JP2014005506A - Austenite stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenite stainless steel having high temperature strength, excellent pitting corrosion resistance and solidification crack resistance.SOLUTION: An austenite stainless steel of this embodiment contains C:less than 0.018%, Si:0.9% or less, Mn:1.8% or less, P:0.04% or less, S:0.01% or less, Cu:2.0-4.5%,Ni:9-16%, Cr:15-19%, Mo:5% or less, sol.Al:0.04% or less, N:0.02-0.3%, Sn:0.002-0.1% and B:0.009% by mass%, further contains one or more selected from a group consisting of Nb:0.9% or less, Ti:0.15% or less and V:0.4% or less, with the balance consisting of Fe with inevitable impurities and satisfies formula (1) to formula (3). 0.2Cu+325Sn≥1.5(1) Cu-10Sn≥1.9(2) Cu+35Sn≤6.5(3)

Description

  The present invention relates to an austenitic stainless steel.

  Equipment such as power boilers, heating furnace tubes of petroleum refining and petrochemical plants operate in a high temperature environment and are further in contact with process fluids containing sulfides and / or chlorides. Therefore, the material used for these facilities is required to have excellent high temperature strength and corrosion resistance.

  The materials used for the above-mentioned facilities are disclosed in Japanese Patent Application Laid-Open No. 50-67215 (Patent Document 1), Japanese Patent Application Laid-Open No. 60-224762 (Patent Document 2), and International Publication No. 2009/044802 (Patent Document 3). ), Ikuo Kudo et al., “Development of 347AP Stainless Steel for Heating Furnace Tubes Excellent in Polythionic Acid SCC Resistance”, Sumitomo Metals, 38 (1986), p. 190 (Non-Patent Document 1) and Tetsuo Ishizuka et al., “Development of Steel Pipes for Austenitic Stainless Boilers that Achieve High-Temperature Strength and Intergranular Corrosion Resistance”, Nippon Steel Technical Report, 380 (2004), p. 91 (Non-Patent Document 2).

  Patent Document 1 proposes a stainless steel that is strong against intergranular corrosion and intergranular stress corrosion cracking (polythionic acid SCC resistance). In Patent Document 1, the C content in stainless steel is set to 0.03% or less. Furthermore, the ratio of the Nb content to the C content and the ratio of the N content to the C content are set to a predetermined value or more. This is stated to improve the intergranular corrosion properties.

  Patent Document 2 proposes a stainless steel excellent in corrosion resistance and stress corrosion cracking resistance (due to sulfide (polythionic acid) and chloride). In Patent Document 2, the Cr content and the Ni content are increased, and the C content is set to 0.02% or less. This describes that the stress corrosion cracking resistance is improved and the corrosion resistance is also improved.

  Patent Document 3 proposes an austenitic stainless steel excellent in polythionic acid SCC resistance and excellent in resistance to brittle cracking in HAZ when used at a high temperature for a long time. In Patent Document 3, the SCC resistance of polythionic acid is improved by making the C content less than 0.05%. Furthermore, C-fixing elements such as Nb and Ti are reduced, and grain boundary embrittlement elements such as P, S, and Sn in the steel are reduced. Thereby, it is described that the brittle cracking resistance of HAZ is improved.

  Non-Patent Document 1 proposes an austenitic stainless steel excellent in high-temperature strength and polythionic acid SCC resistance. In Non-Patent Document 1, the C content is lowered, a specific amount of N is contained, and a specific amount of Nb is contained as a C-immobilizing element. Thus, it is described that excellent high-temperature strength and polythionic acid SCC resistance are obtained, and further, sensitization is suppressed even when aged for a long time without heat treatment after welding.

  Non-Patent Document 2 proposes an austenitic stainless steel for boilers that is excellent in high-temperature strength and intergranular corrosion resistance. In Non-Patent Document 2, the C content is lowered and a specific amount of V is contained. This describes that the high-temperature strength and the intergranular corrosion resistance are improved.

