Stainless steel foil, switch spring, flexible display substrate, and method for manufacturing stainless steel foil

Abstract

The present invention provides a stainless steel foil (1), which is made of stainless steel (110a) and has a non-metallic inclusion (2) with an equivalent circular diameter (R) of less than 3 μm when viewed in section.

Description

Technical Field

[0001] The present invention relates to a stainless steel foil, a spring for a switch, a substrate for a flexible display and a method for manufacturing the stainless steel foil. Background Art

[0002] Currently, stainless steel foil is used in disc springs for switches in communication equipment, audio equipment, and other applications. Repeated operation of the switches in these applications generates repeated stress in the stainless steel foil. Therefore, stainless steel foil with sufficient fatigue strength to withstand repeated stress is required.

[0003] For example, Japanese Patent No. 4401816 describes a metastable austenitic stainless steel strip for metal domes used in portable terminal switches. In Japanese Patent No. 4401816, the 0.2% proof stress of the austenitic stainless steel strip is increased to improve fatigue strength.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent No. 4401816 Summary of the Invention

[0007] Technical problem to be solved by the invention

[0008] Although not disclosed in the aforementioned Japanese Patent No. 4401816, the fatigue strength of stainless steel foil is affected by non-metallic inclusions contained in the foil. Non-metallic inclusions in stainless steel foil, depending on various factors such as their shape, size, and the type and proportion of the compounds they contain, can become the starting point for cracking, thereby reducing the fatigue strength of the stainless steel foil.

[0009] However, non-metallic inclusions in stainless steel foil are generated during the melting process of the stainless steel that constitutes the stainless steel foil and are difficult to completely remove from the stainless steel foil. Therefore, in order to obtain a stainless steel foil with sufficient fatigue strength, it is important to prevent the non-metallic inclusions in the stainless steel foil from becoming the starting point of cracking.

[0010] The present invention has been developed to solve the above-mentioned technical problems, and one object of the present invention is to provide a stainless steel foil that is less likely to have non-metallic inclusions present in the stainless steel foil become starting points of cracking and has sufficient fatigue strength.

[0011] Technical solutions to technical problems

[0012] The inventors of the present invention conducted extensive research focusing on various factors affecting the presence of non-metallic inclusions in stainless steel foil. As a result, they discovered that stainless steel foil with a maximum equivalent circle diameter of non-metallic inclusions of less than 3 μm is less susceptible to cracking. This led to the completion of the present invention. Specifically, the stainless steel foil of the first embodiment of the present invention is made of stainless steel, and the maximum equivalent circle diameter of the non-metallic inclusions, when viewed in cross section, is less than 3 μm.

[0013] The stainless steel foil according to the first embodiment of the present invention is made of stainless steel, and the maximum equivalent circle diameter of the non-metallic inclusions in the stainless steel foil is less than 3 μm when viewed in cross section. By limiting the maximum equivalent circle diameter of the non-metallic inclusions in the stainless steel foil to less than 3 μm, the occurrence of cracks in the stainless steel foil originating from the non-metallic inclusions can be suppressed. As a result, a stainless steel foil can be provided in which the non-metallic inclusions in the stainless steel foil are less likely to become the starting point of cracks and which exhibits sufficient fatigue strength. This effect has been confirmed by experiments (Examples) described below.

[0014] The stainless steel foil of the first embodiment preferably has a fatigue strength of 1550 MPa or greater. This configuration, with a fatigue strength of 1550 MPa or greater, allows the foil to adequately withstand the repetitive stresses required of disc springs in switches for communication equipment and audio devices, for example. This effect has been confirmed by experiments (Examples) described below.

[0015] In the stainless steel foil of the first embodiment described above, the non-metallic inclusions preferably include Mn oxides, with the proportion of Mn oxides in the total non-metallic inclusions preferably being 50% by mass or greater. Mn oxides are more brittle than other non-metallic inclusions (e.g., Al oxides, Mg oxides, etc.) and are more susceptible to breakage during rolling. Therefore, by ensuring that the proportion of brittle Mn oxides is 50% by mass or greater of the total non-metallic inclusions, the maximum equivalent circle diameter of the non-metallic inclusions present in the stainless steel foil can be easily reduced to less than 3 μm.

[0016] In the stainless steel foil of the first embodiment described above, the non-metallic inclusions contain 50% or more by mass of Mn oxides, and may also contain Al oxides and Mg oxides. In this case, the proportion of Al oxides in the total non-metallic inclusions is preferably 10% or less by mass, and the proportion of Mg oxides is preferably 5% or less by mass. Compared to Mn oxide, Al oxides and Mg oxides are less brittle and, therefore, are less likely to break during rolling. Therefore, by reducing the proportion of Al oxides and Mg oxides, which are harder than Mn oxides, in the total non-metallic inclusions and limiting the proportion of brittle Mn oxides to 50% or more by mass of the total non-metallic inclusions, it is easier to achieve a maximum equivalent circle diameter of less than 3 μm for the non-metallic inclusions in the stainless steel foil.

[0017] In the stainless steel foil of the first embodiment described above, the maximum value of the equivalent size of non-metallic inclusions obtained using the extreme value statistics method is preferably less than 3 μm. The extreme value statistics method refers to a method in which the size (e.g., equivalent circle diameter) of multiple inspection reference areas is measured, the maximum value of each of the multiple inspection reference areas is obtained, and the maximum value of the size that can exist in a given area is estimated from the obtained maximum value. Therefore, when the maximum value of the equivalent size of non-metallic inclusions obtained using the extreme value statistics method is less than 3 μm, it can be inferred that the maximum value of the equivalent circle diameter of non-metallic inclusions in the entire stainless steel foil does not exceed 3 μm. This indicates that the maximum value of the equivalent circle diameter of non-metallic inclusions in the stainless steel foil is less than 3 μm, making it less likely that non-metallic inclusions in the stainless steel foil will become cracking starting points, and that the stainless steel foil has sufficient fatigue strength.

[0018] In the stainless steel foil of the first embodiment, the arithmetic mean of the equivalent circle diameters of the non-metallic inclusions in cross-section is preferably less than 2.5 μm, and more preferably less than 2.3 μm. The inventors of the present invention have determined through experiments (Examples) described below that by setting the arithmetic mean of the equivalent circle diameters of the non-metallic inclusions present in the stainless steel foil to less than 2.3 μm, the non-metallic inclusions are sufficiently unlikely to become cracking initiations, thereby enabling the production of a stainless steel foil having sufficient fatigue strength.

[0019] In the first embodiment of the stainless steel foil, the overall thickness of the stainless steel foil can also be 0.1 mm or less. As described above, when the non-metallic inclusions present in the stainless steel foil are in a form with a maximum equivalent circle diameter of less than 3 μm, which is not likely to become a starting point for cracking, and the stainless steel foil has sufficient fatigue strength, the stainless steel foil can have sufficient fatigue strength even if the overall thickness is as small as 0.1 mm or less. In addition, the overall thickness of such a stainless steel foil with sufficient fatigue strength can also be 0.05 mm or less. Therefore, by using a stainless steel foil with an overall thickness of 0.1 mm or less (preferably 0.05 mm or less), it is also possible to fully cope with the practical application of products that require thinness and fatigue strength (such as disc springs for switches in communication equipment or audio equipment, etc.).

[0020] The stainless steel foil of the first embodiment is preferably made of austenitic stainless steel. This configuration, unlike ferritic stainless steel, is less susceptible to rust and magnetism. Therefore, it is possible to suppress rust or magnetization in components using the stainless steel foil. This can, for example, prevent defects caused by rust or magnetization in peripheral electronic components, such as switches used in communications equipment.

[0021] In this case, the austenitic stainless steel is preferably JIS SUS301. JIS SUS301 is more susceptible to work hardening through rolling than JIS SUS304, another austenitic stainless steel. Therefore, it is easier to increase the hardness of the stainless steel foil than JIS SUS304, which is beneficial for improving the fatigue strength of the stainless steel foil.

[0022] The switch spring of the second embodiment of the present invention can be formed using the stainless steel foil of the first embodiment. By using a stainless steel foil having non-metallic inclusions with a maximum equivalent circle diameter of less than 3 μm, which is unlikely to cause cracking, and having sufficient fatigue strength, the durability of the switch spring can be improved.

