Low-temperature-resistant anti-fatigue X65 seamless line pipe and manufacturing method thereof

Abstract

The low-temperature-resistant anti-fatigue X65 seamless line pipe comprises the following components in percentage by weight: 0.06 to 0.11 percent of C, 0.15 to 0.35 percent of Si, 1.41 to 1.65 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, less than or equal to 0.1 percent of Cu, 0.05 to 0.15 percent of Ni, less than or equal to 0.1 percent of Cr, 0.07 to 0.2 percent of Mo, 0.01 to 0.04 percent of Nb, 0.03 to 0.08 percent of V, 0.001 to 0.006 percent of Ti, 0.001 to 0.004 percent of Ca, 0.01 to 0.04 percent of Alt, less than or equal to 0.0005 percent of B, less than or equal to 0.003 percent of O, 0.001 to 0.0055 percent of N, less than or equal to 0.0002 percent of H and the balance of Fe and impurities, Ni / (C + Mn) The yield strength Rt0.5 of the X65 seamless line pipe is 450-570 MPa, the tensile strength is 535-760 MPa, the yield ratio is smaller than or equal to 0.9, the total elongation A50 is larger than or equal to 30%, the uniform elongation Ag is larger than or equal to 8%, the impact energy KV8 at the temperature of-80 DEG C is larger than or equal to 200 J, the stress ratio R is equal to-1, and the fatigue life is larger than or equal to 10 million times under the tension-compression fatigue condition.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel, specifically to a low-temperature resistant and fatigue-resistant X65 seamless pipeline pipe and its manufacturing method. Background Technology

[0002] Some low-temperature and alternating load service conditions require pipeline pipes to possess low-temperature fatigue resistance. Conventional pipeline pipes do not incorporate targeted steel composition, microstructure, and processing design to address these performance requirements, thus failing to meet them. Strengthening with C and Mn elements, as well as microalloying elements Nb, V, and Ti, to improve pipeline pipe strength is a common approach; however, these strengthening methods may reduce toughness while increasing strength. Ensuring fatigue performance requires a good balance between strength and toughness; solely considering strength improvement while neglecting toughness will not yield satisfactory fatigue performance. Therefore, to ensure the safe use of pipeline pipes under low-temperature and alternating load service conditions, new solutions for steel grades and processing are urgently needed.

[0003] Chinese Patent No. CN201610035265.4 discloses a "manufacturing method of seamless pipeline pipe with high strain and corrosion resistance for R-Lay laying in marine environment". This patent adopts a design to produce pipeline steel with high C, low Mn, high Cr and high Ti. The invention object is seamless steel pipe products. The upper limit of Ti and N control is also relatively high. This is not conducive to controlling the occurrence of coarse TiN in steel, which is not good for fatigue resistance and low temperature toughness. It is easy to cause fatigue and toughness properties to be unstable and uncontrollable.

[0004] Chinese Patent No. CN201110179945.0 discloses "An X65 Pipeline Steel with Excellent Low-Temperature Toughness and Its Manufacturing Method." This patent employs an ultra-low carbon and low-temperature rolling design to produce low-temperature resistant X65 grade pipeline steel, with the invention object being steel coil products. The ultra-low carbon design utilizes high Mn, high Cr, high Nb, and composite addition of Ti and V to promote phase deformation nucleation, ensure hardenability, and hinder austenite growth during deformation, thereby obtaining a fine acicular ferrite structure and achieving high toughness. However, the addition of Ti easily forms TiN. While fine TiN can effectively hinder grain growth, it also easily forms coarse TiN, equivalent to inclusions, thus negatively impacting fatigue resistance and low-temperature toughness, easily leading to unstable and uncontrollable fatigue and toughness properties.

[0005] Chinese patent CN201210492187.2 discloses "Thick-walled high DWTT performance X65-70 submarine pipeline steel and its manufacturing method." This patent employs an ultra-low carbon design with added Cr, Cu, and Ni, but without Ti, to produce low-temperature resistant high DWTT performance (i.e., high toughness) pipeline steel. The invention focuses on steel plate products. The ultra-low carbon toughness steel design necessitates the addition of Cr, Cu, and high Mn for strengthening to ensure strength performance. High Mn content easily leads to segregation, increasing microstructure inhomogeneity and thus hindering fatigue. To prevent Cu brittleness, Ni must be added to improve toughness. However, increasing toughness through Ni addition results in high costs.

