Ferritic stainless steel for ammonia combustion environment

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

Ferritic stainless steel for ammonia combustion environments

[0001] The present invention relates to NH ３ The present invention relates to a ferritic stainless steel for use in an ammonia combustion environment. In particular, the present invention relates to a Cr-containing steel, and is suitable for use in an ammonia combustion environment such as a heating furnace or an incinerator. ３ Excellent high temperature resistance, suitable for use in parts exposed to high temperatures of 300°C or higher in structures where NH4 is burned. ３ This invention relates to a ferritic stainless steel having excellent properties.

[0002] Ceramic insulation materials, Ni-based alloys, and stainless steel plates are used for the walls of various furnaces, such as heating furnaces and incinerators, to provide insulation and prevent heat loss within the furnace. Ceramics have excellent insulation properties, but they have issues such as the need for a large amount of energy to heat up, their susceptibility to cracking due to temperature differences, and their tendency to produce fine peeling. Furthermore, Ni-based alloys containing pure Ni pose significant issues in terms of resource depletion and cost, as Ni is a rare metal. On the other hand, stainless steel plates have slightly inferior insulation properties compared to ceramic insulation materials, but they have the advantages of being able to save the energy required for heating due to their excellent electrical conductivity, being able to reuse residual heat through heat exchange, and significantly reducing material costs compared to Ni-based alloys.

[0003] In recent years, with growing awareness of global environmental issues, efforts are being made around the world to create a carbon-neutral society. ２ H that does not emit ２ and NH ３ and CO during manufacturing ２ The use of alcohols containing H as fuel (at least in part) is being considered. Hereinafter, these fuels will also be referred to as low-carbon fuels. ２ and NH ３ is CO during combustion ２ Alcohol fuel does not emit CO when burned. ２ Although CO is emitted during production, ２ By using ２ However, H emissions can be reduced. ２The boiling point of NH is low at -253°C, and there are issues with costs, including the difficulty of transporting and storing it in a liquefied form. ３ The boiling point of H is -33°C. ２ It is easier to liquefy than NH4, and is considered a promising option for energy carriers from the viewpoints of transportation and storage. ３ is then decomposed at high temperature to generate H ２ In addition to burning it and using it to generate electricity, ３ It can be used to generate electricity by burning it as part or all of the fuel.

[0004] High temperature NH ３ Examples of Ni-based alloys exposed to the above 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, not only Ni-based alloys but also SUS310S contain a large amount of Ni, making them very expensive. In addition, austenitic stainless steels such as SUS310S have a thermal expansion coefficient that is approximately 1.5 times larger than that of Ni-based alloys and ferritic stainless steels, which means that they are subject to greater thermal deformation when heated, and there is also a high risk of fracture due to thermal fatigue when the temperature is repeatedly increased and decreased.

[0006] On the other hand, ferritic stainless steels contain almost no Ni, making them less expensive than Ni-based alloys and austenitic stainless steels.Furthermore, ferritic stainless steels have a relatively small thermal expansion coefficient, making them more advantageous than austenitic stainless steels in terms of thermal deformation during temperature rise and thermal fatigue associated with temperature rise and fall.

[0007] As examples of ferritic stainless steels used at high temperatures, Patent Document 1 discloses a ferritic stainless steel with improved oxidation resistance due to the addition of Al. Patent Document 2 discloses a ferritic stainless steel for fuel cells that, in addition to Al, adds one or both of Sn and B to improve creep resistance. Patent Document 3 discloses an Fe—Cr—Al ferritic stainless steel that suppresses high-temperature deformation due to 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-gravity ferritic stainless steel containing Al and with a balance between the Cr and Si contents. Patent Document 7 discloses a heat-resistant ferritic stainless steel containing Al.

