N80 grade oil well pipe and method of manufacturing the same

Abstract

The application discloses an N80 steel grade oil well pipe, each chemical element mass percentage of which is as follows: C: 0.23-0.38%, Si: 0.1-0.5%, Mn: 1.0-1.5%, the rest is Fe and inevitable impurity elements; the microstructure of the N80 steel grade oil well pipe is tempering sorbite. Correspondingly, the application also discloses a manufacturing method of the N80 steel grade oil well pipe, which comprises the following steps: (1) smelting and continuous casting to obtain a pipe blank; (2) pipe blank heating: the pipe blank is heated and kept in an annular furnace at 1180-1230 DEG C; (3) piercing; (4) continuous rolling; (5) continuous rolling post-cooling: the rough pipe after continuous rolling is cooled to 500-560 DEG C and then enters the following re-heating process of step (6); (6) re-heating in a re-heating furnace: keeping in a re-heating furnace at 900-930 DEG C; (7) tension reducing and cooling; (8) quenching and tempering treatment and hot straightening.

Description

Technical Field

[0001] This invention relates to a pipe and its manufacturing method, and more particularly to an oil well pipe and its manufacturing method. Background Technology

[0002] In recent years, with the rapid development of my country's oil and gas industry, the market and users' demand for oil and gas special pipelines has also begun to increase year by year. Among them, N80 steel grade oil well pipes, as the oil and gas pipelines with a large current application share, have been the subject of continuous research by researchers.

[0003] According to API standards, N80 grade oilfield steel pipes have requirements for sulfur and phosphorus content, but no other chemical composition or heat treatment process is specified.

[0004] Therefore, in the current technology, steel mills have produced a large number of N80 grade oil pipes that meet API requirements by formulating different steel composition, rolling process and heat treatment process.

[0005] For example, Chinese patent document CN101020986A, published on August 22, 2007, entitled "Production of N80 Grade Oil Pipes from Non-Quenched and Tempered Steel and its Process," discloses a process for producing N80 grade oil pipes from non-quenched and tempered steel. This process, through research on the effects of V and N microalloying elements and controlled rolling and cooling processes on performance, invented a new non-quenched and tempered steel, 34Mn2VN. Although this technical solution eliminates the heat treatment process, reducing production steps and cycle time, its alloy content is relatively high. While the performance of the resulting non-quenched and tempered steel meets standard requirements, the performance improvement is not significant.

[0006] For example, Chinese patent document CN101319292A, published on December 10, 2008, entitled "High-Toughness N80 Oil Casing and Manufacturing Method Thereof," discloses a high-toughness N80 oil casing and manufacturing method. This method uses traditional processes and controls the alloy content to achieve high-toughness N80 oil casing while reducing costs. However, the performance of the high-toughness N80 oil casing obtained by this technical solution is only slightly improved compared to the standard requirements.

[0007] For example, Chinese patent document CN102534375A, published on July 4, 2012, entitled "A Type of N80 Grade Nb-Containing Bainitic Oil Casing Steel and its Manufacturing Method", discloses a method for manufacturing N80 grade Nb-containing bainitic oil casing steel and its pipe. By adding more Nb, the toughness, weldability and corrosion resistance of the pipe are improved. However, the manufacturing process adopted by this technical solution has a long cycle, and slow cooling and heat preservation for 48 hours are required after air cooling. Summary of the Invention

[0008] One of the objectives of this invention is to provide a new N80 grade oil well pipe. Compared with the same grade oil well pipe produced by traditional processes, this N80 grade oil well pipe has a finer and more uniform grain size, superior and more stable mechanical properties, and lower alloy cost. Its yield strength is ≥680MPa, tensile strength is 770~860MPa, elongation is ≥21%; and impact energy at 0℃ KV2 is ≥150J.

[0009] The N80 grade steel well pipe has a wide range of applications and is inexpensive. It can be effectively used in the field of oil and gas development and can be used as tubing, casing and drill pipe in oil and gas extraction, which has great practical significance.

[0010] To achieve the above objectives, the present invention provides an N80 grade oil well pipe, wherein the mass percentage content of each chemical element is as follows:

[0011] C: 0.23–0.38%, Si: 0.1–0.5%, Mn: 1.0–1.5%, balance being Fe and unavoidable impurity elements;

[0012] The microstructure of the N80 grade oil well pipe is tempered sorbite.

