Patent Publication Number: US-2022238248-A1

Title: Copper-coated steel wire, spring, stranded wire, insulated electric wire, and cable

Description:
TECHNICAL FIELD 
     The present disclosure relates to a copper-coated steel wire, a spring, a stranded wire, an insulated electric wire, and a cable. 
     BACKGROUND ART 
     A copper-coated steel wire, with the surface of a steel material coated with copper, may be adopted in applications where both conductivity and strength are required (see, for example, Patent literatures 1 and 2). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-Open No. 2002-270039 
     Patent Literature 2: Japanese Patent Application Laid-Open No. H01-289021 
     SUMMARY OF INVENTION 
     A copper-coated steel wire according to the present disclosure includes: a core wire made of a stainless steel; and a coating layer made of copper or a copper alloy and covering an outer peripheral surface of the core wire. In a cross section perpendicular to a longitudinal direction of the core wire, the outer peripheral surface of the core wire has a value of an arithmetic mean roughness Ra of not less than 25% and not more than 90% of a thickness of the coating layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing the structure of a copper-coated steel wire in Embodiment 1; 
         FIG. 2  is a flowchart illustrating an outline of a method of producing a copper-coated steel wire; 
         FIG. 3  is a schematic cross-sectional view illustrating the method of producing the copper-coated steel wire; 
         FIG. 4  is a schematic cross-sectional view illustrating the method of producing the copper-coated steel wire: 
         FIG. 5  is a schematic cross-sectional view illustrating the method of producing the copper-coated steel wire; 
         FIG. 6  is a schematic cross-sectional view showing a first modification of the copper-coated steel wire in Embodiment 1; 
         FIG. 7  is a schematic cross-sectional view showing a second modification of the copper-coated steel wire in Embodiment 1; 
         FIG. 8  is a perspective view showing the structure of a spring in Embodiment 2; 
         FIG. 9  is a perspective view showing the structure of a stranded wire in Embodiment 3; 
         FIG. 10  is a schematic cross-sectional view showing the structure of an insulated electric wire in Embodiment 4; 
         FIG. 11  is a schematic cross-sectional view showing the structure of a cable in Embodiment 5; 
         FIG. 12  shows the results of a fatigue strength test; 
         FIG. 13  shows the results of the fatigue strength test; 
         FIG. 14  shows the results of the fatigue strength test; and 
         FIG. 15  shows a relationship between the number of repeated stress cycles and corrosion weight loss. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Problems to be Solved by the Present Disclosure 
     The aforementioned copper-coated steel wire includes a core wire and a coating layer made of copper or a copper alloy. Copper-coated steel wire may be used in applications where stress is applied repeatedly. Such repeatedly applied stress may cause cracking at the interface of the coating layer with the core wire, leading to a decreased conductivity or breakage of the steel wire. In addition, the above copper-coated steel wire is required to suppress the occurrence of corrosion in the core wire. 
     In view of the foregoing, one of the objects is to provide a copper-coated steel wire which can suppress the occurrence of cracking at the interface of the coating layer with the core wire and also suppress the occurrence of corrosion in the core wire. 
     Advantageous Effects of the Present Disclosure 
     According to the copper-coated steel wire of the present disclosure, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed, and the occurrence of corrosion in the core wire can also be suppressed. 
     DESCRIPTION OF EMBODIMENTS OF THE PRESENT DISCLOSURE 
     Firstly, embodiments of the present disclosure will be listed and described. A copper-coated steel wire of the present disclosure includes a core wire made of a stainless steel and a coating layer made of coper or a copper alloy and covering an outer peripheral surface of the core wire. In a cross section perpendicular to a longitudinal direction of the core wire, the outer peripheral surface of the core wire has a value of an arithmetic mean roughness Ra of not less than 25% and not more than 90% of a thickness of the coating layer. 
