High-strength heat-resistant oil casing and manufacturing method therefor

Abstract

Disclosed in the present invention is a high-strength heat-resistant oil casing, comprising Fe and inevitable impurities, and further comprising the following chemical elements in percentage by mass: 0.20-0.35% of C, 0.2-0.4% of Si, 0.9-1.1% of Mn, 0.5-1.0% of Cr, 0.1-0.25% of Mo, 0.5-1.0% of W, 0.006-0.015% of B, and 0.01-0.03% of Al. Also disclosed in the present invention is a manufacturing method for the high-strength heat-resistant oil casing, comprising the following steps: smelting and continuous casting; heating, piercing, and continuous rolling of a continuous casting billet; online controlled cooling: carrying out online water mist cooling on the rolled pierced billet to 500-600°C; reheating: the reheating temperature being 930-980°C, and maintaining the temperature for 20-40 min; sizing; quenching and tempering heat treatment; and straightening.

Description

A High-strength Heat-resistant Oil Casing and Manufacturing Method Thereof Technical Field

[0001] The present invention relates to a steel pipe and a manufacturing method thereof, in particular to an oil casing and a manufacturing method thereof. Background Art

[0002] With the continuous increase of global energy demand, the development difficulty of conventional oil and gas resources keeps increasing, and unconventional oil and gas resources have received increasing attention. As an important unconventional oil and gas resource, the development and utilization of heavy oil are crucial to ensuring stable energy supply.

[0003] Compared with conventional crude oil, heavy oil is characterized by high viscosity and low fluidity, and steam huff and puff is the most widely used extraction method with high economic benefits. However, during the steam injection process, the average temperature of steam can reach 320°C, and the maximum temperature can exceed 350°C, accompanied by a high pressure of 10-15 MPa, which exposes the oil casing to a harsh high-temperature and high-pressure environment. Meanwhile, stress changes during long-term service easily cause casing deformation, so heat-resistant casings are required to have excellent comprehensive mechanical properties and good service performance at 300-400°C.

[0004] In the prior art, existing seamless heat-resistant casings are strengthened by adding various alloy elements. To avoid high-temperature grain boundary weakening, coarse grain design is adopted, and advanced online controlled rolling and controlled cooling technology is not used. For example:

[0005] The invention disclosed in publication No. CN112143972A, publication date December 29, 2020, and titled "a high-strength heat-resistant oil casing material and a preparation method thereof", provides a high-strength heat-resistant oil casing material and a preparation method thereof, with the chemical composition ratio as follows: 0.25% ≤ C ≤ 0.30%, 0.15% ≤ Si ≤ 0.35%, 0.9% ≤ Mn ≤ 1.1%, 0.9% ≤ Cr ≤ 1.1%, 0.04% ≤ V ≤ 0.08%, 0.01% ≤ Ti ≤ 0.03%, 0 < Al ≤ 0.008%; continuously cast round billets are heated in an annular furnace and perforated to prepare hollow shells, the hollow shells are rolled to obtain rough tubes, the rough tubes are heated and subjected to micro-tension sizing to obtain rolled steel tubes, and the rolled steel tubes are quenched and tempered to obtain the high-strength heat-resistant oil casing material.

[0006] The publication number is CN103627962A, the publication date is March 12, 2014, and the title is "Seamless Steel Pipe for Heavy Oil Thermal Recovery Wells and its Preparation Method". The chemical composition is as follows: C: 0.24-0.30%, Si: 0.08-0.28%, Mn: 0.45-0.85%, Cr: 1.05-1.15%, Mo: 0.20-0.30%, V: 0.07-0.10%, Al: 0.01-0.04%, P≤0.015%, S≤0.006%, and rare earth La: 0.005-0.01%. The process includes billet heating, billet piercing, pipe rolling, tension reduction, cooling, and tempering heat treatment. Summary of the Invention

[0007] One of the objectives of this invention is to provide a high-strength heat-resistant oil casing. This high-strength heat-resistant oil casing, through the design of high W and high B, can improve high-temperature mechanical properties while reducing alloy costs and significantly increasing economic benefits.

