JP7525322B2 - Cu-Ni-Co-Si copper alloy sheet material, its manufacturing method and conductive spring member - Google Patents

Description

The present invention relates to a high-strength Cu-Ni-Co-Si copper alloy sheet material with excellent durability and a manufacturing method thereof. It also relates to a conductive spring member using the Cu-Ni-Co-Si copper alloy sheet material. Note that the "Cu-Ni-Co-Si copper alloy" referred to in this specification includes a "Cu-Ni-Si copper alloy" that contains only Ni out of Ni and Co, and a "Cu-Co-Si copper alloy" that contains only Co.

Among copper alloys, Cu-Ni-Co-Si copper alloys have a relatively good balance between strength and conductivity, and are useful for conductive spring members such as connectors. In recent years, as electronic devices become smaller and lighter, connector materials have become thinner, and copper alloy sheets that are the raw materials for these connectors are required to have good conductivity and high strength. On the other hand, in order to improve the reliability of conductive spring members such as connectors, it is important to improve durability such as "fatigue resistance" and "sag resistance". In the past, fatigue resistance was generally evaluated based on fatigue strength when repeated stress was applied 10 ⁷ times. However, in order to further improve the reliability of conductive spring members for vehicle use, it has become desirable to have not only high fatigue strength at 10 ⁷ times, but also performance that maintains high fatigue strength even at 10 ⁸ times. Conventionally, Cu-Ni-Co-Si copper alloy sheets have not been able to meet such high-level durability needs. In addition, it is also important to evaluate the sag resistance performance using test pieces with a shape similar to that of a mounted part having a bent part.

Patent Document 1 describes a Cu-Ni-Co-Si copper alloy sheet material in which the strength, bending workability, and fatigue strength at high cycles are improved by controlling the half-value width of the (220) plane. The fatigue strength is evaluated at 10 ⁷ times. Patent Document 2 describes a Cu-Ni-Co-Si copper alloy sheet material in which bending workability, strength, electrical conductivity, and stress relaxation properties are improved by utilizing a method for optimizing the proportion of Cube orientation. Patent Document 3 describes a Cu-Ni-Co-Si copper alloy sheet material in which high strength is achieved by utilizing a method for controlling the cooling rate in solution treatment. Patent Document 4 describes a Cu-Ni-Co-Si copper alloy sheet material in which sag resistance is improved by controlling the size and number density of precipitates. However, in the manufacturing processes described in these documents, it is difficult to maintain a high fatigue strength at 10 ⁸ times, and further improvement in reliability is desired regarding durability under severe conditions.

JP 2012-224898 A JP 2012-246549 A JP 2015-187308 A International Publication No. 2010/064547

The present invention aims to provide a high-strength Cu-Ni-Co-Si-based copper alloy sheet material having excellent durability, in which the fatigue strength at ¹⁰⁸ cycles evaluated in an alternating-swing fatigue test and the resistance to settling evaluated using a test piece having a shape similar to that of a mounted part having a bent portion are significantly improved, and to provide a conductive spring member using the same.

In order to increase the strength of Cu-Ni-Co-Si copper alloys, it is common to utilize a precipitation strengthening mechanism using fine (Ni,Co)-Si precipitates with a particle size of about 5 to 10 nm. The inventors have found that it is possible to significantly improve the durability by utilizing this precipitation strengthening mechanism and creating a structure in which clusters, which are aggregates of solute atoms, are dispersed. Such a structure can be realized by a process that combines two stages of aging treatment at low and high temperatures, cold rolling, and low-temperature annealing, based on the technical idea of clustering solute atoms via a structure in which some precipitates are considered to be in pseudo-solid solution.
In order to achieve the above object, the present specification discloses the following invention.

