High-strength Anti-non-uniform-load oil casing and manufacturing method therefor

Abstract

The present disclosure discloses an oil casing. In addition to Fe and inevitable impurities, the oil casing also comprises the following chemical elements in percentage by mass: C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%. The oil casing has a microstructure of ferrite+granular bainite+ austenite. The present disclosure also discloses a method for manufacturing the above-mentioned oil casing.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a pipe and a manufacturing method therefor, particularly to an oil casing and a manufacturing method therefor.BACKGROUND

[0002] Shale gas is an exploitable clean natural gas energy that is stored in shale formations and mud formations. Compared to conventional natural gas exploitation, shale gas exploitation has challenges such as complex formations and complex fracturing and perforation processes. Currently, the special exploitation process of horizontal wells + multi-stage sectional fracturing is adopted for shale gas exploitation, which invloves that the casing perforation is carried out in 15 to 30 stages, with the horizontal section up to 3000 meters in length, then the large-volume hydraulic fracturing is performed, resulting in high formation pressure (90-120MPa) and thus shale fracture development and formation creep. Therefore, the high-strength casing used is extremely prone to deformation, with an average casing deformation rate of 30% or more. A typical characteristic of such casing deformation is that the casing does not collapse, but deforms in the radial direction under the influence of non-uniform loads of the formations. As multi-stage fracturing progresses, the degree of casing deformation gradually increases, and the bridge plugs used for fracturing operations cannot pass through the deformed positions, causing difficulties in hydraulic fracturing and seriously affecting the recovery efficiency of shale gas.

[0003] Chinese patent document with publication number CN101532113A, published on September 16, 2009, and entitled "anti-collapse oil casing and manufacturing method therefor" discloses a technical solution for obtaining anti-collapse casing with ultra-high anti-collapse abilityand excellent performance through the design and control of texture and microstructure.

[0004] Chinese patent document with publication number CN102296239A, published on December 28, 2011, and entitled "high-strength anti-collapse oil casing string and manufacturing method therefor" adopts a casing string with an outer diameter of <p141.3mm to <p142.7mm and a wall thickness of 10.7mm-12.9mm, but it does not mention the performance of anti-non-uniform external load.

[0005] Chinese patent document with publication number CN104651723A, published on May 27, 2015, and entitled "rare earth-containing oil casing with resistance to hydrogen sulfide corrosion and high collapse resistanceand production method therefor" involves an oil casing with a yield strength of 780-870 MPa and a transverse impact value at 0°Cof ≥70 J / cm 2< , which features high resistance to hydrogen sulfide corrosion and high anti-collapse performance, etc.

[0006] Although the oil casings mentioned in the above-mentioned patent documents all have high anti-collapse performance, they are limited to the research of the anti-collapse performance of casings under uniform loads, and do not address the technical problem of poor anti-deformation ability of oil casing under non-uniform loads.SUMMARY

[0007] One of the objectives of the present disclosure is to provide a high-strength oil casing, which has high work hardening index and toughness, as well as low yield-to-tensile ratio, and exhibits excellent anti-deformation ability under non-uniform loads. Since the material strength is significantly enhanced through work hardening, thereby improving the anti-deformation ability of the casing, it ensures that the bridge plug can pass through to smoothly carry out fracturing operations.

[0008] To achieve the above-mentioned objective, the present disclosure provides an oil casing comprising, in additon to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%; the oil casing has a microstructure of ferrite+granular bainite+ austenite.

[0009] Specifically, in the oil casing according to the present disclosure, the austenite is retained austenite.

[0010] Preferably, the present disclosure provides an oil casing comprising the following chemical elements in percentage by mass: C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%; and the balance being Fe and inevitable impurities.

[0011] Due to rational composition design, preferably in combination with heat treatment processes, the oil casing according to the present disclosure has a mixed microstructure of ferrite, granular bainite and austenite, increased work hardening index n, significantly enhanced anti-deformation ability under non-uniform loads, and good strength and toughness.

[0012] Specifically, the design principles of chemical elements in the oil casing according to the present disclosure are as follows: C: In the oil casing according to the present disclosure, the C element is a precipitate-forming element, which can enhance the strength of the steel. It should be noted that when the C element content is less than 0.05%, the hardenability of the steel will be reduced, and at the same time, the strength of the steel cannot be ensured. When the C element content is higher than 0.15%, the formation of ferrite structure will be inhibited, significantly reducing the toughness of the steel. In addition, C element can form a large amount of coarse precipitates with Cr and Mo elements, and significantly aggravate the segregation of the steel, result in a significant decrease of toughness. Therefore, in the oil casing according to the present disclosure, the mass percentage of C element is limited to between 0.05% and 0.15%.

