Method for processing 25cr2ni4mov steel rotor forgings

Abstract

The application discloses a 25Cr2Ni4MoV steel rotor forging machining method, which comprises the following steps: a smelting and ingot casting process, a process route of EBT primary smelting-LF furnace refining-VD furnace degassing-full-process argon protection casting is adopted to prepare a 25Cr2Ni4MoV bottom pouring single vacuum steel ingot; a forging process, after the steel ingot is solidified and demoulded, the steel ingot is heated for hot forging, the forging temperature range is controlled to be 1260-850 DEG C, and the forging process comprises the following steps: first fire forging, steel ingot pressure jaw, chamfering, bottom cutting and round pressing; second fire forging, rough upsetting, first WHF lengthening and eight side pressing; third fire forging, rough upsetting, second WHF lengthening and flat side pressing; fourth fire forging, round pressing, number printing, step forging and product finishing; a heat treatment process, which comprises post-forging heat treatment and performance heat treatment. The application can meet the requirements of high purity and high homogeneity of the rotor on the material, the requirements of the rotor on flaw detection, the requirements of the rotor on comprehensive mechanical properties and mechanical properties, and the target of saving cost, improving the material yield and processing efficiency is achieved.

Description

Technical Field

[0001] This invention belongs to the field of forging technology, and more specifically relates to a method for processing 25Cr2Ni4MoV steel rotor forgings. Background Technology

[0002] Generator rotors are key large forgings in power generation equipment, and their manufacturing process involves hot working processes such as smelting and casting, forging, and heat treatment. Under extreme operating conditions, rotors require excellent toughness, strength, and corrosion resistance. Therefore, rotor forgings must have high-purity and high-homogeneity chemical composition, a refined microstructure to meet flaw detection requirements, and excellent comprehensive mechanical properties. However, during the hot working process of rotor manufacturing, strong grain and grain boundary inheritance leads to mixed grains and coarse grains in the rotor's internal microstructure, resulting in unstable rotor performance and posing significant challenges to rotor forging manufacturing. Given these special operating conditions and manufacturing difficulties, conventional forging methods using existing technologies result in long manufacturing cycles, coarse grains, and costly repeated normalizing, leading to low yield and pass rates. Summary of the Invention

[0003] To address some or all of the technical problems existing in the prior art, the present invention provides a method for processing 25Cr2Ni4MoV steel rotor forgings, comprising the following steps:

[0004] 1) Smelting and casting processes

[0005] In the smelting and casting process, the process route of electric arc furnace (EBT) primary refining → LF furnace refining → VD furnace degassing → argon-protected casting is adopted to prepare 25Cr2Ni4MoV casting single vacuum steel ingots. The chemical composition of the steel ingots is controlled by mass percentage as follows: C≤0.25%, Mn≤0.35%, Si: 0.15~0.35%, P≤0.012%, S≤0.010%, Cr: 1.50~2.00%, Ni: 3.50~4.00%, Mo: 0.20~0.50%, V: 0.05~0.13%, Cu≤0.20%, Al≤0.010%, As≤0.020%, Sn≤0.020%, Sb≤0.0020. The gas content of the steel ingots is controlled as follows: [H]≤1.0ppm, [O]≤40ppm, [N]≤70ppm.

[0006] (2) Forging process

[0007] After the steel ingot solidifies and is demolded, the surface temperature of the steel ingot is controlled to be no less than 700℃ before hot forging. The forging temperature range is controlled to be 1260~850℃, and the total forging ratio is controlled to be greater than 5. The specific process of forging is as follows:

[0008] a. First forging: pressing the steel ingot into jaws, chamfering, cutting the bottom, and rounding;

[0009] b. Second forging: roughing, first WHF drawing and pressing in all directions;

[0010] c. Third forging: upsetting, second WHF drawing and flattening;

[0011] d. Fourth forging: pressing into a round shape, marking, forging steps, and finishing the product;

[0012] (3) Heat treatment process

[0013] The heat treatment process includes post-forging heat treatment and performance heat treatment. Post-forging heat treatment includes: air cooling the rotor and then performing a first low-temperature normalizing treatment and a high-temperature normalizing treatment, followed by air cooling again and then performing a second low-temperature normalizing treatment and a high-temperature normalizing treatment, followed by air cooling again and then tempering treatment. Performance heat treatment is carried out by quenching and tempering and water cooling, including quenching treatment plus high-temperature tempering treatment.

