Surface-treated steel foil

Abstract

[Problem] To provide a surface-treated steel foil endowed with hydrogen barrier properties suitable for a bipolar battery. [Solution] A surface-treated steel foil having a first surface and a second surface positioned on the opposite side from the first surface, wherein the surface-treated steel foil is characterized by having a base material composed of low-carbon steel or ultra-low-carbon steel, and an iron-nickel alloy layer laminated on the base material on the first surface and / or the second surface, Fe1Ni1 being included as an alloy phase in the iron-nickel alloy layer, the orientation index in X-ray diffraction of the (220) plane of the Fe1Ni1 being 1.0 or greater in the surface having the iron-nickel alloy layer, and the ratio of the maximum value of diffraction intensity of the (220) plane of the Fe1Ni1 and the maximum value of diffraction intensity of the Fe (200) plane satisfying expression (1).　Expression (1): I(Fe1Ni1 (220)) / I(Fe (200))≥0.5

Description

[Technical Field]

[0001] The present invention relates to a surface-treated steel foil particularly suitable for use in current collectors such as secondary batteries. [Background technology]

[0002] Conventionally, nickel-metal hydride batteries and lithium-ion batteries are known as secondary batteries used in vehicles and other applications. The types of electrodes for these secondary batteries include monopolar electrodes, in which a positive electrode layer or a negative electrode layer is formed on both sides of the current collector, and bipolar electrodes, in which a positive electrode layer (positive electrode active material layer) and a negative electrode layer (negative electrode active material layer) are formed on both sides of the current collector.

[0003] A bipolar battery is constructed by stacking the bipolar electrodes described above with an electrolyte, separator, etc., in between, and housing them in a single battery case. This configuration allows each electrode to be stacked in a series circuit, which reduces the internal resistance of the battery and makes it easier to increase the operating voltage and output. In addition to battery performance, compared to conventional batteries using monopolar electrodes, the number of components such as tab leads for extracting current can be omitted or reduced through battery design, thereby reducing the battery volume and weight, and thus improving the volume and gravimetric energy density of the battery.

[0004] For example, Patent Document 1 below discloses the use of metal foil such as nickel foil as a current collector in a bipolar battery. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-053401 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] While developing nickel-plated surface-treated steel foil suitable for battery applications, the inventors discovered that suppressing hydrogen permeation in the surface-treated steel foil can reduce the degradation of battery performance.

[0007] For example, in nickel-metal hydride batteries, hydrogen is used as the active material for the negative electrode, and generally, a hydrogen storage alloy is used. With conventional monopolar electrodes, it was sufficient for battery components such as the current collector to have electrolyte resistance on their surface according to the type of battery. However, in the case of bipolar electrodes as described above, hydrogen present on the negative electrode side is prone to moving through the metal material and permeating to the positive electrode side. When such permeation occurs, it becomes apparent that battery performance tends to deteriorate.

[0008] This invention has been made in view of solving such problems, and aims to provide a surface-treated steel foil with hydrogen barrier properties. [Means for solving the problem]

[0009] To solve the problems illustrated above, the surface-treated steel foil in one embodiment of the present invention is characterized in that (1) a surface-treated steel foil having a first surface and a second surface located on the opposite side of the first surface, comprising a base material made of low-carbon steel or ultra-low-carbon steel, and an iron-nickel alloy layer laminated on the base material on at least one side of the first surface and the second surface, wherein the iron-nickel alloy layer contains Fe1Ni1 as an alloy phase, and on the surface having the iron-nickel alloy layer, the orientation index in X-ray diffraction of the (220) plane of Fe1Ni1 is 1.0 or more, and the ratio of the maximum diffraction intensity of the (220) plane of Fe1Ni1 to the maximum diffraction intensity of the Fe(200) plane satisfies the following formula (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1)

[0010] In the surface-treated steel foil described in (1) above, (2) it is preferable that the ratio of the maximum diffraction intensity of the Fe (211) plane to the maximum diffraction intensity of the Fe (200) plane among the crystal planes of Fe contained in the iron-nickel alloy layer satisfies the following formula (2). I(Fe(211)) / I(Fe(200))≧1.7···(2)

[0011] Furthermore, in the surface-treated steel foil described in (1) or (2) above, (3) the substrate has an iron-nickel alloy layer on both the first surface and the second surface, and the iron-nickel alloy layer on at least one of the first surface or the second surface contains Fe1Ni1 as an alloy phase, and in the surface having the iron-nickel alloy layer containing Fe1Ni1, the orientation index in X-ray diffraction of the (220) plane of Fe1Ni1 is 1.0 or more, and the ratio of the maximum diffraction intensity of the (220) plane of Fe1Ni1 to the maximum diffraction intensity of the Fe(200) plane satisfies the following formula (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1)

[0012] In the surface-treated steel foil described in (3) above, it is preferable that the surface having the iron-nickel alloy layer containing Fe1Ni1 satisfies the following formula (3). I(Fe1Ni1(220)) / I(Fe(200))≧0.6···(3)

[0013] In any of the surface-treated steel foils described in (1) to (4) above, (5) it is preferable that the total thickness of the surface-treated steel foil is 200 μm or less.

[0014] In any of the surface-treated steel foils described in (1) to (5) above, (6) the amount of nickel deposited in the iron-nickel alloy layer is 2.22 to 26.7 g / m² per side. 2 It is preferable that this be the case.

[0015] In any of the surface-treated steel foils described in (1) to (6) above, (7) it is preferable that the steel foil further comprises a metal layer formed on the iron-nickel alloy layer, wherein the metal layer is a nickel layer.

[0016] In the surface-treated steel foil described in (7) above, (8) the total amount of nickel deposited in the iron-nickel alloy layer and the nickel layer is 2.22 to 53.4 g / m². 2 It is preferable that this be the case.

[0017] In any of the surface-treated steel foils described in (1) to (8) above, (9) the electrochemically measured hydrogen permeation current density is 55 μA / cm². 2 The following is preferable: However, hydrogen permeation current density is defined as the increase in oxidation current measured on the hydrogen detection side when a potential of -1.5V is applied to the hydrogen generation side, under conditions where the reference electrodes for the potentials of the hydrogen detection side and the hydrogen generation side are Ag / AgCl, the potential of the hydrogen detection side is +0.4V, and the electrolyte is at 65°C.

[0018] In any of the surface-treated steel foils described in (1) to (9) above, (10) it is preferable that a roughened nickel layer is formed on the outermost surface of at least one of the first surface side and the second surface side, and that the three-dimensional surface property parameter Sa of the roughened nickel layer is 0.2 to 1.3 μm.

[0019] The surface-treated steel foil of any of the above (1) to (10) is preferably for use as a current collector for a battery (11).

[0020] The surface-treated steel foil described in (11) above is preferably used for the current collector of a bipolar battery (12).

[0021] The surface-treated steel foil described in (11) or (12) above is a surface-treated steel foil having a first surface on which a hydrogen storage alloy is arranged and a second surface located on the opposite side of the first surface, comprising a base material made of low-carbon steel or ultra-low-carbon steel, and an iron-nickel alloy layer laminated on the base material on at least one side of the first surface and the second surface to suppress the permeation or diffusion of hydrogen within the surface-treated steel foil, wherein the iron-nickel alloy layer contains Fe1Ni1 as an alloy phase, and preferably, on the surface having the iron-nickel alloy layer, the orientation index in X-ray diffraction of the (220) plane of Fe1Ni1 is 1.0 or greater, and the ratio of the maximum diffraction intensity of the (220) plane of Fe1Ni1 to the maximum diffraction intensity of the Fe(200) plane satisfies the following formula (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1) [Effects of the Invention]

[0022] According to the present invention, a surface-treated steel foil with hydrogen barrier properties can be provided. [Brief explanation of the drawing]

[0023] [Figure 1(a)] This figure schematically shows the surface-treated steel foil of this embodiment. [Figure 1(b)] This figure schematically shows the surface-treated steel foil of this embodiment. [Figure 1(c)] This figure schematically shows the surface-treated steel foil of this embodiment. [Figure 2(a)] This is a schematic diagram of an apparatus for measuring the hydrogen barrier properties of the surface-treated steel foil 10 of this embodiment. [Figure 2(b)] This is a schematic diagram of an apparatus for measuring the hydrogen barrier properties of the surface-treated steel foil 10 of this embodiment. [Figure 2(c)] This is an explanatory diagram of a method for measuring the hydrogen barrier properties of the surface-treated steel foil 10 of this embodiment. [Figure 2(d)] This is an explanatory diagram of a method for measuring the hydrogen barrier properties of the surface-treated steel foil 10 of this embodiment. [Figure 2(e)]This is an explanatory diagram of a method for measuring the hydrogen barrier properties of the surface-treated steel foil 10 of this embodiment. [Figure 3] This diagram illustrates a method for calculating the thickness of the iron-nickel alloy layer in this embodiment. [Figure 4] This figure illustrates a method for calculating the thickness of an iron-nickel alloy layer using glow discharge surface spectroscopy (GDS) in this embodiment. [Figure 5] This figure schematically shows a surface-treated steel foil of another embodiment. [Figure 6(a)] This figure schematically shows a surface-treated steel foil of another embodiment. [Figure 6(b)] This figure schematically shows a surface-treated steel foil of another embodiment. [Figure 7] This figure schematically shows a surface-treated steel foil of another embodiment. [Figure 8(a)] This figure illustrates the manufacturing method for surface-treated steel foil according to this embodiment. [Figure 8(b)] This figure illustrates the manufacturing method for surface-treated steel foil according to this embodiment. [Modes for carrying out the invention]

[0024] ≪Surface-treated steel foil 10≫ The following describes embodiments for implementing the surface-treated steel foil of the present invention. Figure 1 is a schematic diagram showing one embodiment of the surface-treated steel foil 10 of the present invention. The surface-treated steel foil 10 of this embodiment can be applied to the current collector of a bipolar battery, as well as to the positive or negative electrode current collector of a monopolar battery. The battery type can be either a secondary battery or a primary battery.

[0025] The surface-treated steel foil 10 of this embodiment has a base material 20 and an iron-nickel alloy layer 30. The surface-treated steel foil 10 has a first surface 10a and a second surface 10b opposite to the first surface. When the surface-treated steel foil 10 of this embodiment is used as a battery current collector for a battery containing a hydrogen storage alloy, the hydrogen storage alloy, which will be used as the negative electrode material when the battery is assembled, is placed on the side of the first surface 10a. On the other hand, the positive electrode material is placed on the side of the second surface 10b, for example, in the case of a nickel-metal hydride battery with a bipolar electrode structure.

[0026] The surface-treated steel foil 10 of this embodiment is characterized by having an iron-nickel alloy layer 30 as described above. The iron-nickel alloy layer 30 may be arranged on the side of the second surface 10b as shown in Figure 1(a), or on either side of the first surface 10a as shown in Figure 1(b). It may also be arranged on both the side of the first surface 10a and the side of the second surface 10b as shown in Figure 1(c). Furthermore, the iron-nickel alloy layer 30 may be placed on the outermost surface of the surface-treated steel foil 10 as shown in Figures 1(a) to (c), or it may be placed inside (in the middle) of the surface-treated steel foil 10 as shown in Figure 5. The iron-nickel alloy layer 30 has the function of suppressing the permeation or diffusion of hydrogen within the surface-treated steel foil for the current collector.

[0027] <Base material 20> In this embodiment, the type of steel foil used for the base material 20 in the surface-treated steel foil 10 is preferably low-carbon steel (carbon content 0.01 to 0.15 wt%), such as low-carbon aluminum-killed steel, extremely low-carbon steel with a carbon content of less than 0.01 wt%, or non-aging extremely low-carbon steel obtained by adding Ti, Nb, etc. to extremely low-carbon steel.

[0028] The thickness of the substrate 20 used in the surface-treated steel foil 10 of this embodiment is preferably in the range of 10 μm to 200 μm. When used as a current collector for a battery, where volume and gravimetric energy density are important, the thickness is more preferably 25 μm to 100 μm, and even more preferably 10 μm to 80 μm, from the viewpoint of strength and desired battery capacity. The thickness of the substrate 20 can be measured by cross-sectional observation with an optical microscope or a scanning electron microscope (SEM).

[0029] <Iron-nickel alloy layer 30> The iron-nickel alloy layer 30 contained in the surface-treated steel foil 10 of this embodiment is an alloy layer containing iron (Fe) and nickel (Ni), and is an alloy layer containing an alloy made of iron and nickel (also referred to as "iron-nickel alloy" or "Fe-Ni alloy"). The state of this iron-nickel alloy may be a solid solution, eutectoid / eutectic, or compound (intermetallic compound), or these may coexist.

[0030] The iron-nickel alloy layer 30 contained in the surface-treated steel foil 10 of this embodiment may contain other metallic elements and unavoidable impurities, as long as the problems of the present invention are solved. For example, the iron-nickel alloy layer 30 may contain metallic elements such as cobalt (Co) and molybdenum (Mo), and additive elements such as boron (B). The proportion of metallic elements other than iron (Fe) and nickel (Ni) in the iron-nickel alloy layer 30 is preferably 10% by weight or less, more preferably 5% by weight or less, and even more preferably 1% by weight or less. Since the iron-nickel alloy layer 30 may be a binary alloy composed substantially only of iron and nickel, the lower limit of the content of other metallic elements excluding unavoidable impurities is 0% by weight. The types and amounts of other metallic elements present can be measured by known means such as X-ray fluorescence (XRF) analyzers and GDS (glow discharge surface spectroscopy).

[0031] The iron-nickel alloy layer 30 contained in the surface-treated steel foil 10 of this embodiment is formed through the following steps: forming a nickel plating layer on a base plate to create a nickel-plated material (nickel plating step), applying heat treatment to the nickel-plated material (first heat treatment step), rolling the nickel-plated material after heat treatment (first rolling step), and applying a second heat treatment (second heat treatment step), in this order. Furthermore, the rolling process in the "first rolling process" described above will also be referred to as "re-rolling" to distinguish it from the rolling of the base material (cold rolling from a hot coil). Furthermore, the heat treatment in the "second heat treatment process" described above will also be simply referred to as the "second heat treatment."

[0032] After the second heat treatment process, a second rolling process (second rolling process) may be performed, in which the rolling process does not deviate from the constituent range of equation (1) described later. Nickel plating methods include, for example, electrolytic plating, electroless plating, hot-dip plating, and dry plating. Of these, electrolytic plating is particularly preferred from the viewpoint of cost and film thickness control. The method for manufacturing the surface-treated steel foil of this embodiment will be described in detail later.

