Conductive wire

Abstract

A conductive wire includes: a core wire made of metal; and a clad layer made of stainless steel that covers a surface of the core wire. The core wire includes a diffusion layer that is disposed to constitute the surface of the core wire and contains 0.5 mass% or more of Fe. The diffusion layer has a thickness of 0.4% or more and 5% or less of the diameter of the core wire.

Description

Technical Field

[0001] This disclosure relates to conductive wires.

[0002] This application claims priority based on Japanese Application No. 2021-130221, filed on August 6, 2021, and Japanese Application No. 2022-88815, filed on May 31, 2022, the entire contents of which are incorporated herein by reference. Background Technology

[0003] As a material constituting conductive wire, Be-Cu alloy (hereinafter also referred to as Be-Cu) is known, which is a Cu (copper) alloy containing 0.2 to 2% by mass of Be (beryllium). Be-Cu has high strength, but on the other hand, it has the problem of high price due to the presence of Be. As a conductive wire that does not contain Be, conductive wire comprising a Cu core wire and a stainless steel cladding layer covering the surface of the core wire is known (for example, see Japanese Patent Application Publication No. 59-205105 (Patent Document 1) and International Publication No. 2010 / 129293 (Patent Document 2)).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 59-205105

[0007] Patent Document 2: International Publication No. 2010 / 129293 Summary of the Invention

[0008] The conductive wire according to this disclosure comprises: a metallic core wire; and a stainless steel sheathing layer covering the surface of the core wire. The conductivity of the metal constituting the core wire is greater than that of the stainless steel. The core wire includes a diffusion layer configured to constitute the surface of the core wire and containing 0.5% by mass or more of Fe (iron). The thickness of the diffusion layer is 0.4% or more and 5% or less of the diameter of the core wire. Attached Figure Description

[0009] Figure 1 It is a schematic cross-sectional view showing a section of a conductive wire perpendicular to its length.

[0010] Figure 2 This is a schematic cross-sectional view showing the structure near the interface between the core wire and the cladding layer.

[0011] Figure 3 This is a flowchart illustrating a general method for manufacturing conductive wires.

[0012] Figure 4This is a graph showing the relationship between tensile strength and electrical conductivity. Detailed Implementation

[0013] [The technical problem this disclosure aims to solve]

[0014] For conductive wires, there are situations where, in applications such as movable parts of equipment, not only sufficient conductivity and strength are required, but also durability against repeated bending fatigue. In the conductive wires disclosed in Patent Document 1, there is a tendency for a decrease in the reduction of area during tensile testing. A decrease in the reduction of area implies a high work hardening rate, which reduces durability against repeated bending fatigue.

[0015] Furthermore, even from the perspective of strength (tensile strength), it cannot necessarily be said to be higher than Be-Cu, nor can it be said to have superior strength.

[0016] Therefore, one of the objectives is to provide a conductive wire that ensures the same conductivity as Be-Cu, has higher strength than Be-Cu, and has excellent durability against repeated bending fatigue.

[0017] [The Effects of This Disclosure]

[0018] Based on the above-mentioned conductive wire, it is possible to provide a conductive wire that has the same conductivity as Be-Cu, higher strength than Be-Cu, and excellent durability against repeated bending fatigue.

[0019] [Description of embodiments of this disclosure]

[0020] First, embodiments of this disclosure will be described. The conductive wire of this disclosure comprises: a metallic core wire; and a stainless steel sheathing layer covering the surface of the core wire. The conductivity of the metal constituting the core wire is greater than that of the stainless steel. The core wire includes a diffusion layer configured to constitute the surface of the core wire and contains 0.5% by mass or more of Fe. The thickness of the diffusion layer is 0.4% or more and 5% or less of the diameter of the core wire.

[0021] The present inventors investigated why conductive wires with high conductivity cores covered by a stainless steel cladding, including those disclosed in Patent Document 1, cannot achieve higher strength than Be-Cu and superior durability against repeated bending fatigue. The results showed that a diffusion layer (containing more than 0.5% by mass of Fe) formed near the surface of the core by Fe diffusing from the cladding layer affects the durability and strength against repeated bending fatigue.

[0022] That is, in the absence of a diffusion layer and when the thickness of the diffusion layer is less than 0.4% of the core wire diameter, the conductive wire exhibits low durability against repeated bending fatigue. This can be attributed to the fact that, for example, when the conductive wire is bent, one of the sheathing and the core wire does not adequately follow the deformation of the other, thus causing strain to concentrate in the sheathing.

[0023] On the other hand, when the thickness of the diffusion layer exceeds 5% of the core wire diameter, the strength of the conductive wire decreases. This can be attributed to the fact that, although the diffusion layer is included within the core wire made of a highly conductive material, the conductivity is substantially reduced compared to the area of ​​the core wire outside the diffusion layer. Therefore, to ensure conductivity equivalent to Be-Cu, the area ratio of the core wire in the cross-section perpendicular to the length direction of the conductive wire needs to be increased. As a result, the area ratio of the stainless steel cladding layer, which contributes to increased strength, becomes relatively smaller, leading to a decrease in strength.

