Ferritic stainless steel for solid oxide electrochemical cells

Abstract

Provided is a stainless steel having excellent oxidation resistance, Cr poisoning resistance, and electric conductivity, not only in use environments for solid oxide fuel cells (SOFCs) but also in use environments for solid oxide electrolysis cells (SOECs). In the present invention, the composition of components is appropriately controlled, and, in particular, is set to contain 0.05-0.40 mass% Si, 1.01-3.00 mass% Mn, 10.0-22.0 mass% Cr, 0.10-1.10 mass% Al, and 0.20-0.45 mass% Nb, and to satisfy the relationships of formulae (1), (2), and (3).　Formula (1): [Mn] + 0.50 × [Cr] ≥ 7.00. Formula (2): [Mn] +1.67 × [Al] ≤ 4.34. Formula (3): [Si] + 0.33 × [Al] ≤ 0.67.

Description

Ferritic stainless steel for solid oxide electrochemical cells

[0001] The present invention relates to a ferritic stainless steel for solid oxide electrochemical cells (hereinafter also referred to as SOC) that has excellent oxidation resistance, resistance to Cr poisoning, and electrical conductivity in environments containing high-temperature water vapor.

[0002] Solid oxide fuel cells (SOFCs) are attracting attention as a next-generation energy source. SOFCs operate at high temperatures of 500 to 900°C. SOFCs have the following outstanding features: - No catalyst is required for the electrode reaction. - A variety of fuel gases, including reformed coal gas, can be used. - They can be combined with gas turbines or steam turbines that utilize high-temperature waste heat.

[0003] As shown in FIG. 1, an SOFC is composed of an electrolyte 1, an air electrode 2 and an anode electrode 3, which are electrodes, and an interconnector 4 (sometimes called a separator). Typically, an ion-conductive solid electrolyte such as yttria-stabilized zirconia (YSZ) is used as the electrolyte 1. One side of the electrolyte 1 is coated with (La, Sr)MnO ３ An air electrode 2 such as Ni / YSZ (cermet of Ni and yttria-stabilized zirconia) is attached to the other surface of the electrolyte 1. An anode 3 such as Ni / YSZ (cermet of Ni and yttria-stabilized zirconia) is attached to the other surface of the electrolyte 1. Power is generated by supplying a fuel gas 5 such as hydrogen gas to one side and an oxidizing gas 6 such as air to the other side, using the electrolyte 1 as a partition. The interconnector 4 supports the three layers of the electrolyte 1, air electrode 2, and anode 3, and forms a gas flow path 7. In addition, the interconnector 4 also serves to pass current.

[0004] As materials for the above-mentioned SOFC members, particularly for the interconnectors, metal materials such as stainless steels disclosed in Patent Documents 1 to 7 have been proposed.

[0005] JP 7-166301 JP 7-145454 JP 9-157801 JP 10-280103 JP 2003-187828 JP 2005-206884 International Publication 2018 / 008658

[0006] Recently, solid oxide electrolysis cells (hereinafter also referred to as SOECs), which apply SOFC technology, have been attracting attention.

[0007] Similar to SOFCs, SOECs are configured as shown in Figure 1. In SOECs, oxidized species are electrochemically reduced using electricity or heat. For example, if the oxidized species is water, hydrogen is produced at the anode. If the oxidized species is a mixture of water and carbon dioxide, a mixture of carbon monoxide and hydrogen is produced, and in one example, the mixture is methanated.

[0008] Materials used in SOEC components, particularly interconnectors, are required to have excellent oxidation resistance, resistance to Cr poisoning, and electrical conductivity, similar to materials used in SOFC components. Here, Cr poisoning refers to the deterioration of electrode performance due to the adhesion of Cr-based oxides. For example, Cr-based oxides volatilize on the surface of stainless steel, which contains a large amount of Cr in its composition, and these oxides adhere to the electrode, thereby deteriorating the performance of the electrode.

[0009] However, the environments in which SOFCs and SOECs are used, particularly the amount of water vapor in the atmosphere, are significantly different from each other, and therefore the stainless steels and other metallic materials developed for SOFCs and disclosed in Patent Documents 1 to 7 do not provide sufficient oxidation resistance and resistance to Cr poisoning in the environments in which SOECs are used.

[0010] The present invention was developed in view of the above-mentioned current situation, and aims to provide a stainless steel, particularly a ferritic stainless steel for SOC, that has excellent oxidation resistance, resistance to Cr poisoning, and electrical conductivity not only in the usage environment of SOFCs but also in the usage environment of SOECs.

[0011] Here, an SOC (solid oxide electrochemical cell) is a cell having an electrolyte, air electrode, and fuel electrode made of solid oxide. In addition to the SOFCs and SOECs described above, SOCs also include solid oxide reversible cells (SORCs) that combine the functions of both SOFCs and SOECs. In this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively, except when described as "greater than" or "less than."

