Steel plate for 4500 cubic liquid carbon dioxide sphere and method for manufacturing the same

Abstract

This invention relates to the field of steel technology, and more particularly to a steel plate for a 4500 cubic meter liquid carbon dioxide spherical tank and its manufacturing method. The steel plate composition by weight percentage is as follows: C: 0.08%~0.10%, Si: 0.15%~0.25%, Mn: 1.50%~1.60%, P≤0.012%, S≤0.003%, Ni: 0.50%~0.60%, Mo: 0.15%~0.25%, Nb: 0.040%~0.050%, V: 0.05%~0.06%, Cr: 0.20%~0.30%, Alt: 0.020%~0.040%, with the remainder being Fe and unavoidable impurities. This invention employs a low Pcm value and low carbon equivalent chemical composition design, along with online controlled rolling and cooling (TMCP), multi-stage straightening, and QLT treatment processes. These techniques result in products with excellent full-thickness strength and toughness matching, plate flatness, weldability, and mechanical properties, making them widely applicable in the manufacture of extra-large 4500 cubic meter cryogenic liquid carbon dioxide spherical tanks.

Description

Technical Field

[0001] This invention relates to the field of steel technology, and more particularly to a steel plate for a 4500 cubic meter liquid carbon dioxide spherical tank and its manufacturing method. Background Technology

[0002] With increasing global demands for greenhouse gas emission reduction, the demand for large-scale liquid carbon dioxide storage has increased significantly. As a crucial pressure vessel for storing liquefied gases, the increasing size of spherical tanks is an effective way to reduce unit storage costs, improve land utilization, and enhance operational economics. Currently, the mainstream volume of liquid carbon dioxide spherical tanks in China is 1500 cubic meters and below. The design and manufacture of larger-volume spherical tanks face numerous technical challenges, including tank wall thickness, low-temperature impact toughness, weldability, and safe service life.

[0003] Liquid carbon dioxide is typically stored at temperatures ranging from -20°C to -50°C, corresponding to relatively low saturated vapor pressures. However, this places specific demands on the low-temperature toughness and resistance to brittle fracture of the steel plates. When the volume of the spherical tank is increased from 1500 cubic meters to 4500 cubic meters, the hydrostatic and gas pressure loads on the shell plates increase exponentially. If conventional strength-grade steel plates for cryogenic pressure vessels are used, the plate thickness must be significantly increased to meet stress verification requirements. However, excessive wall thickness leads to excessive overall tank weight, a sharp increase in construction costs, and a series of engineering problems, including coarsening of the microstructure in the heat-affected zone of welded joints, increased residual stress, deterioration of toughness, and difficulties in implementing heat treatment processes.

[0004] Therefore, developing specialized steel plates that combine excellent low-temperature toughness, good weldability, and adaptability to large thicknesses is key to overcoming the bottleneck in the construction of 4,500 cubic meter ultra-large liquid carbon dioxide spherical tanks. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned problems and shortcomings, and to provide a steel plate for a 4500 cubic meter liquid carbon dioxide spherical tank and its manufacturing method. This invention employs a low Pcm value, low carbon equivalent chemical composition design, online controlled rolling and cooling (TMCP), multi-stage straightening, and QLT treatment processes, resulting in a product with excellent full-thickness strength and toughness matching, plate flatness, weldability, and mechanical properties. It is widely used in the manufacture of 4500 cubic meter extra-large cryogenic liquid carbon dioxide spherical tanks.

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

[0007] This invention provides a steel plate for a 4500 cubic meter liquid carbon dioxide spherical tank. The composition of the steel plate, by weight percentage, is as follows: C: 0.08%~0.10%, Si: 0.15%~0.25%, Mn: 1.50%~1.60%, P≤0.012%, S≤0.003%, Ni: 0.50%~0.60%, Mo: 0.15%~0.25%, Nb: 0.040%~0.050%, V: 0.05%. %~0.06%, Cr: 0.20%~0.30%, Alt: 0.020%~0.040%, with the remainder being Fe and unavoidable impurities; the Pcm (%) of the steel plate = C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.23; the carbon equivalent CEV = C+Mn / 6+Cr / 5+Mo / 5+V / 5+Ni / 15+Cu / 15≤0.50.

