JPH06158257A - Hot dip zn-al coated steel sheet having excellent appearance, age blackening resistance and corrosion resistance - Google Patents

Abstract

PURPOSE:To produce the hot dip Zn-Al coated steel sheet having excellent appearance, age blackening resistance and corrosion resistance by forming hot dip Zn-Al plating films contg. specific ratios of Al, Ti and Mg and having the spangle size of the surfaces of a specific value on the steel sheet. CONSTITUTION:The plating films of about 100 to 120g/m<2> per surface of coating weight of plating are formed on the steel sheet by immersing the steel sheet into a hot dip coating bath (about 440 deg.C bath temp.) consisting of 3 to 10wt.% Al, 0.01 to 1wt.% Ti, 0.01 to 1wt.% Mg and the balance Zn and inevitable impurities. The cooling rate of the steel sheet is adjusted to about 15 to 100 deg.C/sec during the transition from about 400 deg.C to 380 deg.C to reduce the spangle size on the plating surfaces to <=0.5mm at the time of cooling the steel sheet. As a result, the hot dip Zn-Al coated steel sheet having the excellent appearance, age blackening resistance and corrosion resistance is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot dip Zn-Al plated steel sheet 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 the surface thereof is blackened when exposed to a high temperature and high humidity environment, and it is necessary to improve it. 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
One or two of i or Zr, one or two of Mn, Ni, Co or Fe, TiAl ₃ or ZrAl
A technique of Zn alloy with high corrosion resistance added with one or two of ₃ is disclosed.

Further, in JP-A-2-274851,
Disclosed is a plated steel sheet having excellent corrosion resistance, which uses an alloy obtained by adding one or two of Ti, B and Si, Mg, Mn or Cu to a Zn-Al alloy. However, although these techniques can improve the corrosion resistance, they cannot improve the appearance and the blackening resistance with time. Also,
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. For improvement of blackening resistance with time, the surface of the steel sheet after hot dip coating was subjected to C
A method of depositing a metal such as o or Ni by electroplating or displacement plating 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]

By using the above-mentioned conventional techniques, it is possible to obtain a steel sheet having excellent corrosion resistance and blackening resistance over time, but it is possible to obtain both appearance, blackening resistance over time and corrosion resistance. No good steel plate was obtained. In particular, as to the appearance, as shown in FIG. 2, if the spangle size on the plating surface is large, a problem of poor appearance called a hexagonal pattern occurred.

[0005]

Therefore, the present inventor has found the following solution as a hot-dip Zn-Al plated steel sheet which is excellent in appearance, blackening resistance over time and corrosion resistance. The gist of the invention is that the composition of the plating film is Al: 3.
-10 wt%, Ti: 0.01-1 wt%, Mg: 0.
It is a hot-dip Zn-Al plated steel sheet excellent in appearance, blackening resistance over time, and corrosion resistance, which is characterized by comprising 01 to 1% by weight, balance Zn and unavoidable impurities, and having a spangle size of 0.5 mm or less on the plating surface. .

[0006]

[Function] The plating film composition is Al: 3 to 10% by weight, T
i: 0.01 to 1% by weight, Mg: 0.01 to 1% by weight,
The balance consists of Zn and unavoidable impurities. Al forms a eutectic structure with Zn and exhibits excellent corrosion resistance. In order to obtain this effect, Al: 3 to 10% by weight. 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 is 10% by weight or more, 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 Ti forms an intermetallic compound with Al and finely precipitates in the plating layer, it has the effect of refining the spangle of the plating layer, and therefore reduces the hexagonal pattern caused by coarsening of the spangle. The effect of improving the plating surface appearance is obtained.

To obtain this effect, Ti: 0.01-
1% by weight. If Ti is less than 0.01% by weight, the above effect cannot be obtained, and if it is 1% by weight or more, the viscosity of the plating bath is increased, and it becomes difficult to control the amount of deposited plating. Mg
Has the effect of forming stable rust on the plated surface when the plated steel sheet is exposed to a corrosive environment, and improves the corrosion resistance of the plated steel sheet. To obtain this effect, Mg: 0.0
1 to 1% by weight. If the Mg content is less than 0.01% by weight, the above effect cannot be obtained. If the Mg content is 1% by weight or more, the viscosity of the plating bath increases, which makes it difficult to control the coating amount.

