Patent Publication Number: US-2023154654-A1

Title: Wire Drawing Method and Superconducting Wire

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wire drawing method and a superconducting wire. 
     2. Description of the Related Art 
     A high-temperature superconducting wire is manufactured by filling a metal tube with a mixed powder, further introducing a plurality of the metal tubes filled with the mixed powder into another tube, and processing the tube into an elongated wire by a wire drawing method. In the method, a wire drawing method generally used for a metal tube or a metal rod is applied. A wire pulling-out method, which is an example of the wire drawing method, is described in, for example, JP-A-2013-252565. 
     The above wire pulling-out method is a processing method in which a material to be drawn passes through a die hole having a hole diameter smaller than a maximum diameter of the material, so that a cross-sectional diameter of the material is reduced to a diameter that is the same as the hole diameter. A step of passing the material through a die hole whose die hole diameter gradually decreases is performed a plurality of times until a target cross-sectional diameter is obtained. 
     For example, a high-temperature superconducting wire includes a cylindrical copper tube positioned on a central portion of a cross section perpendicular to a longitudinal direction of the wire, a plurality of cylindrical iron tubes disposed around the copper tube and filled with a mixed powder, and a cylindrical Monel tube disposed outside the plurality of cylindrical iron tubes, and a wire formed of a plurality of materials is drawn. 
     When the wire pulling-out method is used, the step of passing the material through the die hole is repeatedly performed to manufacture an elongated wire. In the wire drawing of a wire including a plurality of cylindrical metal members and a compressive material (for example, the mixed powder), deformation starts from a metal tube positioned on an outermost circumferential side. During the wire drawing, since a location where a pressure is applied from the metal tube positioned on the outermost circumferential side to a material inside is constant, stress is locally concentrated on the location. When a shape of each of the metal tubes filled with the compressive material and disposed around the metal tube positioned at a central portion of a cross section is a cylindrical shape, a contact state between the cylindrical metal tubes is point contact in the middle of the wire drawing. 
     As a result, local stress concentration and a non-uniform average porosity distribution are generated inside the wire, and thus defects such as disconnection and performance deterioration of the wire may occur. In addition, since the contact state between the metal tubes is unstable, compression of the material during the wire drawing becomes non-uniform. 
     For example, when a plurality of cylindrical iron tubes filled with the mixed powder are used for the high-temperature superconducting wire, a decrease in processing ability occurs due to the local deformation and the unstable contact state in the wire during the wire drawing. Therefore, it is an object to ensure processing stability by preventing a shape from deforming non-uniformly. 
     In particular, in the high-temperature superconducting wire, an average porosity distribution of the mixed powder becomes non-uniform due to the non-uniform deformation, and thus performance of the superconducting wire deteriorates, and it is an object to prevent a quality variation of the superconducting wire due to the processing stability. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to ensure the processing stability by preventing a shape from deforming non-uniformly in a wire drawing method. 
     A wire drawing method according to an aspect of the invention includes reducing a cross-sectional diameter of a first wire by wire drawing. The first wire includes a center member, a plurality of first peripheral wires surrounding the center member, and an outer shell disposed outside the first peripheral wires. Each of the first peripheral wires includes a compressive material and a metal sheath covering the compressive material, and a shape of a cross section perpendicular to a longitudinal direction of the first peripheral wire is a substantially isosceles trapezoidal shape including a long side in contact with the outer shell, a short side in contact with the center member, and a first oblique side and a second oblique side that are in contact with the adjacent peripheral wires. 
