Stainless steel sheet having multilayer structure

Abstract

To propose a stainless steel sheet that exhibits both strength and workability while enabling enhanced electromagnetic wave shielding performance in a high frequency range of 10 MHz or higher.SOLUTION: This stainless steel sheet has a multilayer structure in which a first region comprising stainless steel composed predominantly of austenitic phase and a second region comprising stainless steel composed predominantly of ferritic phase are laminated in the sheet thickness direction, wherein in the second region (ferritic phase dominant region), an average KAM value obtained by EBSD measurement is 0.50-1.50, which provides higher electromagnetic wave shielding performance.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a stainless steel sheet having a multi-layer structure. [Background technology]

[0002] Currently, electromagnetic wave countermeasures (EMC countermeasures) for electronic devices use sintered ferrite for absorbing electromagnetic waves, and electromagnetic wave shielding films and electromagnetic wave shielding coating agents for shielding (electromagnetic wave shielding).

[0003] Sintered ferrite has excellent electromagnetic wave absorption properties, and is used by mixing it with resin and applying it to the housing of electronic devices. It is effective in the low frequency band of a few tens of megahertz or less.

[0004] Electromagnetic wave shielding films are made by pressing a metal plate (metal foil) and a resin layer together, or by coating a substrate resin or cloth with a conductive material made by mixing metal or magnetic powder with resin. For example, Patent Document 1 proposes an electromagnetic wave shielding film in which a protective layer, a metal layer, and an adhesive layer are laminated. Patent Document 2 proposes an electromagnetic wave shielding film in which conductive layers are laminated on both sides of a flame-retardant cloth, and an anti-corrosion layer is further laminated on the top surface of those.

[0005] Electromagnetic wave shielding coating agents are applied to the housing of an electronic device to reflect electromagnetic waves and prevent interference from the electromagnetic waves entering the inside of the housing. For example, Patent Document 3 proposes a spray coating agent for electromagnetic wave shielding that contains silver particles dispersed in a solvent, a thermosetting resin, and a curing agent.

[0006] Furthermore, Patent Document 4 proposes a stainless steel sheet in which an austenite phase and a ferrite phase are laminated, as a stainless steel sheet that gives the housing of an electronic device electromagnetic wave shielding properties and also has both strength and workability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-174948 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-031589 [Patent Document 3] Japanese Patent Publication No. 2020-143225 [Patent Document 4] International Publication No. 2023 / 189142 Summary of the Invention [Problem to be solved by the invention]

[0008] In recent years, in response to the explosive growth in communication data volume and the need for faster communication speeds, carrier waves are being used at higher frequencies as a means of increasing the volume and speed of data communication. As a result, the frequency band of electromagnetic waves that must be blocked in electronic devices that receive and transmit high-frequency radio waves is becoming higher. Since conventional electromagnetic wave shielding in the low-frequency band below 10 MHz can cause electronic devices to malfunction, there has been a recent demand for improved electromagnetic wave shielding in the high-frequency band above 10 MHz.

[0009] Furthermore, as electronic devices become smaller and lighter, their shapes become more diverse. This makes it difficult to apply conventional electromagnetic wave shielding films and coatings, making it difficult to completely shield them from electromagnetic waves. Therefore, there is a demand for the housings of electronic devices and components to be made of stainless steel. To be used in electronic device housings, they must be strong and workable when thinned to reduce size and weight. However, electromagnetic wave shielding films and coatings are not suitable for use as strong components. Aluminum foil also lacks strength, and when thinned, it breaks during drawing and other processes, making it unsuitable for housings.

[0010] Therefore, the present invention focuses on stainless steel sheets to meet the need to add electromagnetic wave shielding properties to the housings of electronic devices, and aims to propose a stainless steel sheet that has such properties, while also maintaining strength and workability and further improving the electromagnetic wave shielding properties in the high frequency band of 10 MHz or more. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to achieve the above object and have obtained the following findings. [a] Selection of metal materials suitable for electronic equipment and component housings In order to meet the need for smaller and lighter electronic devices and components (hereinafter collectively referred to as electronic devices), we investigated metallic materials that are strong and easy to process even when extremely thin. Metals with good electromagnetic wave shielding and high conductivity include silver, copper, and aluminum. However, these metals have low strength and are prone to breakage when processed into complex shapes, posing practical problems. Therefore, we came up with the idea of ​​using stainless steel sheet (hereinafter sometimes referred to as "stainless steel"), which has a certain level of strength and processability even when made extremely thin, although its conductivity is inferior to that of aluminum, and we further investigated this idea to make it a reality.

[0012] [b] Improved electromagnetic wave shielding The electromagnetic wave shielding properties of stainless steel are based on the following two mechanisms. External reflection loss: Basically, electromagnetic wave energy is attenuated (lost) due to the reflection of electromagnetic waves on the outer surface of metal materials. Absorption loss: When electromagnetic waves penetrate metal, the energy is absorbed and the waves are attenuated.

[0013] Furthermore, Patent Document 4 proposes that the following electromagnetic wave attenuation effect can be obtained by forming stainless steel into a multi-layer structure in which an austenite phase (sometimes called γ) and a ferrite phase (sometimes called α) are laminated in the plate thickness direction. Interphase interface reflection loss: Electromagnetic waves are reflected at the interface between the austenite and ferrite phases, resulting in attenuation of electromagnetic wave energy. Correlated repeated reflection loss: Repeated reflections at multiple austenite and ferrite phase interfaces further enhance the attenuation effect. Furthermore, by using a multi-layer structure of austenite and ferrite phases, the interface between the two phases is always present in the thickness direction of the plate, so the reflected wave always collides with the interface between the two phases, resulting in repeated correlated reflections and attenuation. This significantly reduces the electromagnetic wave energy that ultimately passes through the stainless steel and reaches the inside of the housing.

