Flexible stainless steel foil and flexible light-emitting devices

Abstract

Provided is a stainless foil having improved durability against repeated bending under stringent bending conditions that are required for foldable displays and rollable displays. The stainless foil is a rolled stainless foil having a thickness of 0.1 mm or less, and is a flexible stainless foil having a tensile strength of 1800 MPa or more and a maximum height roughness Rz of 0.35 μm or less, in which the maximum height roughness Rz is measured from a roughness curve for the surface of the stainless foil which is measured in the same direction as the direction of stretching.

Description

Technical Field

[0001] This invention relates to a stainless steel foil having high durability against repeated bending with a small curvature and a movable light-emitting device including the same.

Background Art

[0002] Flexible light-emitting elements that can be bent, particularly organic EL elements, have been developed. In recent years, electronic devices called foldable devices whose screens can be folded and rollable devices that can be wound up and stored in a roll have been developed. Since organic EL elements do not have their own strength and rigidity, a reinforcing plate is often required on the back side thereof. The reinforcing plate is bonded to the organic EL element and needs to be flexible because the screen is bent, and mainly stainless steel thin plates or stainless steel foils are selected. The characteristic required for such a reinforcing plate is durability against repeated bending. Here, durability specifically means that no bending distortion occurs and it does not break. Bending distortion means permanent deformation that occurs after bending back and unloading. Also, not breaking means that no cracks enter the stainless steel thin plate or stainless steel foil and it does not fracture due to metal fatigue caused by repeated bending.

[0003] As inventions related to stainless steel thin plates or stainless steel foils having excellent fatigue characteristics, a measure for reducing inclusions in a stainless steel foil (Patent Document 1) and a measure for defining the average peak-to-valley interval of the surface cross-sectional curve in a direction perpendicular to the rolling direction of the stainless steel foil (Patent Document 2) are disclosed.

[0004] Also, as a stainless steel foil used for a substrate for a flexible display, a stainless steel foil having an average arithmetic mean roughness (Ra) of 50 nm or less in the rolling direction and a direction perpendicular to the rolling direction of the stainless steel foil has been devised (Patent Document 3). The curvature in this stainless steel foil for a flexible display is assumed to be relatively large from the description of the claims. As a substrate on which a circuit is formed through a thin insulating film with the stainless steel foil as the substrate, smoothness is required so as not to impair the resolution of the display.

[0005] Patent Document 4 proposes an organic light-emitting device that includes a flexible conductive substrate made of stainless steel or titanium and a thin-film transistor formed on the conductive substrate. This light-emitting device relates to a system for applying a bias to the conductive substrate, and the required property of the flexible substrate is conductivity; repeated bending is not considered. Patent Document 5 describes the use of stainless steel as a reinforcing material for light-emitting panels such as organic ELs, but it is generally described in the same category as plastics, aluminum, and silicone rubber, which have low strength, and its use in applications that require the severe repeated bending demands of recent years is not considered.

[0006] In recent years, stainless steel sheets or foils used as reinforcing materials in displays for foldable and rollable devices have been required to withstand bending at small curvatures and large bending angles that were not previously necessary. Furthermore, when used as reinforcing plates rather than substrates, even smaller curvatures may be required. In addition, these electronic devices, like clamshell terminals, slide terminals, and tablet devices, require thinner and lighter reinforcing plates, resulting in increasingly smaller bending curvatures. This demands for more precise metal materials for reinforcing plates, and current materials are becoming insufficient to meet these requirements. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 1-309919 [Patent Document 2] Japanese Patent Publication No. 2005-307295 [Patent Document 3] International Publication No. 2009 / 139495 [Patent Document 4] Japanese Patent Publication No. 2007-11256 [Patent Document 5] Japanese Patent Publication No. 2016-75884 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] To increase the strength of stainless steel foil, a common method is to work-harden the stainless steel by rolling. However, when attempting to obtain high strength by rolling, the surface smoothness of the stainless steel foil is impaired due to irregularities in the rolling rolls and the entrapment of rolling oil. The objective of the present invention is to clarify the requirements for obtaining the necessary durability to improve the durability against repeated bending under the harsh bending conditions required for foldable displays and rollerable displays, and to provide stainless steel foil that satisfies the conflicting conditions of maximum height roughness (Rz) in the bending direction and strength, which have not been required until now. [Means for solving the problem]

[0009] The present invention includes the following embodiments. (1) A rolled stainless steel foil with a thickness of 0.1 mm or less, a tensile strength of 1800 MPa or more, and a flexible stainless steel foil in which the maximum height roughness Rz obtained from the surface roughness curve of the stainless steel foil measured in the same direction as the tensile direction is 0.35 μm or less. (2) The flexible stainless steel foil according to (1), wherein the thickness is 0.05 mm or less, the ratio of the maximum valley depth Rv to Rz (Rv / Rz) is 0.6 or less, and the elongation at break when the tensile strength is measured is 1% or more and 2% or less. (3) When the material is repeatedly bent 180° in the same direction as the tensile direction with a value of R / t = 100 (where R is the bending radius (mm) and t (mm) is the thickness), and then returned to 0°, The number of repeated bending cycles until a crack of 5 mm or longer in length occurs on the surface of the stainless steel foil is 100,000 or more. The flexible stainless steel foil according to (1) or (2) above, wherein the bending tendency is 170° or more at the opening angle. (4) The Rz is 0.30 μm or less, When the bending is repeated in the same direction as the tensile direction, with a value satisfying R / t=75, and then returned to 0°, The number of repeated bending cycles until a crack of 5 mm or longer in length occurs on the surface of the stainless steel foil is 100,000 or more. The flexible stainless steel foil according to (1) or (2) above, wherein the bending tendency when unloaded is 170° or more at the opening angle. (5) The flexible stainless steel foil according to any one of (1) to (4) above, wherein the stainless steel foil's components are, by mass%, C: 0.15% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030% or less, Ni: 6.00 to 8.00%, Cr: 16.00 to 18.00%, N: 0.20% or less, and includes a martensite phase. (6) The flexible stainless steel foil according to any one of (1) to (5) above, wherein the area ratio of the martensite phase in the cross-section of the stainless steel foil is 50% or more. (7) A flexible stainless steel foil according to any one of (1) to (6), wherein a planar flexible light-emitting element is bonded to the surface of the stainless steel foil. (8) The flexible stainless steel foil according to (7), wherein the flexible light-emitting element is an organic EL display element. (9) A flexible light-emitting device comprising a flexible stainless steel foil and a planar flexible light-emitting electronic element as described in any of (1) to (6) above. [Effects of the Invention]

[0010] This invention provides a stainless steel foil that simultaneously satisfies conflicting conditions of maximum height roughness in the bending direction and strength, in order to enhance the durability against repeated bending under the harsh bending conditions required for foldable displays and rollable displays. Specifically, the material has a thickness of 0.1 mm or less, a tensile strength of 1800 MPa or more, and a maximum height roughness Rz of 0.35 μm or less, determined from the surface roughness curve of the stainless steel foil measured in the same direction as the tensile direction, making it possible to provide a reinforcing material for use in organic EL displays and the like that are subjected to particularly severe repeated bending. [Brief explanation of the drawing]

[0011] [Figure 1] This is a diagram showing an example of the usage form of a foldable display. [Figure 2] This is a diagram showing an example of the usage form of a rollable display. [Figure 3] This is a diagram showing an example of the usage form of a rollable display. [Figure 4] This is a diagram showing a clam shell type repetitive bending tester and its operation.

Mode for Carrying Out the Invention

[0012] (Explanation of Usage Forms and Terms) Examples of the usage forms of the stainless steel foil of the invention are shown in FIGS. 1 to 3. FIG. 1 is an example of a display panel for a foldable electronic device, and FIGS. 2 and 3 are examples of display panels for rollable electronic devices. These diagrams are all simplified for explaining the usage forms of the stainless steel foil of the present invention. The stainless steel foil 1 of the present invention is used by bonding a light-emitting element 2 such as an organic EL display element to the surface with an adhesive or the like.

