Copper-coated steel wire and insulated electric wire

Abstract

This copper-coated steel wire comprises: a core wire made of austenitic stainless steel; and a copper or copper alloy coating layer that covers a first outer circumferential surface, which is the outer circumferential surface of the core wire. In a vertical cross-section, which is a cross-section perpendicular to the longitudinal direction of the core wire, the coating layer includes a plurality of first grains having an area exceeding 100μm2. In the vertical cross section, a first area ratio, which is the percentage of the total area of the first grains relative to the area of the coating layer, is 25% or more.

Description

Copper-clad steel wire and insulated electric wire

[0001] This disclosure relates to copper-clad steel wires and insulated electric wires. This application claims priority under Japanese application No. 2024-225444, filed on 20 December 2024, and incorporates all the provisions contained herein.

[0002] A copper-clad steel wire is known, comprising a stainless steel core wire and a copper or copper alloy coating layer (see, for example, Patent Document 1 below). In the copper-clad steel wire described in Patent Document 1, the coating layer covers the outer surface of the core wire. The insulated wire comprises a copper-clad steel wire.

[0003] Japanese Patent Publication No. 2022-157046

[0004] A copper-clad steel wire according to this disclosure comprises a core wire made of austenitic stainless steel and a coating layer made of copper or a copper alloy that covers the first outer surface, which is the outer surface of the core wire. With respect to a vertical cross section, which is a cross section perpendicular to the longitudinal direction of the core wire, the coating layer is 100 μm thick. ２ It contains multiple first crystal grains having an area exceeding [a certain value]. With respect to the vertical cross-section, the first area ratio, which is the percentage of the total area of ​​the first crystal grains relative to the area of ​​the coating layer, is 25% or more.

[0005] Figure 1 is a cross-sectional view of one embodiment of a copper-clad steel wire. Figure 2 is an enlarged cross-sectional view of the coating layer shown in Figure 1. Figure 3 shows the results of EBSD analysis of sample E of the copper-clad steel wire. Figure 4 is a flowchart of the manufacturing method of the copper-clad steel wire. Figure 5 is a cross-sectional view of one embodiment of an insulated wire. Figure 6 is a perspective view of one embodiment of an insulated wire with a crimp terminal. Figure 7 is a schematic diagram illustrating the method of tensile testing. Figure 8 shows the results of EBSD analysis of sample H of the copper-clad steel wire.

[0006] When attaching a crimp terminal to the end of an insulated wire, the insulating layer at the end of the wire is removed, and the crimp terminal is then crimped onto the exposed copper-clad steel wire. At this time, high reliability (hereinafter referred to as high connection reliability) is required for both the physical and electrical connection between the copper-clad steel wire and the crimp terminal.

[0007] This disclosure provides copper-clad steel wires and insulated wires that have high connection reliability when crimp terminals are attached.

[0008] The copper-clad steel wire of this disclosure has high connection reliability when crimp terminals are attached.

[0009] (1) A copper-clad steel wire according to the present disclosure comprises a core wire made of austenitic stainless steel and a coating layer made of copper or a copper alloy that covers the first outer surface which is the outer surface of the core wire. With respect to the vertical cross section which is a cross section perpendicular to the longitudinal direction of the core wire, the coating layer is 100 μm ２ It contains multiple first crystal grains having an area exceeding [a certain value]. With respect to the vertical cross-section, the first area ratio, which is the percentage of the total area of ​​the first crystal grains relative to the area of ​​the coating layer, is 25% or more.

[0010] 100 μm ２ First crystal grains with an area exceeding a certain size have a relatively large tolerance for deformation. Therefore, when the first area ratio, which is the percentage of the total area of ​​the first crystal grains relative to the area of ​​the coating layer in a vertical cross-section, is less than 25%, the coating layer cannot adequately absorb (mitigate) the force from the crimp terminal when it is crimped to the end of the copper-clad steel wire. As a result, the core wire is prone to breakage. This causes the end of the copper-clad steel wire to come loose from the crimp terminal. Consequently, the reliability of the copper-clad steel wire connection is reduced.