[1] A chemical composition consisting of, in mass%, the sum of Ni and Co: 1.00 to 6.00%, Si: 0.30 to 1.40%, Ag: 0 to 0.30%, Al: 0 to 1.00%, B: 0 to 0.20%, Be: 0 to 0.15%, Cr: 0 to 0.50%, Fe: 0 to 1.00%, Mg: 0 to 0.50%, Mn: 0 to 1.00%, P: 0 to 0.20%, S: 0 to 0.20%, Sn: 0 to 1.00%, Ti: 0 to 0.50%, Zn: 0 to 1.00%, Zr: 0 to 0.30%, the balance Cu and unavoidable impurities, and a cluster (aggregate) of solute atoms specified in the following (A) observed by a three-dimensional atom probe field ion microscope is 1.0 x 10 ²³ / m Copper alloy sheet material containing ³ or more in number density.
(A) A cluster containing one or more atoms of Ni, Co, and Si, in which when Ni, Co, and Si atoms are collectively referred to as "atom X", the distance between the atomic centers of the closest atoms X is kept at 0.40 nm or less, and the total number of atoms X is in the range of 10 to 400.
[2] The copper alloy sheet material according to the above [1], having a tensile strength of 750 MPa or more in a direction parallel to the rolling direction.
[3] The copper alloy sheet material according to the above [1] or [2], wherein in a nominal stress-nominal strain curve obtained by a tensile test at a strain rate of 1×10 ⁻⁵ (/s) using a JIS No. 5 tensile test piece with a longitudinal direction parallel to the rolling direction, serrations having 10 or more stress drop portions, in which the stress drop width from when the stress starts to drop until when the stress starts to rise again is 1.0 MPa or more, are observed during the period from when the 0.2% proof stress is reached until the fracture.
[4] In mass%, the total of Ni and Co: 1.00 to 6.00%, Si: 0.30 to 1.40%, Ag: 0 to 0.30%, Al: 0 to 1.00%, B: 0 to 0.20%, Be: 0 to 0.15%, Cr: 0 to 0.50%, Fe: 0 to 1.00%, Mg: 0 to 0.50%, Mn: 0 to 1.00%, P: 0 to 0.20%, S: 0 to 0.20%, Sn: 0 to 1.00%, Ti: 0 to 0.50%, Zn: 0 to 1.00%, Zr: 0 to 0.30%, the balance Cu and unavoidable impurities are contained in the intermediate product plate material of the copper alloy having a chemical composition consisting of, by the steps including solution treatment, aging treatment, finish cold rolling, and low temperature annealing in the above order,
The solution treatment is performed under the following conditions: heating from 500°C to 800°C at an average heating rate of 10 to 150°C/s, holding at 800 to 1050°C for 10 to 600 seconds, and cooling from 800°C to 500°C at an average cooling rate of 50 to 200°C/s;
The aging treatment is carried out under two-stage conditions: first, at 200 to 400°C for 4 to 10 hours, and then at 425 to 550°C for 1 to 10 hours.
The finish cold rolling is performed under the condition that the rolling ratio is 5% or more.
The low-temperature annealing is performed under the following conditions: the temperature is increased from 150°C to 300°C at an average heating rate of 10 to 150°C/s, the temperature is held at 300 to 500°C for 10 to 300 seconds, and the temperature is cooled from 300°C to 150°C at an average cooling rate of 30 to 150°C/s.
A method for manufacturing copper alloy sheet material.
In the above, after the solution treatment and before the aging treatment, cold rolling may be carried out as necessary.
[5] A conductive spring member using the copper alloy sheet material according to any one of [1] to [3] above as a material.

According to the present invention, remarkable improvements in fatigue resistance and settling resistance have been achieved in high-strength plate materials made of Cu-Ni-Co-Si-based copper alloys.

FIG. 13 is a diagram illustrating a portion of a nominal stress-nominal strain curve obtained by a tensile test in the rolling direction for Example No. 5 of the present invention. FIG. 2 is a cross-sectional view of a sag resistance test specimen (a jack that is a connector part) showing the shape and the position where a load is applied.

[Chemical composition]
Hereinafter, "%" regarding alloy components means "mass %" unless otherwise specified.

Ni and Co form Ni-Co-Si, Ni-Si or Co-Si precipitates. In this specification, precipitates of these composition systems are collectively referred to as "(Ni, Co)-Si precipitates". Among (Ni, Co)-Si precipitates, fine ones with a particle diameter (major axis) of about 5 to 10 nm contribute greatly to improving strength and electrical conductivity. In the present invention, precipitation strengthening by (Ni, Co)-Si precipitates is utilized, and the durability of the material is improved by forming clusters of Ni, Co and Si atoms, which are solute atoms. In order to fully exert these effects, the total content of Ni and Co must be 1.00% or more, and more preferably 2.00% or more. The Ni content is more preferably in the range of 0 to 4.00%, and even more preferably in the range of 1.00 to 4.00%. The Co content is preferably in the range of 0 to 3.50%, and more preferably in the range of 0.5 to 3.5%. On the other hand, if the total content of Ni and Co is excessive, the electrical conductivity decreases, and manufacturing becomes difficult and costs increase. The total content of Ni and Co is limited to 6.00% or less, and may be controlled to 5.00% or less.

