Ferritic stainless steel and steel sheet

Abstract

To provide a ferritic stainless steel and a steel sheet thereof, having excellent oxidation resistance and hot workability.SOLUTION: A ferritic stainless steel contains C≤0.03 mass%, Si≤0.05 mass%, 0.30≤Mn≤1.00 mass%, P≤0.05 mass%, S≤0.05 mass%, Cu≤0.10 mass%, 20.0≤Cr≤25.0 mass%, V≤0.10 mass%, Co≤0.10 mass%, Al≤0.10 mass%, 0.01≤Ti≤0.30 mass%, N≤0.03 mass%, and 0.05≤La≤0.30 mass%. The balance is Fe and unavoidable impurities. The ferritic stainless steel further contains 0.10≤W≤2.00 mass% and / or 0.10≤Mo≤2.00 mass%. The ferritic stainless steel may further contain 0≤Ni≤2.00 mass%. A steel sheet is made of the ferritic stainless steel.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to ferritic stainless steel and steel sheets, and more particularly to ferritic stainless steel suitable as a separator for a solid oxide fuel cell (SOFC) or a solid oxide electrolysis cell (SOEC), and a steel sheet using the same.

Background Art

[0002] Ferritic stainless steel is stainless steel having a ferritic structure at normal temperature. Ferritic stainless steel is excellent in heat resistance, oxidation resistance, and cold workability, and has a feature that its coefficient of thermal expansion is smaller than that of other materials. Therefore, the use of ferritic stainless steel as a separator for a solid oxide fuel cell (SOFC) or a solid oxide electrolysis cell (SOEC) has been studied.

[0003] For example, Patent Document 1 discloses a ferritic stainless steel containing a predetermined amount of Cr, C, Si, Mn, P, S, Al, N, Nb, Ni, Mo, Sn, and B, with the balance being Fe and unavoidable impurities. The same document describes (A) In order to obtain good oxidation resistance, the Al content needs to be 1.00 mass% or more, but when the Al content exceeds 3.00 mass%, the toughness of the steel decreases. (B) In order to improve the oxidation resistance in a reforming environment, the Si content needs to be 0.50 mass% or more. (C) Such ferritic stainless steel has high oxidation resistance and high high-temperature strength, and is therefore suitable as a high-temperature member for SOFC. is described.

[0004] Patent Document 2 discloses a stainless steel sheet for a separator of a fuel cell containing a predetermined amount of C, Si, Mn, P, S, Cr, Ni, Ti, Al, and N, with the balance being Fe and unavoidable impurities. The same document describes (A) When an anodic oxidation treatment is performed on a stainless steel sheet containing a predetermined amount of Cr and Ti, fine precipitates containing Cr and Ti can be precipitated on the surface of the steel sheet, and the contact resistance of the steel sheet can be reduced. Also, (B) Although REM is an element effective for deoxidation, when the REM content exceeds 0.100 mass%, the hot workability deteriorates. is described.

[0005] Patent Document 3 discloses a ferritic stainless steel containing a predetermined amount of Cr, C, Si, Mn, Al, P, S, and N, with the balance being Fe and unavoidable impurities. In the same document, (A) In an Al-containing ferritic stainless steel cold-rolled annealed sheet, when the grain structure is optimized, the creep characteristics are improved. Also, (B) When the Al content is less than 0.5%, it becomes difficult to form an Al-based oxide film, and thus the effect of suppressing Cr evaporation cannot be obtained. is described.

[0006] Patent Document 4 discloses a steel for a solid oxide fuel cell separator containing a predetermined amount of C, Si, Mn, Ni, Cr, Cu, and Al, further containing one or more of Y, REM, and Zr, with the balance being Fe and unavoidable impurities. In the same document, (A) Si has the effect of improving the oxidation resistance of the separator, and the preferable lower limit of the Si content is 0.05 mass%. Also, (B) In order to improve the oxidation resistance of the separator, it is necessary to add a Cu content of 0.3 mass% or more. Also, (C) When a small amount of Y, REM, or Zr is added, the oxidation resistance and the electrical conductivity of the oxide film can be significantly improved. is described.

