Niobium plate forging method and forged niobium plate

Abstract

The invention provides a forging method of a niobium plate and a forged niobium plate. The forging method of the niobium plate comprises the steps that primary forging is conducted, specifically, flattening is conducted in the radial direction of a cylindrical niobium cast ingot, a forged part is rotated by 90 + / -5 degrees and then continues to be flattened in the radial direction, the steps are repeated till a niobium bar blank with the target diameter is obtained, and in the primary forging process, the temperature of the forging face is kept to be lower than 200 DEG C; secondary forging is conducted, specifically, flattening is conducted in the radial direction of the niobium bar blank, then a forged part is shaped after being rotated by 90 + / -5 degrees, the process is repeated till a niobium bar blank with the target thickness and width is obtained, and in the secondary forging process, the temperature of the forging face is kept to be lower than 200 DEG C; the niobium plate blank is sequentially subjected to machining and acid pickling, the niobium plate blank subjected to acid pickling is subjected to vacuum heat treatment, the niobium plate is obtained, the highest temperature of vacuum heat treatment is 700-850 DEG C, and the heat preservation time at the highest temperature is 1-3 hours.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of processing superconducting niobium material for niobium radio frequency superconducting cavity, and particularly relates to a forging method of niobium plate and forged niobium plate. BACKGROUND

[0002] Radio frequency superconducting acceleration is one of the key technologies of modern particle accelerators, which can accelerate charged particle beams in continuous wave and long pulse mode. Radio frequency superconducting accelerating cavity has the advantages of low heat loss and high electric conversion efficiency, and has been widely used as a core component in various high-energy accelerators and light source devices. High-purity niobium, as a type II superconductor with a relatively high critical temperature (T c = 9.2K), is the preferred material for manufacturing radio frequency superconducting accelerating cavities, and has the advantages of high quality factor, high accelerating field gradient, small power loss, small impedance, and excellent energy resolution and stability.

[0003] At present, the resonant cavity of the radio frequency superconducting cavity linear accelerator is mainly made of superconducting niobium plate, and the quality of the niobium plate directly affects the performance of the radio frequency superconducting cavity. Therefore, the niobium plate for the radio frequency superconducting cavity has very strict purity and performance requirements, in particular, the niobium plate needs to have a residual resistivity ratio (RRR) of not less than 300. However, in the existing niobium plate forging stage, due to the lack of fine control of the forging process parameters, the niobium ingot has a large deformation amount and a fast deformation speed in the forging process, causing serious loss of the RRR value. It is usually necessary to increase the RRR value of the niobium ingot for forging to offset the reduction of the RRR value in the forging stage, resulting in high smelting manufacturing cost and low product direct yield. SUMMARY

[0004] In view of the problem of serious loss of the RRR value of the niobium plate blank in the forging process, the first aspect of the present application provides a forging method of niobium plate, which comprises: primary forging, the primary forging comprising flattening along the radial direction of a cylindrical niobium ingot, then continuing to flatten along the radial direction after rotating the forged piece by 90°±5°, and repeating until a niobium rod blank with a target diameter is obtained, and in the primary forging process, the temperature of the forging surface is kept below 200℃; secondary forging, the secondary forging comprising flattening along the radial direction of the niobium rod blank, then shaping after rotating the forged piece by 90°±5°, and repeating until a niobium plate blank with a target thickness and width is obtained, and in the secondary forging process, the temperature of the forging surface is kept below 200℃; and sequentially machining and pickling the niobium plate blank, vacuum heat treating the pickled niobium plate blank to obtain a niobium plate, the highest temperature of the vacuum heat treatment being 700℃-850℃, and the holding time at the highest temperature being 1-3 hours.

[0005] In the forging method of the niobium plate, the temperature of the forging surface is kept below 200°C, which greatly reduces the absorption of gaseous elements and effectively prevents the decrease of RRR value and performance degradation. After forging, vacuum heat treatment is performed within the above-mentioned maximum temperature and holding time range to eliminate the residual stress inside the niobium plate blank, so that the niobium plate blank obtains an equiaxed crystal structure with excellent homogenization and refinement degree, and the impurity content of C, N, O, H and the like in the niobium plate blank can be reduced, which is beneficial to further optimize the RRR value of the niobium plate and improve the uniformity of the mechanical properties of the niobium plate.

