Ferritic stainless steel foil

Abstract

Provided is ferritic stainless steel foil that achieves both excellent adhesion to a thin film formed on an oxide film and excellent electrical characteristics. Ferritic stainless steel foil according to the present disclosure comprises a foil body and an oxide film formed on the surface of the foil body. The foil body includes ferritic stainless steel. The oxide film has an intermediate change region in oxygen concentration in the depth direction from the surface of the oxide film, the intermediate change region having a thickness of 3.0-10.0 nm.

Description

Ferritic stainless steel foil

[0001] The present disclosure relates to ferritic stainless steel foil.

[0002] Stainless steel materials have excellent corrosion resistance and excellent fatigue strength. In particular, ferritic stainless steel materials, which are relatively inexpensive among stainless steel materials, have been processed into various shapes and used in a wide range of applications. In recent years, ferritic stainless steel foils, which are ferritic stainless steel materials processed into foil shapes, have also come into use. In this specification, "steel foil" refers to a steel plate having a thickness of 100 μm or less.

[0003] Ferritic stainless steel foils are used in a variety of applications, such as substrate materials for electronic devices, current collector materials for primary and secondary batteries, exterior materials for primary and secondary batteries, and spring materials for supporting magnetic heads in hard disk drives. Specifically, Japanese Patent Laid-Open Publication No. 2014-183254 (Patent Document 1), International Publication No. 2016 / 031192 (Patent Document 2), and International Publication No. 2020 / 004595 (Patent Document 3) propose stainless steel foils for various applications.

[0004] The ferritic stainless steel foil disclosed in Patent Document 1 is a ferritic stainless steel foil for solar cell substrates, containing 14 to 18% Cr by mass and having a Vickers hardness of Hv 250 or higher, and having a Vickers hardness of Hv 250 or higher after a film-forming heat treatment in which the foil is maintained at a temperature range of 450 to 600° C. for 1 minute or longer. Patent Document 1 discloses that this stainless steel foil can suppress the occurrence of wrinkles due to buckling of the substrate, even in a continuous process after the high-temperature process (film-forming heat treatment) when manufacturing solar cells using a roll-to-roll method, and has good sheet passing properties.

[0005] The ferritic stainless steel foil disclosed in Patent Document 2 is a ferritic stainless steel foil used for catalyst carriers for exhaust gas purification devices, and contains, by mass%, 0.020% or less of C, 2.0% or less of Si, 1.0% or less of Mn, 0.010% or less of S, 0.050% or less of P, 10.0 to 25.0% of Cr, 0.05 to 0.50% of Ni, 0.14 to 0.25% of Ti, 0.001 to 0.10% of Al, 0.02 to 0.10% of V, and 0.020% or less of N, with the balance consisting of Fe and impurities, and has a Vickers hardness of more than 200 but less than 350. Patent Document 2 discloses that this stainless steel foil has excellent corrugation processability, resistance to shape deformation at high temperatures, and manufacturability.

[0006] The ferritic stainless steel foil disclosed in Patent Document 3 is a stainless steel foil current collector for a secondary battery positive electrode, and contains, by mass%, C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, Cr: 12 to 18%, and at least one of Sn and Ti, where either Sn or Ti satisfies the following conditions: (a) Sn: 0.01 to 1.00%, (b) Ti: 0.10% or 16 (%C + %N), whichever is greater, or greater, with the balance being Fe and impurities, with the impurities limited to P: 0.050% or less, S: 0.030% or less, and N: 0.0300% or less, and the stainless steel has a thickness of 1 to 20 μm and a surface hardness of Hv 300 or less in Vickers hardness. Patent Document 3 discloses that this stainless steel foil not only has mechanical strength, but also ensures electrical conductivity by increasing the contact area with the positive electrode active material and positive electrode mixture, while also ensuring corrosion resistance against alkalis and electrolytes.

[0007] Patent Document 1: JP 2014-183254 A, International Publication No. 2016 / 031192, International Publication No. 2020 / 004595

[0008] As shown in the above Patent Documents 1 to 3, ferritic stainless steel foils are used in a variety of applications. Here, the corrosion resistance of ferritic stainless steel foils is enhanced by the formation of an oxide film (also called a passive film) on the surface. Ferritic stainless steel foils may also have a thin film formed on the oxide film formed on the surface. On the other hand, if the surface of ferritic stainless steel foil becomes flat, there is a concern that the thin film formed on the surface of the oxide film may peel off. Therefore, ferritic stainless steel foils are sometimes required to have the property of preventing such thin films from peeling off, i.e., to have excellent adhesion to the thin film.

[0009] As mentioned above, ferritic stainless steel foils are sometimes used as substrates for batteries and the like. In such cases, it is preferable that the ferritic stainless steel foil also has excellent electrical properties. In other words, it is preferable that the ferritic stainless steel foil has both excellent adhesion to thin films and excellent electrical properties.

[0010] As described above, Patent Documents 1 to 3 propose ferritic stainless steel foils for various applications. However, Patent Documents 1 to 3 do not consider the excellent adhesion to a thin film formed on an oxide film of the ferritic stainless steel foil, or the excellent electrical properties.

