Method for improving rolling uniformity of multi-core superconducting strip

Abstract

The invention discloses a method for improving rolling uniformity of a multi-core superconducting strip. The method specifically comprises the steps that a plurality of core materials are subjected to tubing treatment, then the composite pipe subjected to tubing treatment is subjected to drawing, rolling and heat treatment, the core materials are regular hexagon core materials, and the core materials are single-core core materials or core materials obtained through tubing treatment. According to the invention, the regular hexagon single core (or multi-core) is used for secondary (or multiple) tubing, so that the deformation of the multi-core round wire in the drawing process is changed, the section of the superconducting core is changed from an original irregular shape to an approximately circular shape, and the stress condition of the single superconducting core in the rolling process is closer to that of a single-core strip; and finally, the superconducting core with uniform density and texture degree is obtained. In addition, geometric stability can be enhanced, and uncertain deformation is avoided.

Description

A method for improving the rolling uniformity of multi-core superconducting strips Technical Field

[0001] This invention relates to the field of superconducting materials technology, and in particular to a method for improving the rolling uniformity of multi-core superconducting strips. Background Technology

[0002] Superconducting materials, substances whose electrical resistance completely disappears at specific low temperatures and which completely repel magnetic fields, have embodied humanity's dreams of cutting-edge technology and efficient energy since their discovery. Their two core characteristics—zero resistance and perfect diamagnetism (Meissner effect)—have broad application prospects in cutting-edge scientific exploration, medicine, energy, and information technology. Through the tireless efforts of researchers, more and more superconducting materials are being discovered, and efforts are being made to transform these laboratory wonders into powerful engines driving social progress. Currently, superconductors that have been put into practical use or have the potential for practical application include NbTi / Nb3Sn, YBCO, Bi-2212 / Bi-2223, and iron-based superconductors. Among them, Bi-based superconductors and iron-based superconductors possess high critical transition temperatures (T0). c ), High Upper Critical Field (H c2 ) and high critical current density (J c Both can be prepared using the low-cost powder-in-tube method (PIT), giving them unique advantages in large-scale applications.

[0003] From a practical standpoint, to prevent flux fluctuations and reduce AC losses, practical superconducting wires and tapes typically employ multi-core filament composite structures, such as NbTi / Nb3Sn and Bi-2212 round wires. However, for Bi-2223 tapes and iron-based superconducting tapes that require rolling to introduce texture, multi-core construction significantly reduces current-carrying capacity. Taking 122-series (AeFe2As2) iron-based superconducting tapes as an example, when the number of cores in Sr-122 / Ag tapes is increased from 7 to 19, the current-carrying capacity at 4.2K and 10T decreases. c From 6.1×10 4 A / cm 2 Reduced to 3.5×10 4 A / cm 2 The attenuation was 43%. Similarly, after increasing the number of cores in the Sr-122 / Ag / Fe tape from 7 to 19, the J at 4.2K and 10T was reduced. c From 1.4×10 4 A / cm 2 Reduced to 8.4×10 3 A / cm 2 The attenuation was 40%. And when further increased to 114 cores, J... c It will decrease to 6.3×103 A / cm 2 The attenuation is 55%. If a thousand-core structure similar to NbTi / Nb3Sn and Bi-2212 superconducting circular wires is fabricated, J c The degradation may be even more severe.

[0004] Therefore, there is an urgent need to develop methods to increase the critical current density of multi-core (core number ≥ 7) superconducting tapes and promote their practical application. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for improving the rolling uniformity of multi-core superconducting strips. This method can significantly improve the deformation uniformity of the superconducting core during rolling, thereby increasing the critical current density of the strip.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for improving the rolling uniformity of multi-core superconducting strips, the method specifically involves loading multiple core materials into a tube, and then drawing, rolling and heat-treating the composite tube after the loading process. The core material is a regular hexagonal core material, and the core material is a single core material or a core material obtained through the loading process.

[0008] During the tube loading process, a filler rod is provided between the metal tube for tube loading and the outermost core material placed inside the metal tube for tube loading. The filler rod is made of the same material as the metal tube of the core material.

[0009] Preferably, the filler rod is a round rod.

[0010] The metal tube used in the tube assembly process is made of the same material as the core material. A groove is provided on the inner wall of the metal tube used for tube assembly, and the shape of the groove matches the external shape of the combination of the multiple core materials.

[0011] And / or, when the core material is a core material obtained through tubing processing, the material of the metal tube used in the tubing processing is the same as the material of the metal tube of the core material.

