Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2002-11-05
2004-06-15
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S650000, C148S651000
Reexamination Certificate
active
06749701
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a high-strength, high-toughness martensitic stainless steel sheet suitable for use in various types of springs, metal gaskets, metal masks, flapper valves, steel belts and the like, a method of inhibiting cold-rolled steel sheet edge cracking during production thereof, and a method of producing the steel sheet.
2. Background Art
Stainless steels conventionally used in metal gaskets, metal masks, and other applications demanding high strength include the following:
(A) Stainless steels work-hardened by cold rolling austenitic stainless steels such as SUS301 and SUS304. Stainless steels of this type utilize the hardness of cold-rolling-induced martensite per se. The asbestos gaskets long used in automobile and motorcycle engines are currently being replaced by metal gaskets employing stainless steel of this type.
(B) Precipitation-hardened stainless steels as typified by SUS630. Stainless steels of this type are low in hardness and excellent in workability before aging and exhibit high hardness owing to precipitation hardening after aging. They are also characterized by high resistance to weld softening. Stainless steel of this type are therefore used extensively for springs and steel belts that require welding. The assignee has developed stainless steels of this type with improved toughness and torsional properties (Japanese Patent Publication JPA No.Hei 7-157850 (1995) and JPA No.Hei 8-74006 (1996)).
(C) Quench-hardened stainless steels having high strength in the annealed state or after skin-pass rolling at a reduction ratio of several percent. Stainless steels of this type achieve high strength by utilizing martensite formed during quenching from the temperature region of austenite phase, or austenite phase+ferrite phase, to normal room temperature. These stainless steels do not require expensive precipitation hardening elements and can be produced with relatively few production steps. They are therefore relatively inexpensive in terms of both raw material cost and production cost. Stainless steels of this type developed by the assignee include the low-carbon martensitic stainless steel for steel belts described in Japanese Patent Publication JPB No.Sho 51-31085 (1976) and the high-ductility, high-strength multiphase structure stainless steel with small in-plane anisotropy described in Japanese Patent Publication JPA No.Sho 63-7338 (1988).
These prior-art stainless steels have the following drawbacks:
The type (A) work-hardened stainless steels require considerably strong cold working in order to form the large amount of martensite needed to attain high-level strength and spring properties. Since martensite is not readily formed at high working temperature, moreover, the cold working must be conducted at low speed to avoid steel temperature increase. Productivity is therefore low. In addition, the amount of martensite generation induced by the working is very sensitive to the austenite stability of the steel. This means that just a slight shift in steel composition makes the amount of martensite generated deviate from the desired constant value, even under a constant amount of cold working. The properties of the product therefore tend to vary.
As explained further later, a stainless steel to be used for cylinder head gaskets, which require high air-tightness, needs superb spring property. Consider, for example, the spring bending elastic limit Kb of a type (A) stainless steel such as SUS301 or SUS304, even if the strength of the stainless steel is increased to a high level by cold working, the Kb
0.1
value after imparting a tensile strain of 0.1% is only about 650 N/mm
2
at best. Better spring property than this is hard to achieve. Aging is sometimes used for imparting outstanding spring property to a metastable austenitic stainless steel. It has been found, however, that in applications to cylinder gaskets and the like, whose bead portion may come under compressive stress exceeding the steel's elastic limit, the spring property maintained after deformation during use in such a case increases with higher spring property of the steel before aging. In other words, the stainless steel should preferably already have excellent spring property before aging and impartation of excellent spring property for the first time by aging is not advisable. Given the present state of the art, therefore, an attempt to boost the performance of stainless steels of this type for use in metal gaskets is unlikely to be successful.
The type (B) precipitation-hardened stainless steels must contain age-hardening elements such as Cu, Al, Ti and Mo. The generally high price of these elements raises the starting material cost. In addition, the need for an aging furnace makes the initial outlay for equipment enormous. Production cost is also high owing to the numerous production processes required.
The type (C) quench-hardened stainless steels are generally lower in strength than the type (A) and (B) stainless steels. An attempt to enhance strength by skin-pass rolling or inclusion of large amounts of C or N is apt to degrade toughness. Achieving a high level of strength as well as good toughness in the type (C) steels is therefore no easy matter. As far as the inventors are aware, no type (C) stainless steel that succeeds on both counts has been made available.
The inventors conducted an extensive study in search of a method enabling low-cost production of a stainless steel excellent in spring property and exhibiting both high strength and toughness. As a result, it was concluded that the type (C) quench-hardened stainless steels still had room for development. A first object of the present invention is therefore to provide a type (C) quench-hardened stainless steel that possesses high strength comparable to SUS301, a typical type (A) work-hardened stainless steel, and further exhibits excellent toughness and spring property capable of meeting the increasingly severe requirements for use in metal gaskets.
The properties required of a stainless steel for use in metal gaskets are particularly demanding. The steel is required to have excellent fatigue property so it can stand up under the high temperature, high pressure, harsh vibration, and repeated temperature and pressure changes peculiar to engines. It must also have excellent shape-retaining property (shape freezing property) so that after being precision-machined to a shape for optimum sealing performance it can retain this shape without change even under the aforesaid severe use environment. While excellent resistance to permanent set can be considered essential for a stainless steel to achieve excellent in fatigue property and shape freezing property, no type (C) stainless steel excellent in resistance to permanent set has yet been developed, wherein the permanent set means a permanent shape change which has been occurred in the usage of the material as a spring or gasket under compressive load, and can be evaluated for instance by specified fatigue test as described in Example 4 hereinafter. A second object of the present invention is therefore to provide a stainless steel sheet having the foregoing properties desirable for use in metal gaskets.
The inventors further discovered that production of a stainless steel sheet enhanced in strength from the foregoing perspective encountered previously unexperienced problems that needed to be solved. Specifically, trouble was encountered during cold rolling. When the rolling loads required during cold rolling were compared between such improved stainless steel sheet in accordance with the present invention and a conventional quench-hardened stainless steel sheet, the rolling load required by the improved stainless steel sheet was markedly greater in proportion to its higher strength. In addition, the improved steel sheet tended to experience edge cracking. Edge cracking must be avoided by all means because it not only degrades product quality but also poses a safety issue during steel sheet production. When edge cra
Hiramatsu Naoto
Isozaki Seiichi
Tomimura Kouki
McDermott & Will & Emery
Nisshin Steel Co. Ltd.
Yee Deborah
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