High-strength oil casing resistant to co2 and microbial corrosion and manufacturing method therefor

Abstract

The present disclosure provides a high-strength oil casing resistant to CO2 and microbial corrosion and a manufacturing method therefor. The oil casing comprises, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.06-0.20%, Cr: 1.5-7.0%, Cu: 0.3-3.5%, P≤0.015, S≤0.007. For the oil casing of the present disclosure, an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of ≥758 MPa, preferably ≥800 MPa, and a full-sized impact energy at 0° C. of ≥80J, preferably ≥95J can be obtained by controlling the amounts of Cr and Cu added and the contents of impurity elements P and S, optimizing the proportion of chemical elements, and combining TMCP manufacturing process, controlling the rolling temperature, the temperature before cooling and the cooling rate.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of metallurgy, specifically to a high-strength oil casing resistant to CO2 and microbial corrosion and a manufacturing method therefor.BACKGROUND

[0002] In the production process in oil and gas fields, water injection operation is a commonly used and efficient process. Due to the advantages of convenience and low cost of river water and lake water, the river water and lake water near the oil field are usually used as water for water injection operation. However, in the exploitation process of oil and gas field, microorganisms such as sulphate-reducing bacteria (SRB) and saprophytic bacteria (TGB) in the water are easy to attach to the oil casing, directly or indirectly accelerating the corrosion or damage of metal materials. In addition, the associated gas in oil and gas fields during water injection operation also contains corrosive gas CO2, which can cause local corrosion and perforation of the oil casing. The coupling effect of CO2 and microorganisms can accelerate the corrosion of oil casing, and increase the incidence of oil casing leakage accidents. With the continuous development of oil and gas extraction, the depth of oil and gas well and formation pressure in water injection process, the requirement for the steel grade of oil casing is higher and higher. Therefore, there is an urgent desire to obtain an oil casing product with high steel grade that is resistant to CO2 and microbial corrosion in the oil and gas field industry.

[0003] The Chinese patent document with the announcement number of CN101289730B, the publication date of May 11, 2011 and the title of “Method for Manufacturing 110 ksi High Steel Grade, High CO2-Corrosion Resistant Oil Casing and Oil Casing Manufactured Using the Method”, discloses an oil casing, which has a chemical element composition of: C: 0.15-0.25%, Si: 0.2-1.0%, Mn: 0.20-1.0%, Cr: 12.0-14.0%, Ni: 0.5-1.5%, Mo: 0.2-1.0%, N: 0.03-0.10%, and the balance being Fe and inevitable impurities. An oil casing with a strength of 110 ksi steel grade and being resistant to CO2 and chloride ion corrosion at temperatures exceeding 150° C. can be obtained. This oil casing has excellent CO2 corrosion resistance, but does not have SRB corrosion resistance.

[0004] The Chinese patent document with the publication number of CN107619994A, the publication date of Jan. 23, 2018 and the title of “Seamless Pipeline Pipe Resistant to CO2 / H2S and Sulphate-reducing Bacteria Corrosion and Manufacturing Method Therefor”, discloses a pipeline pipe and a manufacturing method therefor, the pipeline pipe has a chemical element composition of: C: 0.03-0.10%, Si: 0.1-0.5%, Mn: 0.10-1.5%, Cr: 1.0-4.0%, Ni: 0.1-1.5%, Cu: 0.15-2.0%, Mo: 0.05-0.4%, Ti: 0.01-0.05%, REM: 0.05-0.1%, and the balance being Fe and inevitable impurities. The pipeline pipe obtained by this invention has excellent CO2-H2S-SRB corrosion resistance, but the strength does not satisfy the 110 ksi-125 ksi steel grade.

[0005] The Canadian patent document with the publication number of CA02872342, the publication date of Oct. 31, 2014 and the title of “High-Strength Stainless Steel Seamless Oil Casing and Manufacturing Method Therefor”, discloses a high-strength stainless steel pipe for oil well pipes with a wall thickness exceeding 25.4 mm, a high-strength of yield strength of 110 ksi or more, and excellent toughness and corrosion resistance, which has a chemical element composition of: C: 0.005-0.06%, Si: 0.05-0.50%, Mn: 0.20-1.8%, Cr: 15.5-18.0%, Ni: 1.5-5.0%, V: 0.02-0.2%, Al: 0.002-0.05%, N: 0.01-0.15%, Mo: 1.0-3.5%, W>3.0%, Cu<3.5%, and satisfying the relationship equation (% Cr)+0.65 (% Ni)+0.60 (% Mo)+0.30 (% W)+0.55 (% Cu)−20 (% C)≥19.5, (% Cr)+(% Mo)+0.50 (% W)+0.30 (% Si)−43.5 (% C)−0.4 (% Mn)−(% Ni)−0.3 (% Cu)−9 (% N)≥11.5. This invention can obtain a high-strength and high-toughness oil casing with an excellent resistance to CO2 and Cl− corrosion at 230° C., a yield strength of 110 ksi or more, and an impact toughness of 40 J or more at −10° C., but the oil casing does not have resistance to SRB corrosion.

