High-strength and tough oil tubing resistant to co 2 and h 2s corrosion and manufacturing method therefor

Abstract

A high-strength and tough oil tubing resistant to CO2 and H2S corrosion and a manufacturing method therefor. In addition to Fe and unavoidable impurities, the oil tubing further comprises the following chemical elements in wt%: C: 0.08-0.18%, Si: 0.20-0.40%, Mn: 0.30-0.80%, Alt: 0.02-0.04%, Cr: 4.00-10.50%, Ni: 0.50-2.50%, Mo: 0.10-0.40%, and Cu: 0.50-1.50%; and Cr, Cu, and Ni satisfy the following relationship: 6.35% ≤ (Cr + Cu + Ni) ≤ 12.85%, Ni / Cu ≥ 0.55. The described oil tubing has a room temperature yield strength ≥ 580 MPa, and an impact energy at 0°C KV8 ≥ 150 J.

Description

A high-strength and tough oil casing resistant to CO2 and H2S corrosion and its manufacturing method Technical Field

[0001] This invention relates to the field of oil casing technology, and in particular to a high-strength and tough oil casing resistant to CO2 and H2S corrosion and its manufacturing method. Background Technology

[0002] Pipeline corrosion has long been a serious problem in the oil and gas industry. As many of my country's major oilfields enter the mid-to-late stages of development, the CO2 content and water cut in these fields are increasing. This, coupled with the development of deep, high-temperature, and high-pressure oil and gas fields and the implementation of various oil recovery technologies, such as the widespread use of enhanced oil recovery (EOR) with CO2 reinjection, has led to more demanding production conditions. The interaction of high water cut, high salinity, and large amounts of corrosive gases (mainly CO2 and H2S) exacerbates the corrosion of oil production equipment, casing, and pipelines. Furthermore, chloride ions (Cl-) are frequently present in the produced water flowing into the pipelines. - Pipe sections exposed to carbon dioxide-containing brine are particularly susceptible to severe corrosion.

[0003] Existing technologies mention some corrosion-resistant oil casings. For example, Chinese patent CN115491606B discloses "a low-Cr content CO2 corrosion-resistant oil casing and its preparation method," with the following chemical element composition: C 0.21%-0.29%, Si 0.25%-0.45%, Mn 0.90%-1.10%, Cr 2.95%-5.50%, Mo 0.35%-0.50%, Ni 0.15%-0.25%, Cu 0.15%-0.25%, Nb 0.05%-0.08%, Ti 0.02%-0.05%, N 0.001%-0.010%, Ce 0.006%-0.010%, Al 0.005%-0.020%, Ca 0.005%-0.010%, P≤0.015%, S≤0.005%, O≤0.002%, H≤0.00015%. However, this oil casing can only meet P. (CO2) It does not have corrosion resistance in working conditions with pressure ≤0.2MPa and no H2S gas.

[0004] Despite extensive research both domestically and internationally, several perforation failures of oil casing and tubing operating in CO2+H2S coexisting environments have occurred in domestic oil and gas fields in recent years, causing significant economic losses and environmental problems for oil and gas extraction. Currently, oil casing and tubing that are simply resistant to CO2 corrosion are no longer effective in handling the complex conditions of CO2 and H2S coexistence, and cannot effectively solve the resulting perforation failure problem. Therefore, existing technologies need further improvement and development. To ensure the safety and stability of oil and gas extraction, there is an urgent need to invent a high-strength and tough oil casing and tubing resistant to both CO2 and H2S corrosion. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength and high-toughness oil casing resistant to CO2 and H2S corrosion and its manufacturing method. The oil casing has a room temperature yield strength ≥580MPa (preferably 580-800MPa) and an impact energy KV8 ≥150J at 0℃ (preferably 150-335J). The oil casing shows no cracks after constant load NACE-A method test according to NACE TM0177-2016. The oil casing has a uniform corrosion rate <0.0250mm / a and a maximum pitting corrosion rate <0.0450mm / a as measured according to ASTM G111-97 (2013).

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] An oil casing, in addition to Fe and unavoidable impurities, contains the following chemical elements in wt%: C: 0.08-0.18%, Si: 0.20-0.40%, Mn: 0.30-0.80%, Alt: 0.02-0.04%, Cr: 4.00-10.50%, Ni: 0.50-2.50%, Mo: 0.10-0.40%, Cu: 0.50-1.50%; and Cr, Cu and Ni satisfy the following relationship: 6.35% ≤ (Cr+Cu+Ni) ≤ 12.85%, Ni / Cu ≥ 0.55.

