A kind of extra-thick high homogeneity large single-weight nuclear power pressure-bearing equipment steel and its manufacturing method

Abstract

The application discloses a kind of extra-thick high homogeneity big single weight nuclear power pressure-bearing equipment steel and its manufacturing method, the chemical composition of steel C0.20-0.30, Si0.25-0.45, Mn1.45-1.85, P≤0.010, S≤0.005, Cr0.55-0.75, Ni0.35-0.55, Mo0.85-1.55, Al0.030-0.050, N0.008-0.015, RE0.01-0.04.Manufacturing method includes electroslag remelting, forging, heating rolling, pretreatment, quenching and tempering heat treatment, the tensile strength of steel plate is greater than or equal to 750MPa at room temperature, the tensile strength of 150 DEG C high temperature tensile test is greater than or equal to 620MPa, the tensile strength of 350 DEG C high temperature tensile test is greater than or equal to 570MPa, the impact energy at-40 DEG C is all above 153J, and the tensile fracture shrinkage rate in the thickness direction of steel plate reaches more than 75%.Steel plate has good comprehensive mechanical properties, and meets the demand of nuclear power unit pressure-bearing equipment steel.

Description

Technical Field

[0001] This invention belongs to the field of metal material production technology, and in particular relates to an extra-thick, highly homogeneous, large-single-weight nuclear power pressure-bearing equipment steel and its manufacturing method. Background Technology

[0002] Steel used in nuclear power plant pressure vessels is employed in critical nuclear-grade equipment such as pressure vessels, pressurizers, various pipelines, boxes, tanks, and vessels within nuclear reactors. It plays a crucial role in the safe operation of nuclear power plants. The steel plates must possess not only high strength and toughness but also high-temperature resistance and ease of welding. Given the extremely important application locations, the overall quality and performance stability requirements for the steel plates are very stringent. Their operational safety and performance stability directly impact the safe operation of the equipment and the entire unit. With the national sustainable development strategy, nuclear power plant pressure vessels are gradually developing towards higher quality and integration, necessitating materials with higher strength, greater weight, and superior overall performance.

[0003] Currently, numerous patents have been established both domestically and internationally for steel used in nuclear power plants. The main patents related to this invention include the following:

[0004] The application filed by Northeastern University, entitled "An Ultra-Thick, High-Strength, High-Toughness, High-Homogeneity Extra-Thick Steel Plate for Hydropower and Its Manufacturing Method" (application number: CN202211018305.6), contains the following composition by mass percentage: C 0.02-0.10%, Si≤0.30%, Mn 4.0-8.0%, P≤0.015%, S≤0.005%, Als 0.015-0.040%, Cr≤0.4%, Mo≤0.40%, Ni≤1.0%, Cu≤0.3%, Nb≤0.030%, V 0.04-0.12%, Ca 0.001-0.005%, B≤0.002%, rare earth elements 0.01-0.04%, with the balance being Fe. Manufacturing steps: Blast furnace molten iron smelting - KR pre-desulfurization - converter tapping - VD / RH vacuum decarburization - LF refining - VD / RH vacuum degassing - ingot casting - hot cleaning and charging - ingot heating - rolling - ACC cooling - stack cooling - quenching - critical tempering. The steel produced by this invention has excellent hardenability, meeting the requirements of conventional quenching conditions for 120-300mm thick steel plates. The strength, plasticity, elongation, low-temperature toughness, and uniformity of microstructure of the produced steel plates are far superior to traditional CrNiMo alloy quenched and tempered high-strength steels. However, this invention has a long production process and does not address the mechanical properties and high-temperature properties of the steel plates after simulated post-weld heat treatment, making it unsuitable for the production of steel for large-scale nuclear power plant pressure equipment.

[0005] The application filed by Shougang Group Co., Ltd., entitled "A Large-Size, Extra-Thick Offshore Wind Power Steel and Its Production Method" (application number: CN202010654392.9), has the following chemical composition by weight percentage: C: 0.045-0.075%, Si: 0.20-0.30%, Mn: 1.30-1.60%, P≤0.010%, S≤0.003%, Cr≤0.20%, Nb: 0.025~0.035%, Ti: 0.01~0.02%, with the remainder being Fe and unavoidable impurities. The steel plate meets the requirements for large single weight and extra-thickness while possessing excellent weldability. This invention also provides a production method for large-weight, extra-thick offshore wind power steel. However, the steel plate manufactured by this invention has relatively low strength properties and unstable impact resistance at -40℃, failing to meet the technical requirements of nuclear power pressure-bearing equipment.

