Stainless steel material for solid oxide type water electrolysis

Abstract

This stainless steel material for solid oxide type water electrolysis contains, on a mass basis, 0.030% or less of C, 0.20% or less of Si, less than 0.30% of Mn, 0.050% or less of P, 0.0030% or less of S, 19.0-24.0% of Cr, 2.5% or less of Mo, 0.01-0.15% of Al, 0.0001-0.0100% of Mg, 0.030% or less of N, 0.40% or less of Nb, 0.40% or less of Ti, 1.00% or less of Ni and 1.00% or less of Cu, with the remainder comprising Fe and impurities. In this stainless steel material for solid oxide type water electrolysis, the average particle diameter of inclusions is 0.2-3.0 μm, the abundance of inclusions is 30-150 inclusions / mm2, the aspect ratio of inclusions is more than 1.0 and less than 3.0, and the proportion among inclusions of Mg-containing inclusions in which the Mg concentration is 0.5 mass% or more is 0.30 or more.

Description

Stainless steel material for solid oxide water electrolysis

[0001] The present invention relates to a stainless steel material for solid oxide water electrolysis.

[0002] In recent years, the development and introduction of CO2-free hydrogen production technologies using water electrolysis have accelerated. Among these technologies, water electrolysis technologies using solid oxide electrolysis cells (SOECs) (hereinafter referred to as "solid oxide water electrolysis") have attracted attention due to their highest hydrogen production efficiency (see, for example, Patent Document 1). Solid oxide water electrolysis generates hydrogen by raising water vapor to a high temperature to promote water electrolysis. Therefore, components used in solid oxide water electrolysis are exposed to a high-temperature environment containing water vapor and hydrogen. Stainless steel has been considered as a material for these components, but in a high-temperature environment containing water vapor and hydrogen, stainless steel is prone to oxidation and thermal deformation due to hydrogen (proton) penetration.

[0003] On the other hand, a solid oxide fuel cell (SOFC) is known as a technology similar to solid oxide water electrolysis, which generates electricity from air and hydrogen reformed from city gas, etc. Stainless steel materials with improved electrical conductivity at high temperatures, resistance to Cr poisoning, and thermal deformation are used as components constituting solid oxide fuel cells (e.g., Patent Documents 2 to 5).

[0004] Japanese Patent Publication No. 2022-119078 Japanese Patent No. 5716054 Japanese Patent No. 6643906 International Publication No. 2022 / 153752 Japanese Patent Publication No. 2022-136911

[0005] As described above, stainless steel materials used in solid oxide water electrolysis are exposed to a high-temperature environment containing water vapor and hydrogen, and are therefore prone to oxidation and thermal deformation associated with hydrogen (proton) penetration into the stainless steel material. On the other hand, stainless steel materials used in solid oxide fuel cells are not exposed to a high-temperature environment containing water vapor and hydrogen, and therefore the above problems have not been particularly addressed. The present invention has been made to solve the above problems specific to stainless steel materials used in solid oxide water electrolysis, and aims to provide a stainless steel material for solid oxide water electrolysis that can suppress oxidation and thermal deformation associated with hydrogen (proton) penetration.

[0006] As a result of intensive research conducted by the inventors to solve the above problems, they discovered that the above problems can be solved by controlling the inclusions in the stainless steel material in addition to the composition of the stainless steel material, and thus completed the present invention. That is, the present invention provides a steel sheet containing, by mass, C: 0.030% or less, Si: 0.20% or less, Mn: less than 0.30%, P: 0.050% or less, S: 0.0030% or less, Cr: 19.0 to 24.0%, Mo: 2.5% or less, Al: 0.01 to 0.15%, Mg: 0.0001 to 0.0100%, N: 0.030% or less, Nb: 0.40% or less, Ti: 0.40% or less, Ni: 1.00% or less, and Cu: 1.00% or less, with the balance being Fe and impurities, the average particle size of the inclusions being 0.2 to 3.0 μm, and the abundance ratio of the inclusions being 30 to 150 pieces / mm 2 the inclusions have an aspect ratio of more than 1.0 and less than 3.0, and the proportion of Mg-containing inclusions having a Mg concentration of 0.5 mass% or more among the inclusions is 0.30 or more.

