Copper alloys for use as connector materials having high resistance to stress corrosion cracking and a process for producing the same

Abstract

The improved copper alloy suitable for use as a connector material contains 17-32 wt % of Zn, 0.1-4.5 wt % of Sn and 0.01-2.0 wt % of Si, with Zn and Sn satisfying the relation 54≦3X+Y≦100 where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added. Rolled material of the alloy can be produced by a process comprising the steps of melting a copper alloy of the composition specified above, cooling the melt over a temperature range from the liquidus line to 600° C. at a rate of at least 50° C./min, and subsequently hot rolling the resulting ingot at an elevated temperature of 900° C. or below.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to copper alloys in sheet or strip form that have satisfactory strength, electrical conductivity, stress corrosion cracking resistance and stress relaxation performance that are suitable for use as materials for connectors and other electrical or electronic components and which also have small Young's moduli. The invention also relates to a process for producing such copper alloys.

[0002] With the recent advances in electronics, the wire harnessing in various machines has increased in the degree of complexity and integration and this in turn has led to the growing use of wrought copper articles as materials for connectors and other electrical or electronic components.

[0003] The demands required of materials for connectors and other electrical or electronic components include lightweightness, high reliability, environmental resistance and low cost. To meet these requirements, copper alloy strips for connector materials are becoming smaller in thickness and must allow for smaller pin widths. At the same time, in order to press such copper alloy strips into complex shapes, they must have satisfactory strength, elasticity, electrical conductivity, resistance to stress corrosion cracking, as well as good workability in bending and stamping on press.

[0004] Specifically, electric terminals must have sufficient strength to ensure against buckling or deforming which would otherwise occur due to smaller pin widths resulting from their size reduction of or during connector insertion and withdrawal or upon bending; they are also required to have sufficient strength to withstand crimping, fitting and subsequent holding in position of electrical wires. To meet this need, wrought copper alloy strips for use as electric terminals are required to have a 0.2% yield strength of at least 600 N/mm2, preferably at least 650 N/mm2, more preferably at least 700 N/mm2, and a tensile strength of at least 650 N/mm2, preferably at least 700 N/mm2, more preferably at least 750 N/mm2 in the rolling direction. In addition, in order to prevent cascade deterioration that may occur during pressing into terminals, the strips must also have sufficient strength in a direction perpendicular to the rolling direction. To meet this need, the strips are required to have a 0.2% yield strength of at least 650 N/mm2, preferably at least 700 N/mm2, more preferably at least 750 N/mm2, and a tensile strength of at least 700 N/mm2, preferably at least 750 N/mm2, more preferably at least 800 N/mm2, in the perpendicular direction.

[0005] Further, in order to suppress the generation of Joule's heat due to current impression, the wrought copper alloy strips for use as electric terminals have preferably a conductivity of at least 20% IACS. Another requirement is that the strips have great enough Young's moduli to ensure that connectors of small size can produce great stress in response to small displacement but this has increased rather than reduced the production cost of terminals because the need for closer dimensional tolerances resulting from smaller pin widths has required rigorous control not only in mold technology and pressing operations but also over variations in the thickness of strips to be worked upon as well as the residual stress that develops in them. Under these circumstances, it has become necessary to design structures that use strips of small Young's moduli and which undergo large enough displacements to allow for substantial dimensional variations. To meet this need, the wrought copper alloy strips for use as electric terminals are required to have a Young's modulus of 120 kN/mm2 or less, preferably 115 kN/mm2 or less, in the direction where they were wrought and a Young's modulus of 130 kN/mm2 or less, preferably 125 kN/mm2 or less, more preferably 120 kN/mm2 or less in a direction perpendicular to the direction in which they were wrought.

[0006] The above situation has become complicated by the fact that the frequency of mold maintenance accounts for a substantial portion of the production cost. One of the major causes that require mold maintenance is worn mold tools. Since mold tools such as punches, dies and strippers wear as a result of repeated stamping, bending or other press working operations, burring and dimensional inaccuracy will occur in the workpiece in strip form. The effect of the workpiece material itself on the wear of mold tools is by no means negligible and there is a growing need to reduce the likelihood of the workpiece for causing mold wear.

[0007] The strips for use as electric terminals are further required to have high resistance not only to corrosion but also against stress corrosion cracking. Speaking of female terminals, they are subject to thermal loading, so they must also exhibit good stress relaxation performance. Specifically, their stress corrosion cracking life is preferably at least five times, more preferably at least ten times, as long as the value for the conventional strips of class 1 brass (specified by the Japanese Industrial Standard, or JIS) and their percent stress relaxation at 150° C. must be no more than one half the value for the strips of class 1 brass, preferably 25% or less, more preferably 20% or less.

[0008] The class 1 brass is a Sn-free, Cu-30 zn based copper alloy and corresponds to alloy C2600 specified in JIS H3100. It also corresponds to alloy C26000 specified by the CDA (Copper Development Association, USA) and which was used in Comparative Example 2 to be set forth later in this specification.

[0009] Brasses and phosphor bronzes in strip form have heretofore been used as principal connector materials. Brasses are used to make strips at lower cost but, even if they are in temper H08 (spring), they only have a 0.2% yield strength and a tensile strength of about 570 N/mm2 and 640 N/mm2, respectively, thus failing to satisfy the above-mentioned minimum requirements for 0.2% yield strength (≧600 N/mm2) and tensile strength (≧2 650 N/mm2). Brass strips are also poor not only in resistance to corrosion and resistance to stress corrosion cracking but also in stress relaxation performance. Phosphor bronze strips have good balance between strength, resistance to corrosion, resistance to stress corrosion cracking and stress relaxation performance; on the other hand, phosphor bronze strips have only small electrical conductivities (12% IACS for spring phosphor bronze) and an economic disadvantage also results.

