Preparation method of nuclear-grade hafnium plate and nuclear-grade hafnium plate

Abstract

The invention provides a nuclear-grade hafnium plate preparation method and a nuclear-grade hafnium plate, and belongs to the technical field of nuclear fission reactor control rod neutron absorber material processing.The nuclear-grade hafnium plate preparation method comprises the steps that nuclear-grade sponge hafnium is adopted as a raw material, the oxygen content of a hafnium cast ingot is controlled within a specific range through a smelting technology, and the more obvious strengthening effect is achieved; and the strength of the finished hafnium plate can meet the nuclear pole requirement. Meanwhile, the deformation resistance of the material is increased, so that the problem that the high-strength hafnium plate is difficult to machine is further solved through optimal combination of forging upsetting, hot rolling, annealing, one-pass cold rolling, intermediate annealing, two-pass cold rolling and finished product heat treatment parameters, and the nuclear-grade hafnium plate with various properties meeting the technical requirements is prepared; the tensile strength and the welding strength of the hafnium plate are improved, the process is stable, and the yield reaches 95% or above.

Description

Technical Field

[0001] This application belongs to the technical field of processing nuclear-grade hafnium plates for neutron absorbers of control rods in nuclear fission reactors, and specifically relates to a method for preparing nuclear-grade hafnium plates and nuclear-grade hafnium plates. Background Art

[0002] Hafnium has a large thermal neutron absorption cross-section. The product after the fission reaction is still hafnium, and at the same time, no secondary γ-rays are generated. It is a safe and effective long-life neutron absorption material and is used as a neutron absorber material for control rods in nuclear reactors. Usually, nuclear-grade hafnium plates are prepared from crystal bar hafnium. However, the preparation process of crystal bar hafnium is complex and the cost is high, resulting in expensive nuclear-grade hafnium plates. To reduce production costs, a technology for preparing hafnium plates using sponge hafnium as raw material has been developed.

[0003] For example, Patent CN117646154A discloses a preparation method for improving the tissue performance of hafnium plates. Hafnium plates are prepared from sponge hafnium raw materials through melting, forging, hot rolling, annealing, single-pass cold rolling, and finished product heat treatment. Emphasis is placed on not tempering during the hot rolling process to avoid grain growth and improve the tissue strength of hafnium plates. By matching the heat treatment process and parameters with the plate size, the mechanical strength and tissue uniformity of the finished plates are prevented from decreasing. However, from the perspective of the final performance of the finished plates, there is still room for improvement. Although qualified finished plates are prepared using the single-pass cold rolling process of "multiple passes, small deformation", if the oxygen content in hafnium material fluctuates greatly during the production process, there is a risk of unstable forming performance and low finished product rate for the hafnium plates prepared by this technology. Therefore, it is necessary to further improve the preparation process of hafnium plates to improve performance and process stability. Summary of the Invention

[0004] In order to solve the problems in the above-mentioned prior art, such as unstable performance of the prepared hafnium plates, low finished product rate, and inability to meet the usage requirements of nuclear-grade hafnium plates, this application provides a method for preparing nuclear-grade hafnium plates and nuclear-grade hafnium plates. Using nuclear-grade sponge hafnium as raw material, the oxygen content of the hafnium ingot is controlled within a specific range through the melting process to achieve a more obvious strengthening effect, so that the strength of the finished hafnium plates can meet the nuclear-grade requirements. However, at the same time, the deformation resistance of the material increases, and it is more difficult to process and form the plates into the target size. This application further optimizes the combination of forging upsetting, hot rolling (tempering once in the middle), annealing, single-pass cold rolling - intermediate annealing - two-pass cold rolling, and finished product heat treatment parameters, which not only solves the problem of difficult processing of high-strength hafnium plates, but also prepares nuclear-grade hafnium plates with various performances meeting the design technical requirements, improves the tensile strength and welding strength of hafnium materials, has stable process, and the finished product rate reaches more than 95%. The specific technical solutions are as follows: Melting nuclear-grade sponge hafnium to make a hafnium ingot; Forging the hafnium ingot to make a hafnium plate blank; Hot-rolled hafnium slab billet, annealing the hafnium slab billet to make a hot-rolled slab billet; Cold-rolling the hot-rolled slab billet in multiple passes, and performing intermediate annealing on the cold-rolled sheet to make a cold-rolled hafnium sheet; Performing finish annealing on the cold-rolled hafnium sheet; Finishing the hafnium sheet to make a finished hafnium sheet.

[0005] In a feasible implementation, the nuclear-grade sponge hafnium is melted three times using a vacuum electron beam melting furnace to make a hafnium ingot.

[0006] In a feasible implementation, the oxygen element content of the hafnium ingot is 100 ppm to 400 ppm.

[0007] In a feasible implementation, the steps of forging the hafnium ingot to make a hafnium slab billet include: At a temperature of 1100 °C to 1200 °C for 60 min to 120 min; First upset the hafnium ingot, then draw out the upset hafnium ingot, and then forge the hafnium ingot into a slab billet. During the forging process, selectively reheat in the furnace. The reheat temperature is 1100 °C to 1200 °C, and the reheat holding time is 20 min to 60 min. The final forging temperature is ≥750 °C; Air-cool the forged slab billet to room temperature; Remove the surface oxide layer of the forged slab billet and grind the defective positions to make a hafnium slab billet.

[0008] In a feasible implementation, the steps of hot-rolling the hafnium slab billet and annealing the hafnium slab billet to make a hot-rolled slab billet include: Heat the hafnium slab billet to 800 °C to 850 °C and hold for 60 min to 120 min; After the hafnium slab billet is taken out of the furnace and hot-rolled to a certain thickness, reheat it to 800 °C to 850 °C and hold for 20 min to 60 min; Take the hafnium slab billet out of the furnace again and hot-roll it to a thickness of 10 mm to 12 mm, and control the cumulative hot-rolling deformation amount to be 75% to 85%; Remove the surface oxide layer of the hot-rolled slab billet and grind the defective positions; Perform vacuum annealing treatment to make a hot-rolled slab billet.

[0009] In a feasible implementation, intermediate annealing is performed on the cold-rolled sheet between adjacent cold-rolling passes.

