Stainless steel foil

Abstract

The present invention provides a stainless steel foil which has excellent durability with respect to bending stress. A stainless steel foil according to the present disclosure comprises a stainless steel base material and a siloxane polymer film that is formed on at least one surface of the stainless steel base material. The siloxane polymer film includes a first region that is formed on the surface of the stainless steel base material and a second region that is formed on a surface of the first region. The first region contains 5-30% Si and 5-45% Fe, in terms of at%.

Description

Stainless steel foil

[0001] The present disclosure relates to stainless steel foil, and more particularly to stainless steel foil applicable as a substrate material for electronic devices.

[0002] Until now, resins and glass have often been used as substrate materials for electronic devices. In recent years, attention has been focused on metal materials, which have better heat resistance and gas barrier properties than resins and better toughness than glass. On the other hand, materials for electronic device substrates are also required to have flatness and insulating properties. For this reason, metal foils with insulating layers have been proposed as substrate materials for electronic devices.

[0003] Specifically, Japanese Patent Laid-Open Publication No. 2013-236029 (Patent Document 1) proposes a semiconductor device substrate having an alkali metal silicate layer formed on a metal substrate by a liquid-phase method. This semiconductor device substrate uses a clad material in which one or both sides of an aluminum, stainless steel, or steel plate are integrated with an aluminum plate, and the water contact angle of the alkali metal silicate layer surface is 20 to 90°. Patent Document 1 discloses that this semiconductor device substrate suppresses the formation of fine precipitates in the alkali metal silicate layer, ensuring water resistance.

[0004] Furthermore, Japanese Patent Laid-Open Publication No. 2003-247078 (Patent Document 2) proposes a stainless steel foil coated on one or both sides with an inorganic-organic hybrid film having a skeleton of an inorganic three-dimensional network structure mainly composed of siloxane bonds, in which at least one of the bridging oxygen atoms in the skeleton has been substituted with an organic group and / or a hydrogen atom. Patent Document 2 discloses that this stainless steel foil has excellent heat resistance, processability, flatness, insulation properties, etc., and is flexible when used as a substrate for electronic materials.

[0005] JP 2013-236029 A JP 2003-247078 A

[0006] However, with the development of electronic devices, there is a demand for materials that can be used in harsher environments than ever before. Specifically, the development of deformable thin electronic devices, such as deformable displays, deformable transistors, deformable batteries, and deformable sensors, has been progressing. Such deformable thin electronic devices are repeatedly subjected to bending stress during use. Therefore, it is preferable that deformable thin electronic devices are less likely to crack even when subjected to repeated bending stress.

[0007] Among metal foils, stainless steel foil has excellent durability against bending stress. In this specification, "steel foil" means a steel plate having a thickness of 150 μm or less. In this specification, the term "excellent durability against bending stress" also refers to a material that is less susceptible to cracking even when subjected to repeated bending stress. In other words, there has been a demand for stainless steel foil having an insulating layer and excellent durability against bending stress.

[0008] The above-mentioned Patent Documents 1 and 2 propose metal foils having an insulating layer formed thereon that can be used as substrates for electronic devices, but do not consider increasing durability against bending stress to the extent that they can be used in deformable thin electronic devices.

[0009] An object of the present disclosure is to provide a stainless steel foil that has excellent resistance to bending stress.

[0010] The stainless steel foil according to the present disclosure comprises a stainless steel substrate and a siloxane polymer film formed on at least one surface of the stainless steel substrate, wherein the siloxane polymer film includes a first region formed on the surface of the stainless steel substrate and a second region formed on the surface of the first region, and the first region contains, in atomic %, 5 to 30% Si and 5 to 45% Fe.

[0011] The stainless steel foil according to the present disclosure has excellent resistance to bending stress.

[0012] The present inventors have conducted extensive research into stainless steel foils that have excellent durability against bending stress, and have made the following discoveries.

[0013] First, the inventors investigated an insulating layer to be formed on a stainless steel substrate. Specifically, the inventors discovered that forming a siloxane polymer film on a stainless steel substrate can impart not only insulating properties but also heat resistance and flatness to the stainless steel foil. Furthermore, in deformable, thin electronic devices, an insulating layer may be formed on only one side of the substrate. Therefore, the stainless steel foil according to this embodiment forms a siloxane polymer film on at least one surface of the stainless steel substrate. Here, a siloxane polymer film refers to a film composed of a polymer whose main skeleton is a siloxane bond.

[0014] On the other hand, even stainless steel foils with a siloxane polymer film formed thereon sometimes do not have excellent durability against bending stress. Therefore, the present inventors have investigated various methods for improving the durability against bending stress of stainless steel foils with a siloxane polymer film formed thereon. As a result, the following findings have been obtained.