[0011] In contrast, this copper-clad steel wire has a high first area ratio of 25% or more, which is the percentage of the total area of ​​the first crystal grains, which have a relatively large tolerance for deformation. Therefore, even when a crimp terminal is crimped to the end of the copper-clad steel wire, the coating layer can sufficiently absorb (mitigate) the force from the crimp terminal. Consequently, the core wire is less likely to break. As a result, the end of the copper-clad steel wire is less likely to come loose from the crimp terminal. As a result, the connection reliability of the copper-clad steel wire can be improved.

[0012] (2) In (1) above, the coating layer has a second inner surface that contacts the first outer surface and a second outer surface that is located away from the second inner surface. It includes an inner region defined by the second inner surface and an intermediate surface which is a surface that is located away from the second inner surface by half the thickness of the coating layer in the thickness direction of the coating layer. The first crystal grain includes an inner portion of the first crystal grain that is located within the inner region. With respect to the vertical cross section, the second area ratio, which is the percentage of the total area of ​​the inner portion of the first crystal grain to the area of ​​the inner region, may be 50% or more. The inner portion of the first crystal grain in the inner region of the coating layer contributes greatly to force absorption (mitigation). And in this configuration, since the second area ratio can be made as high as 50% or more, the coating layer can mitigate the above-mentioned forces more effectively, and the core wire is less likely to break. As a result, the end of the copper-coated steel wire is less likely to come out of the crimp terminal.

[0013] (3) In (1) or (2) above, the coating layer has a second inner surface that contacts the first outer surface and a second outer surface that is located away from the second inner surface, and the coating layer includes an inner region which is defined by the second inner surface and an intermediate surface which is a surface that is located away from the second inner surface by half the thickness of the coating layer in the thickness direction of the coating layer, and an outer region which is defined by the second outer surface and the intermediate surface. The first hardness ratio, which is the ratio of the hardness of the inner region measured by the nanoindenter method to the hardness of the outer region measured by the nanoindenter method, may be 0.4 or more and 0.75 or less. With this configuration, even if a crimp terminal is crimped to the end of an insulated wire in the longitudinal direction, the force applied to the core wire can be sufficiently absorbed (mitigated). Therefore, the core wire is less likely to break.

[0014] (4) In any one of (1) to (3) above, the first area ratio may be 75% or less. With this configuration, the tolerance of the coating layer for deformation is appropriately large.

[0015] (5) In any one of (1) to (4) above, the first area ratio may be 50% or less. With this configuration, the tolerance of the coating layer for deformation is appropriately large.

[0016] (6) In any one of (2) to (5) above, the second area ratio may be 90% or less. With this configuration, the tolerance of the coating layer for deformation is appropriately large.

[0017] (7) In any one of (1) to (6) above, the wire diameter may be 0.1 mm or more and 0.8 mm or less. This configuration uses copper-clad steel wire with a wire diameter of 0.1 mm or more and 0.8 mm or less.

[0018] (8) In any one of (1) to (7) above, the thickness of the coating layer may be 25 μm or more and 150 μm or less. With this configuration, copper-coated steel wire can achieve both high conductivity and high mechanical strength.

[0019] (9) In any one of (1) to (8) above, the austenitic stainless steel forming the core wire may be SUS304, SUS301, SUS310S, or SUS316 as specified in JIS (Japanese Industrial Standards). SUS304, SUS301, SUS310S, or SUS316 are suitable for forming the core wire.

[0020] (10) The insulated wire of the present disclosure comprises a copper-clad steel wire as described in any one of (1) to (9) above, and an insulating layer covering the outer surface of the copper-clad steel wire. With this configuration, the connection reliability of the insulated wire is high even when a crimp terminal is crimped to the end of the insulated wire.

[0021] (11) In the above (10), the tensile strength at 25°C of the insulated wire with a crimp terminal attached to a copper-clad steel wire exposed at the end of the insulated wire is 800 N / mm ２ The above configuration may also be acceptable. This configuration allows for increased connection strength of insulated wires with crimp terminals.

[0022] [Details of Embodiments of the Present Invention] Embodiments of copper-clad steel wire and insulated electric wire according to the present disclosure will be described below with reference to the drawings. In the following drawings, the same or corresponding parts will be given the same reference numeral and their descriptions will not be repeated.

