Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-03-15
2004-06-15
Morris, Terrel (Department: 1771)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S646000, C428S655000, C428S668000, C428S678000, C428S679000, C428S680000, C428S681000, C428S682000, C428S687000, C428S446000, C428S450000, C428S704000
Reexamination Certificate
active
06749952
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to surface-treated steel sheets for use in cans such as “drawn and ironed” (DI) cans, food cans, beverage cans, and the like. More particularly, it relates to a tin-plated steel sheet having excellent overcoat adhesion property and superior resistance to discoloration and rust.
2. Description of the Related Art
Tin-plated steel sheets are widely used as surface-treated steel sheets for use in cans. The tin-plated steel sheets are usually produced by first plating a cold-rolled steel sheet with tin and then immersing or electrolyzing the resulting plated steel sheet in an aqueous solution of hexavalent chromium compounds such as chromates or dichromates. Through such immersion or electrolysis, which is known as a chromating process, chromium oxides are formed on the plated tin layer to provide a chromate coating. The chromate coating, which prevents growth of tin oxides, suppresses “yellowing”, i.e., discoloration of the tin-plated steel sheet surface to a yellowish color (hereinafter also referred to as discoloration resistance) and enhances overcoat adhesion property and resistance to rust.
However, chemical conversion treatment using an aqueous solution of hexavalent chromium compounds such as chromates or dichromates requires a significantly high cost for securing the safety of the work environment and for effluent treatment. Moreover, leakage of the liquid used in the chromating process, if caused by an accident or the like, inflicts significant damage upon the ambient environment. The recent trend toward environmental protection has promoted regulations on the use of chromium; thus, there is an increasing need for chromium-free chemical conversion treatment for the surface-treated steel sheets for use in cans having improved resistance to discoloration and rust and overcoat adhesion property.
Examples of chromium-free chemical conversion treatments for surface-treated steel sheets for use in cans which replace conventional chromating processes are as follows. Japanese Unexamined Patent Application Publication No. 55-24516 discloses a method for forming chromium-free chemical conversion coating on a tin-plated steel sheet, the method comprising direct-current electrolysis of the tin-plated steel sheet in a phosphate-system aqueous solution using the tin-plated sheet as a cathode. Japanese Unexamined Patent Application Publication No. 1-32308 discloses a chromium-free electrolytic tin-plated steel sheet for use in seamless cans, comprising a chemical conversion coating formed on a tin plating layer, the chemical conversion coating including either phosphorus (P) alone or phosphorus (P) and aluminum (Al).
However, all of the chemical conversion coatings disclosed in the above-described publications are hardly comparable to the conventional chromate coating formed using dichromic acid or chromic acid when their comprehensive performance is evaluated in terms of overcoat adhesion property and resistance to discoloration and rust.
Accordingly, it is an object of the present invention to provide a tin-plated steel sheet having excellent overcoat adhesion property and resistance to discoloration and rust without having to contain, in its chemical conversion coating, chromium, which is harmful to the environment.
SUMMARY OF THE INVENTION
The present invention provides a tin-plated steel sheet comprising: a base steel sheet; a tin plating layer coating approximately more than 97.0% of the base steel sheet; and a chemical conversion coating having approximately 0.5 to 100 mg/m
2
phosphorus and approximately 0.1 to 250 mg/m
2
silicon formed on the tin plating layer and an unplated region corresponding to approximately less than 3.0%.
Preferably, silicon contained in the chemical conversion coating is derived from a silane coupling agent. More preferably, the silane coupling agent contains an epoxy group.
The tin-plated steel sheet may further comprise an alloy layer disposed on the base steel sheet and at least beneath the tin plating layer.
Preferably, the alloy layer comprises at least one layer selected from the group consisting of a Fe—Sn alloy layer, a Fe—Ni alloy layer, a Sn—Ni alloy layer, and a Fe—Sn—Ni alloy layer. More preferably, the alloy layer comprises a composite alloy layer comprising a Fe—Ni alloy layer having a mass ratio Ni/(Fe+Ni) in the range of approximately 0.02 to 0.50 and a Fe—Sn—Ni alloy layer disposed on the Fe—Ni alloy layer.
Preferably, the total Sn content of the tin plating layer and the alloy layer is in the range of approximately 0.4 to 6.0 g/m
2
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described in detail.
Chromium-free chemical conversion coatings formed on tin plating layers by known methods rarely achieve all of the required overcoat adhesion property and resistance to discoloration and rust, which are the key properties of steel sheets for use in cans.
The present inventors have conducted extensive research to overcome the above problem of tin-plated steel sheets and found that all of the above required properties can be fulfilled by forming a chemical conversion coat containing phosphorus (P) and silicon (Si) on a tin plating layer.
In particular, a chemical conversion solution containing P and a silane coupling agent is used to form a chemical conversion coating containing adequate amounts of P and Si on the tin plating layer. Alignment of the functional groups contained in the silane coupling agent enhances the adhesion property to the overcoat for inner surfaces of cans. That is, the chemical conversion coating improves compatibility and reactivity to the overcoat and thereby yields superior overcoat adhesion property. Moreover, the chemical conversion coating functions as a protective coating to improve resistance to discoloration and rust.
The detailed configuration of the present invention will now be described.
In the tin-plated steel sheet of the present invention, the base steel plate needs to have at least one surface satisfying the requirements of the present invention. No limit is imposed as to the type of the base steel sheet; a cold-rolled steel sheet is generally employed.
The present invention can be applied to a tin-plated steel sheet. The tin-plated steel sheet may be formed by directly plating a base steel sheet with tin or by forming an alloy layer on the base steel sheet and then plating the alloy layer with tin. For example, a tin-plated steel sheet according to an embodiment of the present invention has a tin plating layer directly formed on almost the whole surface of a base steel sheet with a coating coverage exceeding 97%. Another embodiment of a tin-plated steel sheet has an alloy layer between the tin plating layer and the base steel sheet. In this embodiment also, the coating coverage by the tin plating layer exceeds 97%; accordingly, an unplated portion may remain at less than 3.0%. The unplated portion may be the base steel sheet or the alloy layer. In the present invention, the term “coating coverage” refers to the percentage of the surface of the material to be plated covered by the tin plating layer. In this invention, a sufficient resistance to rust can be obtained with a coating coverage, i.e., the percentage of the base steel sheet and/or the alloy layer covered by the tin plating layer, exceeding 97%.
As described above, the present invention includes an embodiment in which an alloy layer is provided on the base steel sheet and at least beneath the tin plating layer. The alloy layer preferably includes at least one selected from a Fe—Sn alloy layer, a Fe—Ni alloy layer, a Sn—Ni alloy layer, and a Fe—Sn—Ni alloy layer. More preferably, the alloy layer is a composite alloy layer comprising a Fe—Ni alloy layer having a Ni/(Fe+Ni) mass ratio of approximately 0.02 to 0.50 and a Fe—Sn—Ni alloy layer on the Fe—Ni alloy layer. This alloy layer, which has also been employed in the conventional tin-plated steel sheets, improves resistance to corrosion and rust. Since the hardne
Kato Chiaki
Nakakoji Hisatada
Shigekuni Tomofumi
JFE Steel Corporation
Morris Terrel
Piziali Andrew T
Young & Thompson
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