Thick normalized seamless line pipe and manufacturing method thereof

Abstract

The invention discloses a thick normalized seamless line pipe, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.18 to 0.21 percent of C, 0.25 to 0.35 percent of Si, 1.32 to 1.50 percent of Mn, 0.02 to 0.035 percent of Alt, 0.13 to 0.25 percent of Cr, 0.08 to 0.10 percent of V, less than or equal to 0.01 percent of Ti, 0.02 to 0.04 percent of Nb and 0.014 to 0.020 percent of N. The invention further discloses a manufacturing method of the thick normalized seamless line pipe. The manufacturing method comprises the steps that a pipe blank is manufactured; the steel pipe is obtained after the pipe blank is subjected to heating, perforating, hollow blank reducing, continuous rolling, reheating, tension reducing and cooling; and normalizing is conducted, specifically, the steel pipe is heated to 900-950 DEG C, heat preservation is conducted for 35-90 min, then the steel pipe is cooled to 400 DEG C or below at the cooling speed Vc of 4-18 DEG C / s, and then air cooling is conducted.

Description

Technical Field

[0001] This invention relates to a steel pipe and its manufacturing method, and more particularly to a seamless pipeline pipe and its manufacturing method. Background Technology

[0002] Pipeline transportation is an economical and efficient method of fluid transport. Currently, the operating and design pressures of natural gas pipelines have reached 15 MPa or even 20 MPa, and they contain corrosive media such as hydrogen sulfide (H2S). When pipeline steel is exposed to a water + H2S environment, the H2S dissolves in the water and ionizes to become acidic, making the material highly susceptible to electrochemical corrosion. Hydrogen atoms generated during corrosion are absorbed by the steel and accumulate in metallurgical defect areas, potentially leading to embrittlement, crack initiation, and eventual cracking. Hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) are the main forms of H2S corrosion. Therefore, using large-diameter, thick-walled, corrosion-resistant seamless pipelines to transport high-pressure, high-density media to improve safety and reliability has become an important development trend.

[0003] In existing technology, patent documents have mentioned corrosion-resistant seamless pipes:

[0004] For example, Chinese patent document CN104862614A, published on August 26, 2015, entitled "An X60N Normalized Pipeline Steel and Its Production Method," discloses an X60N normalized pipeline steel with the following chemical composition: C: 0.07-0.15%, Si: 0.20-0.40%, Mn: 1.55-1.7%, P: ≤0.008%, S: ≤0.015%, Mo: ≤0.10%, Alt: 0.02-0.05%, Ni: ≤0. 0.30%, Cr: ≤0.30%, Cu: ≤0.30%, V: 0.004-0.008%, Nb: 0.03-0.06%, Ti: 0.005-0.025%, with the balance being Fe and unavoidable impurity elements; delivered in the normalized state, yield strength 410-430 MPa, tensile strength 500-600 MPa, elongation ≥40%, full-size impact toughness Akv >350 J at -20℃, and drop weight test (DWTT) average shear area ≥85% at -15℃. It should be noted that this patent document incorporates a relatively high amount of Mn element through a low-carbon design to ensure the strength in the normalized state.

[0005] For example, Chinese patent document CN106756537A, published on May 31, 2017, entitled "A High-Strength and Tough Normalized Pipeline Steel with Excellent H2S Corrosion Resistance," discloses a high-strength and tough normalized pipeline steel with excellent H2S corrosion resistance. Its composition and wt% are: C: 0.05-0.08%, Si: 0.30-0.45%, Mn: 0.85-1.0%, P≤0.010%, S≤0.0020%, Mo: 0.10-0.20%, Nb: 0.06-0.12%, B not exceeding 0.0005%, and RE not exceeding 0.0005%.

