Austenitic stainless steel sheet and member for portable electronic device

Abstract

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

Description

Technical Field

[0001] This invention relates to an austenitic stainless steel sheet and a component for portable electronic devices. Background Technology

[0002] As communication devices such as smartphones and precision equipment such as computers become smaller and more powerful, the structural and functional components used in these devices are becoming increasingly thinner and lighter. Therefore, stainless steel sheets used as raw materials for these components are required to maintain high strength even when thin. For example, in portable electronic devices such as foldable smartphones, the backplate supporting the screen bending function uses a spring material (leaf spring). For spring materials subjected to repeated bending, there is a constant demand for stainless steel sheets with strength (fatigue characteristics) capable of withstanding repeated bending. Furthermore, in components such as the frame of portable electronic devices and the reinforcing plate of the display, there is a continuous demand for stainless steel sheets with the strength to ensure these functions.

[0003] For example, in the case of stainless steel sheets that can be used as spring materials, Patent Document 1 discloses a stainless steel foil in which the maximum value of the equivalent circular diameter of non-metallic inclusions in cross-section is less than 3 μm. This stainless steel foil has high strength due to the processing-induced martensitic structure generated during cold rolling, and its fatigue strength is improved by suppressing cracking by reducing non-metallic inclusions.

[0004] Furthermore, Patent Document 2 discloses a stainless steel foil with a thickness of 0.1 mm or less, a tensile strength of 1800 MPa or more, and a maximum height roughness Rz of 0.35 μm or less, determined by a surface roughness curve measured in the same direction as the stretching direction. This stainless steel foil achieves high strength due to the processing-induced martensitic structure generated during cold rolling, and its fatigue strength is improved by controlling the maximum height roughness Rz.

[0005] In recent years, minimizing electromagnetic interference has become increasingly 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 these materials due to electromagnetic waves can affect adjacent electronic circuits, leading to reduced device performance and malfunctions. Therefore, from the perspective of suppressing electromagnetic interference, using non-magnetic materials is effective. Accordingly, for various components of portable electronic devices, except for components such as sensors or motors that must be made of magnetic materials, it is desirable to use non-magnetic materials as much as possible. In terms of non-magnetic grade, in the past, it was mostly designed with a relative permeability of 1.010 or less. However, recently, the demand for miniaturization, improved performance, and increased reliability of portable electronic devices has increased, and the demand for non-magnetic materials with a relative permeability of 1.007 or less, especially 1.005 or less, in various components of portable electronic devices used by the general public is also continuously increasing.

[0006] However, the stainless steel foils in Patent Documents 1 and 2, whose processing-induced martensitic structure is generated for high strength, are strongly magnetic, making it difficult to achieve a relative permeability of less than 1.007.

[0007] Regarding non-magnetic stainless steel sheets with improved strength and fatigue properties, Patent Document 3 proposes a stainless steel sheet for portable electronic devices, which is a steel sheet with the following chemical composition: by mass%, C: 0.040~0.080%, Si: 0.30~1.00%, Mn: 2.00~4.00%, P: less than 0.050%, S: less than 0.005%, Ni: 11.00~14.00%, Cr: 18.00~20.00%, Cu: less than 0.50%, Mo: less than 0.50%, Ti: less than 0.015%, Co: 0.10~2.00%, N: 0.100~0.300%, Al: less than 0.010%, B: less than 0.0100%, O: 0.0030~0.0100%, with the balance consisting of Fe and unavoidable impurities; the average particle size D of non-metallic inclusions observed on the surface of the steel sheet is... M5 For thicknesses below 15.0 μm, the tensile strength in the rolling direction is 1000 N / mm. 2 The above conditions apply to the relative permeability μr, which is below 1.005.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2022 / 014307

