High-strength and carbon dioxide corrosion-resistant tubing and casing and manufacturing method therefor

Abstract

Disclosed in the present invention is a high-strength and carbon dioxide corrosion-resistant tubing and casing, comprising Fe and inevitable impurities, and further comprising the following chemical elements: C: 0.10-0.20%, Si: 0.10-0.80%, Mn: 0.10-1.50%, Cr: 3-5%, Mo: 0.10-0.70%, Al: 0.01-0.10%, V: 0.10-0.20%, Ni: 0.1-1.0%, Ce: 0.002-0.01%, and La: 0.002-0.01%. Correspondingly, further disclosed in the present invention is a manufacturing method for the high-strength and carbon dioxide corrosion-resistant tubing and casing. In the present invention, by means of reasonable component design of chemical elements, the tubing and casing having both ultrahigh strength and excellent carbon dioxide corrosion resistance can be obtained.

Description

A high-strength carbon dioxide corrosion-resistant oil casing and its manufacturing method Technical Field

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

[0002] In the development of oil and gas, corrosion problems often lead to huge economic losses or even catastrophic consequences. Many oil fields currently have serious CO2 corrosion problems.

[0003] Although a lot of research has been conducted at home and abroad to develop 13Cr steel and high corrosion-resistant Ni-based alloys that are resistant to CO2 corrosion, the above materials contain a large amount of precious metal elements such as Cr, Ni, and Mo, which leads to a significant increase in the cost of steel pipes.

[0004] In the existing technology, most alloy steels used for CO2 corrosion resistance (or combined CO2, H2S, and Cl- corrosion resistance) are medium alloy or high alloy steels such as 13Cr. Low alloy 3Cr series corrosion-resistant oil casing has low strength, not reaching 110ksi steel grade or above.

[0005] For example, the Chinese patent document with publication number CN101899621A, publication date December 1, 2010, and titled “3Cr seamless steel pipe and manufacturing method thereof” has a chemical composition of C: 0.1-0.2%, Si: 0.1-0.5%, Mn: 0.3-0.8%, Cr: 2-4%, Mo: 0.3-0.6%, Ti: 0.0024-0.024%, Al: 0.01-0.04%, P≤0.03%, S≤0.03%, and the remainder is Fe and unavoidable impurities. The yield strength is ≥552 MPa, or 80 ksi, achieved by a quenching and tempering heat treatment process.

[0006] For example, the Chinese patent document with publication number CN103602903A, publication date February 26, 2014, and titled "High-strength carbon dioxide corrosion-resistant oil well pipe and manufacturing method thereof" has a chemical composition of C: 0.30-0.42%, Si: 0.10-1.0%, Mn: 0.80-1.5%, Cr: 0.5-1.5%, V: 0.21-0.4%, Al: 0.01-0.10%, N: 0.031-0.05%, P≤0.015%, S≤0.005%, and the remainder is Fe and unavoidable impurities. The V / N ratio is controlled at 6-11, and the normalizing process is used, and the mechanical properties reach above 95ksi steel grade.

[0007] Summary of the Invention

[0008] One of the objectives of the present invention is to provide a high-strength oil casing and tubing that is resistant to carbon dioxide corrosion. By rationally designing the chemical element composition, an oil casing and tubing that has both ultra-high strength and excellent carbon dioxide corrosion resistance can be obtained.

[0009] To achieve the above object, the present invention provides a high-strength oil casing and tubing resistant to carbon dioxide corrosion, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0010] C: 0.10~0.20%, Si: 0.10~0.80%, Mn: 0.10~1.50%, Cr: 3~5%, Mo: 0.10~0.70%, Al: 0.01~0.10%, V: 0.10~0.20%, Ni: 0.1~1.0%, Ce: 0.002~0.01%, La: 0.002~0.01%.

[0011] Accordingly, the present invention also provides a high-strength oil casing and tubing resistant to carbon dioxide corrosion, wherein the mass percentage of each element is as follows:

[0012] C: 0.10-0.20%, Si: 0.10-0.80%, Mn: 0.10-1.50%, Cr: 3-5%, Mo: 0.10-0.70%, Al: 0.01-0.10%, V: 0.10-0.20%, Ni: 0.1-1.0%, Ce: 0.002-0.01%, La: 0.002-0.01%; the balance is Fe and other inevitable impurities.