JP 50-67215 A JP-A-60-224864 International Publication No. 2009/044802

Kudo Ikuo et al., “Development of 347AP Stainless Steel for Heating Furnace Tubes Excellent in Polythionic Acid SCC Resistance”, Sumitomo Metals, 38 (1986), p. 190 Tetsuo Ishizuka et al., “Development of Steel Pipes for Austenitic Stainless Boilers with High-Temperature Strength and Intergranular Corrosion Resistance”, Nippon Steel Technical Report, 380 (2004), p. 91

  By the way, not only polythionic acid SCC but also pitting corrosion may occur in the steel material in contact with the process fluid under the high temperature environment as described above. Therefore, such steel materials are required to have excellent pitting corrosion resistance and high temperature strength.

  Furthermore, these steel materials may be processed into various shapes or non-filler welding may be performed. Therefore, these steel materials are also required to have excellent solidification cracking resistance.

  However, in the stainless steel disclosed in Patent Documents 1 to 3 and Non-Patent Documents 1 and 2, any of high temperature strength, pitting corrosion resistance and solidification crack resistance may be low.

  An object of the present invention is to provide an austenitic stainless steel having high-temperature strength and excellent pitting corrosion resistance and solidification cracking resistance.

The austenitic stainless steel according to the present embodiment is, in mass%, C: less than 0.018%, Si: 0.9% or less, Mn: 1.8% or less, P: 0.04% or less, S: 0.00. 01% or less, Cu: 2.0 to 4.5%, Ni: 9 to 16%, Cr: 15 to 19%, Mo: 5% or less, sol. Al: 0.04% or less, N: 0.02-0.3%, Sn: 0.002-0.1%, and B: 0.009% or less, and Nb: 0.9 % Or less, Ti: 0.15% or less, and V: 1 type or 2 or more types selected from the group consisting of 0.4% or less, the balance consisting of Fe and impurities, Equation (3) is satisfied.
0.2Cu + 325Sn ≧ 1.5 (1)
Cu-10Sn ≧ 1.9 (2)
Cu + 35Sn ≦ 6.5 (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the expressions (1) to (3).

  The austenitic stainless steel may further contain one or more of W: 5% or less and Co: 1% or less, instead of part of Fe. The austenitic stainless steel is further selected from the group consisting of Ca: 0.02% or less, Mg: 0.02% or less, and rare earth element (REM): 0.1% or less, instead of part of Fe. You may contain 1 type, or 2 or more types.

  The austenitic stainless steel according to the present embodiment has high-temperature strength and is excellent in pitting corrosion resistance and solidification crack resistance.

FIG. 1 is a diagram showing the relationship between F1 = 0.2Cu + 325Sn and the pitting potential. FIG. 2 is a diagram showing the relationship between F2 = Cu-10Sn and the fracture time. FIG. 3 is a diagram showing the relationship between F3 = Cu + 35Sn and the maximum crack length.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, “%” related to elements means “mass%”.

  The present inventors investigated the pitting corrosion resistance, high temperature strength and solidification crack resistance of austenitic stainless steel. As a result, the present inventors obtained the following knowledge.

  (A) When the C content is kept low in order to improve the SCC resistance of polythionic acid (for example, when the C content is 0.02% or less), if Cu and Sn are contained, these elements are synergistic. And increase pitting corrosion resistance. Here, it is defined as F1 = 0.2Cu + 325Sn. If F1 is 1.5 or more, excellent pitting corrosion resistance is obtained due to the synergistic effect of Cu and Sn.

  FIG. 1 is a diagram showing the relationship between F1 and pitting potential in an austenitic stainless steel having a C content of 0.02% or less. FIG. 1 was created by measuring the pitting corrosion potential of a plurality of austenitic stainless steels having different F1 values by the same method as the pitting corrosion potential measurement method in Examples described later.

