JPH09156027A - Rust proof steel plate for fuel tank having excellent press workability and corrosion resistance, and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a rust proof steel plate having excellent press workability and corrosion resistance when it is used as a fuel tank for an automobile. SOLUTION: A steel plate having an alloy layer having 25-50% of Fe, 3-18% of Si, 0.05-0.8% of Cr, 0.1-5% of Mn, a remaining part of Al with a thickness of 5 p or below and a plating layer having 1% or below of Fe, 2-12% of Si, 0.005-0.3% of Mn, 0.002-0.08% of Cr, 1% or below of Zn+Sn, a remaining part of Al has excellent plating adhesion which can endure severe press working and corrosion resistance after working. This steel plate can be manufactured by plating a steel plate with a plating bath of a composition having 3-12% of Si, 0.5-2.5% of Fe, 0.05-1% of Mn, 0.02-0.15% of Cr, 1% or below of Zn+Sn, and a remaining part of Al. When it is annealed at 300-500 deg.C for 5-20hr., the performance is further improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention does not cause press cracking even under extremely harsh processing conditions such as an automobile gasoline tank, has good corrosion resistance after molding, and has an environment in which gasoline is oxidized and deteriorated, such as methanol and methanol. The present invention relates to a method for producing a rust-preventive steel plate for a fuel tank, which has excellent workability and corrosion resistance without causing perforations or clogging of a circulation system filter even when used in an environment containing alcohol such as ethanol.

[0002]

2. Description of the Related Art In the recent automobile industry, the functionality and design of the vehicle body are becoming more important than ever before. The design of automobile fuel tanks is usually designed last according to the design of the vehicle body, and its shape is becoming more and more complicated. Since the fuel tank is an important safety component for automobiles, a material required for the fuel tank has a high corrosion resistance that does not cause pitting corrosion. However, recently, in addition to that, workability capable of withstanding a high degree of press molding has become more important.

Conventionally, a Pb-Sn alloy-plated steel sheet (Japanese Patent Publication No. 57-61833) called a turn sheet has been mainly used as a material for automobile fuel tanks. This material has stable chemical properties for gasoline, has excellent press formability, and has a flexible plating film that does not cause cracks in the plating after forming, thus providing excellent corrosion resistance after processing. Have In addition to this, a steel plate obtained by subjecting a galvanized steel plate to a thick chromate treatment (Japanese Patent Publication No. 53-19981) is also used.
Although it is not as good as Sn alloy, it also has excellent workability and corrosion resistance. However, in recent years, the effect of Pb on the environment has been taken into consideration, and a material that does not use Pb has been sought after from the viewpoint of the load on the environment.

Further, in order to make up for the shortage of gasoline related to the recent crude oil problem, the introduction of alcohol fuel such as methanol or ethanol, which has the advantages that it can be easily blended with gasoline, is inexpensive, and produces no harmful gas even when burned, The rate tends to increase year by year. However, when using these alcohols or alcohol-blended fuels as fuels for automobiles, since these fuels have different corrosion characteristics from gasoline, conventional materials for gasoline tanks may not be usable due to corrosion. strong.

Specifically, in the case of a Pb-Sn alloy-plated steel sheet, the problem is that the Pb-Sn alloy itself has the property of being very easily dissolved in alcohol such as methanol or ethanol. On the other hand, in the case of zinc chromated steel sheet subjected to thick chromate treatment, zinc is a metal with a base potential lower than that of iron. Although the generation of perforations is suppressed, zinc has a drawback that the elution rate of zinc itself is large and a large amount of floating white precipitate is generated, which easily causes clogging of the filter in the fuel circulation system.

From such a background, Pb is not used and Z
A material having a corrosion resistance higher than n is required. One of the candidates is an aluminum (Al-Si) plated steel sheet.
Since a stable oxide film is formed on the surface of aluminum,
It has good corrosion resistance and good corrosion resistance against alcohols and mixed gasoline thereof, or organic acids generated when gasoline or the like deteriorates. Therefore, it can be said that the aluminized steel sheet is a very promising material as a fuel tank material. In fact, Japanese Patent Publication No. 4-68399 proposes a steel sheet for a fuel tank for alcohol fuel, which is obtained by subjecting an aluminum-plated steel sheet to a chromate treatment. However, the aluminum-plated steel sheet has not been used as a fuel tank material until now, because it has some weak points when used as a tank material.

