Ferritic stainless steel for ammonia combustion environments

Abstract

The purpose of the present invention is to provide a ferritic stainless steel for an NH3 combustion environment and having excellent high-temperature NH3 resistance in an NH3 combustion environment in which high-temperature NH3 is included.　This ferritic stainless steel for an NH3 combustion environment has a component composition which contains, in mass%, 0.020% or less of C, 0.05-2.50% of Si, 0.05-2.00% of Mn, 0.050% or less of P, 0.010% or less of S, 0.20-6.00% of Al, 0.020% or less of N, and 12.0-25.0% of Cr, and in which the remaining portion includes Fe and unavoidable impurities.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel for an NH3 (ammonia) combustion environment. More specifically, the present invention relates to a Cr-containing steel, and relates to a ferritic stainless steel having excellent high-temperature NH3 resistance, which is suitable for use in a part exposed to a high temperature of 300 °C or higher in a structure for burning NH3, such as a heating furnace or an incinerator.

Background Art

[0002] For the furnace walls of various furnaces such as heating furnaces and incinerators, in order to provide heat insulation so as not to let the heat in the furnace escape, ceramic heat insulation materials, Ni-based alloys, and stainless steel plates are used. In the case of ceramics, although it has excellent heat insulation, it has problems such as requiring a large amount of energy required for temperature rise, being easily cracked due to temperature differences, and being prone to fine peeling. In addition, Ni-based alloys containing pure Ni have major problems in terms of resource depletion and cost because Ni is a rare metal. On the other hand, in the case of stainless steel plates, although the heat insulation is slightly inferior to that of ceramic heat insulation materials, the excellent conductivity can save the energy required for temperature rise, the waste heat can be heat-exchanged and used, and the material cost can be significantly reduced compared to Ni-based alloys.

[0003] In recent years, with growing awareness of global environmental issues, efforts toward a carbon-neutral society are being made worldwide, and the use of H2 and NH3, which do not emit CO2 when burned, and alcohol, which uses CO2 in its production, as fuel (or at least part of it) is being considered. Hereafter, these fuels will also be called low-carbon fuels. H2 and NH3 do not emit CO2 when burned, and although alcohol fuels emit CO2 when burned, the use of CO2 in production can reduce overall CO2 emissions. However, H2 has a low boiling point of -253°C, and there are cost challenges, including the difficulty of transporting and storing it in liquefied form. On the other hand, NH3 has a boiling point of -33°C and is easier to liquefy than H2, making it a promising option as an energy carrier from the perspective of transport and storage. After transport, NH3 can be used to generate H2 by decomposing it at high temperatures, or by burning the resulting H2 for power generation, or by burning NH3 as part or all of the fuel for power generation.

[0004] Examples of Ni-based alloys exposed to high temperatures of NH3 include NCF600 (15 mass%Cr-8 mass%Fe) as specified in JIS G 4902:2019. Examples of stainless steels include austenitic stainless steels such as SUS310S (25 mass%Cr-19 mass%Ni) as specified in JIS G 4305:2021.

[0005] However, because SUS310S, like Ni-based alloys, contains a large amount of Ni, its price is very high. Furthermore, austenitic stainless steels such as SUS310S have a coefficient of thermal expansion that is about 1.5 times greater than that of Ni-based alloys and ferritic stainless steels, resulting in greater thermal deformation when heating, and a high risk of fracture due to thermal fatigue when heating and cooling are repeated.

[0006] On the other hand, ferritic stainless steel contains very little nickel, making it less expensive than nickel-based alloys and austenitic stainless steel. Furthermore, ferritic stainless steel has a relatively small coefficient of thermal expansion, giving it an advantage over austenitic stainless steel in terms of thermal deformation during heating and thermal fatigue associated with heating and cooling.

[0007] As examples of ferritic stainless steel used at high temperatures, Patent Document 1 discloses a ferritic stainless steel with enhanced oxidation resistance due to the addition of Al. Patent Document 2 discloses a ferritic stainless steel for fuel cells in which creep resistance is enhanced by the addition of one or two types of Sn and B in addition to Al. Patent Document 3 discloses an Fe-Cr-Al ferritic stainless steel in which high-temperature deformation is suppressed by the addition of Nb. Patent Document 4 discloses a ferritic stainless steel containing Al to improve oxidation resistance. Patent Document 5 discloses a ferritic stainless steel containing appropriate amounts of Si and Al to improve oxidation resistance. Patent Document 6 discloses a low-density ferritic stainless steel containing Al and considering the balance of Cr and Si content. Patent Document 7 discloses a heat-resistant ferritic stainless steel containing Al. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 6-220587 [Patent Document 2] Patent No. 6113359 [Patent Document 3] Japanese Patent Application Publication No. 10-251810 [Patent Document 4] Japanese Patent Publication No. 2012-102376 [Patent Document 5] Japanese Patent Publication No. 2012-107314 [Patent Document 6] Japanese Patent Publication No. 2018-168457 [Patent Document 7] Japanese Patent Publication No. 2018-188687 [Overview of the project] [Problems that the invention aims to solve]

