High-quality x80 grade pipeline steel plate for offshore drilling riser and a method for manufacturing the same

Abstract

The application discloses a high-quality X80-grade pipeline steel plate for a marine drilling riser and a preparation method thereof. The X80-grade pipeline steel plate for the marine drilling riser is prepared by improving from aspects of component design, casting blank preparation, casting blank heating, controlled rolling and controlled cooling technology and post-rolling straightening, and under the premise of avoiding cost increase and performance deterioration caused by high alloy addition, the X80-grade pipeline steel plate for the marine drilling riser with high cleanliness, high precision size and excellent low-temperature fracture toughness is prepared by accurately controlling contents of C, Mn, Si, Cu, Ni, Ti, Cr and Mo and steelmaking purity, combining with specific dynamic soft reduction, controlled rolling and controlled cooling and straightening technology, so that the high fatigue resistance requirement of the deep-sea riser is met, and the application has far-reaching strategic significance for improving the marine oil and gas resource exploration and development capacity of China.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel plate production technology and manufacturing technology for risers used in marine drilling, and more specifically, to a high-quality X80 grade pipeline steel plate for marine drilling risers and its preparation method. Background Technology

[0002] With global energy demand continuing to rise, fossil fuels still occupy a central position. However, given the gradual depletion of onshore oil and gas resources, the exploration and development of offshore oil and gas resources has become a focus of international attention. Offshore drilling risers, as an indispensable key component of offshore drilling platforms, bear the crucial responsibility of isolating seawater, supporting and controlling pipelines, and transporting drilling fluids. Their dimensional accuracy, metallurgical quality, and service performance directly affect the safety and stability of the entire offshore drilling system.

[0003] As the scope of marine resource development expands from shallow to deep seas, the service environment of marine drilling risers has become increasingly harsh, facing multiple challenges such as seawater corrosion, high pressure, low temperature, wave impact, and sediment erosion during drilling. Therefore, extremely stringent technical standards have been imposed on the steel plates required for manufacturing risers. These steel plates must not only possess excellent longitudinal and transverse strength and high low-temperature toughness to withstand the immense pressure and extreme temperatures of the deep-sea environment, but also exhibit outstanding fatigue resistance. In particular, steel plates for deep-sea risers must demonstrate resistance to hydrogen-induced cracking (HIC). Furthermore, the metallurgical quality of the steel plate is also a key factor affecting riser performance, and the presence of non-metallic inclusions and banded structures must be strictly controlled. These complex technical indicators undoubtedly increase the challenges of technological development.

[0004] Patent CN109234623A discloses an X80M deep-sea strain-resistant pipeline steel plate and its rolling process. The composition employs a high Ni (0.65%–0.85%) and high Mo (0.31%–0.36%) design, resulting in excessively high alloy content and cost. The P content is 0.008%, and the S content is 0.0016%, indicating excessively high levels of harmful elements. Patent CN109423572A discloses a seawater corrosion-resistant steel plate with high crack arrest and resistance to strain-aging embrittlement, along with its manufacturing method. This plate employs a high Ni (0.60%–1.00%) and high Cu (0.90%–1.20%) design, and undergoes over-aging tempering treatment after controlled rolling and cooling. This method involves high alloy additions, numerous processes, and further refinement of the polygonal ferrite grain size to meet the more stringent requirements for low-temperature toughness and strength-toughness matching. Patent CN106367685A discloses a pipeline steel of X80 grade and below for deep-sea drilling risers and its preparation method, but does not clearly specify the metallurgical quality requirements for the subsea pipeline steel plate. Furthermore, the yield strength ratio of the steel is close to 0.85, indicating insufficient performance reserves and strain hardening capacity. The elongation after fracture is around 25%, and the overall comprehensive performance and strength-toughness matching have not yet reached the ideal state. Patent CN112195396A discloses an X80 pipeline steel plate for deep-sea drilling risers that combines HIC resistance and erosion resistance, and its manufacturing method. However, it does not undergo straightening and heat treatment after rolling, failing to guarantee the dimensional accuracy and low stress requirements of the steel plate.

[0005] Therefore, research and technology related to pipeline steel for high-grade deep-sea risers, especially steel plates for deep-sea risers with high cleanliness, high dimensional accuracy, and high longitudinal and transverse strength as well as high toughness at low temperatures, are still incomplete. Summary of the Invention