[0023] The third embodiment of the flexible display substrate of the present invention can be formed using the stainless steel foil of the first embodiment. By using a stainless steel foil in which non-metallic inclusions have a maximum equivalent circle diameter of less than 3 μm, which is unlikely to cause cracking, and which exhibits sufficient fatigue strength, the durability of the flexible display substrate can be improved.

[0024] A fourth embodiment of the present invention provides a method for producing stainless steel foil, comprising a melt refining step of melting and refining stainless steel to produce a stainless steel material, a first rolling step of forming the molten stainless steel material into a stainless steel plate, and a second rolling step of forming the rolled stainless steel plate into stainless steel foil, wherein the second rolling step includes crushing non-metallic inclusions contained in the stainless steel plate so that the maximum equivalent circle diameter of the non-metallic inclusions is less than 3 μm.

[0025] In the fourth aspect of the present invention's stainless steel foil production method, the second rolling step includes a step of crushing non-metallic inclusions contained in the stainless steel sheet to reduce the maximum equivalent circle diameter of the non-metallic inclusions to less than 3 μm. By crushing the non-metallic inclusions in the second rolling step to reduce the maximum equivalent circle diameter of the non-metallic inclusions to less than 3 μm, the inventors of the present invention have determined through experiments (Examples) described below that it is possible to produce a stainless steel foil in which the non-metallic inclusions present in the stainless steel foil have a maximum equivalent circle diameter of less than 3 μm, making them less likely to become cracking starting points, and which exhibits sufficient fatigue strength.

[0026] In the fourth embodiment of the stainless steel foil manufacturing method, the melt refining step preferably includes a step of adjusting the proportion of Mn oxides in the total non-metallic inclusions to 50% by mass or greater. Mn oxides are more brittle than other non-metallic inclusions (e.g., Al oxides, Mg oxides, etc.) and are therefore more easily broken during the first and second rolling steps following the melt refining step. Therefore, by adjusting the proportion of Mn oxides to 50% by mass or greater during the melt refining step, the non-metallic inclusions can be easily reduced to a maximum equivalent circle diameter of less than 3 μm during the second rolling step, making them less likely to become cracking starting points. This allows the non-metallic inclusions in the stainless steel foil to have a maximum equivalent circle diameter of less than 3 mm, making them less likely to become cracking starting points, and allows for the production of a stainless steel foil with sufficient fatigue strength.

[0027] In the fourth embodiment of the stainless steel foil manufacturing method, the non-metallic inclusions contain 50% or more by mass of Mn oxides, and may also contain Al oxides and Mg oxides. In this case, the melt refining step preferably includes a step of adjusting the proportion of Al oxides in the total non-metallic inclusions to 10% or less by mass, and the proportion of Mg oxides to 5% or less by mass. Compared to Mn oxide, Al oxides and Mg oxides are less brittle and more difficult to break during rolling. Therefore, by reducing the proportion of Al oxides and Mg oxides, which are less brittle and difficult to break, in the total non-metallic inclusions, and increasing the proportion of Mn oxides, which are brittle and easily breakable, to 50% or more by mass, it is possible to easily achieve a maximum equivalent circle diameter of less than 3 μm, which makes it less likely that the non-metallic inclusions will become cracking starting points, by performing the first and second rolling steps after the melt refining step.

[0028] In the fourth embodiment of the stainless steel foil manufacturing method, the second rolling step preferably includes rolling the stainless steel sheet at a reduction ratio of 60% or greater. This increased reduction ratio facilitates the formation of non-metallic inclusions in the rolled stainless steel foil with a maximum equivalent circle diameter of less than 3 μm, making them less likely to become cracking initiations. Furthermore, the thickness of the rolled stainless steel foil can be reduced. The reduction ratio refers to the degree of rolling work.

[0029] In the fourth embodiment of the stainless steel foil manufacturing method, the second rolling step preferably includes a step of rolling the stainless steel sheet to a thickness of 0.1 mm or less. With this configuration, by rolling the stainless steel sheet to a thickness of 0.1 mm or less, it is possible to manufacture stainless steel foil suitable for components of products requiring thinness and fatigue strength (e.g., disc springs for switches in communications equipment, audio equipment, etc.).

[0030] Effects of the Invention

[0031] According to the present invention, as described above, it is possible to provide a stainless steel foil in which non-metallic inclusions present in the stainless steel foil are less likely to become cracking starting points and in which a stainless steel foil has sufficient fatigue strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a diagram showing a stainless steel foil according to an embodiment of the present invention.

[0033] Figure 2 This is a diagram used to explain non-metallic inclusions.

[0034] Figure 3 This is a diagram for explaining the equivalent circle diameter.

[0035] Figure 4 This is a graph used to illustrate the extreme value statistics method.

[0036] Figure 5 It is a perspective view showing a communication device according to an embodiment of the present invention.

[0037] Figure 6 This is a diagram showing the configuration of a volume adjustment button of a communication device according to an embodiment of the present invention.

[0038] Figure 7 1 is a diagram illustrating a flexible display according to an embodiment of the present invention.

[0039] Figure 8 It is a figure for demonstrating the manufacturing method of the stainless steel foil of this invention.

[0040] Figure 9 It is a figure for demonstrating a test piece. DETAILED DESCRIPTION

[0041] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0042] (Composition of stainless steel foil)

[0043] First, refer to Figures 1 to 6 A stainless steel foil 1 according to one embodiment of the present invention will be described.

[0044] like Figure 1 As shown, the thickness (total thickness) t1 of the stainless steel foil 1 of this embodiment is 0.1 mm or less. Depending on the intended use of the stainless steel foil 1, the thickness t1 of the stainless steel foil 1 may be set to 0.05 mm or less (eg, 0.04 mm).

[0045] The stainless steel foil 1 is made of stainless steel. The stainless steel constituting the stainless steel foil 1 is not particularly limited, and can be selected from austenitic, ferritic, and martensitic stainless steels, as long as it is stainless steel. However, when used in communications equipment, audio products, computer-related equipment, and precision electronic components, austenitic stainless steel is preferably used because it is resistant to rust and magnetization.

[0046] Austenitic stainless steel contains Fe (iron), Cr (chromium), and Ni (nickel), and is a stainless steel with austenite as its main structure at room temperature. Fe is the main component used to form austenite, Cr contributes to corrosion resistance, and Ni stabilizes austenite. Examples of austenitic stainless steel include JIS standard SUS304, which contains Fe, 18% to 20% Cr, and 8% to 11% Ni, and JIS standard SUS301, which contains Fe, 16% to 18% Cr, and 6% to 8% Ni. SUS301 is easier to work harden than SUS304, which helps improve the fatigue strength of the stainless steel foil 1. In addition, for example, in switch springs, phosphor bronze, beryllium copper, nickel-based alloys, etc. can also be used in addition to stainless steel, but it is preferred to use stainless steel such as SUS301, which is cheaper and has higher fatigue strength than these.

[0047] like Figure 2 As shown, non-metallic inclusions 2 are present in stainless steel foil 1. Examples of non-metallic inclusions that may be present in conventional stainless steel foil include Al oxides such as alumina (aluminum oxide, Al2O3) derived from the deoxidizing material used in melting stainless steel, Mg oxides such as magnesia (magnesium oxide, MgO), Al- and Mg-containing spinels (MgAl2O4, Al2O3·MgO), Si oxides such as silicon oxide (silicon dioxide, SiO2), Ca oxides such as calcium oxide (quicklime, CaO), Ti oxides such as titanium oxide (titanium dioxide, TiO2), Nb oxides, and W oxides. In addition to these oxides, Cr oxides, derived from the main component of stainless steel, are also potentially present in conventional stainless steel foil.

[0048] The non-metallic inclusions 2 present in the stainless steel foil 1 of the present invention are Mn oxides such as manganese oxide (MnO) derived from the deoxidizing material used when melting the stainless steel. In addition to these Mn oxides, the aforementioned Al oxides, Mg oxides, Si oxides, Ca oxides, and Ti oxides may also be present in the stainless steel foil 1 as non-metallic inclusions 2. Figure 2 The non-metallic inclusions 2 are schematically shown and therefore differ from their actual shapes.

