JPWO2018043565A1 - Austenitic stainless steel - Google Patents

Abstract

Provided is an austenitic stainless steel excellent in polythioic acid resistance to SCC and excellent in creep ductility. The austenitic stainless steel according to the present embodiment is, by mass%, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% S: 0.010% or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1% .00%, N: 0.050 to 0.300%, sol. Al: 0.0005 to 0.100%, and B: 0.0010 to 0.0080%, with the balance being Fe and impurities, and having a chemical composition satisfying the formula (1).
B + 0.004-0.9C + 0.017Mo ² 0 0 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1).

Description

  The present invention relates to stainless steel, and more particularly to austenitic stainless steel.

  Among components used in plant equipment such as thermal boilers, heating furnace tubes of petroleum refining and petrochemical plants, corrosive fluids containing sulfides and / or chlorides at high temperatures of 600 to 700 ° C. Some are used in high temperature corrosive environments, including: When such plant equipment is shut down by periodic inspection, air, moisture, and sulfide scale react to form polythionic acid on the surface of the member. This polythioic acid induces stress corrosion cracking at grain boundaries (hereinafter referred to as polythioic acid SCC). Therefore, the members used in the above-mentioned high temperature corrosive environment are required to have excellent resistance to polythioic acid SCC.

The steel which raised polythioic acid resistance SCC property is proposed by Unexamined-Japanese-Patent No. 2003-166039 (patent document 1) and international publication 2009/044802 (patent document 2). Polythioic acid SCC is generated by the precipitation of Cr as M ₂₃ C ₆ type carbides at grain boundaries and the formation of a Cr-deficient layer near the grain boundaries. Therefore, in Patent Document 1 and Patent Document 2, the amount of C is reduced to suppress the formation of M ₂₃ C ₆ type carbides, thereby enhancing the resistance to polythioic acid SCC.

  Specifically, the austenitic heat resistant steel disclosed in Patent Document 1 is, by mass%, C: 0.005 to less than 0.03%, Si: 0.05 to 0.4%, Mn: 0.5 ~ 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%, the balance being Fe and unavoidable It consists of impurities. The total content of Nb and V is 0.6% or more, and the solid solution amount of Nb in the steel is 0.15% or more. Furthermore, N / 14 ≧ Nb / 93 + V / 51 and Cr-16 C-0.5 Nb-V ≧ 17.5 are satisfied. In Patent Document 1, the C content is reduced, and the relationship between Cr and C, Nb and V is defined to enhance the resistance to SIC resistance to polythioic acid.

  The austenitic stainless steel disclosed in Patent Document 2 contains, by mass%, C: less than 0.04%, Si: 1.5% or less, Mn: 2% or less, Cr: 15 to 25%, Ni: 6 to 6 30%, N: 0.02 to 0.35%, Sol. Al: 0.03% or less, Nb: 0.5% or less, Ti: 0.4% or less, V: 0.4% or less, Ta: 0.2% or less, Hf: 0.2% And at least one of Zr: 0.2% or less, with the balance being Fe and impurities. Among 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% Below, and Sb: 0.01% or less. Furthermore, F1 = S + {(P + Sn) / 2} + {(As + Zn + Pb + Sb) / 5} ≦ 0.075 and 0.05 ≦ Nb + Ta + Zr + Hf + 2Ti + (V / 10) ≦ 1.7-9 × F1 are satisfied. In patent document 2, polythionic acid-resistance SCC resistance is improved by making C content less than 0.05%. Furthermore, by reducing C fixing elements such as Nb and Ti and reducing grain boundary embrittlement elements such as P, S and Sn in the steel, the embrittlement cracking resistance in the heat affected zone (HAZ) is enhanced. .

Japanese Patent Application Laid-Open No. 2003-166039 WO 2009/044802

  By the way, recently, high creep ductility is required for members used in the above-mentioned high temperature corrosive environment. In plant facilities, as described above, the facilities may be shut down and periodic inspections may be performed. In periodic inspections, members that need to be replaced are investigated. At this time, if the creep ductility is high, the degree of deformation of the member can be confirmed at the time of periodic inspection, and can be used as a determination criterion of replacement of the member.

  Patent Documents 1 and 2 aim to improve polythioic acid resistance to SCC, but do not aim to improve creep ductility. In the steels proposed in these patent documents, the C content is lowered to enhance polythioic acid resistance to SCC. In this case, high creep ductility may not be obtained.

  An object of the present invention is to provide an austenitic stainless steel which is excellent in polythioic acid resistance to SCC and excellent in creep ductility.

The austenitic stainless steel according to the present invention is, by 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 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1. 00%, N: 0.050-0.300%, sol. Al: 0.0005 to 0.100%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0 to 1.0%, V : 0 to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, and rare earth elements: 0 It contains about 0.10%, and the balance consists of Fe and impurities, and has a chemical composition that satisfies the formula (1).
B + 0.004-0.9C + 0.017Mo ² 0 0 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1).

  The austenitic stainless steel according to the present invention is excellent in polythioic acid SCC resistance and excellent in creep and ductility.

  The present inventors investigated and examined a steel excellent not only in resistance to polythioic acid SCC but also in creep ductility.

