Processing method for stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power

Abstract

The present invention discloses a processing method for a stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power plants, which comprises the following steps: Step 1, refining a TP321 stainless steel raw material ingot; Step 2, forging and blanking: adopting a process route of two-fire forging to control the temperature and deformation amount; Step 3, piercing and extending to produce a rough pipe; Step 4, cold processing and forming to obtain a steel pipe; Step 5, performing degreasing, grinding and shot peening surface treatment on the steel pipe to obtain a finished product. The stainless steel seamless steel pipe prepared by the present invention can meet the technical requirements of the superheated steam pipeline of a nuclear power high-temperature gas-cooled reactor and can provide a safe steel pipe for nuclear power.

Description

Technical Field

[0001] The present invention relates to the technical field of steel pipe forging, and particularly to a processing method for a stainless steel seamless pipe for high-temperature and high-pressure superheated steam in nuclear power plants. Background Art

[0002] Nuclear fission hides a large amount of energy. The energy generated by the fission of 1 kg of U235 is equivalent to the heat generated by the combustion of 2000 tons of coal and there is no carbon emission. Therefore, nuclear power is an important means for China to adjust its energy structure and reduce carbon emissions at present. It is the only way for China's energy strategy and development. Nuclear safety is an important issue that the world's countries are most concerned about and need to solve. Shidao Bay Nuclear Power Plant is a high-temperature gas-cooled reactor nuclear power plant independently developed and built by China. It is the world's first to adopt advanced high-temperature gas-cooled technology and is one of the preferred reactor types of the six fourth-generation nuclear energy systems in the current international nuclear energy field. It has extremely strong adaptability. Even in the face of the most dangerous emergencies such as earthquakes, the nuclear power reactor will not be melted down, and there will be no large amount of radioactive substances leaking out. It is even called an indestructible nuclear reactor by the whole world. However, the high-temperature gas-cooled reactor has a high outlet temperature and high pressure, and can generate high-temperature and high-pressure superheated steam of 19.0 MPa and 535 °C. The thermal efficiency can reach 40%, and it can provide high-temperature process gas of about 900 - 950 °C. Therefore, there are very high requirements for the conveying pipeline. Therefore, under the specific technical requirements and specifications of nuclear power plants, how to forge a steam conveying pipeline that can be used in conjunction with a superheated steam generator has become a technical problem that the applicant urgently needs to solve. Summary of the Invention

[0003] The purpose of the present invention is to provide a processing method for a stainless steel seamless pipe for high-temperature and high-pressure superheated steam in nuclear power plants. The stainless steel seamless pipe prepared by the present invention can meet the technical requirements of the superheated steam pipeline of the high-temperature gas-cooled reactor in nuclear power plants and can provide safe steel pipes for nuclear power.

[0004] The technical solution of the present invention: A processing method for a stainless steel seamless pipe for high-temperature and high-pressure superheated steam in nuclear power plants includes the following steps:

[0005] Step 1: Refine the TP321 stainless steel raw material ingot so that the non-metallic inclusions in the steel meet the following requirements: A ≤ 1.0 grade, B ≤ 1.5 grades, C ≤ 1.5 grades, D ≤ 1.5 grades, and A + B + C + D ≤ 4.5 grades;

[0006] Step 2: Forging and blanking: Adopt a two-fire forging process route with temperature control and deformation amount control. The initial forging temperature is 50 - 70 °C lower than that of non-vacuum smelted steel, the final forging temperature > 800 °C, the heating time is 0.8 mm / min, forging into a square shape in the first fire and sizing into a round shape in the second fire, and the deformation amount each time is not more than 30%;

[0007] Step 3: Piercing and extending to produce a rough tube: A round steel with an outer diameter of 616 mm is pierced once to obtain a billet with an outer diameter of 635 mm and a wall thickness of 100 mm. It is heated at a temperature 50 degrees below the overheating temperature, pierced at a heating rate of 1 mm / min, and then water quenched. Then, it is rolled and extended a second time to roll the billet into a rough tube with an outer diameter of 695 mm and a wall thickness of 43 mm. The heating temperature is a temperature 70 degrees below the overheating temperature, the heating rate is 1.2 mm / min, and then water quenched again.

