Extra-thick high-strength steel plate for containment gate flange of nuclear power plant and its manufacturing method

Abstract

The present invention provides a super-thick high-strength steel plate for the flange of the containment gate of a nuclear power plant and a manufacturing method thereof. The composition of the steel plate is as follows by weight percentage: C: 0.21% - 0.23%; Si: 0.15% - 0.30%; Mn: 1.75% - 1.90%; P ≤ 0.010%; S ≤ 0.005%; Ni: 0.96% - 0.99%; Cr: 0.15% - 0.25%; Mo: 0.50% - 0.80%; V: 0.010% - 0.050%; Cu: 0.10% - 0.30%; Al: 0.010% - 0.040%, and the balance is Fe and unavoidable impurities. The manufacturing method includes smelting, continuous casting, electroslag remelting, forging, rolling, and quenching and tempering treatment; the tensile strength at room temperature of the steel plate produced by the present invention after simulated post-weld heat treatment is > 660 MPa, the yield strength is > 545 MPa, the elongation after fracture is ≥ 22.5%; the reduction of area in the thickness direction is > 65%; the impact energy absorption at -7°C is ≥ 240 J; the tensile strength at 350°C is > 620 MPa, and the yield strength is > 510 MPa.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and particularly relates to an extra-thick high-strength steel plate for the flange of a containment gate of a nuclear power plant and a manufacturing method thereof. Background Art

[0002] Whether it is an advanced passive (safety system) nuclear power plant of AP1000 technology or a reformed active (safety system) nuclear power plant of EPR technology, the high-level safety design requirements of the third-generation pressurized water reactor nuclear power plant mainly include the ability to prevent accidents, prevent core damage, and three aspects. The most important point in mitigating the consequences of accidents is to provide a large-volume, stable containment building or containment system.

[0003] The steel materials used to manufacture the containment building or containment system need to meet the following technical requirements: strict and reasonable chemical compositions, good internal quality, excellent strength and toughness matching, and excellent processing performance. In the actual application process of the materials, the single weight of the materials, the welding performance of the materials, and the corrosion resistance of the materials also need to be considered.

[0004] Compared with the above-mentioned many technical requirements and application requirements for the steel used in the containment of the third-generation nuclear reactor, there are obvious deficiencies in both SA-738Gr.B of the ferrous materials in Part A of ASMEⅡ and steel grades such as Q265HR in the NB standard. Considering both the original innovation of the steel used in the containment of the third-generation nuclear reactor and the reduction of the construction difficulty of the containment of the third-generation pressurized water reactor nuclear reactor, developing a new generation of high-strength steel for the containment not only has significant economic benefits but also has great economic benefits.

[0005] The relevant patents for the steel used in the containment of the currently produced nuclear reactors are as follows:

[0006] Patent for "High-strength steel plate for nuclear reactor containment and its manufacturing method" applied by POSCO of South Korea, with patent application number 200980152846.4 and publication number CN 102264936A. The steel plate contains, by weight: 0.03% - 0.20% C, 0.15% - 0.55% Si, 0.90% - 1.50% Mn, 0.001% - 0.05% Al, 0.030% or less P, 0.030% or less S, 0.30% or less Cr, 0.2% or less Mo, 0.6% or less Ni, 0.07% or less V, 0.04% or less Nb, 5 ppm - 50 ppm Ca, 0.005% - 0.025% Ti, 0.0020% - 0.0060% N, 0.0005% - 0.0020% B, with the balance being Fe and inevitable impurities. The steel plate can be composed of tempered martensite, and the conditions of cooling and recrystallization controlled rolling are optimized to control the average grain size and aspect ratio of the structural grains of the microstructure. The produced steel plate has a tensile strength ≥ 650 MPa at -50°C and an impact toughness of at least 200 J, and thus can be used in nuclear power plants. However, from the ingot rolling production method adopted in this invention, the cost is greatly increased, the maximum thickness of the manufactured steel plate is only 80 mm, the high-temperature tensile index is not provided in the specification, and the mechanical properties after simulated post-weld heat treatment of the steel plate are not provided.