If the C content is reduced to 0.030% or less, the formation of M ₂₃ C _6- type carbides is suppressed and the formation of a Cr-depleted layer near grain boundaries is suppressed during use in a high-temperature corrosive environment . In the present invention, by further containing 0.20 to 1.00% of Nb, C is fixed with Nb to further reduce the amount of solid solution C which is a factor of forming M ₂₃ C ₆ type carbide. In the present invention, further, 0.1 to 5.0% of Mo is contained. Mo suppresses the formation of M ₂₃ C ₆ type carbides. Therefore, the formation of the C deficient layer is reduced. By the above measures, resistance to polythioic acid SCC can be enhanced.

  However, as a result of investigations by the present inventors, it was found that creep ductility is reduced if the C content is reduced to 0.030% or less. The following can be considered as the reason. The precipitates formed at the grain boundaries increase the grain boundary strength. If the grain boundary strength is increased, creep ductility is increased. However, if the C content is reduced to 0.030% or less, precipitates (such as carbides) formed at grain boundaries are also reduced. As a result, it is believed that the grain boundary strength is hardly obtained and the creep ductility is lowered.

  Therefore, the present inventors have further studied an austenitic stainless steel that is compatible with excellent polythioic acid resistance SCC resistance and excellent creep ductility. It is considered that B (boron) can be segregated at grain boundaries in the above-described high temperature corrosive environment at 600 to 700 ° C. to enhance grain boundary strength.

  Therefore, the present inventors, 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 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1.00 %, N: 0.050-0.300%, sol. Al: 0.0005 to 0.100%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0 to 1.0%, V : 0 to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, and rare earth elements: 0 It was thought that if it is an austenitic stainless steel containing ~ 0.10% and the balance being Fe and impurities, it is possible to achieve both excellent resistance to polythioic acid SCC and excellent creep ductility.

  However, as a result of investigating polythioic acid resistance SCC and creep ductility of austenitic stainless steels having the above-mentioned chemical composition, although excellent polythionic acid resistance SCC can be obtained, it may not always be possible to obtain excellent creep ductility. all right. Therefore, the present inventors further studied. As a result, it was found that the following mechanism can be considered for creep ductility.

As described above, in the present embodiment, in order to enhance the resistance to polythioic acid SCC, not only the C content is reduced to 0.030% or less, but 0.22 to 1.00% of Nb is contained and C is added. It fixes to Nb and reduces solid solution C. Specifically, Nb combines with C and precipitates as MX-type carbonitride by solution treatment or aging in a short time. However, in the use environment (high temperature corrosive environment of 600 to 700 ° C.) of the steel material of the present embodiment, the MX type carbonitride is a metastable phase. Therefore, when a steel material having the above chemical composition is used for a long time in a high temperature corrosive environment at 600 to 700 ° C., MX type carbonitrides of Nb become stable phases Z phase (CrNbN) and M ₂₃ C ₆ type carbides. Change. In this case, the B that segregates at the grain _boundaries, is replaced with a portion of the C of _M 23 _{C 6} type carbide _is absorbed by the _M 23 _{C 6} type carbide. Therefore, the amount of B segregated in the grain boundaries is reduced, and the grain boundary strength is reduced. As a result, it is considered that sufficient creep ductility can not be obtained.

  Then, the method of suppressing the reduction of the amount of segregation B in a grain boundary was further examined during use under the high temperature corrosive environment of 600-700 ° C. As a result, it turned out that the following mechanism can be considered.

As described above, Mo suppresses the formation of M ₂₃ C ₆ type carbide itself. Mo is _further replaced with a part of M of _M 23 _{C 6} type _carbide, which may be dissolved in _M 23 _{C 6} type carbide. In the present specification, the M ₂₃ C ₆ type carbide in which Mo is solid-solved is defined as “Mo-solid solution M ₂₃ C ₆ type carbide”. Mo solid solution M ₂₃ C ₆ type carbides do not easily solidify B. Therefore, even when MX type carbonitride containing Nb is changed to Z phase and M ₂₃ C ₆ type carbide during use in a high temperature corrosive environment, M ₂₃ C ₆ type carbide is solid solution of Mo if the M ₂₃ C ₆ type carbide, it is possible to suppress the dissolution of the _M 23 _{C 6} type carbides of B, reducing the segregation amount of B at the grain boundaries is suppressed. As a result, it is considered that both excellent polythioic acid resistance SCC resistance and excellent creep ductility can be achieved.

Therefore, in the austenitic stainless steel having the above-mentioned chemical composition, the MX-type carbonitride containing Nb was changed to the Z phase and the M ₂₃ C _6- type carbide during use in a high temperature corrosive environment at 600 to 700 ° C. Even in this case, a chemical composition capable of suppressing a reduction in the amount of segregation B at grain boundaries was further examined by the formation of Mo solid solution M ₂₃ C ₆ type carbides. As a result, it was found that B, C, and Mo in the above-described chemical composition are closely related to the suppression of the reduction of the amount of segregation B due to the formation of Mo solid solution M ₂₃ C ₆ type carbide. And, when B, C and Mo satisfy the formula (1) in the above chemical composition, excellent SIC resistance against polythioic acid and excellent creep ductility even during use in a high temperature corrosive environment of 600 to 700 ° C. It turned out that it is possible to make it compatible.
B + 0.004-0.9C + 0.017Mo ² 0 0 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1).