[0014] The inventors used stainless steel with a multi-layer structure in which austenite and ferrite phases are stacked in layers to efficiently obtain these electromagnetic wave attenuation effects, and focused on absorption losses within the metal in order to further improve electromagnetic wave shielding properties.

[0015] First, because the ferrite phase has better conductivity than the austenite phase and therefore has a large absorption loss effect in the high-frequency band above 10 MHz, we improved the electromagnetic shielding properties by modifying the ferrite phase. As a result, we found that the average KAM value in the ferrite phase (the average value of the KAM values ​​in the ferrite phase measured by EBSD KAM analysis) correlates with the electromagnetic shielding properties. In other words, we found that higher electromagnetic shielding properties can be obtained by adjusting the average KAM value of the ferrite phase to an appropriate range. The present invention was made based on the above findings, and the gist of the present invention is as follows.

[0016] [1] A plate having a thickness of 5 to 200 μm, in which a first region made of stainless steel having an austenite phase area ratio of 75% or more and a second region made of stainless steel having a ferrite phase area ratio of 95% or more are laminated in the plate thickness direction, A stainless steel plate having a multilayer structure, characterized in that the KAM value obtained by EBSD measurement in the second region is 0.50 to 1.50. The multi-layer structure refers to a structure in which first regions and second regions are alternately stacked in the plate thickness direction, and the number of layers is not particularly limited. It may be a total of two layers, one of which is the first region and one of which is the second region, or a three-layer structure, one of which is one layer and the other is two layers, or a multi-layer structure with more than that. [2] The stainless steel plate according to [1], wherein at least one surface is the first region. [3] The stainless steel plate according to [1], wherein both surfaces are the first region. [4] The stainless steel sheet according to any one of [1] to [3], having a sheet thickness of 5 to 50 μm. [5] The stainless steel sheet according to any one of [1] to [4], wherein the total area ratio of the second regions in a cross section in the sheet thickness direction perpendicular to the surface is 15% or more. [6] The chemical composition of the second region is, in mass %, Cr: 16.00~26.00%, N: 0~0.10%, 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: 0-0.50% Mo: 0-3.00%, Ni: 0~5.00% Ca: 0 to 50 ppm, sol.Al:0-300ppm, The stainless steel sheet according to any one of the above [1] to [5], wherein the balance is Fe and impurities. [7] The chemical composition of the first region is, in mass %, Cr: 16.00~26.00%, N: 0.15~5.00%, 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: 0-0.50% Mo: 0-3.00%, Ni: 0~5.00% Ca: 0 to 50 ppm, sol.Al:0-300ppm, The stainless steel sheet according to [6] above, wherein the balance is Fe and impurities. [Effects of the Invention]

[0017] According to the present invention, it is possible to obtain stainless steel that serves as an electromagnetic shielding material that has good electromagnetic shielding properties against high-frequency electromagnetic waves, as well as strength and processability. This stainless steel can be used to manufacture housings (casings) for electronic devices and components with even higher electromagnetic shielding properties. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a flow diagram showing an example of a method for producing a stainless steel sheet as a stainless steel sheet according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, as one embodiment of the present invention (hereinafter simply referred to as the present invention), the stainless steel sheet according to the present invention will be described using as an example a stainless steel sheet having a γ-α-γ three-layer multilayer structure of austenite phase (hereinafter sometimes referred to as γ)-ferrite phase (hereinafter sometimes referred to as α)-austenite phase (γ).

[0020] [Area 1 (austenitic stainless steel)] The first region is stainless steel (austenitic stainless steel) mainly composed of austenite phase, and is a region mainly composed of austenite phase (area fraction of 75% to 100%) among all phases detectable by EBSD (Electron Backscattered Diffraction) (including iron phases such as ferrite phase, austenite phase, and martensite phase, compound phases such as CrN and CrN, and passive films). The area fraction of the austenite phase is preferably 80% or more, 85% or more, or 90% or more.

[0021] [Second category (ferritic stainless steel)] The second region is stainless steel (ferritic stainless steel) mainly composed of ferrite phase. As with the first region, this region is mainly composed of ferrite phase (area fraction of 95% to 100%) among all phases detectable by EBSD (Electron Backscattered Diffraction) (including iron phases such as ferrite phase, austenite phase, and martensite phase, compound phases such as CrN and CrN, and passive films). The area fraction of the ferrite phase is preferably 92% or more, 94% or more, or 95% or more.

[0022] [KAM value of the second region] The KAM (kernel average misorientation) value is an index measurable by electron backscatter diffraction (EBSD) and is calculated from the average misorientation between a measurement point of interest and adjacent measurement points in an image obtained by EBSD. The KAM value represents the plastic strain gradient in the measurement region and can be used to evaluate damage to the ferrite phase and the strain distribution within the crystal grains. The inventors believed that this KAM value could be an index representing the strain distribution within the ferrite phase. Therefore, they investigated the relationship between the KAM value in the ferrite phase and the electromagnetic shielding properties. They found that the larger the average KAM value obtained by EBSD measurement in the second region (the region mainly composed of ferrite phase), the more eddy currents are suppressed, the better the magnetic permeability is, and the better the electromagnetic shielding properties of the second region. Therefore, the average KAM value should be 0.50 or more, preferably 0.60 or more, 0.70 or more, or 0.80 or more. On the other hand, if the average KAM value is too high, the material becomes too hard, resulting in poor bending workability and making it difficult to use as an electromagnetic wave shielding material. Therefore, the average KAM value should be 1.50 or less, and more preferably 1.45 or less, 1.40 or less, 1.35 or less, 1.30 or less, 1.25 or less, or 1.20 or less. The mechanism by which strain distribution within the crystal grains correlates with electromagnetic wave shielding properties has not been elucidated, but the inventors speculate that when strain is applied to the ferrite phase, the eddy currents that are generated when electromagnetic waves pass through it become smaller, increasing the apparent permeability of the ferrite phase and resulting in the effect of suppressing electromagnetic waves passing through the ferrite phase.