[0006] Existing patents do not provide a technical solution for low-temperature resistant and fatigue-resistant seamless pipeline pipes. Summary of the Invention

[0007] The purpose of this invention is to propose a low-temperature resistant and fatigue-resistant X65 seamless pipeline and its manufacturing method, which can achieve the X65 strength level and has low-temperature resistance and high fatigue resistance. The yield strength Rt0.5 of the X65 seamless pipeline is 450-570MPa, the tensile strength Rm is 535-760MPa, the yield ratio Rt0.5 / Rm≤0.90, the total elongation A50≥30%, the uniform elongation Ag≥8%, the impact energy KV8 at -80℃≥200J, and the fatigue life under tensile and compressive fatigue conditions with a stress ratio of R=-1≥10 million cycles. It is particularly suitable for pipeline applications in marine environments subjected to fatigue loads.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] A low-temperature resistant and fatigue-resistant X65 seamless pipeline tube, with the following chemical composition by weight percentage: C 0.06-0.11%, Si 0.15-0.35%, Mn 1.41-1.65%, P≤0.015%, S≤0.005%, Cu≤0.10%, Ni 0.05-0.15%, Cr≤0.10%, Mo 0.07-0.20%, Nb 0.01-0.04%, V 0.03-0.08%, Ti 0.001-0.006%, Ca 0.0010-0.0040%, Alt 0.010–0.040%, B ≤ 0.0005%, O ≤ 0.0030%, N 0.0010–0.0055%, H ≤ 0.0002%, with the balance including Fe and other unavoidable inclusions, and simultaneously satisfying the following:

[0010] Ni / (C+Mn)≥0.03;

[0011] CE Pcm =0.171~0.215%;

[0012] Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0013] Furthermore, its chemical composition also satisfies: Alt / N≥2 and / or Ca / Alt≥0.10.

[0014] Furthermore, the balance consists of Fe and other unavoidable inclusions.

[0015] The microstructure of the X65 seamless pipeline tube of the present invention is tempered bainite with a fine lath substructure of size ≤0.2μm plus a small amount of ferrite, with a ferrite ratio of ≤5% and a grain size of ≥7; and the percentage of inclusions with a size ≥5μm is 0%, and the percentage of TiN particles with a size ≥2μm is 0%.

[0016] The X65 seamless pipeline pipe of this invention has a yield strength Rt0.5 of 450-570 MPa, a tensile strength Rm of 535-760 MPa, a yield strength ratio Rt0.5 / Rm ≤ 0.90, a total elongation A50 ≥ 30%, a uniform elongation Ag ≥ 8%, an impact energy KV8 ≥ 200 J at -80℃, and a fatigue life ≥ 10 million cycles under tensile and compressive fatigue conditions with a stress ratio R = -1.

[0017] In the composition design of the X65 seamless pipeline tube described in this invention:

[0018] Carbon (C): The most economical strengthening element in steel, enhancing its strength through interstitial solid solution strengthening. Increasing the carbon content can significantly improve the hardenability of steel, reduce the amount of other expensive alloys added, and lower production costs. However, increasing the C content is detrimental to the steel's plasticity, toughness, and weldability. This invention employs an ultra-low C design, with its content controlled between 0.06% and 0.11%.

[0019] Silicon (Si): A solid solution strengthening element and also a deoxidizing element in steel. However, excessive content can adversely affect surface quality and weldability. If the Si content exceeds 0.35%, toughness may decrease. In this invention, the Si content is controlled between 0.15% and 0.35%.

[0020] Manganese (Mn): It enhances the strength of steel through solid solution strengthening and is the most important and economical strengthening element in steel to compensate for the strength loss caused by a decrease in carbon content. However, Mn can also exacerbate central segregation, leading to uneven hardness and microstructure, and affecting low-temperature toughness and fatigue performance. Therefore, the Mn content in this invention is controlled at 1.41–1.65%.

[0021] Chromium (Cr): It has a certain solid solution strengthening effect and can effectively improve the hardenability of steel, but it is detrimental to weldability. Therefore, the Cr content in this invention is controlled at ≤0.10%.

[0022] Copper (Cu): It has a certain solid solution strengthening effect and can effectively improve the hardenability of steel, but it is not good for weldability. Therefore, the Cu content in this invention is controlled at ≤0.10%.

