X65 pipeline steel plate for high plasticity marine drilling riser and a method for manufacturing the same

Abstract

This invention discloses a high-plasticity X65 pipeline steel plate for marine drilling risers and its preparation method. By improving key processes such as composition design, billet preparation and heating, controlled rolling and cooling, and post-rolling straightening, an X65 grade pipeline steel for risers with high plasticity, high cleanliness, high dimensional accuracy, and excellent low-temperature fracture toughness has been developed. The composition of elements such as C, Mn, Si, and Mo is precisely controlled to avoid the cost increase and performance degradation caused by high alloying. At the same time, dynamic light pressing, controlled rolling and cooling, and straightening technologies are integrated into the manufacturing process to comprehensively improve the overall performance and dimensional accuracy of the steel plate. The steel plate has extremely low content of harmful elements, and all kinds of inclusions are less than grade 1.0. It also exhibits excellent mechanical properties, which has profound strategic significance for improving the exploration and development capabilities of marine oil and gas resources.

Description

Technical Field

[0001] This invention relates to the production and manufacturing technology of pipeline steel plates for marine drilling risers, and more specifically, to a high-plasticity X65 pipeline steel plate for marine drilling risers and its preparation method. Background Technology

[0002] Against the backdrop of continuously growing global energy demand, fossil fuels still dominate. However, constrained by the increasing depletion of onshore oil and gas resources, the exploration and development of offshore oil and gas resources has become a strategic focus of international attention. As a key piece of equipment on offshore drilling platforms, the offshore drilling riser plays a crucial role in isolating seawater, supporting control pipelines, and transporting drilling fluids. The metallurgical quality and service performance of its material directly affect the safety and reliability of the entire offshore drilling system.

[0003] As marine resource development moves from shallow to deep seas, the service environment for risers is becoming increasingly harsh, requiring them to withstand multiple challenges such as seawater corrosion, high pressure, low temperature, wave surge impact, and drilling sediment erosion. This places extremely stringent technical requirements on the steel plates used to manufacture risers: they must not only possess excellent plasticity and low-temperature toughness to withstand the high pressure and extreme low temperature environment of the deep sea, but also excellent fatigue resistance. At the same time, the metallurgical quality of the steel plate is a key factor affecting the performance of the riser, and the content and distribution of non-metallic inclusions and banded structures must be strictly controlled.

[0004] Patent CN119592871A discloses an X65MOS-grade submarine acid-resistant pipeline steel plate and its manufacturing method. Regarding composition control precision, the wide range of carbon content leads to significant performance fluctuations, making it difficult to precisely control the strength-toughness balance. The low molybdenum content (0.02%–0.09%) may result in insufficient fracture toughness, which is detrimental to resisting crack propagation risks in deep-sea environments. In terms of process parameters, the patent does not detail the specific parameters during controlled rolling and cooling, affecting the control of the steel microstructure and making it difficult to obtain an ideal microstructure, thus impacting the overall performance of the steel. Regarding performance requirements, the yield strength ratio is ≤0.92; a high yield strength ratio makes the steel plate prone to cracking and increased springback during pipe manufacturing, affecting pipe quality and dimensional accuracy. The DWTT test at -15℃ shows an SA% ≥85%, indicating relatively weak tear resistance. Under impact in deep-sea environments, it is prone to tearing failure, failing to meet the high tear resistance requirements of deep-sea risers. Patent application CN202010944524.1 discloses an X80 pipeline steel plate and its manufacturing method for deep-sea drilling risers that combines HIC resistance and erosion resistance. Although the steel grade is X80, the relatively broad composition control in this patent may lead to poor performance stability of the steel, making it unable to accurately meet the specific needs of deep-sea risers. In terms of manufacturing process, the patent lacks details on the pre-rolling billet preparation process. The quality of the billet is a key factor affecting the final steel plate performance; the absence of this billet preparation process makes it difficult to ensure high cleanliness and homogenization of the billet, thus affecting the overall performance of the steel plate. Regarding mechanical properties, the elongation after fracture is only 23%–25%. This low elongation indicates insufficient plastic deformation capacity of the steel plate. Under external forces in the deep-sea environment, it is difficult to absorb energy through plastic deformation, making it prone to brittle fracture and failing to meet the high plasticity requirements of deep-sea risers. Patent CN109055865A discloses a type of steel for risers with excellent corrosion resistance and its manufacturing method. Regarding strength and toughness matching, the patent has a yield strength ratio of ≤0.88 in both the transverse and longitudinal directions. A higher yield strength ratio reduces the plastic reserve of the steel plate. During pipe manufacturing, the steel plate needs to undergo plastic deformation operations such as bending and diameter expansion. A higher yield strength ratio easily leads to cracking of the steel plate during deformation and also makes it difficult to control the dimensional accuracy after pipe manufacturing. The elongation after fracture in both the transverse and longitudinal directions is less than 25%. A low elongation after fracture means that the steel plate has poor plastic deformation capacity. During the service of deep-sea risers, if subjected to external forces such as surge impacts and drilling vibrations, it cannot buffer the external forces through sufficient plastic deformation, easily leading to structural failure. In terms of process, this patent does not optimize the process for the large thickness requirements of deep-sea risers.Regarding performance indicators, this patent does not explicitly specify metallurgical quality indicators such as the level of non-metallic inclusions and the grade of banded structure. Metallurgical quality indicators have a significant impact on the fatigue performance and corrosion resistance of steel. Non-metallic inclusions and banded structures can become stress concentration sources, easily initiating and propagating cracks under complex deep-sea conditions, reducing the service life and safety of the riser. Patent CN103834874A discloses a thick-walled X65-70 subsea pipeline steel with high DWTT performance and its manufacturing method. In terms of composition control, its C content range is relatively wide, and its Mn content is relatively high. An excessively wide C content range can easily cause fluctuations in the strength and toughness of the steel, while an excessively high Mn content will increase the tendency of the steel to form banded structures and reduce the uniformity of toughness. In terms of process design, the slab heating only specifies the furnace exit temperature, which is difficult to guarantee the uniform heating of thick-walled slabs and easily leads to internal structural segregation. In terms of performance indicators, its SA% ≥ 85% in the DWTT test at -15℃ is insufficient in tear resistance; the HIC performance and CTOD indicators are not clearly specified. These indicators are the core guarantee for deep-sea risers to resist high pressure, corrosion and impact loads. The lack of indicators in this patent makes it unable to fully adapt to the application scenarios of deep-sea risers.