[0010] In a feasible implementation, the steps of performing intermediate annealing on the cold-rolled sheet between adjacent cold-rolling passes include: Perform the first cold-rolling pass, and control the deformation amount of the first cold-rolling pass to be 20% to 29%; Perform intermediate vacuum annealing treatment at a temperature of 740°C to 810°C for 60 min to 120 min of heat preservation. Perform the second-pass cold rolling, control the deformation amount of the second-pass cold rolling to be 25% to 35%, and fabricate a cold-rolled hafnium sheet.

[0011] In a feasible implementation manner, the steps of performing finish annealing treatment on the cold-rolled hafnium sheet include: Install the cold-rolled hafnium sheet on a flattening device; Put the flattening device and the cold-rolled hafnium sheet into a vacuum heat treatment furnace together, and perform annealing treatment at a temperature of 720°C to 800°C and a vacuum degree of 5×10 - 3 Pa for 60 min to 120 min of heat preservation.

[0012] In a feasible implementation manner, the steps of finishing the hafnium sheet to fabricate a finished hafnium sheet include: Grind and polish the hafnium sheet to remove 0.2 mm to 0.5 mm on each side; Remove the scratches and defects on the hafnium sheet to obtain a finished hafnium sheet with a bright surface and no scratches.

[0013] According to the second aspect of the embodiments of the present application, a nuclear-grade hafnium sheet is provided, which is fabricated by the method for fabricating a nuclear-grade hafnium sheet according to any of the above technical solutions.

[0014] A method for fabricating a nuclear-grade hafnium sheet and a nuclear-grade hafnium sheet according to the present application, compared with the prior art, have the beneficial effects that: The method for fabricating a nuclear-grade hafnium sheet provided by the embodiments of the present application fabricates a nuclear-grade hafnium sheet from sponge hafnium by using the process of forging - hot rolling - cold rolling - intermediate annealing - cold rolling - finish annealing; by performing intermediate annealing treatment during the cold rolling process to eliminate the work hardening brought about by the cold rolling treatment, prevent cold rolling cracking, improve the process stability, ensure that multi-pass cold rolling does not crack, solve the manufacturing and processing problems of high-strength hafnium sheets, and make the microstructure of the hafnium sheet uniform and the texture reasonable through the optimized matching of the cold rolling deformation amount and the annealing process parameters, improve the creep performance and welding performance of the finished hafnium sheet, make the hafnium sheet have higher room / high-temperature strength and elongation rate, and enable the various properties of the hafnium sheet to meet the design requirements of the nuclear-grade hafnium sheet; use nuclear-grade sponge hafnium as the raw material for fabricating the hafnium sheet, and through the redesign and optimization of the process steps of the hafnium sheet, fabricate a high-strength hafnium sheet that can meet the nuclear-grade requirements, and significantly improve the comprehensive performance of the hafnium sheet, that is, the present application further improves the strength of the hafnium sheet while improving the comprehensive performance of the hafnium sheet, improves the stability of the hafnium sheet fabrication process, and has a high yield. Description of the Drawings

[0015] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present application. Moreover, throughout the drawings, the same reference numerals are used to denote the same components. In the drawings: Figure 1 It is a schematic step flow chart of a method for preparing nuclear-grade hafnium plates according to an embodiment provided by the present application; Figure 2 It is a schematic step flow chart of the forging process of a method for preparing nuclear-grade hafnium plates according to an embodiment provided by the present application; Figure 3 It is a schematic step flow chart of the multi-pass hot rolling process of a method for preparing nuclear-grade hafnium plates according to an embodiment provided by the present application; Figure 4 It is a schematic step flow chart of the multi-pass cold rolling process of a method for preparing nuclear-grade hafnium plates according to an embodiment provided by the present application; Figure 5 It is a schematic step flow chart of the annealing process after cold rolling of a method for preparing nuclear-grade hafnium plates according to an embodiment provided by the present application; Figure 6 It is a schematic step flow chart of the finishing process of a method for preparing nuclear-grade hafnium plates according to an embodiment provided by the present application. Specific Embodiments

[0016] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation of the present application.

[0017] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present application, "a plurality" means two or more unless otherwise specifically defined.

[0018] In this application, unless otherwise clearly specified and defined, terms such as "installation", "connection", "linkage", "fixation" shall be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0019] The preferred embodiments of the present application will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present application and are not used to limit the present application.

[0020] As Figure 1 shown, according to the first aspect of the embodiments of the present application, a method for preparing nuclear-grade hafnium plates is proposed, including: Step 100: Melting nuclear-grade hafnium sponge to make a hafnium ingot; using nuclear-grade hafnium sponge as the raw material for preparing nuclear-grade hafnium plates can reduce production costs, improve the economy of hafnium plate manufacturing, and make the prepared hafnium plates meet nuclear requirements. Step 200: Forging the hafnium ingot to make a hafnium plate blank; by forging the ingot into a plate blank, the plasticity and toughness of the hafnium ingot can be improved, and the mechanical properties can be enhanced. Step 300: Hot-rolling the hafnium plate blank in multiple passes and annealing the hafnium plate blank to make a hot-rolled plate blank; by reheating in the furnace at least once during the hot-rolling process, the performance of the hot-rolled plate blank after hot-rolling forming can be ensured, the grains can be broken, and rolling cracking can be avoided. Step 400: Cold-rolling the hot-rolled plate blank in multiple passes and performing intermediate annealing on the cold-rolled plate to make cold-rolled hafnium plates; by performing at least one intermediate annealing during the cold-rolling process, on the one hand, it is to eliminate work hardening and ensure the cold-rolling forming performance, and on the other hand, by means of intermediate annealing treatment, the structure is regulated to make the structure more uniform and improve the comprehensive performance. Step 500: Performing finished annealing on the cold-rolled hafnium plates; through the annealing treatment after cold-rolling, the residual stress generated during the cold-rolling process is eliminated, and a uniform recovery and recrystallization structure is obtained, so that the cold-rolled hafnium plates have better mechanical properties and processing properties. Step 600: Finishing the hafnium plates to make finished hafnium plates; by finely processing the plates, the hafnium plates meet the standards of nuclear-grade hafnium plates.