[0008] In the above technical solution, further, by volume percentage, the microstructure of the finished steel plate in the delivery state consists of 60%~80% tempered bainite and 20%~40% ferrite, with a grain size of 9~10 throughout the thickness direction.

[0009] In the above technical solution, the tensile strength of the finished steel plate in the delivery state is 700~830MPa, the yield strength is 590~710MPa, the elongation after fracture is 19%~24%, the impact strength at -50℃ at the thickness T / 4 and T / 2 of the steel plate is 150~250J, and the surface and thickness T / 2 of the drop hammer test at NDT-50℃ did not crack. The tensile strength of the finished steel plate in the simulated post-weld heat-treated state is 700~830MPa, the yield strength is 590~710MPa, the elongation after fracture is 19%~24%, the impact strength at -50℃ at the thickness T / 4 and T / 2 of the steel plate is 150~250J, and the drop hammer test at NDT-50℃ at the surface and thickness T / 2 of the steel plate did not crack.

[0010] Furthermore, in the above technical solution, the unevenness of the entire width direction of the finished steel plate in the delivery state meets the requirement of ≤6 / 2000mm.

[0011] In the above technical solution, the thickness of the finished steel plate is further 40~60mm.

[0012] Another aspect of the present invention provides a method for manufacturing the steel plate for the above-mentioned 4500 cubic meter liquid carbon dioxide spherical tank, comprising the following steps: (1) Online controlled rolling and cooling: After the continuous casting billet is heated, it is rolled. The roughing rolling start temperature is 1020~1050℃, the roughing rolling finish temperature is 950~980℃, the finishing rolling start temperature is 840~865℃, the finishing rolling finish temperature is 800~820℃, the rolling speed is 4.5~5.5m / s, after the finishing rolling is completed, the steel is thrown at a speed of 4~5m / s, and then ACC laminar flow cooling is carried out. ACC laminar flow cooling is carried out in two stages. The first stage starts at a cooling temperature of 760~780℃, the cooling rate is 10~15℃ / s, and the reddening temperature is 600~650℃. The second stage slowly cools to below 450℃ at 5~8℃ / s, and the reddening temperature is 405~435℃.

[0013] (2) Multi-stage straightening: 2~3 passes of hot straightening + 1~2 passes of cold straightening are adopted. The opening temperature of hot straightening is 400~420℃ and the straightening speed is 0.1~0.15m / s. The opening temperature of cold straightening is 300~320℃ and the straightening speed is 0.06~0.1m / s. After cold straightening, the material is cooled on a cooling bed with a temperature ≤270℃. (3) QLT heat treatment: First quenching temperature 900~920℃, net holding 30~60min, second quenching temperature 760~780℃, net holding 40~70min, tempering temperature 620~640℃, net holding 60~80min, heating rate 1.54~2.62℃ / min, after holding, air cool to room temperature.

[0014] In the above technical solution, the rough rolling ensures a reduction rate of 15% to 25% per pass, and the intermediate billet thickness is 2 to 3 times the thickness of the finished steel plate; the finish rolling has a reduction rate of 5% to 15% per pass, up to the final finished product thickness.

[0015] In the above technical solution, furthermore, in the first stage of ACC laminar flow cooling, there are 14 to 16 water groups, a roller speed of 0.8 to 1.0 m / s, and a water flow rate of 570 to 610 m³ / s. 3 / h, water flow rate 1580~1620m³ 3 / h.

[0016] In the above technical solution, furthermore, in the second stage of ACC laminar flow cooling, there are 14 to 16 water groups, a roller speed of 0.8 to 1.0 m / s, and a water flow rate of 470 to 510 m³ / s. 3 / h, water flow rate 1280~1320m³ 3 / h, with the side spray device working throughout. The parameters of the side spray device are: pressure 1.2~2.0MPa, flow rate 30~40m / s.

[0017] In the above technical solution, furthermore, in the ACC laminar flow cooling process, the temperature difference between the head and tail of the same plate is 20~30℃.

[0018] In the above technical solution, further, in the hot straightening process: the final reduction amount of the hot straightening is adjusted according to the type of plate defect. When the plate defect type is a central wave, the final reduction amount of the hot straightening is 0.5~1.5mm. When the plate defect type is an edge wave, the final reduction amount of the hot straightening is 0.2~0.8mm. Before straightening, the cooling water on the lower surface of the previous straightening machine is turned off, and the cooling water volume of the subsequent machine is reduced by 30%~50%.

