Austenitic stainless steel

Abstract

An austenitic stainless steel having a prescribed chemical composition and satisfying the following conditions: [0.010 < = VER + TiER + NbER], [27.0 < = 1.13 (Ni-NiER) + (Cr-CrER) + 1.85 (Mo-MoER) + 1.79 (Nb-NbER) lt; 40.5], [NbERlt; 0.052], and [0.5 < = (Hv40-Hvt / 2) / Hvt / 2].

Description

Technical Field

[0001] The present invention relates to austenitic stainless steel. Background Art

[0002] Boilers installed in factories such as thermal power plants and chemical plants are exposed to high temperatures. Therefore, the heat transfer tubes used inside the boilers (hereinafter simply referred to as "boiler heat transfer tubes") are required to have good high-temperature strength, specifically creep strength.

[0003] Moreover, due to its characteristics, austenitic stainless steel is sometimes used for boiler heat transfer tubes. For example, Patent Documents 1 and 2 disclose austenitic stainless steels having good creep strength.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-268503

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2021-21093 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In addition, in recent years, the usage environment of boiler heat transfer tubes in factories has become increasingly severe. Specifically, during equipment construction or regular maintenance after operation, stress corrosion cracking (hereinafter referred to as "SCC") may occur in the boiler heat transfer tubes. In addition, as the operating temperature increases, the temperature of the water vapor to which the heat transfer tubes are exposed becomes higher, making it easier for water vapor oxidation to occur.

[0010] Therefore, the billets of boiler heat transfer tubes are required to have not only good creep strength but also excellent SCC resistance and water vapor oxidation resistance. However, Patent Documents 1 and 2 do not discuss SCC resistance and water vapor oxidation resistance. Therefore, there is still room for further improvement in the austenitic stainless steels disclosed in the above documents in terms of SCC resistance and water vapor oxidation resistance.

[0011] In view of this, an object of the present invention is to solve the above problems and provide an austenitic stainless steel having good creep strength, SCC resistance, and water vapor oxidation resistance.

[0012] Means for Solving the Problems

[0013] The present invention has been completed to solve the above technical problems, and the gist lies in the following austenitic stainless steel and steel pipe.

[0014] (1) An austenitic stainless steel having a chemical composition in mass % of

[0015] C: 0.002 to 0.020%,

[0016] Si: 0.10 to 0.60%,

[0017] Mn: 0.2 to 2.0%,

[0018] P: 0.035% or less,

[0019] S: 0.010% or less,

[0020] Cu: 2.50 to 4.50%,

[0021] Ni: 9.00 to 16.00%,

[0022] Cr: 15.00 to 20.00%,

[0023] Mo: 0.20 to 1.50%,

[0024] Nb: 0.15 to 0.60%,

[0025] N: 0.05 to 0.15%,

[0026] B: 0.0010 to 0.0060%,

[0027] Balance: Fe and impurities,

[0028] The austenitic stainless steel satisfies the following formulas (i) to (iv).

[0029] 0.010 ≤ V ER +Ti ER +Nb ER (i)

[0030] 27.0 ≤ 1.13(Ni - Ni ER )+(Cr - Cr ER )+1.85(Mo - Mo ER )+1.79(Nb - Nb ER ) < 40.5 (ii)

[0031] Nb ER <0.052 (iii)

[0032] 0.5 ≤ (Hv 40 -Hv t / 2 ) / Hv t / 2 (iv)

[0033] Wherein, the definitions of each symbol in the above formulas are as follows. Each element symbol in the above formulas represents the content (mass %) of each element contained in the steel. When not contained, it is set to zero.

[0034] V ER : V content (mass %) in the precipitate obtained from the extraction residue analysis

[0035] Ti ER : Ti content (mass %) in the precipitate obtained from the extraction residue analysis

[0036] Nb ER : Nb content (mass %) in the precipitate obtained from the extraction residue analysis

[0037] Ni ER : Ni content (mass %) in the precipitate obtained from the extraction residue analysis

[0038] Cr ER : Cr content (mass %) in the precipitate obtained from the extraction residue analysis

[0039] Mo ER : Mo content (mass %) in the precipitate obtained from the extraction residue analysis

[0040] Hv 40 : Vickers hardness measured with a test load of 10 gf at a position 40 μm from the surface in the thickness direction

[0041] Hv t / 2 : Vickers hardness measured with a test load of 10 gf at a position 1 / 2t from the surface in the thickness direction when the total thickness is t

[0042] (2) An austenitic stainless steel, the chemical composition of which is as follows in mass %

[0043] C: 0.002 - 0.020%,

[0044] Si: 0.10 - 0.60%,

[0045] Mn: 0.2 - 2.0%,

[0046] P: 0.035% or less,

[0047] S: 0.010% or less,

[0048] Cu: 2.50 - 4.50%,

[0049] Ni: 9.00 - 16.00%,

[0050] Cr: 15.00 - 20.00%,

[0051] Mo: 0.20 - 1.50%,

[0052] Nb: 0.15 to 0.60%,

[0053] N: 0.05 to 0.15%,

[0054] B: 0.0010 to 0.0060%,

[0055] containing one or more elements selected from the group consisting of the following group A and group B,

[0056] the balance being Fe and impurities,

[0057] the austenitic stainless steel satisfies the following formulas (i) to (iv).

[0058] [Group A] V: 0.50% or less, and Ti: 0.500% or less,

[0059] [Group B] Co: 1.00% or less, W: 1.00% or less, Ta: 0.40% or less, Sn: 0.0300% or less, Al: 0.035% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0800% or less.

[0060] 0.010 ≤ V ER + Ti ER + Nb ER (i)

[0061] 27.0 ≤ 1.13(Ni - Ni ER ) + (Cr - Cr ER ) + 1.85(Mo - Mo ER ) + 1.79(Nb - Nb ER ) < 40.5 (ii)

[0062] Nb ER <0.052 (iii)

[0063] 0.5 ≤ (Hv 40 - Hv t / 2 ) / Hv t / 2 (iv)

[0064] wherein, the definitions of the symbols in the above formulas are as follows, and the element symbols in the above formulas represent the contents (mass%) of the respective elements contained in the steel, and are set to zero when not contained.

[0065] V ER : the V content (mass%) in the precipitate obtained by extraction residue analysis

[0066] Ti ER : the Ti content (mass%) in the precipitate obtained by extraction residue analysis

[0067] Nb ER : Nb content (mass %) in the precipitate obtained by the extraction residue analysis

[0068] Ni ER : Ni content (mass %) in the precipitate obtained by the extraction residue analysis

[0069] Cr ER : Cr content (mass %) in the precipitate obtained by the extraction residue analysis

[0070] Mo ER : Mo content (mass %) in the precipitate obtained by the extraction residue analysis

[0071] Hv 40 : Vickers hardness measured with a test load of 10 gf at a position 40 μm from the surface in the thickness direction

[0072] Hv t / 2 : Vickers hardness measured with a test load of 10 gf at a position 1 / 2t from the surface in the thickness direction when the total thickness is t

[0073] (3) The austenitic stainless steel according to (2) above, wherein the chemical composition contains one or more elements selected from Group A.

[0074] (4) The austenitic stainless steel according to (2) above, wherein the chemical composition contains one or more elements selected from Group B.

[0075] (5) The austenitic stainless steel according to any one of (1) to (4) above, which is a steel pipe, and the surface is the inner surface of the steel pipe.

[0076] Advantages of the Invention

[0077] According to the present invention, an austenitic stainless steel having good creep strength, SCC resistance, and steam oxidation resistance can be obtained. Brief Description of the Drawings

[0078] Figure 1 is a micrograph obtained by observing Nb compounds. Detailed Description of the Invention

[0079] The inventors of the present invention have studied methods for improving the creep strength, SCC resistance, and steam oxidation resistance of austenitic stainless steels and obtained the following insights.

