A non-magnetic stainless steel drill collar, a preparation method and application thereof

Abstract

The present application belongs to the technical field of oil drilling equipment manufacturing, and particularly relates to a non-magnetic stainless steel drill collar, a preparation method and application. The preparation method comprises the following steps: pipe blank production, perforation, hot rolling sizing, reducing sizing, sectional straightening, annealing softening, rotary rolling finishing, quenching and tempering heat treatment and machining. The present application limits the content of each element in Cr-Ni-Mo-N low-carbon austenitic non-magnetic stainless steel, and combines the process route of 'perforation-hot rolling sizing-reducing sizing-sectional straightening-annealing softening-rotary rolling finishing-quenching and tempering heat treatment', especially the rotary rolling finishing process, effectively improves the axial fiber texture and residual stress distribution in the pipe forming process, significantly reduces the longitudinal and transverse performance difference, and obtains a non-magnetic stainless steel drill collar with low magnetic permeability, high strength, high dimensional accuracy, excellent corrosion resistance and low anisotropy.

Description

Technical Field

[0001] This invention belongs to the field of oil drilling and production equipment manufacturing technology, specifically relating to a non-magnetic stainless steel drill collar, its preparation method, and its application. Background Technology

[0002] Drill collars are one of the most important components of oil and gas drill strings, serving to provide drilling pressure to the drill bit and improve the rigidity of the drill string. With economic development, the number of oil drilling and production projects is constantly increasing, leading to a continuous increase in the demand for drill collars in oil drilling and production equipment.

[0003] Currently, most non-magnetic drill collars are manufactured using high-alloy austenitic stainless steel, primarily employing either solid forging or deep-hole machining processes. However, these traditional processes have gradually revealed numerous problems that urgently need to be addressed in practical applications:

[0004] (1) Low material utilization: In the process of deep hole machining, a large amount of raw materials need to be removed in order to achieve specific shape and size requirements. This process inevitably causes serious waste of raw materials, resulting in high manufacturing costs and also has an adverse effect on the rational use of resources.

[0005] (2) Poor dimensional accuracy: As drilling and mining projects place increasingly higher demands on the performance of drill collars, dimensional accuracy standards are becoming more stringent. However, when the length-to-diameter ratio of the drill collar (i.e., the ratio of the hole depth to the hole diameter) is large, traditional deep hole machining technology struggles to achieve high-precision dimensional control, failing to meet the stringent requirements for product dimensional accuracy in actual production. This, to some extent, limits the application range and performance of drill collars.

[0006] (3) Significant anisotropy: During cold working processes such as cold rolling and stretching, a strong fibrous texture forms inside the material, while a large amount of residual stress accumulates. This leads to the orderly arrangement of material grains along the axial direction, resulting in significant differences in the mechanical properties of the drill collar in the longitudinal and transverse directions, exhibiting significant anisotropy. This anisotropy not only adversely affects the corrosion resistance of the drill collar but also reduces its strength and toughness, seriously affecting the service life and reliability of the drill collar in complex and harsh drilling and production environments.

[0007] Therefore, in order to overcome the problems of traditional processes, it is urgent to develop a new manufacturing process to ensure that non-magnetic drill collars not only possess basic properties such as low magnetic permeability and high strength, but also significantly improve the dimensional accuracy of the product and effectively reduce the anisotropy of the material. This will enable the comprehensive performance of non-magnetic drill collars to better meet the needs of practical applications and provide a solid guarantee for the efficient and safe operation of oil drilling and production projects. Summary of the Invention

[0008] To address the shortcomings of the existing technology, this invention provides a non-magnetic stainless steel drill collar, its preparation method, and its application, thereby solving the problems of low material utilization, poor dimensional accuracy, and significant anisotropy in the existing drill collar technology.

[0009] This invention, by limiting the content of each element in Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel and combining it with a process route of "piercing-hot rolling sizing-reduction finishing-segment straightening-annealing softening-spin-roll finishing-quenching and tempering heat treatment," especially the spin-roll finishing process, effectively improves the axial fiber texture and residual stress distribution during the tube forming process, significantly reduces the difference in longitudinal and transverse properties, and obtains non-magnetic stainless steel drill collars with low magnetic permeability, high strength, high dimensional accuracy, excellent corrosion resistance, and low anisotropy. In addition, compared with non-magnetic stainless steel drill collars produced by traditional process routes, the process route provided by this invention effectively improves material utilization, reduces manufacturing costs, and has broad application prospects.

[0010] The first aspect of this invention provides a method for preparing a non-magnetic stainless steel drill collar, comprising the following steps:

[0011] S1. Billet production: After the billet is melted into molten steel in an electric arc furnace, the molten steel is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is Cr-Ni-Mo-N series low carbon austenitic non-magnetic stainless steel.

[0012] S2. Piercing: The tube blank is uniformly heated and then pierced to obtain the tube. After piercing, the tube is cooled to 820-880℃.

[0013] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature.