[0013] Generally speaking, in order to improve the material's resistance to CO2 corrosion, a certain amount of Cr is usually added to form a dense corrosion film to protect the base material. However, Cr and C elements are easy to combine to form carbides C. 23C6 and C7C3 cannot form a Cr(OH)3 corrosion film with anion selectivity. Therefore, the present invention adopts a low-carbon component design to reduce the carbide content of Cr to promote the formation of Cr enrichment in the corrosion film, improve the stability of the corrosion film, and thus improve the corrosion resistance. At the same time, Ce and La elements are added. On the one hand, they have a strong affinity with elements such as O and S, which can be deeply deoxidized to form rare earth compounds that are convenient for smelting and impurity removal, thereby reducing the content of coarse inclusions and preventing the formation of pitting pits at the location of large inclusions. On the other hand, rare earths reduce the crystallization supercooling, resulting in microstructure refinement during the solidification process. Rare earth modification forms uniformly dispersed fine rare earth inclusion particles, which can serve as the second phase nucleation core and can also produce a grain boundary pinning effect, which is conducive to grain refinement and promotes the improvement of comprehensive performance. In addition, the multi-element microalloying component design of V, Ni, and Nb enables the present invention to achieve a high yield strength of 140ksi, while improving the corrosion resistance and the anti-collapse performance of the casing, thereby significantly improving the service life of the casing.

[0014] Specifically, in the high-strength carbon dioxide corrosion-resistant oil casing of the present invention, the design principles of the various chemical elements are as follows:

[0015] C: In the high-strength carbon dioxide corrosion-resistant oil casing described in the present invention, C is the main solid solution strengthening element and can improve the strength of steel. However, too high a content will worsen segregation, reduce toughness, and form carbides, which will impair corrosion resistance. Therefore, the present invention controls the C content to 0.10-0.20%.

[0016] Si: In the high-strength carbon dioxide corrosion-resistant oil casing described in the present invention, Si is a deoxidizer and has a strong solid solution strengthening effect on ferrite, improving hardenability, tempering resistance, and yield strength. However, too high a content will deteriorate the toughness and processing performance of the steel. Therefore, the present invention controls the Si content to 0.10-0.80%.

[0017] Mn: In the high-strength carbon dioxide corrosion-resistant oil casing described in the present invention, Mn is one of the important alloying elements. It strongly improves the hardenability of steel and promotes the formation of martensite to ensure strength. However, too high a content has the unfavorable tendency of promoting grain coarsening and structural segregation. Therefore, the present invention controls the Mn content to 0.10-1.50%.

[0018] Cr: In the high-strength carbon dioxide corrosion-resistant oil casing of the present invention, Cr is a main alloying element that improves corrosion resistance and can increase the hardenability of steel. Therefore, the Cr content is controlled to be 3-5% in the present invention.

[0019] Mo: In the high-strength carbon dioxide corrosion-resistant oil casing described in the present invention, Mo has a strong ability to inhibit the transformation of austenite to pearlite, can suppress temper brittleness, and form dispersed special carbides at higher tempering temperatures to improve strength. Based on this, the present invention controls the Mo content to 0.10-0.70%.

[0020] Al: In the high-strength carbon dioxide corrosion-resistant oil casing of the present invention, Al is mainly used for deoxidation and grain refinement, and has a great solid solution strengthening effect. Based on this, the present invention controls the Al content to be 0.01-0.10%.

[0021] V: In the high-strength, carbon dioxide corrosion-resistant oil casing described in the present invention, V is a strong carbide and nitride-forming element, which improves the tempering resistance of steel and has a strong secondary hardening effect. It is dissolved in ferrite and has a very strong solid solution strengthening effect. However, if the V content is too high, coarse V (C, N) will easily form, which will impair toughness. Therefore, the V content is controlled to 0.10-0.20% in the present invention.

[0022] Ni: Ni has a certain solid solution strengthening effect and can improve corrosion resistance. In the high-strength carbon dioxide corrosion-resistant oil casing described in the present invention, Ni is used in combination with Cr and Mo to improve the thermal strength and corrosion resistance of steel. Based on this, the present invention controls the Ni content to 0.1-1.0%.

[0023] Ce and La: In the high-strength carbon dioxide corrosion-resistant oil casing described in the present invention, trace amounts of Ce and La are added to refine the grains and improve toughness. However, excessive Ce and La content can easily form coarse oxides or sulfur oxides. Therefore, the present invention controls the Ce and La content to 0.002-0.01%.

