Large-thickness steel plate for nuclear power plant reactor pressure vessel and production method thereof

Abstract

The invention discloses a large-thickness steel plate for a reactor pressure vessel of a nuclear power station and a production method of the steel plate. The steel plate comprises the following chemical components: 0.15-0.21% of C, 0.2-0.3% of Si, 0.3-0.5% of Mn, 3.0-4.0% of Ni, 0.5-0.6% of Mo, 1.5-2% of Cr, 0.02-0.04% of Al, less than or equal to 0.01% of P, less than or equal to 0.003% of S, less than or equal to 0.8 ppm of H and the balance of Fe and inevitable impurities. The production method comprises the steps of electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preheating treatment and thermal refining. The tensile strength of the steel plate is 780 MPa or above, the impact absorbing energy at the temperature of-40 DEG C is 150 J or above, the free-plasticity transition temperature of a drop weight experiment is smaller than or equal to-20 DEG C, and the thickness-tensile Z-direction lamellar tearing resistance surface shrinkage of the steel plate is 40% or above.

Description

Technical Field

[0001] The invention belongs to the technical field of medium and thick plate production, and in particular relates to a thick steel plate for a nuclear power plant reactor pressure vessel and a production method thereof. Background Art

[0002] As a relatively clean energy source, nuclear power is experiencing an unprecedented construction boom around the world, especially in my country. Reactor pressure vessels are one of the key components of nuclear reactors. They contain coolants, support the core, seal radioactive materials, and maintain the operating pressure inside the reactor. They are first-level nuclear safety equipment. Damage and leakage of the container are not allowed under any circumstances, so the requirements for their materials are more stringent.

[0003] In order to further improve the efficiency and safety of a single machine, large-scale and integrated equipment is the technical development trend of nuclear pressure vessels. Large-scale and integrated equipment will lead to an increase in the total weight of the pressure vessel and the wall thickness of the steel plate, which poses a severe challenge to the steel used in pressure vessels and its manufacturing technology. In addition, the strength level has gradually developed from 550MPa to 620MPa or even higher, such as 780MPa. With the increase in thickness and high performance requirements, the internal penetration deformation of the steel plate during rolling is insufficient, and the quenching cooling rate is insufficient, resulting in performance that is difficult to meet technical requirements; especially in the nuclear power field, drop hammer RT is required. NDT , and most of them require a drop hammer test at one-quarter of the plate thickness, further increasing the difficulty of production. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a thick steel plate for a reactor pressure vessel and a production method thereof, wherein the steel plate has a thickness of 150 to 200 mm, a tensile strength of more than 780 MPa, an impact absorption energy of more than 150 J at -40°C, a drop hammer test without plastic transition temperature ≤-20°C, and a surface reduction of more than 40% in the Z-direction lamellar tearing resistance of the steel plate through tension, meeting the Z35 standard.

[0005] To solve the above technical problems, the technical solution of the present invention is as follows: A thick steel plate for a reactor pressure vessel, the chemical composition of which is as follows by mass percentage: C 0.15%-0.21%, Si 0.2%-0.3%, Mn 0.3%-0.5%, Ni 3.0%-4.0%, Mo 0.5%-0.6%, Cr 1.5%-2%, Al0.02%-0.04%, P≤0.01%, S≤0.003%, H≤0.8ppm, and the balance is Fe and unavoidable impurities.

[0006] The chemical composition design principle of the steel plate described in the present invention is as follows: C is a strengthening element of steel, which can significantly improve the strength of steel, but at the same time will reduce the plasticity and toughness of steel; Mn and Mo can improve the hardenability and high temperature performance of steel; Ni can improve the toughness of steel, especially the low temperature toughness; Cr can significantly improve the hardenability and corrosion resistance of steel; and Cu, P, S, etc. are impurity elements, which form segregation or precipitation in pressure vessel steel, damaging the neutron radiation resistance of pressure vessel steel.

[0007] The functions of each component and content in the present invention are specifically described as follows: C: It has a significant impact on the yield, tensile strength and welding performance of steel. Carbon can significantly improve the strength of steel plates through interstitial solid solution, but when the carbon content is too high, it will affect the welding performance and toughness of steel.