[0009] According to the present inventors' research, Patent Documents 1 to 7 disclose steels whose oxidation resistance and creep resistance have been improved mainly by additive elements such as Al. However, the nitriding phenomenon, in which nitrogen atoms from the decomposition of NH3, which occurs in high-temperature environments containing NH3, penetrate the steel and combine with Fe, Cr, Al, etc., to form nitrides, has not been considered.

[0010] Thus, conventional technologies have only investigated the nitriding resistance (hereinafter also referred to as high-temperature NH3 resistance) in environments containing high temperatures of NH3 for Ni-based alloys and austenitic stainless steels, and have hardly investigated ferritic stainless steels. The reasons for this are thought to be as follows: Ferritic stainless steels have a BCC (Body-Centered Cubic) crystal structure, and their nitrogen solid solubility limit at high temperatures is smaller compared to Ni-based alloys and austenitic stainless steels, which have an FCC (Face-Centered Cubic) crystal structure. Therefore, it is thought that ferritic stainless steels generate a large amount of nitrides in a short time in environments containing high temperatures of NH3, and significant cracking occurs due to the difference in thermal expansion between the steel substrate and the nitrides.

[0011] Therefore, the present invention aims to provide a ferritic stainless steel for NH3 combustion environments that exhibits excellent resistance to high-temperature NH3 in NH3 combustion environments containing high-temperature NH3.

[0012] Furthermore, the "excellent high-temperature NH3 resistance" of this invention refers to a high-temperature NH3 test simulating NH3 combustion, in which the material is held at 600°C for 100 hours in nitrogen containing 3 vol% NH3 (representing unburned NH3) and 5 vol% water vapor (representing water vapor produced by combustion), with an increase of 40 g / m². 2 This means that the value is less than the limit and no cracks occur. For details on the high-temperature NH3 test, please refer to the description in the examples. [Means for solving the problem]

[0013] The inventors of this invention have diligently studied the high-temperature NH3 resistance of various elements in order to develop a ferritic stainless steel with high-temperature NH3 resistance that is equal to or better than that of austenitic stainless steel (SUS310S).

[0014] As a result, we found that by including Cr in the range of 12.0 to 25.0% by mass and Al in the range of 0.20 to 6.00%, we were able to obtain a ferritic stainless steel with high-temperature NH3 resistance in a high-temperature range of 300°C or higher, containing 3 vol% or more of NH3 and 5 vol% or more of water vapor.

[0015] Based on the above findings, the present invention was completed by considering compositions that include appropriate amounts of Cr and Al, as well as other components. If even one of Cr or Al is not included in an appropriate amount, the excellent high-temperature resistance of NH3 intended by the present invention cannot be obtained. Furthermore, even in an environment where NH3 is burned, the excellent high-temperature resistance of NH3 intended by the present invention cannot be obtained in an environment where the water vapor content is less than 5 vol%.

[0016] As described above, in a high-temperature environment where NH3 is burned as part or all of the fuel, the combustion environment contains NH3 derived from unburned NH3 and water vapor produced by combustion. In this invention, an NH3 combustion environment means a high-temperature environment of 300°C or higher containing 3 vol% or more of NH3 and 5 vol% or more of water vapor. The ferritic stainless steel of this invention is particularly suitable for use in an NH3 combustion environment and has excellent high-temperature NH3 resistance in an NH3 combustion environment.

