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 Kb0.1 value after imparting a tensile strain of 0.1% is only about 650 N/mm2 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 cracking having an effect on later processing steps arises, the only alternative is to cut away the edge portions of the steel sheet by the width of the cracked region using a trimmer or the like. This trimming adds another step to the production process and lowers production yield. It therefore leads to a large increase in production cost. A third object of the invention is therefore to provide a method of markedly inhibiting cold-rolled steel sheet edge cracking in the production of a stainless steel sheet having high strength comparable to SUS301 and also excellent in toughness and spring property.
Regarding the matensitic stainless steels classified under the aforesaid type (C) quench-hardened stainless steels, the inventors learned through the research that by regulating C, N and Ni content and further controlling amount of xcex4 ferrite and amount of residual austenite there can be obtained a high-strength steel that is superior to a conventional quench-hardened stainless steel in strength, toughness and spring property, superior to a work-hardened stainless steel in productivity and uniformity of product properties, and cheaper than a precipitation-hardened stainless steel.
Through further studies regarding optimization for metal gasket applications in particular, it is found that imparting a metallic structure composed of not less than 85 vol % martensite phase in the quenched state, in addition to regulating C, N and Ni content, is very effective for improving the fatigue property of a type (C) steel. As a result of repeated experimentation, it is discovered that it is highly effective for improvement of resistance to permanent set during metal gasket use for the steel to exhibit a high spring bending elastic limit after being imparted with a certain amount of strain. Specifically, it was found that a metal gasket steel capable of satisfying today""s demanding requirements could be obtained when a test specimen imparted with 0.1% tensile strain was made to have a spring bending elastic limit Kb0.1 measured in conformity with JIS (Japanese Industrial Standard) H 3130 of not less than 700 N/mm2. The inventors additionally ascertained that occurrence of microcracks during bead formation can be effectively suppressed by regulating composition and production conditions to regulate uniform elongation or tensile strength to an appropriate level.
Another clear finding is that in order to markedly suppress edge cracking during cold rolling of such a steel it is highly important to 1) reduce the degree of surface roughening at the steel sheet edge portions to the absolute minimum during hot rolling, 2) hold down steel sheet hardness before cold rolling, and 3) suppress grain boundary precipitation of carbides and nitrides during intermediate annealing conducted before cold rolling. For achieving point 1), it was found to be effective to incorporate an appropriate amount of B as an alloying component and to regulate the composition so as to keep the amount of xcex4 ferrite below a certain level. For achieving points 2) and 3), it was found to be effective to strictly control the conditions of the intermediate annealing conducted before cold rolling.
The present invention was accomplished based on the foregoing new knowledge.
Specifically, in a first aspect, the invention provides a high-strength, high-toughness martensitic stainless steel sheet having a chemical composition comprising, in mass percent, more than 0.03 to 0.15% of C, 0.2-2.0% of Si, not more than 1.0% of Mn, not more than 0.06% of P, not more than 0.006% of S, 2.0-5.0% of Ni, 14.0-17.0% of Cr, more than 0.03 to 0.10% of N, 0.0010-0.0070% of B, and the balance of Fe and unavoidable impurities and having an A value defined by Equation (1) of not less than minus(xe2x88x92)1.8:
A value=30(C+N)xe2x88x921.5Si+0.5Mn+Nixe2x88x921.3Cr+11.8xe2x80x83xe2x80x83(1), 
provided that each element symbol on the right side of Equation (1) is replaced by a value representing the content of the element in mass percent.
xe2x80x9cSteel sheetxe2x80x9d as termed with respect to the present invention is defined to include xe2x80x9csteel strip.xe2x80x9d
In a second aspect of the invention, the steel sheet according to the first aspect is a high-strength, high-toughness martensitic stainless steel sheet whose edges at opposite lateral extremities of the steel sheet are edges formed by cold rolling that have no edge cracks of a length greater than 1 mm.
In a third aspect, the invention provides a high-strength, high-toughness martensitic stainless steel sheet for metal gaskets comprising, in mass percent, more than 0.03 to 0.15% of C, 0.2-2.0% of Si, not more than 1.0% of Mn, not more than 0.06% of P, not more than 0.006% of S, 2.0-5.0% of Ni, 14.0-17.0% of Cr, more than 0.03 to 0.10% of N, 0.0010-0.0070% of B, and the balance of Fe and unavoidable impurities and including not less than 85 vol % martensite phase, a test specimen of which imparted with a nominal tensile strain of 0.1% exhibits a spring bending elastic limit Kb0.1 measured in conformity with JIS H 3130 of not less than 700 N/mm2.
Kb0.1 is the spring bending elastic limit when permanent deflection is 0.1 mm in the moment-type test according to JIS H 3130.
In a fourth aspect of the invention, the steel sheet according to the third aspect further comprises one or both of Mo and Cu at a total of not less than 2.0 mass percent.
In a fifth aspect of the invention, the steel sheet according the third or fourth aspect has a chemical composition wherein A value defined by Equation (1) above is not less than xe2x88x921.8.
In a sixth aspect of the invention, the steel sheet according to any of the third to fifth aspects has a uniform elongation of not less than 0.3%.
In a seventh aspect of the invention, the steel sheet according to any of the third to sixth aspects has a tensile strength of 1,400-1,700 N/mm2.
