Ferritic stainless steel for solid oxide fuel cells.

Abstract

DEPCT66 The presence of a chemical element (1) containing 0.15 percent by mass to 1.00 percent by mass of Nb, and (2) which contains Mg0.0005 percent by mass to 0.0100 percent by mass, by (3) amount of Al filling It was controlled within a range of values ​​of 0.55 percent by mass to 2.00 percent by mass, and (4) Consistent with the correlation 0.0004 less than or equal to [Mg] / [Al] less than or equal to 0.0050 -----------------------------------------------------------

Description

Ferritic stainless steel for solid oxide fuel cells The present invention relates to a ferritic stainless steel for solid oxide fuel cells that exhibits excellent electrical conductivity and also has excellent oxidation resistance and chromium toxicity resistance in environments containing high-temperature water vapor. Fuel cells emit fewer harmful gases and have high power generation efficiency. Therefore, fuel cells are expected to be applied to a wide range of power generation systems, including large-scale power generation, cogeneration systems, and automotive power supplies. In particular, solid oxide fuel cells (sometimes referred to as solid electrolyte fuel cells) are attracting attention as a next-generation energy source. Solid oxide fuel cells operate at high temperatures of 500 to 900°C. And solid oxide fuel cells... - No catalyst is required for the electrode reaction. - It can use a variety of fuel gases, such as coal-reformed gas. - It can be combined with gas turbine or steam turbine power generation systems that utilize high-temperature waste heat. It possesses excellent features such as these. Here, a solid oxide fuel cell, as shown in Figure 1 as an example, consists of an electrolyte 1, electrodes consisting of an anode (air electrode) 2 and a cathode (fuel electrode) 3, and an interconnector 4 (hereinafter sometimes referred to as a separator). Typically, an ion-conducting solid electrolyte such as yttria-stabilized zirconia (YSZ) is used for electrolyte 1. On one side of electrolyte 1, (La,Sr)MnO ３ An anode (air electrode) 2 is attached. A cathode (fuel electrode) 3, such as Ni / YSZ (a cermet of Ni and yttria-stabilized zirconia), is attached to the other side of the electrolyte 1. Electricity is then generated by using the electrolyte 1 as a partition and supplying a fuel gas 5, such as hydrogen gas, to one side and an oxidizing gas 6, such as air, to the other side. Furthermore, the interconnector 4 plays a role in supporting the three layers of electrolyte 1, anode (air electrode) 2, and cathode (fuel electrode) 3 to form a gas flow path 7. In addition, the interconnector 4 also plays a role in conducting electric current. The components of the solid oxide fuel cell described above, particularly the interconnectors, require oxidation resistance, electrical conductivity, and thermal expansion compatibility with other components. Various metallic materials have been proposed for use as components in such solid oxide fuel cells. For example, Patent Document 1 contains, "A separator for a solid electrolyte fuel cell, in which a flat plate-shaped single cell, each having a fuel electrode and an air electrode arranged so as to sandwich a solid electrolyte layer, is electrically connected in series and fuel gas or oxidizer gas is distributed to each electrode of the single cell, characterized in that the separator is made of an alloy consisting of 60-82% by weight of Fe and 18-40% by weight of Cr, and additive elements that reduce contact resistance between the separator and the air electrode of the single cell." This has been disclosed. Patent Document 2 contains: "A metal material for solid electrolyte fuel cells, characterized by containing 5-30 wt% Cr, 3-45 wt% Co, 1 wt% or less La, with the remainder being Fe and unavoidable impurities." This has been disclosed. Patent Document 3 contains: "A steel for solid electrolyte fuel cell separators, characterized by containing, by weight percent, one or more of the following elements: C 0.2% or less, Si 0.2-3.0%, Mn 0.2-1.0%, Cr 15-30%, and Y 0.5% or less, rare earth elements 0.2% or less, and Zr 1% or less, with the remainder being Fe and unavoidable impurities." This has been disclosed. Patent Document 4 contains: "A steel for solid electrolyte fuel cell separators, characterized by containing, by weight, C 0.2% or less, Si 3.0% or less, Mn 1.0% or less, Cr 15-30%, Hf 0.5% or less, with the remainder being substantially Fe." This has been disclosed. Patent Document 5 contains: "A ferritic stainless steel for solid oxide fuel cell components, characterized by containing, in mass percent, C: 0.03% or less, Mn: 2.0% or less, Ni: 0.6% or less, N: 0.03% or less, Cr: 10.0 to 32.0%, and at least one of Si: 2.0% or less or Al: 6.0% or less in total amount of 1.5% or more, with the remainder being substantially Fe." This has been disclosed. Patent Document 6 contains, "An Fe-Cr alloy