[0007] According to the present invention, it is possible to provide a stainless steel material for solid oxide water electrolysis that can suppress oxidation and thermal deformation associated with hydrogen (proton) penetration.

[0008] FIG. 2 is a schematic diagram for explaining an oxidation test in the examples.

[0009] Oxidation in a high-temperature environment containing water vapor and hydrogen occurs when hydrogen reacts with the stainless steel material to form protons (H +This is thought to be due to steam oxidation, which occurs when hydrogen diffuses as a proton-trapping site and reacts with oxygen on the surface facing the atmosphere to generate water vapor. Furthermore, thermal deformation in a high-temperature environment containing water vapor and hydrogen is thought to be due to hydrogen dissolved in the stainless steel material reducing the yield stress of the stainless steel material (increasing dislocation mobility). In order to suppress these phenomena, it is effective to trap protons in the stainless steel material with inclusions. It has been found that inclusions containing Mg are particularly effective as proton trapping sites, with carbonitrides being particularly effective. It has also been found that the addition of Mg and control of the heating temperature during hot rolling and the coiling temperature after hot rolling are effective in generating the above-mentioned inclusions. However, the simple addition of Mg may promote the formation of coarse oxides, which may impair hot workability and lead to deterioration of properties due to the formation of hydrogen accumulation sites. The present invention has been completed based on these points.

[0010] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention.

[0011] Herein, the "%" notation for components in this specification means "% by mass" unless otherwise specified. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, unless otherwise specified. In addition, in this specification, when a numerical value is followed by "more than" or "less than," the numerical range means a range that does not include the numerical value as the lower or upper limit. Furthermore, for numerical ranges described in stages in this specification, the upper limit of a certain numerical range may be replaced with the upper limit of another numerical range described in stages or a value shown in an example. Furthermore, for numerical ranges described in stages in this specification, the lower limit of a certain numerical range may be replaced with the lower limit of another numerical range described in stages or a value shown in an example. Furthermore, a numerical range may be a combination of any upper and lower limits described in this specification.

[0012] A stainless steel material for solid oxide water electrolysis (hereinafter abbreviated as "stainless steel material") according to an embodiment of the present invention has a composition containing C: 0.030% or less, Si: 0.20% or less, Mn: less than 0.30%, P: 0.050% or less, S: 0.0030% or less, Cr: 19.0 to 24.0%, Mo: 2.5% or less, Al: 0.01 to 0.15%, Mg: 0.0001 to 0.0100%, N: 0.030% or less, Nb: 0.40% or less, Ti: 0.40% or less, Ni: 1.00% or less, Cu: 1.00% or less, with the balance being Fe and impurities.

[0013] Here, in this specification, "impurities" refer to components that are mixed in during the industrial production of ferritic stainless steel materials due to raw materials such as ores and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities also include unavoidable impurities. Furthermore, in this specification, "stainless steel material" refers to a material formed from stainless steel, and its shape is not particularly limited. Examples of the shape include plate (including strip), rod, and tube. Various cross-sectional shapes, such as T-shaped and I-shaped steel, may also be used. Regarding the content of each element, "xx% or less" means that the content is xx% or less, but includes an amount greater than 0% (particularly, above the impurity level).

[0014] Furthermore, the stainless steel material according to the embodiment of the present invention may further contain, as necessary, one or more selected from B: 0.0050% 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: 0.500% or less, Ga: 0.050% or less, Hf: 0.10% or less, and REM: 0.10% or less. Each component will be described in detail below.

[0015] <C: 0.030% or less> C is an element that affects oxidation and thermal deformation in a high-temperature environment containing water vapor and hydrogen. If the C content is too high, oxidation and thermal deformation due to hydrogen (proton) penetration occur. Therefore, the C content is set to 0.030% or less, preferably 0.020% or less, and more preferably 0.015% or less. On the other hand, the lower limit of the C content is not particularly limited, but the lower the C content, the longer 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.

[0016] <Si: 0.20% or less> Si is useful as a deoxidizer and is an element that improves oxidation resistance, but if the Si content is too high, thermal deformation occurs in a high-temperature environment containing water vapor and hydrogen. Therefore, the Si content is set to 0.20% or less, preferably 0.19% or less, and more preferably 0.18% or less. On the other hand, the lower limit of the Si content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects, it is preferably 0.01% or more, and more preferably 0.02% or more.