[0033] The surface-treated steel foil 10 of this embodiment is characterized in that (a) Fe1Ni1 is included as an alloy phase in the iron-nickel alloy layer 30, (b) the orientation index in X-ray diffraction of the (220) plane of Fe1Ni1 on the surface having the iron-nickel alloy layer 30 is 1.0 or more, and (c) the ratio of the maximum diffraction intensity of the (220) plane of Fe1Ni1 to the maximum diffraction intensity of the Fe(200) plane satisfies the following formula (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1) The following describes the characteristics of (a), (b), and (c) above.

[0034] The first characteristic is that, as a result of the nickel plating, first heat treatment, and re-rolling steps in the manufacturing process described above, the iron-nickel alloy layer 30 formed in the subsequent second heat treatment has a greater presence of crystals of a specific orientation compared to an alloy layer formed by nickel plating and heat treatment alone. Specifically, when X-ray diffraction is performed, the orientation index of the (220) plane is higher. (Characteristic (a) above)

[0035] The second feature is that, in a surface-treated steel foil having an iron-nickel alloy layer 30 with a high orientation index of the (220) plane, the iron-nickel alloy layer 30 further contains an alloy phase with a Fe1Ni1 crystal structure. (Feature (a) above)

[0036] The third feature, which will be described in detail later, is that the (220) plane of Fe1Ni1 is sufficiently present relative to the (200) plane of Fe. This configuration makes it possible to achieve the hydrogen barrier properties suitable for bipolar batteries, which is the objective of the present invention. (Feature (c) above) The reason why, in this embodiment, the iron-nickel alloy layer 30 is specified to contain an alloy phase with a Fe1Ni1 crystal structure is as follows.

[0037] In the process of repeatedly conducting experiments to improve battery performance, the inventors discovered the occurrence of an unexplained voltage drop (self-discharge) phenomenon, and found that suppressing hydrogen permeation in the surface-treated steel foil 10 is effective in resolving this phenomenon. The cause of hydrogen permeation and the reason why the above-mentioned voltage drop (self-discharge) phenomenon can be suppressed by inhibiting hydrogen permeation in the surface-treated steel foil 10 are still unclear, but the inventors have made the following predictions.

[0038] In other words, in this embodiment, when the surface-treated steel foil 10 is used as an electrode for a bipolar battery, the hydrogen storage alloy used as the negative electrode material is placed on at least one side of the surface-treated steel foil 10 (the side of the first surface 10a in the embodiment shown in Figure 1), and the positive electrode material is placed on the opposite side. In this case, a hydrogen-rich environment (negative electrode) and a hydrogen-scarce environment (positive electrode) exist on either side of the surface-treated steel foil 10, resulting in a hydrogen concentration gradient. It was then anticipated that hydrogen would permeate and move through the surface-treated steel foil 10 due to some trigger, causing the permeated hydrogen to react at the positive electrode and resulting in the voltage drop (self-discharge) described above.

[0039] The inventors then obtained various surface-treated steel foils having an iron-nickel alloy layer 30 by changing the plating conditions, rolling conditions, heat treatment conditions, etc. Furthermore, they measured the hydrogen permeation current density (oxidation current value) of each steel foil and analyzed the content of metal elements and the structure of the alloy. Through diligent research and repeated experiments, the inventors discovered that by ensuring a certain level of alloy phase in the Fe1Ni1 crystal structure, it is possible to obtain a surface-treated steel foil with highly stable hydrogen barrier properties, thereby solving the aforementioned hydrogen permeability problem. The reason why the Fe1Ni1 alloy phase in the iron-nickel alloy crystal structure contributes significantly to hydrogen barrier properties is thought to be that the structure of this alloy phase has a low porosity and narrow hydrogen pathways, and that it contains a high density of lattice strain due to the difference in atomic radii between Fe and Ni, resulting in the presence of numerous hydrogen trapping sites. As a result, it is believed that including a large amount of this alloy phase in the iron-nickel alloy layer 30 significantly improves the hydrogen barrier properties of the surface-treated steel foil. Furthermore, the inventors focused even more on the crystal structure of Fe1Ni1 and predicted that orienting the iron-nickel alloy layer to the Fe1Ni1(220) plane, rather than the Fe1Ni1(200) plane which is dominant after nickel plating and heat treatment, would complicate the hydrogen pathways and improve hydrogen barrier properties. To achieve this, they attempted to roll the iron-nickel alloy layer to orient it to the Fe1Ni1(220) plane.

[0040] However, it has been found that when the nickel plating, heat treatment, and re-rolling processes are carried out in this order, the hydrogen barrier properties that should be obtained by Fe1Ni1 formation may decrease. As a result of our diligent research, we have found the following. First, when we investigated the cause of the decrease in hydrogen barrier properties, we found that when hydrogen barrier properties are obtained by the iron-nickel diffusion layer formed by heat treatment after nickel plating, the hydrogen barrier properties decrease when there is a large amount of iron that is exposed when the iron-nickel diffusion layer cracks during re-rolling, or when a large amount of iron is exposed in a way that penetrates the iron-nickel diffusion layer. It should be noted that such a decrease in hydrogen barrier properties cannot occur if iron is detected on the surface due to the formation and diffusion by heat treatment without going through re-rolling, so it is thought to occur during the re-rolling process. Focusing on this point, we repeated experiments and found that it depends on the state before re-rolling, that is, the composition of the soft nickel and iron-nickel diffusion layer formed by nickel plating and heat treatment, but it is especially likely to occur when the reduction ratio during re-rolling is high. Furthermore, it was found that even if iron exposure is suppressed to some extent, hydrogen barrier properties still decrease if the Fe1Ni1 alloy phase formation is insufficient during the heat treatment and the second heat treatment. Conversely, even if iron is exposed, good hydrogen barrier properties can be obtained if the degree of exposure is such that the exposed iron can be sufficiently alloyed with the surrounding FeNi during the second heat treatment. In addition, it was found that in order to obtain even better hydrogen barrier properties in surface-treated steel foil obtained through nickel plating, heat treatment, and re-rolling, it is important to suppress the reduction ratio during re-rolling to control the orientation of iron, and to ensure that the structure has a sufficient Fe1Ni1 alloy phase corresponding to the reduction ratio.

[0041] The presence of Fe1Ni1 in the iron-nickel alloy layer 30 can be confirmed using X-ray diffraction (XRD) measurements. Specifically, if a diffraction intensity is obtained at a diffraction angle 2θ = 75.1 ± 0.11°, the presence of the crystal plane (220) in the crystal structure of Fe1Ni1 contained in the iron-nickel alloy layer 30 can be confirmed, indicating that it contains an alloy phase of the Fe1Ni1 crystal structure.

[0042] Furthermore, in this embodiment, the orientation index of the X-ray diffraction of the Fe1Ni1(220) plane among the crystal planes of Fe1Ni1 contained in the iron-nickel alloy layer 30 is 1.0 or greater, and the ratio of the maximum diffraction intensity of the Fe1Ni1(220) plane to the maximum diffraction intensity of the Fe(200) plane in X-ray diffraction satisfies the following equation (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1) Here, "obtaining diffraction intensity at diffraction angle 2θ = 75.1 ± 0.11°" is defined as "the maximum diffraction intensity at diffraction angle 2θ = 75.1 ± 0.11° being 2.0 times or more the average diffraction intensity at diffraction angle 2θ = 86 ± 0.5°." In other words, the diffraction intensity at diffraction angle 2θ = 86 ± 0.5° is unaffected by either iron or nickel in a sample obtained by forming nickel plating on a steel plate. Therefore, if a diffraction intensity of 2.0 times or more the average diffraction intensity at diffraction angle 2θ = 86 ± 0.5° is obtained at diffraction angle 2θ = 75.1 ± 0.11° in X-ray diffraction measurement, it can be understood that a crystal plane (220) exists in the crystal structure of Fe1Ni1 contained in the iron-nickel alloy layer 30.

[0043] When the above formula (1) is satisfied, it is preferable because it can suppress the decrease in hydrogen barrier properties associated with the exposure of iron that occurs during the aforementioned re-rolling process. As a result, when the surface-treated steel foil 10 is used as a current collector for a bipolar battery, it is possible to obtain excellent hydrogen barrier properties. From the viewpoint of hydrogen barrier properties, it is more preferable that the ratio represented by the above formula (1) is 0.6 or more on at least one side of the surface-treated steel foil. In other words, it is preferable that the following formula (3) is satisfied. I(Fe1Ni1(220)) / I(Fe(200))≧0.6···(3) More preferably, the ratio represented by formula (1) above is 0.8 or more on at least one side of the surface-treated steel foil. Furthermore, from the viewpoint of more stable hydrogen barrier properties, it is preferable that the surface-treated steel foil of this embodiment has an iron-nickel alloy layer 30 on both the first and second surfaces. It is sufficient that formula (1) is satisfied on at least one side when the iron-nickel alloy layer 30 is present on both sides, but it is even more preferable that formula (3) is satisfied on at least one side. There is no particular upper limit to the ratio expressed by formula (1) or (3) above, but considering the thickness and strength balance between the iron-nickel alloy layer and the base iron, it is preferable that it be less than 10. By setting it to less than 10, it becomes possible to control the mechanical properties of the current collector surface-treated steel foil by controlling the state of the base iron, making it easier to control. On the other hand, when the above ratio is 10 or more, a thick iron-nickel alloy layer, which is harder than the base iron, is formed, and it is thought that the iron-nickel alloy layer is more likely to influence the mechanical properties of the current collector surface-treated steel foil.

[0044] In equations (1) or (3) above, "I(Fe1Ni1(220))" represents the maximum diffraction intensity obtained at the diffraction angle 2θ = 75.1 ± 0.11° in the above X-ray diffraction measurement. The diffraction intensity obtained at this diffraction angle represents the (220) plane of Fe1Ni1. (Based on 01-071-8322 in the ICDD PDF-2 2014 database)

[0045] Furthermore, "I(Fe(200))" refers to the maximum diffraction intensity obtained at the diffraction angle 2θ = 65.02 ± 0.11° in X-ray diffraction measurements. The diffraction intensity obtained at the above diffraction angle represents the (200) plane of iron (Fe). (Based on 01-071-3763 from the ICDD PDF-2 2014 database)

[0046] In this application, the reason for using the diffraction intensity ratio between the diffraction intensity of the (220) plane of Fe1Ni1 and the diffraction intensity of the (200) plane of iron (Fe) as an indicator of the hydrogen barrier properties of Fe1Ni1 is as follows: Specifically, as a result of diligent research through repeated experiments, the inventors focused on the fact that when a surface-treated steel foil having an iron-nickel alloy layer is obtained through the aforementioned processes of nickel plating, first heat treatment, re-rolling, and second heat treatment, the diffraction intensity originating from the (200) plane of iron is influenced by the rolling conditions of re-rolling and the conditions of the second heat treatment. By indexing this with the above formula, they found that the resulting value is correlated with the hydrogen barrier properties.

[0047] The Fe(211) plane is the matrix of the original rolled texture of iron, and test samples have confirmed that the Fe(211) plane has a higher orientation index. However, no index was found in the diffraction intensity of the Fe(211) plane that correlates with hydrogen barrier properties. This is probably because the diffraction intensity of the Fe(211) plane is more influenced by the recovery of the iron itself, i.e., the carbon steel substrate, from processing, than by the decrease caused by alloying iron and nickel. Therefore, the inventors used Fe(200) as an index to observe the degree of iron exposure, using the ratio of Fe1Ni1 in the iron-nickel alloy phase to the (220) plane.

[0048] If the left-hand side of equation (1) above is too small, it is thought that this is due to an excessive reduction ratio during re-rolling or insufficient heat treatment during the second heat treatment, resulting in a large remaining portion of exposed iron and thus a decrease in hydrogen barrier properties. If the left-hand side is 0.5 or greater, that is, as in this embodiment, by controlling the reduction ratio in re-rolling and performing sufficient heat treatment in the second heat treatment, it is considered that the exposure of iron in the first place can be suppressed, or even if some iron is exposed, the decrease in hydrogen barrier properties can be suppressed by alloying the iron near the surface with the surrounding iron-nickel alloy layer during the second heat treatment. In addition, if the reduction ratio in the recoiling is too high, even if the secondary heat treatment is sufficiently performed, it is considered that the exposure of iron cannot be sufficiently mitigated and the hydrogen barrier property will deteriorate.

[0049] In addition, when manufactured by the manufacturing steps as described above, since it is characteristic that the orientation index of the X-ray diffraction of the (220) plane of Fe1Ni1 becomes 1.0 or more, the (220) plane of Fe1Ni1 is used as an index for looking at the degree of exposure of iron. Particularly when rolling to less than 100 μm, which is very thin, and making the final thickness of the surface-treated steel foil less than 100 μm, it shows a strong orientation of 2.0 or more. In addition, there is no particular limitation on the upper limit, and it is usually less than 6.0.

[0050] The crystal orientation index Ico_Fe1Ni1(220) of the X-ray diffraction of the (220) plane of Fe1Ni1 was defined and calculated by the following formula. The subscript co means crystal orientation. Ico_Fe1Ni1(220)= [I_Fe1Ni1(220) / [I_Fe1Ni1(111)+I_Fe1Ni1(200)+I_Fe1Ni1(220)+I_Fe1Ni1(311)+I_Fe1Ni1(222)]] / [I S _Fe1Ni1(220) / [I S _Fe1Ni1(111)+I S _Fe1Ni1(200)+I S _Fe1Ni1(220)+I S _Fe1Ni1(311)+I S _Fe1Ni1(222)]] Here, the diffraction intensity of each crystal plane of Fe1Ni1 measured by X-ray diffraction is expressed as follows. I_Fe1Ni(111): The diffraction intensity of the Fe1Ni1(111) crystal plane measured by X-ray diffraction I_Fe1Ni(200): The diffraction intensity of the Fe1Ni1(200) crystal plane measured by X-ray diffraction I_Fe1Ni(220): The diffraction intensity of the Fe1Ni1(220) crystal plane measured by X-ray diffraction I_Fe1Ni1(311): Diffraction intensity of Fe1Ni1(311) crystal planes measured by X-ray diffraction I_Fe1Ni1(222): Diffraction intensity of Fe1Ni1(222) crystal planes measured by X-ray diffraction

[0051] The diffraction intensity referred to here is the maximum value of the diffraction intensity (cps) measured within the range of ±0.11° of each diffraction angle (2θ) as described in JCPDS (Joint Committee on Powder Diffraction Standards, PDF card number: 01-071-8322). Specifically, the (111) plane is the maximum value within the range of 43.83°±0.11°, the (200) plane is the maximum value within the range of 51.05°±0.11°, the (220) plane is the maximum value within the range of 75.10±0.11°, the (311) plane is the maximum value within the range of 91.23±0.11°, and the (222) plane is the maximum value within the range of 96.56±0.11°.