[0049] like Figure 3 As shown, in this embodiment, the stainless steel foil 1 (see Figure 2 ) when the maximum value of the equivalent circle diameter R of the non-metallic inclusion 2 is less than 3 μm. That is, the equivalent circle diameter R of all non-metallic inclusions 2 present in the stainless steel foil 1 is less than 3 μm. The equivalent circle diameter R of one non-metallic inclusion 2 means the diameter of a circle 3 having the same area as the area S of the non-metallic inclusion 2. The equivalent circle diameter R of the non-metallic inclusion 2 present in the stainless steel foil 1 can be obtained by, for example, forming a 20 mm diameter stainless steel foil 1 along the thickness direction of the laminated stainless steel foil 1 by making a plurality of stainless steel foils 1 in a closely laminated state. 2 The cross-section of the above area was observed and calculated by observing the non-metallic inclusions 2 contained in the cross-section. Observation (cross-section) using an electron microscope revealed that the non-metallic inclusions 2 had a maximum equivalent circle diameter R of less than 3 μm. While non-metallic inclusions in conventional stainless steel foils may have granular, round, elliptical, polygonal, or other shapes, or rod-like shapes, the non-metallic inclusions 2 in the stainless steel foil 1 of this embodiment were confirmed to be mostly granular.

[0050] The stainless steel foil 1 preferably has a fatigue strength of 1550 MPa or more. If a stainless steel foil 1 with insufficient fatigue strength is subjected to repeated application of stress exceeding a specified value, it will break (crack), and the break (crack) will become a starting point for damage. The stainless steel foil 1 with a fatigue strength of 1550 MPa or more can fully withstand the stress of the switch spring 11 (see Figure 6 ) required repeated stress or flexible display substrate 21 (refer to Figure 7 ) required repetitive stress.

[0051] Among them, fatigue strength can be obtained by fatigue testing with reference to JIS-Z2273:1978 (General Rules for Fatigue Test Methods of Metallic Materials). Specifically, a pulley-type uniaxial tensile fatigue testing machine is used to repeatedly apply a prescribed tensile stress (test stress) to a test sample cut from a stainless steel foil 1 and having a plate-shaped cross-section (thickness t1, width 3 mm) with a length of 6 mm in the rolling direction. The fatigue test is performed by gradually increasing the test stress. In this case, 5 test samples are prepared for 1 test stress, the number of repetitions (number of cycles) of applying the test stress is set to 1 million times, and 5 fatigue tests are performed. The test stress is adjusted to 11 stages: 1260 MPa, 1400 MPa, 1440 MPa, 1460 MPa, 1490 MPa, 1540 MPa, 1560 MPa, 1590 MPa, 1640 MPa, 1730 MPa, and 1830 MPa. The variation trend of the test stress can be well fitted with the cubic approximation y = 1.31x 3 -22.6x 2 +152.1x+1146.

[0052] In the present invention, based on the fatigue test results described above, the fatigue strength of the stainless steel foil 1 is defined as the test stress (σ1) at which three or more test samples remain unbroken after a cycle count of 1,000,000 cycles. Alternatively, there may be cases where all five test samples remain unbroken at a certain test stress (σ2) after a cycle count of 1,000,000 cycles, but two or fewer test samples remain unbroken at a test stress one level higher (σ3). In this case, instead of the above definition, the average value σa = (σ2 + σ3) / 2 of the test stress σ3 at which two or fewer test samples remain unbroken after a cycle count of 1,000,000 cycles and the test stress σ2 at which all five test samples remain unbroken one level lower is calculated, and this average value σa is defined as the fatigue strength of the stainless steel foil 1.

[0053] In this embodiment, the non-metallic inclusions 2 contain Mn oxides. In this case, the proportion of Mn oxides in the entire non-metallic inclusions 2 is preferably 50% by mass or greater. The greater the proportion of Mn oxides in the entire non-metallic inclusions 2 in the stainless steel foil 1, the better. Furthermore, in this embodiment, in addition to Mn oxides, Al oxides and Mg oxides may also be included. In this case, the proportion of Al oxides in the entire non-metallic inclusions 2 is preferably 10% by mass or less, and the proportion of Mg oxides in the entire non-metallic inclusions 2 is preferably 5% by mass or less. The proportions of Al oxides and Mg oxides in the entire non-metallic inclusions 2 in the stainless steel foil 1 are preferably as low as possible.

[0054] like Figure 4 As shown, the maximum value of the equivalent size of the non-metallic inclusion 2 obtained using the extreme value statistics method is preferably less than 3 μm. The so-called extreme value statistics method refers to a method of measuring the size (e.g., equivalent circle diameter) of multiple inspection reference areas, obtaining the maximum value of the size of each of the multiple inspection reference areas, and inferring the maximum value of the size that can exist in a certain area from the obtained maximum value. In this embodiment, the equivalent circle diameter R is measured for multiple inspection reference areas S0, and the maximum value of the equivalent circle diameter R of each of the multiple inspection reference areas S0 is obtained. The maximum value of the equivalent circle diameter R that can exist in a certain area is inferred from the obtained maximum value of the equivalent circle diameter R. In addition, in the extreme value statistics method, the larger the area for calculation, the larger the calculated value. As a result, the calculated value may sometimes be greater than the measured value (e.g., equivalent circle diameter).

[0055] In this embodiment, the arithmetic mean of the equivalent circle diameters R of the non-metallic inclusions 2 in cross-section is preferably less than 2.5 μm, more preferably less than 2.3 μm. The arithmetic mean is calculated by dividing the sum of the equivalent circle diameters R of the non-metallic inclusions 2 observed in cross-section by the number of non-metallic inclusions 2.

[0056] The stainless steel foil 1 of this embodiment can be used for a switch spring 11 (see Figure 6 ), or flexible display substrate 21 (refer to Figure 7 ). Next, the spring 11 for the switch (refer to Figure 5 ) or flexible display substrate 21 (refer to Figure 7 ) is explained.

[0057] like Figure 5 As shown, the switch spring 11 (refer to Figure 6 ) For example, the volume adjustment button 50 of the communication device 100.

[0058] like Figure 6 As shown, the volume adjustment button 50 is provided with a cover 51 and a switch spring 11 in this order from the outer surface side of the communication device 100. Figure 6 As shown in (a), the switch spring 11 has a dome shape that protrudes toward the cover 51 when the switch is in the off state. The switch spring 11 is, for example, a disc spring. Figure 6 As shown in (b), when the volume adjustment button 50 is operated (pressed from the outer surface), the switch spring 11 is depressed and contacts the volume adjustment circuit 52, thereby turning on the volume adjustment circuit 52. Thus, the volume is adjusted while the volume adjustment circuit 52 is on. Furthermore, after the operation of the volume adjustment button 50 is released, the switch spring 11 returns to its original shape, and the volume adjustment circuit 52 becomes non-connected, turning off the switch. The switch spring 11 is constructed by laminating multiple layers of stainless steel foil 1 having a small thickness t1 (e.g., less than 0.1 mm). This allows the user to experience the tactile sensation of pressing the volume adjustment button 50, and reduces the stress on each stainless steel foil 1 compared to a case where a switch spring 11 is constructed from a single stainless steel sheet of the same thickness as when multiple layers are laminated. Consequently, the switch spring 11 is less likely to break.

[0059] When stainless steel foil 1 is used for the switch spring 11, a metal layer for improving conductivity can be provided on the surface of the stainless steel foil 1, at least on the surface in contact with the volume adjustment circuit 52. The method for forming the metal layer is not particularly limited as long as it is a conventionally known method, and it can be formed by, for example, plating or vapor deposition. In addition, when providing a metal layer, the metal layer can be provided directly on the stainless steel foil 1, or a primer layer can be provided between the metal layer and the stainless steel foil 1. The material of the metal layer is not particularly limited, and as the metal layer, for example, Au plating, Ag plating, Cu plating, or Ni plating can be used. For example, the surface of the switch spring preferably has a metal layer composed of a relatively hard Ag plating. In addition, the material of the primer layer is not particularly limited, and for example, the primer layer can be composed of Ni, and the metal layer can be composed of Au plating. The method for forming the metal primer layer is not particularly limited as long as it is a conventionally known method, and it can be formed by, for example, plating or vapor deposition.