[0012] The inventors conducted extensive research to achieve the above-mentioned object. As a result, they discovered that the above-mentioned object can be achieved by appropriately controlling the composition of ferritic stainless steel, particularly by controlling the composition so as to simultaneously satisfy the following (a) and (b): (a) The Si content, Mn content, Cr content, Al content, and Nb content are controlled to be within the following ranges: Si content: 0.05 to 0.40 mass% Mn content: 1.01 to 3.00 mass% Cr content: 10.0 to 22.0 mass% Al content: 0.10 to 1.10 mass% Nb content: 0.20 to 0.45 mass% (b) The relationships of the following formulas (1), (2), and (3) are satisfied: [Mn] + 0.50 × [Cr] ≥ 7.00 (1) [Mn] + 1.67 × [Al] ≤ 4.34 (2) [Si] + 0.33 × [Al] ≤ 0.67 (3) Here, [Mn], [Cr], [Al] and [Si] are the contents (mass%) of Mn, Cr, Al and Si in the above composition, respectively.

[0013] The inventors believe that the reason for this is as follows: Cr and Al are both elements that form oxides on the surface of ferritic stainless steel. ２ O ３ Al-based oxides such as these are electrically insulating. Therefore, if a ferritic stainless steel contains a large amount of Al, the oxide film formed on the surface of the ferritic stainless steel (hereinafter also referred to as the surface oxide film) will be a film mainly composed of Al-based oxides. As a result, the electrical resistance increases and the electrical conductivity decreases.

[0014] On the other hand, if the surface oxide film becomes a film mainly composed of Cr-based oxides, Cr poisoning occurs. In this regard, by reducing the Cr content to the above range and controlling the Si content and Nb content to the above ranges, Al in the steel is preferentially oxidized even in the use environment of an SOEC where Al-based oxides are unlikely to form. The use environment of an SOEC has a higher oxygen partial pressure and a larger amount of water vapor than the use environment of an SOFC, making it difficult for Al-based oxides to form. Furthermore, by controlling the Al content and Mn content in addition to the Si content and Nb content to the above ranges, the relationship of the above formula (1) is satisfied. As a result, even if the Cr content is reduced to the above range, the surface oxide film, which becomes a film mainly composed of Al-based oxides, becomes thin, while Cr-based oxides, particularly MnCr, are partially present in the surface oxide film. ２ O ４ It is possible to contain spinel-type oxides such as those mentioned above (hereinafter also referred to as Cr-based spinel-type oxides).

[0015] Because the surface oxide film is primarily composed of Al-based oxides, it exhibits excellent oxidation resistance even in harsher environments for SOECs than those for SOFCs, i.e., environments with high temperatures, high oxygen partial pressures, and high levels of water vapor. Furthermore, in the surface oxide film, Cr-based oxides are partially contained within the Al-based oxides, and the volatilization of the Cr-based oxides is inhibited by the nearby Al-based oxides. As a result, excellent resistance to Cr poisoning is achieved even in harsh environments with high temperatures and water vapor. Furthermore, the Cr-based oxides, particularly Cr-based spinel-type oxides, partially formed in the surface oxide film serve as electrical paths, ensuring electrical conductivity.

[0016] However, Cr-based spinel oxides are likely to form in the outermost layer of the surface oxide film. In particular, in an SOEC operating environment where the oxygen partial pressure is high and the amount of water vapor is large, if Cr-based spinel oxides are excessively generated and form a thick film, the surface oxide film will peel off entirely, and the desired oxidation resistance and resistance to Cr poisoning will not be obtained. In addition to the above, satisfying the relationship of the above formula (2) will prevent the surface oxide film from peeling off entirely. Furthermore, particularly in an SOEC operating environment, Si will cause SiO , which has low electrical conductivity, to form near the interface between the surface oxide film and the base material. ２ A continuous coating (hereinafter referred to as SiO ２ In addition to the above, by satisfying the relationship of the above formula (3), SiO ２ As a result, the inventors believe that a ferritic stainless steel whose composition is controlled so as to simultaneously satisfy the above (a) and (b) can simultaneously achieve excellent oxidation resistance and resistance to Cr poisoning, as well as excellent electrical conductivity, not only in the use environment of an SOFC but also in the use environment of an SOEC.

[0017] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0018] 1. A ferritic stainless steel for solid oxide electrochemical cells having a chemical composition, in mass%, of C: 0.030% or less, Si: 0.05 to 0.40%, Mn: 1.01 to 3.00%, P: 0.050% or less, S: 0.010% or less, Cr: 10.0 to 22.0%, Al: 0.10 to 1.10%, Ni: 1.00% or less, Nb: 0.20 to 0.45%, Mo: 2.00 to 5.00%, and N: 0.025% or less, with the balance being Fe and unavoidable impurities, and which satisfies the relationships of the following formulas (1), (2), and (3): [Mn] + 0.50 × [Cr] ≥ 7.00 (1) [Mn] + 1.67 × [Al] ≤ 4.34 (2) [Si] + 0.33 × [Al] ≤ 0.67 (3) Here, [Mn], [Cr], [Al] and [Si] are the contents (mass%) of Mn, Cr, Al and Si in the above composition, respectively.

[0019] 2. The ferritic stainless steel for a solid oxide electrochemical cell according to 1 above, wherein the component composition further contains, in mass %, one or more selected from Cu: 0.50% or less, Co: 1.00% or less, and W: 3.00% or less.

[0020] 3. The ferritic stainless steel for a solid oxide electrochemical cell according to 1 or 2 above, wherein the chemical composition further contains, in mass%, one or more elements selected from Ti: 1.00% or less, V: 1.00% or less, Sn: 0.50% or less, Zr: 0.50% or less, B: 0.0050% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, and REM: 0.20% or less.