[0006] The purpose of this invention is to overcome the limitations of existing technologies in effectively addressing the challenges of synergistic regulation of high-purity smelting, high-dimensional precision control, and fracture toughness in deep-sea riser steel plates. This leads to a trade-off between material safety and cost control in the extreme deep-sea environment. The invention provides a high-quality X80 grade pipeline steel plate for marine drilling risers and its preparation method. Based on the service environment of deep-sea risers, improvements are made in composition design, billet preparation, billet heating, controlled rolling and cooling processes, and post-rolling straightening. While avoiding increased costs and performance degradation due to high-alloy additions, the invention precisely controls the content of elements such as C, Mn, Si, Cu, Ni, Ti, Cr, and Mo, as well as the purity of the steelmaking process, combined with specific dynamic... By employing controlled rolling, controlled cooling, and straightening techniques, pipeline steel for risers with high cleanliness, high dimensional precision, and excellent low-temperature fracture toughness has been produced, meeting the X80 strength level. Its harmful element content is S≤0.001%, P≤0.006%, and the levels of non-metallic inclusions (Class A, B, C, and D) are all ≤1.0. The impact energy (CVN) at -20℃ is ≥300J, the drop hammer tear test (DWTT) shear area (SA%) at -15℃ is ≥85%, the fracture resistance (CTOD) at 0℃ is ≥0.25mm, and the straightness of the entire length after pipe fabrication is ≤8mm (12m). This meets the extremely high fatigue resistance requirements of deep-sea risers and has profound strategic significance for enhancing my country's marine oil and gas resource exploration and development capabilities. Compared with existing technologies, this invention, through the introduction of low-stress control processes such as post-rolling straightening and heat treatment, not only ensures that the steel plate meets the service performance requirements of X80 grade pipeline steel in terms of strength, toughness, and resistance to HIC, but also further improves the dimensional accuracy, shape stability, and residual stress control level of wide and thick steel plates, solving the problem that existing technologies cannot simultaneously achieve high performance, high precision, and low-stress manufacturing.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows: A high-quality X80 grade pipeline steel plate for marine drilling risers comprises the following components by mass percentage: C: 0.046%–0.066%, Mn: 1.65%–1.74%, Si: 0.15%–0.24%, Nb: 0.052%–0.072%, Ti: 0.014%–0.024%, V≤0.048%, S≤0.001%, P≤0.006%, Alt≤0.06%, N≤0.010%, Mo: 0.16%–0.24%, Cu≤0.14%, Ni≤0.14%, Cr: 0.08%–0.16%, Al: 0.025%–0.035%, B≤0.001%, with the balance being Fe and unavoidable impurity elements.

[0008] This invention also discloses a method for preparing X80 grade pipeline steel plates for high-quality marine drilling risers as described above, including: 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 0.80~0.95m / 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 carried out within the austenite recrystallization temperature range; the finishing stage is carried out within the austenite non-recrystallization temperature range. The control parameters for the roughing stage are: the initial roughing temperature is 1140~1170℃, the final roughing temperature is 970~1030℃, and the temperature after roughing cross rolling is 1110~1140℃. After cross rolling, spraying is performed. Cooling and waiting time for 30-70 seconds; longitudinal rolling start temperature is 1090-1120℃; single-pass reduction rate of longitudinal rolling is >15%, and roll speed is ≤1.5m / s; cumulative deformation of the last two passes of roughing rolling is ≥38%; after roughing rolling, waiting time is required, and the thickness of the intermediate billet is set to 3.5t-4.5t, where t is the thickness of the finished steel plate; the control parameters for the finishing rolling stage are: finishing rolling start temperature is 790-900℃, and finishing rolling finish temperature is 760-820℃. In the cooling process, the rolled steel plate is air-cooled and then accelerated water-cooled. The ACC cooling start temperature is 770-790℃, the reddening temperature is 360-420℃, the water-cooling time is 15-35s, and the water-cooling rate is 15-30℃ / s. The head and tail shielding process is adopted, and the shielding length is 2-3m. During the straightening process, the straightening speed is 0.5~0.9m / s, and the temperature of the steel plate entering the temperature straightening machine is 85~125℃.

[0009] Implementing the embodiments of the present invention will have the following beneficial effects: (1) Excellent metallurgical quality and microstructure: This invention, through rigorous composition design and refining processes, effectively controls non-metallic inclusions and banded structures in the steel plate, ensuring its high cleanliness. The levels of non-metallic inclusions (Class A, B, C, and D) in the pipeline steel plate are all ≤1.0, segregation in the continuously cast billet does not exceed Class C 1.0, and the banded structure of the steel plate is no greater than Level 1. This surpasses the requirements of the group standard T / CSTM 01388-2025, "Medium-thick Hot-rolled Steel Plate for Marine Drilling Riser". Reasonable billet preparation and heating processes further improve the metallurgical quality of the steel plate, resulting in a uniform internal structure, reducing quality defects, meeting the high-quality requirements of deep-sea risers, and enhancing fatigue resistance.

[0010] This invention, through unique smelting, heating, rolling, and cooling processes, achieves an ideal microstructure consisting of acicular ferrite, polygonal ferrite, and a small amount of ferrite (M / A). The polygonal ferrite comprises 15%–30% by volume, the acicular ferrite grain diameter is controlled below 8 μm, the polygonal ferrite grain size is below 5 μm, and the grain size level is ≥10. Fine carbonitride precipitates smaller than 10 nm account for ≥30% of all carbonitride precipitates. This microstructure significantly improves the strength, toughness, corrosion resistance, and fatigue resistance of the steel plate.