[0007] Patent for "A steel plate for the reactor containment of the third-generation nuclear power plant and its manufacturing method" applied by Jinan Iron and Steel Group Co., Ltd., with patent application number 201210282831.3 and publication number CN 102776441A. C: 0.08 - 0.12%, Si: 0.15 - 0.55%, Mn: 0.90 - 1.50%, P ≤ 0.007%, S ≤ 0.004%, Ni: 0.10 - 0.50%, Cr: 0.0 - 0.30%, Mo: 0.10 - 0.35%, V: 0.010 - 0.050%, Nb: 0.010 - 0.030%, Ti: 0.008 - 0.035%, Alt: 0.020 - 0.050%, N ≤ 0.006%, Nb + V ≤ 0.08%, with the balance being Fe and inevitable impurities. The steel plate manufactured by the method of this invention has a low carbon content, its tensile strength reaches above 600 MPa, has high-temperature resistance at 200°C, low cost, and excellent welding performance. However, the maximum thickness of the steel plate in the examples in the specification of this comparative document is 45 mm, and the performance after simulated post-weld heat treatment of the steel plate and the bending performance of the steel plate are not provided in the specification.

[0008] The patent titled "Thick Steel Plate for Nuclear Power Plant Containment and Its Manufacturing Method" applied by Baoshan Iron & Steel Co., Ltd. has a patent application number of 201210269122.1 and a publication number of CN 102766805 A. C: 0.06 - 0.15%, Si: 0.10 - 0.40%, Mn: 1.0 - 1.5%, Mo: 0.10 - 0.30%, P ≤ 0.012%, S ≤ 0.003%, Alt: 0.015 - 0.050%, Ni: 0.20 - 0.50%; and at least one of V ≤ 0.050%, Ti ≤ 0.030%, Cr ≤ 0.25%, Nb ≤ 0.030%, Ca: 0.0005 - 0.0050%; the balance is Fe and unavoidable impurities. The thick steel plate for nuclear power plant containment described by the method of this invention has high strength, high toughness, and good impact toughness in the base metal and heat affected zone at low temperatures, and is suitable for application in the field of nuclear power plant containment manufacturing. However, the maximum thickness of the steel plate in the claims of this comparative document is 60 mm, and the high-temperature tensile index is not provided in the specification. Summary of the Invention

[0009] The purpose of the present invention is to overcome the above problems and deficiencies and provide a special thick high-strength steel plate for nuclear power plant containment gate flange and its manufacturing method, which has good mechanical properties, and after long-term simulated post-weld heat treatment, key indicators such as room-temperature tensile, high-temperature tensile, and low-temperature impact still remain good, and can fully meet the usage requirements of the steel for the gate flange of the new generation of pressurized water reactor nuclear power plant containment.

[0010] The purpose of the present invention is achieved as follows:

[0011] A special thick high-strength steel plate for nuclear power plant containment gate flange, the composition of the steel plate is as follows by weight percentage: C: 0.21% - 0.23%; Si: 0.15% - 0.30%; Mn: 1.75% - 1.90%; P ≤ 0.010%; S ≤ 0.005%; Ni: 0.96% - 0.99%; Cr: 0.15% - 0.25%; Mo: 0.50% - 0.80%; V: 0.010% - 0.050%; Cu: 0.10% - 0.30%; Al: 0.010% - 0.040%, and the balance is Fe and unavoidable impurities.

[0012] The thickness of the steel plate is 130 - 180 mm, meeting the requirements of Class I ultrasonic flaw detection in the NB / T47013.3 standard.

[0013] The microstructure of the steel plate is tempered sorbite, and a large number of complex cementites with a size not greater than 60 nm are dispersed in the ferrite matrix.

[0014] The tensile strength of the steel plate at room temperature is > 690 MPa, the yield strength is > 580 MPa, and the elongation after fracture is ≥ 22%; the tensile strength of the steel plate at 350 °C high temperature is > 635 MPa, and the yield strength is > 525 MPa; the reduction of area in the thickness direction is ≥ 65%; the impact energy absorbed at -7 °C is ≥ 250 J.