[0021] The method for preparing nuclear-grade hafnium plates provided by the embodiments of the present application prepares nuclear-grade hafnium plates from hafnium sponge by using the process of forging - hot rolling - cold rolling - intermediate annealing - cold rolling - finish annealing; by performing intermediate annealing treatment during the cold rolling process, the work hardening brought by the cold rolling treatment is eliminated, cold rolling cracking is prevented, the process stability is improved, and multi-pass cold rolling without cracking is ensured, solving the manufacturing and processing problems of high-strength hafnium plates, and through the optimized matching of the cold rolling deformation amount and the annealing process parameters, the microstructure of the hafnium plates is uniform and the texture is reasonable, improving the creep performance and welding performance of the finished hafnium plates, enabling the hafnium plates to have higher room / high-temperature strength and elongation, and enabling the various properties of the hafnium plates to meet the design requirements of the nuclear grade; using nuclear-grade hafnium sponge as the raw material for preparing hafnium plates, through the redesign and optimization of the process steps of the hafnium plates, high-strength hafnium plates that can meet the nuclear grade requirements are prepared, and the comprehensive performance of the hafnium plates is significantly improved, that is, the present application improves the strength of the hafnium plates while further improving the comprehensive performance of the hafnium plates, improving the stability of the hafnium plate preparation process, and having a high yield rate.

[0022] In a feasible implementation manner, a nuclear-grade hafnium sponge is melted three times using a vacuum electron beam melting furnace to form a hafnium ingot.

[0023] In this technical solution, the vacuum electron beam melting is carried out in a high-vacuum environment, which can effectively remove gas elements, non-metallic inclusions, and metal impurities in the hafnium sponge, ensuring the purity of the hafnium ingot; by melting the hafnium sponge multiple times, the hafnium sponge can be further purified, effectively reducing the oxygen element content in the hafnium ingot, and using a small amount of oxygen elements to form a non-metallic interstitial solid solution, significantly improving the strength of the hafnium ingot, so that the strength of the finally prepared hafnium plates can meet the nuclear grade design requirements; on the other hand, melting the hafnium sponge multiple times can also effectively control the content of interstitial impurity elements such as carbon, nitrogen, and hydrogen, thereby reducing the hardness of the hafnium ingot, reducing defects such as pores, porosity, and shrinkage cavities, and preparing for subsequent forging and rolling processes.

[0024] In a feasible implementation manner, the oxygen element content of the hafnium ingot is 100 ppm to 400 ppm.

[0025] In this technical solution, the oxygen content of the hafnium ingot prepared from nuclear-grade hafnium sponge by three times of vacuum electron beam melting is controlled at 100 ppm to 400 ppm. A small amount of oxygen elements form an interstitial solid solution in the hafnium ingot, playing a role of solid solution strengthening and improving the strength of the hafnium ingot, and further improving the strength of the finished hafnium plates; and causing the lattice of hafnium to distort, inhibiting the movement of atomic dislocations, to improve the tensile strength and corrosion resistance of the hafnium ingot, and further enabling the tensile strength and corrosion resistance of the finally prepared hafnium plates to meet the technical requirements of nuclear-grade hafnium plates; by controlling the oxygen content in the hafnium ingot through multiple times of vacuum electron beam melting, brittleness caused by excessive oxygen elements is prevented, and slab cracking and brittle fracture during subsequent rolling processes are prevented.

[0026] Furthermore, impurities in nuclear-grade hafnium sponge can be removed by three times of vacuum electron beam melting, so that other impurity elements in the hafnium ingot can also meet the technical requirements of nuclear-grade hafnium plates.

[0027] It should be noted that the oxygen content of the nuclear-grade hafnium plate prepared from nuclear-grade hafnium sponge is significantly higher than that of the hafnium plate prepared from crystal bar hafnium and lower than that of the hafnium plate after grain refinement. The interstitial solid solution strengthening effect of oxygen element is obvious, which significantly improves the strength and recrystallization temperature of the hafnium plate. However, due to the increase in the strength of the hafnium plate, its processing deformation resistance will also increase significantly, and it is difficult to process and form the plate into the target size. On the basis of the obvious increase in the strength and processing deformation resistance of the hafnium ingot, this application further optimizes the combination of processing deformation and heat treatment parameters such as processing temperature, deformation amount and annealing temperature, improves the tensile strength and welding strength of the hafnium material, reduces the cracking phenomenon in the subsequent preparation process of the hafnium plate, improves the tensile strength, welding strength and creep performance of the hafnium plate, and the finished product rate of the hafnium plate that meets the design technical requirements can reach more than 95% in the subsequent processing, and the process stability is significantly improved.

[0028] As Figure 2 shown, in a feasible implementation manner, the steps of forging a hafnium ingot into a hafnium slab blank include: Step 210: Insulate the hafnium ingot; by insulating the ingot after melting, the metal plasticity of the hafnium ingot is improved and the deformation resistance is reduced, thereby improving the forming ability in the forging process of the hafnium ingot and reducing the forging difficulty of the hafnium ingot; Step 220: First upset the hafnium ingot, then draw out the upset hafnium ingot, and then forge the hafnium ingot into a slab blank; by first upsetting the hafnium ingot, then drawing it out, and then forging it to the required thickness, the microstructure is improved, the uniformity of the hafnium ingot is increased, the anisotropy of the mechanical properties of the slab blank is reduced, the mechanical properties of the slab blank are improved, and the high-strength slab blank is prevented from cracking in the subsequent rolling process, which is beneficial to improving the finished product rate of the nuclear-grade hafnium plate; Step 230: Air-cool the forged slab blank to room temperature; the plasticity and toughness of the hafnium ingot are improved by forging, and the forging stress is reduced by air-cooling, the comprehensive mechanical properties of the slab blank are improved, the forging crack caused by too low temperature is avoided, and the process stability is improved; Step 240: Remove the surface oxide layer of the forged slab blank and grind the defective positions to make a hafnium slab blank; by surface-treating the forged part, the oxide layer generated by forging is removed, the surface quality of the forged blank is ensured, a clean surface is obtained, and the surface defects are prevented from expanding or affecting the performance of the final product in the subsequent processing.