  Referring to FIG. 1, when F1 is less than 1.5, the pitting potential increases remarkably as F1 increases. On the other hand, when F1 exceeds 1.5, the pitting corrosion potential increases as F1 increases, but the degree of increase is smaller than that when F1 is less than 1.5. In short, the pitting potential graph shown in FIG. 1 has an inflection point around F1 = 1.5. Therefore, if F1 is 1.5 or more, excellent pitting corrosion resistance is obtained.

  (B) Cu further increases the high temperature strength (creep strength) in the austenitic stainless steel having a low C content as described above. Specifically, while using austenitic stainless steel at a high temperature for a long time, fine Cu precipitates in the steel, and the precipitated Cu increases the high temperature strength. However, Sn tends to segregate at the grain boundaries and lowers the high temperature strength. Therefore, if the Sn content is excessively large compared to the Cu content, the above effect of Cu cannot be obtained sufficiently.

  It is defined as F2 = Cu-10Sn. If F2 is 1.9 or more, excellent high-temperature strength can be obtained even with an austenitic stainless steel having a low C content.

  FIG. 2 is a diagram showing the relationship between F2 and fracture time in an austenitic stainless steel having a C content of 0.02% or less. The fracture time (h) at 650 ° C. of a plurality of austenitic stainless steels having different F2 values was obtained by the same method as the high temperature strength evaluation test in the examples described later, and FIG. 2 was created.

  Referring to FIG. 2, the fracture time (h) gradually increases as F2 increases. And when F2 becomes 1.9 or more, as F2 becomes larger, the fracture time increases remarkably. Therefore, if F2 = 1.9 or more, excellent high temperature strength (creep strength) can be obtained.

  (C) As described above, Cu and Sn synergistically increase the pitting corrosion potential and increase the pitting corrosion resistance. However, Cu and Sn increase the susceptibility to solidification cracking. It is defined as F3 = Cu + 35Sn. If F3 is 6.5 or less, excellent solidification cracking resistance is obtained.

  FIG. 3 is a diagram showing the relationship between F3 and the maximum crack length obtained by the transbalance train test. The maximum crack length (mm) of a plurality of austenitic stainless steels having different F3 values was determined by the same method as the solidification crack resistance evaluation test described later, and FIG. 3 was created.

  Referring to FIG. 3, when F3 is larger than 6.5, the maximum crack length is remarkably reduced as F3 is reduced. On the other hand, when F3 is 6.5 or less, the maximum crack length is substantially the same even if F3 is reduced. In short, the curve of the maximum crack length has an inflection point around F3 = 6.5. Therefore, if F3 is 6.5 or less, excellent solidification cracking resistance can be obtained.

  Based on the above knowledge, the present inventors completed the austenitic stainless steel by this embodiment. Hereinafter, the austenitic stainless steel according to the present embodiment will be described.

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

C: Less than 0.018% Carbon (C) stabilizes the austenite phase. C further forms intragranular carbides and increases the high temperature strength of the steel. However, if the C content is too high, Cr carbide precipitates and the intergranular corrosion sensitivity increases. Therefore, the SCC resistance of the steel is reduced. Therefore, the C content is less than 0.018%. The upper limit with preferable C content is less than 0.016%, More preferably, it is 0.012%.

Si: 0.9% or less Silicon (Si) deoxidizes steel during melting. Si further increases the oxidation resistance and steam oxidation resistance of the steel. However, since Si stabilizes the ferrite phase, if the Si content is too high, a sigma phase (σ phase) is likely to be generated after aging at a high temperature. Therefore, the Si content is 0.9% or less. The minimum with preferable Si content is 0.02%, More preferably, it is 0.1%. The upper limit with preferable Si content is less than 0.9%, More preferably, it is 0.5%.

