Niobium rod for Nb3Sn superconducting wire niobium material and preparation method thereof

Abstract

The invention provides a niobium rod for an Nb3Sn superconducting wire niobium material and a preparation method of the niobium rod, and the preparation method of the niobium rod comprises the following steps: a, upsetting and drawing forging: carrying out n times of upsetting and drawing forging on a niobium cast ingot, n is any integer in 2-5, and carrying out annealing heat treatment in a gap between two times of upsetting and drawing forging or after the last upsetting and drawing forging, the upsetting and drawing forged round bar is obtained; b, radial forging is conducted, specifically, the upsetting and drawing forging round bar is subjected to radial forging; and c, rotary forging: carrying out rotary forging on the rod blank subjected to radial forging. The niobium rod prepared by the method meets one or more of the following items: the grain size is 7-8 grades, the elongation at break is greater than or equal to 40%, and the hardness (HV / 2.9 N) is greater than or equal to 40 and less than or equal to 65. Namely, the niobium rod prepared by the preparation method has at least one of the characteristics of uniform and fine grains, low hardness and high plasticity, and the comprehensive performance of the niobium rod is improved, so that the strict requirements of the Nb3Sn superconducting wire on the base material in high-field magnet application are met.

Description

Technical Field

[0001] The present application relates to the technical field of nonferrous metal material processing, and in particular to a niobium rod for Nb3Sn superconducting wire niobium material and a preparation method thereof. Background Art

[0002] Nb3Sn superconducting wire is a key material for high-magnetic field superconducting magnets, particularly in fusion experiments. Its ability to withstand high magnetic fields is crucial for controlling the intense plasma generated by nuclear fusion reactors. Therefore, it is primarily used in the toroidal field (TF) coils and central solenoid (CS) of the ITER project. The TF coils generate magnetic fields as high as 11.8 T during operation, placing extremely high demands on Nb3Sn superconducting wire: moderate critical current density and minimal hysteresis losses.

[0003] In addition, the China Fusion Engineering Test Reactor (CFETR) is a large-scale scientific engineering device independently designed and developed by China. The CFETR superconducting magnet system includes 1 CS coil (<8 winding units), 16 TF coils and 7 PF coils. Among them, the Nb3Sn coil with higher operating parameters is an important component of the CFETR superconducting magnet system.

[0004] During the processing of Nb3Sn superconducting wires, the total processing rate of the core wire niobium rod is very high, exceeding 99.9%. This requires the niobium rod to have excellent mechanical properties and high purity, and to be free of cracks, wrinkles, inclusions and other harmful defects. However, in the existing technology, niobium rods still have the following defects: (1) the impurity content of the ingot is high and the purity is low; (2) the grain crushing during the processing is insufficient, the grains of the rod are uneven, the size varies greatly, and the performance deviation is large; and (3) the heat treatment process is unreasonable, the grain growth in the later stage is uneven, and the grain size varies greatly. Therefore, it is urgent to develop a niobium rod that can meet the processing requirements of Nb3Sn superconducting wires, which is of great significance for improving the performance of Nb3Sn superconducting wires and promoting the development and application of clean energy. Summary of the Invention

[0005] In order to meet the processing requirements of Nb3Sn superconducting wires and solve the problems of large and uneven grain size or insufficient mechanical properties of niobium rods, the present application proposes a niobium rod and a preparation method thereof.

[0006] In its first aspect, the present invention provides a niobium rod that meets one or more of the following characteristics: a grain size of 7-8, an elongation at break ≥ 40%, and a hardness (HV / 2.9N) ≤ 65. Specifically, the niobium rod provided herein exhibits at least one of the following characteristics: uniform and fine grains, low hardness, and high plasticity, improving the rod's overall performance and meeting the stringent substrate requirements for Nb3Sn superconducting wire in high-field magnet applications.

[0007] In any embodiment of the first aspect, the niobium rod comprises 0.0006%≤carbon≤0.005%, 0.0020%≤oxygen≤0.01%, 0.00020%≤nitrogen≤0.005%, 0.0002%≤hydrogen≤0.0015%, 0.0003%≤iron≤0.005%, 0.01%≤tantalum≤0.1%, 0.0010%≤silicon≤0.005%, 0.0004%≤nickel≤0.005%, 0.0005%≤molybdenum≤0.005%, 0.0005%≤tungsten≤0.01%, 0.0001%≤titanium≤0.005%, 0.0001%≤zirconium≤0.02%, and 0.001%≤hafnium≤0.02%.