[0014] S4. Reduction and Finishing: The pipe is further reduced and finished by a multi-stand reduction machine. At the same time, a water mist cooling system is used to cool the pipe during the reduction and finishing process.

[0015] S5. Segmented straightening: After the diameter reduction and finishing of the pipe, air cool it to below 550℃, then perform hot straightening on the air-cooled pipe. After the hot straightening is completed, allow the pipe to cool naturally to room temperature, and finally perform final fine straightening on the pipe cooled to room temperature.

[0016] S6. Annealing and softening: The pipes after segmented straightening are subjected to solution annealing, followed by water quenching and cooling to room temperature;

[0017] S7. Spin rolling finishing: A three-roll spin rolling mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process.

[0018] S8. Tempering heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, then water quenched and cooled to room temperature, and then tempered. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank of non-magnetic stainless steel drill collar.

[0019] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0020] Preferably, the chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C≤0.06wt.%, N 0.5~0.8wt.%, Mo 0.5~0.8wt.%, Mn 18~25wt.%, Cr 16~20wt.%, Ni 8~12wt.%, S≤0.02wt.%, P≤0.03wt.%, with the balance being Fe and unavoidable impurity elements.

[0021] Preferably, the uniform heating process involves heating the tube blank to 1150–1250°C, with the heating time determined based on the tube blank thickness, calculated using the following formula:

[0022]

[0023] In the formula, Heating time, in minutes; The thickness of the tube blank is in mm; Heating time per millimeter of tube blank thickness. =3min / mm;

[0024] In step S2, a skew rolling piercing mill is used to pierce the uniformly heated tube blank. The piercing process is as follows: the mandrel temperature is 1050-1200℃, the piercing speed is 30-50mm / s, and the mandrel compression ratio is 1.5-2.0. After piercing, the tube is cooled to 820-880℃ by a constant-speed air cooling system at a cooling rate of 2-8℃ / s.

[0025] Preferably, during hot rolling sizing, the rolling temperature is 950–1100℃, the rolling speed is 10–15 m / min, the roll pressure is 100–200 tons, the number of hot rolling passes is 6, and the reduction per pass is 18–24%; after hot rolling, a water mist cooling system is used to cool the tube to room temperature at a cooling rate of 5–15℃ / s.

[0026] During the diameter reduction finishing process, the single-stand diameter reduction ratio is 1.02 to 1.05, the total diameter reduction ratio is 1.1 to 1.3, the rolling speed is 15 to 20 m / min, and the cooling rate of the water mist cooling system is 8 to 20 ℃ / s.

[0027] Preferably, during segmented straightening, the reduced-diameter finished pipe is air-cooled to below 550°C at a cooling rate of 0.5–3°C / s;

[0028] In step S5, a hot straightener is used to perform hot straightening on the air-cooled pipe. The hot straightening process parameters are: straightening temperature 450-500℃, straightening pressure 50-200 tons, hot straightening times 1-3 times, single reduction 1-6mm, and straightening speed 0.2-1.5m / s. A cold multi-roller straightener is used to perform final fine straightening on the pipe cooled to room temperature. The final fine straightening process parameters are: fine straightening pressure 10-80 tons, fine straightening times 1-2 times, single reduction 0.5-3.0mm, and fine straightening speed 0.1-1.0m / s.

[0029] Preferably, during annealing and softening, the solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is >100℃ / s;

[0030] During the rotary finishing process, the rotary temperature is 60–120℃, the feed rate is 0.2–0.5 mm / r, the rolling force is 50–80 kN, the roll speed is 60–90 rpm, the number of rotary finishing passes is 1–3, the instantaneous cross-sectional compression rate per pass is ≥10%, and the cooling rate of the water mist cooling system is 5–10℃ / s.

[0031] During quenching and tempering heat treatment, the solution annealing temperature is 1050~1100℃, the solution annealing time is 30min, the cooling rate during water quenching is >100℃ / s, the tempering temperature is 450~500℃, and the tempering time is 120min.

[0032] Preferably, after hot rolling and sizing, the cumulative cross-sectional reduction rate of the pipe is 70% to 80%;

[0033] After the air-cooled pipes are hot-straightened, the straightness of the pipes is ≤0.3mm / m; after the pipes are cooled to room temperature and then fine-straightened, the straightness of the pipes is ≤0.3mm / m, and the deviation of the outer diameter and the wall thickness are both controlled within ±0.2mm.

[0034] After annealing and softening, the hardness of the pipe is ≤180HB;

[0035] After the rotary rolling finishing, the longitudinal and transverse yield strength, tensile strength and elongation differences of the pipe are all ≤5%, the residual stress difference is controlled within 30MPa, and the texture strength is controlled below 3 times the randomness.