     In the copper-coated steel wire of the present disclosure, the core wire made of a stainless steel assures high strength. The coating layer made of copper or a copper alloy ensures excellent conductivity. Further, in a cross section perpendicular to the longitudinal direction of the core wire, the value of Ra of the outer peripheral surface of the core wire is set to be not less than 25% and not more than 90% of the thickness of the coating layer. Protrusions and indentations thus formed on the surface of the core wire increase the bonding strength between the core wire and the coating layer. As a result, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed. Setting the value of Ra to be not less than 25% of the thickness of the coating layer can reliably improve the bonding strength between the core wire and the coating layer. Setting the value of Ra to be not more than 90% of the thickness of the coating layer can maintain sufficient strength of the core wire. When protrusions and indentations are formed on the surface of the core wire, the interface of the coating layer with the core wire increases in area, leading to an increased possibility of occurrence of corrosion at the dissimilar metal interface of the core wire with the coating layer. The use of the stainless steel as the material constituting the core wire can suppress the occurrence of corrosion at the dissimilar metal interface. 
     As described above, according to the copper-coated steel wire of the present disclosure, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed, and the occurrence of corrosion in the core wire can also be suppressed. 
     In the copper-coated steel wire described above, the outer peripheral surface of the core wire in the cross section perpendicular to the longitudinal direction of the core wire may have a value of a maximum cross-sectional height Rt of not less than 45% and not more than 300% of the thickness of the coating layer. Setting the value of Rt to be not less than 45% of the thickness of the coating layer can more reliably improve the joining strength between the core wire and the coating layer. If the value of Rt exceeds 300% of the thickness of the coating layer, the conductivity of the coating layer may be reduced. Therefore, the value of Rt of not more than 300% of the thickness of the coating layer is preferable. 
     A copper-coated steel wire of the present disclosure includes: a core wire made of a stainless steel; and a coating layer made of copper or a copper alloy and covering an outer peripheral surface of the core wire. In a cross section perpendicular to a longitudinal direction of the core wire, the outer peripheral surface of the core wire has a value of a maximum cross-sectional height Rt of not less than 45% and not more than 300% of a thickness of the coating layer. 
     In the copper-coated steel wire of the present disclosure, the value of Rt of the outer peripheral surface of the core wire in the cross section perpendicular to the longitudinal direction of the core wire is set to be not less than 45% and not more than 300% of the thickness of the coating layer. Setting the value of Rt to be not less than 45% of the thickness of the coating layer can reliably improve the bonding strength between the core wire and the coating layer. If the value of Rt exceeds 300% of the thickness of the coating layer, the conductivity of the coating layer may be reduced. Therefore, the value of Rt of not more than 300% of the thickness of the coating layer is preferable. When protrusions and indentations satisfying the above conditions are formed on the surface of the core wire, the area of the interface of the coating layer with the core wire increases, leading to an increased possibility of occurrence of corrosion at the dissimilar metal interface of the core wire with the coating layer. The use of the stainless steel as the material constituting the core wire can suppress the occurrence of corrosion at the dissimilar metal interface. According to the copper-coated steel wire of the present disclosure as well, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed, and the occurrence of corrosion in the core wire can also be suppressed. 
     In the copper-coated steel wire of the present disclosure, the steel constituting the core wire may be a ferritic stainless steel. The ferritic stainless steel is a suitable material for constituting the above-described core wire. 
     In the copper-coated steel wire of the present disclosure, the steel constituting the core wire may be an austenitic stainless steel. The austenitic stainless steel is a suitable material for constituting the above-described core wire. 
     In the copper-coated steel wire of the present disclosure, the austenitic stainless steel may have a component composition satisfying the following expression (1). The austenitic stainless steel having a component composition satisfying the following expression (1) is a suitable material for constituting the above-described core wire. 
       −400≥1032−1667×( A+B )−27.8× C− 33× D− 61× E− 41.7× F   [Math. 1]
 
