JP3025397B2 - Zn-Ni alloy plated steel sheet with excellent press formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Zn--Ni alloy-plated steel sheet having excellent press formability and corrosion resistance and suitable as a surface-treated steel sheet used for hard-to-form products in automobiles and home electric appliances.

[0002]

2. Description of the Related Art Various types of surface-treated steel sheets have been used as a countermeasure against corrosion of automobile bodies due to scattered rock salt for preventing roads from freezing in winter in cold regions. The simplest surface treated steel sheet is a galvanized steel sheet, but high corrosion resistance is required for automotive applications, so it is necessary to increase the amount of plating, which causes problems in the body assembly stage such as weldability and workability. . Therefore, many alloy platings having high corrosion resistance even with a small plating amount have been proposed.
Zn-based alloy plating containing an iron group metal such as e, Co, and Ni as an alloy component has been widely used because it is recognized that not only corrosion resistance but also weldability and workability are good.

On the other hand, with the diversification of needs, automobiles and home electric appliances are required to have more advanced and versatile formability, and various techniques have been used to design materials and make use of rolling and annealing conditions. Steel sheets having the following forming characteristics have been developed. However, in the case of a plated steel sheet, even if a plating film is simply provided on a steel sheet which is excellent in forming, since the physical properties of the base steel sheet and the plating film are significantly different, the forming characteristics of the steel sheet itself are not sufficient at the actual forming stage. May not be fully demonstrated.

[0004] This is because the surface that actually contacts the press die becomes a plating film and the effect of its characteristics becomes large.
When a plated steel sheet is applied to a difficult-to-form product requiring deep drawing or a complicated shape, unexpected forming defects such as cracks and galling tend to occur. In order to solve such a problem, as disclosed in Japanese Patent Application Laid-Open Nos. 2-274853-6, there has been proposed a technique for improving the formability of a galvannealed steel sheet by previously adjusting the roughness of the steel sheet surface, JP-A-214877 discloses a technique for improving the slidability by adding an inorganic compound to the surface of a galvanized steel sheet, but none of these techniques has reached a satisfactory level.

[0005]

As described above, the use of Zn-based alloy-plated steel sheets is not fully utilized because of its limited use in terms of press formability while having excellent corrosion resistance. . In particular, Zn-Ni-based alloy-plated steel sheets are thin-plated and have excellent corrosion resistance. Therefore, there is a great need for expanding applications to automobiles and home electric appliances, and improvement in press formability is desired. An object of the present invention is to provide a Zn—Ni-based alloy-plated steel sheet that can satisfy high press formability mainly for use in automobiles and home appliances.

[0006]

[Means for Solving the Problems] In the case of plated steel sheets, it is not possible to follow the advanced forming, no matter how high the quality of the steel sheet is increased, so a technique for modifying the plating film itself to a state excellent in press formability is necessary. It is. JP-A-4-214
As described in Japanese Patent Application Laid-Open No. 877, coating with a different kind of film is one method, but other quality deterioration such as weldability, cost increase, and the like are easily assumed, and it is not always a good idea. Accordingly, the present inventors have conducted intensive studies on a means for improving the press formability of a Zn-Ni-based alloy-coated steel sheet without impairing any excellent quality such as corrosion resistance, and as a result, the surface layer of a specific thickness is modified. It has been found that the purpose can be achieved by forming a film. The present invention has been made based on this finding, and the gist is as follows.

The average Ni content of the surface layer having a thickness of 1 to 20% of the total plating thickness is 9 to 15% by weight, and the average Ni content of the surface layer having a thickness of 1 to 20% of the total plating thickness is 1% of the average Ni content. A Zn-Ni-based alloy-plated steel sheet excellent in press formability, characterized by having a Zn-Ni-based alloy-plated layer having a thickness of 0.01 to 1.5 on both sides or one side of the steel sheet.