[0029] In this technical solution, before multi-pass hot rolling, the hafnium ingot is first forged into a plate shape. After air cooling at room temperature, the surface defects of the slab are removed. The size and uniformity of the grains are controlled through plastic deformation during the forging process, thereby increasing the final hardness and strength of the slab. At the same time, the plasticity and toughness are improved, making the slab easier to be hot-rolled into shape without cracking, which is beneficial to improving the yield rate of the final nuclear-grade hafnium plates. Forging before hot rolling can eliminate defects such as pores and inclusions inside the slab, further improve the mechanical properties of the slab, enhance the fatigue strength and service life of the slab, and ensure that the final hafnium plate product meets the technical requirements of nuclear-grade hafnium plates.

[0030] As a preferred solution, first, the ingot is heated at a temperature of 1100°C to 1200°C for 60 min to 120 min, and then taken out of the furnace for forging. During forging, the ingot is first upset to 2 / 3 of the original height of the ingot, then the ingot is drawn out, and finally forged into a slab with a thickness of 50 mm to 60 mm. The final forging temperature is controlled to be greater than or equal to 750°C, and the forging ratio is controlled to be 2 to 4. After forging, it is air-cooled to room temperature. Finally, the surface oxide layer of the forged slab is removed by machining, and the defective positions are ground.

[0031] Furthermore, during the forging process, if the hardness of the ingot increases or the plasticity decreases, it is allowed to be reheated in the furnace to restore the plasticity of the material, facilitating the smooth progress of subsequent final forging forming.

[0032] In some examples, during the forging process, it is allowed to be reheated in the furnace to 1100°C to 1200°C and held for 20 min to 60 min.

[0033] As Figure 3 shown, in a feasible implementation, the hafnium slab is hot-rolled, and the hafnium slab is annealed to make a hot-rolled slab. The steps include: Step 310: Heat the hafnium slab to 800°C to 850°C and hold for 60 min to 120 min; Step 320: After the hafnium slab is taken out of the furnace and hot-rolled to a certain thickness, it is reheated in the furnace to 800°C to 850°C and held for 20 min to 60 min; Step 330: Once again, the hafnium slab is taken out of the furnace and hot-rolled to a thickness of 10 mm to 12 mm, and the cumulative hot-rolling deformation amount is controlled to be 75% to 85%; Step 340: Remove the surface oxide layer of the hot-rolled slab, and grind the defective positions; Step 350: Perform vacuum annealing treatment to make a hot-rolled slab.

[0034] In this technical solution, the hafnium slab after solution strengthening has a high work hardening rate and is prone to cracking. The hafnium slab is hot-rolled in two passes. After the first pass of hot rolling, it is reheated in the furnace once. After the second pass of hot rolling, annealing treatment is carried out, and heat preservation is carried out before each pass of hot rolling to ensure the hot rolling forming performance and ensure that the high-strength slab can be deformed smoothly; by controlling the cumulative hot rolling deformation amount within 75% - 85%, rolling cracking is avoided, and the yield rate of nuclear-grade hafnium plates is improved; the grains are broken by hot rolling with large deformation, making the structure of the hot-rolled slab more uniform; first, the oxide layer generated by hot rolling is removed and surface defects are eliminated for annealing treatment, forming a clean and bright surface while homogenizing the internal structure, and improving the surface finish of the hot-rolled slab.

[0035] It should be noted that the control of hot rolling temperature and holding time is the key factor affecting the grain size. If the hot rolling temperature and holding time are set improperly, the design of the hot rolling temperature and holding time in this application in combination with the large hot rolling deformation amount is to break the grains by hot rolling with large deformation in the follow-up, so as to prevent rolling cracking, rather than to prevent grain growth.

[0036] In a feasible implementation manner, intermediate annealing treatment is carried out on the cold-rolled sheet between two adjacent cold rolling passes.

[0037] In this technical solution, intermediate annealing treatment is carried out between two cold rolling passes. On the one hand, it is to eliminate work hardening, facilitate the next cold rolling, and ensure the cold rolling forming performance; on the other hand, it is to regulate the structure, further improve the uniformity of the structure, thereby improving the comprehensive performance of the hafnium sheet after cold rolling, and making the finished hafnium sheet meet the nuclear-grade design requirements.

[0038] As Figure 4 shown, in a feasible implementation manner, the steps of carrying out intermediate annealing treatment on the cold-rolled sheet between two adjacent cold rolling passes to make cold-rolled hafnium sheets include: Step 410: Carry out the first pass of cold rolling, and control the deformation amount of the first pass of cold rolling to be 20% - 29%; Step 420: Carry out intermediate vacuum annealing treatment at a temperature of 740°C - 810°C and hold for 60 min - 120 min; Step 430: Carry out the second pass of cold rolling, and control the deformation amount of the second pass of cold rolling to be 25% - 35% to make cold-rolled hafnium sheets.

[0039] In this technical solution, the hot-rolled hafnium sheet is cold-rolled in two passes, and an intermediate annealing is carried out between the two cold-rolling passes. Since the strength and the degree of cold work hardening of the sheet after solution strengthening with oxygen element are very high, the cold-rolling deformation amounts of the two passes are respectively controlled at 20% - 29% and 25% - 35%, and an annealing treatment is carried out in the middle to avoid the situation of immovable cold-rolled slab, ensure the smooth deformation of the slab, avoid cold-rolling cracking, and improve the process stability and the finished product rate.

[0040] It should be noted that on the basis of the obvious improvement of the strength of the hafnium ingot, by redesigning the preparation process and the hot-rolling temperature, hot-rolling deformation amount, cold-rolling passes, cold-rolling pass deformation amount, subsequent heat treatment temperature, the situations of cracking or unable to roll and deform in the subsequent processing of hafnium sheet can be effectively reduced, and the finished product rate of the final nuclear-grade hafnium sheet can be improved.

[0041] As Figure 5 shown, in a feasible implementation manner, the steps of performing finished product annealing treatment on the cold-rolled hafnium sheet include: Step 510: Install the cold-rolled hafnium sheet on the leveling device; Step 520: Put the leveling device and the cold-rolled hafnium sheet into a vacuum heat treatment furnace together, and carry out annealing treatment at a temperature of 720°C - 800°C and a vacuum degree of 5×10 -3 Pa, and keep the temperature for 60 min - 120 min.