[0015] The present inventors specifically focused on the adhesion between a siloxane polymer film and a stainless steel substrate. Here, the siloxane polymer film is a coating whose main skeleton is a siloxane bond (Si—O—Si). Meanwhile, a passive film of chromium oxyhydroxide is formed on the outermost surface of a stainless steel substrate. It has been previously believed that the siloxane polymer film and the Cr element in the chromium oxyhydroxide form Cr—O—Si bonds, enhancing the adhesion of the siloxane polymer film. As a result, it has been thought that stainless steel foils on which a siloxane polymer film is formed may have durability against bending stress.

[0016] Therefore, the present inventors thought that if the adhesion between the siloxane polymer film and the stainless steel substrate could be further improved, the durability of the stainless steel foil against bending stress would be further improved. As a result of detailed studies by the present inventors, it was revealed that if a mixed layer containing Fe atoms could be formed in the siloxane polymer film, the durability of the stainless steel foil against bending stress could be improved. Specifically, as a result of further detailed studies by the present inventors, it was revealed that if a region containing 5 to 30 atomic % of silicon (Si) and 5 to 45 atomic % of iron (Fe) is formed in the siloxane polymer film, the occurrence of cracks in the siloxane polymer film can be suppressed.

[0017] More specifically, in this specification, a region of the siloxane polymer film containing, in atomic percent, 5 to 30% Si and 5 to 45% Fe is also referred to as the "first region." The first region is formed at the interface of the siloxane polymer film with the stainless steel substrate. In other words, the first region is formed on the surface of the stainless steel substrate. Furthermore, in this specification, a region of the siloxane polymer film formed on the surface of the first region is also referred to as the "second region." In short, both the first region and the second region are included in the siloxane polymer film, with the first region formed on the surface of the stainless steel substrate and the second region formed on the surface of the first region. As a result, the stainless steel foil according to this embodiment is able to suppress cracking in the siloxane polymer film and has excellent durability against bending stress.

[0018] The details of why forming a first region on the surface of a stainless steel substrate and forming a second region on the surface of the first region improve the durability of the stainless steel foil against bending stress are not clear. However, the examples described below demonstrate that forming a first region containing, in atomic percent, 5 to 30% Si and 5 to 45% Fe on the surface of a stainless steel substrate and forming a second region on the surface of the first region improves the durability of the stainless steel foil against bending stress.

[0019] The stainless steel foil according to this embodiment, which was completed based on the above findings, has the following features.

[0020] [1] A stainless steel foil comprising: a stainless steel substrate; and a siloxane polymer film formed on at least one surface of the stainless steel substrate, wherein the siloxane polymer film includes a first region formed on the surface of the stainless steel substrate; and a second region formed on the surface of the first region, and the first region contains, in atomic %, 5 to 30% Si and 5 to 45% Fe.

[0021] [2] The stainless steel foil according to [1], wherein the thickness of the first region is 2 to 100 nm.

[0022] [3] The stainless steel foil according to [1], wherein the siloxane polymer film has a thickness of 0.3 to 5.0 μm.

[0023] [4] The stainless steel foil according to [2], wherein the siloxane polymer film has a thickness of 0.3 to 5.0 μm.

[0024] [5] The stainless steel foil according to any one of [1] to [4], wherein the stainless steel substrate has a thickness of 5 to 80 μm.

[0025] The stainless steel foil according to this embodiment will be described below.

[0026] [Stainless Steel Foil] The stainless steel foil according to this embodiment comprises a stainless steel substrate and a siloxane polymer film formed on at least one side of the stainless steel substrate. Here, the stainless steel foil according to this embodiment may include a configuration other than the stainless steel substrate and the siloxane polymer film. For example, another coating may be formed on the siloxane polymer film. For example, another coating may be formed on the surface of the stainless steel substrate on which the siloxane polymer film is not formed. The other coating may be, for example, a resin coating or a coating mainly composed of an inorganic compound.

[0027] [Stainless Steel Substrate] The stainless steel substrate according to this embodiment may be any substrate made of stainless steel. In other words, in this embodiment, the type of stainless steel constituting the stainless steel substrate 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.

[0028] In this embodiment, the thickness of the stainless steel substrate is not particularly limited, but may be, for example, 5 to 80 μm. As described above, in this specification, "steel foil" refers to a steel sheet having a thickness of 150 μm or less. The stainless steel foil according to this embodiment, that is, the thickness of the stainless steel foil according to this embodiment, is 150 μm or less. As described above, the stainless steel foil according to this embodiment comprises at least a stainless steel substrate and a siloxane polymer film. Therefore, if the thickness of the stainless steel substrate is 5 to 80 μm, the thickness of the stainless steel foil can be stably 150 μm or less.