Mn: 1.8% or less Manganese (Mn) stabilizes the austenite phase. Mn further suppresses a decrease in hot workability due to S. Mn further deoxidizes the steel. However, if the Mn content is too high, precipitation of an intermetallic compound phase such as a sigma phase is promoted. Therefore, the structural stability in a high temperature environment is lowered, and the toughness and ductility of the steel are lowered. Therefore, the Mn content is 1.8% or less. The minimum with preferable Mn content is 0.02%, More preferably, it is 0.1%. The upper limit with preferable Mn content is less than 1.8%.

P: 0.04% or less S: 0.01% or less Phosphorus (P) and sulfur (S) are impurities. P and S are segregated at the grain boundaries during welding and solidification to increase the susceptibility to solidification cracking. Accordingly, the P content and the S content are preferably as low as possible. The P content is 0.04% or less, and the S content is 0.01% or less. The upper limit with preferable P content is less than 0.04%, More preferably, it is 0.03%. The upper limit with preferable S content is less than 0.01%, More preferably, it is 0.005%.

Cu: 2.0 to 4.5%
Copper (Cu) precipitates finely in the steel and increases the high temperature strength (creep strength). Cu further enhances the pitting corrosion resistance of steel due to a synergistic effect with Sn. However, Cu is easily segregated at grain boundaries and is an austenite stabilizing element. Therefore, if the Cu content is too high, the formation of ferrite during weld solidification is suppressed, and the solidification cracking sensitivity is increased. Therefore, the Cu content is 2.0 to 4.5%. The minimum with preferable Cu content is higher than 2.0%, More preferably, it is 2.5%. The upper limit with preferable Cu content is less than 4.5%.

Ni: 9-16%
Nickel (Ni) stabilizes austenite in the steel structure. Therefore, Ni stabilizes the steel structure when used for a long time and increases the high temperature strength (creep strength). However, if the Ni content is too high, the cost increases. Therefore, the Ni content is 9-16%. The minimum with preferable Ni content is higher than 9%, More preferably, it is 9.5%. The upper limit with preferable Ni content is less than 16%, More preferably, it is 13%, More preferably, it is 12%.

Cr: 15-19%
Chromium (Cr) enhances the oxidation resistance and corrosion resistance of steel at high temperatures. However, if the Cr content is too high, the stability of austenite at high temperatures decreases, and the high temperature strength (creep strength) of the steel decreases. Therefore, the Cr content is 15 to 19%. The minimum with preferable Cr content is higher than 15%, More preferably, it is 16%. The upper limit with preferable Cr content is less than 19%, More preferably, it is 18%.

Mo: 5% or less Molybdenum (Mo) is dissolved in the matrix to increase the high temperature strength (creep strength). Mo further suppresses grain boundary precipitation of Cr carbides. However, if the Mo content is too high, the stability of austenite decreases and the high-temperature strength decreases. Therefore, the Mo content is 5% or less. The minimum with preferable Mo content is 0.05%, More preferably, it is 0.1%. The upper limit with preferable Mo content is less than 5%, More preferably, it is 2.5%, More preferably, it is 2%.

sol. Al: 0.04% or less Aluminum (Al) deoxidizes steel. However, 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 0.04% or less. The minimum with preferable Al content is 0.0005% or more, More preferably, it is 0.001%. The upper limit with preferable Al content is less than 0.04%, More preferably, it is 0.03%. In this embodiment, Al content means content of acid-soluble Al (sol.Al).

N: 0.02-0.3%
Nitrogen (N) stabilizes austenite by dissolving in the matrix (matrix). N further forms fine carbonitrides in the grains and increases high temperature strength (creep strength). However, if the N content is too high, Cr nitride is formed in the grains, and the polythionic acid SCC resistance in the weld heat affected zone (HAZ) is lowered. Therefore, the N content is 0.02 to 0.3%. The minimum with preferable N content is higher than 0.02%, More preferably, it is 0.04%, More preferably, it is 0.06%. The upper limit with preferable N content is less than 0.3%, More preferably, it is 0.2%, More preferably, it is 0.15%.

