High-strength super-thick steel plate for building structure and method for producing the same

Abstract

This application discloses a high-strength extra-thick steel plate for building structures and its production method. The steel plate contains: C 0.055~0.085%, Si 0.10~0.25%, Mn 1.5~1.7%, Cr 0.4~0.6%, Ni 0.3~0.5%, Mo 0.4~0.6%, Cu 0.25~0.35%, Nb 0.03~0.04%, V 0.03~0.04%, Ti 0.01~0.02%, Al 0.02~0.05%, B 0.0010~0.0016%, S≤0.002%, P≤0.012%, N≤0.0045%, H≤0.00015%; the thickness of the steel plate is 80~120mm; the yield strength at room temperature is 740~800MPa, the tensile strength is 930~1000MPa, and the difference in tensile strength between the surface layer and the core is ≤45MPa.

Description

Technical Field

[0001] This application belongs to the field of steel material preparation technology, and relates to a high-strength extra-thick steel plate for building structures and its production method. Background Technology

[0002] As building structures continue to develop towards super-high, large-span, and complex stress-bearing directions, due to the overall structural safety and stability requirements, core load-bearing components such as the tubes of super-high-rise buildings, nodes of large-span stadiums, heavy industrial columns and beams, and key stress-bearing parts of large transportation hubs generally adopt thick steel plates.

[0003] The application conditions of thick steel plates have extremely high performance requirements. They not only need to have high strength to withstand complex static and dynamic loads, but also need to have uniform and stable mechanical properties across the entire thickness of the steel plate.

[0004] Existing high-strength steel plates struggle to achieve a harmonious balance between large thickness and excellent overall performance. As the thickness of the steel plate increases, the performance differences between its thickness sections become significant. Summary of the Invention

[0005] The purpose of this application is to provide a high-strength, extra-thick steel plate for building structures and a method for producing the same.

[0006] To achieve the aforementioned objectives, one embodiment of this application provides a high-strength, extra-thick steel plate. The chemical composition of the steel plate, by mass percentage, includes: C 0.055~0.085%, Si 0.10~0.25%, Mn 1.5~1.7%, Cr 0.4~0.6%, Ni 0.3~0.5%, Mo 0.4~0.6%, Cu 0.25~0.35%, Nb 0.03~0.04%, V 0.03~0.04%, Ti 0.01~0.02%, Al 0.02~0.05%, B 0.0010~0.0016%, S≤0.002%, P≤0.012%, N≤0.0045%, H≤0.00015%, with the remainder being iron and unavoidable impurities, and satisfying Ni≥Cu;

[0007] The thickness d of the steel plate is 80~120mm; at room temperature, the yield strength of the steel plate is 740~800MPa, the tensile strength is 930~1000MPa, the yield strength ratio is ≤0.82, the elongation is ≥22%, and the difference in tensile strength between the surface layer and the core of the steel plate is ≤45MPa.

[0008] In one embodiment, the microstructure of the steel plate is a multiphase microstructure of ferrite + lath bainite + tempered martensite; wherein the volume percentage of ferrite is 10-25%, the volume percentage of lath bainite is 70-80%, and the volume percentage of tempered martensite is 5-10%.

[0009] In one embodiment, the average grain size of ferrite is ≤5μm, the average width of bainite laths is ≤0.5μm, and the average width of martensite laths is ≤0.2μm.

[0010] In one embodiment, the impact energy KV2 at -60℃ for both the surface layer and the core of the steel plate is ≥200J; the yield strength R of the steel plate at 600℃ is... p0.2 ≥495MPa; at a strain frequency of 1~3Hz and a strain amplitude of 0.8%, the steel plate can withstand at least 1600 cycles without cracking.

[0011] To achieve the above-mentioned application objectives, one embodiment of this application provides a method for producing high-strength extra-thick steel plates, the method comprising the sequentially performed processes of steelmaking, continuous casting, slow cooling by stacking, heating, controlled rolling, controlled cooling, quenching, and tempering. The heating process includes, in sequence, a first preheating section, a second preheating section, a first heating section, a second heating section, and a homogenizing section; the temperature of the first preheating section is ≤900℃, and the residence time is ≤50min; the temperature of the second preheating section is 900~1050℃, and the residence time is ≤50min; the temperature of the first heating section is 1000~1100℃, and the residence time is ≤50min; the temperature of the second heating section is 1150~1160℃, and the residence time is ≥80min; the temperature of the homogenizing section is 1130~1150℃, and the residence time is ≥100min. The controlled rolling process adopts a one-stage rolling process, with an initial rolling temperature of T. nr +35℃~T nr +65℃, final rolling temperature is T nr +5℃~T nr +25℃; In the controlled cooling process, the rolled steel plate is immersed in water for cooling at a rate of 1~4℃ / s, and the final cooling temperature is B. s +50℃~B s +80℃, then air-cooled to room temperature; The quenching process includes sequential heating, first-stage cooling, and second-stage cooling steps, with the heating temperature being A. C3 -65℃~A C3 -15℃, holding time is 1.8~2.0 min / mm; in the first stage cooling step, the cooling rate of the steel plate surface layer is ≥35℃ / s, the cooling rate of the steel plate core is ≥5℃ / s, and the final cooling temperature of the steel plate surface layer is B. s +5℃~B s +25℃; In the two-stage cooling process, the cooling rate of the steel plate surface is ≤15℃ / s, the cooling rate of the steel plate core is ≥10℃ / s, and the final cooling temperature of the steel plate surface is ≤300℃. Among them, the austenite recrystallization temperature Tnr =887+464C-357Si+(6445Nb-644 )+(732V-230 +890Ti +363Al; the starting temperature B for the bainite transformation of austenite. s =830-270C-90Mn-70Cr-37Ni-83Mo; Austenitization completion temperature A C3 =910-320C-14Ni-12Cu-10Mn+5Cr+14Mo+5V+18Si; The element symbols in the formula represent the mass percentage of the corresponding element in the steel plate.

[0012] In one embodiment, in the tempering process, the tempering heating temperature is 540~560℃, the furnace time is 0.8~1.0min / mm, and the furnace exit temperature of the steel plate is 490~510℃.

[0013] In one embodiment, the reduction per pass in the controlled rolling process is 28~45mm.

[0014] In one embodiment, the first-stage cooling in the quenching process uses water cooling, with the high-pressure water pressure being 0.8~1.0 MPa and the water flow rate being 8500~10000 m³ / h. 3 The water flow rate is 40-50° between the water flow and the steel plate surface; the second-stage cooling uses a laminar flow cooling water curtain with a water pressure of 0.2-0.4 MPa and a flow rate of 5000-6500 m³ / h. 3 / h, the angle between the water flow and the steel plate surface is 85~95°.

[0015] In one embodiment, the stacking time of the stacking slow cooling process is ≥48h.

[0016] In one embodiment, during the continuous casting process, electromagnetic stirring is performed in the crystallizer, the secondary cooling zone, and the solidification end, and the electromagnetic stirring frequency of the crystallizer is greater than that of the secondary cooling zone, which is greater than that of the solidification end, while the electromagnetic stirring current of the crystallizer is less than that of the solidification end and the electromagnetic stirring current of the secondary cooling zone is less than that of the secondary cooling zone.

[0017] In one embodiment, during the continuous casting process, the electromagnetic stirring frequency of the crystallizer is 5~8Hz and the electromagnetic stirring current is 200~400A; the electromagnetic stirring frequency of the secondary cooling zone is 3~6Hz and the electromagnetic stirring current is 400~800A; and the electromagnetic stirring frequency at the solidification end is 2~4Hz and the electromagnetic stirring current is 300~600A.

[0018] In one embodiment, during the continuous casting process, the superheat of the molten steel is 15~20°C.

[0019] In one embodiment, during the continuous casting process, the ladle is left to stand on the rotary table for 15-18 minutes before casting begins.

[0020] In one embodiment, the steelmaking process includes sequential steps of pre-desulfurization treatment, converter smelting, LF refining, and RH vacuum refining. In the RH vacuum refining step, the vacuum degree is ≤2mbar, the degassing time is 20~25min, and the net circulation processing time is 8~10min.

[0021] Compared with the prior art, the beneficial effects of one embodiment of this application are as follows: First, the chemical composition design of the steel plate is based on a low-carbon and low-nitrogen design, with titanium for nitrogen fixation and boron for increased hardenability. Furthermore, by adding Nb, V, Ti, Ni, Cr, Cu, and Mo, the strengthening effects of fine grain strengthening, precipitation strengthening, and solid solution strengthening are utilized to synergistically improve the strength, low-temperature toughness, weldability, core toughness, and hardenability of the thick steel plate, thereby improving the uniformity of the mechanical properties of the thick steel plate in the thickness direction. Secondly, in terms of production process, the use of low-temperature heating in the heating process and the control of the rolling process to perform one-stage recrystallization rolling in the low-temperature zone can effectively refine the original austenite grains. Furthermore, by controlling the cooling process, the steel plate can quickly pass through the pearlite and ferrite transformation zone, inhibiting the formation of coarse structures. The formed acicular ferrite structure further divides the original austenite structure. Afterwards, the quenching process controls the heating temperature in the two-phase region and combines it with staged cooling to obtain a uniform structure along the thickness direction, mainly composed of fine lath bainite. This allows the tensile strength difference between the surface layer and the core of the steel plate to be controlled within 40MPa, significantly improving the performance uniformity of the extra-thick steel plate in the thickness direction and greatly increasing the crack propagation resistance. This enables the steel plate to maintain high impact energy at -60℃ and withstand more than 1600 cycles of cyclic loading without fracture at a strain frequency of 1~3Hz and a strain amplitude of 0.8%, demonstrating excellent resistance to dynamic loads and fatigue performance. Attached Figure Description

[0022] Figure 1 This is a micrograph of the surface structure of the steel plate of Embodiment 1 of this application; Figure 2 This is a micrograph of the steel plate in Embodiment 1 of this application at 1 / 2 of the longitudinal section along the thickness direction; Figure 3 This is a micrograph of the surface structure of the steel plate in Embodiment 2 of this application; Figure 4 This is a micrograph of the steel plate at 1 / 2 of its thickness longitudinal section in Embodiment 2 of this application; Figure 5This is a micrograph of the surface structure of the steel plate in Embodiment 3 of this application; Figure 6 This is a micrograph of the steel plate at 1 / 2 of its thickness longitudinal section in Embodiment 3 of this application; Figure 7 This is a micrograph of the surface structure of the steel plate used in this application as a comparative example; Figure 8 This is a micrograph of the steel plate in the comparative example of this application, taken at 1 / 2 of the longitudinal section along the thickness direction. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them.

