Processing methods for high-purity niobium materials with high RRR values ​​and niobium materials for radio frequency superconducting cavities

Abstract

A processing method for high-purity niobium material with high RRR value and niobium material for radio frequency superconducting cavities includes ingot forging, billet sawing, billet surface machining, billet pickling, billet heat treatment, billet blanking, slab trimming and pickling, slab rolling, finished product blanking, polishing and pickling, finished product heat treatment, finished product shearing and one-time fine polishing, and finished product pickling steps. Through this processing method, the grain size of the finished plate is maintained between 4.5 and 5.5, with good uniformity; the mechanical properties all meet the requirements, with transverse and longitudinal differences controlled within 15%, of which tensile strength and yield strength are controlled within 10%, and elongation is controlled within 10%; the RRR value is controlled above 300; the gas element content all meets the requirements; and it improves the direct yield of the product, significantly reducing the manufacturer's manufacturing costs and enabling mass production.

Description

Technical Field

[0001] This invention relates to the field of niobium material processing technology, and in particular to a processing method for high-purity niobium material with high RRR value and niobium material for radio frequency superconducting cavities. Background Technology

[0002] Niobium is the best metal for constructing superconducting accelerator resonators. Past, present, under development or installation, and future projects include: KEK's TRISTAN ring memory; CERN's LEP ring memory; DESY's HERA electron-proton ring memory; Jefferson Laboratory's CEBAF; TESLA Test Facility (TTF); the Split Neutron Source (SNS); a superconducting accelerator resonator for generating strong particle beams (Cornell University); future TESLA projects; synchrotron light sources with superconducting resonators; free-electron lasers with superconducting linear accelerators; proton accelerators for nuclear waste conversion and neutron production; and rare isotope accelerators.

[0003] Superconducting niobium plates (RRR≥300) are mainly used to fabricate resonant cavities for radio frequency superconducting linear accelerators, and are a core component of accelerator engineering. Resonant cavities are mostly pressure vessels, and their molding process often involves pressure forming, which places high demands on the performance of the superconducting niobium plates.

[0004] The main drawbacks of the existing technology are: the RRR of the finished sheet cannot reach more than 300; the grain size is not uniform, with large grains; the gas element cannot meet the corresponding requirements; the difference between transverse and longitudinal properties in mechanical properties exceeds 10%; the strain hardening coefficient n < 0.3; and the product yield is low, basically around 50%. Summary of the Invention

[0005] In view of this, it is necessary to provide a processing method for high-purity niobium materials with high RRR values.

[0006] It is also necessary to provide a niobium material for radio frequency superconducting cavities.

[0007] A method for processing high-purity niobium material with a high RRR value, the method comprising the following steps:

[0008] (1) Ingot forging: Select the superconducting niobium ingot obtained by electron beam melting, and forge the selected superconducting niobium ingot at a temperature of ≤100℃ to obtain the billet;

[0009] (2) Blank sawing: The blank obtained in step (1) is sawn at equal intervals along its length direction according to a specified length L1, wherein the length L1 is calculated as follows: S represents the weight of the finished product, δ1 represents the thickness of the forged billet, B1 represents the width of the forged billet, ρ represents the density of niobium material, L1, δ1, and B1 are in mm, S is in Kg, and Z is the product yield.

[0010] (3) Machining of the billet surface: The billet obtained in step (2) is first machined on a lathe, and then the side of the forged billet in the width direction is milled on a milling machine.

[0011] (4) Pickling of billet: The billet that has been surface machined in step (4) is placed in acid solution for pickling for 5 to 10 minutes.

[0012] (5) Heat treatment of billet: Take out the billet after pickling in step (4), wash it with water, wipe off the liquid on the billet, and then perform stress-relief annealing.

[0013] (6) Billet rolling: The heat-treated billet in step (5) is rolled 3 to 5 times along the long side of the billet, then rotated 90° to roll the width of the billet to (B3+20) mm, and then rotated 90° to roll the thickness of the billet to (N×δ3) mm, where B3 is the width of the finished plate in mm, and N is the introduction coefficient, with a value range of 2.0 to 3.0;

[0014] (7) Slab trimming and pickling: The slab obtained in step (6) is trimmed by an angle grinder and a belt sander to remove surface defects. The trimmed slab is then put back into the acid solution for pickling for 5 to 10 minutes.