[0006] The application submitted by Ansteel Co., Ltd., entitled "High-strength Homogeneous Ferritic Extra-thick Steel Plate for Wind Turbine Structures and its Manufacturing Method" (Application No.: CN202311068588.X), specifies the following chemical composition (mass percentage): C: 0.10%-0.22%, Mn: 0.05%-1.88%, P≤0.022%, S≤0.010%, Cr: 0.01%-0.60%, Ni: 0.001%-0.009%, Mo: 0.001%-0.010%, Cu: 0.001%-0.009%, Nb: 0.010%. The composition is as follows: Fe: 0.001%-0.020%, V: 0.001%-0.027%, Ti: 0.001%-0.028%, CaO: 0.02%-0.05%, La: 5ppm-15ppm, B: 0.0011%-0.0029%, with the balance being Fe and unavoidable impurities. The manufacturing method includes smelting, continuous casting, billet assembly, rolling, and quenching. The wind power structural steel plates produced using this invention have a thickness of 80-150mm. The steel plates at 1 / 2T and 1 / 4T thickness exhibit fine polygonal ferrite and pearlite structures with a grain size ≥8.5. However, the steel plates produced by this invention have relatively low strength properties. Furthermore, the addition of rare elements such as Ca, B, and La makes smelting difficult and increases production costs, making it unsuitable for the production of steel for large-scale nuclear power pressure equipment.

[0007] The application filed by Ansteel Co., Ltd., entitled "A Highly Homogeneous Ultra-Thick Steel Plate for Advanced Pressurized Water Reactor Nuclear Power Plants and Its Manufacturing Method" (application number: CN202111194019.0), specifies the following steel plate composition by weight percentage: C: 0.1%-0.35%; Mn: 0.30%-1.50%; P≤0.010%; S≤0.005%; Cr: 0.10%-3.30%; Ni: 0.10%-3.90%; Mo: 0.01%-0.60%; V: 0.001%-0.020%; Cu: 0.01%-2.10%; Ca≤0.030%; Ti: 0.001%-0.020%, with the balance being Fe and unavoidable impurities. The manufacturing method includes smelting, continuous casting, billet assembly, rolling, and quenching and tempering. The steel plates produced using this invention maintain an impact absorption energy of over 122 J in the quenched and tempered state and over 107 J after simulated post-weld heat treatment at -20℃, respectively; the tensile strength difference between the 1 / 2 and 1 / 4 thickness points of the steel plate is ≤25 MPa. However, the composition design concept and manufacturing process of this invention differ from those of this invention, and its impact energy at -20℃ is relatively low, which cannot meet the technical requirements of nuclear power plant pressure-bearing equipment. Summary of the Invention

[0008] The purpose of this invention is to provide an extra-thick, highly homogeneous, large-unit-weight steel for nuclear power pressure equipment and its manufacturing method. By strictly controlling the chemical composition of the steel, as well as the electroslag remelting, forging, rolling and heat treatment processes, the produced steel plate not only has good comprehensive mechanical properties, but also has the characteristics of large unit weight and high homogeneity, which can fully meet the requirements of steel for nuclear power unit pressure equipment.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] One of the technical solutions of this invention is to provide an extra-thick, highly homogeneous, large-single-unit nuclear power pressure-bearing steel. The chemical composition of the steel by weight percentage is: C: 0.20%-0.30%, Si: 0.25%-0.45%, Mn: 1.45%-1.85%, P≤0.010%, S≤0.005%, Cr: 0.55%-0.75%, Ni: 0.35%-0.55%, Mo: 0.85%-1.55%, Al: 0.030%-0.050%, N: 0.008%-0.015%, RE: 0.01%-0.04%, with the remainder being Fe and unavoidable impurities.

[0011] The reasons for using the above-mentioned components are as follows:

[0012] C: In this invention, C is dissolved in the matrix to provide solid solution strengthening. Furthermore, through interaction with alloying elements, it forms fine alloy carbides, refining the grain size. During deformation, these carbides strongly impede dislocation slip, thereby significantly increasing the steel's strength. From the perspective of improving the low-temperature toughness and weldability of the steel plate, it is desirable to control the C content in the steel to a low level. However, from the perspective of matching the hardenability and strength / toughness of the steel plate, the C content should not be controlled too low. Therefore, the C content in the steel of this invention is designed to be 0.20%-0.30%.

