A high-temperature-resistant pressure vessel steel produced by continuous casting of a round bloom and a manufacturing method thereof

Abstract

This invention discloses a high-temperature resistant pressure vessel steel produced from continuously cast large round billets and its manufacturing method. The chemical composition, by mass percentage, is as follows: C: 0.10–0.18%, Si: 0.35–0.60%, Mn: 0.45%–0.70%, P≤0.008%, S≤0.005%, V: 0.01%–0.05%, Al: 0.010%–0.050%, Nb: ≤0.01%, Ti≤0.003%, Cr≤1.10–1.35%, Cu≤0.05%, Ni: 0.15–0.25%, H≤0.00018%, O≤0.002%, N≤0.007%, As≤0.01%, Sn≤0.01%, Sb≤ 0.01%, Pb≤0.01%, Bi≤0.01%, and As+Sn+Sb+Pb+Bi≤0.035%, with the balance being Fe and unavoidable impurities. Additionally, there are requirements for temper brittleness sensitivity coefficients: X=(10P+5Sb+4Sn+As)×100≤10; J=(Si+Mn)×(P+Sn)×10000≤100. The production process involves: molten iron pretreatment, primary BOF refining, LF ladle refining, vacuum degassing, continuous casting of round billets, slow cooling of round billets, heated forging, air cooling of forgings, normalizing, hydrogen-expanded annealing, and quenching and tempering.

Description

Technical Field

[0001] This invention belongs to the field of special steel and its manufacturing technology, specifically relating to pressure vessel steel and its manufacturing method, and particularly to a high-temperature resistant pressure vessel steel produced from continuously cast large round billets with a diameter of Φ1000mm or more as raw materials and its manufacturing method. Background Technology

[0002] Pressure vessel steel, which operates in high-temperature environments, is an indispensable key material in the nation's core industrial sectors such as energy, chemical, and power. It is widely used in medium- and high-temperature pressure-bearing core equipment in petrochemicals, nuclear power, steam turbine cylinders, and thermal power plants. These devices typically operate for extended periods in extreme environments of high temperature (400°C or even higher) and high pressure, thus placing extremely high demands on the high-temperature strength, creep resistance, toughness, and temper brittleness sensitivity coefficient of the steel used.

[0003] Currently, both domestically and internationally, the production of such products still mainly relies on the traditional process route of "ingot casting → forging / rolling". Although this route is technically mature, it has inherent drawbacks such as long production cycle, complex processes, huge energy consumption, and low yield, resulting in high product costs. This seriously restricts the manufacturing efficiency and market competitiveness of high-end heavy equipment.

[0004] Compared with ingot casting, continuous casting of round billets has a series of significant advantages, such as high machine-hour output, high yield, short production cycle, and low energy consumption. However, directly using large continuously cast round billets in the production of high-performance high-temperature pressure vessel steel, especially when manufacturing high-end products with large dimensions and extra-thick cross-sections, presents significant challenges in controlling the overall purity of the material, the compositional segregation across the entire cross-section, and the core density as the dimensions increase, particularly above Φ1000mm.

[0005] In existing technologies, such as CN102094150A, an extra-thick high-temperature pressure vessel steel and its preparation method are disclosed. However, this method uses a continuous casting billet hot rolling process, resulting in a product thickness of only 80 mm. Furthermore, its chemical composition employs a high V (0.15~0.30%) + Mo (0.25~0.35%) alloy design, leading to high alloy costs. In addition, this technology does not address methods for controlling the unique defects of large continuously cast round billets, nor does it systematically control the tempering brittleness sensitivity coefficient and the five harmful elements.

[0006] Therefore, the novel technical method for stably producing high-purity, highly homogeneous, and high-performance high-temperature pressure vessel steel through continuous casting of large round billets has urgent practical significance and enormous economic value for promoting technological progress in the industry, reducing production costs, and improving the manufacturing level of major equipment in my country. This patent invention is proposed against this background. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for producing high-temperature pressure vessel steel from continuously cast large round billets with a diameter of Φ1000mm or more and the same method, so that the finished billet of the continuously cast large round billet has good purity and the pressure vessel made after forging and tempering meets strict requirements for flaw detection, mechanical properties and grain size.