for fuel cells characterized by containing C: 0.20 mass% or less, Si: 1.0 mass% or less, Mn: 1.1 to 2.0 mass%, Cr: 10 to 40 mass%, Al: 1.0 mass% or less, Mo: 0.03 to 5.0 mass%, and Nb: 0.1 to 3.0 mass%, with Si and Al satisfying the following condition: Si + Al ≤ 1.2 mass%, and the remainder consisting of Fe and unavoidable impurities." This has been disclosed. Patent Document 7 contains: "A ferritic stainless steel characterized by having, in mass percent, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.045% or less, S: 0.0030% or less, Cr: 20.0 to 25.0%, Mo: 0.3 to 2.0%, N: 0.040% or less, Al: 0.50% or less, V: 0.20% or less, containing Nb: 0.001 to 0.500% and / or Ti: 0.001 to 0.50%, with the remainder being Fe and unavoidable impurities." This has been disclosed. JP 7-166301 JP 7-145454 JP 9-157801 JP 10-280103 JP 2003-187828 JP 2005-206884 International Publication 2018 / 008658 However, the metal materials disclosed in Patent Documents 1 and 2 do not exhibit sufficient oxidation resistance, particularly oxidation resistance when exposed to high temperatures for extended periods. Furthermore, the metal material disclosed in Patent Document 5 requires a large amount of Si and / or Al to be included. However, when large amounts of Si and Al are included, insulating oxides are formed on the surface of the metal material. Therefore, if the metal material disclosed in Patent Document 5 is used in an interconnect of a solid oxide fuel cell, the electrical resistance increases and the performance of the battery deteriorates. Furthermore, in order to obtain the above-mentioned characteristics required for the interconnector of a solid oxide fuel cell, the component composition must include Cr, and Cr must be applied to its surface. ２ It is effective to form an oxide film mainly composed of Cr-based oxides such as O3. However, when a large amount of Cr is included in the component composition, there is a problem in that at high temperatures, Cr-based oxides volatilize and adhere to the electrodes, easily causing degradation of the electrode's performance (hereinafter also referred to as "Cr poisoning"). In this regard, Patent Documents 3, 4, and 7 do not take into consideration the suppression of Cr poisoning. Therefore, when the metal materials disclosed in Patent Documents 3, 4, and 7 are used in interconnects for solid oxide fuel cells, there is concern about degradation of electrode performance due to Cr poisoning. Furthermore, in the metallic material disclosed in Patent Document 6, by increasing the amount of Mn in the component composition, Mn is incorporated into the surface oxide. ２ O ３ It forms a structure that improves resistance to chromium toxicity. In this context, the operating environment for interconnectors in solid oxide fuel cells may include high-temperature water vapor. Therefore, it is necessary to enhance oxidation resistance and resistance to chromium toxicity even in environments containing such high-temperature water vapor. However, in Patent Document 6, the evaluation of oxidation resistance and chromium toxicity resistance was conducted in an atmospheric environment, and no consideration was given to oxidation resistance and chromium toxicity resistance in an environment containing high-temperature water vapor. The present invention has been developed in view of the above-mentioned circumstances, and aims to provide a ferritic stainless steel for solid oxide fuel cells that has excellent electrical conductivity, as well as excellent oxidation resistance and resistance to chromium toxicity, particularly in environments containing high-temperature water vapor. Now, the inventors diligently considered various options to achieve the above objective. As a result, in ferritic stainless steel, the component composition can be appropriately controlled, in particular, (1) Contains 0.15 to 1.00% by mass of Nb, (2) Contains 0.0005 to 0.0100% by mass of Mg, (3) Furthermore, the Al content is controlled to be in the range of 0.55 to 2.00 mass%, (4) Furthermore, satisfying the relationship 0.0004 ≤ [Mg] / [Al] ≤ 0.0050, We found that the above objectives are achieved by doing so. Here, [Al] and [Mg] represent the Al content (mass%) and Mg content (mass%) in the component composition, respectively. The inventors believe the reason is as follows: In other words, both Al and Cr are elements that form oxides on the surface of ferritic stainless steel. However, Al ２ O ３ Al-based oxides such as aluminum are insulating. Therefore, when a large amount of aluminum is added to ferritic stainless steel, the oxide film formed on the surface of the ferritic stainless steel (hereinafter also referred to as the surface oxide film) becomes a film mainly composed of Al-based oxides. As a result, electrical resistance increases and electrical conductivity decreases. However, if the surface oxide film is mainly composed of chromium-based oxides, chromium poisoning occurs. In this regard, when