[0015] (8) Slab rolling: The slab after pickling in step (7) is rolled along the short side of the slab to roll the thickness of the slab to (δ3+0.5)mm, where δ3 is the thickness of the product slab in mm;

[0016] (9) Finished product cutting, polishing and pickling: After the slabs rolled in step (8) are pre-cut to a certain size by a shearing machine, the slabs are polished by a belt sander, and then the polished slabs are put into the acid solution again for pickling treatment. The pickling time is 5 to 10 minutes.

[0017] (10) Finished product heat treatment: Take out the pickled plate in step (9), wash it with water, wipe the liquid on the blank, and then anneal the plate.

[0018] (11) Finished product shearing and one-time fine polishing: The annealed plate in step (10) is cut off by a shearing machine to the finished plate size, and then the plate is polished by a belt sander. The thickness removal on one side is at least 0.05mm.

[0019] (12) Pickling of finished product: The plate polished in step (11) is placed into the acid solution for pickling treatment for 5 to 10 seconds. Then the plate after pickling treatment is taken out and washed with water. Finally, the liquid on the plate is wiped off to obtain the finished niobium material.

[0020] A niobium material for radio frequency superconducting cavities prepared using the above-mentioned processing method for high-purity niobium material with high RRR value.

[0021] The present invention adopts the above-mentioned technical solution, and its beneficial effects are as follows: (1) Through the reasonable forging process of the present invention, the grains with a single grain size of more than 200mm in the large diameter ingot are broken, and the radial performance of the ingot is improved by square-making and rounding, thereby improving isotropy; the grains are further broken by a large rolling processing rate of more than 90% between two annealing processes, increasing the yield strength difference, and the transverse processing rate and three-way reversal method are used to make the transverse and longitudinal properties more consistent and optimized; the grain growth rate and size are controlled by the heat treatment process, and the grains are made uniform, improving isotropy and reducing the transverse and longitudinal differences in mechanical properties; the gas elements in the plate are reduced by vacuum annealing, thereby effectively improving the RRR value; the surface treatment process is controlled to remove the hardened layer and contaminants on the plate surface, greatly reducing the introduction of gas elements, improving the RRR value, and reducing the surface hardness of the material to a certain extent; the introduction of gas elements is reduced by the control of auxiliary parameters, thereby improving the RRR value. It can be widely used to make resonant cavities for radio frequency superconducting linear accelerators.

[0022] (2) The grain size of the plate is kept between 4.5 and 5.5 for 90% of the plate, with good uniformity; the mechanical properties meet the requirements, and the difference between the transverse and longitudinal directions is controlled within 10%, of which the tensile strength and yield strength are controlled within 10%, and the elongation is controlled within 15%; the RRR value is controlled above 300; and the content of gas elements meets the requirements.

[0023] (3) Low cost: Through reasonable forging process, side defects are milled and three-way reversal, while the performance is stable, the surface and side defects are reduced, thereby improving the product yield. This can significantly reduce the manufacturer's manufacturing cost and can be mass-produced. Therefore, this process has great market promotion value and protection value. Detailed Implementation

[0024] The specific parameters of a qualified niobium plate are as follows:

[0025] 1. Organizational properties

[0026] • ≥95% recrystallization, with uniform size and equiaxed grains

[0027] • The grain size is mainly concentrated in ASTM #5 (0.064 mm).

[0028] • No grains larger than ASTM #3 (0.127 mm)

[0029] • It does not have a polycrystalline structure.

[0030] 2. Mechanical properties

[0031] The final product should meet the mechanical property requirements in the table below at room temperature (293K):

[0032] project Standard requirements Yield strength, Rp0.2 (MPa) 40≤Rp0.2≤100 Tensile strength, Rm (MPa) Rm≥140 Elongation, A (%) ≥30％ strain hardening coefficient n ≥0.3 Hardness HV (load 10N) ≤60

[0033] Yield strength, tensile strength, and elongation should be measured along the longitudinal (rolling direction) and transverse (perpendicular to the rolling direction) of the niobium disc, using 10... -3 The strain rate should be / second, and the difference between the transverse and longitudinal directions should not exceed 10% (the specific calculation method is: difference between transverse and longitudinal directions = (large value - small value) / small value).

