A large-thickness ultra-low-temperature toughness pipeline steel and a manufacturing method thereof

Abstract

This invention discloses a thick-walled, ultra-low temperature toughness pipeline steel and its manufacturing method. The chemical composition (mass percentage) of the steel is as follows: C 0.025-0.05%, Si 0.1-0.3%, Mn 1.4-1.6%, P≤0.015%, S≤0.002%, Nb 0.03-0.06%, Ti 0.01-0.02%, Cr 0.2-0.3%, Ni 0.3-0.8%, Mo 0.06-0.12%, Al 0.02-0.04%, with the balance being Fe and unavoidable impurities. B must not be intentionally added and B≤0.0005%. The process includes: continuous casting of slabs ≥ 6 tons; heating at 1100–1180℃ and > 1000℃ for ≥ 0.8 t in the furnace; rough rolling at ≤ 1000℃; intermediate slab thickness 3–4 times that of the finished product; finishing rolling at 740–750℃; accelerated cooling at ≤ 300–350℃, 20–40℃ / s; and tempering at 600–680℃. The steel plates produced by this invention have a thickness of 30–50 mm, yield strength ≥ 450 MPa, tensile strength ≥ 535 MPa, Charpy impact energy at -40℃ ≥ 250 J, and DWTT shear area at -30℃ ≥ 85% of the full thickness. The microstructure is mainly composed of ultrafine-grained polygonal ferrite and granular bainite / acicular ferrite.

Description

Technical Field

[0001] This invention belongs to the field of pipeline steel technology, specifically relating to a thick-walled ultra-low temperature toughness pipeline steel and its manufacturing method, mainly used for the production and manufacturing of pipeline steel materials for pipeline engineering in polar regions. Background Technology

[0002] In oil and gas infrastructure in regions such as the Arctic Circle, Alaska, northern Canada, Siberia, and high-latitude seas, long-distance pipelines are facing multiple extreme operating conditions: ultra-low temperature operation ranging from -30°C to -80°C and the impact of start-up and shutdown temperature differences, as well as complex load environments in the deep sea and the sea-land transition zone. To meet the safety margins for high internal pressure and resistance to buckling under external loads, engineering projects are increasingly using pipeline steel with large wall thicknesses (e.g., 30–50 mm, or even thicker), ensuring high Charpy and DWTT performance at -30°C or even -50°C under full thickness conditions. Therefore, the material system must not only achieve high strength and high toughness "base material properties," but also obtain a high degree of consistency in microstructure and properties in the thickness direction to ensure fracture propagation control and full-life reliability.

[0003] In terms of technological approaches, the international mainstream is TMCP (controlled rolling and controlled cooling) for "precise microstructure control." The main approach is low-carbon (C≈0.02–0.06%) + Nb–Ti–V composite microalloying, supplemented with a small amount of Ni and optional Mo / Cu / Cr. The goal is to form fine, uniform polygonal ferrite (PF) / granular bainite (GB) multiphase microstructures in thick sections, suppressing continuous MA islands and banded segregation. In thick plate production, it is also necessary to balance cooling intensity with plate thickness thermal inertia to avoid falling into the unmanufacturable zone of "ultra-high cooling rate / ultra-high equipment burden" in pursuit of toughness.

[0004] In terms of industrial practice and technological trends, domestic and foreign steel companies have developed a series of large-thick-walled TMCP pipeline steel products to serve high-altitude and subsea pipelines. Current R&D focus is shifting from "alloy and cooling strength" to "precision microstructure in the thick section" and "manufacturability robustness": low-temperature toughness control is achieved through low-C-Nb-Ti-V alloys with controllable alloy costs, synergistic addition of trace amounts of Ni / Cr / Cu, and low-temperature final rolling. However, fluctuations are likely to occur in drop hammer tests below -30°C, making stable mass production difficult. High cleanliness and inclusion engineering are also employed to reduce the risk of hydrogen-induced damage. Looking to the future, the coupled control of the thick-section temperature field and phase transformation field, as well as the integrated design of the base material, welding, and service environment, will become key directions for the continuous iteration of large-thick-walled ultra-low temperature toughness pipeline steel.

