Microalloyed seamless steel tube for nuclear power and method for producing the same

Abstract

This invention belongs to the field of seamless steel pipe production technology in the metallurgical industry, specifically relating to a microalloyed seamless steel pipe for nuclear power and its preparation method. The microalloyed seamless steel pipe for nuclear power is composed of the following components by mass fraction: C: 0.19%~0.22%, Si: 0.20%~0.35%, Mn: 1.15%~1.25%, S≤0.008%, P≤0.015%, Cu≤0.1%, Sn≤0.03%, Cr: 0.22%~0.30%, Mo: 0.10%~0.20%, V: 0.06%~0.09%, Nb: 0.050%~0.070%, Ni: 0.2%~0.25%, Al: 0.015%~0.050%, N: 0.008%~0.012%, with the balance being Fe and unavoidable impurities. The microalloyed seamless steel pipe for nuclear power provided by this invention is a microalloyed steel containing Cr, Mo, V, Nb and Ni. It has the advantages of good low-temperature impact toughness, excellent performance after ultra-long simulated welding heat treatment, and low cost.

Description

Technical Field

[0001] This invention belongs to the field of seamless steel pipe production technology in the metallurgical industry, specifically relating to a micro-alloyed seamless steel pipe for nuclear power and its preparation method. Background Technology

[0002] Due to the harsh operating environment of seamless steel pipes used in nuclear power plants, the mechanical properties of the pipes must not only meet the requirements for room temperature performance testing, but also for low-temperature impact performance (0℃), and the performance after simulated heat treatment for an extended period (over 16 hours). Both the pipe body and the overall performance must simultaneously meet the performance indicators. Furthermore, ultrasonic testing of the pipe body and delamination defects must be strictly performed according to the RCC-M standard. Therefore, the quality requirements for seamless steel pipes used in nuclear power plants are extremely high. Seamless steel pipes used for nuclear power plant support are nuclear-grade pipes, made of TU48C material. Nuclear power steel has extremely high requirements for its composition and performance stability. Currently, the commonly used domestic production processes result in low yield and high cost. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a microalloyed seamless steel pipe for nuclear power plants and its preparation method, thus resolving at least one aspect of the above-mentioned technical issues.

[0004] This invention is achieved through the following technical solution: In a first aspect, the present invention provides a microalloyed seamless steel tube for nuclear power plants, comprising the following components by mass fraction: C: 0.19%~0.22%, Si: 0.20%~0.35%, Mn: 1.15%~1.25%, S≤0.008%, P≤0.015%, Cu≤0.1%, Sn≤0.03%, Cr: 0.22%~0.30%, Mo: 0.10%~0.20%, V: 0.06%~0.09%, Nb: 0.035%~0.050%, Ni: 0.2%~0.25%, Al: 0.015%~0.050%, N: 0.008%~0.012%, with the balance being Fe and unavoidable impurities.

[0005] Secondly, the present invention provides a method for preparing the above-mentioned microalloyed seamless steel tube for nuclear power, comprising the following steps: After the steel pipe is made into a round tube blank with through holes according to the preset composition, the round tube blank is subjected to homogenization treatment, rolling and normalizing heat treatment.

[0006] The microalloyed seamless steel pipe for nuclear power plants provided by this invention has at least the following beneficial technical effects compared with the prior art: The microalloyed seamless steel pipe for nuclear power provided by this invention is a microalloyed steel containing Cr, Mo, V, Nb and Ni. It has the advantages of good low-temperature impact toughness, excellent performance after ultra-long simulated welding heat treatment, and low cost.

[0007] The method for preparing microalloyed seamless steel tubes for nuclear power plants provided by this invention has at least the following beneficial technical effects compared with the prior art: (1) The method for preparing microalloyed seamless steel pipes for nuclear power provided by the present invention includes: temperature uniform treatment to soften the billet at high temperature to facilitate deformation and suppress undesirable structures such as overheating and burning; rolling treatment to make the steel pipe size meet the initial set requirements and a large rolling ratio to make the subsequent microstructure more compact; and normalizing heat treatment to make the material have excellent microstructure to meet the material performance requirements.

