JP2938402B2 - Rust-proof steel plate for fuel tanks with excellent press moldability and corrosion resistance after molding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention provides a rustproof steel sheet having both excellent press formability and corrosion resistance after forming 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. Further, since the fuel tank is an important security part of an automobile, the material used for the fuel tank is required to have extremely excellent deep drawing properties and also to be free from cracks due to impact after molding. In addition to this, it is also important that the material is a material that generates less corrosion products that lead to perforated corrosion and filter clogging, and that can be easily and stably bonded.

[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 with respect to gasoline, and is excellent in press formability because plating is excellent in lubricity. 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 them is press moldability. 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. I do. For this reason,
Cracks easily occur due to severe processing.

[0005] Further, there is a disadvantage that plating peeling and plating cracks are likely to occur starting from the alloy layer. If cracks occur in the plating, corrosion from the inner surface proceeds from here, which may lead to perforation in a short period of time. Therefore, corrosion resistance after molding is also a major issue. The present inventors have shown in Japanese Patent Application No. 7-329193 that this problem can be solved by cooling rate after plating and reheating. However, this method has an economical disadvantage because the number of steps is increased.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of press formability and corrosion resistance after molding by providing Pb.
New rust-preventive steel plate for fuel tanks that has excellent press workability that can withstand severe pressing conditions expected to increase in the tank manufacturing process in the future without using steel, and also ensures corrosion resistance to organic substances after molding Is provided.

[0007]

Means for Solving the Problems The present inventors have conducted various studies to improve the press formability and corrosion resistance after forming of an aluminum-plated steel sheet, and have achieved this by optimizing the steel composition and material. Things. Specifically, from the new finding that aluminum-plated steel sheets with high total elongation suppress cracking of plating, we use ultra-low C steel to ensure press formability, and further increase total elongation to increase plating efficiency. Crack generation is also suppressed.

That is, the gist of the present invention is as follows: (1) C: 0.003% or less, Si: 0.0% by weight
3% or less, Mn: 0.3% or less, P: 0.02% or less,
N: 0.006% or less, Ti: 0.1% or less,
The rest is on the surface of the steel plate consisting of Fe and unavoidable impurities,
Press formability, characterized by containing 2 to 13% by weight of Si, the remainder having an aluminum-based coating layer composed of Al and unavoidable impurities, and having a total elongation of 45% or more after plating. Anti-corrosion steel sheet for fuel tanks with excellent corrosion resistance after molding.

(2) The fuel tank according to (1), which has excellent press moldability and corrosion resistance after molding as described in (1), wherein the adhesion amount of the aluminum-based coating layer is 50 g / m ² or less per side. Rust steel plate. (3) At least one of the aluminum-based coating layers has a Cr surface
(1) or (2), which has a chromate treatment layer of 5 to 100 mg / m ² per one side in conversion.
A rust-preventive steel plate for fuel tanks having excellent press moldability and excellent corrosion resistance after molding as described in 1. (4) The rustproof steel plate for a fuel tank according to (1) to (3), which has an organic resin film layer on at least one outermost surface and has excellent press moldability and corrosion resistance after molding. is there.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the steel components will be described. C: In the present invention, it is necessary that the steel sheet has a high deep drawability that can be processed into a complicated shape such as a fuel tank. For this purpose, the C content is preferably as small as possible, and since the material is deteriorated by aluminum plating, the C content is required to have a lower value. If the C content exceeds 0.003%, the desired moldability cannot be obtained, so this value% is made the upper limit. However, considering the increasingly complex shape of the tank in the future, it is desirable that
8% or less, more preferably 0.001% or less.

Si: Si has a strong affinity for oxygen, and easily forms a stable oxide film on the surface in a hot-dip aluminum plating process. When the oxide film is formed, Al-Fe in the plating bath
The reaction is inhibited, and a plating defect called non-plating is easily formed during aluminum plating. Further, since this element is also an element that hardens the steel sheet, it is preferable that the element is small as a steel sheet requiring high formability as in the present invention.
% Or less.