[0006] It should be noted that producing thick-gauge seamless pipes is more difficult than producing thin-gauge seamless pipes, as it presents challenges in hardenability, strength, and low-temperature toughness. Although increasing the C and Mn content in the steel pipe base material can ensure strength, excessively high C and Mn content can lead to severe segregation at the center of the billet and also affect weldability. Summary of the Invention

[0007] One of the objectives of this invention is to provide a thick-gauge normalized seamless pipeline pipe that is not only resistant to high pressure, but also has high strength, excellent low-temperature toughness, and good resistance to H2S corrosion.

[0008] To achieve the above objectives, the present invention provides a thick-gauge normalized seamless pipeline tube containing Fe and unavoidable impurities, and further containing the following chemical elements in the following mass percentages:

[0009] C: 0.18~0.21%, Si: 0.25~0.35%, Mn: 1.32~1.50%, Alt: 0.02~0.035%, Cr: 0.13~0.25%, V: 0.08~0.10%, Ti≤0.01%, Nb: 0.02~0.04%, N: 0.014~0.020%.

[0010] Furthermore, in the thick-gauge normalized seamless pipeline of the present invention, the mass percentage content of each chemical element is as follows:

[0011] C: 0.18–0.21%, Si: 0.25–0.35%, Mn: 1.32–1.50%, Alt: 0.02–0.035%, Cr: 0.13–0.25%, V: 0.08–0.10%, Ti≤0.01%, Nb: 0.02–0.04%, N: 0.014–0.020%; balance Fe and unavoidable impurities.

[0012] The thick-gauge normalized seamless pipeline of this invention adopts a composition design with medium C, medium Mn, controlled N, and micro-alloyed Nb and V to obtain suitable strength and excellent low-temperature toughness. The specific design principle of its chemical elements is as follows:

[0013] C: In the thick-gauge normalized seamless line pipe of this invention, carbon (C) is the most important solid solution strengthening element, which can significantly improve the strength of the steel. C can also combine with microalloying elements Nb and V to form carbides. When the mass percentage content of C is too high, it will significantly reduce the low-temperature toughness and affect the weldability. In addition, when the mass percentage content of C is too high, it is easy for alloying element carbides to precipitate at the grain boundaries, thereby reducing the corrosion resistance of the steel. Therefore, in the thick-gauge normalized seamless line pipe of this invention, due to the thickness of the line pipe, in order to obtain excellent low-temperature toughness, suitable strength and good hardenability, the mass percentage content of C is controlled between 0.18% and 0.21%.

[0014] Si: In the thick-gauge normalized seamless line pipe of the present invention, an appropriate amount of Si can improve strength and hardness, and it is also a strong deoxidizing element. However, when the mass percentage content of Si is too high, it will reduce the surface quality of the steel and affect the low-temperature toughness and weldability of the steel. Based on this, in the thick-gauge normalized seamless line pipe of the present invention, the mass percentage content of Si is controlled between 0.25% and 0.35%.

[0015] Mn: In the thick-gauge normalized seamless line pipe of the present invention, Mn element has the functions of improving hardenability, solid solution strengthening, and grain refinement strengthening. While increasing the strength of steel, Mn element can also improve the toughness and lower the ductile-brittle transition temperature. However, when the mass percentage content of Mn element is too high, it will lead to increased center segregation and deterioration of toughness. Therefore, in the thick-gauge normalized seamless line pipe of the present invention, the mass percentage content of Mn element is controlled between 1.32% and 1.50%.

[0016] Alt: In the thick-gauge normalized seamless line pipe of the present invention, Al is added to the steel as a deoxidizer, which can refine the grains and fix the nitrogen in the steel, thereby improving the impact toughness of the steel and reducing the tendency for cold brittleness. However, when the mass percentage content of Al is too high, coarse precipitates may be formed, which is detrimental to low-temperature toughness. Based on this, in the thick-gauge normalized seamless line pipe of the present invention, the mass percentage content of Al is controlled between 0.02% and 0.035%.