[0017] <Mn: Less than 0.30%> Mn is an element that affects oxidation and thermal deformation in a high-temperature environment containing water vapor and hydrogen. If the Mn content is too high, oxidation and thermal deformation associated with hydrogen (proton) penetration occur. Therefore, the Mn content is less than 0.30%, preferably 0.28% or less, and more preferably 0.26% or less. On the other hand, the lower limit of the Mn content is not particularly limited, but is preferably 0.01% or more, and more preferably 0.02% or more.

[0018] <P: 0.050% or less> P is an element that may reduce the toughness of a stainless steel material. Therefore, the P content is set to 0.050% or less, 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, the longer 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.

[0019] <S: 0.0030% or less> S is an element that may reduce the toughness of a stainless steel material. Therefore, the S content is set to 0.0030% or less, preferably 0.0015% or less. On the other hand, the lower limit of the S content is not particularly limited, but the lower the S content, the longer 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.

[0020] <Cr: 19.0 to 24.0%> Cr is a major element for forming a passive film on the surface of stainless steel material, which improves properties such as corrosion resistance and heat resistance. If the Cr content is too low, oxidation and thermal deformation due to hydrogen (proton) penetration occur, so the Cr content is 19.0% or more, preferably 19.2% or more. On the other hand, if the Cr content is too high, toughness and workability decrease, so the Cr content is 24.0% or less, preferably 23.8% or less.

[0021] <Mo: 2.5% or Less> Mo is a major element for strengthening the passive film of stainless steel material, which improves properties such as corrosion resistance and heat resistance through the passive film. Furthermore, Mo is an effective element for controlling the amount of Mg-containing inclusions produced by dissolving in the stainless steel material, thereby increasing the activity of Mg and contributing to the formation of Mg-containing inclusions. Furthermore, Mo is an element that reduces the thermal expansion coefficient and thus inhibits thermal deformation. Therefore, the Mo content is set to 2.5% or less, preferably 2.4% or less, and more preferably 2.3% or less. Meanwhile, the lower limit of the Mo content is not particularly limited. From the viewpoint of obtaining the above effects, the Mo content is preferably 0.1% or more, more preferably more than 0.5%, even more preferably 0.6% or more, and particularly preferably 0.7% or more.

[0022] <Al: 0.01 to 0.15%> Al is an element that affects oxidation and thermal deformation in high-temperature environments containing water vapor and hydrogen. If the Al content is too low, thermal deformation due to hydrogen (proton) penetration is likely to occur. Therefore, the Al content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the Al content is too high, AlN, which serves as the starting point for abnormal oxidation, is likely to be formed, and the toughness of the stainless steel material and the effect of suppressing thermal deformation may be impaired. Therefore, the Al content is set to 0.15% or less, preferably 0.14% or less.

[0023] <Mg: 0.0001 to 0.0100%> Mg is an element necessary for generating Mg-containing inclusions that function as trap sites for hydrogen (protons). However, if the Mg content is too high, the amount of inclusions generated increases, which may lead to oxidation and thermal deformation in high-temperature environments containing water vapor and hydrogen. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less. On the other hand, from the viewpoint of obtaining the above-mentioned effects, the lower limit of the Mg content is set to 0.0001% or more, preferably 0.0002% or more, and more preferably 0.0003% or more.

[0024] <N: 0.030% or less> N is an element necessary for generating carbonitrides that function as trap sites for hydrogen (protons). However, if the N content is too high, it may combine with Al to form AlN, which serves as a starting point for abnormal oxidation, thereby reducing the toughness of the stainless steel material. Therefore, the N content is set to 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less. On the other hand, the lower limit of the N content is not particularly limited, but the lower the N content, the longer the refining process will take, which may increase the manufacturing cost. Therefore, the N content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0025] <Nb: 0.40% or less> Nb is an element that preferentially bonds with C and N to form Nb carbonitrides. However, if the Nb content is too high, the amount of solute Nb that is not consumed in the formation of Nb carbonitrides increases. As a result, there is a risk that workability and the effect of suppressing thermal deformation may be impaired due to hardening. Therefore, the Nb content is set to 0.40% or less, preferably 0.35% or less. On the other hand, the lower limit of the Nb content is not particularly limited. From the viewpoint of obtaining the above effects, the Nb content is preferably 0.01% or more, more preferably 0.05% or more.