[0008] Preferably, the oil casing contains the following chemical elements in wt%: C: 0.08-0.18%, Si: 0.20-0.40%, Mn: 0.30-0.80%, Alt: 0.02-0.04%, Cr: 4.00-10.50%, Ni: 0.50-2.50%, Mo: 0.10-0.40%, Cu: 0.50-1.50%, with the balance being Fe and unavoidable impurities; and Cr, Cu and Ni satisfy the following relationship: 6.35% ≤ (Cr+Cu+Ni) ≤ 12.85%, Ni / Cu ≥ 0.55, preferably 0.55-3.64, where each element symbol is replaced with the corresponding element's mass percentage content.

[0009] Preferably, unavoidable impurities include P, S, O and H, wherein P ≤ 0.012%, S ≤ 0.0020%, O ≤ 0.006%, and H ≤ 0.0002%; more preferably, P ≤ 0.010%, S ≤ 0.0018%, O ≤ 0.005%, and H ≤ 0.0001%.

[0010] Preferably, the microstructure of the oil casing of the present invention is tempered sorbite, and preferably, its grain size measured according to GB / T6394-2017 is grade 7.5 or above, more preferably grade 7.5-9.5, and more preferably grade 8.5-9.5 (Note: the higher the grain size grade, the smaller the grain size).

[0011] Preferably, the room temperature yield strength of the oil casing of the present invention is ≥580MPa, preferably 580-800MPa, and the 0℃ impact energy KV8 is ≥150J, preferably 150-335J, and more preferably 195-335J.

[0012] Preferably, the oil casing shows no cracks after being tested under constant load using the NACE-A method according to NACE TM0177-2016, wherein the test is conducted under the following conditions: NACE-B solution (pH=3.5), 0.1MPa H2S, load of 80% SMYS, and test time of 720h.

[0013] Preferably, the oil casing has a uniform corrosion rate of <0.0250 mm / a, more preferably 0.0165 mm / a, and a maximum pitting rate of <0.0450 mm / a, more preferably 0.0267 mm / a, as measured according to ASTM G111-97 (2013). The test is conducted under the following conditions: dynamic corrosion testing at 60-80°C in a CO2 and H2S coexisting environment. (CO2) ≤2MPa, P (H2S) ≤0.5MPa, using a 5wt% NaCl solution, a flow rate of 1m / s, and a test time of 168h.

[0014] Preferably, after the oil casing is subjected to a corrosion test according to ASTM G111-97 (2013), the resulting corrosion product film contains:

[0015] The enrichment amount of Cr ({Cr}%) satisfies: enrichment amount ({Cr}%) / nominal Cr% = 3.00-5.50; and / or

[0016] The enrichment level of Ni, {Ni}%, satisfies: enrichment level {Ni}% / nominal Ni% = 1.50-4.50; and / or

[0017] The enrichment of Cu, {Cu}%, satisfies the following condition: enrichment of Cu, {Cu}%, / nominal Cu%, = 1.50 - 4.50.

[0018] Wherein, nominal Cr% is the Cr content in the design composition of the casing and oil pipe, and enrichment {Cr}% is the Cr content in the corrosion product film; nominal Ni% is the Ni content in the design composition of the casing and oil pipe, and enrichment {Ni}% is the Ni content in the corrosion product film; nominal Cu% is the Cu content in the design composition of the casing and oil pipe, and enrichment {Cu}% is the Cu content in the corrosion product film.

[0019] In the composition design of the oil casing described in this invention:

[0020] C: As the most important solid solution strengthening element, C can significantly improve the strength of steel. A low-C design is beneficial for improving the steel's ductility, toughness, and corrosion resistance. When the C content is too low, the steel has poor hardenability, making it difficult to improve strength. When the C content is too high, it will combine with Cr in the steel to form Cr carbides, which precipitate at grain boundaries, creating localized Cr-depleted zones and thus reducing resistance to CO2 corrosion. Simultaneously, the precipitation of carbides easily becomes a "hydrogen trap," attracting hydrogen atoms to accumulate. The edges of these carbides easily become crack initiation sites for hydrogen-induced cracking and sulfide stress corrosion cracking, reducing the steel's resistance to H2S. For these reasons, the C content in this invention is controlled at 0.08-0.18%.