[0011] An object of the present disclosure is to provide a ferritic stainless steel foil that combines excellent adhesion to a thin film formed on an oxide film with excellent electrical properties.

[0012] The ferritic stainless steel foil according to the present disclosure comprises a foil body and an oxide film formed on the surface of the foil body, wherein the foil body is made of ferritic stainless steel, and the oxide film has an intermediate change region of oxygen concentration with a thickness of 3.0 to 10.0 nm in the depth direction from the surface of the oxide film.

[0013] The ferritic stainless steel foil according to the present disclosure can achieve both excellent adhesion to a thin film formed on an oxide film and excellent electrical properties.

[0014] Fig. 1 is a graph showing an example of the relationship between the depth (nm) from the surface of an oxide film of a ferritic stainless steel foil and the relative intensity (%) of Fe atoms and O atoms, as obtained by field emission Auger electron spectroscopy. Fig. 2 is a graph for explaining the definition of the thickness of the intermediate change region of the oxygen concentration in the example shown in Fig. 1.

[0015] The present inventors have investigated various methods for improving the adhesiveness of ferritic stainless steel foil to a thin film formed on an oxide film and improving its electrical properties, and have obtained the following findings.

[0016] Specifically, the inventors focused on the oxide film formed on the surface of ferritic stainless steel foil and investigated various methods for improving the adhesion of thin films formed on the oxide film and the electrical properties. The oxide film (passive film) formed on the surface of ferritic stainless steel foil is a film mainly composed of oxides and / or hydroxides containing iron (Fe) and chromium (Cr). This oxide film is chemically very stable and very thin.

[0017] On the other hand, because oxide films are very thin, they rapidly change the surface condition of ferritic stainless steel foil. Therefore, the inventors focused on the concentration of oxygen (O) in the oxide film, in particular, to study ways to improve the adhesion to a thin film formed on the oxide film and the electrical properties. As a result, the inventors found that the O concentration gradient in the oxide film affects the adhesion to a thin film formed on the oxide film and the electrical properties. This point will be explained in detail below with reference to the drawings.

[0018] Fig. 1 is a graph showing an example of the relationship between the depth (nm) from the surface of an oxide film of a ferritic stainless steel foil and the relative intensity (%) of Fe atoms and O atoms, obtained by field emission Auger electron spectroscopy (FE-AES). Here, a depth of 0 nm from the surface of the oxide film refers to the outermost surface of the oxide film. Fig. 1 was obtained by elemental analysis by FE-AES, performed on the surface of an oxide film of a ferritic stainless steel foil using the method described below.

[0019] The solid line in FIG. 1 indicates the relative intensity of O atoms, and the dashed line in FIG. 1 indicates the relative intensity of Fe atoms. The relative intensity of each atom was determined by defining the maximum intensity of Fe atoms obtained by FE-AES as 100%. Hereinafter, the relative intensity of O atoms with respect to the maximum intensity of Fe atoms will also be simply referred to as the "relative intensity of O atoms." Similarly, the relative intensity of Fe atoms with respect to the maximum intensity of Fe atoms will also be simply referred to as the "relative intensity of Fe atoms." The relative intensity of O atoms corresponds to the O concentration at a corresponding depth position, and the relative intensity of Fe atoms corresponds to the Fe concentration at a corresponding depth position.

[0020] 1, it can be seen that the relative intensity of O atoms corresponding to the O concentration (solid line in FIG. 1) shows a high value in a certain range in the depth direction from the surface of the oxide film, and then suddenly decreases. Thereafter, it can be seen that the relative intensity of O atoms stabilizes within a certain range. Furthermore, it can be seen that the relative intensity of Fe atoms corresponding to the Fe concentration (dashed line in FIG. 1) shows a value lower than the relative intensity of O atoms in a certain range in the depth direction from the surface of the oxide film, and then increases as the relative intensity of O atoms decreases. Thereafter, it can be seen that the relative intensity of Fe atoms stabilizes within a certain range.

[0021] Furthermore, Fig. 2 is a diagram for explaining the definition of the thickness of the intermediate change region of oxygen concentration in the example shown in Fig. 1. Referring to Fig. 2, the maximum value of the relative intensity of O atoms is defined as Oy1 (%). Similarly, the minimum value (%) of the relative intensity (%) of O atoms is defined as Oy6 (%). Further referring to Fig. 2, the range between the maximum value Oy1 (%) of the relative intensity of O atoms and the minimum value Oy6 (%) of the relative intensity of O atoms is divided into five equal parts, and the ranges are defined as Oy2 to Oy5 in ascending order of relative intensity.

[0022] 2 , in this specification, the range from the relative intensity of O atoms Oy2 (%) to the relative intensity of O atoms Oy5 (%) is defined as the intermediate change region R (%) of oxygen (O) concentration. That is, in this specification, the intermediate change region R (%) of O concentration is defined as the region (%) from the second highest relative intensity of O atoms Oy2 (%) to the second lowest relative intensity of O atoms Oy5 (%) when the range from the maximum value Oy1 (%) of the relative intensity of O atoms with respect to the maximum intensity of Fe atoms to the minimum value Oy6 (%) of the relative intensity of O atoms with respect to the maximum intensity of Fe atoms is divided into five equal parts.