[0060] like Figure 7 As shown, flexible display 200 is a foldable or rollable display device. Flexible display 200 includes a resin display 20 made of self-luminous materials such as organic EL. Because flexible display 200 is inherently flexible, a flexible display substrate 21 is used to enhance mechanical strength. Stainless steel foil 1 can be used for flexible display substrate 21. Furthermore, a hinge for folding flexible display 200 can be attached to flexible display substrate 21. In this case, stainless steel foil 1 can also be used for the hinge.

[0061] When stainless steel foil 1 is used for flexible display substrate 21, the surface of stainless steel foil 1 that contacts display 20 can also be treated to form a Cr oxide film. Oxidation of the Cr contained in stainless steel foil 1 is relatively simple in forming Cr oxide. This improves the adhesion between resin display 20 and flexible display substrate 21 formed of stainless steel foil 1.

[0062] (Method for Manufacturing Stainless Steel Foil)

[0063] Reference Figure 8 The method for manufacturing the stainless steel foil 1 of this embodiment will now be described. The method for manufacturing the stainless steel foil 1 includes a melt refining step of melting and refining stainless steel 110a to form a stainless steel material 110; a first rolling step of forming the molten stainless steel material 110 into a stainless steel plate 10; and a second rolling step of forming the rolled stainless steel plate 10 into the stainless steel foil 1.

[0064] In the melting and refining step, the stainless steel 110a is melted and refined to produce the stainless steel material 110. The stainless steel plate 10 is formed to have a thickness of, for example, 10 mm.

[0065] In the melting and refining process, stainless steel 110a is melted in a furnace 30, for example, to form a molten metal. A deoxidizing material containing a sufficient amount of Mn is then preferentially added to the molten stainless steel 110a for the purpose of deoxidation. Following the addition of the Mn-containing deoxidizing material, an Al-containing deoxidizing material, a Mg-containing deoxidizing material, and a deoxidizing material containing both Al and Mg may be selectively added as needed. Furthermore, a Si-containing deoxidizing material, a Ca-containing deoxidizing material, and a Ti-containing deoxidizing material may be selectively added as needed. The total amount of the selectively added deoxidizing materials other than the Mn-containing deoxidizing material is preferably adjusted so that the proportion of Mn oxides in the entire non-metallic inclusions 2 is not less than 50% by mass. By reducing the amount of Mn-containing deoxidizing material and adding other deoxidizing materials as needed, the oxygen contained in the stainless steel 110a reacts with the Mn contained in the added deoxidizing materials within the molten stainless steel 110a (molten metal), thereby deoxidizing (reducing) the stainless steel 110a. Alternatively, the Al and Mg contained in the deoxidizing material, which are selectively added as needed, react with oxygen to deoxidize (reduce) the stainless steel 110a. Alternatively, the Si, Ca, and Ti contained in the deoxidizing material, which are further selectively added as needed, react with oxygen to further deoxidize (reduce) the stainless steel 110a. Subsequently, the slag containing oxides such as Mn oxides floating on the surface of the molten stainless steel 110a (molten metal) is removed as much as possible. At this time, the Mn oxides and other oxides that are not completely removed and remain in the stainless steel 110a become non-metallic inclusions 2.

[0066] The melting and refining step preferably includes a step of adjusting the proportion of Mn oxides in the entire non-metallic inclusions 2 to 50% by mass or greater. This step is performed by preferentially adding a Mn-containing deoxidizing material in a sufficient amount to allow Mn to react with oxygen in the stainless steel 110a to form sufficient Mn oxides. The slag containing oxides such as Mn oxides is then removed as much as possible. This allows the proportion of Mn oxides, which are brittle and easily break during rolling, to be adjusted to 50% by mass or greater in the entire non-metallic inclusions 2.

[0067] Furthermore, the melting and refining step may, if necessary, include a step for adjusting the proportion of Al oxides in the entire non-metallic inclusions 2 to 10% by mass or less and the proportion of Mg oxides to 5% by mass or less. This step is performed by preferentially adding a Mn-containing deoxidizing material in a sufficient amount to fully form Mn oxides, then appropriately adding an Al-containing deoxidizing material and a Mg-containing deoxidizing material to allow Al and Mg to react with residual oxygen in the stainless steel 110a to form Al oxides and Mg oxides, and then removing as much of the slag containing oxides such as Mn oxides, Al oxides, and Mg oxides as possible. This allows the proportion of Mn oxides, which are brittle and easily broken during rolling, to be 50% by mass or more in the entire non-metallic inclusions 2, while the proportion of Al oxides, which are less brittle than Mn oxides and less easily broken during rolling, to be 10% by mass or less, and the proportion of Mg oxides to be 5% by mass or less.

[0068] In the first rolling step, the molten stainless steel material 110 is rolled using the first roller 31 to form the stainless steel plate 10. The thickness t2 of the stainless steel plate 10 is, for example, 0.25 mm. Furthermore, in the first rolling step, until the thickness t2 is reached, soft annealing, pickling, cleaning, or grinding may be performed as needed.

[0069] In the second rolling step, the rolled stainless steel sheet 10 is rolled using the second roller 32 to form a stainless steel foil 1. The thickness t1 of the stainless steel foil 1 is set to 0.10 mm or less. Furthermore, in the second rolling step, until the thickness t1 is reached, soft annealing, pickling, cleaning, or grinding may be performed as needed.

[0070] In the second rolling step, the stainless steel sheet 10 is preferably rolled at a reduction ratio of 60% or more. The reduction ratio refers to the degree of rolling processing and can be obtained by dividing the difference between the thickness t2 before rolling and the thickness t1 after rolling (t1-t2) by the thickness t2 before rolling. For example, if the thickness t2 of the stainless steel sheet 10 before the second rolling step is 0.25 mm and the thickness t1 of the stainless steel foil 1 after the second rolling step is 0.10 mm, the reduction ratio is 60% by (0.25-0.10)÷0.25×100=60. In addition, if the thickness t1 of the stainless steel foil 1 after the second rolling step is 0.05 mm, the reduction ratio is 80% by (0.25-0.05)÷0.25×100=80.

[0071] <Effects of this embodiment>

[0072] In this embodiment, the following effects can be obtained.

[0073] In this embodiment, the stainless steel foil 1 is made of stainless steel 110a, and the maximum equivalent circle diameter R of the non-metallic inclusions 2 in cross-section is less than 3 μm. By setting the maximum equivalent circle diameter R of the non-metallic inclusions 2 in the stainless steel foil 1 to less than 3 μm, the non-metallic inclusions 2 are less likely to serve as cracking initiations, thereby suppressing cracking of the stainless steel foil 1 originating from the non-metallic inclusions 2. As a result, a stainless steel foil 1 can be provided that is less likely to serve as cracking initiations by the non-metallic inclusions 2 in the stainless steel foil 1 and has sufficient fatigue strength.

[0074] In this embodiment, the stainless steel foil 1 preferably has a fatigue strength of 1550 MPa or greater. With this configuration, the stainless steel foil 1 having a fatigue strength of 1550 MPa or greater can sufficiently withstand the repeated stress required of disc springs in switches of communication equipment or audio equipment.

[0075] In this embodiment, the non-metallic inclusions 2 preferably contain Mn oxides, and the proportion of Mn oxides in the entire non-metallic inclusions 2 is preferably 50% by mass or greater. With this configuration, since the proportion of Mn oxides, which are brittle and easily breakable during rolling, is 50% by mass or greater in the entire non-metallic inclusions 2, the non-metallic inclusions 2 present in the stainless steel foil 1 can be easily configured to have a maximum equivalent circle diameter R of less than 3 μm.

[0076] In this embodiment, the non-metallic inclusions 2 contain 50% or more by mass of Mn oxides, and may also contain Al oxides and Mg oxides. In this case, the proportion of Al oxides in the total non-metallic inclusions 2 is preferably 10% or less by mass, and the proportion of Mg oxides is preferably 5% or less by mass. With this configuration, by reducing the proportion of Al oxides and Mg oxides, which are less brittle than Mn oxides and less prone to breakage during rolling, and by reducing the proportion of Mn oxides, which are brittle and easily breakable during rolling, to 50% or more by mass of the total non-metallic inclusions, the non-metallic inclusions in the stainless steel foil can more easily have a maximum equivalent circle diameter of less than 3 μm.