[0007] The SOFC includes a single cell with an anode (fuel electrode) and a cathode (air electrode) joined to both sides of an electrolyte, and separators disposed on both sides of the single cell. The SOEC has the same structure as the SOFC but causes a reaction opposite to that of the SOFC. Also, the operating temperature of the SOFC and SOEC reaches several hundred degrees Celsius. Therefore, the separators are required to have properties such as oxidation resistance, electronic conductivity, and a low coefficient of thermal expansion. Also, in order to reduce the cost of the separators, the materials for the separators are required to have easy processability.

[0008] Regarding this point, the ferritic stainless steel described in Patent Documents 1 and 3 has a large coefficient of thermal expansion because the Al content is excessive. Also, when used in a high-temperature oxidation atmosphere, an Al-based oxide is formed on the surface, and the electronic conductivity is likely to decrease. Also, the steel for separators described in Patent Document 4 may prevent the formation of a dense Mn-based oxide (oxide having Cr volatility resistance) in the outermost layer of the separator because the Si content is excessive.

[0009] Also, Patent Documents 2 and 4 describe that the addition of REM (so-called "misch metal") is effective in improving oxidation resistance, but Ce contained in REM has a great effect of reducing the hot workability of stainless steel. Also, when excessive N is contained in the stainless steel, REM may combine with N to form precipitates, and the effect of adding REM (for example, improvement of oxidation resistance) may be lost. Furthermore, in Patent Document 4, the upper limit of the C content is 0.2 mass%, but the limitation of the addition amount is loose, and the possibility of sensitization is considered.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0011] The problem to be solved by the present invention is to provide a ferritic stainless steel having a relatively small coefficient of thermal expansion and excellent in electrical conductivity, oxidation resistance, and hot workability, and a steel sheet using the same.

Means for Solving the Problems

[0012] In order to solve the above problems, the ferritic stainless steel according to the present invention is C ≦ 0.03 mass%, Si ≦ 0.05 mass%, 0.30 ≦ Mn ≦ 1.00 mass%, P ≦ 0.05 mass%, S ≦ 0.05 mass%, Cu ≦ 0.10 mass%, 20.0 ≦ Cr ≦ 25.0 mass%, V ≦ 0.10 mass%, Co ≦ 0.10 mass%, Al ≦ 0.10 mass%, 0.01 ≦ Ti ≦ 0.30 mass%, N ≦ 0.03 mass%, and, 0.05 ≦ La ≦ 0.30 mass% and the balance consists of Fe and unavoidable impurities, 0.10 ≦ W ≦ 2.00 mass%, and / or, 0.10 ≦ Mo ≦ 2.00 mass% further contains.

[0013] The ferritic stainless steel according to the present invention may further contain 0 ≦ Ni ≦ 2.00 mass%. The steel sheet according to the present invention is made of the ferritic stainless steel according to the present invention.

Effects of the Invention

[0014] Ce contained in REM (mischmetal) has the effect of improving the oxidation resistance of ferritic stainless steel, but has a great effect of deteriorating the hot workability. On the other hand, La has the effect of improving the oxidation resistance of ferritic stainless steel and has a smaller effect of deteriorating the hot workability compared to Ce. Therefore, when an appropriate amount of La is added to ferritic stainless steel instead of Ce, the oxidation resistance can be improved without significantly deteriorating the hot workability of ferritic stainless steel.

[0015] In ferritic stainless steel, when the Al content is limited, an increase in the coefficient of thermal expansion and a decrease in electronic conductivity due to the formation of an insulating oxide film can be suppressed. In addition, Mn and Ti have the effect of forming Cr-Mn spinel-type oxides or Ti oxides when ferritic stainless steel is exposed to a high-temperature oxidation atmosphere, and suppressing the volatilization of CrO3. Furthermore, both W and Mo dissolve in the matrix phase and have the effect of improving the oxidation resistance of the steel. Therefore, in addition to the above elements, when W and / or Mo is further added, the oxidation resistance of ferritic stainless steel is further improved.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, an embodiment of the present invention will be described in detail. [1. Ferritic Stainless Steel] [1.1. First Main Component] The ferritic stainless steel according to the present invention contains the following elements, and the balance consists of Ni and inevitable impurities. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0017] (1) C ≦ 0.03 mass%: When the C content becomes excessive, Cr carbides are formed, which reduces the Cr concentration in the matrix phase and may reduce the oxidation resistance of the steel. Therefore, the Cr content needs to be 0.03 mass% or less. The C content is preferably 0.02 mass% or less, or 0.01 mass% or less. On the other hand, C is also an element that inevitably mixes in, and excessive reduction will lead to higher costs. Therefore, it is preferable to select an optimal value for the C content considering the manufacturing cost.