[0024] One embodiment of this application provides a steel plate, specifically a high-strength, extra-thick steel plate for building structures, to meet the usage requirements of core load-bearing components such as the tubes of super high-rise buildings, nodes of large-span stadiums, heavy industrial columns and beams, and key load-bearing parts of large transportation hubs.

[0025] The chemical composition of the steel plate, by mass percentage, includes: C 0.055~0.085%, Si 0.10~0.25%, Mn 1.5~1.7%, Cr 0.4~0.6%, Ni 0.3~0.5%, Mo 0.4~0.6%, Cu 0.25~0.35%, Nb 0.03~0.04%, V 0.03~0.04%, Ti 0.01~0.02%, Al 0.02~0.05%, B 0.0010~0.0016%, S≤0.002%, P≤0.012%, N≤0.0045%, H≤0.00015%, with the remainder being iron and unavoidable impurities, and satisfying Ni≥Cu.

[0026] The following is a detailed analysis and explanation of the main functions of each element and the selection of its dosage.

[0027] C: Carbon is a fundamental element determining the strength and microstructure of steel plates. This application adopts a low-carbon design with C ≤ 0.085%, which effectively reduces the carbon equivalent Ceq and the weld cold cracking sensitivity index Pcm, ensuring good weldability and heat-affected zone toughness. Furthermore, the low carbon content avoids the toughness deterioration caused by excessive carbide precipitation, which is beneficial for obtaining a bainite-dominated multiphase microstructure. In addition, trace amounts of carbon form fine carbonitrides with microalloying elements such as Nb, V, and Ti, contributing to strength through precipitation strengthening, and synergistically ensuring hardenability with elements such as Mn, Cr, and Mo, enabling uniform bainite microstructure to be obtained even in the core of 80~120mm thick steel plates. In this application, the carbon content is controlled at 0.055~0.085%.

[0028] Si: Silicon mainly acts as a deoxidizer and has a certain solid solution strengthening effect, which can slightly improve the strength of steel. However, excessive Si will deteriorate the surface quality. When the silicon content is high, it will also increase the grain boundary segregation of elements such as phosphorus and sulfur, reducing low-temperature toughness and weldability. In this application, the silicon content is controlled at 0.10~0.25%.

[0029] Manganese (Mn) is an important solid solution strengthening and hardenability element. It can significantly lower the austenite decomposition temperature, delay the transformation of pearlite and ferrite, and promote bainite formation. It is one of the key elements to ensure that the core of 80-120mm thick steel plates obtains a bainitic structure. Simultaneously, Mn can combine with sulfur to form MnS, mitigating the hot brittleness caused by sulfur. However, excessively high Mn content can exacerbate center segregation, reduce toughness, and increase carbon equivalent. In this application, the manganese content is controlled at 1.5-1.7%.

[0030] Cr: Chromium is one of the core elements for improving hardenability. It can significantly delay the transformation of pearlite and ferrite and promote bainite formation, which is crucial for obtaining a uniform bainitic microstructure in the core of 80-120mm thick steel plates. Simultaneously, Cr dissolves in ferrite at high temperatures and, in synergy with elements such as Mo and V, can effectively delay softening at 600℃, making it a major supporting element for ensuring refractory performance. Furthermore, Cr can form fine carbides, assisting in precipitation strengthening. However, Cr increases Pcm and Ceq; to balance weldability, the chromium content is controlled at 0.4-0.6% in this application.

[0031] Ni: Nickel is one of the effective elements for improving low-temperature toughness. It can significantly reduce the ductile-brittle transition temperature and is an important guarantee for steel plates to maintain an impact energy of not less than 200J at -60℃, especially contributing significantly to the core toughness of extra-thick steel plates. At the same time, Ni can enhance hardenability and, in synergy with Cr and Mo, help the core to obtain a bainitic structure. In terms of refractory properties, Ni is dissolved in ferrite, which can delay softening at 600℃, thus improving refractory properties. Furthermore, Ni, in combination with Cu, can inhibit surface enrichment cracking of Cu during heating, i.e., copper brittleness, ensuring the surface quality of Cu-containing steel plates. However, Ni is a precious metal element, and alloy costs must be considered. In this application, the nickel content is controlled at 0.3~0.5%.

[0032] Mo: Molybdenum is a key element with multiple functions in this application. First, Mo can significantly improve hardenability, strongly delay the transformation of pearlite and ferrite, and promote bainite formation, which is an important guarantee for obtaining a uniform bainitic microstructure across the entire cross-section of extra-thick steel plates. Second, Mo has strong solid solution strengthening and precipitation strengthening effects at high temperatures. In synergy with Cr, V, and Nb, it can effectively delay the softening of the microstructure and coarsening of carbides at 600℃, making it a core contributing element to refractory performance. In addition, Mo can refine the microstructure, improve tempering stability, and improve the toughness of the weld heat-affected zone. However, Mo is expensive and will increase Pcm; therefore, in this application, the molybdenum content is controlled at 0.4~0.6%.

[0033] Cu: In this application, copper primarily functions as precipitation strengthening and corrosion resistance. During tempering, Cu precipitates as ε-Cu nanoparticles, increasing the steel strength by 20-40 MPa with minimal impairment to toughness. Simultaneously, Cu promotes the formation of a dense rust layer on the steel surface, enhancing atmospheric corrosion resistance and benefiting the long-term service life of building structures. However, Cu tends to accumulate at grain boundaries during heating, leading to surface network cracking, i.e., copper brittleness. Therefore, it must be combined with Ni, and the content of both must satisfy Ni ≥ Cu. In this application, the copper content is controlled at 0.25-0.35%. Controlling the Cu content to 0.35% or below avoids adverse effects on the toughness of the weld heat-affected zone.

[0034] Niobium (Nb) is an important grain-refining element in steel. During hot rolling, niobium strongly inhibits austenite recrystallization and precipitation within austenite, pinning austenite grain boundaries and refining recrystallized grains. During cooling, dissolved niobium can continue to precipitate as niobium carbonitrides, significantly refining the microstructure after phase transformation and further improving the strength and toughness of the steel. By refining grains and forming fine, dispersed carbonitrides to pin dislocations, it can also effectively delay cyclic softening and extend fatigue life. However, coarse or undissolved NbC can become crack initiation sites, therefore, the Nb content must be strictly controlled. In this application, the niobium content is controlled at 0.03~0.04%.

[0035] Vanadium (V) works synergistically with niobium, primarily acting as precipitation strengthener and grain refiner. At high temperatures, V precipitates as VC and VN, effectively pinning austenite grain boundaries and inhibiting grain growth. It further precipitates during tempering, providing precipitation strengthening. V has a wide precipitation temperature range, maintaining fine dispersion even at 600℃, which is beneficial for refractory properties. Furthermore, V can combine with Ti and Nb to enhance the inhibition of grain growth in the weld heat-affected zone. However, excessive V content can easily form coarse primary precipitates, weakening the strengthening effect and adversely affecting toughness. In this application, the vanadium content is controlled at 0.03~0.04%.

[0036] Ti: Titanium is the key element for controlling nitrogen in this composition design. By strictly controlling the Ti and N content, Ti preferentially combines with N to form fine, high-temperature stable TiN particles. These TiN particles can effectively pin austenite grain boundaries and inhibit grain coarsening during heating, rolling, and welding, which is especially crucial for protecting the toughness of the weld heat-affected zone. Simultaneously, Ti can also form fine carbonitrides with C and N, assisting in precipitation strengthening. However, the Ti content should not be too high, otherwise coarse TiN or TiC will form, which will impair toughness. In this application, the titanium content is controlled at 0.01~0.02%.

[0037] Al: Aluminum is mainly used as a deoxidizer and can combine with nitrogen to form AlN, which helps refine grains. AlN can inhibit austenite grain growth during heating and works synergistically with TiN. Appropriate amounts of Al are beneficial for improving the purity of steel and enhancing low-temperature toughness. However, excessive Al content will form coarse alumina inclusions, reducing toughness. In this application, the aluminum content is controlled at 0.02~0.05%.

[0038] Boron (B) is a highly efficient trace element for improving hardenability. Even trace amounts of B can significantly delay the transformation from pearlite to ferrite and promote bainite formation, particularly noticeable in improving the hardenability of the core of extra-thick steel plates. In synergy with elements such as Mo, Cr, and Mn, B can significantly reduce the critical cooling rate required to obtain bainite, which is beneficial for achieving a uniform microstructure across the entire cross-section of 80-120mm thick steel plates. However, excessive B content can lead to the formation of borides, reducing toughness. Furthermore, the hardenability effect of B is closely related to the nitrogen content in the steel; therefore, it needs to be combined with Ti for nitrogen fixation to ensure the effectiveness of B. In this application, the boron content is controlled at 0.0010-0.0016%.