[0023] An embodiment of copper-clad steel wire will be described with reference to Figures 1 to 3. Figure 1 is a cross-sectional view of an embodiment of copper-clad steel wire. Figure 2 is an enlarged cross-sectional view of the coating layer shown in Figure 1. Figure 3 shows the results of EBSD (Electron Back Scatter Diffraction) analysis of copper-clad steel wire sample E. Note that the dashed lines in Figure 3 indicate the intermediate surface, which is not actually observed by EBSD.

[0024] [Copper-clad steel wire 1] The copper-clad steel wire 1 extends along its longitudinal direction. In a cross section perpendicular to the longitudinal direction (vertical cross section), the shape of the copper-clad steel wire 1 is not limited, but as shown in Figure 1, in one embodiment, the copper-clad steel wire 1 has a circular shape in the vertical cross section. Although not shown, the copper-clad steel wire 1 may have a rectangular shape in the vertical cross section described above. The copper-clad steel wire 1 has an outer circumferential surface 11. The copper-clad steel wire 1 comprises a core wire 2 and a coating layer 3.

[0025] [Core wire 2] In one embodiment, the core wire 2 has a circular shape in a vertical cross-section perpendicular to the longitudinal direction of the core wire 2. Although not shown in the figures, the core wire 2 may have a rectangular shape in the vertical cross-section described above. The core wire 2 has an axis common to the copper-clad steel wire 1. The core wire 2 has a first outer surface 21 which is the outer surface. The core wire 2 is made of austenitic stainless steel. Examples of austenitic stainless steel that form the core wire 2 include SUS304, SUS301, SUS310S, or SUS316. Each of SUS304, SUS301, SUS310S, and SUS316 is specified in JIS, specifically JIS G4303 (2021).

[0026] [Coating Layer 3] The coating layer 3 covers the first outer surface 21. In one embodiment, the coating layer 3 has a ring shape in a vertical cross-section. Although not shown, the coating layer 3 may have a rectangular frame shape in a vertical cross-section. The coating layer 3 has an axis common to the core wire 2. The coating layer 3 has a second inner surface 31 and a second outer surface 32. The second inner surface 31 is in contact with the first outer surface 21. The second inner surface 31 is formed integrally with the first outer surface 21. The second inner surface 31 has the same shape as the first outer surface 21. The second outer surface 32 is located away from the second inner surface 31. The second outer surface 32 corresponds to the outer surface 11 of the copper-clad steel wire 1. The coating layer 3 is made of copper or a copper alloy. Examples of copper alloys include copper-silver alloys. The coating layer 3 may contain unavoidable impurities. The thickness T of the coating layer 3 is the distance from the second inner circumferential surface 31 to the second outer circumferential surface 32. The thickness T of the coating layer 3 is between 25 μm and 150 μm. As shown in Figure 2, the coating layer 3 includes an inner region 33 and an outer region 34. The inner region 33 is defined by the second inner circumferential surface 31 and the intermediate surface 30. The intermediate surface 30 is a surface that is separated from the second inner circumferential surface 31 by half the thickness T of the coating layer 3 in the thickness direction of the coating layer 3. The outer region 34 is defined by the second outer circumferential surface 32 and the intermediate surface 30. The outer region 34 is adjacent to the inner region 33 on the radially outer side.

[0027] (First crystal grain 35) As shown in Figure 3, with respect to the vertical cross section, which is a cross section perpendicular to the longitudinal direction of the core wire 2, the coating layer 3 is 100 μm ２ It contains multiple first crystal grains 35 having an area exceeding 1 μm. With respect to the vertical cross-section, the coating layer 3 is 1 μm ２ The material may further contain a second crystal grain 36 having the following area: With respect to the vertical cross-section, the second crystal grain 36 has an area of ​​0.1 μm. ２ The following areas may be present. The vertical cross section is analyzed using EBSD. The EBSD identifies the first crystal grain 35 and the second crystal grain 36. The first crystal grain 35 is a relatively large crystal formed by the recrystallization of copper in the third step S3 (wire drawing step) of the manufacturing method described later. In contrast, the second crystal grain 36 is significantly smaller than the first crystal grain 35.

