High-strength seamless tube and manufacturing method therefor

Abstract

The present disclosure provides a high-strength seamless tube, comprising, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%. The present disclosure also provides a method for manufacturing the seamless tube, comprising the following steps performed in sequence: smelting and continuous casting; heating, perforating, rolling, and sizing; heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 580-650°C with a holding time of 50-80 min; and hot sizing and straightening.

Description

TECHNICAL FIELD

[0001] This disclosure relates to a steel material and a manufacturing method therefor, and more particularly to a seamless tube and a manufacturing method therefor.BACKGROUND

[0002] High-strength structural tube products are widely used in the field of engineering machinery, serving as core components of truck cranes and crawler cranes. They are required to possess good strength, low-temperature toughness, good surface quality, and weldability.

[0003] Currently, conventional high-strength seamless steel tubes all contain high levels of W and Ni alloying elements. By adding low-carbon equivalent contributing elements such as tungsten and nickel, the low-temperature toughness of the steel can be improved, and a certain strength and a low carbon equivalent level can be guaranteed.

[0004] For example, Chinese patent publication No. CN101586450A, published on November 25, 2009, entitled "HIGH-STRENGTH AND HIGH-TOUGHNESS OIL CASING AND MANUFACTURING METHOD THEREOF" discloses an oil casing, comprising the following components: C: 0.16-0.28%, Si: ≤0.50%, Mn: 0.30-1.10%, Cr: 0.50-1.10%, Mo: 0.60-0.95%, Al: 0.015- 0.060%, wherein acid-soluble Als / Al≥0.80%, Ni<0.60%, Cu: 0.05%-0.25%, V: 0.060-0.20%, Ca>0.0015%, Nb:≤0.05%, Ti:≤0.05%, P<0.010%, S<0.002%, O:<0.0024%, H<0.0002%, N<0.008%, B: 0.0000%-0.005%, and the remainder being Fe. In the oil casing, the Ni element is added, and the Cr content is high.

[0005] However, the cost of elements such as Ni, W, and Cr is relatively high. In the context of a highly homogeneous and fiercely competitive engineering machinery industry, there is a need for a high-strength seamless tube with an innovative composition design, which can reduce the content of high-cost alloy elements or eliminate high-cost alloy elements while maintaining high strength performance, thereby significantly reducing the cost of engineering machinery components.SUMMARY

[0006] One of the objects of the present disclosure is to provide a seamless tube, which, through a reasonable design of the chemical composition, preferably in combination with process design, can achieve higher strength on the basis of reducing the content of high-cost alloy elements or eliminating high-cost alloy elements, while having excellent low-temperature impact toughness and good weldability.

[0007] In order to achieve the above object, the present disclosure provides a seamless tube, comprising, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%.

[0008] In addition, the present disclosure also provides a seamless tube, wherein the seamless tube consists of the following chemical elements in percentage by mass: C: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%; with the balance being Fe and unavoidable impurities.

[0009] In some embodiments, the mass percentage of each element of the seamless tube according to the present disclosure further satisfies at least one of the following: C: 0.15-0.19%; Si: 0.15-0.35%; Mn: 0.6-1.1%; Cr: 0.8-1.0%; Mo: 0.5-0.85%; Ti: 0.015-0.035%.

[0010] In some embodiments, in the seamless tube according to the present disclosure, unavoidable impurities include P, S, and N, and P≤0.01 %, S≤0.002%, and N≤0.007%.

[0011] In some embodiments, the seamless tube according to the present disclosure has a carbon equivalent of ≤0.68, preferably ≤0.67, wherein the carbon equivalent is calculated by the following formula: carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element.

[0012] In some embodiments, the seamless tube according to the present disclosure has a microstructure of tempered sorbite.

[0013] In some embodiments, the seamless tube according to the present disclosure has an average grain size of 8-20 µm.

