Electromagnetic wave shielding material

Abstract

The present invention addresses the problem of providing an electronic equipment casing having electromagnetic shielding properties in a high-frequency band up to the GHz band, as well as having strength and workability even when the thickness is reduced.　The present invention provides an electromagnetic shielding material using stainless steel, characterized in that a first region made of stainless steel mainly composed of an austenite phase and a second region made of stainless steel mainly composed of a ferrite phase are laminated in the sheet thickness direction in order to improve the electromagnetic shielding properties.

Description

Technical Field

[0001] The present invention relates to an electromagnetic shielding material.

Background Art

[0002] Currently, for electromagnetic countermeasures (EMC countermeasures) of electronic devices, sintered ferrite is used for electromagnetic wave absorption, and electromagnetic shielding films, electromagnetic shielding coating agents, etc. are used for electromagnetic shielding (electromagnetic wave shielding). Sintered ferrite is excellent in electromagnetic wave absorption properties, is used by being mixed with resin or the like and applied to an electronic device housing, and is effective in a low frequency band of several tens of MHz or less. Electromagnetic shielding films include those obtained by pressure-bonding a metal plate (metal foil) and a resin layer, or those obtained by applying a conductive material in which metal or magnetic powder is mixed with resin to a resin or cloth serving as a substrate. For example, Patent Document 1 proposes an electromagnetic shielding film in which a protective layer, a metal layer, and an adhesive layer are laminated. Patent Document 2 proposes an electromagnetic shielding film in which a conductor layer is laminated on both sides of a flame-retardant cloth, and a rust preventive layer is further laminated on the upper surface thereof. The electromagnetic shielding coating agent reflects electromagnetic waves by being applied to the housing of an electronic device, and prevents the intrusion and interference of electromagnetic waves into the inside of the housing. For example, Patent Document 3 proposes a spray coating agent for electromagnetic shielding containing silver particles dispersed in a solvent, a thermosetting resin, and a curing agent.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] In recent years, in line with the explosive increase in communication data volume and the need for higher communication speeds, the frequency of carrier radio waves has been increased as a means of achieving high-capacity and high-speed data communication. Along with this, in electronic devices that transmit and receive high-frequency radio waves, the frequency band of electromagnetic waves to be blocked has been shifted to higher frequencies. Since the conventional electromagnetic wave shielding in the low-frequency band of less than 10 MHz causes malfunction of electronic devices, recently, electromagnetic wave shielding in the high-frequency band of 10 MHz or more has been demanded.

[0005] Furthermore, while the miniaturization and weight reduction of electronic devices are progressing, the device shapes are also diversifying. With conventional electromagnetic wave shielding films and coating agents, the workability is poor, and it has become difficult to completely shield electromagnetic waves. Therefore, it is required to configure the housing (casing) of electronic devices and components themselves with an electromagnetic wave shielding material. In order to be applied to the housing of an electronic device, it must have strength and workability when thinned for miniaturization and weight reduction, but electromagnetic wave shielding films and coating agents are not suitable for strength members in the first place. Aluminum foil also lacks strength and is not suitable for the housing because it breaks during drawing processes, especially when thinned.

[0006] Therefore, an object of the present invention is to propose an electromagnetic wave shielding material that has electromagnetic wave shielding properties in a high-frequency band of 10 MHz or more and also has strength and workability even when thinned, in order to meet the need for adding electromagnetic wave shielding characteristics to the housing of electronic devices.

Means for Solving the Problems

[0007] The inventors of the present invention have intensively studied to achieve the above problems and obtained the following findings. [a] Selection of a metal material suitable for electronic devices and component housings In order to meet the needs for miniaturization and weight reduction of electronic devices and components (hereinafter collectively referred to as electronic devices), a metal material with high strength and workability even in an extremely thin thickness was studied. Silver, copper, and aluminum are metals with high conductivity and good electromagnetic shielding properties. However, since these metals have low strength, they are easily damaged when processed into complex shapes, presenting practical problems. Therefore, although the conductivity is inferior to that of aluminum or the like, the application of stainless steel foil (hereinafter sometimes referred to as "stainless steel sheet") which has a certain strength and workability even when extremely thin was envisioned and further studied for realization.

[0008] [b]Improvement of Electromagnetic Shielding Property The electromagnetic shielding property of stainless steel is based on the following two mechanisms. ·Basically, electromagnetic wave energy is attenuated (lost) by reflection of electromagnetic waves on the outer surface of the metal material (outer surface reflection loss).

[0009] ·Furthermore, the electromagnetic waves that penetrate into the metal are absorbed and attenuated in the metal (absorption loss). Absorption loss of electromagnetic waves also occurs in stainless steel, and since ferritic stainless steel has better conductivity than austenitic stainless steel, the absorption loss effect is greater in the high-frequency band of 10 MHz or more.

[0010] These outer surface reflection loss and absorption loss are phenomena that occur in any metal material and depend on the dielectric constant and magnetic permeability of the metal. Therefore, although stainless steel has inferior conductivity compared to copper or aluminum, in order to improve the electromagnetic shielding property, the inventors have intensively studied and obtained the following findings.

[0011] ·When an austenite phase (sometimes referred to as γ) and a ferrite phase (sometimes referred to as α) exist in stainless steel, electromagnetic wave energy is attenuated by reflection of electromagnetic waves at the interface (phase interface reflection loss).

[0012] ·By repeating the reflection at the interfaces of a plurality of austenite phases and ferrite phases, the attenuation effect is further enhanced.

[0013] · On the one hand, when reflection in random directions occurs at the grain boundaries, electromagnetic waves may pass through the grain boundaries, penetrate the stainless steel foil, and leak into the interior of the housing. To prevent this, it was conceived to laminate in layers in the plate thickness direction a region mainly composed of the austenite phase (a region where the area ratio of the austenite phase is 75% or more and 100% or less; also called austenitic stainless steel) and a region mainly composed of the ferrite phase (a region where the area ratio of the ferrite phase is 95% or more and 100% or less; also called ferritic stainless steel). As a result, unlike the interface reflection when one phase exists as a matrix and the other phase exists as crystal grains, since the interface between the two phases always exists in the plate thickness direction, the reflected wave will always collide with the interface between the two phases and be reflected and attenuated. Therefore, it has been found that the electromagnetic wave energy finally reaching the interior of the housing through the stainless steel foil is significantly reduced.

