Reduction of automobile weight has been aggressively promoted from the viewpoint of protection of the global environment. Current trends involve switching the material used from steel to aluminum to reduce the weight of the automobile. In this respect, various types of aluminum alloys have been developed as external automobile body plates. In Japan, the 5000 Series Al--Mg--Zn--Cu alloys (disclosed in JP-A-103914(1978) and JP-A-171547(1983), (the term "JP-A-" referred herein signifies "unexamined Japanese patent publication") and Al--Mg--Cu alloys (disclosed in JP-A-219139(1989)) have been developed as aluminum alloys for external automobile body plates. Several of these aluminum alloy sheets are already in practical application.
In Western countries, 6000 Series Al--Mg--Si alloys such as 6009 alloy, 6111 alloy, and 6016 alloy have been introduced (disclosed in JP-A-19117(1978)). The 6000 Series aluminum. alloys have sufficient formability to be used as external automobile body plates and provide high strength after heat treatment during the coat-baking stage, though they are somewhat inferior in formability to the 5000 Series aluminum alloys. Accordingly, the 6000 Series aluminum alloys are expected to provide thinner and lighter materials than the 5000 Series aluminum alloys, but the product surface quality after forming is inferior to that of the 5000 Series.
Typical defects appearing during the forming stage include stretch-strain marks (hereinafter referred to simply as "SS marks"), orange peel (hereinafter referred to simply as "rough surface"), and ridging marks. SS marks are most likely to appear on a material showing high yield elongation during plastic working, and often become a problem, particularly in the 5000 Series alloys. Rough surface is most commonly observed on a material with a coarse crystal grain size. Ridging marks are a surface irregularity caused by a significant difference in behavior of crystal grains at the boundary of a group of segregated crystal grains with almost identical crystalline orientation relative to each other, even if the size of these segregated crystal grains is sufficiently fine not to induce a rough surface.
For SS marks and rough surface, countermeasures are applied by adopting leveler correction and minimizing the crystal grain size, respectively. For ridging marks, however, insufficient investigation has been carried out because the defect causes a problem only under conditions where exceptional surface quality is needed after forming, as in external automobile body plates. Even where 6000 Series aluminum alloy sheets are formed for use as external automobile body plates, occurrence of ridging marks is often observed, and becomes a problem. In some cases, the 6000 Series aluminum alloys induce corrosion, particularly filiform corrosion, after coat-baking treatment, so preventive measures are also required.
Generally speaking, aluminum alloys often fail to provide satisfactory formability in press-forming compared with steel plates when a lubricant for press-forming is applied thereto. Therefore, further improvements are necessary before aluminum alloys can match the stringent formability requirements applied to steel plates.
A method is disclosed in JP-A-255587(1993) which enables continuous forming without applying lubricant. According to the disclosure, a composition comprising 100 wt. parts of a water-dispersible polyurethane resin, 5 to 50 wt. parts of silica particles, and 0.5 to 30 wt. parts of a lubricant consisting of a polyolefin wax and a fluororesin powder is applied to the surface of the metallic plate to prepare the lubricant-treated metallic plate. This treatment allows the steel plates to be press-formed at a high speed, and creates a lubricant film which provides excellent corrosion resistance and coating adhesiveness. However, this treatment cannot be satisfactorily applied to aluminum alloy sheets.