[0013] In FIG. 1, the light-emitting element 2 is shown as being attached inside the stainless steel foil 1, but the light-emitting element may be attached outside the stainless steel foil 1. In the case of the form of FIG. 1, the stainless steel foil undergoes repetitive bending in a completely closed state and a 180° open state. In this specification, the repetitive bending angle of the stainless steel foil is referred to as the unfolding angle, with the closed state taken as the reference (0°), this being the "closed angle", and the most open state being the "open angle". The minimum value of the closed angle is 0°, and the maximum value of the open angle is 360°. Therefore, the range of the unfolding angle is 0° or more and less than 360°. In the case of FIG. 1, the closed angle is 0°, the open angle is 180°, and the unfolding angle is 180°.

[0014] When the flexible stainless steel foil of the present invention is used, for example, for a foldable display, the closing angle, opening angle, and unfolding angle of the stainless steel foil are not necessarily limited. However, the stainless steel foil of the present invention is a foil having durability against repeated bending at an unfolding angle of 180°. In applications such as foldable terminals represented by mobile phones, in many cases, it is used in a flat state like a smartphone after being completely folded as shown in FIG. 1 and then completely opened. The degree of bending of the stainless steel foil is a closing angle of 0°, an opening angle of 180°, and an unfolding angle of 180°. In the case of a laptop terminal, it is not necessary for the unfolding angle to be 180°. It is better if the opening angle can be unfolded up to about 135° for the visibility of the display, and the flexible stainless steel foil of the present invention can be applied.

[0015] In the bending form as shown in FIG. 1, at the bent portion, the cross-section of the stainless steel foil 1 is bent into an arc shape defined by the bending radius R. Depending on the restraint with other members and the bending radius, it may not form a complete arc. However, the bending radius R defined in the present invention is the radius of the outer peripheral surface when the bend approximates an arc when bent as shown in FIG. 1. If R is reduced, the thickness of the electronic device in the folded state can be reduced, and the stainless steel foil of the present invention is useful.

[0016] In the present invention, when the stainless steel foil plane is folded in half, the line forming the fold is called the ridge line, and the direction of the ridge line is called the ridge line direction. When the stainless steel foil is returned to a plane, the direction perpendicular to the ridge line within the plane is called the bending direction. In the form as shown in FIG. 1, the bent portion is subjected to bending strain at a local location near the ridge line. However, in a rollable display as shown in FIGS. 2 and 3, the area where repeated bending is applied extends over an area close to the entire area of the stainless steel foil and changes. The closing angle of the stainless steel foil used in the rollable display as shown in FIGS. 2 and 3 is 0°, the opening angle is 180°, and the unfolding angle is 180°. As shown in Figure 2, the maximum radius is constant when the display is used with an expanded width. However, when used in a rolled form as shown in Figure 3, the smallest bending radius of the outer surface of the stainless steel foil is defined as R (mm). If the bending radius R can be reduced, the overall thickness of a slide-type rollable display as shown in Figure 2 can be reduced, and the storage space of a roll-type rollable display as shown in Figure 3 can be reduced. The highly durable stainless steel foil of the present invention is extremely useful for rollable displays.

[0017] The present invention relates to a flexible stainless steel foil having a thickness t of 0.1 mm or less, a tensile strength of 1800 MPa or more in at least one of the rolling direction or a direction perpendicular to the rolling direction, and a maximum height roughness Rz of 0.35 μm or less, determined from the surface roughness curve of the stainless steel foil measured in the same direction as the tensile direction.

[0018] Preferably, the flexible stainless steel foil has a thickness of 0.05 mm or less, a ratio of the maximum valley depth Rv to Rz (Rv / Rz) of 0.6 or less, and a fracture elongation of 1% or more and 2% or less when the tensile strength is measured.

[0019] More preferably, in addition to the above, the flexible stainless steel foil is characterized by being bent 180° in the same direction as the tensile direction with a value satisfying R / t = 100 (where R is the bending radius (mm) and t (mm) is the thickness), and then returning to 0°, and the number of repeated bending cycles until a crack of 5 mm or longer occurs on the surface of the stainless steel foil is 100,000 or more, and the bending tendency is 170° or more at the opening angle. The R / t specified for the preferred stainless steel foil in this invention is an index representing the severity of bending, taking into account the stress and strain experienced by the material. Even with the same R, as the thickness (t) of the material increases, the stress and strain experienced locally in the material increase, but on the other hand, the material needs strength and rigidity according to its application. The R / t = 100 index used for the stainless steel foil in this invention has been rarely seen until now in applications of metal foil where repeated bending durability at an unfolding angle of 180° is required.

[0020] (Stainless steel foil) In this invention, the composition of the stainless steel foil is not limited, but since the invention mainly concerns stainless steel foil used in electronic devices, examples include stainless steel foil with good corrosion resistance, primarily composed of iron (Fe) at a concentration of 50% by mass or more, and containing 10.5% by mass or more of chromium (Cr). Hereafter, unless otherwise specified, the composition values ​​of the stainless steel foil will be expressed in mass percent.

[0021] Since the stainless steel foil of the present invention must achieve extremely high strength, there are not many varieties of this material currently used commercially. For example, stainless steel foil containing the following components can be mentioned: C: 0.15% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030% or less, Ni: 6.00~8.00%, Cr: 16.00~18.00%, N: 0.20% or less. Also, austenitic stainless steel foil containing the following elements can be mentioned: C: 0.07~0.10%, Si: 0.75% or less, P: 0.030% or less, S: 0.030% or less, N: 0.25~0.40%, Ni: 5.50~7.00%, Cr: 17.50~20.00%, Mn: 9.0~12.0%.

[0022] The former stainless steel foil is referred to as SUS301 or SUS301L in JIS standards and is sometimes used as a spring material. It has been found that it can be manufactured by diligently devising a manufacturing method to meet the strength and roughness requirements of the stainless steel foil of the present invention. The latter stainless steel foil is a product called NSSC130S from Nippon Steel Stainless Steel Corporation. It has also been found that this can be manufactured by diligently devising a manufacturing method to meet the strength and roughness requirements of the stainless steel foil of the present invention. This material is nonmagnetic and is useful in applications where magnetism is undesirable.

[0023] The microstructure of the stainless steel foil of the present invention is not particularly limited, but austenitic stainless steel with a well-developed martensitic structure is preferred. Some austenitic stainless steels form a stress- or strain-induced martensitic phase upon processing. To ensure strength, it is desirable that the stainless steel foil of the present invention has a microstructure in which this processing-induced, high-strength martensitic structure is well-developed. Furthermore, it is preferable that the martensitic structure accounts for 50% or more, and more preferably 60% or more, of the area, and that it forms a multiphase structure with the austenite phase, thereby obtaining the bending durability within the range specified for the stainless steel foil of the present invention. The area fraction of the martensitic structure shall be measured by EBSD (electron backscatter diffraction) on an FE-SEM (field emission electron microscope) and evaluated on the polished surface of the foil perpendicular to the thickness direction. For foils that undergo martensitic transformation by processing, such as SUS301 foil, the surface shall be smoothed by chemical polishing to prevent martensitic transformation due to mechanical polishing. The measurement conditions shall be a magnification of 1500x, a measurement area of ​​60 × 120 μm, and a measurement interval of 0.08 μm. Measurements shall be performed on three fields of view at different locations, and the average value shall be taken. With EBSD, the crystal structure and its orientation at each measurement point can be determined from the diffraction lines from that point. The area fraction of the martensite phase shall be determined by the proportion of measurement points of the phase determined to be the martensite phase. In the case of SUS301 and SUS301L, phases having a body-centered cubic (bcc) crystal structure may be considered as martensite phases, and the ratio of the number of points of this crystal structure to the total is taken as the area fraction of the martensite phase.

[0024] A coexistence structure of austenite and martensite phases can be obtained, for example, by heavily rolling metastable austenitic stainless steel such as SUS304 or SUS301 to induce a phase transformation from austenite to work-induced martensite. Stainless steel foil produced in this manner is particularly preferable as the stainless steel foil of the present invention because it is composed of a high-strength martensite phase and an austenite phase with accumulated strain. In particular, stainless steel foil containing the following components is suitable as the material for the stainless steel foil of the present invention: C: 0.15% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030% or less, Ni: 6.00~8.00%, Cr: 16.00~18.00%, N: 0.20% or less.