[0018] (2) Si ≤ 0.05 mass%: When the Si content becomes excessive, Si-based oxides may be formed on the outermost surface of the steel during oxidation, increasing the contact resistance of the steel. Therefore, the Si content needs to be 0.05 mass% or less. The Si content is preferably 0.03 mass% or less. On the other hand, Si is also an element that inevitably mixes in, and excessive reduction will cause higher costs. Therefore, it is preferable to select an optimal value for the Si content considering the manufacturing cost.

[0019] (3) 0.30 ≤ Mn ≤ 1.00 mass%: When a ferritic stainless steel containing a large amount of Cr is heated to a high temperature, CrO3 is likely to volatilize from the ferritic stainless steel. The air electrode (cathode) of the SOFC is easily poisoned by the volatilized Cr. In contrast, Mn has the effect of forming an oxide (Mn-based spinel oxide) together with Cr during oxidation and suppressing the volatilization of CrO3. To obtain such an effect, the Mn content needs to be 0.30 mass% or more. The Mn content is preferably 0.40 mass% or more. On the other hand, since Mn itself is easily oxidized, when the Mn content becomes excessive, the oxidation resistance of the steel may decrease. Therefore, the Mn content needs to be 1.00 mass% or less. The Mn content is preferably 0.80 mass% or less.

[0020] (4) P ≤ 0.05 mass%: When the P content becomes excessive, the hot workability and cold workability may decrease. Therefore, the P content should be 0.05 mass% or less. Preferably, the P content is 0.03 mass% or less. On the other hand, P is also an element that inevitably mixes in, and excessive reduction will lead to higher costs. Therefore, it is preferable to select an optimal value for the P content considering the manufacturing cost.

[0021] (5) S ≤ 0.05 mass%: When the S content becomes excessive, the hot workability and cold workability may decrease. Therefore, the S content should be 0.05 mass% or less. Preferably, the S content is 0.03 mass% or less. On the other hand, S is also an element that inevitably mixes in, and excessive reduction will lead to higher costs. Therefore, it is preferable to select an optimal value for the S content considering the manufacturing cost.

[0022] (6) Cu ≤ 0.10 mass%: When the Cu content becomes excessive, the hot workability and cold workability may decrease. Therefore, the Cu content should be 0.10 mass% or less. Preferably, the Cu content is 0.05 mass% or less. On the other hand, excessive reduction of the Cu content may lead to higher costs. Therefore, it is preferable to select an optimal value for the Cu content considering the manufacturing cost.

[0023] (7) 20.0 ≤ Cr ≤ 25.0 mass%: Cr has the effect of improving the oxidation resistance of steel. To obtain such an effect, the Cr content should be 20.0 mass% or more. Preferably, the Cr content is 21.0 mass% or more, or 21.5 mass% or more. On the other hand, when the Cr content becomes excessive, a large amount of CrO3 is generated when the steel is heated in a high-temperature oxidation atmosphere, and CrO3 is likely to volatilize from the surface of the steel. Therefore, the Cr content should be 25.0 mass% or less. Preferably, the Cr content is 24.0 mass% or less, or 23.5 mass% or less.

[0024] (8) V ≤ 0.10 mass%: When the amount of V becomes excessive, the hot workability and cold workability may decrease. Therefore, the amount of V needs to be 0.10 mass% or less. The amount of V is preferably 0.09 mass% or less. On the other hand, reducing the amount of V more than necessary may lead to an increase in cost. Therefore, it is preferable to select an optimal value for the amount of V in consideration of the manufacturing cost.