[0036] Preferably, the obtained non-magnetic stainless steel drill collar has a yield strength ≥980MPa, magnetic permeability μr≤1.005, outer diameter accuracy up to ±0.05mm, surface roughness Ra≤0.8μm, residual stress difference of 18~23MPa, texture strength of 1.9~2.3 times random, average grain size of 16.9~20.1μm, longitudinal and transverse yield, tensile and elongation differences of 3~5%, neutral salt spray test duration of 800~900h, pitting corrosion observation results of no obvious pitting corrosion, G150 critical pitting corrosion temperature of 48~58℃; the worst result in G48 crevice corrosion test is slight crevice corrosion, and the maximum crevice corrosion depth is 5~18μm.

[0037] The second aspect of the present invention provides a non-magnetic stainless steel drill collar, which is manufactured using a method for preparing a non-magnetic stainless steel drill collar.

[0038] The third aspect of this invention provides an application of a non-magnetic stainless steel drill collar in the field of oil drilling and production.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. This invention limits the range of manganese, nickel, nitrogen, chromium, molybdenum, carbon and impurity elements in Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel, so that each element can form a synergistic effect in austenite stability, solid solution strengthening, corrosion resistance improvement and processing stability control, thereby providing a material basis for the low magnetic permeability, high strength and high precision manufacturing of non-magnetic stainless steel drill collars.

[0041] 2. This invention introduces a rotary rolling finishing process, applying multi-directional shear stress during the cold working stage of the material. This alters the stress distribution under traditional cold working methods, effectively breaking down the strong fiber texture in traditional rolling, reducing longitudinal and transverse performance differences (anisotropy) and grain boundary slip, and avoiding the impact of residual stress generated by traditional cold working on the material's corrosion resistance. This results in more uniform mechanical and magnetic properties, thereby enhancing the service performance of the drill collar. Simultaneously, after the rotary rolling finishing process, the material surface is also subjected to strong pressing, forming a more uniform and dense surface layer, further reducing the occurrence of corrosion pits.

[0042] 3. The rotary rolling finishing process introduced in this invention, through the action of triaxial composite stress, can generate large plastic deformation on the surface and inside of the tube, and achieve a large reduction ratio within a short stroke. This process promotes grain refinement and enhances the mechanical properties of the material, especially improving the yield strength.

[0043] 4. This invention, by introducing a rotary rolling finishing process, can effectively control the material texture and break down the fibrous structure formed during traditional cold rolling, thereby making the magnetic properties of the material more uniform and achieving the requirement of low magnetic permeability. Especially for the manufacture of non-magnetic stainless steel drill collars, rotary rolling can significantly reduce the increase in magnetism caused by cold working, ensuring the stability and reliability of the drill collars under high precision requirements. Attached Figure Description

[0044] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0045] Figure 1 A flowchart illustrating a method for preparing a non-magnetic stainless steel drill collar provided by the present invention;

[0046] Figure 2 This is a metallographic diagram of the non-magnetic stainless steel drill collar prepared according to the method described in Example 1 of this invention. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0048] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0049] like Figure 1 As shown, the first aspect of the present invention provides a method for preparing a non-magnetic stainless steel drill collar, comprising the following steps:

[0050] S1. Billet production: After the billet is melted into molten steel in an electric arc furnace, the molten steel is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel.

[0051] Preferably, the chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C≤0.06wt.%, N 0.5~0.8wt.%, Mo 0.5~0.8wt.%, Mn 18~25wt.%, Cr 16~20wt.%, Ni 8~12wt.%, S≤0.02wt.%, P≤0.03wt.%, with the balance being Fe and unavoidable impurity elements.

[0052] The design principles of each chemical element in the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel designed in this invention are as follows:

[0053] Carbon (≤0.06wt.%): Carbon in austenitic stainless steel mainly forms carbides, which can easily affect weldability and corrosion resistance. Therefore, using a low-carbon alloy design can effectively avoid carbide precipitation, thereby improving the strength and corrosion resistance of the welded joint.

[0054] Nitrogen (0.5-0.8 wt.%): Nitrogen can improve the strength, hardness and corrosion resistance of steel, especially its resistance to pitting and crevice corrosion; at the same time, nitrogen can also promote austenite formation, improve the toughness and oxidation resistance of materials, and enhance their high-temperature resistance.

[0055] Molybdenum (Mo) (0.5–0.8 wt.%): Molybdenum significantly improves the corrosion resistance of stainless steel, especially in chlorine-containing environments. It effectively extends the service life of materials by strengthening the acid resistance and pitting corrosion resistance of the steel. In industries such as oil drilling and extraction, the addition of molybdenum significantly enhances the corrosion resistance of stainless steel in seawater and acidic environments.

[0056] Manganese (Mn) (18-25 wt.%): Manganese can stabilize the austenitic structure and improve the plasticity and toughness of materials. In addition, manganese can enhance the oxidation resistance and high temperature resistance of steel, and also help reduce the risk of brittleness caused by the presence of sulfur and phosphorus.

[0057] Chromium (Cr) (16-20 wt.%): Chromium is the core component of austenitic stainless steel. It effectively blocks corrosive media by forming a dense and stable chromium oxide film on the steel surface, thus endowing the material with strong corrosion resistance. For non-magnetic stainless steel, an appropriate amount of chromium ensures its stability in high-temperature and corrosive environments.