[0021] Si: Appropriate amounts of elemental Si can improve strength and hardness, and it is also a strong deoxidizing element. However, when the Si content is too high, it will reduce the surface quality of the steel and affect its toughness and corrosion resistance. For these reasons, the Si content in this invention is controlled at 0.20-0.40%.

[0022] Mn: Element Mn improves hardenability, strengthens through solid solution, and enhances grain refinement. However, excessive Mn content can exacerbate center segregation, thereby reducing the steel's resistance to H2S and CO2 corrosion. Therefore, in this invention, the Mn content is controlled at 0.30-0.80%.

[0023] Total aluminum content (Al): Al is added to steel as a deoxidizer, which can refine the grains and fix nitrogen in the steel, thereby improving the impact toughness and reducing the tendency for cold brittleness. However, when the Al content is too high, it is easy to form coarse oxide inclusions, which is detrimental to toughness and corrosion resistance. For the above reasons, the Al content in this invention is controlled at 0.02-0.04%.

[0024] Cr: Cr is the most economical and essential element for CO2 resistance. Cr forms a dense oxide film on the metal surface, effectively preventing direct contact between CO2, water, and other corrosive media and the metal substrate, thus slowing down corrosion. Simultaneously, Cr contributes to metal surface passivation. When metal is in a CO2-containing environment, Cr promotes the formation of a passivation film, keeping the metal surface in a relatively stable state. Cr also alters the composition of corrosion products, forming a more stable and dense corrosion product layer, thereby enhancing the protective performance against corrosion. When the Cr content is too low, its corrosion resistance is limited; when the Cr content is too high, carbides easily precipitate at grain boundaries, attracting hydrogen atoms and leading to a decrease in sulfur resistance. Furthermore, it also adversely affects weldability. For these reasons, the Cr content in this invention is controlled at 4.00-10.50%, preferably 5.05-10.50%.

[0025] Ni: Element Ni can alter the electrochemical properties of steel surfaces, reduce the corrosion potential of steel in a CO2 environment, decrease anodic dissolution reactions, and thus improve corrosion resistance. Ni combines with hydrogen atoms to form stable compounds, reducing hydrogen diffusion and accumulation in steel. Ni and Cr work together to form a more stable passivation film, and Ni combined with Mo can improve the steel's resistance to pitting corrosion. Furthermore, Ni can balance the "Cu embrittlement" caused by Cu. For these reasons, the Ni content in this invention is controlled at 0.50-2.50%, preferably 1.22-2.50%.

[0026] Mo: Element Mo can improve the stability of the passivation film, enhance the steel's resistance to CO2 and H2S corrosion, and inhibit pitting corrosion. It also improves toughness and weldability. For these reasons, the Mo content in this invention is controlled at 0.10-0.40%.

[0027] Cu (Cu): Element Cu promotes the formation of a denser and more stable protective film on the steel surface. The copper-containing film effectively blocks corrosive media such as CO2 and H2S from contacting the steel substrate, reducing the corrosion rate. It also inhibits anodic dissolution, reduces the release of iron ions, and shifts the anodic reaction potential in the positive direction, thereby slowing down corrosion. Cu dissolved in the steel matrix produces a solid solution strengthening effect, improving the strength and hardness of the steel. Appropriate amounts of Cu can also improve the toughness and ductility of the steel. However, excessive Cu content leads to hot brittleness in the steel, resulting in "Cu brittleness," which makes hot forging and rolling difficult. For these reasons, the Cu content in this invention is controlled at 0.50-1.50%.

[0028] Unavoidable impurities include P, S, O, and H, with the following preferred values: P ≤ 0.012% (preferably P ≤ 0.010%), S ≤ 0.0020% (preferably S ≤ 0.0018%), O ≤ 0.006% (preferably O ≤ 0.005%), and H ≤ 0.0002% (preferably H ≤ 0.0001%). It should be noted that P, S, O, and H are all unavoidable impurity elements in steel, and the lower the impurity content, the better, provided that technical requirements allow.