[0008] (2) The method for preparing microalloyed seamless steel pipes for nuclear power provided in the embodiments of the present invention has a simple preparation process and a high yield. Attached Figure Description

[0009] To more clearly illustrate the technical solutions in the embodiments of this drawing or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this drawing. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0010] Figure 1 Metallographic image of the microalloyed seamless steel tube for nuclear power provided in Embodiment 1 of the present invention; Figure 2 Metallographic image of the microalloyed seamless steel tube for nuclear power provided in Embodiment 2 of the present invention; Figure 3 Metallographic image of the microalloyed seamless steel tube for nuclear power provided in Embodiment 3 of the present invention; Figure 4 Metallographic image of the steel pipe provided in Comparative Example 1 of this invention; Figure 5 Metallographic image of the steel pipe provided in Comparative Example 2 of this invention; Figure 6 The image shows the metallographic structure of the steel pipe provided in Comparative Example 3 of this invention.

[0011] The purpose, features, and advantages of this accompanying drawing will be further explained in conjunction with the embodiments and with reference to the accompanying drawing. Detailed Implementation

[0012] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described and illustrated below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments provided by this invention without inventive effort are within the scope of protection of this invention.

[0013] Obviously, the following description is merely some examples or embodiments of the present invention. Those skilled in the art can apply the present invention to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this invention, modifications to design, manufacturing, or production based on the technical content disclosed in this invention are merely conventional technical means and should not be construed as insufficient disclosure of the present invention.

[0014] However, there may be instances where unnecessary detailed descriptions are omitted. For example, detailed descriptions of well-known matters or repetitive descriptions of essentially the same structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the following description is provided to enable those skilled in the art to fully understand the invention and is not intended to limit the subject matter of the claims.

[0015] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions, and all technical features and optional technical features of the present invention can be combined with each other to form new technical solutions.

[0016] Microalloyed seamless steel tubes for nuclear power applications The first aspect of this invention provides a microalloyed seamless steel tube for nuclear power plants, composed of the following components by mass fraction: C: 0.19%~0.22%, Si: 0.20%~0.35%, Mn: 1.15%~1.25%, S≤0.008%, P≤0.015%, Cu≤0.1%, Sn≤0.03%, Cr: 0.22%~0.30%, Mo: 0.10%~0.20%, V: 0.06%~0.09%, Nb: 0.035%~0.050%, Ni: 0.2%~0.25%, Al: 0.015%~0.050%, N: 0.008%~0.012%, with the balance being Fe and unavoidable impurities.

[0017] The microalloyed seamless steel pipe for nuclear power provided in this embodiment of the invention is a microalloyed steel containing Cr, Mo, V, Nb and Ni. It has the advantages of good low-temperature impact toughness, excellent performance after ultra-long simulated welding heat treatment, and low cost.

[0018] In the embodiments, the mass fraction of C is 0.19%~0.22%. In this case, the strength of the steel pipe is improved (by forming carbides) while ensuring the low-temperature toughness of the steel pipe.

[0019] In the examples, the mass fraction of Si is 0.20%~0.35%. In this case, Si is a deoxidizing element in the steelmaking process, which is very important for reducing the content of harmful element O in the material. Si can also improve strength. In addition to inhibiting Mn segregation when present in steel in a certain proportion with Mn, Si can also inhibit P segregation at grain boundaries.

[0020] In the examples, the mass fraction of Mn is 1.15%~1.25%. In this case, Mn is an austenite stabilizing element and also a matrix strengthening element, which can improve strength through precipitation strengthening. However, if the Mn content is too high, it is easy to form segregation in the material, affecting the toughness of the material. The presence of Si and Mn in a certain proportion in the steel is also beneficial to suppress their segregation.

[0021] In the embodiments, S ≤ 0.008% and P ≤ 0.015%. Under these conditions, sulfur (S) readily forms sulfides with the metallic element Mn, reducing the low-temperature toughness of the material; phosphorus (P) readily segregates at grain boundaries, reducing low-temperature toughness and resistance to crack propagation.

[0022] In this embodiment, Cu ≤ 0.1%. Under these conditions, the material will not be prone to "copper brittleness" during hot working.