     where A represents a carbon content [mass %], B represents a nitrogen content [mass %], C represents a silicon content [mass %], D represents a manganese content [mass %], E represents a nickel content [mass %], and F represents a chromium content [mass %]. 
     In the copper-coated steel wire of the present disclosure, the coating layer may include an alloy layer disposed in a region including an interface with the core wire, the alloy layer containing an alloy of nickel and a metallic element contained in the steel constituting the core wire. The formation of such an alloy layer can increase the bonding force between the core wire and the coating layer and more reliably suppress the occurrence of cracking at the interface of the coating layer with the core wire. 
     The copper-coated steel wire of the present disclosure may have a tensile strength of not less than 300 MPa and not more than 3400 MPa. With the tensile strength set to be 300 MPa or more, sufficient strength can be obtained. With the tensile strength set to be 3400 MPa or less, sufficient workability can be ensured. 
     The copper-coated steel wire of the present disclosure may further include a surface layer disposed to include a surface of the copper-coated steel wire, the surface layer including at least one selected from the group consisting of a gold layer, a silver layer, a tin layer, a palladium layer, a nickel layer, and an alloy layer of these metals. Such a configuration improves the corrosion resistance, solderability, and conductivity on the surface of the copper-coated steel wire. 
     A spring of the present disclosure is made of the copper-coated steel wire described above. According to the spring of the present disclosure, with it being made of the above-described copper-coated steel wire, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed, and the occurrence of corrosion in the core wire can also be suppressed. It is therefore possible to provide a spring having excellent durability. 
     A stranded wire of the present disclosure is composed of a plurality of the above-described copper-coated steel wires twisted together. According to the stranded wire of the present disclosure, with it having the structure of the above-described copper-coated steel wires twisted together, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed, and the occurrence of corrosion in the core wire can also be suppressed. It is therefore possible to provide a stranded wire having excellent durability. 
     An insulated electric wire of the present disclosure includes: the above-described copper-coated steel wire or the above-described stranded wire; and an insulating layer disposed to cover an outer periphery of the copper-coated steel wire or the stranded wire. According to the insulated electric wire of the present disclosure, with it including the above-described copper-coated steel wire or the above-described stranded wire, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed, and the occurrence of corrosion in the core wire can also be suppressed. It is therefore possible to provide an insulated electric wire having excellent durability. 
     A cable of the present disclosure includes: a conductor portion of a wire shape; an insulating layer disposed to cover an outer peripheral surface of the conductor portion; and a shielding portion disposed to surround an outer peripheral surface of the insulating layer. The shielding portion includes a plurality of the above-described copper-coated steel wires. According to the cable of the present disclosure, with the shielding portion including the plurality of the above-described copper-coated steel wires, the durability of the shielding portion can be improved. 
     A cable of the present disclosure includes: the above-described copper-coated steel wire or the above-described stranded wire; an insulating layer disposed to cover an outer periphery of the copper-coated steel wire or the stranded wire; and a shielding portion disposed to surround an outer peripheral surface of the insulating layer. According to the cable of the present disclosure, with it including the above-described copper-coated steel wire or the above-described stranded wire, the occurrence of cracking at the interface of the coating layer with the core wire can be suppressed, and the occurrence of corrosion in the core wire can also be suppressed. It is therefore possible to provide a cable having excellent durability. 
     In the above-described cable, the shielding portion may include a plurality of the above-described copper-coated steel wires. With the shielding portion including the plurality of the above-described copper-coated steel wires, the durability of the shielding portion can be improved. 
     Details of Embodiments of the Present Disclosure 
     Embodiments of a copper-coated steel wire according to the present disclosure will be described below with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and the descriptions thereof will not be repeated. 
     Embodiment 1 
       FIG. 1  is a cross-sectional view perpendicular to the longitudinal direction of a core. Referring to  FIG. 1 , a copper-coated steel wire  1  in the present embodiment includes a core wire  10  and a coating layer  20 . The core wire  10  is made of a stainless steel. The coating layer  20  covers an outer peripheral surface  11  of the core wire  10 . The coating layer  20  is made of copper or a copper alloy. The copper-coated steel wire  1  has a circular cross section perpendicular to the longitudinal direction thereof. 
     In the present embodiment, the stainless steel constituting the core wire  10  is an austenitic stainless steel. The austenitic stainless steel in the present embodiment has a component composition that satisfies the following expression (1). The stainless steel constituting the core wire  10  in the present embodiment is, for example, SUS 304 specified in JIS standard. 
       −400≥1032−1667×( A−+B )−27.8× C− 33× D− 61× E− 41.7× F   [Math. 1]
 