[0077] In this embodiment, the maximum value of the equivalent size of the non-metallic inclusions 2 obtained by the extreme value statistics method is preferably less than 3 μm. With this configuration, the maximum value of the equivalent size of the non-metallic inclusions 2 obtained by the extreme value statistics method is less than 3 μm. It can be inferred that the stainless steel foil 1 as a whole will not have a maximum equivalent circle diameter R of 3 μm or more. It can be inferred that the non-metallic inclusions 2 present in the stainless steel foil 1 have a maximum equivalent circle diameter R of less than 3 μm, making them less likely to become cracking starting points, and that the stainless steel foil 1 has sufficient fatigue strength.

[0078] In this embodiment, the arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 in cross-section is preferably less than 2.5 μm, and more preferably less than 2.3 μm. With this configuration, the arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 is less than 2.3 μm. This makes it sufficiently difficult for the non-metallic inclusions 2 to become the starting point of cracking, thereby enabling the stainless steel foil 1 to have sufficient fatigue strength.

[0079] In this embodiment, the overall thickness t1 of the stainless steel foil 1 can also be 0.1 mm or less. As described above, the stainless steel foil 1 can still exhibit sufficient fatigue strength even when its overall thickness t1 is reduced to 0.1 mm or less. Therefore, by using a stainless steel foil 1 with an overall thickness t1 of 0.1 mm or less, it is possible to adequately support the practical application of products requiring both thinness and fatigue strength (e.g., disc springs for switches in communication equipment and audio equipment).

[0080] In this embodiment, the stainless steel foil 1 is preferably made of austenitic stainless steel. This configuration, unlike ferritic stainless steel, is less susceptible to rust and magnetism. Therefore, it is possible to suppress rust or magnetization in components using the stainless steel foil 1 of the present invention. This can, for example, prevent defects in peripheral electronic components such as those used in switches for communications equipment from occurring due to rust or magnetization.

[0081] In this embodiment, the austenitic stainless steel is preferably JIS standard SUS301. This configuration is advantageous in improving the fatigue strength of the stainless steel foil 1 because SUS301 is more easily work-hardened by rolling than other austenitic stainless steel JIS standard SUS304.

[0082] In this embodiment, switch spring 11 can be formed using the stainless steel foil 1 of the above-described embodiment. Using stainless steel foil 1 with sufficient fatigue strength and a maximum equivalent circle diameter R of less than 3 μm, which makes non-metallic inclusions 2 less likely to become cracking initiations, can improve the durability of switch spring 11.

[0083] In this embodiment, the flexible display substrate 21 can be formed using the stainless steel foil 1 of the above-described embodiment. By using a stainless steel foil 1 having non-metallic inclusions 2 with a maximum equivalent circle diameter R of less than 3 μm, which is less likely to cause cracking, and having sufficient fatigue strength, the durability of the flexible display substrate 21 can be improved.

[0084] In this embodiment, the method for producing a stainless steel foil 1 includes a melt refining step of melting and refining stainless steel 110a to produce a molten stainless steel material 110, a first rolling step of forming the molten stainless steel material 110 into a stainless steel plate 10, and a second rolling step of forming the rolled stainless steel plate 10 into a stainless steel foil 1. The second rolling step includes a step of crushing non-metallic inclusions 2 contained in the stainless steel plate 10 so that the maximum equivalent circle diameter R of the non-metallic inclusions 2 is less than 3 μm. Thus, by crushing the non-metallic inclusions 2 in the second rolling step to form the non-metallic inclusions 2 with a maximum equivalent circle diameter R of less than 3 μm, it is possible to produce a stainless steel foil 1 having sufficient fatigue strength, in which the non-metallic inclusions 2 present in the stainless steel foil 1 have a maximum equivalent circle diameter R of less than 3 μm and are less likely to become cracking starting points.

[0085] In this embodiment, the melt refining step preferably includes a step for adjusting the proportion of Mn oxides in the entire non-metallic inclusions 2 to 50% by mass or greater. This ensures that the proportion of Mn oxides, which are brittle and easily broken during rolling, reaches 50% by mass or greater in the entire non-metallic inclusions 2, making it easier to break the non-metallic inclusions 2 (Mn oxides) during the first and second rolling steps following the melt refining step. Therefore, by adjusting the proportion of Mn oxides to 50% by mass or greater during the melt refining step, the non-metallic inclusions 2 can be easily formed into a form with a maximum equivalent circle diameter R of less than 3 μm, which is less likely to serve as a cracking starting point, during the second rolling step. This allows the non-metallic inclusions 2 present in the stainless steel foil 1 to have a maximum equivalent circle diameter R of less than 3 μm, which is less likely to serve as a cracking starting point, and to have sufficient fatigue strength.

[0086] In this embodiment, during the melt refining step, the non-metallic inclusions 2 contain 50% or more of Mn oxides, and may also contain Al oxides and Mg oxides. In this case, the melt refining step preferably includes a step of adjusting the proportion of Al oxides in the non-metallic inclusions 2 to 10% by mass or less, and the proportion of Mg oxides to 5% by mass or less. This reduces the proportion of Al oxides and Mg oxides, which are less brittle than Mn oxides and less prone to breakage during rolling, in the non-metallic inclusions 2. This makes it easier for the non-metallic inclusions 2 in the stainless steel foil 1 to have a maximum equivalent circle diameter R of less than 3 μm.

[0087] In this embodiment, the second rolling step preferably includes rolling the stainless steel sheet 10 at a reduction ratio of 60% or greater. By increasing the reduction ratio in this manner, the non-metallic inclusions 2 present in the rolled stainless steel foil 1 can be easily formed into a form having a maximum equivalent circle diameter R of less than 3 μm, which makes them less likely to become cracking starting points, and the thickness of the rolled stainless steel foil 1 can be reduced.

[0088] In this embodiment, the second rolling step preferably includes a step of rolling the stainless steel sheet 10 so that the thickness of the stainless steel foil 1 is 0.1 mm or less. Thus, by rolling the stainless steel sheet 10 so that the thickness of the stainless steel foil 1 is 0.1 mm or less, it is possible to produce a stainless steel foil 1 suitable for parts of products requiring thinness and fatigue strength (e.g., disc springs for switches in communication equipment, audio equipment, etc.).

[0089] Example

[0090] Comparative experiments (Examples and Comparative Examples) based on the above-described embodiment are described below. In Examples 1 to 4, stainless steel foil 1 was formed using the same manufacturing method as the above-described embodiment. In the melt refining step, various deoxidizing materials (such as a Mn-containing deoxidizing material, an Al-containing deoxidizing material, and a Mg-containing deoxidizing material) were used to intentionally adjust the proportion of Mn oxides in the entire non-metallic inclusions 2 to 50% by mass or greater. In Comparative Examples 1 to 4, stainless steel foil 1 was formed using the same manufacturing method as the above-described embodiment, except that the above-described adjustment step to adjust the proportion of Mn oxides in the entire non-metallic inclusions 2 to 50% by mass or greater was not performed during the melt refining step. In both Examples and Comparative Examples, the thickness t2 of the stainless steel sheet 10 after the first rolling step was 0.25 mm, and the thickness t1 of the stainless steel foil 1 after the second rolling step was 0.04 mm, which is less than 0.1 mm.

[0091] For the stainless steel plate 10 and the stainless steel foil 1, an electron microscope (FlexSEM1000 manufactured by Hitachi High-Tech Corporation) and particle analysis software (Aztec manufactured by Oxford Instruments) were used to observe the cross-section (image analysis). In the observation of the stainless steel plate 10 and the stainless steel foil 1, a sample was used which was cut out along the rolling direction, embedded in a resin in such a way that the end face (cross section) in the thickness direction was exposed, and the end face was mirror-polished. In addition, for the stainless steel foil 1 with a small thickness t1, a plurality of foils cut out along the rolling direction were laminated in close contact in the thickness direction and then embedded in a resin. The observation magnification was set to 400 times and the observation area was set to 20 mm 2 .

