EP4589049A1 - Method for producing stainless steel, weight reduction processing method, stainless steel and aqueous composition - Google Patents

Abstract

The present invention provides: low-density stainless steel and a method for producing the same; a weight reduction processing method for stainless steel; and an aqueous composition, etc. that is useful in weight reduction of stainless steel. The above problem was solved by a method for producing stainless steel, etc. that includes a weight reduction processing step that reduces the weight of stainless steel using an aqueous composition, the density of the stainless steel that has undergone weight reduction processing being less than 7.4 g/cm³. Reduction of the weight and lowering of the density of stainless steel can be achieved by an easy method in accordance with the present invention.

Description

TECHNICAL FIELD
• The present invention relates to a manufacturing method for stainless steel, a method for weight reduction treatment of stainless steel, stainless steel and an aqueous composition, and particularly relates to low-density stainless steel and a manufacturing method therefor, a weight reduction treatment method, and an aqueous composition useful for weight reduction of stainless steel.
BACKGROUND ART
• Because of its properties excellent in durability and weather resistance, stainless steel has recently been considered for application in various fields. For example, stainless steel is coming into widespread use in electronic components, battery current collector foils and automotive component housings.
• For use in various products, stainless steel is known to be treated to increase its surface area (e.g., Patent Literature 1) and/or treated to form asperities on the surface of stainless steel (e.g., Patent Literature 2), etc.
CITATION LIST Patent Literatures
• Patent Literature 1: JP 2011-168017 A
• Patent Literature 2: JP 2015-183239 A
SUMMARY OF INVENTION Technical Problem
• Stainless steel is desired to be reduced in weight depending on the purpose of its use. In particular, there is a demand in some cases for stainless steel having a certain thickness and being of low density. Moreover, even when using conventional methods for manufacturing or surface treatment of stainless steel, etc., it was difficult to obtain certain stainless steel of low density whose weight has been reduced.
Solution to Problem
• The present invention provides a manufacturing method for stainless steel, a weight reduction treatment method, stainless steel, an aqueous composition, etc., as shown below.
• Namely, the present invention encompasses the following embodiments.
1. [1] A manufacturing method for stainless steel, comprising:
   • a weight reduction treatment step wherein stainless steel is treated using an aqueous composition,
   • wherein the density of the stainless steel after the weight reduction treatment is less than 7.4 g/cm³.
2. [2] The manufacturing method for stainless steel according to [1] above, wherein the density of the stainless steel after the weight reduction treatment is 7.0 g/cm³ or less.
3. [3] The manufacturing method for stainless steel according to [1] or [2] above, wherein the density (g/cm³) of the stainless steel after the weight reduction treatment is 0.95-fold or less of the density (g/cm³) of the stainless steel before the weight reduction treatment.
4. [4] The manufacturing method for stainless steel according to any one of [1] to [3] above, wherein the thickness of the stainless steel after the weight reduction treatment is 100 µm or less.
5. [5] The manufacturing method for stainless steel according to any one of [1] to [4] above, wherein the thickness (µm) of the stainless steel after the weight reduction treatment is 0.80-fold or more of the thickness (µm) of the stainless steel before the weight reduction treatment.
6. [6] The manufacturing method for stainless steel according to any one of [1] to [5] above, wherein the aqueous composition comprises 0.1% to 5% by mass of hydrogen peroxide, 1% to 30% by mass of halide ions and 0% to 40% by mass of copper ions.
7. [7] Stainless steel having a density of less than 7.4 g/cm³ and a thickness of 100 µm or less.
8. [8] A method for weight reduction treatment of stainless steel, comprising:
   • a weight reduction treatment step wherein stainless steel is treated using an aqueous composition,
   • wherein the density of the stainless steel after the weight reduction treatment is less than 7.4 g/cm³.
9. [9] An aqueous composition for reducing the weight of stainless steel, comprising 0.1% to 5% by mass of hydrogen peroxide, 1% to 30% by mass of halide ions and 0% to 40% by mass of copper ions.
ADVANTAGEOUS EFFECTS OF INVENTION
• According to the present invention, it is possible to achieve a manufacturing method for stainless steel, a method for weight reduction treatment of stainless steel and an aqueous composition, each of which allows the weight and density reduction of stainless steel by simple procedures, as well as low-density stainless steel.
• According to the present invention, weight reduction can be attempted preferably without too much thinning of stainless steel which is foil-shaped or plate-shaped. Moreover, the present invention achieves, for example, stainless steel whose density has been reduced while preventing the deterioration of its surface properties, such as the occurrence of pinholes, as well as a manufacturing method for such stainless steel.
