JP5308683B2 - Bronze method Nb3Sn superconducting wire production Nb or Nb-based alloy rod, Nb3Sn superconducting wire production precursor and production method thereof, and Nb3Sn superconducting wire - Google Patents

Description

The present invention relates to an Nb or Nb-based alloy rod used as a raw material when producing an Nb ₃ Sn superconducting wire by the bronze method, a superconducting wire manufacturing precursor constituted using such an Nb or Nb-based alloy rod, and The present invention relates to a manufacturing method and Nb ₃ Sn superconducting wire. Specifically, it exhibits good superconducting properties without causing processing inconveniences during surface-reduction processing such as extrusion during precursor production, and is representative of magnets for high-resolution nuclear magnetic resonance (NMR) analyzers. Liquid helium immersion cooling superconducting magnet, Nb ₃ Sn superconducting wire useful as a material applied to a refrigerator cooling superconducting magnet, a precursor and manufacturing method therefor, and for manufacturing these It relates to the Nb or Nb-based alloy rod used.

A superconducting magnet that generates a strong magnetic field by passing a large current through a coil wound with a superconducting wire has been developed with the aim of applying it to nuclear fusion devices in addition to nuclear magnetic resonance (NMR) analyzers and physical property evaluation devices. Is underway. Among these, the NMR analyzer is the only device that can analyze the molecular structure of biopolymers and proteins that cannot be crystallized, and is a powerful tool for promoting post-genome development. In addition, in this apparatus, the higher the magnetic field generated by the superconducting magnet, the higher the resolution of the analysis and the higher the ratio of the NMR signal and noise, enabling analysis in a shorter time. As a constituent material of the superconducting magnet as described above, a Nb ₃ Sn superconducting wire has been widely used as a representative material.

FIG. 1 is an explanatory view schematically showing a cross-sectional structure of a Nb ₃ Sn superconducting wire manufactured by a bronze method at a secondary multi-core billet stage, in which 1 is an Nb or Nb-based alloy core, Linear Cu—Sn base alloy base material (bronze matrix), 3 diffusion barrier layer, 4 stabilized copper, 5 primary stack material, 6 outer layer case, 7 secondary multi-core billet (Nb ₃ Sn (Precursor for producing superconducting wire).

First, as shown in FIG. 1, a plurality of (7 in this figure) Nb or Nb-based alloy cores 1 are embedded in a Cu—Sn-based alloy base material 2 formed into a hexagonal cross section to form a composite material (primary stack material 5). A plurality of primary stack members 5 are bundled and inserted into a pipe-like Cu-Sn base alloy (outer layer case 6) wound with an Nb sheet or Ta sheet as a diffusion barrier layer 3, or bundled. A secondary multi-core billet 7 is assembled by winding a Nb sheet or a Ta sheet directly around the primary stack material, and further disposing the stabilizing copper 4 on the outside thereof. The diffusion barrier layer 3 exhibits a function of suppressing the outward diffusion of Sn during the heat treatment for generating Nb ₃ Sn. The stabilized copper 4 is arranged as a stabilizing material for the Nb ₃ Sn superconducting wire, and is made of, for example, oxygen-free copper.

The secondary multi-core billet shown in FIG. 1 is extruded under hydrostatic pressure, and subsequently subjected to a surface reduction process such as a drawing process to obtain a multi-core precursor for producing an Nb ₃ Sn superconducting wire. Thereafter, by performing a heat treatment (diffusion heat treatment) at a temperature of about 650 to 720 ° C. for about 100 hours, the vicinity of the surface of the Nb or Nb base alloy core 1 (in this case, the base material 2 made of Cu—Sn base alloy) Nb ₃ Sn phase is formed on the interface of Nb or Nb-based alloy core 1.

  In the above configuration, the stabilizing copper 4 in the secondary multi-core billet 7 has been provided as the outermost layer. However, the position of the stabilizing copper 4 is the center of the secondary multi-core billet 7 (axial core). The structure provided in the part) is also employed. 1 shows a circular cross-sectional shape of the secondary multi-core billet 7, but a rectangular cross-sectional shape (flat wire) as shown in FIG. 2, for example, is also employed.

  In the drawing process after the extrusion of the secondary multi-core billet, the area reduction rate per pass is often set to a low value of about 20% or less in order to process it smoothly. Therefore, the pass schedule for processing to a predetermined wire diameter becomes very long. Further, since bronze is hardened by work, it needs to be annealed and softened, which requires more time. In order to shorten the lengthening pass schedule as much as possible, the extrusion ratio of the secondary billet (= the cross-sectional area of the billet / the cross-sectional area of the member after extrusion) is increased and the subsequent drawing process is shortened. It is demanded. However, at present, the extrusion ratio is limited to 15-20, and if the extrusion ratio is made higher than that, the balance of the pressure inside the billet is lost during extrusion, and the Nb or Nb-based alloy core is deformed thick or thin. (So-called so-called phenomenon), and in some cases, the Nb or Nb-based alloy core may be cut.

