Technical field
[0001]
　The present invention relates to a stainless steel, more particularly, it relates to an austenitic stainless steel.
BACKGROUND
[0002]
　Thermal power boiler, in a member used in plant facilities such as heating furnace tube of the petroleum refining and petrochemical plants, at a high temperature of 600 ~ 700 ° C., and a corrosive fluid containing sulfides and / or chlorides there are those used in high temperature corrosive environments comprising. When such a plant facility is stopped due to periodic inspection or the like, air, water, and the reaction sulfide scale, polythionic acid is produced on the component surface. The polythionic acid, stress corrosion cracking at the grain boundaries (hereinafter referred to polythionic acid SCC) induces. Thus, the members used in the above high-temperature corrosive environments is required excellent polythionic acid SCC resistance.
[0003]
　Resistant polythionic acid SCC resistance enhanced steel is proposed in JP 2003-166039 (Patent Document 1) and WO 2009/044802 (Patent Document 2). Polythionic acid SCC is, Cr is M 23 C 6 generated by Cr-depleted zone is formed near the grain boundary precipitates in the grain boundaries as type carbide. Therefore, Patent Document 1 and Patent Document 2, M to reduce the amount of C 23 C 6 to suppress the formation of type carbide, to enhance the resistance to polythionic acid SCC resistance.
[0004]
　Specifically, austenitic heat-resistant steels disclosed in Patent Document 1, by mass%, C: less than 0.005 ~ 0.03%, Si: 0.05 ~ 0.4%, Mn: 0.5 ~ 2%, P: 0.01 ~ 0.04%, S: 0.0005 ~ 0.005%, Cr: 18 ~ 20%, Ni: 7 ~ 11%, Nb: 0.2 ~ 0.5% , V: 0.2 ~ 0.5%, Cu: 2 ~ 4%, N: 0.10 ~ 0.30%, B: 0.0005 ~ containing 0.0080% the balance being Fe and unavoidable consisting of impurities. The total content of Nb and V is at least 0.6%, Nb solid solution amount in the steel is 0.15% or more. Further, to satisfy the N / 14 ≧ Nb / 93 + V / 51 and Cr-16C-0.5Nb-V ≧ 17.5,. In Patent Document 1, to reduce the C content, Cr and C, by defining the relationship between Nb and V, thereby enhancing the resistance to polythionic acid SCC resistance.
[0005]
　Patent Document 2 discloses austenitic stainless steel, by 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: includes 0.03% or less, furthermore, 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 and Zr: one or more of 0.2% or less, the balance being Fe and impurities. During 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% hereinafter, and Sb: 0.01% or less. Furthermore, satisfies 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. In Patent Document 2, enhancing the resistance to polythionic acid SCC resistance by the C content to less than 0.05%. Furthermore, by reducing the C immobilized elements such Nb and Ti, P in steel, S, by reducing the grain boundary embrittlement elements such as Sn, enhancing embrittlement cracking resistance of the weld heat affected zone (HAZ) .
CITATION
Patent Document
[0006]
Patent Document 1: JP 2003-166039 Patent Publication
Patent Document 2: WO 2009/044802
Summary of the Invention
Problems that the Invention is to Solve
[0007]
　Meanwhile, recently, in member used in the above high temperature corrosive environments, high creep ductility are required. In plant equipment, as described above, there is a case to carry out periodic inspections to stop facilities. In periodic inspections, replacement is necessary member is investigated. At this time, the higher the creep ductility, during periodic inspection, check the degree of deformation of the member may be a criterion for replacement parts.
[0008]
　Patent Document 1 and Patent Document 2, although the purpose of improving the resistance to polythionic acid SCC resistance, does not aim for improvement in creep ductility. In the proposed steel in these patent documents, in order to increase the resistance to polythionic acid SCC resistance, and low C content. In this case, there is a case in which high creep ductility can not be obtained.
[0009]
　An object of the present invention is excellent in resistance to polythionic acid SCC resistance, and excellent in creep ductility is to provide an austenitic stainless steel.
Means for Solving the Problems
[0010]
　Austenitic stainless steel according to the present invention, in mass%, C: 0.030% or less, Si: 0.10 ~ 1.00%, Mn: 0.20 ~ 2.00%, P: 0.040% or less , S: 0.010% or less, Cr: 16.0 ~ 25.0%, Ni: 10.0 ~ 30.0%, Mo: 0.1 ~ 5.0%, Nb: 0.20 ~ 1. 00%, N: 0.050 ~ 0.300 %, sol. Al: 0.0005 ~ 0.100%, B : 0.0010 ~ 0.0080%, Cu: 0 ~ 5.0%, W: 0 ~ 5.0%, Co: 0 ~ 1.0%, V : 0 ~ 1.00%, Ta: 0 ~ 0.2%, Hf: 0 ~ 0.20%, Ca: 0 ~ 0.010%, Mg: 0 ~ 0.010%, and rare earth elements: 0 contains ~ 0.10%, the balance being Fe and impurities, having a chemical composition satisfying the formula (1).
　+ 0.004-0.9C + B 0.017Mo 2 ≧ 0 (1)
　where each element symbol of the formula (1), the content of the corresponding element (mass%) is substituted.
Effect of the invention
[0011]
　Austenitic stainless steel according to the present invention is excellent in resistance to polythionic acid SCC resistance and is also excellent in creep ductility.
DESCRIPTION OF THE INVENTION
[0012]
　The present inventors have found that not only resistance to polythionic acid SCC resistance, were investigated and examined excellent steel creep ductility.
