Austenitic stainless steel material

Abstract

To provide an austenitic stainless steel material for use in a methanation environment having excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength.SOLUTION: An austenitic stainless steel material for a methanation environment contains, in terms of mass percent, C: 0.100% or less, Si: 1.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 30.00%, Ni: 10.00 to 40.00%, N: 0.500% or less, and Mo: 3.00% or less, the balance being Fe and inevitable impurities, wherein a Ni equivalent represented by the following formula (1) is 23.0 to 60.0. Ni equivalent=12.6C+0.35Si+1.05Mn+Ni+0.65Cr+0.98Mo (where, each elemental symbol represents the content (%) of the corresponding element).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel material, and more particularly to an austenitic stainless steel material for methanation environments that can be used in methanation environments. [Background technology]

[0002] There are various known methanation technologies for producing methane from hydrogen, including biomethanation, solid oxide fuel cells (SOEC), and methanation using the Sabatier reaction. In particular, methanation using the Sabatier reaction is known as a method of producing methane by reacting a catalyst such as LaNi5 with carbon dioxide captured from the air and hydrogen obtained using renewable energy under medium-high temperature and pressure conditions at temperatures of 600 to 850°C and pressures of approximately 0.1 to 2 MPa.

[0003] Because methanation reactions utilize hydrogen gas under medium- to high-temperature and high-pressure conditions, materials used in methanation reaction environments must have high steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength. To date, materials such as SUS444 (19Cr-0.5Nb-2Mo) specified in JIS G4305 and Ni-Cr-Mo-Nb alloys (Patent Documents 1 to 3) have been used for applications requiring corrosion resistance and heat resistance, such as solar water heaters, heat exchangers, and exhaust manifolds for automobile exhaust gases. Ferritic stainless steel alloys have also been used as materials with steam oxidation resistance (Patent Document 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-161528 [Patent Document 2] Japanese Patent Publication No. 2021-161534 [Patent Document 3] Japanese Patent Application Publication No. 2023-12078 [Patent Document 4] Japanese Patent Application Laid-Open No. 2014-189826 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the high-temperature creep strength of conventional materials such as SUS444 and those described in Patent Documents 1 to 3 is not sufficient for use in a methanation environment. Furthermore, the hydrogen embrittlement resistance of the material described in Patent Document 4 is not sufficient for use in a methanation environment. Thus, a stainless steel material that combines excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength suitable for a methanation environment is not yet known, and there is room for further research.

[0006] In view of the above problems, an object of the present invention is to provide an austenitic stainless steel material for use in a methanation environment that has excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above problems and have obtained the following findings. Increasing the Cr content in austenitic stainless steel is effective in improving the steam oxidation resistance of the steel. However, too much Cr reduces the stability of the austenite phase (γ phase). Adding elements such as Ni and C is effective in increasing the stability of the austenite phase. These elements not only increase the stability of the austenite phase, but also improve creep strength in the high-temperature environments required for methanation. However, too much Ni and C content increases manufacturing costs and reduces workability.

[0008] In methanation environments, hydrogen is used under medium- to high-temperature and high-pressure conditions, so the issue of hydrogen embrittlement must be taken into consideration. In austenitic stainless steels, if the Ni equivalent is low and the stability of the austenite phase is reduced, hydrogen sensitivity increases, leading to hydrogen embrittlement. On the other hand, if the Ni equivalent is too high, the grain boundary strength, the degree of localization of dislocation slip, and the local vacancy density increase, resulting in increased hydrogen sensitivity and hydrogen embrittlement.

[0009] Taking these circumstances into consideration, the present inventors have conducted studies and found that the above-mentioned problems can be solved by controlling the composition and Ni equivalent of the austenitic stainless steel material within appropriate ranges taking into account the methanation environment, and have thus completed the present invention.

[0010] That is, the present invention provides a steel sheet containing, by mass, C: 0.100% or less, Si: 1.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 30.00%, Ni: 10.00 to 40.00%, N: 0.500% or less, Mo: 3.00% or less, and the balance being Fe and impurities; An austenitic stainless steel material for use in a methanation environment, having a Ni equivalent, as represented by the following formula (1), of 23.0 to 60.0. Ni equivalent=12.6C+0.35Si+1.05Mn+Ni+0.65Cr+0.98Mo (1) In the formula, each element symbol represents the content (%) of each element. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide an austenitic stainless steel material for use in a methanation environment that combines excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0013] In this specification, "austenitic" refers to a metal structure that is primarily austenite at room temperature. Therefore, "austenitic" also includes those that contain small amounts of phases other than austenite (e.g., ferrite or martensite). However, "austenitic" does not include a multi-phase structure of ferrite and austenite, a multi-phase structure of ferrite and martensite, or a multi-phase structure of ferrite, austenite, and martensite. In this specification, "stainless steel material" refers to a material made of stainless steel, and the shape of the material is not particularly limited. Examples of the shape include plate (including strip), rod, and tube. Also, various types of shaped steel may be used, such as T-shaped and I-shaped cross sections.

