is inevitably contained. Namely, the B content exceeds 0%. And B is segregated in the grain boundary when used in a high-temperature corrosion environment at 600-700 ℃, so that the grain boundary strength is improved. As a result, the creep ductility of the weld metal 20 is improved in a high-temperature corrosive environment at 600 to 700 ℃. And the creep strength of the weld metal 20 is further improved under the high-temperature corrosion environment of 600-700 ℃. However, if the B content is too high, solidification cracks occur in the weld metal 20 during welding. Therefore, the B content in the chemical composition of the weld metal 20 is 0.0050% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, further preferably 0.0010%, further preferably 0.0015%, further preferably 0.0020%, and further preferably 0.0030%. When the B content in the weld metal 20 is 0.0030% or more, particularly the weld joint 1 obtains excellent creep strength. The upper limit of the B content is preferably 0.0045%, and more preferably 0.0040%.

The balance of the chemical composition of the weld metal 20 in the austenitic stainless steel welded joint 1 according to the present embodiment is Fe and impurities. Here, the impurities are those which are mixed from the welding material as the raw material and the welding environment when the weld metal 20 is formed, and are allowed within a range where the impurities do not adversely affect the weld metal 20.

[ for any element ]

[ any element of group 1]

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.

sol.Al：0～1.000％

Aluminum (Al) is an arbitrary element, and may not be contained. That is, the Al content may be 0%. When 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 lower limit of the Al content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, even more preferably 0.010%, and even more preferably 0.050%. The upper limit of the Al content is preferably 0.850%, and more preferably 0.800%. In the present embodiment, the Al content means the content of acid-soluble Al (sol.al).

[ any element of group 2]

The chemical composition of the weld metal 20 according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of Cu, W, and Co in place of a part of Fe. Both of these elements improve the creep strength of the weld metal 20.

Cu：0～2.50％

Copper (Cu) is an arbitrary element, and may not be contained. That is, the Cu content may be 0%. When Cu is contained, Cu precipitates as a Cu phase in grains when used in a high-temperature corrosive environment of 600 to 700 ℃, and the creep strength of the weld metal 20 is improved by precipitation strengthening. However, if the Cu content is too high, weldability of the weld metal 20 is lowered, and cracks may occur during welding. Therefore, the Cu content is 0 to 2.50%. The lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%, still more preferably 0.50%, still more preferably 1.00%, and still more preferably 1.20% in order to further effectively improve the creep strength of the weld metal 20. The upper limit of the Cu content is preferably 2.30%, more preferably 2.10%, still more preferably 1.90%, still more preferably 1.80%, and still more preferably 1.70%.

W：0～1.0％

Tungsten (W) is an arbitrary element, and may not be contained. That is, the W content may be 0%. When W is contained, W is dissolved in the weld metal 20 in a solid state, and the creep strength of the weld metal 20 is improved when W is used in a high-temperature corrosive environment of 600 to 700 ℃. However, if the W content is too high, the austenite steady state property decreases, and the creep strength and toughness of the weld metal 20 decrease. Therefore, the W content is 0 to 1.0%. The lower limit of the W content is preferably more than 0%, more preferably 0.1%, and still more preferably 0.2%. The upper limit of the W content is preferably 0.9%, more preferably 0.7%, and still more preferably 0.5%.

Co：0～15.0％

Cobalt (Co) is an arbitrary element, and may or may not be contained. That is, the Co content may be 0%. When contained, Co stabilizes austenite and improves creep strength of the weld metal 20. However, if the Co content is too high, the raw material cost is high. Therefore, the content of Co is 0 to 15.0%. The lower limit of the Co content is preferably more than 0%, more preferably 0.1%, even more preferably 1.0%, even more preferably 2.0%, even more preferably 2.5%. The upper limit of the Co content is preferably 12.0%, and more preferably 11.0%.

[ any element of group 3]

The chemical composition of the weld metal 20 of the austenitic stainless steel weld joint 1 according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of V, Ti and Ta in place of a part of Fe. These elements all improve the resistance of the steel to polythionic acid SCC, as well as creep strength.

V：0～0.10％

Vanadium (V) is an arbitrary element, and may or may not be contained. That is, the V content may be 0%. When V is used in a high-temperature corrosion environment at 600-700 ℃, V bonds with C to generate carbonitride, reduces solid solution C, and improves the polythionic acid (SCC) resistance of the weld metal 20. The resulting 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 lower limit of the V content is preferably more than 0%, more preferably 0.01%, and still more preferably 0.02%. The upper limit of the V content is preferably 0.09%, and more preferably 0.08%.