[0023] 2, the depth from the surface of the oxide film corresponding to the relative intensity Oy2 (%) of O atoms is defined as Ox1 (nm). Similarly, the depth from the surface of the oxide film corresponding to the relative intensity Oy5 (%) of O atoms is defined as Ox2 (nm). In this specification, the thickness D (nm) of the intermediate O concentration change region R (%) is defined as the difference between Ox2 (nm) and Ox1 (nm).

[0024] The present inventors further investigated the thickness D of the intermediate O concentration change region R defined by the above method. As a result, the present inventors found that if the thickness D of the intermediate O concentration change region R is 3.0 to 10.0 nm, the ferritic stainless steel foil can achieve both excellent adhesion to a thin film formed on the oxide film and excellent electrical properties. Therefore, in the ferritic stainless steel foil according to this embodiment, the thickness D of the intermediate O concentration change region R in the depth direction from the surface of the oxide film is 3.0 to 10.0 nm. As a result, the ferritic stainless steel foil according to this embodiment can achieve both excellent adhesion to a thin film formed on the oxide film and excellent electrical properties.

[0025] The reason why the thickness D of the intermediate O concentration change region R is 3.0 to 10.0 nm, and as a result, the ferritic stainless steel foil is able to achieve both excellent adhesion to the thin film formed on the oxide film and excellent electrical properties is not clear in detail. However, the inventors speculate as follows.

[0026] As described above, the oxide film formed on the surface of a ferritic stainless steel foil is very thin, and the element concentrations on the surface of the ferritic stainless steel foil change rapidly. On the other hand, if the thickness D of the intermediate change region R of the O concentration in the oxide film is 3.0 nm or more, the change in the O concentration becomes relatively gradual. As a result, the region where the O concentration changes gradually becomes a layer with a gradient in the concentration of the chemical composition. The inventors speculate that this may result in improved adhesion of a thin film formed on the surface of the oxide film.

[0027] On the other hand, if the thickness D of the intermediate change region R of the O concentration in the oxide film exceeds 10.0 nm, the O concentration changes deep within the oxide film. In other words, the oxide film formed becomes too thick. The inventors speculate that this may result in a deterioration in electrical properties.

[0028] It is possible that, due to a mechanism different from that conjectured by the present inventors, the thickness D of the intermediate O concentration change region R of 3.0 to 10.0 nm allows the ferritic stainless steel foil to have both excellent adhesion to a thin film formed on an oxide film and excellent electrical properties. However, the fact that the thickness D of the intermediate O concentration change region R of 3.0 to 10.0 nm allows the ferritic stainless steel foil to have both excellent adhesion to a thin film formed on an oxide film and excellent electrical properties is proven by the examples described below.

[0029] The ferritic stainless steel foil according to the present embodiment, which was completed based on the above findings, has the following features.

[0030] [1] A ferritic stainless steel foil comprising a foil body and an oxide film formed on the surface of the foil body, wherein the foil body is made of ferritic stainless steel, and the oxide film has an intermediate oxygen concentration change region with a thickness of 3.0 to 10.0 nm in the depth direction from the surface of the oxide film.

[0031] [2] The ferritic stainless steel foil according to [1], wherein the foil body has a thickness of 5 to 60 μm.

[0032] [3] The ferritic stainless steel foil according to [1] or [2], further comprising a resin coating disposed on the surface of the foil body and / or the oxide coating.

[0033] The stainless steel foil according to this embodiment will be described below. In the following description, the ferritic stainless steel foil will also be simply referred to as "steel foil."

[0034] [Ferritic Stainless Steel Foil] The ferritic stainless steel foil according to this embodiment comprises a foil body and an oxide film formed on the surface of the foil body. Here, the foil body is made of ferritic stainless steel. Specifically, ferritic stainless steel refers to a steel having a Cr content of 10.5% or more and a microstructure mainly composed of ferrite. In this specification, a microstructure mainly composed of ferrite means that the volume fraction of ferrite in the microstructure is 95% or more.

[0035] In this embodiment, the ferritic stainless steel may be a well-known ferritic stainless steel, specifically, for example, SUS405, SUS410L, SUS429, SUS430, SUS430LX, SUS430J1L, SUS434, SUS436L, SUS436J1L, SUS443J1, SUS444, SUS445J1, SUS445J2, SUS447J1, or SUSXM27, as specified in JIS G 4305 (2012).

[0036] The ferritic stainless steel foil may further be, for example, 403, 405, 409L, 410, 410L, 410S, 415, 420J1, 420J2, 420, 429, 429J1, 430, 430J1L, 430LX, 430Ti, 434, 436, 436J1L, 439, 441, 444, 445, 445J1, 445J2, 446, 447, or 448 as specified in ASTM A 280 (2006).

[0037] As mentioned above, in this specification, "steel foil" refers to a steel sheet having a thickness of 100 μm or less. Preferably, the thickness of the foil body according to this embodiment is 5 to 60 μm. A foil body thickness of 5 to 60 μm can be widely used in a variety of applications. Furthermore, in recent years, due to the miniaturization of electronic devices and the like, there has been a demand for thinner ferritic stainless steel foils than before. Therefore, the thickness of the foil body according to this embodiment is preferably 5 to 60 μm.