[0024] Furthermore, in the high-strength carbon dioxide corrosion-resistant oil casing of the present invention, the mass percentage of each chemical element may further satisfy at least one of the following conditions:

[0025] Si: 0.1-0.4%;

[0026] Mn: 0.2~1.0%;

[0027] Mo: 0.4~0.6%;

[0028] Al: 0.01~0.04%;

[0029] Ni: 0.1~0.5%;

[0030] Ce: 0.002~0.008%;

[0031] La: 0.002~0.005%.

[0032] Furthermore, the high-strength carbon dioxide corrosion-resistant oil casing of the present invention also contains 0<Nb≤0.15wt%.

[0033] Nb has a strong solid solution strengthening effect and can refine grains. Trace addition can improve the strength of steel without affecting plasticity and toughness. Based on this, in some embodiments of the present invention, its content is controlled to 0<Nb≤0.15wt%, for example, 0.01wt%≤Nb≤0.15wt%.

[0034] Furthermore, among the inevitable impurities in the high-strength carbon dioxide corrosion-resistant oil casing of the present invention: P≤0.015%, S≤0.008%, and N≤0.008%.

[0035] Furthermore, P is controlled to be ≤ 0.012%, S ≤ 0.005%, and N ≤ 0.006%.

[0036] In the above-mentioned technical solution of the present invention, phosphorus (P), sulfur (S), and nitrogen (N) are all harmful impurity elements. P and sulfur (S) segregate severely in steel, adversely affecting both plasticity and toughness. Nitrogen (N) has no significant strengthening effect, forming nitrides that undermine the strengthening effects of microalloying elements. It also easily causes age hardening in low-carbon steel, reducing plasticity and toughness. Therefore, to obtain steel with improved performance and higher quality, the contents of P, S, and N should be reduced as much as possible, technically feasible.

[0037] Furthermore, the microstructure matrix of the high-strength carbon dioxide corrosion-resistant oil casing of the present invention is tempered troostite.

[0038] Furthermore, the grain size of the high-strength carbon dioxide corrosion-resistant oil casing of the present invention is above level 10. In some embodiments, the grain size of the high-strength carbon dioxide corrosion-resistant oil casing of the present invention is level 10-11.

[0039] Furthermore, in the high-strength, carbon dioxide corrosion-resistant oil casing of the present invention, the percentage of large inclusions with an average diameter greater than 5 μm is less than 6% of the total inclusions. In some embodiments, the percentage of large inclusions with an average diameter greater than 5 μm is less than 3% and less than 6% of the total inclusions.

[0040] Furthermore, the Cr content in the corrosion product film of the high-strength CO2 corrosion-resistant oil casing of the present invention is greater than 13 wt%. In some embodiments, the Cr content in the corrosion product film is less than 13 wt% and less than or equal to 21 wt%. The corrosion product film is formed under corrosive environmental conditions of 60°C, 1 MPa CO2 partial pressure, and 20 g / L NaCl concentration.

[0041] Furthermore, the performance of the high-strength carbon dioxide corrosion-resistant oil casing of the present invention meets the following requirements: yield strength of 965-1173 MPa, tensile strength ≥1034 MPa, and 0°C transverse Charpy impact energy ≥80 J.

[0042] In some embodiments, the tensile strength of the high-strength carbon dioxide corrosion-resistant oil casing of the present invention is 1034-1300 MPa.

[0043] In some embodiments, the high-strength carbon dioxide corrosion-resistant oil casing of the present invention has a 0°C transverse Charpy impact energy of 80 to 200 J.

[0044] In some embodiments, the high-strength carbon dioxide corrosion-resistant oil casing of the present invention has an elongation after fracture of 18-25%.

[0045] Furthermore, the high-strength carbon dioxide corrosion-resistant oil casing of the present invention has a uniform corrosion rate of ≤2.5 mm / a, for example, 0.5 to 2.5 mm / a, under the corrosive environmental conditions of a temperature of 60° C., a CO 2 partial pressure of 1 MPa, and a NaCl concentration of 20 g / L.

[0046] Accordingly, another object of the present invention is to provide a method for manufacturing the above-mentioned high-strength carbon dioxide corrosion-resistant oil casing, which has a simple process and low production cost.

