Stainless steel for solid oxide type water electrolysis

Abstract

A solid oxide type stainless steel for water electrolysis, comprising, by weight, less than 0.030% C, 1.6-3.5% Si, 0.10-1.00% Mn, less than 0.050% P, less than 0.0030% S, 16.0-21.0% Cr, less than 1.00% Al, less than 1.00% N, less than 0.030% Nb, less than 1.00% Ti, less than 1.00% Ni, less than 1.00% Cu, with the balance being Fe and impurities.

Description

Technical Field

[0001] This invention relates to stainless steel materials for solid oxide-type water electrolysis. Background Technology

[0002] In recent years, the development and adoption of hydrogen production technologies based on CO2-free water electrolysis have been accelerating. Among them, water electrolysis technology using solid oxide electrolysis cells (SOEC) (hereinafter referred to as "solid oxide water electrolysis") has attracted attention due to its highest hydrogen production efficiency (e.g., patent document 1).

[0003] Solid oxide-based water electrolysis promotes water electrolysis by exposing water vapor to a high temperature (700-800°C), thereby generating hydrogen and oxygen. Therefore, components such as the oxygen electrode (anode) are exposed to a high-temperature environment containing high concentrations of oxygen. Stainless steel has been studied as a material for this component; however, in a high-temperature environment containing high concentrations of oxygen, the oxide coating on stainless steel becomes thicker, which easily reduces its conductivity.

[0004] On the other hand, as a technology similar to solid oxide type water electrolysis, solid oxide fuel cells (SOFCs) are known to generate electricity using reformed hydrogen and air from sources such as city gas. The components of solid oxide fuel cells use stainless steel, which has excellent electrical conductivity at temperatures below 600°C (e.g., Patent Document 2).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2022-119078

[0008] Patent Document 2: Japanese Patent Application Publication No. 2022-136912 Summary of the Invention

[0009] The problem the invention aims to solve

[0010] As mentioned above, the stainless steel used in solid oxide type water electrolysis is exposed to a high-temperature environment containing high concentrations of oxygen, etc. Therefore, it is necessary to suppress oxidation in such an environment and reduce the thickness of the oxide coating to ensure conductivity.

[0011] However, the stainless steel used in solid oxide fuel cells does not take into account its use in environments such as those described above (especially high-concentration oxygen atmospheres). Therefore, when used in solid oxide water electrolysis, the oxide coating sometimes becomes thick and the conductivity decreases.

[0012] This invention was made to solve the aforementioned problems unique to stainless steel materials used in solid oxide water electrolysis. Its purpose is to provide a stainless steel material for solid oxide water electrolysis that can suppress oxidation in high-temperature environments containing high concentrations of oxygen, reduce the thickness of the oxide coating, and ensure conductivity.

[0013] Solution for solving the problem

[0014] In order to solve the above problems, the inventors conducted in-depth research and found that by controlling the composition of stainless steel, the above problems can be solved, thus completing the present invention.

[0015] That is, the present invention is a solid oxide type stainless steel for water electrolysis, which, by mass, contains C: less than 0.030%, Si: 1.6~3.5%, Mn: 0.10~1.00%, P: less than 0.050%, S: less than 0.0030%, Cr: 16.0~21.0%, Al: less than 1.00%, N: less than 0.030%, Nb: less than 1.00%, Ti: less than 1.00%, Ni: less than 1.00%, Cu: less than 1.00%, with the balance being Fe and impurities.

[0016] The effects of the invention

[0017] According to the present invention, a solid oxide type stainless steel for water electrolysis can be provided, which can suppress oxidation in a high-temperature environment containing high concentrations of oxygen, reduce the thickness of the oxide coating, and ensure conductivity. Attached Figure Description

[0018] Figure 1 This is a top view of a test piece used to determine conductivity. Detailed Implementation

[0019] In high-temperature environments containing high concentrations of oxygen, oxide coatings such as Cr2O3, which exhibit excellent oxidation resistance, are formed on the surface of stainless steel. However, since the Cr2O3 coating is a p-type semiconductor, it tends to thicken, leading to a decrease in conductivity. While Al2O3 and SiO2 coatings also possess excellent oxidation resistance, they suffer from low conductivity. Therefore, to ensure conductivity in high-temperature environments containing high concentrations of oxygen, it is desirable to form an oxide coating primarily composed of a thin Cr2O3 coating. Adding Si is effective in forming such an oxide coating, but if a continuous SiO2 coating is formed directly beneath the Cr2O3 coating, conductivity will decrease.

[0020] Therefore, the composition of stainless steel was studied, and the results showed that by appropriately controlling the content of Si, Cr and Mn, the formation of continuous SiO2 coating can be suppressed in a high-temperature environment containing high concentrations of oxygen, and an oxide coating mainly composed of a thin Cr2O3 coating can be formed.

[0021] This invention is based on these ideas.

[0022] The embodiments of the present invention will be described in detail below. It should be understood that the present invention is not limited to the following embodiments. Without departing from the spirit of the present invention, appropriate changes or improvements to the following embodiments based on the common knowledge of those skilled in the art also fall within the scope of the present invention.

[0023] Unless otherwise specified, the "%" in this specification refers to "mass%" when referring to ingredients.

