The materials used for electric and electronic parts as the materials of current-carrying parts, such as connectors, lead frames, relays and switches, are required to have a good electric conductivity in order to suppress the generation of Joule heat due to the carrying of current, as well as such a high strength that the materials can withstand stress applied thereto during the assembly and operation of electric and electronic apparatuses using the parts. The materials used for electric and electronic parts, such as connectors, are also required to have an excellent bending workability since the parts are generally formed by bending after press blanking. Moreover, in order to ensure the contact reliability between electric and electronic parts, such as connectors, the materials used for the parts are required to have an excellent stress relaxation resistance, i.e., a resistance to such a phenomenon (stress relaxation) that the contact pressure between the parts is deteriorated with age.
In recent years, there is a tendency for electric and electronic parts, such as connectors, to be integrated, miniaturized and lightened. In accordance therewith, the sheets of copper and copper alloys serving as the materials of the parts are required to be thinned, so that the required strength level of the materials is more severe. In particular, since connectors for automobiles and so forth are used in environments wherein violent vibrations are repeatedly applied thereto, the materials thereof are required to have a high fatigue strength, i.e., a property which is difficult to cause fatigue failure.
In accordance with the miniaturization and complicated shape of electric and electronic parts, such as connectors, it is required to improve the precision of shape and dimension of products manufactured by bending the sheets of copper alloys. For that reason, there is recently often applied a so-called bending after notching wherein a sheet is bent along a notch which is formed by carrying out notching (working for forming the notch) in a portion of the sheet. However, in the bending after notching, portions near the notch portion are work-hardened by notching, so that cracks are easily produced in the subsequent bending operation. Therefore, the bending after notching is a very severe bending process for materials. However, the materials of electric and electronic parts, such as connectors, are generally bent so that the bending axis thereof is a direction (TD) perpendicular to a rolling direction (LD) and thickness direction.
Moreover, as the increase of cases where electric and electronic parts, such as connectors, are used in severe environments, the requirements for the stress relaxation resistance of the parts are more severe. For example, the stress relaxation resistance of electric and electronic parts, such as connectors, is particularly important when the parts are used for automobiles in high-temperature environments. Furthermore, the stress relaxation resistance is such a kind of creep phenomenon that the contact pressure on the spring portion of the material of electric and electronic parts, such as connectors, is deteriorated with age in a relatively high-temperature (e.g., 100 to 200° C.) environment even if it is maintained to be a constant contact pressure at ordinary temperature. That is, the stress relaxation resistance is such a phenomenon that the stress applied to a metal material is relaxed by plastic deformation produced by the movement of dislocation, which is caused by the self-diffusion of atoms forming a matrix and the diffusion of the solid solution of atoms, in a state that the stress is applied to the metal material.
However, there are generally trade-off relationships between the strength and electric conductivity of the sheet of a copper alloy, between the strength and bending workability thereof, and between the bending workability and stress relaxation resistance thereof, respectively. Therefore, in conventional methods, a sheet having a good electric conductivity, strength, bending workability or stress relaxation resistance is suitably chosen in accordance with the use thereof as a material used for a current-carrying part, such as a connector.
Among the sheets of copper alloys, the sheets of Cu—Ni—Sn—P alloys have a good balance between the electric conductivity, strength, bending workability and stress relaxation resistance, and are easily produced. The sheets of Cu—Ni—Sn—P alloys have the functions of carrying out the solid-solution strengthening (or hardening) thereof by Sn and Ni. In addition, in the sheets of Cu—Ni—Sn—P alloys, the above-described characteristics are improved by finely dispersing Ni—P precipitates. Thus, there are proposed various sheets of Cu—Ni—Sn—P alloys as the materials used for electric and electronic parts, such as connectors (see, e.g., Japanese Patent Laid-Open Nos. 4-154942, 4-236736, 10-226835, 2000-129377, 2000-256814, 2001-262255, 2001-262297 and 2002-294368).
There are also proposed a Cu—Ni—Sn—P alloy sheet wherein a texture having the {420} plane as a principal orientation component is developed to be optimized for the bending after notching (see, e.g., Japanese Patent Laid-Open No. 2008-231492), a Cu—Ni—Sn—P alloy sheet wherein the development of Brass orientation is suppressed to improve the stress relaxation resistance and bending workability thereof (see, e.g., Japanese Patent Laid-Open No. 2009-62592), and sheets of Cu—Ni—Si alloys (so-called Corson alloys) being high-strength copper alloys wherein a texture having the {100} plane as a principal orientation component is developed to improve the bending workability and press blankability thereof (see, e.g., Japanese Patent Laid-Open Nos. 2000-80428 and 2000-73130). These copper alloy sheets are designed so as to avoid the anisotropy of characteristics on the rolled surface thereof to maintain the strength and bending workability thereof.
The sheets of Cu—Ni—Sn—P alloys have a relatively high strength (a tensile strength of 500 to 600 MPa) and a relatively high electric conductivity (30 to 50% IACS) to have an excellent balance between the strength and electric conductivity thereof. The stress relaxation resistance of the sheets of Cu—Ni—Sn—P alloy is far better than that of the sheets of general solid-solution strengthening type copper alloys, such as brass and phosphor bronze, and is equal to or higher than that of the sheets of Cu—Ni—Si alloys (so-called Corson alloys) and the sheets of precipitation strengthening type copper alloys, such as Cu—Ti alloys. Moreover, the sheets of Cu—Ni—Sn—P alloys have an excellent bending workability, and are suitable for the materials of connectors for automobiles.