[0023] The method for measuring the KAM value is described below. The KAM value of the second region in a cross section perpendicular to the surface of a stainless steel sheet (cross section in the sheet thickness direction) is measured using the KAM map function of EBSD. For example, in an analysis example using EBSD (Electron Backscattered Diffraction, JEOL JSM-7001F), the acceleration voltage is 15 kV, the measurement field is 60 μm (horizontal: parallel to the steel sheet surface) × 60 μm (vertical: sheet thickness direction) of the steel sheet, the step size is 0.01 μm, and the orientation difference is 5°. Measurements are performed using a TSL Solutions HIKARI (high-speed detector) and OIM DATA COLLECTION (measurement software). The KAM value of the second region (ferrite-based phase) in the observation field can be obtained by KAM map analysis using OIM Analysis (analysis software). The arithmetic average of the KAM values ​​measured in three or more different observation fields is used as the average KAM value of the ferrite phase.

[0024] [Double-layer structure of the first and second areas] The first region (austenitic stainless steel) and the second region (ferritic stainless steel) are layered (layers roughly parallel to the surface), and these regions are alternately stacked in the thickness direction of the stainless steel plate (a multi-layer structure). Austenitic stainless steels belong to the first region, even if they are of different types (compositions, etc.). Similarly, ferritic stainless steels belong to the second region, even if they are different types of ferritic stainless steel.

[0025] The multi-layer structure is not particularly limited in its configuration, as long as the first and second regions are alternately stacked. For example, it may be a two-layer structure, with the first layer being the first region and the second layer being the second region. For example, it may be a three-layer structure with the order of first region-second region-first region. For example, it may be a four-layer or more multi-layer structure with the order of first region-second region-first region-second region-... In order to obtain the effect of the interface reflection loss between the first and second regions, a multi-layer structure with three or more layers is preferable. Of course, the first region and the second region may be reversed in the layer order described above. On the other hand, since a multi-layer structure complicates the manufacturing process, it is best to determine the number of layers by balancing the electromagnetic wave shielding effect and manufacturing costs.

[0026] [Divided part] It is better for each layer to have as few divided parts that penetrate the layer (parts that are not austenite phase in the first region and parts that are not ferrite phase in the second region; hereafter, these will be referred to as divided parts). This is because if there are divided parts, electromagnetic waves will pass through them. When the stainless steel plate is viewed from above in a direction perpendicular to the surface, it is desirable for the area ratio of divided parts to be 30% or less, 20% or less, 10% or less, 5% or less, 3% or less, 2% or less, 1% or less, or 0% (none).

[0027] [At least one surface is the first region (austenite phase)] Since the austenite phase has better corrosion resistance than the ferrite phase, if at least one of the front and back surfaces of the stainless steel sheet is covered with stainless steel (first region) mainly composed of austenite, better corrosion resistance can be ensured. Also, from the viewpoint of workability, in bending and drawing, it is preferable that the outermost layer where maximum stress occurs is stainless steel (first region) mainly composed of austenite. Therefore, it is preferable that at least one surface portion (surface layer portion) of the surface of the electromagnetic shielding material, preferably both surface portions (both surface layer portions), be the first region.

[0028] [Layer thickness in each region] The thickness of each region is not particularly limited. However, if the second region is thick enough to reach the surface in some places, non-uniform deformation may occur during processing, resulting in a decrease in flexibility. Therefore, the thickness of the second region is preferably (the thickness of the stainless steel plate - 2 μm) or less at its thickest point. More preferably, the thickness of the second region is (the thickness of the stainless steel plate - 4 μm) or less at its thickest point. On the other hand, if the second region is too thin and there are parts where the ferrite phase is divided, there is a risk that electromagnetic waves may be transmitted through the divided parts. Therefore, the thickness of the second region at its thinnest part is preferably 10% or more, and more preferably 20% or more, of the thickness of the stainless steel plate.

[0029] [Second region cross-sectional area ratio] In the stainless steel sheet according to this embodiment, the area ratio of the second region (sometimes referred to as the cross-sectional area ratio) in a cross section perpendicular to the steel sheet surface is preferably 15% or more. If the cross-sectional area ratio of the second region is 15% or more, it is possible to provide both sufficient flexibility and excellent electromagnetic wave shielding properties. The cross-sectional area ratio of the second region is preferably 20% or more. On the other hand, if the cross-sectional area ratio of the second region is too high, the ferrite phase may be exposed on the surface, so the cross-sectional area ratio of the second region is preferably 80% or less, 70% or less, 60% or less, or 50% or less.

[0030] [Method for measuring layer thickness and cross-sectional area ratio] The method for measuring the thickness of each layer is described below. A test piece is taken from the stainless steel plate so that the cross section in the plate thickness direction serves as 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 containing aqua regia and glycerin in a volume ratio of 4:1 can be preferably used.

[0031] The boundary between the first and second regions in a stainless steel plate can be clearly identified by etching. The boundary between the first and second regions can also be confirmed by EBSD (Electron Backscattered Diffraction). The EBSD IPF map allows confirmation of the region (first region) primarily composed of the austenite phase (γ) and the region (second region) primarily composed of the ferrite phase (α). After determining the boundary between the first and second regions using the method described above, the area of ​​each layer is calculated using image analysis software.