[0010] With a view to solving the aforementioned problems, many copper alloys in strip form have been studied, developed and proposed for use as connector materials. Most of them have various elements added in small amounts to copper such that they strike a balance between important characteristics such as strength, electrical conductivity and resistance to stress relaxation. However, their Young's moduli were as high as 120-135 kN/mm2 in the direction where they are wrought (rolled) and in the range of 125-145 kN/mm2 in the perpendicular direction. They could achieve only a small improvement in resistance to stress corrosion cracking (just twice the value for strips of class 1 brass). In addition, their cost was high.

[0011] Turning back to brass and phosphor bronze in strip form, they both have Young's moduli of 110-120 kN/mm2 in the direction where they were wrought and 115-130 kN/mm2 in the perpendicular direction. As already mentioned, the recent trend in the design of connector materials is for reducing their Young's moduli and this has brought about renewed interest in those materials that meet this requirement. Thus, it is strongly desired to develop a strip of copper alloy as an improvement of brass strips that is available at a comparable price to them and which exhibits a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young's modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction in which the alloy was wrought while exhibiting a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to the direction in which it was wrought and which has sufficiently high resistance to stress corrosion cracking that its corrosion cracking life in a 3% NH3 atmosphere is at least five times as long as the value for strips of class 1 brass.

[0012] As typified by brasses, the inclusion of Zn facilitates the production of copper alloys having a good balance between strength, workability and cost. What is more, connector materials are subjected to Sn plating in an increasing number of occasions and the usefulness of copper alloys is enhanced by incorporating Sn since Sn-plated scrap can be recycled as feedstock. From this viewpoint, Cu-Zn-Sn alloys may well be worth attention and known examples are copper alloys having designations ranging from C40000 to C49900 that are specified by the CDA. For example, c42500 is a Cu-9.5 Zn-2.0 Sn-0.2 P alloy and well known as a connector material. C43400 is a Cu-14 Zn-0.7 Sn alloy and used in switches, relays and terminals, though in small amounts. However, little use as connector materials is made of Cu-Zn-Sn alloys having higher Zn contents. This is because increased Zn and Sn levels lower hot workability and various characteristics such as the mechanical ones required by rolled connector materials cannot be developed and, what is more, nothing has been known about the appropriate Zn and Sn levels and the conditions for producing the desired rolled materials.

[0013] Specific examples of copper alloys containing more Zn than C45500 include C43500 (Cu-18 Zn-0.9 Sn), C44500 (Cu-28 Zn-1 Sn-0.05P) and C46700 (Cu-39 Zn-0.8 Sn-0.05 P) and they are wrought into sheets, rods, tubes and other shapes that only find use in musical instruments, ships and miscellaneous goods but not as rolled materials for connectors. In addition, these copper alloy materials fail to satisfy all of the numerical property requirements of connector materials set forth above such as 0.2% yield strength, tensile strength, Young's modulus, electrical conductivity and percent stress relaxation in the direction where the alloy was wrought, as well as 0.2% yield strength, tensile strength and Young's modulus in a direction perpendicular to the direction where the alloy was wrought, plus press formability and resistance to stress corrosion cracking. Particular challenges to these copper alloy materials are how to improve their 0.2% yield strength and resistance to stress corrosion cracking.

SUMMARY OF THE INVENTION

[0014] The present invention has been accomplished under these circumstances and has as an object providing a rolled copper alloy material that has all of the above-mentioned characteristics currently required of materials for connectors and other electrical or electronic components in view of the recent advances in electronics, namely, a copper alloy for use as a connector material that can be manufactured at low cost thanks to the addition of elements less expensive than Cu and which performs well not only in resistance to stress corrosion cracking but also in other qualities including 0.2% yield strength, tensile strength, electrical conductivity, Young's modulus, stress relaxation performance and press formability.

[0015] Another object of the invention is to provide a process for producing the copper alloy.

[0016] As a result of the intensive studies they made in order to attain the above-stated objects, the present inventors found that adding small amounts of Si to Cu—Zn—Sn alloys contributed to refining their microstructures and that forming Si compounds with elements such as Ni, Ti, Cr and Mn also contributed to sufficient refining of the microstructure to achieve an improvement in the 0.2% yield strength of the alloy. Several mechanisms have been proposed to explain stress corrosion cracking and they include an electrochemical phenomenon, adsorption of deleterious ions and the dissolution of the stress promoting slip plane but none of these theories are definitive. The present inventors found that Si or its compounds such as Si—Ni, Si—Ti, Si—Cr and Si—Mn could retard the progress of stress corrosion cracking and realized a marked improvement in that quality. The present inventors also found an optimum composition of Cu—Zn—Sn—Si alloys and optimum conditions for thermo-mechanical treatments that could simultaneously satisfy the characteristics required of rolled copper alloys for use as connector materials.

[0017] Thus, according to the first aspect of the invention, there is provided a copper alloy for use as a connector material having high resistance to stress corrosion cracking that contains 17-32 wt % Zn, Sn and Si, with the balance being copper and incidental impurities, and which has resistance to stress corrosion cracking such that its stress cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass.