[0014] · Furthermore, it has also been found that when precipitates (such as oxides and nitrides) exist in the stainless steel foil, the electromagnetic waves are also attenuated by reflection at the interface with the precipitates (grain boundary interface reflection loss).

[0015] [c] Improvement of workability Stainless steel foil has high strength compared to aluminum foil, copper foil, and silver foil of the same thickness, is resistant to damage during processing (such as cracks and pinhole openings), and has excellent workability. Taking advantage of this characteristic, it has already been applied to the housings of electronic devices and battery cases, etc. To improve the electromagnetic shielding property, it has been explained that it is advisable to laminate the austenite phase and the ferrite phase in layers. It has been found that this two-phase laminated stainless steel foil also has advantages from the viewpoint of improving workability.

[0016] Usually, austenitic stainless steel has better workability than ferritic stainless steel. On the other hand, the ferrite phase has little work hardening and the strain is locally concentrated, resulting in a decrease in the ductility as a foil. Therefore, by laminating these two phases, the ferrite phase can also be deformed following the austenite phase with excellent ductility during deformation, so that the strain can be dispersed and the ductility as a foil can be improved.

[0017] Furthermore, in general casing processing, drawing and bending are frequently performed, and maximum tensile stress is generated on the surface of the plate, making it prone to cracking. Therefore, it has also been found that by making the surface part into an austenite phase, the applicability to casing processing is improved.

[0018] [D] Method for manufacturing laminated stainless steel foil of austenite phase and ferrite phase As described above, there are various methods for manufacturing stainless steel foil with the austenite phase and the ferrite phase laminated in layers. For example, a clad steel plate with an austenitic stainless steel plate and a ferritic stainless steel plate laminated alternately can be created, and stainless steel foil with both phases laminated can be obtained by an ordinary foil rolling process.

[0019] Also, for example, a ferritic stainless steel foil obtained usually is nitrided to contain nitrogen on the surface and made into an austenite phase, thereby obtaining stainless steel foil with the austenite phase and the ferrite phase laminated. By this method, stainless steel foil with one surface or both surfaces being the austenite phase and the central part being the ferrite phase can be obtained. Furthermore, by controlling the heat treatment of the nitriding treatment, precipitation control of precipitates in the austenite phase of the surface layer part becomes possible. With this method, since existing ferritic stainless steel foil only needs to be nitrided, it can be manufactured by utilizing existing processes relatively simply.

[0020] Based on the above findings, it has been found that by applying a multi-layer stainless steel foil with the austenite phase and the ferrite phase laminated in layers in the thickness direction, the electromagnetic shielding effect can be enhanced, and an electromagnetic shielding material having workability and strength equivalent to those of conventional stainless steel foil can be obtained, thus completing the present invention. The gist of the present invention is as follows.

[0021] [1] An electromagnetic shielding material characterized in that a first region made of stainless steel having a plate shape with a plate thickness of 5 to 200 μm and an austenite phase with an area ratio of 75% or more and a second region made of stainless steel having a ferrite phase with an area ratio of 95% or more are laminated in the plate thickness direction. It is preferably structured such that layers are alternately stacked in the plate thickness direction, and the number of layers is not particularly limited. The first region and the second region may each be one layer for a total of two layers, or one region may be one layer and the other region may be two layers for a total of three layers, or there may be a multi-layer structure with more layers. [2] The electromagnetic shielding material according to [1], wherein at least one surface is the first region. [3] The electromagnetic shielding material according to [1], wherein both surfaces are the first region. [4] The electromagnetic shielding material according to [2] or [3], having precipitates containing at least one of carbide, oxide, and nitride in the first region, the precipitates having a particle size of 1 μm or less, and the precipitates having a particle size of 0.1 μm or more having an area ratio of 10% or less in the cross-section in the plate thickness direction. [5] The electromagnetic shielding material according to any one of [1] to [4], having a plate thickness of 5 to 50 μm. [6] The electromagnetic shielding material according to any one of [1] to [5], wherein the total area ratio of the second region in the cross-section in the plate thickness direction is 15% or more. [7] The chemical composition of the second region is, in mass%, Cr: 20.00 to 26.00%, N: 0 to 0.10%, Si: 0 to 2.00%, C: 0 to 0.040%, P: 0.030% or less, S: 0.030% or less, Mn: 0 to 1.50%, Cu: 0 to 0.50%, Mo: 0 to 3.00%, Ni: 0 to 5.00%, Ca: 0 to 50 ppm, sol.Al: 0 to 300 ppm, The electromagnetic shielding material according to any one of [1] to [6], the balance being Fe and impurities. [8] The chemical composition of the first region is, in mass%, Cr: 20.00 - 26.00%, N: 0.15 - 5.00%, Si: 0 - 2.00%, C: 0 - 0.040%, P: 0.030% or less, S: 0.030% or less, Mn: 0 - 1.50%, Cu: 0 - 0.50%, Mo: 0 - 3.00%, Ni: 0 - 5.00%, Ca: 0 - 50 ppm, sol.Al: 0 - 300 ppm, The balance: Fe and impurities, the electromagnetic wave shielding material according to [7] above.

Advantages of the Invention

[0022] According to the present invention, an electromagnetic shielding material having electromagnetic shielding properties for high - frequency band electromagnetic waves and having strength and workability can be obtained. With this electromagnetic shielding material, a housing (casing) of an electronic device or component having electromagnetic shielding properties can be manufactured.

Brief Description of the Drawings

[0023]

Figure 1

Modes for Carrying Out the Invention

[0024] Hereinafter, as an embodiment of the present invention, an embodiment having a three - layer structure of austenite phase (hereinafter, may be denoted as γ in some cases) - ferrite phase (hereinafter, may be denoted as α in some cases) - austenite phase (γ) will be taken as an example to explain the electromagnetic shielding material according to the present invention.

