High-performance x70m pipeline steel for thick-wall spiral seam submerged arc welded steel pipe and manufacturing method

Abstract

The present invention relates to the technical field of pipeline steel production, and particularly relates to a high-performance X70M pipeline steel for thick-walled spiral submerged arc welded steel pipes and a manufacturing method. The chemical composition of the pipeline steel is C 0.070% - 0.085%, Si 0.18% - 0.28%, Mn 1.60% - 1.75%, P≤0.015%, S≤0.002%, V 0.033% - 0.043%, Nb 0.080% - 0.095%, Cr 0.25% - 0.35%, Als 0.020% - 0.050%, and the rest is Fe and inevitable impurities, and 0.115% < Nb + V < 0.130% is satisfied. The present invention adopts low-cost innovative composition design and reasonable element ratio, and combines with appropriate processes to manufacture a thick-specification high-performance pipeline steel hot-rolled coil, which has low production cost, simple and feasible process, and is suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel production technology, specifically to a high-performance X70M pipeline steel for thick-walled spiral submerged arc welded steel pipes and its manufacturing method. Background Technology

[0002] Pipeline transportation is a crucial method for long-distance transport of oil, natural gas, and other commodities. To improve operational efficiency and reduce costs, pipeline transportation is evolving towards larger diameter and higher pressure applications, leading to a surge in demand for thick-walled, high-strength spiral submerged arc welded steel pipes. Spiral submerged arc welded pipes are widely used in oil and natural gas pipelines. They are manufactured from hot-rolled steel coils, formed into a spiral at room temperature, and welded using double-sided submerged arc welding, allowing for the production of large-diameter pipes from narrower coils. Under high pressure, pipelines are prone to fracture. To ensure pipeline safety during service, it is essential to significantly improve the thickness and strength of the pipeline steel. However, with increasing thickness, not only is it difficult to improve the strength of the pipeline steel, but low-temperature toughness also deteriorates significantly, drastically increasing the difficulty of ensuring performance in low-temperature drop hammer tests. Spiral welded pipes have a specific forming angle, therefore the strength testing direction of the raw material steel coil is generally 30° to the rolling direction. However, for straight seam submerged arc welded steel pipes, the strength testing direction of the raw material steel plate is transverse. Due to the anisotropy and texture of the steel, the yield strength is lowest at 30°, approximately 30-60 MPa lower than the transverse yield strength. This significantly increases the difficulty of ensuring the yield strength of pipeline steel used in spiral seam submerged arc welded steel pipes. Furthermore, due to the Bauschinger effect during pipe manufacturing, the yield strength of the pipe after manufacturing is usually lower than that of the steel coil. Therefore, to ensure the performance of both the coil and the pipe is qualified, the strength requirement of the raw material steel coil should be higher than the X70 grade pipeline standard requirement (≥485 MPa). When producing thick-gauge high-strength pipeline steel, it is difficult for the steel coil head to enter the coiling machine, easily causing poor coil shape or even steel jamming accidents.

[0003] Patent CN101525722 A discloses X70 hot-rolled steel plate with excellent toughness and its production method. It adds precious alloying elements Mo and Ni, and has low Nb+V+Ti content. The production method adopts medium and heavy plate rolling, which has low production efficiency and high manufacturing cost. Moreover, the steel plate is not suitable for the manufacture of spiral seam submerged arc welded steel pipes.

[0004] Patent CN101684539 A discloses a thick-gauge, high-toughness X70 needle-shaped ferritic pipeline steel and its manufacturing method. It adds precious alloying elements Mo and Cu, resulting in high alloy costs. It does not contain V and is produced using a short-process hot-charging and hot-delivery process for medium-thin slabs.

[0005] Patent CN101705438 A discloses a pipeline steel X70 hot-rolled coil and its manufacturing method. It adds expensive alloying elements such as Mo, Ni, and Cu, resulting in high alloy costs. Furthermore, it adopts a low-temperature coiling process with a coiling temperature of 350-420℃, which makes it difficult to control the process stability and easily leads to poor coil shape.