[0018] According to the first aspect of the invention, there is also provided a copper alloy for use as a connector material having high resistance to stress corrosion cracking that contains 17-32 wt % Zn, Sn and Si, with the balance being copper and incidental impurities, which has resistance to stress corrosion cracking such that its stress cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass, and which has a 0.2 wt % yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, an electrical conductivity of at least 20% IACS, a Young's modulus of no more than 120 kN/mm2 and a percent stress relaxation of no more than 20%, in the direction where said alloy was wrought.

[0019] Either of the copper alloys described above may consist essentially of 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1):

54 ≦3X+Y≦100  (1)

[0020] where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added.

[0021] According to the first aspect of the invention, there is also provided a copper alloy for use as a connector material having high resistance to stress corrosion cracking that contains 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1):

54 ≦3X+Y≦100  (1)

[0022] where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added, which has resistance to stress corrosion cracking such that its stress cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass, and which has a 0.2 wt % yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, an electrical conductivity of at least 20% IACS, a Young's modulus of no more than 120 kN/mm2 and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought whereas it has a 0.2% yield point of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction.

[0023] Either of the copper alloys described above may further contain at least one element of the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.

[0024] According to the second aspect of the invention, there is provided a process for producing a copper alloy for use as a connector material having high resistance to stress corrosion cracking which comprises the steps of:

[0025] melting a copper alloy that contains 17-32 wt % Zn, Sn and Si, with the balance being copper and incidental impurities;

[0026] cooling the melt over a temperature range from the liquidus line to 600° C. at a rate of at least 50° C./min; and

[0027] subsequently hot rolling the resulting ingot at an elevated temperature of 900° C. or below.

[0028] In this process, the copper alloy to be melted may consist essentially of 17-32 wt % zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1),:

54≦3X+Y≦100  (1)

[0029] where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added.

[0030] According to the second aspect of the invention, there is also provided a process for producing a copper alloy for use as a connector material having high resistance to stress corrosion cracking which comprises the steps of:

[0031] melting an alloy that contains 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1),:

54≦3X+Y≦100  (1)

[0032] where X is the amount in wt % of Z added and Y is the amount in wt % of Sn added;

[0033] cooling the melt over a temperature range from the liquidus line to 600° C. at a rate of at least 50° C./min;

[0034] subsequently hot rolling the resulting ingot at an elevated temperature of 900° C. or below; and

[0035] repeating the process of cold rolling and annealing in a temperature range of 300-650° C. until the as-annealed rolled material has a crystal grain size of no more than 10 &mgr;m.

[0036] According to the second aspect of the invention, there is also provided a process for producing a copper alloy for use as a connector material having high resistance to stress corrosion cracking which comprises the steps of:

[0037] melting an alloy that contains 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being Cu and incidental impurities, provided that Zn and Sn satisfy the following relation (1):

54≦3X+Y≦100  (1)

[0038] where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added;

[0039] cooling the melt over a temperature range from the liquidus line to 600° C. at a rate of at least 50° C./min;

[0040] subsequently hot rolling the resulting ingot at an elevated temperature of 900° C. or below;

[0041] repeating the process of cold rolling and annealing in a temperature range of 300-650° C. until the as-annealed rolled material has a crystal grain size of no more than 10 &mgr;m; and

[0042] further performing cold rolling for a reduction ratio of at least 30% and low-temperature annealing at 450° C. or below so that the rolled material has a stress corrosion cracking property such that the corrosion cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass, and that it has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young's modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought whereas the rolled material has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction.

[0043] In either of the processes described above, said copper alloy may further contain at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.

DETAILED DESCRIPTION OF THE INVENTION

[0044] The present invention is described below in detail.

[0045] The feedstock to be melted and subsequently cast consists essentially of 17-32 wt % Zn, Sn and Si. Preferably, it consists essentially of 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfies the relation 54≦3X+Y≦100 (where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added). The master alloy may optionally be used.

[0046] If scrap having a surface Sn coat, in particular chips resulting from stamping on a press, are used as the feedstock, they are preferably melted after a preliminary heat treatment is performed in air atmosphere or an inert atmosphere at a temperature of 300-600° C. for 0.5-24 hours. If the temperature for the heat treatment is below 300° C., the pressing oil adhering to the chips is not completely burnt; what is more, the moisture that has been adsorbed during storage is not fully dried and if the melting operation is subsequently initiated by rapid temperature elevation, the moisture is decomposed to evolve hydrogen gas which is taken up by the melt to generate blow holes in the ingot formed by subsequent casting.

[0047] If the preliminary heat treatment is done at a temperature higher than 600° C., oxidation proceeds so rapidly as to induce dross formation. If dross forms, the melt becomes viscous and its castability decreases. Therefore, the temperature for the preliminary heat treatment of the feedstock to be melted is specified to lie between 300 and 600° C. If this heat treatment lasts for less than 0.5 hours, combustion of the pressing oil and drying of the moisture are accomplished only incompletely. If the time of the heat treatment is longer than 24 hours, the parent metal Cu diffuses in the Sn surface coat, where it oxidizes to form a Cu—Sn—O type oxide that is not only a dross former but also an economic bottleneck. Therefore, the time of the preliminary heat treatment of the feedstock is specified to lie between 0.5 and 24 hours. The preliminary heat treatment will bring about satisfactory results if it is performed in air atmosphere but providing an inert gas seal is preferred for the purpose of preventing oxidation. However, some disadvantage will result from the use of a reducing gas since at elevated temperatures, the moisture decomposes to evolve hydrogen gas that is taken up by the melt to diffuse in it.