[0047] In order to achieve the above object, the present invention proposes a method for manufacturing the above-mentioned high-strength carbon dioxide corrosion-resistant oil casing, which comprises the following steps:

[0048] (1) smelting and continuous casting to obtain tube blanks;

[0049] (2) Heating, perforating, rolling, and sizing;

[0050] (3) Quenching: The quenching temperature is 880-920℃, and the temperature is kept for 30-60 minutes before water quenching;

[0051] (4) Tempering: The tempering temperature is 550-650℃, and the holding time is 45-75min.

[0052] Furthermore, in step (1) of the manufacturing method of the present invention, the superheat of the molten steel is controlled to ≤30°C, for example, 15-30°C, and the continuous casting speed is controlled to be below 2.0m / min, for example, 1.8-2.0m / min.

[0053] Furthermore, in step (2) of the manufacturing method of the present invention, the mixture is heated to a soaking temperature of 1200-1250° C. and kept warm for 100-300 minutes.

[0054] Furthermore, in step (2) of the manufacturing method of the present invention, the perforation temperature is controlled to be 1150-1250°C.

[0055] Furthermore, in step (2) of the manufacturing method of the present invention, the finishing temperature is controlled to be 900-1000°C.

[0056] Furthermore, in step (2) of the manufacturing method of the present invention, the sizing temperature is controlled to be 850-950°C.

[0057] Furthermore, in step (4) of the manufacturing method of the present invention, air cooling is performed after the heat preservation is completed.

[0058] The high-strength carbon dioxide corrosion-resistant oil casing and its manufacturing method of the present invention have the following advantages and beneficial effects:

[0059] The high-strength carbon dioxide corrosion-resistant oil casing of the present invention adopts a low-carbon component design, adds appropriate Cr elements, and is designed with Ni, Mo, V, Ce, and La elements, so that the produced oil casing has high strength and excellent corrosion resistance.

[0060] In some embodiments, the high-strength carbon dioxide corrosion-resistant oil casing of the present invention has a yield strength of 965-1173 MPa, reaching 140 ksi, a tensile strength ≥1034 MPa, and a 0°C transverse Charpy impact energy ≥80 J, and has excellent mechanical properties.

[0061] In some embodiments, the high-strength carbon dioxide corrosion-resistant oil casing described in the present invention has a uniform corrosion rate of ≤2.5 mm / a under the corrosive environment conditions of a temperature of 60°C, a CO2 partial pressure of 1 MPa, and a NaCl concentration of 20 g / L, and thus has excellent carbon dioxide corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG1 is a microstructure photograph of the oil casing according to Example 1 of the present invention under an optical microscope.

[0063] FIG2 is a microstructure photograph of the oil casing of Example 1 of the present invention under a scanning electron microscope.

[0064] FIG3 is a microstructure photograph of the oil casing of Comparative Example 1 under a scanning electron microscope. DETAILED DESCRIPTION

[0065] The high-strength carbon dioxide corrosion-resistant oil casing and its manufacturing method of the present invention will be further explained and illustrated below in conjunction with specific embodiments and the accompanying drawings. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0066] Examples 1-18 and Comparative Examples 1-5

[0067] The high-strength carbon dioxide corrosion-resistant oil casings of Examples 1-18 were all manufactured by the following steps:

[0068] (1) Smelting: The smelting is carried out by primary refining, refining outside the furnace and vacuum degassing. The tapping temperature can be 1600-1700℃.

[0069] (2) Continuous casting: The round billet is made by continuous casting process, the superheat of molten steel is controlled to be ≤30℃, and the continuous casting speed is controlled below 2.0m / min.

[0070] (3) Heating and piercing: The round billet is heated to an aquifer temperature of 1200-1250°C in a ring heating furnace, kept warm for 100-300 minutes, and then pierced. The piercing temperature is 1150-1250°C.

[0071] (4) Rolling: Continuous rolling is carried out after piercing, and the final rolling temperature is controlled at 900-1000℃.

[0072] (5) Sizing: After a short heating in the heating furnace, sizing is performed and the sizing temperature is controlled at 850-950°C.

[0073] (6) Quenching: Heat to 880-920℃ and keep warm for 30-60min before water quenching.

[0074] (7) Tempering: The tempering temperature is 550-650℃, the holding time is 45-75min, and then air cooling.