[0017] This invention was made after further consideration based on the above findings, and its gist is as follows. [1] In mass%, C: 0.020% or less, Si: 0.05~2.50%, Mn: 0.05~2.00%, P: 0.050% or less, S: 0.010% or less, Al: 0.20 - 6.00%, N: 0.020% or less, and Cr: 12.0 - 25.0% by mass, a ferrite stainless steel for an NH3 combustion environment having a component composition consisting of the balance Fe and inevitable impurities. [2] Further, in mass%, Nb: 0.15 - 2.00%, Ni: 0.05 - 2.00%, Cu: 0.01 - 2.00%, Mo: 0.01 - 3.00%, W: 0.01 - 3.00%, Ti: 0.005 - 0.500%, Zr: 0.005 - 0.300%, V: 0.01 - 0.50%, Co: 0.01 - 0.50%, REM: 0.01 - 0.50%, Hf: 0.01 - 0.50%, Sb: 0.01 - 0.50%, and Sn: 0.01 - 0.50% The ferrite stainless steel for an NH3 combustion environment according to [1], containing one or more selected from among them. [3] Further, in mass%, B: 0.0002 - 0.0050%, Ca: 0.0002 - 0.0050%, and Mg: 0.0002 - 0.0050% The ferrite stainless steel for an NH^3 combustion environment according to [1] or [2], containing one or more selected from among them. [4] The ferrite stainless steel for an NH3 combustion environment according to any one of [1] to [3], used for a structure that burns NH3 as at least part of the fuel.

Advantages of the Invention

[0018] According to the present invention, it is possible to provide a ferritic stainless steel for NH3 combustion environments that exhibits excellent resistance to high-temperature NH3 in NH3 combustion environments containing high-temperature NH3.

[0019] The ferritic stainless steel for NH3 combustion environments of the present invention exhibits excellent high-temperature NH3 resistance and can be suitably used in NH3 combustion environments as a substitute for expensive Ni-based alloys or expensive austenitic stainless steels that exhibit significant thermal deformation.

[0020] The ferritic stainless steel for NH3 combustion environments of the present invention can be suitably used in structures such as furnace walls of various furnaces, such as heating furnaces, that use NH3 as part or all of their fuel. [Modes for carrying out the invention]

[0021] The present invention will be described in detail below.

[0022] The ferritic stainless steel for NH3 combustion environments of the present invention has a composition in mass percent of C: 0.020% or less, Si: 0.05~2.50%, Mn: 0.05~2.00%, P: 0.050% or less, S: 0.010% or less, Al: 0.20~6.00%, N: 0.020% or less, and Cr: 12.0~25.0%, with the remainder being Fe and unavoidable impurities. Hereinafter, the ferritic stainless steel for NH3 combustion environments of the present invention will also be simply referred to as the ferritic stainless steel of the present invention.

[0023] Next, the component composition of the ferritic stainless steel of the present invention will be described. Hereinafter, unless otherwise specified, percentages indicating the content of steel components refer to mass percentages.

[0024] C: 0.020% or less Carbon (C) is an effective element for increasing the strength of steel, but if the C content exceeds 0.020%, the toughness and workability decrease significantly. Therefore, the C content should be 0.020% or less. From the viewpoint of ensuring better workability, the C content is preferably 0.010% or less. From the viewpoint of ensuring strength, the C content is preferably 0.001% or more. More preferably, the C content is 0.003% or more. Furthermore, even more preferably, the C content is 0.008% or less.

[0025] Si: 0.05~2.50% Si is an effective element for improving oxidation resistance. To obtain this effect, a Si content of 0.05% or more is necessary. On the other hand, in steel containing a large amount of Al, as in the present invention, an excess of Si exceeding 2.50% can actually make the oxide scale more prone to peeling, so the upper limit of the Si content is set at 2.50%. Preferably, the Si content is 0.10% or more. Also preferably, the Si content is 1.50% or less. More preferably, the Si content is 0.50% or less.

[0026] Mn: 0.05~2.00% Mn has the effect of improving the resistance to peeling of oxide scale. To obtain this effect, a Mn content of 0.05% or more is necessary. On the other hand, an excessive content of Mn exceeding 2.00% makes oxide scale more likely to grow abnormally, reducing oxidation resistance. Therefore, the Mn content should be between 0.05% and 2.00%. Preferably, the Mn content should be 0.10% or more. Also preferably, the Mn content should be 1.00% or less. Furthermore, more preferably, the Mn content should be 0.50% or less.

[0027] P:0.050% or less P is a harmful element that reduces the toughness of steel, and it is desirable to reduce it as much as possible. Therefore, the P content should be 0.050% or less. Preferably, the P content is 0.040% or less, and more preferably 0.030% or less. There is no particular lower limit to the P content. However, excessive removal of P leads to increased costs, so a P content of 0.005% or more is preferable.