[0014] In this specification, the term "impurities" refers to components that are mixed in during the industrial production of austenitic stainless steel materials due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities also include unavoidable impurities. Regarding the content of each element, "including xx% or less" means that the content is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level).

[0015] In this specification, "for a methanation environment" means being applicable under an environment where a methanation reaction proceeds. Members for a methanation environment can include members other than austenitic stainless steel materials in addition to austenitic stainless steel materials. Examples of members other than austenitic stainless steel materials include members of a methane production apparatus using a methanation reaction. Such members include various members in a methanation environment, such as a methanation reactor, a base of the reactor, a liner part, a pipe contacted by high-temperature and high-pressure hydrogen or steam, a joint for connecting pipes to each other, a connecting member such as a bolt, and a member used for a structure carrying a methanation catalyst.

[0016] Further, the austenitic stainless steel material according to an embodiment of the present invention can be used in various methanation environments such as biomethanation, a solid oxide fuel cell (SOFC), and a methanation environment using a Sabatier reaction. In particular, the austenitic stainless steel material according to an embodiment of the present invention is preferably used in a methanation environment where methane and water are generated from hydrogen and carbon dioxide under medium-high temperature and high-pressure conditions of a temperature of 600 to 850 °C and a pressure of 0.1 to 2 MPa.

[0017] (Composition) The austenitic stainless steel material according to an embodiment of the present invention contains C: 0.100% or less, Si: 1.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 30.00%, Ni: 10.00 to 40.00%, N: 0.500% or less, Mo: 3.00% or less, and the balance consists of Fe and impurities. Hereinafter, each component will be described in detail.

[0018] <C: 0.100% or less> C is an element that contributes to the improvement of creep strength and the stabilization of the austenite phase in the methanation environment. However, if the C content is too high, the workability will decrease. Therefore, the upper limit value of the C content is controlled to 0.100%, preferably 0.080%, more preferably 0.078%, and still more preferably 0.074%. On the other hand, the lower limit value of the C content is not particularly limited, but from the perspective of ensuring the above effects, it is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, and particularly preferably 0.020%.

[0019] <Si: 1.00% or less> Si is an element that contributes to corrosion resistance. However, when added in a large amount, although the high-temperature strength increases, the toughness decreases during processing. Therefore, the upper limit value of the Si content is controlled to 1.00%, preferably 0.70%, more preferably 0.65%, and still more preferably 0.60%. On the other hand, the lower limit value of the Si content is not particularly limited, but it is preferably 0.20%, more preferably 0.25%, and still more preferably 0.30%.

[0020] <Mn: 3.00% or less> Mn is an austenite phase (γ-phase) forming element. However, if the Mn content is too high, the workability will decrease. Therefore, the upper limit value of the Mn content is controlled to 3.00%, preferably 2.50%, more preferably 2.00%, and still more preferably 1.90%. On the other hand, the lower limit value of the Mn content is not particularly limited, but from the perspective of enhancing the stability of the austenite phase, it is preferably 0.25%, more preferably 0.30%, and still more preferably 0.35%.

[0021] <P: 0.100% or less> If the P content is too high, it will not only deteriorate the steam oxidation resistance under the methanation environment, but also cause a decrease in workability and weldability. Therefore, from the perspective of ensuring these effects, the upper limit value of the P content is controlled to 0.100%, preferably 0.050%, more preferably 0.040%, and still more preferably 0.030%. On the other hand, the lower limit value of the P content is not particularly limited, but an excessive reduction in the P content will lead to an increase in steelmaking load and raw material costs. Therefore, the lower limit value of the P content is generally 0.001%, preferably 0.005%, and more preferably 0.010%.

[0022] <S: 0.100% or less> S forms MnS-based non-metallic inclusions. MnS-based inclusions are stretched in the rolling direction, but tend to exist in the steel material as inclusions that are elongated in the rolling direction without being finely divided. This type of inclusion is likely to be a starting point for fatigue fracture with respect to bending stress having the rolling parallel direction as the bending axis. Therefore, from the perspective of ensuring the hydrogen embrittlement resistance under the methanation environment, the upper limit value of the S content is controlled to 0.100%, preferably 0.050%, and more preferably 0.001%. On the other hand, the lower limit value of the S content is not particularly limited, but an excessive reduction in the S content may cause problems such as an increase in steelmaking load and raw material costs, and is likely to be a corrosion starting point in the methanation environment, resulting in a decrease in the toughness of the welded part. The lower limit value of the S content is generally 0.0001%, preferably 0.0002%, and more preferably 0.0005%.

[0023] <Cr: 16.00 - 30.00%> Cr is an element that contributes to the improvement of steam oxidation resistance under the methanation environment. From the perspective of ensuring this effect, the lower limit value of the Cr content is controlled to 16.00%, preferably 17.00%, and more preferably 17.50%. On the other hand, if the Cr content is too high, the formation of intermetallic compounds (σ phase) will be promoted, resulting in a decrease in the workability and toughness of austenitic stainless steel materials. Therefore, the upper limit value of the Cr content is controlled to 30.00%, preferably 28.00%, and more preferably 27.00%.