[0006] Based on patent search, it is found that there is currently no patent involving 110 ksi-125 ksi steel grade oil casing with CO2-SRB-TGB corrosion resistance.SUMMARY

[0007] In this regard, the present disclosure provides a high-strength oil casing resistant to CO2 and microbial corrosion.

[0008] The present disclosure also provides a method for manufacturing a high-strength oil casing resistant to CO2 and microbial corrosion.

[0009] In order to achieve the above-mentioned objectives, the present disclosure relates to an oil casing, the oil casing comprises, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.06%-0.20%, Cr: 1.5%-7.0%, Cu: 0.3%-3.5%, P≤0.050% and S≤0.010%, preferably P≤0.015% and S≤0.007%.

[0010] Preferably, the oil casing further comprises a chemical element REM in percentage by mass: 0.07%-0.75%, preferably 0.10%-0.75%, preferably 0.30%-0.75%.

[0011] Preferably, the REM comprises one or both of La and Ce, and the mass percentages of La and Ce satisfy: 0.07%≤La+Ce≤0.35%, preferably 0.15%≤La+Ce≤0.35%.

[0012] Preferably, the oil casing according to the present disclosure further comprises the following chemical elements in percentage by mass:

[0013] Si: 0.1%-1.0%, Mn: 0.10%-2.5%, Ni: 0.5%-3.5%, Mo: 0.1%-3.5%, Nb: 0.02%-0.15%, V: 0.01%-0.20%, Al: 0.01%-0.08%, B: 0.0010%-0.008%.

[0014] Preferably, the oil casing satisfies one or more of the following:

[0015] Mn: 0.5-2.5%, preferably 1.0-2.5%;

[0016] Ni: 1.0-3.5%;

[0017] Mo: 1.0-3.5%; and

[0018] V: 0.06-0.20%.

[0019] Preferably, the oil casing according to the present disclosure has an average corrosion rate of <0.0250 mm / a and a local corrosion rate of <0.0450 mm / a measured according to ASTM G111-97 (2013) standard (the corrosion test is performed in an environment where CO2, sulfate-reducing bacteria and saprophytic bacteria coexist at 40° C., wherein a CO2 partial pressure is 2.0 MPa, a concentration of sulphate-reducing bacteria is 50,000 / mL, and a concentration of saprophytic bacteria is 20,000 / mL, and a test time is 300 hours).

[0020] The present disclosure further relates to a method for manufacturing the above-mentioned oil casing, comprising the following steps:

[0021] S1: smelting and casting molten steel into an ingot, subsequently forging or rolling to obtain a pipe blank;

[0022] S2: performing heating, heat holding, piercing, continuous-rolling, and stretch reducing or sizing on the pipe blank of step S1, obtaining a pierced billet: in the continuous-rolling step, a rolling temperature is 900-970° C.:

[0023] S3: cooling the pierced billet of step S2 to room temperature, obtaining an oil casing: preferably, water-cooling the pierced billet of step S2, wherein a temperature of the pierced billet before cooling is ≥870° C. and a cooling rate is 20-60° C. / s.

[0024] Preferably, in step S2, a temperature of the heating is 1220-1280° C., and a time of the heat holding is 1-4 hours.

[0025] Preferably, in step S2, a temperature of the piercing is 1170-1250° C.

[0026] Preferably, the method for manufacturing the oil casing according to the present disclosure further comprises:

[0027] S4: performing tempering heat treatment on the oil casing of step S3, wherein a tempering temperature is 530-630° C. and a holding time is 40-60 minutes.

[0028] Preferably, the oil casing according to the present disclosure has a yield strength of ≥758 MPa, preferably ≥800Mpa, and a full-sized impact energy at 0° C. of ≥80 J, preferably ≥95 J.

[0029] Preferably, a microstructure of the oil casing according to the present disclosure is tempered sorbite.