[0011] (2) High strength, high toughness and corrosion resistance: This invention employs a low-C, low-Mn design, combined with the addition of appropriate amounts of Nb, V, Ti, Ni, Cu, Mo, and Cr, effectively controlling the corrosion sensitivity of the steel plate while compensating for strength loss. Through optimized composition design and controlled rolling and cooling processes, the formation of fine-grained ferrite and acicular ferrite is promoted, improving the toughness and plasticity 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. Specifically, the transverse yield strength can reach 555~675MPa, the tensile strength can reach 625~825MPa, and the yield-to-tensile ratio ≤0.85; the longitudinal yield strength is 535~565MPa, the tensile strength is 605~635MPa, and the yield-to-tensile ratio ≤0.89; the elongation of the round bar tensile specimen is ≥25%; the impact energy CVN at -20℃ is ≥300J; the shear area SA% in the drop hammer tear test (DWTT) at -15℃ is ≥85%; and the fracture resistance at 0℃ is... CTOD≥0.25mm; Vickers hardness HV10 at the quarter-thickness position of the steel plate, 1.5mm from the surface along the thickness direction, at 1 / 4 of the thickness, and at 1 / 2 of the thickness are all ≤265; Excellent corrosion resistance due to the multiphase structure: The reasonable combination of soft and hard phases in the steel plate not only improves the strength but also endows the steel plate with excellent corrosion resistance. The HIC corrosion resistance meets the requirements of CSR=0%, CLR=0%, and CTR=0% after 96 hours of corrosion with NACE standard A solution.

[0012] (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 riser in the deep-sea environment. The straightness of the entire length after pipe fabrication is ≤8mm (12m).

[0013] These characteristics ensure that steel plates with thicknesses ranging from 18.4 to 22.4 mm maintain excellent overall mechanical properties and superior metallurgical quality. These performance indicators far exceed existing technologies and standards, meeting the special operating conditions required for offshore drilling riser systems. Attached Figure Description

[0014] Figure 1 This is a microstructure diagram of the plate at 1 / 2 thickness in Embodiment 1 of the present invention. Detailed Implementation

[0015] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0016] This invention discloses a high-quality X80 grade pipeline steel plate for marine drilling risers, comprising the following components by mass percentage: C: 0.046%–0.066%, Mn: 1.65%–1.74%, Si: 0.15%–0.24%, Nb: 0.052%–0.072%, Ti: 0.014%–0.024%, V≤0.048%, S≤0.001%, P≤0.006%, Alt≤0.06%, N≤0.010%, Mo: 0.16%–0.24%, Cu≤0.14%, Ni≤0.14%, Cr: 0.08%–0.16%, Al: 0.025%–0.035%, B≤0.001%, with the balance being Fe and unavoidable impurity elements.

[0017] In one specific embodiment, the mass percentage ratio of Cu and Ni in the pipeline steel plate conforms to the following: Cu:Ni = 0.67~1.00:1.

[0018] In one specific embodiment, the mass percentage ratio of Cr and Mo in the pipeline steel plate conforms to the following: Cr:Mo = 0.35 to 0.8:1.

[0019] The specific reasons for limiting the amounts of each chemical element in steel plates are detailed below: Carbon (C) is an economical strengthening element in steel. Increasing the C content can strengthen the matrix through solid solution strengthening and precipitation strengthening. When the C content is low, the precipitation strengthening effect of microalloying elements such as Nb and V in pipeline steel cannot be fully utilized, and the erosion resistance cannot be met. When the C content is too high, it will deteriorate the plasticity, toughness, and weldability of pipeline steel. Increasing the C content will also increase the segregation tendency in medium and heavy steel plates, which is detrimental to the resistance of pipeline steel to hydrogen embrittlement. Therefore, the C content should be controlled within the range of 0.046% to 0.066%.

[0020] Mn: Mn can strengthen the matrix through solid solution strengthening, compensating for the strength loss caused by low carbon content, and is also an economical and effective strengthening element. Mn is an austenite forming element; increasing the austenite phase region can lower the Ar3 critical temperature for the transformation of austenite to ferrite, helping to obtain fine phase transformation products, improving the toughness of steel, and lowering the ductile-brittle transition temperature. Mn can dissolve in ferrite, thereby strengthening ferrite and increasing the yield strength and tensile strength of the soft / hard combined multiphase structure. However, excessively high Mn content can increase the tendency for center segregation in thick plates of continuously cast slabs, potentially affecting HIC resistance due to obvious bainite segregation bands, and is also detrimental to the forming of steel plates into tubes. This invention designs the Mn content to be controlled between 1.65% and 1.74%.