[0008] In a second aspect of the present invention, a method for preparing a niobium rod is provided, comprising the following steps:

[0009] a. Upsetting forging: The niobium ingot is subjected to upsetting forging n times, where n is any integer from 2 to 5, and an annealing heat treatment is performed between two upsetting forgings or after the last upsetting forging to obtain an upsetting forging round bar;

[0010] b. Radial forging: radial forging of upsetting forged round bars;

[0011] c. Rotary forging: Rotary forging is performed on the bar billet after radial forging.

[0012] By employing the aforementioned combination of upsetting, radial, and rotary forging, the material properties of the niobium rods have been significantly improved, providing key support for the preparation of high-performance superconducting materials for nuclear fusion engineering. Specifically, repeated upsetting and stretching in each upsetting and forging process promotes the full fragmentation of the material's grains, refining the grains within the material, thereby improving its density and uniformity. Radial forging improves the material's internal structure, while rotary forging further enhances its cold working properties. Furthermore, combined with heat treatment processes, the internal structure of the material can be further improved, enhancing its cold working plasticity and toughness, resulting in a product with fine grains, low hardness, and high plasticity.

[0013] In any embodiment of the second aspect, the preparation method satisfies one or more of the following characteristics:

[0014] i. The axial upsetting reduction rate in each upset forging is 47%-59%, and the axial drawing rate is 47%-59%;

[0015] ii. The reduction rate of radial forging is 10%-15%;

[0016] iii. The reduction rate of rotary forging is 10%-15%.

[0017] In any embodiment of the second aspect, the primary annealing heat treatment is performed after the second upset forging.

[0018] In any embodiment of the second aspect, the maximum holding temperature of the single annealing heat treatment in step a is 850°C to 1200°C. Preferably, the single annealing heat treatment includes a step-by-step temperature increase process under vacuum conditions. Further preferably, the single annealing heat treatment includes three stages of heating and holding processes, wherein preferably, the temperature is increased to the first stage holding temperature T1 within 20min-60min and held for 10min-100min, and T1 is 300°C to 700°C; preferably, the temperature is increased to the second stage holding temperature T2 within 20min-60min and held for 10min-100min, and T2 is 600°C to 900°C; preferably, the temperature is increased to the third stage holding temperature T3 within 20min-60min and held for 10min-100min, and T3 is 850°C to 1200°C.

[0019] In any embodiment of the second aspect, the insulation temperature of the first section is 450°C to 550°C, and the insulation time is 20min-50min; preferably, the insulation temperature of the second section is 750°C to 850°C, and the insulation time is 20min-50min; preferably, the insulation temperature of the third section is 900°C to 1000°C, and the insulation time is 80min-100min.

[0020] In any embodiment of the second aspect, the last upsetting forging further includes a rounding process, and the rounding reduction rate is 5%-15%.

[0021] In any embodiment of the second aspect, the niobium ingot has a diameter of 150 mm to 300 mm. Preferably, the niobium ingot comprises the following elements by weight: 0.0006% ≤ carbon ≤ 0.005%, 0.0020% ≤ oxygen ≤ 0.01%, 0.00020% ≤ nitrogen ≤ 0.005%, 0.0002% ≤ hydrogen ≤ 0.0015%, 0.0003% ≤ iron ≤ 0.005%, 0.01% ≤ tantalum ≤ 0.1%, 0.0010% ≤ silicon ≤ 0.005%, 0.0004% ≤ nickel ≤ 0.005%, 0.0005% ≤ molybdenum ≤ 0.005%, 0.0005% ≤ tungsten ≤ 0.01%, 0.0001% ≤ titanium ≤ 0.005%, 0.0001% ≤ zirconium ≤ 0.02%, 0.001% ≤ hafnium ≤ 0.02%, and the balance niobium. Preferably, the niobium ingot is preheated before step a, wherein the preheating temperature is 300° C. to 500° C. and the preheating time is 60 min to 180 min.

[0022] In any embodiment of the second aspect, prior to the heat treatment in step a, the process further includes pickling the bar after the upset forging. Preferably, the pickling solution comprises HF, HCl, and HNO3. Preferably, the volume ratio of HF:HCl:HNO3 in the pickling solution is 1:(1-5):(1-10).

[0023] In any embodiment of the second aspect, after step a and before step b, the method further includes any one or more of turning, surface treatment, and pickling. Preferably, the turning amount of a single side of the turning process is 1 mm to 5 mm. Preferably, the surface treatment equipment is selected from any one of a grinding wheel machine, an angle grinder, and a flap wheel. Preferably, the pickling solution contains HF, HCl, and HNO3. Preferably, the volume ratio of HF:HCl:HNO3 in the pickling solution is 1:(1-5):(1-10).

