Stainless steel foil

Abstract

Provided is a stainless steel foil in which the occurrence of damage is reduced. The stainless steel foil of the present disclosure is provided with a foil body that comprises stainless steel. On a discretionary surface of the foil body, when coarse particles are defined as particles with an equivalent circular diameter of at least 6.0 μm and special particles are defined as coarse particles in which, in the elemental content of the particles, the aluminum content, the magnesium content, or the titanium content is highest and the aluminum content, the magnesium content, and the titanium content fulfill formula (1), the numerical density of the special particles is 40.0 particles / cm2 or less, and the numerical proportion of the special particles with respect to the coarse particles is at least 80%.　Formula (1): 0.3591 ≤ 4.5579Al / (Al + Mg + Ti) − 4.5579Mg / (Al + Mg + Ti) + 1.7321Ti / (Al + Mg + Ti) ≤ 3.0943, where the element symbols in formula (1) are replaced with the aluminum content, the magnesium content, and the titanium content of the coarse particles in mass%.

Description

Stainless steel foil

[0001] The present disclosure relates to steel foil, and more particularly to stainless steel foil.

[0002] Because stainless steel materials have excellent corrosion resistance and fatigue strength, they have been processed into various shapes and used in a wide range of applications. In recent years, stainless steel foils, which are stainless steel materials processed into foil shapes, have come into use. In this specification, "steel foil" refers to a steel plate having a thickness of 150 μm or less.

[0003] 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 use in a variety of applications.

[0004] The 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 continuous processes after the high-temperature process (film-forming heat treatment) during solar cell manufacturing using the roll-to-roll method, and has good sheet passing properties.

[0005] The 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 stainless steel foil disclosed in Patent Document 3 is a stainless steel foil current collector for a secondary battery positive electrode, which 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 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] However, stainless steel foils are sometimes required to have corrosion resistance under severe conditions. For example, when applied to solar cell substrates as proposed in Patent Document 1, corrosion resistance against corrosion from the installation environment is required. Furthermore, when applied to exhaust gas purification devices as proposed in Patent Document 2, corrosion resistance against corrosive gases contained in exhaust gases is required. Furthermore, when applied to current collectors as proposed in Patent Document 3, corrosion resistance against electrolytes and the like is required.

[0009] On the other hand, as mentioned above, stainless steel foil is very thin, with a thickness of 150 μm or less. Therefore, during the manufacturing process of stainless steel foil, minute scratches and irregularities are easily formed on the surface. Hereinafter, in this specification, minute scratches and irregularities formed on the surface or inside of steel foil are also referred to as "damage." Note that, in this specification, damage is distinguished from pinholes (through holes in the foil thickness direction) formed in stainless steel foil. Since damage often contains lattice defects, such as dislocations and point defects, damage may also be formed around pinholes depending on the pinhole formation process. Furthermore, as mentioned above, since damage often contains lattice defects, when damage is formed in stainless steel foil, the damage can become a starting point for corrosion. Furthermore, particularly large damage is likely to become a starting point for corrosion. Herein, large depressions formed on the surface of steel foil are also referred to as "gouges." Therefore, for stainless steel foils that require excellent corrosion resistance under harsh conditions, it is preferable to reduce the formation of damage, especially gouges.

[0010] As described above, Patent Documents 1 to 3 propose stainless steel foils for various applications. However, Patent Documents 1 to 3 do not consider damage formed on the surface of the stainless steel foil.

[0011] An object of the present disclosure is to provide a stainless steel foil with reduced damage formation.

[0012] The stainless steel foil according to the present disclosure comprises a foil body made of stainless steel, and in any surface of the foil body, particles having an equivalent circle diameter of 6.0 μm or more are defined as coarse particles, and among the coarse particles, particles having the highest Al content, Mg content, or Ti content among the element contents contained in the particles, and when the particles whose Al content, Mg content, and Ti content satisfy formula (1) are defined as specific particles, the number density of the specific particles is 40.0 particles / cm ２ or less, and the ratio of the number of the specific particles to the number of the coarse particles is 80% or more. 0.3591≦4.5579Al / (Al+Mg+Ti)−4.5579Mg / (Al+Mg+Ti)+1.7321Ti / (Al+Mg+Ti)≦3.0943 (1) Here, the Al content, Mg content, and Ti content contained in the coarse particles are substituted for the element symbols in formula (1) in mass %. When a corresponding element is not contained in the coarse particles, “0” is substituted for that element symbol.

[0013] Stainless steel foils according to the present disclosure exhibit reduced damage formation.

[0014] The present inventors conducted extensive research into reducing damage formation in stainless steel foil. First, the main body of stainless steel foil may contain coarse particles that differ from the matrix (stainless steel phase), such as metallic precipitates, crystallized particles, and inorganic particles called inclusions, typically compounds of metals and metalloids with oxygen, nitrogen, carbon, or sulfur. Meanwhile, if the main body of the foil contains coarse particles with different hardness and ductility, typically inclusions, damage may be formed in the stainless steel foil. As a result of extensive research by the present inventors, it was found that certain coarse particles, particularly those with an equivalent circle diameter of 6.0 μm or more, are particularly likely to cause damage to the matrix at particle interfaces. Hereinafter, in this specification, particles with an equivalent circle diameter of 6.0 μm or more are also referred to as "coarse particles."

[0015] Next, the inventors focused on the components of coarse particles and conducted a detailed study on coarse particles that are likely to cause damage. As a result, it was found that coarse particles with the highest Al, Mg, or Ti content among the element contents contained in the particles and whose Al, Mg, and Ti contents satisfy formula (1) have a significant effect on damage to stainless steel foil. 0.3591≦4.5579Al / (Al+Mg+Ti)−4.5579Mg / (Al+Mg+Ti)+1.7321Ti / (Al+Mg+Ti)≦3.0943 (1) Here, the element symbols in formula (1) are substituted with the Al content, Mg content, and Ti content contained in the coarse particles in mass percent. If the corresponding element is not contained in the coarse particles, "0" is substituted for that element symbol.

[0016] It is defined as Fn1 = 4.5579Al / (Al + Mg + Ti) - 4.5579Mg / (Al + Mg + Ti) + 1.7321Ti / (Al + Mg + Ti). Ａｌ = Al / (Al + Mg + Ti). Ａｌ means the ratio of the Al content to the total of the Al content, the Mg content, and the Ti content in the coarse particles. Ｍｇ = Mg / (Al + Mg + Ti). Ｍｇ means the ratio of the Mg content to the total of the Al content, Mg content, and Ti content in the coarse particles. Ｔｉ = Ti / (Al + Mg + Ti). Ｔｉ means the ratio of the Ti content to the total of the Al content, Mg content, and Ti content in the coarse particles. Ａｌ -4.5579F Ｍｇ +1.7321F Ｔｉ is equivalent to

[0017] Fn1 defined as above is an index for identifying the type of coarse particles. Here, in the stainless steel foil, coarse particles (particles with an equivalent circle diameter of 6.0 μm or more) are inclusions or metallic precipitates or crystallized particles. Specifically, particles that may have an equivalent circle diameter of 6.0 μm or more contained in the stainless steel foil include, for example, SiO２ , CaO, MgO, Al ２ O ３ , TiN, MgAl ２ O ４ As a result of detailed investigations by the present inventors, among inorganic particles, TiN and MgAl ２ O ４ Compared with other inclusions and precipitates, coarse TiN and MgAl have very high hardness and low ductility. ２ O ４ When the steel foil contains , damage is likely to be formed in the produced stainless steel foil.

