JP7534883B2 - Cu-Ni-Al copper alloy sheet material, its manufacturing method and conductive spring member - Google Patents

Description

The present invention relates to a Cu-Ni-Al-based copper alloy sheet material and a manufacturing method thereof, and also to a conductive spring member using the sheet material.

Cu-Ni-Al copper alloys can be made strong due to Ni-Al precipitates, and have a metallic appearance with a light copper color, even among copper alloys. Copper alloys with this composition are useful as conductive spring parts for lead frames, connectors, etc., and as non-magnetic high-strength parts.

Conductive spring components such as connectors are now required to be smaller and have higher performance than ever before in order to accommodate the miniaturization and high integration of electronic devices that use them. In particular, there is a growing need for high durability, as the repeated load stress on metal parts increases as parts become smaller. In order to improve the durability of metal spring components such as connectors, it is important to improve the fatigue properties. On the other hand, in applications where the white surface appearance unique to Cu-Ni-Al copper alloys is important, it is also important that the material has excellent resistance to tarnish so that the beautiful white tone is not lost.

Various studies have been conducted to date on improving various other properties of Cu-Ni-Al based copper alloys while taking advantage of the high strength properties of the alloys.
For example, Patent Document 1 discloses a technique for obtaining a material having excellent strength, workability, and electrical conductivity by precipitating a γ' phase containing Si with an average grain size of 100 nm or less in a Cu-Ni-Al-based copper alloy containing a predetermined amount of Si through a process of performing a solution treatment at 700 to 1,020° C. and an aging treatment at 400 to 650° C. However, the manufacturing process disclosed in Patent Document 1 cannot be expected to sufficiently improve fatigue properties.

Patent Document 2 discloses a technology for manufacturing a plate material of Cu-Ni-Al system copper alloy that has an excellent "strength-bending workability balance" and excellent discoloration resistance. In the manufacturing process, a method is adopted in which a material that has been subjected to solution treatment is given cold rolling strain as necessary, and then a first aging treatment at a higher temperature range and a second aging treatment at a conventional temperature range are successively performed. This two-stage aging treatment makes it difficult for discontinuous precipitation of grain boundary reaction to occur, and fine second phase particles that contribute to improving strength are sufficiently precipitated within the grains, thereby achieving an excellent balance between strength and bending workability. However, according to the inventors' recent research, although the manufacturing method of Patent Document 2 has an effect of suppressing discontinuous precipitation at grain boundaries, when the amount of grain boundary precipitates themselves is considered, the effect of reducing the amount is limited, and sufficient improvement in fatigue properties is not observed.

Patent Document 3 discloses a manufacturing technology for a Cu-Ni-Al-based copper alloy sheet material with a high Young's modulus. Specifically, cold rolling with intermediate annealing is performed under specific conditions, solution treatment is performed at a slow heating rate, and finish cold rolling is performed under conditions where the rolling ratio is controlled to be low, followed by aging treatment, which results in a specific crystal orientation and achieves a high Young's modulus. However, the manufacturing method in Patent Document 3 does not sufficiently improve fatigue properties.

Patent Document 4 discloses a technique for suppressing grain boundary reaction precipitation in a Cu-Ti copper alloy, which is a different alloy system from that of the present invention, by employing the steps of solution treatment, precursor treatment, cold rolling, and aging treatment. However, the Cu-Ni-Al copper alloy targeted in the present invention contains more alloy components than the Cu-Ti copper alloy, and therefore inherently produces more grain boundary precipitates. Therefore, it is difficult to effectively reduce the amount of grain boundary precipitates simply by applying the method of Patent Document 4, and more strict control of manufacturing conditions is required to improve fatigue properties.

International Publication No. 2012/081573 JP 2020-50923 A JP 2020-79436 A Patent No. 6263333

Cu-Ni-Al copper alloys are useful for conductive spring components such as connectors, but no method has been established for improving fatigue properties that can adequately meet the need for miniaturization of parts. The present invention aims to significantly improve fatigue properties while maintaining discoloration resistance in high-strength Cu-Ni-Al copper alloy sheet materials.