[0013] The design principles of each chemical element in the N80 grade oil well tubing of this invention are as follows:

[0014] C: In the N80 grade oil well tubing described in this invention, carbon (C) is an important element for improving the strength, hardness, and hardenability of the steel. The inventors have found that when the amount of C added to the steel is too low, with a C content less than 0.23%, it is difficult to guarantee the strength of the steel. Conversely, when excessive C is added to the steel, with a C content greater than 0.38%, cracks are prone to occur during quenching, and the hardness (M) will also increase. 23 The tendency for the precipitation of coarse grain boundary carbides such as C6. Based on this, considering the influence of C on the properties of oil well pipe steel, in order to give full play to the beneficial effects of C, the mass percentage of C in the N80 grade oil well pipe of this invention is controlled between 0.23% and 0.38%.

[0015] Si: In the N80 grade oil well tubing of this invention, Si can play a role in solid solution strengthening and deoxidation, which is beneficial to improving the strength of the steel. Furthermore, adding an appropriate amount of Si to the steel can increase its hardenability and inhibit the precipitation of carbides. However, it should be noted that the Si content in the steel should not be too high. When the Si content exceeds 0.5%, it will promote the formation of σ phase and carbides, increase the cold brittleness tendency of the steel, and thus affect the performance of the steel, reducing its plasticity and toughness. Therefore, in order to maximize the beneficial effects of Si, the mass percentage of Si in the N80 grade oil well tubing of this invention is controlled between 0.1% and 0.5%.

[0016] Mn: In the N80 grade oil well tubing of this invention, Mn has many beneficial effects, such as expanding the austenite phase region, increasing hardenability, and refining grains. Adding an appropriate amount of Mn to steel can improve its performance, but the Mn content should not be too high. Excessive Mn can promote the formation of σ phase and carbides, and segregation is prone to occur during solidification, resulting in banded structures in the product that are significantly different from the matrix hardness and precipitated phases. In addition, when the Mn content in the steel is too high, it will increase the overheating sensitivity of the steel. Under slight overheating, the grains will coarsen. Therefore, the Mn content needs to be limited to below 1.5%. Based on this, in order to give full play to the beneficial effects of Mn, the mass percentage of Mn in the N80 grade oil well tubing of this invention is specifically controlled between 1.0% and 1.5%.

[0017] Furthermore, the N80 grade oil well tubing of the present invention also contains residual elements Cu≤0.1%, Ni≤0.1%, Cr≤0.15%, Mo≤0.05%, V≤0.01%, Nb≤0.01%, Al≤0.03%, and Ti≤0.02%.

[0018] In the N80 steel grade oil well pipe described in this invention, Cu, Ni, Cr, Mo, V, Nb, Al, and Ti are all residual associated elements. In order not to affect the performance of the N80 steel grade oil well pipe, the upper limit of the mass percentage content of these elements can be further preferably controlled.

[0019] Cu: In this invention, Cu is a by-product element in steel. When the Cu content in steel is too high, it will affect the mechanical properties of oil well pipe steel. Therefore, the mass percentage of Cu needs to be controlled to be no more than 0.1%.

[0020] Ni: In this invention, Ni is a by-product element in steel. Although Ni is an austenite stabilizing element and has good corrosion resistance, Ni is very expensive, so it is not added separately, and the mass percentage of Ni is controlled to be no more than 0.1%.

[0021] Cr: In this invention, Cr is a by-product element in steel. When the Cr content in steel is too high, there is a risk of σ phase precipitation and it will reduce the hot working performance of oil well pipe steel. Therefore, the mass percentage of Cr needs to be controlled to be no more than 0.15%.

[0022] Mo: In this invention, Mo is a by-product element in steel. When the Mo content in steel is too high, it is very easy to cause the formation of high-temperature δ-ferrite, and reduce the hot working performance of oil well pipe steel, thereby increasing the cost of oil well pipe steel. Therefore, the mass percentage of Mo needs to be controlled to be no more than 0.05%.

[0023] V: In this invention, V is a by-product element in steel. V itself is a strong carbide element. The dispersion precipitation of its carbides can enhance the strength of steel. However, if the V content in steel is too high, coarse carbides will be formed. In addition, V is expensive. Therefore, it is not added separately, and the mass percentage of V is controlled to be no more than 0.01%.