[0008] To achieve the above objectives, this invention proposes a high-strength heat-resistant oil casing, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0009] C: 0.20~0.35%, Si: 0.2~0.4%, Mn: 0.9~1.1%, Cr: 0.5~1.0%, Mo: 0.1~0.25%, W: 0.5~1.0%, B: 0.006~0.015%, Al: 0.01~0.03%.

[0010] In some embodiments, the mass percentage of each chemical element in the high-strength heat-resistant oil casing of the present invention is as follows:

[0011] C: 0.20–0.35%, Si: 0.2–0.4%, Mn: 0.9–1.1%, Cr: 0.5–1.0%, Mo: 0.1–0.25%, W: 0.5–1.0%, B: 0.006–0.015%, Al: 0.01–0.03%; balance Fe and unavoidable impurities.

[0012] This invention improves high-temperature strength by controlling the content of C, Cr, Mo, and W elements and using a conditioning process to promote the formation of dispersed precipitates. At the same time, by adding W element, the amount of Cr and Mo elements added can be appropriately reduced, thereby reducing costs.

[0013] Furthermore, this invention employs a high-boron design, which differs from the traditional approach of using trace amounts of B to improve hardenability. This invention adds B element, which enhances grain boundary strength through B grain boundary segregation, thereby improving high-temperature performance.

[0014] Specifically, in the high-strength heat-resistant oil casing of this invention, the design principles of each chemical element are as follows:

[0015] C: In the high-strength heat-resistant oil casing of this invention, carbon (C) is the main reinforcing element. It combines with microalloying elements to form carbides, which significantly improve high-temperature strength. However, excessively high C content negatively impacts toughness. Therefore, in the high-strength heat-resistant oil casing of this invention, the C content is controlled between 0.20% and 0.35% by mass.

[0016] Si: In the high-strength heat-resistant oil casing of this invention, Si is a commonly used deoxidizer that can improve hardenability and tempering resistance. Therefore, in the high-strength heat-resistant oil casing of this invention, the mass percentage of Si is controlled between 0.2% and 0.4%.

[0017] Mn: In the high-strength heat-resistant oil casing of this invention, Mn is an important alloying element, improving hardenability and having a low cost. When the mass percentage of Mn is too high, it will promote the formation of segregation and MnS, which is detrimental to performance. Therefore, in the high-strength heat-resistant oil casing of this invention, the mass percentage of Mn is controlled between 0.9% and 1.1%.

[0018] Cr: In the high-strength heat-resistant oil casing of this invention, Cr is an important element for improving corrosion resistance and high-temperature oxidation resistance. It belongs to the medium-strength carbide-forming elements and can increase the hot strength of steel. Therefore, in the high-strength heat-resistant oil casing of this invention, the mass percentage of Cr is controlled between 0.5% and 1.0%.

[0019] Mo: In the high-strength heat-resistant oil casing of this invention, Mo is a strong carbide-forming element. High-temperature tempering will form dispersed special carbides, which can improve the hot strength and creep strength of steel. Therefore, in the high-strength heat-resistant oil casing of this invention, the mass percentage of Mo is controlled between 0.1% and 0.25%.

[0020] W: In the high-strength heat-resistant oil casing of this invention, the effect of W on the hardenability, tempering resistance, mechanical properties, and thermal strength of steel is similar to that of molybdenum. W is a strong carbide-forming element with relatively low cost. However, when the mass percentage of W is too high, it is detrimental to high-temperature oxidation resistance. Therefore, in the high-strength heat-resistant oil casing of this invention, the mass percentage of W is controlled between 0.5% and 1.0%.

[0021] B: In the high-strength heat-resistant oil casing of this invention, element B can segregate at grain boundaries, improving grain boundary strength and preventing the segregation of impurity elements at grain boundaries. However, when the mass percentage of element B is too high, it is detrimental to elongation and impact toughness. Therefore, in the high-strength heat-resistant oil casing of this invention, the mass percentage of element B is controlled between 0.006% and 0.015%.

[0022] Al: In the high-strength heat-resistant oil casing of the present invention, Al element is mainly used for deoxidation and grain refinement. Therefore, in the high-strength heat-resistant oil casing of the present invention, the mass percentage content of Al element is controlled between 0.01% and 0.03%.

