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EP-4741155-A1 - FILM-LAMINATED STEEL SHEET AND METHOD FOR MANUFACTURING DRAWN AND IRONED CAN

EP4741155A1EP 4741155 A1EP4741155 A1EP 4741155A1EP-4741155-A1

Abstract

[SUMMARY] [PROBLEM] To further improve punch-withdrawing performance while maintaining ironing formability during DI forming. [SOLUTION] A film-laminated steel sheet according to the present invention includes: a steel sheet serving as a base material; film layers formed by thermoplastic polyester films provided on front and rear surfaces of the steel sheet; and wax layers provided on the film layers, wherein a coating weight of the wax layer is within a range of 0.030 to 0.135 g/m 2 per side, the coating weights of the wax layers are different between the front surface and rear surface of the steel sheet, and a coating weight ratio, which is obtained by dividing the coating weight of the wax layer on a side with a higher coating weight by the coating weight of the wax layer on a side with a lower coating weight, is within a range of 1.05 to 1.35.

Inventors

  • KADOWAKI, NOBUO
  • TAJIMA, YUTA
  • IWAKIRI, KAZUSHI
  • MIZUTANI, TOMOHIRO

Assignees

  • Nippon Steel Corporation

Dates

Publication Date
20260513
Application Date
20240906

Claims (7)

  1. A film-laminated steel sheet, comprising: a steel sheet serving as a base material; a film layer formed by a thermoplastic polyester film provided on front and rear surfaces of the steel sheet; and a wax layer provided on the film layer, wherein a coating weight of the wax layer is within a range of 0.030 to 0.135 g/m 2 per side, and the coating weights of the wax layers are different between the front surface and rear surface of the steel sheet, and a coating weight ratio, which is obtained by dividing the coating weight of the wax layer on a side with a higher coating weight by the coating weight of the wax layer on a side with a lower coating weight, is within a range of 1.05 to 1.35.
  2. The film-laminated steel sheet according to claim 1, wherein a melting point of the thermoplastic polyester film forming the film layer on the side with the lower coating weight is 215°C or higher and less than 255°C, a melting point of the thermoplastic polyester film forming the film layer on the side with the higher coating weight is 220 to 260°C, and the melting point of the thermoplastic polyester film on the side with the higher coating weight is 5°C or more higher than the melting point of the thermoplastic polyester film on the side with the lower coating weight.
  3. The film-laminated steel sheet according to claim 1 or 2, wherein penetration of wax forming the wax layer specified in JIS K2235:2022 is 5 to 20 at a test temperature of 25°C, and a melting point of the wax is 50.0 to 70.0°C.
  4. The film-laminated steel sheet according to any one of claims 1 to 3, wherein a thickness of the film layer on the side with the lower coating weight is 12 to 40 µm.
  5. The film-laminated steel sheet according to any one of claims 1 to 4, wherein surface roughness of the steel sheet on the side with the lower coating weight, measured as arithmetic mean roughness Ra specified in JIS B0601:2013, is 0.10 to 0.50 µm.
  6. The film-laminated steel sheet according to any one of claims 1 to 5, wherein the coating weight of the wax layer is more than 0.120 g/m 2 and 0.135 g/m 2 or lower per side.
  7. A method for manufacturing a DI can, which uses the film-laminated steel sheet according to any one of claims 1 to 6, comprising: arranging the film-laminated steel sheet such that a surface on a side with a lower coating weight of a wax layer becomes a surface of the DI can on an outer surface side, and a surface on a side with a higher coating weight of the wax layer becomes a surface of the DI can on an inner surface side; and subjecting the film-laminated steel sheet to DI forming.