Nb: 0.15 to 1.00% by mass and Mg: 0.0005 to 0.0100% by mass are contained, Al in the steel is preferentially oxidized. Further, while containing a predetermined amount of Nb and Mg, by controlling the Al content within the range of 0.55 to 2.00% by mass, it becomes possible to partially contain a Cr-based oxide in the surface oxide film while thinning the surface oxide film mainly composed of an Al-based oxide. Since this surface oxide film is mainly composed of an Al-based oxide, excellent oxidation resistance can be obtained even in a severe environment containing high-temperature steam. Also, in the above surface oxide film, a state is formed in which a Cr-based oxide is partially contained in the Al-based oxide. Therefore, the volatilization of the Cr-based oxide is inhibited by the Al-based oxide in its vicinity. As a result, excellent Cr poisoning resistance can be obtained even in a severe environment containing high-temperature steam. Furthermore, the Cr-based oxide partially generated in the surface oxide film serves as an electric conduction path, ensuring electrical conductivity. For these reasons, the inventors consider that in a ferritic stainless steel in which the component composition is appropriately controlled as in (1) to (3) above and the relationship in (4) above is satisfied, excellent electrical conductivity, excellent oxidation resistance, and excellent Cr poisoning resistance can be achieved simultaneously. The present invention has been completed through further study based on the above findings. That is, the gist configuration of the present invention is as follows. 1. In mass %, C: 0.025% or less, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.050% or less, S: 0.010% or less, Cr: 14.0 to 32.0%, Al: 0.55 to 2.00%, Ni: 0.01 to 1.00%, Nb: 0.15 to 1.00%, Mo: 1.05 to 3.00%, Mg: 0.0005 to 0.0100% and N: 0.025% or less containing, with the balance consisting of Fe and inevitable impurities, having a component composition of a ferrite stainless steel for a solid oxide fuel cell that satisfies the relationship of 0.0004 ≤ [Mg] / [Al] ≤ 0.0050. Here, [Al] and [Mg] are respectively the Al content (mass%) and the Mg content (mass%) in the component composition. 2. The component composition further contains, in mass%, Cu: 0.01 to 0.50%, Co: 0.01 to 1.00% and W: 0.01 to 3.00% The ferrite stainless steel for a solid oxide fuel cell according to 1 above, containing one or more selected from the above. 3. The component composition further contains, in mass%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, Zr: 0.01 to 0.50%, <00001​​​​​​​​​​​​​​​​​​Carbon (C) has the effect of increasing strength at high temperatures. To obtain this effect, it is preferable to have a C content of 0.001% or more. More preferably, the C content is 0.003% or more. However, if the C content exceeds 0.025%, toughness and moldability decrease. Therefore, the C content should be 0.025% or less. Preferably, the C content is 0.015% or less, and more preferably 0.010% or less. Si: 0.05-1.00% Si has the effect of improving oxidation resistance. To obtain this effect, the Si content should be 0.05% or more. Preferably, the Si content is 0.10% or more. However, if the Si content exceeds 1.00%, SiO, which has low electrical conductivity, will form near the interface between the surface oxide film and the base material. ２ This generates a substance that reduces electrical conductivity. Therefore, the Si content should be 1.00% or less. Preferably, the Si content is 0.40% or less, and more preferably 0.20% or less. Mn: 0.05-1.00% Mn has the effect of improving the peel resistance of oxide scale. To obtain this effect, the Mn content should be 0.05% or more. Preferably, the Mn content is 0.10% or more. However, if the Mn content exceeds 1.00%, the oxide scale may grow abnormally, reducing oxidation resistance. Also, the steel hardens at room temperature, reducing workability. Therefore, the Mn content should be 1.00% or less. Preferably, the Mn content is 0.50% or less, more preferably 0.20% or less. P: 0.050% or less P is a harmful element that reduces the toughness of steel. Therefore, it is desirable to reduce the amount of P as much as possible. Accordingly, 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 0.010% or more is preferable. S: 0.010% or less S is a harmful element that not only has an adverse effect on formability but also reduces the corrosion resistance, which is a basic property of stainless steel. Therefore, it is desirable to reduce S as much as possible. Thus, the S content is set to 0.010% or less. The S content is preferably 0.005% or less. The lower limit of the S content is not particularly limited. However, excessive desulfurization leads to an increase in cost, so the S content is preferably 0.0001% or more. Cr: 14.0 - 32.0% Cr partially forms Cr ２ O ３ films in the surface oxide film mainly composed of Al oxide, having the effect of ensuring electrical conductivity. To