[0028] (First Area Ratio) With respect to the vertical cross-section, the first area ratio, which is the percentage (S1 / S0 × 100) of the total area S1 of the first crystal grains 35 relative to the area S0 of the coating layer 3, is 25% or more, more preferably 30% or more, more preferably 35% or more, and more preferably 40% or more. If the first area ratio is above the lower limit described above, even if a crimp terminal 80 (described later, see Figure 6) is crimped to the end of the copper-coated steel wire 1 in the longitudinal direction, the coating layer 3 can sufficiently absorb (mitigate) the force from the crimp terminal 80 (see Figure 6). Therefore, the core wire 2 is less likely to break. Therefore, the copper-coated steel wire 1 is less likely to come out of the crimp terminal 80 (see Figure 6). The first area ratio is 75% or less, and more preferably 50% or less.

[0029] (Second Area Ratio) The first crystal grain 35 includes an inner portion 351 of the first crystal grain located within the inner region 33. With respect to the vertical cross-section, the second area ratio, which is the percentage (S2 / S0 × 100) of the total area S2 of the inner portion 351 of the first crystal grain to the area S0 of the inner region 33, is 40% or more, more preferably 45% or more, more preferably 50% or more, more preferably 55% or more, and more preferably 60% or more. The second area ratio is 90% or less, and more preferably 80% or less. In the first crystal grain 35, the outer portion 352 of the first crystal grain, which is the portion other than the inner portion 351 of the first crystal grain, may be included in the outer region 34. For example, the first crystal grain 35 is unevenly distributed in the inner region 33. Specifically, the second area ratio is higher than the third area, which is the percentage of the area of ​​the outer portion 352 of the first crystal grain in the area of ​​the outer region 34.

[0030] (First Hardness Ratio) The tolerance of the inner region 33 for deformation is greater than the tolerance of the outer region 34 for deformation. In other words, the hardness of the inner region 33 is lower than the hardness of the outer region 34. Specifically, the first hardness ratio, which is the ratio of the hardness H1 of the inner region 33 to the hardness H2 of the outer region 34 (H1 / H2), is 0.4 or greater, moreover 0.5 or greater, and also less than 1.0, moreover 0.95 or less, moreover 0.9 or less, and moreover 0.75 or less. The hardness H2 of the outer region 34 and the hardness H1 of the inner region 33 are measured by the nanoindenter method.

[0031] [Composition and physical properties of copper-clad steel wire 1] The proportion of copper in copper-clad steel wire 1 is 40% by volume or more and less than 70% by volume. The proportion of copper in copper-clad steel wire 1 is determined from the area ratio in the orthogonal cross section obtained using EBSD for the cross section perpendicular to the axis of the copper-clad steel wire 1 (orthogonal cross section).

[0032] As shown in Figure 1, the wire diameter D of the copper-clad steel wire 1 is 0.1 mm or more and 0.8 mm or less. Here, "wire diameter" is defined as the equivalent diameter of a circle in a cross-section perpendicular to the longitudinal direction of the copper-clad steel wire 1. If the cross-section is circular, the equivalent diameter of a circle means the diameter of the circle. If the cross-section is not circular, the equivalent diameter of a circle means the diameter of a circle having the same area as the area of ​​the cross-section.

[0033] [Method for Manufacturing Copper-Clad Steel Wire 1] The method for manufacturing copper-clad steel wire will be explained with reference to Figure 4. Figure 4 is a flowchart of the method for manufacturing copper-clad steel wire. As shown in Figure 4, the method for manufacturing copper-clad steel wire 1 comprises a first step S1, a second step S2, and a third step S3. In the method for manufacturing copper-clad steel wire 1, the first step S1, the second step S2, and the third step S3 are carried out in order.

[0034] [First Step S1] The first step S1 is a raw material steel wire preparation step. In the first step S1, raw material steel wire is prepared. Specifically, steel wire having a component composition corresponding to SUS304, SUS301, SUS310S, or SUS316 as defined in the JIS above is prepared. The raw material steel wire does not yet contain at least the first crystal grain 35. The wire diameter of the raw material steel wire is 0.2 mm or more and 2.0 mm or less.