[0012] In this process, multiple core materials are assembled into a core material assembly after being processed into tubing. The relationship between the inner diameter *r* of the metal tubing used in the tubing process and the side length *a* of the regular hexagonal core material is as follows: , where n is the number of layers in the core material assembly.

[0013] In this process, multiple core materials are assembled into a core material assembly after being processed into tubes. The core material located at the center of the core material assembly is replaced with a solid rod of the same material as the metal tube of the core material.

[0014] Specifically, the core material located at the center refers to several layers of core material located outward from the center of the core material assembly.

[0015] Specifically, when the core material assembly has 3 or 4 layers, the innermost core material layer is replaced; thereafter, for every 3 additional layers in the core material assembly, one more innermost core material layer is replaced. Specifically, when the core material assembly has 3 layers (19 core materials) or 4 layers (37 core materials), the innermost core material is replaced; when the core material assembly has 5 to 7 layers, the inner two layers are replaced, and so on; when the core material assembly has 8 to 10 layers, the inner three layers are replaced.

[0016] More preferably, after multiple core materials are packaged into tubes, the multiple core materials are combined to form a core material assembly, and the core material at the outermost apex corner of the core material assembly is removed.

[0017] Preferably, after removing the core material at the outermost apex of the core material assembly, the relationship between the inner diameter *r* of the metal tube used in the tube assembly process and the side length *a* of the regular hexagonal core material is as follows: Where n is the number of layers in the core material assembly.

[0018] The method specifically includes the following steps:

[0019] (1) The superconducting powder is loaded into a primary metal tube and the two ends of the metal tube are sealed.

[0020] (2) The tube is drawn to a certain size using a circular mold, and then shaped to a certain size using a regular hexagonal mold several times. After that, it is cut into several pieces of the same length to obtain a regular hexagonal single core material.

[0021] (3) Insert several of the aforementioned regular hexagonal single-core core materials into the secondary tube to complete the secondary tube assembly;

[0022] (4) The secondary tube assembly obtained in step (3) is subjected to a pulling process;

[0023] (5) After drawing, rolling and heat treatment are carried out in sequence;

[0024] (6) If three or more recombinations are required, steps (2) and (3) must be repeated.

[0025] The beneficial effects of this invention are as follows:

[0026] (1) This invention uses a regular hexagonal single core (or multiple cores) for secondary (or multiple) tube assembly, which changes the deformation of the multi-core round wire during the drawing process. The cross-section of the superconducting core changes from its original irregular shape to a near-circular shape. During the rolling process, the stress condition of a single superconducting core is closer to that of a single-core strip, ultimately obtaining a superconducting core with uniform density and texture. In addition, it also enhances geometric stability and avoids uncertain deformation.

[0027] (2) The present invention further optimizes the multi-core superconducting tape composite structure, removes redundant superconducting cores that contribute little to performance, includes the middle superconducting core with excessive deformation and the two end superconducting cores with insufficient deformation, avoids the influence of such superconducting cores on the deformation of surrounding superconducting cores, ensures the deformation uniformity of other superconducting cores, and improves the contribution to critical current density. Attached Figure Description

[0028] Figure 1 shows the cross-sectional morphology of a single-core round wire. In Figure 1, (a) is the cross-sectional morphology of the single-core round wire, (b) is a schematic diagram of the single-core round wire being rolled into a single-core strip, and (c) is the cross-sectional morphology of the single-core strip.

[0029] Figure 2 shows the morphology of multi-core superconducting strips using single-core circular wire in a comparative manner. In Figure 2(a), the cross-sectional morphology of a 37-core circular wire with secondary tubing using single-core circular wire is shown. In Figure 2(b), the cross-sectional morphology of the 37-core strip with secondary tubing using single-core circular wire and the morphology of some abnormally deformed superconducting cores are shown during the intermediate stage of single rolling. In Figure 2(c), the final cross-sectional morphology of the 37-core strip with secondary tubing using single-core circular wire is shown.

[0030] Figure 3 is an overall schematic diagram of an embodiment of optimizing the core material assembly. Figure 3(a) is a schematic diagram of a 37-core structure using a circular single core for secondary tubing, and Figure 3(b) is a schematic diagram of a 30-core structure using a regular hexagon for secondary tubing.

[0031] Figure 4 is an overall schematic diagram of another embodiment of the optimized core material assembly. Figure 4(a) is a schematic diagram of a 91-core structure using a circular single core for secondary tubing, and Figure 4(b) is a schematic diagram of a 78-core structure using a regular hexagon for secondary tubing.

