Stainless steel foil

Abstract

Provided is stainless steel foil having excellent durability against bending stress. The stainless steel foil according to the present disclosure comprises a stainless steel base material and a siloxane polymer film formed on at least one surface of the stainless steel base material. The stainless steel foil further contains, at the interface between the stainless steel base material and the siloxane polymer film, an (Fe, Cr) oxide that has a NaCl-type crystal structure.

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 stainless steel foil further contains an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film.

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

[0012] Figure 1 is a schematic diagram showing a diffraction pattern in a selected area of ​​a TEM for a siloxane polymer film, Figure 2 is a schematic diagram showing a diffraction pattern in a selected area of ​​a TEM for a region including a siloxane polymer film and an (Fe, Cr) oxide, and Figure 3 is a schematic diagram showing a diffraction pattern in a selected area of ​​a TEM for a stainless steel substrate.

[0013] The present inventors have conducted extensive research into stainless steel foils that have excellent durability against bending stress, and have made the following discoveries.

[0014] 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 side 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.

[0015] 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.

[0016] As a result of detailed studies by the present inventors, it has become clear that when cracks occur as a result of applying bending stress to a stainless steel foil having a siloxane polymer film formed thereon, the cracks occur in the siloxane polymer film, not in the stainless steel substrate. In other words, if cracks are less likely to occur in the siloxane polymer film even when bending stress is applied, it may be possible to improve the durability of stainless steel foil against bending stress.

[0017] As a result of further detailed studies by the present inventors, it was found that if an oxide having a specific crystal structure can be dispersed at the interface between the stainless steel foil and the siloxane polymer film, the durability of the stainless steel foil against bending stress can be improved. Specifically, the stainless steel foil according to this embodiment contains an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film. As a result, the stainless steel foil according to this embodiment has excellent durability against bending stress.

[0018] The details of why the inclusion of an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film increases the durability of the stainless steel foil against bending stress are not clear. However, the inventors speculate as follows. As described above, if cracks are less likely to occur in the siloxane polymer film, the durability of the stainless steel foil against bending stress may be improved. Cracks are particularly likely to occur in the siloxane polymer film near the interface with the stainless steel substrate. Therefore, if low-hardness precipitates, i.e., soft precipitates, can be finely dispersed in the siloxane polymer film near the interface with the stainless steel substrate, stress concentration in the siloxane polymer film may be alleviated, potentially suppressing cracking in the siloxane polymer film.

[0019] On the other hand, if precipitates with high hardness are dispersed in the siloxane polymer film, stress may be concentrated around the precipitates with high hardness when bending stress is applied, which may become the starting point of fracture. In other words, if the stress concentration in the siloxane polymer film can be alleviated and precipitates that are less likely to become the starting point of fracture are dispersed, the occurrence of cracks in the siloxane polymer film can be suppressed.

[0020] As defined in JIS G 0203:2009, stainless steel contains chromium (Cr) and iron (Fe). Stable (Fe, Cr) oxides include magnetite ((Fe, Cr)O) and hematite ((Fe, Cr)O). Among these, wustite ((Fe, Cr)O), which has a NaCl-type crystal structure, is particularly well known. 1-yWüstite (O) has a lower hardness than magnetite, hematite, etc., and is less likely to become a starting point for fracture. Therefore, if wüstite is dispersed at the interface between the stainless steel substrate and the siloxane polymer film, it is thought that not only can the wüstite relieve stress in the siloxane polymer film, but it also makes it less likely to become a starting point for fracture, thereby suppressing the occurrence of cracks in the siloxane polymer film.

[0021] It is possible that the inclusion of an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film increases the durability of the stainless steel foil against bending stress through a mechanism different from that conjectured by the inventors. However, the fact that the inclusion of an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film increases the durability of the stainless steel foil against bending stress is proven by the examples described below.

[0022] The stainless steel foil according to this embodiment, which was completed based on the above findings, has the following features.

[0023] [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 stainless steel foil further contains an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film.

[0024] [2] The stainless steel foil according to [1], wherein in the (Fe, Cr) oxide having an NaCl-type crystal structure, the atomic ratio of Cr to Fe satisfies 0.2 to 0.6.

