Austenitic stainless steel pipe

Abstract

To provide an austenitic stainless steel tube which materializes reduction of an expensive platinum element as the component thereof, and nonetheless has excellent fatigue characteristics in a hydrogen environment.SOLUTION: An austenitic stainless steel tube having a weld zone is provided, in which when an amount of hydrogen discharged is measured using a temperature-programmed desorption analysis of hydrogen at a rate of temperature increase of 100°C / h in a temperature range of 25-800°C, the discharged amount of hydrogen [H], is less than 10.0 ppm, the peak temperature of hydrogen discharge [Tp], is more than 350°C, the peak speed of hydrogen discharge [Rmax], is 0.050 ppm / min or less, and the crystal grain size in the weld zone is 8.0 or more in terms of grain size number.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In recent years, hydrogen has been attracting attention as a new energy source to replace fossil fuels. Hydrogen is a clean energy source that does not emit CO2. However, hydrogen 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. However, when the austenitic stainless steel disclosed in Patent Document 1 is used in steel pipes, many expensive alloy elements are added, resulting in high manufacturing costs. In addition, when austenitic stainless steel is used in steel pipes, fatigue properties in a hydrogen environment become an issue, but Patent Document 1 does not consider these fatigue properties. Therefore, there is room for further improvement in the fatigue properties of steel pipes.

[0005] In view of the above, an object of the present invention is to solve the above problems and to provide an austenitic stainless steel pipe that has excellent fatigue properties in a hydrogen environment 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 pipe.

[0007] (1) A steel pipe having a welded portion, When the amount of hydrogen released was measured using thermal desorption hydrogen analysis at a temperature rise rate of 100°C / h in the temperature range of 25 to 800°C, The amount of released hydrogen [H] is less than 10.0 ppm, The peak temperature of hydrogen release [Tp] is 350°C or higher, The peak rate of hydrogen release [Rmax] is 0.050 ppm / min or less; An austenitic stainless steel pipe, wherein the grain size of the weld is 8.0 or more in terms of grain size number.

[0008] (2) The chemical composition of the steel pipe is, in mass%, C: 0.080% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.050% or less, S: 0.020% or less, Cr: 17.0~20.0%, Ni: 8.0-13.0% N: 0.25% or less, Nb: 0 to 0.20% Ti: 0 to 0.20% Mo: 0-1.0% Cu: 0-1.0% Al: 0 to 0.30% Co: 0 to 0.50% V: 0~0.50%, W: 0~0.50%, B: 0~0.0050%, Ca: 0 to 0.010% Mg: 0 to 0.010% Zr: 0 to 0.50% Ga: 0 to 0.05%, Hf: 0 to 0.10% REM: 0~0.10%, The austenitic stainless steel pipe according to (1) above, wherein the balance is Fe and impurities.

[0009] (3) An austenitic stainless steel pipe according to (1) or (2) above, which is used in a high-pressure hydrogen gas environment or a liquefied hydrogen environment.

[0010] (4) An austenitic stainless steel pipe according to (1) or (2) above, which is used as a component of a hydrogen gas production device or a hydrogen gas supply device. [Effects of the Invention]

[0011] According to the present invention, it is possible to obtain an austenitic stainless steel pipe that has excellent fatigue properties in a hydrogen environment while reducing the amount of expensive alloying elements. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram schematically showing a hydrogen release curve (TDA curve). DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have conducted various studies to improve the fatigue properties of austenitic stainless steel pipes in a hydrogen environment while reducing the amount of expensive alloy elements, and have obtained the following findings (a) to (c).

[0014] (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.

[0015] (b) Therefore, the inventors focused on the hydrogen contained in steel in order to improve fatigue properties in a hydrogen environment without increasing the alloying strength. In a typical 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, hydrogen embrittlement is promoted, resulting in a deterioration in the fatigue properties of the steel pipe. Therefore, while it is desirable for hydrogen in the steel to be released, it is difficult to release it to the outside of the steel in the presence of hydrogen at low temperatures, which is the environment in which the steel is used.

[0016] (c) For this reason, it is desirable to suppress the accumulation of hydrogen present inside the steel and make it difficult for hydrogen to move. In other words, it is effective to control the state of hydrogen so that it is trapped inside the steel. In order to control the state of hydrogen, it is preferable to perform a heating treatment during the production of steel pipe, in which heat treatment is performed for a long period of time in a temperature range of 50 to 400°C. In addition, it is preferable to make the crystal grains in the weld finer, with a crystal grain size of 8.0 or more in terms of grain size number, so that hydrogen is more firmly trapped.

