Metal treatment – Stock – Age or precipitation hardened or strengthened
Reexamination Certificate
2001-04-18
2002-10-08
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Age or precipitation hardened or strengthened
C420S534000, C420S535000, C428S610000
Reexamination Certificate
active
06461454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aluminum alloy plate for an automobile such as a panel used by being coated after a chemical conversion treatment and a manufacturing method thereof, and more particularly, relates to an aluminum alloy plate for an automobile made of Al—Mg—Si based aluminum alloy and having excellent filiform rust resistance and a manufacturing method thereof.
2. Description of the Related Art
The weight reduction of an automobile has been recently proceeded from the viewpoints of energy reduction, the control of exhausted gas and the like. Concerning the weight reduction of the automobile, it is studied to adopt an aluminum (Al) alloy plate in place of a steel plate, in addition to make the steel plate thinner.
The aluminum alloy plate for an automobile is generally formed into a predetermined member, a chemical conversion treatment such as zinc phosphate treatment or the like is carried out, and electro-deposition coating, intermediate coating and top coating are further carried out.
An Al alloy material such as A.A or JIS5000-base (hereinafter, referred to as 5000-base) excellent in formability, or A.A or JIS6000-base (hereinafter, referred to as 6000-base) excellent in formability and baking cure property is suitable for an aluminum panel such as the outer plate of an automobile, or the like, and in particular, the 6000-base Al alloy material has an excellent formability.
In order to adopt the Al alloy plate as a panel for transportation instruments, a press forming processing such as deep drawing, overhang, bending and elongating flange or the like is carried out to make the aluminum alloy plate be in a predetermined member form. In this case, it is required to secure high deep drawing property (the limit deep drawing ratio (LDR) is large, or the limit deep drawing height (LDH
0
) is high) and high form-freezing (form-holding) property, in the deep drawing, overhang, bending and elongating flange forming.
Accordingly, it is carried out to control the chemical composition of the 6000-base Al alloy plate as means of improving the formability of the 6000-base Al alloy plate. In particular, the most effective means for improving the formability is the addition of Cu, and such a technique is disclosed in many Japanese Patent Application Laid-Open No.6-2064, No.6-136478, No.8-109428, No.9-209068, No.9-202933 and the like.
When Cu is added, the formability is surely improved, but a thread shape corrosion called as filiform rust is apt to be generated between a coating and the aluminum alloy plate after coating. As one of methods for suppressing such filiform rust, a technique in which the filiform rust is suppressed by preventing intergranular corrosion from occurring disclosed, under an assumption that there is a correlation between the intergranular corrosion of the aluminum alloy plate and the filiform rust (Conventional Example 1).
For example, a technique of suppressing the intergranular corrosion is disclosed in the Lecture Abstract No.99 (edited in 1995) of the 88
th
Meeting of Light Metal Society. In this technique, Mg
2
Si is precipitated both in the grain boundary and in the grain by means of a precipitation process so as to make the intergranular potential and the transgranular potential be the same level each other.
In addition, a technique of suppressing the intergranular corrosion by making the transgranular potential be the same level as the dissolution potential of Mg
2
Si precipitated in the grain boundary by addition of Zn is disclosed in the Lecture Correction No.165 (edited in 1997) of the 92
th
Meeting of Light Metal Society.
Further, as an alternative method, it is studied to improve phosphorous acid treatment property under an assumption that the filiform rust can be suppressed by improving the adhesion of a coating film with a chemical conversion treatment film such as a phosphate treatment film or the like. In JP-A No.6-287672, is disclosed a technique of improving the filiform rust property, by precipitating (concentrating) 0.1 to 10% by mass of Cu on a surface, by way of etching treatment and the like, of the 6000-base Al alloy plate containing 0.01 to 5% by mass of Cu, working the precipitated Cu as a cathode reaction point at a phosphorous acid treatment to improve phosphorous acid treatment property, and improving the adhesion of the Al alloy plate with the coating film (Conventional Example 2).
However, when the difference between the intergranular potential and transgranular potential is made smaller as the technique in Conventional Example 1, the intergranular corrosion property of the aluminum alloy plate is surely improved, but there is a problem that the formability of the aluminum alloy plate is lowered by the precipitation of the above-described Mg
2
Si. Further, even if the precipitation treatment of Mg
2
Si is carried out, the filiform rust still happens, and there is a problem that the rust cannot be effectively suppressed.
Further, in the technique of Conventional Example 2, when Cu was concentrated on the surface of the aluminum alloy plate, the phosphate treatment property of the aluminum alloy plate is surely improved, and the adhesion property of the coating with the aluminum alloy plate treated with chemical conversion treatment is improved. However, there is a problem that the filiform rust resistance is remarkably lowered by having concentrated Cu on the surface. Accordingly, on the contrary, the technique of Conventional Example 2 in which Cu is concentrated on the aluminum alloy plate is reverse effect to improve the filiform rust property of the 6000-base aluminum alloy plate material containing Cu.
Thus, it is status quo that there has been no effective method of improving the filiform rust resistance of the 6000-base aluminum alloy material containing Cu which has remarkably high sensitivity for the generation of the filiform rust after coating.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an aluminum alloy plate for an automobile containing Cu, which has an improved filiform rust resistance while keeping high formability and, and a manufacturing method thereof.
The aluminum alloy plate for an automobile according to the present invention has a chemical composition containing 0.8 to 1.5% by mass of Si, 0.4 to 0.7% by mass of Mg and 0.5 to 0.8% by mass of Cu. The crystal grain size of the aluminum alloy plate is 10 to 40 &mgr;m, and Cu content obtained by analyzing the outermost surface of the aluminum alloy plate with an oxide film according to X-ray photoelectron spectroscopy (XPS) is {fraction (1/10)} to ½ of the Cu content of the bulk of said aluminum alloy plate.
The manufacturing method of the aluminum alloy plate for an automobile according to the present invention comprises the steps of: melting and casting the ingot of an aluminum alloy plate containing 0.8 to 1.5% by mass of Si, 0.4 to 0.7% by mass of Mg and 0.5 to 0.8% by mass of Cu according to a DC casting method and homogenizing; carrying out hot rolling, cold rolling and annealing to obtain a plate of a predetermined thickness; and rapidly cooling after carrying out solution heat treatment during a predetermined time in a heat treatment furnace.
In the present invention, since the contents of Cu, Mg and Si in the Al alloy, the crystal grain size of the Al alloy, and Cu content on the outermost surface are appropriately defined, the aluminum alloy plate for an automobile which has a high filiform rust resistance while keeping high formability and is further excellent in appearance after zinc phosphate treatment, baking cure property and productivity, can be obtained.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is further described in detail below. The inventors of the present invention have intensively studied and conducted experiments in order to solve the above-mentioned problems, and as a result, found that the crystal grain size and the Cu content on the outermost surface of the Al alloy plate with an oxide film are impor
Hattori Tsutomu
Katsukura Masahito
Matsuda Shinji
Sakata Mariko
Takezoe Osamu
Combs-Morillo Janelle
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd).
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wyszomierski George
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