[0008] Si: It acts as a reducing agent and deoxidizer in the steelmaking process. At the same time, Si can play a role in solid solution strengthening. However, when the content is high, it will cause the toughness of the steel to decrease and reduce the welding performance of the steel.

[0009] Mn: It can increase the toughness, strength and hardness of steel, improve the hardenability of steel, improve the hot processing performance of steel, and is low in price, which can reduce the production cost of steel plates. However, when the manganese content is too high, there is a risk of coarse grains during heating, which reduces welding performance.

[0010] P and S: In general, they are harmful elements in steel. They will increase the brittleness of steel. P will deteriorate the welding performance of steel and reduce its plasticity. S will reduce the ductility and toughness of steel and cause cracks during rolling. Therefore, the content of P and S in steel should be reduced as much as possible.

[0011] Ni: It slightly improves the strength and plasticity of the steel plate, but the low-temperature impact toughness is improved to a greater extent. This is because Ni only forms a solid solution in the steel, and the solid solution strengthening effect is not obvious. It mainly improves the plasticity of the material by increasing the lattice slip plane during plastic deformation. Ni can also improve the hardenability of alloy steel and improve the toughness of steel at low temperatures, reducing the ductile-brittle transition temperature.

[0012] Mo: It can greatly improve the strength, plasticity and low-temperature impact toughness of steel plates. This is because when Mo is dissolved in ferrite and austenite, the C curve of steel can be shifted to the right, thereby significantly improving the hardenability of steel. Mo can also significantly increase the recrystallization temperature of steel and improve tempering stability. When Mo carbides are formed, it can play a role of dispersion strengthening.

[0013] Cr: It can significantly improve the hardenability and corrosion resistance of steel, significantly improve the anti-oxidation effect of steel, and increase the corrosion resistance of steel. Chromium can significantly increase the hardenability of steel, but it can also increase the temper brittleness tendency of steel, and needs to be used in combination with elements such as Mo.

[0014] TAl (total aluminum): a commonly used deoxidizer in steel; adding a small amount of Al to steel can refine the grains and improve impact toughness; aluminum also has antioxidant and corrosion resistance, and too high a content will affect the hot working properties, welding properties and cutting properties of the steel.

[0015] H: When the hydrogen content in steel is high, it is easy to cause cracking, which in turn causes the strength, plasticity and impact toughness of the steel to decrease, which is called "hydrogen embrittlement". In addition, it is easy to produce white spots, resulting in dense defects in flaw detection points, and it is easy to cause layered fractures. The layered fractures of alloy steels containing chromium-nickel or chromium-nickel-molybdenum are more serious than those of carbon steel.

[0016] Furthermore, the thickness of the steel plate of the present invention is 150-200 mm.

[0017] Furthermore, the steel plate of the present invention has a transverse -40°C impact energy of more than 150J at 1 / 4 of the thickness, a tensile strength of more than 780MPa, a no-plastic transition temperature ≤-20°C in a drop hammer test, and a surface reduction of more than 40% in the Z-direction lamellar tearing resistance of the steel plate, meeting the Z35 standard.

[0018] Furthermore, the production method of the thick steel plate for reactor pressure vessel of the present invention comprises electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and tempering heat treatment.

[0019] Furthermore, in the VD vacuum refining of the present invention, the vacuum degree of vacuum degassing treatment is not greater than 66 Pa, and the vacuum holding time is not less than 25 minutes.

[0020] Furthermore, the heating described in the present invention is as follows: the steel ingot charging temperature is 400-600°C, preheating is carried out at 700°C for 2 hours to eliminate thermal stress, and then the temperature is increased to 900°C at a rate of ≤80°C / h, and the temperature is kept for 3 hours to eliminate tissue stress. The heating rate above 900°C is not limited, the maximum heating temperature is 1240°C, the soaking temperature is 1220±10°C, the soaking time is 15 to 20 hours depending on the size of the ingot, and the total heating time is not less than 30 hours.