[0013] Si mainly exists in steel as a reducing agent and deoxidizer. It can improve the strength and hardenability of steel, enhance the stability of retained austenite, thereby improving the toughness of steel, and effectively inhibit the initiation and propagation of cracks. However, excessive Si will cause severe segregation, making the steel anisotropic and adversely affecting the uniformity of the steel structure. Therefore, this invention requires the Si content to be 0.25%-0.45%.

[0014] Mn plays a role in solid solution strengthening, with relatively little impact on the plasticity of steel. During high-temperature solid solution treatment, it allows more carbides to dissolve into the matrix and combine with other strong carbide elements to form carbides, fully exerting its beneficial effects. Simultaneously, Mn in steel lowers the critical transformation temperature, refining the grain size and indirectly increasing the strength of the steel. However, increasing the Mn content reduces the plasticity and high-temperature performance of the steel, and easily leads to element enrichment and segregation, resulting in uneven material composition and microstructure. Therefore, this invention requires the Mn content in the steel to be controlled between 1.45% and 1.85%.

[0015] P and S: Harmful elements in steel. Excessive P and S can affect the homogeneity and purity of steel. P leads to microstructure segregation and has a significant adverse effect on low-temperature toughness. It tends to segregate at austenite grain boundaries, weakening the interatomic bonding force at the grain boundaries of the matrix material, resulting in high temper brittleness. S is distributed in steel in the form of MnS. During hot rolling, MnS elongates along the rolling direction, significantly reducing the transverse mechanical properties of the steel, exacerbating the anisotropy of the steel, reducing its ductility and toughness, and causing cracks during rolling. Therefore, the lower the content, the better. However, considering steelmaking conditions and costs, this invention requires controlling P in steel to ≤0.010% and S to ≤0.005%.

[0016] Cr: It can improve the hardenability of steel and has a secondary strengthening effect, promote alloying, and increase the strength of steel without making it brittle, ensuring the production and manufacturing of thick nuclear power steel. Simultaneously, Cr readily combines with carbon to form various carbides. These carbides are distributed in the steel matrix and pin dislocations, delaying the recovery of the martensitic matrix, thereby improving the high-temperature performance of the material. This plays a crucial role in improving the strength and thermal stability of the nuclear power steel of this invention. However, if the Cr content is too high, during high-temperature tempering or simulated post-weld heat treatment, other alloying elements in the carbides are replaced by Cr, forming coarse and soft high-chromium carbides, thus reducing the steel's hot strength. Therefore, this invention requires the Cr content in the steel to be controlled at 0.55%-0.75%.

[0017] Ni (Ni): On the one hand, it can significantly improve the strength of steel, and on the other hand, it maintains a consistently high level of toughness, lowering the ductile-brittle transition temperature of the material. It can prevent grain growth at high temperatures while preserving a fine-grained structure. Furthermore, Ni's lattice constant is similar to γ-Fe, allowing it to form a continuous solid solution, lowering the critical point and increasing the stability of austenite, especially improving the hardenability of thick, large-section materials. However, excessive Ni content increases production costs and causes lattice distortion, reducing the diffusion rate of carbon in the matrix, hindering phase transformation kinetics, and delaying the dissolution of undissolved carbides into austenite. Therefore, this invention requires the Ni content in the steel to be controlled at 0.35%-0.55%.

[0018] Mo (Mo) strengthens the strength and hardness of steel through solid solution strengthening. Mo dissolved in the matrix tends to aggregate around dislocations, hindering dislocation movement and acting as a pinning agent. This increases the tempering stability of the material, allowing parts to be tempered at higher temperatures, thus more effectively eliminating residual stress and improving plasticity. Furthermore, the small size and dispersed distribution of carbides precipitated during tempering enhance the thermal stability of the steel. For thick and large-section materials, it also improves the hardenability of the steel, enabling deeper and more thorough hardening. However, excessive Mo content can lead to larger carbide particles, causing embrittlement and reducing toughness. Therefore, this invention selects a Mo content of 0.85%-1.55%.

[0019] This invention requires controlling the Cr+Mo ratio to 1.50%-2.10%. This is done to improve the hardenability of the steel, ensuring uniformity of microstructure and properties along the thickness direction of large cross-sections, and to guarantee the high-temperature performance of the material after high-temperature tempering and long-term simulated post-weld heat treatment. If Cr and Mo are not controlled, excessive high-Cr carbides will form, reducing the material's thermal strength and affecting its high-temperature strength and tempering resistance. Therefore, the Cr and Mo addition ratio must be controlled to ensure the overall performance of the steel.