[0021] According to the present invention, it is possible to obtain stainless steel that is excellent in oxidation resistance, resistance to Cr poisoning, and electrical conductivity not only in the use environment of SOFCs but also in the use environment of SOECs.

[0022] FIG. 1 is a diagram schematically illustrating an example of an SOFC.

[0023] The present invention will be described based on the following embodiments. First, the chemical composition of a ferritic stainless steel for an SOC according to one embodiment of the present invention will be described. Note that the units for the chemical composition are all "mass %", and hereinafter, unless otherwise specified, will be simply represented as "%".

[0024] C: 0.030% or less C has the effect of increasing strength at high temperatures. To achieve this effect, the C content is preferably 0.001% or more, more preferably 0.003% or more. However, if the C content exceeds 0.030%, toughness and formability decrease. Therefore, the C content is 0.030% or less, preferably 0.010% or less, more preferably 0.008% or less.

[0025] Si: 0.05 to 0.40% Si has the effect of promoting the oxidation of Al and improving oxidation resistance. Si also has the effect of suppressing the evaporation of Cr-based oxides. Furthermore, when Si is simultaneously contained with Cr, it forms a thin and dense surface oxide film in the early stage of oxidation. This reduces the oxygen partial pressure and reduces the Al ２ O ３ In order to obtain such an effect, the Si content is set to 0.05% or more, and preferably 0.10% or more. However, if the Si content exceeds 0.40%, particularly in the use environment of SOEC, SiO 2 with low electrical conductivity is generated near the interface between the surface oxide film and the base material. ２ A thick film is formed, and electrical conductivity is reduced. Therefore, the Si content is 0.40% or less, preferably 0.30% or less, and more preferably 0.20% or less.

[0026] Mn: 1.01 to 3.00% Mn, together with Cr, forms Cr-based oxides, particularly Cr-based spinel-type oxides, in the surface oxide film, which is primarily composed of Al-based oxides. The Cr-based oxides partially formed within the Al-based oxides in the surface oxide film serve as electrical paths, effectively contributing to ensuring electrical conductivity. Mn also reduces the amount of Cr required to form Cr-based oxides. Therefore, Mn effectively contributes to improving resistance to Cr poisoning. To achieve this effect, the Mn content is 1.01% or more, preferably 1.10% or more, and more preferably 1.15% or more. However, if the Mn content exceeds 3.00%, the oxide scale grows abnormally and becomes more prone to spalling. Spalling of the oxide scale may result in a decrease in oxidation resistance and accelerate Cr poisoning. Furthermore, the steel hardens at room temperature, resulting in a decrease in workability. Therefore, the Mn content is 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0027] P: 0.050% or less P is a harmful element that reduces the toughness of steel. Therefore, it is desirable to reduce the P content as much as possible. Therefore, the P content is 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. The lower limit of the P content is not particularly limited and may be 0%. However, since excessive dephosphorization increases costs, the P content is preferably 0.001% or more.

[0028] S: 0.010% or less S is a harmful element that not only adversely affects formability but also reduces corrosion resistance, a basic property of stainless steel. Therefore, it is desirable to reduce S as much as possible. Therefore, the S content is 0.010% or less, preferably 0.005% or less. The lower limit of the S content is not particularly limited and may be 0%. However, since excessive desulfurization increases costs, the S content is preferably 0.0005% or more.

[0029] Cr: 10.0 to 22.0% Cr has the effect of partially generating Cr-based oxides in the surface oxide film, which is a film mainly composed of Al-based oxides, thereby ensuring electrical conductivity. Cr also has the effect of improving oxidation resistance. To achieve this effect, the Cr content is 10.0% or more, preferably 12.5% ​​or more, and more preferably 15.0% or more. However, excessive Cr content, particularly a Cr content exceeding 22.0%, promotes the volatilization of Cr-based oxides. It also leads to deterioration of workability. Therefore, the Cr content is 22.0% or less, preferably 21.0% or less, and more preferably 20.0% or less.

[0030] Al: 0.10 to 1.10% When Nb is simultaneously contained, Al forms oxides preferentially over Cr, thereby improving oxidation resistance. Furthermore, Al's preferential oxide formation can suppress Cr poisoning of electrodes due to the volatilization of Cr-based oxides. To achieve this effect, the Al content is 0.10% or more, preferably 0.50% or more, and more preferably 0.70% or more. On the other hand, if the Al content exceeds 1.10%, the steel becomes hard and its workability deteriorates. Furthermore, it becomes impossible to partially allow Cr-based oxides to exist in the surface oxide film, which is primarily composed of Al-based oxides. Furthermore, the thickness of the surface oxide film increases, resulting in an increase in electrical resistance. Therefore, the Al content is 1.10% or less, preferably 1.05% or less, and more preferably 1.00% or less.

[0031] Ni: 1.00% or less Ni has the effect of improving the toughness and oxidation resistance of steel. To achieve this effect, the Ni content is preferably 0.01% or more, more preferably more than 0.05%. On the other hand, Ni is a γ-phase forming element. Therefore, if the Ni content exceeds 1.00%, the γ-phase is formed at high temperatures, reducing the oxidation resistance and resistance to Cr poisoning. Furthermore, the reduced oxidation resistance also increases the electrical resistance. Therefore, the Ni content is 1.00% or less, preferably less than 0.50%, more preferably less than 0.20%.

