Austenitic stainless steel

Abstract

To provide an austenitic stainless steel having excellent hydrogen embrittlement resistance while reducing the amount of an expensive alloy element.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, 1.0% or less 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 maximum value of the A value [=Cr+4Si+10Al+5(Nb+Ti)] in the oxide film is 30 or more as measured by GDS and the thickness of the oxide film is 5 nm 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] However, the austenitic stainless steel disclosed in Patent Document 1 has a problem in that many expensive alloy elements are added to improve hydrogen gas embrittlement resistance, resulting in high alloy costs. In general, it is difficult to improve hydrogen embrittlement resistance while reducing the amount of expensive alloy elements in austenitic stainless steel.

[0005] The present invention aims to solve the above problems and to provide an austenitic stainless steel that has excellent hydrogen embrittlement resistance while reducing the amount of expensive alloying elements. [Means for solving the problem]

[0006] 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.

[0007] (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: 1.0% or less, 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, In the measurement results of the oxide film, the maximum value of A calculated by the following formula (i) is 30 or more, The thickness of the oxide film is 5 nm or more. Austenitic stainless steel. A value=Cr+4Si+10Al+5(Nb+Ti) (i) However, each element symbol in the above formula (i) represents the content (mass %) of each element measured at each depth position by glow discharge optical emission spectroscopy, and is set to zero if the element is not contained.

[0008] (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. [Effects of the Invention]

[0009] According to the present invention, it is possible to obtain an austenitic stainless steel that has excellent resistance to hydrogen gas embrittlement while reducing the amount of expensive alloying elements. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present inventors have investigated methods for improving hydrogen gas embrittlement resistance and have obtained the following findings.

[0011] (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.

[0012] (b) In conventional steel manufacturing processes and hydrogen environments, hydrogen penetrates into the steel, resulting in trace amounts of hydrogen trapped within the steel. If this hydrogen in the steel accumulates, for example, in areas where strain has accumulated, it accelerates hydrogen embrittlement and reduces the steel's resistance to hydrogen gas embrittlement. Therefore, while it is desirable for the hydrogen in the steel to be released, it is difficult to release it from the steel in the presence of hydrogen at low temperatures, which is the environment in which the steel is used.

[0013] (c) Therefore, the present inventors investigated methods for suppressing hydrogen penetration into steel and found that forming an oxide film mainly composed of Cr2O3 on the surface of steel sheet is effective. Furthermore, the incorporation of Si, Al, Ti, and Nb into the oxide film is also effective. These elements are thought to improve the density of the oxide film and suppress hydrogen penetration.

[0014] (d) In addition, by performing a preliminary oxidation treatment and making the oxide film sufficiently thick, it becomes possible to suppress the penetration of hydrogen.

[0015] 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.

[0016] 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.

[0017] 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."

[0018] 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.

[0019] Si: 1.0% or less Si is an effective element for deoxidation and also contributes to improving hydrogen gas embrittlement resistance. 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. On the other hand, to achieve the above effects, the Si content is preferably set to 0.10% or more, and more preferably set to 0.30% or more.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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. Therefore, 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. Therefore, 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] From the viewpoint of improving the density of the oxide film and further improving resistance to hydrogen gas embrittlement, it is preferable to contain Nb and / or Ti, and the total content of Nb and Ti is preferably 0.10% or more.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 2.Oxide film As mentioned above, in order to suppress the penetration of hydrogen into steel, it is important to form an oxide film of a certain thickness or more that is mainly composed of Cr2O3 and contains at least Si and Al as solid solutions. That is, in order to improve resistance to hydrogen gas embrittlement, it is important to control the chemical composition and thickness of the oxide film. Furthermore, it is preferable that Nb and / or Ti be solid-solved in the oxide film.

[0046] 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 sheet, and the total amount of the other elements excluding O is converted to 100% by mass.

[0047] From the above viewpoint, in this embodiment, in the measurement results of the oxide film, the maximum value of A calculated by formula (i) is set to 30 or more, and the thickness of the oxide film is set to 5 nm or more.

[0048] A value The A value is an index of the ability of an oxide film to suppress hydrogen penetration. In the austenitic stainless steel of this embodiment, in order to improve resistance to hydrogen gas embrittlement, the maximum A value calculated at each depth position in the region from the outermost surface of the steel sheet to a depth of 20 nm is set to be 30 or more. A value=Cr+4Si+10Al+5(Nb+Ti) (i) However, each element symbol in the above formula (i) represents the content (mass %) of each element measured at each depth position by glow discharge optical emission spectroscopy, and is set to zero if the element is not contained.