[0021] Si: Adding Si is beneficial for improving the cleanliness of steel. Si mainly exists in steel in solid solution form, which inhibits the formation of cementite during bainite transformation. It promotes the diffusion of carbon into untransformed austenite, increasing the tendency for M / A island formation. However, when excessive Si is added, it will reduce the plasticity and toughness of the material, which is not conducive to improving weldability. This invention designs the Si content to be controlled between 0.15% and 0.24%.

[0022] Nitrogen (Nb): Nitrogen (Nb) is one of the main microalloying elements in pipeline steel. During the billet heating process, undissolved Nb particles can pin austenite grain boundaries, hindering austenite grain growth. During hot rolling, deformation-induced NbC precipitates can inhibit austenite recrystallization, which is beneficial to the formation of extended austenite structure and fine ferrite grains. The addition of appropriate amounts of Nb, through precipitation strengthening and grain refinement, can improve the strength of the material without sacrificing ductility and toughness, giving the steel high strength and high ductility and toughness. However, if the Nb content is too high, and the Nb solid solution in austenite and ferrite is limited, some Nb will be distributed in elemental form in the steel, forming a blocky structure, which deteriorates the material's ductility and toughness. On the other hand, excessively high Nb content has a strong inhibitory effect on austenite recrystallization, which is detrimental to obtaining a more uniform austenite structure in both the longitudinal and transverse directions. When adding Nb content, it is necessary to take into account the C content in the steel to maximize the role of Nb in ensuring the special requirements of strength and toughness for marine drilling risers. The present invention designs the Nb content to be controlled between 0.052% and 0.072%.

[0023] Ti: As a strong nitrogen-fixing element, Ti easily forms fine TiN precipitates in cast billets. TiN precipitates exhibit good high-temperature stability and can effectively hinder austenite grain growth during subsequent reaustification, improving the toughness of the matrix and weld structure. However, when the Ti content exceeds a certain value, coarse TiN particles easily form in the steel, deteriorating its ductility and toughness. This invention designs the Ti content to be controlled between 0.014% and 0.024%.

[0024] V: V combines with C or N in steel, exhibiting a high precipitation strengthening effect. Compared to Nb, V-containing carbonitrides have a lower dissolution-precipitation equilibrium temperature, which is beneficial for re-dissolution and precipitation under lower heating conditions, thereby achieving the effect of grain refinement at the grain boundaries. In the two-phase region heating, Nb and Ti carbonitrides cannot re-dissolve and tend to grow, while V does not have this problem because it can re-dissolve. Therefore, it is easier to achieve grain refinement and precipitation strengthening effects. However, excessive V addition increases the risk of surface and internal cracks in the continuously cast billet; therefore, the V content in this invention is designed to be controlled below 0.048%.

[0025] Mo: Can be postponed γ→α During phase transformation, the formation of the ferrite phase is the first to occur, and the key element promoting the formation of acicular ferrite plays a crucial role in controlling the phase transformation. It also improves the hardenability of the steel. Adding a certain amount of Mo can obtain a acicular ferrite microstructure with good toughness and plasticity. Considering economic factors, the Mo content should not exceed 0.24%. This invention designs the Mo content to be controlled between 0.16% and 0.24%.

[0026] 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 controlled between 0.08% and 0.16%. When Cr and Mo are added at a ratio of approximately Cr:Mo = 0.35 to 0.8:1, the effect on controlling phase transformation in the microstructure is even better.

[0027] Cu and Ni: Cu and Ni are added synergistically, primarily existing in the microstructure in solid solution form. Adding Cu significantly improves the corrosion resistance of steel, while adding Ni mitigates the hot brittleness caused by Cu. Furthermore, Ni is beneficial for low-temperature toughness and hardenability of thick plates. In this invention, the Cu and Ni contents are both controlled within 0.14%.

[0028] Al: Al has a significant impact on the cleanliness of steel. As a deoxidizer, low Al content results in insufficient deoxidation, while excessive Al content promotes the formation of large-sized oxide inclusions. Therefore, the Al content in this invention is designed to be between 0.025% and 0.035%.

[0029] 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. By controlling the inclusion morphology of sulfides through ultra-low S and Ca treatment, the low-temperature toughness of pipeline steel can be improved.

[0030] B: Considering the impact of B on the toughness of the steel plate, the B content should be controlled to be no more than 0.001%.

[0031] In one specific embodiment, the thickness of the pipeline steel plate is 18.4–22.4 mm.

[0032] 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 15% to 30%, the volume percentage of MA component is 0.5% to 3%, the grain diameter of acicular ferrite is ≤8μm, the grain size of polygonal ferrite is ≤5μm, the grain size level is ≥10, and the fine carbonitride precipitation below 10nm accounts for ≥30% of the total number of carbonitride precipitation.