[0028] S: 0.010% or less Since sulfur (S) is a harmful element that reduces the corrosion resistance, a fundamental property of stainless steel, it is desirable to reduce it as much as possible. Therefore, in this invention, the S content is 0.010% or less. Preferably, the S content is 0.005% or less. The lower limit of the S content is not particularly limited. However, excessive removal of S leads to increased costs, so an S content of 0.0005% or more is preferable.

[0029] Al: 0.20~6.00% Al is an important element in this invention for improving the high-temperature NH3 resistance of steel. At high temperatures, Al preferentially bonds with O over Fe and Cr to form a highly protective Al2O3 film, thereby suppressing the penetration of N atoms generated from the decomposition of NH3 into the steel in an NH3 combustion environment. To obtain this effect, an Al content of 0.20% or more is necessary. If the Al content is less than 0.20%, a highly protective Al2O3 film is not continuously formed in an NH3 combustion environment, and N atoms penetrate from the areas where the film is not formed, causing nitriding to progress and generating a large amount of Fe nitride and Cr nitride, leading to cracking due to the difference in thermal expansion with the steel. Furthermore, since the NH3 combustion environment also contains a large amount of water vapor, if the Al content is less than 0.20%, not only Cr but also Fe oxidizes preferentially, forming a less protective oxide film. As a result, oxidation progresses significantly, leading to thinning of the steel, and N atoms decomposed from NH3 in the atmosphere penetrate into the steel to form nitrides, causing the steel to collapse with significant cracking. However, even in an environment where NH3 is burned, the above-mentioned effects of Al cannot be obtained in an environment where the water vapor content is less than 5 vol%. On the other hand, Al also has the disadvantage of increasing the coefficient of thermal expansion, and if the Al content exceeds 6.00%, it becomes more susceptible to thermal deformation. For these reasons, the Al content should be 0.20 to 6.00%. The Al content is preferably 1.00% or more, and more preferably 2.10% or more. Furthermore, the Al content is preferably 3.50% or less, and more preferably 2.90% or less.

[0030] N: 0.020% or less Nitric oxide (N) is an element that reduces the toughness and workability of steel. If it is present in a concentration exceeding 0.020%, the decrease in toughness and formability becomes significant, and the precipitation of coarse AlN prevents the improvement in high-temperature NH3 resistance due to Al content mentioned above. Therefore, the N content should be 0.020% or less. Furthermore, from the viewpoint of ensuring toughness and workability, it is preferable to reduce N as much as possible, and it is desirable for the N content to be less than 0.010%. There is no particular lower limit to the N content. However, excessive removal of N leads to increased costs, so an N content of 0.004% or more is preferable. Note that the N referred to here means only the N contained in the steel during the manufacturing of stainless steel, and does not include N atoms that enter the steel when NH3 decomposes upon exposure to high-temperature NH3.

[0031] Cr: 12.0~25.0% Cr is an important element that is effective in improving the corrosion resistance and oxidation resistance, which are characteristics of stainless steel. In this invention, it also has the effect of promoting the formation of a highly protective Al2O3 film. If the Cr content is less than 12.0%, even if an appropriate amount of Al is present, a sufficiently protective oxide film cannot be formed, and sufficient high-temperature NH3 resistance cannot be obtained. On the other hand, if the Cr content exceeds 25.0%, a second phase (σ phase) mainly composed of Fe and Cr precipitates, leading to a significant decrease in toughness, so the upper limit of the Cr content is set at 25.0%. The Cr content is preferably 16.0% or more, more preferably 18.0% or more. Furthermore, the Cr content is preferably 22.0% or less, more preferably 20.0% or less.

[0032] In the ferritic stainless steel of the present invention, the remainder consists of Fe and unavoidable impurities.

[0033] In addition to the above essential components, the ferritic stainless steel of the present invention may further contain one or more elements selected from Nb, Ni, Cu, Mo, W, Ti, Zr, V, Co, REM, Hf, Sb, and Sn, within the following ranges.

[0034] Nb: 0.15~2.00% Nb is an element that forms carbonitrides with C and N, fixing C and N and improving the corrosion resistance of welded joints. It is also an element that increases high-temperature strength. When the Nb content is 0.15% or more, Nb readily bonds with C and N in the steel, and the bonding of the remaining C and N with Cr and Al is suppressed, thus suppressing the decrease in high-temperature NH3 resistance. However, an Nb content exceeding 2.00% hardens the steel and significantly reduces its toughness. Therefore, when Nb is included, the Nb content should be between 0.15% and 2.00%. The Nb content is preferably 0.30% or more, more preferably 0.45% or more. Furthermore, the Nb content is preferably 1.25% or less, more preferably 0.70% or less.