Together with Ni and Co, Si leads to the formation of (Ni,Co)-Si precipitates and the formation of solute atom clusters. To fully exert these effects, a Si content of 0.3% or more is required. It is more preferable to ensure a Si content of 0.4% or more. Excessive Si is likely to produce coarse precipitates and cause cracking during hot rolling. The Si content is limited to 1.50% or less. It may also be controlled to 1.20% or less.

Other elements such as Ag, Al, B, Cr, Fe, Mg, Mn, P, S, Sn, Ti, Zn, and Zr can be included as necessary. The content ranges of these elements can be as follows: Ag: 0-0.30%, Al: 0-1.00%, B: 0-0.20%, Be: 0-0.15%, Cr: 0-0.50%, Fe: 0-1.00%, Mg: 0-0.50%, Mn: 0-1.00%, P: 0-0.20%, S: 0-0.20%, Sn: 0-1.00%, Ti: 0-0.50%, Zn: 0-1.00%, and Zr: 0-0.30%.

When adding one or more of Ag, Al, B, Be, Cr, Fe, Mg, Mn, P, S, Sn, Ti, Zn, and Zr, it is more effective to set the total content to 0.01% or more. However, adding a large amount will adversely affect hot or cold workability and will be disadvantageous in terms of cost. It is more preferable to keep the total amount of these optional added elements to 1.0% or less, or 0.5% or less.

[Cluster density]
The copper alloy sheet material of the present invention is structurally characterized in that it contains clusters (aggregates) of solute atoms as defined in (A) below at a number density of 1.0 x ¹⁰²³ /m3 or more as observed by a ^three -dimensional atom probe field ion microscope.
(A) A cluster containing one or more atoms of Ni, Co, and Si, in which when Ni, Co, and Si atoms are collectively referred to as "atom X", the distance between the atomic centers of the closest atoms X is kept at 0.40 nm or less, and the total number of atoms X is in the range of 10 to 400.

By analyzing a three-dimensional atom map based on elemental analysis data by a three-dimensional atom probe field ion microscope, it is possible to identify clusters of a specific size in which atoms of a specific element are close to each other at a specific distance, and to know the density of the clusters. The "maximum separation method" can be applied as the analysis method. The inventors performed analysis by a three-dimensional atom probe field ion microscope for many experimental examples to determine the density of the clusters by the above analysis method, and investigated the relationship between the density of the clusters and the fatigue resistance and sag resistance. As a result, the inventors found that the fatigue resistance and sag resistance of a Cu-Ni-Co-Si-based copper alloy sheet material can be significantly improved by creating ^a structure state in which clusters satisfying the requirement shown in (A) exist at a number density of 1.0 x ¹⁰²³ pieces/m3 or more. It is speculated that this type of cluster exerts an action similar to "sticking hardening" due to chemical interaction between dislocations and solute atoms (the so-called "Suzuki effect"), hindering the movement of dislocations, thereby greatly improving fatigue resistance and sag resistance. In particular, to obtain high fatigue strength, it is more effective for the number density of the clusters specified in (A) above to be 10.0 x ¹⁰²³ pieces/ ^m3 or more. If the total content of Ni and Co in the alloy or the Si content increases, the number density of the clusters specified in (A) above also tends to increase under the same manufacturing conditions. There is no particular limit to the upper limit of the number density, but it is sufficient to adjust it to a range of, for example, 200.0 x ¹⁰²³ pieces/ ^m3 or less.

[Tensile strength, electrical conductivity]
For use in conductive spring members such as connectors, which are required to be small and thin, the tensile strength in the rolling direction is preferably 750 MPa or more, and can be adjusted to 950 MPa or more, or 1100 MPa or more. The electrical conductivity is preferably 20% IACS or more, and more preferably 25% IACS or more.

[Serration]
The occurrence of a phenomenon similar to the above-mentioned "stick hardening" due to the presence of clusters is supported by the occurrence of serrations in the stress-strain curve of a tensile test.
Figure 1 shows an enlarged portion of the nominal stress-nominal strain curve for invention example No. 5, obtained by a tensile test in the rolling direction at a strain rate of ^1x10-5 (/s) using a JIS No. 5 tensile test piece. The serrations in the nominal stress-nominal strain curve are formed by repeated occurrences of "stress drop sections" in which the stress starts to drop and then starts to rise again. Two stress drop sections with stress drop widths of 1.0 MPa or more are seen in Figure 4.

