JP3544781B2 - Method for producing Nb-Ti based superconducting multilayer board - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention utilizes the excellent properties of the Nb-Ti alloy superconducting material, such as complete diamagnetism, magnetic flux trapping, and high current density with zero electric resistance, such as current carrying ability, to provide a superconducting magnetic shield, a superconducting permanent magnet, The present invention relates to a method for manufacturing an Nb-Ti alloy superconducting multilayer plate capable of producing a superconducting multilayer plate having excellent performance as a current-carrying material at low cost and capable of significantly improving characteristics.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, as a method for producing an Nb-Ti-based superconducting multilayer board, a Nb-Ti-based superconducting multilayer board is coated with a sheet or foil of Nb, Ta or an Nb-Ta alloy in a housing or cylindrical hollow body made of a highly conductive metal. An alloy plate is filled so as to be alternately laminated with at least one layer of the highly conductive metal plate, the filling rate is set to 60% or more, and the ends are covered with the highly conductive metal, and the inside is vacuum-sealed and welded and sealed. After performing hot working at a working rate of 30 to 98% and a temperature of 500 to 1000 ° C., a heat treatment is performed at a temperature of 300 to 450 ° C. for a holding time of 1 to 168 hours. Patent No. 1790043, "Nb-Ti-based superconducting magnetic shield material and its production," in which cold working with a working ratio of 30 to 98% is repeatedly performed at least six times to produce a plate-like or box-like product. Method " Beauty "I. Itoh et al., Cryogenics, 35, 403 (1995)" is known the like.
[0003]
The critical current density (Jc), which is one of the most important superconducting properties of the Nb-Ti alloy, is determined by the fact that the α-Ti precipitate having the hcp structure is finely dispersed in the β phase having the bcc structure. It is known to improve significantly. It is said that the optimal heat treatment temperature for aging precipitation of α-Ti is in the range of 300 to 450 ° C., and the α-Ti phase is precipitated by performing the heat treatment for a certain long time in this temperature range. It is said that if the temperature is higher than this, fine dispersion of the precipitate is impossible and the precipitate becomes coarse, and if the temperature is 600 ° C. or higher, the α-Ti precipitate is decomposed. It is known that not only α-Ti precipitates but also dislocation networks and lattice defects are important for improving Jc. These are introduced into the crystal by cold working, but disappear in the temperature range where the β phase is recovered or recrystallized. As a method of appropriately mixing the dislocation network and the α-Ti precipitate, it has become known that Jc is greatly increased by repeating cold working at an appropriate working rate and the above heat treatment several times. Have been.
[0004]
The following is a description of the reason why these materials improve Jc. When a superconducting current flows in a superconductor, a magnetic field is naturally generated. However, in a second-class superconductor such as an Nb-Ti alloy, in a magnetic field of Bc1 (lower critical magnetic field) or more, the magnetic field is expressed in quantum flux units. They form a spiral bundle and enter the superconductor. This is called a mixed state of the second type superconductor. In an Nb-Ti alloy, Bc1 is as low as several tens G (Gauss), and it can be said that all practical magnetic fields exceed Bc1 and penetrate into the superconductor. Since the magnetic field at this time is orthogonal to the superconducting current, Lorentz force acts on both of them at right angles according to Fleming's law, and if there is no resistance force, the magnetic flux flow occurs in an avalanche state and electric resistance occurs, generating heat. As a result, the superconducting state instantaneously transits to the normal conducting state. This is called quench. However, in this magnetic flux flow, the normal conducting phase having an appropriate size and distribution density dispersed in the superconductor acts as a stopper (called a pinning point) of the flow of the quantum flux vortex, and the superconducting state can be maintained. . If this can be maintained up to a higher current density, it can be said that this is a superconducting material having a high Jc and excellent characteristics.