[0032] For example, in the case of a three-layer structure consisting of the first region (γ) - second region (α) - first region (γ), if the area of ​​the second region is A, the area of ​​the first layer of the region 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 can be calculated as A / (A+B1+B2). Similarly, the cross-sectional area ratio of the first layer can be calculated as B1 / (A+B1+B2), and the cross-sectional area ratio of the third layer can be calculated as B2 / (A+B1+B2). Because the first and second regions are layered, the calculated cross-sectional area ratio is the ratio of the layer thicknesses. Therefore, the thickness of each layer can be calculated by multiplying the thickness of the stainless steel plate by the cross-sectional area ratio.

[0033] When the stainless steel plate is long, it is desirable to measure the cross-sectional area ratio at a plurality of cross sections (for example, 10 cross sections) obtained by cutting the stainless steel plate evenly and calculate the average. The area of ​​the precipitates in the first region is determined from the distribution of metal elements such as C, N, O and Cr, and the proportion of precipitates can be calculated from the area of ​​the first region.

[0034] [Precipitate] One or more precipitates containing at least one of carbides, oxides, and nitrides may be dispersed in the first and second regions. Examples of such precipitates include Cr nitrides, Cr oxides, Cr carbides, Fe oxides, and Fe carbides. The presence of such precipitates is expected to reduce the reflection loss of electromagnetic wave energy at the interface. In particular, the presence of precipitates in the outermost layer (the layer on at least one side of the stainless steel sheet) can reduce the reflection loss at the surface layer, and the reflected wave is reflected by the interface between the first and second regions, thereby preventing the wave from penetrating the stainless steel sheet.

[0035] On the other hand, if precipitates are present in stainless steel, strain concentrates around the precipitates during processing, becoming the initiation point for fracture. Coarse crystal grains in particular not only become the initiation point for stress concentration, but also reduce the effective thickness of the stainless steel sheet, causing fracture. Therefore, from the perspective of workability, it is preferable for precipitates to be absent. Considering workability, it is preferable for precipitates to have a grain size of 1.0 μm or less. Smaller precipitates are preferable, with grain sizes of 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 being preferred. Furthermore, the density of precipitates should be 10% or less, 7% or less, 5% or less, or 3.5% or less, in terms of the area ratio of precipitates with a grain size of 0.1 μm or more in the thickness direction cross section of the stainless steel sheet.

[0036] The lower limit of the precipitate density (area ratio) is preferably 0%, but in the case of the outermost layer (surface layer), a reflection loss effect can also be expected at the precipitate interface, so the area ratio of precipitates with a grain size of 0.1 μm or more may be 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more. The grain size of the precipitates herein refers to the diameter of a circle equivalent to the area of ​​the precipitates observed in a cross section of the electromagnetic shielding material in the thickness direction.

[0037] [component] The composition of the stainless steel sheet according to this embodiment is not limited. The composition of the first region is not limited as long as it is stainless steel mainly composed of austenite phase. The composition of the second region is also not limited as long as it is stainless steel mainly composed of ferrite phase.

[0038] Plate Thickness The thickness of the stainless steel sheet according to this embodiment is not particularly limited. However, if the sheet thickness is too thin, it becomes difficult to roll the sheet while maintaining its shape, and it also becomes difficult to ensure the flatness of the steel sheet. Therefore, the sheet thickness should be 5 μm or more. Preferably, it should be 7 μm or more, 8 μm or more, 9 μm or more, or 10 μm or more. On the other hand, if the sheet thickness is too thick, it becomes difficult to perform fine processing as a shielding material for small electronic components such as sensors, and the weight of the electronic device increases, making it unsuitable for practical use. Therefore, the sheet thickness should be 200 μm or less. To accommodate the miniaturization and thinning of electronic devices, a thinner sheet is preferable, and the sheet thickness should be 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.

[0039] [Manufacturing method] The stainless steel sheet according to the present invention can be produced by any method as long as it has the multilayer structure described above. For example, a clad steel sheet can be produced by laminating an austenitic stainless steel sheet and a ferritic stainless steel sheet, and then a stainless steel sheet in which both phases are laminated can be obtained by a conventional foil rolling process.

[0040] Furthermore, for example, a stainless steel sheet with a laminated austenite and ferrite phases can be obtained by nitriding a ferritic stainless steel sheet and allowing nitrogen to penetrate from the surface, converting the surface layer to austenite. This method produces a stainless steel sheet with austenite (γ) on one or both surfaces and ferrite (α) in the center (a two-layer γ-α or three-layer γ-α-γ stainless steel sheet). Since existing ferritic stainless steel sheets can be nitrided, this method is relatively simple and can be manufactured using existing equipment.

[0041] Alternatively, the two methods may be combined. For example, a ferritic stainless steel sheet, an austenitic stainless steel sheet, and a ferritic stainless steel sheet may be laminated to form an α-γ-α clad steel sheet, and this clad steel sheet may then be nitrided. This allows for the production of a γ-α-γ-α-γ five-layer stainless steel sheet.

[0042] As one embodiment of the manufacturing method, a manufacturing method for nitriding a ferritic stainless steel plate as a base material will be described below. Figure 1 is a flow diagram showing one example of a manufacturing method for a stainless steel plate as the stainless steel plate according to this embodiment. This manufacturing method is merely an example, and the manufacturing method of the stainless steel plate according to this embodiment is not limited to this method.

[0043] This manufacturing method includes a step of preparing a slab (step S1), a step of hot-rolling and cold-rolling the slab to obtain a rolled steel sheet with a thickness of 5 to 200 μm (step S2), a step of annealing (nitriding) the rolled steel sheet in a nitrogen-containing gas atmosphere and cooling it (step S3), and a step of imparting strain to the second region (step S4). Each step will be described in detail below.

[0044] [Slab preparation process] Step S1 A slab having the chemical composition of a ferritic stainless steel base material is prepared (Step S1). For example, the slab prepared has a chemical composition, in mass %, of the ferritic stainless steel according to one embodiment: Cr: 16.00 to 26.00%, N: 0.10% or less, Si: 2.00% or less, C: 0.040% or less, P: 0.030% or less, S: 0.030% or less, Mn: 1.50% 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 why the N content in the slab is set to 0.10% or less is that a high N content increases the deformation resistance, making it difficult to form a steel sheet by rolling. The upper limit of the N content in the slab is preferably 0.05%.