[0021] Furthermore, the rolling described in the present invention: adopts a temperature-controlled rolling process, the rolling temperature in stage I is 950-1200°C, wherein the start rolling temperature is 1150-1200°C, the final rolling temperature is 950-980°C, the single-pass reduction is 10%-20%, the cumulative reduction rate is 30%-60%, and then the steel is air-dried, the steel air-dried thickness is 1.5t-2.0t, t is the thickness of the steel plate, and high-pressure water is used to reduce the surface temperature of the steel plate, so that the steel plate forms a temperature gradient from the surface to the inside; the start rolling temperature in stage II is ≤900°C, and the final rolling temperature is ≤850°C.

[0022] Furthermore, the preliminary heat treatment described in the method of the present invention includes normalizing and tempering, wherein the normalizing heating temperature is 900±10°C, the insulation time is 1.5-2min / mm, and the cooling is performed in air; the tempering heating temperature is 660-670°C, the insulation time is ≥4min / mm, and the cooling is performed in air.

[0023] Furthermore, the quenching and tempering heat treatment process of the present invention is as follows: the quenching heating temperature is 880±10°C, the heat preservation is 1.5-2min / mm, the quenching is cooled in a roller bottom thick plate quenching machine, and the water volume is not less than 5000m 3 / h, water temperature ≤26℃, swing time 0.5~1 hour, the red-back temperature of the steel plate after leaving the water is not more than 40℃; tempering heating temperature 640-660℃, holding time ≥3.5min / mm, air cooling.

[0024] The present invention adopts a reasonable ratio of alloy elements in the steel plate and a heat treatment process of preliminary heat treatment + quenching + tempering to make the steel plate obtain a martensite + bainite + a small amount of undissolved ferrite structure with fine grains and a grain size of more than 7.0, thereby ensuring the mechanical properties of the steel plate.

[0025] The method of the present invention has the following beneficial technical effects: (1) The maximum thickness of the steel plate of the present invention is 200 mm, the internal quality meets the first-level standard for forging flaw detection, and the oblique flaw detection meets the first-level requirement of NB / T47013.

[0026] (2) The transverse impact energy at -40℃ at 1 / 4 of the steel plate thickness is more than 150J, the temperature of no plastic transition in the drop hammer test is ≤-20℃, and the thickness of the steel plate is reduced by more than 40% in the Z-direction lamellar tearing resistance, meeting the Z35 standard.

[0027] (3) The steel plate has a low H content and high purity, and the sum of the total inclusion content is less than 1.0. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The metallographic structure is 100 times that of the steel plate in Example 1.

[0029] Figure 2 The metallographic structure is 200 times that of the steel plate in Example 1. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below with reference to specific examples. Example 1

[0031] The thickness of the steel plate for the reactor pressure vessel of the nuclear power plant in this embodiment is 150 mm, and its chemical composition is C 0.15%, Si 0.25%, Mn 0.5%, Ni 3.5%, Mo 0.56%, Cr 1.5%, Al 0.02%, P 0.008%, S 0.001% by mass percentage, and the balance is Fe and unavoidable impurities; its production method includes electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and quenching and tempering treatment; wherein the key process parameters are controlled as follows: (1) VD vacuum refining: The vacuum degree of vacuum degassing treatment is 36 Pa, and the vacuum holding time is 30 minutes.

[0032] (2) Heating: Heat the steel ingot, load it into the furnace at 400°C, and heat it up at a rate of 70°C / h below 900°C. Keep it at 700°C for 2 hours and at 900°C for 3 hours. Heat it up at a rate of 120°C / h above 900°C until it reaches an evenly heated temperature of 1220°C. Keep it at that temperature (evenly heated) for 19 hours. The total heating time is 33 hours.

[0033] (3) Rolling: The temperature-controlled rolling process is adopted, which is divided into two stages of rolling: the rolling temperature in stage I is 950℃~1200℃, of which the starting rolling temperature is 1050℃, the final rolling temperature is 980℃, the single-pass reduction is 10%~20%, and the cumulative reduction rate is 51%. Then the steel is air-dried to a thickness of 1.6t, where t is the thickness of the steel plate. The surface temperature of the steel plate is lowered with the help of high-pressure water, so that the temperature gradient from the surface to the inside of the steel plate is formed; the starting rolling temperature in stage II is 890℃, and the final rolling temperature is 840℃.