[0025] [First Region (Austenitic Stainless Steel)] The first region is a stainless steel mainly composed of an austenite phase (austenitic stainless steel), and is a region mainly composed of an austenite phase (with an area ratio of 75% or more and 100% or less) among all phases detectable by EBSD (Electron Back Scattered Diffraction) (including iron phases such as ferrite phase, austenite phase, martensite phase, compound phases such as Cr2N and CrN, and passive films). The area ratio of the austenite phase is preferably 80% or more, 85% or more, or 90% or more.

[0026] [Second Region (Ferritic Stainless Steel)] The second region is a stainless steel mainly composed of a ferrite phase (ferritic stainless steel). Similar to the first region, it is a region mainly composed of a ferrite phase (with an area ratio of 95% or more and 100% or less) among all phases detectable by EBSD (Electron Back Scattered Diffraction) (including iron phases such as ferrite phase, austenite phase, martensite phase, compound phases such as Cr2N and CrN, and passive films). The area ratio of the ferrite phase is preferably 92% or more, 94% or more, or 95% or more.

[0027] [Laminated Structure of the First Region and the Second Region] The first region (austenitic stainless steel) and the second region (ferritic stainless steel) are each in a layered form (layered parallel to the surface), and have a structure (laminated structure) in which these regions are alternately laminated in the thickness direction of the plate-shaped electromagnetic wave shielding material. As long as it is austenitic stainless steel, even different types of austenitic stainless steel belong to the first region. Similarly, as long as it is ferritic stainless steel, even different types of ferritic stainless steel belong to the second region.

[0028] The structure of the laminated structure is not particularly limited as long as the first region and the second region are alternately laminated. For example, it may have a total of two layers with the first layer being the first region and the second layer being the second region. For example, it may have a three-layer structure in the order of the first region - the second region - the first region. For example, it may also have a multilayer structure of four or more layers such as the first region - the second region - the first region - the second region - ······. In order to obtain the effect of interface reflection loss between the first region and the second region, a multilayer structure of three or more layers is preferred. Of course, in the above-mentioned layer order, the first region and the second region may be interchanged. On the other hand, since the manufacturing process of the multilayer structure becomes complicated, it is advisable to determine the number of layers in consideration of the electromagnetic shielding effect and the manufacturing cost.

[0029] [Partition portion] Each layer preferably has fewer divided portions (portions that are not austenite phase in the first region and not ferrite phase in the second region. Hereinafter, these are referred to as partition portions) that penetrate the layer. This is because if there are partition portions, electromagnetic waves will transmit through those portions. When each layer is viewed from directly above, it is desirable that the area ratio of the partition portions is 30% or less, 20% or less, 10% or less, 5% or less, 3% or less, 2% or less, 1% or less, or 0% (none).

[0030] [At least one surface is the first region] Since the austenite phase has better corrosion resistance than the ferrite phase, if at least one of the front and back surfaces of the electromagnetic shielding material is covered with stainless steel (the first region) mainly composed of the austenite phase, better corrosion resistance can be ensured. Also, from the perspective of workability, in the case of bending or drawing processes, it is preferable that the outermost layer where the maximum stress occurs is stainless steel (the first region) mainly composed of the austenite phase. Therefore, it is preferable that at least one surface portion (surface layer portion), preferably both surface portions (both surface layer portions) of the surface of the electromagnetic shielding material are the first region.

[0031] [Thickness of each layer] The thickness of each layer is not particularly limited.

[0032] [Method for measuring layer thickness and cross-sectional area ratio] The method for measuring the thickness of each layer will be described below. A test piece is taken from the electromagnetic shielding material so that the cross-section in the plate thickness direction becomes the observation surface. The observation surface is polished and etched. The etching solution is not particularly limited as long as it is an etching solution for stainless steel, but an etching solution with a volume ratio of aqua regia to glycerin of 4:1 can be preferably used.

[0033] The boundary between the first region and the second region in the electromagnetic wave shielding material can be clearly distinguished by etching. The boundary between the first region and the second region can also be confirmed by EBSD (Electron Back Scattered Diffraction). From the IPF map of EBSD, the region mainly composed of the austenite phase (γ) (the first region) and the region mainly composed of the ferrite phase (α) (the second region) can be confirmed. After determining the boundary between the first region and the second region by the method described above, the area of each layer is obtained by image analysis software.

[0034] For example, in the case of a three-layer structure in the order of the first region (γ) - the second region (α) - the first region (γ), if the area of the second region is A, the area of the first layer among the regions other than the second region (i.e., the first region) is B1, and the area of the third layer is B2, the cross-sectional area ratio of the second region is obtained from A / (A + B1 + B2). Similarly, the cross-sectional area ratio of the first layer is B1 / (A + B1 + B2), and the cross-sectional area ratio of the third layer is B2 / (A + B1 + B2). Since the first region and the second region are laminated in layers, the obtained cross-sectional area ratio becomes the ratio of the layer thickness. Therefore, if the cross-sectional area ratio is multiplied by the thickness of the electromagnetic wave shielding material, the layer thickness of each layer can be obtained.

[0035] When the electromagnetic shielding material is long, it is desirable to measure the cross-sectional area ratio with a plurality of cross-sections (for example, 10 cross-sections) obtained by evenly cutting the electromagnetic shielding material and obtain the average. The area of the precipitate in the first region is obtained from the distribution of metal elements such as C, N, O, and Cr, and the ratio of the precipitate can be calculated from the area of the first region.

[0036] [Precipitate] In the first region and the second region, one or more of precipitates containing at least any one of carbides, oxides, and nitrides may be dispersed. The precipitates are, for example, Cr nitrides, Cr oxides, Cr carbides, Fe oxides, Fe carbides, etc. The reason is that when precipitates are present, the effect of reflection loss of electromagnetic wave energy at the interface can be expected. In particular, when precipitates are present in the outermost layer (the layer on at least one surface side of the electromagnetic shielding material), reflection loss in the surface layer can be expected, and since the reflected wave is reflected by the interface between the first region and the second region, penetration of the shielding material can be suppressed.