[0024] <Ni: 10.00 to 40.00%> Ni is an element that contributes to the stabilization of the austenite phase and the ensuring of creep strength under medium and high temperature and high pressure conditions suitable for the methanation environment. From the perspective of ensuring these effects, the lower limit value of the Ni content is controlled to 10.00%, preferably 11.00%, more preferably 12.00%. On the other hand, if the Ni content is too high, it will cause α-phase destabilization and lead to an increase in manufacturing costs. Therefore, the upper limit value of the Ni content is controlled to 40.00%, preferably 39.00%, more preferably 32.00%, and even more preferably 20.00%.

[0025] <N: 0.500% or less> N is an element that contributes to high strength and the stabilization of the austenite phase. However, if the N content is too high, the workability will decrease. Therefore, the upper limit value of the N content is controlled to 0.500%, preferably 0.400%, more preferably 0.250%. On the other hand, the lower limit value of the N content is not particularly limited, but from the perspective of ensuring the above effects, it is preferably 0.001%, more preferably 0.003%, even more preferably 0.050%, and particularly preferably 0.060%.

[0026] <Mo: 3.00% or less> Mo is an element effective in improving the water vapor oxidation resistance characteristics. However, since Mo is expensive, if the Mo content is too high, it will lead to an increase in manufacturing costs. Therefore, the upper limit value of the Mo content is controlled to 3.00%, preferably 2.50%, more preferably 2.10%. On the other hand, the lower limit value of the Mo content is not particularly limited, but from the perspective of ensuring the effects of Mo, it is preferably 0.01%, more preferably 0.02%.

[0027] The austenitic stainless steel material according to an embodiment of the present invention may further contain one or more selected from Ti: 1.000% or less, Al: 1.000% or less, Nb: 1.00% or less, V: 1.000% or less, Co: 1.00% or less, W: 1.00% or less, Zr: 1.00% or less, Cu: 1.00% or less, Zn: 1.000% or less, Ta: 1.000% or less, REM: 0.10% or less, Ca: 0.100% or less, Sn: 0.100% or less, Bi: 0.010% or less, Pb: 0.010% or less, B: 0.0100% or less, and Mg: 0.0100% or less.

[0028] <Ti: 1.000% or less> Ti is an element for improving oxidation resistance and fixing C in steel to improve intergranular corrosion resistance. However, if the Ti content is too high, the amount of coarse inclusions generated increases, which becomes a factor in reducing fatigue properties. Therefore, the upper limit value of the Ti content is controlled to 1.000%, preferably 0.800%, more preferably 0.700%. On the other hand, the lower limit value of the Ti content is not particularly limited, but from the viewpoint of ensuring the effect of Ti, it is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%.

[0029] <Al: 1.000% or less> Al is an element that acts as a strong deoxidizer and is effective for improving oxidation resistance. However, if Al is added excessively, the cleanliness of inclusions cannot be controlled within a predetermined range, and high-temperature fatigue properties deteriorate. Therefore, the upper limit value of the Al content is controlled to 1.000%, preferably 0.600%, more preferably 0.500%. On the other hand, the lower limit value of the Al content is not particularly limited, but from the viewpoint of ensuring the effect of Al, it is preferably 0.001%, more preferably 0.005%, and still more preferably 0.010%.

[0030] <Nb: 1.00% or less> Nb is an element with a high affinity for C and N. During hot rolling, it precipitates as carbide or nitride, reducing the dissolved C and N in the matrix phase, and has the effect of improving the workability and oxidation resistance. It is also an element effective in improving the steam oxidation resistance characteristics in a methanation environment. However, if the Nb content is too high, the austenitic stainless steel material will harden and the ductility will decrease. Therefore, the upper limit value of the Nb content is controlled to 1.00%, preferably 0.70%, more preferably 0.60%. On the other hand, the lower limit value of the Nb content is not particularly limited, but from the perspective of ensuring the effect of Nb, it is preferably 0.01%, more preferably 0.05%.

[0031] <V: 1.000% or less> V is an element effective in improving the oxidation resistance of austenitic stainless steel materials. However, if the V content is too high, it will lead to a decrease in workability and toughness and an increase in manufacturing costs. Therefore, the upper limit value of the V content is controlled to 1.000%, preferably 0.800%, more preferably 0.700%. On the other hand, the lower limit values of the V content are not particularly limited, but from the perspective of obtaining the effect of V, it is preferably 0.010%, more preferably 0.030%, still more preferably 0.050%.

[0032] <Co: 1.00% or less> Co is an element effective in improving the oxidation resistance of austenitic stainless steel materials. However, if the Co content is too high, the workability will decrease and it will lead to an increase in manufacturing costs. Therefore, the upper limit value of the Co content is controlled to 1.00%, preferably 0.50%, more preferably 0.10%. On the other hand, the lower limit value of the Co content is not particularly limited, but from the perspective of ensuring the effect of Co, it is preferably 0.01%, more preferably 0.02%.