[0023] In this embodiment, Sn ≤ 0.03%. Under these conditions, excessive Sn content will not lead to reduced high-temperature mechanical properties, hot brittleness, or deterioration of weldability.

[0024] In the examples, the mass fraction of Cr is 0.22%~0.30%. In this case, while ensuring the low-temperature impact toughness of the steel pipe, Cr provides the hardenability and strength of the steel and improves the tempering stability of the steel.

[0025] In the examples, the mass fraction of Mo is 0.10%~0.20%. In this case, molybdenum forms stable carbides, hindering grain growth and refining the grains, resulting in a more uniform and dense microstructure of the steel, thereby improving strength and toughness. It dissolves in ferrite, lowering the critical cooling rate, allowing the steel to obtain a deeper hardened layer during heat treatment, improving the overall uniformity of mechanical properties and increasing hardenability. Molybdenum raises the recrystallization temperature of the steel, and by combining with impurities such as sulfur and phosphorus, it reduces grain boundary segregation, preventing a decrease in toughness during tempering. Simultaneously, an appropriate amount of Mo effectively maintains the strength of the steel above 500°C.

[0026] In the embodiments, the mass fraction of V is 0.06%~0.09%, and the mass fraction of Nb is 0.035%~0.050%. Under these conditions, V and Nb readily form compounds with C / N, hindering grain growth at high temperatures, refining the grains, and significantly improving the strength and low-temperature toughness of the steel. Furthermore, the dispersed distribution of carbonitrides hinders dislocation movement, leading to precipitation strengthening, increasing strength while maintaining good ductility. This also improves tempering stability and optimizes heat treatment performance.

[0027] In the examples, the mass fraction of Ni is 0.2%~0.25%. In this case, Ni is an austenite stabilizing element. Ni significantly improves the strength of steel by strengthening ferrite through solid solution and refining pearlite, with minimal damage to plasticity and toughness. Lowering the low-temperature brittle-to-crystalline transition temperature delays the pearlite phase transformation, shifts the CCT curve to the right, and improves hardenability.

[0028] In the examples, the mass fraction of Al is 0.015%~0.050%. In this case, Al is a strong deoxidizer, combining with oxygen in the steel to form alumina, effectively removing oxygen impurities, purifying the molten steel, reducing inclusions, and improving the purity and fluidity of the steel. It also reduces the austenite grain size and refines the grains, thereby enhancing the strength and toughness of the steel, especially reducing brittleness at low temperatures. Aluminum combines with nitrogen to form aluminum nitride, preventing aging in the steel and improving impact toughness and stability.

[0029] In the examples, the mass fraction of N is 0.008%~0.012%. In this case, N is a solid solution strengthening element that can significantly improve the strength of steel. It can also inhibit grain growth, refine grains, and improve the microstructure, while simultaneously forming nitrides with elements such as Al and V in the steel.

[0030] In some embodiments, the composition of microalloyed seamless steel pipes for nuclear power plants contains Cu+Sn≤0.55%. In this case, the content of Cu must be controlled because it makes the material prone to "copper embrittlement" during hot working. The content of Sn must also be controlled because it reduces high-temperature mechanical properties, causes hot embrittlement, and deteriorates weldability.

[0031] In some embodiments, the mass ratio of Al to N in the composition of microalloyed seamless steel pipes for nuclear power plants is ≥2:1. In this case, the ratio of Al and N elements can form AlN, which plays a role in dispersion strengthening and grain boundary pinning, thereby improving toughness. However, excessive N content can lead to problems such as cold brittleness, reduced toughness, and poor weldability. Therefore, the N content must be controlled within a certain range.

[0032] In some embodiments, the yield strength of the microalloyed seamless steel tube for nuclear power is 313 MPa to 380 MPa.

[0033] In some embodiments, the tensile strength of the microalloyed seamless steel tube for nuclear power is 489 MPa to 530 MPa.

[0034] In some embodiments, the thickness of the microalloyed seamless steel tube for nuclear power is 40 mm to 60 mm.

