Ferritic stainless steel material for cell component, and cell component

Abstract

To provide a ferritic stainless steel material for a cell component excellent in high temperature fatigue characteristics.SOLUTION: A ferritic stainless steel material for a cell component includes, by mass, 0.001-0.020% C, 0.001-0.020% N, 0.01-1.00% Si, 0.01-1.20% Mn, 0.001-0.050% P, 0.0001-0.0014% S, 16.7-24.5% Cr, 0.001-1.000% Ni, 0.001-1.500% Cu, 0.03-0.34% Ti, 0.15-0.80% Nb, 0.001-0.080% Al, and a balance Fe with inevitable impurities. In the ferritic stainless steel material for a cell component, cleanness of oxide based inclusion is 0.004-0.200% and cleanness of B1 based inclusion is 0.100% or less, measured by a point counting method stipulated in appendix JA of JIS G0555:2020, and expressions (1) and (2) are satisfied: C+N≤0.027 ...(1) and Cr+10 Nb≥20.5 ...(2), where each symbol of element shows a content (mass%) of the element.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel material for battery components and a battery component. [Background technology]

[0002] Among various types of batteries, lithium-ion secondary batteries have been used as power sources for mobile phones, smartphones, laptops, and other devices due to their high voltage, light weight, high energy density, and high power density. On the other hand, due to the need to reduce greenhouse gas (CO2) emissions caused by global warming and oil resource issues, regulations on automobile CO2 emissions and fuel efficiency have been put into place, leading to the development of vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (EVs), and fuel cell vehicles (FCVs), and lithium-ion secondary batteries are also used as their power sources.

[0003] Lithium-ion secondary batteries for vehicles are classified by shape into cylindrical, prismatic, and laminated types, with prismatic and laminated types being more commonly used due to their space-saving nature. Prismatic lithium-ion secondary batteries contain a laminated body with a separator between the positive and negative electrodes, housed in a battery case (prismatic case) primarily made of aluminum. A lid, primarily made of aluminum, is placed over the opening of the prismatic case and is welded to the prismatic case. Additionally, battery components adjacent to the battery case, such as the battery module cover, end plates, side plates, spacers, shims, beams, trays, brackets, and restraining parts, as well as coolers and heat exchangers that cool the battery, are often made of aluminum.

[0004] Lithium-ion secondary batteries for vehicles are required to have increased battery capacity, energy density, and power density to extend their driving range. Therefore, the reliability of various battery components, such as battery cases and lids, is also a critical issue. Therefore, battery components are evaluated using nail penetration tests, crash tests, external short-circuit tests, and other tests to assess whether the positive and negative electrodes are short-circuited internally or externally, or whether a sudden temperature rise causes a reaction of the nonaqueous electrolyte to an abnormally high temperature. However, battery components made of aluminum often fail to provide satisfactory results due to their low high-temperature strength and melting point. In particular, for high-capacity lithium-ion secondary batteries, stainless steel battery components are often used because of their higher reliability compared to aluminum battery components.

[0005] For example, Patent Document 1 discloses a method for manufacturing a lithium-ion secondary battery case using austenitic stainless steel foil as a stainless steel material for use in battery components. Patent Document 2 discloses the application of an austenitic stainless steel sheet to a battery case for an electric vehicle, which has excellent heat resistance. Patent Document 3 also discloses a ferritic stainless steel containing 16.0 to 32.0 mass% Cr as an electrode material and electrode case for a large-capacity battery. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6090923 [Patent Document 2] Japanese Patent Application Publication No. 10-188922 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-167486 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the stainless steel materials disclosed in Patent Documents 1 to 3 do not have sufficient high-temperature fatigue properties when used in battery components. Therefore, when a lithium-ion secondary battery is subjected to an impact due to a collision accident or the like, it is prone to breakage, short circuiting, and thermal runaway. Even in lithium-ion secondary batteries that do not experience thermal runaway, thermal runaway is likely to occur if the battery components are deformed due to fatigue caused by temperature rise and vibration. If the temperature rises to approximately 800°C due to thermal runaway, a fire may break out in a short period of time. Thus, high-temperature fatigue properties are important for battery components in order to extend the time between the occurrence of thermal runaway due to a collision accident or the like and a fire, thereby ensuring sufficient time for people to evacuate.

[0008] The present invention has been made to solve the above problems, and an object of the present invention is to provide a ferritic stainless steel material for battery components having excellent high-temperature fatigue properties, and a battery component. [Means for solving the problem]

[0009] As a result of extensive research into ferritic stainless steel materials, the inventors discovered that the composition and the cleanliness of certain inclusions are closely related to high-temperature fatigue properties, and discovered that high-temperature fatigue properties can be improved by appropriately controlling these factors, which led to the completion of the present invention.

