JP4889137B2 - Method for producing aluminum-plated steel sheet for fuel tank - Google Patents

Description

【００１３】
【発明の効果】
以上に説明したように、本発明の燃料タンク用アルミ系めっき鋼板は、ＡｌーＳｉめっき層の表面粗さをＲａ０．３〜２．０μｍの範囲に調整すると共に大きな窪みのある個所での比Ｒｍａｘ／ｔを０．８以下に規制することにより、潤滑油を保持するミクロプールとして機能させながら、Ａｌ−Ｓｉめっき層の有効厚みを確保している。これにより、プレス成形時の加工割れが抑制され、めっき層への応力集中が緩和されるため、伸び，圧縮等が複合した塑性変形を伴う燃料タンク形状への加工に際してもめっき層にクラックが生じることなく、アルミ系めっき鋼鈑本来の優れた耐食性が維持される。よって、深絞り等の加工を施しても加工割れが発生せず、また、めっき層のクラック発生を抑制することで腐食が長期にわたって生じない耐食性に優れた燃料タンクが得られる。
【図面の簡単な説明】
【図１】 自動車用燃料タンクの概略斜視図
【図２】 アルミ系めっき鋼板の断面図
【図３】 めっき層の表面粗さが限界絞り比に及ぼす影響を示したグラフ[0001]
[Industrial application fields]
The present invention relates to a method for producing an aluminum-plated steel sheet for a fuel tank that maintains excellent corrosion resistance by preventing cracking of a plating layer even when processing such as deep drawing is performed. .
[0002]
[Prior art]
Since the fuel tank for automobiles is manufactured by pressing a steel plate, a molten Pb-Sn plated steel plate has been conventionally used with emphasis on workability. A fuel tank made of a molten Pb—Sn plated steel sheet is resistant to internal corrosion caused by fuel or external corrosion caused by salt damage.
However, in recent years, materials that do not use Pb have been demanded from the viewpoint of reducing environmentally hazardous substances. Therefore, an aluminum-plated steel sheet that has been subjected to Al or Al-Si plating that does not contain environmentally hazardous substances has begun to be used as a fuel tank material.
[0003]
[Problems to be solved by the invention]
The fuel tank is manufactured by pressing a steel plate into a complicated shape as shown in FIG. 1, and the fuel tank body 1 has an inlet pipe 2, a fuel pipe 3, a fuel return pipe 4, a sub tank 5, a drain plug 6, and the like. Various members are attached. The press work at this time is a process involving complicated plastic deformation in which elongation, compression, and the like are combined. In an aluminum-based plated steel sheet in which a hard alloy layer is likely to be generated and grown during hot dip plating, cracks tend to occur in the plated layer due to stress concentration during processing. When the generated crack reaches the base steel, the corrosion proceeds from there and the risk of opening the fuel tank in a short time increases.
[0004]
[Means for Solving the Problems]
The present invention has been devised to solve such problems, and by controlling the effective thickness and surface roughness of the Al-Si plating layer, processing cracks can occur even when processing such as deep drawing is performed. An object of the present invention is to provide a method for producing an aluminum-plated steel sheet for a fuel tank that does not occur, relieves stress concentration during press processing, suppresses cracking of the plating layer, and maintains excellent corrosion resistance after processing. To do.
In order to achieve the object of the method for producing an aluminum-plated steel sheet for fuel tanks of the present invention, the hot-plated aluminum-plated steel sheet is temper-rolled by a rolling roll whose surface roughness is adjusted , Si content: 13% by mass or less and an Al—Si plating layer having a thickness of 5 μm or more is passed through an Al—Fe—Si alloy layer, and the surface roughness of the Al—Si plating layer is 0.3 to 2 in terms of Ra. in the range of .0Myuemu, and Al-Fe-Si-based ratio Rmax / t of the surface maximum height Rmax of the Al-Si plating layer to the total thickness t of the alloy layer a plating layer including becomes 0.8 or less It is formed as follows.
[0005]
[Operation and embodiment]
On the surface of the aluminum-plated steel sheet, as schematically shown in FIG. 2, an Al—Si plating layer is formed on the base steel via an Al—Fe—Si alloy layer. Moreover, the plating layer has an uneven surface and has a large depression. When a plated steel sheet having such an Al—Si plating layer is pressed into a fuel tank shape, stress concentrates on a portion having a large depression, and cracks reaching the base steel tend to occur. When an aluminum-plated steel sheet with cracks that reaches the base steel is exposed to a corrosive environment, corrosion starts from the defective part and leads to early opening.
Therefore, the present inventors investigated the relationship between the occurrence of cracks and the surface morphology of the Al—Si plating layer. As a result, the surface roughness of the Al—Si plating layer is adjusted to a range of 0.3 to 2.0 μm with Ra, and the Al—Si with respect to the total thickness t of the plating layer including the Al—Fe—Si based alloy layer. When the ratio Rmax / t of the maximum surface height Rmax of the plating layer is suppressed to 0.8 or less, the stress concentration during press working is relaxed, the holding capability of the processing lubricant is increased, and cracks reaching the base steel are generated. Has been found to be suppressed.
[0006]
In an aluminum-plated steel sheet manufactured by a normal hot-dip aluminum plating line, an Al—Si plating layer having a thickness of 5 to 25 μm is formed through an Al—Fe—Si alloy layer having an average thickness of 2 to 5 μm. In order to satisfy the corrosion resistance required for the fuel tank application, it is necessary to adjust the thickness of the Al—Si plating layer to be slightly larger than 6 μm (preferably 8 to 16 μm). The layer thickness can be easily adjusted by, for example, a method of gas wiping a hot-dip metal adhering to the original plating plate pulled up from the hot-dip plating bath.