[0014] In some embodiments, the seamless tube according to the present disclosure has a yield strength of ≥890 MPa, preferably a yield strength of ≥960 MPa, a tensile strength of 960-1100 MPa, preferably a tensile strength of 1000-1100 MPa, an elongation of ≥23%, and a longitudinal impact toughness KV 8 at -40°C of ≥45 J, preferably KV 8 of ≥70 J.

[0015] In addition, another object of the present disclosure is to provide a method for manufacturing a seamless tube. The manufacturing method employs a unique heat treatment process, together with the composition design of the seamless tube according to the present disclosure. Using this method, it is possible to obtain a seamless tube with high strength, excellent low-temperature impact toughness, and good weldability, while reducing or eliminating high-cost alloy elements.

[0016] In order to achieve the above object, the present disclosure provides a method of manufacturing a seamless tube, comprising the following steps performed in sequence: performing smelting and continuous casting to obtain a tube billet; subjecting the tube billet to heating, perforating, rolling, and sizing to obtain a tube body; subjecting the tube body to a heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 580-650°C with a holding time of 50-80 min; and hot sizing and straightening.

[0017] In some embodiments, in the smelting step, an ingredient formulation of scrap steel + blast furnace molten iron may be adopted. For example, the proportion of the molten iron is 50-60%. The molten steel is smelted in the electric furnace, refined outside the furnace, vacuum degassed and stirred under argon gas, and then subjected to inclusion modification by Ca treatment, thereby reducing the content of O and H.

[0018] In the manufacturing method according to the present disclosure, the reason for using the above process parameters (i.e., austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 580-650°C with a holding time of 50-80 min) in the heat treatment step is that: by subjecting the rolled tube to re-austenitizing and water-quenching, on the one hand, the original austenite grains can be refined, and on the other hand, the element distribution can be made more uniform, and then by performing the tempering, the precipitated strengthening phase is more uniformly dispersed, thereby improving the strength-toughness matching performance.

[0019] In some preferred embodiments, in the continuous casting step of the manufacturing method according to the present disclosure, a superheat of the molten steel is controlled to be 30 °C or below. and a continuous casting speed is controlled to be 1.8-2.2 m / min. Therefore, component segregation can be effectively reduced.

[0020] In some embodiments, in the heating step of the manufacturing method according to the present disclosure, the tube billet is heated to a soaking temperature of 1200-1240 °C.

[0021] In some embodiments, in the perforating step of the manufacturing method according to the present disclosure, a perforating temperature is 1180-1240°C.

[0022] In some embodiments, in the rolling step of the manufacturing method according to the present disclosure, a final rolling temperature is 900-950°C.

[0023] In some embodiments, in the sizing step of the manufacturing method according to the present disclosure, a sizing temperature is 850-900°C.

[0024] In some embodiments, in the manufacturing method according to the present disclosure, a temperature for heat sizing and straightening is 400-550°C.

[0025] The seamless tube and the manufacturing method therefor according to the present disclosure have the advantages and beneficial effects as described below: The seamless tube according to the present disclosure employs low-carbon, Cr-Mo and micro-alloyed steel in its composition design, with B added to increase hardenability. The seamless tube may be free of W and Ni alloying elements, thus significantly reducing the material alloy costs and providing economic advantages.

[0026] The heat treatment process of the seamless tube is simple and easy to implement in large-scale production.

[0027] The seamless tube according to the present disclosure can be used to manufacture structural tube products with a 890 MPa grade, offering excellent strength, low-temperature toughness, and weldability.

[0028] By using the design of low-carbon, Cr-Mo and micro-alloyed steel, the carbon equivalent of the steel of the seamless tube according to the present disclosure can be controlled at a relatively low level, so as to achieve high strength while maintaining excellent low-temperature impact toughness and good weldability.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0030] Herein, carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element.

[0031] Herein, yield strength, tensile strength and elongation are determined in accordance with Standard GB / T 228.1-2000.

[0032] Herein, longitudinal impact toughness KV 8 at -40°C is determined in accordance with Standard GB / T 229-2007 "Metallic materials - Charpy pendulum impact test method".

[0033] Herein, metallographic microscope is used to observe the microstructure.

