A stress corrosion resistant martensitic stainless steel oil sleeve and a method of manufacturing the same

Abstract

The application discloses a martensitic stainless steel oil sleeve with stress corrosion resistance, which contains Fe and inevitable impurities, and also contains the following chemical elements with mass percentage as follows: C: 0.12-0.25%, Si: 0.1-1.0%, Mn: 0.10-1.0%, Cr: 11.0-13.5%, Al: 0.01-0.04%, O: 0.002-0.005%, La+Ce: 0.01-0.04%; and 5≤(La+Ce):O≤8 is satisfied. Correspondingly, the application also discloses a manufacturing method of the above-mentioned martensitic stainless steel oil sleeve, which comprises the following steps: (1) smelting: adding rare earth in a refining process, and adopting vacuum or argon protection atmosphere; (2) pipe rolling: controlling heating temperature to be 1150-1250 DEG C, and total deformation of hot working is >3:1; (3) quenching + tempering heat treatment.

Description

Technical Field

[0001] This invention relates to a type of steel and its manufacturing method, and more particularly to an oil casing and its manufacturing method. Background Technology

[0002] In the current field of corrosion-resistant alloys, martensitic stainless steel, especially pure Cr-based martensitic stainless steel, is the lowest-cost steel that can achieve high strength. It is widely used in various corrosive conditions, such as oil and gas extraction.

[0003] In practical applications, martensitic stainless steel exhibits excellent corrosion resistance when the environment contains high concentrations of CO2. However, improving the resistance of martensitic stainless steel to hydrogen sulfide stress corrosion cracking is a significant challenge for this type of material. Enhancing the resistance of martensitic stainless steel to stress corrosion cracking (SSC) is of great importance for expanding the application range of these products.

[0004] The API 5CT standard specifies a type of martensitic stainless steel oil casing with the grade L80-13Cr, which is mainly used for the exploitation and development of oil and gas resources containing CO2 and a small amount of H2S. Its highest grade, PSL-3, requires excellent resistance to stress corrosion cracking (SSC) under certain H2S environment. The specific test standard is to test in a solution environment with 0.1 bar of H2S and pH=3.5, and no cracking should occur under 80% load. This condition is close to the limit of conventional Cr-based martensitic stainless steels such as 2Cr13.

[0005] In current technology, it is generally believed that reducing the carbon content of Cr-based martensitic stainless steel and adding several elements from Ni, Mo, and Cu can effectively improve its resistance to stress corrosion. However, all of the added alloying elements are precious metals, which will significantly increase the cost.

[0006] For example, Chinese patent document with publication number CN107849658A, publication date March 27, 2018, entitled "Stainless Steel Pipe and Manufacturing Method Thereof", discloses a martensitic stainless steel pipe with a low yield strength ratio and a manufacturing method thereof, wherein the martensitic phase has a pre-austenitic grain with a grain size of less than 8.0 according to ASTM E112. The chemical composition of this martensitic stainless steel pipe is as follows: C: less than 0.02%, Si: 0.05-1.00%, Mn: 0.1-1.0%, P: less than 0.030%, S: less than 0.002%, Ni: 5.5-8%, Cr: 10-14%, Mo: 2-4%, V: 0.01-0.1%, Ti: 0.03-0.3%, Nb: less than 0.1%, Al: 0.001-0.10%, N: less than 0.05%, Cu: less than 0.5%, Ca: 0-0.008%, Mg: 0-0.05%, B: 0-0.005%, with the balance being Fe and impurities.

[0007] For example, Chinese patent document CN108546811A, published on September 18, 2018, entitled "A Controlled Rolling Method for Fine-Grained Martensitic Aging Stainless Steel," discloses a controlled rolling method for fine-grained martensitic aging stainless steel. The finished martensitic aging stainless steel has a grain size grade of 9 or higher, and the specific chemical composition is designed as follows: C: less than 0.03%, Cr: 10.0-13.0%, Ni: 9.0-12.0%, Mo: 0.5-2.5%, Ti: 0.8-1.8%, with the remainder being Fe and impurity elements.