[0018] In the foil body of the stainless steel foil of this embodiment, the coarse particles that satisfy Fn1 of 0.3591 to 3.0943 are mostly TiN, MgAl ２ O ４ , or composite particles thereof. In this specification, therefore, coarse particles whose Fn1 satisfies 0.3591 to 3.0943 are defined as "specific particles." In other words, if the foil main body contains a large number of specific particles, a large number of damages are likely to be formed during the process of manufacturing the stainless steel foil. Therefore, reducing the number density of the specific particles contained in the foil main body may potentially reduce damage to the stainless steel foil.

[0019] Therefore, the present inventors have investigated ways to reduce damage to the stainless steel foil by reducing the number density of the specific particles contained in the foil body. As a result of the investigations by the present inventors, it was found that the number density of the specific particles is 40.0 particles / cm ２ It has been revealed that the formation of damage can be suppressed if the following conditions are met, provided that the other configurations of this embodiment are met.

[0020] On the other hand, in the foil body, the number density of the specific particles is 40.0 particles / cm ２ Therefore, the inventors of the present invention have attempted to reduce the number density of specific particles to 40.0 particles / cm or less. ２Various methods for further reducing damage were investigated for a stainless steel foil having the following foil body. As a result, it was found that damage to the stainless steel foil can be further reduced by increasing the proportion of specific particles in the coarse particles.

[0021] Therefore, in the stainless steel foil according to the present embodiment, the number density of the specific particles in the foil body is set to 40.0 particles / cm ２ The stainless steel foil according to the present embodiment is characterized in that the ratio of the number of specific particles to the number of coarse particles is 80% or more. As a result, the formation of damage is reduced in the stainless steel foil according to the present embodiment.

[0022] In the foil body of the stainless steel foil, the number density of the specific particles is 40.0 particles / cm ２ or less, and further, by making the ratio of the number of specific particles to the number of coarse particles 80% or more, it has been proven by the examples described later that the formation of damage to the stainless steel foil can be reduced.

[0023] The stainless steel foil according to this embodiment, which was completed based on the above findings, has the following features.

[0024] [1] A foil body made of stainless steel, wherein, on any surface of the foil body, particles having an equivalent circle diameter of 6.0 μm or more are defined as coarse particles, and among the coarse particles, particles having the highest Al content, Mg content, or Ti content among the element contents contained in the particles, and the Al content, the Mg content, and the Ti content satisfying formula (1) are defined as specific particles, the number density of the specific particles is 40.0 particles / cm ２ A stainless steel foil having a ratio of the number of the specific particles to the number of the coarse particles of 80% or more, wherein: 0.3591≦4.5579Al / (Al+Mg+Ti)−4.5579Mg / (Al+Mg+Ti)+1.7321Ti / (Al+Mg+Ti)≦3.0943 (1) Here, the element symbols in formula (1) are substituted with the Al content, Mg content, and Ti content contained in the coarse particles, expressed in mass%. When a corresponding element is not contained in the coarse particles, “0” is substituted for that element symbol.

[0025] [2] The stainless steel foil according to [1], wherein, in the foil main body, among the specific particles, when particles having an element content in which the Ti content satisfies formula (2) are defined as specific spinel particles, the number density of the specific spinel particles is 4.5 particles / cm ２ A stainless steel foil having a Ti / (Al+Mg+Ti)≦0.05 (2) where the element symbols in formula (2) are substituted with the Al content, Mg content, and Ti content, expressed in mass%, contained in the coarse particles. When a corresponding element is not contained in the coarse particles, "0" is substituted for the element symbol.

[0026] [3] The stainless steel foil according to [1], wherein the chemical composition of the foil body contains, in mass%, Ti: 0.10 to 1.00%.

[0027] [4] The stainless steel foil according to [2], wherein the chemical composition of the foil body contains, in mass%, Ti: 0.10 to 1.00%.

[0028] [5] The stainless steel foil according to any one of [1] to [4], wherein the foil body is made of ferritic stainless steel or austenitic stainless steel.

[0029] The stainless steel foil according to this embodiment will be described below. In the following description, the stainless steel foil will also be simply referred to as "steel foil."

[0030] [Stainless Steel Foil] In this specification, "steel foil" refers to a steel sheet having a thickness of 150 μm or less. That is, the thickness of the stainless steel foil according to this embodiment is 150 μm or less. Furthermore, the stainless steel foil according to this embodiment has a foil body made of stainless steel. The stainless steel foil may have a configuration other than the foil body. For example, the stainless steel foil may have a resin coating formed on the surface of the foil body, or a coating mainly composed of an inorganic compound.

[0031] [Foil Body] As described above, in this embodiment, the foil body is made of stainless steel. That is, in this embodiment, the type of stainless steel constituting the foil body is not particularly limited. The stainless steel may be, for example, ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, ferritic-martensitic duplex stainless steel, or ferritic-austenitic duplex stainless steel.

[0032] Preferably, the stainless steel according to this embodiment is a ferritic stainless steel or an austenitic stainless steel. When the stainless steel is an austenitic stainless steel, the steel type may be, for example, SUS301, SUS301L, SUS301J1, SUS302B, SUS304, SUS304Cu, SUS304L, SUS304N1, SUS304N2, SUS304LN, SUS304J1, SUS304J2, SUS305, SUS309S, SUS310S, SUS312L, SUS315J1, SUS315J2, as specified in JIS G 4305 (2015). It may be any one of SUS316, SUS316L, SUS316N, SUS316LN, SUS316Ti, SUS316J1, SUS316J1L, SUS317, SUS317L, SUS317LN, SUS317J1, SUS317J2, SUS836L, SUS890L, SUS321, SUS347, SUSXM7, and SUSXM15J1.

[0033] In this case, the chemical composition of the austenitic stainless steel is, in mass %, C: 0.150% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.0300% or less, Cr: 16.00 to 20.00%, Ni: 6.00 to 15.00%, Al: 0.010% or less, N: 0.100% or less, Mo: 0 to 2.50%, Ti: 0 to 0.5 0.00%, Nb: 0-0.12%, V: 0-1.00%, Zr: 0-0.100%, Ta: 0-0.50%, Hf: 0-0.10%, Co: 0-0.50%, B: 0-0.0100%, Ca: 0-0.0200%, Mg: 0-0.0200%, rare earth elements: 0-0.0100%, Cu: 0-3.00%, and the balance being Fe and impurities.

