Austenitic stainless steel sheet and component for portable electronic apparatus

Abstract

This austenitic stainless steel sheet contains, by mass, 0.030-0.120% of C, 0.20-1.00% of Si, 2.00-12.50% of Mn, 0.050% or less of P, 0.0350% or less of S, 4.50-14.50% of Ni, 16.50-21.00% of Cr, 0.50% or less of Cu, 0.70% or less of Mo, and 0.100-0.500% of N, the balance being Fe and impurities. The austenitic stainless steel sheet has an Ni equivalent represented by formula (1) of 18.0 or higher, a relative permeability of 1.007 or lower, and a tensile strength of 1500 MPa or higher.　Formula (1): Ni equivalent = Ni + 0.60Mn + 0.18Cr + 9.69(C + N) - 0.11Si2. In the formula, each element symbol represents the amount of the respective element contained.

Description

Austenitic stainless steel sheets and parts for portable electronic devices

[0001] The present invention relates to an austenitic stainless steel sheet and a part for a portable electronic device.

[0002] As communication devices such as smartphones and precision instruments such as personal computers become smaller and more powerful, various structural and functional components used in these devices are becoming thinner and lighter. Therefore, stainless steel sheets used as materials for these components are required to have high strength despite their small thickness. For example, in portable electronic devices such as foldable smartphones (foldable phones), spring materials (leaf springs) are used in the back plates that support the folding function of the screen. For spring materials exposed to repeated bending, stainless steel sheets with strength (fatigue properties) sufficient to withstand repeated bending are required. Furthermore, stainless steel sheets with strength sufficient to ensure their functionality are also required for components such as the housings of portable electronic devices and reinforcing plates for displays.

[0003] For example, Patent Document 1 proposes a stainless steel foil for use as a spring material, in which the maximum equivalent circle diameter of non-metallic inclusions in a cross-sectional view is less than 3 μm. This stainless steel foil is strengthened by a deformation-induced martensite structure formed during cold rolling, and fatigue strength is improved by suppressing cracking through the reduction of non-metallic inclusions. Patent Document 2 also proposes a stainless steel foil having a thickness of 0.1 mm or less, a tensile strength of 1800 MPa or more, and a maximum roughness height Rz of 0.35 μm or less, as determined from a surface roughness curve measured in the same direction as the tensile direction. This stainless steel foil is strengthened by a deformation-induced martensite structure formed during cold rolling, and fatigue strength is improved by controlling the maximum roughness height Rz.

[0004] In recent years, minimizing electromagnetic interference has become important to ensure the high performance and reliability of portable electronic devices. When magnetic materials are used in various components of portable electronic devices, the magnetic field generated by the materials due to electromagnetic waves can affect adjacent electronic circuits, causing performance degradation and malfunction of the devices. Therefore, using nonmagnetic materials is effective from the perspective of suppressing electromagnetic interference. Therefore, it is desirable to construct various components of portable electronic devices using nonmagnetic materials as much as possible, except for components such as sensors and motors that require magnetic materials. Previously, a relative permeability of 1.010 or less was often considered sufficient to define the level of nonmagnetic properties. However, with the recent increasing demand for miniaturization and improved performance and reliability of portable electronic devices, there is an increasing demand for nonmagnetic materials with a relative permeability of 1.007 or less, especially 1.005 or less, even for various components of consumer portable electronic devices.

[0005] However, in the stainless steel foils of Patent Documents 1 and 2, the deformation-induced martensite structure formed to increase strength is ferromagnetic, making it difficult to achieve a relative magnetic permeability of 1.007 or less. Patent Document 3 describes a stainless steel sheet that is nonmagnetic and has improved strength and fatigue properties, containing, in mass %, C: 0.040 to 0.080%, Si: 0.30 to 1.00%, Mn: 2.00 to 4.00%, P: 0.050% or less, S: 0.005% or less, Ni: 11.00 to 14.00%, Cr: 18.00 to 20.00%, and Cu: 0.50% or less. a steel sheet having a chemical composition consisting of Mo: 0.50% or less, Ti: 0.015% or less, Co: 0.10 to 2.00%, N: 0.100 to 0.300%, Al: 0.010% or less, B: 0.0100% or less, O: 0.0030 to 0.0100%, and the balance being Fe and unavoidable impurities; and M5 is 15.0 μm or less, and the tensile strength in the rolling direction is 1000 N / mm 2 As described above, a stainless steel sheet for portable electronic devices having a relative magnetic permeability μr of 1.005 or less has been proposed.