[0024] In any embodiment of the second aspect, after step b and before step c, a turning step is further included. Preferably, the turning amount of a single side of the turning process is 1 mm to 5 mm.

[0025] In any embodiment of the second aspect, step c is followed by a further surface treatment step. Preferably, the surface treatment equipment is selected from any one of a grinding machine, an angle grinder, and a flap wheel.

[0026] In any embodiment of the second aspect, step c further includes a secondary annealing heat treatment, wherein the maximum holding temperature of the secondary annealing heat treatment is 850°C to 1200°C. Preferably, the secondary annealing heat treatment includes a step-by-step temperature increase process under vacuum conditions. Further preferably, the secondary annealing heat treatment includes two stages of heating and holding processes, wherein preferably, the temperature is increased to the first holding temperature T4 in 20-60 minutes and held for 10-120 minutes, and T4 is 300-700°C; preferably, the temperature is increased to the second holding temperature T5 in 20-60 minutes and held for 10-150 minutes, and T4 is 800-1200°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0028] Figure 1 This is a micrograph of the metallographic structure of the niobium rod product of Example 1 of the present application;

[0029] Figure 2 This is a micrograph of the metallographic structure of the niobium rod product of Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0030] Below with reference to accompanying drawings and examples, embodiments of the present invention are described in detail, but those skilled in the art will appreciate that the following examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention. Where specific conditions are not specified in the examples, the methods are carried out according to conventional conditions or the conditions recommended by the manufacturer. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be obtained commercially.

[0031] To address the issues of overly large and uneven grain sizes, or insufficient mechanical properties, in niobium rods, a first embodiment of the present application provides a niobium rod that meets one or more of the following characteristics: a grain size of 7-8, an elongation at break ≥ 40%, and a hardness (HV / 2.9N) ≤ 65. In other words, the niobium rod provided in the above embodiment of the present application exhibits at least one of the following characteristics: uniform and fine grains, low hardness, and high plasticity, improving the overall performance of the niobium rod and thus meeting the stringent substrate requirements of Nb3Sn superconducting wire for high-field magnet applications.

[0032] In some embodiments, the niobium rod comprises 0.0006% ≤ carbon ≤ 0.005%, 0.0020% ≤ oxygen ≤ 0.01%, 0.00020% ≤ nitrogen ≤ 0.005%, 0.0002% ≤ hydrogen ≤ 0.0015%, 0.0003% ≤ iron ≤ 0.005%, 0.01% ≤ tantalum ≤ 0.1%, 0.0010% ≤ silicon ≤ 0.005%, 0.0004% ≤ nickel ≤ 0.005%, 0.0005% ≤ molybdenum ≤ 0.005%, 0.0005% ≤ tungsten ≤ 0.01%, 0.0001% ≤ titanium ≤ 0.005%, 0.0001% ≤ zirconium ≤ 0.02%, and 0.001% ≤ hafnium ≤ 0.02%. Thus, the prepared niobium rod has a high purity, i.e., the niobium content is high and the contents of other impurity elements are low, resulting in good uniformity of the niobium rod.

[0033] A second embodiment of the present application provides a method for preparing a niobium rod, comprising the following steps:

[0034] a. Upsetting forging: The niobium ingot is subjected to upsetting forging n times, where n is any integer from 2 to 5, and an annealing heat treatment is performed between two upsetting forgings or after the last upsetting forging to obtain an upsetting forging round bar;

[0035] b. Radial forging: radial forging of upsetting forged round bars;

[0036] c. Rotary forging: Rotary forging is performed on the bar billet after radial forging.

[0037] The present invention utilizes the aforementioned combination of upsetting, radial, and rotary forging processes to significantly enhance the material properties of niobium rods, providing key support for the preparation of high-performance superconducting materials for nuclear fusion engineering. Specifically, repeated upsetting and stretching during each upsetting and forging process promotes the full fragmentation of the material's grains, refining the grains within the material, thereby improving its density and uniformity. Radial forging improves the material's internal structure, while rotary forging further enhances its cold working properties. Furthermore, combined with heat treatment processes, this further improves the material's internal structure and enhances its cold working plasticity and toughness, resulting in a product with fine grains, low hardness, and high plasticity.

[0038] For example, the niobium rod obtained by the above preparation method meets one or more of the following characteristics: grain size is 7-8, elongation at break is ≥40%, and hardness is 40≤(HV / 2.9N)≤65.

[0039] In some embodiments, the preparation method meets one or more of the following characteristics:

[0040] i. The axial upsetting reduction rate in each upset forging is 47%-59%, and the axial drawing rate is 47%-59%;

[0041] ii. The reduction rate of radial forging is 10%-15%;

[0042] iii. The reduction rate of rotary forging is 10%-15%.