     A superconducting wire according to another aspect of the invention includes a core, a plurality of peripheral wires surrounding the core, and an outer shell disposed outside the peripheral wires. Each of the peripheral wires includes a porous material and a peripheral-wire cover covering the porous material, a shape of a cross section perpendicular to a longitudinal direction of the peripheral wire is a substantially annular sector including a first side in contact with the outer shell, a second side in contact with the core, and a third side and a fourth side that are in contact with the adjacent peripheral wires, a first average pore diameter of the porous material at a midpoint of the first side is larger than either one of a second average pore diameter of the porous material at a first point that is an intersection of the first side and the third side and a third average pore diameter of the porous material at a second point that is an intersection of the first side and the fourth side, and a fourth average pore diameter of the porous material at a midpoint of the second side is smaller than either one of a fifth average pore diameter of the porous material at a third point that is an intersection of the second side and the third side and a sixth average pore diameter of the porous material at a fourth point that is an intersection of the second side and the fourth side. 
     A superconducting wire according to still another aspect of the invention includes a core, a plurality of peripheral wires surrounding the core, and an outer shell disposed outside the peripheral wires. Each of the peripheral wires includes a porous material and a peripheral-wire cover covering the porous material, a shape of a cross section perpendicular to a longitudinal direction of the peripheral wire is a substantially annular sector including a first side in contact with the outer shell, a second side in contact with the core, and a third side and a fourth side that are in contact with the adjacent peripheral wires, a pore diameter of the porous material is always larger than a minimum value of pore diameters at (1) a first point that is an intersection of the first side and the third side, (2) a second point that is an intersection of the first side and the fourth side, and (3) a midpoint of the second side, and a pore diameter of the porous material at a midpoint of the first side is larger than a maximum value of pore diameters at the first point and the second point. 
     According to an aspect of the invention, in a wire drawing method, the processing stability can be ensured by preventing a shape from deforming non-uniformly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a cross-sectional view of a wire including a center member, peripheral wires each having a substantially isosceles trapezoidal cross-sectional shape, and an outer shell before wire drawing, and  FIG.  1 B  is a cross-sectional view of the peripheral wires each including a compressible material and a metal tube before the wire drawing. 
         FIG.  2 A  is a cross-sectional view of a wire including a center member, circular peripheral wires, and an outer shell before the wire drawing, and  FIG.  2 B  is a cross-sectional view of the peripheral wires each including a compressible material and a metal tube before the wire drawing. 
         FIG.  3    is a simplified view of a wire pulling-out device. 
         FIG.  4 A  is a cross-sectional view of the wire including the center member, the peripheral wires each having the substantially isosceles trapezoidal cross-sectional shape, and the outer shell after the wire drawing, and  FIG.  4 B  is a cross-sectional view of the peripheral wires each including the compressible material and the metal tube after the wire drawing. 
         FIG.  5 A  is a cross-sectional view of the wire including the center member, the circular peripheral wires, and the outer shell after the wire drawing, and  FIG.  5 B  is a cross-sectional view of the peripheral wires each including the compressible material and the metal tube after the wire drawing. 
         FIGS.  6 A,  6 B, and  6 C  are simplified views of a trapezoidal die. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention relate to wire drawing of a high-temperature superconducting wire or a material including a plurality of metal tubes or metal rods and a compressive material (for example, a mixed powder). For example, in a step of manufacturing the high-temperature superconducting wire, in a state in which a metal tube and a plurality of cylindrical metal tubes disposed around the metal tube and filled with a mixed powder are bundled and incorporated into a large metal tube, a cross-sectional diameter of a wire is reduced by using a wire pulling-out method using a die or the like. 
     In the middle of the wire drawing, due to the reduction in the cross-sectional diameter of the wire, a local pressure is applied to the cylindrical metal tubes filled with the mixed powder from the large metal tube positioned on an outermost peripheral side, and thus a shape deforms non-uniformly, and a risk of disconnection may occur. 
     In addition, due to the non-uniform deformation, a local average porosity increases inside the mixed powder, and an average porosity distribution also becomes non-uniform. As a result, a critical current density of the superconducting wire may decrease. In addition, since a contact state between the cylindrical metal tubes filled with the mixed powder is point contact in an initial stage of processing, a large variation in a cross-sectional shape after the wire drawing may occur due to such an unstable processing state. 