In order to solve this problem, it is conceivable to improve the workability by making the deformation resistance of the raw material smaller than before. As a raw material of the precursor for manufacturing the Nb ₃ Sn superconducting wire, it is possible to improve the workability and superconducting characteristics of the Nb bar by controlling the oxygen and nitrogen impurity concentration in the range of 20 to 200 ppm. It has been proposed (for example, Patent Document 1).

  As a result of experiments, the present inventors have confirmed that even if such an Nb rod is used, the extrusion ratio can be extruded only under the same conditions as the conventional one (extrusion ratio of 15 to 20). . When extrusion was performed at an extrusion ratio larger than that, it was found that the Nb core was partially broken and disappeared when the cross section after extrusion was observed. Thus, when the extrusion ratio was set to more than 20, it was difficult to extrude soundly.

  The industry standard for Nb bars is B392-99 from the American Society for Testing and Materials (ASTM), the world's largest private and non-profit international standardization and standardization organization. In this standard, the 0.2% proof stress is 73 MPa or more. However, the upper limit is not stipulated, and those that pass this standard cannot be used for processing with a high extrusion ratio.

Further, the average crystal grain size of the Nb bar is proposed to be 5 to 100 μm in order to maintain the workability (for example, Patent Document 2), but the extrusion ratio exceeds 20 by satisfying these requirements. It was difficult to do.
JP-A-8-138467 JP 2007-141796 A

The present invention has been made under such circumstances, and the object thereof is to improve the workability (particularly, the extrusion ratio) in Nb or Nb-based alloys used when manufacturing a Nb ₃ Sn superconducting wire. Nb or Nb-based alloy rods, superconducting wires that exhibit good superconducting properties (particularly critical current and n value) using such Nb or Nb-based alloy rods, and precursors therefor (for producing superconducting wires) A precursor) and a method for producing the same.

The Nb or Nb-based alloy rod for producing a superconducting wire of the present invention that has achieved the above object is an Nb or Nb-based alloy rod used for producing an Nb ₃ Sn superconducting wire by a bronze method, It has a gist in that the recrystallization rate of the crystal structure in a cross section passing through the center and parallel to the longitudinal direction is 78% or more and the 0.2% proof stress is 220 MPa or less.

In the Nb ₃ Sn superconducting wire manufacturing Nb or Nb-based alloy rod, (a) the average crystal grain size is 10 to 50 μm, and (b) the average Vickers hardness Hv of the outer surface is 50 to 120. It is preferable to satisfy the requirements such as

Using the Nb or Nb base alloy rod as described above, it is combined with a Cu-Sn base alloy to form a composite material, the composite material is extruded, and a plurality of bundles are bundled to reduce the surface to a wire. Thus, a precursor useful for obtaining an Nb ₃ Sn superconducting wire exhibiting good superconducting properties can be produced.

Also by forming a superconducting phase by heat-treating precursor for fabricating a superconducting wire as described above, bronze process Nb ₃ Sn superconducting wire exhibiting good superconducting properties can be obtained.

By applying the present invention, it becomes possible to extrude a precursor for producing a bronze Nb ₃ Sn superconducting wire at an extrusion ratio greater than 20, which has been difficult so far. It becomes possible to manufacture a precursor of an Nb ₃ Sn wire, and an Nb ₃ Sn superconducting wire obtained from such a precursor exhibits good superconducting characteristics.

  The present inventors have studied from various angles in order to solve the above problems. When extruding a secondary multi-core billet, it is often heated to a temperature of about 600 ° C. from the viewpoint of increasing the temperature of the constituent material bronzes within a range that does not melt even by processing heat generation. Bronze of primary stack material which is a constituent material of secondary multi-core billet (for example, Cu-15% Sn-0.3% Ti: “%” means “mass%”, and chemical composition is the same hereinafter) And Nb or an Nb-based alloy core (hereinafter sometimes referred to as “Nb core”) were cut out and subjected to a tensile test near 600 ° C. As a result, compared to the tensile strength of bronze (Cu—Sn-based alloy) It was found that the tensile strength of the Nb core was 20 times or more. The processing balance during extrusion is preferably as small as possible.