[0052] Next, the standard diffraction peak intensity values ​​(I) at each crystal plane of Fe1Ni1 S _Fe1Ni1(111), I S _Fe1Ni1(200), I S _Fe1Ni1(220), I S _Fe1Ni1(311), I S The value of _Fe1Ni1(222)) can be used as specified in JCPDS (Joint Committee on Powder Diffraction Standards, PDF card number: 01-071-8322). The subscript 's' stands for Standard.

[0053] Furthermore, in order to improve hydrogen barrier properties by creating a crystal structure that is not preferentially oriented only on the Fe1Ni1(200) plane but also oriented on the Fe1Ni1(220) plane, the ratio of the orientation index of (220) and the crystal orientation index of the Fe1Ni1(200) plane, calculated in the same manner as above, to Ico_Fe1Ni1(220) / Ico_Fe1Ni1(200) is preferably 1.0 to 5.0, more preferably 1.0 to 4.0, and even more preferably 1.5 to 3.5. In addition, from the perspective of not oriented too much on the (220) plane, Ico_Fe1Ni1(200) is preferably 1.0 to 2.5, and more preferably 1.0 to 2.0. The crystal orientation index Ico_Fe1Ni1(200) for X-ray diffraction of the (200) plane of Fe1Ni1 is defined and calculated by the following formula. The subscript co stands for crystal orientation. Ico_Fe1Ni1(200)= [I_Fe1Ni1(200) / [I_Fe1Ni1(111)+I_Fe1Ni1(200)+I_Fe1Ni1(220)+I_Fe1Ni1(311)+I_Fe1Ni1(222)]] / [I S _Fe1Ni1(200) / [I S _Fe1Ni1(111)+I S _Fe1Ni1(200)+I S _Fe1Ni1(220)+I S _Fe1Ni1(311)+I S _Fe1Ni1(222)]]

[0054] Furthermore, surface-treated steel foil obtained after nickel plating and heat treatment without undergoing a re-rolling process or a second heat treatment process has an orientation index of approximately 0.35 to 0.85 for the (220) plane of Fe1Ni1, regardless of whether the nickel plating was done in a Watt bath or a sulfamic acid bath.

[0055] Furthermore, in the surface-treated steel foil 10 of this embodiment, it is preferable that the ratio of the maximum diffraction intensity of the Fe(211) plane to the maximum diffraction intensity of the Fe(200) plane in X-ray diffraction satisfies the following equation (2). I(Fe(211)) / I(Fe(200))≧1.7···(2)

[0056] The reason why the properties of the surface-treated steel foil 10 of this embodiment are better represented by the above formula (2) is as follows. That is, iron crystals have a BCC structure, and the orientation that becomes preferred by rolling is the Fe{211} plane, and this crystal orientation does not decrease easily even after a second heat treatment. On the other hand, in iron, the Fe{200} plane is an orientation that is easily affected by the rolling conditions of re-rolling and the conditions of the second heat treatment, as described above, and specifically it is an orientation that is easily oriented during rolling and easily decreased during the second heat treatment. Therefore, in the surface-treated steel foil 10 of this embodiment, when the iron-nickel alloy layer 30 has gone through the rolling process, the state of the steel foil satisfies both the above formula (1) and the above formula (2), which means that it has not been over-rolled by re-rolling and has recovered in the second heat treatment, and that hydrogen barrier properties can be stably obtained. From the viewpoint of obtaining hydrogen barrier properties more stably, it is more preferable that "I(Fe(211)) / I(Fe(200))≧2.0" is satisfied. There is no particular upper limit to the ratio expressed by formula (2) above, but it is preferable to have a value of less than 10 from the viewpoint of the strength of the surface-treated steel foil.

[0057] In equation (2) above, "I(Fe(211))" refers to the maximum intensity obtained at the diffraction angle 2θ = 82.33 ± 0.11° during X-ray diffraction measurement. The peak obtained at this diffraction angle represents the (211) plane of iron (Fe). (Based on 01-071-3763 from the ICDD PDF-2 2014 database).

[0058] In this embodiment, the iron-nickel alloy layer 30 may contain, in addition to the alloy phase with a Fe1Ni1 crystal structure, an alloy phase with a Fe1Ni3 and / or Fe3Ni2 crystal structure, etc.

[0059] The above-mentioned X-ray diffraction (XRD) measurement was performed using the X-ray diffraction method with CuKα as the radiation source, and the diffraction intensity was expressed in cps.

[0060] In this embodiment, in order to suppress the voltage drop (self-discharge) described above, the surface-treated steel foil 10 of this embodiment has an electrochemically measured hydrogen permeation current density (oxidation current value) of 55 μA / cm². 2 The following conditions are preferable. The measurement conditions for hydrogen permeation current density (oxidation current value) are as follows: in an electrolyte at 65°C, the cathode side potential is -1.5V and the anode side potential is +0.4V.

[0061] Here, we will explain the evaluation of hydrogen barrier properties. As described above, when hydrogen permeates and moves through the surface-treated steel foil 10, hydrogen atoms that reach the hydrogen detection side from the hydrogen intrusion side are oxidized to hydrogen ions. The value of the oxidation current at this time increases or decreases depending on the amount of hydrogen that reaches the hydrogen detection surface, so it is possible to quantify and evaluate the hydrogen barrier properties of the surface-treated steel foil 10 by the detected current value. (Toru Mizunaga, Tokyo Institute of Technology, Materials and Environment, 63, 3-9 (2014), Measurement of hydrogen intrusion and permeation into steel by electrochemical method) Based on the above predictions, the inventors conducted measurements and evaluations and found that in this embodiment, in order to more stably suppress the occurrence of the voltage drop (self-discharge) described above, the surface-treated steel foil 10 of this embodiment has a hydrogen permeation current density obtained from the electrochemically measured oxidation current of 55 μA / cm². 2 The conclusion was reached that the following is preferable. In this embodiment, the measurement conditions for hydrogen permeation current density are as follows: in an electrolyte solution at 65°C, the reference electrode is Ag / AgCl (silver chloride), the potential on the hydrogen generation side is -1.5V, and the potential on the hydrogen detection side is +0.4V. Note that all potential values ​​used in the hydrogen permeation current density measurement method in this embodiment are based on the reference electrode being Ag / AgCl.

[0062] As a specific example of the hydrogen permeation current density measurement method in this embodiment, the hydrogen barrier properties of the surface-treated steel foil 10 can be quantified and evaluated by detecting the current value (current density) using a measuring device configured as shown in Figure 2(a). The measuring device shown in Figure 2(a) will be described below. In the following description, the hydrogen intrusion side is also referred to as the hydrogen generation side, and is the side on which the hydrogen storage alloy is placed, i.e., the first surface 10a of the surface-treated steel foil 10. The hydrogen detection side is the opposite side of the hydrogen intrusion side, and is the positive electrode side of the bipolar electrode structure, i.e., the second surface 10b of the surface-treated steel foil 10.

[0063] Two cells are prepared: cell X for hydrogen generation and cell Y for detecting permeated hydrogen. A test piece (sample) of surface-treated steel foil 10 is placed between these two measurement cells. Each measurement cell contains an alkaline aqueous solution (alkaline electrolyte), and reference electrodes (RE1 and RE2) and counter electrodes (CE1 and CE2) are immersed in it. An Ag / AgCl electrode made of saturated KCl solution is used as the reference electrode, and platinum (Pt) is used as the counter electrode. The composition of the alkaline electrolyte consists of KOH, NaOH, and LiOH, and the liquid temperature is 65°C. As shown in Figure 2(b), the measurement diameter of the surface-treated steel foil 10 is φ20 mm (measurement area 3.14 cm²). 2 ) Potential control and current measurement on the hydrogen intrusion side and hydrogen detection side are performed using potentiostats as shown in Figure 2(a). For example, the "Multi-Electrochemical Measurement System HZ-Pro" manufactured by Hokuto Denko Co., Ltd. can be used as the potentiostat. The sample of surface-treated steel foil 10 to be evaluated and the connection of each electrode can be performed as shown in Figure 2(a).

[0064] On the hydrogen generation side, the sample is polarized to a cathode (low potential), generating hydrogen on the sample surface and allowing it to penetrate. The potential is applied in steps of -0.7V, -1.1V, and -1.5V, and each potential is applied for 15 minutes. The reason for applying the potential in steps is to minimize the effects of potential changes and obtain a stable plot. The measurement plot is taken every 5 seconds.

[0065] Generally, in nickel-metal hydride batteries using a nickel hydroxide compound as the positive electrode and a hydrogen storage alloy as the negative electrode, the operating potential of the negative electrode during the battery's charge-discharge reaction is around -1.1V. In the measurement method applicable to this embodiment, as a method that can confirm the effect of hydrogen barrier properties without using a hydrogen storage alloy, measurement conditions that generate hydrogen more significantly were investigated. Then, the hydrogen permeation current density I (μA / cm²) was calculated. 2 To calculate the change in oxidation current (hereinafter also referred to as the change in oxidation current) when the applied potential on the hydrogen generation side is -1.5V, we decided to use that value.

[0066] On the hydrogen detection side, when hydrogen atoms permeate from the hydrogen generation side, these permeated hydrogen atoms are oxidized on the hydrogen detection side, generating an oxidation current that can be measured by the potentiostat on the hydrogen detection side. Therefore, this change in oxidation current makes it possible to quantify and evaluate the hydrogen permeability of the surface-treated steel foil 10. On the hydrogen detection side, a potential is applied and maintained to promote the oxidation of hydrogen atoms to hydrogen ions and to clarify the peak of the oxidation current.

[0067] In nickel-metal hydride batteries, which use a nickel hydroxide compound as the positive electrode and a hydrogen storage alloy as the negative electrode, the positive electrode operating potential during the battery's charge-discharge reaction is generally around +0.4V. Therefore, in this measurement method, a potential of +0.4V was applied to the detection side and maintained throughout the measurement. Before applying the hydrogen to the hydrogen generation side, the hydrogen detection side was held at the aforementioned potential for 60 minutes to stabilize the current value. After the hydrogen generation application was completed, that is, after the 15-minute application of -1.5V was terminated and the hydrogen generation side was set to zero, the hydrogen detection side was held to apply +0.4V for 5 minutes to calculate the background. Measurement plots were taken every 5 seconds. In other words, as a preliminary step to the evaluation by the above measurement, the process begins by applying +0.4V to the hydrogen detection side, then stabilizing the current value by applying the voltage for 60 minutes, and finally starting the application on the hydrogen generation side as the actual evaluation (15 minutes at each potential, for a total of 45 minutes).

[0068] From the change in oxidation current on the hydrogen detection side obtained by the above method, the hydrogen permeation current density I (μA / cm²) can be calculated. 2This makes it possible to calculate the oxidation current and hydrogen permeation current density I (μA / cm²). 2 Figures 2(c) to 2(e) show a numerical representation of the value.

[0069] Figure 2(c) shows the total current value measurement including the pre- and post-processing steps for evaluation. Figure 2(d) shows the change in current value for the actual evaluation, and is an enlarged view of the area from around 5300 seconds to 6500 seconds in Figure 2(c). Figure 2(e) is shown for comparison with this embodiment, and shows the change in current value when the same current value measurement as in Figure 2(c) is performed using a surface-treated steel foil with a thickness of 50 μm and a nickel plating layer of 1.0 μm thickness, without heat treatment, i.e., without the iron-nickel alloy layer. According to Figure 2(e), it can be confirmed that the detection-side current value during the application of -1.5V for 15 minutes is clearly higher than that of the metal foil shown in Figure 2(c) in the surface-treated steel foil without the iron-nickel alloy layer, which is a characteristic of this embodiment.

[0070] In this embodiment, the hydrogen permeation current density I (μA / cm²) 2 The following formula can be used to calculate the oxidation current change when the applied potential on the hydrogen generation side is -1.5V, as shown in Figure 2(d). Hydrogen permeation current density I (μA / cm 2 ) = ((Average value of oxidation current from Ib to Ic) / S) — ((Average value of Ia and Id) / S) However, Ia(μA) is the oxidation current 5 seconds before -1.5V is applied, Ib(μA) is the oxidation current 155 seconds after the start of -1.5V application, Ic(μA) is the oxidation current at the end of -1.5V application, Id(μA) is the oxidation current 155 seconds after the end of -1.5V application, S(cm 2 The measurement area (evaluation area) of the test specimen is defined as follows. The hydrogen permeation current density I (μA / cm²) calculated using the above formula 2 When the hydrogen permeation current density (I) is small, hydrogen permeation is suppressed, meaning that the hydrogen barrier property is high, and the hydrogen permeation current density (I) is low (μA / cm²). 2 A larger ) indicates that hydrogen permeability is easier.

[0071] In this embodiment, the hydrogen permeation current density measured electrochemically as described above is 55 μA / cm². 2 We concluded that the surface-treated steel foil 10 is suitable for bipolar electrodes from the viewpoint of more stable hydrogen barrier properties when the following conditions are met: 30 μA / cm² from the viewpoint of further suppressing voltage drop. 2 More preferably, the following is true: 20 μA / cm² 2 It is even more preferable that the following conditions are met: 15 μA / cm² 2 The following is particularly preferable. However, the hydrogen permeation current density is the increase in oxidation current measured at the hydrogen detection side (anode side) when a potential of -1.5V is applied to the hydrogen generation side (cathode side) under the condition that the potential of the hydrogen detection side is +0.4V (vs Ag / AgCl) in an electrolyte at 65°C. If no increase in oxidation current is detected, the hydrogen permeation current density is 0 (zero).