[0017] Cr: In the thick-gauge normalized seamless line pipe of this invention, Cr can improve the thermodynamic stability of steel, has a certain solid solution strengthening effect, and effectively improves the hardenability of the steel. It should be noted that when the mass percentage content of Cr is too high, it will lead to a decrease in weldability. At the same time, the segregation of Cr carbides at grain boundaries can easily lead to a decrease in the sulfur resistance of the steel. Therefore, in the thick-gauge normalized seamless line pipe of this invention, the mass percentage content of Cr is controlled between 0.13% and 0.25%.

[0018] V: In the thick-gauge normalized seamless line pipe of the present invention, the element V has a precipitation strengthening effect and can form carbonitrides V(C,N) with C and N elements. However, excessive V(C,N) content is detrimental to the toughness of the steel, especially its low-temperature toughness. Based on this, in the thick-gauge normalized seamless line pipe of the present invention, the mass percentage content of the element V is controlled between 0.08% and 0.10%.

[0019] Ti: In the thick-gauge normalized seamless pipeline pipe described in this invention, Ti is not an element that is intentionally added, but a residual element whose upper limit needs to be controlled.

[0020] Nb: In the thick-gauge normalized seamless line pipe of this invention, Nb has a strong affinity for C and N to form carbonitrides, which strongly refine grains and strengthen through precipitation, significantly improving the overall mechanical properties of the steel. However, it should be noted that excessively high Nb content can lead to mixed crystal formation in the steel and increase costs. Therefore, in the thick-gauge normalized seamless line pipe of this invention, the Nb content is controlled between 0.02% and 0.04% by mass.

[0021] Nitrogen (N): In the thick-gauge normalized seamless pipeline of this invention, nitrogen (N) can combine with Al, Nb, and V in the steel to form nitrides, which have the effects of grain refinement and precipitation strengthening. However, when the mass percentage of nitrogen is too high, it will negatively impact weldability and exacerbate cold brittleness. Therefore, in the thick-gauge normalized seamless pipeline of this invention, the mass percentage of nitrogen is controlled between 0.014% and 0.020%.

[0022] Furthermore, in the other unavoidable impurities of the thick-gauge normalized seamless pipeline pipe described in this invention: P≤0.015%, S≤0.005%, O≤0.004%, H≤0.0002%.

[0023] It should be noted that in the technical solution described in this invention, P, S, O, and H are all unavoidable impurity elements in steel. Where technical requirements permit, the lower the impurity content, the better.

[0024] Furthermore, in the thick-gauge normalized seamless pipeline pipe described in this invention, the requirement of V+Ti+Nb < 0.15% is also met.

[0025] In this invention, economic efficiency can be ensured by controlling the content of microalloying elements V and Nb.

[0026] Furthermore, the microstructure of the thick-gauge normalized seamless pipeline pipe described in this invention is ferrite + pearlite.

[0027] Furthermore, in the thick-gauge normalized seamless pipeline of the present invention, the volume ratio of pearlite is 25-45%, and the volume ratio of ferrite is 55-75%.

[0028] In this invention, by controlling the volume ratio of pearlite and the volume ratio of ferrite, the strength and low-temperature toughness after normalizing can be effectively guaranteed.

[0029] Furthermore, the average grain size of the thick-gauge normalized seamless pipeline pipe described in this invention is ≤11.9μm, and the grain size rating is higher than 9.5.

[0030] Furthermore, the thickness of the thick-gauge normalized seamless pipeline pipe described in this invention is 18–35 mm.

[0031] Furthermore, in the thick-gauge normalized seamless pipeline pipe described in this invention, its room temperature yield strength is ≥420MPa, tensile strength is ≥550MPa, and full-size impact energy KV8 at -60℃ is ≥250J.

[0032] Furthermore, in the thick-gauge normalized seamless pipeline pipe described in this invention, its hydrogen-induced cracking resistance indices CRL%, CTR%, and CSR% are all 0.