DESCRIPTION OF EMBODIMENTS
• The present invention will be further described in more detail below. The present invention is not limited to the following descriptions, and various modifications may be made without departing from the spirit of the present invention.
[1. Manufacturing method for stainless steel]
• The manufacturing method for stainless steel of the present invention comprises a weight reduction treatment step wherein stainless steel is treated using an aqueous composition. According to the manufacturing method for stainless steel of the present invention, for example, plate-shaped or foil-shaped stainless steel can be reduced in weight and density while mostly maintaining its thickness before the weight reduction treatment.
<1-1. Properties of stainless steel>
• Stainless steel undergoing the weight reduction treatment step, i.e., stainless steel after the weight reduction treatment has a density of less than 7.4 g/cm³ (7.40 g/cm³). Stainless steel according to an embodiment of the present invention, for example, stainless steel obtained by being subjected to the weight reduction treatment has a density of preferably 7.3 g/cm³ or less, 7.29 g/cm³ or less, less than 7.29 g/cm³, 7.28 g/cm³ or less, or 7.2 g/cm³ or less, more preferably 7.18 g/cm³ or less, 7.15 g/cm³ or less, 7.1 g/cm³ or less, or 7.0 g/cm³ or less, and even more preferably 6.9 g/cm³ or less, or 6.8 g/cm³ or less. The lower limit of the density of stainless steel after the weight reduction treatment will vary depending on the intended purpose of use and is not limited in any way, but it is usually preferably 1.3 g/cm³ or more in terms of durability and weather resistance.
• The density D₁ (g/cm³) of stainless steel undergoing the weight reduction treatment compared with the density D₂ (g/cm³) of the same stainless steel without undergoing the weight reduction treatment is preferably 0.95-fold or less (i.e., D₁/D₂ ≤ 0.95), more preferably 0.93-fold or less or 0.91-fold or less, even more preferably 0.90-fold or less or 0.88-fold or less, and particularly preferably 0.85-fold or less, 0.82-fold or less or 0.80-fold or less. The lower limit of the ratio of densities before and after the weight reduction treatment (D₁/D₂) will vary depending on the intended purpose of use and is not limited in any way, but it is usually preferably 0.16-fold or more in terms of durability and weather resistance.
• The difference between the density D₁ (g/cm³) of stainless steel undergoing the weight reduction treatment and the density D₂ (g/cm³) of the same stainless steel without undergoing the weight reduction treatment, i.e., D₂-D₁ (g/cm³) is preferably 0.2 (g/cm³) or more, more preferably 0.4 (g/cm³) or more, 0.5 (g/cm³) or more or 0.6 (g/cm³) or more, even more preferably 0.8 (g/cm³) or more, 0.9 (g/cm³) or more or 1.0 (g/cm³) or more, and particularly preferably 1.2 (g/cm³) or more, 1.4 (g/cm³) or more or 1.6 (g/cm³) or more. The upper limit of the difference between densities before and after the weight reduction treatment (D₂-D₁ (g/cm³)) will vary depending on the intended purpose of use and is not limited in any way, but it is usually preferably 6.7 (g/cm³) or less in terms of durability and weather resistance.
• There is no particular limitation on the shape and thickness of stainless steel to be subjected to the weight reduction treatment, but foil-shaped or plate-shaped stainless steel is preferred for use. When foil-shaped stainless steel, i.e., a stainless foil is subjected to the weight reduction treatment, the thickness is for example 100 µm or less, preferably 50 µm or less or 25 µm or less, more preferably 20 µm or less or 18 µm or less, even more preferably 15 µm or less or 12 µm or less, and particularly preferably 10 µm or less. The lower limit of the above thickness will vary depending on the intended purpose of use and is not limited in any way, but it is preferably 1 µm or more in terms of durability and weather resistance.
• When plate-shaped stainless steel is subjected to the weight reduction treatment, the thickness is for example 2.0 mm or less, preferably 1.5 mm or less, more preferably 1.2 mm or less, or less than 1.2 mm, even more preferably 1.1 mm or less, less than 1.1 mm, or 1.0 mm or less, and particularly preferably 0.8 mm or less. The lower limit of the above thickness will vary depending on the intended purpose of use and is not limited in any way, but it is preferably 0.1 mm or more in terms of durability and weather resistance.
