Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-09-20
2001-11-13
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C148S282000, C148S284000, C428S613000, C428S472100, C428S913000
Reexamination Certificate
active
06316117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a stainless steel sheet having a passive film good of solderability and electric conductivity.
2. Background Art
A stainless steel sheet represented by SUS 430 or SUS 340 has good corrosion resistance due to a passive film present on its surface. The passive film comprises oxides and hydroxides, and contains metallic components such as Si, Mn other than Cr.
Oxides and hydroxides in the passive film are thermally stable but unfavorable for low-temperature bonding such as soldering. In order to improve solderability of a stainless steel sheet, the passive film is dissolved by a fluxing agent containing a strong acid such as hydrofluoric acid, or the stainless steel sheet is precoated with a metal layer such as Cu for good solderability. However, such a corrosive fluxing agent causes contamination of a surface of the stainless steel around the soldered joint, and the soldered stainless steel sheet is necessarily washed to remove the contaminants. Formation of a metal layer of good solderability requires a plating step before soldering which causes an increase in manufacturing cost.
Oxides and hydroxides in the passive film are also electrically insulative. In this regard, the stainless steel sheet can not be used as such for a can body of a battery, a spring member for fixing a battery, a contact part for an electric circuit or an electromagnetic relay, and so on. Although a copper alloy has been used so far as material for an electric contact due to its excellent electric conductivity, it has insufficient corrosion resistance, and a contact part made of the copper alloy loses its electric conductivity due to generation of rust. In this regard, JP 63-145793A disclosed a stainless steel sheet coated with a Ni layer useful as a contact part. The proposed contact part has good of corrosion resistance derived from stainless steel, and defects caused by a passive film are eliminated by the Ni layer. However, formation of a Ni layer means increase in manufacturing steps and requires an expensive electroplating or electroless plating method. Due to the Ni-plating, there is a big burden on processing of liquid wastes. If the Ni-layer is formed on a surface of a stainless steel sheet with poor adhesiveness, it is peeled off during forming or handling the stainless steel sheet.
SUMMARY OF THE INVENTION
The present invention aims at surface-reformation of a stainless steel sheet to a state good of solderability and electric conductivity without degrading excellent corrosion resistance of stainless steel itself. Such reformation is performed by precipitation of Cu-enriched grains in a matrix or condensation of Cu in a passive film or an outermost layer.
The newly proposed stainless steel sheet is characterized by a stainless steel, which contains Cu at a ratio of 1.0 wt. % or more and has the matrix that Cu-enriched grains are precipitated, and a passive film through which the Cu-enriched grains are exposed outside.
Condensation of Cu in the passive film as well as at an outermost layer is also effective for reformation of the passive film, instead of precipitation of Cu-enriched grains. Condensation of Cu up to the level that a mass ratio of Cu/(Cr+Si) is 0.1 or higher with respect to Cr and Si present in the passive film or at the outermost layer, remarkably improves solderability of the stainless steel. Condensation of Cu up to the level that a mass ratio of Cu/(Si+Mn) is 0.5 or higher with respect to Si and Mn present in the passive film or at the outermost layer, remarkably reduces contact resistance of the stainless steel.
A stainless steel sheet having Cu condensed to such a level does not need treatment for precipitation of Cu-enriched grains in its matrix. Of course, combination of Cu condensation with precipitation of Cu-enriched grains are more effective for improvement in solderability and electric conductivity.
There are no restrictions on the type of stainless steel in the present invention, as far as the stainless steel contains 1.0 wt. % or more of Cu. Various kinds of ferritic, austenitic, martensitic and dual-phase stainless steels may be used for the purpose.
Cu-enriched grains are sufficiently precipitated in a matrix of a stainless steel sheet by aging the stainless steel one hour or longer at a temperature about 800° C. at any stage until final annealing in a manufacturing process.
Condensation of Cu in the passive film or at the outermost layer of a stainless steel sheet is performed by bright-annealing the stainless steel sheet in an atmosphere at a dew point of −30° C. or lower at a final stage of a manufacturing process. Condensation of Cu is also performed by pickling a stainless steel sheet with mixed acids such as hydrofluoric-nitric acids or sulfuric-nitric acids. A surface of a stainless steel may be finished to a state suitable for an estimated use without any restrictions on surface-finishing, as far as exposure of Cu-enriched grains or condensation of Cu in a passive film or at an outermost layer is not diminished. For instance, such finishing as BA (cold-rolling and then bright-annealing),
2
B (cold-rolling, heat treatment, pickling or other surface conditioning, and then cold-rolling to a state with proper glossiness) or
2
D (cold-rolling, heat treatment, and then pickling or other surface conditioning to a state with proper glossiness), each regulated by JIS G0203, may be applied to a stainless steel sheet.
REFERENCES:
patent: 5944917 (1999-08-01), Takeda et al.
patent: 63145793 (1988-06-01), None
Hiramatsu Naoto
Kageoka Kazuyuki
Nakamura Sadayuki
Koehler Robert R.
Nisshin Steel Co. Ltd.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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