[0014] Furthermore, in the above-mentioned processing method for 25Cr2Ni4MoV steel rotor forgings:

[0015] In the smelting and casting processes, during the initial refining in the electric arc furnace, the carbon content is controlled to be ≥0.50%, the tapping temperature is controlled to be 1660~1690℃, and the C content and P content in the tapping molten steel are controlled to be ≤0.15% and ≤0.003%, respectively. During the refining process in the LF furnace, when the [O] in the molten steel is ≤12ppm and the temperature is 1650~1660℃, a vacuum degassing treatment is performed in the VD furnace, and the molten steel is kept under a vacuum of ≤67Pa for more than 20 minutes. After the VD treatment is completed and before the argon-protected casting, the molten steel is returned to the LF furnace, and after temperature adjustment, it is gently stirred with argon for more than 20 minutes. Then it is hoisted into a ladle, and the ladle temperature is controlled to be 1560~1570℃. During the casting process, the casting process is protected by an argon curtain of 0.3~0.4MPa throughout the casting process. Finally, after the casting is completed, carbon-free protective slag and a heating agent are added.

[0016] In the first forging process, the hot-transported steel ingot is heated to 1250±10℃ in the heating furnace and held for 12 hours. After the steel ingot is taken out of the furnace, the upper and lower flat anvils are used to press the jaws about 100mm from the ingot body along the riser line. When pressing the jaws, the steel ingot is rotated evenly so that the center line of the jaws is aligned with the center line of the steel ingot. After pressing the jaws, the bottom of the steel ingot is cut off and then the steel ingot is chamfered and rounded.

[0017] In the second forging process, the billet is heated to 1250±10℃ in the furnace and held for 30 hours. The billet is then uplifted using a press, a slotted pan, and a spherical top upsetting cap. After the ingot is upset, the first WHF elongation is performed using upper and lower wide flat anvils. The deformation on both sides is controlled to be 18-22%. After each pass, the billet is rotated 90° and the anvil is staggered by half before another pass. Each pass of the pass begins with the riser end. Through multiple passes of the pass, the billet is compacted into a flat square shape. Then, the edges are beveled to make the billet octagonal.

[0018] In the third forging process, the billet is heated to 1250±10℃ and held for 20 hours. The billet is then uplifted using a press, a slotted plate, and a spherical top upsetting cap. After the steel ingot is upset, it is stretched and flattened for the second time using upper and lower flat anvils. The deformation on both sides is controlled to be 18-22%. After each pressing, the ingot is rotated 90° and the anvils are staggered by half before pressing for another time. Each pressing starts from the riser end.

[0019] In the fourth forging process, the billet is returned to the furnace and heated to 1220±10℃ and held for 10 hours. It is then rounded using upper and lower flat anvils, marked with a marking anvil, and the shaft body and the steps at both ends are forged. After correction and rounding, the rotor forging is finally formed.

[0020] As one specific embodiment, the above-mentioned 25Cr2Ni4MoV steel rotor forging processing method is used to manufacture forgings for rotors used in 150MW generator sets, wherein the outer diameter specification of the rotor for the 50MW generator set is... in:

[0021] In the first forging stage of the forging process, a flat anvil with an upper and lower diameter of 850mm is used to press the jaws of the clamps. After removing the bottom of the steel ingot, lightly press the edges of the ingot with a reduction of 30-50mm to chamfer them. After chamfering, increase the reduction to round the ingot.

[0022] In the second forging process of the forging process, a 125MN press is used. The spherical top cap and the spherical top cap roughen the billet to the desired size. After upsetting the steel ingot, the first WHF (Whole-Handed Drafting) elongation is performed using a 1700mm wide flat anvil. The deformation under double-sided pressing is controlled at 20%. Full anvil pressing is used, with the feed amount of each anvil greater than 90% of the anvil width. There is a 100-200mm joint between each anvil. Through 6 passes of pressing, the billet is compacted to a flat square of 1560mm × 1640mm. Then, the edges are chamfered to make the billet an octagonal shape of 1600mm × 3150mm. The process parameters for the first WHF pressing are controlled according to the table below:

[0023]

[0024] In the third forging stage of the forging process, a 125MN press is used. The spherical top cap and the spherical top cap roughen the billet to the desired size. After the steel ingot is upset, it is drawn and flattened a second time using a flat anvil with a top and bottom diameter of 1700mm. The deformation on both sides is controlled to be 20%. Through 11 passes of pressing, the billet is drawn and flattened to a size of 1300mm × 960mm. The process parameters for the second WHF pressing are controlled according to the following table:

[0025]

[0026] In the fourth forging process, a flat anvil with an upper and lower diameter of 850mm is used to round the billet.

[0027] The post-forging heat treatment process includes: (1) a first low-temperature normalizing treatment and a high-temperature normalizing treatment, where the rotor is air-cooled to 300-350℃ for material preparation, heated to 660-680℃ at a heating rate of ≤40℃ / h, held for 12h, and then heated to 900-950℃, held for 20-22h, and air-cooled out of the furnace; (2) a second low-temperature normalizing treatment and a high-temperature normalizing treatment, where the rotor is air-cooled to 280-32℃. After 0℃, keep it at 22h, then heat it to 660~680℃ at a heating rate of ≤40℃ / h, keep it at 12h, then heat the rotor to 870~900℃, keep it at 20~22h, and air cool it out of the furnace; (3) Tempering treatment: air cool the rotor to 180~220℃ and keep it at 22h, then heat it to 640~660℃ at a heating rate of ≤40℃ / h, keep it at 45h, and then furnace cool it to ≤150℃ before it is taken out of the furnace;

