Oil casing and its preparation method and application

Abstract

The application discloses a petroleum casing pipe and a preparation method and application thereof, and relates to the technical field of petroleum casing pipes. The petroleum casing pipe comprises the following chemical components in percentage by mass: C 0.25-0.32%, Si 0.55-0.70%, Mn 1.25-1.40%, P 0-0.020%, S 0-0.010%, RE 0.0005-0.0020%, B <0.0010%, Al 0.015-0.035%, and the balance of Fe and inevitable impurities. The steel grade of the petroleum casing pipe is 60ksi and 65ksi, the yield strength is 414-621 MPa, the tensile strength is greater than or equal to 552 MPa, and the transverse Charpy impact toughness at 0 DEG C is greater than or equal to 40 J. When the petroleum casing pipe is used as a surface casing pipe instead of a J55 steel grade casing pipe, the wall thickness and weight can be reduced, and the cost of oil and gas development can be lowered.

Description

Technical Field

[0001] This invention relates to the field of oil casing technology, and in particular to an oil casing, its preparation method, and its application. Background Technology

[0002] In oil and gas development, surface casing is used extensively, especially J55 steel grade casing. However, the cost of oil and gas development remains high when using J55 steel grade casing as surface casing. Therefore, developing a high-strength oil casing that also offers good toughness and cost-effectiveness to further reduce oil and gas development costs would be of great significance.

[0003] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0004] Based on the shortcomings of the existing technology, the purpose of this invention is to provide an oil casing, its preparation method and application, in order to solve the problem that the cost of oil and gas development is still relatively high when using J55 steel grade casing as the surface casing.

[0005] The technical solution of the present invention is as follows:

[0006] In a first aspect, the present invention provides an oilfield casing, wherein the oilfield casing comprises, by weight percentage, the following chemical components:

[0007] C 0.25%–0.32%, Si 0.55%–0.70%, Mn 1.25%–1.40%, P 0%–0.020%, S 0%–0.010%, RE 0.0005%–0.0020%, B <0.0010%, Al 0.015%–0.035%, balance being Fe and unavoidable impurities.

[0008] Optionally, the oil casing comprises the following chemical components by weight percentage:

[0009] C 0.25%–0.32%, Si 0.55%–0.70%, Mn 1.25%–1.40%, P 0.015%–0.020%, S 0.005%–0.010%, RE 0.0005%–0.0020%, B 0%–0.0009%, Al 0.015%–0.035%, with the balance being Fe and unavoidable impurities.

[0010] Optionally, the oil casing comprises the following chemical components by weight percentage:

[0011] C 0.29%–0.32%, Si 0.55%–0.62%, Mn 1.25%–1.31%, P 0.018%–0.020%, S 0.008%–0.010%, RE 0.0005%–0.0010%, Al 0.015%–0.020%, with the balance being Fe and unavoidable impurities.

[0012] Optionally, the oil casing comprises the following chemical components by weight percentage:

[0013] C 0.25%–0.28%, Si 0.65%–0.70%, Mn 1.34%–1.40%, P 0.015%–0.017%, S 0.005%–0.007%, RE 0.0015%–0.0020%, B 0.0007%–0.0009%, Al 0.027%–0.035%, with the balance being Fe and unavoidable impurities.

[0014] A second aspect of the present invention provides a method for preparing an oil casing, comprising the following steps:

[0015] According to the present invention, after the chemical composition of the oil casing as described above is prepared, smelted, and continuously cast, a continuously cast billet is obtained;

[0016] After homogenization, hot piercing, and hot continuous rolling of the continuously cast billet, a tube blank is obtained;

[0017] The tube blank is hot-straightened and then threaded to obtain the oil casing.

[0018] Optionally, the temperature used for heat equalization is 1170–1220°C, and the heat equalization time is 90–120 min.

[0019] Optionally, the temperature used for the thermal perforation is 1140–1190°C.

[0020] Optionally, after hot continuous rolling, a cooling step is also included before obtaining the tube blank;

[0021] The hot rolling temperature is 850–1150°C, and the final rolling temperature is controlled at 850–900°C.

[0022] Optionally, the tube blank is further subjected to a water cooling step before thread processing after hot straightening;

[0023] The temperature for thermal straightening is 600–650°C.

[0024] A third aspect of the present invention provides the application of the oil casing of the present invention as described above and / or the oil casing prepared by the preparation method of the present invention as described above in replacing J55 steel grade casing as surface casing for oil and gas development.