[0034] 3. Electrical performance RRR

[0035] Material value requirement of residual resistivity (RRR) ≥ 300

[0036] 4. Chemical composition requirements

[0037] The interstitial element content of niobium 300 should meet the following requirements:

[0038] element Maximum content (by weight) C 10ppm O 10ppm N 10ppm H 2ppm

[0039] Example 1

[0040] This invention provides a method for processing high-purity niobium material with a high RRR value. The method includes the following steps:

[0041] (1) RRR300 superconducting niobium ingots were obtained by electron beam melting. The diameter of the superconducting niobium ingots was 285 mm and the length was 1250 mm. The composition of the ingots is shown in Table 1. Table 1 is the elemental composition data table of superconducting niobium ingots.

[0042] Table 1. Elemental composition data of superconducting niobium ingots

[0043]

[0044] (2) The superconducting niobium ingot obtained in step (1) is subjected to ultrasonic testing. Then, the superconducting niobium ingot is radially squared, axially elongated, and rounded. The specific forging sequence and dimensional changes are as follows: First, the ingot is forged into a square with an end face of 240×Lmm → an equilateral octagon with an end face of 240×Lmm → a square with an end face of 200×Lmm → an equilateral octagon with an end face of 200×Lmm → a square with an end face of 200×Lmm → a square with an end face of 240×Lmm. The dimensions are 170×Lmm → the end face is an equilateral octagon with a size of 170×Lmm → the end face is a square with a size of 160×Lmm → the end face is an equilateral octagon with a size of 160×Lmm → the end face is a circle with a size of φ158×Lmm → resulting in a superconducting niobium forging billet of 55×165×8780mm, with a width of 165mm, a length of 8780mm, and a thickness of 55mm. The maximum temperature of the billet during the forging process is 97℃.

[0045] (3) The superconducting niobium forging blank obtained in step (2) is cut into 13 equal sections along the length direction at equal intervals of L1 = 675 ± 1 mm. One section is taken and the end face and side forging defects are turned and milled. The side of the forged blank in the width direction is milled by a milling machine to remove the folding defects and surface contamination layer caused by forging. The specific milling size is to remove the side defects cleanly. When turning and milling the end face, a cooling lubricant needs to be applied to the end face that needs to be turned and milled. The cooling lubricant is animal fat, preferably lard, which plays a lubricating and cooling role.

[0046] (4) Pickling the superconducting niobium forging obtained in step (3): the volume ratio of hydrofluoric acid, sulfuric acid and nitric acid is 1:3:3, the concentration of hydrofluoric acid is greater than 40%, the concentration of nitric acid is 65-68%, the concentration of sulfuric acid is 95-98%, pickling for 6 minutes, and the metal luster without impurities can be observed by the naked eye.

[0047] (5) The superconducting niobium forging obtained in step (4) is heat-treated in a vacuum annealing furnace: the heating curve is to heat to 500℃ in 30 min and hold for 60 min, then heat to 730℃ in 30 min after the end of the heat treatment, hold for 120 min and then air-cooled to below 80℃ in the furnace before being taken out of the furnace.

[0048] (6) The superconducting niobium forging obtained in step (5) is subjected to oil-free cold rolling. First, it is rolled in three passes along the length direction L1 of the billet to a thickness of 40 mm. Then, it is rolled to a width of 520 mm by reversing the direction by 90° and then rolled to a thickness of 12 mm by reversing the direction by 90°. The rolling rate is 78%, and a superconducting niobium slab with a width of 520-530 mm and a thickness of 12±0.05 mm is obtained.

[0049] (7) After leveling the superconducting niobium slab obtained in step (6), the surface defects and subsequent contamination layers of the slab are removed by an angle grinder and a belt sander, and the thickness is polished to 11.8±0.05mm; then the oil and dirt are removed by acid washing, and the specific acid solution, acid solution ratio and acid washing time are the same as in step 4.