[0028] The heat treatment process includes: (1) Quenching treatment: the rotor is heated to ≤300℃ and put into the furnace. It is heated to 640~660℃ at a heating rate of ≤50℃ / h and held for 7h. Then the rotor is heated to 830~870℃ and held for 16~20h. After air cooling for 2 minutes, it is water cooled for 4~4.5h. The rotor shaft surface temperature is ≤200℃ before it is taken out of the water. (2) High temperature tempering treatment: the rotor is held at 290~310℃ for 6h. Then it is heated to 600~650℃ at a heating rate of ≤35℃ / h and held for 28~35h. Then it is air cooled to ≤200℃ at a cooling rate of ≤30℃ / h before it is taken out of the furnace.

[0029] The processing method for 25Cr2Ni4MoV steel rotor forgings of this invention innovatively solves the problems of manufacturing high-purity homogeneous steel ingots, refining the microstructure and flaw detection of forgings, and improving the mechanical properties of the products, filling a technological gap. Specifically, it has the following advantages and beneficial effects:

[0030] (1) The process route of short-process electric arc furnace primary refining → LF furnace refining → VD furnace degassing → argon-protected casting is adopted. By controlling the material composition and gas content, the high purity and high homogeneity requirements of the rotor forging for steel ingots are met, thereby meeting the strict requirements of rotor flaw detection.

[0031] (2) By utilizing the alternating static grain growth, dynamic and static recovery and recrystallization principles during the forging process, the number of forging fires and appropriate forging and compaction methods are effectively controlled, thereby controlling coarse grain and mixed grain defects, refining the forging structure, and ultimately meeting the rotor flaw detection requirements.

[0032] (3) The heat treatment process includes post-forging heat treatment and performance heat treatment. Post-forging heat treatment adopts two high and low temperature normalizing and tempering treatments, three undercooling and two austenitizing treatments to adjust the structure, refine the grains and cut off the structure inheritance. Performance heat treatment adopts quenching and tempering treatment with high temperature tempering and water cooling to refine the grains and improve the internal structure, so that the rotor surface has martensite and lower bainite structure, and the core has lower bainite structure or lower bainite plus a small amount of upper bainite structure, to meet the requirements of the rotor's comprehensive mechanical properties and mechanical properties, and further meet the rotor flaw detection requirements.

[0033] (4) The manufacturing and forming method of producing one piece from one ingot can be used to mass-produce rotor forgings, ensuring one-time qualification, thereby achieving the comprehensive goals of saving costs, increasing yield and processing efficiency, and reducing carbon emissions. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings:

[0035] Figure 1 This is a schematic flowchart of the processing method for 25Cr2Ni4MoV steel rotor forgings according to the present invention;

[0036] Figure 2 The diagram shows the deformation morphology of the forging in each step of the processing method of the 25Cr2Ni4MoV steel rotor forging of the present invention. (a) is a schematic diagram of the steel ingot, (b) is a schematic diagram of the billet after pressing the jaws, chamfering, cutting the bottom, and pressing into a round shape, (c) is a schematic diagram of the billet after the steel ingot is upsetting, first WHF drawing and pressing into an octagon, (d) is a schematic diagram of the billet after the steel ingot is upsetting, second WHF drawing and pressing into a flat square shape, (e) is a schematic diagram of the billet after pressing into a round shape, marking, and forging into a step, and (f) is a schematic diagram of the finished forging.

[0037] Figure 3This is a schematic diagram of the timing of post-forging heat treatment in the processing method of 25Cr2Ni4MoV steel rotor forgings of the present invention;

[0038] Figure 4 This is a schematic diagram of the timing of the performance heat treatment in the processing method of the 25Cr2Ni4MoV steel rotor forging of the present invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0040] like Figure 1 As shown, the processing method for the 25Cr2Ni4MoV steel rotor forging of the present invention includes the following steps:

[0041] (1) Smelting and casting processes

[0042] In the smelting and casting process, the process route of electric arc furnace EBT primary refining → LF furnace refining → VD furnace degassing → full argon protection casting is adopted to optimize the steel composition, improve the purity of the steel ingot, strictly control the content of impurity elements such as P, As, Sn, and Sb that may cause temper embrittlement of steel, strictly control the content of S that may reduce the toughness of steel, and strictly control the H content in the steel to prevent large internal stress from being generated during solidification, which could cause white spots in the local hydrogen-rich parts of the forging and cause serious longitudinal cracks in the rotor forging. In this way, high-purity, homogeneous, and low-segregation 25Cr2Ni4MoV casting single vacuum steel ingots are prepared.