[0048] After melting, the feedstock is cast. Casting is desirably done by the continuous process which may be either vertical or horizontal, provided that the melt is cooled over a temperature range from the liquidus line to 600° C. at a rate of at least 50° C./min. If the cooling rate is less than 50° C./min, segregation of Zn and Sn is highly likely to occur at grain boundaries and the efficiency of the subsequent hot working step decreases to lower the yield. The cooling rate need only be controlled over the temperature range between the liquidus line and 600° C. There is no sense of controlling the cooling rate at temperatures higher than the liquidus line; below 600° C., the duration of cooling in the casting process is so short that it is insufficient to cause excess segregation of Zn and Sn at grain boundaries. Therefore, the cooling rate suffices to be controlled over the range from the liquidus line to 600° C.

[0049] After casting the melt into an ingot, hot rolling is performed. The heating temperature in the hot rolling step should not be higher than 900° C. Above 900° C., intergranular segregation of Zn and Sn and generation of the second-phase grains cause hot cracking which, in turn, leads to a lower yield. By performing hot rolling at temperatures of 900° C. and below, not only the microsegregations that occurred during the casting step but also the cast structure will disappear and the resulting rolled material has a homogeneous structure which contains Zn, Sn and Si in he amounts defined for the copper alloy according to the first aspect of the invention. Preferably, hot rolling is performed at a temperature of 870° C. or below. The crystal grains in the hot rolled material are desirably sized to 25 &mgr;m or less. If the crystal grain size exceeds 25 &mgr;m, the latitude in control over the reduction ratio for the subsequent cold rolling and the conditions for the annealing that follows is so small that the slightest departure may potentially produce mixed crystal grains, leading to deteriorated characteristics.

[0050] After hot rolling, the surface of the material may be planed as required. Subsequently, cold rolling and annealing in the temperature range of 300-650° C. are repeated until the crystals in the as-annealed material have a grain size of no more than 20 &mgr;m. Below 300° C., it takes an uneconomically prolonged time to control the crystal grains; above 650° C., the crystal grains become coarse in a short time. If the size of the crystal grains in the as-annealed material exceeds 20 &mgr;m, mechanical characteristics, in particular 0.2% yield strength, and workability deteriorate. Preferably, the crystal grain size is reduced to 15 &mgr;m or below, more preferably 10 &mgr;m or below.

[0051] The thus annealed material is subjected to cold rolling for a reduction ratio of at least 30% and low-temperature annealing at 450° C. or below so as to produce a rolled copper alloy material that has a. 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young's modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where the alloy was wrought (rolled) whereas the rolled material has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction. If the reduction ratio in cold rolling is less than 30%, the improvement in strength that is achieved by work hardening is insufficient to achieve the desired improvement in mechanical characteristics. The reduction ratio is preferably at least 60%. Low-temperature annealing is necessary to improve 0.2% yield strength, tensile strength, spring limit value and stress relaxation performance. Beyond 450° C., so large a heat capacity is applied that the workpiece softens in a short time. Another difficulty is that variations in the characteristics of the rolled material are prone to occur in both a batch and a continuous system. Hence, low-temperature annealing should be performed at temperatures not higher than 450° C.

[0052] We now describe in detail the alloying elements in the thus obtained rolled copper alloy material for use as a connector material. Zn: Zinc (Zn) is desirably added in large amounts since it contributes to enhanced strength and spring quality and is available at a lower price than cu. If its addition exceeds 32 wt %, the second-phase grains appear in the presence of Sn, causing significant drop in workability, in particular, hot workability. The crystal structure of brasses containing 17-32 wt % of Zn is solely composed of &agr;-grains (solid solution) having good workability but if the Zn level is increased beyond 32 wt %, &bgr;-grains appear as the second phase to deteriorate workability, in particular, hot workability. Also affected are resistance to corrosion and resistance to stress corrosion cracking. Platability and solderability which are sensitive to moisture and heat are also deteriorated. If the addition of Zn is smaller than 17 wt %, strength and spring quality that are typified by 0.2% yield strength and tensile strength are insufficient and Young's modulus increases. What is more, if scrap that was surface treated with Sn is recycled as the feedstock, the resulting melt will occlude an increased amount of hydrogen gas to produce an ingot in which blow holes are highly likely to occur. Since Zn is an inexpensive element, using less than 17 wt % of it is an economical disadvantage. For these reasons, the Zn level is preferably specified to range from 17 to 32 wt %. A more preferred range is from 23 to 28 wt %. The small range for the Zn level is one of the basic requirements of the present invention. Sn: Tin (Sn) has the advantage that it need be used in a very small amount to be effective in improving mechanical characteristics such as strength (e.g. 0.2% yield strength and tensile strength) and elasticity without increasing Young's modulus. Materials having a surface Sn coat such as tin plate can be put into a recycle path and this is another reason why incorporating Sn is preferred. However, if the Sn content increases, electrical conductivity drops sharply and the second-phase grains are highly likely to appear in the presence of Zn, causing significant drop in hot workability. In order to ensure the desired hot workability and an electrical conductivity of at least 20% IACS, the addition of Sn should not exceed 4.5 wt %. If the addition of Sn is less than 0.1 wt %, it is difficult to achieve the intended improvement in mechanical characteristics and chips or the like that result from the pressing of tin-plated or otherwise tin-coated scrap are difficult to use as the feedstock. Therefore, the content of Sn is preferably specified to range from 0.1 to 4.5 wt %, more preferably from 0.6 to 1.4 wt %. Si: Silicon (Si) has the advantage that it need be used in only a small amount to be effective in improving mechanical characteristics such as strength (e.g. 0.2% yield strength and tensile strength) and elasticity without increasing Young's modulus. Silicon also has the advantage that it binds to other alloying elements such as Ni, Ti, Cr and Mn and forms compounds, thereby refining the crystal grains and contributing to improving the aforementioned mechanical characteristics. As a further advantage, Si and its compounds with Ni, Ti, Cr, Mn, etc. prevent the segregation of Zn to aggregation defects that will be formed in the rolling step, thereby providing improved resistance to stress corrosion cracking. However, if the Si content increases, electrical conductivity drops sharply and the second-phase grains which are not the a-phase grains appear in the presence of Zn, causing significant drop in hot workability. In order to ensure the desired hot workability and an electrical conductivity of at least 20% IACS, the addition of Si should not exceed 2.0 wt %. If the Si content is less than 0.01 wt %, there will be no improvement in stress corrosion cracking resistance and mechanical characteristics and chips or the like that result from the pressing of tin-plated or otherwise tin-coated scrap are difficult to use as the feedstock. Therefore, the content of Si is preferably specified to range from 0.01 to 2.0 wt %, more preferably from 0.2 to 1.0 wt %.