[0032] Nb: 0.20 to 0.45% Nb has the effect of increasing strength at high temperatures. Nb also has the effect of promoting the oxidation of Al and improving oxidation resistance. Furthermore, Nb has the effect of suppressing the evaporation of Cr-based oxides. Therefore, Nb is an important element in the ferritic stainless steel for SOC according to one embodiment of the present invention. In particular, when Nb is simultaneously contained with Cr, it forms a thin and dense surface oxide film in the early stage of oxidation. This reduces the oxygen partial pressure and reduces the Al ２ O ３ The formation of Al-based oxides such as Nb is promoted. To obtain this effect, the Nb content is 0.20% or more, preferably 0.25% or more, and more preferably 0.30% or more. However, if the Nb content exceeds 0.45%, the steel becomes hard and workability deteriorates. Therefore, the Nb content is 0.45% or less, preferably 0.40% or less, and more preferably 0.35% or less.

[0033] Mo: 2.00 to 5.00% Mo has the effect of increasing strength at high temperatures and improving oxidation resistance. To achieve these effects, the Mo content is 2.00% or more, preferably 2.10% or more, and more preferably 2.20% or more. On the other hand, if Mo is contained in excess, particularly if the Mo content exceeds 5.00%, the steel becomes hard and workability deteriorates. Therefore, the Mo content is 5.00% or less, preferably 3.00% or less, more preferably 2.50% or less, and even more preferably 2.30% or less.

[0034] N: 0.025% or less N is an element that reduces the toughness and formability of steel, and is therefore preferably reduced as much as possible. In particular, if the N content exceeds 0.025%, the toughness and formability of the steel may be significantly reduced. Therefore, the N content is 0.025% or less, preferably 0.020% or less, and more preferably less than 0.015%. The lower limit of the N content is not particularly limited and may be 0%. However, since excessive denitrification increases costs, the N content is preferably 0.001% or more.

[0035] Furthermore, in the ferritic stainless steel for an SOC according to one embodiment of the present invention, it is important that, in addition to the above-mentioned component composition, the Mn content and the Cr content satisfy the relationship of the above formula (1), the Mn content and the Al content satisfy the relationship of the above formula (2), and the Si content and the Al content satisfy the relationship of the above formula (3).

[0036] [Mn] + 0.50 × [Cr] ≥ 7.00 (1) In a ferritic stainless steel for SOC according to one embodiment of the present invention, it is important to satisfy the relationship of the above formula (1), i.e., to make [Mn] + 0.50 × [Cr] ≥ 7.00. This allows the surface oxide film, which is primarily composed of Al-based oxides, to be thinned even when the Cr content is reduced to the above range, while allowing the surface oxide film to partially contain Cr-based oxides, particularly Cr-based spinel-type oxides. As a result, excellent resistance to Cr poisoning is achieved even in harsh environments such as those containing high-temperature steam. Furthermore, the Cr-based spinel-type oxides partially formed in the surface oxide film serve as electrical conduction paths, ensuring electrical conductivity. Therefore, [Mn] + 0.50 × [Cr] is ≥ 7.00, preferably ≥ 9.00, and more preferably ≥ 10.00. The upper limit of [Mn] + 0.50 × [Cr] is not particularly limited. For example, from the viewpoint of resistance to Cr poisoning and workability, [Mn] + 0.50 × [Cr] is preferably 13.00 or less, and more preferably 12.00 or less.

[0037] [Mn] + 1.67 × [Al] ≦ 4.34 (2) In a ferritic stainless steel for an SOC according to one embodiment of the present invention, it is important to satisfy the relationship in formula (2), i.e., to make [Mn] + 1.67 × [Al] 4.34 or less. As described above, Cr-based spinel oxides are likely to form in the outermost layer of a surface oxide film. In particular, in an SOEC operating environment with a high oxygen partial pressure and a large amount of water vapor, excessive Cr-based spinel oxides may form, resulting in a thick film. This can lead to the overall peeling of the surface oxide film, making it impossible to achieve the desired oxidation resistance and Cr-poisoning resistance. In this regard, by making [Mn] + 1.67 × [Al] 4.34 or less, the overall peeling of the surface oxide film can be prevented. Therefore, [Mn] + 1.67 × [Al] is 4.34 or less, preferably 4.00 or less, and more preferably 3.60 or less. The lower limit of [Mn] + 1.67 × [Al] is not particularly limited. For example, from the viewpoint of oxidation resistance and resistance to Cr poisoning, [Mn]+1.67×[Al] is preferably 2.00 or more, and more preferably 2.30 or more.

[0038] [Si]+0.33×[Al]≦0.67 (3) In the ferritic stainless steel for SOC according to one embodiment of the present invention, it is important to satisfy the relationship of the above formula (3), that is, to make [Si]+0.33×[Al] 0.67 or less. As described above, particularly in the use environment of SOEC, Si generates SiO 2 , which has low electrical conductivity, near the interface between the surface oxide film and the base material. ２ In this regard, by making [Si] + 0.33 × [Al] 0.67 or less, SiO ２ The formation of a coating is suppressed, and excellent electrical conductivity can be obtained. Therefore, [Si] + 0.33 × [Al] is 0.67 or less, preferably 0.60 or less, and more preferably 0.50 or less. The lower limit of [Si] + 0.33 × [Al] is not particularly limited. For example, from the viewpoint of oxidation resistance and resistance to Cr poisoning, [Si] + 0.33 × [Al] is preferably 0.15 or more, and more preferably 0.25 or more.