[0006] In any embodiment of the first aspect, the Nb content of the cylindrical niobium ingot is not less than 99.95%.

[0007] In any embodiment of the first aspect, the single-side deformation amount Δh of the primary forging and / or the secondary forging is 15mm-40mm, preferably 15mm-30mm.

[0008] In any embodiment of the first aspect, the maximum temperature of the vacuum heat treatment is 750°C-800°C, and the holding time at the maximum temperature is 1.5-2 hours.

[0009] In any embodiment of the first aspect, the vacuum heat treatment further comprises, before heating to the maximum temperature: heating to 450°C-550°C at a heating rate R1, holding for 45min-75min, and then heating to the maximum temperature at a heating rate R2, R1>R2.

[0010] In any embodiment of the first aspect, the heating rate R1 is 10°C / min-20°C / min, and the heating rate R2 is 6°C / min-15°C / min.

[0011] In any embodiment of the first aspect, the machining includes rough machining and finish machining, the rough machining includes sawing, face turning and edge milling, and the finish machining includes surface grinding; optionally, the surface roughness Ra of the niobium plate blank after machining is ≤1.6μm.

[0012] In any embodiment of the first aspect, the acid solution for pickling includes hydrofluoric acid with a concentration greater than or equal to 40%, hydrochloric acid with a concentration greater than or equal to 35%, and nitric acid with a concentration of 65%-68%, and the volume ratio of the acid solution is preferably hydrofluoric acid:hydrochloric acid:nitric acid=(1-2):(1-3):(1-3).

[0013] In any embodiment of the first aspect, the cooling method after the vacuum heat treatment is completed is furnace cooling.

[0014] In any embodiment of the first aspect, the diameter of the cylindrical niobium ingot is 250mm-600mm, and the target diameter is 160mm-200mm.

[0015] In any embodiment of the first aspect, the target thickness in the secondary forging is 55mm-85mm.

[0016] In any embodiment of the first aspect, the width of the ni plate blank obtained by the secondary forging is 210mm-450mm.

[0017] The second aspect of the present application provides a ni plate material, which can be obtained by any one of the ni plate material forging methods of the first aspect.

[0018] The third aspect of the present application provides a ni plate material, which has an equiaxed crystal structure with a grain size of 40μm-120μm and a residual resistivity RRR≥300.

[0019] In any embodiment of the third aspect, the content of each of C, H, N and O in the ni plate material is not more than 10ppm. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the specific embodiments of the present application, the drawings required to be used in the specific embodiments will be briefly introduced as follows. Obviously, the drawings described below are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort.

[0021] Figure 1 The metallographic microstructure picture obtained by sampling in the direction perpendicular to the thickness section of the ni plate material in some embodiments of the present application. DETAILED DESCRIPTION

[0022] The embodiments of the present application will be further described in detail below with reference to the examples. The detailed description of the following examples is used to exemplarily illustrate the principles of the present application, but cannot be used to limit the scope of the present application, i.e. the present application is not limited to the described examples.

[0023] If not specifically stated, all the embodiments and optional embodiments of the present application can be combined to form new technical solutions.

[0024] If not specifically stated, all the technical features and optional technical features of the present application can be combined to form new technical solutions.

[0025] In the present application, the residual resistivity ratio (RRR) represents the ratio of the resistance value of a material at room temperature (~293K) to the resistance value just above the superconducting transition temperature.

[0026] As analyzed in the background of the present application, the existing forging method often seriously wastes the RRR value of the niobium slab blank formed by forging due to the lack of fine control of the forging process parameters and the surface temperature of the forged piece, inevitably leading to problems such as high production cost of superconducting niobium plate and low direct yield of products.

[0027] To this end, the first embodiment of the present application provides a forging method for a niobium plate, which comprises: primary forging, the primary forging comprising radial flattening along a cylindrical niobium ingot, and then continuing to flatten radially after rotating the forged piece by 90°±5°, and repeating until a niobium rod blank with a target diameter is obtained, and during the primary forging process, the temperature of the forging surface is kept below 200°C; secondary forging, the secondary forging comprising radial flattening along the above-mentioned niobium rod blank, and then continuing to flatten radially after rotating the forged piece by 90°±5°, and repeating until a niobium slab blank with a target thickness is obtained, and during the secondary forging process, the temperature of the forging surface is kept below 200°C; and sequentially machining and pickling the niobium slab blank, and then vacuum heat treating the pickled niobium slab blank to obtain a niobium plate, the highest temperature of the vacuum heat treatment being 700-850°C, and the holding time at the highest temperature being 1-3 hours.

