JP3344518B2 - Method for producing hot-dip galvanized steel sheet with excellent resistance to aging and blackening - Google Patents

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 galvanized steel sheet having improved adhesion with time and resistance to blackening, which are particularly problematic when used in building materials and home appliances.

[0002]

2. Description of the Related Art In recent years, with the diversification of needs and the sophistication, many products with surface-treated steel sheets have been developed. Of these, hot-dip galvanized steel sheets are widely used as surface-treated steel sheets having sacrificial corrosion protection. By the way, a general hot-dip galvanized steel sheet usually has Fe which impairs the adhesion of the plated layer.
−Al is added to the plating bath in 0.05% so that Zn alloy is not formed.
~ 0.3wt%, Pb 0.01 ~ 0.3wt for spangle adjustment
% Added. However, when such a hot-dip galvanized steel sheet is stored in a coil state for a long period of time, or when used in a high-temperature and high-humidity environment such as indoors, the plating layer peels off over time due to intergranular corrosion, , And a phenomenon of locally discoloring black (hereinafter referred to as black discoloration) occurs, often losing commercial value.

Therefore, several countermeasures for preventing such deterioration of plating adhesion and blackening over time have been disclosed. For example, the former method of preventing the plating adhesion from deteriorating with time is described in JP-A-62-89856 and JP-A-57-89856.
Japanese Patent Application Laid-open No. Sho 56-105447 discloses a method of removing Pb in a zinc plating bath, which is harmful to plating adhesion, or adding Sb instead of reducing the amount thereof. A method of adding Mg and one of Cu and Sb to a bath is disclosed. However, these Sb, Mg, Cu
The addition of such a compound is effective in preventing the deterioration of plating adhesion only when the amount of addition is large, so that it is disadvantageous in terms of cost, and has disadvantages in that spangles are nonuniform and it is difficult to obtain a beautiful plating appearance. Moreover, it does not show any improvement effect in preventing blackening of plating.

With respect to blackening prevention, a method of applying a metal hydroxide immediately after plating is disclosed in Japanese Patent Application Laid-Open No. 63-65088.
(1986) 13, disclosed in S1307, etc., but the inventors of the present invention have conducted detailed investigations and found that none of the effects are sufficient, and they show an effect of preventing deterioration of plating adhesion over time. is not.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet in which the anti-aging property and the resistance to blackening are dramatically improved at the same time.

[0006]

SUMMARY OF THE INVENTION Generally used
Hot-dip galvanized steel sheet manufactured in Al-Pb-Zn plating bath is
The present inventors examined the bath composition and the post-plating post-treatment because Al and Pb in the plating layer deteriorated the adhesion with time and the blackening resistance due to Al and Pb. As a result, Al: 0.06 to 0.20 wt%, P
b: 20 ° C / s immediately after plating in a zinc plating bath containing 0.001 to 0.01 wt% and then re-melting the plated layer
The hot-dip galvanized steel sheet obtained by quenching at the above cooling rate was found to be excellent in anti-aging adhesion and extremely excellent in blackening resistance.

That is, the present invention relates to Al: 0.06 to 0.20 wt.
%, Pb: 0.001 to 0.01 wt%, with the balance being Zn and unavoidable impurities. After re-melting the plated layer of the galvanized steel sheet, immediately quenching at a cooling rate of 20 ° C / s or more. This is a method for producing a hot-dip galvanized steel sheet having excellent anti-aging properties and blackening resistance.

[0008]

The present invention will be specifically described below. First, the reason why the Al and Pb contents in the plating layer are limited to the above ranges will be described. Al in the plating layer is limited to 0.06 to 0.20 wt%. If the Al content is less than 0.06 wt%, the formation of an Al-enriched layer during zinc plating becomes insufficient, and an Fe-Zn alloy layer is locally formed to significantly reduce the plating adhesion. Conversely, if the Al content exceeds 0.20 wt%, Although plating adhesion is excellent, blackening is promoted, so the upper limit was made 0.20 wt%.

[0009] Pb in the plating layer is limited to 0.001 to 0.01 wt%. The present inventors investigated in detail the effect of Al and Pb on the deterioration of the coating adhesion and the blackening phenomenon with time of the hot-dip galvanized steel sheet, and found that the deterioration with time increases with the addition amount of Al and Pb in the bath. However, it was found that Pb had a greater effect on the adhesion with time and the resistance to blackening although blackening was more likely to occur. From this, it is desirable that the Pb in the bath be as low as possible, but if it is less than 0.001 wt%, the wettability of the plating decreases,
Non-plating occurs and plating quality is impaired, which is not desirable.
On the other hand, if the content of Pb is more than 0.01 wt%, it is not desirable because adhesion deterioration and blackening are liable to occur with time. Therefore, in the present invention, the content range of Pb which does not cause non-plating and does not deteriorate the adhesion with time and the resistance to blackening is 0.001 to 0.01 wt.
%.