[0024] Nb: In this invention, Nb is a by-product element in steel. Nb is an effective grain-refining element that can improve the strength and toughness of steel. However, Nb is expensive, so it is not added separately, and the mass percentage of Nb is controlled to be no more than 0.01%.

[0025] Al: In this invention, Al is a by-product element in steel. When the content of Al in steel is too high, it will have an adverse effect on the casting process. Therefore, it is necessary to limit its content and control the mass percentage of Al to be no more than 0.03%.

[0026] Ti: In this invention, Ti is a by-product element in steel. When the Ti content in steel is too high, it will significantly reduce the impact toughness of the steel. Therefore, its content needs to be limited and the mass percentage of Ti should be controlled to be no more than 0.02%.

[0027] Furthermore, in the N80 steel grade oil well tubing described in this invention, among the unavoidable impurity elements, P≤0.02% and S≤0.015%.

[0028] In the above technical solution, P and S are both impurity elements in the N80 grade oil well pipe of the present invention. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the N80 grade oil well pipe should be reduced as much as possible.

[0029] When steel contains excessive amounts of phosphorus (P) and sulfur (S), it negatively impacts the steel's hot workability, purity, and toughness. Therefore, in the N80 grade oil well tubing described in this invention, the mass percentages of P and S are controlled as follows: P ≤ 0.02%, S ≤ 0.015%.

[0030] Furthermore, in the N80 grade oil well tubing described in this invention, the tempered sorbite grain size is ≥9.

[0031] Furthermore, in the N80 steel grade oil well pipe described in this invention, its yield strength is ≥680MPa, tensile strength is 770~860MPa, elongation is ≥21%; and impact energy at 0℃ KV2 is ≥150J.

[0032] Accordingly, another object of the present invention is to provide a method for manufacturing the N80 grade oil well tubing of the present invention. This manufacturing method employs an optimized process that can reduce energy consumption and CO2 emissions during production. The N80 grade oil well tubing obtained by this manufacturing method not only has excellent and stable mechanical properties, but also has uniform and fine grain size.

[0033] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned N80 grade oil well tubing, comprising the following steps:

[0034] (1) Smelting and continuous casting to produce tube blanks;

[0035] (2) Tube billet heating: The tube billet is heated and kept warm in an annular furnace at 1180℃~1230℃;

[0036] (3) Perforation;

[0037] (4) Continuous rolling;

[0038] (5) Cooling after continuous rolling: Cool the rolled tube to 500℃~560℃ before proceeding to the following step (6);

[0039] (6) Reheating in a reheating furnace: Keep warm in a reheating furnace at 900℃~930℃;

[0040] (7) Tension reduction and cooling;

[0041] (8) Quenching and tempering heat treatment and hot straightening.

[0042] In the above-described technical solution of the present invention, the present invention adopts a reasonable manufacturing method, which optimizes the manufacturing process and can reduce energy consumption and CO2 emissions during the production process.

[0043] In the manufacturing method of the present invention, compared with other patent documents, the present invention reduces the furnace temperature of the annular furnace during the billet heating in step (2), and the furnace temperature of the annular furnace is controlled between 1180°C and 1230°C, which can effectively suppress the growth of steel grains at high temperature while ensuring smooth production.

[0044] Furthermore, in other existing technologies, when preparing N80 grade oil well tubing, the raw tubing after continuous rolling is directly fed into a reheating furnace for heat preservation. However, this invention adds a cooling process after continuous rolling in step (5) after the continuous rolling process in step (4), so that the steel grade can undergo phase transformation, and the coarse austenite grains can be re-nucleated, which can effectively reduce the grain size.

[0045] In addition, compared with other existing technologies, in the reheating process of step (6) of the present invention, the inventors designed the reheating furnace temperature to be 900℃~930℃, which reduces the reheating furnace temperature and thus effectively suppresses the growth of steel grains at high temperature while ensuring smooth production.

[0046] It should be noted that after the reheated rough pipe undergoes tension reduction, it can be air-cooled to room temperature on a cooling bed. Subsequently, the rough pipe needs to undergo tempering heat treatment and hot straightening to finally obtain N80 grade oil well pipe with tempered martensitic structure.

