JPH10330957A - Rust preventive steel sheet for fuel tank excellent in resistance weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rust preventive steel sheet combining excellent resistance weldability, press formability and corrosion resistance as the stock for an automotive fuel tank. SOLUTION: A coating layer of Al or an Al-(3 to 15%)Si series alloy is formed on the surface of a steel sheet, and inorganic and organic composite chromate coating film contg. chromic acid, an organic high polymer compd. of 5 to 1600 wt.% to the Cr content in chromic acid, 100 to 600 wt.% phosphoric acid compd. and 100 to 1000 wt.% colloidal silica composed of one or more kinds of silica and silicate is formed on at least one side thereof by 10 to 200 mg/m<2> per side expressed in terms of the Cr content. Moreover, 5 to 200 wt.% phosphonic acid or the salt thereof is incorporated into the coating film. Furthermore, the coating is formed on one side, and chromate coating film contg. silica is formed on the other face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rustproof steel sheet having excellent resistance weldability, press formability, and corrosion resistance as a material for a fuel tank of an automobile.

[0002]

2. Description of the Related Art In many cases, a fuel tank of an automobile has a complicated shape, so that excellent workability (deep drawing property) is required. In addition, since it is an important safety component for automobiles, the material used is free from corrosion products that could lead to filter clogging, free from concerns about perforated corrosion, and a material that can be easily and stably welded. It is important that there is. Materials having these various properties include Pb-Sn
Alloy plated steel sheets (Japanese Patent Publication No. 57-61833) have been widely used as a material for automobile fuel tanks.
This material has stable chemistry to gasoline,
In addition, since the plating is excellent in lubricity, it is excellent in press formability,
It also has excellent resistance weldability such as spot welding and seam welding. However, in recent years, materials that do not use lead have been demanded from the viewpoint of load on the environment.

[0003] One of the materials which does not use such Pb and has good corrosion resistance and workability is an Al-based plated steel sheet. Since Al forms a stable oxide film on its surface, it exhibits good corrosion resistance to gasoline, alcohol, and organic acids generated when gasoline is deteriorated. However, when manufacturing an Al-plated steel sheet into a fuel tank, the problem is weldability. A of coated metal
l has a high affinity for Cu which is usually used as an electrode,
Al-Cu or Al-Cu brittle on electrode surface during welding
-There is a problem that an Fe alloy is formed, which is gradually broken during continuous operation, resulting in early welding failure.

[0004] Conventionally, Al-based plated steel sheets are generally used after being subjected to a chromate treatment mainly composed of chromic acid and silica for the purpose of improving corrosion resistance. For example, Japanese Patent Publication No. 4-68399 discloses the disclosed example. JP-A-58-
6976, JP-A-58-48679, and JP-A-60-56072. However, in these methods, the reaction with the electrode occurs in the same manner as in the case of the untreated material, and does not contribute much to the improvement of the continuous workability. Japanese Patent Publication No. 4-68399 discloses that the Cr equivalent is 35-7.
Although it is characterized by forming 0 mg / m ^2, although the corrosion resistance as a fuel tank can be obtained with this amount of adhesion, in the spot welding or seam welding, the Al in the plating layer is alloyed with the electrode Cu like the untreated material. This is disadvantageous in that the end of the electrode is easily alloyed during continuous operation and the life of the electrode is shortened.

Japanese Patent Application Laid-Open Nos. 58-6976 and 58-48679 disclose examples characterized in that the amount of chromate adhering is 5 to 40 mg / m ^2. In addition to the problem of resistance weldability as in the publication, if it is less than 10 mg / m ² , the corrosion resistance as a fuel tank material is poor, and the corrosion resistance to organic acids generated when the gasoline fuel is deteriorated is insufficient. In addition, Japanese Unexamined Patent Publication
No. 0-56072 discloses a chromate adhesion amount of 10 mg /
Since the features less than m ^2, weldability require the fuel tank also not be obtained corrosion, has a problem, continuous workability in these prior art required when the fuel tank manufacturing, and corrosion resistance At present it was difficult to meet.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a conventional Al
To provide an Al-based plated steel sheet for fuel tank materials that has improved resistance weldability and has good press-formability and corrosion resistance, compared to rust-preventive steel sheets for fuel tanks, which were difficult to apply with plated steel sheets. With the goal.

