Austenitic stainless steel

Abstract

Provided is an austenitic stainless steel having a predetermined chemical composition and satisfying [0.010 ≤ VER + TiER + NbER], [27.0 ≤ 1.13 (Ni - NiER) + (Cr - CrER) + 1.85 (Mo - MoER) + 1.79 (Nb - NbER) < 40.5], [NbER < 0.052], and [0.5 ≤ (Hv40 - Hvt / 2) / Hvt / 2].

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an austenitic stainless steel.BACKGROUND ART

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

[0003] Austenitic stainless steels are sometimes used, due to their characteristics, for boiler heat transfer tubes. For example, Patent Documents 1 and 2 disclose austenitic stainless steels with good creep strength.LIST OF PRIOR ART DOCUMENTSPATENT DOCUMENT

[0004] Patent Document 1: JP2003-268503A Patent Document 2: JP2021-21093A SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0005] In addition, in recent years, the environment in which boiler heat transfer tubes are used in plants has become more severe. Specifically, stress corrosion cracking (hereinafter, referred to as "SCC") may occur in boiler heat transfer tubes during equipment installation or during periodic maintenance after operation. In addition, as operating temperatures rise, the steam temperature to which heat transfer tubes are exposed becomes higher, making steam oxidation more likely to occur.

[0006] Therefore, not only good creep strength but also excellent SCC resistance and steam oxidation resistance are increasingly required in materials for boiler heat transfer tubes. However, SCC resistance and steam oxidation resistance were not investigated in Patent Documents 1 and 2. Therefore, the austenitic stainless steels disclosed in the above documents have room for further improvement in SCC resistance and steam oxidation resistance.

[0007] Based on the above, an objective of the present disclosure is to solve the above problems and provide an austenitic stainless steel having good creep strength, SCC resistance, and steam oxidation resistance.SOLUTION TO PROBLEM

[0008] The present disclosure has been made to solve the above-mentioned problems and the gist thereof is the following austenitic stainless steel and steel tube. (1) An austenitic stainless steel having a chemical composition consisting, by mass percent, 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 to 20.00%, Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N: 0.05 to 0.15%, B: 0.0010 to 0.0060%, and a balance being Fe and impurities; and satisfying the following Inequalities (i) to (iv): 0.010 ≤ V ER + Ti ER + Nb ER 27.0 ≤ 1.13 Ni − Ni ER + Cr − Cr ER + 1.85 Mo − Mo ER + 1.79 Nb − Nb ER < 40.5 Nb ER < 0.052 0.5 ≤ Hv 40 − Hv t / 2 / Hv t / 2 where, each symbol in the above Inequalities is defined as follows, and each element symbol in the above Inequalities represents the content (mass %) of each element in the steel, or zero if not contained. V ER : V content in precipitates obtained by extraction residue analysis (mass %) Ti ER : Ti content in precipitates obtained by extraction residue analysis (mass %) Nb ER : Nb content in precipitates obtained by extraction residue analysis (mass %) Ni ER : Ni content in precipitates obtained by extraction residue analysis (mass %) Cr ER : Cr content in precipitates obtained by extraction residue analysis (mass %) Mo ER : Mo content in precipitates obtained by extraction residue analysis (mass %) 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 / 2)t from the surface in the thickness direction when the total thickness is t (2) An austenitic stainless steel having a chemical composition consisting, by mass percent, 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 to 20.00%, Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N: 0.05 to 0.15%, B: 0.0010 to 0.0060%, one or more elements selected from the group consisting of following groups A and B, and a balance being Fe and impurities; and satisfying the following Inequalities (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 27.0 ≤ 1.13 Ni − Ni ER + Cr − Cr ER + 1.85 Mo − Mo ER + 1.79 Nb − Nb ER < 40.5 Nb ER < 0.052 0.5 ≤ Hv 40 − Hv t / 2 / Hv t / 2 where, each symbol in the above Inequalities is defined as follows, and each element symbol in the above Inequalities represents the content (mass %) of each element in the steel, or zero if not contained. V ER : V content in precipitates obtained by extraction residue analysis (mass %) Ti ER : Ti content in precipitates obtained by extraction residue analysis (mass %) Nb ER : Nb content in precipitates obtained by extraction residue analysis (mass %) Ni ER : Ni content in precipitates obtained by extraction residue analysis (mass %) Cr ER : Cr content in precipitates obtained by extraction residue analysis (mass %) Mo ER : Mo content in precipitates obtained by extraction residue analysis (mass %) 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 / 2)t from the surface in the thickness direction when the total thickness is t (3) The austenitic stainless steel as described in (2) above, wherein the chemical composition contains one or more elements selected from Group A. (4) The austenitic stainless steel as described in (2) above, wherein the chemical composition contains one or more elements selected from Group B. (5) The austenitic stainless steel as described in any one of (1) to (4) above, wherein the steel is a steel tube and the surface is the inner surface of the steel tube.

[0009] According to the present disclosure, an austenitic stainless steel with good creep strength, SCC resistance, and steam oxidation resistance can be obtained.BRIEF DESCRIPTION OF DRAWINGS

[0010] [Figure 1] Figure 1 shows a micrograph of an observed Nb compound.DESCRIPTION OF EMBODIMENTS

[0011] The present inventors investigated methods to improve the creep strength, SCC resistance, and steam oxidation resistance of austenitic stainless steel and made the following findings. (a) C has an effect of increasing creep strength. Therefore, the austenitic stainless steels disclosed in Patent Document 1 contain 0.03% or more of C. However, in austenitic stainless steels, C can be a cause of SCC. For example, when austenitic stainless steels are heated to approximately 600 to 700°C, Cr in the steel combines with C to form Cr carbides. As a result, a Cr-depleted layer is formed, and SCC occurs in a corrosive environment where chlorides and the like are present. Therefore, it is necessary to reduce the C content from the viewpoint of suppressing SCC. (b) In 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 the dissolved C is heated in the service environment, etc., it leaves a solid solution state and combines with Cr to form Cr carbides. These Cr carbides reduce the SCC resistance. Therefore, it is desirable to reduce the dissolved C by combining C with elements such as V, Ti, and Nb and precipitating them in advance at the pre-use stage. (c) On the other hand, when the C content is reduced, it becomes difficult to ensure creep strength. Therefore, it is effective to include elements such as Ni, Cr, Mo, and Nb to improve creep strength. These elements improve creep strength by forming precipitates in the service environment where they are exposed to high temperatures. However, if the above elements form precipitates before being used at high temperatures, the creep strength cannot be sufficiently improved. Therefore, in order to obtain good creep strength, it is desirable for these elements to be dissolved as much as possible in the matrix phase at the pre-use stage and to form precipitates in the service environment. Therefore, it is necessary to adjust the dissolved amount of the above elements to an appropriate range in advance at the pre-use stage. (d) In order to improve steam oxidation resistance, it is common practice to form a machined layer near the surface of the steel by shot processing and the like. However, in order to improve creep strength and SCC resistance, it is difficult to improve steam oxidation resistance simply by forming a machined layer in steel with a controlled chemical composition. The inventors then have found that it is important to reduce Nb precipitates and ensure a certain amount of dissolved Nb.