[0053] If the elements specified above are contained in the specified amounts and if Zn and Sn satisfy the following relation (1):

54≦3X+Y≦100  (1)

[0054] where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added, the Zn- and Sn-rich phases that precipitate at grain boundaries under elevated temperatures such as those which are encountered during casting or hot rolling can be effectively controlled to produce a rolled copper alloy material that has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young's modulus of no more than 120 kN/mm2 and an electrical conductivity of at least 20% IACS in the direction where the alloy was wrought, and which has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction. This copper alloy material has the other characteristics required for use as connector materials, as exemplified by high resistance to corrosion and high resistance to stress corrosion cracking (having a cracking life in 3% NH3 vapor which is at least five times the value for class 1 brass) and good stress relaxation performance (a percent stress relaxation of no more than 20%, or the percent stress relaxation at 150° C. being no more than one half the value for class I brass and paralleling phosphor bronze), as well as efficient stamping on a press.

[0055] The contents of Zn and Sn more preferably satisfy the following relation (2):

75≦3X+Y≦90  (2)

[0056] The content of S as an impurity is desirably held to a minimum. Even a small amount of s will markedly reduce the working capacity, or deformability, in hot rolling. Two typical sources for the entrance of S is scrap that has been plated with tin in a sulfate bath and oils for working such as pressing; controlling the value of S content is effective for preventing cracking in the process of hot rolling. In order to have this effect come into being, S should not be present in an amount greater than 30 ppm, preferably no more than 15 ppm.

[0057] Besides Zn, Sn and Si, a fourth alloying element may be added and it is at least one element of the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of these elements being 0.01-5 wt %.

[0058] These elements either independently or, as already mentioned, in the form of compounds with Si can enhance strength and resistance to stress corrosion cracking without substantial deterioration in electrical conductivity, Young's modulus and machinability. If the ranges for the contents of the respective elements are not observed, the stated effect is not attained or, alternatively, disadvantages will result in various aspects such as resistance to stress corrosion cracking, hot workability, cold workability, press formability, electrical conductivity, Young's modulus and cost.

[0059] Connector materials made of the thus obtained rolled material may optionally be subjected to surface treatments to provide a cu underplate 0.3 -2.0 &mgr;m thick and a Sn plate 0.5-5.0 &mgr;m thick before they are put to service. If the Cu underplate is thinner than 0.3 &mgr;m, it is by no means effective in preventing the Zn in the alloy from diffusing into the Sn surface coat and to the surface where it is oxidized to increase contact resistance while reducing solderability. If the Cu underplate is thicker than 2.0 &mgr;m, its effect is saturated and there is no economic advantage. The Cu underplate need not be solely made of pure copper but may be composed of a copper alloy such as Cu—Fe, Cu—Ni, Cu—Ni—P, Cu—Zn or Cu—Cr.

[0060] If the Sn surface coat is thinner than 0.5 &mgr;m, the desired resistance to corrosion, particularly to hydrogen sulfide, is not obtained. If the Sn surface coat is thicker than 5.0 &mgr;m, its effect is saturated and an economic disadvantage will simply result. To secure uniformity in film thickness and economy, the surface treatments for providing the Cu underplate and the Sn surface coat are preferably performed by electroplating. The Sn surface coat may be reflowed to improve its gloss. This treatment is also effective as a means of preventing Sn whiskers.

[0061] The rolled material thus obtained is pressed into electric terminals, which may subsequently be heat treated at a temperature of 100-280° C. for a duration of 1-180 minutes. This heat treatment is not only effective for improving on the spring limit value and stress relaxation performance that have deteriorated as the result of press working but also instrumental to the prevention of whiskers. Below 100° C., these effects of the heat treatment are not fully attained; above 280° C., diffusion and subsequent oxidation not only increase the contact resistance but also lower the solderability and workability. If the duration of the heat treatment is shorter than 1 minute, its effects are not fully attained; if it continues longer than 180 minutes, diffusion and subsequent oxidation bring about the unwanted results just mentioned above and, in addition, there is no economic advantage.