[0005] Chinese patent CN103834874A discloses a thick-walled X65-70 grade submarine pipeline steel, employing a C-Mn-Nb-Cr composition system and produced using a hot-rolled TMCP process. By using a relatively low C content (0.03-0.05%) and a relatively high Mn content (1.47-1.70%), combined with the appropriate addition of Cr, Cu, and Ni elements, pipeline steel with a thickness greater than 25.4 mm is achieved, simultaneously meeting the performance requirements of ≥320J impact energy at -20℃ and ≥85% DWTT at -15℃. Compared to this patent, the microstructure of this patent is dominated by acicular ferrite, rather than ultrafine-grained polygonal ferrite and a composite microstructure of granular bainite / acicular ferrite, and it does not employ high-temperature tempering heat treatment to improve the uniformity of the microstructure in the thickness direction, thus failing to effectively eliminate the M / A microstructure.

[0006] Chinese patent CN112143968A discloses an ultra-thick X70M-grade dual-resistance pipeline steel plate. This patent provides an ultra-thick X70M-grade dual-resistance (HIC and SSCC resistance) pipeline steel plate. Through reasonable composition design and matching of the ultra-fast cooling rolling process, it achieves X70M-grade pipeline steel with a thickness of 19-38mm, simultaneously possessing excellent resistance to hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), as well as excellent atmospheric corrosion resistance. The composition design employs a relatively high carbon content (0.05-0.08%) and a moderate manganese content (1.15-1.30%), with the composite addition of Ni, Cr, and Cu to improve the steel plate's strength. Compared to this patent, the carbon content of this patent is relatively high, which may adversely affect weldability, and its cooling process uses a UFC+ACC combined cooling mode, placing higher demands on the cooling equipment.

[0007] Chinese patent CN105950973B discloses a thick-gauge X80 pipeline steel with excellent ultra-low temperature impact performance, mainly used for the production of high-grade pipeline steel with a thickness of 30mm or more. Through a design with low C (0.02-0.06%) and high Mn (1.45-1.6%), combined with the addition of microalloying elements Nb, V, and Ti, and appropriate amounts of Mo, Cu, and Ni, and employing intermediate billet cooling technology and a two-stage rolling process, it achieves excellent low-temperature toughness with a DWTT shear area ≥85% at -25℃. Compared to this patent, which focuses more on the toughness control of ultra-thick X80 high-strength steel with a thickness of 30mm or more, this patent emphasizes stability under ultra-low temperature conditions.

[0008] Therefore, developing a thick-walled pipeline steel that can meet the requirements of ultra-low temperature environments below -30℃, has excellent full-thickness DWTT performance, and good microstructure uniformity has become an urgent technical problem to be solved in this field. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a thick-walled ultra-low temperature toughness pipeline steel and its manufacturing method in light of the above-mentioned prior art, so as to solve the technical problems of insufficient toughness and uneven microstructure in the thickness direction of 30-50mm thick-walled pipeline steel in ultra-low temperature environment.

[0010] The technical solution adopted by this invention to solve the above problems is as follows: a thick-walled ultra-low temperature toughness pipeline steel, whose microstructure is mainly composed of ultrafine-grained polygonal ferrite and granular bainite / acicular ferrite, with ultrafine-grained polygonal ferrite accounting for 30%-50%; its chemical composition by mass percentage is: C: 0.025-0.05%, Si: 0.1-0.3%, Mn: 1.4-1.6%, P≤0.015%, S≤0.002%, Nb: 0.03-0.06%, Ti: 0.01-0.02%, Cr: 0.2-0.3%, Ni: 0.3-0.8%, Mo: 0.06-0.12%, Al: 0.02-0.04%, with the remainder being Fe and unavoidable impurities. B must not be intentionally added to the composition and B≤0.0005%.

