Austenitic stainless steel

Abstract

To provide an austenitic stainless steel having excellent fatigue properties under hydrogen environments.SOLUTION: There is provided an austenitic stainless steel which has a base material and an oxide film and has a base material chemical composition comprising, by mass%, 0.080% or less of C, 0.10 to 1.0% of Si, 2.00% or less of Mn, 0.050% or less of P, 0.020% or less of S, 17.0 to 20.0% of Cr, 8.0 to 13.0% of Ni, 0.300% or less of Al, 0.250% or less of N, an optional element and the balance Fe with impurities, wherein the Si content at the depth of 1 nm from the surface of the oxide film is 15% or more.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, hydrogen gas has been attracting attention as a new energy source to replace fossil fuels. Hydrogen gas is a clean energy source that does not emit CO2. However, hydrogen gas can cause hydrogen embrittlement, which weakens materials. Therefore, Patent Document 1 discloses an austenitic stainless steel with improved resistance to hydrogen gas embrittlement. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-196842 Summary of the Invention [Problem to be solved by the invention]

[0004] Parts manufactured in a hydrogen environment include, for example, steel pipes used in hydrogen gas production equipment, etc. When austenitic stainless steel is used in hydrogen gas production equipment, etc., fatigue properties in a hydrogen environment become an issue.

[0005] In the austenitic stainless steel disclosed in Patent Document 1, sufficient consideration has not been given to fatigue properties in a hydrogen environment, and there is still room for improvement.

[0006] An object of the present invention is to solve the above problems and to provide an austenitic stainless steel that has excellent fatigue properties in a hydrogen environment. [Means for solving the problem]

[0007] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following austenitic stainless steel.

[0008] (1) An austenitic stainless steel having a base material and an oxide film formed on the surface of the base material, The chemical composition of the base material is, in mass%, C: 0.080% or less, Si: 0.10 to 1.0% Mn: 2.00% or less, P: 0.050% or less, S: 0.020% or less, Cr: 17.0~20.0%, Ni: 8.0-13.0% Al: 0.300% or less, N: 0.250% or less, Nb: 0 to 0.20% Ti: 0 to 0.20% Mo: 0-1.00%, Cu: 0-1.0% Co: 0 to 0.50% V: 0~0.50%, W: 0~0.50%, B: 0~0.0050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.50% Ga: 0 to 0.050%, Hf: 0 to 0.10% REM: 0~0.10%, The balance is Fe and impurities. Using glow discharge optical emission spectroscopy, the concentration changes of O, Fe, Cr, Mn, Ni, Mo, Nb, Ti, Si, Al and N were measured in the depth direction from the outermost surface of the austenitic stainless steel, and when the total amount of other elements excluding O was converted to 100% by mass, The Si content at a depth of 1 nm from the surface of the oxide film is 15% or more. Austenitic stainless steel.

[0009] (2) The chemical composition is in mass%: Nb: 0.01 to 0.20%, Ti: 0.01 to 0.20%, Mo: 0.05 to 1.00%, Cu: 0.05 to 1.0% Co: 0.01 to 0.50%, V: 0.05 to 0.50%, W: 0.05 to 0.50%, B: 0.0002~0.0050%, Ca: 0.0002 to 0.0100%, Mg: 0.0002 to 0.0100%, Zr: 0.01 to 0.50%, Ga: 0.001 to 0.050%, Hf: 0.01 to 0.10%, and REM: 0.01~0.10%, Contains one or more selected from The austenitic stainless steel according to (1) above.

[0010] (3) The chemical composition is in mass%: Si: 0.40% or more, and Nb+Ti: 0.10% or more, The austenitic stainless steel according to (2) above. [Effects of the Invention]

[0011] According to the present invention, an austenitic stainless steel having excellent fatigue properties in a hydrogen environment can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have investigated methods for improving the fatigue properties of austenitic stainless steel pipes in a hydrogen environment while reducing the amount of expensive alloying elements, and have obtained the following findings.

[0013] (a) Hydrogen embrittlement of austenitic stainless steels is likely to occur in hydrogen environments at temperatures between -100 and -40°C. This is thought to be because the austenite phase (hereinafter also referred to as the "γ phase") becomes unstable in this temperature range and transforms into the brittle and weak α' phase due to strain accumulation, etc. Therefore, in order to improve the stability of the γ phase, it is conceivable to increase the content of additive elements such as Ni, Cu, and Mn. However, increasing the content of such elements and increasing the alloying level increases the alloying cost.