[0005] Therefore, there are still significant shortcomings in the research and technology related to pipeline steel for thick-walled deep-sea risers with high safety requirements, especially deep-sea riser steel plates that combine high cleanliness, high dimensional accuracy, and high toughness and plasticity. Summary of the Invention

[0006] The purpose of this invention is to overcome the aforementioned deficiencies in the existing technology and provide a high-ductility X65 pipeline steel plate for marine drilling risers that meets the stringent service requirements of deep-sea risers, along with its preparation method. Addressing the extremely harsh service environment of deep-sea risers, this invention improves key processes such as composition design, billet preparation and heating, controlled rolling and cooling, and post-rolling straightening. This results in an X65 grade pipeline steel for risers that combines high ductility, high cleanliness, high dimensional accuracy, and excellent low-temperature fracture toughness. The steel plate has extremely low levels of harmful elements, with all inclusions below grade 1.0, and exhibits superior mechanical properties: the longitudinal elongation after fracture is 57%–68%. The steel plate exhibits the following properties: uniform elongation ≥12%, impact energy (CVN) ≥250J at -20℃, drop hammer tear test (DWTT) shear area (SA%) ≥90% at -15℃, CTOD characteristic value (δm) ≥0.3mm at -10℃, and overall straightness control of the entire length after pipe fabrication ≤7mm / 12m. This invention precisely controls the content of elements such as C, Mn, Si, and Mo in its composition, avoiding the cost increases and performance degradation caused by high alloying. Furthermore, it integrates dynamic light pressing, controlled rolling and cooling, and straightening technologies during manufacturing, comprehensively improving the overall performance and dimensional accuracy of the steel plate. This has profound strategic significance for enhancing the exploration and development capabilities of marine oil and gas resources.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows: An X65 pipeline steel plate for high-plasticity marine drilling risers comprises the following components by mass percentage: C: 0.038%~0.045%, Mn: 1.30%~1.40%, Si: 0.15%~0.20%, Nb: 0.015%~0.035%, Ti: 0.012%~0.015%, S≤0.001%, P≤0.006%, Alt≤0.06%, N≤0.010%, Mo: 0.10%~0.15%, Ni≤0.12%, Cr: 0.06%~0.12%, Al: 0.025%~0.035%, B≤0.001%, with the balance being Fe and unavoidable impurity elements.

[0008] The present invention also discloses a method for preparing X65 pipeline steel plate for high plasticity marine drilling risers as described above, comprising: hot metal desulfurization pretreatment, BOF top and bottom blowing converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, heating, rolling, cooling and straightening. In the continuous casting process, a covering agent is added during tundish pouring for protective purposes; the superheat is controlled at 25~35℃; the casting speed is 0.80~1.00m / min; dynamic light reduction is adopted for the final fan-shaped section of the continuous casting; and the casting billet is slowly cooled in the pit. In the rolling process, the heated billet is descaled after exiting the furnace and then rolled. The rolling process adopts a two-stage controlled rolling process: the roughing stage is rolled in the austenite recrystallization zone; the finishing stage is rolled in the austenite non-recrystallization zone. The control parameters for the roughing stage are: the roughing start temperature is 1125~1155℃, the roughing finish temperature is 975~1030℃, the temperature after roughing cross rolling is controlled at 1115~1145℃, and the longitudinal rolling start temperature is controlled in the range of 1095~1125℃; the longitudinal rolling single-pass reduction rate is >15%, and the roll speed is ≤1.5m / s; the cumulative deformation of the last two passes of roughing is ≥38%; and the intermediate billet thickness is set at 110~150mm. The control parameters for the finishing stage are: the finishing start temperature is 810~850℃, and the finishing finish temperature is 790~810℃. In the cooling process, the rolled steel plate is air-cooled and then accelerated water-cooled. The ACC cooling start temperature is 740-750℃, the reddening temperature is 520-550℃, the water-cooling time is 16-24s, and the water-cooling rate is 12-15℃ / s. The head and tail shielding process is adopted, and the shielding length is 2-3m. During the straightening process, the straightening speed is 0.6~0.8m / s, and the temperature of the steel plate entering the temperature straightening machine is 80~140℃.

[0009] Implementing the embodiments of the present invention will have the following beneficial effects: (1) Excellent metallurgical quality and microstructure: This invention effectively controls non-metallic inclusions and banded structures in steel plates through strict component design and refining processes, ensuring the high cleanliness of the steel plates. The levels of non-metallic inclusions (Class A, B, C, and D) in pipeline steel plates are all ≤1.0, segregation in continuously cast billets does not exceed Class C 1.0, and the banded structure of the steel plates is ≤1.

[0010] Reasonable billet preparation and heating processes further improve the metallurgical quality of steel plates, making the internal structure of steel plates uniform, reducing quality defects, and meeting the high-quality requirements of deep-sea risers for steel plates.

[0011] Through unique smelting, heating, rolling, and cooling processes, this invention achieves an ideal microstructure comprising acicular ferrite, polygonal ferrite, and MA components. The volume percentage of polygonal ferrite is 50%–60%, the grain diameter of acicular ferrite is controlled below 10 μm, the grain size of polygonal ferrite is below 7 μm, and the grain size level is ≥9. Fine carbonitride precipitates below 10 nm account for ≥25% of all carbonitride precipitates. This microstructure significantly improves the strength, toughness, corrosion resistance, and fatigue resistance of the steel plate.

[0012] (2) Excellent combination of strength and toughness with corrosion resistance: This invention employs a low-C, low-Mn design, combined with the addition of appropriate amounts of Nb, Ti, Ni, Mo, Cr, and other elements, effectively controlling the corrosion sensitivity of the steel plate while compensating for strength loss. Through optimized composition design and controlled rolling and cooling processes, a fine and uniform microstructure is promoted, improving the toughness of the steel plate. Especially at low temperatures, it maintains high impact toughness and fracture toughness, effectively resisting the effects of extreme low temperatures in deep-sea environments on the performance of the riser. The excellent corrosion resistance brought by the multiphase microstructure: the reasonable combination of soft and hard phases in the steel plate not only improves strength but also endows it with excellent corrosion resistance. Its HIC corrosion resistance meets the requirements of 0% CSR, 0% CLR, and 0% CTR after 96 hours of corrosion in NACE standard A solution.