obtain such an effect, the Cr content is set to 14.0% or more. The Cr content is preferably 16.0% or more, more preferably 18.5% or more. However, when Cr is contained excessively, especially when the Cr content exceeds 32.0%, the volatilization of Cr-based oxides is promoted. Also, deterioration of workability is caused. Therefore, the Cr content is set to 32.0% or less. The Cr content is preferably 24.0% or less, more preferably 22.0% or less, still more preferably 20.5% or less. Al: 0.55 - 2.00% Al, by simultaneously containing Nb and Mg, preferentially forms oxides over Cr, having the effect of improving oxidation resistance. Also, by preferentially forming oxides by Al, Cr poisoning of the electrode due to volatilization of Cr-based oxides can be suppressed. To obtain such an effect, the Al content is set to 0.55% or more. The Al content is preferably 0.85% or more, more preferably 1.00% or more. On the other hand, when the Al content exceeds 2.00%, the steel hardens and workability deteriorates. Also, Cr-based oxides cannot be partially present in the surface oxide film. Furthermore, the thickness of the surface oxide film mainly composed of Al-based oxides increases. As a result, the electrical resistance increases. Therefore, the Al content is set to 2.00% or less. The Al content is preferably 1.60% or less, more preferably 1.25% or less. Ni: 0.01 - 1.00% Ni has the effect of improving the toughness and oxidation resistance of steel. To obtain such effects, the Ni content should be 0.01% or more. Preferably, the Ni content is greater 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, and the oxidation resistance decreases. Furthermore, the decrease in oxidation resistance also increases electrical resistance. For this reason, the Ni content should be 1.00% or less. Preferably, the Ni content is less than 0.50%, more preferably less than 0.20%. Nb: 0.15-1.00% Nb has the effect of increasing strength at high temperatures. Furthermore, Nb promotes the oxidation of Al, improving oxidation resistance, and suppresses the volatilization of Cr oxide. Therefore, Nb is an important element. To obtain these effects, the Nb content should be 0.15% or more. Preferably, the Nb content is 0.25% or more, more preferably 0.30% or more. However, if the Nb content exceeds 1.00%, the steel hardens, reducing its workability. Therefore, the Nb content should be 1.00% or less. Preferably, the Nb content is 0.60% or less, more preferably 0.50% or less, and even more preferably 0.40% or less. Mo: 1.05-3.00% Mo has the effect of increasing strength at high temperatures and improving oxidation resistance. To obtain these effects, the Mo content should be 1.05% or more. Preferably, the Mo content is 1.50% or more, more preferably 1.80% or more, and even more preferably 2.00% or more. On the other hand, if the Mo content is excessive, especially if it exceeds 3.00%, the steel hardens and its workability decreases. Therefore, the Mo content should be 3.00% or less. Preferably, the Mo content is 2.80% or less, more preferably 2.40% or less, and even more preferably 2.30% or less. Mg: 0.0005-0.0100% Mg is an essential element for preferentially oxidizing Al in steel to form a surface oxide film mainly composed of Al-based oxides. Furthermore, Mg is essential for partially incorporating Cr-based oxides into the surface oxide film. In other words, Mg is an essential element for achieving the effect of simultaneously possessing excellent electrical conductivity, excellent oxidation resistance, and resistance to Cr toxicity. To obtain these effects, the Mg content should be 0.0005% or more. Preferably, the Mg content is 0.0010% or more, more preferably 0.0015% or more. On the other hand, if Mg is present in excess, especially if the Mg content exceeds 0.0100%, the thickness of the surface oxide film mainly composed of Al-based oxides increases, and the electrical resistance increases. Therefore, the Mg content should be 0.0100% or less. Preferably, the Mg content is 0.0050% or less, more preferably 0.0025% or less. N: 0.025% or less Nitrogen (N) is an element that reduces the toughness and formability of steel, and it is preferable to reduce it as much as possible. In particular, if the N content exceeds 0.025%, it may lead to a significant decrease in toughness and formability. Therefore, the N content should be 0.025% or less. Preferably, the N content is 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 0.001% or more is preferable. The basic component composition of ferritic stainless steel for solid oxide fuel cells according to one embodiment of the present invention has been described above. Furthermore, it is important to satisfy the relationship 0.0004 ≤ [Mg] / [Al] ≤ 0.0050. Here, [Al] and [Mg] represent the Al content (mass%) and Mg content (mass%) in the component composition, respectively. 