[0013]
　If reducing the C content to 0.030% or less, during use under high-temperature corrosion environment, M 23 C 6 generates the type carbide is suppressed, generation of Cr-depleted layers in the vicinity of the grain boundary is suppressed . Further, in the present invention, by containing 0.20 to 1.00% of Nb, fixed C-in Nb, M 23 C 6 further reduce the amount of solute C as the generation source of the type carbide. The present invention further containing Mo 0.1 ~ 5.0%. Mo is, M 23 C 6 inhibiting the production of type carbide. Therefore, the generation of C-deficient layer is reduced. By the above measures, it is possible to increase the resistance to polythionic acid SCC resistance.
[0014]
　However, the present inventors have investigated, if reducing the C content to 0.030% or less, creep ductility was found to be reduced. The reason for this, is considered the following matters. It precipitates generated in the grain boundaries, increasing the grain boundary strength. If the grain boundary strength is Takamare, creep ductility increases. However, if reducing the C content to 0.030% or less, precipitates produced in grain boundaries (carbide, etc.) is also reduced. As a result, hardly grain boundary strength can be obtained, it is considered to creep ductility is decreased.
[0015]
　Accordingly, the present inventors have for the austenitic stainless steels which can achieve both excellent polythionic acid SCC resistance and excellent creep ductility was further investigated. B (boron) in the high-temperature corrosion environment of above 600 ~ 700 ° C., segregates to grain boundaries, it is considered possible to increase the grain boundary strength.
[0016]
　Accordingly, the present inventors have, by mass%, C: 0.030% or less, Si: 0.10 ~ 1.00%, Mn: 0.20 ~ 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 16.0 ~ 25.0%, Ni: 10.0 ~ 30.0%, Mo: 0.1 ~ 5.0%, Nb: 0.20 ~ 1.00 %, N: 0.050 ~ 0.300%, sol. Al: 0.0005 ~ 0.100%, B: 0.0010 ~ 0.0080%, Cu: 0 ~ 5.0%, W: 0 ~ 5.0%, Co: 0 ~ 1.0%, V : 0 ~ 1.00%, Ta: 0 ~ 0.2%, Hf: 0 ~ 0.20%, Ca: 0 ~ 0.010%, Mg: 0 ~ 0.010%, and rare earth elements: 0 contains ~ 0.10%, was considered balance if austenitic stainless steel consisting of Fe and impurities, it is possible to achieve both excellent polythionic acid SCC resistance and excellent creep ductility.
[0017]
　However, the results of the examination of resistance to polythionic acid SCC and creep ductility of austenitic stainless steel having the above chemical composition, although excellent polythionic acid SCC resistance is obtained, that in some cases excellent creep ductility can not necessarily be obtained all right. Accordingly, the present invention have conducted a further investigation. As a result, the creep ductility, was found to be considered the following mechanism.
[0018]
　As described above, in the present embodiment, in order to increase the resistance to polythionic acid SCC resistance, as well as the C content to 0.030% or less, the C containing from 0.20 to 1.00% of Nb fixed to nb, reducing the solid solution C. Specifically, Nb is solution heat treatment, or by aging in a short time, precipitates as MX type carbonitrides combines with C. However, in the steel of the use environment of the present embodiment (600 ~ 700 ° C. hot corrosive environment), MX type carbonitrides are metastable phases. Therefore, when it has been used for a long time the steel having the above chemical composition at 600 ~ 700 ° C. in hot corrosion environment, MX type carbonitrides of Nb is, Z-phase (CrNbN) and M is a stable phase 23 C 6 in type carbide Change. In this case, the B that segregates at the grain boundaries, M 23 C 6 is replaced with a portion of the C of the type carbide, M 23 C 6 are absorbed by the type carbide. Therefore, to reduce the amount of B which is segregated at the grain boundaries, the grain boundary strength is reduced. As a result, sufficient creep ductility is considered to not be obtained.
[0019]
　Therefore, during use in high temperature corrosive environment of 600 ~ 700 ° C., for a method of suppressing the reduction of segregation amount of B at the grain boundaries, it was further studied. As a result, it was found that to be considered the following mechanism.
[0020]
　Mo is as described above, M 23 C 6 inhibiting the production itself of type carbide. Mo further, M 23 C 6 is replaced with a part of M of type carbide, M 23 C 6 which may be dissolved in type carbide. In this specification, M Mo in a solid solution 23 C 6 a type carbide, "Mo solid solution M 23 C 6 is defined as type carbide". Mo solid solution M 23 C 6 type carbide is hard to solid solution B. Accordingly, during use under high-temperature corrosion environment, MX type carbonitrides containing Nb is Z phase and M 23 C 6 even when the change in type carbide, M 23 C 6 type carbide is Mo solid solution M 23 C 6 if type carbide, M of B 23 C 6 can suppress solid solution of the type carbide, reduction of segregation B amounts at the grain boundary is suppressed. As a result, it is considered that it is possible to achieve both excellent polythionic acid SCC resistance and excellent creep ductility.
[0021]
　Accordingly, the austenitic stainless steel having the above chemical composition, during use at a high temperature corrosive environment of 600 ~ 700 ° C., MX type carbonitrides containing Nb is Z phase and M 23 C 6 was changed to type carbide even if, Mo solid solution M 23 C 6 by-type carbides are formed and was further examined the chemical composition capable of suppressing the reduction of segregation B amounts at the grain boundaries. As a result, Mo solid solution M 23 C 6 in the reduced suppression of segregation B amount by generating the type carbides, and B in the chemical composition, and C, and it was found that the Mo is closely related. Then, in the above chemical composition, B, satisfies the C and Mo is the formula (1), even during use at a high temperature corrosive environment of 600 ~ 700 ° C., excellent polythionic acid SCC resistance and excellent creep ductility it was found that it is possible to achieve both.