[0024] In this specification, unless otherwise specified, the numerical range represented by "~" refers to the range including the values ​​before and after "~" as both the lower and upper limits. Furthermore, in this specification, numerical ranges containing "more than" or "less than" refer to ranges excluding values ​​used as lower or upper limits.

[0025] Furthermore, regarding the numerical ranges described in segments in this specification, the upper limit of a certain numerical range can be replaced with the upper limit of other numerical ranges or the value shown in the embodiments. Additionally, the lower limit of a certain numerical range described in segments in this specification can be replaced with the lower limit of other numerical ranges or the value shown in the embodiments. Moreover, a numerical range can also be defined as a combination of any upper and lower limits described in this specification.

[0026] The solid oxide type stainless steel for water electrolysis (hereinafter referred to as "stainless steel") according to embodiments of the present invention has the following composition: containing C: less than 0.030%, Si: 1.6~3.5%, Mn: 0.10~1.00%, P: less than 0.050%, S: less than 0.0030%, Cr: 16.0~21.0%, Al: less than 1.00%, N: less than 0.030%, Nb: less than 1.00%, Ti: less than 1.00%, Ni: less than 1.00%, Cu: less than 1.00%, with the balance being Fe and impurities.

[0027] In this specification, "impurity" refers to components that are mixed in during the industrial manufacturing of ferritic stainless steel due to various reasons such as raw materials (ore, waste, etc.) and manufacturing processes, and are permissible within the scope that do not adversely affect the present invention. For example, impurities also include unavoidable impurities.

[0028] In addition, in the present specification, "stainless steel material" refers to a material formed of stainless steel, and the shape of the material is not particularly limited. Examples of material shapes include plate shape (including strip shape), rod shape, tubular shape, etc. In addition, the material may also be various shaped steels having a cross-sectional shape such as T-shape, I-shape, or the like.

[0029] It should be noted that, regarding the content of each element, the expression "xx% or less" means an amount that is not more than xx% but exceeds 0% (particularly exceeds the impurity level).

[0030] In addition, the stainless steel material according to the embodiment of the present invention may further optionally contain one or more selected from the group consisting of Mo: 0.50% or less, B: 0.0050% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, V: 0.50% or less, Co: 0.50% or less, W: 0.50% or less, Sn: 0.50% or less, Zr: 0.50% or less, Ga: 0.05% or less, Hf: 0.10% or less, and REM: 0.10% or less.

[0031] Hereinafter, each component will be described in detail.

[0032] <C: 0.030% or less>

[0033] C is an element that affects the electrical conductivity of stainless steel materials in high-temperature environments (particularly 700 to 800°C). If the C content is too high, the electrical conductivity will decrease. Therefore, the C content is set to 0.030% or less, preferably 0.029% or less, and more preferably 0.028% or less. On the other hand, there is no particular limitation on the lower limit of the C content. However, the lower the C content is reduced, the more time the refining process takes, which may increase the manufacturing cost. Therefore, the C content is preferably 0.001% or more, and more preferably 0.002% or more.

[0034] <Si: 1.6~3.5%>

[0035] Si is an element that improves the heat resistance of stainless steel materials and is effective for forming an oxide film. If the Si content is too low, oxidation in a high-temperature environment containing high concentration of oxygen cannot be suppressed, and the oxide film will become thick. Therefore, the Si content is set to 1.6% or more, preferably 1.7% or more, and more preferably 1.8% or more. On the other hand, if the Si content is too high, in a high-temperature environment containing high concentration of oxygen, a continuous SiO₂ film is easily formed directly below the Cr₂O₃ film, thereby reducing electrical conductivity. From the viewpoint of suppressing the formation of the continuous SiO₂ film, the Si content is set to 3.5% or less, preferably 3.4% or less, and more preferably 3.3% or less.

[0036] <Mn: 0.10~1.00%>

[0037] In a high-temperature environment containing high-concentration oxygen and the like, Mn itself forms a spinel-based oxide to improve electrical conductivity, and inhibits the formation of a continuous coating film of SiO₂ by forming a composite oxide with Si (such as MnSi₂O₅). From the viewpoint of ensuring this effect, the Mn content is set to 0.10% or more, preferably 0.13% or more, more preferably 0.15% or more. On the other hand, if the Mn content is too high, a large amount of (FeMnCr)₃O₄ will be generated, the oxide coating will become thicker, resulting in a decrease in electrical conductivity. In addition, heat resistance may also decrease. Therefore, the Mn content is set to 1.00% or less, preferably 0.95% or less, more preferably 0.90% or less.

[0038] <P: 0.050% or less>

[0039] P is an element that may reduce the toughness of stainless steel. Therefore, the P content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, the lower limit of the P content is not particularly limited, but the lower the P content is, the more time the refining process takes, which may increase the manufacturing cost. Therefore, the P content is preferably 0.001% or more, more preferably 0.010% or more.

[0040] <S: 0.0030% or less>

[0041] S is an element that forms sulfide-based inclusions and may degrade the performance of SOEC due to evaporation / poisoning to components such as electrodes. Therefore, the S content is set to 0.0030% or less, preferably 0.0028% or less, more preferably 0.0025% or less. On the other hand, the lower limit of the S content is not particularly limited, but the lower the S content is, the more time the refining process takes, which may increase the manufacturing cost. Therefore, the S content is preferably 0.0001% or more, more preferably 0.0002% or more.

