Preparation method of uniform fine-grain small-size Nb47Ti rod

Abstract

The application relates to the field of superconducting materials, and particularly relates to a preparation method of a uniform fine-grain small-size Nb47Ti rod. The preparation method comprises the following steps: heating by an electric furnace, radial forging to phi 50-80 mm, material repairing, cold rolling, heat treatment, machining, nondestructive testing and ultrasonic flaw detection; the average grain size is greater than or equal to 9, and the acid pickling, anodic oxidation, vacuum annealing and other multiple processes in the prior art are omitted, the production cycle is greatly shortened, the energy consumption and the cost are reduced, and the method is suitable for stable mass production.

Description

Technical Field

[0001] This invention relates to the field of superconducting materials, and more particularly to a method for preparing uniform, fine-grained, small-sized Nb47Ti rods. Background Technology

[0002] Among existing superconducting technologies, Nb47Ti superconducting alloy rods are the most widely used superconducting material. Nb-Ti alloy rods with a near 1:1 Nb to Ti mass ratio exhibit excellent superconducting properties. Niobium-titanium superconducting alloy rods, as a high-performance material, can be processed into superconducting wires and strips through traditional smelting, machining, and heat treatment processes. However, the production process faces numerous technical challenges, most notably its complex preparation process and high technological barriers. Niobium-titanium alloys belong to the β-alloy family. Because β-titanium alloys have a single body-centered cubic structure above the phase transition point, they exhibit good hot working plasticity and low deformation resistance. Their forging temperature is consistently above the phase transition point; however, higher temperatures can lead to abnormal grain growth, which negatively impacts the superconducting properties of the superconducting wires.

[0003] Current bar processing methods all employ high-temperature forging. High-temperature forging can reduce deformation resistance, improve material plasticity, facilitate the forming of large-sized billets, and eliminate some casting defects. However, the high temperature can cause abnormal grain growth, failing to achieve the goal of grain refinement. Cold working can introduce a large number of dislocations and substructures at room temperature, effectively refining the grains. It also offers high dimensional accuracy, good surface quality, and requires no heating energy. Cold working of bars uses a combination of roll forming and rotary forging. This process requires anodizing of the billet before roll forming and manual trimming afterward. The bar also requires vacuum annealing heat treatment. Therefore, the entire preparation process is time-consuming, energy-intensive, and costly, making it unsuitable for mass production. Furthermore, the deformation amount of the existing roll forming + rotary forging method is small and discontinuous, resulting in insufficient deformation energy stored within the material. This leads to a weak recrystallization driving force during subsequent heat treatment, causing grains to grow excessively and unevenly. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a method for preparing uniform, fine-grained, small-sized Nb47Ti rods, which solves one of the problems in the prior art, such as coarse and uneven grains, complicated processes, long preparation cycles, high energy consumption, and high costs, making it unsuitable for mass production.

[0005] On one hand, the present invention provides a method for preparing uniform, fine-grained, small-sized Nb47Ti rods, comprising the following steps: S1. Electric furnace heating: Heat to 500-700℃ and hold for 60-150 minutes; S2. Low-temperature radial forging to φ50-80mm, with deformation controlled at 10-30% per pass; S3. Repairing materials; S4. Cold rolling: The product is directly rolled to the finished product through 6-8 passes, with the total processing rate controlled at 50-80%; S5. Heat treatment: Annealing is carried out in a box-type resistance furnace and cooled by water. S6. Machining: The surface is treated using a centerless grinder and a centerless cylindrical polisher.

[0006] Furthermore, in S1, the electric furnace heating includes: using a box-type resistance furnace for heating, and the heating method is to load the furnace at the specified temperature and start timing when the temperature is reached.

[0007] Furthermore, in S2, the low-temperature radial forging includes: using an SXP-13 radial forging machine, with a total machining rate of 30-70%, and straightening after forging, with a straightness ≤5mm / m.

[0008] Furthermore, in S3, the finishing process includes: machining the bar after low-temperature radial forging using a centerless lathe with a single-sided machining allowance controlled at 1-2mm to remove surface wrinkles, resulting in a metallic luster on the bar surface, and then using an angle grinder to remove surface black skin and micro-cracks.

[0009] Furthermore, in S4, the cold rolling process includes: after surface repair, the cold rolling is performed using a 250-tandem rolling mill, with a deformation amount of 10-50% per pass.

[0010] Furthermore, in step S5, the annealing process is performed at an annealing temperature of 700-850℃ for 30-90 minutes.

[0011] Furthermore, in S5, the water cooling method involves a water temperature of <50℃, and the time from when the bar is taken out of the furnace to when it enters the water is controlled within 8-12 seconds.

[0012] Furthermore, the S6 machining process includes: using the centerless grinder, with the grinding amount controlled at 1-1.5mm, and then using the centerless cylindrical polishing machine, with the abrasive belt mesh size increasing from small to large as follows: 240 mesh → 400 mesh → 600 mesh → 800 mesh, and the polishing amount controlled at 0.05-0.015mm.

[0013] Furthermore, the step S6 is followed by the following steps: S7. Non-destructive testing using penetrant staining; S8. Ultrasonic testing: The titanium rod is subjected to ultrasonic testing using an automated phased array ultrasonic testing system: DPL-AZBC-JHO1.

[0014] Secondly, the present invention provides a uniform fine-grained small-sized Nb47Ti rod obtained by any of the preparation methods described in any one of the claims, wherein the average grain size of the rod is ≥9, the grain size is uniform, and the metallographic structure is a fully recrystallized equiaxed structure.