     In order to obtain a high-temperature superconducting wire with less disconnection and high performance, a deformation behavior inside the wire is required to be uniform in the middle of the wire drawing to improve the processing stability. 
     Therefore, in the embodiment, for a wire in a state in which a metal tube and a plurality of peripheral wires disposed around the metal tube and each including a mixed powder and a metal tube are bundled and incorporated into a large metal tube, a cross-sectional shape of each of the plurality of peripheral wires including the mixed powder and the metal tube is not circular but is a substantially isosceles trapezoidal shape. 
     For example, computer aided engineering (CAE) is used to examine equivalent strain and an average porosity after the wire drawing based on the cross-sectional shapes of the plurality of peripheral wires disposed in the wire, and results of the examination are used. In the CAE examination, CAE is used to examine wire drawing of reducing a maximum cross-sectional diameter of the wire before the wire drawing by 35% or more. 
     The equivalent strain and the average porosity generated in the peripheral wires when the cross-sectional diameter of the wire is smaller than an initial cross-sectional diameter by 35% or more are measured, and the measurement results are used to evaluate, by CAE, an effect generated by processing the peripheral wires before the wire drawing into a trapezoidal shape. 
     As a result, according to the above embodiment, the deformation behavior inside the wire is made uniform in the wire drawing by processing the peripheral wires before the wire drawing into the trapezoidal shape. Accordingly, defects such as the disconnection can be reduced, a cross sectional region in which an average porosity of the compressive material is 30% or less can be enlarged, and performance of the wire can be improved. In addition, by stabilizing the contact state between the peripheral wires, variations in a shape after the wire drawing can be reduced, and a manufacturing cost can be reduced. 
     Hereinafter, embodiments will be described with reference to the drawings. 
     Embodiment 1 
     Hereinafter, a wire drawing method in Embodiment 1 will be described with reference to the drawings. 
     A wire  100  before the wire drawing, which is a material including a center member  106 , peripheral wires  103  each having a substantially isosceles trapezoidal shape, and an outer shell  109 , will be described with reference to  FIG.  1   . 
       FIG.  1 A  shows the wire  100  in which ten peripheral wires  103  each having a substantially isosceles trapezoidal shape are disposed around one center member  106 , and the peripheral wires  103  and the center member  106  are incorporated into the outer shell  109 . A longitudinal length of the wire is denoted by L 1 . In the wire  100  including ten peripheral wires  103 , the center member  106  includes a core member  104  that is formed by a circular metal rod, and a covering material  105  that is formed by a circular metal tube covering the core member  104 . The outer shell  109  includes two layers of an outer layer  107  and an inner layer  108 , the inner layer  108  is formed by a circular metal tube, and the outer layer  107  is formed by a circular metal tube having an outer diameter D 1 . 
     As shown in  FIG.  1 B , in each of the peripheral wires  103 , a compressive material (for example, a mixed powder)  101  having a substantially isosceles trapezoidal shape is covered with a metal sheath  102 . A cross-sectional shape of each of the peripheral wires  103  is a substantially isosceles trapezoidal shape including a long side I 1 , a short side I 2 , a first oblique side I 3 , and a second oblique side I 4 . The long side I 1  is positioned on an inner circumferential side of the outer shell  109 , and the short side I 2  is positioned on the outer circumferential side of the center member  106 . 
     A wire before the wire drawing, whose peripheral wire has a circular cross-sectional shape, is also prepared as a comparative wire. 
     A wire  100  before the wire drawing, which is a material including the center member  106 , circular peripheral wires  103 , and the outer shell  109 , will be described with reference to  FIG.  2   . Configurations, shapes, and arrangements of the center member  106  and the outer shell  109  are the same as those of the wire  100  before the wire drawing in  FIG.  1   . On the other hand, in the wire in  FIG.  2   , a cross-sectional shape of each of the peripheral wires  103  is a circle having an outer diameter D 3 . 