[0024] In the conductive wire of this disclosure, the thickness of the diffusion layer is set to be 0.4% or more and 5% or less of the diameter of the core wire. By making the thickness of the diffusion layer 0.4% or more of the diameter of the core wire, the durability against repeated bending fatigue is improved. On the other hand, by making the thickness of the diffusion layer 5% or less of the diameter of the core wire, the thickness of the coating layer can be sufficiently ensured, and strength greater than Be-Cu can be easily obtained. Thus, according to the conductive wire of this disclosure, a conductive wire that ensures conductivity equivalent to Be-Cu while having higher strength than Be-Cu and excellent durability against repeated bending fatigue can be provided.

[0025] Here, the diameter of the core wire refers to the equivalent circle diameter of the core wire in a cross-section perpendicular to the length direction of the conductive wire. If the core wire in this cross-section is circular, the equivalent circle diameter is the diameter of the core wire. If the core wire in this cross-section has a shape other than a circle, the equivalent circle diameter is the diameter of the circle corresponding to the area of ​​the core wire. Furthermore, the thickness of the diffusion layer can be determined, for example, by using line analysis with an EPMA (Electron Probe Micro Analyzer). Specifically, firstly, the conductive wire is cut along a cross-section perpendicular to the length direction. Line analysis is then performed in a direction perpendicular to the interface between the core wire and the coating layer in this cross-section to determine the Fe concentration. Then, the thickness of the portion where the Fe content is 0.5% by mass or more can be determined as the thickness of the diffusion layer.

[0026] In the aforementioned conductive wires, the thickness of the diffusion layer can be 0.85% or more of the core wire diameter. This configuration improves the resistance to permanent strain from static deformation.

[0027] In the aforementioned conductive wires, the core wire can also be composed of at least one of Cu, Ag (silver), Al (aluminum), Cu alloys, Ag alloys, and Al alloys. These materials have high conductivity and are therefore suitable as materials for constituting the core wire.

[0028] In the aforementioned conductive wires, the core wire can also be made of Cu. Cu is inexpensive among materials with high conductivity, making it particularly suitable as a material for the core wire.

[0029] In the aforementioned conductive wires, the stainless steel can also be austenitic stainless steel. Austenitic stainless steel, with its sufficient workability, is suitable as the stainless steel constituting the cladding layer.

[0030] In the aforementioned conductive wires, the core wire can also be made of Cu. The aforementioned stainless steel can also be JIS standard (Japanese Industrial Standards) SUS304. In a cross-section of the conductive wire perpendicular to its length, when the elastic modulus is measured at 0.5 μm intervals across the interface between the core wire and the cladding, the maximum value of the change in elastic modulus can be 1500 GPa or more and 16250 GPa or less. Cu and SUS304 are particularly suitable as materials for the core wire and cladding, respectively. Therefore, by satisfying the above-mentioned elastic modulus conditions, it is possible to balance improved resistance to permanent strain from static deformation with high conductivity.

[0031] [Details of the embodiments of this disclosure]

[0032] Next, embodiments of the conductive wire of this disclosure will be described with reference to the accompanying drawings. It should be noted that in the following drawings, the same or equivalent parts are labeled with the same reference numerals, and repeated descriptions are omitted.

[0033] Reference Figure 1 In this embodiment, the conductive wire 1 includes a core wire 11 and a sheathing layer 12. The core wire 11 is made of metal. The sheathing layer 12 is made of stainless steel. The sheathing layer 12 covers the surface (outer peripheral surface) 11A of the core wire 11. The conductivity of the metal constituting the core wire 11 is greater than the conductivity of the stainless steel constituting the sheathing layer 12.

[0034] Preferably, the core wire 11 is made of a metal with excellent electrical conductivity. The core wire 11 is, for example, made of at least one of Cu, Ag, Al, Cu alloys, Ag alloys, and Al alloys (e.g., Cu (pure copper)). The shape of the core wire 11 in a cross-section perpendicular to the length direction is not particularly limited, such as... Figure 1 As shown, in this embodiment, it is circular. The shape of the core wire 11 in the cross-section perpendicular to the length direction can also be a shape other than circular, such as ellipse.

[0035] In a cross-section perpendicular to the length direction, the cladding layer 12 has a shape along the outer peripheral surface of the core wire 11. In this embodiment, the cladding layer 12 has a hollow cylindrical shape. In a cross-section perpendicular to the length direction, the thickness of the cladding layer 12 is constant. The stainless steel constituting the cladding layer 12 can also be, for example, an austenitic stainless steel (e.g., SUS304) with excellent workability and corrosion resistance, such as JIS standard SUS304 or SUS316.