[0032] Figure 5 is a schematic diagram of the structure of the multi-core superconducting tape obtained in Example 2 of the present invention. In Figure 5(a), it is a cross-sectional view of the secondary tube using a regular hexagonal single core; in Figure 5(b), it is a cross-sectional view of the tape in the intermediate rolling stage; and in Figure 5(c), it is the final morphology of the multi-core superconducting tape obtained in Example 2.

[0033] Figure 6 is a schematic diagram showing the positions of the hardness analysis of the multi-core superconducting tapes obtained in Example 2 and the comparative example. In Figure 6(a), the hardness analysis position of the 37-core superconducting tape obtained in the comparative example is shown; in Figure 6(b), the hardness analysis position of the 30-core superconducting tape obtained in Example 2 is shown; and in Figure 6(c), the positions of abcde in a single superconducting core are shown.

[0034] Figure 7 shows a comparison of the standard deviation and range of hardness of the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example. In Figure 7(a), the standard deviation of hardness of the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example is compared, and in Figure 7(b), the range of hardness of the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example is compared.

[0035] Figure 8 shows the aspect ratio statistics of the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example. In Figure 8, (a) and (d) show the aspect ratio statistics of the 37-core tape in the comparative example; (b) and (e) show the aspect ratio statistics of the 37-core tape in Example 1; and (c) and (f) show the aspect ratio statistics of the 30-core tape in Example 2.

[0036] Figure 9 is a schematic diagram of the inner diameter of a metal tube after post-assembly using a regular hexagonal core material;

[0037] Figure 10 is a schematic diagram of the inner diameter of a metal tube with an optimized core material assembly using a regular hexagonal core material. Figure 10(a) is a schematic diagram of a 30-core structure with a secondary tube assembly using a regular hexagonal core material, and Figure 10(b) is a schematic diagram of a 78-core structure with a secondary tube assembly using a regular hexagonal core material.

[0038] Figure 11 is a schematic diagram of the inner diameter of a metal tube with an optimized core material assembly using regular hexagonal core material. Figure 11(a) is a schematic diagram of a 30-core structure with secondary tube assembly using regular hexagonal core material, and Figure 11(b) is a schematic diagram of a 78-core structure with secondary tube assembly using regular hexagonal core material.

[0039] The attached figures are labeled as follows: 101 - central core material, 102 - central second layer core material, 103 - filler rod. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0041] The inventors discovered that, with an increase in the core count of Bi-2223 tape and iron-based superconducting tape, J... c The main reason for the reduction is that the uniformity of deformation in the downward pressing direction of the superconducting core cannot be guaranteed during the rolling process. This is mainly reflected in two aspects: first, the non-uniformity of deformation of a single superconducting core; and second, the significant differences in deformation at different locations within the superconducting core. This non-uniform deformation of the superconducting core directly leads to non-uniformity in texture and density, ultimately resulting in J... c This reduces [the yield]. Therefore, obtaining a uniformly deformed superconducting core is crucial for achieving high J [performance / performance]. c The necessary conditions for superconducting multi-core tapes.

[0042] The inventor has provided a detailed analysis of the above reasons, as follows:

[0043] (1) With the increase of the number of cores, the deformation of a single superconducting core is uneven.

[0044] Currently, the PIT preparation methods for Bi-2223 and iron-based superconducting multicore strips are to use a drawn circular single core for secondary tube assembly (7 cores, 19 cores, 37 cores, etc.), or a circular 19 core for tertiary tube assembly (114 cores, etc.), and then continue to draw to a specific size before rolling.

[0045] Typically, as shown in Figure 1, after a single-core circular wire is rolled into a single-core strip, the cross-sectional morphology of the superconducting core changes from circular to approximately spindle-shaped. Specifically, the original cross-sectional morphology of the single-core circular wire is shown in Figure 1(a), with a circular interface, an inner layer of superconducting material, and an outer layer of metal tube; it can be rolled into a single-core strip as shown in Figure 1(b), and the cross-sectional morphology of the formed single-core strip is shown in Figure 1(c), where the cross-sectional morphology of the superconducting core changes from circular to approximately spindle-shaped.

[0046] However, for multi-core strips, only the central superconducting core in the pre-rolling circular wire is nearly circular, while the other superconducting cores are irregularly shaped. For example, the six superconducting cores surrounding a 7-core circular wire tend to be trapezoidal, and their deformation will be uneven when subjected to downward pressure not perpendicular to the parallel sides during rolling. For strips with more cores, such as 37 cores, the superconducting cores in different layers of the circular wire will exhibit various irregular shapes, as shown in Figure 2. This results in the superconducting cores in the strip exhibiting triangular, irregular quadrilateral, or even irregular polygonal shapes during rolling, ultimately leading to significantly lower uniformity of superconducting core deformation compared to strips with fewer cores. Therefore, the inventors believe it is necessary to optimize the deformation of the circular wire before rolling to ensure that the strip has uniform superconducting core deformation.