[0034] (4) Preliminary heat treatment: including normalizing and tempering, normalizing heating temperature 910℃, holding temperature 1.5min / mm, air cooling, tempering heating temperature 665℃, holding temperature 4min / mm, air cooling; (5) Quenching and tempering heat treatment process: quenching heating temperature 870℃, heat preservation 2min / mm, quenching cooling in roller bottom thick plate quenching machine, water volume 5100m 3 / h, water temperature 18℃, swing time 0.5 hour, the steel plate returns to red temperature 30℃ after leaving the water; tempering is carried out in a car bottom furnace, heating temperature 650℃, holding time 3.5min / mm, air cooling.

[0035] The metallographic structures of the steel plate at 100 times and 200 times magnification in this embodiment are shown in Figure 1. Figure 1 and Figure 2 ;Depend on Figure 1 and Figure 2 It can be seen that its metallographic structure is martensite, bainite and a small amount of undissolved ferrite, the grains are fine and uniform, and the grain size is 7.5.

[0036] The internal quality of the steel plate in this embodiment meets the first-level standard for forging flaw detection, and the oblique flaw detection meets the first-level requirements of NB / T47013. The transverse impact energy of the steel plate at -40°C at 1 / 4 of the thickness is 186J, the tensile strength is 810MPa, the drop hammer test has no plastic transition temperature of -20°C, and the thickness of the steel plate is reduced by 50% in the Z-direction lamellar tearing resistance, meeting the Z35 standard. The hydrogen content of the steel plate is low, at 0.6ppm, the purity of the steel plate is high, and the sum of the total inclusion content is 0.5. Example 2

[0037] The thickness of the steel plate for the reactor pressure vessel of the nuclear power plant in this embodiment is 180 mm, and its chemical composition is C 0.17%, Si 0.3%, Mn 0.4%, Ni 3.8%, Mo 0.55%, Cr 2%, Al 0.035%, P 0.007%, S0.002% by mass percentage, and the balance is Fe and inevitable impurities; its production method includes electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and quenching and tempering treatment; wherein the key process parameters are controlled as follows: (1) VD vacuum refining: The vacuum degree of vacuum degassing treatment is 43pa, and the vacuum holding time is 35 minutes.

[0038] (2) Heating: Heat the steel ingot, load it into the furnace at 500°C, and heat it up at a rate of 70°C / h below 900°C. Keep it at 700°C for 2 hours and at 900°C for 3 hours. Heat it up at a rate of 110°C / h above 900°C until it reaches an evenly heated temperature of 1210°C. Keep it at that temperature (evenly heated) for 18 hours. The total heating time is 31.5 hours.

[0039] (3) Rolling: The temperature-controlled rolling process is adopted, which is divided into two stages of rolling: the starting rolling temperature of the first stage is 1150℃, the final rolling temperature is 990℃, the single-pass reduction is 10%-20%, and the cumulative reduction rate is 54%. Then the steel is air-dried to a thickness of 1.5t, where t is the thickness of the steel plate. The surface temperature of the steel plate is lowered with the help of high-pressure water, so that a temperature gradient is formed from the surface to the inside of the steel plate. The starting rolling temperature of the second stage is 890℃, and the final rolling temperature is 840℃.

[0040] (4) Preliminary heat treatment: including normalizing and tempering. Normalizing heating temperature is 900℃, holding temperature is 1.8min / mm, and air cooling. Tempering heating temperature is 670℃, holding temperature is 4min / mm, and air cooling.

[0041] (5) Quenching and tempering heat treatment: quenching heating temperature 880℃, heat preservation 1.5min / mm, quenching cooling in roller bottom thick plate quenching machine, water volume 5200m 3 / h, water temperature 18℃, swing time 0.8h, steel plate reddening temperature after water exit 30℃. Tempering is carried out in the bottom furnace, heating temperature 660℃, holding time 3.5min / mm.

[0042] The metallographic structure of the steel plate in this embodiment is martensite, bainite and a small amount of undissolved ferrite, the grains are fine and uniform, and the grain size is 8.0.