• The thickness T₁ (µm) of stainless steel undergoing the weight reduction treatment compared with the thickness T₂ (µm) of the same stainless steel without undergoing the weight reduction treatment is preferably 0.80-fold or more (i.e., T₁/T₂ ≥ 0.80), more preferably 0.85-fold or more or 0.90-fold or more, even more preferably 0.95-fold or more or 0.98-fold or more, and particularly preferably 1.0-fold, i.e., the thickness before the treatment is maintained.
<1-2. Type of stainless steel>
• There is no particular limitation on the type of stainless steel to be subjected to the weight reduction treatment using the aqueous composition of the present invention, but examples include the following.
• Namely, examples include those defined in JIS G4305, as exemplified by chromium-nickel stainless steel including SUS304, SUS316, SUS321, SUS347, and SUS329J1; ferrite stainless steel (chromium stainless steel) including SUS405, SUS430, SUS434, SUS444, SUS447, and SUSXM27; and precipitation hardening stainless steel (chromium-nickel stainless steel) including SUS630, SUS631, and SUH660.
• Among the above stainless steels, SUS304, SUS430 series (e.g., SUS430 and SUS430LX) and SUS444 are more preferred as those to be subjected to the weight reduction treatment using the aqueous composition.
<1-3. Aqueous composition>
• The aqueous composition for use in the manufacturing method for stainless steel preferably comprises 0.1% to 5% by mass of hydrogen peroxide, 1% to 30% by mass of halide ions and 0% to 40% by mass of copper ions, each based on the total amount of the aqueous composition. Details on the components of the aqueous composition will be described later. The aqueous composition forms fine hollows on the surface of stainless steel, whereby weight reduction is attempted without extremely reducing the thickness of stainless steel.
• In the method for weight reduction treatment for reducing the surface weight of stainless steel according to the present invention, a weight reduction treatment step is conducted. Namely, the method for weight reduction treatment of the present invention comprises a weight reduction treatment step in which the above aqueous composition is used to perform weight reduction treatment on the surface of stainless steel. In the context of the present invention, performing weight reduction treatment on the surface of stainless steel using the aqueous composition is intended to mean bringing the aqueous composition for use in the treatment into contact with the surface of stainless steel. As can be seen from this, the method for weight reduction treatment of stainless steel of the present invention comprises at least the step of bringing the aqueous composition into contact with the surface of stainless steel.
<1-4. Conditions for weight reduction treatment>
• In the weight reduction treatment step, the above aqueous composition is brought into contact with stainless steel to be treated.
• In the weight reduction treatment step, the temperature required for weight reduction treatment is preferably 20°C to 60°C, more preferably 25°C to 55°C, and particularly preferably 30°C to 50°C. Thus, the method for weight reduction treatment of stainless steel of the present invention is advantageous in that the surface weight reduction of stainless steel proceeds even at not so high temperature, for example, even at room temperature of 25°C. In the context of the present invention, the temperature required for weight reduction treatment is intended to mean the temperature at which the aqueous composition is brought into contact with the surface of stainless steel, particularly the solution temperature of the aqueous composition to be brought into contact with the surface of stainless steel.
• Moreover, in the weight reduction treatment step, the time required for weight reduction treatment is preferably 30 seconds to 120 seconds, more preferably 40 seconds to 100 seconds, and particularly preferably 50 seconds to 90 seconds. Thus, the method for weight reduction treatment of stainless steel of the present invention is advantageous in that the weight reduction of stainless steel proceeds even within not so long time. In the context of the present invention, the time required for weight reduction treatment is intended to mean the time during which the aqueous composition is in contact with the surface of stainless steel. For example, it means the time during which stainless steel is soaked in the aqueous composition, or the time required from spraying the aqueous composition over the surface of stainless steel until removing the aqueous composition by water washing, etc.
• Any method may be used to bring the aqueous composition into contact with the surface of stainless steel. For example, it is possible to select a method in which the aqueous composition is brought into contact with stainless steel in a dropwise manner or in a spraying (spray treatment) manner, etc., or a method in which stainless steel is soaked in the aqueous composition. In the present invention, either of these methods may be selected for this purpose. For example, the aqueous composition may be sprayed over stainless steel processed into a specific shape to obtain weight-reduced stainless steel, or alternatively, an apparatus for adding dropwise, spraying or soaking the aqueous composition is provided between stainless steel foil rolls, and a stainless steel foil is passed near the above apparatus to supply the aqueous composition during being conveyed in a roll-to-roll fashion from the roll around which the untreated stainless steel foil has been wound, and the weight-reduced stainless steel foil is then wound around the other roll.
• It should be noted that the weight-reduced stainless steel obtained in the weight reduction treatment step may be subjected to water washing or other treatment (i.e., a washing step).