[0025] SUS301 and SUS301L are materials having the above components, with the remainder being iron and unavoidable impurities. SUS301L is a material in which nitrogen is intentionally added in a range of 0.2% or less while reducing the carbon content to 0.03%. SUS301L is easier to increase in strength, and it is an even more desirable material if it can be manufactured to meet the roughness range specified in this invention. Materials with added Nb, which are derived from these JIS standard steel grades, have high fatigue strength, and are particularly desirable if they can be manufactured to meet the strength and maximum height roughness Rz range specified in this invention.

[0026] In order to form the austenite phase and martensite phase in the preferred stainless steel foil of the present invention, this can be achieved by controlling the total rolling ratio, rolling speed, etc. Details of the manufacturing conditions will be described later.

[0027] By utilizing processing-induced transformation to form the phase structure of the stainless steel foil of the present invention, and by setting the area ratio of the martensite phase to 50% or more, preferably 60% or more, the crystal grains become lamellar in shape, with longer grains in the rolling direction and shorter grains in the foil thickness direction. When the structure within the foil surface is viewed from the foil thickness direction, the austenite phase is distributed in an island-like manner. The formation of such a multiphase structure inhibits crack propagation, thereby improving bending durability.

[0028] The stainless steel foil of the present invention can be repeatedly bent and used with a bending radius R of 1 mm to 10 mm. The thickness t (mm) of the foil used under these conditions is 0.1 mm or less, the R / t is 30 to 120, and it is intended for use in applications where it is repeatedly bent at an unfolding angle of 135° or more. When used in applications where R / t is less than 30, the foil will develop a bending curvature, making it difficult to meet the required break life. However, when used in applications where R / t is greater than 120, conventional stainless steel foils that do not meet the tensile strength and maximum height roughness (Rz) requirements of the present invention can satisfy the requirements for bending curvature and break life. Hereafter, in this specification, unless it is necessary to distinguish between foldable devices and rollable devices, both will be collectively referred to as foldable devices. While there is no lower limit on the thickness t of the stainless steel foil used in foldable devices, it is generally 0.02 mm or more for the purpose of reinforcing the light-emitting element.

[0029] The number of bending cycles required for a foldable device varies depending on the device's application, but a minimum of 100,000 cycles is necessary. In the present invention, the preferred stainless steel foil is required to have a crack of 5 mm or less introduced on its surface when subjected to 100,000 repeated bending tests at R / t=100 and an unfolding angle of 180°. The flexible stainless steel foil of the present invention can be bonded to a flexible light-emitting display element, such as an organic EL display element used in a foldable device, and used as an integrated unit. If there is a crack on the surface of the bent portion of the stainless steel foil, a large local deformation will occur, causing abnormalities in the display of the light-emitting display element, and in some cases, damage. Therefore, it is desirable that no cracks occur, but due to the buffering effect of the adhesive layer between the light-emitting display element and the stainless steel foil, cracks up to a maximum of 5 mm are acceptable.

[0030] Foldable devices are expected to become smaller, thinner, and lighter in the future. When the stainless steel foil of the present invention is subjected to a repeated bending test of 100,000 cycles with R / t=75 and an unfolding angle of 180°, it is desirable that the crack introduced into the surface of the stainless steel foil be 5 mm or less. Furthermore, when the repeated bending test of 100,000 cycles with R / t=67 and an unfolding angle of 180° is performed, it is desirable that the crack introduced into the surface of the stainless steel foil be 5 mm or less. When the repeated bending test of 100,000 cycles with R / t=50 and an unfolding angle of 180° is performed, it is desirable that the crack introduced into the surface of the stainless steel foil be 5 mm or less.

[0031] Furthermore, permanent deformation (bending habit) after repeated bending is also necessary as an indicator of durability. When bending is repeated, if the bending habit that occurs in the bending direction is small, it is not a problem because it is corrected by the hinges and frames that make up the electronic device. However, if the bending habit is large, it will cause problems such as distortion in the display. Considering the application of reinforcing plates for foldable displays, the acceptable range of bending habit required for the stainless steel foil of the present invention is preferably 170° or more when the opening angle caused by plastic deformation along the bending edge of the stainless steel foil is measured after performing a repeated bending test 100,000 times at an unfolding angle of 180° or more, and when this is taken as the free opening angle, while ensuring that no force such as gravity is applied in the bending direction of the stainless steel foil. 175° or more is desirable, and 180° with no bending habit at all is even more desirable. The bending tendency is measured by placing a stainless steel foil upright on a flat table so that the bend is perpendicular to the tabletop surface, taking a picture with a digital camera from directly above, focusing on the top edge, and then measuring the angle on the stainless steel foil using that image.

[0032] The repeated bending test of stainless steel foil is performed using a clamshell-type repeated bending tester under conditions of a closing angle of 0° and an opening angle of 180°. Figure 4 schematically shows the bending operation of the repeated bending test. The clamshell-type repeated bending tester is equipped with a pair of holding plates 3, to which the stainless steel foil 1 is attached and the holding plates are tilted to apply a forced bending displacement to the stainless steel foil 1. Figure 4 shows state A with an opening angle of 180° and state C with a closing angle of approximately 90°. One of the two holding plates rotates and tilts around the drive shaft 4, while the other holding plate maintains the same angle, and the edges of both holding plates that contact the stainless steel foil move in a way that keeps them parallel and the distance between them constant, as shown by the dotted lines in Figure 4. This operation allows for repeated bending without applying any load other than bending to the stainless steel foil. An example of a commercially available evaluation device that performs this operation is the Yuasa System Equipment Co., Ltd. unloaded clamshell bending tester, model DR11MR.

[0033] When the distance between the two holding plates is 2R, the stainless steel foil undergoes a bending displacement that forms an arc with a bending radius R. Depending on the thickness and mechanical properties of the stainless steel foil, the bent portion may not form a perfect arc. However, in this invention, R is the radius of the outer surface of the stainless steel foil when the bent portion, determined by the gap 2R at a closed angle of 0° during repeated bending tests, is considered to be an arc.

[0034] For the repeated bending test, the stainless steel foil shall be cut to a size of 40 mm wide x 150 mm long, and measurements shall be taken with the center of the long side and the width direction aligned with the bending edge direction. The width and length of the stainless steel foil shall be measured using a scale with a minimum scale of 1 mm, reading to 1 / 10 of the minimum scale division, and cut to a tolerance of ±0.5 mm. The thickness shall be measured using a single-sphere micrometer with a minimum reading accuracy of less than a micrometer, measuring 10 points at different locations within the sample, and taking the average value to 0.1 μm. The gap length shall be set with an accuracy of less than 0.1 mm so that the predetermined R / t is within ±3%. In addition, both ends of the bending edge shall be polished with emery paper of #1500 or higher before attaching the stainless steel foil to the holding plate to prevent cracking from the edges of the stainless steel foil. The frequency of repeated bending, which determines the bending speed, shall be 1 Hz.

[0035] The stainless steel foil of this invention requires a bending strength of 1800 MPa or higher. This is a necessary condition for both preventing bending deformation and failure due to repeated bending, but it is especially essential for suppressing bending deformation. If the Young's modulus of the stainless steel foils is approximately the same, the presence or absence of bending deformation can be directly determined by the yield strength. However, in the case of the stainless steel foil of this invention, the yield strength is often unclear, so it is determined by the maximum strength. The strength of the stainless steel foil is 1800 MPa or higher, and more preferably 2000 MPa or higher. If the strength is less than 1800 MPa, a large bending deformation will occur under the conditions specified for the stainless steel foil of this invention, which is undesirable.

