A pressure vessel steel plate for 720MPa grade extra-large crude oil storage tanks with a capacity of 160,000-200,000 cubic meters and its manufacturing method.

Abstract

This invention relates to the field of steel technology, and more particularly to a pressure vessel steel plate for 160,000-200,000 cubic meter extra-large crude oil storage tanks with a capacity of 720 MPa, and its manufacturing method. The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.070%-0.100%, Si: 0.20%-0.30%, Mn: 1.45%-1.60%, P≤0.012%, S≤0.002%, Nb: 0.030%-0.050%, Ti: 0.015%-0.025%, V: 0.040%-0.060%, Ni: 0.80%-0.90%, Cr: 0.40%-0.55%, Mo: 0.15%-0.25%, Cu: 0.30%-0.45%, Alt: 0.020%-0.050%, with the remainder being Fe and unavoidable impurities. This invention obtains a three-phase microstructure of martensite, retained austenite, and carbides through quenching-carbon distribution-tempering heat treatment. The carbides are dispersed and the microstructure is uniform. The product has the characteristics of high strength, high toughness, and high plasticity, and is widely used in key areas such as national strategic crude oil reserves and local commercial crude oil storage projects.

Description

Technical Field

[0001] This invention relates to the field of steel technology, and more particularly to a pressure vessel steel plate for 160,000-200,000 cubic meter extra-large crude oil storage tanks of 720MPa grade and its manufacturing method. Background Technology

[0002] With the continuous growth of global energy demand, the demand for ultra-large crude oil storage tanks is increasing. Currently, the manufacturing of ultra-large crude oil storage tanks abroad mainly uses steel plates with yield strength of 550 MPa and 620 MPa. Due to the limited yield strength of steel plates, in order to ensure the structural integrity and safety of the storage tank under complex working conditions, it is necessary to use thicker steel plates, which are usually 60 to 80 mm thick. This increase in thickness directly leads to the difficulty of engineering welding and manufacturing costs.

[0003] Furthermore, inadequate protective measures during the industrial production of steel plates are another pressing issue. If molten steel is not effectively protected during casting, secondary oxidation is highly likely. This not only introduces a large amount of non-metallic inclusions, reducing the purity of the steel, but also adversely affects the mechanical properties and corrosion resistance of the steel plate. Decreased steel purity directly impacts the service life and safety performance of storage tanks, increasing the costs of subsequent maintenance and replacement. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned problems and deficiencies, and to provide a pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks and its manufacturing method.

[0005] This invention employs a process of nickel-molybdenum-chromium-niobium-vanadium-titanium microalloying + continuous casting billet smelting + controlled rolling and cooling + quenching - carbon distribution - tempering treatment, resulting in products with high strength, high toughness, and high plasticity. Simultaneously, the internal quality and steel purity of the continuous casting billet are significantly improved, with smaller deviations in mechanical properties at the beginning and end of the steel plate and good uniformity of properties throughout the plate. Even after simulated post-weld heat treatment, it still exhibits a low yield strength ratio and good strength-toughness balance.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a pressure vessel steel plate for extra-large crude oil storage tanks with a capacity of 160,000-200,000 cubic meters and a pressure rating of 720MPa. The chemical composition of the steel plate, by weight percentage, is as follows:

[0008] C: 0.070%–0.100%, Si: 0.20%–0.30%, Mn: 1.45%–1.60%, P≤0.012%, S≤0.002%, Nb: 0.030%–0.050%, Ti: 0.015%–0.025%, V: 0.040%–0.060%, Ni: 0.80%–0.90%, Cr: 0.40%–0.55%, Mo: 0.15%–0.25%, Cu: 0.30%–0.45%, Alt: 0.020%–0.050%, with the remainder being Fe and unavoidable impurities.

[0009] The reasons for using the above-mentioned components are as follows:

[0010] (1) C: C is the main component element of steel. The strength of steel mainly depends on the C content. Excessive C content will lead to poor toughness, plasticity and weldability of steel; low C content will lead to lower strength and performance after simulated stress relief treatment. In order to ensure that the steel plate has a good match of low-temperature impact toughness, strength and weldability during use, the C content in the steel of this invention is controlled at 0.070% to 0.100%.