[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 impact energy CVN at -20℃ is ≥300J; the drop hammer tear test shear area SA% at -15℃ is ≥85%; and the fracture resistance CTOD at 0℃ is ≥0.25mm.

[0035] In one specific embodiment, the straightness of the entire length of the pipe after manufacturing (12m) is ≤8mm. Straightness refers to the 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) ≤8mm mentioned in this invention means that the straightness of the entire 12m finished pipe is less than 8mm.

[0036] In one specific embodiment, the transverse yield strength is 555~675MPa, the tensile strength is 625~825MPa, and the yield strength ratio is ≤0.85; the longitudinal yield strength is 535~565MPa, the tensile strength is 605~635MPa, and the yield strength ratio is ≤0.89; the elongation of the round bar tensile specimen is ≥25%.

[0037] In one specific embodiment, the Vickers hardness HV10 at the locations one-quarter of the plate width, approximately 1.5 mm from the surface along the thickness direction of the steel plate, at one-quarter of the thickness, and at one-half the thickness are all ≤265.

[0038] 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%.

[0039] This invention also discloses a method for preparing X80 grade pipeline steel plates for high-quality marine drilling risers as described above, including: 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.

[0040] 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.

[0041] Specifically, this invention selects CaO and Mg as desulfurization powders based on 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.

[0042] 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 15-20 minutes per hour. Slag-forming materials are added during the converter smelting process. When the carbon content in the molten steel is ≤0.06%, the oxygen content reaches 0.05%-0.09%, the phosphorus content is ≤0.01%, and the steel temperature 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 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.

[0043] In the S3 and LF refining processes, molten steel from the ladle is hoisted to an external refining furnace, i.e., the LF furnace, for refining. Electrodes are used to heat the molten steel while simultaneously adding slagging agents and strong deoxidizers to the ladle. Argon is blown from the bottom of the ladle to agitate the molten steel, homogenizing its composition and temperature. Subsequently, silicon alloys are added, and the steel temperature is controlled at 1630–1650°C before entering the vacuum refining process. 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 min; the argon flow rate is 80–150 L / min; and the LF refining time is 35–55 min.

[0044] In one specific embodiment, the silicon alloy is ferrosilicon.

[0045] 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 1630–1650℃ 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.

[0046] In S4 and RH vacuum refining, an RH vacuum circulation device is used for degassing, controlling the vacuum degree to ≤0.2 kPa and the processing time to 20–30 min. When the molten steel temperature reaches 1610–1640℃, the RH vacuum circulation device is shut off after 10–15 min of net circulation. The vacuum is broken when the content and temperature of various components in the molten steel meet the target requirements. Hydrogen and nitrogen are harmful elements in steel, reducing its plasticity and toughness and increasing the crack susceptibility of steel plates. RH vacuum refining removes hydrogen, nitrogen, and other gases, as well as inclusions, from the molten steel through a vacuum circulation degassing device, improving the purity of the steel. Appropriate vacuum degree and processing time ensure degassing efficiency while avoiding overcooling or overheating of the molten steel. The RH vacuum refining parameters are set based on the characteristics of the RH vacuum refining process and the product cleanliness requirements, aiming to ensure optimal degassing during the refining process.

[0047] In S5, calcium treatment involves feeding calcium alloy cored wire into molten steel at a feeding speed of 300-320 m / min, controlling the Ca / S weight ratio in the molten steel to be 2.5-4.5, and the calcium treatment time to be 15-25 min. After feeding, the steel is allowed to stand for 15-25 min to promote the floating of inclusions. Before tapping, the hydrogen content is controlled to be ≤0.0002%, and argon is blown cleanly for 5-8 min after feeding.

[0048] 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. Controlling the wire feeding speed and the Ca / S weight ratio in the molten steel is based on the requirements of pipeline steel calcium treatment processes and inclusion size, as well as experimental optimization, aiming to ensure effective control of sulfide morphology while avoiding fluctuations in molten steel composition.

[0049] 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 0.80~0.95m / min; dynamic light reduction is adopted in the fan-shaped section at the end of continuous casting, and the reduction range is adjusted according to the casting speed to improve the internal quality of the billet and reduce center segregation and porosity; the billet is slowly cooled in the pit.

[0050] 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.

[0051] 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.

[0052] In one specific embodiment, the low-magnification center segregation of the continuously cast billet does not exceed Class C, Grade 1.0.

[0053] S7. During heating, the billet is placed in a heating furnace and heated in multiple stages, passing through a first heating section, a second heating section, a third heating section, and a soaking section before being removed from the furnace. Specifically, the heating temperature of the first heating section is controlled at 1150~1200℃, and the heating time is 90~210min; the heating temperature of the second heating section is controlled at 1210~1240℃, and the heating time is 90~210min; the heating temperature of the third heating section is controlled at 1200~1230℃, and the heating time is 90~210min; and the temperature of the soaking section is controlled at 1190~1200℃, and the heating time is 90~210min. The total time the billet spends in the heating furnace is ≥6h, preferably 6~7h.