[0011] The pipeline steel of this invention has a steel grade of X65-X70, a plate thickness of 30-50mm, a yield strength ≥450MPa, a tensile strength ≥535MPa, a full-size Charpy impact energy ≥250J at -40℃, and a full-thickness DWTT fracture shear area percentage ≥85% at -30℃. It is mainly used for the production and manufacturing of pipeline steel for pipeline engineering in extremely cold areas or deep-sea environments below -30℃.

[0012] In the design of the chemical composition of this invention: Carbon (C): As a fundamental strengthening element, excessively high C content can easily lead to localized C segregation in continuously cast billets, worsening toughness. High C content also hinders weldability. This invention controls the C content to be 0.025–0.05%.

[0013] Si: Si is a solid solution strengthening element and also a deoxidizing element in steel. However, if the content is too high, it will reduce weldability and is not conducive to improving the surface quality of hot-rolled steel plates. Therefore, this invention controls the Si content to be between 0.1% and 0.3%.

[0014] Mn: The Mn design of this invention adopts a high Mn content design, controlling the Mn content range to 1.40-1.60%. The relatively high Mn content achieves solid solution strengthening to improve strength, compensating for the strength reduction caused by the ultra-low C content control and the high proportion of ferrite microstructure. However, the Mn content should not be controlled too high, as high Mn can easily lead to a deterioration in the weldability of the material and reduce the inhomogeneity of the steel plate microstructure caused by Mn segregation.

[0015] Cr: Cr can improve hardenability, enhance the uniformity of microstructure in the thickness direction, and improve the corrosion resistance of steel. However, excessive addition of chromium and manganese to steel simultaneously can lead to the formation of low-melting-point Cr-Mn composite oxides, which easily cause surface cracks and reduce weldability. Therefore, the Cr content in this invention is controlled at 0.1% to 0.3%.

[0016] Ni: Ni can prolong the incubation period of pearlite and lower the Ar3 phase transformation temperature, which is beneficial to compensate for the strength decrease caused by the increase in thickness. At the same time, it can promote the transformation of bainite at a lower cooling rate, effectively improve the stability of the strength properties of steel plates, and improve the stability of DWWTT. Mo: Mo expands the γ-phase region, lowers the Ar3 phase transformation temperature, which is beneficial for increasing the finishing rolling temperature. It also improves the hardenability of thicker gauge steels and enhances the uniformity of properties in the thickness direction of thick-walled pipeline steel. However, Mo is a precious alloy, resulting in high costs. Furthermore, excessively high Mo content can lead to brittle structures, which is detrimental to improving toughness. In this invention, the Mo content is controlled between 0.08% and 0.18%.

[0017] Nb: Nb is one of the most important elements in controlled rolling and controlled cooling of low-carbon microalloyed steel. Combined with a two-stage rolling process, it can effectively reduce mill load and has a significant effect on grain refinement. In the rapid cooling stage, dissolved Nb can effectively delay the ferrite phase transformation, which is beneficial to the production of equiaxed ferrite. In the slow cooling stage, dissolved Nb precipitates in the form of NbC, which can simultaneously improve strength and toughness. Too low an Nb content is not conducive to the grain refinement and precipitation strengthening effects; therefore, the Nb content in this invention is controlled at 0.03% to 0.06%.

[0018] Ti: Ti is a strong carbonitride forming element. Undissolved Ti carbonitrides can inhibit the growth of austenite grains when steel is heated. During pipe welding, TiN particles in the steel can effectively improve the toughness of the weld heat-affected zone. However, too high a Ti content can easily lead to the formation of large Ti inclusions, which deteriorates the steel plate properties. Therefore, the Ti content in this invention is controlled between 0.01% and 0.02%.

[0019] Al: An element added to steel for deoxidation. Adding an appropriate amount of Al helps to refine the grains and improve the strength and toughness of the steel.

[0020] S, P: Strictly control the S and P element content of steel to improve the cleanliness of steel, reduce the content of inclusions in steel, and modify inclusions through Ca treatment to prevent toughness deterioration caused by inclusions.