[0010] That is, the present invention provides a steel sheet containing, by mass, C: 0.001 to 0.020%, N: 0.001 to 0.020%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.20%, P: 0.001 to 0.050%, S: 0.0001 to 0.0014%, Cr: 16.7 to 24.5%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.500%, Ti: 0.03 to 0.34%, Nb: 0.15 to 0.80%, Al: 0.001 to 0.080%, and the balance being Fe and impurities; The cleanliness of oxide inclusions measured by the point counting method specified in Appendix JA of JIS G0555:2020 is 0.004 to 0.200%, and the cleanliness of B1 inclusions is 0.100% or less, The ferritic stainless steel material for battery components satisfies the following formulas (1) and (2). C+N≦0.027 (1) Cr+10Nb≧20.5 (2) In the formula, each element symbol represents the content (mass %) of each element.

[0011] The present invention also provides a battery component comprising the ferritic stainless steel material for battery components. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a ferritic stainless steel material for battery components and a battery component having excellent high-temperature fatigue properties. DETAILED DESCRIPTION OF THE INVENTION

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

[0014] (1) Ferritic stainless steel for battery components The ferritic stainless steel material for battery components according to an embodiment of the present invention (hereinafter sometimes abbreviated as "ferritic stainless steel material") contains C: 0.001 to 0.020%, N: 0.001 to 0.020%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.20%, P: 0.001 to 0.050%, S: 0.0001 to 0.0014%, Cr: 16.7 to 24.5%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.500%, Ti: 0.03 to 0.34%, Nb: 0.15 to 0.80%, Al: 0.001 to 0.080%, with the remainder being Fe and impurities.

[0015] In this specification, the term "impurities" refers to components that are mixed in during the industrial production of ferritic stainless steel materials due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities include unavoidable impurities. Examples of impurities include O, As, and Pb. It is preferable to reduce impurities as much as possible. In this specification, "stainless steel material" refers to a material made of stainless steel, and the shape of the material is not particularly limited. Examples of the shape include plate (including strip), rod, and tube. Also, various types of shaped steel may be used, such as T-shaped and I-shaped cross sections.

[0016] Furthermore, in this specification, "ferritic" means that the metal structure at room temperature is mainly ferrite phase. Therefore, "ferritic" also includes those that contain small amounts of phases other than ferrite phase (e.g., austenite phase, martensite phase, etc.). However, "ferritic" does not include a multi-phase structure of ferrite phase and austenite phase, a multi-phase structure of ferrite phase and martensite phase, or a multi-phase structure of ferrite phase, austenite phase, and martensite phase.

[0017] The ferritic stainless steel material according to the embodiment of the present invention may contain, as needed, Mo: 0.01 to 2.50%, V: 0.01 to 0.50%, Zr: 0.01 to 0.50%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0100%, W: 0.01 to 2.50%, Sn: 0.01 to 0.50%, Co: 0.01 to 0.2 It may further contain one or more selected from the group consisting of 0.5%, Mg: 0.0001 to 0.0100%, Sb: 0.001 to 0.300%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.3000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, and Bi: 0.001 to 0.200%. Each component will be described in detail below.

[0018] <C:0.001~0.020%> If excessive C is added, the immobilization of C is insufficient, resulting in sensitization and failure to ensure fatigue properties at high temperatures (e.g., 600°C). Furthermore, adding a large amount of C reduces corrosion resistance and increases the Ti content required to immobilize C. Therefore, the C content is set to 0.020% or less, preferably 0.018% or less, and more preferably 0.015% or less. However, since excessive reduction of C leads to increased refining costs, the C content is set to 0.001% or more, and preferably 0.002% or more.

[0019] <N:0.001~0.020%> As with C, excessive addition of N results in sensitization due to insufficient N fixation, making it impossible to ensure fatigue properties at high temperatures (e.g., 600°C). Furthermore, adding a large amount of N reduces corrosion resistance and increases the Ti content required to fix N. Therefore, the N content is set to 0.020% or less, preferably 0.018% or less, and more preferably 0.015% or less. However, since excessive reduction of N leads to increased refining costs, the lower limit of the N content is set to 0.001% or more, and preferably 0.002% or more.

[0020] <Si:0.01~1.00%> Although the addition of a large amount of Si increases the high-temperature strength, it also reduces toughness and high-temperature fatigue properties when subjected to impacts such as in a collision accident or during processing. Therefore, the Si content is set to 1.00% or less, preferably 0.60% or less, and more preferably 0.50% or less. On the other hand, Si is an element effective for deoxidation and solid-solution strengthening. To obtain these effects, the Si content is set to 0.01% or more, preferably 0.02% or more.