[0008] Step 4: Cold working forming: The rough tube with an outer diameter of 695 mm and a wall thickness of 43 mm is cold-expanded to an outer diameter of 775 mm, then cold-drawn to a steel tube with an outer diameter of 762 mm and a wall thickness of 34 mm. Then, solution heat treatment is carried out for reinforcement. The intermediate heat treatment temperature is 1120 °C, the holding time is 70 min, the final heat treatment temperature is 1100 °C, and the holding time is 70 min to obtain the steel tube.

[0009] Step 4: The steel tube is subjected to surface treatment such as degreasing, grinding, and shot peening to obtain the finished product.

[0010] In the above processing method of the stainless steel seamless steel tube for nuclear power high-temperature and high-pressure superheated steam, in Step 1, the refining includes the control of nuclear harmful elements. The processes are in sequence: three-removal hot metal, intermediate frequency furnace pre-melting treatment, AOD treatment, RH vacuum treatment, and ingot casting treatment, so that the dehydrogenation rate of the TP321 stainless steel raw material ingot reaches more than 80%, the harmful metal elements of P, V, and Cu are reduced by more than 60%, and the reduction rate of Co element reaches more than 93%.

[0011] In the aforementioned processing method of the stainless steel seamless steel tube for nuclear power high-temperature and high-pressure superheated steam, in Step 1, the refining includes smelting with a vacuum induction furnace and vacuum casting. The required vacuum degree is 0.05 - 100 Pa; after the furnace charge is melted, the molten metal is kept boiling under vacuum for 5 - 30 min. The oxygen content is reduced through the vacuum carbon oxidation reaction, so that the total oxygen content in the steel is reduced to less than 15 ppm, the inclusions in the molten steel are reduced, and the size of the inclusions in the molten steel is controlled to be less than 15 μm.

[0012] In the aforementioned processing method of the stainless steel seamless steel tube for nuclear power high-temperature and high-pressure superheated steam, during the RH vacuum treatment, dehydrogenation is accompanied by an oxygen lance accurately used for decarburization and temperature increase, so that the carbon content in the smelting process is accurately controlled.

[0013] In the aforementioned processing method of the stainless steel seamless steel tube for nuclear power high-temperature and high-pressure superheated steam, in Step 1, the refining includes secondary smelting of ingots and electroslag remelting, using the resistance heat generated when the current passes through the slag as the heat source for secondary melting, improving the strength of TP321 stainless steel, forming a protective layer on the surface of the ingot at the same time, and obtaining large-sized ingots.

[0014] In the aforementioned processing method of stainless steel seamless steel pipe for nuclear power high-temperature and high-pressure superheated steam, in step 3, a Ф720 perforating unit is used for perforation treatment, the roller shape is conical, the rolling angle β=15°, the advancing angle α=12°, and the rotation speed is 12r / min.

[0015] In the aforementioned processing method of stainless steel seamless steel pipe for nuclear power high-temperature and high-pressure superheated steam, in step 4, the degreasing treatment is to mix a degreasing agent with water in a ratio of 1:2 in a degreasing cylinder, heat the temperature to 70°C, soak the steel pipe for 40-60 minutes, and rinse it with clean water after leaving the cylinder.

[0016] In the aforementioned method for processing stainless steel seamless pipes for nuclear power high-temperature and high-pressure superheated steam, in step 4, the cold working forming adopts cold drawing deformation to reduce the work hardening caused by dislocation, avoid the deformation martensitic structure transformation, and produce a sudden increase in internal stress.

[0017] In the aforementioned method for processing stainless steel seamless pipes for nuclear power high-temperature and high-pressure superheated steam, in step 5, the shot peening uses mixed shots, stainless steel shots and cast steel shots in a ratio of 1:2, and the shot size is 0.7-1.2 mm in diameter.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The present invention refines the raw materials of TP321 stainless steel seamless pipes, and through ultra-low hydrogen content, deep deoxidation, inclusion content control, and nuclear harmful elements control, the prepared TP321 stainless steel seamless pipes become the basic conditions for realizing high-quality nuclear power stainless steel production.