  As a result of further investigations by the present inventors, when Cu, which is an optional element, is contained in the austenitic stainless steel, if 5.0% or less of Cu is contained, excellent creep strength can be obtained and creep ductility is also obtained. Although it is maintainable, when the upper limit of Cu content is made into 1.9% or less, it turned out that high creep ductility can be maintained, improving creep strength further. The following can be considered as the reason. Cu precipitates in the grains to form a Cu phase during use in a high temperature corrosive environment. Although the Cu phase increases creep strength, it may reduce creep ductility. Therefore, in the austenitic stainless steel which has the above-mentioned chemical composition and satisfies the formula (1), more preferably, the Cu content is 1.9% or less. If the Cu content is 1.9% or less, excellent creep ductility can be more effectively maintained.

  As a result of further investigation by the inventor, it was found that creep ductility is further enhanced if the Mo content is 0.5% or more. The reason is not clear, but the following can be considered. In the above chemical composition (satisfying the formula (1)), when the Mo content is 0.5% or more, Mo is further segregated at grain boundaries during use in a high temperature corrosive environment of 600 to 700 ° C. Form intermetallic compounds. Grain boundary strength is further enhanced by grain boundary segregation and intermetallic compounds. As a result, creep ductility is further enhanced. Therefore, the lower limit of the preferable Mo content is 0.5%. The lower limit of the preferable Mo content for further enhancing the creep ductility is 0.8%, more preferably 1.0%, further preferably 2.0%.

The austenitic stainless steel according to the present invention completed based on the above findings is, by 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 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1.00%, N: 0.050 to 0.300%, sol. Al: 0.0005 to 0.1000%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0 to 1.0%, V : 0 to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, and rare earth elements: 0 It contains about 0.10%, and the balance consists of Fe and impurities, and has a chemical composition that satisfies the formula (1).
B + 0.004-0.9C + 0.017Mo ² 0 0 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1).

  The above chemical composition is selected from the group consisting of Cu: 0.1 to 5.0%, W: 0.1 to 5.0%, and Co: 0.1 to 1.0% by mass. It may contain species or two or more species.

  The above chemical composition is selected from the group consisting of V: 0.1 to 1.00%, Ta: 0.01 to 0.2%, and Hf: 0.01 to 0.20% by mass. It may contain species or two or more species.

  The chemical composition is selected from the group consisting of, by mass%, Ca: 0.0005 to 0.010%, Mg: 0.0005 to 0.010%, and a rare earth element: 0.001 to 0.10% It may contain one or two or more.

  The said chemical composition may contain Cu: 0-1.9% by mass%.

  The said chemical composition may contain Mo: 0.5-5.0% by mass%.

  Hereinafter, the austenitic stainless steel of the present embodiment will be described in detail. The term "%" with respect to an element means mass% unless otherwise noted.

[Chemical composition]
The chemical composition of the austenitic stainless steel of the present embodiment contains the following elements.

C: 0.030% or less Carbon (C) is inevitably contained. While using the austenitic stainless steel of the present embodiment in a high temperature corrosive environment at 600 to 700 ° C., C forms M ₂₃ C ₆ type carbides at grain boundaries and reduces the resistance to polythioic acid SCC. Therefore, the C content is 0.030% or less. The upper limit of the C content is preferably 0.020%, more preferably 0.015%. It is preferable that the C content be as low as possible. However, as described above, since C is contained inevitably, at least 0.0001% of C may be contained in industrial production. Therefore, the preferable lower limit of the C content is 0.0001%.

Si: 0.10 to 1.00%
Silicon (Si) deoxidizes the steel. Si further enhances the oxidation and steam oxidation resistance of the steel. If the Si content is too low, the above effect can not be obtained. On the other hand, if the Si content is too high, sigma phase (σ phase) precipitates in the steel and the toughness of the steel decreases. Therefore, the Si content is 0.10 to 1.00%. The upper limit of the Si content is preferably 0.75%, more preferably 0.50%.

Mn: 0.20 to 2.00%
Manganese (Mn) deoxidizes the steel. Mn further stabilizes austenite and enhances creep strength. If the Mn content is too low, the above effect can not be obtained. On the other hand, if the Mn content is too high, the creep strength of the steel decreases. Therefore, the Mn content is 0.20 to 2.00%. The preferable lower limit of the Mn content is 0.40%, and more preferably 0.50%. The upper limit of the Mn content is preferably 1.70%, more preferably 1.50%.

P: 0.040% or less Phosphorus (P) is an impurity. P reduces the hot workability and toughness of the steel. Therefore, the P content is 0.040% or less. The upper limit of the P content is preferably 0.035%, more preferably 0.032%. The P content is preferably as low as possible. However, P is unavoidably contained, and for industrial production, P may be contained at least 0.0001%. Therefore, the preferable lower limit of P content is 0.0001%.

S: 0.010% or less Sulfur (S) is an impurity. S reduces the hot workability and creep ductility of the steel. Therefore, the S content is 0.010% or less. The preferred upper limit of the S content is 0.005%. The S content is preferably as low as possible. However, S is contained inevitably, and in industrial production, S may be contained at least 0.0001%. Therefore, the preferable lower limit of S content is 0.0001%.

