Stainless steel sheets for lithium-ion batteries and surface-modified stainless steel parts for lithium-ion batteries

Abstract

This invention provides stainless steel sheets for lithium-ion batteries that offer excellent corrosion resistance in non-aqueous electrolyte environments containing fluorine compounds, and that ensure corrosion resistance even when current flows due to short circuits, etc., enabling the manufacture of lithium-ion battery components. [Solution] A stainless steel sheet for lithium-ion batteries, on a mass basis, containing C: 0.0200% or less, Si: 0.02~1.90%, Mn: 0.02~1.00%, P: 0.040% or less, S: 0.0002~0.0100%, Al: 0.004~0.100%, Cr: 10.0~25.0%, Mo: 0.02~2.50%, total of one or more Nb and Ti: 0.01~0.55%, N: 0.0500% or less, with the remainder being Fe and impurities, and the ratio of Fe concentration to Cr concentration in the film formed on the surface (Fe / Cr) being 3.0 or more.

Description

[Technical Field]

[0001] This invention relates to stainless steel sheets for lithium-ion batteries and surface-modified stainless steel parts for lithium-ion batteries. [Background technology]

[0002] Stainless steel sheets, due to their chromium content, form a passive film on their surface containing chromium and oxygen. Stainless steel sheets with this passive film exhibit excellent oxidation and corrosion resistance, and are therefore used in applications such as automotive exhaust system components and roofing / building materials. On the other hand, in non-aqueous electrolyte environments such as lithium-ion batteries, stainless steel sheets have the drawback of poor corrosion resistance because they contain less H2O, which is necessary for the formation of the passive film. To address the above issues, Patent Document 1 describes a passive film in which the average Cr cation fraction is 20-60%, the average Ni cation fraction is less than 5%, and in a non-aqueous electrolyte with a water content of 20 ppm-10%, the current value when held at the natural potential is 10 μA·cm. -2 A stainless steel sheet with a low elution rate in non-aqueous electrolytes has been proposed. This stainless steel sheet ensures elution resistance by optimizing the passive film formed on its surface through electrolytic treatment, immersion treatment, or atmospheric annealing to suit the non-aqueous electrolyte environment. Furthermore, Patent Document 2 proposes a ferritic stainless steel sheet for a button-type lithium secondary battery case, containing, by mass%, Cr: 10.0-32.0%, C: 0.08% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, and S: 0.03% or less, with the remainder being iron and unavoidable impurities. This stainless steel sheet ensures corrosion resistance to electrolytes containing fluorine compounds by limiting its composition. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2015-86470 [Patent Document 2] Japanese Patent Publication No. 2003-257384 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Although the stainless steel sheet described in Patent Document 1 ensures corrosion resistance by controlling the average Cr cation fraction and average Ni cation fraction in the passivation film, the influence of the composition of the stainless steel sheet on corrosion resistance has not been sufficiently studied. Furthermore, while the stainless steel plate described in Patent Document 2 has good corrosion resistance to electrolytes containing fluorine compounds, its corrosion resistance when an electric current flows through it is not sufficient. Therefore, corrosion may occur when an electric current flows through the stainless steel plate due to a short circuit or other reasons.

[0005] The present invention was made to solve the above-mentioned problems, and aims to provide a stainless steel sheet for lithium-ion batteries that can be used to manufacture lithium-ion battery components that have excellent corrosion resistance in a non-aqueous electrolyte environment containing fluorine compounds and that ensure corrosion resistance even when current flows due to a short circuit or the like. Furthermore, the present invention aims to provide surface-modified stainless steel components for lithium-ion batteries that exhibit excellent corrosion resistance in non-aqueous electrolyte environments containing fluorine compounds, and that ensure corrosion resistance even when current flows due to short circuits or other reasons. [Means for solving the problem]

[0006] The inventors of the present invention conducted diligent research to solve the above problems and found that by controlling the composition of the stainless steel plate and the ratio of the Fe concentration to the Cr concentration in the film formed on its surface to a predetermined range, it is possible to manufacture lithium-ion battery components that have excellent corrosion resistance in non-aqueous electrolyte environments containing fluorine compounds and that ensure corrosion resistance even when current flows due to a short circuit. Furthermore, the inventors found that by controlling the composition of surface-modified stainless steel components for lithium-ion batteries and the total concentration of fluoride in the film formed on its surface to a predetermined range, excellent corrosion resistance in non-aqueous electrolyte environments containing fluorine compounds and that ensure corrosion resistance even when current flows due to a short circuit can be achieved. The present invention was completed based on these findings.

[0007] In other words, the present invention contains, by mass, C: 0.0200% or less, Si: 0.02~1.90%, Mn: 0.02~1.00%, P: 0.040% or less, S: 0.0002~0.0100%, Al: 0.004~0.100%, Cr: 10.0~25.0%, Mo: 0.02~2.50%, total of one or more Nb and Ti: 0.01~0.55%, N: 0.0500% or less, with the remainder being Fe and impurities. This is a stainless steel sheet for lithium-ion batteries, in which the ratio of Fe concentration to Cr concentration in the film formed on the surface (Fe / Cr) is 3.0 or higher.

[0008] Furthermore, the present invention contains, by mass, C: 0.0200% or less, Si: 0.02~1.90%, Mn: 0.02~1.00%, P: 0.040% or less, S: 0.0002~0.0100%, Al: 0.004~0.100%, Cr: 10.0~25.0%, Mo: 0.02~2.50%, total of one or more Nb and Ti: 0.01~0.55%, N: 0.0500% or less, with the remainder being Fe and impurities. This is a surface-modified stainless steel component for lithium-ion batteries, in which the total concentration of Cr fluoride and Mo fluoride in the film formed on the surface is 0.05 atomic percent or more. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a stainless steel sheet for lithium-ion batteries that can be used to manufacture lithium-ion battery components that exhibit excellent corrosion resistance in a non-aqueous electrolyte environment containing fluorine compounds and that ensure corrosion resistance even when current flows due to a short circuit or the like. Furthermore, according to the present invention, it is possible to provide surface-modified stainless steel parts for lithium-ion batteries that exhibit excellent corrosion resistance in a non-aqueous electrolyte environment containing fluorine compounds, and that ensure corrosion resistance even when current flows due to a short circuit or the like. [Modes for carrying out the invention]

[0010] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and it should be understood that modifications, improvements, etc., made to the embodiments described below, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention, also fall within the scope of the present invention. In this specification, unless otherwise specified, any "%" indication for ingredients refers to "mass%".