[0008] The technical solution adopted by this invention to solve the above problems is as follows: a high-temperature pressure vessel steel produced by continuous casting large round billets, with Fe as the base element, and containing the following chemical composition by mass percentage: C: 0.10~0.18%, Si: 0.35~0.60%, Mn: 0.45%~0.70%, P≤0.008%, S≤0.005%, V: 0.01%~0.05%, Al: 0.010%~0.050%, Nb: ≤0.01%, Ti≤0.003%, Cr≤1.10~1.35%, Cu≤0.05%, Ni: 0.15~0.25%, H≤0.00018%, O≤0.002%, N≤0.007%, As≤0.01%, Sn≤0.01%, Sb≤ 0.01%, Pb≤0.01%, Bi≤0.01%, and As+Sn+Sb+Pb+Bi≤0.035%, with the balance being Fe and unavoidable impurities.

[0009] Temper brittleness sensitivity coefficient requirements: X=(10P+5Sb+4Sn+As)×100≤10; J=(Si+Mn)×(P+Sn)×10000≤100.

[0010] The mechanical properties of the steel meet the requirements of Rm: 520~670MPa, Rel≥330MPa, A≥20%, and KV2(-10℃)≥50J; the flaw detection meets the requirements of EN10228-3 standard level 4 (equivalent to a single defect flat-bottomed hole ≤φ3mm), and the grain size is ≥5. The steel is inspected according to GB / T 10561 standard method A and conforms to the provisions of Table 1 below. Table 1. Acceptance Levels for Non-metallic Inclusions

[0011] The following details the function and dosage selection of the main elemental components contained in this invention: Carbon (C) is the most economical and basic strengthening element in steel. It can significantly improve the strength of steel through solid solution strengthening and precipitation strengthening. However, excessively high C content can adversely affect the toughness and ductility of steel. In this invention, the C content is determined to be in the range of 0.10–0.18%. Si is a deoxidizing element in steel. It improves the hardness and strength of steel through solid solution strengthening. In this invention, the Si content is controlled to be 0.35-0.60%.

[0012] Mn (Mn) is a deoxidizing element in the steelmaking process, improves the hardenability of steel, and plays a role in solid solution strengthening. Mn can also fix the form of sulfur in steel and form MnS and (Fe,Mn)S, which are less harmful to steel properties, reducing or inhibiting the production of FeS and improving the purity and performance of steel. Excessive Mn content will coarsen the grains and increase the tendency for temper brittleness; therefore, it should not exceed 1.0%. In this invention, the Mn content is controlled at 0.45%–0.70%.

[0013] Phosphorus (P): In steel, P significantly causes segregation during solidification. P dissolves in ferrite, leading to grain distortion and coarsening, and increasing cold brittleness. To reduce the material's temper brittleness sensitivity coefficient, the P content is strictly controlled. In this invention, the P content is defined as ≤0.008%.

[0014] S: S mostly exists in steel in the form of FeS, which causes hot brittleness and reduces the ductility and toughness of steel. The S content range is determined to be ≤0.005% in this invention.

[0015] Cr: a carbide-forming element that improves the hardenability, wear resistance, and corrosion resistance of steel. In steel, some Cr replaces iron to form alloyed cementite, improving the tempering stability of the steel; others dissolve into ferrite, resulting in solid solution strengthening and increasing the strength and hardness of the ferrite. In this invention, the Cr content is determined to be in the range of 1.10–1.35%.

[0016] V: V is an element that plays a significant role in grain refinement. It can also generate V(C,N) composite grains, thereby refining the grains and improving the low-temperature impact toughness of steel. V can overcome the cold brittleness and severely deteriorated weldability caused by P in steel. In this invention, the V content is controlled at 0.01% to 0.05%.

[0017] Nb: The primary carbide melting point of Nb is high, and it is not easily dissolved during the forging heating process, so it cannot play a role in refining the grains. In addition, Nb is a precious metal, which increases the production cost of steel. This invention controls its content to ≤0.01%, only using the residue and not adding it in order to reduce costs.