[0007]
The Al—Si plating layer is softer and more malleable than the Al—Fe—Si alloy layer. When the thickness of the Al-Si plating layer is 5 μm or more, the metal flow of the Al-Si plating layer following the plastic deformation during the press working is ensured, and the processed product surface is covered with the Al-Si plating layer. Is called. However, generally, as the plating layer becomes thicker, the corrosion resistance is improved, but the weldability and plating peel resistance are lowered. Therefore, although depending on the application, the Al—Si plating layer is preferably 25 μm or less.
The surface unevenness of the Al—Si plating layer functions as a micropool that holds lubricating oil used for press forming of an aluminum-based plated steel sheet. The lubricating oil retention function becomes significant when the surface roughness of the Al—Si plating layer is Ra ≧ 0.3 μm. However, when the surface roughness Ra exceeds 2 μm, the lubricating oil is excessively retained in the surface layer recesses of the Al—Si plating layer, so that the effect of the lubricating oil is reduced.
[0008]
It is necessary to adjust the surface roughness of the Al—Si plating layer so that the ratio Rmax / t of the maximum surface height Rmax to the total thickness t of the plating layer is 0.8 or less. By setting the ratio Rmax / t to be 0.8 or less, the depression where stress is likely to concentrate during press forming becomes shallow, and the stress concentration is relaxed accordingly, and the generation of cracks reaching the base steel is suppressed. The ratio Rmax / t ≦ 0.8 also means that an effective thickness of the malleable Al—Si plating layer is secured, and is effective in preventing exposure of the base steel after press forming. Since the unevenness on the surface of the Al-Si plating layer is formed by transferring the unevenness on the surface of the rolling roll during temper rolling after hot dipping, the surface roughness of the Al-Si plating layer depends on the surface roughness of the temper rolling roll. You can adjust the degree.
[0009]
【Example】
A normal steel plate with a thickness of 0.8 mm is used as the plating base plate, and after reduction annealing, it is introduced into a molten aluminum plating bath having a bath temperature of 660 ° C. and Si: 3 to 13 mass% at a line speed of 90 m / min. An aluminized steel sheet with an adhesion amount adjusted to 20 to 60 g / m ² per side was produced. Aluminum having Al-Si plating layers with different surface roughness Ra and ratio Rmax / t by changing the surface roughness of the temper rolling roll used in temper rolling after hot dipping and the total thickness t of the plating layer A galvanized steel sheet was produced.
[0010]
A flat test piece was cut out from each of the obtained aluminum-based plated steel sheets, and the workability was investigated from the limit drawing ratio at which a work crack occurred by a workability test. Moreover, the corrosion resistance of the test piece after processing was investigated by an inner surface corrosion test and an outer surface corrosion test assuming a fuel tank application.
[Workability test]
Using a punch with a radius of 20 mm and a shoulder radius of 5 mm, a test piece with a blank diameter of 84 mm was drawn into a flat bottom cylindrical shape, and a limit drawing ratio at which a working crack would occur was determined.
[Internal corrosion test]
Corrosion test prepared by using a punch with a radius of 20 mm and a shoulder radius of 5 mm, drawing a test piece with a blank diameter of 84 mm into a flat bottom cylindrical shape with a drawing ratio of 2, and then mixing water containing 350 ppm of formic acid with an equal amount of gasoline. Immerse in the liquid. The test piece was continuously immersed for 10 weeks while changing the corrosion test solution every week. At the time when 10 weeks passed, the test piece was pulled up and the inner surface corrosion resistance was evaluated by the thickness reduction rate of the corroded portion.
[External corrosion test]
Similarly, salt water having a concentration of 5% by mass was continuously sprayed on a test piece drawn into a flat bottom cylindrical shape with a drawing ratio of 2, and the surface of the test piece was observed after 240 hours had elapsed from the start of spraying. The outer surface corrosion resistance was evaluated by the area ratio of red rust generated on the surface of the test piece.
[0011]
As can be seen from the investigation results in Table 1, the surface roughness of the Al—Si plating layer was adjusted to Ra = 0.3 to 2.0 μm, and the ratio Rmax / t was suppressed to 0.8 or less. In each of the examples, good workability was exhibited, and both the inner surface corrosion resistance and the outer surface corrosion resistance were good because the base steel was covered with the Al—Si plating layer even after the processing. Further, when the surface roughness was maintained in the range of Ra = 0.3 to 2.0 μm, the limit drawing ratio showed a high value as shown in FIG.
On the other hand, in the comparative example in which the surface roughness Ra is out of the range specified in the present invention, a low limit drawing ratio of 1.7 or less is shown, and in the comparative example in which the ratio Rmax / t is out of the range specified in the present invention Corrosion occurred starting from the part where the plating layer was peeled off by molding.
[0012]