0.0004≦[Mg] / [Al]≦0.0050 When the ratio of Mg content to Al content in the component composition, [Mg] / [Al], becomes small, especially below 0.0004, the amount of Cr-based oxides contained in the surface oxide film increases. As a result, Cr poisoning occurs. On the other hand, when [Mg] / [Al] exceeds 0.0050, the thickness of the surface oxide film mainly composed of Al-based oxides increases. As a result, electrical resistance increases. Therefore, the condition 0.0004 ≤ [Mg] / [Al] ≤ 0.0050 is satisfied. [Mg] / [Al] is preferably 0.0010 or higher. Also, [Mg] / [Al] is preferably 0.0030 or lower. Furthermore, the component composition of the ferritic stainless steel for solid oxide fuel cells according to one embodiment of the present invention may further contain one or both of the following (a) and (b). (a) One or more elements selected from Cu: 0.01-0.50%, Co: 0.01-1.00%, and W: 0.01-3.00%. (b) One or more selected from Ti: 0.01–0.50%, V: 0.01–0.50%, Zr: 0.01–0.50%, B: 0.0002–0.0050%, Ca: 0.0002–0.0050%, and REM: 0.01–0.20%. Cu: 0.01~0.50% Cu has the effect of improving the corrosion resistance of steel. To obtain this effect, it is preferable that the Cu content be 0.01% or more. More preferably, the Cu content is 0.05% or more, and even more preferably 0.10% or more. On the other hand, if the Cu content exceeds 0.50%, the oxide scale becomes more prone to peeling, leading to a decrease in oxidation resistance. Therefore, when Cu is included, the Cu content should be 0.50% or less. More preferably, the Cu content is 0.30% or less, and more preferably 0.10% or less. Co:0.01~1.00% Co has the effect of improving the toughness of steel. To obtain this effect, the Co content is preferably 0.01% or more. More preferably, the Co content is 0.03% or more, and even more preferably 0.05% or more. On the other hand, if the Co content exceeds 1.00%, it leads to a decrease in the toughness and workability of the steel. Therefore, when Co is included, the Co content should be 1.00% or less. More preferably, the Co content is less than 0.30%, and more preferably 0.10% or less. W: 0.01~3.00% Like Mo, W has the effect of improving strength at high temperatures through solid solution strengthening. To obtain this effect, the W content is preferably 0.01% or more. More preferably, the W content is 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 hardens and its workability decreases. Also, a strong scale is formed during the annealing process in manufacturing, making descaling during pickling difficult. Therefore, when W is included, the W content should be 3.00% or less. More preferably, the W content is 2.00% or less, and more preferably 1.50% or less. Ti: 0.01~0.50% Ti has the effect of improving the workability and oxidation resistance of steel. To obtain such effects, the Ti content is preferably 0.01% or more. More preferably, the Ti content is 0.03% or more, and even more preferably 0.05% or more. However, if the Ti content exceeds 0.50%, it leads to the precipitation of coarse Ti(C,N), which not only reduces toughness but also deteriorates the surface properties. Therefore, when Ti is included, the Ti content should be 0.50% or less. More preferably, the Ti content is 0.35% or less, and more preferably 0.20% or less. V:0.01~0.50% V has the effect of improving the workability and oxidation resistance of steel. To obtain such effects, the V content is preferably 0.01% or more. More preferably, the V content is 0.03% or more, and even more preferably 0.05% or more. However, if the V content exceeds 0.50%, it leads to the precipitation of coarse V(C,N), which not only reduces toughness but also deteriorates the surface properties. Therefore, when V is included, the V content should be 0.50% or less. More preferably, the V content is 0.30% or less, and more preferably 0.15% or less. Zr: 0.01-0.50% Zr has the effect of improving oxidation resistance. To obtain this effect, it is preferable that the Zr content be 0.01% or more. More preferably, the Zr content is 0.05% or more. However, if the Zr content exceeds 0.50%, Zr intermetallic compounds precipitate, causing the steel to become brittle. Therefore, when Zr is included, the Zr content should be 0.50% or less. The Zr content is preferably 0.25% or less, and more preferably 0.10% or less. B: 0.0002-0.0050% B has the effect of improving the workability of steel, particularly secondary workability. To obtain this effect, the B content is preferably 0.0002% or more. More preferably, the B content is 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, BN is generated, and the workability decreases. Therefore, when B is included, the B content should be 0.0050% or less. The B content is preferably 0.0020% or less, and more preferably 0.0010% or less. Ca: 0.0002-0.0050% Ca has the effect of preventing nozzle blockage due to inclusion precipitation, which is likely to occur during continuous casting. To obtain this effect, the Ca content is preferably 0.0002% or more. More preferably, the Ca content is 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, surface