[0037] On the other hand, when precipitates are present in stainless steel, strain concentrates around the precipitates during processing and becomes a starting point for fracture. In particular, coarse crystal grains not only become a starting point for stress concentration but also cause a reduction in the substantial thickness of the stainless steel foil, leading to fracture. Therefore, from the perspective of workability, it is preferable that there are no precipitates. Considering workability, the precipitates preferably have a particle size of 1.0 μm or less. The smaller the precipitates, the better, and the particle size is preferably 0.8 μm or less, 0.6 μm or less, 0.4 μm or less, 0.2 μm or less, or 0.1 μm or less. Also, the density of the precipitates is preferably 10% or less, 7% or less, 5% or less, or 3.5% or less in terms of the area ratio of precipitates with a particle size of 0.1 μm or more in the cross-section in the plate thickness direction of the electromagnetic shielding material.

[0038] The lower limit of the precipitate density (area ratio) is preferably 0%, but in the case of the outermost layer (surface layer), since the reflection loss effect at the precipitate interface can also be expected, it may be 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more in terms of the area ratio of precipitates with a particle size of 0.1 μm or more. Here, the particle size of the precipitates refers to the equivalent diameter of a circle with an area equal to that of the precipitates observed in the cross-section in the plate thickness direction of the electromagnetic shielding material.

[0039] [Components] The components of the electromagnetic shielding material according to this embodiment are not limited. The first region is not limited in components as long as it is stainless steel mainly composed of an austenite phase. The second region is also not limited in components as long as it is stainless steel mainly composed of a ferrite phase.

[0040] [Thickness of the plate] The thickness of the electromagnetic shielding material according to the present embodiment is not particularly limited. However, if the thickness is too thin, it becomes difficult to roll while maintaining a good shape, and it also becomes difficult to ensure foil flatness. Therefore, the thickness is preferably 5 μm or more. Preferably, it is 7 μm or more, 8 μm or more, 9 μm or more, or 10 μm or more. On the other hand, if the thickness is too thick, it becomes difficult to perform fine processing as a shielding material for small electronic device components such as sensors, and in addition, the weight of the electronic device increases and it cannot withstand practical use. Therefore, the thickness is preferably 200 μm or less. Preferably, it is 150 μm or less, 100 μm or less, 70 μm or less, 60 μm or less, 50 μm or less, 40 μm or less, 30 μm or less, 20 μm or less, or 10 μm or less.

[0041] [Manufacturing method] The manufacturing method of the electromagnetic shielding material according to the present invention is not limited as long as it has the above structure. For example, a clad steel plate in which an austenitic stainless steel plate and a ferritic stainless steel plate are laminated can be created, and a stainless steel foil in which both phases are laminated can be obtained by a normal foil rolling process.

[0042] Also, for example, by nitriding a ferritic stainless steel foil and infiltrating nitrogen from the surface to transform the surface layer part into an austenite phase, a stainless steel foil in which an austenite phase and a ferritic phase are laminated can be obtained. In this method, a stainless steel foil (a two-layer γ-α or a three-layer γ-α-γ stainless steel foil) in which one surface or both surfaces are the austenite phase (γ) and the central part is the ferritic phase (α) can be obtained. Since an existing ferritic stainless steel foil can be nitrided, it can be manufactured relatively simply by utilizing existing equipment.

[0043] Further, for example, the above two methods may be combined. For example, a ferrite stainless steel sheet - an austenite stainless steel sheet - a ferrite stainless steel sheet may be laminated to form an α-γ-α clad steel sheet, and this clad steel sheet may be nitrided. Thereby, a 5-layer stainless steel foil of γ-α-γ-α-γ can be obtained.

[0044] As an example of an embodiment of the manufacturing method, a manufacturing method for nitriding a ferrite stainless steel foil as a base material will be described below. FIG. 1 is a flowchart showing an example of a manufacturing method of a stainless steel foil as an electromagnetic shielding material according to this embodiment. This manufacturing method is merely an example, and the manufacturing method of the stainless steel sheet according to this embodiment is not limited to this method. This manufacturing method includes a step of preparing a slab (step S1), a step of obtaining a rolled steel sheet with a thickness of 5 to 200 μm by hot rolling and cold rolling the slab (step S2), and a step of annealing (nitriding treatment) the rolled steel sheet in a gas atmosphere containing nitrogen and then cooling it (step S3). Each step will be described in detail below.

[0045] [Slab Preparation Step] Step S1 Prepare a slab having the chemical composition of a ferrite stainless steel as the base material (step S1). For example, prepare a slab in which the chemical composition of the ferrite stainless steel according to one embodiment is, by mass%, Cr: 20 to 26%, N: 0.1% or less, Si: 2.0% or less, C: 0.040% or less, P: 0.030% or less, S: 0.030% or less, Mn: 1.5% or less, Cu: 0.50% or less, Mo: 3.00% or less, Ni: 5.00% or less, Ca: less than 50 ppm, sol.Al: less than 300 ppm, and the balance: Fe and impurities. The reason for setting the N content of the slab to 0.1% or less is that when the N content exceeds 0.1%, the deformation resistance becomes high and it becomes difficult to form a steel sheet by rolling. The upper limit of the N content of the slab is preferably 0.05%.

[0046] The chemical composition of the ferritic stainless steel used as the base material according to the above embodiment will be described. In the following description, “%” and “ppm” of the element content mean mass % and mass ppm, respectively, unless otherwise specified.

[0047] (Cr: 20.00 to 26.00%) Chromium (Cr) has the effect of forming a Cr2O3 passive film on the surface of stainless steel to improve corrosion resistance. When the Cr content is low, the martensite phase may be contained in a large amount in the structure austenitized by nitrogen absorption. Also, when the Cr content is low, work-induced martensite phase may be generated when severe working is performed. Therefore, the lower limit of Cr is set to 20.00%. On the other hand, as the Cr content increases, the deformation resistance increases. Therefore, in order to ensure more stable manufacturability (especially flatness of thin steel sheets), the upper limit of the Cr content is set to 26.00%. The lower limit of the Cr content is preferably 21.00%, 22.00%, or 23.00%. The upper limit of the Cr content is preferably 25.00% or 24.00%.