[0008] For example, Chinese patent document CN106399829A, published on February 15, 2017, entitled "High-strength, high-toughness, corrosion-resistant martensitic stainless steel oil well pipe and its manufacturing method," discloses a high-strength, high-toughness, corrosion-resistant martensitic stainless steel oil well pipe with the following chemical composition: C ≤ 0.03%, Si: 0.2-0.5%, Mn: 0.20-1.50%, Cr: 9.0-12.5%, Ni: 0.5-3.0%, Mo: 0.1-1.0%, V: 0.01-0.2%, Nb: 0.01-0.08%, W: 0.01-0.50%, Al: 0.005-0.050%, P: less than 0.02%, S: less than 0.005%, with the remainder consisting of Fe and impurities. This martensitic stainless steel oil well pipe has a yield strength of over 862 MPa, its microstructure is a single martensite, and its ductile-brittle transition temperature is below -80℃.

[0009] As can be seen from the above existing technologies, all of the above technical solutions have designed and added precious alloying elements such as Ni, Mo, Cu, and W to achieve the high strength and stress corrosion resistance of martensitic stainless steel pipes. However, these technical solutions cannot avoid the problem of a significant increase in cost caused by adding such precious alloying elements.

[0010] Therefore, in view of the problems existing in the prior art, the present invention aims to study and obtain a new martensitic stainless steel oil casing with lower production cost, which does not contain Ni, Mo, Cu and W elements, but still has the characteristics of high strength and resistance to stress corrosion. Summary of the Invention

[0011] One of the objectives of this invention is to provide a martensitic stainless steel oil casing resistant to stress corrosion. This martensitic stainless steel oil casing still exhibits excellent performance without the addition of Ni, Mo, Cu, and W elements. Its yield strength is greater than 552 MPa (i.e., 80 Ksi) and it is suitable for CO2 environments of any concentration below 100°C. It has excellent resistance to stress corrosion and can pass stress corrosion tests under the conditions of 1 bar H2S and 80% SMYS (SMYS is the specified minimum yield strength). It has good economic benefits and a very broad application prospect.

[0012] To achieve the above objectives, this invention provides a stress corrosion resistant martensitic stainless steel oil casing, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0013] C: 0.12~0.25%, Si: 0.1~1.0%, Mn: 0.10~1.0%, Cr: 11.0~13.5%, Al: 0.01~0.04%, O: 0.002~0.005%, La+Ce: 0.01~0.04%;

[0014] And it satisfies 5≤(La+Ce):O≤8.

[0015] Furthermore, in the martensitic stainless steel oil casing described in this invention, the mass percentage content of each chemical element is as follows:

[0016] C: 0.12–0.25%, Si: 0.1–1.0%, Mn: 0.10–1.0%, Cr: 11.0–13.5%, Al: 0.01–0.04%, O: 0.002–0.005%, La+Ce: 0.01–0.04%; balance Fe and other unavoidable impurities.

[0017] And it satisfies 5≤(La+Ce):O≤8.

[0018] Studies have shown that refining the grain size can effectively improve the resistance of martensitic stainless steel to H2S stress corrosion cracking. Therefore, without intentionally adding other precious metal elements, this invention, by rationally adding rare earth elements to pure Cr-based martensitic stainless steel and utilizing their oxide precipitation characteristics, combined with process control, can control the average size of the original austenite grains to below 10 μm, achieving a yield strength of 80 ksi steel grade. Furthermore, the steel designed in this invention also possesses resistance to stress corrosion cracking in environments with H2S partial pressure ≤1 bar. Based on the alloying properties of martensitic stainless steel, this steel is also suitable for environments with any concentration of carbon dioxide (CO2) below 100°C.