[0020] 2. The TP321 stainless steel seamless pipe forged by the present invention can be used in a high temperature environment of 650°C, and has sufficient creep strength, endurance strength and high toughness. The present invention is based on the high temperature endurance strength limit as the main basis, and then uses the creep limit to check the superheated steam pipeline strength calculation, which can ensure safe operation under creep conditions, and on the other hand, can avoid difficulties in processing and operation caused by excessively thick pipe walls.

[0021] 3. The TP321 stainless steel seamless pipe forged by the present invention has good performance in terms of pipe wall thickness full thickness organization, performance uniformity and stability. The present invention can minimize the possibility of grain change, reduce the change in grain size and uniformity due to its change, avoid the phenomenon of unqualified grain size or "mixed crystal" that is easy to occur in the production process, and the grain size of any position in all wall thickness ranges of the forged TP321 stainless steel seamless pipe is controlled within 4-7 levels, and the level difference does not exceed 2 levels.

[0022] 4. The present invention uses an efficient alkaline degreasing agent, reasonably controls the use temperature and time to meet the requirements for the surface purity of the pipeline before heat treatment, and at the same time, in combination with the actual situation on site, adopts a surface treatment technology of surface grinding + shot peening to achieve the purpose of eliminating residual stress. Detailed implementation mode

[0023] The following further illustrates the present invention in conjunction with embodiments, but it shall not be used as a basis for limiting the present invention.

[0024] Embodiment: A processing method for a stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power plants includes the following steps: Step 1: Refine the TP321 stainless steel raw material ingot so that the non-metallic inclusions in the steel meet the following requirements: A≤1.0 level, B≤1.5 levels, C≤1.5 levels, D≤1.5 levels, and A + B + C + D≤4.5 levels, where A type (sulfide), B type (aluminum oxide type); C type (silicate type), D type (spherical oxide); the mass fraction of its raw material components meets the following requirements: 0.08% > C > 0.04%, S < 0.015%, P < 0.025%, 12% > Ni ≥ 10%, 19% > Cr > 17.5%, 0.7% > Ti > 5×(C + N) maximum mass fraction of 0.7%, N≤0.1%, Co < 0.025%, Cu < 0.05%;

[0025] Step 2: Forging and blanking: Adopt a two-fire forging process route to control the temperature and deformation amount. The starting forging temperature is 50 - 70°C lower than that of non-vacuum smelted steel, the final forging temperature > 800°C, the heating time is 0.8mm / min, forging into a square shape in the first fire and sizing into a round shape in the second fire, and the deformation amount each time is not more than 30%;

[0026] Step 3: Piercing and extending to make a rough pipe: A round steel with an outer diameter of 616mm is pierced once to obtain a rough pipe with an outer diameter of 635mm and a wall thickness of 100mm, heated at a temperature 50 degrees lower than the overheating temperature, and pierced at a heating speed of 1mm / min, and then water quenched; then the rough pipe is rolled into a rough pipe with an outer diameter of 695mm and a wall thickness of 43mm by secondary extension rolling, the heating temperature is a temperature 70 degrees lower than the overheating temperature, the heating speed is 1.2mm / min, and then water quenched;

[0027] Step 4: Cold processing and forming: The rough pipe with an outer diameter of 695mm and a wall thickness of 43mm is cold-expanded to an outer diameter of 775mm, and then cold-drawn to a steel pipe with an outer diameter of 762mm and a wall thickness of 34mm, and then solution heat treatment is strengthened. The intermediate heat treatment temperature is 1120°C, the holding time is 60 - 80min, the final heat treatment temperature is 1100°C, and the holding time is 60 - 80min to obtain the steel pipe;

[0028] Step 4: Degrease, grind and shot peen the surface of the steel pipe to obtain the finished product.

[0029] Specifically, the refining of the TP321 stainless steel raw material ingot in Step 1 needs to comply with the nuclear requirement steel principle and can be applied to the superheated steam pipeline of the high-temperature gas-cooled reactor. The control requirements for ultra-low hydrogen, oxygen, nuclear harmful elements, inclusions, and high-temperature creep involved in its specification content. In this embodiment, good control has been achieved in three aspects in the smelting, especially in the refining technology:

