A steel for a super-large tube plate forging of a pressure vessel of a thermal power plant and a method for manufacturing the same

Abstract

The present application relates to a kind of power plants such as pressure vessel super large tube plate forging steel and its preparation method, the component of the tube plate forging steel is as follows according to percentage by weight: C:0.17%‑0.23%, Si:0.20%‑0.30%, Mn:1.30%‑1.60%, P≤0.015%, S≤0.010%, Al:0.020%‑0.040%, Nb:0.030%‑0.045%, Cr:0.20%‑0.35%, Ni:0.20%‑0.35%, Mo:0.45%‑0.65%, the balance is Fe and inevitable impurities.Preparation method: including converter→LF refining furnace refining→RH furnace vacuum degassing→thick slab continuous casting→continuous casting slab slow cooling→heat treatment annealing.The steel prepared in the application is [H], [O], [N] respectively≤1.0ppm, 10ppm, 40ppm;Harmful residual elements: Sn≤0.003%, Pb≤0.001%, As≤0.003%, Sb≤0.001%, Bi≤0.0040%;Compared with traditional production smelting process, the method has the advantages of low cost, high quality, efficient production process, while customer forging, and the like advantages of high material yield.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a steel for ultra-large tube sheet forgings for pressure vessels such as those used in thermal power plants and its preparation method. Background Technology

[0002] Currently, the steel used in pressure vessels such as deep-sea pressure testing equipment, oil and gas transportation equipment, ships, and thermal power plants for manufacturing welded ultra-large tube sheet forgings is usually 20MnMoNb low-alloy high-strength steel. Its preparation method is as follows: electric furnace (or converter) → refining → vacuum degassing → die casting of steel ingots and electric furnace (or converter) → refining → vacuum degassing → die casting of steel ingots → electroslag smelting into electroslag ingots.

[0003] The invention "Post-forging heat treatment process for welded ultra-large tube sheet forgings of pressure vessels" (application number: CN201310262401.X) discloses that the ultra-large tube sheet for pressure vessels is made of 20MnMoNb steel, with the following composition by mass fraction: C: 0.17%-0.23%, Si: 0.15%-0.40%, Mn: 1.30%-1.60%, P≤0.025%, S≤0.010%, Nb: 0.030%-0.045%, Cr: ≤0.30%, Ni: ≤0.20%, Mo: 0.45%-0.65%. The preparation method is electric furnace → refining → vacuum degassing → die casting of steel ingots.

[0004] The inventions “A Heat Treatment Process for Ultra-Large Tube Sheet Forgings for Pressure Vessels” (application number: CN201310262384.X) and “A Heat Treatment Process for Welded Ultra-Large Tube Sheet Forgings for Pressure Vessels” (application number: CN201310262402.4) both use 20MnMoNb steel as their material, and the preparation method is electric furnace → refining → vacuum degassing → die casting of steel ingots.

[0005] To meet market demand, reduce billet procurement costs, increase the yield of forgings, and improve the quality of tube sheet forgings, thereby enhancing their comprehensive mechanical properties, especially impact toughness, is a common goal pursued by steel mills and forging plants.

[0006] The reason why traditional tube sheet forgings did not use continuously cast billets as raw materials in the past was:

[0007] ① Forging enterprises that have steelmaking capabilities and provide their own billets are not large steel enterprises; they can only produce large steel ingots or electroslag ingots.

[0008] ② Forging enterprises that purchase billets as raw materials can acquire thick (450mm) continuously cast slabs. However, currently only a few large steel plate enterprises, such as Liaoning Yingkou, Hunan Xianggang, and Nanjing Steel, can provide thick (450mm) and heavy (approximately 30 tons) continuously cast slabs. These enterprises are ordinary steel plate producers, and their thick plate continuous casting machines lack equipment such as light or heavy pressing and electromagnetic stirring.

[0009] The company is unable to produce extra-thick plates with high quality requirements. A few companies, such as Nanjing Iron & Steel, have production lines for 450mm thick special steel slabs, but steel mills are unwilling to produce them due to low order volumes (each casting of a continuous casting slab requires at least three heats of steel, and the initial order volume must be at least 450 tons, while forging users cannot reach this volume for large tube sheet forgings using 20MnMoNb forgings). In addition, Nanjing Iron & Steel's 450mm continuous casting machine was only put into use in 2023, and its continuous casting production technology and other parameters are still being explored.