[0030] The above-mentioned technical solutions according to the present disclosure have at least one of the following beneficial effects:

[0031] Regarding the oil casing according to the present disclosure, an oil casing with resistance to CO2, sulfate-reducing bacteria and saprophytic bacteria (CO2-SRB-TGB) corrosion can be obtained by designing the composition combined with optimizing the manufacturing process, i.e, by controlling the amounts of Cr and Cu added in the steel, as well as controlling the contents of impurity elements P and S in the steel. On this basis, by further adding other chemical elements with the specific contents combined with controlling the manufacturing process of microstructure control technology (TMCP) for rolling and cooling, an oil casing with a yield strength of ≥758 MPa, preferably ≥800Mpa, and a full-sized impact energy at 0° C. of ≥80 J, preferably ≥95 J is obtained.BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 is the metallographic diagram of the oil casing of Example 1 according to the present disclosure.DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present disclosure clearer, the technical solutions of the embodiments according to the present disclosure will be clearly and completely described below. Obviously, the described embodiments are a part of the embodiments of the present disclosure, not all the embodiments. All other embodiments obtained by a person of ordinary skill in the art based on the described embodiments of the present invention are within the protection scope of the present disclosure.

[0034] Unless otherwise defined, the technical or scientific terms used in the present disclosure shall have the usual meanings as understood by persons with ordinary skills in the field to which the present disclosure belongs. The words “first”, “second” and similar words used in the present disclosure do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, the words such as “an” or “a” do not indicate a limit on quantity, but rather indicate the existence of at least one. Words such as “connection” or “attachment” are not limited to physical or mechanical connections, but may include the electrical connection, whether direct or indirect. “Up”, “down”, “left”, “right”, etc. are only used to represent relative position relationships. When the absolute position of the object described changes, the relative position relationship also changes accordingly.

[0035] The oil casing according to the first aspect of the present disclosure is described specifically below.

[0036] The oil casing according to the present disclosure comprises, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.06%-0.20%, Cr: 1.5%-7.0%, Cu: 0.3%-3.5%, P≤0.050%, S≤0.010%, preferably P≤0.015% and S≤0.007%.

[0037] The present disclosure can obtain an oil casing with resistance to CO2-SRB-TGB corrosion by controlling the amounts of Cr and Cu added in steel and controlling the contents of impurity elements P and S.

[0038] Preferably, the oil casing further comprises REM in percentage by mass: 0.07%-0.75%. In other words, the present disclosure can further improve the CO2-SRB-TGB corrosion resistance of the oil casing by adding REM to steel.

[0039] Preferably, REM comprises one or both of La and Ce, and the contents of La and Ce in percentage by mass satisfy the inequality: 0.07%≤La+Ce≤0.35%. In other words, the present disclosure further improves the CO2-SRB-TGB corrosion resistance of the oil casing by using REM containing La and / or Ce and controlling the amounts of La and Ce added.

[0040] Preferably, the oil casing according to the present disclosure further comprises the following chemical elements in percentage by mass: Si: 0.1%-1.0%, Mn: 0.10%-2.5%, Ni: 0.5%-3.5%, Mo: 0.1%-3.5%, Nb: 0.02%-0.15%, V: 0.01%-0.20%, Al: 0.01%-0.08%, B: 0.0010%-0.008%. The present disclosure can improve the strength and toughness of the oil casing by further adding the above-mentioned chemical elements with specific contents.

[0041] Specifically, the design principles of each chemical element in the oil casing according to the present disclosure are as follows:

[0042] C: C is a necessary component to ensure the strength at room temperature and hardenability of steelpipes. When the carbon content is less than 0.06%, the hardenability is low, the strength is difficult to be guaranteed, and the toughness will also be decreased: When the carbon content is higher than 0.20%, the steel will crack due to the influence of deformation stress during quenching process.

[0043] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of 758 MPa or more and a full-sized impact energy at 0° C. of ≥80 J, the mass percentage of C in the oil casing is controlled to be 0.06%-0.20%.

[0044] Si: Si is an important deoxidizer in the steelmaking process. In addition, Si can also improve high-temperature oxidation resistance and acid resistance. When the Si content is less than 0.1%, the deoxidation effect of the steel cannot be guaranteed. When the Si content exceeds 1.0%, the toughness and plasticity of the steel will be decreased.

[0045] Therefore, in the present disclosure, in order to obtain an oil casing with a yield strength of ≥758 MPa and a full-sized impact energy at 0° C. of >80 J, the mass percentage of Si in the oil casing is controlled to be 0.1%-1.0%.