[0006] International Publication No. 2022 / 014307 International Publication No. 2022 / 210918 Patent No. 7215938

[0007] As described above, due to the increasing need for miniaturization of portable electronic devices, there is a demand for stainless steel sheets that have high strength even when the thickness is reduced. However, the stainless steel sheet of Patent Document 3 does not necessarily have properties suitable for use as a component for portable electronic devices when the thickness is reduced. For example, the stainless steel sheet of Patent Document 3 does not necessarily have sufficient strength when the thickness is reduced. For example, the fatigue properties required for spring materials are better as the strength (particularly tensile strength) increases, so further increases in strength are required as a method for improving fatigue properties.

[0008] The present invention has been made to solve the above problems, and has an object to provide a non-magnetic, high-strength austenitic stainless steel sheet. Another object of the present invention is to provide a component for a portable electronic device that can be made smaller while improving its performance and reliability.

[0009] As a result of extensive research into austenitic stainless steel sheets, the inventors have discovered that the above-mentioned problems can be solved by controlling the composition, Ni equivalent, relative magnetic permeability, and tensile strength, and have thus completed the present invention.

[0010] Specifically, the present invention provides an austenitic stainless steel sheet containing, by mass, 0.030 to 0.120% C, 0.20 to 1.00% Si, 2.00 to 12.50% Mn, 0.050% or less P, 0.0350% or less S, 4.50 to 14.50% Ni, 16.50 to 21.00% Cr, 0.50% or less Cu, 0.70% or less Mo, and 0.100 to 0.500% N, with the balance being Fe and impurities, having a Ni equivalent of 18.0 or more, a relative magnetic permeability of 1.007 or less, and a tensile strength of 1500 MPa or more, as expressed by the following formula (1): Ni equivalent = Ni + 0.60Mn + 0.18Cr + 9.69(C + N) - 0.11Si 2 ... (1) In the formula, each element symbol represents the content of each element.

[0011] The present invention also provides a part for a portable electronic device that includes a member having the above-mentioned austenitic stainless steel sheet.

[0012] According to the present invention, it is possible to provide a non-magnetic, high-strength austenitic stainless steel sheet, and also to provide a part for a portable electronic device that can be made smaller while also improving its performance and reliability.

[0013] Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, as appropriate, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the present invention, also fall within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0014] An austenitic stainless steel sheet according to an embodiment of the present invention contains C: 0.030 to 0.120%, Si: 0.20 to 1.00%, Mn: 2.00 to 12.50%, P: 0.050% or less, S: 0.0350% or less, Ni: 4.50 to 14.50%, Cr: 16.50 to 21.00%, Cu: 0.50% or less, Mo: 0.70% or less, N: 0.100 to 0.500%, and the balance being Fe and impurities.

[0015] Here, in this specification, "stainless steel sheet" refers to a plate-shaped (including strip-shaped) material formed from stainless steel. Stainless steel sheet also includes thin foil-shaped materials. The thickness of the stainless steel sheet is not particularly limited, but is preferably 0.30 mm or less, more preferably 0.20 mm or less, and even more preferably 0.01 to 0.10 mm. Furthermore, in this specification, "austenitic" refers to a stainless steel sheet whose metal structure at room temperature is primarily austenite, preferably a single austenite phase. Therefore, "austenitic" also includes a stainless steel sheet containing small amounts of phases other than austenite (e.g., extremely small martensite phases), precipitates such as intermetallic compounds, inclusions, etc. If phases other than austenite, precipitates, inclusions, etc. are contained, the amount thereof is not particularly limited as long as it does not impair the effects of the present invention. Furthermore, in this specification, "impurities" refer to components that are mixed in during the industrial production of stainless steel sheet due to raw materials such as ores and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, the impurities include unavoidable impurities such as O (oxygen). When O is contained as an impurity, the O content is 0.0150% or less. With regard to the content of each element, "xx% or less" means that the content is xx% or less, but includes an amount exceeding 0% (particularly, above the impurity level).