[0042] In this technical solution, the cold-rolled hafnium sheet is further subjected to finished product annealing treatment. The cold-rolled hafnium sheet is clamped by the leveling device and annealed to eliminate the stress and work hardening generated by cold rolling, release the stress in the hafnium sheet, make the hafnium sheet flat under the pressure of the leveling device, and obtain a uniform recovery and recrystallization structure, restore the plasticity and toughness of the sheet, adjust the grain structure and mechanical properties of the sheet, make the surface of the sheet flat and smooth, improve the surface quality, and improve the corrosion resistance to meet the application requirements of the nuclear-grade hafnium sheet.

[0043] As Figure 6 shown, in a feasible implementation manner, the steps of finishing the hafnium sheet to make a finished product hafnium sheet include: Step 610: Grind and polish the hafnium sheet to remove 0.2 mm - 0.5 mm on one side; Step 620: Remove the scratches and defects on the hafnium sheet to obtain a finished product hafnium sheet with a bright surface and no scratches.

[0044] In this technical solution, after multi-pass cold rolling, the hafnium sheet is finished to ensure the standardization of the hafnium sheet, which is convenient for subsequent unified processing and forming.

[0045] Further, before surface grinding and polishing, the hafnium plates need to be leveled to ensure the flatness of the surface of the hafnium plates after grinding and polishing.

[0046] Further, the final finished hafnium plates also need to undergo ultrasonic flaw detection and performance testing, and can be used to prepare nuclear equipment such as neutron absorbers for control rod assemblies only after passing the tests.

[0047] Example 1 Step 100': Ingot preparation Using nuclear-grade hafnium sponge as the raw material, hafnium is smelted three times on a vacuum electron beam melting furnace to prepare a finished nuclear-grade hafnium ingot, the impurity elements of which all meet the technical requirements, and the oxygen content is controlled at 260 ppm.

[0048] Step 200': Forging Heat the hafnium ingot to 1150 °C, hold for 90 min and then take it out of the furnace. Use a 3t forging hammer to upset and draw out the ingot to a slab with dimensions of about δ (thickness) 52 mm × W (width) 200 mm × L (length) (thickness of 52 mm, width of 200 mm, any length). During the forging process, heat it in the furnace once, control the reheating temperature at 1150 °C, hold for 30 min, and then perform final forging at a temperature greater than or equal to 750 °C; after the forging blank is cooled, grind the surface of the forging blank to remove the oxide scale.

[0049] Step 300': Hot rolling and annealing Use a resistance furnace to heat the forged slab to 820 °C, hold for 90 min, and then transfer it to a 1300t four-high hot rolling mill for hot rolling. After multiple repeated hot rollings in one pass to δ18 mm × W200 mm × L, heat it back to 820 °C, hold for 40 min, take it out of the furnace and perform multiple repeated hot rollings in the second pass to δ10.4 mm × W200 mm × L, and control the cumulative hot rolling deformation amount at 80%. Grind the surface of the hot-rolled hafnium plate to remove the surface oxide scale, then heat it to 790 °C, hold for 120 min, and perform vacuum annealing treatment.

[0050] Step 400': Cold rolling and intermediate annealing After annealing the hot-rolled hafnium plates, carry out 2 passes of cold rolling. After multiple repeated cold rollings in one pass to δ7.4 mm × W200 mm × L, control the cumulative cold rolling deformation amount in the first pass at 29%. The surface quality after the first pass of cold rolling is good and there is no cracking phenomenon. After degreasing and cleaning and vacuum annealing at a temperature of 790 °C for 120 min, then perform finished cold rolling. After multiple repeated cold rollings to the target thickness, control the cumulative cold rolling deformation amount in the second pass at 26%. The surface quality of the finished cold-rolled plate is good and there are no cracks and defects visible to the naked eye.

[0051] Step 500': Finished annealing After degreasing and cleaning, the finished cold-rolled hafnium sheet is subjected to vacuum annealing at 780 °C for 120 min. After the annealing treatment, a uniform equiaxed crystal structure is obtained. When annealing the finished product, a special fixture is used to clamp the hafnium sheet to ensure the flatness of the sheet after annealing.

[0052] Step 600': Finishing The hafnium sheet after finished annealing is subjected to surface polishing and leveling treatment to obtain the finished hafnium sheet. The unilateral removal amount of surface polishing is about 0.25 mm. The surface of the hafnium sheet after finishing is bright and free of defects such as scratches. The final finished hafnium sheet passes ultrasonic flaw detection and performance testing.

[0053] Example 2 The difference between this example and Example 1 is that the oxygen content of the hafnium ingot prepared in Example 2 and the process parameters of each step are different, as follows: Step 100': Ingot preparation Using nuclear-grade sponge hafnium as raw material, hafnium is melted three times on a vacuum electron beam melting furnace to prepare a finished nuclear-grade hafnium ingot. Its impurity elements all meet the technical requirements, and the oxygen content is controlled at 290 ppm.

[0054] Step 200': Forging The hafnium ingot is heated to 1200 °C, held for 60 min and then taken out of the furnace. A 3t forging hammer is used to upset and draw out the ingot to a slab with dimensions of about δ60 mm × W160 mm × L. During the forging process, it is reheated in the furnace once. The reheating temperature is controlled at 1200 °C and the reheating holding time is 20 min. Then finish forging at a temperature greater than or equal to 750 °C. After the forging blank is cooled, the surface of the forging blank is ground to remove the oxide scale.

[0055] Step 300': Hot rolling and annealing The forged slab is heated to 850 °C in a resistance furnace, held for 60 min, and then transferred to a 1300t four-high hot rolling mill for hot rolling. It is repeatedly hot rolled multiple times in one pass to δ22 mm × W160 mm × L, then reheated to 850 °C, held for 30 min, taken out of the furnace and repeatedly hot rolled multiple times in the second pass to δ11 mm × W160 mm × L. The cumulative hot rolling deformation amount is controlled at 82%. The surface of the hot-rolled hafnium plate is ground to remove the surface oxide scale, then heated to 800 °C, held for 60 min, and subjected to vacuum annealing treatment.