Sn: 0.002-0.1%
Tin (Sn) is present as an oxide in the passive film on the steel surface. Further, Sn is concentrated and present in a solid solution state in the outermost layer of the matrix under the passive film. Therefore, Sn increases the pitting corrosion resistance of steel, and particularly increases the pitting corrosion resistance in a chloride environment. However, Sn tends to cause strong segregation. Therefore, if the Sn content is too high, the solid segregation cracking sensitivity increases due to strong segregation of Sn, and the high-temperature strength also decreases. Therefore, the Sn content is 0.002% to 0.1%. The minimum with preferable Sn content is higher than 0.002%, More preferably, it is 0.004%. The upper limit with preferable Sn content is less than 0.1%, More preferably, it is 0.03%.

B: 0.009% or less Boron (B) segregates at the grain boundaries and finely disperses the grain boundary carbides. Therefore, B strengthens the grain boundary. However, if the B content is too high, the susceptibility to solidification cracking increases. Therefore, the B content is 0.009% or less. A preferable lower limit of the B content is 0.0005%. The upper limit with preferable B content is less than 0.009%, More preferably, it is 0.005%.

  The austenitic stainless steel according to the present embodiment further contains one or more selected from the group consisting of Nb, Ti and V.

Nb: 0.9% or less Ti: 0.15% or less V: 0.4% or less Niobium (Nb), titanium (Ti), and vanadium (V) all form carbides in the grains. Thereby, the production | generation of Cr carbide | carbonized_material at a grain boundary is suppressed and the intergranular corrosion resistance of steel increases. Furthermore, Nb carbide, Ti carbide and V carbide formed in the grains increase the high temperature strength (creep strength) of the steel. If one or more of these elements are contained, the above effect can be obtained to some extent. In order to obtain the above effect remarkably, the preferable lower limit of the Nb content is 0.01%, the preferable lower limit of the Ti content is 0.005%, and the preferable lower limit of the V content is 0.005%. .

  However, if the content of these elements is too high, the phase stability is lowered, and coarse carbides are formed, and the strength and corrosion resistance are lowered. Therefore, the Nb content is 0.9% or less, the Ti content is 0.15% or less, and the V content is 0.4% or less. The upper limit with preferable Nb content is 0.5%. The upper limit with preferable Ti content is 0.1%. The upper limit with preferable V content is 0.2%.

  The balance of the austenitic stainless steel according to the present embodiment is iron (Fe) and impurities. The impurities here refer to ores and scraps used as raw materials for steel, or elements mixed in from the environment of the manufacturing process.

  The austenitic stainless steel according to the present embodiment may further contain one or more of W and Co instead of part of Fe. W and Co are not essential elements but selective elements. All of these elements increase the high temperature strength of the steel.

W: 5% or less Tungsten (W) is dissolved in the matrix to increase the high temperature strength (creep strength) of the steel. If W is contained even a little, the above effect can be obtained. However, if the W content is too high, the stability of austenite decreases and the high-temperature strength decreases. Therefore, the W content is 5% or less. The minimum with preferable W content is 0.01%, More preferably, it is 0.05%. The upper limit with preferable W content is less than 5%, More preferably, it is 2%.

Co: 1% or less Cobalt (Co), like Ni, stabilizes austenite and increases the high temperature strength of steel. If Co is contained even a little, the above effect can be obtained. However, if the Co content is too high, the cost increases. Therefore, the Co content is 1% or less. A preferable lower limit of the Co content is 0.03%. The upper limit with preferable Co content is less than 1%.

  The austenitic stainless steel according to the present embodiment further contains one or more selected from the group consisting of Ca, Mg and rare earth elements (REM) instead of a part of Fe. All of these elements are not essential elements but are selective elements. All of these elements enhance the hot workability of steel.