[0016] The austenitic stainless steel sheet according to the embodiment of the present invention may further contain, as necessary, one or more selected from Al: 0.080% or less, Ti: 0.050% or less, Co: 0.50% or less, B: 0.0100% or less, Nb: 0.060% or less, Ca: 0.0100% or less, V: 0.200% or less, Sn: 0.050% or less, W: 0.100% or less, Pb: 0.009% or less, and Mg: 0.0030% or less. Each component will be described in detail below.

[0017] <C: 0.030 to 0.120%> C is an element that contributes to increasing strength and stabilizing the austenite phase. From the viewpoint of ensuring these effects, the lower limit of the C content is controlled to 0.030%, preferably 0.040%, more preferably 0.070%, and even more preferably 0.080%. On the other hand, if the C content is too high, workability decreases, so the upper limit of the C content is controlled to 0.120%, preferably 0.110%, and more preferably 0.100%.

[0018] <Si: 0.20 to 1.00%> Si is an element that contributes to increasing strength. From the viewpoint of ensuring this effect, the lower limit of the Si content is controlled to 0.20%, preferably 0.30%. On the other hand, if the Si content is too high, workability decreases, so the upper limit of the Si content is controlled to 1.00%, preferably 0.90%, more preferably 0.75%, and even more preferably 0.60%.

[0019] <Mn: 2.00 to 12.50%> Mn is an element that contributes to increasing strength and stabilizing the austenite phase. From the viewpoint of ensuring these effects, the lower limit of the Mn content is controlled to 2.00%, preferably 2.70%, more preferably 9.00%, and even more preferably 11.30%. On the other hand, if the Mn content is too high, workability decreases, so the upper limit of the Mn content is controlled to 12.50%, preferably 12.00%, and more preferably 11.70%.

[0020] <P: 0.050% or less> An excessively high P content can cause a decrease in corrosion resistance. Therefore, from the viewpoint of ensuring corrosion resistance, the upper limit of the P content is controlled to 0.050%, preferably 0.045%, more preferably 0.040%, and even more preferably 0.030%. On the other hand, the lower limit of the P content is not particularly limited, but an excessive reduction in the P content leads to an increase in the steelmaking load and raw material costs. Therefore, the lower limit of the P content is generally 0.001%, preferably 0.005%, and more preferably 0.010%.

[0021] <S: 0.0350% or less> S forms MnS-based nonmetallic inclusions. MnS-based inclusions are elongated in the rolling direction, but tend to exist in the steel sheet as elongated inclusions in the rolling direction without being broken down into small pieces. This type of inclusion is likely to be the initiation point of fatigue fracture due to bending stress with the bending axis parallel to the rolling direction. Therefore, from the viewpoint of ensuring fatigue properties, the upper limit of the S content is controlled to 0.0350%, preferably 0.0300%, more preferably 0.0030%, and even more preferably 0.0010%. On the other hand, the lower limit of the S content is not particularly limited, but excessive reduction of the S content leads to increased steelmaking load and raw material costs. Therefore, the lower limit of the S content is generally 0.0001%, preferably 0.0002%, and more preferably 0.0003%.

[0022] <Ni: 4.50 to 14.50%> Ni is an element that contributes to stabilizing the austenite phase and improving corrosion resistance. From the viewpoint of ensuring these effects, the lower limit of the Ni content is controlled to 4.50%, preferably 5.50%, and more preferably 6.50%. On the other hand, because Ni is expensive, an excessively high Ni content leads to an increase in manufacturing costs. Therefore, the upper limit of the Ni content is controlled to 14.50%, preferably 14.00%, more preferably 13.50%, and even more preferably 7.00%.