[0056] Step 400': Cold rolling and intermediate annealing After annealing the hot-rolled hafnium sheet, two passes of cold rolling are carried out. In the first pass, it is repeatedly cold-rolled many times to δ8.05mm×W160mm×L. The cumulative deformation of the first pass of cold rolling is controlled at 27%. After the first pass of cold rolling, the surface quality is good and there is no cracking. After degreasing, cleaning, and vacuum annealing at 800°C for 60 minutes, finish cold rolling is carried out. It is repeatedly cold-rolled many times to the target thickness, and the cumulative deformation of the second cold rolling is controlled at 27%. The surface quality of the finish cold-rolled sheet is good and there are no cracks or defects visible to the naked eye.

[0057] Step 500’: Finish annealing After degreasing and cleaning, the finish cold-rolled hafnium sheet is subjected to vacuum annealing at 790°C for 60 minutes. A uniform equiaxed crystal structure is obtained after the annealing treatment. When performing finish annealing, a special fixture is used to clamp the hafnium sheet to ensure the flatness of the sheet after annealing.

[0058] Step 600’: Finishing The hafnium sheet after finish annealing is processed by surface polishing and leveling to obtain the finished hafnium sheet. The unilateral removal amount of surface polishing is about 0.25mm. The surface of the finished hafnium sheet after finishing is bright and there are no defects such as scratches. The final finished hafnium sheet passes ultrasonic flaw detection and performance testing.

[0059] Example 3 The difference between this example and Example 1 is that the oxygen content of the hafnium ingot prepared in Example 3 and the process parameters of each step are different, as follows: Step 100’: Ingot preparation Using nuclear-grade hafnium sponge as the raw material, hafnium is melted three times on a vacuum electron beam melting furnace to prepare a finished nuclear-grade hafnium ingot. Its impurity elements all meet the technical requirements, and the oxygen content is controlled at 220ppm.

[0060] Step 200’: Forging The hafnium ingot is heated to 1100°C, held for 120 minutes and then taken out of the furnace. A 3t forging hammer is used to upset and draw the ingot into a slab with dimensions of about δ50mm×W200mm×L. The ingot is reheated once during the forging process. The reheating temperature is controlled at 1100°C and the reheating holding time is 60 minutes. Then finish forging is carried out at a temperature greater than or equal to 750°C; after the forging blank is cooled, the surface of the forging blank is ground to remove the oxide scale.

[0061] Step 300’: Hot rolling and annealing The forged slab is heated to 800°C in a resistance furnace and held for 120 minutes. It is then transferred to a 1300t four-roll hot rolling mill for hot rolling. The first pass is repeatedly hot-rolled to δ24mm×W200mm×L, then returned to the furnace and heated to 800°C, held for 60 minutes, and then repeatedly hot-rolled to δ12mm×W160mm×L in the second pass. The cumulative deformation of hot rolling is controlled at 76%. The surface of the hot-rolled hafnium plate is ground to remove the surface oxide scale, then heated to 780°C, held for 120 minutes, and vacuum annealed.

[0062] Step 400': Cold rolling and intermediate annealing After annealing, the hot-rolled hafnium plate undergoes two cold-rolling passes. Each pass is repeatedly cold-rolled to δ8.5mm×W200mm×L. The cumulative deformation of the first cold-rolling pass is controlled at 29%. The surface quality after the first cold-rolling pass is good, with no cracking. After degreasing and cleaning and vacuum annealing at 780°C for 120 minutes, the finished product is cold-rolled. Repeated cold-rolling is performed to the target thickness, with the cumulative deformation of the second cold-rolling pass controlled at 35%. The finished cold-rolled plate has a good surface quality, with no visible cracks or defects.

[0063] Step 500': Finished product annealing After degreasing and cleaning, the finished cold-rolled hafnium sheet undergoes a vacuum annealing treatment at 760°C for 120 minutes. This annealing treatment results in a uniform equiaxed grain structure. During the annealing process, the hafnium sheet is clamped in a special fixture to ensure smoothness after annealing.

[0064] Step 600': Finishing After annealing, the hafnium sheets are polished and leveled to obtain finished hafnium sheets. The surface removal per side of the polishing process is approximately 0.25mm. The finished hafnium sheets have a bright surface and are free of scratches or other defects. The final hafnium sheets pass ultrasonic testing and performance testing.

[0065] The finished plate produced in Example 1 was subjected to room / high-temperature tensile testing, welding testing, and corrosion testing, as required for control rod production and use. All performance characteristics met the technical requirements for nuclear hafnium plate. Furthermore, multiple production runs revealed minimal cracking in the hafnium plate, with a yield rate of approximately 97%.

[0066] Comparative Example 1 Step 1: Ingot preparation Nuclear-grade hafnium sponge is used as raw material, and hafnium is smelted three times in a vacuum electron beam melting furnace to produce finished nuclear-grade hafnium ingots. The impurity elements meet the technical requirements and the oxygen content is controlled to 340ppm.

[0067] Step 2: Forging Heat the hafnium ingot to 1150 °C, hold for 90 min and then take it out of the furnace. Use a 3t forging hammer to upset and draw out the ingot to a slab with a thickness (δ) of 52 mm. During forging, heat it in the furnace once, control the reheat temperature at 1150 °C, hold for 30 min, and then perform final forging at a temperature greater than or equal to 750 °C; after the forged blank cools, grind the surface of the forged blank to remove the oxide scale.

[0068] Step 3: Hot rolling and annealing Use an electric resistance furnace to heat the forged slab to 820 °C, hold for 90 min, and then transfer it to a 1300t four-high hot rolling mill for multiple repeated hot rolling. During hot rolling, heat it in the furnace and hold for one time, the heating temperature is 820 °C, and the holding time is 40 min. The cumulative hot rolling deformation is controlled at 80%. Grind the surface of the hot-rolled hafnium plate to remove the surface oxide scale. Perform vacuum annealing treatment at 790 °C for 120 min.