[0010] Therefore, forging plants mainly sell 20MnMoNb billets using large steel ingots or electroslag steel ingots, which have relatively mature processes and many purchasing manufacturers.

[0011] In summary, in order to promote the development and research of new materials in my country's thermal power and other fields, and by combining our own advantages in special steel technology, we will develop proprietary materials that enable the steel used in ultra-large tube sheet forgings for pressure vessels in my country's thermal power and other industries to achieve efficient production processes, better quality, and lower costs. Summary of the Invention

[0012] The technical problem to be solved by the present invention is to provide a steel for ultra-large tube sheet forgings for pressure vessels such as thermal power plants and a method for preparing the same, which takes into account both cost and performance, and gives it excellent comprehensive performance, especially high tangential and high-axis impact toughness and efficient production process.

[0013] The technical solution adopted by the present invention to solve the above problems is as follows: a steel for ultra-large tube sheet forgings for pressure vessels such as those used in thermal power plants, wherein the composition of the steel by weight percentage is as follows: C: 0.17%-0.23%, Si: 0.20%-0.30%, Mn: 1.30%-1.60%, P≤0.015%, S≤0.010%, Al: 0.020%-0.040%, Nb: 0.030%-0.045%, Cr: 0.20%-0.35%, Ni: 0.20%-0.35%, Mo: 0.45%-0.65%, with the balance being Fe and unavoidable impurities.

[0014] The rationale for the design of the components in this invention is as follows:

[0015] This material, due to the addition of appropriate alloying elements such as Mo and Nb, has a certain tendency to harden. It can achieve excellent comprehensive mechanical properties through quenching and tempering. However, due to the high content of alloying elements such as Mo and Nb, it has a high tendency to crack, and the risk of cracking during blanking, cutting, and forging is high. At the same time, the material itself has a certain tendency to harden, and the welding of thick-walled barrel-shaped structures is difficult, which will also generate large residual welding stress. Meanwhile, to ensure the performance of the ultra-large tube sheet forgings produced by users after quenching and tempering, especially the superior tangential and high-axis impact performance, the composition of 20MnMoNb is optimized by adding appropriate amounts of Cr and Ni alloying elements.

[0016] The reasons for adding Cr and Ni are as follows:

[0017] Cr: Chromium can increase the hardenability of steel and has a secondary hardening effect, which can improve the hardness and wear resistance of steel without making it brittle.

[0018] The main role of chromium in quenched and tempered structural steel is to improve hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering. It can also form chromium-containing carbides, thereby improving the wear resistance of the material surface.

[0019] Chromium partially dissolves into the matrix, providing solid solution strengthening, while the remainder combines with carbon to form carbides. During quenching heating, chromium dissolves in austenite and then solidifies in martensite after quenching, improving the steel's resistance to temper softening. During tempering, it precipitates from the matrix, generally forming alloy carbides, which tend to coarsen with increasing tempering temperature and time. In this invention, the chromium content is selected to be 0.20%-0.35%.

[0020] Nickel (Ni): In steel, nickel strengthens ferrite and refines pearlite, resulting in an overall increase in strength, with little effect on plasticity. Generally speaking, a certain amount of nickel can increase the strength of steel without significantly reducing its toughness. Statistics show that every 1% increase in nickel increases strength by approximately 29.4 MPa. With increasing nickel content, the yield strength of steel increases faster than its tensile strength. While increasing steel strength, nickel has less impact on the toughness, plasticity, and other industrial properties compared to other alloying elements. Because nickel lowers the pearlite transformation temperature, it refines the pearlite; and because nickel lowers the carbon content at the eutectoid point, it produces more pearlite than carbon steel with the same carbon content, resulting in higher strength for nickel-containing pearlitic-ferritic steel compared to carbon steel with the same carbon content. Conversely, to maintain the same strength, the carbon content of nickel-containing steel can be appropriately reduced, thus improving its toughness and plasticity. Nickel can improve the fatigue resistance of steel and reduce its notch sensitivity.