[0047] Of course, in practical applications, in order to meet specific application needs, the obtained N80 steel grade oil well pipe can be further threaded according to requirements.

[0048] Furthermore, in the manufacturing method described in this invention, in step (2), the heat preservation time is 1.5-2.5h.

[0049] Furthermore, in the manufacturing method described in this invention, in step (6), the heat preservation time is 10-30 min.

[0050] Furthermore, in the manufacturing method described in this invention, in step (7), after tension reduction, the material is air-cooled to room temperature on a cooling bed.

[0051] Furthermore, the manufacturing method described in this invention also includes step (9) thread machining.

[0052] Compared with the prior art, the N80 steel grade oil well pipe and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0053] This invention provides a new N80 grade oil well pipe. Compared with the same grade oil well pipe produced by traditional processes, the N80 grade oil well pipe has finer and more uniform grain size, better and more stable mechanical properties, and lower alloy cost. Its yield strength is ≥680MPa, tensile strength is 770~860MPa, elongation is ≥21%; and impact energy at 0℃ KV2≥150J.

[0054] Accordingly, the present invention also optimizes the manufacturing method of N80 steel grade oil well pipe with low cost of the above alloy. This manufacturing method reduces energy consumption and CO2 emissions by lowering the furnace temperature of the ring furnace and reheat furnace, while making the product grains finer and more uniform, thereby improving the strength, toughness and other mechanical properties of N80 steel grade oil well pipe products and making their performance more stable.

[0055] The N80 grade steel well pipe has a wide range of applications and is inexpensive. It can be effectively used in the field of oil and gas development and can be used as tubing, casing and drill pipe in oil and gas extraction, which has great practical significance. Detailed Implementation

[0056] The N80 steel grade oil well pipe and its manufacturing method described in this invention will be further explained and described below with reference to specific embodiments. However, this explanation and description do not constitute an improper limitation on the technical solution of this invention.

[0057] Examples 1-6 and Comparative Examples 1-6

[0058] Table 1 lists the mass percentage of each chemical element for the N80 steel grade oil well tubing of Examples 1-6 and the comparative oil well tubing of Comparative Examples 1-6.

[0059] Table 1. (wt%, balance Fe and other unavoidable impurities besides P and S)

[0060]

[0061]

[0062] In this invention, the N80 steel grade oil well tubing of Examples 1-6 and the comparative oil well tubing of Comparative Examples 1-6 are all prepared using the following steps:

[0063] (1) Smelting and continuous casting are carried out according to the chemical composition shown in Table 1 to obtain tube blanks.

[0064] (2) Heating of tube blank: The tube blank is heated and kept warm in a ring furnace at 1180℃~1230℃, and the holding time is controlled to be 1.5-2.5h to ensure that the tube blank is heated evenly.

[0065] (3) Piercing: The heated tube blank is pierced on a piercing machine to obtain a tube blank.

[0066] (4) Continuous rolling: The tube is rolled by a continuous rolling mill to obtain a rough tube.

[0067] (5) Cooling after continuous rolling: The rolled blank is cooled to 500℃~560℃ on a spiral cooling bed and then enters a reheating furnace for the following steps (6).

[0068] (6) Reheating in a reheating furnace: The raw tube is kept at 900℃~930℃ in a reheating furnace for 10-30 minutes.

[0069] (7) Tension reduction and cooling: After heating, the rough tube is reduced by tension. The outer surface temperature of the rough tube before tension reduction is controlled at about 810℃, and then air-cooled to room temperature on a cooling bed.

[0070] (8) After quenching and tempering (holding at 880-950℃ for 30-60 minutes, water quenching, tempering at 550-650℃ for 40-80 minutes), hot straightening is performed at 400-500℃.

[0071] (9) Threading is performed to obtain the finished oil well pipe.

[0072] In this invention, the chemical composition design and related processes of the N80 steel grade oil well tubing in Examples 1-6 all meet the design specifications of this invention. To demonstrate the superiority of the composition design and manufacturing process of the N80 steel grade oil well tubing designed in this invention, the inventors designed six comparative oil well tubings, namely Comparative Examples 1-6, for comparison.

[0073] While the comparative oil well tubing of Comparative Examples 1-6 was also prepared using the process flow of steps (1)-(6) above, it had parameters in its chemical composition design and related processes that did not meet the design requirements of this invention.