[0036] The strength of the stainless steel foil of this invention is measured using a sample cut to a shape conforming to JIS No. 13B with a length of 150 mm. The test shall be conducted in accordance with JIS 2241 "Tensile Testing Method for Metallic Materials," with the load applied to the load cell being read and the test performed at a crosshead speed of 50 mm / min. The strength shall be defined as the value obtained by dividing the maximum load until fracture by the cross-sectional area of ​​the test piece. When measuring elongation, an extensometer shall be used. If there is a difference in tensile strength depending on the cutting direction within the stainless steel foil surface, it is sufficient if the maximum value is 1800 MPa or higher. When used as stainless steel foil for foldable devices, the direction in which the tensile strength is 1800 MPa or higher will be used as the bending direction.

[0037] The flexible stainless steel foil of the present invention is used as a reinforcing material for light-emitting devices such as lighting and displays, which can be folded (bent) or rolled up after bonding light-emitting elements to the foil surface. These devices are not only subjected to the formation of curved surfaces but also to repeated bending at small curvatures. In particular, organic EL display elements, one type of light-emitting element, are elements that can display high color rendering and high definition, and are used in the displays of high-end televisions and mobile phones. High durability is especially required because bending, cracking, and delamination of the reinforcing material from the adhesive degrade the display quality in those areas. For this reason, high tensile strength and low roughness are required in a specific direction, specifically in the bending direction. The stainless steel foil of the present invention is not a substrate on which light-emitting elements are directly formed, so it does not need to be perfectly smooth as a surface. However, the roughness measured in the bending direction has a significant impact on bending durability. A key requirement in this invention is the coexistence of strength and roughness, and the direction in which roughness is measured must coincide with the direction in which strength is measured in the tensile test. Strength and roughness measured in the same direction are a paired requirement. It is normal for roughness to differ in the measurement direction within the surface of rolled stainless steel foil, and the stainless steel foil of this invention must satisfy the specified strength and roughness in one of the directions within the surface. When used as stainless steel foil for foldable devices, the direction that satisfies the specified values ​​of this invention will be used as the bending direction. The specified values ​​for roughness in the bending direction will be explained below.

[0038] Under the severe repeated bending conditions assumed for the stainless steel foil of this invention, the maximum height roughness Rz of the stainless steel foil surface measured in the bending direction must be 0.35 μm or less, preferably 0.30 μm or less, more preferably 0.25 μm or less, and ideally 0.20 μm or less. When there are no inclusions such as oxides that could act as fracture initiation points near the surface of the stainless steel foil, cracks often originate from the valleys of the surface irregularities when viewing a cross-section parallel to the bending direction and perpendicular to the foil surface. This is because, when bent, the maximum stress (principal bending stress) is generated in the bending direction of the surface of the stainless steel foil that forms the bending radius R, and if there are valleys in the roughness curve measured in the same direction as the principal bending stress on the surface of the stainless steel foil, stress concentration occurs at these locations. Furthermore, when a fine crack that has been introduced propagates in a direction perpendicular to the principal stress direction during repeated bending, if there are a series of valleys in that direction, the crack will propagate along those valleys. Under the repeated bending conditions assumed for the stainless steel foil of this invention, when Rz, determined from the roughness curve measured in the same direction as the bending direction, is 0.35 μm or greater, cracks tend to propagate particularly easily, reaching a length of 5 mm.

[0039] Typical indicators of roughness include the maximum height roughness Rz and the arithmetic mean roughness Ra, as defined in JIS B0601 (2001). While there is some correlation between the maximum height roughness Rz and Ra, the durability of the stainless steel foil under repeated bending conditions in this invention is strongly correlated with the maximum height roughness Rz. Furthermore, repeated bending durability is particularly strongly dependent on Rz in the bending direction, with less correlation to Rz in other directions. Roughness in directions other than the bending direction is desirable because if the adhesion strength between the light-emitting element and the adhesive decreases, delamination will occur in the bent region, causing display malfunctions or damage to the light-emitting element. In particular, two-dimensional irregularities perpendicular to the bending direction have little effect on the propagation of cracks that propagate along the ridges forming folds within the foil, and therefore have little effect on the bending durability of the stainless steel foil itself. However, they increase the contact area with the adhesive, which has a positive effect on the durability of the light-emitting panel itself against adhesive delamination.

[0040] In the stainless steel foil of the present invention, roughness in one direction perpendicular to the ridge line is important from the viewpoint of bending durability. There is no specific lower limit for Rz, but from the standpoint of industrial feasibility and ensuring adhesion strength with adhesives, an Rz of 0.05 μm or higher is desirable. This is because if the Rz in the bending direction is reduced to this level, it will also decrease in other directions, making it difficult to ensure adhesion strength with adhesives due to the anchoring effect. In other words, the stainless steel foil of the present invention does not need to have good strength and smoothness in all directions; rather, the Rz of the stainless steel foil surface should be 0.35 μm or less, and there should be a direction within the stainless steel foil surface in the same direction as the direction in which the Rz is measured, where the tensile strength is 1800 MPa or higher. Based on this idea, the stainless steel foil of the present invention can be realistically manufactured as an industrial material.

[0041] The Rz value of the stainless steel foil in this invention shall be measured according to JIS B0601 (2001), using the stylus method. The measurement conditions are: measurement length 1.25 mm, cutoff (λc) 0.25 mm, cutoff (λs) 0.0025 mm, stylus scanning speed 0.3 mm / sec, and measurement load 0.7 mN. The measuring probe shall be a cone with a radius of 2 μmR and a tip opening angle of 60°. Measurements shall be taken at five or more different locations on each side of the stainless steel foil, and the average value shall be adopted. If there is a significant difference in roughness between the two sides of the stainless steel foil, the roughness measured on the side with the worse roughness shall be adopted.

[0042] More preferably in the present invention is a stainless steel foil having a thickness of 0.05 mm or less, a ratio of the maximum valley depth Rv to the Rz (Rv / Rz) of 0.6 or less, and a fracture elongation of 1% or more and 2% or less when the tensile strength is measured. The stainless steel foil of the present invention is particularly suitable for applications where the bending curvature is large, R is 5 mm or less, and is subjected to repeated bending, and is especially effective when it is necessary to reduce the foil thickness to 0.05 mm or less. Also, when considering the yield of the material, it is preferable to cut and use the foil with the rolling direction as the bending direction. The smaller the foil thickness, the greater the influence of Rz, but in the case of a high-strength foil like the present invention, oil marks are what particularly affect Rz in the rolling direction. Oil marks are marks left when oil is discharged perpendicular to the rolling direction during rolling, and thus form depressions. Therefore, the maximum valley depth Rv that constitutes the maximum height roughness Rz tends to be larger in the rolling direction, but the size of this Rv is sensitive to bending durability, and it is desirable to keep Rv / Rz at 0.6 or less while ensuring strength.

[0043] Furthermore, in the low-cycle fatigue region addressed by this invention, a high elongation at break is generally desirable. However, when a tensile test is performed in the rolling direction on the stainless steel foil of this invention, it is desirable that the elongation at break be in the range of 1% to 2%. Generally, stainless steel foils with a large elongation at break have low strength, but even high-strength stainless steel foils that meet the specified strength of this invention can have an elongation at break exceeding 2%. Such stainless steel foils have a clear yield point, and if a strain beyond that point is applied, a large plastic deformation occurs, resulting in a large bending tendency. This is not a problem if the yield strength is sufficiently high relative to the bending curvature, but the stainless steel foil of this invention is intended for applications where bending and straightening are repeatedly performed under conditions where the curvature is larger than conventionally observed relative to the foil thickness. Moreover, even if the magnitude of the strain estimated from the bending curvature is smaller than the yield strain in the tensile test, repeated bending in foldable devices is expected to exceed that value. Foldable device displays are generally composed of reinforcing metal foil, organic EL elements, and adhesives to bond them together. However, the rigidity of the elements and adhesives other than the metal foil is low, and the metal foil is used to compensate for this lack of rigidity. Since the bendable portion of the display is designed to bend in free space, it is conceivable that large instantaneous strains may be applied due to vibrations, etc., exceeding the static design strain. For example, in the case of stainless steel foil with an upper yield point, if a strain exceeding the yield strain is applied, there is a risk of large plastic deformation and the development of a large bending habit. Therefore, in the stainless steel foil of the present invention, even if the strength is the same, a material in which the stress increases significantly in response to strain even above the yield point is preferable to a material with a clear yield point and large elongation at break, and it has been found that a material with an elongation at break of 2% or less is preferable. However, materials with poor ductility of less than 1% are not very preferable.