[0034] Specifically, the design principle of each chemical element in the seamless tube according to the present disclosure is described as follows: Carbon (C): C is a basic element to ensure the strength and hardenability. At the same time, the C content affects the carbon equivalent and weldability. If the C content in the steel is less than 0.14%, the hardenability of the steel is relatively poor, and the strength of the steel cannot meet the requirements. If the C content in the steel exceeds 0.20%, i.e., the C content is too high, the weldability of the steel is relatively poor, and its plasticity and toughness also deteriorate. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the C element is controlled between 0.14-0.20%.

[0035] In some embodiments, the mass percentage of C may be further controlled between 0.15-0.19%.

[0036] Si: In the seamless tube according to the present disclosure, Si is an important deoxidizer in the steel. The Si element can be solid-solved in ferrite to increase the yield strength of the steel. However, it should be noted that the content of the Si element in the steel should not be too high. When the Si content in the steel is too high, for example, higher than 0.55%, the processability and toughness of the steel will deteriorate. If the Si content in the steel is too low, for example, lower than 0.15%, the steel is easily oxidized. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the Si element is controlled between 0.15-0.55%.

[0037] In some embodiments, the mass percentage of Si may be further controlled between 0.15%-0.35%.

[0038] Mn: In the seamless tube according to the present disclosure, the Mn element is a deoxidizing and desulfurizing agent, which has a significant effect on the hardenability and toughness of the steel. When the Mn content in the steel is 0.5% or above, a better effect can be produced. However, if the Mn content is too high, the hardenability of the steel will be too high, the toughness of a welding heat-affected zone will be reduced, and center segregation will occur during continuous casting, the low-temperature impact toughness of the base material will deteriorate. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the Mn element is controlled between 0.5-1.3%.

[0039] In some embodiments, the mass percentage of Mn may be further controlled between 0.6-1.1%.

[0040] Cr: In the seamless tube according to the present disclosure, the Cr element can increase the strength and hardenability of the steel, and the combination of the Cr and Mo elements have a better effect. However, it should be noted that if the Cr content in the steel is too high, the carbon equivalent will be significantly increased, resulting in increased susceptibility to welding cracking and reduced toughness in the welding heat affected zone. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the Cr element is controlled between 0.5-1.0%.

[0041] In some embodiments, the mass percentage of Cr may be further controlled between 0.8-1.0%.

[0042] Mo: In the seamless tube according to the present disclosure, Mo is one of the main added elements, which can improve the hardenability of steel. Mo in combination with Cr more effectively improves hardenability. In addition, the Mo element also has good precipitation strengthening and solid solution strengthening effects. Mo has a good interaction with micro-alloying elements, which can effectively refine the precipitation phase, increase the stability and volume fraction of the precipitation phase, and improve the toughness of the welding heat affected zone. However, it should be noted that the Mo element is expensive. If the added Mo content in the steel is too high, not only will the carbon equivalent be too high, but the cost of alloy will also increase. Therefore, in the seamless tube according to the present disclosure, the mass percentage of Mo is controlled between 0.35%-0.85%.

[0043] In some embodiments, the mass percentage of Mo may be further controlled between 0.5-0.85%.

[0044] V: In the seamless tube according to the present disclosure, V can refine the grains in the steel, and carbides formed with V significantly improve the strength of the steel. However, when the added V content in the steel reaches a certain level, further increasing the V content has little additional strengthening effect and may reduce the stability of low-temperature impact toughness. Moreover, V is a relatively expensive alloy element and should not be added in excess. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the V element is controlled between 0.06-0.1%.

[0045] Nb: In the seamless tube according to the present disclosure, Nb is a grain-refining and precipitation-strengthening element, which compensates for the decrease in strength caused by the reduction in carbon content. When the Nb content is less than 0.02%, it is insufficient to achieve the desired effect. When the Nb content is higher than 0.06%, coarse Nb (CN) are easily formed, thereby reducing toughness. Therefore, in the seamless tube according to the present disclosure, the mass percentage of Nb is controlled between 0.02%-0.06%.