[0028] During the forging process, the surface deformation of the forged piece is accompanied by a rise in surface temperature, and gas elements such as C, N, O and H in the air are more likely to enter the forged piece through the high-temperature surface, directly leading to a decrease in the RRR value of the niobium slab blank after forging. The above-mentioned multiple forging method of first forming a rod blank and then forming a slab blank with continuous rotation of about 90° is used to prepare the niobium slab blank, and the temperature of the forging surface is kept below 200°C, which greatly reduces the absorption of gas elements and effectively prevents the RRR value from decreasing and the performance from deteriorating. After forging, vacuum heat treatment is carried out within the above-mentioned highest temperature and holding time range to eliminate the residual stress inside the niobium slab blank, so that the niobium slab blank obtains an equiaxed crystal structure with excellent homogenization and refinement degree, and the impurity content of C, N, O and H in the niobium slab blank can also be reduced, which is conducive to further optimizing the RRR value of the niobium plate and improving the uniformity of the mechanical properties of the niobium plate.

[0029] In some embodiments, the Nb content of the above-mentioned cylindrical niobium ingot is not less than 99.95%. Using a high-purity niobium ingot as the forging raw material can better meet the application requirements of superconducting niobium plate. The cylindrical niobium ingot used in the present application can be prepared by a vacuum electron beam melting method, but is not limited thereto. Considering that niobium elements are extremely prone to oxidation as the temperature rises, inducing the formation of a Nb2O5 powdery oxide film on the surface of the forged piece, and the powdery oxide continuously peeling off further exacerbates the oxidation of the forged piece, therefore controlling the temperature of the forging surface below 200°C has a significant effect on preventing the RRR value from decreasing and the performance from deteriorating during the processing of the cylindrical niobium ingot with the above-mentioned Nb content.

[0030] In some embodiments, the single-side deformation amount Δh of the primary forging and / or the secondary forging is 15mm-40mm, preferably 15mm-30mm. During the forging, the single-side deformation amount Δh is controlled within the above range, and the required cumulative deformation amount is slowly accumulated by a small single deformation amount, so as not to make the surface temperature too high, and to obtain a uniform and grain-size suitable equiaxed crystal structure. Too small single-side deformation amount requires an increased number of forging strokes, which reduces the production efficiency; too large single-side deformation amount affects the plastic flow inside the forged piece, and also causes the temperature of the surface of the forged piece to rise, which increases the difficulty of controlling the surface temperature, and inevitably increases the probability of the penetration of gaseous elements such as O, C, H, and N, and further causes the RRR value of the niobium plate blank to be lost to a greater extent.

[0031] In order to eliminate the residual stress during the forging and obtain a uniform and fine grain structure, the niobium plate blank obtained by the forging needs to be subjected to vacuum heat treatment (annealing treatment). In some embodiments, the highest temperature of the vacuum heat treatment is 750°C-800°C, and the holding time at the highest temperature is 1.5-2 hours. Since the niobium plate blank has a very high purity and a relatively low recrystallization temperature (about 650°C), the highest temperature of the vacuum heat treatment is maintained within the range of 750°C-800°C, so that the niobium plate blank is more fully recovered and recrystallized, and a uniform equiaxed fine grain structure is obtained. It should be understood that the holding time is related to the size of the niobium plate blank, and a person skilled in the art can appropriately adjust the holding time at the highest temperature according to the actual situation.

[0032] During the vacuum heat treatment, controlling the temperature rising process (for example, the temperature rising rate) is beneficial to the regulation and control of the release of the residual stress of the forging. In some embodiments, the vacuum heat treatment further comprises, before rising to the highest temperature: rising to 450°C-550°C at a temperature rising rate R1, holding for 45min-75min, and then rising to the highest temperature at a temperature rising rate R2, R1>R2. Holding at a lower temperature before rising to the highest temperature enables the residual stress of the niobium plate blank to be more fully released, which is beneficial to obtaining a niobium plate blank with a more uniform structure, and more balanced toughness and strength. Moreover, during the process of rising to 450°C-550°C, the target temperature is lower than the temperature at which the niobium plate material undergoes recrystallization transformation, so the temperature rising rate can be appropriately increased and the heat treatment time can be shortened to improve the processing efficiency, as far as the equipment allows.

