JP2000290763A - Aluminum plated steel sheet for automotive fuel tank excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain workability, weldability and corrosion resistance of extremely high levels by incorporating an Mg2Si layer into aluminum plating layer contg. respectively specified range ratios of Mg and Si, and the balance Al with inevitable impurities on the surface of a steel sheet. SOLUTION: The content of Mg to be incorporated into an aluminum plating layer is controlled to, by weight, 0.5 to 15%, Si to 2 to 15%, preferably, Mg to 3 to 9%, and Si to 6 to 10%, and the coating weight of the plating is controlled to 10 to 60 g/m2 per one side. The one contributing to its corrosion resistance is Mg2Si, and Mg2Si remarkably contributes the corrosion from places where the plating has, not been applied such as cracks of the plating, the edge faces and weld zones. Mg2Si is easy to dissolve even in a soln. being close to the neutral state, and the dissolved Mg forms a stable protective coating film on the iron face or plated face. For crystallizing out Mg2Si by a desirable amt., the control of the cooling rate is effective, and desirably, it is rapidly cooled after the plating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides an aluminum-plated steel sheet having extremely excellent corrosion resistance as a steel sheet for a fuel tank of an automobile, and having both weldability, workability, and airtightness at a welded portion.

[0002]

2. Description of the Related Art A fuel tank of an automobile is usually designed last according to the design of a vehicle body, and its shape tends to be more and more complicated in recent years. Further, since the fuel tank is an important security part of an automobile, the material used is required not only to have excellent deep drawing properties but also to be free from cracks due to impact after molding. In addition to this, it is also important that the material does not generate corrosion products that may lead to filter clogging, has no concern about perforated corrosion, and can be easily and stably welded.

[0003] As a material having these various characteristics, a Pb-Sn alloy-plated steel sheet (Japanese Patent Publication No. 57-61833), which is conventionally called a turn sheet, has been mainly used. This material has stable chemical properties for gasoline, and has excellent press formability due to excellent plating lubricity, and also has excellent spot welding and seam welding properties. In addition, a steel sheet obtained by subjecting a galvanized steel sheet to a thick chromate treatment is also used. Although not as good as a Pb-Sn alloy, it also has excellent workability and corrosion resistance. However, in recent years, materials that do not use Pb have been sought from the viewpoint of load on the environment.

One of the candidate materials for automotive fuel tank materials not using Pb is an aluminum (Al-Si) plated steel sheet. Since aluminum has a stable oxide film formed on its surface, it has good corrosion resistance to organic acids generated when gasoline, alcohol, gasoline and the like are deteriorated. However, there are some problems when using an aluminum-plated steel sheet as a fuel tank material. One of these is workability. Aluminum-plated steel sheets are very hard Fe-Al-Si intermetallic compound layers (hereinafter referred to as alloy layers) generated at the interface between the coating layer and the steel sheets. It is easy to cause plating peeling and plating cracks.

[0005] In response to this problem, the present inventors have disclosed Japanese Patent Application Laid-Open No.
No. 53166 discloses that the problem can be solved by cooling rate after plating and reheating. Another issue is weldability. In other words, aluminum-plated steel sheets can be subjected to resistance welding such as spot welding and seam welding, but since the aluminum of the plating layer is easily alloyed with Cu, which is the welding electrode, the electrode tip is made of an Al-Cu-based alloy. It is converted into an intermetallic compound, and since this intermetallic compound is brittle, there is a problem that the intermetallic compound is gradually lost and the life of the electrode is inferior. To solve this problem, Japanese Patent Application Laid-Open No. 9-53166 discloses that a resin chromate treatment improves weldability.

[0006] The workability and the weldability are disadvantageous if the coating amount is large, and it is desirable to set the coating amount to a low value in fuel tank applications where these characteristics are required. However, when the plating adhesion amount is reduced, the corrosion resistance tends to deteriorate. Aluminum-plated steel sheet has good corrosion resistance to fuel, and has extremely good corrosion resistance in a salt-damage environment even in a dry / humid environment. However, in an environment where it is constantly wet due to salt damage, plating is easier than at places where the plating is not covered, such as end faces and welds. And corrosion of the ground iron is started.