[0014] (b) In the normal steel manufacturing process and in a hydrogen environment, hydrogen penetrates into the steel, resulting in trace amounts of hydrogen being trapped inside the steel. If this hydrogen in the steel accumulates, for example, in areas where strain has accumulated, it promotes hydrogen embrittlement and reduces the fatigue properties of the steel pipe. Therefore, it is desirable for the hydrogen in the steel to be released, but it is difficult to release it outside the steel in the presence of hydrogen at low temperatures, which is the environment in which the steel will be used.

[0015] (c) Therefore, the present inventors have investigated methods for suppressing the penetration of hydrogen into steel, and have found that forming an oxide film mainly composed of Cr2O3 with Si oxide concentrated in the outermost layer on the surface of the base material is effective in suppressing hydrogen penetration.

[0016] (d) In order to form such an oxide film, it is effective to perform the final annealing in a hydrogen gas atmosphere, which has a higher dew point than usual, and then to control the subsequent cooling rate to a low value.

[0017] An embodiment of the present invention has been made based on the above findings. Each requirement of this embodiment will be described in detail below.

[0018] 1. Overall structure The austenitic stainless steel according to this embodiment has a base material and an oxide film formed on the surface of the base material.

[0019] 2. Chemical composition of the base material The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0020] C: 0.080% or less C is an element effective in stabilizing the austenite phase and also contributes to improving hydrogen gas embrittlement resistance. However, excessive C content promotes grain boundary precipitation of Cr-based carbides, making them more likely to form fracture initiation points. As a result, hydrogen gas embrittlement resistance actually decreases. For this reason, the C content is set to 0.080% or less. The C content is preferably set to 0.075% or less, and more preferably set to 0.070% or less. On the other hand, to obtain the above effects, the C content is preferably set to 0.010% or more, and more preferably set to 0.020% or more.

[0021] Si: 0.10 to 1.0% Si is an effective element for deoxidation, and by concentrating in the outermost layer of the oxide film, it also contributes to improving resistance to hydrogen gas embrittlement. Therefore, the Si content is set to 0.10% or more. The Si content is preferably set to 0.20% or more, and more preferably set to 0.40% or more. However, excessive Si content promotes the formation of intermetallic compounds such as the σ phase, reducing hot workability and toughness. Therefore, the Si content is set to 1.0% or less. The Si content is preferably set to 0.80% or less, and more preferably set to 0.60% or less.

[0022] Mn: 2.00% or less Mn is an element effective in stabilizing the austenite phase and contributes to improving hydrogen gas embrittlement resistance. However, excessive Mn content promotes the formation of the ε phase, which is highly susceptible to hydrogen embrittlement, thereby reducing hydrogen gas embrittlement resistance. Furthermore, excessive precipitation of MnS leads to a decrease in hydrogen gas embrittlement resistance. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably set to 1.80% or less, and more preferably set to 1.60% or less. To achieve the above effects, the Mn content is preferably set to 0.50% or more, more preferably set to 0.70% or more, and preferably set to 1.00% or more.

[0023] P:0.050% or less P is an element contained in steel as an impurity, and causes segregation, reducing hydrogen gas embrittlement resistance. Therefore, the P content is set to 0.050% or less. The P content is preferably set to 0.045% or less, and more preferably set to 0.040% or less. On the other hand, excessive reduction of P leads to an increase in manufacturing costs, so the P content is preferably set to 0.010% or more.

[0024] S: 0.020% or less S is an element contained in steel as an impurity, and forms MnS, which reduces hydrogen gas embrittlement resistance. Therefore, the S content is set to 0.020% or less. The S content is preferably set to 0.010% or less, and more preferably set to 0.005% or less. However, excessive reduction of the S content increases manufacturing costs. Therefore, the S content is preferably set to 0.0005% or more.

[0025] Cr: 17.0~20.0% Cr is an element contained in a certain amount in stainless steel and has the effect of improving corrosion resistance. Cr also has the effect of promoting the concentration of Si in the outermost layer. For this reason, the Cr content is set to 17.0% or more. The Cr content is preferably set to 17.5% or more, and more preferably set to 18.0% or more. However, Cr is a ferrite-forming element. Therefore, excessive Cr content destabilizes the austenite phase and reduces hydrogen gas embrittlement resistance. For this reason, the Cr content is set to 20.0% or less. The Cr content is preferably set to 19.5% or less, and more preferably set to 19.0% or less.

