Production method of high-temperature-resistant die welding SA516Gr70 nuclear power steel

Abstract

The invention discloses a production method of high-temperature-resistant die welding SA516Gr70 nuclear power steel, which adopts two-stage high-pressure rolling to ensure that grains of a steel plate are fully refined in the rolling process, and controls the self-tempering temperature and the plate shape of the steel plate by adjusting the water entering temperature, the water volume and the water pressure of the steel plate in the online quenching process. 70-80% of lath-shaped bainite and 10-15% of ferrite can be generated on the steel plate, meanwhile, off-line quenching of the steel plate is avoided, and the production cost is reduced. The flatness of the steel plate produced by adopting the scheme is greatly improved, and the unevenness can reach 3mm / m. After 13.5 hours of 675 DEG C die welding heat treatment, the yield strength at room temperature is greater than or equal to 420 MPa, the high-temperature tensile yield strength at 300 DEG C is greater than 340 MPa, the tensile strength at 300 DEG C and room-temperature tensile test is stabilized at 540-620 MPa, the elongation is greater than or equal to 25%, the impact energy at-20 DEG C is 170-230 J, and the plate shape and physical properties of the steel completely meet the use requirements of nuclear power steel.

Description

Technical Field

[0001] The invention belongs to the technical field of wide and thick plate metallurgy, and in particular relates to a production method of high-temperature resistant die-welded SA516Gr70 nuclear power steel. Background Art

[0002] As third-generation nuclear power technology increases its requirements for equipment service environments, the performance limitations of traditional SA516Gr70 steel in high-temperature die welding scenarios are becoming increasingly prominent, becoming a potential risk factor for the long-term safe operation of nuclear power equipment. On-site welding and local repairs of large components often require high temperatures (>300°C). Traditional SA516Gr70 is susceptible to abnormal grain coarsening and carbide segregation along grain boundaries in the heat-affected zone (HAZ) under the thermal cycling of high-temperature welding, resulting in a significant decrease in local toughness. Among them, SA516Gr70 nuclear power steel is required to have its room temperature yield strength not degraded and still meet ≥380MPa after heat treatment at a mold welding temperature of 675℃ and mold welding for 13.5 hours, and the high-temperature tensile yield strength at 300℃ is greater than 280MPa, the tensile strength of the tensile test at 300℃ and room temperature is between 485 and 620MPa, the elongation is ≥25%, and the impact energy at -20℃ is ≥57J. However, the SA516Gr70 steel produced by the existing scheme is prone to form brittle structure at high temperature, which not only reduces the crack resistance of the welded joint, but may also induce stress corrosion cracking (SCC) under subsequent high temperature and high-pressure service conditions, seriously threatening the structural integrity.

[0003] Patent application number 2023112049821 discloses “SA516Gr70 steel plate with excellent high temperature strength at temperatures above 400-525°C and its manufacturing method”, with the chemical composition of the steel plate being C: 0.16-0.20%, Si: 0.20-0.40%, Mn: 1.20-1.50%, P: ≤0.006%, S: ≤0.001%, Ni: 0.25-0.35%, Mo: 0.06-0.12%, Cr: 0. The yield strength and tensile strength margin of the obtained steel plate are above 50 MPa, the single value of the transverse Charpy impact energy at -40°C in the core is ≥150J, and the yield strength and tensile strength of the steel plate under high temperature tensile test at >400-525°C meet the requirements specified in Table Y and Table U of Part D of Section II of ASME. This solution uses a large amount of alloying elements such as Nb, V, Cr, and Ni to precipitate carbides under high temperature conditions to improve the high temperature performance of the steel, but its performance cannot meet the die welding performance requirements at a high temperature of 675°C for a long time.