[2. Method for weight reduction treatment of stainless steel]
• The above weight reduction treatment step not only serves as a part of the manufacturing method for stainless steel, but is also useful as a post-treatment for the manufactured stainless steel, etc., by way of example. The method for weight reduction treatment of stainless steel of the present invention comprises the weight reduction treatment step described above. Thus, in the weight reduction treatment used in the weight reduction treatment method, the properties and type of stainless steel, the components of the aqueous composition, and conditions for the weight reduction treatment are as described above.
[3. Aqueous composition]
• The aqueous composition of the present invention is preferred for use in the manufacturing method for stainless steel or the method for weight reduction treatment of stainless steel described above. The aqueous composition of the present invention comprises 0.1% to 5% by mass of hydrogen peroxide, 1% to 30% by mass of halide ions and 0% to 40% by mass of copper ions, each based on the total amount of the aqueous composition. In addition to these components, the aqueous composition may comprise water, etc., especially ion exchanged water or ultrapure water.
• Each component of the aqueous composition will be explained below.
<3-1. Hydrogen peroxide>
• Hydrogen peroxide to be contained in the aqueous composition is usually provided as an aqueous solution of appropriate concentration for admixture with other components. The concentration of hydrogen peroxide in an aqueous hydrogen peroxide solution for use in the preparation of the aqueous composition is not limited in any way, and it may be for example 10% to 90%, and is preferably 35% to 60% in line with the industrial standards.
• Moreover, hydrogen peroxide may contain a stabilizer in an amount up to around 0.01% by mass, and acceptable stabilizers include sulfuric acid, phosphoric acid and so on. Hydrogen peroxide may be prepared in any manner and may be available through any route. For example, hydrogen peroxide prepared by the anthraquinone method may be used.
• The concentration of hydrogen peroxide in the aqueous composition is 0.1% to 5.0% by mass based on the total amount (total mass) of the aqueous composition, but it is preferably 0.2% to 4.5% by mass, more preferably 0.3% to 4.5% by mass or 0.2% to 4.0% by mass, even more preferably 0.35% to 3.0% by mass or 0.3% to 3.5% by mass, and particularly preferably 0.4% to 2.0% by mass, 0.45% to 1.5% by mass, 0.45% to 1.2% by mass or 0.5% to 1.0% by mass, based on the total amount (total mass) of the aqueous composition.
• Moreover, the lower limit of the concentration of hydrogen peroxide in the aqueous composition may be, for example, 0.001% by mass, 0.01% by mass, 0.05% by mass, 0.15% by mass, 0.2% by mass, 0.25% by mass, 0.3% by mass, 0.35% by mass, 0.4% by mass, 0.45% by mass or 0.5% by mass, based on the total amount (total mass) of the aqueous composition, while the upper limit of the concentration of hydrogen peroxide contained in the aqueous composition may be, for example, 4.8% by mass, 4.5% by mass, 4.2% by mass, 4.0% by mass, 3.8% by mass, 3.5% by mass, 3.2% by mass, 3.0% by mass, 2.8% by mass, 2.5% by mass, 2.2% by mass, 2.0% by mass, 1.8% by mass, 1.5% by mass, 1.2% by mass, 1.0% by mass, etc., based on the total amount (total mass) of the aqueous composition.
• The concentration range of hydrogen peroxide may be selected as appropriate from the above lower and upper limits combined.
• As a result of selecting the concentration of hydrogen peroxide from among the ranges mentioned above, the effect of the present invention tends to be achieved in a more preferred manner, and even when copper ions, halide ions and others described later are also contained in the aqueous composition, the possibility of heat generation or bubble formation associated with the decomposition of hydrogen peroxide can be suppressed to thereby ensure the safety of operation.
<3-2. Halide ions (halogen ions)>
• Halide ions to be contained in the aqueous composition may be of any type, and examples include fluoride ions, chloride ions, bromide ions and iodide ions, with chloride ions being more preferred in terms of easy handling and cost effectiveness.
• Halogen compounds supplying halide ions are not limited in any way, and examples include hydrochloric acid, alkali metal halides (e.g., sodium halides and potassium halides), alkaline earth metal halides (e.g., calcium halides), ammonium halides, copper halides, and hydrogen halides. Among them, preferred are hydrochloric acid, alkali metal halides or hydrogen halides in terms of more effectively and reliably providing the effect of the present invention, and more preferred is hydrochloric acid or sodium chloride.