Mn: Mn is an element effective for increasing the strength of a steel sheet, but the present invention aims at a soft steel sheet, and the smaller the number, the better. If Mn exceeds 0.3%, the steel is hardened and it is difficult to produce a steel sheet with high ductility. Therefore, Mn is set to 0.3% or less. P: P is an element that segregates the grain boundary to embrittle the grain boundary, and is an element that inhibits the ductility and deep drawability of the steel sheet. Therefore, in the present invention, it is limited to 0.02% or less. More preferably, it is 0.01% or less. N: For the same reason as for C, it is preferable that N is small, and the upper limit of N is set to 0.006% from the viewpoint of ensuring moldability.

Ti: This element is known as an element fixing C and N, and a steel sheet in which C and N are fixed with these elements to substantially eliminate solid solution C and N is known as IF steel. Such IF steel is not only soft but also excellent in deep drawability. In the present invention, Ti is added for this purpose.
It is desirable that the amount of addition be equal to or more than the atomic equivalent of (C + N), but when the amounts of C and N are extremely small, the amount of Ti may be at the impurity level. Therefore, there is no particular lower limit. If the addition amount is too large, the effect is saturated and Ti
An element that promotes the l-Fe reaction. If the amount is large, the alloy layer tends to be thick, and the workability of the steel sheet tends to be impaired.
Therefore, the upper limit is set to 0.1%.

Generally, Ti is used as an element for fixing C and N.
Besides, Nb is used, but in the present invention, only Ti is added. Nb is an element that raises the recrystallization temperature of a cold-rolled steel sheet, and raising the annealing temperature leads to impairment of the productivity of the hot-dip coating process, and also increases the residence time in the bath, which leads to growth of the alloy layer. Cheap. Therefore, Nb is at the impurity level.

Next, the reasons for limiting the coating layer will be described. The amount of Si added to the plating coating layer is usually about 10% for the purpose of thinning the alloy layer. As described above, the alloy layer formed by hot-dip aluminum plating is very hard and brittle, so it easily becomes a starting point of destruction,
It also impairs the ductility of the steel sheet itself. Even with an ordinary alloy layer having a thickness of about 2 to 3 μm, the ductility of the steel sheet is reduced by about 2 to 5 points (2 to 5%). Therefore, the thinner this alloy layer is, the more advantageous it is 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. For this reason, the amount of Si is limited to 2 to 13%.

As the amount of the aluminum plating increases, the corrosion resistance increases, and the plating adhesion and the weldability tend to deteriorate. As a material for an automobile fuel tank that requires severe molding and various types of welding, it is desirable that the weight be 50 g / m ² or less per side. On the other hand, the thickness of the alloy layer is preferably thinner, as described above, because it adversely affects the ductility of the aluminum-plated steel sheet.

In the present invention, a new finding has been obtained that improving the ductility of the steel sheet is effective in suppressing cracks generated in the aluminum plating layer. If the total elongation of the steel sheet after the aluminum plating is 45% or more, cracks in the plating hardly occur even if severe molding is performed, and therefore, the corrosion resistance after the molding is improved. For this reason, the total elongation is limited to 45% or more. Although the reason for suppressing the generation of cracks is unclear, it is considered that some stress concentration is reduced. The larger the upper limit, the better, but the production of a steel sheet exceeding 60% is uneconomical, so 60% is the practical upper limit.

In the post-plating process, the primary rust prevention chrome
Heat treatment, temper rolling for adjustment of surface condition and material, lubrication
There may be a resin coating or the like for imparting lubricity. In the present invention
It is desirable to apply a chromate film after plating.
Good. Chromate types are inorganic and organic.
System, etc., and coating methods, reaction methods, etc.
However, any of them may be known. Chromate treatment
Mainly improves the weldability and, of course, the corrosion resistance
Also improve. At this time, the amount of attached chromate is
5-100mg / m per side^TwoAnd For weldability
5mg / m ^TwoLess than 100m is less effective
g / m^TwoThis is because the effect saturates above. Sa
Further, it is desirable to provide a resin film on the outermost surface.