Cr: 16.0 to 25.0%
Chromium (Cr) enhances the polythioic acid resistance SCC of the steel. Cr further enhances oxidation resistance, water vapor oxidation resistance, high temperature corrosion resistance and the like. If the Cr content is too low, the above effect can not be obtained. On the other hand, if the Cr content is too high, creep strength and toughness of the steel will be reduced. Therefore, the Cr content is 16.0 to 25.0%. The preferable lower limit of the Cr content is 16.5%, more preferably 17.0%. The upper limit of the Cr content is preferably 24.0%, more preferably 23.0%.

Ni: 10.0 to 30.0%
Nickel (Ni) stabilizes austenite and enhances creep strength. If the Ni content is too low, the above effect can not be obtained. On the other hand, if the Ni content is too high, the above effect is saturated and the manufacturing cost is further increased. Therefore, the Ni content is 10.0 to 30.0%. The lower limit of the Ni content is preferably 11.0%, more preferably 13.0%. The upper limit of the Ni content is preferably 25.0%, more preferably 22.0%.

Mo: 0.1 to 5.0%
Molybdenum (Mo) suppresses the formation of M ₂₃ C ₆ type carbides at grain boundaries during use in a high temperature corrosive environment at 600 to 700 ° C. Mo further dissolves B in M ₂₃ C ₆ type carbide when MX type carbonitride of Nb changes to M ₂₃ C ₆ type carbide during use in a high temperature corrosive environment at 600 to 700 ° C. To reduce the amount of segregation B at grain boundaries in a high temperature corrosive environment. Thereby, sufficient creep ductility can be obtained in a high temperature corrosive environment. If the Mo content is too low, the above effect can not be obtained. On the other hand, if the Mo content is too high, the stability of austenite decreases. Therefore, the Mo content is 0.1 to 5.0%. The preferable lower limit of the Mo content is 0.2%, and more preferably 0.3%.

  If the Mo content is 0.5% or more, Mo segregates in grain boundaries or forms intermetallic compounds to further enhance grain boundary strength. In this case, even better creep strength can be obtained in a high temperature corrosive environment. Therefore, a further preferable lower limit of the Mo content is 0.5%, more preferably 0.8%, further preferably 1.0%, further preferably 1.5%, further preferably It is 2.0%. If the Mo content is 1.5% or more, the creep strength is also enhanced. The preferable upper limit of the Mo content is 4.5%, and more preferably 4.0%. If the Mo content is 1.5% or more, the creep strength is also enhanced.

Nb: 0.20 to 1.00%
Niobium (Nb) combines with C to form MX-type carbonitrides during use in a high temperature corrosive environment at 600 to 700 ° C. to reduce the amount of solid solution C in the steel. This enhances the resistance to polythioic acid SCC of the steel. The formed Nb MX carbonitrides also enhance creep strength. If the Nb content is too low, the above effect can not be obtained. On the other hand, if the Nb content is too high, δ-ferrite is formed and the long-term creep strength, toughness and weldability of the steel are reduced. Therefore, the Nb content is 0.20 to 1.00%. The preferred lower limit of the Nb content is 0.25%. The preferable upper limit of the Nb content is 0.90%, and more preferably 0.80%.

N: 0.050 to 0.300%
Nitrogen (N) dissolves in the matrix (matrix phase) to stabilize austenite and enhance creep strength. N further forms fine carbonitrides in the grains to increase the creep strength of the steel. That is, N contributes to creep strength both in solid solution strengthening and precipitation strengthening. If the N content is too low, the above effect can not be obtained. On the other hand, if the N content is too high, Cr nitrides are formed at grain boundaries, and the resistance to SIC of polythioic acid in the heat-affected zone (HAZ) decreases. If the N content is too high, the workability of the steel is further reduced. Therefore, the N content is 0.050 to 0.300%. The preferable lower limit of the N content is 0.070%. The upper limit of the N content is preferably 0.250%, more preferably 0.200%.

sol. Al: 0.0005 to 0.100%
Aluminum (Al) deoxidizes the steel. If the Al content is too low, the above effect can not 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 0.0005 to 0.100%. The preferred lower limit of the Al content is 0.001%, and more preferably 0.002%. The upper limit of the Al content is preferably 0.050%, more preferably 0.030%. In the present embodiment, the Al content means the content of acid-soluble Al (sol. Al).

B: 0.0010 to 0.0080%
Boron (B) segregates at grain boundaries and enhances grain boundary strength during use in a high temperature corrosive environment at 600 to 700 ° C. As a result, creep ductility is enhanced. If the B content is too low, the above effect can not be obtained. On the other hand, if the B content is too high, the weldability and the hot workability at high temperatures are reduced. Therefore, B content is 0.0010 to 0.0080%. The preferable lower limit of the B content is 0.0015%, and more preferably 0.0020%. The upper limit of the B content is preferably less than 0.0060%, more preferably 0.0050%.

  The balance of the chemical composition of the austenitic stainless steel according to the present embodiment consists of Fe and impurities. Here, when the austenitic stainless steel is manufactured industrially, the impurities are mixed from the ore as a raw material, scrap, or the manufacturing environment, etc., and adversely affect the austenitic stainless steel of the present embodiment. Means what is acceptable without giving.