[0006] Patent CN 101994059 A discloses a thick-walled X70 pipeline steel coil and its production method. It adds precious alloying elements such as Mo and Ni, resulting in high alloy costs. It does not contain V and is produced using a short-process hot charging and hot delivery technology for medium and thin slabs.

[0007] Patent CN 109402354 A discloses a method for efficiently producing thick X70M pipeline hot-rolled strip with excellent low-temperature drop hammer performance. It adds precious alloying elements Mo and Ni, resulting in high alloy costs. It does not contain V, and the coiling temperature is 400℃-440℃. The process is difficult to control and stable, and it is easy to cause poor coil shape.

[0008] Patent CN 110961456 A discloses a low-cost 550MPa X70 hot rolling production method, which adds the precious alloying element Ni and has a thinner steel coil thickness (≤12mm).

[0009] Patent CN 112680659 A discloses a low compression ratio economical X70 pipeline steel and its production method. The 150mm thin slab used in the steel is produced by a furnace rolling mill. The yield strength allowance is relatively low, and the yield strength of the steel pipe is prone to decrease due to the Baosinger effect after pipe making, which may result in the steel pipe failing to meet the standard.

[0010] In summary, to achieve the high performance requirements of thick-gauge pipeline steel in existing technologies, a large amount of expensive alloying elements such as Cu, Ni, and Mo are typically added to the composition. This results in high alloy costs, demanding requirements for rolling and cooling processes, significant production control difficulties, and low yield rates. There is an urgent need to develop cost-effective and simple-process hot-rolled coils for thick-gauge high-performance pipeline steel to better meet the manufacturing and pipeline construction needs of spiral submerged arc welded pipes for oil and natural gas transportation. Summary of the Invention

[0011] To address the technical challenges of high alloy costs, stringent requirements for rolling and cooling processes, difficult production control, and low yield rates in pipeline steel for thick-walled spiral submerged arc welded steel pipes, this invention provides a high-performance X70M pipeline steel for thick-walled spiral submerged arc welded steel pipes and its manufacturing method. This method employs a low-cost, innovative composition design and reasonable element ratios, combined with appropriate smelting, continuous casting, heating, rolling, cooling, and coiling processes to produce hot-rolled coils of thick-gauge high-performance pipeline steel. Thick-walled, large-diameter steel pipes can be manufactured using spiral submerged arc welding, thereby reducing production costs and better meeting the manufacturing and construction needs of oil and gas pipelines.

[0012] In a first aspect, the present invention provides a high-performance X70M pipeline steel for thick-walled spiral submerged arc welded steel pipes, wherein the chemical composition and mass percentages are as follows: C: 0.070%~0.085%, Si: 0.18%~0.28%, Mn: 1.60%~1.75%, P≤0.015%, S≤0.002%, V: 0.033%~0.043%, Nb: 0.080%~0.095%, Cr: 0.25%~0.35%, Als: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities, and satisfying 0.115%. <Nb+V<0.130%;

[0013] The thickness of the hot-rolled pipeline steel coil is 14~22mm. The microstructure is mainly composed of fine and uniform acicular ferrite with a grain size ≥12 grade. There is no banded structure and non-metallic inclusions ≤0.5 grade. The yield strength Rt0.5 in the 30° direction is 535~585MPa, the tensile strength is 640~675MPa, the elongation is ≥35%, the yield strength ratio is ≤0.86, the impact energy at -40℃ is ≥250J, the drop shear area at -20℃ is ≥90%, and the hardness HV10 is ≤230.

[0014] Chemical composition has a significant impact on product cost and performance, and the chemical composition of this invention is limited as follows.