[0075] It should be noted that the comparative oil casings of Comparative Examples 1-5 were manufactured with reference to the steps of Example 1-18, but the chemical element content and / or process parameters in the manufacturing process of Comparative Examples 1-5 were not consistent with the design of the present invention.

[0076] Table 1 lists the mass percentages of the chemical elements in the high-strength carbon dioxide corrosion-resistant oil casings of Examples 1-18 and the comparative oil casings of Comparative Examples 1-5.

[0077] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, and N)

[0078] Table 2 lists the specific process parameters of the high-strength carbon dioxide corrosion-resistant oil casing of Examples 1-18 and the comparative oil casing of Comparative Examples 1-5 in the above process steps.

[0079] Table 2.

[0080] Samples of the high-strength, CO2-corrosion-resistant oil casing and tubing from Examples 1-18 and the comparative tubing from Comparative Examples 1-5 were collected, and large-sized inclusion particles with an average diameter greater than 5 μm were counted, as well as grain size measurements. The test results are listed in Table 3. Inclusion statistics were analyzed using scanning electron microscopy (SEM) energy-dispersive spectroscopy (EDS), with reference to the standard T / CSTM 00346-2021, "Automatic Classification and Statistics of Inclusions in Steel," to count inclusion sizes and counts. Grain size statistics were determined using the intercept method, with reference to the standard GB / T 6394-2017, "Method for Determination of Average Grain Size of Metals," to calculate the grain size grade G.

[0081] Table 3.

[0082] As can be seen from Table 3, the grain sizes of all examples of the present invention are all above Grade 10, and the percentage of large inclusions with an average diameter greater than 5 μm accounts for less than 6% of the total inclusions. Comparative Examples 4 and 5, on the other hand, have grain sizes below Grade 10, and the proportion of large inclusions is also much higher than that of the examples of the present invention.

[0083] In addition, Figure 1 shows a microscopic photograph of the oil casing and tubing of Example 1 of the present invention, taken under an optical microscope. As can be seen from Figure 1, the microstructure of Example 1 at room temperature is tempered bainite. Optical microscope observation also reveals that the microstructures of the oil casing and tubing of Examples 2-18 are tempered bainite.

[0084] Figure 2 shows a scanning electron microscope microstructure photograph of the oil casing of Example 1 of the present invention. As can be seen from Figure 2, the microstructure of Example 1 is distributed with a small amount of fine and dispersed carbides. Scanning electron microscope observation also reveals that the microstructures of the oil casings of Examples 2-18 are also distributed with a small amount of fine and dispersed carbides.

[0085] Figure 3 shows a scanning electron microscope microstructure photograph of the oil casing of Comparative Example 1. As can be seen from Figure 3, the microstructure of Comparative Example 1 contains a large amount of coarse carbides, which is not conducive to corrosion resistance.

[0086] In addition, samples of the high-strength carbon dioxide corrosion-resistant oil casing of Examples 1-18 and the comparative pipes of Comparative Examples 1-5 were taken for mechanical property tests and corrosion resistance tests, and the test results are listed in Table 4. The relevant test procedures are as follows:

[0087] Referring to the standard GB / T 228.1-2021 "Tensile tests on metallic materials - Part 1: Room temperature tensile test method", room temperature tensile performance tests were carried out to measure the yield strength, tensile strength and elongation after fracture.

[0088] V-shaped impact specimens were processed and the 0°C impact performance test was carried out according to the standard GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials", and the 0°C impact energy was measured.

[0089] With reference to the standard JB / T 7901-1999 “Test Method for Laboratory Uniform Corrosion of Metallic Materials by Full Immersion Test”, corrosion specimens were sampled and processed. The test was conducted in a high-temperature and high-pressure autoclave under the following corrosive environment conditions: temperature of 60°C, CO2 partial pressure of 1 MPa, NaCl concentration of 20 g / L, rotation speed of 1 m / s, and test period of 168 h. The uniform corrosion rate was measured, and the cross-section of the corrosion specimen was analyzed by SEM+EDS to determine the Cr content of the corrosion product film.

[0090] Table 4 lists the test results of the high-strength carbon dioxide corrosion-resistant oil casings of Examples 1-18 and the comparative oil casings of Comparative Examples 1-5.

[0091] Table 4.