[0080] (a) C has the effect of improving creep strength. Therefore, the austenitic stainless steel disclosed in Patent Document 1 contains 0.03% or more of C. However, in austenitic stainless steel, C sometimes becomes a factor causing SCC to occur. For example, when austenitic stainless steel is heated to around 600 - 700 °C, Cr in the steel combines with C to form Cr carbide. As a result, a Cr-depleted layer is formed, and SCC occurs in a corrosive environment where chlorides, etc. are present. Therefore, from the perspective of suppressing SCC, it is necessary to reduce the C content.

[0081] (b) In terms of suppressing SCC, it is effective not only to reduce the C content but also to control the amount of C dissolved in the matrix phase (hereinafter referred to as "dissolved C") within an appropriate range. When dissolved C is heated in the use environment, etc., it will deviate from the dissolved state and combine with Cr to form Cr carbide. This Cr carbide reduces the SCC resistance. Therefore, it is desirable to combine C with elements such as V, Ti, Nb, etc. and precipitate it in advance to reduce the dissolved C at the pre-use stage.

[0082] (c) On the other hand, if the C content is reduced, it is difficult to ensure creep strength. Therefore, in order to improve creep strength, it is effective to contain elements such as Ni, Cr, Mo, Nb, etc. These elements form precipitates in a use environment exposed to high temperatures, thereby improving creep strength. However, if the above elements form precipitates before high-temperature use, creep strength cannot be sufficiently improved. Therefore, in order to obtain good creep strength, it is desirable that these elements are dissolved in the matrix phase as much as possible at the pre-use stage and form precipitates in the use environment. Therefore, it is necessary to adjust the dissolved amount of the above elements to an appropriate range at the pre-use stage.

[0083] (d) In terms of improving steam oxidation resistance, shot peening, etc. is usually carried out to form a processed layer near the surface of the steel. However, in steel with controlled chemical composition in order to improve creep strength and SCC resistance, it is difficult to improve steam oxidation resistance only by forming a processed layer. In addition, the present inventors have clarified that it is important to reduce Nb precipitates and ensure a solid solution Nb of a certain amount or more.

[0084] The reason is not yet clear, but it is considered that dissolved Nb promotes the formation of Cr oxide by suppressing the excessive penetration of O into the steel in the use environment. Although the formation rate of Cr oxide is slow, it helps to improve steam oxidation resistance. Therefore, it is effective to pre-form a state in which Nb is dissolved not only from the perspective of creep strength but also from the perspective of steam oxidation resistance. And preferably, shot peening, etc. is carried out thereafter to form a processed layer near the surface.

[0085] (e) When forming the above-mentioned working layer, if the degree of working applied is increased, nucleation and growth will occur during use at high temperatures, that is, recrystallization will occur. The recrystallized structure has a very fine grain size, so the grain boundary diffusion of Cr becomes easy. Therefore, the supply of Cr is sufficient, and it is easy to form Cr oxides, such as Cr oxide scales mainly containing Cr2O3. This Cr oxide scale is extremely thin and forms uniformly, which will improve the steam oxidation resistance.

[0086] In addition, if a working layer is pre-formed and the surface vicinity is made to have a recrystallized structure in the use environment, even if defects occur in the Cr oxide scale, a protective oxide scale will re-form at this position. This is called oxide scale repair. If the Cr oxide scale is repaired, abnormal oxidation will not develop. As a result, the steam oxidation resistance characteristics can be maintained for a long time.

[0087] In view of the above situation, it is desirable to appropriately control the chemical composition and manufacturing conditions, adjust the balance of solid solution and precipitation of each element in the steel, and form a working layer in the surface vicinity.

[0088] One embodiment of the present invention is completed based on the above insights. Hereinafter, each component of the austenitic stainless steel and steel pipe of this embodiment will be described in detail.

[0089] 1. Chemical composition

[0090] The reasons for limiting each element are as follows. It should be noted that "%" regarding the content in the following description means "mass %".

[0091] C: 0.002 - 0.020%

[0092] C (carbon) is an element necessary to ensure high-temperature strength, especially creep strength. Therefore, the C content is 0.002% or more. The C content is preferably 0.003% or more, more preferably 0.004% or more. However, if C is contained in excess, the SCC resistance decreases. Therefore, the C content is 0.020% or less. The C content is preferably 0.015% or less, more preferably 0.010% or less.

[0093] Si: 0.10 - 0.60%

[0094] Si (silicon) is an element having a deoxidation effect. Therefore, the Si content is 0.10% or more. The Si content is preferably 0.12% or more, more preferably 0.14% or more. However, if Si is contained in excess, the workability decreases. Therefore, the Si content is 0.60% or less. The Si content is preferably 0.50% or less, more preferably 0.40% or less.

[0095] Mn: 0.2 - 2.0%

[0096] Mn (manganese) combines with the impurity S contained in steel to form MnS, which has the effect of improving hot workability. Therefore, the Mn content should be 0.2% or more. The Mn content is preferably 0.4% or more, and more preferably 0.6% or more. However, if the steel contains an excessive amount of Mn, it will become hard and brittle, and instead, the workability and weldability will decrease. Therefore, the Mn content is 2.0% or less. The Mn content is preferably 1.5% or less, and more preferably 1.3% or less.

[0097] P: 0.035% or less

[0098] P (phosphorus) is an element contained as an impurity in steel, which reduces the SCC resistance. In addition, P also reduces the hot workability and toughness of steel. Therefore, the P content is 0.035% or less. The P content is preferably 0.030% or less, and more preferably 0.025% or less. It is preferable to minimize the P content as much as possible, but if its content is excessively reduced, the manufacturing cost will increase. Therefore, the P content is preferably 0.010% or more.

[0099] S: 0.010% or less

[0100] S (sulfur) is an element contained as an impurity in steel, which reduces the SCC resistance. In addition, S also reduces the hot workability and creep ductility of steel. Therefore, the S content is 0.010% or less. The S content is preferably 0.009% or less, and more preferably 0.008% or less. It is preferable to minimize the S content as much as possible, but if its content is excessively reduced, the manufacturing cost will increase. Therefore, the S content is preferably 0.0001% or more.

[0101] Cu: 2.50 - 4.50%

[0102] Cu (copper) precipitates as a Cu phase within the grains, thereby improving the creep strength and creep ductility of steel through precipitation strengthening. Therefore, the Cu content is 2.50% or more. The Cu content is preferably 2.70% or more, and more preferably 2.90% or more. However, if the steel contains an excessive amount of Cu, the hot workability and weldability will decrease. Therefore, the Cu content is 4.50% or less. The Cu content is preferably 4.00% or less, and more preferably 3.50% or less.

[0103] Ni: 9.00 - 16.00%

[0104] Ni (Nickel) is an element that stabilizes austenitic structure and has the effect of improving SCC resistance and corrosion resistance. In addition, Ni also has the effect of improving creep strength. Therefore, the Ni content is 9.00% or more. The Ni content is preferably 10.00% or more, and more preferably 10.50% or more. However, if Ni is contained in excess, the manufacturing cost will increase. In addition, the creep strength will instead decrease. Therefore, the Ni content is 16.00% or less. The Ni content is preferably 15.00% or less, and more preferably 14.00% or less.

[0105] Cr: 15.00 - 20.00%

[0106] Cr (Chromium) is an element necessary to ensure corrosion resistance. In addition, Cr also has the effect of improving creep strength. Therefore, the Cr content is 15.00% or more. The Cr content is preferably 15.50% or more, and more preferably 16.00% or more. However, if Cr is contained in excess, the stability of the austenitic structure will decrease and the weldability will also decrease. Therefore, the Cr content is set to 20.00% or less. The Cr content is preferably 19.75% or less, and more preferably 19.50% or less.

[0107] Mo: 0.20 - 1.50%

[0108] Mo (Molybdenum) has the effect of improving creep strength. Therefore, the Mo content is 0.20% or more. The Mo content is preferably 0.35% or more, and more preferably 0.50% or more. However, if Mo is contained in excess, the stability of the austenitic structure will decrease. Therefore, the Mo content is 1.50% or less. The Mo content is preferably 1.25% or less, and more preferably 1.00% or less.