[0026] <Ti: 0.40% or less> Like Nb, Ti is an element that preferentially bonds with C and N to form Ti carbonitrides. However, if the Ti content is too high, the Ti carbonitrides become coarse, which acts as a starting point to reduce workability and the effect of suppressing thermal deformation. Therefore, the Ti content is set to 0.40% or less, preferably 0.35% or less. On the other hand, the lower limit of the Ti content is not particularly limited. From the viewpoint of obtaining the above-mentioned effects, the Ti content is preferably 0.01% or more, more preferably 0.05% or more.

[0027] <Ni: 1.00% or less> Ni is an element that improves the corrosion resistance of stainless steel materials and suppresses a decrease in toughness. However, because Ni is an austenite phase stabilizing element, if the Ni content is too high, the thermal expansion coefficient increases and the effect of suppressing thermal deformation decreases. Therefore, the Ni content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less. On the other hand, the lower limit of the Ni content is not particularly limited. From the viewpoint of obtaining the above effects, the Ni content is preferably 0.01% or more, and more preferably 0.05% or more.

[0028] <Cu: 1.00% or less> Cu is an element that improves the corrosion resistance and electrical conductivity of stainless steel materials. However, because Cu is an austenite phase stabilizing element, if the Cu content is too high, the thermal expansion coefficient increases and the effect of suppressing thermal deformation decreases. Therefore, the Cu content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less. On the other hand, the lower limit of the Cu content is not particularly limited. From the viewpoint of obtaining the above effects, the Cu content is preferably 0.01% or more, and more preferably 0.03% or more.

[0029] <B: 0.0050% or less> B is an effective element for increasing grain boundary strength and improving secondary workability by preferentially concentrating at grain boundaries, and is included in stainless steel materials as needed. However, excessive B content causes boride (CrB) at grain boundaries to coarsen, thereby reducing the effect of suppressing thermal deformation. Therefore, the B content is set to 0.0050% or less, preferably 0.0030% or less. On the other hand, the lower limit of the B content is not particularly limited. From the viewpoint of obtaining the above effects, the B content is preferably 0.0001% or more, more preferably 0.0002% or more.

[0030] <Ca: 0.0100% or less> Ca is an element that fixes S to improve oxidation resistance and promotes the formation of an oxide film, and is contained in the stainless steel material as needed. However, if the Ca content is too high, the amount of inclusions formed increases, reducing the thermal deformation suppression property. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, the lower limit of the Ca content is not particularly limited. From the viewpoint of obtaining the effects of Ca, the Ca content is preferably 0.0005% or more, more preferably 0.0010% or more.

[0031] <V: 0.50% or less> V is an element that improves the strength of a stainless steel material without impairing its toughness, and is contained in the stainless steel material as needed. However, if the V content is too high, there is a risk of deterioration in workability and toughness, and costs increase. Therefore, the V content is set to 0.50% or less, preferably 0.40% or less. On the other hand, the lower limit of the V content is not particularly limited. From the viewpoint of obtaining the effects of V, the V content is preferably 0.01% or more, more preferably 0.05% or more.

[0032] <Co: 0.50% or less> Co is an element that improves the strength of a stainless steel material without impairing its toughness, and is contained in the stainless steel material as needed. However, if the Co content is too high, there is a risk of reduced workability and toughness, and costs increase. Therefore, the Co content is set to 0.50% or less, preferably 0.40% or less. On the other hand, the lower limit of the Co content is not particularly limited. From the viewpoint of obtaining the effects of Co, the Co content is preferably 0.01% or more, more preferably 0.05% or more.

[0033] <W: 0.50% or less> W is an element that improves the strength of a stainless steel material without impairing its toughness, and is contained in the stainless steel material as needed. However, if the W content is too high, there is a risk of reduced workability and toughness, and costs will increase. Therefore, the W content is set to 0.50% or less, preferably 0.40% or less. On the other hand, the lower limit of the W content is not particularly limited. From the viewpoint of obtaining the effects of W, the W content is preferably 0.01% or more, more preferably 0.05% or more.