[0042] <Cr: 16.0~21.0%>

[0043] Cr is the main element for forming an oxide coating mainly composed of a Cr₂O₃ coating in a high-temperature environment containing high-concentration oxygen and the like. If the Cr content is too low, a large amount of (FeMnCr)₃O₄ will be generated, the oxide coating will become thicker, resulting in a decrease in electrical conductivity. Therefore, the Cr content is set to 16.0% or more, preferably 16.5% or more, more preferably 17.0% or more. On the other hand, if the Cr content is too high, the Cr₂O₃ coating constituting the oxide coating becomes thicker, resulting in a decrease in electrical conductivity. Therefore, the Cr content is set to 21.0% or less, preferably 20.5% or less, more preferably 20.0% or less.

[0044] <Al: 1.00% or less>

[0045] Al is an element effective for promoting the formation of an oxide film and improving electrical conductivity. However, if the Al content is excessively high, a continuous oxide layer of Al₂O₃ is formed, which reduces electrical conductivity. In addition, AlN, which serves as an initiation site for abnormal oxidation, is easily formed, and may impair the toughness of the stainless steel. Therefore, the Al content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less. On the other hand, the lower limit of the Al content is not particularly limited. From the viewpoint of obtaining the aforementioned effects provided by Al, the Al content is preferably 0.01% or more, and more preferably 0.02% or more.

[0046] <N: 0.030% or less>

[0047] N is an element that combines with Al to form AlN, which serves as an initiation site for abnormal oxidation, and may reduce the toughness of the stainless steel. Therefore, the N content is set to 0.030% or less, preferably 0.028% or less, and more preferably 0.025% or less. On the other hand, the lower limit of the N content is not particularly limited. However, the lower the N content, the longer the refining process takes, which may increase manufacturing costs. Therefore, the N content is preferably 0.001% or more, and more preferably 0.005% or more.

[0048] <Nb: 1.00% or less>

[0049] Nb preferentially combines with C and N to form Nb carbonitrides, promotes the formation of an oxide film, and contributes to the improvement of electrical conductivity. However, if the Nb content is excessively high, the amount of solute Nb that is not consumed in the formation of Nb carbonitrides increases. As a result, toughness may be impaired due to hardening. Therefore, the Nb content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less. On the other hand, the lower limit of the Nb content is not particularly limited. From the viewpoint of obtaining the aforementioned effects provided by Nb, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.

[0050] <Ti: 1.00% or less>

[0051] Similar to Nb, Ti preferentially combines with C and N to form Ti carbonitrides, promotes the formation of an oxide film, and contributes to the improvement of electrical conductivity. However, if the Ti content is excessively high, Ti carbonitrides coarsen, which act as an initiation site to reduce toughness. Therefore, the Ti content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less. On the other hand, the lower limit of the Ti content is not particularly limited. From the viewpoint of obtaining the aforementioned effects provided by Ti, the Ti content is preferably 0.01% or more, and more preferably 0.02% or more.

[0052] <Ni: 1.00% or less>

[0053] Ni is an element that improves the corrosion resistance of stainless steel and inhibits the reduction of toughness. However, Ni is an austenite phase stabilizing element, so if the Ni content is excessively high, the coefficient of thermal expansion will increase, and properties such as thermal shock resistance will decrease. Therefore, the Ni content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less. On the other hand, the lower limit of the Ni content is not particularly limited. From the perspective of obtaining the above effects brought by Ni, the Ni content is preferably 0.01% or more, and more preferably 0.02% or more.

[0054] <Cu: 1.00% or less>

[0055] Cu is an element that improves the corrosion resistance and electrical conductivity of stainless steel. However, Cu is an austenite phase stabilizing element, so if the Cu content is excessively high, the coefficient of thermal expansion will increase, and properties such as thermal shock resistance will decrease. Therefore, the Cu content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less. On the other hand, the lower limit of the Cu content is not particularly limited. From the perspective of obtaining the above effects brought by Cu, the Cu content is preferably 0.01% or more, and more preferably 0.02% or more.

[0056] <Mo: 0.50% or less>

[0057] Mo is a main element for strengthening the oxide film of stainless steel, and is contained in the stainless steel as required. In particular, Mo can improve properties such as corrosion resistance and heat resistance by means of the oxide film. In addition, Mo can also promote the formation of the oxide film on stainless steel to improve electrical conductivity. Generally, the Cr oxide constituting the oxide film contains Fe, so the electrical conductivity is low. However, by allowing Mo to exist in the Cr oxide, the electrical conductivity can be improved. Nevertheless, if the Mo content is excessively high, properties such as toughness may be impaired due to hardening. Therefore, the Mo content is set to 0.50% or less, preferably 0.45% or less. On the other hand, the lower limit of the Mo content is not particularly limited. From the perspective of obtaining the above effects brought by Mo, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more.

[0058] <B: 0.0050% or less>

[0059] B is an element effective for improving secondary workability by preferentially segregating at grain boundaries to increase grain boundary strength, and is included in stainless steel as required. However, if the B content is excessive, borides (Cr₂B) at grain boundaries are coarsened, which results in reduced Cr poisoning resistance. Therefore, the B content is set to 0.0050% or less, preferably 0.0045% or less. On the other hand, the lower limit of the B content is not particularly limited. From the viewpoint of obtaining the effect brought by B, the B content is preferably 0.0001% or more, more preferably 0.0005% or more.