The spangle size of the plating surface is 0.5 m
m or less. As shown in FIG. 2, when it exceeds 0.5 mm, the individual spangles become visually conspicuous and the grain boundary depth between the spangles becomes deep, causing a problem of poor appearance called a hexagonal pattern. On the contrary, as shown in FIG.
When the length is less than or equal to mm, individual spangles cannot be visually identified, and the grain boundary depth between spangles becomes shallow, so that a problem of poor appearance called a hexagonal pattern does not occur.

[0009]

EXAMPLES Next, examples will be described. In a continuous hot-dip galvanizing line, an Al-killed steel plate having a plate thickness of 0.4 mm was immersed in a hot-dip bath having a predetermined composition to produce a plated steel plate. The coated amount of coated steel sheet is 100 to 1 on one side.
It was adjusted to be 20 g / m ² . The temperature of the hot dip bath was 440 ° C. When the steel sheet is cooled after immersion plating in the hot dip plating bath, the spangle size of the plating surface is 0 by adjusting the cooling rate in the range of about 15 to 100 ° C./sec during the transition from the plate temperature of 400 ° C. to 380 ° C. .5m
It was made to be m or less. Cooling of the plated steel sheet was performed by the mist spray method. However, among the materials manufactured as comparative examples, there was a steel sheet whose spangle size did not become 0.5 mm or less even though the cooling rate was adjusted. Table 1 shows the results of the appearance, blackening resistance and corrosion resistance of the obtained plated steel sheet evaluated by the following methods.

Inventive Examples 1 to 4 are influenced by the Al composition of the plating film, Inventive Examples 5 and 6 are influenced by the Ti composition, Inventive Example 7,
No. 8 shows the effect of Mg composition on the surface appearance,
Good blackening resistance over time and corrosion resistance. On the other hand, in Comparative Example 1, when Ti was not added, the spangle size was large 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 Examples 3 and 4 are currently commercially available Zn-A.
Evaluations were made on the 1-system hot-dip galvanized steel sheet. Comparative Example 4 is extremely inferior in blackening resistance over time, and Comparative Example 5 is inferior in surface appearance and corrosion resistance.

[0011]

[Table 1]

Surface Appearance Evaluation Method The surface appearance was evaluated according to the following criteria. ○: No turtle pattern. Δ: A slight glaze pattern is observed. X: A tortoiseshell pattern is noticeable.

Evaluation Method of Black Deterioration Resistance over Time After subjecting the sample surface to chromate treatment, 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. [Measuring instrument used: Suga Test Instruments Co., Ltd. color computer-Model SM-4] ○: Less than 3 △: 3 or more and less than 10 ×: 10 or more

Corrosion resistance A salt water spray test based on JIS Z2371 was carried out for 3 days, corrosion products were washed and removed using chromic acid, and the plating corrosion loss (g / m ² · day) before and after the test was measured by a gravimetric method. , Was 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

[0015]

As described above, the use of the present invention makes it possible to obtain a molten Z excellent in appearance, blackening resistance with time and corrosion resistance.
An n-Al-plated steel sheet is obtained, and the applications of the hot-dip Zn-Al-plated steel sheet centering on building materials and home appliances are expanded, and a large industrial effect is achieved.

[Brief description of drawings]

FIG. 1 is a micrograph showing the surface of a plated steel sheet according to the present invention.

FIG. 2 is a micrograph showing the surface of a conventional plated steel sheet.

Claims (1)

[Claims] 1. The composition of the plating film is as follows: Al: 3 to 10% by weight, Ti: 0.01 to 1% by weight, Mg: 0.01 to 1%.
Molten Z having excellent appearance, blackening resistance over time, and corrosion resistance, characterized in that the spangle size of the plating surface is 0.5 mm or less, which is composed of wt%, balance Zn, and unavoidable impurities.
n-Al plated steel sheet.