[0025] (9) Co ≤ 0.10 mass%: When the amount of Co becomes excessive, the hot workability and cold workability may decrease. Therefore, the amount of Co needs to be 0.10 mass% or less. The amount of Co is preferably 0.09 mass% or less. On the other hand, reducing the amount of Co more than necessary may lead to an increase in cost. Therefore, it is preferable to select an optimal value for the amount of Co in consideration of the manufacturing cost.

[0026] (10) Al ≤ 0.10 mass%: When the amount of Al becomes excessive, the thermal expansion coefficient of the steel may increase. Therefore, the amount of Al needs to be 0.10 mass% or less. The amount of Al is more preferably 0.05 mass% or less. On the other hand, reducing the amount of Al more than necessary may lead to an increase in cost. Therefore, it is preferable to select an optimal value for the amount of Al in consideration of the manufacturing cost.

[0027] (11) 0.01 ≤ Ti ≤ 0.30 mass%: Ti has the effect of improving the oxidation resistance of steel by forming Ti carbide and / or Ti nitride and suppressing the formation of Cr carbide and / or Cr nitride. Also, Ti has the effect of forming Ti oxide during oxidation and suppressing the volatilization of CrO3. In order to obtain such effects, the amount of Ti needs to be 0.01 mass% or more. The amount of Ti is preferably 0.05 mass% or more. On the other hand, if the Ti content is excessive, hot workability and cold workability may be deteriorated. Therefore, the Ti content must be 0.30 mass% or less. The Ti content is preferably 0.20 mass% or less.

[0028] (12)N≦0.03mass%: If the N content is excessive, Cr nitrides are formed, which reduces the Cr concentration in the matrix and may reduce the oxidation resistance of the steel. Therefore, the N content must be 0.03 mass% or less. The N content is preferably 0.02 mass% or less. On the other hand, N is an element that is inevitably mixed in, and reducing the amount more than necessary will lead to high costs. Therefore, it is preferable to select an optimal value for the N content, taking into consideration production costs.

[0029] (13) 0.05≦La≦0.30mass%: La has the effect of improving the oxidation resistance of steel without significantly deteriorating the hot workability and cold workability of the steel. To obtain such an effect, the La content must be 0.05 mass% or more. The La content is preferably 0.07 mass% or more. On the other hand, if the La content is excessive, hot workability and cold workability may be deteriorated. Therefore, the La content must be 0.30 mass% or less. The La content is preferably 0.20 mass% or less.

[0030] [1.2. Second principal component] The ferritic stainless steel according to the present invention further contains at least one of the following elements in addition to the elements described above. The types of the added elements, their component ranges, and the reasons for their limitations are as follows:

[0031] (14) 0.10≦W≦2.00mass%: W has the effect of improving the oxidation resistance of steel by dissolving in the matrix. When W is contained in steel, in order to obtain such an effect, the W content is preferably 0.10 mass% or more. The W content is more preferably 0.20 mass% or more. On the one hand, when the W content is excessive, the hot workability and cold workability may decrease. Therefore, the W content is preferably 2.00 mass% or less. More preferably, the W content is 1.50 mass% or less. Also, W has the effect of reducing the thermal expansion coefficient of steel. Therefore, it is preferable to adjust the thermal expansion coefficient of steel by adding it within the range of 0.10 to 2.00 mass%.

[0032] (15) 0.10 ≦ Mo ≦ 2.00 mass%: Mo has the effect of dissolving in the matrix phase and improving the oxidation resistance of steel. When Mo is contained in steel, in order to obtain such an effect, the Mo content is preferably 0.10 mass% or more. More preferably, the Mo content is 0.20 mass% or more. On the one hand, when the Mo content is excessive, the hot workability and cold workability may decrease. Therefore, the Mo content is preferably 2.00 mass% or less. More preferably, the Mo content is 1.50 mass% or less. Also, Mo has the effect of reducing the thermal expansion coefficient of steel. Therefore, it is preferable to adjust the thermal expansion coefficient of steel by adding it within the range of 0.10 to 2.00 mass%.