[0021] <Mn:0.01~1.20%> Like Si, adding excessive Mn increases high-temperature strength, but reduces toughness and high-temperature fatigue properties when subjected to impacts such as in a collision accident or during processing. Therefore, the Mn content is set to 1.20% or less, preferably 0.65% or less, more preferably 0.60% or less, and even more preferably 0.40% or less. However, since excessive reduction of Mn leads to increased refining costs, the Mn content is set to 0.01% or more, preferably 0.05% or more.

[0022] <P:0.001~0.050%> If excessive P is added, high-temperature strength increases, but toughness decreases when subjected to impacts such as in a collision accident or during processing, resulting in poor high-temperature fatigue properties. Therefore, the P content is set to 0.050% or less, preferably 0.040% or less. However, since an excessive reduction in P content increases steelmaking costs due to factors such as raw material selection, the P content is set to 0.001% or more, preferably 0.005% or more.

[0023] <S:0.0001~0.0014%> If excessive S is added, high-temperature strength increases, but toughness decreases when subjected to impacts such as in a collision accident or during processing, resulting in poor high-temperature fatigue properties. Therefore, the S content is set to 0.0014% or less, preferably 0.0013% or less, and more preferably 0.0012% or less. However, since excessive reduction in S content increases steelmaking costs due to factors such as raw material selection, the S content is set to 0.0001% or more, and preferably 0.0002% or more.

[0024] <Cr:16.7~24.5%> Cr is an element necessary for ensuring corrosion resistance, a fundamental characteristic of ferritic stainless steel materials. However, excessive Cr addition increases high-temperature strength, but reduces toughness and high-temperature fatigue properties when subjected to impacts such as in a collision accident or during processing. Therefore, the Cr content is set to 24.5% or less, preferably less than 24.0%, and more preferably less than 23.0%. On the other hand, excessive reduction in Cr reduces high-temperature strength and high-temperature fatigue properties, and corrosion resistance cannot be sufficiently ensured. Therefore, the Cr content is set to 16.7% or more, preferably 17.0% or more, and more preferably more than 17.0%.

[0025] <Ni:0.001~1.000%> If excessive Ni is added, high-temperature strength increases, but toughness decreases when subjected to impacts such as in a collision accident or during processing, resulting in poor high-temperature fatigue properties. Therefore, the Ni content is set to 1.000% or less, preferably 0.600% or less, more preferably 0.550% or less, and even more preferably 0.400% or less. However, excessive reduction in Ni content reduces high-temperature strength and poor high-temperature fatigue properties. Therefore, the Ni content is set to 0.001% or more, preferably 0.002% or more.

[0026] <Cu:0.001~1.500%> If excessive Cu is added, high-temperature strength increases, but toughness decreases and high-temperature fatigue properties deteriorate when subjected to impacts such as in a collision accident or during processing. Therefore, the Cu content is set to 1.500% or less, preferably 1.200% or less, and more preferably 0.600% or less. However, excessive reduction in Cu content reduces high-temperature strength and high-temperature fatigue properties. Therefore, the Cu content is set to 0.001% or more, preferably 0.002% or more.

[0027] <Ti:0.03~0.34%> Excessive addition of Ti increases the amount of coarse carbonitrides, degrading high-temperature fatigue properties. Therefore, the Ti content is set to 0.34% or less, preferably 0.30% or less, and more preferably less than 0.26%. On the other hand, Ti is also an element that combines with C and N to form precipitates and improve corrosion resistance. To obtain the effects of Ti, the Ti content is set to 0.03% or more, preferably 0.05% or more, and more preferably 0.10% or more.

[0028] <Nb:0.15~0.80%> If Nb is added in excess, the number of coarse carbonitrides increases, deteriorating high-temperature fatigue properties. Therefore, the Nb content is set to 0.80% or less, preferably 0.70% or less, and more preferably less than 0.60%. On the other hand, if the addition of Nb is too little, the fixation of C and N is insufficient, resulting in sensitization and inability to ensure fatigue properties at high temperatures (e.g., 600°C). Therefore, the Nb content is set to 0.15% or more, preferably more than 0.20%.

[0029] <Al:0.001~0.080%> If excessive Al is added, the cleanliness of oxide-based inclusions and B1-based inclusions cannot be controlled within a predetermined range, resulting in a deterioration in high-temperature fatigue properties. Therefore, the Al content is set to 0.080% or less, preferably 0.040% or less, and more preferably 0.020% or less. On the other hand, if too little Al is added, although the oxide-based inclusions are reduced, deoxidation becomes insufficient, the S content increases, and high-temperature fatigue properties deteriorate. Therefore, the Al content is set to 0.001% or more, preferably 0.003% or more.