[0060] <Mg: 0.0100% or less>

[0061] Mg is an element effective for refining stainless steel, and is included in stainless steel as required. However, if the Mg content is excessive, the amount of inclusions formed increases and reduces electrical conductivity. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0090% or less. On the other hand, the lower limit of the Mg content is not particularly limited. From the viewpoint of obtaining the effect brought by Mg, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more.

[0062] <Ca: 0.0100% or less>

[0063] Ca is an element that fixes S to improve oxidation resistance and promote the formation of oxide film, and is included in stainless steel as required. However, if the Ca content is excessive, the amount of inclusions formed increases and reduces electrical conductivity. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0095% or less, more preferably 0.0090% or less. On the other hand, the lower limit of the Ca content is not particularly limited. From the viewpoint of obtaining the effect brought by Ca, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more.

[0064] <V: 0.50% or less>

[0065] V is an element that improves the strength without impairing the toughness of stainless steel, and is included in stainless steel as required. However, if the V content is excessive, composite oxides with low melting points are formed, which significantly reduces oxidation resistance and increases production cost. Therefore, the V content is set to 0.50% or less, preferably 0.45% or less. On the other hand, the lower limit of the V content is not particularly limited. From the viewpoint of obtaining the effect brought by V, the V content is preferably 0.01% or more, more preferably 0.02% or more.

[0066] <Co: 0.50% or less>

[0067] Co is an element that improves the strength of stainless steel without impairing the toughness thereof, and is included in the stainless steel as required. However, if the Co content is excessive, the workability and toughness may be reduced, and the cost will increase. Therefore, the Co content is set to 0.50% or less, preferably 0.45% or less. On the other hand, the lower limit of the Co content is not particularly limited. From the viewpoint of obtaining the effect brought by Co, the Co content is preferably 0.01% or more, more preferably 0.02% or more.

[0068] <W:0.50%以下>

[0069] W is an element that improves the strength of stainless steel without impairing the toughness thereof, and is included in the stainless steel as required. However, if the W content is excessive, the workability and toughness may be reduced, and the cost will increase. Therefore, the W content is set to 0.50% or less, preferably 0.45% or less. On the other hand, the lower limit of the W content is not particularly limited. From the viewpoint of obtaining the effect brought by W, the W content is preferably 0.01% or more, more preferably 0.02% or more.

[0070] <Sn:0.50%以下>

[0071] Sn is an element effective for improving corrosion resistance and electrical conductivity, and is included in the stainless steel as required. However, if the Sn content is excessive, hot workability and toughness will be reduced. Therefore, the Sn content is set to 0.50% or less, preferably 0.40% or less. On the other hand, the lower limit of the Sn content is not particularly limited. From the viewpoint of obtaining the effect brought by Sn, the Sn content is preferably 0.01% or more, more preferably 0.02% or more.

[0072] <Zr:0.50%以下>

[0073] Zr is an element that fixes C and increases the effective Cr content of the stainless steel, and is included in the stainless steel as required. However, if the Zr content is excessive, the workability of the stainless steel will be reduced. Therefore, the Zr content is set to 0.50% or less, preferably 0.45% or less. On the other hand, the lower limit of the Zr content is not particularly limited. From the viewpoint of obtaining the effect brought by Zr, the Zr content is preferably 0.01% or more, more preferably 0.02% or more.

[0074] <Ga:0.05%以下>

[0075] Ga is an element that improves the hot workability of stainless steel, and is included in stainless steel as required. However, if the Ga content is excessively high, it will reduce manufacturability. Therefore, the Ga content is set to 0.05% or less, preferably 0.04% or less. On the other hand, the lower limit of the Ga content is not particularly limited. From the viewpoint of obtaining the effect brought by Ga, the Ga content is preferably 0.01% or more.

[0076] <Hf: 0.10% or less>

[0077] Hf is an element that fixes C and increases the effective Cr content in stainless steel, and is included in stainless steel as required. However, if the Hf content is excessively high, the workability of stainless steel will decrease. Therefore, the Hf content is set to 0.10% or less, preferably 0.09% or less. On the other hand, the lower limit of the Hf content is not particularly limited. From the viewpoint of obtaining the effect brought by Hf, the Hf content is preferably 0.01% or more, more preferably 0.02% or more.

[0078] <REM: 0.10% or less>

[0079] REM (rare earth elements) preferentially combine with S and P to form compounds, so it can inhibit the reduction of conductivity caused by S and P. REM is included in stainless steel as required. However, if the REM content is excessively high, stainless steel may be hardened, and toughness and workability may decrease. Therefore, the REM content is set to 0.10% or less, preferably 0.09% or less. On the other hand, the lower limit of the REM content is not particularly limited. From the viewpoint of obtaining the effect brought by REM, the REM content is preferably 0.01% or more, more preferably 0.02% or more.

[0080] It should be noted that REM is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements from lanthanum (La) to lutetium (Lu) (lanthanide elements). They may be used alone or in a mixture. Furthermore, among REM, La and Y are preferred.