[0046] Mn: Mn can expand the austenite phase region and improve the hardenability of steel. But Mn is prone to segregation during solidification, affecting the toughness of steel. When the Mn content is less than 0.10%, the hardenability of steel will significantly decrease; When the Mn content is higher than 2.5%, component segregation and quenching cracks are likely to occur.

[0047] Therefore, in the present disclosure, in order to obtain an oil casing with a yield strength of >758 MPa and a full-sized impact energy at 0° C. of >80 J, the mass percentage of Mn in the oil casing is controlled to be 0.10%-2.5%, preferably 1.0-2.5%.

[0048] P: P is a harmful element that reduces the resistance to CO2 corrosion, and has an adverse effect on the thermal processing performance. If the content of P exceeds 0.050%, the corrosion resistance performance cannot meet the requirements of CO2 environment.

[0049] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, the mass percentage of P in the oil casing is ≤0.050%, preferably ≤0.015%.

[0050] S: S is a harmful element which reduces the thermal processing performance and has an adverse effect on the impact toughness.

[0051] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of 758 MPa or more and a full-sized impact energy at 0° C. of ≥80 J, the mass percentage of S in the oil casing is ≤0.010%, preferably ≤0.007%.

[0052] Cr: Cr can significantly improve the local CO2 corrosion resistance and average CO2 corrosion resistance in steel. This is because the addition of Cr element can improve the passivation film on the steel surface and form Cr (OH)3, increasing the protection ability of corrosion products and thereby improving the CO2 corrosion resistance. When the Cr content is less than 1.5%, the excellent CO2 corrosion resistance of the steel cannot be guaranteed. However, it does not mean that the higher the Cr, the better, because the segregation of Cr carbides at grain boundaries can easily lead to a decrease of the corrosion resistance of the steel.

[0053] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, the mass percentage of Cr in the oil casing is controlled to be 1.5%-7.0%, preferably 3.0-7.0%, and more preferably 3.0-5.0%.

[0054] Ni: Ni can significantly improve the performance of passivation film and enhance the corrosion resistance of steel. Ni can also improve the cracking problem of steel slabs during steelmaking and reduce the situation where cracks are easily formed in the TMCP process.

[0055] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of 758 MPa or more and a full-sized impact energy at 0° C. of ≥80 J, the mass percentage of Ni in the oil casing is controlled to be 0.5%-3.5%, preferably 1.0-3.5%.

[0056] Cu: in the present disclosure, Cu is a key alloying element and is a necessary condition for ensuring the microbial corrosion resistance. This is because Cu has a biological toxicity to microorganisms and can improve the microbial corrosion resistance of steel. Cu is evenly dispersed in the matrix as a copper-rich phase. In a service environment that is rich in microorganisms, the copper-rich phase in the matrix can be continuously dissolved in the form of Cu ion and Cu-rich phase which are adsorbed on the steel surface, preventing SRB, TGB and other microorganisms from adsorbing and growing on the steel surface, thereby playing a role in killing bacteria and ensuring the long-lasting microbial corrosion resistance of the steel. However, excessive Cu will lead to the precipitation of coarse copper-rich phases, affecting the impact toughness and thermal processing performance of the steel.

[0057] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of 758 MPa or more, and a full-sized impact energy at 0° C. of ≥80 J, the mass percentage of Cu in the oil casing is controlled to be 0.3%-3.5%, preferably 1.5%-3.5%.

[0058] Mo: Mo can enhance the strength of steel and effectively increase the pitting corrosion resistance of steel by carbides and solid solution strengthening. When the Mo content is less than 0.1%, the strengthening effect and pitting corrosion resistance are relatively weak: When the Mo content exceeds 3.5%, quenching cracks are prone to occur in the TMCP process.

[0059] Therefore, in the present disclosure, in order to obtain an oil casing with a yield strength of 758 MPa or more and a full-sized impact energy at 0° C. of ≥80 J, the mass percentage of Mo in the oil casing is controlled to be 0.1%-3.5%, preferably 1.0-3.5%.

[0060] Nb: Nb is a relatively strong strengthening element that can enhance the strength of steel through precipitation strengthening. In addition, the Nb precipitate phase will form many nucleation sites, thereby refining the austenite grains and improving the toughness of steel. When the Nb content is less than 0.02%, the strengthening effect is not significant: When the Nb content exceeds 0.15%, the precipitate phase is in a relatively large amount and coarse, which affects the toughness and corrosion resistance of the steel.