[0058] Nickel (8-12 wt.%): Nickel is an indispensable key element in austenitic stainless steel. It not only significantly improves the plasticity and toughness of the steel, but also ensures that the material maintains excellent performance in extreme temperature environments (from low to high temperatures) by stabilizing the austenitic structure. Especially for non-magnetic stainless steel, the addition of nickel can effectively reduce the magnetic permeability, meeting the stringent requirements of low-magnetic applications.

[0059] Sulfur (S) (≤0.02 wt.%) and Phosphorus (P) (≤0.03 wt.%): Sulfur and phosphorus are harmful impurity elements. The stricter the control of their content, the better it is to reduce the risk of hot working cracks, brittle fracture, and local structural defects, thereby improving the stability of piercing, hot rolling sizing, diameter reduction finishing, rotary rolling finishing, and subsequent machining processes, and ensuring the dimensional consistency and surface quality of the finished product. Therefore, this application adopts a low-sulfur, low-phosphorus design to reduce brittleness and optimize the processing performance of the pipe.

[0060] In this application, by controlling the carbon content to a low level, it is possible to reduce the tendency for harmful precipitation and ensure microstructure stability. Specifically, controlling the carbon content to ≤0.06 wt.% helps reduce the tendency for carbide precipitation, avoids the decrease in corrosion resistance caused by grain boundary precipitation, and reduces the risk of microstructure instability during heat treatment and subsequent processing. The low-carbon design can also reduce local magnetic property fluctuations and mechanical property inhomogeneities caused by precipitated phases, thereby contributing to the achievement of the low magnetic permeability and high overall performance goals described in this application.

[0061] In this application, manganese, nickel, and nitrogen are key elements for achieving low magnetic permeability. Manganese and nickel are both austenite-stabilizing elements; nitrogen not only further promotes austenite formation but also enhances austenite stability. The synergistic effect of these three elements results in a stable austenite-dominated microstructure in the material, reducing the formation of ferrite, deformed martensite, and other microstructures that easily increase magnetic permeability during processing. This allows the material to maintain low magnetic permeability even after hot working, rotary rolling, and subsequent heat treatment, meeting the magnetic performance stability requirements of non-magnetic drill collars. Especially under the conditions of the diameter reduction finishing, rotary rolling finishing, and two solution treatment processes employed in this application, the above-mentioned composition system helps maintain microstructure stability and reduces the risk of increased magnetism induced by cold working.

[0062] In this application, nitrogen is the primary strengthening element for achieving high strength. After nitrogen dissolves in the austenitic matrix, it significantly improves the yield strength and hardness of the material, while also enhancing corrosion resistance. Manganese, on the one hand, helps improve the solid solution capacity of nitrogen in steel, and on the other hand, works with nickel to maintain the stability of the austenitic structure, allowing the high-nitrogen strengthening effect to be effectively exerted. Nickel, in addition to stabilizing the austenitic structure, also improves the material's plasticity and toughness, avoiding a decrease in toughness due to the pursuit of high strength alone. Thus, manganese, nickel, and nitrogen form a synergistic relationship, enabling the material to achieve high strength and a good strength-toughness balance while maintaining low magnetic permeability.

[0063] In this application, chromium and molybdenum are primarily used to ensure corrosion resistance and maintain overall service performance. Chromium is a fundamental element for the corrosion resistance of stainless steel, forming a stable passivation film on the material surface and improving its corrosion resistance in drilling fluids, chlorine-containing media, and corrosive environments. Molybdenum further enhances the material's resistance to pitting and crevice corrosion in chlorine-containing environments. The combination of chromium, molybdenum, and nitrogen can improve the material's corrosion adaptability in the complex environments of oil drilling and production while maintaining low magnetic permeability and high strength, thereby ensuring the overall service performance of the drill collar.

[0064] It should be noted that the high dimensional accuracy of the drill collar in this application is not solely determined by the composition of the billet, but rather by the combined effect of the billet's composition system and the drill collar's forming process. The billet's composition system provides a stable austenitic structure, good plasticity and toughness, and low defect sensitivity, thus providing a suitable material basis for subsequent processes such as piercing, hot rolling sizing, diameter reduction finishing, segmented cooling and straightening, rotary rolling finishing, and machining. The outer diameter accuracy, wall thickness accuracy, straightness, and surface roughness of the finished product are primarily guaranteed by the drill collar's forming process. In other words, the billet's composition system mainly addresses the question of whether the material is suitable for stable high-precision forming, while the high dimensional accuracy is ultimately achieved through the combined effect of the billet's composition system and the drill collar's forming process.

[0065] In this embodiment, the steel is refined by AOD refining and VOD vacuum degassing processes, which can remove impurities, especially harmful elements such as sulfur and phosphorus, and ensure the purity and stability of the material.

[0066] S2. Piercing: The tube blank is uniformly heated and then pierced to obtain the tube. After piercing, the tube is cooled to 820-880℃.