[0039] S, P, N, and H are all impurity elements in steel that can cause the deterioration of steel plate properties. In this application, S is controlled to be ≤0.002%, P ≤0.012%, N ≤0.0045%, and H ≤0.00015%.

[0040] In summary, this chemical composition design scheme, through low carbon and low nitrogen design, uses titanium to fix nitrogen and boron to enhance hardenability, and further adds Nb, V, Ti, Ni, Cr, Cu, and Mo to utilize their grain refinement strengthening, precipitation strengthening, and solid solution strengthening effects to synergistically improve the strength, low-temperature toughness, weldability, core toughness, and hardenability of thick steel plates, thereby improving the uniformity of the mechanical properties of the cross-section in the thickness direction of the thick steel plate.

[0041] The thickness d of the steel plate is 80~120mm.

[0042] According to GB / T 2975-2018 "Sampling Location and Specimen Preparation for Mechanical Properties Testing of Steel and Steel Products" and GB / T228.1-2021 "Metallic Materials - Tensile Testing - Part 1: Test Method at Room Temperature", the steel was sampled and its mechanical properties were tested.

[0043] At room temperature, the steel plate has a yield strength of 740~800MPa, a tensile strength of 930~1000MPa, a yield-to-tensile ratio ≤0.82, an elongation ≥22%, and a tensile strength difference between the surface and core of the steel plate ≤45MPa. This not only meets the requirements for thick steel plates in core load-bearing components such as high-rise building tubes, large-span stadium nodes, heavy industrial columns and beams, and key load-bearing parts of large transportation hubs, but also provides high strength, enabling it to withstand complex static and dynamic loads, and exhibits good uniformity of mechanical properties across the thickness section of the steel plate.

[0044] The microstructure of the steel plate is a multiphase structure consisting of ferrite, lath bainite, and tempered martensite; wherein ferrite accounts for 10-25% of the volume, lath bainite accounts for 70-80%, and tempered martensite accounts for 5-10%. Thus, the microstructure of the steel plate is dominated by high-density dislocation lath bainite, with a small amount of ferrite as a soft phase, and high-strength tempered martensite dispersed throughout, resulting in high strength. During deformation, the soft ferrite phase yields first, while the hard bainite + martensite network provides strong constraint and continuous work hardening capability, giving the material both high yield strength and high tensile strength, a low yield-to-tensile ratio, and extremely high seismic ductility and deformation resistance.

[0045] Full-thickness samples were ground and polished, then etched with 4% nitric acid alcohol. The average grain size was measured according to ASTM E1181, "Standard Test Method for Characterization of Double Grain Size". The average grain size of ferrite was ≤5μm, the average width of bainite laths was ≤0.5μm, and the average width of martensite laths was ≤0.2μm.

[0046] According to GB / T 229-2020 "Metallic Materials Charpy Pendulum Impact Test Method", the steel was sampled and its low-temperature performance was tested.

[0047] The impact energy (KV2) at -60℃ for both the surface and core of the steel plate is ≥200J.

[0048] High-temperature tensile tests were conducted in accordance with GB / T 228.2-2015 "Metallic materials - Tensile testing - Part 2: High-temperature test method".

[0049] The steel plate was tested after being held at 600℃ for 3 hours to determine its yield strength R at 600℃. p0.2 ≥495MPa.

[0050] According to GB / T 15248-2008 "Metallic Materials Axial Constant Amplitude Low Cyclic Fatigue Test Method", axial constant amplitude low cyclic fatigue test was carried out on metallic materials.

[0051] At a strain frequency of 1~3Hz and a strain amplitude of 0.8%, the steel plate can withstand at least 1600 cycles without breaking, that is, the steel plate can withstand at least 1600 loading-unloading cycles without breaking, and the steel plate has good strain fatigue life.

[0052] In addition, the steel plate has excellent shape. The flatness of the steel plate was tested according to GB / T 709-2019 "Dimensions, shape, weight and permissible deviations of hot-rolled steel plates and strips".

[0053] The unevenness of the steel plate is ≤3mm / m.

[0054] This application also provides a method for producing the steel plate. The method includes, in sequence, a steelmaking process, a continuous casting process, a stacking and slow cooling process, a controlled rolling process, a controlled cooling process, a quenching process, and a tempering process. Each process will be described in detail below in order.

[0055] <Steelmaking process> The steelmaking process includes sequential steps of pre-desulfurization treatment, converter smelting, LF refining, and RH vacuum refining, thereby smelting the steelmaking raw materials into molten steel.

[0056] It is understandable that the chemical composition of the molten steel obtained from the steelmaking process is consistent with the chemical composition of the continuously cast billet obtained from the subsequent continuous casting process and the final steel plate, so it will not be elaborated further here.

[0057] In the RH vacuum refining step, the vacuum degree is ≤2 mbar, the degassing time is 20~25 min, and the net circulation processing time is 8~10 min. Thus, through RH vacuum refining, deep degassing and inclusion flotation can be achieved, greatly improving the purity of the molten steel and reducing the H content to H≤0.00015%. This reduces the generation of white spots and hydrogen-induced cracks from the source, ensuring the internal quality safety of thick steel plates, especially high-strength thick steel plates.

[0058] In one embodiment, the pre-desulfurization step involves desulfurizing the molten iron. After desulfurization, the temperature of the molten iron is ≥1300℃, the S content in the molten iron is ≤0.003%, and the slag removal rate is ≥95%.

[0059] In the converter smelting process, the amount of slag left is 50% to 60% of the total slag from the previous furnace. During the dephosphorization period, the molten pool temperature is controlled at 1350 to 1400℃, the slag basicity is controlled at 1.8 to 2.0, and the total iron content in the slag is controlled at 12% to 15%. The final slag basicity is controlled at 3.5 to 4.0, and the final slag total iron content is controlled at 15% to 18%. The temperature of the molten steel at the end of the converter smelting process is controlled at 1640 to 1660℃, and the P content in the molten steel is ≤0.006%.

[0060] In the LF refining step, a slag surface deoxidizer is used for deoxidation during the refining process. Argon is blown into the ladle throughout the refining process. During deoxidation and slag formation, the argon flow rate is controlled at 600-800 NL / min, with a stirring time ≤5 min. During alloy addition, the argon flow rate is controlled at 600-800 NL / min, with a stirring time ≤3 min. At other times, the argon flow rate is controlled at 200-400 NL / min. Thus, during the initial deoxidation and slag formation after the molten steel enters the station, the strong stirring with a high-flow-rate argon flow rate disrupts the concentration balance at the steel-slag interface, promoting the diffusion of oxygen from the molten steel to the slag phase, thereby improving deoxidation efficiency and accelerating the deoxidation reaction.

[0061] <Continuous casting process> The molten steel obtained from the steelmaking process is fed into a continuous casting machine and continuously cast into billets. The thickness of the billets is 310~330mm.

[0062] Electromagnetic stirring is implemented in the crystallizer, the secondary cooling zone, and the solidification end, with the electromagnetic stirring frequency in the crystallizer > the electromagnetic stirring frequency in the secondary cooling zone > the electromagnetic stirring frequency at the solidification end, and the electromagnetic stirring current in the crystallizer < the electromagnetic stirring current at the solidification end < the electromagnetic stirring current in the secondary cooling zone. On the one hand, segmented electromagnetic stirring effectively promotes the flotation of inclusions, increases the proportion of equiaxed crystals, reduces center segregation and porosity, and significantly improves the uniformity and purity of the internal structure of the continuously cast billet. On the other hand, by controlling the electromagnetic stirring current in the crystallizer to be relatively low and the frequency relatively high, violent fluctuations in the molten steel surface can be avoided, preventing slag entrapment. The relatively high electromagnetic stirring current in the secondary cooling zone ensures sufficient magnetic field penetration depth, allowing the stirring force to reach the central area of ​​the continuously cast billet. At the solidification end, the solidified billet shell is relatively thick, requiring a lower electromagnetic stirring frequency to ensure the magnetic field can penetrate into the remaining fine liquid core. This improves the consistency of the final steel plate's performance in the thickness direction.

[0063] In one embodiment, the electromagnetic stirring frequency of the crystallizer is 5~8Hz, the electromagnetic stirring frequency of the secondary cooling zone is 3~6Hz, and the electromagnetic stirring frequency of the solidification end is 2~4Hz.

[0064] In one embodiment, the electromagnetic stirring current of the crystallizer is 200~400A, the electromagnetic stirring current of the secondary cooling zone is 400~800A, and the electromagnetic stirring current of the solidification end is 300~600A.

[0065] In one embodiment, the superheat of the molten steel during casting is 15-20°C. By using low superheat casting, the molten steel can begin to solidify quickly after entering the crystallizer, forming a large number of fine equiaxed crystal nuclei. This lays a fine-grained foundation for the entire subsequent solidification process, thereby effectively expanding the equiaxed crystal zone and inhibiting the growth of coarse columnar crystals, thus reducing center segregation and porosity at the source.

[0066] In one embodiment, the ladle is left to stand on a rotary table for 15-18 minutes before the molten steel is poured. This standing time promotes further homogenization of the composition and further floating of large inclusions, laying the foundation for obtaining a high-cleanliness continuously cast billet, and thus laying the foundation for the steel plate to obtain excellent impact toughness and high strain fatigue life.