[0048] (N in the second region: 0.10% or less) In this embodiment, a ferritic stainless steel is used as the base material, and the surface layer portion is made into austenitic stainless steel (the first region) by surface nitriding treatment. Therefore, the ferritic stainless steel serving as the base material remains as the second region as it is. Nitrogen (N) is also an element that promotes austenitization of stainless steel. In order to obtain a structure mainly composed of a ferrite phase in an Fe-Cr-N series stainless steel, the nitrogen (N) content is preferably 0.10% or less. In addition, when the ferritic stainless steel serving as the base material is rolled to a predetermined plate thickness and then the first region is formed by surface nitriding treatment, if the N content in the base material exceeds 0.10%, the deformation resistance increases, and it becomes difficult to form the steel sheet by rolling. From these facts, the N content of the ferritic stainless steel as the base material is preferably 0.10% or less, more preferably 0.09% or less, 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less.

[0049] (N in the first region: 0.15 to 5.00%) In the case of this embodiment, the surface of the ferritic stainless steel as the base material is nitrided to austenitize the surface layer portion. Therefore, in the Fe-Cr-N stainless steel, the N content required to obtain a structure (first region) mainly composed of an austenite phase in the surface layer portion is preferably 0.15% or more. On the other hand, since the workability deteriorates when nitrides such as Cr2N and CrN are generated in the crystal grains, the upper limit of the N content is preferably 5.00% to suppress this. Therefore, the nitrogen (N) in the first region is preferably 0.15% or more and 5.00% or less. The lower limit of the N content in the first region can preferably be 0.17%, 0.19%, 0.21%, 0.23%, 0.25%, 0.27%, 0.29%, 0.31%, 0.33%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55% or 0.60%. The upper limit of the N content can preferably be 4.75%, 4.50%, 4.25%, 4.00%, 3.75%, 3.50%, 3.25%, 3.00%, 2.80%, 2.60%, 2.40%, 2.20%, 2.00%, 1.80%, 1.60%, 1.50%, 1.40%, 1.30%, or 1.20%. Naturally, in the case of this embodiment, the N content in the first region is higher than the N content in the second region (base material).

[0050] (Si: 0 to 2.00%) Silicon (Si) may not be contained. Since Si is an element that deteriorates the workability of stainless steel, it is not usually an element that is actively added. On the other hand, when stainless steel is exposed to a highly passive corrosion environment, Si generates SiO2 on the surface and plays a role of protecting by covering the Cr2O3 passive film. On the other hand, when the Si content is high, the workability deteriorates, and brittle σ-phase is likely to precipitate during manufacturing, and cracks may occur in the processing step of the steel sheet, or the flatness may be poor and it may not be suitable for press working. Therefore, when Si is contained, the upper limit of the Si content is set to 2.00%. The upper limit of the Si content is preferably 1.90%, 1.80%, 1.70%, 1.60%, or 1.50%. The preferable lower limit of the Si content is more than 0%, and more preferably 0.10% or more.

[0051] (C: 0 to 0.040%) Carbon (C) may not be contained. In the stainless steel sheet of this embodiment, since a certain amount or more of N is contained, solid solution strengthening by N is sufficient, and C may not be added. On the other hand, C is a solid solution strengthening element and contributes to the improvement of the strength of stainless steel. However, if the C content is too high, a large number of carbides are generated during the manufacturing process, and these carbides become the starting points of fracture, resulting in a decrease in the formability of the steel. Therefore, the C content is set to 0.040% or less. The upper limit of the C content is preferably 0.038%, 0.036%, 0.034%, 0.032%, or 0.030%. The preferable lower limit of the C content is more than 0%, and more preferably 0.001%.

[0052] (P: 0.030% or less) Phosphorus (P) is an impurity. P segregates at the grain boundaries during solidification and increases the solidification cracking susceptibility. Therefore, it is preferable that the P content is as low as possible. Therefore, the P content is set to 0.030% or less. The lower limit of the P content is 0%, but an excessive decrease requires a higher refining load or the use of expensive raw materials, so in reality, it may be 0.001%.

[0053] (S: 0.030% or less) Sulfur (S) is an impurity. S segregates at grain boundaries during solidification, increasing the susceptibility to solidification cracking. Therefore, the S content is preferably as low as possible. For this reason, the S content is set to 0.030% or less. The lower limit of the S content is 0%, but an excessive decrease would increase the refining load or require the use of expensive raw materials, so in reality, it may be 0.001%.

[0054] (Mn: 0 to 1.50%) Manganese (Mn) may not be contained. On the other hand, Mn suppresses the decrease in hot workability due to S. Mn also deoxidizes stainless steel. However, when the Mn content increases, the precipitation of intermetallic compound phases such as the σ phase is promoted. The precipitation of the σ phase reduces the tissue stability and also reduces the toughness and ductility of stainless steel. Therefore, the Mn content is set to 1.50% or less. The upper limit of the Mn content is preferably 1.40%, 1.30%, 1.20%, 1.10%, 1.00%, 0.90%, 0.80%, 0.70%, 0.60%, or 0.50%. The preferred lower limit of the Mn content is more than 0%, and more preferably 0.01%, 0.05%, or 0.10%.

[0055] (Cu: 0 to 0.50%) Copper (Cu) may not be contained. Cu tends to segregate at grain boundaries and is also an austenite stabilizing element. Cu acts as a solid solution strengthening element and contributes to an increase in the high-temperature strength required for structural materials, so it may be contained as needed. When the Cu content increases, ferrite formation is suppressed during solidification during casting, increasing the susceptibility to solidification cracking. Also, when the Cu content is high, there is a risk of a decrease in hot workability. Therefore, the Cu content is set to 0.50% or less. The upper limit of the Cu content is preferably 0.47%, 0.43%, 0.40%, 0.37%, 0.33%, 0.30%, 0.27%, 0.23%, 0.20%. The preferred lower limit of the Cu content is more than 0%, and more preferably 0.01%.

[0056] (Mo: 0 to 3.00%) Molybdenum (Mo) may not be contained. On the other hand, especially when it is desired to enhance the corrosion resistance, Mo has the effect of enhancing the corrosion resistance of stainless steel. However, Mo is an expensive element classified as a rare metal, and it is not preferable from the viewpoint of providing a material with excellent economic efficiency. Also, if the Mo content is too high, the hot workability deteriorates, and there may be a case where a structure mainly composed of an austenite phase cannot be obtained on the surface layer. Therefore, the Mo content is set to 3.00% or less. The upper limit of the Mo content is preferably 2.75%, 2.50%, 2.25%, 2.00%, 1.80%, 1.60%, 1.50%, 1.30%, 1.10%, 1.00%, 0.90%, 0.80%, 0.70%, 0.60%, or 0.50%. The preferable lower limit of the Mo content is more than 0%, and more preferably 0.01%, 0.05%, 0.10%, 0.20%, or 0.30%.

