JP2002129300A - Surface treated steel sheet having excellent corrosion resistance and workability, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-dip metal coated steel sheet having excellent workability and corrosion resistance and mainly used for automotive members, building materials, home electric appliances, etc., and also to provide its manufacturing method. SOLUTION: In the surface treated steel sheet having excellent corrosion resistance and workability, a plating layer has a composition containing, by weight, 0.5-2% Mg, 0.2-5% Si, 40-65% Al and 30-60% Zn and Mg is allowed to form a solid solution completely in the plating layer. Its manufacturing method is also provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an Al-Zn-Mg-Si hot-dip coated steel sheet having excellent corrosion resistance and workability, which is used for various materials for automobiles, home appliances, metal construction materials such as roof walls, and the like. is there.

[0002]

2. Description of the Related Art Al-Zn-Mg obtained by immersion in a plating bath containing appropriate amounts of Si and Mg in Al-Zn.
-Si-based plated steel sheets are known to exhibit excellent corrosion resistance. Examples include the following. Tokuhei 3
No. 21627 discloses that 3 to 20% by mass of Mg,
A plated steel sheet comprising 3 to 15% of Si, the balance being Al and Zn, and having [% Al] / [% Zn] of 1 to 1.5 is disclosed. However, a plated steel sheet produced with this component system shows excellent corrosion resistance in an accelerated corrosion test such as a corrosion resistance test. However, despite its excellent corrosion resistance, there is a problem in workability and it has not yet been commercialized as an industrial product.

[0003]

SUMMARY OF THE INVENTION Conventional Al-Zn-M
g-Si-plated steel sheet is brittle plating layer, the dissolution rate in a corrosive environment is earlier Mg intermetallic compound (Mg ₂ S
Since it contains i), the above-mentioned intermetallic compound is preferentially dissolved in a mildly corrosive environment such as workability and an air environment, so that long-term corrosion resistance is poor.

[0004]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that, by dissolving Mg in a plating layer, the effect of Mg on the corrosion resistance is maintained for a long time and a brittle intermetallic compound is added to the plating layer. It was found that corrosion resistance and workability can be achieved at the same time by preventing crystallization. That is,
The gist of the present invention is as follows: (1) The component of the plating layer is mass%, Mg: 0.5 to 2%,
Si: 0.2 to 5%, Al: 40 to 65%, Zn: 30
A surface-treated steel sheet having excellent corrosion resistance and workability, characterized in that it contains about 60% and that 99% or more of the total Mg content of Mg is dissolved in the plating layer.

(2) In a steel strip, in terms of mass%, Mg: 0.5 to 2
%, Si: 0.2 to 5%, Al: 40 to 65%, Zn:
A plating containing 30 to 60% is applied, and the maximum value of the cooling rate after the plated steel strip leaves the bath is 30 ° C./sec.
A method for producing a surface-treated steel sheet having excellent corrosion resistance and workability, characterized by being controlled as described above. (3) Ni, Fe, Cu, C on steel strip immersed in plating bath
The method for producing a surface-treated steel sheet having excellent corrosion resistance and workability according to the above (2), wherein the hot-dip plating is performed after the pre-plating. (4) The above (2) or (3), wherein one or more of Ca, Be, Bi, Cr, and Co are added to the plating layer in a total amount of 0.01 to 1.0%. The method for producing a surface-treated steel sheet having excellent corrosion resistance and workability described in 1 above.

Next, the reasons for limitation of the present invention will be described in detail. First, with respect to the components of the plating layer, it is necessary to add 0.5% or more of Mg to improve the corrosion resistance. 0.5%
If it is less than 1, the formation of basic zinc chloride and basic zinc carbonate contributing to the improvement of corrosion resistance is insufficient, so that sufficient corrosion resistance cannot be obtained. If it exceeds 2%, brittle intermetallic compounds such as Mg ₂ Si having high solubility are crystallized, so that workability is deteriorated and a long-term anticorrosion effect cannot be obtained. Therefore, the lower limit of Mg is 0.5% and the upper limit is 2%.

[0007] Si is required to suppress the growth of a brittle alloy layer generated at the interface between the plating layer and the steel sheet during the production of the plated steel sheet and to improve the workability, and it is necessary to add 0.2% or more. If it exceeds 5%, brittle intermetallic compounds such as Mg ₂ Si are crystallized in the plating layer and a large number of hard needle-like Si crystals are crystallized, so that workability is deteriorated and a long-term anticorrosion effect cannot be obtained. . Therefore, the lower limit of Si is set to 0.2% and the upper limit is set to 2%.