　+ 0.004-0.9C + B 0.017Mo 2 ≧ 0 (1)
　where each element symbol of the formula (1), the content of the corresponding element (mass%) is substituted.
[0022]
　The present inventors have made further investigation, if Cu is an optional element in the austenitic stainless steel is contained, if containing Cu 5.0% or less, while provides excellent creep strength even creep ductility It can be maintained, the upper limit of the Cu content further if 1.9% or less, while increasing the creep strength was found to be maintained even higher creep ductility. The reason for this, is considered the following matters. Cu is during use under high temperature corrosive environment, to form a Cu phase precipitated in the grains. Although Cu phase increases the creep strength, it may decrease the creep ductility. Therefore, an above chemical composition, the austenitic stainless steels satisfying the formula (1), more preferably, Cu content is less 1.9%. If Cu content is less 1.9%, it can maintain excellent creep ductility more effectively.
[0023]
　The inventors of the present inventors have further studied, if the Mo content is 0.5% or more, it was found that further enhances the creep ductility. The reason for this is not clear, it is believed the following matters. The chemical composition (satisfying the formula (1)) In yet, when the Mo content is 0.5% or more, during use at a high temperature corrosive environment of 600 ~ 700 ° C., Mo is further segregated at the grain boundaries or generate an intermetallic compound or. The grain boundary segregation and intermetallic compounds, further enhanced grain boundary strength. As a result, further enhanced creep ductility. Therefore, the lower limit of the preferred Mo content is 0.5%. The lower limit of the preferred Mo content to further enhance the creep ductility was 0.8%, more preferably 1.0%, more preferably 2.0%.
[0024]
　Austenitic stainless steel according to the present invention has been completed based on the above findings, by mass%, C: 0.030% or less, Si: 0.10 ~ 1.00%, Mn: 0.20 ~ 2.00% , P: 0.040% or less, S: 0.010% or less, Cr: 16.0 ~ 25.0%, Ni: 10.0 ~ 30.0%, Mo: 0.1 ~ 5.0%, Nb: 0.20 ~ 1.00%, N : 0.050 ~ 0.300%, sol. Al: 0.0005 ~ 0.1000%, B : 0.0010 ~ 0.0080%, Cu: 0 ~ 5.0%, W: 0 ~ 5.0%, Co: 0 ~ 1.0%, V : 0 ~ 1.00%, Ta: 0 ~ 0.2%, Hf: 0 ~ 0.20%, Ca: 0 ~ 0.010%, Mg: 0 ~ 0.010%, and rare earth elements: 0 contains ~ 0.10%, the balance being Fe and impurities, having a chemical composition satisfying the formula (1).
　+ 0.004-0.9C + B 0.017Mo 2 ≧ 0 (1)
　where each element symbol of the formula (1), the content of the corresponding element (mass%) is substituted.
[0025]
　The chemical composition, in mass%, Cu: 0.1 ~ 5.0%, W: 0.1 ~ 5.0%, and Co: 1 selected from the group consisting of 0.1 to 1.0% it may contain more species or in combination.
[0026]
　The chemical composition, by mass%, V: 0.1 ~ 1.00%, Ta: 0.01 ~ 0.2%, and Hf: 1 selected from the group consisting of 0.01 to 0.20% it may contain more species or in combination.
[0027]
　The chemical composition, by mass%, Ca: 0.0005 ~ 0.010%, Mg: 0.0005 ~ 0.010%, and rare earth element: is selected from the group consisting of 0.001 to 0.10% it may contain one or more that.
[0028]
　The chemical composition, in mass%, Cu: 0 may contain ~ 1.9%.
[0029]
　The chemical composition, by mass%, Mo: may contain 0.5 to 5.0%.
[0030]
　It described in detail below austenitic stainless steel of the present embodiment. "%" Related elements, unless otherwise specified, it means mass%.
[0031]
　[Chemical composition]
　The chemical composition of the austenitic stainless steel of the present embodiment contains the following elements.
[0032]
　C: 0.030% or less
　carbon (C) is inevitably contained. C, during use austenitic stainless steel of the present embodiment in a high temperature corrosive environment of 600 ~ 700 ° C., M in the grain boundary 23 C 6 generates a type carbide, decreases the resistance to polythionic acid SCC resistance. Therefore, C content is 0.030% or less. The preferable upper limit of the C content is 0.020%, more preferably 0.015%. C content is preferably as small as possible. However, as described above, since C is unavoidably contained, industrial production, C is at least, is 0.0001% may be contained. Therefore, a preferred lower limit of the C content is 0.0001%.
[0033]
　Si: 0.10 ~ 1.00%
　silicon (Si), the deoxidizing steel. Si further enhance the oxidation resistance and steam oxidation resistance of steel. If Si content is too low, not the effect. On the other hand, if the Si content is too high, the sigma phase in steel (sigma phase) precipitated and the toughness of the steel is lowered. Therefore, Si content is 0.10 to 1.00%. The preferable upper limit of the Si content is 0.75%, more preferably from 0.50%.
[0034]
　Mn: 0.20 ~ 2.00%
　of manganese (Mn) of deoxidizing steel. Mn further stabilizes the austenite, increase the creep strength. If the Mn content is too low, not the effect. On the other hand, if the Mn content is too high, the creep strength of the steel is lowered. Therefore, Mn content is 0.20 to 2.00%. The preferable lower limit of the Mn content is 0.40%, more preferably 0.50%. The preferable upper limit of the Mn content is 1.70%, more preferably 1.50%.
[0035]
　P: 0.040% or less
　phosphorus (P) is an impurity. P decreases the hot workability and toughness of the steel. Accordingly, P content is 0.040% or less. The preferable upper limit of the P content is 0.035%, more preferably 0.032%. P content is preferably as small as possible. However, P is is unavoidably contained, industrial production, P is at least, is 0.0001% may be contained. Therefore, a preferred lower limit of the P content is 0.0001%.