[0007]

Means for Solving the Problems The present inventors have developed weldability,
As a result of various studies on post-treatment of Al-plated steel sheets with excellent formability and corrosion resistance, the chromate film composition was composed of an inorganic and organic composite chromate in which an appropriate amount of an inorganic component such as chromic acid and silica and an organic component such as a resin were blended. It has been found that by forming a suitable amount of on the surface, the above-mentioned problem of the continuous workability at the time of welding is solved and that the fuel tank has excellent characteristics. Specifically, a coating layer of Al or an Al- (3 to 15%) Si-based alloy is formed on the surface of a steel sheet, and at least one surface thereof includes chromic acid and C of chromic acid.
5 wt% to 1600 wt% of an organic polymer compound, 100 wt% to 600 wt% of a phosphoric acid compound, and 100 wt% to 1000 wt% of colloidal silica composed of at least one of silica and silicate, based on the amount of r. The composite chromate film of inorganic and organic contained contains 1
Formed at 0 mg / m ² or more and 200 mg / m ² or less.

[0008] Alternatively, Al or Al-
(3 to 15%) A coating layer of a Si-based alloy is formed, and at least one surface thereof is composed of chromic acid and an organic polymer compound in an amount of 5% by weight to 1600% by weight based on the amount of Cr in the chromic acid.
100% to 600% by weight of a phosphoric acid compound, silica,
100% by weight to 1000% by weight of colloidal silica composed of at least one silicate, and 5% by weight or more of at least one type of phosphonic acid or phosphonate compound
An inorganic / organic composite chromate film containing 200% by weight contains 10 mg / m ² or more per one side in terms of Cr amount.
mg / m ² or less, or the above-mentioned respective inorganic and organic composite chromate films are formed on one surface,
The chromate film containing silica on the other surface is Cr
10 mg / m ² or more and 100 mg / m per side in terms of quantity
² or less.

Hereinafter, the present invention will be described in detail. As described above, Al in the metal coated with the steel sheet easily reacts with Cu of the electrode, and has a problem that electrode wear is accelerated and continuous workability is reduced. Therefore, in order to improve the continuous workability, it is important to suppress the electrode wear and to increase the contact resistance between the steel plates to form an efficient nugget. The present inventors have found that an inorganic and organic composite chromate film works effectively for these purposes, and have completed the present invention.

That is, as shown in FIG. 1, in a coating composed only of an inorganic component such as chromic acid and silica, the contact resistance between steel sheets is not so different from that of a non-treated material with respect to the adhesion amount of the conventional chromate treatment. As in the case of the material, a reaction between Al of the plating and Cu of the electrode occurs during welding, and the life of the electrode is not improved. Conversely, if the amount of adhesion is increased, it is hard and brittle because it is an inorganic film, and the contact resistance increases.However, the film is locally broken, and the formation of the current-carrying point between the plate and the electrode is uneven. The resistance values vary widely,
Electrode wear control cannot be expected. In addition, there is a problem that dust is easily generated due to local excessive energization between the plate and the electrode.

On the other hand, by adding an organic component, the toughness of the film is increased, there is no local destruction of the film, and there is no variation in the formation of a current-carrying point. It is considered that the method has a feature that a uniform current-carrying point can be easily formed. Therefore, even if a high contact resistance value is obtained between the plates, the contact resistance value between the plates and the electrodes is uniformly low (FIG. 1), and it is considered that a good nugget forming ability and an effect of suppressing electrode wear can be obtained. . These effects are most effective in the case of double-sided processing, but the effects are exhibited even in the case of single-sided processing.