Generally, Cu—Ni—Sn—P alloys have a good castability since they are basically solid-solution strengthening type alloys and since the amounts of easily oxidized elements, such as Si, Ti, Mg and Zr, can be decreased even if the elements are added to carry out the precipitation strengthening and to fine the cast structure thereof. Moreover, the sheets of Cu—Ni—Sn—P alloys can be produced at relatively low costs since it is possible to omit complicated heat treatment steps, such as solution and ageing treatments, which are required to produce the sheets of precipitation strengthening type copper alloys.
However, in recent years, electric and electronic parts, such as connectors, are severely required to be thinned and miniaturized. In order to meet such severe requirements, it is required to further enhance the strength level of the sheets of Cu—Ni—Sn—P alloys. For example, when the sheets are required to be high strength sheets having a tensile strength of not less than 600 MPa, and further, not less than 650 MPa, it is very difficult for conventional Cu—Ni—Sn—P alloys to have a higher strength without increasing the producing costs while maintaining the excellent stress relaxation resistance and bending workability.
As general methods for enhancing the strength of Cu—Ni—Sn—P alloys, there are known a method for adding a large amount of solute elements, such as Ni and Sn, and a method for increasing a finish rolling (temper rolling) reduction. However, in the method for adding a large amount of solute elements, the electric conductivity of the sheets of the alloys is remarkably deteriorated, and the amount of relatively expensive Ni, Sn or the like to be added is increased to be uneconomical. In the method for increasing the finish rolling reduction, the bending workability of the sheets of the alloys is deteriorated as the extent of work hardening is enhanced. For that reason, even if the strength level and the electric conductivity are high, there are some cases where the sheets can not be used for electric and electronic parts, such as female terminals, which are required to be manufactured by box-bending. On the other hand, there is a method for adding a large amount of elements, such as Ni and P, which contribute to the amount of precipitates. However, there are some cases where the addition of the large amount of these elements to form coarse precipitates which serve as the origins of the production of cracks to deteriorate the bending workability and fatigue strength of the sheets. In addition, if it is controlled so as to form fine precipitates even if a large amount of these elements are added, the number of heat treatments is increased, and/or the producing conditions are limited, so that the producing costs are increased.
In order to improve the bending workability of a sheet of a copper alloy, a method for fining the crystal grains of the copper alloy is generally adopted. As the crystal grain size of the copper alloy is smaller, the area of grain boundaries existing per a unit volume thereof is larger. The grain boundaries function as interfaces which allow boundary sliding and rotation of crystal grains on both sides thereof during bending. Therefore, as the area of grain boundaries is larger, there is a tendency for local stress concentration to be avoided to improve the bending workability of the sheet. However, the increase of the area of grain boundaries due to grain refining causes to promote the stress relaxation which is a kind of creep phenomenon. Particularly, in connectors for automobiles and so forth which are used in high-temperature environments, the diffusion rate along the grain boundaries of atoms is far higher than that in the grains, so that the deterioration of the stress relaxation resistance due to grain refining causes a serious problem. Moreover, there are some cases where grain boundaries serve as the origins of fatigue fracture since they act as storage portions for dislocation during repeated bending operations to cause work hardening. In such temperature environments, grain refining is not always suitable for the improvement of fatigue strength. In addition, there are some cases where connectors for automobiles are influenced by vibrations of engines in accordance with the connecting portions and connecting methods thereof, so that fatigue failure is caused in and around electric cable crimping portions. Such fatigue failure is caused if work hardening and partial stress concentration portions are caused by methods for forming serrations and crimping electric cables while collapsing them in order to strongly crimp the electric cables and in order to improve the tight fitting of the electric cables into connectors. In addition, since the spring portions of female terminals are narrow and severely work-hardened by the 180° bending, the contact pressure applied thereto is deteriorated by stress relaxation due to fatigue and heat caused by vibrations, so that a critical problem is capable of being caused. In order to solve these problems, there has been taken measures, such as the improvement of the structures of connectors and the structures supported by housings, and the prevention of vibrations of electric cables. However, from the standpoint of costs and the degree of freedom of design, it is greatly expected to improve the characteristics of the materials of connectors. Therefore, it is considered that a method for causing the materials of connectors to have appropriate texture is effective in order to prevent excessive work hardening in serrations and crimping portions, since it reasonably suppresses work hardening.
In recent years, as a method for solving the problems on both of the strength and bending workability of the sheets, there are proposed a method for developing a predetermined texture of the sheets, and a method for suppressing the development of a predetermined texture of the sheets. For example, Japanese Patent Laid-Open No. 2008-231492 discloses a method for developing a texture having the {420} plane as a principal orientation component, and Japanese Patent Laid-Open No. 2009-62592 discloses a method for suppressing the development of Brass orientation. However, in the method for developing a texture having the {420} plane as a principal orientation component, there is a problem in that the producing loads at rolling steps are increased since the number of heat treatments is extremely limited until a sheet is obtained as a final product. In the method for suppressing the development of Brass orientation, it is not possible to increase the rolling reduction in final rolling, so that it is difficult to sufficiently improve the strength of the sheet by utilizing work hardening.
Thus, it is difficult to improve both of the bending workability and stress relaxation resistance of the sheets of Cu—Ni—Sn—P alloys while improving the strength and fatigue strength thereof. Particularly, in recent years, in order to use electric and electronic parts, such as connectors for automobiles, in severe environments, it is desired to produce a copper alloy sheet which has an excellent strength, electric conductivity, bending workability and stress relaxation resistance and which is difficult to cause fatigue failure.