[0047] (2) The deformation of the superconducting core varies greatly at different locations.

[0048] Since the rolling process involves rolling a round wire into a flat strip, an increase in the number of cores inevitably leads to a larger deformation in the central superconducting core and a smaller deformation in the surrounding superconducting cores, as shown in Figure 2(c). Larger deformation can cause core breakage along the length of the superconducting core, while smaller deformation results in insufficient texture. Therefore, it is essential to control the deformation of all superconducting cores within a certain range. Current multi-core composite structures cannot solve this problem; therefore, optimization of the multi-core composite structure is needed to reduce the difference in deformation between superconducting cores at different locations.

[0049] Specifically, the present invention provides a method for improving the rolling uniformity of multi-core superconducting strips. The method involves loading multiple core materials into a tube, and then drawing, rolling, and heat-treating the composite tube after loading. The core material is a regular hexagonal core material, which can be a single core material or a core material obtained through loading.

[0050] It should be noted that the method of this invention first prepares single-core wires using the powder in-tube (PIT) method, and then performs tube-packing, drawing, rolling, and heat treatment on the single-core wires to obtain multi-core wires. The tube-packing process can be performed once or multiple times. The drawing, rolling, and heat treatment processes in this embodiment are the same as other existing technologies. Drawing uses common cemented carbide or diamond dies, with a reduction in area of ​​4%-20%, a drawing speed of 1 m / min to 30 m / min, and a final diameter of 0.3 mm to 3 mm. Rolling involves a single-pass reduction of 5%-40%, 3-20 rolling passes, and a final thickness of 0.2 mm to 1 mm. Heat treatment is performed in an Ar environment or vacuum sintering at a temperature of 400℃-1200℃ for 10 min to 24 h, and cooling is typically achieved through furnace cooling.

[0051] The improvement of this invention lies in replacing the existing circular core material with a regular hexagonal core material. Using a regular hexagonal single core for composite tubing ensures that all superconducting cores except the outermost superconducting core can be packed as tightly as possible, improving the geometric stability of the composite structure. After drawing using the composite round wire of this invention, the cross-sectional morphology of the superconducting core changes from an irregular shape to an approximately spherical shape. During rolling, a superconducting core morphology close to that of a single-core strip is obtained, improving the deformation uniformity of individual superconducting cores in multi-core structures. Furthermore, similar to NbTi / Nb3Sn and Bi-2212 multi-core round wires, this method improves geometric stability, reduces the probability of abnormal deformation during wire drawing, and allows for better control of the core-to-superconducting ratio.

[0052] Furthermore, during the tubing process, a filler rod is placed between the tubing metal tube and the outermost core material placed inside the tubing metal tube. The filler rod is made of the same material as the metal tube of the core material. To eliminate the gap between the outermost single core and the outer sheath, a round wire, i.e., a filler rod, made of the same material as the single core sheath, is filled into the gap between the outermost single core and the outer sheath to maximize the filling density. Generally, the higher the filling density, the better.

[0053] Preferably, the filler rod is a round rod.

[0054] Furthermore, the metal tube used for tubing is made of the same material as the core material, and grooves are provided on the inner wall of the metal tube, the shape of which matches the overall shape of the combined core materials. If the outer sheath and inner sheath are made of the same material, the outer sheath can be replaced from a round tube with a custom tube having specific grooves on its inner wall.

[0055] In this process, multiple core materials are assembled into a core material assembly after being processed into tubing. The relationship between the inner diameter *r* of the metal tubing used in the tubing process and the side length *a* of the regular hexagonal core material is as follows: , where n is the number of layers in the core material assembly.

[0056] Referring to Figure 9, taking a 37-core core as an example, after replacing the circular core material with a regular hexagonal core material, the core material assembly has 4 layers, i.e., n=4. Therefore, in Figure 9, OA=r, OB= , ;according to Calculations show that At this point, the diameter of the 37-core metal tube... .

[0057] Wherein, when the core material is obtained through tube assembly, the material of the metal tube used in the tube assembly is the same as the material of the metal tube of the core material.

[0058] As a preferred embodiment, referring to Figure 3, after multiple core materials are assembled into a tube, the core materials are combined to form a core material assembly. The core material located at the center of the core material assembly is replaced with a solid rod of the same material as the metal tube of the core material. The purpose of this method is to reduce the deformation of the central superconducting core through structural optimization, thereby reducing the difference in deformation of the superconducting core.

