Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
1999-11-05
2002-05-21
King, Roy (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C148S605000, C148S609000, C420S040000, C164S459000
Reexamination Certificate
active
06391253
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a stainless steel, in particular to a stainless steel having excellent antibacterial property and being suitable for the applications of, for example, kitchen utensils and other daily utensils, medical devices, electrical equipment, chemical instruments and construction materials. The steels in the present invention include steel sheets, steel strips, steel pipes and steel wires.
BACKGROUND ART
Silver and copper have been known to have effects of suppressing growth of pathogenic bacteria typically including
Escherichia coli
and salmonellae and hence preventing food poisoning linked to such pathogenic bacteria.
Recently, materials obtained by using these metals and having inhibitory effect on bacterial growth (hereinafter referred to as “antibacterial property”) have been proposed.
By way of illustration, Japanese Unexamined Patent Publication No. 8-49085 discloses a stainless steel sheet having excellent antibacterial property obtained by forming a metal layer or alloy later of Cr, Ti, Ni, Fe or the like containing Ag and/or Cu on the surface of a stainless steel matrix through magnet sputtering. This steel sheet is preferably obtained by forming a metal layer or alloy layer containing 19 to 60 wt % of Ag.
Separately, Japanese Unexamined Patent Publication No. 8-156175 proposes a coated steel sheet obtained by applying a pigment containing silver to suppress bacterial growth.
However, the aforementioned process of forming a metal layer or alloy layer containing an antibacterial metal onto the surface of a steel sheet and the process of applying a pigment containing an antibacterial metal have the following problems: The surface layer containing the antibacterial metal is peeled or removed through drawing or grinding of the surface, and the effects of the surface layer are no longer provided. In addition, the antibacterial property cannot be retained for a long duration in the applications where the surface of the steel is always rubbing such as in a steel sheet used for interior trim of washing machines or in the applications where the surface of steel is always rubbed by cleansing as in kitchen utensils. According to the above processes, extra manufacturing steps for coating or for forming a metal or alloy layer are required than conventional processes, and with a decreasing thickness of sheet the surface area per unit weight increases and hence the coating amount or the amount of the metal layer or alloy layer per unit weight increases, which results in unfavorably increasing costs.
Japanese Unexamined Patent Publication No. 8-239726 discloses an antibacterial and anti-maricolous-organism material comprising, by weight, Fe: 10 to 80%, Al: 1 to 10%, or in addition, 1 to 15% of at least one member of Cr, Ni, Mn, Ag with the balance being copper and incidental impurities. This material is, however, a copper-based alloy or iron-based alloy containing 1 to 10% Al, has low workability and is still problematic for the application as thin steel sheets as in eating utensils, kitchen utensils and parts of electrical equipment.
To solve the aforementioned problems, Japanese Unexamined Patent Publication No. 8-104953 proposes an austenitic stainless steel having improved antibacterial property obtained by adding 1.1 to 3.5 wt % Cu, and Japanese Unexamined Patent Publication No. 8-104952 proposes a martensitic stainless steel having improved antibacterial property obtained by adding 0.3 to 5 wt % Cu.
According to the technologies described in Japanese Unexamined Patent Publication No. 8-104953 and Japanese Unexamined Patent Publication No. 8-104952, however, Cu as ions must be eluted from the surface of the steel sheet to develop antibacterial property. The elution of Cu as ions means the destruction of a passivation film at the same site, and hence the corrosion resistance is extremely deteriorated although the antibacterial property is improved. According to such a Cu-added stainless steel, therefore, the antibacterial property can hardly be compatible with the corrosion resistance.
It is an object of the present invention to provide both a stainless steel and a method of producing the same, which stainless steel can advantageously solve the problems of conventional technologies and has excellent workability and corrosion resistance, and in addition has still excellent antibacterial property even when subjected to currently-employed surface finishing inclusive of grinding.
DISCLOSURE OF INVENTION
The present inventors made intensive investigations on the relation between the chemical composition of the surface of a stainless steel sheet and the antibacterial property, using analyzers such as a field emission type Auger electron spectroscope and an electron beam microanalyzer in order to develop a stainless steel sheet compatibly having antibacterial property, and excellent workability and corrosion resistance. Consequently, they newly found that stainless steel sheets having high antibacterial property and, add to this, excellent workability and corrosion resistance can be obtained by adding a proper amount of Ag to a stainless steel and making one or more members of silver particles, silver oxides and silver sulphides to occur on the surface of resultant stainless steel sheets. They further found that these stainless steel sheets have stable antibacterial property even in the applications to be subjected to forming or grinding or in the applications where the surfaces are rubbed or abraded.
The present invention has been accomplished based upon the above findings and further investigations.
(1) A stainless steel having excellent antibacterial property and containing 10 wt % or more Cr and 0.0001 to 1 wt % Ag, wherein the steel includes a total of 0.001 or more in area percentage of one or more members selected from a silver particle, a silver oxide and a silver sulphide.
(2) The stainless steel having excellent antibacterial property according to (1), wherein the stainless steel contains one or more members selected from: Sn: 0.0002 to 0.02 wt %, Zn: 0.0002 to 0.02 wt %, Pt: 0.0002 to 0.01 wt %.
(3) The stainless steel having excellent antibacterial property according to (1) or (2), wherein the silver particle, silver oxide and silver sulphide each have a mean grain diameter of 500 &mgr;m or less.
(4) A method of producing a stainless steel material having excellent antibacterial property, which method comprises continuously casting a molten stainless steel containing Cr: 10 wt % or more, Ag: 0.0001 to 1 wt % to give a steel material, wherein the casting rate of the continuous casting ranges from 0.8 to 1.6 m/min.
(5) The method of producing a stainless steel material having excellent antibacterial property according to (4), wherein the molten stainless steel contains one or more members selected from Sn: 0.0002 to 0.02 wt %, Zn: 0.0002 to 0.02 wt %, Pt: 0.0002 to 0.01 wt %.
(6) A method of producing a cold-rolled stainless sheet steel, wherein the stainless steel obtained according to (4) or (5) is further subjected to hot-rolling, cold-rolling.
BEST MODE FOR CARRYING OUT THE INVENTION
The content limits of the chemical composition of the steel according to the present invention will now be described.
The steel of the invention can advantageously be applied to any of austenitic stainless steels, ferritic stainless steels, martensitic stainless steels and a variety of other stainless steels.
The austenitic stainless steel preferably has a chemical composition of: C: 0.01 to 0.1 wt %, Si: 2.0 wt % or less, Mn: 2.0 wt % or less, P: 0.08 wt % or less, S: 0.02 wt % or less, Cr: 10 to 35 wt %, Ni: 6 to 15 wt %, N:0.01 to 0.1 wt % with the balance being Fe and incidental impurities. The steel may further comprise one or more members selected from: Mo: 3.0 wt % or less, Cu: 1.0 wt % or less, W: 0.30 wt % or less, V: 0.30 wt % or less, Al: 0.3 wt % or less, Ti: 1.0 wt % or less, Nb: 1.0 wt % or less, Zr: 1.0 wt % or less, B: 0.01 wt % or less.
The ferritic stainless steel preferably has a chemical composition of: C: 0.01 wt % or less, Si: 1.
Hasuno Sadao
Satoh Susumu
Tochihara Misako
Yokoto Takeshi
Kawasaki Steel Corporation
King Roy
Schnader Harrison Segal & Lewis LLP
Wilkins, III Harry D.
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