[0026] Ni: 8.0 to 13.0% Ni, together with Mn, is an element necessary for ensuring hydrogen gas embrittlement resistance. For this reason, the Ni content is set to 8.0% or more. However, excessive Ni content increases manufacturing costs. Also, segregation is more likely to occur. For this reason, the Ni content is set to 13.0% or less. The Ni content is preferably set to 12.5% ​​or less, more preferably 12.0% or less, and even more preferably 11.5% or less.

[0027] Al: 0.300% or less Al is an effective deoxidizing element, and also has the effect of suppressing grain boundary segregation of low-melting point elements and strengthening the grain boundaries. However, Al is a ferrite-forming element. Therefore, if Al is contained in excess, the austenite phase becomes unstable. Therefore, the Al content is set to 0.300% or less. The Al content is preferably 0.250% or less, and more preferably 0.200% or less. On the other hand, to obtain the above effect, the Al content is preferably 0.001% or more, and more preferably 0.005% or more.

[0028] N: 0.250% or less Like Mn and Ni, N is an element effective in improving hydrogen gas embrittlement resistance. However, excessive N content may cause internal defects such as blowholes during melting, which may actually decrease hydrogen gas embrittlement resistance. Therefore, the N content is set to 0.250% or less. The N content is preferably set to 0.200% or less, and more preferably set to 0.100% or less. On the other hand, to obtain the above effects, the N content is preferably set to 0.010% or more.

[0029] In addition to the above elements, one or more elements selected from Nb, Ti, Mo, Cu, Co, V, W, B, Ca, Mg, Zr, Ga, Hf, and REM may be contained within the ranges shown below. The reasons for limiting the amount of each element will be explained below.

[0030] Nb: 0 to 0.20% Nb forms carbonitrides, refines crystal grains, and strengthens grain boundaries. Furthermore, Nb dissolves in the oxide film formed on the surface of the base material, improving the density of the oxide film. Therefore, Nb may be added as needed. However, excessive Nb content reduces manufacturability and workability during hot rolling. Therefore, the Nb content is set to 0.20% or less. The Nb content is preferably set to 0.15% or less, more preferably 0.10% or less, and even more preferably 0.05% or less. To achieve the above effects, the Nb content is preferably set to 0.01% or more, and more preferably 0.05% or more.

[0031] Ti: 0 to 0.20% Ti forms carbonitrides, refines crystal grains, and strengthens grain boundaries. Furthermore, Ti dissolves in the oxide film formed on the surface of the base material, thereby improving the density of the oxide film. Therefore, Ti may be added as needed. However, excessive Ti content reduces manufacturability during hot rolling. Therefore, the Ti content is set to 0.20% or less. The Ti content is preferably set to 0.15% or less, more preferably 0.10% or less, and even more preferably 0.05% or less. Meanwhile, to achieve the above effects, the Ti content is preferably set to 0.01% or more, and more preferably 0.05% or more.

[0032] From the viewpoint of improving the density of the oxide film and further improving the fatigue properties in a hydrogen environment, it is preferable to contain Nb and / or Ti, and the total content of Nb and Ti is preferably 0.10% or more.

[0033] Mo: 0 to 1.00% Mo has the effect of improving strength. Therefore, it may be contained as necessary. However, excessive content of Mo promotes the formation of the δ-ferrite phase and reduces hydrogen gas embrittlement resistance. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably set to 0.50% or less, and more preferably set to 0.25% or less. On the other hand, in order to obtain the above effect, the Mo content is preferably set to 0.05% or more.

[0034] Cu: 0 to 1.0% Cu is an element that is mixed in from raw materials such as scrap, and is effective in stabilizing the austenite phase. Therefore, it may be added as needed. However, Cu is a low-melting-point element that segregates at grain boundaries, making fracture initiation points more likely to occur. For this reason, the Cu content is set to 1.0% or less. The Cu content is preferably set to 0.70% or less, and more preferably set to 0.50% or less. On the other hand, to obtain the above effects, the Cu content is preferably set to 0.05% or more.

[0035] Co: 0 to 0.50% Co has the effect of improving corrosion resistance and stabilizing the austenite phase. It also has the effect of improving hydrogen gas embrittlement resistance. Therefore, it may be added as needed. However, excessive Co content reduces toughness and workability. For this reason, the Co content is set to 0.50% or less. The Co content is preferably set to 0.40% or less, and more preferably set to 0.30% or less. On the other hand, to obtain the above effects, the Co content is preferably set to 0.01% or more, more preferably set to 0.05% or more, and even more preferably set to 0.10% or more.