[0035] Ni: 0.05~2.00% Ni is an element that improves the toughness and oxidation resistance of steel. To obtain these effects, the Ni content should be 0.05% or more. However, since Ni is a strong γ-phase forming element, an excess of Ni exceeding 2.00% will generate the γ-phase at high temperatures, leading to a larger coefficient of thermal expansion, which causes the oxide film to peel off and reduces oxidation resistance. Furthermore, as it is a rare metal, the raw material cost will also be high. Therefore, when Ni is included, the Ni content should be between 0.05% and 2.00%. The Ni content is preferably 0.10% or more. Moreover, the Ni content is preferably less than 0.30%, and more preferably less than 0.20%.

[0036] Cu: 0.01~2.00% Cu is an element that improves the corrosion resistance of steel. This effect can be achieved with a Cu content of 0.01% or more. On the other hand, if the Cu content exceeds 2.00%, the oxide scale becomes more prone to peeling, and the oxidation resistance decreases. Therefore, when Cu is included, the Cu content should be between 0.01% and 2.00%. The Cu content is preferably 0.30% or more, more preferably 1.00% or more. Furthermore, the Cu content is preferably 1.50% or less.

[0037] Mo: 0.01~3.00% Mo is an element that improves the corrosion resistance and high-temperature strength of steel. This effect can be obtained with a Mo content of 0.01% or more. On the other hand, an excessive Mo content exceeding 3.00% hardens the steel and reduces its workability. Therefore, when Mo is included, the Mo content should be between 0.01% and 3.00%. The Mo content is preferably 1.00% or more, and more preferably over 2.00%. Furthermore, the Mo content is preferably 2.50% or less.

[0038] W: 0.01~3.00% W, like Mo, is an element that significantly improves high-temperature strength. This effect can be obtained with a W content of 0.01% or more. On the other hand, excessive W content exceeding 3.00% not only significantly hardens the steel but also generates a strong oxide scale during the annealing process in manufacturing, making descaling during pickling difficult. Therefore, when W is included, the W content should be between 0.01% and 3.00%. The W content is preferably 0.30% or more, more preferably 1.00% or more. Furthermore, the W content is preferably 2.00% or less, more preferably 1.50% or less.

[0039] Ti: 0.005~0.500% Ti is an element that enhances oxidation resistance by bonding with C and N, preventing Al from bonding with N to form AlN. This effect can be obtained with a Ti content of 0.005% or more. However, a Ti content exceeding 0.500% generates coarse TiN, leading to a decrease in the toughness of the steel. Therefore, when Ti is included, the Ti content should be between 0.005% and 0.500%. The Ti content is preferably less than 0.300%, and more preferably 0.050% or less.

[0040] Zr: 0.005~0.300% Zr is an element that improves oxidation resistance. To obtain this effect, it is preferable to have a Zr content of 0.005% or more. However, if the Zr content exceeds 0.300%, Zr intermetallic compounds precipitate, causing the steel to become brittle. Therefore, when Zr is included, the Zr content should be between 0.005% and 0.300%. The Zr content is preferably 0.030% or more. Furthermore, the Zr content is preferably 0.150% or less.

[0041] V: 0.01~0.50% V is an effective element for improving the workability of steel, as well as its oxidation resistance. By adding V, V combines with N in the steel, making it easier to suppress the formation of coarse nitrides by which Al combines with N. By suppressing the formation of coarse nitrides by Al, high-temperature NH3 resistance can be easily obtained. This effect becomes significant when the V content is 0.01% or higher. However, an excessive V content exceeding 0.50% leads to the precipitation of coarse V(C,N) and reduces toughness. Therefore, when V is included, the V content should be between 0.01% and 0.50%. The V content is preferably 0.05% or higher. Also, the V content is preferably 0.10% or lower.

[0042] Co: 0.01~0.50% Co is known as an element effective in improving the toughness of steel. Furthermore, in this invention, Co also has the effect of improving thermal fatigue characteristics by reducing the coefficient of thermal expansion that has increased due to the Al content. To obtain these effects, it is preferable that the Co content be 0.01% or more. On the other hand, an excessive amount of Co exceeding 0.50% not only reduces the toughness of the steel but also reduces its workability. Therefore, when Co is included, the Co content should be 0.01 to 0.50%. The Co content is preferably 0.03% or more. In addition, the Co content is preferably less than 0.30%, and more preferably less than 0.10%.