     where A represents a carbon content [mass %], B represents a nitrogen content [mass %], C represents a silicon content [mass %], D represents a manganese content [mass %], E represents a nickel content [mass %], and F represents a chromium content [mass %]. 
     In the cross section perpendicular to the longitudinal direction, the outer peripheral surface  11  of the core wire  10  has a value of the arithmetic mean roughness Ra of not less than 25% and not more than 90% of a thickness t of the coating layer  20 . The value of Ra of the outer peripheral surface  11  of the core wire  10  is preferably not less than 27% and not more than 75%, and more preferably not less than 30% and not more than 60%. Here, for measuring the Ra, the following method, for example, is carried out. Firstly, a sample is taken from the copper-coated steel wire  1 . Next, a cross section perpendicular to the longitudinal direction of the obtained sample is polished. Then, the interface of the core wire  10  with the coating layer  20  in the polished surface is observed to derive the Ra of the outer peripheral surface  11  of the core wire  10 . The Ra is determined, in accordance with JIS B 0601:2013, by measuring the entire outer peripheral surface  11  of the core wire  10 . The thickness t of the coating layer  20  can be determined in the following manner. Firstly, the area of the core wire  10  in the cross section perpendicular to the longitudinal direction is measured. Next, for a circle (indicated by the broken line in  FIG. 1 ) corresponding to the obtained area, its radius (equivalent circle radius) is calculated. Then, the difference between the radius of the copper-coated steel wire  1  and the equivalent circle radius of the core wire  10  is regarded as the thickness t of the coating layer  20 . 
     In the present embodiment, in the cross section perpendicular to the longitudinal direction, the outer peripheral surface  11  of the core wire  10  has a value of the maximum cross-sectional height Rt of not less than 45% and not more than 300% of the thickness t of the coating layer  20 . The value of Rt of the outer peripheral surface  11  of the core wire  10  is preferably not less than 50% and not more than 250%, and more preferably not less than 100% and not more than 200%. Here, for measuring the Rt, the following method, for example, is carried out. Firstly, a sample is taken from the copper-coated steel wire  1 . Next, a cross section perpendicular to the longitudinal direction of the obtained sample is polished. Then, the interface of the core wire  10  with the coating layer  20  in the polished surface is observed to derive the Rt of the outer peripheral surface  11  of the core wire  10 . The Rt is determined, in accordance with JIS B 0601:2013, by measuring the entire outer peripheral surface  11  of the core wire  10 . 
     An exemplary method of producing the copper-coated steel wire  1  will now be described.  FIG. 3  is a cross-sectional view of a material steel wire in its cross section perpendicular to the longitudinal direction. Referring to  FIG. 2 , in the method of producing the copper-coated steel wire  1  of the present embodiment, firstly, a material steel wire preparing step is conducted as a step S 10 . In this step S 10 , referring to  FIG. 3 , a material steel wire  90  to be the core wire  10  is prepared. In the present embodiment, the steel constituting the material steel wire  90  is SUS 304. 
     Next, referring to  FIG. 2 , a first drawing step is conducted as a step S 20 . In this step S 20 , the material steel wire prepared in step S 10  is subjected to drawing. Next, referring to  FIG. 2 , a surface roughening step is conducted as a step S 30 . In this step S 30 , the material steel wire  90  that has undergone drawing in step S 20  is subjected to a surface roughening process to increase the surface roughness. Specifically, referring to  FIG. 3 , the material steel wire  90  has its surface  91  brought into contact with an acid such as hydrochloric acid or sulfuric acid for increasing the surface roughness. For example, hydrochloric acid with a concentration of 35% can be used. The concentration of sulfuric acid can be, for example, 65%. In the process of producing a steel wire, a pickling process may be carried out for the purpose of cleaning the surface of the steel wire or removing the oxide coating. However, the surface roughening process in the step S 30  differs from the general pickling process in that a highly concentrated acid or a highly corrosive acid is used, or the time of contact with the acid is increased, to achieve the surface roughening. The arithmetic mean roughness Ra at this point in time can be 0.8 μm or more, for example. The surface roughening process may include, instead of or in addition to the process of making the wire surface contact the acid, a process of mechanically achieving the surface roughening by, for example, pressing a polishing non-woven fabric against the surface  91  of the material steel wire  90  and moving the fabric relative to the surface. In this manner, referring to  FIG. 4 , the core wire  10  in the present embodiment having protrusions and indentations formed on the outer peripheral surface  11  is obtained. 