[0033] <W: 1.00% or less> W is an element for improving high-temperature strength without impairing ductility at room temperature. However, if the W content is too high, coarse eutectic carbides will be formed, causing a decrease in ductility. Therefore, the upper limit value of the W content is controlled to 1.00%, preferably 0.90%, more preferably 0.80%. On the other hand, the lower limit value of the W content is not particularly limited, but from the viewpoint of ensuring the effect of W, it is preferably 0.01%, more preferably 0.10%.

[0034] <Zr: 1.00% or less> Zr is an element that contributes to improving the oxidation resistance and corrosion resistance of austenitic stainless steel materials. However, if the content of Zr is too high, the workability and toughness of the austenitic stainless steel material will decrease, leading to an increase in manufacturing cost. Therefore, the upper limit value of the Zr content is controlled to 1.00%, preferably 0.80%, more preferably 0.50%. On the other hand, the lower limit value of the Zr content is not particularly limited, but from the viewpoint of obtaining the effect of Zr, it is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0035] <Cu: 1.00% or less> Cu is an element that suppresses work hardening of the austenite phase. Therefore, it is advantageous to contain Cu when the degree of processing by press forming is large or when cold forging is performed. However, if the Cu content is too high, it will cause a decrease in corrosion resistance. Therefore, the upper limit value of the Cu content is controlled to 1.00%, preferably 0.70%, more preferably 0.50%. On the other hand, the lower limit value of the Cu content is not particularly limited, but from the viewpoint of ensuring the effect of Cu, it is preferably 0.01%, more preferably 0.10%.

[0036] <Zn: 1.000% or less> Zn is an element that contributes to the hot workability and weldability of austenitic stainless steel materials. From the perspective of obtaining the effect of Zn, the upper limit value of the Zn content is controlled to be 1.000%, preferably 0.700%, more preferably 0.500%. On the other hand, the lower limit value of the Zn content is not particularly limited, but from the perspective of ensuring the effect of Zn, it is preferably 0.001%, more preferably 0.002%.

[0037] <Ta: 1.000% or less> Ta is an element for improving corrosion resistance. However, if the content of Ta is too high, the workability and toughness of austenitic stainless steel materials will decrease, leading to an increase in manufacturing costs. Therefore, the upper limit value of the Ta content is controlled to be 1.000%, preferably 0.100%, more preferably 0.010%. On the other hand, the lower limit value of the Ta content is not particularly limited, but from the perspective of obtaining the effect of Ta, it is preferably 0.001%, more preferably 0.002%.

[0038] <REM: 0.10% or less> REM is an element for improving productivity (hot workability). However, since REM is expensive, if the content of REM is too high, it will lead to an increase in manufacturing costs. Therefore, the upper limit value of the REM content is controlled to be 0.10%, preferably 0.08%, more preferably 0.05%. On the other hand, the lower limit value of the REM content is not particularly limited, but from the perspective of obtaining the effect of REM, it is preferably 0.01%, more preferably 0.02%. Note that REM refers to the general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements from lanthanum (La) to lutetium (Lu) (lanthanoids). These may be used alone or as a mixture.

[0039] <Ca: 0.100% or less> Ca is an element for improving hot workability. However, if the Ca content is too high, the toughness of austenitic stainless steel materials will decrease. Therefore, the upper limit value of the Ca content is controlled to be 0.100%, preferably 0.050%, more preferably 0.030%. On the other hand, the lower limit value of the Ca content is not particularly limited, but from the viewpoint of ensuring the effect of Ca, it is preferably 0.0001%, more preferably 0.001%.

[0040] <Sn: 0.100% or less> Sn is an element for improving workability by promoting the generation of deformation bands during rolling. However, if the Sn content is too high, the effect of Sn will saturate and the workability will decrease. Therefore, the upper limit value of the Sn content is controlled to be 0.100%, preferably 0.050%. On the other hand, the lower limit value of the Sn content is not particularly limited, but from the viewpoint of ensuring the effect of Sn, it is preferably 0.001%, more preferably 0.005%.

[0041] <Bi: 0.010% or less> Bi is an element effective for improving machinability. However, excessive addition of Bi leads to an increase in manufacturing cost, so the upper limit value of the Bi content is controlled to be 0.010%, preferably 0.008%, more preferably 0.005%. On the other hand, the lower limit value of the Bi content is not particularly limited, but from the viewpoint of obtaining the effect of Bi, it is preferably 0.001%, more preferably 0.002% or more.

[0042] <Pb: 0.010% or less> Pb is an element for improving machinability. However, if the Pb content is too high, there is a concern of causing deterioration of hot workability such as liquation cracking based on grain boundary melting by lowering the melting point of grain boundaries and reducing the bonding force of grain boundaries. Therefore, the upper limit value of the Pb content is controlled to be 0.010%, preferably 0.008%, still more preferably 0.005%. On the other hand, the lower limit value of the Pb content is not particularly limited, but from the viewpoint of ensuring the effect of Pb, it is preferably 0.001%.

[0043] <B: Below 0.0100%> B is an element effective for suppressing the occurrence of surface defects, improving productivity, and improving weldability. However, if the B content is too high, these properties will deteriorate conversely. Therefore, the upper limit value of the B content is controlled to 0.0100%, preferably 0.0060%. On the other hand, the lower limit value of the B content is not particularly limited, but from the viewpoint of ensuring the effect of B, it is preferably 0.0001%, more preferably 0.0010%.