[0025] [3] The stainless steel foil according to [1], wherein the siloxane polymer film has a thickness of 0.3 to 1.0 μm.

[0026] [4] The stainless steel foil according to [2], wherein the siloxane polymer film has a thickness of 0.3 to 1.0 μm.

[0027] [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.

[0028] The stainless steel foil according to this embodiment will be described below.

[0029] [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.

[0030] [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.

[0031] 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.

[0032] 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 less than 5 μm, it may be difficult to manufacture a stainless steel substrate with a consistent thickness. On the other hand, if the thickness of the stainless steel substrate exceeds 60 μm, the rigidity of the stainless steel substrate may become too high, and the deformation resistance of the stainless steel foil may become too high. Therefore, in this embodiment, the thickness of the stainless steel substrate is preferably 5 to 60 μm.

[0033] [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.

[0034] 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.

[0035] 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.

[0036] In this embodiment, the thickness of the siloxane polymer film is not particularly limited, but is, for example, 0.3 to 5.0 μm. The thickness of the siloxane polymer film here refers to 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.

[0037] If the thickness of the siloxane polymer film is less than 0.3 μm, the above-mentioned insulation 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 0.3 to 1.0 μm. In this case, the stainless steel foil has even better durability against bending stress.

[0038] 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. The measurement can be performed by the following method.

[0039] [(Fe, Cr) Oxide Having NaCl-Type Crystal Structure] The stainless steel foil according to this embodiment contains an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film. As described above, the (Fe, Cr) oxide having an NaCl-type crystal structure essentially means wüstite ((Fe, Cr)O).

[0040] Wüstite is formed as fine crystals at the interface between the stainless steel substrate and the siloxane polymer film. Therefore, when bending stress is applied, it can suppress the occurrence of cracks in the siloxane polymer film. Wüstite also has a lower hardness than other stable (Fe, Cr) oxides. Therefore, even when bending stress is applied, stress concentration is less likely to occur and it is less likely to become the starting point of fracture. In the stainless steel foil according to this embodiment, the distribution state of wüstite is not particularly limited. Specifically, wüstite may be distributed in a layered form or in a particulate form.

[0041] Therefore, the stainless steel foil according to this embodiment contains (Fe, Cr) oxides having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film, and as a result, the stainless steel foil according to this embodiment has excellent durability against bending stress.

[0042] Furthermore, the (Fe, Cr) oxide contains Fe and Cr as metal elements, but the ratio of Fe to Cr is not particularly limited. The Cr / Fe atomic ratio in the (Fe, Cr) oxide may be, for example, 0.2 to 0.7, or 0.3 to 0.6. Preferably, the Cr / Fe atomic ratio in the (Fe, Cr) oxide is 0.3 to 0.5. In this case, the durability of the stainless steel foil against bending stress is further improved.

[0043] The (Fe, Cr) oxide has a NaCl-type crystal structure and is not particularly limited as long as it contains Fe and Cr as metal elements. However, the element contents in the (Fe, Cr) oxide according to this embodiment are substantially Fe: 5 to 50 atomic %, Cr: 5 to 50 atomic %, and the remainder is oxygen (O) and impurities.

[0044] In this embodiment, the (Fe, Cr) oxide having an NaCl-type crystal structure is identified by the following method. The stainless steel foil according to this embodiment is cut to obtain an observation surface that includes the thickness direction of the siloxane polymer film. The observation surface is observed using a transmission electron microscope (TEM), and five locations of the siloxane polymer film's interface with the stainless steel substrate are identified and used as measurement points. The interface between the stainless steel substrate and the siloxane polymer film can be identified from the contrast.