[0023] <Cr: 16.50 to 21.00%> Cr is an element necessary for ensuring corrosion resistance. From the viewpoint of ensuring this effect, the lower limit of the Cr content is controlled to 16.50%, preferably 17.00%, more preferably 17.50%, and even more preferably 17.70%. On the other hand, if the Cr content is too high, the formation of intermetallic compounds (σ phases) is promoted, thereby reducing the workability of the austenitic stainless steel material. Therefore, the upper limit of the Cr content is controlled to 21.00%, preferably 20.00%, and more preferably 18.00%.

[0024] <Cu: 0.50% or less> Cu is an element that suppresses work hardening of the austenite phase. Therefore, it is advantageous to add Cu when the degree of working by press forming is large or when cold forging is performed. However, if the Cu content is too high, it can cause a decrease in corrosion resistance. Therefore, the upper limit of the Cu content is controlled to 0.50%, preferably 0.35%, and more preferably 0.30%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.01%, and more preferably 0.02%, from the viewpoint of ensuring the effects of Cu.

[0025] <Mo: 0.70% or less> Mo is an element effective in improving corrosion resistance. However, because Mo is expensive, if the Mo content is too high, it leads to an increase in production costs. Therefore, the upper limit of the Mo content is controlled to 0.70%, preferably 0.50%, and more preferably 0.30%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.01%, and more preferably 0.02%, from the viewpoint of ensuring the effects of Mo.

[0026] <N: 0.100 to 0.500%> N is an element that contributes to increasing strength and stabilizing the austenite phase. From the viewpoint of ensuring these effects, the lower limit of the N content is controlled to 0.100%, preferably 0.130%, more preferably 0.250%, and even more preferably 0.290%. On the other hand, if the N content is too high, workability decreases, so the upper limit of the N content is controlled to 0.500%, preferably 0.400%, and more preferably 0.320%.

[0027] <Al: 0.080% or less> Al is an element that acts as a powerful deoxidizer. However, if the Al content is too high, the amount of Al2O3-based inclusions produced increases, which may result in a deterioration in quality. Therefore, the upper limit of the Al content is controlled to 0.080%, preferably 0.070%, and more preferably 0.060%. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of ensuring the effects of Al, it is preferably 0.003%, more preferably 0.005%, and even more preferably 0.010%.

[0028] <Ti: 0.050% or less> Ti is an element effective in fixing C and improving intergranular corrosion resistance. However, if the Ti content is too high, the amount of coarse inclusions generated increases, which causes deterioration of fatigue properties. Therefore, the upper limit of the Ti content is controlled to 0.050%, preferably 0.040%, and more preferably 0.030%. On the other hand, the lower limit of the Ti content is not particularly limited, but from the viewpoint of ensuring the effects of Ti, it is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0029] <Co: 0.50% or less> Co is an element that contributes to improving corrosion resistance. However, if the Co content is too high, workability decreases and manufacturing costs increase. Therefore, the upper limit of the Co content is controlled to 0.50%, preferably 0.40%. On the other hand, the lower limit of the Co content is not particularly limited, but from the viewpoint of ensuring the effects of Co, it is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%.

[0030] <B: 0.0100% or less> B is an element effective in suppressing the occurrence of surface defects, improving manufacturability, and improving weldability. However, if the B content is too high, these properties will deteriorate. Therefore, the upper limit of the B content is controlled to 0.0100%, preferably 0.0060%. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of ensuring the effects of B, it is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0031] <Nb: 0.060% or less> Nb is an element with a high affinity for C and N. It precipitates as carbides or nitrides during hot rolling, reducing the amount of dissolved C and N in the matrix and improving workability. However, if the Nb content is too high, the austenitic stainless steel sheet becomes hard and its ductility decreases. Therefore, the upper limit of the Nb content is controlled to 0.060%, preferably 0.050%. On the other hand, the lower limit of the Nb content is not particularly limited, but is preferably 0.001%, more preferably 0.005%, from the viewpoint of ensuring the effects of Nb.