[0109] Nb: 0.15 - 0.60%

[0110] Nb (Niobium) has the effect of improving creep strength and SCC resistance. Therefore, the Nb content is 0.15% or more. The Nb content is preferably 0.20% or more, and more preferably 0.25% or more. However, if Nb is contained in excess, significantly coarse precipitates will form and the creep strength will instead decrease. Therefore, the Nb content is 0.60% or less. The Nb content is preferably 0.55% or less, and more preferably 0.50% or less.

[0111] N: 0.05 - 0.15%

[0112] N (nitrogen) has the effect of increasing strength through solid solution strengthening and precipitation strengthening based on Nb carbonitrides. Therefore, the N content is 0.05% or more. The N content is preferably 0.06% or more, more preferably 0.07% or more. However, if N is contained in excess, massive nitrides will be formed, resulting in a decline in the quality of the steel. As a result, the strength may decrease. Therefore, the N content is 0.15% or less. The N content is preferably 0.13% or less, more preferably 0.12% or less.

[0113] B: 0.0010 - 0.0060%

[0114] B (boron) has the effect of improving creep ductility. Therefore, the B content is 0.0010% or more. The B content is preferably 0.0015% or more, more preferably 0.0020% or more. However, if B is contained in excess, the weldability and hot workability at high temperatures will decline. Therefore, the B content is 0.0060% or less. The B content is preferably 0.0050% or less, more preferably 0.0045% or less.

[0115] Any element of Group A

[0116] In the chemical composition of the austenitic stainless steel of the present embodiment, in addition to the above elements, one or more selected from V and Ti (hereinafter referred to as "Group A") may be further contained within the following ranges. The reasons for limiting each element will be described.

[0117] V: 0.50% or less

[0118] V (vanadium) has the effect of reducing the dissolved C and improving the SCC resistance. In addition, V also has the effect of improving the creep strength. Therefore, V can be contained as needed. However, if V is contained in excess, δ-ferrite will be formed, and the creep strength, toughness, and weldability of the steel will decline. Therefore, the V content is 0.50% or less. The V content is preferably 0.40% or less, more preferably 0.30% or less. The V content may be 0% or more, but in order to obtain the above effects, for example, the V content is preferably 0.01% or more, more preferably 0.02% or more.

[0119] Ti: 0.500% or less

[0120] Ti (titanium), like V, has the effect of reducing the dissolved C and improving the SCC resistance. In addition, Ti also has the effect of improving the creep strength. Therefore, Ti can be contained as needed. However, if Ti is contained in excess, the creep strength will instead decline. Therefore, the Ti content is 0.500% or less. The Ti content is preferably 0.400% or less, more preferably 0.100% or less, further preferably 0.050% or less. The Ti content may be 0% or more, but in order to obtain the above effects, for example, the Ti content is preferably 0.001% or more, more preferably 0.002% or more.

[0121] Any element of Group B

[0122] In the chemical composition of the austenitic stainless steel of the present embodiment, in addition to the above elements, one or more selected from Co, W, Ta, Sn, Al, Ca, Mg, and REM (hereinafter referred to as "Group B") may be further contained within the following ranges. The reasons for limiting each element will be described.

[0123] Co: 1.00% or less

[0124] Co (cobalt) has the effects of stabilizing the austenite structure and improving the creep strength. Therefore, Co can be contained as needed. However, if Co is contained in excess, the manufacturing cost increases. Therefore, the Co content is 1.00% or less. The Co content is preferably 0.50% or less, more preferably 0.30% or less. The Co content may be 0% or more, but in order to obtain the above effects, for example, the Co content is preferably 0.02% or more.

[0125] W: 1.00% or less

[0126] W (tungsten) has the effect of dissolving in the matrix phase to improve the creep strength of the steel. Therefore, W can be contained as needed. However, if W is contained in excess, the stability of the austenite phase will decrease, and the creep strength and toughness will instead decrease. Therefore, the W content is 1.00% or less. The W content is preferably 0.50% or less, more preferably 0.30% or less. The W content may be 0% or more, but in order to obtain the above effects, for example, the W content is preferably 0.01% or more.

[0127] Ta: 0.40% or less

[0128] Ta (tungsten) combines with C to form carbonitrides, reducing the dissolved C. As a result, Ta has the effect of improving the SCC resistance. In addition, Ta also has the effect of improving the creep strength. Therefore, Ta can be contained as needed. However, if Ta is contained in excess, δ-ferrite will be formed, resulting in a decrease in the creep strength, toughness, and weldability of the steel. Therefore, the Ta content is 0.40% or less. The Ta content is preferably 0.30% or less, more preferably 0.10% or less. The Ta content may be 0% or more, but in order to obtain the above effects, for example, the Ta content is preferably 0.01% or more.

[0129] Sn: 0.0300% or less

[0130] Sn (tin) has the effect of improving corrosion resistance and high-temperature properties. Therefore, Sn can be contained as needed. However, if Sn is contained in excess, the weldability and manufacturability will deteriorate. Therefore, the Sn content is 0.0300% or less. The Sn content is preferably 0.0200% or less, more preferably 0.0100% or less. The Sn content may be 0% or more, but in order to obtain the above effects, for example, the Sn content is preferably 0.0010% or more.

[0131] Al: 0.035% or less

[0132] Al (aluminum) is an element with a deoxidizing effect and has the effect of improving hot workability by fixing O as inclusions. Therefore, Al can be contained as needed. However, if Al is contained in excess, too many inclusions will be formed and the surface properties will deteriorate. In addition, the hot workability will also deteriorate instead. Therefore, the Al content is 0.035% or less. The Al content is preferably 0.030% or less, more preferably 0.025% or less. The Al content may be 0% or more, but in order to obtain the above effects, for example, the Al content is preferably 0.0005% or more.

[0133] Ca: 0.0100% or less

[0134] Ca (calcium) has the effect of fixing S and O as inclusions and improving the hot workability and creep ductility of steel. Therefore, Ca can be contained as needed. However, if Ca is contained in excess, the hot workability and creep ductility will deteriorate instead. Therefore, the Ca content is 0.0100% or less. The Ca content is preferably 0.0050% or less, more preferably 0.0030% or less. The Ca content may be 0% or more, but in order to obtain the above effects, for example, the Ca content is preferably 0.0010% or more.

[0135] Mg: 0.0100% or less

[0136] Mg (magnesium), like Ca, has the effect of fixing S and O as inclusions and improving the hot workability and creep ductility of steel. Therefore, Mg can be contained as needed. However, if Mg is contained in excess, the hot workability and long-term creep ductility will deteriorate instead. Therefore, the Mg content is 0.0100% or less. The Mg content is preferably 0.0050% or less, more preferably 0.0030% or less. The Mg content may be 0% or more, but in order to obtain the above effects, for example, the Mg content is preferably 0.0002% or more.

[0137] REM: 0.0800% or less

[0138] Like Ca and Mg, rare earth elements (REM) have the effect of fixing S and O as inclusions, improving the hot workability and creep ductility of steel. Therefore, REM can be contained as needed. However, if REM is contained in excess, the hot workability and long-term creep ductility will instead decrease. Therefore, the REM content is 0.0800% or less. The REM content is preferably 0.0600% or less, more preferably 0.0400% or less. The REM content may be 0% or more, but in order to obtain the above effects, for example, the REM content is preferably 0.0010% or more.

[0139] It should be noted that REM refers to a total of 17 elements including Sc, Y, and lanthanide elements, and the above REM content refers to the total content of these elements. Industrially, REM is often added in the form of mischmetal.

[0140] In addition, among the optional elements, as described above, one or more elements selected from the group consisting of the above Group A and Group B can be contained as needed.

[0141] In the chemical composition of the austenitic stainless steel of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components mixed in due to various factors such as raw materials such as ores and scraps and manufacturing processes during industrial steel manufacturing, and components allowed within the range that does not adversely affect the properties of the austenitic stainless steel.

[0142] 2. Formula (i)

[0143] As described above, in a use environment exposed to high temperatures, the solid-solution C combines with Cr to form Cr carbides. As a result, the SCC resistance decreases. Therefore, before using it as a heat transfer tube for boilers, it is desirable to reduce the amount of solid-solution C in advance. Specifically, C can be fixed in the form of precipitates (compounds) using V, Ti, and Nb. Therefore, the amounts of V, Ti, and Nb that combine with C and exist in the form of precipitates are controlled, that is, the right-hand value of formula (i) is controlled.