[0011] Patent Document 2: International Publication No. 2022 / 210918

[0012] Patent Document 3: Japanese Patent No. 7215938 Summary of the Invention

[0013] The problem the invention aims to solve

[0014] As mentioned above, the increasing demand for miniaturization in portable electronic devices necessitates a stainless steel sheet that maintains high strength even with a thinner profile. However, reducing the thickness of the stainless steel sheet in Patent Document 3 does not necessarily guarantee that it possesses characteristics suitable for components in portable electronic devices. For instance, reducing the thickness of the stainless steel sheet in Patent Document 3 does not necessarily guarantee sufficient strength. For example, regarding the fatigue characteristics required for spring materials, higher strength (especially tensile strength) results in superior fatigue characteristics; therefore, pursuing higher strength has consistently been a method to improve fatigue characteristics.

[0015] This invention was made to solve the above-mentioned problems, and its purpose is to provide a non-magnetic and high-strength austenitic stainless steel sheet.

[0016] Another object of the present invention is to provide a component for portable electronic devices that is miniaturized while improving performance and reliability.

[0017] Solution for solving the problem

[0018] The inventors conducted in-depth research on austenitic stainless steel sheets and found that the above-mentioned problems could be solved by controlling the composition, Ni equivalent, relative magnetic permeability and tensile strength, thus completing the present invention.

[0019] That is, the present invention is an austenitic stainless steel plate, which, by weight, contains C: 0.030~0.120%, Si: 0.20~1.00%, Mn: 2.00~12.50%, P: less than 0.050%, S: less than 0.0350%, Ni: 4.50~14.50%, Cr: 16.50~21.00%, Cu: less than 0.50%, Mo: less than 0.70%, N: 0.100~0.500%, with the balance consisting of Fe and impurities;

[0020] The austenitic stainless steel sheet has a Ni equivalent of 18.0 or more, a relative permeability of 1.007 or less, and a tensile strength of 1500 MPa or more, as shown in the following formula (1).

[0021] Ni equivalent = Ni + 0.60Mn + 0.18Cr + 9.69(C + N) - 0.11Si 2 ···(1)

[0022] In the formula, the symbols of each element represent the content of each element.

[0023] The present invention is also a component for a portable electronic device, which comprises a component having the aforementioned austenitic stainless steel plate.

[0024] The effects of the invention

[0025] According to the present invention, a non-magnetic and high-strength austenitic stainless steel sheet can be provided.

[0026] According to the present invention, a component for portable electronic devices that can be miniaturized while improving performance and reliability can also be provided. Detailed Implementation

[0027] The following describes specific embodiments of the present invention. The present invention is not limited to these embodiments. It should be understood that any modifications or improvements to the following embodiments based on ordinary knowledge of those skilled in the art, without departing from the spirit and scope of the present invention, also fall within the scope of the present invention.

[0028] In addition, unless otherwise stated, the percentage designation of ingredients in this specification means "mass %".

[0029] The austenitic stainless steel plate of the present invention comprises C: 0.030~0.120%, Si: 0.20~1.00%, Mn: 2.00~12.50%, P: less than 0.050%, S: less than 0.0350%, Ni: 4.50~14.50%, Cr: 16.50~21.00%, Cu: less than 0.50%, Mo: less than 0.70%, N: 0.100~0.500%, with the balance consisting of Fe and impurities.

[0030] In this specification, "stainless steel sheet" means a sheet (including strip) material formed of stainless steel. Stainless steel sheet also includes thin foil-like 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.

[0031] Furthermore, in this specification, the term "austenite system" refers to a metallographic structure that is primarily austenitic at room temperature, preferably a single austenitic phase. Accordingly, "austenite system" also encompasses trace amounts of phases other than austenitic (e.g., extremely small amounts of martensite), intermetallic compounds, precipitates, inclusions, etc. When phases other than austenitic, precipitates, inclusions, etc., are present, their amounts are not particularly limited if they do not impair the effectiveness of the invention.