     Examples of a processing method for drawing a wire include wire pulling-out, cassette roll processing, and groove roll processing, and among these processing methods, the wire pulling-out will be described as an example in Embodiment 1. 
     A configuration of a wire pulling-out device, which is an example of a wire drawing device, will be described with reference to  FIG.  3   . 
     As shown in  FIG.  3   , the wire pulling-out device includes a die  210  including a hole  230 , and a gripping unit (chuck unit)  220 . The wire  100  including an end portion B 5  with an initial diameter of D 1  is moved by pulling the gripping unit  220  in a B 4  direction through a predetermined tensile force in a state in which an end portion B 6  of the wire  100  is gripped by the gripping unit  220 . Accordingly, the cross-sectional diameter D 1  of the end portion B 5  is reduced to a cross-sectional diameter D 2  of the end portion B 6 . 
     Specifically, the wire  100  is pulled in the B 4  direction by the gripping unit  220  to pass the wire  100  through the hole  230  of the die  210 . In the wire  100  that passes through the hole  230  of the die  210 , the initial diameter D 1  is smaller than a diameter of the die and is reduced to the cross-sectional diameter D 2 . As a result, the longitudinal length of the wire  100  that passes through the hole  230  is increased while the cross-sectional diameter of the wire  100  is reduced. 
     For the wire  100  having the cross-sectional diameter of D 1  before the wire drawing, which is shown in  FIG.  1    and is a material including the center member  106 , the peripheral wires  103  each having a substantially isosceles trapezoidal shape, and the outer shell  109 , and the wire  100  having the cross-sectional diameter of D 1  before the wire drawing, which is shown in  FIG.  2    and is a material including the center member  106 , the circular peripheral wires  103 , and the outer shell  109 , the cross-sectional diameters are reduced from D 1  to D 2  by the wire pulling-out device in  FIG.  3    to produce a wire  100  having the cross-sectional diameter of D 2 . 
     A result of examining, by the CAE, wire drawing of reducing a maximum cross-sectional diameter of the wire  100  before the wire drawing by 35% or more will be described below. 
     For example, as metal materials having different deformation resistances, the core member  104  in the center members  106  in  FIGS.  1  and  2    is a copper rod, the outer layer  107  in the outer shell  109  is a Monel tube, the covering material  105  in the center member  106 , the metal sheath  102  in each of the peripheral wires  103 , and the inner layer  108  in the outer shell  109  are low carbon steel, and among the three metal materials, the deformation resistance of the outer layer  107  is the maximum and the deformation resistance of the core member  104  is the minimum. In addition, the compressive material  101  in each of the peripheral wires  103  is a mixed powder of Mg and B having an average porosity of 50%. 
     The wire  100  having the length of L 1  and the cross-sectional diameter of D 2 , which includes the center member  106 , the peripheral wires  103 , and the outer shell  109 , is subjected to the wire pulling-out, so that the initial cross-sectional diameter D 1  is reduced by 35% to obtain the cross-sectional diameter D 2 . The length of the wire  100  after the wire drawing is changed from L 1  to L 2 . This examination is performed by measuring, for example, a contact state between the peripheral wires in the middle of the wire pulling-out, and an equivalent strain distribution and an average porosity distribution after the wire pulling-out. 
     The wire  100  having the length of L 2  and the cross-sectional diameter of D 2  after the wire drawing, which is a material including the center member  106 , the peripheral wires  103  each having the substantially isosceles trapezoidal shape, and the outer shell  109 , will be described with reference to  FIGS.  4 A and  4 B . 
     As shown in  FIG.  4 B , it is confirmed that a cross-sectional shape of the compressive material  101  after the processing is deformed to a shape similar to a shape before the wire drawing by reducing the shape before the wire drawing in a similar shape, and a cross section of each of the peripheral wires  103  after the wire drawing is changed from the substantially isosceles trapezoidal shape to a substantially annular sector including a first side I 11  in contact with the outer shell  109 , a second side I 21  in contact with the center member  106 , and a third side I 31  and a fourth side I 41  that are in contact with the adjacent peripheral wires  103 . 