[0015] C: Carbon is the main solid solution strengthening element in steel, which can significantly improve the strength of steel. It can also form nanoscale carbides with microalloying elements such as niobium and vanadium, playing a role in grain refinement and precipitation strengthening. Therefore, the carbon content cannot be too low, but if the carbon content is too high, it will have an adverse effect on the toughness and weldability of steel. In this invention, the carbon content is controlled at 0.070%~0.085%.

[0016] Si: Silicon is an important solid solution strengthening element and has a deoxidizing effect, but excessive silicon content will lead to a decrease in the weldability and toughness of steel. The silicon content of this invention is controlled at 0.18%~0.28%.

[0017] Mn: Manganese has a significant solid solution strengthening effect, and can also reduce the phase transformation temperature of steel and refine the steel structure. However, Mn is also an element that is prone to segregation. When the content is too high, it can have an adverse effect on the toughness and weldability of steel. In this invention, the manganese content is controlled at 1.60%~1.75%.

[0018] P: Phosphorus increases the cold brittleness of steel, causes banded structure and central segregation, and deteriorates the toughness, weldability and corrosion resistance of steel. It is a harmful element and its content should be reduced as much as possible. In this invention, the phosphorus content is controlled at ≤0.015%.

[0019] S: Sulfur is also a harmful element that increases the hot brittleness of steel and reduces its toughness. The sulfur content in steel should be strictly controlled. In this invention, the sulfur content is controlled at ≤0.002%.

[0020] V: Vanadium is also a microalloying element, and it is used in combination with niobium in this invention. Vanadium can precipitate a large number of fine vanadium nitrides during and after the winding stage, resulting in a significant precipitation strengthening effect and thus improving the yield strength. However, it will increase the ductile-brittle transition temperature. Therefore, the vanadium content in this invention is controlled at 0.033%~0.043%.

[0021] Niobium (Nb) is a microalloying element that can increase the recrystallization temperature of austenite in steel, expand the rolling range of the non-recrystallized region, and facilitate rolling in the non-recrystallized region. Niobium carbonitride precipitates at defects such as dislocation grain boundaries, which can inhibit austenite recrystallization and prevent grain growth, thereby refining the grains. It can also promote the formation of acicular ferrite structure, significantly improving the strength and toughness of steel. However, niobium iron is expensive. Based on its function and cost considerations, the niobium content in this invention is controlled at 0.080%~0.095%.

[0022] Cr: Chromium has a strong solid solution strengthening effect and can improve the hardenability of steel. It can effectively improve the uniformity of the microstructure in the thickness direction of steel, promote the formation of acicular ferrite, and improve the strength and toughness of steel. However, excessive chromium content will increase the sensitivity to welding cracks and reduce the weldability of steel. In this invention, the chromium content is controlled at 0.25%~0.35%.

[0023] Al: Aluminum is an important deoxidizing element and can also refine the microstructure, but its content should not be too high, otherwise it will easily cause cracks in the casting. In this invention, the aluminum content is controlled at 0.020%~0.050%.

[0024] Secondly, the present invention provides a method for manufacturing high-performance X70M pipeline steel for the aforementioned thick-walled spiral submerged arc welded steel pipe, comprising smelting, continuous casting, heating, rolling, cooling, and coiling processes, wherein...

[0025] In the heating process, the temperature of the soaking zone is controlled at 1210~1250℃;

[0026] In the rolling process, the thickness of the intermediate slab is controlled at 64~70mm, the entry temperature of the finishing mill is 960~1010℃, the finishing mill finishing temperature is 790~840℃, and the finishing mill adopts a seven-stand continuous rolling process. The reduction rate of the first three passes in the finishing mill stage is ≤30%, and the reduction rate of the last pass is ≥9%.

[0027] In the winding process, the winding temperature is controlled at 470~530℃, and after winding into a roll, it is air-cooled to room temperature.

[0028] Furthermore, in the steelmaking process, converter smelting and LF+RH furnace double refining are adopted, and the pure degassing time is ≥6min.