[0011] (1) Stainless steel plate for lithium-ion batteries The stainless steel sheet for lithium-ion batteries according to an embodiment of the present invention (hereinafter abbreviated as "stainless steel sheet") contains C: 0.0200% or less, Si: 0.02 to 1.90%, Mn: 0.02 to 1.00%, P: 0.040% or less, S: 0.0002 to 0.0100%, Al: 0.004 to 0.100%, Cr: 10.0 to 25.0%, Mo: 0.02 to 2.50%, total of one or more Nb and Ti: 0.01 to 0.55%, N: 0.0500% or less, with the remainder being Fe and impurities.

[0012] Herein, in this specification, "impurities" means components that are mixed in during the industrial manufacture of stainless steel sheets due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and which are acceptable as long as they do not adversely affect the present invention. For example, impurities include unavoidable impurities. Examples of impurities include oxygen (O). Regarding the content of each element, "xx% or less" includes being xx% or less but including an amount exceeding 0% (especially exceeding the impurity level).

[0013] In addition, the stainless steel sheet according to an embodiment of the present invention can further contain one or more selected from Ni: 0.02 to 0.40%, Cu: 0.02 to 1.60%, and Sn: 0.005 to 0.500% as required. Furthermore, the stainless steel sheet according to an embodiment of the present invention can further contain one or more selected from B: 0.0001 to 0.0050%, V: 1.00% or less, Zr: 0.005 to 0.500%, Ga: 0.030% or less, REM: 0.010% or less, Ta: 0.10% or less, W: 1.60% or less, Co: 1.00% or less, and Sb: 0.005 to 0.500% as required. Hereinafter, each component will be described in detail.

[0014] <C: 0.0200% or less> Since C reduces the formability and corrosion resistance of the stainless steel sheet, it is necessary to keep its content low. For this reason, the C content is set to 0.0200% or less. On the other hand, the lower limit of the C content is not particularly limited, but excessive reduction of the C content increases the refining cost, so the C content is preferably 0.0020% or more. From the viewpoint of stably ensuring the above effects, the C content is preferably 0.0030 to 0.0130%, more preferably 0.0040 to 0.0120%, and still more preferably 0.0050 to 0.0110%.

[0015] <Si: 0.02 to 1.90%> Si is useful as a deoxidizer and is also an element effective for high-temperature strength and oxidation resistance. However, excessive reduction of Si increases the refining cost, so the Si content is set to 0.02% or more. On the other hand, Si deteriorates the corrosion resistance in a non-aqueous electrolyte environment (lithium-ion battery environment) containing a fluorine compound, so the Si content is set to 1.90% or less. From the viewpoint of stably ensuring the above effects, the Si content is preferably 0.02 to 1.75%, more preferably 0.03 to 1.60%, and still more preferably 0.04 to 1.55%.

[0016] <Mn: 0.02 to 1.00%> Mn is an element added as a deoxidizer, but when added excessively, the corrosion resistance deteriorates. Therefore, the Mn content is set to 1.00% or less. Also, setting the Mn content to less than 0.01% causes an increase in manufacturing cost, so the Mn content is set to 0.02% or more. From the viewpoint of stably ensuring the above effects, the Mn content is preferably 0.02 to 0.90%, more preferably 0.03 to 0.80%, and still more preferably 0.04 to 0.70%.

[0017] <P: 0.040% or less> P impairs the toughness of the base material and the welded part, so the P content is set to 0.040% or less. On the other hand, the lower limit of the P content is not particularly limited, but extremely reducing the P content of the stainless steel sheet causes an increase in manufacturing cost, so the P content is preferably 0.010% or more. From the viewpoint of stably ensuring the above effects, the P content is preferably 0.012 to 0.038%, more preferably 0.014 to 0.035%, and still more preferably 0.015 to 0.032%.

[0018] <S: 0.0002 to 0.0100%> Since sulfur (S) is a harmful element that adversely affects corrosion resistance and high-temperature cracking of welded joints, the S content should be 0.0100% or less. Furthermore, since the improvement in corrosion resistance due to a decrease in S content saturates at 0.0002%, the S content should be 0.0002% or more. From the viewpoint of stably ensuring the above effects, the S content is preferably 0.0002 to 0.0050%, more preferably 0.0002 to 0.0025%, and even more preferably 0.0002 to 0.0015%.

[0019] <Al:0.004~0.100%> Al is added as a deoxidizing element and also as an element that improves oxidation resistance. If the Al content is less than 0.004%, it leads to an increase in manufacturing costs related to deoxidation, so the Al content should be 0.004% or more. On the other hand, excessive addition of Al reduces processability, so the Al content should be 0.100% or less. From the viewpoint of stably ensuring the above effects, the Al content is preferably 0.006 to 0.090%, more preferably 0.008 to 0.085%, and even more preferably 0.008 to 0.080%.

[0020] <Cr:10.0~25.0%> Cr is an essential element for achieving the basic corrosion resistance of stainless steel sheets. If the Cr content is less than 10.0%, a stable passive film will not be formed, so the Cr content should be 10.0% or more. On the other hand, excessive addition of Cr can reduce corrosion resistance and yield strength, so the Cr content should be 25.0% or less. From the viewpoint of stably ensuring the above effects, the preferred Cr content is 10.4 to 24.4%, more preferably 10.6 to 23.5%, and even more preferably 11.1 to 22.8%.