In order to achieve the above object, the present specification discloses the following invention.
[1] In mass%, Ni: 10.0 to 30.0%, Al: 1.00 to 6.50%, Ag: 0 to 0.50%, B: 0 to 0.10%, Co: 0 to 2.0%, Cr: 0 to 0.5%, Fe: 0 to 2.0%, Mg: 0 to 2.0%, Mn: 0 to 2.0%, P: 0 to 0.2%, Si: 0 to 2.0%, Sn: 0 to 2.0%, Ti: 0 to 2.0%, Zn: 0 to 2.0%, Zr: 0 to 0.3%, the balance being Cu and unavoidable impurities. A copper alloy sheet material having a chemical composition that satisfies the following formula (1), in which the area ratio of precipitates having an area of 0.1 μm ² or more on an observation surface parallel to the sheet surface is 2.0% or less, and the Vickers hardness is 270 HV or more.
Ni/Al≦9.0…(1)
Here, the content value of the element expressed as mass % is substituted for the element symbol in formula (1).
[2] The copper alloy sheet material according to the above [1], wherein the number density of coarse precipitate particles having a major axis of 1.0 μm or more in an observation plane parallel to the sheet surface is 3.0 × 10 ⁴ particles/mm ² or less.
[3] The copper alloy sheet material according to the above [1] or [2], wherein the number density of fine precipitate particles having a major axis of 5 to 50 nm is 1.0 × 10 ⁷ particles/mm ² or more in an observation plane parallel to the sheet surface.
[4] The copper alloy sheet material according to any one of [1] to [3] above, having a tensile strength in a direction perpendicular to the rolling direction of 900 MPa or more.
[5] A step of heating a cast slab having a chemical composition, in mass%, of Ni: 10.0 to 30.0%, Al: 1.00 to 6.50%, Ag: 0 to 0.50%, B: 0 to 0.10%, Co: 0 to 2.0%, Cr: 0 to 0.5%, Fe: 0 to 2.0%, Mg: 0 to 2.0%, Mn: 0 to 2.0%, P: 0 to 0.2%, Si: 0 to 2.0%, Sn: 0 to 2.0%, Ti: 0 to 2.0%, Zn: 0 to 2.0%, Zr: 0 to 0.3%, the balance being Cu and unavoidable impurities, and satisfying the following formula (1), at 1000 to 1150 ° C. (cast slab heating step);
A process of performing hot rolling under conditions in which the rolling temperature in the final rolling pass is 800 ° C. or higher, and then cooling under conditions in which the average cooling rate from 700 ° C. to 600 ° C. is 40 ° C./s or higher (hot rolling process);
A process of carrying out a heat treatment at 950 to 1100 ° C for 30 to 360 seconds while applying a tension of 10.0 to 20.0 N / mm ² (solution treatment process);
A step of subjecting the plate material after the solution treatment step to a heat treatment at 700 to 900 ° C. for 10 to 300 seconds (first aging treatment step);
A step of performing cold rolling at a rolling ratio in the range of 5 to 50% or less (aging cold rolling step);
A step of subjecting the plate material after the aging cold rolling step to a heat treatment at 400 to 620 ° C. for 0.5 to 75 hours (second aging treatment step);
A method for producing a copper alloy sheet material, comprising the steps of: obtaining a sheet material having an area ratio of precipitates having an area of ^0.1 μm2 or more of 2.0% or less in an observation plane parallel to the sheet surface, by a production process including the steps of:
Ni/Al≦9.0…(1)
Here, the content value of the element expressed as mass % is substituted for the element symbol in formula (1).
[6] Between the hot rolling process and the solution treatment process,
A step of performing cold rolling at a rolling ratio of 50% or more (cold rolling step);
The method for producing the copper alloy sheet material according to [5] above, comprising:
[7] A conductive spring member using the copper alloy sheet material according to any one of [1] to [4] above as its material.

In this specification, the "sheet surface" refers to a surface perpendicular to the sheet thickness direction of the sheet material. The "sheet surface" is also called the "rolled surface." The "area ratio of precipitates having an area of ^0.1 μm2 or more" can be determined as follows.

[How to determine the area ratio of precipitates with an area of ^0.1 μm2 or more]
The plate surface is electrolytically polished under the following electrolytic polishing conditions, and then ultrasonically cleaned in ethanol for 20 minutes to obtain an observation surface. An observation field of 2000x magnification is randomly set using an FE-SEM (field emission scanning electron microscope) to obtain an observation image with an area S0 ( ^μm2 ). The obtained image is processed using image analysis software to obtain the total area _S1 (μm2) of all precipitate regions, excluding those regions corresponding to precipitates (hereinafter referred to as "precipitate regions") in which the area of one independent precipitate region ^is less than _0.1 ^μm2 , and the value of ( _S1 / _S0 ) x 100 is defined as the precipitate area ratio A (%) in this observation field. This operation is performed for five or more different non-overlapping fields to obtain the arithmetic mean value of the precipitate area ratio A of each field, and this is defined as the "area ratio (%) of precipitates with an area of ^0.1 μm2 or more" of the plate material.
(Electrolytic polishing conditions)
・Electrolyte: Distilled water, phosphoric acid, ethanol, 2-propanol mixed in a ratio of 10:5:5:1 ・Liquid temperature: 20℃
Voltage: 15V
Electrolysis time: 20 seconds

The Vickers hardness may be the Vickers hardness of the plate surface of the plate material measured in accordance with JIS Z2244:2009.

The "major axis" of a particle is defined as the diameter (μm or nm) of the smallest circle that surrounds the particle. The "number density of coarse precipitate particles with a major axis of 1.0 μm or more" and the "number density of fine precipitate particles with a major axis of 5 to 50 nm" can each be determined as follows.

[Method of determining the number density of coarse precipitate particles]
The plate surface was electrolytically polished under the following electrolytic polishing conditions to dissolve only the Cu base to expose the precipitate particles, and then ultrasonically cleaned in ethanol for 20 minutes to obtain an observation surface, which was then observed with a FE-SEM (field emission scanning electron microscope). The total number of precipitate particles with a major axis of 1.0 μm or more observed on the FE-SEM image was divided by the total observation area (mm ² ) to obtain the particle number density (particles/mm ² ) of the coarse precipitate particles. The total observation area was set to a total of 0.1 mm ² or more for multiple randomly set non-overlapping observation fields. Precipitate particles that protrude from the observation field were counted if the major axis of the portion appearing within the observation field was 1.0 μm or more.
(Electrolytic polishing conditions)
・Electrolyte: Distilled water, phosphoric acid, ethanol, 2-propanol mixed in a ratio of 10:5:5:1 ・Liquid temperature: 20℃
Voltage: 15V
Electrolysis time: 20 seconds

[Method of determining the number density of fine precipitate particles]
The plate surface is electrolytically polished under the electrolytic polishing conditions described above in "How to determine the area ratio of precipitates with an area of 0.1 μm2 or ^more " and then ultrasonically cleaned in ethanol for 20 minutes to obtain an observation surface, which is observed at a magnification of 100,000 times using an FE-SEM (field emission scanning electron microscope), and an observation field in which some or all of the particles with an area of ^0.1 μm2 or more are not included is set at random. For each observation field, the number of precipitate particles with a major axis of 5 to 50 nm among particles whose entire outlines are visible is counted. This operation is performed for 10 or more observation fields with no overlapping areas, and the total number of counts N _TOTAL in all observed fields is divided by the total area of the observation fields to convert it to the number per ^mm2 , which is the number density of fine precipitate particles (pieces/ ^mm2 ).