[0029] Furthermore, in the continuous casting process, argon gas is used for the entire casting process, the superheat is controlled at 15~25℃, and the billet casting speed is a constant casting speed of 0.9~1.2m / min.

[0030] Furthermore, in the rolling process, the thicker the target steel strip, the lower the finishing rolling temperature should be. Specifically, for a target steel strip thickness of 14~18mm, the finishing rolling inlet temperature should be controlled at 970~1010℃, and the finishing rolling end temperature should be controlled at 800~840℃.

[0031] For steel strips with a target thickness of 18~22mm, the entry temperature of the finishing mill is controlled at 960~1000℃, and the finishing mill finishing temperature is controlled at 790~830℃.

[0032] Furthermore, in the cooling process, a segmented cooling mode is adopted, consisting of ultra-fast cooling, front-end laminar flow cooling for temperature control, and rear-end laminar flow cooling for fine-tuning. First, the number of ultra-fast cooling water-cooling manifolds and the water pressure are adjusted according to different thicknesses to achieve an ultra-fast cooling rate of 20~30℃ / s, ensuring that the ultra-fast cooling outlet temperature of the steel strip is 550~590℃. Then, the front-end laminar flow cooling water continuously cools and controls the temperature return of the steel strip, ensuring that the temperature return of the steel strip does not exceed 600℃. Finally, the rear-end laminar flow cooling water fine-tunes the temperature of the steel strip to ensure that the set coiling temperature is reached.

[0033] Furthermore, in the winding process, for target steel strip thickness of 14~18mm, the winding temperature is controlled at 490~530℃;

[0034] For steel strips with a thickness of 18~22mm, the winding temperature should be controlled at 470~510℃.

[0035] The manufacturing process is crucial to the overall performance of the product, and the manufacturing process of this invention is subject to limitations, as explained below.

[0036] Steelmaking process: In order to fully ensure the purity of molten steel, the molten steel undergoes LF+RH double refining after converter smelting, and specific process parameters are restricted.

[0037] Continuous casting process: In order to prevent secondary oxidation of molten steel, full-process protective casting is adopted. Constant casting speed control is to ensure the stability of billet quality, while limiting superheat is mainly to ensure billet quality and avoid or reduce segregation. The use of a chamfered crystallizer is to ensure the quality of the steel coil edge and prevent edge crack defects.

[0038] Heating process: The solid solution temperature is calculated based on the content of the microalloying element niobium. The heating temperature and time are limited according to different furnace loading methods to ensure that the billet is fully austenitized and the alloying elements are fully dissolved, while avoiding coarsening of austenite grains.

[0039] Rolling process: Based on the influence of niobium content on the recrystallization temperature of austenite, reasonable roughing and finishing rolling temperatures are determined. Roughing is conducted in the recrystallization zone, utilizing repeated austenite recrystallization to refine the grains. Finishing is conducted in the non-recrystallization zone to prevent mixed grain formation. Simultaneously, the finishing reduction rate is limited to ensure sufficient flattening of the austenite grains in thick-gauge pipeline steel, achieving austenite grain refinement and homogenization, and forming numerous subgrain boundaries, dislocations, and other deformed structures, creating conditions for the subsequent formation and refinement of acicular ferrite. The intermediate billet thickness is determined based on the finished product thickness to ensure sufficient reduction during finishing rolling to fully refine the grains.

[0040] Cooling Process: The purpose of ultra-rapid cooling after rolling is to rapidly cool the steel strip to a lower temperature, increase undercooling, lower the phase transformation temperature, promote the formation of acicular ferrite, further refine the microstructure, and improve strength and toughness. A low cooling rate cannot form the ideal acicular ferrite microstructure for pipeline steel and will also lead to insufficient cooling penetration in the thickness direction, resulting in uneven microstructure, especially pearlite in the thickness center with coarse grains. This makes it difficult to meet requirements for strength and low-temperature drop hammer performance. However, an excessively high cooling rate can easily cause a camber at the coil head, making coiling difficult, affecting coil quality, and increasing control difficulty, leading to instability in process, microstructure, and properties. Therefore, the ultra-rapid cooling process parameters are reasonably limited. Continuous cooling and temperature control of the steel strip using laminar flow cooling water is used to prevent excessively high reheat temperatures, which can adversely affect the steel's microstructure type, grain size, and the precipitation of microalloying elements.