[0033] [1.3. Minor components] The ferritic stainless steel according to the present invention may further contain the following elements in addition to the above-described elements. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0034] (16) 0 ≦ Ni ≦ 2.00 mass%: Ni is an element that has the effect of improving corrosion resistance and can be contained as necessary. In order to obtain such an effect, the Ni content is preferably more than 0.1 mass%. More preferably, the Ni content is 1.0 mass% or more. In addition, Ni has the effect of increasing the thermal expansion coefficient of steel. Therefore, when the amount of Ni becomes excessive, the thermal expansion coefficient of steel may increase. Accordingly, it is preferable to adjust the thermal expansion coefficient of steel by adding Ni in an amount of 2.00 mass% or less. The amount of Ni is preferably 1.70 mass% or less.

[0035] [1.4. Inevitable impurities] Inevitable impurities mean elements mixed in from ores, scraps used as raw materials for steel, or the environment of the manufacturing process, etc. Specifically, in addition to C, Si, P, and S described above, other elements may be further included as inevitable impurities. Specifically, Ni < 0.1 mass% can be mentioned.

[0036] [2. Steel sheet] The steel sheet according to the present invention is made of the ferritic stainless steel according to the present invention.

[0037] [2.1. Ferritic stainless steel] Details of the ferritic stainless steel are as described above, so the description will be omitted.

[0038] [2.2. Thickness] The thickness of the steel sheet according to the present invention is not particularly limited, and an optimal thickness can be selected according to the purpose. For example, when the steel sheet according to the present invention is used for a separator for SOFC or a separator for SOEC, if the thickness becomes too thick, there is a risk that SOFC or SOEC will become large-sized. Accordingly, the thickness is preferably 3.5 mm or less. The thickness is more preferably 3.0 mm or less. On the other hand, if the thickness becomes too thin, the strength and / or oxidation resistance of the steel sheet may decrease. Accordingly, the thickness is preferably 0.1 mm or more. The thickness is more preferably 0.5 mm or more.

[0039] [2.3. Applications] The steel sheet according to the present invention can be used in any application that requires a low coefficient of thermal expansion, heat resistance, electronic conductivity, oxidation resistance, or hot workability. The steel sheet according to the present invention is particularly suitable as a material for a separator for a solid oxide fuel cell (SOFC) or a separator for a solid oxide electrolysis cell (SOEC).

[0040] [3. Manufacturing method of steel sheet] The steel sheet according to the present invention is (a) melting and casting raw materials blended to have predetermined components, (b) performing homogenization heat treatment on the steel ingot as necessary, (c) performing hot working on the steel ingot, (d) performing primary annealing on the hot-worked body as necessary, (e) performing cold working on the hot-worked body as necessary, (f) performing secondary annealing on the hot-worked body or the cold-worked body to manufacture it.

[0041] [3.1. Melting and casting process] First, raw materials blended to have predetermined components are melted and cast. The melting conditions and casting conditions are not particularly limited, and optimal conditions can be selected according to the purpose.

[0042] [3.2. Homogenization heat treatment process] Next, homogenization heat treatment is performed on the steel ingot as necessary. The homogenization heat treatment is performed to homogenize the components of the steel ingot. When the component segregation of the steel ingot does not pose a problem, the homogenization heat treatment can be omitted. The conditions of the homogenization heat treatment are not particularly limited, and optimal conditions can be selected according to the purpose.

[0043] [3.3. Hot working process] Next, hot working is performed on the steel ingot. The "steel ingot" to be hot-worked refers to a steel ingot as it is melted and cast, or a steel ingot that has undergone homogenization heat treatment. Hot working is performed to process the steel ingot into the final product shape or to process the steel ingot into a rough-shaped material suitable for cold working. The conditions of hot working are not particularly limited, and optimal conditions can be selected according to the purpose.

[0044] [3.4. Primary annealing process] Next, if necessary, primary annealing is performed on the hot-worked body. When further cold working is to be performed on the hot-worked body, it is preferable to perform primary annealing on the hot-worked body. Primary annealing is performed to remove the strain contained in the hot-worked body and improve the cold workability. Therefore, when cold working is not performed or when the cold workability of the hot-worked body is high, primary annealing can be omitted. The conditions of primary annealing are not particularly limited, and optimal conditions can be selected according to the purpose.