[0029] It should be noted that the Cr, Cu, and Ni elements in the steel not only need to compensate for the strength loss caused by the low-carbon design, but also need to be enriched in large quantities to form a fully encapsulated, dense, and uniform corrosion product film on the matrix interface. In order to ensure that the alloying elements in the steel can be fully distributed and to have a certain degree of economy, the composition design of this invention also satisfies: 6.35% ≤ (Cr+Cu+Ni) ≤ 12.85%, preferably 7.37% ≤ (Cr+Cu+Ni) ≤ 12.85%, where the symbols of each element are substituted with the corresponding mass percentage content of the element.

[0030] It should also be noted that in order to form a Cu-rich film, 0.50-1.50% Cu was added to the steel in this invention. The amount of copper has a significant impact on copper embrittlement. When the Cu content exceeds 1.50%, selective oxidation occurs on the surface during heating, causing Fe to oxidize Cu before Fe, resulting in a relatively increased surface Cu content and the formation of a thin film. This film then diffuses inward, leading to copper embrittlement. To avoid copper embrittlement and to fully utilize the beneficial effects of Cu, the composition design of this invention also satisfies: Ni / Cu ≥ 0.55. Preferably, 0.55 ≤ Ni / Cu ≤ 3.64, where each element symbol corresponds to its mass percentage content.

[0031] The corrosion-resistant, high-strength, and tough oil casing of this invention employs a composition design with a synergistic effect of low to medium Cr, Cu, and Ni to achieve excellent strength, toughness, and resistance to CO2 and H2S corrosion. The main design concept is as follows: During corrosion, the Cu, Ni, and Cr alloying elements in the steel undergo redistribution, enriching within the corrosion product film and enhancing its density; simultaneously, the electrochemical behavior of the corrosion system is altered, further improving corrosion resistance.

[0032] Ni is present in divalent oxides (Ni 2+ The form of ) exists in the spinel structure NiFe2O4, which helps to refine and densify the spinel structure, thereby reducing the defect density of the film and Fe 2+ / Fe 3+ The increased proportion and thickening of the internal barrier oxide film also promote the formation of nanoscale superparamagnetic α-FeOOH, further enhancing the film's density and thus hindering the formation of Cl. -The entry and diffusion of H2S and CO2 are facilitated. Secondary Cu precipitation primarily accumulates at the interface between the corrosion product film and the substrate, promoting passivation of the steel substrate (acting as the anode) to form copper oxide (CuO) or copper carbonate (CuCO3) films, enhancing the barrier against corrosive media. Simultaneously, Cu hinders the crystallization of α-FeOOH, causing the α-FeOOH grains to distort and deform, tightly binding together and thus improving the film's density. Cr is the most economical corrosion-resistant element, accumulating in the inner layer of the corrosion product film. The Cr-rich layer reduces the film's conductivity and exhibits some cation selectivity, effectively preventing anions from penetrating the film and contacting the metal substrate. During secondary distribution, Cr replaces some of the Fe in α-FeOOH, accumulating in large quantities at microcracks and grain boundaries, reducing film defects, accelerating defect healing, and blocking the direct contact channels between the corrosive media and the substrate, preventing pitting corrosion. When Cu, Cr, and Ni coexist, their interaction promotes the stability of the corrosion product film and improves the overall corrosion resistance.

[0033] It should be noted that the enrichment degree of Cu, Cr, and Ni in the film layer has a significant impact on corrosion resistance, especially resistance to localized corrosion. When the enrichment degree of alloying elements is low, the film layer is not dense enough and is prone to "gaps," causing pitting corrosion. Through research, it has been found that the enrichment amount of alloying elements needs to reach a certain ratio to form a continuous and dense film layer in order to effectively cut off the connection between the medium and the substrate. Preferably, the oil casing corrosion product film of the present invention has certain requirements on the enrichment amount of Cr, Ni, and Cu: {Cr}%, {Ni}%, and {Cu}%. A high enrichment degree of alloying elements forms a uniform and dense film layer, which has higher chemical and thermodynamic stability, changes the electrochemical system, prevents the medium from entering the substrate, improves resistance to uniform corrosion, and greatly reduces the tendency to pitting corrosion.