[0045] Crystal structure analysis is performed using selected-area electron diffraction (TEM) on the five identified measurement points. The TEM can be, for example, a 200 kV-field emission transmission electron microscope (JEM-2100F) manufactured by JEOL Ltd. In the selected-area electron diffraction, a selected-area aperture is inserted to set the observation field to 3 nm, and a diffraction pattern is obtained using electron beam diffraction. Furthermore, elemental analysis of the selected-area area is performed using energy dispersive X-ray spectrometry (EDS) to identify the elements present within the field of view. Based on the elements within the obtained selected-area area and the diffraction pattern estimated based on the crystal structure, it is identified as an (Fe, Cr) oxide having a NaCl-type crystal structure. A database of elements and diffraction patterns, such as an ICDD card, may be used for identification. Furthermore, the concentrations of Cr and Fe are calculated in atomic percent from the results of elemental analysis of the selected-area area using EDS. The obtained Cr concentration (atomic %) is divided by the Fe concentration (atomic %) to obtain the Cr / Fe atomic ratio in the (Fe, Cr) oxide.

[0046] The diffraction patterns will be explained in more detail with reference to the drawings. Figures 1 to 3 were created using a stainless steel foil in which a siloxane polymer film was formed on the surface of a ferritic stainless steel substrate, and an (Fe, Cr) oxide having an NaCl-type crystal structure was formed at the interface between the stainless steel substrate and the siloxane polymer film.

[0047] FIG. 1 is a schematic diagram showing a diffraction pattern in a TEM selected field of view for a siloxane polymer film. In the selected field of view of FIG. 1, a large white dot in the center and a thin whitish area spreading around it can be seen. Due to the amorphous structure that accounts for the majority of the siloxane polymer film, the whitish area is seen instead of a specific diffraction pattern. FIG. 2 is a schematic diagram showing a diffraction pattern in a TEM selected field of view for a region containing a siloxane polymer film and (Fe, Cr) oxide. In the selected field of view of FIG. 2, a diffraction pattern is seen in addition to a large white dot in the center and a thin whitish area spreading around it. FIG. 3 is a schematic diagram showing a diffraction pattern in a TEM selected field of view for a stainless steel substrate. In the selected field of view of FIG. 3, a diffraction pattern of

[011] ferrite-Fe is seen.

[0048] 2 and 3, the diffraction pattern in Fig. 2 is clearly different from the diffraction pattern in Fig. 3. Specifically, the diffraction patterns scattered around the white area in the center of Fig. 2 are different from the diffraction pattern of the stainless steel substrate confirmed in Fig. 3. Furthermore, by combining this diffraction pattern with the results of elemental analysis by EDS, one skilled in the art would be able to identify the material as an (Fe, Cr) oxide having an NaCl-type crystal structure.

[0049] Furthermore, in this embodiment, if it is determined that NaCl-type (Fe, Cr) oxides are present at at least three of the five measurement points, it is determined that the interface between the stainless steel substrate and the siloxane polymer film contains (Fe, Cr) oxides having an NaCl-type crystal structure.

[0050] In the stainless steel foil according to this embodiment, wüstite exists as very fine precipitates. Therefore, it is extremely difficult to identify wüstite from the contrast observed under a microscope. Therefore, in this embodiment, the interface between the stainless steel substrate and the siloxane polymer film is identified from the contrast, and then the interface is identified by selected-area electron diffraction using a TEM. However, since the interface is identified from the contrast, the measurement points may deviate from the interface. In this case, wüstite may not be detected at the measurement points. Therefore, in this embodiment, if wüstite is detected at at least three of the five measurement points, it is determined that the interface between the stainless steel substrate and the siloxane polymer film contains (Fe, Cr) oxides having an NaCl-type crystal structure.

[0051] In this embodiment, the siloxane polymer film is identified by the following method. For the same measurement points identified from the same observation surface as in the method for identifying the (Fe, Cr) oxide having the NaCl-type crystal structure described above, a pattern specific to amorphous materials (a so-called "halo pattern") is confirmed in the diffraction pattern obtained by electron beam diffraction. Furthermore, for the above measurement points, Si and O are detected by elemental analysis of a selected-field area using EDS. Based on these results, the presence of a siloxane polymer film can be identified.

[0052] 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 -1If 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. -1 If 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.

[0053] [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, and contains an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film. As a result, the stainless steel foil according to this embodiment has excellent durability against bending stress.

[0054] 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.

[0055] 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.