[0011] (2) Si: Si is a common solid solution strengthening alloying element in steel. It is essential for the strength, toughness, hardenability and even deoxidation of steel. However, a high content will also lead to a decrease in the toughness of steel. Therefore, the Si content in the steel of this invention is controlled at 0.20% to 0.30%.

[0012] (3) Mn: Mn can strengthen pearlite in steel through solid solution strengthening. C-Mn strengthening is also the main way to improve the strength of low carbon steel. However, if the Mn content is too high, it will increase the production cost. Mn is easy to combine with S to form MnS, which reduces the material's resistance to hydrogen-induced cracking. At the same time, the Mn content will reduce the activity of carbon. Therefore, the Mn content in the steel of this invention is controlled at 1.45% to 1.60%.

[0013] (4) P: Phosphorus is a harmful element in steel, increasing its cold brittleness, worsening its weldability, reducing its plasticity, and worsening its cold bending performance. Furthermore, P is particularly sensitive to radiation embrittlement. Therefore, the lower the P content in steel, the better; this invention controls it to below 0.012%.

[0014] (5) S: Sulfur is a harmful element under normal circumstances. S usually readily forms brittle sulfides with alloying elements in steel, causing hot brittleness and reducing the ductility and toughness of the steel. At the same time, S also tends to accelerate irradiation embrittlement. Therefore, the S content in the steel of this invention is controlled below 0.002%.

[0015] (6) V: V is a microalloying element. V microalloying in steel can form fine second phase particles, which can play the role of pinning grain boundaries and precipitation strengthening. It can effectively refine grains and greatly improve the comprehensive mechanical properties of steel such as strength, toughness, ductility and thermal fatigue resistance. The V content in the steel of this invention is controlled at 0.040% to 0.060%.

[0016] (7) Nb: As a strong carbide-forming element, Nb forms a highly dispersed NbC phase with good high-temperature stability in steel, playing a precipitation strengthening role. Through multi-stage rolling, it can effectively refine the grains and improve the reduction in toughness caused by precipitation strengthening, thereby enabling the steel plate to obtain comprehensive properties of high strength and high toughness. In addition, in Nb-Mo composite steel, Mo can also agglomerate at the NbC matrix interface, preventing the coarsening of NbC particles, thereby greatly improving the high-temperature strength of the steel. Therefore, the Nb content in the steel of this invention is controlled at 0.030% to 0.050%.

[0017] (8) Ti: Adding an appropriate amount of Ti can form a large number of dispersed fine TiN or Ti2O3 particles, which can serve as heterogeneous nucleation sites for acicular pearlite during solidification, thereby refining the microstructure. Ti also has a deoxidizing effect, ensuring that B is not oxidized or nitrided. B can lower the transformation temperature from austenite to pearlite, promoting the formation of acicular pearlite within the grains and refining the grains. However, when w(Ti)≥0.09%, the content of acicular pearlite will decrease, causing the low-temperature toughness of the steel plate to deteriorate. Therefore, the Ti content in the steel of this invention is controlled at 0.020% to 0.050%.

[0018] (9) Alt: Adding a small amount of Al to steel can effectively refine the austenite grains, thereby refining the ferrite grains and microstructure, and improving the impact toughness of the steel. However, Al has the disadvantage of affecting the hot working performance, weldability, and machinability of the steel. Therefore, the Alt content in the steel of this invention is controlled at 0.015% to 0.045%.

[0019] (10) Ni: Ni is a solid solution strengthening element in steel that can improve the strength of steel. Ni reduces the resistance to dislocation movement in steel, relaxes stress, and changes the substructure of the matrix, thereby improving the toughness of steel, especially the low-temperature toughness. However, excessively high Ni content in medium carbon steel will increase the phase transformation temperature. Therefore, the Ni content in this invention is controlled at 0.80% to 0.90%.