[0021] B: B is a highly hardenable element and is prone to precipitating at grain boundaries, which can lead to a decrease in the plasticity and toughness of the material. Therefore, this invention controls B to be ≤0.0005% and B should not be intentionally added during smelting.

[0022] The manufacturing method of the thick-walled, high-toughness deep-sea pipeline steel of the present invention includes the following steps: 1) Smelting and continuous casting The slabs are smelted and continuously cast into slabs according to the above chemical composition. The smelting is carried out in a converter or electric furnace, followed by LF refining and RH vacuum treatment, and Ca treatment before continuous casting. The thickness t of the continuously cast slab and the thickness T of the finished steel plate satisfy t≥6T, where T is the thickness of the finished steel plate, i.e., the compression ratio ≥6. By using a large-thickness continuously cast slab with a high compression ratio, the grains are fully refined and the toughness of the steel plate is improved. 2) Slab heating The continuously cast slab is heated to a temperature of 1100–1180℃. The slab's time in the furnace at temperatures above 1000℃ is no less than 0.8*t, where t is the slab thickness in mm and the time in minutes. This relatively low-temperature heating regime helps suppress the coarsening of the original austenite grains, laying the foundation for subsequent grain refinement and improved toughness. Simultaneously, it ensures sufficient time for the slab to remain in the high-temperature section of the furnace, guaranteeing thorough heating and uniform temperature, and reducing rolling control fluctuations caused by uneven or incomplete heating of the slab.

[0023] 3) Rough rolling (first stage rolling) The initial rolling temperature of roughing is ≤1000℃. Lowering the initial rolling temperature allows the material to undergo repeated recrystallization at a lower temperature during the roughing stage, thus refining the grains more thoroughly. The average reduction rate of the last three passes in roughing is ≥16%. By ensuring the reduction rate of the last three passes in roughing, repeated recrystallization of the material is promoted to refine the grains.

[0024] 4) Thickness to be heated After rough rolling, the intermediate billet thickness at the heating stage should be controlled to be 3-4 times the thickness of the finished steel plate. This heating thickness must be strictly controlled within this range. If the heating thickness is too low, the deformed austenite height will be insufficient during the finishing rolling stage, which is detrimental to the formation of fine ferrite grains. If the heating thickness is too high, firstly, the repeated recrystallization during the rough rolling stage will reduce the effect of refining austenite grains; secondly, the deformation during finishing rolling will be too large, leading to strain-induced proeutectoid ferrite production or secondary recrystallization, resulting in coarse grains and deteriorated performance.

[0025] 5) Finish rolling (second stage rolling) The initial rolling temperature of the finishing mill is controlled at 730-760℃, and the final rolling temperature is controlled at 740-750℃. Low-temperature rolling ensures the accumulated deformation energy of austenite, promoting the formation of fine equiaxed ferrite grains after rolling. The temperature difference between the initial and final rolling temperatures is strictly controlled within a small range of 5-10℃ to ensure uniform deformation throughout the thickness direction.

[0026] 6) Control cooling After rolling, the steel plate is accelerated and cooled at a rate of 25-40℃ / s, with a final cooling temperature of ≤350℃. Through high-speed cooling and control of the final cooling temperature, the formation of granular bainite or acicular ferrite with good toughness is ensured.

[0027] 7) High-temperature tempering heat treatment The steel plate is finally subjected to a high-temperature tempering heat treatment at 600-680℃, with a tempering coefficient of 2.0-6.0 min / mm (i.e., holding at that temperature for 2.0-6.0 minutes per millimeter of thickness). This high-temperature tempering treatment improves the thickness-direction microstructure inhomogeneity caused by rapid cooling, and eliminates the brittle M / A (martensite / austenite) microstructure that easily forms in the core area, thereby significantly improving the material's high toughness control stability at low temperatures.

[0028] The manufacturing process design of this invention mainly adopts low-temperature heating, low-temperature rough rolling with large reduction, intermediate billet thickness control, low-temperature finishing rolling and rapid cooling. By controlling the grain morphology and phase transformation process of the original austenite, the target microstructure, mainly composed of ultrafine-grained polygonal ferrite and granular bainite / acicular ferrite, is obtained, thereby obtaining a large thick-walled pipeline steel with high toughness at ultra-low temperatures.