[0034] When the stainless steel is a ferritic stainless steel, the steel type is not particularly limited, and well-known ferritic stainless steels can be used. The steel type of the ferritic stainless steel may be, for example, any one of SUS405, SUS410L, SUS429, SUS430, SUS430LX, SUS430J1L, SUS434, SUS436L, SUS436J1L, SUS443J1, SUS444, SUS445J1, SUS445J2, SUS447J1, and SUSXM27 specified in JIS G 4305 (2015). The steel grade of the ferritic stainless steel may also be, for example, any one of 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, and 448 as specified in ASTM A 240 (2006).

[0035] In this case, the chemical composition of the ferritic stainless steel is, in mass %, C: 0.150% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.030% or less, Cr: 12.00 to 20.00%, Ni: 0.01 to 0.50%, Mo: 0.01 to 2.50%, Al: 0.010% or less, N: 0.100% or less, Ti: 0 to 1.0 0%, Nb: 0 to 0.80%, V: 0 to 1.00%, Zr: 0 to 0.80%, Ta: 0 to 0.50%, Hf: 0 to 0.10%, Co: 0 to 0.50%, B: 0 to 0.0100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, rare earth elements: 0 to 0.0100%, Cu: 0 to 3.00%, and the balance being Fe and impurities.

[0036] More preferably, the chemical composition of the foil body according to this embodiment contains, by mass%, 0.10 to 1.00% Ti. In this case, the corrosion resistance of the stainless steel foil is further improved. On the other hand, if the chemical composition of the foil body contains, by mass%, 0.10 to 1.00% Ti, the amount of coarse TiN contained in the foil body increases. As a result, the number density of the specific particles increases, which may make the stainless steel foil more susceptible to damage. On the other hand, in the stainless steel foil according to this embodiment, even if the chemical composition of the foil body contains, by mass%, 0.10 to 1.00% Ti, the number density of the specific particles can be kept at 40.0 particles / cm. ２ By setting the ratio of the number of specific particles to the number of coarse particles to 80% or more, it is possible to reduce the formation of damage to the stainless steel foil.

[0037] The thickness of the foil body according to this embodiment is not particularly limited, but is, for example, 3 to 80 μm. If the foil body is too thin, it is difficult to manufacture a foil body with a uniform thickness. On the other hand, if the foil body is too thick, it may not be possible to obtain the desired characteristics depending on the application. Preferably, the foil body has a thickness of 5 to 60 μm.

[0038] [Specific particles] In the stainless steel foil according to the present embodiment, among the coarse particles (particles having a circle-equivalent diameter of 6.0 μm or more) in the foil body, when the particles have the highest Al content, Mg content, or Ti content in terms of the element content contained in the particles and the Al content, Mg content, and Ti content satisfy the formula (1), the number density of the specific particles is 40.0 particles / cm ２ or less, and the ratio of the number of specific particles to the number of coarse particles is 80% or more. 0.3591≦4.5579Al / (Al+Mg+Ti)−4.5579Mg / (Al+Mg+Ti)+1.7321Ti / (Al+Mg+Ti)≦3.0943 (1) Here, the element symbols in formula (1) are substituted with the Al content, Mg content, and Ti content contained in the coarse particles in mass%. When a corresponding element is not contained in the coarse particles, “0” is substituted for that element symbol.

[0039] Fn1 (= 4.5579Al / (Al+Mg+Ti) - 4.5579Mg / (Al+Mg+Ti) + 1.7321Ti / (Al+Mg+Ti)) is an index for identifying the type of coarse particles. Ａｌ (=Al / (Al+Mg+Ti)), F Ｍｇ (=Mg / (Al+Mg+Ti)), F Ｔｉ (=Ti / (Al+Mg+Ti)), Fn1 = 4.5579F Ａｌ -4.5579F Ｍｇ +1.7321F Ｔｉ It can be written as follows.

[0040] In this specification, coarse particles having an Fn1 of 0.3591 to 3.0943 are defined as "specific particles." As described above, in the main body of the stainless steel foil, the specific particles having an Fn1 of 0.3591 to 3.0943 are mostly TiN, MgAl ２ O ４ Also, among the coarse particles, TiN, MgAl ２ O ４These composite particles have significantly higher hardness and lower ductility than other coarse particles. Therefore, damage is more likely to occur at the interfaces of the specific particles than with other coarse particles. Therefore, in this embodiment, the number density of the specific particles is reduced.

[0041] Specifically, in the stainless steel foil according to the present embodiment, the number density of the specific particles in the foil body is 40.0 particles / cm ２ As a result, damage formation can be suppressed, provided that the other configurations of this embodiment are satisfied. The preferred upper limit of the number density of the specific particles is 39.5 particles / cm. ２ and more preferably 35.0 particles / cm ２ and more preferably 30.0 particles / cm ２ and more preferably 25.0 particles / cm ２ and more preferably 20.0 particles / cm ２ In this embodiment, it is preferable that the number density of the specific particles is low. That is, the lower limit of the number density of the specific particles is not particularly limited, and is 0.1 particles / cm. ２ The lower limit of the number density of the specific particles may be 1.0 particles / cm ２ 3.0 pieces / cm ２ 5.0 pieces / cm ２ may be.

[0042] The stainless steel foil according to this embodiment further defines the ratio of the number of specific particles to the number of coarse particles. As described above, the foil body of the stainless steel foil contains, for example, SiO ２ , CaO, MgO, Al ２ O ３ , TiN, MgAl ２ O ４ , and composite particles thereof. Among these, specific particles (TiN, MgAl ２ O ４ In other words, the coarse particles other than the specific particles are softer than the specific particles and are thought to have less influence on the formation of damage than the specific particles.

[0043] On the other hand, coarse particles other than the specific particles can also be the starting point of damage. Furthermore, if an attempt is made to excessively reduce the number density of the specific particles, there is a concern that damage may be formed in the stainless steel foil depending on the manufacturing process. Therefore, in the stainless steel foil according to this embodiment, the number of coarse particles other than the specific particles is reduced as much as possible. As a result, the ratio of the number of specific particles to the number of coarse particles increases. In other words, by increasing the number ratio of specific particles to the number of coarse particles and reducing the number of coarse particles other than the specific particles, the formation of damage in the stainless steel foil is reduced.

[0044] Specifically, in the stainless steel foil according to the present embodiment, the number density of the specific particles in the foil body is 40.0 particles / cm ２ The ratio of the number of specific particles to the number of coarse particles is further reduced to 80% or more after the ratio is reduced to 81% or less. As a result, the stainless steel foil according to this embodiment can suppress the formation of damage. A preferred lower limit of the ratio of the number of specific particles to the number of coarse particles is 81%, and more preferably 82%. The upper limit of the ratio of the number of specific particles to the number of coarse particles is not particularly limited, and may be 100%.