[0004] Chinese patent 2022107958868 discloses “SA516Gr70 steel plate with excellent high temperature strength at 200-400°C and its manufacturing method”, whose chemical composition is C: 0.16-0.20%, Si: 0.20-0.40%, Mn: 1.20-1.50%, P: ≤0.006%, S: ≤0.001%, Ni: 0.10-0.20%, Mo: 0.04-0.10%, Cr :0.10~0.20%,Nb:0.01~0.02%,V:0.01~0.03%,Ti:0.01~0.03%,The steel plate has a yield strength and tensile strength margin of 30MPa or more under simulated post-weld heat treatment conditions at 620±10℃×12h, a core transverse Charpy impact energy value of -30℃ ≥100J, a yield strength of 320MPa or more and a tensile strength of 500MPa or more under high-temperature tensile treatment at 200~400℃. However, the die welding temperature of this solution is low, and the tensile strength margin is not large. At the same time, the use of normalizing + tempering process also increases the cost of heat treatment.

[0005] To ensure that the steel plate maintains high tensile strength and low-temperature impact toughness during prolonged high-temperature die welding, existing solutions, whether adding large amounts of alloying elements or using the traditional normalizing and tempering heat treatment route, increase production costs and still fail to meet the requirements of high-temperature die welding. There are no reports on how to produce SA516Gr70 nuclear power steel that meets the technical requirements of high-temperature die welding while simplifying the production process and reducing costs.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of the present invention is to provide a low-cost method for producing high-temperature resistant die-welding SA516Gr70 nuclear power steel. This solution has low alloy input, short process flow, excellent steel plate roughness, and meets the requirements of long-term high-temperature die welding.

[0008] In order to achieve the above object, the specific scheme adopted by the present invention is as follows: A method for producing high-temperature resistant die-welding SA516Gr70 nuclear power steel, comprising the following chemical composition by mass percentage (unit: wt%): C: 0.14-0.17, Si: 0.2-0.3, Mn: 1.4-1.6, P≤0.02, S≤0.003, Nb: 0.01-0.02, V: 0.02-0.03, Cr: 0.010-0.020, Ti: 0.008-0.015, Alt: 0.025-0.04, [H]≤1.0 ppm, [O]≤15 ppm, and the remainder being Fe and residual elements; The production of nuclear power steel includes rolling, online quenching and tempering, as follows: ① Rolling: The heated billet is subjected to two-stage controlled rolling and controlled cooling. The first-stage rolling temperature is greater than 980°C, the first three passes are reduced by 45-55mm, and the second-stage rolling temperature is controlled at 800-850°C. After the rolling is completed, the cumulative reduction rate is 55%-60%, and the finished product thickness is 40-80mm. ② Online quenching: After rolling, the steel plate is quenched online, and the water temperature of the steel plate is controlled to be 780-820℃, the cooling rate is ≥30℃ / s, and the red-back temperature of the steel plate after quenching is ≤420℃; in order to meet the cooling rate requirements, after the steel plate enters the quenching equipment, 9-12 groups of manifold nozzles are opened, and the water temperature of the quenching equipment is controlled to be 10-15℃, the water pressure of the manifold nozzle is 0.3-0.7MPa, and the water volume is 260-350m 3 / h, the ratio of water supply to water supply is 1.5 to 1.7; ③ Tempering: After quenching, the steel plate is sent to the tempering furnace for tempering, with a holding temperature of 680℃~720℃ and a holding time of 3.0~4.0min / mm, and then air-cooled to room temperature.

[0009] Compared with the prior art, the present invention has the following beneficial effects: In terms of composition design, by adding a small amount of micro-alloying elements such as Nb, V, and Ti, the tendency of the original austenite grain size to grow is reduced, the alloy cost is reduced, and the precipitation of a large amount of carbides or other brittle structures during long-term high-temperature mold welding is avoided, which affects the low-temperature impact toughness.