     Next, referring to  FIG. 2 , a coating layer forming step is conducted as a step S 40 . In this step S 40 , referring to  FIGS. 4 and 5 , a coating layer  20  made of copper or a copper alloy is formed to cover the outer peripheral surface  11  of the core wire  10  that has undergone the surface roughening process in step S 30 . The coating layer  20  formed in the step S 40  has a thickness of, for example, not less than 30 μm and not more than 90 μm. The coating layer  20  may be formed by plating, for example, or may be formed as a cladding layer, which is obtained by separately preparing a member to be the coating layer  20  and mechanically integrating the member with the core wire  10 . 
     Next, referring to  FIG. 2 , a second drawing step is conducted as a step S 50 . In this step S 50 , referring to  FIG. 5 , the core wire  10  with the coating layer  20  formed thereon in step S 40  is subjected to drawing. With this, a copper-coated steel wire  1  having a desired wire diameter is obtained. The degree of working (reduction of area) and the true strain in step S 50  can be, for example, 90% or more and 2.3 or more, respectively. The above procedure completes the production of the copper-coated steel wire  1  in the present embodiment. 
     Here, in the copper-coated steel wire  1  in the present embodiment, the value of Ra of the outer peripheral surface  11  of the core wire  10  in a cross section perpendicular to the longitudinal direction of the core wire  10  is set to be not less than 25% and not more than 90% of the thickness of the coating layer  20 . Setting the value of Ra to be not less than 25% of the thickness of the coating layer  20  reliably improves the bonding strength between the core wire  10  and the coating layer  20 . Setting the value of Ra to be not more than 90% of the thickness of the coating layer  20  can maintain sufficient strength of the core wire  10 . When protrusions and indentations are formed on the outer peripheral surface  11  of the core wire  10 , an interface  20 A (see  FIG. 1 ) of the coating layer  20  with the core wire  10  increases in area, leading to an increased possibility of the occurrence of corrosion at the dissimilar metal interface of the core wire  10  with the coating layer  20 . The use of a stainless steel as the material constituting the core wire  10  can suppress the occurrence of corrosion at the dissimilar metal interface. As described above, according to the copper-coated steel wire  1  in the present embodiment, the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  can be suppressed, and the occurrence of corrosion in the core wire  10  can also be suppressed. 
     In the above embodiment, the value of Rt of the outer peripheral surface  11  of the core wire  10  in the cross section perpendicular to the longitudinal direction of the core wire  10  is not less than 45% and not more than 300% of the thickness of the coating layer  20 . Although it is not essential to set the value of Rt within the above-described range, setting the value of Rt to be not less than 45% of the thickness of the coating layer  20  can more reliably improve the joining strength between the core wire  10  and the coating layer  20 . If the value of Rt exceeds 300% of the thickness of the coating layer  20 , the conductivity of the coating layer  20  may be reduced. Therefore, the value of Rt of not more than 300% of the thickness of the coating layer  20  is preferable. 
     In the above embodiment, the description was made of the case where, in a cross section perpendicular to the longitudinal direction of the core wire  10 , the value of Ra of the outer peripheral surface  11  of the core wire  10  is not less than 25% and not more than 90% of the thickness of the coating layer  20  and the value of Rt of the outer peripheral surface  11  of the core wire  10  is not less than 45% and not more than 300% of the thickness of the coating layer  20 . However, the configuration is not limited to the above case; only one of the values of Ra and Rt may be set to fall within the above-described range. When protrusions and indentations satisfying the above conditions are formed on the surface of the core wire, the area of the interface  20 A of the coating layer  20  with the core wire  10  increases, leading to an increased possibility of the occurrence of corrosion at the dissimilar metal interface of the core wire  10  with the coating layer  20 . The use of a stainless steel as the material constituting the core wire can suppress the occurrence of corrosion at the dissimilar metal interface. With such a copper-coated steel wire  1  as well, the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  can be suppressed, and the occurrence of corrosion in the core wire  10  can also be suppressed. 
     In the above embodiment, the description was made of the case where the steel constituting the core wire  10  is an austenitic stainless steel. However, not limited thereto, the steel constituting the core wire  10  may be a ferritic stainless steel. 
     In the copper-coated steel wire  1  of the above embodiment, the tensile strength may be not less than 300 MPa and not more than 3400 MPa. With the tensile strength set to be 300 MPa or more, sufficient strength can be obtained. With the tensile strength set to be 3400 MPa or less, sufficient workability can be ensured. The tensile strength is measured, for example, in accordance with JIS Z 2241. 
     In the copper-coated steel wire  1  of the above embodiment, the electrical conductivity may be not less than 5% IACS and not more than 80% IACS, where IACS is an abbreviation for International Annealed Copper Standard. This ensures sufficient conductivity in various applications. 