[0019] Through extensive research, the inventors discovered that adding rare earth elements La and Ce in a vacuum or argon-protected refining process allows them to combine with residual oxygen in the molten steel, effectively improving the properties of inclusions. Simultaneously, the rare earth oxides can be dispersed throughout the molten steel. Correspondingly, by combining this with appropriate heating temperatures and deformation amounts during hot working, 2Cr13 martensitic stainless steel with an average austenitic grain size of less than 10 μm can be obtained. After quenching and tempering heat treatment, this martensitic stainless steel achieves a yield strength of 80 ksi (yield strength ≥ 552 MPa) and exhibits good resistance to stress corrosion cracking in environments with H2S partial pressure ≤ 1 bar.

[0020] The design principles of each chemical element in the martensitic stainless steel oil casing of this invention are as follows:

[0021] C: In the martensitic stainless steel oil casing of this invention, carbon (C) can act as an austenite-forming element. By increasing the C content in the steel, the percentage of austenitization of stainless steel at high temperatures can be increased, thereby obtaining a martensitic structure at room temperature and improving the strength of the steel. However, for this technical solution, the C content in the steel should not be too high. When the C content in the steel is too high, the corrosion resistance of the stainless steel will decrease, and the toughness will also decrease. Therefore, in order to ensure the strength of the material and reduce the risk of ferrite precipitation, the mass percentage of C in the martensitic stainless steel oil casing of this invention is controlled between 0.12% and 0.25%.

[0022] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element C can be preferably controlled between 0.17% and 0.22%.

[0023] Si: In the martensitic stainless steel oil casing of this invention, Si is an important deoxidizer in the steelmaking process, which can play a deoxidizing role. However, Si in stainless steel with high Cr content carries the risk of promoting the formation of σ phase and ferrite phase, which have adverse effects on the toughness and corrosion resistance of stainless steel. Based on this, in order to ensure the performance of the material, the mass percentage of Si in the martensitic stainless steel oil casing of this invention is controlled between 0.1% and 1.0%.

[0024] Mn: In the martensitic stainless steel oil casing of this invention, adding an appropriate amount of Mn can improve the strength of the stainless steel. To ensure that the stainless steel has the required strength when used as an oil casing, at least 0.1% Mn needs to be added to the steel. However, it should be noted that the Mn content in the steel should not be too high; when the Mn content exceeds 1.0%, the toughness of the steel decreases. Therefore, in the martensitic stainless steel oil casing of this invention, the mass percentage of Mn is controlled between 0.10% and 1.0%.

[0025] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mn element can be preferably controlled between 0.2% and 0.5%.

[0026] Cr: In the martensitic stainless steel oil casing of this invention, Cr is an important element for improving the corrosion resistance of the steel. The addition of Cr allows the stainless steel surface to quickly form a corrosion-resistant passivation film even in air, thereby improving the oil casing's resistance to CO2 corrosion in high-temperature environments. Therefore, to obtain CO2 corrosion resistance above 500℃, the amount of Cr added to the steel should reach more than 11.0%. However, it should be noted that the Cr content in the steel should not be too high. When the Cr content in the steel exceeds 13.5%, it will increase the risk of ferrite precipitation and adversely affect the hot working performance and corrosion resistance of the product. Therefore, in the martensitic stainless steel oil casing of this invention, the mass percentage of Cr is controlled between 11.0% and 13.5%.

[0027] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Cr element can preferably be controlled between 11.5% and 13.50%.

[0028] Al: In the martensitic stainless steel oil casing of this invention, Al is an element added during the smelting process as a deoxidizer. To achieve the deoxidation effect, the amount of Al added to the steel needs to be controlled above 0.01%. However, it should be noted that the Al content in the steel should not be too high either. When the Al content in the steel exceeds 0.04%, the toughness of the steel will decrease. Therefore, in the martensitic stainless steel oil casing of this invention, the mass percentage of Al is controlled between 0.01% and 0.04%.