[0015] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1. In the preparation method of the present invention, electric furnace heating (500) is performed sequentially. 700℃), low-temperature radial forging to φ50 80mm (deformation 10 per pass) 30%), repair materials, 6 8-pass cold rolling (total processing rate 50%) The process involves 80% atmospheric pressure annealing and water cooling, centerless grinding, and centerless cylindrical polishing. Low-temperature radial forging after electric furnace heating achieves a combination of warm forging pre-refinement and 6-8 passes of cold rolling, resulting in large deformation and energy storage. This effectively suppresses grain growth and stores sufficient deformation energy. Combined with rapid water-cooling annealing, the average grain size of the bar material stabilizes at ≥9 levels, achieving a fully recrystallized equiaxed structure. Furthermore, it eliminates multiple processes such as pickling, anodizing, and vacuum annealing found in existing technologies, significantly shortening the production cycle, reducing energy consumption and costs, and making it suitable for mass production. Simultaneously, dual inspection using penetrant coloring and automated ultrasonic flaw detection ensures that both the surface and interior of the bar material are free of defects.

[0016] This invention refines the grain size through low-temperature radial forging followed by cold rolling. Heat treatment induces recovery and recrystallization of the deformed microstructure, and water cooling ensures complete recrystallization, preventing abnormal grain growth due to residual heat in the bar. This results in a fine, fully recrystallized equiaxed microstructure. This fully recrystallized equiaxed microstructure prevents abnormal grain growth during subsequent heat treatments, achieving a completely recrystallized equiaxed microstructure. The Nb47Ti bars produced by this invention have an average grain size ≥9 and uniform grain size, with a fully recrystallized equiaxed microstructure. This microstructure indicates the absence of residual deformed fibrous structures or mixed grains within the bar, completely eliminating work hardening.

[0017] 2. Existing high-temperature forging techniques operate above the phase transformation point, typically exceeding 850℃. These high temperatures cause abnormal grain growth, failing to achieve the desired grain refinement. This invention limits the electric furnace heating parameters to 500-700℃, holding for 60-150 minutes, and charging at the desired temperature. This low-temperature heating method keeps Nb47Ti in a warm forging state, ensuring sufficient plasticity to withstand the large deformation of subsequent radial forging while avoiding grain coarsening caused by high temperatures. Simultaneously, precise temperature uniformity control creates conditions for consistent deformation of the bar's core and surface, and retains the necessary deformation energy for subsequent cold rolling, thus laying the microstructural foundation for obtaining a final fine-grained structure of ≥9.

[0018] 3. This invention defines the specific process of low-temperature radial forging: deformation per pass is 10-30%, total machining rate is 30-70%, and straightness after forging is ≤5mm / m, with significantly higher deformation strength: achieving a more thorough effect of breaking the original grains. Difference in deformation strategy: The existing technology is "one high-temperature forging + one cold working forming", while this invention is a phased "warm forging pre-refining (steps S1+S2) + cold rolling large energy storage (step S4)", which achieves (1) suppressing grain growth caused by deformation heat; (2) avoiding forging cracks; (3) achieving uniform refinement in each pass; and (4) ensuring sufficient cumulative deformation, which can significantly break the cast or initial coarse grain structure and form a fibrous deformed structure. High straightness ensures the stability of cold rolling feed, reduces machining allowance, shortens the production cycle, and significantly improves production efficiency.

[0019] 4. This invention specifies the finishing method: centerless lathe machining is used, with the single-sided machining allowance controlled at 1-2mm to remove surface wrinkles, resulting in a visible metallic luster on the bar surface. Then, an angle grinder is used to remove surface black scale and micro-cracks. This mechanical finishing method completely replaces the pickling and anodizing processes in existing technologies, eliminating waste acid pollution and chemical treatment costs; it maximizes raw material savings; and the localized angle grinder finishing further ensures surface quality, providing defect-free billets for subsequent cold rolling.

[0020] 5. Existing rolling and rotary forging techniques result in small and discontinuous deformation, insufficient deformation energy stored within the material, and weak recrystallization driving force during subsequent heat treatment, leading to easy grain growth and uneven grain size. This invention employs 6-8 passes of direct rolling to the finished product, with each pass involving a deformation of 10-50%, and a total processing rate controlled at 50-80%. Large deformation cold rolling stores significant deformation energy within the grains, providing numerous tiny recrystallization nuclei for subsequent heat treatment, which is crucial for obtaining fine grains. This invention uses a 250 tandem rolling mill for cold rolling, accumulating dislocations and substructures. These defects provide numerous nuclei for recrystallization nucleation during subsequent heat treatment, ultimately forming fine and uniform equiaxed grains (≥9 grade). The 6-8 passes of direct rolling to the finished product ensure progressive deformation, with each pass penetrating deep into the core, avoiding the mixed grain phenomenon of "fine outer layer, coarse core."

[0021] Cold working employs a combination of roll forming and rotary forging. This method requires anodizing the billet before roll forming and manual trimming after roll forming. When the amount of cold deformation is large, intermediate vacuum annealing is required to restore plasticity. Compared to the roll forming and rotary forging method, this invention controls the deformation amount per pass to no more than 50%, ensuring that the work hardening of the material after 6-8 consecutive cold rolling passes remains within an acceptable range (without cracking). This eliminates the need for the annealing process, significantly shortening the production cycle and reducing costs.