[0033] In some embodiments, the heating rate R1 is 10-20 ℃ / min, and the heating rate R2 is 6-15 ℃ / min. The heating rate R2 of the high-temperature section is lower than the heating rate R1 of the low-temperature section, and the more gentle heating process is conducive to the full recrystallization of the niobium plate and better control of the size of the recrystallized grains. For example, the heating rate R1 is 10 ℃ / min, and the heating rate R2 is 6 ℃ / min; the heating rate R1 is 12 ℃ / min, and the heating rate R2 is 6 ℃ / min; the heating rate R1 is 10 ℃ / min, and the heating rate R2 is 8 ℃ / min; the heating rate R1 is 15 ℃ / min, and the heating rate R2 is 10 ℃ / min; the heating rate R1 is 16 ℃ / min, and the heating rate R2 is 9 ℃ / min; the heating rate R1 is 18 ℃ / min, and the heating rate R2 is 12.5 ℃ / min; the heating rate R1 is 20 ℃ / min, and the heating rate R2 is 15 ℃ / min; but not limited to this.

[0034] In some embodiments, the machining includes rough machining and finish machining. The rough machining includes sawing, face turning, and edge milling. The finish machining includes surface grinding. The rough machining is mainly used to remove the visible forging defects, surface hardening layer, and contaminants on the surface of the niobium plate blank. The finish machining is mainly used to further optimize the surface quality of the niobium plate blank, remove the surface defects such as folds, cracks, and press pits caused by forging, and improve the surface finish.

[0035] In some embodiments, the surface roughness Ra of the niobium plate blank after machining is less than or equal to 1.6 μm. After finish machining, the surface of the niobium plate blank is smoother, which is conducive to improving the surface corrosion resistance and facilitating the subsequent processing of the niobium plate blank.

[0036] In some embodiments, the acid solution composition for pickling includes hydrofluoric acid with a concentration greater than or equal to 40%, hydrochloric acid with a concentration greater than or equal to 35%, and nitric acid with a concentration of 65%-68%. The volume ratio of the above-mentioned acid solution composition is preferably hydrofluoric acid: hydrochloric acid: nitric acid = (1-2):(1-3):(1-3), and more preferably the volume ratio of the acid solution composition is HF:HCl:HNO3 = 1:2:3. Exemplarily, the machined niobium plate blank can be placed in the prepared acid solution for pickling until the surface of the niobium plate blank presents a metallic luster and no speckles are observed.

[0037] In some embodiments, the cooling method after the vacuum heat treatment is completed is furnace air cooling. Exemplarily, after the vacuum heat treatment is completed, the niobium plate blank is furnace air cooled to 80 ℃ and then discharged from the furnace to obtain the niobium plate.

[0038] The purpose of the first forging is to obtain a small-diameter niobium rod blank by multiple times of forging of a large-diameter niobium ingot, to keep the isotropy of the forged piece as much as possible, and to optimize the uniformity of the grain structure. In some embodiments, the diameter of the cylindrical niobium ingot is 250 mm to 600 mm, and the target diameter is 160 mm to 200 mm. For example, the diameter of the cylindrical niobium ingot is 285 mm, and the target diameter can be 160 mm; the diameter of the cylindrical niobium ingot is 480 mm, and the target diameter can be 240 mm; but not limited to this.

[0039] The purpose of the second forging is to obtain a niobium plate blank with a required size by multiple times of forging of the small-diameter niobium rod blank. In some embodiments, the target thickness in the second forging is 55 mm to 85 mm; and optionally, the width of the niobium plate blank obtained by the second forging is 210 mm to 450 mm. Those skilled in the art can appropriately adjust the above-mentioned target diameter, target thickness, and width according to the actual situation.

[0040] The second embodiment of the present application provides a forged niobium plate, which can be obtained by any one of the niobium plate forging methods in the first embodiment. The niobium plate obtained by the niobium plate forging method of the present application has a high RRR value, a moderate grain size, and high isotropy.