[0035] [Second Step S2] The second step S2 is a coating layer formation step. In the second step S2, a coating layer 3 is formed so as to cover the first outer surface 21 of the raw steel wire. In the second step S2, for example, copper plating or copper alloy plating is performed. Therefore, the coating layer 3 is also a copper plating layer or a copper alloy plating layer.

[0036] [Third Step S3] The third step S3 is carried out after the second step S2. The third step S3 is a wire drawing step. In the third step S3, wire drawing (drawing) processing is carried out on the raw material steel wire and the coating layer 3. In the third step S3, wire drawing (drawing) is carried out two or more times. At this time, it is considered that recrystallization of copper in the coating layer 3 is promoted based on the heat generation caused by wire drawing (drawing), and the first crystal grains 35 are organized. Specifically, in the third step S3, at least one of the following methods A to C is adopted.

[0037] Method A: Set the area reduction rate of one-time wire drawing (drawing) to be as large as 20% or more. The upper limit of the area reduction rate is 30% from the viewpoint of improving processing accuracy. The "area reduction rate" is the percentage obtained by dividing the difference between the area before wire drawing processing and the area after wire drawing processing by the area before wire drawing processing with respect to the cross-section perpendicular to the longitudinal direction of the raw material steel wire. A high area reduction rate means a high degree of processing.

[0038] Method B: Set the approach angle (half angle) of the die used for wire drawing to be as narrow as 8° or less. The lower limit of the approach angle of the die is 3° from the viewpoint of improving production efficiency. When the approach angle of the die is narrowed, the frictional force applied to the raw material steel wire during wire drawing (drawing) processing becomes large.

[0039] Method C: Set the wire drawing speed to be as high as 500 m / min or more. The upper limit of the wire drawing speed is 800 m / min from the viewpoint of improving production efficiency.

[0040] In addition, the true strain in the third step S3 is 1 or more and 3 or less.

[0041] By the third step S3, the first crystal grains 35 and the second crystal grains 36 are organized in the raw material steel wire, and the raw material steel wire becomes the core wire 2. The raw material steel wire becomes the core wire 2 containing at least the first crystal grains 35. Thereby, the copper-coated steel wire 1 shown in FIG. 1 is manufactured.

[0042] [Insulated Wire] Referring to FIG. 5, an embodiment of an insulated wire will be described. FIG. 5 is a cross-sectional view of an embodiment of an insulated wire.

[0043] As shown in Figure 5, the insulated wire 5 comprises the copper-clad steel wire 1 described above and an insulating layer 4. The insulating layer 4 covers the outer circumferential surface 11 of the copper-clad steel wire 1. The insulating layer 4 is in contact with the outer circumferential surface 11. In one embodiment, the insulating layer 4 has a ring shape in a cross section perpendicular to the longitudinal direction of the copper-clad steel wire 1. The insulating layer 4 has an axis common to the copper-clad steel wire 1. The insulating layer 4 is flexible. Examples of materials for the insulating layer 4 include insulating polymers. Examples of polymers include olefin polymers, halogen-containing polymers, elastomers, and rubbers. The thickness of the insulating layer 4 is, for example, 0.1 mm or more and 0.25 mm or less. To manufacture the insulated wire 5, the outer circumferential surface 11 of the copper-clad steel wire 1 is covered with the insulating layer 4.

[0044] [Connection between the end 12 of the insulated wire 5 and the crimp terminal 80] An embodiment of an insulated wire with a crimp terminal (connection between the end of the insulated wire and the crimp terminal) will be described with reference to Figure 6. Figure 6 is a perspective view of an embodiment of an insulated wire with a crimp terminal.

[0045] The insulated wire 5 is connected to the crimp terminal 80. The crimp terminal 80 includes a main body 83, a conductor barrel 81, and an insulating barrel 82. The conductor barrel 81 is connected to the main body 83. The insulating barrel 82 is connected to the side of the conductor barrel 81 opposite to the side connected to the main body 83. The main body 83, the conductor barrel 81, and the insulating barrel 82 are arranged in that order.