[0032] Furthermore, in this specification, "impurity" refers to components that are mixed in during the industrial manufacturing of stainless steel sheets due to various factors related to raw materials such as ores and waste, as well as the manufacturing process, and are permissible within the scope that does not adversely affect the present invention. For example, impurities also include unavoidable impurities such as O (oxygen). When O is included as an impurity, the O content is 0.0150% or less.

[0033] In addition, regarding the content of each element, the phrase "below ××%" means: containing less than ××% but greater than 0% (especially greater than the impurity level).

[0034] The austenitic stainless steel sheet of the embodiments of the present invention may, as needed, further include one or more of the following: 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.

[0035] The following is a detailed description of each ingredient.

[0036] <C: 0.030~0.120%>

[0037] Carbon (C) is an element that contributes to high strength and stabilizes the austenitic phase. From the viewpoint of ensuring these effects, the lower limit of the C content is controlled at 0.030%, preferably 0.040%, more preferably 0.070%, and even more preferably 0.080%. On the other hand, excessive C content reduces processability; therefore, the upper limit of the C content is controlled at 0.120%, preferably 0.110%, and more preferably 0.100%.

[0038] <Si: 0.20~1.00%>

[0039] Si is an element that contributes to high strength. From the viewpoint of ensuring this effect, the lower limit of Si content is controlled at 0.20%, preferably 0.30%. On the other hand, excessive Si content reduces processability, therefore the upper limit of Si content is controlled at 1.00%, preferably 0.90%, more preferably 0.75%, and even more preferably 0.60%.

[0040] <Mn: 2.00~12.50%>

[0041] Mn is an element that contributes to high strength and stabilizes the austenitic phase. From the viewpoint of ensuring these effects, the lower limit of the Mn content is controlled at 2.00%, preferably 2.70%, more preferably 9.00%, and even more preferably 11.30%. On the other hand, excessive Mn content reduces processability; therefore, the upper limit of the Mn content is controlled at 12.50%, preferably 12.00%, and more preferably 11.70%.

[0042] <P: below 0.050%>

[0043] Excessive phosphorus (P) content can lead to reduced corrosion resistance. Therefore, from the viewpoint of ensuring corrosion resistance, the upper limit of P content is controlled at 0.050%, preferably 0.045%, more preferably 0.040%, and even more preferably 0.030%. On the other hand, the lower limit of P content is not particularly limited, but excessively low P content will lead to increased steelmaking load and raw material costs. Therefore, the lower limit of P content is generally 0.001%, preferably 0.005%, and more preferably 0.010%.

[0044] <S: below 0.0350%>

[0045] S forms MnS-based nonmetallic inclusions. MnS-based inclusions tend to exist in steel plates as elongated inclusions extending along the rolling direction without fracturing. These inclusions, when faced with bending stress with the rolling direction parallel to the bending axis, easily become the starting point for fatigue failure. Therefore, from the viewpoint of ensuring fatigue characteristics, the upper limit of S content is controlled at 0.0350%, preferably 0.0300%, more preferably 0.0030%, and even more preferably 0.0010%. On the other hand, the lower limit of S content is not particularly limited, but excessively low S content will lead to increased steelmaking load and raw material costs. Therefore, the lower limit of S content is generally 0.0001%, preferably 0.0002%, and more preferably 0.0003%.

[0046] <Ni: 4.50~14.50%>

[0047] Ni is an element that helps stabilize the austenitic phase and improve corrosion resistance. From the viewpoint of ensuring these effects, the lower limit of the Ni content is controlled at 4.50%, preferably 5.50%, and more preferably 6.50%. On the other hand, Ni is relatively expensive, so an excessive Ni content will lead to increased manufacturing costs. Therefore, the upper limit of the Ni content is controlled at 14.50%, preferably 14.00%, more preferably 13.50%, and even more preferably 7.00%.