[0045] The chemical composition of the ferritic stainless steel used as the base material according to the embodiment will now be described. In the following description, unless otherwise specified, "%" and "ppm" used for the content of an element refer to mass % and mass ppm, respectively. Furthermore, a lower limit of 0% also includes the case where the element is not present (0%).

[0046] (Cr:16.00~26.00%) Chromium (Cr) forms a Cr2O3 passive film on the surface of stainless steel, improving corrosion resistance. A low Cr content may result in a large amount of martensite in the austenitized structure due to nitrogen absorption. Furthermore, a low Cr content may result in the formation of strain-induced martensite during severe deformation. Therefore, the lower limit of Cr is set to 16.00%. On the other hand, as the Cr content increases, deformation resistance increases. Therefore, 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 18.00%, 20.00%, 21.00%, 22.00%, or 23.00%. The upper limit of the Cr content is preferably 25.00% or 24.00%.

[0047] (N in the second region: 0.10% or less) In this embodiment, a ferritic stainless steel base material is used, and the surface layer is converted to an austenitic stainless steel (first region) by surface nitriding. Therefore, the ferritic stainless steel base material remains as it is as the second region. Nitrogen (N) is also an element that promotes the austenitization of stainless steel. To obtain a structure mainly composed of ferrite phase in Fe-Cr-N stainless steel, it is recommended that the nitrogen (N) content be 0.10% or less. Furthermore, when the ferritic stainless steel base material is rolled to a predetermined plate thickness and then the first region is formed by surface nitriding, if the N content in the base material exceeds 0.10%, the deformation resistance increases, making it difficult to form the base material into a steel plate by rolling. For these reasons, the N content of the ferritic stainless steel base material should be 0.10% or less, and preferably 0.09% or less, 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less.

[0048] (N in the first area: 0.15 to 5.00%) In this embodiment, the surface of the ferritic stainless steel base material is austenitized by nitriding. 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 region should be 0.15% or more. However, since the formation of nitrides such as CrN and CrN within the grains deteriorates workability, the upper limit of the N content should be set to 5.00% to suppress this. Therefore, the nitrogen (N) content in the first region should be set to 0.15% or more and 5.00% or less. The lower limit of the N content in the first region can be preferably 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 may be preferably 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 this embodiment, the N content of the first region is higher than the N content of the second region (base metal).

[0049] (Si: 0 to 2.00%) Silicon (Si) may not be included. Si is an element that deteriorates the workability of stainless steel, and is therefore not typically added. When stainless steel is exposed to a transpassive corrosive environment, Si generates SiO2 on the surface, which forms a protective Cr2O3 passivation film. However, a high Si content can lead to deterioration in workability and the precipitation of a brittle σ phase during manufacturing, which can cause cracks during the steel sheet processing process or result in poor flatness and a shape unsuitable for press processing. Therefore, if Si is included, 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 lower limit of the Si content is preferably greater than 0%, and more preferably 0.10% or greater.

[0050] (C: 0 to 0.040%) Carbon (C) may not be included. Since the stainless steel sheet of this embodiment contains a certain amount of N, solid solution strengthening by N is sufficient, and C does not need to be added. Meanwhile, C is a solid solution strengthening element and contributes to improving the strength of stainless steel. However, if the C content is too high, many carbides are formed during the manufacturing process, and these carbides become the origin of fracture, reducing 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 lower limit of the C content is preferably greater than 0%, more preferably 0.001%.

[0051] (P:0.030% or less) Phosphorus (P) is an impurity. P segregates at grain boundaries during solidification, increasing solidification cracking susceptibility. Therefore, it is preferable to keep the P content as low as possible. Therefore, the P content should be 0.030% or less. The lower limit for P content is 0%, but an excessive decrease would increase the load during refining or require the use of expensive raw materials, so in reality, 0.001% is acceptable.

[0052] (S:0.030% or less) Sulfur (S) is an impurity. S segregates at grain boundaries during solidification and increases solidification cracking susceptibility. Therefore, it is preferable to keep the S content as low as possible. Therefore, 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 load during refining or require the use of expensive raw materials, so in reality, 0.001% is acceptable.

[0053] (Mn: 0 to 1.50%) Manganese (Mn) may not be included. On the other hand, Mn suppresses the deterioration of hot workability due to S. Mn also deoxidizes stainless steel. However, a high Mn content promotes the precipitation of intermetallic compound phases such as the σ phase. The precipitation of the σ phase reduces structural stability and the toughness and ductility of the 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 lower limit of the Mn content is preferably greater than 0%, and more preferably 0.01%, 0.05%, or 0.10%.

[0054] (Cu: 0 to 0.50%) Copper (Cu) may not be included. Cu tends to segregate at grain boundaries and is an austenite-stabilizing element. Cu acts as a solid-solution strengthening element and contributes to increasing the high-temperature strength required for structural materials, so it may be included as needed. A high Cu content suppresses ferrite formation during solidification during casting, increasing solidification cracking susceptibility. Furthermore, a high Cu content may degrade 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%, or 0.20%. The lower limit of the Cu content is preferably greater than 0%, and more preferably 0.01%.

[0055] (Mo: 0-3.00%) Molybdenum (Mo) may not be included. On the other hand, when corrosion resistance is particularly desired, Mo has the effect of increasing the corrosion resistance of stainless steel. However, Mo is an expensive element classified as a rare metal, and is not preferable from the perspective of providing an economically excellent material. Furthermore, if the Mo content is too high, hot workability will decrease and a structure mainly composed of austenite phase may not be obtained in 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 lower limit of the Mo content is preferably more than 0%, and more preferably 0.01%, 0.05%, 0.10%, 0.20% or 0.30%.