[0067] <Stacking and Slow Cooling Process> The continuously cast billets obtained from the continuous casting process are stacked for slow cooling. Preferably, the stacking time is ≥48 hours. Slow cooling by stacking allows sufficient time for hydrogen in the continuously cast billets to diffuse and escape, further reducing the risk of hydrogen-induced delayed cracking; moreover, slow cooling helps release internal stress and stabilize the microstructure, providing billets with a uniform state for subsequent heating and controlled rolling.

[0068] <Heating Process> The continuously cast billets, after being stacked and slowly cooled, are sent into a heating furnace for heating.

[0069] The heating process includes, in sequence, a first preheating section, a second preheating section, a first heating section, a second heating section, and a soaking section; the temperature of the first preheating section is ≤900℃, and the residence time is ≤50min; the temperature of the second preheating section is 900~1050℃, and the residence time is ≤50min; the temperature of the first heating section is 1000~1100℃, and the residence time is ≤50min; the temperature of the second heating section is 1150~1160℃, and the residence time is ≥80min; the temperature of the soaking section is 1130~1150℃, and the residence time is ≥100min.

[0070] Thus, low-temperature heating significantly improves the removability of surface oxide scale during the heating process of continuously cast billets, thereby significantly improving the surface quality of Ni and Cu-containing steel plates. Furthermore, low-temperature heating prevents abnormal growth of austenite. Further, the rapid gradient heating through the first preheating section, second preheating section, and first heating section retains more austenite nucleation sites, resulting in a finer austenitized microstructure. The prolonged holding time in the second heating section and soaking section ensures thorough heating of the continuously cast billet, improving temperature uniformity and allowing for complete dissolution of alloying elements.

[0071] <Controlling the rolling process> The continuously cast billet after the heating process is sent to the rolling mill and rolled into steel plate.

[0072] Specifically, the rolling process is controlled to use a one-stage rolling process, with an initial rolling temperature of T. nr +35℃~T nr +65℃, final rolling temperature is T nr +5℃~T nr +25℃.

[0073] Among them, T nr The recrystallization temperature of austenite is given by the formula T. nr =887+464C-357Si+(6445Nb-644 )+(732V-230 The calculation is performed using )+890Ti+363Al, with the unit being ℃.

[0074] In the formula, the element symbol represents the mass percentage of the corresponding element. For example, if the content of element C in the steel plate is 0.06%, then C in the formula is substituted with a mass percentage of 0.06 for calculation. The same applies below, and will not be repeated hereafter.

[0075] Thus, by rolling in the recrystallization zone at low temperatures, austenite can be repeatedly recrystallized, refining the original austenite grain size, suppressing the growth of recrystallized austenite, and providing more nucleation sites for subsequent cooling phase transformation.

[0076] In one embodiment, the reduction per pass is 28-45 mm, meaning that the reduction in each pass of a single-stage rolling process is within the range of 28-45 mm. This ensures that the rolling deformation fully penetrates into the core of the steel plate, refining the austenite structure in the core, and also avoids excessive reduction in a single pass, which could affect the safety of the rolling mill.

[0077] <Controlled Cooling Process> The rolled steel plate is cooled in water at a rate of 1~4℃ / s, with a final cooling temperature of Bs+50℃~Bs+80℃, and then air-cooled to room temperature.

[0078] Among them, B s The starting temperature for the bainite transformation of austenite is given by formula B. s The calculation is performed using 830-270C-90Mn-70Cr-37Ni-83Mo, with the unit being °C.

[0079] In this way, by controlling the cooling rate and the final cooling temperature, the steel plate can quickly pass through the pearlite and ferrite transformation zone, suppressing the formation of coarse structures and forming acicular ferrite structures. Acicular ferrite mainly nucleates within austenite grains, and its orientation is random and interwoven, thus making the segmentation of the original austenite structure more effective and providing a refined microstructure basis for phase transformation during subsequent quenching and tempering heat treatment.

[0080] <Quenching Process> The steel plate after the controlled cooling process is then subjected to quenching treatment.

[0081] The quenching process includes sequential heating, first-stage cooling, and second-stage cooling steps. The heating temperature in the heating step is A. C3 -65℃~A C3 -15℃, holding time is 1.8~2.0 min / mm; in the first stage cooling step, the cooling rate of the steel plate surface layer is ≥35℃ / s, the cooling rate of the steel plate core is ≥5℃ / s, and the final cooling temperature of the steel plate surface layer is B. s +5℃~B s +25℃; In the two-stage cooling process, the cooling rate of the steel plate surface is ≤15℃ / s, the cooling rate of the steel plate core is ≥10℃ / s, and the final cooling temperature of the steel plate surface is ≤300℃.

[0082] Among them, the austenitization completion temperature A C3 =910-320C-14Ni-12Cu-10Mn+5Cr+14Mo+5V+18Si.

[0083] Thus, by controlling the heating temperature in the two-phase region, a small amount of fine, existing ferrite is retained, while the re-austenitic structure transforms into fine lath bainite and a small amount of martensite during the subsequent first-stage and second-stage cooling processes. This results in a multiphase structure of soft phase (ferrite) and hard phase (lath bainite + a small amount of martensite) in the steel plate. Ferrite provides good plasticity and toughness, while bainite and martensite ensure high strength. During deformation, the soft phase ferrite yields first, while the hard phase bainite and martensite constrain its deformation. This gives the steel plate continuous yielding behavior and high work hardening capacity, significantly reducing the yield strength ratio and imparting excellent strain fatigue performance. As a result, the steel plate can withstand at least 1600 cycles without fracture at a strain frequency of 1~3Hz and a strain amplitude of 0.8%. Furthermore, by using staged cooling, excessive cooling of the steel plate surface can be avoided, while accelerating the cooling of the steel plate core. This reduces the temperature difference between the steel plate core and surface, achieving a uniform microstructure transformation in the thickness direction. This results in a uniform microstructure along the thickness direction, dominated by fine lath bainite, allowing the tensile strength difference between the steel plate surface and core to be controlled within 40 MPa. This significantly improves the performance uniformity of extra-thick steel plates in the thickness direction.

[0084] In one embodiment, the first-stage cooling uses water cooling, with the high-pressure water pressure being 0.8~1.0 MPa and the water flow rate being 8500~10000 m³ / h. 3 The water flow rate is 40-50° between the water flow and the steel plate surface; the second-stage cooling uses a laminar flow cooling water curtain with a water pressure of 0.2-0.4 MPa and a flow rate of 5000-6500 m³ / h. 3 / h, the angle between the water flow and the steel plate surface is 85~95°.

[0085] Thus, by employing dense, high-pressure water jets in the first-stage cooling process and controlling the angle between the water flow and the steel plate surface, the water and vapor film on the steel plate surface can be swept away, enhancing heat transfer and reducing water flow rebound interference. This instantly breaks down the vapor film on the steel plate surface, achieving extremely high heat transfer efficiency and providing short-term, intense cooling to the steel plate surface. Furthermore, it rapidly surpasses the nose temperature, suppressing the formation of proeutectoid ferrite and pearlite, creating conditions for obtaining a uniform bainitic structure. In addition, it allows the surface temperature of the steel plate to decrease rapidly, while the core of the steel plate remains at an extremely high austenitizing temperature. This results in a large temperature gradient along the thickness direction of the steel plate cross section. Combined with the chemical composition design scheme, it is possible to achieve no proeutectoid ferrite or pearlite precipitation across the entire thickness of the steel plate.

[0086] The reasons are as follows: The alloying elements in the chemical composition design of this application shift the pearlite C-curve to the right, significantly prolonging the incubation period of diffusion-type phase transformation (i.e., ferrite and pearlite phase transformation), and raising the minimum cooling rate of the entire cross section in the thickness direction, i.e. the core cooling rate, to above the critical cooling rate corresponding to this composition; even if the cooling rate gradually decreases from the surface of the steel plate to the core, the cooling of each region does not fall into the ferrite + pearlite phase transformation region of the C-curve, and the austenite is only retained with different degrees of undercooling. Combined with the fine ferrite generated by heating the two-phase region, the steel plate will not generate additional ferrite and pearlite throughout its entire thickness.

[0087] Regarding the nasal tip temperature, the distance between the line where the transformation begins and the vertical axis in the C-curve represents the incubation period, indicating the stability of supercooled austenite under different degrees of supercooling. The temperature with the shortest incubation period and the lowest stability of supercooled austenite is called the nasal tip temperature.

[0088] The second-stage cooling employs water curtain cooling. By controlling the angle between the water flow and the steel plate surface, the laminar water curtain can uniformly and completely cover the steel plate surface, achieving stable and uniform cooling. During the slow cooling process in the second stage, the high-temperature core continuously conducts heat to the low-temperature surface region, which slows down further cooling of the surface layer. Due to the reduced cooling intensity of the steel plate surface, the resistance to heat conduction from the core to the outside is reduced, and the actual cooling rate of the core is significantly improved compared to the traditional quenching process. This allows the steel plate to complete the phase transformation almost entirely within the bainitic transformation temperature range from the surface to the core, resulting in a uniform bainitic structure across the entire cross-section and greatly reducing performance differences in the thickness direction.

[0089] <Tempering Process> Tempering is performed on steel plates that have undergone quenching.

[0090] The tempering heating temperature is 540~560℃, the furnace time is 0.8~1.0 min / mm, and the steel plate exiting the furnace temperature is 490~510℃. This high tempering temperature allows for rapid tempering, enabling the steel plate to quickly reach the target temperature. This further eliminates quenching stress without significantly reducing the steel plate's strength, promotes the dispersion and precipitation of fine carbides, improves the low-temperature toughness of the steel plate, and thus avoids warping deformation that easily occurs in subsequent cutting and welding processes, improving the flatness of the steel plate. Furthermore, the tempered bainite and martensite exhibit excellent high-temperature stability; after prolonged holding at 600℃, the microstructure is less prone to coarsening and severe degradation.