[0023] Nickel (Ni): It can improve the toughness of steel and effectively enhance its hardenability. However, the alloy cost is high, and adding Ni will reduce the austenitic phase transformation temperature and narrow the heat treatment process window, which is not conducive to the use of high-temperature tempering process. Therefore, in order to improve the toughness of C-Mn steel, the Ni content is controlled at 0.05-0.15%, and Ni / (C+Mn)≥0.03.

[0024] Simultaneously considering the hardenability and the cumulative value of elements, the overall effect is related. To avoid deterioration of the steel's weldability, CE also needs to be controlled. Pcm ≤0.215%; at the same time, it is necessary to ensure good hardenability, ensure uniformity of microstructure and properties in the thickness direction, and also control CE. Pcm ≥0.171%.

[0025] Niobium (Nb) is an important element in steel microalloying. During high-temperature rolling, Nb dissolved in solid solution precipitates under strain to form Nb carbonitrides, which pin grain boundaries and inhibit the growth of deformed austenite, resulting in fine austenite grains. After quenching and tempering, dissolved Nb precipitates as NbC, a second-phase particle, dispersed in the matrix, playing a role in precipitation strengthening. However, excessive Nb can lead to increased anisotropy in the material, resulting in uneven properties and decreased ductility and toughness, as well as reduced toughness and fatigue resistance. In this invention, the Nb content is controlled at 0.01–0.04%.

[0026] Vanadium (V): Vanadium exists primarily in steel as carbides. In steel, vanadium refines the grain structure, increases the grain coarsening temperature, and enhances strength through grain refinement and precipitation strengthening. It also improves the softening problem in the weld heat-affected zone, thus improving weldability. However, excessive V content leads to decreased toughness and an increased ductile-brittle transition temperature. In this invention, the V content is controlled at 0.03–0.08%.

[0027] Titanium (Ti): A good deoxidizer and degassing agent, and an effective element for fixing nitrogen and carbon. Undissolved Ti carbonitrides can inhibit austenite grain growth during steel heating. TiN and TiC precipitated during rough rolling in the high-temperature austenitic region can effectively suppress austenite grain growth, thus refining the grains. Furthermore, precipitation during welding can also inhibit high-temperature grain growth, thereby improving weldability. However, excessive Ti content can lead to the formation of large TiN particles larger than 2 μm in the steel, which is detrimental to low-temperature toughness and fatigue resistance. In this invention, the Ti content is controlled at 0.001–0.006%.

[0028] Molybdenum (Mo): A highly hardenable element that significantly delays the ferrite phase transformation, inhibits the formation of ferrite and pearlite, and effectively promotes bainite transformation to strengthen the matrix, resulting in a finer microstructure. It also provides more nucleation sites for precipitation strengthening, enhancing its effectiveness. A certain amount of Mo has significant benefits for improving the microstructure and properties of materials; however, excessive Mo content reduces the plasticity of steel, and Mo is also expensive. Therefore, in this invention, the Mo content is controlled at a low level, ranging from 0.07% to 0.20%.

[0029] Sulfur and phosphorus (S and P): These are the main impurity elements in steel. Phosphorus easily leads to cold brittleness in steel, while sulfur easily causes hot brittleness, resulting in unstable steel properties. In particular, as the S content increases, the number of MnS inclusions increases, significantly reducing the low-temperature toughness of the material. Therefore, the phosphorus and sulfur content in steel should be minimized. In this invention, S ≤ 0.005% and P ≤ 0.015%.

[0030] Calcium (Ca): Ca treatment can control the morphology of sulfides, transforming strip-shaped MnS inclusions into spherical CaS inclusions, improving the anisotropy of the steel plate. It can also transform high-melting-point alumina inclusions into low-melting-point calcium aluminate inclusions, thereby improving low-temperature toughness. However, excessive Ca content can easily lead to an increase in the size and density of inclusions such as CaO in the steel. Therefore, in this invention, the Ca content is controlled at 0.0010–0.0040%, preferably, Ca / Alt ≥ 0.10.

[0031] Aluminum (Al): Al is used for deoxidation of steel. Appropriate amounts of Al can solidify nitrogen (N), forming AlN, which refines the grain size and improves strength and toughness. However, if the Al content exceeds 0.040%, coarse precipitates may form, reducing the steel's low-temperature toughness. Therefore, the Al content should be controlled between 0.010% and 0.040%. To ensure a good effect of solidifying N and improving toughness, it is preferable to control the Al / N ratio to be ≥2.