[0045] In the stainless steel foil according to the present embodiment, the number density of the specific particles in the foil body is 40.0 particles / cm ２ The number density of the coarse particles is not particularly limited as long as the ratio of the number of specific particles to the number of coarse particles is 80% or more. ２ The lower limit of the number density of the coarse particles is not particularly limited, and is 0.1 particles / cm ２ may be.

[0046] In this embodiment, the number density of specific particles and the ratio of the number of specific particles to coarse particles are determined by the following method. First, a test piece is prepared from the stainless steel foil according to this embodiment, with the surface or cross section (polished surface) of the foil body as the observation surface. The observation surface of the prepared test piece is observed using an SEM and subjected to elemental analysis using EDS (EDS analysis). First, particles are detected based on the contrast of the image. Specifically, particles exposed on the surface are detected by controlling and scanning the SEM stage over an arbitrary observation area set on the observation surface. Note that particle detection can be performed using an automated particle detection system or a commercially available method such as an MQA (Metal Quality Analyzer). More specifically, an AZtec manufactured by Oxford Instruments, Inc. can be used as the MQA. Below, the measurement and analysis methods are described.

[0047] Particle detection by a scanning electron microscope (SEM) is carried out based on the contrast of a backscattered electron image. In this embodiment, particles with a circle equivalent diameter of 6.0 μm or more are defined as "coarse particles." Therefore, the size of one pixel of the backscattered electron image to be analyzed must be at least 0.7 μm. ２ The observation magnification and other factors are set so that the following conditions are met. Furthermore, to ensure measurement resolution, a field emission scanning microscope (FE-SEM) is preferably used. Specifically, the JSM-IT500HR manufactured by JEOL Ltd. can be used as the FE-SEM. When using this FE-SEM, typical conditions for satisfying the above measurement conditions are: detector: backscattered electron detector BED-C, observation magnification: 80x, acceleration voltage: 20.0 kV, working distance (WD): 10.0 mm, and irradiation current: 80%.

[0048] The particles detected above are subjected to elemental concentration analysis (EDS analysis). In the EDS analysis, the resolution is 2048, the dwell time (collection time per pixel): 3 μs, the input signal is a backscattered electron image (BSE), the number of frames (number of scans in the same field of view): 1, and the lower limit of the detected particle size: 6 pixels. For the EDS, an ULTIM MAX 65 manufactured by Oxford Instruments Ltd. can be used.

[0049] The equivalent circle diameter of each particle can be determined by image analysis. The elements detected in the EDS analysis are Cr, Fe, Ni, Mo, C, N, O, Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl. That is, for particles identified based on the contrast, the contents of Cr, Fe, Ni, Mo, C, N, O, Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl are measured by EDS analysis.

[0050] Here, in the stainless steel foil according to this embodiment, particles may be erroneously detected due to contamination, etc. Therefore, in this embodiment, when elements other than Cr, Fe, Ni, Mo, C, N, and O among the above-mentioned detected elements, i.e., Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl are detected in an amount of 1 mass % or more in the EDS analysis, the element is identified as a "particle." That is, in this embodiment, "coarse particles" are defined as particles that are identified based on contrast as having an equivalent circle diameter of 6.0 μm or more, and in which elements other than Cr, Fe, Ni, Mo, C, N, and O, i.e., Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl, are detected in an amount of 1 mass % or more.

[0051] The measurement is repeated by changing the field of view until the total number of coarse particles defined as above reaches 100 or more. Furthermore, Fn1 is calculated from the Al content, Mg content, and Ti content obtained by the EDS analysis described above. Fn1 is calculated by rounding the obtained value to the nearest tenth.

[0052] Among the coarse particles defined as above, particles having the highest Al content, Mg content, or Ti content among the elements other than Cr, Fe, Ni, Mo, C, N, and O among the detected elements, i.e., Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl, and having an Fn1 of 0.3591 to 3.0943, are defined as "specific particles." Measurements are repeated with different fields of view until 100 or more coarse particles are counted, and the coarse particles and specific particles are counted separately. From the number of specific particles obtained and the total area of ​​the fields of view where EDS analysis was performed, the number density of the specific particles (particles / cm) is calculated. ２ ) is calculated. The number density of specific particles (particles / cm ２ ) is calculated by rounding off the obtained number to one decimal place. The ratio (%) of the number of specific particles to the number of coarse particles is calculated from the obtained number of specific particles and the number of coarse particles. The ratio (%) of the number of specific particles to the number of coarse particles is calculated by rounding off the obtained number to one decimal place.

[0053] [Specific Spinel Particles] The stainless steel foil according to the present embodiment further has a number density of the specific spinel particles of 4.5 particles / cm when the specific spinel particles are defined as particles in which the Ti content satisfies the formula (2) in terms of the element content of the particles among the specific particles in the foil main body. ２ Preferably, the ratio is equal to or less than Ti / (Al+Mg+Ti)≦0.05 (2). Here, the Al content, Mg content, and Ti content contained in the coarse particles are substituted for the element symbols in formula (2) in mass %. When the corresponding element is not contained in the coarse particles, "0" is substituted for the element symbol.

[0054] As described above, in the foil body of the stainless steel foil of this embodiment, the specific particles are mostly TiN, MgAl ２ O ４ The specific spinel particles are specific particles having a low Ti content. ２ O ４In other words, Fn2 is an index indicating the proportion of specific spinel particles in the specific particles.

[0055] As a result of investigations by the present inventors, it has been found that among the specific particles, specific spinel particles (MgAl ２ O ４ ) are likely to become the starting point of damage in the stainless steel foil. Furthermore, there is a concern that the specific spinel particles may become the starting point of forming composite particles with TiN. Therefore, in the stainless steel foil according to this embodiment, the specific spinel particles are arranged in the foil body at a density of 4.5 particles / cm. ２ It is preferable that the number density of the specific particles in the foil body is reduced to 40.0 particles / cm or less. ２ the ratio of the number of specific particles to the number of coarse particles is 80% or more, and the specific spinel particles are 4.5 particles / cm ２ If the temperature is less than 1000°C, the formation of damage in the stainless steel foil can be further reduced.

[0056] A more preferable upper limit of the number density of the specific spinel particles is 4.4 particles / cm ２ and more preferably 4.3 particles / cm ２ The lower limit of the number density of the specific spinel particles is not particularly limited, and is 0.0 particles / cm ２ The lower limit of the number density of the specific particles may be 0.1 particles / cm ２ 0.5 pieces / cm ２ 1.0 particles / cm ２ may be.

[0057] In this embodiment, the number density of the specific spinel particles is determined by the following method. Fn2 (=Ti / (Al+Mg+Ti)) is determined from the Al content, Mg content, and Ti content obtained in the method for determining the number density of the specific particles described above. Specific particles having an Fn2 of 0.05 or less are defined as "specific spinel particles." The number density of the specific spinel particles (particles / cm) is determined from the number of specific spinel particles obtained and the total area of ​​the field of view where the EDS analysis was performed. ２ ) is determined. The number density (particles / cm ２ ) is calculated by rounding the obtained number to one decimal place.