[0010] The steel is rolled in a two-stage controlled rolling and controlled cooling method with large reduction. The rough rolling stage is carried out in the recrystallization zone to refine the austenite grain size. The finishing rolling stage is carried out in the non-recrystallization zone, where the austenite grains are flattened and elongated. The deformed austenite structure has a stronger driving force for phase transformation, and the degree of refinement of its structure is further improved, thereby ensuring the strength and toughness of the steel plate.

[0011] During the subsequent direct quenching process, the microstructure can maintain the corresponding lath morphology. By controlling the water temperature of the steel plate, the water flow rate and pressure during the online quenching process, the number of manifold nozzles, and the roller speed, the main microstructure of the steel plate is guaranteed to be 70-80% lath bainite and 10-15% ferrite. At the same time, the microstructure dislocation density under online quenching conditions is higher than that of offline quenching processes, making the steel plate more resistant to high-temperature die welding after high-temperature tempering. Research has found that if the temperature of the steel plate entering the quenching equipment is low, the cooling rate may be faster, and the austenite will directly transform into lath bainite. The strength of the steel plate will increase but the toughness will decrease, resulting in excessive internal stress and increasing the risk of deformation or cracking. If the water temperature of the steel plate is too high, mixed crystals may be generated, and the insufficient cooling rate will cause most of the austenite to transform into ferrite or pearlite, thereby reducing the bainite content and affecting the strength of the steel plate.

[0012] In addition, the appropriate top-to-bottom water ratio not only effectively controls the plate shape, avoiding secondary cold straightening or flattening, but also thoroughly hardens the core structure of the steel plate, ensuring its excellent high-temperature performance. The flatness of the steel plate produced using this solution can reach 2-3mm / 1m.

[0013] After heat treatment at a mold welding temperature of 675℃ and mold welding for 13.5 hours, its room temperature yield strength is ≥420MPa, the high temperature tensile yield strength at 300℃ is >340MPa, the tensile strength of the tensile test at 300℃ and room temperature is stable at 540~620MPa, the elongation is ≥25%, and the impact energy at -20℃ is 170J~230J. Its plate shape and physical properties meet the requirements for nuclear power steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The technical features of the present invention are further described below in conjunction with the accompanying drawings and embodiments.

[0015] Figure 1 Schematic diagram of the metallographic structure of the steel plate obtained in Example 1 of the present invention. DETAILED DESCRIPTION Example

[0016] A 75 mm thick high-temperature resistant die-weldable SA516Gr70 nuclear power steel is produced by mixing KR desulfurized molten iron with high-quality scrap steel, smelting the steel in a converter, an LF furnace, and a VD vacuum furnace, and then casting the resulting ingot. The ingot has the following chemical composition (in wt%): C: 0.143, Si: 0.23, Mn: 1.45, P ≤ 0.010, S ≤ 0.003, Nb: 0.015, V: 0.023, Cr: 0.018, Ti: 0.015, Alt: 0.030, [H] ≤ 1.0 ppm, [O] ≤ 15 ppm, with the remainder being Fe and residual elements. After the steel billet is heated, it is rolled using a two-stage controlled rolling and controlled cooling method with large reduction: the starting rolling temperature of the first stage is 1120℃, and the reductions of the first three passes of rough rolling are 55mm, 50mm, and 48mm respectively; the starting rolling temperature of the second stage is 820℃, and the finished steel plate thickness is 75mm.

[0017] After rolling, the steel plate is quenched online: the temperature of the steel plate before entering the water is 785℃, the length of the online quenching roller is 12000mm, the quenching water temperature is 13.5℃, the roller speed is 1.3m / s, 12 groups of header nozzles are opened, and the water volume is controlled to 330m 3 / h, the ratio of lower water to upper water is 1.5, and the temperature of the steel plate after quenching is 380℃.