[0048] <Cr: 16.50~21.00%>

[0049] Cr is an element required to ensure corrosion resistance. From the viewpoint of ensuring this effect, the lower limit of Cr content is controlled at 16.50%, preferably 17.00%, more preferably 17.50%, and even more preferably 17.70%. On the other hand, excessive Cr content promotes the formation of intermetallic compounds (σ phase), thus reducing the workability of austenitic stainless steel. Therefore, the upper limit of Cr content is controlled at 21.00%, preferably 20.00%, and more preferably 18.00%.

[0050] <Cu: less than 0.50%>

[0051] Cu is an element that inhibits work hardening of the austenite phase. Therefore, a Cu content is beneficial for applications requiring high machinability during pressing or for cold forging. However, excessive Cu content can reduce corrosion resistance. Therefore, the upper limit of Cu content is controlled at 0.50%, preferably 0.35%, and more preferably 0.30%. On the other hand, the lower limit of Cu content is not particularly limited, but from the viewpoint of ensuring the effectiveness of Cu, the lower limit is preferably 0.01%, and more preferably 0.02%.

[0052] <Mo: 0.70% or less>

[0053] Mo is a very effective element for improving corrosion resistance. However, Mo is relatively expensive, so excessive Mo content will increase manufacturing costs. Therefore, the upper limit of Mo content is controlled at 0.70%, preferably 0.50%, and more preferably 0.30%. On the other hand, the lower limit of Mo content is not particularly limited, but from the viewpoint of ensuring the effect of Mo, the lower limit is preferably 0.01%, and more preferably 0.02%.

[0054] <N: 0.100~0.500%>

[0055] Nitrogen (N) is an element that contributes to increased strength and stabilization of the austenitic phase. From the viewpoint of ensuring these effects, the lower limit of the N content is controlled at 0.100%, preferably 0.130%, more preferably 0.250%, and even more preferably 0.290%. On the other hand, excessive N content reduces processability; therefore, the upper limit of the N content is controlled at 0.500%, preferably 0.400%, and more preferably 0.320%.

[0056] <Al: Below 0.080%>

[0057] Al is an element with strong deoxidizing properties. However, excessive Al content can increase the formation of Al₂O₃ inclusions, potentially reducing quality. Therefore, the upper limit of Al content is controlled at 0.080%, preferably 0.070%, and more preferably 0.060%. On the other hand, the lower limit of Al content is not particularly limited, but from the viewpoint of ensuring the effectiveness of Al, the lower limit is preferably 0.003%, more preferably 0.005%, and even more preferably 0.010%.

[0058] <Ti: below 0.050%>

[0059] Ti is an element that can effectively improve resistance to intergranular corrosion by fixing carbon. However, excessive Ti content increases the formation of coarse inclusions, which reduces fatigue properties. Therefore, the upper limit of Ti content is controlled at 0.050%, preferably 0.040%, and more preferably 0.030%. On the other hand, the lower limit of Ti content is not particularly limited, but from the viewpoint of ensuring the effect of Ti, the lower limit is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0060] <Co: less than 0.50%>

[0061] Co is an element that helps improve corrosion resistance. However, excessive Co content reduces processability and increases manufacturing costs. Therefore, the upper limit of Co content is controlled at 0.50%, preferably 0.40%. On the other hand, the lower limit of Co content is not particularly limited, but from the viewpoint of ensuring the effect of Co, the lower limit is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%.

[0062] <B: Below 0.0100%>

[0063] Boron (B) is an element that is very effective in suppressing surface defects, improving manufacturability, and improving weldability. However, when the B content is too high, these properties will deteriorate. Therefore, the upper limit of the B content is controlled at 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 effect brought by B, the lower limit is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0064] <Nb: below 0.060%>

[0065] Nitrogen (Nb) is an element with a high affinity for both carbon (C) and nitrogen (N). Its effect is that during hot rolling, it precipitates as carbides or nitrides, reducing the amount of dissolved C and N in the parent phase and improving processability. However, excessive Nb content can cause austenitic stainless steel sheets to harden and reduce ductility. Therefore, the upper limit of Nb content is controlled at 0.060%, preferably 0.050%. On the other hand, the lower limit of Nb content is not particularly limited, but from the viewpoint of ensuring the effects of Nb, the lower limit is preferably 0.001%, more preferably 0.005%.