[0033] Another objective of this invention is to provide a method for manufacturing a thick-gauge normalized seamless pipeline. This method is simple, low-cost, and can produce a thick-gauge normalized seamless pipeline with high strength, excellent low-temperature toughness, and good resistance to H2S corrosion.

[0034] To achieve the above objectives, the present invention provides a method for manufacturing a thick-gauge normalized seamless pipeline pipe, comprising the following steps:

[0035] A tube blank is obtained;

[0036] The tube blank is heated, pierced, reduced in diameter by hollow billet, continuously rolled, reheated, reduced in diameter by tension and cooled to obtain a steel tube;

[0037] Normalizing: Heat the steel pipe to 900-950℃ and hold for 35-90 minutes, then cool it to ≤400℃ at a cooling rate of 4-18℃ / s, followed by air cooling.

[0038] In the normalizing step of the manufacturing method described in this invention, if the heating temperature is too low, the alloying elements in the steel cannot be fully dissolved, and a good strengthening effect cannot be achieved; if the heating temperature is too high, the austenite grains in the steel will continuously coarsen, which is detrimental to the mechanical properties of the steel. Therefore, in the manufacturing method described in this invention, the heating temperature of the steel pipe is controlled between 900 and 950°C.

[0039] In the normalizing step of the manufacturing method described in this invention, the holding time has a significant impact on grain size. When the holding time is too long, the mobility and diffusion of atoms (especially grain boundary atoms) are continuously enhanced, exacerbating grain coagulation and leading to grain coarsening. When the holding time is too short, it results in an uneven internal microstructure of the steel, which is detrimental to its performance. Therefore, in the manufacturing method described in this invention, the holding time is controlled between 35 and 90 minutes.

[0040] In the normalizing step of the manufacturing method described in this invention, the cooling rate affects the grain size and microstructure type and proportion. When the cooling rate Vc is too low, an equilibrium microstructure is obtained, but the interlamellar spacing of pearlite in the equilibrium microstructure is often relatively large, which is detrimental to both strength and toughness. When the cooling rate Vc is too high, a certain proportion of non-equilibrium microstructure, such as bainite or even martensite, will be generated, which has high strength but poor ductility and toughness. Therefore, in the manufacturing method described in this invention, the cooling rate Vc is controlled between 4 and 18 °C / s. Using this cooling rate can refine the grains and obtain a microstructure with a suitable proportion.

[0041] In the normalizing step of the manufacturing method described in this invention, since the ferrite and pearlite structures have been completely transformed at temperatures above 400°C, the temperature is further reduced to room temperature at 400°C or below in order to save costs.

[0042] Furthermore, in the normalizing step of the manufacturing method described in this invention, in addition to controlling the cooling rate Vc within the range of 4 to 18°C / s, the cooling rate Vc can further satisfy 10Vc = 0.11D. 2 +(15~35), where D represents the thickness of the steel pipe, and its unit parameter is mm.

[0043] In this embodiment, the present invention can design a specific heat preservation and cooling rate Vc for tubes of different thicknesses to obtain a suitable pearlite + ferrite microstructure.

[0044] Furthermore, in the normalizing step of the manufacturing method described in this invention, in addition to controlling the holding time t within the range of 35 to 90 minutes, the holding time t further satisfies: t = 2 × α × k × D + (5 to 20), where α represents the steel grade coefficient, which ranges from 0.8 to 1.5, for example, a value of 1.0 in a specific instance; k represents the furnace loading coefficient, which ranges from 0.5 to 1.0, for example, a value of 0.83 based on the actual equipment capacity in a specific instance; and D represents the steel pipe thickness, with the unit parameter being mm.

