JP2012092361A - Austenitic stainless steel foil for laminate case of lithium ion secondary battery and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metallic foil for obtaining a laminate sheet for use in a lithium ion secondary battery, the laminate sheet being good in press formability, preventing a whitening phenomenon of a resin film layer in press forming, having excellent peeling resistance at a thermal fusion part when ear parts of a press molding are thermally fused with each other, and having strength higher than that of a laminate sheet prepared using an aluminum foil.SOLUTION: An austenitic stainless steel foil has: a composition having an Md value represented by the following formula (1) of 0.0-40.0; a thickness of 40-150 μm; an average number of crystal grains existing in the thickness direction of 5.0 pieces or more; and an average nitrogen concentration in a surface layer region from the surface to a depth of 2 nm of 5.0-9.5 mass%. Md value=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo...(1).

Description

  The present invention relates to a metal foil used for a laminate-type case for housing a cell of a lithium ion secondary battery, and a method for manufacturing the metal foil.

  Lithium ion secondary batteries have high energy density and high output characteristics, and are therefore widely used as small batteries for notebook computers, mobile phones, mobile devices and the like. Since a battery is required to be small and light, a laminate type case is often adopted as a case for storing a battery cell.

  A typical laminate-type battery case is a member (pressed) that is formed by press-molding a laminate sheet in which an acid-resistant resin film layer such as polypropylene is formed on the surface of a metal foil, leaving an ear in the periphery. Formed body). The battery is prepared with two such press-molded bodies, the battery cells are accommodated between the cup parts (recesses) of both members, and the ears are heat-sealed with the electrode tabs exposed to the outside. It is manufactured by doing. As the material for the metal body, an aluminum alloy is generally used.

JP 2004-52100 A

  Recently, studies on mounting lithium ion secondary batteries on electric vehicles and hybrid vehicles are in progress, and they are already in practical use. In addition, lithium ion secondary batteries are expected to be used in the future for power applications for storing electrical energy generated by solar cells. In lithium ion secondary batteries used in these applications, the active electrolyte content increases as the output capacity increases, so the battery case has better robustness and durability than small batteries. Required.

  In a conventional general laminate type battery case based on an aluminum alloy foil, it is effective to change the material of the foil to stainless steel as a means for improving the strength. Patent Document 1 describes a battery case using an austenitic stainless steel foil. However, it is taught that nitriding of the surface should be prevented as much as possible in the final annealing of the stainless steel foil in order to impart press formability.

  According to the technique of Patent Document 1, the strength of the laminated battery case is improved as compared with the case of using an aluminum alloy foil. However, according to the investigation by the inventors, in a battery case using a laminate sheet based on austenitic stainless steel foil, the peel strength (heat seal strength) of the heat-sealed portion may be lower than before. I understood. Further, it was found that when the laminate sheet is press-molded, a problem (whitening phenomenon) that the resin film layer on the surface turns white is likely to occur. The whitening phenomenon is a phenomenon that occurs when fine cracks occur in the resin film.

  In the laminate sheet used for the case of the lithium ion secondary battery, the present invention has good press moldability, the whitening phenomenon of the resin film layer is suppressed during press molding, and the ears of the press molded body are heat-sealed. In this case, it is intended to realize a material having excellent peeling resistance at the heat-sealed portion and higher strength than that using an aluminum foil.

The purpose is mass%, C: 0.03 to 0.15%, Si: 1.00% or less, Mn: 1.50% or less, Ni: 5.0 to 10.0%, Cr: 15. 0 to 20.0%, Cu: 0.05 to 0.50%, Mo: 0 to 0.50%, N: 0.10% or less, balance Fe and inevitable impurities, and the following formula (1) A stainless steel foil having a composition having a Md value of 0.0 to 40.0 and a thickness of 40 to 150 μm, and the average number of crystal grains existing in the thickness direction is 5.0 or more. Yes, it is achieved by an austenitic stainless steel foil for a lithium ion secondary battery laminate case having an average nitrogen concentration in the surface layer 2 nm region of 5.0 to 9.5% by mass.
Md value = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7Cr-18.5Mo (1)

  Here, the lower limit “0%” of the Mo content means that Mo is not added (it is not detected by a normal analysis method in the steel making process). The value of the content of the element represented by mass% is substituted for the element symbol in the formula (1). For an additive-free element, 0 (zero) is substituted.