• Such halogen compounds are used either alone or in combination. It should be noted that halogen compounds may overlap with copper compounds described later. For example, when a copper halide is used as a source of halide ions, this copper halide also falls within copper compounds described later, which serve as a copper ion source. Copper chloride is preferred as a copper halide. Halogen compounds (halide ions) are deemed to cause pitting corrosion on the passive film during the weight reduction treatment of the stainless steel surface.
• The concentration of halide ions in the aqueous composition is 1% to 30% by mass based on the total amount (total mass) of the aqueous composition, but it is preferably 2.0% to 25% by mass, more preferably 4.0% to 22% by mass or 3.0% to 20% by mass, and particularly preferably 5.0% to 15% by mass, 8% to 15% by mass, or 10% to 15% by mass, based on the total amount (total mass) of the aqueous composition.
• Moreover, as to the concentration range of halide ions in the aqueous composition, the lower limit may be set to any of 0.01% by mass, 0.1% by mass, 0.5% by mass, 1.0% by mass, 2.0% by mass, 3.0% by mass, 5.0% by mass, 8.0% by mass, 10.0% by mass and 12.0% by mass, based on the total amount (total mass) of the aqueous composition, while the upper limit may be set to any of 29% by mass, 28% by mass, 27% by mass, 26% by mass and 25% by mass, based on the total amount of the aqueous composition.
• The concentration range of halide ions may be selected as appropriate from the above lower and upper limits combined as appropriate.
• As a result of selecting the concentration of halide ions from among the ranges mentioned above, the effect of the present invention tends to be achieved in a more preferred manner. In more detail, in the aqueous composition whose concentration of halide ions is within the above range, fine hollows can be formed on the surface of stainless steel while preventing the progression of pitting corrosion reaction on stainless steel, and the decomposition reaction of hydrogen peroxide can also be prevented to ensure safety.
<3-3. Copper ions>
• The concentration of copper ions in the aqueous composition is 0% to 40% by mass based on the total amount (total mass) of the aqueous composition. Namely, the aqueous composition intended in the present invention is free from copper ions or has a copper ion content of 40% by mass or less based on the total amount (total mass) of the aqueous composition.
• Copper ions in the aqueous composition can be generated by mixing a copper compound serving as a copper ion source with other components. The copper ion source may be of any type, as long as it is a copper compound capable of supplying copper ions in the aqueous composition.
• Examples of such a copper compound include copper sulfate (e.g., cupric sulfate), copper chloride (e.g., cupric chloride), copper tetrafluoroborate, cupric bromide, cupric oxide, copper phosphate, copper acetate, copper formate, copper nitrate and so on, which may be in anhydride form or in pentahydrate form. Among them, preferred is copper sulfate or copper chloride in terms of more effectively and reliably providing the effect of the present invention and in terms of easy handling and cost effectiveness, more preferred is cupric sulfate or cupric chloride, and even more preferred is cupric sulfate. These members may be used either alone or in combination.
• It is inferred that copper ions contained in the aqueous composition will cause substitution reaction for nickel and chromium, which are components of stainless steel, during the weight reduction treatment, and substitution reaction products derived from copper ions are then removed to form hollows on the surface.
• Moreover, in terms of the quality of stainless steel after the weight reduction treatment, particularly when the aqueous composition is used in the treatment of a stainless steel foil whose thickness is small, the content of copper ions is preferably adjusted to prevent the occurrence of extremely thin regions or pinholes, as described later.
• The concentration of copper ions contained in the aqueous composition is 40% by mass or less, preferably 25% by mass or less, 20% by mass or less or 15% by mass or less, and more preferably 12% by mass or less or 10% by mass or less, and may be 7.0% by mass or less, 5.0% by mass or less, or 3.0% by mass or less. A preferred concentration range of copper ions is preferably 0% to 3.0% by mass, preferably 0% to 2.5% by mass, more preferably 0% to 1.5% by mass, and even more preferably 0.1% to 1.5% by mass, and may be 0.3% to 1.5% by mass or 0.5% to 1.5% by mass.
• The lower limit of the concentration of copper ions in the aqueous composition is 0% by mass based on the total amount (total mass) of the aqueous composition, but may be for example 0.00001% by mass (0.1 mass ppm), 0.0001% by mass (1 mass ppm), 0.001% by mass, 0.01% by mass, 0.02% by mass, 0.03% by mass, 0.05% by mass, 0.07% by mass, or 0.1% by mass, based on the total amount (total mass) of the aqueous composition.
• Likewise, the upper limit of the concentration of copper ions in the aqueous composition may be, for example, 30% by mass, 25% by mass, 20% by mass, 17% by mass, 15% by mass, 10% by mass, 7.0% by mass, 5.0% by mass, 3.0% by mass, 2.5% by mass, 2.0% by mass, 1.8% by mass, etc., based on the total amount of the aqueous composition.