The resin film contributes to lubricity, suppression of the reaction between the electrode and the steel plate during resistance welding, and the like, thereby improving the performance such as moldability and weldability, and overall providing excellent performance as a fuel tank. . At this time, when the thickness of the organic film is small, the organic film may be directly provided on the steel sheet, or chromate may be added to the organic film. The manufacturing method of the plating base sheet is a normal 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 or the like and hot rolled. Hot rolling,
The conditions of the subsequent cold rolling affect the deep drawability of the steel sheet. In order to provide particularly excellent deep drawability, the heating temperature during hot rolling should be as low as about 1150 ° C, the finishing temperature of hot rolling should be about 900 ° C, and the winding temperature should be as high as 600 ° C or more. Is preferably as high as about 80%.

[0020]

Next, the present invention will be described in more detail by way of examples. (Example 1) The steels shown in Table 1 were melted by a normal converter-vacuum degassing process to obtain steel slabs, and then heated at 1130 to 1
170 ° C, finishing temperature 870-920 ° C, winding temperature 60
Hot rolling at 0 to 630 ° C, cold rolling at a cold rolling rate of about 80%,
A cold-rolled steel strip having a thickness of 0.8 mm was obtained. For some materials, the hot rolling conditions were adjusted so as not to elongate. Using these as materials, hot-dip aluminum plating was performed. The 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. Annealing temperature is 800-85
0 ° C. After plating, the plating thickness was adjusted to about 60 g / m ² on both sides by a gas wiping method. The plating temperature at this time was 660 ° C., and Si was added to Al as a plating bath composition. In this bath, Fe may be mixed as impurities from plating equipment or strips in the bath. The performance of the aluminum-plated steel sheet manufactured in this manner as a fuel tank was evaluated. The evaluation method at this time is as follows:
Table 2 shows the plating conditions and performance evaluation results.

[0021]

[Table 1]

[0022]

[Table 2]

(1) Evaluation of Appearance The appearance after plating was visually judged. [Evaluation Criteria] ○: No abnormality △: Fine spot-like non-plated ×: Non-plated (2) Material after plating A tensile test was performed on a JIS No. 5 test piece according to JIS Z2241, and the total elongation was measured.

(3) Evaluation of press moldability A hydraulic molding tester was used to conduct a molding test using a cylindrical punch having a diameter of 50 mm at a drawing ratio of 2.4. The wrinkle suppressing pressure at this time was 500 kg, and the evaluation of formability was based on the following index. [Evaluation criteria]: Formable, no defect in plating layer △: Formable, cracking in plating layer ×: Formable, peeling of plating layer −: Not formable (cracks on original plate)

(4) Evaluation of corrosion resistance after molding A hydraulic die with a bead was attached to a tensile tester, and the bead was pulled out by pressing the plate with hydraulic pressure. Bead is diameter 4
mm, semicircular, and the applied pressure is 600 kgf. The sample from which the beads had been extracted in this manner was sealed in a glass container together with the fuel, and the corrosion resistance was evaluated. The test liquid is gasoline + distilled water 10% + formic acid 200 ppm, the period is 3 months, and the temperature is room temperature. The corrosion state after the test was visually observed. [Evaluation criteria] ：: Less than 0.1% of red rust generated △: 0.1 to 5% of red rust generated or white rust generated ×: More than 5% of red rust generated or remarkable white rust

As shown in Table 2, when C and N in steel are high (Comparative Examples 10 and 14) and P and Mn are high and ductility is insufficient (Comparative Examples 12 and 13), the press It is inferior in workability, and it is difficult to perform deep drawing such as in a fuel tank. In addition, when the element that inhibits hot-dip aluminum plating such as Si in steel is high (Comparative Example 11), there is much unplating, and corrosion proceeds from the unplated part, so that the corrosion resistance naturally deteriorates. Moreover, when Ti in steel is too high (Comparative Example 15),
When the amount of Si in the aluminum plating is small (Comparative Example 16), the alloy layer develops thickly, the plating tends to peel off during pressing, and the corrosion resistance also deteriorates. While plating
Even if there is too much Si (Comparative Example 17), the corrosion resistance deteriorates. Even if the steel component is appropriate, if the elongation is low due to the hot rolling conditions (Comparative Example 18), the steel cannot withstand severe molding. If the steel component and the plating composition are proper, a hot-dip aluminized steel sheet excellent in all of press formability, appearance, and corrosion resistance after forming can be obtained.