[0015] The tensile strength of the steel plate at room temperature after simulated post-weld heat treatment is > 660 MPa, the yield strength is > 545 MPa, and the elongation after fracture is ≥ 22.5%; the tensile strength of the steel plate at 350 °C high temperature is > 620 MPa, and the yield strength is > 510 MPa; the reduction of area in the thickness direction is > 65%; the impact energy absorbed at -7 °C is ≥ 240 J.

[0016] The size of inclusions in the full-thickness section of the steel plate is controlled below grade 0.5.

[0017] The reasons for the composition design of the present invention are as follows:

[0018] In the present invention, by adding higher contents of C (wt 0.21% - 0.23%) and Mn (wt 1.75% - 1.90%), the addition of alloying elements such as Cr, Ni, and Mo is reduced, while ensuring the strength and toughness of the extra-thick steel plate. At the same time, Ni can reduce the low-temperature brittle transition temperature of the steel and can also improve the hardenability of the steel. When used in combination with chromium, molybdenum, etc., the steel can obtain good strength and toughness after heat treatment. Through the simplified design of the total composition, while ensuring good strength and toughness matching of the extra-thick steel plate, it also has economy at the same time.

[0019] C: C is the most effective element for strengthening structural steel and is also the most economical element. If the C content in the steel is too low, the strength cannot meet the requirements; if the C content is too high, it will have an adverse impact on the ductility, toughness, and weldability of the steel, and hardening will also occur in the heat-affected zone of the steel during welding, resulting in the generation of welding cold cracks. Therefore, in the design of the composition of the steel in the present invention, the C content in the steel is required to be controlled within the range of 0.21% - 0.23%.

[0020] Si: Si is a reducing agent and deoxidizer in the steel. If the content is too high, silicate inclusions are likely to be generated, damaging the welding performance of the steel plate. Therefore, the Si content is controlled at 0.15 - 0.30%.

[0021] Mn: Mn is a good deoxidizer and desulfurizer. A certain amount of manganese in the steel can eliminate or weaken the hot brittleness of the steel caused by sulfur; Mn can strengthen ferrite, has a solid solution strengthening effect, can improve the strength and hardness of the steel plate, and does not affect the plasticity and toughness of the steel; when used in combination with Cr and Mo, it improves the high-temperature resistance of the steel plate. Therefore, in actual production, the Mn content is controlled at 1.75% - 1.90%.

[0022] P: P dissolves in ferrite, increasing the temper brittleness of steel, significantly reducing the plasticity and toughness of steel, and also having an adverse effect on welding. Therefore, the lower the content of P, the better. In this invention, it is required to control P ≤ 0.010% in the steel.

[0023] S: In steel, it is easy to form sulfide inclusions, reducing the impact toughness of steel, damaging the welding performance, and at the same time aggravating defects such as central segregation and porosity, and increasing irradiation embrittlement. Therefore, in this invention, it is required that S ≤ 0.005%.

[0024] Ni: It can improve the low-temperature toughness and strength of the steel plate, and at the same time can also improve the hardenability of the steel, and has the effect of refining the martensite structure. In this invention, the Ni content in the steel is controlled to be 0.96% - 0.99%.

[0025] Cr: It improves the hardenability and high-temperature oxidation resistance of the steel plate; when used in combination with Mn and Mo, it improves the high-temperature performance of the steel plate. At the same time, Cr also acts as a "cleaner" to reduce the harm of interstitial elements to irradiation. Therefore, in this invention, the Cr content in the steel is required to be controlled within 0.15% - 0.25%.

[0026] Mo: Mo can refine the grains of the steel, improve the hardenability, so that the core of the thick plate can be hardened even when the cooling rate is relatively slow during quenching. At the same time, molybdenum is a strong carbide-forming element. When the content is relatively low, it forms complex cementite and can improve the heat resistance and reduce the temper brittleness; when used in combination with Mn and Cr, it improves the high-temperature performance of the steel plate. Therefore, in this invention, the Mo content is required to be controlled within 0.50% - 0.80%.