  “The average number of crystal grains existing in the thickness direction” is the thickness direction on the metallographic observation image in which the entire thickness of the cross section (L cross section) perpendicular to the rolling direction and thickness direction of the foil enters the field of view. N straight lines are drawn at equal intervals (where n ≧ 10), and for each straight line, the number m of intersections of the straight line and the grain boundary is obtained, and the value of m + 1 is the crystal grain in the thickness direction at the straight line position. And the sum of a values for each straight line is divided by the number n of straight lines. However, the measurement distance in the rolling direction (distance in the rolling direction of the straight lines at both ends) is at least five times the average thickness of the foil. Further, the surface of the foil is not regarded as a crystal grain boundary.

  The “average nitrogen concentration in the surface layer of 2 nm region” is obtained by quantitatively calculating the “light intensity-time” profile in the depth direction measured using a high-frequency glow discharge emission spectrometer (GDS) by a calibration curve using a standard sample. It can be determined by creating a (mass%)-depth (nm) "profile and calculating the average value of the nitrogen concentration from the outermost surface to 2 nm.

  In addition, as a method for producing the above-mentioned austenitic stainless steel foil for a lithium ion secondary battery laminate case, a final annealing in a non-oxidizing atmosphere is performed on a stainless steel foil having a thickness adjusted by cold rolling. In addition, the nitrogen concentration in the atmosphere gas is 15 to 30% by volume, the furnace temperature is 1000 to 1100 ° C., the holding time at which the foil temperature is 980 ° C. or more is secured for 3 seconds or more, and the crystal grains exist in the thickness direction There is provided a method of setting the in-furnace time in a range in which the average number of the reactors becomes 5.0 or more. The in-furnace time is more preferably 3 to 15 seconds as the holding time for the foil temperature to be 980 ° C. or higher.

  Here, the furnace temperature is a temperature at which the material put in the furnace for a long time finally reaches. The temperature of the foil is the material temperature of the foil itself, and in the case of a foil, the temperature difference between the surface and the interior during the temperature raising process is very small, so the surface temperature can be adopted.

  The laminate sheet provided by utilizing the present invention has good press formability despite the use of stainless steel foil, and the resin film layer whitening phenomenon (occurrence of fine cracks) is remarkable during pressing. Is deterred. A battery case using this laminate sheet has higher strength than a conventional general laminate-type battery case, and sufficiently secures the peel resistance of the heat-sealed portion. Therefore, the present invention can contribute to the widespread use of lithium ion secondary batteries in applications for supplying power to large-sized devices such as electric vehicles and hybrid vehicles, and for storing power in solar cell power generation systems.

The figure which showed typically the cross section containing the central axis of the cup shape | molded for the heat seal intensity | strength measurement. The figure which showed typically the cross section of the tensile test piece for heat seal strength measurement.

  When a laminate sheet based on austenitic stainless steel foil is used, the first problem that must be overcome is an improvement in press formability. As a result of detailed investigations by the inventors, it was confirmed that adopting an austenitic steel type having a metastable composition is more advantageous for press forming of a battery case than adopting a steel type having high austenite stability. It was done. The press forming of the battery case is a process in which the overhanging element is strong, and the metastable steel type is advantageous for improving the forming height.

  Further, the press forming of the battery case is not a complete overhanging process but a composite forming having elements of deep drawing. Therefore, it is important for obtaining good moldability that the flange portion is smoothly drawn immediately after the punch hits the material. As a result of various studies, it has been found that a nitride layer sufficiently formed on the surface layer of the foil is extremely effective for pulling in the flange. The surface-induced nitrided layer suppresses deformation-induced martensite transformation at the initial stage of deformation, and the surface is pulled in with a high level of smoothness, so the flange is pulled in more smoothly, which effectively functions to improve the machining height. To do.