[0036] V: 0 to 0.50% V precipitates in steel as a solid solution or carbonitride and has the effect of improving strength. Therefore, V may be added as needed. However, excessive V content will result in excessive formation of carbonitrides, reducing manufacturability during hot rolling. Therefore, the V content is preferably 0.50% or less. The V content is preferably 0.30% or less. On the other hand, to obtain the above effects, the V content is preferably 0.05% or more.

[0037] W: 0 to 0.50% W has the effect of improving strength and corrosion resistance. Therefore, it may be contained as needed. However, excessive W content increases manufacturing costs. Therefore, the W content is set to 0.50% or less. The W content is preferably set to 0.30% or less. On the other hand, in order to obtain the above effects, the W content is preferably set to 0.05% or more.

[0038] B: 0 to 0.0050% B has the effect of strengthening grain boundaries and improving strength. Therefore, it may be added as needed. However, even if an excessive amount of B is added, the effect saturates. Therefore, the B content is set to 0.0050% or less. The B content is preferably set to 0.0030% or less. On the other hand, in order to obtain the above effect, the B content is preferably set to 0.0002% or more.

[0039] Ca: 0 to 0.0100% Ca has the effect of suppressing the grain boundary segregation of low-melting point elements and strengthening the grain boundaries. Therefore, it may be added as needed. However, if an excessive amount of Ca is added, segregation is likely to occur and become the starting point of fracture. Therefore, the Ca content is set to 0.0100% or less. The Ca content is preferably set to 0.0050% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably set to 0.0002% or more.

[0040] Mg: 0 to 0.0100% Mg has the effect of suppressing the grain boundary segregation of low-melting point elements and strengthening the grain boundaries. Therefore, it may be added as needed. However, excessive Mg content will result in the formation of a large amount of inclusions, which will easily become the starting point of fracture. For this reason, the Mg content is set to 0.0100% or less. The Mg content is preferably set to 0.0050% or less. On the other hand, to obtain the above effect, the Mg content is preferably set to 0.0002% or more.

[0041] Zr: 0 to 0.50% Zr has a deoxidizing effect and also has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, excessive Zr content reduces toughness and workability. Furthermore, a large amount of inclusions is formed, which easily become the starting point of fracture. For this reason, the Zr content is set to 0.50% or less. The Zr content is preferably set to 0.30% or less. On the other hand, to obtain the above effects, the Zr content is preferably set to 0.01% or more.

[0042] Ga: 0 to 0.050% Ga has the effect of improving hot workability. Therefore, it may be contained as necessary. However, excessive Ga content reduces manufacturability. Therefore, the Ga content is set to 0.050% or less. The Ga content is preferably set to 0.030% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably set to 0.001% or more.

[0043] Hf: 0 to 0.10% Hf has the effect of improving strength and hydrogen gas embrittlement resistance. Therefore, it may be contained as needed. However, excessive Hf content reduces workability. Therefore, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.07% or less. On the other hand, in order to obtain the above effects, the Hf content is preferably set to 0.01% or more.

[0044] REM: 0 to 0.10% REM has the effect of improving hot workability. It also has the effect of improving corrosion resistance. Therefore, it may be added as needed. However, if REM is added in excess, not only will the effect saturate, but the hot workability will also decrease. Therefore, the REM content is set to 0.10% or less. The REM content is preferably set to 0.07% or less. On the other hand, to obtain the above effects, the REM content is preferably set to 0.01% or more.

[0045] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0046] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of austenitic stainless steel due to various factors in raw materials such as ore and scrap, and in the production process, and are acceptable within a range that does not adversely affect this embodiment.

[0047] 2.Oxide film As mentioned above, to prevent hydrogen penetration into steel, it is important to form an oxide film that is mainly composed of Cr2O3 and has a concentrated Si outermost layer. In other words, to improve fatigue properties in a hydrogen environment, it is important to control the chemical composition of the oxide film.

[0048] In the present invention, the chemical composition and thickness of the oxide film are measured by glow discharge optical emission spectroscopy (GDS). Specifically, GDS is used to measure the changes in the concentrations of O, Fe, Cr, Mn, Ni, Mo, Nb, Ti, Si, Al, and N in the depth direction from the outermost surface of the steel, and the total amount of the other elements excluding O is converted to 100% by mass.