[0022] 6. Existing technology requires vacuum annealing heat treatment for bar stock. This invention uses an atmospheric pressure box-type resistance furnace for rapid annealing, combined with water cooling. Compared with the existing vacuum annealing heat treatment, this invention simplifies the production process, reduces energy and maintenance costs, and is suitable for mass production.

[0023] The heat treatment of this invention employs a box-type resistance furnace for annealing, with water cooling: the annealing temperature is 700-850℃ to achieve complete recrystallization while avoiding grain coarsening; the holding time is 30-90 minutes to ensure sufficient recrystallization and avoid excessive time leading to grain growth; and grain coarsening is prevented. Water cooling is used with a water temperature <50℃, and the time from when the bar exits the furnace to when it enters the water is controlled at 8-12 seconds. Precise control of the bar's immersion time and water temperature ensures a certain cooling rate, preventing abnormal grain growth, rapidly freezing the recrystallized structure, and inhibiting grain growth.

[0024] 7. This invention limits the machining method, using a grinding machine + polishing method to process the surface, thereby obtaining precise diameter tolerance and excellent straightness; eliminating minor deformation caused by heat treatment; removing an extremely thin defect layer on the surface; and reducing surface roughness. Attached Figure Description

[0025] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0026] Figure 1 is a metallographic image of a φ19.0mm bar provided in Embodiment 1 of the present invention. Detailed Implementation

[0027] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0028] The preparation of existing Nb47Ti superconducting alloy rods typically employs high-temperature forging (temperatures above the phase transformation point, usually >850℃). While this can reduce deformation resistance, improve plasticity, and eliminate some casting defects, the high temperature easily leads to abnormal grain growth, making it impossible to obtain a fine-grained structure. Subsequently, the cold working of the rods often adopts a combination of roll forming and rotary forging. This process requires anodizing of the billet before roll forming, manual trimming after roll forming, and vacuum annealing heat treatment of the finished product. This process is not only cumbersome, time-consuming, energy-intensive, and costly, but also results in small and discontinuous deformation due to roll forming and rotary forging. The deformation energy stored inside the material is insufficient, and the driving force during subsequent recrystallization annealing is small, leading to easy grain growth and uneven distribution. This makes it difficult to meet the requirements of high-performance superconducting wires for fine and uniform structures, and is also unsuitable for large-scale stable production.

[0029] Therefore, on the one hand, the present invention provides a method for preparing uniform, fine-grained, small-sized Nb47Ti rods (φ10mm-φ30mm), comprising the following steps: S1. Electric furnace heating: Heat to 500-700℃ and hold for 60-150 minutes; S2. Low-temperature radial forging to φ50-80mm, with deformation controlled at 10-30% per pass; S3. Repairing materials; S4. Cold rolling: The product is directly rolled to the finished product through 6-8 passes, with the total processing rate controlled at 50-80%; S5. Heat treatment: Annealing is carried out in a box-type resistance furnace and cooled by water. S6. Machining: The surface is treated using a centerless grinder and a centerless cylindrical polisher.

[0030] Furthermore, in S1, the electric furnace heating includes: using a box-type resistance furnace for heating, and the heating method is to load the furnace at the specified temperature and time the temperature is reached.

[0031] The electric furnace heating plays a crucial role in subsequent grain control and finished product performance. Existing high-temperature forging techniques operate above the phase transformation point, typically exceeding 850°C. These high temperatures cause abnormal grain growth, failing to achieve the desired grain refinement. Therefore, this invention limits the electric furnace heating parameters to: 500-700°C, holding time 60-150 minutes, and loading into the furnace at the desired temperature. This low-temperature heating method keeps Nb47Ti in a warm forging state, ensuring sufficient plasticity to withstand the large deformation of subsequent radial forging while avoiding grain coarsening caused by high temperatures. Simultaneously, precise temperature uniformity control creates conditions for consistent deformation of the bar core and surface, and retains the necessary deformation energy for subsequent cold rolling, thus laying the microstructural foundation for obtaining a final fine-grained structure of ≥9. Specifically, it can achieve (1) suppress abnormal grain growth; the heating temperature in this step is controlled at 500-700℃, which is much lower than the conventional forging heating temperature (usually >850℃). In the warm forging range of 500-700℃, the structure will be mainly composed of deformed grains and subgrains that have not yet recrystallized. This structure provides high-density dislocations and subgrains for subsequent radial forging to achieve fine grains. The temperature is much lower than the recrystallization temperature, which suppresses grain growth from the source and ensures that the grains of the bar will not grow due to overheating before subsequent radial forging, laying the foundation for finally obtaining a fine grain structure of ≥9. (2) achieve “warm forging” instead of “hot forging”; the heating temperature is 500-700℃. This temperature range puts the material in a warm working state (between cold working and hot working), which not only retains enough plasticity to withstand 10-30% of the deformation amount in each pass, but also avoids the rapid growth of grains at high temperatures. (3) Temperature uniformity control: The box-type resistance furnace + temperature-controlled loading + temperature-controlled timing method can ensure that the core and surface temperature of the entire bar is consistent. If the temperature rises with the furnace, the temperature difference between the inside and outside of the bar is large, and the outer layer may be overheated while the core is not, resulting in uneven deformation and local grain growth during forging. (4) Reserve sufficient cold deformation energy for subsequent cold rolling.