[0067] In this application, the uniform heat treatment process specifically involves heating the tube blank to 1150–1250°C, with the heating time determined based on the tube blank thickness, as shown in the following formula:

[0068]

[0069] In the formula, Heating time, in minutes; The thickness of the tube blank is in mm; Heating time per millimeter of tube blank thickness. =3min / mm.

[0070] It should be noted that precise control of the heating temperature is necessary when uniformly heating the tube blank. When the heating temperature is below 1150℃, the plasticity of the material will decrease significantly, making it difficult to fully meet the requirements of subsequent processing (such as piercing, hot rolling sizing, etc.), thus leading to forming difficulties. In addition, excessively low heating temperatures may also cause uneven grain structure distribution, adversely affecting the subsequent mechanical properties of the material. When the heating temperature is above 1250℃, the material will soften excessively, resulting in a decrease in strength and toughness. More seriously, excessively high temperatures may promote grain coarsening, interfering with subsequent processing and damaging the overall performance of the material. Furthermore, high-temperature environments may also cause the evaporation or oxidation of certain elements, altering the chemical composition of the material and reducing its corrosion resistance. Therefore, this application controls the heating temperature within the range of 1150~1250℃, which not only ensures that the material has sufficient plasticity, allowing it to exhibit excellent processing performance in subsequent piercing, hot rolling sizing, and other heat treatment processes, but also helps to optimize the material's microstructure, making it more uniform and effectively avoiding microstructural defects during the forming process.

[0071] It is important to emphasize that when uniformly heating the tube blank, the heating temperature and heating time are not controlled in isolation, but rather jointly determine the overall thermal uniformity, microstructure stability, and adaptability to subsequent hot deformation. The heating temperature ensures sufficient plasticity, while the heating time ensures uniform temperature distribution along the cross-section. This synergistic matching avoids excessive internal and external temperature differences, insufficient plasticity, and difficulties in subsequent piercing caused by insufficient temperature or inadequate holding time. It also avoids grain coarsening, localized overheating, and microstructure inhomogeneity caused by excessively high temperature or excessively long holding time. This provides a uniformly structured and appropriately plastic initial tube blank for subsequent skew rolling piercing and hot rolling sizing.

[0072] In this application, a skew rolling piercing mill is used to pierce the uniformly heated tube blank. The piercing process is as follows: the mandrel temperature is 1050-1200℃, the piercing speed is 30-50mm / s, and the mandrel compression ratio is 1.5-2.0.

[0073] It should be noted that, in this application, the compression ratio of the mandrel is the ratio of the initial cross-section to the final cross-section of the material during the forming process.

[0074] In this application, the mandrel temperature is controlled within the range of 1050 to 1200°C to ensure that the material has sufficient machinability, allowing the piercing process to proceed stably and efficiently, while avoiding various problems caused by excessively high or low temperatures, thus laying a solid foundation for producing high-quality pipes.

[0075] It is important to note that precise control of the mandrel temperature is a key factor in ensuring product quality during the tube blank piercing process. If the mandrel temperature is below 1050℃, the material's plasticity will significantly decrease. Under these conditions, piercing will easily lead to cracks or uneven shapes. If the mandrel temperature is above 1200℃, the material will soften excessively, making the piercing process highly unstable and potentially damaging the material's grain structure, resulting in coarse grains and reduced strength and toughness. In contrast, the temperature range of 1050–1200℃ ensures that the material retains appropriate plasticity during piercing, allowing it to be successfully processed into the desired tube shape.

[0076] If the piercing speed is too high, the piercing head may not have enough time to evenly compress the material to the correct position, resulting in uneven pipe wall thickness and even cracks in severe cases. If the piercing speed is too slow, it will reduce production efficiency and may cause the material to overheat, affecting subsequent processes. Therefore, in this application, the piercing speed is controlled between 30 and 50 mm / s to ensure that the piercing process is carried out efficiently and orderly, improving production efficiency, while also ensuring the quality of the pipe and avoiding problems such as uneven wall thickness, cracks, and irregular hole walls, thus balancing piercing efficiency and quality.

[0077] During the piercing process, if the mandrel compression ratio is too low, insufficient material flow will result in inadequate inner diameter of the pipe, potentially leading to unsuccessful piercing in severe cases. Conversely, if the mandrel compression ratio is too high, excessive material flow will cause uneven pipe wall thickness, resulting in inconsistent thickness. Therefore, selecting an appropriate mandrel compression ratio, i.e., 1.5 to 2.0, ensures the shape and dimensional accuracy of the pipe while effectively preventing material loss and deformation due to excessive compression.