[0030] (1) Developed a refining process of "three-decarburized hot metal + medium-frequency furnace pre-solution → AOD treatment → RH vacuum treatment → ingot casting treatment". The main feature of this process route is the control of nuclear harmful elements. Ordinary austenitic stainless steel is smelted with scrap steel as raw materials, and the Co≈0.25%, V≈0.15%, Cu≈0.10%, P≈0.045% in the steel are much higher than the specified values of TP321 for the superheated steam pipeline of the high-temperature gas-cooled reactor. After use, a breakthrough in the control of stainless steel purity has been achieved, reducing the harmful metal elements of P, V, Cu, and Co in the stainless steel by more than 60%, and the reduction rate of Co element can reach 93%. To meet the high requirements of TP321 for the superheated steam pipeline of the high-temperature gas-cooled reactor on the oxygen content and inclusions in the steel, a refining technology for ultra-low oxygen and fine and dispersed inclusions of stainless steel has been developed. Vacuum induction furnace smelting and vacuum casting are used, and the required vacuum degree is 0.05 - 100 Pa; after the furnace charge melts, the molten metal is kept boiling under vacuum for 5 - 30 min, and the oxygen content is reduced through the vacuum carbon oxidation reaction. The main feature is that a high-content MgO + Al2O3 slag system is prepared in the first LF and strong aluminum deoxidation is used. The total oxygen content in the steel is reduced from more than 30 ppm to less than 15 ppm. The size of the inclusions in the molten steel is controlled below 15 μm and the number is significantly reduced, eliminating large-size inclusions. RH vacuum treatment is used for dehydrogenation of stainless steel, and the dehydrogenation rate can reach more than 80%.

[0031] (2) Control of the high-temperature strength and intergranular corrosion resistance in the sensitized state of TP321 stainless steel seamless pipes. Currently, generally two methods are adopted to improve the intergranular corrosion resistance of steel grades: reducing the C content or adding stabilizing elements Nb and Ti. However, reducing the C content will decrease the strength of the steel. Adding Nb and Ti elements in proportion can improve the intergranular corrosion resistance of the steel grade, but according to the nuclear power product specifications, the content of Nb shall not exceed 0.15%, and the addition of Ti element will form inclusions. If a steel grade with a certain amount is selected, such as TP347, the Ni content is C%×10 - 1.10%, which is obviously much higher than the nuclear power specifications, so it cannot be selected. Therefore, when selecting TP321 stainless steel containing Ti element, for improving the intergranular corrosion resistance of TP321 in the superheated steam pipeline of the high-temperature gas-cooled reactor and meeting the requirement of high-temperature strength, only the control and matching of the contents of C element and Ti element can be studied. Therefore, in the RH vacuum treatment process of this embodiment, dehydrogenation is accompanied by an oxygen lance for precise decarburization and heating up, so that the carbon content in the smelting process is precisely controlled, and the control accuracy of the carbon content reaches ±0.002%.

[0032] (3) The refining in this embodiment includes the secondary smelting type ingot + ESR (electroslag remelting) technology, which uses the resistance heat generated when the current passes through the slag as the heat source for secondary melting. The purpose is to improve the strength of TP321 stainless steel for the superheated steam pipeline of the high-temperature gas-cooled reactor, improve the crystallization condition of the ingot, make the steel meet the nuclear power specifications with higher purity, lower sulfur content, and further reduce the size and quantity of inclusions. At the same time, a dense, uniform, smooth and clean protective layer is formed on the surface of the steel ingot, and the metallographic structure and chemical composition are promoted to be in a uniform state. More importantly, through the secondary remelting, large-sized steel ingots are obtained, laying a good foundation for the manufacture of extra-large-sized finished products.

[0033] Furthermore, the key point of the hot working process of forging blank in Step 2 is the control of the thermoplastic properties of the TP321 stainless steel seamless pipe. The actual process of Step 2 in this embodiment can be summarized as temperature-controlled forging + deformation amount control. Temperature-controlled forging mainly means that on the premise of not having overheating and burning, the starting forging temperature should be increased as much as possible to increase the plasticity of the stainless steel, reduce the hot deformation resistance, and is beneficial to the forging forming of the stainless steel; the determination of the final forging temperature is a control technology to ensure that the stainless steel still has sufficient plasticity before the end of forging and can obtain a recrystallized structure. Due to the extra-large specification TP321 stainless steel ingot (Ф950mm, weight 6000kg), the control of the heating temperature and time is necessarily more difficult than that of the normal specification ingot. Also, because the starting overheating temperature of the electroslag remelted steel is lower than that of the non-vacuum smelted steel with the same chemical composition, this is because there are very few non-metallic inclusions in the steel, and the ultra-pure steel is prone to grain growth. That is, the process route of "two-fire forging" for temperature control and deformation amount control proposed in Step 2 has a starting forging temperature 50 - 70°C lower than that of the non-vacuum smelted steel, the final forging temperature > 800°C, the heating time is 0.8mm / min, forging into a square shape in the first fire and sizing into a round shape in the second fire, and the deformation amount per time is not more than 30%.