[0035] In some embodiments, the inner diameter of the microalloyed seamless steel tube for nuclear power is 310 mm to 320 mm.

[0036] In some embodiments, the elongation at break of microalloyed seamless steel tubes for nuclear power is 31.5% to 33%.

[0037] In some embodiments, the impact energy of microalloyed seamless steel pipes for nuclear power plants at 0°C is 82J~108J.

[0038] [Preparation Method of Microalloyed Seamless Steel Tubes for Nuclear Power Plants] A second aspect of this invention provides a method for preparing a microalloyed seamless steel tube for nuclear power plants, comprising the following steps: S10. After preparing a round tube blank with through holes according to the preset composition of the steel pipe, the round tube blank is subjected to homogenization treatment, rolling and normalizing heat treatment.

[0039] The method for preparing microalloyed seamless steel pipes for nuclear power plants provided in this invention includes: isothermal treatment to soften the billet at high temperature, facilitating deformation and suppressing undesirable structures such as overheating and burning; rolling treatment to ensure the steel pipe dimensions meet the initial requirements, and a larger rolling ratio to make the subsequent microstructure more compact; and normalizing heat treatment to obtain an excellent microstructure that meets the material performance requirements.

[0040] The preparation method provided in this invention is simple and has a high yield.

[0041] It should be noted that the preparation of round tube billets with through holes is a conventional process in the art. For example, in the continuous casting process of steel billets, liquid core reduction technology, lowering the superheating temperature of molten steel, controlling the amount of secondary cooling water, and alternating stirring technology with electromagnetic stirrers are used to reduce central defects, make the composition more uniform, expand the equiaxed crystal zone, and reduce the segregation of major elements in the continuously cast billet. After the continuously cast billet is made, it is only necessary to drill holes in the continuously cast billet to obtain round tube billets with through holes. Drilling can remove central porosity, central shrinkage cavities, central cracks, etc., and reduce or avoid defects such as internal surface cracks, folds, scale, rolling folds, and delamination in the rolled steel pipe. Unlike conventional techniques in the art, the composition uses the steel pipe composition provided in the embodiments of this invention. Therefore, the preparation process of round tube billets with through holes is not particularly limited in the embodiments of this invention.

[0042] In some embodiments, the temperature equalization process in step S10 above includes the following steps: S101. The round tube blank is heated and kept at a uniform temperature after four stages of heating and heat preservation. The four-stage heating and insulation system includes first heating and insulation, second heating and insulation, third heating and insulation, and fourth heating and insulation.

[0043] In the above-mentioned temperature equalization process, the four-stage heating and heat preservation process ensures that the steel billet material has a uniform and stable structure during the heating process, and does not produce harmful structures such as overheating or burning. The temperature equalization and heat preservation process ensures that the steel billet is heated evenly throughout the entire circular cross-section, and that the temperature at every position of the entire cross-section is consistent.

[0044] In some embodiments, in step S101 above, the heating rate of the first heating and heat preservation is 20°C / min to 30°C / min.

[0045] In some embodiments, in step S101 above, the heat preservation temperature of the first heating and heat preservation is 1020℃~1040℃.

[0046] In some embodiments, in step S101 above, the heat preservation time for the first heating and heat preservation is 1h to 2h.

[0047] In some embodiments, in step S101 above, the heating rate of the second heating and heat preservation is 20°C / min to 30°C / min.

[0048] In some embodiments, in step S101 above, the heat preservation temperature of the second heating and heat preservation is 1120℃~1140℃.

[0049] In some embodiments, in step S101 above, the heat preservation time for the second heating and heat preservation is 1.5h to 2h.

[0050] In some embodiments, in step S101 above, the heating rate of the third heating and heat preservation is 20°C / min to 30°C / min.

[0051] In some embodiments, in step S101 above, the heat preservation temperature of the third heating and heat preservation is 1175℃~1195℃.

[0052] In some embodiments, in step S101 above, the heat preservation time for the third heating and heat preservation is 1.5h to 2h.

[0053] In some embodiments, in step S101 above, the heating rate of the fourth heating and heat preservation is 20°C / min to 30°C / min.