[0029] Preferably, the thickness of the stainless steel substrate according to this embodiment is 5 to 60 μm, more preferably 5 to 50 μm, even more preferably 5 to 35 μm, and even more preferably 5 to 20 μm. If the thickness of the stainless steel substrate is 5 μm or more, a stainless steel substrate having a stable thickness can be stably produced. On the other hand, if the thickness of the stainless steel substrate is 20 μm or less, the rigidity of the stainless steel substrate decreases, and the deformation resistance of the stainless steel foil can be reduced. Therefore, in this embodiment, the thickness of the stainless steel substrate is preferably 5 to 20 μm. In this case, stable production and ease of deformation can both be achieved.

[0030] [Siloxane Polymer Film] The stainless steel foil according to this embodiment includes a siloxane polymer film formed on at least one side of a stainless steel substrate. The siloxane polymer film has excellent insulating properties, excellent heat resistance, excellent chemical stability, and excellent flatness. Therefore, by forming a siloxane polymer film on the surface of the stainless steel substrate, the stainless steel foil can be endowed with the performance required for a substrate of a deformable, thin electronic device.

[0031] In this embodiment, the siloxane polymer film may be formed on both sides of the stainless steel substrate, or on only one side. Furthermore, in this specification, the term "siloxane polymer film" refers to a coating having siloxane bonds developed into a three-dimensional network structure as the main skeleton. In other words, in this embodiment, the siloxane polymer film may have a portion of the siloxane bonds, which form the main skeleton, substituted with functional groups (organic groups) or other atoms.

[0032] In this embodiment, the siloxane polymer film is preferably composed of a polymer in which a portion of the bridging oxygen in the siloxane skeleton is substituted with an organic group. In other words, the oxygen concentration [O] (mol / L) and the silicon concentration [Si] (mol / L) in the siloxane polymer film preferably satisfy the relationship 2.5<[O] / [Si]<3.9. In this case, the toughness of the siloxane polymer film is increased, and its durability against bending stress is further improved. Furthermore, the "organic group" in the siloxane polymer film is one or more selected from an alkyl group, a phenyl group, and an amino group.

[0033] In this embodiment, the thickness of the siloxane polymer film is not particularly limited, but is, for example, 0.3 to 5.0 μm. Here, in the stainless steel foil according to this embodiment, the siloxane polymer film includes a first region and a second region. In other words, the thickness of the siloxane polymer film means the sum of the thicknesses of the first region and the second region. Furthermore, the thickness of the siloxane polymer film here means the thickness of the siloxane polymer film formed on one side of the stainless steel substrate. In other words, when siloxane polymer films are formed on both sides of the stainless steel substrate, the total thickness may be 0.6 to 10.0 μm.

[0034] If the thickness of the siloxane polymer film is less than 0.3 μm, the above-mentioned insulating properties, heat resistance, and flatness may not be sufficiently obtained. On the other hand, if the thickness of the siloxane polymer film exceeds 5.0 μm, the toughness of the siloxane polymer film may decrease, and durability against bending stress may not be obtained. More preferably, the thickness of the siloxane polymer film is 0.5 to 3.0 μm. In this case, the stainless steel foil has excellent insulating properties.

[0035] In this embodiment, the thickness of the siloxane polymer film is measured using a scanning electron microscope (SEM). A cross section is formed on the stainless steel foil on which the siloxane polymer film is formed using a cross-section polisher method. The formed cross section is observed using the SEM. The thickness of the siloxane polymer film is measured from the obtained observation image.

[0036] [First Region] The siloxane polymer film according to this embodiment includes a first region formed on the surface of the stainless steel substrate and a second region formed on the surface of the first region. The first region according to this embodiment contains, in atomic percent, 5 to 30% Si and 5 to 45% Fe. In this embodiment, the first region also contains elements other than Si and Fe. For example, the first region may also contain O, Cr, C, N, etc. The content of elements other than Si and Fe in the first region is not particularly limited.

[0037] The Fe contained in the first region may be contained by any method. For example, the first region may be formed by diffusing Fe from the stainless steel substrate into the siloxane polymer film. When Fe diffuses from the stainless steel substrate into the siloxane polymer film, Cr contained in the stainless steel substrate may also diffuse. In other words, Cr may be contained in a portion of the interface with the stainless steel substrate in the first region. Even in this case, the effect of increasing the durability of the stainless steel foil against bending stress, which is achieved by having the first region, is not lost.

[0038] As described above, it is believed that the siloxane polymer film and the Cr element in the chromium oxyhydroxide form Cr—O—Si bonds. Meanwhile, the stainless steel foil according to this embodiment has a first region formed in the siloxane polymer film containing 5 to 45 atomic % of Fe. The inventors speculate that this further enhances the adhesion of the siloxane polymer film to the stainless steel substrate, further increasing its durability against bending stress.