[0034] <Sn: 0.50% or less> Sn is an element effective in improving corrosion resistance and electrical conductivity, and is contained in stainless steel materials as needed. However, if the Sn content is too high, toughness and workability decrease. Therefore, the Sn content is set to 0.50% or less, preferably 0.30% or less. On the other hand, the lower limit of the Sn content is not particularly limited. From the viewpoint of obtaining the effects of Sn, the Sn content is preferably 0.01% or more, more preferably 0.05% or more.

[0035] <Zr: 0.500% or less> Zr is an element that fixes C and increases the effective Cr content of the stainless steel material, and is contained in the stainless steel material as needed. However, if the Zr content is too high, the workability of the stainless steel material will decrease. Therefore, the Zr content is set to 0.500% or less, preferably 0.400% or less. On the other hand, the lower limit of the Zr content is not particularly limited. From the viewpoint of obtaining the effects of Zr, the Zr content is preferably 0.001% or more, more preferably 0.005% or more.

[0036] <Ga: 0.050% or less> Ga is an element that improves the hot workability of a stainless steel material and is contained in the stainless steel material as needed. However, if the Ga content is too high, manufacturability will be reduced. Therefore, the Ga content is set to 0.050% or less, preferably 0.030% or less. On the other hand, the lower limit of the Ga content is not particularly limited. From the viewpoint of obtaining the effects of Ga, the Ga content is preferably 0.001% or more, more preferably 0.002% or more.

[0037] <Hf: 0.10% or less> Hf is an element that fixes C and increases the effective Cr content of the stainless steel material, and is contained in the stainless steel material as needed. However, if the Hf content is too high, the workability of the stainless steel material will decrease. Therefore, the Hf content is set to 0.10% or less, preferably 0.08% or less. On the other hand, the lower limit of the Hf content is not particularly limited. From the viewpoint of obtaining the effects of Hf, the Hf content is preferably 0.001% or more, more preferably 0.005% or more.

[0038] <REM: 0.10% or Less> REM (rare earth elements) preferentially bond with S and P to form compounds, thereby suppressing the reduction in the thermal deformation suppression effect caused by S and P. REM is included in stainless steel materials as needed. However, if the REM content is too high, the stainless steel material may harden, resulting in reduced toughness and workability. Therefore, the REM content is set to 0.10% or less, preferably 0.08% or less. On the other hand, the lower limit of the REM content is not particularly limited. From the viewpoint of obtaining the effects of REM, the REM content is preferably 0.001% or more, more preferably 0.005% or more. REM refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These elements may be used alone or as a mixture. Among REMs, La and Y are preferred.

[0039] The stainless steel material according to the embodiment of the present invention contains inclusions, which in this specification refer to non-metallic inclusions such as oxides and carbonitrides.

[0040] The average particle size of the inclusions is 0.2 to 3.0 μm, preferably 0.5 to 2.8 μm, and more preferably 0.8 to 2.5 μm. The proportion of the inclusions is 30 to 150 pieces / mm 2 , preferably 35 to 120 pieces / mm 2 , more preferably 40 to 100 pieces / mm 2 Furthermore, the aspect ratio of the inclusions is greater than 1.0 and less than 3.0, preferably 1.1 to 2.8, and more preferably 1.2 to 2.5. By controlling the average particle size, abundance, and aspect ratio of the inclusions within the above ranges, oxidation and thermal deformation associated with hydrogen (proton) penetration can be suppressed without deteriorating properties such as workability.

[0041] Here, the average particle size, abundance ratio, and aspect ratio of inclusions are measured in accordance with the microscope test method specified in JIS G0555:2020. Specifically, the measurements are performed as follows. First, a sample is taken by exposing an arbitrary cross section (observation area) parallel to the rolling direction of the stainless steel material. Next, this cross section is mirror-polished, and inclusions present in the polished cross section are identified using an optical microscope. The particle sizes of the identified inclusions are measured, and their average value is taken as the average particle size of the inclusions. Furthermore, the longest and shortest diameters of the inclusions are measured, and the longest diameter relative to the shortest diameter (longest diameter / shortest diameter) is measured, and this average value is taken as the aspect ratio of the inclusions. Furthermore, the total number of inclusions in the observation area is counted, and the value obtained by dividing the total number of inclusions by the area of ​​the observation area is taken as the abundance ratio of the inclusions. The average particle size, abundance ratio, and aspect ratio of the inclusions can be measured using a commercially available automatic inclusion analyzer that complies with the microscope test method specified in JIS G0555:2020.