[0054] Specifically, multi-stage heating of continuously cast billets 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 solution requirements while preventing excessive austenite grain growth; and controlling the soaking time ensures the overall temperature uniformity of the continuously cast billet. The parameter settings here are based on the characteristics and requirements of the billet heating process, aiming to ensure sufficient alloy element solution while avoiding excessive austenite grain growth, thereby improving the overall performance of the steel plate.

[0055] 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 rough rolling stage is carried out within the austenite recrystallization temperature range to promote the recrystallization of austenite grains. The finish rolling stage is carried out within the austenite non-recrystallization temperature range to refine the ferrite grains. The control parameters for the rough rolling stage are: rough rolling start temperature 1140~1170℃, rough rolling finish temperature 970~1030℃, rough rolling cross-rolling widening temperature 1110~1140℃, spray cooling and waiting for 30~70s after cross-rolling, and longitudinal rolling start temperature 1090~110℃. Within the 20℃ range; the single-pass reduction rate in longitudinal rolling is >15%, and the roll speed is ≤1.5m / s; the cumulative deformation of the last two passes in roughing rolling is ≥38%. Using rapid cooling between transverse and longitudinal rolling, combined with low-temperature large deformation in longitudinal rolling and low-speed rolling, can promote the penetration of rolling deformation towards 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. After roughing rolling, a waiting-warming period is implemented, with the intermediate billet thickness set at 3.5t~4.5t, where t is the thickness of the finished steel plate. The control parameters for the finishing rolling stage are: the starting temperature of finishing rolling is 790~900℃, and the finishing rolling temperature is 760~820℃. A suitable intermediate waiting-warming billet thickness combined with a low-temperature finishing rolling process can achieve sufficient accumulation of deformation energy and promote the induced precipitation of fine precipitates, increasing the nucleation sites for ferrite, which is beneficial to improving plasticity and toughness.

[0056] 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.

[0057] S9. During cooling, the rolled steel plate is air-cooled and then subjected to accelerated water cooling (ACC). The ACC cooling start temperature is 770-790℃, the reddening temperature is 360-420℃, the water cooling time is 15-35s, and the water cooling rate is 15-30℃ / s. A head and tail shielding process is adopted to avoid uneven temperature at the head and tail, thereby ensuring consistent steel plate performance. The shielding length is 2-3m.

[0058] Specifically, the combination of short-term air cooling after steel plate rolling and low-temperature final rolling can effectively control the precipitation of fine particles and the further formation of soft-phase polygonal ferrite, thereby improving toughness, plasticity and deformation capacity; the combination of rapid cooling and final cooling temperature can promote the formation of hard-phase granular bainite and a small amount of M / A, and ensure sufficient hardness difference between the soft phase and the hard phase, thereby obtaining sufficient tensile strength and a suitable yield strength ratio.

[0059] S10. During straightening, the straightening speed is 0.5~0.9m / s, and the temperature of the steel plate entering the temperature straightening machine is 85~125℃.

[0060] In one specific embodiment, a 9-roll quadruple reversible straightener is used for straightening.

[0061] 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 setting of these parameters is 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.

[0062] The following are specific embodiments. Examples 1-8 The preparation method of X80 grade pipeline steel plate for high-quality marine drilling risers 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.

[0063] 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.

[0064] 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 and smelt it into molten steel. During the converter smelting process, slag-forming materials are added to the converter, namely lime and lightly calcined dolomite. When the carbon content in the molten steel in the converter is ≤0.06%, the oxygen content reaches 0.05% to 0.09%, and the phosphorus content is ≤0.01%, and the temperature of the molten steel in the converter is controlled to reach the target temperature before being tapped into the ladle, slag-blocking tapping is adopted, and the slag thickness of the molten steel is ≤100mm. During the tapping process, aluminum-iron alloy is added for deoxidation.

[0065] 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 slag-forming agents and strong deoxidizers are added to the ladle simultaneously. The slag-forming agents are lime and pre-melted refining slag, and the strong deoxidizers are aluminum granules and calcium carbide. 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 before it is transferred to the vacuum refining process.

[0066] 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.

[0067] In S5, calcium treatment involves feeding calcium alloy cored wire into molten steel to perform calcium treatment, controlling the Ca / S weight ratio in the molten steel; after feeding the wire, the steel is allowed to stand to promote the floating of inclusions; before tapping the steel, the hydrogen content is controlled to be ≤0.0002%, and argon is blown cleanly after feeding the wire.

[0068] S6. In continuous casting, a continuous casting mold billet with a cross-sectional thickness of 300mm is used for casting. When casting in the tundish, a covering agent is added for protection. The covering agent is the tundish covering agent. 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 billet is slowly cooled in the pit. After the temperature drops to below 400℃, it is taken out of the furnace and used as a hot-rolled billet.