[0039] In this embodiment, the first region only needs to be formed to a certain extent, and the thickness of the first region is not particularly limited. However, the thickness of the first region is 2 to 100 nm to more effectively obtain the above-mentioned effect. If the thickness of the first region is large, the durability of the stainless steel foil against repeated bending is further increased. Therefore, the preferred lower limit of the thickness of the first region is 3 nm, more preferably 5 nm, even more preferably 8 nm, and even more preferably 10 nm. On the other hand, if the thickness of the first region is too large, the color of the siloxane polymer film may change. Therefore, the preferred upper limit of the thickness of the first region is 80 nm.

[0040] The thickness of the first region can be determined by the following method. The stainless steel foil according to this embodiment is cut to obtain an observation surface including the thickness direction of the siloxane polymer film. Specifically, a test piece is prepared by a microsampling method using a focused ion beam (FIB) device. The size of the test piece is, for example, 10 μm on a side and 100 nm thick. For example, an NB5000 manufactured by Hitachi High-Tech Corporation can be used as the FIB device.

[0041] The observation surface of the prepared test piece is observed using a transmission electron microscope (TEM), and five points are identified at the interface between the stainless steel substrate and the siloxane polymer film as measurement points. The five measurement points are not particularly limited, but are identified so that each measurement point is spaced at least 2 μm apart. For example, a spherical aberration corrected transmission electron microscope (CS-TEM) NOEARAM manufactured by JEOL Ltd. can be used as the TEM. The accelerating voltage for TEM observation is 200 kV. The interface between the stainless steel substrate and the siloxane polymer film can be identified from the contrast.

[0042] Rectangular continuous analysis is performed in the vicinity of the identified measurement point by energy dispersive X-ray spectrometry (EDS). Rectangular continuous analysis means arranging a plurality of rectangular analysis regions side by side and continuously performing elemental analysis on each rectangular analysis region. Specifically, a rectangular analysis region measuring 1 nm in the vertical direction and 10 nm in the horizontal direction relative to the surface of the stainless steel substrate is identified in the observation field of the identified measurement point. The rectangular analysis regions are arranged vertically so as to include the siloxane polymer film from the stainless steel substrate. The number of rectangular analysis regions is not particularly limited, but may be, for example, 20.

[0043] Elemental analysis is performed by focusing an electron beam to approximately 0.1 nmφ on a rectangular analysis region and integrating the characteristic X-ray spectral intensity generated when scanning the measurement area. To perform elemental analysis continuously on multiple rectangular analysis regions, measurements are performed while performing image-based drift correction. The target elements are quantified as Si, O, Fe, Cr, Ni, Cu, Mn, and Mo. C is excluded from the target elements due to its poor quantitation and concerns about the influence of contamination. From the obtained elemental analysis results, the Si content (atomic %) and Fe content (atomic %) are calculated. Rectangular analysis regions that satisfy the following results are identified as belonging to the first region: Si: 5-30% and Fe: 5-45%. The thickness (nm) of the first region at that measurement point is defined based on the number of rectangular analysis regions belonging to the first region. Similarly, the thickness (nm) of the first region is measured at five measurement points. The arithmetic mean value of the thicknesses of the first region obtained at the five measurement points is defined as the thickness (nm) of the first region. If the first region is thick and the thickness of the first region cannot be accurately measured within the observation field, multiple observation fields are combined to determine the thickness of the first region. In this case, the thickness of the first region is determined by arranging multiple rectangular analysis regions using multiple observation fields. Those skilled in the art can determine the thickness of the first region from multiple observation fields.

[0044] [Second Region] The siloxane polymer film according to this embodiment includes a first region formed on the surface of the stainless steel substrate and a second region formed on the surface of the first region. As described above, the first region according to this embodiment contains, in atomic percent, 5 to 30% Si and 5 to 45% Fe. That is, in this embodiment, the second region refers to the region of the siloxane polymer film other than the first region. Specifically, the second region according to this embodiment is, in atomic percent, a region of 5% or more Si and 0 to less than 5% Fe, or a region of 30% or more Si and 45% or less Fe. In the second region, the Fe content may be 0%.

[0045] Like the first region, the second region may also contain elements other than Si and Fe. Specifically, the second region may also contain O, C, N, etc. The amount of elements other than Si and Fe contained in the second region is not particularly limited.

[0046] In this embodiment, the siloxane polymer film is identified by the following method. Crystal structure analysis is performed by selected-area electron diffraction using a TEM. For example, a 200 kV-field emission transmission electron microscope (JEM-2100F) manufactured by JEOL Ltd. can be used as the TEM. In the selected-area electron diffraction, a selected-area aperture is inserted, the observation field is set to 3 nm, and a diffraction pattern is obtained by electron beam diffraction. Furthermore, elemental analysis of the selected-area area is performed by EDS to identify the elements present within the field of view. If a siloxane polymer film is included at the measurement point, a pattern specific to amorphous materials (a so-called "halo pattern") is confirmed in the obtained diffraction pattern. Furthermore, Si and O are detected by elemental analysis of the selected-area area by EDS at the above-mentioned measurement point. Based on these results, the presence of a siloxane polymer film can be identified.