[0032] Boron (B): The main role of B in steel is to increase its hardenability and strength, thereby saving on other rarer and more expensive metals. However, the addition of B will have a significant adverse effect on the low-temperature toughness of the material. In this invention, B ≤ 0.0005%.

[0033] Nitrogen (N): In steel, N can form nitrides with Ti, Nb, and V, increasing strength and inhibiting grain coarsening and growth, while reducing toughness. High N and Ti contents easily lead to the formation of large TiN particles with a size ≥2 μm, which is detrimental to low-temperature toughness and fatigue resistance. In this invention, the N content is controlled between 0.0010 and 0.0055%.

[0034] Oxygen and hydrogen (O, H): Excessive N and H content is detrimental to the plasticity of the material. Therefore, in this invention, O ≤ 0.0030% and H ≤ 0.0002%.

[0035] In summary, this invention achieves its goals through:

[0036] (1) Based on the low C content, the material is designed with high Mn and high Mo, and the grain refinement is promoted by microalloying with Nb, V and Ti, so that the material can obtain good strength and toughness and meet the X65 strength level.

[0037] (2) By adding trace amounts and controlling the Ti content (0.001-0.006%) and the N content (0.0010-0.0055%), the grain size of the continuously cast billet can be improved by fixing N with Ti, thereby improving the grain size of the final steel pipe product (reaching ≥7 grade). At the same time, excessive Ti addition should be avoided to prevent the appearance of large TiN particles with a size ≥2μm in the steel, which would have an adverse effect on plasticity, fatigue resistance and low temperature toughness. There are requirements for the maximum amount of Ti added. Therefore, the design of Ti needs to control the upper and lower limits, and the design of N also needs to control the upper and lower limits.

[0038] (3) By adding a certain amount of Ni while ensuring the Ni / (C+Mn) ratio, the effect of C and Mn in reducing the toughness of the material can be suppressed, while the toughening effect of Ni can be maximized.

[0039] (4) By controlling CEpcm, the hardenability and weldability of steel are guaranteed, and the size of the tempered bainitic lath substructure is guaranteed to be ≤0.2μm, thereby obtaining good low-temperature toughness and fatigue resistance.

[0040] (5) By adding Ca and controlling the Ca / Alt ratio, the inclusions are dispersed and finely distributed, while the inclusions are spherical, reducing the number of angular or sharp inclusions, and ensuring that the percentage of inclusions with a size ≥5μm is 0% (i.e., no inclusions with a size ≥5μm appear), thereby reducing the influence of inclusions on low-temperature toughness and fatigue resistance.

[0041] (6) By controlling the quenching heating temperature, the grain refinement effect of Nb-V-Ti makes the initial austenite grain size small (reaching ≥7 level), thereby obtaining a fine microstructure during quenching, resulting in good low-temperature toughness and fatigue resistance. Moreover, the Nb-V composite addition can obtain composite precipitates with granular Nb(C,N) as the core and V(C,N) on both sides, further improving the microstructure refinement effect of the steel.

[0042] (7) High-temperature tempering promotes the rapid and uniform precipitation of carbides and nitrides, avoiding preferential precipitation in a few locations that would reduce the uniformity of the structure, thereby obtaining good fatigue resistance.

[0043] The manufacturing method of the low-temperature resistant and fatigue-resistant X65 seamless pipeline pipe of the present invention includes the following steps:

[0044] 1) A tube blank is obtained by smelting and casting according to the above composition;

[0045] 2) The tube blank is made into a steel pipe;

[0046] 3) Quenching treatment, quenching temperature: 880~940℃, quenching time: 0.3~1.2h;

[0047] 4) Tempering treatment, tempering temperature: 620~700℃, tempering time: 0.3~1.2h.

[0048] In the manufacturing method described in this invention:

[0049] Quenching temperature: 880–940℃. If the quenching temperature is too high, the austenite grains coarsen and grow, with a grain size <7. Large austenite grains are not conducive to the formation of a fine microstructure, resulting in poor deformation compatibility under load and negatively impacting low-temperature toughness and fatigue resistance. If the quenching temperature is too low, the austenite grains become too fine, with a grain size >9, easily forming a high proportion of ferrite soft phase (ferrite proportion ≥5%), which fails to meet strength requirements. Furthermore, if the quenching temperature is too low, the hardenability of the material is insufficient, and the X65 strength target cannot be achieved. Therefore, this invention controls the quenching temperature to be between 880 and 940℃.