[0019] The beneficial effects of this invention are as follows: 1. This invention ensures strength and toughness by controlling C (0.08%~0.10%), Mn (1.50%~1.60%), etc., limits Pcm, CEV, etc. to ensure weldability, strengthens with microalloying such as Nb, V, Cr, Ni, Mo, etc., and improves tear resistance.

[0020] 2. This invention promotes recrystallization and refinement through TMCP finishing rolling, promotes bainite nucleation through final rolling energy storage, initiates ACC cooling through cooling, controls microstructure stability through segmented controlled cooling, and ensures uniform performance of the steel plate throughout its thickness by precisely controlling the cooling rate and rolling speed, including the amount of water supplied to the top and bottom of the plate.

[0021] 3. This invention uses 2-3 passes of hot straightening + 1-2 passes of slow parallel multi-stage straightening with dynamic pressure adjustment, combined with coupling modification, to control the cooling water volume of the straightening machine, ensuring the uniformity of temperature on the upper and lower surfaces of the steel plate, and ensuring that the unevenness is ≤6 / 2000mm.

[0022] 4. This invention employs QLT heat treatment (quenching + two-phase sub-temperature quenching + tempering). Through two-phase sub-temperature quenching, reverse transformation austenite is generated, refining the effective grain size, increasing the number of large-angle grain boundaries, effectively preventing crack propagation, and improving low-temperature toughness. After QLT treatment, a uniform mixed structure of bainite and ferrite is formed, reducing the yield strength ratio and improving the strength-toughness match.

[0023] 5. This invention utilizes optimized steel plate composition design, online controlled rolling and cooling, multi-stage straightening, and QLT treatment to refine the grain size of the steel plate across its entire thickness, improving uniformity and significantly enhancing weldability. This allows for the production of pressure vessel steel plates for liquid carbon dioxide spherical tanks with a maximum thickness of 60mm, overcoming the bottleneck of inconsistent low-temperature impact performance in the core after traditional die welding. Simultaneously, the product exhibits excellent strength and toughness matching across its entire thickness, good plate straightness, and simulated post-weld heat treatment performance. The finished steel plates, in both the delivered state and simulated post-weld heat-treated state, exhibit tensile strength of 700-830MPa, yield strength of 590-710MPa, elongation after fracture of 19-24%, and impact energy of 150-250J at -50℃ at thicknesses T / 4 and T / 2. No cracking was observed in the NDT-50℃ drop hammer test at the surface and thickness T / 2, resulting in a significant improvement in production efficiency. The flatness of the steel plate in the width direction of the entire plate in the delivery state meets ≤6 / 2000mm, the straightening efficiency is improved by 40%, and the grain size in the thickness direction of the steel plate in the delivery state meets grade 9~10. Attached Figure Description

[0024] Figure 1 The image shows a metallographic photograph (100X) of the steel plate of Embodiment 1 of the present invention. Detailed Implementation

[0025] 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.

[0026] This invention provides a steel plate for a 4500 cubic meter liquid carbon dioxide spherical tank, the composition of which, by weight percentage, is as follows: C: 0.08%~0.10%, Si: 0.15%~0.25%, Mn: 1.50%~1.60%, P≤0.012%, S≤0.003%, Ni: 0.50%~0.60%, Mo: 0.15%~0.25%, Nb: 0.040%~0.050%, V: 0.05%. ~0.06%, Cr: 0.20%~0.30%, Alt: 0.020%~0.040%, the remainder being Fe and unavoidable impurities; Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.23; Carbon equivalent CEV=C+Mn / 6+Cr / 5+Mo / 5+V / 5+Ni / 15+Cu / 15≤0.50.

[0027] The reasons for using the above-mentioned components are as follows: (1) C: C is the main component element of steel. The strength of steel mainly depends on the C content in the steel. Too high a C content will lead to poor toughness, plasticity and weldability of steel; too low a 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 0.08%~0.10%.

[0028] (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 this invention is controlled at 0.15%~0.25%.

[0029] (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 not only increase the production cost, but also Mn will easily combine with S to form MnS, which will reduce the material's resistance to hydrogen-induced cracking. At the same time, too high Mn content will reduce the activity of carbon. Therefore, the Mn content in the steel of this invention is controlled at 1.50%~1.60%.