[0091] In summary, one embodiment of this application employs low-temperature heating in the heating process and controls the rolling process to perform a one-stage recrystallization rolling in the low-temperature zone, which effectively refines the original austenite grains. Furthermore, by controlling the cooling process, the steel plate rapidly passes through the pearlite and ferrite transformation zone, suppressing the formation of coarse structures. The resulting acicular ferrite structure further divides the original austenite structure. Subsequently, the quenching process controls the heating temperature in the two-phase region and combines it with staged cooling to obtain a uniform structure along the thickness direction, mainly composed of fine lath bainite. This allows the tensile strength difference between the surface and core of the steel plate to be controlled within 40 MPa, significantly improving the performance uniformity of the extra-thick steel plate in the thickness direction and greatly increasing the crack propagation resistance. This enables the steel plate to maintain high impact energy at -60°C and withstand more than 1600 cycles of cyclic loading without fracture at a strain frequency of 1~3 Hz and a strain amplitude of 0.8%, exhibiting excellent resistance to dynamic loads and fatigue performance.

[0092] The technical intent and basic details of various embodiments of this application have been described above. Several embodiments are provided below to demonstrate the beneficial effects of this application. Of course, these embodiments are only a part of the numerous variations contained in this application, and not all of them.

[0093] Example 1 This embodiment provides a steel plate whose chemical composition, by mass percentage, includes: C 0.055%, Si 0.10%, Mn 1.50%, Cr 0.40%, Ni 0.30%, Mo 0.4%, Cu 0.25%, Nb 0.030%, V 0.030%, Ti 0.01%, Al 0.02%, B 0.0010%, S 0.0015%, P 0.010%, N 0.0040%, H 0.00012%, with the remainder being Fe and unavoidable impurities.

[0094] The following sections will describe each step of the steel plate production process in sequence.

[0095] <Steelmaking process> The steelmaking process includes sequential steps of pre-desulfurization treatment, converter smelting, LF refining, and RH vacuum refining, thereby smelting the steelmaking raw materials into molten steel.

[0096] In the pre-desulfurization step, the molten iron is desulfurized. After desulfurization, the temperature of the molten iron is 1320℃, the S content in the molten iron is 0.002%, and the slag removal rate is 96%.

[0097] In the converter smelting process, the amount of slag left is 50% of the total slag amount of the previous furnace. During the dephosphorization period, the temperature of the molten pool is controlled at 1400℃, the basicity of the molten slag is controlled at 1.8, and the total iron content in the slag is controlled at 12%. The basicity of the final slag is controlled at 3.5, and the total iron content in the final slag is controlled at 15%. The temperature of the molten steel at the end of the converter smelting process is controlled at 1640℃, and the P content in the molten steel is 0.006%.

[0098] In the LF refining process, deoxidation is carried out using a slag surface deoxidizer. Argon is blown into the ladle throughout the refining process. During deoxidation and slag formation, the flow rate of the bottom-blown argon is controlled at 600 NL / min, and the stirring time is 5 min. When adding alloys, the flow rate of the bottom-blown argon is controlled at 600 NL / min, and the stirring time is 3 min. At other times, the flow rate of the bottom-blown argon is controlled at 200 NL / min.

[0099] In the RH vacuum refining step, the vacuum level is 2 mbar, the degassing time is 20 min, and the net circulation processing time is 8 min.

[0100] <Continuous casting process> The molten steel obtained from the steelmaking process is sent to the continuous casting machine. Before pouring, the ladle is left to stand on the rotary table for 15 minutes. Then, it is continuously cast into a continuous casting billet with a thickness of 320 mm. The superheat of the molten steel during casting is 15°C.

[0101] Electromagnetic stirring is implemented in the crystallizer, the secondary cooling zone, and the solidification end. The electromagnetic stirring frequency in the crystallizer is 5Hz and the current is 200A; the electromagnetic stirring frequency in the secondary cooling zone is 3Hz and the current is 400A; and the electromagnetic stirring frequency at the solidification end is 2Hz and the current is 300A.

[0102] <Stacking and Slow Cooling Process> The continuously cast billets obtained from the continuous casting process are stacked for slow cooling, and the stacking time is 48 hours.

[0103] <Heating Process> The continuously cast billets, after being stacked and slowly cooled, are sent into a heating furnace for heating.

[0104] The heating process includes, in sequence, a first preheating section, a second preheating section, a first heating section, a second heating section, and a soaking section. The temperature of the first preheating section is 900℃, and the residence time is 50 min; the temperature of the second preheating section is 1000℃, and the residence time is 50 min; the temperature of the first heating section is 1100℃, and the residence time is 50 min; the temperature of the second heating section is 1160℃, and the residence time is 80 min; the temperature of the soaking section is 1150℃, and the residence time is 100 min.

[0105] <Controlling the rolling process> The continuously cast billet after the heating process is fed into a rolling mill for one-stage rolling to produce a steel plate with a thickness of 80 mm. The initial rolling temperature is 1040℃, and the final rolling temperature is 1002℃. The reductions in each pass are 30 mm, 45 mm, 40 mm, 35 mm, 33 mm, 29 mm, and 28 mm, respectively.

[0106] <Controlled Cooling Process> The rolled steel plate was cooled in water at a rate of 4°C / s, with a final cooling temperature of 675°C, and then air-cooled to room temperature.

[0107] <Quenching Process> The steel plate after the controlled cooling process is then subjected to quenching treatment.

[0108] The quenching process includes sequential heating, first-stage cooling, and second-stage cooling steps.

[0109] The heating temperature for the heating step is 820℃, and the holding time is 160 minutes.

[0110] The first-stage cooling process uses high-pressure water cooling, with a water pressure of 0.8 MPa and a flow rate of 8500 m³ / h. 3 The water flow rate is 42℃ / s, and the angle between the water flow and the steel plate surface is 45°. The surface of the steel plate is rapidly cooled to the final cooling temperature of 618℃. The cooling rate of the surface of the steel plate is 42℃ / s, and the cooling rate of the core of the steel plate is 10℃ / s.

[0111] The second-stage cooling process uses a laminar flow cooling water curtain, with low-pressure water at a pressure of 0.2 MPa and a flow rate of 5000 m³ / h. 3 The water flow rate is 15℃ / s, and the cooling rate of the steel plate surface is 13℃ / s. The angle between the water flow and the steel plate surface is 90°. The surface of the steel plate is rapidly cooled to the final cooling temperature of 253℃.

[0112] <Tempering Process> Tempering is performed on steel plates that have undergone quenching.

[0113] The tempering heating temperature is 550℃, the furnace time is 80 minutes, and the steel plate exit temperature is 506℃.

[0114] The finished steel plates undergo performance testing, as detailed below: (1) The microstructure of the steel plate is a multiphase microstructure of ferrite + lath bainite + tempered martensite; wherein, the volume percentage of ferrite is 15%, the volume percentage of lath bainite is 77%, and the volume percentage of tempered martensite is 8%.

[0115] Full-thickness samples were ground and polished, then etched with 4% nitric acid alcohol, and the average grain size was measured according to ASTM E1181, "Standard Test Method for Characterization of Double Grain Size".

[0116] The average grain size of ferrite was measured to be 2.5 μm, the average width of bainite laths to be 0.25 μm, and the average width of martensite laths to be 0.1 μm.

[0117] (2) According to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", the steel was sampled and its mechanical properties were tested. The test results are as follows: At room temperature, the yield strength of the surface layer of the steel plate is 775 MPa, and the yield strength of the core layer is 748 MPa. At room temperature, the tensile strength of the surface layer of the steel plate is 970 MPa, and the tensile strength of the core layer is 940 MPa. At room temperature, the yield strength ratio of the surface layer of the steel plate is 0.80, and the yield strength ratio of the core layer is 0.80. At room temperature, the elongation of the surface layer of the steel plate is 25%, and the elongation of the core layer is 27%.

[0118] (3) According to GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials", the steel was sampled and tested for low-temperature performance. The test results are as follows: The impact energy (KV2) at -60℃ on the surface of the steel plate is 325J, and the impact energy (KV2) at -60℃ on the core is 288J.

[0119] (4) High-temperature tensile tests were conducted according to GB / T 228.2-2015 "Metallic materials - Tensile testing - Part 2: High-temperature test method", and the test results are as follows: The steel plate surface layer was tested after being held at 600℃ for 3 hours to determine the yield strength R at 600℃. p0.2 The yield strength R of the core at 600℃ is 529 MPa. p0.2 It is 502 MPa.

[0120] (5) In accordance with GB / T 15248-2008 "Metallic Materials - Axial Constant Amplitude Low Cyclic Fatigue Test Method", axial constant amplitude low cyclic fatigue tests were conducted on metallic materials, and the test results are as follows: At a strain frequency of 1-3 Hz and a strain amplitude of 0.8%, the steel plate can withstand 1800 cycles without fracturing. In other words, the steel plate has good strain fatigue life.

[0121] (6) The flatness of the steel plate was tested according to GB / T 709-2019 "Dimensions, shape, weight and permissible deviations of hot-rolled steel plates and strips".

[0122] The unevenness of the steel plate is ≤2mm / m.

[0123] Example 2 This embodiment provides a steel plate whose chemical composition, by mass percentage, includes: C 0.085%, Si 0.25%, Mn 1.70%, Cr 0.60%, Ni 0.50%, Mo 0.6%, Cu 0.35%, Nb 0.040%, V 0.040%, Ti 0.02%, Al 0.05%, B 0.0016%, S 0.0015%, P 0.010%, N 0.0040%, H 0.00012%, with the remainder being Fe and unavoidable impurities.