     As a result of measuring the average porosity by the CAE, it is confirmed that an average porosity of the compressive material  101  at a midpoint P 11  of the first side I 11  is larger than either one of an average porosity of the compressive material  101  at a first point P 12  that is an intersection of the first side I 11  and the third side I 31  and an average porosity of the compressive material  101  at a second point P 13  that is an intersection of the first side I 11  and the fourth side I 41 . This is a characteristic inherent to such a manufacturing process. This is because pores are compacted since the first point P 12  and the second point P 13  are locations to which a pressure is applied from the outer shell  109  during the wire drawing. 
     It is confirmed that an average porosity of the compressive material  101  at a midpoint P 14  of the second side I 21  is smaller than, by about 10%, either one of an average porosity of the compressive material  101  at a third point P 15  that is an intersection of the second side I 21  and the third side I 31  and an average porosity of the compressive material  101  at a fourth point P 16  that is an intersection of the second side I 21  and the fourth side I 41 . This is also a characteristic inherent to such a manufacturing process. This is because the pores are compacted by compressing the second side I 21  more uniformly than the comparative wire (the wire  100  using the circular peripheral wires  103 ) due to the reduction of the outer shell  109  during the wire drawing. 
     Based on the two characteristics described above, it is confirmed that a cross-sectional region in which the average porosity of the compressive material  101  in the wire  100  is 0.3 or less increases since the average porosity distribution of the compressive material  101  during the wire drawing is made uniform, and in particular, the average porosity at the first point P 12  and the average porosity at the second point P 13  become smaller than those of the comparative wire. 
     For example, it is considered that the critical current density, which is performance of the superconducting wire, depends on the average porosity of the compressive material  101  in the wire, and a critical current property of the wire  100  is improved by processing the peripheral wires  103  from a shape before the wire drawing into a trapezoidal shape. 
     In addition, by processing the peripheral wires  103  from the shape before the wire drawing into the trapezoidal shape, the contact state between the peripheral wires becomes a line contact state in the middle of the wire drawing. Therefore, a positional deviation is less likely to occur along with the deformation, and the compression of the compressive material  101  in the peripheral wires  103  during the wire drawing is made uniform when the cross-sectional shape of the peripheral wires  103  is compared with that of the peripheral wires  103  of the comparative wire (wire  100  using the circular peripheral wires  103 ). Therefore, it is considered that the wire  100  manufactured by processing the peripheral wires  103  into the trapezoidal shape has improved processing stability during molding. Since variation can be reduced by improving the processing stability, stability of quality can be ensured during mass production, and a manufacturing cost can be reduced. 
     In addition, by processing the peripheral wires  103  from the shape before the wire drawing into the trapezoidal shape, strain is preferentially introduced from the outer shell  109  to both left and right sides of the long side I 1  of each of the peripheral wires  103  in the middle of the wire drawing, so that the cross-sectional shapes of the peripheral wires  103  are reduced in a shape similar to the cross-sectional shape before the wire drawing. Therefore, based on the results of the CAE, it is confirmed that strain concentration on the metal sheaths (covering materials)  102  of the peripheral wires  103  metal sheath during the wire drawing can be avoided as compared with the comparative wire. In addition, defects such as disconnection can be reduced by uniform deformation, the stability of the quality can be ensured during the mass production, and the manufacturing cost can be reduced. 
     The wire  100  having the length of L 2  and the cross-sectional diameter of D 2  after the wire drawing, which is a material including the center member  106 , the circular peripheral wires  103 , and the outer shell  109 , will be described with reference to  FIG.  5   . 