[0039] The chemical composition of the ferritic stainless steel for SOC according to one embodiment of the present invention may optionally contain one or both of the following (A) and (B): (A) one or more selected from Cu: 0.50% or less, Co: 1.00% or less, and W: 3.00% or less (B) one or more selected from Ti: 1.00% or less, V: 1.00% or less, Sn: 0.50% or less, Zr: 0.50% or less, B: 0.0050% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, and REM: 0.20% or less

[0040] Cu: 0.50% or less Cu has the effect of improving the corrosion resistance of steel. To achieve this effect, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, if the Cu content exceeds 0.50%, oxide scale tends to peel off, resulting in a decrease in oxidation resistance. Therefore, when Cu is contained, the Cu content is preferably 0.50% or less, more preferably 0.35% or less, and even more preferably 0.20% or less.

[0041] Co: 1.00% or less Co has the effect of improving the toughness of steel. To achieve this effect, the Co content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more. On the other hand, if the Co content exceeds 1.00%, the toughness and workability of the steel will be reduced. Therefore, when Co is contained, the Co content is preferably 1.00% or less, more preferably less than 0.30%, even more preferably 0.10% or less, and even more preferably 0.07% or less.

[0042] W: 3.00% or less Like Mo, W has the effect of improving strength at high temperatures through solid solution strengthening. To achieve this effect, the W content is preferably 0.01% or more, more preferably 0.30% or more, and even more preferably 1.00% or more. On the other hand, if the W content exceeds 3.00%, the steel becomes hard and its workability deteriorates. In addition, strong scale is formed during the annealing process during production, making descaling during pickling difficult. Therefore, when W is contained, the W content is preferably 3.00% or less, more preferably 2.50% or less, and even more preferably 2.00% or less.

[0043] Ti: 1.00% or less Ti has the effect of improving the workability and oxidation resistance of steel. To achieve this effect, the Ti content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more. However, if the Ti content exceeds 1.00%, coarse Ti(C,N) precipitates are formed, which not only reduces toughness but also degrades surface properties. Therefore, when Ti is contained, the Ti content is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less.

[0044] V: 1.00% or less V has the effect of improving the workability and oxidation resistance of steel. To achieve this effect, the V content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more. However, if the V content exceeds 1.00%, coarse V(C,N) precipitates are formed, which not only reduces toughness but also degrades surface properties. Therefore, when V is contained, the V content is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.20% or less.

[0045] Sn: 0.50% or less Sn has the effect of increasing strength at high temperatures. To achieve this effect, the Sn content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Sn content exceeds 0.50%, the manufacturability of the steel decreases. Therefore, when Sn is contained, the Sn content is preferably 0.50% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.

[0046] Zr: 0.50% or less Zr has the effect of improving oxidation resistance. To achieve this effect, the Zr content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Zr content exceeds 0.50%, the steel becomes embrittled due to the precipitation of Zr intermetallic compounds. Therefore, when Zr is contained, the Zr content is preferably 0.50% or less, more preferably 0.25% or less, and even more preferably 0.10% or less.

[0047] B: 0.0050% or less B has the effect of improving the workability of steel, particularly secondary workability. To achieve this effect, the B content is preferably 0.0002% or more, more preferably 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, BN is formed and workability deteriorates. Therefore, when B is contained, the B content is preferably 0.0050% or less, more preferably 0.0020% or less, and even more preferably 0.0010% or less.

[0048] Mg: 0.0050% or less Mg has the effect of preventing nozzle clogging due to inclusion precipitation, which is likely to occur during continuous casting. To achieve this effect, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more. On the other hand, if the Mg content exceeds 0.0050%, surface defects are likely to occur. Therefore, when Mg is contained, the Mg content is preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less.

[0049] Ca: 0.0050% or less Ca has the effect of preventing nozzle clogging due to inclusion precipitation, which is likely to occur during continuous casting. To achieve this effect, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, surface defects are likely to occur. Therefore, when Ca is contained, the Ca content is preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less.

[0050] REM: 0.20% or less REM is a collective term for Sc, Y, La, Ce, Pr, Nd, Pm, Sm, and Hf. REM has the effect of improving the adhesion of a surface oxide film and improving oxidation resistance. To achieve this effect, the REM content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the REM content exceeds 0.20%, surface defects are more likely to occur. Therefore, when REM is contained, the REM content is preferably 0.20% or less, more preferably 0.10% or less. The REM content is the total content of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, and Hf.

[0051] The balance of the composition other than the above elements is Fe and unavoidable impurities.

[0052] Furthermore, the structure of the ferritic stainless steel for SOC according to one embodiment of the present invention is a structure mainly composed of ferrite phase. Specifically, the structure of the ferritic stainless steel for SOC according to one embodiment of the present invention has a ferrite phase that accounts for 90% or more (preferably 95% or more, more preferably 98% or more) of the volume fraction of the entire structure, and the remaining structure other than the ferrite phase (hereinafter simply referred to as the remaining structure) accounts for 10% or less (preferably 5% or less, more preferably 2% or less) of the volume fraction of the entire structure. Furthermore, the structure of the ferritic stainless steel for SOC according to one embodiment of the present invention may be a single ferrite phase (100% of the volume fraction of the entire structure). The remaining structure mainly includes a martensite phase. Note that precipitates and inclusions are not included in the calculation of the volume fraction.