[0036] Reference Figure 1 The outer diameter D (wire diameter) of the conductive wire 1 is not particularly limited, but may be, for example, 10 μm or more and 60 mm or less. The outer diameter D of the conductive wire 1 may also be 20 μm or more. The outer diameter D of the conductive wire 1 may be 30 mm or less, or 10 mm or less. In a cross-section perpendicular to the length direction, the ratio of the cross-sectional area of ​​the core wire 11 to the cross-sectional area of ​​the conductive wire 1 can be appropriately determined considering the required strength and conductivity, and may be, for example, 10% or more and 90% or less. The ratio of the cross-sectional area of ​​the core wire 11 to the cross-sectional area of ​​the conductive wire 1 may also be 15% or more, and further 20% or more. The ratio of the cross-sectional area of ​​the core wire 11 to the cross-sectional area of ​​the conductive wire 1 may also be 85% or less, 80% or less, and further 75% or less.

[0037] Reference Figure 2 The core wire 11 includes a diffusion layer 11D, which is configured to form a surface 11A of the core wire 11 and contains 0.5% by mass or more of Fe. (Refer to...) Figure 2 and Figure 1 The thickness t of the diffusion layer 11D is 0.4% or more and 5% or less of the diameter D1 of the core wire 11. More preferably, the thickness t of the diffusion layer 11D is 0.7% or more, 0.85% or more, and further 1% or more of the diameter D1 of the core wire 11. More preferably, the thickness t of the diffusion layer 11D is 4% or less of the diameter D1 of the core wire 11.

[0038] Next, an example of a method for manufacturing conductive wire 1 will be described. (Refer to...) Figure 3 In the manufacturing method of the conductive wire 1 according to this embodiment, the cladding process is first performed as a process (S10). In this process (S10), a stainless steel tube with a hollow cylindrical shape, which will become the cladding layer 12, and a metal rod, which will become the core wire 11, are first prepared. In this embodiment, a tube made of JIS standard SUS304 austenitic stainless steel and a rod made of oxygen-free copper (pure Cu) are prepared. Then, by inserting the rod into the tube, a cladding material is obtained in which a copper metal rod is inserted into a stainless steel tube. In this embodiment, the outer diameter of the tube is set to 4.91 mm.

[0039] Next, an interface bonding process is performed as process (S20). In this process (S20), the interface between the tube and the rod constituting the cladding material produced in process (S10) is bonded. Specifically, the cladding material obtained in process (S10) is subjected to wire drawing (drawing) with a small processing rate (reduction of area). Specifically, the outer diameter of the cladding material is reduced to 3 mm by wire drawing. Then, the interface between the tube and the rod constituting the cladding material is bonded by heat treatment. The heat treatment can be performed, for example, by heating to 950°C or higher and 1050°C or lower and holding for about 1 to 10 minutes.

[0040] Next, a first wire drawing process is performed as process (S30). In this process (S30), the cladding material obtained by joining the interface of the tube and the rod in process (S20) is drawn into wires. Specifically, in this embodiment, the wire drawing process is performed to achieve an outer diameter of 1.88 mm for the cladding material. The wire drawing process is performed by passing the cladding material through a through hole formed in a die. The wire drawing process can be performed in one operation using a single die, or in multiple operations using multiple dies. As a result, a conductive wire 1 with an outer diameter D of 1.88 mm is obtained. The rod becomes the core wire 11, and the tube becomes the cladding layer 12.

[0041] Next, a first solution treatment process is performed as process (S40). In this process (S40), a solution treatment is performed on the wire with an outer diameter D of 1.88 mm obtained in process (S30). Specifically, the conductive wire 1 obtained in process (S30) is subjected to a heat treatment, for example, heated to a temperature range of 900°C or higher and 1100°C or lower, held for 5 seconds or more and 20 minutes or less, and then rapidly cooled. As a result, in the microstructure of the stainless steel constituting the cladding layer 12, the grains stretched by the wire drawing process in process (S30) recrystallize, and the martensitic structure generated by the wire drawing process disappears. As a result, the work-hardened cladding layer 12 in process (S30) softens and becomes a state that can be wire drawn again. The heating temperature in the solution treatment is preferably set at the A value of the stainless steel constituting the cladding layer. C3 Temperatures above a certain point. Stainless steel A. C3For example, the point can be calculated using the following formula: 937.2-436.5×(C%)+56×(Si%)-19.7×(Mn%)-16.3×(Cu%)-26.6×(Ni%)-4.9×(Cr%)+38.1×(Mo%)+124.8×(V%)+136.3×(Ti%)-19.1×(Nb%)+198.4×(Al%)+3315×(B%) (unit: °C). Here, (C%), (Si%), (Mn%), (Cu%), (Ni%), (Cr%), (Mo%), (V%), (Ti%), (Nb%), (Al%), and (B%) refer to the percentage content of C (carbon), Si (silicon), Mn (manganese), Cu (copper), Ni (nickel), Cr (chromium), Mo (molybdenum), V (vanadium), Ti (titanium), Nb (niobium), Al (aluminum), and B (boron) in the stainless steel, respectively. When the stainless steel constituting the cladding layer is an austenitic stainless steel such as JIS standard SUS304, the heating temperature during solution treatment is preferably 900°C or higher, more preferably 950°C or higher.