[0008] Japanese Patent Application Laid-Open No. 6-220587 Japanese Patent No. 6113359 Japanese Patent Application Laid-Open No. 10-251810 Japanese Patent Application Laid-Open No. 2012-102376 Japanese Patent Application Laid-Open No. 2012-107314 Japanese Patent Application Laid-Open No. 2018-168457 Japanese Patent Application Laid-Open No. 2018-188687

[0009] According to the research of the present inventors, Patent Documents 1 to 7 disclose steels in which the oxidation resistance and creep resistance are improved mainly by adding elements such as Al. However, the oxidation resistance and creep resistance are improved by adding elements such as Al to steels in high-temperature NH ３ NH ３ The nitriding phenomenon in which nitrogen atoms decomposed from the SiO2 penetrate into the steel and combine with Fe, Cr, Al, etc. to form nitrides is not taken into consideration.

[0010] Thus, in the conventional technology, high-temperature NH ３ Nitriding resistance in environments containing ３The N solubility (also called N solubility) has only been studied for Ni-based alloys and austenitic stainless steels, and has hardly been studied for 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 have a smaller N solid solubility limit at high temperatures than Ni-based alloys and austenitic stainless steels, which have FCC (Face Centered Cubic) crystal structures. Therefore, ferritic stainless steels have a high N solubility limit at high temperatures. ３ This is thought to be because, in an environment containing nitrogen, a large amount of nitrides are formed in the steel in a short period of time, and the difference in thermal expansion between the steel base and the nitrides causes significant cracking.

[0011] Therefore, the present invention provides a method for producing NH ３ Including NH ３ High temperature resistant in combustion environment ３ Excellent in NH ３ The object is to provide a ferritic stainless steel for use in combustion environments.

[0012] In addition, the "high temperature resistant NH ３ "Excellent in quality" means NH ３ Simulating combustion, unburned NH ３ Assuming 3 vol% NH ３ and high-temperature NH3, which is maintained at 600°C for 100 hours in nitrogen containing 5 vol% water vapor, which is assumed to be water vapor generated by combustion. ３ In the test, the weight gain was 40 g / m ２ This means that the temperature is less than 100°C and no cracks occur. ３ For details of the test, please refer to the description of the Examples.

[0013] The present inventors have developed a high-temperature resistant NH ３ In order to develop a ferritic stainless steel with properties equivalent to or better than austenitic stainless steel (SUS310S), we investigated the high temperature resistance of various elements. ３ We conducted thorough research into gender.

[0014] As a result, by containing Cr in the range of 12.0 to 25.0% and Al in the range of 0.20 to 6.00% by mass, NH３ High temperature resistant in the high temperature range of 300℃ or higher containing 3vol% or more of NH ３ It has been found that a ferritic stainless steel having the above properties can be obtained.

[0015] Based on the above findings, the present invention was completed by examining compositions containing appropriate amounts of Cr and Al and other components. When the appropriate amount of either Cr or Al is not contained, the excellent high-temperature resistance of the present invention cannot be obtained. ３ Sexuality is not obtained. ３ Even in an environment where water vapor is less than 5 vol%, the excellent high-temperature resistance of the present invention is not obtained. ３ Sex is not available.

[0016] As mentioned above, NH ３ In a high-temperature environment where NH is burned as part or all of the fuel, unburned NH ３ NH derived from ３ and water vapor produced by combustion. ３ The combustion environment is NH ３ The ferritic stainless steel of the present invention is a high-temperature environment of 300°C or higher containing 3 vol% or more of NH ３ It is particularly suitable for combustion environments, and NH ３ Excellent resistance to high temperatures in combustion environments ３ It has sexuality.

