A steel for a containment vessel of a pressurized water reactor nuclear power plant and a method for manufacturing the same

Abstract

The application discloses a kind of pressurized water reactor nuclear power unit containment steel and manufacturing method thereof. Steel plate includes the following chemical components according to weight percentage: C: 0.07%~0.13%, Si: 0.1%~0.4%, Mn: 0.7%~1.5%, P≤0.010%, S≤0.005%, Ni: 0.35%~0.8%, Cu≤0.4%, Cr: 0.2%~0.65%, Mo: 0.31%~0.85%, V: 0.08%~0.15%, Ti: 0.04%~0.09%, Al: 0.04%~0.08%, [N]<20ppm, the balance is Fe and inevitable impurities. Steel plate manufacturing process is: smelting-continuous casting-heating-thermoforming and controlled cooling-off-line tempering heat treatment. The steel plate of the application still maintains good mechanical properties after simulated postweld heat treatment at holding temperature 625 DEG C for 15h, meets the high standard requirements of China's larger power CAP1400, CAP1700 nuclear power unit for containment steel.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, specifically relating to a type of steel for the containment vessel of a pressurized water reactor nuclear power unit and its manufacturing method. Background Technology

[0002] Third-generation pressurized water reactor nuclear power units such as AP1000 and CAP1000 employ a double-containment design, with an outer reinforced concrete structure and an inner steel containment structure, forming the final safety line of the entire nuclear power unit. The steel containment structure is primarily constructed using ASME SA-738Gr.B, a steel grade required for engineering applications to possess both medium-to-high strength and good low-temperature toughness. However, with the introduction and application of higher-power CAP1400 and CAP1700 nuclear power technologies, the requirements for steel materials needed for nuclear power plant construction have also increased. The thickness of steel plates used has increased from a maximum of 100mm to 130mm, the high-temperature tensile test temperature has increased from 150℃ to 200℃, and the simulated post-weld heat treatment temperature has increased from 605℃ to 625℃. To ensure equipment safety, engineering applications require a minimum tempering temperature at least 15℃ higher than the simulated post-weld heat treatment temperature. Therefore, the minimum tempering temperature in production has also increased. These changes mean that the original composition and properties of ASME SA-738Gr.B steel can no longer meet the construction requirements of my country's new high-power pressurized water reactor nuclear power units. Existing invention patents for this type of steel have problems such as the inability to guarantee simulated post-weld heat treatment performance and the lack of coverage of high-temperature tensile performance indicators.

[0003] Patent CN102264936A discloses a high-strength steel plate for nuclear reactor containment and its manufacturing method. The chemical composition of the steel is designed as follows: C: 0.03%~0.20%; Si: 0.15%~0.55%; Mn: 0.9%~1.5%; Al: 0.001%~0.05%; P≤0.030%; S≤0.030%; Cr≤0.30%; Mo≤0.20%; Ni≤0.60%; V≤0.07%; Nb≤0.04%; Ti: 0.005%~0.025%; N: 0.0020%~0.0060%; B: 0.0005%~0.0020%; Ca: 5ppm~50ppm, with the balance being Fe and unavoidable impurities. This invention achieves the manufacture of a high-strength and high-toughness steel plate with a thickness of less than 60mm by designing a C+Mn micro-alloying composition and combining it with recrystallization zone controlled rolling + offline quenching and tempering process. However, the invention does not specify the high-temperature tensile properties and simulated post-weld heat treatment properties of the steel plate. Because a small amount of Cr and Mo are added to its composition, it cannot effectively inhibit the aggregation and coarsening of precipitates in the steel plate during high-temperature, long-term simulated post-weld heat treatment. Therefore, the simulated post-weld heat treatment properties of the steel plate cannot meet the needs of actual engineering applications. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a steel for the containment vessel of a pressurized water reactor nuclear power unit and its manufacturing method. The steel plate maintains good mechanical properties after simulated post-weld heat treatment at a temperature of 625°C and a holding time of 15 hours, which can meet the high standard requirements of my country's larger power CAP1400 and CAP1700 nuclear power units for containment vessel steel.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] This invention provides a steel for the containment vessel of a pressurized water reactor nuclear power unit, comprising the following chemical composition by weight percentage:

[0007] C: 0.07%–0.13%, Si: 0.1%–0.4%, Mn: 0.7%–1.5%, P≤0.010%, S≤0.005%, Ni: 0.35%–0.8%, Cu≤0.4%, Cr: 0.2%–0.65%, Mo: 0.31%–0.85%, V: 0.08%–0.15%, Ti: 0.04%–0.09%, Al: 0.04%–0.08%, [N] <20ppm, balance being Fe and unavoidable impurities.