First, the workability of an aluminized steel sheet is generally not very good. This is attributed to an Fe-Al-Si intermetallic compound layer called an alloy layer which is formed at the interface between the aluminum plating layer and the steel sheet. This layer is a very hard layer and, if subjected to severe processing, breakage will occur at this portion, resulting in plating delamination. Another weakness is that since the aluminum plating layer is an Al-Si alloy and is not as soft and ductile as the Pb-Sn alloy, countless cracks are likely to occur in the plating layer during processing. Moreover, since aluminum does not have sacrificial anticorrosion ability against iron, corrosion of iron from cracks easily progresses, and generally, corrosion resistance after processing is not so good, and perforation easily occurs.

Therefore, the applicant of the present invention has filed Japanese Patent Application Laid-Open No. 6-12871.
As shown in Japanese Patent Publication No. 3, a method has been proposed in which heat treatment is performed after hot dip aluminum plating. The publication discloses that an aluminum plating layer having a certain thickness or more is applied, and a heat treatment is performed to soften the aluminum plating layer, and a hard Fe-Al-
This is a method for producing a coated aluminized steel sheet in which crack propagation from the Si-based alloy layer is stopped at the aluminized layer, resulting in improved corrosion resistance after working. However, this method involves an increase in the number of steps, which means an increase in cost. Further, although the corrosion resistance after processing was almost cleared by this method, the problem of plating adhesion to withstand severe processing still remained.

[0009]

Therefore, the present invention solves the above-mentioned problems of corrosion resistance and adhesion after processing, and not only ordinary gasoline and oxidatively deteriorated gasoline but also methanol,
It has excellent corrosion resistance to alcohol fuels such as ethanol or gasoline mixed with these alcohols, and has excellent press workability that can sufficiently withstand the severe pressing conditions expected to increase in the tank manufacturing process in the future, It is intended to provide a method for producing a rust-proof steel sheet for a fuel tank, which hardly causes deterioration of corrosion resistance after working and does not use Pb.

[0010]

Means for Solving the Problems As a result of various studies to develop the above-mentioned rustproof steel sheet, the present inventors have found that it is sufficient to use aluminum plating as the base for the non-working corrosion resistance, and the adhesion For the corrosion resistance after processing, by optimizing the composition of the plating layer, we have completed a fuel tank rust preventive steel sheet with excellent press workability (adhesion and post-processing corrosion resistance) and corrosion resistance, and its manufacturing method. Is.

Next, the present invention will be described in detail. The present inventors have repeatedly conducted various experiments on the properties of the plated layer and the alloy layer that affect various properties of the hot-dip aluminized steel sheet, and have obtained the following findings. That is, Mn and C in the aluminum plating bath
It was found that, when r is added in combination, these elements are not uniformly dispersed in the plating layer but are significantly concentrated in the alloy layer. This is a phenomenon that is first observed when the elements are added in combination. Specifically, the concentration of these elements in the plating layer is about 1/5 to 1/10 of the addition amount, and the rest is concentrated in the alloy layer. Turn into. These elements are particularly concentrated in the upper part of the alloy layer, that is, at the interface between the plating layer and the alloy layer. It has been found that the hot-dip aluminized steel sheet having a composition which becomes even harder has much better corrosion resistance than the conventional steel sheet, particularly, the corrosion resistance after working. It is presumed that this is due to the effect of elements such as Mn and Cr that are significantly concentrated on the plated layer side of the alloy layer.