[0050] (8) Roll the superconducting niobium slab obtained in step (7) to a thickness of 4.48±0.03 mm along the original rolling direction.

[0051] (9) The superconducting niobium plate obtained in step (8) is pre-cut to a length of 530±5mm using a shearing machine. The plate is then polished using a belt sander. The thickness of the plate after polishing is 4.35±0.05mm. Finally, the plate is pickled. The specific acid solution, acid solution ratio, pickling time, and pickling time are the same as in step 4.

[0052] (10) Heat treatment of the superconducting niobium plate obtained in step (9): The heating curve is to heat to 500℃ in 30 min and hold for 60 min. After the end, heat to 750℃ in 30 min and hold for 120 min. Then air cool with the furnace to below 80℃ and take it out of the furnace.

[0053] (11) The superconducting niobium plate obtained in step (10) is cut into finished product and polished once: the plate is cut into 500×500×4.35mm by a shearing machine, and then the plate is polished by a belt sander. The thickness after polishing is 4.25±0.05mm.

[0054] (12) The superconducting niobium plate obtained in step (11) is acid-washed: the acid-washing solution is preferably a mixed solution of hydrofluoric acid, phosphoric acid and nitric acid; the hydrofluoric acid is commercially available hydrofluoric acid with a concentration greater than 40%, the nitric acid is commercially available nitric acid with a concentration of 65-68%, the phosphoric acid is commercially available phosphoric acid with a concentration of 99.5%, the volume ratio of hydrofluoric acid, phosphoric acid and nitric acid is 1:1:1, the acid washing time is 5-10 seconds, and a superconducting niobium material with a high RRR value is obtained.

[0055] The performance of the superconducting niobium material prepared in this embodiment was tested, and the results showed that:

[0056]

[0057]

[0058] Example 2

[0059] (1) RRR300 superconducting niobium ingots were obtained by electron beam melting. The diameter of the superconducting niobium ingots was 285 mm and the length was 400 mm. The composition of the ingots is shown in Table 2.

[0060] Table 2 Elemental composition data of superconducting niobium ingots

[0061]

[0062] (2) The superconducting niobium ingot obtained in step (1) is ultrasonically tested, and then the superconducting niobium ingot is radially squared, axially elongated and rounded. The specific forging sequence and size changes are as follows: First, the ingot is forged into a square with an end face of 240×Lmm → an equilateral octagon with an end face of 240×Lmm → a square with an end face of 200×Lmm → an equilateral octagon with an end face of 200×Lmm → a square with an end face of 170×Lmm → an equilateral octagon with an end face of 170×Lmm → a square with an end face of 160×Lmm → an equilateral octagon with an end face of 160×Lmm → a round with an end face of φ158×Lmm → a superconducting niobium forging billet of 55×165×2810mm, 165mm wide, 2810mm long and 55mm thick is obtained. The maximum temperature of the billet during the forging process is 90℃.

[0063] (3) The superconducting niobium forging blank obtained in step (2) is cut into 16 sections at equal intervals along the length direction according to the length of each piece L1=661±1mm. One section is taken and the end face and side forging defects are turned and milled. The side of the forged blank in the width direction is milled by a milling machine to remove the folding defects and surface contamination layer caused by forging. The specific milling size is to remove the side defects cleanly. When turning and milling the end face, a cooling lubricant needs to be applied to the end face that needs to be turned and milled. The cooling lubricant is animal fat, preferably lard, which plays a lubricating and cooling role.

[0064] (4) Pickling the superconducting niobium forging obtained in step (3): the volume ratio of hydrofluoric acid, sulfuric acid and nitric acid is 1:3:3, the concentration of hydrofluoric acid is greater than 40%, the concentration of nitric acid is 65-68%, the concentration of sulfuric acid is 95-98%, pickling for 6 minutes, and the metal luster without impurities can be observed by the naked eye.

[0065] (5) The superconducting niobium forging obtained in step (4) is heat-treated in a vacuum annealing furnace: the heating curve is to heat to 500℃ in 30 min and hold for 60 min, then heat to 690℃ in 30 min after the end of the heating process, hold for 120 min and then air-cooled to below 80℃ in the furnace before being taken out of the furnace.