[0023] In some embodiments, in the high-strength heat-resistant oil casing of the present invention, the mass percentage content of Mo and W satisfies: 3Mo+2W≥1.5%, where each chemical element is substituted into its mass percentage content.

[0024] In this invention, Mo and W are important alloying elements that improve thermal strength. Appropriately replacing Mo with W can reduce costs, while ensuring a certain tungsten and molybdenum content can guarantee the high-temperature strength of the steel.

[0025] In some embodiments, in the high-strength heat-resistant oil casing of the present invention, the mass percentage content of Mo and W satisfies: 1.5% ≤ 3Mo + 2W ≤ 2.75%, where each chemical element is substituted into its mass percentage content.

[0026] In some embodiments, the high-strength heat-resistant oil casing of the present invention further contains at least one of the following chemical elements:

[0027] 0 < V ≤ 0.05%;

[0028] 0 < Ti ≤ 0.03%.

[0029] In this invention, V and Ti elements are optional or preferred additions. By adding trace amounts of V and Ti to form vanadium nitride or titanium nitride, the bonding of nitrogen and boron can be avoided, thus preventing the weakening effect of boron. Simultaneously, the dispersed distribution of vanadium nitride or titanium nitride can refine the grain size and improve performance. Therefore, in the high-strength heat-resistant oil casing of this invention, the mass percentage of V is controlled to V ≤ 0.05%, and the mass percentage of Ti is controlled to Ti ≤ 0.03%.

[0030] In this invention, both P and S are conventional harmful impurity elements that are detrimental to mechanical properties, so the content of P and S should be minimized.

[0031] In some embodiments, among other unavoidable impurities in the high-strength heat-resistant oil casing of the present invention, P ≤ 0.015% and S ≤ 0.005% can be controlled.

[0032] In some embodiments, among other unavoidable impurities in the high-strength heat-resistant oil casing of the present invention: 0 < P ≤ 0.015%, 0 < S ≤ 0.005%.

[0033] In some embodiments, the microstructure of the high-strength heat-resistant oil casing of the present invention is tempered sorbite.

[0034] In some embodiments, the high-strength heat-resistant oil casing of the present invention has a grain size level of 8.5 to 9.5.

[0035] In some embodiments, the high-strength heat-resistant oil casing of the present invention satisfies at least one of the following:

[0036] The yield strength at room temperature is 750-1070 MPa, the tensile strength at room temperature is ≥860 MPa, the transverse Charpy impact energy at 0℃ is ≥120 J, and the longitudinal Charpy impact energy at 0℃ is ≥150 J.

[0037] The yield strength at at least 300℃ is ≥700MPa, and the decrease in yield strength at room temperature is <25%;

[0038] The yield strength at at least 400°C is ≥650MPa, and the yield strength at room temperature decreases by <35%.

[0039] In some embodiments, the high-strength heat-resistant oil casing of the present invention has a tensile strength of 860–1070 MPa at room temperature.

[0040] In some embodiments, the high-strength heat-resistant oil casing of the present invention has a transverse Charpy impact energy of 120-140 J at 0°C.

[0041] In some embodiments, the longitudinal Charpy impact energy at 0°C of the high-strength heat-resistant oil casing of the present invention is 150–180 J.

[0042] In some embodiments, the high-strength heat-resistant oil casing of the present invention has a yield strength at 300°C ≥ 700 MPa, for example 700-800 MPa; preferably, the yield strength at 300°C is greater than or equal to the yield strength at room temperature by > 0 and < 25%.

[0043] In some embodiments, the high-strength heat-resistant oil casing of the present invention has a yield strength at 400°C ≥ 650 MPa, for example 650–750 MPa; preferably, the yield strength at 400°C is greater than or equal to the yield strength at room temperature, with a decrease of >0 and <35%.

[0044] Another objective of this invention is to provide a method for manufacturing a high-strength heat-resistant oil casing. This method employs an online controlled cooling process, which can help improve product performance, avoid quality abnormalities, and enhance manufacturing stability.