[0046] To connect the insulated wire 5 and the crimp terminal 80, first, the insulating layer 4 at the end of the insulated wire 5 in the longitudinal direction is removed to expose the end 1E of the copper-clad steel wire 1. Then, the end 1E of the copper-clad steel wire 1 is held by the conductor barrel 81, and the insulating layer 4 is held by the insulating barrel 82. Specifically, the end 1E of the copper-clad steel wire 1 is crimped by the conductor barrel 81. More specifically, the end 1E of the copper-clad steel wire 1 is pressed by the conductor barrel 81 from the outside of the outer peripheral surface 11. In other words, the end 1E is caulked. The crimp terminal 80 is crimped to the end 1E of the insulated wire 5. That is, the end 1E of the insulated wire 5 is attached to the crimp terminal 80. Thereby, an insulated wire 6 with a crimp terminal including the insulated wire 5 and the crimp terminal 80 attached to the end 1E of the insulated wire 5 is obtained.

[0047] The tensile strength of the insulated wire 6 with a crimp terminal at 25°C is 800 N / mm ２ or more, further, 900 N / mm ２ or more, further, 1000 N / mm ２ or more, further, 1200 N / mm ２ or more, further, 1300 N / mm ２ or more. The upper limit of the tensile strength of the insulated wire 6 with a crimp terminal at 25°C is 2500 N / mm ２ That is. The tensile strength of the copper-clad steel wire 1 at 25°C is measured based on the description of the examples described later.

[0048] Hereinafter, examples and comparative examples are shown to more specifically explain the copper-clad steel wire 1 of the present disclosure.

[0049] The copper-clad steel wire 1 and the insulated wire 5 were manufactured in order according to the manufacturing method of one embodiment (manufacture of samples A to P). Samples A, B, E, F, I, J, M, and N are examples. Samples C, D, G, H, K, L, O, and P are comparative examples.

[0050] In the first step S1, steel wire made of SUS304 or SUS301 was prepared as the raw material. As shown in Table 1, for samples A to L, SUS304 steel wire was prepared as the raw material. As shown in Table 1, for samples M to P, SUS301 steel wire was prepared as the raw material.

[0051] In the second step S2, a coating layer 3 made of pure copper was formed on the first outer surface 21 of the raw steel wire by copper plating.

[0052] In the third step S3, the raw steel wire and coating layer 3 were drawn using a die. In the third step S3, the approach angle (half angle) of the die was adjusted within the range of 3 to 8 degrees. The drawing speed was adjusted within the range of 80 m / min to 600 m / min. The reduction ratio per die was adjusted within the range of 14.5% to 25%. As a result, sample P (copper-clad steel wire 1) was produced from sample A. Specifically, in the third step S3, sample E was produced by setting the approach angle (half angle) of the die to 3 degrees, the drawing speed to 600 m / min, and the reduction ratio per die to 25%. In the third step S3, sample P was produced by setting the approach angle (half angle) of the die to 8 degrees, the drawing speed to 80 m / min, and the reduction ratio per die to 14.5%.

[0053] Subsequently, the outer surface 11 of the copper-clad steel wire 1 was covered with an insulating layer 4 to produce multiple samples (insulated wires 5).

[0054] (Evaluation) Samples A through P were evaluated according to the following method. The results are shown in Table 1.

[0055] [Wire diameter D of copper-clad steel wire 1 and thickness T of coating layer 3] The wire diameter D of copper-clad steel wire 1 and the thickness T of coating layer 3 were measured by laser outer diameter measurement and SEM (Scanning Electron Microscope) analysis of the orthogonal cross-section.

[0056] [First Area Ratio and Second Area Ratio] The vertical cross-section of the copper-clad steel wire 1 with its outer surface 11 exposed was observed using EBSD. As a pretreatment for EBSD, the copper-clad steel wire 1 was sequentially subjected to resin embedding and polishing with a cross-section polisher. For EBSD, the equipment used was a ZEISS Gemini 450, and the detector was an Oxford Symmetry. The acceleration voltage was 15 kV, and the step size was 0.1 μm. The first crystal grain 35 was identified, and the first area ratio and the second area ratio were determined.