[0012] The reason for this is not certain, but it is considered that dissolved Nb promotes the formation of Cr oxides, which contribute to the improvement of steam oxidation resistance, although the formation rate is slow, by suppressing the excessive penetration of O into the steel in the service environment. Therefore, it is effective to leave Nb in a solid solution state in advance, not only from the viewpoint of creep strength, but also from the viewpoint of steam oxidation resistance. Then, it is preferable to perform shot processing, etc. afterwards to form a machined layer near the surface.

[0013] (e) When the degree of processing given is increased in forming the machined layer described above, nucleation and growth, i.e., recrystallization, occur during use at high temperatures. The recrystallized microstructure has a very fine grain size, which facilitates intergranular diffusion of Cr. Thus, Cr is supplied sufficiently to facilitate the formation of Cr oxide, for example, Cr oxide scale containing mainly Cr 2 O 3 . This Cr oxide scale forms very thin and uniformly, and improves the resistance to steam oxidation.

[0014] In addition, when a machined layer is formed and the vicinity of the surface is recrystallized in the service environment, even if a defect occurs in the Cr oxide scale, a protective oxide scale will be reformed at that location. This is called scale repair; once the Cr oxide scale is repaired, abnormal oxidation does not occur. As a result, steam oxidation resistance can be maintained for a long period of time.

[0015] Based on the above, it is desirable to adjust the balance between dissolution and precipitation of each element in the steel by appropriately controlling the chemical composition and manufacturing conditions, as well as forming a machined layer near the surface.

[0016] One embodiment of the present invention is based on the above findings. The following is a detailed description of each requirement of the austenitic stainless steel and steel tube of the present embodiment.1. Chemical Composition

[0017] The reasons for the limitation of each element are as follows. In the following explanation, "%" for content means "mass percent."C: 0.002 to 0.020%

[0018] C (carbon) is an element necessary to ensure high temperature strength, especially creep strength. Therefore, the C content is to be 0.002% or more. The C content is preferably 0.003% or more, and more preferably 0.004% or more. However, if C is contained in excess, the SCC resistance will be reduced. Therefore, the C content is to be 0.020% or less. The C content is preferably 0.015% or less, and more preferably 0.010% or less.Si: 0.10 to 0.60%.

[0019] Si (silicon) is an element that has a deoxidizing effect. For this reason, the Si content is to be 0.10% or more. The Si content is preferably 0.12% or more, and more preferably 0.14% or more. However, an excessive Si content reduces workability. Therefore, the Si content is to be 0.60% or less; the Si content is preferably 0.50% or less, and more preferably 0.40% or less.Mn: 0.2 to 2.0%.

[0020] Mn (manganese) combines with impurity S contained in steel to form MnS, thereby exhibiting an effect of improving hot workability. For this reason, the Mn content is to be 0.2% or more. The Mn content is preferably 0.4% or more, and more preferably 0.6% or more. However, if the Mn content is excessive, the steel will become hard and brittle, resulting in reduced workability and weldability. Therefore, the Mn content is to be 2.0% or less; the Mn content is preferably 1.5% or less, and more preferably 1.3% or less.P: 0.035% or less

[0021] P (phosphorus) is an element contained in steel as an impurity that reduces SCC resistance. P also reduces the hot workability and toughness of steel. Therefore, the P content is to be 0.035% or less. The P content is preferably 0.030% or less, and more preferably 0.025% or less. It is preferable to reduce the P content as much as possible, but if the content is reduced excessively, production cost will increase. Therefore, the P content is preferably 0.010% or more.S: 0.010% or less

[0022] S (sulfur) is an element contained in steel as an impurity that reduces SCC resistance. S also reduces the hot workability and creep ductility of the steel. Therefore, the S content is to be 0.010% or less. The S content is preferably 0.009% or less, and more preferably 0.008% or less. It is preferable to reduce the content as much as possible, but an excessive reduction of the content causes an increase in production cost. Therefore, it is preferable that the S content be 0.0001% or more.Cu: 2.50 to 4.50%.

[0023] Cu (copper) precipitates as Cu phase in grains and enhances the creep strength and creep ductility of the steel by precipitation strengthening. For this reason, the Cu content is to be 2.50% or more; The Cu content is preferably 2.70% or more, and more preferably 2.90% or more. However, an excessive Cu content reduces the hot workability and weldability. Therefore, the Cu content is to be 4.50% or less; the Cu content is preferably 4.00% or less, and more preferably 3.50% or less.Ni: 9.00 to 16.00%.

[0024] Ni (nickel) is an element that stabilizes the austenite structure and has an effect of improving SCC resistance and corrosion resistance. Ni also has the effect of increasing creep strength. Therefore, the Ni content is to be 9.00% or more. The Ni content is preferably 10.00% or more, and more preferably 10.50% or more. However, an excessive Ni content will increase production cost, and in addition, the creep strength will be adversely reduced. Therefore, the Ni content is to be 16.00% or less; the Ni content is preferably 15.00% or less, and more preferably 14.00% or less.Cr: 15.00 to 20.00%.

[0025] Cr (chromium) is an element necessary to ensure corrosion resistance. Cr also has the effect of increasing creep strength. Therefore, the Cr content is to be 15.00% or more. The Cr content is preferably 15.50% or more, and more preferably 16.00% or more. However, an excessive Cr content reduces the stability of the austenite structure, and also reduces the weldability. Therefore, the Cr content is to be 20.00% or less. The Cr content is preferably 19.75% or less, and more preferably 19.50% or less.Mo: 0.20 to 1.50%.

[0026] Mo (molybdenum) has the effect of increasing creep strength. Therefore, the Mo content is to be 0.20% or more. The Mo content is preferably 0.35% or more, and more preferably 0.50% or more. However, an excessive Mo content reduces the stability of the austenite structure. Therefore, the Mo content is to be 1.50% or less. The Mo content is preferably 1.25% or less, and more preferably 1.00% or less.Nb: 0.15 to 0.60%.

[0027] Nb (niobium) has the effect of improving creep strength and SCC resistance. Therefore, the Nb content is to be 0.15% or more. The Nb content is preferably 0.20% or more, and more preferably 0.25% or more. However, an excessive Nb content will cause significantly coarse precipitates, resulting in a decrease in creep strength. Therefore, the Nb content is to be 0.60% or less. The Nb content is preferably 0.55% or less, and more preferably 0.50% or less Nb.N: 0.05 to 0.15%

[0028] N (nitrogen) has an effect of increasing strength through solid solution strengthening, and precipitation strengthening by Nb carbonitride. Therefore, the N content is to be 0.05% or more. The N content is preferably 0.06% or more, and more preferably 0.07% or more. However, an excessive N content will result in the formation of lumpy nitrides, which deteriorate the steel quality. As a result, the strength may be reduced. Therefore, the N content is to be 0.15% or less. The N content is preferably 0.13% or less, and more preferably 0.12% or less.B: 0.0010 to 0.0060%.