[0050] Quenching heating time: 0.3–1.2 h. If the quenching time is too long, the austenite grains coarsen and grow, with a grain size <7. Large austenite grains are not conducive to the formation of a fine microstructure, resulting in poor deformation coordination under load and negatively impacting low-temperature toughness and fatigue resistance. If the quenching temperature is too low, the austenite grains become too fine, with a grain size >9, easily forming a high proportion of ferrite soft phase (ferrite proportion ≥5%), which fails to meet strength requirements. Furthermore, if the quenching temperature is too low, the hardenability of the material is insufficient, and the X65 strength target cannot be achieved. Therefore, the quenching heating time should be controlled within 0.3–1.2 h.

[0051] Tempering temperature: 620–700℃. If the tempering temperature is too high, a large amount of the bainite structure formed during quenching will soften and transform into ferrite (ferrite proportion ≥ 5%), resulting in the material's strength failing to meet the X65 target. If the tempering temperature is too low, carbon and nitrogen will not precipitate sufficiently or rapidly and uniformly, leading to weakened microstructure uniformity. Simultaneously, the strength will be too high, the toughness too low, and a good strength-toughness balance cannot be achieved to obtain optimal fatigue resistance. Therefore, the tempering temperature should be controlled between 620 and 700℃.

[0052] Tempering heating time: 0.3–1.2 h. If the tempering time is too long, a large amount of the bainite structure formed during quenching will soften and transform into ferrite (ferrite proportion ≥ 5%), resulting in the material's strength failing to meet the X65 target. If the tempering time is too short, carbon and nitrogen will not precipitate sufficiently or rapidly and uniformly, leading to weakened microstructure uniformity. Simultaneously, the strength will be too high, the toughness too low, and a good strength-toughness balance cannot be achieved to obtain optimal fatigue resistance. Therefore, the tempering heating time should be controlled within 0.3–1.2 h.

[0053] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0054] This invention controls CE based on low Ti, low N, low Ni, controlling the Ni / (C+Mn) ratio, adding Ca, and controlling the Ca / Alt ratio. Pcm The method utilizes TiN to suppress grain size coarsening, thereby refining the microstructure and reducing its adverse effects on low-temperature toughness and fatigue resistance. Simultaneously, by determining the amount of Ni added based on the C and Mn contents, i.e., controlling the Ni / (C+Mn) ratio, low-temperature toughness is ensured. By adding Ca and controlling the Ca / Alt ratio, inclusions are modified to obtain dispersed, fine, and spherical inclusions, reducing the adverse effects of large-sized and sharp inclusions on toughness and fatigue. This results in a seamless pipeline tube that is resistant to low temperatures and fatigue.

[0055] This invention, based on the optimized alloy content or element combination design of multiple elements such as Ti, N, Ni, Ca, Al, C, and Mn, and combined with appropriate quenching and tempering processes, achieves grain and microstructure refinement, avoids the formation of high proportion of ferrite phase, and modifies inclusions to a dispersed, fine, and spherical morphology, avoiding the formation of large-sized, sharp inclusions. Ultimately, this significantly improves the low-temperature toughness and fatigue performance of pipeline pipes, achieving high low-temperature toughness and high fatigue resistance at high steel grades. It solves technical problems not addressed by existing technologies, thus meeting the requirements for use under complex marine operating conditions subjected to fatigue loads and ensuring pipeline safety. Detailed Implementation

[0056] The present invention will be further described below with reference to the embodiments.

[0057] The components of the embodiments and comparative examples of this invention are shown in Table 1. The process parameters are shown in Table 2. The performance results are shown in Table 3.

[0058] As can be seen from Tables 1 to 3:

[0059] Comparative Example 1: Both Ti and N are relatively high, and there are large TiN particles with a size greater than 2μm in the steel, which leads to a decrease in low-temperature impact toughness and fatigue performance.