The rolling ratio from a certain plate thickness t ₀ (mm) to a certain plate thickness t ₁ (mm) is calculated by the following formula (2).
Rolling ratio (%) = [(t ₀ -t ₁ )/t ₀ ] × 100 ... (2)

According to the present invention, it is now possible to provide a Cu-Ni-Al-based copper alloy sheet material in a composition range that exhibits a whitish metallic appearance, has high strength, significantly improved fatigue properties, and excellent resistance to discoloration.

FIG. 13 is a diagram illustrating an example of a "Threshold" screen in the image analysis software. FIG. 2 shows an example of an FE-SEM observation image (upper part) at a magnification of 2000 times for Comparative Example No. 35, and a mapping image (lower part) obtained by image processing the image, showing the locations of "precipitates having an area of ^0.1 μm2 or more." FIG. 13 is a diagram illustrating an example of an FE-SEM observation image (upper part) at a magnification of 2000 times for Inventive Example No. 6, and a mapping image (lower part) obtained by image processing the image, showing the locations of "precipitates having an area of ^0.1 μm2 or more."

[Chemical composition]
In the present invention, the Cu-Ni-Al system copper alloy is the subject of the present invention. Hereinafter, "%" regarding the alloy components means "mass %" unless otherwise specified.

Ni, together with Cu, is a major element that constitutes the matrix (metal base) of Cu-Ni-Al copper alloys. In addition, a portion of the Ni in the alloy combines with Al to form Ni-Al precipitates, and these fine particles contribute to improving strength. In order to obtain sufficient strength, it is desirable to ensure a Ni content of 10% or more. In addition, as the Ni content increases, the alloy exhibits a whiter metallic appearance compared to other general copper alloys. However, like other copper alloys, when exposed to a high humidity environment, a thin oxide film forms on the metal surface, which can cause noticeable discoloration. In that case, the beautiful white appearance is lost. In particular, when emphasis is placed on discoloration resistance, it is more preferable to increase the Ni content to 12.0% or more and ensure the Al content as described below. It is more effective to have a Ni content of 15.0% or more. On the other hand, if the Ni content is high, the hot workability deteriorates. The Ni content is limited to 30.0% or less, and may be limited to 25.0% or less. The Ni content may also be controlled to 18.0% or more and 22.0% or less.

Al is an element that forms Ni-Al precipitates. If the Al content is too low, the strength improvement will be insufficient. Also, by increasing the Al content along with an increase in the Ni content, discoloration resistance can be improved. Furthermore, it has been found that in order to improve fatigue properties while obtaining a sufficient level of strength, it is important to adjust the components so that the Ni/Al ratio does not become too high. As a result of various investigations, it is necessary that the Al content be 1.00% or more and that the Ni/Al ratio satisfy the following formula (1). It is more preferable that the following formula (1)' be satisfied.
Ni/Al≦9.0…(1)
2.0≦Ni/Al≦8.0…(1)'
Here, the content value of the element expressed as mass % is substituted for the element symbol in formula (1) and formula (1)'.
On the other hand, if the Al content is too high, the hot workability deteriorates, so the Al content is limited to 6.50% or less.

Other elements such as Ag, B, Co, Cr, Fe, Mg, Mn, P, Si, Sn, Ti, Zn, and Zr can be added as necessary. The content ranges of these elements are Ag: 0-0.50%, B: 0-0.10%, Co: 0-2.0%, Cr: 0-0.5%, Fe: 0-2.0%, Mg: 0-2.0%, Mn: 0-2.0%, P: 0-0.2%, Si: 0-2.0%, Sn: 0-2.0%, Ti: 0-2.0%, Zn: 0-2.0%, and Zr: 0-0.3%. The total amount of these optional added elements is preferably 2.0% or less, and may be 1.2% or less, or 0.5% or less.

[Area ratio of precipitates with an area of ^0.1 μm2 or more]
In the Cu-Ni-Al copper alloys targeted in the present invention, a second phase mainly composed of Ni-Al intermetallic compounds is easily generated. Among the second phases, particles that are finely precipitated within the crystal grains by aging treatment ("fine precipitates" described below) contribute to improving strength. However, in this alloy system, discontinuous precipitation occurs at the grain boundaries during aging treatment, and the grain boundary precipitates present at the grain boundaries are a factor that prevents the improvement of fatigue properties. In addition to discontinuous precipitation at the grain boundaries, the second phase that was not fully dissolved by solution treatment and the precipitates generated by continuous precipitation within the crystal grains may coarsen, forming coarse particles on the order of microns ("coarse precipitates" described below).

The inventors have conducted detailed research into the size and amount of precipitates that affect fatigue properties in this alloy system. As a result, they have found that it is extremely effective to reduce the amount of precipitates with an area of ^0.1 μm2 or more observed in the metal structure. Specifically, a remarkable improvement in fatigue properties can be obtained by making the microstructure such that the area ratio of precipitates with an area of ^0.1 μm2 or more is 2.0% or less on an observation surface parallel to the plate surface. It is more effective to make the area ratio of precipitates with an area of ^0.1 μm2 or more 1.6% or less, and even more effective to make it 1.0% or less.

Precipitates with an area of ^0.1 μm2 or more include both "grain boundary precipitates" and "intragranular precipitates", but in terms of area ratio in the metal structure, the majority of precipitates with an area of ^0.1 μm2 or more are "grain boundary precipitates". Fatigue properties are particularly affected by the size and amount of grain boundary precipitates. Therefore, reducing the area ratio specified by the above-mentioned "method for determining the area ratio of precipitates with an area of 0.1 ^μm2 or more" is an extremely effective means for improving the fatigue properties of plate materials of this alloy system.