[0017] The present invention has been made based on the above findings and after further investigation, and provides the following: [1] A NH alloy having a component composition containing, by mass%, C: 0.020% or less, Si: 0.05 to 2.50%, Mn: 0.05 to 2.00%, P: 0.050% or less, S: 0.010% or less, Al: 0.20 to 6.00%, N: 0.020% or less, and Cr: 12.0 to 25.0%, with the balance being Fe and unavoidable impurities. ３[2] The ferritic stainless steel according to [1], further containing, by mass%, one or more selected from Nb: 0.15 to 2.00%, Ni: 0.05 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.01 to 3.00%, W: 0.01 to 3.00%, Ti: 0.005 to 0.500%, Zr: 0.005 to 0.300%, V: 0.01 to 0.50%, Co: 0.01 to 0.50%, REM: 0.01 to 0.50%, Hf: 0.01 to 0.50%, Sb: 0.01 to 0.50%, and Sn: 0.01 to 0.50%. ３ [3] The ferritic stainless steel according to [1] or [2], further containing, by mass%, one or more selected from the group consisting of B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, and Mg: 0.0002 to 0.0050%. ３ Ferritic stainless steel for combustion environments. [4] NH ３ The NH according to any one of [1] to [3], which is used in a structure that burns as at least a part of the fuel. ３ Ferritic stainless steel for combustion environments.

[0018] According to the present invention, high temperature NH ３ Including NH ３ High temperature resistant in combustion environment ３ Excellent in NH ３ It is possible to provide a ferritic stainless steel for combustion environments.

[0019] NH of the present invention ３ Ferritic stainless steel for combustion environments is resistant to high temperatures and NH ３ It has excellent mechanical properties and can be used as an alternative to expensive Ni-based alloys and expensive austenitic stainless steels that are subject to large thermal deformation. ３ It can be suitably used in a combustion environment.

[0020] NH of the present invention ３ Ferritic stainless steel for combustion environments is NH ３ The present invention can be suitably used for structures such as furnace walls of various furnaces, such as heating furnaces, which use the above-mentioned material as a part or all of the fuel.

[0021] The present invention will be specifically described below.

[0022] NH of the present invention ３ The ferritic stainless steel for combustion environments has a composition containing, by mass%, C: 0.020% or less, Si: 0.05 to 2.50%, Mn: 0.05 to 2.00%, P: 0.050% or less, S: 0.010% or less, Al: 0.20 to 6.00%, N: 0.020% or less, and Cr: 12.0 to 25.0%, with the balance being Fe and unavoidable impurities. ３ The ferritic stainless steel for combustion environments is also simply referred to as the ferritic stainless steel of the present invention.

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

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

[0025] Si: 0.05 to 2.50% Si is an element effective in improving oxidation resistance. To achieve this effect, a Si content of 0.05% or more is necessary. On the other hand, in steels containing a large amount of Al, such as in the present invention, an excessive Si content of more than 2.50% actually makes oxide scale more likely to peel off, so the upper limit of the Si content is set to 2.50%. Preferably, the Si content is set to 0.10% or more. Also, preferably, the Si content is set to 1.50% or less. More preferably, the Si content is set to 0.50% or less.

[0026] Mn: 0.05 to 2.00% Mn has the effect of increasing the resistance to exfoliation of oxide scale. To achieve this effect, a Mn content of 0.05% or more is necessary. On the other hand, an excessive Mn content of more than 2.00% makes oxide scale more likely to grow abnormally, reducing oxidation resistance. Therefore, the Mn content is set to 0.05% or more and 2.00% or less. Preferably, the Mn content is set to 0.10% or more. Also, preferably, the Mn content is set to 1.00% or less. And, more preferably, the Mn content is set to 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 is set to 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. There is no particular lower limit for the P content. However, since excessive dephosphorization leads to increased costs, the P content is preferably 0.005% or more.

[0028] S: 0.010% or less S is a harmful element that reduces the corrosion resistance, which is a basic property of stainless steel, so it is desirable to reduce the S content as much as possible. Therefore, in the present invention, the S content is set to 0.010% or less. Preferably, the S content is 0.005% or less. There is no particular lower limit for the S content. However, since excessive desulfurization leads to an increase in costs, the S content is preferably 0.0005% or more.