[0020] (11) Mo: Mo mainly relies on solid solution strengthening and grain boundary strengthening to improve the strength of steel; secondly, Mo increases the stability of supercooled austenite, causing the austenite to pearlite transformation curve to shift to the right, resulting in a finer pearlite structure after phase transformation; in addition, Ti and Mo combine to precipitate a large amount of nano-sized Ti-Mo(CN) carbides in the steel, and the refined carbides pin dislocations, greatly improving the strength and toughness of the steel. Therefore, the Mo content in the steel of this invention is controlled at 0.15% to 0.25%.

[0021] (12) Cu: The prominent role of Cu in steel is to improve the corrosion resistance of plain carbon low-alloy steel, and it can also increase the strength and yield strength ratio of steel, without adversely affecting the weldability. At the same time, its role is similar to that of nickel, which can play a certain role in saving nickel and reducing costs. However, when the content is high, it will lead to copper embrittlement during hot deformation processing. Therefore, the Cu content in the steel of this invention is controlled at 0.30% to 0.45%.

[0022] (13) Cr: Chromium is an element that stabilizes carbides. The main role of chromium in quenched and tempered steel is to improve hardenability, resulting in better comprehensive mechanical properties after quenching and tempering, including increased strength, toughness, and elongation. In carburizing steel, it can also form chromium-containing carbides, thereby improving the wear resistance of the material surface. Therefore, the Cr content in the steel of this invention is controlled at 0.40%–0.55%.

[0023] In the above technical solution, the thickness of the finished steel plate is further 10-50mm.

[0024] In the above technical solution, further, the thickness of the finished steel plate is 10mm≤ and <30mm, the tensile strength at 1 / 4 of the thickness of the finished steel plate in the delivery state is 860~890MPa, the yield strength is 740~770MPa, the elongation after fracture is 29%~32%, the yield ratio is 0.85~0.87, the impact energy at -70℃ is 160~230J, the lateral expansion value LE is 1.1~1.9mm, the welding heat input is 140~150KJ / cm, the impact energy at -50℃ in the heat-affected zone after welding is 120~190J, and the lateral expansion value LE after welding is 1.0~2.0mm;

[0025] For finished steel plates with a thickness of 30mm or less and 50mm or less, the tensile strength in the transverse direction at 1 / 4 and 1 / 2 of the thickness in the delivery state of the finished steel plate is 840-890MPa, the yield strength is 720-760MPa, the elongation after fracture is 30%-33%, the yield ratio is 0.85-0.87, the impact energy at -70℃ is 160-250J, the lateral expansion value LE is 1.2-1.5mm, the welding heat input energy is 140-150KJ / cm, the impact energy at -50℃ in the heat-affected zone after welding is 110-200J, and the lateral expansion value LE after welding is 1.3-1.7mm.

[0026] In the above technical solution, further, the thickness of the finished steel plate is 10mm≤30mm, the tensile strength of the finished steel plate at 1 / 4 of its thickness in the simulated post-weld heat-treated state is 850~880MPa, the yield strength is 730~760MPa, the elongation after fracture is 29%~32%, the yield ratio is 0.85~0.87, the impact energy at -70℃ is 150~250J, and the lateral expansion value LE is 1.0~2.0mm;

[0027] The thickness of the finished steel plate is 30mm≤50mm. The tensile strength of the finished steel plate at 1 / 4 and 1 / 2 of the thickness in the transverse direction is 850~880MPa, the yield strength is 730~760MPa, the elongation after fracture is 24%~33%, the yield ratio is 0.85~0.86, the impact energy at -70℃ is 190~270J, and the lateral expansion value LE is 1.3~1.7mm.