[0018] Al has a strong affinity for oxygen. As a deoxidizing element in steel, it is added not only to reduce dissolved oxygen in molten steel, but also to form fine, dispersed aluminum nitride inclusions with nitrogen, which can refine the grain size. However, a high Al content can easily lead to the formation of brittle inclusions such as Al₂O₃ during steelmaking, reducing the purity of the molten steel. In this invention, the Al content is determined to be in the range of 0.010–0.050%.

[0019] N, H, O: The gaseous elements oxygen, nitrogen, and hydrogen are all harmful elements in steel. Oxygen forms oxide inclusions in steel, which have a significant impact on the strength, plasticity, and fatigue strength of steel. Nitrogen has poor solubility in ferrite and reduces the plasticity of steel at high temperatures. Hydrogen can cause hydrogen embrittlement and white spot defects. The range of N content in this invention is determined to be N≤0.0070%, H≤0.00018%, and O≤0.0020%.

[0020] As, Sn, Sb, Pb, and Bi are all trace elements and low-melting-point non-ferrous metals that easily cause soft spots and uneven hardness on the surface of parts. Therefore, they are considered harmful elements in steel. In this invention, the content range of these elements is defined as As ≤ 0.01%, Sn ≤ 0.01%, Sb ≤ 0.01%, Pb ≤ 0.01%, Bi ≤ 0.01%, and Sn + As + Sb + Pb + Bi ≤ 0.035%.

[0021] This invention also provides a method for manufacturing the above-mentioned high-temperature pressure vessel steel, the production steps of which involve: hot metal pretreatment, initial BOF refining, LF ladle refining, vacuum degassing, continuous casting of round billets, slow cooling of round billets, heated forging, air cooling of forgings, normalizing, hydrogen diffusion annealing, and quenching and tempering treatment, specifically achieved through the following processes: 1. Hot metal pretreatment and converter primary smelting After pretreatment, the molten iron must meet the following requirements: Si ≤ 0.08% and S ≤ 0.002%. The molten iron and scrap steel should be mixed in a weight ratio of 4:1 or higher to form the smelting raw materials for the converter. The smelting raw materials should be fed into the converter for initial smelting. The converter should ensure that the P in the molten steel is ≤ 0.006% at the time of tapping (the P may increase during subsequent smelting processes, so the converter should allow for some margin at tapping). At the same time, C ≥ 0.05% should be provided to prevent the molten steel from over-oxidizing. The tapping temperature should be ≥ 1610℃, and the five harmful elements Sn, As, Sb, Pb, and Bi should all be less than 0.006%, and Sn + As + Sb + Pb + Bi ≤ 0.025%. When tapping the steel from the converter, 1 ton of ferrochrome and 200 kg of ferroaluminum should be added appropriately. The tapping time should be controlled at 5-8 minutes. Slag-blocking plugs should be used during the tapping process to prevent oxidized slag from entering the next process with the molten steel, which would lead to the return of P and S and the generation of oxide inclusions.

[0022] 2. LF external refining Molten steel enters the LF refining furnace for slag formation, stirring, deoxidation, and alloying. Lime and fluorite are added to the furnace during the smelting process. SiC is used for diffusion deoxidation on the surface of the refining slag. The viscosity and stability of the slag are adjusted to control the FeO+MnO content in the slag to be less than 1%. The chemical composition is adjusted to the design requirements by mass percentage. Argon gas is used to stir the steel during the refining process to make the solid white slag float to the surface of the molten steel and be removed from the slag. The time for white slag removal is controlled to be ≥25 min, and the total [O] content in the steel is ensured to be ≤20 ppm.

[0023] 3. RH vacuum degassing Molten steel is hoisted to the RH vacuum degassing station via a ladle. During the vacuuming process, the processing time is greater than 15 minutes while maintaining a vacuum level of ≤100Pa. No adjustment of alloy element content is made during the vacuum degassing process. The atmosphere analysis of the molten steel after vacuum degassing should meet the following requirements: H≤1.0ppm, O≤15ppm, N≤60ppm.