  Furthermore, by sufficiently forming the nitride layer on the surface layer portion of the foil, the processing-induced martensitic transformation in the surface layer portion is remarkably suppressed until the end of press molding. For this reason, the resin film layer that is in close contact with the foil surface is released from the local tensile stress due to the unevenness caused by the martensitic transformation of the underlying stainless steel foil in the press molding process, and the formation of fine cracks. It is suppressed. Suppressing fine cracks in the resin film layer means that the whitening phenomenon is prevented at the same time. Moreover, the adhesiveness between the surface of the base stainless steel foil and the resin film layer is also kept high. As a result, a decrease in peel resistance (heat seal strength) is prevented at the heat-sealed portion between the ear portions of the press-formed body.

  According to the teaching of Patent Document 1, it is important to prevent nitriding of the austenitic stainless steel foil surface as much as possible in order to improve press formability. However, in this case, the whitening phenomenon resulting from the work-induced martensitic transformation in the surface layer part and the decrease in the peel resistance at the heat-sealed part remain unsolved. In the present invention, by combining a method of employing a foil made of an austenitic steel grade having a metastable composition and a method of sufficiently nitriding the surface layer portion of the foil, the press formability is improved and the whitening phenomenon is reduced. Preventing the deterioration of the peel resistance at the heat-sealed part at once.

[Chemical composition]
The component composition of the austenitic stainless steel that is the subject of the present invention will be briefly described. Hereinafter, “%” regarding the composition of steel means “% by mass” unless otherwise specified.
C is an element necessary for maintaining high strength of steel. In the present invention, since the foil is a target, if the C content is too low, the foil is easily “breached” due to insufficient strength in a press forming process at a mass production site where the deformation speed is large, resulting in formability. Leading to a decline. As a result of various studies, the C content needs to be 0.03% or more. On the other hand, excessive C content causes a decrease in formability and a decrease in corrosion resistance due to curing, so the content is made 0.15% or less. You may manage to 0.12% or less.

  Si is an element added as a deoxidizer. However, excessive Si content causes a decrease in moldability. As a result of various studies, the Si content is limited to 1.00% or less. The lower limit of the Si content is not particularly limited, but may be controlled to, for example, 0.03% or more, or 0.10% or more.

  Mn is effective as a deoxidizer and is effective in improving hot workability and corrosion resistance by the action of fixing S. In order to sufficiently exhibit these effects, it is more effective to ensure the Mn content of 0.05% or more, and it is more effective to set the content to 0.10% or more. You may manage to 0.50% or more. However, excessive Mn content causes a decrease in moldability. As a result of various studies, the Mn content is limited to a range of 1.50% or less.

  Ni is an essential element for ensuring press formability and corrosion resistance as a metastable austenitic stainless steel. For that purpose, it is necessary to secure a Ni content of 5.0% or more. However, since a large amount of Ni causes an increase in cost and causes excessive stabilization of the austenite stability, it is limited to a range of 10.0% or less.

  Cr is an essential element for maintaining the corrosion resistance of stainless steel, and here, steel with a Cr content of 15.0% or more is targeted. However, excessive Cr content causes a decrease in steel plate manufacturability and press formability, so it is limited to 20.0% or less. A more preferable Cr content range is 16.0 to 19.0%.

  Cu has the effect of softening the work-induced martensite phase and is effective in improving the press formability to the battery case. As a result of various studies, it is necessary to secure a Cu content of 0.05% or more. It is more effective to set it to 0.10% or more, and it is more effective to set it to 0.15% or more. However, excessive Cu content is a factor that hinders press formability, and is limited to a range of 0.50% or less. You may manage to 0.35% or less.

  Mo is effective in improving the corrosion resistance, and can be added as necessary. In that case, it is more effective to secure a Mo content of 0.05% or more. However, excessive Mo addition increases costs, so the Mo content is 0.50% or less. You may manage in the range of 0.25% or less.

  N is effective in improving the strength and corrosion resistance of steel, and it is more effective to secure an N content of 0.005% or more, for example. However, since excessive N content causes a decrease in press formability, the N content is limited to a range of 0.10% or less. You may manage to 0.05% or less.