[0025] Beneficial Effects: The oil casing provided by this invention has a steel grade of 60ksi and 65ksi, a yield strength of 414–621 MPa, a tensile strength greater than or equal to 552 MPa, and a transverse Charpy impact toughness at 0°C greater than or equal to 40 J. It is evident that the strength of the oil casing provided by this invention is higher than that of J55 grade oil casing (yield strength 379–552 MPa). Therefore, when the oil casing provided by this invention, with its higher strength and balanced toughness and economy, is used as a surface casing to replace J55 grade oil casing, the casing wall thickness can be reduced, the casing string weight can be decreased, and oil and gas development costs can be further reduced. Detailed Implementation

[0026] This invention provides an oilfield casing, its preparation method, and its application. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0027] 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 invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0028] This invention provides an oilfield casing, wherein the oilfield casing comprises the following chemical components by mass percentage:

[0029] C 0.25%–0.32%, Si 0.55%–0.70%, Mn 1.25%–1.40%, P 0%–0.020%, S 0%–0.010%, RE 0.0005%–0.0020%, B <0.0010%, Al 0.015%–0.035%, balance being Fe and unavoidable impurities.

[0030] RE stands for rare earth element, and in some specific embodiments, RE can be Ce (cerium).

[0031] In this embodiment of the invention, fewer types of chemical elements are used. Specifically, a low to medium C content is proposed, with the addition of Mn and Si, and trace amounts of rare earth (RE) elements. A small amount of B element may also be added. Al and Si fully deoxidized killed steel is used to achieve a casing that balances strength, toughness, and economy. The oil casing steel grades provided by this invention are 60ksi and 65ksi, with a yield strength of 414-621MPa, a tensile strength greater than or equal to 552MPa, and a transverse Charpy impact toughness at 0℃ greater than or equal to 40J. The strength of the oil casing provided by this invention is higher than that of J55 grade oil casing (55ksi grade, yield strength of 379-552MPa, tensile strength greater than or equal to 517MPa, and transverse Charpy impact toughness at 0℃ greater than or equal to 20J). Therefore, when the oil casing provided by this invention, which has higher strength and takes into account toughness and economy, replaces J55 grade oil casing as a surface casing in oil and gas development, the casing wall thickness can be reduced and the weight of the casing string can be reduced while ensuring the same pressure bearing capacity of the casing string, which can further reduce the cost of oil and gas development. In other words, this invention proposes to replace the J55 steel grade casing with lower strength and larger wall thickness with a solution that appropriately increases the casing strength level and reduces the casing wall thickness without significantly increasing the unit weight manufacturing cost, thereby reducing casing weight and oil and gas development costs.

[0032] Carbon (C) is the main strengthening element in steel. Using medium to low carbon content is beneficial for balancing the strength and toughness of steel. However, too low a carbon content is detrimental to improving the hardenability of steel, which is also detrimental to improving its strength; too high a carbon content is detrimental to improving the toughness of steel. Taking all factors into consideration, it is advisable to control the carbon content within the range of 0.25% to 0.32%.

[0033] Silicon (Si) is mainly used to improve the strength of steel. Considering all factors, its content should ideally be controlled within the range of 0.55% to 0.70%.

[0034] Manganese (Mn) is mainly used to improve the hardenability of steel, thereby increasing its strength. Too low a content will not have the desired effect, while too high a content will increase the tendency for segregation. Considering all factors, it is advisable to control the content within the range of 1.25% to 1.40%.

[0035] Boron (B): Adding trace amounts of boron can improve the hardenability of steel, thereby further increasing its strength. Considering all factors, it is advisable to control the boron content within the range of <0.0010% (i.e., the mass percentage of boron is less than 0.0010%).

[0036] Rare earth elements (RE) have multiple functions, including purifying molten steel, refining grains, modifying inclusions, and alloying. Their concentration should be controlled within the range of 0.0005% to 0.0020%.

[0037] Al (aluminum) forms fine, uniformly distributed oxides with oxygen, which can refine grains and improve strength and toughness. It is also an important deoxidizer. Considering all factors, it is advisable to control it within the range of 0.015% to 0.035%.

[0038] Phosphorus (P) is a harmful element that mainly affects the ductility and toughness of steel. It is advisable to control P to ≤0.020% (i.e., P mass content less than or equal to 0.020%).

[0039] Sulfur (S) is a harmful element that mainly affects the ductility and toughness of steel. It is advisable to control S to ≤0.010% (i.e., the mass content of S is less than or equal to 0.020%).