[0078] It should be noted that the mandrel temperature, piercing speed, and mandrel compression ratio in this application are not controlled in isolation, but rather work together in the piercing process. The mandrel temperature ensures sufficient thermoplasticity of the material; the piercing speed controls the deformation loading rate and the high-temperature residence time; and the mandrel compression ratio controls the actual degree of material deformation and the sufficiency of metal flow. The coordinated matching of these three factors allows the tube blank to form a stable and uniform plastic flow state during piercing. This avoids cracks, uneven hole walls, and wall thickness fluctuations caused by excessively low temperature, excessively high speed, or excessively high compression ratio, as well as overheating, softening, insufficient deformation, and dimensional instability caused by excessively high temperature, excessively slow speed, or excessively low compression ratio. This improves piercing quality and provides tubes with more stable microstructure and dimensions for subsequent hot rolling sizing and reduction finishing.

[0079] In this embodiment of the application, after the perforation is completed, the pipe is cooled to 820-880°C by a constant-speed air cooling system, with a cooling rate of 2-8°C / s.

[0080] It should be noted that if the pipe temperature is below 820℃ after cooling, the excessively low temperature may cause the material to form an overly hard structure (such as martensite), increasing the difficulty of subsequent processing and even leading to cracks. If the pipe temperature is above 880℃ after cooling, the excessively high temperature may cause the pipe shape to deform or its dimensions to become unstable, affecting subsequent processes. Therefore, in this application, the pipe is cooled to 820–880℃ after piercing to prevent overheating and maintain the shape and dimensional accuracy of the pipe, laying the foundation for subsequent hot rolling and sizing.

[0081] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature. The rolling temperature is 950-1100℃, the rolling speed is 10-15m / min, the roll pressure is 100-200 tons, the number of hot rolling passes is 6, and the reduction per pass is 18-24%.

[0082] Rolling temperature is a critical factor in the hot rolling process. Maintaining it within the range of 950–1100℃ ensures the material possesses appropriate plasticity, allowing it to pass through the rolling process smoothly. This avoids the material becoming brittle due to excessively low temperatures, or experiencing grain growth and performance degradation due to excessively high temperatures. If the rolling temperature is below 950℃, the pipe's plasticity and ductility are poor, deformation during rolling may be insufficient, leading to uneven pipe dimensions, rolling difficulties, and even cracking. If the rolling temperature exceeds 1100℃, it can cause excessive grain growth, affecting the pipe's mechanical properties; excessively high temperatures can also lead to over-softening, affecting subsequent process control and potentially causing surface defects.

[0083] Rolling speed determines the contact time between the material and the rolls during the rolling process. Higher rolling speeds help improve production efficiency, but excessively fast rolling speeds may lead to uneven material deformation and surface defects; excessively slow rolling speeds will affect production efficiency and may cause excessively high temperatures, affecting material quality. Therefore, in this application, the rolling speed is controlled within the range of 10–15 m / min.

[0084] Similarly, if the roll pressure exceeds 200 tons, it may cause the material to plastically flow too quickly, making it difficult to control dimensions and even causing cracks or defects; if the roll pressure is less than 100 tons, it may cause incomplete deformation, resulting in the material failing to achieve the required dimensions and precision, and the uniformity of outer diameter and wall thickness cannot be guaranteed. Therefore, in this application, the roll pressure is controlled within the range of 100 to 200 tons.

[0085] In this application, after hot rolling and sizing, the cumulative section reduction rate of the pipe is 70% to 80%, and in the preferred state, the cumulative section reduction rate of the pipe is 75% to 80%.

[0086] In this application, the formula for calculating the cumulative cross-sectional reduction rate of the pipe is as follows:

[0087]

[0088] In the formula, R is the cumulative cross-sectional reduction rate of the pipe, b is the single-pass reduction during hot rolling sizing, and B is the number of hot rolling cycles.

[0089] During the hot rolling sizing stage, rolling temperature, rolling speed, roll pressure, and multiple reduction passes all contribute to the thermoplastic deformation of the tube. Rolling temperature primarily determines the material's plasticity, rolling speed mainly affects the deformation rhythm and heat dissipation, and roll pressure and multiple reduction passes together determine the sufficiency of metal flow and dimensional correction capability. When these parameters are coordinated and matched, the material maintains a stable and uniform plastic flow state during hot rolling. This avoids insufficient deformation, dimensional inconsistencies, and cracks caused by low temperature, high speed, or insufficient pressure, as well as grain coarsening, localized softness, and surface defects caused by excessively high temperature, slow speed, or excessive pressure. This ensures a stable transition of the tube to a high-precision tube body and creates a good dimensional foundation for subsequent diameter reduction and finishing.

[0090] In this application, after hot rolling, a water mist cooling system is used to cool the pipe to room temperature, with a cooling rate of 5 to 15°C / s.

[0091] It is important to note that after hot rolling, the pipe remains at a relatively high temperature. Therefore, water mist cooling is necessary to control temperature changes and prevent material deformation or surface defects caused by excessive heat. Furthermore, controlling the pipe temperature through a water mist cooling system ensures that the pipe's shape and dimensions do not deform due to overcooling or overheating, thereby improving the product's dimensional accuracy and surface quality.

