An austenitic stainless steel tubing for sour oil and gas wells and a method of processing the same

Abstract

The application relates to the technical field of oil and gas pipes, in particular to an austenitic stainless steel oil pipe for acid corrosion oil and gas wells and a processing method. The austenitic stainless steel oil pipe for acid corrosion oil and gas wells is prepared from the following chemical elements in percentage by weight: C: 0.03% or less, Si: 0.15%-0.3%, P: 0.010% or less, S: 0.005% or less, Cr: 20.0%-24.0%, Mn: 14.0%-20%, Ni: 5.0%-10.0%, Mo: 3.0%-5.0%, Cu: 0.25%-0.50%, N: 0.4%-0.6%, V: 0.1% or less, Nb: 0.1% or less, Ca: 0.001%-0.003%, Re: 0.01-0.03%, and the balance of Fe and inevitable impurities. The oil pipe meets the mechanical property requirements of 110ksi steel grade oil pipes, has a yield strength of 760-880 MPa, a tensile strength of 900-1050 MPa, an elongation of 40% or more, a full-size transverse impact energy at -10 DEG C of 110 J or more, a full-size longitudinal impact energy of 130 J or more, an austenitic structure, high strength and toughness matching, excellent corrosion resistance, and is used in the harsh service conditions of acid oil and gas wells containing hydrogen sulfide, carbon dioxide and chloride ions and other complex corrosion media.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil and gas pipes, in particular to an austenitic stainless steel oil pipe for acid corrosion oil and gas wells and a processing method thereof. BACKGROUND

[0002] With the increasingly harsh environment of oil and gas resources exploitation, the mechanical properties and corrosion resistance of the pipe string are increasingly required by oil and gas wells, especially for the corrosion environment of high temperature, high pressure, high concentration of carbon dioxide, a small amount of hydrogen sulfide or chloride ions in the downhole. The conventional carbon steel or low alloy steel oil well pipe is prone to corrosion and cracking during long-term work in such environment, and even causes major safety accidents. In view of such harsh corrosion service conditions, stainless steel or nickel-based alloy products are generally required to meet the corrosion resistance requirements, but the limited nature of nickel resources and the high cost of corrosion-resistant oil well pipes limit further widespread application. Therefore, developing oil well pipe products with good strength and toughness, good corrosion resistance and relative economy has important engineering significance for solving the corrosion problem of the pipe string under harsh corrosion conditions.

[0003] The existing stainless steel oil well pipe generally adopts martensitic structure. International patent WO2017 / 162160 A1 discloses a martensitic stainless steel oil sleeve resistant to H2S stress corrosion cracking, and the main characteristics of the chemical elements are as follows: C: <0.05%, Cr: 11-14%, Ni: 4-7%, Mo: 1.5-2.5%; the metal structure is mainly tempered martensite, which can be used in oil wells and gas wells of crude oil or natural gas in a strong corrosion environment coexisting with high concentration of CO2 and Cl - , but its H2S applicable environment is limited to 0.01 MPa, and the strength is only 95 ksi. International patent WO2005 / 007915A1 discloses a martensitic stainless steel with superior H2S stress corrosion resistance than super 13Cr steel, and the main characteristics are as follows: by adding Mo with a maximum of 10%, by controlling the solid solution Mo content in the range of 3.5-7%, the corrosion resistance and H2S (hydrogen embrittlement) stress corrosion resistance are improved. This kind of martensitic stainless steel has high mechanical strength and excellent corrosion resistance in CO2 environment, but the applicable H2S partial pressure is only 0.003 MPa. SUMMARY

[0004] In view of the above problems, the purpose of the present application is to provide an austenitic stainless steel oil pipe for acid corrosion oil and gas wells and a processing method thereof. The austenitic stainless steel oil pipe has high strength and toughness, excellent corrosion resistance, and economy, and can be used in acid oil and gas wells with complex corrosion media such as hydrogen sulfide, carbon dioxide and chloride ions.