[0034] The present invention also provides a method for manufacturing the oil casing, comprising the following steps performed sequentially:

[0035] 1) Based on the above components, a tube blank is obtained through smelting, casting, and rolling;

[0036] 2) The tube blank is heated, pierced, reduced in diameter by hollow billet, continuously rolled, reduced in diameter by tension and air cooled to obtain a rough tube;

[0037] 3) Quenching and tempering heat treatment, including quenching and tempering, wherein,

[0038] Quenching: Quenching temperature is 890-950℃, holding time is 30-80min, and cooling rate is 20-60℃ / s;

[0039] Tempering: Tempering temperature is 580-650℃, holding time is 60-120min, and air cooling is performed.

[0040] Preferably, in step 2), the billet heating temperature is 1240-1280℃, the holding time is 60-120min; and / or the piercing temperature is 1200-1230℃, the piercing speed is 0.3-0.8m / s; and / or the continuous rolling temperature is 1050-1100℃, the rolling speed is 2.5-3.5m / s.

[0041] In the manufacturing method described in this invention

[0042] High-grade steel oil casing has been widely and successfully applied in CO2 corrosion resistance, but its resistance to CO2+H2S environmental corrosion remains weak. The oil casing of this invention has a yield strength ≥580MPa. As the yield strength increases, the ratio of critical stress to yield strength decreases, meaning the material is more prone to cracking under H2S stress corrosion. Addressing this technical challenge, this invention primarily controls the heating temperature, holding time, and cooling rate during quenching to ensure sufficient austenitization, microstructure transformation, and carbide dissolution, avoiding austenite grain coarsening and obtaining an ideal quenched martensite structure. Controlling the tempering temperature and holding time reduces internal stress and promotes carbide dispersion and spheroidization precipitation, resulting in a tempered sorbite structure that balances excellent mechanical properties and corrosion resistance.

[0043] Excessive heating temperature of the tube blank can cause coarse grains, overheating, or burning, while excessively low temperature can make subsequent processes difficult. Therefore, to obtain good plasticity and machinability, it is preferable to control the heating temperature of the tube blank at 1240-1280℃ and the holding time at 60-120min.

[0044] The piercing temperature is mainly affected by the distance between the piercing machine and the billet heating furnace. In order to improve the working efficiency of the piercing machine and reduce the workload of the machine, it is preferable to control the piercing temperature at 1200-1230℃ and the piercing speed at 0.3-0.8m / s.

[0045] Excessively high or low continuous rolling temperatures can lead to abnormal microstructure, and excessively high temperatures can cause the rolls to overheat, affecting roll life and product surface quality. To ensure that the rolling temperature does not exceed the equipment's capacity and to obtain high dimensional accuracy and surface quality, it is preferable to control the rolling temperature at 1050-1100℃ and the rolling speed at 2.5-3.5m / s.

[0046] In quenching and tempering heat treatment, if the quenching temperature is too low or the holding time is too short, the alloying elements in the steel cannot be fully dissolved, and a good strengthening effect cannot be achieved. If the quenching temperature is too high or the holding time is too long, the austenite grains in the steel will continuously coarsen, which is detrimental to the mechanical properties of the steel. If the cooling rate is too low or too high, different types of mixed microstructures will be obtained. Due to their different physical properties, these microstructures are prone to distortion under load. Therefore, in order to obtain a suitable austenite grain size, a good strengthening effect, and an ideal microstructure, this invention controls the quenching temperature of the quenched steel pipe to 890-950℃, the holding time to 30-80min, and the cooling rate to 20-60℃ / s.

[0047] In the tempering process of heat treatment, in order to fully eliminate stress and achieve a yield strength of over 580MPa while maintaining good toughness, the tempering temperature of this invention is controlled at 580-650℃ and the holding time is controlled at 60-120min.

[0048] Compared with the prior art, the present invention shows the following advantages and beneficial effects:

[0049] Currently, many patented corrosion-resistant oil casing systems employ a 13Cr composition to achieve high strength and excellent CO2 corrosion resistance. However, with increasing yield strength, the susceptibility to H2S stress cracking also increases, making cracking and failure more likely to occur in CO2 / H2S environments. Some patented oil casing systems control the Cr content to below 4%, limiting their service life to conditions with low CO2 partial pressure and only trace amounts of H2S.