  Therefore, a plurality of Nb or Nb-based alloy bars (hereinafter, may be represented by “Nb bars”) having different 0.2% proof stress are prepared, and a primary stack material for a secondary multi-core billet is prepared using them. A Nb core was prepared and subjected to a tensile test at 600 ° C. to examine the relationship between tensile strength and 0.2% yield strength. By reducing the 0.2% yield strength, the Nb core at 600 ° C. It has been found that the tensile strength of can be reduced. In other words, by reducing the 0.2% yield strength of the Nb bar, it is possible to reduce the difference in tensile strength between the Nb core and bronze at 600 ° C, and it is clear that the processing balance during extrusion can be improved. It became. The “0.2% proof stress” in the present invention is a value at room temperature (25 ° C.).

  And the recrystallization rate in the crystal structure in the cross section (longitudinal section) parallel to the longitudinal direction passing through the axial center (cross-sectional center point) is 78% or more, and the value of 0.2% proof stress is 220 MPa or less. By using Nb rods (or Nb-based alloy rods), while maintaining the workability improvement effect of Nb crystal grains, it is possible to maintain the work balance with bronze and to perform strong work with an extrusion ratio exceeding 20 It became.

  The 0.2% proof stress is a stress necessary for plastically deforming a material. As described above, in the case of a composite material, by bringing the 0.2% proof stress of different materials close to each other, it is similar to a single material. It becomes possible to process with balance. Therefore, the appropriate range of 0.2% proof stress required for soundly processing an Nb bar (or Nb-based alloy bar) is determined by the relationship with the 0.2% proof stress of bronze. It has been confirmed that the lower limit of the 0.2% proof stress of the Nb bar can be used up to 55 MPa (see Example 7 below).

  By setting the value of 0.2% proof stress of the Nb bar to 220 MPa or less, the difference in tensile strength from bronze is reduced, and the processing balance during extrusion is improved. The mechanical property value in the tensile test varies depending on the strain increase rate. From such a viewpoint, the strain increase rate in the tensile test at room temperature for measuring the 0.2% proof stress is 5 to 10% / min.

  On the other hand, the recrystallization rate of the crystal structure indicates a margin for processing the Nb bar as a single material. Even if the 0.2% proof stress of the Nb bar is within an appropriate range, if the recrystallization rate is low, the processing limit is reached and processing becomes impossible. By setting the recrystallization rate of the Nb bar to 78% or more, the ratio of recrystallized grains that are not distorted by processing increases, and the Nb bar has a sufficient margin for processing.

  Thus, by setting the values of the 0.2% proof stress and recrystallization rate of the Nb bar as described above, the extrusion process was performed with the extrusion ratio being larger than 20, as shown in the examples below. However, no disconnection occurs in the Nb core. In the Nb bar of the present invention, it is more preferable that the recrystallization rate of the crystal structure is 93% or more and the 0.2% proof stress value is 172 MPa or less. The “crystal structure recrystallization rate” is a value obtained by the method described in Examples below.

  In the conventional Nb bar, it has been shown that it is useful to make the average crystal grain size 5 to 100 μm in order to maintain the workability (Patent Document 2). However, in the Nb bar of the present invention, the required range for the average crystal grain size of the Nb bar is narrowed from the viewpoint of increasing the extrusion ratio. That is, by setting the average crystal grain size of the Nb bar to 10 to 50 μm, elongation is maintained while maintaining uniform workability (“elongation” is the initial value of the distance between the gauges attached to the test piece during the tensile test. It is also found that it can be increased to a large value of 50% or more.

  When the average crystal grain size in the Nb bar exceeds 50 μm, the elongation decreases. On the other hand, when the average crystal grain size is less than 10 μm, the 0.2% proof stress value becomes large, the processing balance with the bronze during extrusion is disturbed, and abnormalities such as partial disconnection of the Nb core occur. This is because when the crystal grain size is reduced, the crystal grain boundary increases, and when plastic working occurs, the crystal grain boundary hinders the movement of atoms, and high stress is required. A more preferable range of the average crystal grain size of the Nb bar is about 10 to 30 μm. The “average crystal grain size” is a value obtained by the method described in Examples below.

  By the way, in order to improve the superconducting characteristics such as critical current density and n value in the superconducting wire, surface-reducing processing such as cold drawing after extrusion is also important. When performing surface-reducing processing, an important factor for realizing sound processing is the hardness of the Nb core. In such surface reduction processing (cold processing), the hardness of the bronze exceeds the hardness of the Nb bar, and when the average Vickers hardness Hv of the outer surface of the raw material Nb bar is less than 50, the hardness of both Since the difference in thickness increases, the cold working balance is disturbed, and the above superconducting characteristics are deteriorated.