     As shown in  FIG.  5   , it is confirmed that the cross-sectional shape of the compressive material  101  after the wire drawing is not reduced in a shape similar to the shape before the wire drawing and is locally deformed to a shape different from that before the wire drawing, and the cross section of each of the peripheral wires  103  after the wire drawing is changed from the circular shape to a substantially trapezoidal shape including a first side Z 1  in contact with the outer shell  109 , a second side Z 2  in contact with the center member  106 , and a third side Z 3  and a fourth side Z 4  that are in contact with the adjacent peripheral wires  103 . 
     In addition, as a result of measuring the average porosity by the CAE, it is confirmed that an average porosity of the compressive material  101  at a midpoint P 1  of the first side Z 1  is smaller than either one of an average porosity of the compressive material  101  at a first point P 2  that is an intersection of the first side Z 1  and the third side Z 3  and an average porosity of the compressive material  101  at a second point P 3  that is an intersection of the first side Z 1  and the fourth side Z 4 . This is a characteristic inherent to such a manufacturing process. This is because the pores are locally compacted since the midpoint P 1  of the first side Z 1  is a location to which a pressure is applied from the outer shell  109  during the wire drawing. 
     It is confirmed that an average porosity of the compressive material  101  at a midpoint P 4  of the second side Z 2  is smaller than, by about 20%, either one of an average porosity of the compressive material  101  at a third point P 5  that is an intersection of the second side Z 2  and the third side Z 3  and an average porosity of the compressive material  101  at a fourth point P 6  that is an intersection of the second side Z 2  and the fourth side Z 4 . This is also a characteristic inherent to such a manufacturing process. This is because the pores are compacted since the second side Z 2  is compressed non-uniformly as compared with the wire (wire  100  using the peripheral wires  103  each having a substantially isosceles trapezoidal shape) due to the reduction of the outer shell  109  during the wire drawing. 
     Based on the two characteristics described above, it is confirmed that the average porosity distribution of the compressive material  101  at the time of the wire drawing is non-uniform. In addition, the contact state between the peripheral wires becomes point contact in the middle of the wire drawing, and thus the positional deviation is likely to occur along with the deformation, and the compression of the compressive material  101  in the peripheral wires  103  is non-uniform. 
     In addition, since the cross-sectional shape of each of the peripheral wires  103  before the wire drawing is circular, the cross-sectional shape is locally deformed to a shape different from that before the wire drawing by preferentially introducing strain from the outer shell  109  to a center side of the long side I 1  of each of the peripheral wires  103 . Therefore, it is found that strain concentration on the metal sheath (covering material)  102  of the peripheral wires  103  occurs, and in particular, equivalent strain of the metal sheath (covering material)  102  at the midpoint P 1  of the first side Z 1  is larger than that at other portions of the covering material  102 . 
     A procedure for producing the peripheral wires  103  each having a substantially isosceles trapezoidal shape, which are used in the invention, will be described below. 
       FIGS.  6 A,  6 B, and  6 C  show trapezoidal dies for producing the peripheral wires  103  each having the substantially isosceles trapezoidal shape. 
     A composite including the metal sheath  102  and the compressive material  101  is drawn by the die  210  (see  FIG.  3   ) including the circular hole  230 . The inner compressive material  101  is densified by repeating reduction in a cross-sectional area by the wire drawing. 
     In a final processing stage, the composite passes through a substantially isosceles trapezoidal die  240  shown in  FIG.  6 A , which includes a substantially isosceles trapezoidal hole  250  on an inlet side and a substantially isosceles trapezoidal hole  260  on an outlet side, so that the peripheral wires  103 , whose cross-sectional shape shown in  FIG.  1 B  is the substantially isosceles trapezoidal shape including the long side I 1 , the short side I 2 , the first oblique side I 3 , and the second oblique side I 4 , are produced. 