[0021] <Mo:0.02~2.50%> Mo is an element that improves corrosion resistance. When the Mo content is less than 0.02%, no improvement in corrosion resistance is expected, so the Mo content is set to 0.02% or more. On the other hand, excessive addition of Mo reduces workability and oxidation resistance and leads to an increase in alloy cost, so the Mo content is set to 2.50% or less. Considering manufacturability, scale adhesion, and alloy cost, the Mo content is preferably 0.02 - 2.40%, more preferably 0.02 - 2.30%, still more preferably 0.02 - 2.20%, and most preferably 0.02 - 2.10%.

[0022] <Total of one or more of Nb and Ti: 0.01 - 0.55%> Ti and Nb are elements that combine with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, normal temperature ductility, and deep drawing properties. When the total content of Ti and Nb is less than 0.01%, no improvement in the above properties is expected, so the total content of Ti and Nb is set to 0.01% or more. On the other hand, when the total content of Ti and Nb exceeds 0.55%, workability decreases, so the total content of Ti and Nb is set to 0.55% or less. From the perspective of stably ensuring the above effects, the total content of Ti and Nb is preferably 0.03 - 0.38%, more preferably 0.03 - 0.35%, and still more preferably 0.03 - 0.33%. In addition, the respective contents of Ti and Nb are not particularly limited as long as the total content of Ti and Nb is within the above range. For example, the Ti content and the Nb content are each typically 0 - 0.55%, preferably 0 - 0.35%, more preferably 0 - 0.30%, and still more preferably 0 - 0.25%.

[0023] <N: 0.0500% or less> Since N, like C, reduces formability and corrosion resistance, the N content is set to 0.0500% or less. On the other hand, the lower limit of the N content is not particularly limited, but excessive reduction of N increases refining cost, so the N content is preferably 0.0020% or more. From the perspective of stably ensuring the above effects, the N content is preferably 0.0020 - 0.0430%, more preferably 0.0020 - 0.0400%, and still more preferably 0.0030 - 0.0380%.

[0024] <Ni:0.02~0.40%> Ni is an element that improves corrosion resistance and can be added as needed. If the Ni content is less than 0.02%, no improvement in corrosion resistance can be expected, so the Ni content should be 0.02% or more. On the other hand, if the Ni content exceeds 0.40%, oxidation resistance and processability will decrease, so the Ni content should be 0.40% or less. From the viewpoint of stably ensuring the above effects, the Ni content is preferably 0.02 to 0.38%, more preferably 0.03 to 0.35%, and even more preferably 0.03 to 0.32%.

[0025] <Cu:0.02~1.60%> Cu is an element that improves corrosion resistance and high-temperature strength, and can be added as needed. If the Cu content is less than 0.02%, no improvement in corrosion resistance can be expected, so the Cu content should be 0.02% or more. On the other hand, if the Cu content exceeds 1.60%, the ductility decreases significantly, so the Cu content should be 1.60% or less. From the viewpoint of stably ensuring the above effects, the Cu content is preferably 0.02 to 1.50%, more preferably 0.02 to 1.40%, and even more preferably 0.02 to 1.30%.

[0026] <Sn:0.005~0.500%> Sn has the effect of further enhancing the corrosion resistance of stainless steel sheets and can be added as needed. To achieve this effect, the Sn content should be 0.005% or more. On the other hand, excessive addition of Sn leads to a decrease in workability, so the Sn content should be 0.500% or less. From the viewpoint of stably ensuring the above effect, the Sn content is preferably 0.010 to 0.400%, more preferably 0.030 to 0.350%, and even more preferably 0.050 to 0.300%.

[0027] <B:0.0001~0.0050%> B is an element that enhances the strength of grain boundaries and contributes to the improvement of workability, and can be added as needed. To achieve this effect, the B content should be 0.0001% or more. On the other hand, excessive addition of B will instead cause poor workability due to a decrease in elongation, so the B content should be 0.0050% or less. From the perspective of stably ensuring the above effects, the B content is preferably 0.0003 - 0.0045%, more preferably 0.0005 - 0.0040%, and even more preferably 0.0005 - 0.0035%.

[0028] <V: 1.00% or less> V has the effect of improving corrosion resistance and can be added as needed. However, if V is contained in excess, the toughness will decrease due to coarse carbonitrides. Therefore, the V content should be 1.00% or less. From the perspective of stably ensuring the above effects, the V content is preferably 0.90% or less, more preferably 0.80% or less, and even more preferably 0.70% or less. The lower limit value of the V content is not particularly limited, but for example, it is 0.01%.

[0029] <Zr: 0.005 - 0.500%> Zr can be added as needed to improve corrosion resistance. To achieve this effect, the Zr content should be 0.005% or more. However, although Zr is an important element for suppressing the corrosion rate, excessive addition will deteriorate the manufacturability and cost, so the Zr content should be 0.500% or less. From the perspective of stably ensuring the above effects, the Zr content is preferably 0.008 - 0.450%, more preferably 0.020 - 0.400%, and even more preferably 0.050 - 0.350%.

[0030] <Ga: 0.030% or less> Since Ga has the effect of improving corrosion resistance, it can be added as necessary. However, if Ga is contained in excess, the hot workability will decrease, so the Ga content should be 0.030% or less. From the viewpoint of stably ensuring the above effects, the Ga content is preferably 0.020% or less, more preferably 0.015% or less, and still more preferably 0.012% or less. The lower limit of the Ga content is not particularly limited, but for example, it is 0.001%.

[0031] <REM:0.010% or less> REM (rare earth element) is an element effective for improving hot workability and the cleanliness of steel and for improving the corrosion resistance of the present invention, and can be added as necessary. However, excessive addition of REM leads to an increase in alloy cost and a decrease in productivity, so the REM content should be 0.010% or less. On the other hand, the lower limit of the REM content is not particularly limited, but in order to exhibit the above effects, it is preferably 0.001% or more. From the viewpoint of stably ensuring the above effects, the REM content is preferably 0.001 - 0.008%, more preferably 0.001 - 0.006%, and still more preferably 0.001 - 0.005%. Note that REM follows the general definition. That is, REM refers to the general name of two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). REM may be added alone or as a mixture.