Ti：0～0.50％

Titanium (Ti) is an arbitrary element, and may not be contained. That is, the Ti content may be 0%. When Ti is contained, Ti is bonded with C to generate carbide in the use under the high-temperature corrosion environment of 600-700 ℃, solid solution C is reduced, and the resistance of the welding metal 20 to polythionic acid SCC is improved. The resulting Ti carbides also improve 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 lower limit of the Ti content is preferably more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Ti content is preferably 0.45%, and more preferably 0.40%.

Ta：0～0.20％

Tantalum (Ta) is an arbitrary element, and may not be contained. That is, the Ta content may be 0%. When the Ta is contained, in the use of the high-temperature corrosion environment at 600-700 ℃, the Ta is bonded with C to generate carbonitride, solid solution C is reduced, and the polythionic acid SCC resistance of the weld metal 20 is improved. The resulting Ta carbonitride also improves creep strength. However, if the Ta content is too high, the creep strength, toughness, and weldability of the weld metal 20 decrease. Therefore, the Ta content is 0 to 0.20%. The lower limit of the Ta content is preferably more than 0%, more preferably 0.01%, and still more preferably 0.02% in order to further effectively improve the polythionic acid SCC resistance and the creep strength. The upper limit of the Ta content is preferably 0.18%, and more preferably 0.16%.

[ any element of group 4]

The chemical composition of the weld metal 20 of the austenitic stainless steel weld joint 1 according to the present embodiment may further contain 1 element or 2 or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM) in place of a part of Fe. Both of these elements improve the deformability of the weld metal 20 at high temperatures (hot conditions).

Ca：0～0.010％

Calcium (Ca) is an arbitrary element, and may or may not be contained. That is, the Ca content may be 0%. If it is contained, Ca fixes O (oxygen) and S (sulfur) as inclusions, thereby improving the resistance to the reheat cracking of the weld metal 20. However, if the Ca content is too high, the solidification cracking resistance of the weld metal 20 is rather lowered. Therefore, the Ca content is 0 to 0.010%. The lower limit of the Ca content is preferably more than 0%, more preferably 0.001%, and still more preferably 0.002%. The upper limit of the Ca content is preferably 0.008%, and more preferably 0.006%.

Mg：0～0.010％

Magnesium (Mg) is an arbitrary element, and may or may not be contained. That is, the Mg content may be 0%. If contained, Mg fixes O (oxygen) and S (sulfur) as inclusions, and improves the resistance to reheat cracking of the weld metal 20. However, if the Mg content is too high, the solidification cracking resistance of the weld metal 20 under hot conditions is lowered. Therefore, the Mg content is 0 to 0.010%. The lower limit of the Mg content is preferably more than 0%, and more preferably 0.001%. The upper limit of the Mg content is preferably 0.008%, and more preferably 0.006%.

Rare earth elements: 0 to 0.100 percent

The rare earth element (REM) is an arbitrary element and may or may not be contained. That is, the REM content may be 0%. When contained, REM fixes O (oxygen) and S (sulfur) as inclusions to improve the resistance of the weld metal 20 to reheat cracking. However, if the REM content is too high, the solidification cracking resistance of the weld metal 20 is lowered. Therefore, the REM content is 0 to 0.100%. The lower limit of the REM content is preferably more than 0%, more preferably 0.001%, and still more preferably 0.002%. The upper limit of the REM content is preferably 0.080%, and more preferably 0.060%.

The austenitic stainless steel welded joint 1 according to the present embodiment includes a base material 10 having the above chemical composition and a weld metal 20 having the above chemical composition. Therefore, the polythionic acid SCC resistance is excellent, and the naphthenic acid corrosion resistance is excellent. Further, the base material 10 has excellent creep ductility in a high temperature corrosive environment at 600 to 700 ℃.

[ for formula (2) ]

The chemical composition of the region P at the center of the width and the center of the thickness of the weld metal 20 preferably satisfies the following formula (2).

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

Wherein the content (mass%) of the corresponding element is substituted into the symbol of the element in the formula (2).

Definition of F2 ═ 0.012Cr-0.005Ni +0.013Mo +0.023Nb +0.02Al-0.004 Co. As described above, when the austenitic stainless steel welded joint 1 according to the present embodiment is used as a member of a welded structure of chemical plant equipment, it is used in a high-temperature corrosive environment of 600 to 700 ℃ during operation, as described above. Therefore, the weld metal 20 of the austenitic stainless steel weld joint 1 is subjected to high temperature aging. On the other hand, the temperature of the austenitic stainless steel welded joint 1 is lowered to normal temperature when the equipment is stopped. The weld metal 20 is significantly affected by high-temperature aging at 600 to 700 ℃ as compared with the base metal 10 due to reheating and solidification segregation during welding. Therefore, in the austenitic stainless steel welded joint 1 used for applications where polythionic acid corrosion resistance and naphthenic acid corrosion resistance are required, the weld metal 20 preferably has excellent toughness even after high-temperature aging.