[0054] In some embodiments, in step S101 above, the heat preservation temperature of the fourth heating and heat preservation is 1230℃~1250℃.

[0055] In some embodiments, in step S101 above, the heat preservation time for the fourth heating and heat preservation is 1.5h to 2h.

[0056] In some embodiments, in step S101 above, the temperature for uniform heat preservation is 1230℃~1250℃.

[0057] In some embodiments, in step S101 above, the heat preservation time for uniform temperature preservation is 1.5h to 2h.

[0058] In some embodiments, during step S10 above, the rolling ratio is ≥3. In this case, a rolling ratio ≥3 makes the material's microstructure denser and its grain size smaller, thereby improving the material's overall performance. It should be noted that the process steps for turning round tube billets into steel pipes are conventional process steps in the art, and therefore, they are not described in detail or specifically in the embodiments of the present invention.

[0059] In some embodiments, the normalizing heat treatment in step S10 above includes the following steps: S102. The rolled steel pipe is pre-normalized and then air-cooled, and then normalized.

[0060] In some embodiments, in step S102 above, the pre-normalizing holding temperature is 880°C to 900°C.

[0061] In some embodiments, in step S102 above, the relationship between the pre-normalizing holding time and the wall thickness is 2 min / mm to 2.5 min / mm.

[0062] In some embodiments, in step S102 above, the pre-normalizing holding time is 130 minutes or more.

[0063] In some embodiments, in step S102 above, the holding temperature for normalizing is 900°C to 920°C.

[0064] In some embodiments, in step S102 above, the relationship between the holding time of normalizing and the wall thickness is 3 min / mm to 3.5 min / mm.

[0065] In some embodiments, in step S102 above, the holding time for normalizing is 200 minutes or more.

[0066] The following description, in conjunction with specific embodiments, provides further details.

[0067] For ease of explanation, the following embodiments and comparative examples involve: (1) The preparation steps of the round tube blank with through hole are as follows: Electric furnace smelting (tap temperature 1610℃) - LF furnace refining (tap temperature 1605℃) - vacuum furnace (vacuum holding time 8min) - arc continuous casting (casting start temperature 1600℃) - Φ500 continuous casting round tube billet enters the slow cooling pit, slow cooling time ≥40 hours.

[0068] (2) The length of the finished steel pipe is 7100mm.

[0069] (3) The thickness of the finished steel pipe is 50mm.

[0070] Example 1 Example 1 provides a microalloyed seamless steel tube for nuclear power plants, composed of the following components by mass fraction: C: 0.2%, Si: 0.3%, Mn: 1.2%, S: 0.002%, P: 0.01%, Cu: 0.1%, Sn: 0.03%, Cr: 0.25%, Mo: 0.15%, V: 0.08%, Nb: 0.040%, Ni: 0.2%, Al: 0.03%, N: 0.010%, with the balance being Fe and unavoidable impurities.

[0071] This embodiment also provides a method for preparing the above-mentioned microalloyed seamless steel tube for nuclear power, the steps of which are as follows: E10. Preparation of round tube blanks with through holes A round tube blank with through holes is prepared according to the composition of the microalloyed seamless steel tube for nuclear power in this embodiment.

[0072] E20. Temperature equalization treatment E201. First heating and holding: The round tube blank is heated to 1030℃±10℃ at a heating rate of 25℃ / min and held for 1.5h.

[0073] E202. Second heating and holding: The round tube blank is heated to 1130℃±10℃ at a heating rate of 25℃ / min and held for 1.5h.

[0074] E203. Third heating and heat preservation: The round tube blank is heated to 1185℃±10℃ at a heating rate of 25℃ / min and kept at that temperature for 1.5h.

[0075] E204. Fourth heating and heat preservation: Heat the round tube blank to 1240℃±10℃ at a heating rate of 25℃ / min and keep it at that temperature for 1.5h.

[0076] E205. Uniform Temperature Insulation: After the round tube billet is heated and insulated in four stages, it is uniformly insulated at 1230℃±10℃ for 1.5h.

[0077] E30. Rolling The round tube billet after temperature homogenization is rolled at a rolling ratio of 3 to obtain the finished steel pipe.