The Md value represented by the following formula (1) is an index of austenite stability. The lower the Md value, the more stable the austenite. Conversely, the higher the Md value, the easier the processing-induced martensitic transformation occurs.
Md value = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7Cr-18.5Mo (1)

  In the present invention, as described above, the peel resistance (heat seal strength) at the heat-sealed portion is ensured by the method of sufficiently nitriding the surface. It has been found that press formability (particularly stretch workability), which is considered to be inhibited by the nitrided layer, can be sufficiently avoided by using a limited amount of metastable austenitic steel in which austenite is unstable to some extent. As a result of detailed examination, it is necessary to use steel whose Md value is adjusted to 0.0 to 40.0. If steel whose Md value is out of the above range is used, it is difficult to stably secure sufficient press formability even when the following appropriate manufacturing conditions are followed. The Md value may be managed in the range of 0.0 to 30.0, or 0.0 to 25.0.

[Thickness of foil]
In a battery case using a metastable austenitic stainless steel foil having the above chemical composition, it is necessary to secure a thickness of 40 μm or more at the stage before press molding. If it is thinner than that, it will be difficult to ensure the average number of crystal grains in the thickness direction within a predetermined range in the final annealing, and the formability will deteriorate. In addition, the strength level of the battery case also decreases. On the other hand, when it becomes excessively thick, the load on the molding process increases, and the mass of the battery case increases. As a result of various studies, it is desired to use a foil having a thickness of 150 μm or less.

[Average number of crystal grains in the thickness direction]
In the processing of foil, the size of crystal grains tends to affect workability. According to the inventors' research, when a foil having the above metastable composition and the above thickness range is employed, the average number of crystal grains existing in the thickness direction is 5.0 or more. By doing so, it becomes possible to ensure the workability required for processing the battery case. More preferably, the number is 6.0 or more. The size of the crystal grains can be controlled mainly by the final annealing conditions.

(Nitride layer)
As described above, by forming a nitride layer on the surface layer portion of the foil, martensitic transformation of the surface layer portion during press molding is suppressed, and formation of surface irregularities associated with the transformation is largely avoided. As a result, it is possible to avoid a decrease in peel resistance at the heat-sealed portion and a whitening phenomenon of the resin film layer, which have been a problem with conventional laminate sheets using austenitic stainless steel foil. It also leads to an improvement in formability (especially workability related to deep drawing elements). As a result of detailed studies, it is necessary to set the average nitrogen concentration in the surface layer 2 nm region to 5.0% by mass or more on the foil surface on the side to be bonded to the resin film. However, if the surface layer portion is excessively nitrided, the moldability (particularly the workability related to the overhanging element) decreases, and it becomes difficult to stably obtain a sufficient molding height. Therefore, it is desirable to keep the average nitrogen concentration in the surface layer 2 nm region within a range of 9.5% by mass or less. The nitrogen concentration in the surface layer can be controlled by the conditions of final annealing performed in a non-oxidizing atmosphere.

〔Manufacturing process〕
The austenitic stainless steel foil having the above-described configuration can be manufactured using manufacturing equipment conventionally used for manufacturing stainless steel foil. Generally, it is manufactured by subjecting a steel strip of stainless steel whose components have been adjusted to intermediate annealing and cold rolling a plurality of times to obtain a foil having a predetermined thickness, and then performing final annealing in a non-oxidizing atmosphere. At that time, it is important to strictly adjust the final annealing conditions. In addition, temper rolling can also be performed as needed after the final annealing.
Specific conditions for the final annealing are as follows.

[Atmosphere gas]
As the non-oxidizing atmosphere, a hydrogen + nitrogen gas atmosphere can be applied as in the conventional BA annealing. However, it is necessary to set the nitrogen concentration in the atmosphere to a range of 15 to 30% by volume. It is more preferable to set it as 20-30 volume%. If the nitrogen concentration is too low, it is difficult to perform nitriding such that the average nitrogen concentration in the surface layer 2 nm region is 5.0% by mass or more in a range where the crystal grains are not coarsened. On the other hand, if the nitrogen concentration in the atmospheric gas is too high, nitriding becomes excessive and a foil having good moldability cannot be obtained.

[In-furnace temperature]
The furnace temperature of final annealing shall be 1000-1100 degreeC. If the temperature is lower than this, the maximum temperature of the material becomes too low, and nitriding tends to be insufficient. On the other hand, when the ambient temperature exceeds 1100 ° C., the coarsening of crystal grains is very likely to proceed. That is, when the ambient temperature is outside the range of 1000 to 1100 ° C., it is difficult to find an appropriate in-furnace time for achieving both formation of a predetermined nitride layer and prevention of crystal grain coarsening.

