Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-07-08
2004-01-06
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S259000, C148S261000, C428S600000, C428S601000, C428S682000, C428S219000, C428S469000, C428S472300
Reexamination Certificate
active
06673470
ABSTRACT:
TECHNICAL FIELD
The present invention relates to surface-treated tin-plated steel sheets having superior paint-adhesion characteristics, corrosion resistance after coating, antirust properties, and workability, and relates to surface treatment solutions for imparting the above-mentioned properties to steel sheets. These surface-treated tin-plated steel sheets are suitably used for DI (drawn and ironed) cans, food-cans, beverage-cans, and the like.
BACKGROUND ART
Tin-plated steel sheets have been widely used as surface-treated steel sheets used for cans. Generally, after tin plating is performed on cold-rolled steel sheets, these tin-plated steel sheets are immersed or electrolyzed in a dichromic acid solution. The immersion treatment or the electrolytic treatment is called chemical conversion treatment and it forms a chromium oxide layer on the tin-plated layer. The chromium oxide film thus formed prevents the growth of Sn oxide and functions to improve adhesion to paint provided thereon and antirust properties.
However, in view of recent environmental conservation measures, restrictions are being increasingly placed on the use of chromium in various industrial fields, and surface-treated steel sheets for cans have also been increasingly required to be chromium-free.
Chromium-free techniques applied to surface-treated tin-plated steel sheets for cans have been disclosed in, for example, the following patents. In Japanese Examined Patent Application Publication No. 55-24516, a method is disclosed in which a chemical conversion film containing no chromium is formed on a tin-plated steel sheet by performing DC electroplating in a solution primarily containing phosphoric acid using the tin-plated steel sheet as a cathode. In Japanese Examined Patent Application Publication No. 1-32308, an electrolytic tin-plated steel sheet which is provided with a chemical conversion film containing phosphorus (P) and/or aluminum (Al) on the tin-plated layer and which is used for seamless cans containing no chromium (Cr) is disclosed.
Furthermore, in Japanese Examined Patent Application Publication No. 58-41352, a chemical conversion solution, which is used for treating metal surfaces, containing tin ions and at least one of phosphate ions, chlorate ions, and bromate ions and having a pH of 3 to 6 is disclosed.
However, when properties, such as paint-adhesion characteristics and antirust properties, are considered together, the chemical conversion films described in the publications or films obtained by the chemical conversion method described above cannot be sufficiently superior to the chemical conversion film obtained by using a conventional dichromatic acid solution.
As requirements for the surface-treated steel sheets for cans, in addition to the chromium-free techniques, cost-reduction of surface-treated steel sheets as a starting material for cans has also been requested. In particular, since the tin used in tin-plated steel sheets is an expensive metal, reduction in the coating weight of tin formed by plating has been attempted.
However, because tin has superior lubricative properties, when the coating weight of tin formed by plating is reduced, the workability of the steel sheet is decreased, and hence, the coating weight by which tin can be reduced has been limited to a certain level.
In addition, in the case of the conventional tin-plated steel sheets, a chromium oxide film formed on the tin-plated layer advantageously serves to improve the corrosion resistance. However, since the chromium oxide film itself is a hard film, when the coating weight of tin obtained by plating is reduced, galling is likely to occur in a can-forming step. Accordingly, in order to maintain the workability, the coating weight of tin cannot be decreased, and as a result, a film having the most preferable composition has not always been created.
An object of the present invention is to provide a tin-plated steel sheet at an inexpensive cost, which does not contain unfavorable chromium in view of environment conservation and which has superior paint-adhesion characteristics, corrosion resistance after coating, antirust properties, and workability. In addition, the present invention also provides a surface treatment solution, which does not contain Cr, for forming the tin-plated steel sheets described above.
DISCLOSURE OF INVENTION
A surface-treated tin-plated steel sheet of the present invention comprises (1) an alloy layer on a surface of a steel sheet, (2) a tin-plated layer which is provided on the alloy layer so that the alloy layer is exposed at an areal rate of 3.0% or more; and (3) a film comprising P and Si as coating weight of 0.5 to 100 mg/m
2
and 0.1 to 250 mg/m
2
, respectively, provided on the exposed portions of the alloy layer and the tin-plated layer.
The film of the surface-treated tin-plated steel sheet described above preferably further comprises Sn in addition to the predetermined coating weight of the P and the Si described above.
In both of the surface-treated tin-plated steel sheets described above, the Si in the film is preferably derived from a silane coupling agent having an epoxy group.
Each of the surface-treated tin-plated steel sheets described above, the alloy layer is preferably at least one selected from the group consisting of a Fe—Sn alloy layer, a Fe—Ni alloy layer, and a Fe—Sn—Ni alloy layer. In particular, it is more preferable that the alloy layer be a composite alloy layer comprising a Fe—Ni alloy layer having a ratio Ni/(Fe+Ni) of 0.02 to 0.50 on a mass basis and a Fe—Sn—Ni alloy layer provided thereon.
Each of the surface-treated tin-plated steel sheets described above, the coating weight of the tin plating is preferably in the range of 0.05 to 2.0 g/m
2
.
Furthermore, the present invention provides a chemical conversion solution which contains phosphate ions, tin ions, and a silane coupling agent, and which has a pH of 1.5 to 5.5. The silane coupling agent preferably comprises an epoxy group.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
When a chemical conversion film containing no Cr is formed on a tin-plated steel sheet by a conventional technique, both paint-adhesion characteristics and corrosion-resistance, which are the major properties required for the steel sheets for cans, are difficult to obtain.
Accordingly, the inventors of the present invention made intensive research in order to solve the problems of the tin-plated steel sheets described above. As a result, the inventors discovered that when a steel sheet was tin-plated so that parts of an alloy layer on the steel sheet are exposed to the surface and was then processed by immersion treatment or electrolytic treatment using a chemical conversion solution composed of an acidic solution containing phosphate ions and tin ions and a silane coupling agent added to the acidic solution, superior paint-adhesion characteristics, corrosion resistance, and workability could be obtained.
Based on the discovery described above, the inventors of the present invention made further detailed research. Consequently, as the alloy layer mentioned above, it was found that at least one alloy layer selected from the group consisting of a Fe—Sn alloy layer, a Fe—Ni alloy layer, and a Fe—Sn—Fe alloy layer was preferable. In addition, it was also found that when a chemical conversion film containing an appropriate coating weight of P and Si was formed on the tin-plated steel sheet and on exposed portions of this alloy layer, superior workability and adhesion to a paint applied to the inside of cans could be obtained. In the case described above, it was discovered that this chemical conversion film was preferably a composite film composed of a phosphate film and a silane film formed by dehydrating condensation of silanol groups. In addition, it was also discovered that when a silane coupling agent was dissolved in an acidic solution containing phosphate ions, a silane coupling agent having an epoxy group was particularly preferable since it could be more homogeneously
Kato Chiaki
Mochizuki Kazuo
Nakakoji Hisatada
Shigekuni Tomofumi
Kawasaki Steel Corporation
Koehler Robert R.
LandOfFree
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