[0081] The stainless steel according to the embodiment of the present invention may have a passivation coating on the surface thereof.

[0082] It should be noted that this passivation coating is different from the oxide coating formed in a solid oxide water electrolysis environment described later, and refers to a coating formed in a normal atmospheric atmosphere.

[0083] The passivation coating preferably measures the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti, and Al from the surface to the depth direction using glow discharge emission spectroscopy (hereinafter referred to as "GDS"). When converted with the total amount of elements other than O as 100% by mass, the maximum Si concentration is 25-40% by mass. If the maximum Si concentration is within this range, the formation of a continuous SiO2 coating is suppressed under the condition of forming an oxide coating in a solid oxide-type water electrolysis environment, and an oxide coating mainly composed of a thin Cr2O3 coating is easily formed.

[0084] Here, the passivation coating is determined in GDS as follows. In the depth-direction Cr concentration distribution based on GDS, the average Cr concentration is calculated using the maximum Cr concentration and the Cr concentration value at a depth of 1 μm. The initial position in the depth direction smaller than this average value is taken as the boundary between the passivation coating and the base material. Therefore, the region from the surface to the initial position in the depth direction where the Cr concentration is smaller than the average value is taken as the passivation coating, and the region deeper than the initial position in the depth direction where the Cr concentration is smaller than the average value is taken as the base material. It should be noted that in the Si concentration measurement in GDS, peaks caused by contamination are easily generated in the region from the surface to a depth of 0.5 nm; therefore, data from the position from the surface to a depth of 0.5 nm are excluded.

[0085] When the stainless steel of the present invention is oxidized in a solid oxide-type water electrolysis environment, the surface resistivity at 700°C is preferably 100 mΩ / cm. 2 The following is more preferably 50 mΩ / cm 2 The following applies. If the surface resistance is within this range, then it can be said that the conductivity is excellent under solid oxide-type water electrolysis conditions.

[0086] Here, the surface resistance can be measured by the method described in the embodiments below.

[0087] In addition, in this specification, "solid oxide type water electrolysis environment" refers to environmental conditions with an oxygen concentration of 50% or more by volume and a temperature of 700~800℃.

[0088] In the embodiments of the present invention, the preferred stainless steel material is one where, for the oxide coating formed on the surface during oxidation in a solid oxide-type water electrolysis environment, the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti, and Al along the depth direction from the surface are measured by glow discharge emission spectroscopy analysis. When the total amount of elements other than O is taken as 100% by mass, the maximum Si concentration is 10% by mass or less, the maximum Mn concentration is 5% by mass or more, and the thickness is 3.0 μm or less. If the oxide coating has such a range of maximum Si concentration, maximum Mn concentration, and thickness, the formation of a continuous SiO2 coating can be suppressed, and an oxide coating mainly composed of a thin Cr2O3 coating can be formed. From the viewpoint of forming an oxide coating with better electrical conductivity, it is preferable that the maximum Si concentration is 5% by mass or less, the maximum Mn concentration is 15% by mass or more, and the thickness is 2.0 μm or less. It should be noted that the lower limit of the maximum Si concentration is not particularly limited, for example, it can be 1% by mass. Furthermore, there is no particular upper limit to the maximum value of Mn concentration, for example, 50% by mass. Similarly, there is no particular lower limit to the thickness, for example, 0.3 μm.

[0089] Here, the oxide coating is determined in the GDS as follows. In the depth-direction Cr concentration distribution based on the GDS, the average Cr concentration is calculated using the maximum Cr concentration and the Cr concentration value at a depth of 10 μm. The initial position in the depth direction where the Cr concentration is smaller than this average is taken as the boundary between the oxide coating and the base material. Therefore, the region from the surface to the initial position in the depth direction where the Cr concentration is smaller than the average is taken as the oxide coating, and the region deeper than the initial position in the depth direction where the Cr concentration is smaller than the average is taken as the base material.

[0090] The stainless steel material of the embodiments of the present invention may have an oxide coating on its surface. This oxide coating is formed in a solid oxide-type water electrolysis environment.

[0091] The surface resistivity of stainless steel with an oxide coating at 700°C is preferably 100 mΩ / cm. 2 The following is more preferably 50 mΩ / cm 2 the following.

[0092] Furthermore, for stainless steel with an oxide coating on its surface, when the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti, and Al in the depth direction from the surface are measured by glow discharge emission spectroscopy analysis, and the total amount of elements other than O is converted to 100% by mass, it is preferable that the maximum Si concentration of the oxide coating is 10% by mass or less, the maximum Mn concentration is 5% by mass or more, and the thickness is 3.0 μm or less. More preferably, the maximum Si concentration of the oxide coating is 5% by mass or less, the maximum Mn concentration is 15% by mass or more, and the thickness is 2.0 μm or less.

[0093] The shape of the stainless steel material in the embodiments of the present invention is not particularly limited, but it is preferably in the form of a plate or foil. When the stainless steel material is in the form of a plate or foil, its thickness is, for example, 0.1 to 5.0 mm, preferably 0.1 to 3.0 mm, more preferably 0.1 to 1.0 mm, and even more preferably 0.1 to 0.5 mm.

[0094] The stainless steel material of the embodiments of the present invention can be manufactured according to known methods, except that a slab having the above-described composition is used.