[Vickers hardness]
The Cu-Ni-Al-based copper alloy sheet material of the present invention is strengthened by aging treatment. Vickers hardness can be used as an index of strength by aging treatment. In order to fully meet the needs for miniaturization of conductive spring members, it is desirable for the Vickers hardness to be 270 HV or more. It is more preferable for the Vickers hardness to be 300 HV or more, and even more preferable for the Vickers hardness to be 320 HV or more, and it may be 330 HV or more.

[Number density of coarse precipitate particles having a major axis of 1.0 μm or more]
Coarse precipitate particles with a major axis of 1.0 μm or more account for a part of the above-mentioned "area ratio of precipitates with an area of 0.1 μm2 ^or more". When the area ratio of precipitates with an area of ^0.1 μm2 or more is large, the number density of coarse precipitate particles with a major axis of 1.0 μm or more tends to be large. However, coarse particles resulting from the second phase remaining after the solution treatment and coarse precipitates in crystal grains caused by coarse growth during aging treatment do not have a significant adverse effect on fatigue properties. Rather, they become a factor in reducing bending workability. Since coarse precipitate particles with a major axis of 1.0 μm or more do not contribute to improving strength, it is desirable that their amount is small. The density of coarse precipitate particles with a major axis of 1.0 μm or more is preferably 3.0 × ¹⁰⁴ particles/ ^mm2 or less. In consideration of applications in which bending workability is important, it is more preferable that the density of coarse precipitate particles having a major axis of 1.0 μm or more is extremely small, at 0.3 × 10 ⁴ particles/mm ² or less.

[Number density of fine precipitate particles with major axis of 5 to 50 nm]
Fine precipitate particles with a major axis of 5 to 50 nm contribute to improving strength by being dispersed in the matrix (metal base). Among them, those with a major axis of about 20 nm or more also contribute to improving bending workability. The fine precipitates formed in the Cu-Ni-Al copper alloy of the present invention are Ni-Al precipitate phases mainly composed of the intermetallic compound Ni ₃ Al. From the viewpoint of improving strength and bending workability, the number density of fine precipitate particles with a major axis of 5 to 50 nm is preferably 1.0×10 ⁷ particles/mm ² or more. In particular, in order to stably obtain a fairly high strength level for a copper alloy, such as a tensile strength of 1000 MPa or more in the transverse direction (TD), the number density of fine precipitate particles with a major axis of 5 to 50 nm is preferably 5.0×10 ⁷ particles/mm ² or more, and may be 8.0×10 ⁷ particles/mm ² or more.

[Tensile strength]
As a copper alloy sheet material used as a material for conductive spring members that are becoming smaller and thinner, the tensile strength in the direction perpendicular to the rolling direction (TD) is desirably 900 MPa or more, and more preferably 950 MPa or more. In addition, as shown in the examples described later, the tensile strength in the direction perpendicular to the rolling direction can be adjusted to 1000 MPa or more or 1100 MPa or more, and a strength level according to the application can be realized. Copper alloy sheets with such a high strength level, combined with their whitish appearance, are also suitable as a replacement for iron-based conductive spring members that are inherently less conductive than copper-based materials.

[Production method]
The copper alloy sheet material described above can be produced, for example, by the following manufacturing process.
Melting/casting → heating the slab → hot rolling → cold rolling → (intermediate annealing → cold rolling) → solution treatment → → first aging treatment → aging cold rolling → second aging treatment 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 piece may be produced by continuous casting, semi-continuous casting, etc. From the viewpoint of preventing oxidation of Al, it is preferable to carry out melting in a chamber under an inert gas atmosphere or under vacuum.

[Cast heating]
The slab is heated and held at 1000 to 1150°C. This heating can be performed by utilizing the slab heating process during hot rolling. Conventionally, slab heating of Cu-Ni-Al-based copper alloys has often been performed at a temperature of 950°C or less. In the present invention, in a composition range with high Ni and Al contents, it is necessary to promote continuous precipitation by aging treatment to sufficiently generate fine precipitates in the crystal grains that contribute to strength, and to suppress discontinuous precipitation at the crystal grain boundaries that hinders improvement of fatigue properties. For this purpose, it is effective to heat the slab to the above-mentioned high temperature to dissolve as much of the coarse second phase present in the cast structure as possible. However, if the temperature exceeds 1150°C, the parts of the cast structure with a low melting point become brittle, and there is a risk of cracking during hot rolling. It is more effective to set the heating holding time in the above temperature range to 1.5 hours or more, and even more effective to set it to 2 hours or more. Considering economic efficiency, it is desirable to set the slab heating time in the above temperature range to a range of 5 hours or less.

[Hot rolling]
In hot rolling, the rolling temperature of the final pass is set to 800°C or higher. The temperature of each rolling pass can be expressed by the surface temperature of the material immediately after it leaves the work roll in that rolling pass. After the rolling in the final pass is completed, forced cooling is started as soon as possible, and cooling is performed under conditions where the average cooling rate from 700°C to 600°C is 40°C/s or higher. By completing the hot rolling process under such conditions, it is possible to obtain a hot rolled material with reduced unnecessary precipitation, which has been found to be extremely effective in suppressing grain boundary precipitation in the second aging treatment process described below, that is, in significantly reducing the above-mentioned "area ratio of precipitates with an area of 0.1 ^μm2 or more". From the viewpoint of stably achieving a final pass rolling temperature of 800°C or higher in the operation of mass production sites, the plate thickness after hot rolling (finished plate thickness) is preferably in the range of, for example, 5 to 15 mm, and more preferably in the range of 7 to 15 mm.