[0045] [3.5. Cold working process] Next, if necessary, cold working is performed on the hot-worked body. The "hot-worked body" to be cold-worked refers to a hot-worked body on which only hot working has been performed or a hot-worked body on which primary annealing has been performed after hot working. Cold working is particularly performed when high dimensional accuracy is required. Therefore, when high dimensional accuracy is not required for the steel plate, cold working can be omitted. The conditions of cold working are not particularly limited, and optimal conditions can be selected according to the purpose.

[0046] [3.6. Secondary annealing process] Next, secondary annealing is performed on the hot-worked body or the cold-worked body. The "hot-worked body" to be secondary-annealed refers to a hot-worked body on which only hot working has been performed. The secondary annealing is performed to remove the strain contained in the hot-worked body or cold-worked body. The conditions for the secondary annealing are not particularly limited, and the optimum conditions can be selected according to the purpose. The steel sheet thus obtained is used for various applications after additional processing is performed as necessary.

[0047] [4. Action] Ce contained in REM (mischmetal) has the effect of improving the oxidation resistance of ferritic stainless steel, but has a great effect of deteriorating the hot workability. On the other hand, La has the effect of improving the oxidation resistance of ferritic stainless steel and has a smaller effect of deteriorating the hot workability compared to Ce. Therefore, when an appropriate amount of La is added to ferritic stainless steel instead of Ce, the oxidation resistance can be improved without significantly deteriorating the hot workability of ferritic stainless steel.

[0048] In ferritic stainless steel, when the Al content is limited, an increase in the coefficient of thermal expansion and a decrease in electronic conductivity due to the formation of an insulating oxide film can be suppressed. In addition, Mn and Ti have the effect of forming Cr-Mn spinel-type oxides or Ti oxides when ferritic stainless steel is exposed to a high-temperature oxidation atmosphere, and suppressing the volatilization of CrO3. Furthermore, both W and Mo dissolve in the matrix phase and have the effect of improving the oxidation resistance of the steel. Therefore, in addition to the above elements, when W and / or Mo is further added, the oxidation resistance of ferritic stainless steel is further improved.

Example

[0049] (Examples 1 to 16, Comparative Examples 1 to 20) [1. Preparation of Samples] The raw materials formulated to have the compositions shown in Table 1 and Table 2 were melted in a vacuum induction heating furnace to obtain 15 kg ingots. Subsequently, a homogenization heat treatment was performed by heating the ingots at 1200 °C for 4 hours. Thereafter, hot forging was carried out at 1100 °C to form a plate shape with a thickness of 30 mm. Further, hot rolling was carried out at 1100 °C to form a plate shape with a thickness of 10 mm.

[0050] For the obtained plate-shaped hot-formed body, annealing treatment (primary annealing) was performed by holding at 1100 °C for 2 hours and then air-cooling. Subsequently, cold rolling was carried out at room temperature to obtain a cold-formed body with a thickness of 2.5 mm. Further, vacuum annealing treatment (secondary annealing) was performed by heating the obtained cold-formed body at 800 °C for 4 hours.

[0051]

Table 1

[0052]

Table 2

[0053] [2. Test Method] [2.1. Workability] The workability was evaluated based on the degree of cracks generated in the obtained formed body during the above-mentioned hot rolling and cold rolling.

[0054] [2.2. Continuous Oxidation Test] Test pieces for the continuous oxidation test were cut out from the formed body after the vacuum annealing treatment with dimensions of 2.5 mm × 15 mm × 20 mm. An oxidation test was carried out for 100 hours in the atmosphere at 800 °C in accordance with the high-temperature continuous oxidation test method for metallic materials defined in JIS Z2281.

[0055] [2.3. Cr Evaporation Test] The test pieces for the Cr evaporation test were cut from the formed body after vacuum annealing, with dimensions of 2.5 mm × 15 mm × 50 mm. The cut test pieces were placed in a quartz tube and exposed to an atmosphere of 800°C with a dew point of 80°C and humidified for 100 hours. After the test, the Cr oxide adhering to the inside of the quartz tube was dissolved in dilute sulfuric acid, and the amount of Cr was measured by ICP emission spectrometry to calculate the Cr evaporation amount.