[0092] For the stainless steel plate 10, non-metallic inclusions 2 with an equivalent circle diameter R of 2 μm or more were extracted based on the image analysis data. The number of non-metallic inclusions 2 was counted at 1 μm intervals, and the maximum value and arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 were calculated. Furthermore, for the stainless steel foil 1, the number, maximum value, and arithmetic mean were calculated in the same manner as for the stainless steel plate 10, and the inspection reference area S0 was set to 0.505 mm. 2 , set the number of inspections to 40, and set the area of the maximum equivalent size predicted by extreme value statistics to 2 mm 2 , calculate the maximum value of equivalent size by extreme value statistics method.

[0093] The observation results of the stainless steel plate 10 are shown in Table 1, and the observation results of the stainless steel foil 1 are shown in Table 2.

[0094] [Table 1]

[0095]

[0096] [Table 2]

[0097]

[0098] As shown in Table 1, the maximum value of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel sheet 10 after the first rolling step was 6.45 μm in Example 1 and 6.58 μm in Example 2. Furthermore, the maximum value of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel sheet 10 after the first rolling step was 3.84 μm in Example 3 and 5.37 μm in Example 4. Meanwhile, the maximum value of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel sheet 10 after the first rolling step was 10.90 μm in Comparative Example 1, 4.11 μm in Comparative Example 2, 5.74 μm in Comparative Example 3, and 5.39 μm in Comparative Example 4. Furthermore, the arithmetic mean value of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel sheet 10 after the first rolling step was 2.43 μm in Example 1 and 2.51 μm in Example 2. Furthermore, the arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel sheet 10 after the first rolling step was 2.46 μm in Example 3 and 2.50 μm in Example 4. Meanwhile, the arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel sheet 10 after the first rolling step was 2.82 μm in Comparative Example 1, 2.47 μm in Comparative Example 2, 2.93 μm in Comparative Example 3, and 2.64 μm in Comparative Example 4. This confirms that non-metallic inclusions 2 having a maximum equivalent circle diameter R of 3 μm or greater are present in all of the stainless steel sheets 10 after the first rolling step. Furthermore, it was confirmed that the maximum value of the equivalent circle diameter R of the non-metallic inclusions 2 in Examples 1 and 2 is greater than that in Comparative Examples 2 to 4.

[0099] When the stainless steel sheet 10, rolled in the first rolling step, was further rolled in the second rolling step to form the stainless steel foil 1, as shown in Table 2, the maximum equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step was 2.53 μm in Example 1 and 2.45 μm in Example 2. Furthermore, the maximum equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step was 2.68 μm in Example 3 and 2.54 μm in Example 4. Therefore, the maximum equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step was less than 3 μm in all the Examples, reaching 2.7 μm or less. On the other hand, the maximum equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step was 4.99 μm in Comparative Example 1, 3.12 μm in Comparative Example 2, 4.81 μm in Comparative Example 3, and 4.33 μm in Comparative Example 4, all of which were not less than 3 μm but were 3 μm or greater. Furthermore, the arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step was 2.21 μm in Example 1 and 2.18 μm in Example 2. Furthermore, the arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step was 2.42 μm in Example 3 and 2.31 μm in Example 2. The arithmetic mean of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step was 2.45 μm or less in the examples. On the other hand, in Comparative Example 1, where the maximum value of the equivalent circle diameter of the non-metallic inclusions was 3 μm or greater, it was 2.72 μm, in Comparative Example 2 it was 2.32 μm, in Comparative Example 3 it was 2.74 μm, and in Comparative Example 4 it was 2.57 μm; none of these values were less than 2.3 μm.

[0100] The above results confirm that the manufacturing method of this embodiment can reduce the maximum value of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 to less than 3 μm. In other words, it is clear that the manufacturing method of this embodiment, which includes at least a melt refining step including a step of adjusting the Mn oxide ratio in the entire non-metallic inclusions 2 to 50 mass% or more (and may also include a step of adjusting the Al oxide ratio to 10 mass% or less and the Mg oxide ratio to 5 mass% or less in the entire non-metallic inclusions 2), and a second rolling step including a step of crushing the non-metallic inclusions 2 contained in the stainless steel sheet 10 formed by the first rolling step to reduce the maximum value of the equivalent circle diameter R of the non-metallic inclusions 2 to less than 3 μm, can reduce the maximum value of the equivalent circle diameter R of the non-metallic inclusions 2 present in the stainless steel foil 1 to less than 2.3 μm.

[0101] Furthermore, as shown in Table 2, the maximum equivalent size of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step, as determined by the extreme value statistics method, was 2.86 μm in Example 1 and 2.65 μm in Example 2. Furthermore, the maximum equivalent size of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step, as determined by the extreme value statistics method, was 2.95 μm in Example 3 and 2.83 μm in Example 4. Therefore, the maximum equivalent size of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step, as determined by the extreme value statistics method, was less than 3 μm in the Examples. On the other hand, the maximum equivalent size of the non-metallic inclusions 2 present in the stainless steel foil 1 after the second rolling step, as determined by the extreme value statistics method, was 4.92 μm in Comparative Example 1, 4.42 μm in Comparative Example 2, 4.69 μm in Comparative Example 3, and 4.32 μm in Comparative Example 4, all of which were not less than 3 μm but were 4.3 μm or greater. From these results, it was confirmed that the production method of the present embodiment can make the non-metallic inclusions 2 present in the stainless steel foil 1 have a maximum value of less than 3 μm in equivalent size obtained by the extreme value statistics method.

[0102] Next, the composition of the non-metallic inclusions 2 observed in the cross-sections of the stainless steel foils 1 of Examples 1 to 4 and Comparative Examples 1 to 4 was determined. The composition of the non-metallic inclusions 2 present in the stainless steel foils 1 was determined by performing energy dispersive X-ray spectroscopy (EDX) analysis of the oxides of the non-metallic inclusions 2 extracted using the particle analysis software described above. The mass concentrations of the analyzed elements (Mn, Al, Mg, Si, Ca, and Ti) were converted to 100 mass % based on the total mass of the non-metallic inclusions 2 (MnO, Al2O3, MgO, SiO2, CaO, and TiO2). The results are shown in Table 3.

[0103] [Table 3]

[0104]

[0105] Furthermore, the fatigue strength, Vickers hardness, tensile strength, and work-induced martensite (amount of martensite generated) of the stainless steel foils 1 of Examples 1 to 4 and Comparative Examples 1 to 4 were measured.

[0106] The fatigue strength of the stainless steel foil 1 was measured by fatigue testing with reference to JIS-Z2273:1978. Specifically, a pulley-type uniaxial tensile fatigue testing machine was used to repeatedly apply a prescribed tensile stress (test stress) to a test sample having a plate-shaped cross-section (thickness t1, width 3 mm) with a length of 6 mm in the rolling direction cut out from the stainless steel foil 1. The fatigue test was performed by gradually increasing the test stress. In addition, 5 test samples were prepared for 1 test stress, and the number of repetitions (cycles) of applying the test stress was set to 1 million times, and 5 fatigue tests were performed. The test stress was adjusted to 11 stages: 1260 MPa, 1400 MPa, 1440 MPa, 1460 MPa, 1490 MPa, 1540 MPa, 1560 MPa, 1590 MPa, 1640 MPa, 1730 MPa, and 1830 MPa. The results are shown in Table 4.

[0107] The Vickers hardness of the stainless steel foil 1 was measured by polishing the rolled surface of the stainless steel foil 1 to expose the cross section using an automatic hardness tester (AMT-X7FS manufactured by Matsuzawa Co., Ltd.). Figure 9 The results are shown in Table 4.

[0108] The tensile strength of the stainless steel foil 1 was measured using a Tensilon universal testing machine (RTC-1310A manufactured by A&D Co., Ltd.). The test piece was a JIS No. 5 test piece, and the test direction was the rolling direction. The results are shown in Table 4.

[0109] The process-induced martensite (martensite generation amount) of the stainless steel foil 1 was measured by polishing the rolled surface of the stainless steel foil 1 to expose the cross section, and then using a ferrite meter FMP30 manufactured by Helmut Fisher. The ferrite measurement value refers to the ratio of the process-induced martensite structure in the structure of the stainless steel foil 1. The test piece was taken from the stainless steel foil 1 with a width of B (refer to Figure 9 The results are shown in Table 4.