[0007]

SUMMARY OF THE INVENTION The present invention provides a new aluminum-plated steel sheet for a fuel tank, which achieves the above-mentioned characteristics of workability, weldability and corrosion resistance at an extremely high level.

[0008]

Means for Solving the Problems The present inventors have made various studies to drastically improve the corrosion resistance of aluminum-plated steel sheets. As a result, Mg was added to the plating layer, and Mg was added to the plating layer.
_The present inventors have found that extremely excellent corrosion resistance can be obtained by the presence of the ₂ Si phase, and have completed the present invention. JP-A-56-
In 127,762, the applicants have already reported that Si,
A method for producing an aluminized steel sheet containing Mg is disclosed.

Furthermore detailed study the result of, for contributing to the corrosion resistance is Mg ₂ Si, cracking of the plating, the end face, Mg ₂ Si to corrosion from locations uncoated plating such welds Has greatly contributed to dramatically improving the corrosion resistance of fuel tanks. M
g ₂ Si is also easily dissolved in a solution near neutrality, and it is considered that the dissolved Mg forms a stable protective film on the iron surface or the plated surface. In order to crystallize a desired amount of Mg ₂ Si, it is effective to control the cooling rate. For example, rapid cooling after plating is preferable.

The gist of the present invention is as follows. (1) Mg: 0.5 to 15% by weight on the steel sheet surface;
i: having an aluminum plating layer composed of 2 to 15%, with the balance being Al and unavoidable impurities, wherein Mg ₂ Si
Aluminized steel sheet for fuel tanks with excellent corrosion resistance characterized by containing a phase. (2) The aluminum plated steel sheet for a fuel tank having excellent corrosion resistance according to the above (1), wherein Mg: 3 to 9% and Si: 6 to 10%.

(3) The aluminized steel sheet for a fuel tank according to the above (1) or (2), characterized in that the plating layer contains Mg ₂ Si. (4) At the interface between the plating layer and the steel sheet, Al-
Aluminized steel sheets for fuel tanks having excellent corrosion resistance according to the above (1) to (3), comprising an Si-Fe alloy layer.

(5) The aluminized steel sheet for a fuel tank according to the above (4), wherein the alloy layer contains Mg. (6) The composition of the steel sheet is% by weight, C: 0.01% or less, S
i: 0.5% or less, B: 0.0001 to 0.008%,
The corrosion-resistant aluminum-plated steel sheet for a fuel tank according to (1) to (5), wherein one or more of Ti, Nb, and V are contained in a total amount of 0.1% or less.

(7) The aluminized steel sheet for a fuel tank according to the above (1) to (6), further comprising a post-treatment film containing at least one of Si and C on the surface of the plating layer. (8) The aluminized steel sheet for a fuel tank according to the above (1) to (7), wherein the coating weight is 10 to 60 g / m ² per one side.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the plating layer will be described. The plating layer is Mg: 0.5-1
5%, Si: 2 to 15%, balance Al and inevitable impurities. By adding Mg and Si in combination, Mg ₂ Si is generated in the plating layer, and the corrosion resistance is greatly improved. If Mg is less than 0.5%, the effect of improving corrosion resistance is not exhibited, while if added more than 15%, the hardness of the plating layer is increased and the workability of plating is impaired. Even if Si is added in an amount of less than 2%, the effect of improving corrosion resistance cannot be obtained, and if it exceeds 15%, the workability of plating is impaired. Therefore, the amounts of Mg and Si are limited to this range. More preferably, Mg: 3-9
%, Si: 6 to 10%. Mg ₂ Si contributes most to corrosion resistance, and the greater the amount, the greater the effect of improving corrosion resistance.