[0027] V: It has the effect of pinning dislocations and refining grains, and can improve the strength of steel at normal and high temperatures. In this invention, the V content is designed to be 0.010% - 0.050%.

[0028] Cu: It can improve the tissue performance of the core of the steel plate and is also used to improve the corrosion resistance of the steel in marine atmosphere. However, too high a Cu content is not conducive to the processing and welding of the steel plate. At the same time, Cu is an element harmful to irradiation embrittlement. Therefore, the Cu content in the steel is controlled within 0.10% - 0.30%.

[0029] Al: Aluminum plays a certain role in deoxidation during steelmaking and is also beneficial to grain refinement. Although too much aluminum will produce Al2O3 inclusions, affecting the hot working performance and welding performance of the steel, through electroslag remelting technology, the morphology and distribution of inclusions are effectively improved, maximizing the role of the Al element and reducing its harmful effects. Therefore, in this invention, the Als content is required to be controlled within 0.010% - 0.040%.

[0030] The second technical solution of this invention is to provide a manufacturing method for extra-thick high-strength steel plates for nuclear power plant containment gate flanges, including smelting, continuous casting, electroslag remelting, forging, rolling, and quenching and tempering treatment;

[0031] Smelting: In order to accurately control the chemical composition and inclusion level, a two-stage pure steel smelting technology of smelting, continuous casting + electroslag remelting is adopted;

[0032] The first stage utilizes smelting and continuous casting, with the molten steel produced using a dual-process (blast furnace hot metal - hot metal desulfurization pretreatment - converter dephosphorization - converter desulfurization) method. This minimizes the content of impurities P and S in the steel. Meanwhile, necessary off-furnace refining treatments such as LF and VD further guarantee the concentrations of gases such as H (≤1.0ppm), O (≤20ppm), and N (≤40ppm) in the steel. Furthermore, a period of argon blowing, stirring, and sedation after refining effectively promotes the floating of inclusions, thereby ensuring the purity of the molten steel and controlling the inclusion level to 0.5 or below, with uniform and dispersed distribution.

[0033] forging:

[0034] In order to improve the internal quality and resistance to lamellar tearing of extra-thick steel plates, forging technology is adopted for extra-thick electroslag billets; longitudinal forging + transverse forging is adopted in two dimensions, with the starting forging temperature above 1250°C and the final forging temperature above 950°C, to eliminate central porosity and reduce central segregation to the greatest extent, form a forged steel billet, and then perform surface cleaning.

[0035] Rolling:

[0036] The billet heating temperature is ≥1220°C, the starting rolling temperature is ≥1150°C, the average single-pass reduction is ≥15%, and the final rolling temperature is 900-950°C. The rolling process is followed by natural cooling. High-permeability rolling technology is employed to ensure an average single-pass reduction of ≥15%, maximally refining the grain size of the steel plate and effectively controlling its low-temperature impact toughness. The rolling process preferably utilizes asynchronous rolling combined with surface spraying to achieve flatness control during rolling. Asynchronous rolling requires a top-to-bottom roll speed ratio of 1.05-1.3. The surface spraying technology cools the slab by spraying water, controlling the surface temperature drop by 5-10°C / s.

[0037] Quenching and tempering treatment:

[0038] High-precision group-spacing heat treatment technology. Heat treatment is carried out in a roller-hearth continuous heat treatment furnace, protected by a non-oxidizing atmosphere throughout the entire process. During the heat treatment holding phase, the temperature deviation is controlled within a ±5°C range, and the water temperature is approximately 25°C.

[0039] Quenching temperature 940℃~960℃ holding time 3-5min / mm;

[0040] Tempering temperature 650℃~680℃, holding time 6-10min / mm.

[0041] The second-phase particles in the steel are mainly V(C N), which are uniformly and dispersedly distributed on the matrix. The size of the precipitates is no more than 25 nm, and it has a very strong precipitation strengthening effect. In addition, a large number of composite cementites with a size of no more than 60 nm are uniformly precipitated at the ferrite grain boundaries and within the grains. On the one hand, the precipitation of the composite carbide will have a good strengthening effect, and on the other hand, it also indicates that the high-temperature tempering treatment of the steel plate is relatively sufficient, which plays a good role in improving the toughness of the steel plate after long-term simulated post-weld heat treatment and ensuring a reasonable match between the strength and toughness of the steel plate.