[0038] [Oxide Film] In the oxide film according to this embodiment, the thickness D of the intermediate change region R of the oxygen concentration in the depth direction from the surface of the oxide film is 3.0 to 10.0 nm. The thickness of the oxide film is not particularly limited, but may be, for example, 10 to 30 nm.

[0039] As described above, in this specification, the intermediate change region R (%) of oxygen (O) concentration refers to the region (%) from the second highest relative intensity Oy2 (%) of O atoms to the second lowest relative intensity Oy5 (%) of O atoms, when the maximum intensity of Fe atoms is defined as 100% and the range from the maximum value Oy1 (%) of O atoms to the minimum value Oy6 (%) of O atoms is equally divided into five, based on elemental analysis by field emission Auger electron spectroscopy (FE-AES) performed under the conditions described below.

[0040] In other words, the intermediate change region R in O concentration refers to a region where the change is large within the region where the O concentration decreases. The smaller the thickness D of the intermediate change region R in O concentration, the more abrupt the decrease in O concentration. In this case, the change in O concentration at the interface between the oxide film and the foil body becomes more abrupt. On the other hand, the larger the thickness D of the intermediate change region R in O concentration, the more gradual the decrease in O concentration. In this case, the change in O concentration at the interface between the oxide film and the foil body becomes more gradual.

[0041] Specifically, if the thickness D of the intermediate O concentration change region R is 3.0 nm or more, the adhesion of the thin film formed on the surface of the oxide film is improved. On the other hand, if the thickness D of the intermediate O concentration change region R exceeds 10.0 nm, the electrical properties of the ferritic stainless steel foil are degraded. Therefore, in the ferritic stainless steel foil according to this embodiment, the thickness D of the intermediate O concentration change region R in the depth direction from the surface of the oxide film is 3.0 to 10.0 nm.

[0042] The thickness D of the O concentration intermediate change region R is preferably 3.3 nm, more preferably 3.5 nm, and even more preferably 3.8 nm. The thickness D of the O concentration intermediate change region R is preferably 9.5 nm, more preferably 9.0 nm, and even more preferably 8.8 nm.

[0043] In this embodiment, the thickness D of the intermediate change region R of the O concentration in the oxide film is determined by the following method. Specifically, a test piece is prepared from the ferritic stainless steel foil according to this embodiment. The size of the test piece is not particularly limited. One surface of the test piece is used as the measurement surface. Elemental analysis is performed on the measurement surface by field emission Auger electron spectroscopy (FE-AES). For elemental analysis by FE-AES, for example, an instrument manufactured by JEOL Ltd. under the trade name of JAMP-9500F can be used. The measurement conditions are an acceleration voltage of 10 kV and an acceleration current of 10 nA. For etching with Ar ions, an acceleration voltage of 1 kV is used. The measurement region is, for example, a square with one side of 20 μm. A standard sample for converting the sputtering rate and the depth position is SiO ２By FE-AES performed under the above conditions, an intensity profile in the depth direction from the measurement surface (surface of the oxide film) is obtained.

[0044] The end point of the FE-AES measurement is determined by the following method. Specifically, the end point of the measurement is the point at which the change in the relative intensity of O becomes within 5% over a depth of 5.0 nm in the O intensity spectrum obtained by the FE-AES measurement performed under the above conditions. Here, as will be described later, in the intensity profile obtained by the FE-AES measurement, the maximum value of the relative intensity of O atoms is confirmed near the measurement surface of the test piece (a depth of approximately 0 μm from the surface of the oxide film). Therefore, the end point of the FE-AES measurement is determined in a region deeper than the depth at which the maximum value of the relative intensity of O atoms is confirmed. In other words, the FE-AES measurement is performed from the measurement surface of the test piece to a point deeper than the maximum value of the relative intensity of O atoms and at which the change in the relative intensity of O becomes within 5% over a depth of 5.0 nm.

[0045] From the obtained intensity profile, the thickness D of the intermediate change region R of the O concentration in the oxide film is determined. Specifically, as described above, from the obtained intensity profile, the maximum intensity of Fe atoms is defined as 100%, and the range from the maximum value Oy1 (%) of the relative intensity of O atoms to the minimum value Oy6 (%) of the relative intensity of O atoms is divided into five equal parts. The region (%) from the second highest relative intensity Oy2 (%) of O atoms to the second lowest relative intensity Oy5 (%) of O atoms is defined as the intermediate change region R (%) of the O concentration in the oxide film.

[0046] In FE-AES measurements, the relative intensity of O atoms may be extremely large or extremely small (i.e., an outlier) due to measurement errors, the inclusion of impurities, or the like. Therefore, in this embodiment, the minimum value Oy6 (%) of the relative intensity of O atoms is identified near the end point of the FE-AES measurement, as defined by the above-mentioned conditions. Furthermore, in this embodiment, the maximum value Oy1 (%) of the relative intensity of O atoms is identified near the measurement surface of the test piece on which the FE-AES measurement was performed (near a depth of 0 μm from the surface of the oxide film).