[About any element]
The austenitic stainless steel according to the present embodiment may further contain one or more selected from the group consisting of Cu, W and Co, instead of part of Fe. All these elements increase the creep strength of the steel.

Cu: 0 to 5.0%
Copper (Cu) is an optional element and may not be contained. When it is contained, Cu precipitates as Cu phase in the grains during use in a high temperature corrosive environment at 600 to 700 ° C. to enhance the creep strength of the steel by precipitation strengthening. However, if the Cu content is too high, the hot workability and weldability of the steel will be reduced. Therefore, the Cu content is 0 to 5.0%. The preferable lower limit of the Cu content to further effectively increase the creep strength is 0.1%, more preferably 2.0%, and still more preferably 2.5%. The upper limit of the Cu content is preferably 4.5%, more preferably 4.0%. On the other hand, the preferable Cu content for maintaining more excellent creep ductility is 0 to 1.9%, and the upper limit of the more preferable Cu content is 1.8%.

W: 0 to 5.0%
Tungsten (W) is an optional element and may not be contained. When it is contained, W dissolves in the matrix (matrix phase) to increase the creep strength of the steel. However, if the W content is too high, the stability of austenite decreases and the creep strength and toughness of the steel decrease. Therefore, the W content is 0 to 5.0%. The preferable lower limit of the W content is 0.1%, more preferably 0.2%. The upper limit of the W content is preferably 4.5%, more preferably 4.0%.

Co: 0 to 1.0%
Cobalt (Co) is an optional element and may not be contained. When contained, Co stabilizes austenite and enhances creep strength. However, if the Co content is too high, the raw material cost will increase. Therefore, the Co content is 0 to 1.0%. The preferable lower limit of the Co content is 0.1%, more preferably 0.2%.

  The austenitic stainless steel according to the present embodiment may further contain one or more selected from the group consisting of V, Ta and Hf instead of part of Fe. All of these elements enhance the SIC resistance and the creep strength of the steel against polythioic acid.

V: 0 to 1.00%
Vanadium (V) is an optional element and may not be contained. When contained, V combines with C to form carbonitrides during use in a high-temperature corrosive environment at 600 to 700 ° C. to reduce solid solution C, and SIC resistance to steel polythionic acid Raise. The formed V carbonitrides also enhance creep strength. However, if the V content is too high, δ ferrite will be formed and the creep strength, toughness and weldability of the steel will be reduced. Therefore, the V content is 0 to 1.00%. The preferable lower limit of the V content for further effectively enhancing polythioic acid resistance SCC resistance and creep strength is 0.10%. The upper limit of the V content is preferably 0.90%, more preferably 0.80%.

Ta: 0 to 0.2%
Tantalum (Ta) is an optional element and may not be contained. When contained, Ta combines with C to form carbonitrides during use in a high temperature corrosive environment at 600 to 700 ° C. to reduce solid solution C, and to reduce SIC resistance to steel polythionic acid. Raise. The formed Ta carbo-nitrides also enhance creep strength. However, if the Ta content is too high, δ-ferrite is formed and the creep strength, toughness and weldability of the steel are reduced. Therefore, the Ta content is 0 to 0.2%. The preferable lower limit of the Ta content for further effectively enhancing the polythioic acid resistance SCC resistance and the creep strength is 0.01%, more preferably 0.02%.

Hf: 0 to 0.20%
Hafnium (Hf) is an optional element and may not be contained. If contained, Hf combines with C to form carbonitrides during use in a high temperature corrosive environment at 600 to 700 ° C. to reduce solid solution C, and SIC resistance to steel polythionic acid Raise. The Hf carbonitrides formed also increase the creep strength. However, if the Hf content is too high, δ ferrite will be formed and the creep strength, toughness and weldability of the steel will be reduced. Therefore, the Hf content is 0 to 0.20%. The lower limit of the Hf content is preferably 0.01%, more preferably 0.02%.

  The austenitic stainless steel according to the present embodiment may further contain one or more selected from the group consisting of Ca, Mg and a rare earth element instead of part of Fe. All of these elements enhance the hot workability and creep ductility of the steel.

Ca: 0 to 0.010%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca fixes O (oxygen) and S (sulfur) as inclusions to enhance the hot workability and creep ductility of the steel. However, if the Ca content is too high, the hot workability and creep ductility of the steel are reduced. Therefore, the Ca content is 0 to 0.010%. The preferable lower limit of the Ca content is 0.0005%, and more preferably 0.001%. The upper limit of the Ca content is preferably 0.008%, more preferably 0.006%.

Mg: 0 to 0.010%
Magnesium (Mg) is an optional element and may not be contained. When contained, Mg fixes O (oxygen) and S (sulfur) as inclusions to enhance the hot workability and creep ductility of the steel. However, if the Mg content is too high, the hot workability and long-time creep ductility of the steel are reduced. Therefore, the Mg content is 0 to 0.010%. The preferable lower limit of the Mg content is 0.0005%, more preferably 0.001%. The upper limit of the Mg content is preferably 0.008%, more preferably 0.006%.