[0124] The following sections will describe each step of the steel plate production process in sequence.

[0125] <Steelmaking process> The steelmaking process includes sequential steps of pre-desulfurization treatment, converter smelting, LF refining, and RH vacuum refining, thereby smelting the steelmaking raw materials into molten steel.

[0126] In the pre-desulfurization step, the molten iron is desulfurized. After desulfurization, the temperature of the molten iron is 1320℃, the S content in the molten iron is 0.002%, and the slag removal rate is 96%.

[0127] In the converter smelting process, the amount of slag left is 60% of the total slag amount of the previous furnace. During the dephosphorization period, the temperature of the molten pool is controlled at 1400℃, the basicity of the molten slag is controlled at 2.0, and the total iron content in the slag is controlled at 15%. The basicity of the final slag is controlled at 4.0, and the total iron content in the final slag is controlled at 18%. The temperature of the molten steel at the end of the converter smelting process is controlled at 1660℃, and the P content in the molten steel is 0.005%.

[0128] In the LF refining process, a slag surface deoxidizer is used for deoxidation. Argon is blown into the ladle throughout the refining process. During deoxidation and slag formation, the flow rate of the bottom-blown argon is controlled at 800 NL / min, and the stirring time is 4 min. When adding alloys, the flow rate of the bottom-blown argon is controlled at 800 NL / min, and the stirring time is 2 min. At other times, the flow rate of the bottom-blown argon is controlled at 400 NL / min.

[0129] In the RH vacuum refining step, the vacuum level is 2 mbar, the degassing time is 25 min, and the net circulation processing time is 10 min.

[0130] <Continuous casting process> The molten steel obtained from the steelmaking process is sent to the continuous casting machine. Before pouring, the ladle is left to stand on the rotary table for 18 minutes. Then, it is continuously cast into a continuous casting billet with a thickness of 320 mm. The superheat of the molten steel during casting is 20°C.

[0131] Electromagnetic stirring is implemented in the crystallizer, the secondary cooling zone, and the solidification end. The electromagnetic stirring frequency in the crystallizer is 8Hz and the current is 400A; the electromagnetic stirring frequency in the secondary cooling zone is 6Hz and the current is 800A; and the electromagnetic stirring frequency at the solidification end is 4Hz and the current is 600A.

[0132] <Stacking and Slow Cooling Process> The continuously cast billets obtained from the continuous casting process are stacked for slow cooling for 50 hours.

[0133] <Heating Process> The continuously cast billets, after being stacked and slowly cooled, are sent into a heating furnace for heating.

[0134] The heating process includes, in sequence, a first preheating section, a second preheating section, a first heating section, a second heating section, and a soaking section. The temperature of the first preheating section is 890℃, and the residence time is 45 min; the temperature of the second preheating section is 1050℃, and the residence time is 50 min; the temperature of the first heating section is 1000℃, and the residence time is 45 min; the temperature of the second heating section is 1150℃, and the residence time is 80 min; the temperature of the soaking section is 1140℃, and the residence time is 120 min.

[0135] <Controlling the rolling process> The continuously cast billet after the heating process is fed into a rolling mill for one-stage rolling to produce a steel plate with a thickness of 100 mm. The initial rolling temperature is 1040℃, and the final rolling temperature is 992℃. The reductions in each pass are 30 mm, 40 mm, 35 mm, 30 mm, 29 mm, 28 mm, and 28 mm, respectively.

[0136] <Controlled Cooling Process> The rolled steel plate was cooled in water at a rate of 3°C / s, with a final cooling temperature of 611°C, and then air-cooled to room temperature.

[0137] <Quenching Process> The steel plate after the controlled cooling process is then subjected to quenching treatment.

[0138] The quenching process includes sequential heating, first-stage cooling, and second-stage cooling steps.

[0139] The heating temperature for the heating step is 835℃, and the holding time is 240 minutes.

[0140] The first-stage cooling process uses high-pressure water cooling, with a water pressure of 0.9 MPa and a flow rate of 9000 m³ / h. 3 The water flow rate is 38℃ / s, and the angle between the water flow and the steel plate surface is 45°. The surface of the steel plate is rapidly cooled to a final cooling temperature of 550℃. The cooling rate of the surface of the steel plate is 38℃ / s, and the cooling rate of the core of the steel plate is 7℃ / s.

[0141] The second-stage cooling process uses a laminar flow cooling water curtain, with a water pressure of 0.3 MPa and a flow rate of 6200 m³ / h. 3 The water flow rate is 15℃ / s, and the cooling rate of the steel plate surface is 12℃ / s. The angle between the water flow and the steel plate surface is 90°. The surface of the steel plate is rapidly cooled to the final cooling temperature of 272℃.

[0142] <Tempering Process> Tempering is performed on steel plates that have undergone quenching.

[0143] The tempering heating temperature is 550℃, the furnace time is 120 minutes, and the steel plate exit temperature is 500℃.

[0144] The finished steel plates undergo performance testing, as detailed below: (1) The microstructure of the steel plate is a multiphase microstructure of ferrite + lath bainite + tempered martensite; wherein, the volume percentage of ferrite is 17%, the volume percentage of lath bainite is 76%, and the volume percentage of tempered martensite is 7%.

[0145] Full-thickness samples were ground and polished, then etched with 4% nitric acid alcohol, and the average grain size was measured according to ASTM E1181, "Standard Test Method for Characterization of Double Grain Size".

[0146] The average grain size of ferrite was measured to be 3.8 μm, the average width of bainite laths to be 0.37 μm, and the average width of martensite laths to be 0.13 μm.

[0147] (2) According to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", the steel was sampled and its mechanical properties were tested. The test results are as follows: At room temperature, the yield strength of the surface layer of the steel plate is 777 MPa, and the yield strength of the core layer is 740 MPa. At room temperature, the tensile strength of the surface layer of the steel plate is 978 MPa, and the tensile strength of the core layer is 938 MPa. At room temperature, the yield strength ratio of the surface layer of the steel plate is 0.79, and the yield strength ratio of the core layer is 0.79. At room temperature, the elongation of the surface layer of the steel plate is 26%, and the elongation of the core layer is 28%.

[0148] (3) According to GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials", the steel was sampled and tested for low-temperature performance. The test results are as follows: The impact energy (KV2) at -60℃ on the surface of the steel plate is 318J, and the impact energy (KV2) at -60℃ on the core is 270J.

[0149] (4) High-temperature tensile tests were conducted according to GB / T 228.2-2015 "Metallic materials - Tensile testing - Part 2: High-temperature test method", and the test results are as follows: The steel plate surface layer was tested after being held at 600℃ for 3 hours to determine the yield strength R at 600℃. p0.2 The yield strength R of the core at 600℃ is 530 MPa. p0.2 It is 500 MPa.

[0150] (5) In accordance with GB / T 15248-2008 "Metallic Materials - Axial Constant Amplitude Low Cyclic Fatigue Test Method", axial constant amplitude low cyclic fatigue tests were conducted on metallic materials, and the test results are as follows: At a strain frequency of 1-3 Hz and a strain amplitude of 0.8%, the steel plate can withstand 1690 cycles without fracturing. In other words, the steel plate has good strain fatigue life.

[0151] (6) The flatness of the steel plate was tested according to GB / T 709-2019 "Dimensions, shape, weight and permissible deviations of hot-rolled steel plates and strips".

[0152] The unevenness of the steel plate is ≤2mm / m.

[0153] Example 3 This embodiment provides a steel plate whose chemical composition, by mass percentage, includes: C 0.085%, Si 0.25%, Mn 1.70%, Cr 0.60%, Ni 0.50%, Mo 0.6%, Cu 0.35%, Nb 0.040%, V 0.040%, Ti 0.02%, Al 0.05%, B 0.0016%, S 0.0015%, P 0.010%, N 0.0040%, H 0.00012%, with the remainder being Fe and unavoidable impurities.

[0154] The following sections will describe each step of the steel plate production process in sequence.

[0155] <Steelmaking process> The steelmaking process includes sequential steps of pre-desulfurization treatment, converter smelting, LF refining, and RH vacuum refining, thereby smelting the steelmaking raw materials into molten steel.

[0156] In the pre-desulfurization step, the molten iron is desulfurized. After desulfurization, the temperature of the molten iron is 1340℃, the S content in the molten iron is 0.002%, and the slag removal rate is 96%.

[0157] In the converter smelting process, the amount of slag left is 50% of the total slag amount of the previous furnace. During the dephosphorization period, the temperature of the molten pool is controlled at 1380℃, the basicity of the molten slag is controlled at 1.9, and the total iron content in the slag is controlled at 14%. The basicity of the final slag is controlled at 3.8, and the total iron content in the final slag is controlled at 17%. The temperature of the molten steel at the end of the converter smelting process is controlled at 1650℃, and the P content in the molten steel is 0.006%.

[0158] In the LF refining process, a slag surface deoxidizer is used for deoxidation. Argon is blown into the ladle throughout the refining process. When deoxidizing and forming slag, the flow rate of the bottom-blown argon is controlled at 800 NL / min and the stirring time is 5 min. When adding alloys, the flow rate of the bottom-blown argon is controlled at 800 NL / min and the stirring time is 3 min. At other times, the flow rate of the bottom-blown argon is controlled at 400 NL / min.

[0159] In the RH vacuum refining step, the vacuum level is 2 mbar, the degassing time is 25 min, and the net circulation processing time is 10 min.