[0008]

The Zn-Ni-based alloy-coated steel sheet having excellent press formability according to the present invention has an average Ni content of 9 to 15% by weight, which is a main component of the alloy, and has an average Ni content of the surface layer. It is characterized in that it is modified to be higher than the Ni content. As a method for modifying the plating film, Zn-N
After the i-based alloy plating, a method of preferentially dissolving Zn on the plating surface by immersion in a weakly acidic aqueous solution or anodic electrolysis, and a method of re-plating the plating surface with a high Ni content on the plating surface can be mentioned. Is a more advantageous method in terms of stability and economy.

Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 shows the relationship between the average Ni content of the Zn—Ni alloy plated steel sheet and press formability in the case of no treatment (open circles).
And a case where the surface layer of the present invention is modified (black circles). The test conditions are as follows. A Ti-added ultra-low carbon steel was used as a plating base plate, and the amount of adhesion was 20
g / m ² , a Zn—Ni alloy plating with a Ni content of 5 to 20% by weight, and then a surface layer modification treatment.
It was subjected to a press molding test. The degree of modification of the surface layer was confirmed by Auger electron spectroscopy.

In FIG. 1, the degree of modification of the surface layer is such that the thickness of the surface layer is 5 to 15% and the Ni content of the surface layer is 1.1 average.
範 囲 1.4 times. As a press forming test, low-viscosity oil was applied under the conditions of a punch diameter of 50 mm and a drawing ratio of 2.1, and 10 cylindrical press formings were continuously performed. The number of cracks, punch load, and powdering due to side wall taping of the pressed product were measured. Was evaluated.

5: No cracks, punch load almost at the level of the original plate, powdering minor 4: No cracks, punch load slightly higher than original plate, powdering minor 3: No. of cracks 1-2, punch load higher than original plate Powdering minor 2: More than 3 cracks or more powdering 1: More than 3 cracks or significantly more powdering

First, the case of no treatment will be described with reference to FIG. 1. The press formability depends on the Ni content. If Ni is less than 10%, cracks occur, and if Ni exceeds 16%, powdering increases. With Ni of 10 to 16%, the press formability was relatively good, but one or two cracks occurred, and the punch load was at a higher level than the original plate. In the case of the modified surface layer, the press formability is improved from about 8% Ni. However, in the range of 9% to 15% Ni, cracks are eliminated, and the punch load is reduced to a level close to the original sheet. Stable press formability was obtained. At a higher Ni content, a powdering problem occurred as in the case of no treatment. As described above, the effect of modifying the surface layer on the press formability is large when the average Ni content of the plating film is in the range of 9 to 15% by weight. If it is lower than this range, the effect is insufficient, and if it is higher, the problem of powdering cannot be avoided.

Next, the degree of modification of the surface layer will be described.
FIG. 2 shows the ratio of the surface layer thickness / total plating thickness, and the surface layer N
It is a correlation diagram of the ratio of i content / average Ni content, press formability, and corrosion resistance. 20 g /
m ³ , the average Ni content is 9 to 15% by weight. The press formability was evaluated by the same test as in FIG. 1, and the corrosion resistance was evaluated by a salt spray test (based on JIS Z2371). The evaluation criteria are as follows.

：: Press formability is improved and corrosion resistance is equivalent to that of no treatment. Δ: Press formability is improved but corrosion resistance is slightly reduced as compared to the case of no treatment. In comparison, press formability is not improved, or press formability is improved, but corrosion resistance is greatly reduced

As apparent from FIG. 2, in the region where the surface layer thickness ratio exceeds 20% or the Ni content ratio exceeds 1.5,
Although the press formability is improved, the evaluation is low because the corrosion resistance is reduced. Conversely, if the surface layer thickness ratio is less than 1% or the Ni content ratio is less than 1.01, press formability is not improved. As a range in which the press formability can be improved without impairing the corrosion resistance as described above, the surface layer thickness is 1 to 20% of the total plating thickness, and the Ni content of the surface layer is 1.01 to 1.1 of the average Ni content. Limited to 5 times. In a more preferred range, the thickness of the surface layer is 5 to 15, and the Ni content of the surface layer is 1.1 to 1.4 times.