[0057] (Ni: 0 to 5.00%) Nickel (Ni) may not be contained. On the other hand, Ni is an element that promotes the austenitization of stainless steel and also contributes to the improvement of corrosion resistance. Therefore, it may be contained especially when it is desired to enhance the corrosion resistance or improve the workability. However, Ni is an element belonging to rare metals, and it is not preferable from the viewpoint of providing a material with excellent economic efficiency. Also, there is a risk that the oxygen reduction reaction rate at the interface between the platinum catalyst and the polymer electrolyte membrane may be decreased due to the elution of Ni ions. Therefore, the Ni content is set to 5.0% or less. The upper limit of the Ni content is preferably 4.50%, 4.00%, 3.50%, 3.00%, 2.50%, 2.00%, 1.50%, 1.00%, 0.80%, 0.70%, 0.6%, 0.5%, 0.4%, 0.3%, or 0.2%. The preferable lower limit of the Ni content is more than 0%, and more preferably 0.01%, 0.05%, or 0.10%.

[0058] (Ca: 0 to less than 50 ppm) Calcium (Ca) is preferably not contained as it is an impurity. As non-metallic inclusions that can be the starting point of stainless steel corrosion, CaS and MnS are generally known. In order not to generate a large amount of CaS that serves as the corrosion starting point, the Ca content is preferably less than 50 ppm.

[0059] (sol.Al: 0 to less than 300 ppm) Aluminum (Al) may not be contained. On the other hand, Al deoxidizes stainless steel. However, if the Al content is too high, the cleanliness of the steel decreases, and the workability and ductility of the stainless steel deteriorate. Therefore, the Al content is less than 300 ppm. The preferable lower limit of the Al content is more than 0%, more preferably 1 ppm, 10 ppm, 50 ppm, or 100 ppm. In this specification, the Al content means the content of acid-soluble Al (sol.Al).

[0060] The remainder of the chemical composition of the stainless steel sheet according to this embodiment is Fe and impurities. The impurities here refer to elements mixed from ores and scraps used unintentionally as steel raw materials, or elements mixed from the manufacturing process environment, etc.

[0061] The process of preparing the slab is not limited to this, but for example, it can be done as follows. Melt the raw materials. As raw materials, ferrochrome and ferrosilicon for stainless steel production, cast iron, and scraps of ferritic stainless steel can be used. Melting is mainly carried out in an electric furnace. At the laboratory level, it can also be carried out in a vacuum induction heating furnace. Refining is carried out to reduce the carbon content, gas components, and metallic inclusions. For refining, the AOD (Argon-Oxygen-Decarburization) method, VOD (Vacuum-Oxygen-Decarburization) method, V-AOD method, etc. are applicable. Then, by continuous casting equipment or casting into a case, it is made into a slab with a shape suitable for rolling. The chemical composition of the slab can be adjusted according to the blending of raw materials and the refining conditions.

[0062] [Rolling Process] Step S2 By hot rolling and cold rolling the slab, a rolled steel sheet with a thickness of 5 to 200 μm is obtained (Step S2). The hot rolling and cold rolling may be repeated respectively, and intermediate heat treatment such as annealing and pickling may be performed as necessary. Further, in addition to hot rolling and cold rolling, hot forging or cutting may be further performed as necessary.

[0063] The rolling process is not limited to this, but for example, it can be as follows. Hot roll the slab with a tandem mill or a Steckel mill to make it a hot coil. Anneal and pickle this hot coil. Further, cold roll it with a multi-stage roll cold rolling machine to make it a rolled steel sheet with a thickness of 5 to 200 μm.

[0064] [Annealing (Nitriding Treatment) Process] Step 3 Anneal and cool the rolled steel sheet in a gas atmosphere containing nitrogen (Step S3). By this process, nitrogen is infiltrated from the surface of the steel sheet, and the structure of the surface layer of both surfaces or one surface of the steel sheet is made into a structure mainly composed of an austenite phase (the first region). When making only the surface layer of one surface into the austenite phase, for example, the opposite surface (the surface not made into the austenite phase) may be masked and nitriding treatment may be performed.

[0065] The N content in the surface layer part (the first region) of the steel sheet after the annealing process can be adjusted according to the N content of the slab, the annealing conditions, etc. Specifically, the N content in the surface layer part (the first region) of the steel sheet can be increased by increasing the N content of the slab, increasing the nitrogen partial pressure during annealing, increasing the annealing temperature, increasing the annealing holding time, or controlling the passing speed of the steel sheet.

[0066] The ratio of the partial pressure of nitrogen to the total pressure of the processing gas is preferably 0.2 to 0.9. When the ratio of the partial pressure of nitrogen to the total pressure is less than 0.2, nitrogen is not sufficiently supplied from the surface, and when the thickness of the steel sheet is thick, it becomes difficult to form the first region so as to cover the entire surfaces of the front and back of the steel sheet. On the other hand, when the ratio of the partial pressure of nitrogen to the total pressure of the processing gas is higher than 0.9, excessive Cr nitride is generated on the surface, which may become a starting point for the occurrence of processing cracks. The upper limit of the ratio of the partial pressure of nitrogen to the total pressure of the processing gas is preferably 0.75. As the gas to be mixed with nitrogen, hydrogen is preferably used so as not to oxidize the steel sheet. Instead of hydrogen, or in addition to hydrogen, argon may be used.

[0067] The annealing temperature is preferably set to 950 to 1200 °C. When the annealing temperature is less than 950 °C, not only the austenite phase but also the Cr2N phase exists in the equilibrium state, so there is a possibility that the austenite phase fraction in the first region cannot be increased. On the other hand, when the annealing temperature exceeds 1200 °C, especially when Si is contained, a liquid phase is generated in the vicinity of the grain boundary, and melting and embrittlement may occur. Although the annealing temperature varies depending on the Cr content, 1050 to 1150 °C is more preferable.