[In-furnace time]
As a result of various studies, in the final annealing, it is necessary to secure a holding time of 3 seconds or more at which the foil temperature (hereinafter sometimes referred to as “foil temperature”) is 980 ° C. or more. When the time during which the foil temperature is maintained in this temperature range is shorter than 3 seconds, it becomes difficult to perform stable and sufficient nitriding. In actual operation, according to the temperature rise curve peculiar to the furnace to be used, the in-furnace time (time in which the material stays in the furnace) is set so that the holding time at which the foil temperature becomes 980 ° C. or more is secured for 3 seconds or more. Just control. On the other hand, if the in-furnace time is too long, the crystal grains tend to be excessive. Therefore, it is important to set the in-furnace time in a range where the average number of crystal grains existing in the thickness direction is 5.0 or more. Although the specific in-furnace time range varies depending on the ambient temperature, an appropriate in-furnace time can be set by conducting preliminary experiments under various conditions in advance. If the in-furnace time is set so that the holding time at which the foil temperature is 980 ° C. or higher is usually 3 to 15 seconds within the above-mentioned furnace temperature range, excessive coarsening of crystal grains can be prevented.

  Stainless steel shown in Table 1 was melted, and a foil (cold rolled finish) having a thickness of 30 to 100 μm was produced in a normal process. The foil was subjected to final annealing in a non-oxidizing atmosphere under various conditions to obtain test materials.

  Each sample material was subjected to surface analysis by GDS (manufactured by Rigaku Corporation; GDA750), and the nitrogen concentration in the surface layer of 2 nm region was measured. Further, the metal structure of the L cross section was observed, and the average number of crystal grains in the thickness direction was determined in accordance with the above-described method of drawing n straight lines in the thickness direction at equal intervals from the L cross section image. Here, the number of straight lines is n = 10.

  A resin film layer was formed on the surface of each test material on the side where the nitrogen concentration was measured to obtain a laminate sheet. The resin film layer has a two-layer structure of an acid-modified polypropylene film (thickness 30 μm) and a polypropylene film (thickness 30 μm) from the stainless steel side. Using this laminate sheet, “molding height” and “heat seal strength” were measured by the following methods.

[Molding height]
Cylindrical drawing was performed under the following conditions, the limit drawing depth at which material breakage did not occur was determined, and this was taken as the molding height. At that time, the surface against which the punch was pressed was defined as the surface on the resin film layer side of the laminate sheet.
(Conditions) Blank: φ70 mm, Punch: φ40 mm, R = 2 mm, Die: R = 2 mm, Clearance: 30%, Wrinkle presser: 10 kN, Punch speed: 500 mm / min

  When a molding height of 10 mm or more is obtained under these conditions, it can be evaluated that the laminate case for a lithium ion secondary battery has good press formability. Therefore, a product having a molding height of 10 mm or more was determined to be acceptable.

[Heat seal strength]
Cylindrical squeezing was performed under the following conditions to obtain a molded body (cup with remaining ears) having a squeezing depth of 8 mm. At that time, the surface against which the punch was pressed was defined as the surface on the resin film layer side of the laminate sheet.
(Conditions) Blank: φ70 mm, Punch: φ40 mm, R = 2 mm, Die: R = 2 mm, Clearance: 30%, Wrinkle holding: 10 kN, Punch speed: 5 mm / min, Drawing depth: 8 mm

  FIG. 1 schematically shows a cross section including the central axis of the cup. A cup having the resin film layer 2 on the inside is formed by drawing a laminate sheet having the resin film layer 2 on one surface of the stainless steel foil 1. In this figure, the thicknesses of the stainless steel foil 1 and the resin film layer 2 are exaggerated (the same applies to FIG. 2 described later). An ear 3 is present on the cup.