[0032] <Ca: 0.0100% or less> Ca is an element that improves hot workability. However, if the Ca content is too high, the toughness of the austenitic stainless steel sheet decreases. Therefore, the upper limit of the Ca content is controlled to 0.0100%, preferably 0.0070%, and more preferably 0.0050%. On the other hand, the lower limit of the Ca content is not particularly limited, but is preferably 0.0001%, and more preferably 0.0003%, from the viewpoint of ensuring the effects of Ca.

[0033] <V: 0.200% or less> V is an element that has the effect of enhancing age hardenability. However, if the V content is too high, it leads to an increase in manufacturing costs. Therefore, the upper limit of the V content is controlled to 0.200%, preferably 0.150%. On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of ensuring the effect of V, it is preferably 0.001%, more preferably 0.003%.

[0034] <Sn: 0.050% or less> Sn is an element that improves workability by promoting the formation of deformation bands during rolling. However, if the Sn content is too high, the effect of Sn saturates and workability decreases. Therefore, the upper limit of the Sn content is controlled to 0.050%, preferably 0.040%. On the other hand, the lower limit of the Sn content is not particularly limited, but is preferably 0.001%, more preferably 0.003%, from the viewpoint of ensuring the effect of Sn.

[0035] <W: 0.100% or less> W is an element that improves high-temperature strength without impairing ductility at room temperature. However, if the W content is too high, coarse eutectic carbides are formed, causing a decrease in ductility. Therefore, the upper limit of the W content is controlled to 0.100%, preferably 0.080%. On the other hand, the lower limit of the W content is not particularly limited, but is preferably 0.001%, more preferably 0.003%, from the viewpoint of ensuring the effects of W.

[0036] <Pb: 0.009% or less> Pb is an element that improves machinability. However, if the Pb content is too high, it lowers the melting point of the grain boundary and reduces the bonding strength of the grain boundary, which may lead to deterioration of hot workability, such as liquation cracking due to grain boundary melting. Therefore, the upper limit of the Pb content is controlled to 0.009%, preferably 0.008%. On the other hand, the lower limit of the Pb content is not particularly limited, but is preferably 0.001% from the viewpoint of ensuring the effects of Pb.

[0037] <Mg: 0.0030% or less> Mg is an element that forms Mg oxide together with Al in molten steel and acts as a deoxidizer. However, if the Mg content is too high, the toughness of the austenitic stainless steel sheet decreases. Therefore, the upper limit of the Mg content is controlled to 0.0030%, preferably 0.0020%. On the other hand, the lower limit of the Mg content is not particularly limited, but is preferably 0.0001%, more preferably 0.0003%, from the viewpoint of ensuring the effects of Mg.

[0038] The austenitic stainless steel sheet according to the embodiment of the present invention has a Ni equivalent, expressed by the following formula (1), of 18.0 or more, preferably 18.5 or more, and more preferably 19.0 or more: Ni equivalent = Ni + 0.60Mn + 0.18Cr + 9.69(C + N) - 0.11Si 2 ... (1) In the formula, each element symbol represents the content of each element. Here, the Ni equivalent is an index related to the stability of non-magnetic properties. The larger the Ni equivalent value, the more the formation of a deformation-induced martensite structure during cold rolling is suppressed, and the amount of ferromagnetic deformation-induced martensite structure formed can be reduced. By controlling the Ni equivalent within the above range, a non-magnetic austenitic stainless steel sheet can be obtained. The upper limit of the Ni equivalent is not particularly limited, but is generally 30.0.