[0020] Al (Al) is a major deoxidizing element in steel and also helps refine grains. When used in conjunction with N (Nitrogen) in this invention, it can fix the N in the steel. Al also has antioxidant properties, and when used in conjunction with Cr, Si, Mo, etc., it can significantly improve the high-temperature resistance of steel. However, if the Al content is too high, not only will the effect be insignificant, but it will also easily lead to an increase in inclusions in the steel. Therefore, this invention requires that the Al content in the steel be controlled at 0.030%-0.050%.

[0021] This invention requires controlling the Al / Cr ratio in the steel to be 0.045 ≤ Al / Cr ≤ 0.085. Al in the steel refines the austenite grains by forming AlN, while Cr enhances strength through solid solution strengthening and carbide precipitation. The combination of these two factors balances the steel's strength and toughness. However, excessive Al preferentially combines with N to form AlN, reducing the precipitation of Cr carbonitrides and weakening the precipitation strengthening effect of Cr. Therefore, the Al and Cr addition ratio needs to be controlled.

[0022] This invention requires controlling the Al / Si ratio in steel to be 0.10 ≤ Al / Si ≤ 0.16. Both Al and Si are strong deoxidizers; their combined use can more thoroughly remove oxygen from molten steel, reduce oxide inclusions, and improve the purity of the steel. Simultaneously, Si dissolved in ferrite can significantly improve the strength of the steel, while Al indirectly enhances toughness through grain refinement. The combination of the two optimizes the balance between strength and plasticity. However, an improper Al / Si ratio may lead to the formation of low-melting-point aluminosilicate inclusions, deteriorating the hot working properties of the steel.

[0023] This invention requires controlling the Mo / Al content in steel to be 20 ≤ Mo ≤ 40. On one hand, Mo enhances the high-temperature strength of steel through solid solution strengthening and the formation of stable carbides, while Al's oxidation resistance delays high-temperature oxidation; the combination of these two factors helps ensure the high-temperature performance of the steel. On the other hand, Mo can inhibit the segregation of impurity elements such as P at grain boundaries, while Al refines the grains and reduces the grain boundary area, together reducing temper brittleness sensitivity. However, excessively high Mo content may promote Mo-C bonding, reduce AlN formation, and affect the grain refinement effect.

[0024] Nitrogen (N): As an interstitial solid solution strengthening element, it improves the strength of steel. It can also form carbonitrides with carbon, while reducing the diffusion rate of carbon and chromium, thus improving the thermal stability of the material. During tempering and simulated post-weld heat treatment, it promotes the fine and dispersed distribution of carbides, resulting in strengthening without compromising its plasticity. However, excessive N will significantly reduce the toughness of steel, worsen its weldability, and exacerbate cold brittleness. In this invention, its use in combination with Al can reduce its adverse effects and improve material properties. Therefore, this invention selects an N content of 0.008%-0.015%.

[0025] This invention requires controlling the Al / N ratio to be 2 ≤ Al / N ≤ 4. On one hand, the addition of Al and N can reduce the oxygen content in the steel, effectively improving its purity. Simultaneously, it can promote grain refinement, facilitate the formation of nano-precipitates, enhance the steel's toughness and strength, and improve processing performance. On the other hand, Al and N can alter the morphology of sulfides in the steel, transforming them into spherical or cellular structures, thereby reducing the impact of sulfides and improving the uniformity of the microstructure. If the addition of Al and N is controlled within an appropriate range, uncontrolled addition will lead to the formation of a large number of oxide inclusions in the steel, affecting its quality. Therefore, to ensure the overall performance of this steel, the Al and N addition ratio must be controlled.

[0026] Adding an appropriate amount of rare earth elements (RE) to steel can transform oxide and sulfide inclusions into fine, dispersed spherical inclusions, thereby eliminating the harmful effects of inclusions such as MnS. Furthermore, rare earth elements in steel can deoxidize, desulfurize, and their microalloying can alter the deformability of rare earth inclusions, especially by modifying brittle Al2O3 to a certain extent. Simultaneously, it improves the fluidity of steel, reduces non-metallic inclusions, and makes the steel structure denser and purer. When used in conjunction with elements such as Si and Al in steel, it significantly improves the strength and toughness of steel through inclusion modification, grain refinement, and phase transformation regulation. Therefore, this invention selects an RE content of 0.01%-0.04%.