[0043] The internal quality of the steel plate in this embodiment meets the first-level standard for forging flaw detection, and the oblique flaw detection meets the first-level requirements of NB / T47013. The transverse impact energy of the steel plate at -40°C at 1 / 4 of the thickness is 173J, the tensile strength is 860MPa, the drop hammer test has no plastic transition temperature of -25°C, and the thickness of the steel plate is tensile Z-direction lamellar tearing resistance surface shrinkage is 55%, meeting the Z35 standard. The hydrogen content of the steel plate is low, at 0.5ppm, the purity of the steel plate is high, and the total inclusion content is 0.5. Example 3

[0044] The thickness of the steel plate for the reactor pressure vessel of the nuclear power plant in this embodiment is 200 mm, and the mass percentage content of its chemical composition is C 0.21%, Si 0.2%, Mn 0.3%, Ni 3.2%, Mo 0.51%, Cr 1.7%, Al 0.025%, P 0.005%, S0.002%, and the balance is Fe and inevitable impurities; its production method includes electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and quenching and tempering treatment; wherein the key process parameters are controlled as follows: (1) VD vacuum refining: The vacuum degree of vacuum degassing treatment is 60pa, and the vacuum holding time is 32 minutes.

[0045] (2) Heating: The steel ingot is heated and loaded into the furnace at 600°C. The heating rate is 50°C / h below 900°C, and it is kept at 700°C for 2 hours and 900°C for 3 hours. The heating rate above 900°C is 130°C / h. It is heated to an evenly heated temperature of 1230°C and kept (evenly heated) for 17 hours. The total heating time is 30.5 hours.

[0046] (3) Rolling: The temperature-controlled rolling process is adopted, which is divided into two stages of rolling: the starting rolling temperature of the first stage is 1180℃, the final rolling temperature is 970℃, the single-pass reduction is 10%-20%, and the cumulative reduction rate is 36%. Then the steel is air-dried to a thickness of 2.0t, where t is the thickness of the steel plate. The surface temperature of the steel plate is lowered with the help of high-pressure water to form a temperature gradient from the surface to the inside of the steel plate. The starting rolling temperature of the second stage is 900℃, and the final rolling temperature is 850℃.

[0047] (4) Preliminary heat treatment: including normalizing and tempering. Normalizing heating temperature is 900℃, holding temperature is 2min / mm, air cooling. Tempering heating temperature is 660℃, holding temperature is 4min / mm, air cooling.

[0048] (5) Quenching and tempering heat treatment: quenching heating temperature 890℃, heat preservation 1.8min / mm, quenching cooling in roller bottom thick plate quenching machine, water volume 5250m 3 / h, water temperature 25℃, swing time 1 hour, the red-back temperature of the steel plate after leaving the water is 36℃; tempering is carried out in the bottom furnace, the heating temperature is 655℃, and the holding time is 3.5min / mm.

[0049] The metallographic structure of the steel plate in this embodiment is martensite, bainite and a small amount of undissolved ferrite, the grains are fine and uniform, and the grain size is 7.0.

[0050] The internal quality of the steel plate in this embodiment meets the first-level standard for forging flaw detection, and the oblique flaw detection meets the first-level requirements of NB / T47013. The transverse impact energy at -40°C at 1 / 4 of the thickness of the steel plate is 165J, the tensile strength is 830MPa, the drop hammer test has no plastic transition temperature of -25°C, and the thickness of the steel plate is tensile Z-direction lamellar tearing resistance surface shrinkage is 45%, meeting the Z35 standard. The hydrogen content of the steel plate is low, at 0.7ppm, the purity of the steel plate is high, and the sum of the total inclusion content is 1.0. Example 4

[0051] The thickness of the steel plate for the reactor pressure vessel of the nuclear power plant in this embodiment is 160 mm, and the mass percentage content of its chemical composition is C 0.17%, Si 0.22%, Mn 0.38%, Ni 3.0%, Mo 0.6%, Cr 1.5%, Al 0.020%, P 0.006%, S0.001%, and the balance is Fe and inevitable impurities; its production method includes electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and quenching and tempering treatment; wherein the key process parameters are controlled as follows: (1) VD vacuum refining: The vacuum degree of vacuum degassing treatment is 39pa, and the vacuum holding time is 25 minutes.