[0144] 0.010 ≤ V ER + Ti ER + Nb ER (i)

[0145] Among them, the definitions of the symbols in the above formula are as follows.

[0146] V ER : The V content (mass%) in the precipitate obtained by extraction residue analysis

[0147] Ti ER : The Ti content (mass%) in the precipitate obtained by extraction residue analysis

[0148] Nb ER: Nb content (mass %) in the precipitate obtained by the extraction residue analysis

[0149] When the right - hand side value of formula (i) is less than 0.010, the solid - solution C increases. In the use environment, the solid - solution C combines with Cr to form Cr carbide. As a result, a Cr - depleted layer is formed and the SCC resistance decreases. Therefore, the right - hand side value of formula (i) is 0.010 or more. The right - hand side value of formula (i) is preferably 0.012 or more, more preferably 0.015 or more, and further preferably 0.020 or more.

[0150] It should be noted that, from the perspective of weldability, the right - hand side value of formula (i) is preferably 0.120 or less. In addition, from the perspective that excessive fixation of N will lead to a decrease in strength, the right - hand side value of formula (i) is preferably 0.100 or less.

[0151] 3. Formula (ii)

[0152] In the austenitic stainless steel of the present embodiment, the solid - solution amounts of Ni, Cr, Mo, and Nb in the matrix phase are controlled. This is because, by reducing C which is effective for creep strength and ensuring the solid - solution amounts of the above - mentioned elements, an improvement in creep strength can be achieved. For the solid - solution amount in the matrix phase, it can be calculated by the difference between the content (mass %) of each element and the content (mass %) of each element in the precipitate obtained by the extraction residue analysis. And the austenitic stainless steel of the present embodiment must satisfy formula (ii).

[0153] 27.0 ≤ 1.13(Ni - Ni ER )+(Cr - Cr ER )+1.85(Mo - Mo ER )+1.79(Nb - Nb ER ) < 40.5 (ii)

[0154] Among them, the definitions of each symbol in the above formula are as follows. Each element symbol in the above formula represents the content (mass %) of each element contained in the steel, and is set to zero when not contained.

[0155] Nb ER : Nb content (mass %) in the precipitate obtained by the extraction residue analysis

[0156] Ni ER : Ni content (mass %) in the precipitate obtained by the extraction residue analysis

[0157] Cr ER : Cr content (mass %) in the precipitate obtained by the extraction residue analysis

[0158] Mo ER : Mo content (mass %) in the precipitate obtained by the extraction residue analysis

[0159] When the intermediate value of formula (ii) is less than 27.0, Ni, Cr, Mo, and Nb will not be sufficiently dissolved, and the creep strength cannot be improved. Therefore, the intermediate value of formula (ii) is 27.0 or more. The intermediate value of formula (ii) is preferably 29.0 or more, more preferably 31.0 or more.

[0160] On the other hand, when the intermediate value of formula (ii) is 40.5 or more, too much Nb is dissolved in the above elements, and the amount of Nb that can combine with dissolved C to form precipitates cannot be sufficiently ensured, and it may not be possible to satisfy formula (i). Therefore, the intermediate value of formula (ii) is less than 40.5. The intermediate value of formula (ii) is preferably 39.0 or less, more preferably 37.0 or less, and further preferably 35.0 or less.

[0161] 4. Formula (iii)

[0162] Nb ER <0.052 (iii)

[0163] Among them, the definitions of each symbol in the above formula are as follows.

[0164] Nb ER : The Nb content (mass%) in the precipitate obtained by the extraction residue analysis

[0165] For the austenitic stainless steel of this embodiment, in order to improve the steam oxidation resistance, it is made into a state in which a certain amount of Nb is dissolved. Generally, forming a processed layer by shot peening or the like is effective for improving the steam oxidation resistance, but when the composition is set to improve the creep strength and SCC resistance, only setting the processed layer cannot sufficiently improve the steam oxidation resistance.

[0166] It is considered that by limiting the amount of Nb in the precipitated state, that is, Nb ER to a certain amount and ensuring the amount of dissolved Nb, in the use environment, dissolved Nb promotes the formation of Cr oxides. The growth rate of Cr oxides is slow. On the other hand, dissolved Nb inhibits the excessive penetration of O into the steel, thereby inhibiting the formation of other oxides. Therefore, it is also considered that the growth time of Cr oxides can be sufficiently ensured. Therefore, on the basis of ensuring the amount of dissolved Nb, setting a processed layer by shot peening or the like is effective for improving the steam oxidation resistance.

[0167] In addition, as described above, in the austenitic stainless steel of this embodiment, at the stage before use, C is combined with elements such as V, Ti, and Nb and precipitated in the form of precipitates in advance. In particular, for Nb, from the viewpoints of creep strength and steam oxidation resistance, it is preferably present in a certain amount in the form of dissolved Nb, and is precipitated in advance in the form of Ti-containing Nb carbonitride. Figure 1This is a micrograph obtained by collecting Nb compounds from the precipitates obtained by extracting residues from the austenitic stainless steel of this embodiment and observing them. Analysis of this micrograph reveals that the above Nb compounds are Ti-containing Nb carbonitrides.

[0168] Here, Nb sometimes combines with N, Cr, etc. in the steel to form fine NbCr nitrides. When such fine NbCr nitrides are formed, the ER value of Nb increases, and the amount of Cr in solid solution decreases. As a result, it is difficult for the austenitic stainless steel to satisfy formula (ii). Therefore, it is necessary to control Nb so that it becomes the above Nb carbonitrides rather than NbCr nitrides.

[0169] In view of the above situation, the austenitic stainless steel of this embodiment satisfies formula (iii). If the austenitic stainless steel does not satisfy formula (iii), that is, when ER Nb is 0.052 or more, it is difficult to improve the steam oxidation resistance even after forming a processed layer. In addition, NbCr nitrides are formed, and it is difficult to form the desired Nb carbonitrides. Nb ER is preferably 0.050 or less, more preferably 0.045 or less, further preferably 0.040 or less, and most preferably 0.035 or less.

[0170] It should be noted that the content (mass%) of each element in the precipitates obtained by the extraction residue analysis described in the above items 2 to 4 can be measured according to the following steps. Specifically, using 10% acetylacetone - 1% tetramethylammonium chloride / methanol, electrolyze about 0.4 g of the sample at a current value of 20 mA / cm 2 . Then, filter the solution of the sample that has undergone this electrolysis through a 0.2 μm filter, and then decompose the residue with acid. Then, for the above elements, use an ICP emission spectrometer to calculate the amount (mass%) analyzed as the electrolytic extraction residue.

[0171] 5. Formula (iv)

[0172] For the austenitic stainless steel of this embodiment, in order to sufficiently improve the steam oxidation resistance, a processed layer is provided near the surface by shot peening or the like. As described above, by making ER Nb less than 0.052 and providing a processed layer, a recrystallized structure is formed during use at high temperatures. This recrystallized structure has a very fine particle size, so Cr easily diffuses along the grain boundaries.

[0173] Accordingly, Cr required for forming Cr oxide can be sufficiently supplied, and Cr oxide scale mainly containing Cr oxide, i.e., Cr2O3, is easily formed in the initial stage of use. This Cr oxide scale is extremely thin and uniformly formed, so the surface has high protection and is not easily peeled off. In addition, even if there is a position where the oxide scale peels off, the oxide scale at this position will be reformed and repaired. Therefore, the steam oxidation resistance can be improved.

[0174] The processed layer is a hardened structure near the surface. Therefore, the following formula (iv) is satisfied.

[0175] 0.5 ≤ (Hv 40 -Hv t / 2 ) / Hv t / 2 (iv)

[0176] Among them, the definitions of the symbols in the above formula are as follows.