[0008] The reasons for using the above-mentioned components are as follows:

[0009] Carbon (C) promotes bainite formation and strengthens the matrix structure. It forms fine carbides with V and Ti, playing a precipitation strengthening role during continuous casting, heating, rolling, and cooling, thereby improving the strength and toughness of the steel plate. If the carbon content is too low, the formation of carbides and other compounds will decrease, affecting the precipitation strengthening effect; if the carbon content is too high, it will promote the formation of retained austenite during cooling, impairing the low-temperature impact performance of the steel plate. Therefore, the C content in the steel of this invention is designed to be 0.07%–0.13%.

[0010] Si: Silicon is mainly used as a reducing agent and deoxidizer in the steel of this invention. Excessive silicon content easily leads to the formation of large silicate inclusions, impairing the steel's ductility and toughness. The Si content in the steel of this invention is 0.1% to 0.4%.

[0011] Mn: Used to expand the austenite phase region, increase the density of deformation dislocation substructure and deformation bands. It lowers the martensitic transformation temperature of steel, promotes bainite formation, and refines the lath bainite structure during cooling, improving the plasticity and toughness of the steel plate. Used in conjunction with Cr and Mo, it eliminates the temper brittleness of the steel during high-temperature tempering and inhibits the aggregation and coarsening of precipitates during high-temperature, long-term simulated post-weld heat treatment, improving the strength and toughness of the steel plate. Excessive manganese content will promote the formation of retained austenite, impairing the low-temperature impact performance of the steel plate; insufficient manganese content will prevent it from exerting its effect. Therefore, the Mn content in this invention is designed to be 0.7%–1.5%.

[0012] P and S are both harmful elements in the steel of this invention. The lower the content, the better. However, considering the steelmaking conditions and costs, this invention controls the P content in the steel to be ≤0.010% and the S content to be ≤0.005%.

[0013] Ni: Used to expand the austenite phase region, improve the effect of controlled rolling deformation, and promote the formation of fine deformation structures. It lowers the martensite formation temperature and promotes bainite formation. It improves the hardenability of steel plates and enhances the uniformity of the microstructure across the thickness of the steel plate during controlled cooling. The Ni content in this invention is designed to be 0.35%–0.8% as needed.

[0014] Cu: Due to the influence of hardenability, proeutectoid ferrite tends to form at half the thickness of thicker steel plates. Adding an appropriate amount of Cu can suppress the γ→α transformation and avoid the formation of proeutectoid ferrite. Precipitates in the range of 10-30 nm can be formed, refining the grains and improving strength. Therefore, the Cu content in this invention is designed to be ≤0.4%.

[0015] Cr: Used to improve the thermodynamic stability of alloying penetrants such as (FeCr)3C precipitates in steel during tempering, and to reduce the aggregation rate of carbides during tempering; used in combination with Mn and Mo, it eliminates the temper brittleness of the steel in this invention during high-temperature tempering, and inhibits the aggregation and coarsening of precipitates during high-temperature, long-term simulated post-weld heat treatment, thereby improving the strength and toughness of the steel plate. In this invention, excessive Cr content will reduce the low-temperature toughness and weldability of the steel. Therefore, the Cr content in the steel of this invention is 0.2% to 0.65%.

[0016] Mo, used in combination with Mn and Cr, eliminates the temper brittleness of the steel during high-temperature tempering and inhibits the aggregation and coarsening of precipitates during high-temperature, long-term simulated post-weld heat treatment, thus improving the strength and toughness of the steel plate. It can lower the critical cooling rate for bainite transformation, which is beneficial for promoting bainite transformation over a wider cooling rate range, resulting in better process adaptability for thicker steel plates. Excessive molybdenum can impair the toughness of the heat-affected zone formed during welding, reducing the weldability of the steel; therefore, the Mo content in this invention is designed to be 0.31%–0.85%.

[0017] V: The steel of this invention is produced using a controlled rolling + controlled cooling + offline tempering process in the austenitic region. After rolling in the austenitic region at a relatively high temperature, V is essentially in a solution-treated state, which improves the hardenability of the steel plate. This is particularly beneficial for extra-thick steel plates over 50mm, effectively ensuring the uniformity of the phase transformation structure along the thickness section. Furthermore, during cooling, a large amount of V is dispersed and precipitated, which not only refines the bainite structure but also plays a significant role in precipitation strengthening. Further precipitation of V during tempering and simulated post-weld heat treatment effectively improves the heat treatment resistance of the steel plate. Therefore, the V content in the steel of this invention is designed to be 0.08%–0.15%.