[0041] In some embodiments, by controlling the surface temperature, deformation amount, and other forging process parameters in multiple times of forging, the forged niobium plate can have RRR≥300, and exhibit a uniform equiaxed grain structure with excellent processing performance.

[0042] The third embodiment of the present application provides a niobium plate having an equiaxed grain structure with a grain size of 40 μm to 120 μm and a residual resistivity RRR≥300. The grain size of the niobium plate affects its mechanical properties and residual resistivity. For example, if the grain size is too small and the uniformity is poor, it is difficult to maintain an RRR value of not less than 300; and if the grain size is too large, the fine-grain strengthening effect cannot be achieved, and the tensile strength and yield strength of the niobium plate are deteriorated.

[0043] Figure 1 A metallographic microstructure picture obtained by sampling in the direction perpendicular to the thickness section of the niobium plate in some embodiments of the present application is shown. By comparison with a standard rating chart, it can be roughly judged that the grain size of the niobium plate in this direction is at the level of 3-5, and the grain structure is nearly equiaxed and uniform.

[0044] In some embodiments, the content of each of C, H, N, and O elements in the niobium plate is not more than 10 ppm. Controlling the above-mentioned gaseous elements in a lower range helps to reduce various types of crystal defects in the niobium plate, suppresses electron scattering, and reduces the loss of residual resistivity RRR.

[0045] [Examples]

[0046] The beneficial effects of the present application will be further illustrated below in conjunction with examples and comparative examples, but the scope of the present application is not limited to these examples.

[0047] Table 1

[0048]

[0049] Example 1

[0050] The niobium plate forging method in the present example includes the following steps:

[0051] 1) First forging: using a fast forging machine to flatten the cylindrical niobium ingot (the parameters are shown in Table 1) along the radial direction, the single-sided deformation amount Δh is 40 mm, when the thickness of the forged piece reaches 245 mm, the forged piece is rotated 90° along the axial direction and flattened along the radial direction, the above operation is repeated until a □165 mm square billet is forged, then chamfering and round forging are performed to obtain a niobium rod billet with a diameter of 160 mm; in this process, a temperature measuring device is used to monitor the surface temperature of the forged piece in real time, if the forging surface temperature approaches 200℃, stop processing and let it cool naturally to ensure that the temperature of the forging surface is below 200℃;

[0052] 2) Second forging: flatten the niobium rod billet obtained in step 1) along the radial direction, the single-sided deformation amount Δh is 40 mm, when the thickness reaches 80 mm, the forged piece is rotated 90° along the axial direction for width forging, the above operation is repeated until a niobium plate billet with a thickness of 55 mm and a width of 210 mm is forged, similarly, in this process, a temperature measuring device is used to monitor the surface temperature of the forged piece in real time, if the forging surface temperature approaches 200℃, stop processing and let it cool naturally to ensure that the temperature of the forging surface is below 200℃;

[0053] 3) Machining: the niobium plate billet obtained in step 2) is cut into multiple sections along the length direction at equal intervals with a length of 500 mm each, one of the niobium plate billets is taken and subjected to face turning and edge milling to remove defects such as folds, cracks and indentation pits on the surface caused by forging, and to ensure that the surface roughness Ra of the niobium plate billet is ≤1.6 μm;

[0054] 4) Pickling: the machined niobium plate billet is subjected to pickling treatment with a mixed acid solution, the volume ratio of hydrofluoric acid, hydrochloric acid and nitric acid in the mixed acid solution is 1:2:3, and the concentration of hydrofluoric acid is 40%, the concentration of hydrochloric acid is 35%, and the concentration of nitric acid is 65%, pickling until the surface of the niobium plate billet presents a metallic luster and no speckles are observed;

[0055] 5) Vacuum heat treatment: after pickling, the niobium plate blank is transferred into a vacuum annealing furnace for heat treatment, the initial temperature is room temperature (about 25℃), first heated to 500℃ at a rate of 15.8℃ / min, and then heated to 750℃ at a rate of 10℃ / min for 2 hours, and then air-cooled to below 80℃ in the furnace, and the niobium plate is obtained.