[0047] Using the maximum value Oy1 of the relative intensity of O atoms and the minimum value Oy6 of the relative intensity of O atoms determined by the above method, the relative intensity Oy2 (%) and the relative intensity Oy5 (%) of O atoms are determined. Furthermore, the depth Ox1 (nm) from the surface of the oxide film corresponding to the relative intensity Oy2 (%) and the depth Ox2 (nm) from the surface of the oxide film corresponding to the relative intensity Oy5 (%) of O atoms are determined. The difference between the obtained Ox2 (nm) and Ox1 (nm) is defined as the thickness D (nm) of the intermediate O concentration change region R (%). The thickness D (nm) of the intermediate O concentration change region R (%) is determined by rounding the obtained value to one decimal place.

[0048] [Resin Coating] The ferritic stainless steel foil according to this embodiment may further include a resin coating disposed on the surface of the foil body and / or the oxide film. By disposing a resin coating on the surface of the foil body and / or the oxide film, the insulation properties, heat resistance, and flatness of the ferritic stainless steel foil can be improved. In this embodiment, the resin coating may be disposed on both sides of the ferritic stainless steel foil, on one side, or not at all. The resin coating may also be disposed on a portion of one side of the ferritic stainless steel foil, or on the entirety of the entire surface. In this embodiment, the thickness of the resin coating is not particularly limited, but may be, for example, 0.3 to 5.0 μm.

[0049] The resin coating is not particularly limited, but may be, for example, an inorganic-organic hybrid resin coating. The inorganic-organic hybrid resin coating refers to a resin coating formed by combining an inorganic component and an organic component. Specifically, the inorganic-organic hybrid resin coating may be a siloxane coating having an inorganic skeleton with siloxane bonds developed into a three-dimensional network structure as the main skeleton, in which at least one bridging oxygen in the skeleton is substituted with an organic group and / or a hydrogen atom. That is, in this embodiment, the resin coating may be a siloxane coating in which the oxygen concentration [O] (mol / L) and the silicon concentration [Si] (mol / L) in the coating satisfy the relationship 1<[O] / [Si]<2.

[0050] [Uses of Ferritic Stainless Steel Foil] The ferritic stainless steel foil according to this embodiment can be used in a variety of applications. Specifically, the ferritic stainless steel foil according to this embodiment is suitable as a substrate for applications in which a thin film is formed on an oxide film by a dry process. Examples of thin film formation by a dry process include vacuum deposition, sputtering, ion plating, and plasma CVD (chemical vapor deposition).

[0051] An example of an application for forming a thin film by a dry process is an all-solid-state secondary battery. In this case, the ferritic stainless steel foil according to this embodiment can be used as a substrate for a positive electrode current collector of the all-solid-state secondary battery. In this case, a positive electrode conductor layer may be formed on the surface of the ferritic stainless steel foil by sputtering, a positive electrode active material layer may be formed on the positive electrode conductor layer by sputtering, a solid electrolyte layer may be formed on the positive electrode active material layer by sputtering, and a negative electrode current collector layer may be formed on the solid electrolyte layer by sputtering.

[0052] That is, when the ferritic stainless steel foil according to this embodiment is used as a substrate for a current collector of an all-solid-state secondary battery, it not only has excellent adhesion to the positive electrode conductor layer formed on the surface of the oxide film, but also has high electrical properties. Therefore, the ferritic stainless steel foil according to this embodiment is suitable for use as a substrate for a current collector of an all-solid-state secondary battery. However, the use of the ferritic stainless steel foil according to this embodiment is not limited to use as a substrate for a current collector of an all-solid-state secondary battery. For example, it can also be used as a substrate for a solar cell, a substrate for an electronic device, or a catalyst carrier for an exhaust gas purification system.

[0053] [Method for manufacturing ferritic stainless steel foil] An example of a method for manufacturing a ferritic stainless steel foil according to this embodiment will be described. The method for manufacturing a ferritic stainless steel foil described below is one example for manufacturing a ferritic stainless steel foil according to this embodiment. In other words, the ferritic stainless steel foil according to this embodiment may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a ferritic stainless steel foil according to this embodiment.

[0054] An example of a method for producing the ferritic stainless steel foil of this embodiment includes a material preparation step, an intermediate cold rolling step, an intermediate annealing step, a final cold rolling step, and a final heat treatment step.

[0055] [Material Preparation Step] In the material preparation step, a ferritic stainless steel sheet having a thickness of several hundred μm to several mm is prepared as the material for producing the ferritic stainless steel foil according to this embodiment. The material is, for example, a cold-rolled coil obtained by cold-rolling a hot-rolled coil. The material may be prepared by manufacturing or by purchasing from a third party. In other words, the material preparation step is not particularly limited.

[0056] When manufacturing a raw material, for example, it is manufactured by the following method: Produce molten steel having a desired chemical composition. Produce a slab using the molten steel by continuous casting. Produce the slab by hot working and cold rolling to produce a steel plate with a thickness of several hundred μm to several mm. Through the above steps, the raw material for the ferritic stainless steel foil according to this embodiment is prepared.