[0042] Through solution treatment, Fe and Ni (nickel) contained in the stainless steel constituting the cladding layer 12 diffuse into the core wire 11. As a result, a diffusion layer 11D containing more than 0.5% by mass of Fe is formed (see reference). Figure 2 The Ni content in the diffusion layer 11D is, for example, 0.2% by mass. In the conductive wire 1 of this embodiment, the thickness t of the diffusion layer 11D needs to be 0.4% to 5% of the diameter D1 of the core wire 11, preferably 0.85% to 5%. The thickness t of the diffusion layer 11D is mainly adjusted by the diffusion layer thickness adjustment process (S70) described later. However, the diffusion of Fe from the coating layer 12 to the core wire 11 caused by the solution treatment also affects the thickness t of the diffusion layer 11D. Therefore, it is also necessary to consider the conditions of the wire drawing process and the solution treatment in the subsequent process to determine the conditions of the solution treatment in this process (S40). The diffusion rate of Fe (and Ni) (diffusion distance per unit time) increases with the increase of the heating temperature in the solution treatment. The heating temperature and heating time are determined by considering the production efficiency and the ease of controlling the thickness t of the diffusion layer 11D.

[0043] Next, a second wire drawing process is performed as process (S50). In this process (S50), the conductive wire 1, which underwent solution treatment in process (S40), is drawn. Specifically, in this embodiment, the wire drawing process is performed to achieve an outer diameter D of 0.7 mm for the conductive wire 1. Similar to process (S30), the wire drawing process can be performed using a single die in one operation, or using multiple dies in multiple operations. Thus, a conductive wire 1 with an outer diameter D of 0.7 mm is obtained.

[0044] Next, a second solution treatment process is performed as process (S60). In this process (S60), the wire with an outer diameter D of 0.7 mm obtained in process (S50) is subjected to solution treatment. Specifically, for the conductive wire 1 obtained in process (S50), for example, similar to process (S40), a heat treatment is performed, heating to a temperature range of 900°C or higher and 1100°C or lower, holding for 5 seconds or more and 20 minutes or less, followed by rapid cooling. As a result, similar to process (S40), the stainless steel grains constituting the cladding layer 12 recrystallize, and the martensitic structure disappears. Consequently, the work-hardened cladding layer 12 in process (S50) softens, becoming a state that can be drawn again.

[0045] Similar to step (S40), a diffusion layer 11D is formed through solution treatment. Step (S60) differs from step (S40), which involves two wire drawing processes and one solution treatment in subsequent steps, in that it significantly affects the ratio of the thickness t of the diffusion layer 11D to the diameter D1 of the core wire 11. This is because the thickness of the diffusion layer 11D formed in step (S40) is reduced by the two wire drawing processes in steps (S50) and (S70). To set the thickness t of the diffusion layer 11D within a narrow range of 0.4% to 5% (and further, 0.85% to 5%) of the diameter D1 of the core wire 11, it is necessary to consider facilitating the adjustment of the thickness t of the diffusion layer 11D in the subsequent diffusion layer thickness adjustment step (S70) and determine the conditions for the solution treatment in step (S60).

[0046] Next, a diffusion layer thickness adjustment process is performed as process (S70). In this process (S70), the conductive wire 1, which has undergone solution treatment in process (S60), is subjected to a heat treatment to adjust the thickness t of the diffusion layer 11D. Specifically, a heat treatment is performed to heat the wire 1 to a temperature range of 800°C or higher and 1100°C or lower, and then rapidly cooled after holding the temperature for 5 seconds or more and 20 minutes or less. In order to set the thickness t of the diffusion layer 11D within a narrow range of 0.4% or higher and 5% or lower (and further 0.85% or higher and 5% or lower) of the diameter D1 of the core wire 11, the diffusion distance of Fe needs to be strictly controlled. By allowing a certain degree of reduction in production efficiency while selecting a low heating temperature with a low Fe diffusion rate and increasing the allowable heating time, a diffusion layer 11D of appropriate thickness can be easily formed. It should be noted that this process (S70) is not a necessary process and can be omitted by appropriately setting the conditions of the solution treatment in processes (S40) and (S60). However, by performing process (S70) with the thickness t of the diffusion layer 11D formed in processes (S40) and (S60) being sufficiently small in advance, it is easy to set the thickness t of the diffusion layer 11D within a narrow range of 0.4% to 5% (and further 0.85% to 5%) of the diameter D1 of the core wire 11.