[0029] O: In existing steel smelting processes, oxygen (O) is usually an unavoidable impurity element. To obtain high-performance steel, deoxidation treatment is typically required to minimize the O content. However, in this invention, rare earth element oxides are used to refine austenite grains, thus requiring a certain O content. The O content does not need to be reduced to very low levels, effectively reducing the cost of the deoxidation process during smelting. However, it is important to note that the O content in the steel should not be too high. When the O content is too high, the number of inclusions increases significantly, and even the addition of La and Ce cannot prevent the formation of large inclusions, thereby reducing the steel's performance. Therefore, in the martensitic stainless steel oil casing described in this invention, the mass percentage of O is controlled between 0.002% and 0.005%.

[0030] La+Ce: La and Ce are the most common and inexpensive rare earth elements. In the martensitic stainless steel oil casing of this invention, the added La+Ce rare earth elements are a mixture with no limit on the proportion. By adding the La+Ce mixture, the inclusions in the steel can be modified and refined. The oxides of La and Ce can be dispersed in the molten steel, forming dispersed crystal nuclei during solidification, inhibiting excessive dendrite growth, thereby achieving the purpose of refining the as-cast structure. Therefore, to achieve the above purpose, the total addition of La+Ce should be ≥0.01%; however, it should be noted that when the addition of La+Ce is too high, it is easy to form coarse inclusions, which will reduce the corrosion resistance of the steel. Therefore, its total content should be limited to no more than 0.04%. Based on this, in the martensitic stainless steel oil casing of this invention, the addition of La+Ce is specifically controlled between 0.01% and 0.04%.

[0031] Additionally, it is important to note that the addition of La+Ce requires effective synergy with O to form a dispersed oxide, thus preventing excessively large oxide sizes or insufficient oxide precipitation. Therefore, to ensure the performance of the obtained steel, in the martensitic stainless steel oil casing described in this invention, while controlling the mass percentage content of a single chemical element, it is also necessary to control the mass percentage content of the element to meet the condition 5 ≤ (La+Ce):O ≤ 8.

[0032] Furthermore, in the martensitic stainless steel oil casing of the present invention, if it contains at least one of Ni, Mo, Cu, and W, then they are all residual elements in the smelting process, and Ni+Mo+Cu+W≤0.5%.

[0033] In this invention, a martensitic stainless steel oil casing that is as inexpensive as possible is designed, which does not contain precious metal elements such as Ni, Mo, Cu, and W. Therefore, even if the addition of these precious alloying elements is proven to be beneficial, they will not be added intentionally.

[0034] However, in the actual preparation process, during the smelting process, a small amount of the above-mentioned Ni, Mo, Cu and W elements may inevitably be introduced from the raw materials. Generally speaking, the total content of Ni+Mo+Cu+W is ≤0.5%.

[0035] Furthermore, in the martensitic stainless steel oil casing of the present invention, the mass percentage content of each chemical element also satisfies at least one of the following conditions:

[0036] C: 0.17–0.22%;

[0037] Mn: 0.2-0.5%;

[0038] Cr: 11.5%–13.50%.

[0039] Furthermore, in the martensitic stainless steel oil casing of the present invention, among the unavoidable impurities, P≤0.02%, N≤0.02%, and S≤0.01%.

[0040] In the above technical solution of the present invention, P, N and S are all impurity elements in steel. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the material should be reduced as much as possible.

[0041] P: In this invention, phosphorus (P) is a harmful element that reduces the CO2 corrosion resistance of stainless steel oil casing at high temperatures and also adversely affects the hot working properties of the steel. When the P content in the steel exceeds 0.02%, the corrosion resistance of the steel cannot meet the requirements of high-temperature environments. Therefore, in this invention, the mass percentage of P is controlled to P ≤ 0.02%. In some preferred embodiments, it can be preferably controlled to P ≤ 0.015%.