[0008] Al is important for obtaining long-term corrosion resistance of the plated portion for a long time. As the amount of Al increases, the long-term corrosion resistance of the plated part improves.However, when the amount of Al increases, it leads to an increase in the plating bath temperature during the production of a plated steel sheet, and as a result, the brittle alloy layer grows thicker. At the same time, the corrosion resistance of the plated steel sheet deteriorates. Therefore, A
The lower limit of 1 is 40% and the upper limit is 65%.

[0009] Zn is an essential element for exhibiting the end face rust-preventive ability, and when dissolved in a corrosive environment together with Mg, the corrosion of basic zinc carbonate or basic zinc chloride having an anticorrosion protective action on the exposed surface of the base iron. Form product. If the amount of Zn is small, the amount of dissolution is small, so that it is insufficient to form a sufficient protective film. If the amount is too large, the amount of dissolution increases and the corrosion resistance of the plating itself deteriorates. From the results of atmospheric exposure, the optimum range of Zn is set at a lower limit of 30% and an upper limit of 60%.

[0010] The components of the plating layer can be specified by the following method. First, the post-treatment film on the plating surface is lightly polished and removed. Next, a plated steel sheet having an arbitrary area is immersed in a 5 to 10% NaOH solution at room temperature and left until the reaction is completed (bubbles are not generated). The solution is measured by ICP or atomic absorption,
The contents of various elements are calculated from the amount of the solution. At this time, a part of the alloy layer may be melted out or a small amount of Fe present in the plating layer may be included, but this is determined as an inevitable impurity.

The point of obtaining good corrosion resistance in the long term is
That is, 99% or more of Mg is dissolved in the plating layer. For this purpose, the steel plate is immersed in a plating bath and the maximum cooling rate after the plated steel strip leaves the bath is 30 ° C. /
It is desirable that the time be equal to or longer than sec. If the cooling rate is reduced, intermetallic compounds such as Mg ₂ Si are likely to crystallize, thus deteriorating long-term corrosion resistance. The maximum value of the cooling rate is
Determined from the cooling curve of the steel sheet. In the cooling curve, if the cooling rate satisfies 30 ° C./sec or more even at one point, the range of claim 2 is satisfied. Note that it is extremely desirable that the amount of Mg dissolved in the plating layer be 100% (complete solid solution).

[0012] Further, when there is a pinhole where plating is not normally formed on the steel sheet during plating, corrosion of the steel sheet proceeds from that portion. It is desirable to apply Ni, Fe, Cu, Cr plating on the steel plate before plating to reduce pinholes. The pre-plating adhesion amount is not particularly limited, but is preferably 0.5 to 10 g / m ² . The Ni plating method is not particularly limited, and examples thereof include displacement plating and electrolytic plating in a Watt bath. Further, one or more of Ca, Be, Bi, Cr, and Co are desirably added in a total amount of 0.01 to 1.0% for the purpose of further improving corrosion resistance and design. If the added amount is less than 0.01%, the effect cannot be sufficiently obtained, and if it exceeds 1.0%, it becomes a starting point of corrosion, and the corrosion resistance deteriorates conversely.

In the present invention, the post-treatment of plating is not particularly limited, but it is desirable to have a post-treatment film containing at least one of Si and C. Specifically, a chemical conversion treatment such as chromate treatment, a resin film, a resin chromate treatment, or the like can be performed. As the chemical conversion treatment, phosphoric acid, silica, or the like can be contained, and an Mg-based compound may be added.
Examples of the resin type include acrylic acid or methacrylic acid ester, carboxylic acid vinyl ester, vinyl ether, styrene, acrylamide, acrylonitrile,
Examples include ethylenically unsaturated compounds such as vinyl halides, epoxies, urethanes and polyesters. Recently, various post-treatments that do not use Cr are being developed, but it is naturally possible to apply these.

[0014] In addition to the chemical conversion treatment and the resin film, post-treatments such as surface rolling after hot-dip plating and temper rolling for adjusting the material may be used. However, in the present invention, these are not particularly limited. It is also possible. In hot-dip plating, wiping is most commonly performed by gas wiping. The wiping gas species is usually N ₂ , air or the like, but carbon dioxide gas or the like can also be used. The method of cooling the plated steel strip includes air cooling, brackish water, etc., but is not particularly limited in the present invention.