[0061] Therefore, in the present disclosure, in order to obtain an oil casing with a yield strength of 758 MPa or more and a full-sized impact energy at 0° C. of >80 J, the mass percentage of Nb in the oil casing is controlled to be 0.02%-0.15%.

[0062] V: V is a typical precipitation strengthening element that can enhance the strength of steel. When the V content is less than 0.01%, the strengthening effect is not significant: When the V content exceeds 0.20%, the precipitate phase is in a relatively large amount and coarse, which affects the toughness and corrosion resistance of the steel.

[0063] Therefore, in the present disclosure, in order to obtain an oil casing with a yield strength of 758 MPa or more and a full-sized impact energy at 0° C. of >80 J, the mass percentage of V in the oil casing is controlled to be 0.01%-0.20%, preferably 0.06-0.20%.

[0064] Al: Al is a relatively good deoxidizing element.

[0065] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of 758 MPa or more, and a full-sized impact energy of ≥80 J, the mass percentage of Al in the oil casing is controlled to be 0.01%-0.08%.

[0066] B: B is an element that significantly improves the hardenability of steel. When the B content is less than 0.0010%, the effect of improving hardenability is not significant: When the B content exceeds 0.008%, it is easy to form a BN brittle phase.

[0067] Therefore, in the present disclosure, in order to obtain an oil casing with a yield strength of 758 MPa or more and a full-sized impact energy at 0° C. of >80 J, the mass percentage of B in the oil casing is controlled to be 0.0010%-0.008%.

[0068] REM: REM can effectively improve the toughness and CO2 corrosion resistance of steel: However, excessive REM content will produce too many coarse inclusions, affecting the toughness and corrosion resistance of the steel.

[0069] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of 758 MPa or more, and a full-sized impact energy at 0° C. of ≥80 J, the mass percentage of REM in the oil casing is controlled to be 0.07%-0.75%, preferably 0.30-0.75%.

[0070] Preferably, the rare earth metal REM can be selected from one or both of La and Ce. La and Ce have toxic effects on microorganisms such as SRB and TGB. REM can also improve the CO2 corrosion resistance of steel. However, excessive of La and Ce will generate more coarse-inclusions, affecting the toughness of the steel.

[0071] Therefore, in the present disclosure, in order to obtain an oil casing with CO2-SRB-TGB corrosion resistance, a yield strength of 758 MPa or more, and a full-sized impact energy at 0° C. of ≥80 J, the contents of La and Ce in the oil casing should satisfy: 0.07%≤La+Ce≤0.35%, preferably 0.15%≤La+Ce≤0.35%.

[0072] Preferably, the oil casing according to the present disclosure has an average corrosion rate of less than 0.0250 mm / a and a local corrosion rate of less than 0.0450 mm / a measured according to ASTM G111-97 (2013) standard (the corrosion test is performed in an environment where CO2, sulfate-reducing bacteria and saprophytic bacteria coexist at 40° C., wherein a CO2 partial pressure is 2.0 MPa, a concentration of sulphate-reducing bacteria is 50,000 / mL, and a concentration of saprophytic bacteria is 20,000 / mL, and a test time is 300 hours). The present disclosure obtains an oil casing with excellent CO2-SRB-TGB corrosion resistance by the above-mentioned composition design.

[0073] The present disclosure further relates to a method for manufacturing the above-mentioned oil casing, comprising the following steps:

[0074] S1: smelting and casting molten steel into an ingot, subsequently forging or rolling to obtain a pipe blank;

[0075] S2: performing heating, heat holding, piercing, continuous-rolling, and stretch reducing or sizing on the pipe blank of step S1, obtaining a pierced billet: in the continuous-rolling step, a rolling temperature is 900-970° C.;

[0076] S3: cooling the pierced billet of step S2 to room temperature, obtaining an oil casing: preferably, water-cooling the pierced billet of step S2, wherein a temperature of the pierced billet before cooling is ≥870° C. and a cooling rate is 20-60° C. / s.

[0077] In the present disclosure, if the cooling rate is lower than 20° C. / s, it is difficult to obtain a complete martensitic structure: If the cooling rate is higher than 60° C. / s, quenching cracks are prone to occur. by the above-mentioned composition design combined with the TMCP manufacturing process, controlling the rolling temperature during the rolling process and the temperature and cooling rate during the cooling process, the present disclosure can obtain an oil casing with a yield strength of >758 MPa, preferably ≥800 MPa, and a full-sized impact energy at 0° C. of ≥80 J, preferably ≥95 J, ultimately achieving energy saving and consumption reduction, green manufacturing, and good economic benefits.