[0092] As can be seen from Table 4, the high-strength, CO2-corrosion-resistant oil casing and tubing of Examples 1-18 of the present invention all exhibited yield strengths exceeding 965 MPa, tensile strengths exceeding 1034 MPa, and transverse Charpy impact energies exceeding 80 J at 0°C. Furthermore, under the experimental corrosion environment, the Cr content in the corrosion product films of each of the examples of the present invention was greater than 13 wt%, and the uniform corrosion rate was less than 2.5 mm / a.

[0093] However, since the chemical element contents of comparative examples 1 to 5 do not meet the design requirements of the present invention, their corrosion resistance is inferior to that of the present invention, and their mechanical properties are also poor.

[0094] It can be seen that by adopting the technical solution described in the present invention, an oil casing with both ultra-high strength and carbon dioxide corrosion resistance can be obtained.

[0095] It should be noted that the combination of the various technical features in the present invention is not limited to the combination described in the claims or the combination described in the specific embodiments. All technical features described in the present invention may be freely combined or combined in any manner unless they conflict with each other.

[0096] 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 therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A high-strength carbon dioxide corrosion-resistant oil casing, containing Fe and inevitable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.10~0.20%, Si: 0.10~0.80%, Mn: 0.10~1.50%, Cr: 3~5%, Mo: 0.10~0.70%, Al: 0.01~0.10%, V: 0.10~0.20%, Ni: 0.1~1.0%, Ce: 0.002~0.01%, La: 0.002~0.01%.

2. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1, characterized in that: The mass percentage of each element of the oil casing is: C: 0.10~0.20%, Si: 0.10~0.80%, Mn: 0.10~1.50%, Cr: 3~5%, Mo: 0.10~0.70%, Al: 0.01~0.10%, V: 0.10~0.20%, Ni: 0.1~1.0%, Ce: 0.002~0.01%, La: 0.002~0.01%; the balance is Fe and other inevitable impurities.

3. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The mass percentage of each chemical element of the oil casing further satisfies at least one of the following items: Si: 0.1-0.4%; Mn: 0.2-1.0%; Mo: 0.4-0.6%; Al：0.01～0.04％； Ni: 0.1-0.5%; Ce: 0.002~0.008%; La: 0.002~0.005%.

4. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The oil casing also contains 0 <Nb≤0.15wt%。 5. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: Among the unavoidable impurities: P ≤ 0.015%, S ≤ 0.008%, N ≤ 0.008%.

6. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The microstructure matrix of the oil casing is tempered troostite.

7. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The grain size of the oil casing is above grade 10.

8. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The percentage of large inclusions with an average diameter greater than 5 μm is less than 6% of all inclusions.

9. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The Cr content in the corrosion product film of the oil casing is higher than 13wt%.

10. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The oil casing has the following properties: yield strength of 965-1173 MPa, tensile strength of ≥1034 MPa, and 0°C transverse Charpy impact energy of ≥80 J.

11. The high-strength carbon dioxide corrosion-resistant oil casing according to claim 1 or 2, characterized in that: The uniform corrosion rate of the oil casing under the corrosive environment conditions of temperature 60° C., CO2 partial pressure 1 MPa, and NaCl concentration 20 g / L is ≤2.5 mm / a.

12. The method for manufacturing a high-strength carbon dioxide corrosion-resistant oil casing according to any one of claims 1 to 11, characterized in that: It includes the steps of: (1) smelting and continuous casting to obtain tube blanks; (2) Heating, perforating, rolling and sizing; (3) Quenching: The quenching temperature is 880-920°C, and the temperature is kept for 30-60 minutes before water quenching; (4) Tempering: The tempering temperature is 550-650℃ and the holding time is 45-75min.

13. The manufacturing method according to claim 12, characterized in that: In step (1), during the casting process, the superheat of the molten steel is controlled to be ≤30°C, and the continuous casting speed is controlled to be below 2.0 m / min.

14. The manufacturing method according to claim 12, characterized in that: In step (2), the mixture is heated to an average temperature of 1200 to 1250° C. and kept at this temperature for 100 to 300 minutes.

15. The manufacturing method according to claim 12, characterized in that: The manufacturing method has one or more of the following characteristics: In step (2), the perforation temperature is controlled to be 1150-1250°C; In step (2), the final rolling temperature is controlled to be 900-1000°C; In step (2), the sizing temperature is controlled to be 850-950°C.

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