Water cooling is a common method of forced cooling after hot rolling. For example, Patent Document 2 also states that after hot rolling of Cu-Ni-Al-based copper alloys, "it is preferable to rapidly cool the alloy by water cooling or the like" (paragraph 0033). Water cooling can also be used in the present invention. However, in the normal hot rolling carried out at mass production sites, it is not necessarily easy to strictly control the above-mentioned fast cooling rate over the entire width of a relatively thick hot-rolled plate, for example, a plate thickness of 5 to 15 mm, due to equipment constraints, and the cooling rate required in the present invention was not obtained by "rapid cooling" using normal water cooling. The technology of Patent Document 2 does not require strict control of the above-mentioned fast cooling rate. By using equipment with enhanced water cooling capacity, it is possible to achieve both a high final pass rolling temperature of 800°C or more and a fast cooling rate of 40°C/s or more.

[Cold rolling]
Before the solution treatment, cold rolling can be performed to adjust the plate thickness. If necessary, the process of "intermediate annealing → cold rolling" can be added once or multiple times. The reduction ratio in the cold rolling performed before the solution treatment (if intermediate annealing is performed, the reduction ratio in the cold rolling after the final intermediate annealing) can be, for example, 50% or more. The upper limit of the reduction ratio may be set, for example, in the range of 99.5% or less depending on the capacity of the mill.

[Solution treatment]
The main purpose of the solution treatment is to sufficiently dissolve the Ni-Al second phase before aging treatment (solution treatment). In the present invention, the material is heated to a temperature higher than the solution treatment temperature (about 800 to 900°C) of a general Cu-Ni-Al copper alloy. Specifically, the time during which the material is held in the temperature range of 950 to 1100°C is set to 30 to 360 seconds. When heated to such a high temperature range, the second phase can be sufficiently dissolved even if the holding time is as short as described above. The technique of Patent Document 2 also employs a method of performing a short solution treatment in a similar high temperature range, but in the present invention, this solution treatment is performed in a state in which a high tension of 10.0 to 20.0 N/ ^mm2 is applied to the material. It has been found that following these solution treatment conditions is extremely effective in stably obtaining a remarkable improvement effect on fatigue properties. Although the mechanism is not entirely clear at present, it is believed that a recrystallized structure with moderate strain is formed by a high-temperature, short-time solution treatment under high tension, and the strain within the crystal grains functions as the starting site for aging precipitation, promoting fine precipitation within the crystal grains, which in turn suppresses grain boundary reactions and improves fatigue properties. If the tension is too low or too high, the amount of grain boundary precipitates increases, increasing the "area ratio of precipitates with an area of 0.1 ^μm2 or more," and no significant improvement in fatigue properties can be achieved. Therefore, the inventors believe that the introduction of moderate strain during solution treatment functions as the nucleus for intragranular precipitation.

The tension can be controlled, for example, by the driving force of the bridle rolls at both ends of the heating zone while the sheet is passing through a continuous heat treatment furnace. The direction of the tension is the rolling direction. Following the solution treatment, the sheet is subjected to a first aging treatment, but the first aging treatment can also be performed during the cooling process of the solution treatment. When cooling to near room temperature after the solution treatment, it is preferable to rapidly cool the sheet so that the average cooling rate from 900°C to 300°C is 100°C/s or more.

[First aging treatment]
After the solution treatment, the steel is directly subjected to the first aging treatment without being subjected to processing strain by cold rolling. The first aging treatment is performed under the condition of holding at 700 to 900 ° C for 10 to 300 seconds. It is considered that the recrystallized structure to which the above-mentioned strain is given is obtained by the solution treatment under tension. When the above-mentioned heating conditions are applied to such a structure state, it is presumed that a large number of nuclei of continuous precipitation are formed in the crystal grains, which consequently function effectively in suppressing the generation of grain boundary precipitates in the second aging treatment. After the heating and holding of the first aging treatment, it is preferable to cool the steel under the condition that the average cooling rate from 700 ° C to 600 ° C is 70 ° C / s or more. Since the first aging treatment is performed for a short time, it is efficient to perform it in a continuous heat treatment furnace.

[Aging cold rolling]
Cold rolling is performed between the first aging treatment and the second aging treatment. This cold rolling is referred to as "aging cold rolling" in this specification. This cold rolling can be the last cold rolling to obtain the final plate thickness. For example, it is preferable to finish the plate to a thickness of 0.03 to 0.5 mm. The rolling reduction ratio in the aging cold rolling needs to be in the range of 5 to 50%, and more preferably in the range of 5 to 40%. By performing this relatively light cold rolling on the material that has completed the first aging treatment, processing strain (i.e., dislocation) is appropriately introduced, and continuous precipitation within the crystal grains is promoted in the second aging treatment. If the rolling reduction ratio of the aging cold rolling is too high, the structure will have many grain boundary precipitates, and no significant improvement in fatigue properties can be expected.

[Second aging treatment]
Next, the "second aging treatment" is performed as the final aging treatment. The second aging treatment is performed on the material in the structure state after the above-mentioned aging cold rolling, within the condition range of 400 to 620 °C for 0.5 to 75 hours. Within this condition range, the aging conditions can be set so that the Vickers hardness is 270 HV or more depending on the target strength level. The second aging treatment obtains a structure in which fine precipitates are dispersed within the crystal grains, realizing high strength. At the same time, the formation of grain boundary precipitates is significantly suppressed, resulting in a structure state in which the "area ratio of precipitates with an area of ^0.1 μm2 or more" is small, and a significant improvement effect on fatigue properties is stably obtained. The second aging treatment is preferably performed in a nitrogen atmosphere using a batch-type heat treatment furnace.

The plate material according to the present invention obtained in the above manner can be used as a material for processing, including press forming and bending, to obtain a highly durable conductive spring component.