  In addition, when the average Vickers hardness Hv of the outer surface of the Nb bar exceeds 120, the Vickers hardness variation increases, and the distribution of the superconducting characteristics in the longitudinal direction of the wire increases, causing a problem. By making the average Vickers hardness Hv of the outer surface of the Nb rod within the range of 50 to 120, it is possible to improve the superconducting characteristics with a small distribution in the longitudinal direction of the wire while maintaining a good cold work balance. . The more preferable lower limit of the average Vickers hardness Hv of the outer surface of the Nb bar is 60, and the more preferable upper limit is 90. Further, the average Vickers hardness Hv of the outer surface of the Nb bar is a value obtained by the method shown in Examples described later.

  In order to produce the Nb rod (or Nb-based alloy rod) as described above, the hydrogen concentration as an impurity is 10 ppm or less, the nitrogen concentration is 30 ppm or less, and the total concentration of carbon and oxygen is 60 ppm or less. It is preferable to use a certain high purity ingot. For the production of a high purity ingot, a vacuum melting method such as electron beam melting is applied. For example, in the case of electron beam melting, the melting is repeated twice or three times instead of once. It is possible to produce an ingot having a higher purity than conventional (for example, Patent Document 1).

  After processing such a high-purity ingot into a rod shape, and subsequently performing final annealing at a temperature of 900 to 1800 ° C. for a predetermined time, cold processing is not performed at all, or cold processing with a processing rate of about 5% or less Processing. In some cases, re-annealing is also performed. Conventionally known ones can be applied to other conditions and processes. By such a process, an Nb rod or an Nb-based alloy rod having a predetermined 0.2% yield strength, recrystallization rate, average crystal grain size, average Vickers hardness Hv, and the like can be produced.

  Next, specific manufacturing conditions for controlling each requirement defined by the Nb bar of the present invention will be described. First, in order to increase the recrystallization rate to 78% or more, Nb ingot is hot-extruded, cold-rolled and processed into a final shape with an area reduction after extrusion of about 20% or more, and then about 950 ° C. The final annealing may be performed for about 1 hour or longer. At this time, the purity of the Nb ingot does not significantly affect the recrystallization rate.

  In order to further increase the recrystallization rate of the Nb bar to 93% or more, after the Nb ingot is hot-extruded, cold-rolled and processed into a final shape with a reduction in area after extrusion of about 50% or more The final annealing may be performed at about 1100 ° C. for about 1 hour or longer.

  In order to reduce the 0.2% proof stress of the Nb bar to 220 MPa or less, the Nb ingot is hot-extruded, cold-rolled and processed into a final shape in a state where the area reduction after extrusion is about 20% or more, The final annealing may be performed at about 1200 ° C. for about 0.2 hours or longer. If the purity of the Nb ingot at this time is low, the 0.2% yield strength tends to increase.

  In order to further reduce the 0.2% proof stress of the Nb bar to 172 MPa or less, after the Nb ingot is hot extruded, cold rolled and processed into a final shape with a reduction in area after extrusion of about 20% or more The final annealing may be performed at about 1200 ° C. for about 2 hours or more. At this time, if the purity of the Nb ingot is low, the 0.2% yield strength tends to be high.

  In order to produce an Nb bar having a recrystallization rate of 78% or more and a 0.2% proof stress of 220 MPa or less, at least a Nb ingot is hot-extruded, and cold rolling is performed, and the area reduction after extrusion is about 20% or more. After being processed into the final shape in this state, the final annealing may be performed at about 1200 ° C. for about 1 hour. Further, from the viewpoint of lowering the 0.2% proof stress, it is necessary to increase the purity of the Nb ingot (reducing the impurity elements as much as possible).

  In order to produce an Nb bar having a recrystallization ratio of 93% or more and a 0.2% proof stress of 172 MPa or less, at least a Nb ingot is hot-extruded, cold-rolled, and the area reduction after extrusion is about 50 After being processed into a final shape in a state of% or more, final annealing is performed at about 1200 ° C. for about 2 hours or more. At this time, if the purity of the Nb ingot is low, the 0.2% yield strength tends to increase.

  The above effect is basically for the Nb rod, but Nb contains about 0.01 to 5.0% of one or more selected from the group consisting of Ti, Ta, Zr, and Hf. Even if the Nb-based alloy bar is used, the same effect as that obtained when the pure Nb bar is used can be obtained.