[0030] (4) P: Phosphorus is a harmful element in steel, which increases the cold brittleness of steel, worsens the weldability, reduces plasticity, and worsens the cold bending performance. In addition, P is also particularly sensitive to radiation embrittlement. Therefore, the lower the P content in steel, the better. In this invention, it is controlled to be below 0.012%.

[0031] (5) S: Sulfur is a harmful element under normal circumstances. S usually forms brittle sulfides with alloying elements in steel, causing hot brittleness and reducing the ductility and toughness of 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 0.003%.

[0032] (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. Therefore, the V content in the steel of this invention is controlled at 0.050%~0.060%.

[0033] (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 this invention is controlled at 0.04%~0.05%.

[0034] (8) Cr: Adding an appropriate amount of Cr mainly enhances the strength and hardness of the steel through solid solution strengthening. In addition, Cr can improve the hardenability of the steel, optimize the microstructure after heat treatment, and form a dense oxide film, thereby improving the steel's oxidation resistance and corrosion resistance. However, the Cr content must be strictly controlled. Excessive Cr will reduce the plasticity and toughness of the steel, increase the tendency for temper brittleness, and affect the welding performance. Therefore, the Cr content in the steel of this invention is controlled at 0.20%~0.30%.

[0035] (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, the disadvantage of Al is that it affects the hot working performance, weldability and machinability of the steel. Therefore, the Alt content in the steel of this invention is controlled at 0.020%~0.040%.

[0036] (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 should be controlled at 0.50%~0.60%.

[0037] (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 bainite transformation curve to shift to the right, resulting in a finer bainite structure after phase transformation; at the same time, as a strong carbide forming element, Mo can refine grains, suppress temper brittleness, improve the tempering stability of steel, and ensure that it maintains excellent mechanical properties after high-temperature tempering. Therefore, the Mo content in the steel of this invention is controlled at 0.15%~0.25%.

[0038] (12) Pcm: Pcm (Weld Crack Sensitivity Index) is a value calculated by analyzing the composition of the smelting material and using a formula. It is used to measure the crack sensitivity of steel during the welding process. When the Pcm value is small, the weldability of the steel is good and the risk of cracking during welding is low. The requirement of Pcm≤0.23 can ensure that it has good weldability during the welding process, thereby meeting the high requirements of engineering structural components for welding quality. Therefore, this invention requires that Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.23 in steel.

[0039] (13) Carbon Equivalent (CEV): Carbon equivalent is the equivalent carbon content of alloying elements, including carbon, in steel that affect hardening, cold cracking, and embrittlement. This invention relates to a steel plate for a 4500 cubic meter pressure vessel, which has high requirements for weldability. Therefore, its carbon equivalent (CEV) is specified to be <0.50 to ensure that the steel is not prone to cracking during welding and to improve welding quality.