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 heated and held at the temperatures and times shown in Tables 2 and 3, then extracted, hot rolled, and water-cooled. The total hot rolling reduction was 85 to 95%. The rolling temperature of the final pass, the average cooling rate from 700°C to 600°C, and the finished plate thickness after hot rolling are shown in Tables 2 and 3. In the water cooling after hot rolling, a cooling facility with an increased amount of water was used so that a sufficient cooling rate could be obtained over the entire plate width. The average cooling rate from 700°C to 600°C was calculated from the plate surface temperature T ₀ (°C) just before the start of water cooling and the plate surface temperature T ₁ (°C) just after the end of water cooling, and the elapsed time t (s) between them. In each example, it was confirmed that T ₀ was 700°C or more, T ₁ was 600°C or less, and no increase in surface temperature due to reheating from the center of the plate thickness was observed after water cooling. Therefore, the calculated value of (T ₀ -T ₁ )/t was rounded down in 5°C increments (for example, the calculated value was 68°C/s, converted to 65°C/s) and listed in Tables 2 and 3 as the average cooling rate from 700°C to 600°C. If this value is 40°C/s or more, it can be considered that a cooling rate of at least 40°C/s was ensured from 700°C to 600°C during cooling.

In some cases (No. 37 to 39) where cracks occurred during hot rolling, production was stopped at that point. After hot rolling, the surface oxide layer was removed by mechanical polishing (face grinding), and cold rolling was performed to the plate thickness shown in Tables 2 and 3. In No. 47, intermediate annealing at 675°C for 10 hours was performed during this cold rolling process. The rest were straight rolled without intermediate annealing. The obtained cold-rolled materials were subjected to solution treatment using a continuous annealing furnace. The solution treatment was performed under the conditions shown in Tables 2 and 3, while applying a specified tension to the plate while passing through the furnace. The tension was controlled by the driving force of the bridle rolls on the inlet and outlet sides of the heating zone. After heating, the plate was water-cooled under conditions where the average cooling rate from 900°C to 300°C was 100°C/s or more. After the solution treatment, except for some examples (Nos. 35, 47, and 48), the first aging treatment described below was directly performed without adding cold rolling strain. In Nos. 35 and 48, the plate was subjected to final cold rolling to a thickness of 0.1 mm after the solution treatment and then subjected to the first aging treatment. In No. 47, the plate was subjected to final cold rolling to a thickness of 0.1 mm after the solution treatment and then directly subjected to final aging treatment equivalent to the second aging treatment described below. In some of the above examples, the cold rolling performed immediately after the solution treatment is listed in the table as "cold rolling before aging."

In the first aging treatment, a continuous annealing furnace was used to hold the samples at the temperatures listed in Tables 2 and 3 for the times listed in the tables, followed by water cooling under conditions that resulted in an average cooling rate from 700°C to 600°C that was the value listed in the tables. Next, with the exception of some examples (Nos. 35 and 48), final cold rolling was performed to the final plate thickness listed in Tables 2 and 3 at the rolling reduction ratios listed in the tables. This cold rolling is performed between the first and second aging treatments, and is therefore referred to as "aging cold rolling" in this specification. Nos. 35 and 48 did not undergo aging cold rolling.

Next, except for some examples (No. 35, 47, 48), the plate material after the aging cold rolling was subjected to the final aging treatment, "second aging treatment". In No. 35 and 48, the plate material after the first aging treatment was directly subjected to the second aging treatment. In No. 47, the plate material after the pre-aging cold rolling was directly subjected to the final aging treatment. These final aging treatments were performed using a batch-type annealing furnace under the conditions of holding the temperature and time shown in the "second aging treatment" columns of Tables 2 and 3. The treatment was performed in a nitrogen atmosphere, and cooling was performed by air cooling. In this way, plate products (test materials) with the final plate thicknesses shown in Tables 2 and 3 were obtained. The following investigations were performed on each test material.

(Area ratio of precipitates with an area of ^0.1 μm2 or more)
According to the above-mentioned "Method of determining the area ratio of precipitates with an area of ^0.1 μm2 or more", the observation surface prepared by electrolytic polishing and ultrasonic cleaning was observed with an FE-SEM (manufactured by JEOL Ltd.; JSM-7200F) at an acceleration voltage of 15 kV and an irradiation current of 14, and observation images were obtained for five different fields of view that were randomly set and did not overlap. The image size was saved at 1280 pixels x 960 pixels. The electrolytic polishing was performed using an electrolytic polishing device (ElectroMet 4) manufactured by BUEHLER. The ultrasonic cleaning was performed for 20 minutes using an ultrasonic cleaner "BRANSONIC M2800-J". Image J (National Institutes of Health (NIH), Version 1.52a) was used as image analysis software. The analysis conditions by the software are as follows: on the "Set Scale" screen, which is displayed by sequentially selecting Analyze and Set Scale, measure the number of pixels of the scale bar length of the captured FE-SEM image, and input it into Distance in pixels. Next, input the length of the scale bar (μm) into Known distance, set Pixel aspect ratio to 1.0, and Unit of length to μm, so that the size of the precipitates can be recognized on the software. Then, on the "Resize Image Canvas" screen, set Width to 1280 pixels, Height to 950 pixels, and Position to Top-Center, and display the image of the FE-SEM image portion excluding the portion of the scale bar. After setting this scale and deleting the scale bar display portion, the upper limit of the threshold was set to 255 and the lower limit was set to the value that would make the pixel number ratio closest to 5% on the "Threshold" screen displayed by sequentially selecting Image, Adjust, and Threshold. Figure 1 shows an example of the "Threshold" screen. Specifically, the upper limit of the threshold at the P position in Figure 1 was set to 255, and the lower limit of the threshold was set so that the value displayed at the Q position would be closest to 5%. After that, on the "Analyze Particles" screen of the same software, Size was set to "0.10-Infinity" as a condition for excluding independent precipitate regions with an area of less than 0.1 ^μm2 . Furthermore, under the conditions of Circularity being 0.00-1.00, Show being MASK, and only the items Display results, Clear results, Summarize, and Include holes being checked, particle analysis was performed, and the area ratio A of precipitates having an area of ^0.1 μm2 or more for that visual field was determined. This operation was performed for five visual fields, and the arithmetic mean value of the precipitate area ratios A for each visual field was calculated, and this was defined as the "area ratio (%) of precipitates having an area of ^0.1 μm2 or more" for that sheet material.