[0095] Here, an example of a typical manufacturing method for stainless steel according to an embodiment of the present invention will be described. It should be noted that the manufacturing method for stainless steel according to an embodiment of the present invention is not limited to the method described below.

[0096] The stainless steel material of the present invention can be manufactured by hot rolling a slab having the above composition, followed by cold rolling and annealing.

[0097] There are no particular limitations on the conditions for hot rolling and cold rolling, and they can be carried out according to known methods. In addition, there are no particular limitations on the annealing conditions, but it is preferred to carry them at a temperature of 900~1100°C.

[0098] After annealing, descaling is preferably performed by pickling and / or grinding, more preferably by pickling. Pickling is more effective at improving conductivity than grinding. Pickling can be performed, for example, by using a mixture of nitric acid (10-30%) and hydrofluoric acid (1-10%) at 30-60°C. Grinding can be performed using abrasive materials with a grit size of #100-1000.

[0099] It should be noted that annealing and pickling can also be performed after hot rolling.

[0100] Furthermore, stainless steel with an oxide coating on its surface can be manufactured by using it in a solid oxide-type water electrolysis environment. Alternatively, stainless steel with an oxide coating on its surface can be manufactured by heat treatment under the same conditions as the solid oxide-type water electrolysis environment. Specifically, stainless steel with an oxide coating on its surface can be manufactured by heating it to 700-800°C in an atmosphere containing more than 50% by volume of oxygen. By heat treatment under such conditions, an oxide coating with the aforementioned properties can be stably formed.

[0101] The composition of the stainless steel (particularly the content of Si, Cr, and Mn) in the embodiments of the present invention is appropriately controlled, thus suppressing the formation of a continuous SiO2 coating in a high-temperature environment containing high concentrations of oxygen, and forming an oxide coating mainly composed of a thin Cr2O3 coating. Therefore, the stainless steel of the embodiments of the present invention is suitable for use in solid oxide type water electrolysis exposed to a high-temperature environment containing high concentrations of oxygen.

[0102] In the case of using the stainless steel material in the embodiments of the present invention for solid oxide type water electrolysis, it can be used for various components such as separators, interconnects, current collectors, and metal supports.

[0103] Example

[0104] The following examples illustrate the content of the present invention in detail, but the present invention is not limited to these examples.

[0105] The slabs with the composition shown in Table 1 (balance: Fe and impurities) were melted, hot-rolled to a thickness of 3.5 mm, and then annealed and pickled. Next, the hot-rolled sheet was cold-rolled to a thickness of 0.3 mm and annealed at 980°C. Then, the annealed sheet was pickled with a mixture of 15% nitric acid and 3% hydrofluoric acid at 50°C, or ground with a grinding material of grit #600, to remove oxide scale and obtain the cold-rolled annealed sheet (stainless steel sheet). Table 2 shows the post-annealing treatments, denoted as "pickling" for pickling and "grinding" for grinding.

[0106] [Table 1]

[0107]

[0108] Next, the obtained cold-rolled annealed sheet is evaluated as follows.

[0109] (Maximum Si concentration in passivation coating)

[0110] Glow discharge emission spectroscopy (GDS) analysis was performed on cold-rolled annealed steel sheets to determine the concentration variations of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti, and Al along the depth direction from the surface. From the elemental concentration distribution along the depth direction of the passivation coating obtained through this analysis, the maximum Si concentration (sometimes expressed as "Si(Max)") was calculated when the total amount of all elements except O was taken as 100% by mass. Data from the surface to a depth of 0.5 nm were excluded in this determination due to the significant influence of contamination.

[0111] GDS was performed under the conditions of Ar gas pressure 600Pa, power 35W, frequency 100Hz, and duty cycle 0.25.

[0112] Next, test pieces were cut from the cold-rolled annealed sheet and subjected to oxidation treatment at 800°C for 200 hours in a gaseous atmosphere containing 60% by volume O2 and 40% by volume N2, thereby forming an oxide coating on the surface.

[0113] The following evaluation was performed on the test pieces after oxidation treatment.

[0114] (Oxidation resistance)

[0115] After calculating the increase in mass [mg] of the oxidized test piece relative to the unoxidized test piece, divide the increase in mass by the surface area [cm²] of the oxidized test piece. 2 From this, the oxidation increment can be calculated.

[0116] In this evaluation, the oxidation increment was set at 0.5 mg / cm³. 2 The following is designated as A (excellent oxidation resistance), where the oxidation increment exceeds 0.5 mg / cm³. 2 And it is 1.0 mg / cm 2 The following cases are designated as B (good oxidation resistance), where the oxidation increment exceeds 1.0 mg / cm³. 2 The condition is represented as C (insufficient oxidation resistance).

[0117] (Maximum Si concentration, maximum Mn concentration, and thickness of the oxide coating)

[0118] Glow discharge emission spectroscopy (GDS) was performed on the oxidized test pieces to determine the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti, and Al along the depth direction from the surface. From the elemental concentration distribution along the depth direction of the oxide coating obtained through this analysis, the maximum Si concentration (sometimes expressed as "Si(Max)") and the maximum Mn concentration (sometimes expressed as "Mn(Max)") were determined when the total amount of all elements except O was taken as 100% by mass. Furthermore, the thickness of the oxide coating was calculated based on the elemental concentration distribution along this depth direction as described above.