[0047] In this embodiment, the organic groups in the siloxane polymer film are further identified by the following method. Specifically, an absorption spectrum is obtained by Fourier transform infrared spectroscopy (FT-IR). For FT-IR, for example, an infrared spectrophotometer: Frontier manufactured by PerkinElmer Japan LLC can be used. More specifically, an infrared absorption spectrum is obtained by total reflection measurement (ATR method: Attenuated Total Reflection method) for the surface of the siloxane polymer film. As a result, for example, an absorption spectrum at 1263 cm -1 If an absorption peak of Si-CH is confirmed in the vicinity, it can be determined that the siloxane polymer film contains a methyl group as an organic group. -1If an absorption peak of Si-C6H5 is confirmed nearby, it can be determined that the siloxane polymer film contains a phenyl group as an organic group. In this way, it is naturally possible for a person skilled in the art to identify organic groups from the infrared absorption spectrum of the surface of a siloxane polymer film.

[0048] [Durability Against Bending Stress] The stainless steel foil according to this embodiment comprises a stainless steel substrate and a siloxane polymer film formed on at least one side of the stainless steel substrate, the siloxane polymer film including a first region formed on the surface of the stainless steel substrate and a second region formed on the surface of the first region, the first region containing, in atomic %, 5 to 30% Si and 5 to 45% Fe. As a result, the stainless steel foil according to this embodiment has excellent durability against bending stress. In this embodiment, excellent durability against bending stress is defined as follows:

[0049] A repeated bending test is performed on the stainless steel foil according to this embodiment. Specifically, a test specimen is prepared from the stainless steel foil according to this embodiment. The size of the test specimen is 50 mm x 100 mm. The prepared test specimen is placed in a clamshell-type reverse bending tester. For example, a clamshell-type reverse bending tester, model DR11MR manufactured by Yuasa System Co., Ltd., can be used. The test conditions are as follows: the bending radius of the stainless steel foil is set so that R / t = 150, where R is the bending radius of the stainless steel foil and t is the thickness of the stainless steel foil. The bending angle is set to 0 to 180°. Here, R / t is a dimensionless quantity with the same units for the numerator and denominator, and is an index that represents the severity of bending taking into account the stress and strain experienced by the material. Even if R is the same, the stress and strain experienced by the material increase as the thickness t of the material increases.

[0050] The number of repeated bending cycles under the above conditions until cracks appear on the siloxane polymer film surface is defined as the number of bending cycles. If the number of bending cycles defined above is 5,000 or more, it is judged to have excellent durability against bending stress. Furthermore, if the number of bending cycles is 20,000 or more, it is judged to have even better durability against bending stress.

[0051] [Method for manufacturing stainless steel foil] An example of a method for manufacturing stainless steel foil according to this embodiment will be described. The manufacturing method described below is one example for manufacturing stainless steel foil according to this embodiment, and the manufacturing method for stainless steel foil according to this embodiment may be 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 stainless steel foil according to this embodiment. The method for manufacturing stainless steel foil according to this embodiment includes a substrate preparation step, a coating step, a diffusion preparation step, and a heat treatment step.

[0052] [Substrate Preparation Step] In the substrate preparation step, a stainless steel substrate that is the substrate for the stainless steel foil is prepared. The method for preparing the stainless steel substrate is not particularly limited and may be a well-known method. For example, the method for preparing the stainless steel substrate may involve preparing an intermediate steel material having a desired chemical composition and cold working the intermediate steel material. This case will be specifically described below.

[0053] The intermediate steel material having the desired chemical composition can be appropriately set depending on the mechanical properties of the stainless steel substrate to be obtained. Here, the intermediate steel material refers to a stainless steel plate having a thickness of several hundred μm to several mm. When preparing the stainless steel substrate by cold working the intermediate steel material, the preferred cold working method is cold rolling. Cold rolling can be performed using well-known equipment. For example, a multiple reversing cold rolling mill may be used. In this case, the degree of reduction in cold rolling is not particularly limited. The degree of reduction can be appropriately set depending on the thickness of the stainless steel substrate to be obtained. Furthermore, the stainless steel substrate after cold rolling may be appropriately heat treated.

[0054] As described above, in the substrate preparation step, a stainless steel substrate is prepared. The stainless steel substrate may be manufactured by the preferred process described above, or a stainless steel substrate manufactured by a third party, or a stainless steel substrate manufactured in a factory or business establishment other than the factory where the coating process described below is carried out, may be prepared. In short, the substrate preparation step in this embodiment is not particularly limited and may be a well-known method.