It is necessary to limit the contents of Al and Pb and, after plating, immediately re-melt the coating layer of the galvanized steel sheet and immediately cool it at a cooling rate of 20 ° C./s or more. The reason for re-melting the plating layer and immediately quenching is as follows. The present inventors investigated the plating layer of a commercially available hot-dip galvanized steel sheet, and found that Al in the plating layer was about 0.25 wt%, Pb was about 0.05 wt%, and Al in the plating layer was an Al-enriched layer. In addition to the formation of Pb, on the relatively outermost surface of the η layer, Pb is present in the vicinity of the Al-enriched layer at the interface of the plating layer or at the grain boundaries and high index planes of the plating layer surface (here, the orientation is investigated by X-ray diffraction ( 001) The one with a deviation angle of about 5 degrees or more from the plane was defined as a high index plane, and the one with a deviation angle of less than 5 degrees was defined as a low index plane. The blackening phenomenon occurs when Al and Pb present in the plating layer diffuse and concentrate on the plating surface and grain boundaries.
What happens to promote the oxidation of Zn, and the deterioration of adhesion with time, the Pb segregated on the grain boundaries and high index planes forms a local battery for Zn, intergranular corrosion progresses, and It reached a certain steel plate interface and found that the preferential oxidation of the Al-enriched layer caused the plating layer to peel off. That is,
Al and Pb in the hot-dip galvanized layer are present at the interface of the steel sheet, which is the base material of the coating, at the grain boundaries of the coating surface and at high index planes. It was considered that the adhesion and the deterioration of the adhesion with time occurred.

Therefore, the present inventors have shown above Al and Pb in the plating layer for the purpose of suppressing the grain boundary segregation of Al and Pb and having a low index surface in which Al and Pb are hardly segregated. It has been found that it is necessary to regulate the temperature within the range, re-melt after plating, and immediately quench at a cooling rate of 20 ° C./s or more. The remelting temperature may be at least the melting point of Zn at which the plating surface remelts, and the cooling rate is set to 20 ° C./s or more at a rate lower than this cooling rate because there are many high index surfaces, and as a result, Al, This is because Pb is easily segregated and concentrated. In producing a hot-dip galvanized steel sheet with such a plating layer composition and a post-treatment technique, other conditions can be manufactured in accordance with the usual manufacturing conditions of a hot-dip galvanized steel sheet. That is, the bath temperature during hot-dip Zn plating is preferably 430 to 500 ° C, and the intruding plate temperature is preferably within the range of ± 20 ° C.

[0012]

【Example】

(Example 1) A coil having a thickness of 0.7 mm and a width of 1250 mm was passed through a zinc plating bath having a bath temperature of 470 ° C. and a line temperature of 70 mpm in a zinc plating bath containing 0.15 wt% of Al and 0.005 wt% of Pb.
A hot-dip galvanized steel sheet of 80 g / m ² was obtained. This hot-dip galvanized steel sheet is re-melted at 430 ° C for 10 seconds and immediately 30 ° C / s
And quenched. Two galvanized steel sheets were brought into close contact with each other and subjected to a blackening and a time-dependent adhesion promotion test in an atmosphere at 70 ° C. and a relative humidity of 95%.

As comparative materials, Al: 0.15 wt%, Pb: 0.04 wt%
A hot-dip galvanized steel sheet plated under the same plating conditions in a galvanizing bath containing it was subjected to an accelerated test. The comparative material had no cracks or peeling of the plating layer before the accelerated test.
After 20 hours, the entire surface turned black, and after 10 days, almost all of the plating layer was peeled off by a 180 ° adhesion bending test. On the other hand, the material produced by the method of the present invention did not show blackening and no deterioration in adhesion with time.

The Al content in the plating layer is 0.17 wt%,
The Pb content was 0.005 wt%. (Example 2) In a zinc plating bath in which the addition amounts of Al and Pb were variously changed, other plating conditions were the same as in Example 1 and the basis weight was about 80 g / m2.
² was plated. Table 1 shows the results of examining the blackening resistance and the plating resistance over time of the plated steel sheet.

[0015]

[Table 1]

The blackening acceleration test and the adhesion promoting test with time and the evaluation thereof were as follows. (Blackening Acceleration Test) Plates plated under various plating conditions
After 10 hours and 20 hours of blackening acceleration test at 95 ° C. and 95% relative humidity, the change in lightness (ΔL) of the plating surface before and after the blackening acceleration test was measured with a color difference meter.

A: ΔL = 0 No change ○: ΔL = 1-2 Light grayish black Δ: ΔL = 3-5 Slight blackening (20%) ×: ΔL = 6-10 Blackening 50% or more *: ΔL = 11 or more Blackening over time (adhesion promotion test over time) The plated plate was left at 70 ° C. and 95% relative humidity for 10 days and 20 days, and after 180 ° adhesion bending test, the cracks and peeling degree of the bent portion were visually observed. .

◎: No change in plating layer ○: Slight cracking △: Many cracks ×: Partial peeling of plating layer *: Fully peeling

[0019]

According to the present invention, the amount of Al and Pb in the galvanized layer is limited, and the remelting and cooling rate after the galvanizing are controlled so that the hot-dip galvanized steel sheet has excellent anti-adhesion properties and blackening resistance. Was improved. As a result, a further increase in the use of the hot-dip galvanized steel sheet, particularly for building materials and home electric parts, can be expected.

Continuation of the front page (72) Inventor Kazuo Mochizuki 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Steel Development and Production Division Steel Research Laboratory (56) References JP-A-2-85347 (JP, A) JP-A-6-256857 (JP, A) JP-A-62-149861 (JP, A) JP-A-57-126977 (JP, A) JP-B-35-15673 (JP, B1) (58) (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. Al: 0.06 to 0.20 wt%, Pb: 0.001 to 0.01
After remelting the plated layer of a hot-dip galvanized steel sheet having a plated layer containing wt% and the balance consisting of Zn and unavoidable impurities, immediately quenched at a cooling rate of 20 ° C./s or more. A method for producing a hot-dip galvanized steel sheet having excellent aging adhesion and blackening resistance.