[0036]
　S: 0.010% or less
　Sulfur (S) is an impurity. S lowers the hot workability and creep ductility of the steel. Thus, S content is 0.010% or less. The preferable upper limit of the S content is 0.005%. S content is preferably as small as possible. However, S is being unavoidably contained, industrial production, S is at least, is 0.0001% may be contained. Therefore, a preferred lower limit of the S content is 0.0001%.
[0037]
　Cr: 16.0 ~ 25.0%
　chromium (Cr) increases the resistance to polythionic acid SCC resistance of the steel. Cr further oxidation resistance, steam oxidation resistance, increasing the high temperature corrosion resistance and the like. If the Cr content is too low, not the effect. On the other hand, if the Cr content is too high, it decreases the creep strength and toughness of the steel. Therefore, Cr content is 16.0 to 25.0%. A preferable lower limit of Cr content is 16.5%, more preferably 17.0%. The preferable upper limit of the Cr content is 24.0%, still more preferably 23.0%.
[0038]
　Ni: 10.0 ~ 30.0%
　nickel (Ni) is to stabilize the austenite and increases the creep strength. If the Ni content is too low, not the effect. On the other hand, if the Ni content is too high, the effect is saturated, further, the manufacturing cost is high. Therefore, Ni content is 10.0 to 30.0%. A preferable lower limit of Ni content is 11.0%, still more preferably 13.0%. The preferable upper limit of the Ni content is 25.0%, more preferably 22.0%.
[0039]
　Mo: 0.1 ~ 5.0%
　of molybdenum (Mo), during use at a high temperature corrosive environment of 600 ~ 700 ° C., M in the grain boundary 23 C 6 suppress to generate the type carbide. Mo is further during use under high temperature corrosive environment at 600 ~ 700 ° C., MX type carbonitride Nb is M 23 C 6 when changing the type carbide, M 23 C 6 B solid solution in the type carbide to the suppressed by, restrain the segregation B of grain boundaries in a high temperature corrosive environment is reduced. Thus, in a high temperature corrosive environment, sufficient creep ductility can be obtained. If the Mo content is too low, not the effect. On the other hand, if Mo content is too high, the stability of the austenite is lowered. Therefore, Mo content is 0.1 to 5.0%. A preferable lower limit of Mo content is 0.2%, and more preferably 0.3%.
[0040]
　Mo content is more long than 0.5%, Mo or segregates at the grain boundaries, or by generating an intermetallic compound, further increasing the grain boundary strength. In this case, in a high temperature corrosive environment, better creep strength. Thus, more preferred lower limit of the Mo content is 0.5%, more preferably 0.8%, more preferably 1.0%, more preferably 1.5%, more preferably it is 2.0%. If Mo content is 1.5% or more, the creep strength increase. The preferable upper limit of the Mo content is 4.5%, more preferably from 4.0%. If Mo content is 1.5% or more, the creep strength increase.
[0041]
　Nb: 0.20 ~ 1.00%
　niobium (Nb), during use at a high temperature corrosive environment of 600 ~ 700 ° C., to produce an MX type carbonitride bonded by C, the solid solution in steel to reduce the amount of C. This increases the resistance to polythionic acid SCC resistance of the steel. MX type carbonitrides of the resulting Nb also enhances the creep strength. If the Nb content is too low, not the effect. On the other hand, if the Nb content is too high, [delta] ferrite is generated, long-term creep strength of the steel, toughness, and reduces the weldability. Therefore, Nb content is 0.20 to 1.00%. The preferable lower limit of Nb content is 0.25%. The preferable upper limit of Nb content is 0.90%, more preferably 0.80%.
[0042]
　N: 0.050 ~ 0.300%
　nitrogen (N) is to stabilize the austenite solid solution in the matrix (mother phase), increasing the creep strength. N further forms fine carbonitrides in the grains, increasing the creep strength of the steel. That, N represents contributes to creep strength in both solid solution strengthening and precipitation. If the N content is too low, not the effect. On the other hand, if the N content is too high, Cr nitrides are formed at the grain boundaries, anti polythionic acid SCC resistance of the weld heat affected zone (HAZ) is reduced. Further if the N content is too high, the workability of the steel is lowered. Therefore, N content is 0.050 to 0.300%. The preferable lower limit of the N content is 0.070%. The preferable upper limit of the N content is 0.250%, more preferably 0.200%.
[0043]
　sol. Al: 0.0005 ~ 0.100%
　of aluminum (Al) is deoxidized steel. If the Al content is too low, not the effect. On the other hand, if the Al content is too high, it decreases the cleanliness of the steel, which lowers the workability and ductility of the steel. Therefore, Al content is from 0.0005 to 0.100 percent. A preferable lower limit of Al content is 0.001%, more preferably from 0.002%. The preferable upper limit of Al content is 0.050%, more preferably 0.030%. Al content in the present embodiment means a content of acid-soluble Al (sol. Al).
[0044]
　B: 0.0010 ~ 0.0080%
　boron (B), during use under high-temperature corrosion environment at 600 ~ 700 ° C., segregated in the grain boundary, increasing the grain boundary strength. As a result, increase the creep ductility. If the B content is too low, not the effect. On the other hand, if the B content is too high, the hot workability in weldability and high temperature decreases. Therefore, B content is 0.0010 to 0.0080 percent. The preferable lower limit of the B content is 0.0015%, more preferably 0.0020%. The preferable upper limit of the B content is less than 0.0060%, more preferably 0.0050%.