     Now, a first modification of the copper-coated steel wire  1  in Embodiment 1 will be described.  FIG. 6  is an enlarged view of the vicinity of the interface  20 A of the coating layer  20  with the core wire  10  in a cross section perpendicular to the longitudinal direction of the core wire  10 . Referring to  FIG. 6 , the coating layer  20  in the present modification includes an alloy layer  19  disposed in a region including the interface  20 A with the core wire  10 . The alloy layer  19  includes an alloy of nickel and a metallic element contained in the steel constituting the core wire  10 . Although the presence of the alloy layer  19  is not essential in the copper-coated steel wire of the present application, the formation of the alloy layer  19  can increase the bonding force between the core wire  10  and the coating layer  20  and more reliably suppress the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10 . 
     Next, a second modification of the copper-coated steel wire  1  in Embodiment 1 will be described.  FIG. 7  is across-sectional view of the core wire  10  in its cross section perpendicular to the longitudinal direction. Referring to  FIG. 7 , the copper-coated steel wire  1  in the present modification includes a surface layer  30  disposed to include the surface of the copper-coated steel wire  1 . The surface layer  30  includes at least one selected from the group consisting of a gold layer, a silver layer, a tin layer, a palladium layer, a nickel layer, and an alloy layer of these metals. Although the presence of the alloy layer  19  is not essential in the copper-coated steel wire of the present application, the inclusion of the surface layer  30  can improve the corrosion resistance, solderability, and conductivity on the surface of the copper-coated steel wire  1 . 
     Embodiment 2 
     A description will now be made, as Embodiment 2, of an embodiment of a spring of the present disclosure. Referring to  FIG. 8 , a spring  100  in the present embodiment is made of the copper-coated steel wire  1  of Embodiment 1 described above. The spring  100  is a copper-coated steel wire  1  of the above-described Embodiment 1 coiled into a spring shape. The spring  100  in the present embodiment is a helical spring which has a structure in which the copper-coated steel wire  1  is wound inclined with respect to a plane perpendicular to the direction along a central axis P. The spring  100  in the present embodiment is a canted coil spring that is used such that a load is applied in a direction perpendicular to the axial direction. According to the spring  100  in the present embodiment, as being made of the above-described copper-coated steel wire, the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  can be suppressed, and the occurrence of corrosion in the core wire  10  can also be suppressed. It is therefore possible to provide a spring  100  that is excellent in durability. While the description was made in the present embodiment of the case where the spring  100  is a canted coil spring, it may be a spring that is used such that a load is applied in the axial direction of the spring  100 . 
     Embodiment 3 
     A description will now be made, as Embodiment 3, of an embodiment of a stranded wire of the present disclosure. In  FIG. 9 , cross sections of copper-coated steel wires  1  perpendicular to the longitudinal direction are illustrated as well. Referring to  FIG. 9 , a stranded wire  200  in the present embodiment is composed of a plurality of the copper-coated steel wires  1  of the above-described Embodiment 1 twisted together. In the present embodiment, the stranded wire has a structure in which seven copper-coated steel wires  1  are twisted together. Each copper-coated steel wire  1  included in the stranded wire  200  is the copper-coated steel wire of the above-described Embodiment 1. With the stranded wire  200  in the present embodiment having the structure in which the copper-coated steel wires  1  of the above-described Embodiment 1 are twisted together, the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  can be suppressed, and the occurrence of corrosion in the core wire  10  can also be suppressed. It is therefore possible to provide a stranded wire  200  that is excellent in durability. 
     Embodiment 4 
     A description will now be made, as Embodiment 4, of an embodiment of an insulated electric wire of the present disclosure.  FIG. 10  is a cross-sectional view of a copper-coated steel wire  1  in its cross section perpendicular to the longitudinal direction. Referring to  FIG. 10 , an insulated electric wire  300  in the present embodiment includes the copper-coated steel wire  1  of Embodiment 1 described above, and an insulating layer  40  disposed to cover an outer periphery  1 A of the copper-coated steel wire  1 . According to the insulated electric wire  300  of the present disclosure, with it including the copper-coated steel wire  1  of the above-described Embodiment 1, the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  can be suppressed, and the occurrence of corrosion in the core wire  10  can also be suppressed. It is therefore possible to provide an insulated electric wire  300  that is excellent in durability. While the case of using the copper-coated steel wire  1  was described in the present embodiment, the wire is not limited thereto; the stranded wire  200  of Embodiment 3 may be used in place of the copper-coated steel wire  1 . 