[0053] Here, the identification of the structure (calculation of the volume fraction of the ferrite phase) is performed, for example, as follows. Specifically, a test piece for cross-sectional observation is prepared from the ferritic stainless steel for SOC and subjected to an etching treatment using a picric acid saturated hydrochloric acid solution. Next, the test piece for cross-sectional observation is observed under an optical microscope at a magnification of 100x in 10 fields of view, and the ferrite phase and the remaining structure (such as the martensite phase) are identified from the structure shape and etching intensity. Next, the volume fraction of the ferrite phase is determined by image processing, and the average value is calculated.

[0054] Furthermore, the shape of the ferritic stainless steel for SOC according to one embodiment of the present invention may be, for example, a plate (steel plate) or a part. The thickness of the ferritic stainless steel plate for SOC in the form of a plate (steel plate) or part (hereinafter simply referred to as the thickness of the stainless steel) is not particularly limited. However, from the viewpoints of oxidation resistance, resistance to Cr poisoning, and workability, the thickness of the stainless steel is preferably 0.01 to 10.0 mm. The thickness of the stainless steel is more preferably 0.02 mm or more, and even more preferably 0.03 mm or more. The thickness of the stainless steel is more preferably 8.0 mm or less, and even more preferably 2.0 mm or less.

[0055] The SOEC operates at high temperatures of 500 to 1000°C. The electrolyte of the SOEC is an ion-conductive solid electrolyte, such as yttria-stabilized zirconia (YSZ) or gadolinia-doped ceria (CGO). A fuel electrode made of Ni / YSZ (cermet of Ni and yttria-stabilized zirconia) or Ni / CGO (cermet of Ni and gadolinia-doped ceria) is attached to one side of the electrolyte. A fuel electrode made of (La,Sr)MnO is attached to the other side of the electrolyte. ３ or (La, Sr) (Co, Sr) O ３ An air electrode such as the above is attached.

[0056] Next, a preferred method for producing a ferritic stainless steel for SOC according to one embodiment of the present invention will be described. Specifically, molten steel is produced in a melting furnace such as a converter or an electric furnace. The molten steel is then subjected to secondary refining by ladle refining or vacuum refining to adjust the composition to the above-described composition. The molten steel is then formed into a slab by a continuous casting method or an ingot casting-blooming rolling method. From the standpoints of productivity and quality, the continuous casting method is preferred. The slab is then hot-rolled to produce a hot-rolled steel sheet. The hot-rolled steel sheet may be further subjected to hot-rolled sheet annealing and / or pickling. Hereinafter, when simply referring to a hot-rolled steel sheet, it includes not only an as-hot-rolled steel sheet (including a steel sheet obtained by subjecting an as-hot-rolled steel sheet to pickling or the like) but also a so-called hot-rolled annealed sheet (including a steel sheet obtained by subjecting an as-hot-rolled steel sheet to hot-rolled annealing, and a steel sheet obtained by further subjecting the steel sheet obtained by the hot-rolled annealing to pickling or the like). Depending on the application, the hot-rolled steel sheet can be used as a product (hereinafter also referred to as a hot-rolled product) without being subjected to cold rolling or the like. For example, when manufacturing a housing for an SOC, the hot-rolled steel sheet can be used as is.

[0057] Next, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Next, the cold-rolled steel sheet is subjected to processes such as finish annealing (cold-rolled sheet annealing) and pickling to obtain a product such as a cold-rolled annealed sheet (hereinafter also referred to as a cold-rolled product). Note that the cold-rolled annealed sheet includes a steel sheet obtained by subjecting an as-cold-rolled steel sheet to finish annealing (cold-rolled sheet annealing), and a steel sheet obtained by further subjecting a steel sheet obtained by subjecting a finish annealing (cold-rolled sheet annealing) to pickling or the like. The atmosphere for the finish annealing is not particularly limited. As the finish annealing, BA (bright) annealing in a reducing atmosphere such as hydrogen may be performed, and pickling may be omitted. Note that before pickling, scale may be removed by shot blasting or mechanical descaling.

[0058] As described above, the ferritic stainless steel for SOC according to one embodiment of the present invention can be produced.

[0059] Here, the conditions for each of the above-mentioned steps may follow conventional methods. For example, when a steel slab is heated before hot rolling, the temperature is preferably 1050 to 1250°C. Hot-rolled sheet annealing is preferably performed by continuous annealing in a temperature range of 900 to 1150°C. Cold rolling may be performed once or two or more times with intermediate annealing in between. From the standpoint of productivity and required quality, two or more times with intermediate annealing in between are preferable. Furthermore, the total reduction ratio of cold rolling is preferably 50% or more, more preferably 60% or more. Finish annealing (cold-rolled sheet annealing) is preferably performed by continuous annealing in a temperature range of 900 to 1150°C. A more preferable temperature range is 950 to 1100°C. 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.

[0060] The hot-rolled product and cold-rolled product obtained as described above are then subjected to processes such as cutting, bending, stretching, and drawing depending on the application, and are formed into SOC-related components such as interconnectors and reformers.

[0061] In addition, when forming these members, for example, 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, high-frequency resistance welding such as electric resistance welding, and high-frequency induction welding can be applied.