[0074] Table 2 lists some of the process parameters of the N80 steel grade oil well tubing of Examples 1-6 and the comparative oil well tubing of Comparative Examples 1-6 in the above manufacturing method.

[0075] Table 2.

[0076]

[0077]

[0078] It is easy to see from Tables 1 and 2 above that, in this invention, the chemical element composition of the comparative oil well pipe in Comparative Example 1 is the same as that of the N80 steel grade oil well pipe in Example 1, but its reheat furnace temperature does not meet the design requirements; the chemical element composition of the comparative oil well pipe in Comparative Example 2 is the same as that of the N80 steel grade oil well pipe in Example 2, but its temperature before entering the reheat furnace does not meet the design requirements; the chemical element composition of the comparative oil well pipe in Comparative Example 3 is the same as that of the N80 steel grade oil well pipe in Example 3, but its annular furnace temperature does not meet the design requirements.

[0079] Unlike Comparative Examples 1-3, Comparative Examples 4-6 had deficiencies in their chemical composition design. Specifically, the C content of the control well tubing in Comparative Example 4 was below the design range; the Si content of the control well tubing in Comparative Example 5 exceeded the design range; and the Mn content of the control well tubing in Comparative Example 6 exceeded the design range.

[0080] Samples of the N80 steel grade oil well pipes of Examples 1-6 and the comparative oil well pipes of Comparative Examples 1-6 obtained through the above process steps were taken and observed and analyzed to obtain the microstructure and tempered sorbite grain size of each example and comparative example oil well pipe. The results of the observation and analysis are listed in Table 3 below.

[0081] Table 3 lists the microstructure and tempered sorbite grain size of the N80 steel grade oil well tubing of Examples 1-6 and the comparative oil well tubing of Comparative Examples 1-6.

[0082] Table 3.

[0083]

[0084]

[0085] Microscopic observation of the N80 steel grade oil well pipes in Examples 1-6 shows that the microstructure of the N80 steel grade oil well pipes in Examples 1-6 is tempered sorbite, and the grain size of the tempered sorbite reaches level 9.

[0086] Accordingly, after completing the above observations and analyses, it is necessary to resample the N80 steel grade oil well pipes of finished examples 1-6 and the comparative oil well pipes of comparative examples 1-6, and test the mechanical properties of the oil well pipe samples of each example and comparative example. The results of the mechanical property test are listed in Table 4.

[0087] The relevant performance testing methods are as follows:

[0088] (1) Room temperature tensile test: According to GB / T 228.1-2010 Metallic materials, tensile testing - Part 1: Room temperature tensile test method, the yield strength, tensile strength and elongation of the oil well pipes of Examples 1-6 and Comparative Examples 1-6 at room temperature were obtained. The test results are shown in Table 4.

[0089] (2) Charpy impact test at 0℃: According to GB / T 229-2020 Metallic Materials Charpy Pendulum Impact Test Method, the full-size specimen was controlled to be 10×10×55mm to test the impact performance AKV of the oil well pipes of Examples 1-6 and Comparative Examples 1-6 at 0℃, that is, the impact energy at 0℃ KV2. The test results are shown in Table 4.

[0090] Table 4 lists the mechanical property test results of the N80 steel grade oil well pipes of Examples 1-6 and the comparative oil well pipes of Comparative Examples 1-6.

[0091] Table 4.

[0092]

[0093]

[0094] In Table 4 above, the “range” for each column of mechanical properties in each embodiment and comparative example refers to the range of multiple values ​​obtained by performing multiple measurements on the same embodiment.

[0095] As can be seen from Table 4, compared with the comparative oil well pipes of Examples 1-6, the N80 steel grade oil well pipes of Examples 1-6 have significantly better comprehensive mechanical properties. The N80 steel grade oil well pipe products of Examples 1-6 of the present invention have fine and uniform grains, and their mechanical properties are excellent and stable, making them suitable for use as tubing, casing, and drill pipe in oil and gas extraction.

[0096] As shown in Table 4, in this invention, the average yield strength of N80 steel grade oil well pipes in Examples 1-6 is between 683-741 MPa, the average tensile strength is between 778-853 MPa, the average elongation is between 21.5-25%, and the average impact energy KV2 at 0℃ is between 151-169 J.