[0040] In some embodiments, the oil casing comprises the following chemical components by weight percentage:

[0041] C 0.25%–0.32%, Si 0.55%–0.70%, Mn 1.25%–1.40%, P 0.015%–0.020%, S 0.005%–0.010%, RE 0.0005%–0.0020%, B 0%–0.0009%, Al 0.015%–0.035%, with the balance being Fe and unavoidable impurities.

[0042] Currently, neither the API (American Petroleum Institute) standards nor relevant Chinese standards include D60 grade oil casing, and M65 grade oil casing, which offers a reasonable balance of strength, toughness, and economy, is rarely used. In this embodiment, the provided oil casing has yield strength levels equivalent to both D60 and M65 grade oil casing. The D60 grade oil casing has a yield strength of 414–586 MPa (corresponding to 60–85 ksi), a tensile strength greater than or equal to 552 MPa, and a transverse Charpy impact toughness at 0°C greater than or equal to 40 J. The M65 grade oil casing has a yield strength of 448–621 MPa (corresponding to 65–90 ksi), a tensile strength greater than or equal to 586 MPa, and a transverse Charpy impact toughness at 0°C greater than or equal to 60 J. Both its strength and toughness are superior to J55 grade oil casing. The oil casing provided by this invention is not only high in strength but also in toughness, and can replace the J55 steel grade surface casing currently in widespread use, thereby reducing the cost of oil and gas development.

[0043] In some embodiments, the oil casing comprises the following chemical components by weight percentage:

[0044] C 0.29%–0.32%, Si 0.55%–0.62%, Mn 1.25%–1.31%, P 0.018%–0.020%, S 0.008%–0.010%, RE 0.0005%–0.0010%, Al 0.015%–0.020%, with the balance being Fe and unavoidable impurities.

[0045] In this embodiment, the oil casing is specifically a D60 grade oil casing (60ksi grade), with a yield strength of 461-468MPa, a tensile strength of 608-617MPa, an elongation of 29%, a longitudinal Charpy V-notch impact toughness of 63-69J at 0℃, and a transverse Charpy V-notch impact toughness of 44-49J at 0℃.

[0046] In some embodiments, the oil casing comprises the following chemical components by weight percentage:

[0047] C 0.25%–0.28%, Si 0.65%–0.70%, Mn 1.34%–1.40%, P 0.015%–0.017%, S 0.005%–0.007%, RE 0.0015%–0.0020%, B 0.0007%–0.0009%, Al 0.027%–0.035%, with the balance being Fe and unavoidable impurities.

[0048] In this embodiment, the oil casing is specifically an M65 grade oil casing (grade 65ksi), with a yield strength of 498-503MPa, a tensile strength of 646-653MPa, an elongation of 28%, a longitudinal Charpy V-notch impact toughness of 83-88J at 0℃, and a transverse Charpy V-notch impact toughness of 64-68J at 0℃.

[0049] Under the same outer diameter and pressure-bearing capacity, the casing strength σ is inversely proportional to the wall thickness t, with the following relationship: σ1t1=σ2t2. Therefore, when D60 grade oil casing replaces J55 grade oil casing as surface casing, the wall thickness can be reduced by 8.3%; when M65 grade oil casing replaces J55 grade oil casing as surface casing, the wall thickness can be reduced by 15.4%. Reducing the casing wall thickness reduces the weight of the casing string, thereby significantly reducing oil and gas extraction costs.

[0050] This invention develops a manufacturing process to match the aforementioned chemical composition, primarily through steelmaking, continuous casting, hot rolling in the austenitic region, and hot straightening, to achieve a fine and uniform pearlite + ferrite microstructure in the material. This results in a reasonable balance between the strength and toughness of the casing, giving it excellent overall performance. Specifically, this invention provides a method for preparing an oilfield casing, comprising the following steps:

[0051] S1. According to the present invention, after the chemical composition of the oil casing as described above is prepared, smelted, and continuously cast, a continuously cast billet is obtained.

[0052] S2. After homogenization, hot piercing, and hot continuous rolling of the continuously cast billet, a tube blank is obtained;

[0053] S3. After hot straightening the tube blank, thread processing is performed to obtain the oil casing.