[0005] The technical scheme of the present application is that an austenitic stainless steel oil pipe for acid corrosion oil and gas well, the chemical element composition of the oil pipe is as follows in percentage by weight: C≤0.03%, Si: 0.15%-0.3%, P: ≤0.010%, S: ≤0.005%, Cr: 20.0%-24.0%, Mn: 14.0%-20%, Ni: 5.0%-10.0%, Mo: 3.0%-5.0%, Cu: 0.25%-0.50%, N: 0.4%-0.6%, V: ≤0.1%, Nb: ≤0.1%, Ca: 0.001%-0.003%, Re: 0.01-0.03%, and the balance is Fe and inevitable impurities.

[0006] A processing method of the austenitic stainless steel oil pipe for acid corrosion oil and gas well, comprising the following steps:

[0007] S1: preparing austenitic stainless steel ingot: under the protection of 0.1-0.5 MPa nitrogen atmosphere, using micro-carbon chromium iron, metal manganese, nickel-iron alloy, molybdenum iron and industrial pure iron as raw materials, adding aluminum or silicon steel deoxidizer, smelting by vacuum induction furnace, controlling the smelting temperature in the range of 1500-1550 DEG C, adding nitrogen alloy, then controlling the pouring temperature in the range of 1450-1480 DEG C for nitrogen protection pouring, to prepare the austenitic stainless steel ingot;

[0008] S2: electroslag remelting: preparing the austenitic stainless steel ingot into consumable electrode for electroslag remelting, and carrying out electroslag remelting under nitrogen protection, selecting corresponding slag system and using Si or Ca for deoxidation in the process of electroslag remelting, and carrying out nondestructive testing and inclusion analysis after electroslag remelting;

[0009] S3: austenitic stainless steel ingot forging: forging the austenitic stainless steel ingot after electroslag remelting, wherein the austenitic stainless steel ingot heating temperature is 1200-1250 DEG C, the holding time is 2-3 hours, the austenitic stainless steel ingot with a diameter of 300-350 mm is forged into a steel ingot with a diameter of 200-220 mm, and the final forging temperature is controlled to be not lower than 1050 DEG C;

[0010] S4: pipe preparation: preparing the forged austenitic stainless steel ingot into oil pipe blank by hot extrusion, preparing the oil pipe blank with required specifications by multi-pass cold rolling, and obtaining the finished oil pipe blank with austenitic structure by full pipe body solid solution treatment, wherein annealing is carried out between each cold rolling pass;

[0011] S5: carrying out nondestructive testing on the finished oil pipe blank, carrying out pipe end threading, coupling screwing, hydrostatic test, protecting ring mounting, marking spraying and painting after the nondestructive testing is qualified, to prepare the finished oil pipe.

[0012] After the S1 austenitic stainless steel ingot is prepared, element analysis, flaw detection and machining treatment are carried out on the austenitic stainless steel ingot to remove surface defects and black skin.

[0013] The pitting corrosion equivalent value PREN of the austenitic stainless steel ingot material in the S1 is greater than or equal to 40.

[0014] In the S4 pipe manufacturing process, the austenitic stainless steel blank pipe is cold rolled by 5 passes, wherein the reduction ratio of the first 3 passes is 18% to 25%, the reduction ratio of the fourth pass is 5% to 10%, and the reduction ratio of the fifth pass is 10% to 15%.

[0015] In the S4 pipe manufacturing process, the temperature of the austenitic stainless steel blank pipe needs to be controlled at 450 DEG C to 550 DEG C during cold rolling, and the annealing temperature between cold rolling passes is 650 DEG C to 700 DEG C, and the time is 1 to 3 hours.

[0016] In the S4 pipe manufacturing process, the solid solution treatment heat treatment parameters of the pipe blank are that the temperature is greater than 1100 DEG C, the time is greater than 4 hours, and the air cooling is to room temperature.

[0017] In the S4 pipe manufacturing process, the microstructure of the finished oil pipe blank after the pipe blank solid solution treatment is austenitic structure, and the content of delta phase is less than 3%.

[0018] In the S4 pipe manufacturing process, the yield strength of the processed oil pipe blank is 760 MPa to 880 MPa, the tensile strength is 900 MPa to 1050 MPa, the elongation is greater than or equal to 40%, the full-size transverse impact energy at-10 DEG C is greater than or equal to 110 J, and the full-size longitudinal impact energy is greater than or equal to 130 J.

[0019] The micro-carbon chromium iron used in the application contains less than 0.15% of carbon, and is mainly used to improve the oxidation resistance and corrosion resistance of steel, so that a strong oxide film is formed on the surface of the steel in the oxidation atmosphere.