[0047] Next, a third wire drawing process is performed as process (S80). In this process (S80), the conductive wire 1, whose thickness t of the diffusion layer 11D was adjusted in process (S70), is drawn. Specifically, in this embodiment, the wire drawing process is performed so that the outer diameter D of the conductive wire 1 reaches 0.3 mm. Similar to processes (S30) and (S50), the wire drawing process can be performed using one die in one operation, or using multiple dies in multiple operations. Thus, a conductive wire 1 with an outer diameter D of 0.3 mm and a diffusion layer 11D thickness t of 0.4% to 5% (moreover, 0.85% to 5%) of the diameter D1 of the core wire 11 is completed.

[0048] According to the manufacturing method of the conductive wire of this embodiment described above, the conductive wire 1 of this embodiment can be easily manufactured.

[0049] [Example]

[0050] (1) Evaluation of durability under repeated bending fatigue

[0051] An experiment was conducted to confirm that the formation of the diffusion layer improves the durability against repeated bending fatigue. A conductive wire 1 with an outer diameter D of 0.3 mm was fabricated according to the manufacturing method described in the above embodiment, and a repeated bending fatigue test was carried out. By changing the heating temperature and heating time in step (S70), the thickness t of the diffusion layer 11D was varied within the range of 0.4% to 5% with respect to the diameter D1 of the core wire 11. For comparison, samples were also fabricated by further changing the heating temperature and heating time in step (S70) such that the thickness t of the diffusion layer 11D was outside the range of 0.4% to 5% with respect to the diameter D1 of the core wire 11. Then, the obtained conductive wires 1 were subjected to the repeated bending fatigue test. The operation of bending the conductive wire 1 from a straight state by 90° and then restoring it to the straight state and further bending it by 90° to the opposite side was repeated, and the number of repetitions (durability number) until the maximum bending strain became 0.5% or more was measured. The maximum bending strain is a quantity represented by D / (D + 2R)×100% when the wire diameter is D and the bending curvature is R. Then, if the durability number is 200×10 3 times or more, it is judged as qualified. The experimental results are shown in Table 1.

[0052] [Table 1]

[0053]

[0054] In Table 1, Samples B and C, in which the ratio of the thickness t of the diffusion layer 11D to the diameter D1 of the core wire 11, i.e., t / D1, is within the range of 0.4% to 5%, are examples of the conductive wire of the present disclosure. On the other hand, Samples A and D, in which t / D1 is outside the range of 0.4% to 5%, are comparative examples. As shown in Table 1, Samples B to D are qualified for the durability against repeated bending fatigue, while Sample A is unqualified. Then, focusing on Sample A and Sample B, although only the value of t / D1 slightly increased from 0.36 to 0.40, the durability number increased to more than 3 times. Thus, it can be confirmed that making the value of t / D1 0.4 or more is very important for improving the durability against repeated bending fatigue.

[0055] (2) Evaluation of the influence of the diffusion layer on the conductivity

[0056] An experiment was conducted to confirm the effect of the diffusion layer formation on conductivity. For samples C and D from (1) above, the manufacturing process was temporarily stopped after reaching step (S20) (outer diameter D is 3 mm), and the conductivity of the cladding material and the area ratio of the copper core wire 11 in the cross section perpendicular to the length direction were measured. As a result, the area ratio of the copper core wire 11 in the cross section perpendicular to the length direction was 33%. In addition, the conductivity was 32.1% IACS (International Annealed Copper Standard). It can be said that at the stage of step (S20), a conductivity approximately corresponding to the area ratio of the copper core wire 11 was obtained.

[0057] Next, the tensile strength and electrical conductivity of samples C and D, which were completed in process (S80) above, were measured. For comparison, the tensile strength and electrical conductivity of samples with the same outer diameter (0.3 mm) as samples C and D and composed of Be-Cu were also measured. The experimental results are shown in Table 2.

[0058] [Table 2]

[0059]

[0060] As shown in Table 2, regarding tensile strength, samples C and D are both higher than Be-Cu. On the other hand, regarding electrical conductivity, sample C is higher than Be-Cu, while sample D is lower than Be-Cu.

[0061] Here, to confirm that the difference in conductivity between sample C and sample D is not caused by the change in the area ratio of the core wire 11 in the cross-section perpendicular to the length direction, the area ratio was measured for each wire drawing process included in the manufacturing method of samples C and D. The measurement results are shown in Table 3.

[0062] [Table 3]

[0063]

[0064] The area fractions of the core wire 11 in samples C and D are shown in Table 3. With the conductivity measured at an outer diameter D of 0.3 mm, the area fraction of the core wire 11 in both samples C and D is 33.8%. This confirms that the difference in conductivity between samples C and D is not caused by the change in the area fraction of the core wire 11 in the cross-section perpendicular to the length direction.

[0065] Next, for samples C and D, the thickness of the diffusion layer 11D, with an Fe content of 0.5% by mass or more, was determined by line analysis using EPMA. Specifically, a field emission type EPMA (JXA-8530F) manufactured by Nippon Electronics Corporation was used, and line analysis was performed in the radial direction of the conductive wire 1 near the interface between the core wire 11 and the cladding layer 12 under the conditions of accelerating voltage: 15 kV, irradiation current: 100 nA, sampling time: 1 s, and probe diameter: 0.1 μm. The measurement results are shown in Table 4.

