Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2001-07-20
2003-05-20
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S258000, C148S259000, C148S264000, C148S267000, C148S268000, C106S014120, C252S389500, C252S389300, C428S472000, C428S472300
Reexamination Certificate
active
06565671
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an unleaded surface-treated steel sheet for fuel tanks of an automobile, more specifically, a surface-treated steel sheet coated with a chromate layer having excellent cosmetic corrosion resistance and fuel corrosion resistance as well as seam processibility and a method of fabricating the same.
(b) Description of the Related Art
Generally, it is required that fuel tanks for storing fuel of an automobile should have resistance to corrosion on its outer surface to be exposed to the atmosphere (hereinafter, referred to as the “cosmetic corrosion resistance”) as well as resistance to corrosion on its inner surface to contact fuel such as gasoline (hereinafter, referred to as the “fuel corrosion resistance”).
A fuel tank is usually made by press-forming steel sheets into cup-shaped upper and lower tank bodies, and welding the bodies to each other by resistance welding method such as spot welding or seam welding, or joining method such as soldering or brazing. In this respect, because two parts are joined each other in a fuel tank, good weldability and good processibility are required for the steel sheet to be applied for use in forming fuel tanks.
A ternesheet that is a cold-rolled steel sheet plated with lead-tin alloy has widely been used for fuel tanks. However, the ternesheet has been limited in its use because it contains lead that causes environmental pollution.
Korean Patent Application No. 97-703448 and Japanese Patent Laid Open Publication No. Hei9-59783 disclose unleaded surface-treated steel sheet that can replace ternesheet. The above inventions relate to coating special chromate solution on a zinc-nickel (Zn—Ni) alloy plated steel sheet. However, the above technique requires a minute crack-format on process on the zinc-nickel alloy plated layer before treating chromate, thus the manufacturing process is complicated. Furthermore, in such a structure, chrome is liable to elute even in contact with minimum amount of moisture contained in fuel, and this results in deteriorated fuel corrosion resistance.
To solve these problems, the present inventors provided chromate solution for a surface-treated steel sheet that does not need to form minute cracks on the plated layer and can prevent elution of chrome in the international patent publication No. WO 00/32843.
As an automobile company using surface-treated steel sheet for fuel tank tries to develop the appearance of an automobile variously, necessity of seam processing increases when manufacturing fuel tanks. When fuel tanks are manufactured by seam processing, a coating layer of surface-treated steel sheet which is subjected to seam-process forms cracks and peels off as well.
In addition, recent automobile companies have a tendency to replace electroplated steel sheet with melted zinc plated steel sheet in order to reduce cost and to increase cosmetic corrosion resistance of an automobile. Melted zinc plated steel sheet, however, is not appropriate to use as a fuel tank of automobile because it has problems such as cracking or exfoliation at the coating during seam processing even if it has good cosmetic corrosion resistance.
Thus, there is a need for developing chromate solution and a process for treating the same that satisfy cosmetic corrosion resistance and fuel corrosion resistance as well as seam processibility at the same time.
SUMMARY OF THE INVENTION
Therefore, the present invention is derived to solve the above problems. The present invention provides an unleaded surface-treated steel sheet coated with chromate which prevents cracks of coating layer during seam processing thereby improving cosmetic corrosion resistance and fuel corrosion resistance after processing.
It is another object of the present invention to provide chromate solution used for manufacturing surface-treated steel sheet coated with chromate which can improve cosmetic corrosion resistance and fuel corrosion resistance after processing.
It is another object of the present invention to provide chromate solution that adheres into melted zinc plated steel sheet and can prevent cracks or exfoliation due to abrasion between press plane during seam processing.
It is another object of the present invention to provide a process for fabricating unleaded surface-treated steel sheet coated with chromate layer which has excellent cosmetic corrosion resistance and fuel corrosion resistance after processing.
In order to achieve the above objects, the present invention provides a resin solution comprising: (a) a main solution prepared by adding 50 to 80 w % of phosphoric acid, 20 to 40 w % of fluoric acid, 300 to 2000 w % of colloidal silica, 5 to 15 w % of sulfuric acid, and 25 to 40 w % of sodium phosphate on the basis of chrome in a chrome solution having 0.4 to 0.8 of trivalent chrome rate and 7 to 50 g/l of dissolved chrome; and (b) a hardener aqueous solution comprising one of epoxy silane or glycidyl silane of 2 to 10 w % on the basis of total hardener solution, the pH of which being controlled between 2 and 3, said hardener aqueous solution of 10 to 60 w % being added to the main solution. Wherein, pH of said hardener aqueous solution is controlled by phosphoric acid.
In order to achieve the above objects, the present invention provides a method of fabricating surface-treated steel sheet for a fuel tank of an automobile comprising the steps of: plating zinc/zinc-based alloy on surface of low carbon content steel; coating the chromate solution of claim 1 on steel sheet plated with zinc/zinc-based alloy so as to have 30-150 mg/m
2
of chrome content of coating layer after drying; and baking said steel sheet coated with chromate solution at 140-250° C. Wherein, said plating step comprises soaking zinc/zinc-based alloy into steel sheet in electroplated or melted state.
In order to achieve the above objects, the present invention provides an unleaded surface-treated steel sheet coated with chromate solution.
According to the present invention, it is possible to provide an unleaded surface-treated steel sheet coated with chromate solution which compactly adheres melted zinc plated layer or zinc/zinc-based alloy electric plated layer so that it can reinforce adhesion between steel sheet and coating layer while seam processing, and improve cosmetic corrosion resistance and fuel corrosion resistance even after processing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An unleaded surface-treated steel sheet according to the present invention is prepared by electroplated zinc or zinc-nickel alloy, or plating melted zinc on cold-rolled steel sheet and coating chromate solution on the above steel sheet plated with eletroplating or melted zinc plating.
Low-carbon steel sheet having less than or equal to 0.03% of carbon content is used as a cold-rolled steel sheet in the present invention.
Hereinafter, chromate solution used for chromate layer coating is explained in detail.
Characteristics of chromate solution used in the present invention are as follows:
First, in order to maintain secure adhesion between coated chromate layer and zinc or zinc alloy layer, content of hardener solution added to the chromate solution is optimized. Thus, it can remarkably prevent destruction of chromate coating layer when processing surface-treated steel sheet coated with chromate layer. Accordingly, it is not necessary to form minute cracks on the surface of plated layer as prior art so that the process become simple and cost is reduced.
Second, excellent fuel corrosion resistance is obtained by drastically reducing elution of six-valent chrome while increasing hydrophobicity of chromate coating layer.
Third, since structure of chromate layer is improved by adding additives into chromate solution, the surface-treated steel sheet according to the present invention can maintain excellent cosmetic corrosion resistance and fuel corrosion resistance even after processing.
The chromate solution of the present invention comprises main solution and hardener solution.
The main solution comprises phosp
Chang Sam-Kyu
Cho Soo-Hyoun
Lee Jae-Ryung
Noh Sang-Geol
Baker & Botts L.L.P.
Oltmans Andrew L.
Pohang Iron & Steel Co. Ltd.
Sheehan John
LandOfFree
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