[0059] Specifically, the core material located at the center refers to one or two layers of core material located at the center of the core material assembly. The rule is that when the core material assembly has 3 to 4 layers, the innermost core material layer (i.e., the very innermost core piece) is replaced; when the core material assembly has 5 to 7 layers, the innermost two core materials layers (i.e., 7 core materials) are replaced. Furthermore, for every 3 additional layers in the core material assembly, one more innermost core material layer is replaced. Additionally, when the core material assembly has fewer than 3 layers, replacement is not required.

[0060] Referring to Figure 3, Figure 3(a) is a schematic diagram of a 37-core structure using a circular single core for secondary tubing, i.e., 37 circular core materials form a core material assembly, and the overall core material assembly is a regular hexagon. Figure 3(b) is a schematic diagram of a 30-core structure using a regular hexagon for secondary tubing. In this embodiment, the central core material 101 of the 37-core core material assembly is replaced with a solid rod of the same material as the metal tube of the core material.

[0061] Referring to Figure 4, Figure 4(a) is a schematic diagram of a 91-core structure using a circular single core for secondary tubing, i.e., 91 circular core materials form a core material assembly, and the overall core material assembly is a regular hexagon. Figure 4(b) is a schematic diagram of a 78-core structure using a regular hexagon for secondary tubing. In this embodiment, the two central core layers of the 91-core structure are replaced with solid rods of the same material as the metal tube of the core material. Specifically, the central core material 101 and the central second core material 102 are replaced with solid rods.

[0062] More preferably, after multiple core materials are assembled into a tube, the core materials are combined to form a core material assembly, and the outermost corner core material of the core material assembly is removed. The purpose of this method is to improve the deformation of the superconducting cores at both ends through structural optimization, thereby reducing the difference in deformation of the superconducting cores. It should be noted that after removing the outermost corner core material, a filler rod is required during the tube assembly process.

[0063] More preferably, the outermost corner of the core material assembly has 6 core materials. Specifically, referring to Figures 3 and 4, the 6 core materials removed from the outermost corner are filled with filler rods 103. Referring to Figures 3 and 4, after replacing the center core material and removing the 6 core materials from the outermost corner, the original 37-core core material assembly contains 30 regular hexagonal core materials, while the original 91-core core material assembly contains 78 regular hexagonal core materials.

[0064] After removing the outermost apex of the core material in the core material assembly, the relationship between the inner diameter *r* of the metal tube used in the tube assembly process and the side length *a* of the regular hexagonal core material is as follows: Where n is the number of layers in the core material assembly.

[0065] Referring to Figure 10, taking a 30-core core as an example (the core material assembly after replacement contains 30 regular hexagonal core materials), after replacing the circular core materials with regular hexagonal core materials, the core material assembly has 4 layers, i.e., n=4. Therefore, in Figure 9, OA=r, OB= , ;according to Calculations show that .

[0066] The inner diameter of the metal tube of the 30-core core material assembly in Figure 10 The inner diameter of the metal tube of the 78-core core material assembly in Figure 10 .

[0067] More preferably, the relationship between the diameter d of the filler rod, the side length a of the regular hexagonal core material, and the inner diameter r of the metal tube is as follows: Specifically, referring to Figure 11, the relationship between the diameter d of the filler rod of the 30-core core assembly in Figure 11 and the side length a of the regular hexagonal core material and the inner diameter r of the metal tube is as follows: The relationship between the diameter d of the filler rod of the 78-core core assembly in Figure 11 and the side length a of the regular hexagonal core material and the inner diameter r of the metal tube is as follows: .

[0068] The specific operation steps of the method for improving the rolling uniformity of multi-core superconducting strips according to the present invention are as follows:

[0069] (1) Load the superconducting powder into a primary metal tube (e.g., Ag and its alloys, Cu and its alloys, etc.) and seal both ends.

[0070] (2) The tube is first drawn to a specific size using a circular mold, and then shaped to a specific size several times using a regular hexagonal mold, and then cut into several pieces of the same length. The specific size in this step is determined according to actual needs.

[0071] (3) A number of hexagonal single-core tubes (e.g., 30) are densely packed into a secondary tube of a specific size (Ag and its alloys, Cu and its alloys, etc.). The secondary tube can be of two types. The first type can be made of the same or different material as the primary tube, with a circular inner wall. Its dimensions can be calculated using the side lengths of the hexagonal single-core tubes. For example, with 30 cores, the relationship between the inner diameter r of the secondary tube and the side length a of the hexagonal single-core tube is r = a. At this point, the six gaps between the outermost hexagonal single core and the secondary tube are filled with round bars of the same material as the primary tube. The second type of material is the same as the primary tube, but its inner wall has specific grooves, the shape of which matches the assembled single core.