     As shown in  FIG.  6 B , a cross section of the hole  250  on the inlet side has a substantially isosceles trapezoidal shape including a long side I 10 , a short side I 20 , a first oblique side I 30 , and a second oblique side I 40 . In addition, as shown in  FIG.  6 C , a cross section of the hole  260  on the outlet side has a substantially isosceles trapezoidal shape including the long side I 1 , the short side I 2 , the first oblique side I 3 , and the second oblique side I 4 . 
     On the other hand, when the peripheral wires  103  pass through the substantially isosceles trapezoidal die  240 , a gap is generated between the metal sheath  102  and the compressive material  101  in the peripheral wires  103 . When the peripheral wires  103  are used for the wire  100  with the gap left, non-uniform deformation or molding failure of the compressive material  101  may occur in the middle of the wire drawing. 
     As a countermeasure against the above problem, dimensions of the long side I 1 , the short side I 2 , the first oblique side I 3 , and the second oblique side I 4  of the hole  260  on the outlet side of the substantially isosceles trapezoidal die  240  are made smaller than the maximum diameter of the compressive material  101  in the peripheral wires  103  whose cross-sectional shape before the wire drawing is circular, so that the gap generated when the peripheral wires  103  pass through the substantially isosceles trapezoidal die  240  can be eliminated. 
     In order to allow the circular peripheral wires  103  to pass through an inside of the substantially isosceles trapezoidal die  200 , a dimension of the hole  250  on the inlet side of the substantially isosceles trapezoidal die  240  is required to be larger than a maximum diameter of the metal sheath  102  in the peripheral wires  103  each having a circular cross-sectional shape before the wire drawing. 
     Embodiment 2 
     Next, a superconducting wire according to Embodiment 2 will be described with reference to  FIGS.  4 A and  4 B . 
     The superconducting wire according to Embodiment 2 shown in  FIGS.  4 A and  4 B  is a wire obtained by drawing the material (see  FIGS.  1 A and  1 B ) including the center member  106 , the peripheral wires  103  each having the substantially isosceles trapezoidal shape, and the outer shell  109 . The wire after the wire drawing is the wire  100  having the length of L 2  and the cross-sectional diameter of D 2 . 
     As shown in  FIGS.  4 A and  4 B , a cross-sectional shape of the compressive material  101  after the wire drawing is deformed to a shape similar to that before the wire drawing by reducing the shape before the wire drawing in a similar shape, and the cross section of each of the peripheral wires  103  after the wire drawing is changed from the substantially isosceles trapezoidal shape to the substantially annular sector including the first side I 11  in contact with the outer shell  109 , the second side I 21  in contact with the center member  106 , and the third side I 31  and the fourth side I 41  that are in contact with the adjacent peripheral wires  103 . 
     A specific configuration of an aspect of the superconducting wire according to Embodiment 2 will be described with reference to  FIG.  4 A . 
     As shown in  FIG.  4 A , an aspect of the superconducting wire according to Embodiment 2 includes the center member  106  that is a core, the plurality of peripheral wires  103  surrounding the center member  106 , and the outer shell  109  disposed outside the peripheral wires  103 . 
     Each of the peripheral wires  103  includes the compressive material (for example, the mixed powder)  101  and the metal sheath (peripheral-wire cover)  102  covering the compressive material  101 . Here, the compressive material  101  includes a porous material. 
     A shape of a cross section perpendicular to the longitudinal direction of the peripheral wire  103  is a substantially annular sector including the first side I 11  in contact with the outer shell  109 , the second side I 21  in contact with the center member  106 , and the third side I 31  and the fourth side I 41  that are in contact with the adjacent peripheral wires  103 . 
     A first pore diameter of the compressive material (porous material)  101  at the midpoint P 11  of the first side I 11  is larger than either one of a second average pore diameter of the compressive material (porous material)  101  at the first point P 12  that is the intersection of the first side I 11  and the third side I 31  and a third average pore diameter of the compressive material (porous material)  101  at the second point P 13  that is the intersection of the first side I 11  and the fourth side I 41  (Configuration (1)). 