(Example 2) Hot-rolled steel strip having the components shown in Table 1 of Example 1 was used as a base plate to perform hot-dip aluminum plating. The conditions for the hot-dip aluminum plating are the same as those in the first embodiment. However, the component of the plating was Al-9.4% Si, the amount of plating applied was uniform on both sides, and was changed to 50 to 120 g / m ² on both sides.
Chromic acid-silica sol-phosphoric acid-organic resin-based chromate treatment was performed on a part of the manufactured aluminum-plated steel sheet, and a part thereof was covered with an epoxy-based resin film. The performance of these materials as a fuel tank was evaluated in the same manner as in Example 1, and the weldability was evaluated. Table 3 shows the film configuration and the results of the performance evaluation. In Table 3, the adhesion amount is a value on both sides, C
The r adhesion amount and the film thickness are values per one surface.

(1) Method of Evaluating Weldability Spot welding was performed under the following welding conditions, and the number of continuous hitting points until the nugget system fell below 4 Δt (t: plate thickness) was evaluated. In the case of single-sided coating, the evaluation was made such that the resin surfaces were inside and outside when they were overlaid. [Welding conditions] Welding current: 10 KA, pressure: 200 kg, welding time:
12 cycles, electrode: Cu-based 6 mm diameter [Evaluation criteria] ：: Continuous hit points more than 1200 points △: Continuous hit points 400 to 1200 points ×: Continuous hit points less than 400 points

As shown in Table 3, when the adhesion amount of aluminum plating was 120 g / m ^{2 on} both sides (Example 9) and when the adhesion amount of chromate was small (Example 10), the weldability was slightly lowered. To avoid these conditions,
A material with excellent productivity can be obtained. Further, the corrosion resistance and the weldability after molding are stabilized by the chromate and the resin.

[0030]

[Table 3]

[0031]

According to the present invention, it is possible to provide a hot-dip aluminized steel sheet having both press-formability and post-mold corrosion resistance required for a fuel tank material for automobiles. Future Pb
The system 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.

────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code FI C23C 28/00 C23C 28/00 C (72) Inventor Nobuyoshi Okada 1-1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation (56) References JP-A-6-306637 (JP, A) JP-A-62-230987 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00-38/60 B32B 15/08 C23C 2/12 C23C 22/30 C23C 28/00

Claims (4)

(57) [Claims] 1. In% by weight, C: 0.003% or less, Si: 0.03% or less, Mn: 0.3% or less, P: 0.02% or less, N: 0.006% or less, Ti : On the surface of steel sheet containing 0.1% or less, the balance being Fe and unavoidable impurities,
Press moldability, characterized in that it contains 2 to 13% by weight of Si and the balance has an aluminum-based coating layer composed of Al and unavoidable impurities, and the total elongation after plating is 45% or more. Anti-corrosion steel sheet for fuel tanks with excellent corrosion resistance after molding. 2. The adhesion amount of the aluminum-based coating layer is 50 per side.
excellent fuel tank anticorrosive steel sheet corrosion resistance after molding and press molding of claim 1, characterized in that g / m ² or less. 3. The press formability according to claim 1, wherein at least one of the aluminum-based coating layers has a chromate treatment layer of 5 to 100 mg / m ² per one side in terms of Cr. Anti-corrosion steel sheet for fuel tanks with excellent corrosion resistance after molding. 4. The rust-preventive steel sheet for a fuel tank according to claim 1, which has an organic resin film layer on at least one outermost surface and has excellent press moldability and corrosion resistance after molding.