As a result of various experiments, the present inventors found that the resin component contained 5% by weight based on the amount of Cr in chromate in the chromate film.
As a result, it has been found that the effect appears clearly. On the other hand, if the content of the resin component exceeds 1600% by weight, the film thickness inevitably increases in order to secure the amount of Cr necessary for obtaining good corrosion resistance, and the contact resistance between steel sheets becomes too high, resulting in poor current flow. Occurs. Therefore, the content is set to 1600% by weight or less. As the resin component used in the present invention, a water-soluble organic polymer compound, specifically, an anionic polyacrylic acid having a carboxyl group and a copolymer thereof, a maleic copolymer, a vinyl acetate copolymer, polyethylene Compounds, polyurethane compounds, epoxy resin compounds and the like are used. These organic polymer compounds are mainly added and used alone, but two or more kinds may be added in combination.

The chromic acid used in the present invention is at least either chromic anhydride or a reduced chromic acid aqueous solution obtained by reacting an aqueous solution of chromic anhydride with a reducing agent to adjust the composition ratio of Cr ³⁺ / Cr ^6+. use. When reducing chromic acid is used, an organic compound such as starch, saccharides and alcohol, or an inorganic compound such as hydrazine and hypophosphorous acid is used as the reducing agent.

Further, the chromate treatment liquid of the present invention includes a colloidal silica comprising a phosphate compound and at least one of silica and silicate in order to improve the uniform coating property of the treatment liquid, the corrosion resistance of the chromate film, and the improvement of coating performance. Is added. The addition amount of the phosphoric acid compound is in the range of 100% by weight to 600% by weight based on the Cr amount of chromic acid. 100
If the amount is less than 600% by weight, the effect of the addition is not sufficient.
If it is excessive, the chromate film tends to absorb water, and the corrosion resistance deteriorates. The amount of colloidal silica composed of at least one of silica and silicate is in the range of 100% by weight to 1000% by weight based on the amount of Cr in chromic acid. 10
When the amount is less than 0% by weight, uniform coating properties are deteriorated, so that it is difficult to secure the corrosion resistance and the coating performance. When the amount is more than 1000% by weight, the effect is saturated.

Further, in order to form a chromate film having further excellent corrosion resistance and paint adhesion, phosphonic acid or a chloride thereof can be added to the inorganic and organic composite chromate film in the present invention. The addition amount of these phosphonic acids is from 5% by weight to the Cr amount of chromic acid.
Preferably, it is 200% by weight. If the amount of the phosphonic acid is less than 5% by weight, the effect of cleaning the surface by the etching action of the phosphonic acid, the uniform formation of the film against the effect, the anticorrosion effect by inclusion in the film, and the effect of improving the paint adhesion are small. On the other hand, if the amount of the phosphonic acid exceeds 200% by weight, the effect of adding the phosphonic acid is saturated and the stability of the treatment bath is lowered, which is not preferable.

The amount of adhesion of the inorganic and organic composite chromate film is 10 mg / m 2 in terms of the amount of Cr in consideration of corrosion resistance.
^If it is less than ² , the effect is insufficient, and there is a concern that corrosion from a crack in the plating layer in the processed portion. Further, the plating metal easily adheres to the electrode during spot welding, and the continuous workability is reduced. With the adhesion amount of 10 mg / m ² or more, the fuel tank has corrosion resistance and resistance weldability.
When the content is not less than mg / m ² , the resistance weldability is further improved. On the other hand, when the adhesion amount is more than 200 mg / m ² , the corrosion resistance is good, but the resistance between steel sheets increases due to the increase in the film thickness.
Poor energization or local excessive energization is likely to occur, causing problems such as a decrease in continuous workability. Preferably 140 mg
/ M ² or less. Therefore, from these viewpoints, the present inventors set the range from 10 mg / m ^{2 to} 200 mg / m ² , more preferably from 80 mg / m ^{2 to} 140 mg / m ^2.
m ² or less.

When the inorganic and organic composite chromate film is on one side, a chromate film containing silica can be formed on the other side to ensure better corrosion resistance. This chromate film may be formed by a conventionally known method, and the adhesion amount is 10 mg / m ² or more.
00 mg / m ² or less. If it is less than 10 mg / m ² , sufficiently good corrosion resistance as a fuel tank cannot be obtained,
If it exceeds 100 mg / m ² , the effect is saturated. In the present invention, a chromate treatment is performed in a post-plating step, and the production method can be a known method such as coating, dipping, or spraying.