[0013] Si: In the oil casing according to the present disclosure, the Si element promotes the diffusion of carbon into austenite and inhibits the precipitation of cementite in austenite, which not only has a solid solution strengthening effect but also promotes the enrichment of carbon in austenite and simultaneously improves the stability of austenite. However, it should be noted that the addition amount of Si element in the steel should not be excessively high, as an overly high Si element content will deteriorate the workability and toughness of the steel. At the same time, the addition amount of Si should not be excessively low. When the mass percentage of Si element is less than 0.4%, the effect ofimproving the stability of austenite is not significant. Therefore, in the oil casing according to the present disclosure, the mass percentage of Si element is controlled to be0.4%-0.8%.

[0014] Mn: In the oil casing according to the present disclosure, the Mn element is anaustenite-forming element, and it not only stabilize the austenite in the steel, but also slows down the transformation rate of ferrite, and reduces the diffusion of carbon from ferrite into austenite. At the same time, Mn increases the amount of austenite, which is beneficial for enhancing the work-hardening effect of the material. In the present disclosure, in order to obtain the desired microstructure, the content of Mn element in the casing needs to be controlled to be 3% or more. It should be noted that when the Mn element content is greater than 5%, structural segregation in the steel will be significantly increased, significantly affecting the uniformity of the hot-rolled microstructure and the impact properties. Therefore, in the high-strength anti-non-uniform-load oil casing according to the present disclosure, the mass percentage of Mn element is limited to be 3-5%.

[0015] Ni: In the oil casing according to the present disclosure, the Ni element can promote the formation of austenite structure to ensure the presence of austenite in the rolled steel, facilitating to increase the work hardening index of the material. It should be noted that when the Ni content is higher than 0.5%, the cost is high and the effect of increasing the austenite proportion is not significant; meanwhile, when the Ni content is lower than 0.2%, the austenite proportion cannot be effectively increased. Therefore, in the oil casing according to the present disclosure, the mass percentage of Ni element is controlled to be 0.2%-0.5%.

[0016] Ca: Ca can purify molten steel, promote the spheroidization of MnS, and enhance impact toughness. However, when the Ca content is excessively high, coarse non-metallic inclusions are prone to form. Based on this, in the oil casing according to the present disclosure, the mass percentage of Ca element is controlled to be 0.0005-0.005%.

[0017] Ti: In the oil casing according to the present disclosure, the Ti element is a strong carbonitride-forming element, which can significantly refine austenite grains and compensate for the decrease in strength caused by the reduction of carbon content. It should be noted that when the Ti element content is higher than 0.04%, coarse TiN is prone to form, significantly reducing the toughness of the material. Therefore, in the oil casing according to the present disclosure, the mass percentage of Ti element is controlled to be 0.01-0.04%.

[0018] Al: In the oil casing according to the present disclosure, the Al element serves as a deoxidizing and nitrogen-fixing element, which is capable of refining grain size. It is important to note that the content of Al element should not be excessively high, as an excessively high content of Al will significantly reduce the yield-to-tensile ratio of the material. Therefore, in the oil casing according to the present disclosure, the mass percentage of Al element is limited to between 0.01% and 0.05%.

[0019] Nb: In the oil casing according to the present disclosure, the Nb element is a grain rrefining and precipitation strengthening element, which can compensate for the decrease in strength caused by reduction of carbon content. When the Nb element content is higher than 0.04%, coarse Nb(CN) is prone to form, thereby reducing the toughness. Meanwhile, when the content of Nb element is lower than 0.02%, the strengthening effect is not obvious. Therefore, in the oil casing according to the present disclosure, the mass percentage of Nb element is limited to be 0.02%-0.04%.

[0020] Preferably, the oil casing according to the present disclosure further satisfies at least one of the following: 0< Cr ≤1%; 0 < Mo≤0.5%; 0< V≤0.15%.