[0013] (3) High-precision dimensional control: This invention employs advanced rolling processes and straightening techniques to achieve high-precision control of steel plate dimensions. Whether it's the thickness, width, or flatness of the steel plate, it meets the stringent dimensional accuracy requirements of deep-sea risers, ensuring the stability and reliability of the risers in the deep-sea environment. The appropriate intermediate billet thickness combined with a low-temperature finishing rolling process, along with short-term air cooling after rolling and low-temperature final rolling, effectively improves the dimensional accuracy and performance uniformity of the steel plate.

[0014] This ensures that the steel plate maintains excellent comprehensive mechanical properties and superior metallurgical quality even with a thickness of 22.4~36.5mm. Specifically: at room temperature tension, the transverse yield strength is 450~570MPa, the tensile strength is 535~755MPa, and the yield-to-tensile ratio is ≤0.85; the longitudinal yield strength is ≥427MPa, the tensile strength is ≥508MPa, and the yield-to-tensile ratio is ≤0.85; the longitudinal elongation after fracture is 57%~68%, and the uniform elongation is ≥12%; the impact energy (CVN) at -20℃ is ≥250J; the drop hammer tear test (DWTT) shear area (SA%) at -15℃ is ≥90%; and the Vickers hardness (HV10) at the quarter-thickness position along the thickness direction of the steel plate is ≤240 at 1.5mm from the surface, 1 / 4 of the thickness, and 1 / 2 of the thickness.

[0015] HIC test: Crack length ratio CLR% = 0, crack width ratio CTR% = 0, crack sensitivity ratio CSR% = 0; Crack tip opening displacement test: CTOD characteristic value δm ≥ 0.3 mm at -10℃. These performance indicators far exceed existing technologies and standards, meeting the special working conditions requirements of marine drilling riser systems. Attached Figure Description

[0016] Figure 1 This is a microstructure diagram of the plate at 1 / 2 thickness in Embodiment 1 of the present invention. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0018] This invention discloses an X65 pipeline steel plate for high-plasticity marine drilling risers, comprising the following components by mass percentage: C: 0.038%~0.045%, Mn: 1.30%~1.40%, Si: 0.15%~0.20%, Nb: 0.015%~0.035%, Ti: 0.012%~0.015%, S≤0.001%, P≤0.006%, Alt≤0.06%, N≤0.010%, Mo: 0.10%~0.15%, Ni≤0.12%, Cr: 0.06%~0.12%, Al: 0.025%~0.035%, B≤0.001%, with the balance being Fe and unavoidable impurity elements.

[0019] In one specific embodiment, the mass percentage ratio of C and Nb in the pipeline steel plate conforms to the following: C:Nb=1~1.5.

[0020] In one specific embodiment, the mass percentage ratio of Cr and Mo in the pipeline steel plate conforms to the following: Cr:Mo = 0.4~1.2.

[0021] The specific reasons for limiting the amounts of each chemical element in steel plates are detailed below: C: As the most basic and cost-effective strengthening element in steel, its content control has a significant impact on performance. Increasing the carbon content can enhance the matrix strength through a dual mechanism of solid solution strengthening and precipitation strengthening. However, if the carbon content is too low, the precipitation strengthening effect of microalloying elements such as niobium (Nb) cannot be fully released, and the erosion resistance will not meet the stringent requirements of the deep-sea environment. If the carbon content is too high, it will lead to a significant deterioration in plasticity, toughness, and weldability, while also exacerbating the tendency of compositional segregation in medium and thick plates, easily causing banded structures in the core, thereby weakening resistance to hydrogen embrittlement and fatigue. Therefore, this technical solution precisely controls the carbon content within a narrow range of 0.038% to 0.045%, which ensures both the microalloying strengthening effect and corrosion resistance, while avoiding the performance degradation risk caused by high carbon content.

[0022] Mn: As an economical and efficient solid solution strengthening element, Mn can effectively compensate for strength loss under low-carbon design. Its austenitic stabilizing properties can expand the phase region, lower the Ar3 critical temperature, and promote the formation of fine-grained ferrite, thereby improving toughness and lowering the ductile-brittle transition temperature. Mn dissolved in ferrite can directly strengthen the matrix, enhancing yield and tensile strength through the synergistic effect of soft and hard phases. However, the content must be strictly controlled: too high a content will exacerbate center segregation in continuously cast billets, forming obvious bainite segregation bands, leading to a decrease in HIC resistance and deterioration of tube formability; too low a content will not fully utilize the solid solution strengthening and phase transformation refining effects. Therefore, this scheme precisely limits the Mn content to the range of 1.30% to 1.40%, achieving a balance between strengthening effect and process stability.

[0023] Si: Adding Si helps improve the cleanliness of steel. Si mainly exists in steel in the form of solid solution. Too low a content will not fully exert its deoxidizing and strengthening effects, while too high a content will lead to a decrease in plasticity / toughness, deterioration of weldability, and increased segregation. The Si content in this invention is designed to be controlled between 0.15% and 0.20%.

[0024] Niobium (Nb) is the most important microalloying element in pipeline steel. During the billet heating stage, it effectively inhibits abnormal grain growth by pinning austenite grain boundaries with undissolved particles. During hot rolling, deformation-induced NbC precipitates can hinder austenite recrystallization and promote the transformation of deformed austenite microstructure into fine ferrite grains. Appropriate amounts of Nb enhance strength through a dual mechanism of precipitation strengthening and grain refinement, while maintaining a good balance between plasticity and toughness. However, excessive Nb leads to solid solubility saturation, forming blocky elemental niobium phases that deteriorate plasticity and toughness. Furthermore, excessive inhibition of austenite recrystallization is detrimental to the uniformity of the microstructure in both the longitudinal and transverse directions. Therefore, to meet the stringent requirements for strength and toughness of marine drilling risers, it is necessary to optimize the efficiency of Nb by combining the carbon content in the steel. When the carbon / niobium ratio is too low, the NbC precipitation drive is insufficient, and the strengthening effect is weakened. When the carbon / niobium ratio is too high, it will lead to NbC coarsening (>50nm). This solution precisely controls the Nb content in the range of 0.015% to 0.035% and the carbon / niobium ratio is 1 to 1.5, so as to avoid segregation and the risk of blocky elemental niobium while ensuring the balance of strength and toughness.