In the present invention, the method for producing the aluminum-plated steel sheet is not particularly limited, and a melting method, an electric method from a non-aqueous solvent, a vapor deposition method, a clad method, and the like can be applied. Currently, the most industrially widespread are hot-dip aluminized steel sheets. At this time, an alloy layer composed of an intermetallic compound is generated at the interface between the plating layer and the ground iron. Al by melting method
-Si-Mg plating produces an alloy layer as a matter of course, but at this time, the composition of the alloy layer does not change significantly and Al-F
It is an e-Si system. However, when the amount of Mg increases, Al-Fe
Formation of -Si-Mg based intermetallic compounds is also observed. The thickness of the alloy layer is desirably 5 μm or less. This is because the alloy layer is hard and brittle, and if the alloy layer is thick, the workability of the steel sheet is greatly impaired. By adding Mg to the plating bath, an effect of reducing the thickness of the alloy layer can be obtained, and an alloy layer of 2 μm or less can be obtained.

[0016] The composition of the plating base sheet to be used is not particularly limited, but it is desirable to apply IF steel excellent in workability only to a portion where high workability is required. A steel sheet to which B is added at a few ppm is desirable in order to ensure secondary workability and the like. Desirable components are: C: 0.01% or less, Si: 0.5% or less, B:
It contains 0.0001 to 0.008% and one or more of Ti, Nb and V in a total of 0.1% or less.
C and Si have a great influence on the workability of the steel sheet, and Si is also an element that inhibits the plating property during hot-dip plating. Ti,
Nb and V are both carbide forming elements, and are necessary elements for IF conversion.

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, resin coating, 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, ethylenically unsaturated compounds such as styrene, acrylamide, acrylonitrile, and vinyl halide, and epoxy, urethane, and polyester. Recently, Cr
Various post-treatments that do not use are being developed, but it is naturally possible to apply these.

As the post-treatment of the hot-dip aluminized steel sheet,
In addition to chemical conversion treatment and resin coating, there may be zero spangle treatment which is an appearance uniforming treatment after hot-dip plating, annealing treatment which is a plating modification treatment, surface condition, temper rolling for adjusting the material, etc. In the present invention, these are not particularly limited and can be applied. Lastly, as to the amount of plating, as described at the beginning, the present invention is a material having a high degree of compatibility between weldability, workability, and corrosion resistance, and these properties are greatly affected by the amount of coating. In the present invention, it is desirable that the total coating amount of the aluminum coating layer and the intermetallic compound layer (hereinafter, referred to as plating adhesion amount) is 10 to 60 g / m ² per one surface. If the amount is small, the corrosion resistance is insufficient. If the amount is large, weldability and workability are insufficient.

[0019]

Next, the present invention will be described in more detail by way of examples. (Example 1) Steel having the components shown in Table 1 was melted by a normal converter-vacuum degassing process to obtain a billet, which was then subjected to a hot rolling and cold rolling process under normal conditions to obtain a cold rolled steel sheet. (Plate thickness 0.8mm)
I got Using this as a material, hot-dip aluminum plating was performed. For the hot-dip aluminum plating, a non-oxidizing furnace-reduction furnace type line was used, and annealing was also performed in this hot-dip plating line.
The annealing temperature was 800 to 850 ° C. After plating, the amount of plating was adjusted by a gas wiping method. The plating bath composition was an Al-Mg-Si system. At this time, the bath contained about 2% of Fe supplied from the plating equipment or the strip. The bath temperature was a value obtained by adding 60 ° C to the melting point read from the Al-Mg-Si ternary phase diagram. The plating appearance was good with no plating or the like. By devising the cooling rate after plating, it was confirmed from the plating layer structure by cross-sectional microscopy that the required amount of Mg ₂ Si was generated as a granular phase of about 1 to ₂ μm or a coarse phase of about 10 μm. By controlling the invading plate temperature, cooling rate after plating, etc.
The alloy layer was manufactured with a low thickness.