[0066] [Table 4]

[0067]

[0068] As shown in Table 4, it can be confirmed that reducing the thickness of the diffusion layer 11D by adjusting the conditions for adjusting the diffusion layer thickness can suppress the decrease in conductivity. It should be noted that, in order to reduce the thickness of the diffusion layer 11D, measures such as placing the Ni foil between the core wire 11 and the cladding layer 12 to suppress diffusion can also be considered. However, as described above, reducing the thickness of the diffusion layer 11D by adjusting the conditions for adjusting the diffusion layer thickness can suppress the increase in working hours and achieve high production efficiency.

[0069] Next, samples G and H of conductive wires 1 with different solution treatment conditions obtained from the first solution treatment step (S40) of the above embodiment, and conductive wires 1 (with outer diameter D of 1.88 mm) that were subjected to step (S40) in samples C and D, were prepared, and the thickness of the layer (diffusion layer 11D) with an Fe content of 0.5% by mass or more formed on the surface including the core wire 11 was measured. Then, the ratio t / D1 of the thickness t of the diffusion layer to the diameter D1 of the core wire 11 was calculated. In addition, the thickness t of the layer with a Ni content of 0.2% by mass or more was also measured. Ni Then, calculate t. Ni The ratio t relative to the diameter D1 of core wire 11 Ni / D1. Then, based on the diffusion equation, t / D1 and t are calculated. Ni / D1. The results are shown in Tables 5 and 6.

[0070] [Table 5]

[0071]

[0072] [Table 6]

[0073]

[0074] Referring to Tables 5 and 6, the thicknesses of the layers with Fe content of 0.5% by mass or more and the layers with Ni content of 0.2% by mass or more are approximately consistent with the thicknesses of the diffusion layers calculated based on the diffusion equation. Therefore, it can be confirmed that the layers with Fe content of 0.5% by mass or more and the layers with Ni content of 0.2% by mass or more are layers formed through elemental diffusion (diffusion layers).

[0075] The conductivity of samples C, D, G, and H was evaluated. Specifically, the resistance between two points 150 mm apart was measured using a 3540 mΩ Hitester manufactured by Hioki Electric Co., Ltd., and the conductivity was calculated based on the obtained resistance values. In addition, the conductivity was calculated when the thickness of the diffusion layer 11D was considered to be reduced by the amount by which the core wire 11 was thinned. The results are shown in Table 7.

[0076] [Table 7]

[0077]

[0078] Referring to Table 7, the measured conductivity values ​​are roughly consistent with the calculated conductivity values. This confirms that the diffusion layer 11D does not function as a conductive region. Therefore, it can be said that by adjusting the solution treatment conditions to suppress the formation of the diffusion layer, the decrease in conductivity can be suppressed.

[0079] (3) Verification of its superiority over Be-Cu

[0080] According to the manufacturing method described above, samples J to N with different tensile strengths and electrical conductivitys were produced by changing the area ratio of the core wire 11 in the cross-section perpendicular to the length direction and adjusting the diffusion layer thickness. For each of the obtained samples and sample C, the ratio (t / D1) of tensile strength, electrical conductivity, and diffusion layer thickness was investigated. Then, the relationship between tensile strength and electrical conductivity was compared with that of Be-Cu. The results are shown in Table 8 and... Figure 4 .

[0081] [Table 8]

[0082]

[0083] exist Figure 4 In the diagram, the horizontal axis corresponds to tensile strength, and the vertical axis corresponds to electrical conductivity. Figure 4 For comparison, the relationship between the tensile strength and conductivity of Be-Cu (hollow triangle) is also shown. Additionally, in... Figure 4 The calculation values ​​for t / D1 of 0% and t / D1 of 5% are also displayed (for hollow quadrilaterals and solid quadrilaterals).

[0084] As the proportion of the core wire 11's cross-section in the conductive wire 1 increases in a section perpendicular to the length direction, conductivity increases and tensile strength decreases. Conversely, as the proportion of the core wire 11's cross-section decreases, conductivity decreases and tensile strength increases. Figure 4 In the diagram, calculated values ​​for t / D1 of 5% (solid quadrilateral) are shown within the range of 10% to 90% of the cross-sectional area of ​​the core wire 11. For example... Figure 4 As shown, within the range of 10% to 90% of the cross-sectional area of ​​the core wire 11, if the tensile strength is the same, the conductive wire of this disclosure with a t / D1 of 5% or less has a conductivity exceeding that of Be-Cu. Furthermore, in all samples C and J to N, which are embodiments of the conductive wire of this disclosure, conductivity exceeding that of Be-Cu at the same tensile strength was measured. Based on the above results, the superiority of the conductive wire of this disclosure over Be-Cu can be confirmed from the perspective of the relationship between tensile strength and conductivity.