[0020] The beneficial effects of the present application are: 1. The present application replaces part of the expensive nickel element with inexpensive manganese and nitrogen elements, and adds appropriate alloying elements, which not only reduces the cost of stainless steel alloy, but also ensures the high temperature hot plasticity, hot working performance, good microstructure stability of austenitic stainless steel, avoids the cracking problem of the oil pipe blank during the extrusion process, reduces the harmful phase precipitation sensitivity of the oil pipe during the heat treatment process, and ensures that the oil pipe product has good strength and toughness and corrosion resistance; 2. The present application adopts 5-pass cold rolling reduction during pipe manufacturing, wherein the reduction ratio of the first 3 passes is 18%~25% to avoid the problem of pipe blank cracking caused by excessive reduction, the reduction ratio of the fourth pass is 5%~10%, and the reduction ratio of the fifth pass is 10%~15%. The reduction ratio of the fifth pass is slightly increased compared with the reduction ratio of the fourth pass, and combined with the subsequent full pipe body solid solution heat treatment process, the grain size can be refined, the specific surface area of the grain boundary can be increased, and good comprehensive mechanical properties and corrosion properties can be obtained; 3. The present application adopts temperature control cold rolling at 450℃~550℃ during cold rolling reduction, and annealing at 650℃~700℃ close to the recrystallization temperature between passes. On the one hand, temperature control cold rolling can reduce the high dislocation density formed in the cold rolling reduction due to the addition of nitrogen element to enhance the work hardening capacity of austenitic stainless steel, thereby reducing the problem of pipe blank cracking in the cold rolling process due to the change of lattice structure of metastable austenite caused by partial martensite transformation; on the other hand, high temperature annealing can reduce or eliminate the deformed structure and high density dislocations, and prepare for the next pass of cold rolling; 4. The oil pipe of the present application meets the mechanical property requirements of 110ksi steel grade oil pipe, the yield strength is 760MPa~880MPa, the tensile strength is 900MPa~1050MPa, the elongation is ≥40%, the full-size transverse impact energy at-10℃ is ≥110J, the full-size longitudinal impact energy is ≥130J, the microstructure is austenitic structure, the product has high strength and toughness, excellent corrosion resistance, and can be applied to harsh service conditions of acid oil and gas wells containing hydrogen sulfide, carbon dioxide and chloride ions and other complex corrosion media. DETAILED DESCRIPTION

[0021] The present application will be further described in detail below in combination with examples:

[0022] Example 1

[0023] An austenitic stainless steel tubing for sour gas well corrosion, the chemical element composition of the tubing is as follows in percentage by weight: C: 0.03% or less, Si: 0.15%-0.3%, P: 0.010% or less, S: 0.005% or less, Cr: 20.0%-24.0%, Mn: 14.0%-20%, Ni: 5.0%-10.0%, Mo: 3.0%-5.0%, Cu: 0.25%-0.50%, N: 0.4%-0.6%, V: 0.1% or less, Nb: 0.1% or less, Ca: 0.001%-0.003%, Re: 0.01-0.03%, and the balance of Fe and inevitable impurities.

[0024] The present application replaces part of the expensive nickel element with inexpensive manganese and nitrogen elements, and adds appropriate alloying elements, thereby not only reducing the cost of the stainless steel alloy, but also ensuring the high-temperature hot plasticity, hot working performance, good microstructure stability of the austenitic stainless steel, avoiding cracking problems in the extrusion process of the tubing blank, reducing the harmful phase precipitation sensitivity in the heat treatment process of the tubing, and ensuring good strength and toughness and corrosion resistance of the tubing product.

