Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
2000-01-28
2001-10-23
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S070000, C420S071000, C148S325000, C148S542000
Reexamination Certificate
active
06306341
ABSTRACT:
TECHNICAL FIELD
The present invention relates to stainless steel. More particularly, the present invention relates to stainless steel having antibacterial properties which is suitably used for apparatuses such as kitchen fixtures, medical apparatuses, electrical appliances, chemical apparatuses, and building materials, and also relates to a manufacturing method therefor. The steel according to the present invention are in forms including sheets, strips, pipes, and wires.
BACKGROUND ART
It is well known that silver and copper have inhibitory effects on pathogenic bacteria growth, typically represented by
Escherichia coli
and
salmonella,
and acts for preventing food poisoning caused by pathogenic bacteria.
Recently, materials provided with inhibitory effects on bacterial growth (hereinafter referred to as antibacterial characteristics) by using these metals have been proposed. In Japanese Unexamined Patent Application Publication No. 8-49085, for example, a stainless steel sheet having superior antibacterial properties is disclosed; on the surfaces of the stainless steel sheet, metal layers or alloy layers of chromium, titanium, nickel, iron and the like containing silver and/or copper are formed by magnetic sputtering. In this steel sheet, metal layers or alloy layers formed containing 19 to 60 percent by weight of silver is preferable.
In Japanese Unexamined Patent Application Publication No. 8-156175, a steel sheet coated by pigments containing silver, which can suppress bacterial growth, is proposed.
However, in the methods described above for forming the metal layers or the alloy layers on the steel sheet surfaces including metals having the antibacterial properties, and in the methods for coating the pigments including the metals having the antibacterial properties, the layers including the metals having the antibacterial properties are stripped or removed by drawing and surface polishing, and the problems are that the effects cannot thereby be anticipated. In applications, such as steel sheets used for the interiors of washing machines, which are continually abraded, and steel sheets used for kitchens which are frequently scrubbed for cleaning, the problem is that the antibacterial properties do not last over long periods of time. In the methods described above, additional steps for manufacturing the steel sheets are required to form coating layers, metal layers, and alloy layers. In addition, when steel sheets are made thinner, since the amounts of coating, metal layers, and alloy layers per weight increase concomitant with an increase of surface area per weight, there is a disadvantage in terms of cost.
In order to solve the problems described above, there have been proposed austenitic stainless steel enhancing antibacterial properties by adding 1.1 to 3.5 percent by weight of copper as disclosed in Japanese Unexamined Patent Application Publication No. 8-104953; martensitic stainless steel enhancing antibacterial properties by adding 0.3 to 5 percent by weight of copper as disclosed in Japanese Unexamined Patent Application Publication No. 8-104952; and ferritic stainless steel enhancing antibacterial properties by adding 0.4 to 3.0 percent by weight of copper as disclosed in Japanese Unexamined Patent Application Publication No. 9-170053.
However, in the technologies disclosed in Japanese Unexamined Patent Application Publications Nos. 8-104953, 8-104952, and 9-170053, copper ions must leach from the surfaces of the steel sheets to produce the antibacterial properties. The leaching of copper in ionic form is due to breakage of passivation layers at the leaching points, and corrosion resistance is therefore seriously degraded, even though antibacterial properties are improved. Accordingly, it is difficult for stainless steel having copper therein to have antibacterial properties and corrosion resistance at the same time.
Objects of the present invention are to provide stainless steel and a manufacturing method therefor by solving the problems in the conventional technologies. The stainless steel of the present invention has superior antibacterial properties and corrosion resistance, and continues to have superior antibacterial properties even after surface processing is performed, such as polishing.
DISCLOSURE OF INVENTION
In order to develop stainless steel sheets having superior antibacterial properties and corrosion resistance, intensive research of the relationship between chemical compositions of stainless steel sheet surfaces and antibacterial properties were made by the inventors of the present invention by fully utilizing analytical instruments, such as a field emission Auger electron spectroscope and an electron beam microanalyser. Consequently, by adding an optimum amount of silver to stainless steel and by dispersing optimum amounts of silver on the surface and the inside of the stainless steel, the inventors found that stainless steel having superior antibacterial properties and also superior corrosion resistance could be obtained. In addition, the inventors of the present invention found that a continuous casting rate and an addition of vanadium had a substantial effect on the homogeneous dispersion of the silver. Furthermore, the inventors of the present invention found that the steel provided with the optimum amount of silver homogeneously dispersed therein had stable antibacterial properties for applications in which the steel was subject to mold pressing and polishing, and in which the surfaces of the steel were scrubbed or abraded during use.
The present invention was accomplished based on the above knowledge in conjunction with further research therefor.
Accordingly, a first aspect of the invention is that a stainless steel, having antibacterial properties, comprises not less than 10 percent by weight of chromium, 0.001 to 0.30 percent by weight of silver, and not less than 0.0005 percent by weight of a silver oxide, the amount of the silver oxide being not more than 1.1 times that of the silver.
A second aspect of the invention is that the stainless steel, having the antibacterial properties according to the first aspect of the invention, further comprises 0.001 to 1.0 percent by weight of vanadium.
A third aspect of the invention is that the stainless steel, having the antibacterial properties according to the first aspect and the second aspect of the invention, further comprises not more than 0.015 percent by weight of sulfur.
A fourth aspect of the invention is the stainless steel, having the antibacterial properties according to the first aspect to the third aspect of the invention, wherein the silver content is not less than 0.001 and is less than 0.05 percent by weight of the stainless steel.
A fifth aspect of the invention is the stainless steel, having the antibacterial properties according to the second aspect of the invention, wherein the vanadium content is 0.001 to 0.30 percent by weight of the stainless steel.
A sixth aspect of the invention is the stainless steel having the antibacterial properties according to the first aspect to the fifth aspect of the invention, wherein the stainless steel is in the form of any one of a sheet, a strip, a pipe, and a wire.
A seventh aspect of the invention is a method for manufacturing a stainless steel raw material, comprising the steps of controlling amounts of not less than 10 percent by weight of chromium, 0.001 to 0.30 percent by weight of silver, and not more than 0.015 percent by weight of sulfur in molten stainless steel, and performing continuous casting of the molten stainless steel at a casting rate of 0.8 to 1.6 m/min.
A eighth aspect of the invention is the method for manufacturing the stainless steel according to the seventh aspect of the invention, in which the molten stainless steel further comprises 0.001 to 1.0 percent by weight of vanadium.
A ninth aspect of the invention is that the method for manufacturing the stainless steel, having antibacterial properties according to the seventh aspect and the eighth aspect of the invention, further comprises the steps of hot rolling and
Hasuno Sadao
Satoh Susumu
Tochihara Misako
Yokota Takeshi
Kawasaki Steel Corporation
Schnader Harrison Segal & Lewis LLP
Yee Deborah
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