[0042] N: In this invention, although N is an element that improves the pitting corrosion resistance of stainless steel, its application is mainly reflected in aqueous solution systems. Since N can act as interstitial atoms, filling the alloy's crystal lattice, it reduces the toughness of stainless steel while increasing its hardness. Therefore, in this invention, the mass percentage of N is controlled to be N ≤ 0.02%.

[0043] S: In this invention, sulfur (S) not only reduces the hot working performance of martensitic stainless steel oil casing, but also adversely affects its impact toughness. When the sulfur content in the steel exceeds 0.01%, the steel pipe cannot be manufactured normally. Therefore, in this invention, the mass percentage of sulfur is controlled to S ≤ 0.01%. Of course, in some preferred embodiments, it can be preferably controlled to S ≤ 0.005%.

[0044] Furthermore, in the martensitic stainless steel oil casing of the present invention, the average size of the original austenite grains is less than 10 μm.

[0045] In this technical solution designed in this invention, by controlling the addition and morphology of rare earth oxides in steel, the average size of the original austenite grains in the prepared martensitic stainless steel sleeve can be made to be below 10 μm, thereby obtaining excellent resistance to H2S stress corrosion and resistance to carbon dioxide and chloride ion corrosion.

[0046] Furthermore, in the martensitic stainless steel oil casing of the present invention, rare earth oxides are dispersedly distributed, the size of which is 100-300 nm and the number of which is 200-500 per cm. 2 .

[0047] Furthermore, in the martensitic stainless steel oil casing of the present invention, its yield strength is ≥552MPa, it has the ability to resist stress corrosion cracking in an environment with H2S partial pressure ≤1bar, and it is suitable for CO2 environments of any concentration below 100℃.

[0048] Accordingly, another objective of the present invention is to provide a method for manufacturing martensitic stainless steel oil casing. This method is simple to produce, and the resulting martensitic stainless steel oil casing not only has excellent strength but also good resistance to stress corrosion cracking, which has a very broad application prospect.

[0049] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned martensitic stainless steel oil casing, comprising the following steps:

[0050] (1) Smelting: Rare earth elements are added during the refining process, and a vacuum or argon protective atmosphere is used.

[0051] (2) Rolling: The heating temperature is controlled at 1150~1250℃, and the total deformation of hot working is >3:1;

[0052] (3) Quenching and tempering heat treatment.

[0053] In the above technical solution of the present invention, in step (1), in order to obtain the combination effect of La+Ce with oxygen in steel, rare earth needs to be added in the refining process during the smelting process, and it needs to be added in a vacuum or argon protective atmosphere.

[0054] Furthermore, in the rolling process of step (2) above, in order to obtain fine-grained microstructure and properties, the heating temperature of the steel ingot needs to be controlled between 1150 and 1250°C. When the heating temperature is too low, the deformation resistance of the steel is too high, resulting in excessive processing difficulty. In addition, excessively high heating temperature will cause grain coarsening, which will reduce the performance of the steel.

[0055] In addition, in this tube rolling process, it is also necessary to control the total hot working deformation to be greater than 3:1. When the total hot working deformation is less than 3:1, it will lead to grain coarsening and affect the final performance.

[0056] Furthermore, in the manufacturing method described in this invention, in step (3), the quenching heating temperature is 900-1000℃, and then cooled to room temperature.

[0057] Furthermore, in the manufacturing method described in this invention, in step (3), the tempering heating temperature is 650-750°C, and then the temperature is cooled to room temperature.

[0058] This invention does not impose any special limitations on the quenching and tempering heat treatment process. Operators can use conventional quenching and tempering heat treatment processes in the field for tempering treatment.

[0059] Of course, in some embodiments, the quenching heating temperature can preferably be controlled at 900-1000℃, and then cooled to room temperature by air cooling, oil cooling or other cooling methods; at the same time, the tempering heating temperature can preferably be controlled at 650-750℃, and then cooled to room temperature by air cooling.