• The concentration range of copper ions may be selected from the above lower and upper limits combined as appropriate.
• As a result of selecting the concentration of copper ions from among the ranges mentioned above, the effect of the present invention tends to be achieved in a more preferred manner.
• On the other hand, if the concentration of copper ions in the aqueous composition is too high, pinholes or extremely thin regions are more likely to occur on stainless steel after the weight reduction treatment. If the concentration of copper ions is too low, the formation of hollows on the surface and the weight reduction treatment will probably not proceed efficiently.
<3-4. Additives contained in the aqueous composition>
• The aqueous composition of the present invention may comprise additives as components other than hydrogen peroxide, copper ions and halide ions mentioned above, as long as the effect of the present invention is exerted. Examples of such additives include heterocyclic nitrogen compounds (azole compounds), organic solvents and so on. These additives are used either alone or in combination. Moreover, additives also include a surfactant, a pH adjuster and so on, but they are preferably not contained in the aqueous composition of the present invention.
• The concentration of additives which may be contained in the aqueous composition is preferably 10% by mass or less, more preferably 5.0% by mass or less, even more preferably 2.0% by mass or less, and particularly preferably 1.0% by mass or less.
<3-5. Water>
• The aqueous composition of the present invention may comprise water, and preferably comprises water. The water intended here is not limited in any way, but it is preferably water which has been treated to remove metal ions, organic impurities, particles and so on by distillation, ion exchange treatment, filter treatment, various adsorption treatments, etc., and it is more preferably pure water, and particularly preferably ultrapure water.
• The content of water in the aqueous composition of the present invention is the balance of the composition (i.e., other than the individual components described above and additives described in detail later), and is not limited in any way, but it is preferably 50% to 98% by mass, more preferably 60% to 95% by mass, even more preferably 75% to 93% by mass, and particularly preferably 85% to 90% by mass, based on the total amount (total mass) of the aqueous composition.
• It should be noted that the aqueous composition of the present invention is preferably in the form of a solution, and is preferably free from components which are non-soluble in the composition in the form of a solution, as exemplified by solid particles such as abrasive particles.
<3-6. Function and properties of the aqueous composition>
• The aqueous composition comprising the above individual components is considered to efficiently form hollows on the surface of stainless steel and thereby allow weight reduction.
• Halide ions are responsible for pitting corrosion of the oxide film which is usually formed on the surface of stainless steel. Copper ions have the effect of causing substitution reaction for nickel and chromium, which are components of stainless steel, as described above, and substitution reaction products derived from copper ions are then removed to form relatively large hollows on the surface of stainless steel.
• Likewise, hydrogen peroxide serves to remove the above substitution reaction products derived from copper ions after the substitution reaction.
• Moreover, the aqueous composition comprising copper ions and others whose content is adjusted within an appropriate range can prevent the occurrence of extremely thin regions or pinholes on stainless steel after the weight reduction treatment, particularly on a stainless steel foil after the weight reduction treatment.
• The aqueous composition of the present invention may be prepared by stirring the hydrogen peroxide-containing component, the halide ion-supplying component and water mentioned above, optionally together with the copper ion-supplying component and other components, until a uniform mixture is obtained.
• The properties of the aqueous composition are not limited in any way, but its pH value is preferably -1.0 to 4.0, more preferably -0.5 to 3.0, even more preferably - 0.25 to 2.5, and particularly preferably -0.1 to 2.0, 0.0 to 1.5, 0.005 to 1.0, or 0.01 to 0.5. The pH value may be measured by the method described in the Example section.
[4. Stainless steel]:
• Stainless steel according to the present invention has a density of less than 7.4 (g/cm³) (7.40 g/cm³). Stainless steel according to an embodiment of the present invention, for example, stainless steel obtained by being subjected to the weight reduction treatment has a density of preferably 7.3 g/cm³ or less, 7.29 g/cm³ or less, less than 7.29 g/cm³, 7.28 g/cm³ or less, or 7.2 g/cm³ or less, more preferably 7.18 g/cm³ or less, 7.15 g/cm³ or less, 7.1 g/cm³ or less or 7.0 g/cm³ or less, and even more preferably 6.9 g/cm³ or less or 6.8 g/cm³ or less.
