Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-11-06
2004-05-04
La Villa, Michael (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S651000, C428S681000, C428S633000, C428S450000, C428S702000
Reexamination Certificate
active
06730414
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chemically processed steel sheet having a converted layer, which is excellent in workability and corrosion resistance at both a flat plane and a worked or machined part, generated on a surface of an Al—Si alloy plating layer.
2. Description of Related Art
Al-coated steel sheets have been used as steel material excellent in the corrosion-resistance. But, when the Al-coated steel sheet is held as such in a humid atmosphere, exhaust gas or an environment subjected to dispersion of sea salt grains for a long time, its external appearance is worsened due to generation of white rust on the Al plating layer. Chromating effectively inhibits generation of white rust on a surface of the Al-coated steel sheet from the following reasons.
A chromate layer generated on a surface of a steel base is composed of complex oxides and hydroxides of trivalent and hexavalent Cr. Scarcely-soluble compounds of Cr(III) such as Cr
2
O
3
acts as a barrier against a corrosive atmosphere and protects a steel base from corroding reaction. Compounds of Cr(VI) are dissolved as oxoatic anions such as Cr
2
O
7
2−
from the converted layer and re-precipitated as scarcely-soluble compounds of Cr(III) due to reducing reaction with exposed parts of a steel base formed by working or machining. Re-precipitation of Cr(III) compounds autogenously repairs defective parts of the converted layer, so that a corrosion-preventing effect of the converted layer is still maintained after working or machining.
Although chromating is effective for corrosion prevention of a steel sheet, it obliges a big load on post-treatment of Cr ion-containing waste fluid. In this regard, chemical liquors containing compounds such as titanium compounds, zirconium compounds or phosphates have been developed for generation of converted layers (hereinafter referred to as “Cr-free layers), which do not contain chromium compounds or Cr ion, and some are already applied to aluminum DI (drawn and ironed) cans. For instance, JP 9-20984 Al proposed an aqueous solution containing titanium compound, sulfuric phosphate, fluorides and an accelerator for coating an Al-containing metal part with a chemically converted (titanium compound) layer.
Titanium compound, zirconium compound or phosphate-containing converted layers, which have been proposed instead of the conventional chromate layer, do not exhibit such a self-repairing faculty as the chromate layer. For instance, a titanium compound layer does not exhibit a self-repairing faculty due to insolubility, although it is uniformly generated on a surface of a steel base in the same way as the chromate layer. As a result, the titanium compound layer is ineffective for suppression of corrosion starting at defective parts formed during chemical conversion or plastic deformation of a steel sheet. The other Cr-free layers are also insufficient for corrosion prevention due to poor self-repairing faculty.
When a small amount of a Cr-free chemical liquor is spread on an Al-coated steel sheet by a conventional method using an applicator roll or a spray wringer, an Al plating layer is not uniformly coated with a converted layer. The un-coated parts, i.e. surface parts where the Al plating layer is exposed to an atmosphere, act as starting points for corrosion or scratching during working, resulting in occurrence of damages in the converted layer or the Al plating layer. When a relatively thick converted layer is generated so as to completely cover the plating layer by spreading an excessive amount of a Cr-free chemical liquor on the contrary, defects such as cracks easily occur in the converted layer during press-working, since the converted layer cannot follow to deformation of a steel base. The defects in addition to an insufficient self-repairing faculty cause degradation of corrosion-resistance.
SUMMARY OF THE INVENTION
The present invention aims at provision of a chemically processed steel sheet remarkably improved in corrosion resistance by generating a converted layer, which contains both soluble and scarcely-soluble metal compounds, with a self-repairing faculty on an Al—Si alloy plating layer formed on a steel base.
The present invention proposes a new chemically processed steel sheet having a steel base coated with an Al—Si alloy plating layer containing 5-13 mass % Si. A surface of the plating layer is preferably reformed to a rugged state by concentration of Si so as to distribute Si-rich particles as convex parts thereon. Such distribution of Si-rich particles is attained concentration of Si to 7-80 mass % at a surface of the plating layer.
A converted layer, which is generated on the rugged surface, contains a complex compound of Ti and Mn. The complex compound may be one or more of oxides, hydroxides, fluorides and organic acid salts. The converted layer may further contain one or more of phosphates, complex phosphates and lubricants. Concentration of Si at a surface of the plating layer is preferably controlled under the condition such that Si content within a range from the surface to at least 100 nm depth is adjusted to 7-80 mass %.
Another converted layer, which contains one or more oxides or hydroxides of valve metals together with fluorides, is also effective for corrosion prevention. The valve metal has the feature that its oxide exhibits high insulation resistance. The valve metal is selected from Ti, Zr, Hf, V, Nb, Ta, Mo and W. The self-repairing faculty of the converted layer is typically noted by addition of one or more fluorides to the converted layer at an F/O atomic ratio not less than 1/100. The converted layer optionally contains organic or inorganic lubricants.
The converted layer may further contain one or more of soluble or scarcely-soluble metal phosphates or complex phosphates. The soluble metal phosphate or complex phosphate may be a salt of alkali metal, alkaline earth metal or Mn. The scarcely-soluble metal phosphate or complex phosphate may be a salt of Al, Ti, Zr, Hf or Zn.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Manganese compounds and valve metal fluorides are effective components other than chromium compound, which give a self-repairing faculty to a converted layer, since these compounds are dissolved in water and then re-precipitated as scarcely-soluble compounds at defective parts of the converted layer.
The manganese compound in the converted layer is partially changed to a soluble component with a self-repairing faculty. Accounting the self-repairing faculty of the manganese compound, the inventors experimentally added various kinds of chemical agents to a liquor for generation of a converted layer containing the manganese compound, and researched effects of the chemical agents on corrosion-resistance of the converted layer. In the course of the researches, the inventors discovered that addition of a titanium compound to the chemical liquor is effective for suppressing dissolution of the converted layer and for bestowing the converted layer with a self-repairing faculty, as disclosed in JP Application No. 3302677B.
The titanium compound improves stability and corrosion-resistance of a converted layer containing a manganese compound. On the basis of such the effect of the titanium compound, the inventors have further researched for a method which can inhibit exposure of an Al plating layer through a converted layer generated even at a relatively small ratio, and discovered that a substrate suitable for improvement of corrosion-resistance is an Al—Si alloy-coated steel sheet with concentration of Si at a surface of a plating layer. It is assumed that increase of Si content in at surface improves corrosion-resistance of the converted layer from the following reason:
When an Al—Si alloy-coated steel sheet having Si concentrated at its surface is held in contact with a chemical liquor, Al is selectively etched away from the surface of the Al—Si plating layer, so that the surface of the plating layer is reformed to a rugged state having convex parts composed of metallic S
Asabuki Mitsuo
Furukawa Shinya
Morikawa Shigeyasu
Taketsu Hirofumi
Yamamoto Masaya
La Villa Michael
Nisshin Steel Co. Ltd.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
LandOfFree
Chemically processed steel sheet excellent in corrosion... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Chemically processed steel sheet excellent in corrosion..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Chemically processed steel sheet excellent in corrosion... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3248206