[0054] The chemical composition design and manufacturing process of this invention combine to improve strength and enhance ductility and toughness. The preparation method of this invention, targeting the above-mentioned chemical composition of the oil casing, achieves the desired microstructure and properties, fully utilizing the performance of the oil casing. Furthermore, the process parameters are easily controlled, resulting in stable oil casing with steel grades of 60ksi and 65ksi, a yield strength of 414–621 MPa, a tensile strength greater than or equal to 552 MPa, and a transverse Charpy impact toughness at 0°C greater than or equal to 40 J. Its strength is higher than that of J55 grade oil casing (yield strength 379–552 MPa), and it can replace the currently widely used J55 grade surface casing. In other words, by appropriately increasing strength, the casing wall thickness can be reduced, thereby reducing the weight of the casing string and lowering oil and gas development costs.

[0055] In step S2, in some embodiments, the temperature used for heat equalization is 1170–1220°C (e.g., 1170°C, 1180°C, 1190°C, 1200°C, 1210°C, or 1220°C, etc.); the heat equalization time is 90–120 min (e.g., 90 min, 100 min, 110 min, or 120 min, etc.).

[0056] In some embodiments, the temperature used for thermal perforation is 1140–1190°C (e.g., 1140°C, 1150°C, 1160°C, 1170°C, 1180°C, or 1190°C).

[0057] In some implementations, a cooling step (e.g., air cooling or wind cooling) is included after hot continuous rolling and before obtaining the tube blank.

[0058] In some embodiments, the hot rolling temperature is 850–1150°C, and the final rolling temperature is controlled at 850–900°C.

[0059] In step S3, in some embodiments, a water cooling step is included before thread processing after hot straightening of the tube blank.

[0060] In some embodiments, the temperature of the thermal straightening is 600-650°C (e.g., 600°C, 610°C, 620°C, 630°C, 640°C, or 650°C).

[0061] In step S3, API standard threads or special threads can be machined.

[0062] This invention also provides an application of the oil casing described above and / or the oil casing prepared by the method described above in this invention as a substitute for J55 grade steel casing in oil and gas development surface casing. This invention utilizes the high strength of the aforementioned oil casing to replace J55 grade steel casing as a surface casing, thereby reducing casing wall thickness, casing string weight, and oil and gas development costs.

[0063] The present invention will be further described below through specific embodiments.

[0064] Table 1. Chemical composition of oil casing in Examples 1-6

[0065]

[0066] In Examples 1-6 of Table 1, the content of each chemical component is by mass percentage, with the remainder being Fe and unavoidable impurities up to 100%.

[0067] Example 1

[0068] This embodiment provides a method for preparing D60 grade oil casing, including the following steps:

[0069] Steelmaking: According to the chemical composition of D60 grade oil casing in Example 1 shown in Table 1, the raw materials are batched and then sequentially processed through oxygen blowing converter steelmaking, Ce wire feeding, ladle refining and vacuum degassing to obtain molten steel.

[0070] Continuous casting: casting molten steel into rod-shaped billets.

[0071] Homogenization, piercing, and hot rolling: The bar-shaped continuous casting billet is homogenized in an annular heating furnace at a temperature of 1200℃ for 120 minutes; then hot piercing is performed at 1160℃, followed by hot rolling at 880℃~1150℃, with an initial rolling temperature of 1150℃ and a final rolling temperature of 880℃. After air cooling, the billet is sawn to a suitable length to obtain a tube billet (whose microstructure is: pearlite + ferrite microstructure).

[0072] Hot straightening: The tube blank is hot straightened at a temperature of 650℃ and then water-cooled.

[0073] Threading: Machining API standard threads or special threads on tube blanks.

[0074] Example 2

[0075] This embodiment provides a method for preparing D60 grade oil casing, including the following steps:

[0076] Steelmaking: According to the chemical composition of D60 grade oil casing in Example 2 shown in Table 1, the raw materials are batched and then sequentially processed through oxygen blowing converter steelmaking, Ce wire feeding, ladle refining and vacuum degassing to obtain molten steel.

[0077] Continuous casting: Same as Example 1.

[0078] Heat homogenization, piercing, and hot continuous rolling: Same as in Example 1.

[0079] Thermal straightening: Same as in Example 1.

[0080] Thread machining: Same as in Example 1.

[0081] Example 3

[0082] This embodiment provides a method for preparing D60 grade oil casing, including the following steps:

[0083] Steelmaking: According to the chemical composition of D60 grade oil casing in Example 3 shown in Table 1, the raw materials are batched and then successively passed through oxygen blowing converter steelmaking, Ce wire feeding, ladle refining and vacuum degassing to obtain molten steel.

[0084] Continuous casting: Same as Example 1.

[0085] Heat homogenization, piercing, and hot continuous rolling: Same as in Example 1.