[0029] Al: 0.20 to 6.00% Al contributes to the high temperature NH resistance of steel. ３ Al is an important element in the present invention, as it bonds with O preferentially to Fe and Cr at high temperatures, providing high protection. ２ O ３ By forming a film, NH ３ NH in the combustion environment ３ In order to obtain this effect, the Al content must be 0.20% or more. When the Al content is less than 0.20%, NH ３ Highly protective Al in combustion environments ２ O ３The film is not formed continuously, and N atoms penetrate into the areas where the film is not formed, causing nitriding to proceed, forming large amounts of Fe nitrides and Cr nitrides, which cause cracks due to the difference in thermal expansion with the steel. ３ Since the combustion environment contains a large amount of water vapor, when the Al content is less than 0.20%, not only Cr but also Fe is preferentially oxidized, resulting in the formation of an oxide film with low protective properties. As a result, oxidation progresses significantly, resulting in thickness reduction, and the NH ３ The N atoms decomposed from the NH3 penetrate into the steel and form nitrides, causing the steel to break down with significant cracks. ３ Even in an environment where aluminum is burned, the above-mentioned effects of aluminum cannot be obtained in an environment where the water vapor content is less than 5 vol%. On the other hand, aluminum has the drawback of increasing the thermal expansion coefficient, and if the aluminum content exceeds 6.00%, the material becomes more susceptible to thermal deformation. For these reasons, the aluminum content is set to 0.20 to 6.00%. The aluminum content is preferably 1.00% or more, and more preferably 2.10% or more. The aluminum content is also preferably 3.50% or less, and more preferably 2.90% or less.

[0030] N: 0.020% or less N is an element that reduces the toughness and workability of steel. If the content of N exceeds 0.020%, the toughness and workability are significantly reduced, and the high-temperature NH3 resistance due to the Al content described above is reduced due to the precipitation of coarse AlN. ３ If the N content is too low, the effect of improving the properties will not be obtained. Therefore, the N content is set to 0.020% or less. From the viewpoint of ensuring toughness and workability, it is preferable to reduce the N content as much as possible, and it is desirable that the N content be less than 0.010%. There is no particular lower limit for the N content. However, since excessive denitrification leads to an increase in costs, the N content is preferably 0.004% or more. Note that N here refers only to N contained in the steel during the production of stainless steel, and does not include N that is removed by high-temperature NH ３ When exposed to NH ３ This does not include N atoms that have decomposed and penetrated into the steel.

[0031] Cr: 12.0 to 25.0% Cr is an important element that is effective in improving the corrosion resistance and oxidation resistance that are characteristics of stainless steel.２ O ３ If the Cr content is less than 12.0%, even if an appropriate amount of Al is contained, a highly protective oxide film cannot be sufficiently formed, and sufficient high-temperature NH3 resistance cannot be obtained. ３ On the other hand, if the Cr content exceeds 25.0%, a second phase (σ phase) mainly composed of Fe and Cr will precipitate, resulting in a significant decrease in toughness, so the upper limit of the Cr content is set to 25.0%. The Cr content is preferably 16.0% or more, and more preferably 18.0% or more. Furthermore, the Cr content is preferably 22.0% or less, and more preferably 20.0% or less.

[0032] In the ferritic stainless steel of the present invention, the balance 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 in the ranges shown below.

[0034] Nb: 0.15 to 2.00% Nb is an element that forms carbonitrides with C and N to fix C and N, thereby improving the corrosion resistance of welds. It is also an element that improves high-temperature strength. If the Nb content is 0.15% or more, Nb will easily bond with C and N in the steel, making it easier to prevent the remaining C and N from bonding with Cr or Al, improving high-temperature NH3 resistance. ３ This makes it easier to suppress deterioration in toughness. However, a Nb content exceeding 2.00% hardens the steel and significantly reduces toughness. Therefore, when Nb is contained, the Nb content is set to 0.15 to 2.00%. The Nb content is preferably 0.30% or more, and more preferably 0.45% or more. Furthermore, the Nb content is preferably 1.25% or less, and more preferably 0.70% or less.