[0050] The corrosion-resistant, high-strength, and tough oil casing of this invention, through reasonable composition design, controls the addition of alloying elements such as Cr, Ni, and Cu in the Fe-based system, and controls the content of impurity elements P and S. Through the self-precipitation and secondary distribution of alloying elements, the electrochemical behavior of the corrosion system is improved, affecting the structure and type of film phases, blocking defects such as microcracks and vacancies, and thus forming a dense corrosion product film. Ultimately, an oil casing with excellent resistance to CO2 and H2S corrosion can be obtained.

[0051] Based on this, by controlling the alloy element ratio and combining the heat treatment process of quenching and tempering to refine the grains, disperse carbides are precipitated and the structural stability is increased, thereby obtaining oil casing with a yield strength ≥580MPa (preferably 580-800MPa), an impact energy of 0℃ KV8 ≥150J (preferably 150-335J) and excellent resistance to CO2 / H2S corrosion. Detailed Implementation

[0052] The present invention will be further described below with reference to specific embodiments.

[0053] The composition of the oil casings obtained in the embodiments and comparative examples of this invention is shown in Table 1, with the balance being Fe and other unavoidable impurities. Table 2 lists the manufacturing process parameters used in the embodiments and comparative examples. Table 3 lists the product performance parameters of the embodiments and comparative examples of this invention. Table 4 lists the elemental enrichment degree of the corrosion product film of the steel pipes in the embodiments and comparative examples of this invention.

[0054] The chemical composition of Comparative Examples 1-3 does not meet the design requirements of this invention, and the tempering temperature and holding time in the tempering step of the control tube of Comparative Example 4 do not meet the technical requirements described in this invention.

[0055] In Comparative Example 1, the contents of Cr and Ni are both low, and Cr+Cu+Ni is only 3.80%. Although the Cu content of 1.5% meets the requirements, the Ni / Cu ratio is only 0.07 (lower than the lower limit of protection of this invention, 0.55). Therefore, during high-temperature hot rolling, Cu is prone to segregation at the high-temperature austenite grain boundaries, resulting in surface hot cracking and reducing the toughness of the material.

[0056] In Comparative Example 2, the Cr content was only 1.20%, and the Cr+Cu+Ni content was only 4.50%, which easily resulted in insufficient strength and poor corrosion resistance.

[0057] In Comparative Example 3, the Cu and Ni contents were both 0.10% (each lower than the lower limit of protection claimed by this invention, 0.50%). Although this did not cause copper embrittlement (Ni / Cu = 1.00), it resulted in insufficient toughness and poor corrosion resistance.

[0058] In Comparative Example 4, if the tempering temperature and holding time do not meet the requirements, the grains will coarsen, resulting in reduced toughness and poorer corrosion resistance.

[0059] The final manufactured oil casing was sampled and subjected to elemental, microstructure, and performance testing. The specific testing methods are as follows:

[0060] (1) After grinding and polishing, the sample was etched with 4% nitric acid alcohol and then the microstructure was observed under a Zeiss Axio Scope A1 optical microscope.

[0061] (2) Tensile tests were conducted using a 2000KN tensile testing machine in accordance with GB / T 228.1-2021 “Metallic materials, tensile testing - Part 1: Tensile testing at room temperature”; and in accordance with GB / T 229-2020 “Metallic materials, Charpy pendulum impact test”, a 750J instrumented impact testing machine and a low-temperature impact testing chamber were used for the test. The sample size was 10mm×10mm×55mm and the test temperature was 0℃.

[0062] (3) In accordance with NACE TM0177-2016 "Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments", the constant load NACE-A method test was conducted. The load was 80% SMYS, NACE-B solution (pH=3.5), 0.1 MPa H2S, and the sample immersion time was 720 hours. The samples of the examples and comparative examples were placed under a 10x field of view to observe whether the samples fractured or had destructive cracks.

[0063] (4) Conduct CO2 and H2S coexistence corrosion tests according to ASTM G111-97(2013) "Standard Guide for Corrosion Tests in High Temperature or High Pressure Environment, or Both". Specific test conditions: temperature 80℃, P (CO2) =2MPa, P (H2S) =0.5 MPa, using a 5 wt% NaCl solution, a flow rate of 1 m / s, and a test time of 168 h. The weights of the samples from the examples and comparative examples before and after the test were weighed. The "uniform corrosion rate" was calculated by dividing the total mass loss by the material density, total exposed area, and total exposure time. The depth of the deepest pit in the pitting corrosion of the samples from the examples and comparative examples was measured and divided by the exposure time to calculate the "maximum pitting rate".