[0032] If the heating temperature is too high and the holding time is too long, the grains will begin to grow and the grain size will decrease; if the heating temperature is too low and the holding time is too short, forging cracks will occur and the scrap rate will increase. Therefore, the heating temperature is 500-700℃, and the holding time is 60-150 minutes; for example, the heating temperatures are 500℃, 520℃, 540℃, 560℃, 580℃, 600℃, 620℃, 640℃, 660℃, 680℃, and 700℃; and the holding times are 60 minutes, 70 minutes, 80 minutes, 90 minutes, 100 minutes, 110 minutes, 120 minutes, 130 minutes, 140 minutes, and 150 minutes.

[0033] Furthermore, in S2, low-temperature radial forging includes: using an SXP-13 radial forging machine, with a total machining rate of 30-70%, and straightening after forging, with a straightness ≤5mm / m.

[0034] Specifically, radial forging controls the deformation amount per pass (10-30%), with a total processing rate of 30-70%, resulting in significantly higher deformation strength and achieving a more thorough effect of breaking down the original grains. Difference in deformation strategy: Existing technology is "one high-temperature forging + one cold working forming", while this invention is a phased "warm forging pre-refining (steps S1+S2) + cold rolling large energy storage (step S4)", which achieves (1) suppressing grain growth caused by deformation heat; (2) avoiding forging cracks; (3) achieving uniform refinement in each pass; and (4) ensuring sufficient cumulative deformation, which can significantly break down the cast or initial coarse-grained structure and form a fibrous deformed structure.

[0035] If the deformation amount per pass is too small, it will not be able to effectively break the original grains; if the deformation amount per pass is too large, surface or internal cracks may occur due to stress concentration. Therefore, the deformation amount per pass should be controlled between 10% and 30%, with examples being 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, and 30%.

[0036] If the total processing rate is less than 30%, the grain refinement effect is insufficient, and the grain size decreases; if the total processing rate is greater than 70%, the cumulative temperature rise leads to grain coarsening, and the grain size decreases. Therefore, the total processing rate should be 30-70%, with examples being 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, and 70%.

[0037] Post-forging straightening, with a straightness ≤5mm / m, ensures the stability of cold rolling feed; reduces subsequent machining allowance, shortens the production cycle, and significantly improves production efficiency; and avoids uneven deformation during cold rolling. If the straightness >5mm / m, the bent bar will be under tension on the outer side and compression on the inner side during cold rolling, resulting in uneven deformation distribution and potentially causing inconsistent grain refinement in certain areas.

[0038] Furthermore, in S3, the finishing process includes: the radially forged bar stock is machined using a centerless lathe with the single-sided machining amount controlled at 1-2mm to remove surface wrinkles, so that the surface of the bar stock can be seen to have a metallic luster, and then the surface black skin and micro-cracks are removed using an angle grinder.

[0039] Specifically, S3 trimming includes: After radial forging, the bar stock is machined on a centerless lathe to remove forging surface defects (wrinkles, microcracks, oxide scale); ensuring consistent bar diameter; improving efficiency; and making it suitable for mass production. The radially forged bar stock is machined on a centerless lathe with a single-sided machining allowance controlled at 1-2mm to remove surface wrinkles. Then, an angle grinder is used to remove surface defects, ensuring surface quality while minimizing the use of expensive Nb47Ti material. This also provides dimensionally accurate and surface-perfect billets for subsequent cold rolling. This mechanical trimming method completely replaces pickling and anodizing in existing technologies, eliminating waste acid pollution and chemical treatment costs; maximizing raw material savings; and localized angle grinder trimming further ensures surface quality, providing defect-free billets for subsequent cold rolling.

[0040] The single-sided machining allowance is controlled at 1-2mm to maximize material utilization and avoid waste while ensuring quality; it also avoids introducing new defects due to excessive cutting depth. If the single-sided cutting amount is too large (>2mm), a large amount of base metal will be removed, reducing the yield; if it is too small, surface defects will not be completely removed. Therefore, the single-sided machining allowance is controlled at 1-2mm, with examples being 1 mm, 1.2 mm, 1.4 mm, 1.6 mm, 1.8 mm, and 2mm.

[0041] Furthermore, in S4, cold rolling includes: after surface repair, cold rolling is performed using a 250-tandem rolling mill, with a deformation amount of 10-50% per pass.

[0042] Existing rolling and rotary forging techniques result in small, discontinuous deformation, insufficient strain energy stored within the material, and weak recrystallization driving force during subsequent heat treatment, leading to easy grain growth and uneven grain size. This invention utilizes large-deformation cold rolling to store significant strain energy within the grains, providing numerous tiny recrystallization nuclei for subsequent heat treatment—a key to achieving fine grains. This invention employs a 250-row transverse rolling mill for cold rolling, accumulating dislocations and substructures. These defects provide numerous nuclei for recrystallization during subsequent heat treatment, ultimately forming fine, uniform equiaxed grains (≥9 grade). After 6-8 passes of direct rolling to the finished product, each pass involves progressive deformation, ensuring the deformation penetrates deep into the core and avoiding the mixed grain phenomenon of "fine outer layer, coarse core." Cold working adopts a combination of roll forming and rotary forging. This method requires anodizing of the billet before roll forming and manual trimming after roll forming. When the amount of cold deformation is large, intermediate vacuum annealing is required to restore plasticity. However, this invention controls the amount of deformation per pass to no more than 50%, so that the work hardening of the material after 6-8 consecutive cold rolling passes is still within an acceptable range (no cracking). This eliminates the need for the annealing process, significantly shortens the production cycle, and reduces costs.