[0035] Ni: 0.05 to 2.00% Ni is an element that improves the toughness and oxidation resistance of steel. To achieve these effects, the Ni content is set to 0.05% or more. However, because Ni is a strong γ-phase forming element, an excessive Ni content of more than 2.00% generates γ-phase at high temperatures, which increases the thermal expansion coefficient and leads to peeling of the oxide film, thereby reducing oxidation resistance. Furthermore, because Ni is a rare metal, raw material costs are also high. Therefore, if Ni is contained, the Ni content is set to 0.05 to 2.00%. The Ni content is preferably 0.10% or more. The Ni content is preferably less than 0.30%, more preferably less than 0.20%.

[0036] Cu: 0.01 to 2.00% Cu is an element that has the effect of improving the corrosion resistance of steel. This effect is obtained with a Cu content of 0.01% or more. On the other hand, if the Cu content exceeds 2.00%, oxide scale becomes more likely to peel off, and oxidation resistance decreases. Therefore, when Cu is contained, the Cu content is set to 0.01 to 2.00%. The Cu content is preferably 0.30% or more, and more preferably 1.00% or more. Furthermore, the Cu content is preferably 1.50% or less.

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

[0038] W: 0.01 to 3.00% Like Mo, W is an element that significantly improves high-temperature strength. This effect is achieved with a W content of 0.01% or more. On the other hand, an excessive W content of more than 3.00% not only significantly hardens the steel, but also forms a strong oxide scale during the annealing process during manufacturing, making descaling during pickling difficult. Therefore, when W is contained, the W content is set to 0.01 to 3.00%. The W content is preferably 0.30% or more, and more preferably 1.00% or more. The W content is also preferably 2.00% or less, and more preferably 1.50% or less.

[0039] Ti: 0.005 to 0.500% Ti is an element that combines with C and N, preventing Al from combining with N to form AlN, thereby enhancing oxidation resistance. This effect is achieved with a Ti content of 0.005% or more. However, a Ti content of more than 0.500% generates coarse TiN, resulting in a decrease in the toughness of the steel. Therefore, if Ti is contained, the Ti content is set to 0.005 to 0.500%. The Ti content is preferably less than 0.300%, and more preferably 0.050% or less.

[0040] Zr: 0.005 to 0.300% Zr is an element that improves oxidation resistance. To achieve this effect, the Zr content is preferably 0.005% or more. However, if the Zr content exceeds 0.300%, Zr intermetallic compounds precipitate, embrittling the steel. Therefore, when Zr is contained, the Zr content is set to 0.005 to 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 to 0.50% V is an element that is effective in improving the workability of steel, and is also effective in improving oxidation resistance. By adding V, V bonds with N in the steel, which makes it easier to prevent Al from bonding with N to form coarse nitrides. By preventing Al from forming coarse nitrides, high-temperature NH3 resistance is improved. ３This effect becomes more pronounced when the V content is 0.01% or more. However, an excessive V content of more than 0.50% leads to the precipitation of coarse V(C,N) and reduces toughness. Therefore, when V is contained, the V content is set to 0.01 to 0.50%. The V content is preferably 0.05% or more. Furthermore, the V content is preferably 0.10% or less.

[0042] Co: 0.01 to 0.50% Co is known as an element effective in improving the toughness of steel. Furthermore, in the present invention, Co also has the effect of reducing the thermal expansion coefficient, which is increased by the inclusion of Al, and improving thermal fatigue properties. To achieve these effects, the Co content is preferably 0.01% or more. On the other hand, an excessive Co content exceeding 0.50% not only reduces the toughness of the steel but also reduces its workability. Therefore, when Co is contained, the Co content is set to 0.01 to 0.50%. The Co content is preferably 0.03% or more. The Co content is preferably less than 0.30%, and more preferably less than 0.10%.