[0057] [Intermediate Cold Rolling Step] In the intermediate cold rolling step, cold rolling is performed on the prepared material to produce an intermediate steel plate having a thickness of several tens of μm to several hundreds of μm. In the intermediate cold rolling step, for example, cold rolling may be performed using a continuous rolling mill equipped with multiple rolling stands, or cold rolling may be performed using a reverse rolling mill. The cumulative reduction rate in the intermediate cold rolling step is not particularly limited.

[0058] [Intermediate Annealing Step] In the intermediate annealing step, the intermediate steel sheet after the intermediate cold rolling step is subjected to bright annealing. Bright annealing is an annealing treatment carried out in an extremely low oxygen atmosphere. The intermediate steel sheet that has been subjected to bright annealing has its surface hardly oxidized and can maintain its surface gloss. The extremely low oxygen atmosphere in bright annealing is preferably an H ２ Gas and N ２ The atmosphere is a mixed gas of N and N gas. ２ The volume fraction is, for example, 35 to 65%. The heating temperature for bright annealing is, for example, 800 to 1200°C.

[0059] The intermediate cold rolling step and the intermediate annealing step may be alternately repeated multiple times. For example, when the intermediate cold rolling step and the intermediate annealing step are alternately repeated twice, a first intermediate cold rolling step and a first intermediate annealing step are performed, and then a second intermediate cold rolling step and a second intermediate annealing step are performed.

[0060] [Final Cold Rolling Step] In the final cold rolling step, the intermediate steel sheet after the intermediate annealing step is subjected to final cold rolling. In the final cold rolling step, for example, cold rolling may be performed using a continuous rolling mill equipped with multiple rolling stands, or cold rolling may be performed using a reverse rolling mill. The cumulative reduction rate in the final cold rolling step is not particularly limited.

[0061] [Final Heat Treatment Step] In the final heat treatment step, the intermediate steel sheet after the final cold rolling step is subjected to a heat treatment. In this embodiment, the heat treatment is performed by passing the steel sheet through a heat treatment furnace.

[0062] Preferably, the atmosphere in the heat treatment furnace in the final heat treatment step has a dew point of -50 to less than 0°C and an oxygen concentration of 30 to 150 ppm. If the dew point is too low, oxidation by water vapor is suppressed, making it difficult for an oxide film to grow. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film in the produced ferritic stainless steel foil may be thin. In this case, excellent adhesion to the thin film formed on the oxide film may not be obtained. On the other hand, if the dew point is too high, oxidation by water vapor may be promoted, causing the oxide film to grow too large. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film in the produced ferritic stainless steel foil may be thick. In this case, excellent electrical properties may not be obtained. Therefore, it is preferable that the dew point of the atmosphere in the heat treatment furnace in the final heat treatment step be -50 to less than 0°C.

[0063] Furthermore, if the oxygen concentration is too low, oxidation is suppressed on the surface of the intermediate steel sheet, making it difficult for an oxide film to grow. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film in the manufactured ferritic stainless steel foil may be thin. In this case, excellent adhesion to the thin film formed on the oxide film may not be obtained. On the other hand, if the oxygen concentration is too high, oxidation is promoted on the surface of the intermediate steel sheet, causing the oxide film to grow too much. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film in the manufactured ferritic stainless steel foil may be thick. In this case, excellent electrical properties may not be obtained. Therefore, it is preferable that the oxygen concentration in the atmosphere in the heat treatment furnace in the final heat treatment step be 30 to 150 ppm.

[0064] Preferably, the heat treatment temperature in the final heat treatment step is 350 to 500°C, and the heat treatment time is 5 seconds to 20 minutes. If the heat treatment temperature is too low, the growth of the oxide film may be insufficient. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film may be thin in the produced ferritic stainless steel foil. In this case, excellent adhesion to the thin film formed on the oxide film may not be obtained. On the other hand, if the heat treatment temperature is too high, the oxide film may grow too much. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film may be thick in the produced ferritic stainless steel foil. In this case, excellent electrical properties may not be obtained. Therefore, the heat treatment temperature in the final heat treatment step is preferably 350 to 500°C.

[0065] Furthermore, if the heat treatment time is too short, the oxide film may not grow sufficiently. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film may become thin in the manufactured ferritic stainless steel foil. In this case, excellent adhesion to the thin film formed on the oxide film may not be obtained. On the other hand, if the heat treatment time is too long, the oxide film may grow too much. Therefore, the heat treatment time in the final heat treatment step is preferably 5 seconds to 20 minutes.

[0066] The ferritic stainless steel foil according to this embodiment is manufactured by the above-described manufacturing method. The ferritic stainless steel foil according to this embodiment will be described in more detail below with reference to examples. Note that the examples described below are examples for confirming the effects of the ferritic stainless steel foil according to this embodiment, and do not limit the present invention.

[0067] Materials of the steel types shown in Table 1 were prepared. Note that "SUS444" in the "Steel type" column in Table 1 refers to a ferritic stainless steel equivalent to SUS444 specified in JIS G 4305:2012. Similarly, "SUS430" in the "Steel type" column in Table 1 refers to a ferritic stainless steel equivalent to SUS430 specified in JIS G 4305:2012.