[0028] Another aspect of the present invention provides a method for manufacturing the above-mentioned steel plate, comprising the following steps:

[0029] (1) Continuous casting billet smelting:

[0030] The RH inlet temperature is 1590–1720℃, and the RH oxygen blowing rate is 5–25 m³ / h. 3 / h, circulating oxygen blowing time 30-40min, effectively preventing secondary oxidation of molten steel. At the same time, a two-step aluminum addition method is adopted for deoxidation, namely, adding aluminum cakes for pre-deoxidation during the converter tapping process + adding aluminum wires for deoxidation during the LF furnace refining process. The continuous casting process is protected during pouring, and the thickness of the continuous casting billet is 250-300mm.

[0031] (2) Controlled rolling and controlled cooling:

[0032] After heating, the continuously cast billet undergoes rough rolling at an initial temperature of 1100–1130℃ and a final temperature of 980–1020℃. Following rough rolling, the intermediate billet is air-cooled via a reciprocating oscillating roller table until it reaches a temperature of 905–935℃, then undergoes continuous multi-pass finishing rolling at a final temperature of 835–875℃. After finishing rolling, pre-straightening is performed, followed by ACC laminar flow cooling at an initial cooling temperature of 810–860℃ to preserve a significant amount of stored energy in the austenite grains. The cooling rate is 15–25℃ / s. Water cooling reaches a red-hot temperature of 650–720℃. The number of ACC manifolds is 10–12, the roll speed is 1.2–1.4 m / s, and the water flow rate is 270–290 m³ / s. 3 / h, water temperature 24~30℃, and at the same time, the head and tail of the pre-straightened steel plate are shielded during ACC laminar flow cooling, so that the temperature difference between the head and tail of the steel plate is less than 20℃, the temperature uniformity of the steel plate is good, the flatness of the steel plate after ACC is ≤8mm / m, and the steel plate shape is straight, which meets the requirements of the steel plate entering the furnace.

[0033] (3) Quenching-Carbon Distribution-Tempering Heat Treatment:

[0034] The quenching temperature is 850-870℃, the heating rate is 0.5-1.5 min / mm, the net holding time is 20-80 min, after the holding time is completed, the water is cooled to 100-120℃, the chamber furnace is held for 10-20 min, the tempering temperature is 420-440℃, the heating rate is 1.5-2.5 min / mm, the net holding time is 30-130 min, after the holding time is completed, the air is cooled to room temperature.

[0035] (4) Straightening:

[0036] The flatness of the finished steel plate after straightening is ≤5mm / m.

[0037] In the above technical solution, further, in step (1), during converter smelting, the size of the furnace charge is 50-100mm, the hot charging temperature of the furnace charge is 1150-1200℃, and the net heat preservation time is 3-6min;

[0038] The amount of aluminum ingot added is 1-5 kg / ton of steel, and the amount of aluminum wire added is 1-5 kg / ton of steel.

[0039] In the above technical solution, further, in step (1), the mold protective slag is composed of the following components by weight percentage: CaO 35%~50%, Al2O3 30%~40%, MnO 0~10%, SiO2 5%~10%, with the balance being unavoidable impurities. The thickness of the slag film is 0.1~1.5mm, ensuring that the inclusions are fully floated and removed, and preventing the generation of linear defects in the billet.

[0040] In the above technical solution, further, in step (1), the casting temperature is 1530~1550℃ and the casting speed is 0.9~1.0m·min. -1 The steel billet is placed in a slow cooling pit after it leaves the production line. The slow cooling temperature is 300-400℃ and the holding time is 12-24 hours.

[0041] In the above technical solution, further, in step (1), the continuous casting process adopts a two-stage protective casting method, namely, protective casting from ladle to tundish + protective casting from tundish to crystallizer; wherein:

[0042] The method for protective casting from ladle to tundish is as follows:

[0043] a. Adding a covering agent to the tundish reduces heat loss from the molten steel, isolates air, reduces secondary oxidation of the molten steel, and absorbs inclusions that float to the surface of the molten steel.

[0044] b. The protective casting adopts the protective sleeve method. The molten steel in the ladle flows into the tundish through a long refractory protective pipe. Argon gas is introduced at the connection between the long water nozzle protective pipe and the ladle water outlet to maintain positive pressure and prevent air from entering.