[0043] The process involves 6-8 passes of direct rolling to the finished product, with each pass yielding a deformation of 10-50%, and a total processing rate controlled at 50-80%. Large deformation cold rolling stores significant deformation energy within the grains, providing numerous tiny recrystallization nuclei for subsequent heat treatment—a crucial step in achieving fine grains. A deformation of 10-50% per pass allows for rapid diameter reduction, significantly shortening the production cycle and improving efficiency, while also ensuring grain refinement. If the deformation per pass is too small, the refinement effect is insufficient, and the production cycle is long; if the deformation per pass is too large, work hardening occurs, leading to localized grain coarsening. Therefore, a deformation of 10-50% per pass is recommended, with examples including 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, and 50%. Maintaining a total processing rate of 50-80% ensures sufficient cold deformation energy storage, guaranteeing a subsequent recrystallization grain size of ≥9, and preventing cracking due to over-processing. If the total machining rate is too low, the grain refinement will be insufficient; if the total machining rate is too high, work hardening will occur, cracks will appear in large numbers, and the scrap rate will increase. Therefore, the total machining rate should be controlled between 50-80%, with examples being 50%, 55%, 60%, 65%, 70%, 75%, and 80%.

[0044] Furthermore, in S5, the annealing process is as follows: annealing temperature is 700-850℃, and holding time is 30-90 minutes.

[0045] Furthermore, in S5, the water cooling method is used to cool the bar at a water temperature of <50℃, and the time from the bar exiting the furnace to entering the water is controlled within 8-12 seconds.

[0046] Existing technologies require vacuum annealing heat treatment for bar stock. This invention employs a heat treatment method including: atmospheric pressure annealing in a box-type resistance furnace at 700-850℃ to achieve complete recrystallization while preventing grain coarsening. After the Nb47Ti alloy undergoes a previous cold rolling process (total processing rate 50-80%), it stores a large amount of deformation energy. During annealing at 700-850℃, complete recrystallization occurs: new equiaxed grains nucleate and grow, replacing the deformed structure. Within the 700-850℃ range, the recrystallization nucleation rate is much higher than the grain growth rate, thus obtaining fine and uniform equiaxed grains. If the temperature exceeds 850℃, the recrystallization rate is too fast, and the grains will rapidly coarsen, failing to achieve a fine grain size of grade 9 or higher, and thus failing to obtain fine and uniform equiaxed grains. Holding at this temperature for 30-90 minutes ensures sufficient recrystallization and avoids excessive time leading to grain growth and coarsening. If the temperature is too high and the holding time is too long, the grains will slowly grow through grain boundary migration, leading to a decrease in grain size. If the temperature is too low and the holding time is too short, recrystallization will be incomplete, leaving deformed structures (fibrous grains) and resulting in mixed crystals. Therefore, the annealing temperature is 700-850℃, and the holding time is 30-90 minutes. Exemplary annealing temperatures are 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, 810℃, 820℃, 830℃, 840℃, and 850℃, with holding times of 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, 65 minutes, 70 minutes, 75 minutes, 80 minutes, 85 minutes, and 90 minutes, respectively. This invention uses an atmospheric pressure box-type resistance furnace for rapid annealing, combined with water cooling. Compared with the vacuum annealing heat treatment of the prior art, this invention simplifies the production process, reduces energy and maintenance costs, and is suitable for mass production.

[0047] Water cooling is employed, with the water temperature below 50℃. The time from when the bar exits the furnace to when it enters the water is controlled within 8-12 seconds. Precise control of the bar's entry time and water temperature ensures a certain cooling rate, preventing abnormal grain growth and rapidly freezing recrystallization to inhibit grain growth. If the time from when the bar exits the furnace to when it enters the water is too long, severe grain coarsening and a decrease in grain size will occur; if the time is too short, surface grains will grow, and the grain size will decrease. Therefore, the time from when the bar exits the furnace to when it enters the water is controlled within 8-12 seconds, with examples of 8 seconds, 9 seconds, 10 seconds, 11 seconds, and 12 seconds. This invention uses an atmospheric pressure box-type resistance furnace for rapid annealing, combined with water cooling, significantly reducing energy consumption and production cycle time.

[0048] Furthermore, the S6 machining process includes: using the centerless grinder, with the grinding amount controlled at 1-1.5mm, and then using the centerless cylindrical polishing machine, with the abrasive belt mesh size increasing from small to large as follows: 240 mesh → 400 mesh → 600 mesh → 800 mesh, and the polishing amount controlled at 0.05-0.015mm.

[0049] Machining includes: grinding and polishing the surface of the bar stock to obtain precise diameter tolerances and excellent straightness; eliminating minor deformations caused by heat treatment; removing extremely thin defect layers from the surface; and reducing surface roughness.

[0050] This machining step uses a centerless grinder to precisely control the single-sided grinding amount of 1-1.5mm, which can efficiently remove the extremely thin oxide layer, minor deformation, and microscopic defects on the surface of the heat-treated bar, while ensuring that the diameter tolerance and straightness of the finished product meet the requirements for drawing superconducting wires. Subsequently, centerless outer cylindrical polishing is carried out by progressively refining abrasive belts of 240 mesh → 400 mesh → 600 mesh → 800 mesh, with the final polishing amount controlled at 0.05-0.015mm. This can gradually eliminate grinding marks and obtain a mirror-level surface roughness (Ra≤0.2μm). This avoids dimensional deviations or grain damage caused by over-polishing and provides a clean, scratch-free inspection interface for subsequent penetrant staining and ultrasonic testing, thereby ensuring that there are no open defects on the surface of the bar and meeting the requirements for mass production.