[0056] [2.4. Coefficient of Thermal Expansion Measurement] The test pieces for the coefficient of thermal expansion measurement were cut from the formed body after vacuum annealing, with dimensions of 2.5 mm × 6 mm × 19 mm. In accordance with the method for measuring the linear expansion coefficient of metallic materials specified in JIS Z2285, the coefficient of thermal expansion from 28°C to 800°C was measured.

[0057] [2.5. Contact Resistance Measurement] The test pieces for the contact resistance measurement were cut from the formed body after vacuum annealing, with dimensions of 2.5 mm × 15 mm × 20 mm. The cut test pieces were oxidized in air at 800°C for 100 hours. After oxidation, the test pieces were sandwiched and fixed between 10 mm × 10 mm Pt plates with Pt wires joined for current application and voltage measurement, and the contact resistance was measured by the four-terminal method.

[0058] [3. Results] The results are shown in Table 3. From Table 3, the following can be understood.

[0059] Regarding workability, 「◎」 indicates that a cold-formed body without cracks could be produced, 「○」 indicates that although slight cracks occurred during hot rolling, a cold-formed body could be produced without polishing treatment, 「△」 indicates that cracks occurred during hot rolling, so a cold-formed body could not be produced without polishing treatment, but a cold-formed body could be produced by polishing the hot-formed body, 「×」 indicates that it cracked during hot rolling, so it could not be cold-rolled.

[0060] Regarding oxidation resistance, 「◎」 means that the oxidation weight gain is 0.4 mg / cm 2represents the following, "○" represents that the oxidation weight gain is 0.4 mg / cm 2 exceeding 0.6 mg / cm 2 represents the following, "△" represents that the oxidation weight gain is 0.6 mg / cm 2 exceeding 0.8 mg / cm 2 represents the following, "×" represents that the oxidation weight gain is 0.8 mg / cm 2 exceeding.

[0061] Regarding the Cr poisoning resistance, "◎" represents that the Cr evaporation amount is 80 μg / cm 2 represents the following, "○" represents that the Cr evaporation amount is 80 μg / cm 2 exceeding 90 μg / cm 2 represents the following, "△" represents that the Cr evaporation amount is 90 μg / cm 2 exceeding 100 μg / cm 2 represents the following, "×" represents that the Cr evaporation amount is 100 μg / cm 2 exceeding.

[0062] Regarding the thermal expansion coefficient, "◎" represents that the thermal expansion coefficient is 12.25×10 -6 / °C or less, "○" represents that the thermal expansion coefficient is 12.25×10 -6 / °C exceeding 12.30×10 -6 / °C or less, "△" represents that the thermal expansion coefficient is 12.30×10 -6 / °C exceeding 12.35×10 -6 / °C or less, "×" represents that the thermal expansion coefficient is 12.35×10 -6 / °C exceeding.

[0063] Regarding the contact resistance, "◎" represents that the contact resistance is 40 mΩ·cm 2 represents the following, "○" represents that the contact resistance is 40 mΩ·cm 2Exceeding 45 mΩ·cm 2 represents that it is as follows, "△" indicates that the contact resistance is 45 mΩ·cm 2 Exceeding 50 mΩ·cm 2 represents that it is as follows, "×" indicates that the contact resistance is 50 mΩ·cm 2 represents that it exceeds.

[0064] (1) In Comparative Example 1, the oxidation resistance decreased. This is presumably because the excessive C content consumed Cr in the matrix phase for the formation of Cr carbides. (2) In Comparative Example 2, the contact resistance slightly increased. This is presumably because the excessive Si content formed Si oxide on the outermost surface. (3) In Comparative Example 3, the Cr poisoning resistance decreased. This is presumably because the low Mn content made it difficult to form Mn-Cr-based oxides. On the other hand, in Comparative Example 4, the oxidation resistance decreased. This is presumably because of the excessive Mn content.