[0058] [Uses of Stainless Steel Foil] The uses of the stainless steel foil according to this embodiment are not particularly limited. On the other hand, as described above, the stainless steel foil according to this embodiment can reduce the formation of damage, thereby suppressing corrosion initiated by the damage and achieving excellent corrosion resistance. Therefore, the stainless steel foil according to this embodiment is suitable for use in thin film materials that require corrosion resistance. Specifically, the stainless steel foil according to this embodiment can be used in a wide range of fields, 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. It can also be used as a current collector material for solid-state batteries, which are expected to be used in harsh environments.

[0059] [Manufacturing method] An example of a method for manufacturing a stainless steel foil according to this embodiment will be described. Note that the stainless steel foil according to this embodiment may be manufactured by a manufacturing method other than the example manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a stainless steel foil according to this embodiment. The method for manufacturing a stainless steel foil according to this embodiment includes a material preparation step, an intermediate cold rolling step, an annealing step, and a foil rolling step.

[0060] [Material Preparation Step] In the material preparation step, a material having the same chemical composition as the foil main body, which is the base material of the stainless steel foil, is prepared. Here, the material refers to an intermediate product for manufacturing the foil main body according to this embodiment, and is a steel plate having a thickness of several hundred μm to several mm. The material is, for example, a hot-rolled coil. The material may also be a cold-rolled coil. The material may be prepared by manufacturing or by purchasing from a third party. In other words, the process for preparing the material is not particularly limited.

[0061] When preparing a raw material, it can be produced, for example, by the following method: Molten steel having a desired chemical composition is produced; A cast piece (slab, bloom, or billet) is produced using the molten steel by a continuous casting method; A steel ingot (ingot) may be produced using the molten steel by an ingot casting method; If necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet.

[0062] The produced cast or steel ingot (slab, bloom, billet, or ingot) is hot worked to produce a steel plate (raw material) having a thickness of several hundred μm to several mm. The hot working method is not particularly limited, and any well-known method may be used. Preferably, the raw material after hot working is subjected to a surface treatment such as pickling as appropriate. In this case, impurities are removed from the surface of the raw material. The raw material according to this embodiment is prepared by the above steps.

[0063] [Intermediate Cold Rolling Process] In the intermediate cold rolling process, cold rolling is performed on the prepared material to produce an intermediate steel material. Here, the intermediate steel material is an intermediate product for producing the foil body according to this embodiment, and refers to a steel sheet having a thickness of several tens to several hundreds of μm. In the intermediate cold rolling process, cold rolling and annealing are repeatedly performed. At this time, cold rolling may be performed using, for example, a continuous rolling mill or a reverse rolling mill equipped with multiple cold rolling stands.

[0064] Preferably, the total rolling ratio in the intermediate cold rolling process is 95.0% or more. The total rolling ratio refers to the ratio (%) of the thickness of the intermediate steel material after the intermediate cold rolling process to the thickness of the material before the intermediate cold rolling process. If the total rolling ratio is 95.0% or more, the specific particles can be divided and the number density of the specific particles can be reduced. On the other hand, if the total rolling ratio is too low, the number density of the specific particles in the manufactured stainless steel foil may be too high, making it impossible to reduce the formation of damage. Therefore, in the intermediate cold rolling process according to this embodiment, the total rolling ratio is preferably 95.0% or more.

[0065] Preferably, the number of passes in the intermediate cold rolling process is 8 or more. The number of passes in cold rolling refers to the number of times a material passes through a cold rolling mill. Specifically, when cold rolling is performed with 8 passes, the material is cold rolled 8 times by the cold rolling mill. If the number of passes is 8 or more, the specific particles can be divided and the number density of the specific particles can be reduced. On the other hand, if the number of passes is too small, the number density of the specific particles in the manufactured stainless steel foil may be too high, making it impossible to reduce the formation of damage. Therefore, in the intermediate cold rolling process according to this embodiment, the number of passes is preferably 8 or more.

[0066] More preferably, the number of passes is 10 or more. In this case, the specific spinel particles can be divided and the number density of the specific spinel particles can be reduced. Therefore, in the intermediate cold rolling step according to this embodiment, the number of passes is more preferably 10 or more.

[0067] Preferably, in the intermediate cold rolling process, the rolling ratio of each pass is 30% or more. The rolling ratio of each pass in cold rolling refers to the average value of the rolling ratios in each pass. Furthermore, the rolling ratio of each pass refers to the ratio (%) of the thickness of the material after the pass to the thickness of the material before the pass. If the rolling ratio of each pass is 30% or more, the specific particles can be divided and the number density of the specific particles can be reduced. On the other hand, if the rolling ratio of each pass is too low, the number density of the specific particles in the produced stainless steel foil may be too high, making it impossible to reduce the formation of damage. Therefore, in the intermediate cold rolling process according to this embodiment, the rolling ratio of each pass is preferably 30% or more. Through the above steps, the intermediate steel material according to this embodiment is produced.

[0068] [Annealing step] In the annealing step, the produced intermediate steel material is subjected to an annealing treatment. As the annealing treatment, bright annealing is preferably performed. Bright annealing is an annealing treatment performed in an extremely low-oxygen atmosphere. The surface of the intermediate steel material that has been subjected to bright annealing is hardly oxidized, and the surface gloss can be maintained. The extremely low-oxygen atmosphere in bright annealing is H ２ Gas and N ２It is preferable to use a mixed gas atmosphere with N gas. ２ The fraction is not particularly limited, but is, for example, 35 to 65% by volume.

[0069] The intermediate cold rolling step and the annealing step may be alternately repeated multiple times. For example, cold rolling and annealing treatment may be repeated the same number of times as the number of passes in the intermediate cold rolling step. For example, one or more passes in the intermediate cold rolling step may be followed by one annealing treatment. That is, in this embodiment, since the number of passes in the intermediate cold rolling step is eight or more, the annealing step may also be performed eight or more times, the annealing step may be performed seven or less times, or the annealing step may be performed only once.

[0070] Here, in the intermediate cold rolling process, there is a concern that damage may be formed at the interface between the base material and the specific particles due to cold rolling. On the other hand, since the intermediate cold rolling process and the annealing process are repeatedly performed, the formed damage is repaired to some extent by the annealing treatment. Therefore, in the intermediate cold rolling process, the rolling ratio of each pass can be set high to reduce the number density of coarse particles, especially specific particles.

[0071] [Foil Rolling Step] In the foil rolling step, the intermediate steel material that has been subjected to the annealing treatment is subjected to cold rolling to produce a foil main body having a desired thickness.