In the Cu-Ni-Co-Si copper alloy sheet material obtained by the manufacturing process described below, which shows a remarkable improvement in fatigue resistance and sag resistance, serrations having 10 or more large "stress drop parts" with a stress drop width of 1.0 MPa or more are observed in the nominal stress-nominal strain curve obtained by a tensile test in the rolling direction using a JIS No. 5 tensile test piece at a strain rate of 1×10 ⁻⁵ (/s) from the time when 0.2% proof stress is reached until fracture. The stress drop width indicates the strength of the interaction between the cluster and the dislocation, and it is presumed that the number of "stress drop parts" is correlated with the density of the cluster. From such a serration phenomenon, it is confirmed that the copper alloy sheet material according to the present invention has a structure state in which the above-mentioned solute atom clusters are dispersed.

[Fatigue strength]
The Cu-Ni-Co-Si based copper alloy sheet material according to the present invention not only has high fatigue strength at ¹⁰⁷ times in an alternating fatigue test, but also has a small decrease in fatigue strength at ¹⁰⁸ times. Specifically, when the fatigue strengths (MPa) at ¹⁰⁷ times and ¹⁰⁸ times are _σ7 and _σ8 , respectively, the sheet material exhibits excellent durability with _σ8 being 200 MPa or more and a damping ratio _σ8 / _σ7 being 0.8 or more.

For example, in the invention example No. 2 shown in the examples described later, _σ7 and _σ8 are 280 MPa and 260 MPa, respectively, and the damping ratio _σ8 / _σ7 is 0.93. On the other hand, in the comparative example No. 46, _σ7 and _σ8 are 260 MPa and 170 MPa, respectively, and the damping ratio _σ8 / _σ7 is 0.65. Such excellent fatigue resistance is considered to be an effect caused by the above-mentioned structure state in which the clusters are dispersed. The densely dispersed clusters hinder the movement of dislocations and suppress the occurrence of striations (slip bands). It is presumed that this hinders the growth of cracks in the fatigue test, resulting in excellent fatigue resistance.

[Resistance to settling]
The Cu-Ni-Co-Si-based copper alloy sheet material according to the present invention exhibits excellent sag resistance in test pieces having a shape similar to that of a mounting part having a bent portion. Here, the results of the sag resistance test performed on the invention example No. 12 and the comparative example No. 38 described later are shown as examples. The sag resistance test was performed as follows.

(Settling resistance test method)
A test piece 1 (a jack, which is a connector part) having the shape shown in Fig. 2 was prepared, one end of the test piece 1 was fixed to a plate surface 3 by a restraining jig 2, and a sag resistance test was performed 100,000 times by repeatedly applying a load P to the maximum height position (referred to as the "top") from the plate surface 3. The test piece had a bent part with the rolling direction as the bending axis, the width of the test piece in the bending axis direction (direction perpendicular to the drawing) was 1.2 mm, the bending radius R of the U-shaped bent part (the part marked with "R" in Fig. 2) was 0.8 mm, and the top height h before the start of the test was 2.85 mm. The top height position before the start of the test was set as the origin, and the pressing jig was used to repeatedly apply a pressing amount of 1.45 mm (constant) downward from the origin with a time per cycle of 15.5 seconds, and the load (N) applied to the pressing jig during the displacement in each cycle was measured.

The thickness of the test pieces was 0.20 mm for both Inventive Example No. 12 and Comparative Example No. 38. When the top height before the test was _h0 (mm) and the top height after the test was _h1 (mm), the amount of settling was expressed as _h0 - _h1 . The amount of settling for Inventive Example No. 12 was 0.20 mm, and the amount of settling for Comparative Example No. 38 was 0.37 mm.

When the "sag" of the material of the conductive spring member progresses, the contact force between the mating member cannot be sufficiently secured, and the connector cannot function. The load curves of the above-mentioned invention example No. 12 and comparison example No. 38 were examined for the second cycle, 50,000 cycles, and 100,000 cycles. As a result, in comparison example No. 38, the load during displacement dropped considerably at 50,000 times, and at 100,000 times, the load during displacement was almost zero. In contrast, in invention example No. 12, the load was maintained at the same level as the initial (second cycle) even at 50,000 times, and at 100,000 times, a load exceeding 0.2 N was observed when a displacement of 1.45 mm was applied, and a significant improvement in sag resistance was observed. Such excellent sag resistance is also thought to be an effect of the structure state in which the clusters are densely dispersed as described above.