[0049] From the above viewpoints, in this embodiment, the Si content at a depth of 1 nm from the surface of the oxide film is set to 15% or more. The Si content at a depth of 1 nm from the surface of the oxide film is preferably 20% or more, and more preferably 23% or more. As will be described later, in GDS analysis, measurements are taken at a depth pitch of 0.8 to 1.2 nm in the depth direction, so the measured value at a depth position closest to 1.0 nm from the surface of the oxide film is used as the Si content at a depth of 1 nm.

[0050] Oxide film thickness There is no particular limitation on the thickness of the oxide film, but from the viewpoint of more reliably suppressing the penetration of hydrogen, the thickness of the oxide film is preferably 3 nm or more, and more preferably 5 nm or more.

[0051] GDS analysis is performed at a single point selected from an area with few surface defects. At the measurement point, sputtering is performed from the outermost surface of the steel to a depth of 50 nm, while measuring the elemental concentrations of O, Fe, Cr, Mn, Ni, Mo, Nb, Ti, Si, Al, and N at a pitch of 0.8 to 1.2 nm. This determines the content (mass%) of each of the above elements at each depth. At this time, the total amount of other elements excluding O is converted to 100% by mass. GDS analysis is also performed at a depth of 1000 nm from the outermost surface of the steel, and the content of each element is determined in the same way.

[0052] Next, the obtained Cr concentration profile is referenced to identify the depth position where the Cr content is maximum. Then, the average value of the maximum Cr content and the Cr content at a depth position 1000 nm from the outermost surface is calculated. Next, the Cr content is confirmed in the depth direction (toward the base material) from the depth position where the Cr content is maximum, and the depth position where the Cr content first falls below the average value is defined as the boundary between the base material and the oxide film. Based on this result, the thickness of the oxide film is calculated.

[0053] As a GDS measuring device, for example, a GD-Profiler2 device manufactured by Horiba Ltd. can be used, and the measurement conditions can be 35 W, argon pressure 600 Pa, frequency 100 Hz, and measurement diameter 4 mmφ.

[0054] 3. Thickness There are no particular limitations on the thickness of the austenitic stainless steel of this embodiment, but when used as a material for hydrogen gas piping, the thickness is preferably 6.0 mm or less, and more preferably less than 4.5 mm.

[0055] 4.Applications The austenitic stainless steel of this embodiment includes austenitic stainless steel plate or austenitic stainless steel pipe. Furthermore, since the austenitic stainless steel of this embodiment has excellent fatigue properties in a hydrogen environment, it is preferably used in a high-pressure hydrogen gas environment or a liquefied hydrogen environment. For example, it is preferably used as a component of a hydrogen gas production device or a hydrogen gas supply device. Note that components of a hydrogen gas production device or a hydrogen gas supply device include, for example, a tank body, a nozzle, a liner, a valve, a heat exchanger, a dispenser, and other instruments, or piping used in a flow path.

[0056] 5. Manufacturing method A preferred method for producing the austenitic stainless steel of this embodiment will now be described. The austenitic stainless steel of this embodiment can be stably produced, for example, by the following production method.

[0057] Stainless steel having the above chemical composition is melted and produced into billets such as slabs. Next, the billets are heated to a predetermined temperature and hot rolled (hot rolling process). The heating temperature in hot rolling is preferably in the range of 1050 to 1250°C, and the rolling reduction is preferably 40% or more. This is because by setting the heating temperature and rolling reduction in the above ranges during hot rolling, it becomes easier to control the plate thickness to the desired value.

[0058] After hot rolling, annealing may be performed to adjust the structure, if necessary. The annealing conditions are not particularly limited, but for example, the annealing temperature is preferably in the range of 950 to 1150°C. The annealing time is preferably in the range of 0.5 to 15 minutes. The annealing atmosphere may be air. After hot rolling, or after annealing if annealing has been performed after hot rolling, pickling is performed to remove scale. The conditions for pickling are also not particularly limited, and may be in accordance with conventional methods.

[0059] Subsequently, the obtained hot-rolled sheet is subjected to cold rolling to obtain a cold-rolled sheet. The conditions for this are not particularly limited, but a cold rolling reduction ratio of 40% or more is preferable. Cold rolling may be performed multiple times. Intermediate annealing may also be performed between cold rolling steps. The conditions for intermediate annealing are not particularly limited, but it is preferable to perform the annealing for 10 seconds to 10 minutes in a temperature range of 950 to 1150°C.