[0032] <Ta: 0.10% or less> Since Ta has the effect of improving corrosion resistance, it can be added as necessary. However, if Ta is contained in excess, the toughness will decrease, so the Ta content should be 0.10% or less. On the other hand, the lower limit of the Ta content is not particularly limited, but in order to exhibit the above effects, it is preferably 0.01% or more. From the viewpoint of stably ensuring the above effects, the Ta content is preferably 0.04 - 0.10%, more preferably 0.06 - 0.10%, and still more preferably 0.08 - 0.10%.

[0033] <W: 1.60% or less> Since W has the effect of improving corrosion resistance, it can be added as necessary. However, if W is contained in excess, toughness will decrease due to coarse carbonitrides or the like, so the W content should be 1.60% or less. From the viewpoint of stably ensuring the above effects, the W content is preferably 1.50% or less, more preferably 1.40% or less, and still more preferably 1.30% or less. The lower limit of the W content is not particularly limited, but for example, it is 0.01%.

[0034] <Co: 1.00% or less> Since Co has the effect of improving the corrosion resistance of steel materials, it can be added as necessary. However, if Co is contained in excess, toughness will decrease, so the Co content should be 1.00% or less. From the viewpoint of stably ensuring the above effects, the Co content is preferably 0.90% or less, more preferably 0.80% or less, and still more preferably 0.70% or less. The lower limit of the Co content is not particularly limited, but for example, it is 0.01%.

[0035] <Sb: 0.005 - 0.500%> Sb is an important element for suppressing the corrosion rate and can be added as necessary to improve corrosion resistance. To exhibit this effect, the Sb content should be 0.005% or more. On the other hand, excessive addition of Sb deteriorates productivity and cost, so the Sb content should be 0.500% or less. From the viewpoint of stably ensuring the above effects, the Sb content is preferably 0.005 - 0.400%, more preferably 0.020 - 0.350%, and still more preferably 0.050 - 0.300%.

[0036] The microstructure of the stainless steel sheet according to the embodiment of the present invention is not particularly limited, but is ferritic. Herein, "ferritic" means a microstructure in which the ferrite phase is mainly present at room temperature. Therefore, "ferritic" also includes those that contain small amounts of phases other than the ferrite phase (for example, austenite phase or martensite phase). However, "ferritic" does not include multiphase structures of ferrite and austenite phases, multiphase structures of ferrite and martensite phases, or multiphase structures of ferrite, austenite, and martensite phases.

[0037] In the embodiment of the present invention, the ratio of Fe concentration to Cr concentration (Fe / Cr) in the film (passivation film) formed on the surface is 3.0 or higher. Within this Fe / Cr range, Fe is concentrated in the film (especially the outermost layer). When a stainless steel sheet having such a film is used in a non-aqueous electrolyte environment containing fluorine compounds (lithium-ion battery environment), the low proportion of Cr in the film suppresses over-passivation dissolution, forming a film containing fluorides (Cr fluoride, Mo fluoride, and Sn fluoride), thereby improving corrosion resistance. From the viewpoint of stably ensuring this effect, Fe / Cr is preferably 3.1 or higher, more preferably 3.2 or higher. The upper limit of Fe / Cr is not particularly limited, but for example, it is 10.0.

[0038] Here, the ratio of Fe concentration to Cr concentration in the film formed on the surface of a stainless steel plate (Fe / Cr) can be determined by glow discharge emission spectroscopy (hereinafter referred to as "GDS"). Specifically, using GDS, elemental concentration profiles in the depth direction are created at measurement intervals of 1 nm or less, and the concentrations of Fe and Cr are determined at positions from the surface to a depth of 20 nm. Then, the ratio of Fe concentration to Cr concentration is calculated using the Fe and Cr concentrations at each position, and the maximum value of this ratio is taken as the Fe / Cr result.

[0039] Furthermore, the non-aqueous electrolyte containing a fluorine compound is not particularly limited as long as it can be used in lithium-ion batteries, but it is generally an electrolyte containing an electrolyte and an organic solvent. The electrolyte can be any compound containing lithium and fluorine, for example, lithium hexafluoride phosphate (LiPF6) and lithium borofluoride (LiBF4) are used. The organic solvent can be ethylene carbonate or ethyl methyl carbonate.

[0040] In the embodiment of the present invention, when the stainless steel sheet is modified by holding it at a constant potential of 4.2V for 48 hours in a non-aqueous electrolyte containing a fluorine compound, it is preferable that the total concentration of fluorides in the film is 0.05 atomic percent or more. Here, the total concentration of fluorides in the film means the total concentration of Cr fluoride and Mo fluoride if the stainless steel sheet does not contain Sn, and if either Cr fluoride or Mo fluoride is not present, the concentration is calculated as zero. If the stainless steel sheet contains Sn, it means the total concentration of Cr fluoride, Mo fluoride and Sn fluoride, and if one or two of Cr fluoride, Mo fluoride and Sn fluoride are not present, the concentration is calculated as zero. If a film containing each fluoride with the above total concentration is formed, it can be said that it has excellent corrosion resistance in a non-aqueous electrolyte environment containing a fluorine compound. From the viewpoint of stably ensuring this effect, the total concentration of each fluoride in the film is more preferably 0.08 atomic% or more, and even more preferably 0.10 atomic% or more. The upper limit of the total concentration of each fluoride in the film is not particularly limited, but is, for example, 1.00 atomic%.