[0042] The inclusions include Mg-containing inclusions with a Mg concentration of 0.5% or more. Examples of inclusions include, but are not limited to, oxide-based inclusions such as MgO (magnesium oxide) and MgO-Al2O3. The ratio of Mg-containing inclusions to all inclusions (number of Mg-containing inclusions / total number of inclusions) is 0.30 or more, preferably 0.32 or more, and more preferably 0.35 or more. By controlling the ratio of Mg-containing inclusions within this range, sufficient Mg-containing inclusions that function as proton trapping sites can be present in the stainless steel material, thereby suppressing oxidation and thermal deformation in high-temperature environments containing water vapor and hydrogen. The upper limit of the ratio of Mg-containing inclusions is not particularly limited, but is preferably 0.80 or less, more preferably 0.75 or less, and even more preferably 0.70 or less. The Mg-containing inclusions can be identified by elemental analysis of the inclusions. Specifically, elemental analysis of the inclusions may be performed using EDX (energy dispersive X-ray fluorescence spectroscopy), etc. Furthermore, Mg-containing inclusions may be identified using a commercially available automatic inclusion analyzer capable of performing elemental analysis.

[0043] The ratio of carbonitrides in the inclusions (number of carbonitrides / total number of inclusions) is preferably 0.40 to 0.80, more preferably 0.40 to 0.75, and even more preferably 0.40 to 0.70. By controlling the ratio of carbonitrides within this range, the presence of the carbonitrides makes it easier to suppress oxidation and thermal deformation in high-temperature environments containing water vapor and hydrogen. Examples of carbonitrides include chromium carbide, chromium nitride, carbides of Nb or Ti, and composite carbonitrides of these elements with nitrides. The carbonitrides can be identified by elemental analysis of the inclusions as described above. In this elemental analysis, inclusions with an oxygen concentration of 0.5% or less are considered to be carbonitrides.

[0044] The shape of the stainless steel material according to the embodiment of the present invention is not particularly limited, but is preferably 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.

[0045] The method for producing a stainless steel material according to an embodiment of the present invention is not particularly limited as long as it is a method capable of producing a stainless steel material having the above-described characteristics. An example of a typical method for producing a stainless steel material according to an embodiment of the present invention will be described below. However, the method for producing a stainless steel material according to an embodiment of the present invention is not limited to the following method.

[0046] The stainless steel material according to the embodiment of the present invention can be produced by using a slab having the composition described above and controlling the heating temperature during hot rolling to 1150° C. or less and the coiling temperature after hot rolling to 500° C. or less. By setting these conditions, the above-mentioned inclusions can be produced in the stainless steel material.

[0047] A slab having the above-described composition can be produced by melting and casting stainless steel. In this case, increasing the cooling rate during casting can facilitate the formation of the above-described inclusions. Next, the slab is hot-rolled. Hot-rolling may be performed according to a known method, but the heating temperature is set to 1150°C or lower. If the heating temperature exceeds 1150°C, the inclusions tend to coarsen, making it difficult to obtain inclusions with a particular aspect ratio, and the amount of Mg-containing inclusions and carbonitrides produced also decreases. The lower limit of the heating temperature is not particularly limited, but is typically 1000°C or higher.

[0048] The hot-rolled material obtained by hot rolling is wound into a coil at a coiling temperature of 500° C. or less. If the coiling temperature exceeds 500° C., the amount of carbonitrides produced decreases. The lower limit of the coiling temperature is not particularly limited, but is typically 300° C. or higher.

[0049] The hot-rolled material wound into a coil is annealed. Annealing conditions are not particularly limited and are carried out in accordance with known methods. After annealing, pickling may be carried out as necessary. When the stainless steel material is a cold-rolled material, the hot-rolled annealed material obtained as described above is cold-rolled. Cold-rolling conditions are not particularly limited and are carried out in accordance with known methods. After cold-rolling, annealing and pickling may be carried out as necessary.