[0072] It is generally known that different types of metallic materials have different hydrogen diffusion coefficients. Depending on the application of the metallic material, there may be a need for a metallic material that suppresses hydrogen intrusion in order to suppress defects and hydrogen embrittlement phenomena caused by hydrogen in the metal. Examples include the use of high-alloy steel to suppress delayed fracture of high-strength bolts, and the use of titanium welded members to suppress cracking in pressure reaction vessels. However, these materials and applications do not anticipate hydrogen intrusion under conditions where the amount of hydrogen is actively increased, such as when hydrogen storage alloys are placed on the surface. Furthermore, the challenge with these technologies is that hydrogen remaining in the metal affects the mechanical properties of the metal itself, but the problem of hydrogen permeating through the metal material and affecting the opposite side does not occur. Furthermore, regarding hydrogen permeability in battery components, it is known that, for example, hydrogen impermeability is required as gas impermeability in the separator of a fuel cell. However, in fuel cells, hydrogen permeability is mainly a problem with carbon separators, and it was thought that there was no hydrogen permeability and therefore no problem when stainless steel or aluminum separators were used. In addition, corrosion resistance in a sulfuric acid atmosphere is essential for fuel cell separators, and steel plates are difficult to apply, so no problems had been identified assuming the application of steel plates. On the other hand, it was found that in current collectors with a bipolar electrode structure in which one side of the current collector is the negative electrode active material layer and the other side is the positive electrode active material layer, hydrogen permeation is more likely to occur compared to fuel cells, and this can affect battery performance. This is thought to be a problem that was discovered precisely because the battery structure, target parts, internal environment, etc., are different from those of fuel cells.

[0073] The voltage drop caused by hydrogen permeation, as described above, is thought to accelerate as the conditions under which hydrogen permeation is more favorable in the battery's operating environment increase, leading to a shorter time until the voltage drop occurs, and thus a faster deterioration of battery performance. It is thought that hydrogen permeation becomes easier as the hydrogen concentration gradient increases. In addition to the hydrogen concentration gradient, it is thought that hydrogen permeation is further promoted when voltage is applied to both sides of the surface-treated steel foil. In other words, in batteries using hydrogen storage alloys, batteries with high concentration gradients such as nickel-metal hydride batteries, and secondary batteries with frequent charging and discharging, hydrogen permeation may be one of the causes of the gradual decline in battery performance over time. On the other hand, the gradual decline in battery performance is also greatly influenced by other factors, and the phenomenon of hydrogen permeation is difficult to grasp. Therefore, it had not been clarified in the use and development of conventional monopolar batteries. However, through repeated experiments in the development of surface-treated steel foil for bipolar batteries, the inventors realized that improving the hydrogen barrier properties of the iron-nickel alloy layer contributes to suppressing the deterioration of battery performance. Therefore, the surface-treated steel foil of this embodiment is particularly suitable for use as a current collector in bipolar batteries, especially batteries using hydrogen storage alloys. However, even in batteries that do not use other hydrogen storage alloys, if the battery contains hydrogen or generates hydrogen, there is a possibility of gradual deterioration of battery performance due to hydrogen permeation, which has not been previously recognized, and the surface-treated steel foil of this embodiment can be suitably used. For example, in alkaline secondary batteries, the battery components are almost the same as in nickel-metal hydride batteries, except that nickel-zinc batteries use zinc for the negative electrode and nickel-cadmium batteries use cadmium for the negative electrode, and they use an alkaline electrolyte mainly composed of potassium hydroxide. They also have the characteristic of easily generating hydrogen on the negative electrode side. Therefore, although not to the same extent as nickel-metal hydride batteries, which store a large amount of hydrogen within the hydrogen storage alloy, when these batteries are bipolar batteries with a bipolar structure, hydrogen movement between the front and back surfaces of the current collector may occur, and similarly, it is thought that battery performance is likely to deteriorate due to hydrogen permeation. Accordingly, the surface-treated steel foil of this embodiment can be suitably used in bipolar alkaline secondary batteries as well.

[0074] Furthermore, from the viewpoint of suppressing hydrogen permeation as described above, the thickness of the iron-nickel alloy layer 30 contained in the surface-treated steel foil 10 of this embodiment is preferably 1.0 μm or more, and more preferably 1.6 μm or more.

[0075] In this embodiment, the method for calculating the thickness of the iron-nickel alloy layer 30 will be described. In this embodiment, the method for calculating the thickness of the iron-nickel alloy layer 30 is to perform quantitative analysis of Ni and Fe at a depth of 10 μm from the surface in the thickness direction using SEM-EDX (energy-dispersive X-ray spectroscopy).

[0076] An example of how to obtain the thickness of the iron-nickel alloy layer 30 from a graph obtained by SEM-EDX is shown. In the graph in Figure 3, the horizontal axis represents the distance in the depth direction from the surface (μm), and the vertical axis represents the X-ray intensity of Ni and Fe. The graph in Figure 3 shows that the shallower parts in the thickness direction have a high nickel content and a low iron content. On the other hand, the iron content increases as the thickness increases.

[0077] In this embodiment, in the portion before and after the intersection of the nickel curve and the iron curve, the distance between 1 / 10 of the maximum values ​​of nickel and iron respectively is defined as the iron-nickel alloy layer 30, and its thickness can be read from the graph.

[0078] While there is a known method for measuring the thickness of the iron-nickel alloy layer using GDS, as shown in Figure 4, in this embodiment, GDS cannot accurately measure the thickness when a roughened nickel layer is provided on top of the iron-nickel alloy layer 30, as described later. Therefore, the SEM-EDX measurement method described above is recommended. In this embodiment, the second heat treatment promotes alloying of the exposed iron on the surface and provides a sufficient amount of Fe1Ni1. As a result, it was confirmed that the region of the iron-nickel alloy layer where Ni is 5-50% by mass, as measured by GDS, becomes significantly thicker after the second heat treatment process, exceeding 80% of the thickness of the region above it. Furthermore, while a certain level of hydrogen barrier performance can be obtained when the iron-nickel alloy layer thickness is 1.0 μm or more, the problem of this invention is that, when the process includes re-rolling, the expected improvement in hydrogen barrier performance due to the increase in the iron-nickel alloy layer thickness cannot be obtained. In other words, as described above, the partially exposed iron is not uniformly present on the surface but is localized, so even if the average value of the overall iron-nickel alloy layer thickness is 1.0 μm or more, the thickness measured by GDS or EDS alone cannot control the exposure of iron, and therefore the problem of this invention cannot be anticipated or solved.

[0079] In the surface-treated steel foil 10 of this embodiment, the amount of nickel deposited in the iron-nickel alloy layer 30 is 2.2 to 26.7 g / m². 2 This is preferable from the viewpoint of hydrogen barrier properties and electrolyte resistance suitable for bipolar electrodes. Furthermore, in the surface-treated steel foil 10 of this embodiment, the iron-nickel alloy layer 30 may be formed on both sides of the base material 20 as shown in Figure 1(c). In that case, the total amount of nickel deposited in the iron-nickel alloy layer on both sides may be 4.4 to 53.4 g / m². 2 It is preferable that this be the case. The amount of nickel deposited as described above can be determined by measuring the total amount of nickel in the iron-nickel alloy layer 30 using an X-ray fluorescence apparatus. However, this method is not limited to this method, and other known measurement methods can also be used.

[0080] In this embodiment, the iron-nickel alloy layer 30 may be a layer without a brightener added, or it may be a layer formed by adding a brightener (including a brightener for semi-gloss finishes). It should be noted that the terms "glossy" and "matte" used above are based on visual evaluation of the appearance, and it is difficult to distinguish them precisely using numerical values. Furthermore, the degree of gloss may change depending on other parameters such as the bath temperature, which will be discussed later. Therefore, the terms "glossy" and "matte" used in this embodiment are defined solely in terms of the presence or absence of a glossing agent.

[0081] Next, the overall thickness of the surface-treated steel foil 10 in this embodiment will be described. In this embodiment, the overall thickness of the surface-treated steel foil 10 is preferably 200 μm or less if it does not have the roughened nickel layer 50 described later. Furthermore, from the viewpoint of strength and the desired battery capacity, it is more preferably 10 μm to 100 μm, even more preferably 25 μm to 90 μm, and particularly preferably 25 μm to 70 μm. On the other hand, when the roughened nickel layer 50 described later is present on the outermost surface, the overall thickness of the surface-treated steel foil 10 in this embodiment is preferably 210 μm or less. Furthermore, from the viewpoint of strength and the desired battery capacity, it is more preferably 20 μm to 110 μm, even more preferably 35 μm to 100 μm, and particularly preferably 35 μm to 80 μm. If the thickness exceeds the upper limit of the above thickness range, it is undesirable from the viewpoint of the volume and gravimetric energy density of the manufactured battery, and is particularly undesirable when aiming for a thinner battery. On the other hand, if the thickness is below the lower limit of the above thickness range, it becomes difficult to have sufficient strength against the effects of charging and discharging the battery, and the possibility of tearing, ripping, or wrinkling during manufacturing or handling of the battery increases.

[0082] In this embodiment, the "thickness of the surface-treated steel foil 10" is preferably measured using a micrometer.

[0083] As shown in Figure 5, the surface-treated steel foil 10 in this embodiment may further have a metal layer 40 formed on the iron-nickel alloy layer 30. Examples of metal materials constituting the metal layer 40 include nickel, chromium, titanium, copper, cobalt, iron, etc. Of these, nickel or nickel alloys are particularly preferred due to their excellent corrosion resistance and strength.

[0084] In other words, the following are the advantages of forming the metal layer 40 on the iron-nickel alloy layer 30 in the surface-treated steel foil 10 of this embodiment. Specifically, by forming the metal layer 40 in addition to the iron-nickel alloy layer 30, the conductivity, corrosion resistance, strength, etc. of the surface-treated steel foil 10 as a whole can be adjusted, making it possible to manufacture a surface-treated steel foil as a current collector material with desired properties.

[0085] In the current collector surface-treated steel foil 10 of this embodiment, if the metal layer 40 is a nickel layer, the total amount of nickel deposited in the iron-nickel alloy layer 30 and the metal layer 40 (nickel layer) is 3.0 g / m². 2 ~53.4g / m 2 This is preferable from the viewpoint of hydrogen barrier properties and electrolyte resistance. More preferably 3.0 g / m² 2 ~26.7g / m 2 The total amount of nickel deposited in the iron-nickel alloy layer 30 and the metal layer 40 can be measured by X-ray fluorescence analysis (XRF) or the like.

[0086] Furthermore, the thickness of the metal layer 40 is preferably 0.1 μm to 8.0 μm. In addition, regarding the thickness ratio of the iron-nickel alloy layer 30 to the metal layer 40 in the surface-treated steel foil 10, particularly when the metal layer 40 is made of nickel, it is preferable that the iron-nickel alloy layer 30:metal layer 40 = 3:10 to 60:1, and more preferably that the iron-nickel alloy layer 30:metal layer 40 = 3:4 to 35:1, from the viewpoint of improving hydrogen barrier properties and electrolyte resistance. Similar to the iron-nickel alloy layer 30, the thickness of the metal layer 40 can be measured by SEM-EDX (energy-dispersive X-ray spectroscopy) analysis of the cross-section of the surface-treated steel foil.

[0087] In the surface-treated steel foil 10 of this embodiment, a roughened nickel layer 50 may be formed on the outermost surface, as shown in Figure 6. The metal layer 40 described above may also be a roughened nickel layer. Furthermore, a roughened nickel layer may be formed on the metal layer 40 described above, as shown in Figure 7. The roughened nickel layer 50 may be formed on the second surface 10b side of the surface-treated steel foil 10 as shown in Figure 6(a), or as shown in Figure 6(b) First surface 10a The roughened nickel layer may be formed on one side or on both sides. Details regarding the roughened nickel layer are omitted here, as they are described in, for example, the applicants' own application (WO2021 / 020338, etc.). However, from the viewpoint of improving adhesion to the active material, it is preferable that the three-dimensional surface property parameter Sa of the roughened nickel layer is 0.2 μm to 1.3 μm. More preferably, it is 0.36 to 1.2 μm. This three-dimensional surface property parameter Sa is preferably measured using a laser microscope. Furthermore, when forming the roughened nickel layer 50, from the viewpoint of adhesion between the roughened nickel layer 50 and the underlying layer, a base nickel layer may be formed before applying the roughened nickel plating, and then a coating nickel plating may be applied after the roughened nickel plating to form the roughened nickel layer. That is, a nickel plating applied as a metal layer 40 on top of the iron-nickel alloy layer may be used as the base nickel layer, and the roughened nickel layer 50 may be formed on top of it. Alternatively, in the heat treatment when forming the iron-nickel alloy layer, a nickel layer in which iron has hardly diffused may be left on top of the iron-nickel alloy layer, and then nickel plating may be applied to form a metal layer 40 which may be used as the base nickel layer, and the roughened nickel layer 50 may be formed on top of it. In addition, the description of the metal layer 40 as "roughened nickel layer 50" in this specification may include a coating nickel layer. Details of the base nickel layer, roughened nickel layer, and coating nickel layer will be described later.

[0088] When a roughened nickel layer 50 is formed, the total amount of nickel deposited in the iron-nickel alloy layer 30 and the roughened nickel layer 50 is 9 g / m². 2 ~106g / m 2 Preferably, and more preferably, 15 g / m² 2 ~70g / m 2 And more preferably 27 g / m² 2 ~60g / m 2 That is the case. When a roughened nickel layer 50 is formed and the roughened nickel layer 50 is formed on a metal layer 40 made of nickel, the total amount of nickel deposited in the iron-nickel alloy layer 30, the metal layer 40, and the roughened nickel layer 50 is 9 g / m². 2 ~106g / m 2 Preferably, and more preferably, 15 g / m² 2 ~70g / m 2 And more preferably 27 g / m² 2 ~60g / m 2 That is the case. Furthermore, as a method for measuring the amount of nickel deposited on the roughened nickel layer 50, methods such as those described in International Publication No. WO2020 / 017655 or International Publication No. WO2021 / 020338 can be appropriately adopted. In other words, it can be determined by measuring the total amount of nickel on the surface-treated steel foil 10 for the current collector using X-ray fluorescence analysis (XRF) or the like.