[0029] B (boron) has the effect of increasing creep ductility. Therefore, the B content is to be 0.0010% or more. The B content is preferably 0.0015% or more, and more preferably 0.0020% or more. However, an excessive B content will reduce weldability and hot workability at high temperatures. Therefore, the B content is to be 0.0060% or less. The B content is preferably 0.0050% or less, and more preferably 0.0045% or less.Group A optional elements

[0030] In the chemical composition of the austenitic stainless steel of the present embodiment, in addition to the above elements, one or more elements selected from V and Ti (hereinafter referred to as "Group A") may be included within the ranges shown below. The reasons for the limitation of each element are explained below.V: 0.50% or less

[0031] V (vanadium) has an effect of reducing dissolved C and enhancing SCC resistance. V also has the effect of increasing creep strength. For this reason, V may be included as necessary. However, if V is included in excess, δ-ferrite is formed, and the creep strength, toughness, and weldability of the steel will be reduced. Therefore, the V content is to be 0.50% or less. The V content is preferably 0.40% or less, and more preferably 0.30% or less. The V content may be 0% or more, but to achieve the above effect, for example, the V content is preferably 0.01% or more, and more preferably 0.02% or more.Ti: 0.500% or less

[0032] Ti (titanium), like V, has the effect of reducing dissolved C and increasing SCC resistance. Ti also has the effect of increasing creep strength. For this reason, Ti may be included as necessary. However, an excessive Ti content will result in a decrease in creep strength. Therefore, the Ti content is to be 0.500% or less. The Ti content is preferably 0.400% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The Ti content may be 0% or more, but to achieve the above effect, for example, the Ti content is preferably 0.001% or more, and more preferably 0.002% or more.Group B optional elements

[0033] In the chemical composition of the austenitic stainless steel of the present embodiment, in addition to the above elements, one or more elements selected from Co, W, Ta, Sn, Al, Ca, Mg, and REM (hereinafter referred to as "B group") may be included within the ranges shown below. The reasons for the limitation of each element are explained below.Co: 1.00% or less

[0034] Co (cobalt) stabilizes the austenite structure and has the effect of increasing creep strength. Therefore, Co may be included as necessary. However, an excessive Co content will increase production cost. Therefore, the Co content is to be 1.00% or less. The Co content is preferably 0.50% or less, and more preferably 0.30% or less. The Co content may be 0% or more, but to achieve the above effect, for example, the Co content is preferably 0.02% or more.W: 1.00% or less

[0035] W (tungsten) is dissolved in the matrix phase and has the effect of increasing the creep strength of the steel. Therefore, W can be included as necessary. However, if W is included in excess, the stability of the austenite phase will be reduced, resulting in a decrease in creep strength and toughness. Therefore, the W content is to be 1.00% or less. The W content is preferably 0.50% or less, and more preferably 0.30% or less. The W content may be 0% or more, but to achieve the above effect, for example, the W content is preferably 0.01% or more.Ta: 0.40% or less

[0036] Ta (tantalum) combines with C to form carbonitride and reduce dissolved C. As a result, Ta has the effect of increasing SCC resistance. Ta also has the effect of increasing creep strength. For this reason, Ta can be included as necessary. However, if Ta is contained in excess, δ-ferrite will be formed and the creep strength, toughness, and weldability of the steel will be reduced. Therefore, the Ta content is 0.40% or less. The Ta content is preferably 0.30% or less, and more preferably 0.10% or less. The Ta content may be 0% or more, but to achieve the above effect, for example, the Ta content is preferably 0.01% or more.Sn: 0.0300% or less

[0037] Sn (tin) has the effect of improving corrosion resistance and high temperature properties. Therefore, Sn may be included as necessary. However, an excessive Sn content will reduce weldability and producibility. Therefore, the Sn content is to be 0.0300% or less. The Sn content is preferably 0.0200% or less, and more preferably 0.0100% or less. The Sn content may be 0% or more, but to achieve the above effect, for example, the Sn content is preferably 0.0010% or more.Al: 0.035% or less

[0038] Al (aluminum) is an element that has a deoxidizing effect and improves hot workability by fixing O as inclusions. For this reason, Al can be included as necessary. However, if Al is included in excess, an excessive amount of inclusions will be formed and the surface properties will deteriorate. In addition, the hot workability will also be reduced. Therefore, the Al content is 0.035% or less. The Al content is preferably 0.030% or less, and more preferably 0.025% or less. The Al content may be 0% or more, but to achieve the above effect, for example, the Al content is preferably 0.0005% or more.Ca: 0.0100% or less

[0039] Ca (calcium) fixes S and O as inclusions and has the effect of increasing hot workability and creep ductility of steel. Therefore, Ca may be included as necessary. However, an excessive Ca content will reduce hot workability and creep ductility. Therefore, the Ca content is to be 0.0100% or less. The Ca content is preferably 0.0050% or less, and more preferably 0.0030% or less. The Ca content may be 0% or more, but to achieve the above effect, for example, the Ca content is preferably 0.0010% or more.Mg: 0.0100% or less

[0040] Mg (magnesium), like Ca, has the effect of fixing S and O as inclusions and improving the hot workability and creep ductility of steel. For this reason, Mg may be included as necessary. However, if Mg is contained in excess, the hot workability and long-term creep ductility will be reduced. For this reason, the Mg content is to be 0.0100% or less. The Mg content is preferably 0.0050% or less, and more preferably 0.0030% or less. The Mg content may be 0% or more, but to achieve the above effect, for example, the Mg content is preferably 0.0002% or more.REM: 0.0800% or less

[0041] REM (rare earth element), like Ca and Mg, has the effect of fixing S and O as inclusions and enhancing the hot workability and creep ductility of steel. For this reason, REM may be included as necessary. However, if REM is contained in excess, the hot workability and long-term creep ductility will be reduced. Therefore, the REM content is to be 0.0800% or less. The REM content is preferably 0.0600% or less, and more preferably 0.0400% or less. The REM content may be 0% or more, but to achieve the above effect, for example, the REM content is preferably 0.0010% or more.

[0042] Note that "REM" refers to a total of 17 elements of Sc, Y, and lanthanides, and the above "REM content" means a total content of these elements. REM is often added in the form of mischmetal in industry.

[0043] As for the optional elements, as described above, one or more elements selected from the group consisting of the above-mentioned Group A and Group B may be included as necessary.