[0066] <Ca: below 0.0100%>

[0067] Ca is an element that improves hot workability. However, excessive Ca content reduces the toughness of austenitic stainless steel sheets. Therefore, the upper limit of Ca content is controlled at 0.0100%, preferably 0.0070%, and more preferably 0.0050%. On the other hand, the lower limit of Ca content is not particularly limited, but from the viewpoint of ensuring the effect of Ca, the lower limit is preferably 0.0001%, and more preferably 0.0003%.

[0068] <V: below 0.200%>

[0069] V is an element that improves age-hardening properties. However, excessive V content leads to increased manufacturing costs. Therefore, the upper limit of V content is controlled at 0.200%, preferably 0.150%. On the other hand, the lower limit of V content is not particularly limited, but from the viewpoint of ensuring the effect of V, the lower limit is preferably 0.001%, more preferably 0.003%.

[0070] <Sn: below 0.050%>

[0071] Sn is an element that promotes the formation of deformable bands during rolling, thereby improving processability. However, when the Sn content is too high, the effect of Sn reaches saturation, and processability decreases. Therefore, the upper limit of the Sn content is controlled at 0.050%, preferably 0.040%. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of ensuring the effect of Sn, the lower limit is preferably 0.001%, more preferably 0.003%.

[0072] <W: Below 0.100%>

[0073] W is an element that can improve high-temperature strength without compromising room-temperature ductility. However, excessive W content can lead to the formation of coarse eutectic carbides, which can reduce ductility. Therefore, the upper limit of W content is controlled at 0.100%, preferably 0.080%. On the other hand, the lower limit of W content is not particularly limited, but from the viewpoint of ensuring the effect brought by W, the lower limit is preferably 0.001%, more preferably 0.003%.

[0074] <Pb: below 0.009%>

[0075] Pb is an element that improves machinability. However, excessive Pb content lowers the melting point of grain boundaries and reduces their bonding strength, potentially leading to deterioration in hot workability, such as liquefaction cracks caused by grain boundary melting. Therefore, the upper limit of Pb content is controlled at 0.009%, preferably 0.008%. On the other hand, the lower limit of Pb content is not particularly limited, but from the viewpoint of ensuring the benefits of Pb, the lower limit is preferably 0.001%.

[0076] <Mg: less than 0.0030%>

[0077] Mg combines with Al in molten steel to form Mg oxide, which acts as a deoxidizer. However, excessive Mg content reduces the toughness of austenitic stainless steel sheets. Therefore, the upper limit of Mg content is controlled at 0.0030%, preferably 0.0020%. On the other hand, the lower limit of Mg content is not particularly limited, but from the viewpoint of ensuring the effect of Mg, the lower limit is preferably 0.0001%, more preferably 0.0003%.

[0078] The austenitic stainless steel sheet of the embodiment of the present invention has a Ni equivalent of 18.0 or more, preferably 18.5 or more, and more preferably 19.0 or more, as shown in the following formula (1).

[0079] Ni equivalent = Ni + 0.60Mn + 0.18Cr + 9.69(C + N) - 0.11Si 2 ···(1)

[0080] In the formula, the symbols of each element represent the content of each element.

[0081] Here, Ni equivalent is an indicator related to non-magnetic stability. A higher Ni equivalent value better suppresses the formation of work-induced martensite during cold rolling, thus reducing the amount of strongly magnetic work-induced martensite. By controlling the Ni equivalent within the aforementioned range, a non-magnetic austenitic stainless steel sheet can be obtained. Furthermore, the upper limit of the Ni equivalent is not specifically limited, but it is generally 30.0.