[0089] In this embodiment, when the roughened nickel layer 50 is not formed on the surface-treated steel foil, it is preferable that the surface roughness Sz is 1.0 μm or more. In other words, it is preferable that the surface roughness Sz of the alloy layer 30 surface or metal layer 40 on the side where the roughened nickel layer 50 is not formed when the roughened nickel layer 50 is formed on only one side, or the surface roughness Sz of the iron-nickel alloy layer 30 or metal layer 40 on the surface-treated steel foil surface when the roughened nickel layer 50 is not formed on both sides, is 1.0 μm or more. The reason for this is that in order to achieve a surface roughness Sz of less than 1.0 μm, it is necessary to reduce not only the roll roughness of the final finish but also the roll roughness of the intermediate finishes, making it difficult to obtain the desired thickness of the steel foil. Furthermore, regarding the surface roughness Sz mentioned above, especially when used in current collector applications, it is desirable to have a certain level of adhesion, even if it is not as strong as that of a roughened nickel layer. Therefore, a surface roughness of 1.5 μm or more is more desirable. On the other hand, if the surface roughness Sz is too high, there is a concern about the effects of surface non-uniformity, so it is preferably 15 μm or less, and more preferably 10 μm or less.

[0090] ≪Method for manufacturing surface-treated steel foil≫ An example of a manufacturing method for the surface-treated steel foil 10 of this embodiment will be explained with reference to Figure 8.

[0091] As an example of the manufacturing method of this embodiment, as shown in Figure 8(a), the process involves forming a nickel plating layer on a base plate to create a nickel-plated material (STEP A: Nickel Plating Process), and then subjecting the nickel-plated material to heat treatment (STEP B: First Heat Treatment Process). The process involves rolling the nickel-plated material after heat treatment (STEP C: First Rolling Process), followed by a second heat treatment (STEP D: Second Heat Treatment Process), in that order.

[0092] The surface-treated steel foil obtained by the manufacturing method of this embodiment contains Fe1Ni1 as an alloy phase in the iron-nickel alloy layer, and the orientation index in the X-ray diffraction of the (220) plane of Fe1Ni1 on the surface having the iron-nickel alloy layer is 1.0 or more, and (c) the ratio of the maximum diffraction intensity of the (220) plane of Fe1Ni1 to the maximum diffraction intensity of the Fe(200) plane satisfies the following equation (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1)

[0093] Alternatively, you may repeat STEP C and STEP D after STEP D. Furthermore, the rolling process described in the "First Rolling Process" above will also be referred to as "re-rolling" to distinguish it from the rolling of the original sheet. Furthermore, the heat treatment in the "second heat treatment process" described above will also be simply referred to as the "second heat treatment."

[0094] Furthermore, as shown in Figure 8(b), a second rolling step (STEP E) may be added sequentially for purposes such as further thickness adjustment or tempering. It is preferable that the above formula (1) is still satisfied even after going through this second rolling step. After STEP D or STEP E, a recoating process (STEP F) or a roughened nickel layer forming process (STEP F) may be included. The following describes each process in detail.

[0095] <Pre - process> First, prepare a steel sheet as the original plate. The original plate here refers to the steel plate before rolling described below, which becomes the base material part when it becomes a surface - treated steel foil after going through each process described later. Therefore, similar to the base material, the steel plate as the original plate is preferably low - carbon steel or extra - low - carbon steel. Also, the original plate is preferably a cold - rolled steel sheet.

[0096] The thickness of the original plate is not particularly limited. However, in order to obtain a thickness such that it is called a steel foil after the first rolling process described later, the original plate is preferably 150 - 500 μm. In order to obtain a foil of 120 μm or less after the first rolling process described later, the thickness of the original plate is more preferably 400 μm or less. This is because when the original plate is thinner, it is easier to relieve the rolling reduction during rolling and prevent the exposure of iron. In order to obtain a foil of less than 100 μm after the first rolling process described later, the thickness of the original plate is even more preferably 350 μm or less, and particularly preferably 300 μm or less. When a cold - rolled steel sheet is used as the original plate, generally, the "annealing" performed to remove the work hardening of the cold - rolled steel sheet can be carried out before the nickel plating process described later. Also, in this embodiment, it is possible to omit the "annealing" of this cold - rolled steel sheet. This is because in the first heat - treatment process mainly aimed at softening the nickel plating described later, the work hardening of the cold - rolled steel sheet can be removed simultaneously. [[ID=2�]]

[0097] <STEP A: Nickel plating process> The nickel plating process is a process of applying nickel, which is necessary for forming the iron - nickel alloy layer 30 formed by the second heat - treatment described later, as a nickel plating layer on at least one side of the above - mentioned original plate. In this nickel plating process, the amount of nickel plating applied to the base plate is 7.2 g / m² per side. 2 More than ~89.0g / m 2 The following is preferable. More preferably, 7.2 g / m² per side on both sides. 2 More than ~89.0g / m 2 The following nickel plating is applied, and at least one side is 10 g / m² per side. 2 It is even more preferable to have a value of 13.0 g / m² or more. 2 It is particularly preferable to keep it above this level. The upper limit is 72.0 g / m². 2 The following is more preferable: 63.0 g / m 2 The following are even more preferable. Nickel plating thickness: 89.0 g / m² 2 If the value exceeds this, productivity will be poor, and even after the first heat treatment process, the foil may break during the first rolling process due to insufficient elongation of the entire foil. On the other hand, the amount of nickel plating deposited was 7.2 g / m². 2 If the value is less than the required amount, the nickel in the iron-nickel alloy layer 30 obtained after the second heat treatment process may be insufficient, resulting in insufficient amounts of Fe1Ni1 being obtained, or the required hydrogen barrier properties may not be achieved due to the inability to suppress iron exposure.

[0098] The amount of nickel plating can be converted to the thickness of the nickel plating by dividing it by the specific gravity of nickel, which is 8.9. Therefore, by adding the thickness of the base sheet and the thickness of the nickel plating, the thickness before the first rolling can be determined.

[0099] In the nickel plating process described above, known conditions can be applied to the electroplating conditions. Examples of plating conditions are shown below.

[0100] [Example of nickel plating bath and plating conditions] • Bath composition: Known watt bath Nickel sulfate hexahydrate: 200~300g / L Nickel chloride hexahydrate: 20-60 g / L Boric acid: 10-50 g / L Bath temperature: 40~70°C pH: 3.0~5.0 Stirring: Air stirring or jet stirring Current density: 5~30 A / dm 2 Regarding the bath composition, in addition to the above-mentioned Watts bath, known nickel sulfamate baths or citrate baths may also be used. Furthermore, additives such as known brighteners may be added to the plating bath to obtain bright nickel plating or semi-bright nickel plating.

[0101] <STEP B: First heat treatment process> Next, the first heat treatment process will be described. The first heat treatment process is a heat treatment process that is first performed after the above-mentioned nickel plating process and is carried out in a reducing atmosphere. The main purpose of this first heat treatment process is to soften the nickel plating layer formed in the above-mentioned nickel plating process prior to the subsequent rolling process.

[0102] When rolling without heat treatment after nickel plating, there is no problem as long as it is temper rolling. However, in the production of foils, when obtaining a surface-treated metal foil with a thickness of 10 μm to 200 μm by applying a reduction exceeding 35% from a base plate of 0.15~2.0 mm, the nickel plating layer is too hard to produce a foil or the nickel plating layer peels off, so a surface-treated steel foil having the target iron-nickel alloy layer cannot be produced. Therefore, heat treatment is performed for the purpose of softening the nickel plating layer.

[0103] As the heat treatment conditions for the first heat treatment process, conditions under which the nickel in the nickel plating layer is sufficiently softened to the extent that the subsequent first rolling process is possible can be applied. For example, the heat treatment conditions in known batch annealing (box annealing) or continuous annealing can be applied.

[0104] As an example of the temperature and time for continuous annealing, it is preferable to perform the process at a temperature of 600°C to 950°C with a soaking time of 15 to 150 seconds. Lower temperatures or shorter times than this may result in insufficient softening, which is undesirable as it may make it difficult to form the foil during the subsequent first rolling process. On the other hand, higher temperatures or longer times than the above heat treatment range are undesirable from a cost perspective, as they may lead to significant changes in the mechanical properties of the base material, such as the steel foil, resulting in a substantial decrease in strength. Furthermore, a soaking time of 20 to 150 seconds is more preferable for sufficient softening.

[0105] As an example of temperature and time for batch annealing (box annealing), it is preferable to perform the process at a temperature of 450°C to 690°C, with a soaking time of 1.5 to 20 hours, and a total time of 4 to 80 hours including heating, soaking, and cooling. Lower temperatures or shorter times are undesirable because the softening will be insufficient, making it difficult to form the foil during the subsequent first rolling process. On the other hand, higher temperatures or longer times than the above heat treatment range are undesirable because they may cause significant changes in the mechanical properties of the base material, such as the steel foil, potentially leading to a substantial decrease in strength, or from a cost perspective.

[0106] However, the amount of nickel plating is 54.0 g / m² per side. 2 Below, in particular, the single-sided side is 27.0 g / m². 2 In the following cases of low levels, high-temperature or long-duration heat treatment may result in insufficient nickel to alloy the exposed iron during the second heat treatment. Therefore, continuous annealing at a temperature below 780°C is preferred, and more preferably below 750°C.

[0107] Note that when the first heat treatment step is completed, the nickel in the iron of the base plate and the nickel plating layer diffuses mutually due to heat, and an iron-nickel diffusion layer is formed. That is, the surface where nickel plating was performed in the above-described nickel plating step forms an iron-nickel diffusion layer, or an iron-nickel diffusion layer and a soft nickel layer, when the first heat treatment step is completed. In other words, in the present embodiment, the iron-nickel diffusion layer refers to an alloy layer obtained by heat treatment of iron and nickel that does not satisfy either of the above characteristics (a) or (c). Also, in the present embodiment, the soft nickel layer refers to a layer of softened nickel in which the iron of the base plate has not diffused into the nickel of the nickel plating layer by heat treatment.

[0108] Note that in the present embodiment, the Fe1Ni1 alloy phase required for hydrogen barrier properties may be formed when the second heat treatment step described later is completed. Therefore, at the time when the first heat treatment step is completed, the Fe1Ni1 alloy phase may or may not be formed.

[0109] Note that the thickness of the steel plate after heat treatment after the first heat treatment step is the same as the thickness of the nickel-plated steel plate after the nickel plating step.

[0110] <STEP C: First Rolling Step> Next, the first rolling step in the manufacturing method of the present embodiment will be described. The first rolling step in the present embodiment is a step of rolling the nickel-plated material after heat treatment after the above nickel plating step and the first heat treatment step. The purpose of this first rolling step is to obtain a desired foil thickness, or to obtain a thickness that is not problematic in advance in order to obtain a foil of a desired thickness when the second rolling step described later is completed.

[0111] In this first rolling process, the reduction ratio is preferably 35% or more. By setting it to 35% or more, a large processing strain can be imparted to the iron-nickel diffusion layer that has an orientation toward the Fe1Ni1(220) plane that is sufficient to prevent collapse even after the second heat treatment. As mentioned above, by orienting not only the Fe1Ni1(200) plane but also the Fe1Ni1(220) plane, the hydrogen pathway can be made more complex and the hydrogen barrier properties can be improved. Furthermore, the structure oriented toward Fe1Ni1(220) will retain its Fe1Ni1(220) orientation even when the iron-nickel alloy crystals recrystallize and the crystal grains coarse during the second heat treatment, or when alloying progresses and the thickness of the iron-nickel alloy layer increases. However, if the reduction ratio is less than 35%, the iron exposure described above is less likely to occur, so the problem of reduced hydrogen barrier properties does not occur, and the crystal orientation of the iron-nickel alloy layer is less likely to be oriented toward Fe1Ni1(220) to the extent that it remains after the second heat treatment. To suppress iron exposure, a lower reduction ratio is preferable, but to retain the orientation toward the Fe1Ni1(220) surface after heat treatment, a reduction ratio of 35% or more is preferable, and more preferably 50% or more. Furthermore, when rolling into foil, the thickness used as the denominator of the reduction ratio, i.e., the thickness before rolling, is thinner compared to rolling from a normal thick plate to a thin plate, resulting in a higher reduction ratio, and especially when forming foil less than 100 μm thick, the reduction ratio is 50% or more. However, as the amount of exposed iron increases with a higher reduction ratio, a reduction ratio of 85% or less is preferable, more preferably 80% or less, even more preferably 78% or less, and particularly preferably 75% or less.

[0112] In this first rolling process, there may be one set of rolling rolls or multiple sets. A typical rolling mill consists of multiple sets of upper and lower rolls, i.e., rolling rolls, that directly act to thin the sheet, and rolls for passing the sheet through. During rolling, there may be one set of rolling rolls acting on the rolling process, or multiple sets of rolling rolls may be acting. In this embodiment, there may be one set or multiple sets of rolling rolls acting in the first rolling process, and for example, three sets of rolling rolls may be used to pass the sheet through twice, resulting in a total of six sets of rolling rolls for rolling. Generally, as the number of times the sheet passes through the rolling rolls increases, problems due to work hardening are more likely to occur during rolling. Therefore, it is preferable to have six sets or fewer rolling rolls acting on the rolling process, and more preferably four sets or fewer. Here, one set of rolling rolls refers to the upper and lower rolls that directly touch the sheet and whose thickness changes before and after passing through them.

[0113] Furthermore, the reduction ratio mentioned above refers to the reduction ratio obtained from the thickness before and after the first rolling process. In other words, when the sheet metal is passed through three sets of rolling rolls twice, it refers to the reduction ratio obtained from the thickness before the first pass and the thickness after the second pass.

[0114] In the first rolling process, the reduction ratio by the first set of rolling rolls is not particularly limited, but it is preferable to set it to 35% or more from the viewpoint that thinning the material in its initial, softest state makes it easier to suppress the exposure of iron. However, since the first set has the thickest thickness before rolling, it is preferable to set it to less than 50% from the viewpoint that if the reduction amount is too large, it becomes difficult to control the uniformity of the thickness.

[0115] Furthermore, the amount of nickel deposited on the steel foil after the first rolling process, that is, the amount of nickel per unit area after the nickel plating process has been stretched by rolling, should be at least 5.0 g / m² on one side from the viewpoint of hydrogen barrier properties. 2 It is preferable that it exceeds this value, and more preferably 6.0 g / m². 2 The above is preferable, and more preferably 6.5 g / m 2This is the case. Also, in order to obtain a more stable hydrogen barrier property, it is preferable that both sides of the steel foil are each 5.0 g / m 2 or more.