[0043] The control of the above-mentioned reduction rates and drawing rates is conducive to achieving the processability of each step, and the control of the above-mentioned reduction rates is used to fully refine the grains inside the material, and then gradually process it into the required rods through radial forging and rotary forging.

[0044] In some embodiments, the axial upset reduction ratio and the axial drawing ratio in each upset forging are independently 47%-59%, for example, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, or within the range of any two of the above ratios.

[0045] In some embodiments, the radial forging reduction ratio is 10%-15%, such as 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, or within the range of any two of the above ratios.

[0046] In some embodiments, the reduction ratio of rotary forging is 10%-15%, such as 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, or within the range of any two of the above ratios.

[0047] The primary annealing treatment is primarily used to eliminate defects such as cracks and pores that may have occurred during the upsetting forging process, thereby improving the overall quality of the material. Therefore, the primary annealing treatment is performed after at least one upsetting forging process. In some embodiments, n is any integer between 2 and 4, such as 2, 3, or 4, and is preferably any integer between 2 and 3.

[0048] In some embodiments, the primary annealing heat treatment is performed after the second upset forging, so as to promptly and thoroughly eliminate cracks or porosity defects that may be generated during the upset forging.

[0049] In some embodiments, the maximum holding temperature of the primary annealing heat treatment in step a is 850° C. to 1200° C. This temperature range is below the melting point of niobium to ensure that the niobium rod material does not melt during the heating process while eliminating defects and improving its uniformity.

[0050] In order to further improve the degree of grain refinement and grain uniformity, in some embodiments, the primary annealing heat treatment includes a step-by-step temperature increase process under vacuum conditions. In a preferred embodiment, the primary annealing heat treatment includes a three-stage temperature increase and heat preservation process. In a preferred embodiment, the temperature is increased to the first heat preservation temperature T1 in 20min-60min and kept warm for 10min-100min, T1 is 300℃ to 700℃; the temperature is increased to the second heat preservation temperature T2 in 20min-60min and kept warm for 10min-100min, T2 is 600℃ to 900℃; the temperature is increased to the third heat preservation temperature T3 in 20min-60min and kept warm for 10min-100min, T3 is 850℃ to 1200℃.

[0051] In a preferred embodiment, the insulation temperature of the first section is 450°C to 550°C, and the insulation time is 20min-50min; the insulation temperature of the second section is 750°C to 850°C, and the insulation time is 20min-50min; the insulation temperature of the third section is 900°C to 1000°C, and the insulation time is 80min-100min.

[0052] The above-mentioned vacuum annealing process is carried out at three different temperatures. The staged holding process can fully and slowly release the thermal stress formed by the temperature increase, improve the uniformity of nucleation within the structure, and avoid secondary nucleation and abnormal nucleation. It makes the crystal nuclei in the structure more evenly dispersed and effectively controls grain growth. Among them, the first stage of heating and holding achieves the initial elimination of the original structure stress and promotes the recovery process; the second stage of heating and holding achieves partial recrystallization and the transition to uniform structure; and the final stage of holding process achieves complete grain recrystallization, with a larger distribution of fine equiaxed crystals in the microstructure, and a smaller difference in the size of the circumferential and core grains, achieving better grain refinement and dispersion, thereby improving the plasticity of the ingot.

[0053] In some embodiments, the final upsetting and drawing forging process also includes a rounding process to smooth the surface of the product, reduce burrs and sharp edges, and reduce stress concentration. The rounding reduction ratio is 5%-15%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or within the range of any two of the above ratios.

[0054] In some embodiments, according to the requirements of the niobium rod preparation process, the diameter of the niobium ingot is selected to be 150 mm-300 mm, for example, 150 mm, 160 mm, 170 mm, 180 mm, 190 mm,

[0055] 200mm, 210mm, 220mm, 230mm, 240mm, 250mm, 260mm, 270mm,

[0056] 280mm, 290mm, 300mm or within the range of any two of the above specifications.

[0057] In some embodiments, the niobium ingot contains the following elements by weight: 0.0006%≤carbon≤0.005%, 0.0020%≤oxygen≤0.01%, 0.00020%≤nitrogen≤0.005%, 0.0002%≤hydrogen≤0.0015%, 0.0003%≤iron≤0.005%, 0.01%≤tantalum≤0.1%, 0.0010%≤silicon≤0.005%, 0.0004%≤nickel≤0.005%, 0.0005%≤molybdenum≤0.005%, 0.0005%≤tungsten≤0.01%, 0.0001%≤titanium≤0.005%, 0.0001%≤zirconium≤0.02%, 0.001%≤hafnium≤0.02%, and a balance of niobium to ensure a niobium rod with higher purity.