[0044] (Manufacturing of stainless steel foil according to the present invention) In the case of flexible stainless steel foil, it has been conventionally difficult to achieve both the required strength and roughness for the stainless steel foil of the present invention. An effective strategy for achieving both the required strength and roughness for the stainless steel foil of the present invention requires consideration of all factors, including the rolling speed, the grinding grit of the work rolls, the number of passes, and the tension heat treatment conditions after rolling. Stainless steel foil for foldable devices is often roughly rectangular in shape. When cutting it from a rolled material to make a product, in practice, when cutting a rectangular material from a rolled material to make a product, from the standpoint of yield, in this invention, the material is cut so that the four sides of the rectangle are aligned with the rolling (RD) direction and its perpendicular (TD) direction.

[0045] One way to increase the tensile strength of the foil against bending is to increase the total rolling ratio. Furthermore, by applying heat treatment at 300°C or higher after rolling, carbon and nitrogen in the stainless steel foil are rearranged in defects such as dislocations formed by processing strain, improving yield strength and tensile strength. This suppresses bending tendency with repeated bending, and allows for a greater number of bends before the crack length reaches 5 mm.

[0046] On the other hand, applying strong rolling to achieve tensile strength increases defects that worsen the surface roughness, such as increased oil marks and transfer of work roll grinding marks. Oil marks primarily create valleys perpendicular to the rolling direction, while grinding marks primarily create irregularities in the rolling direction. The irregular shape caused by oil marks and grinding marks particularly affects the durability of the stainless steel foil of this invention. Therefore, it is necessary to obtain strength while suppressing these. Meeting the specified thickness, strength, and roughness of the stainless steel foil in this invention requires special ingenuity, but the specification of the roughness of the stainless steel foil in this invention does not require two-dimensional roughness; rather, it is sufficient to reduce roughness in at least one specific direction depending on the bending direction. When the rolling direction is the bending direction, a process that primarily focuses on reducing oil marks should be adopted. Furthermore, if the bending direction is to be perpendicular to the rolling direction, measures can be taken to reduce the transfer of grinding marks. To suppress oil marks, the oil film thickness on the roll bite can be reduced by lowering the rolling speed or increasing the rolling rate per pass. To suppress irregularities perpendicular to the rolling direction, the grinding grit of the work roll can be increased to create a roll with a small work roll roughness (arithmetic mean roughness Ra), and an additional polishing process can be added to create a work roll with even less roughness. In addition, work rolls with a dimpled surface created by blast processing can be used to eliminate grinding marks.

[0047] When manufacturing stainless steel foil having a high-strength multiphase structure with well-developed processing-induced martensite, as in the present invention, streaky irregularities parallel to the rolling direction tend to form, making it particularly difficult to achieve both strength and smoothness. However, with the flexible stainless steel foil of the present invention, this can be resolved by improving the roughness in the bending direction (maximum height roughness Rz). For example, if the material is designed so that the rolling direction is the bending direction, the main focus of countermeasures should be on reducing streaky irregularities that run perpendicular to the rolling direction due to oil marks. Irregularities other than those in the bending direction resulting from the multiphase structure of the martensite and austenite phases are advantageous because they can be used to increase the adhesion strength with adhesives.

[0048] The method for manufacturing the flexible stainless steel foil of the present invention will be described below based on the above manufacturing guidelines. The manufacturing of stainless steel foil according to the present invention requires control during the foil rolling process, particularly for foils with a thickness of 0.4 mm or less. For foil rolling, multi-stage rolling mills are commonly used to control plate thickness and roughness, and to stabilize mechanical properties, including strength. Cold rolling mills with 12 or more stages are particularly desirable for rolling. To achieve a tensile strength of 1800 MPa or more, the total rolling ratio in the final cold rolling process must be 45% or more. Here, the total rolling ratio refers to the percentage reduction in the thickness of the steel material before and after the final cold rolling process.

[0049] The total rolling ratio is defined by the following equation (A): Total rolling ratio (%) = 100 - (thickness of steel after the final cold rolling process) / (thickness of steel before the final cold rolling process) × 100 (A) The total rolling ratio is more preferably 50% or more, and even more preferably 55% or more. If the total rolling ratio is less than 45%, it becomes difficult to obtain a tensile strength of 1800 MPa or more. There is no particular upper limit on the total rolling ratio, but since a higher total rolling ratio generally increases the number of passes and deepens the oil marks, it is preferable to keep the total rolling ratio at 80% or less. In addition, it is preferable to have 4 to 10 passes in the final cold rolling process. A higher number of passes leads to a reduction in processing heat generation and promotes the phase transformation of processing-induced martensite. As a result, the tensile strength increases. On the other hand, the oil marks become deeper. Conversely, if the number of passes is too low, it is effective in suppressing oil marks, but sufficient tensile strength may not be obtained. To prevent this, measures are taken to suppress the rolling speed.

[0050] Oil marks are also controlled by the average rolling speed in the final cold rolling process. To obtain a smooth surface, it is preferable to keep the average rolling speed across all passes below 210 mpm. More preferably below 150 mpm, and even more preferably below 100 mpm. If the average rolling speed is too high, the entrainment of rolling oil increases, promoting the formation of oil marks, and the surface roughness measured in the rolling direction may increase.

[0051] The surface roughness of the steel material in the final cold rolling process, particularly the roughness in the TD direction, is also controlled by the roughness perpendicular to the rolling rotation direction of the work rolls. To obtain a steel material surface roughness Rz = 0.35 μm or less, the work roll roughness Ra perpendicular to the rolling rotation direction of the work rolls is preferably 0.40 μm or less, more preferably 0.30 μm or less, even more preferably 0.10 μm, and ideally 0.08 μm or less. If the work roll roughness is too high, the steel material surface roughness Rz will also increase, and sufficient bending durability may not be obtained. The lower limit is preferably a work roll roughness Ra = 0.01 μm or more in order to shorten the time required for grinding or polishing the work rolls.

[0052] The heat treatment process refers to either a bright annealing process or a low-temperature heat treatment process. Bright annealing is performed when cold rolling is not performed in the subsequent process, and low-temperature heat treatment is performed after the final cold rolling process. The purpose of the bright annealing process is to soften the material. A key feature is that by annealing in a reducing atmosphere containing hydrogen, such as ammonia decomposition gas, the formation of oxide films is suppressed, thereby suppressing surface defects in the subsequent cold rolling process. The heat treatment temperature in the bright annealing process is preferably 900 to 1200°C. If the heat treatment temperature is too low, the steel will not soften sufficiently, leading to an increase in the number of cold rolling passes. If the heat treatment temperature is too high, coarse crystal grains may result, and sufficient durability may not be obtained. As mentioned above, the purpose of the low-temperature heat treatment process is to strengthen the structure by rearranging carbon and nitrogen in lattice defects. The heat treatment temperature in this process is preferably 300 to 800°C. While heat treatment after cold working is not mandatory, if the heat treatment temperature is too low, carbon and nitrogen will not diffuse easily, and the structural strengthening will not proceed sufficiently. In addition, the elongation value may exceed the preferred elongation value at fracture for this invention. Conversely, if the heat treatment temperature is too high, the martensite phase, which is a factor in high strength, will disappear, and the processing strain will be released, making it impossible to obtain the required tensile strength.

[0053] The flexible stainless steel foil of the present invention can be used as a reinforcing material to increase the mechanical strength and durability of light-emitting electronic elements, such as planar organic EL display elements, which are bonded to the surface of foldable devices that can be folded or rolled up for storage. Currently, there are no applications other than flexible light-emitting devices including organic EL display elements that require the durability to withstand repeated bending as defined by the present invention, but if there is an application that requires similar severe repeated bending in one direction, the flexible stainless steel foil of the present invention can be applied. [Examples]

[0054] The flexible stainless steel foil of the present invention will be described in more detail below with reference to examples. It should be noted that the examples shown below are merely examples of the flexible stainless steel foil of the present invention, and the flexible stainless steel foil of the present invention is not limited to the examples shown below.