[0177] Hv 40 : Vickers hardness measured with a test load of 10 gf at a position 40 μm from the surface in the thickness direction

[0178] Hv t / 2 : When the total thickness is set to t, Vickers hardness measured with a test load of 10 gf at a position 1 / 2t from the surface in the thickness direction

[0179] The right - hand value of formula (iv) is called the hardness increase rate. The larger this value is, the higher the introduced degree of processing. And if the right - hand value of formula (iv) is 0.5 or more, recrystallization will occur in the processed layer formed within the range from the surface to 40 μm in the thickness direction during use at high temperature. As a result, even if the oxide scale peels off during use at high temperature, the peeled position can be repaired. Therefore, the right - hand value of formula (iv) is 0.5 or more.

[0180] The higher the degree of processing imparted near the surface of the steel, that is, the larger the right - hand value of formula (iv), the finer the formed crystal structure. And the fine crystal structure makes it easier to supply Cr to the surface at high temperature. Accordingly, abnormal oxidation is prevented especially in the initial stage of use, contributing to the uniform formation of Cr oxide scale. Based on this, the right - hand value of formula (iv) is preferably 0.6 or more, more preferably 0.7 or more.

[0181] It should be noted that the upper limit of the right - hand value of formula (iv) is not particularly limited. However, the larger the right - hand value of formula (iv), the harder the surface will be significantly, and problems may occur in forming processes, welding, etc. Therefore, the right - hand value of formula (iv) is preferably 2.0 or less.

[0182] In addition, for Hv t / 2, in order to obtain good creep strength, for example, it is preferably to set the Vickers hardness in the range of 100 to 245 Hv.

[0183] For the above Hv 40 and Hv t / 2 , the measurement can be carried out according to the following steps. Cut a test piece with a side length of 15 mm, embed this test piece in resin, cut the cross-section and perform mirror polishing. For this test piece, in a way that it is in a straight line in the thickness direction, at a position 40 μm away from the surface in the thickness direction and at a position t / 2 away from the surface in the thickness direction (when the total thickness is set as t), measure the Vickers hardness with a load of 10 gf. Repeat this measurement at different positions of the test piece and at positions not affected by other measurements, and the number of measurements at each position is 5 times. Then, take the average value of the measurement results of 5 points at the position 40 μm in the thickness direction as Hv 40 . Similarly, take the average value of the measurement results of 5 points at the position t / 2 in the thickness direction as Hv t / 2 .

[0184] In the case of a steel plate, measure at a position 40 μm away from the surface in the plate thickness direction and at a position t / 2 away from the surface in the plate thickness direction in a way that it is in a straight line in the plate thickness direction, and also perform this measurement at other positions, and perform the above measurement in such a way that the total number of measurements at each position is 5 times. In addition, in the case of a steel pipe, perform the above measurement at a position 40 μm away from the inner surface of the steel pipe in the wall thickness direction and at a position t / 2 away from the inner surface of the steel pipe in the wall thickness direction (when the total thickness is set as t). It should be noted that the hardness of the inner surface of the steel pipe is measured because when it is used as a heat transfer pipe for a boiler, high-temperature steam will pass through the inside of the pipe, so the inner surface of the pipe is particularly required to have steam oxidation resistance.

[0185] 6. Thickness and type

[0186] For the thickness of the austenitic stainless steel of this embodiment, considering its use, it is preferably in the range of 2 to 100 mm. It should be noted that when the austenitic stainless steel is in the shape of a pipe, it is preferably to set the wall thickness in the range of 2 to 95 mm. In addition, when the austenitic stainless steel is in the shape of a steel plate, it is preferably to set the plate thickness in the range of 2 to 35 mm. It should be noted that the austenitic stainless steel of this embodiment can be, for example, a steel plate or a steel pipe.

[0187] 7. Manufacturing method

[0188] The manufacturing method of the austenitic stainless steel of this embodiment will be described. The inventors of the present invention have confirmed through research so far that the austenitic stainless steel of this embodiment can be manufactured by the following method.

[0189] 7-1. Melting

[0190] The steel having the above chemical composition is melted, and an ingot is manufactured by continuous casting or the like. The manufactured ingot is subjected to blooming to manufacture billets. Various conditions in the manufacture of the billets can be adjusted according to conventional methods.

[0191] 7-2. Hot working

[0192] Next, the obtained billets are hot-worked. The hot-working conditions are not particularly limited, but in order to prevent harmful defects from occurring during the pipe manufacturing process, for example, it is heated to the range of 900 to 1300 °C before hot working. In addition, there is no particular limitation on the type of hot working. When manufacturing a steel plate, it can be set as hot rolling. In addition, when manufacturing a steel pipe, it can be formed into a pipe shape by hot extrusion.

[0193] Here, after hot working, the steel is rapidly cooled under the following conditions.

[0194] Time from the end of hot working to the start of rapid cooling: 5.0 minutes or less

[0195] Temperature of the steel at the start of rapid cooling: 700 °C or more

[0196] Cooling rate from the end of hot working to the start of rapid cooling: 15 °C / minute or more

[0197] The time (in minutes) from the end of hot working to the start of rapid cooling is referred to as the "holding time". When the steel is rapidly cooled after hot working, the steel is usually rapidly cooled (water-cooled) using a water-cooling device. In other words, this "holding time" refers to the time from the end of hot working until it is transported to the water-cooling device and water cooling starts. If the holding time exceeds 5.0 minutes, coarse precipitates will be generated. In addition, it is easy to form NbCr nitrides rather than the required Nb carbonitrides.

[0198] Moreover, the precipitates cannot be dissolved during the softening treatment described later. This will lead to the formation of coarse precipitates such as nitrides, Cr carbides, and NbCr nitrides, not satisfying formula (ii). As a result, the creep strength decreases. Therefore, the holding time is set to 5.0 minutes or less. The holding time is preferably set to 4.5 minutes or less, more preferably set to 4.0 minutes or less, and further preferably set to 3.5 minutes or less.

[0199] The temperature (°C) of the steel at the start of rapid cooling is referred to as the "rapid cooling start temperature". When the rapid cooling start temperature is lower than 700 °C, coarse precipitates such as Cr carbides are formed. In addition, it is easy to form NbCr nitrides rather than the required Nb carbonitrides. And even if softening treatment is carried out, the precipitates cannot be dissolved in the matrix phase, and formula (ii) is not satisfied. As a result, the creep strength decreases. Therefore, the rapid cooling start temperature is set to 700 °C or higher. The rapid cooling start temperature is preferably set to 750 °C or higher, more preferably set to 780 °C or higher, further preferably set to higher than 790 °C, and extremely preferably set to 800 °C or higher.

[0200] When the cooling rate (°C / min) from the end of hot working to the start of rapid cooling is less than 15 °C / min, coarse precipitates such as nitrides and Cr carbides are formed. And formula (ii) is no longer satisfied. As a result, the creep strength (or SCC resistance) decreases. Therefore, the cooling rate from the end of hot working to the start of rapid cooling is set to 15 °C / min or higher, preferably set to 18 °C / min or higher, more preferably set to 20 °C / min or higher. It should be noted that the above cooling rate is the value obtained by dividing the difference between the surface temperature of the steel just after hot working and the surface temperature of the steel before the start of rapid cooling by the holding time.

[0201] 7-3. Softening treatment

[0202] By performing softening treatment, the non-uniformity of the steel quality in the longitudinal direction and the wall thickness direction generated during hot working is reduced. Without softening treatment, the coarse precipitates generated during hot working cannot be sufficiently dissolved in the matrix phase. And in the subsequent solution heat treatment, there will be parts where the precipitation of Ti, V, and Nb is insufficient. As a result, the right side value of formula (i) is less than the left side value, and the SCC resistance may decrease. Therefore, softening treatment is performed as a process after hot working to make the steel quality uniform. The holding temperature in the softening treatment (hereinafter referred to as "softening treatment temperature T1") only needs to be above the recrystallization temperature and below the grain boundary melting temperature. In the softening treatment, soaking can be carried out at 1040 - 1300 °C.