[0045] The thick-gauge normalized seamless pipeline pipe of the present invention has the following advantages and beneficial effects compared with the prior art:

[0046] The thick-gauge normalized seamless pipeline of the present invention utilizes a reasonable design of chemical elements C, Mn, Nb, V, and N. By combining C and N elements with microalloying elements Nb and V, finely dispersed carbides, nitrides, and carbonitrides are precipitated. This compositional system design has both strong precipitation strengthening and fine grain strengthening effects to ensure strength and toughness, and can also form hydrogen traps to fix hydrogen, thereby improving a certain resistance to hydrogen-induced cracking.

[0047] In some embodiments, the room temperature yield strength Rt0.5 of the thick-gauge normalized seamless pipeline pipe of the present invention is 420-465 MPa, the room temperature tensile strength Rm is 520-620 MPa, and the KV8 of a full-size 10mm×10mm×55mm sample at -60℃ is ≥250 J; its average grain size is ≤11.9μm, and the grain size rating is higher than 9.5; the hydrogen-induced cracking (HIC) resistance indices CLR%, CTR%, and CSR% are all 0 and there is no hydrogen blistering; the hydrogen sulfide stress corrosion four-point bending test (SSC) sample does not crack and there is no hydrogen blistering. It can be effectively applied to natural gas pipeline transportation under complex working conditions and has good promotion prospects and application value.

[0048] The manufacturing method of thick-gauge normalized seamless pipeline of the present invention obtains thick-gauge normalized seamless pipeline with excellent performance by controlling process parameters, especially normalizing process parameters. Detailed Implementation

[0049] The following will further explain and illustrate the thick-gauge normalized seamless pipeline and its manufacturing method according to the present invention with reference to specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0050] Examples 1-12 and Comparative Examples 1-6

[0051] The thick-gauge normalized seamless pipeline tubes of Examples 1-12 of the present invention and the comparative tubes of Comparative Examples 1-6 were all prepared using the following steps:

[0052] (1) Smelting and continuous casting to produce tube blanks;

[0053] (2) After heating, piercing, hollow billet reduction, continuous rolling, reheating, tension reduction and cooling, steel pipe is obtained;

[0054] (3) Normalizing: Heat the steel pipe to 900-950℃ and hold for 35-90 minutes, then...

[0055] The temperature of Vc is cooled to 400°C or below at a cooling rate of 4–18°C / s, followed by air cooling.

[0056] It should be noted that the thick-gauge normalized seamless pipelines in Examples 1-12 of this invention were all prepared using the above steps, and their chemical composition and manufacturing process parameters met the design specifications of this invention. The control tubes in Comparative Examples 1-6 were also prepared using the above manufacturing process; however, the chemical composition of Comparative Examples 1, 3, and 5 did not meet the design requirements of this invention, and the relevant process parameters of the control tubes in Comparative Examples 2, 4, and 6 did not meet the technical requirements described in this invention.

[0057] Tables 1-1 and 1-2 list the mass percentage of each chemical element in the thick-gauge normalized seamless pipelines of Examples 1-12 of the present invention and the comparative pipes of Comparative Examples 1-6.

[0058] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides P, S, O, and H)

[0059] serial number C Si Mn Alt Cr V Nb N Example 1 0.191 0.35 1.45 0.025 0.22 0.080 0.028 0.018 Example 2 0.187 0.28 1.47 0.020 0.19 0.086 0.028 0.016 Example 3 0.205 0.25 1.32 0.028 0.16 0.092 0.022 0.016 Example 4 0.198 0.29 1.4 0.031 0.2 0.088 0.03 0.015 Example 5 0.182 0.32 1.43 0.033 0.23 0.097 0.038 0.020 Example 6 0.209 0.33 1.47 0.035 0.24 0.098 0.037 0.018 Example 7 0.187 0.27 1.32 0.024 0.13 0.081 0.022 0.015 Example 8 0.195 0.34 1.41 0.03 0.2 0.09 0.031 0.016 Example 9 0.208 0.28 1.50 0.031 0.25 0.1 0.020 0.018 Example 10 0.21 0.34 1.33 0.03 0.22 0.09 0.04 0.015 Example 11 0.185 0.32 1.48 0.023 0.16 0.082 0.037 0.014 Example 12 0.195 0.31 1.42 0.028 0.24 0.098 0.029 0.019 Comparative Example 1 0.1 0.31 1.41 0.031 0.16 0.09 0.015 0.015 Comparative Example 2 0.181 0.33 1.47 0.026 0.2 0.094 0.035 0.02 Comparative Example 3 0.196 0.28 1.16 0.025 0.24 0 0.03 0.007 Comparative Example 4 0.195 0.25 1.42 0.028 0.18 0.082 0.025 0.017 Comparative Example 5 0.15 0.31 1.42 0.028 0.24 0 0 0.019 Comparative Example 6 0.209 0.33 1.47 0.03 0.2 0.09 0.031 0.016