[0025] Titanium (Ti), as a highly efficient nitrogen-fixing element, forms fine, high-temperature stable TiN precipitates in the billet. These nanoscale TiN particles continuously pin grain boundaries during subsequent austenitization, effectively inhibiting abnormal austenite grain growth and simultaneously improving the toughness of the steel matrix and the low-temperature toughness of the weld heat-affected zone (HAZ). However, when the Ti content is too high, coarse TiN particles (size > 50 nm) are easily formed, leading to stress concentration and crack initiation, significantly worsening the ductility-toughness match of the steel. Therefore, this scheme strictly controls the Ti content within the range of 0.012% to 0.015%, which fully utilizes its nitrogen-fixing and grain-refining effects while avoiding the risk of coarsening, achieving synergistic optimization of the strength-toughness match and weldability of the riser.

[0026] Mo: Mo significantly promotes the formation of acicular ferrite structure and improves the fracture toughness of materials by inhibiting the formation of the ferrite phase that precipitates first during the γ→α phase transformation. While playing a role in phase transformation control, it also acts as a hardenability strengthening element to enhance the strength of steel. Studies have shown that Mo, in synergy with microalloying elements such as Nb, can refine grains and form nanoscale precipitates, resulting in an interleaved distribution of acicular ferrite grain boundaries, effectively preventing crack propagation. In terms of economics, to balance cost and performance, the Mo content is strictly controlled within the range of 0.10% to 0.15%. This avoids both the cost surge caused by excessively high addition levels and the potential for insufficient fracture toughness in steel due to low Mo content, which would be detrimental to resisting crack propagation risks in deep-sea environments. Simultaneously, it ensures that the carbon equivalent and welding cold cracking sensitivity index of the material are within safe ranges.

[0027] Cr: The addition of Cr can improve the hardenability of steel and is beneficial for obtaining moderate hardness. In this invention, the Cr content is designed to be controlled between 0.06% and 0.12%. At the same time, when the Cr and Mo components are added at a ratio of Cr:Mo = 0.4 to 1.2, the effect on controlling phase transformation in the microstructure is even better.

[0028] Ni: Ni is beneficial to low-temperature toughness and hardenability of thick plates. In this invention, the Ni content is controlled within 0.12%.

[0029] Al: Al has a significant impact on the cleanliness of steel. As a deoxidizer, insufficient Al content leads to inadequate deoxidation, while excessive Al content promotes the formation of large-sized oxide inclusions, affecting toughness and fatigue resistance. Therefore, the Al content in this invention is designed to be between 0.025% and 0.035%.

[0030] P and S: Due to limitations in smelting costs and production technology, the upper limits for P and S content are set at 0.006% and 0.001%, respectively. Controlling the inclusion morphology of sulfides through ultra-low S and Ca treatment can improve the low-temperature toughness of steel plates.

[0031] B: Considering the adverse effect of boron addition on the toughness of steel plates, the residual boron content should not exceed 0.001%.

[0032] In one specific embodiment, the thickness of the pipeline steel plate is 22.4~36.5mm.

[0033] In one specific embodiment, the non-metallic inclusions of types A, B, C and D in the pipeline steel plate are all ≤1.0 grade; the banded structure is ≤1 grade.

[0034] In one specific embodiment, the microstructure of the pipeline steel plate consists of acicular ferrite, polygonal ferrite, and MA component; wherein, the volume percentage of polygonal ferrite is 50% to 60%, the volume percentage of MA component is 0.5% to 1%, the grain diameter of acicular ferrite is ≤10μm, the grain size of polygonal ferrite is ≤7μm, the grain size level is ≥9, and the fine carbonitride precipitation below 10nm accounts for ≥25% of the total number of carbonitride precipitation.

[0035] In one specific embodiment, the resistance to HIC corrosion meets the following requirements after 96 hours of corrosion with NACE standard A solution: CSR=0%, CLR=0%, CTR=0%.

[0036] In one specific embodiment, at room temperature, the transverse yield strength is 450–570 MPa, the tensile strength is 535–755 MPa, and the yield-to-tensile ratio is ≤0.85; the longitudinal yield strength is ≥427 MPa, the tensile strength is ≥508 MPa, and the yield-to-tensile ratio is ≤0.85; the longitudinal elongation after fracture of the steel plate is 57%–68%, and the uniform elongation is ≥12%.

[0037] In one specific embodiment, the impact energy (CVN) at -20℃ is ≥250J; the drop hammer tear test (DWTT) shear area (SA%) at -15℃ is ≥90%; the Vickers hardness (HV10) at the quarter-thickness position along the thickness direction of the steel plate (approximately 1.5mm from the surface), 1 / 4 of the thickness, and 1 / 2 of the thickness are all ≤240; the CTOD characteristic value (δm) at -10℃ is ≥0.3mm; and the straightness of the entire length of the pipe after manufacturing (12m) is ≤7mm. Straightness refers to the degree of straightness of the steel pipe axis and is an important indicator for measuring the quality of the steel pipe. The straightness of the entire length of the pipe after manufacturing (12m) ≤7mm mentioned in this invention means that the straightness of the entire 12m finished pipe is less than 7mm.

[0038] The present invention also discloses a method for preparing X65 pipeline steel plate for high plasticity marine drilling risers as described above, comprising: hot metal desulfurization pretreatment, BOF top and bottom blowing converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, heating, rolling, cooling and straightening.

[0039] S1. In the desulfurization pretreatment of molten iron, a desulfurizing agent is injected into the molten iron for deep desulfurization, and the sulfur content in the molten iron is controlled to be ≤0.002%. The desulfurizing agent is CaO and Mg, wherein the amount of CaO added is 6-10 kg / t steel and the amount of Mg added is 0.3-0.8 kg / t steel.

[0040] Specifically, this invention selects CaO and Mg as desulfurization powders because of their high desulfurization efficiency and fast reaction speed, which can effectively reduce the sulfur content in molten iron. Given the strict requirements for sulfur content in steel plates used in deep-sea risers, desulfurization pretreatment of molten iron can ensure that the steel plates maintain good toughness and plasticity during subsequent smelting and processing.