[0058] In some embodiments, the niobium ingot is preheated before step a to preliminarily eliminate residual stress inside the niobium ingot and reduce the risk of deformation and cracking. The temperature of the preheat treatment is 300°C to 500°C, and the time is 60min-180min. For example, the temperature of the preheat treatment can be 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C,

[0059] The preheat treatment time may be 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, or within the range of any two of the above temperatures. The preheat treatment time may be 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, 130 min, 140 min, 150 min, 160 min, 170 min, 180 min, or within the range of any two of the above times.

[0060] In some embodiments, before the heat treatment in step a, the step of pickling the bar blank after upsetting and forging is further included to remove surface oxides and impurities, improve surface quality, and provide a better foundation for subsequent processing.

[0061] In some embodiments, the pickling solution comprises HF, HCl, and HNO3. In some embodiments, the volume ratio of HF:HCl:HNO3 in the pickling solution is 1:(1-5):(1-10). In a preferred embodiment, the volume ratio of HF:HCl:HNO3 in the pickling solution is 1:(1-3):(2-5).

[0062] In some embodiments, after step a and before step b, any one or more processing steps of turning, surface treatment and pickling are further included.

[0063] In some embodiments, after step a and before step b, a turning process is further included to remove excess material, form the desired shape and size, and ensure machining accuracy and quality. In a preferred embodiment, the turning process has a single-sided turning depth of 1 mm to 5 mm, such as 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, or within a range of any two of the foregoing values.

[0064] In some embodiments, after step a and before step b, a surface treatment step is also included to remove remaining defective single points without affecting the overall surface quality, which can save materials, make the surface smoother and flatter, improve the direct yield, and meet design requirements.

[0065] In a preferred embodiment, the surface treatment equipment is selected from any one of a grinding machine, an angle grinder, and a flap wheel.

[0066] In some embodiments, after step a and before step b, an acid wash step is included to further remove surface oxides and impurities and improve surface quality. In some embodiments, the acid wash solution comprises HF, HCl, and HNO3. In a preferred embodiment, the volume ratio of HF:HCl:HNO3 in the acid wash solution is 1:(1-5):(1-10).

[0067] In some embodiments, after step b and before step c, a turning step is further included. In a preferred embodiment, the turning amount of a single side is 1 mm to 5 mm, for example, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, or within the range of any two of the above values.

[0068] In some embodiments, step c is followed by a further surface treatment step. In a preferred embodiment, the surface treatment equipment is selected from any one of a grinding machine, an angle grinder, and a flap wheel.

[0069] In some embodiments, step c further includes a secondary annealing heat treatment, wherein the maximum holding temperature of the secondary annealing heat treatment is 850° C. to 1200° C.

[0070] In some embodiments, the secondary annealing heat treatment includes a step-wise temperature increase process under vacuum conditions. In a preferred embodiment, the secondary annealing heat treatment includes a two-stage temperature increase and temperature holding process. In a preferred embodiment, the temperature is increased to the first holding temperature T4 in 20-60 minutes and held for 10-120 minutes, with T4 being 300°C to 700°C; the temperature is increased to the second holding temperature T5 in 20-60 minutes and held for 10-150 minutes, with T4 being 800°C to 1200°C.

[0071] Example

[0072] The following examples and comparative examples will further illustrate the beneficial effects of the present application, but the scope of the present invention is not limited to these examples.

[0073] The surface quality, grain size, hardness and elongation of niobium rods are tested using the following methods:

[0074] Surface quality: Observe with the naked eye whether the overall surface of the bar is smooth, and whether there are wrinkles, cracks and inclusions;

[0075] Grain size test: According to ASTM E112 standard, the grain size is tested using the Olympus GX51f metallographic instrument. Grade definition: The grain size grade number G is determined by the number of grains N per square inch of the viewing field under standard magnification (100×). 100× OK: N 100× =2 G-1 Generally speaking, the larger the grain size, the finer the grains, and the material tends to have a uniform and dense organizational morphology.

[0076] Hardness test: According to GB / T 4340.1-2009 standard, use a hardness tester to perform Vickers hardness test (HV / 2.9N);

[0077] Elongation test: Tensile strength and yield strength were tested using an INSTRON 4505 testing machine. During the test, a non-proportional tensile specimen with a gauge length of 25 mm was selected. Elongation = ((length after test - length before test) / length before test) × 100%.

[0078] Example 1

[0079] The following steps are performed in sequence to prepare a niobium rod.