[0021] In preferred embodiments of the present disclosure, at least one of Cr, Mo and V may be further added into the oil casing. Wherein: Cr: Cr is an element that strongly enhances hardenability, and is also a strong precipitate-forming element, thus precipitates can be precipitated during the cooling phase transformation process, improving the strength of the steel. It should be noted that when the content of Cr element is higher than 1%, coarse M 23 C 6 precipitates are prone to be precipitated at grain boundaries, reducing toughness. Therefore, in the oil casing according to the present disclosure, the mass percentage of Cr element is preferably limited to be 0<Cr<1%.

[0022] Mo: Mo element enhances the strength of the steel and the stability during the cooling phase transformation process of the steel primarily by precipitation and solid solution strengthening. The oil casing according to the present disclosure has a low carbon content; when the Mo element content is higher than 0.5%, its effect on improving steel strength is not significant, thereby resulting in alloy waste and increased manufacturing costs. Therefore, in the oil casing according to the present disclosure, the mass percentage of Mo element is preferably limited to 0 < Mo ≤ 0.5%.

[0023] V: V element is a typical precipitation strengthening element, which can compensate for the decrease in strength caused by reduction of carbon content. However, when the content of V element is higher than 0.15% in the steel, coarse V(CN) particles tend to form, thereby reducing the toughness. Therefore, in the oil casing according to the present disclosure, the mass percentage of V element is preferably limited to 0 < V < 0.15%.

[0024] Preferably, in the high-strength anti-non-uniform-load oil casing according to the present disclosure, the inevitable impurities comprise P and S, wherein P ≤ 0.015%; S ≤ 0.005%.

[0025] In the oil casing according to the present disclosure, the main impurity elements are P and S, and their contents are expected to be as low as possible where the technical conditions permit. In some embodiments, the contents of P and S are preferably controlled to: P≤0.015%, S≤0.005%.

[0026] Preferably, in the oil casing according to the present disclosure, the granular bainite has a volume fraction of 6-10%.

[0027] Preferably, in the oil casing according to the present disclosure, the austenite has a volume fraction of 8-20%.

[0028] Preferably, the oil casing according to the present disclosure has at least one of the following properties: a yield strength of 758-965 MPa; a tensile strength of ≥ 930 MPa; a yield-to-tensile ratio of ≤ 0.85; an elongation of ≥ 35%; an uniform elongation of ≥ 20%; a transverse Charpy impact energy at 0°C of ≥ 60J; a work hardening index n of ≥ 0.15; and an increase of 30% or more in yield strength after 5% deformation of material compared to that before deformation.

[0029] Another objective of the present disclosure is to provide a method for manufacturing the oil casing. The oil casing manufactured by this method not only has high strength, toughness, uniform elongation and work hardening index, but also has low process cost, thereby solving the problem of poor anti-deformation ability of oil casings under non-uniform loads in the prior art, enhancing the pressure resistance of the oil casing in underground service, and meeting the demand for high-strength oil casing in the field of oilfield exploitation.

[0030] To achieve the above-mentioned objective, the present disclosure provides a method for manufacturing the above-mentioned oil casing, which comprises the following steps performed in sequence: (1) smelting and continuous casting to obtain a round billet; (2) perforating; (3) rolling and stretch reducing to obtain a steel pipe; (4) intercritical normalizing heat treatment: heating the steel pipe to an austenitizing temperature of Ac1+60°C to Ac3-60°C, holding for 30-60 minutes and then air cooling; wherein Ac1=723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ac3=910-203C 1 / 2< -15.2Ni+44.7Si+104V+31.5Mo, wherein each chemical element is substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element; (5) hot straightening.

[0031] The method according to the present disclosure, by adopting a specific intercritical normalizing heat treatment process that matches the above-mentioned element content ranges of the casing, obtains a mixed microstructure of ferrite, granular bainite and austenite, increases the work hardening index n, and improves the anti-deformation ability of the oil casing under non-uniform loads.

[0032] Preferably, in step (l) of the method according to the present disclosure, the the continuous casting is performed with a casting speed of 1.6-2.0 m / min.

[0033] Preferably, in step (2) of the method according to the present disclosure, the round billet is soaked at 1220-1270°C, and then perforated at a perforating temperature of 1150-1200°C.

[0034] More preferably, in step (3) of the method according to the present disclosure, the rolling is performed with a finishing rolling temperature of 920-980°C.

[0035] Preferably, in step (3) of the method according to the present disclosure, the stretch reducing is performed at a temperature of 900-950°C.