[0060] Compared with the prior art, the martensitic stainless steel oil casing and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0061] In this invention, without intentionally adding other precious metal elements, the inventors rationally added rare earth elements to pure Cr-based martensitic stainless steel. Utilizing the precipitation characteristics of their oxides and combined with process control, the average size of the original austenite grains can be effectively controlled to below 10 μm. This allows the yield strength to reach 80 ksi steel grade (yield strength ≥ 552 MPa), while also possessing resistance to stress corrosion cracking in environments with H2S partial pressure ≤ 1 bar. Furthermore, based on the alloying properties of martensitic stainless steel, the martensitic stainless steel oil casing designed in this invention can be effectively applied to environments with any concentration of carbon dioxide (CO2) below 100℃.

[0062] Therefore, by adopting the technical solution designed in this invention, high-performance 2Cr13 oil casing can be obtained. It has low production cost and features high strength and resistance to stress corrosion, thus having good economic benefits and a very broad application prospect. Attached Figure Description

[0063] Figure 1 The image shows the metallographic structure of the martensitic stainless steel oil casing of Example 5 under a 500x optical microscope. Detailed Implementation

[0064] The martensitic stainless steel oil casing and its manufacturing method described in this invention will be further explained and described below with reference to specific embodiments. However, this explanation and description do not constitute an improper limitation on the technical solution of this invention.

[0065] Examples 1-6 and Comparative Examples 1-5

[0066] The martensitic stainless steel oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-5 were all prepared using the following steps:

[0067] (1) Smelting according to the chemical composition shown in Table 1 below: rare earth is added in the refining process and a vacuum or argon protective atmosphere is used.

[0068] (2) Rolling: The tube is fed into a ring furnace for heating, and the heating temperature is controlled at 1150~1250℃. Then, it is made into a seamless steel pipe through processes such as piercing and continuous rolling, and the total deformation of hot working is controlled to be >3:1.

[0069] (3) Quenching and tempering heat treatment: In the quenching step, the quenching heating temperature is controlled at 900-1000℃, and then air cooling, oil cooling or other cooling methods can be used to cool to room temperature; In the tempering step, the tempering heating temperature is controlled at 650-750℃, and then air cooling can be used to cool to room temperature.

[0070] In Examples 1-6 of the present invention, the chemical composition design and related processes of the martensitic stainless steel oil casings in Examples 1-6 all meet the design specifications of the present invention. Furthermore, to verify the superiority of the technical solution designed in this invention, comparative oil casings of Comparative Examples 1-5 are also provided in this invention. The comparative oil casings of Comparative Examples 1-5 all have parameters in their chemical composition design that do not meet the design specifications of the present invention.

[0071] Table 1 lists the mass percentage of each chemical element in the martensitic stainless steel oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-5.

[0072] Table 1. (Balance represents Fe and other unavoidable impurities besides P, N, and S)

[0073]

[0074] Note: In Table 1 above, Ni, Mo, Cu, and W are all residual elements from the smelting process; in the formula (La+Ce):O, the mass percentages of "La+Ce" and "O" are respectively included.

[0075] Table 2 lists the specific process parameters of the martensitic stainless steel oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-5 in the above process steps.

[0076] Table 2.

[0077]

[0078]

[0079] It should be noted that, in this invention, after completing the above manufacturing process, samples of the martensitic stainless steel oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-5 can be taken respectively, and the microstructure of the sample pipes of Examples 1-6 and Comparative Examples 1-5 can be observed. It can be found that the pipes of each example and comparative example have diffusely distributed rare earth oxides, and the size and quantity of rare earth oxides obtained in the sample pipes of Examples 1-6 and Comparative Examples 1-5 can be further obtained.