[0064] (5) Using a Zeiss GeminiSEM 450, EDS analysis was performed on the corrosion product films of the examples and comparative examples to obtain the mass percentages of element enrichment in the corrosion product films: {Cr}%, {Ni}%, and {Cu}%.

[0065] Referring to Table 3, the microstructure of the high-strength and tough oil casing resistant to CO2 and H2S corrosion obtained by adopting the technical requirements scheme designed in this invention is tempered sorbite, and the grain size is 7.5-9.5 grade as measured by GB / T 6394-2017.

[0066] From the mechanical properties, it can be seen that:

[0067] In Comparative Example 1, the amounts of Cr and Ni are relatively small, and Cr+Cu+Ni=3.80, which meets the strength requirements. However, the loss of toughness caused by "copper brittleness" results in an impact energy of less than 150J.

[0068] In Comparative Example 2, the amount of Cr is relatively small, and Cr+Cu+Ni=4.50. Although the impact energy is greater than 150J, the Cr content cannot compensate for the strength loss caused by the low-carbon design.

[0069] In Comparative Example 3, the amounts of Cu and Ni were relatively small, resulting in impact energy of less than 150 J.

[0070] In Comparative Example 4, due to its excessively low tempering temperature and long holding time, the grains became coarse (the grain size measured according to GB / T6394-2017 was grade 6.5), resulting in an impact energy of less than 10J.

[0071] The high-strength and high-toughness oil casings of Examples 1-7 of this invention have even better performance in resisting CO2 and H2S corrosion. Their yield strength is stably controlled above 580MPa (within the range of 580-800MPa) and their impact energy at 0℃ is KV8≥150J.

[0072] Furthermore, the high-strength and tough oil casing resistant to CO2 and H2S corrosion described in this invention exhibits excellent corrosion resistance. The uniform corrosion rate, measured according to ASTM G111-97 (2013), is <0.0250 mm / a, and the maximum pitting corrosion rate is <0.0450 mm / a. The oil casing of this embodiment demonstrates excellent sulfur resistance; after immersion in 80% SMYS, NACE-B solution (pH = 3.5), and 0.1 MPa H2S for 720 h, no cracks were observed under 10x magnification. In contrast, Comparative Examples 1, 3, and 4 all showed obvious cracks under the same test conditions at 10x magnification.

[0073] Table 4 shows the enrichment degree of corrosion product film elements in the steel pipes of the embodiments and comparative examples of the present invention.

[0074] EDS results show that in the corrosion product film obtained from the oil casing of this invention, the mass percentage of Cr ({Cr} / nominal Cr content) ranges from 3.00 to 5.50; the mass percentage of Ni ({Ni} / nominal Ni content) ranges from 1.50 to 4.50; and the mass percentage of Cu ({Cu} / nominal Cu content) ranges from 1.50 to 4.50. This demonstrates that the oil casing of this invention exhibits a high enrichment of alloying elements, resulting in a uniform and dense film layer. This film layer possesses higher chemical and thermodynamic stability, alters the electrochemical system, prevents the medium from penetrating the substrate, improves resistance to uniform corrosion, and significantly reduces the tendency for pitting corrosion.

[0075] In Examples 1-7 of this invention, the corrosion product film contains 3.00≤{Cr}% / Cr%≤5.50, 1.50≤{Ni}% / Ni%≤4.50, and 1.50≤{Cu}% / Cu%≤4.50, which indicates that the enrichment of Cr, Ni, and Cu elements is relatively high, which is beneficial to the formation of a dense and continuous film.

[0076] In Comparative Example 1, the enrichment levels of Cr, Cu, and Ni elements were all low. In Comparative Example 2, the enrichment level of Cr element was low. In Comparative Example 3, the enrichment levels of Ni and Cu elements were low. This would result in a relatively loose and insufficiently dense corrosion product film, increasing the tendency for pitting corrosion. As can be seen from Table 3, the uniform corrosion rate and maximum pitting corrosion rate of Comparative Examples 1-3 were all unqualified.

[0077] In summary, the high-strength and high-toughness oil casing resistant to CO2 and H2S corrosion described in this invention has excellent mechanical properties and corrosion resistance. It can be effectively applied in the later stages of oil and gas development to avoid accidents such as perforation failure, and has excellent prospects for promotion and application value.