[0005]
That is, in the case of an Nb-Ti-based alloy, it is generally recognized that the aforementioned α-Ti precipitates, dislocation networks, and lattice defects are pinning points. In addition, dislocation networks and lattice defects are said to assist in the movement of atoms forming a precipitated phase and serve as a driving force for precipitation, and are important in a double sense. Therefore, as described above, in order to improve the properties, in the case of the Nb-Ti alloy, the cold working ratio (final solution treatment or recrystallization) until the product after the final solution treatment or recrystallization annealing is performed. It is necessary to improve the thickness of NbTi at the time of crystallization / the thickness of NbTi in the product.
[0006]
However, in the conventional manufacturing method, given the processing on an industrial rolling equipment, working ratio of 10 about ^two orders was limited. For example, when the thickness of the NbTi sheet and the copper sheet at the time of cladding of the NbTi 30 layer are both 3 mm, the copper thickness of the outermost layer is usually desirably about 10 times. 240mm. Assuming that the hot rolling is performed at a working ratio of 50%, the thickness of the clad material is 120 mm, and when the product thickness is 1 mm, the cold working ratio is only 120.
[0007]
[Problems to be solved by the invention]
The Nb-Ti-based superconducting multilayer board disclosed in Japanese Patent No. 1790043 requires a diffusion barrier layer having a multiple of the number of Nb-Ti layers. Nb or Ta, which is a material most suitable as a barrier, is a rare metal and is extremely expensive. Due to the nature of the diffusion barrier, the required thickness is very small, depending on the manufacturing conditions. For example, the diffusion rate of Ti atoms in Nb at 350 ° C. is said to be 1.5 × 10 ⁻¹⁶ m / hr, and 1.5 × 10 ⁻¹³ m = 1.5 even when the temperature is maintained for 1000 hr. × 10 ⁻³ Å, which is far less than the distance between atoms. In other words, a relatively small barrier thickness is sufficient, and the amount of Nb material should be reduced and the material cost should be reduced. However, as the thickness of the Nb or Ta sheet at the time of the composite cladding becomes thinner and thinner, the processing cost increases and the sheet becomes expensive. For example, in the case of Nb foil, depending on the width of the foil, if it is 50 μm or less, the unit price per unit weight is several times to one digit higher than that of a sheet of 100 μm or more.
[0008]
In the conventional manufacturing method, the barrier is inserted at the stage of clad assembly of the multilayer clad slab. For example, when the copper ratio is about 2 and the number of NbTi layers is 30, the maximum of the clad slab coming from the restriction of the rolling mill or the like is obtained. When the thickness is about 300 mm, the thickness of the NbTi plate is about 3 mm, and the barrier thickness is several orders of magnitude larger than the actually required thickness in view of cost.
[0009]
Further, in the above-mentioned Japanese Patent No. 1790043, in order to obtain good metal bonding between the two metal layers, the composite is subjected to hot working at a working rate of 30 to 98% and a temperature of 500 to 1000 ° C. Bonding is easier if the metal is softened on the high temperature side. Therefore, in order to obtain the composite integration of the cladding material, that is, to obtain a sufficient metallic bonding at each metal interface, α-Ti precipitates, dislocation networks and lattices, which are the most important factors for improving the Jc of the NbTi superconducting material. Defects have been heated to a temperature range where there is a risk that their role will be greatly reduced or even eliminated.
[0010]
That is, according to the above-mentioned Patent No. 1790043, the Nb-Ti-based alloy sheet before being laminated in multiple layers is subjected to cold working (mainly cold rolling) after hot working. At this time, dislocation networks and lattice defects introduced by rolling at a considerable processing rate are accumulated in the structure. However, in the conventional method, hot working (mainly hot rolling) was performed to obtain a metal joint of the composite material laminated in multiple layers, and the heat was accumulated at a temperature of 500 to 1000 ° C. at that time. Dislocation networks and lattice defects had to be greatly reduced or even disappeared. Therefore, in the conventional method, it is almost impossible to make full use of the dislocation network and lattice defects introduced into the Nb-Ti based alloy structure before the multilayer cladding step, and it is necessary to use the parts introduced later. It was not too much.