[0119] It should be noted that the conditions for GDS are the same as those described above.

[0120] (Surface resistivity)

[0121] Using two oxidized test pieces, prepare as follows: Figure 1 The test piece shown was measured using the four-terminal method of a potentiostat. Specifically, the procedure is as follows.

[0122] First, a conductive paste (Ag paste) is applied in a square shape (10 mm on one side, 10 μm thick) to the center of two test pieces 10 and dried to form a conductive portion 20. Next, the conductive portions 20 of the two test pieces 10 are overlapped and arranged in a cross shape, clamped with an alumina plate, and a weight (200 g) is placed on top. The conductive portion 20 is then sintered in an electric furnace. Next, the surface is cut using a precision cutter until the metal substrate is exposed, forming a wiring mounting portion 30. Next, a silver wire 40 (φ0.3 mm) is wound around the wiring mounting portion 30 and coated with conductive paste to obtain a test piece for measurement. Next, this test piece is placed in a high-temperature electrochemical measurement apparatus, and the voltage-current curve is obtained using the four-terminal method of a potentiostat. In this measurement, the measurement temperature is set to 700 °C, and the voltage is scanned to 10 mV. Furthermore, the resistance value is calculated from the slope of the voltage-current curve.

[0123] In this evaluation, if the resistance value is 50 mΩ / cm 2 The following can be considered as having excellent conductivity; if the resistance value exceeds 50 mΩ / cm 2 And it is 100mΩ / cm 2 The following can be considered as good conductivity; if the resistance value exceeds 100 mΩ / cm 2 If so, it can be determined that the conductivity is insufficient.

[0124] The evaluation results are shown in Table 2.

[0125] [Table 2]

[0126]

[0127] As shown in Table 2, the cold-rolled annealed sheets (stainless steel sheets) of Examples 1 to 14 have good oxidation resistance and low surface resistance and good electrical conductivity due to their specified composition.

[0128] In contrast, for the cold-rolled annealed sheet of Comparative Example 1, the oxide coating is thicker due to the insufficient Si content. As a result, the surface resistivity is high, and the conductivity becomes poor.

[0129] For the cold-rolled annealed sheet of Comparative Example 2, due to the excessive Si content, a continuous SiO2 coating forms directly beneath the Cr2O3 coating. As a result, the surface resistivity is high, and the conductivity becomes poor.

[0130] For the cold-rolled annealed sheet of Comparative Example 3, due to the insufficient Mn content, a composite oxide with Si cannot be fully formed, resulting in a continuous SiO2 coating. Consequently, the surface resistivity is high, and the conductivity becomes poor.

[0131] For the cold-rolled annealed sheet of Comparative Example 4, due to the excessive Mn content, a large amount of (FeMnCr)3O4 is generated, resulting in a thicker oxide coating. As a result, the surface resistivity is high, and the conductivity becomes poor.

[0132] For the cold-rolled annealed sheet of Comparative Example 5, due to the insufficient Cr content, a large amount of (FeMnCr)3O4 is generated, resulting in a thicker oxide coating. As a result, the surface resistivity is high, and the conductivity becomes poor.

[0133] For the cold-rolled annealed sheet of Comparative Example 6, due to the excessive Cr content, the Cr2O3 coating constituting the oxide coating became thicker. As a result, the surface resistivity was high, and the conductivity became poor.

[0134] For the cold-rolled annealed sheet of Comparative Example 7, since it does not contain Ti, the fixation of C and N achieved by the formation of carbonitrides is insufficient, resulting in a thicker oxide coating. As a result, the surface resistivity is high, and the conductivity becomes poor.

[0135] For the cold-rolled annealed sheet of Comparative Example 8, since it does not contain Ni, its corrosion resistance is reduced and the oxide coating becomes thicker. As a result, its surface resistivity is high and its electrical conductivity becomes poor.

[0136] For the cold-rolled annealed sheet of Comparative Example 9, since it does not contain Cu, its corrosion resistance is reduced and the oxide coating becomes thicker. As a result, its surface resistivity is high and its electrical conductivity becomes poor.

[0137] As can be seen from the above results, according to the present invention, a solid oxide type stainless steel for water electrolysis can be provided that can suppress oxidation in high-temperature environments containing high concentrations of oxygen, reduce the thickness of the oxide coating, and ensure conductivity.

[0138] Therefore, embodiments of the present invention can be configured as follows.

[0139] <1> A solid oxide type stainless steel for water electrolysis, comprising, by weight, less than 0.030% C, 1.6-3.5% Si, 0.10-1.00% Mn, less than 0.050% P, less than 0.0030% S, 16.0-21.0% Cr, less than 1.00% Al, less than 1.00% N, less than 0.030% Nb, less than 1.00% Ti, less than 1.00% Ni, less than 1.00% Cu, with the balance being Fe and impurities.

[0140] <2> according to <1> The solid oxide type stainless steel for water electrolysis further contains, by weight, one or more of the following: Mo: less than 0.50%, B: less than 0.0050%, Mg: less than 0.0100%, Ca: less than 0.0100%, V: less than 0.50%, Co: less than 0.50%, W: less than 0.50%, Sn: less than 0.50%, Zr: less than 0.50%, Ga: less than 0.05%, Hf: less than 0.10%, and REM: less than 0.10%.