[0069] Step 4: Cold rolling After annealing the hot-rolled hafnium plate, carry out cold rolling. After multiple passes of cold rolling without intermediate annealing, the cumulative deformation is 36%. After cold rolling, the hafnium plate cracks from the long side position, the cracking length is about 2 / 5 of the total length, and the width is about 1 / 3 of the width, making it impossible to carry out subsequent processing and testing, so it is scrapped.

[0070] Comparative example 2 Step 1: Ingot preparation Prepare a nuclear-grade hafnium ingot by vacuum electron beam melting, control the oxygen content to be less than 100 ppm, and other impurity elements all meet the technical requirements.

[0071] Step 2: Forging Heat the hafnium ingot to 1100 °C, hold for 60 min and then take it out of the furnace for forging to δ (thickness) 55 mm, air-cool to room temperature, and grind the surface of the forged blank to remove the oxide scale.

[0072] Step 3: Hot rolling and annealing Heat the forged blank to 800 °C, hold for 90 min, and then transfer it to a four-high hot rolling mill for one-pass multiple repeated hot rolling. During hot rolling, do not heat it in the furnace and hold, the cumulative hot rolling deformation is 85%. Grind the surface of the hot-rolled hafnium plate to remove the surface oxide scale. Perform vacuum annealing treatment at 740 °C for 60 min.

[0073] Step 4: Cold rolling For the annealed hot-rolled hafnium plate, carry out cold rolling. After multiple passes of cold rolling without intermediate annealing, the cumulative deformation is 37%. After cold rolling, the surface quality is good, and there are no cracks and defects visible to the naked eye.

[0074] Step 5: Final annealing After degreasing and cleaning, the finished cold-rolled hafnium sheet is subjected to vacuum annealing at 730 °C for 60 min.

[0075] Step 6: Finishing The annealed hafnium sheet is subjected to surface polishing and leveling to obtain the finished hafnium sheet. The surface of the finished hafnium sheet is bright and free of defects such as scratches. According to the production and use requirements of control rods, the finished hafnium sheet is subjected to room / high-temperature tensile property tests, welding property tests, and corrosion property tests, and the results are shown in Tables 1-3.

[0076] Comparative Example 3 Step 1: Ingot preparation Using hafnium sponge as the raw material, it is melted twice on a vacuum electron beam melting furnace to prepare a finished nuclear-grade hafnium ingot, and its impurity elements all meet the technical requirements, and the oxygen content is not controlled.

[0077] Step 2: Forging The hafnium ingot is heated to 1080 °C and kept warm for 90 min, then taken out of the furnace and forged to δ (thickness) 45 mm. It is reheated in the furnace once, the reheating temperature is 1080 °C, and the holding time is 40 min. After forging, it is air-cooled to room temperature, and the surface of the forged blank is ground to remove the oxide scale.

[0078] Step 3: Hot rolling and annealing The forged blank is heated to 820 °C and kept warm for 60 min, then transferred to a four-high hot rolling mill for one-pass multiple repeated hot rolling. The hot rolling process does not involve reheating and holding in the furnace, and the cumulative deformation amount of hot rolling is 60%. The surface of the hot-rolled hafnium sheet is ground to remove the surface oxide scale. It is subjected to vacuum annealing at 800 °C for 90 min.

[0079] Step 4: Cold rolling The annealed hot-rolled hafnium sheet is cold-rolled. After multiple passes of cold rolling without annealing in the middle, the cumulative deformation amount is 40%. The surface quality after cold rolling is good, without cracks and defects visible to the naked eye.

[0080] Step 5: Finished product annealing After degreasing and cleaning, the finished cold-rolled hafnium sheet is subjected to vacuum annealing at 680 °C for 240 min.

[0081] Step 6: Finishing The annealed hafnium sheet is subjected to surface polishing and leveling to obtain the finished hafnium sheet. The surface of the finished hafnium sheet is bright and free of defects such as scratches. According to the production and use requirements of control rods, the finished hafnium sheet is subjected to room / high-temperature tensile property tests, welding property tests, and corrosion property tests, and the results are shown in Tables 1-3.

[0082] Table 1 Tensile test results of the sheet at room temperature and high temperature

[0083] Table 2 Room temperature tensile test results of the weld seam

[0084] Table 3 Corrosion performance test results

[0085] It can be seen from Comparative Example 1 that for hafnium plates within the oxygen content range of this technology, the cumulative cold rolling deformation amount in one pass is higher than 30%. Without intermediate annealing, obvious cracking will occur during cold rolling, and it is impossible to continue preparing finished nuclear-grade hafnium plates that meet the technical requirements.

[0086] The tensile properties of hafnium plates at room temperature and high temperature are shown in Table 1, the room temperature tensile properties of the weld seam are shown in Table 2, and the corrosion properties are shown in Table 3. It can be seen from Table 1 that for the finished hafnium plates prepared by the method for preparing nuclear-grade hafnium plates of this application, the transverse and longitudinal tensile strengths of the hafnium plates at room temperature can both be maintained at about 570 MPa, and the tensile strength at 350 °C is about 370 MPa, showing a significant improvement compared with the strength of hafnium plates in the prior art; the transverse and longitudinal yield strengths of the hafnium plates at room temperature both exceed 330 MPa, and the transverse yield strength is close to 500 MPa; the yield strength at 350 °C exceeds 260 MPa. It can be seen from Table 2 that the minimum tensile strength of the welded weld seam of the finished hafnium plates prepared by the method for preparing nuclear-grade hafnium plates of this application at room temperature can reach 480 MPa, and the yield strength at room temperature can be maintained at about 400 MPa, showing a significant improvement in the yield strength of the weld seam. It can be seen from Table 3 that the corrosion weight gain of the finished hafnium plates prepared by the method for preparing nuclear-grade hafnium plates of this application is 50% of that of hafnium plates prepared by the existing hafnium plate preparation method, with a slow corrosion rate and significantly improved corrosion resistance. On the basis of the significant improvement in tensile strength and yield strength, other properties of the finished hafnium plates are also improved, and all meet the design requirements of nuclear-grade hafnium plates.

[0087] According to the second aspect of this application, a nuclear-grade hafnium plate is proposed, which is prepared by the method for preparing nuclear-grade hafnium plates according to any one of the above technical solutions.

