Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-04-29
2001-08-21
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
Reexamination Certificate
active
06277215
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a martensitic-ferritic duplex-phase stainless steel which is inexpensive but has high hardness, good workability, and good corrosion resistance. It also relates to a method for the production of a sheet of such a stainless steel. The stainless steel is particularly suitable for use in the fabrication of engine gaskets for automobiles or the like.
BACKGROUND OF THE INVENTION
Engine gaskets are important parts of automotive engines. The gaskets are positioned between a cylinder head and an engine block which define a combustion chamber of an automotive engine. As shown in FIGS.
1
(
a
) and
1
(
b
), an engine gasket
1
is a sealing member having an opening
2
, which generally has a circular shape with the same diameter as the cylinder of the engine, and an annular bead
3
which is a ridge formed by beading so as to surround the opening. The bead
3
functions as a seal since it is compressed between the cylinder head and engine block and blocks the interstice therebetween to prevent leakage of combustion gas, cooling water, and lubricating oil from the combustion chamber.
A material for fabricating such a gasket is, therefore, required to have high strength (high hardness) sufficient to retain a bead against compression, along with good workability and good corrosion resistance.
In order to meet the above-described requirements, a metastable austenitic stainless steel, such as SUS 301 stainless steel which is a Cr- and Ni-added stainless steel, has been used to fabricate engine gaskets. Deformation of such a steel by cold working, such as cold rolling and beading, causes the metastable austenite in the deformed area to transform to martensite which has a greater hardness. Thus, the steel can exhibit a high work hardenability with good workability.
However, such a stainless steel has the disadvantage that its properties, particularly hardness may fluctuate greatly, since the increased hardness of the steel obtained by working may vary significantly depending on the working ratio of the steel and the temperature at which the steel is subjected to working. Therefore, the quality, particularly sealing quality of gaskets made from the steel may fluctuate significantly. Another disadvantage is that the metastable austenitic steel is susceptible to stress corrosion cracking. Furthermore, the steel contains a large amount of nickel, which is expensive, thereby adding to the production costs of the gaskets.
In order to cope with these problems, a Cr-based martensitic stainless steel having a tempered martensitic structure has been proposed for the fabrication of engine gaskets in Japanese Patent Application Laid-Open No. 7-278758(1995). In general, martensitic stainless steel has improved resistance to stress corrosion cracking over the above-described metastable austenitic stainless steel. Moreover, it is relatively easy to achieve a high hardness with martensitic stainless steel by means of quenching, which causes transformation to form hard martensitic phases. Furthermore, martensitic steel is less expensive since it contains a very limited amount of expensive Ni.
However, since martensitic stainless steel as quenched has a decreased elongation and is difficult to work, it is essential that the quenched martensitic steel be subjected to heat treatment for tempering after quenching. Such heat treatment adds to the production costs of the steel and may cause embrittlement of the steel due to deposition of carbides or a loss of corrosion resistance due to the formation of Cr-deficient phases resulting from the deposition of carbides.
U.S. Pat. No. 5,624,504 discloses a martensitic-ferritic duplex-phase stainless steel which contains C, Si, Mn, P, S, Ni, Cr, N, B and Cu as essential alloying elements. The fraction of the martensite in the steel structure is selected so as to provide the steel with high strength, and the grain size of the martensite is as small as 10 &mgr;m or less to assure good workability. The steel has a low carbon content of up to 0.10% by weight. This patent does not teach that the steel is suitable for use in the fabrication of gaskets.
Thus, there is a need for a high-performance, less expensive stainless steel for engine gaskets which can be produced in a stable manner.
These and other objects and advantages of the present invention will be apparent from the description as set forth below.
SUMMARY OF THE INVENTION
The present invention provides a less expensive martensitic-ferritic duplex-phase stainless steel suitable for use in the fabrication of engine gaskets, the steel improved in that it can exhibit high strength with good workability and good corrosion resistance as quenched (without tempering), contrary to the above-described martensitic stainless steel.
The present inventors found that when quenching occurs starting from a temperature in the two-phase region of austenite plus ferrite rather than in the higher-temperature, austenitic single-phase region in such a manner that the austenite in the austenitic-ferritic duplex-phase structure is transformed to martensite having high hardness to form a two-phase structure of martensite plus ferrite, the resulting as-quenched martensitic-ferritic duplex-phase steel exhibits good workability which is sufficient to fabricate gaskets, and it still maintains high hardness and good corrosion resistance, provided that the hardness and the fraction of martensite of the steel are within specific ranges.
The present invention provides for a less expensive stainless steel suitable for use in the fabrication of gaskets, with addition of a minimized amount of expensive metals such as nickel.
In one aspect, the present invention relates to a stainless steel suitable for use in the fabrication of gaskets, particularly engine gaskets, which has a chemical composition comprising on a weight basis:
C+N: 0.1%-0.3%,
Si: not greater than 0.5%,
Mn: not greater than 0.7%,
Cr: 10%-17%, and
Ni: 0-0.6%,
the steel having a structure consisting essentially of from 40% to 80% by volume of martensite, the balance being ferrite, and a Vickers hardness in the range of from 300 to 500. Preferably, the martensite (martensitic phases) in the steel structure has a Vickers hardness in the range of from 300 to 600.
Preferably, the steel has a chemical composition consisting essentially, on a weight basis, of:
C+N: 0.1%-0.3%,
Si: not greater than 0.5%,
Mn: not greater than 0.7%,
Cr: 10%-17%,
Ni: 0-0.6%, and
one or more elements selected from the group consisting of Nb, V and Ti: 0 to 2.0% in total,
the balance being iron and inevitable impurities.
In another aspect, the present invention provides a method for producing a stainless steel sheet suitable for use in the fabrication of gaskets, which comprises the steps of:
preparing a steel having the above-described chemical composition,
applying working to the steel to form a sheet having a predetermined thickness, and
subjecting the steel sheet to final heat treatment at a temperature in the range of from 850° C. to 1000° C. followed by quenching.
The term “steel sheet” used herein encompasses a steel strip, coil, and the like.
In still another aspect, the present invention provides a gasket, particularly engine gasket, having at least one bead, the gasket being made from the above-described stainless steel.
In a broad sense, the present invention relates to a stainless steel gasket having at least one bead for sealing, the stainless steel having an as-quenched martensitic-ferritic duplex-phase structure. More specifically, the present invention provides such a gasket, particularly engine gasket, which is made from a steel having a martensitic-ferritic duplex-phase structure in which martensite comprises from 40% to 80% by volume of the structure, the steel having a Vickers hardness in the range of from 300 to 500. Preferably the martensitic phases in the steel have a Vickers hardness in the range of 300 to 600.
REFERENCES:
patent: 4564566 (1986-01-01), Jerlich et al.
patent: 5118576 (1992-06-01), Imae et al.
patent: 5624504 (1997-04-01
Adachi Kazuhiko
Fujisawa Kazuyoshi
Goshokubo Kenichi
Kinoshita Yuuichi
Yamada Yoshio
Burns Doane , Swecker, Mathis LLP
Sumitomo Metal Industries Ltd.
Yee Deborah
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