[0062] The following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting.

EXAMPLE 1

[0063] Copper alloy sample Nos. 1-7 having the compositions (wt %) shown in Table 1 were melted at temperatures 70° C. higher than their liquidus lines, fed into a small vertical continuous casting machine and cast into ingots measuring 30×70×1000 (mm). The rate of cooling from the liquidus line to 600° C. was adjusted to be in great excess of 50° C./min by controlling the primary cooling with the mold and the secondary cooling with a shower of water.

[0064] The ingots were heated to 800-840° C., hot rolled to strips in a thickness of 5 mm and checked for surface or edge cracks to evaluate their hot workability. The samples are rated O if no cracks are found under examination with an optical microscope (x50) after pickling; otherwise, rating X is given. Hot rolling was allowed to end at about 600° C. and by subsequent quenching, the size of the crystal grains in the as-rolled strips was controlled to about 25 &mgr;m. The strips were then cold rolled to a thickness of 1 mm and annealed at temperatures of 450-520° C. so that the crystal grain size was adjusted to about 10 &mgr;m. After pickling, the strips were cold rolled to a thickness of 0.25 mm and low-temperature annealed at 250° C. in the final step.

[0065] From each of thus produced strips, test pieces were sampled and measured for 0.2% yield strength, tensile strength, Young's modulus, electrical conductivity, percent stress relaxation and stress corrosion cracking life. The first three parameters were measured by the test methods described in JIS Z2241, provided that small (70 mm long) test pieces were used for measurements in a direction perpendicular to the rolling direction. Electrical conductivity was measured by the method described in JIS H0505. In the stress relaxation test, a bending stress representing 80% of the 0.2% yield strength was applied to the surface of each sample, which was held at 150° C. for 500 hours to measure the amount of bend. The percent stress relaxation was calculated by the following equation (3):

Stress relaxation (%)=[(L1−L2)/(L1−L0)]×100   (3)

[0066] where L0: length (mm) of the jig

[0067] L1: initial length (mm) of a sample

[0068] L2: horizontal distance (mm) between ends of the bent sample

[0069] In the stress corrosion cracking test, a bending stress representing 80% of the 0.2% yield strength was applied to the surface of each sample, which was exposed and held in a desiccator containing 3% aqueous ammonia. At 10-min intervals, each sample was checked for cracks. The point in time 10 minutes before any crack was observed was designated the “stress corrosion cracking life”.

[0070] The results of measurements are shown in Table 2 which also show the result of evaluation of workability. 1 TABLE 1 Alloy composition (wt %) Value of Sample relation No. Zn Sn (1) Si Others Example 1 1 25.03 0.80 75.89 0.074 — 2 25.11 0.75 76.08 0.180 — 3 25.18 0.78 76.32 0.290 — 4 24.80 0.75 75.15 0.460 — 5 25.00 0.78 75.78 0.210 Ni 0.21 6 25.31 0.85 76.78 0.190 Ti 0.05 Mn 0.36 7 24.81 0.75 75.18 0.180 Fe 0.18 Cr 0.08 Comparative 8 24.81 0.06 74.49 — — Example 1 9 34.26 0.87 103.7 — — Comparative 10 29.8 — — — — Example 2 11 — 8.11 — — P 0.19

[0071] 2 TABLE 2 0.2% yield Tensile Young's strength strength Modulus Stress (N/mm2) (N/mm2) (kN/mm2) Electri- relaxa- Rolling Rolling Rolling cal Stress corrosion tion Sam- direction direction direction conduc- cracking life (%, Hot ple Perpendicular Perpendicular Perpendicular tivity (min, in) 150° C. × work- No. direction direction direction (% IACS) 3% NH3 atmosphere) 1000 h) ability Example 1 1 670 755 108 24.7 1520 10.8 ◯ 710 829 118 5 times the value for class 1 brass 2 741 817 108 23.2 1820 10.6 ◯ 825 935 117 6 times the value for class 1 brass 3 735 804 110 21.7 2030 10.2 ◯ 822 910 118 6 times the value for class 1 brass 4 758 818 109 20.1 2410 10.3 ◯ 829 930 117 8 times the value for class 1 brass 5 741 816 107 22.8 1900 10.9 ◯ 828 936 117 6 times the value for class 1 brass 6 742 818 108 23.8 1880 10.6 ◯ 825 938 117 6 times the value for class 1 brass 7 743 819 106 25.2 1980 10.8 ◯ 822 935 118 6 times the value for class 1 brass Compar- 8 648 712 118  29.82  980 12.0 ◯ ative 690 800 125 3 times the value Example for class 1 brass 1 9 680 753 105  24.88  950 12.1 X 720 849 119 3 times the value for class 1 brass Compar- 10  570 640 112 27.2  300 48.9 — ative 550 650 119 Example 11  725 784 116 13.1 — 13.0 — 2 808 911 128

[0072] As can be seen from Table 2, the rolled strips of copper alloy sample Nos. 1-7 according to the present invention had good enough workability to allow for efficient manufacture of electric and electronic components, exhibited good balance between 0.2% yield strength, tensile strength, Young's modulus and electrical conductivity, and featured satisfactory stress relaxation performance and high resistance to stress corrosion cracking. Obviously, the addition of Si was effective in improving 0.2% yield strength and resistance to stress corrosion cracking. Hence, the rolled strips of the copper alloy according to the present invention have excellent characteristics for use as the materials of connectors and other electrical or electronic components.