[0030] <Mo:0.01~2.50%> Like Cr, Mo is an element effective in ensuring corrosion resistance, a basic property of ferritic stainless steel. To obtain the effects of Mo, the Mo content is set to 0.01% or more, preferably 0.05% or more. However, since excessive addition of Mo increases steelmaking costs and leads to increased strength and reduced elongation, the Mo content is set to 2.50% or less, preferably 2.30% or less, and more preferably 1.30% or less.

[0031] <V:0.01~0.50%、Zr:0.01~0.50%> Like Ti and Nb, V and Zr are elements that bond with C and N and suppress the formation of Cr carbonitrides. To obtain the effects of V and Zr, the V content and Zr content are each set to 0.01% or more, preferably 0.05% or more. However, since excessive addition of V and Zr reduces workability, the V content and Zr content are each set to 0.50% or less, preferably 0.40% or less, and more preferably 0.20% or less.

[0032] <B:0.0001~0.0050%> B is an element that is effective in increasing strength and also suppresses secondary processing cracks. To obtain the effects of B, the B content is set to 0.0001% or more, preferably 0.0003% or more. However, since excessive addition of B can cause void formation and reduce formability and toughness, the B content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0033] <Ca:0.0001~0.0100%> Ca is an element effective in fixing S and improving hot workability. To obtain the effects of Ca, the Ca content is set to 0.0001% or more, preferably 0.0002% or more. However, since excessive addition of Ca leads to a decrease in corrosion resistance, the Ca content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.

[0034] <W:0.01~2.50%> W is an element that improves corrosion resistance and also acts as a solid-solution strengthening element. To obtain the effects of W, the W content is set to 0.01% or more, preferably 0.05% or more. However, since excessive addition of W leads to a decrease in workability and toughness and an increase in cost, the W content is set to 2.50% or less, preferably 2.00% or less, and more preferably 0.50% or less.

[0035] <Sn:0.01~0.50%> Sn is an element that enhances the repair ability of the passive film of ferritic stainless steel materials and improves corrosion resistance. To obtain the effects of Sn, the Sn content is set to 0.01% or more, preferably 0.02% or more. However, since excessive addition of Sn leads to an increase in strength and a decrease in ductility, the Sn content is set to 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0036] <Co:0.01~0.25%> Co is an element that contributes to improving high-temperature strength. To obtain the effects of Co, the Co content is set to 0.01% or more, preferably 0.03% or more. However, since excessive addition of Co leads to reduced toughness during manufacturing, increased costs, and reduced workability, the Co content is set to 0.25% or less, preferably 0.20% or less, and more preferably 0.10% or less.

[0037] <Mg:0.0001~0.0100%> Mg is an element added as a deoxidizing element. Furthermore, Mg contributes to improving manufacturability by refining ferrite grains, improving surface defects known as ridging, and improving the workability of welds. To obtain the effects of Mg, the Mg content is set to 0.0001% or more, preferably 0.0003% or more. However, excessive addition of Mg significantly reduces corrosion resistance and also leads to reduced workability due to coarse MgO. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.

[0038] <Sb:0.001~0.300%> Sb is an element that segregates at grain boundaries and has the effect of increasing high-temperature strength. To obtain the effects of Sb, the Sb content is set to 0.001% or more, preferably 0.005% or more. However, since excessive addition of Sb leads to grain boundary cracking during processing and cracking during welding due to Sb segregation, the Sb content is set to 0.300% or less, preferably 0.200% or less, and more preferably 0.100% or less.

[0039] <REM:0.001~0.500%> REM (rare earth elements) are elements effective in improving oxidation resistance. To obtain the effects of REM, the REM content is set to 0.001% or more, preferably 0.005% or more. However, if REM is added in excess, the effect saturates and REM sulfides can lead to a decrease in corrosion resistance, so the REM content is set to 0.500% or less, preferably 0.400% or less, and more preferably 0.300% or less. REM follows the general definition. That is, REM refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). REM may be added alone or in the form of a mixture.

[0040] <Ga:0.0001~0.3000%> Ga is an element effective in improving corrosion resistance and suppressing hydrogen embrittlement. To obtain the effects of Ga, the Ga content is set to 0.0001% or more, preferably 0.0005% or more. However, since excessive addition of Ga generates coarse sulfides, which leads to a decrease in workability, the Ga content is set to 0.3000% or less, preferably 0.2000% or less, and more preferably 0.1000% or less.

[0041] <Ta:0.001~1.000%、Hf:0.001~1.000%> Ta and Hf are elements effective in improving high-temperature strength. To obtain the effects of Ta and Hf, the Ta content and Hf content are each set to 0.001% or more, preferably 0.005% or more. However, since excessive addition of Ta and Hf leads to an increase in manufacturing costs, the Ta content and Hf content are each set to 1.000% or less, preferably 0.800% or less, and more preferably 0.500% or less.