[0056] [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 discharge treatment step, a coating step, and a heat treatment step.

[0057] [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.

[0058] 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.

[0059] 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.

[0060] [Discharge Treatment Step] In the discharge treatment step, a prepared stainless steel substrate is subjected to discharge treatment. By performing the discharge treatment step, impurities such as oxides and oil films on the surface of the stainless steel substrate are sufficiently removed. It is presumed that hydroxyl groups (OH groups) are also formed on the surface during this process. Although the mechanism is unclear, it is presumed that the formation of hydroxyl groups on the surface leads to the stable formation of (Fe, Cr) oxides with an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film in the manufactured stainless steel foil.

[0061] Specifically, in the discharge treatment process, one of a pair of metal electrodes is covered with an insulator. A high-frequency, high-voltage is applied between the metal electrodes in the atmosphere. As a result, a corona discharge occurs between the pair of metal electrodes. Next, the prepared stainless steel substrate is placed on the metal electrodes covered with the insulator. The effect of the discharge treatment is generally considered to be proportional to the input power per unit area, i.e., the discharge amount. The discharge amount is expressed as P / (L x v). Here, P is the discharge power (W), L is the discharge electrode length (m), and v is the substrate speed (m / min) passing under the metal electrode. The discharge amount is not particularly limited, but is preferably 800 to 2000 (W x min / m 2 )

[0062] [Coating Step] In the coating step, a siloxane polymer film composition is applied to the discharge-treated 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).

[0063] When preparing using the above-described method, the organoalkoxysilane can be selected depending on 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 organic group of the organoalkoxysilane contains a methyl group, an ethyl group, and / or a phenyl group. In this case, the thermal decomposition temperature of the organoalkoxysilane becomes higher, making it easier to control the amount of organic groups remaining after the heat treatment step.

[0064] 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.

[0065] 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.

[0066] [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 a nitrogen atmosphere containing a trace amount of oxygen.

[0067] As described above, (Fe, Cr) oxides having an NaCl-type crystal structure are essentially wüstite. In other words, the stainless steel foil according to this embodiment selectively forms wüstite, rather than magnetite or hematite, thereby enhancing durability against bending stress. Furthermore, it is presumed that the Fe and Cr contained in the (Fe, Cr) oxide originate from the stainless steel substrate. In other words, it is presumed that, during the heat treatment process, oxygen contained in the atmosphere and permeating the siloxane polymer film reacts with the Fe and Cr of the stainless steel substrate to produce wüstite. Based on the above presumption, in the heat treatment process according to this embodiment, the heat treatment atmosphere preferably contains 50 to 1000 ppm of oxygen, with the remainder consisting of nitrogen and impurities. The Fe:Cr concentration ratio in the (Fe, Cr) oxide tends to vary depending on the oxygen concentration during heat treatment. Specifically, the higher the oxygen concentration in the atmosphere, the higher the Cr ratio tends to be.

[0068] That is, in the method for producing stainless steel foil according to this embodiment, it is preferable to remove impurities from a prepared stainless steel substrate by discharge treatment, form hydroxyl groups on the surface, apply a siloxane polymer film composition, and then hold the mixture at 350 to 500°C for 5 to 300 minutes in an atmosphere of 50 to 1000 ppm oxygen, the remainder being nitrogen and impurities. In this case, stainless steel foil containing (Fe, Cr) oxides with an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film can be stably produced.

[0069] 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.

[0070] 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).

[0071]

[0072] Discharge treatment was carried out on the stainless steel substrates of each test number except for test number 17 (in Table 1, the "Discharge treatment" column is marked as "Executed"). The discharge treatment was carried out at a discharge rate of 800 to 2000 (W × min / m 2 On the other hand, for test number 17, no discharge treatment was performed (indicated as "-" in the "Discharge treatment" column in Table 1).

[0073] 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.

[0074] 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.