[0051] Furthermore, the above-mentioned method for preparing uniform, fine-grained, small-sized Nb47Ti rods further includes the following steps after step S6: S7. Non-destructive testing using penetrant staining; S8. Ultrasonic testing: The titanium rod is subjected to ultrasonic testing using an automated phased array ultrasonic testing system: DPL-AZBC-JHO1.

[0052] The non-destructive testing of S7 penetrant staining includes: cleaning the surface of the polished bar with water, then spraying red penetrant for 10-20 minutes; rinsing the surface penetrant with water, uniformly spraying developer, and letting it stand for 5-10 minutes.

[0053] Step S7 involves cleaning the surface of the bar with water to remove oil and impurities, then spraying a red penetrant and allowing it to penetrate for 10-20 minutes to ensure effective penetration into microscopic surface defects (such as microcracks, folds, and wrinkles). Excess penetrant is then rinsed off with water, followed by uniform spraying of a developer and allowing it to stand for 5-10 minutes. Capillary action draws any remaining penetrant from the defects to the surface, forming a red color mark that can be clearly identified under white light. This method is simple to operate, low in cost, and can detect surface defects invisible to the naked eye. Combined with ultrasonic testing, it provides a complementary, multi-dimensional inspection, ensuring the bar surface is free of defects that could affect subsequent wire drawing or superconducting properties. This provides an efficient and reliable quality control method for mass production.

[0054] The S8 step flaw detection level is AMS 2631 AA. Existing technologies all employ manual contact methods for flaw detection, which are prone to false positives and missed negatives. The DPL-AZBC-JHO1 automated phased array ultrasonic flaw detection system for titanium rods addresses these shortcomings by using automated equipment to perform ultrasonic flaw detection on the rods, overcoming the low efficiency, poor accuracy, and insufficient reliability of traditional methods. Ultrasonic flaw detection utilizes the propagation characteristics of high-frequency sound waves in materials to efficiently detect volumetric defects such as cracks, pores, and inclusions within the rods, achieving an AMS 2631 AA level. Penetrant dye penetrant testing, on the other hand, involves penetrating a penetrant into surface open defects and developing the color with a developer, clearly revealing microcracks, folds, and skinning defects on the rod surface. The combination of these two methods enables comprehensive quality inspection of the rods, both internally and externally. The dual detection of penetrant dye penetrant and automated ultrasonic flaw detection ensures that both the surface and internal parts of the rods are free of defects.

[0055] On the other hand, the present invention provides a uniform fine-grained small-sized Nb47Ti rod obtained by the above preparation method with an average grain size ≥ 9, uniform grain size, and a metallographic structure of fully recrystallized equiaxed structure.

[0056] The Nb47Ti rods obtained by this invention have an average grain size ≥ 9 and uniform grain size, and the metallographic structure is a fully recrystallized equiaxed structure. This structure indicates that there are no residual deformed fibrous structures or mixed crystals inside the rods, completely eliminating work hardening, and is a fully recrystallized equiaxed structure.

[0057] In summary, the preparation method of this invention, through a complete preparation process of "electric furnace heating → radial forging to φ50-80mm → trimming → cold rolling → heat treatment → machining → non-destructive testing and ultrasonic flaw detection", organically combines warm forging, cold rolling and rapid heat treatment, and introduces dual non-destructive testing of ultrasonic flaw detection and penetrant dyeing. This not only effectively inhibits abnormal grain growth during processing, but also ensures that the finished bar is free of defects inside and out, achieving a balance between fine grains (≥9 grade) and high reliability. At the same time, it eliminates multiple processes such as pickling, anodizing and vacuum annealing in the prior art, greatly shortening the production cycle, reducing energy consumption and costs, and is suitable for large-scale stable production.

[0058] The advantages of the processing method of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0059] Example 1 The method for preparing uniform, fine-grained, small-sized (φ19mm) Nb47Ti rods in this embodiment includes: 1) Electric furnace heating: Box-type resistance furnace is used for heating. The heating temperature is 500℃, and the holding time is 60 minutes. The heating method is to load the furnace when the temperature is reached and start timing when the temperature is reached. 2) Radial forging: SXP-13 radial forging machine is used, and the deformation is carried out in 2 passes. The deformation amount of each pass is controlled at 30%, and the total processing rate is 60%. After forging, the straightening is performed and the straightness is ≤5mm / m. 3) Finishing: After radial forging, the bar is machined on a centerless lathe with the single-sided machining amount controlled at 1mm to remove the surface wrinkles. The surface of the bar can be seen with a metallic luster. Then, the surface black skin and micro-cracks are removed with an angle grinder. 4) Rolling: After surface repair, cold rolling is carried out using a 250 tandem rolling mill. The product is directly rolled in 8 passes, with a deformation of 10% per pass and a total processing rate controlled at 50%. 5) Heat treatment: A box-type resistance furnace is used, with an annealing temperature of 700℃ and a holding time of 30 minutes. Water cooling is employed. The water temperature is <50℃, and the time from when the bar is taken out of the furnace to when it enters the water is controlled within 10 seconds.

[0060] 6) Machining: The bar stock is ground using a centerless grinder with a grinding amount controlled at 1-1.5mm. Then, a centerless cylindrical polishing machine is used with abrasive belt grits increasing from 240 to 400 to 600 to 800, and the polishing amount is controlled at 0.05-0.015mm. 7) Penetration coloring: Clean the surface of the polished bar with water, then spray red penetrant for 10-20 minutes; wash the surface penetrant with water, spray developer evenly, and let stand for 5-10 minutes. The penetrant will be precipitated and diffused at the defect. Observe under white light. The red area is the defect area and the white area is the normal area.