[0055] (Example 1) Stainless steel foil samples with thicknesses ranging from 30 μm to 100 μm were manufactured, and their durability was investigated through repeated bending tests. The manufactured stainless steel foils were ferritic SUS430 (steel grade 430: Fe-17.0%Cr-0.07%C-0.4%Si-0.6%Mn-0.02%P-0.001%S), austenitic stainless steel SUS301 (steel grade 301: Fe-17.0%Cr-6.5%Ni-0.12%C-0.06%N-0.5%Si-0.6%Mn-0.03%P-0.0008%S), and austenitic stainless steel NSSC130S (steel grade 130S: Fe-17.7%Cr-6.5%Ni-11.6%Mn-0.09%C-0.31%N-0.48%Si-0.022%P).

[0056] Stainless steel foil samples were manufactured as follows: Ferritic and austenitic stainless steel foils with a thickness of 300-400 μm were purchased, and the austenitic stainless steel foils underwent one or two cold rolling and bright annealing processes. The bright annealing process was intended to soften the material, and it was performed at a temperature of 900°C to 1200°C in a reducing atmosphere containing hydrogen, such as ammonia decomposition gas.

[0057] The final cold rolling process resulted in a thickness of 30 μm to 100 μm. Subsequently, evaluation samples were produced through a washing process and a low-temperature heat treatment (TA) process. A 12-stage reverse cold rolling mill was used for rolling. The total rolling percentage in the cold rolling process was set to 50% to 87%, and the number of passes was set to 5 to 15. The average rolling speed for all passes was set to 50 to 300 mpm. Three types of work rolls were prepared, with work roll roughness Ra perpendicular to the rolling rotation direction controlled to 0.08 or less, 0.2 μm or more, and less than 0.3 μm.

[0058] Low-temperature heat treatment (TA) was performed after the final cold rolling. This process is abbreviated as TA because it involves continuously annealing the stainless steel foil by applying tension along its length. The heat treatment temperature in the TA process was set to 350-900°C, and evaluations were also conducted on samples that were finished without some low-temperature heat treatment. The stainless steel foils manufactured as described above were subjected to tensile tests, surface roughness measurements, and repeated bending tests in various directions. Furthermore, the SUS301 foils were evaluated for their microstructure using EBSD (electron beam backscatter diffraction). In the EBSD microstructural evaluation of SUS301 foil, the proportion of the martensite phase in the stainless steel foil was determined. Crystalline phases within the sample were identified using an EBSD analyzer mounted on a field emission electron microscope (FE-SEM) on the stainless steel foil surface, which had been chemically polished to remove surface strain. Measurement conditions were 1500x magnification, a measurement area of ​​60 × 120 μm, and a measurement interval of 0.08 μm. Measurements were performed on three fields of view at different locations. The FE-SEM used was a Hitachi High-Technologies Corporation SU70, and the EBSD analyzer was a TSL Solutions Corporation OIM.

[0059] For the tensile test, a 150 mm long specimen conforming to the JIS No. 13B test specimen shape was cut from the stainless steel foil manufactured as described above, and the test was conducted using a contact strain gauge with a gauge length of 50 mm at a crosshead speed of 50 mm / min. The test direction was either the rolling (RD) direction or its perpendicular (TD) direction, the same direction as the repeated bending test. The load until fracture was monitored with a load cell, and the strength was defined as the maximum load divided by the sample cross-sectional area before the test. The elongation at fracture was defined as the elongation measured by the contact strain gauge at the time of fracture, expressed as a percentage of the initial measured length (gauge length). The strength and elongation at fracture were the average of the values ​​measured from five test specimens.

[0060] The surface roughness of the stainless steel foil manufactured as described above was measured using a stylus-type surface roughness meter in the same direction as the tensile direction in the tensile test. The measuring device used was a Tokyo Seimitsu stylus-type surface roughness meter (with tabletop vibration isolation stand), model: SURFCOM130A, and evaluation was performed according to JIS B0601 (2001). The measurement conditions were: measurement length 1.25 mm, cutoff (λc) 0.25 mm, cutoff (λs) 0.0025 mm, stylus scanning speed 0.3 mm / sec, and measurement load 0.7 mN. The probe used was a cone with a tip radius of 2 μm and an opening angle of 60°.

[0061] In this roughness measurement, a roughness curve is obtained from the contour curve, which is a displacement profile corresponding to the unevenness of the foil surface measured by the measuring probe in one direction. From this, the maximum height roughness (Rz) and maximum valley depth (Rv), which are roughness indices for the stainless steel foil of the present invention, are derived. Five arbitrary locations on different parts of the stainless steel foil surface were measured, and the average value of the five points was used as the measured value. In this embodiment, no significant difference in roughness was observed between the two sides of the stainless steel foil used, but in this embodiment, the worse value was used as the index.

[0062] For the repeated bending test, samples measuring 40 mm wide x 100 mm long were cut from stainless steel foil manufactured as described above, with the length oriented in either the RD direction or the TD direction. The repeated bending test was performed using a Yuasa Systems Equipment, no-load clamshell bending tester, model DR11MR. With the 100 mm length of the sample as the bending direction, the sample was repeatedly bent 180° in the center and then unbent 180°. By adjusting the gap when the sample was bent and closed, the bending curvature could be changed. As shown in Figure 4, setting the gap to 2R formed a bent section with a bending radius R. The bending cycle was set to 1 Hz. The test was continued until the number of repeated bending cycles reached 100,000.

[0063] Samples that showed no crack formation after 100,000 repeated bending cycles were rated A for crack evaluation, samples with at least one crack of 5 mm or longer were rated D, samples with a maximum crack length of 3 mm or more but less than 5 mm were rated C, and samples with cracks but a maximum crack length of less than 3 mm were rated B. In particular, samples judged as D and C sometimes showed multiple cracks, but based on the intended use of the stainless steel foil of this invention, the maximum crack length was used as the criterion for evaluation, with D-rated samples being rejected and all others being passed. Through the evaluation, it was found that most samples judged as B had cracks of less than 1 mm, and cracks propagated relatively quickly when the crack length exceeded 1 mm.

[0064] For samples that did not completely break after 100,000 cycles, the specimen was removed from the test fixture without applying excessive force, and the remaining free opening angle of the stainless steel foil was measured. Since the opening angle changes due to the weight of the stainless steel foil when it is laid flat, the stainless steel foil was placed upright on a flat table with the bent edge perpendicular to the tabletop surface, and a digital camera was used to photograph the top edge from directly above. The angle attached to the stainless steel foil was then measured using the image. If the bending curvature was small and the stainless steel foil could not stand on its own, the bending curvature of the stainless steel foil was not altered, and plates were applied to both sides of the stainless steel foil for measurement. Samples that returned to an opening angle of 180° without any bending curvature were designated as A, those with an opening angle of 175° or more but less than 180° were designated as B, those with an opening angle of 170° or more but less than 175° were designated as C, and those with an opening angle of less than 170°, i.e., those with a bending curvature of 10° or more, were designated as D.

[0065] Table 1 summarizes the manufacturing methods and evaluation results for stainless steel foil. The following parameters are important for the manufacturing process: total rolling rate (%), arithmetic mean roughness Ra (μm) representing the work roll roughness, number of passes in the final rolling process, rolling speed (m / min.), and annealing temperature after the low-temperature heat treatment process (TA temperature: (°C)).

[0066] In Table 1, the bending direction in the repeated bending test was the same for both the tensile test direction and the roughness measurement direction. RD indicates that the measurement direction was the rolling direction, and TD indicates that it was perpendicular to the thickness direction. Furthermore, the EBSD measurements were performed and analyzed on the premise that the material consists of two phases: an austenite phase with a face-centered cubic structure and a martensite phase with a body-centered cubic structure. The number of points determined to be martensite relative to the total number of measurement points, i.e., the area ratio of the martensite phase, was presented as the M phase ratio (%). The M phase ratio was calculated by rounding the average value from three different locations to the nearest whole number.