[0062] Steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a vacuum melting furnace and cast into a 30 kg steel ingot. The steel ingot was heated to 1250°C and then hot-rolled to form a sheet bar with a thickness of 30 mm. The sheet bar was heated to 1150°C and then hot-rolled to form a hot-rolled steel sheet. The hot-rolled steel sheet was then subjected to hot-rolled annealing at a temperature of 900 to 1150°C and then ground to obtain a hot-rolled annealed sheet with a thickness of 4 mm. The hot-rolled annealed sheet was then cold-rolled to form a cold-rolled steel sheet. The cold-rolled steel sheet was then finish-annealed at a temperature of 750 to 1100°C to obtain a cold-rolled annealed sheet with a thickness of 1 mm. The front and back surfaces of the cold-rolled and annealed steel sheets were then polished with #800 emery paper, and the polished cold-rolled and annealed steel sheets were evaluated for oxidation resistance, Cr-poisoning resistance, and electrical conductivity in the following manner. The evaluation results are shown in Table 2.

[0063] <Evaluation of oxidation resistance> Samples measuring 1 mm x 20 mm x 20 mm were cut out from the above cold-rolled annealed sheets. After degreasing with acetone, the samples were subjected to an oxidation test in a heating furnace in the following environments (temperature and atmosphere) for 100 hours. Environment 1: Simulated environment of the air electrode in a power generation environment (SOFC usage environment) Atmosphere: 15 vol. % H ２ O + 85 vol. % air Temperature: 700°C Environment 2: Simulated environment of the air electrode in an electrolytic environment (SOEC usage environment) Atmosphere: 80 vol. % H ２ O + 20 vol. % air Temperature: 700°C Environment 3: Simulated environment of the fuel electrode in an electrolytic environment (an environment using SOEC) Atmosphere: 80 vol. % H ２ O + 20 vol. % hydrogen Temperature: 700 °C

[0064] The oxidation weight gain of the sample was measured before and after the test, and the oxidation resistance in each environment was evaluated according to the following criteria: The oxidation weight gain was calculated by multiplying the mass gain (mg) of the sample before and after the test by the surface area (cm ２ ) is the value obtained by dividing the weight gain by the weight gain of the anode in the power generation environment (SOFC usage environment). Since oxidation does not progress in the anode in the power generation environment (SOFC usage environment), evaluation in the simulated anode environment in the power generation environment (SOFC usage environment) was omitted. A (pass, particularly excellent): Oxidation weight gain of 0.125 mg / cm ２B (pass, excellent): Oxidation gain 0.125 mg / cm ２ Super 0.200mg / cm ２ F (Fail): Oxidation gain is 0.200 mg / cm or less ２ super

[0065] <Cr poisoning resistance> A sample having the same shape as the sample used in the oxidation resistance evaluation was prepared from the above cold-rolled annealed sheet. The prepared sample was loaded into a quartz sample holder in a quartz tubular furnace. The sample holder was placed in the center of the tubular furnace, and quartz wool was placed downstream of the sample holder to capture Cr evaporating from the sample. Next, as in Environment 1 for the oxidation resistance evaluation, the tubular furnace was heated to 15 vol. % H ２ The temperature inside the tubular furnace was set to 700°C and maintained for 100 hours while flowing 85% H2O+ air. Separately, similar samples were prepared and stored in the tubular furnace under the same conditions as in Environments 2 and 3 for the evaluation of oxidation resistance. ２ O + 20 vol. % air and 80 vol. % H ２ While flowing O + 20 vol. % hydrogen, the temperature inside the tubular furnace was maintained at 700°C for 100 hours. Air was collected from the periphery of the tubular furnace. After the maintenance period, all of the Cr adhering to the sample holder and quartz wool was dissolved in an acid solution, and the Cr concentration in the acid solution was measured using an ICP-MS (inductively coupled plasma mass spectrometer). Next, the amount of Cr contained in the acid solution was calculated from the Cr concentration and the amount of acid solution. Next, the amount of Cr evaporated from the sample was calculated by dividing the amount of Cr contained in the acid solution by the surface area of ​​the sample. Then, the resistance to Cr poisoning in each environment was evaluated according to the following criteria: A (pass, excellent): Cr evaporation amount 2.0 μg / cm ２ Below B (pass, excellent): Cr evaporation amount is 2.0 μg / cm ２ Super 5.0μg / cm ２ F (Fail): Cr evaporation amount is 5.0 μg / cm ２ super

[0066] <Evaluation of Electrical Conductivity> After the oxidation resistance evaluation, 5 mm x 5 mm Pt paste was applied to the front and back surfaces of the sample used in the oxidation resistance evaluation, and the sample was dried by holding it at 675°C for 30 minutes. The temperature increase rate during heating before holding and the temperature decrease rate during cooling after holding were both 4°C / min. Next, Pt meshes (10 mm x 10 mm) to which Pt wires for applying current and Pt wires for measuring voltage were spot-welded were placed on the front and back surfaces of the sample where the Pt paste had been applied. These samples were then subjected to a pressure of 0.1 kgf / cm. ２ The specimen was held for 30 minutes in a heating furnace heated to 700°C under a load of 1000 kJ / s. The temperature was increased at a rate of 4°C / min during heating before the holding. During the holding at 700°C, the current density was 0.5 A / cm. ２ A current was passed through the sample so that the voltage was measured to determine the electrical resistance (area resistivity). The electrical resistance of each sample was measured three times, and the electrical conductivity in each environment was evaluated based on the average value of the measured values ​​according to the following criteria: A (pass, particularly excellent): The average electrical resistance was 0.20 Ω cm 2 Below B (pass, excellent): The average electrical resistance value is 0.20 Ω cm 2 Super 0.25Ω・cm 2 F (Fail): The average electrical resistance value is 0.25 Ω cm or less 2 super

[0067]

[0068]

[0069] As shown in Table 2, all of the invention examples had excellent electrical conductivity in all of the environments 1 to 3, and also had excellent oxidation resistance and resistance to Cr poisoning in an environment containing high-temperature water vapor.