[0078] It is worth noting that the TMCP manufacturing process of the present disclosure utilizes the residual heat after hot rolling to directly quench and achieve further strengthening by deformation-induced phase transformation effect, which can not only further improve the comprehensive mechanical performances of the oil casing, but also save production processes and reduces production costs.

[0079] More specifically, in step S2, a temperature of heating is 1220-1280° C. and a time of heat holding is 1-4 hours. In the present disclosure, for the oil casing composed of the above-mentioned chemical elements, controlling the temperature of heating to 1220-1280° C. and the time of heat holding to 1-4 hours can make carbon and alloying elements fully diffuse at the composite interface, achieving metallurgical bonding, and also facilitating the homogenization of carbon and alloying elements.

[0080] Preferably, in step S2, the temperature of piercing is 1170-1250° C. This is because, for the oil casing composed of the above-mentioned chemical elements, when the temperature of piercing is below: 1170° C., it will result in high deformation resistance and difficult biting in the obtained oil casing. When the temperature of piercing is higher than 1250° C., it will lead to thermal deformation defects.

[0081] Preferably, the manufacturing method of the oil casing according to the present disclosure may further comprise S4: performing tempering heat treatment on the oil casing of step S3 wherein, a tempering temperature is 530-630° C. and a holding time is 40-60 min. In other words, by tempering heat treatment and controlling the tempering temperature to 530-630° C. and the holding time to 40-60 minutes, the refinement and strengthening effect of the structure can be further improved, resulting in a high-strength and high-toughness oil casing.

[0082] Preferably, the oil casing according to the present disclosure has a yield strength of >758 MPa, preferably ≥800 MPa, and a full-sized impact energy at 0° C. of ≥80 J, preferably ≥95 J.

[0083] In other words, through the above-mentioned composition design, controlling and optimizing the contents of chemical elements in the steel, the oil casing with excellent comprehensive mechanical performances can be obtained.

[0084] In order to make the objectives, technical solutions and advantages of the present disclosure clearer, the Examples of the present disclosure will be further described in detail below.Examples 1-5 and Comparative Examples 1-4

[0085] The oil casing of Examples 1-5 of the present disclosure is manufactured by the following steps:

[0086] S1: the molten steel is smelted, cast and forged according to the formula shown in Table 1, to obtain a pipe blank;

[0087] S2: the pipe blank is heated for 3 hours, performing piercing and rolling on the pipe blank in sequence, and finally obtaining a pierced billet by stretch reducing;

[0088] S3: performing controlled cooling treatment on the pierced billet of step S2 to room temperature, obtaining an oil casing;

[0089] S4: performing heat treatment on the oil casing of step S3 with a tempering time of 60 minutes, obtaining the oil casings of Examples 1-5.

[0090] The oil casings of Comparative Examples 1-4 are manufactured using a manufacturing process similar to that of Examples 1-5. However, one or more of the chemical element compositions and / or process parameters of the oil casings of Comparative Examples 1-4 do not fall within the scope of the present disclosure.

[0091] Table 1 lists the chemical element compositions of the oil casings of Examples 1-5 and Comparative Examples 1-4.TABLE 1Chemical element compositions of oil casings in Examples 1-5 and Comparative Examples1-4 (wt %, the balance is Fe and inevitable impurities except for P and S)REMTotalNo.CSiMnCrNiCuMoNbVAlBPSamountLaCeExample 10.100.50.13.01.51.50.10.020.200.010.0010.0150.0020.300.080.07Example 20.200.11.04.00.52.02.00.120.100.060.0040.0150.0020.500.150.10Example 30.150.82.57.03.03.03.00.100.150.050.0050.0500.0100.700.200.15Example 40.180.31.65.03.53.53.50.150.010.080.0030.0150.0020.750.140.12Example 50.061.00.51.51.00.31.00.060.060.030.0080.0150.0020.100.050.02Comparative0.050.11.04.00.52.02.00.010.010.060.0020.0150.0020.500.150.10example 1Comparative0.200.11.01.00.51.00.10.120.100.060.0040.0150.0020.080.020.03example 2Comparative0.180.31.65.01.50.10.20.150.010.080.0030.0150.0020.090.030.03example 3Comparative0.050.11.00.80.20.120.20.010.010.060.0030.0150.0020.0950.040.02example 4