[0066] (6) The superconducting niobium forging obtained in step (5) is subjected to oil-free cold rolling. First, it is rolled for 3 passes along the length direction L1 of the billet, and the thickness is rolled to 40 mm. Then, it is rolled to 420 mm by widening the width B2 by 90° and then rolled to 12 mm by 90°. The rolling rate is 78%, and a superconducting niobium slab with a width of 420-430 mm and a thickness of 12±0.05 mm is obtained.

[0067] (7) After leveling the superconducting niobium slab obtained in step (6), surface defects and subsequent contamination layers are removed by using an angle grinder and a belt sander, and the thickness is polished to 11.8±0.05mm; then, oil and dirt are removed by acid washing, and the specific acid solution, acid solution ratio and acid washing time are the same as in step 4.

[0068] (8) The superconducting niobium slab obtained in step (7) is rolled to a thickness of 5.58±0.03 mm along the original rolling direction.

[0069] (9) The superconducting niobium plate obtained in step (8) is pre-cut to a length of 430±5mm using a shearing machine. The plate is polished using a belt sander. The thickness of the plate after polishing is 5.45±0.05mm. Finally, it is pickled. The specific acid solution, acid solution ratio, pickling time and step 4 are the same.

[0070] (10) Heat treatment of the superconducting niobium plate obtained in step (9): The heating curve is to heat to 500℃ in 30 min and hold for 60 min. After the end, heat to 740℃ in 30 min and hold for 120 min. Then air cool with the furnace to below 80℃ and take it out of the furnace.

[0071] (11) The superconducting niobium plate obtained in step (10) is cut into finished products and polished once: the plate is cut into 400×400×5.45mm by a shearing machine, and then the plate is polished by a belt sander. The thickness after polishing is 5.35±0.05mm.

[0072] (12) The superconducting niobium plate obtained in step (11) is acid-washed: the acid-washing solution is preferably a mixed solution of hydrofluoric acid, phosphoric acid and nitric acid; the hydrofluoric acid is commercially available hydrofluoric acid with a concentration greater than 40%, the nitric acid is commercially available nitric acid with a concentration of 65-68%, the phosphoric acid is commercially available phosphoric acid with a concentration of 99.5%, the volume ratio of hydrofluoric acid, phosphoric acid and nitric acid is 1:1:1, the acid washing time is 5-10 seconds, and a superconducting niobium material with a high RRR value is obtained.

[0073] The performance of the superconducting niobium material prepared in this embodiment was tested, and the results showed that:

[0074]

[0075] The above-disclosed embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of the invention. Those skilled in the art will understand that implementing all or part of the above-described embodiments and making equivalent changes in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A method for processing high RRR value high purity niobium material, characterized by: The processing method of the high RRR value high-purity niobium material comprises the following steps: ​ (1) Ingot forging: selecting the superconducting niobium ingot obtained by electron beam melting, and forging the selected superconducting niobium ingot at a temperature of 90-100 DEG C to obtain a blank; (2) blank sawing: the blank prepared in step (1) is sawed at equal intervals along the length direction of the blank according to a specified length L1, wherein the length L1 is calculated in the following manner S represents the weight of the finished product, δ1 represents the thickness of the blank after forging, B1 represents the width of the blank after forging, ρ represents the density of the niobium material, L1, δ1 and B1 are in units of mm, S is in units of Kg, and Z is the product yield; (3) Surface machining of the blank: the blank obtained in step (2) is first turned on the surface of the blank by a lathe, and then the side edges of the forged blank in the width direction are milled by a milling machine; (4) Pickling of the blank: the blank subjected to surface machining in step (4) is placed in an acid solution for pickling, and the pickling time is 5-10 minutes; (5) Heat treatment of the blank: the blank subjected to pickling in step (4) is taken out, washed with water, and then the liquid on the blank is wiped off, and stress relief annealing is performed; (6) Blooming of the blank: the blank subjected to heat treatment in step (5) is rolled along the long edge direction of the blank for 3-5 passes, then turned by 90 DEG, rolled to (B3+20) mm in width, and then turned by 90 DEG, rolled to (N*delta3) mm in thickness, wherein B3 is the width of the finished plate, unit: mm, N is the introduction coefficient, and the value of N ranges from 2.0 to 3.0; (7) Repairing and pickling of the plate blank: the plate blank obtained in step (6) is repaired by an angle grinder and a sand belt machine to remove surface defects, and then the repaired blank is placed in an acid solution again for pickling treatment, and the pickling time is 5-10 minutes; (8) Plate blank rolling: the blank subjected to pickling in step (7) is rolled along the short edge direction of the blank, and the thickness of the blank is rolled to (delta3+0.5) mm, wherein delta3 is the thickness of the product plate, unit: mm; (9) Finished product cutting, polishing and pickling: the plate blank subjected to rolling in step (8) is pre-cut to a certain size by a plate shearing machine, polished by a sand belt machine, and then placed in an acid solution again for pickling treatment, and the pickling time is 5-10 minutes; (10) Heat treatment of the finished product: the plate subjected to pickling in step (9) is taken out, washed with water, and then the liquid on the blank is wiped off, and the plate is subjected to annealing treatment; (11) Finished product cutting and first polishing: the plate subjected to annealing treatment in step (10) is cut to the size of the finished product by a plate shearing machine, and then polished by a sand belt machine, and the single-side thickness removal amount is at least 0.05 mm; (12) Pickling of the finished product: the polished plate in step (11) is placed in an acid solution for pickling treatment, and the pickling time is 5-10 seconds, then the pickled plate is taken out and washed with water, and finally the liquid on the plate is wiped off, thereby obtaining the finished niobium material.