[0044] <Mg: Below 0.0100%> Mg is an element that forms Mg oxide together with Al in molten steel and acts as a deoxidizer. However, if the Mg content is too high, the toughness of austenitic stainless steel materials will decrease. Therefore, the upper limit value of the Mg content is controlled to 0.0100%, preferably 0.0050%. On the other hand, the lower limit value of the Mg content is not particularly limited, but from the viewpoint of ensuring the effect of Mg, it is preferably 0.0001%, more preferably 0.0003%.

[0045] The austenitic stainless steel material for a methanation environment according to an embodiment of the present invention has a Ni equivalent represented by the following formula (1) of 23.0 to 60.0. Ni equivalent = 12.6C + 0.35Si + 1.05Mn + Ni + 0.65Cr + 0.98Mo ···(1) In the formula, each element symbol represents the content (%) of each element.

[0046] Hydrogen embrittlement is a phenomenon in which a metal material interacts with hydrogen and as a result becomes brittle. Hydrogen embrittlement in austenitic stainless steel materials is understood in relation to the stability of the austenite phase. When the Ni equivalent is too low and the stability of the austenite phase becomes low, the hydrogen sensitivity increases and hydrogen embrittlement occurs. On the other hand, if the Ni equivalent is too high, the grain boundary strength, the localization degree of dislocation slip, and the local pore density increase, and conversely, as a result of hydrogen embrittlement, the hydrogen embrittlement resistance property deteriorates.

[0047] By controlling the Ni equivalent within the above range, an austenitic stainless steel material that satisfies steam oxidation resistance suitable for a methanation environment can be obtained. Therefore, the upper limit of the Ni equivalent is 60.0, preferably 50.0, more preferably 40.0, and even more preferably 30.0. The lower limit of the Ni equivalent is 23.0, preferably 25.0, and more preferably 28.0.

[0048] In the austenitic stainless steel material for use in a methanation environment according to an embodiment of the present invention, Y, represented by the following formula (2), is −5.00 or less. Y=(-0.35Cr)+(-0.8Al)+7.8×Solute C content...(2) In the formula, each element symbol represents the content (%) of each element.

[0049] According to the austenitic stainless steel material for use in a methanation environment according to an embodiment of the present invention, in addition to the steel composition and Ni equivalent, Y in formula (2) is -5.00 or less, thereby obtaining an austenitic stainless steel material for use in a methanation environment that combines excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength. The upper limit of Y is preferably -5.50, more preferably -6.60, and even more preferably -7.00. The lower limit of Y is preferably -20.00, more preferably -15.00.

[0050] (Solute C amount) Carbon in austenitic stainless steel materials forms Cr carbides in high-temperature environments, resulting in Cr deficiency. To prevent Cr deficiency, the amount of solute carbon can be reduced by adding Ti and Nb to form TiC or NbC. If the solute carbon content is too high, the steam oxidation resistance required in methanation environments may not be sufficiently achieved. The upper limit of the solute carbon content is 0.020%, preferably 0.010%, and more preferably 0.008%. The lower limit of the solute carbon content is 0.0001%, preferably 0.0005%, and more preferably 0.001%. In this specification, the "solute carbon content" can be measured by dissolving the surface by electrolytic etching to extract Ti and Nb carbides, subjecting the extracted carbides to X-ray diffraction (XRD) analysis, determining the peak intensities of Ti carbide and Nb carbide from the obtained XRD profile, calculating the amount of consumed carbon using the formula described below, and subtracting the amount of consumed carbon from the C content of the steel.

[0051] (Oxidation increase) "Oxidation weight gain" is an index used to evaluate steam oxidation resistance suitable for a methanation environment. The oxidation weight gain refers to the weight gain per unit area when a steel material measuring 10 mm (thickness direction) x 25 mm (width direction) x 25 mm (rolling direction) is oxidized in a methanation environment.