The object of the present invention is that the average Ni content is 9-1.
5% by weight, which is a Zn—Ni-based alloy-plated steel sheet having a modified surface layer as described above, and in addition to Zn and Ni as plating components, for example, Co, Fe, Mn, Cr,
Pb, a metal such as _{Sn; SiO 2, Al 3 O} 3, BaCr
Insoluble inorganic compounds such as O ₄ ; and those containing one or more kinds of organic substances are included. The total amount of components other than Ni is preferably 5% by weight or less, and within this range, the effects of the present invention can be exhibited without any problem. In the present invention, the amount of plating is not particularly limited, but is preferably 10 to 60 g / m ^{2 in} terms of corrosion resistance and production cost.
Is a practical range. As in the present invention, the Zn—Ni-based alloy plating film having the modified surface layer may be formed on only one side or both sides of the steel sheet, and can be appropriately selected according to the forming object.

[0017]

[Example] Ti-added ultra-low carbon steel sheets were plated with Zn-Ni-based alloys of various Ni contents, and the surface layer was modified by treatment in a weakly acidic aqueous solution. Then, press formability and corrosion resistance were evaluated. . The press formability was evaluated by the same test as in FIG. 1, and the corrosion resistance was evaluated by a salt spray test (JIS Z2371).
Compliant). The evaluation criteria for press formability were the same as those in FIG. 2, and the evaluation criteria for corrosion resistance were ○: equivalent to no treatment, ×: lower than non-treatment. Table 1 summarizes the contents of the test materials and the evaluation results.

[0018]

[Table 1]

The comparative example in Table 1 will be described. In Comparative Example 1, cracks frequently occur in a press molding test even when the surface layer is modified because the average Ni content is low, and Comparative Example 2 shows an average Ni content.
Since the content was high, cracking did not occur, but powdering was remarkable. Comparative Example 3 has a thickness ratio of the surface layer and Ni%.
Although the press formability was good due to the high ratio, the corrosion resistance was greatly reduced.
Due to the low ratio, the effect of improving press formability was hardly recognized. In Comparative Example 5, the surface layer was not modified, and many cracks were generated by press molding. On the other hand, the examples of the present invention can improve the press formability without impairing the corrosion resistance even when components other than Ni are contained, and the effect of the surface layer is clear.

[0020]

As described above, the present invention provides a Zn-Ni
By improving the surface of a system-alloy-plated steel sheet to a surface layer of a specific thickness with a high Ni content, the press-formability of the Zn-Ni-based alloy plating is improved without impairing the originally excellent corrosion resistance. Successful
It is indeed effective in expanding the use of Zn-Ni alloy-plated steel sheets to difficult-to-form parts of automobiles and home electric appliances.

[Brief description of the drawings]

FIG. 1 shows the relationship between the average Ni content of a Zn—Ni alloy-plated steel sheet and press formability, comparing a case where no treatment is performed (open circles) and a case where the surface layer of the present invention is modified (black circles). Diagram,

FIG. 2 is a correlation diagram of the ratio of surface layer thickness / total plating thickness, the ratio of surface layer Ni content / average Ni content, and press formability and corrosion resistance.

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Yoshio Ishii 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Masayuki Fujii 1 Kimitsu, Kimitsu City, Chiba Prefecture New Japan Kimitsu Works (58) Fields surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] An average Ni content of the entire plating thickness is 9-1.
5% by weight, and the Ni content of the surface layer having a thickness of 1 to 20% of the total plating thickness is 1.01 of the average Ni content.
Zn-Ni-based alloy plated steel sheet excellent in press formability, characterized by having a Zn-Ni-based alloy plated layer of up to 1.5 times on both sides or one side of the steel sheet