[0065] (4) In Comparative Example 5, the workability slightly decreased. This is presumably because of the excessive Cu content. (5) In Comparative Example 6, the coefficient of thermal expansion increased. This is presumably because of the excessive Ni content. (6) In Comparative Example 7, the oxidation resistance slightly decreased. This is presumably because of the low Cr content. On the other hand, in Comparative Example 8, the Cr poisoning resistance decreased. This is presumably because of the excessive Cr content.

[0066] (7) In Comparative Example 9, the workability slightly decreased. This is presumably because the excessive V content generated V-based carbonitrides. (8) In Comparative Example 10, the workability slightly decreased. This is presumably because the excessive Co content caused an excessive increase in the hardness of the material. (9) In Comparative Example 11, the coefficient of thermal expansion increased. This is presumably because of the excessive Al content. Also, in Comparative Example 11, the contact resistance slightly increased. This is presumably because Al-based oxide was formed on the outermost surface of the sample.

[0067] (10) In Comparative Example 12, the oxidation resistance was slightly reduced. This is considered to be due to the small amount of Ti. On the other hand, in Comparative Example 13, the workability was slightly reduced. This is considered to be due to the excessive amount of Ti. (11) In Comparative Example 14, the oxidation resistance was slightly reduced. This is considered to be due to the small amount of La. On the other hand, in Comparative Example 15, the workability was slightly reduced. This is considered to be due to the excessive amount of La.

[0068] (12) In Comparative Example 16, the oxidation resistance was slightly reduced. This is considered to be because it contains substantially neither W nor Mo. On the other hand, in Comparative Examples 17 and 18, the workability of both was slightly reduced. This is considered to be due to the excessive amount of W or Mo. (13) In Comparative Example 19, the oxidation resistance was reduced. This is considered to be because the Cr in the matrix phase was consumed for the formation of carbonitrides due to the excessive amount of N. (14) In Comparative Example 20, the workability was slightly reduced. This is considered to be due to the excessive amount of Ce.

[0069] (15) In Examples 1 to 16, all of them had excellent workability, oxidation resistance, and Cr poisoning resistance. Also, in Examples 1 to 16, the coefficient of thermal expansion and the contact resistance were both reduced. This is considered to be because in addition to the optimized content of each component, an appropriate amount of W and / or Mo is contained. (16) In Example 6, although the amount of La was 0.07 mass%, it showed oxidation resistance equivalent to that of Comparative Example 20 (Ce amount = 0.06 mass%) and better workability than Comparative Example 20. Similarly, in Example 1, although the amount of La was 0.12 mass%, it showed oxidation resistance equivalent to that of Comparative Example 20 and better workability than Comparative Example 20.

[0070]

Table 3

[0071] As described above, the embodiments of the present invention have been described in detail. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

Industrial Applicability

[0072] The ferrite-based stainless steel according to the present invention can be used as a separator for a solid oxide fuel cell and a separator for a solid oxide electrolysis cell.

Claims

1. C ≤ 0.03 mass%, Si ≤ 0.05 mass%, 0.30 ≤ Mn ≤ 1.00 mass%, P ≤ 0.05 mass%, S ≤ 0.05 mass%, Cu ≤ 0.10 mass%, 20.0 ≤ Cr ≤ 25.0 mass%, V ≤ 0.10 mass%, Co ≤ 0.10 mass%, Al ≤ 0.10 mass%, 0.01 ≤ Ti ≤ 0.30 mass%, N ≤ 0.03 mass%, and, 0.05 ≤ La ≤ 0.30 mass% comprising, the balance being composed of Fe and inevitable impurities, 0.10 ≤ W ≤ 2.00 mass%, and / or, 0.10 ≤ Mo ≤ 2.00 mass% further comprising a ferritic stainless steel.

2. 0 ≤ Ni ≤ 2.00 mass% The ferritic stainless steel according to claim 1, further comprising.

3. A steel sheet made of the ferritic stainless steel according to claim 1 or 2.

4. The steel sheet according to claim 3, having a thickness of 3.5 mm or less.

5. The steel sheet according to claim 3, used for a separator for a solid oxide fuel cell (SOFC) or a separator for a solid oxide electrolysis cell (SOEC).

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