[0036] The high-strength anti-non-uniform-load oil casing and manufacturing method therefor according to the present disclosure have the following advantages and beneficial effects: From the perspective of composition design, the rational composition design adopted in the present disclosure enables the oil casing to have high strength, toughness, uniform elongation and work hardening index, and low cost,thereby solving the problem of of poor anti-deformation ability of oil casings under non-uniform loads in prior art, enhancing the pressure resistance of the oil casing in underground service, and meeting the demand for high-strength oil casing in the field of oilfield exploration.

[0037] In addition, the manufacturing method according to the present disclosure employs a specific intercritical normalizing heat treatment that matches the element contents, thereby obtaining a mixed microstructure of ferrite, granular bainite and austenite, improving the work hardening index n, and ultimately enhancing the anti-deformation ability of the oil casing under non-uniform loads, while achieving a good match between strength and toughness.

[0038] In some embodiments, the oil casing according to the present disclosure has a yield strength of 758-965 MPa, a tensile strength of ≥ 930 MPa, a yield-to-tensile ratio of ≤ 0.85, an elongation of ≥ 35%, a uniform elongation of ≥20%, a transverse Charpy impact energy at 0°C of ≥ 60J, a work hardening index n of ≥ 0.15, and an increase of 30% or more in yield strength after 5% deformation of material compared to that before deformation.DETAILED DESCRIPTION

[0039] The high-strength anti-non-uniform-load oil casing and manufacturing method therefor according to the present disclosure will be further explained and illustrated below with reference to specific embodiments. However, the explanation and illustration do not constitute undue limitations on the technical solution of the present disclosure.Examples 1-6 and Comparative Examples 1-8

[0040] The oil casings of Examples 1-6 are manufactured by the following steps: (1) Smelting and continuous casting: Smelting and continuous casting were carried out with the casting speed controlled to be 1.6-2.0 m / min, to obtain round billets. The smelting of each Example and Comparative Example was carried out according to the chemical compositions shown in Table 1. (2) Perforating: the round billets were soaked at 1220-1270°C and then perforated at the perforating temperature of 1 150-1200°C. (3) Rolling and stretch reducing: The finishing rolling temperature was 920-980°C, the stretch reducing was carried out at the temperature of 900-950°C after finishing rolling; (4) intercritical normalizing heat treatment: The steel pipes were heated to the austenitizing temperature of Ac1+60°C to Ac3-60°C, held for 30-60 minutes and then air-cooled; wherein Ac1=723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ac3=910-203C 1 / 2< -15.2Ni+44.7Si+104V+31.5Mo, wherein each chemical element symbol was substituted with the numerical value before the percent sign of the mass percentage content of corresponding chemical element. For example, when the Mn element content in the steel pipe was 3.5%, the numerical value 3.5 was substituted for calculation. (5) Hot straightening.

[0041] It should be noted that the chemical compositions and relevant process parameters of the oil casings in Examples 1-6 of the present disclosure all met the limitations of the present disclosure. The oil casings in Comparative Examples 1-8 were manufactured by processes similar to the above-mentioned processes, but there are some parameters of the chemical compositions and / or manufacturing method processes of the oil casings in Comparative Examples 1-8 did not meet the design requirements of the present disclosure.

[0042] Table 1 lists the mass percentages of chemical elements in the oil casings of Examples 1-6 and Comparative Examples 1-8 of the present disclosure. Table 1. (wt%, the balance being Fe and inevitable impurities other than P, S)No.CMnSiPSCrMoVNiCaTiAlNbExample 10.0530.40.0090.002 / / / 0.20.00050.010.010.02Example 20.1140.50.0100.0010.40.20.070.30.0010.020.020.04Example 30.1350.80.0100.00310.50.10.30.0020.040.050.03Example 40.153.50.70.0120.005 / / 0.150.40.0050.030.040.04Example 50.1340.60.0150.002 / / / 0.50.0040.040.030.03Example 60.143.50.80.0110.0020.30.20.080.250.0030.0250.050.02Comparative Example 10.111.5 0.50.0100.0030.40.20.070.30.0010.020.020.04Comparative Example 20.116 0.50.0100.0030.40.20.070.30.0010.020.020.04Comparative Example 30.25 40.1 0.0100.0030.40.20.070.30.0050.030.040.04Comparative Example 40.03 40.80.0100.0030.40.20.070.30.0050.030.040.04Comparative Example 50.1140.50.0100.0030.40.20.070.1 0.0050.030.040.04Comparative Example 60.1140.50.0100.0030.40.20.070.8 0.0040.040.030.03Comparative Example 70.1140.50.0100.0030.40.20.070.30.0030.0250.050.02Comparative Example 80.1140.50.0100.0030.40.20.070.30.0020.040.050.03