[0067] Since both microcrack resistance and bending curvature are necessary characteristics for the durability of the stainless steel foil of this invention, any foil that receives a D rating in either the crack evaluation or the bending curvature evaluation will be rejected.

[0068] [Table 1]

[0069] Samples 1-11 are stainless steel foils with a thickness of 40 μm. Sample 1, made of steel 430, is a ferritic stainless steel that is relatively inexpensive and easy to smooth the surface of. However, compared to other stainless steel foils, its strength did not increase even when the total rolling ratio was increased. Steels 130S and 301 are austenitic stainless steels. Steel 130S is a stainless steel foil mainly composed of the austenite phase, but steel 301 was found to have a composite structure of austenite phase containing a large amount of martensite phase, as revealed by X-ray diffraction and electron backscatter diffraction. This is because the austenite phase underwent martensitic transformation due to strong cold working. Repeated bending tests were performed on these stainless steel foils at a bending radius of 4.0 mm.

[0070] Stainless steel foils of sample numbers 2, 3, and 6-11, which satisfy the material requirements of the present invention in terms of tensile strength and maximum roughness Rz, are stainless steel foils of the present invention with excellent durability as foil double devices. On the other hand, sample number 1, which had a lower Rz than sample numbers 2 and 3 and was superior, exhibited crack elongation and significant bending tendency, resulting in inferior bending durability and failing to meet the criteria of the present invention. This is because steel material 430, which is mainly composed of the ferrite phase, is easy to finish with a smooth surface, but did not provide sufficient strength. While steel materials 130S and 301 offer sufficient strength, unlike steel material 430, the Rz value could not be reduced without significantly lowering the rolling speed. Only by reducing the rolling speed, using smooth rolls, and ensuring an appropriate number of passes could durable stainless steel foils like those shown in samples 2, 3, and 6-11 be obtained.

[0071] Sample No. 2 is an austenitic stainless steel, characterized by its non-magnetic properties. Therefore, it is useful as a reinforcing stainless steel foil for foldable devices where non-magnetic properties are required. There was almost no permanent deformation (bending curvature) at the ridges. On the other hand, even though Sample No. 4 is the same steel type, it developed cracks after fewer bending cycles than Sample No. 2, despite having higher strength. This was due to the roughness in the bending direction. The roughness of the work rolls used in the final rolling was the same, but the higher rolling speed resulted in larger oil marks, which in turn increased Rz, especially in the rolling direction, and also increased the maximum valley depth Rv. When repeated bending tests were performed on Sample No. 4 under the same conditions with a bending radius of 5 mm, the crack evaluation result was B, indicating that under conditions where R / t exceeds 125, durability of 100,000 cycles can be obtained without paying too much attention to the manufacturing method. When repeated bending is performed at a severe bending curvature, especially with thin foils, the effects of Rz and Rv / Rz become strong, at a level that is not a problem with sheets.

[0072] The influence of Rz and Rv / Rz on repeated bending durability is similar for 301 steel materials of different grades. Furthermore, in 301 steel foil, the microstructure is also affected by differences in manufacturing conditions. Among the 301 steel materials, sample number 5 showed no cracking at all, but exhibited a significantly increased bending tendency. This is because the TA temperature was too high, causing softening and a decrease in strength. As a result, the bending strain under the bending conditions in this test exceeded the yield point, leading to large plastic deformation.

[0073] Samples 5 and 6, like sample 1, exhibit high ductility. While it is generally believed that greater elongation at break indicates better durability in low-cycle fatigue conditions like those of this test, the tests conducted here revealed that for the stainless steel foil of the present invention, an elongation at break of 2% or less is preferable. Samples 5 and 6 also have a small proportion of the martensite phase (M phase). This is because increasing the TA temperature after rolling transformed these materials into the equilibrium phase, the austenite phase.

[0074] In this embodiment, represented by sample number 3, the 301 steel foil exhibiting excellent durability has a martensite phase proportion of over 60%, and EBSD analysis revealed that the martensite structure viewed from the foil surface appears to have island-like austenite phases distributed within the martensite phase. The size of the austenite phases was 20 μm or less in the TD direction, but they had an elongated shape in the RD direction. This structure enhances strength and yield strength, and further inhibits the propagation of extremely fine fatigue cracks that occur due to repeated bending.

[0075] Even among samples 7 to 11, which met the requirements of the present invention, there were differences in their durability as reinforcing stainless steel foil for foldable devices. Samples 7 and 8 were subjected to the same cold rolling conditions, but sample 7 was finished without low-temperature heat treatment (TA) after cold rolling, while sample 8 was finished with TA at a temperature of 350°C. No significant differences were observed in strength or roughness, but the proportion of martensite phase was lower in sample 7. This is thought to be due to the smaller total rolling ratio, but the proportion of martensite phase increased after performing TA at 350°C. As a result, although the difference in strength was not large, the stress-strain obtained in tensile tests differed significantly. Sample 7 showed clear yielding and a large elongation at fracture, while sample 8 did not show clear yielding and a small elongation at fracture. The strain indicating yield was smaller than the strain estimated from the thickness and curvature of the stainless steel foil in this test. However, in more dynamic repeated bending tests, the stainless steel foil is not constrained near the bending ridges, so vibrations cause localized strain increases, leading to localized yielding. This is thought to increase plastic deformation and reduce durability. In the foldable device envisioned by this invention, the same situation is expected to occur because the rigidity of the organic EL element is smaller than that of the stainless steel foil. Even with stainless steel foils of similar high strength, contrary to the metallurgical idea that a higher elongation at fracture is desirable in low-cycle fatigue, it was found that samples like sample number 8, in which the processed martensitic structure is well-developed, resulting in a smaller elongation at fracture and an unclear yield point, are preferable.

[0076] Sample No. 9, like Sample No. 7, is SUS301 finished without low-temperature heat treatment (TA) after cold rolling. However, by increasing the total rolling ratio, the martensite ratio could be increased even without low-temperature heat treatment, resulting in a stainless steel foil with relatively high bending durability.

[0077] The results for sample number 10 and sample number 11 show that the difference in durability was due to the different pass schedules during cold rolling. Sample 10 and sample number 11 differed in the number of passes during the final cold working stage. Sample number 10, with its increased number of passes, had larger oil marks, resulting in a larger Rz, especially Rv, which increased its bending sensitivity in the RD direction, and consequently, increased crack occurrence frequency and propagation speed, which are aspects of bending durability. On the other hand, sample number 11, with its reduced number of passes, resulted in a stainless steel foil with higher strength and a smoother surface, thus improving durability under the bending conditions required for foldable devices. Attempts to further reduce the number of passes resulted in an increase in fracture during rolling.

[0078] Samples 12, 13, and 14 are 301 steel foils with a thickness of approximately 100 μm. X-ray diffraction and EBSD (electron backscatter diffraction) revealed that they have a composite structure of austenite and martensite phases, similar to sample 3. The proportion of martensite phase was smaller than that of sample 3. Under the rolling conditions used to produce samples 12 and 13, strengths of 1800 MPa or higher were achieved only in the direction perpendicular to the rolling direction. Samples 12 and 13 differed in the roughness (Ra) of the rolling rolls, and the roll roughness particularly affects the roughness in the direction perpendicular to the rolling direction. As a result, the durability in the direction perpendicular to the rolling direction differed significantly. Despite sample 13 having higher strength than sample 12, sample 13 showed inferior durability due to crack elongation because its Rz value was greater than the standard value.

[0079] Sample No. 12 does not meet the strength criteria in some directions other than the TD direction, including the RD direction, but it satisfies the strength and roughness conditions in the TD direction and is a stainless steel foil of the present invention that is durable against 100,000 repeated bending cycles in the TD direction. On the other hand, the foil of sample 13, which had a larger roll gauge, had increased surface roughness in the TD direction and no longer met the specifications of the present invention, resulting in a particularly reduced break life.