By using the Nb bar (or Nb-based alloy bar) as described above, it is combined with a Cu—Sn-based alloy to form a composite material, the composite material is extruded, and a plurality of bundles are bundled to reduce the surface. In the case of a wire (see FIGS. 1 and 2), a superconducting wire precursor that is efficiently stable while maintaining a high extrusion ratio can be obtained, and it is good by heat-treating such a precursor to form a superconducting phase. A bronze Nb ₃ Sn superconducting wire that exhibits superconducting properties can be realized.

Of course, the structure of the bronze Nb ₃ Sn wire to which the present invention can be applied is not limited to the cross-sectional structure of the secondary multi-core billet shown in FIG. For example, the cross-sectional structure shown in FIG. 1 is a secondary multi-core billet for obtaining an externally stabilized Nb ₃ Sn precursor in which the stabilized copper is in the outermost layer of the cross section. The same effect can be expected for the secondary multi-core billet for obtaining the internally stabilized bronze method Nb ₃ Sn precursor present in FIG. Moreover, the cross-sectional structure of the secondary multi-core billet may be that shown in FIG.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention. The recrystallization rate, 0.2% proof stress, elongation, average crystal grain size and average Vickers hardness Hv, and superconducting properties (critical current density and n value) in the superconducting wire shown in the following experiments are as follows: It is measured by.

[Measurement method of recrystallization rate of crystal structure]
A plane (longitudinal section) that passes through the axis and is parallel to the longitudinal direction is randomly extracted from 100 to 110 crystal grains from the microstructure image observed with a microscope, and the length of the major axis is the length of the minor axis. Divide the crystal grains whose aspect ratio (long axis length / short axis length) is 2.5 or less into recrystallized grains, and then divide the number of recrystallized grains by the total number of extracted crystal grains. The crystal ratio was taken. Here, the “long axis length” is the length when the distance between the surface of the crystal grain and the two points where the line intersects is longest when a straight line is drawn in the crystal cross section. The “length” is the length when the distance between the surface of the crystal grain and the two points where the straight line intersects is the shortest when a straight line is drawn so as to pass through the midpoint of the long axis in the crystal cross section.

[Measurement method of 0.2% yield strength]
A tensile test was conducted to evaluate the stress-strain curve and Young's modulus, and the stress (normal temperature) at which the plastic strain becomes 0.2% was determined as the 0.2% yield strength. At this time, the strain increase rate in the tensile test at room temperature was adjusted to 5 to 10% / min.

[Measurement method of elongation]
According to the Japanese Industrial Standard metal material tensile test method (JIS Z 2241), the value obtained by dividing the permanent elongation of the gauge distance after breaking the test piece by the original gauge distance is shown as a percentage.

[Measurement method of average grain size]
One or more lines are drawn at random on the microstructure (cross-section passing through the axis and parallel to the longitudinal direction) under a microscope, and the equivalent length of the line intersects the line. The value divided by the number was determined as the average crystal grain size. The total number of crystal grains intersecting with the straight line at this time was 30-50.

[Measurement method of average Vickers hardness Hv of outer surface of Nb bar]
Four to six indentations are made on the surface of the Nb bar in the longitudinal direction, and the respective Vickers hardnesses are obtained from the size of each indentation, and the sum is averaged. The load when hitting the impression was 10 kgf (98 N).

[Measurement method of superconducting properties (critical current density, n value)]
The superconducting properties of the superconducting wire are as follows: the critical current at a temperature of 4.2K and an external magnetic field of 19T is measured by a DC four-terminal method using an electric field standard of 10 μV / m, and this is divided by the cross-sectional area other than copper of the wire As a result, the critical current density of the non-copper portion was determined as nonCu-Jc (= critical current / area of the portion excluding the stabilized copper). The n value was determined from the values of two critical currents determined using electric field standards of 10 μV / m and 100 μV / m.

  The “n value” is an index indicating the uniformity of the current flowing in the wire direction in the superconducting wire, that is, the uniformity of the superconducting filament in the longitudinal direction of the wire. The larger this value, the higher the superconducting characteristics ( That is, the current uniformity is said to be excellent.

(Comparative Example 1)
The number of times of Nb electron beam melting was set to 3 to prepare ingots having oxygen and nitrogen impurity concentrations of 50 ppm and 10 ppm, respectively. This Nb ingot was hot-extruded, cold-rolled and processed into a final shape with a reduction in area of 10% after extrusion, followed by final annealing at 950 ° C. for 30 minutes, recrystallization rate: 57% 0.2% proof stress: 231 MPa, average crystal grain size: 120 μm, average Vickers hardness Hv: 132, elongation: 38% Nb bar was produced.