[0017] One embodiment of the present invention has been made based on the above findings. The austenitic stainless steel pipe of this embodiment is a steel pipe having a welded portion. More specifically, the austenitic stainless steel pipe of this embodiment has a welded portion formed by solidifying molten metal through welding. In addition to the welded portion, the pipe also has a base metal portion. The base metal portion includes a weld heat-affected zone that is affected by heat input from welding.

[0018] 1. State of hydrogen In the austenitic stainless steel pipe of this embodiment, the state of hydrogen is controlled so that hydrogen is less likely to move inside the steel, i.e., is trapped. In order to quantitatively grasp the state of hydrogen, the austenitic stainless steel pipe of this embodiment uses thermal desorption hydrogen analysis.

[0019] Thermal desorption spectroscopy is a technique in which stainless steel material is heated at a constant temperature increase rate and the released hydrogen is detected using a gas chromatograph or quadrupole mass spectrometer. Hydrogen is trapped in many defects, such as atomic vacancies, dislocations, lattice defects such as grain boundaries, and interfaces between precipitates and inclusions. Thermal desorption spectroscopy can quantitatively evaluate the amount of hydrogen trapped in these defects.

[0020] Here, the hydrogen desorption curve (hereinafter also referred to as "TDA curve") obtained by thermal desorption analysis is explained with reference to Figure 1. In a TDA curve, the horizontal axis represents temperature and the horizontal axis represents hydrogen desorption rate. In the TDA curve of steel materials, steel pipes, etc., the hydrogen desorption rate generally peaks at a certain temperature. In this TDA curve, the temperature at which the hydrogen desorption rate reaches its peak is called the peak temperature of hydrogen desorption [Tp]. The hydrogen desorption rate at this point is called the peak rate of hydrogen desorption [Rmax]. Furthermore, by integrating this curve, the amount of released hydrogen [H] can also be calculated.

[0021] The amount of released hydrogen [H], the peak temperature of hydrogen release [Tp], and the peak rate of hydrogen release [Rmax] described above are indices that indicate the state of hydrogen present in the steel.

[0022] When the amount of hydrogen released is measured using thermal desorption hydrogen analysis at a temperature rise rate of 100°C / h in the temperature range of 25 to 800°C, the amount of released hydrogen [H] is less than 10.0 ppm, the peak temperature of hydrogen release [Tp] is 350°C or higher, and the peak rate of hydrogen release [Rmax] is 0.050 ppm / min or less.

[0023] If the amount of released hydrogen [H] measured by the above method is 10.0 ppm or more, excessive hydrogen is taken up into the steel, making it difficult to improve hydrogen embrittlement resistance. Therefore, the amount of released hydrogen [H] is set to 10.0 ppm or less. The amount of released hydrogen [H] is preferably 8.5 ppm or less, more preferably 6.5 ppm or less, and even more preferably less than 5.0 ppm. The lower limit of the amount of released hydrogen [H] is not particularly limited, but is usually about 0.5 ppm.

[0024] Furthermore, if the peak temperature [Tp] of hydrogen release is less than 350°C, the degree of hydrogen trapping is low and hydrogen moves easily within the steel. Therefore, the peak temperature [Tp] of hydrogen release is set to 350°C or higher. The peak temperature [Tp] of hydrogen release is preferably set to 370°C or higher, more preferably 400°C or higher, and even more preferably 450°C or higher. The upper limit of the peak temperature [Tp] of hydrogen release is not particularly limited, but is usually about 500°C.

[0025] Furthermore, if the peak rate of hydrogen desorption [Rmax] exceeds 0.050 ppm / min, the degree of hydrogen trapping is low and hydrogen is more likely to move within the steel. Therefore, the peak rate of hydrogen desorption [Rmax] is set to 0.050 ppm / min or less. The peak rate [Rmax] is preferably set to 0.045 ppm / min or less, more preferably set to 0.035 ppm / min or less, and even more preferably set to 0.020 ppm / min or less. The lower limit of the peak rate of hydrogen desorption [Rmax] is not particularly limited, but is usually about 0.005 ppm / min.