[0045] To achieve the above objectives, the present invention proposes a method for manufacturing a high-strength heat-resistant oil casing, comprising the following steps:

[0046] Smelting and continuous casting;

[0047] Continuous casting billet heating, piercing, and continuous rolling;

[0048] Online controlled cooling: The rolled rough tubes are cooled by online water mist to 500-600℃;

[0049] Reheating: The reheating temperature is 930-980℃, and the temperature is maintained for 20-40 minutes;

[0050] sizing;

[0051] Tempering heat treatment;

[0052] Straightening.

[0053] In this invention, online controlled cooling is performed after continuous rolling and before entering the reheating furnace to induce a microstructure transformation in the rolled tube. After entering the reheating furnace, the tube is re-austenitized, resulting in a refined microstructure and thus improving the strength and toughness of the oil casing.

[0054] In summary, this invention, through chemical composition design combined with online controlled cooling process, can produce high-strength, high-toughness seamless heat-resistant oil casing. Unlike the traditional approach of improving high-temperature strength through coarse grains, this invention employs a fine-grained structure + high-temperature strengthened precipitates + grain boundary strengthening approach to achieve superior comprehensive mechanical properties.

[0055] In some embodiments, the heating temperature in the continuous casting billet heating step of the manufacturing method described in this invention is 1220–1270°C.

[0056] In some implementations, during the continuous casting billet heating step, the superheat of the molten steel can be controlled to ≤35°C, for example, 0-35°C, and the casting speed can be controlled to 2.0~2.2m / min.

[0057] In some implementations, the piercing temperature of the continuously cast billet after heating can be controlled to be ≥1200℃, for example, 1200~1250℃.

[0058] In some embodiments, the final rolling temperature is 950–1050°C in the continuous rolling step of the manufacturing method described in this invention.

[0059] In some embodiments, in the sizing step of the manufacturing method described in this invention, the tension reduction inlet temperature can be controlled to be ≥880℃, for example, 880~950℃; after sizing, a cooling bed rolling + air cooling can be used.

[0060] In some embodiments, the tempering heat treatment step of the manufacturing method of the present invention includes quenching, tempering and cooling in sequence.

[0061] In some embodiments, in the quenching and tempering heat treatment step of the manufacturing method described in this invention, the quenching temperature is 900-930°C, held for 30-50 minutes and then water-cooled to room temperature, and the tempering temperature is 620-660°C, held for 60-80 minutes and then air-cooled.

[0062] In some embodiments, in the straightening step of the manufacturing method described in this invention, the straightening temperature is controlled to be ≥500°C, for example, 500–550°C.

[0063] The high-strength heat-resistant oil casing and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0064] The high-strength heat-resistant oil casing of the present invention, through the design of high W and high B, can improve high-temperature mechanical properties while reducing alloy costs, thus significantly increasing economic benefits.

[0065] The manufacturing method of the high-strength heat-resistant oil casing described in this invention adopts an online controlled cooling process, which can help improve product performance, avoid quality abnormalities, and improve production stability. Attached Figure Description

[0066] Figure 1 shows a microstructure photograph of the high-strength heat-resistant oil casing of Embodiment 1 of the present invention. Detailed Implementation

[0067] The high-strength heat-resistant oil casing 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.

[0068] Examples 1-6 and Comparative Examples 1-4

[0069] The high-strength heat-resistant oil casings of Embodiments 1-6 of the present invention are obtained by the following steps:

[0070] (1) Smelting and continuous casting: According to the design composition, the steel is smelted by converter steelmaking, ladle refining and vacuum degassing, and then continuously cast to obtain a continuously cast billet;

[0071] (2) Heating, piercing, and continuous rolling of continuously cast billets;

[0072] In the continuous casting billet heating step, a ring furnace is used to heat the continuous casting billet, and the heating temperature is controlled at 1220~1270℃.

[0073] In the continuous casting billet heating step, the superheat of the molten steel is controlled to be ≤35℃, and the casting speed is controlled to be 2.0-2.2m / min.

[0074] After heating the continuously cast billet, piercing is performed, and the piercing temperature is controlled to be ≥1200℃.

[0075] The pierced tube is then subjected to continuous rolling. During the continuous rolling process, the final rolling temperature is controlled at 950–1050℃. After continuous rolling, a rough tube is obtained.