[0060] Table 1-2. (wt%, balance Fe and other unavoidable impurities besides P, S, O, and H)

[0061]

[0062]

[0063] Table 2 lists the specific process parameters of the thick-gauge normalized seamless pipelines of Examples 1-12 of the present invention and the comparative pipes of Comparative Examples 1-6.

[0064] Table 2.

[0065]

[0066] Samples were taken from the thick-gauge normalized seamless pipelines of Examples 1-12 and the control pipes of Comparative Examples 1-6, and various tests were performed on them. The microstructure test results are listed in Table 3, and the performance test results are listed in Table 4. Among them:

[0067] After grinding and polishing, the samples were etched with 4% nitric acid alcohol and then the microstructure was observed under a Zeiss Axio Scope A1 optical microscope. The grain size was evaluated according to "GB / T 6394-2017 Method for Determination of Average Grain Size of Metals".

[0068] Tensile tests were conducted using a 2000 kN tensile testing machine in accordance with "GB / T 228.1-2021 Metallic materials, tensile testing—Part 1: Tensile testing at room temperature".

[0069] According to "GB / T 229-2020 Metallic Materials Charpy Pendulum Impact Test Method", the test was conducted using a 750J instrumented impact testing machine and a low-temperature impact test chamber. The sample size was 10mm×10mm×55mm, and the test temperature was -60℃.

[0070] According to "NACE TM0284-2016 Evaluation of Pipeline and Pressure Vesselsteels for Resistance to Hydrogen Induced Cracking", HIC hydrogen-induced cracking test was conducted, with NACE-A solution and sample immersion time of 96 hours.

[0071] The hydrogen sulfide stress corrosion four-point bending test was conducted according to "GB / T 4157-2017 Laboratory test method for metals in hydrogen sulfide environment to resist sulfide stress cracking and stress corrosion cracking". The solution was NACE-A solution, the sample immersion time was 720 hours, and the load was 90% AYS.

[0072] Table 3 lists the microstructure test results of the thick-gauge normalized seamless pipelines of Examples 1-12 of the present invention and the comparative pipes of Comparative Examples 1-6.

[0073] Table 3.

[0074]

[0075]

[0076] As can be seen from Table 3 above, the microstructure of the thick-gauge normalized seamless pipelines of Examples 1-12 prepared by the manufacturing method described in this invention is ferrite + pearlite, and the volume ratio of pearlite is between 25% and 45%, the volume ratio of ferrite is between 55% and 75%, the average grain size is less than 11.9 μm, and the grain size rating is higher than 9.5.

[0077] Table 4 lists the performance test results of the thick-gauge normalized seamless pipelines of Examples 1-12 of the present invention and the comparative pipes of Comparative Examples 1-6.

[0078] Table 4.

[0079]

[0080] Note: The three data points for impact energy KV8 (J) in Table 3 above represent the results of three experiments.