[0072] Preferably, the foil rolling process involves four or more passes. As described above, the number of passes in cold rolling refers to the number of times a material passes through a cold rolling mill. Specifically, when cold rolling is performed with four passes, the intermediate steel material is cold rolled four times by the cold rolling mill. If the number of passes is four or more, the specific particles can be divided and the number density of the specific particles can be reduced. If the number of passes is four or more, coarse particles other than the specific particles can be divided and the ratio of the number of the specific particles to the coarse particles can be increased. On the other hand, if the number of passes is too small, the number density of the specific particles in the manufactured stainless steel foil may be too high, making it impossible to reduce the formation of damage. In this case, the ratio of the number of the specific particles to the coarse particles may not be sufficiently increased. Therefore, in the foil rolling process according to this embodiment, the number of passes is preferably four or more.

[0073] Preferably, in the foil rolling process, the rolling ratio of each pass is 20 to 30%. As described above, the rolling ratio of each pass in cold rolling means the average value of the rolling ratios in each pass. Furthermore, the rolling ratio of each pass means the ratio (%) of the thickness of the intermediate steel material after the pass to the thickness of the intermediate steel material before the pass. If the rolling ratio of each pass is 20% or more, the specific particles can be divided and the number density of the specific particles can be reduced. On the other hand, if the rolling ratio of each pass is too high, there is a concern that damage may be formed. Therefore, in the foil rolling process according to this embodiment, the rolling ratio of each pass is preferably 20 to 30%.

[0074] According to the above rolling conditions, soft inclusions other than the specific particles are stretched and broken in a manner that suppresses damage to the base material, and the number of coarse particles of 6.0 μm or more is extremely reduced, making it possible to produce a stainless steel foil in which the number ratio of specific particles to the coarse particles of this embodiment is 80% or more.

[0075] [Other Steps] The foil body according to this embodiment is manufactured by the above steps. The stainless steel foil according to this embodiment may consist of only the foil body. In this case, the stainless steel foil according to this embodiment can be manufactured by the above manufacturing steps. On the other hand, the stainless steel foil according to this embodiment may include components other than the foil body. As described above, for example, an insulating protective coating such as a resin coating may be formed on the surface of the foil body. In this case, for example, the resin coating may be formed by applying a composition that is the raw material of the resin coating to the surface of the manufactured foil body and then curing it by heat treatment. For example, a resin coating may further be formed on the surface of the manufactured foil body by vapor deposition. In other words, the stainless steel foil may be manufactured by performing a well-known surface treatment on the foil body manufactured by the above foil rolling step.

[0076] The stainless steel foil according to this embodiment is manufactured by the above manufacturing process. The 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 stainless steel foil according to this embodiment, and do not limit the present invention.

[0077] A material having each test number was prepared. The material had a chemical composition made of stainless steel equivalent to SUS430LX (steel type "A" in Table 1), SUS444 (steel type "B" in Table 1), or SUS304 (steel type "C" in Table 1) specified in JIS G 4305 (2015).

[0078]

[0079] The material of each test number was subjected to an intermediate cold rolling process under the conditions shown in Table 1 to produce an intermediate steel material of each test number. The intermediate steel material of each test number was subjected to bright annealing a total of four times between cold working processes. Furthermore, the intermediate steel material that had been subjected to the fourth bright annealing was subjected to a foil rolling process under the conditions shown in Table 1 to produce a stainless steel foil of each test number.

[0080] [Evaluation Tests] The produced stainless steel foils were subjected to a coarse particle observation test and a damage evaluation test. Note that in this example, none of the stainless steel foils used in each evaluation test were subjected to prior polishing.

[0081] [Coarse particle observation test] The stainless steel foils of each test number were evaluated using the automatic particle detection system (AZtec manufactured by Oxford) using the SEM and EDS analysis described above. SEM (JSM-IT500HR manufactured by JEOL Ltd.) was used to obtain images of inorganic particles (inclusions), and EDS (ULTIM MAX 65 manufactured by Oxford Instruments Ltd.) was used to analyze the inorganic particles. The measurement conditions and SEM and EDS settings used for the measurement are shown below. Measurement area per measurement: 20 mm x 50 mm Detector: BED-C backscattered electron detector Observation magnification: 80x Acceleration voltage: 20.0 kV Working distance (WD): 10.0 mm Exposure current: 80% Resolution: 2048 Dwell time (collection time per pixel): 3 μs Input signal: backscattered electron image (BSE) Number of frames (number of scans in the same field of view): 1 Lower limit of detected particle size: 6 pixels

[0082] In addition to the above settings, the brightness and contrast of the SEM were appropriately adjusted to identify detected objects (substances different from the parent phase that may be inclusions) with a circle equivalent diameter (diameter of a circle equivalent to an area of ​​2.22 μm or more) from the SEM image. EDS analysis was performed on each of the identified detected objects.

[0083] The elements detected in the EDS analysis were Cr, Fe, Ni, Mo, C, N, O, Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl. From the component analysis results for each detected object, detected objects in which Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl were detected at 1% by mass or more were defined as "particles." Based on the results of the EDS analysis, a database containing the size and component values ​​of each particle within the measurement area was created. From this database, coarse particles with a circle equivalent diameter of 6.0 μm or more were extracted. This series of operations was repeated with the field of view changed to suppress variations, until a total of 100 coarse particles having a circle equivalent diameter of 6.0 μm or more were detected, and analysis was performed using this database.

[0084] Among the coarse particles in the database, particles having the highest Al content, Mg content, or Ti content and further having an Fn1 of 0.3591 to 3.0943 were defined as "specific particles." Here, particles having the highest Al content, Mg content, or Ti content refer to particles having the highest Al content, Mg content, or Ti content among Al, Mg, Ti, Si, Ca, V, Mn, Cu, Nb, P, S, Na, Cl, K, Zn, Br, Ag, Ba, Lu, W, and Tl. Furthermore, among the specific particles, particles having an Fn2 of 0.05 or less were defined as "specific spinel particles."

[0085] The number density of the specific particles was calculated from the number of specific particles and the total area of ​​the observation field where EDS analysis was performed. Furthermore, the ratio of the number of specific particles to the number of coarse particles was calculated from the number of coarse particles and the number of specific particles. Furthermore, the number density of the specific spinel particles was calculated from the number of specific spinel particles and the total area of ​​the observation field where EDS analysis was performed. For each test number, the number density of the specific particles (particles / cm ２ ), the ratio (%) of the number of specific particles to the number of coarse particles, and the number density (particles / cm ２ ) are shown in Table 2.

[0086]

[0087] [Damage Evaluation Test] The stainless steel foils of each test number were observed with an SEM to evaluate the degree of damage. Specifically, a visual test to visually evaluate the surface of the stainless steel foil and a quantitative gouge test to evaluate the degree of large dents were performed. The specimens used for observation were selected from those with the specific particle number density values ​​closest to those of each test number shown in Table 2.