  19 holes with a diameter of 8.0 mm are drilled in a Cu-15% Sn-0.3% Ti ingot with a diameter of 67 mm, 19 Nb bars are inserted, and electron beam welding is performed. Billets were made. This was extruded at 600 ° C. under hydrostatic pressure, and extruded while being annealed in the middle, to be processed into a primary stack material having a hexagonal cross section with a distance between opposite sides of 2.5 mm. 433 bundles of this primary stack material, Nb sheet winding with a thickness of 0.2 mm on the outer periphery thereof, integrated them and inserted into a pure copper pipe with an outer diameter of 67 mm and an inner diameter of 60 mm, and performing electron beam welding, A billet of secondary stack material (secondary multi-core billet) was produced.

The secondary multi-core billet was heated to 600 ° C. and subjected to hot isostatic pressing to a diameter of 14 mm (extrusion ratio 22.9). When the cross section of the extruded member was observed, it was found that there were 19 Nb cores in the primary stack material unit, but they were disconnected and missing. Thus, even if the precursor in which the Nb core is disconnected inside generates the Nb ₃ Sn phase by performing heat treatment, the Nb ₃ Sn filament is missing in the longitudinal direction. And cannot be applied to an NMR magnet or the like that operates in the permanent current mode.

(Comparative Example 2)
The Nb ingot produced in Comparative Example 1 was hot-extruded, cold-rolled and processed into a final shape with an area reduction of 10% after extrusion, and then subjected to final annealing at 950 ° C. for 1 hour, A Nb rod having a recrystallization ratio of 74%, 0.2% proof stress: 210 MPa, average crystal grain size: 81 μm, average Vickers hardness Hv: 128, and elongation: 42% was prepared.

  19 holes with a diameter of 8.0 mm are drilled in a Cu-15% Sn-0.3% Ti ingot with a diameter of 67 mm, 19 Nb bars are inserted, and electron beam welding is performed. Billets were made. This was hydrostatically extruded at 600 ° C., and subjected to surface reduction while annealing in the middle, and processed into a primary stack material having a hexagonal cross section with a distance between opposite sides of 2.5 mm. 433 bundles of this primary stack material are wound around the outer periphery of an Nb sheet having a thickness of 0.2 mm, and they are integrated and inserted into a pure copper pipe having an outer diameter of 67 mm and an inner diameter of 60 mm, and electron beam welding is performed. The billet (secondary multi-core billet) of the next stack material was produced.

  The secondary multi-core billet was heated to 600 ° C. and subjected to hot isostatic pressing to a diameter of 14 mm (extrusion ratio 22.9). When the cross section of the member after extrusion was observed, it was found that the Nb core was disconnected inside and was missing as in Comparative Example 1.

(Comparative Example 3)
The Nb ingot produced in Comparative Example 1 was hot-extruded, cold-rolled and processed into a final shape with a reduction in area after extrusion of 50%, and then subjected to final annealing at 1100 ° C. for 1 hour. An Nb bar having a recrystallization ratio of 93%, 0.2% proof stress: 224 MPa, average crystal grain size: 117 μm, average Vickers hardness Hv: 125, and elongation: 40% was prepared.

  19 holes with a diameter of 8.0 mm are drilled in a Cu-15% Sn-0.3% Ti ingot with a diameter of 67 mm, 19 Nb bars are inserted, and electron beam welding is performed. Billets were made.

  The billet for the primary stack material was hydrostatically extruded at 600 ° C., and surface-reduced while annealing in the middle, and processed into a primary stack material having a cross-section with a hexagonal cross section of 2.5 mm across the distance. 433 bundles of this primary stack material, Nb sheet winding with a thickness of 0.2 mm on the outer periphery thereof, integrated them and inserted into a pure copper pipe with an outer diameter of 67 mm and an inner diameter of 60 mm, and performing electron beam welding, A billet of secondary stack material (secondary multi-core billet) was produced. This was heated to 600 ° C. and subjected to hot isostatic pressing to a diameter of 14 mm (extrusion ratio 22.9). When the cross section of the member after extrusion was observed, it was found that the Nb core was disconnected inside and was missing as in Comparative Example 1.

(Comparative Example 4)
The number of times of electron beam melting of Nb was set to 2 to produce ingots having oxygen and hydrogen impurity concentrations of 192 ppm and 19 ppm (higher than the ingots used in Comparative Examples 1 to 3, respectively). The ingot was subjected to cold rolling with a surface reduction ratio of 30% after hot extrusion, and a final annealing was attempted in a temperature range of 900 to 1800 ° C. for 1 to 20 hours [the following (a) to (c) The recrystallization rate: 78% or more and the 0.2% proof stress: 220 MPa or less could not be produced.