[0068]

[0069] The prepared material having a thickness of 300 μm was subjected to an intermediate cold rolling process. The obtained intermediate steel sheet was subjected to bright annealing as an intermediate annealing process. ２ Gas and H ２ The bright annealed intermediate steel sheet was subjected to a final cold rolling process to produce an intermediate steel sheet having a thickness of 10 μm.

[0070] The produced intermediate steel sheets having a thickness of 10 μm were subjected to a final heat treatment process. In the final heat treatment process, the sheets were maintained in an atmosphere with a dew point (°C) and oxygen concentration (ppm) shown in Table 1 for the temperature (°C) and time (minutes) shown in Table 1. Note that for test number 1, the final heat treatment process was not performed (indicated as "-" in Table 1). Through the above process, ferritic stainless steel foils of each test number were produced.

[0071] [Evaluation Tests] The ferritic stainless steel foils of each test number were subjected to a surface measurement test using field emission Auger electron spectroscopy, a cycle characteristic evaluation test, and a steel foil conductivity measurement test.

[0072] [Surface measurement test by field emission Auger electron spectroscopy] A surface measurement test by field emission Auger electron spectroscopy was performed on the ferritic stainless steel foil of each test number using the method described above, and intensity profiles of Fe atoms and O atoms in the oxide film in the depth direction of the oxide film were obtained. From the obtained intensity profiles, the thickness D (nm) of the intermediate change region R of the O concentration in the oxide film was determined using the method described above. The thickness D of the intermediate change region R of the O concentration in the oxide film for the ferritic stainless steel foil of each test number obtained is shown in Table 1 as "thickness D (nm) of the intermediate change region of O."

[0073] [Cycle characteristic evaluation test] A cycle characteristic evaluation test was performed on the ferritic stainless steel foil of each test number to evaluate the adhesion of the thin film formed on the surface of the oxide film. Specifically, a mock all-solid-state secondary battery for cycle characteristic evaluation was produced from the ferritic stainless steel foil of each test number.

[0074] The simulants were prepared by the following method. First, a steel foil was cut out to form a test piece. A positive electrode conductor layer was formed on the surface of the test piece by sputtering using a DC sputtering device. The target material was Pt, and sputtering was performed without heating in an Ar atmosphere to a film thickness of 1.0 μm. A positive electrode active material layer was formed on the formed positive electrode conductor layer of the test piece by sputtering using an RF sputtering device. The active material of the positive electrode active material layer was lithium cobalt oxide (LiCoO ２ The test piece on which the positive electrode active material layer was formed was subjected to an annealing treatment in which the test piece was heat-treated at 600°C.

[0075] On the annealed test piece, a solid electrolyte layer was formed on the positive electrode active material layer by sputtering using an RF sputtering device. The electrolyte of the solid electrolyte layer was lithium oxynitride phosphate (Li ３ PON ４ ), and the film thickness of the solid electrolyte layer was 1.4 μm. On the solid electrolyte layer formed on the test piece, sputtering was performed using a DC sputtering device to form a negative electrode current collector layer. The conductive material of the negative electrode current collector layer was copper (Cu), and the film thickness of the negative electrode current collector layer was 1.0 μm. On the negative electrode current collector layer formed on the test piece, sputtering was performed using an RF sputtering device to form a protective layer. The protective layer was made of silicon dioxide (SiO ２ The protective layer had a thickness of 0.05 to 1.0 μm. The simulated products prepared as described above were used to evaluate the cycle characteristics of each test number.

[0076] Specifically, the simulated samples of each test number were subjected to constant current charging at a rate of 0.1 C from the rest potential to 4.2 V. Thereafter, constant voltage charging at 4.2 V was performed for 20 hours. The simulated samples charged in the above manner were subjected to constant current discharging at a rate of 0.1 C to 3.0 V. In this example, the discharge capacity obtained by this constant current discharging was defined as the initial discharge capacity Qd1 (mAh).

[0077] In this example, one cycle consisted of 0.1 C constant current charging up to 4.2 V, 20 hours of constant voltage charging at 4.2 V, and 0.1 C constant current discharging down to 3.0 V. The simulated battery for which the initial discharge capacity Qd1 (mAh) was measured was subjected to nine more charge / discharge cycles. In this example, when the initial discharge capacity Qd1 (mAh) was defined as the first cycle, the discharge capacity obtained by constant current discharging at the 10th cycle was defined as the 10th cycle discharge capacity Qd10 (mAh). Furthermore, in this example, the capacity retention rate was defined by the following formula (1): Capacity retention rate (%) = 100 × Qd10 / Qd1 (1)

[0078] In this example, the obtained capacity retention rate (%) of each test number was further evaluated using a relative value. Specifically, the capacity retention rate (%) of test number 1 was used as a reference value, and the capacity retention rate (%) of each test number was evaluated relative to it. For the ferritic stainless steel foil of each test number, if the capacity retention rate was greater than the reference value, it was judged to have excellent cycle characteristics and excellent adhesion to thin films (denoted as "E (Excellent)" in Table 1). On the other hand, for the ferritic stainless steel foil of each test number, if the capacity retention rate was similar to or smaller than the reference value, it was judged to not have excellent cycle characteristics and excellent adhesion (denoted as "NA (Not Acceptable)" in Table 1).

