Aluminum alloy plate excelling in filiform corrosion...

Details Aluminum alloy plate excelling in filiform corrosion... Aluminum alloy plate excelling in filiform corrosion...

2001-03-26

2002-10-15

Wyszomierski, George (Department: 1742)

Metal treatment

Stock

Age or precipitation hardened or strengthened

C420S534000, C420S531000

Reexamination Certificate

active

06464805

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aluminum alloy plate excelling in filiform corrosion resistance. More particularly, the present invention relates to a bake-hardenable Al—Mg—Si—Cu aluminum alloy plate, excelling in filiform corrosion resistance, which is suitably used as a material for transportation devices such as an automotive outer body panel, and to a method of fabricating the same.
2. Description of Background Art
In recent years, reduction of the weight of automobiles has been demanded in order to improve fuel consumption from the viewpoint of environmental protection, etc. To deal with this demand, an aluminum alloy plate has been partly used for an automotive outer body panel, in place of a cold-rolled steel plate has conventionally been used.
As examples of aluminum alloys currently used for an automotive outer body panel in practice, Al—Mg alloys such as A5022, A5023, and A5182, and Al—Mg—Si alloys such as A6111, A6016, and A6022 can be given. Al—Mg alloys excel in formability. However, Al—Mg alloys do not exhibit bake-hardenability, thereby exhibiting inferior dent resistance.
Since Al—Mg—Si alloys exhibit excellent bake-hardenability, Al—Mg—Si alloys exhibits superior dent resistance. However, formability of Al—Mg—Si alloys is insufficient. The addition of Cu provides improved formability to Al—Mg—Si alloys due to an increased r value (Lankford value). However, addition of Cu tends to cause intergranular corrosion to occur, thereby resulting in decrease in corrosion resistance, in particular, filiform corrosion resistance. Therefore, the Cu content in the A6016 alloy and the A6022 alloy is limited to 0.20% or less and 0.11% or less, respectively. In addition, A6111 alloys which contain 0.50-0.9% of Cu may exhibit inferior corrosion resistance.
Japanese Patent Application Laid-open No. 10-176233 proposes a method for preventing intergranular corrosion by adding Zn to an Al—Mg—Si alloy containing Cu. The addition of Zn decreases the electric potential of an electrochemical matrix, thereby decreasing the potential difference between Mg
2
Si and the matrix. This prevents Mg
2
Si precipitated on the grain boundaries from being dissolved under corrosive environment, whereby intergranular corrosion can be prevented. In this method, the Cu content is also limited to 0.8% or less. If the Cu content exceeds 0.8%, corrosion resistance decreases.
Japanese Patent Application Laid-open No. 10-237576 proposes an Al—Mg—Si alloy containing 0.25-1.0% of Cu and exhibiting improved corrosion resistance which is used for an automotive outer body panel, wherein Pb, As, Sn, and other impurity concentrations in a Zn substrate plating layer used for zinc phosphate treatment and paint treatment are limited. However, the object of this technique is to improve corrosion resistance of the material by use of the substrate plating layer for conversion treatment, but not to improve corrosion resistance of the Al—Mg—Si—Cu alloy itself. This method involves difficulty in managing the plating solution.
SUMMARY OF THE INVENTION
The present inventors have conducted extensive experiments and studies to solve the above problems relating to an Al—Mg—Si—Cu alloy used for an automotive outer body panel by improving corrosion resistance of the alloy, and to produce an Al—Mg—Si—Cu alloy exhibiting excellent formability, excellent intergranular corrosion resistance, and improved filiform corrosion resistance after painting. The present inventors have conducted studies from the viewpoint of clarifying the relation between intergranular/filiform corrosion resistance and intermetallic compounds precipitated inside or boundaries of the crystal grains during the fabrication process. Accordingly, an object of the present invention is to provide an Al—Mg—Si—Cu alloy suitable for an automotive outer body panel which excels in strength and formability and exhibits improved filiform corrosion resistance, and a method of fabricating the same.