By cutting this cup, a strip-shaped sample having a width of 15 mm was collected. At that time, the cross section shown in FIG. 1 was positioned in the center in the width direction of the strip-shaped sample. This strip-shaped sample was divided into two at the center in the longitudinal direction to obtain two samples having the ears 3. The resin film layers 2 of the ear portions 3 of these two samples were brought into close contact with each other and heat-sealed at 150 ° C. to obtain tensile test pieces for measuring heat seal strength. FIG. 2 schematically shows a cross-sectional structure of the tensile test piece. Using this test piece, a tensile test was performed until it broke at a tensile speed of 2 mm / min in the direction of the arrow in FIG. 2, and the value (N) of the maximum load recorded at that time was defined as the heat seal strength. The test was conducted with the number of tests n = 3, and the lowest value among the obtained heat seal strengths was adopted as the heat seal strength performance value for the test material. In this test, when a heat seal strength of 20 N or more is obtained, it can be evaluated that the laminate case for a lithium ion secondary battery has a good peel resistance at the heat-sealed portion. Therefore, the heat seal strength 20N or more was determined to be acceptable.
These results are shown in Table 2.

  As can be seen from Table 2, in the case of using a stainless steel foil satisfying the chemical composition, thickness, average nitrogen concentration in the surface layer 2 nm region and average number of crystal grains in the thickness direction (example of the present invention) defined in the present invention, Good molding height and heat seal strength were realized. In these samples, no whitening phenomenon during press molding was observed.

  On the other hand, in No. A5, the crystal grains became coarse due to the atmospheric temperature being too high, and the molding height was small. A6 had an inferior molding height and heat-seal strength as a result of insufficient holding time at a foil temperature of 980 ° C. or more because the ambient temperature was too low and the average nitrogen concentration in the surface layer of 2 nm was insufficient. In A7, although the atmosphere temperature was high, the crystal grains became coarse due to excessively long holding time of the foil temperature of 980 ° C. or more, and the molding height and heat seal strength were inferior. A8 was inferior in molding height and heat seal strength because the average number of crystal grains in the thickness direction was small due to the foil being too thin and the strength of the foil was insufficient. A9, B2, and C2 were all subjected to final annealing in a hydrogen atmosphere, and because the formation of the nitride layer was insufficient, the molding height and heat seal strength were inferior. In A10, since the nitrogen concentration in the annealing atmosphere was excessive, the average nitrogen concentration in the surface layer 2 nm region was too high, and the molding height was small. Since D1 and E1 employ steels each having an Md value outside the range specified in the present invention, a sufficient forming height could not be obtained. Since F1 employs steel with excessive Cu content and H1 employs steel with excessive Si, Mn, and Cu contents, these all have insufficient molding height. G1 adopted a steel having an excessively low C content, so that the strength level of the foil was lowered, and tearing occurred at the time of molding, so that a sufficient molding height could not be obtained.

1 Stainless steel foil 2 Resin film layer 3 Ear

Claims (3)

By mass%, C: 0.03 to 0.15%, Si: 1.00% or less, Mn: 1.50% or less, Ni: 5.0 to 10.0%, Cr: 15.0 to 20. 0%, Cu: 0.05 to 0.50%, Mo: 0 to 0.50%, N: 0.10% or less, remaining Fe and inevitable impurities, and represented by the following formula (1) A stainless steel foil having a composition with an Md value of 0.0 to 40.0 and a thickness of 40 to 150 μm, the average number of crystal grains existing in the thickness direction is 5.0 or more, and a surface layer of 2 nm An austenitic stainless steel foil for a lithium ion secondary battery laminate case having an average nitrogen concentration in the region of 5.0 to 9.5% by mass.
Md value = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7Cr-18.5Mo (1)   When the final annealing in a non-oxidizing atmosphere is performed on the stainless steel foil whose thickness is adjusted by cold rolling, the nitrogen concentration in the atmosphere gas is 15 to 30% by volume, and the furnace temperature is 1000 to 1100 ° C. The in-furnace time is set in a range in which the holding time at which the foil temperature is 980 ° C. or more is ensured for 3 seconds or more and the average number of crystal grains present in the thickness direction is 5.0 or more. The manufacturing method of the austenitic stainless steel foil for lithium ion secondary battery laminated cases of 1.   When the final annealing in a non-oxidizing atmosphere is performed on the stainless steel foil whose thickness is adjusted by cold rolling, the nitrogen concentration in the atmosphere gas is 15 to 30% by volume, and the furnace temperature is 1000 to 1100 ° C. The method for producing an austenitic stainless steel foil for a lithium ion secondary battery laminate case according to claim 1, wherein the holding time at which the foil temperature is 980 ° C or higher is 3 to 15 seconds.

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