[0029] Compared with the prior art, the present invention has the following beneficial effects: 1) This invention effectively eliminates the brittle and hard M / A structure formed during accelerated cooling by adding a high-temperature tempering treatment of 600-680℃ to the TMCP process, making the full-thickness DWTT shear area ≥85% at -30℃ and the Charpy impact energy ≥250J at -40℃, meeting the stringent requirements for use in extremely cold regions and deep-sea environments.

[0030] 2) By combining the processes of high compression ratio continuous casting billet (t≥6T), precise control of intermediate billet thickness (3-4 times the finished product thickness), small temperature difference control of finishing rolling (5-10℃), and high temperature tempering, the microstructure uniformity of 30-50mm thick-walled pipeline steel in the thickness direction is significantly improved, and the performance difference between the core and the surface is eliminated.

[0031] 3) This invention adopts a low carbon + low Mo (0.06-0.12%) composition design to avoid the use of expensive alloying elements. The amount of Ni, Cr and Mo is controlled within a reasonable range, which has a lower alloy cost compared with the comparative scheme with high Mo or high Ni content. Attached Figure Description

[0032] Figure 1 This is a typical microstructure diagram of the pipeline steel plate in TMCP state in Embodiment 1 of the present invention.

[0033] Figure 2 This is a typical microstructure diagram of the pipeline steel plate after tempering at 650℃ in Embodiment 1 of the present invention.

[0034] Figure 3 This is a SEM image of the fracture morphology (TMCP state) of the DWTT fracture surface at -30℃ in this invention.

[0035] Figure 4 This is a SEM image of the fracture morphology of the DWTT fracture surface at -30℃ (tempered at 650℃) of the present invention. Detailed Implementation

[0036] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0037] Description of relevant performance testing methods involved in the embodiments Charpy impact test: conducted according to GB / T 229-2020 standard, using full-size Charpy V-notch impact specimens (10mm×10mm×55mm), test temperature -40℃, measuring the impact absorption energy of three specimens and calculating the average value.

[0038] DWTT drop hammer tear test: conducted according to GB / T 8363-2020 standard, using full-thickness specimens (original steel plate thickness, width 75±1.5mm), test temperature -30℃, measuring the percentage of shear area at the fracture surface, and taking the average value of two specimens for each condition.

[0039] Tensile test: conducted according to GB / T 228.1-2021 standard, using transverse rectangular specimens, to measure yield strength Rt0.5, tensile strength Rm, yield ratio YR and elongation after fracture A.

[0040] Table 1 shows the composition of the steel in Example 1 of the present invention, Table 2 shows the manufacturing process parameters of the steel in Example 1 of the present invention, and Table 3 shows the properties of the steel in Example 1 of the present invention.

[0041] Figure 1 and Figure 2 The images show typical microstructures of the steel of Example 1 of the present invention in the TMCP state and the tempered state at 650°C.

[0042] As shown in Table 3, the pipeline steel obtained according to the composition and manufacturing process designed in this invention can meet the target performance requirements and has good comprehensive mechanical properties.

[0043] Table 1

[0044] Manufacturing method: (1) Smelting and continuous casting: After smelting and Ca treatment according to the above chemical composition, the slab is continuously cast into a slab with a thickness t of 365mm (t=365mm ≥ 6T=240mm, compression ratio about 9.1).

[0045] (2) Slab heating: The heating temperature is 1160℃, and the time in the furnace at >1000℃ is 200 minutes (0.8×t=292 minutes. The actual holding time in this embodiment is slightly lower than the theoretical value, but uniform heating can still be achieved by optimizing the heating regime).

[0046] (3) Rough rolling: The initial rolling temperature of rough rolling is 980℃, and the average reduction rate of the last three passes of rough rolling is ≥16%.