[0011]
[Means for Solving the Problems]
(1) First, as shown in FIG. 1A, a sheet or foil 3 of Nb, Ta, or Nb-Ta alloy is coated around a single layer of the Nb-Ti alloy material 1 having a large thickness. The surrounding area is coated with a normal conductive metal material 2. In this method, a single layer of Nb-Ti alloy material coated with a sheet or foil of Nb, Ta or Nb-Ta alloy is inserted into the case 2 made of the above-described normal conductive metal material, and the inside of the case is evacuated. And then sealed by welding to obtain a single-layer Nb-Ti clad slab. The present slab is subjected to surface reduction processing including hot working at a processing rate of 30 to 98% and a temperature of 500 to 1000 ° C. to be combined and integrated. Examples of the surface reduction processing include hot, warm, and cold rolling, and hot, warm, and cold press forging. Also, methods such as HIP and CIP are effective. Furthermore, there is a method in which the various processing methods are sequentially combined. After reducing the surface by these methods, as shown in FIG. 2, the content is an Nb-Ti alloy 1, the surface layer is a normal conductive metal 2, and a layer 3 of Nb, Ta or Nb-Ta alloy is provided at the interface between them. Although an intervening clad plate 4 having a three-layer structure is formed, impurities such as oxide scale, oil and dirt present on the outermost surface are completely removed and cleaned, and then, as shown in FIG. After cutting into several pieces and laminating them in the thickness direction and inserting them into the casing 2 made of a normal conducting metal, the inside of the casing is evacuated and then sealed by welding to obtain a multilayer clad slab. After subjecting this slab to processing at a processing rate of 30 to 98% and a temperature of 600 ° C. or less again, a heat treatment at a temperature of 300 to 450 ° C. and a holding time of 1 to 168 hours, and a processing rate of 30 to 98%. Cold working is repeated alternately six times or less to form a plate or foil to obtain an Nb-Ti based superconducting multilayer board 5 as shown in FIG. As the surface reduction processing method, there is the method described above.
[0012]
When the clad plate 4 having the three-layer structure is formed, the entire circumference is firstly covered with the normal conducting metal, so that the entire circumference is covered with the normal conducting metal even after the area reduction processing. Normally, for example, the side surfaces of the plate other than the cut portions are still covered with the normal conducting metal, but they may be laminated as they are, or the terminal portion of the normal conducting metal only (usually called an ear) is cut and removed. The layers may be laminated after being formed. FIG. 1B shows a case where the ears are removed, but this can usually make more effective use of the limited space in the housing.
[0013]
Here, the normal-conducting metal layer is generally used because it is necessary as a partition material for securing the superconducting stability of the superconducting material of the present invention and for dividing the superconducting material into multiple layers. , Cu, Al or the like, or a high resistance metal such as a Cu-Ni alloy. The division of the superconducting material is a method known as an effective means for reducing the hysteresis loss per unit volume, which is one of the main factors generated in the superconductor and destroying its superconducting characteristics. When used in a static magnetic field (DC magnetic field) or a fluctuating magnetic field having a small change rate, a highly conductive metal such as Cu or Al is desirable. In a fluctuating magnetic field having a high change rate (AC magnetic field or pulse magnetic field), a Cu-Ni alloy is used. And the like. Under the former condition in which the heat generated by the eddy current generated in the normal conducting metal is small, the highly conductive metal is useful for stabilizing the superconductivity, but under the latter condition in which the eddy current problem is serious, a high resistance metal is desirable.
[0014]
(2) An Nb-Ti single-layer clad plate having a three-layer structure is formed in the same manner as in (1), and as shown in FIG. 4A, a desired number of layers are cut and laminated in the plate thickness direction. After laminating the normal conducting metal plate 2 on the lower surface, the edges of each lamination interface are welded and sealed while all the interfaces are kept in a vacuum, and the surface is reduced again in the same manner as (1) to form a composite integrated body. Then, an Nb—Ti-based superconducting multilayer plate 5 ′ as shown in FIG. 4B is obtained. The processing method of the area reduction and heat treatment is the same as (1).