[0041] Here, the total concentration of fluorides in the modified film can be determined by X-ray photoelectron spectroscopy (hereinafter referred to as "XPS"). Specifically, the photoelectron spectrum is obtained by XPS of the modified film, the peaks at the binding energy of each fluoride are separated, and these are then fitted onto the Cr2P photoelectron spectrum, Mo2P photoelectron spectrum, and Sn2P photoelectron spectrum to calculate the concentration of each fluoride, and these concentrations are then summed up to determine the total concentration. In XPS, a commercially available X-ray photoelectron spectrometer can be used, with mono-AlK as the X-ray source and a photoelectron extraction angle (TOA) of 45°.

[0042] In an embodiment of the present invention, the stainless steel plate exhibits a current density of 5.0 μA / cm² in a constant potential holding test in which it is maintained at a constant potential of 4.2 V for 48 hours in a non-aqueous electrolyte containing a fluorine compound. 2 The following is preferable. With such a current density, excellent corrosion resistance can be achieved in a non-aqueous electrolyte environment containing fluorine compounds. From the viewpoint of stably ensuring this effect, the current density in the constant potential holding test is more preferably 4.0 μA / cm². 2 More preferably, 3.0 μA / cm 2 The following applies. Note that the current density in the constant potential holding test is not particularly limited, as a lower value indicates better corrosion resistance, and is 0 μA / cm². 2 The following is also acceptable.

[0043] Here, the non-aqueous electrolyte used in the above constant potential holding test contains 1 M LiPF6 as the electrolyte and a mixture of ethylene carbonate and ethyl methyl carbonate in a 1:3 volume ratio as the organic solvent. The above low potential holding test can be performed using a commercially available charge / discharge device, and is carried out using a three-electrode cell with a stainless steel plate as the working electrode and metallic lithium as the counter electrode and reference electrode. The electrolytic area of ​​the stainless steel plate is 1 cm². 2 (Seats should be applied to areas not subjected to electrolysis as needed.) The temperature of the non-aqueous electrolyte should be set to 25°C, and the stainless steel plate of the working electrode should be maintained at 4.2V for 48 hours based on lithium metal, during which time the current density flowing should be measured.

[0044] The thickness of the stainless steel plate according to the embodiment of the present invention is not particularly limited and can be appropriately set according to the type of component of the lithium-ion battery in which the stainless steel plate is used. For example, considering the weight reduction of the lithium-ion battery components, the thickness of the stainless steel plate is preferably 2.0 mm or less. Furthermore, considering the rigidity of the lithium-ion battery components, the thickness of the stainless steel plate is more preferably 0.2 mm or more. Furthermore, considering the fire resistance and cost of the lithium-ion battery components, the thickness of the stainless steel plate is even more preferably 0.2 to 1.8 mm. Furthermore, considering the weldability of the lithium-ion battery components, the thickness of the stainless steel plate is even more preferably 0.3 to 1.5 mm.

[0045] The method for manufacturing a stainless steel sheet according to the embodiment of the present invention is not particularly limited as long as it is a method capable of manufacturing a stainless steel sheet having the above-described features. For example, a method for manufacturing a stainless steel sheet according to an embodiment of the present invention generally includes a steelmaking process, a hot rolling process, a pickling process, a cold rolling process, an annealing process, and an electrolytic pickling process.

[0046] <Steelmaking Process> In the steelmaking process, a preferred method involves melting steel having the above composition in a converter, followed by secondary refining. The molten steel is then formed into slabs according to a known casting method (continuous casting). The conditions of the steelmaking process are not particularly limited, and known conditions can be adopted. <Hot rolling process> The slab is heated to a predetermined temperature and hot-rolled to a predetermined thickness by continuous rolling. The conditions for hot rolling are not particularly limited, and known conditions can be used. <Acid washing process> Hot-rolled steel sheets are pickled. The pickling conditions are not particularly limited, and known conditions can be used. However, pickling may be omitted. Annealing may also be performed after hot rolling.

[0047] <Cold rolling process> The conditions for cold rolling are not particularly limited, but for example, reverse rolling can be performed in a Zenzimir rolling mill with a roll diameter of about 50 to 100 mm, or unidirectional rolling can be performed in a tandem rolling mill with a roll diameter of 400 mm or more. <Annealing process> The conditions for the annealing process are not particularly limited, and known conditions can be adopted. Specifically, appropriate conditions should be selected according to the material level. Furthermore, after the cold rolling and annealing processes, temper rolling or tension leveling may be applied as needed.

[0048] <Electrolytic pickling process> An oxide film, mainly composed of Fe and Cr, exists on the surface of stainless steel sheets after annealing. The electrolytic pickling process for stainless steel sheets is a process to remove the oxide film. By removing the oxide film through the electrolytic pickling process, a desired film (passivation film) can be formed, and furthermore, the film can be modified to have excellent corrosion resistance when exposed to a non-aqueous electrolyte environment containing fluorine compounds. The electrolytic pickling process involves holding the stainless steel plate in an acidic solution within the overpassivation potential range of Cr, thereby selectively dissolving Cr. This process allows for the concentration of Fe in the coating (especially the outermost layer), resulting in the formation of a coating with a Fe / Cr ratio of 3.0 or higher. Electrolytic pickling process is 0.05~1.00A / cm 2 It is preferable to perform anode electrolysis with a current density of 1 to 300 seconds. Current density is 1.00 A / cm² 2 If the current density exceeds 0.05 or the electrolysis time exceeds 300 seconds, excessive dissolution of the oxide film occurs, resulting in significant surface irregularities, and it becomes difficult to control the Fe / Cr ratio of the film within the specified range. Also, if the current density is less than 0.05 or the electrolysis time is less than 3 seconds, insufficient dissolution of the oxide film occurs, making it difficult to control the Fe / Cr ratio of the film within the specified range. The acidic solution used in the electrolytic pickling process is not particularly limited, but for example, a 15% aqueous HNO3 solution can be used. Furthermore, if necessary, a molten salt treatment may be performed before the electrolytic pickling process.