[0160] <Continuous casting process> The molten steel obtained from the steelmaking process is sent to the continuous casting machine. Before pouring, the ladle is left to stand on the rotary table for 18 minutes. Then, it is continuously cast into a continuous casting billet with a thickness of 320 mm. The superheat of the molten steel during casting is 20°C.

[0161] Electromagnetic stirring is implemented in the crystallizer, the secondary cooling zone, and the solidification end. The electromagnetic stirring frequency in the crystallizer is 7Hz and the current is 300A; the electromagnetic stirring frequency in the secondary cooling zone is 5Hz and the current is 700A; and the electromagnetic stirring frequency at the solidification end is 3Hz and the current is 500A.

[0162] <Stacking and Slow Cooling Process> The continuously cast billets obtained from the continuous casting process are stacked for slow cooling for 50 hours.

[0163] <Heating Process> The continuously cast billets, after being stacked and slowly cooled, are sent into a heating furnace for heating.

[0164] The heating process includes, in sequence, a first preheating section, a second preheating section, a first heating section, a second heating section, and a soaking section. The temperature of the first preheating section is 900℃, and the residence time is 50 minutes; the temperature of the second preheating section is 900℃, and the residence time is 50 minutes; the temperature of the first heating section is 1000℃, and the residence time is 50 minutes; the temperature of the second heating section is 1155℃, and the residence time is 100 minutes; the temperature of the soaking section is 1150℃, and the residence time is 100 minutes.

[0165] <Controlling the rolling process> The continuously cast billet after the heating process is fed into a rolling mill for one-stage rolling to produce a steel plate with a thickness of 120 mm. The initial rolling temperature is 1030℃, and the final rolling temperature is 995℃. The reductions in each pass are 42 mm, 45 mm, 40 mm, 38 mm, and 35 mm, respectively.

[0166] <Controlled Cooling Process> The rolled steel plate was cooled in water at a rate of 2°C / s, with a final cooling temperature of 605°C, and then air-cooled to room temperature.

[0167] <Quenching Process> The steel plate after the controlled cooling process is then subjected to quenching treatment.

[0168] The quenching process includes sequential heating, first-stage cooling, and second-stage cooling steps.

[0169] The heating temperature for the heating step is 850℃, and the holding time is 240 minutes.

[0170] The first-stage cooling process uses high-pressure water cooling, with a water pressure of 1.0 MPa and a flow rate of 10,000 m³ / h. 3 The water flow rate is 35℃ / s, and the angle between the water flow and the steel plate surface is 45°. The surface of the steel plate is rapidly cooled to a final cooling temperature of 554℃. The cooling rate of the surface of the steel plate is 35℃ / s, and the cooling rate of the core of the steel plate is 5℃ / s.

[0171] The second-stage cooling process uses a laminar flow cooling water curtain, with low-pressure water at a pressure of 0.4 MPa and a flow rate of 6500 m³ / h. 3 The water flow rate is 14℃ / s, and the cooling rate of the steel plate surface is 10℃ / s. The angle between the water flow and the steel plate surface is 90°. The surface of the steel plate is rapidly cooled to the final cooling temperature of 288℃.

[0172] <Tempering Process> Tempering is performed on steel plates that have undergone quenching.

[0173] The tempering heating temperature is 550℃, the furnace time is 120 minutes, and the steel plate exit temperature is 500℃.

[0174] The finished steel plates undergo performance testing, as detailed below: (1) The microstructure of the steel plate is a multiphase microstructure of ferrite + lath bainite + tempered martensite; wherein, the volume percentage of ferrite is 20%, the volume percentage of lath bainite is 74%, and the volume percentage of tempered martensite is 6%.

[0175] Full-thickness samples were ground and polished, then etched with 4% nitric acid alcohol, and the average grain size was measured according to ASTM E1181, "Standard Test Method for Characterization of Double Grain Size".

[0176] The average grain size of ferrite was measured to be ≤5μm, the average width of bainite laths to be ≤0.5μm, and the average width of martensite laths to be ≤0.2μm.

[0177] (2) According to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", the steel was sampled and its mechanical properties were tested. The test results are as follows: At room temperature, the yield strength of the surface layer of the steel plate is 788 MPa, and the yield strength of the core layer is 746 MPa. At room temperature, the tensile strength of the surface layer of the steel plate is 981 MPa, and the tensile strength of the core layer is 938 MPa. At room temperature, the yield strength ratio of the surface layer of the steel plate is 0.80, and the yield strength ratio of the core layer is 0.80. At room temperature, the elongation of the surface layer of the steel plate is 24%, and the elongation of the core layer is 28%.

[0178] (3) According to GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials", the steel was sampled and tested for low-temperature performance. The test results are as follows: The impact energy (KV2) at -60℃ on the surface of the steel plate is 316J, and the impact energy (KV2) at -60℃ on the core is 269J.

[0179] (4) High-temperature tensile tests were conducted according to GB / T 228.2-2015 "Metallic materials - Tensile testing - Part 2: High-temperature test method", and the test results are as follows: The steel plate surface layer was tested after being held at 600℃ for 3 hours to determine the yield strength R at 600℃. p0.2 The yield strength R of the core at 600℃ is 532 MPa. p0.2 It is 499 MPa.

[0180] (5) In accordance with GB / T 15248-2008 "Metallic Materials - Axial Constant Amplitude Low Cyclic Fatigue Test Method", axial constant amplitude low cyclic fatigue tests were conducted on metallic materials, and the test results are as follows: At a strain frequency of 1-3 Hz and a strain amplitude of 0.8%, the steel plate can withstand 1600 cycles without fracturing. In other words, the steel plate has good strain fatigue life.

[0181] (6) The flatness of the steel plate was tested according to GB / T 709-2019 "Dimensions, shape, weight and permissible deviations of hot-rolled steel plates and strips".

[0182] The unevenness of the steel plate is ≤2mm / m.

[0183] Comparative Example This comparative example provides a steel plate whose chemical composition, by mass percentage, includes: C 0.085%, Si 0.25%, Mn 1.60%, Cr 0.50%, Ni 0.40%, Mo 0.3%, Cu 0.35%, Nb 0.040%, V 0.030%, Ti 0.02%, Al 0.05%, B 0.0009%, S 0.0015%, P 0.010%, N 0.0040%, H 0.00012%, with the remainder being Fe and unavoidable impurities.

[0184] The following sections will describe each step of the steel plate production process in sequence.

[0185] <Steelmaking process> The steelmaking process includes sequential steps of pre-desulfurization treatment, converter smelting, LF refining, and RH vacuum refining, thereby smelting the steelmaking raw materials into molten steel.

[0186] In the pre-desulfurization step, the molten iron is desulfurized. After desulfurization, the temperature of the molten iron is 1340℃, the S content in the molten iron is 0.002%, and the slag removal rate is 96%.

[0187] In the converter smelting process, the amount of slag left is 50% of the total slag amount of the previous furnace. During the dephosphorization period, the temperature of the molten pool is controlled at 1380℃, the basicity of the molten slag is controlled at 1.9, and the total iron content in the slag is controlled at 14%. The basicity of the final slag is controlled at 3.8, and the total iron content in the final slag is controlled at 17%. The temperature of the molten steel at the end of the converter smelting process is controlled at 1650℃, and the P content in the molten steel is 0.006%.

[0188] In the LF refining process, a slag surface deoxidizer is used for deoxidation. Argon is blown into the ladle throughout the refining process. When deoxidizing and forming slag, the flow rate of the bottom-blown argon is controlled at 800 NL / min and the stirring time is 5 min. When adding alloys, the flow rate of the bottom-blown argon is controlled at 800 NL / min and the stirring time is 3 min. At other times, the flow rate of the bottom-blown argon is controlled at 400 NL / min.

[0189] In the RH vacuum refining step, the vacuum level is 2 mbar, the degassing time is 25 min, and the net circulation processing time is 10 min.

[0190] <Continuous casting process> The molten steel obtained from the steelmaking process is sent to the continuous casting machine. Before pouring, the ladle is left to stand on the rotary table for 18 minutes. Then, it is continuously cast into a continuous casting billet with a thickness of 320 mm. The superheat of the molten steel during casting is 20°C.

[0191] Electromagnetic stirring is implemented in the crystallizer, the secondary cooling zone, and the solidification end. The electromagnetic stirring frequency in the crystallizer is 8Hz and the current is 400A; the electromagnetic stirring frequency in the secondary cooling zone is 6Hz and the current is 800A; and the electromagnetic stirring frequency at the solidification end is 4Hz and the current is 600A.

[0192] <Heating Process> The continuously cast billets, after being stacked and slowly cooled, are sent into a heating furnace for heating.

[0193] The heating process includes, in sequence, a first preheating section, a second preheating section, a first heating section, a second heating section, and a soaking section. The temperature of the first preheating section is 900℃, and the residence time is 50 min; the temperature of the second preheating section is 1050℃, and the residence time is 50 min; the temperature of the first heating section is 1100℃, and the residence time is 50 min; the temperature of the second heating section is 1160℃, and the residence time is 80 min; the temperature of the soaking section is 1150℃, and the residence time is 100 min.

[0194] <Controlling the rolling process> The continuously cast billet after the heating process is fed into a rolling mill for one-stage rolling to produce a steel plate with a thickness of 120 mm. The initial rolling temperature is 1100℃, and the final rolling temperature is 1060℃. The reductions in each pass are 20 mm, 26 mm, 25 mm, 24 mm, 23 mm, 22 mm, 20 mm, 17 mm, 13 mm, and 10 mm, respectively.

[0195] Cooling Process The rolled steel plate is air-cooled to room temperature.