[0018] The online quenched steel plate was sent to the tempering furnace for tempering, with a holding temperature of 700°C and a holding time of 4.0 min / mm. After the holding period, the plate was air-cooled to room temperature. Example

[0019] A 70 mm thick high-temperature resistant die-weldable SA516Gr70 nuclear power steel is produced by mixing KR desulfurized molten iron with high-quality scrap steel, smelting the steel in a converter, an LF furnace, and a VD vacuum furnace, and then casting the resulting ingot. The ingot has the following chemical composition (in wt%): C: 0.147, Si: 0.25, Mn: 1.42, P ≤ 0.010, S ≤ 0.003, Nb: 0.02, V: 0.025, Cr: 0.015, Ti: 0.012, Alt: 0.035, [H] ≤ 1.0 ppm, [O] ≤ 15 ppm, with the remainder being Fe and residual elements. After the steel billet is heated, it is rolled using a two-stage controlled rolling and controlled cooling method with large reduction: the starting rolling temperature of the first stage is 1140℃, and the reductions of the first three passes of rough rolling are 50mm, 50mm, and 45mm respectively; the starting rolling temperature of the second stage is 820℃, and the finished steel plate thickness is 70mm.

[0020] After rolling, the steel plate is quenched online: the temperature of the steel plate before entering the water is 800℃, the length of the online quenching roller is 13000mm, the quenching water temperature is 12℃, the roller speed is 1.0m / s, 10 sets of header nozzles are opened, and the water volume is controlled to 280m 3 / h, the ratio of lower water to upper water is 1.7, and the temperature of the steel plate after quenching is 410℃.

[0021] The online quenched steel plate was sent to the tempering furnace for tempering, with a holding temperature of 690°C and a holding time of 3.5 min / mm. After the holding period, the plate was air-cooled to room temperature.

[0022] The steel plates obtained in Examples 1 and 2 were tested for appearance and mechanical properties. The test results are shown in Table 1 below.

[0023]

[0024] Table 1 shows that the roughness of the high-temperature-resistant SA516Gr70 nuclear power steel obtained by this solution can reach 3mm / m. After heat treatment at a die welding temperature of 675°C for 13.5 hours, its room temperature yield strength meets the requirements of ≥420MPa, the high-temperature tensile yield strength at 300°C is greater than 340MPa, and the tensile strength in tensile tests at 300°C and room temperature is stable at 540-620MPa with a large margin. The elongation is ≥25%, and the impact energy at -20°C is between 170J and 230J. Its plate shape and physical properties meet the requirements for nuclear power steel.

[0025] Metallographic examination of the steel plate structure obtained in Example 1 showed that the main structures were lamellar bainite and ferrite. The overall structure was relatively uniform and fine, which was an ideal structure for this grade of steel.

[0026] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limiting. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in this field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for producing high temperature resistant die-welded SA516Gr70 nuclear power steel, characterized in that: The steel has the following chemical composition by mass percentage (unit: wt%): C: 0.14-0.17, Si: 0.2-0.3, Mn: 1.4-1.6, P≤0.02, S≤0.003, Nb: 0.01-0.02, V: 0.02-0.03, Cr: 0.010-0.020, Ti: 0.008-0.015, Alt: 0.025-0.04, [H]≤1.0ppm, [O]≤15ppm, and the balance is Fe and residual elements; The production of nuclear power steel includes rolling, online quenching and tempering, as follows: ① Rolling: The heated billet is subjected to two-stage controlled rolling and controlled cooling. The first-stage rolling temperature is greater than 980°C, the first three passes are reduced by 45-55mm, and the second-stage rolling temperature is controlled at 800-850°C. After the rolling is completed, the cumulative reduction rate is 55%-60%, and the finished product thickness is 40-80mm. ② Online quenching: After rolling, the steel plate is quenched online, the water temperature of the steel plate is controlled at 780-820℃, the cooling rate is ≥30℃ / s, and the red-return temperature of the steel plate after quenching is ≤420℃; ③ Tempering: After quenching, the steel plate is sent to the tempering furnace for tempering, with a holding temperature of 680℃~720℃ and a holding time of 3.0~4.0min / mm, and then air-cooled to room temperature.

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