[0027] This invention requires controlling the content of RE+Si+Al to be 0.32%≤0.51%. On the one hand, Si is a strong deoxidizer, generating SiO2 deoxidation products. The chain-like or clustered distribution of SiO2 easily becomes a crack initiation point. RE can react with SiO2 to form rare earth silicates, transforming brittle SiO2 into fine, spherical, and dispersed inclusions, reducing stress concentration and improving toughness. Al refines austenite grains by generating AlN, but AlN tends to coarsen. RE adsorbs on the AlN surface, inhibiting its growth and promoting the uniform distribution of fine AlN, further refining the grains. RE, Al, and Si synergistically promote the spheroidization and refinement of inclusions, reducing the number and size of harmful inclusions, and improving the cleanliness and mechanical properties of the steel. On the other hand, excessive Si exacerbates the hot brittleness of the steel and consumes too much RE to form rare earth silicates, reducing the purification effect of RE on other harmful elements. Excessive Al also excessively consumes N, reducing AlN formation and increasing competitive oxidation between RE and Al, thus reducing the modification effect of RE on inclusions. Therefore, the addition ratio of RE, Si, and Al needs to be controlled to achieve the synergistic effect of the three in refining grains and solid solution strengthening, thereby achieving a balance between high strength and high toughness.

[0028] The second technical solution of this invention provides a method for manufacturing steel for extra-thick, highly homogeneous, large-capacity nuclear power plant pressure-bearing equipment, comprising electroslag remelting, forging, hot rolling, pretreatment, and quenching and tempering heat treatment, wherein:

[0029] Electroslag remelting: Before arc ignition, the inlet water temperature of the crystallizer should be ≤45℃, the outlet water temperature ≤60℃, and the argon flow rate ≥30m³ / h. 3 / h; After arc ignition, the melting rate during the smelting stage is controlled at 1.31-1.45t / h. After power outage for 60-90 minutes, demolding begins and electroslag ingot slow cooling is carried out.

[0030] Forging: The electroslag ingot is heated to 1220-1250℃ in the soaking section for 10-12 hours for forging. After forging, it is placed in the furnace for holding at 620-650℃ for 10-15 hours, and then cooled with the furnace.

[0031] Heating and rolling: The heating temperature of the electroslag ingot is controlled at 1150-1200℃. The reduction rate of the first three passes during rolling is 20%-24%. The final rolling temperature is 920-970℃. The steel plate temperature after rolling is 350-450℃. After rolling, the steel plate is stacked and cooled slowly for 36-48 hours.

[0032] Pretreatment: The steel plate is heated to 1020-1050℃ and held for 3-4 hours. It is then removed from the furnace and air-cooled to room temperature. Subsequently, the steel plate is heated to 850-880℃ and held for 2-3 hours. It is then furnace-cooled to 300-350℃ and air-cooled to room temperature.

[0033] Quenching and tempering heat treatment: Quenching heat treatment, heat the steel plate to 900-950℃, hold for 4-7 hours, remove from the furnace and water cool to room temperature, then perform tempering heat treatment, heat the steel plate to 620-650℃, hold for 2-3 hours, and air cool to room temperature.

[0034] Furthermore, the consumable electrode for electroslag remelting is assembled by welding 2-3 continuously cast billets. The size of the consumable electrode is (600-750)×(1600-1900)×L, mm.

[0035] Furthermore, continuously cast billets at least 3m away from the head and tail billets are selected as consumable electrodes. The thickness of the continuously cast billets is 250-300mm. After the continuously cast billets are removed from the line, they are stacked and cooled slowly. The stacking temperature is ≥500℃ and the stacking time is ≥48h.

[0036] Furthermore, before arc initiation, the cooling water is turned on, with the flow rate of the crystallizer cooling water being 70-80 m³ / h. 3 / h, bottom tank flow rate 35-45m³ 3 / h.

[0037] Furthermore, the size of the electroslag ingot is (800-1000)×(1900-2200)×L, mm. The slow cooling pit is preheated to 200-250℃ and heated to 400-450℃ at a heating rate of 40-50℃ / h. The ingot is held at this temperature for 16-20h and then cooled in the pit. It is removed from the pit when the temperature is ≤150℃.

[0038] This invention provides an extra-thick, highly homogeneous, large-capacity steel for nuclear power plant pressure-bearing equipment and its manufacturing method. The produced steel plates have a thickness of 100-180 mm, a single weight of 30-50 tons, and the width and length can be produced according to actual needs. Compared with the prior art, the advantages are as follows:

[0039] (1) The present invention adopts the design concept of high Cr and high Mo, with Al and N used in combination, and Cr, Ni and Mo working together. No precious metal elements such as V, W and Co are added, which ensures that the steel plate with large interface thickness has good hardenability. Various carbides formed during tempering and simulated post-weld heat treatment are dispersed in the steel matrix, thereby inhibiting the aggregation and growth of carbides at high temperature. This plays an important role in improving the strength and thermal stability of the nuclear power steel of the present invention.