(A) Final annealing for 1 hour at 1200 ° C. Recrystallization rate: 92%, 0.2% yield strength: 253 MPa
(B) Final annealing at 1200 ° C. for 5 hours Recrystallization rate: 95%, 0.2% yield strength: 243 MPa
(C) Final annealing at 1200 ° C. for 20 hours Recrystallization rate: 96%, 0.2% yield strength: 243 MPa

Example 1
The Nb ingot produced in Comparative Example 1 was hot-extruded, cold-rolled and processed into a final shape with an area reduction after extrusion of about 20% or more, and then subjected to final annealing at about 1200 ° C. for about 1 hour. Then, an Nb bar having a recrystallization ratio of 78%, 0.2% proof stress: 205 MPa, average crystal grain size: 63 μm, average Vickers hardness Hv: 122, and elongation: 45% was produced.

  19 holes with a diameter of 8.0 mm are drilled in a Cu-15% Sn-0.3% Ti ingot with a diameter of 67 mm, 19 Nb bars are inserted, and electron beam welding is performed. Billets were made.

  The billet for the primary stack material was hydrostatically extruded at 600 ° C., and surface-reduced while annealing in the middle, and processed into a primary stack material having a cross-section with a hexagonal cross section of 2.5 mm across the distance. 433 bundles of this primary stack material, Nb sheet winding with a thickness of 0.2 mm on the outer periphery thereof, integrated them and inserted into a pure copper pipe with an outer diameter of 67 mm and an inner diameter of 60 mm, and performing electron beam welding, A billet of secondary stack material (secondary multi-core billet) was produced. This was heated to 600 ° C. and subjected to hot isostatic pressing to a diameter of 14 mm (extrusion ratio 22.9). When the cross section of the extruded member was observed, it was found that all the Nb cores were processed smoothly.

This member was cold drawn to produce a precursor wire having a cross-sectional size of 1.50 × 2.50 mm ² . The precursor was heat-treated at 720 ° C. for 150 hours to generate an Nb ₃ Sn phase. When the superconducting properties were evaluated after the heat treatment, the critical current density of the non-copper part nonCu-Jc (= area of the part excluding critical current / stabilized copper) was 130 A / mm ² , and the n value was 30; It was confirmed that the film had sufficient characteristics as a wire for NMR under the conditions of this temperature and external magnetic field.

(Example 2-6)
Using the Nb ingot produced in Comparative Example 1, cold rolling with an appropriate area reduction rate and final annealing at an appropriate temperature and time, various recrystallization rates as shown in Table 1 below, 0.2 Various Nb bars having% proof stress, average crystal grain size, and average Vickers hardness Hv were prepared.

  For a primary stack material, 19 holes with a diameter of 8.0 mm are made in a Cu-15% Sn-0.3% Ti ingot with a diameter of 67 mm, 19 Nb bars are inserted, and electron beam welding is performed. Billet was prepared.

  The billet for the primary stack material was hydrostatically extruded at 600 ° C., and surface-reduced while annealing in the middle, and processed into a primary stack material having a cross-section with a hexagonal cross section of 2.5 mm across the distance. 433 bundles of this primary stack material are wound around the outer periphery of an Nb sheet having a thickness of 0.2 mm, and they are integrated and inserted into a pure copper pipe having an outer diameter of 67 mm and an inner diameter of 60 mm, and electron beam welding is performed. The billet (secondary multi-core billet) of the next stack material was produced.

  This was heated to 600 ° C. and subjected to hot isostatic pressing to different diameters (in the order of Example 2-6, φ13.5 mm, φ13.0 mm, φ12.7 mm, φ12.5 mm, φ12.2 mm). (The extrusion ratio is 24.6, 26.6, 27.8, 28.7, 30.2 in the order of Example 2-6). Observation of the cross-section of the extruded member revealed that all the Nb cores were processed smoothly in all examples.

Each member after extrusion was cold drawn to produce a precursor wire having a cross-sectional size of 1.50 × 2.50 mm ² . The precursor was heat-treated at 720 ° C. for 150 hours to generate an Nb ₃ Sn phase. When the superconducting properties (critical current density, n value) were evaluated after the heat treatment, the critical current density nonCu-Jc and n value of the non-copper part as shown in Table 1 below were obtained. It has been confirmed that it has sufficient characteristics as an NMR wire.

  Despite the fact that the extrusion ratio increased and the extrusion conditions became stricter in the order of Examples 1-6, the superconducting characteristics were almost the same as or higher than those of Example 1.