[0044] In the chemical composition of the austenitic stainless steel of the present embodiment, the balance is Fe and impurities. Here, "impurities" means constituents that are mixed in from ores and scrap as raw materials or from the production environment in the industrial production of the steel material and that are allowed within a range that does not adversely affect the austenitic stainless steel.2. Inequality (i)

[0045] As mentioned above, dissolved C combines with Cr to form Cr carbides in service environments where the tube is exposed to high temperatures. As a result, the SCC resistance is reduced. Therefore, it is desirable to reduce the amount of dissolved C in advance before the tubes are used as boiler heat transfer tubes. Specifically, it is recommended that C be fixed in advance as precipitates (compounds) by V, Ti, and Nb. Therefore, the amount of V, Ti, and Nb that combine with C and exist as precipitates, i.e., the value of the right side of Inequality (i), is controlled. 0.010 ≤ V ER + Ti ER + Nb ER

[0046] Here, each symbol in the above Inequality is defined as follows: V ER : V content in precipitates obtained by extraction residue analysis (mass %) Ti ER : Ti content in precipitates obtained by extraction residue analysis (mass %) Nb ER : Nb content in precipitates obtained by extraction residue analysis (mass %)

[0047] If the value on the right side of Inequality (i) is less than 0.010, dissolved C increases, and in the service environment, dissolved C and Cr combine to form Cr carbides. As a result, a Cr-depleted layer is formed, and the SCC resistance decreases. Therefore, the value on the right side of Inequality (i) is to be 0.010 or more. The value on the right side of Inequality (i) is preferably 0.012 or more, more preferably 0.015 or more, and even more preferably 0.020 or more.

[0048] From the viewpoint of weldability, the value on the right side of Inequality (i) is preferably 0.120 or less. From the viewpoint of strength reduction due to excessive N fixation, it is preferable that the value on the right side of Inequality (i) be 0.100 or less.3. Inequality (ii)

[0049] In the austenitic stainless steel of the present embodiment, the amount of Ni, Cr, Mo, and Nb dissolved in the matrix phase is controlled. This is because the improvement in creep strength can be achieved by ensuring the amount of the above elements dissolved in the matrix while reducing C, which is effective for creep strength. The amount of each element dissolved in the matrix can be calculated from the difference between the content (mass%) of each element and the content (mass%) of each element in the precipitates obtained by the extraction residue analysis. The austenitic stainless steel of the present embodiment must satisfy Inequality (ii).

[0050] Here, each symbol in the above Inequality is defined as follows, and each element symbol in the above Inequality represents the content (mass %) of each element in the steel, or zero if not contained. Nb ER : Nb content in precipitates obtained by extraction residue analysis (mass %) Ni ER : Ni content in precipitates obtained by extraction residue analysis (mass %) Cr ER : Cr content in precipitates obtained by extraction residue analysis (mass %) Mo ER : Mo content in precipitates obtained by extraction residue analysis (mass %)

[0051] If the value on the middle side of Inequality (ii) is less than 27.0, Ni, Cr, Mo, and Nb will not be sufficiently dissolved, and the creep strength will not be improved. Therefore, the value on the middle side of Inequality (ii) is to be 27.0 or more. The value on the middle side of Inequality (ii) is preferably 29.0 or more, and more preferably 31.0 or more.

[0052] On the other hand, if the value on the middle side of Inequality (ii) is 40.5 or more, Nb will be too much dissolved among the above elements, and the amount of Nb that combines with dissolved C to form precipitates cannot be sufficiently ensured, and Inequality (i) may not be satisfied. Therefore, the value on the middle side of Inequality (ii) is to be less than 40.5. The value on the middle side of Inequality (ii) is preferably 39.0 or less, more preferably 37.0 or less, and even more preferably 35.0 or less.4. Inequality (iii)

[0053] Nb ER < 0.052

[0054] Here, each symbol in the above Inequality is defined as follows Nb ER : Nb content in precipitates obtained by extraction residue analysis (mass %)

[0055] In the austenitic stainless steel of the present embodiment, a certain amount of Nb is left dissolved in the steel to improve the steam oxidation resistance. Normally, the formation of a machined layer by shot processing, etc. is effective in improving the steam oxidation resistance. However, when a composition with improved creep strength and SCC resistance is to be produced, simply adding a machined layer does not sufficiently improve the steam oxidation resistance.

[0056] It is considered that by limiting the amount of Nb ER , which is the amount of Nb in a precipitated state, to a certain amount and ensuring the amount of dissolved Nb, the dissolved Nb promotes the formation of Cr oxide in the service environment. Cr oxide grows slowly. On the other hand, the dissolved Nb suppresses excessive penetration of O into the steel, thereby suppressing the formation of other oxides. Therefore, it is considered that sufficient time can be provided for the growth of Cr oxide. Therefore, in order to improve the steam oxidation resistance, it is effective to provide a machined layer by shot processing or the like after ensuring the amount of dissolved Nb.

[0057] In addition, as mentioned above, in the austenitic stainless steel of the present embodiment, C is combined with elements such as V, Ti, and Nb at the pre-use stage and precipitated in advance as precipitates. In particular, in terms of creep strength and steam oxidation resistance, it is preferable for Nb to be precipitated as Nb carbonitride with Ti, while a certain amount of Nb is present as dissolved Nb. Figure 1 shows a micrograph of the Nb compound collected and observed from precipitates obtained by extractive residue analysis of the austenitic stainless steel of the present embodiment. Analysis of this micrograph reveals that the above Nb compound is an Nb carbonitride with Ti.

[0058] Here, Nb may combine with N, Cr, etc. in steel to form fine NbCr nitrides. When these fine NbCr nitrides are formed, the Nb ER value increases and the amount of dissolved Cr decreases. As a result, the austenitic stainless steels are less likely to satisfy Inequality (ii). Therefore, Nb should be controlled so that Nb becomes Nb carbonitride as described above instead of NbCr nitride.

[0059] Based on the above, the austenitic stainless steel of the present embodiment satisfies Inequality (iii). If the austenitic stainless steel does not satisfy Inequality (iii), i.e., if the Nb ER is 0.052 or more, it will be difficult to improve the steam oxidation resistance even if a machined layer is subsequently formed. In addition, NbCr nitrides will be formed, making it difficult to form the desired Nb carbonitrides. The Nb ER is preferably 0.050 or less, more preferably 0.045 or less, even more preferably 0.040 or less, and most preferably 0.035 or less.

[0060] Note that the content (mass %) of each element in the precipitates obtained by extraction residue analysis, as described in items 2 to 4 above, can be determined by the following procedure. Specifically, approximately 0.4 g of the sample is electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride / methanol at a current value of 20 mA / cm 2< . The electrolyzed solution of the sample is then filtrated through a 0.2 µm filter, and the residue is acid-decomposed. The amount (mass %) analyzed as electrolytic extraction residue is then calculated for the above elements using an ICP optical emission spectrometer.5. Inequality (iv)

[0061] In the austenitic stainless steel of the present embodiment, in order to sufficiently improve the steam oxidation resistance, a machined layer is provided near the surface by shot processing or the like. In addition to reducing the Nb ER to less than 0.052 as mentioned above, providing a machined layer causes a recrystallized microstructure to be formed during use at high temperatures. This recrystallized microstructure has a very fine grain size, which facilitates grain boundary diffusion of Cr.

