JPH05125515A - Manufacture of hot-dip zn-al plated steel sheet excellent in appearance, secular blackening resistance and corrosion resistance - Google Patents

Abstract

PURPOSE:To provide a hot-dip Zn-Al plated steer sheet excellent in appearance, secular blackening resistance and workability and its manufacturing method. CONSTITUTION:In a continuous galvanizing line, the steel sheet to be plated is plated by using a plating bath of 410 to 470 deg.C having a compsn. constituted of, by weight, 3 to 10% Al, 0.01 to 1% Ti and the balance Zn. After that, at the time of cooling the above steel sheet, its cooling rate is regulated to 15 to 100 deg.C/sec in a process in which the sheet temp. reduces from 400 to 380 deg.C, by which the hot-dip Zn-Al plated steel sheet excellent in appearance, secular blackening resistance and workability can be manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip Zn-Al plated steel sheet having excellent appearance, blackening resistance over time, and corrosion resistance, which are mainly used for building materials and home appliances.

[0002]

2. Description of the Related Art Hot-dip Zn-Al plated steel sheets are
Compared with galvanized steel sheet, it has more excellent bare corrosion resistance and post-painting corrosion resistance, so the amount of its use mainly in building materials and home appliances has been increasing recently. However, the hot-dip Zn-Al plated steel sheet has a problem of poor appearance called a hexagonal pattern and a problem of blackening over time when its surface is blackened when exposed to high temperature and high humidity environment, and its improvement is necessary. There is also a demand for further improvement in corrosion resistance. To solve these problems, for example, in Japanese Patent Publication No. 63-11420, Zn-Al alloys with T
i or Zr, one or two, Mn, Ni, Co, or Fe, one or two, TiAl ₃ or ZrAl ₃
Of producing a plated steel sheet having excellent corrosion resistance using an alloy containing one or two of the above, Japanese Patent Application Laid-Open No. 2-274851
In the publication, the same Zn-Al alloy is added to Ti, B and S.
A technique for producing a plated steel sheet having excellent corrosion resistance using an alloy containing one or two of i, Mg, Mn, or Cu is disclosed.

However, although these techniques can improve the corrosion resistance, they cannot improve the appearance and the blackening resistance with time. Further, the method of adding various kinds of elements to the plating bath causes a problem that it is difficult to control the bath components in the actual operation. Further, with respect to the improvement in blackening resistance over time, a method of depositing a metal such as Co or Ni by electroplating or displacement plating on the surface of a steel sheet after hot dipping is applied. However, this method causes a reduction in corrosion resistance because it deposits a metal, which is more noble to Zn, on the plating surface, and there is a problem that the manufacturing cost increases because it needs to be treated after hot dipping. there were.

[0004]

Although it is possible to individually improve the corrosion resistance and the blackening resistance over time by using the above-mentioned conventional techniques, it is necessary to improve both the appearance, the blackening resistance over time and the corrosion resistance. I couldn't. Further, when the method of combining these conventional techniques is used, there is a problem that the manufacturing cost is significantly increased.

[0005]

Therefore, the present inventor has found the following solution as a method for producing a hot-dip Zn-Al plated steel sheet having excellent appearance, blackening resistance over time, and corrosion resistance. In a continuous hot dip Zn plating line, a steel sheet to be plated is plated with a bath composition of Al: 3 to 10% by weight, Ti: 0.01
After plating using a plating bath having a plating bath temperature of 410 to 470 ° C., the plate temperature is 40% when cooled.
Cooling rate is 15 to 10 while decreasing from 0 ° C to 380 ° C.
Appearance characterized by 0 ° C / sec, blackening resistance over time,
A method for producing a hot-dip Zn-Al plated steel sheet having excellent corrosion resistance.

[0006]

[Function] In the plating bath composition, Al 3 to 10% by weight,
0.01 to 1% by weight of Ti and the balance Zn are used. Al
Forms a eutectic structure with Zn and exhibits excellent corrosion resistance.
In order to obtain this effect, Al 3 to 10% by weight is used.
If Al is less than 3% by weight, the effect of improving the corrosion resistance due to the formation of a eutectic structure cannot be obtained. On the other hand, if it exceeds 10% by weight, the hardness of the plating layer increases, and problems such as peeling of the plating are likely to occur at the site where the steel sheet is bent and used. Ti has the effects of improving corrosion resistance and blackening resistance over time by forming a Ti oxide film layer on the surface of the plating layer. Further, since it has the effect of making the spangle of the plating layer fine, it is possible to reduce the hexagonal pattern caused by the coarsening of the spangle and improve the appearance of the plated surface. To obtain this effect, Ti is 0.01 to 1% by weight. If the Ti content is less than 0.01% by weight, the above effect cannot be obtained. If the Ti content exceeds 1% by weight, the viscosity of the plating bath increases, which makes it difficult to control the coating amount. The plating bath temperature is 410 to 470 ° C. If the temperature is lower than 410 ° C., a ripple-like appearance defect will occur during the wiping of the plating, and the smoothness of the plating surface will be lost. If the temperature exceeds 470 ° C, Fe-
This causes a problem that the Al-based alloy layer develops and the workability of the plated layer decreases.