[0043] During the smelting and casting processes, the chemical composition of the steel ingot is controlled by mass percentage as follows: C≤0.25%, Mn≤0.35%, Si: 0.15~0.35%, P≤0.012%, S≤0.010%, Cr: 1.50~2.00%, Ni: 3.50~4.00%, Mo: 0.20~0.50%, V: 0.05~0.13%, Cu≤0.20%, Al≤0.010%, As≤0.020%, Sn≤0.020%, Sb≤0.0020; the gas content of the steel ingot is controlled as follows: [H]≤1.0ppm, [O]≤40ppm, [N]≤70ppm.

[0044] In the smelting and casting processes, during the initial smelting in the electric arc furnace (EBT), the charging process is strictly followed. The carbon content of the raw materials is controlled to be ≥0.50%. During the oxidation period, slag removal and new slag formation are carried out to ensure the amount of slag replacement. At the same time, reasonable oxygen blowing for melting, reasonable decarburization, and low-temperature dephosphorization are adopted. Steel and slag retention are adopted during tapping to prevent oxidized slag from entering the refining ladle. The tapping temperature is controlled at 1660~1690℃, and the C content in the tapped steel is controlled to be ≤0.15% and the P content is controlled to be ≤0.003%.

[0045] During the smelting and casting processes, in the LF furnace refining and VD furnace vacuum degassing, the refining slag system is adjusted to effectively reduce the level and quantity of inclusions. All alloys and auxiliary materials entering the LF furnace are strictly baked to reduce the hydrogen content in the steel. At the same time, high-basicity, strong reducing slag is used for desulfurization, and carbon powder diffusion deoxidation is adopted to form refining foam slag. During the refining process, the argon pressure and flow rate are controlled to uniformly adjust the temperature and composition of the molten steel. The refining time is carefully controlled. After the alloy is added, the refining white slag should be kept for a certain period of time, and diffusion deoxidizer should be added to ensure good deoxidation. When the [O] in the molten steel is ≤12ppm and the temperature is 1650~1660℃, VD furnace vacuum degassing is carried out. The molten steel is kept under a vacuum of ≤67Pa for at least 20 minutes.

[0046] In the smelting and ingot casting processes, after VD treatment and before argon-protected casting, the molten steel is returned to the LF furnace. After temperature adjustment based on the steel's temperature, it is gently stirred with argon for more than 20 minutes. Then, the ladle is hoisted, and the ladle temperature is controlled at 1560–1570℃. Strict adherence to the ingot casting process specifications is maintained, ensuring the gating system, dedicated base, and ingot mold remain clean and dry. During argon curtain protection, the argon pressure is controlled at 0.3–0.4 MPa to prevent secondary oxidation of the molten steel. Finally, after casting, an appropriate amount of carbon-free protective slag and exothermic agent are added.

[0047] Through the above measures in the smelting and casting processes, the finished steel ingots have [H]: 0.84ppm, [O]: 15ppm, [N]: 65ppm, and inclusions A, B, C, and D (both coarse and fine series) are controlled at level 0 to 0.5, and DS is controlled at level ≤1.0, laying a solid foundation for subsequent forgings to meet the flaw detection requirements.

[0048] (2) Forging process

[0049] After the steel ingot solidifies and is demolded, the surface temperature of the ingot should not be lower than 700℃. It should then be loaded into a heat-insulating cart for hot forging and heating. See the schematic diagram of the steel ingot. Figure 2(a) The forging temperature range is controlled at 1260–850℃, and the total forging ratio is controlled to be greater than 5. Through effective and reasonable forging processes, inherent casting defects in steel ingots can be improved, such as macroscopic segregation, shrinkage porosity, etc. Forging within the forging temperature range can fully eliminate coarse dendrites in the cast state of the steel ingot, refine the grains, weld internal porosity defects, and simultaneously ensure that the steel has good plasticity and low deformation resistance. The specific process of forging is as follows:

[0050] a. First Forging: The steel ingot is clamped, chamfered, undercut, and rounded. After the hot-transported steel ingot is heated to 1250±10℃ in the furnace, it is held at that temperature for 12 hours. After exiting the furnace, the ingot is clamped approximately 100mm from the riser line using upper and lower flat anvils. During clamping, the ingot is rotated evenly to ensure the center line of the clamps aligns with the center line of the ingot. After clamping, the bottom of the ingot is cut off, and then the ingot is chamfered and rounded. See the schematic diagram of the billet for clamping, chamfering, undercutting, and rounding of the steel ingot. Figure 2 (b) in the middle.