Claims

1. An oil casing, in addition to Fe and unavoidable impurities, the oil casing contains, in wt%, the following chemical elements: C: 0.08-0.18%, Si: 0.20-0.40%, Mn: 0.30-0.80%, Alt: 0.02-0.04%, Cr: 4.00-10.50%, Ni: 0.50-2.50%, Mo: 0.10-0.40%, Cu: 0.50-1.50%; and Cr, Cu and Ni satisfy the following relationship: 6.35% ≤ (Cr+Cu+Ni) ≤ 12.85%, Ni / Cu ≥ 0.55, preferably 0.55-3.64, where each element symbol is replaced with the corresponding element's mass percentage content.

2. The oil casing according to claim 1, wherein, The oil casing contains the following chemical elements in wt%: C: 0.08-0.18%, Si: 0.20-0.40%, Mn: 0.30-0.80%, Alt: 0.02-0.04%, Cr: 4.00-10.50%, Ni: 0.50-2.50%, Mo: 0.10-0.40%, Cu: 0.50-1.50%, with the balance being Fe and unavoidable impurities; and Cr, Cu and Ni satisfy the following relationship: 6.35% ≤ (Cr+Cu+Ni) ≤ 12.85%, Ni / Cu ≥ 0.55, preferably 0.55-3.64, where each element symbol is replaced with the corresponding element's mass percentage content.

3. The oil casing according to claim 1 or 2, wherein, Unavoidable impurities include P, S, O and H, wherein P ≤ 0.012%, S ≤ 0.0020%, O ≤ 0.006%, and H ≤ 0.0002%; preferably, P ≤ 0.010%, S ≤ 0.0018%, O ≤ 0.005%, and H ≤ 0.0001%.

4. The oil casing according to any one of claims 1-3, wherein, The microstructure of the oil casing is tempered sorbite, preferably with a grain size of 7.5 or higher, more preferably 7.5-9.5, and more preferably 8.5-9.5, as measured according to GB / T 6394-2017.

5. The oil casing according to any one of claims 1-4, wherein, The oil casing has a room temperature yield strength ≥580MPa, preferably 580-800MPa, as measured according to GB / T 228.1-2021; and a 0℃ impact energy KV8 ≥150J, preferably 150-335J, more preferably 195-335J, as measured according to GB / T 229-2020; and / or The oil casing showed no cracks after being tested under constant load using the NACE-A method according to NACE TM0177-2016; and / or The uniform corrosion rate of the oil casing, as measured according to ASTM G111-97(2013), is <0.0250 mm / a, preferably 0.0165 mm / a, and the maximum pitting rate is <0.0450 mm / a, preferably 0.0267 mm / a.

6. The oil casing according to any one of claims 1-5, wherein, After corrosion testing of the oil casing according to ASTM G111-97 (2013), the resulting corrosion product film contained: The enrichment amount of Cr ({Cr}%) satisfies: enrichment amount ({Cr}%) / nominal Cr% = 3.00-5.50; and / or The enrichment level of Ni, {Ni}%, satisfies: enrichment level {Ni}% / nominal Ni% = 1.50-4.50; and / or The enrichment of Cu, {Cu}%, satisfies the following condition: enrichment of Cu, {Cu}%, / nominal Cu%, = 1.50 - 4.

50.

7. A method for manufacturing an oil casing according to any one of claims 1-6, wherein, The method includes the following steps performed sequentially: 1) A tube blank is obtained by smelting, casting, and rolling based on the composition described in claim 1, 2, or 3; 2) The tube blank is heated, pierced, reduced in diameter by hollow billet, continuously rolled, reduced in diameter by tension and air cooled to obtain a rough tube; 3) Quenching and tempering heat treatment, including quenching and tempering. in, Quenching: Quenching temperature is 890-950℃, holding time is 30-80min, and cooling rate is 20-60℃ / s; Tempering: Tempering temperature is 580-650℃, holding time is 60-120min, and air cooling is performed.

8. The method according to claim 7, wherein, In step 2), the billet heating temperature is 1240-1280℃, the holding time is 60-120min; and / or the piercing temperature is 1200-1230℃, the piercing speed is 0.3-0.8m / s; and / or the continuous rolling temperature is 1050-1100℃, the rolling speed is 2.5-3.5m / s.

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