Rare earth element: 0 to 0.10%
The rare earth element (REM) is an optional element and may not be contained. When included, REM fixes O (oxygen) and S (sulfur) as inclusions to enhance the hot workability and creep ductility of the steel. However, if the REM content is too high, the hot workability and long-term creep ductility of the steel are reduced. Therefore, the REM content is 0 to 0.01%. The preferable lower limit of the REM content is 0.001%, and more preferably 0.002%. The upper limit of REM content is preferably 0.08%, more preferably 0.06%.

  REM in the present specification contains at least one or more of Sc, Y, and lanthanoids (Lu of atomic number 57 to Lu of 71), and the REM content means the total content of these elements. Do.

[About formula (1)]
The above chemical composition further satisfies the formula (1).
B + 0.004-0.9C + 0.017Mo ² 0 0 (1)
The content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

As described above, in the present embodiment, in order to enhance the resistance to polythioic acid SCC, not only the C content is reduced to 0.030% or less, but 0.22 to 1.00% of Nb is contained and 600 During use in a high temperature corrosive environment at ~ 700 ° C, form MX type carbonitrides of Nb and reduce the amount of solid solution C. However, since the MX type carbonitride of Nb is a metastable phase, it changes to Z phase and M ₂₃ C ₆ type carbide during use in the above-mentioned high temperature use environment. At this time, B segregated in grain boundaries dissolves in the M ₂₃ C ₆ type carbide, and the amount of B segregation in grain boundaries is reduced. As a result, creep ductility is reduced.

However, if Mo is generating a "Mo solid solution _M 23 _{C 6} type carbides" a solid solution in the _{M 23} type _{C 6} type carbide, the Mo solid solution _M 23 _{C 6} type carbides B is hardly dissolved. Therefore, the amount of B segregation at grain boundaries is maintained, and not only excellent resistance to polythioic acid SCC can be obtained, but also excellent creep ductility can be obtained.

It is defined as ^{F1 = B + 0.004-0.9C + 0.017Mo 2} . F1, among the plurality of the _M 23 _{C 6} type carbide to produce in the steel in use in a high temperature corrosive environment, is an index showing the proportion of Mo solid solution _M 23 _{C 6} type carbide. If F1 is 0 or more, even if a plurality of M ₂₃ C ₆ type carbides are formed in the steel during use in a high temperature corrosive environment, the proportion of Mo solid solution M ₂₃ C ₆ type carbides is high. Therefore, B segregated in the grain boundaries is hard to form a solid solution in the M ₂₃ C ₆ type carbide, and the amount of segregation B in the grain boundaries is maintained. Therefore, it is possible to achieve both excellent resistance to polythioic acid SCC and excellent creep ductility. Therefore, F1 is 0 (0.00000) or more. Preferably, F1 is 0.00100 or more, more preferably 0.00200 or more, still more preferably 0.00400 or more, still more preferably 0.00500, more preferably 0.00800 or more. And most preferably 0.01000.

  Preferably, when the chemical composition of the austenitic stainless steel contains Cu, as described above, the upper limit of the Cu content is 1.9% or less. That is, in consideration of obtaining excellent creep ductility while enhancing creep strength, the preferable Cu content is 0% to 1.9%. If the Cu content is 1.9% or less, excellent creep ductility can be maintained while obtaining excellent creep strength by precipitation strengthening of the Cu phase.

  In the chemical composition of the austenitic stainless steel, the lower limit of the Mo content is preferably 0.5%. In this case, during use in a high temperature corrosive environment at 600 to 700 ° C., Mo further segregates at grain boundaries and forms intermetallic compounds. Grain boundary strength is further enhanced by grain boundary segregation and intermetallic compounds. As a result, creep ductility is further enhanced. Therefore, the lower limit of the preferable Mo content is 1.0%. When the lower limit of the Mo content is 1.0% or more, the preferable F1 value is 0.00500 or more, more preferably 0.00800 or more, and still more preferably 0.01000 or more.

[Production method]
An example of the method for producing an austenitic stainless steel of the present invention will be described. This manufacturing method cold-works the steel material after the hot-working process, if necessary, preparing the material, performing the hot-working on the material to produce the steel material, and if necessary It comprises a cold working step and a solution treatment step of carrying out a solution treatment on the steel if necessary. Hereinafter, the manufacturing method will be described.

[Preparation process]
A molten steel satisfying the above-mentioned chemical composition and satisfying the formula (1) is manufactured. For example, the molten steel is manufactured using an electric furnace, an AOD (Argon Oxygen Decarburization) furnace, or a VOD (Vacuum Oxygen Decarburization) furnace. A well-known degassing process is implemented with respect to the manufactured molten steel as needed. A raw material is manufactured from the molten steel which implemented the degassing process. The method of producing the material is, for example, a continuous casting method. A continuous casting material (material) is manufactured by a continuous casting method. The continuous cast materials are, for example, slabs, blooms and billets. The molten steel may be made into an ingot by the ingot method.

[Hot working process]
The prepared material (continuously cast material or ingot) is hot-worked to produce an austenitic stainless steel material. For example, the material is hot-rolled to produce steel plates, bars, and wires. In addition, an austenitic stainless steel pipe is manufactured by hot extrusion, hot piercing rolling, or the like. The specific method of hot working is not particularly limited, and hot working may be performed according to the shape of the final product. The finish temperature of hot working is, for example, 1050 ° C. or higher. The processing end temperature mentioned here means the temperature of the steel material immediately after the final hot working is completed.