[0056] Example 2

[0057] The difference between the niobium plate forging method in this example and that in Example 1 is only that:

[0058] 1) One-time forging: using a fast forging machine to flatten the cylindrical niobium ingot (the parameters are shown in Table 1) along the radial direction, the single-sided deformation amount Δh is 30mm, when the thickness reaches 245mm, the forged piece is rotated 90° along the axial direction and continues to be flattened along the radial direction, and the above operation is repeated until a □200mm square billet is forged, and then chamfering and round drop forging are performed to obtain a niobium rod blank with a diameter of 200mm. In this process, a temperature measuring device is used to monitor the surface temperature of the forged piece in real time, and if the forging surface temperature approaches 200℃, the processing is stopped to allow it to cool naturally, ensuring that the temperature of the forging surface is below 200℃;

[0059] The remaining steps and settings remain the same as in Example 1.

[0060] Example 3

[0061] The difference between the niobium plate forging method in this example and that in Example 1 is only that:

[0062] 1) One-time forging: using a fast forging machine to flatten the cylindrical niobium ingot (the parameters are shown in Table 1) along the radial direction, the single-sided deformation amount Δh is 15mm, when the thickness reaches 245mm, the forged piece is rotated 90° along the axial direction and continues to be flattened along the radial direction, and the above operation is repeated until a □165mm square billet is forged, and then chamfering and round drop forging are performed to obtain a niobium rod blank with a diameter of 160mm. In this process, a temperature measuring device is used to monitor the surface temperature of the forged piece in real time, and if the forging surface temperature approaches 200℃, the processing is stopped to allow it to cool naturally, ensuring that the temperature of the forging surface is below 200℃;

[0063] 2) Two-time forging: flattening the niobium rod blank obtained in step 1) along the radial direction, the single-sided deformation amount Δh is 40mm, when the thickness reaches 80mm, the forged piece is rotated 90° along the axial direction for width forging, and the above operation is repeated until a niobium plate blank with a thickness of 55mm and a width of 210mm is forged. Similarly, in this process, a temperature measuring device is used to monitor the surface temperature of the forged piece in real time, and if the forging surface temperature approaches 200℃, the processing is stopped to allow it to cool naturally, ensuring that the temperature of the forging surface is below 200℃;

[0064] The remaining steps and settings remain the same as in Example 1.

[0065] Example 4

[0066] The difference between the niobium plate forging method in this example and that in Example 1 is only that:

[0067] 5) Vacuum heat treatment: the pickled niobium plate blank is transferred into a vacuum annealing furnace for heat treatment, the initial temperature is room temperature (about 25°C), first heated to about 500°C at a heating rate of 15.8°C / min, and then kept for 1 hour, then heated to 800°C at a heating rate of 10°C / min, and kept for 2 hours, and then air-cooled to below 80°C in the furnace and discharged, to obtain the niobium plate;

[0068] The remaining steps and settings remain the same as in Example 1.

[0069] Example 5

[0070] The difference between the niobium plate forging method in this example and that in Example 1 is only that:

[0071] 5) Vacuum heat treatment: the pickled niobium plate blank is transferred into a vacuum annealing furnace for heat treatment, the initial temperature is room temperature (about 25°C), first heated to about 500°C at a heating rate of 15.8°C / min, and then kept for 1 hour, then heated to 700°C at a heating rate of 10°C / min, and kept for 2 hours, and then air-cooled to below 80°C in the furnace and discharged, to obtain the niobium plate;

[0072] The remaining steps and settings remain the same as in Example 1.

[0073] Example 6

[0074] The difference between the niobium plate forging method in this example and that in Example 1 is only that:

[0075] 5) Vacuum heat treatment: the pickled niobium plate blank is transferred into a vacuum annealing furnace for heat treatment, the initial temperature is room temperature (about 25°C), first heated to about 500°C at a heating rate of 15.8°C / min, and then kept for 1 hour, then heated to 850°C at a heating rate of 10°C / min, and kept for 2 hours, and then air-cooled to below 80°C in the furnace and discharged, to obtain the niobium plate;

[0076] The remaining steps and settings remain the same as in Example 1.

[0077] Example 7

[0078] The difference between the niobium plate forging method in this example and that in Example 1 is only that:

[0079] 5) Vacuum heat treatment: the niobium plate blank after pickling was transferred into a vacuum annealing furnace for heat treatment, and heated to about 800℃ at a heating rate of 10℃ / min, and kept for 1 hour, and then furnace-cooled to below 80℃ to discharge the furnace, to obtain the niobium plate;

[0080] The remaining steps and settings were consistent with Example 1.