[0051] b. Second forging: roughing, first WHF (wide anvil high-pressure forging method) drawing and pressing in eight directions. After the billet is reheated to 1250±10℃ in the furnace, it is held for 30 hours. The billet is then upsetting using a press, a slotted pan, and a spherical top cap. This upsetting deformation increases the cross-sectional area of ​​the steel ingot billet, increasing the forging ratio for subsequent drawing. It also closes internal pores, breaks up coarse grains, improves segregation, enhances the transverse properties of the rotor forging, and reduces the anisotropy of mechanical properties. After upsetting, the steel ingot is first drawn using wide and flat anvils. The double-sided pressing deformation is controlled at 18-22%. After each pass, the ingot is rotated 90° and the anvils are staggered by half before another pass. Each pass begins from the riser end to ensure that every area of ​​the billet receives strong pressure deformation, guaranteeing the uniformity of the billet deformation. This results in a forging effect that is compacted, has fine and uniform grains, and consistent properties. Through multiple passes, the billet is compacted into a flat square shape, and then chamfered to make it octagonal. During the drawing process, defects such as cracks, pits, and folds are cleaned using an oxygen lance. See the schematic diagram of the billet for the first WHF drawing and octagonal pressing of the steel ingot. Figure 2 (c) in the middle.

[0052] c. Third forging: Upsetting and second WHF drawing and flattening. After the billet is returned to the furnace and heated to 1250±10℃, it is held for 20 hours. The billet is upset using a press, a spool, and a spherical top upsetting cap. After the ingot is upset, a second WHF drawing and flattening is performed using upper and lower flat anvils. The deformation on both sides is controlled to be 18-22%. After each pass, the ingot is rotated 90°, and the anvils are staggered by half before the next pass. Each pass begins with the riser end. During the drawing process, defects such as cracks, pits, and folds are cleaned hot with an oxygen lance. See the schematic diagram of the billet for the ingot upsetting and second WHF drawing and flattening process. Figure 2 (d) in the middle.

[0053] d. Fourth forging: Rounding, marking, forging steps, and finishing. After the billet is returned to the furnace and heated to 1220±10℃, it is held for 10 hours. Rounding is performed using upper and lower flat anvils, followed by marking with a marking anvil. The shaft body and steps at both ends are forged, and the rounding is corrected to achieve the final rotor forging. See the schematic diagram of the billet rounding, marking, and forging steps for more details. Figure 2 (e) In the diagram, see the schematic diagram of the finished forging after finishing. Figure 2 (f) in the middle.

[0054] (3) Heat treatment process

[0055] The heat treatment process includes post-forging heat treatment and performance heat treatment. After the rotor forging is formed, the excess material at both ends is cut off according to the required dimensions of the finished rotor. Post-forging heat treatment is then performed, including: air cooling the rotor followed by a first low-temperature normalizing treatment and a high-temperature normalizing treatment, then air cooling again followed by a second low-temperature normalizing treatment and a high-temperature normalizing treatment, followed by air cooling again followed by tempering. Through three undercoolings and two austenitizing processes, the microstructure is adjusted, the grains are refined, and the microstructure inheritance is cut off. After post-forging heat treatment, the rotor forging is subjected to performance heat treatment, which adopts a tempering and water cooling method, including quenching and high-temperature tempering, so that the rotor surface has martensite and lower bainite microstructure, and the core has lower bainite microstructure or lower bainite with a small amount of upper bainite microstructure, to meet the comprehensive mechanical performance requirements of the rotor.

[0056] The following detailed description, with reference to specific embodiments, illustrates the processing method for 25Cr2Ni4MoV steel rotor forgings according to the present invention. The processing method for 25Cr2Ni4MoV steel rotor forgings of this invention is used to manufacture forgings of rotors for 150MW generator sets using a one-piece-per-ingot manufacturing method. The outer diameter specification of the rotor for the 150MW generator set is as follows: The requirement is that there must be no severe crystal mixing, and internal UT testing must not exceed or equal to [a certain value]. The dense defects of equivalent diameter belong to the typical long shaft forging with large cross-section.

[0057] The processing method for the 25Cr2Ni4MoV steel rotor forging of this invention is carried out according to the above-mentioned smelting and casting process, forging process, and heat treatment process, and specifically:

[0058] In the first forging stage of the forging process, the steel ingot is tapered using a flat anvil with an 850mm diameter at the top and bottom. The anvils are aligned, and initial light pressure is applied to eliminate surface defects such as riser taper, heavy scale, and inclusions at the riser root. After the riser taper is eliminated, the pressure is increased while ensuring uniform rotation of the ingot and consistent pressure control. If the center line of the clamping jaws is not aligned with the center line of the ingot, it is promptly corrected and straightened. After removing excess riser material, round the ingot to ensure it can fit into the upsetting tray in the next pass. After cutting off the bottom of the ingot, lightly press the edges of the ingot with a reduction of 30-50mm to chamfer them. After chamfering, increase the reduction to round the ingot.

[0059] In the second forging stage of the forging process, a 125MN press is used. The spherical top cap and the spherical top cap roughen the billet to the desired size. After upsetting the steel ingot, the first WHF (whole-face high-low) drawing is performed using a 1700mm wide flat anvil. The anvil radius R is 170mm, and the double-sided pressing deformation is controlled at 20%. Full anvil pressing is used, with the feed amount of each anvil greater than 90% of the anvil width. There is a 100-200mm joint between each anvil. Through 6 passes of pressing, the billet is compacted to a flat square of 1560mm × 1640mm. Then, the edges are chamfered to make the billet an octagonal shape of 1600mm × 3150mm. The process parameters for the first WHF pressing are controlled according to Table 1.