[0042] The beneficial effects of the present invention are as follows:

[0043] (1) For the steel plate produced by the process technology of the present invention, through the optimization of chemical composition and the reasonable design of process parameters, it has excellent low-temperature toughness indexes. After the steel plate is rolled + quenched and tempered + simulated post-weld heat treatment, the impact energy absorbed at -7°C remains above 240 J.

[0044] (2) After the steel grade of the present invention is quenched and tempered and simulated post-weld heat treated, it has good strength and toughness in different states. After quenching and tempering heat treatment + simulated post-weld heat treatment (holding temperature 610°C, holding time 10 h), the tensile strength of the steel plate at room temperature > 660 MPa, and the tensile strength at 350°C > 620 MPa.

[0045] (3) For the steel plate produced by the process technology of the present invention, the inclusion level of the whole cross-section is controlled at 0.5 level and below, meeting the requirements of Grade I ultrasonic flaw detection in the NB / T47013.3 standard. Description of the Drawings

[0046] Figure 1 It is the metallographic diagram of the microstructure of the steel plate in Example 1 of the present invention. Detailed Embodiments

[0047] The present invention will be further described below through examples.

[0048] In the embodiments of the present invention, smelting, continuous casting, electroslag remelting, forging, rolling, quenching and tempering treatment are carried out according to the component ratios of the technical solutions.

[0049] Forging:

[0050] The forging start temperature is above 1250°C, and the forging end temperature is above 950°C;

[0051] Rolling:

[0052] The heating temperature of the steel billet ≥ 1220°C, the rolling start temperature ≥ 1150°C, the average single-pass reduction rate ≥ 15%, the final rolling temperature is 900 - 950°C, and it is naturally cooled after rolling;

[0053] Quenching and tempering treatment:

[0054] The quenching temperature is 940°C to 960°C, and the holding time is 3 - 5 min / mm;

[0055] The tempering temperature is 650°C to 680°C, and the holding time is 6 - 10 min / mm.

[0056] Furthermore, the forging is carried out in a two - dimensional manner of longitudinal forging + transverse forging.

[0057] Furthermore, in the rolling process, asynchronous rolling + surface spraying is adopted. For asynchronous rolling, the roll speed ratio of the upper and lower rolls is 1.05 - 1.3. The surface spraying technology is to cool the slab by spraying water, and the temperature drop rate of the slab surface layer is controlled at 5 - 10°C / s.

[0058] The chemical composition of the steel in the embodiments of the present invention is shown in Table 1. The main process parameters of the steel in the embodiments of the present invention are shown in Table 2. The main heat treatment process parameters of the steel in the embodiments of the present invention are shown in Table 3. The inclusion level of the steel in the embodiments of the present invention is shown in Table 4. The properties of the steel in the embodiments of the present invention are shown in Table 5.

[0059] Table 1 Chemical composition of the steel in the embodiments of the present invention (wt%)

[0060]

[0061] Table 2 Main process parameters of the steel in the embodiments of the present invention

[0062]

[0063]

[0064] Table 3 Inclusion level and microstructure of the steel in the embodiments of the present invention

[0065]

[0066] Table 5 Properties of the steel in the embodiments of the present invention

[0067]

[0068]

[0069] As can be seen from the above, for the steel plate of the present invention, the room - temperature tensile strength > 690 MPa, the yield strength > 580 MPa, and the elongation after fracture ≥ 22%; the 350°C high - temperature tensile strength > 635 MPa, the yield strength > 525 MPa; the reduction of area in the thickness direction ≥ 65%; the impact energy absorption at - 7°C ≥ 250 J. After simulated post - weld heat treatment, the room - temperature tensile strength of the steel plate > 660 MPa, the yield strength > 545 MPa, and the elongation after fracture ≥ 22.5%; the 350°C high - temperature tensile strength > 620 MPa, the yield strength > 510 MPa; the reduction of area in the thickness direction > 65%; the impact energy absorption at - 7°C ≥ 240 J.