[0045]
　The remainder of the chemical composition of the austenitic stainless steel according to the embodiment consists of Fe and impurities. Here, the impurity, when the industrial production of austenitic stainless steels, the ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, adverse effects on the austenitic stainless steel of the present embodiment It means what is allowed in a range which does not give.
[0046]
　[For any element]
　Austenitic stainless steels according to the present embodiment further includes, in place of part of Fe, may contain one or more selected Cu, from the group consisting of W and Co. Both of these elements, increasing the creep strength of the steel.
[0047]
　Cu: 0 ~ 5.0%
　copper (Cu) is an optional element and may not be contained. If contained, Cu in during the use at 600 ~ 700 ° C. hot corrosion environment, precipitates as Cu phase in the grains, increasing the creep strength of the steel by precipitation strengthening. However, if the Cu content is too high, decrease the hot workability and weldability of the steel. Therefore, Cu content is 0 to 5.0%. The preferable lower limit of the Cu content for increasing the creep strength more effective is 0.1%, more preferably 2.0%, more preferably from 2.5%. The preferable upper limit of Cu content is 4.5%, more preferably from 4.0%. On the other hand, the preferred Cu content in order to maintain a more excellent creep ductility is 0 to 1.9% upper limit for further Konomashikui Cu content is 1.8%.
[0048]
　W: 0 ~ 5.0%
　tungsten (W) are optional elements may not be contained. If contained, W is a solid solution in the matrix (mother phase), increasing the creep strength of the steel. However, if the W content is too high, the stability of the austenite is lowered, to decrease the creep strength and toughness of the steel. Therefore, W content is 0 to 5.0%. The preferable lower limit of the W content is 0.1%, more preferably 0.2%. The preferable upper limit of the W content is 4.5%, more preferably from 4.0%.
[0049]
　Co: 0 ~ 1.0%
　cobalt (Co) is an arbitrary element, it may not be contained. If contained, Co is stable austenite, increase the creep strength. However, if the Co content is too high, it increases the material cost. Therefore, Co content is 0 to 1.0%. The preferable lower limit of the Co content is 0.1%, more preferably 0.2%.
[0050]
　Austenitic stainless steel according to the present embodiment further, in place of part of Fe, V, may contain one or more members selected from the group consisting of Ta and Hf. Both of these elements enhance the resistance to polythionic acid SCC resistance and creep strength of the steel.
[0051]
　V: 0 ~ 1.00%
　vanadium (V) are optional elements may not be contained. If contained, V is, 600 during use at a high temperature corrosive environment of ~ 700 ° C., to produce a carbonitride bonded as C, to reduce the solute C, resistance to polythionic acid SCC resistance of the steel increased. V carbonitrides generated also enhances the creep strength. However, if the V content is too high, [delta] ferrite is generated, the creep strength of the steel, toughness, and weldability is deteriorated. Therefore, V content is from 0 to 1.00%. The preferable lower limit of V content to increase further the effective resistance polythionic acid SCC resistance and creep strength is 0.10%. The preferable upper limit of the V content is 0.90%, more preferably 0.80%.
[0052]
　Ta: 0 ~ 0.2%
　tantalum (Ta) is any element, it may not be contained. If contained, Ta is 600 during use at a high temperature corrosive environment of ~ 700 ° C., to produce a carbonitride bonded as C, to reduce the solute C, resistance to polythionic acid SCC resistance of the steel increased. Ta carbonitride generated also enhances the creep strength. However, if Ta content is too high, [delta] ferrite is generated, the creep strength of the steel, toughness, and weldability is deteriorated. Therefore, Ta content is 0-0.2%. The preferable lower limit of the Ta content to increase further the effective resistance polythionic acid SCC resistance and creep strength is 0.01%, more preferably 0.02%.
[0053]
　Hf: 0 ~ 0.20%
　hafnium (Hf) is an arbitrary element, it may not be contained. If contained, Hf is 600 during use at a high temperature corrosive environment of ~ 700 ° C., to produce a carbonitride bonded as C, to reduce the solute C, resistance to polythionic acid SCC resistance of the steel increased. Hf carbonitride generated also enhances the creep strength. However, if Hf content is too high, [delta] ferrite is generated, the creep strength of the steel, toughness, and weldability is deteriorated. Accordingly, Hf content is 0 to 0.20%. The preferable lower limit of the Hf content is 0.01%, more preferably 0.02%.
[0054]
　Austenitic stainless steel according to the present embodiment further, in place of part of Fe, Ca, may contain one or two or more selected from the group consisting of Mg and rare earth elements. Both of these elements enhances the hot workability and creep ductility of the steel.
[0055]
　Ca: 0 ~ 0.010%
　of calcium (Ca) is an arbitrary element, it may not be contained. If contained, Ca is, O (oxygen) and S (sulfur) were fixed as inclusions, increase the hot workability and creep ductility of the steel. However, if the Ca content is too high, it decreases the hot workability and creep ductility of the steel. Therefore, Ca content is from 0 to 0.010%. The preferable lower limit of the Ca content is 0.0005%, more preferably from 0.001%. The preferable upper limit of Ca content is 0.008%, more preferably 0.006%.
[0056]
　Mg: 0 ~ 0.010%
　magnesium (Mg) is an arbitrary element, it may not be contained. If contained, Mg is, O (oxygen) and S (sulfur) were fixed as inclusions, increase the hot workability and creep ductility of the steel. However, if the Mg content is too high, it decreases the hot workability and long-term creep ductility of the steel. Thus, Mg content is 0 to 0.010%. A preferable lower limit of the Mg content is 0.0005%, more preferably from 0.001%. The preferable upper limit of the Mg content is 0.008%, more preferably 0.006%.