[0049] The oxide film is usually composed mainly of oxides of Fe and Cr depending on the chemical composition of the base material. As a result of research by the inventors, it was found that by optimizing the manufacturing conditions, it is possible to increase the ratio of Cr oxide and to dissolve Si, Al, Ti, and Nb in the oxide film.

[0050] If the maximum value of A is less than 30, the ratio of Fe oxides is excessive, and the effect of suppressing hydrogen penetration into the steel cannot be sufficiently obtained. The maximum value of A is preferably 35 or more, more preferably 40 or more, and even more preferably 45 or more. Since a higher maximum value of A is preferable, there is no need to set an upper limit, but 80 is the upper limit that can be practically produced.

[0051] Oxide film thickness As described above, in order to suppress the penetration of hydrogen, the oxide film needs to be sufficiently thick. From this viewpoint, in this embodiment, the oxide film is set to have a thickness of 5 nm or more. The thickness of the oxide film is preferably 10 nm or more, more preferably 15 nm or more, and even more preferably 20 nm or more.

[0052] 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 sheet 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 sheet, and the content of each element is determined in the same way.

[0053] Next, the obtained Cr concentration profile is referenced to identify the depth position where the Cr content is maximum. The average value of the maximum Cr content and the Cr content at a depth position 1000 nm from the outermost surface is then calculated. The Cr content is then 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 these results, the thickness of the oxide film is calculated. Additionally, the A value is calculated at each depth position in the region identified as the oxide film, and the maximum A value is determined from all the calculated measurements.

[0054] 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.

[0055] 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, from the standpoints of workability and weight reduction, the thickness is preferably 6.0 mm or less, and preferably less than 4.5 mm.

[0056] 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 resistance to hydrogen gas embrittlement, 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.

[0057] 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 by, for example, subjecting a hot-rolled annealed sheet, a cold-rolled annealed sheet, or a cold-rolled annealed pipe to a descaling step and a preliminary oxidation step, which will be described later.

[0058] Stainless steel having the above chemical composition is melted and produced into billets such as slabs. The billets are then heated to a predetermined temperature and hot-rolled to produce hot-rolled sheets. The heating temperature in hot rolling is preferably in the range of 1050 to 1250°C, and the reduction is preferably 40% or more. This is because by setting the heating temperature and reduction during hot rolling within the above ranges, it becomes easier to control the sheet thickness to the desired value.

[0059] The obtained hot-rolled sheet is annealed to adjust the structure. 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. In this way, a hot-rolled annealed sheet is obtained.

[0060] When a cold-rolled annealed sheet is to be obtained, the obtained hot-rolled annealed sheet is subsequently descaled by pickling and then cold-rolled 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. Note that cold rolling may be performed multiple times. Intermediate annealing may also be performed between cold rolling operations. The conditions for intermediate annealing are not particularly limited, but it is preferable to perform, for example, intermediate annealing in a temperature range of 950 to 1150°C for 10 seconds to 10 minutes.

[0061] The obtained cold-rolled sheet is then annealed. 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. This results in a cold-rolled annealed sheet.

[0062] When a cold-rolled annealed pipe is to be obtained, the hot-rolled annealed sheet or the cold-rolled annealed sheet is formed into a tubular shape to be used as a pipe material. The forming method is not particularly limited, but typically, roll forming is used, in which the sheet is bent using rolls with various curvatures to form the tubular shape.

[0063] Next, the ends of the formed 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.

[0064] The obtained welded pipe is further annealed as necessary, pickled, and cold drawn to obtain a cold-rolled 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.

[0065] The cold-rolled pipe after cold drawing is annealed. 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.

[0066] The hot-rolled annealed sheet, cold-rolled annealed sheet, or cold-rolled annealed pipe obtained by the above steps is subjected to a descaling step and a pre-oxidation step. In the descaling step, scale formed during hot rolling or annealing is removed by pickling. Pickling is performed by immersing the sheet in a nitric-hydrofluoric acid aqueous solution at 40 to 80°C for 1 to 30 minutes.

[0067] In the pre-oxidation step following the descaling step, heating is performed for 1 to 24 hours at a temperature range of 100°C to 400°C in air, an inert gas such as Ar gas, or an LNG atmosphere. By performing the pre-oxidation step following the descaling step, the maximum A value can be increased to 30 or more, and the thickness of the oxide film can be increased to 5 nm or more.