[0110] [Table 4]

[0111]

[0112] As shown in Table 3, the non-metallic inclusions 2 present in the stainless steel foil 1 of the examples are: in Example 1, the proportion of MnO was 65.84 mass%, the proportion of Al₂O₃ was 5.18 mass%, the proportion of MgO was 1.46 mass%, and the proportion of other oxides totaled 27.52 mass%. In Example 2, the proportion of MnO was 53.22 mass%, the proportion of Al₂O₃ was 6.30 mass%, the proportion of MgO was 3.87 mass%, and the proportion of other oxides totaled 36.61 mass%. Regarding the non-metallic inclusions 2 present in the stainless steel foil 1, in Example 3, the proportion of MnO was 76.56 mass%, the proportion of Al₂O₃ was 9.43 mass%, the proportion of MgO was 1.30 mass%, and the proportion of other oxides totaled 12.7 mass%. In Example 4, the proportion of MnO was 75.07 mass%, the proportion of Al₂O₃ was 8.56 mass%, the proportion of MgO was 1.95 mass%, and the proportion of other oxides totaled 14.42 mass%. This confirmed that in the stainless steel foils 1 of Examples 1 to 4, the proportion of Mn oxide in the entire non-metallic inclusions 2 was 50 mass % or more, the proportion of Al oxide was 10 mass % or less, and the proportion of Mg oxide was 5 mass % or less.

[0113] On the other hand, regarding the non-metallic inclusions 2 present in the stainless steel foil 1 of the comparative examples, in Comparative Example 1, the proportion of MnO was 2.53% by mass, the proportion of Al₂O₃ was 37.38% by mass, the proportion of MgO was 41.62% by mass, and the proportion of other oxides totaled 18.47% by mass. In Comparative Example 2, the proportion of MnO was 11.31% by mass, the proportion of Al₂O₃ was 38.15% by mass, the proportion of MgO was 16.24% by mass, and the proportion of other oxides totaled 34.30% by mass. In Comparative Example 3, the proportion of MnO was 6.51% by mass, the proportion of Al₂O₃ was 53.40% by mass, the proportion of MgO was 26.61% by mass, and the proportion of other oxides totaled 13.48% by mass. In Comparative Example 4, the proportion of MnO was 1.38% by mass, the proportion of Al₂O₃ was 12.00% by mass, the proportion of MgO was 52.88% by mass, and the proportion of other oxides totaled 33.74% by mass. This confirmed that in the stainless steel foils 1 of Comparative Examples 1 to 4, the proportion of Mn oxide in the entire non-metallic inclusions 2 was less than 50 mass %, the proportion of Al oxide exceeded 10 mass %, and the proportion of Mg oxide exceeded 5 mass %.

[0114] As shown in Table 4, the fatigue strength of the stainless steel foil 1 was 1590 MPa in Example 1, 1615 MPa in Example 2, 1615 MPa in Example 3, and 1590 MPa in Example 4, all of which were 1550 MPa or higher. On the other hand, the fatigue strengths of Comparative Example 1 were 1443 MPa, 1492 MPa in Comparative Example 2, 1474 MPa in Comparative Example 3, and 1404 MPa in Comparative Example 4, all of which were less than 1550 MPa and less than 1500 MPa. These results confirm that the manufacturing method of this embodiment can produce a stainless steel foil 1 having a sufficient fatigue strength of 1550 MPa or higher.

[0115] As shown in Table 2, the stainless steel foils 1 of Examples 1 to 4 have a maximum equivalent circle diameter R of less than 3 μm for the non-metallic inclusions 2 present in the stainless steel foils 1. Considering this, it was confirmed that the stainless steel foils 1 in which the non-metallic inclusions 2 have a maximum equivalent circle diameter R of 3 μm can exhibit a fatigue strength of 1550 MPa or greater.

[0116] Furthermore, as shown in Table 3, the non-metallic inclusions 2 present in the stainless steel foils 1 of Examples 1 to 4 contain Mn oxides at a ratio of 50 mass% or greater within the entire non-metallic inclusions 2. This fact confirms that stainless steel foils 1 containing non-metallic inclusions 2 having a maximum equivalent circle diameter R of 3 μm and containing Mn oxides at a ratio of 50 mass% or greater within the entire non-metallic inclusions 2 can exhibit fatigue strength of 1550 MPa or greater.

[0117] Alternatively, the non-metallic inclusions 2 present in the stainless steel foils 1 of Examples 1 to 4 are shown in Table 3. The Mn oxide ratio of the non-metallic inclusions 2 as a whole is 50 mass% or greater, the Al oxide ratio is 10 mass% or less, and the Mg oxide ratio is 5 mass% or less. Considering this, it can be seen that stainless steel foils 1 in which the non-metallic inclusions 2 are present in a form with a maximum equivalent circle diameter R of 3 μm, and the Mn oxide ratio of the non-metallic inclusions 2 as a whole is 50 mass% or greater, the Al oxide ratio is 10 mass% or less, and the Mg oxide ratio is 5 mass% or less can exhibit a fatigue strength of 1550 MPa or greater.

[0118] As shown in Table 4, the Vickers hardness of the stainless steel foil 1 was 574 HV in Example 1 and 585 HV in Example 2. Furthermore, the Vickers hardness of the stainless steel foil 1 was 580 HV in Example 3 and 580 HV in Example 4. Meanwhile, the Vickers hardness was 584 HV in Comparative Example 1, 581 HV in Comparative Example 2, 572 HV in Comparative Example 3, and 578 HV in Comparative Example 4. Therefore, it was confirmed that, as in the manufacturing method of this embodiment, even a stainless steel foil 1 containing non-metallic inclusions 2 having a Mn oxide ratio of 50 mass% or greater within the entire non-metallic inclusions 2 can exhibit a sufficient Vickers hardness of 570 HV or greater. Alternatively, it was confirmed that even a stainless steel foil 1 containing non-metallic inclusions 2 having a Mn oxide ratio of 50 mass% or greater, an Al oxide ratio of 10 mass% or less, and a Mg oxide ratio of 5 mass% or less within the entire non-metallic inclusions 2 can exhibit a sufficient Vickers hardness of 570 HV or greater. This confirmed that Examples 1 to 4 could ensure a Vickers hardness of 570 HV or higher, equivalent to that of Comparative Examples 1 to 4, while ensuring a fatigue strength of 1550 MPa or higher, greater than that of Comparative Examples 1 to 4.

[0119] As shown in Table 4, the tensile strength of the stainless steel foil 1 was 1802 MPa in Example 1 and 1820 MPa in Example 2. Furthermore, the tensile strength of the stainless steel foil 1 was 1843 MPa in Example 3 and 1818 MPa in Example 4. Meanwhile, the tensile strengths of the stainless steel foil 1 were 1856 MPa in Comparative Example 1, 1836 MPa in Comparative Example 2, 1826 MPa in Comparative Example 3, and 1795 MPa in Comparative Example 4. Therefore, it was confirmed that, as in the production method of this embodiment, even a stainless steel foil 1 containing non-metallic inclusions 2 having a Mn oxide ratio of 50% by mass or greater within the entire non-metallic inclusions 2 can have a sufficient tensile strength of 1800 MPa or greater. Alternatively, it was confirmed that even stainless steel foil 1 containing non-metallic inclusions 2 containing Mn oxides at a ratio of 50 mass% or more, Al oxides at a ratio of 10 mass% or less, and Mg oxides at a ratio of 5 mass% or less within the entire non-metallic inclusions 2 can still exhibit a sufficient tensile strength of 1800 MPa or more. This confirms that Examples 1 and 2 can maintain a tensile strength of 1800 MPa or more, comparable to Comparative Examples 1 to 4, while also maintaining a fatigue strength of 1550 MPa or more, which is greater than that of Comparative Examples 1 to 4.