[0045] The method for protective casting from the tundish to the crystallizer is as follows: a liquid protection method is adopted, a nozzle ring is installed at the bottom of the tundish, and liquid nitrogen is sprayed around the molten steel and onto the surface of the crystallizer. The spraying pressure of liquid nitrogen is 0.5 to 2 MPa, forming a nitrogen curtain to prevent secondary oxidation. The pressure range of the nitrogen curtain is 0.1 to 0.5 MPa.

[0046] In the above technical solution, further, in step (2), the heating adopts a four-stage heating method: the temperature of heating stage I is 600-650℃, the temperature of heating stage II is 1000-1050℃, the temperature of heating stage III is 1170-1210℃, the temperature of the heat soaking stage is 1150-1190℃, the heat soaking stage time is 30-40 min, and the total heating time is H*(0.9-1.2) min·mm. -1 H represents the thickness of the continuously cast billet, in mm; appropriate heating regime effectively controls the original austenite grain size and ensures sufficient solid solution of alloying elements, thus guaranteeing good final performance of the product.

[0047] In the above technical solution, further, in step (2), rolling is carried out on a double-stand rolling mill. The reduction rate of each pass in roughing is 10% to 15%, the thickness of the intermediate billet is 3 to 4 times the thickness of the finished steel plate, and the reduction rate of each pass in finishing is 8% to 13%, until the final finished thickness. In the roughing stage, the deformation amount should be appropriately increased to promote the plastic deformation and recrystallization process of the steel. In the finishing stage, the deformation amount should be controlled to avoid excessive work hardening of the steel. By reasonably allocating the deformation amount, the microstructure of the steel is optimized, thereby reducing its yield strength ratio.

[0048] In the above technical solution, further, in step (2), during the pre-straightening process, the roller gap setting parameters of the pre-straightening machine system are adjusted according to the thickness of the finished steel plate: the tilt correction range is 0 to +1.5 mm, the roller gap correction range is -1.5 to +0.5 mm, and the load reduction range is +0.5 to +1.0 mm, so that the steel plate after pre-straightening is straight and can smoothly enter the ACC area, avoiding a series of problems such as uneven temperature and plate warping caused by local water accumulation, asymmetrical cooling, and uneven cooling during the water cooling process.

[0049] In the above technical solution, further, in step (3), the straightening speed is 150-200 r / min.

[0050] The beneficial effects of this invention are as follows:

[0051] 1. The use of niobium-vanadium-titanium-nickel-molybdenum-chromium microalloying controls the expected transformation of the microstructure, laying the foundation for the strength and toughness of the steel plate.

[0052] 2. During the continuous casting billet smelting process, the inclusion content in the steel is reduced by optimizing the protective casting, slag system ratio and RH circulation process, thereby improving the low-magnification quality of the continuous casting billet. This lays the foundation for improving the flaw detection pass rate and reducing the tensile delamination rate. The oxygen-aluminum control method is adopted in the smelting process to fully improve the purity of the steel, reduce the inclusion content in the steel, and control the composition segregation. The center segregation (grade) is ≤C1.0 grade, the center porosity is ≤0.5 grade, and the non-metallic inclusions such as A-class (sulfides), B-class (alumina), C-class (silicates), D-class (spherical oxides), and DS-class (single-particle spherical inclusions) shall not exceed grade 0.5.

[0053] 3. By adopting controlled rolling and controlled cooling processes, products with fully uniform and refined microstructure and good flatness are obtained, and the flatness after cold straightening is ≤5mm / m.

[0054] 4. A three-phase microstructure of 20-40% martensite, 30-40% retained austenite, and 30-40% carbides is obtained through quenching-carbon distribution-tempering heat treatment. The carbides are dispersed and the microstructure is uniform. The product also possesses high strength, high toughness, and high plasticity. This ensures that the product exhibits good mechanical properties and a low yield strength ratio in the mold-welded state. Both the delivered state and the simulated post-weld heat-treated state of the steel plate demonstrate good strength-toughness matching, with yield strength and tensile strength fluctuations within 50 MPa and a yield strength ratio of 0.85-0.87. Attached Figure Description

[0055] Figure 1 The image shows the metallographic structure of the steel plate in Example 1. Detailed Implementation

[0056] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0057] Examples 1-7

[0058] The chemical composition of the steel plates in Examples 1-7 of this invention is shown in Table 1.