[0026] The amount of released hydrogen [H], the peak temperature of hydrogen release [Tp], and the peak rate of hydrogen release [Rmax] are measured by thermal desorption hydrogen analysis, and specifically, they may be measured by the following procedure.

[0027] After cutting a 30 mm long (in the elongated direction) test piece from the steel pipe, the test piece was degreased and cleaned with an organic solvent. Next, this test piece was evaluated using thermal desorption analysis (TDA) to measure the hydrogen released. In TDA, the test piece was heated in an argon atmosphere from 25 (room temperature) to 800°C at a heating rate of 100°C / h, and the hydrogen released (desorbed) from the test piece was measured using a chromatograph. The chromatograph used was capable of detecting hydrogen with an accuracy of 0.01 ppm. Based on the TDA measurement results, a TDA curve like that shown in Figure 1 was created, and the amount of released hydrogen [H], peak temperature of hydrogen release [Tp], and peak velocity of hydrogen release [Rmax] were calculated. The amount of released hydrogen was calculated by integrating the curve.

[0028] 2. Grain size of the weld In the austenitic stainless steel pipe of this embodiment, the grain size of the welded portion is further refined and controlled to trap hydrogen. Specifically, the grain size of the welded portion is set to a grain size number of 8.0 or more. If the grain size number of the welded portion is less than 8, the metal structure of the welded portion will be coarse-grained, making it difficult to trap hydrogen. For this reason, the grain size number of the welded portion is set to 8.0 or more, and preferably 10 or more. The upper limit of the grain size number is not particularly limited, but is usually around 13.

[0029] The grain size number of the welded portion can be measured using the following procedure. First, an embedded specimen is prepared that includes the welded portion of the steel pipe and has a cross section perpendicular to the pipe elongation direction as the observation surface. Next, the structure of this specimen is observed using an optical microscope, and the grain size of the welded portion is measured. The grain size is measured in accordance with the intercepting method in the microscope test method described in JIS G 0551:2020.

[0030] 3.Chemical composition In the austenitic stainless steel pipe of this embodiment, the hydrogen embrittlement resistance is improved by controlling the state of hydrogen existence, as described above. Therefore, there is no need to particularly limit the chemical composition of the austenitic stainless steel pipe. However, the effect of the steel pipe of this embodiment is particularly pronounced in a component design that reduces the amount of expensive alloying elements. Therefore, the austenitic stainless steel pipe of this embodiment preferably has the chemical composition shown below. The reasons for limiting each element are as follows. In the following description, "%" for the content means "% by mass."

[0031] C: 0.080% or less C (carbon) is an element effective in stabilizing the austenite phase and has the effect of improving strength. However, if C is contained in excess, toughness decreases. For this reason, the C content is preferably 0.080% or less. The C content is more preferably 0.070% or less, and even more preferably 0.065% or less. On the other hand, in order to obtain the above effects, the C content is preferably 0.010% or more.

[0032] Si: 1.0% or less Silicon (Si) is an effective element for deoxidation and improves hydrogen embrittlement resistance. However, excessive Si content promotes the formation of intermetallic compounds such as the σ phase, reducing toughness and other properties. For this reason, the Si content is preferably 1.0% or less. The Si content is more preferably 0.9% or less. On the other hand, to achieve the above effects, the Si content is preferably 0.1% or more.

[0033] Mn: 2.0% or less Manganese (Mn) is an element effective in stabilizing the austenite phase and improving hydrogen embrittlement resistance, but excessive Mn content increases alloy costs. Therefore, the Mn content is preferably 2.0% or less. The Mn content is more preferably 1.9% or less. On the other hand, to obtain the above effects, the Mn content is preferably 0.5% or more.

[0034] P:0.050% or less P (phosphorus) is an impurity element contained in steel and reduces mechanical properties. For this reason, the P content is preferably 0.050% or less. It is preferable to reduce the P content as much as possible, but excessive reduction of P increases refining costs. For this reason, the P content is preferably 0.010% or more.

[0035] S: 0.020% or less S (sulfur) is an impurity element contained in steel and reduces mechanical properties. For this reason, the S content is preferably 0.020% or less. It is preferable to reduce the S content as much as possible, but excessive reduction of S increases refining costs. For this reason, the S content is preferably 0.0002% or more.