[0072] (4) The subsequent drawing process of the round wire after the secondary tube is completed is the same as the conventional drawing process.

[0073] (5) The rolling and heat treatment processes after drawing are the same as those of conventional rolling and heat treatment.

[0074] (6) If three or more composite processes are required, steps (2) and (3) must be repeated, and the material of the outer tube should be the same as that of the previous stage during the composite process.

[0075] Example 1

[0076] This embodiment uses a regular hexagonal single core for secondary tubing to prepare a 37-core 122 series iron-based superconducting tape. The specific operation steps are as follows:

[0077] (1) Load 122 series iron-based superconducting powder into an Ag tube (primary tube) with an outer diameter of 6 mm and an inner diameter of 5 mm, and seal both ends.

[0078] (2) Pull the Ag tube to a diameter of 1.5mm, then use a regular hexagonal mold to shape it 5 times. The final side length a of the regular hexagon is 0.657mm. Then cut it into 37 pieces of the same length.

[0079] (3) Insert 37 regular hexagonal single cores into a tube with an outer diameter of 10 mm and an inner diameter of 8 mm (inner diameter r = In a silver tube (secondary tube) where 'a' is the side length of a regular hexagon, both ends are sealed.

[0080] (4) After the composite, the round wire is drawn using a carbide die. The surface reduction rate of a single drawing is 12%, the drawing speed is 5m / min, and the final diameter is 1.8mm after several drawings.

[0081] (5) The 30-core round wire with a diameter of 1.8mm is rolled by flat rolls with a single rolling pressure of 31% and the final strip thickness is 0.4mm.

[0082] (6) The strip with a thickness of 0.4 mm was sintered at 800℃ for 2 hours and then cooled in the furnace after sintering.

[0083] Example 2

[0084] This embodiment uses a regular hexagonal single core for secondary tube assembly and optimization of the tube assembly structure to prepare a 30-core 122 series iron-based superconducting tape. Its specific structure is shown in Figure 3(b), and the specific operation steps are as follows:

[0085] (1) Load 122 series iron-based superconducting powder into an Ag tube (primary tube) with an outer diameter of 6 mm and an inner diameter of 5 mm, and seal both ends.

[0086] (2) Pull the Ag tube to a diameter of 1.5mm, then use a regular hexagonal mold to shape it 5 times. The final side length a of the regular hexagon is 0.718mm. Then cut it into 30 pieces of the same length.

[0087] (3) Insert 30 regular hexagonal single cores into a tube with an outer diameter of 10mm and an inner diameter of 8mm (inner diameter r= In a silver tube (secondary tube) where 'a' is the side length of a regular hexagon, both ends are sealed.

[0088] (4) After the composite, the round wire is drawn using a carbide die. The surface reduction rate of a single drawing is 12%, the drawing speed is 5m / min, and the final diameter is 1.8mm after several drawings.

[0089] (5) The 30-core round wire with a diameter of 1.8mm is rolled by flat rolls with a single rolling pressure of 31% and the final strip thickness is 0.4mm.

[0090] (6) The strip with a thickness of 0.4 mm was sintered at 800℃ for 2 hours and then cooled in the furnace after sintering.

[0091] Figure 5 is a schematic diagram of the multi-core superconducting tape obtained in Example 2. Figure 5(a) shows the cross-sectional morphology of the superconducting core after secondary tube assembly using a regular hexagonal single core, showing that the cross-section of the superconducting core changes from its original irregular shape to a near-circular shape. Figure 5(b) shows the cross-sectional morphology of the tape during the intermediate rolling stage; it can be seen that the stress condition of a single superconducting core during rolling is closer to that of a single-core tape. Figure 5(c) shows the final morphology of the multi-core superconducting tape obtained in Example 2, showing that the final multi-core superconducting tape has a superconducting core with uniform density and texture.

[0092] Comparative Example

[0093] This comparative example demonstrates a 37-core 122 series iron-based superconducting tape prepared using a circular single-core secondary tube assembly process. Its structure is shown in Figure 3(a), and the specific operational steps are as follows:

[0094] (1) Load 122 series iron-based superconducting powder into an Ag tube (primary tube) with an outer diameter of 6 mm and an inner diameter of 5 mm, and seal both ends.

