Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-10-23
2002-04-23
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C148S250000, C148S254000, C148S255000, C148S261000, C148S262000, C148S263000, C148S273000, C428S472200, C428S659000, C428S681000, C428S471000, C428S684000, C428S628000
Reexamination Certificate
active
06376092
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a surface-treated steel sheet excellent in corrosion resistance and formability applicable mainly for automobile body uses.
BACKGROUND ART
There is at present an increasing demand for improvement of both corrosion resistance and formability of steel sheets for automobile body uses. Particularly as to corrosion resistance, a problem is that pitting corrosion is produced in a joint portion between steel sheets known as a hem flange. Since painting, if any, does not cause the paint to adhere to the hem flange, a steel sheet is demanded to be corrosion-resistant for this portion in a non-painted state. For the purpose of improving corrosion resistance of steel sheet to satisfy this demand, a steel sheet manufactured by plating the steel sheet with a Zn—Ni alloy of a thin coating weight of 20 to 30 g/m
2
, and further forming a chromate film and an organic film on the alloy film is now widely in use. While such a steel sheet has sufficient performance in corrosion resistance as well as in formability, the presence of an upper organic film acting as an insulating layer poses problems of easy occurrence of poor appearance upon ED-painting and difficulty to obtain a uniform appearance of painting. In addition, use of expensive nickel and containing detrimental hexavalent chromium are another problems. While a galvanized steel sheet having an increased coating weight or a Zn—Fe alloy coated steel sheet is also used, an increase in coating weight of plating generally results in an improved corrosion resistance but in a poorer formability. It is therefore very difficult to satisfy requirements for both corrosion resistance and formability.
Japanese Examined Patent Publication No. 3-28509 discloses a highly corrosion-resistant plated steel sheet having a magnesium plating layer formed on a galvanizing layer, and Japanese Unexamined Patent Publication No. 2-254178 discloses a highly corrosion-resistant plated steel sheet having a composite film, comprising a metal magnesium and an oxide thereof, formed on a galvanizing layer. These steel sheets, having a high corrosion resistance, permit reduction of the coating weight, and an improvement to some extent is observed in formability, but has not as yet a performance sufficient to satisfy the general requirements.
(WO85/103089 and U.S. Pat. No. 4,722,753 describe corrosion-resistant coated metal objects and methods for producing the same by phosphate conversion coating, wherein said phosphate conversion coating is an improved zinc phosphate conversion coating method. The phosphating solution comprises first and second divalent cations, first metal cations selected from magnesium and transition metals having a hydroxide with lower solubility in alkaline solution than zinc hydroxide and zinc cations.)
DISCLOSURE OF INVENTION
The present invention has therefore an object to provide a coated steel sheet which solves the aforementioned drawbacks, satisfies requirements for both corrosion resistance and formability, and satisfies other basic properties required for a steel sheet mainly for automobile body uses, and a manufacturing method thereon.
In summary, the present invention provides:
(1) A surface-treated steel sheet comprising an amorphous inorganic film containing at least 5% magnesium and having a weight within a range of from 0.1 to 2.0 g/m
2
, formed on the surface of a zinc or zinc alloy plated steel sheet; wherein the inorganic film is soluble in an acidic solution and hardly soluble in a neutral or alkaline solution.
(2) A surface-treated steel sheet comprising a phosphate film formed on the surface of a zinc or zinc alloy plated steel sheet, and an amorphous inorganic film containing at least 5% magnesium and having a weight of at least 0.1 g/m
2
formed on the phosphate film; wherein the inorganic film is soluble in an acidic solution and hardly soluble in a neutral or alkaline solution, and the inorganic film and the phosphate film have a total film weight of up to 2.0 g/m
2
.
(3) A surface-treated steel sheet according to item (2) above, wherein the phosphate film is a zinc phosphate film modified with one or more selected from the group consisting of nickel, magnesium, manganese, calcium, cobalt and copper.
(4) A surface-treated steel sheet according to item (3) above, wherein the amorphous inorganic film and the phosphate film have a total film weight within a range of from over 2.0 g/m
2
to 3.0 g/m
2
.
(5) A surface-treated steel sheet according to any one of items (1) to (4) above, wherein the inorgarnic film comprises one or more selected from the group consisting of phosphoric acid, phosphates, biphosphates, condensed phosphoric acids, condensed phosphates, organic phosphoric acids, and organic phosphates.
(6) A surface-treated steel sheet according to any one of items (1) to (5) above, wherein a solution is coated onto the surface of the steel sheet having a clean surface; the steel sheet is a zinc or zinc alloy plated steel sheet or a zinc or zinc alloy plated steel sheet coated with a phosphate film; the aqueous solution contains magnesium dihydrogenphosphate as an essential component in a magnesium concentration in nonvolatile matters of at least 5%; and the steel sheet is baked at a temperature within a range of from 90 to 150° C., and air-cooled.
BEST MODE FOR CARRYING OUT THE INVENTION
The surface-treated steel sheet of the present invention comprises an amorphous inorganic film containing magnesium as an upper layer on a galvanized steel sheet, wherein this film is hardly soluble in a neutral or alkaline solution and soluble in an acidic solution.
Magnesium contained in the inorganic film has a function of stabilizing corrosion products of zinc, thereby inhibiting progress of rust, and is therefore primarily necessary for improving corrosion resistance.
The morphology of magnesium compound in the inorganic film also has an effect on corrosion resistance. Morphology of magnesium compound in a metallic form, while being favorable for corrosion resistance, poses a problem in formability as described later, and further, causes very difficult problems in manufacturing technology as well as in manufacturing cost. A film mainly comprising crystalline magnesium cannot give a sufficiently satisfactory corrosion resistance because of a high porosity. For these reasons, the most preferable morphology of magnesium is in an amorphous form which permits formation of a tight layer. Whether amorphous or not can be determined through observation of crystal by surface SEM and presence of diffraction patterns in an X-ray diffraction.
In order to improve formability, the inorganic film of the invention must be an amorphous film. A film comprising metallic magnesium, magnesium oxide or magnesium phosphate has not effect of improving formability. Particularly when the coating weight is increased, the resultant steel sheet cannot withstand high-speed pressing for automobile. The amorphous inorganic film covers the soft galvanizing layer to serve as a hard barrier film, thereby inhibiting flaking of the galvanizing layer. The film itself has an excellent lubricating effect. Further, even upon generation of heat from the steel sheet subjected to press forming, the film does not lose this excellent effect, thus giving a very good formability.
The amorphous inorganic film containing magnesium, serving as a barrier film against corrosive factors, is favorable for improving corrosion resistance. However, when the film acts as a barrier against reactions in the chemical conversion treatment (phosphate treatment) carried out in automotive coating, the chemical conversion film does not adhere, thus causing problems in coating appearance and paint adhesion. The inorganic film of the invention must necessarily be solved in a weak acidic solution environment of such a chemical conversion solution (usually having a pH within a range of from 2 to 3), and this is the very point of the invention. Being soluble in an acidic solution means that application of the aforementioned chemical conversion treatment
Hayashi Kimitaka
Ishizuka Kiyokazu
Ito Daisuke
Shindo Hidetoshi
Jones Deborah
Osorio Enrique F.
Wenderoth , Lind & Ponack, L.L.P.
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