[Production method]
The copper alloy sheet material described above can be produced, for example, by the following manufacturing process.
Melting/casting → hot rolling → cold rolling → (intermediate annealing → cold rolling) → solution treatment → (cold rolling before aging) → aging treatment → finish cold rolling → low temperature annealing Although not mentioned in the above steps, facing is performed as necessary after hot rolling, and pickling, polishing, or further degreasing is performed as necessary after each heat treatment. Each step will be explained below.

[Melting and Casting]
The cast pieces may be produced by continuous casting, semi-continuous casting, etc. To prevent oxidation of Si, Mg, etc., it is preferable to carry out the casting in an inert gas atmosphere or in a vacuum melting furnace.

[Hot rolling]
It is preferable to heat the slab at 920 to 1,060° C. for 1 to 10 hours or more, and then to perform hot rolling. The rolling temperature in the final pass is preferably 700° C. or higher. After the hot rolling is completed, it is preferable to rapidly cool the slab by water cooling or the like.

[Cold rolling]
Cold rolling is performed by a conventional method to obtain an intermediate product sheet material for the next step of solution treatment. If necessary, multiple cold rolling steps with intermediate annealing can be performed. The reduction ratio in cold rolling (the reduction ratio after the final intermediate annealing step if intermediate annealing steps are performed) is preferably set in the range of 90.0 to 99.5%.
The rolling ratio from a certain plate thickness t ₀ (mm) to a certain plate thickness t ₁ (mm) is determined by the following formula (1).
Rolling ratio (%) = [(t ₀ -t ₁ )/t ₀ ] × 100 ... (1)

[Solution treatment]
The "intermediate product sheet material" obtained as described above is subjected to solution treatment. The solution treatment is performed under the conditions of increasing the temperature from 500°C to 800°C at an average heating rate of 10 to 150°C/s, holding at 800 to 1050°C for 10 to 600 seconds, and cooling from 800°C to 500°C at an average cooling rate of 50 to 200°C/s. When the solution treatment is performed with such a heat pattern, the aforementioned structure state in which clusters of solute atoms are dispersed can finally be obtained.

[Cold rolling before aging]
Before the aging treatment, cold rolling can be performed as necessary. In this specification, the cold rolling at this stage is called "pre-aging cold rolling". The rolling ratio of the pre-aging cold rolling is preferably set in the range of 50% or less. It may be 35% or less.

[Aging treatment]
Next, aging treatment is performed. The clusters of solute atoms mentioned above are considered to be something like precipitates, and can be regarded as ultrafine precipitates. Although such clusters can be formed to some extent by aging treatment of solution-treated material, it is difficult to form them at high density by aging treatment alone. Therefore, the solution-treated material is first aged to form precipitates, and then the clusters are densified by a process that combines finish cold rolling and low-temperature annealing. Although the mechanism of clustering is not clear at this time, it is thought that after the precipitates are pseudo-solid-solved, the solute atoms that constituted those precipitates probably gather together to form clusters. Therefore, this aging treatment needs to generate fine precipitates that contribute to strength, and also to realize a precipitation form that is thought to contribute to the structural change from "pseudo-solid-solution to clustering" in the subsequent process of finish cold rolling and low-temperature annealing.

The inventors conducted detailed research on aging treatment conditions. As a result, a two-stage temperature holding condition is adopted, in which the first stage is performed at a low temperature and the second stage is performed at a higher temperature. Specifically, the first stage is performed at 200 to 400°C for 4 to 10 hours, and the second stage is performed at 425 to 550°C for 1 to 10 hours. It is efficient to apply a heat pattern in which the temperature is raised to the second stage holding temperature in the furnace after the first stage is held. In the first stage of aging, it is sufficient to hold for 4 to 10 hours at a temperature within a range of, for example, ±20°C (but not outside the range of 200 to 400°C) with respect to the aging temperature T ₁ (°C) set within the range of 200 to 400°C. In the second stage aging, the aging temperature T ₂ (°C) is set in the range of 425 to 550°C, and the temperature is maintained at a temperature within the range of ±20°C (but not outside the range of 425 to 550°C) for 1 to 10 hours. It is more effective to set the difference T ₂ -T ₁ between the aging temperature T ₁ (°C) in the first stage and the aging temperature T ₂ (°C) in the second stage to 50°C or more. It is economical to raise the temperature from the first stage to the second stage quickly, for example, so that the average heating rate from 400 to 425°C is 30°C/h or more. The atmosphere in the furnace can be, for example, a nitrogen gas atmosphere. It is considered that when "pre-aging cold rolling" is performed, the strain introduced thereby becomes a nucleation site, and the effect of promoting the formation of clusters is obtained.