[0056] (Ni: 0 to 5.00%) Nickel (Ni) may not be included. However, Ni promotes the austenitization of stainless steel and contributes to improving corrosion resistance. Therefore, Ni may be included when particularly enhancing corrosion resistance or workability is desired. However, Ni is a rare metal, and is therefore undesirable from the perspective of providing an economically efficient material. Furthermore, leaching of Ni ions may reduce the oxygen reduction reaction rate at the interface between the platinum catalyst and the polymer electrolyte membrane. Therefore, the Ni content is limited to 5.0% or less. The upper limit of the Ni content is preferably 4.50%. Examples of Ni content include 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%, and 0.2%. The lower limit of the Ni content is preferably greater than 0%, and more preferably 0.01%, 0.05%, or 0.10%.

[0057] (Ca: 0 to less than 50 ppm) Calcium (Ca) is an impurity and should preferably not be included. CaS and MnS are generally known non-metallic inclusions that can be the starting point for corrosion in stainless steel. To prevent the formation of large amounts of CaS, which can be the starting point for corrosion, the Ca content should be less than 50 ppm.

[0058] (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 decrease. Therefore, the Al content is less than 300 ppm. The preferred lower limit of the Al content is more than 0%, and more preferably 1 ppm, 10 ppm, 50 ppm, or 100 ppm. In this specification, the Al content refers to the content of acid-soluble Al (sol.Al).

[0059] The balance of the chemical composition of the stainless steel sheet according to this embodiment is Fe and impurities, which refer to elements unintentionally mixed in from ores or scrap used as raw materials for steel, or elements mixed in from the environment during the manufacturing process.

[0060] The process of preparing the slab can be, for example, but not limited to, the following. The raw materials are melted. Ferrochrome and ferrosilicon for stainless steel production, cast iron, and scrap of ferritic stainless steel can be used as raw materials. Melting is mainly carried out in an electric furnace. At the laboratory level, it can also be carried out in a vacuum induction furnace. Refining is carried out to reduce the carbon content, gas components, and metal inclusions. Refining can be carried out using the AOD (Argon-Oxygen-Decarburization) method, VOD (Vacuum-Oxygen-Decarburization) method, V-AOD method, etc. The material is then cast into a continuous casting machine or case to form a shape suitable for rolling. The chemical composition of the slab can be adjusted by the blending of raw materials and the refining conditions.

[0061] [Rolling process] Step S2 The slab is hot-rolled and cold-rolled to obtain a rolled steel sheet having a thickness of 5 to 200 μm (step S2). Hot-rolling and cold-rolling may be repeated, and intermediate heat treatment such as annealing and pickling may be performed as needed. In addition to hot-rolling and cold-rolling, hot-forging and cutting may be performed as needed.

[0062] The rolling process is not limited to this, but may be performed, for example, as follows. The slabs are hot-rolled using a tandem mill or a Steckel mill to form hot coils. These hot coils are then annealed and pickled. They are then cold-rolled using a multi-roll cold rolling mill to form rolled steel sheets with thicknesses of 5 to 200 μm.

[0063] [Annealing (nitriding) process] Step 3 The rolled steel sheet is annealed in a nitrogen-containing gas atmosphere and cooled (step S3). This process allows nitrogen to penetrate from the surface of the steel sheet, converting the surface layer structure of both or one of the surfaces of the steel sheet into a structure (first region) mainly composed of austenite phase. If only the surface layer of one surface is to be made into austenite phase, for example, it is advisable to mask the opposite surface (the surface not to be made into austenite phase) and then perform nitriding treatment.

[0064] The N content in the surface layer portion (first region) of the steel sheet after the annealing process can be adjusted by the N content of the slab, the annealing conditions, etc. Specifically, the N content in the surface layer portion (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, or lengthening the annealing holding time, or controlling the steel sheet passing speed.

[0065] The ratio of the nitrogen partial pressure to the total pressure of the processing gas is preferably 0.2 to 0.9. If the ratio of the nitrogen partial pressure to the total pressure is less than 0.2, nitrogen is not supplied sufficiently from the surface, and when the steel sheet is thick, it becomes difficult to form the first region so as to cover the entire front and back surfaces of the steel sheet. On the other hand, if the ratio of the nitrogen partial pressure to the total pressure of the processing gas is higher than 0.9, Cr nitrides are formed in excess on the surface, which may become the starting point for cracking during processing. The upper limit of the ratio of the nitrogen partial pressure to the total pressure of the processing gas is preferably 0.75. Hydrogen is preferably used as the gas to be mixed with nitrogen to prevent oxidation of the steel sheet. Argon may be used instead of or in addition to hydrogen.

[0066] The annealing temperature is preferably 950 to 1200°C. If the annealing temperature is less than 950°C, not only the austenite phase but also the CrN phase exists in the equilibrium state, and therefore the austenite phase fraction of the first region may not be increased. On the other hand, if the annealing temperature exceeds 1200°C, particularly when Si is contained, a liquid phase may be generated near the grain boundaries, which may cause melting and embrittlement. The annealing temperature varies depending on the Cr content, but is more preferably 1050 to 1150°C.

[0067] The annealing holding time needs to be controlled within a narrow range depending on the thickness and nitrogen partial pressure of the steel sheet. Austenitization due to nitrogen absorption progresses from the surface to the interior of the sheet over time, but in the manufacturing of the stainless steel sheet according to this embodiment, it is necessary to stop the progress of austenitization midway. If the holding time is too short, there is a risk that ferrite phase will remain on the surface even if the sheet thickness is thin. On the other hand, if the holding time is too long, there is a risk that the cross-sectional area ratio of the second region will be too low.