[0092] It should be noted that traditional water cooling may cause the material surface temperature to drop too quickly, generating excessive thermal stress. Water mist cooling, due to the evaporation process, effectively avoids this rapid cooling, ensuring a smoother and more uniform cooling process. Therefore, in this embodiment, a water mist cooling system is used to cool the hot-rolled pipe to room temperature.

[0093] S4. Reduction Finishing: The tube is further reduced and finished by a multi-stand reduction mill. At the same time, a water mist cooling system is used to cool the tube during the reduction finishing process. The single-stand reduction ratio is 1.02 to 1.05, the total reduction ratio is 1.1 to 1.3, the rolling speed is 15 to 20 m / min, and the cooling rate of the water mist cooling system is 8 to 20℃ / s.

[0094] In this application, the single-stand reduction ratio refers to the percentage change in the diameter or thickness of the tube after being pressed down through each mill stand; the total reduction ratio refers to the total reduction ratio of the entire reduction process.

[0095] In this application, the formula for calculating the overall reduction ratio is as follows:

[0096]

[0097] In the formula, denoted as the total reduction ratio, r as the reduction ratio per stand, and n as the total number of mill stands.

[0098] It should be noted that if the reduction ratio per stand is too large, the material may be over-compressed in each process, leading to significant deformation or cracks. Excessive pressure also increases the material's hardness, affecting subsequent processing. Conversely, if the reduction ratio per stand is too low, insufficient reduction will result in poor dimensional accuracy of the tube, uneven outer diameter or wall thickness, and failure to achieve the target precision. Therefore, this application sets the reduction ratio per stand to 1.02–1.05, which ensures uniform material deformation while avoiding defects caused by excessive reduction, thus ensuring the accuracy of the outer diameter and wall thickness, ultimately achieving a control precision of ±0.2 mm. Furthermore, maintaining the rolling speed within the range of 15–20 m / min ensures both good production efficiency and uniform material cooling, preventing deformation or surface defects caused by overheating.

[0099] It is important to note that in the embodiments of this application, the reduction amount in each pass of the hot rolling sizing process remains consistent, and the reduction ratio per stand of each mill stand in the reducing finishing process also remains the same. In actual operation, those skilled in the art can flexibly adjust the reduction amount in each pass and the reduction ratio per stand of each mill stand according to specific needs. However, it must be noted that if the reduction amount in each pass of the hot rolling sizing process is inconsistent, and the reduction ratio per stand of each mill stand in the reducing finishing process is also inconsistent, then the cumulative section reduction rate calculation formula and the total reduction ratio calculation formula in the reducing finishing process given in the embodiments of this application need to be adaptively adjusted based on common knowledge in the art.

[0100] During the reduction and finishing process, the temperature of the tube is controlled by a water mist cooling system to prevent uneven performance caused by overheating. By uniformly spraying water mist, the surface temperature of the material can be rapidly reduced during rolling, avoiding performance degradation or surface defects caused by excessively high surface temperatures. This creates excellent conditions and lays a solid foundation for subsequent segmented straightening and rotary rolling finishing processes.

[0101] During the reduction and finishing stage, there is a clear synergistic relationship between the single-stand reduction ratio, the total reduction ratio, the rolling speed, and the cooling control parameters. The single-stand reduction ratio primarily determines the local deformation intensity of each stand, the total reduction ratio determines the overall dimensional correction range, the rolling speed affects the stability of the continuous deformation process, and the cooling control parameters are used to control temperature rise and thermal stress accumulation. After the parameters are coordinated and matched, the tube can achieve uniform and gradual plastic deformation during continuous reduction. This avoids local instability, wall thickness fluctuations, and surface damage caused by excessive single-stand reduction, excessive total reduction, or excessively high rolling speed. It also avoids insufficient dimensional correction and decreased production stability caused by insufficient reduction or excessively low rolling speed. This effectively improves the accuracy of outer diameter and wall thickness control and reduces residual stress and the burden on subsequent straightening.

[0102] In this embodiment, during the reduction finishing stage, the number of mill stands can be flexibly determined based on the total reduction ratio and the reduction ratio per stand, preferably 3 to 8 stands. When the number of mill stands is less than 3, it is easy to cause uneven pipe wall thickness, surface scratches and cracks, and a significant increase in residual stress. When the number of mill stands is more than 8, equipment investment and operating costs increase significantly, production efficiency decreases, and excessive dispersion of deformation weakens the dimensional correction effect of reduction finishing.

[0103] In this application, by implementing the diameter reduction finishing process, the outer diameter deviation and wall thickness deviation of the pipe can be controlled within ±0.2mm.

[0104] S5. Segmented straightening: After the pipe is reduced in diameter and finished, it is air-cooled to below 550℃. Then, the air-cooled pipe is hot-straightened. After the hot straightening is completed, the pipe is naturally cooled to room temperature. Finally, the pipe cooled to room temperature is finely straightened.

[0105] In this application, the reduced-diameter finished pipe is air-cooled to below 550°C at a cooling rate of 0.5 to 3°C / s.

