Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1998-02-26
2004-01-06
La Villa, Michael (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C220S004140, C428S654000, C428S682000, C428S684000, C428S341000, C428S334000
Reexamination Certificate
active
06673472
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides a rust-preventive steel sheet, having corrosion resistance as well as excellent air-tightness after welding, as a steel sheet for automobile fuel tanks.
BACKGROUND OF THE INVENTION
A fuel tank of an automobile is usually designed in accordance with the design of the body in the final stage, and the shape has tended to become more and more complicated in recent years. Moreover, since the fuel tank must be safe in an automobile, the material to be used for the fuel tank is required to have an extremely excellent deep drawability and not to crack, due to an impact, subsequent to forming. In addition to the requirements, it is also important that the material form a decreased amount of corrosion product leading to pitting corrosion and filter clogging, and that the material can be easily and stably welded.
A Pb—Sn alloy-plated steel sheet, which is called a terne steel sheet, (Kokoku (Japanese Examined Patent Publication) No. 57-61833) has heretofore been principally used as a material having such various properties. The steel sheet has chemical properties stabilized against gasoline, and shows excellent press formability due to the excellent lubricity of the plating. In addition to the Pb—Sn alloy-plated steel sheet, a Zn-plated steel sheet which is thickly chromated is also used. The steel sheet also has excellent formability and corrosion resistance though it is not as excellent as the Pb—Sn alloy-plated steel sheet. However, a material not using Pb is desired from the standpoint of decreasing environmental pollution.
One of the prospective fuel tank materials of automobiles in which Pb is not used is an Al—(Al—Si) plated steel sheet. Since Al forms a stabilized oxidized film on its surface, Al shows excellent resistance to corrosion caused by organic acids formed by the deterioration of alcohol, gasoline, etc. as well as to gasoline. However, there are several problems with using the Al-plated steel sheet as a fuel tank material. One of the problems is poor press formability. Since the Al-plated steel sheet has a very hard Fe—Al—Si intermetallic compound layer (referred to as an alloy layer hereinafter) formed at the interface between the plating layer and the steel sheet, the material quality is lowered compared with that of a steel sheet having no Al-plated layer. The Al-plated steel sheet, therefore, tends to crack when severely formed.
The Al-plated steel sheet also has the disadvantage that the peeling of the plating and crack formation therein tend to take place from a starting point in the alloy layer. When cracks are formed in the plating, inner corrosion tends to proceed from the cracks, and pitting may result in a short period of time. Accordingly, corrosion resistance subsequent to forming is a serious problem.
Another problem is weldability. Although an Al-plated steel sheet may be resistance welded, the welding lacks stability to some degree. Moreover, the Al-plated steel sheet has a problem in that the weld zone shows poor air-tightness though the steel sheet may be subjected to resistance welding such as spot welding and seam welding. Although a fuel tank material is required to have air-tightness after welding so that the fuel neither leaks nor volatilizes, the Al-plated steel sheet has the problem that its weld zone tends to fracture when an internal pressure is applied after welding, and the steel sheet shows poor air-tightness after welding. This is a phenomenon which substantially does not take place in other plated steel sheets such as a terne steel sheet and a Zn-plated steel sheet but which appears significant in the Al-plated steel sheets alone. Though the reasons are not definite, it is thought that Al in the plating layer diffuses into the steel to exert undesirable effects.
DISCLOSURE OF THE INVENTION
The present invention solves the problems as mentioned above, and provides a new rust-preventive steel sheet for fuel tanks having excellent corrosion resistance without using Pb in an organic acid environment and excellent press formability with which the steel sheet may satisfy anticipated sever press conditions in the production process of the tanks, showing no poor air-tightness in the weld zone, and ensuring resistance to corrosion caused by an organic acid after forming.
Furthermore, the chemical composition of the steel is optimized in the present invention to ensure the air-tightness in the weld zone. Specifically, the properties of the steel sheet has been significantly improved by restricting the P content and adding B.
The present applicant has disclosed a hot-dip Al-plated steel sheet to which up to 30 ppm of B is added in Kokai (Japanese Unexamined Patent Publication) No. 60-165366, and a hot-dip Al-plated steel sheet to which 0.01% of B is added in Kokai (Japanese Unexamined Patent Publication) No. 60-103167. These inventions have intended to provide steel sheets having a high temperature strength or oxidation resistance at high temperatures, and B has been added to fulfill the object. Moreover, the steel sheets are naturally anticipated to be used for automobile exhaust system materials, etc. to be employed in a high temperature environment. In contrast to the inventions mentioned above, the present inventor has discovered that the optimization of the contents of P and B in the steel has significant effects on the improvement of the air-tightness after welding which is an essential property of fuel tank materials.
On the other hand, the present invention provides two methods for greatly improving the corrosion resistance of a steel sheet subsequent to forming, namely the other problem to be solved.
First, the present inventors have investigated the progress of corrosion in fuel tanks subsequent to forming, and found a corrosion behavior as described below. A principal corrosion component in the inner environment of fuel tanks is formic acid formed by decomposition of the fuel. Corrosion of the base steel begins from cracks in the plating and the alloy layer as starting points, and the corrosion proceeds at the interface between the base steel and the alloy layer, resulting in gradual floating of the plating from the base steel and entire corrosion. The corrosion proceeds at the interface between the base steel and the alloy layer because the potential of the alloy layer is nobler than that of the base steel in the presence of formic acid and consequently corrosion of the base steel near the alloy layer is promoted.
There are two methods for diminishing the corrosion based on such discoveries. One of the methods is to inhibit cracks in the alloy layer, and the other is to decrease the potential difference between the alloy layer and the base steel.
Accordingly, the present invention provides two methods as described below. One is based on the discovery that an Al-plated steel sheet having a high total elongation inhibits crack formation in the plating, and intends to inhibit the formation of cracks in the alloy layer by optimizing the chemical composition of the steel. The other one is based on the discovery that when the potential difference between the alloy layer and the base steel measured in an environment containing 100 ppm of formic acid and the balance water at 20° C. is up to 0.35V, corrosion hardly proceeds, and tends to inhibit the progress of corrosion even when cracks are formed in the alloy layer by optimizing the chemical compositions of the steel and the plating. The potential of the alloy layer-base steel can be controlled by adjusting the chemical compositions of the steel and the plating bath, or by pre-plating before hot-dip plating. For example, the following procedures may be practiced: Cr is added to the steel; the steel surface is pre-plated with Cr; or a clad steel is used; and Zn, etc., is added to the plating bath.
That is, the aspects of the present invention are as described below.
(1) A rust-preventive steel sheet for fuel tanks excellent in air-tightness after welding and corrosion resistance subsequent to forming, which comprises
a steel sheet comprising
Fuda Masahiro
Izaki Teruaki
Maki Jun
Ohmori Takayuki
Okada Nobyoshi
Kenyon & Kenyon
La Villa Michael
Nippon Steel Corporation
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