[0082] The austenitic stainless steel sheet of the embodiment of the present invention has a relative magnetic permeability of 1.007 or less, preferably 1.005 or less. If the relative magnetic permeability is within this range, it can be called a non-magnetic austenitic stainless steel sheet. Furthermore, regarding the lower limit of the relative magnetic permeability, since the closer it is to 1, the more non-magnetic it is considered, the lower limit of the relative magnetic permeability is 1.000.

[0083] Here, relative permeability can be calculated by dividing permeability by free permeability. Permeability can be obtained by measuring the slope of the magnetic field-magnetization curve using a commercially available magnetometer.

[0084] The austenitic stainless steel sheet of 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 such a range, good strength can still be ensured even with a thin thickness. Furthermore, the upper limit of the tensile strength is not particularly limited, but is typically 3000 MPa.

[0085] Here, the tensile strength of austenitic stainless steel can be determined according to JIS Z2241:2022.

[0086] The Vickers hardness of the austenitic stainless steel sheet according to the embodiments of the present invention is preferably 400 HV or higher, more preferably 420 HV or higher, and even more preferably 450 HV or higher. By controlling the Vickers hardness within such a range, good strength can still be ensured even with a thin sheet. Furthermore, the upper limit of the Vickers hardness is not particularly limited, but is typically 800 HV.

[0087] Here, the Vickers hardness of austenitic stainless steel sheets can be determined according to JIS Z2244-1:2020.

[0088] The elongation at break (EL) of the austenitic stainless steel sheet according to the embodiments of the present invention is preferably 1.0% or more, more preferably 1.2% or more, and even more preferably 1.3% or more. By controlling the elongation at break within such a range, the ductility of the austenitic stainless steel sheet can be ensured. In addition, there is no particular limitation on the upper limit of the elongation at break, but it is typically 15.0%.

[0089] Here, the elongation at break of austenitic stainless steel sheets can be determined according to JIS Z2241:2022.

[0090] Regarding the method for manufacturing the austenitic stainless steel sheet according to an embodiment of the present invention, the method is not particularly limited if it can produce an austenitic stainless steel sheet with the above-described characteristics. Hereinafter, an example of a method for manufacturing the austenitic stainless steel sheet according to an embodiment of the present invention will be described.

[0091] A typical manufacturing method for austenitic stainless steel sheets according to embodiments of the present invention includes an intermediate rolling annealing process and a quenching and tempering rolling process. Additionally, this manufacturing method may, depending on the requirements, further include a tension annealing process after the quenching and tempering rolling process.

[0092] The intermediate rolling annealing process is a process in which the following steps are repeatedly performed twice or more on a hot-rolled annealed sheet having the above composition. These steps involve sequentially performing cold rolling and annealing. By performing the intermediate rolling annealing process, the grain size can be refined, thereby increasing the strength.

[0093] Hot-rolled annealed sheets can be manufactured by melting and forging or casting stainless steel with the above-mentioned composition, followed by hot rolling and then annealing. The conditions for hot rolling and annealing can be adjusted appropriately according to the composition of the stainless steel and are not particularly limited. In addition, pickling or other treatments may be performed after annealing as needed.

[0094] The steps of cold rolling and annealing are repeated more than twice. For example, when performing this step three times, the sequence is cold rolling-annealing-cold rolling-annealing-cold rolling-annealing. Accordingly, even if the number of times this step is performed increases, the initial step is still cold rolling and the final step is still annealing. There is no particular limit to the number of times this step is performed, but for example, it is 10 times.

[0095] The cold rolling conditions in each step can be adjusted appropriately according to the composition of the stainless steel and are not particularly limited. However, the total rolling yield of two or more cold rolling passes is preferably 50% or more, more preferably 55% or more, and even more preferably 60% or more. Controlling the total rolling yield within such a range makes it easier to refine the grain size. In addition, the upper limit of the total rolling yield is not particularly limited, but for example, it is 99%.