[0070] On the other hand, in comparative example Steel No. B1, the Cr content did not satisfy the appropriate range, so sufficient oxidation resistance in Environment 2 and sufficient electrical conductivity in Environments 2 and 3 were not obtained. In Steel No. B2, the Al content did not satisfy the appropriate range, so sufficient oxidation resistance in Environments 2 and 3 and sufficient resistance to Cr-poisoning in Environments 1 and 2 were not obtained. In Steel No. B3, the relationship of formula (1) above was not satisfied, so sufficient oxidation resistance in Environment 2, sufficient resistance to Cr-poisoning in Environment 2, and sufficient electrical conductivity in Environments 2 and 3 were not obtained. In Steel No. B4, the relationship of formula (2) above was not satisfied, so sufficient oxidation resistance in Environment 2 and sufficient resistance to Cr-poisoning in Environment 2 were not obtained. In Steel No. B5, the relationship of formula (3) above was not satisfied, so sufficient electrical conductivity in Environments 2 and 3 were not obtained. In Steel No. B6, the Si content exceeded the appropriate range, so sufficient electrical conductivity was not obtained in Environments 2 and 3. In Steel No. B7, the Mn content exceeded the appropriate range, so sufficient oxidation resistance in Environment 2 and sufficient resistance to Cr poisoning in Environment 2 were not obtained. In Steel No. B8, the Al content exceeded the appropriate range, so sufficient electrical conductivity was not obtained in Environments 2 and 3. In Steel No. B9, the Nb content was below the appropriate range, so sufficient oxidation resistance in Environments 2 and 3 and sufficient resistance to Cr poisoning in Environment 2 were not obtained. In Steel No. B10, the Mo content was below the appropriate range, so sufficient oxidation resistance in Environments 2 and 3, sufficient resistance to Cr poisoning in Environment 2, and sufficient electrical conductivity in Environments 2 and 3 were not obtained. In Steel No. B11, the Ni content exceeded the appropriate range, so sufficient oxidation resistance in Environments 2 and 3, sufficient resistance to Cr poisoning in Environment 2, and sufficient electrical conductivity in Environment 2 were not obtained. In Steel No. B12, the Cr content exceeded the appropriate range, so sufficient resistance to Cr poisoning was not obtained in Environments 1 and 2. In Steel No. B13, the Mn content was below the appropriate range, so sufficient electrical conductivity was not obtained in Environments 2 and 3.

[0071] The ferritic stainless steel for SOC according to one embodiment of the present invention can be used for SOC (solid oxide electrochemical cells), particularly for SOC interconnectors and peripheral components such as heat exchangers and reformers. Furthermore, the ferritic stainless steel for SOC of the present invention can be suitably used not only for SOCs, but also for materials for automobiles and the like, as well as materials for boilers and gas turbines where material degradation due to volatilization of Cr-based oxides is a problem.

[0072] 1: Electrolyte 2: Air electrode 3: Fuel electrode 4: Interconnector (separator) 5: Fuel gas (hydrogen gas) 6: Oxidizing gas (air) 7: Gas flow path

Claims

1. A ferritic stainless steel for solid oxide electrochemical cells having a chemical composition, in mass%, of C: 0.030% or less, Si: 0.05 to 0.40%, Mn: 1.01 to 3.00%, P: 0.050% or less, S: 0.010% or less, Cr: 10.0 to 22.0%, Al: 0.10 to 1.10%, Ni: 1.00% or less, Nb: 0.20 to 0.45%, Mo: 2.00 to 5.00%, and N: 0.025% or less, with the balance being Fe and unavoidable impurities, and which satisfies the relationships of the following formulas (1), (2), and (3). [Mn] + 0.50 × [Cr] ≥ 7.00 (1) [Mn] + 1.67 × [Al] ≤ 4.34 (2) [Si] + 0.33 × [Al] ≤ 0.67 (3) Here, [Mn], [Cr], [Al] and [Si] are the contents (mass%) of Mn, Cr, Al and Si in the above composition, respectively.

2. A ferritic stainless steel for solid oxide electrochemical cells according to claim 1, wherein the composition further contains, in mass %, one or more selected from Cu: 0.50% or less, Co: 1.00% or less, and W: 3.00% or less.

3. A ferritic stainless steel for solid oxide electrochemical cells as described in claim 1 or 2, wherein the chemical composition further contains, in mass %, one or more selected from the following: Ti: 1.00% or less, V: 1.00% or less, Sn: 0.50% or less, Zr: 0.50% or less, B: 0.0050% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, and REM: 0.20% or less.

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