[0024] 4. Continuous casting of round billets The refined molten steel is cast into continuously cast round billets of 1000mm or larger. The fluctuation of the liquid level in the crystallizer is controlled within ±5mm, the superheat is controlled between 15 and 45℃, the casting speed is 0.12 to 0.17m / min, and a three-stage electromagnetic stirring is adopted: the electromagnetic stirring in the secondary cooling section, the crystallizer, and the end is 200A / 2Hz, 300A / 1Hz, and 300A / 2Hz, respectively. The secondary cooling water ratio is 0.20 L / kg. After the continuously cast round billets are cut into sections according to the required length, they are slowly cooled in the pit for more than 72 hours. The temperature in the pit is not lower than 550℃, and the temperature when they come out of the pit is not higher than 300℃.

[0025] 5. Heating and forging The continuously cast round billet is heated in a furnace to 1200℃ and held for more than 20 hours to ensure uniform heating and complete dissolution of alloying elements. Forging is then performed using a hydraulic press of 8000t or higher. The initial forging temperature is controlled between 1100 and 1180℃. The billet undergoes two upsetting and two drawing operations (ensuring an upsetting ratio of 3 or higher, and the ratio of the upset cross-sectional area to the unupset cross-sectional area). A center punch is then performed, strictly adhering to alignment requirements (the alignment deviation between the punch center and the geometric center of the continuously cast billet ≤ 5mm) to ensure complete removal of porosity and shrinkage cavities in the billet's core. The final forging temperature is not lower than 850℃. The billet is then returned to the furnace for reheating, with a second heating time exceeding 6 hours. After the second heating, the billet is expanded and rolled into a round shape. The entire forging process is completed in two heating operations, with a total forging ratio greater than 6. After forging, the billet is air-cooled to room temperature.

[0026] 6. Normalizing + Hydrogen diffusion annealing After forging, the steel undergoes a normalizing heat treatment: the normalizing temperature is 930℃, the holding time is at least 15 hours, and it is then air-cooled after being removed from the furnace. The purpose of normalizing is to refine the grains and homogenize the microstructure, preparing the microstructure for subsequent quenching and tempering. After normalizing, hydrogen diffusion annealing is performed: the annealing temperature is 650℃, the holding time is at least 24 hours, and it is then furnace-cooled. The purpose of hydrogen diffusion annealing is to fully remove hydrogen from the steel, prevent the formation of white spot defects, and eliminate the internal stress generated during forging and normalizing.

[0027] 7. Tempering heat treatment Quenching: Temperature 910℃, holding time 15h or more, medium is water, water outlet temperature controlled below 100℃; Tempering: Temperature 700℃, tempering time 24h or more, cooling to room temperature in furnace. The purpose of quenching and tempering heat treatment is to obtain sufficient tensile strength, and also to obtain a uniform, fine and stable grain structure. After quenching and tempering, a ferrite + pearlite structure can be obtained, and the grain structure is finer, reaching level 5 or above.

[0028] After heat treatment, the high-temperature pressure vessel blanks were sampled and tested for performance and inclusions to meet the performance requirements of Rm: 520~670MPa, Rel≥330MPa, A≥20%, KV2(-10℃)≥50J; after machining, the flaw detection met the requirements of level 4 (equivalent of flat bottom hole of single defect ≤φ3mm) in EN10228-3 standard.

[0029] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention adopts the alloy design concept of "low V (0.01~0.05%) + no Mo added + Ni (0.15~0.25%)", which significantly reduces the alloy cost while ensuring high temperature performance and hardenability. It has obvious economic advantages compared with the high V+Mo scheme in the prior art.

[0030] (2) By strictly controlling P≤0.008% and the five harmful elements, and introducing dual temper embrittlement sensitivity coefficients of X≤10 and J≤100, the present invention enables the steel to have excellent resistance to temper embrittlement and is suitable for long-term service in high-temperature environments.

[0031] (3) In response to the unique problems of core porosity, shrinkage cavities and compositional segregation of continuous casting large round billets with diameters of Φ1000mm and above, this invention has developed a special forging scheme of "high temperature long-term heat preservation (1200℃×>20h) + two upsetting and two drawing (upsetting ratio ≥3) + center punching + hole expansion and rounding", with a total forging ratio >6, which effectively eliminates the internal defects of continuous casting billets.