[0039] The austenitic stainless steel sheet according to the embodiment of the present invention has a relative permeability of 1.007 or less, preferably 1.005 or less. A relative permeability within this range can be said to be a non-magnetic austenitic stainless steel sheet. The lower limit of the relative permeability is 1.000, since the closer to 1 the value, the more non-magnetic the sheet is. Here, the relative permeability is calculated by dividing the permeability by the permeability in a vacuum. The permeability can be obtained by determining the slope of the magnetic field-magnetization curve measured using a commercially available magnetometer.

[0040] The austenitic stainless steel sheet according to the embodiment of the present invention has a tensile strength (TS) of 1500 MPa or more, preferably 1600 MPa or more, and more preferably 1800 MPa or more. By controlling the tensile strength within this range, good strength can be ensured even when the thickness is reduced. The upper limit of the tensile strength is not particularly limited, but is typically 3000 MPa. The tensile strength of the austenitic stainless steel material can be measured in accordance with JIS Z2241:2022.

[0041] The austenitic stainless steel sheet according to the embodiment of the present invention preferably has a Vickers hardness of 400 HV or more, more preferably 420 HV or more, and even more preferably 450 HV or more. By controlling the Vickers hardness within this range, good strength can be ensured even when the thickness is reduced. The upper limit of the Vickers hardness is not particularly limited, but is typically 800 HV. The Vickers hardness of the austenitic stainless steel sheet can be measured in accordance with JIS Z2244-1:2020.

[0042] The austenitic stainless steel sheet according to the embodiment of the present invention preferably has a fracture elongation (EL) of 1.0% or more, more preferably 1.2% or more, and even more preferably 1.3% or more. By controlling the fracture elongation within this range, the ductility of the austenitic stainless steel sheet can be ensured. The upper limit of the fracture elongation is not particularly limited, but is typically 15.0%. The fracture elongation of the austenitic stainless steel sheet can be measured in accordance with JIS Z2241:2022.

[0043] The method for producing an austenitic stainless steel sheet according to an embodiment of the present invention is not particularly limited as long as it is a method capable of producing an austenitic stainless steel sheet having the above-described characteristics. An example of the method for producing an austenitic stainless steel sheet according to an embodiment of the present invention will be described below. A typical method for producing an austenitic stainless steel sheet according to an embodiment of the present invention includes an intermediate rolling annealing step and a temper rolling step. Furthermore, this production method may further include a tension annealing step after the temper rolling step, if necessary.

[0044] The intermediate rolling annealing process is a process in which the steps of cold rolling and annealing are sequentially performed on a hot-rolled annealed sheet having the above composition are repeated two or more times. By performing the intermediate rolling annealing process, the crystal grains can be refined, thereby improving strength. The hot-rolled annealed sheet can be produced by melting and forging or casting stainless steel having the above composition, followed by hot rolling and annealing. The conditions for hot rolling and annealing are not particularly limited and can be adjusted appropriately depending on the composition of the stainless steel. After annealing, pickling or other processes may be performed as needed.

[0045] The step of sequentially performing cold rolling and annealing is repeated two or more times. For example, when this step is performed three times, it is performed in the order of cold rolling - annealing - cold rolling - annealing - cold rolling - annealing. Therefore, even if the number of times this step is performed increases, the first step is cold rolling and the last step is annealing. The upper limit of the number of times this step can be performed is not particularly limited, but is, for example, 10 times.

[0046] The cold rolling conditions in each step are not particularly limited and may be adjusted appropriately depending on the composition of the stainless steel, but the total rolling reduction ratio of two or more cold rolling passes is preferably 50% or more, more preferably 55% or more, and even more preferably 60% or more. By controlling the total rolling reduction ratio within this range, crystal grains can be easily refined. The upper limit of the total rolling reduction ratio is not particularly limited, but is, for example, 99%.

[0047] Similarly, the annealing conditions in each step may be adjusted appropriately depending on the composition of the stainless steel, and are not particularly limited. However, each annealing temperature is preferably 900°C or higher, more preferably 950°C or higher, and even more preferably 1000°C or higher. By controlling the temperature within such a temperature range, crystal grains can be easily refined. The annealing temperatures in each step may be the same or different. The upper limit of each annealing temperature is not particularly limited, and is, for example, 1200°C or 1300°C. The annealing time may be adjusted appropriately depending on the annealing temperature, and is, for example, 1 to 10 seconds.