[0116] <STEP D: Second heat treatment process> Next, the second heat treatment process in the manufacturing method of this embodiment will be described. The second heat treatment process is a process of annealing the material after the first rolling process in a reducing atmosphere. This second heat treatment process aims to form a Fe1Ni1 alloy phase in the iron-nickel alloy layer, make the orientation index of the X-ray diffraction of the (220) plane of Fe1Ni1 1.0 or more, or make the ratio of the diffraction intensity of the (220) plane of Fe1Ni1 to the diffraction intensity of the Fe(200) plane satisfy the following formula (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1)

[0117] More specifically, first, the iron-nickel diffusion layer or the iron-nickel diffusion layer and the soft nickel layer formed on the surface by the first heat treatment described above are rolled together with the original plate in the first rolling process. As the material thickness becomes thinner by this rolling, the orientation of the Fe1Ni1(220) direction increases. Also, there is a tendency for portions where the iron-nickel diffusion layer or the iron-nickel diffusion layer and the soft nickel layer become extremely thin to occur, and the iron of the original plate may be exposed. Therefore, at the time when the first rolling process is completed, the effective hydrogen barrier property obtained in the first heat treatment process may decrease.

[0118] In this second heat treatment process, while sufficiently forming Fe1Ni1 effective for the hydrogen barrier property, alloying of extremely thin portions and portions where the iron of the original plate is exposed (hereinafter, also referred to as "alloying of deficient portions") is achieved, and by adopting a configuration that satisfies the above formula (1), it is possible to recover the hydrogen barrier property.

[0119] The heat treatment conditions in the second heat treatment process differ depending on the state of the steel foil before the second heat treatment in order to satisfy formula (1). As an example, when the second heat treatment step is continuous annealing, it is carried out within the range of 680°C to 950°C and a soaking time of 30 seconds to 150 seconds. On the other hand, in the case of batch annealing (box annealing), it is carried out at 500°C to 650°C with a soaking time of 1.5 hours to 20 hours, and the total time including heating, soaking, and cooling is within the range of 4 hours to 80 hours.

[0120] When the temperature is lower than the above heat treatment temperature or the time is shorter, it is not preferable from the viewpoint that sufficient Fe1Ni1 cannot be formed, and / or the alloying at the locations extremely thinned by rolling or where the iron of the base material is exposed is insufficient, resulting in poor hydrogen barrier properties.

[0121] Also, there is no limitation as long as it satisfies the conditions of the configuration of formula (1). However, particularly when the reduction ratio of the first rolling step is 50% or more, in order to sufficiently form Fe1Ni1 in this second heat treatment step and for alloying of the deficient locations, in the case of continuous annealing, it is preferable to have a soaking temperature of 700°C to 750°C with a soaking time of 60 seconds to 150 seconds or conditions of 760°C or higher, and in the case of box annealing, it is preferable to have a soaking temperature of 500°C or higher and less than 540°C with a soaking time of 4 hours or more or conditions of 540°C or higher.

[0122] The nickel adhesion amount of the surface-treated steel foil obtained at the time when the second heat treatment step has passed is the same as the nickel adhesion amount after passing through the above first rolling step.

[0123] Particularly in the case of a continuous steel strip, before the second heat treatment step, a surface treatment for preventing the adhesion of nickel plating may be performed. Examples of this surface treatment for preventing the adhesion of nickel plating include, for example, the formation of a silicon oxide layer in a bath mainly composed of sodium orthosilicate disclosed in JP-A-08-333689. Note that this surface treatment for preventing the adhesion of nickel plating may be removed after the second heat treatment step.

[0124] <STEP E: Second Rolling Step> Next, the second rolling process after the second heat treatment process will be described. This second rolling process is a process for further adjusting the thickness and tempering of the surface-treated steel foil. Note that this second rolling process is not an essential process and can be omitted as appropriate.

[0125] In this second rolling process, the rolling reduction (the rolling reduction calculated from the difference in thickness before and after the second rolling process) is preferably less than 35%, more preferably 33% or less, and even more preferably 25% or less. There is no particular lower limit, and it is 0% or more including temper rolling where the actual thickness does not change.

[0126] Note that at the time of passing through this second rolling process, it is necessary to satisfy the above formula (1).

[0127] Also, since the nickel deposition amount decreases according to the rolling reduction of the second rolling process, when passing through the second rolling process, it is necessary to set the nickel deposition amount to a preferable amount after the second rolling. The preferable nickel deposition amount after the second rolling is preferably at least 5.0 g / m on at least one side from the viewpoint of hydrogen barrier property. 2 More preferably, it is 6.0 g / m. 2 Or more, and even more preferably 6.5 g / m. 2 Or more. Also, in order to obtain a more stable hydrogen barrier property, it is preferable that both sides of the steel foil are each more than 5.0 g / m. 2

[0128] <STEP F: Re - plating process> The surface - treated steel foil 10 may further have a metal layer 40 on the iron - nickel alloy layer 30. There are mainly two formation methods for this metal layer 40. The first is a method of forming by leaving a nickel layer with almost no iron diffusion as the metal layer 40 in the above - mentioned first heat treatment process and second heat treatment process. ​The second method is a method of forming the metal layer 40 by subjecting it to plating after at least one of the first rolling process, the second heat treatment process, and the second rolling process (re-plating process). Note that the metal layer 40 may be formed using both the first method and the second method.

[0129] In the above re-plating process, examples of the metal layer 40 include a nickel layer and a chromium layer. When forming a nickel layer as the metal layer 40 in the re-plating process, it can be formed by a known nickel bath such as the Watts bath, nickel sulfamate bath, or citrate bath described above.

[0130] Note that when a nickel layer is formed by both the above first method and the second re-plating process, it can be treated as one nickel layer. When a metal layer made of a metal other than nickel, such as a chromium layer, is formed in the second re-plating process, the metal layer may be a plurality of layers. Note that after the formation of this metal layer, it is preferable not to perform heat treatment from the viewpoint of adhesion to the roughened nickel layer described later.

[0131] When nickel plating is performed in the re-plating process, the total nickel deposition amount of the surface-treated steel foil including the deposition amount in the re-plating is 2.22 to 53.4 g / m 2 which is preferable from the viewpoints of hydrogen barrier properties and electrolyte resistance suitable for bipolar electrodes. More preferably, it is 2.22 to 26.7 g / m 2 Note that the deposition amount of nickel in the iron-nickel alloy layer 30 and the metal layer 40 can be measured by fluorescence X-ray analysis (XRF) or the like.

[0132] <STEP G: Roughened Nickel Layer Formation Step> Furthermore, the manufacturing method of the surface-treated steel foil 10 of this embodiment may include a step of forming a roughened nickel layer 50 on the outermost surface. The plating bath for forming the roughened nickel layer preferably has a chloride ion concentration of 3 to 90 g / L, more preferably 3 to 75 g / L, and even more preferably 3 to 50 g / L, and the ratio of nickel ions to ammonium ions is preferably 0.05 to 0.75, more preferably 0.05 to 0.60, even more preferably 0.05 to 0.50, and even more preferably 0.05 to 0.30 in weight ratio of "nickel ions / ammonium ions", and the bath conductivity at 50°C is preferably 5.00 to 30.00 S / m, more preferably 5.00 to 20.00 S / m, and even more preferably 7.00 to 20.00 S / m. When the chloride ion concentration is 10 g / L or higher, it is easier to achieve a good roughened plating state even if the amount of roughened nickel plating is relatively small. The method for adjusting the chloride ion concentration, nickel ion to ammonium ion ratio, and bath conductivity of the plating bath to the above range is not particularly limited, but for example, one method is to make the plating bath contain nickel sulfate hexahydrate, nickel chloride hexahydrate, and ammonium sulfate, and adjust the amounts of these in the mixture as appropriate. An example of plating conditions is as follows.

[0133] ≪Example of conditions for roughened nickel plating≫ Bath composition: Nickel sulfate hexahydrate 10-100 g / L, nickel chloride hexahydrate 1-90 g / L, ammonium sulfate 10-130 g / L pH 4.0~8.0 Bath temperature 25~70℃ Current density 4~40A / dm 2 Plating time: 10 seconds to 150 seconds Agitation: Air agitation or jet agitation Furthermore, instead of ammonium sulfate, ammonia water or ammonium chloride may be used to add ammonia to the nickel plating bath. The ammonia concentration in the plating bath is preferably 6 to 35 g / L, more preferably 10 to 35 g / L, even more preferably 16 to 35 g / L, and even more preferably 20 to 35 g / L. In addition, basic nickel carbonate compounds, hydrochloric acid, sodium chloride, or potassium chloride may be used to control the chloride ion concentration.

[0134] The three-dimensional surface property parameter Sa of the roughened nickel layer 50 is preferably 0.2 μm to 1.3 μm, as described above. In order to bring the value of the three-dimensional surface property parameter Sa of the roughened nickel layer 50 within this range, for example, this can be achieved by controlling the surface roughness of the substrate 20, adjusting the roughened nickel plating conditions and thickness, adjusting the undercoat nickel plating conditions and thickness, and so on.

[0135] Furthermore, as disclosed in International Publication No. WO2020 / 017655, a coated nickel plating layer may be formed as a post-step after roughened nickel plating. Since the coated nickel plating conditions can be applied as described in International Publication No. WO2020 / 017655, a detailed explanation is omitted here.

[0136] In this embodiment, the manufacturing method for the surface-treated steel foil 10 can be a continuous manufacturing method (for example, a roll-to-roll method), or it can be a batch manufacturing method using cut plates, for example.

[0137] The surface-treated steel foil obtained by the manufacturing method described above has a hydrogen permeation current density (oxidation current value) of 55 μA / cm². 2 The following characteristics are preferable for bipolar electrodes from the viewpoint of hydrogen barrier properties. In this embodiment, the hydrogen permeation current density (oxidation current value) refers to the current value on the hydrogen detection side when measured using the apparatus described in Figures 2(a) and (b) in an electrolyte at 65°C under the conditions that the cathode side potential is -1.5V and the anode side potential is +0.4V.

[0138] Examples The present invention will be described in more detail below with reference to examples. First, the measurement method used in the examples will be described.

[0139] [X-ray diffraction (XRD) measurement] The alloy phase in the iron-nickel alloy layer was identified by X-ray diffraction. From the measurement results obtained by performing X-ray diffraction on surface-treated steel foil, the orientation index and peak intensity ratio (ratio of the maximum diffraction intensities) were obtained. A Rigaku SmartLab was used as the X-ray diffraction measurement device. The sample used was a surface-treated steel foil cut into 20 mm x 20 mm sections. The diffraction intensity of the Fe1Ni1(220) crystal plane was confirmed at the following diffraction angle 2θ. Fe1Ni1(220) crystal plane: diffraction angle 2θ = 75.1 ± 0.11° The diffraction intensity of each crystal plane of iron was confirmed at the following diffraction angle 2θ. Fe(200) crystal plane: diffraction angle 2θ = 65.02 ± 0.11° Fe(211) crystal plane: diffraction angle 2θ = 82.33 ± 0.11° Furthermore, in order to calculate the orientation index, the diffraction intensity of each crystal plane of Fe1Ni1 was confirmed at the following diffraction angle 2θ. Fe1Ni1(111) crystal plane: diffraction angle 2θ = 43.83 ± 0.11° Fe1Ni1(200) crystal plane: diffraction angle 2θ = 51.05 ± 0.11° Fe1Ni1(311) crystal plane: diffraction angle 2θ = 91.23 ± 0.11° Fe1Ni1(222) crystal plane: diffraction angle 2θ = 96.56 ± 0.11° Furthermore, in order to determine the presence of the (220) crystal plane in the Fe1Ni1 crystal structure, the diffraction intensity was confirmed at the following diffraction angle 2θ. Diffraction angle 2θ = 86 ± 0.5° The specific measurement conditions for X-ray diffraction were as follows:

[0140] <Device configuration> ·X-ray source: CuKα • Goniometer radius: 300mm ·Optical system: Concentration method (Induction-side slit system) • Solar slit: 5° • Longitudinal limiting slit: 5mm • Divergent slit: 2 / 3° (Light-receiving slit system) • Scattering slit: 2 / 3° • Solar slit: 5° • Light-receiving slit: 0.3mm • Monochromatic method: Counter monochromator method • Detector: Scintillation counter <Measurement parameters> • Tube voltage-current: 45kV 200mA ·Scanning axis: 2θ / θ • Scanning mode: Continuous • Measurement range: 2θ 40~100° • Scanning speed: 10° / min Step: 0.02°

[0141] The ratio of the diffraction intensity of the Fe1Ni1(220) plane to the diffraction intensity of the Fe(200) plane in the Fe1Ni1 crystal structure obtained at the above diffraction angles is shown in the "Fe1Ni1(220) / Fe(200)" column of Tables 1 to 4. The same applies to Fe(211) / Fe(200). Furthermore, the presence of Fe1Ni1 was determined to be present if the maximum diffraction intensity at the diffraction angle 2θ = 75.1 ± 0.11° was more than twice the average diffraction intensity at 2θ = 86 ± 0.5°, and absent if it was less than twice, and this is indicated as "-" in Tables 1 to 4.

[0142] The crystal orientation index Ico_Fe1Ni1(220) for X-ray diffraction of the (220) plane of Fe1Ni1 was calculated using the following formula and is shown in the "Fe1Ni1(220) Orientation Index" column of Tables 1 to 4. [I_Fe1Ni1(220) / [I_Fe1Ni1(111)+I_Fe1Ni1(200)+I_Fe1Ni1(220)+I_Fe1Ni1(311)+I_Fe1Ni1(222)]] / [I S _Fe1Ni1(220) / [I S _Fe1Ni1(111)+I S _Fe1Ni1(200)+I S _Fe1Ni1(220)+I S _Fe1Ni1(311)+I S _Fe1Ni1(222)]] Here, the diffraction intensity of each crystal plane of Fe1Ni1 in the above calculation formula is the maximum value of the diffraction intensity confirmed at each diffraction angle 2θ, as follows. I_Fe1Ni1(111): Diffraction intensity of Fe1Ni1(111) crystal planes measured at diffraction angle 2θ = 43.83 ± 0.11° I_Fe1Ni1(200): Diffraction intensity of Fe1Ni1(200) crystal planes measured at diffraction angle 2θ = 51.05 ± 0.11° I_Fe1Ni1(220): Diffraction intensity of Fe1Ni1(220) crystal planes measured at diffraction angle 2θ = 75.1 ± 0.11° I_Fe1Ni1(311): Diffraction intensity of Fe1Ni1(311) crystal planes measured at diffraction angle 2θ = 91.23 ± 0.11° I_Fe1Ni1(222): Diffraction intensity of Fe1Ni1(222) crystal planes measured at diffraction angle 2θ = 96.56 ± 0.11° Furthermore, in the above formula for calculating the crystal orientation index, S _Fe1Ni1(111), I S _Fe1Ni1(200), I S _Fe1Ni1(220), I S _Fe1Ni1(311), I S_Fe1Ni1(222) is the standard diffraction peak intensity value for each crystal plane ((111) plane, (200) plane, (220) plane, (311) plane, and (222) plane) of Fe1Ni1 as described in JCPDS (Joint Committee on Powder Diffraction Standards, PDF card number: 01-071-8322).