[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 1230°C and hot-rolled to obtain a hot-rolled sheet having a thickness of 4.0 mm. After hot rolling, the hot-rolled sheet was annealed by holding at 1100°C for 3 minutes to obtain a hot-rolled annealed sheet having a thickness of 4.0 mm. The hot-rolled annealed sheet was then shot-blasted, and then subjected to a descaling process in which it was immersed in a nitric hydrofluoric acid aqueous solution at 60°C for 3 minutes.

[0070] Furthermore, a portion of the hot-rolled and annealed sheet after this descaling process was further cold-rolled to obtain a cold-rolled sheet with a thickness of 2.0 mm, which was then held at 1080°C for 0.5 minutes for cold-rolled sheet annealing, followed by salt immersion and water washing to obtain a cold-rolled and annealed sheet with a thickness of 2.0 mm. The cold-rolled and annealed sheet was then subjected to a descaling process in which it was immersed in a nitric-hydrofluoric acid aqueous solution at 60°C for 1 minute.

[0071] After the descaling process, the 4.0 mm thick hot-rolled and annealed sheets and the 2.0 mm thick cold-rolled and annealed sheets were each subjected to a pre-oxidation process under the conditions shown in Table 2 to obtain austenitic stainless steel. Note that "LNG" in the table refers to an LNG combustion simulated atmosphere of 1% O2-72% N2-10% CO2-17% HO.

[0072] [Table 1]

[0073] [Table 2]

[0074] Measurement of the chemical composition and thickness of oxide films Each austenitic stainless steel specimen was analyzed by GDS at a random point selected from an area with minimal surface defects. At the measurement point, sputtering was performed from the outermost surface of the steel sheet to a depth of 50 nm, measuring the elemental concentrations of O, Fe, Cr, Mn, Ni, Mo, Nb, Ti, Si, Al, and N at approximately 1 nm intervals, and the content (mass%) of each of the above elements at each depth was determined. The total amount of all elements excluding O was converted to 100% by mass. GDS analysis was also performed at a depth of 1000 nm from the outermost surface of the steel sheet, and the content of each element was determined in the same manner.

[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 these results, the thickness of the oxide film was calculated. In addition, the above-mentioned A value was calculated at each depth position in the region identified as the oxide film, and the maximum A value was determined from all the calculated measurements.

[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] Hydrogen gas embrittlement resistance evaluation test Tensile tests were conducted to evaluate hydrogen gas embrittlement resistance. Tensile test specimens with a parallel section width of 4 mm, length of 20 mm, and original thickness were taken from the steel plate, with the longitudinal direction perpendicular to the rolling direction. These tensile test specimens were subjected to a descaling process in which they were immersed in a nitric hydrofluoric acid solution at 60°C for 0.5 minutes, and then reheated under the pre-oxidation conditions shown in Table 2. They were then placed in a pressure vessel, the inside of which was purged with hydrogen gas, and the vessel was then heated and pressurized to 1 MPa and 300°C for 100 hours, at which point they were hydrogen-charged.

[0078] The specimens were then removed from the pressure vessel and immediately placed in an air environment at -70°C, where tensile stress was applied until fracture. The tensile speed was 0.09 mm / min up to 0.2% proof stress, and 1.2 mm / min thereafter. Furthermore, in order to calculate the RTS (relative tensile strength) for each specimen, as described below, the tensile strength of the tensile test specimens without hydrogen charging was also measured separately.

[0079] The test results were evaluated using RTS (relative tensile strength), which is calculated using the following formula: RTS = tensile strength at -70°C in air after hydrogen charging / tensile strength at -70°C in air without hydrogen charging

[0080] When the RTS was 0.90 or more, the hydrogen gas embrittlement resistance was judged to be good and marked with ○. When the RTS was 0.95 or more, the hydrogen gas embrittlement resistance was judged to be even better and marked with ⊚. The results are also shown in Table 2. [Industrial Applicability]

[0081] According to the present invention, it is possible to obtain an austenitic stainless steel that has excellent resistance to hydrogen gas embrittlement while reducing the amount of expensive alloying elements.

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: 1.0% or less, 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, In the measurement results of the oxide film, the maximum value of A calculated by the following formula (i) is 30 or more, The thickness of the oxide film is 5 nm or more. Austenitic stainless steel. A value=Cr+4Si+10Al+5(Nb+Ti)...(i) However, each element symbol in the above formula (i) represents the content (mass %) of each element measured at each depth position by glow discharge optical emission spectroscopy, and if the element is not contained, it is set to zero.

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.

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