[0050] The stainless steel material according to the embodiment of the present invention produced as described above can suppress oxidation and thermal deformation associated with hydrogen (proton) penetration in a high-temperature environment containing water vapor and hydrogen. Therefore, this stainless steel material is suitable for use in solid oxide water electrolysis. When the stainless steel material according to the embodiment of the present invention is used in solid oxide water electrolysis, the stainless steel material can be used for various components such as separators, interconnectors, and current collectors.

[0051] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0052] A slab having the composition shown in Table 1 (the balance being Fe and impurities) was melted and hot rolled at the heating temperature shown in Table 2 to form a 4.0 mm hot-rolled sheet, which was then coiled at the coiling temperature shown in Table 2. Next, the hot-rolled sheet wound into a coil was annealed at 1000°C for 1 minute and pickled. Next, the hot-rolled sheet was cold-rolled to 2.0 mm, annealed at 980°C for 30 seconds and pickled, and then cold-rolled (finish cold-rolled) to 0.5 mm, annealed at 950°C for 5 seconds and pickled, thereby obtaining a cold-rolled annealed sheet (stainless steel material).

[0053]

[0054]

[0055] The cold-rolled and annealed sheets obtained above were subjected to the following evaluations.

[0056] (Average particle size, abundance ratio, and aspect ratio of inclusions) The average particle size, abundance ratio, and aspect ratio of inclusions were measured according to the above-mentioned method. Specifically, a sample was taken from the cold-rolled annealed sheet, exposing a thickness direction cross section (L cross section; observation area) parallel to the rolling direction, and then the cross section was mirror-polished. The average particle size, abundance ratio, and aspect ratio of inclusions present in the polished cross section were measured using an automatic inclusion analyzer. Explorer 4 MQA (manufactured by Thermo Fisher Scientific) was used as the automatic inclusion analyzer, and a measurement area of ​​225 mm was used. 2 The lower limit of the particle size of inclusions measured by this device is approximately 0.2 μm.

[0057] (Proportions of Mg-containing inclusions and carbonitrides in inclusions) The proportions of Mg-containing inclusions and carbonitrides in inclusions were measured according to the method described above. Specifically, the inclusions were identified using an automatic inclusion analyzer in the same manner as described above, and then elemental analysis of the inclusions was performed using EDX attached to the automatic inclusion analyzer. The number of Mg-containing inclusions with a Mg concentration of 0.5% or more was counted and divided by the total number of inclusions to calculate the proportion of Mg-containing inclusions in the inclusions. In the elemental analysis of inclusions using EDX, C, N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Nb, and Mo were measured as the elements. The proportion of carbonitrides was calculated by counting the number of carbonitrides, assuming that inclusions with an oxygen concentration of 0.5% or less were carbonitrides, and dividing the number by the total number of inclusions.

[0058] (Oxidation Test) A sample with a diameter of 24 mm was cut out from the cold-rolled annealed sheet and placed in an oxidation test jig as shown in FIG. 1. A gasket was placed between the oxidation test jig and the sample to ensure airtightness. An oxidation test was performed in which a mixed gas of 50% by volume of hydrogen and 50% by volume of water vapor was passed through the flow path in the oxidation test jig at 650°C for 1000 hours, as shown in FIG. 1. After the oxidation test, abnormal oxidation was evaluated in a 15 mm diameter region on the air-side surface of the sample. In this evaluation, a case where no nodular oxides were observed was designated A (no abnormal oxidation occurred), a case where nodular oxides were observed but in two or fewer spots was designated B (little abnormal oxidation), and a case where nodular oxides were observed in more than two spots or thick scale was formed was designated C (lots of abnormal oxidation).

[0059] (Thermal Deformation Test) The thermal deformation of the cold-rolled annealed sheet was evaluated by the high-temperature bending strength test method specified in JIS R1604:2008. Specifically, a three-point bending method with an external support distance of 30 mm was used, and a stainless steel material was cut to prepare a 4 mm x 40 mm test piece. Next, the test piece and the three-point bending tester were placed in a muffle furnace and heated to 650 ° C in air, and the bending strength at which the test piece developed thermal deformation was measured. A bending strength of 150 MPa or more was evaluated as A (high thermal deformation suppression effect), and a bending strength of less than 150 MPa was evaluated as C (low thermal deformation suppression effect). The evaluation results are shown in Table 3.