[0120] As shown in Table 4, the strain-induced martensite content of the stainless steel foil 1 was 40.8% by mass in Example 1 and 40.7% by mass in Example 2. Furthermore, the strain-induced martensite content of the stainless steel foil 1 was 40.6% by mass in Example 3 and 40.8% by mass in Example 4. Meanwhile, the strain-induced martensite content was 40.6% by mass in Comparative Example 1, 40.9% by mass in Comparative Example 2, 40.7% by mass in Comparative Example 3, and 40.7% by mass in Comparative Example 4. This confirms that even when the proportion of Mn oxides in the entire non-metallic inclusions 2 is adjusted to 50% by mass or greater, as in the manufacturing method of this embodiment, the strain-induced martensite content in the structure of the stainless steel foil 1 is approximately 40% by mass, which is less than 50% by mass and is comparable to a case where the proportion of Mn oxides in the entire non-metallic inclusions 2 is not adjusted to 50% by mass or greater.

[0121] [Modification]

[0122] The embodiments and examples disclosed herein are to be considered as illustrative in all respects and should not be construed as limiting. The scope of the present invention is not represented by the description of the embodiments and examples described above, but is represented by the scope of the claims, and includes all modifications (variations) within the meaning and scope equivalent to the scope of the claims.

[0123] For example, this embodiment shows the use of stainless steel foil for switch springs and flexible display substrates, but the present invention is not limited to this. Stainless steel foil can be used for, for example, chassis, ribs, hinges, flexible wiring substrates, and auxiliary parts for heat dissipation components of portable devices.

[0124] In addition, in this embodiment, the austenitic stainless steel is exemplified as JIS standard SUS301, but the present invention is not limited thereto. In the present invention, for example, the austenitic stainless steel may be JIS standard SUS304, SUS316, or SUS316L, or other stainless steels.

[0125] In addition, in this embodiment, examples (Examples 1 and 2) are shown in which the oxides contained in the non-metallic inclusions include Mn oxide, Al oxide, Mg oxide, Si oxide, Ca oxide, and Ti oxide. However, the present invention is not limited to this. In the present invention, as long as the non-metallic inclusions include Mn oxide, for example, any other oxide may be included, and oxides other than the oxides listed above may be included.

[0126] Explanation of symbols

[0127] 1: Stainless steel foil; 2: Non-metallic inclusion; 10: Stainless steel plate; 11: Switch spring; 21: Flexible display substrate; 110: Stainless steel material; 110a: Stainless steel; R: Equivalent circle diameter.

Claims

1. A stainless steel foil (1), characterized in that: It is made of austenitic stainless steel. The maximum value of the equivalent circle diameter (R) of the non-metallic inclusions (2) when viewed in cross section is 2.45 μm or more and 2.68 μm or less, and the arithmetic mean value of the equivalent circle diameters of the non-metallic inclusions when viewed in cross section is 2.18 μm or more and less than 2.42 μm, The non-metallic inclusions include Mn oxide, Al oxide, Mg oxide, Si oxide, Ca oxide, and Ti oxide. In the entire non-metallic inclusions, the proportion of the Mn oxide is 65.84 mass% to 76.56 mass%, the proportion of the Al oxide is 5.18 mass% to 9.43 mass%, and the proportion of the Mg oxide is 1.30 mass% to 3.87 mass%. The proportions of the Mn oxide, the Al oxide, and the Mg oxide are obtained by converting the mass concentrations of the elements Mn, Al, Mg, Si, Ca, and Ti analyzed by energy dispersive X-ray analysis of oxides of the non-metallic inclusions so that the total of the non-metallic inclusions MnO, Al2O3, MgO, SiO2, CaO, and TiO2 becomes 100 mass%. The fatigue strength is above 1590 MPa.

2. The stainless steel foil according to claim 1, wherein: The arithmetic mean of the equivalent circle diameters of the non-metallic inclusions is less than 2.3 μm when viewed in section.

3. The stainless steel foil according to claim 1, wherein: The overall thickness of the stainless steel foil is 0.1 mm or less.

4. The stainless steel foil according to claim 1, wherein: The austenitic stainless steel is SUS301 according to the JIS standard.

5. A switch spring (11), characterized in that: A stainless steel foil (1) is used which is made of austenitic stainless steel and has a maximum value of the equivalent circle diameter of non-metallic inclusions of 2.45 μm or more and 2.68 μm or less when viewed in cross section, and an arithmetic mean value of the equivalent circle diameter of the non-metallic inclusions of 2.18 μm or more and less than 2.42 μm when viewed in cross section. The non-metallic inclusions include Mn oxide, Al oxide, Mg oxide, Si oxide, Ca oxide, and Ti oxide. In the entire non-metallic inclusions, the proportion of the Mn oxide is 65.84 mass% to 76.56 mass%, the proportion of the Al oxide is 5.18 mass% to 9.43 mass%, and the proportion of the Mg oxide is 1.30 mass% to 3.87 mass%. The proportions of the Mn oxide, the Al oxide, and the Mg oxide are obtained by converting the mass concentrations of the elements Mn, Al, Mg, Si, Ca, and Ti analyzed by energy dispersive X-ray analysis of oxides of the non-metallic inclusions so that the total of the non-metallic inclusions MnO, Al2O3, MgO, SiO2, CaO, and TiO2 becomes 100 mass%. The fatigue strength is above 1590 MPa.

6. A substrate (20) for a flexible display, characterized in that: The invention uses a stainless steel foil made of austenitic stainless steel, wherein the maximum value of the equivalent circle diameter of non-metallic inclusions in cross-section is 2.45 μm or more and 2.68 μm or less, and the arithmetic mean value of the equivalent circle diameter of the non-metallic inclusions in cross-section is 2.18 μm or more and less than 2.42 μm. The non-metallic inclusions include Mn oxide, Al oxide, Mg oxide, Si oxide, Ca oxide, and Ti oxide. In the entire non-metallic inclusions, the proportion of the Mn oxide is 65.84 mass% to 76.56 mass%, the proportion of the Al oxide is 5.18 mass% to 9.43 mass%, and the proportion of the Mg oxide is 1.30 mass% to 3.87 mass%. The proportions of the Mn oxide, the Al oxide, and the Mg oxide are obtained by converting the mass concentrations of the elements Mn, Al, Mg, Si, Ca, and Ti analyzed by energy dispersive X-ray analysis of oxides of the non-metallic inclusions so that the total of the non-metallic inclusions MnO, Al2O3, MgO, SiO2, CaO, and TiO2 becomes 100 mass%. The fatigue strength is above 1590 MPa.

7. A method for manufacturing a stainless steel foil (1), characterized in that: include: A melting and refining process of melting and refining austenitic stainless steel to produce a stainless steel material (110); A first rolling step of forming the molten stainless steel material into a stainless steel plate; and The second rolling step is to form the rolled stainless steel sheet into stainless steel foil. The second rolling step includes crushing the non-metallic inclusions (2) contained in the stainless steel plate so that the maximum value of the equivalent circle diameter (R) of the non-metallic inclusions is greater than or equal to 2.45 μm and less than or equal to 2.68 μm, and the arithmetic mean value of the equivalent circle diameters of the non-metallic inclusions in cross-section is greater than or equal to 2.18 μm and less than or equal to 2.42 μm. The melt refining step includes the step of causing the non-metallic inclusions to contain Mn oxide, Al oxide, Mg oxide, Si oxide, Ca oxide, and Ti oxide, and adjusting the proportion of the Mn oxide in the entire non-metallic inclusions to be 65.84 mass% to 76.56 mass%, the proportion of the Al oxide to be 5.18 mass% to 9.43 mass%, and the proportion of the Mg oxide to be 1.30 mass% to 3.87 mass%. The ratio of the Mn oxide, the Al oxide, and the Mg oxide is determined by performing energy dispersive X-ray analysis of the oxides of the non-metallic inclusions, and converting the mass concentrations of the analyzed elements Mn, Al, Mg, Si, Ca, and Ti so that the total of the non-metallic inclusions MnO, Al2O3, MgO, SiO2, CaO, and TiO2 becomes 100 mass%. The second rolling step includes a step of forming a stainless steel foil having a fatigue strength of 1590 MPa or more.

8. The method for producing a stainless steel foil according to claim 7, wherein: The second rolling step includes rolling the stainless steel sheet at a rolling reduction of 60% or more.

9. The method for manufacturing a stainless steel foil according to claim 7, wherein: The second rolling step includes a step of rolling the stainless steel plate so that the thickness of the stainless steel foil becomes 0.1 mm or less.

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