[0078] E40. Normalizing heat treatment The rolled steel pipe is pre-normalized and then air-cooled, and then normalized again to obtain a micro-alloyed seamless steel pipe for nuclear power.

[0079] The pre-normalizing holding temperature is 890℃, and the relationship between the holding time and the wall thickness is 2min / mm; The holding temperature for normalizing is 910℃, and the relationship between holding time and wall thickness is 3 min / mm.

[0080] Example 2 Example 2 provides a microalloyed seamless steel tube for nuclear power plants, composed of the following components by mass fraction: C: 0.2%, Si: 0.3%, Mn: 1.2%, S: 0.002%, P: 0.01%, Cu: 0.1%, Sn: 0.03%, Cr: 0.30%, Mo: 0.20%, V: 0.09%, Nb: 0.050%, Ni: 0.25%, Al: 0.03%, N: 0.010%, with the balance being Fe and unavoidable impurities.

[0081] This embodiment also provides a method for preparing the above-mentioned microalloyed seamless steel tube for nuclear power plants. The steps are basically the same as in Embodiment 1, except that: In the first heating and heat preservation step E201, the heating rate is 30℃ / min.

[0082] In the second heating and heat preservation step E202, the heating rate is 30℃ / min.

[0083] In the third heating and heat preservation step E203, the heating rate is 30℃ / min.

[0084] In the fourth heating and heat preservation step E204, the heating rate is 30℃ / min.

[0085] Example 3 Example 3 provides a microalloyed seamless steel tube for nuclear power plants, composed of the following components by mass fraction: C: 0.2%, Si: 0.3%, Mn: 1.2%, S: 0.002%, P: 0.01%, Cu: 0.1%, Sn: 0.03%, Cr: 0.22%, Mo: 0.10%, V: 0.06%, Nb: 0.035%, Ni: 0.2%, Al: 0.03%, N: 0.010%, with the balance being Fe and unavoidable impurities.

[0086] This embodiment also provides a method for preparing the above-mentioned microalloyed seamless steel tube for nuclear power plants. The steps are basically the same as in Embodiment 1, except that: In the first heating and heat preservation step E201, the heating rate is 20℃ / min.

[0087] In the second heating and heat preservation step E202, the heating rate is 20℃ / min.

[0088] In the third heating and heat preservation step E203, the heating rate is 20℃ / min.

[0089] In the fourth heating and heat preservation step E204, the heating rate is 20℃ / min.

[0090] Comparative Example 1 Comparative Example 1 provides a steel pipe composed of the following components by mass fraction: C: 0.2%, Si: 0.3%, Mn: 1.2%, S: 0.002%, P: 0.01%, Cu: 0.1%, Sn: 0.03%, Al: 0.03%, N: 0.010%, with the balance being Fe and unavoidable impurities.

[0091] This comparative example also provides a method for preparing the above-mentioned steel pipe, the steps of which are basically the same as those in Example 1.

[0092] Comparative Example 2 Comparative Example 2 provides a steel pipe composed of the following components by mass fraction: C: 0.2%, Si: 0.3%, Mn: 1.2%, S: 0.002%, P: 0.01%, Cu: 0.1%, Sn: 0.03%, Cr: 0.20%, Mo: 0.08%, V: 0.03%, Nb: 0.020%, Ni: 0.1%, Al: 0.03%, N: 0.010%, with the balance being Fe and unavoidable impurities.

[0093] This comparative example also provides a method for preparing the above-mentioned steel pipe, the steps of which are basically the same as those in Example 1.

[0094] Comparative Example 3 Comparative Example 3 provides a method for preparing a steel pipe, the steps of which are as follows: D10. Preparation of round tube blanks with through holes A round tube blank with through holes was prepared according to the composition of the microalloyed seamless steel tube for nuclear power provided in Example 1.

[0095] D20. Temperature equalization treatment D201. Heating and holding: Heat the round tube blank to 1240℃±10℃ at a heating rate of 25℃ / min and hold for 6 hours.

[0096] D202. Uniform temperature insulation: After the round tube billet is heated and kept at a constant temperature of 1230℃±10℃ for 1.5h.