[0052] In this specification, the methanation environment refers to an environment having a temperature of 600 to 850°C, a pressure of 0.1 MPa or more, and containing 20.0 mol % or more of hydrogen and 15.0 mol % or more of water vapor. More specifically, it refers to an environment with a temperature of 600 to 850°C, a pressure of 0.10 to 20.0 MPa, and containing, on a molar basis, 20.0 to 65.0% H2, 15.0 to 50.0% H2O, 1.0 to 20.0% CO, 3.0 to 10.0% CO2, and 0.1 to 30.0% CH4. More specifically, the methanation environment in this embodiment is: - An environment at 600°C and 0.1 MPa, with a mixed gas composition of H2: 45.0%, H2O: 20.4%, CO: 5.7%, CO2: 6.9%, and CH4: 7.3%. - An environment at 600°C, 2.0 MPa, with a mixed gas composition of H2: 23.7%, H2O: 47.1%, CO: 1.0%, CO2: 5.2%, and CH4: 16.7%. - 700℃, 0.1MPa, mixed gas composition: H2: 61.0%, H2O: 16.0%, CO: 13.0%, CO2: 6.0%, CH4: 2.0% - 700℃, 2.0MPa, mixed gas composition H2: 36.2%, H2O: 37.1%, CO: 1.0%, CO2: 5.2%, CH4: 16.7% - An environment where the mixed gas composition is H2: 63.7%, H2O: 15.9%, CO: 15.6%, CO2: 4.2%, CH4: 0.2% at 800℃ and 0.1MPa. - 800℃, 2.0MPa, mixed gas composition: H2: 48.5%, H2O: 27.7%, CO: 8.9%, CO2: 5.5%, CH4: 9.4% - An environment where the mixed gas composition is H2: 48.5%, H2O: 27.7%, CO: 8.9%, CO2: 5.5%, and CH4: 9.4% at 850°C and 2.0 MPa. The mixed gas may further contain oxygen and nitrogen. The upper limit of the oxidation mass gain of the austenitic stainless steel material for use in a methanation environment according to the embodiment of the present invention is 5.0 mg / cm 2 , preferably 2.5 mg / cm 2 , more preferably 2.0 mg / cm 2 , and even more preferably 2.0 mg / cm 2 The lower limit of the oxidation mass gain is 0.1 mg / cm 2 is.

[0053] (oxide film) An austenitic stainless steel material for a methanation environment according to an embodiment of the present invention has an oxide film on the surface of the austenitic stainless steel material, the oxide film containing 20% ​​or more of Cr and 50% or less of O. This oxide film is obtained by oxidizing the austenitic stainless steel material for a methanation environment in a methanation environment.

[0054] According to the austenitic stainless steel material for use in a methanation environment according to an embodiment of the present invention, a Cr-containing oxide film is formed on the steel surface, which makes it possible to reduce the effects of hydrogen embrittlement and steam oxidation caused by a mixed gas containing hydrogen and steam, even in a medium- to high-temperature and high-pressure methanation environment at a temperature of 600 to 850°C and a pressure of about 0.1 to 2 MPa, and also to reduce creep damage caused by long-term exposure to high-temperature and high-pressure gas.

[0055] The oxide film preferably contains 30% or more of Cr, and more preferably 35% or more. The upper limit of the Cr content in the oxide film is 45%, and more preferably 38%. If the oxide film is too thin, the desired effect cannot be obtained, but if it is too thick, excess oxide film deposited on the steel surface may peel off, and the peeled parts may become corrosion initiation points, causing hydrogen embrittlement. The upper limit of the thickness of the oxide film is preferably 2.00 μm, more preferably 1.50 μm, and even more preferably 1.00 μm. The lower limit of the thickness of the oxide film is preferably 0.01 μm, more preferably 0.02 μm, and even more preferably 0.03 μm.

[0056] The thickness of the oxide film and the concentrations of Cr and O contained in the oxide film are measured based on a profile obtained by depth analysis using a GDS (GD-Profiler2 manufactured by HORIBA France SAS). The thickness of the oxide film is defined as the depth from the outermost surface of the steel material to the position where the O (oxygen) concentration drops to half of its peak value. The Cr concentration in the oxide film is defined as the cation fraction at the depth from the outermost surface of the steel material where the O concentration reaches its peak value. The O concentration is defined as the cation fraction at the depth from the outermost surface of the steel material where the Cr concentration reaches its peak value.

[0057] (Creep rupture strength) Creep rupture strength is an index for evaluating the high-temperature creep strength of austenitic stainless steel materials. The austenitic stainless steel materials for use in methanation environments according to embodiments of the present invention have a creep rupture strength of 15 MPa or more at 800°C when oxidized in a methanation environment. The creep rupture strength is preferably 20 MPa or more, and more preferably 30 MPa or more. Creep rupture strength is measured in accordance with JIS Z2271:2019 by taking a test specimen (round bar, parallel portion 6 mmφ × 80 mm (rolling direction)) and determining the strength at rupture of the steel material at 800°C using a direct-loading creep testing machine.

[0058] (Charpy impact value) The Charpy impact value is an index for evaluating hydrogen embrittlement of austenitic stainless steel materials. The austenitic stainless steel material for a methanation environment according to an embodiment of the present invention has a Charpy impact value of 200 J / cm at −196°C when oxidized in a methanation environment. 2 The Charpy impact value is measured by determining the absorbed energy at -196°C in a Charpy impact test in accordance with JIS Z2242:2018. The lower limit of the Charpy impact value is preferably 205 J / cm 2 , more preferably 230 J / cm 2 The upper limit of the Charpy impact value is preferably 300 J / cm 2 , more preferably 250 J / cm 2 More preferably, 230 J / cm 2 is.

[0059] The method for producing an austenitic stainless steel material according to an embodiment of the present invention is not particularly limited. For example, in a method for producing an austenitic stainless steel material according to an embodiment of the present invention, a stainless steel having the above-described composition is vacuum melted, forged, heated, and then hot-rolled to produce a hot-rolled sheet. The hot-rolled sheet is then annealed and pickled to obtain the product. Finally, finish polishing may be performed. By performing finish polishing, diffusion of Cr from the steel surface can be promoted when a heating test is performed in a methanation atmosphere, and a dense Cr oxide film can be formed on the surface. This results in an austenitic stainless steel material that combines excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength in a methanation environment.