[0036] Cr: 17.0~20.0% Cr (chromium) is an element necessary for maintaining the corrosion resistance of stainless steel. Cr also has the effect of improving strength. For this reason, the Cr content is preferably 17.0% or more. The Cr content is more preferably 17.5% or more, and even more preferably 18.0% or more. However, if Cr is contained in excess, toughness decreases. For this reason, the Cr content is preferably 20.0% or less. The Cr content is more preferably 19.5% or less.

[0037] Ni: 8.0 to 13.0% Ni (nickel) has the effect of improving hydrogen embrittlement resistance. It also has the effect of improving strength. For this reason, the Ni content is preferably 8.0% or more. The Ni content is more preferably 8.5% or more. However, since Ni is an expensive element, excessive Ni content increases the alloy cost. For this reason, the Ni content is preferably 13.0% or less, and more preferably 12.0% or less.

[0038] N: 0.25% or less Like Mn and Ni, N (nitrogen) is an element effective in improving hydrogen embrittlement resistance. However, excessive N content can cause internal defects such as blowholes during melting, making fracture initiation points more likely to occur. As a result, impact resistance is reduced. For this reason, the N content is set to 0.25% or less. The N content is preferably set to 0.24% or less, and more preferably set to 0.20% or less. On the other hand, to obtain the above effects, the N content is preferably set to 0.04% or more.

[0039] In addition to the above elements, one or more elements selected from Nb and Ti may be contained within the ranges shown below. The reasons for limiting the amount of each element will be explained below.

[0040] Nb: 0 to 0.20% Nb (niobium) has the effect of forming fine precipitates and enhancing the hydrogen trapping function. Therefore, it may be contained as needed. However, since Nb is an expensive element, excessive Nb content increases the alloy cost. Furthermore, excessive formation of precipitates reduces toughness. Therefore, the Nb content is preferably 0.20% or less. The Nb content is more preferably 0.18% or less, and even more preferably 0.15% or less. On the other hand, in order to obtain the above effects, the Nb content is more preferably 0.03% or more.

[0041] Ti: 0 to 0.20% Like Nb, Ti (titanium) also forms fine precipitates and has the effect of enhancing the hydrogen trapping function. Therefore, it may be contained as needed. However, since Ti is an expensive element, excessive Ti content increases the alloy cost. Therefore, the Ti content is preferably 0.20% or less. The Ti content is more preferably 0.18% or less, and even more preferably 0.15% or less. On the other hand, in order to obtain the above effect, the Ti content is more preferably 0.01% or more.

[0042] In addition to the above elements, one or more elements selected from Mo, Cu, Al, 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.

[0043] Mo: 0 to 1.0% Mo (molybdenum) has the effect of improving strength and corrosion resistance. Therefore, it may be contained as needed. However, Mo is an expensive element, and if Mo is contained in excess, the alloy cost increases. For this reason, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.5% or less. On the other hand, in order to obtain the above effects, the Mo content is preferably 0.1% or more.

[0044] Cu: 0 to 1.0% Cu (copper) has the effect of improving strength and corrosion resistance. Therefore, it may be contained as needed. However, Cu is an expensive element, and if Cu is contained in excess, the alloy cost increases. Furthermore, the steel becomes excessively hard, resulting in a decrease in mechanical properties such as toughness. For this reason, the Cu content is preferably 1.0% or less. The Cu content is more preferably 0.9% or less, and even more preferably 0.6% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.1% or more.

[0045] Al: 0 to 0.30% Al (aluminum) is an element that has a deoxidizing effect. Therefore, it may be contained as needed. However, if Al is contained in excess, excessive inclusions are formed, degrading the surface properties. Hot workability is also degraded. Therefore, the Al content is preferably 0.30% or less. The Al content is more preferably 0.25% or less, and even more preferably 0.10% or less. On the other hand, in order to obtain the above effects, the Al content is preferably 0.01% or more.

[0046] Co: 0 to 0.50% Co (cobalt) has the effect of improving strength and corrosion resistance. It also has the effect of improving hydrogen embrittlement resistance by stabilizing the austenite phase. Therefore, it may be contained as needed. However, Co is an expensive element, and excessive Co content increases alloy costs. It also reduces workability and toughness. For this reason, the Co content is preferably 0.50% or less. On the other hand, to obtain the above effects, the Co content is preferably 0.1% or more.