[0062] This allows sufficient supply of Cr necessary for the formation of Cr oxides and facilitates the formation of Cr oxide scale, which consists mainly of Cr oxide Cr 2 O 3 , in the initial stage of use. Since this Cr oxide scale is formed very thin and uniformly, the surface is highly protected and difficult to peel off. Even if there are areas where the scale peels off, the scale in these areas will reform and repair itself. Therefore, the steam oxidation resistance can be improved.

[0063] The machined layer is a structure the surface vicinity of which is hardened. Therefore, the following Inequality (iv) is satisfied. 0.5 ≤ Hv 40 − Hv t / 2 / Hv t / 2 wherein, each symbol in the above Inequality is defined as follows: Hv 40 : Vickers hardness measured at a position 40 µm from the surface in the thickness direction with a test load of 10 gf. Hv t / 2 : Vickers hardness measured with a test load of 10 gf at a position (1 / 2)t from the surface in the thickness direction when the total thickness is t.

[0064] The value on the right side of Inequality (iv) is called the hardness increase rate, and the larger this value is, the more processing has been introduced. If the value on the right side of Inequality (iv) is 0.5 or more, recrystallization will occur in the machined layer formed in the range from the surface to 40 µm in the thickness direction during use at high temperatures. As a result, even if peeling occurs on the scale during use at high temperatures, the peeled area can be repaired. Therefore, the value on the right side of Inequality (iv) is to be 0.5 or more.

[0065] The greater the degree of processing applied to the vicinity of the steel surface, i.e., the larger the value on the right side of Inequality (iv), the finer the crystalline structure that is formed. The fine crystalline structure also facilitates the supply of Cr to the surface at high temperatures. This prevents abnormal oxidation, especially in the initial stage of use, and contributes to the uniform formation of Cr oxide scale. In this respect, it is preferred that the value on the right side of Inequality (iv) be 0.6 or more, more preferably 0.7 or more.

[0066] Note that the upper limit of the right side of Inequality (iv) is not particularly limited. However, the larger the value on the right side of Inequality (iv) becomes, the harder the surface becomes, which may cause problems in forming, welding, and other processes. For this reason, it is preferable that the value on the right side of Inequality (iv) be 2.0 or less.

[0067] As for Hv t / 2 , in order to obtain good creep strength, for example, the Vickers hardness preferably ranges from 100 to 245Hv.

[0068] The above-mentioned Hv 40 and Hv t / 2 can be measured by the following procedure. A square test specimen of 15 mm is cut out, embedded in resin, and the cross section is cut and mirror-polished. The test specimen is measured for Vickers hardness with a load of 10 gf at a position 40 µm from the surface in the thickness direction and at a position t / 2 from the surface in the thickness direction (when the total thickness is t) so that they are on the same straight line in the thickness direction. This measurement is repeated at different locations where the measurement at other locations on the specimen is not affected, so that the number of measurements at each position is five. The average of the five measurements at 40 µm in the thickness direction is then taken as Hv 40 . Similarly, the average of the five measurement results at a position t / 2 in the thickness direction is Hv t / 2 .

[0069] In the case of a steel plate, measurements are taken at a position 40 µm from the surface in the thickness direction and at a position t / 2 from the surface in the thickness direction so that they are on the same straight line in the thickness direction, and these measurements are also taken at other locations so that the total number of measurements at each position is 5 times. In the case of a steel tube the above measurements are taken at a position 40 µm from the inner surface of the tube in the direction of the wall thickness and at a position t / 2 from the inner surface of the tube in the direction of the wall thickness (when the total thickness is t). The reason for measuring the hardness of the inner surface of the steel tube is that when the tube is used as a boiler heat transfer tube, high-temperature steam passes through the inner surface of the tube; therefore, the inner surface of the tube is required to be especially resistant to steam oxidation.6. Thickness and Type

[0070] The thickness of the austenitic stainless steel of the present embodiment is preferably in a range of 2 to 100 mm, considering its application. When the austenitic stainless steel is in the form of a tube, the wall thickness is preferably in a range of 2 to 95 mm. When the austenitic stainless steel is in the form of steel plate, the thickness is preferably in a range of 2 to 35 mm. The austenitic stainless steel may be, for example, a steel plate or a steel tube.7. Manufacturing Method

[0071] A method for manufacturing the austenitic stainless steel of the present embodiment is described below. As a result of research conducted to date, the present inventors have confirmed that the austenitic stainless steel of the present embodiment can be manufactured by the following method.7-1. Melting

[0072] Steel having the above-mentioned chemical composition is melted, an ingot is produced by continuous casting or the like, and the produced ingot is bloomed into a billet. Various conditions for producing a billet can be set according to a conventional method.7-2. Hot Working

[0073] The resulting billet is then hot worked. There are no restrictions on the hot working conditions, but in order to prevent harmful defects from occurring during tube making, for example, the billet is heated to a temperature in a range of 900 to 1300°C before hot working. In addition, there are no restrictions on the type of hot working. Hot rolling can be used to manufacture steel plates, and hot extrusion can be used to manufacture steel tubes.

[0074] Here, the hot-worked steel is quenched under the following conditions.

[0075] Time from the completion of hot working to start of quenching: 5.0 minutes or less Temperature of the steel at the start of quenching: 700°C or higher Cooling rate from the completion of hot working to the start of quenching: 15°C / min or more

[0076] The time (minutes) between the completion of hot working and the start of quenching is called "leaving time." When steel is quenched after hot working, it is usually quenched (water cooled) by a water cooling system. In other words, the "leaving time" refers to the time from the completion of hot working to the time when the steel is transported to a water cooling device and water cooling is started. If the leaving time exceeds 5.0 minutes, coarse precipitates will be formed. It is also more likely to form NbCr nitrides rather than the desired Nb carbonitrides.

[0077] The precipitates cannot be dissolved even in the softening process described below; as a result, precipitates such as nitrides, Cr carbides, and NbCr nitrides are coarsely formed, and Inequality (ii) is not satisfied. As a result, the creep strength is reduced. Therefore, the leaving time should be 5.0 minutes or less. The leaving time is preferably 4.5 minutes or less, more preferably 4.0 minutes or less, and even more preferably 3.5 minutes or less.

[0078] The temperature (°C) of the steel at the start of quenching is referred to as the "quench start temperature." If the quench start temperature is less than 700°C, precipitates such as Cr carbides will be coarsely formed. Also, NbCr nitrides are more likely to be formed than the desired Nb carbonitrides. Then, even with softening treatment, the precipitates cannot be dissolved in the matrix phase, and Inequality (ii) is no longer satisfied. As a result, the creep strength decreases. Therefore, the quench start temperature should be 700°C or higher. The quench start temperature is preferably set at 750°C or higher, more preferably 780°C or higher, even more preferably higher than 790°C, and extremely preferably at 800°C or higher.