[0048] The temper rolling step is a step in which the cold-rolled annealed sheet obtained in the intermediate rolling annealing step is temper-rolled at a rolling reduction rate of 50% or more to adjust the thickness to 0.30 mm or less. By performing the temper rolling step, strain can be accumulated in the austenite phase, thereby improving strength. From the viewpoint of stably obtaining the above-mentioned effects, the rolling reduction rate of the temper rolling is preferably 55% or more, more preferably 60% or more. The upper limit of this rolling reduction rate is not particularly limited, but is, for example, 80% or 90%.

[0049] The tension annealing process is performed by applying a tension of several kgf / mm to the temper rolled sheet obtained in the temper rolling process. 2 (For example, 5 kgf / mm 2 This is a process in which a heat treatment is performed at a temperature of 450 to 600°C for a few seconds while applying a tension of 100 MPa (100 psi). By performing the tension annealing process, residual stress can be removed and shape correction can be performed. After the tension annealing process, the material can be cooled either by force or by natural cooling.

[0050] The austenitic stainless steel sheet according to the embodiment of the present invention is nonmagnetic and has excellent strength and fatigue properties, and therefore can be used in a variety of applications where these properties are required. In particular, this austenitic stainless steel sheet is suitable for use in components for portable electronic devices, which require miniaturization and improved performance and reliability. In particular, this austenitic stainless steel sheet is suitable for use in components such as spring materials (leaf springs) used as back plates that support the folding function of portable electronic devices such as foldable smartphones (foldable phones), housings, and display reinforcing plates.

[0051] A portable electronic device according to an embodiment of the present invention includes a member having the above-described austenitic stainless steel sheet. Because the austenitic stainless steel sheet is nonmagnetic and has excellent strength and fatigue properties, the portable electronic device component can be made smaller while improving its performance and reliability.

[0052] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0053] Stainless steel having the composition shown in Table 1 was melted, and after adjusting the components, it was continuously cast into a 140 mm to 200 mm continuously cast slab. This slab was then subjected to hot rolling, annealing, and pickling to obtain a hot-rolled annealed sheet having a thickness of 3 to 6 mm. The hot-rolled annealed sheet was then subjected to an intermediate rolling annealing step, a temper rolling step, and a tension annealing (hereinafter abbreviated as "TA") step (although some sheets did not undergo the TA step) in that order to obtain an austenitic stainless steel sheet. The conditions for the intermediate rolling annealing step and the temper rolling step, as well as whether or not the TA step was performed, are shown in Table 2. The annealing conditions in the intermediate rolling annealing step were 1050°C x 5 seconds. The TA step was performed at 5 kgf / mm 2 With a tension of 1.0 mm applied, the wire was subjected to a heat treatment at 500° C. for 1 second.

[0054]

[0055]

[0056] The austenitic stainless steel sheets obtained as described above were evaluated as follows.

[0057] <Tensile strength (TS) and elongation at break (EL)> JIS No. 13B test pieces were cut out from the austenitic stainless steel plate, and the TS and EL in the rolling direction were measured using these test pieces in accordance with JIS Z2241:2022. In this evaluation, if the TS was 1500 MPa or more, it could be said that the strength was good. Also, if the EL was 1.0% or more, it could be said that the ductility was good.

[0058] <Vickers Hardness> The Vickers hardness of the surface (rolled surface) of the austenitic stainless steel plate was measured in accordance with JIS Z2244-1: 2020 using a Vickers hardness tester (MVK-G2 manufactured by Akashi Seisakusho Co., Ltd.) In this evaluation, if the Vickers hardness was 400 HV or more, it could be said that the strength was good.