[0096] Similarly, the annealing conditions in each step can be adjusted appropriately according to the composition of the stainless steel and are not particularly limited. However, the annealing temperature is preferably 900°C or higher, more preferably 950°C or higher, and even more preferably 1000°C or higher. Controlling the temperature within this range makes it easier to refine the grain size. Furthermore, the annealing temperatures in each step can be the same or different. Additionally, the upper limit of each annealing temperature is not particularly limited, but may be, for example, 1200°C or 1300°C.

[0097] In addition, the annealing time can be adjusted appropriately according to the annealing temperature, for example, 1 to 10 seconds.

[0098] The temper rolling process is a process in which cold-rolled annealed sheets obtained from intermediate rolling and annealing processes are temper rolled at a rolling rate of 50% or more to adjust the thickness to below 0.30 mm. By performing the temper rolling process, strain can be accumulated in the austenite phase, thereby increasing the strength.

[0099] From the viewpoint of consistently achieving the aforementioned effects, the rolling ratio for quenching and tempering is preferably 55% or more, more preferably 60% or more. The upper limit of this rolling ratio is not particularly limited, but may be, for example, 80% or 90%.

[0100] Tension annealing is a process performed on tempered rolled plates obtained from tempering rolling, where several kgf / mm² is applied. 2 (e.g., 5kgf / mm) 2 Under tension, the material undergoes a heat treatment process at a temperature of 450~600℃ for approximately a few seconds. This tension annealing process removes residual stress and corrects the shape. Furthermore, after tension annealing, forced cooling or natural cooling can be performed.

[0101] The austenitic stainless steel sheet of this invention is non-magnetic and possesses excellent strength and fatigue properties, making it suitable for various applications requiring these characteristics. Among these, the austenitic stainless steel sheet is particularly suitable for components in portable electronic devices requiring miniaturization and improved performance and reliability. Specifically, the austenitic stainless steel sheet is suitable for use as spring material (leaf spring) for backplates that withstand bending in portable electronic devices such as foldable smartphones, as well as for components such as frames and reinforcing plates for displays.

[0102] The portable electronic device according to an embodiment of the present invention includes a component having the aforementioned austenitic stainless steel sheet. Because the aforementioned austenitic stainless steel sheet is non-magnetic and possesses excellent strength and fatigue properties, the component for this portable electronic device can be miniaturized while improving performance and reliability.

[0103] Example

[0104] The following examples illustrate the invention in detail, but the invention is not limited to these interpretations.

[0105] After melting and adjusting the composition of stainless steel with the composition shown in Table 1, continuous casting is performed to produce continuously cast slabs of 140mm to 200mm in diameter. These slabs are then subjected to hot rolling, annealing, and pickling processes to obtain hot-rolled annealed sheets with a thickness of 3 to 6mm. Next, the hot-rolled annealed sheets are sequentially subjected to intermediate rolling annealing, tempering rolling, and tension annealing (hereinafter referred to as "TA") processes (some may not undergo TA), thereby obtaining austenitic stainless steel sheets. The conditions for the intermediate rolling annealing and tempering rolling processes, and the sequence of whether or not TA is performed, are shown in Table 2. Furthermore, the annealing conditions in the intermediate rolling annealing process are set at 1050℃ for 5 seconds. Additionally, the TA process involves applying 5 kgf / mm² of heat. 2 Under tension, it is heat-treated at 500°C for 1 second.

[0106] [Table 1]

[0107]

[0108] [Table 2]

[0109]

[0110] The following evaluation is conducted on the austenitic stainless steel sheets obtained by the above method.

[0111] <Tensile Strength (TS) and Elongation at Break (EL)>

[0112] JIS 13B test pieces were cut from austenitic stainless steel sheets, and the TS and EL in the rolling direction were determined using these test pieces according to JIS Z2241:2022.

[0113] In this evaluation, a strength of TS above 1500MPa is considered good. Additionally, an EL of EL above 1.0% is considered good ductility.