[0088] The nuclear-grade hafnium plate provided by the embodiment of this application is prepared by the method for preparing nuclear-grade hafnium plates according to any one of the above technical solutions, so the nuclear-grade hafnium plate has all the beneficial effects of the method for preparing nuclear-grade hafnium plates of the above technical solutions.

[0089] The hafnium plates provided by the embodiments of the present application are prepared from hafnium sponge into nuclear-grade hafnium plates by means of a process of forging - hot rolling - cold rolling - intermediate annealing - cold rolling - final annealing; through intermediate annealing treatment during the cold rolling process, the work hardening caused by the cold rolling treatment is eliminated, cold rolling cracking is prevented, the process stability is improved, multi-pass cold rolling without cracking is ensured, the manufacturing and processing problems of high-strength hafnium plates are solved, and the microstructure of the hafnium plates is uniform and the texture is reasonable, improving the creep performance and welding performance of the finished hafnium plates, enabling the hafnium plates to have higher room / high-temperature strength and elongation, and enabling the various properties of the hafnium plates to meet the design requirements of the nuclear grade; using nuclear-grade hafnium sponge as the raw material for the preparation of hafnium plates, through the redesign and optimization of the technological steps of the hafnium plates, high-strength hafnium plates that can meet the nuclear grade requirements are prepared, and the comprehensive performance of the hafnium plates is significantly improved, that is, while further improving the strength of the hafnium plates, the comprehensive performance of the hafnium plates is improved, the stability of the hafnium plate preparation process is improved, and the yield rate is high.

[0090] It is easy for those skilled in the art to understand that, on the premise of no conflict, the above advantageous methods can be freely combined and superimposed.

[0091] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present application shall be included within the protection scope of the present application. The above is only the preferred implementation manner of the present application. It should be noted that for those of ordinary skill in the art in this technical field, several improvements and variations can be made without departing from the technical principle of the present application, and these improvements and variations should also be regarded as the protection scope of the present application.

Claims

1. A method for preparing nuclear-grade hafnium plates, characterized in that, The method for preparing nuclear-grade hafnium plates includes: Melting nuclear-grade hafnium sponge to make a hafnium ingot; Forging the hafnium ingot to make a hafnium slab; Hot-rolling the hafnium slab and annealing the hafnium slab to make a hot-rolled slab; Cold-rolling the hot-rolled slab in multiple passes and performing intermediate annealing on the cold-rolled sheet to make cold-rolled hafnium plates; Performing finish annealing on the cold-rolled hafnium plates; Finishing the hafnium plates to make finished hafnium plates.

2. The method for preparing a nuclear-grade hafnium plate according to claim 1, wherein The nuclear-grade hafnium sponge is melted three times using a vacuum electron beam melting furnace to make the hafnium ingot.

3. The method for preparing a nuclear-grade hafnium plate according to claim 2, wherein The oxygen element content of the hafnium ingot is 100 ppm to 400 ppm.

4. A method for preparing a nuclear-grade hafnium sheet according to claim 1, characterized in that, The step of forging the hafnium ingot to make a hafnium slab includes: Heating the hafnium ingot to 1100°C to 1200°C and holding for 60 min to 120 min; First upsetting the hafnium ingot, then drawing out the upset hafnium ingot, and then forging the hafnium ingot into a slab. During the forging process, selectively reheating in the furnace, the reheating temperature is 1100°C to 1200°C, the reheating holding time is 20 min to 60 min, and the final forging temperature is ≥750°C; Air-cooling the forged slab to room temperature; Removing the surface oxide layer of the forged slab and grinding the defective positions to make the hafnium slab.

5. A method for preparing a nuclear-grade hafnium sheet according to claim 1, characterized in that, The steps of hot-rolling the hafnium slab and annealing the hafnium slab to make a hot-rolled slab include: Heating the hafnium slab to 800°C to 850°C and holding for 60 min to 120 min; After the hafnium slab is taken out of the furnace and hot-rolled to a certain thickness, reheating it to 800°C to 850°C and holding for 20 min to 60 min; Taking the hafnium slab out of the furnace again and hot-rolling it to a thickness of 10 mm to 12 mm, and controlling the cumulative hot-rolling deformation amount to be 75% to 85%; Removing the surface oxide layer of the hot-rolled slab and grinding the defective positions; Performing vacuum annealing treatment to make the hot-rolled slab.

6. The method for preparing a nuclear-grade hafnium plate according to claim 1, wherein Performing intermediate annealing on the cold-rolled sheet between adjacent cold-rolling passes.

7. A method for preparing nuclear-grade hafnium plates according to claim 6, characterized in that, The steps of performing intermediate annealing on the cold-rolled sheet between adjacent cold-rolling passes include: Performing the first cold-rolling pass and controlling the deformation amount of the first cold-rolling pass to be 20% to 29%; Performing intermediate vacuum annealing treatment at a temperature of 740°C to 810°C and holding for 60 min to 120 min; Performing the second cold-rolling pass and controlling the deformation amount of the second cold-rolling pass to be 25% to 35% to make the cold-rolled hafnium plates.

8. A method for preparing a nuclear-grade hafnium sheet according to claim 1, characterized in that, The steps of performing finish annealing on the cold-rolled hafnium plates include: Installing the cold-rolled hafnium plates on a leveling device; Put the leveling device and the cold-rolled hafnium sheet into a vacuum heat treatment furnace, and perform annealing treatment at a temperature of 720°C to 800°C and a vacuum degree of 5×10 -3 Pa for 60 min to 120 min.

9. A method for preparing nuclear-grade hafnium plates according to claim 1, characterized in that, The steps of finishing the hafnium plates to make finished hafnium plates include: Grinding and polishing the hafnium plates so that the unilateral removal amount is 0.2 mm to 0.5 mm; Removing the scratches and defects of the hafnium plates to obtain the finished hafnium plates with a bright surface and no scratches.

10. A nuclear-grade hafnium sheet, characterized in that, The nuclear-grade hafnium sheet is prepared by the method for preparing a nuclear-grade hafnium sheet according to any one of claims 1 to 9.

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