[0080] Furthermore, based on this observation and analysis, the average size of the original austenite grains in the sample tubes of Examples 1-6 and Comparative Examples 1-5 can also be obtained. The relevant observation and analysis results are listed in Table 3 below.

[0081] Table 3 lists the microstructure observation results of the martensitic stainless steel oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-5.

[0082] Table 3.

[0083]

[0084] As can be seen from Table 3 above, in this invention, the average size of the original austenite grains in the martensitic stainless steel oil casings of Examples 1-6 is all below 10 μm. Furthermore, the martensitic stainless steel oil casings of Examples 1-6 also have dispersed rare earth oxides, the size of which is specifically between 120-290 nm, and the number of rare earth oxides is 210-480 per cm³. 2 .

[0085] The average size of the original austenite grains in the comparative oil casings prepared in Comparative Examples 1-5 all significantly exceeded 10 μm, surpassing the design requirements of this invention; furthermore, the size of their rare earth oxides also did not meet the design requirements of this invention. While the amount of rare earth oxides in Comparative Examples 3-4 met the design requirements of this invention, the amount of rare earth oxides in Comparative Examples 1, 2, and 5 still did not meet the design requirements of this invention.

[0086] Accordingly, after completing the above observation and analysis of the metallographic structure, in order to further illustrate the excellent performance of the martensitic stainless steel oil casing prepared by the present invention, the inventors took samples of the martensitic stainless steel oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-5 again, and further tested the performance of the sample pipes of Examples 1-6 and Comparative Examples 1-5 to obtain the corresponding strength and corrosion resistance. The relevant test results are listed in Table 4 below.

[0087] The relevant testing methods are as follows:

[0088] Strength test: The manufactured steel pipes are processed into API arc-shaped specimens, tested according to API standards, and the average values ​​are taken to obtain the yield strength and tensile strength values ​​of the steel pipes in each embodiment and comparative example.

[0089] CO2 and Cl at high temperatures - Coexistence corrosion test: The sample pipes of Examples 1-6 and Comparative Examples 1-5 were immersed in liquid in an autoclave, and the temperature in the autoclave was specifically controlled at 100℃, the partial pressure of CO2 at 4MPa, and the Cl... - The concentration was 100,000 mg / L, the liquid flow rate was 1 m / s, the test time was controlled at 240 h, and the sample weights before and after the test were compared to calculate the uniform corrosion rate of the steel pipes in each embodiment and comparative example.

[0090] H2S stress corrosion resistance test: Method A of NACE TM0177 standard was used to test the sample pipes of each example and comparative example. The test solution contained 5% NaCl and 0.4% CH3COONa, with the pH adjusted to 4.0 using CH3COOH. The H2S partial pressure was controlled at 1 bar, the test load was 80% SMYS, and the test period was 30 days. After the test, the sample pipes of Examples 1-6 and Comparative Examples 1-5 were taken out and observed for the presence of macroscopic and microscopic cracks to determine their resistance to stress corrosion cracking.

[0091] Table 4 lists the test results of the martensitic stainless steel oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-5.

[0092] Table 4.

[0093]

[0094] As can be seen from Table 4, compared with the comparative oil casings of Comparative Examples 1-5, the martensitic stainless steel oil casings of Examples 1-6 of the present invention have significant advantages and superior performance.

[0095] The martensitic stainless steel oil casings of Examples 1-6 of this invention have a yield strength between 568-652 MPa and a tensile strength between 629-721 MPa, which meets the requirements of 80 ksi steel grade. Correspondingly, the martensitic stainless steel oil casings of Examples 1-6 exhibit good resistance to CO2 and Cl at 100℃. - The uniform corrosion rate in the coexistence environment is between 0.015 and 0.04 mm / a, and it exhibits excellent resistance to uniform corrosion in an environment containing CO2 and high Cl ion concentration at 100℃.