[0049] In the embodiment of the present invention, the stainless steel sheet has its composition and the ratio of Fe concentration to Cr concentration in the film formed on its surface controlled within a predetermined range. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, overpassive dissolution is suppressed, the film is modified to contain fluoride, and corrosion resistance is improved. Therefore, it is possible to manufacture lithium-ion battery components that have excellent corrosion resistance in a non-aqueous electrolyte environment containing fluorine compounds and can ensure corrosion resistance even when current flows due to a short circuit or the like. The components of a lithium-ion battery are not particularly limited, but include cases, battery packs, battery modules, lids, battery module covers, end plates, side plates, and spacers.

[0050] (2) Surface-modified stainless steel parts for lithium-ion batteries A surface-modified stainless steel component for a lithium-ion battery according to an embodiment of the present invention (hereinafter abbreviated as "surface-modified stainless steel component") contains C: 0.0200% or less, Si: 0.02~1.90%, Mn: 0.02~1.00%, P: 0.040% or less, S: 0.0002~0.0100%, Al: 0.004~0.100%, Cr: 10.0~25.0%, Mo: 0.02~2.50%, total of one or more Nb and Ti: 0.01~0.55%, N: 0.0500% or less, with the remainder being Fe and impurities. Furthermore, the surface-modified stainless steel parts according to the embodiment of the present invention may further contain, if necessary, one or more selected from Ni: 0.02 to 0.40%, Cu: 0.02 to 1.60%, and Sn: 0.005 to 0.500%. Furthermore, the surface-modified stainless steel parts according to the embodiment of the present invention may further contain, if necessary, one or more selected from B: 0.0001 to 0.0050%, V: 1.00% or less, Zr: 0.005 to 0.500%, Ga: 0.030% or less, REM: 0.010% or less, Ta: 0.10% or less, W: 1.60% or less, Co: 1.00% or less, and Sb: 0.005 to 0.500%. Since the composition of the surface-modified stainless steel part according to the embodiment of the present invention is the same as the composition of the stainless steel sheet described above, a detailed explanation is omitted.

[0051] Herein, in this specification, "surface-modified stainless steel part" means a part obtained by processing the above-mentioned stainless steel sheet into a predetermined shape and using the part in a non-aqueous electrolyte environment containing a fluorine compound. The part is not particularly limited, but examples include cases, battery packs, battery modules, lids, battery module covers, end plates, side plates, and spacers.

[0052] In the embodiment of the present invention, the surface-modified stainless steel part preferably has a total fluoride concentration of 0.05 atomic percent or more in the film formed on its surface. Here, the total fluoride concentration in the film means the total concentration of Cr fluoride and Mo fluoride if the surface-modified stainless steel part does not contain Sn, and if either Cr fluoride or Mo fluoride is not present, its concentration is calculated as zero. If the surface-modified stainless steel part contains Sn, it means the total concentration of Cr fluoride, Mo fluoride and Sn fluoride, and if one or two of Cr fluoride, Mo fluoride and Sn fluoride are not present, their concentrations are calculated as zero. If a film containing each fluoride with the above total concentrations is formed, it exhibits excellent corrosion resistance in a non-aqueous electrolyte environment containing fluorine compounds, and corrosion resistance can be ensured even when current flows due to a short circuit, etc. From the viewpoint of stably ensuring this effect, the total concentration of each fluoride in the film is preferably 0.08 atomic percent or more, more preferably 0.10 atomic percent or more. The upper limit of the total concentration of each fluoride in the coating is not particularly limited, but is, for example, 1.00 atomic percent.

[0053] Here, the total concentration of fluorides in the film can be determined by XPS. Specifically, the photoelectron spectrum is obtained by XPS of the film, the peaks at the binding energy of each fluoride are separated, and these are then fitted onto the Cr2P photoelectron spectrum, Mo2P photoelectron spectrum, and Sn2P photoelectron spectrum to calculate the concentration of each fluoride, and then these concentrations are summed up to determine the total concentration. In XPS, a commercially available X-ray photoelectron spectrometer can be used, with mono-AlK as the X-ray source and a photoelectron extraction angle (TOA) of 45°.

[0054] The surface-modified stainless steel part according to an embodiment of the present invention can be manufactured by processing the above-mentioned stainless steel sheet into a part of a predetermined shape and using the part in a non-aqueous electrolyte environment containing a fluorine compound. The processing method is not particularly limited, and various known processing methods can be used.

[0055] The surface-modified stainless steel parts according to the embodiment of the present invention have excellent corrosion resistance in non-aqueous electrolyte environments containing fluorine compounds, and corrosion resistance can be ensured even when current flows due to a short circuit, etc., because the composition and the total concentration of Cr fluoride and Mo fluoride in the film formed on the surface are controlled within a predetermined range. [Examples]

[0056] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0057] (Example 1) Steel melt having the composition shown in Table 1 (the balance being Fe and impurities) was cast, and the obtained ingot was hot-rolled, pickled, cold-rolled and annealed to obtain a cold-rolled and annealed sheet with a thickness of 1.0 mm. Next, after subjecting the cold-rolled and annealed sheet to molten salt treatment as a pretreatment, electrolytic pickling was carried out to obtain a stainless steel sheet. For the molten salt treatment, a NaOH + NaNO3 mixed solution adjusted so that NaNO3 was 30% was used as the molten salt, and it was immersed in the molten salt heated to 450 °C for 10 s. For the electrolytic pickling, a 15% HNO3 aqueous solution was used as the acidic solution, and the electrolysis was carried out with a current density of 0.10 A / cm 2 and an electrolysis time of 16 s. Note that the electrolytic pickling was carried out while changing the direction of the current. After electrolyzing at a current density of -0.10 A / cm 2 for 2 s, an operation of electrolyzing at a current density of +0.10 A / cm 2 for 2 s was taken as one set, and four sets were carried out.