     Embodiment 5 
     A description will now be made, as Embodiment 5, of an embodiment of a cable of the present disclosure. In  FIG. 11 , cross sections of stranded wire, insulating layer, shielding portion, and protective layer perpendicular to the longitudinal direction are illustrated as well. Referring to  FIG. 11 , a cable  400  includes the stranded wire  200  of Embodiment 3, an insulating layer  40  disposed to cover an outer periphery  200 A of the stranded wire  200 , a shielding portion  50  disposed to surround an outer peripheral surface  40 A of the insulating layer  40 , and a protective layer  60  disposed to cover an outer periphery  50 A of the shielding portion  50 . The shielding portion  50  includes a plurality of the copper-coated steel wires  1  of Embodiment 1 above. The shielding portion  50  in the present embodiment has a shape of the plurality of copper-coated steel wires  1  of the above-described Embodiment 1 woven together. According to the cable  400  of the present disclosure, with it having the structure including the stranded wire  200 , the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  can be suppressed, and the occurrence of corrosion in the core wire  10  can also be suppressed. In addition, with the shielding portion  50  including a plurality of the above-described copper-coated steel wires  1 , the durability of the shielding portion  50  can be improved. It is therefore possible to provide a cable  400  that is excellent in durability. Although the case of using the stranded wire  200  as a conductor portion was described in the present embodiment, the conductor portion is not limited thereto, the copper-coated steel wire  1  in Embodiment 1 may be used in place of the stranded wire  200 . Further, while the case where the shielding portion  50  includes a plurality of copper-coated steel wires  1  was described, the configuration is not limited thereto; the shielding portion  50  may be composed of a wire material other than in the present embodiment. Furthermore, the conductor portion may be composed of a wire material other than in the present embodiment, and only the shielding portion  50  may include a plurality of the copper-coated steel wires  1  of the above-described Embodiment 1. 
     Examples 
     Experiments were conducted to investigate how the value of the arithmetic mean roughness Ra and the maximum cross-sectional height Rt of the core wire  10  with respect to the thickness of the coating layer  20  in a cross section perpendicular to the longitudinal direction affect the properties of the copper-coated steel wire  1 . Firstly, the steps S 10  to S 50  of the above embodiment were performed to prepare a sample of the copper-coated steel wire  1 . For the steel constituting the material steel wire prepared in step S 10 , SUS 304 was adopted. A sample A was thus obtained. The sample A had a wire diameter of 2 mm, a core wire diameter of 0.8 mm, and a coating layer thickness t of 200 μm. The value of Ra of the outer peripheral surface  1 I of the core wire  10  in the sample A was 33% of the thickness t of the coating layer  20 . The value of Rt of the outer peripheral surface  11  of the core wire  10  in the sample A was 58% of the thickness t of the coating layer  20 . 
     Samples B to H were prepared which differed from the sample A in at least one of the diameter of the core wire  10 , the thickness of the coating layer  20 , the value of Ra of the outer peripheral surface  11  of the core wire  10 , and the value of Rt of the outer peripheral surface  11  of the core wire  10 . For comparison, samples I to L were prepared for which SWP-B was adopted as the material steel wire. 
     Next, the tensile strength was measured for the samples A to L. The measurement results are shown in Table 1. As a fatigue test, a Hunter fatigue test was conducted.  FIGS. 12 to 14  are S-N diagrams showing the relationship between the number of stress cycles to breakage and the stress amplitude in the fatigue test. In  FIGS. 12 to 14 , the vertical axis represents stress amplitude, and the horizontal axis represents number of stress cycles. The stress amplitude is expressed in MPa. For each sample, a maximum stress amplitude at which the copper-coated steel wire  1  did not break even after 1×10 7  cycles of repeated stress in a fatigue test was measured. Further, on the samples A, C, D, F, G, and H, a fatigue test was conducted by repeatedly loading the above-described maximum stress amplitude. Then, corrosion weight loss was measured by spraying salt water on the samples that had undergone 100, 10000, or 1000000 cycles of repeated stress. The corrosion weight loss of each sample before being subjected to the fatigue test was also measured after spraying salt water. The salt water spraying was conducted in accordance with JIS Z 2371.  FIG. 15  shows the relationship between the number of repeated stress cycles and the corrosion weight loss.  FIG. 15  shows the corrosion weight loss of each sample when 0, 100, 10000, or 1000000 cycles of repeated stress were applied. In  FIG. 15 , the vertical axis represents corrosion weight loss, and the horizontal axis represents number of repeated stress cycles. The corrosion weight loss is expressed in mg/mm 2 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Wire 
                 Core Wire 
                 Coating Layer 
                 Percentage of 
                 Percentage of Maximum 
                 Tensile 
               