[0092] Table 2 lists the main process parameters of the hot-piercing step, rolling step, controlled cooling step, and heat treatment step of Examples 1-5 and Comparative Examples 1-4.TABLE 2TemperingHot-piercingRollingControlled coolingheat treatmentHeatingPiercingRollingTemperatureTemperingtemperaturetemperaturetemperaturebefore coolingCoolingtemperatureNo.° C.° C.° C.° C.rate ° C. / s° C.Example 11280125097091020600Example 21240119093088050580Example 31220117090087060530Example 41270124096090035550Example 51250122095089040630Comparative1260120094590035650example 1Comparative1250119595086015570example 2Comparative1280125096090040545example 3Comparative1230118592089050640example 4

[0093] FIG. 1 shows a metallographic diagram of the oil casing of Example 1 of the present disclosure. It can be seen from the FIGURE that the microstructure of tempered sorbite can be obtained through the optimization of the above-mentioned composition design and manufacturing process parameters, and the structure of tempered sorbite can ensure the high strength and excellent impact performance of the oil casing.

[0094] The oil casings of the examples of the present disclosure can be used for exploitation of oil and gas field. To further verify the mechanical performances, the obtained oil casings of Examples 1-5 and Comparative Examples 1-4 were sampled respectively.

[0095] The yield strength data is obtained by processing the oil casings of Examples 1-5 and Comparative Examples 1-4 into API arc specimens, and taking the average after testing the specimens according to API standards.

[0096] The impact test was performed in accordance with GB / T 229-2020. Specifically, the full-size V-shaped impact specimens with a cross-sectional dimension of 10 mm*10 mm*55 mm were taken from the oil casings of Examples 1-5 and Comparative Examples 1-4, respectively. The impact test was performed at 0° C., and the average value was taken after the test.

[0097] The corrosion test was performed according to ASTM G111-97 (2013). The specific conditions are: the corrosion test was performed in an environment where CO2, sulfate reducing bacteria and saprophytic bacteria coexist at 40° C., and the CO2 partial pressure was 2.0 MPa, the concentration of sulfate reducing bacteria was 50000 / mL, the concentration of saprophytic bacteria is 20000 / mL, and the test time was 300 hours. The oil casings of Examples 1-5 and Comparative Examples 1-4 were weighed before and after the test, and the average corrosion rate was obtained after conversion. Performing cross-section analysis on the pitting pits of the oil casings of Examples 1-5 and the steelpipes of Comparative Examples 1-4, and the local corrosion rate was obtained after conversion.

[0098] The yield strength, full-sized impact energy at 0° C., average corrosion rate, and local corrosion rate of the oil casings of Examples 1-5 and Comparative Examples 1-4 are shown in Table 3.TABLE 3Performance test resultsComprehensive mechanicalCorrosion performanceperformance test resultstest resultsFull-sizedAverageLocalimpact energycorrosioncorrosionYield strengthat 0° C.raterateNo.MPaJmm / amm / aExample 18451200.01990.0387Example 29051100.01820.0328Example 31034950.00890.0158Example 49501050.01040.0207Example 58001400.02010.0415Comparative7301800.02010.0363example 1Comparative885603.09104.2023example 2Comparative9201002.01814.8262example 3Comparative7251764.62915.7210example 4

[0099] It can be seen with reference to Tables 1-3 that since the oil casings in Examples 1-5 have the mass percentage ratios of chemical elements specified by the present disclosure, and are processed and produced according to the manufacturing process parameters provided by the present disclosure, thereby the oil casings with an average corrosion rate of <0.0250 mm / a, a local corrosion rate of <0.0450 mm / a, a yield strength of ≥800 MPa and a full-scale impact energy at 0° C. of ≥95 J are obtained. In other words, the oil casings in Examples 1-5 of the present disclosure have excellent CO2-SRB-TGB corrosion resistance and good mechanical strength and toughness.

[0100] Regarding the oil casings of the examples of the present disclosure, oil casings with CO2-SRB-TGB corrosion resistance, a yield strength of ≥800 MPa and a full-sized impact energy at 0° C.≥95 J can be obtained by controlling the addition amounts of Cr and Cu and the contents of impurity elements P and S, optimizing the proportion of alloy elements, and combining the TMCP manufacturing process, controlling the rolling temperature, the temperature before cooling and the cooling rate.

[0101] The above are preferred embodiments of the present disclosure. It should be pointed out that for a person of ordinary skill in the art, several improvements and modifications can be made without departing from the principles of the present disclosure, and these improvements and modifications should also be regarded as within the scope of protection of the present disclosure.