[0075] Heat treatment was performed on the stainless steel substrate with the dry film of each test number under the heat treatment conditions listed in Table 1. Under heat treatment condition "A," the substrate was held at 350 to 380°C for 10 to 200 minutes in an atmosphere with an oxygen concentration of 10 to 100 ppm, the remainder consisting of nitrogen and impurities. Under heat treatment condition "B," the substrate was held at 390 to 430°C for 10 to 200 minutes in an atmosphere with an oxygen concentration of 500 to 1000 ppm, the remainder consisting of nitrogen and impurities. Under heat treatment condition "C," the substrate was held at 430 to 450°C for 10 to 200 minutes in an atmosphere with an oxygen concentration of 1000 to 5000 ppm. Under heat treatment condition "D," the substrate was held at 430 to 450°C for 10 to 200 minutes in an atmosphere with an oxygen concentration of 0 ppm, consisting of hydrogen and impurities.

[0076] [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, an NaCl-type (Fe, Cr) oxide identification test, a repeated bending test, an insulation test, and a deformability test.

[0077] [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.

[0078]

[0079] [NaCl-type (Fe, Cr) oxide identification test] For each stainless steel foil test number, (Fe, Cr) oxides having an NaCl-type crystal structure were identified using the method described above. Crystal structure analysis was performed on the observation surface obtained by the method described above using selected-area electron diffraction (TEM). A selected-area aperture was inserted to set the observation field to 3 nm, and a diffraction pattern was obtained by electron beam diffraction. Furthermore, elemental analysis of the selected-area area was performed using EDS to identify the elements present within the field of view. Based on the elements within the obtained selected-area area and the diffraction pattern estimated based on the crystal structure, the oxide was identified as an (Fe, Cr) oxide having an NaCl-type crystal structure. An ICDD card was used for identification. The ratio of Cr to Fe was calculated by dividing the Cr concentration (atomic %) obtained from the elemental analysis results of the selected-area area by the Fe concentration (atomic %). The Cr / Fe atomic ratio in the obtained (Fe, Cr) oxide is shown in Table 2. The element contents of the (Fe, Cr) oxides having the NaCl-type crystal structure for each test number were Fe: 5 to 50 atomic %, Cr: 5 to 50 atomic %, and the remainder was oxygen (O) and impurities.

[0080] [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.

[0081] 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).

[0082] [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).

[0083] [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).

[0084] [Evaluation Results] The stainless steel foils of test numbers 1 to 15 contained an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between 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.

[0085] The stainless steel foils of test numbers 3 to 5 and 7 to 15 further had a Cr to Fe ratio of 0.3 to 0.5 in the (Fe, Cr) oxide having an NaCl-type crystal structure. 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.

[0086] The stainless steel foils of test numbers 2 to 15 also had a siloxane polymer film thickness of 0.5 μm or more, and as a result, these stainless steel foils had even better insulating properties in the insulation test.

[0087] Furthermore, the stainless steel foils of test numbers 1 to 7, 10 to 12, 14, and 15 had a stainless steel substrate thickness of 30 μm or less. As a result, these stainless steel foils had even better deformability in the deformability test.

[0088] On the other hand, the oxygen concentration in the atmosphere during the heat treatment for the stainless steel foil of test number 16 was too low. As a result, this stainless steel foil did not contain an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film. As a result, the number of bending cycles in the repeated bending test was less than 50,000, and the foil did not have excellent durability against bending stress.

[0089] The stainless steel foil of test number 17 was not subjected to discharge treatment. As a result, this stainless steel foil did not contain an (Fe, Cr) oxide having an NaCl-type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film. As a result, the number of bending cycles in the repeated bending test was less than 50,000, and the foil did not have excellent durability against bending stress.

[0090] 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, the stainless steel foil further comprising an (Fe, Cr) oxide having a NaCl type crystal structure at the interface between the stainless steel substrate and the siloxane polymer film.

2. The stainless steel foil according to claim 1, wherein in the (Fe, Cr) oxide having a NaCl type crystal structure, the atomic ratio of Cr to Fe satisfies 0.2 to 0.

6.

3. The stainless steel foil according to claim 1, wherein the siloxane polymer film has a thickness of 0.3 to 1.0 μm.

4. The stainless steel foil according to claim 2, wherein the siloxane polymer film has a thickness of 0.3 to 1.0 μm.

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

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