     In addition, a fourth pore diameter of the compressive material (porous material)  101  at the midpoint P 14  of the second side I 21  is smaller than either one of a fifth average pore diameter of the compressive material (porous material)  101  at the third point P 15  that is the intersection of the second side I 21  and the third side I 31  and a sixth average pore diameter of the compressive material (porous material)  101  at the fourth point P 16  that is the intersection of the second side I 21  and the fourth side I 41  (Configuration (2)). 
     According to the superconducting wire of an aspect of Embodiment 2, compression of the compressive material (porous material)  101  is made uniform. In addition, since the strain concentration on the metal sheath (peripheral-wire cover)  102  can be avoided, the disconnection can be avoided. 
     One characteristic of the wire drawing method according to Embodiment 1 described above is Configuration (1) described above. This is because pores of the compressive material (porous material)  101  are compressed since the first point P 12  and the second point P 13  are locations to which a pressure is applied from the outer shell  109  during the wire drawing. 
     Similarly, another characteristic of the wire drawing method according to Embodiment 1 described above is Configuration (2) described above. This is because the pores of the compressive material (porous material)  101  are compressed by compressing the midpoint P 14  of the second side I 21  during wire drawing of the peripheral wires  103  each having the substantially isosceles trapezoidal shape. 
     In addition, as shown in  FIG.  4 A , another aspect of the superconducting wire according to Embodiment 2 includes the center member  106  that is a core, the plurality of peripheral wires  103  surrounding the center member  106 , and the outer shell  109  disposed outside the peripheral wires  103 . 
     Each of the peripheral wires  103  includes the compressive material  101  and the metal sheath (peripheral-wire cover)  102  covering the compressive material  101 . Here, the compressive material  101  is formed of a porous material. 
     A shape of a cross section perpendicular to the longitudinal direction of the peripheral wire  103  is a substantially annular sector including the first side I 11  in contact with the outer shell  109 , the second side I 21  in contact with the center member  106 , and the third side I 31  and the fourth side I 41  that are in contact with the adjacent peripheral wires  103 . 
     Here, a pore diameter of the compressive material (porous material)  101  is always larger than a minimum value of pore diameters at (1) the first point P 12  that is the intersection of the first side I 11  and the third side I 31 , (2) the second point P 13  that is the intersection of the first side I 11  and the fourth side I 41 , and (3) the midpoint P 14  of the second side I 21 . In addition, a pore diameter of the compressive material (porous material)  101  at the midpoint of the first side I 11  is larger than a maximum value of the pore diameters at the first point P 12  and the second point P 13 . 
     According to the superconducting wire of another aspect of Embodiment 2, the critical current property of the wire  100  can be improved by forming the configuration described above. 
     In particular, regarding the periphery of the first side I 11 , the pore diameter of the compressive material (porous material)  101  at the midpoint P 11  of the first side I 11  is larger than the maximum value of the pore diameters at the first point P 12  and the second point P 13 , and thus an effect thereof is higher than that of the comparative wire shown in  FIG.  5   . This is because the reduction in the pore diameter due to compression is small since the first point P 12  and the second point P 13  are originally easily deformed and an amount of the mixed powder used as a material of the compressive material (porous material)  101  is also small. 
     According to the embodiments described above, non-uniform deformation of the material generated inside the wire can be prevented, and the non-uniform porosity distribution and disturbance of the shape of the cross-section perpendicular to the longitudinal direction of the wire can be prevented. 
     The invention is not limited to the above embodiments and includes various modifications and equivalent configurations within the spirit of the claims. For example, the above embodiments have been described in detail for easy understanding of the invention, and the invention is not necessarily limited to those having all the configurations described. A part of a configuration of one embodiment may be replaced with a configuration of another embodiment. Further, the configuration of another embodiment may be added to the configuration of one embodiment. Further, a part of the configuration of the embodiments may be added to, deleted from, or replaced with another configuration.