Further, Sn, Zn and the like in the plating bath are elements which markedly impair the corrosion resistance of the aluminized steel sheet. Therefore, Mn and Cr are contained as described above, and S in impurities is contained.
A hot-dip aluminized steel sheet in which n and Zn are limited to a certain amount or less has extremely excellent post-working corrosion resistance. On the other hand, in the above-mentioned plated steel sheet, by adding a specific amount of Mn, Cr, Fe, Si in the plating bath, or by adding a predetermined amount of Mn, Cr, Fe, Si, Sn and Zn as impurities to a specific amount. It can be manufactured with restrictions.

Further, as described in JP-A-6-128713, by annealing after plating, solid solution Fe and Si precipitate in the plating layer to soften the plating,
Generation of cracks during processing is suppressed. M for plating bath
When n and Cr are added and then annealed, the occurrence of cracks is suppressed by the effect of annealing, and the corrosion resistance of the slightly remaining cracks is also greatly improved by the addition of Mn and Cr, so the overall corrosion resistance after processing is further improved. To do.

[0014] Also, the other is the adhesion of plating, but the adhesion of the plating film generally tends to be weak against compression deformation, and there is no concern of plating peeling in bending processing of building materials or the like that are only tensile deformation. Almost no, but in severe processing such as fuel tank material where both compression and tension are applied and sliding is applied, the adhesion of plating is an important index of workability. This is also greatly improved by adding Mn and Cr. The reason for this is not clear yet, but it is presumed that the ductility of the alloy layer is slightly improved by the effect of Mn and Cr concentrated in the alloy layer.

The gist of the present invention is as follows. (1) Fe: 2 on the surface of the steel sheet, the average composition of which is% by weight.
5 to 50%, Si: 3 to 18%, Mn: 0.1 to 5%,
Cr: 0.05 to 0.8%, the balance consisting of Al and inevitable impurities, and having an intermetallic compound coating layer having a thickness of 5 μm or less, and on the surface of the intermetallic compound coating layer,
Si: 2 to 12% by weight, Fe: 1% or less, Mn:
0.005-0.3%, Cr: 0.002-0.08
%, The balance consisting essentially of Al, and Z in the impurities
An anticorrosive steel sheet for a fuel tank having excellent press workability and corrosion resistance, which has a coating layer having a total content of n and Sn of 1% or less.

(2) On a steel sheet, Si: 3-12%, Fe:
0.5-2.5%, Mn: 0.05-1.0%, Cr:
0.02 to 0.15%, the balance being Al and unavoidable impurities, and Zn and Sn contents in the impurities are 1% or less in total, and hot dip aluminum plating is performed. A method for manufacturing rust-proof steel plates for fuel tanks that have excellent press workability and corrosion resistance. (3) The method for producing an anticorrosive steel sheet for a fuel tank, which is excellent in press workability and corrosion resistance according to claim 2, characterized in that annealing treatment is performed at 300 to 500 ° C for 5 to 20 hours after plating.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the present invention will be described below. First, the composition of the coating layer and the bath composition of the manufacturing method will be described. Si: A plated layer and an alloy layer are formed on the hot-dip aluminized steel sheet as described above, and since the alloy layer is hard and brittle, the plating adhesion is impaired. In order to reduce this effect, Si is usually added to the plating bath in an amount of about 10% to suppress the thickness of the alloy layer. Also in the present invention, Si is added for the same purpose. For this purpose, the minimum amount of Si in the plating bath is 3%, and the amount of Si in the plating layer at this time is 2%.
That is all. On the other hand, if too much Si is added, coarse primary crystal Si
Is generated and adversely affects the corrosion resistance, so the upper limit is made 12%. The amount of Si in the plating layer at this time is also about 12%.