[0081] Comparative Example 1

[0082] The difference between the niobium plate forging method in this comparative example and Example 1 is only that:

[0083] 1) Primary forging: the cylindrical niobium ingot (the parameters are shown in Table 1) was flattened along the radial direction using a fast forging machine, and the single-sided deformation amount Δh was 15mm, when the thickness reached 245mm, the forged piece was rotated 90° along the axial direction and continued to be flattened along the radial direction, and the above operation was repeated until a □165mm square billet was forged, and then chamfered and rounded to a niobium rod blank with a diameter of 160mm;

[0084] 2) Secondary forging: the niobium rod blank obtained in step 1) was flattened along the radial direction, and the single-sided deformation amount Δh was 40mm, when the thickness reached 80mm, the forged piece was rotated 90° along the axial direction and continued to be flattened along the radial direction, and the above operation was repeated until a niobium plate blank with a thickness of 55mm and a width of 210mm was forged;

[0085] In the above primary forging and secondary forging processes, the forging surface temperature was not controlled, and the highest forging surface temperature could reach 400℃. The remaining steps and settings were consistent with Example 1.

[0086] Comparative Example 2

[0087] The difference between the niobium plate forging method in this comparative example and Example 1 is only that:

[0088] 5) Vacuum heat treatment: the niobium plate blank after pickling was transferred into a vacuum annealing furnace for heat treatment, and the initial temperature was room temperature (about 25℃), first heated to about 500℃ at a heating rate of 15.8℃ / min, and kept for 1 hour, then heated to 1000℃ at a heating rate of 10℃ / min, and kept for 2 hours, and then furnace-cooled to below 80℃ to discharge the furnace, to obtain the niobium plate;

[0089] The remaining steps and settings were consistent with Example 1.

[0090] [Performance Test]

[0091] The niobium plate obtained in the above examples and comparative examples was tested for grain size, gas element content and residual resistivity RRR value in the following manner.

[0092] Grain size determination, the thickness section parallel to the niobium plate and the thickness section perpendicular to the niobium plate are selected for grain size test, which can be carried out according to methods well known to those skilled in the art, for example intercept method, and the specific operation can refer to the corresponding content in the standard test method for determining average grain size in ASTM E112-13(2021).

[0093] The determination of O, H, C, N gas element content in the niobium plate can refer to the current relevant national standards, including GB / T15076.14-2008 "Tantalum and Niobium Chemical Analysis Method for Determination of Oxygen Content", GB / T15076.15-2008 "Tantalum and Niobium Chemical Analysis Method for Determination of Hydrogen Content", GB / T 15076.8-2008 "Tantalum and Niobium Chemical Analysis Method for Determination of Carbon and Sulfur Content" and GB / T15076.13-2017 "Tantalum and Niobium Chemical Analysis Method Part 13: Determination of Nitrogen Content by Inert Gas Fusion Thermal Conductivity Method".

[0094] The determination method of residual resistivity ratio RRR of the niobium plate can refer to IEC 61788.23 "Measurement of Residual Resistivity Ratio of Niobium Material for RF Superconducting Cavities".

[0095] The niobium plates of each example and comparative example are tested by using the above determination method, in order to improve the accuracy, the average value is taken after determination under multiple(≥5) fields of view, and the results are recorded in Table 2.

[0096] Table 2

[0097]

[0098]

[0099] By comparing the test results of the niobium plates of each example and comparative example in Table 2, it can be found that the temperature of the forging surface in the forging process of Comparative Example 1 is not controlled, the increase of the temperature of the forging surface leads to a large amount of gas absorption, which makes the content of gas impurity elements in the niobium plate increase sharply, and the RRR value is significantly deteriorated. The vacuum heat treatment temperature of Comparative Example 2 is as high as 1000℃, which leads to grain coarsening in the recrystallization process, seriously affecting the strength and hardness of the niobium plate. By strictly controlling the temperature of the forging surface in the forging process and the temperature and time of vacuum heat treatment after forging, the grain size and RRR value of the prepared niobium plate can meet the performance requirements of niobium plate for RF superconducting cavities.