[0041] In S2 and BOF top-and-bottom blown converter smelting, desulfurized molten iron is added to the converter, and oxygen is blown into the converter to reduce carbon and smelt it into molten steel. The oxygen flow rate is 18,000–26,000 m³ / h. 3 The oxygen blowing time is 18-20 minutes per hour. Slag-forming materials are added during the converter smelting process. When the carbon content in the molten steel in the converter is ≤0.046%, the oxygen content reaches 0.05%-0.09%, the phosphorus content is ≤0.01%, and the temperature of the molten steel in the converter reaches 1650-1690℃, the steel is tapped into the ladle using a slag-blocking tapping method. The slag thickness is ≤120mm. During tapping, 2.5-4.0 kg / t of aluminum-iron alloy is added for deoxidation. The slag-forming materials are lime and lightly calcined dolomite, with lime added at 30-50 kg / t of steel and lightly calcined dolomite added at 8-15 kg / t of steel. Specifically, appropriate oxygen blowing time ensures that the carbon content in the molten steel is reduced to the required level while avoiding overheating or undercooling. Controlling the oxygen and phosphorus content also helps improve the purity of the steel plate and the crack sensitivity of the billet.

[0042] In the S3 and LF refining processes, molten steel from the ladle is hoisted into the LF furnace for refining. Electrodes are used to heat the molten steel while slagging agents and strong deoxidizers are simultaneously added to the ladle. Argon is blown from the bottom of the ladle to agitate the molten steel, homogenizing its composition and temperature. A silicon alloy is then added, and the steel temperature is controlled at 1625–1640°C before proceeding to the vacuum refining process. The LF refining time is 40–50 minutes. The slagging agents are lime and pre-melted refining slag, with lime added at 3–8 kg / t of steel and pre-melted refining slag at 2–6 kg / t of steel. The strong deoxidizers are aluminum granules and calcium carbide, with aluminum granules added at 0.5–1.5 kg / t of steel and calcium carbide at 1–3 kg / t of steel. The argon blowing time is 8–15 minutes, and the argon flow rate is 80–150 L / min.

[0043] Specifically, electrode heating and bottom-blowing argon stirring in the ladle can homogenize the composition and temperature of the molten steel, remove inclusions and gases, and improve the purity of the steel plate. Appropriate molten steel temperature ensures smooth reactions during refining while preventing component segregation and performance degradation caused by overheating. The selection of 1625–1640℃ as the molten steel temperature for transitioning to RH vacuum refining is based on the refining process's temperature requirements and the temperature adaptability of the subsequent RH vacuum refining process.

[0044] In S4 and RH vacuum refining, an RH vacuum circulation device is used for degassing, with the vacuum degree controlled to ≤0.2 kPa and the processing time to 18-28 min. When the temperature of the molten steel reaches 1610-1630℃, the RH vacuum circulation device is shut off after a net circulation of 12-16 min. The vacuum is broken when the content and temperature of various components in the molten steel meet the target requirements.

[0045] Specifically, hydrogen and nitrogen are harmful elements in steel, reducing its plasticity and toughness and increasing its susceptibility to cracking. RH vacuum refining removes gases such as hydrogen and nitrogen, as well as inclusions, from molten steel through a vacuum circulation degassing device, improving the steel's purity. Appropriate vacuum levels and processing time ensure degassing efficiency while preventing the molten steel from becoming too cold or too hot. The RH vacuum refining parameters are set based on the characteristics of the RH vacuum refining process and product cleanliness requirements, aiming to ensure optimal degassing during the refining process.

[0046] In S5, calcium treatment involves feeding calcium alloy cored wire into molten steel for calcium treatment for 10-20 minutes; the feeding speed is 300-320 m / min, and the weight ratio of Ca / S in the molten steel is controlled at 2.5-4.5; after feeding, the steel is allowed to stand for 15-25 minutes to promote the floating of inclusions; before tapping, the hydrogen content is controlled to be ≤0.0002%, and argon is blown cleanly for 5-20 minutes after feeding.

[0047] Specifically, calcium treatment involves feeding calcium alloy cored wire into molten steel, where it reacts with sulfur to form calcium sulfide, thereby altering the morphology of sulfides and reducing the crack susceptibility of the steel plate. Appropriate calcium treatment parameters can ensure effective control of sulfide morphology, improving the purity and toughness of the steel plate. The selection of a feeding speed of 300–320 m / min for the calcium alloy cored wire and the control of the Ca / S weight ratio in the molten steel are based on the requirements of pipeline steel calcium treatment processes and inclusion size, as well as experimental optimization. This aims to ensure effective control of sulfide morphology while avoiding fluctuations in the molten steel composition.

[0048] S6. During continuous casting, a covering agent is added to protect the casting during tundish pouring; the superheat is controlled at 25~35℃, as too low a temperature affects the flow of molten steel, and too high a temperature easily leads to segregation in the center; the casting speed of the billet is 0.80~1.00m / min; dynamic light reduction is adopted in the fan-shaped section at the end of continuous casting, and the reduction range is automatically adjusted according to the casting speed, which improves the internal quality of the billet and reduces center segregation and porosity; the billet is slowly cooled in the pit.

[0049] Specifically, pouring superheat and casting speed are crucial factors affecting billet quality. Appropriate pouring superheat can prevent premature solidification of molten steel in the crystallizer, which can lead to center segregation and porosity, while also ensuring the surface quality of the billet. Controlling the casting speed affects the solidification rate and internal quality of the billet; an appropriate casting speed can prevent cracks and defects. The parameter settings here aim to ensure billet quality while improving production efficiency.

[0050] In one specific embodiment, during continuous casting, a continuous casting mold billet with a cross-sectional thickness of 300 mm is used for pouring to increase the compression ratio of the billet to the steel plate; the covering agent is a tundish covering agent; the amount of covering agent added is 3~5 kg / t steel; the reduction rate is 0.8~1.2 mm / m, and the reduction amount is 8~15 mm; the billet is slowly cooled in the pit, and after the temperature drops to below 400℃, it is taken out of the furnace and used as a hot-rolled billet. This process refines the grain size of the billet.

[0051] In one specific embodiment, the low-magnification center segregation of the continuously cast billet does not exceed Class C, Grade 1.0.

[0052] S7. During heating, the billet is placed in the heating furnace and heated in multiple stages, passing through the first heating section, the second heating section, the third heating section, and the soaking section before being removed from the furnace. The heating temperature of the first heating section is controlled at 1050~1080℃; the heating temperature of the second heating section is controlled at 1220~1240℃; the heating temperature of the third heating section is controlled at 1215~1235℃; and the temperature of the soaking section is controlled at 1195~1205℃. The heating time of the first and second heating sections is no less than 40 minutes each, and the total heating time of the third and soaking sections is 90~150 minutes. The total time the billet spends in the heating furnace is ≥4 hours. Multi-stage heating of the continuously cast billet is beneficial for improving heating efficiency and uniformity. The heating temperature is designed based on the content of elements such as niobium and carbon to meet the alloy solid solution requirements while preventing excessive austenite grain growth. The temperature control of the soaking section ensures the overall temperature uniformity of the continuously cast billet. The parameters here are set based on the characteristics and requirements of the billet heating process, aiming to ensure that alloying elements are fully dissolved while avoiding excessive growth of austenite grains, thereby improving the overall performance of the steel plate.