defects are more likely to occur. Therefore, when Ca is included, the Ca content should be 0.0050% or less. The Ca content is preferably 0.0030% or less, and more preferably 0.0020% or less. REM: 0.01~0.20% 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 oxide films and improving oxidation resistance. To obtain such effects, the REM content is preferably 0.01% or more. The REM content is 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 included, the REM content should be 0.20% or less. The REM content is preferably 0.10% or less. The remainder of the components other than those listed above consists of Fe and unavoidable impurities. Furthermore, the microstructure of the ferritic stainless steel for solid oxide fuel cells according to one embodiment of the present invention is a single-phase ferrite structure (volume fraction: 100%). Here, the identification of the structure (calculation of the volume fraction of the ferrite phase) is performed as follows. Specifically, a test specimen for cross-sectional observation is prepared from ferritic stainless steel for solid oxide fuel cells and etched with a picric acid-saturated hydrochloric acid solution. Then, the test specimen for cross-sectional observation is observed with an optical microscope at 100x magnification for 10 fields of view, and the ferrite phase is identified from the microstructure and etching intensity. Next, the volume fraction of the ferrite phase is determined by image processing, and its average value is calculated. Note that the volume fraction of the ferrite phase is calculated excluding intermetallic compounds, precipitates, and inclusions. The shape of the ferritic stainless steel for solid oxide fuel cells according to one embodiment of the present invention may be, for example, a plate (steel sheet) or a component shape. Furthermore, the thickness of the ferritic stainless steel sheet in the form of a plate (steel sheet) or component (hereinafter also simply referred to as the thickness of the steel sheet) is not particularly limited. However, from the viewpoint of oxidation resistance, resistance to chromium toxicity, and workability, the thickness of the steel sheet is preferably 0.01 to 10.0 mm. The thickness of the steel sheet is preferably 0.03 mm or more. Also, the thickness of the steel sheet is preferably 8.0 mm or less, and more preferably 2.0 mm or less. Next, a preferred method for manufacturing ferritic stainless steel for solid oxide fuel cells 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 electric furnace. Then, the molten steel is subjected to secondary refining by ladle refining or vacuum refining to adjust it to the above-mentioned component composition. Next, the molten steel is formed into steel billets (slabs) by continuous casting or ingot-part rolling. Continuous casting is preferred in terms of productivity and quality. Next, the steel billets (slabs) are subjected to hot rolling to produce hot-rolled steel sheets. The hot-rolled steel sheets may be further subjected to hot-rolled annealing and / or pickling. Hereinafter, when simply referring to hot-rolled steel sheets, it shall include not only as-hot-rolled steel sheets (including steel sheets obtained by pickling hot-rolled steel sheets, etc.) but also so-called hot-rolled annealed sheets (including steel sheets obtained by hot-rolled annealing hot-rolled steel sheets, and steel sheets obtained by further pickling hot-rolled annealed steel sheets). Depending on the application, it may be possible to use hot-rolled steel sheets as products (hereinafter also referred to as hot-rolled products) without cold rolling or other processes. For example, when manufacturing the casing of a solid oxide fuel cell, hot-rolled steel sheets can be used as is. Next, the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. Then, the cold-rolled steel sheet is subjected to various processes such as finish annealing (cold-rolled sheet annealing) and pickling to produce products such as cold-rolled annealed sheets (hereinafter also referred to as cold-rolled products). Cold-rolled annealed sheets include steel sheets obtained by performing finish annealing (cold-rolled sheet annealing) on ​​the as-cold-rolled steel sheet, and steel sheets obtained by further pickling on the steel sheet obtained by performing finish annealing (cold-rolled sheet annealing). The atmosphere for cold-rolled sheet annealing is not particularly limited, and BA (bright) annealing may be performed in a reducing atmosphere such as hydrogen, and pickling may be omitted. Before pickling, scale may be removed by shot blasting or mechanical descaling. As described above, ferritic stainless steel for solid oxide fuel cells according to one embodiment of the present invention can be manufactured. Here, the conditions for each of the above processes should follow standard practices. For example, when heating steel billets (slabs) before hot