[0032] (4) The finished product of the present invention meets the mechanical property requirements of Rm 520~670MPa, Rel≥330MPa, A≥20%, KV2(-10℃)≥50J, the flaw detection meets the requirements of EN10228-3 Level 4 (φ3mm equivalent), and the grain size is ≥5. Attached Figure Description

[0033] Figure 1 This is a low-magnification tissue image of Embodiment 1 of the present invention.

[0034] Figure 2 This is a low-magnification tissue image of Embodiment 2 of the present invention. Detailed Implementation

[0035] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0036] Examples 1-2 illustrate the chemical composition and manufacturing method of the high-temperature pressure vessel steel of the present invention.

[0037] The chemical composition (wt%) of each embodiment is shown in Table 2-3.

[0038] Table 2

[0039] Table 3

[0040] The inclusions in the steel of each embodiment are shown in Table 4.

[0041] Table 4

[0042] Table 5 shows a comparison of the mechanical properties of each embodiment.

[0043] Table 5

[0044] The flaw detection performance of each embodiment met the requirements of Level 4 (equivalent to flat-bottomed hole with single defect ≤ φ3mm) in EN10228-3 standard, and no defects exceeding the standard were found.

[0045] The low-magnification microstructure of each embodiment is good, and the central crack, shrinkage cavity, and porosity all show a high level of control, as shown in the appendix. Figure 1 , 2 .

[0046] The manufacturing process of high-temperature pressure vessel steel in each embodiment is as follows: KR molten iron pretreatment—100t BOF furnace primary refining—100t LF ladle refining—100t RH vacuum degassing—Φ1000mm round billet continuous casting—slow cooling—heat forging—air cooling—normalizing—hydrogen diffusion annealing—quenching and tempering treatment—machining—flaw detection—inspection and warehousing.

[0047] In the specific smelting process, high-quality molten iron, scrap steel, and raw and auxiliary materials are selected, and residual elements in the steel are controlled. High-quality deoxidizers and refractory materials are selected to control oxygen content and inclusions, and high-grade alloys are selected to control the residual Ti content to less than 0.003%. After pretreatment, the molten iron must meet the following requirements: Si ≤ 0.08% and S ≤ 0.002%. The molten iron and scrap steel should be mixed in a weight ratio of 4:1 or higher to form the smelting raw materials for the converter. The smelting raw materials should be fed into the converter for initial smelting. The converter should ensure that the P in the molten steel is ≤ 0.006% at the time of tapping (the P may increase during subsequent smelting processes, so the converter should allow for some margin at tapping). At the same time, C ≥ 0.05% should be provided to prevent the molten steel from over-oxidizing. The tapping temperature should be ≥ 1610℃, and the five harmful elements Sn, As, Sb, Pb, and Bi should all be less than 0.006%, and Sn + As + Sb + Pb + Bi ≤ 0.025%. When tapping the steel from the converter, 1 ton of ferrochrome and 200 kg of ferroaluminum should be added appropriately. The tapping time should be controlled at 5-8 minutes. Slag-blocking plugs should be used during the tapping process to prevent oxidized slag from entering the next process with the molten steel, which would lead to the return of P and S and the generation of oxide inclusions. Molten steel enters the LF refining furnace for slag formation, stirring, deoxidation, and alloying. Lime and fluorite are added to the furnace during the smelting process. SiC is used for diffusion deoxidation on the surface of the refined slag. The slag viscosity and stability are adjusted to control the FeO+MnO content in the slag to be <1%. The chemical composition is adjusted to the design requirements by mass percentage. Argon gas is used for stirring during the refining process to float solid white slag to the surface of the molten steel and remove it from the slag. The time for white slag removal is controlled to be ≥25 min, ensuring that the total [O] content in the steel is ≤20 ppm. RH vacuum degassing is performed, with the vacuum level controlled at ≤100 Pa for a processing time greater than 15 min. No alloying element content is adjusted during vacuum degassing. The atmosphere analysis of the molten steel after vacuum degassing should meet the following requirements: H ≤1.0 ppm, O ≤15 ppm, N ≤60 ppm.