[0203] When the softening treatment temperature T1 is lower than 1040 °C, formula (i) is not satisfied and the SCC resistance decreases. Therefore, the softening treatment temperature T1 is set to 1040 °C or higher. The softening treatment temperature T1 is preferably set to 1100 °C or higher, more preferably set to 1150 °C or higher. On the other hand, when the softening treatment temperature T1 exceeds 1300 °C, the grains tend to become coarse. Therefore, the softening treatment temperature T1 is set to 1300 °C or lower. The softening treatment temperature T1 is preferably set to 1290 °C or lower, more preferably set to 1280 °C or lower.

[0204] It should be noted that there is no particular limitation on the time of the softening treatment. However, from the perspectives of recrystallization in the wall thickness direction and manufacturing cost, it is preferably 1 to 10 minutes. When the time of the softening treatment is less than 1 minute, the SCC resistance may decrease. On the other hand, when the time of the softening treatment exceeds 10 minutes, the grains may become coarse.

[0205] 7-4. First Cooling

[0206] After the softening treatment, the steel is cooled. This cooling is regarded as the first cooling. In the first cooling, the steel is cooled by water cooling or at a cooling rate equivalent to or higher than that of water cooling. If water cooling is not carried out or the cooling is not carried out at a cooling rate equivalent to or higher than that of water cooling, an uneven structure will be formed due to the generation and growth of precipitates. As a result, the non-uniformity may continue to subsequent cold working and solution heat treatment, and the product quality is unstable. Therefore, in the first cooling, water cooling or a cooling rate equivalent to or higher than that of water cooling is adopted. This cooling is usually carried out to below 700°C. It should be noted that the cooling rate in water cooling is usually 2 to 8°C / s.

[0207] 7-5. Cold Working

[0208] After the first cooling, the steel is cold worked. Cold working is a process necessary for finishing to the dimensional accuracy of the specification. Similar to hot working, there is no particular limitation on the type of processing. There is no particular regulation on the cross-sectional shrinkage rate of the steel accompanying the processing, but in normal processing, the cross-sectional shrinkage rate is about 90% or less.

[0209] It should be noted that in the case of manufacturing a steel pipe, for example, the cold working can be set as drawing to form a specified steel pipe shape.

[0210] 7-6. Solution Heat Treatment

[0211] After the above cold working, the steel is solution heat treated at 1100 to 1200°C. In the following description, the soaking holding temperature in the solution heat treatment is recorded as the solution temperature T2. This is because when the solution temperature T2 is lower than 1100°C, the solution of the elements effective in improving the creep strength is insufficient and does not satisfy formula (ii), resulting in a decrease in creep strength. In addition, when the solution temperature T2 exceeds 1200°C, the desired precipitation elements are over-solutionized, so sometimes formula (i) and / or formula (ii) are not satisfied. It should be noted that in order to improve the SCC resistance and make the right side value of formula (i) 0.012 or more, the solution temperature T2 is preferably set to 1170°C or less.

[0212] When comparing the softening treatment temperature T1 and the solution temperature T2, adjustments are made so that the softening treatment temperature T1 is higher than the solution temperature T2. That is, control is carried out to satisfy the following formula (a). This is because when the solution temperature T2 is higher than the softening treatment temperature T1, the SCC resistance cannot be sufficiently improved.

[0213] T1 > T2 (a)

[0214] Among them, the definitions of the symbols in the above formula (a) are as follows.

[0215] T1: Softening treatment temperature (°C)

[0216] T2: Solution temperature (°C)

[0217] It should be noted that the difference between the softening treatment temperature T1 and the solution temperature T2 is set to 150 °C or less. That is, control is carried out to satisfy the following formula (b).

[0218] T1 - T2 ≤ 150 (b)

[0219] Among them, the definitions of the symbols in the above formula (b) are as follows.

[0220] T1: Softening treatment temperature (°C)

[0221] T2: Solution temperature (°C)

[0222] This is because when the difference between the softening treatment temperature T1 and the solution temperature T2 exceeds 150 °C, it is difficult to satisfy formula (iii), and the steam oxidation resistance is likely to decrease.

[0223] The time of solution heat treatment is preferably less than 10 minutes. When the time of solution heat treatment is 10 minutes or more, the desired precipitated elements will be overly dissolved, so sometimes formula (i) and / or formula (ii) cannot be satisfied. It should be noted that the lower limit of the time of solution heat treatment is not particularly limited and is usually 1 minute.

[0224] 7 - 7. Second cooling

[0225] After solution heat treatment, the steel is cooled. This cooling is regarded as the second cooling. In the second cooling, like the first cooling, the steel is cooled by water cooling or at a cooling rate equivalent to or higher than water cooling. If water cooling is not carried out or the cooling rate is not equivalent to or higher than water cooling, the creep strength will decrease due to the formation and growth of precipitates, and the product quality will also be unstable. It should be noted that this cooling is usually preferably carried out to below 600 °C.

[0226] 7 - 8. Formation of the processed layer

[0227] Next, a processed layer is formed near the surface of the steel that has undergone the second cooling. The method of forming the processed layer is not particularly limited. For example, various spraying methods such as known shot peening, peening, shot blasting, sandblasting, abrasive blasting, air jetting, and water jetting can be employed. Additionally, there are no restrictions on the material, shape, etc. of the spraying particles. As the material, for example, steel, cast steel, stainless steel, glass, silica sand, alumina, amorphous, etc. can be used.

[0228] Furthermore, as the shape of the above-mentioned particles, for example, spherical, linear (cut wire), angular (grit), etc. can be used. Compressed air, the centrifugal force of an impeller, high-pressure water, ultrasonic waves, etc. can also be used to jet the particles. Additionally, the particles can be mixed with a liquid and jetted using compressed air, etc. (this is also referred to as "liquid honing").

[0229] Alternatively, a processed layer can also be imparted through grinding, ball milling, lapping, honing, or impact machining using ultrasonic waves, etc. Particularly when it is necessary to stably ensure the steam oxidation resistance characteristics at high temperatures for a long time, it is preferable to form the processed layer by particle jetting that is easy to uniformly process over the entire surface. It should be noted that the processed layer is formed on the pickled surface. That is, the processed layer is formed after pickling. This is because by performing pickling, the surface state becomes good, making it easier for strain to enter the surface.

[0230] Hereinafter, the present embodiment will be described in more detail through examples, but the present embodiment is not limited to these examples.

[0231] Examples

[0232] Steel having the chemical composition shown in Table 1 was melted, subjected to primary rolling, and made into a hollow bloom. This bloom was hot-worked to form a tube with dimensions (outer diameter 56 mm × wall thickness 10.5 mm × length 6684 mm), and rapidly cooled under the conditions shown in Table 2. Then, except for the example of No. 23, a softening treatment with a soaking hold for 3 minutes was performed under the conditions shown in Table 2, followed by water cooling, and cold working was carried out with a cross-sectional shrinkage rate of 35.7%. That is, cold working was performed to make the wall thickness 8.35 mm. The softening treatment temperature T1 was all above the recrystallization temperature and below the grain boundary melting temperature. Next, solution heat treatment was performed under the conditions shown in Table 2, followed by water cooling, thereby obtaining a steel pipe. For the obtained steel pipe, in some examples, shot blasting was performed on the inner surface of the steel pipe to form a processed layer. For the processed layer, conditions such as the spraying pressure, spraying amount, spraying angle, and nozzle shape were controlled and adjusted. Additionally, except for No. 33, the surface of the steel pipe during shot blasting was the pickled surface.

[0233] [Table 1]

[0234]

[0235] [Table 2]

[0236] Table 2

[0237]

[0238] It means beyond the scope defined by this embodiment.

[0239] Underline: It means beyond the preferred manufacturing conditions of this embodiment (excluding the examples of double underlines below).

[0240] Double underline: It means beyond the preferred manufacturing conditions of this embodiment, although it satisfies the preferred temperature range of heat treatment but does not satisfy the condition of T1 > T2 or T1 - T2 ≤ 150.

[0241] For the obtained steel pipe, the determination of electrolytic extraction residue, hardness (Hv 40 and Hv t / 2 ), SCC test, creep test, and steam oxidation test are carried out in the following order.