[0043] REM: 0.01~0.50% REM (rare earth metals) are elements that improve oxidation resistance. To obtain this effect, it is preferable to have a REM content of 0.01% or more. However, if the REM content exceeds 0.50%, the steel becomes brittle. Therefore, when REM is included, the REM content should be between 0.01% and 0.50%. The REM content is preferably 0.03% or more. Also, the REM content is preferably 0.15% or less. Note that REM (rare earth metals) is a collective term for 17 elements in total, including Sc, Y, and lanthanide elements. One or more of these 17 elements may be included, and the REM content referred to here is the total content of these elements.

[0044] Hf: 0.01~0.50% Hf is an element that improves oxidation resistance. To obtain this effect, it is preferable to have an Hf content of 0.01% or more. However, if the Hf content exceeds 0.50%, the steel becomes brittle. Therefore, when Hf is included, the Hf content should be between 0.01% and 0.50%. The Hf content is preferably 0.03% or more. Also, the Hf content is preferably 0.15% or less.

[0045] Sb: 0.01~0.50% Sb is an element that improves the toughness of steel. This effect can be obtained with an Sb content of 0.01% or more. On the other hand, excessive Sb content exceeding 0.50% actually reduces toughness, so the Sb content should be limited to 0.50%. Therefore, when Sb is included, the Sb content should be between 0.01% and 0.50%. The Sb content is preferably 0.03% or more. Furthermore, the Sb content is preferably 0.30% or less.

[0046] Sn: 0.01~0.50% Sn (Sin) is an element that improves the corrosion resistance and high-temperature strength of steel. This effect can be obtained with a Sn content of 0.01% or more. On the other hand, excessive Sn content exceeding 0.50% reduces the workability of the steel, so the Sn content should be limited to 0.50%. Therefore, when Sn is included, the Sn content should be between 0.01% and 0.50%. The Sn content is preferably 0.03% or more. Furthermore, the Sn content is preferably 0.30% or less.

[0047] The ferritic stainless steel of the present invention may further contain one or more elements selected from B, Ca, and Mg within the following ranges.

[0048] B: 0.0002~0.0050% B is an effective element for improving the workability of steel, particularly its secondary workability. This effect can be obtained with a B content of 0.0002% or more. On the other hand, an excessive B content exceeding 0.0050% generates BN, which reduces workability. Therefore, when B is included, the B content should be between 0.0002% and 0.0050%. Preferably, the B content is 0.0005% or more. Furthermore, preferably, the B content is 0.0020% or less, and more preferably 0.0010% or less.

[0049] Ca: 0.0002~0.0050% Ca is an effective component for preventing nozzle blockage caused by inclusion precipitation, which is likely to occur during continuous casting. This effect is achieved when the Ca content is 0.0002% or higher. On the other hand, to obtain good surface properties without generating surface defects, it is preferable to have a Ca content of 0.0050% or less. Therefore, when Ca is included, the Ca content should be 0.0002 to 0.0050%. The Ca content is preferably 0.0005% or higher. Furthermore, the Ca content is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0050] Mg: 0.0002~0.0050% Mg is an element that improves the equiaxity of the slab, and is effective in improving workability and toughness. This effect can be obtained with an Mg content of 0.0002% or more. On the other hand, if the Mg content exceeds 0.0050%, there is a risk of deteriorating the surface properties of the steel. Therefore, when Mg is included, the Mg content should be between 0.0002% and 0.0050%. The Mg content is preferably 0.0004% or more. Furthermore, the Mg content is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0051] Furthermore, if the content of any of the optional components described above—Nb, Ni, Cu, Mo, W, Ti, Zr, V, Co, REM, Hf, Sb, Sn, B, Ca, Mg—is below the lower limit, that component shall be considered an unavoidable impurity.

[0052] Next, the method for producing ferritic stainless steel according to the present invention will be described.

[0053] The method for manufacturing ferritic stainless steel of the present invention can be suitably used in any conventional method for manufacturing ferritic stainless steel, and is not particularly limited. For example, steel may be melted in a known melting furnace such as a converter or electric furnace, or further refined through ladle refining or vacuum refining to obtain steel having the component composition of the present invention as described above, and then formed into steel billets (slabs) by continuous casting or ingot-split rolling. Subsequently, it can be manufactured in a manufacturing process that involves hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, and pickling to produce cold-rolled annealed sheets. The cold rolling may be performed once or two or more times with intermediate annealing in between, and the cold rolling, finish annealing, and pickling steps may be repeated. Pickling may be omitted by performing finish annealing in a reducing atmosphere or by removing oxide scale by polishing after annealing in an oxidizing atmosphere. Pickling may also be omitted after finishing annealing in an oxidizing atmosphere, and the sheet may be used with an oxide film formed. Furthermore, hot-rolled sheet annealing may be omitted, and if surface gloss or roughness adjustment of the steel sheet is required, skin pass rolling may be performed after cold rolling or finish annealing. In addition, depending on the application, it is possible to use hot-rolled sheets or hot-rolled annealed sheets as they are.