[0079] [Steel Foil Conductivity Measurement Test] A steel foil conductivity measurement test was performed on the ferritic stainless steel foil of each test number to evaluate its electrical properties. Specifically, the conductivity of the ferritic stainless steel foil of each test number was measured using a conductivity meter. The conductivity meter used was a SIGMATEST2.069 manufactured by Nippon Foerster Co., Ltd.

[0080] The conductivity was evaluated using the International Annealed Copper Standard (IACS), where copper (Cu) is 100%. For the ferritic stainless steel foils with each test number, if the conductivity IACS was 2.5% or higher, they were judged to have high conductivity and excellent electrical properties (denoted as "E (Excellent)" in Table 1). On the other hand, for the ferritic stainless steel foils with each test number, if the conductivity IACS was 2.4% or lower, they were judged not to have high conductivity and excellent electrical properties (denoted as "NA (Not Acceptable)" in Table 1).

[0081] [Evaluation Results] Referring to Table 1, the ferritic stainless steel foils of test numbers 2 to 7 had a thickness D of 3.0 to 10.0 nm of the intermediate change region R of the O concentration in the oxide film. As a result, the ferritic stainless steel foils of these test numbers exhibited excellent cycle characteristics in the cycle characteristic evaluation test and had excellent adhesion to thin films. Furthermore, the ferritic stainless steel foils of these test numbers exhibited high conductivity in the steel foil conductivity measurement test and had excellent electrical properties.

[0082] On the other hand, the final heat treatment step was not performed for Test No. 1. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film of the ferritic stainless steel foil of this Test No. was less than 3.0 nm. As a result, the ferritic stainless steel foil of this Test No. did not exhibit excellent cycle characteristics in the cycle characteristic evaluation test, and did not have excellent adhesion to thin films.

[0083] For test numbers 8 to 12, the dew point in the atmosphere during the final heat treatment step was 0°C or higher, and the oxygen concentration exceeded 150 ppm. As a result, the ferritic stainless steel foils with these test numbers had a thickness D of the intermediate change region R of the O concentration in the oxide film exceeding 10.0 nm. As a result, the ferritic stainless steel foils with these test numbers did not exhibit high conductivity in the steel foil conductivity measurement test, and did not have excellent electrical properties.

[0084] For test number 13, the dew point was 0°C or higher in the atmosphere used in the final heat treatment. As a result, the thickness D of the intermediate change region R in the O concentration in the oxide film of the ferritic stainless steel foil of this test number exceeded 10.0 nm. As a result, the ferritic stainless steel foil of this test number did not exhibit high conductivity in the steel foil conductivity measurement test, and did not have excellent electrical properties.

[0085] For test number 14, the dew point in the atmosphere used in the final heat treatment step was less than −50° C. As a result, the thickness D of the intermediate change region R in the O concentration in the oxide film of the ferritic stainless steel foil of this test number was less than 3.0 nm. As a result, the ferritic stainless steel foil of this test number did not exhibit excellent cycle characteristics in the cycle characteristic evaluation test, and did not have excellent adhesion to thin films.

[0086] In test number 15, the oxygen concentration in the atmosphere during the final heat treatment exceeded 150 ppm. As a result, the thickness D of the intermediate change region R in the O concentration in the oxide film of the ferritic stainless steel foil of this test number exceeded 10.0 nm. As a result, the ferritic stainless steel foil of this test number did not exhibit high conductivity in the steel foil conductivity measurement test, and did not have excellent electrical properties.

[0087] In test number 16, the oxygen concentration in the atmosphere during the final heat treatment step was less than 30 ppm. As a result, the thickness D of the intermediate change region R of the O concentration in the oxide film of the ferritic stainless steel foil of this test number was less than 3.0 nm. As a result, the ferritic stainless steel foil of this test number did not exhibit excellent cycle characteristics in the cycle characteristic evaluation test, and did not have excellent adhesion to thin films.

[0088] For test numbers 17 and 18, the heat treatment temperature exceeded 500°C in the final heat treatment step. As a result, the ferritic stainless steel foils with these test numbers had a thickness D of the intermediate change region R of the O concentration in the oxide film exceeding 10.0 nm. As a result, the ferritic stainless steel foils with these test numbers did not exhibit high conductivity in the steel foil conductivity measurement test, and did not have excellent electrical properties.

[0089] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A ferritic stainless steel foil comprising a foil body and an oxide film formed on the surface of the foil body, wherein the foil body is made of ferritic stainless steel, and the oxide film has an intermediate oxygen concentration change region with a thickness of 3.0 to 10.0 nm in the depth direction from the surface of the oxide film.

2. A ferritic stainless steel foil according to claim 1, wherein the foil body has a thickness of 5 to 60 μm.

3. A ferritic stainless steel foil according to claim 1 or 2, further comprising a resin coating disposed on the surface of the foil body and / or the oxide film.

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