[0242] (Electrolytic extraction residue)

[0243] For V, Ti, Nb, Ni, Cr, and Mo, calculate the content (mass%) of each element in the precipitate obtained by extraction residue analysis according to the following steps, and calculate the right - hand value of formula (i), the intermediate value of formula (ii), and the left - hand value of formula (iii).

[0244] Specifically, use 10% acetylacetone - 1% tetramethylammonium chloride / methanol, and electrolyze about 0.4 g of the sample at a current value of 20 mA / cm 2 . Then, filter the solution of the electrolyzed sample with a 0.2 - μm filter, and decompose the residue with a mixed acid of sulfuric acid + phosphoric acid + nitric acid + perchloric acid. Then, for the above elements, use an ICP emission spectrometer to calculate the amount (mass%) analyzed as electrolytic extraction residue.

[0245] (Hardness)

[0246] Cut a test piece with a size of 15 mm square, embed the test piece in resin, cut the cross-section and perform mirror polishing. For each test piece, measure the Vickers hardness at a position 40 μm away from the inner surface of the steel pipe in the wall thickness direction and at a position t / 2 away from the surface of the steel pipe in the wall thickness direction in a straight line in the wall thickness direction of the steel pipe. It should be noted that the test load is set to 10 gf. Repeat this measurement at different positions on the test piece and at positions not affected by other measurements, and measure 5 times at each position. Then, take the average value of the measurement results at 5 points at the position 40 μm in the wall thickness direction as Hv 40 , and take the average value of the measurement results at 5 points at the position t / 2 in the wall thickness direction as Hv t / 2 .

[0247] (SCC test)

[0248] For SCC resistance, perform an SCC test according to ASTM A262 Method E for evaluation. Specifically, collect a test piece of 15 mm × 2 mm × 70 mm from one end of the obtained austenitic stainless steel. After subjecting the test piece to sensitization heat treatment at 700 °C for 30 minutes, put about 70 g of copper chips into the sulfuric acid / cupric sulfate aqueous solution specified by the above specifications, immerse the test piece in the boiling solution for 24 hours, and perform a bending test. After the test, if no cracks appear on the test piece and no cracks appear even when the immersion time is further set to 48 hours and a bending test is performed, it is evaluated as having very good SCC resistance (A). For the case where cracks appear when the immersion time is 48 hours but no cracks appear when the immersion time is 24 hours, it is evaluated as having good SCC resistance (B). The case where cracks appear even when the immersion time is 24 hours is evaluated as having poor SCC resistance (D).

[0249] (Creep test)

[0250] To evaluate the creep strength, a creep test was carried out. Specifically, collect round bar creep test pieces and perform a creep rupture test. In the evaluation, for the case where the rupture strength is 210 MPa or more when performing a creep test at 600 °C for 10,000 hours and the rupture strength is 150 MPa or more when performing a creep test at 650 °C for 10,000 hours, it is evaluated as having very good creep characteristics (A).

[0251] In addition, except for the case where the creep property evaluation is very good (A) as described above, the case satisfying any of the following is evaluated as having good creep property (B): when performing a creep test at 600 °C for 10,000 hours, the fracture strength is 210 MPa or more; or when performing a creep test at 650 °C for 10,000 hours, the fracture strength is 150 MPa or more. In addition, the case other than the above cases of very good creep property (A) and good creep property (B) is evaluated as having poor creep property (D).

[0252] (Steam oxidation test)

[0253] Cut a rectangular test piece with a thickness of 2 mm × width of 10 mm × length of 25 mm from the obtained steel pipe, and make the inner surface of the pipe a part of the test piece surface. Hold the test piece in a hanging manner with a fixture and insert it into a horizontal tubular heating furnace, and perform a 1,000-hour oxidation test in a steam atmosphere with a dissolved oxygen content of 100 ppb at 650 °C. After cooling in the furnace, embed the taken-out test piece in resin, cut the cross-section and perform mirror polishing, then observe the cross-section of the oxide scale formed on the inner surface of the steel pipe with an optical microscope and measure the thickness of the oxide scale. Measure the thickness of the oxide scale in 10 arbitrary fields of view at 500 times magnification and calculate the average value of these thicknesses. The case where the thickness of the oxide scale is 10 μm or less is evaluated as having very good steam oxidation resistance (B); the case where the oxide scale thickness exceeds 10 μm and is 50 μm or less is evaluated as having slightly poor steam oxidation resistance (C); the case where the oxide scale thickness is 50 μm or more is evaluated as having poor steam oxidation resistance (D). Hereinafter, the results are summarized in Table 3.

[0254] [Table 3]

[0255] Table 3

[0256]

[0257] means beyond the scope defined in this embodiment. Nb ER <0.052…(iii)

[0258] 0.010 ≤ V ER +Ti ER +Nb ER …(i) (Hv 40 -Hv t / 2 / Hv t / 2 …(iv)

[0259] 27.0 ≤ 1.13(Ni - Ni ER )+(Cr - Cr ER )+1.85(Mo - Mo ER )+1.79(Nb - Nb ER ) < 40.5…(ii)

[0260] In Tests No. 1 to 20, 21 to 23, 24, 29, 31 to 33, 26, 37, and 38, regardless of whether the immersion time was 24 hours or 48 hours, no cracks appeared in the SCC resistance test and they were evaluated as "A". It is considered that in these examples, since the right-hand value of formula (i) was 0.012 or more, better SCC resistance was exhibited. In addition, in Tests No. 5 and 6, when the immersion time was 24 hours, no cracks appeared in the SCC resistance test and they were evaluated as "B". It is considered that although the right-hand value of formula (i) was less than 0.012 but 0.010 or more, due to the precipitation of V, Ti, and Nb, C was still fixed even after sensitization heat treatment, thus suppressing the formation of grain boundary Cr carbides and preventing cracks.

[0261] On the other hand, cracks appeared in the SCC resistance tests of Tests No. 25 to 28, 30, and 34 to 36 and they were evaluated as "D". It is considered that in these examples, since formula (i) was not satisfied, the precipitation of V, Ti, and Nb was insufficient, Cr carbides were formed at the grain boundaries after sensitization heat treatment, and a Cr-depleted layer was generated around them, so cracks appeared in the SCC resistance test. The reason is considered to be that the chemical composition, softening treatment temperature, or solution temperature exceeded the preferred range.

[0262] In the creep tests of Tests No. 1, 3, 8, 10, 12 to 17, 20 to 22, and 31, they were evaluated as "A". It is considered that in these examples, the intermediate value of formula (ii) was above the more preferred lower limit value, so the creep strength was excellent. In addition, in the creep tests of Tests No. 2, 4 to 7, 9, 11, 18, 23, 25, 29, 30, 32, and 33, they were evaluated as "B". It is considered that in these examples, although the intermediate value of formula (ii) was not above the more preferred lower limit value, since formula (ii) was satisfied, the elements contributing to creep strength were sufficiently dissolved, and good creep strength capable of withstanding the present invention was obtained.

[0263] On the other hand, in the creep tests of Tests No. 19, 24, 26 to 28, they were evaluated as "D". It is considered that in these examples, the intermediate value of formula (ii) was less than 27.0, and the dissolution of Ni, Cr, Mo, and Nb for improving creep strength was insufficient, so sufficient creep strength was not obtained. It should be noted that among them, it is considered that No. 24, 26, and 27 did not satisfy formula (iii), so NbCr nitrides were formed and the creep strength decreased.

[0264] In the steam oxidation tests of Test Nos. 1 to 23, 25, and 28 to 30, they were evaluated as "B". It is considered that these examples satisfy formulas (iii) and (iv), and thus have good steam oxidation resistance. In the steam oxidation tests of Test Nos. 24, 26, 27, and 31, they were evaluated as "C". It is considered that although these examples satisfy formula (iv), they do not satisfy formula (iii), and thus a good processing layer was not formed, resulting in a decrease in steam oxidation resistance. In the steam oxidation tests of Test Nos. 32 and 33, they were evaluated as "D". It is considered that in these examples, the conditions during the formation of the processing layer were not within the preferred range, and the processing layer was not fully formed.