[0025] Example 2

[0026] A processing method of an austenitic stainless steel tubing for sour gas well corrosion, comprising the following steps:

[0027] S1: preparing an austenitic stainless steel ingot: under a nitrogen protective atmosphere of 0.1-0.5 MPa, using micro-carbon chromium iron, metallic manganese, nickel-iron alloy, molybdenum iron, and industrial pure iron as raw materials, adding aluminum or silicon steel deoxidizer, smelting by a vacuum induction furnace, controlling the smelting temperature in the range of 1500-1550 DEG C, adding a nitrogen alloy, then controlling the pouring temperature in the range of 1450-1480 DEG C for nitrogen protection pouring, and preparing the austenitic stainless steel ingot;

[0028] S2: electroslag remelting: preparing the austenitic stainless steel ingot into an electroslag remelting consumable electrode, and performing electroslag remelting under nitrogen protection, selecting a corresponding slag system and using Si or Ca for deoxidation during the electroslag remelting process, and performing nondestructive testing and inclusion analysis after electroslag remelting;

[0029] S3: austenitic stainless steel ingot forging: forging the austenitic stainless steel ingot after electroslag remelting, wherein the austenitic stainless steel ingot heating temperature is 1200-1250 DEG C, the holding time is 2-3 hours, the austenitic stainless steel ingot with a diameter of 300-350 mm is forged into a steel ingot with a diameter of 200-220 mm, and the final forging temperature is controlled to be not lower than 1050 DEG C;

[0030] S4: pipe making: the forged austenitic stainless steel ingot is prepared into the blank pipe of the oil pipe by hot extrusion, the blank pipe is prepared into the oil pipe blank of the required specification by multi-pass cold rolling, and the full pipe body solid solution treatment is carried out to obtain the finished oil pipe blank with austenitic structure.

[0031] S5: the finished oil pipe blank is subjected to nondestructive testing, and after the testing is qualified, the pipe end is threaded, the coupling is screwed, the hydrostatic test is carried out, the protective ring is mounted, the mark is sprayed and the paint is applied to prepare the finished oil pipe.

[0032] After the preparation of the austenitic stainless steel ingot in S1 is completed, element analysis, testing and machining treatment are carried out on the austenitic stainless steel ingot to remove surface defects and black skin.

[0033] The pitting resistance equivalent value PREN of the austenitic stainless steel ingot in S1 is greater than or equal to 40.

[0034] In the pipe making process of S4, the austenitic stainless steel blank pipe is cold rolled by 5 passes, wherein the reduction ratio of the first 3 passes is 18% to 25%, the reduction ratio of the fourth pass is 5% to 10%, and the reduction ratio of the fifth pass is 10% to 15%.

[0035] In the pipe making process, the austenitic stainless steel blank pipe is cold rolled by 5 passes, wherein the reduction ratio of the first 3 passes is 18% to 25% to avoid the problem of pipe blank cracking caused by excessive reduction, the reduction ratio of the fourth pass is 5% to 10%, and the reduction ratio of the fifth pass is 10% to 15%. Compared with the reduction ratio of the fourth pass, the reduction ratio of the fifth pass is slightly increased, which, in combination with the subsequent full pipe body solid solution heat treatment process, can refine the grain size, increase the specific surface area of the grain boundary, and obtain good comprehensive mechanical properties and corrosion performance.

[0036] In the pipe making process of S4, the temperature of the austenitic stainless steel blank pipe needs to be controlled in the range of 450 DEG C to 550 DEG C during cold rolling, and the annealing temperature between cold rolling passes is in the range of 650 DEG C to 700 DEG C, and the time is 1 to 3 hours.

[0037] The introduction of nitrogen element can significantly improve the work hardening capacity of austenitic stainless steel, resulting in a high cold deformation work hardening index. The temperature control cold rolling in the range of 450 DEG C to 550 DEG C reduces the dislocation density. From the surface, the metastable austenite can be controlled due to cold deformation, part of the martensite transformation occurs, and the pipe blank cracking problem in the cold rolling process is reduced. The annealing temperature is close to the recrystallization temperature between 650 DEG C and 700 DEG C, which reduces or eliminates the deformed structure and high density dislocation, and prepares for the next cold rolling.

[0038] In the pipe making process of S4, the pipe blank solid solution treatment heat treatment parameters are: the temperature is greater than 1100 DEG C, the time is greater than 4 hours, and the air cooling is carried out to room temperature.

[0039] In the S4 pipe manufacturing process, the microstructure of the finished oil pipe blank after the pipe blank solid solution treatment is austenite, and the delta phase content is less than 3%.

[0040] In the S4 pipe manufacturing process, the yield strength of the processed oil pipe blank is 760MPa~880MPa, the tensile strength is 900MPa~1050MPa, the elongation is ≥40%, the full-size transverse impact energy at-10℃ is ≥110J, and the full-size longitudinal impact energy is ≥130J.