[0043] REM: 0.01 to 0.50% REM is an element that improves oxidation resistance. To achieve this effect, the REM content is preferably 0.01% or more. However, if the REM content exceeds 0.50%, the steel becomes embrittled. Therefore, if REM is contained, the REM content is set to 0.01 to 0.50%. The REM content is preferably 0.03% or more. The REM content is preferably 0.15% or less. REM (rare earth metals) is a collective term for a total of 17 elements: Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained, and the REM content referred to here is the total content of these elements.

[0044] Hf: 0.01 to 0.50% Hf is an element that improves oxidation resistance. To achieve this effect, the Hf content is preferably 0.01% or more. However, if the Hf content exceeds 0.50%, the steel becomes embrittled. Therefore, if Hf is contained, the Hf content is set to 0.01 to 0.50%. The Hf content is preferably 0.03% or more. Furthermore, the Hf content is preferably 0.15% or less.

[0045] Sb: 0.01 to 0.50% Sb is an element that has the effect of improving the toughness of steel. This effect is obtained with an Sb content of 0.01% or more. On the other hand, an excessive Sb content of more than 0.50% actually reduces toughness, so the upper limit of the Sb content is set to 0.50%. Therefore, when Sb is contained, the Sb content is set to 0.01 to 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 to 0.50% Sn is an element that has the effect of improving the corrosion resistance and high-temperature strength of steel. This effect is obtained with an Sn content of 0.01% or more. On the other hand, an excessive Sn content of more than 0.50% reduces the workability of steel, so the upper limit of the Sn content is set to 0.50%. Therefore, when Sn is contained, the Sn content is set to 0.01 to 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 in the following ranges.

[0048] B: 0.0002 to 0.0050% B is an element effective in 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 of more than 0.0050% generates BN, reducing workability. Therefore, when B is contained, the B content is set to 0.0002 to 0.0050%. The B content is preferably 0.0005% or more. The B content is also preferably 0.0020% or less, and more preferably 0.0010% or less.

[0049] Ca: 0.0002 to 0.0050% Ca is an effective component for preventing nozzle clogging due to inclusion precipitation, which is likely to occur during continuous casting. This effect can be achieved with a Ca content of 0.0002% or more. On the other hand, in order to obtain good surface quality without generating surface defects, the Ca content is preferably 0.0050% or less. Therefore, when Ca is contained, the Ca content is set to 0.0002 to 0.0050%. The Ca content is preferably 0.0005% or more. Furthermore, the Ca content is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0050] Mg: 0.0002 to 0.0050% Mg is an element that is effective in improving the equiaxed crystal ratio of the slab and improving workability and toughness. This effect is obtained with a Mg content of 0.0002% or more. On the other hand, if the Mg content exceeds 0.0050%, the surface quality of the steel may deteriorate. Therefore, if Mg is contained, the Mg content is set to 0.0002 to 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] In addition, when the content of Nb, Ni, Cu, Mo, W, Ti, Zr, V, Co, REM, Hf, Sb, Sn, B, Ca, or Mg, which are described as optional components above, is less than the lower limit value, the component is considered to be contained as an unavoidable impurity.

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

[0053] The method for producing the ferritic stainless steel of the present invention can be basically any conventional method for producing ferritic stainless steel, and is not particularly limited. For example, steel is melted in a known melting furnace such as a converter or electric furnace, or further subjected to secondary refining such as ladle refining or vacuum refining to produce a steel having the above-described composition of the present invention. This steel is then formed into a slab by continuous casting or ingot casting and blooming. The steel can then be produced by a manufacturing process that involves hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, pickling, and other processes to produce a cold-rolled annealed sheet. The cold rolling may be performed once or twice or more times with an intermediate annealing in between. Furthermore, the cold rolling, finish annealing, and pickling processes may be repeated. Finish annealing may be performed in a reducing atmosphere, or annealing in an oxidizing atmosphere followed by polishing or other methods to remove oxide scale, thereby eliminating the need for pickling. After finish annealing in an oxidizing atmosphere, pickling may be omitted and the steel may be used in the state in which an oxide film is formed. Furthermore, the hot-rolled sheet annealing may be omitted, and when adjustment of the surface gloss or roughness of the steel sheet is required, skin-pass rolling may be performed after cold rolling or finish annealing. Also, depending on the application area, it is possible to use the hot-rolled sheet or the hot-rolled annealed sheet as it is.