[0061] 8) Ultrasonic testing: The titanium rod is subjected to ultrasonic testing using the DPL-AZBC-JHO1 automated phased array ultrasonic testing system. The testing level is AMS 2631 AA.

[0062] 9) Finished bars: The average grain size of the finished bars is ≥9, the grain size is uniform, and the metallographic structure is a fully recrystallized equiaxed structure.

[0063] Example 2 The method for preparing uniform, fine-grained, small-diameter Nb47Ti rods (φ10mm) in this embodiment includes: 1) Electric furnace heating: Box-type resistance furnace is used for heating. The heating temperature is 550℃, and the holding time is 150 minutes. The heating method is to load the furnace when the temperature is reached and start timing when the temperature is reached. 2) Radial forging: SXP-13 radial forging machine is used, and the deformation is carried out in 5 passes. The deformation amount of each pass is controlled at 14%, the total processing rate is 70%, and the straightening is performed after forging. The straightness is ≤5mm / m. 3) Finishing: After radial forging, the bar is machined on a centerless lathe with the single-sided machining amount controlled at 2mm to remove surface wrinkles and make the bar surface have a metallic luster. Then, the surface defects are removed with an angle grinder. 4) Rolling: After surface repair, cold rolling is carried out using a 250 tandem rolling mill. The product is directly rolled through 7 passes, with a deformation of 11% per pass and a total processing rate controlled at 80%. 5) Heat treatment: A box-type resistance furnace is used, with an annealing temperature of 850℃ and a holding time of 90 minutes, followed by water cooling. The water temperature is controlled below 50℃, and the time from when the bar is taken out of the furnace to when it enters the water is controlled within 12 seconds.

[0064] 6) Machining: The bar stock is ground using a centerless grinder with a grinding amount controlled at 1-1.5mm. Then, a centerless cylindrical polishing machine is used with abrasive belt grits increasing from 240 to 400 to 600 to 800, and the polishing amount is controlled at 0.05-0.015mm. 7) Penetration coloring: Clean the surface of the polished bar with water, then spray red penetrant for 10-20 minutes; wash the surface penetrant with water, spray developer evenly, and let stand for 5-10 minutes. The penetrant will be precipitated and diffused at the defect. Observe under white light. The red area is the defect area and the white area is the normal area.

[0065] 8) Ultrasonic testing: The titanium rod is subjected to ultrasonic testing using the DPL-AZBC-JHO1 automated phased array ultrasonic testing system. The testing level is AMS 2631 AA.

[0066] 9) Finished bars: The average grain size of the finished bars is ≥9, the grain size is uniform, and the metallographic structure is a fully recrystallized equiaxed structure.

[0067] Example 3 The method for preparing uniform, fine-grained, small-diameter Nb47Ti rods (φ30mm) in this embodiment includes: 1) Electric furnace heating: Box-type resistance furnace is used for heating. The heating temperature is 600℃, and the holding time is 100 minutes. The heating method is to load the furnace when the temperature is reached and start timing when the temperature is reached. 2) Radial forging: SXP-13 radial forging machine is used, and the deformation is carried out in 5 passes. The deformation amount of each pass is controlled at 10%, the total processing rate is 50%, and the straightening is performed after forging. The straightness is ≤5mm / m. 3) Finishing: The radially forged bar is machined on a centerless lathe with the single-sided machining allowance controlled at 1.5mm to remove surface wrinkles, and then surface defects are removed with an angle grinder. 4) Rolling: After surface repair, cold rolling is carried out using a 250 tandem rolling mill. The product is directly rolled to the finished product in 6 passes, with a deformation of 10% per pass and a total processing rate controlled at 60%. 5) Heat treatment: A box-type resistance furnace is used, with an annealing temperature of 800℃ and a holding time of 60 minutes. Water cooling is employed. The water temperature is below 50℃, and the time from when the bar is removed from the furnace to when it enters the water is controlled within 10 seconds.

[0068] 6) Machining: The bar stock is ground using a centerless grinder with a grinding amount controlled at 1-1.5mm. Then, a centerless cylindrical polishing machine is used with abrasive belt grits increasing from 240 to 400 to 600 to 800, and the polishing amount is controlled at 0.05-0.015mm. 7) Penetration coloring: Clean the surface of the polished bar with water, then spray red penetrant for 10-20 minutes; wash the surface penetrant with water, spray developer evenly, and let stand for 5-10 minutes. The penetrant will be precipitated and diffused at the defect. Observe under white light. The red area is the defect area and the white area is the normal area.

[0069] 8) Ultrasonic testing: The titanium rod is subjected to ultrasonic testing using the DPL-AZBC-JHO1 automated phased array ultrasonic testing system. The testing level is AMS 2631 AA.

[0070] 9) Finished bars: The average grain size of the finished bars is ≥9, the grain size is uniform, and the metallographic structure is a fully recrystallized equiaxed structure. The results are shown in Table 1.

[0071] Comparative Example 1 The processing method for small-diameter Nb47Ti bars (φ19mm) in this comparative example includes: 1) Electric furnace heating: A box-type resistance furnace was used for heating at 850℃ for 50 minutes; the rest was the same as in Example 1. The results are shown in Table 1.

[0072] Comparative Example 2 The processing method for small-diameter Nb47Ti bars (φ19mm) in this comparative example includes: 2) Radial forging: An SXP-13 radial forging machine was used, with the deformation per pass controlled at 5% and the total machining rate at 20%. After forging, the material was straightened, and the straightness was greater than 5 mm / m. The rest was the same as in Example 1. The results are shown in Table 1.