[0095] (2) Pull the Ag tube to a diameter of 1.14 mm and then cut it into 37 pieces of the same length.

[0096] (3) Insert 37 circular single cores into a silver tube (secondary tube) with an outer diameter of 10 mm and an inner diameter of 8 mm, and seal both ends.

[0097] (4) After the composite, the round wire is drawn using a carbide die. The surface reduction rate of a single drawing is 12%, the drawing speed is 5m / min, and the final diameter is 1.8mm after several drawings.

[0098] (5) The 30-core round wire with a diameter of 1.8mm is rolled by flat rolls with a single rolling pressure of 31% and the final strip thickness is 0.4mm.

[0099] (6) The strip with a thickness of 0.4 mm was sintered at 800℃ for 2 hours and then cooled in the furnace after sintering.

[0100] Figure 2 shows a 37-core 122 series iron-based superconducting strip prepared using a circular single core for secondary tubing. Figure 2(a) shows the cross-sectional morphology of the 37-core circular strip prepared using a circular single core for secondary tubing; Figure 2(b) shows the cross-sectional morphology of the 37-core strip prepared using a circular single core for secondary tubing during the intermediate stage of single rolling, as well as the morphology of some abnormally deformed superconducting cores; Figure 2(c) shows the final cross-sectional morphology of the 37-core strip prepared using a circular single core for secondary tubing. As can be seen from Figure 2(a), the superconducting cores in different layers of the circular strip exhibit various irregular shapes; consequently, during the rolling process, the superconducting cores of the strip will exhibit triangles, irregular quadrilaterals, or even irregular polygons as shown in Figure 2(b), ultimately resulting in the morphology shown in Figure 2(c) where the deformation of the central superconducting core is larger, while the deformation of the surrounding superconducting cores is smaller.

[0101] Hardness analysis was performed on the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example. The analysis locations are shown in Figure 6. Figure 6(a) shows the hardness analysis locations of the 37-core superconducting tape obtained in the comparative example; Figure 6(b) shows the hardness analysis locations of the 30-core superconducting tape obtained in Example 2; and Figure 6(c) is a schematic diagram of the abcde positions in a single superconducting core. The analysis results are shown in Tables 1, 2, and 3.

[0102] Table 1. Hardness analysis results of the 37-core superconducting tape in the comparative example.

[0103]

[0104] Table 2. Hardness analysis results of the 37-core superconducting tape in Example 1

[0105]

[0106] Table 3. Hardness analysis results of the 30-core superconducting tape in Example 2

[0107]

[0108] Figure 7 shows a comparison of the standard deviation and range of hardness of the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example. Figure 7(a) shows the comparison of the standard deviation of hardness of the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example, and Figure 7(b) shows the comparison of the range of hardness of the multi-core superconducting tapes obtained in Examples 1, 2, and the comparative example. As can be seen from Figure 7, compared to the conventional 37-core tape using circular single cores for secondary tubing, the average hardness of the 37-core tape using hexagonal single cores for secondary tubing and the 30-core tape using hexagonal single cores for secondary tubing remain essentially unchanged, but their standard deviation and range are significantly reduced, indicating higher density uniformity of the superconducting cores.

[0109] The ratio of the width to the thickness of the superconducting core after rolling can be used to measure the texture of the superconducting core. The greater the texture, the greater the aspect ratio. For multi-core strips, the aspect ratio should be uniformly distributed within a certain range. For multi-core strips, the aspect ratio is suitable to be distributed between 10 and 30. Figure 8 shows the statistical results of the aspect ratio of the multi-core superconducting strips obtained in Examples 1, 2, and the comparative example. Among them, (a) and (d) in Figure 8 are the statistical results of the aspect ratio of the 37-core strip in the comparative example, (b) and (e) in Figure 8 are the statistical results of the aspect ratio of the 37-core strip in Example 1, and (c) and (f) in Figure 8 are the statistical results of the aspect ratio of the 30-core strip in Example 2. As shown in Figure 8, compared to the conventional 37-core strip using a circular single core for secondary tubing and the 37-core strip using a regular hexagonal single core for secondary tubing, the aspect ratio of the superconducting core of the 30-core strip using a regular hexagonal secondary tubing is 83.3%, which is higher than the 67.5% and 62.1% of the 37-core strip. Furthermore, the standard deviation of the aspect ratio of the optimized 30-core strip superconducting core decreased from 11.4 and 10.9 to 6.4, indicating improved deformation uniformity of the overall superconducting core. The range also decreased from 68.2 and 59.2 to 25, indicating an improvement in the phenomena of excessive and insufficient deformation of the superconducting core. The above analysis shows that the optimized 30-core strip superconducting core has an appropriate and uniform texture.