[0079] If the cooling rate (°C / min) from the completion of hot working to the start of quenching is less than 15°C / min, precipitates such as nitrides and Cr carbides will be coarsely formed. Then, Inequality (ii) is no longer satisfied. As a result, the creep strength (or SCC resistance) decreases. Therefore, the cooling rate from the completion of hot working to the start of quenching should be 15°C / min or more, preferably 18°C / min or more, and more preferably 20°C / min or more. The above cooling rate is the difference in the surface temperature between the immediately after the completion of hot working and immediately before the start of quenching, divided by the leaving time.7-3. Softening Treatment

[0080] Variations in steel quality in the longitudinal and wall thickness directions caused by hot working are reduced by softening treatment. Without softening treatment, coarse precipitates formed by hot working are not sufficiently dissolved in the matrix phase. Then, in the solution heat treatment described below, insufficient precipitation of Ti, V, and Nb occurs in some areas. As a result, the value on the right side of Inequality (i) becomes smaller than that of the left side, and the SCC resistance may decrease. Therefore, softening treatment should be performed as a process after hot working to homogenize the steel quality. The holding temperature in the softening treatment (hereinafter referred to as "softening treatment temperature T 1 ") is above the recrystallization temperature and below the grain boundary melting temperature. In the softening process, it is recommended to perform soaking at 1040 to 1300°C.

[0081] If the softening treatment temperature T 1 is less than 1040°C, Inequality (i) will not be satisfied and the SCC resistance will be reduced. Therefore, the softening temperature T 1 should be 1040°C or higher. The softening treatment temperature T 1 is preferably 1100°C or higher, and more preferably 1150°C or higher. On the other hand, if the softening treatment temperature T 1 exceeds 1300°C, the crystal grains tend to become coarse. Therefore, the softening treatment temperature T 1 is should be 1300°C or less. The softening treatment temperature T 1 is preferably 1290°C or less, and more preferably 1280°C or less.

[0082] The time of softening treatment is not limited, but from the viewpoint of recrystallization in the thickness direction and production cost, 1 to 10 minutes is preferred. If the softening treatment time is less than 1 minute, the SCC resistance may be reduced. On the other hand, if the softening treatment time exceeds 10 minutes, the crystal grains may become coarse.7-4. First Cooling

[0083] After softening treatment, the steel is cooled. This cooling is referred to as first cooling. In the first cooling, the steel is cooled with water or at a cooling rate equal to or faster than water cooling. If the steel is not cooled with water or at a cooling rate equal to or faster than water cooling, a non-uniform microstructure will be formed due to the formation and growth of precipitates. As a result, subsequent cold working and solution heat treatment may also drag on the non-uniformity, resulting in inconsistent product quality. For this reason, water cooling or a cooling rate equal to or faster than water cooling should be used in the first cooling. This cooling is usually done to a temperature of 700°C or less. The cooling rate in water cooling is usually 2 to 8°C / s.7-5. Cold Working

[0084] After the first cooling, the steel is cold worked. Cold working is necessary to achieve dimensional accuracy according to specification. As with hot working, there are no restrictions on the type of working. The cross-sectional area reduction rate of the steel due to cold working is not limited, but in normal cold working, the cross-sectional area reduction rate is approximately 90% or less.

[0085] In steel tube manufacturing, for example, drawing can be used as a cold working process to produce a specified tube shape.7-6. Solution Heat Treatment

[0086] After the above cold working, the steel is subjected to solution heat treatment at 1100 to 1200°C. In the following description, the soaking temperature in the solution heat treatment is referred to as the solution treatment temperature T 2 . If the solution treatment temperature T 2 is less than 1100°C, the dissolution of elements effective for improving creep strength will be insufficient, and Inequality (ii) will not be satisfied, resulting in lower creep strength. If the solution treatment temperature T 2 exceeds 1200°C, the elements to be precipitated may not satisfy Inequality (i) and / or Inequality (ii) due to excessive dissolution. In order to improve SCC resistance and to increase the value on right side of Inequality (i) to 0.012 or more, the solution treatment temperature T 2 is preferably 1170°C or less.

[0087] Comparing the softening treatment temperature T 1 and the solution treatment temperature T 2 , the softening treatment temperature T 1 is to be controlled to be higher than the solution treatment temperature T 2 . In other words, the softening temperature T 1 is controlled to satisfy the following Inequality (a). If the solution treatment temperature T 2 is higher than the softening temperature T 1 , it will be difficult to improve the SCC resistance sufficiently. T 1 > T 2

[0088] Here, each symbol in the above Inequality (a) is defined as follows. T 1 : Softening treatment temperature (°C) T 2 : Solution treatment temperature (°C)

[0089] The difference between the softening temperature T 1 and the solution temperature T 2 should be 150°C or less. That is, the temperature should be controlled so as to satisfy the following Inequality (b). T 1 − T 2 ≤ 150

[0090] Here, each symbol in the above Inequality (b) is defined as follows. T 1 : Softening treatment temperature (°C) T 2 : Solution treatment temperature (°C)

[0091] If the difference between the softening treatment temperature T 1 and the solution treatment temperature T 2 exceeds 150°C, it will be difficult to satisfy Inequality (iii), and the steam oxidation resistance is likely to decrease.

[0092] The solution heat treatment time is preferably less than 10 minutes. If the solution heat-treatment time is 10 minutes or more, the elements to be precipitated will be excessively dissolved, and Inequality (i) and / or Inequality (ii) may not be satisfied. The lower limit of the solution heat treatment time is not particularly limited, but is usually 1 minute.7-7. Second Cooling

[0093] After solution heat treatment, the steel is cooled. This cooling is referred to as second cooling. In the second cooling, the steel is cooled with water or at a cooling rate equal to or faster than water cooling, as in the first cooling. This is because if the steel is not cooled with water or at a cooling rate equal to or faster than water cooling, the creep strength will be reduced and product quality may not be consistent due to the formation and growth of precipitates. It is usually preferable to cool the steel to 600°C or less.7-8. Machined Layer Formation

[0094] Next, a machined layer is formed in the vicinity of the surface of the steel that has undergone the second cooling. The method of forming a machined layer is not limited. For example, various known spraying methods such as shot peening, shot blasting, shot processing, sand blasting, sanding, air blasting, water jetting, etc. can be used. There are also no restrictions on the material, shape, etc. of the particles to be sprayed. For example, steel, cast steel, stainless steel, glass, silica sand, alumina, amorphous, etc. can be used as materials.

[0095] The shape of the above particles can be, for example, spherical, cut wire, grit, etc. The particles may be blown by compressed air, centrifugal force using an impeller (impeller type), high-pressure water, ultrasonic waves, etc. Particles can also be mixed with liquid and sprayed with compressed air, etc. (also called "liquid honing").

[0096] In addition, it is also possible to provide a machined layer by polishing, ball milling, grinding, honing, or ultrasonic impact processing. Especially when stable steam oxidation resistance is required over a long period of time at high temperatures, it is preferable to form a machined layer by particle spraying, which facilitates uniform processing over the entire surface. The machined layer is formed on a pickled surface. In other words, the machined layer is formed after pickling. This is because pickling improves the surface condition and facilitates surface deformation.EXAMPLE

[0097] The present embodiment is described in more detail below with reference to examples, but the present embodiment is not limited to these examples.