[0041] The oil pipe meets the mechanical property requirements of 110ksi steel grade oil pipe, the yield strength is 760MPa~880MPa, the tensile strength is 900MPa~1050MPa, the elongation is ≥40%, the full-size transverse impact energy at-10℃ is ≥110J, and the full-size longitudinal impact energy is ≥130J, the microstructure is austenite, the product strength and toughness are matched, the corrosion resistance is excellent, and the oil pipe can be applied to the harsh service conditions of sour oil and gas wells containing hydrogen sulfide, carbon dioxide and chloride ions and other complex corrosion media.

[0042] Examples 3, 4 and 5 all use the processing method of the austenitic stainless steel oil pipe for sour corrosion oil and gas wells described in Example 2 to process the austenitic stainless steel oil pipe for sour corrosion oil and gas wells, the melting specific chemical components of the austenitic stainless steel oil pipe in Examples 3~5 are shown in Table 1, and the balance is iron and inevitable impurities.

[0043] Table 1 Melting chemical components of the austenitic stainless steel oil pipe in Examples 3~5 (% by weight)

[0044]

[0045] Note: PREN= w(Cr)+3.3*w(Mo)+16*w(N)

[0046] As can be seen from Table 1, by adding N and Mn inexpensive elements to replace expensive nickel elements, the PREN≥40 of the three components of Cr, manganese and nitrogen not only ensures the austenitic structure of the stainless steel, but also reduces the alloy cost of the stainless steel.

[0047] The solid solution treatment process and the mechanical property test results of the austenitic stainless steel pipe blanks in Examples 3-5 are shown in Table 2, wherein three test samples are taken from each test steel after the solid solution treatment to test the tensile strength, yield strength and toughness, and the average value of the three test samples is recorded in Table 2 as the result. The pitting and uniform corrosion performance test results of the austenitic stainless steel oil pipes in Examples 3-5 are shown in Table 3, three test samples are taken to test the corrosion rate, and the average value of the three test samples is recorded in Table 3 as the result, wherein the uniform corrosion condition is that the total test pressure is 15 MPa, the CO2 partial pressure is 7.6 MPa, the H2S partial pressure is 1.0 MPa, and the test temperature is 160℃.

[0048] Table 2 The solid solution treatment process and the mechanical property test results of the austenitic stainless steel pipe blanks in Examples 3-5

[0049]

[0050] Table 3 The pitting and uniform corrosion performance of the austenitic stainless steel oil pipes in Examples 3-5

[0051]

[0052] As shown in Table 2, after the chemical composition is treated by the appropriate solid solution process, the yield strength of the austenitic stainless steel oil pipe is 765 MPa-875 MPa, the tensile strength is 930 MPa-1020 MPa, the elongation is 40%-55%, the full-size transverse impact energy at-10℃ is 115 J-134 J, and the full-size longitudinal impact energy at-10℃ is 137 J-156 J, so the product has excellent strength and toughness matching, and can fully meet the performance requirements of P110 steel grade oil pipes.

[0053] As shown in Table 3, the austenitic stainless steel oil pipe produced by the present application is immersed in FeCl3 solution for 168 hours according to ASTM G48 standard, and the test temperature is 50℃, and the average corrosion rate after the test is less than 0.6 mm / a. Under the conditions of a total test pressure of 15 MPa, a CO2 partial pressure of 7.6 MPa, a H2S partial pressure of 1.0 MPa, and a test temperature of 160℃, the average corrosion rate is less than 0.01 mm / a, so the product has excellent corrosion resistance and can be applied to harsh service conditions of acidic oil and gas wells containing hydrogen sulfide, carbon dioxide and chloride ions and other complex corrosion media.

[0054] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. An austenitic stainless steel oil pipe for acidic corrosion oil and gas wells, characterized by: The chemical element composition of the oil pipe is as follows by weight: C ≤ 0.03%, Si: 0.15%-0.3%, P: ≤ 0.010%, S: ≤ 0.005%, Cr: 20.0%-24.0%, Mn: 14.0%-20%, Ni: 5.0%-10.0%, Mo: 3.0%-5.0%, Cu: 0.25%-0.50%, N: 0.4%-0.6%, V: ≤ 0.1%, Nb: ≤ 0.1%, Ca: 0.001%-0.003%, Re: 0.01-0.03%, with the balance being Fe and unavoidable impurities. The oil pipe has a yield strength of 760 MPa-880 MPa, a tensile strength of 900 MPa-1050 MPa, an elongation of ≥ 40%, a full-scale transverse impact energy of ≥ 110 J at -10°C, and a full-scale longitudinal impact energy of ≥ 130 J.