[0040] (2) The steel plate of this invention has a thickness of 100-180mm and a single weight of 30-50t. Under conventional hot rolling + quenching and tempering heat treatment production processes, the steel plate is difficult to meet the technical requirements for steel used in nuclear power pressure equipment, and the homogeneity of the steel plate cannot be guaranteed. This invention improves the homogeneity of the steel and reduces segregation by electroslag remelting + forging, while increasing the deformation in the thickness direction, so that the thickness T / 2 is fully deformed, fundamentally eliminating the original defects of the continuous casting billet, so that the steel of this invention has a stable and uniform structure, and has excellent mechanical properties and thermal stability.

[0041] (3) The heat treatment system adopted in this invention enables carbides to diffuse completely into the austenite grains at high temperature, resulting in a more uniform microstructure, finer grains, and fine and uniformly dispersed carbides. This microstructure increases the resistance to dislocation slip and significantly improves the thermal stability of steel, especially for steel plates with large cross-section thickness, which significantly improves the room temperature and high temperature performance at different thickness positions.

[0042] (4) Through chemical composition optimization and process parameter design, the present invention has a uniform and refined internal structure. The steel plate has fine tempered sorbite structure at thickness T / 2 and thickness T / 4, with a grain size ≥ 8.5.

[0043] (5) The steel grade of this invention exhibits high mechanical properties under both quenched and tempered heat treatment and simulated post-weld heat treatment conditions. At different thicknesses and under different conditions, the steel plate exhibits a tensile strength ≥750 MPa at room temperature, ≥620 MPa at 150℃, and ≥570 MPa at 350℃. The impact absorption energy at -40℃ is above 153 J, and the tensile reduction of area in the thickness direction can reach over 75%. The results show that the steel plate has a uniform and stable microstructure and properties, and maintains good strength even after simulated post-weld heat treatment, fully meeting the technical requirements for steel used in nuclear power plant pressure equipment. Attached Figure Description

[0044] Figure 1 Metallographic diagram of tempered sorbite structure of steel plate with thickness T / 4 in Example 5;

[0045] Figure 2 The image shows the metallographic structure of tempered sorbite in steel plate with a thickness of T / 2 in Example 5. Detailed Implementation

[0046] The chemical composition of various embodiments of an extra-thick, highly homogeneous, large-single-weight nuclear power plant pressure-bearing steel is shown in Table 1.

[0047]

[0048] A method for manufacturing an extra-thick, highly homogeneous, large-single-weight nuclear power plant pressure-bearing steel, comprising the following technical measures:

[0049] (1) Consumable electrode assembly: 2-3 continuously cast billets are welded together. The thickness of the continuously cast billets is 250-300mm. In order to ensure the high homogeneity of the original billet, the continuously cast billet at least 3m away from the head and tail billets should be selected as the consumable electrode. After the continuously cast billets are removed from the line, they are stacked and cooled slowly. The stacking temperature is ≥500℃ and the stacking time is ≥48h to fully ensure the internal and external quality of the continuously cast billet. The consumable electrode size is (600-750)×(1600-1900)×L, mm. The consumable electrode assembly process parameters are shown in Table 2.

[0050]

[0051] (2) Electroslag remelting: Before ignition and arc initiation, turn on the cooling water, with the flow rate of the crystallizer cooling water being 70-80 m³ / h. 3 / h, bottom tank flow rate 35-45m³ 3 / h, crystallizer inlet water temperature ≤45℃, outlet water temperature ≤60℃, argon flow rate ≥30m³ / h 3 / h.

[0052] After arc ignition, the melting rate during the smelting stage needs to be controlled at 1.31-1.45 t / h. After a power outage of 60-90 minutes, demolding and slow cooling begin. To ensure the surface quality of the electroslag ingot and prevent cracks caused by stress, the slow cooling pit is preheated to 200-250℃ and heated to 400-450℃ at a heating rate of 40-50℃ / h, held at that temperature for 16-20 hours, and then cooled in the pit. The ingot is removed from the pit when the temperature is ≤150℃. The dimensions of the electroslag ingot are (800-1000) × (1900-2200) × L, mm. The electroslag remelting process parameters are shown in Table 3.