Ca: 0.02% or less Mg: 0.02% or less REM: 0.1% or less Calcium (Ca), magnesium (Mg), and rare earth elements (REM) all have high affinity with S. Therefore, these elements increase the hot workability of steel. These elements further reduce the susceptibility to solidification cracking due to S grain boundary segregation. If at least one of Ca, Mg and REM is contained, the above effect can be obtained. However, if the content of these elements is too high, the cleanliness of the steel decreases due to bonding with oxygen, and the hot workability decreases. Therefore, the Ca content is 0.02% or less, the Mg content is 0.02% or less, and the REM content is 0.1% or less.

  A preferable lower limit of the Ca content is 0.0001%. The upper limit with preferable Ca content is less than 0.02%, More preferably, it is 0.01%. A preferable lower limit of the Mg content is 0.0001%. The upper limit with preferable Mg content is less than 0.02%, More preferably, it is 0.01%. A preferable lower limit of the REM content is 0.001%. The upper limit with preferable REM content is less than 0.1%, More preferably, it is 0.05%.

  REM is a general term for 17 elements in the periodic table, in which yttrium (Y) and scandium (Sc) are added to lanthanum (La) of atomic number 57 to lutetium (Lu) of atomic number 71. The content of REM means the total content of one or more of these elements.

[Formula (1) to Formula (3)]
The chemical composition of the austenitic stainless steel of the present embodiment further satisfies formulas (1) to (3).
0.2Cu + 325Sn ≧ 1.5 (1)
Cu-10Sn ≧ 1.9 (2)
Cu + 35Sn ≦ 6.5 (3)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) to (3).

[Regarding Formula (1)]
It is defined as F1 = 0.2Cu + 325Sn. As described above, Cu and Sn synergistically contribute to the improvement of pitting corrosion resistance. Therefore, F1 is an index of pitting corrosion resistance. As shown in FIG. 1, when the F1 value 1 is 1.5 or more, excellent pitting corrosion resistance is obtained. Preferably, F1 is higher than 1.5, and more preferable F1 is 1.7 or more.

[Regarding Formula (2)]
It is defined as F2 = Cu-10Sn. As described above, Cu increases the high temperature strength by precipitation strengthening, but Sn segregates at the grain boundaries and decreases the high temperature strength. Therefore, F2 is an index of high temperature strength. As shown in FIG. 2, if F2 is 1.9 or more, excellent high-temperature strength can be obtained. Preferred F2 is higher than 1.9, more preferred F2 is 2.0 or more, and further preferred F2 is 2.2 or more.

[Regarding Formula (3)]
It is defined as F3 = Cu + 35Sn. As described above, Cu contained excessively increases the susceptibility to solidification cracking, and Sn also increases the susceptibility to solidification cracking. Therefore, F3 is an index of resistance to solidification cracking. As shown in FIG. 3, when F3 is 6.5 or less, excellent susceptibility to solidification cracking is obtained. Preferred F3 is 6.0 or less, more preferred F3 is 5.5 or less, and further preferred F3 is 5.0 or less.

[Production method]
An example of the manufacturing method of the austenitic stainless steel of this embodiment is demonstrated. A molten steel having the above-described chemical composition and satisfying the formulas (1) to (3) is manufactured. For example, the molten steel is produced by using an electric furnace, an AOD (Argon Oxygen Decarburization) furnace, or a VOD (Vacuum Oxygen Decarburization) furnace.

  An ingot is manufactured from the manufactured molten steel by an ingot-making method. The ingot is hot-worked (hot forging, etc.) to produce steel materials such as slabs, blooms and billets. Steel materials such as slabs, blooms and billets may be manufactured from the manufactured molten steel by a continuous casting method.

  The manufactured steel material is hot-worked to produce an austenitic stainless steel material. For example, a steel material is hot-rolled to produce a steel plate, a steel bar, or a wire rod. Also, an austenitic stainless steel pipe is manufactured by hot extrusion, hot piercing and rolling. As described above, the specific method of hot working is not particularly limited, and hot working corresponding to the shape of the final product may be performed.