[0080] 1) Melting and casting: Use EB melting furnace to melt ingots The raw materials include the following elements: carbon 0.0006%, oxygen 0.002%, nitrogen 0.00020%, hydrogen 0.0002%, iron 0.0003%, tantalum 0.01%, silicon 0.0010%, nickel 0.0004%, molybdenum 0.0005%, tungsten 0.0005%, titanium 0.0001%, zirconium 0.0001%, hafnium 0.001%, and the balance niobium.

[0081] 2) Sawing: The initial ingot is sawn into ingots.

[0082] 3) Heating: First, heat the box-type resistance furnace to 410℃ and load the materials into the furnace; when the box-type resistance temperature rises to 410℃ again, start timing, and after 120 minutes, take it out of the furnace for forging;

[0083] 4) Forging: The raw material is placed vertically between the upper and lower anvils of the rapid forging machine, and axially roughened to H = 220 ± 5mm (the reduction rate is about 48%), and then the blank is straightened between the two anvils and axially stretched and squared to (The drawing rate is about 58%), and the chamfer is square. Then, the billet is placed vertically on the upper and lower anvils of the rapid forging machine for the second time, and the axial upset is The blank is then placed flat between two anvils and axially stretched to a square. Flip the edges and then return to the circle

[0084] 5) Pickling: Then, use pickling with a mixture of HF:HCl:HNO3=1:2:3 (volume ratio) to clean the surface; remove surface impurities and oil stains, and observe with the naked eye that the niobium metal is shiny and free of spots;

[0085] 6) Primary annealing heat treatment:

[0086] ① Rise to 500℃ in 30 minutes and keep warm for 30 minutes;

[0087] ②Heat to 800℃ for 30 minutes and keep warm for 30 minutes;

[0088] ③15min heating to 950℃ and keeping temperature for 90min;

[0089] At vacuum degree ≤3.0×10 -3 The temperature starts to rise at 1200 Pa, and argon is not filled during the cooling process, and the furnace is taken out when the temperature is ≤80℃.

[0090] 7) Forging: The blank obtained after heat treatment in step 6) is subjected to upsetting forging again. The blank is placed vertically between the upper and lower anvils of the rapid forging machine at room temperature and axially upset to The blank is then straightened between the two anvils and axially stretched and squared to Then further The pressing amount is 12mm, chamfering the edges to return to roundness Then use two semicircular anvils, place the material between the two semicircular anvils, and pound it into a round shape. The pressing amount is 8mm.

[0091] 8) Machining: Using a CW6163C lathe, the bar obtained in step 7) was surface turned, with a single-side turning depth of 2 mm, a speed of 720 rpm, and a feed depth of 1 mm;

[0092] 9) Surface treatment: Use a grinding wheel machine to remove any remaining defects on the surface of the bar;

[0093] 10) Pickling: repeat step 5);

[0094] 11) Radial forging: The bar obtained in step 10) is forged at room temperature using a radial forging machine (also known as a precision forging machine) with four hammers hitting the bar blank at high frequency. The radial forging reduction rate is 10% each time. The final forging is of rods;

[0095] 12) Machining: Using a CW6163C lathe, the bar obtained in step 11) was surface turned, with a single-side turning amount of 1.5 mm, a speed of 720 rpm, and a feed amount of 1.0 mm; of rods;

[0096] 13) Rotary forging: The bar obtained in step 12) was subjected to rotary forging using a DH rotary forging machine with a reduction rate of 10% each time, and finally obtained of rods;

[0097] 14) Sawing: The bars obtained in step 13) were cut using a GB40 band saw at a sawing pressure of 2 mm / min. of cut-to-length bars;

[0098] 15) Surface treatment: The rod obtained in step 14) is polished using a homemade cylindrical polishing device to give the surface a metallic luster;

[0099] 16) Secondary annealing heat treatment: The bar obtained in step 15) is subjected to the following annealing treatment:

[0100] ①Heat to 500℃ in 30 minutes and keep warm for 90 minutes;

[0101] ②Heat to 900℃ in 30 minutes and keep warm for 120 minutes;

[0102] At vacuum degree ≤3.0×10 -3 The temperature starts to rise at 1200 Pa, and argon is not filled during the cooling process, and the furnace is taken out when the temperature is ≤60℃.

[0103] 17) Then a niobium rod product is obtained.

[0104] Comparative Example 1

[0105] The difference from Example 1 is that only the first upsetting forging is performed in step 4), and no upsetting forging is performed in step 7, that is, only the first upsetting forging in step 4 is performed in the entire preparation process.