[0098] Steels having the chemical compositions shown in Table 1 were melted, bloomed, and formed into hollow billets. The billet was hot worked into a tube shape of one size (outer diameter 56 mm × wall thickness 10.5 mm × length 6684 mm) and quenched under the conditions shown in Table 2. Then, except for the case of No. 23, the tube was subjected to softening treatment by soaking for 3 minutes under the conditions shown in Table 2, water cooled, and cold worked at a cross-sectional reduction ratio of 35.7%. In other words, cold working was performed to obtain a wall thickness of 8.35 mm. All softening treatment temperatures T 1 were above the recrystallization temperature and below the grain boundary melting temperature. Solution heat treatment was then performed under the conditions shown in Table 2, followed by water cooling to obtain a steel tube. For the obtained steel tubes, in some cases, shot processing was performed on the inner surface of the tubes to form a machined layer. For the machined layer, conditions such as injection pressure, injection amount, injection angle, and nozzle shape were controlled and adjusted. The surface of the steel tube when shot processing was performed was the surface after pickling, except for No. 33.

[0099] The obtained steel tubes were subjected to electrolytic extraction residue measurement, hardness measurement (Hv 40 and Hv t / 2 ), SCC test, creep test, and steam oxidation test according to the procedures described below.(Electrolytic extraction residue)

[0100] The following procedure was used to determine and the content (mass %) of each element in the precipitates obtained by extraction residue analysis for V, Ti, Nb, Ni, Cr, and Mo, and to calculate the value on the right side of Inequality (i), the value on the middle side of Inequality (ii), and the value on the left side of Inequality (iii).

[0101] Specifically, approximately 0.4 g of the specimen was electrolyzed using 10% acetylacetone-1% tetramethylammonium chloride / methanol at a current value of 20 mA / cm 2< . The solution of the electrolyzed sample was then filtrated through a 0.2 µm filter, and the residue was acid decomposed with a mixture of sulfuric acid + phosphoric acid + nitric acid + perchloric acid. The amount (mass%) of each element in the electrolytic extraction residue was then calculated using an ICP emission spectrometer.(Hardness)

[0102] A 15 mm square specimen was cut out, the specimen was embedded in resin, and a cross section was cut and mirror polished. The Vickers hardness of each specimen was measured at a position 40 µm from the inner surface of the steel tube in the direction of the wall thickness and at a position t / 2 from the inner surface of the tube in the direction of the wall thickness in such a way that the positions are on the same straight line. The test load was 10 gf. The measurements were repeated at different locations on the specimen where other measurements were not affected, and the number of measurements at each position was five. The average of the five measurements at a position 40 µm in the wall thickness direction was determined as Hv 40 , and the average of the five measurements at a position t / 2 in the wall thickness direction was determined as Hv t / 2 .(SCC test)

[0103] SCC resistance was evaluated by performing SCC tests in accordance with ASTM A262 Method E. Specifically, a 15 mm × 2 mm × 70 mm test specimen was taken from one end of the resulting austenitic stainless steel. The test specimen was subjected to a sensitization heat treatment at 700°C for 30 minutes, then approximately 70 g of copper chips were placed in a sulfuric acid / copper sulfate solution as specified in the above standard, and the test specimen was immersed in this boiling solution for 24 hours to perform a bending test. If there were no cracks in the test specimen after the test, and furthermore, even if the immersion time was set to 48 hours and a bending test was performed without any cracks, the SCC resistance was rated as very good (A). If cracking occurred after 48 hours of immersion, but no cracking occurred after 24 hours of immersion, the SCC resistance was rated as good (B). If cracks occurred even after 24 hours of immersion, the SCC resistance was rated as poor (D).(Creep test)

[0104] A creep test was performed to evaluate creep strength. Specifically, a round bar creep test specimen was taken and a creep rupture test was performed. The creep properties were rated as very good (A) if the rupture strength was 210 MPa or more in a creep test at 600°C for 10,000 hours and also the rupture strength was 150 MPa or more in a creep test at 650°C for 10,000 hours.

[0105] Except for those whose creep properties were rated as very good (A) above, the creep properties were rated as good (B) if only one of the following was satisfied: either the ultimate tensile strength was 210 MPa or higher in a creep test at 600°C for 10,000 hours, or the ultimate tensile strength was 150 MPa or higher in a creep test at 650°C for 10,000 hours. In addition, the creep properties were rated as poor (D) except for the cases of very good creep properties (A) and good creep properties (B) as described above.(Steam oxidation test)

[0106] A rectangular test specimen measuring 2 mm thick × 10 mm wide × 25 mm long was cut from the obtained steel tube so that the inner surface of the tube became a part of the specimen surface. The test specimen was held suspended on a jig and placed in a tubular horizontal heating furnace, where it was subjected to an oxidation test at 650°C for 1000 hours in a steam atmosphere containing 100 ppb of dissolved oxygen. After cooling in the furnace, the test specimens were taken out and embedded in resin, and the cross sections were cut and mirror-polished. The cross sections of the oxide scale formed on the inner surfaces of the steel tubes were then observed under an optical microscope to measure the thickness of the scale. The scale thickness was measured at 500× magnification in 10 optional fields of view, and the average of the thicknesses was calculated. When the scale thickness was 10 µm or less, the steam oxidation resistance was rated as very good (B), when it was more than 10 µm and 50 µm or less, the steam oxidation resistance was rated as slightly poor (C), and when it was 50 µm or more, the steam oxidation resistance was rated as poor (D). The results are summarized in Table 3 below.

[0107] Test Nos. 1 to 20, 21 to 23, 24, 29, 31 to 33, 26, 37, and 38 were rated "A" with no cracking in the SCC resistance test, whether the immersion time was 24 hours or 48 hours. These examples are considered to have exhibited better SCC resistance because the value on the right side of Inequality (i) was 0.012 or more. Test Nos. 5 and 6 did not crack in the SCC resistance test when the immersion time was 24 hours, and were rated "B". This is considered to be because the value on the right side of Inequality (i) was less than 0.012 but 0.010 or more, which means that C was fixed even after the sensitization heat treatment by precipitation of V, Ti and Nb, which suppressed the formation of grain boundary Cr carbides and prevented cracking.

[0108] On the other hand, Test Nos. 25 to 28, 30, and 34 to 36 cracked in the SCC resistance test and were rated "D". These examples did not satisfy Inequality (i), and therefore it is considered that the precipitation of V, Ti, and Nb was insufficient, and after the sensitization heat treatment, Cr carbides were formed at the grain boundaries, and a Cr-depleted layer was formed around the carbides, causing cracks in the SCC resistance test. The reason for this is considered to be that the chemical composition, softening treatment temperature, or solution treatment temperature was out of the desired range.