[0060] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0061] (Examples 1 to 13 and Comparative Examples 2 to 6) Stainless steel having the composition shown in Table 1 (the balance being Fe and impurities) was melted, continuously cast, and then hot-rolled to obtain a hot-rolled sheet with a thickness of 10 mm. The hot-rolled sheet was annealed at 1000°C and pickled to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was then pickled to obtain an annealed and pickled sheet. The annealed and pickled sheet was then polished as necessary to obtain a polished and finished sheet. (Comparative Example 1) A commercially available annealed and pickled rolled sheet of nickel-chromium-molybdenum-niobium alloy (Inconel 625, NCF625 specified in JIS G4902:2019) (thickness: 10 mm) was used.

[0062] [Table 1]

[0063] The plate materials of Examples 1 to 13 and Comparative Examples 1 to 6 were evaluated as follows.

[0064] <Evaluation of hydrogen embrittlement resistance (Charpy impact value)> The hydrogen embrittlement resistance of the plates was evaluated using a Charpy impact test in accordance with JIS Z2242:2018. For the Charpy impact test, full-size Charpy test specimens measuring 10 mm (thickness direction) x 10 mm (width direction) x 55 mm (rolling direction) were taken from each plate. A V-notch was created in these specimens, and they were then hydrogen-charged in a high-temperature, high-pressure gas autoclave at 400°C and 4 MPa for 96 hours, a hydrogen charge equivalent to that in a methanation atmosphere of 700°C and 2 MPa. After hydrogen charging, the specimens were immediately immersed in liquid nitrogen to prevent dehydrogenation. The Charpy impact test was performed on the specimens at -196°C, and the absorbed energy of each specimen was determined. This was then divided by the cross-sectional area of ​​the specimen to determine the impact energy (Charpy impact value) (J / cm). 2 ) was obtained. The Charpy impact value was 200 J / cm 2 The above specimens were evaluated as having good hydrogen embrittlement resistance.

[0065] <Evaluation of steam oxidation resistance (oxidation gain, oxide film)> The hydrogen embrittlement resistance of each plate material was evaluated by conducting the steam oxidation test described below. Test pieces measuring 10 mm (thickness direction) × 25 mm (width direction) × 25 mm (rolling direction) were taken from the plate material, and the surface area (cm) of the test piece was calculated from the lengths in the thickness direction, width direction, and rolling direction before the test. 2 ) was measured, and the weight of each test piece was measured. After the weight measurement, each test piece was ultrasonically cleaned with acetone. The steam oxidation test was carried out for 100 hours in a gas atmosphere simulating a methanation environment at a temperature of 600°C to 850°C and a pressure of 0.1 MPa to 2 MPa. Details of the test conditions and gas atmosphere are shown in Table 2.

[0066] The oxidation weight gain was calculated by subtracting the weight before the test from the weight after the test, and the average value was calculated for N = 2. The thickness and composition of the oxide film were measured based on the profile obtained by depth analysis using GDS. The thickness of the oxide film was defined as the depth from the outermost surface of the steel material to the position where the O (oxygen) concentration decreased to half of its peak value. The Cr concentration in the oxide film was defined as the cation fraction at the depth from the outermost surface of the steel material where the O concentration reached its peak value. The O concentration was defined as the cation fraction at the depth from the outermost surface of the steel material where the Cr concentration reached its peak value.

[0067] <Evaluation of the amount of solute C> The amount of Ti and Nb carbides formed was measured as the amount of dissolved carbon in each sheet material using the following procedure. Test pieces measuring 10 mm (thickness direction) × 50 mm (width direction) × 50 mm (rolling direction) were cut from the sheet material, and the entire surface of the test piece was wet-polished with a #600 polishing machine. These test pieces were then electrolytically etched using the SPEED method. Electrolytic etching was performed in a 10% acetylacetone solution at a constant potential of 400 mV until the charge reached 5,000 coulombs. The electrolytic solution was filtered through a filter with a 0.05 μm mesh size to collect the carbides. The dissolved amount and carbide amount were determined by measuring the mass of the carbides. Next, the collected carbides were subjected to X-ray diffraction (XRD) analysis. The peak intensities of Ti carbide and Nb carbide (Ti carbide peak intensity, Nb carbide peak intensity) were determined from the XRD profiles obtained by XRD analysis. The peak positions (diffraction angle 2θ) of each carbide used in the calculation were Ti carbide (TiC): 48.838°, and Nb carbide (NbC): 40.557°. Next, the amount of carbon consumed in forming Ti carbide and Nb carbide (hereinafter referred to as "amount of consumed carbon") was calculated using the following formula. Amount of carbon consumed [mass%] = Amount of carbide collected [g] × (0.21 × Ti carbide peak intensity + 0.12 × Nb carbide peak intensity) / (Ti carbide peak intensity + Nb carbide peak intensity) / Amount dissolved [g] × 100 Next, the amount of dissolved C [mass %] was calculated by subtracting the amount of consumed C [mass %] from the total C content [mass %].