rolling, it is preferable to heat them to a temperature of 1050 to 1250°C. Hot-rolled sheet annealing is preferably carried out by continuous annealing in a temperature range of 900 to 1150°C. Cold rolling may be performed once or two or more times with an intermediate annealing in between. From the viewpoint of productivity and required quality, two or more cold rolling with an intermediate annealing in between is preferable. Furthermore, the total reduction ratio of cold rolling is preferably 50% or more, and more preferably 60% or more. Finish annealing (cold-rolled sheet annealing) is preferably carried out by continuous annealing in the temperature range of 900 to 1150°C. A more preferable temperature range is 950 to 1100°C. Furthermore, depending on the application, the shape, surface roughness, and material properties of the steel sheet may be adjusted by skin pass rolling or other processes after finish annealing. The hot-rolled and cold-rolled products obtained as described above are then subjected to processing such as cutting, bending, stretching, and drawing, depending on the application, to form solid electrolyte fuel cell-related components, such as interconnects, cell frames, cell seals, end plates, and reformers. Furthermore, in forming these components, various welding methods can be applied, such as arc welding (MIG, MAG, TIG) and Tungsten Inert Gas, resistance welding (spot welding and seam welding), high-frequency resistance welding (electric resistance welded welding), and high-frequency induction welding. Steel having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a vacuum melting furnace and cast into a 30 kg steel ingot. This steel ingot was heated to 1250°C and then hot-rolled to a sheet bar with a thickness of 30 mm. This sheet bar was heated to 1150°C and then hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was hot-rolled annealed at a temperature of 800 to 1100°C, and then ground to obtain a hot-rolled annealed sheet with a thickness of 4.0 mm. Next, this hot-rolled annealed sheet was cold-rolled to obtain a cold-rolled steel sheet. Next, this cold-rolled steel sheet was finished annealed at a temperature of 750 to 1100°C to obtain a cold-rolled annealed sheet with a thickness of 1.0 mm. Next, the front and back surfaces of the cold-rolled annealed sheet were polished with #800 emery paper, and the oxidation resistance, chromium toxicity resistance, and electrical conductivity of the polished cold-rolled annealed sheet were evaluated according to the following procedure. The evaluation results are shown in Table 2. Furthermore, when the microstructure of each cold-rolled and annealed sheet was identified using the method described above, it was found that all of them had a single-phase ferrite structure (volume fraction of ferrite phase: 100%). <Evaluation of oxidation resistance> A 1.0 mm x 20 mm x 20 mm sample was cut from the cold-rolled and annealed sheet mentioned above. This sample was degreased with acetone and then subjected to a high-temperature steam environment, specifically, an atmosphere of 15 vol. %H. ２ An oxidation test was conducted by holding the samples in a heating furnace with oxygen (O) and air at a temperature of 850°C for 100 hours. The oxidation weight gain of the samples before and after the test was measured, and the oxidation resistance was evaluated according to the following criteria. ◎ (Pass, particularly excellent): Oxidation increase of 0.05 mg / cm³ ２ below ○ (Pass, Excellent): Oxidation increase of 0.05 mg / cm³ ２ Super 0.10mg / cm ２ below × (Failure): Oxidation increase of 0.10 mg / cm² ２ super <Cr toxicity resistance> From the cold-rolled and annealed sheets described above, samples of the same shape as those used in the oxidation resistance evaluation were prepared. The prepared samples were loaded into a quartz sample holder inside 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 collect Cr evaporating from the sample. Then, 15 vol. %H was added to the tubular furnace. ２ The temperature inside the tubular furnace was maintained at 850°C for 100 hours while circulating oxygen-rich air. After maintenance, the total amount of 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 by 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. Then, 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. Finally, the Cr toxicity tolerance was evaluated according to the following criteria. ○ (Pass): Cr evaporation rate is 1.0 mg / cm² ２ below × (Failure): Cr evaporation rate is 1.0 mg / cm ２ super <Evaluation of electrical conductivity> After the oxidation resistance evaluation described above, a 5 mm x 5 mm Pt paste was applied to both the front and back surfaces of the sample used in the oxidation resistance evaluation, and the sample was dried by holding it at 825°C for 30 min. The heating rate before holding and the cooling rate after holding were both set to 4°C / min. Next, a Pt mesh (10 mm x 10 mm) with Pt wires for