               
                   
                 Steel 
                 Diameter 
                 Diameter 
                 Thickness 
                 Arithmetic Mean 
                 Cross-Sectional Height 
                 Strength 
               
               
                   
                 Grade 
                 (mm) 
                 (mm) 
                 (μm) 
                 Roughness Ra (%) 
                 Rt (%) 
                 (MPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Sample A 
                 SUS304 
                 1 
                 0.8 
                 200 
                 33 
                 58 
                 1440 
               
               
                 Sample B 
                 SUS304 
                 1 
                 0.8 
                 200 
                 27 
                 53 
                 1450 
               
               
                 Sample C 
                 SUS304 
                 0.5 
                 0.45 
                 75 
                 45 
                 102 
                 1980 
               
               
                 Sample D 
                 SUS304 
                 0.25 
                 0.18 
                 35 
                 75 
                 180 
                 1160 
               
               
                 Sample E 
                 SUS304 
                 0.25 
                 0.18 
                 35 
                 34 
                 45 
                 1170 
               
               
                 Sample F 
                 SUS304 
                 0.1 
                 0.085 
                 7.5 
                 57 
                 132 
                 2015 
               
               
                 Sample G 
                 SUS304 
                 1 
                 0.8 
                 200 
                 21 
                 32 
                 1470 
               
               
                 Sample H 
                 SUS304 
                 0.25 
                 0.18 
                 35 
                 92 
                 320 
                 1155 
               
               
                 Sample I 
                 SWP-B 
                 1 
                 0.8 
                 200 
                 18 
                 40 
                 1520 
               
               
                 Sample J 
                 SWP-B 
                 0.25 
                 0.18 
                 35 
                 12 
                 32 
                 1170 
               
               
                 Sample K 
                 SWP-B 
                 0.1 
                 0.085 
                 7.5 
                 15 
                 39 
                 7025 
               
               
                 Sample L 
                 SWP-B 
                 0.05 
                 0.042 
                 4 
                 9 
                 25 
                 2220 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, it is confirmed that as to the tensile strength, the samples A to F, having the percentage of Ra within the range of 25% or more and 90% or less and the percentage of Rt within the range of 45% or more and 300% or less, exhibit the values of 300 MPa or more and 3400 MPa or less, which is an appropriate range. Referring to  FIGS. 12 to 14 , as to the maximum stress amplitude, the samples A to F clearly surpass the samples G to L having the percentages of Ra and Rt falling outside the above-described ranges. This is conceivably because the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  has been suppressed in the samples A to F having the percentages of Ra and Rt within the above-described ranges. Further, referring to  FIG. 15 , as to the corrosion weight loss, the increase in corrosion weight loss is suppressed in the samples A, C, D, and F having the percentages of Ra and Rt within the above-described ranges, as compared to the samples G and H having the percentages of Ra and Rt falling outside the above-described ranges. This is more remarkable with an increasing number of repeated stress cycles. 
     The above experimental results demonstrate that according to the copper-coated steel wire  1  of the present disclosure, it is possible to provide a copper-coated steel wire that is capable of suppressing the occurrence of cracking at the interface  20 A of the coating layer  20  with the core wire  10  and also suppressing the occurrence of corrosion in the core wire  10 . 
     It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1 : copper-coated steel wire;  1 A,  50 A,  200 A: outer periphery;  10 : core wire;  11 ,  40 A: outer peripheral surface;  19 : alloy layer;  20 : coating layer;  20 A: interface;  30 : surface layer;  40 : insulating layer;  50 : shielding portion;  60 : protective layer;  90 : material steel wire;  91 : surface;  100 : spring;  200 : stranded wire;  300 : insulated electric wire;  400 : cable; P: central axis; t: thickness; and A, B, C, D, E, F, G, H, I, J, K, L: sample.