[0076] (3) Online controlled cooling: The rolled rough tube is cooled by online water mist to 500-600℃;

[0077] (4) Reheating: The reheating temperature is 930-980℃, and the temperature is maintained for 20-40 minutes;

[0078] (5) Sizing;

[0079] Control the inlet temperature of tension reduction to ≥880℃, and then use a rolling cooling bed and air cooling after sizing.

[0080] (6) Quenching and tempering heat treatment;

[0081] In the quenching and tempering heat treatment step, the quenching temperature is controlled at 900-930℃, held for 30-50 minutes and then water-cooled to room temperature. The tempering temperature is controlled at 620-660℃, held for 60-80 minutes and then air-cooled.

[0082] (7) Straightening.

[0083] During the straightening process, the straightening temperature is controlled to be ≥500℃ to avoid generating high stress that could lead to cracks.

[0084] The comparative steel pipes of Comparative Examples 1-4 were prepared using the same process as the steel pipes in the aforementioned embodiments, except that the process parameters listed in Tables 2-1 and 2-2 were used, and some of these process parameters did not meet the requirements of this invention.

[0085] Tables 1-1 and 1-2 list the mass percentage of each chemical element in the high-strength heat-resistant oil casings of Examples 1-6 of the present invention and the comparative steel pipes of Comparative Examples 1-4.

[0086] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides P and S)

[0087] Table 1-2. (wt%, balance Fe and other unavoidable impurities besides P and S)

[0088] Tables 2-1 and 2-2 list the specific process parameters of the high-strength heat-resistant oil casings of Examples 1-6 of the present invention and the comparative steel pipes of Comparative Examples 1-4.

[0089] Table 2-1.

[0090] Table 2-2.

[0091] Samples were taken from the high-strength heat-resistant oil casings of Examples 1-6 and the comparative steel pipes of Comparative Examples 1-4, and their microstructure was observed. The observation results are listed in Table 3. Among them:

[0092] Microstructure observation: Take a longitudinal section metallographic sample, grind and polish it, then etch it with 4% nitric acid alcohol solution, and observe the microstructure using a scanning electron microscope.

[0093] Grain size testing: Referencing standard GB / T 6394-2017 "Method for determination of average grain size of metals", the intercept method is used for measurement, and the grain size level G is calculated and determined.

[0094] Table 3 lists the microstructure observation results of the high-strength heat-resistant oil casings of Examples 1-6 of the present invention and the comparative steel pipes of Comparative Examples 1-4.

[0095] Table 3.

[0096] As can be seen from Table 3 above, the microstructure of the high-strength heat-resistant oil casings in Examples 1-6 of the present invention is tempered sorbite, and their grain size is between 8.5 and 9.5.

[0097] Figure 1 shows the microstructure of the high-strength heat-resistant oil casing of Embodiment 1 of the present invention.

[0098] As shown in Figure 1, the microstructure of Example 1 of the present invention at room temperature is tempered sorbite with fine dispersed precipitates.

[0099] The high-strength heat-resistant oil casings of Examples 1-6 and the comparative steel pipes of Examples 1-4 were sampled again, and their mechanical properties were tested. The observation results are listed in Table 4. Among them:

[0100] Tensile properties: Take longitudinal arc-shaped tensile specimens and conduct room temperature and high temperature tensile tests in accordance with standards GB / T 228.1-2021 and GB / T228.2-2015 to test yield strength and tensile strength.

[0101] Impact toughness: Take V-type impact specimens and perform 0℃ impact KV8 test according to standard GB / T 229-2020.

[0102] It should be noted that, in this invention, high-temperature strength attenuation = (room temperature yield strength - high-temperature yield strength) / room temperature yield strength.

[0103] Table 4 lists the performance test results of the high-strength heat-resistant oil casings of Examples 1-6 of the present invention and the comparative steel pipes of Comparative Examples 1-4.

[0104] Table 4.