F2 is an index of toughness of the weld metal 20 after high-temperature aging. Ni and Co in F2 improve the toughness of the weld metal 20 when the elements in the chemical composition of the weld metal 20 fall within the ranges of the present embodiment. 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 are all precipitated by high temperature aging at 600 to 700 ℃. The formation of precipitates reduces the toughness by increasing 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.

In F2, Ni and Co, which are elements that improve the toughness of the weld metal, are negative values, and Cr, Mo, Nb, and Al, which are elements that reduce the toughness of the weld metal, are positive values. If F2 is greater than 0.176, the ratio of the element that decreases the toughness of the weld metal to the element that increases the toughness of the weld metal increases. In this case, the austenitic stainless steel welded joint 1 is excellent in polythionic acid corrosion resistance and naphthenic acid corrosion resistance, and also excellent in creep ductility of the base metal 10, and the weld metal 20 is low in toughness after high-temperature aging. Specifically, the weld metal 20 after aging the austenitic stainless steel weld joint 1 of the present embodiment at 700 ℃ for 3000 hours has a charpy value at room temperature (25 ℃. + -. 15 ℃) of less than 20J/cm²。

If F2 is 0.176 or less, it is assumed that the chemical composition of the base material 10 of the austenitic stainless steel welded joint 1 has the respective element contents within the range of the present embodiment and satisfies the formula (1), and that the respective element contents of the chemical composition at the width center position and the thickness center position of the weld metal 20 are within the range of the present embodiment, the austenitic stainless steel welded joint 1 is excellent in the polythio acid corrosion resistance and the naphthenic acid corrosion resistance, and the ductility of the base material 10 is excellent, and the toughness of the weld metal 20 after high-temperature aging is high. Specifically, the weld metal 20 after aging the austenitic stainless steel welded joint 1 of the present embodiment at 700 ℃ for 3000 hours has a charpy value of 20J/cm at normal temperature (25 ℃ ± 15 ℃)²The above.

The upper limit of the F2 value is preferably 0.174, more preferably 0.172, still more preferably 0.170, and yet more preferably 0.165.

As described above, in the austenitic stainless steel welded joint 1 according to the present embodiment, the contents of the respective elements of the chemical composition of the base material 10 are within the above-described ranges and satisfy the formula (1), and further, the contents of the respective elements of the chemical composition at the width center position and the thickness center position of the weld metal 20 are within the above-described ranges. Therefore, the austenitic stainless steel welded joint 1 of the present embodiment is excellent in both the polythionic acid SCC resistance and the naphthenic acid corrosion resistance of the base metal 10 and the weld metal 20. Further, the base material 10 has excellent creep ductility in a high temperature corrosive environment at 600 to 700 ℃. Preferably, the chemical composition at the widthwise central position and the thickness central position of the weld metal 20 further satisfies the formula (2). In this case, the austenitic stainless steel welded joint 1 is excellent in polythionic acid corrosion resistance and naphthenic acid corrosion resistance, and also excellent in creep ductility of the base metal 10, and further excellent in toughness after high-temperature aging of the weld metal 20.

[ production method ]

An example of the method for producing the austenitic stainless steel welded joint 1 according to the present embodiment will be described. One example of the manufacturing method includes a step of preparing a base material 10 (base material preparation step) and a step of welding the base material 10 to form an austenitic stainless steel welded joint 1 (welding step). Hereinafter, each step will be described in detail.

[ base material preparation Process ]

In the base material preparation step, a base material 10 having the chemical composition described above and satisfying formula (1) is prepared. As described above, the base material 10 may be a steel plate or a steel pipe. The base material 10 may be obtained as a product of a material used for performing a welding step described later, or may be manufactured as a material used for performing a welding step.

When manufacturing the base material 10, the base material preparation step includes: a preparation step of preparing a blank; a hot working step of performing hot working on the blank to manufacture the base material 10; a cold working step of cold working the base material 10 after the hot working step, as required; and a solution treatment step of performing solution treatment on the base material 10 as necessary. The following describes a base material preparation step in the case of manufacturing the base material 10.