[0021] Furthermore, the addition of nickel to steel not only enhances its resistance to acids but also alkalis, and provides corrosion resistance to atmospheric conditions and salts. This invention selects a Ni content of 0.20%-0.35%.

[0022] This invention also provides a method for preparing steel for ultra-large tube sheet forgings for pressure vessels such as those used in thermal power plants. The specific process flow is as follows: converter → LF refining furnace refining → RH furnace vacuum degassing → thick slab continuous casting → slow cooling of the continuously cast slab → heat treatment annealing, wherein:

[0023] Converter: Raw materials are smelted in a converter (BOF furnace), and a suitable proportion of converter-specific dephosphorizing and slag-forming agent is added (4 kg of dephosphorizing and slag-forming agent per ton of steel, i.e., 4 kg / t steel) to obtain low-phosphorus steel with P≤0.015%. The specific proportion of the special dephosphorizing and slag-forming agent added is as follows by weight: active lime: dephosphorizing and slag-forming agent = (70%-95%): (5%-30%), where the proportion of the special dephosphorizing and slag-forming agent is: calcium ferrite: magnesium bricks or powder: high alumina powder = (60%-95%): (5%-30%): (2%-10%).

[0024] LF refining furnace refining: The molten steel is refined in a refining furnace, specifically including the following steps:

[0025] 1. Refining and deoxidation: The deoxidizer used can be aluminum granules, silicon carbide, or carbon-free deoxidizer. Deoxidation should be performed when the molten steel reaches the LF furnace and is heated for about 5 minutes, at which point the above-mentioned deoxidizers should be added. Adding any more deoxidizers later is strictly prohibited.

[0026] 2. Use desulfurizing agent and carbon-free deoxidizer. Composition: CaO: 45-55%, Al: 12-18%, Al2O3: 18-25%, Si2O: 6-10%, MgO: 3-6%, CaSi: 3-6%. Addition amount: 15-2 kg / ton of steel. Add it 5 minutes after energizing in the early stage of LF. To maintain the white slag time, use ladle argon blowing and LF heating.

[0027] 3. A special slag conditioner is then added to obtain refined molten steel with low sulfur and low inclusion content. The dosage of the special slag conditioner is as follows: 400-800Kg The dosage per ton of molten steel, the composition of the special slag conditioner is as follows by weight: Al2O3:CaO:MgO:SiO2 = (40%-80%):(10%-40%):(5%-20%):(2%-5%), particle size: 2-5mm, moisture ≤0.5%, ensuring slag system activity: 350, basicity: 1.8-2.2.

[0028] Thick slab continuous casting: The slab thickness is 450mm, and a heavy-pressure continuous casting process is adopted. Heavy-pressure process parameters: Reduction position: from 17.8m to 35m of the slab length (i.e., starting when the solid fraction fs: 0.20 and ending when the solid fraction fs: 0.85). Light and heavy-pressure reduction should begin in the solid-liquid two-phase region and end before the solidification end. Total reduction: 12-20mm. The purpose of implementing heavy-pressure reduction is to improve center segregation and center porosity of the continuously cast slab.

[0029] Slow cooling of continuously cast billets: Slow cooling is carried out using a hood or pit. Process requirements: slow cooling time: ≥24 hours, temperature of billet exiting the hood or pit: ≥250℃.

[0030] Heat treatment annealing: After the continuously cast billets come off the production line, they are first slowly cooled under a cover at the billet storage area of ​​the steelmaking plant for ≥24 hours before being sent to the heat treatment branch for annealing, specifically as follows:

[0031] 1) Annealing furnace loading temperature ≥ 200℃;

[0032] 2) Annealing process: Furnace temperature upon entering the furnace: ≤600℃, heating rate: ≤100℃ / h, heating temperature: 650-750℃, holding time: ≥18 hours (≥2.2min / mm); cooling rate: ≤40℃ / h, furnace temperature upon exiting the furnace: ≤300℃; cooling method after exiting the furnace: air cooling.