2. The method of processing high RRR value high purity niobium material according to claim 1, characterized in that: In step (1), the superconducting niobium ingot is forged by radial square punching, axial lengthening and circular dropping.

3. The method of processing high RRR value high purity niobium material of claim 1, wherein: In step (3), the turning thickness is 1.5-2.0 mm on one side, and the milling size is to remove the defects on the side edges completely.

4. The method of processing high RRR value high purity niobium material of claim 1, wherein: In the step (5), the billet is first heated to 500℃ within 30 minutes, and then kept at 500℃ for 60 minutes. Then the billet is heated to 650-850℃ within 30 minutes, and kept at 650-850℃ for 60-150 minutes. Finally, the billet is air-cooled to below 80℃.

5. The method of processing high RRR value high purity niobium material of claim 1, wherein: In the step (10), the plate after pickling in the step (9) is first heated to 650-850℃ within 30 minutes, and kept at 650-800℃ for 60-150 minutes. Finally, the plate is air-cooled to below 80℃.

6. The method of processing high RRR value high purity niobium material of claim 1, wherein: The acid solution in the steps (4), (7) and (9) is a mixture of hydrofluoric acid with a concentration greater than 40%, nitric acid with a concentration of 65-68%, and sulfuric acid with a concentration of 95-98%, in a volume ratio of 1:3:

3. The acid solution in the step (12) is a mixture of hydrofluoric acid with a concentration greater than 40%, nitric acid with a concentration of 65-68%, and phosphoric acid with a concentration of 99.5%, in a volume ratio of 1:1:

1.

7. The method of processing high RRR value high purity niobium material of claim 1, wherein: In the step (7), the single-side thickness removal of the plate by the angle grinder and the belt sander is at least 0.1mm.

8. The method of processing high RRR value high purity niobium material of claim 1, wherein: The processing method of high RRR value high purity niobium material according to claim 1 is characterized in that the single-side thickness removal of the plate by the belt sander in the step (9) is at least 0.05mm.

9. The method of claim 1, wherein the high RRR value high purity niobium material is characterized by: In the step (1), when forging the superconducting niobium ingot, the ratio of the transverse deformation coefficient M1 to the longitudinal deformation coefficient M2 must be controlled within 0.5-1.5, i.e. wherein, M1 is the transverse deformation coefficient, M2 is the longitudinal deformation coefficient, L1 is the sawing length, and L3 is the length after forging.

10. A niobium material for radio frequency superconducting cavities, characterized by: The niobium material for the radio frequency superconducting cavity is prepared by the processing method of high RRR value high purity niobium material according to any one of claims 1-9.

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