2. A method for processing austenitic stainless steel tubing for acidic corrosion oil and gas wells according to claim 1, characterized in that: The following steps are involved: S1: Preparation of austenitic stainless steel ingots: Using low-carbon ferrochrome, metallic manganese, nickel-iron alloy, ferromolybdenum, and industrial pure iron as raw materials, and adding aluminum or silicon steel deoxidizer, smelting in a vacuum induction furnace under a nitrogen protective atmosphere of 0.1-0.5 MPa, adding nitride alloy when the smelting temperature is controlled in the range of 1500-1550°C, and then controlling the pouring temperature in the range of 1450-1480°C for nitrogen protective pouring to prepare austenitic stainless steel ingots; S2: Electroslag remelting: Austenitic stainless steel ingots are prepared into consumable electrodes for electroslag remelting, and electroslag remelting is performed under nitrogen protection. During the electroslag remelting process, the corresponding slag system is selected and Si or Ca is used for deoxidation. After electroslag remelting, non-destructive testing and inclusion analysis are performed; S3: Austenitic stainless steel ingot forging: The austenitic stainless steel ingot after electroslag remelting is forged, wherein the austenitic stainless steel ingot is heated to a temperature of 1200℃~1250℃ and kept at this temperature for 2~3 hours. The austenitic stainless steel ingot with a diameter of 300mm~350mm is forged into an ingot with a diameter of 200mm~220mm, and the final forging temperature is controlled to be not less than 1050℃; S4: Pipemaking: The forged austenitic stainless steel ingot is prepared into a rough oil pipe billet by hot extrusion, and then the billet is prepared into the required specifications by multiple cold rolling processes. The finished oil pipe billet is then subjected to full-body solution treatment to obtain a finished austenitic oil pipe billet. Annealing is performed between each cold rolling pass. During the S4 pipemaking process, the austenitic stainless steel billet billet needs to be controlled at a temperature between 450°C and 550°C during cold rolling, and the annealing temperature between cold rolling passes is between 650°C and 700°C for 1 to 3 hours. S5: Perform non-destructive testing on the finished oil pipe billet. After passing the testing, the pipe ends are threaded, couplings are screwed, hydrostatic pressure test is performed, protective rings are installed, marking is sprayed and painted to produce the finished oil pipe.

3. The method for processing austenitic stainless steel oil pipe for acid corrosion oil and gas wells according to claim 2, characterized in that: After the preparation of the austenitic stainless steel ingot in S1 is completed, the austenitic stainless steel ingot is subjected to element analysis, flaw detection, and machining to remove surface defects and black scale.

4. The method for processing austenitic stainless steel oil pipe for acid corrosion oil and gas wells according to claim 2, characterized in that: The pitting corrosion equivalent value PREN of the austenitic stainless steel ingot material in S1 is ≥40.

5. The method for processing austenitic stainless steel oil pipe for acid corrosion oil and gas wells according to claim 2, characterized in that: In the S4 pipe making process, the austenitic stainless steel billet blank pipe is cold-rolled for 5 passes to reduce the diameter, wherein the diameter reduction ratio of the first 3 passes is in the range of 18% to 25%, the diameter reduction ratio of the 4th pass is in the range of 5% to 10%, and the diameter reduction ratio of the 5th pass is in the range of 10% to 15%.

6. The method for processing austenitic stainless steel oil pipe for acid corrosion oil and gas wells according to claim 2, characterized in that: In the S4 pipe making process, the parameters of the solution treatment heat treatment of the tube blank are: temperature greater than 1100° C., time greater than 4 hours, and air cooling to room temperature.

7. The method for processing austenitic stainless steel oil pipe for acid corrosion oil and gas wells according to claim 2, characterized in that: In the S4 pipe making process, the microstructure of the finished oil pipe billet after the billet is solution treated is austenite, and the δ phase content is less than 3%.

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