• Thus, according to the manufacturing method and the method for weight reduction treatment, each comprising the weight reduction treatment step described above, low-density stainless steel can be easily manufactured, although low-density stainless steel is difficult to manufacture by commonly used manufacturing methods or treatment methods. Namely, according to the weight reduction treatment step described above, weight-reduced stainless steel whose thickness remains almost unchanged can be obtained simply by a substantially single-step treatment, i.e., weight reduction treatment alone in which the aqueous composition of the present invention is brought into contact with the surface of stainless steel to be treated, or optionally in combination with appropriate washing (e.g., water washing).
• Moreover, as described above, the treatment conditions used in the weight reduction treatment are mild, and the time required for this treatment is also short, so that weight-reduced stainless steel can be efficiently obtained.
• Weight- and density-reduced stainless steel thus obtained can be used, for example, in battery current collector foils for solid-state batteries or lithium-ion batteries, etc., solar battery substrates, flexible substrates for electronic devices, substrates for electric accumulator devices, carriers for exhaust gas purification catalysts, etc., electromagnetic wave shielding members, heat radiating members, and other applications. Weight-reduced stainless steel foils obtained by the manufacturing method for stainless steel of the present invention are preferred for use as battery current collector foils, by way of example.
EXAMPLES
• The present invention will be further described in more detail by way of the following examples, which are not intended to limit the scope of the invention.
<Thickness>
• The surface-treated stainless foils in the following examples and comparative examples, and the untreated stainless foils in Reference Examples 1 to 4 were measured for their thickness with a digital micrometer (manufactured by Mitutoyo Corporation, Japan; MDC-25MXT).
<Density>
• The surface-treated stainless foils in the examples and comparative examples, and the untreated stainless foils in Reference Examples 1 to 4 were calculated for their density by the following equation (1) using the weight (D) of the stainless foil used, the area (S) of the stainless foil used, and the thickness (T) of the stainless foil measured as described above.
  [Math. 1] $$\mathrm{Density}\left(\mathrm{g}/{\mathrm{cm}}^3\right)=\frac{\mathrm{Weight}\mathrm{D}\left(\mathrm{g}\right)\mathrm{of}\mathrm{stainless}\mathrm{foil}}{\mathrm{Area}\mathrm{S}\left({\mathrm{cm}}^2\right)\mathrm{of}\mathrm{stainless}\mathrm{foil}\times \mathrm{Thickness}\mathrm{T}\left(\mathrm{\mu m}\right)\mathrm{of}\mathrm{stainless}\mathrm{foil}}\times {10}^4$$
<Confirmation of the presence or absence of pinholes>
• The surface-treated foils obtained in the examples and comparative examples were visually observed for their surface pattern. A foil showing no occurrence of pinholes (fine through holes) was evaluated as "good" while a foil showing the occurrence of pinholes was evaluated as "poor."
[Example 1]
• A stainless steel (foil) was provided which had a thickness of 10 µm and a length and width of 30 mm × 30 mm and whose material was SUS444 (Reference Example 1).
• To 55 ml of ultrapure water, hydrogen peroxide in a final amount of 0.5% by mass (1.7 g of a 60 wt% aqueous hydrogen peroxide solution) and 72% by mass (143 g) of a 35 wt% aqueous hydrochloric acid solution were added to prepare an aqueous composition. The concentration of halide ions (Cl^-) derived from hydrochloric acid in this composition was 25% by mass based on the total amount of the aqueous composition.
• The above stainless steel foil was soaked for 60 seconds in the above aqueous composition at a solution temperature of 30°C. Then, the soaked stainless steel foil was washed well with ultrapure water, and then dried well to obtain the surface-treated foil. The resulting surface-treated foil was found to have a thickness of 9 µm and a density of 6.6 g/cm³, as measured in accordance with the procedures described above. Moreover, when visually observed for its surface pattern in accordance with the above section (Confirmation of the presence or absence of pinholes), the resulting surface-treated foil showed no occurrence of pinholes (fine through holes) and was evaluated as "good."
[Examples 2 to 7 and Comparative Examples 1 to 4]
• The same weight reduction treatment as shown in Example 1 was performed on stainless steel foils to obtain surface-treated foils, except that the stainless steel to be treated, the properties of the aqueous composition and/or the conditions for the weight reduction treatment method were changed as indicated in Table 1 below.
• It should be noted that copper sulfate pentahydrate (CuSO₄·5H₂O) was used as a source of copper ions (Cu²⁺) in an amount of 3.9 g and 12 g, respectively, in Examples 4 and 7, while iron(III) chloride hexahydrate (FeCl₃·6H₂O) was used in an amount of 133 g and 133 g, respectively, in Comparative Examples 2 and 3.