current application and Pt wires for voltage measurement joined by spot welding was placed on the areas on both the front and back surfaces of the sample where the Pt paste had been applied. Finally, this sample was subjected to a voltage test of 0.1 kgf / cm². ２ The object was held in a heating furnace heated to 850°C for 30 minutes under the applied load. The heating rate before holding was 4°C / min. During this holding at 850°C, the current density was 0.5 A / cm². ２Current was passed through the samples in such a manner, and the voltage was measured to determine the electrical resistance (area resistivity). Then, the electrical resistance was measured for each sample with n=3, and the average value of these measurements was used to evaluate the electrical conductivity according to the following criteria. ◎ (Pass, particularly excellent): Average electrical resistance value is 0.1 Ω·cm 2 below ○ (Pass, Excellent): Average electrical resistance value is 0.1 Ω·cm 2 Super 0.2Ω・cm 2 below × (Fail): The average value of the electrical resistance is 0.2 Ω·cm 2 super As shown in Table 2, all of the inventive examples exhibited excellent electrical conductivity, as well as excellent oxidation resistance and resistance to chromium toxicity in environments containing high-temperature water vapor. On the other hand, in the comparative example, steel No. B1, sufficient oxidation resistance and resistance to Cr toxicity were not obtained because the Mn content exceeded the appropriate range. In steel No. B2, sufficient oxidation resistance and electrical conductivity could not be obtained because the Cr content was below the appropriate range. For steels No. B4 and B12, sufficient electrical conductivity could not be obtained because the Al content exceeded the appropriate range. In the case of steel No. B5, sufficient oxidation resistance and resistance to Cr toxicity could not be obtained because the Mo content was not within the appropriate range. For steels No. B6 and B13, sufficient oxidation resistance and resistance to Cr toxicity were not obtained because the Nb content did not fall within the appropriate range. For steel No. B7, the Mg content was below the appropriate range, and the [Mg] / [Al] ratio was also below the appropriate range, resulting in insufficient resistance to chromium toxicity. For steel No. B8, sufficient resistance to chromium toxicity was not achieved because the [Mg] / [Al] ratio did not meet the appropriate range. For steel No. B9, sufficient electrical conductivity could not be obtained because the [Mg] / [Al] ratio exceeded the limit. In the case of steel No. B10, sufficient oxidation resistance and resistance to chromium toxicity could not be obtained because the Al content was not within the appropriate range. In the case of steel No. B11, sufficient resistance to chromium toxicity was not obtained because the Mg content did not fall within the appropriate range. The ferritic stainless steel for solid oxide fuel cells of the present invention can be used in solid oxide fuel cells, particularly in their interconnectors, cell frames, cell seals, end plates, and surrounding components such as heat exchangers and reformers. Furthermore, the ferritic stainless steel for solid oxide fuel cells of the present invention can be suitably used in other fuel cells, automotive materials, and materials for boilers and gas turbines where material degradation due to the volatilization of chromium oxides is a problem. 1: Electrolyte 2: Electrodes (anode, air electrode) 3: Electrode (cathode, fuel electrode) 4: Interconnector (separator) 5: Fuel gas (hydrogen gas) 6. Oxidizing gas (air) 7: Gas flow path (groove)

Claims

DEPCT661. Ferritic stainless steel for solid oxide fuel cells, which contains the following chemical elements, in percentage by mass: C: 0.025% or less, Si: 0.05% to 1.00%, Mn: 0.05% to 1.00%, P: 0.050% or less, S: 0.010% or less, Cr: 14.0% to 32.0%, Al: 0.55% to 2.00%, Ni: 0.01% to 1.00%, Nb: 0.15% to 1.00%, Mo: 1.05% to 3.00%, Mg: 0.0005% to 0.0100%, and N: 0.025% or less, with amounts that achieve a balance of Fe and unavoidable impurities, which corresponds to the relationship 0.0004 less than or equal to [Mg] / [Al] less than or equal to 0.0050, where [Al] and [Mg] are the percentage by mass of Al and the percentage by mass of Mg in the chemical composition, respectively.2.Ferritic stainless steel for solid oxide fuel cells under claim 1, where the chemical composition is additional in percentage by mass with at least one selected element from the group comprising Cu: 0.01% to 0.50%, Co: 0.01% to 1.00%, and W: 0.01% to 3.00%.

3. Ferritic stainless steel for solid oxide fuel cells under claim 1 or 2, where the chemical composition is additional in percentage by mass with at least one selected element from the group comprising Ti: 0.01% to 0.50%, V: 0.01% to 0.50%, Zr: 0.01% to 0.50%, B: 0.0002% to 0.0050%, Ca: 0.0002% to 0.0050%, and REM: 0.01% to 0.20%.