Fe: Fe is eluted from the steel sheet to be plated or the equipment in the bath and is not particularly positively added in the present invention. Usually, the plating layer also contains about 0.3 to 0.8%. Fe is preferable because it has a bad effect on the corrosion resistance, and the upper limit is 1% in the plating layer. Originally, the smaller the amount, the better. However, as described above, it is difficult to completely remove the element that is inevitably mixed. It is an unavoidable element even in the bath, and it is almost impossible to remove it. If it is forcibly removed, the equipment in the bath will be easily melted, so the lower limit value in the bath is set to 0.5%. On the other hand, the upper limit value in the bath is 2.5% because appearance stains due to corrosion resistance inhibition or dross appear.
And

Mn: This element is of particular importance in the present invention. It is an element that is concentrated in the alloy layer and has a significant effect on corrosion resistance, and in order to exert its effect, at least 0.
05% is required. When plating with this plating bath,
Since 0.005% is contained in the plating layer, this concentration is set as the lower limit value in the plating layer. On the other hand, the solubility of Mn in the plating bath is about 0.
6%. Although the solubility of Mn is about 1% in the Al-Mn binary system phase diagram, it seems that the solubility decreases in the system containing Si of about 10%. Mn to 0.
It is necessary to raise the bath temperature to dissolve 6% or more, which causes a problem that the alloy layer grows thick and the plating adhesion deteriorates. Therefore, the upper limit of the Mn concentration in the bath is 1%. The maximum Mn concentration in the plating layer when plating is performed in this bath is about 0.3%, which is the upper limit of Mn in the plating layer.

Like Cr: Mn, it is also an important element in the present invention. Cr has a particularly large effect on the corrosion resistance and has the effect of concentrating Mn in the alloy layer. In order to expect these effects, 0.002% or more is necessary in the plating layer. For this purpose, it is necessary to add 0.02% or more to the bath. Similar to Mn, Cr also has a low solubility in a plating bath, which is about 0.1% at 650 ° C., and if it is attempted to dissolve Cr further, the bath temperature must be raised. Then, the alloy layer grows thickly, so 0.15% is made the upper limit of the Cr content in the bath. Since the amount of Cr in the plating layer at this time is about 0.08%, this value is made the upper limit of the amount of Cr in the plating layer.
In the phase diagram, the solubility of Cr in Al-Cr is 0.4%, but it seems that the solubility is lowered for the same reason as Mn.

The reason why both elements are concentrated in the alloy layer when Cr and Mn are added in combination is not clear at present, but a stable metal of Cr-Mn-Fe (-Al-Si) system is used. It is considered that Cr and Mn move to the side of the original plate having a high Fe concentration due to the formation of intermetallic compounds. Zn, Sn: All of these are impurity elements that greatly impair the corrosion resistance of Al and accelerate the occurrence of white rust. Therefore, the sum of these elements is limited to 1% or less in both the plating layer and the bath.

Next, the reasons for limiting the composition of the intermetallic compound layer (hereinafter referred to as the alloy layer) will be described. Si: As described above, 3 to 12% of Si is added to the plating bath for the purpose of suppressing alloy layer growth. At this time, the Si concentration in the alloy layer is 3 to 18%. Therefore Si in the alloy layer
Is limited to this range. The Fe: alloy layer is formed mainly by the reaction between Al of the plating bath and Fe of the original plate. At this time, the Fe concentration in the alloy layer becomes 25 to 50%. Therefore, Fe in the alloy layer is limited to this range.

Mn: Mn added to the bath is concentrated in the alloy layer by the effect of Cr as described above. Due to this effect, various properties such as corrosion resistance and adhesion are significantly improved. A minimum of 0.1% Mn is required for these effects to appear. On the other hand, the Mn concentration in the bath also has an upper limit because the Mn concentration in the bath has an upper limit as described above. This is 5%.

Cr: Like Mn, Cr also concentrates in the alloy layer. Cr also has an effect on corrosion resistance, and the effect is 0.05%
The above is effective. The upper limit of Cr in the alloy layer also depends on the amount of Cr that can be added to the plating bath and is 0.8%. Regarding the thickness of the alloy layer, if it is too thick, the plating adhesion is impaired, so the upper limit is made 5 μm. Since the alloy layer hinders the plating adhesion, it is preferable that the alloy layer be thin.
Under normal operating conditions, the alloy layer thickness is 2-3 μm.