[Cold working process]
If necessary, cold working may be performed on the austenitic stainless steel material after hot working. When the austenitic stainless steel material is a bar, wire or steel pipe, cold working is, for example, cold drawing or cold rolling. When the austenitic stainless steel material is a steel plate, it is cold rolling or the like.

[Solution treatment process]
After hot working or cold working, solution treatment may be performed if necessary. In the solution treatment process, homogenization of the structure and dissolution of carbonitride are performed. The preferred solution treatment temperature is as follows.

Preferred solution treatment temperature: 1000 to 1250 ° C.
When the solution treatment temperature is 1000 ° C. or more, carbonitrides of Nb are sufficiently dissolved to further increase creep strength. When the heat treatment temperature is 1250 ° C. or less, excessive solid solution of C can be suppressed, and the resistance to SIC of polythioic acid further increases.

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

  In addition, it may replace with the above-mentioned solution treatment with respect to the steel materials manufactured by the hot-working process, and you may carry out rapid cooling immediately after hot working. In this case, the processing end temperature of the hot working is preferably 1000 ° C. or more. When the hot working end temperature is 1000 ° C. or more, carbonitrides of Nb are sufficiently dissolved, and excellent SIC resistance against polythioic acid and excellent creep during use in a high temperature corrosive environment of 600 to 700 ° C. The combination of ductility is possible, and the formation of Nb carbonitrides during use in a high temperature environment also provides sufficient creep strength.

  The shape of the austenitic stainless steel of the present embodiment is not particularly limited. The austenitic stainless steel of the present embodiment may be a steel plate, a steel pipe, a bar steel or a wire rod, or a shaped steel.

  A molten steel having the chemical composition of Table 1 was produced.

  In the “F1” column of Table 1, the F1 value of the steel of each test number is entered. In addition, the element symbol in the "other" column in the "chemical composition" column and the numerical value attached in front of the element symbol mean an optional element contained and its content (% by mass). Of the chemical compositions of the test numbers, the balance other than the elements listed in Table 1 was Fe and impurities.

  Using molten steel, an ingot with an outer diameter of 120 mm and a weight of 30 kg was produced. The ingot was subjected to hot forging to obtain a steel plate having a thickness of 40 mm. Furthermore, hot rolling was performed to obtain a steel plate with a thickness of 15 mm. The final processing temperature at the time of hot rolling was 1050 ° C. or higher. The steel plate after hot rolling was further subjected to cold rolling to produce a steel plate having a thickness of 10.5 mm, a width of 50 mm, and a length of 100 mm. The solution treatment was performed on each steel plate after cold rolling. The solution treatment temperature of each steel plate of each test number was 1150 ° C., and the solution treatment time was 10 minutes. The steel plate after solution treatment was water-cooled. An austenitic stainless steel material was manufactured by the above steps.

  The plate thickness of the manufactured austenitic stainless steel plate is defined as t (mm), and using a sample at any position from the surface at a depth of t / 4, the well-known component analysis method (combustion for infrared rays and infrared rays-infrared rays The absorption method, the high temperature desorbed gas analysis method for N, and the ICP analysis method for other alloy elements were performed. As a result, the chemical composition of the austenitic stainless steel plate of each test number was in agreement with Table 1.

[Polythionic acid resistant SCC evaluation test]
The steel plate of each test number was subjected to an aging treatment at 600 ° C. for 5000 hours, assuming use in a high temperature environment. From this aging-treated material, plate-like test pieces having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm were collected. According to JIS G 0576 (2001) “Method for testing stress corrosion cracking of stainless steel”, the polythioic acid SCC resistance evaluation test was conducted. Specifically, the test piece was bent around a punch with an inner radius of 5 mm to form a U-bend. The U-bend test piece was immersed in a Wackenroder solution (a solution in which a large amount of H ₂ S gas was blown into a H ₂ SO ₃ saturated aqueous solution prepared by blowing SO ₂ gas into distilled water) at normal temperature for 100 hours. With respect to the test pieces after immersion, the occurrence of cracks was observed with a microscope at a magnification of 500 times to confirm the presence or absence of cracks.

  When a crack was not confirmed, it was judged to be excellent in polythioic acid SCC resistance ("E" (Excellent) in the "polythionic acid SCC resistance" column in Table 2). When even one crack was confirmed, it was judged that polythioic acid resistance SCC resistance is low ("NA" (Not Accepted) in "polythionic acid resistance SCC resistance" column in Table 2).

[Creep ductility and creep strength evaluation test]
The creep rupture test piece based on JIS Z2271 (2010) was produced from the steel plate of each test number. 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 parallel portion was parallel to the rolling direction of the steel plate. The creep rupture test based on JIS Z2271 (2010) was implemented using the produced creep rupture test piece. Specifically, the creep rupture test was carried out after the creep rupture test piece was heated at 750 ° C. The test stress was 45 MPa, and the creep rupture time (hour) and the creep rupture reduction (%) were determined.