[0100] Compared with Example 1, the single-sided deformation amount of the first forging in Example 2 and 3 is reduced to 30mm and 15mm respectively, the degree of grain crushing is lower, and the grain size of the finally obtained niobium plate is larger. Compared with Example 1, the maximum temperature of vacuum heat treatment in Example 5 is lower, the driving force for grain growth in the recrystallization process is smaller, and the grain size obtained is significantly reduced.

[0101] The highest temperature of vacuum heat treatment of Example 4 and Example 7 is 800℃, the difference is that the vacuum heat treatment of Example 4 is first kept at an intermediate temperature (500℃) for a certain time and then slowly heated to the highest temperature, and Example 7 omits the operation of keeping at the intermediate temperature and directly heats to the highest temperature. Through microscopic morphology observation, it is found that, compared with Example 7, Example 4 has better grain equiaxialization degree and grain size uniformity, and the RRR value is slightly higher than that of Example 7.

[0102] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the gist of the present application, various modifications that can be thought of by those skilled in the art are applied to the embodiments, and other modes constructed by combining part of the constituent elements of the embodiments are also included in the scope of the present application.

Claims

1. A method of forging a niobium plate material, characterized by, The method comprises: primary forging, which comprises radial flattening of a cylindrical niobium ingot, and then continues to flatten radially after rotating the forged piece by 90°±5°, and repeats until a niobium rod blank with a target diameter is obtained, and the temperature of the forging surface is kept below 200℃ during the primary forging; secondary forging, which comprises radial flattening of the niobium rod blank, and then shaping after rotating the forged piece by 90°±5°, and repeats until a niobium plate blank with a target thickness and width is obtained, and the temperature of the forging surface is kept below 200℃ during the secondary forging; the niobium plate blank is sequentially machined and pickled, and the pickled niobium plate blank is vacuum heat treated to obtain a niobium plate, the highest temperature of the vacuum heat treatment is 700-850℃, and the holding time at the highest temperature is 1-3 hours.

2. The niobium plate forging method according to claim 1, characterized by, The Nb content of the cylindrical niobium ingot is not less than 99.95%.

3. The niobium plate forging method according to claim 1 or 2, characterized by, The single-side deformation Δh of the primary forging and / or the secondary forging is 15-40mm, preferably 15-30mm.

4. The niobium plate forging method according to any one of claims 1 to 3, characterized by, The highest temperature of the vacuum heat treatment is 750-800℃, and the holding time at the highest temperature is 1.5-2 hours.

5. The niobium plate forging method according to any one of claims 1 to 4, characterized by, The vacuum heat treatment further comprises: heating to 450-550℃ at a heating rate R1, holding for 45-75min, and then heating to the highest temperature at a heating rate R2, R1>R2, before heating to the highest temperature. Optionally, the heating rate R1 is 10-20℃ / min, and the heating rate R2 is 6-15℃ / min.

6. The niobium plate forging method according to any one of claims 1 to 5, characterized by, The machining comprises rough machining and finish machining, the rough machining comprises sawing, face turning, and edge milling, and the finish machining comprises surface polishing. Optionally, the surface roughness Ra of the machined niobium plate blank is ≤1.6μm.

7. The niobium plate forging method according to any one of claims 1 to 6, characterized by, The acid solution for pickling comprises hydrofluoric acid with a concentration of ≥40%, hydrochloric acid with a concentration of ≥35%, and nitric acid with a concentration of 65-68%, and the volume ratio of the acid solution is preferably hydrofluoric acid:hydrochloric acid:nitric acid=(1-2):(1-3):(1-3).

8. The niobium plate forging method according to any one of claims 1 to 7, characterized by, The cooling mode after the vacuum heat treatment is furnace air cooling.

9. The niobium plate forging method according to any one of claims 1 to 8, characterized by, The diameter of the cylindrical niobium ingot is 250-600mm, and the target diameter is 160-200mm. Optionally, the target thickness in the secondary forging is 55-85mm. Optionally, the width of the niobium plate blank obtained by the secondary forging is 210-450mm. 10.A forged niobium plate obtained by the niobium plate forging method of any one of claims 1-9.

11. A niobium plate material, characterized by, The niobium plate has an equiaxed crystal structure with a grain size of 40-120μm, and a residual resistivity RRR≥300.

12. The niobium plate of claim 11, wherein The content of each of C, H, N, and O in the niobium plate is not more than 10ppm.

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