US-12617951-B2 - Steel sheet and method for producing the same

Abstract

A steel sheet includes a base steel sheet and a film disposed on at least one side of the base steel sheet. In the steel sheet, the film includes an organic resin and a wax, the arithmetic average roughness Ra of the base steel sheet is 0.4 μm or more, the organic resin is at least one resin selected from acrylic resins, epoxy resins, urethane resins, phenolic resins, vinyl acetate resins, and polyester resins, the wax is a polyolefin wax with a melting point of 120° C. or above and 140° C. or below and an average particle size of 3.0 μm or less, the fraction of the wax in the film is 10 mass % or more, and the standard deviation σ of the film coating weight distribution per side is less than 0.9 g/m 2 .

Inventors

• Tomohiro Aoyama
• Shun Koibuchi
• Shinichi Furuya
• Takeshi Matsuda
• Takashi Kawano

Assignees

• JFE STEEL CORPORATION

Dates

Publication Date

20260505

Application Date

20230315

Priority Date

20220325

Claims (13)

1. 1 . A steel sheet comprising a base steel sheet and a film disposed on at least one side of the base steel sheet, wherein the film comprises an organic resin and a wax, an arithmetic average roughness Ra of the base steel sheet is 0.4 μm or more, the organic resin is at least one resin selected from acrylic resins, epoxy resins, urethane resins, phenolic resins, vinyl acetate resins, and polyester resins, the wax is a polyolefin wax with a melting point of 120° C. or above and 140°° C. or below and an average particle size of 3.0 μm or less, a fraction of the wax in the film is 10 mass % or more, and a standard deviation σ of a film coating weight distribution per side is less than 0.9 g/m 2 .
2. 2 . The steel sheet according to claim 1 , wherein a film coating weight W per side is 2.0 g/m 2 or less.
3. 3 . The steel sheet according to claim 2 , wherein the film coating weight W per side is 0.9 g/m 2 or less.
4. 4 . The steel sheet according to claim 1 , wherein the arithmetic average roughness Ra of the base steel sheet is 0.4 μm or more and 2.5 μm or less.
5. 5 . The steel sheet according to claim 1 , wherein the average particle size of the wax is 0.01 μm or more and 0.5 μm or less.
6. 6 . The steel sheet according to claim 1 , wherein the fraction of the wax in the film is less than 50 mass %.
7. 7 . The steel sheet according to claim 1 , wherein the organic resin is an alkali-soluble resin.
8. 8 . A steel sheet production method for producing the steel sheet described in claim 1 , comprising applying a paint comprising the organic resin and the wax described in claim 1 to at least one side of the base steel sheet, and drying the paint.
9. 9 . The steel sheet production method according to claim 8 , wherein the paint has a viscosity of 1.5 mPa·s or more and 8.0 mPa·s or less.
10. 10 . The steel sheet production method according to claim 9 , wherein the maximum temperature of the base steel sheet reached during the drying is 60° C. or above and 140° C. or below, and the drying time in the drying is 2 seconds or more and 150 seconds or less.
11. 11 . The steel sheet production method according to claim 8 , wherein the maximum temperature of the base steel sheet reached during the drying is 60° C. or above and 140° C. or below, and the drying time in the drying is 2 seconds or more and 150 seconds or less.
12. 12 . The steel sheet according to claim 1 , wherein the steel sheet has a coefficient of friction of 0.130 or less.
13. 13 . The steel sheet according to claim 1 , wherein the film may be removed from the steel sheet within 120 seconds after being immersed in a degreasing liquid having a degreasing agent concentration of 20 g/L and a temperature of 40° C. and washed with tap water.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This is the U.S. National Phase application of PCT/JP2023/010198, filed Mar. 15, 2023 which claims priority to Japanese Patent Application No. 2022-049831, filed Mar. 25, 2022, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes. FIELD OF THE INVENTION The present invention relates to a steel sheet that excels in sliding characteristics at press forming and to a method for producing such a steel sheet. In particular, aspects of the present invention relates to a steel sheet with a lubricant film that exhibits excellent formability even under severe drawing conditions, and to a method for producing such a steel sheet. BACKGROUND OF THE INVENTION Cold rolled steel sheets and hot rolled steel sheets are widely used in a broad range of fields mainly based on automobile bodies. In such applications, those steel sheets are used after being press formed. In recent years, parts are integrated to simplify the process and more sophisticated designs are demanded. Thus, steel sheets require more complex formability. Complex press forming may cause significant adverse effects on automobile productivity. For example, the steel sheet may fail to withstand the forming and be ruptured, or die galling may occur during continual press forming. Surface treatment on a die is a method widely used to enhance the press formability of cold rolled steel sheets and hot rolled steel sheets. Unfortunately, this method has drawbacks in that the die cannot be adjusted after surface-treated and that the costs are high. Thus, improved press formability of steel sheets themselves is strongly demanded. The use of highly viscous lubricating oils is an approach to obtain enhanced press formability without die surface treatment. In this case, however, press-formed steel sheets may not be degreased completely and may exhibit low paintability. Thus, various lubricant surface-treated steel sheets are being studied as techniques that enable press forming without die surface treatment or without using a highly viscous lubricating oil. Patent Literature 1 describes a metal sheet that is coated with a lubricant film in which a solid lubricant protrudes 0.01 to 1.5 μm from the surface of a resin film. Patent Literature 2 describes a surface-lubricated metal article with excellent press formability that is coated with a 0.5 to 5 μm film containing a polyurethane resin and a lubricant. Patent Literature 3 describes a technique in which an alkali-soluble organic film containing an epoxy resin and a lubricant is formed on a steel sheet. PATENT LITERATURE PTL 1: Japanese Unexamined Patent Application Publication No. 10-52881 PTL 2: Japanese Unexamined Patent Application Publication No. 2000-309747 PTL 3: Japanese Unexamined Patent Application Publication No. 2000-167981 SUMMARY OF THE INVENTION Although the techniques disclosed in Patent Literatures 1 to 3 offer lubricity by the lubricating effect of, for example, the lubricant that is contained, the press formability is not necessarily sufficient when the forming is complicated. Aspects of present invention have been made in view of the circumstances discussed above. Therefore, an object of aspects of the present invention is to provide a steel sheet for complicated and difficult press forming which exhibits low sliding resistance when there is a risk of cracking during press forming and which exhibits excellent press formability when a high contact pressure is applied and thus die galling is expected to occur. Steel sheets used for automobiles are required to be sufficiently cleaned of a lubricant film in an alkaline degreasing step during a painting process. It is a further object of aspects of the present invention that the steel sheet used for such an application also has good film removability. The present inventors carried out intensive studies directed to solving the problems discussed above. As a result, the present inventors have found that press formability is dramatically improved and the problems described above can be solved by forming on a surface of a base steel sheet an organic resin film that contains a polyolefin wax with a melting point of 120° C. or above and 140° C. or below and an average particle size of 3.0 μm or less and by controlling the surface roughness of the base steel sheet and the film coating weight distribution to specific ranges so that the standard deviation σ of the film coating weight distribution of the organic resin film per side will be less than 0.9 g/m2. Furthermore, the present inventors have found that press formability can be further improved by controlling the film coating weight, the arithmetic average roughness Ra of the base steel sheet, the average particle size of the wax, and the fraction of the wax in the film to suitable ranges. Aspects of the present invention have been completed based on the above findings, and are as follows. Incidentally, the base