Next, the reasons for limiting the plating layer will be described. The amount of Si added to the plating coating layer is usually about 10% by weight for the purpose of thinning the alloy layer. As described above, the alloy layer formed by hot-dip Al plating is very hard and brittle, so that it is likely to be a starting point of destruction, and also impairs the ductility of the steel sheet itself. Normal 2-3μ
Even with an alloy layer of about m, the ductility decreases by about 3 points. Therefore, the thinner this alloy layer is, the better it works for processing. If Si is not added in an amount of 3% by weight or more, the effect of reducing the alloy layer is small, and if it exceeds 15% by weight, the effect is saturated and, in addition, Si tends to be a cathode electrochemically. Leads to deterioration of corrosion resistance. For this reason, the amount of Si is limited to 3 to 15% by weight.

In general, as the amount of plating increases, corrosion resistance improves, but plating adhesion and weldability decrease.
When a hot-dip Al-plated steel sheet is applied to a fuel tank material that requires various types of welding, the upper limit of the amount of adhesion is set to 50 g / m ² on one side because it is important to ensure weldability.
Desirably, it is 40 g / m ² or less on one side. Other conditions of the Al plating are not particularly limited.
However, as the thickness of the alloy layer increases, the processing characteristics deteriorate.

As post-treatments after hot-dip plating, in addition to the treatment of the present invention, there are zero spangle treatment which is an appearance uniforming treatment after hot-dip plating, annealing treatment which is a plating modification treatment, surface condition and material adjustment. There may be temper rolling for
In the present invention, these are not particularly limited and can be applied. The composition of the plating base plate to be used is not particularly limited. However, it is desirable to apply IF steel excellent in workability only to the parts where high workability is required, and furthermore, B is added by several ppm to secure airtightness after welding, secondary workability, and the like. Steel plates are preferred.

[0021] The steel sheet is manufactured by an ordinary method. The steel component is adjusted and melted by, for example, a converter-vacuum degassing process, and the steel slab is manufactured by a continuous casting method and hot-rolled. The conditions of hot rolling and subsequent cold rolling affect the deep drawability of the steel sheet. In order to impart particularly excellent deep drawability, the heating temperature during hot rolling is set to 11
The temperature is as low as 50 ° C and the finishing temperature of hot rolling is 800 ° C
It is preferable to set the winding temperature as high as 600 ° C. or higher and the rolling reduction of cold rolling as high as about 80%.

[0022]

Next, the present invention will be described in more detail by way of examples. A steel having the components shown in Table 1 was melted by a normal converter-vacuum degassing process to obtain a billet, which was then hot-rolled and cold-rolled under normal conditions to obtain a cold-rolled steel sheet (sheet thickness 0.8 mm). ) Got.
Using this as a material, hot-dip Al plating was performed. Molten Al
Plating was performed using a non-oxidizing furnace-reduction furnace type line, and annealing was also performed in this hot-dip plating line. Annealing temperature is 800 ~
850 ° C. After plating, the amount of plating was adjusted by a gas wiping method. The plating temperature at this time was 660 ° C.
The composition of the plating bath was basically Al-2% Fe, to which Si was added. The Fe in the bath is supplied from plating equipment or strips in the bath. A predetermined amount of the chromate treatment liquid having the composition shown in Table 2 was applied to the Al-plated steel sheet manufactured as described above using a roll coater or a ringer roll after immersion, and baked and dried with hot air at 150 ° C. The suitability of the steel sheet thus manufactured as a fuel tank was evaluated by the following method. The results are shown in Tables 3 and 4.

[0023]

[Table 1]

[0024]

[Table 2]

(1) Evaluation of Weldability Weldability was evaluated by continuous spot welding and seam weldability. (Spot welding) Welding current 10
Spot welding was performed at kA, a pressure of 200 kg, and a welding time of 12 cycles, and the number of continuous hitting points until the nugget diameter fell below 4 at was evaluated. Ｅｘex: Continuous hit points of 1500 points or more １０００: Continuous hit points of 1000 to less than 1500 points ：: Continuous hit points of 500 to less than 1000 points Δ: Continuous hit points of 250 to less than 500 points ×: Continuous hit points of less than 250 points