For reference, Figures 2 and 3 show an FE-SEM observation image at a magnification of 2000 times (top) and a mapping image (bottom) obtained by image processing to show the locations of "precipitates with an area of ^0.1 μm2 or more." Figure 2 shows Comparative Example No. 35, and Figure 3 shows Invention Example No. 6. The length of the white scale bar shown at the bottom of the FE-SEM observation image corresponds to 10 μm.

(Number density of coarse precipitate particles)
According to the above-mentioned "Method of determining the number density of coarse precipitate particles", the observation surface prepared by electrolytic polishing and ultrasonic cleaning was observed by FE-SEM to determine the number density of coarse precipitate particles with a major axis of 1.0 μm or more. The electrolytic polishing was performed using an electrolytic polishing device (ElectroMet 4) manufactured by BUEHLER. The ultrasonic cleaning was performed for 20 minutes using an ultrasonic cleaner "BRANSONIC M2800-J".

(Number density of fine precipitate particles)
According to the above-mentioned "Method of determining the number density of fine precipitate particles", the observation surface prepared by electrolytic polishing and ultrasonic cleaning was observed with an FE-SEM (JSM-7200F manufactured by JEOL Ltd.) to determine the number density (pieces/mm ² ) of fine second phase particles having a major axis of 5 to 50 nm. The electrolytic polishing was performed using an electrolytic polishing device (ElectroMet 4) manufactured by BUEHLER. The ultrasonic cleaning was performed for 20 minutes using an ultrasonic cleaner "BRANSONIC M2800-J".

(Fatigue strength)
A fatigue test was performed by a resonance method using a fatigue tester (manufactured by Nippon Technoplus Co., Ltd.; RF-RT) using a test piece with a width of 3 mm and a length of 15 to 25 mm, with the width direction being the rolling direction and the length direction being the direction perpendicular to the rolling direction. The position of the laser spot for measuring the amplitude was 1 mm from the end of the test piece. When the Young's modulus became 98% of the initial (before test) Young's modulus, the material was judged to be "fatigued". Measurements were performed 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 (MPa). This operation was performed five times for one test material, and the average value of the five fatigue strengths (rounded off to the nearest 1 digit) was adopted as the fatigue strength of the test material. A fatigue strength of 200 MPa or more is evaluated as having very excellent fatigue properties as a high-strength Cu-Ni-Al alloy plate material. The Young's modulus was determined by performing a tensile test at a crosshead displacement speed v of 0.02 mm/s based on JIS Z 2241:2011 on a JIS No. 5 tensile test piece taken from the plate material, with the longitudinal direction parallel to the rolling direction, and recording the strain and stress values every 0.1 seconds. The Young's modulus was determined by determining a regression line in a stress-strain orthogonal coordinate system by the least squares method using all of the strain and stress data recorded in the stress range of 100 MPa to 400 MPa.

(Hardness)
The Vickers hardness of the plate surface was measured by a method conforming to JIS Z2244: 2009. Seven points were measured with a test force F (N) such that the average value d (mm) of the diagonal lengths _d1 and _d2 of the formed indentation (indentation) was 2/3 or less of the sample plate thickness, and the average value of the five points excluding the maximum and minimum values was adopted as the hardness of the test material.

(Tensile strength)
Tensile test pieces (JIS No. 5) were taken from each test material in the direction perpendicular to the rolling direction (TD), and tensile tests were performed in accordance with JIS Z2241 with the number of tests being N = 3 to measure the tensile strength. The average value of N = 3 was taken as the performance value of the test material.

(Discoloration resistance)
A sample of 10 mm wide x 65 mm long was taken from the test material, and the plate surface (rolled surface) was dry-polished with abrasive paper of number 1200 (grain size P1200 as specified in JIS R6010:2000), and then the polishing powder was wiped off and removed with Kimwipe (registered trademark) soaked in ethanol, and the test piece was dried to prepare a weather-resistant test piece. The weather resistance test was performed by exposing the test piece to an atmosphere of temperature 50 ° C. and relative humidity 95% for 24 hours. The L ^* a ^* b ^* of the test piece surface before and after the weather resistance test was measured, and the color difference ΔE ^* _ab was obtained according to the L ^* a ^* b ^* display color as specified in JIS Z8730:2009. If the color difference ΔE ^* _ab is less than 5.0, it can be determined that the conductive spring member has good discoloration resistance. Therefore, a color difference ΔE ^* _ab of less than 5.0 was judged to be acceptable (good color resistance). For reference, weather resistance tests were also conducted under the same conditions on each sheet material of oxygen-free copper (C1020), 70-30 brass (C2600), and Nepal brass (C4622). As a result, the color difference ΔE ^* _ab was 11.0 for oxygen-free copper, 10.5 for 70-30 brass, and 10.7 for Nepal brass.
The results of these investigations are shown in Tables 4 and 5.

All of the Cu-Ni-Al based copper alloy sheets of the present invention are excellent in strength, fatigue properties, and resistance to discoloration. Among them, No. 13 has a relatively high Ni/Al composition and a relatively long aging treatment time, resulting in a metal structure with more coarse precipitates in the crystal grains than the other examples of the present invention. However, the area ratio of precipitates with an area of ^0.1 μm2 or more was kept low, and a significant improvement in fatigue properties was observed compared to the comparative examples.