[0047] V: 0 to 0.50% V (vanadium) precipitates in steel as a solid solution or carbonitride, and has the effect of improving strength. Therefore, it may be added as needed. However, if excessive V is added, excessive carbonitrides are formed, which reduces manufacturability during hot rolling. Therefore, the V content is preferably 0.50% or less. The V content is more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the V content is preferably 0.05% or more.

[0048] W: 0 to 0.50% W (tungsten) has the effect of improving strength and corrosion resistance. Therefore, it may be contained as needed. However, excessive W content increases the alloy cost. Therefore, the W content is preferably 0.50% or less. The W content is more preferably 0.30% or less. On the other hand, in order to obtain the above effects, the W content is preferably 0.05% or more.

[0049] B: 0 to 0.0050% B (boron) has the effect of strengthening grain boundaries and improving strength. Therefore, it may be contained as needed. However, if B is contained in excess, workability decreases. Therefore, the B content is preferably 0.0050% or less. The B content is more preferably 0.0030% or less. On the other hand, in order to obtain the above effects, the B content is preferably 0.0002% or more.

[0050] Ca: 0 to 0.010% Ca (calcium) has the effect of suppressing grain boundary segregation of low-melting point elements and strengthening the grain boundaries. Therefore, it may be added as needed. However, excessive Ca content makes segregation more likely to occur and reduces toughness. Therefore, the Ca content is preferably 0.010% or less. The Ca content is more preferably 0.005% or less. On the other hand, to obtain the above effects, the Ca content is preferably 0.0002% or more.

[0051] Mg: 0 to 0.010% Mg (magnesium) 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 may result in the formation of a large amount of inclusions, which may easily become the starting point of fracture, resulting in a decrease in toughness. For this reason, the Mg content is preferably 0.010% or less. The Mg content is more preferably 0.005% or less. On the other hand, to obtain the above effect, the Mg content is preferably 0.0002% or more.

[0052] Zr: 0 to 0.50% Zr (zirconium) has a deoxidizing effect and also has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if Zr is contained in excess, toughness and workability decrease. Therefore, the Zr content is preferably 0.50% or less. The Zr content is more preferably 0.30% or less. On the other hand, in order to obtain the above effects, the Zr content is preferably 0.01% or more.

[0053] Ga: 0 to 0.05% Ga (gallium) 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 preferably 0.05% or less. The Ga content is more preferably 0.02% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably 0.001% or more.

[0054] Hf: 0 to 0.10% Hf has the effect of improving strength and hydrogen 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.

[0055] 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.

[0056] 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.

[0057] In the chemical composition of the steel pipe of this embodiment, the balance is preferably Fe and impurities. Here, "impurities" refers 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 manufacturing process, and are acceptable within a range that does not adversely affect the steel pipe of this embodiment.

[0058] 4. Use and shape The austenitic stainless steel pipe of this embodiment 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. Components for a hydrogen gas production device or a hydrogen gas supply device include, for example, piping used in the flow paths of instruments such as tank bodies, nozzles, liners, valves, heat exchangers, and dispensers. In view of the above applications, the wall thickness of the steel pipe is preferably in the range of 0.25 to 6.0 mm. The outer diameter of the steel pipe is preferably 25.4 mm (1 inch) or less, but is not limited to this.

[0059] 5. Manufacturing method The austenitic stainless steel pipe of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0060] 5-1. Manufacturing of steel pipe materials Stainless steel is melted to produce a steel billet, which is then subjected to hot rolling, cold rolling, cold-rolled sheet annealing, pickling, etc., as described below, to produce a steel plate, which is then used as a steel pipe material. The method for producing the steel pipe material is not particularly limited, but it is preferable to produce it by the method described below.

[0061] As described above, stainless steel is melted and a slab is produced. The chemical composition of the slab is preferably within the range described above. The resulting slab is hot-rolled to form a hot-rolled sheet. The conditions for hot-rolling are not particularly limited, but for example, the heating temperature of the slab is preferably within the range of 1150 to 1250°C. After hot-rolling, annealing and pickling of the hot-rolled sheet may be performed as needed to adjust the structure. The conditions for annealing the hot-rolled sheet are not particularly limited, but for example, the annealing temperature is preferably within the range of 950 to 1150°C and the annealing time is preferably within the range of 0.5 to 15 minutes. After hot-rolling or after hot-rolled sheet annealing, pickling may be performed as needed.