[0042] <Bi:0.001~0.200%> Bi is an element effective in improving machinability. To obtain the effects of Bi, the Bi content is set to 0.001% or more, preferably 0.005% or more. However, since excessive addition of Bi increases manufacturing costs, the Bi content is set to 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less.

[0043] The ferritic stainless steel material according to the embodiment of the present invention satisfies the following formulas (1) and (2). C+N≦0.027 (1) Cr+10Nb≧20.5 (2) In the formula, each element symbol represents the content (mass %) of each element. By satisfying formula (1), coarse carbonitrides can be reduced and high-temperature fatigue properties can be improved. The upper limit of C + N is preferably 0.025. The lower limit of C + N is not particularly limited, but is preferably 0.002, more preferably 0.005. Furthermore, by satisfying formula (2), although toughness decreases, the effect of improving high-temperature strength is significant, and fatigue properties at high temperatures (e.g., 600°C) can be ensured. The lower limit of Cr + 10Nb is preferably 21.5. The upper limit of Cr + 10Nb is not particularly limited, but is preferably 32.0, more preferably 29.0.

[0044] In the ferritic stainless steel material according to the embodiment of the present invention, the cleanliness of oxide-based inclusions, as measured by the point counting method specified in Appendix JA of JIS G0555:2020, is 0.004 to 0.200%, preferably 0.004 to 0.150%, and more preferably 0.004 to 0.125%. By controlling the cleanliness of the oxide-based inclusions within this range, high-temperature fatigue properties can be improved. If the cleanliness of the oxide-based inclusions exceeds 0.200%, the oxide-based inclusions become fracture initiation sites, resulting in a decrease in high-temperature fatigue properties. On the other hand, if the cleanliness of the oxide-based inclusions is less than 0.004%, deoxidation becomes insufficient, resulting in a high S content and a decrease in high-temperature fatigue properties.

[0045] The types of inclusions contained in the ferritic stainless steel material according to the embodiment of the present invention are classified into A-type inclusions, B-type inclusions, and C-type inclusions, as described in Annex JA of JIS G0555:2020. A-type inclusions are those (sulfides, silicates, etc.) that have undergone viscous deformation due to processing. Sulfide-based inclusions are called A1-type inclusions, and oxide-based inclusions such as silicates are called A2-type inclusions. B-type inclusions are granular inclusions that are arranged discontinuously in clusters in the processing direction. Oxide-based inclusions such as alumina are called B1-type inclusions, and Nb, Ti, and Zr carbonitride-based inclusions are called B2-type inclusions. C-type inclusions are irregularly dispersed without undergoing viscous deformation. Oxide-based inclusions are called C1-type inclusions, and Nb, Ti, and Zr carbonitride-based inclusions are called C2-type inclusions. Therefore, in this specification, "oxide-based inclusions" refers to the total of A2-based inclusions, B1-based inclusions, and C1-based inclusions, among the above classifications. In this specification, the term "oxide-based inclusions" includes both deoxidation products produced in the steelmaking process and oxides mixed in from raw materials. Examples of oxide-based inclusions include CaO, MgO, Al2O3, SiO2, TiO2, MnO, Cr2O3, FeO, and MgOAl2O3.

[0046] The ferritic stainless steel material according to the embodiment of the present invention has a B1-based inclusion cleanliness of 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less, as measured by the point counting method specified in Appendix JA of JIS G0555:2020. Since B1-based inclusions, among oxide-based inclusions, are particularly likely to become fracture initiation sites, controlling the cleanliness of the B1-based inclusions within the above range can reliably improve high-temperature fatigue properties. Therefore, if the cleanliness of the B1-based inclusions exceeds 0.100%, the B1-based inclusions will become fracture initiation sites, resulting in a deterioration of high-temperature fatigue properties. The lower limit of the B1-based inclusion cleanliness is not particularly limited, but is generally 0.001%.

[0047] The thickness of the ferritic stainless steel material according to the embodiment of the present invention is not particularly limited and may be appropriately set depending on the required characteristics of the battery components in which the ferritic stainless steel material is used. For example, in consideration of the weight reduction of the ferritic stainless steel material and the battery components formed therefrom, the thickness of the ferritic stainless steel material is preferably 1.00 mm or less. In consideration of the rigidity of the ferritic stainless steel material and the battery components formed therefrom, the thickness of the ferritic stainless steel material is more preferably 0.20 mm or more. Furthermore, in consideration of the fire resistance and cost of the ferritic stainless steel material and the battery components formed therefrom, the thickness of the ferritic stainless steel material is even more preferably 0.20 to 0.80 mm. Furthermore, in consideration of the weldability of the ferritic stainless steel material and the battery components formed therefrom, the thickness of the ferritic stainless steel material is even more preferably 0.30 to 0.60 mm.