[0073] Comparative Example 3 The processing method for small-diameter Nb47Ti bars (φ19mm) in this comparative example includes: 4) Rolling: After surface repair, cold rolling was performed using a 250 tandem rolling mill. The product was directly rolled to the finished product in 10 passes, with a deformation of 8% per pass and a total processing rate controlled at 40%. The rest was the same as in Example 1. The results are shown in Table 1.

[0074] Comparative Example 4 The processing method for small-diameter Nb47Ti bars (φ19mm) in this comparative example includes: 5) Heat treatment: A box-type resistance furnace was used, with an annealing temperature of 600℃ and a holding time of 28 minutes. The rest was the same as in Example 1. The results are shown in Table 1.

[0075] Comparative Example 5 The processing method for small-diameter Nb47Ti bars (φ19mm) in this comparative example includes: 5) Heat treatment: Air cooling was used. The rest was the same as in Example 1. The results are shown in Table 1.

[0076] Table 1 Performance results of Examples 1-3 and Comparative Examples 1-5

[0077] As can be seen from Table 1, the Nb47Ti bars prepared in Examples 1-3 of this invention all have a grain size of 9.5 or higher (up to 10), and the metallographic structure is a fully recrystallized equiaxed structure with a Vickers hardness of 138-148 HV10 (Vickers hardness is considered a method for evaluating grain uniformity). However, Comparative Examples 1, 2, 3, and 5 have a grain size of only 4-6 and a Vickers hardness of less than 138 HV10 due to excessively high electric furnace heating temperature, insufficient radial forging deformation, low total cold rolling processing rate, or heat treatment parameters deviating from the range. Comparative Example 4 has a non-recrystallized structure with a Vickers hardness of 150-160 HV10 because the heat treatment parameters deviated from the range. This indicates that the core technologies of this invention, namely radial forging (500-700℃, 10-30% deformation per pass, 30-70% total processing rate) + large deformation cold rolling (50-80% total processing rate, 6-8 passes) + rapid water-cooled annealing (700-850℃, 30-90min, 8-12 seconds from furnace exit to water immersion), can effectively suppress grain growth, provide sufficient deformation energy storage, and rapidly freeze recrystallized structures, thereby stably obtaining a uniform and fine fully recrystallized equiaxed structure with a grain size ≥9, which is significantly superior to existing processes.

[0078] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing uniform, fine-grained, small-sized Nb47Ti rods, characterized in that, Includes the following steps: S1. Electric furnace heating: Heat to 500-700℃ and hold for 60-150 minutes; S2. Low-temperature radial forging to φ50-80mm, with deformation controlled at 10-30% per pass, total machining rate of 30-70%, straightened after forging, with straightness ≤5mm / m; S3. Finishing: The bar stock after low-temperature radial forging is machined on a centerless lathe with a single-sided machining amount controlled at 1-2mm to remove surface wrinkles. The bar stock surface has a metallic luster. Then, an angle grinder is used to remove the surface black skin and micro-cracks. S4. Cold rolling: The product is directly rolled to the finished product through 6-8 passes, with a deformation of 10-50% per pass and a total processing rate controlled at 50-80%. S5. Heat treatment: Annealing is carried out under normal pressure in a box-type resistance furnace and cooled by water; the annealing temperature is 700-850℃, and the holding time is 30-90 minutes; the water cooling method is used when the water temperature is <50℃, and the time from the bar exiting the furnace to entering the water is controlled within 8-12 seconds. S6. Machining: The surface is treated using a centerless grinder and a centerless cylindrical polisher.

2. The preparation method according to claim 1, characterized in that, In S1, the electric furnace heating includes: using a box-type resistance furnace for heating, and the heating method is to load the furnace when it reaches the set temperature and to start timing when the temperature is reached.

3. The preparation method according to claim 1, characterized in that, In S2, the low-temperature radial forging includes: using an SXP-13 radial forging machine, with a total machining rate of 30-50%, and straightening after forging, with a straightness ≤5mm / m.

4. The preparation method according to claim 1, characterized in that, In S4, cold rolling includes: after surface repair, cold rolling is performed using a 250-tandem rolling mill, with a deformation amount of 10-11% per pass.

5. The preparation method according to claim 1, characterized in that, In step S5, the annealing process is performed at an annealing temperature of 800-850℃ for 30-60 minutes.

6. The preparation method according to claim 1, characterized in that, In S5, the water cooling method is used when the water temperature is <50℃, and the time from when the bar is taken out of the furnace to when it enters the water is controlled within 10-12 seconds.

7. The preparation method according to claim 1, characterized in that, S6 machining includes: using the centerless grinder, with the grinding amount controlled at 1-1.5mm, and then using the centerless cylindrical polishing machine, with the abrasive belt mesh size increasing from small to large as follows: 240 mesh → 400 mesh → 600 mesh → 800 mesh, and the polishing amount controlled at 0.05-0.015mm.

8. The preparation method according to claim 1, characterized in that, Following step S6, the following steps are also included: S7. Non-destructive testing using penetrant staining; S8. Ultrasonic testing: The titanium rod is subjected to ultrasonic testing using an automated phased array ultrasonic testing system: DPL-AZBC-JHO1.

9. A uniform, fine-grained, small-sized Nb47Ti rod obtained by the preparation method according to any one of claims 1-8, characterized in that, The average grain size of the bar is ≥9, the grain size is uniform, and the metallographic structure is a fully recrystallized equiaxed structure.

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