Description

TECHNICAL FIELD The present invention relates to film-laminated steel sheets and a method for manufacturing DI cans. BACKGROUND ART In recent years, paper-wrapped cans and distortion-printing cans have been primary types used for food packaging. Particularly for distortion-printing cans, it is crucial that a printed pattern does not distort during forming. Therefore, a can height is set relatively low, and forming methods most commonly used are drawn (DR) forming and drawing-and-redrawing (DRD) forming. On the other hand, since paper-wrapped cans do not require printing on an outer surface of a can body, drawn-and-ironing (DI) forming, which offers superior productivity, has become increasingly used in recent years. The DI forming is a forming method that reduces a thickness of a can wall to approximately 40 to 60% of a thickness of a steel sheet used as a material by drawn forming the steel sheet into a cup shape and then iron forming a part of the obtained cup corresponding to the can wall to increase a can height. The DI forming is characterized in that deformation amounts in both a sheet-thickness direction of the steel sheet and a can height direction are significantly large, along with a faster forming speed compared to the DRD forming. A temperature of the steel sheet during DI forming rises to nearly 200°C. Therefore, for example, in the DI forming of a tin plate, a type of tin-plated steel sheet, a coolant containing lubricant is sprayed onto an ironing die during forming to cool it. Since the coolant sprayed onto the ironing die adheres to the formed DI can, a process is required to wash the coolant off the formed DI can and then dry the washed DI can. Furthermore, a treatment facility for such coolant has been necessary as part of a manufacturing facility. On the other hand, film-laminated steel sheets enable wax to be coated on a film surface during a manufacturing process of the laminated steel sheets. Therefore, when using the film-laminated steel sheets to manufacture cans, there is no need to apply lubricant during forming. This eliminates the need for lubricant washing and drying processes, as well as a lubricant treatment facility. Due to these advantages, the film-laminated steel sheets have been widely used in DRD food cans in recent years. However, when applying the film-laminated steel sheets to forming processes such as the DI forming, where the can wall is rapidly ironed, the steel sheet temperature rises during forming. This causes the film to soften, making the film prone to abrasion and increasing a probability of fracture of a can body during forming. Furthermore, the softened film on a can inner surface closely adheres to a punch used during processing, making it difficult to withdraw the punch and resulting in poor punch-withdrawing performance. Poor punch-withdrawing performance may cause a can end to buckle and get caught between the punch and a stopper, while also making the can body prone to buckling. In such cases, a canmaking machine must be stopped to remove the jammed can end between the punch and stopper, significantly deteriorating productivity. From this perspective, various film-laminated steel sheets suitable for the DI forming have been proposed. For example, Patent Document 1 below discloses a laminated metal sheet for a two-piece can body, wherein both surfaces of the metal sheet have polyester resin film layers, a crystallization temperature of the polyester resin film layer on an outer surface side of the can body is 60 to 100°C, and centerline surface roughness (Ra) of the surface is 0.25 to 1.8 µm. Furthermore, Patent Document 1 below discloses a laminated metal sheet for a two-piece can body wherein the polyester resin film layer becoming the outer surface side of the can body is composed of: a resin having buthylene terephthalate as a main constituent unit: 40 to 100 mass%, and a resin having ethylene terephthalate as a main constituent unit: 0 to 60 mass%, and the centerline surface roughness (Ra) on a surface of a film layer becoming an inner surface side of the can body is 0.2 to 1.8 µm. Furthermore, Patent Document 2 below discloses a laminated steel sheet for a container with excellent blanking and drawing canmaking workability, in which protrusions with a height of 2 to 10 µm and a circle equivalent diameter d of 0.010 to 0.10 mm are formed on a film surface at a density of 15 pieces/mm2 or more, and a space containing air or inert gas exists between a resin film directly beneath the protrusion and the steel sheet. PRIOR ART DOCUMENT PATENT DOCUMENT Patent Document 1: Japanese Laid-open Patent Publication No. 2009-184262Patent Document 2: Japanese Laid-open Patent Publication No. 2009-23193 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION As described above, to enhance ironing formability of the film-laminated steel sheet, conventional methods have involved creating surface irregularities on the film surface to reduce a contact area an