[0043] Table 2 lists the specific process parameters of the oil casings in Examples 1-6 and Comparative Examples 1-8. Table 2.Continuous castingPerforatingRollingIntercritical normalizingHot straighteningCasting speed (m / min)Soaking temperature (°C)Perforating temperature (°C)Finishing rolling temperature (°C)Stretch reducing temperature (°C)Ac1 (°C)Ac1+60 (°C)Ac3 (°C)Ac3-60 (°C)Austenitizing temperature (°C)Holding time (min)Heat-sizing temperature (°C)Example 12.01220115092090069975987981979050500Example 21.81270116094092069675687481479030510Example 31.61240119093093070576589483480060520Example 41.81250118096094069975987281278060530Example 51.81260117098095068974985679677040550Example 61.61240120097093070876888482479050510Comparative Example 12.01220115092090071677687481479050500Comparative Example 21.81270116094092066872887481479030510Comparative Example 31.61240119093093067873882276275060520Comparative Example 41.81250118096094069875892086080060530Comparative Example 51.81260117098095069375387781778040550Comparative Example 61.81250118094093068174186680677060530Comparative Example 71.812501180940930690750874814720 40550Comparative Example 81.812501180940930690750874814850 40550

[0044] The finally manufactured oil casings of Examples 1-6 and Comparative Examples 1-8 were sampled respectively. The samples were polished and then observed for their microstructures under an optical microscope. The test results are listed in Table 3. Table 3.MicrostructureVolume fraction of granular bainite (%)Volume fraction of austenite (%)Example 1ferrite+granular bainite+austenite69Example 2ferrite+granular bainite+austenite712Example 3ferrite+granular bainite+austenite1020Example 4ferrite+granular bainite+austenite69Example 5ferrite+granular bainite+austenite812Example 6ferrite+granular bainite+austenite78Comparative Example 1bainite+ferrite00Comparative Example 2ferrite+granular bainite+austenite1020Comparative Example 3ferrite+granular bainite+austenite57Comparative Example 4bainite+ferrite00Comparative Example 5ferrite+granular bainite+austenite611Comparative Example 6ferrite+granular bainite+austenite915Comparative Example 7ferrite+granular bainite+austenite47Comparative Example 8ferrite+granular bainite+austenite63

[0045] The finally manufactured oil casings of Examples 1-6 and Comparative Examples 1-8 were sampled respectively for property test, and the test results are listed in Table 4. Wherein: (1) The yield strength, tensile strength, elongation, and uniform elongation were determinedaccording to the GB / T 228.1-2000 standard. (2) The transverse Charpy impact energy at 0°C was determined according to GB / T 229-2007 "Metallic Materials - Charpy Pendulum Impact Test Method" standard. (3) The yield strength after 5% deformation and the work hardening index n of the oil casings were determined according to GB / T 228.1-2000 standard.

[0046] Table 4 lists the property test results of the oil casings of Examples 1-6 and Comparative Examples 1-8. Table 4.Yield strength (MPa)Tensile strength (MPa)Elongation (%)Uniform elongation (%)Transverse impact energy, 0°C (J)Yield strength after 5% deformation (MPa)Yield-to-tensile ratioWork hardening indexExample 1760950372510210340.800.16Example 282098039249210820.840.18Example 3860104040238811520.830.25Example 489010903822.57811660.820.18Example 593011103921.57612090.840.2Example 688010703520.58511200.810.17Comparative Example 1820101023 8 1029180.810.05Comparative Example 28501020382140 11220.830.22Comparative Example 378085021 755 8420.92 0.07Comparative Example 4554 710 28 18 1096980.780.15Comparative Example 5835910 26 10 879270.92 0.12Comparative Example 6690 780 33 26989320.88 0.19Comparative Example 7890101023 8 45 9700.88 0.08Comparative Example 81050111015 5 35 10340.800.06

[0047] As can be seen from Table 4, the properties of the oil casings of each example of the present disclosure all meets: a yield strength of 760-930 MPa; a tensile strength of ≥ 950 MPa; a yield-to-tensile ratio of < 0.85; an elongation of ≥ 37%; an uniform elongation of ≥ 21.5%; a transverse Charpy impact energy at 0°C of > 76J; and an increase of 30% or more in yield strength after 5% deformation of material compared to that before deformation. This indicates that the oil casings of each example of the present disclosure all have high strength, toughness, uniform elongation and work hardening index, and are suitable for use under non-uniform loads with excellent anti-deformation ability.