[0055] [Coating Step] In the coating step, a siloxane polymer film composition is applied to the prepared stainless steel substrate. The siloxane polymer film composition can be prepared according to the siloxane polymer film to be obtained. For example, it may be prepared by diluting one or more organoalkoxysilanes with an organic solvent and carrying out hydrolysis. Here, the organoalkoxysilane is a compound represented by the general formula R n Si(OR′) 4-n (R and R' are hydrogen atoms and / or organic groups, and n is a natural number of 3 or less).

[0056] When preparing using the above-described method, the organoalkoxysilane can be selected according to the siloxane polymer film to be obtained. For example, one or more selected from diethoxydimethylsilane, dimethoxydimethylsilane, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, isobutyltrimethoxysilane, isobutyltriethoxysilane, dimethoxymethyl-3,3,3-trifluoropropylsilane, diisobutyldimethoxysilane, trimethylmethoxysilane, dimethoxydimethylsilane, diethoxydimethylsilane, phenyltriethoxysilane, methacryloxypropyltrimethoxysilane, methacryloxypropyltriethoxysilane, aminopropyltriethoxysilane, and aminoethylaminopropyltriethoxysilane can be used. Preferably, the organoalkoxysilane contains a methyl group, an ethyl group, or a phenyl group as the organic group. In this case, the thermal decomposition temperature of the organoalkoxysilane is increased, making it easier to control the amount of organic groups remaining after the heat treatment process.

[0057] The method for applying the composition is not particularly limited and may be any well-known method. For example, the composition may be applied using a bar coater, a roll coater, a spray method, a dip coating method, or a spin coating method. In this case, an appropriate solvent may be selected and used depending on the application method. Furthermore, by appropriately adjusting the amount of coating in the application step, the thickness of the siloxane polymer film on the manufactured stainless steel foil can be appropriately adjusted.

[0058] In the coating process according to this embodiment, the coated composition is further dried. Specifically, it is preferable to dry the composition by heating at a low temperature of about 100 to 180°C in the atmosphere. The drying time is not particularly limited, but is, for example, 0.5 to 30 minutes. Through the above process, the composition is coated on the prepared stainless steel substrate and dried.

[0059] [Diffusion Preparation Step] In the diffusion preparation step, the stainless steel substrate coated with the composition is subjected to a heat treatment for preparation for diffusion. "Diffusion" here refers to diffusing Fe in the stainless steel substrate into the composition. Specifically, in the diffusion preparation step, the stainless steel substrate coated with the composition is maintained at a temperature of 40 to 80°C, a humidity of 60 to 100%, and a time of 10 to 30 days. Maintaining the substrate in a humid environment may cause Cr ions in the passive film (chromium oxyhydroxide) formed on the surface of the stainless steel substrate to form a chelate with the residual acid catalyst in the coated composition. As a result, the passive film may be destroyed, potentially making it easier for Fe ions to leach out of the stainless steel substrate.

[0060] In other words, by making it easier for Fe to diffuse in the diffusion preparation step, the first region can be stably formed in the heat treatment step described below. Therefore, in this embodiment, the diffusion preparation step preferably involves maintaining the stainless steel substrate to which the composition has been applied at a temperature of 40 to 80°C, a humidity of 60 to 100%, and a time of 10 to 30 days.

[0061] [Heat Treatment Step] In the heat treatment step, the stainless steel substrate coated with the composition is heat treated to cure the siloxane polymer film. In the heat treatment step according to this embodiment, the substrate is preferably maintained at 350 to 500°C for 5 to 300 minutes. In the heat treatment step, the siloxane polymer film is cured and the first region is formed at the same time.

[0062] As described above, the stainless steel substrate that has undergone the diffusion preparation process is prone to elution of Fe ions from the passive film. Therefore, during the heat treatment process, the Fe ions eluted from the passive film diffuse into the siloxane polymer film. Based on the above speculation, it is believed that the first region is formed in the siloxane polymer film during the heat treatment process. Therefore, the first region tends to be thicker when the heat treatment temperature is higher and the heat treatment time is longer.

[0063] The stainless steel foil according to this embodiment can be manufactured by the above steps. As mentioned above, the manufacturing steps described above are a preferred example for manufacturing the stainless steel foil according to this embodiment, and the manufacturing method of the stainless steel foil according to this embodiment is not limited to the above method. The stainless steel foil according to this embodiment will be described in more detail below by way of examples.

[0064] Stainless steel substrates were prepared for each test number, having the thickness shown in Table 1. The steel type of the stainless steel substrate for each test number was a ferritic stainless steel equivalent to SUS444 or an austenitic stainless steel equivalent to SUS304 as specified in JIS G 4305 (2012).