[0086] Thermal straightening: Same as in Example 1.

[0087] Thread machining: Same as in Example 1.

[0088] Example 4

[0089] This embodiment provides a method for preparing M65 grade oil casing, including the following steps:

[0090] Steelmaking: According to the chemical composition of the M65 grade oil casing in Example 4 shown in Table 1, the raw materials are batched and then sequentially processed through oxygen blowing converter steelmaking, Ce wire feeding, ladle refining and vacuum degassing to obtain molten steel.

[0091] Continuous casting: casting molten steel into rod-shaped billets.

[0092] Homogenization, piercing, and hot rolling: The bar-shaped continuous casting billet is homogenized in an annular heating furnace at a temperature of 1200℃ for 120 minutes. Then, it is hot-pierced at 1160℃ and hot-rolled at 860℃ to 1150℃, with an initial rolling temperature of 1150℃ and a final rolling temperature of 860℃. After air cooling, it is sawn to a suitable length to obtain a tube billet (whose metallographic structure is: pearlite + ferrite microstructure).

[0093] Hot straightening: The tube blank is hot straightened at a temperature of 600℃ and then water-cooled.

[0094] Threading: Machining API standard threads or special threads on tube blanks.

[0095] Example 5

[0096] This embodiment provides a method for preparing M65 grade oil casing, including the following steps:

[0097] Steelmaking: According to the chemical composition of the M65 grade oil casing in Example 5 shown in Table 1, the raw materials are batched and then sequentially processed through oxygen blowing converter steelmaking, Ce wire feeding, ladle refining and vacuum degassing to obtain molten steel.

[0098] Continuous casting: Same as Example 4.

[0099] Heat homogenization, piercing, and hot continuous rolling: Same as in Example 4.

[0100] Thermal straightening: Same as in Example 4.

[0101] Thread machining: Same as in Example 4.

[0102] Example 6

[0103] This embodiment provides a method for preparing M65 grade oil casing, including the following steps:

[0104] Steelmaking: The raw materials are prepared according to the chemical composition of the M65 grade oil casing in Example 6 shown in Table 1, and then the steel is successively processed through oxygen blowing converter steelmaking, Ce wire feeding, ladle refining and vacuum degassing to obtain molten steel.

[0105] Continuous casting: Same as Example 4.

[0106] Heat homogenization, piercing, and hot continuous rolling: Same as in Example 4.

[0107] Thermal straightening: Same as in Example 4.

[0108] Thread machining: Same as in Example 4.

[0109] The oil casings prepared in Examples 1-6 were subjected to yield strength test, tensile strength test, elongation test and Charpy V-notch impact toughness test at 0℃, and the results are shown in Table 2.

[0110] Table 2. Performance of the oil casings prepared in Examples 1-6

[0111]

[0112] As can be seen, the D60 and M65 grade oil casings provided in the embodiments have excellent comprehensive performance. Specifically, the D60 grade oil casing has a room temperature yield strength of 461-468 MPa, a tensile strength of 608-617 MPa, an elongation of 29%, and a transverse Charpy impact toughness of 44-49 J at 0°C; the M65 grade oil casing has a room temperature yield strength of 498-503 MPa, a tensile strength of 646-653 MPa, an elongation of 28%, and a transverse Charpy impact toughness of 64-68 J at 0°C.

[0113] If D60 grade steel casing is used to replace J55 grade steel casing (244.48mm×8.94mm outer diameter, 8.94mm wall thickness) as the surface casing for oil and gas development, the wall thickness can be reduced to 8.20mm without changing the strength (or pressure bearing capacity), which is about 8.3% less. The weight of steel is reduced by about 8.0%, thus significantly reducing the cost of oil and gas development.

[0114] If M65 steel grade casing (outer diameter 244.48mm, wall thickness 7.92mm) with specifications of 244.48mm×7.92mm is used instead of J55 steel grade casing (outer diameter 244.48mm, wall thickness 8.94mm) with specifications of 244.48mm×7.92mm as the surface casing for oil and gas development, the strength increases by about 4.7%, the wall thickness decreases by about 11.4%, and the steel weight decreases by about 11.0%, thereby significantly reducing the cost of oil and gas development.

[0115] Therefore, it can be seen that using D60 and M65 grade oil casing in this invention to replace J55 grade oil casing of the same outer diameter results in smaller wall thickness and lower weight, which is significantly more economical.