[0143] [Method for measuring the thickness of an iron-nickel alloy layer after heat treatment] The thickness of the iron-nickel alloy layer was calculated using SEM-EDX (energy-dispersive X-ray spectroscopy) (instrument names: Hitachi High-Technologies SU8020 and AMETEK EDAX). Elemental analysis of Ni and Fe was performed by line analysis at a depth of 10 μm from the surface in the thickness direction. The measurement conditions were: acceleration voltage: 15 kV, observation magnification: 5000x, measurement step: 0.1 μm. As shown in Figure 3, the horizontal axis represents the distance in the depth direction from the surface (μm), and the vertical axis represents the X-ray intensity of Ni and Fe. The thickness of the iron-nickel alloy layer 30 was read from the graph, defined as the distance between 1 / 10 of the maximum values ​​of nickel and iron, before and after the intersection of the nickel and iron curves.

[0144] [Method for measuring hydrogen permeation current density] Using the device described in FIG. 2, with the evaluation sample as the working electrode, the reference electrode as Ag / AgCl, the potential on the hydrogen generation side (cathode side) was -1.5 V, and the potential on the hydrogen detection side (anode side) was +0.4 V, and the measurement was carried out under these conditions. As for the detailed measurement method, it was carried out using the device shown in FIG. 2(a) as described above. As the electrolyte, an alkaline aqueous solution composed of KOH, NaOH, and LiOH containing 6 mol / L of KOH as the main component at 65 °C and having a total concentration of KOH, NaOH, and LiOH of 7 mol / L was used. As the potentiostat, the "Multi Electrochemical Measurement System HZ-Pro" manufactured by Hokuto Denko Corporation was used. First, a potential of +0.4 V was applied to the hydrogen detection side and maintained for 60 minutes for the current value to stabilize. The hydrogen detection side was continuously maintained at the same potential. Then, the potential on the hydrogen intrusion side was applied step by step to -0.7 V, -1.1 V, and -1.5 V, and each potential was applied for 15 minutes. The change in oxidation current during the period when the potential on the hydrogen intrusion side was -1.5 V was used as the hydrogen permeation current density and was the evaluation object of this example and the comparative example. The measurement diameter was φ20 mm, and the measurement area was 3.14 cm 2 was used. The hydrogen permeation current density I (μA / cm 2 ) obtained by the following formula (1) is shown in Table 1. The hydrogen permeation current density I (μA / cm 2 ) = ((Average value of oxidation current from Ib to Ic) / S) - ((Average of Ia and Id) / S) ··· (1) However, Ia (μA) is the oxidation current 5 seconds before the application of -1.5 V, Ib (μA) is the oxidation current 155 seconds after the start of the application of -1.5 V, Ic (μA) is the oxidation current at the end of the application of -1.5 V, Id (μA) is the oxidation current 155 seconds after the end of the application of -1.5 V, and S (cm 2 ) is the measurement area (evaluation area). When measuring the hydrogen permeation current density of samples other than Examples 9 to 11 and Comparative Example 1, after forming the following nickel film for measurement with a thickness of 1 μm on each surface of both sides of the surface-treated steel foil, the hydrogen permeation current density was measured. <Nickel plating conditions for measurement> Bath composition: Nickel sulfate hexahydrate 250 g / L, Nickel chloride hexahydrate 45 g / L, Boric acid 30 g / L pH 4.0~5.0 Bath temperature 60℃ Current density 10A / dm 2

[0145] [Method for measuring three-dimensional surface properties (Sa)] For the surface of the roughened nickel layer 50 of the surface-treated steel foil, each three-dimensional surface texture parameter (arithmetic mean height Sa) was measured using a laser microscope (Olympus Corporation, 3D measuring laser microscope LEXT OLS5000) in accordance with ISO 25178-2:2012. Specifically, an analysis image with a field of view of 128 μm × 128 μm was first obtained using a 100x objective lens (lens name: MPLAPON100XLEXT). Next, the obtained analysis image was subjected to automatic correction processes, namely noise reduction and tilt correction, using an analysis application. Afterward, the surface roughness measurement icon was clicked to perform the analysis and obtain the 3D surface texture parameters (arithmetic mean height Sa). Furthermore, the analysis was performed without setting any filter conditions (F-operation, S-filter, L-filter). The arithmetic mean height Sa was taken as the average of the three fields of view. The results obtained are shown in the "Roughened Ni Surface Sa" column of Table 4.

[0146] <Example 1> First, a cold-rolled steel sheet (260 μm thick) of low-carbon aluminum-killed steel having the chemical composition shown below was prepared as the base material 20. C: 0.04 wt%, Mn: 0.32 wt%, Si: 0.01 wt%, P: 0.012 wt%, S: 0.014 wt%, remainder: Fe and unavoidable impurities

[0147] Next, the prepared raw material was subjected to electrolytic degreasing and sulfuric acid pickling, followed by nickel plating under the following conditions, resulting in a target thickness of 3.0 μm and a nickel deposition amount of 26.7 g / m². 2 A nickel plating layer was formed on both sides (nickel plating process). The nickel plating conditions were as follows. (Nickel plating conditions) Bath composition: Watt bath Nickel sulfate hexahydrate: 250 g / L Nickel chloride hexahydrate: 45 g / L Boric acid: 30 g / L Bath temperature: 60 °C pH: 4.0 - 5.0 Stirring: Air stirring or jet stirring Current density: 10 A / dm 2

[0148] The nickel deposition amount was measured using a fluorescent X-ray apparatus. In addition, after the second heat treatment process and after the second rolling process, which will be described later, the nickel deposition amount was also determined by measuring with a fluorescent X-ray apparatus in the same manner. In addition, after the re-plating process or after the roughened plating layer formation process of Examples 9 to 11 described later, the measurement was also performed in the same manner. The fluorescent X-ray apparatus used was ZSX100e manufactured by Rigaku Corporation.

[0149] Next, the steel sheet having the nickel plating layer formed above was heat-treated under the conditions of a heat treatment temperature of 780 °C, a soaking time of 60 seconds, and a reducing atmosphere by continuous annealing to obtain a treated steel sheet (first heat treatment process). The obtained treated steel sheet is designated as "e1". Table 1 shows the results of measuring the X-ray diffraction and the hydrogen permeation current density of this treated steel sheet e1. In the treated steel sheet e1, the orientation index of the Fe1Ni1(220) plane obtained by performing X-ray diffraction was 0.42. That is, it can be confirmed that an iron-nickel diffusion layer is formed in the treated steel sheet e1, but the orientation index of the Fe1Ni1(220) plane is as small as 0.42, and the hydrogen permeation current density was 3 μA / cm 2 It was confirmed that e1 is not suitable for a battery that emphasizes the volume energy density because it is thick.

[0150] Next, in order to obtain a thin foil, the treated steel sheet e1 was rolled to obtain a rolled steel foil (first rolling process). The rolling conditions at this time were cold rolling with a reduction ratio of 75 - 80%. The obtained rolled steel foil is designated as "e2". Table 1 shows the results of X-ray diffraction and hydrogen permeation current density measurements for this rolled steel foil e2. In rolled steel foil e2, the orientation index of the (220) plane of Fe1Ni1 was 2.79. In other words, rolled steel foil e2 exhibits characteristics of the iron-nickel diffusion layer formed by the first heat treatment process described above, after which rolling is performed. Furthermore, in the case of rolled steel foil e2, equation (1) was not satisfied; specifically, the left side of equation (1) was 0.08, far below 0.5. Also, the hydrogen permeation current density was 90 μA / cm². 2 As a result, the hydrogen barrier properties were significantly reduced.

[0151] Next, the rolled steel foil e2 was annealed at 560°C for a soaking time of 6 hours, for a total of 80 hours, to obtain a surface-treated steel foil (second heat treatment step). The obtained surface-treated steel foil is designated as "e3". Table 1 shows the results of X-ray diffraction and hydrogen permeation current density measurements for this surface-treated steel foil e3. Furthermore, the nickel deposition amount for surface-treated steel foil e3 was 5.8 g / m². 2 The orientation index of the (220) plane of Fe1Ni1 was 2.47. In other words, it became clear that even after a second heat treatment process, the characteristics acquired through the first rolling process remain. As shown in Table 1, surface-treated steel foil e3 satisfied equation (1). In other words, it had a configuration in which the value on the left side of equation (1) was 0.5 or greater. The hydrogen permeation current density was 39 μA / cm². 2 This revealed that the hydrogen barrier properties were restored.

[0152] Next, the surface-treated steel foil e3 was rolled under conditions of a reduction ratio of 10-15% (second rolling process). The total reduction ratio calculated from the thickness before the first rolling process and the thickness after the second rolling process is 81.2%. The resulting surface-treated steel foil is designated as "e4". Table 1 shows the results of X-ray diffraction and hydrogen permeation current density measurements for this surface-treated steel foil e4. The nickel deposition amount was 5.0 g / m². 2In surface-treated steel foil e4, the orientation index of the (220) plane of Fe1Ni1 was 3.34, and the thickness was 50 μm. As shown in Table 1, surface-treated steel foil e4 satisfied equation (1). In other words, it had a configuration in which the value on the left side of equation (1) was 0.5 or greater. Furthermore, the hydrogen permeation current density is 55 μA / cm². 2 Therefore, although the hydrogen barrier properties decreased slightly compared to after the second heat treatment process, the reduction ratio in this second rolling process was less than 35%, so the decrease in hydrogen barrier properties was not as significant as that caused by the first rolling process.

[0153] From the above results, it was confirmed that a surface-treated steel foil with good hydrogen barrier properties can be obtained when the orientation index of the (220) plane of Fe1Ni1 is 1.0 or higher and satisfies the following equation (1). I(Fe1Ni1(220)) / I(Fe(200))≧0.5···(1)

[0154] Furthermore, evaluations were conducted by varying the following: raw plate thickness, nickel deposition amount in the nickel plating process, heat treatment conditions in the first heat treatment process, rolling conditions in the first rolling process, and annealing conditions in the second heat treatment process. Furthermore, evaluations were also conducted on samples that had undergone a second rolling process and a roughened nickel layer formation process. The results of the X-ray diffraction measurements and hydrogen permeation current densities for each sample are shown in Table 2. Furthermore, sample e3 in Table 1, confirmed in Example 1 above, is the same sample as in Example 1-1 in Table 2, and sample e4 is the same sample as in Example 1-2 in Table 2.

[0155] <Example 2> First, a cold-rolled steel sheet (200 μm thick) of low-carbon aluminum-killed steel having the chemical composition shown below was prepared as the base material 20. C: 0.04 wt%, Mn: 0.32 wt%, Si: 0.01 wt%, P: 0.012 wt%, S: 0.014 wt%, remainder: Fe and unavoidable impurities

[0156] Next, the prepared raw material was subjected to electrolytic degreasing and sulfuric acid pickling, followed by nickel plating, resulting in a target thickness of 5.0 μm and a nickel deposition rate of 44.5 g / m². 2 A nickel plating layer was formed on both sides (nickel plating process). The nickel plating conditions were the same as in Example 1, except for the amount of plating. Next, the steel sheet having the nickel plating layer formed above was subjected to continuous annealing under the conditions of a heat treatment temperature of 780°C, a soaking time of 40 seconds, and a reducing atmosphere (first heat treatment step) to obtain a treated steel sheet. The treated steel sheet obtained as described above was subjected to rolling (first rolling step) to obtain rolled steel foil. The rolling was carried out using cold rolling with a reduction ratio of 70-75%.

[0157] The rolled steel foil described above, after the first rolling, was annealed in a reducing atmosphere at 560°C for a soaking time of 6 hours, for a total of 80 hours (second heat treatment process). After the second heat treatment process, the nickel deposition amount was 12.3 g / m². 2 The surface-treated steel foil has a thickness of 58 μm, and its hydrogen permeation current density (oxidation current value) is 4.7 μA / cm². 2 The results are shown in Table 1.

[0158] <Example 3> The thickness of the cold-rolled steel sheet used as the base material was set to 180 μm. The target thickness of the nickel plating layer in the nickel plating process was set to 3.0 μm, and the nickel deposition amount was 26.7 g / m². 2 The conditions for continuous annealing in the first heat treatment process were set to 680°C with a soaking time of 40 seconds. The reduction ratio in the first rolling process was set to 65-70%. All other conditions were the same as in Example 2. The nickel content of the surface-treated steel foil after the second heat treatment process was 8.3 g / m². 2 The hydrogen permeation current density (oxidation current value) is 5.3 μA / cm². 2 The results are shown in Table 2.

[0159] <Example 4> The heat treatment temperature for the second heat treatment step was set to 620°C. All other procedures were the same as in Example 3. The nickel content of the surface-treated steel foil after the second heat treatment process was 8.3 g / m². 2 The hydrogen permeation current density (oxidation current value) is 5.3 μA / cm². 2 The results are shown in Table 2.

[0160] <Example 5> The sample, which had undergone the second heat treatment process under the same conditions as in Example 2, was then rolled (second rolling process). The rolling conditions for the second rolling process were cold rolling with a reduction ratio of 10-15%. The reduction ratio for the second rolling process was calculated from the thickness before and after the second rolling process. On the other hand, the total reduction ratio was 76.2%. The total reduction ratio is calculated from the thickness before the first rolling process and the thickness after the second rolling process. The nickel content of the surface-treated steel foil after the second rolling process is 10.6 g / m². 2 The hydrogen permeation current density (oxidation current value) is 7.6 μA / cm². 2 The results are shown in Table 2.