[0097] D30. Rolling The round tube billet after temperature homogenization is rolled at a rolling ratio of 3 to obtain the finished steel pipe.

[0098] D40. Normalizing heat treatment The rolled steel pipe is pre-normalized and then air-cooled, and then normalized again to obtain a micro-alloyed seamless steel pipe for nuclear power.

[0099] The pre-normalizing holding temperature is 890℃, and the relationship between the holding time and the wall thickness is 2min / mm; The holding temperature for normalizing is 910℃, and the relationship between holding time and wall thickness is 3 min / mm.

[0100] To verify the advancement of the microalloyed seamless steel pipe for nuclear power plants and its preparation method provided in this invention, the steel pipes obtained in the embodiments and comparative examples of this invention were subjected to yield strength, tensile strength, elongation at break, and impact energy at 0°C. The results are shown in Table 1 below. Metallographic images of the steel pipes in the embodiments and comparative examples are also provided. Figures 1-6 As shown.

[0101] Table 1

[0102] Note: In the table above, the simulated post-weld heat treatment steps are as follows: the simulated stress-relief heat treatment holding temperature is 615℃±5℃, the holding time is 8h~8.5h, the heating rate above 300℃ is ≤55℃ / h, and the cooling rate above 300℃ is ≤55℃ / h.

[0103] From the table above and the accompanying drawings in the instruction manual, at least the following conclusions can be drawn: (1) The steel pipe of Comparative Example 1 does not contain Cr, Mo, V, Nb, and Ni in its composition. Therefore, its yield strength, tensile strength, elongation at break, and impact energy are significantly lower than those of the Example. The steel pipe of Comparative Example 2 contains a lower content of Cr, Mo, V, Nb, and Ni in its composition. It can be seen that although its yield strength, tensile strength, and elongation at break are increased, its impact energy at room temperature and after simulated post-weld heat treatment is significantly reduced, far lower than that of the Example. It can be seen that the microalloyed seamless steel pipe for nuclear power provided by the present invention has a suitable proportion of Cr, Mo, V, Nb, and Ni in its composition, which makes the microalloyed seamless steel pipe have good low-temperature impact toughness and still have excellent performance after ultra-long simulated post-weld heat treatment. Moreover, Cr, Mo, V, Nb, and Ni are all common metals with low cost.

[0104] (2) In Comparative Example 3, without the four-stage heating step, the tensile strength and elongation at break of the steel pipe were not significantly reduced, but the yield strength and impact energy were significantly reduced, especially the impact energy, which was halved after simulated post-weld heat treatment compared to room temperature. Therefore, the method for preparing microalloyed seamless steel pipes for nuclear power plants provided in this embodiment of the invention uses four-stage heating and heat preservation to ensure that the steel billet material has a uniform and stable microstructure during heating, without generating harmful structures such as overheating or burning. The uniform heat preservation ensures that the steel billet is heated evenly throughout the entire circular cross-section, and the temperature at each location of the entire cross-section is consistent, thereby improving the impact toughness of the steel pipe.

[0105] It should be noted that the present invention is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments that have the same structure and perform the same effects as the technical concept within the scope of the present invention are included within the scope of the present invention. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of the present invention, are also included within the scope of the present invention.

Claims

1. A microalloyed seamless steel pipe for nuclear power plants, characterized in that, Composed of the following components by mass fraction composition: C: 0.19%~0.22%, Si: 0.20%~0.35%, Mn: 1.15%~1.25%, S≤0.008%, P≤0.015%, Cu≤0.1%, Sn≤0.03%, Cr: 0.22%~0.30%, Mo: 0.10%~0.20%, V: 0.06%~0.09%, Nb: 0.050%~0.070%, Ni: 0.2%~0.25%, Al: 0.015%~0.050%, N: 0.008%~0.012%, with the balance being Fe and unavoidable impurities.

2. The microalloyed seamless steel pipe for nuclear power plants according to claim 1, characterized in that, The composition of the microalloyed seamless steel pipe for nuclear power plants has Cu+Sn≤0.55%.