Table 2 shows the manufacturing conditions at this time. The plating adhesion amount was uniform on both sides, and the indication was the adhesion amount on both sides. The performance of the hot-dip aluminized steel sheet thus manufactured was evaluated by the method shown below. The types of post-treatment are: α: chromic acid-silica, β: resin chromate (resin / chromium = 8, using acrylic resin in dry weight ratio), γ: α-treated and silica-containing epoxy resin on both sides The coating was 1 μm. Chromate and coating thickness are indicated per side. The performance of the steel plate manufactured as described above as a fuel tank was evaluated. The evaluation method at this time was as shown below, and the plating conditions and performance evaluation results are also shown in Table 2.

[0021]

[Table 1]

[0022] (1) plating layer, the alloy layer composition, thickness analytical methods plating layer: 3% NaOH + 1% AlCl 3 · 6H 2 O
Only the plating layer was peeled off by constant current electrolytic peeling. M
Since g was insoluble in the alkaline solution, after electrolytic stripping, it was further treated with 20% nitric acid and mixed with an electrolytic stripping solution to obtain a plating layer composition analysis solution. The analysis of each element was performed by ICP. When analyzing a steel sheet subjected to a chemical conversion treatment containing Cr and Si, it is necessary to lightly polish the surface to reduce these effects. Alloy layer thickness: The alloy layer thickness was measured from a 400 times cross-sectional microscopic photograph.

(2) Evaluation of Corrosion Resistance Corrosion Resistance to Salt Damage JIS Z 23 for a sample having a size of 70 × 150 mm
The salt spray test according to No. 71 was performed for 30 days, and the corrosion products were peeled off 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] ◎: corrosion weight loss 5 g / m ² or less ○: less than corrosion loss 10 g / m ² △: Corrosion weight loss 10 to 25 g / m ² ×: corrosion weight loss 25 g / m ² greater

Corrosion resistance after painting A melamine-based black paint 20
μm and baked at 140 ° C. for 20 minutes. Thereafter, a cross cut was made and subjected to a salt spray test. The appearance after 60 days was visually observed. [Evaluation Criteria] ：: No red rust generated Δ: No red rust generated except for cross cut △: Red rust generation rate 5% or less ×: Red rust generation rate 5% or more

Corrosion resistance to fuel Corrosion resistance to gasoline was evaluated. The method is as follows, using the above-mentioned hydraulic forming tester, flange width 20 mm, diameter 50 m
m, 25 mm deep flat-bottomed cylindrical sample
The test solution was charged and covered with glass via a ring made of silicone rubber. The corrosion state after this test was visually determined. [Test conditions] Test solution: gasoline + distilled water 10% + formic acid 200 ppm +
Acetic acid 400 ppm Test period: Left at 40 ° C. for 4 months [Evaluation criteria] 〇: Red rust generation less than 0.1% △: Red rust generation 0.1 to 5% or white rust ×: Red rust generation more than 5% or white rust remarkable

(2) Weldability Spot welding was carried out under the following welding conditions, and the number of continuous hitting points up to the point where the nugget diameter fell below 4 t (t: plate thickness) was evaluated. [Welding conditions] Welding current: 10 kA Pressure: 220 kg Welding time: 12 cycles Electrode diameter: 6 mm Electrode shape: dome shape, tip 6φ
-40R [Evaluation Criteria] 連 続: More than 700 continuous hit points △: 400 to 700 continuous hit points ×: Less than 400 continuous hit points

(3) Workability Using a hydraulic forming tester, cup forming was performed using a cylindrical punch having a diameter of 50 mm at a drawing ratio of 2.25. The test was performed by applying oil, and the wrinkle suppressing 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

[0028]

[Table 2]

Comparative Example No. When Mg is not contained as in No. 18, the corrosion resistance tends to be poor in a severe environment. Also in the present invention example, the present invention example No. M like 1
g when the amount is small. 5, the corrosion resistance tends to decrease when the amount of Mg is large.
Is preferably about 4 to 9% from the viewpoint of corrosion resistance. In addition, the present invention example No. When the amount of Si is low, as in 6 and 9, the alloy layer is easily grown and is slightly inferior in workability. Invention Example No. Even if the amount of Si is too large as in 8, the corrosion resistance tends to deteriorate. Invention Example No. If the amount of plating applied is too large as in No. 12, the weldability tends to decrease. M
When the amounts of g and Si are in appropriate ranges, extremely excellent corrosion resistance, weldability, and workability are exhibited.