[0060]

[0061] As shown in Table 3, the cold-rolled annealed sheets of Test Nos. 1, 2, 4 to 7, and 10 (inventive examples) had appropriate compositions and inclusion states, resulting in good results in the oxidation test and heat deformation test. In contrast, the cold-rolled annealed sheets of Test Nos. 3 and 8 (comparative examples) had large inclusion aspect ratios and small amounts of Mg-containing inclusions because the heating temperature and hot-rolling coiling temperature during hot rolling were too high. Furthermore, the cold-rolled annealed sheet of Test No. 17 (comparative example) did not contain Mo, resulting in large inclusion aspect ratios and small amounts of Mg-containing inclusions. Therefore, the results of the oxidation test and heat deformation test were poor. The cold-rolled annealed sheet of Test No. 9 (comparative example) had large inclusion average particle diameters and a low presence rate because the heating temperature and hot-rolling coiling temperature during hot rolling were too high. Therefore, the results of the oxidation test and heat deformation test were poor. The cold-rolled and annealed sheets Nos. 11 to 16 (comparative examples) had inappropriate compositions, and therefore showed poor results in at least one of the oxidation test and the heat deformation test.

[0062] As can be seen from the above results, the present invention can provide a stainless steel material for solid oxide water electrolysis that can suppress oxidation and thermal deformation associated with hydrogen (proton) penetration.

[0063] Therefore, the embodiment of the present invention can have the following aspects.

[0064] <1> A steel sheet comprising, on a mass basis, C: 0.030% or less, Si: 0.20% or less, Mn: less than 0.30%, P: 0.050% or less, S: 0.0030% or less, Cr: 19.0 to 24.0%, Mo: 2.5% or less, Al: 0.01 to 0.15%, Mg: 0.0001 to 0.0100%, N: 0.030% or less, Nb: 0.40% or less, Ti: 0.40% or less, Ni: 1.00% or less, and Cu: 1.00% or less, with the balance being Fe and impurities; the average particle size of the inclusions is 0.2 to 3.0 μm; and the abundance ratio of the inclusions is 30 to 150 pieces / mm 2 wherein the inclusions have an aspect ratio of more than 1.0 and less than 3.0, and the proportion of Mg-containing inclusions having a Mg concentration of 0.5 mass% or more among the inclusions is 0.30 or more.

[0065] <2> The stainless steel material for solid oxide water electrolysis according to <1>, further comprising, by mass, one or more selected from B: 0.0050% 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: 0.500% or less, Ga: 0.050% or less, Hf: 0.10% or less, and REM: 0.10% or less.

[0066] <3> The stainless steel material for solid oxide water electrolysis according to <1> or <2>, wherein the proportion of carbonitrides in the inclusions is 0.40 to 0.80.

[0067] <4> The stainless steel material for solid oxide water electrolysis according to any one of <1> to <3>, which is used for one or more members selected from a separator, an interconnector, and a current collector.

Claims

1. The steel sheet contains, by mass, C: 0.030% or less, Si: 0.20% or less, Mn: less than 0.30%, P: 0.050% or less, S: 0.0030% or less, Cr: 19.0 to 24.0%, Mo: 2.5% or less, Al: 0.01 to 0.15%, Mg: 0.0001 to 0.0100%, N: 0.030% or less, Nb: 0.40% or less, Ti: 0.40% or less, Ni: 1.00% or less, Cu: 1.00% or less, with the balance being Fe and impurities, the average particle size of the inclusions being 0.2 to 3.0 μm, and the proportion of the inclusions being 30 to 150 pieces / mm 2 wherein the inclusions have an aspect ratio of more than 1.0 and less than 3.0, and the proportion of Mg-containing inclusions having a Mg concentration of 0.5 mass% or more among the inclusions is 0.30 or more.

2. The stainless steel material for solid oxide water electrolysis according to claim 1, further comprising, by mass, one or more elements selected from B: 0.0050% 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: 0.500% or less, Ga: 0.050% or less, Hf: 0.10% or less, and REM: 0.10% or less.

3. The stainless steel material for solid oxide water electrolysis according to claim 1 or 2, wherein the proportion of carbonitrides in the inclusions is 0.40 to 0.

80.

4. The stainless steel material for solid oxide water electrolysis according to claim 1 or 2, which is used for one or more members selected from a separator, an interconnector, and a current collector.

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