[0057]
　Rare earth element: 0 to 0.10%
　rare earth elements (REM) are optional elements may not be contained. If contained, REM is, O (oxygen) and S (sulfur) were fixed as inclusions, increase the hot workability and creep ductility of the steel. However, if REM content is too high, it decreases the hot workability and long-term creep ductility of the steel. Therefore, REM content is 0 to 0.01%. The preferable lower limit of the REM content is 0.001%, more preferably from 0.002%. The preferable upper limit of the REM content is 0.08%, more preferably 0.06%.
[0058]
　REM herein, Sc, Y, and contains at least one or more kinds of lanthanoid (atomic number 57 No. La ~ 71 No. Lu), REM content means the total content of these elements to.
[0059]
　[For formula (1)]
　the chemical composition further satisfying the equation (1).
　+ 0.004-0.9C + B 0.017Mo 2 ≧ 0 (1)
　The formula (1) each element symbol in the content of the corresponding element (mass%) is substituted.
[0060]
　As described above, in the present embodiment, in order to increase the resistance to polythionic acid SCC resistance, as well as the C content to 0.030% or less, contain 0.20 to 1.00% of Nb, 600 It generates MX type carbonitrides of Nb during use in high-temperature corrosion environment of ~ 700 ° C., to reduce the amount of solute C. However, since MX type carbonitrides of Nb is metastable phase, during use under the high temperature environment of use, Z phase and M 23 C 6 varies type carbide. In this case, the B that segregates at the grain boundaries M 23 C 6 was dissolved in type carbide, to reduce the B segregation at grain boundaries. As a result, the creep ductility is decreased.
[0061]
　However, Mo is M 23 type C 6 "Mo solid solution M in solid solution in type carbide 23 C 6 When generating the type carbides", Mo solid solution M 23 C 6 in type carbide B is hardly dissolved. Therefore, B segregation at the grain boundaries is maintained, not only excellent polythionic acid SCC resistance can be obtained, resulting excellent creep ductility.
[0062]
　= B + F1 0.004-0.9C + 0.017Mo 2 is defined as. F1 is a plurality of M to be generated in the steel in use in a high temperature corrosive environment 23 C 6 of the type carbide, Mo solid solution M 23 C 6 , which is an index showing the proportion of type carbide. If F1 is 0 or more, during use under high-temperature corrosion environment, a plurality of M in the steel 23 C 6 and generate the type carbide, Mo solid solution M 23 C 6 is high proportion of type carbide. Therefore, the B that segregates at the grain boundaries M 23 C 6 hardly dissolves in the type carbide segregation amount of B at the grain boundaries is maintained. Therefore, it is possible to achieve both good and excellent polythionic acid SCC resistance creep ductility. Thus, F1 is 0 (0.00000) or more. Preferably, F1 is at 0.00100 or more, more preferably 0.00200 or more, more preferably not 0.00400 or more, more preferably from 0.00500, more preferably 0.00800 or more , most preferably from 0.01000.
[0063]
　Preferably, if the chemical composition of the austenitic stainless steel containing Cu, as described above, the upper limit of the Cu content is less 1.9%. In other words, while increasing the creep strength, further, considering that obtaining good creep ductility, the preferred Cu content is 0% to 1.9%. If Cu content is less 1.9%, while obtaining excellent creep strength by precipitation strengthening of Cu phase, it can maintain excellent creep ductility.
[0064]
　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 at a high temperature corrosive environment of 600 ~ 700 ° C., Mo further or generate an intermetallic compound or segregated at grain boundaries. The grain boundary segregation and intermetallic compounds, further enhanced grain boundary strength. As a result, further enhanced creep ductility. Therefore, the lower limit of the preferred Mo content is 1.0%. In the case the lower limit of the Mo content is 1.0% or more, preferably F1 value is at 0.00500 or more, more preferably not 0.00800 or more, more preferably 0.01000 or more.
[0065]
　[Manufacturing Method]
　An example of a manufacturing method of austenitic stainless steels of the present invention will be described. This manufacturing method includes a preparation step of preparing a material, the hot working step of performing hot working to produce the steel, the steel after hot working step optionally cold working against material comprising a cold working step, and a solution treatment step of performing solution treatment against steel if necessary. Hereinafter, a manufacturing method will be described.
[0066]
　[Preparation Step]
　A above chemical composition, to produce a molten steel satisfying the equation (1). For example, an electric furnace or AOD (Argon Oxygen Decarburization) furnace, using a VOD (Vacuum Oxygen Decarburization) furnace, to produce the molten steel. Relative produced molten steel, carrying out the known degassing treatment if necessary. From molten steel was carried out degassing treatment, to produce the material. Method for producing a material for example, a continuous casting method. By continuous casting, to produce continuously cast material (Material). Continuous casting material is for example, slabs, blooms and billets and the like. Molten steel may be ingot by ingot-making method.
[0067]
　[Hot working step]
　prepared material (continuous cast material or ingot) by hot working to produce a austenitic stainless steel. For example, to produce steel sheets and steel bars, wire rods and hot rolling materials. Moreover, the production of austenitic stainless steel tube by hot extrusion or hot piercing rolling and the like. Specific methods of the hot working is not particularly limited, the hot working in accordance with the shape of the final product may be performed. Machining end temperature of the hot working, for example, is 1050 ° C. or higher. The working end temperature here means the temperature of the steel material immediately after the final hot working is completed.
[0068]
　[Cold working step]
　with respect to austenitic stainless steel after hot working, if necessary, it may be carried out cold working. Austenitic stainless steel is steel bar, wire rod, if a steel pipe, cold working, for example, a cold drawing or cold rolling. When austenitic stainless steel is steel, a cold rolled like.