[0141] <3> according to <1> or <2> The aforementioned solid oxide type stainless steel for water electrolysis has a passivation coating on its surface.

[0142] For the aforementioned passivation coating, the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti and Al from the surface to the depth direction were measured by glow discharge emission spectroscopy. When the total amount of the elements other than O is converted to 100% by mass, the maximum value of Si concentration is 25~40% by mass.

[0143] <4> according to <1> ~ <3> The stainless steel for solid oxide-type water electrolysis as described in any one of the above examples, wherein, under oxidation conditions in a solid oxide-type water electrolysis environment, the surface resistivity at 700°C is 100 mΩ / cm. 2 the following.

[0144] <5> according to <1> ~ <4> As described in any one of the solid oxide type water electrolysis stainless steel materials, for the oxide coating formed on the surface during oxidation in a solid oxide type water electrolysis environment, when the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti and Al in the depth direction from the surface are measured by glow discharge emission spectroscopy analysis, and when converted with the total amount of the elements other than O as 100% by mass, the maximum value of Si concentration is less than 10% by mass, the maximum value of Mn concentration is more than 5% by mass, and the thickness is less than 3.0 μm.

[0145] <6> according to <1> ~ <5> The solid oxide type stainless steel for water electrolysis described in any one of the above descriptions has an oxide coating on its surface.

[0146] <7> according to <6> The aforementioned solid oxide type stainless steel for water electrolysis has a surface resistivity of 100 mΩ / cm at 700℃. 2 the following.

[0147] <8> according to <6> or <7> The aforementioned solid oxide type stainless steel for water electrolysis, wherein, for the aforementioned oxide coating, the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti and Al from the surface to the depth direction are measured by glow discharge emission spectroscopy analysis. When converted with the total amount of the elements other than O as 100% by mass, the maximum value of Si concentration is less than 10% by mass, the maximum value of Mn concentration is more than 5% by mass, and the thickness is less than 3.0 μm.

[0148] <9> according to <1> ~ <8> The solid oxide type stainless steel for water electrolysis as described in any one of the following methods is used in one or more components selected from separators, interconnects, current collectors, and metal supports.

[0149] Explanation of reference numerals in the attached figures

[0150] 10 test pieces

[0151] 20 Conductive parts

[0152] 30. Cabling Installation Department

[0153] 40 silver thread.

Claims

1. A solid oxide type stainless steel for water electrolysis, comprising, by weight, less than 0.030% C, 1.6-3.5% Si, 0.10-1.00% Mn, less than 0.050% P, less than 0.0030% S, 16.0-21.0% Cr, less than 1.00% Al, less than 1.00% N, less than 0.030% Nb, less than 1.00% Ti, less than 1.00% Ni, less than 1.00% Cu, with the balance being Fe and impurities.

2. The solid oxide type stainless steel for water electrolysis according to claim 1, further comprising, by weight, one or more of the following: Mo: 0.50% or less, B: 0.0050% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, V: 0.50% or less, Co: 0.50% or less, W: 0.50% or less, Sn: 0.50% or less, Zr: 0.50% or less, Ga: 0.05% or less, Hf: 0.10% or less, and REM: 0.10% or less.

3. The solid oxide type stainless steel for water electrolysis according to claim 1 or 2, wherein it has a passivation coating on its surface. For the passivation coating, the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti and Al from the surface to the depth direction were measured by glow discharge emission spectroscopy analysis. When the total amount of the elements other than O is converted to 100% by mass, the maximum value of Si concentration is 25~40% by mass.

4. The stainless steel material for solid oxide type water electrolysis according to claim 1 or 2, wherein, Under oxidation conditions in a solid oxide-based water electrolysis environment, the surface resistivity at 700℃ is 100 mΩ / cm. 2 the following.

5. The stainless steel material for solid oxide type water electrolysis according to claim 1 or 2, wherein, For the oxide coating formed on the surface during oxidation in a solid oxide-type water electrolysis environment, the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti, and Al from the surface to the depth direction were measured by glow discharge emission spectroscopy. When the total amount of all elements except O was converted to 100% by mass, the maximum Si concentration was less than 10% by mass, the maximum Mn concentration was more than 5% by mass, and the thickness was less than 3.0 μm.

6. The solid oxide type stainless steel for water electrolysis according to claim 1 or 2, wherein the surface has an oxide coating.

7. The solid oxide type stainless steel for water electrolysis according to claim 6, wherein the surface resistivity at 700℃ is 100 mΩ / cm 2 the following.

8. The stainless steel material for solid oxide type water electrolysis according to claim 6, wherein, For the aforementioned oxide coating, the concentration changes of O, Fe, Cr, Si, Mn, Ni, Mo, Nb, Ti, and Al from the surface to the depth direction were determined by glow discharge emission spectroscopy analysis. When the total amount of the elements other than O is converted to 100% by mass, the maximum Si concentration is less than 10% by mass, the maximum Mn concentration is more than 5% by mass, and the thickness is less than 3.0 μm.

9. The solid oxide type stainless steel for water electrolysis according to claim 1 or 2, wherein it is used in one or more components selected from separators, interconnects, current collectors and metal supports.

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