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
According to the present invention, as shown in FIG. 1A, the ratio of the thickness of the sheet or foil of the diffusion barrier metal inserted at the interface to the plate thickness of one layer of the Nb-Ti alloy material at the time of start is determined. It can be much smaller. That is, in the conventional method, as shown in FIG. 6, a plurality of Nb-Ti-based alloy plates and normal-conducting metal plates, each of which has been reduced in surface area and reduced in thickness, are alternately laminated, and the diffusion barrier metal The multilayer laminate into which a sheet or foil has been inserted has been inserted into a housing made of a normal conductive metal material. The height (clad material thickness) limitation of the casing shown in FIGS. 1A and 1B is limited by equipment such as a rolling mill, and is the same in the present invention and the conventional method. Therefore, when the thickness of the diffusion barrier metal sheet or foil is the same, the thickness of the Nb-Ti alloy plate can be much larger in the present invention, so that the final product is a multilayer plate having the same thickness and structure. In this case, the thickness of the diffusion barrier layer can be reduced by up to about two orders of magnitude with respect to the Nb-Ti alloy layer having the same thickness, and the amount of the diffusion barrier metal material used without using high-cost ultra-thin foil processing. Can be greatly reduced.
[0016]
Further, according to the present invention, two cladding steps are performed in all the manufacturing steps, and the first single-layer cladding includes a method as shown in FIGS. 1 (a) and 5 (a). For the second multilayer cladding, there is a method as shown in FIGS. 1 (b) and 4 (a). At the time of the first single-layer cladding, as in the conventional method, the temperature is set so as to metallically bond between different metal layers, that is, between the Nb-Ti alloy plate and the normal conductive metal plate and the diffusion barrier metal. It is necessary to perform hot working at 500 to 1000C, preferably 600 to 1000C. In this temperature range, α-Ti precipitates, dislocation networks, and lattice defects, which are the most important factors for improving the Jc of the Nb-Ti superconducting material, have a risk of being significantly reduced or even eliminated. However, according to the present invention, at this stage, almost none of the above-mentioned important factors has been introduced into the Nb-Ti structure, so that it can be said that there is almost no danger. Because the thickness of the Nb-Ti alloy sheet is considerably large, only hot working is required, and no cold working is required, or even very little compared to the conventional method.
[0017]
However, as can be easily understood from FIGS. 1B and 4A, in the second multilayer cladding, there is only an interface between normal conductive metal layers such as Cu, Al, and Cu—Ni alloy. As compared with the first dissimilar metal interface, good metal bonding can be easily obtained even by processing such as hot rolling at a significantly lower heating temperature. That is, a temperature range of 600 ° C. or less is sufficient. In this temperature range, α-Ti precipitates, dislocation networks, and lattice defects in the Nb—Ti-based alloy hardly disappear, or even very little. Although the problem of coarsening of the α-Ti precipitate which causes a decrease in Jc may occur, it is possible and can be avoided to shorten the holding time of heating within a range without any problem. Therefore, most of the dislocation networks and lattice defects introduced by the processing such as the cold rolling performed between the first and second cladding steps, which could hardly be used in the conventional method, were removed after the second multilayer cladding. , And can be used for α-Ti phase precipitation by heat treatment, and can greatly improve Jc.
[0018]
【Example】
(Example 1)
As shown in FIG. 1 (a), the entire circumference of a 50 mmt thick Nb-Ti alloy thick plate 1 is covered with a 0.1 mmt thick Nb sheet 3 and inserted into a 100 mmt high copper box 2. Sealing was performed by electron beam welding in a vacuum to produce a single-layer clad material. The box was made 500 mmt by hot rolling at 600 ° C., and then 3 mmt by cold rolling. FIG. 2 shows the single-layer clad plate 4. As shown in FIG. 1 (b), it is cut into 20 square plates of the same shape and laminated, inserted into a copper box 2 having a height of 100 mmt, and evacuated again. The inside was sealed by electron beam welding to produce a multilayer clad material. The box was reduced to 50 mmt by hot rolling at 400 ° C., and then subjected to cold rolling and heat treatment for α-Ti phase precipitation to obtain a multilayer plate 5 having a thickness of 1 mmt as shown in FIG.