[0052] (2) Heating: The steel ingot is heated and loaded into the furnace at 480°C. The heating rate is 60°C / h below 900°C, and it is kept at 700°C for 2 hours and 900°C for 3 hours. The heating rate above 900°C is 125°C / h. It is heated to an evenly heated temperature of 1225°C and kept (evenly heated) for 16 hours. The total heating time is 30.6 hours.

[0053] (3) Rolling: The temperature-controlled rolling process is adopted, which is divided into two stages of rolling: the starting rolling temperature of the first stage is 1150℃, the final rolling temperature is 950℃, the single-pass reduction is 10%-20%, and the cumulative reduction rate is 47%. Then the steel is air-dried to a thickness of 1.8t, where t is the thickness of the steel plate. The surface temperature of the steel plate is lowered with the help of high-pressure water, so that a temperature gradient is formed from the surface to the inside of the steel plate. The starting rolling temperature of the second stage is 890℃, and the final rolling temperature is 835℃.

[0054] (4) Preliminary heat treatment: including normalizing and tempering. Normalizing heating temperature is 905℃, holding temperature is 1.5min / mm, air cooling. Tempering heating temperature is 660℃, holding temperature is 4min / mm, air cooling.

[0055] (5) Quenching and tempering heat treatment: quenching heating temperature 870℃, heat preservation 1.7min / mm, quenching cooling in roller bottom thick plate quenching machine, water volume 5150m 3 / h, water temperature 23℃, swing time 0.9 hour, the red-back temperature of the steel plate after leaving the water is 33℃; tempering is carried out in the bottom furnace, the heating temperature is 645℃, and the holding time is 3.8min / mm.

[0056] The metallographic structure of the steel plate in this embodiment is martensite, bainite and a small amount of undissolved ferrite, the grains are fine and uniform, and the grain size is 7.5.

[0057] The internal quality of the steel plate in this embodiment meets the first-level standard for forging flaw detection, and the oblique flaw detection meets the first-level requirements of NB / T47013. The transverse impact energy of the steel plate at -40°C at 1 / 4 of the thickness is 195J, the tensile strength is 850MPa, the drop hammer test has no plastic transition temperature of -25°C, and the thickness of the steel plate is tensile Z-direction lamellar tearing resistance surface shrinkage is 55%, meeting the Z35 standard. The hydrogen content of the steel plate is low, at 0.6ppm, the purity of the steel plate is high, and the total inclusion content is 1.0. Example 5

[0058] The thickness of the steel plate for the reactor pressure vessel of the nuclear power plant in this embodiment is 190 mm, and the mass percentage content of its chemical composition is C 0.19%, Si 0.3%, Mn 0.5%, Ni 4.0%, Mo 0.53%, Cr 1.9%, Al 0.025%, P 0.004%, S0.001%, and the balance is Fe and inevitable impurities; its production method includes electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and tempering treatment; wherein the key process parameters are controlled as follows: (1) VD vacuum refining: The vacuum degree of vacuum degassing treatment is 33pa, and the vacuum holding time is 28 minutes.

[0059] (2) Heating: Heat the steel ingot, charge the furnace at 430°C, and heat it up at a rate of 40°C / h below 900°C. Keep it at 700°C for 2 hours and 900°C for 3 hours. Heat it up at a rate of 135°C / h above 900°C until the average temperature reaches 1230°C. Keep it at this temperature (average temperature) for 15 hours. The total heating time is 34 hours.

[0060] (3) Rolling: The temperature-controlled rolling process is adopted, which is divided into two stages of rolling: the starting rolling temperature of the first stage is 1200℃, the final rolling temperature is 980℃, the single-pass reduction is 10%-20%, and the cumulative reduction rate is 43%. Then the steel is air-dried to a thickness of 1.9t, where t is the thickness of the steel plate. The surface temperature of the steel plate is lowered with the help of high-pressure water, so that a temperature gradient is formed from the surface to the inside of the steel plate. The starting rolling temperature of the second stage is 880℃, and the final rolling temperature is 840℃.