(Example 2) The composition of the plating bath and the operating conditions were fixed, and the influence of the original plating was investigated. The plating bath composition was Al-9% Si-6% Mg-2% Fe, the coating weight was 60 g / m ^{2 on} both sides, and the post-treatment was a treatment not containing silica-phosphoric acid chromium. 70
mg / m ² . Evaluation items and methods are basically the same as those of Example No. Same as 1. At this time, the composition of the plating layer was approximately 8.5% for Si, 5.7% for Mg, and 0.1% for Fe.
Numerical values of 5% and alloy layer thickness of 1.8 to 2.8 μm were obtained. The steel types used are shown in Table 3, and the evaluation results at that time are summarized in Table 4. Excellent characteristics were obtained regardless of the type of steel used.

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

The present invention provides a hot-dip aluminized steel sheet having an extremely high balance of corrosion resistance, press workability, and corrosion resistance required for a fuel tank material for automobiles. It is very promising as a new fuel tank material when it becomes difficult, and has a great industrial contribution.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C23C 28/00 C23C 28/00 C (72) Inventor Teruaki Izaki 1-1 Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Nippon Steel Corporation Yawata Works (72) Inventor Masao Kurosaki 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Hisaaki Sato Tobata-ku, Kitakyushu-shi, Fukuoka 1-1 Futaba-cho Nippon Steel Corporation Yawata Works F-term (reference) 4K027 AA02 AA23 AB02 AB05 AB13 AB26 AB32 AB48 AC64 AC82 AE03 AE23 4K044 AA02 AB02 BA10 BA14 BA15 BA21 BB03 BB04 BC02 BC05 BC08 CA11 CA16 CA53 CA62

Claims (8)

[Claims] 1. A steel sheet having a Mg content of 0.5 to 1% by weight on a steel sheet surface.
5%, Si: 2 to 15%, the balance has an aluminum plating layer composed of Al and unavoidable impurities.
An aluminized steel sheet for a fuel tank having excellent corrosion resistance, characterized by containing a g ₂ Si phase. 2. The aluminized steel sheet for a fuel tank having excellent corrosion resistance according to claim 1, wherein Mg: 3 to 9% and Si: 6 to 10%. 3. The aluminized steel sheet for fuel tanks having excellent corrosion resistance according to claim 1, wherein the plating layer contains Mg ₂ Si. 4. An aluminum-plated steel sheet for a fuel tank having excellent corrosion resistance according to claim 1, further comprising an Al—Si—Fe-based alloy layer having a thickness of 5 μm or less at an interface between the plating layer and the steel sheet. 5. The aluminum plated steel sheet for a fuel tank according to claim 4, wherein the alloy layer contains Mg. 6. The composition of the steel sheet is% by weight, C: 0.01%
Hereinafter, Si: 0.5% or less, B: 0.0001 to 0.0
The corrosion-resistant aluminum-plated steel sheet for a fuel tank according to any one of claims 1 to 5, wherein the aluminum-plated sheet has excellent corrosion resistance of at least 08% and at least one of Ti, Nb, and V in an amount of 0.2% or less. 7. The method according to claim 1, further comprising a post-treatment film containing at least one of Si and C on the surface of the plating layer.
7. An aluminum-plated steel sheet for a fuel tank according to any one of items 1 to 6. 8. A coating weight of 10 to 60 g per one side.
/ M ² , wherein the aluminized steel sheet for a fuel tank according to any one of claims 1 to 7, wherein