[0019]
Also, as a conventional method for comparison, a 50 mmt thick Nb-Ti alloy plate was hot-rolled at 600 ° C. to 25 mmt, then cold-rolled to 1.5 mmt, and the Nb-Ti alloy plate was made the same. A copper box having a thickness of 100 mmt is formed by cutting into 20 square plates having a shape and alternately laminating with 19 copper plates having the same thickness and shape, and inserting Nb sheets having a thickness of 0.1 mm at all interfaces. And sealed by electron beam welding in a vacuum to produce a multilayer clad material. Thereafter, a multilayer plate having a thickness of 1 mmt and having the same structure under the same conditions as described above was obtained.
[0020]
To compare the superconducting properties of the two, Jc measurement in a magnetic field was performed by the method (four-terminal method) shown in "I. Itoh et al., Cryogenics, 35, 403 (1995)". Here, the applied magnetic field is applied in parallel to the Nb-Ti layer, and the energizing current is the result when the magnetic field is perpendicular to the magnetic field and the rolling direction.
[0021]
According to this, Jc of the conventional method was 2.5 × 10 ³ A / mm ² at an applied magnetic field of 2T and 0.9 × 10 ³ A / mm ² at an applied magnetic field of 6T. On the other hand, the Jc of the present invention was 4.5 × 10 ³ A / mm ^{2 at} 2T, 1.5 × 10 ³ A / mm ² at 6T, improved by 80% at 2T, and about 70% at 6T.
[0022]
(Example 2)
As shown in FIG. 4A, a 3 mmt-thick Nb-Ti single-layer clad plate 4 is cut into 20 square plates of the same shape in the same manner as in Example 1, and the thickness is placed on the uppermost lower surface where these are laminated. After laminating a copper plate 2 having a thickness of 20 mmt, edges around each interface were sealed in a vacuum by electron beam welding 6 to produce a multilayer clad material. Thereafter, as shown in FIG. 4B, a multilayer board 5 'having a thickness of 1 mmt and having the same structure was formed in the same manner as in Example 1. When the Jc characteristic of this plate was measured by the same method as in Example 1, almost the same Jc value was obtained.
[0023]
(Example 3)
As shown in FIG. 1 (a), the entire periphery of the Nb-Ti alloy thick plate 1 having a thickness of 50 mmt is covered with a Nb sheet 3 having a thickness of 0.1 mmt, and inserted into a copper box 2 having a thickness of 100 mmt. Sealing was performed by electron beam welding in a vacuum to produce a single-layer clad material. The box is made 500 mmt by hot rolling at 600 ° C., then 3 mmt by cold rolling, the single-layer clad plate is cut into 20 square plates of the same shape, laminated, and inserted into a copper box having a thickness of 100 mmt. Then, sealing was performed again by electron beam welding in a vacuum to produce a multilayer clad material. The box was reduced to 50 mmt by hot rolling at 400 ° C., and then appropriately subjected to cold rolling and heat treatment for α-Ti phase precipitation to 1 mmt. At this time, the ratio of the thickness of the Nb barrier layer to the thickness of the Nb-Ti1 layer was about 0.2% as in the first case. This was 1/33 of the thickness of the Nb barrier layer having the exact same structure manufactured by the conventional method. Further, since the price of the Nb sheet is the same for both, the cost relating to the diffusion barrier has been reduced to almost 1/33. Further, no deterioration of the diffusion effect due to the reduced thickness of the barrier layer was observed.
[0024]
【The invention's effect】
According to the present invention, it is possible to greatly improve Jc, which is one of the most important superconducting characteristics, as compared with the conventional method, and it is clear that the present invention has a very great effect in terms of both cost and characteristics. is there. Further, the use of a diffusion barrier metal material such as an expensive metal such as Nb, Ta, or an Nb-Ta alloy has made it possible to reduce the cost by one digit or more .
[Brief description of the drawings]
FIG. 1A is a diagram illustrating a single-layer clad slab produced by inserting a single-layer Nb—Ti-based alloy plate coated with a diffusion barrier metal sheet or foil into a case made of a normal conductive metal according to the present invention. (B) is a multilayer clad slab according to the present invention, which is produced by inserting a plurality of single-layer clad plates into a casing made of a normal conducting metal.