[0073] Comparative Example 1

[0074] Comparative copper alloy sample Nos. 8 and 9 having compositions outside the invention ranges shown in Table 1 were cast and worked under the same conditions as in Example 1 to produce rolled strips. From each of the strips, test pieces were sampled and measured for the same items by the same methods as in Example 1. The results are also shown in Table 2.

[0075] The rolled strips of comparative alloy sample No. 8 containing less Sn than does the copper alloy of the invention was inferior in Young's modulus, 0.2% yield strength, tensile strength, stress relaxation performance and resistance to stress corrosion cracking. The rolled strips of comparative sample No. 9 which contained Zn in such a large amount as to exceed the upper limit its relation to the Sn level [see relation (1)] was inferior in hot workability and suffered the problem of cost increase due to lower yield. It was also unsatisfactory in terms of resistance to stress corrosion cracking.

[0076] Comparative Example 2

[0077] Test pieces were sampled from rolled strips of commercial class 1 brass (C26000-H08) and spring phosphor bronze (C52100-H08) and measured for 0.2% yield strength, tensile strength, Young's modulus, electrical conductivity, stress corrosion cracking life and percent stress relaxation by the same methods as in Example 1. No evaluation was made of hot workability. The commercial samples used in this comparative example had the temper grade H08 (spring) which was of higher strength than any other grades of the same composition. The compositions of class 1 brass and spring phosphor bronze are identified in Table 1 as sample Nos. 10 and 11, respectively.

[0078] The results of measurements are shown in Table 2.

[0079] As one can see from Table 2, the copper alloy of the invention is improved, particularly in terms of 0.2% yield strength, tensile strength, resistance to stress corrosion cracking and stress relaxation performance, as compared with class 1 brass (sample No. 10) which is a representative material for electrical or electronic components such as connectors. It is also superior to spring phosphor bronze (sample No. 11) in terms of Young's modulus and electrical conductivity. Spring phosphor bronze has a problem with electrical conductivity. Since it contains as much as 8% of expensive Sn, the materials cost of spring phosphor bronze is liable to frequent increases. In addition, being not amenable to hot rolling, spring phosphor bronze can be produced by only limited methods and it is less advantageous in terms of total cost including production cost.

[0080] Therefore, one may safely conclude that the copper alloy of the invention has practical superiority over the existing brasses and phosphor bronzes.

EXAMPLE 2

[0081] Copper alloy sample No. 2 (Cu-25.11 Zn-0.75 Sn-0.18 Si; see Table 1) of the composition within the scope of the invention was subjected to continuous casting under varying conditions for primary and secondary cooling at varied withdrawing speeds. The cooling rate was measured with thermocouples which were eventually cast into ingots. The alloy had a liquidus line of about 950° C. and the average rate of cooling from this temperature to 600° C. was measured.

[0082] The ingots were subsequently heated to 840° C. and subjected to 9 passes of hot rolling for a reduction ratio of about 15% per pass; the hot rolled sheet metals were checked for surface and edge cracks. The sheet metals from the ingots cast at average cooling rates of 50° C./min and above experienced no cracking at all during hot rolling. In particular, the sheet metals from the ingots cast at average cooling rates of 80° C./min and above had a greater latitude in the conditions for hot rolling in terms of both temperature and reduction ratio. On the other hand, the sheet metals from the ingots cast at cooling rates slower than 50° C./min experienced cracking during hot rolling; it was therefore clear that even an alloy composition within the scope of the invention may sometimes experience cracking during hot rolling to suffer occasional decreases in yield if the average cooling rate in the casting process is not appropriate.

EXAMPLE 3

[0083] Rolled strips of sample No. 1 prepared in Example 1 according to the invention were provided a Cu underplate 0.45 &mgr;m thick and a reflowed Sn plate 1.2 &mgr;m thick. The strips were worked into spring-loaded female terminals in box shape and heat treated at 190° C. for 60 minutes. These terminals and non-heat treated (but as-pressed) test pieces of the same sample were each fitted with a male terminal and the assemblies were exposed and held in a thermostatic vessel at 125° C. for 330 hours. The low-voltage low-current resistance and contact load were measured both at the initial stage and after exposure in the thermostatic vessel. The results are shown in Table 3. 3 TABLE 3 Low-voltage low-current resistance (m&OHgr;) Contact load (N) After After Sample No. 2 Initial exposure Initial exposure With heat 1.90 5.33 7.88 7.11 treatment Without heat 1.79 6.87 7.69 5.92 treatment (as-pressed)

[0084] As can be seen from Table 3, heat treatment of press-formed terminals is effective for preventing the increase in low-voltage low-current resistance and the decrease in contact load that would otherwise occur after standing at elevated temperatures. This contributes to improving the reliability of terminals made from the copper alloy according to the first aspect of the invention which is produced by the manufacturing process according to its second aspect.

EXAMPLE 4 and Comparative Example 3

[0085] Rolled strips were fabricated from sample No. 2 (see Table 1) of the invention (in Example 4) and from comparative sample Nos. 8 and 9 (in Comparative Example 3). The strips were then shaped into sawtoothed terminals (tooth-to-tooth pitch: 1.25 mm) by stamping on a press using a superhard punch and a die made of tool steel. The clearance was adjusted to 8% of the strip thickness.