[0047] (4) Thickness to be heated: After rough rolling, the thickness of the intermediate billet is 148mm, which is 3.7 times the thickness of the finished product of 40mm.

[0048] (5) Finishing rolling: The starting temperature of finishing rolling is 760℃, the finishing rolling temperature is 755℃, and the temperature difference is 5℃.

[0049] (6) Controlled cooling: accelerated cooling, cooling rate 32℃ / s, final cooling temperature 320℃.

[0050] (7) High temperature tempering: tempering temperature 650℃, holding time 160 minutes (tempering coefficient 4.0 min / mm).

[0051] Table 2 Table 3

[0052] Note: T, Z, W: Head, Middle, Tail; Tr: Lateral; YR: Yield-to-tensile ratio; A: Elongation; DWTT: Drop weight test As shown in Table 3, the steel plate of Example 1 has a yield strength of 490-510 MPa, a tensile strength of 600-615 MPa, a yield-to-tensile ratio of 0.81-0.85, and an elongation of 46-47%. The average Charpy impact energy at -40℃ is 347-364 J, far exceeding the target requirement of 250 J. The average DWTT shear area of ​​the entire plate thickness at -30℃ is 90-94%, meeting the target requirement of ≥85%. The properties of the steel plate are basically consistent at the head, middle, and tail sections, and the properties are uniform in the thickness direction.

[0053] Performance testing method description Charpy impact test: conducted according to GB / T 229-2020 standard, using full-size Charpy V-notch impact specimens (10mm×10mm×55mm), test temperature -40℃, measuring the impact absorption energy of three specimens and calculating the average value.

[0054] DWTT drop hammer tear test: conducted according to GB / T 8363-2020 standard, using full-thickness specimens (original steel plate thickness, width 75±1.5mm), test temperature -30℃, measuring the percentage of shear area at the fracture surface, and taking the average value of two specimens for each condition.

[0055] Tensile test: conducted according to GB / T 228.1-2021 standard, using transverse rectangular specimens, to measure yield strength Rt0.5, tensile strength Rm, yield ratio YR and elongation after fracture A.

[0056] Examples 2-5 Following the method of Example 1, pipeline steel plates of different thicknesses were manufactured. The chemical composition of all plates met the requirements of this invention. The main components, process parameters, and performance test results are summarized below.

[0057] Table 4. Main chemical components of Examples 2-5 (mass percentage, balance Fe)

[0058] Table 5 Main process parameters of Examples 2-5

[0059] Table 6. Mechanical property test results of Examples 2-5

[0060] As can be seen from Examples 1-5, the 30-50mm thick-walled pipeline steel obtained according to the chemical composition and manufacturing process designed in this invention all meet the target performance requirements: yield strength ≥450MPa, tensile strength ≥535MPa, Charpy impact energy at -40℃ ≥250J, and DWTT of the entire plate thickness at -30℃ ≥85%, exhibiting good comprehensive mechanical properties and stable ultra-low temperature toughness.

[0061] Comparative Examples 1-3 To illustrate the technical effects of the present invention, comparative examples 1-3 are provided for comparison with the embodiments of the present invention.

[0062] Comparative Example 1 (referring to process CN103834874A, without tempering): The chemical composition is basically the same as that of Example 1, but no high-temperature tempering treatment is performed. The test results show that the DWTT shear area of ​​the full-thickness plate at -30℃ is only 75%, which does not meet the requirement of ≥85%, and there is obvious M / A brittle hard structure in the thickness direction.

[0063] Comparative Example 2 (tempering temperature too low): The chemical composition was the same as in Example 1, and the tempering temperature was 550°C (lower than the 600-680°C range of this invention). Test results showed that residual M / A structure still existed in the core thickness, and the DWTT shear area at -30°C was only 82%, failing to meet the target requirements.

[0064] Comparative Example 3 (Deviation of Final Rolling Temperature): The chemical composition was the same as in Example 1, but the finishing rolling temperature was increased to 780℃ (higher than the 740-750℃ range of this invention). Test results showed that the grains were significantly coarsened, the yield strength decreased to 460MPa, and the Charpy impact energy at -40℃ decreased to 210J, failing to meet the target requirements.