[0033] Compared with the prior art, the advantages of the present invention are as follows:

[0034] This invention employs the addition of appropriate amounts of Cr and Ni alloying elements to the original 20MnMoNb material. The production process involves converter → LF refining furnace refining → RH furnace vacuum degassing → thick slab continuous casting → slow cooling of the continuously cast slab → heat treatment annealing. This process results in forgings of this type of steel used for ultra-large tube sheet forgings in thermal power and other pressure vessels exhibiting superior overall performance. When the user employs quenching and tempering treatment (quenching: 900±20℃, holding time 120-420min, water cooling to ≤100℃, tempering: 650±20℃, holding time 180-660min, air cooling), the tangential and high-axis impact values ​​can reach over 200J, far exceeding the preparation method using 20MnMoNb material and die-cast steel ingots. The method of using continuous casting of thick slabs instead of die casting steel ingots reduces the preparation cost of the steel by about 5%-10%. At the same time, the yield rate during the user's forging process is increased from 65%-70% when using die casting steel ingots to 92%-95% when using continuous casting slabs.

[0035] The continuous casting process of this invention employs special control methods, primarily: heavy pressure reduction technology. By applying heavy pressure at the end of solidification, the solute flows in the opposite direction, causing the positively and negatively segregated molten steel before the solidification endpoint to mix, thus reducing the degree of segregation. This improves center segregation and center porosity.

[0036] Ultimately, this improved the internal quality of the continuously cast billet, achieving center segregation ≤ B0.5 and center porosity ≤ 0.5 grade. Regarding inclusion control: specialized dephosphorizing and desulfurizing agents were used to control P and S content at low levels (P ≤ 0.015%, minimum ≤ 0.005%; S ≤ 0.005%, minimum ≤ 0.001%). Special steel inclusion production control technology was utilized, and a specialized slag-forming agent was employed to increase the LF furnace slag's ability to remove and adsorb inclusions, achieving inclusion content of ≤ 0.5 for A coarse and A fine; B coarse and B fine; C coarse and C fine; and D coarse and D fine. This ensured that the forging plant's flaw detection met the following requirements: ultrasonic testing of the user's forgings, according to JB / T5000.15-2007, achieved Grade I; and the final mechanical properties, impact properties, and corresponding high-temperature properties of the user's forgings met or far exceeded the user's target requirements.

[0037] The present invention relates to a steel for ultra-large tube sheet forgings for pressure vessels such as thermal power plants and its preparation method, which conforms to the future development direction and is a steel for ultra-large tube sheet forgings for pressure vessels such as thermal power plants and its preparation method with development potential. Attached Figure Description

[0038] Figure 1 A flowchart illustrating a method for preparing ultra-large tube sheet forgings for pressure vessels such as those used in thermal power plants, provided in Embodiment 1 of the present invention:

[0039] Figure 2 This is a flowchart illustrating the specific operation of step 102 in Embodiment 1 of the present invention;

[0040] Figure 3 This is a flowchart illustrating the specific operation of step 103 in Embodiment 1 of the present invention.

[0041] Figure 4 This is a flowchart illustrating the specific operation of step 104 in Embodiment 1 of the present invention. Detailed Implementation

[0042] The technical solution of the present invention will be described in more detail 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 invention.

[0043] The embodiments of the present invention, based on the component ratio of the technical solution, involve the following steps: converter → LF refining furnace refining → RH furnace vacuum degassing → thick slab continuous casting → slow cooling of the continuously cast slab → heat treatment annealing. Please refer to the following steps. Figure 1 :

[0044] Step 101: The raw materials are smelted in a converter (BOF furnace) to obtain low-phosphorus molten steel.

[0045] In step 101, the raw materials are smelted in a converter (BOF furnace) to obtain low-phosphorus steel. Specifically, the raw materials used can be blast furnace hot metal and refined scrap steel. Furthermore, in this step, a converter-specific dephosphorizing slag-forming agent is added. This agent is specifically developed for the production of this type of steel and manufactured by a Shanghai auxiliary material plant. It contains an appropriate proportion of calcium ferrite and a certain proportion of slag flux, resulting in better foamed slag formation in the converter slag and improved dephosphorization (calcium ferrite reacts better with phosphorus in the steel to form calcium phosphate, which then enters the slag, achieving a good dephosphorization effect), resulting in lower phosphorus levels. Simultaneously, attention should be paid to controlling the content of residual elements As, Sn, Pb, Sb, and Bi in the raw materials during the smelting process.