[0060] Table 1. Process parameters for the first WHF pressing down.

[0061]

[0062] In the third forging stage of the forging process, a 125MN press is used. The spherical top cap and the spherical top cap roughen the billet to the desired size. After the steel ingot is upset, a second WHF drawing and flattening process is carried out using a flat anvil with a top and bottom diameter of 1700mm. The deformation on both sides is controlled to be 20%. Through 11 passes of pressing, the billet is drawn and flattened to 1300mm×960mm. The process parameters for the second WHF pressing are controlled according to Table 2.

[0063] Table 2. Second WHF pressing process parameters

[0064]

[0065] In the fourth forging stage of the forging process, a flat anvil with an upper and lower diameter of 850mm is used to round the billet.

[0066] In the heat treatment process, according to Figure 3 The timing diagram shown indicates that post-forging heat treatment is performed according to... Figure 4The timing diagram shown is used for performance heat treatment. The post-forging heat treatment specifically includes: (1) the first low-temperature normalizing treatment and high-temperature normalizing treatment, the rotor is air-cooled to 300-350℃ for material preparation, heated to 660-680℃ at a heating rate of ≤40℃ / h, held for 12h, and then the rotor is heated to 900-950℃, held for 20-22h, and air-cooled out of the furnace; (2) the second low-temperature normalizing treatment and high-temperature normalizing treatment, the rotor is air-cooled to 280-320℃. After holding the temperature for 22 hours, heat the rotor to 660-680℃ at a heating rate of ≤40℃ / h, hold for 12 hours, then heat the rotor to 870-900℃, hold for 20-22 hours, and air-cool it out of the furnace; (3) Tempering treatment: air-cool the rotor to 180-220℃, hold for 22 hours, then heat it to 640-660℃ at a heating rate of ≤40℃ / h, hold for 45 hours, and then furnace-cool to ≤150℃ before taking it out of the furnace. The specific heat treatment for performance includes: (1) Quenching treatment: the rotor is heated to ≤300℃ and put into the furnace, heated to 640~660℃ at a heating rate of ≤50℃ / h, held for 7h, and then heated to 830~870℃, held for 16~20h, air-cooled for 2 minutes and then water-cooled for 4~4.5h. The rotor shaft surface temperature is ≤200℃ before being taken out of the water; (2) High temperature tempering treatment: the rotor is held at 290~310℃ for 6h, then heated to 600~650℃ at a heating rate of ≤35℃ / h, held for 28~35h, and then air-cooled to ≤200℃ at a cooling rate of ≤30℃ / h before being taken out of the furnace.

[0067] A 25Cr2Ni4MoV steel rotor forging manufactured using the embodiments of the present invention was used. Samples were taken from the shaft of the rotor forging and chemical composition was tested. All test results met the technical requirements. The specific chemical composition test results of the rotor are shown in Table 3.

[0068] Table 3. Results of rotor chemical composition analysis (mass percentage, %)

[0069] element Required value Measured value C ≤0.25 0.24 Mn ≤0.35 0.31 Si 0.15～0.35 0.21 S ≤0.010 0.001 P ≤0.012 0.006 Cr 1.50～2.00 1.70 Ni 3.50～4.00 3.73 Mo 0.20～0.50 0.37 V 0.05～0.13 0.09 Cu ≤0.20 0.05 Al ≤0.010 0.005 Sn ≤0.020 ＜0.005 Sb ≤0.0020 ＜0.0015 As ≤0.020 0.005

[0070] The 25Cr2Ni4MoV steel rotor forging manufactured using the embodiments of the present invention was sampled in the axial, tangential, radial, and core directions for mechanical property testing. All test results met the technical requirements. The specific mechanical property test results of the rotor are shown in Table 4.

[0071] Table 4. Rotor mechanical performance test results

[0072]

[0073] In the table above, P1 refers to the sampling position at the small shaft end of the rotor nozzle, P4 refers to the sampling position at the small shaft end of the rotor riser, P2 refers to the sampling position at the shaft end of the rotor nozzle, and P3 refers to the sampling position at the shaft end of the rotor riser.

[0074] Furthermore, testing revealed that the 25Cr2Ni4MoV steel rotor forging manufactured using the embodiments of this invention exhibits a grain size of grade 5.0 at positions P2 and P3, meeting the technical requirements. The ultrasonic testing results for the entire rotor forging meet the requirements of the JB / T8708-2014 standard.