[0058]

Table 1

[0059] The following evaluations were performed on the stainless steel sheet obtained above. <Ratio of Fe concentration to Cr concentration (Fe / Cr) in the film formed on the surface> Using GDS (GD-profiler 2 (registered trademark) manufactured by Horiba, Ltd.), an elemental concentration profile in the depth direction was created at a measurement interval of 1 nm or less, and the concentrations of Fe and Cr at positions from the surface to a depth of 20 nm were determined. Then, the ratio of the Fe concentration to the Cr concentration at each position was calculated, and the maximum value was taken as the result of Fe / Cr.

[0060] <Total concentration of fluorides in the film when the film was modified by holding at a constant potential of 4.2 V for 48 h in a non-aqueous electrolyte containing a fluorine compound> A stainless steel plate was maintained at a constant potential of 4.2V for 48 hours in a non-aqueous electrolyte (25°C) containing a fluorine compound to modify its coating. The non-aqueous electrolyte used consisted of 1M LiPF6 as the electrolyte and a 1:3 volume ratio mixture of ethylene carbonate and ethyl methyl carbonate as the organic solvent. A commercially available charge / discharge device (Scribner Associates 580, manufactured by Toyo Technica Co., Ltd.) was used for constant potential maintenance. A three-electrode cell was used, with a stainless steel plate as the working electrode and metallic lithium as the counter electrode and reference electrode. Photoelectron spectra were obtained from the modified film using XPS, and the peaks at the binding energies of each fluoride were separated. These were then fitted onto Cr2P, Mo2P, and Sn2P photoelectron spectra to calculate the concentration of each fluoride, which was then summed up. For the XPS, a commercially available X-ray photoelectron spectrometer (VersaProve III, ULVAC-PHI, Inc.) was used, with a mono-AlK X-ray source and a photoelectron extraction angle (TOA) of 45°.

[0061] <Current density in a constant potential holding test where a constant potential of 4.2V is maintained for 48 hours in a non-aqueous electrolyte containing a fluorine compound> A constant potential holding test was performed on a stainless steel plate in a non-aqueous electrolyte containing a fluorine compound (25°C) to maintain a constant potential of 4.2V for 48 hours, and the current density was determined. The same non-aqueous electrolyte containing fluorine compounds as described above was used. The same charge / discharge apparatus as described above was used for the constant potential holding test. A three-electrode cell was used, with a stainless steel plate as the working electrode and metallic lithium as the counter electrode and reference electrode, and the electrolytic area of ​​the stainless steel plate was 1 cm². 2 (The parts that were not subjected to electrolysis were sealed with O-rings.)

[0062] The results of each of the above evaluations are shown in Table 2.

[0063] [Table 2]

[0064] As shown in Table 2, the stainless steel sheets of Examples No. 1-1 to 1-25 had a composition and a ratio of Fe concentration to Cr concentration (Fe / Cr) in the film formed on the surface that was within the specified range. Therefore, when used in a non-aqueous electrolyte environment containing fluorine compounds, the total fluoride concentration of the film could be modified to an appropriate range, and the current density in the constant potential holding test was also low (good corrosion resistance).

[0065] In contrast, the stainless steel plate of Comparative Example No. 1-1 had too much carbon content, and the ratio of Fe concentration to Cr concentration in the film formed on the surface was also low. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (corrosion resistance was insufficient). The stainless steel plates of Comparative Examples No. 1-2 had an excessively high Si content, and the ratio of Fe concentration to Cr concentration in the film formed on the surface was also low. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was high (insufficient corrosion resistance). This is thought to be because Si oxide remained in the film formed on the surface of the stainless steel plates of Comparative Examples No. 1-2, inhibiting fluoride formation when used in a non-aqueous electrolyte environment containing fluorine compounds. The stainless steel plates of Comparative Examples No. 1-3 had an excessively high Mn content, and the ratio of Fe concentration to Cr concentration in the film formed on the surface was also low. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (corrosion resistance was insufficient). The stainless steel sheets of Comparative Examples No. 1-4 had too much phosphorus (P) and too little sulfur (S) content, and the ratio of Fe concentration to Cr concentration in the film formed on the surface was also low. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was high (corrosion resistance was insufficient). The stainless steel plates of Comparative Examples No. 1-5 had an excessively high sulfur content, and the ratio of Fe concentration to Cr concentration in the film formed on the surface was also low. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (insufficient corrosion resistance).

[0066] In Comparative Example No. 1-6, the stainless steel plate had too little Cr content, resulting in a low ratio of Fe concentration to Cr concentration in the film formed on the surface. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (indicating insufficient corrosion resistance). The stainless steel sheets of Comparative Examples No. 1-7 had too little Mo content and lacked Ti and Nb, resulting in a low ratio of Fe concentration to Cr concentration in the film formed on the surface. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was high (indicating insufficient corrosion resistance). The stainless steel plates of Comparative Examples No. 1-8 had an excessively high Al content, and the ratio of Fe concentration to Cr concentration in the film formed on the surface was also low. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was high (insufficient corrosion resistance). This is thought to be because Al oxide remained in the film formed on the surface of the stainless steel plates of Comparative Examples No. 1-8, inhibiting fluoride formation when used in a non-aqueous electrolyte environment containing fluorine compounds. The stainless steel sheets of Comparative Examples No. 1-9 had excessively high N content and combined Nb and Ti levels, as well as a low ratio of Fe concentration to Cr concentration in the film formed on the surface. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (indicating insufficient corrosion resistance). The stainless steel sheets of Comparative Examples No. 1-10 had insufficient content of Ni, Cu, and Sn, and the ratio of Fe concentration to Cr concentration in the film formed on the surface was also low. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was high (corrosion resistance was insufficient).

[0067] (Example 2) In Example 2, stainless steel sheets were produced by varying the current density and electrolysis time in the electrolytic pickling process as shown in Table 3. Note that Comparative Examples 2-3 did not undergo the electrolytic pickling process. The stainless steel plates obtained above were evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0068] [Table 3]

[0069] As shown in Table 3, the stainless steel sheets of Examples No. 2-1 to 2-20 had a composition and a ratio of Fe concentration to Cr concentration (Fe / Cr) in the film formed on the surface that was within the specified range. Therefore, when used in a non-aqueous electrolyte environment containing fluorine compounds, the total fluoride concentration of the film could be modified to an appropriate range, and the current density in the constant potential holding test was low (good corrosion resistance).