[0069] 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.

[0070] S8. During rolling, the heated billet is descaled after exiting the furnace and then rolled. The roughing mill and finishing mill are four-roll reversible mills. The rolling process adopts two-stage controlled rolling. The roughing stage is carried out in the austenite recrystallization temperature range; the finishing stage is carried out in the austenite non-recrystallization temperature range.

[0071] S9. During cooling, the rolled steel plate is air-cooled and then accelerated water cooling (ACC) is performed; a head and tail shielding process is adopted to avoid uneven temperature at the head and tail.

[0072] S10. During straightening, a 9-roller quadruple reversible straightening machine is used.

[0073] The microstructure of the pipeline steel plate prepared according to this embodiment consists of acicular ferrite, polygonal ferrite and MA component; wherein, the diameter of acicular ferrite grains is ≤8μm, the size of polygonal ferrite grains is ≤5μm, the grain size level is ≥10, and the percentage of fine carbonitride precipitation below 10nm accounts for ≥30% of the total number of carbonitride precipitation.

[0074] Table 1 shows the chemical composition of the steel in the embodiments of the present invention. Table 2 shows the main process parameters for steel smelting and continuous casting billet heating in the embodiments of the present invention. Table 3 shows the main process parameters for the controlled rolling and controlled cooling process in the embodiments of the present invention. Table 4 shows the main performance parameters of the steel plate in the embodiments of the present invention.

[0075] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0076] Note: In Examples 1-8, Fe and its unavoidable impurity elements are in the balance.

[0077] Table 2. Main process parameters for smelting and continuous casting billet heating in the embodiments of the present invention.

[0078] 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.

[0079] (2) The calcium treatment time is the total time from the start of feeding the calcium alloy cored wire to the end of feeding.

[0080] (3) The superheat of continuous casting refers to the difference between the temperature of molten steel in the tundish and the temperature of the liquidus.

[0081] (4) Continuous casting speed refers to the speed at which the billet is pulled out during the continuous casting process.

[0082] (5) The second heating section temperature, the third heating section temperature, and the soaking section temperature refer to the temperatures reached by the continuously cast billet at different stages in the heating furnace.

[0083] (6) The total heating time refers to the total time from when the continuously cast billet enters the heating furnace to when the heating ends.

[0084] Table 2 details the main process parameters for steel smelting and continuous casting billet heating in the embodiments of the present invention. These parameters are crucial for controlling the cleanliness, microstructure, and final properties of the steel plate. By precisely controlling these process parameters, it is possible to ensure the production of high-quality X80 pipeline steel plates for marine drilling risers that meet technical requirements.

[0085] Table 3 Main process parameters of the controlled rolling and cooling process in the embodiments of the present invention

[0086] Table 4. Steel performance parameters of the embodiments of the present invention

[0087] Note: DWTT specimens are full-thickness specimens.

[0088] Table 4 details the main performance parameters of the steel plates in the embodiments of this invention. Specific composition design and optimized manufacturing processes effectively improve the comprehensive performance, metallurgical quality, and dimensional accuracy of the steel plates. For conventional X80 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.

[0089] 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-quality X80 grade pipeline steel plate for marine drilling risers, characterized in that, Includes the following components by mass percentage: C: 0.046%~0.066%, Mn: 1.65%~1.74%, Si: 0.15%~0.24%, Nb: 0.052%~0.072%, Ti: 0.014%~0.024%, V≤0.048%, S≤0.001%, P≤0.006%, Alt≤0.06%, N≤0.010%, Mo: 0.16%~0.24%, Cu≤0.14%, Ni≤0.14%, Cr: 0.08%~0.16%, Al: 0.025%~0.035%, B≤0.001%, with the balance being Fe and unavoidable impurity elements.

2. The X80 grade pipeline steel plate for high-quality marine drilling risers according to claim 1, characterized in that, The mass percentage ratio of Cu and Ni in the pipeline steel plate conforms to the following: Cu:Ni = 0.67~1.00:1; The chemical composition of Cr and Mo in the pipeline steel plate is in the following mass percentage ratio: Cr:Mo = 0.35~0.8:

1.