[0054] Preferred production conditions for the above production method will be described below.

[0055] In the steelmaking process for producing smelted steel, steel melted in a converter or electric furnace is preferably subjected to secondary refining using a VOD method or the like to produce steel containing the above-mentioned essential elements and optional elements added as needed. The molten steel can be produced into a steel material by known methods, but continuous casting is preferred from the perspectives of productivity and quality. The steel material is then heated, preferably to 1050 to 1250°C, and hot-rolled to a desired thickness (3 mm to 10 mm). Of course, hot processing other than plate material is also possible. The hot-rolled sheet is then subjected to continuous annealing at a temperature of 850 to 1150°C as needed to produce a hot-rolled annealed sheet, which is then preferably descaled by pickling or the like to produce a hot-rolled product. The annealing step may be omitted. The pickling step may be omitted and the product may be used as is, or a polished finish may be used instead of pickling. If necessary, scale may be removed by shot blasting or brush grinding before pickling.

[0056] Furthermore, the hot-rolled annealed sheet or hot-rolled sheet may be subjected to a process such as cold rolling to produce a cold-rolled product with a sheet thickness of 0.1 mm to 3 mm. In this case, cold rolling may be performed once, but from the viewpoint of improving productivity and required quality, cold rolling may be performed twice or more times with intermediate annealing in between. The total reduction ratio of one or more cold rolling passes 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 subjected to continuous annealing (finish annealing) at a temperature of preferably 850 to 1150°C, more preferably 900 to 1100°C, followed by pickling to produce a cold-rolled product. Furthermore, depending on the application location, etc., 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 pickling. Furthermore, finish annealing is performed in a reducing atmosphere (e.g., 25 vol% N ２ -75vol%H ２ ) and descaling processes such as pickling may be omitted. Finish annealing may be omitted, or, provided that this does not cause problems in subsequent processing or welding, pickling may be omitted after annealing in an oxidizing atmosphere, and the steel may be used in the state in which an oxide film is formed. A polished finish may be used instead of pickling.

[0057] The hot-rolled or cold-rolled product obtained as described above can then be cut, bent, drilled, or otherwise processed for use in various furnace structures, such as heating furnaces. Examples of applications include the inner walls of furnaces, burners, radiant tubes, and the inner walls of cooling zones. The product may be lined inside the furnace body. Fixing methods include bolting or welding. The welding method for these components is not particularly limited. Examples of welding methods include conventional arc welding, such as MIG (Metal Inert Gas), MAG (Metal Active Gas), and TIG (Tungsten Inert Gas). Resistance welding, such as spot welding and seam welding, and high-frequency resistance welding, such as electric resistance welding, high-frequency induction welding, and laser welding may also be used.

[0058] The ferritic stainless steel of the present invention has high temperature resistance and ３ Because of its excellent ３ The ferritic stainless steel of the present invention is suitable for use in structures such as furnaces that use unburned NH as at least a part of the fuel. ３ Excellent resistance to high temperatures in high-temperature environments containing water vapor ３ It has sexuality.

[0059] As described above, in the present invention, NH ３ The combustion environment is NH ３ It means a high temperature environment of 300°C or higher containing 3 vol% or more of NH and 5 vol% or more of water vapor. ３ An example of a combustion environment is NH ３ Examples include an environment of 600°C or higher containing 3 to 10 vol% of NH3 and 5 to 20 vol% of water vapor. ３ The higher the amount of NH ３ Sex is necessary.