[0048] The ferritic stainless steel material according to the embodiment of the present invention has excellent high-temperature fatigue properties because the composition and the cleanliness of the oxide-based inclusions and B1-based inclusions are controlled. Therefore, the ferritic stainless steel material according to the embodiment of the present invention has excellent resistance to thermal runaway in lithium-ion secondary batteries when the lithium-ion secondary battery is subjected to an impact due to a collision accident or the like. Therefore, the time from the occurrence of thermal runaway due to a collision accident or the like to the outbreak of a fire can be extended, ensuring sufficient time for people to evacuate. The ferritic stainless steel material according to the embodiment of the present invention can be used as a material for battery components in transportation equipment (particularly, automobiles, buses, trains, etc.), and is particularly suitable for use in lithium ion secondary battery cases, lids, battery module covers, end plates, side plates, spacers, shims, beams, trays, brackets, restraining parts, coolers for cooling batteries, heat exchangers, etc.

[0049] (2) Manufacturing method of ferritic stainless steel material for battery parts The method for producing a ferritic stainless steel material according to the embodiment of the present invention is not particularly limited as long as it is a method capable of producing a ferritic stainless steel material having the above-described characteristics. For example, a method for producing a ferritic stainless steel material according to an embodiment of the present invention includes a melting and refining step and a casting step. In the casting step, raw materials, auxiliary materials, and Al as a deoxidizer are added to a molten steel furnace and heated to produce molten steel, which is then adjusted to a target composition by acid blowing or the like. In the casting step, the molten steel with the adjusted composition is solidified to produce a cast slab. In the melting and refining process and the casting process, the time from the end of heating in the melting and refining process to the start of casting in the casting process is controlled to 30 to 120 minutes, and the temperature of the molten steel is lowered at a rate of 0.20 to 2.50°C / min during that time. By carrying out the melting and refining process and the casting process under these conditions, the cleanliness of the oxide-based inclusions and B1-based inclusions can be controlled within a predetermined range.

[0050] If the time from the end of heating in the melting and refining process to the start of casting in the casting process is less than 30 minutes, there is not enough time for oxides in the molten steel to float and be removed, resulting in an increase in oxide-based inclusions.Also, if the time from the end of heating in the melting and refining process to the start of casting in the casting process exceeds 120 minutes, an increase in alumina (B1-based inclusions) is generated due to a decrease in the temperature of the molten steel. If the temperature reduction rate of the molten steel exceeds 2.50°C / min, the amount of alumina (B1-based inclusions) produced by the temperature reduction of the molten steel increases. If the temperature reduction rate of the molten steel is less than 0.20°C / min, the reaction between the refractories constituting the molten steel furnace and the molten steel is accelerated, resulting in an increase in oxide-based inclusions.

[0051] The slab obtained by the casting process can be processed into various shapes by processing using known methods. For example, when producing a ferritic stainless steel sheet, it is processed into a sheet shape as follows. First, the slab is hot-rolled to obtain a hot-rolled sheet. Next, the hot-rolled sheet is annealed, pickled, and cold-rolled in this order to obtain a cold-rolled sheet. After cold rolling, existing treatments (e.g., surface polishing, temper rolling, treatment using a tension leveler, annealing) may be performed as needed. Here, each step can be performed using existing equipment, and the conditions are not particularly limited and may be adjusted appropriately depending on the composition of the stainless steel, etc.

[0052] (3) Battery parts The battery component according to the embodiment of the present invention includes the ferritic stainless steel material described above. Because the ferritic stainless steel material has excellent high-temperature fatigue properties, the battery component has excellent resistance to thermal runaway when the battery is subjected to an impact due to a collision accident or the like. Battery components are not particularly limited, but examples include battery (particularly, lithium ion secondary battery; the same applies below) cases, battery packs, battery modules, lids, battery module covers, end plates, side plates, spacers, shims, beams, trays, brackets, restraining parts, coolers for cooling batteries, and heat exchangers. The manufacturing method of the battery components is not particularly limited, and known methods such as deep drawing, welding, brazing, etc. The type of welding in the welding method is also not particularly limited, and examples thereof include TIG welding, laser welding, and electric resistance welding. [Example]

[0053] The present invention will be specifically explained below with reference to examples, but the present invention should not be construed as being limited to these examples. A ferritic stainless steel sheet was prepared according to the following procedure. First, raw materials, auxiliary materials, and Al as a deoxidizer were added to a molten steel furnace and heated to produce molten steel, which was then refined by acid blowing or other methods to the composition shown in Tables 1A and 1B (the balance being Fe and impurities). A casting process was then carried out under the conditions shown in Tables 2A and 2B for the time from the end of heating in the refining process to the start of casting, and the rate of temperature reduction of the molten steel during that time. The slab was then heated at 1150°C for 1 hour and hot-rolled, followed by pickling, cold-rolling, annealing at 950°C for 1 minute, and pickling to produce a cold-rolled annealed sheet (ferritic stainless steel sheet) with a thickness of 0.50 mm.