  Preferably, the processing end temperature of the hot processing is 1050 ° C. or higher. The processing end temperature here means the temperature of the steel material immediately after the final hot working step is completed. This is because if the processing end temperature is 1050 ° C. or higher, Nb, Ti and V are sufficiently dissolved in the matrix, and thus a higher temperature strength can be obtained.

  Cold working may be performed on the austenitic stainless steel material after hot working. When the austenitic stainless steel material is a steel pipe, the cold working is, for example, cold drawing or cold rolling. When the austenitic stainless steel material is a steel plate, cold rolling or the like is performed.

Preferably, in the cold working, the cross-sectional reduction rate in the final working is set to 10% or more. Here, the cross-sectional reduction rate (%) is defined by the following equation.
Cross-sectional reduction rate = (cross-sectional area of steel before final processing−cross-sectional area of steel after final processing) / cross-sectional area of steel before final processing × 100

  When the cross-section reduction rate in the final processing is set to 10% or more, since the heat treatment in the post-process is performed on the steel material to which processing strain is applied, recrystallization or sizing becomes easy to proceed.

  Heat treatment may be performed after hot working or after cold working. The heat treatment may be performed a plurality of times. The heat treatment temperature in the final heat treatment is preferably 1050 ° C. or higher.

  The austenitic stainless steel produced by the above production method has excellent high temperature strength, pitting corrosion resistance and solidification cracking resistance.

[Investigation method]
[Manufacture of test materials]
Molten steels having test numbers A1 to A7 and B1 to B7 shown in Table 1 were produced using an electric furnace.

  In the “F1”, “F2” and “F3” columns in Table 1, the F1 value, F2 value and F3 value of the steel of each test number are entered, respectively.

  Ingots were produced from each molten steel. Each ingot was hot-worked and cold-worked to produce a steel plate having a thickness of 5 or 10 mm, a width of 60 mm, and a length of 300 mm. A solution heat treatment was performed on each steel plate. The heat treatment temperature was 1050 ° C., and the steel plate after the heat treatment was rapidly cooled (water cooled).

[High temperature strength evaluation test]
A creep rupture test based on JIS Z2271 (2010) was conducted to evaluate the high temperature strength. Specifically, a creep rupture test piece based on JIS Z2271 (2010) was sampled from the steel plate of each test number. The creep rupture test piece was a circular cross-section test piece, the parallel part diameter was 6 mm, and the parallel part length was 60 mm.

The test piece was heated to 650 ° C., and a creep rupture test was performed to obtain the rupture time (h) of the steel of each test number. The test stress was 200 MPa. In this example, it was judged that excellent high-temperature strength was obtained when the breaking time was 0.8 × 10 ³ h or more.

[Pitting corrosion resistance evaluation test]
The pitting corrosion resistance measurement test based on JIS G0577 (2005) was implemented and pitting corrosion resistance was evaluated. Specifically, a polarization test piece having a diameter of 15 mm was collected from a steel plate having a thickness of 5 mm for each test number. A polarization test piece was attached to a crevice corrosion prevention electrode. An anodic polarization curve was measured using a crevice corrosion prevention electrode, and the highest potential at which the current density exceeded 100 μA / cm ² was determined as the pitting corrosion potential V. In addition, a saturated candy electrode was used as the reference electrode.

When the pitting potential V was 1.0 × 10 ⁻¹ V or more, it was judged that excellent pitting corrosion resistance was obtained.

[Evaluation test for solidification cracking resistance]
A transbalance test was performed to evaluate the solidification cracking resistance. Specifically, a test piece having a thickness of 4 mm, a width of 100 mm, and a length of 60 mm was collected from a steel plate having a thickness of 5 mm for each test number. In the transbalance test, bead-on-plate welding (no filler material) was performed with a TIG welding torch, and a part of the test piece surface was melted. The TIG welding conditions were a welding current of 100 A, a welding voltage of 15 V, and a welding speed of 15 cm / min. During welding, bending deformation (additional strain 2%) was forcibly applied to the test piece in a direction perpendicular to the welding progress direction, and the maximum crack length among the generated cracks was determined.