[0054] The preferred manufacturing conditions for the above manufacturing method will now be described.

[0055] In the steelmaking process for melting steel, it is preferable to molten the steel in a converter or electric furnace and then secondary refine it using the VOD method or the like to obtain steel containing the above essential components and optional components added as needed. The molten steel can be processed into steel material by known methods, but continuous casting is preferred in terms of productivity and quality. The steel material is then preferably heated to 1050 to 1250°C and hot-rolled into hot-rolled sheets of the desired thickness (3 mm to 10 mm). Of course, other forms of hot processing can also be performed. The hot-rolled sheets are then preferably subjected to continuous annealing at a temperature of 850 to 1150°C as needed to become hot-rolled annealed sheets, and then descaled by pickling or the like to become hot-rolled products. The above annealing may be omitted. The above pickling may be omitted and the product may be used as is, or polishing may be used instead of pickling. If necessary, scale may be removed before pickling by shot blasting or brush grinding.

[0056] Furthermore, the above-mentioned hot-rolled annealed sheet or hot-rolled sheet may be subjected to processes such as cold rolling to produce a cold-rolled product with a thickness of 0.1 mm to 3 mm. In this case, cold rolling may be performed only once, but from the viewpoint of productivity and improvement of required quality, it may be performed two or more times with intermediate annealing in between. The total reduction ratio of one or more cold rollings is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more. The cold-rolled steel sheet is then preferably continuously annealed (finish annealed) at a temperature of preferably 850 to 1150°C, more preferably 900 to 1100°C, pickled, and produced as a cold-rolled product. Furthermore, depending on the application, after finish annealing, skin pass rolling or the like may be performed to adjust the shape, surface roughness, and material properties of the steel sheet. Brush grinding may be performed before the above pickling. Furthermore, finish annealing may be performed in a reducing atmosphere (e.g., 25 vol% N2 - 75 vol% H2), and descaling processes such as pickling may be omitted. Finish annealing may be omitted, or, assuming that it does not cause problems in subsequent machining or welding processes, pickling may be omitted after annealing in an oxidizing atmosphere, and the material may be used with an oxide film formed. Polishing may be used instead of pickling.

[0057] The hot-rolled or cold-rolled products obtained as described above can then be processed by cutting, bending, drilling, etc., and used as structural elements in various furnaces, such as heating furnaces. Examples of applications include the inner walls of furnaces, burners, radiant tubes, and the inner walls of cooling zones. They may also be used as lining for the furnace body. For fastening, bolts are possible, and welding is also possible. The method of welding these components is not particularly limited. As for welding methods, conventional arc welding such as MIG (Metal Inert Gas), MAG (Metal Active Gas), and TIG (Tungsten Inert Gas) can be applied. In addition, resistance welding such as spot welding and seam welding, and laser welding such as high-frequency resistance welding such as electric resistance welding and high-frequency induction welding may also be applied.

[0058] The ferritic stainless steel of the present invention has excellent high-temperature NH3 resistance and is therefore suitable for use in structures such as furnaces that use NH3 as at least part of their fuel. The ferritic stainless steel of the present invention has excellent high-temperature NH3 resistance in high-temperature environments containing unburned NH3 and water vapor.

[0059] As described above, in this invention, the NH3 combustion environment refers to a high-temperature environment of 300°C or higher containing 3 vol% or more of NH3 and 5 vol% or more of water vapor. An example of an NH3 combustion environment is an environment of 600°C or higher containing 3 to 10 vol% of NH3 and 5 to 20 vol% of water vapor. The higher the amount of NH3 in the environment, the higher the required high-temperature resistance of NH3. [Examples]

[0060] The present invention will be described in detail below with reference to examples.