[0046] Al: In the seamless tube according to the present disclosure, Al is a good deoxidizing element. However, If the added Al content in the steel is too high, aluminum oxide inclusions are easily formed. It is desirable to increase the proportion of acid-soluble aluminum in the total aluminum as much as possible. Therefore, Al wire is typically fed in an appropriate amount after vacuum degassing. In addition, in the seamless tube according to the present disclosure, Al can also be combined with N to form AlN, thereby ensuring the hardenability effect of B. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the Al element is controlled between 0.01-0.05%.

[0047] Ca: In the seamless tube according to the present disclosure, Ca can purify the molten steel, promote the spheroidization of MnS, and improve the impact toughness of the material. However, it should be noted that the Ca content in the steel should not be too high. When the Ca content in the steel is too high, coarse non-metallic inclusions are easily formed, which have an adverse effect on the performances of the steel. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the Ca element is controlled between 0.0005-0.005%.

[0048] Ti: In the seamless tube according to the present disclosure, Ti is a strong carbonitride-forming element, which can significantly refine austenite grains at high temperatures. During the rapid cooling of the steel, the carbonitrides of Ti in the steel can act as nucleation points to promote the refinement of the rapid cooling structure, thereby refining the final quenched and tempered structure, and at the same time, preventing the coarsening of the structure of the heat-affected zone during welding. In addition, the addition of an appropriate amount of Ti element to the steel also contributes to the strength of the steel, which can make up for the decrease in the strength of the steel caused by the reduction in the C content in the steel. It should be noted that the Ti content in the steel should not be too high. When the Ti content in the steel is higher than 0.05%, coarse TiN is easily formed, thereby reducing the toughness of the material. However, when the Ti content in the steel is too low, the Ti element cannot effectively play a refining effect. In the present disclosure, the Ti element is coordinated with the B element, and the Ti element is combined with the N element, thereby ensuring that the B element plays an effect of improving the hardenability and preventing B and N from forming a brittle phase. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the Ti element is controlled between 0.01-0.05%.

[0049] In some embodiments, the mass percentage of the Ti element may be further controlled between 0.015-0.035%.

[0050] B: In the seamless tube according to the present disclosure, B is an element that strongly improves hardenability, and its hardenability effect can replace a certain amount of Mn, Mo, and Cr elements, and thus the B element can significantly reduce the alloy cost. The B element is used in combination with Al and Ti to ensure the hardenability effect. However, if the B content is too high, an embrittled phase will be formed, the impact toughness will deteriorate. Therefore, in the seamless tube according to the present disclosure, the mass percentage of the B element is controlled between 0.001-0.005%.

[0051] It should be noted that the unavoidable impurity elements in the seamless tube according to the present disclosure mainly comprise S, P, and N. Within technically feasible limits, to obtain seamless steel tubes with better performance and higher quality, the content of the impurity elements in the seamless tube should be minimized as much as possible, i.e., the lower, the better.EXAMPLES

[0052] The seamless tube and the manufacturing method therefor disclosed in the present disclosure will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution disclosed in the present disclosure.Examples 1-6 and Comparative Examples 1-5

[0053] The seamless tubes in Examples 1-6 were obtained by the following steps: (1) performing smelting and continuous casting to obtain a tube billet: (2) subjecting the tube billet to heating, perforating, rolling, and sizing to obtain a tube body; (3) subjecting the tube body to a heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 580-650°C with a holding time of 50-80 min; and (4) hot sizing and straightening.

[0054] The comparative tubes of Comparative Examples 1-5 were manufactured using the same steps described above, except for the compositional contents and / or specific process parameters.