[0096] Furthermore, the martensitic stainless steel oil casings of Examples 1-6 designed in this invention also have excellent resistance to stress corrosion cracking. They can pass the stress corrosion test under the conditions of 1 bar H2S and 80% SMYS (SMYS is the specified minimum yield strength), and the pipes after the test are all free of cracks.

[0097] Accordingly, by analyzing Comparative Examples 1-5 designed in this invention in conjunction with Tables 1, 2, 3, and 4, it is not difficult to find that:

[0098] In Comparative Examples 1 and 2, the chemical element composition design of "(La+Ce):O" exceeded the scope defined by this invention, which led to a decrease in the stress corrosion resistance of the final product pipe and caused it to crack after the H2S stress corrosion test.

[0099] While the chemical composition of Comparative Example 3 met the design requirements, the heating temperature used in its tube blank heating process was too high, at 1270℃, which exceeded the design range of this invention. This also reduced the stress corrosion resistance of the finished tube, and it fractured after the H2S stress corrosion test.

[0100] Although the chemical composition of Comparative Example 4 meets the design requirements, its total deformation during hot working in the rolling process is insufficient, resulting in coarse grains. Furthermore, the final finished pipe will fracture after the H2S stress corrosion test.

[0101] While the chemical composition of Comparative Example 5 met the design requirements, the tempering temperature used in the quenching and heat treatment process was too high, resulting in the final product pipe failing to reach a strength of 80 Ksi.

[0102] Figure 1The image shows the metallographic structure of the martensitic stainless steel oil casing of Example 5 under a 500x optical microscope.

[0103] like Figure 1 As shown, in this embodiment, the martensitic stainless steel oil casing of Example 5 has a fine grain size with an average grain size of 9.5 μm.

[0104] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0105] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A martensitic stainless steel oil casing resistant to stress corrosion, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.12–0.25%, Si: 0.1–1.0%, Mn: 0.10–1.0%, Cr: 11.0–13.5%, Al: 0.01–0.04%, O: 0.002–0.005%, La+Ce: 0.01–0.04%; the balance is Fe and other unavoidable impurities. And it satisfies 5≤(La+Ce): 0≤8; The martensitic stainless steel oil casing has dispersed rare earth oxides with a size of 100-300 nm and a quantity of 200-500 oxides / cm². 2 ; The martensitic stainless steel oil casing has a yield strength ≥552MPa, is resistant to stress corrosion cracking in an environment with H2S partial pressure ≤1bar, and is suitable for CO2 environments of any concentration below 100℃.

2. The martensitic stainless steel oil casing as described in claim 1, characterized in that, If it contains at least one of Ni, Mo, Cu, and W, then they are all residual elements from the smelting process, and Ni+Mo+Cu+W≤0.5%.

3. The martensitic stainless steel oil casing as described in claim 1, characterized in that, The mass percentage content of each chemical element also satisfies at least one of the following conditions: C：0.17～0.22%； Mn: 0.2–0.5%; Cr：11.5～13.50%。 4. The martensitic stainless steel oil casing as described in claim 1, characterized in that, In unavoidable impurities, P ≤ 0.02%, N ≤ 0.02%, S ≤ 0.01%.

5. The martensitic stainless steel oil casing as described in claim 1, characterized in that, Its original austenite grain size is less than 10 μm.

6. The method for manufacturing a martensitic stainless steel oil casing as described in any one of claims 1-5, characterized in that, It includes the following steps: (1) Smelting: Rare earth elements are added during the refining process, and a vacuum or argon protective atmosphere is used; (2) Rolling: The heating temperature is controlled at 1150~1250℃, and the total deformation of hot working is >3:1; (3) Quenching and tempering heat treatment.

7. The manufacturing method as described in claim 6, characterized in that, In step (3), the quenching heating temperature is 900-1000℃, and then it is cooled to room temperature.

8. The manufacturing method as described in claim 6, characterized in that, In step (3), the tempering heating temperature is 650-750℃, and then it is cooled to room temperature.

Images