[0061] Steels with the component compositions of No. 1 to 29 shown in Table 1 were melted in a vacuum melting furnace and cast into 50 kg steel ingots. After heating at 1170°C, they were hot-rolled to form 35 mm thick sheet bars. A 200 mm length was cut from these sheet bars, heated at 1150°C, and hot-rolled to form 5.0 mm thick hot-rolled sheets. After annealing at a temperature in the range of 850 to 1150°C, the sheets were ground down to a thickness of 4.5 mm to form hot-rolled annealed sheets. Subsequently, cold rolling with a reduction ratio of 67% was performed, followed by finish annealing at a temperature of 850 to 1150°C to form cold-rolled annealed sheets with a thickness of 1.5 mm. After removing the oxide scale from the surface by polishing, the sheets were subjected to a high-temperature NH3 test. For reference, cold-rolled annealed sheets were also prepared for SUS310S (No. 26) in the same manner as described above and subjected to a high-temperature NH3 test. Regarding the annealing temperature, the temperature was determined for each steel while checking the microstructure within the above temperature range.

[0062] <High-temperature NH3 test> A 30mm x 20mm test specimen was cut from the cold-rolled and annealed sheet obtained as described above. The edges were cut, ground, and polished (emery paper #320), and a 4mm diameter hole was drilled in the upper part of the 30mm direction of the test specimen. After degreasing, the test specimen was suspended in a furnace with two different atmospheres, (1) and (2) below, heated and held at 600°C, and held for 100 hours. (1)92vol%N2-3vol%NH3-5vol%H2O (dew point +33℃, flow rate 0.5L / min) (2)70vol%N2-10vol%NH3-20vol%H2O (dew point +60℃, flow rate 0.5L / min)

[0063] After the test, the test specimen is visually inspected for cracks and its weight is measured. The difference between this weight and the pre-test weight is then calculated to determine the weight increase (g / m²). 2 The following was calculated. High-temperature resistance of NH3 was evaluated as follows.

[0064] [High temperature NH3 property judgment criteria] ◎ (Pass, better): (1) and (2) both increased by 3g / m 2 Less than, and no cracks. ○ (Pass, Excellent): (2) Increased to 3g / m 2 The above was the case, but both (1) and (2) were increased to 40g / m². 2 Less than, and no cracks. × (Failure): Either (1), (2), or both, with an increase of 40g / m². 2 Above or above, or cracked.

[0065] The results obtained are shown in Table 1.

[0066] [Table 1]

[0067] As shown in Table 1, all of the steels No. 1 to 25 in the present invention example had excellent high-temperature NH3 resistance. All of the steels No. 1 to 25 in the present invention example had high-temperature NH3 resistance equivalent to or better than SUS310S (steel No. 26).

[0068] On the other hand, steel No. 27 had a Si content exceeding 2.50%, resulting in a failure of its high-temperature NH3 resistance. Steel No. 28 had an Al content of less than 0.20%, resulting in a failure of its high-temperature NH3 resistance. Steel No. 29 had a Cr content of less than 12.0%, resulting in a failure of its high-temperature NH3 resistance. [Industrial applicability]

[0069] The ferritic stainless steel of the present invention is suitable for use in structures such as heating furnaces, particularly in parts exposed to high temperatures of NH3, such as the inner walls of the furnace.

Claims

1. In mass percent, C: 0.020% or less, Si: 0.05-2.50%, Mn: 0.05-2.00%, P: 0.050% or less, S: 0.010% or less, Al: 0.20-6.00%, N: 0.020% or less, and Contains Cr: 12.0-25.0%, NH has a component composition in which the remainder consists of Fe and unavoidable impurities. ３ Ferritic stainless steel for combustion environments.

2. Furthermore, in mass percent, Nb: 0.15-2.00%, Ni: 0.05-2.00%, Cu: 0.01-2.00%, Mo: 0.01-3.00%, W: 0.01-3.00%, Ti: 0.005-0.500%, Zr: 0.005-0.300%, V: 0.01-0.50%, Co: 0.01 to 0.50%, REM: 0.01-0.50%, Hf: 0.01-0.50%, Sb: 0.01–0.50%, and Sn: 0.01~0.50% The NH according to claim 1, which contains one or more selected from among ３ Ferritic stainless steel for combustion environments.

3. Furthermore, in mass percent, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, and Mg: 0.0002-0.0050% The NH according to claim 1 or 2, which contains one or more selected from among ３ Ferritic stainless steel for combustion environments.

4. NH ３ The NH according to claim 1 or 2, used in a structure that burns at least a portion of the fuel ３ Ferritic stainless steel for combustion environments.

5. The ferritic stainless steel for an NH3 combustion environment according to claim 3, used in a structure that burns NH3 as at least part of the fuel.

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