[0080] Sample No. 14 is a foil in which the strength in the RD direction also satisfies the present invention as a result of increasing the total rolling ratio. Compared with Sample No. 12, the strength is easier to increase and a material with greater elongation at break is obtained in the TD direction, perpendicular to the plane, rather than in the rolling (RD) direction. However, by increasing the total rolling ratio, strength equivalent to that in the TD direction of Sample No. 12 and Sample No. 13 was obtained in the RD direction. It was found that if the strength and surface properties are equivalent, the bending durability is better when bent in the RD direction. Therefore, it was found that when used in foldable devices, it is preferable to use it so that the bending direction is in the RD direction. At this time, controlling the unevenness caused by oil marks is important, and special consideration is needed for the total rolling ratio and the number of passes to satisfy the strength requirements.

[0081] Samples 15 and 16 are foils of steel 301 with a thickness of approximately 50 μm. X-ray diffraction and EBSD (electron backscatter diffraction) revealed that they have a similar microstructure to sample 3, and are composite structures with an austenite phase containing a large amount of martensite phase. The difference in manufacturing methods between sample 15 and sample 16 is the number of rolling cycles. As a result, the Rz value of sample 16 was significantly different. The reason why the Rz value of sample 15 was smaller than that of sample 16 is that reducing the number of rolling cycles reduced the unevenness caused by oil residue, and in particular the unevenness when measured in the rolling direction was reduced. As a result, the crack evaluation result of sample 15 was excellent, and it is the stainless steel foil of the present invention.

[0082] Controlling surface irregularities by adjusting the roll roughness and pass count is effective for controlling irregularities in the rolling (RD) direction and its perpendicular direction (TD), respectively. Conversely, these have relatively little effect on irregularities in other directions. Since the stainless steel foil of this invention is intended as a reinforcing material used in foldable devices to which planar light-emitting electronic elements are attached with adhesive, low surface roughness makes it difficult to ensure adhesion with the adhesive. From the viewpoint of reducing roughness in the bending direction while maintaining roughness in other directions to ensure adhesion, controlling surface irregularities in one direction by adjusting the roll roughness and pass count is effective.

[0083] Sample numbers 17-19 are examples of materials suitable for implementation within the scope of the present invention, with a thickness of 30 μm, which were manufactured and evaluated. By manufacturing the product in a way that balances strength in the bending direction with maximum roughness, it was found that it is possible to provide stainless steel foil with excellent durability even under the demanding conditions of repeated bending with a bending radius of 1.5 mm to 2.25 mm. By using work rolls with a work roll roughness of Ra of 0.1 μm or less, reducing the total rolling rate to 50%, and keeping the rolling speed below 80 mpm, processing heat generation during cold working was suppressed, resulting in a stainless steel foil with a tensile strength of 1900 MPa or more and an Rz of 0.30 μm or less in that direction, achieving a balance between strength and roughness. As a result, it was possible to ensure durability of 100,000 cycles with minimal bending memory under the demanding conditions of R / t of 75 and an unfolding angle of 180°. Furthermore, by suppressing the rolling speed to 50 mpm to ensure strength, and by limiting the final rolling pass count to 5, the work roll roughness was reduced to 0.1 μm or less, and even to 0.08 μm or less. This allowed for an Rz of 0.25 μm, and even 0.20 μm or less, ensuring durability of 100,000 cycles under the demanding conditions of an R / t of 75, 67, and even 50 with a 180° unfolding angle.

[0084] Samples 17-19 are stainless steel foils composed of austenite and martensite phases, particularly those containing a large amount of martensite. The crystal grains are elongated in the rolling direction, and the foil has a lamellar structure with small grain sizes in the foil pressure direction. However, EBSD results show that the phase distribution from the foil surface direction appears to be a structure in which the austenite phase is dispersed in island-like manner within the martensite phase. The test was stopped when cracks began to form, and the surface of the stainless steel foil was polished and observed. The phases were identified by EBSD, and it was found that the cracks generally propagated in the direction of the bending ridge, but the direction of propagation changed or branched into two along the way, and the austenite phase was observed beyond the points where the crack propagation was hindered. Furthermore, the areas on both sides through which the cracks passed were martensite phase. It was found that the cracks propagated while transforming the austenite phase into the martensite phase, and that the austenite phase had the effect of hindering crack propagation. The high-strength martensite phase formed as a result of strong cold working plays an important role in suppressing bending habits under harsh repeated bending conditions. However, in addition to reducing the roughness in the bending direction perpendicular to the crack propagation direction, the presence of an austenite phase coexisting with the martensite phase is considered to be even more effective in suppressing crack propagation.

[0085] While it is difficult to reduce the surface roughness of stainless steel foil, which has high strength and is largely composed of martensite phase, by focusing on lowering the Rz in one direction (the bending direction) without aiming for an overall reduction in roughness, we were able to achieve stainless steel foil with excellent bending durability, which is the objective of this invention. As described above, it was difficult to achieve both the strength and surface properties required by the present invention under conditions that are industrially viable for the rolling speed. However, based on a carefully considered approach, in this embodiment, the stainless steel foil of the present invention was obtained by using rolls with a work roll roughness of 0.3 μm or less (Ra), a total rolling rate of 50% or more and 80% or less, a number of passes of 5 or more and 10 or less, and an average rolling speed of 205 mpm or less, and either producing the final product as a cold-rolled material or finishing it by tempering it under conditions of a TA temperature of 700°C or less. More preferably, it was found that it is sufficient to use rolls with a work roll roughness controlled to 0.1 μm or less (Ra), a total rolling rate of 65% or more and 80% or less, a number of passes of 5 or more and 8 or less, an average rolling speed of 205 mpm or less, and a TA temperature of 350°C or more and 700°C or less.

Claims

1. A flexible stainless steel foil having a thickness of 0.1 mm or less, a tensile strength of 1800 MPa or more, and a maximum height roughness Rz of 0.35 μm or less, determined from the surface roughness curve of the stainless steel foil measured in the same direction as the tensile direction.

2. The flexible stainless steel foil according to claim 1, wherein the thickness is 0.05 mm or less, the ratio of the maximum valley depth Rv to Rz (Rv / Rz) is 0.6 or less, and the elongation at break when the tensile strength is measured is 1% or more and 2% or less.

3. When the material is repeatedly bent 180° in the same direction as the tensile direction, satisfying R / t = 100 (where R is the bending radius (mm) and t (mm) is the thickness), and then returned to 0°, The number of repeated bending cycles until a crack of 5 mm or longer occurs on the surface of the stainless steel foil is 100,000 or more. The flexible stainless steel foil according to claim 1 or claim 2, wherein the bending curvature after 100,000 repeated bending tests at an unfolding angle of 180° or more is 170° or more at an opening angle.

4. The Rz is 0.30 μm or less, When the bending is repeated in the same direction as the tensile direction, with a value satisfying R / t = 75, and then returned to 0°, The number of repeated bending cycles until a crack of 5 mm or longer occurs on the surface of the stainless steel foil is 100,000 or more. The flexible stainless steel foil according to claim 1 or claim 2, wherein the bending curvature after 100,000 repeated bending tests at an unfolding angle of 180° or more is 170° or more at an opening angle.

5. The flexible stainless steel foil according to any one of claims 1 to 4, wherein the stainless steel foil comprises, by mass%, C: 0.15% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030% or less, Ni: 6.00 to 8.00%, Cr: 16.00 to 18.00%, and N: 0.20% or less, and includes a martensite phase.

6. The flexible stainless steel foil according to any one of claims 1 to 5, wherein the area ratio of the martensite phase in the cross-section of the stainless steel foil is 50% or more.

7. The flexible stainless steel foil according to any one of claims 1 to 6, wherein a planar flexible light-emitting electronic element is bonded to the surface of the stainless steel foil.

8. The flexible stainless steel foil according to claim 7, wherein the flexible light-emitting element is an organic EL display element.

9. A flexible light-emitting device comprising a flexible stainless steel foil and a planar flexible light-emitting electronic element according to any one of claims 1 to 6.

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