Among the comparative examples, Nos. 31 to 36 and 41 to 48 are examples in which copper alloys having chemical compositions satisfying the specifications of the present invention were used to produce sheet materials by manufacturing processes outside the manufacturing conditions specified in the present invention. Specifically, No. 31 had a low slab heating temperature. No. 32 had a low rolling temperature in the final pass of hot rolling. No. 33 had a low solution treatment temperature. No. 34 had a small average cooling rate from 700°C to 600°C after the final pass of hot rolling. No. 35 had final cold rolling performed between the solution treatment and the first aging treatment. No. 36 had a short first aging treatment time. No. 41 had solution treatment performed at a low tension level that is conventionally common. No. 42 had a high rolling ratio in the cold rolling between aging treatments. No. 43 had a high tension during solution treatment. In No. 44, the temperature of the first aging treatment was high. In No. 45, the temperature of the first aging treatment was low. In No. 46, the time of the first aging treatment was long. In No. 47, the average cooling rate from 700°C to 600°C after the final pass of hot rolling was small, and after the cold rolling process with intermediate annealing, a solution treatment was performed at a low tension level, which is conventionally common, and a process corresponding to the first aging treatment was not performed. In No. 48, the average cooling rate from 700°C to 600°C after the final pass of hot rolling was small, and a final cold rolling was performed between the solution treatment at a low tension level, which is conventionally common, and the first aging treatment. In all of these examples, a large amount of grain boundary precipitates were generated, resulting in a structure state in which the area ratio of precipitates with an area of ^0.1 μm2 or more was large, and the improvement in fatigue properties was insufficient.

Nos. 37 to 39 are examples where cracks occurred during hot rolling, and production was discontinued at that point. Of these, No. 37 had a slab heating temperature that was too high. No. 38 had too high a Ni content. No. 39 had too high an Al content.

No. 40 had a low Al content and a high Ni/Al ratio in its chemical composition, resulting in poor discoloration resistance and low strength levels.

Claims (7)

A copper alloy sheet material having a chemical composition, in mass%, of Ni: 10.0 to 30.0%, Al: 1.00 to 6.50%, Ag: 0 to 0.50%, B: 0 to 0.10%, Co: 0 to 2.0%, Cr: 0 to 0.5%, Fe: 0 to 2.0%, Mg: 0 to 2.0%, Mn: 0 to 2.0%, P: 0 to 0.2%, Si: 0 to 2.0%, Sn: 0 to 2.0%, Ti: 0 to 2.0%, Zn: 0 to 2.0%, Zr: 0 to 0.3%, with the balance being Cu and unavoidable impurities, and satisfying the following formula (1), in which the area ratio of precipitates having an area of 0.1 ^μm2 or more on an observation surface parallel to the sheet surface is 2.0% or less, and the Vickers hardness is 270 HV or more.
Ni/Al≦9.0…(1)
Here, the content value of the element expressed as mass % is substituted for the element symbol in formula (1). 2. The copper alloy sheet material according to claim 1, wherein the number density of coarse precipitate particles having a major axis of 1.0 μm or more is 3.0 × 10 ⁴ particles/mm ² or less in an observation plane parallel to the sheet surface. 3. The copper alloy sheet material according to claim 1, wherein the number density of fine precipitate particles having a major axis of 5 to 50 nm is 1.0 x 10 ⁷ particles/mm ² or more in an observation plane parallel to the sheet surface. The copper alloy sheet material according to any one of claims 1 to 3, having a tensile strength in the direction perpendicular to the rolling direction of 900 MPa or more. A step of heating a slab having a chemical composition, in mass%, of Ni: 10.0 to 30.0%, Al: 1.00 to 6.50%, Ag: 0 to 0.50%, B: 0 to 0.10%, Co: 0 to 2.0%, Cr: 0 to 0.5%, Fe: 0 to 2.0%, Mg: 0 to 2.0%, Mn: 0 to 2.0%, P: 0 to 0.2%, Si: 0 to 2.0%, Sn: 0 to 2.0%, Ti: 0 to 2.0%, Zn: 0 to 2.0%, Zr: 0 to 0.3%, with the balance being Cu and unavoidable impurities, and satisfying the following formula (1), at 1000 to 1150 ° C. (slab heating step);
A process of performing hot rolling under conditions in which the rolling temperature in the final rolling pass is 800 ° C. or higher, and then cooling under conditions in which the average cooling rate from 700 ° C. to 600 ° C. is 40 ° C./s or higher (hot rolling process);
A process of carrying out a heat treatment at 950 to 1100 ° C for 30 to 360 seconds while applying a tension of 10.0 to 20.0 N / mm ² (solution treatment process);
A step of subjecting the plate material after the solution treatment step to a heat treatment at 700 to 900 ° C. for 10 to 300 seconds (first aging treatment step);
A step of performing cold rolling at a rolling ratio in the range of 5 to 50% or less (aging cold rolling step);
A step of subjecting the plate material after the aging cold rolling step to a heat treatment at 400 to 620 ° C. for 0.5 to 75 hours (second aging treatment step);
A method for producing a copper alloy sheet material, comprising the steps of: obtaining a sheet material having an area ratio of precipitates having an area of ^0.1 μm2 or more of 2.0% or less in an observation plane parallel to the sheet surface , and a Vickers hardness of 270 HV or more , by the steps comprising the steps of:
Ni/Al≦9.0…(1)
Here, the content value of the element expressed as mass % is substituted for the element symbol in formula (1). Between the hot rolling process and the solution treatment process,
A step of performing cold rolling at a rolling ratio of 50% or more (cold rolling step);
The method for producing a copper alloy sheet according to claim 5, comprising: A conductive spring member made from the copper alloy sheet material described in any one of claims 1 to 4.