JPH10265967A - Rust preventive steel sheet for fuel tank, excellent in press formability and corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip aluminum coated steel sheet for automobile fuel tank, excellent in press formability and corrosion resistance and having superior weldability. SOLUTION: An organic resin chromate film, containing 0.5 to 20 wt.% lubricant and having 0.1 to 2 μm film thickness, is formed on one side of both sides of a hot dip aluminum coated steel sheet containing 2 to 13 wt.% Si, and the coating weight of aluminum is regulated to <=60 g/m<2> per side. Further, in the case where the resin chromate film is formed on one side, an inorganic chromate film treatment, e.g. of (chromic acid)-silica as a typical example can be applied to the other side. The resultant aluminum coated steel sheet is a stock for automobile fuel tank, having excellent weldability as well as superior press formability and high corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a rustproof steel sheet having excellent press formability and corrosion resistance as a steel sheet for a fuel tank of an automobile.

[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. In addition, since the fuel tank is an important safety component of an automobile, the material used is required to have not only excellent deep drawing properties but also good resistance to cracking due to impact after molding. In addition to this, it is also important that the material be a material which has little generation of corrosion products which may lead to filter clogging, has good corrosion resistance to perforation, and can be easily and stably welded. As a material having these various properties, 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 with respect to gasoline, and is excellent in press formability due to rich lubricity of plating, and also excellent in spot welding and seam welding. In addition to these materials, galvanized steel sheets are also made of thick chromate.
Although not as good as the -Sn alloy, it still has excellent workability and corrosion resistance. However, in recent years, the development of materials that do not use Pb is urgently required to reduce the burden 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. Since the aluminum-plated steel sheet is a very hard Fe-Al-Si intermetallic compound layer (hereinafter referred to as an alloy layer) generated at the interface between the coating layer and the steel sheet, this part is the starting point. As a result, plating peeling and plating layer cracks are likely to occur. In order to solve this problem, the present inventors have disclosed in Japanese Patent Application No. 7-329193 that the property modification of the plating layer is effective by cooling after plating and reheating. Although this method is effective in improving the ductility of the aluminum plating layer and reducing cracks penetrating from the ground iron to the plating surface layer, it is not sufficient for improving the continuous press formability of the actual tank.

[0004]

An object of the present invention is to provide a new aluminum-plated steel sheet for a fuel tank, which has improved press formability and excellent corrosion resistance.

[0005]

Means for Solving the Problems The present inventors have disclosed in Japanese Patent Application No.
As proposed in -287997, we have already developed a coating treatment that improves the resistance weldability of aluminum-plated steel sheets and has excellent corrosion resistance. However, it was found that further improvement is necessary in the continuous press formability of the actual tank, and as a result of continuing research, it was found that a certain amount of lubricant was added to the resin chromate film to impart lubricity to the film. It was found that continuous press formability can also be improved by the method. In addition, compared to a standard organic coating process, a resin coating process after a chromate process, the process can be performed in one step, so that the process is economically excellent. Furthermore, by using a resin that can be cured at a low temperature, there is also an advantage that a special drying furnace is not required and the resin can be processed with conventional chromate processing equipment.

The present invention has been made based on the above-described ideas and findings. The gist of the present invention is as follows: (1) Double-sided hot-dip aluminum plating containing 2 to 13% by weight of Si in a plating layer A fuel excellent in press formability and corrosion resistance, characterized in that one or both surfaces of a steel sheet are coated with an organic resin chromate film containing 0.5 to 20% by weight of a lubricant and having a film thickness of 0.1 to 2 μm. Rust-proof steel plate for tank. (2) The amount of aluminum coating on the hot-dip
The rust-preventive steel sheet for a fuel tank according to the above (1), which is 60 g / m ² or less per side, excellent in press formability and corrosion resistance. (3) A fuel excellent in press moldability and corrosion resistance as described in (1) above, wherein one side is an organic resin chromate film, and the other side is an inorganic chromate treatment, an organic film treatment on an inorganic chromate, or an organic film treatment. It is a rust-proof steel plate for tanks.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the plating layer will be described. This element, which is the amount of Si added to the plating coating layer, is usually 10 to reduce the thickness of the alloy layer.
% Is added. As described above, the alloy layer formed by hot-dip aluminum 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. Steel sheet ductility is 3 even with normal alloy layer of about 2-3 μm.
About a point. Therefore, the thinner this alloy layer is, the better it works for processing. Unless Si is added in an amount of 2% or more, the effect of reducing the alloy layer is small, and if it exceeds 13%, the effect is saturated and, in addition, Si is easily electrochemically used as a cathode. Leads to deterioration of corrosion resistance. Therefore, the amount of Si is 2 to 13
%.