[0086] After 106 shots of stamping operation, the development of burrs was evaluated by examining the stamped surfaces with an optical microscope in both the rolling direction and the direction perpendicular to it. The terminals made from sample No. 2 had no burrs higher than 10 &mgr;m; on the other hand, the terminals made from comparative sample Nos. 8 and 9 had burrs higher than 20 &mgr;m, particularly in areas parallel to the rolling direction.

[0087] Thus, it can be seen that alloy sample No. 2 of the invention is also advantageous for preventing mold wear.

[0088] As is clear from the foregoing description, the rolled copper alloy material according to the first aspect of the invention as well as the rolled copper alloy material produced by the process according to the second aspect of the invention are superior to rolled copper material.; of the conventional brasses and phosphor bronzes not only in terms of the balance between 0.2% yield strength, tensile strength, electrical conductivity and Young's modulus but also in stress relaxation performance and resistance to stress corrosion cracking, as well as in press formability. What is more, those rolled copper alloy materials can be produced at low cost. Hence, they are an optimum alternative to brasses and phosphor bronzes as materials for connectors and other electrical or electronic components.

Claims

1. A copper alloy for use as a connector material having high resistance to stress corrosion cracking that contains 17-32 wt % Zn, Sn and Si, with the balance being copper and incidental impurities, and which has resistance to stress corrosion cracking such that its stress cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass.

2. A copper alloy for use as a connector material having high resistance to stress corrosion cracking that contains 17-32 wt % Zn, Sn and Si, with the balance being copper and incidental impurities, which has resistance to stress corrosion cracking such that its stress cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass, and which has a 0.2 wt % yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, an electrical conductivity of at least 20 % IACS, a Young's modulus of no more than 120 kN/mm2 and a percent stress relaxation of no more than 20%, each in rolling direction.

3. The copper alloy according to claim 1 or 2, which consists essentially of 17-32 wt % Zn, 0.1-41.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1):

54≦3X+Y≦100  (1)

where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added.

4. A copper alloy for use as a connector material having high resistance to stress corrosion craking that contain 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1):

54≦3X+Y≦100  (1)

where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added, which has resistance to stress corrosion cracking such that its stress cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass, and which has a 0.2 wt % yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, an electrical conductivity of at least 20% IACS, a Young's modulus of no more than 120 kN/mm2 and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought whereas it has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction.

5. The copper alloy according to claim 1 or claim 2, which further contains at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.

6. The copper alloy according to claim 3, which further contains at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.

7. The copper alloy according to claim 4, which further contains at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.

8. A process for producing a copper alloy for use as a connector material having high resistance to stress corrosion cracking which comprises the steps of:

melting a copper alloy that contains 17-32 wt % Zn, Sn and Si, with the balance being copper and incidental impurities;

cooling the melt over a temperature range from the liquidus line to 600° C. at a rate of at least 50° C./min; and

subsequently hot rolling the resulting ingot at an elevated temperature of 900° C. or below.

9. The process according to claim 8, wherein said copper alloy to be melted consists essentially of 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1),:

54≦3X+Y≦100  (1)

where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added.

10. A process for producing a copper alloy for use as a connector material having high resistance to stress corrosion cracking which comprises the steps of:

melting an alloy that contains 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being copper and incidental impurities, provided that Zn and Sn satisfy the following relation (1),:

54≦3X+Y≦100  (1)

where X is the amount in wt % of Z added and Y is the amount in wt % of Sn added;

cooling the melt over a temperature range from the liquidus line to 600° C. at a rate of a,: least 50° C./min;

subsequently hot rolling the resulting ingot at an elevated temperature of 900° C. or below; and

repeating the process of cold rolling and annealing in a temperature range of 300-650° C. until the as-annealed rolled material has a crystal grain size of no more than 10 &mgr;m.

11. A process for producing a copper alloy for use as a connector material having high resistance to stress corrosion cracking which comprises the steps of:

melting an alloy that contains 17-32 wt % Zn, 0.1-4.5 wt % Sn and 0.01-2.0 wt % Si, with the balance being Cu and incidental impurities, provided that 2n and Sn satisfy the following relation (l):

54≦3X+Y≦100  (1)

where X is the amount in wt % of Zn added and Y is the amount in wt % of Sn added;

cooling the melt over a temperature range from the liquidus line to 600° C. at a rate of at least 50° C./min;

subsequently hot rolling the resulting ingot at an elevated temperature of 900° C. or below;

repeating the process of cold rolling and annealing in a temperature range of 300 -650° C. until the as-annealed rolled material has a crystal grain size of no more than 10 &mgr;m; and

further performing rolling for a reduction ratio of at least 30% and low-temperature annealing at 450° C. or below so that the rolled material has a stress corrosion cracking property such that the corrosion cracking life in a 3% NH3 atmosphere is at least five times as long as the value for class 1 brass, and that it has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young's modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought whereas the alloyed material has a 0.2% yield strength of at least 650N/mm2, a tensile strength of at least 700 N/mm2 and a Young's modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction.

12. The process according to claim 8, wherein said copper alloy to be melted further contains at: least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-10 w% Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.

13. The process according to claim 9, wherein said copper alloy to be melted further contains at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01- 10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.

14. The process according to claim 10 or claim 11, wherein said copper alloy to be melted further contains at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01- 2 wt % Mg, 0.01-2 wt % Zr, 0.01- 1wt % Ca, 0.01- 10 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.