[0105] As can be seen from Table 4 above, the high-strength heat-resistant oil casings of Examples 1-6 of the present invention have yield strengths between 750 and 1070 MPa at room temperature, tensile strengths greater than 860 MPa, transverse Charpy impact energy at 0℃ greater than 120 J, and longitudinal Charpy impact energy at 0℃ greater than 150 J. The high-strength heat-resistant oil casings of Examples 1-6 of the present invention have yield strengths greater than 700 MPa at at least 300℃, and the yield strength at room temperature decreases by less than 25%. The high-strength heat-resistant oil casings of Examples 1-6 of the present invention have yield strengths greater than 650 MPa at at least 400℃, and the yield strength at room temperature decreases by less than 35%.

[0106] In summary, the high-strength heat-resistant oil casing and its manufacturing method described in this invention, through reasonable component design and optimized advanced online controlled cooling manufacturing process, can produce high-strength, high-toughness seamless heat-resistant oil casing, which is expected to be widely used in unconventional oil and gas extraction fields and has considerable economic potential.

[0107] It should be noted that the prior art portion of the protection scope of this invention is not limited to the prior art portion in 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 protection scope of this invention.

[0108] 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.

[0109] 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 high-strength heat-resistant oil casing, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following percentages by mass: C: 0.20~0.35%, Si: 0.2~0.4%, Mn: 0.9~1.1%, Cr: 0.5~1.0%, Mo: 0.1~0.25%, W: 0.5~1.0%, B: 0.006~0.015%, Al: 0.01~0.03%.

2. The high-strength heat-resistant oil casing as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.20–0.35%, Si: 0.2–0.4%, Mn: 0.9–1.1%, Cr: 0.5–1.0%, Mo: 0.1–0.25%, W: 0.5–1.0%, B: 0.006–0.015%, Al: 0.01–0.03%; balance Fe and unavoidable impurities.

3. The high-strength heat-resistant oil casing as described in claim 1, characterized in that, Its Mo and W mass percentage content satisfies: 3Mo + 2W ≥ 1.5%.

4. The high-strength heat-resistant oil casing as described in claim 1, characterized in that, It also contains at least one of the following chemical elements: 0＜V≤0.05％； 0 < Ti ≤ 0.03%.

5. The high-strength heat-resistant oil casing as described in claim 1, characterized in that, In other unavoidable impurities: P ≤ 0.015%, S ≤ 0.005%.

6. The high-strength heat-resistant oil casing as described in claim 1, characterized in that, Its microstructure is tempered sorbite.

7. The high-strength heat-resistant oil casing as described in claim 1, characterized in that, Its grain size is 8.5 to 9.

5.

8. The high-strength heat-resistant oil casing as described in claim 1, characterized in that, Its performance satisfies at least one of the following: The yield strength at room temperature is 750-1070 MPa, the tensile strength at room temperature is ≥860 MPa, the transverse Charpy impact energy at 0℃ is ≥120 J, and the longitudinal Charpy impact energy at 0℃ is ≥150 J. The yield strength at at least 300℃ is ≥700MPa, and the decrease in yield strength at room temperature is <25%; The yield strength at at least 400°C is ≥650MPa, and the yield strength at room temperature decreases by <35%.

9. The method for manufacturing a high-strength heat-resistant oil casing as described in any one of claims 1-8, characterized in that, Including the following steps: Smelting and continuous casting; Continuous casting billet heating, piercing, and continuous rolling; Online controlled cooling: The rolled rough tubes are cooled by online water mist to 500-600℃; Reheating: The reheating temperature is 930-980℃, and the temperature is maintained for 20-40 minutes; sizing; Tempering heat treatment; Straightening.

10. The manufacturing method as described in claim 9, characterized in that, In the continuous casting billet heating step, the heating temperature is 1220~1270℃.

11. The manufacturing method as described in claim 9, characterized in that, In the continuous rolling process, the final rolling temperature is 950–1050℃.

12. The manufacturing method as described in claim 9, characterized in that, In the quenching and tempering heat treatment step, the quenching temperature is 900-930℃, and after holding at that temperature for 30-50 minutes, it is water-cooled to room temperature. The tempering temperature is 620-660℃, and after holding at that temperature for 60-80 minutes, it is air-cooled.

13. The manufacturing method as described in claim 9, characterized in that, During the straightening process, the straightening temperature is controlled to be ≥500℃.

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