[0088] First, a visual inspection was performed. Specifically, the magnification of the SEM observation was set to 80x to 5000x, and different magnifications were used for the overall evaluation and the detailed evaluation. The presence or absence of damage was judged from the contrast based on defects such as gouges, scratches, and dislocations in the foil body. As a result, if the number of damages was large and the degree of damage was judged to be significant, the damage formation was evaluated as not reduced ("NA (Not Acceptable)" in Table 2). If the number of damages was small and the degree of damage was judged to be small, the damage formation was evaluated as reduced ("G (Good)" in Table 2). If the number of damages was very small and the degree of damage was judged to be minor, the damage formation was evaluated as further reduced ("E (Excellent)" in Table 2).

[0089] Next, a gouge quantification test was carried out. Here, gouges, which are particularly large depressions, are formed so as to surround coarse particles. Therefore, in this example, coarse particles were identified by the above-mentioned coarse particle observation test, and the gouges formed around the coarse particles were quantified. More specifically, of the coarse particles identified by the coarse particle observation test, five coarse particles with large equivalent circle diameters were identified. A surface profile was obtained using a three-dimensional SEM, with the identified coarse particles as the center. In this case, the area from which the surface profile was obtained was 47.6 μm long x 63.5 μm wide, and the depth was set to an appropriate depth. From the obtained surface profile, the area of ​​the gouge (μm ２The area of ​​the cavities was identified from the SEM image and the surface profile. The lowest position of the surface profile was defined as the bottom of the cavities. Furthermore, the position near the bottom of the cavities where the surface profile showed a maximum value was defined as the outer periphery of the cavities. For each test number, the same measurements were carried out on each of the five identified coarse particles to determine the area and depth of the cavities formed around each inclusion. For each test number, the maximum area of ​​the cavities (μm ２ ) and the average area (μm ２ ), maximum depth (μm), and average depth (μm) are shown in Table 3.

[0090]

[0091] [Evaluation Results] Referring to Tables 1 to 3, the stainless steel foils of test numbers 1 to 7 were manufactured by the above-mentioned preferred manufacturing method. As a result, these stainless steel foils had a specific particle number density of 40.0 particles / cm in the foil body made of stainless steel. ２ The ratio of specific particles to coarse particles was 80% or more. As a result, it was determined by visual inspection that these stainless steel foils had reduced damage formation. These stainless steel foils further had a maximum gouge area of ​​600 μm or less. ２ and the average area of ​​the gouge is 400 μm or less. ２ The results were as follows: the degree of gouging, which is a large depression, was reduced.

[0092] Furthermore, the stainless steel foils of test numbers 2 to 7 were subjected to 10 or more passes of the intermediate cold rolling process. As a result, it was found that the number density of the specific spinel particles in these stainless steel foils was 4.5 particles / cm ２ As a result, it was determined by visual inspection that these stainless steel foils had even less damage. These stainless steel foils also had a maximum gouge depth of 3.00 μm or less and an average gouge depth of 2.00 μm or less, further reducing gouges, which are large depressions.

[0093] On the other hand, the stainless steel foil of test number 8 had a rolling ratio that was too low in each pass in the intermediate cold rolling process. Furthermore, the number of passes in the foil rolling process was too small, and the rolling ratio in each pass was too low. As a result, the number density of the specific particles in this stainless steel foil was 40.0 particles / cm. ２ The ratio of specific particles to coarse particles was less than 80%. As a result, it was determined by visual inspection that the formation of damage was not reduced in this stainless steel foil. This stainless steel foil further had a maximum gouge area of ​​600 μm ２ and the average area of ​​the gouge is 400 μm ２ The degree of gouging, which is a large dent, was not reduced.

[0094] The stainless steel foil of test number 9 had a total rolling ratio that was too low in the intermediate cold rolling process, and a rolling ratio that was too low in each pass. As a result, the number density of the specific particles in this stainless steel foil was 40.0 particles / cm ２ As a result, it was determined by visual inspection that the formation of damage in this stainless steel foil was not reduced. The maximum area of ​​the gouge in this stainless steel foil was 600 μm. ２ and the average area of ​​the gouge is 400 μm ２ The degree of gouging, which is a large dent, was not reduced.

[0095] The stainless steel foil of test number 10 had too few passes in the intermediate cold rolling process. Furthermore, the number of passes in the foil rolling process was too few. As a result, the ratio of specific particles to coarse particles in this stainless steel foil was less than 80%. As a result, it was determined by visual inspection that the formation of damage in this stainless steel foil was not reduced. Furthermore, this stainless steel foil had a maximum gouge area of ​​600 μm. ２ and the average area of ​​the gouge is 400 μm ２ The degree of gouging, which is a large dent, was not reduced.

[0096] The stainless steel foil of test number 11 had too few passes in the intermediate cold rolling process. As a result, the number density of the specific particles in this stainless steel foil was 40.0 particles / cm ２As a result, it was determined by visual inspection that the formation of damage in this stainless steel foil was not reduced. The maximum area of ​​the gouge in this stainless steel foil was 600 μm. ２ and the average area of ​​the gouge is 400 μm ２ The degree of gouging, which is a large dent, was not reduced.

[0097] The stainless steel foil of test number 12 had too few passes in the foil rolling process. As a result, the number density of the specific particles in this stainless steel foil was 40.0 particles / cm ２ The ratio of specific particles to coarse particles was less than 80%. As a result, it was determined by visual inspection that the formation of damage was not reduced in this stainless steel foil. This stainless steel foil further had a maximum gouge area of ​​600 μm ２ and the average area of ​​the gouge is 400 μm ２ The degree of gouging, which is a large dent, was not reduced.

[0098] The stainless steel foil of test number 13 had a too low rolling ratio for each pass in the intermediate cold rolling process. Furthermore, the number of passes in the foil rolling process was too small. As a result, the ratio of specific particles to coarse particles in this stainless steel foil was less than 80%. As a result, it was determined by visual inspection that the formation of damage in this stainless steel foil was not reduced. Furthermore, this stainless steel foil had a maximum gouge area of ​​600 μm. ２ and the average area of ​​the gouge is 400 μm ２ The degree of gouging, which is a large dent, was not reduced.

[0099] In Example 2, the relationship between the number density of specific particles, the ratio of the number of specific particles to the number of coarse particles, and the corrosion resistance of stainless steel foil was investigated. Specifically, the corrosion resistance was investigated using stainless steel foils of test numbers 2, 4, 8, and 10 in Example 1. The steel type of the stainless steel foil of each test number and the number density of specific particles (particles / cm ２ ), the ratio (%) of the number of specific particles to the number of coarse particles, and the evaluation results of damage are shown in Table 4.

[0100]

[0101] [Corrosion Resistance Evaluation Test] Corrosion resistance was evaluated by cyclic voltammetry (CV) assuming application to a current collector for an all-solid-state secondary battery. First, a test piece with a diameter of 11 mm was taken from the main body of the stainless steel foil. One surface of the test piece was used as the evaluation surface. The developed area ratio Sdr, as defined in ISO 25178-2:2012, was measured on the evaluation surface of the test piece. The developed area ratio Sdr is an index representing the rate of increase in the surface area of ​​the measured surface compared to a flat surface. Since the corrosion reaction begins on the surface of the stainless steel foil and proceeds, the greater the developed area ratio Sdr of the test piece, the faster the corrosion progresses.