[0053] S8. During rolling, the heated billet is descaled after exiting the furnace and then rolled. The rolling process adopts a two-stage controlled rolling process: the roughing stage is rolled in the austenite recrystallization zone; the finishing stage is rolled in the austenite non-recrystallization zone. The control parameters for the roughing stage are: the roughing start temperature is 1125~1155℃, the roughing finish temperature is 975~1030℃, the temperature after roughing cross rolling is controlled at 1115~1145℃, and the longitudinal rolling start temperature is controlled in the range of 1095~1125℃; the longitudinal rolling single-pass reduction rate is >15%, and the roll speed is ≤1.5m / s; the cumulative deformation of the last two passes of roughing is ≥38%; the use of rapid cooling between cross and longitudinal rolling + low-temperature large deformation in longitudinal rolling + low rolling speed can promote the penetration of rolling deformation into the center of the billet thickness, refine the microstructure near the thickness center, promote austenite refinement, and improve the uniformity of the microstructure and properties of the thickness section. The intermediate billet thickness is set at 110~150mm; the control parameters for the finishing rolling stage are: the initial finishing rolling temperature is 810~850℃, and the final finishing rolling temperature is 790~810℃. A suitable intermediate billet thickness combined with a low-temperature finishing rolling process can achieve sufficient accumulation of deformation energy, promote the induced precipitation of fine precipitates, increase ferrite nucleation sites, and improve plasticity and toughness.

[0054] In one specific embodiment, during rolling, the roughing mill and finishing mill are four-high reversible mills with high roll stiffness, which can ensure the dimensional accuracy of the steel used for submarine pipelines.

[0055] S9. During cooling, the rolled steel plate undergoes accelerated water cooling (ACC) after air cooling. The ACC cooling start temperature is 740–750℃, the reddening temperature is 520–550℃, the water cooling time is 16–24 seconds, and the water cooling rate is 12–15℃ / s. A head-and-tail shielding process is used to avoid uneven temperatures at the head and tail ends, thereby ensuring consistent steel plate performance. The shielding length is 2–3 meters. The combination of short-term air cooling after rolling and low-temperature final rolling can effectively control the precipitation of fine particles and the further formation of soft-phase polygonal ferrite, improving toughness, plasticity, and deformation capacity, thus obtaining sufficient tensile strength and a suitable yield strength ratio.

[0056] S10. During straightening, the straightening speed is 0.6~0.8m / s, and the temperature of the steel plate entering the temperature straightening machine is 80~140℃.

[0057] In one specific embodiment, a 9-roll quadruple reversible straightener is used for straightening.

[0058] Specifically, during the straightening process, straightening speed and steel plate temperature are crucial factors affecting the straightening effect. An appropriate straightening speed ensures uniform deformation of the steel plate during straightening, while preventing cracks and defects caused by excessive straightening speed. Controlling the steel plate temperature helps ensure the straightening effect and improves the dimensional accuracy and flatness of the steel plate. The parameter settings here are based on the characteristics and requirements of the straightening process, aiming to ensure the steel plate achieves ideal dimensional accuracy and flatness during straightening, while avoiding cracks and defects caused by improper straightening.

[0059] The following are specific embodiments. Examples 1-8 The preparation method of X65 pipeline steel plate for high plasticity marine drilling riser in this embodiment includes: hot metal desulfurization pretreatment, BOF top and bottom blowing converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, heating, rolling, cooling and straightening.

[0060] S1. In the desulfurization pretreatment of molten iron, a desulfurizing agent is injected into the molten iron for deep desulfurization, and the sulfur content in the molten iron is controlled to be ≤0.002%; the desulfurizing agent is CaO and Mg.

[0061] In the S2 and BOF top-and-bottom combined blowing converter smelting process, desulfurized molten iron is added to the converter, and oxygen is blown into the converter to reduce carbon content and smelt it into molten steel. During the converter smelting process, slag-forming materials are added, namely lime and lightly calcined dolomite. When the carbon content in the molten steel in the converter is ≤0.046%, the oxygen content reaches 0.05%~0.09%, the phosphorus content is ≤0.01%, and the temperature of the molten steel in the converter reaches the target temperature, the steel is tapped into the ladle. Slag-blocking tapping is adopted, and the slag thickness of the molten steel is ≤120mm. During the tapping process, aluminum-iron alloy is added for deoxidation. The slag-forming materials are lime and lightly calcined dolomite.

[0062] In the S3 and LF refining processes, molten steel from the ladle is hoisted into the LF furnace for refining. Electrodes are used to heat the molten steel while slagging agents and strong deoxidizers are added to the ladle simultaneously. Argon is blown from the bottom of the ladle to stir the molten steel to homogenize its composition and temperature. Subsequently, silicon alloys are added to control the temperature of the molten steel, which is then transferred to the vacuum refining process. The slagging agents are lime and pre-melted refining slag; the strong deoxidizers are aluminum granules and calcium carbide.

[0063] In S4 and RH vacuum refining, an RH vacuum circulation device is used for degassing, controlling the vacuum level and processing time. When the molten steel temperature reaches the target temperature, the RH vacuum circulation device is shut off after net circulation. When the content of various components and temperature in the molten steel reach the target molten steel requirements, the vacuum is broken.

[0064] S5. In the calcium treatment process, calcium alloy cored wire is fed into the molten steel for calcium treatment; after the wire feeding is completed, the steel is allowed to stand to promote the floating of inclusions; before tapping, the hydrogen content is controlled to be ≤0.0002%, and argon is blown cleanly after the wire feeding.

[0065] S6. In continuous casting, a 300mm thick continuous casting mold billet is used for pouring. During tundish pouring, a tundish covering agent is added for protection. The superheat and casting speed of the billet are controlled. Dynamic light reduction is used in the fan-shaped section at the end of the continuous casting. The reduction range is automatically adjusted according to the casting speed, which improves the internal quality of the billet and reduces center segregation and porosity. The billet is slowly cooled in the pit and used as a hot-rolled billet. This process refines the grain size of the billet.