[0068] <Evaluation of high-temperature creep strength (creep rupture strength)> The high-temperature creep strength of the plate materials was evaluated using a creep rupture strength test in accordance with JIS Z2271:2019. For the creep rupture strength test, test specimens (round bar shape, parallel section 6 mmφ x 80 mm (rolling direction)) were taken from the plate materials and subjected to a 100-hour steam oxidation test under the test conditions shown in Table 2 in a gas atmosphere. The strength at rupture of each test specimen at 800°C was determined using a direct-load creep testing machine. A creep rupture strength of 15 MPa or more, which is the creep rupture strength of the methanation vessel, was considered to be acceptable. The results of each of the above evaluations are shown in Table 3.

[0069] [Table 2]

[0070] [Table 3]

[0071] In Examples 1 to 6, the composition and Ni equivalent of the steel plate were within appropriate ranges, and therefore the Charpy impact value, which indicates hydrogen embrittlement resistance, the creep rupture strength, which indicates high-temperature creep strength, and the thickness of the oxide film and the oxidation weight gain, which indicate hydrogen embrittlement resistance, were all good. In Examples 3, 4, and 7 to 12, the Cr content was within the appropriate range and Al was added, so the thickness of the oxide film, which indicates steam oxidation resistance, was thin, the oxidation weight gain was small, and the addition of Ti resulted in a small amount of dissolved C, which was good. In Example 13, the Cr content was within the appropriate range and Al was added, so the thickness of the oxide film, which indicates steam oxidation resistance, was thin and the oxidation weight gain was small. In addition, the addition of Nb resulted in a small amount of dissolved C, which was good. In contrast, in Comparative Examples 1 to 6, the composition and Ni equivalent of the steel sheet were not within the appropriate range, and therefore austenitic stainless steel sheets having excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength in a methanation environment could not be obtained.

[0072] As can be seen from the above results, the present invention can provide an austenitic stainless steel sheet that combines excellent steam oxidation resistance, hydrogen embrittlement resistance, and high-temperature creep strength.

Claims

1. On a mass basis, the alloy contains C: 0.100% or less, Si: 1.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 30.00%, Ni: 10.00 to 40.00%, N: 0.500% or less, Mo: 3.00% or less, and the balance being Fe and impurities; An austenitic stainless steel material for use in a methanation environment, having a Ni equivalent represented by the following formula (1) of 23.0 to 60.

0. Ni equivalent = 12.6C + 0.35Si + 1.05Mn + Ni + 0.65Cr + 0.98Mo ... (1) In the formula, each element symbol represents the content (%) of each element.

2. 2. The austenitic stainless steel material for use in a methanation environment according to claim 1, further comprising, by mass, one or more selected from Ti: 1.000% or less, Al: 1.000% or less, Nb: 1.00% or less, V: 1.000% or less, Co: 1.00% or less, W: 1.00% or less, Zr: 1.00% or less, Cu: 1.00% or less, Zn: 1.000% or less, Ta: 1.000% or less, REM: 0.10% or less, Ca: 0.100% or less, Sn: 0.100% or less, Bi: 0.010% or less, Pb: 0.010% or less, B: 0.0100% or less, and Mg: 0.0100% or less.

3. The austenitic stainless steel material for use in a methanation environment according to claim 1 or 2, wherein Y represented by the following formula (2) is −5.00 or less. Y=(-0.35Cr)+(-0.8Al)+7.8×Solute C amount...(2) In the formula, each element symbol represents the content (%) of each element.

4. 3. The austenitic stainless steel material for use in a methanation environment according to claim 1, wherein the amount of solute C is 0.020% or less by mass.

5. 3. The austenitic stainless steel material for use in a methanation environment according to claim 1, which has a creep rupture strength of 15 MPa or more at 800°C when oxidized in a methanation environment.

6. When oxidized in a methanation environment, the Charpy impact value at -196°C is 200 J / cm 2 The austenitic stainless steel material for use in a methanation environment according to claim 1 or 2.

7. When oxidized in a methanation environment, the oxidation gain was 5.0 mg / cm 2 The austenitic stainless steel material for use in a methanation environment according to claim 1 or 2, wherein:

8. 3. The austenitic stainless steel material for use in a methanation environment according to claim 1, wherein the thickness of an oxide film formed on the surface of the steel sheet when oxidized in a methanation environment is 2.00 μm or less.

9. 3. An austenitic stainless steel material for use in a methanation environment, comprising the austenitic stainless steel material according to claim 1 or 2, having an oxide film on the surface thereof, the oxide film containing, by mass, 20% or more of Cr and 50% or less of O.

10. The austenitic stainless steel material for use in a methanation environment according to claim 9, wherein the oxide film has a thickness of 2.00 μm or less.

11. 10. The austenitic stainless steel material for use in a methanation environment according to claim 9, which has a creep rupture strength of 15 MPa or more at 800°C.

12. Charpy impact value at -196°C is 200 J / cm 2 The austenitic stainless steel material for use in a methanation environment according to claim 9.

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