[0054] [Table 1A]

[0055] [Table 1B]

[0056] [Table 2A]

[0057] [Table 2B]

[0058] The ferritic stainless steel sheets obtained above were subjected to the following evaluations.

[0059] <Cleanliness of oxide inclusions and B1 inclusions> Test pieces were taken from ferritic stainless steel sheets in accordance with JIS G0555:2020, and the cleanliness of oxide and B1 inclusions was determined using the point counting method specified in Appendix JA of JIS G0555:2020. The point counting method involves inserting a glass plate with 20 vertical and 20 horizontal grid lines into the eyepiece of a microscope, randomly examining the test surface, and counting the number of grid point centers occupied by inclusions. Sixty fields were randomly selected from the cross section of the ferritic stainless steel sheet in the rolling direction, and the microscope magnification was 400x. The area percentage (%) occupied by each inclusion was calculated using the following formula, based on the total number of grid points on the glass plate within the field of view, the number of fields, and the number of grid point centers occupied by each inclusion, and this was used to determine the cleanliness of each inclusion. Cleanliness of oxide inclusions or B1 inclusions = n / (p × f) × 100 In the formula, p is the total number of lattice points on the glass plate within the field of view, f is the number of fields of view, and n is the number of lattice point centers occupied by all inclusions (all oxide-based inclusions or all B1-based inclusions) in f fields of view.

[0060] <High-temperature fatigue properties> To simulate the introduction of strain due to processing or a collision accident, ferritic stainless steel sheets were bent and unbent along the width direction: (a) 90° bending, (b) returning to a flat (initial state), (c) reverse 90° bending (i.e., bending 90° in the opposite direction to (a)), and (d) returning to a flat (initial state). No. 1 test specimens, as specified in JIS Z 2275:1978, were taken from the bent and unbent ferritic stainless steel sheets so that the rolling direction was the longitudinal direction and the bent and unbent portion was located at the longitudinal center of the specimen. High-temperature fatigue tests were performed using a Schenck-type plane bending fatigue tester manufactured by Tokyo Koki Testing Instruments Co., Ltd. The high-temperature fatigue test conditions were an amplitude stress of 180 MPa or more and a load repetition rate of 1700 cycles per minute. The high-temperature fatigue tests were also performed at 300°C and 600°C, simulating the process in which a thermal runaway battery heats surrounding batteries, causing deformation due to high-temperature fatigue, and further thermal runaway occurs and spreads. In this evaluation, at both 300°C and 600°C, those that did not break after 300,000 cycles were represented by a ◎, those that broke after 300,000 cycles but did not break after 100,000 cycles were represented by a ○, and those that broke after 100,000 cycles were represented by an ×. The results of the above evaluations are shown in Tables 3A and 3B.

[0061] [Table 3A]

[0062] [Table 3B]

[0063] As shown in Tables 3A and 3B, Test Nos. 1 to 17 (invention examples) had good high-temperature fatigue properties because the composition, cleanliness of oxide inclusions and B1 inclusions were within the specified ranges.

[0064] In contrast, in Test No. 18 (Comparative Example), the C content was too high, which resulted in insufficient fixation of C, resulting in sensitization and a deterioration in high-temperature fatigue properties. In Test No. 19 (Comparative Example), the N content was too high, which resulted in insufficient fixation of N, resulting in sensitization and a deterioration in high-temperature fatigue properties. Test No. 20 (comparative example) had an excessively high Si content, which resulted in a decrease in toughness and a decrease in high-temperature fatigue properties. Test No. 21 (comparative example) did not contain Si, and therefore the high-temperature fatigue properties were reduced. In Test No. 22 (Comparative Example), the contents of Mn, P, and S were too high, and therefore the toughness and high-temperature fatigue properties were reduced. In Test No. 23 (Comparative Example), the Cr content was too high, which resulted in a decrease in toughness and a decrease in high-temperature fatigue properties. In Test No. 24 (Comparative Example), the Cr content was too low, so that sufficient high-temperature strength could not be ensured, and the high-temperature fatigue properties were reduced. Test No. 25 (comparative example) had an excessively high Ni content, which resulted in a decrease in toughness and a decrease in high-temperature fatigue properties.