[0041] Coiling process: When the coiling temperature is too high, the steel grain size is coarse, with more pearlite and larger precipitates in the microstructure, resulting in a significant decrease in the impact toughness of the strip. When the coiling temperature is too low, it promotes the formation of fine needle-like ferrite, but the low temperature inhibits the precipitation of VC, resulting in insufficient precipitation strengthening and lower strength. It also increases the difficulty of coiling and makes production unstable. Therefore, it is necessary to set a reasonable coiling temperature according to the thickness of the steel coil.

[0042] The beneficial effects of this invention are as follows:

[0043] This invention employs a C-Si-Mn-Nb-Cr-V composition design system, strictly controlling the content of elements such as P and S, and refraining from adding precious alloying elements such as Cu, Ni, and Mo. Through the design of the micro-alloying elements Nb and V ratio, combined with clean steel smelting processes and appropriate controlled rolling and cooling processes, 14-22mm thick hot-rolled pipeline steel coils are produced on a 2050mm hot continuous rolling production line. The products have high strength, low yield strength ratio, low hardness, excellent low-temperature toughness, and strong crack arrest ability, among other high-performance characteristics. Moreover, the production cost is low, the cost-effectiveness is high, the process is simple and easy to implement, and it is suitable for large-scale production. It is applicable to the manufacturing of thick-walled, large-diameter spiral submerged arc welded pipes for oil and gas transportation and their pipeline construction. Attached Figure Description

[0044] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0045] Figure 1 This is a microstructure diagram of the center of thickness of the hot-rolled steel coil produced in Example 3.

[0046] Figure 2 This is a microstructure diagram of the center of thickness of the hot-rolled steel coil produced in Comparative Example 3. Detailed Implementation

[0047] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0048] The embodiments and comparative examples of this invention adopt the following steps: hot metal pretreatment → steelmaking (converter steelmaking, LF+RH double refining) → continuous casting → heating → rolling → cooling → coiling.

[0049] The chemical composition and mass percentage of the steel are as follows: C: 0.070%~0.085%, Si: 0.18%~0.28%, Mn: 1.60%~1.75%, P≤0.015%, S≤0.002%, V: 0.033%~0.043%, Nb: 0.080%~0.095%, Cr: 0.25%~0.35%, Al: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities, and satisfying 0.115%. <Nb+V<0.130%。