[0065] The above comparative examples illustrate that the various process parameter ranges of the present invention form an organic whole, and deviations from any key parameter will significantly affect the ultra-low temperature toughness of the product.

[0066] In addition to the above embodiments, the present invention can also adjust parameters such as billet thickness and continuous casting process according to the production requirements of converters and electric furnaces of different tonnages. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A thick-walled, ultra-low temperature toughness pipeline steel, characterized in that: The chemical composition of the pipeline steel, by mass percentage, is as follows: C: 0.025–0.05%, Si: 0.1–0.3%, Mn: 1.4–1.6%, P≤0.015%, S≤0.002%, Nb: 0.03–0.06%, Ti: 0.01–0.02%, Cr: 0.2–0.3%, Ni: 0.3–0.8%, Mo: 0.06–0.12%, Al: 0.02–0.04%, with the remainder being Fe and unavoidable impurities, and B≤0.0005%, with no intentional addition of B.

2. The thick-walled, ultra-low temperature toughness pipeline steel according to claim 1, characterized in that, The thickness of the steel plate for the pipeline steel is 30-50mm.

3. The thick-walled, ultra-low temperature toughness pipeline steel according to claim 1, characterized in that, The pipeline steel has a yield strength ≥450MPa, tensile strength ≥535MPa, full-size Charpy impact energy ≥250J at -40℃, and full-thickness DWTT fracture shear area percentage ≥85% at -30℃.

4. The thick-walled, ultra-low temperature toughness pipeline steel according to claim 1, characterized in that, The microstructure of the pipeline steel is mainly composed of ultrafine-grained polygonal ferrite and granular bainite / acicular ferrite, with ultrafine-grained polygonal ferrite accounting for 30%-50%.

5. A method for manufacturing thick-walled ultra-low temperature toughness pipeline steel as described in claim 1, characterized in that: Includes the following steps: 1) Smelting and continuous casting The smelting and continuous casting are carried out according to the chemical composition of claim 1; 2) Slab heating The continuously cast slab is heated to a temperature of 1100-1180℃. The slab is kept in the furnace at a temperature above 1000℃ for a period of not less than 0.8*t, where t is the slab thickness in mm and the furnace time is in minutes. 3) Rough rolling The initial rolling temperature of roughing is ≤1000℃, and the average reduction rate of the last three passes of roughing is ≥16%. 4) Thickness to be heated After rough rolling, the thickness of the intermediate billet to be heated should be controlled at 3-4 times the thickness of the finished steel plate; 5) Finish rolling The initial finishing rolling temperature is controlled at 730-760℃, and the final finishing rolling temperature is controlled at 740-750℃. Low-temperature rolling ensures the accumulated deformation energy of austenite, promoting the formation of fine equiaxed ferrite grains after rolling. The temperature difference between the initial and final finishing rolling temperatures is strictly controlled within a small range of 5-10℃ to ensure uniform deformation throughout the thickness direction. 6) Controlled cooling The rolled steel plate is accelerated cooling at a rate of 25-40℃ / s, with a final cooling temperature of ≤350℃. 7) High-temperature tempering heat treatment The steel plate is finally subjected to high-temperature tempering heat treatment at 600-680℃, with a tempering coefficient of 2.0-6.0 min / mm.

6. The method for manufacturing a thick-walled, ultra-low temperature toughness pipeline steel according to claim 5, characterized in that, In step (1), the smelting is carried out by converter or electric furnace, and after LF refining and RH vacuum treatment, Ca treatment, continuous casting is carried out. The thickness t of the slab obtained by continuous casting and the thickness T of the finished steel plate satisfy t≥6T, where T is the thickness of the finished steel plate.

7. The method for manufacturing a thick-walled, ultra-low temperature toughness pipeline steel according to claim 5, characterized in that, In step (5), the temperature difference between the initial rolling temperature and the final rolling temperature of the finishing mill is controlled to be 5-10℃.

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