[0059] Table 1. Chemical composition (wt%) of the steel plates in Examples 1-7 of this invention.

[0060] Example C Si Mn P S V Nb Ti Ni Cr Mo Cu Alt 1 0.074 0.26 1.49 0.010 0.001 0.045 0.034 0.017 0.82 0.42 0.17 0.32 0.027 2 0.082 0.29 1.47 0.012 0.002 0.051 0.042 0.022 0.85 0.51 0.20 0.38 0.033 3 0.091 0.24 1.54 0.011 0.001 0.043 0.037 0.025 0.87 0.48 0.21 0.42 0.045 4 0.077 0.26 1.52 0.008 0.002 0.059 0.048 0.018 0.81 0.54 0.18 0.36 0.039 5 0.095 0.25 1.56 0.007 0.001 0.040 0.031 0.020 0.89 0.45 0.19 0.40 0.030 6 0.088 0.30 1.51 0.006 0.001 0.056 0.045 0.019 0.84 0.49 0.24 0.34 0.026 7 0.099 0.25 1.50 0.009 0.002 0.050 0.049 0.020 0.86 0.50 0.20 0.44 0.040

[0061] Table 2 Main process parameters for continuous casting billet smelting in Examples 1-7 of the present invention

[0062]

[0063] Table 3 Chemical composition and slag film thickness of crystallizer protective slag in Examples 1-7 of the present invention

[0064]

[0065]

[0066] Table 4 Liquid protective casting and casting process parameters of Examples 1-7 of the present invention

[0067]

[0068] Table 5. Low-magnification evaluation grades and gaseous element content of continuously cast billets in Examples 1-7 of the present invention.

[0069]

[0070]

[0071] Table 6 Heating process parameters for continuous casting billets in Examples 1-7 of the present invention

[0072]

[0073] Table 7 Steel plate rolling process parameters of Examples 1-7 of the present invention

[0074]

[0075]

[0076] Table 8. Steel plate pre-straightening process parameters in Examples 1-7 of the present invention.

[0077] Example Tilt correction / mm Roll gap correction / mm Load reduction / mm 1 0.9 -1.4 0.6 2 1.0 -1.0 0.7 3 1.1 -0.6 0.9 4 1.2 0.1 0.8 5 1.3 0.2 0.6 6 1.4 0.4 0.9 7 1.0 -1.1 0.7

[0078] Table 9. ACC laminar flow cooling process parameters for steel plates in Examples 1-7 of the present invention.

[0079]

[0080]

[0081] Table 10. Process parameters for steel quenching-carbon distribution-tempering heat treatment in embodiments of the present invention.

[0082]

[0083] Table 11 Comprehensive mechanical properties of steel plates in delivery state of Examples 1-7 of the present invention

[0084]

[0085]

[0086] The entire steel plate was subjected to ultrasonic testing according to NB / T 47013.3. The scanning method was as follows: the probe was used to scan parallel lines perpendicular to and parallel to the rolling direction of the steel plate, with a spacing of 190-200 mm. 100% scanning was performed within a 40-50 mm range on both sides of the predetermined bevel line of the steel plate. Defect assessment was based on T1 grade as specified in NB / T 47013.3, with a pass rate of 100%. The flaw detection performance test results are shown in Table 14.