[0075] In summary, compared with the prior art, the processing method for 25Cr2Ni4MoV steel rotor forgings of the present invention innovatively solves the problems of manufacturing high-purity homogeneous steel ingots, refining the microstructure and flaw detection of forgings, and improving the mechanical properties of the products, thus filling a technological gap. Specifically, it has the following advantages and beneficial effects:

[0076] (1) The process route of short-process electric arc furnace primary refining → LF furnace refining → VD furnace degassing → argon-protected casting is adopted. By controlling the material composition and gas content, the high purity and high homogeneity requirements of the rotor forging for steel ingots are met, thereby meeting the strict requirements of rotor flaw detection.

[0077] (2) By utilizing the alternating static grain growth, dynamic and static recovery and recrystallization principles during the forging process, the number of forging fires and appropriate forging and compaction methods are effectively controlled, thereby controlling coarse grain and mixed grain defects, refining the forging structure, and ultimately meeting the rotor flaw detection requirements.

[0078] (3) The heat treatment process includes post-forging heat treatment and performance heat treatment. Post-forging heat treatment adopts two high and low temperature normalizing and tempering treatments, three undercooling and two austenitizing treatments to adjust the structure, refine the grains and cut off the structure inheritance. Performance heat treatment adopts quenching and tempering treatment with high temperature tempering and water cooling to refine the grains and improve the internal structure, so that the rotor surface has martensite and lower bainite structure, and the core has lower bainite structure or lower bainite plus a small amount of upper bainite structure, to meet the requirements of the rotor's comprehensive mechanical properties and mechanical properties, and further meet the rotor flaw detection requirements.

[0079] (4) The manufacturing and forming method of producing one piece from one ingot can be used to mass-produce rotor forgings, ensuring one-time qualification, thereby achieving the comprehensive goals of saving costs, increasing yield and processing efficiency, and reducing carbon emissions.

[0080] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Additionally, the terms "front," "back," "left," "right," "upper," and "lower" in this document refer to the placement shown in the accompanying drawings.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for machining 25Cr2Ni4MoV steel rotor forgings, characterized in that, The process includes the following steps: (1) Smelting and casting processes In the smelting and ingot casting process, the following process route is adopted: electric arc furnace (EBT) primary smelting → LF furnace refining → VD furnace degassing → argon-protected casting throughout the process. This is used to prepare 25Cr2Ni4MoV cast single-vacuum steel ingots. The chemical composition of the ingots is controlled by mass percentage as follows: C≤0.25%, Mn≤0.35%, Si: 0.15~0.35%, P≤0.012%, S≤0.010%, Cr: 1.50~2.00%, Ni: 3.50~4.00%, Mo: 0.20~0.50%, V: 0.05~0.13%, Cu≤0.20%, Al≤0.010%, As≤0.020%, Sn≤0.020%, Sb≤0.0020%. The gas content of the ingots is controlled as follows: [H]≤1.0ppm, [O]≤40ppm, [N]≤0.0020%. The carbon content in the process is controlled to be ≤70ppm. During the initial refining in the electric arc furnace, the carbon content is controlled to be ≥0.50%, the tapping temperature is controlled to be 1660~1690℃, the C content in the tapping steel is controlled to be ≤0.15%, and the P content is controlled to be ≤0.003%. During the refining process in the LF furnace, when the [O] in the molten steel is ≤12ppm and the temperature is 1650~1660℃, the VD furnace vacuum degassing treatment is carried out. The molten steel is kept under a vacuum of ≤67Pa for more than 20 minutes. After the VD treatment is completed and before the argon-protected casting, the molten steel is returned to the LF furnace. After temperature adjustment, it is softly stirred with argon for more than 20 minutes. Then it is hoisted into a ladle, and the ladle temperature is controlled to be 1560~1570℃. During the casting, the casting process is protected by an argon curtain of 0.3~0.4MPa throughout the casting process. Finally, after the casting is completed, carbon-free protective slag and heating agent are added. (2) Forging process After the steel ingot solidifies and is demolded, the surface temperature of the steel ingot is controlled to be no less than 700℃ before hot forging. The forging temperature range is controlled to be 1260~850℃, and the total forging ratio is controlled to be greater than 5. The specific process of forging is as follows: a. First forging: pressing the steel ingot jaws, chamfering, cutting the bottom, and rounding. Among them, the hot-transported steel ingot is heated to 1250±10℃ in the heating furnace and held for 12 hours. After the steel ingot is taken out of the furnace, the jaws are pressed along the ingot riser line about 100mm from the ingot body using upper and lower flat anvils. When pressing the jaws, the steel ingot is rotated evenly to make the center line of the jaws consistent with the center line of the steel ingot. After pressing the jaws, the bottom of the steel ingot is cut off and then chamfered and rounded. b. Second forging: upsetting, first WHF drawing and octagonal pressing. The billet is heated to 1250±10℃ in the furnace and held for 30 hours. The billet is upsetting using a press, a slotted pan and a spherical top upsetting cap. After the ingot is upsetting, the first WHF drawing is carried out using a wide and flat anvil. The deformation on both sides is controlled to be 18~22%. After each pressing, the billet is rotated 90° and the anvil is staggered by half. Each pressing starts from the riser end. Through multiple pressings, the billet is compacted into a flat square shape. Then the edges are beveled to make the billet octagonal. c. Third forging: upsetting, second WHF drawing and flattening. The billet is heated to 1250±10℃ and held for 20 hours. The billet is upsetting using a press, a slotted plate and a spherical top upsetting cap. After the steel ingot is upset, it is drawn and flattened for the second time using upper and lower flat anvils. The deformation on both sides is controlled to be 18~22%. After each pressing, the ingot is rotated 90° and the anvil is staggered by half. Each pressing starts from the riser end. d. Fourth forging: pressing, marking, forging steps, and finishing the finished product. The billet is returned to the furnace and heated to 1220±10℃ and held for 10 hours. It is then pressed into round shape using upper and lower flat anvils. After marking with a marking anvil, the shaft body and the steps at both ends are forged, and then the round shape is corrected and rolled to form the final rotor forging. (3) Heat treatment process The heat treatment process includes post-forging heat treatment and performance heat treatment. Post-forging heat treatment includes: air cooling the rotor and then performing a first low-temperature normalizing treatment and a high-temperature normalizing treatment, followed by air cooling again and then performing a second low-temperature normalizing treatment and a high-temperature normalizing treatment, followed by air cooling again and then tempering treatment. Performance heat treatment is carried out by quenching and tempering and water cooling, including quenching treatment plus high-temperature tempering treatment.