Next, the annealing conditions after plating will be described. The present invention has required corrosion resistance and adhesion after working as a fuel tank material because the plating layer contains Mn and Cr, but the performance is further improved by annealing after plating. This is because Fe and Si solid-dissolved in the aluminum plating layer are precipitated by annealing and crack generation during processing is suppressed, and the condition is 300 to 500.
C, limited to 5 to 40 hours. The reason for this is that the precipitation rate of Fe and Si is extremely slow below 300 ° C., and it takes a very long time. Further, above 500 ° C., the Fe—Al alloying reaction proceeds and the adhesion of the plating is rather impaired. Since annealing of less than 5 hr does not sufficiently precipitate Fe and Si, annealing of more than 40 hr is productive,
This is because it is disadvantageous in terms of cost.

In the present invention, the original plating plate is not particularly specified. However, as described above, since it is a fuel tank material that requires complicated processing, it is desirable that the material be a material having excellent deep drawability and overhanging property, specifically, IF steel such as Ti-IF. Naturally C is 10p even if it is not IF steel
Ultra low carbon steel such as pm or less may be used.
Further, depending on the mounting position of the vehicle fuel tank, the vehicle will always sway while the vehicle is running, so it is desirable that the fatigue strength is also excellent, and the addition of B or the like is effective. Next, the present invention will be described in more detail with reference to examples.

[0027]

[Examples] Hot-rolled aluminum sheets were hot-rolled and cold-rolled, and cold-rolled steel sheets (sheet thickness 0.8 mm) having the steel components shown in Table 1 were used for hot-dip aluminum plating. The hot-dip aluminum plating used a non-oxidizing furnace-reducing furnace type line, the plating thickness was adjusted by the gas wiping method after plating, and then the cooling rate was adjusted by air cooling. The composition of the plating bath at this time is basically Al-2% Fe, in which Si, Mn, C
r was added. The Fe in the bath at this time is supplied from the plating equipment or strip in the bath. The plating appearance was good with no plating. Further, after plating, some samples were annealed in air using a box annealing furnace. The molten aluminum plating conditions and annealing conditions at this time are shown in Tables 2 and 3.
Shown in The performance of the hot-dip aluminized steel sheet produced as described above as a fuel tank was evaluated. The evaluation method at this time was based on the method shown below.

[0028]

[Table 1]

(1) Plating layer, alloy layer composition, thickness analysis method Plating layer: 3% NaOH + 1% AlCl ₃ .6H ₂ O
Only the plating layer was peeled off by electrolytic peeling, and each element was quantitatively analyzed as a plating layer composition analysis liquid. Alloy layer: After the above electrolytic stripping, the alloy layer was stripped with a chemical conversion soda to obtain an alloy layer composition analysis liquid, and quantitative analysis of each element was performed. Alloy layer thickness: The alloy layer thickness was measured from a cross-sectional micrograph of 400 times.

(2) Evaluation of press workability A forming test was conducted by a hydraulic forming tester using a cylindrical punch having a diameter of 50 mm and a drawing ratio of 2.3. The wrinkle suppressing pressure at this time was 500 kg / cm ² , and the moldability was evaluated according to the following index. [Evaluation Criteria] 成形: Formable, no defect in plated layer 〇: Formable, cracked in plated layer △: Formable, peeling of plated layer ×: Unable to form (crack on original plate)

(3) Evaluation of Corrosion Resistance of Inner Surface after Processing Using the above hydraulic forming tester, 20 cc of the following 6 types of fuel was put into a sample of the flat-bottomed cylinder having a flange width of 20 mm, a diameter of 50 mm, and a depth of 25 mm. The glass was capped via a rubber ring. After leaving this for 3 months at room temperature, the corrosion state of the material was observed. It is known that the fuel is oxidatively deteriorated during use to generate an organic acid. To simulate this condition, deteriorated gasoline was created. The production method is to put oxygen and gasoline in a container and hold at 100 ° C. and 7 atmospheres for 10 hours. Further, when the amount of fuel in the fuel tank decreases, water in the air that has entered the tank during refueling may condense in the gas phase portion of the tank and mix in the fuel. In order to understand the effect of this water content and the effect of gasoline deterioration, we also evaluated fuel using distilled water.