(Seam Welding) Using an electrode wheel of R6 mm-φ250 m, seam welding of 10 m was performed at a welding current of 13 kA, a pressure of 400 kg, and a current of 2 on-2off.
A test piece shown in JIS-Z-3141 was prepared and subjected to a leak test. ○: No leakage ×: Leakage occurred

(2) Evaluation of press workability A molding test was carried out with a hydraulic molding tester using a cylindrical punch having a diameter of 50 mm at a drawing ratio of 2.3. The wrinkle suppressing pressure at this time was 500 kg, and the evaluation of formability was based on the following index. ◎: Moldable, no defects in plating layer. ：: Moldable and slightly flawed in plating layer. Δ: Moldable, peeling occurred on plating layer. ×: Molding not possible.

(3) Evaluation of corrosion resistance The corrosion resistance to gasoline was evaluated. The method is as follows: Hydraulic molding tester, flange width 20 mm, diameter 50 mm, depth 2
A test solution is placed in a 5 mm flat-bottom cylindrical deep-drawn sample,
Glass was capped via a silicone rubber ring. The corrosion state after this test was visually observed. (Test conditions) Test solution: Gasoline + distilled water 10% + formic acid 200 ppm Test period: Leave at 40 ° C for 3 months (Evaluation criteria) ：: No change ：: White rust generation 0.1% or less △: Red rust generation 5% or less Or white rust generation 0.1% to 50
% ×: Red rust occurrence is more than 5% or white rust is remarkable. As shown in Tables 3 and 4, all of the present invention show good resistance weldability, workability, and corrosion resistance.

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

Industrial Applicability The present invention is a material having good resistance weldability required for an automobile fuel tank material, and excellent in press formability and corrosion resistance. It is very promising as a new tank material when it becomes old and has a great industrial contribution.

[Brief description of the drawings]

FIG. 1 is a diagram showing contact resistance values of a conventional inorganic chromate film and a composite chromate film of the present invention and an inorganic and organic compound.

Continuation of the front page (72) Inventor Nobuyoshi Okada 1-1 Niwahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works

Claims (4)

[Claims] 1. The method according to claim 1, wherein the surface of the steel sheet has Al or Al- (3-1 to 1).
5%) A coating layer of a Si-based alloy is formed, and on at least one surface thereof, chromic acid and an organic polymer compound in an amount of 5 to 1600% by weight and a phosphoric acid compound in an amount of 100% by weight based on the amount of Cr in the chromic acid are 100%. An inorganic / organic composite chromate film containing 100% to 1000% by weight of colloidal silica composed of at least one of silica and silicate in an amount of 10 mg / m ^{2 to} 200 mg per side in terms of Cr amount. / M ² or less, a rust-preventive steel sheet for fuel tanks having excellent resistance weldability, characterized by being formed at a density of not more than / m ² . 2. The method according to claim 1, wherein the surface of the steel sheet has Al or Al- (3-1 to 1).
5%) A coating layer of a Si-based alloy is formed, and on at least one surface thereof, 5% to 1600% by weight of the organic high molecular compound and 100% by weight of the phosphoric acid compound with respect to the chromic acid and the Cr content of the chromic acid. % To 600% by weight, 100% to 1000% by weight of colloidal silica composed of at least one of silica and silicate, and 5% to 20% by weight of at least one of phosphonic acid or a phosphonate compound.
An inorganic and organic composite chromate film containing 0% by weight
10 mg / m ² or more and 200 mg per side in terms of Cr amount
/ M ² or less, a rust-preventive steel sheet for fuel tanks having excellent resistance weldability, characterized by being formed at a density of not more than / m ² . 3. A composite chromate film of inorganic and organic shown in claim 1 is formed on one side, forming a chromate film containing silica per surface 10 mg / m ² or more 100 mg / m ² or less in Cr content in terms on the other side Rust prevention steel plate for fuel tanks with excellent resistance weldability. 4. A composite chromate film of inorganic and organic shown in claim 2 is formed on one side, forming a chromate film containing silica per surface 10 mg / m ² or more 100 mg / m ² or less in Cr content in terms on the other side Rust prevention steel plate for fuel tanks with excellent resistance weldability.