[0161] <Example 6> The cold-rolled steel sheet thickness of the base material was set to 180 μm, the continuous annealing conditions for the first heat treatment were set to 660°C for a soaking time of 40 seconds, the reduction ratio for the first rolling was set to 65-70%, and the heat treatment temperature for the second heat treatment was set to 590°C, all other than the same conditions as in Example 5. The total reduction ratio was 73.7%. The nickel content of the surface-treated steel foil after the second rolling process is 11.7 g / m². 2 The hydrogen permeation current density (oxidation current value) is 3.0 μA / cm². 2 The results are shown in Table 2.

[0162] <Example 7> The target thickness of the nickel plating layer in the nickel plating process is 3.0 μm, and the nickel deposition amount is 26.7 g / m². 2 This was done. Otherwise, the procedure was the same as in Example 5. The total reduction ratio was 75.7%. The nickel content of the surface-treated steel foil after the second rolling process is 6.48 g / m². 2 The hydrogen permeation current density (oxidation current value) is 27.5 μA / cm².2 It was. The results are shown in Table 2.

[0163] <Comparative Example 1> A cold-rolled steel sheet (thickness: 50 μm) of a low-carbon aluminum-killed steel having the chemical composition shown below was prepared. C: 0.04% by weight, Mn: 0.32% by weight, Si: 0.01% by weight, P: 0.012% by weight, S: 0.014% by weight, balance: Fe and unavoidable impurities After subjecting the prepared cold-rolled steel sheet to electrolytic degreasing and pickling in sulfuric acid, nickel plating was performed to form nickel plating layers with a target thickness of 0.5 μm and a nickel adhesion amount of 4.5 g / m 2 on both sides, respectively. The conditions for nickel plating were the same as those in Example 1 except for the adhesion amount. For the obtained surface-treated steel foil, X-ray diffraction and hydrogen permeation current density were measured. As a result of X-ray diffraction analysis, the presence of an iron-nickel alloy layer and Fe1Ni1 was not confirmed. The hydrogen permeation current density (oxidation current value) was 273.0 μA / cm 2 It was. The results are shown in Table 2.

[0164] <Comparative Example 2> Samples that had gone through the process under the same conditions as in Example 1-1 (e3) up to the first rolling process were annealed (second heat treatment process). As the heat treatment conditions for the second heat treatment process, soaking was performed at 600 °C for 60 seconds. For the obtained surface-treated steel foil, X-ray diffraction and hydrogen permeation current density were measured. The presence of Fe1Ni1 was confirmed, but Equation (1) was not satisfied. The nickel adhesion amount was 5.82 g / m 2 and the hydrogen permeation current density (oxidation current value) was 100.0 μA / cm 2 It was. The results are shown in Table 2.

[0165] <Comparative Example 3> The thickness of the cold-rolled steel sheet of the base plate was set to 200 μm. The target thickness of the nickel plating layer in the nickel plating process was set to 1.9 μm, and the nickel adhesion amount was 16.91 g / m 2The conditions for the first heat treatment process were 700°C for 40 seconds for continuous annealing, 75-80% for the first rolling, and 480°C for the second heat treatment. All other conditions were the same as in Example 2. The presence of Fe1Ni1 was confirmed, but equation (1) was not satisfied. The hydrogen permeation current density (oxidation current value) was 80.0 μA / cm². 2 The results are shown in Table 2.

[0166] <Example 8> For the nickel plating layer in the nickel plating process, one side has a target thickness of 5.0 μm and a nickel deposition amount of 44.5 g / m². 2 This was done (Example 8-1). On the other side, the target thickness was 1.0 μm and the nickel deposition amount was 8.9 g / m². 2 This was done (Example 8-2). The reduction ratio in the first rolling was set to 65-70%, and the conditions for continuous annealing in the first heat treatment process were 680°C for 40 seconds. Otherwise, the sample was obtained in the same manner as in Example 6. The obtained sample was subjected to rolling (second rolling process). The rolling conditions for the second rolling process were room temperature with a reduction ratio of 10-15%. The total reduction ratio was 73.1%. The nickel content of the surface-treated steel foil after the second rolling process was 12.0 g / m². 2 (Example 8-1), 2.4 g / m 2 (Example 8-2) The hydrogen permeation current density (oxidation current value) measured using each surface as the detection surface was 15.0 μA / cm² for both Example 8-1 and Example 8-2. 2 The results are shown in Table 3.

[0167] <Example 9> A sample obtained under the same conditions as in Example 6 was nickel-plated to a target thickness of 1.0 μm on both sides (re-plating process). X-ray diffraction analysis was performed on the resulting surface-treated steel foil. In addition, the hydrogen permeation current density was measured without forming a nickel film for measurement. The hydrogen permeation current density (oxidation current value) was 3.0 μA / cm². 2 The results are shown in Table 4.

[0168] <Example 10> A sample prepared under the same conditions as in Example 6 was subjected to a nickel undercoat plating to a target thickness of 1.0 μm on both sides (re-plating step). The nickel undercoat plating conditions were as follows. Next, a roughened nickel plating was applied to one side under the following conditions (roughened nickel layer formation step). This roughened nickel layer formation step also included coating nickel plating. (Undercoat nickel plating conditions) Bath composition: Nickel sulfate hexahydrate 250g / L, Nickel chloride hexahydrate 45g / L, Boric acid 30g / L pH 4.2 Bath temperature 60℃ Current density 10A / dm 2 Plating time: 30 seconds (Conditions for roughened nickel plating) Nickel sulfate hexahydrate concentration in the plating bath: 10 g / L Nickel chloride hexahydrate concentration in the plating bath: 10 g / L Chloride ion concentration in the plating bath: 3 g / L Ratio of nickel ions to ammonium ions in the plating bath: Nickel ions / Ammonium ions (by weight) = 0.17 pH: 6 Bath temperature: 50℃ Current density: 12A / dm 2 Plating time: 60 seconds (Conditions for nickel plating) Bath composition: Nickel sulfate hexahydrate 250g / L, Nickel chloride hexahydrate 45g / L, Boric acid 30g / L pH: 4.0~5.0 Bath temperature: 60℃ Current density: 5A / dm 2 Plating time: 36 seconds X-ray diffraction analysis was performed on the roughened nickel layer of the obtained surface-treated steel foil. Furthermore, without forming a nickel film for measurement, the hydrogen permeation current density was measured using the roughened nickel layer as the detection side. The hydrogen permeation current density (oxidation current value) was 3.0 μA / cm². 2 The results are shown in Table 4.

[0169] <Example 11> Except that the plating time in the roughened nickel layer formation step was 85 seconds, Example 10 the same procedure as [Example 5] was followed. X-ray diffraction analysis was performed on the obtained surface-treated steel foil on the side of the roughened nickel layer. Also, with the roughened nickel layer as the detection side, the hydrogen permeation current density was measured. The hydrogen permeation current density (oxidation current value) was 3.0 μA / cm 2 . The results are shown in Table 4.

[0170]

Table 1

[0171]

Table 2

[0172]

Table 3

[0173]

Table 4

[0174] It was confirmed that each of the examples had preferable hydrogen barrier properties. On the other hand, it was confirmed that in Comparative Example 1, the objective could not be achieved from the viewpoint of hydrogen barrier properties.

[0175] Specifically, in each of Examples 1 to 11, the orientation index of the (220) plane of Fe1Ni1, which is a feature of recoiling, was 1.0 or more, and a structure satisfying Formula (1) and having Fe1Ni1(220) / Fe(200) of 0.5 or more was obtained, so that good hydrogen barrier properties were obtained. This is presumably because even if the iron-nickel diffusion layer became locally thin and iron was exposed during recoiling, the exposed portion was suppressed and an iron-nickel alloy layer having a sufficient Fe1Ni1 alloy phase could be formed in the subsequent process.

[0176] On the other hand, the surface-treated steel foil in Comparative Example 1, which was simply nickel-plated, did not exhibit hydrogen barrier properties. Furthermore, in Comparative Examples 2 and 3, equation (1) was not satisfied, and the left-hand side was less than 0.5, indicating that the hydrogen barrier properties could not be restored. This is thought to be because the exposed iron portions formed in the first rolling process could not be alloyed in the second heat treatment process and remained.

[0177] Furthermore, the examples where Fe1Ni1(220) / Fe(200) was 0.6 or higher (Examples 2-12) showed a higher recovery of hydrogen barrier properties. Furthermore, in the examples where Fe(211) / Fe(200) is 2.0 or higher (Examples 2-6), 8-1、8-2、9~11 ) showed particularly good hydrogen barrier properties.

[0178] Furthermore, in this embodiment, no peak indicating dissolution appeared when measuring hydrogen permeation current density in a strongly alkaline environment and with a potential of +0.4V applied to the hydrogen detection side, and the background oxidation current remained stable. Therefore, this embodiment can be said to also possess electrolyte resistance. The trend of the background oxidation current was similar even when the nickel coating for measurement was absent.

[0179] Furthermore, the embodiments described above and each example can be modified in various ways without departing from the spirit of the present invention. Furthermore, although the surface-treated steel foils in the embodiments and examples described above were primarily used as current collectors for bipolar batteries, they are not limited to this use and can also be applied to other uses, such as heat dissipation materials and electromagnetic shielding materials. [Industrial applicability]

[0180] As described above, the surface-treated steel foil of the present invention can be applied to a wide range of industries, including automobiles and electronic equipment. Furthermore, when the surface-treated steel foil of the present invention is used in automotive batteries and the like, it can particularly contribute to improving fuel efficiency. [Explanation of symbols]

[0181] 10 Surface-treated steel foil 10a First face 10b Second face 20 Base material 30 Iron-nickel alloy layer 40 metal layer 50 Roughened nickel layer Ch1 Potentiostat Ch2 potentiostat

Claims

1. A surface-treated steel foil having a first surface and a second surface located opposite to the first surface, A base material made of low-carbon steel or ultra-low-carbon steel, The present invention comprises an iron-nickel alloy layer laminated on the substrate on at least one of the first and second surfaces, The aforementioned iron-nickel alloy layer contains Fe as an alloying phase. １ Ni １ It includes, In the surface having the iron-nickel alloy layer, the Fe １ Ni １ The orientation index in the X-ray diffraction of the (220) plane is 1.0 or greater, and The Fe １ Ni １ A surface-treated steel foil characterized in that the ratio of the maximum diffraction intensity of the (220) plane to the maximum diffraction intensity of the Fe(200) plane satisfies the following equation (1). I (Fe) １ Yes １ (220)) / / (Fe (2000))≧0.5・・・ (1)

2. The surface-treated steel foil according to claim 1, wherein the ratio of the maximum diffraction intensity of the Fe (211) plane to the maximum diffraction intensity of the Fe (200) plane among the Fe crystal planes contained in the iron-nickel alloy layer satisfies the following formula (2). I(Fe(211)) / I(Fe(200))≧1.7...(2)

3. The substrate has an iron-nickel alloy layer on both the first and second surfaces, The Fe １ Ni １ is included as an alloy phase in the iron-nickel alloy layer on at least one of the surfaces of the first surface or the second surface. The Fe １ Ni １ In a surface having an iron-nickel alloy layer containing the Fe １ Ni １ The orientation index in the X-ray diffraction of the (220) plane is 1.0 or greater, and The Fe １ Ni １ The surface-treated steel foil according to claim 1, wherein the ratio of the maximum diffraction intensity of the (220) plane to the maximum diffraction intensity of the Fe(200) plane satisfies the following formula (1). I (Fe) １ Yes １ (220)) / / (Fe (2000))≧0.5・・・ (1)

4. The Fe １ Ni １ The surface-treated steel foil according to claim 3, wherein the surface having an iron-nickel alloy layer containing satisfies the following formula (3). I (Fe) １ Yes １ (220)) / I(Fe (2000))≧0.6・・・ (3)

5. The surface-treated steel foil according to any one of claims 1 to 4, wherein the overall thickness of the surface-treated steel foil is 200 μm or less.

6. The amount of nickel deposited in the aforementioned iron-nickel alloy layer is 2.22 to 26.7 g / m² per side. ２ The surface-treated steel foil according to any one of claims 1 to 4.

7. The surface-treated steel foil according to any one of claims 1 to 4, further comprising a metal layer formed on the iron-nickel alloy layer, wherein the metal layer is a nickel layer.

8. The total amount of nickel deposited in the iron-nickel alloy layer and the nickel layer is 2.22 to 53.4 g / m². ２ The surface-treated steel foil according to claim 7.

9. The electrochemically measured hydrogen permeation current density was 55 μA / cm². ２ The surface-treated steel foil according to any one of claims 1 to 4, which is as follows: However, hydrogen permeation current density is defined as the increase in oxidation current measured on the hydrogen detection side when a potential of -1.5V is applied to the hydrogen generation side, under conditions where the reference electrodes for the potentials of the hydrogen detection side and the hydrogen generation side are Ag / AgCl, the potential of the hydrogen detection side is +0.4V in an electrolyte solution at 65°C, and the potential of the hydrogen detection side is +0.4V.

10. A surface-treated steel foil according to any one of claims 1 to 4, wherein a roughened nickel layer is formed on the outermost surface of at least one of the first surface and the second surface, and the three-dimensional surface property parameter Sa of the roughened nickel layer is 0.2 to 1.3 μm.

11. A surface-treated steel foil according to any one of claims 1 to 4, for use as a current collector in a battery.

12. The surface-treated steel foil according to claim 11, for use as a current collector in a bipolar battery.

13. A surface-treated steel foil having a first surface on which a hydrogen storage alloy is arranged, and a second surface located opposite to the first surface, A base material made of low-carbon steel or ultra-low-carbon steel, The surface-treated steel foil has an iron-nickel alloy layer laminated on the substrate on at least one of the first surface and the second surface to suppress the permeation or diffusion of hydrogen within the surface-treated steel foil. The aforementioned iron-nickel alloy layer contains Fe as an alloying phase. １ Ni １ It includes, In the surface having the iron-nickel alloy layer, the Fe １ Ni １ The orientation index in the X-ray diffraction of the (220) plane is 1.0 or greater, and The Fe １ Ni １ A surface-treated steel foil for current collectors, wherein the ratio of the maximum diffraction intensity of the (220) plane to the maximum diffraction intensity of the Fe(200) plane satisfies the following equation (1). I (Fe) １ Yes １ (220)) / / (Fe (2000))≧0.5・・・ (1)

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