  With respect to creep strength, when the creep rupture time is 5000 to 10000 h or less, it is judged that the creep strength is excellent (denoted by "G" (Good) in the "creep strength" column in Table 2). When the creep rupture time exceeded 10000 hours, it was judged that the creep strength was remarkably excellent (denoted by "E" (Excellent) in the "creep strength" column in Table 2). When the creep rupture time was less than 5000 hours, it was judged that the creep strength was low (denoted by "NA" (Not Accepted) in the "creep strength" column in Table 2). When the creep rupture time was G or E, it was judged that sufficient creep strength was obtained.

  With respect to creep ductility, when the creep rupture reduction was 20.0% to 30.0% or less, it was judged that the creep ductility was good (represented by "P" (Passing) in the "creep ductility" column in Table 2). It was judged that creep ductility is excellent when the creep rupture reduction is more than 30.0% and 50.0% or less (denoted by “G” (Good) in the “creep ductility” column in Table 2). Furthermore, when the creep rupture reduction exceeded 50.0%, it was judged that the creep ductility was remarkably excellent (denoted by “E” (Excellent) in the “creep ductility” column in Table 2). When the creep rupture reduction is less than 20.0%, it is judged that the creep ductility is low ("NA" (Not Accepted) in the "creep ductility" column in Table 2. The creep rupture restriction is P, G, or E) In the case, it was judged that sufficient creep ductility was obtained.

[Test results]
Table 2 shows the test results.

  Referring to Tables 1 and 2, the content of each element in the chemical composition of the steels of test numbers 1 to 16 was appropriate, and F1 also satisfied the formula (1). Therefore, in the steel plates of these test numbers, excellent resistance to polythioic acid SCC was obtained. Furthermore, the rupture time was 5000 hours or more, and excellent creep strength was obtained. Furthermore, the creep rupture reduction was 20.0% or more, and excellent creep ductility was obtained. Furthermore, in the test numbers 2 to 4, 6 to 12, and 15, the creep time in the creep rupture test is longer than the test numbers 1, 5, 13, 14 and 16 because it contains Cu and contains a large amount of Mo. It was over 10000 hours and excellent creep strength was obtained.

  Furthermore, Test Nos. 3 and 4 containing 1.9% or less of Cu and containing 0.5% or more of Mo, and Test Nos. Containing 1.0% or more of Mo even without Cu. In 5-7, 11 and 12, while sufficient creep strength was obtained, the outstanding creep ductility was also obtained.

  On the other hand, in the test numbers 17 and 18, F1 did not satisfy the formula (1). As a result, the creep rupture reduction was less than 20%, and the creep ductility of the steel was low. It is considered that the grain boundary strengthening effect by grain boundary segregation of B was not sufficiently obtained. Also, the creep strength was low.

  In Test No. 19, the C content was too high. As a result, resistance to polythioic acid SCC was low.

  In Test No. 20, since it contained Cu, although creep strength was high, F1 did not satisfy Formula (1). As a result, the creep rupture reduction was less than 20.0%, and the creep ductility of the steel was low.

  Test No. 21 contained no Mo. Furthermore, F1 was less than the lower limit of Formula (1). As a result, the reduction in area was less than 20.0%, and the creep ductility of the steel was low. Also, the creep strength was low.

  In Test No. 22, the B content was low. As a result, the creep rupture reduction was less than 20.0%, and the creep ductility of the steel was low. Also, the creep strength was low.

  Test No. 23 contained no Nb. As a result, resistance to polythioic acid SCC was low. Furthermore, the rupture time was less than 5000 hours, and the creep strength of the steel was low.

  The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the scope of the invention.

Claims (5)

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 to 25.0%,
Ni: 10.0 to 30.0%,
Mo: 0.1 to 5.0%,
Nb: 0.20 to 1.00%,
N: 0.050 to 0.300%,
sol. Al: 0.0005 to 0.100%,
B: 0.0010 to 0.0080%,
Cu: 0 to 5.0%,
W: 0 to 5.0%,
Co: 0 to 1.0%,
V: 0 to 1.00%,
Ta: 0 to 0.2%,
Hf: 0 to 0.20%,
Ca: 0 to 0.010%
Mg: 0 to 0.010%, and
Rare earth element: containing 0 to 0.10%, the balance being Fe and impurities,
An austenitic stainless steel having a chemical composition satisfying the formula (1).
B + 0.004-0.9C + 0.017Mo ² 0 0 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1). The austenitic stainless steel according to claim 1, wherein
The chemical composition is
Cu: 0.1 to 5.0%,
The austenitic stainless steel containing 1 type, or 2 or more types selected from the group which consists of W: 0.1-5.0%, and Co: 0.1-1.0%. The austenitic stainless steel according to claim 1 or 2, wherein
The chemical composition is
V: 0.1 to 1.00%,
An austenitic stainless steel containing one or more selected from the group consisting of Ta: 0.01 to 0.2% and Hf: 0.01 to 0.20%. The austenitic stainless steel according to any one of claims 1 to 3, wherein
The chemical composition is
Ca: 0.0005 to 0.010%,
Mg: 0.0005 to 0.010%, and
An austenitic stainless steel containing one or more selected from the group consisting of rare earth elements: 0.001 to 0.10%. The austenitic stainless steel according to claim 1, wherein
The chemical composition is
Cu: Austenitic stainless steel containing 0 to 1.9%.