[0059] <Relative magnetic permeability> A sample measuring 7 mm in width and 7 mm in length was cut out from an austenitic stainless steel plate and subjected to electrolytic polishing. Then, using a vibrating sample magnetometer (BHV-525 manufactured by Riken Denshi Co., Ltd.), a magnetic field of 5 kOe (397.9 kA / m) was applied at a sweep rate of 1 kOe / min to magnetize the sample. The magnetic permeability was calculated from the slope of the magnetic field-magnetization curve obtained, and the relative magnetic permeability was compared with the magnetic permeability in vacuum (4π×10 -7 The relative permeability was calculated by dividing the measured value by the tensile strength (H / m). Three tests (n = 3) were performed to calculate the relative permeability, and the average value of the three tests was used as the relative permeability result. In this evaluation, if the relative permeability is 1.007 or less, it can be said to be non-magnetic. The results of the above evaluations are shown in Table 3.

[0060]

[0061] As shown in Table 3, the austenitic stainless steel sheets of Examples 1 to 9 were confirmed to be nonmagnetic and high-strength, with their composition, Ni equivalent, relative permeability, and tensile strength (TS) within appropriate ranges. The austenitic stainless steel sheets of Examples 1 to 9 also had good results in EL and Vickers hardness. In contrast, the austenitic stainless steel sheets of Comparative Examples 1 and 2 had insufficient TS. The austenitic stainless steel sheets of Comparative Examples 3 and 4 had low N contents and too low Ni equivalents, resulting in insufficient TS and Vickers hardness. The austenitic stainless steel sheet of Comparative Example 3 also had high relative permeability. The austenitic stainless steel sheet of Comparative Example 5 had low Mn and N contents and too low Ni equivalents, resulting in high relative permeability and insufficient TS. The austenitic stainless steel sheet of Comparative Example 6 had low contents of C, Mn, and N and too low Ni equivalent, so it had a high relative permeability and insufficient TS and Vickers hardness. The austenitic stainless steel sheet of Comparative Example 7 did not contain Cu, so it had insufficient TS. The austenitic stainless steel sheet of Comparative Example 8 did not contain Mo, so it had insufficient TS.

[0062] As can be seen from the above results, the present invention can provide a non-magnetic, high-strength austenitic stainless steel sheet. Furthermore, the present invention can provide components for portable electronic devices that can be made smaller while also improving performance and reliability.

Claims

1. An austenitic stainless steel sheet containing, by mass, C: 0.030-0.120%, Si: 0.20-1.00%, Mn: 2.00-12.50%, P: 0.050% or less, S: 0.0350% or less, Ni: 4.50-14.50%, Cr: 16.50-21.00%, Cu: 0.50% or less, Mo: 0.70% or less, N: 0.100-0.500%, with the balance being Fe and impurities, with a Ni equivalent expressed by the following formula (1) of 18.0 or more, a relative magnetic permeability of 1.007 or less, and a tensile strength of 1500 MPa or more. Ni equivalent = Ni + 0.60Mn + 0.18Cr + 9.69 (C + N) - 0.11Si 2 In the formula (1), each element symbol represents the content of each element.

2. The austenitic stainless steel sheet according to claim 1, further comprising, by mass, one or more selected from Al: 0.080% or less, Ti: 0.050% or less, Co: 0.50% or less, B: 0.0100% or less, Nb: 0.060% or less, Ca: 0.0100% or less, V: 0.200% or less, Sn: 0.050% or less, W: 0.100% or less, Pb: 0.009% or less, and Mg: 0.0030% or less.

3. The austenitic stainless steel sheet according to claim 1 or 2, having a thickness of 0.01 to 0.30 mm.

4. The austenitic stainless steel sheet according to any one of claims 1 to 3, having a Vickers hardness of 400 HV or more.

5. An austenitic stainless steel sheet according to any one of claims 1 to 4, having a breaking elongation of 1.0% or more.

6. The austenitic stainless steel sheet according to any one of claims 1 to 5, which is used for parts of portable electronic devices.

7. A part for a portable electronic device comprising a member having the austenitic stainless steel sheet according to any one of claims 1 to 6.

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