[0008] In general, as the amount of plating increases, the corrosion resistance improves, but the plating adhesion and weldability decrease. When applying a hot-dip aluminized steel sheet to a fuel tank material that requires various types of welding, it is important to ensure weldability, so the upper limit of the amount of adhesion is 60 g / side.
m ² is desirable. More preferably, it is 40 g / m ² or less per side. If the amount is too small, the corrosion resistance deteriorates greatly. Therefore, the amount is preferably 20 g / m ² or more. Other conditions of the aluminum plating are not particularly limited. However, the thickness of the alloy layer is preferably thin as described above. In the present invention, a resin chromate treatment containing a lubricant is performed in a post-plating step. This treatment is intended mainly for improving press formability, weldability and corrosion resistance. In addition to the resin chromate, silica can be added for the purpose of improving corrosion resistance, and phosphoric acid or the like can be added for the purpose of reducing the yellowness of the chromate.

Although the effect of the resin chromate film according to the present invention has not been completely elucidated, it has been found that not only the improvement of the weldability due to the increase in the contact resistance of the steel sheet surface by the application of the resin, but also the barrier effect of the resin film and the improvement of the corrosion resistance due to the inclusion of Cr. is there.
The thickness of the resin chromate film is limited to 0.1 to 2 μm. If the thickness is less than 0.1 μm, it is impossible to form a sound film as a resin, so that no improvement in press formability can be expected and the contact resistance value does not increase. On the other hand, with a coating having a thickness of more than 2 μm, the resistance value is excessively increased and conduction between the electrode and the steel plate or between the steel plate and the steel plate is hindered, so that welding itself becomes impossible.

[0010] Next, the lubricant is added for the purpose of improving the press formability. However, since the resin is water-based, it must be easily dispersed and dissolved in water. As the lubricant, there are ester type, wax type, stearic acid type, silicone type, special olefin type and paraffin wax type. In the experiments of the inventors, any lubricant exerts a corresponding effect, but among them, stearic acid was effective. Next, FIG.
The change in the Bowden friction coefficient when the stearic acid-based lubricant was used and the amount added in the film was changed (measurement conditions: 10 m
m diameter steel ball, load 500g, average value of n = 5) and diameter 70
It shows the degree of blackening of the tape when taping the outer surface of a sample having been subjected to a cylindrical drawing of mm-depth of 40 mm. The effect is recognized from the added amount of 0.5% by weight, and the workability is improved as the added amount of the lubricant increases. However, when the content exceeds 20% by weight, the effect tends to be saturated, and it is difficult to disperse and dissolve in the resin chromate solution, and a phenomenon that the solution gels is also observed. Therefore, the amount of the lubricant added is 0.5% by weight.
% To 20% by weight, preferably 0.5 to 15%
Is desirable.

In the present invention, the composition of the resin chromate film is not particularly defined, but if the resin / chromic acid ratio is different, the performance of the film changes. For example, if the resin / chromic acid ratio is small,
There is a tendency that an appropriate contact resistance value is not obtained and the weldability is poor. On the other hand, if the resin / chromic acid ratio is large, the corrosion resistance is poor,
Also, the weldability is slightly deteriorated. Therefore, the resin / chromic acid ratio is preferably about 1 to 8. Resins usable in the present invention include, for example, acrylic acid or / and methacrylic acid ester, carboxylic acid vinyl ester, vinyl ether,
Examples include ethylenically unsaturated compounds such as styrene, acrylamide, acrylonitrile, and vinyl halide, and epoxies, urethanes, and polyesters. As described above, among these, especially when using a conventional chromate facility, an emulsion-type resin that can be baked at a low temperature is desirable.

Other post-treatments of the hot-dip aluminum-plated steel sheet include a zero spangle process for uniform appearance after hot-dip plating, an annealing process for a degrading process of plating,
There may be temper rolling for adjusting the surface condition and the material, but the present invention is not particularly limited to these 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 with excellent workability and low content of infiltration type elements such as C and N to only parts where high workability is required, and furthermore, welding airtightness after welding, secondary workability, etc. B in order to secure
The added steel sheet is desirable. In addition, a normal steel plate manufacturing method is used. 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 or the like 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 as low as about 1150 ° C., the finishing temperature of hot rolling is as low as about 800 ° C., and the winding temperature is about 60 ° C.
The rolling reduction of the cold rolling is preferably as high as 0 ° C. or more and as high as about 80%. By the way, the treatment of the other side of the resin chromate one-sided treatment material may be selected as necessary. That is, the inorganic chromate film can be used for parts having less severe formability, for example, parts such as a separator and a subtank used on the inner surface of the tank. An organic film treatment can be used on a portion requiring lubrication and weldability, for example, an outer surface of a tank, or an organic film treatment can be used on an inorganic chromate. Since the outer surface of the tank is coated with a thick film in the final step, the requirement for corrosion resistance for the thin film on the plating is small. However, paint adhesion is required, and the treatment with an organic film on inorganic chromate is more stable than with a single layer of organic film. The inorganic chromate referred to here may be any of a commonly known coating type, reaction type, and electrolytic type. Further, the above-mentioned lubricant can be added to the organic film.