[0050] The steelmaking process employs a dual refining process using LF+RH furnaces, with vacuum holding time ≥12 min, pure degassing time ≥6 min, and soft blowing time ≥15 min. Continuous casting utilizes argon-protected casting throughout the process, with tundish superheat controlled at 15~25℃, and low constant casting speed (0.9~1.2 m / min) for production, resulting in a billet thickness of 230 mm. The billet is then heated, with hot-delivered billet heating time controlled at 180~260 min and cold-delivered billet heating time at 200~280 min. The soaking zone temperature is 1210~1250℃, with a soaking time of 30~60 min, and the furnace exit temperature is 1200~1240℃. After exiting the furnace, the steel strip undergoes two-stage controlled rolling. In the roughing stage, the strip is rolled in the recrystallization zone, with the roughing finishing temperature (R2DT) controlled at 1020~1060℃ and the intermediate slab thickness at 64~70mm. The finishing entry temperature (FET) is 960~1010℃, and the finishing finishing temperature (FDT) is 790~840℃. The finishing stage is performed using a seven-stand continuous rolling mill. The reduction rate of the first three passes in the finishing stage is ≤30%, and the reduction rate of the last pass is ≥9%. The steel strip thickness after finishing is 14~22mm. For target steel strip thickness of 14~18mm, the finishing mill entry temperature (FET) is controlled at 970~1010℃ and the finishing mill finishing temperature (FDT) is controlled at 800~840℃; for target steel strip thickness of 18~22mm, the finishing mill entry temperature (FET) is controlled at 960~1000℃ and the finishing mill finishing temperature (FDT) is controlled at 790~830℃. The thicker the target steel strip, the lower the rolling temperature. After rolling, a segmented cooling mode is adopted, consisting of ultra-fast cooling, front-stage laminar flow cooling for temperature control, and rear-stage laminar flow cooling for fine adjustment. The number of ultra-fast cooling water-cooling manifolds and water pressure are adjusted according to different thicknesses to achieve an ultra-fast cooling rate of 20~30℃ / s, ensuring that the ultra-fast cooling outlet temperature (MT1) of the steel strip is 550~590℃. Subsequently, the front-stage laminar flow cooling water continuously cools and controls the temperature return of the steel strip, ensuring that the temperature return does not exceed 600℃. Finally, the rear-stage laminar flow cooling water finely adjusts the temperature of the steel strip to ensure that the set coiling temperature (CT) is reached, and coiling is carried out at 470~530℃. For target steel strip thicknesses of 14~18mm, the coiling temperature is controlled at 490~530℃; for target steel strip thicknesses of 18~22mm, the coiling temperature is controlled at 470~510℃. After coiling, the steel coil is air-cooled to room temperature.

[0051] The chemical composition designs of each embodiment and comparative example are shown in Table 1 below.

[0052] Table 1 Chemical composition of each example and comparative example (wt.%, balance is Fe and unavoidable impurities)

[0053] project C Si Mn P S V Nb Cr Als Nb+V Example 1 0.073 0.21 1.67 0.010 0.0008 0.033 0.083 0.29 0.032 0.116 Example 2 0.080 0.20 1.68 0.009 0.0010 0.039 0.084 0.26 0.036 0.123 Example 3 0.078 0.24 1.65 0.008 0.0012 0.040 0.089 0.31 0.037 0.129 Comparative Example 1 0.088 0.23 1.62 0.015 0.0012 0.049 0.071 0.15 0.034 0.120 Comparative Example 2 0.062 0.19 1.68 0.009 0.0027 0.021 0.092 0.22 0.029 0.113 Comparative Example 3 0.073 0.22 1.66 0.010 0.0011 0.038 0.088 0.26 0.034 0.126

[0054] The main process parameters of each embodiment and comparative example are shown in Tables 2 and 3 below.

[0055] Table 2. Main parameters of steelmaking, continuous casting, and heating processes in each embodiment and comparative example.

[0056]

[0057] Table 3. Main parameters of rolling, cooling, and coiling processes for each embodiment and comparative example.

[0058]

[0059] The mechanical property test data of the steel coils manufactured in each embodiment and comparative example are shown in Table 4 below.

[0060] Table 4 Mechanical property test data of each embodiment and comparative example

[0061]

[0062] The metallographic test data of the steel coils manufactured in each embodiment and comparative example are shown in Table 5 below.

[0063] Table 5 Metallographic test data for each embodiment and comparative example

[0064]

[0065] Comparing the mechanical properties and metallographic characteristics of the comparative examples and the specific examples, it can be seen that the thick-gauge hot-rolled pipeline steel coils produced according to the composition and process of this invention have outstanding characteristics such as cost-effectiveness and excellent comprehensive performance, making them suitable for the manufacture and construction of thick-walled, large-diameter spiral submerged arc welded pipes for oil and gas transportation. In contrast, the comparative examples, due to differences in composition, process, and microstructure, exhibit less than ideal strength and low-temperature drop hammer performance, failing to meet the requirements.