[0068] When cooling annealed steel sheet, controlled cooling according to the temperature range is recommended. If the annealed steel sheet is slowly cooled, the austenite phase formed by absorbing nitrogen will change to a two-phase structure of ferrite and Cr nitride. On the other hand, if the cooling rate is too fast, it will be impossible to form an austenite phase in the surface layer, which has a structure in which one or more precipitates consisting of carbides, oxides, and nitrides are dispersed. Specifically, controlled cooling is recommended, in which the steel sheet is cooled from the annealing temperature to approximately 500°C at a fast cooling rate (10 to 15°C / min), and then cooled from 500°C to 300°C at a relatively slow cooling rate (5 to 8°C / sec). Alternatively, the steel sheet may be cooled to 500°C at a fast cooling rate (10 to 15°C / sec), held at 500°C ± 10°C for approximately 1 to 30 seconds, and then cooled at a fast cooling rate (10 to 15°C / sec).

[0069] The annealing step may be performed in any form using an annealing furnace as long as the above annealing conditions are satisfied. For example, the annealing step may be performed by passing the steel sheet through an annealing line called a continuous bright annealing line.

[0070] This annealing process forms a first region mainly composed of austenite phase in the surface layer of the steel sheet. The surface layer of the steel sheet after the annealing process has an N content of 0.15 to 5.0 mass %, and is adjusted so that the cross-sectional area ratio of the second region is 15% or more. When only the surface layer on one side is to be the first region (austenite phase), the opposite side (the side not to be made into austenite phase) can be masked.

[0071] The N content of the steel sheet after the annealing process can be adjusted by the N content of the slab and the annealing conditions. Specifically, the N content of the first region, which becomes the surface of the stainless steel sheet, can be adjusted by increasing the N content of the slab, increasing the nitrogen partial pressure in the annealing atmosphere, increasing the annealing temperature, or lengthening the annealing holding time, or controlling the steel sheet passing speed.

[0072] [Straining the second region] Step 4 After annealing to form a multi-layered stainless steel, cold rolling is performed to impart strain to the stainless steel sheet. The reduction ratio in cold rolling is preferably 0.2 to 0.6. Since the ferrite phase (second region) has poorer ductility than the austenite phase (first region), strain is easily imparted to the ferrite phase by rolling. There are no particular restrictions on the number of rolls. The number of rolls in cold rolling can be adjusted to achieve a predetermined average KAM value. After cold rolling, annealing may be performed to further adjust strain and relieve local strain. When annealing, heat treatment is preferably performed in a temperature range of 400 to 500°C with a holding time of 1 to 5 minutes. There are no particular restrictions on the cooling method after annealing; air cooling (natural cooling) is also possible. [Example]

[0073] [Test material] Steels of the eight chemical compositions shown in Table 1 were melted in a 30 kg vacuum melting furnace equipped with high-frequency induction heating to produce cast ingots of roughly truncated cone shape with diameters of 125 to 115 mm and heights of 320 mm. The black surface of the truncated cone shape of these cast ingots was ground off with a grinder, and after grinding, they were heated and held at 1250°C for 3 hours, then hot forged to a finished thickness of 25 mm and ground to a thickness of 20 mm to produce slabs.

[0074] The slab was then held at 1200°C for 2 hours and hot rolled to a thickness of 4 mm to obtain a hot-rolled steel sheet. After rolling, the hot-rolled steel sheet was ground to a thickness of 3 mm to remove the black scale, and then cold-rolled to a thickness of 0.5 mm to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheets were repeatedly subjected to intermediate annealing in an argon atmosphere at 800°C for 10 minutes and cold rolling to obtain test materials with thicknesses shown in Table 2.

[0075] [Annealing treatment (nitriding treatment)] Test pieces measuring 70 mm wide x 200 mm long were cut out from each test material and subjected to annealing treatment (nitriding treatment) in a solid phase using a continuous annealing simulator under the annealing treatment (nitriding treatment) conditions shown in Table 2. The total pressure was set to 1 atmosphere. After annealing, the specimens were cooled under the cooling conditions shown in Table 2. Some test specimens (TP7 and 8 in Table 2) were held at 500°C for 10 seconds and then cooled to 300°C at an average cooling rate of 14°C / second. As a result, a stainless steel plate (test material) was obtained.

[0076] [Strain application] The annealed (nitrided) test materials were cold-rolled and then annealed to impart strain to each test material. The rolling and annealing conditions are shown in Table 2.

[0077] [Investigation of organizations, etc.] (1) Measurement of N content in stainless steel sheets The N content in the steel after nitriding treatment was measured by an inert gas conveying fusion thermal conductivity method using an analytical sample taken from the entire thickness of each test piece.

[0078] (2) Confirmation that the first and second areas are included A sample was taken from each test piece so that the cross section perpendicular to the rolling direction (cross section in the plate thickness direction) served as the observation surface, embedded in resin, mirror-polished, and then etched using an etching solution made of aqua regia and glycerin in a volume ratio of 4:1 until the metal structure became apparent. When the test material had a first region and a second region, the boundary between them could be clearly distinguished by etching.

[0079] The region consisting mainly of ferrite (region 2) was etched more deeply than the region consisting mainly of austenite (region 1). Within the region consisting mainly of ferrite, the difference in etching between grains was small, the etching at the grain boundaries was thin, and the etching within the grains was relatively smooth. This is thought to be because no nitrogen penetration occurred, and the original ferrite phase remained, only being subjected to the thermal history of high-temperature annealing.

[0080] In contrast, the region consisting mainly of austenite (region 1) was etched more shallowly than the region consisting mainly of ferrite (region 2). Within the region consisting mainly of austenite, the etching differed greatly between grains, with the grain boundaries being etched relatively clearly and the grain interiors being etched relatively coarsely. This is thought to be due to the accumulation of strain within the grains as the ferrite phase transformed into the austenite phase due to nitrogen enrichment caused by nitrogen penetration while the surroundings were constrained in the solid state.