[0265] (Note)

[0266] (1) An austenitic stainless steel having a chemical composition in mass % of C: 0.002 to 0.020%,

[0267] Si: 0.10 to 0.60%,

[0268] Mn: 0.2 to 2.0%,

[0269] P: 0.035% or less,

[0270] S: 0.010% or less,

[0271] Cu: 2.50 to 4.50%,

[0272] Ni: 9.00 to 16.00%,

[0273] Cr: 15.00 to 20.00%,

[0274] Mo: 0.20 to 1.50%,

[0275] Nb: 0.15 to 0.60%,

[0276] N: 0.05 to 0.15%,

[0277] B: 0.0010 to 0.0060%,

[0278] V: 0 to 0.50%,

[0279] Ti: 0 to 0.500%,

[0280] Co: 0 to 1.00%,

[0281] W: 0 to 1.00%,

[0282] Ta: 0 to 0.40%,

[0283] Sn: 0 to 0.0300%,

[0284] Al: 0 to 0.035%,

[0285] Ca: 0 to 0.0100%,

[0286] Mg: 0 to 0.0100%,

[0287] REM: 0 to 0.0800%,

[0288] Balance: Fe and impurities,

[0289] The austenitic stainless steel satisfies the following formulas (i) to (iv).

[0290] 0.010 ≤ V ER + Ti ER + Nb ER (i)

[0291] 27.0 ≤ 1.13(Ni - Ni ER ) + (Cr - Cr ER ) + 1.85(Mo - Mo ER ) + 1.79(Nb - Nb ER ) < 40.5 (ii)

[0292] Nb ER <0.052 (iii)

[0293] 0.5 ≤ (Hv 40 - Hv t / 2 ) / Hv t / 2 (iv)

[0294] Among them, the definitions of each symbol in the above formulas are as follows. Each element symbol in the above formulas represents the content (mass %) of each element contained in the steel. If not contained, it is set to zero.

[0295] V ER : The V content (mass %) in the precipitate obtained by extraction residue analysis

[0296] Ti ER : The Ti content (mass %) in the precipitate obtained by extraction residue analysis

[0297] Nb ER : The Nb content (mass %) in the precipitate obtained by extraction residue analysis

[0298] Ni ER : The Ni content (mass %) in the precipitate obtained by extraction residue analysis

[0299] Cr ER : The Cr content (mass %) in the precipitate obtained by extraction residue analysis

[0300] Mo ER : Mo content (mass %) in the precipitate obtained from the extraction residue analysis

[0301] Hv 40 : Vickers hardness measured with a test load of 10 gf at a position 40 μm from the surface in the thickness direction

[0302] Hv t / 2 : Vickers hardness measured with a test load of 10 gf at a position 1 / 2t from the surface in the thickness direction when the total thickness is t

[0303] (2) The austenitic stainless steel according to (1) above, wherein the chemical composition contains, by mass %, one or more selected from

[0304] V: 0.01 - 0.50%, and

[0305] Ti: 0.001 - 0.500%.

[0306] (3) The austenitic stainless steel according to (1) or (2) above, wherein the chemical composition contains, by mass %, one or more selected from

[0307] Co: 0.02 - 1.00%,

[0308] W: 0.01 - 1.00%,

[0309] Ta: 0.01 - 0.40%,

[0310] Sn: 0.0010 - 0.0300%,

[0311] Al: 0.0005 - 0.035%,

[0312] Ca: 0.0010 - 0.0100%,

[0313] Mg: 0.0002 - 0.0100%, and

[0314] REM: 0.0010 - 0.0800%.

[0315] (4) The austenitic stainless steel according to any one of (1) to (3) above, which is a steel pipe, and the surface is the inner surface of the steel pipe.

Claims

1. An austenitic stainless steel having a chemical composition by mass % of C: 0.002 to 0.020%, Si: 0.10 to 0.60%, Mn: 0.2 to 2.0%, P: 0.035% or less, S: 0.010% or less, Cu: 2.50 to 4.50%, Ni: 9.00 to 16.00%, Cr：15.00～20.00％、 Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N：0.05～0.15％、 B：0.0010～0.0060％、 Balance: Fe and impurities, The austenitic stainless steel satisfies the following formulas (i) to (iv), 0.010 ≤ V ER + Ti ER + Nb ER (i) 27.0 ≤ 1.13(Ni - Ni ER )+(Cr - Cr ER )+1.85(Mo - Mo ER )+1.79(Nb - Nb ER ) < 40.5 (ii) Nb ER <0.052 (iii) 0.5 ≤ (Hv 40 -Hv t / 2 ) / Hv t / 2 (iv) wherein the definitions of the respective symbols in the above formulas are as shown below, and each element symbol in the above formulas represents the content by mass % of each element contained in the steel, and is set to zero when not contained, V ER : The V content by mass % in the precipitate obtained from the extraction residue analysis; Ti ER : The Ti content by mass % in the precipitate obtained from the extraction residue analysis; Nb ER : The Nb content in mass % in the precipitate obtained from the extraction residue analysis; Ni ER : The Ni content in mass % in the precipitate obtained from the extraction residue analysis; Cr ER : The Cr content in mass % in the precipitate obtained by the extraction residue analysis; Mo ER : The Mo content in mass % in the precipitate obtained by the extraction residue analysis; Hv 40 : Vickers hardness measured with a test load of 10 gf at a position 40 μm from the surface in the thickness direction; Hv t / 2 : Vickers hardness measured with a test load of 10 gf at a position 1 / 2t from the surface in the thickness direction when the total thickness is set to t.

2. An austenitic stainless steel having a chemical composition by mass % of C：0.002～0.020％、 Si: 0.10 to 0.60%, Mn: 0.2 to 2.0%, P: 0.035% or less, S: 0.010% or less, Cu: 2.50 to 4.50%, Ni: 9.00 to 16.00%, Cr：15.00～20.00％、 Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N：0.05～0.15％、 B：0.0010～0.0060％， Contains one or more elements selected from the group consisting of the following Group A and Group B, Balance is Fe and impurities, The austenitic stainless steel satisfies the following formulas (i) to (iv), [Group A] V: 0.50% or less, and Ti: 0.500% or less, [Group B] Co: 1.00% or less, W: 1.00% or less, Ta: 0.40% or less, Sn: 0.0300% or less, Al: 0.035% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0800% or less, 0.010 ≤ V ER + Ti ER + Nb ER (i) 27.0 ≤ 1.13(Ni - Ni ER ) + (Cr - Cr ER ) + 1.85(Mo - Mo ER ) + 1.79(Nb - Nb ER ) < 40.5 (ii) Nb ER <0.052 (iii) 0.5 ≤ (Hv 40 -Hv t / 2 ) / Hv t / 2 (iv) wherein the definitions of the respective symbols in the above formulas are as shown below, and each element symbol in the above formulas represents the content by mass % of each element contained in the steel, and is set to zero when not contained, V ER : The V content in mass % in the precipitate obtained from the extraction residue analysis; Ti ER : The Ti content in mass % in the precipitate obtained from the extraction residue analysis; Nb ER : The Nb content by mass % in the precipitate obtained from the analysis of the extraction residue; Ni ER : The Ni content by mass % in the precipitate obtained from the extraction residue analysis; Cr ER : The Cr content in mass % in the precipitate obtained from the extraction residue analysis; Mo ER : The Mo content in mass % in the precipitate obtained by the extraction residue analysis; Hv 40 : Vickers hardness measured with a test load of 10 gf at a position 40 μm from the surface in the thickness direction; Hv t / 2 : Vickers hardness measured with a test load of 10 gf at a position 1 / 2t from the surface in the thickness direction when the total thickness is set to t.

3. The austenitic stainless steel according to claim 2, wherein, The chemical composition contains one or more elements selected from Group A.

4. The austenitic stainless steel according to claim 2, wherein, The chemical composition contains one or more elements selected from Group B.

5. The austenitic stainless steel according to any one of claims 1 to 4, which is a steel pipe, and the surface is the inner surface of the steel pipe.

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