[0110] The critical current density of round single-core wire, 7-core wire (existing), multi-core superconducting wire obtained from Example 1, Example 2 and comparative examples was tested using the four-lead method. The results are shown in Table 4.

[0111] Table 4 Critical current densities of different superconducting tapes

[0112]

[0113] The test results of the critical current density in Table 4 show that after optimizing the single-core shape and secondary tube structure, the critical current density of the strip is reduced from 3.2 × 10⁻⁶. 4 A / cm 2Increased to 5.5×10 4 A / cm 2 Compared to the 6.2×10 of the 7-core strip... 4 A / cm 2 The critical current density decreased by only 11.3%, far lower than the 48.4% of conventional 37-core tape. It is also lower than the results reported in the literature (a reduction of about 40% when the number of cores increases from 7 to 19).

[0114] Therefore, the method of the present invention for improving the rolling uniformity of multi-core superconducting strip improves the uniformity of deformation of individual superconducting cores in the rolling direction of the strip and reduces the difference in overall superconducting core deformation, thereby improving the density uniformity and texture of superconducting cores in multi-core strips.

[0115] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0116] The parts of this invention not described in detail are well-known in the art. The above embodiments are provided merely for the purpose of describing the invention and are not intended to limit the scope of the invention. The scope of the invention is defined by the appended claims. All equivalent substitutions and modifications made without departing from the spirit and principles of the invention should be covered within the scope of the invention.

Claims

1. A method for improving the rolling uniformity of multi-core superconducting strips, characterized in that, The method specifically involves assembling multiple core materials into tubes, and then drawing, rolling, and heat-treating the composite tube after the tube assembly process. The core material is a regular hexagonal core material, which can be a single core material or a core material obtained through tube assembly.

2. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 1, characterized in that, During the tube loading process, a filler rod is provided between the metal tube for tube loading and the outermost core material placed inside the metal tube for tube loading. The filler rod is made of the same material as the metal tube of the core material.

3. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 2, characterized in that, The filler rod is a round rod.

4. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 1, characterized in that, The metal tube used in the tube assembly process is made of the same material as the core material. A groove is provided on the inner wall of the metal tube used for tube assembly, and the shape of the groove matches the external shape of the combination of the multiple core materials; and / or, when the core material is a core material obtained through tube assembly, the metal tube used in the tube assembly process is made of the same material as the metal tube of the core material.

5. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 1, characterized in that, After multiple core materials are assembled into a tube, the core materials are combined to form a core material assembly. The relationship between the inner diameter *r* of the metal tube used in the tube assembly process and the side length *a* of the regular hexagonal core material is as follows: , where n is the number of layers in the core material assembly.

6. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 1, characterized in that, After multiple core materials are assembled into a tube, the multiple core materials are combined to form a core material assembly. The core material located at the center of the core material assembly is replaced with a solid rod material of the same metal tube material as the core material. Specifically, the core material located at the center consists of several layers of core material located outward from the center of the core material assembly.

7. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 6, characterized in that, When the core material assembly has 3 or 4 layers, the innermost core material layer is replaced; thereafter, for every 3 additional layers of the core material assembly, the replaced innermost core material layer is increased by one.

8. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 1, characterized in that, After multiple core materials are assembled into a tube, the core materials are combined to form a core material assembly, and the outermost core material at the top corner of the core material assembly is removed.

9. The method for improving the rolling uniformity of multi-core superconducting strips according to claim 1, characterized in that, After removing the outermost apex of the core material in the core material assembly, the relationship between the inner diameter *r* of the metal tube used in the tube assembly process and the side length *a* of the regular hexagonal core material is as follows: Where n is the number of layers in the core material assembly.

10. The method for improving the rolling uniformity of multi-core superconducting strips according to any one of claims 1 to 9, characterized in that, The method specifically includes the following steps: (1) superconducting powder is loaded into a primary metal tube and the two ends of the metal tube are sealed; (2) the primary tube is drawn to a certain size using a circular mold, and then shaped to a certain size several times using a regular hexagonal mold, and then cut into several pieces of the same length to obtain a regular hexagonal single core material; (3) several pieces of the regular hexagonal single core material are loaded into a secondary tube to complete the secondary tube assembly; (4) the secondary tube assembly obtained in step (3) is drawn; (5) after the drawing process, rolling and heat treatment are performed in sequence; (6) if three or more composite processes are required, steps (2) and (3) need to be repeated.

Images