[0062] Next, the hot-rolled sheet is preferably cold-rolled to obtain a cold-rolled sheet. The conditions for cold rolling are not particularly limited and may be conventional. Cold rolling may be performed multiple times, or heat treatment may be performed between cold rollings. When heat treatment is performed, it is preferably performed at a temperature in the range of 950 to 1150°C for 10 seconds to 10 minutes.

[0063] 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 the annealing, the sheet is cooled to obtain an austenitic stainless steel sheet. After annealing, pickling may be performed as necessary. The pickling conditions are not particularly limited. Conventional methods may be used.

[0064] 5-2. Steel pipe manufacturing The obtained steel sheet, i.e., the steel pipe material, is formed into a tubular shape. The forming method is not particularly limited, but usually, so-called roll forming is used, in which the steel sheet is bent using rolls with various curvatures to form the tubular shape.

[0065] Next, it is preferable to weld the ends of the formed steel pipe material in the plate width direction. 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.

[0066] The welded steel pipe material may be cold drawn. The conditions for the drawing are not particularly limited, but for example, it is preferable that the wall thickness reduction rate during the drawing be 20% or less. The drawing may be performed multiple times.

[0067] Furthermore, after cold drawing, heat treatment may be performed as necessary. The heat treatment temperature for the final heat treatment after drawing is preferably in the range of 950 to 1050°C. If the heat treatment temperature is less than 950°C, the structure cannot be sufficiently homogenized. The heat treatment temperature is more preferably 1000°C or higher. On the other hand, if the heat treatment temperature exceeds 1050°C, the crystal grains in the weld become coarse, the grain size number becomes less than 8.0, and fatigue properties deteriorate.

[0068] The heat treatment time is preferably in the range of 1 to 30 minutes. When drawing is performed multiple times, the drawing and heat treatment may be repeated multiple times. After the final heat treatment, the pipe is cooled at a cooling rate within an appropriate range to obtain an austenitic stainless steel pipe.

[0069] The heat treatment is preferably carried out in an air atmosphere or an LNG combustion atmosphere, but may also be carried out in a reducing atmosphere containing hydrogen gas. A typical atmosphere containing hydrogen gas is a 100% hydrogen gas atmosphere or an ammonia decomposition gas (75% hydrogen gas + 25% nitrogen gas). When heat treatment is carried out in an air atmosphere, pickling is preferably carried out. The conditions for pickling are not particularly limited; they may be carried out in accordance with conventional methods. Other heat treatment conditions are also not particularly limited.

[0070] 5-3. Heating treatment The obtained steel pipe is subjected to a heat treatment. The heat treatment is a heat treatment in which the temperature is maintained in the range of 50 to 400°C for 0.5 to 360 hours. By performing the heat treatment at a low temperature for a long time in this manner, the state of hydrogen existence can be controlled, and the amount of released hydrogen [H], the peak temperature of hydrogen release [Tp], and the peak rate of hydrogen release [Rmax] can be within the ranges of this embodiment.

[0071] The atmosphere in which the heat treatment is performed is not particularly limited, and may be an air atmosphere, or an atmosphere of N2 gas or Ar gas at atmospheric pressure.

[0072] The austenitic stainless steel pipe 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]

[0073] Austenitic stainless steel sheets with a thickness of 2.0 mm were produced by hot rolling, cold rolling, annealing, pickling, etc. This austenitic stainless steel sheet was used as a steel pipe material and formed into a 70 mm wide hoop, adjusted to an outer diameter of 22 mm. Both ends of the width were then TIG welded. The TIG-welded steel pipe material was repeatedly subjected to drawing and heat treatment, ultimately obtaining an austenitic stainless steel pipe with an outer diameter of 6.35 mm (1 / 4 inch) and a wall thickness of 1.3 mm. The conditions for the final heat treatment are as shown in Table 2. Some of the obtained steel pipes were subjected to heating treatment. The chemical compositions of the obtained steel pipes are as shown in Table 1.

[0074] [Table 1]

[0075] In Table 2 described later, "Air" in the item of the atmosphere in the heat treatment indicates that the heat treatment was performed in the air atmosphere for 10 minutes or less, followed by pickling, and "BA" indicates that the annealing was performed in a 0.1 MPa H atmosphere for 10 minutes or less.

[0076] In Table 2, "Yes 1" in the heating treatment column indicates that heating treatment was performed in an air atmosphere at 80°C for 7 days, and "Yes 2" indicates that heating treatment was performed in an N2 gas or Ar gas atmosphere at atmospheric pressure at 300°C for 4 hours.