[0070] In order to describe the present invention, the present invention has been appropriately and fully described by way of examples above. The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can also make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent replacements, improvements, etc. should be included within the protection scope of the present invention. The patent protection scope of the present invention shall be defined by the claims.

Claims

1. A super-thick high-strength steel plate for the flange of the containment gate of a nuclear power plant, characterized in that, The composition of the steel plate is as follows by weight percentage: C: 0.21% - 0.23%; Si: 0.15% - 0.30%; Mn: 1.75% - 1.90%; P ≤ 0.010%; S ≤ 0.005%; Ni: 0.96% - 0.99%; Cr: 0.15% - 0.25%; Mo: 0.50% - 0.80%; V: 0.010% - 0.050%; Cu: 0.10% - 0.30%; Al: 0.010% - 0.040%, and the balance is Fe and inevitable impurities; the microstructure of the steel plate is tempered sorbite, and a large number of complex cementites with a size not greater than 60 nm are dispersed in the ferrite matrix.

2. The extra-thick high-strength steel plate for the containment gate flange of a nuclear power plant according to claim 1, characterized in that, The thickness of the steel plate is 130 - 180 mm, meeting the requirements of Class I ultrasonic flaw detection in the NB / T47013.3 standard.

3. The extra-thick high-strength steel plate for the containment gate flange of a nuclear power plant according to claim 1, characterized in that, The tensile strength of the steel plate at room temperature in tensile test > 690 MPa, yield strength > 580 MPa, elongation after fracture ≥ 22%; reduction of area in the thickness direction ≥ 65%; impact energy absorption at - 7°C ≥ 250 J; tensile strength of high-temperature tensile test at 350°C > 635 MPa, yield strength > 525 MPa.

4. The extra-thick high-strength steel plate for the containment gate flange of a nuclear power plant according to claim 1, characterized in that, After simulated post-weld heat treatment, the tensile strength of the steel plate at room temperature in tensile test > 660 MPa, yield strength > 545 MPa, elongation after fracture ≥ 22.5%; reduction of area in the thickness direction > 65%; impact energy absorption at - 7°C ≥ 240 J; tensile strength of high-temperature tensile test at 350°C > 620 MPa, yield strength > 510 MPa.

5. The extra-thick high-strength steel plate for the containment gate flange of a nuclear power plant according to claim 1, characterized in that, The size of inclusions in the full-thickness section of the steel plate is controlled at Grade 0.5 and below.

6. A manufacturing method of an extra-thick high-strength steel plate for the flange of a nuclear power plant containment gate as described in claim 1, including smelting, continuous casting, electroslag remelting, forging, rolling, and quenching and tempering treatment; characterized in that: Forging: The starting forging temperature is above 1250°C, and the final forging temperature is above 950°C. Rolling: The heating temperature of the steel billet ≥ 1220°C, the starting rolling temperature ≥ 1150°C, the average single-pass reduction rate ≥ 15%, the final rolling temperature is 900 - 950°C, and it is naturally cooled after rolling. Quenching and tempering treatment: The quenching temperature is 940°C - 960°C, and the holding time is 3 - 5 min / mm. The tempering temperature is 650°C - 680°C, and the holding time is 6 - 10 min / mm.

7. The manufacturing method of an extra-thick high-strength steel plate for the flange of a containment gate in a nuclear power plant according to claim 6, characterized in that, The forging is carried out in a two-dimensional manner of longitudinal forging + transverse forging.

8. The manufacturing method of an extra-thick high-strength steel plate for the containment gate flange of a nuclear power plant according to claim 6, characterized in that During the rolling process, asynchronous rolling + surface spraying is adopted. For asynchronous rolling, the roll speed ratio of the upper and lower rolls is 1.05 - 1.3, and the surface spraying technology is to cool the slab by spraying water, controlling the temperature drop on the slab surface at 5 - 10°C / s.

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