[0060] The obtained cold-rolled sheet is subjected to cold-rolled sheet annealing. The annealing temperature is preferably in the range of 950 to 1150°C. The annealing time is preferably in the range of 5 seconds to 3 minutes. By setting the annealing temperature and annealing time within the above ranges, recrystallization can be promoted and a homogeneous structure can be obtained. After annealing, pickling may be performed as necessary. The pickling conditions are not particularly limited. Conventional methods may be used.

[0061] In this way, a steel plate is obtained. On the other hand, when a steel pipe is to be obtained, the obtained steel plate is formed into a tubular shape to be used as a steel pipe material. The forming method is not particularly limited, but usually, so-called roll forming is used, in which the steel plate is bent using rolls with various curvatures to form it into a tubular shape.

[0062] Next, the ends of the formed steel pipe material in the plate width direction are welded to form a welded pipe. The welding method is not particularly limited, but may be, for example, high-frequency electric resistance welding (also called "ERW"), inert gas arc welding (also called "TIG welding"), or laser welding. Other welding conditions may be adjusted as appropriate.

[0063] The obtained welded pipe is further annealed as necessary, pickled, and cold drawn to obtain a steel pipe of predetermined dimensions. The conditions for cold drawing are not particularly limited, but for example, the wall thickness reduction rate is preferably 20% or less. Note that cold drawing may be performed multiple times.

[0064] The steel sheet or steel pipe obtained as described above is subjected to final annealing, whereby an oxide film is formed on the surface of the base material. In order to form an oxide film mainly composed of Cr2O3 with Si oxide concentrated in the outermost layer, it is important to control the conditions in the final annealing.

[0065] In this embodiment, the final annealing is a heat treatment in an inert gas atmosphere, so-called bright annealing. Typically, in bright annealing, the dew point is set as low as possible, for example, to −60° C. or lower. However, as a result of investigations, the inventors of the present invention have found that in order to concentrate Si oxide in the outermost layer of the oxide film, it is necessary to perform the heat treatment in a hydrogen gas atmosphere, which has a relatively high dew point.

[0066] Therefore, the dew point during final annealing is set to be above -60°C. Preferably, the dew point during final annealing is set to be above -50°C. Furthermore, the heat treatment temperature during final annealing is preferably 1000 to 1200°C, and the heat treatment time is preferably 10 seconds to 30 minutes. Furthermore, the atmosphere during the heat treatment is a 100% H2 atmosphere.

[0067] In addition, it is important to keep the cooling rate after heat treatment low. This ensures sufficient residence time in the temperature range of 700 to 900°C, where Si oxides and Cr oxides are likely to form. Therefore, the cooling rate after heat treatment must be kept at 20°C / s or less.

[0068] EXAMPLES The austenitic stainless steel according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0069] Stainless steel having the chemical composition shown in Table 1 was melted and produced into a slab. The slab was then heated to a temperature range of 1050 to 1250°C and hot-rolled. After hot rolling, the slab was cold-rolled and then intermediate-annealed to obtain a cold-rolled annealed sheet having a thickness of 2.0 mm. The obtained cold-rolled annealed sheet was then formed and welded to produce a welded pipe having an outer diameter of 22 mm. TIG welding was used for welding.

[0070] The obtained welded pipe was then annealed and pickled, and then cold drawn to obtain a cold-rolled steel pipe having an outer diameter of 6.35 mm and a wall thickness of 1.3 mm.

[0071] The cold-rolled steel pipes thus obtained were then subjected to final annealing by heat treatment at 1,050°C for 30 minutes in the atmosphere shown in Table 2. In the "Atmosphere" column of "Annealing Conditions" in the table, "100% H" means that bright annealing was performed in a 100% H atmosphere, and "LNG" means that heat treatment was performed in an LNG combustion simulated atmosphere (1% O, 72% N, 10% CO, 17% H), followed by pickling. In the "Heat Treatment" column, a circle indicates that the pipe was held in air at 400°C for 1 hour after final annealing.