[0013]
【Effect of the invention】
As described above, the aluminum-based plated steel sheet for fuel tanks of the present invention has a surface roughness of the Al-Si plating layer adjusted to a range of Ra 0.3 to 2.0 [mu] m and a ratio at a portion having a large depression. By regulating Rmax / t to 0.8 or less, the effective thickness of the Al—Si plating layer is ensured while functioning as a micropool that holds the lubricating oil. As a result, processing cracks during press forming are suppressed, and stress concentration on the plating layer is alleviated, so cracks occur in the plating layer even when processing into a fuel tank shape with plastic deformation combined with elongation, compression, etc. Therefore, the original excellent corrosion resistance of the aluminum-based plated steel sheet is maintained. Therefore, even if processing such as deep drawing is performed, a processing tank does not occur, and by suppressing the generation of cracks in the plating layer, a fuel tank excellent in corrosion resistance that does not cause corrosion over a long period of time can be obtained.
[Brief description of the drawings]
1 is a schematic perspective view of a fuel tank for automobiles. FIG. 2 is a cross-sectional view of an aluminum-based plated steel sheet. FIG. 3 is a graph showing the influence of the surface roughness of a plating layer on the limit drawing ratio.

Claims (1)

The hot-plated aluminum-based plated steel sheet is temper-rolled with a rolling roll whose surface roughness is adjusted, and an Al-Si plated layer having a Si content of 13 mass% or less and a layer thickness of 5 μm or more is formed on the steel sheet surface. The surface roughness of the Al—Si plating layer is in the range of 0.3 to 2.0 μm in Ra through the Fe—Si alloy layer, and the plating layer including the Al—Fe—Si alloy layer method for manufacturing a fuel tank for an aluminum-plated steel sheet, characterized in that the ratio Rmax / t of the surface maximum height Rmax of the Al-Si plating layer to the total thickness t is formed so as to 0.8.

Images