MOLD FOR HOT-FORMING Zn-PLATED STEEL SHEET

Abstract

A forming tool for use in a hot forming of a Zn-plated steel sheet is provided. At least a portion of the forming tool to be in contact with the Zn-plated steel sheet during the hot forming is covered with an amorphous-carbon coating film. The amorphous-carbon coating film satisfies the following conditions (1) to (4): (1) a film thickness is 0.5 μm or larger; (2) a surface has an arithmetic mean roughness Ra of 2.0 μm or less; (3) the surface has a root-mean-square slope RΔq of 0.50° or less; and (4) a hydrogen content is 30 atom % or less.

Description

TECHNICAL FIELD[0001]The present invention relates to a forming tool for hot forming of Zn-plated steel sheets. The present invention particularly relates to a forming tool which is used in performing a hot pressing, after a Zn-plated steel sheet is heated at a high temperature in the austenite range. The hot pressing is referred to also as hot stamping, die quenching, press quenching, or hot pressing.BACKGROUND ART[0002]The hot pressing method has problems such as the following. The hot pressing method is a technique in which a blank that is a steel sheet is heated usually to a temperature range of austenite of 800 to 900° C. and then rapidly cooled and simultaneously formed into a desired part shape by means of a water-cooled forming tool. The steps from the heating of the steel sheet to the pressing are conducted in the air from the standpoint of cost. Consequently, for the purpose of inhibiting scale formation due to steel sheet oxidation, a Zn-plated steel sheet which has a pla...

AI Technical Summary

Benefits of technology

[0020]According to the present invention, it is possible to provide a forming tool which is for use in a hot forming of a Zn-plated steel sheet and to which the Zn of the Zn-plated steel sheet is less apt to adhere in the hot forming.MODES FOR CARRYING OUT THE INVENTION

[0021]The present inventor diligently made investigations in order to solve the problem. As a result, the inventor has found that in the cases when, in a forming tool to be used for a hot-forming of a Zn-plated steel sheet, at least a portion thereof to be in contact with the Zn-plated steel sheet during the hot forming is covered with a coating film in which control elements that are divided roughly into the following two are within specified ranges, then the Zn adhesion to the forming tool surface is remarkably inhibited when the forming tool comes into contact with the Zn of the Zn-plated steel sheet. The present invention has been thus completed.

[0022]The first control element is the surface profile of the coating film. Since the Zn-plated steel sheet is heated to a high temperature and formed in hot pressing as stated above, the Zn present in the surface of the Zn-plated steel sheet which is in contact with the forming tool has softened. In the cases when the surface of the forming tool has a shape with projections, Zn transfers and adheres to the forming tool surface due to the Zn-digging effect by the projections. Consequently, in the present invention, it has been first found that for inhibiting the digging and the accompanying transfer and adhesion of Zn, it is necessary that the arithmetic mean roughness Ra of the surface should satisfy 2.0 μm or less. The Ra is preferably 1.0 μm or less and more preferably 0.5 μm or less. The smaller the Ra, the more preferred. For example, it can be reduced to about 0.01 μm, which indicates a supermirror surface.

[0023]The regulation of Ra alone is not sufficient for inhibiting Zn adhesion, and it is necessary that the root-mean-square slope RΔq of the surface should also be regulated. The RΔq indicates the slope of roughness, and there is a tendency that the smaller the RΔq, the gentler the slope. It has been found that by regulating the RΔq to 0.50° or less along with the Ra, Zn adhesion can be inhibited without fail in the present invention. The RΔq is preferably 0.10° or less and more preferably 0.05° or less. The smaller the RΔq, the more preferred. However, a lower limit of RΔq is about 0.001° when producibility, etc. are taken into account.

[0024]Zn is a metal which is highly prone to adhere, as stated above. Physical control, such as controls of roughness and surface state, is not sufficient for inhibiting the adhesion of Zn. Consequently, from the standpoint of reducing reactivity with Zn when in contact with Zn, it is necessary that the material of the surface to be in contact with Zn should be investigated as the second element to be controlled. The present inventor, from this standpoint, has discovered that amorphous-carbon coating films are suitable as substances showing reduced reactivity with Zn, and that among those, an amorphous-carbon coating film which has a hydrogen content, i.e., a proportion of hydrogen to the sum of carbon and hydrogen, of 30 atom % or less is suitable. The amorphous-carbon coating film is hereinafter sometimes referred to as DLC film.

[0025]In the case where the hydrogen content of the DLC film exceeds 30 atom %, this DLC film has reduced heat resistance to show a considerable alteration or deterioration due to the heat during the forming. As a result, Zn adhesion is prone to occur. The hydrogen content is preferably 25 atom % or less and more preferably 10 atom % or less. Meanwhile, in the case where hydrogen is not contained at all, there is a tendency that Zn adhesion is rather enhanced. It is hence preferable that the hydrogen content is 5 atom % or higher.

Problems solved by technology

The hot pressing method has problems such as the following.

However, since the melting point of Zn is lower than the heating temperature of the steel sheet, the Zn softens during the high-temperature forming.

In the case where this adhesion of Zn is serious, the shape of the forming tool is changed, arousing problems concerning product shape and the surface quality of the formed steel sheet.

However, the ceramic coating film is not considered to be sufficient for inhibiting the Zn adhesion to the forming tools.

However, in the case of Zn, which becomes soft at a lower temperature than the magnesium, only the means of Patent Document 1 and Patent Document 2 cannot sufficiently inhibit adhesion to the forming tool.