[0196] <Quenching Process> The steel plate after the controlled cooling process is then subjected to quenching treatment.

[0197] The quenching process includes sequential heating and cooling steps.

[0198] The heating temperature for the heating step is 910℃, and the holding time is 240min.

[0199] The cooling process employs high-pressure water cooling, with a water pressure of 1.0 MPa and a flow rate of 10,000 m³ / h. 3 / h, the angle between the water flow and the steel plate surface is 45°, which rapidly cools the surface of the steel plate to the final cooling temperature of 450℃.

[0200] <Tempering Process> Tempering is performed on steel plates that have undergone quenching.

[0201] The tempering heating temperature is 550℃, the furnace time is 120 minutes, and the steel plate exit temperature is 500℃.

[0202] The finished steel plates undergo performance testing, as detailed below: (1) The surface structure of the steel plate is tempered martensite, and the core structure is a multiphase structure of ferrite + pearlite + bainite.

[0203] (2) According to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", the steel was sampled and its mechanical properties were tested. The test results are as follows: At room temperature, the yield strength of the surface layer of the steel plate is 800 MPa, and the yield strength of the core is 660 MPa. At room temperature, the tensile strength of the surface layer of the steel plate is 940 MPa, and the tensile strength of the core layer is 815 MPa. At room temperature, the yield strength ratio of the surface layer of the steel plate is 0.85, and the yield strength ratio of the core layer is 0.81. At room temperature, the elongation of the surface layer of the steel plate is 15%, and the elongation of the core layer is 22%.

[0204] (3) According to GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials", the steel was sampled and tested for low-temperature performance. The test results are as follows: The impact energy (KV2) at -60℃ on the surface of the steel plate is 86J, and the impact energy (KV2) at -60℃ on the core is 18J.

[0205] (4) High-temperature tensile tests were conducted according to GB / T 228.2-2015 "Metallic materials - Tensile testing - Part 2: High-temperature test method", and the test results are as follows: The steel plate surface layer was tested after being held at 600℃ for 3 hours to determine the yield strength R at 600℃. p0.2 The yield strength R of the core at 600℃ is 490 MPa. p0.2 It is 410 MPa.

[0206] (5) In accordance with GB / T 15248-2008 "Metallic Materials - Axial Constant Amplitude Low Cyclic Fatigue Test Method", axial constant amplitude low cyclic fatigue tests were conducted on metallic materials, and the test results are as follows: At a strain frequency of 1-3 Hz and a strain amplitude of 0.8%, the steel plate fractured within 600 cycles. In other words, the steel plate has a poor strain fatigue life.

[0207] (6) The flatness of the steel plate was tested according to GB / T 709-2019 "Dimensions, shape, weight and permissible deviations of hot-rolled steel plates and strips".

[0208] The unevenness of the steel plate is 8 mm / m, and the flatness of the steel plate is poor.

Claims

1. A high-strength, extra-thick steel plate, characterized in that, The chemical composition of the steel plate, by mass percentage, includes: C 0.055~0.085%, Si 0.10~0.25%, Mn 1.5~1.7%, Cr 0.4~0.6%, Ni 0.3~0.5%, Mo 0.4~0.6%, Cu 0.25~0.35%, Nb 0.03~0.04%, V 0.03~0.04%, Ti 0.01~0.02%, Al 0.02~0.05%, B 0.0010~0.0016%, S≤0.002%, P≤0.012%, N≤0.0045%, H≤0.00015%, with the remainder being iron and unavoidable impurities, and satisfying Ni≥Cu; The thickness d of the steel plate is 80~120mm; at room temperature, the yield strength of the steel plate is 740~800MPa, the tensile strength is 930~1000MPa, the yield strength ratio is ≤0.82, the elongation is ≥22%, and the difference in tensile strength between the surface layer and the core of the steel plate is ≤45MPa.

2. The high-strength extra-thick steel plate according to claim 1, characterized in that, The microstructure of the steel plate is a multiphase microstructure consisting of ferrite, lath bainite, and tempered martensite; wherein the volume percentage of ferrite is 10-25%, the volume percentage of lath bainite is 70-80%, and the volume percentage of tempered martensite is 5-10%.

3. The high-strength extra-thick steel plate according to claim 2, characterized in that, The average grain size of ferrite is ≤5μm, the average width of bainite laths is ≤0.5μm, and the average width of martensite laths is ≤0.2μm.

4. The high-strength extra-thick steel plate according to claim 1, characterized in that, The -60℃ impact energy KV2 of the surface layer and the core of the steel sheet is all ≥200J; the yield strength R p0.2 ≥495MPa at 600℃; the steel sheet can withstand at least 1600 cycles without breaking at a strain frequency of 1~3Hz and a strain amplitude of 0.8%.

5. A method for producing high-strength, extra-thick steel plates as described in any one of claims 1 to 4, characterized in that, The production method includes the sequential processes of steelmaking, continuous casting, stacking and slow cooling, heating, controlled rolling, controlled cooling, quenching, and tempering. The heating process includes, in sequence, a first preheating section, a second preheating section, a first heating section, a second heating section, and a homogenizing section; the temperature of the first preheating section is ≤900℃, and the residence time is ≤50min; the temperature of the second preheating section is 900~1050℃, and the residence time is ≤50min; the temperature of the first heating section is 1000~1100℃, and the residence time is ≤50min; the temperature of the second heating section is 1150~1160℃, and the residence time is ≥80min; the temperature of the homogenizing section is 1130~1150℃, and the residence time is ≥100min. The controlled rolling process uses one stage rolling, the rough rolling temperature is T nr + 35°C ~ T nr + 65°C, the finish rolling temperature is T nr + 5°C ~ T nr + 25°C; In the controlled cooling step, the steel sheet obtained after rolling is water-cooled at a cooling rate of 1 to 4°C / s and a final cooling temperature of B s + 50°C to B s + 80°C, and then air-cooled to room temperature; The quenching process includes sequential heating, first-stage cooling, and second-stage cooling steps, with the heating temperature being A. C3 -65℃~A C3 -15℃, holding time is 1.8~2.0 min / mm; in the first stage cooling step, the cooling rate of the steel plate surface layer is ≥35℃ / s, the cooling rate of the steel plate core is ≥5℃ / s, and the final cooling temperature of the steel plate surface layer is B. s +5℃~B s +25℃; In the two-stage cooling process, the cooling rate of the steel plate surface is ≤15℃ / s, the cooling rate of the steel plate core is ≥10℃ / s, and the final cooling temperature of the steel plate surface is ≤300℃. Among them, the austenite recrystallization temperature T nr =887+464C-357Si+(6445Nb-644 )+(732V-230 +890Ti +363Al; the starting temperature B for the bainite transformation of austenite. s =830-270C-90Mn-70Cr-37Ni-83Mo; Austenitization completion temperature A C3 =910-320C-14Ni-12Cu-10Mn+5Cr+14Mo+5V+18Si; The element symbols in the formula represent the mass percentage of the corresponding element in the steel plate.

6. The method for producing high-strength extra-thick steel plates according to claim 5, characterized in that, In the tempering process, the tempering heating temperature is 540~560℃, the furnace time is 0.8~1.0min / mm, and the furnace exit temperature of the steel plate is 490~510℃.

7. The method for producing high-strength extra-thick steel plates according to claim 5, characterized in that, In the controlled rolling process, the reduction per pass is 28~45mm.

8. The method for producing high-strength extra-thick steel plates according to claim 5, characterized in that, In the quenching process, the first stage of cooling is water cooling, using high-pressure water with a pressure of 0.8~1.0MPa and a flow rate of 8500~10000m³. 3 The water flow rate is 40-50° between the water flow and the steel plate surface; the second-stage cooling uses a laminar flow cooling water curtain with a water pressure of 0.2-0.4 MPa and a flow rate of 5000-6500 m³ / h. 3 / h, the angle between the water flow and the steel plate surface is 85~95°.

9. The method for producing high-strength extra-thick steel plates according to claim 5, characterized in that, The stacking time for the slow cooling process is ≥48 hours.

10. The method for producing high-strength extra-thick steel plates according to claim 5, characterized in that, In the continuous casting process, electromagnetic stirring is performed in the crystallizer, the secondary cooling zone, and the solidification end, and the electromagnetic stirring frequency of the crystallizer is greater than that of the secondary cooling zone, which is greater than that of the solidification end. The electromagnetic stirring current of the crystallizer is less than that of the solidification end, which is less than that of the secondary cooling zone.

11. The method for producing high-strength extra-thick steel plates according to claim 10, characterized in that, In the continuous casting process, the electromagnetic stirring frequency of the crystallizer is 5~8Hz and the electromagnetic stirring current is 200~400A; the electromagnetic stirring frequency of the secondary cooling zone is 3~6Hz and the electromagnetic stirring current is 400~800A; the electromagnetic stirring frequency of the solidification end is 2~4Hz and the electromagnetic stirring current is 300~600A.

12. The method for producing high-strength extra-thick steel plates according to claim 10, characterized in that, In the continuous casting process, the superheat of the molten steel during casting is 15~20℃.

13. The method for producing high-strength extra-thick steel plates according to claim 5, characterized in that, In the continuous casting process, the ladle is left to stand on the rotary table for 15-18 minutes before casting begins.

14. The method for producing high-strength extra-thick steel plates according to claim 5, characterized in that, The steelmaking process includes the following steps in sequence: pre-desulfurization treatment, converter smelting, LF refining, and RH vacuum refining. In the RH vacuum refining step, the vacuum degree is ≤2mbar, the degassing time is 20~25min, and the net circulation processing time is 8~10min.

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