Next, the present invention will be described in more detail with reference to examples.

EXAMPLES Steel having the components shown in Table 1 was melted by a normal converter-vacuum degassing process to obtain billets, which were then subjected to hot rolling and cold rolling processes under normal conditions to obtain cold rolled steel sheets ( (Plate thickness 0.8 mm). 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 temperature at this time was 660 ° C., and the plating bath composition was basically Al-2
% Fe, and Si was added thereto. The Fe in the bath is supplied from plating equipment or strips in the bath. The aluminum-plated steel sheet thus manufactured was subjected to resin chromate treatment and inorganic chromate treatment using the baths shown in Tables 2 and 3 as standard compositions. The thickness (Cr adhesion amount) of any chromate film is adjusted by a ringer roll.
Drying and film formation were performed with hot air at 80 ° C. Furthermore, the baking type of an epoxy resin, an acrylic resin, or a polyethylene resin, which is generally performed for an organic film treatment, was also performed. The performance of the steel plate manufactured as described above as a fuel tank was evaluated.
The evaluation method at this time was based on the method shown below. Table 4 shows the plating conditions and performance evaluation results.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

(1) Evaluation of press formability Cylindrical drawing test Using a hydraulic forming tester, a forming test was performed at a drawing ratio of 2.3 using a cylindrical punch having a diameter of 70 mm. The wrinkle suppressing pressure at this time was 1000 kg, and the evaluation of the moldability was visually determined by the appearance of the molded cylinder and the degree of blackening of the tape by taping. (Evaluation criteria) ：: Formable, no plating layer defect, no blackening of tape 〇: Formable, no plating layer defect, but tape slightly blackened △: Formable, plating layer Slight scratches and blackening of the tape. ×: Formable, peeling occurs in the plating layer

Measurement of Bowden Friction Coefficient Measurement was performed by a Bowden method using a 10 mmφ stainless steel ball under a load of 500 g. The measurement is performed on the same
Scanning was performed 0 times, and the average value was evaluated. (Evaluation Criteria) ：: Coefficient of friction ≦ 0.1 〇: 0.1 <Coefficient of friction ≦ 0.25 Δ: 0.25 <Coefficient of friction ≦ 0.4 ×: 0.4 <Coefficient of friction

(2) Evaluation of corrosion resistance The corrosion resistance to gasoline was evaluated. The method was as follows. A test solution was poured into a sample obtained by drawing a flat-bottomed cylinder having a flange width of 20 mm, a diameter of 50 mm, and a depth of 25 mm using a hydraulic molding tester, and the sample was covered with glass via a silicon rubber ring.
The corrosion state after this test was visually observed. The test surface was a resin chromate treated surface. (Test conditions) Test solution: Gasoline + distilled water 10% + formic acid 200 ppm Test period: Left at 40 ° C for 1 month (Evaluation criteria) 〇: Less than 0.1% of red rust △: 0.1 to 5% of red rust or white Rust generation ×: Red rust generation is more than 5% or white rust is remarkable

[0021]

[Table 4]

[0022]

The present invention provides a hot-dip aluminized steel sheet having the press formability and corrosion resistance required for an automobile fuel tank material and excellent welding characteristics.
The b-based material is very promising as a new fuel tank material when it becomes difficult to use due to environmental problems, and has a great industrial contribution.

[Brief description of the drawings]

FIG. 1 is a diagram showing the amount of stearic acid-based lubricant added, the Bowden friction coefficient, and the blackened state of the tape after taping the outer surface portion subjected to cylindrical drawing.

Claims (3)

[Claims] 1. A double-sided hot-dip aluminized steel sheet containing 2 to 13% by weight of Si in a plating layer and having a thickness of 0.1 to 20% by weight containing a lubricant on one or both sides.
A rustproof steel plate for a fuel tank having excellent press formability and corrosion resistance, characterized by being coated with an organic resin chromate film having a thickness of up to 2 μm. 2. The rust-preventive steel sheet for a fuel tank according to claim 1, wherein the hot-dip aluminized steel sheet has an aluminum plating adhesion amount of 60 g / m ² or less per side. 3. The press moldability and corrosion resistance according to claim 1, wherein one surface is treated with an organic resin chromate film, and the other surface is treated with an inorganic chromate or an organic film or an organic film on the inorganic chromate. Rustproof steel plate for fuel tank.