[0048] In contrast, the Mn contents in Comparative Examples 1 and 2 exceed the range defined by the present disclosure, the C and Si contents in Comparative Example 3 exceed the range defined by the present disclosure, the C content in Comparative Example 4 exceeds the range defined by the present disclosure, the Ni contents in Comparative Examples 5 and 6 exceed the range defined by the present disclosure, and the temperatures of the intercritical normalizing heat treatment in Comparative Examples 7 and 8 exceed the range defined by the present disclosure. As a result, at least one mechanical property of the oil casings in Comparative Examples 1-8 fails to achieve the required range of the oil casing of the present disclosure, and their overall performances are inferior to those of the high-strength anti-non-uniform-load oil casings of each example of the present disclosure.

[0049] It should be noted that the prior art part within in protection scope of the present disclosure is not limited to the examples provided in the present application document.

[0050] The combinations of technical features in the present disclosure are not limited to the combinations recited in the Claims or the combinations recited in Detailed Description of the present disclosure. All technical features recited in the present disclosure can be freely combined or integrated in any way, unless they contradict each other.

[0051] It should also be noted that the above-listed examples are merely specific embodimentsof the present disclosure. Obviously, the present disclosure is not limited to the above examples, and numerous similar variations are possible accordingly. All modifications directly derived or conceived by those skilled in the art from the disclosed content of the present disclosure shall fall within the protection scope of the present disclosure.

Claims

1. An oil casing comprising, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%; the oil casing has a microstructure of ferrite+granular bainite+ austenite.

2. The oil casing according to claim 1, wherein the oil casing comprises the following chemical elements in percentage by mass: C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%; and the balance being Fe and inevitable impurities.

3. The oil casing according to claim 1 or 2, wherein the oil casing further satisfies at least one of the following: 0 < Cr & ≤ 1 % ; 0 < Mo ≤ 0.5 % ; 0 < V ≤ 0.15 % .

4. The oil casing according to claim 1 or 2, wherein the inevitable impurities comprise P and S, wherein P≤0.015%, S ≤0.005%.

5. The oil casing according to claim 1 or 2, wherein the granular bainite has a volume fraction of 6-10%.

6. The oil casing according to claim 1 or 2, wherein the austenite has a volume fraction of 8-20%.

7. The oil casing according to claim 1 or 2, wherein the oil casing has at least one of the following properties: a yield strength of 758-965 MPa; a tensile strength of ≥ 930 MPa; a yield-to-tensile ratio of ≤ 0.85; an elongation of ≥ 35%; an uniform elongation of ≥ 20%; a transverse Charpy impact energy at 0°C of ≥ 60J; a work hardening index n of ≥ 0.15; and an increase of 30% or more in yield strength after 5% deformation of material compared to that before deformation.

8. A method for manufacturing the oil casing according to any one of claims 1 to 7, wherein the method comprises the following steps performed in sequence: (1) smelting and continuous casting to obtain a round billet; (2) perforating; (3) rolling and stretch reducing to obtain a steel pipe; (4) intercritical normalizing heat treatment: heating the steel pipe to an austenitizing temperature of Ac1+60°C to Ac3-60°C, holding for 30-60 minutes and then air-cooling; wherein Ac1=723-10.7Mn-16.9Ni+29.1Si+16.9Cr, Ac3=910-203C1 / 2-15.2Ni+44.7Si+104V+31.5Mo, wherein each chemical element is substituted with the numerical value before the percent sign of mass percentage content of corresponding chemical element; (5) hot straightening.

9. The method according to claim 8, wherein in step (1), the continuous casting is performed with a casting speed of 1.6-2.0 m / min.

10. The method according to claim 8, wherein in step (2), the round billet is soaked at 1220-1270°C, and then perforated with a perforating temperature of 1150-1200°C.

11. The method according to claim 8, wherein in step (3), the rolling is performed with a finishing rolling temperature of 920-980°C.

12. The method according to claim 8, wherein in step (3), the stretch reducing is performed at a temperature of 900-950°C.

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