[0081] As can be seen from Table 4 above, the thick-gauge normalized seamless pipelines of Examples 1-12, manufactured by the manufacturing method described in this invention, have good mechanical properties. Their room temperature yield strength is ≥420MPa, their tensile strength is >550MPa, their full-size impact energy KV8 at -60℃ is >250J, their hydrogen-induced cracking resistance indices CRL%, CTR%, and CSR% are all 0, and they did not crack or blister after the hydrogen sulfide stress corrosion four-point bending test (SSC).

[0082] It should be noted that the scope of protection of the prior art in this invention is not limited to the embodiments given in this application. All prior art that does not contradict the solution of this invention, including but not limited to prior patent documents, prior publications, prior public uses, etc., can be included in the scope of protection of this invention.

[0083] Furthermore, the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0084] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A thick-gauge, normalized, seamless pipeline pipe, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following percentages by mass: C: 0.18~0.21%, Si: 0.25~0.35%, Mn: 1.32~1.50%, Alt: 0.02~0.035%, Cr: 0.13~0.25%, V: 0.08~0.10%, Ti≤0.01%, Nb: 0.02~0.04%, N: 0.014~0.020%.

2. The thick-gauge normalized seamless pipeline as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.18~0.21%, Si: 0.25~0.35%, Mn: 1.32~1.50%, Alt: 0.02~0.035%, Cr: 0.13~0.25%, V: 0.08~0.10%, Ti≤0.01%, Nb: 0.02~0.04%, N: 0.014–0.020%; balance is Fe and unavoidable impurities.

3. The thick-gauge normalized seamless pipeline as described in claim 1 or 2, characterized in that, Among other unavoidable impurities: P ≤ 0.015%, S ≤ 0.005%, O ≤ 0.004%, H ≤ 0.0002%.

4. The thick-gauge normalized seamless pipeline as described in claim 1 or 2, characterized in that, It also satisfies V+Ti+Nb<0.15%.

5. The thick-gauge normalized seamless pipeline as described in claim 1 or 2, characterized in that, Its microstructure consists of ferrite and pearlite.

6. The thick-gauge normalized seamless pipeline as described in claim 5, characterized in that, The volume fraction of pearlite is 25–45%, and the volume fraction of ferrite is 55–75%.

7. The thick-gauge normalized seamless pipeline as described in claim 1 or 2, characterized in that, Its average grain size is ≤11.9μm, and its grain size rating is higher than 9.

5.

8. The thick-gauge normalized seamless pipeline as described in claim 1 or 2, characterized in that, Its thickness is 18-35mm.

9. The thick-gauge normalized seamless pipeline as described in claim 1 or 2, characterized in that, Its room temperature yield strength is ≥420MPa, tensile strength is ≥550MPa, and full-size impact energy KV8 at -60℃ is ≥250J.

10. The thick-gauge normalized seamless pipeline as described in claim 9, characterized in that, Its resistance to hydrogen-induced cracking, CRL%, CTR%, and CSR%, are all 0.

11. The method for manufacturing thick-gauge normalized seamless pipeline pipe as described in any one of claims 1-10, characterized in that, It includes the following steps: A tube blank is obtained; The tube blank is heated, pierced, reduced in diameter by hollow billet, continuously rolled, reheated, reduced in diameter by tension and cooled to obtain a steel tube; Normalizing: Heat the steel pipe to 900-950℃ and hold it for 35-90 minutes. Then cool it to ≤400℃ at a cooling rate of 4-18℃ / s, followed by air cooling.

12. The manufacturing method as described in claim 11, characterized in that, During the normalizing step, the cooling rate satisfies 10Vc = 0.11D 2 +(15~35), where D represents the thickness of the steel pipe, and its unit parameter is mm.

13. The manufacturing method as described in claim 11, characterized in that, In the normalizing step, the holding time t also satisfies: t=2×α×k×D+(5~20), where α represents the steel grade coefficient, and its value ranges from 0.8 to 1.5; k represents the furnace loading coefficient, and its value ranges from 0.5 to 1.0; D represents the steel pipe thickness, and its unit parameter is mm.

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