[0055] Table 1 lists the contents of the chemical elements in percentage by mass for the seamless tubes of Examples 1-6 and the comparative tubes of Comparative Examples 1-5. Table 1. (wt %, the balance being Fe and unavoidable impurities other than P, S and N)CSiMnPSNCrMoVNbTiBAlCaCarbon equivalentExample 10.140.161.30.0060.0020.0060.50.40.060.020.020.0010.010.00050.55Example 20.150.351.10.0050.00180.0070.60.350.070.0350.0150.00250.0250.0020.53Example 30.190.30.60.0040.00150.00510.480.090.0250.050.0030.050.00150.60Example 40.150.550.50.0030.00090.00450.850.50.10.060.040.00250.030.0050.53Example 50.170.450.70.0090.0020.00410.60.10.0450.0350.0050.0250.0040.63Example 60.20.250.80.0060.00150.0050.80.850.090.0350.0150.00150.030.0010.68comparative Example 10.23 0.250.60.0080.00160.0060.80.50.0750.0250.0250.0030.030.0020.61Comparative Example 20.12 0.240.90.0090.00180.00350.90.550.0850.0350.030.0030.020.0010.58Comparative Example 30.20.311.30.0060.00110.00410.850.10.040.0350.00250.0350.00150.81 Comparative Example 40.190.3210.0060.00120.00380.80.50.080.0250.040.00250.030.00150.63Comparative Example 50.160.231.10.0080.00080.00350.80.550.070.020.020.0030.040.0020.63Note: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element.

[0056] Tables 2-1 and Table 2-2 list the process parameters for the seamless tubes of Examples 1-6 and the comparative tubes of Comparative Examples 1-5. Table 2-1.No.Superheat of molten steel (°C)Continuous casting speed (m / min)Soaking temperature (°C)Perforating temperature (°C)Final rolling temperature (°C)Sizing temperature (°C)Example 125212201180910850Example 2102.212301210900860Example 3202.112401220940870Example 4301.812301190950880Example 5251.812001240920890Example 620212401220940900Comparative Example 1201.912301210920900Comparative Example 2152.212401220940880Comparative Example 3201.912101230950870Comparative Example 4201.912201240920890Comparative Example 5231.912201240930890 Table 2-2. No.Austenitizing temperature (°C)Holding time (min)Tempering temperature (°C)Holding time (min)Hot straightening temperature (°C)Example 19205058050400Example 29304060060450Example 39106061060480Example 49206059080500Example 59005063070520Example 69103065060550Comparative Example 19104062070450Comparative Example 29306061060480Comparative Example 39204562060500Comparative Example 4960 4063060510Comparative Example 5870 4063060520

[0057] Samples were taken from the seamless tubes of Examples 1-6 and the comparative tubes of Comparative Examples 1-5 respectively, and subjected to testing. The obtained test results are listed in Table 3. The testing procedures for the relevant properties are described below: (1) Tensile performances: Room-temperature tensile performances were measured in accordance with Standard GB / T 228.1-2000. (2) Microstructure: Cross-section of the tube was taken for microstructure observation. The samples were ground and polished, and then the metallographic structure was etched using 4% nitric acid + alcohol. After etching, the microstructure was observed using a metallographic microscope. (3) Impact toughness: Longitudinal impact energy KV 8 at -40°C was measured in accordance with Standard GB / T 229-2007 "Metallic materials - Charpy pendulum impact test method". (4) Weldability: Weldability was evaluated in accordance with NB / T-47014-2011 "Welding procedure qualification for pressure equipment".

[0058] Table 3 lists the test results of the seamless tubes of Examples 1-6 and the comparative tubes of Comparative Examples 1-5. Table 3.No.Yield strength (MPa)Tensile strength (MPa)Elongatio n (%)-40°C longitudinal impact / KV 8 (J)WeldabilityAverage grain size (µm)Example 1103010902470Good12Example 299010502380Good14Example 398010402590Good15Example 4995105823100Good8Example 5960102523102Good16Example 696510802395Good18Comparative Example 1102010852450Poor 18Comparative Example 2880 952 25110Good17Comparative Example 3101010662590Poor 19Comparative Example 498510532341 Poor 25Comparative Example 5860 993 2338 Poor 15

[0059] As can be seen from Table 3, all the seamless tubes in Examples 1-6 had a yield strength of ≥960 MPa, a tensile strength of 1000 MPa or above, an impact toughness KV 8 at -40°C of ≥70 J, an elongation of ≥23%, a grain size of 8-18 µm, and had good weldability. In contrast, Comparative Examples 1-5 cannot achieve a balance among high strength, excellent low-temperature impact toughness, and good weldability.