[0069]
　[Solution treatment step]
　After hot working, or after cold working, if necessary, it may be carried out solution treatment. The solution treatment step, the solid solution homogenizing, and carbonitrides of tissue. Preferred solution treatment temperature is as follows.
[0070]
　Preferred solution treatment temperature: 1000 ~ 1250 ° C.
　if the solution treatment temperature is 1000 ° C. or higher, carbonitride Nb is sufficiently dissolved, further enhanced creep strength. If less 1250 ° C. heat treatment temperature, it is possible to suppress excessive solute and C, anti polythionic acid SCC resistance increases further.
[0071]
　Retention time at the solution treatment temperature during solution treatment is not particularly limited, for example, 2 to 60 minutes.
[0072]
　Incidentally, with respect to the steel produced by hot working process, in place of the solution treatment described above may be carried out rapidly immediately after the hot working. In this case, the machining end temperature of the hot working is preferably set to 1000 ° C. or higher. If hot working finishing temperature is 1000 ° C. or higher, it carbonitride Nb is sufficiently dissolved, during use in 600 ~ 700 ° C. in hot corrosion environment, excellent polythionic acid SCC resistance and excellent creep it is possible to achieve both ductility and by the formation of Nb carbonitrides during use in a high temperature environment, sufficient creep strength is also obtained.
[0073]
　The shape of the austenitic stainless steel of the present embodiment is not particularly limited. Austenitic stainless steel of the present embodiment may be a steel plate, may be a steel pipe, may be a steel bar or wire, may be in the form steel.
Example
[0074]
　Chemical composition of Table 1 was prepared molten steel having.
[0075]
[Table 1]

[0076]
　The "F1" column in Table 1, F1 value of the steel of each test number is entered. The numerical values ​​attached to the front of the "other" column element symbol and each element symbol in the column "chemical composition" includes any element contained means the content (% by mass). Among the chemical composition of each test number, the balance other than the elements described in Table 1 was Fe and impurities.
[0077]
　With molten steel, an outer diameter of 120 mm, a 30kg ingot was produced. By carrying out hot forging relative ingot was steel plate having a thickness of 40 mm. Further, by carrying out hot rolling, and the steel plate having a thickness of 15 mm. Any final working temperature of the hot rolling were 1050 ° C. or higher. Against steel sheet after hot rolling, further by carrying out cold rolling, a thickness of 10.5 mm, width 50 mm, were manufactured steel plate length 100 mm. For each steel sheet after cold rolling was performed to a solution treatment. Solution treatment temperature of the steel sheet of each test number is either 1150 ° C., the solution treatment time was 10 minutes both. The steel sheet after solution treatment was water-cooled. Through the above process, it was prepared austenitic stainless steel.
[0078]
　The thickness of the produced austenitic stainless steel sheet is defined as t (mm), using a sample of any position of t / 4 depth from the surface, burning the known component analysis (C and S - infrared absorption method, hot leaving gas analysis for N, for other alloying elements was performed ICP analysis). As a result, the chemical composition of the austenitic stainless steel sheet of each test number was consistent with Table 1.
[0079]
　[Resistant polythionic acid SCC resistance evaluation test]
　with respect to the steel sheet of each test number, intended for use in a high temperature environment, was carried out aging treatment for 5000 hours at 600 ° C.. This aging treatment material, thickness 2 mm, width of 10 mm, a plate specimen of length 75mm was collected. In accordance with JIS G 0576 (2001) "Stress corrosion cracking test method stainless steel", was carried out resistance polythionic acid SCC resistance evaluation test. Specifically, the test piece was U-bend-shaped by bending the punch around the inner radius of 5 mm. The U-bend-shaped test piece, Wackenroder solution (in distilled water SO 2 H was prepared by blowing gas 2 SO 3 large amount of H in a saturated aqueous solution 2 was immersed for 100 hours at room temperature the solution was bubbled S gas). On specimens after the immersion, and microscopic observation whether cracks occurred in 500 times magnification, to confirm the presence or absence of cracks.
[0080]
　If cracks were not confirmed was judged to be excellent in resistance to polythionic acid SCC resistance (in the "resistance to polythionic acid SCC resistance" column in Table 2, "E" (Excellent)). If the crack is confirmed even one was judged to be low resistance to polythionic acid SCC resistance (in the "resistance to polythionic acid SCC resistance" column in Table 2, "NA" (Not Accepted)).
[0081]
　[Creep ductility and creep strength evaluation test]
　a steel plate of each test number, to produce a creep rupture test specimen conforming to JIS Z2271 (2010). A cross-section perpendicular to the axial direction of the creep rupture test piece is circular, the outer diameter of the creep rupture test piece is 6 mm, the parallel portion was 30 mm. Parallel portion was parallel to the rolling direction of the steel sheet. Using the produced creep rupture test specimens were carried out creep rupture test according to JIS Z2271 (2010). Specifically, after a creep rupture test piece was heated at 750 ° C., it was carried out creep rupture test. Test stress and 45 MPa, was determined creep rupture time (hours) and the throttle creep rupture (%).
[0082]
　Respect creep strength, when the creep rupture time is less than 5000 ~ 10000h, was judged to be excellent in creep strength (denoted at "creep strength" column in Table 2 with "G" (Good)). If the creep rupture time exceeds 10000 hours, it is determined that the creep strength superior significantly (denoted at "creep strength" column in Table 2 in the "E" (Excellent)). If the creep rupture time is less than 5000 hours, (denoted by "NA" in "creep strength" column in Table 2 (Not Accepted)) where creep strength is determined to be low. If the creep rupture time of the G or E, it is determined that the sufficient creep strength is obtained.