Claims

1. An oil casing, wherein the oil casing comprises, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.06%-0.20%, Cr: 1.5%-7.0%, Cu: 0.3%-3.5%, P≤0.050% and S≤0.010%, preferably P≤0.015% and S≤0.007%.

2. The oil casing according toclaim 1, wherein the oil casing further comprises REM in percentage by mass: 0.07%-0.75%, preferably 0.10%-0.75%.

3. The oil casing according to claim 2, wherein the REM comprises one or both of La and Ce, and contents of La and Ce in percentage by mass satisfy the following inequality: 0.07%≤La+Ce≤0.35%, preferably 0.15%≤La+Ce≤0.35%.

4. The oil casing according to claim 1, wherein a Cr content is 3.0-7.0%, preferably 3.0-5.0%; and / or a Cu content is 1.5%-3.5%.

5. The oil casing according to claim 1, wherein the oil casing further comprises the following chemical elements in percentage by mass: Si: 0.1%-1.0%, Mn: 0.10%-2.5%, Ni: 0.5%-3.5%, Mo: 0.1%-3.5%, Nb: 0.02%-0.15%, V: 0.01%-0.20%, Al: 0.01%-0.08%, B: 0.0010%-0.008%.

6. The oil casing according to claim 5, wherein the oil casing satisfies one or more of the following:Mn: 0.5-2.5%;Ni: 1.0-3.5%;Mo: 1.0-3.5%; andV: 0.06-0.20%.

7. The oil casing according to claim 1, wherein the oil casing has an average corrosion rate of <0.0250 mm / a and a local corrosion rate of <0.0450 mm / a measured according to ASTM G111-97 (2013) standard.

8. The oil casing according to claim 1, wherein the oil casing has a yield strength of ≥758 MPa, preferably ≥800 MPa, a full-sized impact energy at 0° C. of ≥80 J, preferably ≥95 J; and / or, a microstructure of tempered sorbite.

9. A method for manufacturing the oil casing according to claim 1, wherein the method comprises the following steps:S1: performing smelting and casting on molten steel, and then forging or rolling to obtain a pipe blank;S2: performing heating, heat holding, piercing, continuous-rolling, and stretch reducing or sizing on the pipe blank of step S1, obtaining a pierced billet; wherein a rolling temperature in the continuous-rolling step is 900-970° C.;S3: cooling the pierced billet of step S2 to room temperature, obtaining an oil casing; preferably, water-cooling the pierced billet of step S2, wherein a temperature of the pierced billet before cooling is ≥870° C. and a cooling rate is 20-60° C. / s.

10. The method according to claim 9, wherein in step S2, a temperature of the heating is 1220-1280° C., and a time of the heat holding is 1-4 hours.

11. The method according to claim 9, wherein in step S2, a temperature of the piercing is 1170-1250° C.

12. The method according to claim 9, wherein the method further comprises the following steps:S4: performing a tempering heat treatment on the oil casing of step S3, wherein a tempering temperature is 530-630° C. and a holding time is 40-60 minutes.

13. The method according to claim 9, wherein the oil casing further comprises REM in percentage by mass: 0.07%-0.75%, preferably 0.10%-0.75%.

14. The method according to claim 13, wherein the REM comprises one or both of La and Ce, and contents of La and Ce in percentage by mass satisfy the following inequality: 0.07%≤La+Ce≤0.35%, preferably 0.15%≤La+Ce≤0.35%.

15. The method according to claim 9, wherein a Cr content is 3.0-7.0%, preferably 3.0-5.0%; and / or a Cu content is 1.5%-3.5%.

16. The method according to claim 9, wherein the oil casing further comprises the following chemical elements in percentage by mass: Si: 0.1%-1.0%, Mn: 0.10%-2.5%, Ni: 0.5%-3.5%, Mo: 0.1%-3.5%, Nb: 0.02%-0.15%, V: 0.01%-0.20%, Al: 0.01%-0.08%, B: 0.0010%-0.008%.

17. The method according to claim 16, wherein the oil casing satisfies one or more of the following:Mn: 0.5-2.5%;Ni: 1.0-3.5%;Mo: 1.0-3.5%; andV: 0.06-0.20%.

18. The method according to claim 9, wherein the oil casing has an average corrosion rate of <0.0250 mm / a and a local corrosion rate of <0.0450 mm / a measured according to ASTM G111-97 (2013) standard.

19. The method according to claim 9, wherein the oil casing has a yield strength of ≥758 MPa, preferably ≥800 MPa, a full-sized impact energy at 0° C. of ≥80 J, preferably ≥95 J; and / or, a microstructure of tempered sorbite.

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