[0034] Furthermore, the piercing and extending to produce the rough pipe in Step 3 is to produce the pipe blank for the superheated steam pipe of the TP321 stainless steel for the high-temperature gas-cooled reactor, which is the necessary blank for cold processing of pipes. At present, there are two methods for preparing stainless steel rough pipes in China, namely hot piercing and hot extrusion. Since the pipes used in the project are of extra-large size and the single-piece weight is too heavy (the single-piece weight after peeling and boring of the round billet is 5663kg), which has exceeded the range of hot extrusion, so the Ф720 piercing mill, one of the largest piercing mills in China, is selected in the present invention. The roll shape is conical, the rolling angle β = 15°, the forward angle α = 12°, and the designed rotational speed is 12r / min. The process route adopts "piercing a rough pipe + secondary extending and rolling the rough pipe", which can reduce the transverse deformation force, increase the axial extension amount, make the inner and outer surfaces of the rough pipe smooth and defect-free and the wall thickness uniform (the wall thickness deviation < 2mm). Reasonable temperature control not only endows the piercing and secondary extension performance of the rough pipe with thermoplastic processing characteristics, but also conforms to the content of the nuclear power safety code. The temperature-controlled piercing and secondary extension + solution treatment process is a control technology for the grain size requirement, and overheating and burning are not allowed either. On this premise, when reaching the plastic deformation temperature, the grain growth is controlled, so that the grain size of the TP321 stainless steel for the superheated steam pipe of the high-temperature gas-cooled reactor nuclear power reaches 5 - 7 levels and is evenly distributed when the rough pipe is obtained. The main process is that in Step 3, a round steel with an outer diameter of 616mm is pierced once to obtain a rough pipe with an outer diameter of 635mm and a wall thickness of 100mm, heated at a temperature 50 degrees lower than the overheating temperature, and pierced at a heating speed of 1mm / min, and then water-quenched; then the rough pipe is rolled into a rough pipe with an outer diameter of 695mm and a wall thickness of 43mm by secondary extending and rolling, the heating temperature is a temperature 70 degrees lower than the overheating temperature, the heating speed is 1.2mm / min, and then water-quenched.

[0035] Furthermore, the cold forming process in Step 4 uses cold drawing deformation. Cold drawing deformation can reduce work hardening caused by dislocations, avoid the transformation of deformation martensite structure, and prevent a sudden increase in internal stress. The heat treatment during the cold working process is not only a process of softening deformation for each pass, but more importantly, through the control of temperature and heat treatment holding time, phenomena such as grain distortion, stretching, and fragmentation during processing can be well restored. Therefore, the heat treatment for each pass must be fully controlled. The cold forming process route for TP321 stainless steel seamless steel pipes is "cold expansion + cold drawing + solution treatment", that is, in Step 4, a rough pipe with an outer diameter of 695 mm and a wall thickness of 43 mm is cold-expanded to an outer diameter of 775 mm, then cold-drawn to a steel pipe with an outer diameter of 762 mm and a wall thickness of 34 mm, and then solution heat treatment is carried out. The intermediate heat treatment temperature is 1120 °C and the holding time is 70 min. The final heat treatment temperature is 1100 °C and the holding time is 70 min to obtain the steel pipe. The deformation amount for each pass is less than 3 - 10%. The main purpose of the intermediate heat treatment temperature is to soften the steel body and enlarge the grain size. The control of the final solution heat treatment temperature is a technology for grain homogenization and grading while also eliminating internal stress. Due to the extremely large size, during the cold deformation process, the deformation resistance increases significantly, which increases the drawing force. The present invention selects a hydraulic cold drawing machine with a drawing force of 1000 tons, which is characterized by stable deformation, uniform stress, and can retrieve at least about 40% on one side, making the grain changes during the cold forming process of the product uniform.