[0070] In contrast, the stainless steel sheet of Comparative Example No. 2-1 had a low current density during the electrolytic pickling process, resulting in a low ratio of Fe concentration to Cr concentration in the film formed on its surface. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (resulting in insufficient corrosion resistance). In Comparative Example No. 2-2, the current density during the electrolytic pickling process was too high, resulting in a low ratio of Fe concentration to Cr concentration in the film formed on the surface. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (insufficient corrosion resistance). This is thought to be because excessive pickling generated Fe-containing deposits (pickling smut) on the surface of the stainless steel sheet in Comparative Example No. 2-2, which inhibited fluoride formation when used in a non-aqueous electrolyte environment containing fluorine compounds. In Comparative Example No. 2-3, the stainless steel sheet did not undergo an electrolytic pickling process, resulting in a lower ratio of Fe concentration to Cr concentration in the film formed on the surface. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a lower total fluoride concentration, and the current density in the constant potential holding test was also high (indicating insufficient corrosion resistance). In Comparative Example No. 2-4, the electrolytic time in the electrolytic pickling process was too long, resulting in a low ratio of Fe concentration to Cr concentration in the film formed on the surface. As a result, when used in a non-aqueous electrolyte environment containing fluorine compounds, the film was modified to have a low total fluoride concentration, and the current density in the constant potential holding test was also high (insufficient corrosion resistance). This is thought to be because excessive pickling created Fe-containing deposits (pickling smut) on the surface of the stainless steel sheet in Comparative Example No. 2-4, which inhibited fluoride formation when used in a non-aqueous electrolyte environment containing fluorine compounds.

[0071] As can be seen from the above results, the present invention provides a stainless steel sheet for lithium-ion batteries that has excellent corrosion resistance in a non-aqueous electrolyte environment containing fluorine compounds and ensures corrosion resistance even when current flows due to a short circuit or the like, enabling the manufacture of lithium-ion battery components. Furthermore, the present invention provides a surface-modified stainless steel component for lithium-ion batteries that has excellent corrosion resistance in a non-aqueous electrolyte environment containing fluorine compounds and ensures corrosion resistance even when current flows due to a short circuit or the like.

Claims

1. By mass, it contains C: 0.0200% or less, Si: 0.02 to 1.90%, Mn: 0.02 to 1.00%, P: 0.040% or less, S: 0.0002 to 0.0100%, Al: 0.004 to 0.100%, Cr: 10.0 to 25.0%, Mo: 0.02 to 2.50%, total of one or more Nb and Ti: 0.01 to 0.55%, N: 0.0500% or less, with the remainder being Fe and impurities. A stainless steel sheet for lithium-ion batteries, wherein the ratio of Fe concentration to Cr concentration in the film formed on the surface (Fe / Cr) is 3.0 or higher.

2. A stainless steel sheet for lithium-ion batteries according to claim 1, further containing one or more selected by mass from Ni: 0.02 to 0.40%, Cu: 0.02 to 1.60%, and Sn: 0.005 to 0.500%.

3. A stainless steel sheet for lithium-ion batteries according to claim 1 or 2, further containing, by mass, one or more selected from B: 0.0001 to 0.0050%, V: 1.00% or less, Zr: 0.005 to 0.500%, Ga: 0.030% or less, REM: 0.010% or less, Ta: 0.10% or less, W: 1.60% or less, Co: 1.00% or less, and Sb: 0.005 to 0.500%.

4. The stainless steel sheet for lithium-ion batteries according to claim 1, wherein when the film is modified by being held at a constant potential of 4.2 V for 48 hours in a non-aqueous electrolyte containing a fluorine compound, the total concentration of Cr fluoride and Mo fluoride in the film is 0.05 atomic percent or more.

5. The stainless steel sheet for lithium-ion batteries according to claim 2, wherein when the film is modified by being held at a constant potential of 4.2 V for 48 hours in a non-aqueous electrolyte containing a fluorine compound, the total concentration of Cr fluoride, Mo fluoride, and Sn fluoride in the film is 0.05 atomic percent or more.

6. In a constant potential holding test, where a fluorine compound was maintained at a constant potential of 4.2 V for 48 hours in a non-aqueous electrolyte solution, the current density was 5.0 μA / cm². 2 The stainless steel plate for lithium-ion batteries according to claim 1 or 2, which is as follows:

7. By mass, it contains C: 0.0200% or less, Si: 0.02 to 1.90%, Mn: 0.02 to 1.00%, P: 0.040% or less, S: 0.0002 to 0.0100%, Al: 0.004 to 0.100%, Cr: 10.0 to 25.0%, Mo: 0.02 to 2.50%, total of one or more Nb and Ti: 0.01 to 0.55%, N: 0.0500% or less, with the remainder being Fe and impurities. Surface-modified stainless steel parts for lithium-ion batteries, wherein the total concentration of Cr fluoride and Mo fluoride in the film formed on the surface is 0.05 atomic percent or more.

8. It further contains one or more elements selected from Ni: 0.02-0.40% by mass, Cu: 0.02-1.60%, and Sn: 0.005-0.500%. The surface-modified stainless steel part for lithium-ion batteries according to claim 7, wherein the total concentration of Cr fluoride, Mo fluoride, and Sn fluoride in the film formed on the surface is 0.05 atomic percent or more.

9. Surface-modified stainless steel parts for lithium-ion batteries according to claim 7 or 8, further containing one or more selected from, by mass, B: 0.0001 to 0.0050%, V: 1.00% or less, Zr: 0.005 to 0.500%, Ga: 0.030% or less, REM: 0.010% or less, Ta: 0.10% or less, W: 1.60% or less, Co: 1.00% or less, and Sb: 0.005 to 0.500%.

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