Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-08-06
2003-11-11
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C420S034000
Reexamination Certificate
active
06645318
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a fuel tank, for an automobile, demonstrating excellent corrosion resistance to internal gasoline degradation and external salt damage and made of a ferritic stainless steel sheet excellent in formability.
BACKGROUND ART
A steel material for a fuel tank, to contain automobile gasoline, requires corrosion resistance to the environment of the fuel tank and workability to allow pressing into a fuel tank. The internal environment of a fuel tank becomes a severe corrosive environment for a steel material when organic acid such as formic acid, acetic acid, etc., generated by the degradation of gasoline, dissolves in an aqueous phase contained in gasoline. On the other hand, the corrosion on the outer surface of a fuel tank is mainly caused by salt damage resulting from snowmelt salt, sea salt, etc. As a steel material suitable for these internal and external corrosive environments, a terne sheet produced by the surface treatment of Pb—Sn alloy has been used. Since a fuel tank is produced by press forming and has a partially complicated shape, the steel material also requires press formability. A terne sheet secures workability by using a highly formable steel sheet of deep drawing quality (DDQ) called IF (Interstitial Free) steel as its substrate. In addition, since the Pb—Sn alloy plated layers of a terne sheet have good workability and excellent lubricity, a terne sheet is usable as a material for a fuel tank also in terms of workability and has been widely used, accordingly.
However, since a terne sheet contains Pb, there is a movement to reconsider its use for a fuel tank from the recent need to suppress the elution of substances with environmental impact. For that reason, some terne sheets are being replaced with plated steel materials not containing Pb (for example, Al plated steel materials (Japanese Unexamined Patent Publication No. H9-156027)) or resins. Further, recently, as a part of the global environment improvement, the idea of extending the assurance period of a fuel tank from the current 10 years to 15 years and of regulating fuel penetration loss from a fuel tank to zero have surfaced internationally. With regard to these new requirements, it is thought that a conventional terne sheet or an Al plated steel sheet which is a substitute therefor cannot assure a 15 year life and resins cannot suppress the fuel penetration to zero.
Most recently, as a material to satisfy both requirements, the application of stainless steel to a fuel tank has been studied. With regard to steel grades, SUS304L, which is excellent in workability, and austenitic stainless steel, which has more improved corrosion resistance than SUS304L, are the candidates, taking long-term corrosion resistance and formability into consideration. However, the largest problem in using an austenitic stainless steel is that its reliability against stress corrosion cracking, generated by salt damage, cannot be secured. In an environment where stress corrosion cracking occurs, it is effective to adopt ferritic stainless steel which has the same level of corrosion resistance as that of austenitic stainless steel, instead of austenitic stainless steel. However, ferritic stainless steel has lower punch stretchability than that of austenitic stainless steel and therefore generally has low press working limit.
As a steel material excellent in press formability while maintaining the corrosion resistance of stainless steel, a clad steel sheet, for a fuel tank, the surface layers of which are composed of stainless steel and the inner layer of which is composed of carbon steel (IF steel) has been proposed in Japanese Unexamined Patent Publication Nos. H6-158221 and H6-293978. Though austenitic stainless steel is also proposed for the clad steel sheet described here, stress corrosion cracking can hardly be avoided as described above. It is stated that, in case of using ferritic stainless steel for a clad steel sheet, the clad steel sheet can provide as good a press formability as low carbon steel if soft steel excellent in workability is used as the inner layer. However, even in a clad steel sheet, it is necessary to improve the formability of ferritic stainless steel itself of the surface layers for securing excellent formability. To do so, it is conceivable, for example, to apply high temperature annealing to a clad steel sheet and soften the ferritic stainless steel of the surface layers. In this case, the annealing temperature is too high for the carbon steel of the inner layer and deterioration of the strength of the carbon steel, namely the clad steel sheet, becomes unavoidable. Further, in case of cutting a clad steel sheet and using it for forming parts, the carbon steel at the edges is exposed to a corrosive environment and therefore cumbersome anticorrosive measures have to be taken. For the aforementioned reasons, a fuel tank made of a clad steel sheet is not practically usable.
Therefore, for applying a ferritic stainless steel capable of avoiding a stress corrosion cracking problem to a fuel tank, it is necessary to develop ferritic stainless steel having a workability required to easily form a fuel tank as well as excellent corrosion resistance in a fuel tank environment.
The object of the present invention is to provide a fuel tank made of a ferritic stainless steel sheet which overcomes the problems of conventional materials, is excellent in long-term corrosion resistance, is usable for a fuel tank without fuel penetration, has no stress corrosion cracking, is excellent in corrosion resistance in the internal and external environments of the fuel tank, and is excellent in workability.
SUMMARY OF THE INVENTION
The present inventors investigated the workability required for press forming by carrying out press forming, into various shapes, of fuel tanks and then studied chemical compositions satisfying the required workability, with regard to a ferritic stainless steel having excellent corrosion resistance in the internal and external environments of a fuel tank.
As a result, the present inventors discovered that the workability of ferritic stainless steel sheet required for the press forming to form various shapes of fuel tanks were the average r-value of the steel sheet being 1.9 or larger, the r-value in-plane anisotropy &Dgr;r thereof being 1.0 or smaller, and the total elongation thereof being 30% or larger. In addition to these properties, the present inventors confirmed that excellent workability in forming a fuel tank could be secured by controlling the texture and surface friction coefficient of a steel sheet to appropriate conditions. Further, the present inventors discovered that, for satisfying the above workability, it was effective to add Ti or Nb after C and N were lowered as the basic components. Furthermore, the present inventors discovered that, to not cause perforation for a long time in a salt damage environment and, further, to suppress rusting in a fuel environment, it was effective to add 10 to 25 mass % of Cr, add Ti or Nb after C and N were lowered as the basic components, and additionally add appropriate amounts of Mo, Cu and Ni. The present inventors have completed the present invention based on the above findings.
The gist of the present invention is as follows:
(1) A fuel tank characterized by being made of a ferritic stainless steel sheet containing 10 to 25 mass % of Cr and having an average r-value of 1.9 or larger, an r-value in-plane anisotropy &Dgr;r of 1.0 or smaller, and a total elongation of 30% or larger.
(2) A fuel tank according to the item (1), characterized by being made of a ferritic stainless steel sheet having a plane intensity ratio I(111)/I(100) of 10 or larger.
(3) A fuel tank according to the item (1), characterized by being made of a ferritic stainless steel sheet having lubricant films on the surfaces of the steel sheet and a surface friction coefficient of 0.10 or less.
(4) A fuel tank characterized by being made of a ferritic stainless steel sheet: comprising, in mass,
10 to 25% of Cr,
0.015% or less of C,
0
Abe Masayuki
Kato Katsuhiko
Komori Tadashi
Koyama Yuji
Ono Naoto
Kenyon & Kenyon
Nippon Steel Corporation
Yee Deborah
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