[0066] S7. During heating, the billet is placed in the heating furnace and heated in multiple stages, passing through the first heating section, the second heating section, the third heating section and the soaking section before being taken out of the furnace.

[0067] S8. During rolling, the heated billet is descaled after exiting the furnace and then rolled. The rolling process adopts a two-stage controlled rolling process. The roughing stage is rolled in the austenite recrystallization zone; the finishing stage is rolled in the austenite non-recrystallization zone. The roughing mill and the finishing mill are four-high reversible mills.

[0068] S9. During cooling, the rolled steel plate is air-cooled and then accelerated water cooling (ACC) is performed. The head and tail shielding process is used to avoid uneven temperature at the head and tail, thereby ensuring consistent steel plate performance.

[0069] S10. During straightening, a 9-roller quadruple reversible straightening machine is used.

[0070] The microstructure of the pipeline steel plate prepared according to the embodiment consists of acicular ferrite, polygonal ferrite and MA component; wherein, the volume percentage of polygonal ferrite is 50% to 60%, the volume percentage of MA component is 0.5% to 1%, the grain diameter of acicular ferrite is ≤10μm, the grain size of polygonal ferrite is ≤7μm, the grain size level is ≥9, and the fine carbonitride precipitation below 10nm accounts for ≥25% of the total number of carbonitride precipitation.

[0071] Table 1 shows the chemical composition of the steel in the embodiments of the present invention. Table 2 shows the main process parameters for smelting, continuous casting, and heating in the embodiments of the present invention. Table 3 shows the main process parameters for the controlled rolling and controlled cooling stages of the steel in the embodiments of the present invention. Table 4 shows the performance parameters of the steel in the embodiments of the present invention.

[0072] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0073] Note: In all embodiments, Fe and its unavoidable impurity elements are in balance.

[0074] Table 2. Main process parameters for smelting and continuous casting billet heating in the embodiments of the present invention.

[0075] Remark: (1) LF refining time is the total time from the moment the molten steel is hoisted into the LF furnace until the end of refining.

[0076] (2) RH vacuum degassing time is the total time from the start of molten steel entering the RH device to the end of degassing.

[0077] (3) The calcium treatment time is the total time from the start of feeding the calcium alloy cored wire to the end of feeding.

[0078] (4) The superheat of continuous casting refers to the difference between the temperature of molten steel in the tundish and the temperature of the liquidus.

[0079] (5) Continuous casting speed refers to the speed at which the billet is pulled out during the continuous casting process.

[0080] (6) The temperature of the second heating section, the temperature of the third heating section, and the temperature of the soaking section refer to the temperatures reached by the continuous casting billet at different stages in the heating furnace.

[0081] (7) The total heating time refers to the total time from when the continuously cast billet enters the heating furnace to when the heating ends.

[0082] Table 2 details the main process parameters for steel smelting, continuous casting, and heating in the embodiments of the present invention. These parameters are crucial for controlling the chemical composition, microstructure, and final properties of the steel plate. By precisely controlling these process parameters, it is possible to ensure the production of high-quality X65 pipeline steel plates for marine drilling risers that meet technical requirements.

[0083] Table 3. Main process parameters of the controlled rolling and controlled cooling process in the embodiments of the present invention.

[0084] Table 4. Steel performance parameters of the embodiments of the present invention

[0085] Note: DWTT specimens are full-thickness specimens.

[0086] Table 3 details the main performance parameters of the steel plates in the embodiments of the present invention. Specific composition design and optimized manufacturing processes effectively improve the comprehensive performance, metallurgical quality, and dimensional accuracy of the steel plates. For conventional X65 grade pipeline steel plates of the same thickness produced using conventional processes, the various non-metallic inclusions can be controlled at a level of 1.0~2.0, and the banded structure at level 1~3. The steel plates produced using this patent are superior to similar products in both inclusion control and strength-toughness-plasticity matching.

[0087] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A high-plasticity X65 pipeline steel plate for marine drilling risers, characterized in that, Includes the following components by mass percentage: C: 0.038%~0.045%, Mn: 1.30%~1.40%, Si: 0.15%~0.20%, Nb: 0.015%~0.035%, Ti: 0.012%~0.015%, S≤0.001%, P≤0.006%, Alt≤0.06%, N≤0.010%, Mo: 0.10%~0.15%, Ni≤0.12%, Cr: 0.06%~0.12%, Al: 0.025%~0.035%, B≤0.001%, with the balance being Fe and unavoidable impurity elements.

2. The X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 1, characterized in that, The mass percentage ratio of C and Nb in the pipeline steel plate conforms to the following: C:Nb = 1~1.5; The chemical composition of Cr and Mo in the pipeline steel plate is in the following mass percentage ratio: Cr:Mo = 0.4~1.

2.

3. The X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 1, characterized in that, The thickness of the pipeline steel plate is 22.4~36.5mm; The pipeline steel plate contains non-metallic inclusions of grades A, B, C, and D, all ≤ grade 1.0; and banded structures ≤ grade 1. The microstructure of the pipeline steel plate consists of acicular ferrite, polygonal ferrite, and MA component; wherein the volume percentage of polygonal ferrite is 50% to 60%, the volume percentage of MA component is 0.5% to 1%, the grain diameter of acicular ferrite is ≤10μm, the grain size of polygonal ferrite is ≤7μm, the grain size level is ≥9, and the fine carbonitride precipitation below 10nm accounts for ≥25% of the total number of carbonitride precipitation; The HIC corrosion resistance meets the following requirements after 96 hours of corrosion with NACE standard A solution: CSR=0%, CLR=0%, CTR=0%. At room temperature, the transverse yield strength is 450–570 MPa, the tensile strength is 535–755 MPa, and the yield-to-tensile ratio is ≤0.

85. Longitudinal yield strength ≥ 427 MPa, tensile strength ≥ 508 MPa, yield-to-tensile ratio ≤ 0.85; The longitudinal elongation after fracture of the steel plate is 57% to 68%, and the uniform elongation is ≥12%. Impact energy CVN at -20℃ ≥250J; -15℃ drop hammer tear test (DWTT) shear area (SA%) ≥ 90%; The Vickers hardness HV10 at the quarter-width position along the thickness direction of the steel plate, 1.5mm from the surface, 1 / 4 of the thickness, and 1 / 2 of the thickness are all ≤240. The CTOD characteristic value δm at -10℃ is ≥0.3mm; The straightness of the entire length of the pipe after manufacturing (12m) is ≤7mm.