[0085] (4) Summary of relevant experimental results for conductive wires with copper cores

[0086] Based on the above experimental results, from the perspective of improving durability against repeated bending fatigue, the value of t / D1 needs to be 0.4% or higher. On the other hand, from the perspective of ensuring conductivity exceeding that of Be-Cu with the same tensile strength, the value of t / D1 needs to be 5% or lower. It can be confirmed that the conductive wire of this disclosure, with a t / D1 value set between 0.4% and 5%, can provide a conductive wire that ensures conductivity equivalent to Be-Cu while having higher strength than Be-Cu and excellent durability against repeated bending fatigue.

[0087] (5) Verification of materials for core wires other than copper

[0088] A conductive wire was fabricated using JIS standard SUS316 austenitic stainless steel as the cladding layer 12 and a material other than Cu as the core wire 11. Then, the durability under repeated bending fatigue was evaluated in the same manner as described in (1). The experimental results are shown in Table 9.

[0089] [Table 9]

[0090]

[0091] Referring to Table 9, it can be confirmed that by setting the value of t / D1 to 0.4% or higher, excellent durability against repeated bending fatigue is obtained, similar to the case where Cu is used as the core wire material 11. It should be noted that if the value of t / D1 exceeds 5%, the conductivity becomes insufficient, which is considered to be the same regardless of the core wire material 11. Based on the above prototype results, it can be confirmed that even when using materials other than copper as the core wire material, the conductive wire according to this disclosure can provide a conductive wire that ensures both high conductivity and high strength, as well as excellent durability against repeated bending fatigue.

[0092] It should be noted that the excellent adhesion between the core wire 11 and the cladding layer 12, which is used to obtain excellent durability against repeated bending fatigue, is achieved through the diffusion of Fe, which is the main component of stainless steel. Therefore, the composition of the stainless steel constituting the cladding layer 12 is not particularly limited, but from the perspective of processability, austenitic stainless steel is preferred as the stainless steel constituting the cladding layer 12.

[0093] (6) Evaluation of resistance to permanent strain

[0094] Experiments were conducted to confirm that the formation of the diffusion layer improves the resistance to permanent strain. The resistance to torsional stress was evaluated by manufacturing a conductive wire 1 with an outer diameter D of 0.3 mm according to the manufacturing method described in the above embodiment and performing a high-temperature helical spring permanent strain test. Samples were prepared in which the thickness t of the diffusion layer 11D was varied relative to the diameter D1 of the core wire 11 by changing the heating temperature and heating time in step (S70). For comparison, Be-Cu wire with an outer diameter D of 0.3 mm was also prepared. These wires were then formed into helical springs with an average coil diameter of 3.2 mm and a pitch of 0.36 mm. For this helical spring, it was subjected to torsional stress in a nitrogen atmosphere at 200°C in a manner that caused the shear stress to reach 80% of the withstand strength of 0.2%, and the residual strain was measured after holding for 100 hours. Here, the residual strain is the value obtained by dividing the change in spring length before and after the test by the spring length before the test. A residual strain smaller than that of Be-Cu was considered acceptable. The experimental results are shown in Table 10.

[0095] [Table 10]

[0096]

[0097] Referring to Table 10, samples B', C, and D passed the test for permanent strain resistance to torsional stress, while sample A failed. This is likely because the value of t / D1 is small in sample A, indicating a small diffusion layer thickness. Specifically, the reduced resistance to permanent strain resistance to torsional stress is attributed to insufficient bonding between the core wire and the cladding layer due to the thin diffusion layer. This results in strain concentration in the cladding layer and increased deformation of the cladding layer.

[0098] Next, the resistance to permanent strain under bending stress was evaluated by conducting a high-temperature bending permanent strain test. Similar to the evaluation of resistance to permanent strain under torsional stress described above, samples were prepared in which the thickness t of the diffusion layer 11D was varied relative to the diameter D1 of the core wire 11 by changing the heating temperature and heating time in step (S70). For comparison, Be-Cu wire with an outer diameter D of 0.3 mm was also prepared. These wires were then formed into rings with a diameter of 100 mm, both ends were fixed, and the wires were deformed such that the surface of the wire furthest from the fixed portion was subjected to a bending stress of 0.2% of the resistance (80%), and held in a nitrogen atmosphere at 200°C. After 100 hours, the stress was released, and the residual strain was measured. Here, the residual strain is the value obtained by dividing the radius of curvature after the test by the radius of curvature before the test (50 mm). A residual strain smaller than Be-Cu was rated A, equal to Be-Cu was rated B, and larger than Be-Cu was rated C. The experimental results are shown in Table 11.

[0099] [Table 11]

[0100]

[0101] Referring to Table 11, the residual strain of sample A is greater than that of Be-Cu, while the residual strain of sample B' is the same as that of Be-Cu. This is considered to be due to the smaller values ​​of t / D1 in samples A and B', indicating a smaller thickness of the diffusion layer. Specifically, the reduced resistance to permanent strain under bending stress is attributed to the insufficient bonding between the core wire and the cladding layer due to the smaller thickness of the diffusion layer. Consequently, the strain is concentrated in the cladding layer, resulting in greater deformation of the cladding layer.