An aluminum alloy plate excelling in filiform corrosion resistance and a method of fabricating the same according to the present invention are characterized as follows.
(1) An aluminum alloy plate excelling in filiform corrosion resistance, comprising 0.25-0.6% of Mg (mass %, hereinafter the same), 0.9-1.1% of Si, 0.6-1.0% of Cu, and at least one of 0.20% or less of Mn and 0.10% or less of Cr, with the balance consisting of Al and impurities, wherein the number of Q phases (Cu—Mg—Si—Al phases) with a size of 2 &mgr;m or more in diameter present in a matrix is 150 per mm
2
or more.
(2) A method of fabricating an aluminum alloy plate excelling in filiform corrosion resistance, comprising: homogenizing an ingot of an aluminum alloy which comprises 0.25-0.6% of Mg, 0.9-1.1% of Si, 0.6-1.0% of Cu, and at least one of 0.20% or less of Mn and 0.10% or less of Cr, with the balance consisting of Al and impurities, at a temperature of 530° C. or more; cooling the ingot to 450° C. or less at a cooling rate of 30° C./hour or less; hot-rolling the ingot; cold-rolling the hot-rolled product; and providing the cold-rolled product with a solution heat treatment.
(3) A method of fabricating an aluminum alloy plate excelling in filiform corrosion resistance, comprising: homogenizing an ingot of an aluminum alloy which comprises 0.25-0.6% of Mg, 0.9-1.1% of Si, 0.6-1.0% of Cu, and at least one of 0.20% or less of Mn and 0.10% or less of Cr, with the balance consisting of Al and impurities, at a temperature of 530° C. or more; cooling the ingot to room temperature; heating the ingot to 500° C. or more and allowing the ingot to stand for 30 minutes or more; cooling the ingot to 450° C. or less at a cooling rate of 30° C./hour or less; hot-rolling the ingot; cold-rolling the hot-rolled product; and providing the cold-rolled product with a solution heat treatment.
(4) In the method of fabricating an aluminum alloy plate excelling in filiform corrosion resistance described in the above (2) or (3), the solution heat treatment may be carried out at 550° C. or less for 30 seconds or less.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT
The effects of alloy components of the Al—Mg—Si—Cu alloy plate of the present invention and reasons for the limitations thereof are described below. Mg bonds to Si to form intermetallic compounds (Mg
2
Si), thereby improving the strength of the alloy. The Mg content is preferably 0.25-0.6%. If the Mg content is less than 0.25%, the effect may be insufficient. If the Mg content exceeds 0.6%, bendability may decrease. The Mg content is still more preferably 0.30-0.55%.
Si forms an intermetallic compound (Mg
2
Si) in the presence of Mg, thereby improving the strength of the alloy. The Si content is preferably 0.9-1.1%. If the Si content is less than 0.9%, the effect of the strength improvement may be insufficient. If the Si content exceeds 1.1%, bendability may decrease.
Cu improves the strength and formability. The Cu content is preferably 0.6-1.0%. If the Cu content is less than 0.6%, formability may be insufficient. If the Cu content exceeds 1.0%, corrosion resistance may decrease.
Mn and Cr miniaturize the crystal grain. The Mn content and the Cr content are preferably 0.20% or less and 0.10% or less, respectively. If the Mn content and the Cr content respectively exceed the upper limits, elongation may decrease. This results in a decrease in bendability and formability. The Mn content and the Cr content are still more preferably less than 0.10% and 0.07% or less, respectively.
Note that the effects of the present invention are not impaired if elements generally included in an Al—Mg—Si—Cu alloy, such as 0.2% or less of Ti, 0.1% or less of B, 1.0% or less of Fe, 0.5% or less of Zn, and 0.05% or less of Zr, are present in the Al—Mg—Si—Cu alloy.
In a matrix of the alloy plate of the present invention having the above composition, it is important that 150 per mm
2
or more of Q phases (Cu—Mg—Si—Al phases) with a size of 2 &mgr;m or more in diameter are present. This precipitation configuration provides pre

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