[0072] [Table 1]

[0073] [Table 2]

[0074] Measurement of the chemical composition and thickness of oxide films Each austenitic stainless steel pipe was cut to an appropriate size and cold-pressed into a smooth plate. GDS analysis was performed at a randomly selected point within the area with minimal surface defects. Sputtering was performed at each measurement point from the outermost surface of the steel pipe to a depth of 50 nm, with the element concentrations of O, Fe, Cr, Mn, Ni, Mo, Nb, Ti, Si, Al, and N measured at approximately 1 nm intervals. The content (mass%) of each of these elements at each depth was calculated. The total mass% of all elements, excluding O, was calculated to be 100%. GDS analysis was also performed at a depth of 1,000 nm from the outermost surface of the steel pipe, and the content of each element was similarly calculated. The measured value at the depth closest to 1.0 nm from the surface of the oxide film was taken as the Si content at a depth of 1 nm.

[0075] Next, the depth position where the Cr content was maximum was identified by referring to the obtained Cr concentration profile, and the average value of the maximum Cr content and the Cr content at a depth position 1000 nm from the outermost surface was calculated. Next, the Cr content was confirmed in the depth direction (toward the base material) from the depth position where the Cr content was maximum, and the depth position where the Cr content first fell below the average value was determined as the boundary between the base material and the oxide film. Based on this result, the thickness of the oxide film was calculated.

[0076] The GDS measurement device used was a GD-Profiler2 manufactured by Horiba Ltd., and the measurement conditions were 35 W, argon pressure 600 Pa, frequency 100 Hz, and measurement diameter 4 mmφ.

[0077] Fatigue property evaluation test in a hydrogen environment The following test was conducted to evaluate fatigue properties in a hydrogen environment. Specifically, the obtained steel pipe (500 mm long) was placed in a test specimen chamber. The chamber was kept at -70°C, and the H gas pressure inside the steel pipe was increased from 0 MPa to 10 MPa or 70 MPa, and then reduced from 10 MPa or 70 MPa to 0 MPa (one cycle). This was repeated until hydrogen leakage occurred or the outer diameter of the pipe changed. One cycle lasted 30 seconds, i.e., the frequency was 0.03, and the test was repeated up to 10,000 cycles.

[0078] In Table 2, for each case where the H2 gas pressure was 10 MPa or 70 MPa, if there was no hydrogen leakage or change in outer diameter after 10,000 cycles, it was judged as ○. On the other hand, if there was hydrogen leakage or a change in the outer diameter of the tube before 10,000 cycles had elapsed, it was judged as ×. The results are also shown in Table 2. [Industrial Applicability]

[0079] According to the present invention, an austenitic stainless steel having excellent fatigue properties in a hydrogen environment can be obtained.

Claims

1. An austenitic stainless steel having a base material and an oxide film formed on a surface of the base material, The chemical composition of the base material is, in mass%, C: 0.080% or less, Si: 0.10-1.0%, Mn: 2.00% or less, P: 0.050% or less, S: 0.020% or less, Cr: 17.0-20.0%, Ni: 8.0 to 13.0%, Al: 0.300% or less, N: 0.250% or less, Nb: 0 to 0.20%, Ti: 0 to 0.20%, Mo: 0-1.00%, Cu: 0 to 1.0%, Co: 0 to 0.50%, V: 0-0.50%, W: 0-0.50%, B: 0 to 0.0050%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.50%, Ga: 0 to 0.050%, Hf: 0-0.10%, REM: 0-0.10%, The balance is Fe and impurities. Using glow discharge optical emission spectroscopy, changes in the concentrations of O, Fe, Cr, Mn, Ni, Mo, Nb, Ti, Si, Al, and N were measured in the depth direction from the outermost surface of the austenitic stainless steel, and when the total amount of other elements excluding O was converted to 100% by mass, the Si content at a depth of 1 nm from the surface of the oxide film is 15% or more; Austenitic stainless steel.

2. The chemical composition is, in mass %, Nb: 0.01-0.20%, Ti: 0.01-0.20%, Mo: 0.05-1.00%, Cu: 0.05-1.0%, Co: 0.01 to 0.50%, V: 0.05-0.50%, W: 0.05-0.50%, B: 0.0002 to 0.0050%, Ca: 0.0002-0.0100%, Mg: 0.0002 to 0.0100%, Zr: 0.01-0.50%, Ga: 0.001-0.050%, Hf: 0.01 to 0.10%, and REM: 0.01-0.10%, Contains one or more selected from The austenitic stainless steel according to claim 1.

3. The chemical composition is, in mass %, Si: 0.40% or more, and Nb + Ti: 0.10% or more; The austenitic stainless steel according to claim 2.

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