[0061] (4) Preliminary heat treatment: including normalizing and tempering. Normalizing heating temperature is 910℃, holding temperature is 2min / mm, air cooling. Tempering heating temperature is 665℃, holding temperature is 4min / mm, air cooling.

[0062] (5) Quenching and tempering heat treatment: quenching heating temperature 885℃, holding temperature 1.6min / mm, quenching cooling in roller bottom thick plate quenching machine, water volume 5200m 3 / h, water temperature 21℃, swing time 0.5 hour, the steel plate returns to red temperature 30℃ after leaving the water; tempering is carried out in the bottom furnace, heating temperature 640℃, holding time 3.6min / mm.

[0063] The metallographic structure of the steel plate in this embodiment is martensite, bainite and a small amount of undissolved ferrite, the grains are fine and uniform, and the grain size is 8.5.

[0064] The internal quality of the steel plate in this embodiment meets the first-level standard for forging flaw detection, and the oblique flaw detection meets the first-level requirements of NB / T47013. The transverse impact energy at -40°C at 1 / 4 of the thickness of the steel plate is 215J, the tensile strength is 820MPa, the drop hammer test has no plastic transition temperature of -30°C, and the thickness of the steel plate is tensile Z-direction lamellar tearing resistance surface shrinkage is 62%, meeting the Z35 standard. The hydrogen content of the steel plate is low, at 0.8ppm, the purity of the steel plate is high, and the total inclusion content is 1.0. Example 6

[0065] The thickness of the steel plate for the reactor pressure vessel of the nuclear power plant in this embodiment is 200 mm, and the mass percentage content of its chemical composition is C 0.20%, Si 0.28%, Mn 0.48%, Ni 3.2%, Mo 0.58%, Cr 2.0%, Al 0.04%, P 0.008%, S0.001%, and the balance is Fe and inevitable impurities; its production method includes electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and tempering treatment; wherein the key process parameters are controlled as follows: (1) VD vacuum refining: The vacuum degree of vacuum degassing treatment is 50 Pa, and the vacuum holding time is 33 minutes.

[0066] (2) Heating: Heat the steel ingot, charge the furnace at 520°C, and heat it up at a rate of 50°C / h below 900°C. Keep it at 700°C for 2 hours and 900°C for 3 hours. Heat it up at a rate of 140°C / h above 900°C until the average temperature reaches 1210°C. Keep it at this temperature (average temperature) for 20 hours. The total heating time is 35 hours.

[0067] (3) Rolling: The temperature-controlled rolling process is adopted, which is divided into two stages of rolling: the starting rolling temperature of the first stage is 1170℃, the final rolling temperature is 960℃, the single-pass reduction is 10%-20%, and the cumulative reduction rate is 39%. Then the steel is air-dried to a thickness of 1.6t, where t is the thickness of the steel plate. The surface temperature of the steel plate is lowered with the help of high-pressure water, so that a temperature gradient is formed from the surface to the inside of the steel plate. The starting rolling temperature of the second stage is 870℃, and the final rolling temperature is 820℃.

[0068] (4) Preliminary heat treatment: including normalizing and tempering. Normalizing heating temperature is 895℃, holding temperature is 1.8min / mm, and air cooling is performed. Tempering heating temperature is 660℃, holding temperature is 4min / mm, and air cooling is performed.

[0069] (5) Quenching and tempering heat treatment: quenching heating temperature 870℃, heat preservation 1.9min / mm, quenching cooling in roller bottom thick plate quenching machine, water volume 5310m 3 / h, water temperature 22℃, swing time 1 hour, the red-hot temperature of the steel plate after leaving the water is 34℃; tempering is carried out in a car bottom furnace, the heating temperature is 660℃, and the holding time is 3.8min / mm.

[0070] The metallographic structure of the steel plate in this embodiment is martensite, bainite and a small amount of undissolved ferrite, the grains are fine and uniform, and the grain size is 7.5.