[0018] Ti: Ti is a strong carbonitride forming element, but to avoid the formation of large TiN particles, this invention controls the N content in the steel to below 20 ppm through refining and the addition of a certain amount of Al, thereby promoting the formation of more TiC. TiC has strong bonding force, is stable, and does not easily decompose. In steel, it only slowly dissolves into the solid solution when heated to above 1100℃. Before dissolving, TiC particles can effectively prevent grain growth, thus playing a positive role in suppressing the growth and coarsening of the original austenite grains during the heating process of continuously cast billets. In addition, the steel of this invention is rolled at a relatively high temperature, and a large number of fine TiC particles can effectively suppress the recrystallization and growth of austenite during deformation, refining the grains. The steel of this invention requires welding in engineering applications, and the addition of Ti has an important impact on improving the weldability of the steel plate. Therefore, this invention controls the Ti content to be 0.04% to 0.09% as needed.

[0019] Al: Used for deoxidation and nitrogen fixation. It also plays a role in solid solution strengthening. It improves the high-temperature oxidation resistance of the steel plate, allowing it to maintain good high-temperature tensile properties even after simulated post-weld heat treatment. The Al content in the steel of this invention is controlled at 0.04%–0.08%.

[0020] N: This is a harmful element in the steel of this invention. The nitrogen content is reduced to less than 20 ppm by refining and adding a certain amount of Al.

[0021] In the above technical solution, the thickness of the steel plate is ≤130mm.

[0022] In the above technical solution, the steel plate has a room temperature tensile strength > 640 MPa, a yield strength > 570 MPa; a 200℃ tensile strength > 600 MPa, a yield strength > 520 MPa; an impact absorption energy at -45℃ > 250 J; and a zero plastic transition temperature (NDT) ≤ -60℃.

[0023] In the above technical solution, further, after simulated post-weld heat treatment at a heat treatment temperature of 625℃ and a heat treatment time of 15h, the steel plate has a room temperature tensile strength > 620MPa, a yield strength ≥ 560MPa; a 200℃ tensile strength > 590MPa, a yield strength > 510MPa; an impact absorption energy of -45℃ > 250J; and a zero plastic transformation temperature NDT ≤ -65℃.

[0024] Another aspect of the present invention provides a method for manufacturing steel for the containment vessel of the above-mentioned pressurized water reactor nuclear power unit, wherein the manufacturing process of the steel plate is: smelting—continuous casting—heating—controlled rolling and controlled cooling—offline tempering heat treatment;

[0025] The controlled rolling and cooling process is as follows: before rolling, the surface of the continuously cast billet is sprayed with water for 5-9 seconds to cool it down. When the temperature drops to 1100-1160℃, rolling begins. The single-pass reduction rate is >11%. Water mist cooling is applied between each rolling pass at a rate of 4-8℃ / s. The final rolling temperature is 870-910℃. After final rolling, the steel plate is cooled to 440-490℃ at a cooling rate of 30-45℃ / s, and then air-cooled to room temperature.

[0026] The offline tempering heat treatment process is as follows: the tempering temperature is 650-680℃, the holding time is 1-12 min / mm, and after taking it out of the furnace, it is air-cooled to room temperature.

[0027] In the above technical solution, the smelting process is further described as follows: smelting is carried out using a converter + LF + RH method, with the LF furnace producing white slag to refine the molten steel, the RH furnace performing vacuum circulation degassing for 10-20 minutes, soft blowing for 4-11 minutes before exiting the station, the soft blowing flow rate being 12-22 L / min, and the steel being allowed to stand for 3-7 minutes after soft blowing, with the tapping temperature being 1520-1550℃.

[0028] In the above technical solution, the continuous casting process is further described as follows: during casting, the superheat of the tundish is controlled to be 10-20°C, heavy pressure is applied at the end of the continuous casting process, the reduction is 17-28 mm, and the billet pulling speed is controlled to be 0.6-1.1 m / min, and the thickness of the continuously cast billet is 250-400 mm.

[0029] In the above technical solution, the heating process is further described as follows: the heating rate of the continuous casting billet is 3-7 min / cm, the holding temperature is 1280-1350℃, and the holding time is 20-60 min.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1. The manufacturing process of this invention is refined and efficient. Through the control of smelting and continuous casting processes, the steel plate obtained has a non-metallic inclusion content of no more than grade 1.0, high purity, gas [H] < 2ppm, [O] < 15ppm, low gas content, and the central porosity and segregation of the billet are no more than grade 1.5, resulting in excellent performance.

