Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-04-25
2003-03-25
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S450000, C428S615000, C106S014050
Reexamination Certificate
active
06537674
ABSTRACT:
TECHNICAL FIELD
The present invention relates to coated steel sheets intended for use mainly as steel sheets as automobile bodies, and more particularly to coated steel sheets having superior perforative corrosion resistance, paint adhesion, electrodeposition paintability, press formability, and weldability.
BACKGROUND ART
Steel sheets with zinc-based plating are widely used as automobile bodies in order to prevent the strength of automobile bodies from decreasing due to use in a corrosive environment for a long time, and mainly zinc-alloy plated steel sheets such as zinc-nickel-alloy plated steel sheets and zinc-iron-alloy plated steel sheets are used in Japan.
Although zinc-based alloy plating can impart high corrosion resistance to steel sheets by alloying nickel or iron with zinc, it has several problems attributable to being alloy plating.
For example, zinc-nickel alloy plated steel sheets are manufactured by electrogalvanizing and the cost is high because nickel is expensive. Furthermore, the nickel content should be normally adjusted in an extremely narrow range (for example, 12±1 mass %), which makes its manufacture difficult.
On the other hand, zinc-iron-alloy steel sheets can be produced by either electrogalvanizing or hot-dip galvanizing.
However, when zinc-iron-alloy plated steel sheets are manufactured by electrogalvanizing, it entails difficulty in alloy control, i.e. adjusting the iron content in the zinc plating layer in an extremely narrow range, similar to zinc-nickel alloy plated steel sheets. In addition, Fe
2+
ions in the plating solution are prone to be oxidized, which destabilizes plating and makes the manufacture difficult. As a result, the cost is high.
Generally, zinc-iron-alloy plated steel sheets are often manufactured by hot-dip galvanizing. When zinc-iron-alloy plated steel sheets are manufactured by hot-dip galvanizing, molten zinc is allowed to adhere to the surface of steel sheets, which are kept at a high temperature to alloy the steel sheets and zinc. However, the quality of plating produced by this method is susceptible to the Al concentration in molten zinc plating baths and to the temperature and duration of the alloying process, and an advanced technology is necessary to manufacture uniform alloy plating layers. As a result, the cost is high also in this case.
As mentioned above, zinc-based-alloy plating has problems that any form of them is difficult to manufacture and the cost is high.
On the other hand, galvanized steel sheets, which have only zinc plating, are less expensive and can be manufactured by either electrogalvanizing or hot-dip galvanizing. However, they are rarely used as automobile bodies. The reason is that zinc plating alone cannot provide enough corrosion resistance, especially when galvanized steel sheets are exposed to a corrosive environment for a long time, the steel sheets tend to develop perforation by corrosion, which causes a problem in terms of guarantee of the strength of automobile bodies. Furthermore, galvanized steel sheets have problems that a quantity of zinc tends to deposit on electrodes in spot welding, which shortens the life of electrodes, and the plates have poor press formability.
Generally, in manufacturing automobile bodies, steel sheets or plated steel sheets are subjected to press forming and welding, and are subjected to a chemical conversion treatment, electrodeposition painting, and spray painting in succession before being used as automobile bodies. It is said the lower parts of doors are most likely to develop perforation by corrosion. The reasons are that steel sheets are folded at the lower parts of doors, and water entering through slits of windows etc. pools in the folded parts, which accelerates the corrosion compared to the other parts of automobile bodies.
Among the treatments applied after the press forming of bodies, the chemical conversion treatment and the electroplating can be applied to the inner surface of doors, but in the spray painting, which is conducted afterwards, the paint cannot reach the inner surface. Therefore, the anti-corrosive effect of the spray painting cannot be expected to be obtained, and corrosion resistance after the electrodeposition painting is important. Among the parts in the door, the folded part (a hem structure) at the lower end of the door that is exposed to the harshest environment does not receive electrodeposition painting and is exposed to the corrosive environment without the protection of electrodeposition painting. Therefore, perforative corrosion resistance is important in both cases where no electrodeposition painting is conducted (no coating) and only electrodeposition painting is conducted (after electrodeposition painting).
With this situation as a background, a technology in which a coating containing magnesium is formed on zinc plating is disclosed as a method of improving corrosion resistance of galvanized steel sheets. For example, Japanese Unexamined Patent Application Publication No. 1-312081 discloses coated metal materials wherein a phosphate coating containing 0.1 mass % or more of magnesium is formed on an electrogalvanized layer.
However, although the coated metal materials which have a phosphate coating containing only magnesium as described in the above-mentioned publication are resistant to rust formation in a salt spray corrosion test, they have insufficient perforative corrosion resistance in a composite cycle corrosion resistance test, which yields similar results to the actual corrosion in automobile bodies.
Furthermore, as surface-treated steel sheets with improved corrosion resistance, there may be mentioned an organic composite coated steel sheet comprising a chromate layer and an organic coating layer containing silica are formed on a zinc-nickel alloy electroplated steel sheet with a coating weight of 20 to 30 g/m
2
, a zinc hot-dip galvanized steel sheet with a heavy coating weight of 60 g/m
2
, and an electrogalvanized steel sheet with a heavy coating weight of 60 g/m
2
. These surface-treated steel sheets have sufficient corrosion resistance with insufficient electrodeposition painting, which is applied after assembly of automobile bodies, and therefore contribute to prolonged lives of automobiles.
However, the above-mentioned organic composite coated steel sheet has a chromate layer, and if chromium is emitted, it has a great impact on the environment. Therefore, extremely strict wastewater treatment is necessary when the steel sheet is used, which increases the cost.
On the other hand, the heavy coated zinc hot-dip galvanized steel sheet in which chromium is not used has several quality problems, namely, it has poor press formability, and it tends to develop craters during electrodeposition painting and has poor electrodeposition paintability. Furthermore, the heavy coated electrogalvanized steel sheet has poor press formability similar to the thick-coated zinc hot-dip galvanized steel sheet and is too expensive in Japan, where the electricity costs are high. In short, when a steel sheet has a chromate layer which greatly contributes to perforative corrosion resistance, the base plating layer can be thin but it requires measures for environmental protection. On the other hand, when a steel sheet has no chromate layer, the plating layer needs to be thick, which makes the steel sheet inferior in press formability and electrodeposition paintability.
In this connection, a technology to improve perforative corrosion resistance and paint adhesion without increasing the thickness of the plating layer is required. Japanese Unexamined Patent Application Publication No. 52-80239 and Japanese Unexamined Patent Application Publication No. 63-219587 disclose such technologies.
Japanese Unexamined Patent Application Publication No. 52-80239 describes a technology in which a steel sheet or galvanized steel sheet with a coating weight of 10 g/m
2
or less is subjected to an iron-based or zinc-based phosphate treatment and is then subjected to a sealing treatment with a silane coupling agent for the purpose of
Hamahara Kyoko
Mochizuki Kazuo
Nakakoji Hisatada
Dawson Robert
Feely Michael J
Kawasaki Steel Corporation
Young & Thompson
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