[0015] Finally, regarding the amount of plating, as the amount of plating increases, corrosion resistance generally improves, and workability, weldability, etc., decrease. The present invention is a plating composition having excellent corrosion resistance, and the coating amount can be reduced. The total coating amount of the plating layer and the alloy layer (hereinafter referred to as the coating amount) is 10 to 150 g / m ² per side. Is desirable.

[0016]

Next, the present invention will be described in more detail with reference to examples. A steel having the components shown in Table 1 was melted by a normal converter and vacuum degassing to obtain a billet, which was then subjected to hot rolling and cold rolling steps under normal conditions to obtain a cold rolled steel sheet (with a thickness of 0.1 mm). 8 mm). Using this as a material, hot-dip plating was performed at 90 mpm. Hot-dip aluminum plating was performed using a non-oxidizing furnace / reduction furnace type line, and annealing was also performed in this hot-dip plating line.
The annealing temperature was 800 to 850 ° C. The plating bath composition is
The amounts of Al, Mg, and Si were variously changed. In addition, Fe supplied from plating equipment or strip as impurities
Was contained in an amount of 0.1 to 1%. Both the temperature of the plate that entered the plating bath and the bath temperature were 580 ° C. The plating weight was adjusted to about 150 g / m ² on both sides by N ₂ gas wiping method after plating, and a cooling device and non-cloning treatment of a silane coupling agent system as post-treatment was about 100 mg on one side in terms of SiO _2.
/ M ^{2 and} temper rolling at 0.5%. The cross section of the thus manufactured sample was observed, and EPMA was performed to confirm the thickness of the alloy layer and the presence of Mg in the plating layer. The characteristics of the plated steel sheet thus manufactured were evaluated by the method described below. Table 2 shows the results.

[0017]

[Table 1]

(1) Evaluation of corrosion resistance (atmospheric exposure test) Dimension 7 after chromate treatment (Cr conversion: 20 g / m ² )
A sample of 0 × 150 mm (all of the sheared end faces are upper burrs) was exposed for 3 years at a 30 ° inclination on the south side in a coastal industrial zone environment.
The occurrence of red rust and the corrosion products were peeled off from the sheared end face, and the corrosion weight loss was measured. The indication of the corrosion weight loss is a value for one side of the plating.

[Evaluation Criteria] Red rust generation rate (%) at the sheared end portion ◎: 5% or less 〇: 20% or less Δ: 50% or less ×: 100%

[0020] The corrosion weight loss ◎: corrosion weight loss 5g / m ² or less ○: less than corrosion weight loss 10g / m ² △: corrosion weight loss 10~25g / m ² ×: corrosion weight loss 25g / m ² more than

(2) Workability Using a hydraulic forming tester, cup forming was performed at a drawing ratio of 2.25 using a cylindrical punch having a diameter of 50 mm. The test was performed by applying oil, and the wrinkle pressing force was 500 kg. The evaluation of workability was based on the following index. [Evaluation criteria] 〇: No abnormality △: Cracking of plating ×: Peeling of plating

[0022]

[Table 2]

[0023]

The present invention is a hot-dip coated steel sheet having sufficient workability and corrosion resistance far exceeding conventional various coated steel sheets. The present invention has a great effect on various industrial fields where the need for improving corrosion resistance is high.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masao Kurosaki 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division of Nippon Steel Corporation (reference) 4K027 AA02 AA22 AB02 AB05 AB26 AB44 AB48 AC15 AC64 AE03 AE18 AE21

Claims (4)

[Claims] Claims: 1. A plating layer component in which the content of Mg is 0.5
２2%, Si: 0.2 to 5%, Al: 40 to 65%, Z
n: 30 to 60%, and Mg in the plating layer
A surface-treated steel sheet having excellent corrosion resistance and workability, characterized in that 99% or more of the total Mg content is dissolved. 2. The steel strip in mass%, Mg: 0.5-2%,
Si: 0.2 to 5%, Al: 40 to 65%, Zn: 30
Surface treatment with excellent corrosion resistance and workability, characterized in that plating containing up to 60% is applied and the maximum value of the cooling rate after the plated steel strip leaves the bath is controlled to 30 ° C./sec or more. Steel plate manufacturing method. 3. A steel strip immersed in a plating bath contains Ni, Fe,
3. The method for producing a surface-treated steel sheet having excellent corrosion resistance and workability according to claim 2, wherein hot-dip plating is performed after Cu, Cr pre-plating. 4. The method according to claim 1, wherein Ca, Be, Bi, C
one or more of r and Co, in total of 0.01 to
The method for producing a surface-treated steel sheet having excellent corrosion resistance and workability according to claim 2 or 3, wherein 1.0% is added.