[0066] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for manufacturing high-performance X70M pipeline steel for thick-walled spiral submerged arc welded steel pipes, characterized in that, This includes smelting, continuous casting, heating, rolling, cooling, and coiling processes, among which... In the heating process, the temperature of the soaking zone is controlled at 1210~1250℃; In the rolling process, the thickness of the intermediate slab is controlled at 64~70mm, the entry temperature of the finishing mill is 960~1010℃, the finishing mill finishing temperature is 790~840℃, and the finishing mill adopts a seven-stand continuous rolling process. The reduction rate of the first three passes in the finishing mill stage is ≤30%, and the reduction rate of the last pass is ≥9%. In the winding process, the winding temperature is controlled at 470~530℃, and after winding into a roll, it is air-cooled to room temperature; The chemical composition and mass percentage of the high-performance X70M pipeline steel for thick-walled spiral submerged arc welded steel pipes are as follows: C: 0.070%~0.085%, Si: 0.18%~0.28%, Mn: 1.60%~1.75%, P≤0.015%, S≤0.002%, V: 0.033%~0.043%, Nb: 0.080%~0.095%, Cr: 0.25%~0.35%, Als: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities, and satisfying 0.115%. <Nb+V<0.130%; The thickness of the hot-rolled pipeline steel coil is 14~22mm. The microstructure is mainly composed of fine and uniform acicular ferrite with a grain size ≥12 grade. There is no banded structure and non-metallic inclusions ≤0.5 grade. The yield strength Rt0.5 in the 30° direction is 535~585MPa, the tensile strength is 640~675MPa, the elongation is ≥35%, the yield strength ratio is ≤0.86, the impact energy at -40℃ is ≥250J, the drop shear area at -20℃ is ≥90%, and the hardness HV10 is ≤230.

2. The manufacturing method as described in claim 1, characterized in that, In the steelmaking process, converter smelting and LF+RH furnace double refining are adopted, and the pure degassing time is ≥6min.

3. The manufacturing method as described in claim 1, characterized in that, In the continuous casting process, argon gas is used for the entire casting process, the superheat is controlled at 15~25℃, and the billet casting speed is a constant casting speed of 0.9~1.2m / min.

4. The manufacturing method as described in claim 1, characterized in that, In the rolling process, for steel strips with a target thickness of 14~18mm, the finishing mill inlet temperature is controlled at 970~1010℃, and the finishing mill final rolling temperature is controlled at 800~840℃.

5. The manufacturing method as described in claim 1, characterized in that, In the rolling process, for steel strips with a target thickness of 18~22mm, the entry temperature of the finishing mill is controlled at 960~1000℃, and the finishing mill finishing temperature is controlled at 790~830℃.

6. The manufacturing method as described in claim 1, characterized in that, In the cooling process, a segmented cooling mode is adopted, consisting of ultra-fast cooling, front-end laminar flow cooling for temperature control, and rear-end laminar flow cooling for fine-tuning. First, the number of ultra-fast cooling water-cooling manifolds and the water pressure are adjusted according to different thicknesses to achieve an ultra-fast cooling rate of 20~30℃ / s, ensuring that the ultra-fast cooling outlet temperature of the steel strip is 550~590℃. Then, the front-end laminar flow cooling water continuously cools and controls the temperature return of the steel strip, ensuring that the temperature return of the steel strip does not exceed 600℃. Finally, the rear-end laminar flow cooling water fine-tunes the temperature of the steel strip to ensure that the set coiling temperature is reached.

7. The manufacturing method as described in claim 1, characterized in that, In the winding process, for target steel strips with a thickness of 14~18mm, the winding temperature is controlled at 490~530℃.

8. The manufacturing method as described in claim 1, characterized in that, In the winding process, for target steel strip thickness of 18~22mm, the winding temperature is controlled at 470~510℃.

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