[0077] The obtained steel pipes were subjected to thermal desorption hydrogen analysis at a heating rate of 100°C / h from 25°C to 800°C to examine the state of hydrogen (amount of released hydrogen [H], peak temperature of hydrogen release [Tp], peak velocity of hydrogen release [Rmax]). The grain size number of the weld was also measured. Each value was measured using the following procedure.

[0078] (state of hydrogen) After cutting a 30 mm long (in the elongation direction) test piece from the steel pipe, the test piece was degreased and cleaned with an organic solvent. Next, the test piece was used to measure the hydrogen desorbed by thermal desorption analysis (TDA). In TDA, the test piece was heated in an argon atmosphere from 25 (room temperature) to 800°C at a heating rate of 100°C / h, and the hydrogen released (desorbed) from the test piece was measured by chromatography. Based on the TDA measurement results, a TDA curve was created, and the amount of released hydrogen [H], peak temperature of hydrogen desorption [Tp], and peak velocity of hydrogen desorption [Rmax] were calculated.

[0079] (grain size of welded joint) First, a mounted specimen was prepared, including the welded portion of the steel pipe, with the cross section perpendicular to the pipe extension direction as the observation surface. The specimen was then subjected to microstructural observation using an optical microscope, and the grain size was measured. The grain size measurement was performed in accordance with the intercepting method in the microscope testing method described in JIS G 0551:2020.

[0080] (Evaluation of fatigue properties) Similar to the measurements of the physical properties described above, fatigue properties in a hydrogen environment were also evaluated. Specifically, the resulting steel pipe (500 mm long) was placed in a test specimen chamber. The chamber was kept at -70°C, and the H2 gas pressure inside the steel pipe was increased from 0 MPa to 70 MPa, and then reduced from 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.

[0081] In Table 2, the number of fatigue cycles indicates the number of cycles at which hydrogen leakage occurred, and if there was no hydrogen leakage after 10,000 cycles, it was marked as 10,000. In addition, in the fatigue property evaluation, if there was no hydrogen leakage after 10,000 cycles, it was rated as ○, and if there was no hydrogen leakage and no change in outer diameter after 10,000 cycles, it was rated as ⊚. On the other hand, if hydrogen leakage occurred before 10,000 cycles, it was rated as ×. The results are summarized below in Table 2.

[0082] [Table 2]

[0083] Nos. 1, 4, 7, 9, 11, 13, 15, and 17, which satisfied the requirements of this embodiment, had good fatigue properties in a hydrogen environment, while Nos. 2, 3, 5, 6, 8, 10, 12, 14, 16, and 18, which did not satisfy the requirements of this embodiment, had poor fatigue properties in a hydrogen environment.

Claims

1. A steel pipe having a weld, When the amount of hydrogen released was measured using thermal desorption hydrogen analysis at a temperature rise rate of 100°C / h in the temperature range of 25 to 800°C, The amount of released hydrogen [H] is less than 10.0 ppm, The peak temperature [Tp] of hydrogen release is 350°C or higher, The peak rate of hydrogen release [Rmax] is 0.050 ppm / min or less; An austenitic stainless steel pipe, wherein the grain size of the welded portion is 8.0 or more in terms of grain size number.

2. The chemical composition of the steel pipe is, in mass%, C: 0.080% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.050% or less, S: 0.020% or less, Cr: 17.0-20.0%, Ni: 8.0 to 13.0%, N: 0.25% or less, Nb: 0 to 0.20%, Ti: 0 to 0.20%, Mo: 0-1.0%, Cu: 0 to 1.0%, Al: 0-0.30%, Co: 0 to 0.50%, V: 0-0.50%, W: 0-0.50%, B: 0 to 0.0050%, Ca: 0-0.010%, Mg: 0 to 0.010%, Zr: 0 to 0.50%, Ga: 0-0.05%, Hf: 0-0.10%, REM: 0-0.10%, The austenitic stainless steel pipe according to claim 1, wherein the balance is Fe and impurities.

3. 3. The austenitic stainless steel pipe according to claim 1, which is used in a high-pressure hydrogen gas environment or a liquefied hydrogen environment.

4. 3. The austenitic stainless steel pipe according to claim 1, which is used as a component of a hydrogen gas production device or a hydrogen gas supply device.

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