[ preparation Process ]

Molten steel having the chemical composition described above and satisfying the formula (1) is produced. The molten steel is produced, for example, by using an electric furnace, an AOD (argon Oxygen Decarburization) furnace, and a VOD (Vacuum Oxygen Decarburization) furnace. The molten steel produced is subjected to a known degassing treatment as necessary. A billet is produced from the molten steel subjected to the degassing treatment. The method of manufacturing the billet is, for example, a continuous casting method. A continuous casting material (billet) was produced by a continuous casting method. Examples of the continuous casting material include a slab, a billet, and a bar. The molten steel may be cast into an ingot by an ingot casting method.

[ Hot working Process ]

The prepared billet (continuous casting material or ingot) is hot worked to produce a base material. For example, the billet is hot-rolled to produce a steel sheet as the base material 10. Further, the billet is subjected to hot extrusion, hot piercing, or the like to manufacture a steel pipe as the base material 10. The specific method of the heat treatment is not particularly limited, and the heat treatment may be performed in accordance with the shape of the final product. The working termination temperature of the hot working is, for example, 1000 ℃ or higher, and more preferably 1050 ℃ or higher. Here, the machining end temperature means a temperature of the base material 10 immediately after the final hot machining is completed.

[ Cold working Process ]

The base material after the hot working step may be cold worked as necessary. When the base material 10 is a steel pipe, cold working is cold drawing or cold rolling, for example. When the base material 10 is a steel sheet, cold working is, for example, cold rolling.

[ solution treatment Process ]

After the hot working step or after the cold working step, the base material 10 may be subjected to solution treatment as necessary. In the solution treatment step, the structure is made uniform and carbonitride is made to dissolve. The solution treatment temperature is preferably as follows.

Preferable solution treatment temperature: 1000-1250 deg.C

When the solution treatment temperature is 1000 ℃ or higher, the carbonitride of Nb is sufficiently dissolved in a solid state, and the creep strength is further improved when used in a high-temperature corrosive environment. When the heat treatment temperature is 1250 ℃ or lower, excessive solid solution of C is suppressed, and the resistance to polythionic acid SCC is further improved.

The holding time at the solution treatment temperature in the solution treatment is not particularly limited, and is, for example, 2 to 60 minutes.

In place of the solution treatment, the base material 10 produced in the hot working step may be rapidly cooled immediately after the hot working. In this case, the finishing temperature of the hot working is preferably 1000 ℃ or higher. When the hot working end temperature is 1000 ℃ or higher, the carbonitride of Nb is sufficiently dissolved in solid solution, and excellent polythionic acid SCC resistance and excellent creep ductility are obtained when used in a high-temperature corrosion environment of 600 to 700 ℃. Further, sufficient creep strength is obtained by Nb carbonitride generation during use in a high-temperature corrosive environment.

[ welding Process ]

The prepared base material 10 is welded to produce an austenitic stainless steel welded joint 1. A groove is formed at an end of the base material 10. 2 base materials 10 on which grooves are formed are prepared. The prepared grooves of the base material 10 are butted against each other. Then, welding is performed on the pair of butted bevel portions using a welding material to form a weld metal 20 having the above-described chemical composition. The chemical composition of the welding material used for welding is not particularly limited as long as the chemical composition of the weld metal 20 to be formed is within the above range. Examples of welding methods are Gas Tungsten Arc Welding (GTAW), shielded arc welding (SMAW), Flux Cored Arc Welding (FCAW), Gas Metal Arc Welding (GMAW), and Submerged Arc Welding (SAW).

The welding material can be melted, for example, in the same manner as the base material 10 described above. At this time, the melted welding material is cast to form an ingot. The ingot is hot worked to produce a weld material. The welding material after hot working may be further subjected to cold working. Further, a known heat treatment may be performed for the welding material. The heat treatment is, for example, solution treatment similar to the base material 10. The heat treatment may not be performed. The welding material may be rod-shaped or small block-shaped.

In the welding step, when the base material 10 is a steel plate, a groove is formed on an end face or a side face of the steel plate, for example. When the base material 10 is a steel pipe, a groove is formed at an end portion of the steel pipe in the axial direction. When the base material 10 is a steel pipe, for example, the austenitic stainless steel welded joint 1 is formed by performing girth welding. In the welding step, the chemical composition of the welding material and the dilution amount of the base material 10 are adjusted, whereby the weld metal 20 having the above-described chemical composition and preferably satisfying the formula (2) can be produced.

Through the above-described manufacturing steps, the austenitic stainless steel welded joint 1 according to the present embodiment can be manufactured. The method for manufacturing the austenitic stainless steel welded joint 1 according to the present embodiment is not limited to the above-described manufacturing method. The austenitic stainless steel welded joint 1 having the weld metal 20 of the above chemical composition can be produced by another method if welding is performed using the base material 10 of the above chemical composition satisfying the formula (1).