[0048] The superheat of continuous casting is controlled within 15–45℃. Example 1 uses a Ф1000mm round billet, with a casting speed controlled at 0.14–0.17 m / min. Example 2 also uses a Ф1000mm round billet, with the same casting speed. The continuous casting process employs three-stage electromagnetic stirring: electromagnetic stirring in the crystallizer (frequency 2Hz, current 200A); stirring in the second cooling section (frequency 1Hz, current 300A); and electromagnetic stirring at the end (frequency 2Hz, current 300A). After continuous casting, the billet is flame-cut into sections according to the user's specified length. The billet is then slowly cooled in a pit at 550℃–580℃ for 78–82 hours, and then sent to a forging plant's furnace to be heated to 1200℃ and held for 25 hours to allow the alloying elements in the steel to fully dissolve and exert their strengthening and toughening effects, ensuring the composition and performance of the final product. The initial forging temperature is 1100–1180℃, and the final forging temperature is 870–880℃. A two-upsetting and two-drawing process is used for center punching and reaming, with a total forging ratio of 8–10. Forging is followed by air cooling. The steel undergoes normalizing, hydrogen diffusion annealing, and quenching and tempering heat treatment according to the set heat treatment process. As shown in Tables 2, 3, 4, and 5, after forging heat treatment, the purity, yield strength, tensile strength, and low-temperature impact value of the high-temperature pressure vessel steel in the above embodiments of the present invention all meet the design requirements, indicating that the present invention is effective.

[0049] In addition to the above embodiments, the present invention can also adjust parameters such as billet thickness and continuous casting process according to the production requirements of converters and electric furnaces of different tonnages. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A high-temperature pressure vessel steel produced from continuously cast large round billets, with Fe as the base element, characterized in that: The chemical composition contains the following percentages by mass: C: 0.10–0.18%, Si: 0.35–0.60%, Mn: 0.45%–0.70%, P≤0.008%, S≤0.005%, V: 0.01%–0.05%, Al: 0.010%–0.050%, Nb: ≤0.01%, Ti≤0.003%, Cr≤1.10–1.35%, Cu≤0.05%, Ni: 0.15–0.25%, H≤0.00018%, O≤0.002%, N≤0.007%, As≤0.01%, Sn≤0.01%, Sb≤ 0.01%, Pb≤0.01%, Bi≤0.01%, and As+Sn+Sb+Pb+Bi≤0.035%, with the balance being Fe and unavoidable impurities, and the temper brittleness sensitivity coefficient of the steel satisfies: X=(10P+5Sb+4Sn+As)×100≤10; J=(Si+Mn)×(P+Sn)×10000≤100.

2. The high-temperature pressure vessel steel produced from continuously cast large round billets according to claim 1, characterized in that, The non-metallic inclusions in the steel, when tested according to GB / T 10561 standard method A, meet the following requirements: Class A fine inclusions ≤ 1.5, Class A coarse inclusions ≤ 1.0, Class B fine inclusions ≤ 1.5, Class B coarse inclusions ≤ 1.0, Class C fine inclusions ≤ 1.0, Class C coarse inclusions ≤ 1.0, Class D fine inclusions ≤ 1.5, Class D coarse inclusions ≤ 1.0, DS ≤ 1.5, and the sum of A+B+C+D fine inclusions ≤ 3.5 and the sum of A+B+C+D coarse inclusions ≤ 3.

5.

3. The high-temperature pressure vessel steel produced from continuously cast large round billets according to claim 1, characterized in that, The mechanical properties of the steel meet the requirements of Rm: 520~670MPa, Rel≥330MPa, elongation after fracture A≥20%, and KV2(-10℃)≥50J; the flaw detection meets the requirements of level 4 in EN10228-3 standard, that is, the equivalent of a single defect flat bottom hole ≤φ3mm and the grain size ≥5.