[0097] In contrast, the reheat furnace temperature of the oil well pipe in Comparative Example 1 was higher than the range designed in this invention, which easily led to grain growth and a decrease in mechanical properties.

[0098] In Comparative Example 2, the temperature of the oil well tubing before entering the reheat furnace was higher than the range designed in this invention. Its material did not undergo a phase change, did not play a role in refining and homogenizing the grains, and its mechanical properties decreased significantly and fluctuated greatly.

[0099] In Comparative Example 3, the temperature of the annular furnace of the oil well tubing was higher than the range designed in this invention, which easily led to grain growth and a decrease in mechanical properties.

[0100] The C content of the oil well tubing in Comparative Example 4 was lower than that designed in this invention, and its strengthening effect on the steel grade was insufficient. Although the average yield strength and average tensile strength were within the standard required performance range, the strength of some individual cases was lower than the API 5CT standard.

[0101] The Si content of the comparative oil well tubing in Comparative Example 5 was higher than the range designed in this invention, which increased the steel's tendency to become brittle in cold, made it prone to quenching cracks, and resulted in an elongation rate lower than the API 5CT standard.

[0102] The Mn content of the control well tubing in Comparative Example 6 is higher than the range described in this invention, which easily promotes the formation of σ phase and carbides, and segregation causes banded structures in the product, resulting in plasticity and toughness lower than the API 5CT standard.

[0103] In summary, the manufacturing process of the N80 steel grade oil well pipe designed in this invention reduces energy consumption and CO2 emissions. At the same time, the N80 steel grade oil well pipe produced has low alloy cost, excellent and stable mechanical properties, and can be well applied to oil and gas extraction, showing good application prospects.

[0104] It should be noted that 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.

[0105] 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. An N80 grade steel oil well pipe, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.23~0.38%, Si: 0.1~0.5%, Mn: 1.0~1.5%, balance being Fe and unavoidable impurity elements; The microstructure of the N80 grade oil well pipe is tempered sorbite, and the tempered sorbite grain size is ≥9. The N80 grade oil well pipe has a yield strength ≥680MPa, a tensile strength of 770~860MPa, an elongation ≥21%, and an impact energy of 0℃ KV2 ≥150J. The N80 grade oil well tubing is manufactured using the following steps: Smelting and continuous casting to produce tube blanks; Tube billet heating: The tube billet is heated and held at 1180℃~1230℃ in an annular furnace; perforation; Continuous rolling; Cooling after continuous rolling: The rolled tube is cooled to 500℃~560℃ before proceeding to the next step; Reheating in a reheating furnace: holding the temperature in a reheating furnace at 900℃~930℃; Tension reduction and cooling; Quenching and tempering heat treatment and hot straightening.

2. The N80 grade oil well tubing as described in claim 1, characterized in that, It also contains residual elements Cu≤0.1%, Ni≤0.1%, Cr≤0.15%, Mo≤0.05%, V≤0.01%, Nb≤0.01%, Al≤0.03%, and Ti≤0.01%.

3. The N80 grade oil well tubing as described in claim 1, characterized in that, Among the unavoidable impurity elements, P ≤ 0.02% and S ≤ 0.015%.

4. The method for manufacturing N80 grade oil well tubing as described in any one of claims 1-3, characterized in that, It includes the following steps: (1) Smelting and continuous casting to produce tube blanks; (2) Tube billet heating: The tube billet is heated and kept warm in a ring furnace at 1180℃~1230℃; (3) Perforation; (4) Continuous rolling; (5) Cooling after continuous rolling: Cool the rolled tube to 500℃~560℃ before proceeding to the following step (6); (6) Reheating in a reheating furnace: keep warm in a reheating furnace at 900℃~930℃; (7) Tension reduction and cooling; (8) Quenching and tempering heat treatment and hot straightening.

5. The manufacturing method as described in claim 4, characterized in that, In step (2), the heat preservation time is 1.5-2.5h.

6. The manufacturing method as described in claim 4, characterized in that, In step (6), the heat preservation time is 10-30 minutes.

7. The manufacturing method as described in claim 4, characterized in that, In step (7), after tension reduction, the material is air-cooled to room temperature on a cooling bed.

8. The manufacturing method according to any one of claims 4-7, characterized in that, It also includes step (9) thread machining.

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