• The results including the values of thickness and density obtained for the resulting surface-treated foils are shown in Table 1. [Table 1]

  	Metal foil	Aqueous composition				Treatment method		Properties of stainless foil obtained
  		H2O2	Cu2+	Cl-	FeCl3	Temperature	Time	Thickness	Density		Occurrence of pinholes
  		[wt%]	[wt%]	[wt%]	[wt%]	[°C]	[sec]	[µm]	[g/cm3]	Change [%]	[-]
  Example 1	SUS444	0.5	0.0	25	0	30	60	9	6.6	86.1	No
  Example 2	SUS444	0.5	0.0	25	0	30	90	8	5.7	74.5	No
  Example 3	SUS444	1.0	0.0	12	0	30	60	10	6.4	83.0	No
  Example 4	SUS444	0.5	0.5	12	0	30	60	10	6.9	90.4	No
  Example 5	SUS430	0.5	0.0	25	0	30	30	8	6.8	86.9	No
  Example 6	SUS430LX	0.5	0.0	25	0	30	30	8	6.6	88.3	No
  Example 7	SUS304	0.5	1.5	12	0	35	60	8	6.3	79.3	No
  Comparative Example 1	SUS444	0.0	0.0	25	0	30	60	10	7.6	99.2	No
  Comparative Example 2	SUS444	0.0	0.0	0	40	30	30	10	7.4	96.6	Yes
  Comparative Example 3	SUS444	0.0	0.0	0	40	30	60	10	5.3	68.8	Yes
  Comparative Example 4	SUS304	9.0	0.0	9	0	35	60	10	7.9	98.3	Yes
  Reference Example 1 (untreated)	SUS444	-	-	-	-	-	-	10	7.7	-	No
  Reference Example 2 (untreated)	SUS430	-	-	-	-	-	-	10	7.8	-	No
  Reference Example 3 (untreated)	SUS430LX	-	-	-	-	-	-	10	7.4	-	No
  Reference Example 4 (untreated)	SUS304	-	-	-	-	-	-	10	8.0	-	No

INDUSTRIAL APPLICABILITY
• In each example mentioned above using an aqueous composition of particular composition, it was confirmed that stainless steel could be efficiently reduced in weight with few steps and in a short time, and the thickness of stainless steel remained almost unchanged before and after the treatment. Moreover, the treated stainless steel was found to have an almost uniform thickness and showed no defects such as pinholes. When stainless steel, particularly a stainless steel foil, thus reduced in density while maintaining a good appearance is used in battery current collector foils, automotive component housings, etc., dielectric substances or organic materials can be effectively adhered or held. Also in the case of members for heat radiation, weight- and density-reduced stainless steel may be preferred for use.
• Accordingly, the present invention is recognized to have industrial applicability in the field of treating stainless steel as a material member, e.g., as a material member for use in the above products.

Claims (9)

1. A manufacturing method for stainless steel, comprising:

   a weight reduction treatment step wherein stainless steel is treated using an aqueous composition,

   wherein the density of the stainless steel after the weight reduction treatment is less than 7.4 g/cm³.
2. The manufacturing method for stainless steel according to claim 1, wherein the density of the stainless steel after the weight reduction treatment is 7.0 g/cm³ or less.
3. The manufacturing method for stainless steel according to claim 1, wherein the density (g/cm³) of the stainless steel after the weight reduction treatment is 0.95-fold or less of the density (g/cm³) of the stainless steel before the weight reduction treatment.
4. The manufacturing method for stainless steel according to claim 1, wherein the thickness of the stainless steel after the weight reduction treatment is 100 µm or less.
5. The manufacturing method for stainless steel according to claim 1, wherein the thickness (µm) of the stainless steel after the weight reduction treatment is 0.8-fold or more of the thickness (µm) of the stainless steel before the weight reduction treatment.
6. The manufacturing method for stainless steel according to claim 1, wherein the aqueous composition comprises 0.1% to 5% by mass of hydrogen peroxide, 1% to 30% by mass of halide ions and 0% to 40% by mass of copper ions.
7. Stainless steel having a density of less than 7.4 g/cm³ and a thickness of 100 µm or less.
8. A method for weight reduction treatment of stainless steel, comprising:

   a weight reduction treatment step wherein stainless steel is treated using an aqueous composition,

   wherein the density of the stainless steel after the weight reduction treatment is less than 7.4 g/cm³.
9. An aqueous composition for reducing the weight of stainless steel, comprising 0.1% to 5% by mass of hydrogen peroxide, 1% to 30% by mass of halide ions and 0% to 40% by mass of copper ions.