[Fuel used] Gasoline 90% deteriorated gasoline + 10% distilled water 15% methanol + 85% gasoline + 10% distilled water [Evaluation Criteria] ⊚: Red rust occurred less than 0.1% and no change ◯: Red rust occurred 0 1% or more and less than 1% or slight white rust △: Red rust generation 1% or more and less than 5% or white rust X: Red rust generation 5% or more and less than 15% or white rust remarkable XX: Red rust generation on the entire surface

[0033]

[Table 2]

[0034]

[Table 3]

Table 3 shows the results of these evaluations. If the aluminum plating bath composition is low in Si (Comparative Example 1) or if the annealing temperature after plating is too high (Comparative Example 8),
The alloy layer grows too thick and causes plating separation during press forming. Further, at this time, naturally, the corrosion resistance after processing is extremely deteriorated. If the amount of Si in the plating bath (Comparative Example 2) is too large, the ductility of the plating layer deteriorates, the adhesion deteriorates, and the corrosion resistance itself deteriorates. Due to both effects, the corrosion resistance after processing also deteriorates. If the amounts of Mn and Cr in the bath are too small (Comparative Examples 3 and 5), the concentration of these elements in the alloy layer is insufficient and the corrosion resistance after processing is also insufficient.

On the contrary, if these elements are too much (Comparative Examples 4 and 6), they will not dissolve unless the bath temperature is raised, and if so, the alloy layer develops too much and the performance deteriorates. If the amount of Sn or Zn in the bath is too large (Comparative Example 7), the corrosion resistance of the plating layer deteriorates.
With conventional materials such as Pb-Sn alloy plating and zinc plating (Comparative Examples 9 and 10), the corrosion resistance of the plating layer itself is insufficient and the performance is also inferior. As shown in Examples 1 to 16 of the present invention, when all the bath component conditions are appropriate, good processability (adhesion, corrosion resistance after processing) is exhibited. Further, if this is annealed, the performance is further improved (Invention Examples 17 and 18). When the annealing temperature is low, the annealing time is short, and the like, the annealing is insufficient (invention example 19,
20) The effect of improving performance by annealing is insufficient.

[0037]

INDUSTRIAL APPLICABILITY The present invention exhibits excellent corrosion resistance not only for ordinary gasoline and gasoline that has been oxidized and deteriorated, but also for alcohol fuels such as methanol and ethanol or gasoline blended with these alcohols, and will be used in the tank manufacturing process in the future. It has excellent press workability that can sufficiently withstand the severe press conditions that are expected to increase, there is almost no deterioration in corrosion resistance after processing, and it does not use Pb and has a low environmental load for fuel tanks. It provides a method for manufacturing rusted steel sheets, and is a major contribution to the development of the industry.

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Claims (3)

[Claims] 1. The surface of a steel sheet having an average composition of, by weight%, Fe: 25 to 50%, Si: 3 to 18%, Mn: 0.1.
˜5%, Cr: 0.05 to 0.8%, the balance consisting of Al and unavoidable impurities, and having an intermetallic compound coating layer having a thickness of 5 μm or less, and on the surface of the intermetallic compound coating layer. , Si: 2 to 12% by weight, Fe: 1% or less,
Mn: 0.005 to 0.3%, Cr: 0.002 to 0.
08%, the balance being substantially Al, and having a coating layer in which the total content of Zn and Sn in the impurities is 1% or less, which is excellent in press workability and corrosion resistance Rusted steel plate. 2. A steel sheet having Si: 3 to 12% and Fe: 0.
5 to 2.5%, Mn: 0.05 to 1.0%, Cr: 0.
02-0.15%, the balance being Al and unavoidable impurities, and the total content of Zn and Sn in the impurities is 1%
A method for producing an anticorrosive steel sheet for a fuel tank, which is excellent in press workability and corrosion resistance, which is characterized by performing hot dip aluminum plating in the following plating bath. 3. After plating, 5 to 20 at 300 to 500 ° C.
The method for producing an anticorrosive steel sheet for a fuel tank, which is excellent in press workability and corrosion resistance according to claim 2, characterized by performing an hr annealing treatment.