[0109] Test Nos. 1, 3, 8, 10, 12 to 17, 20 to 22, and 31 were rated "A" in the creep test. These examples are considered to have excellent creep strength because the value on the middle side of Inequality (ii) was equal to or greater than the more preferable lower limit. Test Nos. 2, 4 to 7, 9, 11, 18, 23, 25, 29, 30, 32, and 33 were rated "B" in the creep test. In these examples, although the value on the middle side of Inequality (ii) is less than the more preferable lower limit, it is considered that the Inequality (ii) is satisfied and therefore the elements contributing to the creep strength are sufficiently dissolved in solid solution, thereby achieving a good creep strength sufficient to be worthy of the present invention.

[0110] On the other hand, Test Nos. 19, 24, and 26 to 28 were rated "D" in the creep test. In these cases, the value on the middle side of Inequality (ii) was less than 27.0, and it is considered that sufficient creep strength was not obtained due to insufficient dissolution of Ni, Cr, Mo, and Nb, which improves creep strength. Among these, Test Nos. 24, 26, and 27 did not satisfy Inequality (iii), which is considered to have resulted in the formation of NbCr nitrides and a decrease in creep strength.

[0111] Test Nos. 1 to 23, 25, and 28 to 30 were rated "B" in the steam oxidation test. These examples satisfied Inequalities (iii) and (iv), and are therefore considered to have good steam oxidation resistance. Test Nos. 24, 26, 27, and 31 were rated "C" in the steam oxidation test. These examples satisfied Inequality (iv), but did not satisfy Inequality (iii), and are therefore considered to have failed to form a good machined layer, resulting in poor steam oxidation resistance. Test Nos. 32 and 33 were rated "D" in the steam oxidation test. These examples were considered to have failed to form a sufficient machined layer because the conditions for forming the machined layer were not within the preferred range.(Appendix)

[0112] (1) An austenitic stainless steel having a chemical composition consisting, by mass percent, 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 to 20.00%, Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N: 0.05 to 0.15%, B: 0.0010 to 0.0060%, V: 0 to 0.50%, Ti: 0 to 0.500%, Co: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.40%, Sn: 0 to 0.0300%, Al: 0 to 0.035%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0800%, and a balance being Fe and impurities; and satisfying the following Inequalities (i) to (iv): 0.010 ≤ V ER + Ti ER + Nb ER 27.0 ≤ 1.13 Ni − Ni ER + Cr − Cr ER + 1.85 Mo − Mo ER + 1.79 Nb − Nb ER < 40.5 Nb ER < 0.052 0.5 ≤ Hv 40 − Hv t / 2 / Hv t / 2 where, each symbol in the above Inequalities is defined as follows, and each element symbol in the above Inequalities represents the content (mass %) of each element in the steel, or zero if not contained. V ER : V content in precipitates obtained by extraction residue analysis (mass %) Ti ER : Ti content in precipitates obtained by extraction residue analysis (mass %) Nb ER : Nb content in precipitates obtained by extraction residue analysis (mass %) Ni ER : Ni content in precipitates obtained by extraction residue analysis (mass %) Cr ER : Cr content in precipitates obtained by extraction residue analysis (mass %) Mo ER : Mo content in precipitates obtained by extraction residue analysis (mass %) 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 / 2)t from the surface in the thickness direction when the total thickness is t. (2) The austenitic stainless steel as described in (1) above, wherein the chemical composition contains, by mass percent, one or more elements selected from: V: 0.01 to 0.50%, and Ti: 0.001 to 0.500%. (3) The austenitic stainless steel as described in (1) or (2) above, wherein the chemical composition contains, by mass percent, one or more elements selected from: Co: 0.02 to 1.00%, W: 0.01 to 1.00%, Ta: 0.01 to 0.40%, Sn: 0.0010 to 0.0300%, Al: 0.0005 to 0.035%, Ca: 0.0010 to 0.0100%, Mg: 0.0002 to 0.0100%, and REM: 0.0010 to 0.0800%. (4) The austenitic stainless steel as described in any one of (1) to (3) above, wherein the steel is a steel tube and the surface is the inner surface of the steel tube.

Claims

1. An austenitic stainless steel having a chemical composition consisting, by mass percent, 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 to 20.00%, Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N: 0.05 to 0.15%, B: 0.0010 to 0.0060%, and a balance being Fe and impurities; and satisfying the following Inequalities (i) to (iv): 0.010 ≤ V ER + Ti ER + Nb ER 27.0 ≤ 1.13 Ni − Ni ER + Cr − Cr ER + 1.85 Mo − Mo ER + 1.79 Nb − Nb ER < 40.5 Nb ER < 0.052 0.5 ≤ Hv 40 − Hv t / 2 / Hv t / 2 where, each symbol in the above Inequalities is defined as follows, and each element symbol in the above Inequalities represents the content (mass %) of each element in the steel, or zero if not contained. VER: V content in precipitates obtained by extraction residue analysis (mass %) TiER: Ti content in precipitates obtained by extraction residue analysis (mass %) NbER: Nb content in precipitates obtained by extraction residue analysis (mass %) NiER: Ni content in precipitates obtained by extraction residue analysis (mass %) CrER: Cr content in precipitates obtained by extraction residue analysis (mass %) MoER: Mo content in precipitates obtained by extraction residue analysis (mass %) Hv40: Vickers hardness measured with a test load of 10 gf at a position 40 µm from the surface in the thickness direction. Hvt / 2: Vickers hardness measured with a test load of 10 gf at a position (1 / 2)t from the surface in the thickness direction when the total thickness is t.

2. An austenitic stainless steel having a chemical composition consisting, by mass percent, 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 to 20.00%, Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N: 0.05 to 0.15%, B: 0.0010 to 0.0060%, one or more elements selected from the group consisting of following groups A and B, and a balance being Fe and impurities; and satisfying the following Inequalities (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 27.0 ≤ 1.13 Ni − Ni ER + Cr − Cr ER + 1.85 Mo − Mo ER + 1.79 Nb − Nb ER < 40.5 Nb ER < 0.052 0.5 ≤ Hv 40 − Hv t / 2 / Hv t / 2 where, each symbol in the above Inequalities is defined as follows, and each element symbol in the above Inequalities represents the content (mass %) of each element in the steel, or zero if not contained. VER: V content in precipitates obtained by extraction residue analysis (mass %) TiER: Ti content in precipitates obtained by extraction residue analysis (mass %) NbER: Nb content in precipitates obtained by extraction residue analysis (mass %) NiER: Ni content in precipitates obtained by extraction residue analysis (mass %) CrER: Cr content in precipitates obtained by extraction residue analysis (mass %) MoER: Mo content in precipitates obtained by extraction residue analysis (mass %) Hv40: Vickers hardness measured with a test load of 10 gf at a position 40 µm from the surface in the thickness direction. Hvt / 2: Vickers hardness measured with a test load of 10 gf at a position (1 / 2)t from the surface in the thickness direction when the total thickness is 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, wherein the steel is a steel tube and the surface is the inner surface of the steel tube.

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