When the plated steel sheet is cooled after plating, 400
The cooling rate is 15 to 100 during the transition from ℃ to 380 ℃.
C / sec. That is, when the plating layer obtained by the bath composition of the present invention is solidified, the primary crystal β-Zn phase or the α-Al rich phase is precipitated at around 400 ° C. to start solidification.
The ternary eutectic temperature of Zn-Al-Ti is reached near 0 ° C, and solidification is completed. By setting the cooling rate from the start of solidification of the plating layer to the end to 15 to 100 ° C./sec, the effect of improving the appearance of the plating surface by refining the spangle of the plating layer and the primary crystal β- Zn phase or α-A
The effect of improving the corrosion resistance can be obtained by refining the precipitation size of the l-rich phase. If the cooling rate is less than 15 ° C./sec, the above effect cannot be obtained, and if it exceeds 100 ° C./sec, problems such as shape collapse of the steel sheet occur. As a method of controlling the cooling rate within this range, for example, a gas spray type forced cooling device or a mist spray or fog type forced cooling device can be used after the plating wiping portion.

[0008]

EXAMPLES Next, examples will be described. In the Zenzimer system continuous hot dip galvanizing line, the thickness of Al-0.4mm
The killed steel sheet was passed through the plating bath and cooling conditions shown in Table 1 to produce a hot dip Zn-Al plated steel sheet. Table 1 also shows the results of evaluation of the surface appearance, blackening resistance over time, and corrosion resistance of the obtained plated steel sheet by the methods described below. Inventive examples 1 to 4 are influenced by the Al composition of the plating bath, inventive examples 5 and 6 are influenced by the Ti composition, inventive examples 7 and 8 are influenced by the plating bath temperature, and inventive examples 9 and 10 are after the plating. The effects of the cooling rate were examined, and the surface appearance, blackening resistance over time, and corrosion resistance were all good. Comparative Example 1 is a case where Ti was not added to the Examples, and the surface appearance, blackening resistance over time, and corrosion resistance were slightly inferior. In Comparative Example 2, the Al composition is low, and the corrosion resistance is significantly reduced. Comparative Example 3 is inferior in surface appearance when it is not quenched after plating. Comparative Examples 4 and 5 are evaluations of Zn-Al-based hot-dip galvanized steel sheets that are currently on the market, and Comparative Example 4 is significantly inferior in blackening resistance over time.
Has poor surface appearance and corrosion resistance.

[0009]

[Table 1]

Surface Appearance Evaluation Method The surface appearance was evaluated by the following criteria. ○: No turtle shell pattern. Δ: A slight glaze pattern is observed. X: A tortoiseshell pattern is conspicuous. Black aging resistance evaluation method After the chromate treatment was applied to the sample surface, the temperature was set to 50.
1 in a packing stack test in a constant temperature and humidity chamber at ℃ and RH 95%
The amount of change in lightness of the steel sheet surface before and after the test (Δ
The L value) was measured and evaluated according to the following criteria. [Measurement equipment used: Color Computer Model SM-4 manufactured by Suga Test Instruments Co., Ltd.] ○: Less than 3 △: 3 or more and less than 10 ×: 10 or more Corrosion resistance A salt spray test based on JIS / Z2371 was performed for 3 days to obtain a corrosion product. Was washed and removed with chromic acid, and the amount of plating corrosion loss (g / m ² · day) before and after the test was measured by a gravimetric method and rated according to the following criteria. 5: 5 g / m less than ² · day 4: 5 g / m ² · day or more 10 g / m less than ² · day 3: 10 g / m ² · day or more 20 g / m ² · day less than 2: 20 g / m ² · day or more Less than 40 g / m ² · day 1: 40 g / m ² · day or more

[0011]

As described above, by using the present invention, the performance of the hot-dip Zn-Al plated steel sheet can be greatly improved in appearance, blackening resistance and corrosion resistance. The application of the Al-plated steel sheet is expanded and a great industrial effect is achieved.

Claims (1)

[Claims] 1. A steel sheet to be plated in a continuous hot dip Zn plating line has a plating bath composition of Al: 3 to 10% by weight, T
i: 0.01 to 1% by weight, the balance consisting of Zn and unavoidable impurities, and after plating using a plating bath having a plating bath temperature of 410 to 470 ° C., when the plated steel sheet is cooled, the plate temperature is 400. Appearance characterized by a cooling rate of 15 to 100 ° C./sec while the temperature falls from ℃ to 380 ° C.,
A method for producing a hot-dip Zn-Al plated steel sheet which is excellent in blackening resistance over time and corrosion resistance.