[0046] In the specific processing, preferably, in order to ensure the performance of the steel, the P content in the steel is ≤0.015%, and the harmful residual elements in the steel are: Sn≤0.003%, Pb≤0.001%, As≤0.003%, Sb≤0.001%, Bi≤0.0040%.

[0047] Step 102: The low-phosphorus molten steel is refined in a refining furnace to obtain refined molten steel.

[0048] Preferably, step 102 in Embodiment 1 may include the following steps, please refer to Figure 2 :

[0049] Step 201: Pre-deoxidize the molten steel from the converter to obtain deoxidized molten steel. Since the molten steel obtained from converter smelting contains a large amount of oxygen, it is necessary to perform a pre-deoxidation operation. Specifically, the deoxidizer is aluminum-iron. After adding aluminum-iron to the molten steel, it can act as a reducing agent to remove the large amount of oxygen contained in the molten steel.

[0050] Step 202: The deoxidized molten steel is then refined in a refining furnace;

[0051] Among them: 1. In addition to using desulfurizing agents, special slag conditioners are added to make the LF slag system very different from the traditional slag system. The activity, basicity and flowability of the slag system are stably controlled as required, thereby ensuring better and more stable deoxidation, desulfurization and removal of inclusions. It can ensure that the oxygen content in the steel is controlled ≤10ppm and the S content in the steel is ≤0.003%.

[0052] 2. Then, diffusion deoxidation is carried out by using 1.5-2.0Kg / t steel composite deoxidizer and SiC powder, etc., and a certain proportion of lime and refining slag are added according to the desulfurization situation; and the white slag retention time and the good fluidity of the slag are extended.

[0053] 3. Ensure refining time is 30-50 minutes.

[0054] 4. After the residue turns yellowish-white, feed it with Al wire; the Al content during the process should be controlled between 0.010-0.050%.

[0055] 5. Argon control: Used after adding carbon and alloying. 500-600 NL / min argon flow rate Normal power supply and use 200-300 NL / min Argon quantity Use during non-power supply periods 50-80 NL / min argon flow rate .

[0056] 6. LF lifting grate temperature: 1600-1680℃

[0057] Step 103: The refined molten steel is processed in a vacuum degassing furnace to obtain vacuum degassed molten steel.

[0058] Preferably, step 103 in Embodiment 1 may include the following steps, please refer to Figure 3

[0059] Step 301: Perform RH vacuum degassing treatment on the refined steel in the LF refining furnace to obtain degassed steel: wherein: ensure that the ultimate vacuum degree is maintained for ≥15 minutes.

[0060] Step 302: After degassing, determine the hydrogen content, adjust the composition, feed the wire, and perform soft argon blowing. [H] ≤ 1.0 ppm; the finished product sample needs to undergo smelting nitrogen content analysis; adjust the finished product [Al] to the target range, then feed in CaSi wire: 0.5-1.0 m / t steel, and adjust the finished product [Ca] to the target range.

[0061] Step 303: Suitable temperature for the hanging bag: Hanging bag temperature: 1550-1650℃.

[0062] Step 104: The vacuum-treated molten steel is continuously cast into a thick slab to obtain a continuously cast slab with pure molten steel and low content of harmful residual elements.

[0063] Preferably, in step 104 of Example 1: the molten steel after vacuum treatment is continuously cast to obtain a hydrogen content ≤1.0ppm, an oxygen content ≤10ppm, and a nitrogen content ≤40ppm; the harmful residual elements in the steel are: Sn≤0.003%, Pb≤0.001%, As≤0.003%, Sb≤0.001%, and Bi≤0.0040%.

[0064] Specifically, this may include the following steps; please refer to [link / reference]. Figure 4 :

[0065] Step 401: Replace the ladle with argon and lift the ladle onto the continuous casting turret.