[0114] <Vickers Hardness>

[0115] For the surface (rolled surface) of austenitic stainless steel sheets, Vickers hardness was determined according to JIS Z2244-1:2020 using a Vickers hardness tester (MVK-G2 manufactured by Akashi Seisakusho Co., Ltd.).

[0116] In this evaluation, a Vickers hardness of 400 HV or higher is considered to indicate good strength.

[0117] <Relative permeability>

[0118] Samples measuring 7mm wide x 7mm long were cut from austenitic stainless steel plates. After electrolytic polishing, a vibrating sample magnetometer (Riken Electronics Co., Ltd. BHV-525) was used to magnetize the samples at a scanning speed of 1kOe / min with a magnetic field of 5kOe (397.9kA / m). The permeability was calculated based on the slope of the resulting magnetic field-magnetization curve and then divided by the vacuum permeability (4π × 10⁻⁶). -7 The relative permeability is calculated from the H / m value. When calculating the relative permeability, a test number of n=3 is used, and the average of the three measurements is taken as the result of the relative permeability.

[0119] In this evaluation, those with a relative permeability of less than 1.007 can be considered non-magnetic.

[0120] The evaluation results are listed in Table 3.

[0121] [Table 3]

[0122]

[0123] As shown in Table 3, the austenitic stainless steel sheets of Examples 1-9 have compositions, Ni equivalents, relative permeability, and tensile strength (TS) within appropriate ranges, confirming them as non-magnetic and high-strength. Furthermore, the EL and Vickers hardness results for the austenitic stainless steel sheets of Examples 1-9 are also good.

[0124] In contrast, the TS of the austenitic stainless steel sheets in Comparative Examples 1 and 2 is insufficient.

[0125] The austenitic stainless steel sheets in Comparative Examples 3 and 4 have low nitrogen content and low Ni equivalent, resulting in insufficient TS and Vickers hardness. Furthermore, the austenitic stainless steel sheet in Comparative Example 3 also has high relative magnetic permeability.

[0126] The austenitic stainless steel sheet of Comparative Example 5 has low Mn and N content and a low Ni equivalent, resulting in high relative permeability and insufficient TS.

[0127] The austenitic stainless steel sheet of Comparative Example 6 has low C, Mn and N content and a low Ni equivalent, resulting in high relative permeability and insufficient TS and Vickers hardness.

[0128] The austenitic stainless steel sheet of Comparative Example 7 does not contain Cu, therefore its total TS is insufficient.

[0129] The austenitic stainless steel sheet of Comparative Example 8 does not contain Mo, therefore its total TS content is insufficient.

[0130] As can be seen from the above results, the present invention provides a non-magnetic, high-strength austenitic stainless steel sheet. The present invention also provides a miniaturized component for portable electronic devices that improves performance and reliability.

Claims

1. An austenitic stainless steel sheet, comprising, by weight, 0.030-0.120% C, 0.20-1.00% Si, 2.00-12.50% Mn, less than 0.050% P, less than 0.0350% S, 4.50-14.50% Ni, 16.50-21.00% Cr, less than 0.50% Cu, less than 0.70% Mo, and 0.100-0.500% N, with the balance consisting of Fe and impurities; The austenitic stainless steel plate shown in the following formula (1) has 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; Ni equivalent = Ni + 0.60Mn + 0.18Cr + 9.69(C + N) - 0.11Si 2 ···(1) In the formula, the symbols of each element represent the content of each element.

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

3. The austenitic stainless steel sheet according to claim 1 or 2, wherein the thickness is 0.01~0.30mm.

4. The austenitic stainless steel sheet according to any one of claims 1 to 3, wherein the Vickers hardness is 400 HV or higher.

5. The austenitic stainless steel sheet according to any one of claims 1 to 4, wherein the elongation at break is 1.0% or more.

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

7. A component for a portable electronic device, comprising a member having an austenitic stainless steel sheet as described in any one of claims 1 to 6.

Images