[0057] Figure 3 shows an EBSD observation image of sample E, a copper-clad steel wire. Figure 8 shows an EBSD observation image of sample H, a copper-clad steel wire. Note that the dashed line in Figure 8 indicates the intermediate surface 30, which is not actually observed by EBSD. As shown in Figure 3, in sample E, the coating layer 3 contains first crystal grains 35 and second crystal grains 36. The second area ratio in sample E was 55% (see Table 1). In contrast, as shown in Figure 8, in sample H, the coating layer 3 does not contain first crystal grains 35 but contains second crystal grains 36. Therefore, the second area ratio in sample H was 0% (see Table 1).

[0058] [First Hardness Ratio] For the vertical cross-section of the copper-clad steel wire 1 with its outer surface 11 exposed, the hardness H1 of the inner region 33 and the hardness H2 of the outer region 34 were measured using the nanoindenter method. As a pretreatment for the nanoindenter method, the copper-clad steel wire 1 was subjected to resin embedding and polishing with a cross-section polisher in sequence. The apparatus used for the nanoindenter method was a Bruker Hystron TI980 triboindenter. The indenter was a Berkovich indenter, with a maximum load of 0.5 mN, a loading time of 5 seconds, a holding time of 2 seconds, and an unloading time of 5 seconds. The ambient temperature for the nanoindenter method was room temperature (23°C), and the atmosphere was air.

[0059] [Tensile strength of insulated wire with crimp terminals and connection test with crimp terminals] According to one embodiment, each end 1E of sample A to sample P was connected to a crimp terminal 80. The temperature during the above test was 25°C. The compression ratio of the coating layer 3 was set to 15%. The compression ratio of the coating layer 3 is 100 - (area of ​​coating layer 3 before crimping) / (area of ​​coating layer 3 after crimping) × 100. If the compression ratio is at least 15%, the core wire 20 will not break and sufficient crimping strength can be obtained. In other words, the copper-clad steel wire 1 was crimped to the extent that sufficient crimping strength could be obtained.

[0060] Figure 7 is a schematic diagram illustrating the tensile test method. As shown in Figure 7, the main body 83 of the crimp terminal 80 of the insulated wire 6 with a crimp terminal was held by the first chuck 91 of the tensile testing machine, and the insulated wire 5 was held by the second chuck 92 of the tensile testing machine. A tensile test was then performed, and the load at the time of fracture was measured. The crimping fracture rate was determined as the percentage of the load at the time of fracture of the insulated wire with a crimp terminal (crimped insulated wire 5) relative to the load at the time of fracture of the copper-clad steel wire 1. A crimping fracture rate of 60% or more indicates high connection reliability of the connection structure, and the connection structure is considered acceptable. A crimping fracture rate of less than 60% indicates a failure of the connection structure. From the viewpoint of obtaining high connection reliability, a crimping fracture rate of 90% or more is appropriate.

[0061]

[0062] [Summary of Evaluation] (First Area Ratio) Refer to Table 1 and compare samples A to D. Samples C and D, each with a first area ratio of less than 25%, failed the connection test, while samples A and B, each with a first area ratio of 25% or more, passed the connection test. Compare samples E to H. Samples G and H, each with a first area ratio of less than 25%, failed the connection test, while samples E and F, each with a first area ratio of 25% or more, passed the connection test. Compare samples I to L. Samples K and L, each with a first area ratio of less than 25%, failed the connection test, while samples I and J, each with a first area ratio of 25% or more, passed the connection test. Compare samples M to P. Samples O and P, each with a first area ratio of less than 25%, failed the connection test, while samples M and N, each with a first area ratio of 25% or more, passed the connection test.

[0063] (Second Area Ratio) Samples A and B were compared. The crimping fracture rate in the connection test of Sample A, which had a second area ratio of 50% or more, was higher than the crimping fracture rate in the connection test of Sample B, which had a second area ratio of less than 50%. Samples E and F were compared. The crimping fracture rate in the connection test of Sample E, which had a second area ratio of 50% or more, was higher than the crimping fracture rate in the connection test of Sample F, which had a second area ratio of less than 50%.

[0064] (First Hardness Ratio) Samples A and B were compared. The fracture rate during crimping in the connection test of Sample A, which had a first hardness ratio of 0.75 or less, was higher than the fracture rate during crimping in the connection test of Sample B, which had a first hardness ratio greater than 0.75. Samples E and F were compared. The fracture rate during crimping in the connection test of Sample E, which had a first hardness ratio of 0.75 or less, was higher than the fracture rate during crimping in the connection test of Sample F, which had a first hardness ratio greater than 0.75.