[0065] Test No. 26 (comparative example) did not contain Ni, and therefore was unable to ensure sufficient high-temperature strength, resulting in a decrease in high-temperature fatigue properties. Test No. 27 (comparative example) had an excessively high Cu content, which resulted in a decrease in toughness and a decrease in high-temperature fatigue properties. Test No. 28 (comparative example) did not contain Cu, and therefore was unable to ensure sufficient high-temperature strength, resulting in a decrease in high-temperature fatigue properties. In Test No. 29 (Comparative Example), the Ti content was too high, which resulted in an increase in coarse carbonitrides and a decrease in high-temperature fatigue properties. In Test No. 30 (Comparative Example), the Nb content was too high, which resulted in an increase in coarse carbonitrides and a decrease in high-temperature fatigue properties. In Test No. 31 (Comparative Example), the Nb content was too low, which resulted in insufficient fixation of C and N, resulting in sensitization and a deterioration in high-temperature fatigue properties. In Test No. 32 (Comparative Example), the Al content was too high, so the cleanliness of the oxide-based inclusions and B1-based inclusions could not be controlled within the predetermined range, and the high-temperature fatigue properties were deteriorated. Test No. 33 (comparative example) did not contain Al, and therefore deoxidation was insufficient, resulting in an increased S content and a deterioration in high-temperature fatigue properties. In Test No. 34 (Comparative Example), the amount of C+N was too high, so that the amount of coarse carbonitrides increased, and the high-temperature fatigue properties deteriorated. In Test No. 35 (Comparative Example), the amount of Cr+10Nb was too small, so that the high-temperature strength could not be sufficiently ensured, and the high-temperature fatigue properties were deteriorated.

[0066] In Test Nos. 36 and 37 (comparative examples), the time from the start of casting to the casting process was not within an appropriate range, so the cleanliness of oxide-based inclusions or B1-based inclusions could not be controlled within the specified range, and high-temperature fatigue properties were reduced. In Test Nos. 38 and 39 (Comparative Examples), the temperature lowering rate of the molten steel was not within an appropriate range, so the cleanliness of oxide-based inclusions or B1-based inclusions could not be controlled within the specified range, and the high-temperature fatigue properties deteriorated. In Test Nos. 40 to 43 (comparative examples), both the time until the start of casting in the casting process and the rate of temperature reduction of the molten steel were not within appropriate ranges, so the cleanliness of the oxide-based inclusions and B1-based inclusions could not be controlled within the specified range, and the high-temperature fatigue properties were reduced.

[0067] As can be seen from the above results, the present invention can provide a ferritic stainless steel material for battery components and battery components that are excellent in high-temperature fatigue properties.

Claims

1. The alloy contains, on a mass basis, C: 0.001 to 0.020%, N: 0.001 to 0.020%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.20%, P: 0.001 to 0.050%, S: 0.0001 to 0.0014%, Cr: 16.7 to 24.5%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.500%, Ti: 0.03 to 0.34%, Nb: 0.15 to 0.80%, Al: 0.001 to 0.080%, and the balance being Fe and impurities; The cleanliness of oxide inclusions measured by the point counting method specified in Appendix JA of JIS G0555:2020 is 0.004 to 0.200%, B 1 The cleanliness of the system inclusions is 0.100% or less, A ferritic stainless steel material for battery components that satisfies the following formulas (1) and (2). C+N≦0.027...(1) Cr+10Nb≧20.5 (2) In the formula, each element symbol represents the content (mass %) of each element.

2. On a mass basis, Mo: 0.01 to 2.50%, V: 0.01 to 0.50%, Zr: 0.01 to 0.50%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0100%, W: 0.01 to 2.50%, Sn: 0.01 to 0.50%, Co: 0.01 to 0.25%, Mg: 0.0001 to 0.0100%, Sb:

2. The ferritic stainless steel material for battery components according to claim 1, further comprising one or more selected from the group consisting of 0.001 to 0.300%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.3000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, and Bi: 0.001 to 0.200%.

3. The ferritic stainless steel material for battery components according to claim 1 or 2, having a thickness of 0.20 to 1.00 mm.

4. 3. The ferritic stainless steel material for battery components according to claim 1 or 2, wherein the battery component is a case, a lid, a cover of a battery module, an end plate, a side plate, a spacer, a shim, a beam, a tray, a bracket, a restraining part, a cooler for cooling a battery, or a heat exchanger.

5. A battery component comprising the ferritic stainless steel material for battery components according to claim 1 or 2.

6. 6. The battery component according to claim 5, which is a case, a lid, a cover of a battery module, an end plate, a side plate, a spacer, a shim, a beam, a tray, a bracket, a restraining part, a cooler for cooling a battery, or a heat exchanger.

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