4. A method for manufacturing high-temperature pressure vessel steel produced from continuously cast large round billets as described in claim 1, characterized in that, The production process includes: molten iron pretreatment, primary BOF refining, LF ladle refining, vacuum degassing, continuous casting of round billets, slow cooling of round billets, heated forging, air cooling of forgings, normalizing, hydrogen-expanded annealing, and quenching and tempering treatment, specifically including the following steps: (1) Hot metal pretreatment After pretreatment, the molten iron must meet the following requirements: Si≤0.08% and S≤0.002%. Molten iron and scrap steel should be mixed in a weight ratio of 4:1 or higher to form the smelting raw materials for the converter. (2) Primary smelting in a converter The raw materials are fed into the converter for primary refining. When tapping the steel, the P content in the molten steel is ≤0.006%, the C content is ≥0.05%, the tapping temperature is ≥1610℃, and it is ensured that the five harmful elements: Sn, As, Sb, Pb, and Bi are all less than 0.006%, and Sn+As+Sb+Pb+Bi≤0.025%; the tapping time is controlled at 5-8 minutes, and slag-blocking plugs are used to prevent slag from entering the steel. (3) LF ladle refining Molten steel enters the LF refining furnace for slag formation, stirring, deoxidation and alloying. Lime and fluorite are added. SiC is used for diffusion deoxidation on the surface of the refined slag. FeO+MnO in the slag is controlled to be <1%; white slag time is ≥25min to ensure that the total [O] in the steel is ≤20ppm. (4) Vacuum degassing The RH vacuum degassing method is used, with a vacuum degree ≤100Pa and a processing time greater than 15min. The atmosphere analysis of the molten steel after vacuum degassing should meet the following requirements: H ≤1.0ppm, O ≤15ppm, N ≤60ppm. (5) Continuous casting of round billets Molten steel is poured into continuously cast round billets with a diameter ≥1000mm. After the continuously cast round billets are cut into sections according to the required length, they are slowly cooled in the pit for more than 72 hours. The temperature in the pit is not lower than 550℃ and the temperature when they come out of the pit is not higher than 300℃. (6) Heating forging The continuously cast round billet is heated to 1200℃ in the furnace and held for more than 20 hours. It is then forged using a hydraulic press of 8000t or more. The initial forging temperature is controlled at 1100-1180℃. After that, the center hole is punched. The final forging temperature is not lower than 850℃. It is then reheated in the furnace for a second forging. The second forging time is more than 6 hours. After the second forging, the hole is expanded and the billet is rolled into shape. The total forging ratio is greater than 6. After forging, it is air-cooled to room temperature. (7) Normalizing treatment After forging and air cooling, normalizing is carried out: normalizing temperature is 930℃, holding time is more than 15 hours, and air cooling is performed after taking it out of the furnace. (8) Hydrogen diffusion annealing After normalizing, hydrogen diffusion annealing is performed: annealing temperature 650℃, holding time 24 hours or more, furnace cooling; (9) Tempering heat treatment Quenching temperature is 910℃, holding time is more than 15 hours, quenching medium is water, and the water outlet temperature is controlled below 100℃; tempering temperature is 700℃, tempering time is more than 24 hours, and the furnace is cooled to room temperature.

5. The method for manufacturing high-temperature pressure vessel steel from continuously cast large round billets according to claim 4, characterized in that: In step (1), when the converter taps steel, add 1 ton of ferrochrome and 200 kg of ferroaluminum.

6. The method for manufacturing high-temperature pressure vessel steel from continuously cast large round billets according to claim 4, characterized in that: In step (5), the fluctuation of the liquid level in the crystallizer is controlled within ±5mm, the superheat is controlled between 15 and 45℃, the casting speed is 0.12 to 0.17m / min, and three-stage electromagnetic stirring is adopted: the electromagnetic stirring of the secondary cooling section, the crystallizer, and the end is 200A / 2Hz, 300A / 1Hz, and 300A / 2Hz, respectively, and the secondary cooling water volume is 0.20 L / kg.

7. The method for manufacturing high-temperature pressure vessel steel from continuously cast large round billets according to claim 4, characterized in that: In step (5), the forging process first adopts two upsetting and two drawing processes, and the ratio of the cross-sectional area of ​​the round billet after upsetting to the cross-sectional area of ​​the round billet before upsetting is ≥3.

8. The method for manufacturing high-temperature pressure vessel steel produced from continuously cast large round billets according to claim 4, characterized in that: In the center punching process of step (5), the alignment deviation between the punching center and the geometric center of the continuously cast round billet is ≤10mm.

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