  The Alloy 800H weld metal whose structure becomes 100% austenite when solidified exhibits excellent solidification cracking resistance. The maximum crack length of Alloy 800H by the above-described transbalance test is 1.3 mm. Therefore, in this example, it was judged that excellent solidification cracking resistance was obtained when the maximum crack length was 1.3 mm or less.

[Test results]
Table 2 shows the test results.

With reference to Table 1 and Table 2, content of each element of the steel plates of test numbers A1 to A7 was appropriate, and the chemical composition satisfied Formulas (1) to (3). Therefore, the breaking time was 0.8 × 10 ³ h or more, and excellent high temperature strength was exhibited. Furthermore, the pitting corrosion potential V of these test numbers was 1.0 × 10 ⁻¹ V or more, and excellent pitting corrosion resistance was exhibited. Furthermore, the maximum crack length of these test numbers was 1.3 mm or less, and showed excellent solidification crack resistance.

On the other hand, Cu content of test number B1 was low, and F2 did not satisfy Formula (2). Therefore, the breaking time was less than 0.8 × 10 ³ h and the high temperature strength was low.

The Cu content of test number B2 was low, and F2 did not satisfy formula (2). Therefore, the breaking time was less than 0.8 × 10 ³ h and the high temperature strength was low. Furthermore, Sn content of test number B2 was high, and F3 did not satisfy Formula (3). Therefore, the maximum crack length exceeded 1.3 mm, and the solidification crack resistance was low.

The Sn content of test number B3 was low, and F1 did not satisfy formula (1). Therefore, the pitting corrosion potential V was less than 1.0 × 10 ⁻¹ V, and the pitting corrosion resistance was low.

The Sn content of test number B4 was high, F2 did not satisfy formula (2), and F3 did not satisfy formula (3). Therefore, the breaking time was less than 0.8 × 10 ³ h and the high temperature strength was low. Furthermore, the maximum crack length exceeded 1.3 mm, and the solidification crack resistance was low.

The content of each element of test number B5 was appropriate. However, F2 did not satisfy the formula (2). Therefore, the breaking time was less than 0.8 × 10 ³ h and the high temperature strength was low.

  The content of each element of test number B6 was appropriate. However, F3 did not satisfy the formula (3). Therefore, the maximum crack length exceeded 1.3 mm, and the solidification crack resistance was low.

The content of each element of test number B7 was appropriate. However, F1 did not satisfy the formula (1). Therefore, the pitting corrosion potential V was less than 1.0 × 10 ⁻¹ V, and the pitting corrosion resistance was low.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (3)

% By mass
C: less than 0.018%,
Si: 0.9% or less,
Mn: 1.8% or less,
P: 0.04% or less,
S: 0.01% or less,
Cu: 2.0 to 4.5%
Ni: 9 to 16%
Cr: 15-19%,
Mo: 5% or less,
sol. Al: 0.04% or less,
N: 0.02-0.3%
Sn: 0.002-0.1% and
B: contains 0.009% or less,
Nb: 0.9% or less,
Ti: 0.15% or less, and
V: contains one or more selected from the group consisting of 0.4% or less, the balance consists of Fe and impurities,
An austenitic stainless steel that satisfies formulas (1) to (3).
0.2Cu + 325Sn ≧ 1.5 (1)
Cu-10Sn ≧ 1.9 (2)
Cu + 35Sn ≦ 6.5 (3)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) to (3). The austenitic stainless steel according to claim 1, further comprising:
Instead of a part of the Fe,
W: 5% or less, and
Co: An austenitic stainless steel containing one or more of 1% or less. The austenitic stainless steel according to claim 1 or 2, further comprising:
Instead of a part of the Fe,
Ca: 0.02% or less,
Mg: 0.02% or less, and
Rare earth element (REM): An austenitic stainless steel containing one or more selected from the group consisting of 0.1% or less.

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