[0106] Comparative Example 2

[0107] The difference from Example 1 is that there is no upsetting forging step, that is, there are no steps (4) to (10).

[0108] Comparative Example 3

[0109] The difference from Example 1 is that there are no radial forging and rotary forging steps, that is, there are no steps (11) to (13).

[0110] The product performance was tested using the method described above, and the test results are recorded in Table 1.

[0111] Table 1

[0112]

[0113] It can be seen from the data in Table 1 that compared with Comparative Examples 1-4, Example 1 has better surface quality after three upsetting forgings, radial forgings and rotary forgings, and reaches a grain size of 7.5, with refined and uniform grains. Figure 1 As shown in Figure 2, the niobium rod has a Vickers hardness of 54.5 and an elongation of 50.4%, which meets the processing requirements of Nb3Sn superconducting wire. In Comparative Examples 1 and 2, since only one upsetting forging process is performed or no upsetting forging process is performed, the material grains are not fully broken, and the hardness and elongation are significantly reduced. Figure 2 It can also be seen that the grains in Comparative Example 1 are very uneven, indicating that they have not yet been fully crystallized. Compared with Example 1, Comparative Example 3 has folds on the surface and poor quality due to the lack of rotary forging and radial forging. The grains are coarse and uneven in size.

[0114] The following examines the effects of the number of upsetting and forging passes, the heating and holding parameters of the primary heat treatment on the surface quality, grain size, hardness and elongation of the niobium rod.

[0115] Example 2

[0116] The only difference from Example 1 is that the heating process of the heat treatment is changed to a single direct heating process: directly heating to 950° C. for 15 minutes and keeping the temperature for 90 minutes.

[0117] Example 3

[0118] The only difference from Example 1 is that step 4) includes three upsetting forgings and step 7) includes two upsetting forgings, that is, a total of five upsetting forgings are performed.

[0119] Example 4

[0120] The difference from Example 1 is that the holding temperature of the heat treatment is changed. The temperature is first raised to 550°C and held for 20 minutes over 30 minutes; then raised to 850°C and held for 20 minutes over 30 minutes; and finally raised to 1000°C and held for 60 minutes over 15 minutes.

[0121] Example 5

[0122] The only difference from Example 1 is that the holding temperature of the heat treatment is changed. The temperature is first raised to 450°C for 30 minutes and held for 50 minutes; then raised to 750°C for 30 minutes and held for 50 minutes; and finally raised to 900°C for 15 minutes and held for 100 minutes.

[0123] The product performance was tested using the method described above, and the test results are recorded in Table 2.

[0124] Table 2

[0125]

[0126] The data in Table 2 show that Example 1 achieves better grain refinement than Example 2 due to the staged heating and holding treatment, resulting in more complete grain recrystallization. In Example 3, increasing the number of upsetting and forging cycles to five resulted in more uniform grains due to repeated deformation, which further reduced hardness and increased elongation. Examples 4 and 5, which varied the holding temperature during heat treatment, both achieved better grain size, hardness, and elongation characteristics.

[0127] Although the specific embodiments of the present invention have been described in detail, it will be understood by those skilled in the art that various modifications and substitutions may be made to those details based on all the teachings disclosed, and these changes are all within the scope of protection of the present invention. The full scope of the present invention is given by the appended claims and any equivalents thereof.

Claims

1. A niobium rod, wherein the niobium rod satisfies one or more of the following characteristics: The grain size is grade 7-8, the elongation at break is ≥40%, and the hardness (HV / 2.9N) is 40≤65.

2. The niobium rod according to claim 1, wherein: The niobium rod comprises 0.0006%≤carbon≤0.005%, 0.0020%≤oxygen≤0.01%, 0.00020%≤nitrogen≤0.005%, 0.0002%≤hydrogen≤0.0015%, 0.0003%≤iron≤0.005%, 0.01%≤tantalum≤0.1%, 0.0010%≤silicon≤0.005%, 0.0004%≤nickel≤0.005%, 0.0005%≤molybdenum≤0.005%, 0.0005%≤tungsten≤0.01%, 0.0001%≤titanium≤0.005%, 0.0001%≤zirconium≤0.02%, and 0.001%≤hafnium≤0.02%.

3. A method for preparing a niobium rod, comprising the following steps: a. Upsetting forging: The niobium ingot is subjected to n upset forgings, where n is any integer from 2 to 5, and an annealing heat treatment is performed between the two upset forgings or after the last upset forging to obtain an upset forged round bar; b. Radial forging: radial forging of upsetting forged round bars; c. Rotary forging: Rotary forging is performed on the bar billet after radial forging.