[0081] Furthermore, an EBSD orientation map confirmed that the surface region was a region (region 1) primarily composed of austenite (γ) phase (γ area ratio of 75% or more), while the center region was a region (region 2) primarily composed of ferrite (α) phase (α area ratio of 95% or more). The cross-sectional structure of a sample removed from the test material was also analyzed using EBSD (Electron Backscattered Diffraction, JEOL JSM-7001F). Measurements were performed using an accelerating voltage of 15 kV, a measurement field of view of 150 μm (horizontal) × 50 μm (vertical), and a step size of 0.01 μm. The measurements were performed using a TSL Solutions HIKARI (high-speed detector) and OIM DATA COLLECTION (measurement software), and analyzed using OIM Analysis (analysis software). The EBSD IPF map confirmed that the surface layer is a region (region 1) mainly composed of the austenite phase (γ), and the center is a region (region 2) mainly composed of the ferrite phase (α). The "Layer Structure" column in Table 2 lists the phases in order from the surface layer.

[0082] (3) Measurement of the cross-sectional area ratio of the second region (ferritic stainless steel phase (α)) After defining the boundary between the first and second regions, the area of ​​each was calculated using image analysis software. The area of ​​the second region was designated A, and the area of ​​the region other than the second region was designated B. The cross-sectional area ratio of the second region was calculated as A / (A+B).

[0083] (4) Measurement of the average KAM value in the second region The KAM value of the second region was confirmed by EBSD KAM map measurement. The cross-sectional structure of the test material was measured using EBSD (Electron Backscattered Diffraction, JEOL JSM-7001F) with an acceleration voltage of 15 kV, a measurement field of view of 60 μm (horizontal) × total thickness (vertical), a step size of 0.01 μm, and an orientation difference of 5°. Measurements were performed using TSL Solutions' HIKARI (high-speed detector) and OIM DATA COLLECTION (measurement software), and KAM map analysis was performed using OIM Analysis (analysis software). Measurements were taken at three different observation fields, and the average KAM value for each sample was calculated. The measurement results are shown in Table 2.

[0084] [Evaluation of test material characteristics] For each test material, the electromagnetic wave shielding properties in the high frequency range, as well as formability (elongation) and strength (tensile strength) were investigated and evaluated. The results are shown in Table 2.

[0085] (1) Evaluation of electromagnetic wave shielding characteristics at high frequencies The higher the magnetic permeability of a material at the test frequency, the better its electromagnetic shielding properties. To evaluate 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 test material was punched using a mold to prepare test pieces (TP) with outer dimensions of 6.93 to 6.96 mm and inner diameters of 3.06 to 3.10 mm. The magnetic permeability of each TP was measured at 100 MHz. The permeability was measured using a vector network analyzer (Keysight ENA E5071C) over the frequency range of 100 kHz to 8.5 GHz. The measured magnetic permeability was measured as complex permeability, and a complex permeability (imaginary part) of 0.5 or higher at 100 MHz indicated excellent electromagnetic shielding properties. As shown in Table 2, TP1 failed in its electromagnetic shielding properties because the surface layer was a ferrite and martensite structure (α') rather than austenite due to the low Cr concentration. TP17 was not cold-rolled, so its KAM value was small, indicating that its electromagnetic shielding properties were poor.

[0086] (2) Evaluation of formability and strength The formability (elongation) and strength (tensile strength) of each test material were evaluated. ASTM half-specimens (parallel section: 6.25 mm wide, 32 mm long) were subjected to tensile tests at room temperature at 2 mm / min. The elongation (EL) was used as a representative value of ductility. If the measured EL is 5% or more, the moldability is at a satisfactory level, and those with 5% or more were judged to have good moldability. Strength (tensile strength) was defined as being superior if it was 500 MPa or higher. For reference, the strength of aluminum foil is only about 100 MPa at most.

[0087] [Table 1]

[0088] [Table 2] [Industrial Applicability]

[0089] The present invention can be used as a stainless steel sheet in a variety of industrial fields, particularly for housings of electronic equipment and electronic devices.

Claims

1. A plate having a thickness of 5 to 200 μm, in which a first region made of stainless steel having an austenite phase area ratio of 75% or more and a second region made of stainless steel having a ferrite phase area ratio of 95% or more are laminated in the plate thickness direction, A stainless steel plate having a multilayer structure, characterized in that the average KAM value obtained by EBSD measurement in the second region is 0.50 to 1.

50.

2. The stainless steel plate having a multi-layer structure according to claim 1, wherein at least one surface is the first region.

3. 2. The stainless steel plate having a multi-layer structure according to claim 1, wherein both surfaces are the first region.

4. The stainless steel plate having the multilayer structure according to any one of claims 1 to 3, having a plate thickness of 5 to 50 µm.

5. 4. A stainless steel plate having a multilayer structure according to claim 1, wherein the sum of the area ratios of the second regions 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: 16.00-26.00%, N: 0 to 0.10%, Si: 0-2.00%, 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: 0-3.00%, Ni: 0-5.00%, Ca: 0-50ppm, sol. Al: 0 to 300 ppm, The stainless steel plate having a multilayer structure according to any one of claims 1 to 3, wherein the balance is Fe and impurities.

7. The chemical composition of the first region is, in mass %, Cr: 16.00-26.00%, N: 0.15-5.00%, Si: 0-2.00%, 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: 0-3.00%, Ni: 0-5.00%, Ca: 0-50ppm, sol. Al: 0 to 300 ppm, The stainless steel sheet having a multilayer structure according to claim 6, wherein the balance is Fe and impurities.

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