[0066] Step 402: Rotate the ladle to the pouring position, attach the long nozzle, purge with argon, and begin pouring;

[0067] Step 403: Argon is introduced into the tundish, and about 10-25 tons of molten steel are added. Then, a covering agent is added to the tundish, and about 20-35 tons of molten steel are added. The stopper is opened, and the molten steel is poured into the crystallizer.

[0068] Step 404: When the molten steel in the crystallizer is about 450-750mm away from the upper edge of the crystallizer, add protective slag until it reaches 350-650mm, and start casting.

[0069] Step 405: The billet is pulled and poured into the heavy pressure area, and the heavy pressure is started.

[0070] The preferred parameters for casting under heavy pressure are shown in Table 1.

[0071] Table 1. Parameter selection under heavy pressure in this embodiment.

[0072]

[0073] Step 106: Slow cooling of continuously cast slab

[0074] In step 106 of Example 1: the slow cooling process parameters for the thick slab are shown in Table 2.

[0075] Table 2 Slow Cooling Process Parameters for Continuously Cast Thick Slabs

[0076] Slow cooling form Slow cooling time Temperature outside the hood or pit Cover or pit cooling ≥24 hours ≥250℃

[0077] Step 107: Heat treatment and annealing of continuously cast slabs

[0078] In step 107 of Example 1: the annealing process parameters are shown in Table 3.

[0079] Table 3 Heat Treatment Annealing Process

[0080]

[0081] The controlled range of chemical composition of the steel in this embodiment of the invention is shown in Table 4, and the composition of the steel in this embodiment of the invention is shown in Table 5. The main process parameters for heat treatment annealing of the steel in this embodiment of the invention are shown in Table 6. The impact properties of forgings from a certain forging plant in this embodiment of the invention are shown in Table 7.

[0082] Table 4. Chemical composition control range (by weight) of the steel in this embodiment.

[0083]

[0084]

[0085] Table 5. Composition (wt%) of steel in the embodiments of the present invention

[0086]

[0087]

[0088] Table 6 Main process parameters for heat treatment annealing of steel in embodiments of the present invention.

[0089]

[0090] Table 7 Impact performance of forgings from a forging plant according to embodiments of the present invention

[0091]

[0092]

[0093]

[0094] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing steel for ultra-large tube sheet forgings for thermal power pressure vessels, characterized in that, The steel used for tube sheet forgings has the following composition by weight percentage: C: 0.17%-0.23%, Si: 0.20%-0.30%, Mn: 1.30%-1.60%, P≤0.015%, S≤0.010%, Al: 0.020%-0.040%, Nb: 0.030%-0.045%, Cr: 0.20%-0.35%, Ni: 0.20%-0.35%, Mo: 0.45%-0.65%, with the balance being Fe and unavoidable impurities; the [H], [O], and [N] in the steel used for tube sheet forgings are ≤1.0ppm, 10ppm, and 40ppm, respectively; harmful residual elements: Sn≤0.003%, Pb≤0.001%, As≤0.003%, Sb≤0.001%, Bi≤0.0040%; the method specifically includes the following steps: Step 1: Converter The raw materials are smelted in a converter, and a suitable proportion of converter-specific dephosphorizing and slagging agent is added. 4 kg of dephosphorizing and slagging agent is added per ton of steel to obtain low-phosphorus steel with P≤0.015%. The specific proportion of the special dephosphorization slag-forming agent added is as follows by weight: active lime: dephosphorization slag-forming agent = (70%-95%): (5%-30%), wherein the proportion of the special dephosphorization slag-forming agent is: calcium ferrite: magnesium bricks or powder: high alumina powder = (60%-95%): (5%-30%): (2%-10%). Step 2: Refining in the LF refining furnace The molten steel is refined in a refining furnace for 30-50 minutes, specifically including the following steps: 1) Refining and deoxidation: The deoxidizer used is aluminum granules, silicon carbide, or a carbon-free deoxidizer. Deoxidation should be performed 5 minutes after the molten steel reaches the LF furnace. Adding deoxidizing materials later is strictly prohibited. 2) Add the desulfurizing agent 5 minutes after energizing the LF in the early stage. To maintain the white slag time, use argon blowing in the ladle and LF heating. 3) Add a special slag conditioner to obtain refined molten steel with low sulfur and low inclusion content. The dosage of the special slag conditioner is 400-800 kg per ton of molten steel. Step 3: Continuous casting of thick slabs The slab thickness is 450mm. It adopts the heavy pressure continuous casting process. The heavy pressure process parameters are as follows: the pressing position is from 17.8m to 35m of the slab length, that is, the pressing starts when the solid fraction fs is 0.20 and ends when the solid fraction fs is 0.