[0053]

[0054] (3) Forging: To fully eliminate defects such as porosity, shrinkage cavities, and dendritic crystals inside the electroslag ingot, and to refine coarse grains through plastic deformation and recrystallization of the metal to obtain a dense structure, the electroslag ingot is heated to 1220-1250℃ in the soaking section for 10-12 hours for forging. The alloy content of this invention is relatively high, which usually requires a higher heating temperature and a longer holding time to allow the alloying elements to fully dissolve in the matrix, improve the compositional inhomogeneity caused by the large thickness, reduce compositional segregation, and thus reduce subsequent microstructure segregation, allowing large-size eutectic carbides to dissolve. After forging, it is placed in the furnace for holding at 620-650℃ for 10-15 hours, and then cooled with the furnace.

[0055] (4) Heating and rolling: The heating temperature is controlled at 1150-1200℃. After the billet is taken out of the furnace, it is descaled by high pressure water. The reduction rate of the first three passes during steel plate rolling is 20%-24%. Using a large reduction can improve the deformation penetration depth, so that the coarse columnar crystals can be broken and fine and uniform grains can be formed, and the central structural defects can be welded together. The final rolling temperature is 920-970℃, and the steel plate temperature after exiting the line is 350-450℃. After exiting the line, the steel plates are immediately stacked and slow cooled using the "bottom-up, top-down" method. The slow cooling time is 36-48 hours. The thickness of the produced steel plates is 100-180mm, and the single weight is 30-50t.

[0056] (5) Pretreatment: The steel plate is pretreated by normalizing and annealing. It is heated to 1020-1050℃ and held for 3-4 hours. After being removed from the furnace, it is air-cooled to room temperature. Then, the steel plate is heated to 850-880℃ and held for 2-3 hours. After being furnace-cooled to 300-350℃, it is air-cooled to room temperature. This pretreatment can significantly improve the strength and toughness of the steel in the subsequent tempering temperature range. At the same time, it can eliminate undissolved carbides and banded carbides distributed along the grain boundaries, increase the alloying degree in austenite, refine the grains, and make them more uniformly distributed. It increases the resistance to dislocation slip and increases the dispersion of carbides after tempering. Especially for steel plates with large cross-section thickness, it significantly improves the room temperature and high temperature performance of different thickness positions.

[0057] (6) Quenching and tempering heat treatment: Quenching heat treatment, heating the steel plate to 900-950℃, holding for 4-7h, water cooling to room temperature after removal from the furnace, and then tempering heat treatment, heating the steel plate to 620-650℃, holding for 2-3h, and air cooling to room temperature. The alloy content of this invention is relatively high. After adopting this quenching process, the carbides at the grain boundaries can completely diffuse into the austenite grains at high temperature, resulting in a more uniform microstructure, finer grains, and fine and uniformly dispersed carbides. This microstructure increases the resistance to dislocation slip, significantly improving the thermal stability of the steel. The role of the tempering process is to eliminate residual stress, homogenize the entire microstructure, and allow alloying elements to precipitate from the solid solution and disperse in the matrix. These precipitated phases have high thermal stability, which is beneficial to improving the high-temperature strength of the material while also providing good toughness. The forging, heating, rolling, and heat treatment process parameters are shown in Table 4.

[0058]

[0059] Table 4. Forging, heating, rolling, and heat treatment process parameters (continued)

[0060]

[0061] The mechanical properties of the steel plate are shown in Table 5.

[0062]

[0063] Table 5 Mechanical Properties of Steel Plates (Continued)

[0064]

[0065] Figure 1 , Figure 2 The metallographic structures at different thicknesses of the tempered steel plate in Example 5 show that the metallographic structure is tempered sorbite, and the grain size is also grade 9.0, indicating that the steel plate has good hardenability. Similarly, the uniform and fine grains also lay the foundation for the excellent mechanical properties of the steel plate.

[0066] Mechanical property tests on steel plates with thicknesses ranging from 100 to 180 mm under different conditions and locations showed that the results met the requirements of the specifications and fully satisfied the requirements for manufacturing nuclear power pressure equipment.

[0067] It is hereby noted that the above embodiments are only for illustrating the technical concept and features of the present invention, and are not intended to limit the present invention. Any equivalent substitutions or modifications made without departing from the essence of the present invention fall within the protection scope of the present invention.