3. The X80 grade pipeline steel plate for high-quality marine drilling risers according to claim 1, characterized in that, The thickness of the pipeline steel plate is 18.4–22.4 mm; The microstructure of the pipeline steel plate consists of acicular ferrite, polygonal ferrite and MA component; wherein the volume percentage of polygonal ferrite is 15% to 30%, the volume percentage of MA component is 0.5% to 3%, the grain diameter of acicular ferrite is ≤8μm, the grain size of polygonal ferrite is ≤5μm, the grain size level is ≥10, and the fine carbonitride precipitation below 10nm accounts for ≥30% of the total number of carbonitride precipitation; The pipeline steel plate contains non-metallic inclusions of grades A, B, C, and D, all ≤ grade 1.0; and banded structures ≤ grade 1. Impact energy CVN at -20℃ ≥300J; -15℃ drop hammer tear test (DWTT) shear area (SA%) ≥ 85%; Fracture resistance at 0℃: CTOD ≥ 0.25 mm; The straightness of the entire length of the pipe after manufacturing (12m) is ≤8mm; The transverse yield strength is 555~675MPa, the tensile strength is 625~825MPa, and the yield-to-tensile ratio is ≤0.85; The longitudinal yield strength is 535~565MPa, the tensile strength is 605~635MPa, and the yield strength ratio is ≤0.89; The elongation of the round bar tensile specimen is ≥25%; 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 ≤265. 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%.

4. A method for preparing X80 grade pipeline steel plate for high-quality 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 0.80~0.95m / 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 carried out within the austenite recrystallization temperature range; the finishing stage is carried out within the austenite non-recrystallization temperature range. The control parameters for the roughing stage are: the initial roughing temperature is 1140~1170℃, the final roughing temperature is 970~1030℃, and the temperature after roughing cross rolling is 1110~1140℃. After cross rolling, spraying is performed. Cooling and waiting time for 30-70 seconds; longitudinal rolling start temperature is 1090-1120℃; single-pass reduction rate of longitudinal rolling is >15%, and roll speed is ≤1.5m / s; cumulative deformation of the last two passes of roughing rolling is ≥38%; after roughing rolling, waiting time is required, and the thickness of the intermediate billet is set to 3.5t-4.5t, where t is the thickness of the finished steel plate; the control parameters for the finishing rolling stage are: finishing rolling start temperature is 790-900℃, and finishing rolling finish temperature is 760-820℃. In the cooling process, the rolled steel plate is air-cooled and then accelerated water-cooled. The ACC cooling start temperature is 770-790℃, the reddening temperature is 360-420℃, the water-cooling time is 15-35s, and the water-cooling rate is 15-30℃ / s. The head and tail shielding process is adopted, and the shielding length is 2-3m. During the straightening process, the straightening speed is 0.5~0.9m / s, and the temperature of the steel plate entering the temperature straightening machine is 85~125℃.

5. The method for preparing X80 grade pipeline steel plate for high-quality 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 X80 grade pipeline steel plate for high-quality 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 / h, oxygen blowing time is 15-20min; during the converter smelting process, slag-forming materials are added to it. When the carbon content in the molten steel in the converter is ≤0.06%, 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, 2.5-4.0kg / t steel of aluminum-iron alloy is added; the slag-forming materials are lime and lightly calcined dolomite, wherein the amount of lime added is 30-50kg / t steel, and the amount of lightly calcined dolomite added is 8-15kg / t steel.

7. The method for preparing X80 grade pipeline steel plate for high-quality 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, and the steel temperature is controlled at 1630–1650°C. The slagging agent consists of lime and pre-melted refining slag, with lime added at a rate of 3–8 kg / t steel and pre-melted refining slag added at a rate of 2–6 kg / t steel. The strong deoxidizer consists of aluminum granules and calcium carbide, with aluminum granules added at a rate of 0.5–1.5 kg / t steel and calcium carbide added at a rate of 1–3 kg / t steel. The argon blowing time is 8–15 min, the argon flow rate is 80–150 L / min, and the LF refining time is 35–55 min.

8. The method for preparing X80 grade pipeline steel plate for high-quality 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 20 to 30 minutes. When the temperature of the molten steel reaches 1610 to 1640℃, the vacuum is broken after a net circulation of 10 to 15 minutes. In the calcium treatment, calcium alloy cored wire is fed into the molten steel at a feeding speed of 300-320 m / min, and the weight ratio of Ca / S in the molten steel is controlled at 2.5-4.

5. The calcium treatment time is 15-25 min. After feeding, the steel is left to stand for 15-25 min to promote the floating of inclusions. Before tapping, the hydrogen content is controlled to be ≤0.0002%, and argon is blown cleanly for 5-8 min after feeding.

9. The method for preparing X80 grade pipeline steel plate for high-quality 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. Specifically, the heating temperature in the first heating section is controlled at 1150~1200℃, and the heating time is 90~210 min; the heating temperature in the second heating section is controlled at 1210~1240℃, and the heating time is 90~210 min; the heating temperature in the third heating section is controlled at 1200~1230℃, and the heating time is 90~210 min; the temperature in the soaking section is controlled at 1190~1200℃, and the heating time is 90~210 min; the total time the billet spends in the heating furnace is ≥6 hours.

10. The method for preparing X80 grade pipeline steel plate for high-quality 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 pouring; the covering agent is a tundish covering agent; the amount of the 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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