[0116] In summary, this invention provides an oilfield casing, its manufacturing method, and its application. The oilfield casing is made of 60ksi or 65ksi steel, with a yield strength of 414–621 MPa, a tensile strength greater than or equal to 552 MPa, and a transverse Charpy impact toughness at 0°C greater than or equal to 40 J. The oilfield casing provided by this invention has higher strength than J55 grade oilfield casing (yield strength of 379–552 MPa). Therefore, when the high-strength, high-toughness oilfield casing provided by this invention is used as a surface casing to replace J55 grade casing, the wall thickness can be reduced, and the weight can be decreased, thus further reducing oil and gas development costs.

[0117] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. An oilfield casing, characterized in that, The oil casing comprises the following chemical components by weight percentage: C 0.29%~0.32%, Si 0.55%~0.62%, Mn 1.25%~1.31%, P 0.018%~0.020%, S 0.008%~0.010%, RE 0.0005%~0.0010%, Al 0.015%~0.020%, balance being Fe and unavoidable impurities; The oil casing is a D60 grade oil casing with a yield strength of 461-468 MPa, a tensile strength of 608-617 MPa, an elongation of 29%, a longitudinal Charpy V-notch impact toughness of 63-69 J at 0℃, and a transverse Charpy V-notch impact toughness of 44-49 J at 0℃. The method for preparing the oil casing includes the following steps: According to the chemical composition of the oil casing, after batching, smelting, and continuous casting, a continuously cast billet is obtained. After homogenization, hot piercing, and hot continuous rolling of the continuously cast billet, a tube blank is obtained; The tube blank is hot-straightened and then threaded to obtain the oil casing. The temperature used for heat equalization is 1170–1220℃, and the heat equalization time is 90–120 min. The temperature used for the thermal perforation is 1140–1190°C; After hot continuous rolling, the process before obtaining the tube blank also includes an air cooling step; The hot rolling temperature is 850–1150°C, and the final rolling temperature is controlled to be 850–900°C. The process of hot straightening the tube blank and then threading it includes a water cooling step. The temperature for thermal straightening is 600–650°C.

2. An oilfield casing, characterized in that, The oil casing comprises the following chemical components by weight percentage: C 0.25%~0.28%, Si 0.65%~0.70%, Mn 1.34%~1.40%, P 0.015%~0.017%, S 0.005%~0.007%, RE 0.0015%~0.0020%, B 0.0007%~0.0009%, Al 0.027%~0.035%, balance Fe and unavoidable impurities; The oil casing is an M65 grade oil casing with a yield strength of 498–503 MPa, a tensile strength of 646–653 MPa, an elongation of 28%, a longitudinal Charpy V-notch impact toughness of 83–88 J at 0°C, and a transverse Charpy V-notch impact toughness of 64–68 J at 0°C. The preparation method of the oil casing includes the following steps: After batching, smelting, and continuous casting according to the chemical composition of the oil casing, a continuously cast billet is obtained. After homogenization, hot piercing, and hot continuous rolling of the continuously cast billet, a tube blank is obtained; The tube blank is hot-straightened and then threaded to obtain the oil casing. The temperature used for heat equalization is 1170–1220℃, and the heat equalization time is 90–120 min. The temperature used for the thermal perforation is 1140–1190°C; After hot continuous rolling, the process before obtaining the tube blank also includes an air cooling step; The hot rolling temperature is 850–1150°C, and the final rolling temperature is controlled to be 850–900°C. The process of hot straightening the tube blank and then threading it includes a water cooling step. The temperature for thermal straightening is 600–650°C.

3. A method for preparing an oilfield casing, characterized in that, Includes the following steps: The oil casing according to any one of claims 1-2 is subjected to batching, smelting, and continuous casting to obtain a continuously cast billet; After homogenization, hot piercing, and hot continuous rolling of the continuously cast billet, a tube blank is obtained; The tube blank is hot-straightened and then threaded to obtain the oil casing. The temperature used for heat equalization is 1170–1220℃, and the heat equalization time is 90–120 min. The temperature used for the thermal perforation is 1140–1190°C; After hot continuous rolling, before obtaining the tube blank, a cooling step is also included, wherein the cooling is air cooling or wind cooling; The hot rolling temperature is 850–1150°C, and the final rolling temperature is controlled to be 850–900°C. The process of hot straightening the tube blank and then threading it includes a water cooling step. The temperature for thermal straightening is 600–650°C.

4. The application of an oil casing according to any one of claims 1-2 and / or an oil casing prepared by the preparation method according to claim 3 in replacing J55 steel grade casing as a surface casing for oil and gas development.

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