[0060] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated herein by reference in their entirety.

[0061] Although the disclosure has been illustrated and described by reference to certain preferred embodiments of the disclosure, it should be understood by those skilled in the art that the above content is a further detailed description of the disclosure with specific embodiments and should not be construed as limiting the disclosure to these descriptions. Those skilled in the art may perform various changes in formality and details, including performing several simple deductions or substitutions, without departing from the essence and scope of the present disclosure.

Claims

1. A seamless tube, wherein the seamless tube comprises, in addition to Fe and unavoidable impurities, the following chemical elements in percentage by mass: C: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%.

2. The seamless tube according to claim 1, wherein the seamless tube consists of the following chemical elements in percentage by mass: C: 0.14-0.20%, Si: 0.15-0.55%, Mn: 0.5-1.3%, Cr: 0.5-1.0%, Mo: 0.35-0.85%, V: 0.06-0.1%, Nb: 0.02-0.06%, Ti: 0.01-0.05%, B: 0.001-0.005%, Al: 0.01-0.05%, Ca: 0.0005-0.005%; with the balance being Fe and unavoidable impurities.

3. The seamless tube according to claim 1 or 2, wherein the mass percentage of each element of the seamless tube further satisfies at least one of the following: C: 0.15-0.19%; Si: 0.15-0.35%; Mn: 0.6-1.1%; Cr: 0.8-1.0%; Mo: 0.5-0.85%; and Ti: 0.015-0.035%.

4. The seamless tube according to any one of claims 1 to 3, wherein the unavoidable impurities include P, S and N, and wherein P ≤ 0.01%, S ≤ 0.002%, and N ≤ 0.007%.

5. The seamless tube according to any one of claims 1 to 4, wherein the seamless tube has a carbon equivalent of ≤0.68, preferably ≤0.67, wherein the carbon equivalent is calculated by the following formula: carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element.

6. The seamless tube according to any one of claims 1 to 5, wherein the seamless tube has a microstructure of tempered sorbite.

7. The seamless tube according to any one of claims 1 to 6, wherein the seamless tube has an average grain size of 8-20 µm.

8. The seamless tube according to any one of claims 1 to 7, wherein the seamless tube has a yield strength of ≥890 MPa, preferably a yield strength of ≥960 MPa, a tensile strength of 960-1100 MPa, preferably a tensile strength of 1000-1100 MPa, an elongation of ≥23%, and a longitudinal impact toughness KV8 at -40°C of ≥45 J, preferably KV8 of ≥70 J.

9. A method of manufacturing the seamless tube according to any one of claims 1 to 8, wherein the method comprises the following steps performed in sequence: - performing smelting and continuous casting to obtain a tube billet; - subjecting the tube billet to heating, perforating, rolling, and sizing to obtain a tube body; - subjecting the tube body to a heat treatment: austenitizing at a temperature of 900-930°C with a holding time of 30-60 min, quenching, and then tempering at a temperature of 580-650°C with a holding time of 50-80 min; and - hot sizing and straightening.

10. The method according to claim 9, wherein in the continuous casting step, a superheat of a molten steel is 30°C or below, and a continuous casting speed is 1.8-2.2 m / min.

11. The method according to claim 9 or 10, wherein in the heating step, the tube billet is heated to a soaking temperature of 1200-1240 °C.

12. The method according to any one of claims 9 to 11, wherein in the perforating step, a perforating temperature is 1180-1240 °C.

13. The method according to any one of claims 9 to 12, wherein in the rolling step, a final rolling temperature is 900-950 °C.

14. The method according to any one of claims 9 to 13, wherein in the sizing step, a sizing temperature is 850-900 °C.

15. The method according to any one of claims 9 to 14, wherein a temperature for hot sizing and straightening is 400-550 °C.

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