[0060] Comparative Example 2: Both Ca and Ca / Al are relatively low. The inclusions in the steel are mainly Al2O3 inclusions (hard and brittle, not easy to deform, easy to stress concentration, and easy to become crack initiation in the matrix). Unmodified calcium aluminate inclusions that form fine xCaO*yAl2O3 (hard and brittle core but soft CaO outer coating, improved deformation coordination, and reduced risk of becoming crack initiation) result in low low-temperature impact toughness and reduced fatigue performance.

[0061] Comparative Example 3: Both Ni and Ni / (C+Mn) are relatively low. The effect of Ni in improving the toughness of C-Mn steel is insufficient, resulting in a decrease in low-temperature impact toughness. Insufficient toughness also leads to a decrease in fatigue performance.

[0062] Comparative Example 4: The quenching temperature was too low, which resulted in the formation of a high proportion of ferrite after quenching and cooling, reaching 7% and exceeding 5%. This led to a decrease in yield strength, which could not meet the strength requirements of X65 steel grade. It also resulted in a decrease in low-temperature impact toughness and fatigue performance.

[0063] Comparative Example 5: Excessive quenching temperature resulted in low grain size of the microstructure after quenching and cooling, only level 6, which means coarse grains. This led to poor deformation coordination of the microstructure under load conditions, resulting in reduced low-temperature impact toughness and fatigue performance.

[0064] Comparative Example 6: Low C content, CE Pcm The low hardenability and insufficient hardenability make the microstructure prone to unevenness after quenching, and the lath substructure in the microstructure is relatively large, >0.2μm, which makes it impossible to meet the strength requirements of X65 steel grade. It also reduces the low temperature impact toughness and fatigue performance.

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

Claims

1. A low-temperature resistant and fatigue-resistant X65 seamless pipeline tube, the chemical composition by weight percentage of which is: C 0.06-0.11%, Si 0.15-0.35%, Mn 1.41-1.65%, P≤0.015%, S≤0.005%, Cu≤0.10%, Ni 0.05-0.15%, Cr≤0.10%, Mo 0.07-0.20%, Nb 0.01-0.04%, V 0.03-0.08%, Ti 0.001-0.006%, Ca 0.0010~0.0040%, Alt 0.010~0.040%, B≤0.0005%, O≤0.0030%, N0.0010~0.0055%, H≤0.0002%, with the balance including Fe and other unavoidable inclusions, and must simultaneously satisfy: Ni / (C+Mn)≥0.03; THIS Pcm <0.171~0.215%; Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

2. The low-temperature resistant and fatigue-resistant X65 seamless pipeline pipe as described in claim 1, characterized in that, Its chemical composition also satisfies: Alt / N≥2 and / or Ca / Alt≥0.

10.

3. The low-temperature resistant and fatigue-resistant X65 seamless pipeline as described in claim 1 or 2, characterized in that, The balance consists of Fe and other unavoidable inclusions.

4. The low-temperature resistant and fatigue-resistant X65 seamless pipeline as described in claim 1, 2, or 3, characterized in that, The microstructure of the X65 seamless pipeline tube is tempered bainite with a fine lath substructure of ≤0.2μm and a small amount of ferrite, with a ferrite ratio of ≤5% and a grain size of ≥7; and the percentage of inclusions with a size of ≥5μm is 0%, and the percentage of TiN particles with a size of ≥2μm is 0%.

5. The low-temperature resistant and fatigue-resistant X65 seamless pipeline as described in claim 1, 2, 3, or 4, characterized in that, The X65 seamless pipeline has a yield strength Rt0.5 of 450-570 MPa, a tensile strength Rm of 535-760 MPa, a yield strength ratio Rt0.5 / Rm ≤ 0.90, a total elongation A50 ≥ 30%, a uniform elongation Ag ≥ 8%, an impact energy KV8 ≥ 200 J at -80℃, and a fatigue life ≥ 10 million cycles under tensile and compressive fatigue conditions with a stress ratio R = -1.

6. The method for manufacturing the low-temperature resistant and fatigue-resistant X65 seamless pipeline pipe as described in any one of claims 1 to 5, characterized in that, Includes the following steps: 1) A tube blank is obtained by smelting and casting according to the composition described in claim 1, 2 or 3; 2) The tube blank is made into a steel pipe; 3) Quenching treatment, quenching temperature: 880~940℃, quenching time: 0.3~1.2h; 4) Tempering treatment, tempering temperature: 620~700℃, tempering time: 0.3~1.2h.

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