[0036] Furthermore, the surface treatment of the steel pipe in Step 4 includes degreasing, grinding, and shot peening. The nuclear power safety code has very strict requirements for the surface of stainless steel, such as surface polishing, consistent color, and no any oil stains. The strong degreasing agent used in this embodiment is a chemical degreasing method, which uses an alkaline chemical substance solution to remove the oil stains on the surface of stainless steel to achieve the effect of cleaning the surface. The degreasing process of the degreasing agent is a strong emulsification effect. It can emulsify the oil stains on the surface of stainless steel to form an emulsion. Its hydrophobic group adsorbs on the oil surface to produce an affinity effect, while its hydrophilic group combines with water molecules. Under the action of the directional arrangement of degreasing agent molecules, the surface tension at the oil-solution interface is greatly reduced. Under the action of solution convection and stirring, the oil stains can be detached from the surface and dispersed in the degreasing solution in the form of tiny oil droplets. At this time, the emulsifying molecules surround the small oil droplets on the surface to prevent the small oil droplets from re-adhering to the surface and becoming an emulsion. This emulsification effect plays the role of degreasing. This degreasing method reduces the risk of steel surface corrosion to a great extent compared to the method of removing oil by corroding the surface of stainless steel with general waste acid solution, and the degreasing is thorough and clean. Degreasing process: The ratio of the degreasing cylinder is 1:2 (degreasing agent to water), the heating temperature is 70 °C, soak for 40 - 60 minutes, and rinse with clean water after taking out of the cylinder.

[0037] Grinding and shot peening on the stainless-steel surface are the final treatment processes for nuclear power pipes. Grinding is the pretreatment of stainless steel, aiming to eliminate surface defects to obtain a flat and smooth surface, enabling the shot peening process to fully cover the entire surface. Shot peening treatment is a process of strengthening the stainless-steel surface. It involves spraying a high-speed shot stream onto the pipe surface, causing plastic deformation on the surface layer to form a strengthening layer with a certain thickness, eliminating surface residual stress, making the surface of the stainless-steel pipe dull, and improving the surface strength, fatigue resistance, wear resistance, and corrosion resistance of the pipe. The mixing ratio of the shots used is 1:2 (stainless-steel shots + cast-steel shots), and the shot size is 0.7 - 1.2 mm in diameter. The final treatment process: grinding → shot peening → passivation.

[0038] After the above forging and processing steps, the TP321 stainless-steel seamless pipes processed by the present invention can meet the corresponding product standard requirements and additional special technical requirement indicators for use through tests, including the following:

[0039] (1) The mechanical and technological properties, etc. should comply with the contents specified in the product standard ASME SA-213 / SA-213M "Seamless Ferritic and Austenitic Alloy Tubes for Boilers and Superheaters".

[0040] (2) The chemical components (mass fractions) contained in TP321 should meet the following technical requirements: 0.08% > C > 0.04%, S < 0.015%, P < 0.025%, 12% > Ni ≥ 10%, 19% > Cr > 17.5%, 0.7% > Ti > 5×(C + N) maximum mass fraction of 0.7%, N ≤ 0.1%, Co < 0.025%, Cu < 0.05%;

[0041] (3) Non-metallic inclusions in the steel: A ≤ 1.0 grade, B ≤ 1.5 grade, C ≤ 1.5 grade, D ≤ 1.5 grade, and A + B + C + D ≤ 4.5 grades.

[0042] (4) The grain size meets grades 5 - 7, and the grade difference ≤ 2 grades.

[0043] (5) The product size is Ф762×34 mm, outside diameter tolerance: +4 / -0.8 mm, wall thickness tolerance: +0 / -3.4 mm.

[0044] (6) Intergranular corrosion test: qualified.

[0045] In summary, the stainless-steel seamless pipes prepared by the present invention can meet the technical requirements for the superheated steam pipes of nuclear power high-temperature gas-cooled reactors and can provide safe steel pipes for nuclear power.