3. The microalloyed seamless steel pipe for nuclear power plants according to claim 1 or 2, characterized in that, In the composition of microalloyed seamless steel pipes for nuclear power plants, the mass ratio of Al:N is ≥2:

1.

4. The microalloyed seamless steel tube for nuclear power plants according to any one of claims 1 to 3, characterized in that, It satisfies at least one of the following characteristics (1) to (6): (1) The yield strength of the microalloyed seamless steel pipe for nuclear power is 313MPa~380MPa; (2) The tensile strength of the microalloyed seamless steel pipe for nuclear power is 489MPa~530MPa; (3) The thickness of the microalloyed seamless steel pipe for nuclear power is 50 mm; (4) The inner diameter of the microalloyed seamless steel pipe for nuclear power is 319 mm; (5) The elongation at break of the microalloyed seamless steel tube for nuclear power is 31.5%~33%; (6) The impact energy of the microalloyed seamless steel pipe for nuclear power at 0℃ is 82J~108J.

5. A method for preparing a microalloyed seamless steel tube for nuclear power plants as described in any one of claims 1 to 4, characterized in that, Includes the following steps: After the steel pipe is made into a round tube blank with through holes according to the preset composition, the round tube blank is subjected to homogenization treatment, rolling and normalizing heat treatment.

6. The method for preparing microalloyed seamless steel tubes for nuclear power plants according to claim 5, characterized in that, The temperature equalization process includes the following steps: The round tube blank is heated and kept at a uniform temperature after four stages of heating and heat preservation. The four-stage heating and heat preservation includes a first heating and heat preservation, a second heating and heat preservation, a third heating and heat preservation, and a fourth heating and heat preservation.

7. The method for preparing microalloyed seamless steel tubes for nuclear power plants according to claim 6, characterized in that, It satisfies at least one of the following characteristics (1) to (15): (1) The heating rate of the first heating and heat preservation is 20℃ / min~30℃ / min; (2) The heat preservation temperature of the first heating and heat preservation is 1020℃~1040℃; (3) The heat preservation time of the first heating and heat preservation is 1h~2h; (4) The heating rate of the second heating and heat preservation is 20℃ / min~30℃ / min; (5) The heat preservation temperature of the second heating and heat preservation is 1120℃~1140℃; (6) The heat preservation time for the second heating and heat preservation is 1.5h~2h; (7) The heating rate of the third heating and heat preservation is 20℃ / min~30℃ / min; (8) The heat preservation temperature of the third heating and heat preservation is 1175℃~1195℃; (9) The heat preservation time for the third heating and heat preservation is 1.5h to 2h; (10) The heating rate of the fourth heating and heat preservation is 20℃ / min~30℃ / min; (11) The heat preservation temperature of the fourth heating and heat preservation is 1230℃~1250℃; (12) The heat preservation time of the fourth heating and heat preservation is 1.5h~2h; (13) The temperature for uniform heat preservation is 1230℃~1250℃; (14) The heat preservation time for the uniform temperature preservation is 1.5h~2h; (15) In the rolling process, the rolling ratio is ≥3.

8. The method for preparing microalloyed seamless steel tubes for nuclear power plants according to any one of claims 5 to 7, characterized in that, The normalizing heat treatment includes the following steps: The rolled steel pipe is pre-normalized and then air-cooled before being normalized.

9. The method for preparing microalloyed seamless steel tubes for nuclear power plants according to claim 8, characterized in that, It satisfies at least one of the following characteristics (1) to (3): (1) The heat preservation temperature of the pre-normalizing is 880℃~900℃; (2) The relationship between the heat preservation time and the wall thickness during pre-normalizing is 2 min / mm to 2.5 min / mm; (3) The heat preservation time of the pre-normalizing is more than 130 minutes.

10. The method for preparing microalloyed seamless steel tubes for nuclear power plants according to claim 9, characterized in that, It satisfies at least one of the following characteristics (1) to (3): (1) The holding temperature of the normalizing is 900℃~920℃; (2) The relationship between the heat preservation time and the wall thickness during normalizing is 3 min / mm to 3.5 min / mm; (3) The holding time for normalizing is more than 200 minutes.

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