[Finish cold rolling]
The aged material after the aging treatment is subjected to cold rolling at a reduction ratio of 5% or more. This cold rolling is called finishing cold rolling because it is a rolling to finish the product plate thickness. By applying a rolling ratio of 5% or more, the phenomenon of clustering finally occurs. It is more preferable to set the rolling ratio at 15% or more. There is no particular upper limit for the rolling ratio, but it is usually set to a range of 95% or less, and it may be set to a range of 80% or less, or 70% or less. The inventors speculate that some precipitates will be pseudo-solid-dissolved by this finishing cold rolling. The final plate thickness can be, for example, 0.03 to 0.40 mm.

[Low temperature annealing]
The plate material after the finish cold rolling is subjected to low-temperature annealing. The low-temperature annealing is performed under the conditions of increasing the temperature from 150°C to 300°C at an average heating rate of 10 to 150°C/s, holding at 300 to 500°C for 10 to 300 seconds, and cooling from 300°C to 150°C at an average cooling rate of 30 to 150°C/s. By performing low-temperature annealing under these conditions, it is possible to obtain a structure in which the clusters of solute atoms as defined in (A) above exist at a number density of 1.0 x ¹⁰²³ /m3 or ^more .

Copper alloys having the chemical compositions shown in Table 1 were melted and cast using a vertical semi-continuous casting machine. The resulting cast pieces were hot rolled and then cold rolled to obtain intermediate product sheets. The intermediate product sheets were then subjected to solution treatment, aging treatment, finish cold rolling, and low-temperature annealing in that order to obtain final product sheets. In some examples (Nos. 7 and 10), "pre-aging cold rolling" was performed between the solution treatment and the aging treatment. The manufacturing conditions for each process and the sheet thicknesses after each rolling process are shown in Tables 2, 3, 4, and 5 (hereinafter abbreviated as "Tables 2 to 5").
The aging treatment was carried out using a batch-type annealing furnace, with two stages of holding temperature, except for some examples (Comparative Examples Nos. 31 to 34, 46, and 47). The atmospheric gas in the furnace for the aging treatment was nitrogen. In the first and second stages, the temperatures shown in Tables 2 to 5 were held (almost constant) for the times shown in the same tables. The average heating rate from the holding temperature in the first stage to the holding temperature in the second stage was in the range of 10 to 100°C/h. Low-temperature annealing was carried out in a continuous heat treatment furnace.
The product plate material (test material) thus obtained was subjected to the following investigation.

(Cluster density)
Elemental analysis was performed using a three-dimensional atom probe field ion microscope (LEAP-4000HR, manufactured by AMETEK Corporation) to obtain the element distribution inside the sample three-dimensionally at the atomic level.
The sample was processed using a focused ion beam (Hitachi High-Tech, FB2100).
The measurement was performed at a sample temperature of 80 K and a sample size of 40×40×10 nm. Analysis software: IVAS, analysis method: A three-dimensional atom map was analyzed using the maximum separation method to determine the number density (clusters/m ³ ) of clusters that satisfied the following condition (A).
(A) A cluster containing one or more atoms of Ni, Co, and Si, in which when Ni, Co, and Si atoms are collectively referred to as "atom X", the distance between the atomic centers of the closest atoms X is kept at 0.40 nm or less, and the total number of atoms X is in the range of 10 to 400.
Measurements were carried out for each test material, N=5 times, and the average value of the cluster number density obtained in each measurement was used as the performance value of the test material.