2. The method for processing 25Cr2Ni4MoV steel rotor forgings according to claim 1, used for manufacturing forgings of rotors for 150MW generator sets, wherein the outer diameter of the rotor for the 150MW generator set is Ø1014mm, characterized in that: In the first forging process, a flat anvil with an upper and lower 850mm is used to press the jaws. The jaws are pressed to Ø1000mm×1100mm. After the bottom of the steel ingot is cut off, the edges of the steel ingot are lightly pressed with a pressing amount of 30~50mm to bevel them. After beveling, the pressing amount is increased to round the steel ingot to Ø1600mm×3150mm. In the second forging stage of the forging process, a 125MN press, a Ø1000 sprue, and a spherical top upsetting cap are used to upset the billet to Ø2440mm×1465mm. After upsetting, the billet undergoes the first WHF (whole-face high-pressure) drawing using a 1700mm wide anvil. The double-sided deformation is controlled at 20%, and full-anvil high-pressure is used. The feed amount of each anvil is greater than 90% of the anvil width, and there is a 100~200mm joint between each anvil. Through 6 passes of pressing, the billet is compacted to a flat square of 1560mm×1640mm. Then, the edges are chamfered to make the billet octagonal of 1600mm×3150mm. The process parameters for the first WHF pressing are controlled according to the following table: ； In the third forging stage of the forging process, a 125MN press, a Ø1000 die, and a spherical top upsetting cap are used to upset the billet to Ø2390mm×1435mm. After upsetting, the billet is drawn and flattened a second time using a 1700mm flat anvil. The deformation on both sides is controlled at 20%. Through 11 passes of pressing, the billet is drawn and flattened to 1300mm×960mm. The process parameters for the second WHF pressing are controlled according to the following table: ； In the fourth forging process, the billet is rounded to Ø1100mm using a flat anvil with an upper and lower diameter of 850mm. The post-forging heat treatment process includes: (1) a first low-temperature normalizing treatment and a high-temperature normalizing treatment, where the rotor is air-cooled to 300~350℃ for material preparation, heated to 660~680℃ at a heating rate of ≤40℃ / h, held for 12h, and then heated to 900~950℃, held for 20~22h, and air-cooled out of the furnace; (2) a second low-temperature normalizing treatment and a high-temperature normalizing treatment, where the rotor is air-cooled to 280~32℃. After 0℃, keep it warm for 22h, then heat it to 660~680℃ at a heating rate of ≤40℃ / h, keep it warm for 12h, then heat the rotor to 870~900℃, keep it warm for 20~22h, and air cool it out of the furnace; (3) Tempering treatment: air cool the rotor to 180~220℃ and keep it warm for 22h, then heat it to 640~660℃ at a heating rate of ≤40℃ / h, keep it warm for 45h, and then furnace cool it to ≤150℃ before it is taken out of the furnace; The heat treatment process includes: (1) Quenching treatment: the rotor is heated to ≤300℃ and put into the furnace. It is heated to 640~660℃ at a heating rate of ≤50℃ / h and held for 7h. Then the rotor is heated to 830~870℃ and held for 16~20h. After air cooling for 2 minutes, it is water cooled for 4~4.5h. The rotor shaft surface temperature is ≤200℃ before it is taken out of the water. (2) High temperature tempering treatment: the rotor is held at 290~310℃ for 6h. Then it is heated to 600~650℃ at a heating rate of ≤35℃ / h and held for 28~35h. Then it is air cooled to ≤200℃ at a cooling rate of ≤30℃ / h before it is taken out of the furnace.

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