Claims

1. A type of extra-thick, highly homogeneous, large-unit-weight nuclear power plant pressure-bearing steel, characterized in that, The chemical composition of the steel by weight percentage is as follows: C: 0.20%-0.30%, Si: 0.25%-0.45%, Mn: 1.45%-1.85%, P≤0.010%, S≤0.005%, Cr: 0.55%-0.75%, Ni: 0.35%-0.55%, Mo: 0.85%-1.55%, Al: 0.030%-0.050%, N: 0.008%-0.015%, RE: 0.01%-0.04%, with the remainder being Fe and unavoidable impurities.

2. The steel for extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing equipment according to claim 1, characterized in that, Cr+Mo=1.50%-2.10%, 2≤Al / N≤4, 0.045≤Al / Cr≤0.085, 0.10≤Al / Si≤0.16, 20≤Mo / Al≤40, 0.32%≤RE+Si+Al≤0.51%.

3. The steel for extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing equipment according to claim 1, characterized in that, The steel plate thickness is 100-180mm, and the single weight is 30-50t.

4. The steel for extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing equipment according to claim 1, characterized in that, The steel plate has a fine tempered sorbite structure at thicknesses T / 2 and T / 4, with a grain size ≥ 8.

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5. The steel for extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing equipment according to claim 1, characterized in that, Under quenched and tempered heat treatment and simulated post-weld heat treatment conditions, the steel plate has a tensile strength of ≥750MPa at room temperature, a tensile strength of ≥620MPa at 150℃, a tensile strength of ≥570MPa at 350℃, and an impact absorption energy of ≥153J at -40℃. The tensile reduction of area in the thickness direction of the steel plate reaches ≥75%.

6. A method for manufacturing the extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing steel according to any one of claims 1 to 5, characterized in that, This includes electroslag remelting, forging, heated rolling, pretreatment, and quenching and tempering heat treatment, among which: Electroslag remelting: Before arc ignition, the inlet water temperature of the crystallizer should be ≤45℃, the outlet water temperature ≤60℃, and the argon flow rate ≥30m³ / h. 3 / h; After arc ignition, the melting rate during the smelting stage is controlled at 1.31-1.45t / h. After power outage for 60-90 minutes, demolding begins and electroslag ingot slow cooling is carried out. Forging: The electroslag ingot is heated to 1220-1250℃ in the soaking section for 10-12 hours for forging. After forging, it is placed in the furnace for holding at 620-650℃ for 10-15 hours, and then cooled with the furnace. Heating and rolling: The heating temperature of the electroslag ingot is controlled at 1150-1200℃. The reduction rate of the first three rolling passes is 20%-24%. The final rolling temperature is 920-970℃. The steel plate temperature after rolling is 350-450℃. After rolling, the steel plate is stacked and cooled slowly for 36-48 hours. Pretreatment: The steel plate is heated to 1020-1050℃ and held for 3-4 hours. It is then removed from the furnace and air-cooled to room temperature. Subsequently, the steel plate is heated to 850-880℃ and held for 2-3 hours. It is then furnace-cooled to 300-350℃ and air-cooled to room temperature. Quenching and tempering heat treatment: Quenching heat treatment, heat the steel plate to 900-950℃, hold for 4-7 hours, remove from the furnace and water cool to room temperature, then perform tempering heat treatment, heat the steel plate to 620-650℃, hold for 2-3 hours, and air cool to room temperature.

7. The method for manufacturing an extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing steel according to claim 6, characterized in that, The consumable electrode for electroslag remelting is assembled by welding 2-3 continuously cast billets. The size of the consumable electrode is (600-750)×(1600-1900)×L, mm.

8. A method for manufacturing an extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing steel according to claim 7, characterized in that, Select a continuously cast billet at least 3m away from the first and last billets of the continuous casting process as the consumable electrode. The thickness of the continuous casting billet is 250-300mm. After the continuous casting billet is removed from the line, it is stacked and cooled slowly. The stacking temperature is ≥500℃ and the stacking time is ≥48h.

9. A method for manufacturing an extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing steel according to claim 6, characterized in that, Before arc initiation, turn on the cooling water, with the flow rate of the crystallizer cooling water being 70-80 m³ / h. 3 / h, bottom tank flow rate 35-45m³ 3 / h.

10. A method for manufacturing an extra-thick, highly homogeneous, large-unit nuclear power plant pressure-bearing steel according to claim 6, characterized in that, Electroslag ingot dimensions: (800-1000)×(1900-2200)×L, mm. The slow cooling pit is preheated to 200-250℃ and heated to 400-450℃ at a heating rate of 40-50℃ / h. The temperature is held for 16-20h and cooled with the pit. The ingot is removed from the pit when the temperature is ≤150℃.

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