[0071] The internal quality of the steel plate in this embodiment meets the first-level standard for forging flaw detection, and the oblique flaw detection meets the first-level requirements of NB / T47013. The transverse impact energy of the steel plate at -40°C at 1 / 4 of the thickness is 195J, the tensile strength is 810MPa, the drop hammer test has no plastic transition temperature of -30°C, and the thickness of the steel plate is tensile Z-direction lamellar tearing resistance surface shrinkage is 58%, meeting the Z35 standard. The hydrogen content of the steel plate is low, at 0.7ppm, the purity of the steel plate is high, and the total inclusion content is 1.0.

Claims

1. A thick steel plate for a reactor pressure vessel, characterized in that: Its chemical composition by mass percentage is: C 0.15% ~ 0.21%, Si 0.2% ~ 0.3%, Mn 0.3% ~ 0.5%, Ni 3.0% ~ 4.0%, Mo 0.5% ~ 0.6%, Cr 1.5% ~ 2%, Al 0.02% ~ 0.04%, P ≤ 0.01%, S ≤ 0.003%, H ≤ 0.8ppm, and the balance is Fe and unavoidable impurities.

2. The thick steel plate for reactor pressure vessel according to claim 1, characterized in that: The thickness of the steel plate is 150-200 mm.

3. The thick steel plate for reactor pressure vessel according to claim 1, characterized in that: The transverse -40°C impact energy of the steel plate at 1 / 4 of the thickness is more than 150J, the tensile strength is more than 780MPa, the no-plastic transition temperature in the drop hammer test is ≤-20°C, the steel plate thickness is tensile in the Z-direction and the lamellar tearing resistance is reduced by more than 40%, meeting the Z35 standard.

4. The method for producing a thick steel plate for a reactor pressure vessel according to any one of claims 1 to 3, characterized in that: It includes electric furnace smelting, LF refining, VD vacuum refining, die casting, slow cooling, cleaning, heating, rolling, preliminary heat treatment and quenching and tempering heat treatment.

5. The method for producing a thick steel plate for a reactor pressure vessel according to claim 4, characterized in that: The VD vacuum refining: the vacuum degree of vacuum degassing treatment is not greater than 66 Pa, and the vacuum holding time is not less than 25 minutes.

6. The method for producing a thick steel plate for a reactor pressure vessel according to claim 4, characterized in that: The heating method comprises the following steps: the steel ingot charging temperature is 400-600°C, preheating at 700°C for 2 hours, then heating to 900°C at a rate of ≤80°C / h, keeping warm for 3 hours, the maximum heating temperature is 1240°C, the absorptive temperature is 1220±10°C, the absorptive holding time is 15-20 hours, and the total heating time is not less than 30 hours.

7. The method for producing a thick steel plate for a reactor pressure vessel according to claim 4, characterized in that: The rolling process adopts a temperature-controlled rolling process, wherein the rolling temperature in the first stage is 950-1200°C, wherein the start rolling temperature is 1150-1200°C, the final rolling temperature is 950-980°C, the single-pass reduction is 10%-20%, the cumulative reduction rate is 30%-60%, and then the steel is air-dried to a thickness of 1.5t-2.0t, where t is the thickness of the steel plate, and high-pressure water is used to reduce the surface temperature of the steel plate so that a temperature gradient is formed from the surface to the inside of the steel plate; the start rolling temperature in the second stage is ≤900°C, and the final rolling temperature is ≤850°C.

8. The method for producing a thick steel plate for a reactor pressure vessel according to claim 4, characterized in that: The preliminary heat treatment includes normalizing and tempering, wherein the normalizing heating temperature is 900±10°C, the heat preservation is 1.5-2min / mm, and the air is cooled; the tempering heating temperature is 660-670°C, the heat preservation is ≥4min / mm, and the air is cooled.

9. The method for producing a thick steel plate for a reactor pressure vessel according to claim 4, characterized in that: The quenching and tempering heat treatment process: quenching heating temperature 880±10℃, heat preservation 1.5~2min / mm, quenching cooling in roller bottom thick plate quenching machine, water volume not less than 5000m 3 / h, water temperature ≤26℃, swing time 0.5~1 hour, the red-back temperature of the steel plate after leaving the water is not more than 40℃; tempering heating temperature 640-660℃, holding time ≥3.5min / mm, air cooling.

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