[0068] The holding time of annealing needs to be controlled within a narrow range depending on the thickness of the steel sheet and the nitrogen partial pressure. This is because although austenitization due to nitrogen absorption progresses from the surface into the sheet thickness with time, in the production of the stainless steel sheet according to this embodiment, it is necessary to stop the progress of austenitization halfway. If the holding time is too short, even when the sheet thickness is thin, there is a risk that a ferrite phase remains on the surface. On the other hand, if the holding time is too long, the cross-sectional area ratio of the second region may become too low.

[0069] When cooling the annealed steel plate, it is advisable to perform controlled cooling according to the temperature range. If the annealed steel plate is slowly cooled, the austenite phase formed by absorbing nitrogen will change into a two-phase structure of ferrite phase and Cr nitride. On the other hand, if the cooling rate is too fast, it is impossible to form an austenite phase with a structure in which one or more of precipitates composed of carbide, oxide, and nitride are dispersed on the surface layer. Specifically, it is advisable to perform controlled cooling with a fast cooling rate (10 - 15 °C / min) from the annealing treatment temperature to about 500 °C, and a relatively slow cooling rate (5 - 8 °C / s) from 500 °C to 300 °C. Also, after cooling at a fast cooling rate (10 - 15 °C / s) to 500 °C, it may be held in the range of 500 °C ± 10 °C for about 1 - 30 seconds, and then cooled at a fast cooling rate (10 - 15 °C / s).

[0070] The annealing process is not particularly limited in the form of the annealing furnace as long as the above annealing conditions are satisfied. For example, it can be carried out by passing the steel plate through an annealing line called a continuous bright annealing line.

[0071] By this annealing process, a first region mainly composed of austenite phase is formed on the surface layer of the steel plate. The surface layer of the steel plate after the annealing process has an N content of 0.15 - 5.0 mass%, and is adjusted so that the cross-sectional area ratio of the second region is 15% or more.

Examples

[0072] [Test materials] Steels with 8 kinds of chemical compositions shown in Table 1 were melted in a 30 kg vacuum melting furnace of high-frequency induction heating method to produce casting ingots in a substantially frustum shape with a diameter of 125 - 115 mm and a height of 320 mm. The surface black skin on the side surface of these casting ingots in frustum shape was ground by a grinder, heated after grinding, held at 1250 °C for 3 hours, and then hot forged until the finish thickness became 25 mm, and ground until the thickness became 20 mm to produce slabs.

[0073] Next, the slab was held at 1200°C for 2 hours, then hot-rolled until the thickness reached 4 mm to obtain a hot-rolled steel sheet. For removing the scale after rolling, the hot-rolled steel sheet was ground until the thickness reached 3 mm, and then cold-rolled until the thickness reached 0.5 mm to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was repeatedly subjected to intermediate annealing in an argon atmosphere at 800°C for 10 minutes and cold rolling to obtain test specimens with the thicknesses shown in Table 2.

[0074] [Annealing treatment (nitriding treatment)] Test pieces with a width of 70 mm and a length of 200 mm were cut out from each test specimen, and annealing treatment (nitriding treatment) was carried out under the annealing treatment (nitriding treatment) conditions shown in Table 2 by a continuous annealing simulator device to perform nitrogen absorption treatment (nitriding treatment) in the solid phase state. The total pressure was 1 atm. After annealing, it was cooled under the cooling conditions shown in Table 2. For some test specimens (TP7 and 8 in Table 2), after being held at 500°C for 10 seconds, it was cooled from 500°C to 300°C with an average cooling rate of 14°C / second. From the above, a stainless steel sheet (test specimen) was obtained.

[0075] [Investigation of microstructure, etc.] (1) Measurement of N content in the stainless steel sheet The N content in the steel after nitriding treatment was measured by the inert gas carrier fusion thermal conductivity method by collecting an analysis sample from the total thickness of each test piece.

[0076] (2) Confirmation of having a first region and a second region Samples were collected from each test piece so that the cross-section perpendicular to the rolling direction (cross-section in the thickness direction) was the observation surface, embedded in resin and mirror-polished, and then etched with an etching solution having a volume ratio of aqua regia to glycerin of 4:1 until the metal microstructure appeared. When the test specimen had a first region and a second region, the boundary could be clearly discriminated by etching.

[0077] The region mainly composed of the ferrite phase (the second region) was etched deeper than the region mainly composed of the austenite phase (the first region). Within the region mainly composed of the ferrite phase, the etching difference between crystal grains was small, the grain boundary etching was also thin, and the interior of the grains was etched relatively smoothly. This is considered to be because nitrogen intrusion did not occur and it only received the thermal history due to high-temperature annealing while remaining in the original ferrite phase.

[0078] In contrast, the region mainly composed of the austenite phase (the first region) was etched shallower than the region mainly composed of the ferrite phase (the second region). Within the region mainly composed of the austenite phase, the etching difference between crystal grains was large, the grain boundaries were etched relatively clearly, and the interior of the grains was also etched relatively coarsely. This is considered to be due to the accumulation of strain within the grains because of the transformation from the ferrite phase to the austenite phase caused by nitrogen enrichment due to nitrogen intrusion while being constrained in the solid state.

[0079] Furthermore, it was confirmed by the EBSD orientation map that the surface layer part is the region (the first region) mainly composed of the austenite phase (γ) (γ area ratio of 75% or more), and the central part is the region (the second region) mainly composed of the ferrite phase (α) (α area ratio of 95% or more). The cross-sectional structure of the sample taken from the test material was also analyzed by EBSD (Electron Back Scattered Diffraction, JSM-7001F manufactured by JEOL Ltd.). The acceleration voltage was 15 kV, the measurement field of view was 150 μm (horizontal) × 50 μm (vertical), the StepSize was 0.01 μm, and the measurement was carried out using HIKARI (high-speed detector) and OIM DATA COLLECTION (measurement software) manufactured by TSL Solutions, and the analysis was performed using OIM Analysis (analysis software). From the EBSD IPF map, it was confirmed that the surface layer part is the region (the first region) mainly composed of the austenite phase (γ), and the central part is the region (the second region) mainly composed of the ferrite phase (α). In the "Layer Structure" column of Table 2, it was described which phase it becomes in order from the surface layer.