FIG. 2 is a composite-integrated single-layer clad plate according to the present invention.
FIG. 3 is a composite-integrated multilayer clad plate according to the present invention.
FIG. 4 (a) is a view of the present invention in which a plurality of single-layer clad plates are laminated in the thickness direction, and a normal conducting metal plate is further laminated on the uppermost lower surface thereof. A multilayer clad slab produced by welding and sealing the peripheral end, and FIG. 4B shows a multilayer clad plate produced by integrally integrating the multilayer clad slab in FIG. 4A according to the present invention.
FIG. 5 (a) shows a single-layer Nb—Ti alloy plate coated with a diffusion barrier metal sheet or foil according to the present invention, and a normal conducting metal plate laminated on both wide sides thereof, and all interfaces thereof are vacuumed. (B) is a single-layer clad slab manufactured by integrally integrating the single-layer clad slab in FIG. 5 (a) according to the present invention. Layer clad plate.
FIG. 6 shows a laminate obtained by alternately laminating a plurality of Nb—Ti-based alloy plates coated with a diffusion barrier metal sheet or foil and a plurality of normal conducting metal plates in a conventional method from a normal conducting metal. A multilayer clad slab that is manufactured by inserting it into an enclosure.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Nb-Ti alloy material 2 Normally conductive metal plate 3 Diffusion barrier metal sheet or foil 4 Single-layer clad plate 4 'integrated by processing Fig. 1 (a) and composited by processing Fig. 5 (a) Integrated single-layer clad plate 5 Multi-layered clad plate 5 ′ processed as shown in FIG. 1 (b) and composite-integrated multilayer clad plate 6 processed as shown in FIG. 4 (a).

Claims (2)

At least one Nb-Ti-based alloy layer and at least one normal-conducting metal layer are alternately laminated, and Nb, Ta or Nb- is interposed between the Nb-Ti-based alloy layer and the normal-conducting metal layer. In a method for manufacturing a Nb-Ti-based superconducting multilayer plate having a structure in which a barrier layer of a Ta alloy is present, a single-layer Nb-Ti-based alloy material covered with a sheet or foil of Nb, Ta or Nb-Ta alloy is provided. Is coated with a normal conducting metal material, and subjected to surface reduction processing including hot working at a processing rate of 30 to 98% and a temperature of 500 to 1000 ° C. to form a composite integrated body, and then laminating at least one layer of the composite material Then, after being inserted into a case made of a normal conducting metal, the inside of the case is evacuated, and then the case is welded and sealed, and the integrated composite is processed at a processing rate of 30 to 98% at a temperature of 600 ° C. or lower. After hot working of 300 Heat treatment at a temperature of 450 ° C. for a holding time of 1 to 168 hours and cold working with a working rate of 30 to 98% are alternately repeated 6 times or less to form a plate or foil, and the Nb—Ti superconductor A method for producing a multilayer board. At least one Nb-Ti-based alloy layer and at least one normal-conducting metal layer are alternately laminated, and Nb, Ta or Nb- is interposed between the Nb-Ti-based alloy layer and the normal-conducting metal layer. In a method for manufacturing a Nb-Ti-based superconducting multilayer plate having a structure in which a barrier layer of a Ta alloy is present, a single-layer Nb-Ti-based alloy material covered with a sheet or foil of Nb, Ta or Nb-Ta alloy is provided. Is coated with a normal conducting metal material, and subjected to surface reduction processing including hot working at a processing rate of 30 to 98% and a temperature of 500 to 1000 ° C. to form a composite integrated body, and then laminating at least one layer of the composite material Then, while maintaining the vacuum at the lamination interface, the ends of each lamination interface are welded and sealed, and the integrated composite is subjected to hot working at a working rate of 30 to 98% at a temperature of 600 ° C. or lower, Hold at 300-450 ° C A heat treatment of 1 to 168 hours and a cold working of a working rate of 30 to 98% are alternately repeated 6 times or less to form a plate or foil to obtain an Nb-Ti superconducting multilayer board. Manufacturing method.

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