[0065]

[0066] The compositions listed in Table 1 were applied to the stainless steel substrates of each test number. Composition "A" was prepared by adding dropwise over 2 to 3 hours a hydrolyzed solution prepared by mixing 0.3 g of acetic acid and 35 g of water to a mixed solution of 89 g of methyltriethoxysilane, 76 g of tetramethoxysilane, and 67 g of 2-ethoxyethanol. Composition "B" was prepared by adding dropwise over 2 to 3 hours a hydrolyzed solution prepared by mixing 0.3 g of acetic acid and 35 g of water to a mixed solution of 120 g of phenyltriethoxysilane and 65 g of ethanol.

[0067] The composition was applied by the following method. Specifically, the composition was dropped onto the stainless steel substrate of each test number, and then the substrate was rotated at 400 rpm using a spin coating method. The stainless steel substrate of each test number onto which the composition had been applied was heated in a heat treatment furnace at 150°C for 1 minute to dry the composition. Hereinafter, the dried composition will also be referred to as a dried film.

[0068] A diffusion preparation step was carried out on the stainless steel substrate with the dry film of each test number under the conditions shown in Table 1. In the diffusion preparation step "A", the substrate was held at a temperature of 40°C and a humidity of 80% for 10 days. In the diffusion preparation step "B", the substrate was held at a temperature of 30°C and a humidity of 10% for 10 days. In the diffusion preparation step "C", the substrate was held at a temperature of 40°C and a humidity of 60% for 1 day. A "-" in the diffusion preparation step means that the diffusion preparation step was not carried out.

[0069] Furthermore, the stainless steel substrate with the dry film of each test number was subjected to heat treatment under the heat treatment conditions shown in Table 1. Atmosphere "A" refers to an atmosphere with an oxygen concentration of 300 to 1000 ppm, with the remainder consisting of nitrogen and impurities. Atmosphere "B" refers to an atmosphere with an oxygen concentration of 0 ppm, containing hydrogen, with the remainder consisting of nitrogen and impurities. Under these atmospheres, the substrate was maintained at the temperature (°C) shown in Table 1 for the time (minutes) shown in Table 1.

[0070] [Evaluation Tests] Stainless steel foils of each test number were manufactured by the above steps. The manufactured stainless steel foils of each test number were subjected to a siloxane polymer film thickness measurement test, a first region thickness measurement test, a repeated bending test, an insulation test, a deformability test, and a color test.

[0071] [Siloxane polymer film thickness measurement test] The siloxane polymer film thickness was measured for each stainless steel foil with each test number using an SEM. A cross section was formed for each stainless steel foil with each test number using a cross-section polisher method. The formed cross section was observed with an SEM. The thickness of the siloxane polymer film was measured from the obtained observation image. The thickness of the obtained siloxane polymer film is shown in Table 2.

[0072]

[0073] [First Region Thickness Measurement Test] The thickness of the first region was measured for each stainless steel foil with each test number using the method described above. Five measurement points were identified on the observation surface obtained using the method described above. The measurement points were located at positions at least 2 μm apart. Continuous rectangular analysis was performed using the method described above. Specifically, 20 rectangular analysis regions, each measuring 1 nm in the vertical direction and 10 nm in the horizontal direction relative to the surface of the stainless steel substrate, were arranged vertically to include the siloxane polymer film from the stainless steel substrate. Elemental analysis was performed by integrating the characteristic X-ray spectral intensity generated when the electron beam was narrowed to approximately 0.1 nm diameter and scanned within the measurement area. The thickness (nm) of the first region at each measurement point was calculated from the number of rectangular analysis regions that satisfied the atomic percentages of 5 to 30% Si and 5 to 45% Fe. The arithmetic mean value of the thicknesses of the first region obtained at the five measurement points was calculated as the thickness (nm) of the first region. The resulting thicknesses (nm) of the first region are shown in Table 2.

[0074] [Repeated bending test] A repeated bending test was carried out on the stainless steel foil of each test number. Test pieces of 50 mm x 100 mm were prepared using the method described above. The prepared test pieces were placed in a clamshell type reverse bending tester. The clamshell type reverse bending tester used was a DR11MR model manufactured by Yuasa System Co., Ltd. The test conditions were as follows: the bending radius of the stainless steel foil was set to R / t = 150, where R is the bending radius of the stainless steel foil and t is the thickness of the stainless steel foil.

[0075] The number of repeated bending cycles under the above conditions until cracks appeared on the siloxane polymer film surface was defined as the number of bending cycles. When the number of bending cycles defined above was less than 50,000, it was determined that the sample did not have excellent durability against bending stress ("NA (Not Acceptable)" in Table 2). When the number of bending cycles was between 50,000 and less than 200,000, it was determined that the sample had excellent durability against bending stress ("G (Good)" in Table 2). Furthermore, when the number of bending cycles was 200,000 or more, it was determined that the sample had even better durability against bending stress ("E (Excellent)" in Table 2).