(Fatigue strength)
A test piece (without drilling) with a width of 3 mm and a length of 15 to 25 mm, in which the width direction is the rolling direction and the length direction is the direction perpendicular to the rolling direction, was used to carry out a reciprocating fatigue test using the resonance method with a fatigue tester (manufactured by Nippon Technoplus Co., Ltd.; RF-RT). The position of the laser spot for measuring the amplitude was 1 mm from the end of the test piece. It was determined that the material was "fatigued" when the Young's modulus became 98% of the initial (before test) Young's modulus. Measurements were carried out at various stresses in increments of 20 MPa, and the maximum stress at which fatigue did not occur after 10 ⁷ times was taken as the fatigue strength σ ₇ (MPa), and the maximum stress at which fatigue did not occur after 10 ⁸ times was taken as the fatigue strength σ ₈ (MPa). This operation was carried out five times for one test material, and the average values of σ ₇ and σ ₈ (rounded off to the nearest 1) for the five times were adopted as the fatigue strengths σ ₇ and σ ₈ of the test material, respectively. Further, the damping ratio σ ₈ /σ ₇ of the fatigue strength was calculated from σ ₇ and σ ₈ .

In this test, a Cu-Ni-Co-Si copper alloy sheet material having a fatigue strength of 200 MPa or more at 10 ⁸ cycles and a damping ratio σ ₈ /σ ₇ of 0.80 or more can be evaluated as having significantly improved fatigue resistance compared to conventional materials. The Young's modulus was determined by performing a tensile test at a crosshead displacement speed vc of 0.02 mm/s based on JIS Z 2241:2011 on a JIS No. 5 tensile test piece taken from the sheet material and having a longitudinal direction parallel to the rolling direction, recording the strain and stress values every 0.1 seconds, and determining the regression line in a stress-strain orthogonal coordinate system by the least squares method using all the strain and stress data recorded between 100 MPa and 400 MPa. The Young's modulus was determined as the slope of the regression line.

(Durability of connector parts)
Connector parts having the shape shown in FIG. 2 were made from each test material, and tests were performed according to the method described in the "sag resistance test method" above. However, the top height h ₀ (initial height) before the start of the test was 2.75 mm for a plate thickness of 0.10 mm, 2.80 mm for a plate thickness of 0.15 mm, 2.85 mm for a plate thickness of 0.20 mm, 2.90 mm for a plate thickness of 0.25 mm, and 2.92 mm for a plate thickness of 0.27 mm. The top height h ₁ after 100,000 tests was measured, and the amount of sag was calculated as h ₀ -h _1. In addition, after 100,000 tests, the load (N) when a displacement of 1.45 mm (constant) downward from the origin was applied by the pressing jig was rated as 0.2 N or more; the load was rated as ◯, and the rest were rated as ×. The Cu-Ni-Co-Si based copper alloy sheet material having the above-mentioned sag amount of 0.25 mm or less and the load evaluation of ◯ can be evaluated as having significantly improved durability as a conductive spring member compared to conventional materials.

(Tensile strength)
Tensile test pieces (JIS No. 5) in the rolling direction were taken from each test material, and a tensile test was performed in accordance with JIS Z2241 with the number of tests N=3 to measure the tensile strength. The average value of N=3 was taken as the performance value of the test material. Test pieces with a tensile strength of 750 MPa or more in the rolling direction were judged to pass.

(conductivity)
The electrical conductivity was measured by the double bridge average cross-sectional area method in accordance with JIS H0505. A sample having an electrical conductivity of 25% IACS or more was judged to be acceptable.

(Serration)
A nominal stress-nominal strain curve was obtained by a tensile test in the rolling direction at a strain rate of 1×10 ⁻⁵ (/s) using a JIS No. 5 tensile test piece. A tensile test was performed in accordance with the method of Z2241, except that the strain rate was set under the above conditions. In the obtained nominal stress-nominal strain curve, it was examined how many stress drop parts (see FIG. 1) appeared in which the stress drop width from when the stress turned to a drop to when the stress turned to an increase again was 1.0 MPa or more from when the 0.2% proof stress was reached to when the specimen broke. In the Cu-Ni-Co-Si-based copper alloy sheet material in which such stress processing parts appeared in 10 or more places from when the 0.2% proof stress was reached to when the specimen broke, the serration evaluation was given as ○, since it is considered that the dislocation fixing action by clusters occurs. The other ones were given as × for the serration evaluation.
The results are shown in Tables 2 to 5.