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

[0061] Steels having the chemical compositions Nos. 1 to 29 shown in Table 1 were melted in a vacuum melting furnace, cast into 50 kg steel ingots, heated to 1170 ° C, and hot-rolled into 35 mm thick sheet bars. 200 mm lengths were cut out from these sheet bars, heated to 1150 ° C, and hot-rolled into 5.0 mm thick hot-rolled sheets. These were annealed at temperatures ranging from 850 to 1150 ° C, and then ground to a thickness of 4.5 mm to obtain hot-rolled annealed sheets. Subsequently, cold rolling was performed at a reduction rate of 67%, and finish annealing was performed at temperatures of 850 to 1150 ° C, resulting in cold-rolled annealed sheets with a thickness of 1.5 mm. Surface oxide scale was removed by polishing, and then high-temperature NH ３ For reference, a cold-rolled annealed sheet of SUS310S (No. 26) was prepared in the same manner as above, and subjected to high-temperature NH ３ The annealing temperature was determined for each steel while checking the structure within the above temperature range.

[0062] <High temperature NH ３ Test> A test piece measuring 30 mm x 20 mm was cut out from the cold-rolled annealed sheet obtained as described above. The end faces were finished by cutting, grinding, and polishing (emery paper #320), and a 4 mm diameter hole was drilled at the top of the test piece in the 30 mm direction. After degreasing, the test piece was hung in a furnace heated to 600°C and held in the following two atmospheres (1) and (2), and a test was performed in which the test piece was held for 100 hours. (1) 92 vol% N ２ -3 vol% NH ３ -5vol%H ２ O (dew point +33℃, flow rate 0.5L / min) (2) 70vol%N ２ -10vol% NH ３ -20vol%H ２ O (dew point +60℃, flow rate 0.5L / min)

[0063] After the test, the test piece was visually inspected for cracks and the weight of the test piece was measured. The difference between the weight of the test piece measured before the test and the weight of the test piece before the test was calculated, and the weight gain (g / m ２ ) was calculated. ３ The properties were evaluated as follows:

[0064] [High temperature resistant NH ３ Evaluation criteria] ◎ (pass, better): (1) and (2) both increased by 3 g / m２ Less than 3g / m and no cracks ○ (pass, excellent): (2) ２ However, both (1) and (2) were increased by 40 g / m ２ Less than 40g / m² and no cracks × (Fail): Either (1) or (2), or both, and an increase in weight of 40g / m² ２ or more, or cracked

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

[0066]

[0067] From Table 1, it can be seen that all of the steels No. 1 to 25 of the present invention have excellent resistance to high-temperature NH ３ All of the steels Nos. 1 to 25 of the present invention had high-temperature NH resistance equal to or better than that of SUS310S (steel No. 26). ３ had sexuality.

[0068] On the other hand, Steel No. 27 has a Si content of more than 2.50% and is resistant to high-temperature NH ３ Steel No. 28 had an Al content of less than 0.20% and was found to have poor high-temperature NH ３ Steel No. 29 had a Cr content of less than 12.0% and was found to have poor high-temperature NH ３ The gender was a failure.

[0069] The ferritic stainless steel of the present invention is used in structures such as heating furnaces, and is suitable for use in structures such as furnace inner walls, where high-temperature NH ３ It is suitable for use in areas exposed to

Claims

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 to 25.0%; The balance is Fe and unavoidable impurities. ３ Ferritic stainless steel for combustion environments.

2. Furthermore, 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 to 0.50%, REM: 0.01-0.50%, Hf: 0.01-0.50%, Sb: 0.01 to 0.50%, and Sn: 0.01~0.50% NH according to claim 1, which contains one or more selected from ３ Ferritic stainless steel for combustion environments.

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

4. NH ３ NH3 according to claim 1 or 2, which is used in a structure that burns as at least a part of the fuel. ３ Ferritic stainless steel for combustion environments.

5. A ferritic stainless steel for use in an NH 3 combustion environment as described in claim 3, which is used in a structure that burns NH 3 as at least a part of the fuel.