[0087] Table 12 Results of flaw detection performance tests on steel plates in delivery condition in Examples 1-7 of the present invention

[0088]

[0089]

[0090] Table 13 Non-metallic inclusions in steel plates delivered in Examples 1-7 of the present invention

[0091]

[0092] Simulated post-weld heat treatment tests were conducted on the samples from Examples 1-7. The simulated post-weld heat treatment regime was as follows: temperature 630–650℃, net holding time 40–240 min, heating rate 2–8℃ / min above 400℃ (no heating rate control required before reaching 400℃), cooling rate 2–10℃ / min below 400℃, and natural cooling to room temperature after reaching 400℃. The simulated post-weld heat treatment process parameters are shown in Table 14, and the mechanical properties of the steel plates in the simulated post-weld heat-treated state are shown in Table 15.

[0093] Table 14 Main process parameters for post-weld heat treatment of simulated steel in Examples 1-7 of the present invention

[0094]

[0095]

[0096] Table 15 Comprehensive mechanical properties of steel plates in the mold-welded state in Examples 1-7 of the present invention

[0097]

[0098] like Figure 1 As shown, the steel plate has a three-phase microstructure of 20-40% martensite, 30-40% retained austenite and 30-40% carbides, with the carbides dispersed and the microstructure uniform.

[0099] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A pressure vessel steel plate for extra-large crude oil storage tanks with a capacity of 160,000-200,000 cubic meters and a pressure rating of 720MPa, characterized in that... The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.070%~0.100%, Si: 0.20%~0.30%, Mn: 1.45%~1.60%, P≤0.012%, S≤0.002%, Nb: 0.030%~0.050%, Ti: 0.015%~0.025%, V: 0.040%~0.060%, Ni: 0.80%~0.90%, Cr: 0.40%~0.55%, Mo: 0.15%~0.25%, Cu: 0.30%~0.45%, Alt: 0.020~0.050%, with the remainder being Fe and unavoidable impurities; The method for manufacturing the steel plate includes the following steps: (1) Continuous casting billet smelting: The RH inlet temperature is 1590~1720℃, and the RH oxygen blowing rate is 5~25m³. 3 / h, circulating oxygen blowing time 30~40min, and at the same time, a two-step aluminum addition method is adopted for deoxidation, namely, adding aluminum cake for pre-deoxidation during the converter tapping process + adding aluminum wire for deoxidation during the LF furnace refining process, continuous casting with full protection pouring, and continuous casting billet thickness of 250~300mm. (2) Controlled rolling and controlled cooling: After heating, the continuously cast billet undergoes rough rolling at an initial temperature of 1100-1130℃ and a final temperature of 980-1020℃. Following rough rolling, the intermediate billet is air-cooled via a reciprocating oscillating roller table until it reaches a temperature of 905-935℃, then undergoes continuous multi-pass finishing rolling at a final temperature of 835-875℃. After finishing rolling, pre-straightening is performed, followed by ACC laminar flow cooling at an initial cooling temperature of 810-860℃ to preserve a significant amount of stored energy in the austenite grains. The cooling rate is 15-25℃ / s. Water cooling reaches a red-hot temperature of 650-720℃. The number of ACC manifolds is 10-12, the roll speed is 1.2-1.4 m / s, and the water flow rate is 270-290 m³ / s. 3 / h, water temperature 24~30℃, and at the same time, the head and tail of the pre-straightened steel plate are shielded during ACC laminar flow cooling, so that the temperature difference between the head and tail of the steel plate is less than 20℃. After ACC laminar flow cooling, the flatness of the steel plate is ≤8mm / m. (3) Quenching-Carbon Distribution-Tempering Heat Treatment: The quenching temperature is 850~870℃, the heating rate is 0.5~1.5min / mm, the net holding time is 20~80min, after the holding time is completed, the water is cooled to 100~120℃, the chamber furnace is held for 10~20min, the tempering temperature is 420~440℃, the heating rate is 1.5~2.5min / mm, the net holding time is 30~130min, after the holding time is completed, the air is cooled to room temperature. (4) Straightening: The flatness of the finished steel plate after straightening is ≤5mm / m.

2. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 1, characterized in that, The thickness of the finished steel plate is 10~50mm.

3. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 2, characterized in that, For finished steel plates with a thickness of 10mm or less and a thickness of less than 30mm, the tensile strength at 1 / 4 of the thickness in the transverse direction of the finished steel plate in the delivery state is 860~890MPa, the yield strength is 740~770MPa, the elongation after fracture is 29%~32%, the yield ratio is 0.85~0.87, the impact energy at -70℃ is 160~230J, the lateral expansion value LE is 1.1~1.9mm, the welding heat input energy is 140-150KJ / cm, the impact energy at -50°C in the heat-affected zone after welding is 120~190J, and the lateral expansion value LE after welding is 1.0~2.0mm. For finished steel plates with a thickness of 30mm or less and 50mm or less, the tensile strength in the transverse direction at 1 / 4 and 1 / 2 of the thickness in the delivery state of the finished steel plate is 840~890MPa, the yield strength is 720~760MPa, the elongation after fracture is 30%~33%, the yield ratio is 0.85~0.87, the impact energy at -70℃ is 160~250J, the lateral expansion value LE is 1.2~1.5mm, the welding heat input energy is 140~150KJ / cm, the impact energy at -50°C in the heat-affected zone after welding is 110~200J, and the lateral expansion value LE after welding is 1.3~1.7mm.

4. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 2, characterized in that, For finished steel plates with a thickness of 10mm or less and a thickness of less than 30mm, the tensile strength at 1 / 4 of the thickness of the finished steel plate in the simulated post-weld heat-treated state is 850~880MPa, the yield strength is 730~760MPa, the elongation after fracture is 29%~32%, the yield ratio is 0.85~0.87, the impact energy at -70℃ is 150~250J, and the lateral expansion value LE is 1.0~2.0mm. The thickness of the finished steel plate is 30mm≤50mm. The tensile strength of the finished steel plate at 1 / 4 and 1 / 2 of the thickness in the transverse direction at simulated post-weld heat treatment state is 850~880MPa, the yield strength is 730~760MPa, the elongation after fracture is 24%~33%, the yield ratio is 0.85~0.86, the impact energy at -70℃ is 190~270J, and the lateral expansion value LE is 1.3~1.7mm.

5. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 1, characterized in that, In step (1), during converter smelting, the furnace charge size is 50~100mm, the furnace charge hot charging temperature is 1150~1200℃, and the net heat preservation time is 3~6min; The amount of aluminum ingot added is 1~5 kg / ton of steel, and the amount of aluminum wire added is 1~5 kg / ton of steel.

6. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 1, characterized in that, In step (1), the pouring temperature is 1530~1550℃ and the casting speed is 0.9~1.0 m·min. -1 The steel billet is placed in a slow cooling pit after it leaves the production line. The slow cooling temperature is 300~400℃ and the holding time is 12~24h. The mold flux is composed of the following components by weight percentage: CaO 35%~50%, Al2O3 30%~40%, MnO 0~10%, SiO2 5%~10%, with the balance being unavoidable impurities. The thickness of the flux film is 0.1~1.5mm.

7. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 1, characterized in that, In step (2), a four-stage heating method is used: stage I heating temperature is 600~650℃, stage II heating temperature is 1000~1050℃, stage III heating temperature is 1170~1210℃, and the soaking temperature is 1150~1190℃. The soaking time is 30~40min, and the total heating time is H×(0.9~1.2)min·mm. -1 H is the thickness of the continuously cast billet, in mm; The reduction rate for each pass of rough rolling is 10% to 15%, and the thickness of the intermediate billet is 3 to 4 times the thickness of the finished steel plate. The reduction rate for each pass of finish rolling is 8% to 13%, until the final finished product thickness is reached.

8. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 1, characterized in that, In step (2), during the pre-straightening process, the roll gap setting parameters of the pre-straightening machine system are: tilt correction range of 0~+1.5mm, roll gap correction range of -1.5~+0.5mm, and load reduction range of +0.5~+1.0mm.

9. The pressure vessel steel plate for 720MPa grade 160,000-200,000 cubic meter extra-large crude oil storage tanks according to claim 1, characterized in that, In step (3), the straightening speed is 150~200 r / min.

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