All of the examples of the present invention had solute atom clusters as specified in (A) above at a number density of 1.0 x ¹⁰²³ /m3 or ^more , and a significant improvement in fatigue resistance and sag resistance was observed. In contrast, the comparative examples had some chemical compositions and manufacturing conditions that did not satisfy the specifications of the present invention, and did not achieve a structural state having solute atom clusters as specified in (A) above at a number density of 1.0 x ¹⁰²³ /m3 or ^more . As a result, the improvement in fatigue resistance and sag resistance was insufficient.

1 Jack-shaped test piece, which is a connector part 2 Retaining jig 3 Panel surface

Claims (5)

The alloy has a chemical composition, in mass%, of Ni and Co: 1.00 to 6.00%, Si: 0.30 to 1.40%, Ag: 0 to 0.30%, Al: 0 to 1.00%, B: 0 to 0.20%, Be: 0 to 0.15%, Cr: 0 to 0.50%, Fe: 0 to 1.00%, Mg: 0 to 0.50%, Mn: 0 to 1.00%, P: 0 to 0.20%, S: 0 to 0.20%, Sn: 0 to 1.00%, Ti: 0 to 0.50%, Zn: 0 to 1.00%, Zr: 0 to 0.30%, with the balance being Cu and unavoidable impurities, and has a cluster of solute atoms specified in the following (A) of 1.0 x ¹⁰²³ /m2 as observed by a three-dimensional atom probe field ion microscope. Copper alloy sheet material containing ³ or more in number density.
(A) A cluster containing one or more atoms of Ni, Co, and Si, in which when Ni, Co, and Si atoms are collectively referred to as "atom X", the distance between the atomic centers of the closest atoms X is kept at 0.40 nm or less, and the total number of atoms X is in the range of 10 to 400. The copper alloy sheet material according to claim 1, having a tensile strength in the direction parallel to the rolling of 750 MPa or more. 3. The copper alloy sheet material according to claim 1 or 2, wherein in a nominal stress-nominal strain curve obtained by a tensile test at a strain rate of 1×10 ⁻⁵ (/s) using a JIS No. 5 tensile test piece with a longitudinal direction parallel to the rolling direction, serrations having 10 or more stress drop portions, in which the stress drop width from when the stress starts to drop until when the stress starts to rise again is 1.0 MPa or more, are observed during the period from when the 0.2% proof stress is reached until when the sheet material is broken. In mass%, the sum of Ni and Co: 1.00 to 6.00%, Si: 0.30 to 1.40%, Ag: 0 to 0.30%, Al: 0 to 1.00%, B: 0 to 0.20%, Be: 0 to 0.15%, Cr: 0 to 0.50%, Fe: 0 to 1.00%, Mg: 0 to 0.50%, Mn: 0 to 1.00%, P: 0 to 0.20%, S: 0 to 0.20%, Sn: 0 to 1.00%, T A plate material containing clusters of solute atoms as defined in the following (A) at a number density of 1.0×10 23 /m 3 or more as observed by a three-dimensional atom probe field ion microscope is produced from an intermediate product plate material of a copper alloy having a chemical composition consisting of Cu: 0-0.50%, Zn: 0-1.00 %, Zr: ^{0-0.30 %} , the balance Cu and unavoidable impurities, by a process including a solution treatment, an aging treatment, a finish cold rolling, and a low-temperature annealing in the above order .
The solution treatment is performed under the following conditions: heating from 500°C to 800°C at an average heating rate of 10 to 150°C/s, holding at 800 to 1050°C for 10 to 600 seconds, and cooling from 800°C to 500°C at an average cooling rate of 50 to 200°C/s;
The aging treatment is carried out under two-stage conditions: first, at 200 to 400°C for 4 to 10 hours, and then at 425 to 550°C for 1 to 10 hours.
The finish cold rolling is performed under the condition that the rolling ratio is 5% or more.
The low-temperature annealing is performed under the following conditions: the temperature is increased from 150°C to 300°C at an average heating rate of 10 to 150°C/s, the temperature is held at 300 to 500°C for 10 to 300 seconds, and the temperature is cooled from 300°C to 150°C at an average cooling rate of 30 to 150°C/s.
A method for manufacturing copper alloy sheet material.
(A) A cluster containing one or more atoms of Ni, Co, and Si, in which when Ni, Co, and Si atoms are collectively referred to as "atom X", the distance between the atomic centers of the closest atoms X is kept at 0.40 nm or less, and the total number of atoms X is in the range of 10 to 400. A conductive spring member made from the copper alloy sheet material described in any one of claims 1 to 3.

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