[0076] [Insulation Test] The insulating properties of the siloxane polymer film were evaluated for each stainless steel foil. Specifically, a platinum (Pt) film with a diameter of 1 mm was formed on the siloxane polymer film of each test number using an ion coater to form an electrode. A voltage of 100 V was applied between the electrode and the stainless steel substrate, and the leakage current value was measured. Measurement points where the leakage current value was 10 mA or more were judged to have a short circuit. If four or more of the ten measurement points were judged to have a short circuit, the sample was judged to have poor insulating properties ("NA (Not Acceptable)" in Table 2). If two or three of the ten measurement points were judged to have a short circuit, the sample was judged to have excellent insulating properties ("G (Good)" in Table 2). Furthermore, if one or fewer of the ten measurement points were judged to have a short circuit, the sample was judged to have even better insulating properties ("E (Excellent)" in Table 2).

[0077] [Deformability Test] The deformability of the stainless steel foil of each test number was evaluated. Specifically, a test piece measuring 100 mm in length and 50 mm in width was cut from the stainless steel foil of each test number. One short side of each test piece was fixed and held horizontally. The test piece of each test number was observed from the side, and the difference in height between the unfixed short side and the fixed short side (sagging amount) was measured. When the sagging amount was less than 10 mm, it was determined that the specimen did not have excellent deformability ("NA (Not Acceptable)" in Table 2). When the sagging amount was 10 to less than 50 mm, it was determined that the specimen had excellent deformability ("G (Good)" in Table 2). Furthermore, when the sagging amount was 50 mm or more, it was determined that the specimen had even better deformability ("E (Excellent)" in Table 2).

[0078] [Color Test] A color test was conducted on the stainless steel foil of each test number. Specifically, image analysis was performed on the image of the siloxane polymer film on the stainless steel foil of each test number, and the color was quantified. More specifically, the siloxane polymer film on the stainless steel foil of each test number was scanned to create an electronic data image. The scanner used was a TS8330 manufactured by Canon Inc. Note that the scanner was used with a gap of 1 to 2 mm between the glass surface (document table) of the scanner and the siloxane polymer film. Image analysis was performed on the obtained image using Image J, and the color was quantified in the L*a*b* space. If the obtained a* value and b* value satisfied the relational expression b*≦a*−20, it was determined that the color did not have excellent color ("NA (Not Acceptable)" in Table 2). When the obtained a* and b* values ​​satisfied the relationship b*>a*-20, the color was judged to be excellent (G (Good) in Table 2). When the obtained a* and b* values ​​satisfied the relationship b*>a*-10, the color was judged to be even more excellent (E (Excellent) in Table 2).

[0079] [Evaluation Results] The stainless steel foils of test numbers 1 to 17 contained a first region in the stainless steel substrate and the siloxane polymer film formed on one surface of the stainless steel substrate. As a result, these stainless steel foils were bent 50,000 times or more in a repeated bending test, and had excellent durability against bending stress.

[0080] Furthermore, the thickness of the first region of the stainless steel foils of test numbers 3 to 17 was 8 nm or more. As a result, these stainless steel foils were bent 200,000 times or more in a repeated bending test, and had even better durability against bending stress.

[0081] The stainless steel foils of test numbers 1 to 8 and 10 to 17 also had a first region thickness of 80 nm or less, and as a result, these stainless steel foils had even more excellent color in the color test.

[0082] The stainless steel foils of test numbers 1 to 14 and 16 to 17 also had siloxane polymer films with thicknesses of 0.5 μm or more, and as a result, these stainless steel foils had even better insulating properties in the insulation test.

[0083] Furthermore, the stainless steel foils of test numbers 1 to 12 and 15 to 17 had a stainless steel substrate thickness of 30 μm or less, and as a result, these stainless steel foils had even better deformability in the deformability test.

[0084] On the other hand, the stainless steel foils of test numbers 18 to 20 were not subjected to the above-mentioned preferable diffusion preparation step. As a result, these stainless steel foils did not include the first region in the stainless steel substrate and the siloxane polymer film formed on one surface of the stainless steel substrate. As a result, the number of bending cycles in the repeated bending test was less than 50,000, and the foils did not have excellent durability against bending stress.

[0085] 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 stainless steel foil comprising a stainless steel substrate and a siloxane polymer film formed on at least one surface of the stainless steel substrate, wherein the siloxane polymer film includes a first region formed on the surface of the stainless steel substrate and a second region formed on the surface of the first region, and the first region contains, in atomic %, Si: 5 to 30% and Fe: 5 to 45%.

2. The stainless steel foil according to claim 1, wherein the thickness of the first region is 2 to 100 nm.

3. The stainless steel foil according to claim 1, wherein the thickness of the siloxane polymer film is 0.3 to 5.0 μm.

4. The stainless steel foil according to claim 2, wherein the thickness of the siloxane polymer film is 0.3 to 5.0 μm.

5. The stainless steel foil according to any one of claims 1 to 4, wherein the thickness of the stainless steel substrate is 5 to 80 μm.

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