[0102] In contrast, samples B", C, and D with a t / D1 value of 0.85 or higher showed a significant decrease in residual strain, reaching values ​​clearly smaller than Be-Cu. Based on the above experimental results, from the perspective of improving resistance to permanent strain, it can be said that a t / D1 value of 0.85 or higher is preferable.

[0103] (7) Verification of the permanent strain resistance of materials other than copper core wire

[0104] A conductive wire was fabricated using JIS standard SUS316 austenitic stainless steel as the cladding layer 12 and a material other than Cu as the core wire 11. Then, the resistance to permanent strain was evaluated in the same manner as described in (6) above. The experimental results are shown in Tables 12 and 13.

[0105] [Table 12]

[0106]

[0107] [Table 13]

[0108]

[0109] Referring to Tables 12 and 13, it can be confirmed that by making the value of t / D1 greater than 0.85%, the resistance to permanent strain is improved, similar to the case where Cu is used as the material of the core wire 11.

[0110] (8) The effect of the change in elastic modulus on durability

[0111] The effect of the change in elastic modulus in the direction perpendicular to the interface between the core wire and the cladding layer on durability was evaluated. Specifically, the elastic modulus was measured at 0.5 μm intervals in a transverse manner in the direction perpendicular to the interface. The elastic modulus was measured using a Hysitron TI980 TriboIndenter manufactured by Bruker Corporation. A Berkovich indenter was used. Then, the maximum value of the change in elastic modulus at adjacent measurement points (the maximum absolute value of the difference in elastic modulus) was calculated. This value was compared with the results of the repeated bending fatigue test in (1), the conductivity measurement in (2), and the high-temperature bending permanent strain test in (6) to study the effect of the maximum value of the change in elastic modulus on durability. The experimental results are shown in Table 14.

[0112] [Table 14]

[0113]

[0114] Referring to Table 14, it can be confirmed that samples with a small maximum change in elastic modulus exhibit excellent durability against repeated bending fatigue and resistance to permanent strain. Specifically, if the maximum change in elastic modulus is below 16250 GPa, both durability against repeated bending fatigue and resistance to permanent strain clearly increase. Therefore, it can be said that the maximum change in elastic modulus is preferably below 16250 GPa. This is because the change in elastic modulus is gradual near the interface between the core wire and the cladding layer, thus preventing deformation from concentrating only on either the core wire or the cladding layer when deformation occurs near the interface due to external loads. On the other hand, sufficient diffusion near the interface makes the change in elastic modulus gradual, but excessive diffusion reduces the maximum change in elastic modulus to less than 1500 GPa, resulting in decreased conductivity (refer to sample D). Therefore, it can be said that the maximum change in elastic modulus is preferably above 1500 GPa.

[0115] It should be understood that the embodiments and examples disclosed herein are illustrative in all respects and are not restrictive in any way. The scope of the invention is defined not by the foregoing description but by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0116] Explanation of reference numerals in the attached figures

[0117] 1: Conductive wire; 11: Core wire; 11A: Surface; 11D: Diffusion layer; 12: Coating layer; D: Outer diameter; D1: Diameter; t: Thickness; t Ni :thickness.

Claims

1. An electrically conductive wire, comprising: a core wire made of a metal; and a cladding layer made of stainless steel that covers a surface of the core wire, the electrical conductivity of the metal constituting the core wire being greater than the electrical conductivity of the stainless steel, the core wire including a diffusion layer disposed to constitute the surface of the core wire and containing 0.5 mass% or more of Fe, the thickness of the diffusion layer being 0.4% or more and 5% or less of the diameter of the core wire.

2. The electrically conductive wire according to claim 1, wherein the thickness of the diffusion layer is 0.85% or more of the diameter of the core wire.

3. The electrically conductive wire according to claim 1 or 2, wherein the core wire is made of at least one of Cu, Ag, Al, a Cu alloy, an Ag alloy, and an Al alloy.

4. The electrically conductive wire according to claim 1 or 2, wherein the core wire is made of Cu.

5. The electrically conductive wire according to claim 1 or 2, wherein the stainless steel is an austenitic stainless steel.

6. The electrically conductive wire according to claim 3, wherein the stainless steel is an austenitic stainless steel.

7. The electrically conductive wire according to claim 4, wherein the stainless steel is an austenitic stainless steel.

8. The electrically conductive wire according to claim 1 or 2, wherein the core wire is made of Cu, the stainless steel is JIS standard SUS304, and the maximum value of the variation in the elastic modulus measured at intervals of 0.5 μm in a direction perpendicular to the interface between the core wire and the cladding layer in a cross section of the electrically conductive wire perpendicular to the length direction in a manner that traverses the interface is 1500 GPa or more and 16250 GPa or less. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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