[0080] (3) Measurement of the cross-sectional area ratio of the second region (ferritic stainless steel) After determining the boundary between the first region and the second region, the respective areas were obtained using image analysis software. Let the area of the second region be A and the area of the region other than the second region be B, and the cross-sectional area ratio of the second region was determined from A / (A + B).

[0081] (4) Identification of precipitates dispersed in the first region (austenitic stainless steel) The identification of precipitates dispersed in the first region was carried out by XRD. When the thickness of the test material was thicker than 30 μm, XRD measurement was performed while polishing about 10 μm from one side. The precipitates formed in the first region were identified from the obtained XRD peaks.

[0082] (5) Measurement of the ratio of precipitates dispersed in the first region The cross-sectional structure of the test material was analyzed by FE-EPMA (JXA-8530F manufactured by JEOL Ltd.). The details of the analysis method are as follows. In the first region of the test material, elemental mapping of the non-metallic element (e.g., C for carbide, N for nitride, O for oxide) corresponding to the type of precipitate identified by XRD and the alloy elements contained in the base material was carried out, and from the overlap of the non-metallic element and the metal element and the XRD analysis results, the distribution of each precipitate was identified. From the obtained mapping data, the area of each precipitate with a particle size (equivalent circle diameter in terms of area) of 0.1 μm or more was obtained using image analysis software. Let the area of the first region be A and the area of the precipitate be C, and the area ratio of the precipitate was determined from C / A.

[0083] [Evaluation of Test Material Properties] For each test material, the electromagnetic wave shielding properties in the high-frequency region, formability (elongation), and strength (tensile strength) were investigated and evaluated, and the results are shown in Table 2. In each TP, the maximum diameter of the precipitate was 1.0 μm or less.

[0084] (1) Evaluation of Electromagnetic Wave Shielding Properties at High Frequencies The higher the magnetic permeability of the material at the test frequency, the better the electromagnetic shielding characteristics. To evaluate the magnetic permeability in the high-frequency band, the test frequency was set to 100 MHz, and the electromagnetic shielding properties of each test material were evaluated by comparing the magnetic permeability at 100 MHz. Each prepared test material was punched out with a mold to an outer diameter of 6.93 - 6.96 mm and an inner diameter of 3.06 - 3.10 mm to prepare a test piece (TP) for measurement. The magnetic permeability of each TP at 100 MHz was measured. The magnetic permeability measurement was performed with a vector network analyzer (Keysight ENA E5071C) in the frequency range of 100 kHz - 8.5 GHz. The measured magnetic permeability was measured as complex magnetic permeability, and those with a complex magnetic permeability (imaginary part) of 0.5 or more at 100 MHz were considered to have excellent electromagnetic shielding characteristics (〇: excellent). As shown in Table 2 here, since TP1 had a low Cr concentration, the surface layer was not austenite but a ferrite and martensite structure (α’), so the electromagnetic shielding characteristics were unqualified.

[0085] (2) Evaluation of formability and strength The formability (elongation) and strength (tensile strength) of each test material were evaluated. An ASTM half test piece (parallel part; 6.25 mm wide, 32 mm long) was subjected to a tensile test at room temperature at 2 mm / min. The elongation value (EL) was used as a representative value of ductility. For reference, the property evaluations of aluminum foil (aluminum foil) and SUS304 stainless steel foil are described in Table 2. If the measured EL is 5% or more, the formability is at a satisfactory level. Therefore, those with 5% or more and less than 7% were considered to have good formability (△; good), and those with 7% or more were considered to have excellent formability (〇: excellent). The formability of TP16, TP17, and TP19 was evaluated as good (△) because the density of precipitates increased, resulting in a smaller elongation value (EL) than other TPs. For strength (tensile strength), those with 500 MPa or more were considered to have excellent strength (〇: excellent). Since the strength of the aluminum foil was only about 100 MPa at most, it was considered inferior (×: insufficient strength).

[0086]

Table 1

[0087]

Table 2

Industrial Applicability

[0088] The present invention can be used as an electromagnetic shielding material in all industrial fields. In particular, it can be used for the housings of electronic devices and electronic apparatuses.

Claims

1. A plate-shaped electromagnetic shielding material with a plate thickness of 5 to 200 μm, comprising a first region made of stainless steel with an austenite phase of 75% or more in area ratio and a second region made of stainless steel with a ferrite phase of 95% or more in area ratio, laminated in the plate thickness direction.

2. The electromagnetic shielding material according to Claim 1, wherein at least one surface is the first region.

3. In the first region, there are precipitates containing at least one of carbide, oxide, and nitride. The precipitates have a particle size of 1.0 μm or less, and the precipitates with a particle size of 0.1 μm or more have an area ratio of 10% or less in the cross-section in the plate thickness direction. The electromagnetic shielding material according to Claim 2.

4. The electromagnetic shielding material according to any one of Claims 1 to 3, with a plate thickness of 5 to 50 μm.

5. The electromagnetic shielding material according to any one of Claims 1 to 3, wherein the total area ratio of the second region in the cross-section in the plate thickness direction is 15% or more.

6. The chemical composition of the second region is, in mass%, Cr: 20.00 - 26.00%, N: 0 - 0.10%, Si: 0 - 2.00%, C: 0 - 0.040%, P: 0.030% or less, S: 0.030% or less, Mn: 0 - 1.50%, Cu: 0 - 0.50%, Mo: 0 - 3.00%, Ni: 0 - 5.00%, Ca: 0 - 50 ppm, sol. Al: 0 - 300 ppm, The balance: Fe and impurities. The electromagnetic shielding material according to any one of Claims 1 to 3.

7. The chemical composition of the first region is, in mass%, Cr: 20.00 - 26.00%, N: 0.15 - 5.00%, Si: 0 - 2.00%, C: 0 - 0.040%, P: 0.030% or less, S: 0.030% or less, Mn: 0 - 1.50%, Cu: 0 - 0.50%, Mo: 0 - 3.00%, Ni: 0 - 5.00%, Ca: 0 - 50 ppm, sol. Al: 0 - 300 ppm, The balance: Fe and impurities. The electromagnetic shielding material according to Claim 6.

8. The electromagnetic shielding material according to Claim 4, wherein the total area ratio of the second region in the cross-section in the plate thickness direction is 15% or more.

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