[0065] The embodiments disclosed herein should be understood to be illustrative in all respects and not restrictive in any way. The scope of the invention is defined by the claims and not by the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included. It should also be understood that at least one configuration described in the embodiments and examples can be combined or modified in various ways as appropriate.

[0066] 1 Copper-clad steel wire, 1E End, 2 Core wire, 3 Coating layer, 4 Insulation layer, 5 Insulated wire, 6 Insulated wire with crimp terminal, 11 Outer surface, 12 End, 20 Core wire, 21 First outer surface, 30 Intermediate surface, 31 Second inner surface, 32 Second outer surface, 33 Inner region, 34 Outer region, 35 First crystal grain, 36 Second crystal grain, 80 Crimp terminal, 81 Conductor barrel, 82 Insulation barrel, 83 Main body, 91 First chuck, 92 Second chuck, 351 Inner portion of first crystal grain, 352 Outer portion of first crystal grain, H1 Hardness (inner region), H2 Hardness (outer region), S0 Area (coating layer), S1 First process, S2 Second process, S3 Third process, T Thickness (coating layer).

Claims

1. A core wire made of austenitic stainless steel, and a coating layer made of copper or a copper alloy covering the first outer surface which is the outer surface of the core wire, wherein with respect to a vertical cross section which is a cross section perpendicular to the longitudinal direction of the core wire, the coating layer is 100 μm thick. ２ A copper-coated steel wire comprising a plurality of first crystal grains having an area exceeding a certain value, wherein, with respect to the vertical cross-section, the first area ratio, which is the percentage of the total area of ​​the first crystal grains relative to the area of ​​the coating layer, is 25% or more.

2. The copper-coated steel wire according to claim 1, wherein the coating layer has a second inner surface in contact with the first outer surface and a second outer surface located away from the second inner surface, and includes an inner region defined by the second inner surface and an intermediate surface which is a surface located at half the thickness of the coating layer from the second inner surface in the thickness direction of the coating layer, the first crystal grain includes an inner portion of the first crystal grain arranged within the inner region, and with respect to the vertical cross-section, the second area ratio, which is the percentage of the total area of ​​the inner portion of the first crystal grain to the area of ​​the inner region, is 50% or more.

3. The copper-coated steel wire according to claim 1 or 2, wherein the coating layer has a second inner surface in contact with the first outer surface and a second outer surface located away from the second inner surface, and the coating layer includes an inner region defined by the second inner surface and an intermediate surface which is a surface located at half the thickness of the coating layer from the second inner surface in the thickness direction of the coating layer, and an outer region defined by the second outer surface and the intermediate surface, and the first hardness ratio, which is the ratio of the hardness of the inner region measured by the nanoindenter method to the hardness of the outer region measured by the nanoindenter method, is 0.4 or more and 0.75 or less.

4. The copper-clad steel wire according to any one of claims 1 to 3, wherein the first area ratio is 75% or less.

5. The copper-clad steel wire according to any one of claims 1 to 4, wherein the first area ratio is 50% or less.

6. The copper-clad steel wire according to claim 2, wherein the second area ratio is 90% or less.

7. A copper-clad steel wire according to any one of claims 1 to 6, wherein the wire diameter is 0.1 mm or more and 0.8 mm or less.

8. The copper-coated steel wire according to any one of claims 1 to 7, wherein the thickness of the coating layer is 25 μm or more and 150 μm or less.

9. The copper-clad steel wire according to any one of claims 1 to 8, wherein the austenitic stainless steel forming the core wire is SUS304, SUS301, SUS310S, or SUS316 as specified in JIS.

10. An insulated electric wire comprising: a copper-clad steel wire according to any one of claims 1 to 9; and an insulating layer covering the outer surface of the copper-clad steel wire.

11. The tensile strength at 25°C of the insulated wire with a crimp terminal attached, in which a crimp terminal is attached to the copper-clad steel wire exposed at the end of the insulated wire, is 800 N / mm². ２ The insulated wire according to claim 10.

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