[0102] The developed area ratio Sdr of the evaluation surface of the test piece was measured using a laser microscope. The laser microscope used was a VK-X100 manufactured by Keyence Corporation. The observation magnification was set to 2000x, and a measurement area of ​​280 μm x 200 μm was measured. Specifically, four locations within the measurement area were measured with an observation field of 145 μm x 109 μm so that the entire range within the measurement area could be measured, and a surface image was obtained. The four obtained surface images were linked using the attached image linking software to obtain a surface image of a measurement area of ​​280 μm x 200 μm. From the surface shape profile obtained by analyzing the surface image of the measurement area, a reference plane was set by cutting 0.3% of the area from the low brightness side, and a three-dimensional image was generated. At this time, filtering was performed using a high-pass filter with a cutoff wavelength of 50 μm. A Gaussian filter was used for the filtering method. The developed area ratio Sdr defined in ISO25178-2:2012 was determined in a 200 μm × 150 μm region of the obtained three-dimensional image.

[0103] Using the test pieces with the respective test numbers for which the developed area ratio Sdr was measured, evaluation cells simulating sulfide-based all-solid-state batteries were fabricated. Specifically, 0.12 g of the sulfide-based solid electrolyte was placed in a mold with an inner diameter of 11 mm, and then pressurized at 300 MPa to form a cylindrical sulfide-based solid electrolyte pellet with a thickness of 700 μm and a bottom diameter of 11 mm. The sulfide-based solid electrolyte contained β-Li ３ P.S. ４The prepared pellet was placed on the evaluation surface of the test piece of each test number. Furthermore, a cylindrical metal Li foil having a thickness of 1.2 mm and a bottom diameter of 11 mm was placed on top of the sulfide-based solid electrolyte pellet. These were placed in a measurement cell with an inner diameter of 11 mm that could block the outside air, and a pressure of 5 MPa was applied by tightening bolts. The measurement cell was then sealed. Note that the process from preparing the sulfide-based solid electrolyte pellet to sealing the evaluation cell was carried out in a glove box in an Ar atmosphere. Through the above process, an evaluation cell of each test number was prepared.

[0104] Cyclic voltammetry (CV) measurements were performed using the evaluation cells of each test number. In the CV measurements, the test piece was used as the working electrode, and metal Li foil was used as the counter electrode and reference electrode. Specifically, the voltage was measured for the evaluation cell held at 60 ° C for 24 hours as an open-circuit voltage V ＯＣ After sweeping from (V) to 3.0 V, it was swept to 0.005 V, and then the open circuit voltage V ＯＣ (V), and the current density (μA / cm ２ The open-circuit voltage V ＯＣ was 2.0 to 2.5 V for all evaluation cells of all test numbers.

[0105] Based on the measurement results, the open circuit voltage V ＯＣ The current density (μA / cm) flowing during the sweep from 0.0 V to 3.0 V ２ ) was integrated by the time (seconds) required for the sweep. Furthermore, the integrated value was divided by the developed area ratio Sdr to determine a value in order to eliminate the influence of minute irregularities on the evaluation surface of the test piece. The obtained value was used as the integrated current value C (μC / cm ２ ) and was used as an index representing the progress of the corrosion reaction in the test piece. ２ ) is the open circuit voltage V ＯＣ The time elapsed from the start of the sweep from (V) to 3.0 V is defined as the test time t (seconds), and the time when 3.0 V is reached is defined as t ３．０ (seconds), and the current density flowing through the evaluation cell at test time t (seconds) is I(t) (μA / cm ２ ) and the developed area ratio of the test piece is Sdr, it can be calculated by the following formula.

[0106]

[0107] The obtained integrated current value was 1000 μC / cm ２ In the following cases, the test number was evaluated as having excellent corrosion resistance ("E (Excellent)" in the corrosion resistance column in Table 4): ２ If the value exceeded 0.05, the specimen with that test number was evaluated as not having excellent corrosion resistance (in Table 4, the corrosion resistance column is marked "NA (Not Acceptable)").

[0108] [Evaluation Results] Referring to Table 4, the stainless steel foils of test numbers 2 and 4 had a specific particle number density of 40.0 particles / cm in the foil body made of stainless steel. ２ The ratio of the number of specific particles to the number of coarse particles was 80% or more. As a result, these stainless steel foils had reduced damage formation and excellent corrosion resistance.

[0109] On the other hand, the stainless steel foil of test number 8 has a specific particle number density of 40.0 particles / cm in the foil body made of stainless steel. ２ The number ratio of specific particles to coarse particles was less than 80%. As a result, this stainless steel foil did not reduce the formation of damage, and furthermore, did not have excellent corrosion resistance.

[0110] In the stainless steel foil of test number 10, the ratio of specific particles to coarse particles in the stainless steel foil body was less than 80%. As a result, this stainless steel foil did not reduce the formation of damage, and furthermore, did not have excellent corrosion resistance.

[0111] 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 foil body made of stainless steel, wherein particles having an equivalent circle diameter of 6.0 μm or more on any surface of the foil body are defined as coarse particles, and among the coarse particles, particles having the highest Al content, Mg content, or Ti content among the element contents contained in the particles, and the Al content, the Mg content, and the Ti content satisfying formula (1) are defined as specific particles, the number density of the specific particles is 40.0 particles / cm ２ A stainless steel foil having a ratio of the number of the specific particles to the number of the coarse particles of 80% or more, wherein: 0.3591≦4.5579Al / (Al+Mg+Ti)−4.5579Mg / (Al+Mg+Ti)+1.7321Ti / (Al+Mg+Ti)≦3.0943 (1) Here, the element symbols in formula (1) are substituted with the Al content, Mg content, and Ti content contained in the coarse particles, expressed in mass%. When a corresponding element is not contained in the coarse particles, “0” is substituted for that element symbol.

2. A stainless steel foil according to claim 1, wherein, in the foil main body, among the specific particles, when particles whose Ti content satisfies formula (2) in the element content contained in the particles are defined as specific spinel particles, the number density of the specific spinel particles is 4.5 particles / cm ２ A stainless steel foil having a Ti / (Al+Mg+Ti)≦0.05 (2) where the element symbols in formula (2) are substituted with the Al content, Mg content, and Ti content, expressed in mass%, contained in the coarse particles. When a corresponding element is not contained in the coarse particles, "0" is substituted for the element symbol.

3. A stainless steel foil according to claim 1, wherein the chemical composition of the foil body contains, in mass %, Ti: 0.10 to 1.00%.

4. A stainless steel foil according to claim 2, wherein the chemical composition of the foil body contains, in mass %, Ti: 0.10 to 1.00%.

5. A stainless steel foil according to any one of claims 1 to 4, wherein the foil body is made of ferritic stainless steel or austenitic stainless steel.

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