  That is, in Example 1, by making the recrystallization rate: 78% or more and 0.2% proof stress: 220 MPa or less, sound processing is possible even at an extrusion ratio of 22.9. In this way, by setting the average crystal grain size to 50 μm or less, superconducting characteristics equivalent to or higher than those in Example 1 can be obtained even with a larger extrusion ratio. In Example 3, it can be seen that by setting the average Vickers hardness Hv to 120 or less, superconducting characteristics equal to or higher than those in Example 2 can be obtained even at a larger extrusion ratio.

  On the other hand, in Example 4, recrystallization rate: 93% or more, 0.2% yield strength: 172 MPa or less, even at an extrusion ratio larger than that of Examples 1 to 3, it is equal to or more than that of Examples 1 to 3. Superconducting properties have been obtained. Moreover, in Example 5, superconducting characteristics equal to or higher than those in Example 4 can be obtained by setting the average crystal grain size to 30 μm or less. In Example 6, the average Vickers hardness Hv is 90 or less. Thus, it can be seen that superconducting characteristics equal to or higher than those obtained in Example 5 were obtained with a larger extrusion ratio.

(Example 7)
The number of times of Nb electron beam melting was set to 4 to produce ingots having oxygen and nitrogen impurity concentrations of 10 ppm and 8 ppm, respectively. This Nb ingot was hot-extruded, cold-rolled and processed into a final shape with an area reduction after extrusion of about 20% or more, and then subjected to final annealing at about 1200 ° C. for about 1 hour. An Nb bar having a crystal ratio of 79%, 0.2% proof stress: 55 MPa, average crystal grain size: 61 μm, average Vickers hardness Hv: 76, and elongation: 61% was prepared.

  Using this Nb bar, a billet of secondary stack material (secondary multi-core billet) was produced in the same manner as in Example 1. This was heated to 600 ° C. and hot isostatically extruded to a diameter of 12.7 mm (extrusion ratio 27.8). When the cross section of the extruded member was observed, it was found that all the Nb cores were processed smoothly.

This member was cold drawn to produce a precursor wire having a cross-sectional size of 1.50 × 2.50 mm ² . The precursor was heat-treated at 720 ° C. for 150 hours to generate an Nb ₃ Sn phase. When the superconducting properties were evaluated after the heat treatment, the critical current density nonCu-Jc of the non-copper part was 135 A / mm ² , and the n value was 32, and the characteristics sufficient for NMR wires under the conditions of this temperature and external magnetic field. I was able to confirm.

The cross-sectional structure of the secondary multi-core billet stage of Nb ₃ Sn superconducting wire produced by the bronze process is an explanatory view schematically showing. Another example of a sectional structure of the secondary multi-core billet stage of Nb ₃ Sn superconducting wire produced by the bronze process is an explanatory view schematically showing.

Explanation of symbols

1 Nb or Nb base alloy core 2 Cu-Sn base alloy base material (bronze matrix)
3 Diffusion barrier layer 4 Stabilized copper 5 Primary stack material 6 Outer case 7 Secondary multi-core billet

Claims (6)

A Nb or Nb-based alloy rod used for producing a Nb ₃ Sn superconducting wire by a bronze method,
A plane (longitudinal section) that passes through the axis and is parallel to the longitudinal direction is randomly extracted from 100 to 110 crystal grains from the microstructure image observed with a microscope, and the length of the major axis is the length of the minor axis. Divide the crystal grains whose aspect ratio (long axis length / short axis length) is 2.5 or less into recrystallized grains, and then divide the number of recrystallized grains by the total number of extracted crystal grains. Nb or Nb-based alloy rod for producing a bronze Nb ₃ Sn superconducting wire , wherein the recrystallization rate is 78% or more and the 0.2% proof stress is 220 MPa or less .   The Nb or Nb-based alloy rod according to claim 1, wherein the average crystal grain size is 10 to 50 µm.   The Nb or Nb-based alloy bar according to claim 1 or 2, wherein the outer surface has an average Vickers hardness Hv of 50 to 120. A Nb or Nb-based alloy rod according to any one of claims 1 to 3 is combined with a Cu-Sn-based alloy to form a composite material, the composite material is extruded, and a plurality of them are bundled to reduce the surface area. method for manufacturing a Nb ₃ Sn superconducting wire precursor for manufacturing which is characterized in that the wire by that. A Nb or Nb-based alloy rod according to any one of claims 1 to 3 is combined with a Cu-Sn-based alloy to form a composite material, the composite material is extruded, and a plurality of them are bundled to reduce the surface area. A precursor for producing a Nb ₃ Sn superconducting wire, which is made into a wire by doing so. A bronze Nb ₃ Sn superconducting wire obtained by heat-treating the precursor for producing a Nb ₃ Sn superconducting wire according to claim 5 to form a superconducting phase.

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