[0040] The manufacturing method of the steel plate for the aforementioned 4500 cubic meter liquid carbon dioxide spherical tank includes the following steps: (1) Online controlled rolling and cooling: After heating, the continuously cast billet is rolled on a two-stand mill. First, the billet is rough rolled at a temperature of 1020~1050℃ and a final rolling temperature of 950~980℃, ensuring a reduction rate of 15%~25% per pass. The thickness of the intermediate billet is 2~3 times the thickness of the finished steel plate. The finishing rolling temperature is 840~865℃, ensuring a reduction rate of 5%~15% per pass, until the final finished thickness. The finishing rolling temperature is 800~820℃. The rolling speed is 4.5~5.5m / s throughout the rolling process. After finishing rolling, the steel is thrown at a speed of 4~5m / s to reduce the temperature drop of the steel plate. Then, ACC laminar flow cooling is carried out. ACC laminar flow cooling is carried out in two stages. The first stage starts at a cooling temperature of 760~780℃, with 14~16 water groups, a roll speed of 0.8~1.0m / s, and a water flow of 570~610m³. 3 / h, water flow rate 1580~1620m³ 3 The cooling rate is 10~15℃ / s, and the reddening temperature is 600~650℃. In the second stage, the temperature is slowly cooled to below 450℃ at a rate of 5~8℃ / s. This strategy can reduce the temperature difference between the surface and the core, reduce the thermal stress gradient, and avoid martensitic phase transformation embrittlement. The reddening temperature is 405~435℃. In the second stage, there are 14~16 water groups, the roller speed is 0.8~1.0m / s, and the water application rate is 470~510m³. 3 / h, water flow rate 1280~1320m³ 3 / h, with the side spray device (pressure 1.2~2.0MPa, flow rate 30~40m / s) throughout the process to remove residual water on the surface, reduce uneven secondary cooling, and ensure a temperature difference of 20~30℃ between the head and tail of the same plate throughout the entire cooling process; (2) Multi-stage straightening: 2~3 passes of hot straightening + 1~2 passes of cold straightening at slow speed. The hot straightening speed is controlled at 0.1~0.15m / s, the hot straightening start temperature is 400~420℃, and the cold straightening speed is controlled at 0.06~0.1m / s to avoid plate shape fluctuation caused by high speed. The cold straightening start temperature is 300~320℃. After cold straightening, the plate is cooled on a cooling bed with a temperature ≤270℃. By combining with slow straightening, the metal deformation rate is controlled, and the risk of work hardening is reduced. The reduction amount in the final pass of hot straightening is dynamically adjusted based on the type of plate defect. When the plate defect type is a middle wave, the reduction amount is 0.5~1.5mm (the wave amplitude is detected in real time by a laser thickness gauge, and the algorithm automatically calculates the optimal value). When the plate defect type is an edge wave, the reduction amount is 0.2~0.8mm to avoid over-straightening. At the same time, before hot straightening, the cooling water on the lower surface of the previous straightening machine is turned off, and the amount of cooling water in the subsequent pass is reduced by 30%~50% to improve temperature uniformity. (3) QLT heat treatment: First quenching temperature 900~920℃, net holding 30~60min, second quenching temperature 760~780℃, net holding 40~70min, tempering temperature 620~640℃, net holding 60~80min, heating rate 1.54~2.62℃ / min, after holding, air cool to room temperature.

[0041] After the above treatment, a microstructure of 60%~80% bainitic tempered structure + 20%~40% ferrite can be obtained, and the steel plate has a good strength and toughness match, and the mechanical properties of the steel plate are well uniform at both ends.

[0042] Examples 1-8 The parameters of embodiments 1-8 of the present invention are shown in Tables 1-8.

[0043] Table 1. Composition (wt%) of steel plates in embodiments and comparative examples of the present invention

[0044] Table 2. Main process parameters for controlled rolling of steel plates in the embodiments and comparative examples of the present invention.

[0045] Table 3. Main process parameters of the first stage controlled cooling heat treatment of steel plates in the embodiments and comparative examples of the present invention.

[0046] Table 4. Main process parameters of the second-stage controlled cooling heat treatment of steel plates in the embodiments and comparative examples of the present invention.

[0047] Table 5. Main process parameters for multi-stage straightening of steel plates in embodiments and comparative examples of the present invention.

[0048] Table 6. Main process parameters for QLT heat treatment of steel plates in the embodiments and comparative examples of the present invention.

[0049] Table 7. Comprehensive mechanical and technological properties of steel plates in the delivery state of the embodiments and comparative examples of the present invention.

[0050] Table 8. Comprehensive mechanical properties of steel plates in simulated post-weld heat-treated state according to embodiments and comparative examples of the present invention.

[0051] like Figure 1 As shown, the microstructure of the steel plate consists of 60%~80% tempered bainite and 20%~40% ferrite.

[0052] 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 steel plate for a 4500 cubic meter liquid carbon dioxide spherical tank, characterized in that, The steel plate has the following composition by weight percentage: C: 0.08%~0.10%, Si: 0.15%~0.25%, Mn: 1.50%~1.60%, P≤0.012%, S≤0.003%, Ni: 0.50%~0.60%, Mo: 0.15%~0.25%, Nb: 0.040%~0.050%, V: 0.05%~0.06%, Cr: 0 0.20%~0.30%, Alt: 0.020%~0.040%, the remainder being Fe and unavoidable impurities; Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.23; Carbon equivalent CEV=C+Mn / 6+Cr / 5+Mo / 5+V / 5+Ni / 15+Cu / 15≤0.

50.

2. The steel plate for the 4500 cubic meter liquid carbon dioxide spherical tank according to claim 1, characterized in that, By volume percentage, the microstructure of finished steel plates in the delivery state consists of 60%~80% tempered bainite and 20%~40% ferrite, with a grain size of 9~10 throughout the thickness direction.