4. A method for preparing X65 pipeline steel plate for high-plasticity marine drilling risers as described in any one of claims 1-3, characterized in that, include: Hot metal desulfurization pretreatment, BOF top and bottom blowing converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, heating, rolling, cooling and straightening; In the continuous casting process, a covering agent is added during tundish pouring for protective purposes; the superheat is controlled at 25~35℃; the casting speed is 0.80~1.00m / min; dynamic light reduction is adopted for the final fan-shaped section of the continuous casting; and the casting billet is slowly cooled in the pit. In the rolling process, the heated billet is descaled after exiting the furnace and then rolled. The rolling process adopts a two-stage controlled rolling process: the roughing stage is rolled in the austenite recrystallization zone; the finishing stage is rolled in the austenite non-recrystallization zone. The control parameters for the roughing stage are: the roughing start temperature is 1125~1155℃, the roughing finish temperature is 975~1030℃, the temperature after roughing cross rolling is controlled at 1115~1145℃, and the longitudinal rolling start temperature is controlled in the range of 1095~1125℃; the longitudinal rolling single-pass reduction rate is >15%, and the roll speed is ≤1.5m / s; the cumulative deformation of the last two passes of roughing is ≥38%; and the intermediate billet thickness is set at 110~150mm. The control parameters for the finishing stage are: the finishing start temperature is 810~850℃, and the finishing finish temperature is 790~810℃. In the cooling process, the rolled steel plate is air-cooled and then accelerated water-cooled. The ACC cooling start temperature is 740-750℃, the reddening temperature is 520-550℃, the water-cooling time is 16-24s, and the water-cooling rate is 12-15℃ / s. The head and tail shielding process is adopted, and the shielding length is 2-3m. During the straightening process, the straightening speed is 0.6~0.8m / s, and the temperature of the steel plate entering the temperature straightening machine is 80~140℃.

5. The method for preparing X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 4, characterized in that, In the hot metal desulfurization pretreatment, a desulfurizing agent is injected into the hot metal for deep desulfurization, and the sulfur content in the hot metal is controlled to be ≤0.002%. The desulfurizing agent is CaO and Mg, wherein the amount of CaO added is 6-10 kg / t steel and the amount of Mg added is 0.3-0.8 kg / t steel.

6. The method for preparing X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 4, characterized in that, In the BOF top-and-bottom blown converter smelting process, desulfurized molten iron is added to the converter, and oxygen is blown into the converter to reduce carbon content and smelt it into molten steel. The oxygen flow rate is 18,000–26,000 m³ / h. 3 The oxygen blowing time is 18-20 minutes per hour. Slag-forming materials are added to the converter during the smelting process. When the carbon content in the molten steel in the converter is ≤0.046%, the oxygen content reaches 0.05%-0.09%, the phosphorus content is ≤0.01%, and the temperature of the molten steel in the converter reaches 1650-1690℃, the steel is tapped into the ladle. Slag-blocking tapping is adopted, and the slag thickness of the molten steel is ≤120mm. During the tapping process, 2.5-4.0 kg / t of aluminum-iron alloy is added. The slag-forming materials are lime and lightly calcined dolomite, of which the amount of lime added is 30-50 kg / t of steel, and the amount of lightly calcined dolomite added is 8-15 kg / t of steel.

7. The method for preparing X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 4, characterized in that, In the LF refining process, molten steel from the ladle is hoisted into the LF furnace for refining. Electrodes are used to heat the molten steel while a slagging agent and a strong deoxidizer are simultaneously added to the ladle. Argon is blown from the bottom of the ladle, followed by the addition of a silicon alloy, controlling the steel temperature to 1625–1640°C. The LF refining time is 40–50 minutes. The slagging agent consists of lime and pre-melted refining slag, with lime added at 3–8 kg / t of steel and pre-melted refining slag added at 2–6 kg / t of steel. The strong deoxidizer consists of aluminum granules and calcium carbide, with aluminum granules added at 0.5–1.5 kg / t of steel and calcium carbide added at 1–3 kg / t of steel. The argon blowing time is 8–15 minutes, and the argon flow rate is 80–150 L / min.

8. The method for preparing X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 4, characterized in that, In the RH vacuum refining process, an RH vacuum circulation device is used for degassing, the vacuum degree is controlled to be ≤0.2 kPa, and the processing time is 18 to 28 minutes. When the temperature of the molten steel reaches 1610 to 1630℃, the vacuum is broken after a net circulation of 12 to 16 minutes. In the calcium treatment process, calcium alloy cored wire is fed into the molten steel for calcium treatment for 10-20 minutes; the feeding speed is 300-320 m / min, and the weight ratio of Ca / S in the molten steel is controlled at 2.5-4.5; after feeding, the steel is allowed to stand for 15-25 minutes to promote the floating of inclusions; before tapping, the hydrogen content is controlled to be ≤0.0002%, and argon is blown cleanly for 5-20 minutes after feeding.

9. The method for preparing X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 4, characterized in that, During the heating process, the billet is placed in a heating furnace and passes through a first heating section, a second heating section, a third heating section, and a soaking section before being removed from the furnace. The heating temperature of the first heating section is controlled at 1050–1080℃; the heating temperature of the second heating section is controlled at 1220–1240℃; the heating temperature of the third heating section is controlled at 1215–1235℃; and the temperature of the soaking section is controlled at 1195–1205℃. The heating time for the first and second heating sections is no less than 40 minutes each, and the total heating time for the third heating section and the soaking section is 90–150 minutes. The total time the billet spends in the heating furnace is ≥4 hours.

10. The method for preparing X65 pipeline steel plate for high-plasticity marine drilling risers according to claim 4, characterized in that, In the continuous casting process, a continuous casting mold billet with a cross-sectional thickness of 300 mm is used for casting; the covering agent is a tundish covering agent; the amount of covering agent added is 3~5 kg / t steel; the reduction rate is 0.8~1.2 mm / m, and the reduction amount is 8~15 mm; the billet is slowly cooled in the pit, and is taken out of the furnace after the temperature drops below 400℃; the low magnification center segregation of the continuous casting billet does not exceed Class C 1.0 grade; In the rolling process, the roughing mill and the finishing mill are four-high reversible mills; The straightening process uses a 9-roll quadruple reversible straightening machine.

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