[0083]
　Respect creep ductility, when the creep rupture diaphragm is less than 20.0% to 30.0% creep ductility was judged to be good (denoted at "creep ductility" column in Table 2 by "P" (Passing)). If the creep rupture diaphragm is less than 50.0% exceeded 30.0%, it is determined that the creep ductility is excellent (denoted at "creep ductility" column in Table 2 with "G" (Good)). Furthermore, if the creep rupture diaphragm exceeds 50.0%, (denoted by "E" in the "creep ductility" column in Table 2 (Excellent)) creep ductility was determined that excellent remarkably. If the creep rupture diaphragm is less than 20.0%, it was determined to have low creep ductility (in Table 2 of "NA" in "creep ductility" column (Not Accepted). Creep rupture diaphragm, P, G, or E case, it is determined that sufficient creep ductility was obtained.
[0084]
　[Test Results]
　The test results are shown in Table 2.
[0085]
[Table 2]

[0086]
　See Table 1 and Table 2, the content of each element in the chemical composition of the steel with test Nos. 1 to 16 are suitable, F1 also satisfies the equation (1). Therefore, in the steel of these test numbers were obtained excellent polythionic acid SCC resistance. Furthermore, the rupture time of not less than 5000 hours, were obtained excellent creep strength. Furthermore, the creep rupture diaphragm is 20.0% or more were obtained excellent creep ductility. Further, in Test Nos. 2 to 4 and 6 to 12 and 15, to or containing a large amount of Mo or contain Cu, rupture time in creep rupture test is longer than the test No. 1,5,13,14 and 16 is 10,000 hours or more, excellent creep strength was obtained.
[0087]
　Furthermore, Test No. 3, 4 Cu content contains content vital Mo of 0.5% or more of the following 1.9%, and the test numbers without containing Cu containing Mo 1.0% or more In 5-7,11,12, while obtained sufficient creep strength and excellent creep ductility was also obtained.
[0088]
　On the other hand, in Test No. 17 and 18, F1 does not satisfy the equation (1). As a result, the creep rupture diaphragm is less than 20%, had low creep ductility of the steel. Grain boundary strengthening effect due to grain boundary segregation of B is considered because it was not sufficiently obtained. In addition, the creep strength was also low.
[0089]
　In Test No. 19, the C content is too high. As a result, resistance to polythionic acid SCC resistance was low.
[0090]
　In Test No. 20, since the containing Cu, although the creep strength was high, F1 does not satisfy the equation (1). As a result, the creep rupture diaphragm is less than 20.0%, lower creep ductility of the steel.
[0091]
　In Test No. 21 contained no Mo. Furthermore, F1 was less than the lower limit of the formula (1). As a result, fracture aperture is less than 20.0%, lower creep ductility of the steel. In addition, the creep strength was also low.
[0092]
　In Test No. 22, B content is low. As a result, the creep rupture diaphragm is less than 20.0%, lower creep ductility of the steel. In addition, the creep strength was also low.
[0093]
　In test number 23, it did not contain Nb. As a result, resistance to polythionic acid SCC resistance was low. In addition, break time is less than 5000 hours, the creep strength of the steel was low.
[0094]
　It has been described an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the above-described embodiments without departing from the scope and spirit thereof as appropriate.

The scope of the claims
[Requested item 1]
　で
　mass%, C: 0.030% or
　less,
　Si: 0.10 ~ 1.00%, Mn: 0.20
　~ 2.00%, P: 0.040% or
　less, S: 0.010% or less, 　cr:
　16.0 ~ 25.0%, of Ni: 10.0 　~ 30.0%, of Mo: 0.1 ~ 5.0%, of Nb: 0.20 　~ 1.00%, N: 0.050 ~ 　% 0.300, Al: 　0.0005 　~ 0.100%, B: 0.0010 ~ 　0.0080%, a Cu: 0 ~ 5.0%, 　W is: 0 ~ 5.0%, of Co: 0 ~ 　1.0%, V : 　0 　~ 1.00%, 　Ta: 0 ~ 0.2%, Hf: 0 ~ 0.20%, of 　Ca: 0 ~ 0.010%, of Mg: 0 ~ 0.010%, andび, 　rare earth elements: 0 wo shi containing 0.10%, and Fe remnants gaびkara na ri impurities, 　formula (1) wo wo chemical composition of the Man su ta has suru, BioーイSuites na su Te based Toray su su Tensilon steel. 　0.004-0.9C + 0.017Mo + B 2 ≧ 0 (1) 　ko koでformula (1), each element symbol ni waの,のDui Applied suru element content (mass%) substituting Connecticut DomNode- ga ru.

[Requested item 2]
　A austenitic stainless steel according to claim 1,
　wherein the chemical
　composition, Cu:
　0.1 ~ 5.0%, W: 0.1 ~ 5.0%, and
　Co: 0.1 ~ 1. one or containing two or more, austenitic stainless steel is selected from the group consisting of 0%.
[Requested item 3]
　A austenitic stainless steel according to claim 1 or claim 2,
　wherein the chemical
　composition,
　V: 0.1 ~ 1.00%, Ta: 0.01 ~ 0.2%, and
　Hf: 0. containing one or more members selected from the group consisting of from 01 to 0.20%, an austenitic stainless steel.
[Requested item 4]
　A austenitic stainless steel according to any one of claims 1 to 3,
　wherein the chemical
　composition,
　Ca: 0.0005 - 0.010%, Mg: 0.0005 - 0.010%, and
　rare earth elements: containing one or more members selected from the group consisting of 0.001 to 0.10%, an austenitic stainless steel.
[Requested item 5]
　A austenitic stainless steel according to claim 1,
　wherein the chemical composition,
　Cu: 0 containing ~ 1.9%, austenitic stainless steel.