3. The steel plate for the 4500 cubic meter liquid carbon dioxide spherical tank according to claim 1, characterized in that, The tensile strength of the finished steel plate in the delivery state is 700~830MPa, the yield strength is 590~710MPa, the elongation after fracture is 19%~24%, the impact strength at -50℃ at the thickness T / 4 and T / 2 of the steel plate is 150~250J, and the surface and thickness T / 2 of the drop hammer test at NDT-50℃ did not crack. The tensile strength of the finished steel plate in the simulated post-weld heat-treated state is 700~830MPa, the yield strength is 590~710MPa, the elongation after fracture is 19%~24%, the impact strength at -50℃ at the thickness T / 4 and T / 2 of the steel plate is 150~250J, and the drop hammer test at NDT-50℃ at the surface and thickness T / 2 of the steel plate did not crack.

4. The steel plate for the 4500 cubic meter liquid carbon dioxide spherical tank according to claim 1, characterized in that, The flatness of the finished steel plate in the width direction of the entire plate must be ≤6 / 2000mm when delivered.

5. A method for manufacturing a steel plate for a 4500 cubic meter liquid carbon dioxide spherical tank according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Online controlled rolling and cooling: After the continuous casting billet is heated, it is rolled. The roughing rolling start temperature is 1020~1050℃, the roughing rolling finish temperature is 950~980℃, the finishing rolling start temperature is 840~865℃, the finishing rolling finish temperature is 800~820℃, the rolling speed is 4.5~5.5m / s, after the finishing rolling is completed, the steel is thrown at a speed of 4~5m / s, and then ACC laminar flow cooling is carried out. ACC laminar flow cooling is carried out in two stages. The first stage starts at a cooling temperature of 760~780℃, the cooling rate is 10~15℃ / s, and the reddening temperature is 600~650℃. The second stage slowly cools to below 450℃ at a speed of 5~8℃ / s, and the reddening temperature is 405~435℃. (2) Multi-stage straightening: 2~3 passes of hot straightening + 1~2 passes of cold straightening are adopted. The opening temperature of hot straightening is 400~420℃ and the straightening speed is 0.1~0.15m / s. The opening temperature of cold straightening is 300~320℃ and the straightening speed is 0.06~0.1m / s. After cold straightening, the material is cooled on a cooling bed with a temperature ≤270℃. (3) QLT heat treatment: First quenching temperature 900~920℃, net holding 30~60min, second quenching temperature 760~780℃, net holding 40~70min, tempering temperature 620~640℃, net holding 60~80min, heating rate 1.54~2.62℃ / min, after holding, air cool to room temperature.

6. The manufacturing method according to claim 5, characterized in that, Rough rolling ensures a reduction rate of 15% to 25% per pass, with the intermediate billet thickness being 2 to 3 times the finished steel plate thickness; finish rolling ensures a reduction rate of 5% to 15% per pass, up to the final finished product thickness.

7. The manufacturing method according to claim 5, characterized in that, In the first stage of ACC laminar flow cooling, there are 14-16 water groups, a roller speed of 0.8-1.0 m / s, and a water flow rate of 570-610 m³ / s. 3 / h, water flow rate 1580~1620m³ 3 / h.

8. The manufacturing method according to claim 5, characterized in that, In the second stage of ACC laminar flow cooling, there are 14 to 16 water groups, a roller speed of 0.8 to 1.0 m / s, and a water flow rate of 470 to 510 m³ / s. 3 / h, water flow rate 1280~1320m³ 3 / h, with the side spray device working throughout. The parameters of the side spray device are: pressure 1.2~2.0MPa, flow rate 30~40m / s.

9. The manufacturing method according to claim 5, characterized in that, In the ACC laminar flow cooling process, the temperature difference between the head and tail of the same plate is 20~30℃.

10. The manufacturing method according to claim 5, characterized in that, During the hot straightening process: the reduction amount of the final hot straightening pass is adjusted according to the type of plate defect. When the plate defect type is a central wave, the reduction amount of the final hot straightening pass is 0.5~1.5mm. When the plate defect type is an edge wave, the reduction amount of the final hot straightening pass is 0.2~0.8mm. Before straightening, the cooling water on the lower surface of the previous straightening machine is turned off, and the cooling water volume of the subsequent straightening machine is reduced by 30%~50%.

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