85. The total pressing amount is 12-20mm. Step 4: Slow cooling of the continuously cast billet Slow cooling is employed using a cover or pit. Process requirements: slow cooling time: ≥24 hours, temperature exiting the cover or pit: ≥250℃. Step 5: Heat treatment annealing After the continuously cast billets come off the production line, they are first slowly cooled under a cover at the billet storage area of ​​the steelmaking plant for ≥24 hours before being sent to the heat treatment branch for annealing; the heat treatment annealing process is as follows: 1) Annealing furnace loading temperature ≥ 200℃; 2) Annealing process: Furnace temperature upon entering the furnace: ≤600℃, heating rate: ≤100℃ / h, heating temperature: 650-750℃, holding time: ≥18 hours, holding time rate: ≥2.2min / mm; cooling rate: ≤40℃ / h, furnace temperature upon exiting the furnace: ≤300℃; cooling method after exiting the furnace: air cooling.

2. The method for preparing steel for ultra-large tube sheet forgings for thermal power pressure vessels according to claim 1, characterized in that: In step two, the argon gas control is as follows: after adding carbon and alloys, use 500-600 NL / min of argon; during normal power supply, use 200-300 NL / min of argon; during non-power supply time, use 50-80 NL / min of argon; LF ladle temperature: 1600-1680℃.

3. The method for preparing steel for ultra-large tube sheet forgings for thermal power pressure vessels according to claim 1, characterized in that: The composition of the special slag conditioner mentioned in step two is as follows by weight: Al2O3:CaO:MgO:SiO2 = (40%-80%):(10%-40%):(5%-20%):(2%-5%), particle size: 2-5mm, moisture ≤0.5%, ensuring the activity of the slag system: 350, alkalinity: 1.8-2.

2.

4. The method for preparing steel for ultra-large tube sheet forgings for pressure vessels such as those used in thermal power plants according to claim 1, characterized in that: The refined molten steel is processed in a vacuum degassing furnace to obtain vacuum degassed molten steel, specifically including the following steps: 1) The molten steel refined in the LF refining furnace is subjected to RH vacuum degassing treatment to obtain degassed molten steel: wherein the ultimate vacuum degree is maintained for ≥15 minutes; 2) After degassing, determine the hydrogen content, adjust the composition, feed the wire, and gently blow argon, where: [H] ≤ 1.0 ppm, and the finished product sample needs to be analyzed for smelting nitrogen content; adjust the finished product [Al] to the target range, and then feed in CaSi wire: 0.5-1.0 m / t steel, and adjust the finished product [Ca] to the target range; 3) Suitable temperature for the hanging bag: Hanging bag temperature: 1550-1650℃.

5. The method for preparing steel for ultra-large tube sheet forgings for thermal power pressure vessels according to claim 1, characterized in that: Step three involves continuously casting the vacuum-treated molten steel into a thick slab to obtain a continuously cast slab with pure steel and low levels of harmful residual elements. The continuous casting process specifically includes the following steps: 1) Replace the ladle with argon and lift the ladle onto the continuous casting turret. 2) Rotate the ladle to the pouring position, attach the long nozzle, purge with argon, and begin pouring; 3) Argon is introduced into the tundish, 10-25 tons of molten steel are added to the tundish, 20-35 tons of molten steel are added to the tundish, the stopper is opened, and the molten steel from the tundish is poured into the crystallizer; 4) When the molten steel in the crystallizer reaches 450-750mm from the upper edge of the crystallizer, add protective slag; when it reaches 350-650mm, start billet drawing. 5) The billet is pulled and poured into the heavy pressure zone, and the heavy pressure is started.

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