Claims

1. A processing method for a stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power, characterized in that: It includes the following steps: Step 1: Refine the TP321 stainless steel raw material ingot so that the non-metallic inclusions in the steel meet the following requirements: A ≤ 1.0 grade, B ≤ 1.5 grade, C ≤ 1.5 grade, D ≤ 1.5 grade, and A + B + C + D ≤ 4.5 grades; Step 2: Forging and blanking: Adopt a two-fire forging process route with temperature control and deformation control. The starting forging temperature is 50 - 70 °C lower than that of non-vacuum smelted steel, the final forging temperature > 800 °C, the heating time is 0.8 mm / min, forging into a square shape in the first fire and sizing into a round shape in the second fire, and the deformation amount each time is not more than 30%; Step 3: Piercing and extending to produce a rough tube: For a round steel with an outer diameter of 616 mm, obtain a rough tube with an outer diameter of 635 mm and a wall thickness of 100 mm through one-time piercing. Heat it at a temperature 50 degrees lower than the overheating temperature, and pierce it at a heating speed of 1 mm / min, and then water quench; then perform secondary extension rolling to roll the rough tube into a rough tube with an outer diameter of 695 mm and a wall thickness of 43 mm. The heating temperature is a temperature 70 degrees lower than the overheating temperature, the heating speed is 1.2 mm / min, and then water quench; Step 4: Cold processing and forming: Cold-expand the rough tube with an outer diameter of 695 mm and a wall thickness of 43 mm to an outer diameter of 775 mm, and then cold-draw it to a steel tube with an outer diameter of 762 mm and a wall thickness of 34 mm. Then perform solution heat treatment for reinforcement. The intermediate heat treatment temperature is 1120 °C, the holding time is 70 min, the final heat treatment temperature is 1100 °C, and the holding time is 70 min to obtain a steel tube; Step 5: Perform degreasing, grinding, and shot peening surface treatment on the steel tube to obtain a finished product; In Step 1, the refining includes the control of nuclear harmful elements. The processes are in sequence: three-removal hot metal, intermediate frequency furnace pre-melting treatment, AOD treatment, RH vacuum treatment, and ingot casting treatment, so that the dehydrogenation rate of the TP321 stainless steel raw material ingot reaches more than 80%, the harmful metal elements of P, V, and Cu are reduced by more than 60%, and the reduction rate of Co element reaches more than 93%; In Step 1, the refining includes smelting with a vacuum induction furnace and vacuum casting. The required vacuum degree is 0.05 - 100 Pa; after the furnace charge melts, keep the molten metal boiling under vacuum for 5 - 30 min, reduce the oxygen content through vacuum carbon oxidation reaction, reduce the total oxygen content in the steel to below 15 ppm, and control the size of inclusions in the molten steel to below 15 µm; In Step 1, the refining includes secondary smelting of ingots and electroslag remelting, and uses the resistance heat generated when current passes through the slag as the heat source for secondary melting.

2. The processing method of the stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power plants according to claim 1, characterized in that: During the RH vacuum treatment process, dehydrogenation is accompanied by an oxygen lance precisely used for decarburization and heating up, so that the carbon content in the smelting process is precisely controlled.

3. The processing method of the stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power plants according to claim 1, wherein: In Step 3, use a Ф720 piercing mill for piercing treatment. The roll shape is conical, the rolling angle β = 15°, the forward angle α = 12°, and the rotation speed is 12 r / min.

4. The processing method of the stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power plants according to claim 1, characterized in that: In Step 5, the degreasing treatment is to mix a degreasing agent and water with a ratio of 1:2 in a degreasing cylinder, heat it to a temperature of 70 °C, soak the steel tube for 40 - 60 minutes, and rinse it with clean water after taking it out of the cylinder.

5. The processing method of the stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power plants according to claim 1, wherein: In Step 4, the cold processing and forming adopt cold drawing deformation to reduce work hardening caused by dislocations, avoid the transformation of deformation martensite structure, and prevent a sudden increase in internal stress.

6. The processing method of the stainless steel seamless steel pipe for high-temperature and high-pressure superheated steam in nuclear power plants according to claim 1, characterized in that: In step 5, the shot peening treatment uses a mixed shot of stainless steel shot and cast steel shot with a ratio of 1:2, and the size of the shot is 0.7 - 1.2 mm in diameter.