[0032] 2. The steel plate of this invention has a maximum thickness of 130mm, high hardenability, and can obtain a uniform and fine bainitic structure across the entire thickness section.

[0033] 3. The steel plate of this invention has a room temperature tensile strength > 640 MPa and a yield strength > 570 MPa; a 200℃ tensile strength > 600 MPa and a yield strength > 520 MPa; an impact absorption energy of -45℃ > 250 J; and a zero plastic transition temperature (NDT) ≤ -60℃. Moreover, the steel plate maintains good mechanical properties after simulated post-weld heat treatment at a holding temperature of 625℃ for 15 hours, meeting the high standard requirements for containment steel in my country's larger power CAP1400 and CAP1700 nuclear power units. Attached Figure Description

[0034] Figure 1 This is a metallographic diagram of Embodiment 4 of the present invention;

[0035] Figure 2 This is a non-metallic inclusion rating chart for Embodiment 4 of the present invention. Detailed Implementation

[0036] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0037] The chemical composition of the steel used for the containment vessel of the pressurized water reactor nuclear power unit provided in Examples 1-8 is shown in Table 1.

[0038] Table 1. Chemical composition (%) of steels in Examples 1-8

[0039]

[0040] The manufacturing method of the steel used for the containment vessel of the above-mentioned pressurized water reactor nuclear power unit includes the following steps:

[0041] (1) Smelting: The smelting process is carried out by converter + LF + RH. The LF furnace produces white slag to refine the molten steel. The RH furnace is vacuum circulated for degassing for 10 to 20 minutes. Before leaving the station, the steel is soft-blown for 4 to 11 minutes at a flow rate of 12 to 22 L / min. After soft blowing, the steel is left to stand for 3 to 7 minutes. The tapping temperature is 1520 to 1550℃. Soft blowing refers to continuously blowing the molten steel from the bottom of the ladle with inert gas. Through soft blowing, the inclusions in the molten steel are promoted to float and aggregate, and the inclusions are removed to improve the purity of the molten steel. Through refining, the content of non-metallic inclusions in the steel is reduced to no more than grade 1.0, and the gas content is reduced by [N] < 20 ppm, [H] < 2 ppm, and [O] < 15 ppm.

[0042] (2) Continuous casting: During casting, the superheat of the tundish is controlled at 10-20℃. At the end of the continuous casting process, heavy pressure is applied, with a reduction of 17-28mm. The billet pulling speed is controlled at 0.6-1.1m / min. This continuous casting process control improves the equiaxed crystal ratio of the billet, reduces the porosity and segregation in the center of the billet to no more than level 1.5, and greatly improves the quality of the billet, which is conducive to obtaining excellent mechanical properties of the steel plate. The thickness of the continuously cast billet is 250-400mm to ensure a compression ratio greater than 3.

[0043] (3) Heating: The heating rate of the continuous casting billet is 3 to 7 min / cm, the holding temperature is 1280 to 1350℃, and the holding time is 20 to 60 min. The control of this continuous casting process ensures that the austenitization of the continuous casting billet is uniform while avoiding grain growth caused by a long heating time.

[0044] (4) Controlled rolling and cooling: Before rolling, the surface of the continuously cast billet is sprayed with water for 5-9 seconds. When the temperature drops to 1100-1160℃, rolling begins. The single-pass reduction rate is >11%. Water mist cooling is used during the rolling interval. The cooling rate is 4-8℃ / s. The final rolling temperature is 870-910℃. The thickness of the rolled steel plate is ≤130mm. The control of this rolling process is conducive to the full refinement and homogenization of the steel plate structure, and promotes the dispersion of high-density dislocations and precipitates, thereby improving the uniformity and stability of the steel plate properties. The Ms temperature of the steel plate is about 400℃. In order to obtain a full-section bainitic structure and avoid the generation of proeutectoid ferrite, after the final rolling, the steel plate is cooled to 440-490℃ at a cooling rate of 30-45℃ / s, and then air-cooled to room temperature.

[0045] (5) Heat treatment: The tempering temperature is 650~680℃, the holding time is 1~12min / mm, and the plate is air-cooled to room temperature after being taken out of the furnace. High-temperature tempering further homogenizes the microstructure and eliminates the microstructure stress. At the same time, it promotes the precipitation of carbonitrides between bainite laths and improves the comprehensive performance of the steel plate.

[0046] The manufacturing process parameters of the steel in Examples 1-8 of this invention are shown in Table 2.

[0047] Table 2 Manufacturing process parameters for steels in Examples 1-8

[0048]

[0049]

[0050] The mechanical properties of the steel plates in Examples 1-8 of this invention are shown in Table 3.