4. The preparation method according to claim 3, wherein The preparation method meets one or more of the following characteristics: i. The axial upsetting reduction rate of each of the upset forging is 47% -59%, and the axial drawing rate is 47% -59%; ii. The radial forging reduction rate is 10%-15%; iii. The reduction rate of the rotary forging is 10%-15%.

5. The preparation method according to claim 3 or 4, wherein The primary annealing heat treatment is performed after the second upset forging.

6. The preparation method according to any one of claims 3 to 5, wherein The maximum holding temperature of the primary annealing heat treatment in step a is 850°C to 1200°C; preferably, the primary annealing heat treatment includes a step-by-step temperature increase process under vacuum conditions, and further preferably, the primary annealing heat treatment includes three stages of temperature increase and holding processes, wherein Preferably, the temperature is raised to the first holding temperature T1 within 20-60 minutes and kept at this temperature for 10-100 minutes, with T1 being 300°C to 700°C; Preferably, the temperature is raised to the second holding temperature T2 in 20-60 minutes and kept for 10-100 minutes, where T2 is 600°C to 900°C; Preferably, the temperature is raised to the third insulation temperature T3 within 20-60 minutes and maintained for 10-100 minutes, and T3 is 850°C to 1200°C.

7. The preparation method according to claim 6, wherein The first stage of holding temperature is 450℃ to 550℃, and the holding time is 20min-50min; Preferably, the second stage holding temperature is 750°C to 850°C, and the holding time is 20min-50min; Preferably, the third stage insulation temperature is 900° C. to 1000° C., and the insulation time is 80 min-100 min.

8. The preparation method according to any one of claims 3 to 7, wherein The last upsetting forging also includes a rounding process, and the rounding reduction rate is 5%-15%.

9. The preparation method according to any one of claims 3 to 8, wherein The diameter of the niobium ingot is 150mm-300mm. Preferably, the niobium ingot comprises the following elements by weight: 0.0006%≤carbon≤0.005%, 0.0020%≤oxygen≤0.01%, 0.00020%≤nitrogen≤0.005%, 0.0002%≤hydrogen≤0.0015%, 0.0003%≤iron≤0.005%, 0.01%≤tantalum≤0.1%, 0.0010%≤silicon≤0.005%, 0.0004%≤nickel≤0.005%, 0.0005%≤molybdenum≤0.005%, 0.0005%≤tungsten≤0.01%, 0.0001%≤titanium≤0.005%, 0.0001%≤zirconium≤0.02%, 0.001%≤hafnium≤0.02%, and the balance niobium; Preferably, the niobium ingot is preheated before step a, wherein the preheating temperature is 300° C. to 500° C. and the preheating time is 60 min to 180 min.

10. The preparation method according to any one of claims 3 to 9, wherein Before the heat treatment in step a, the process further includes pickling the bar blank after the upset forging. Preferably, the pickling solution comprises HF, HCl and HNO3, Preferably, the volume ratio of HF:HCl:HNO3 in the pickling solution is 1:(1-5):(1-10).

11. The preparation method according to any one of claims 3 to 10, wherein After step a and before step b, the method further comprises any one or more processing steps of turning, surface treatment and pickling. Preferably, the turning amount of a single side of the turning process is 1 mm to 5 mm; Preferably, the surface treatment equipment is selected from any one of a grinding machine, an angle grinder, and a flap wheel; Preferably, the pickling solution comprises HF, HCl and HNO3; Preferably, the volume ratio of HF:HCl:HNO3 in the pickling solution is 1:(1-5):(1-10).

12. The preparation method according to any one of claims 3 to 11, wherein After step b and before step c, the method further includes a turning step. Preferably, the single-side turning amount of the turning process is 1 mm to 5 mm.

13. The preparation method according to any one of claims 3 to 12, wherein The step c also includes a further surface treatment step, Preferably, the surface treatment equipment is selected from any one of a grinding machine, an angle grinder, and a flap wheel.

14. The preparation method according to any one of claims 3 to 13, wherein After step c, a secondary annealing heat treatment is further performed, wherein the maximum holding temperature of the secondary annealing heat treatment is 850° C. to 1200° C.; Preferably, the secondary annealing heat treatment includes a step-by-step temperature increase process under vacuum conditions. Further preferably, the secondary annealing heat treatment includes two stages of temperature increase and temperature holding processes, wherein Preferably, the temperature is raised to the first holding temperature T4 in 20-60 minutes and kept at this temperature for 10-120 minutes, with T4 being 300°C to 700°C; Preferably, the temperature is raised to the second insulation temperature T5 in 20-60 minutes and maintained for 10-150 minutes, and T4 is 800°C to 1200°C.

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