In an eighth aspect, the invention provides a method of inhibiting cold-rolled steel sheet edge cracking of a high-strength, high-toughness martensitic stainless steel sheet, which method is applied with respect to a hot-rolled steel sheet of matensitic stainless steel having a chemical composition comprising, in mass percent, more than 0.03 to 0.15% of C, 0.2-2.0% of Si, not more than 1.0% of Mn, not more than 0.06% of P, not more than 0.006% of S, 2.0-5.0% of Ni, 14.0-17.0% of Cr, more than 0.03 to 0.10% of N, 0.0010-0.0070% of B, and the balance of Fe and unavoidable impurities and having an A value defined by Equation (1) below of not less than xe2x88x921.8:
A value=30(C+N)xe2x88x921.5Si+0.5Mn+Nixe2x88x921.3Cr+11.8xe2x80x83xe2x80x83(1), 
and comprises a step of subjecting the sheet to a single cycle or multiple repeated cycles of a process (intermediate annealing and cold rolling process) consisting of intermediate-annealing the sheet at a soaking temperature of 600-800xc2x0 C. for a soaking period of not more than 10 hr to adjust steel hardness to Vickers hardness (Hv) of not greater than 380, followed by cold rolling.
Conceptually, xe2x80x9csoaking temperaturexe2x80x9d means the constant temperature maintained by the steel sheet once its temperature has become uniform in the thickness direction in the course of temperature rise during heating. Actually, however, accurate determination of this temperature is difficult. As the steel sheet temperature approaches the furnace temperature, moreover, the rate of temperature increase slows to such an extent as to reach a metallurgical state that is substantially no different from that of the temperature being uniform in the direction of sheet thickness. In this invention, therefore, the soaking temperature is defined as: average of temperature T1 (xc2x0 C.) and temperature T2 (xc2x0 C.), i.e., temperature (T1+T2)/2, where T1 (xc2x0 C.) is the steel sheet surface temperature when, in the course of temperature increase during steel sheet heating, the rate of temperature increase at the steel sheet surface becomes not greater than 2xc2x0 C./sec and T2 (xc2x0 C.) is the ultimate steel sheet surface temperature reached thereafter prior to the start of cooling. The steel sheet surface temperature can be measured by, for instance, a thermocouple spot welded on the steel sheet surface.
Conceptually, xe2x80x9csoaking periodxe2x80x9d means the time period during which the steel sheet maintains a constant temperature once its temperature has become uniform in the thickness direction in the course of temperature rise during heating. In this invention, however, the soaking period is defined as: period between the time point at which, in the course of temperature increase during steel sheet heating, the rate of temperature increase at the steel sheet surface becomes not greater than 2xc2x0 C./sec and the time point at the start of cooling. xe2x80x9cSoaking period of not more than 10 hrxe2x80x9d is defined to include the case in which cooling starts as soon as the rate of temperature increase at the steel sheet surface becomes not greater than 2xc2x0 C./sec (zero-second soaking).
A ninth aspect of the invention provides a method according to the eighth aspect, wherein, in addition to adjusting steel hardness after intermediate annealing to Vickers hardness (Hv) of not greater than 380, the soaking temperature is a temperature in a range of x (xc2x0 C.) satisfying Z valuexe2x89xa6380 in Equation (2):
Z value=61Cxe2x88x926Sixe2x88x927Mnxe2x88x921.3Nixe2x88x924Crxe2x88x9236Nxe2x88x927.927xc3x9710xe2x88x926x3+1.854xc3x9710xe2x88x922x2xe2x88x9213.74x+3663xe2x80x83xe2x80x83(2), 
provided that each element symbol on the right side of Equation (2) is replaced by a value representing the content of the element in mass percent and x is soaking temperature (unit: xc2x0 C.).
A tenth aspect of the invention provides a method according to the eighth or ninth aspect, wherein the intermediate annealing soaking period in each cycle of the intermediate annealing and cold rolling process is not greater than 300 sec.
An eleventh aspect of the invention provides a method according to any of the eighth to tenth aspects, wherein the cold rolling reduction ratio in each cycle of the intermediate annealing and cold rolling process is not greater than 85%. When multiple repeated cycles of the intermediate annealing and cold rolling process are conducted, the cold rolling reduction ratio is made not greater than 85% in every cycle. However, the cold rolling reduction ratio need not be the same in every cycle.
A twelfth aspect of the invention provides a method of producing a high-strength, high-toughness martensitic stainless steel sheet while inhibiting cold-rolled steel sheet edge cracking, which method comprises subjecting a cold-rolled sheet produced according to and having undergone the intermediate annealing and cold rolling process of the method of any of the eighth to eleventh aspects to finish annealing at a soaking temperature of 950-1,050xc2x0 C. for a soaking period of not greater than 300 sec, without first subjecting it to trimming of edges at opposite lateral extremities.
The finish annealing here is annealing imparted at the end of the process for producing a steel sheet exhibiting high strength, high toughness and excellent spring property. The soaking temperature and soaking period are defined in the same manner as in the earlier intermediate annealing. The finish annealing also includes the case of zero-second-soaking.
A thirteenth aspect of the invention provides a method according to the twelfth aspect, wherein skin-pass rolling is effected at a reduction ratio of 1-10% after the finish annealing.