[0051] Table 3. Properties of Steel Plates in Quenched and Tempered State

[0052]

[0053]

[0054] Samples from Examples 1-8 were used for simulated post-weld heat treatment tests. The process was as follows: holding temperature 625℃, holding time 15h, and heating / cooling rate above 425℃ not exceeding 60℃ / h. The performance test results are shown in Table 4.

[0055] Table 4 Simulated Post-Weld Heat Treatment Properties of Steel Plates

[0056]

[0057] Figure 1 The metallographic diagram of Embodiment 4 of the present invention shows the obtained tempered bainite structure; Figure 2 This is the non-metallic inclusion rating chart of Embodiment 4 of the present invention, by... Figure 2 It can be seen that the non-metallic inclusions are of Class D, Grade 0.5.

[0058] As shown in Tables 3 and 4, the steel plate of this invention exhibits a room temperature tensile strength > 640 MPa and a yield strength > 570 MPa; a 200℃ tensile strength > 600 MPa and a yield strength > 520 MPa; an impact absorption energy at -45℃ > 250 J; and a zero plastic transition temperature (NDT) ≤ -60℃. Furthermore, the steel plate maintains excellent mechanical properties even after simulated post-weld heat treatment at a holding temperature of 625℃ for 15 hours, meeting the high standards for containment steel required for larger-capacity CAP1400 and CAP1700 nuclear power units in my country.

[0059] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A type of steel for the containment vessel of a pressurized water reactor nuclear power unit, characterized in that, By weight percentage, it includes the following chemical components: C: 0.07%~0.13%, Si: 0.1%~0.4%, Mn: 0.7%~1.5%, P≤0.010%, S≤0.005%, Ni: 0.35%~0.8%, Cu≤0.4%, Cr: 0.2%~0.65%, Mo: 0.31%~0.85%, V: 0.08%~0.15%, Ti: 0.04%~0.09%, Al: 0.04%~0.08%, [N]<20ppm, balance being Fe and unavoidable impurities; After simulated post-weld heat treatment at a temperature of 625℃ for 15 hours, the steel plate exhibits the following characteristics: room temperature tensile strength > 620 MPa, yield strength ≥ 560 MPa; 200℃ tensile strength > 590 MPa, yield strength > 510 MPa; -45℃ impact absorbed energy > 250 J; and zero plastic transition temperature (NDT) ≤ -65℃.

2. The steel for the containment vessel of a pressurized water reactor nuclear power unit according to claim 1, characterized in that, The thickness of the steel plate is ≤130mm.

3. A method for manufacturing steel for the containment vessel of a pressurized water reactor nuclear power unit as described in any one of claims 1-2, characterized in that, The manufacturing process of steel plates is as follows: smelting—continuous casting—heating—controlled rolling and controlled cooling—offline tempering heat treatment; The controlled rolling and cooling process is as follows: before rolling, the surface of the continuously cast billet is sprayed with water for 5-9 seconds to cool it down. When the temperature drops to 1100-1160℃, rolling begins. The single-pass reduction rate is >11%. Water mist cooling is used between each rolling pass. The cooling rate is 4-8℃ / s. The final rolling temperature is 870-910℃. After final rolling, the steel plate is cooled to 440-490℃ at a cooling rate of 30-45℃ / s, and then air-cooled to room temperature. The offline tempering heat treatment process is as follows: tempering temperature is 650~680℃, holding time is 1~12min / mm, and after taking it out of the furnace, it is air-cooled to room temperature.

4. The manufacturing method according to claim 3, characterized in that, The smelting process is as follows: the smelting process is carried out by converter + LF + RH. The LF furnace produces white slag to refine the molten steel. The RH furnace is used for vacuum circulation degassing for 10 to 20 minutes. Before leaving the station, the steel is soft-blown for 4 to 11 minutes with a soft-blown flow rate of 12 to 22 L / min. After soft-blowing, the steel is allowed to stand for 3 to 7 minutes. The tapping temperature is 1520 to 1550℃.

5. The manufacturing method according to claim 3, characterized in that, The continuous casting process is as follows: during casting, the superheat of the tundish is controlled at 10~20℃, heavy pressure is applied at the end of the continuous casting, the reduction is 17~28mm, and the billet pulling speed is controlled at 0.6~1.1m / min, and the thickness of the continuously cast billet is 250~400mm.

6. The manufacturing method according to claim 3, characterized in that, The heating process is as follows: the heating rate of the continuous casting billet is 3~7 min / cm, the holding temperature is 1280~1350℃, and the holding time is 20~60 min.

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