[0106] It should be noted that when air-cooling the reduced-diameter finished pipe to below 550°C, if the cooling is too rapid, a relatively hard structure, such as martensite, may form on the pipe surface, leading to increased internal stress and affecting the pipe's subsequent processing performance. If the cooling is too slow, the pipe may maintain a high temperature, resulting in excessive thermal stress and affecting subsequent hot straightening and fine straightening processes. Therefore, in this application, the reduced-diameter finished pipe is air-cooled to below 550°C at a cooling rate of 0.5–3°C / s.

[0107] In this application, a hot straightening machine is used to hot straighten the air-cooled pipe. The hot straightening process parameters are as follows: straightening temperature is 450-500℃, straightening pressure is 50-200 tons, hot straightening times are 1-3 times, single pressing amount is 1-6mm, and straightening speed is 0.2-1.5m / s.

[0108] In this embodiment, the straightening pressure is preferably 80 to 150 tons, the single pressing amount is preferably 2 to 4 mm, and the straightening speed is preferably 0.4 to 1.0 m / s.

[0109] When hot straightening air-cooled pipes, if the straightening temperature exceeds 500℃, it may lead to grain coarsening, affecting the material's mechanical properties and structural stability. If the straightening temperature is below 450℃, it may result in insufficient plasticity, poor straightening effect, and inability to completely eliminate internal stress and bending. Therefore, this application uses a temperature range of 450–500℃ to hot straighten air-cooled pipes.

[0110] It should be emphasized that the straightening temperature is the set temperature of the straightening machine, not the temperature of the pipe. In the embodiments of this application, the pipe after diameter reduction and finishing is first air-cooled to below 550°C, and then the pipe is directly placed in the straightening machine, the straightening temperature of the straightening machine is set, and hot straightening is performed.

[0111] Furthermore, the straightening pressure determines the strength of the material under stress. Too low a straightening pressure may fail to correct the bending or internal stress of the pipe, while too high a straightening pressure may cause material damage or surface defects. Therefore, in this application, the straightening pressure is set to 50–200 tons to ensure uniform stress on the pipe and to effectively eliminate internal stress and deformation within this pressure range.

[0112] In this application, a cold multi-roller straightener is used to perform final fine straightening on the pipe cooled to room temperature. The final fine straightening process parameters are as follows: fine straightening pressure is 10 to 80 tons, fine straightening times are 1 to 2 times, single fine straightening reduction is 0.5 to 3.0 mm, and fine straightening speed is 0.1 to 1.0 m / s.

[0113] In this embodiment of the application, the fine straightening pressure is preferably 20 to 50 tons, the single fine straightening reduction is preferably 1.0 to 2.0 mm, and the fine straightening speed is preferably 0.2 to 0.6 m / s.

[0114] Preferably, after the air-cooled pipe is hot-straightened, the straightness of the pipe is ≤0.3mm / m; after the pipe is cooled to room temperature and then finely straightened, the straightness of the pipe is ≤0.3mm / m, and the deviation of the outer diameter and the deviation of the wall thickness are both controlled within ±0.2mm.

[0115] In this embodiment of the application, after the pipe is cooled to room temperature and then subjected to final fine straightening, the straightness of the pipe shall at least satisfy ≤0.3mm / m, and preferably ≤0.2mm / m.

[0116] It should be noted that although hot straightening helps eliminate most of the internal stress and deformation of the pipe, minor bending or deformation may occur during the cooling process to room temperature due to changes in temperature and pressure. Therefore, after the hot-straightened pipe has been cooled to room temperature, it needs to undergo final fine straightening to further ensure the accuracy of the pipe's straightness and shape.

[0117] In this application, when using a hot straightener to perform hot straightening on air-cooled pipes, the pipes undergo controlled elastoplastic bending deformation by adjusting the straightening temperature, straightening pressure, number of hot straightening cycles, single compression amount, and straightening speed. This eliminates bending and releases some residual stress, bringing the overall shape of the pipes back to a relatively straight state. When using a cold multi-roller straightener to perform final fine straightening on the hot-straightened pipes, the fine straightening pressure, number of fine straightening cycles, single compression amount, and fine straightening speed are adjusted to correct cooling springback and residual minor bending, further improving straightness stability and dimensional consistency, and providing a geometrically stable pipe foundation for subsequent processes.

[0118] It is important to note that during the segmented straightening stage, the pipe temperature, hot straightening process parameters, and final fine straightening process parameters are not independent parameters, but rather collectively affect the release of internal stress, plastic recovery, and straightness correction. By controlling these parameters in a coordinated manner, straightness correction and structural stability can be balanced while avoiding abnormal structural transformation and excessive thermal stress. This prevents cracks, surface damage, and localized stress concentration caused by excessively rapid cooling, low straightening temperature, or excessive straightening pressure, as well as poor straightening results and unstable dimensional recovery caused by insufficient cooling, excessively high straightening temperature, or insufficient straightening pressure. This provides a pipe body with lower stress and a stable shape for subsequent processes.

[0119] S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is >100℃ / s.

[0120] During the annealing and softening stage, if the solution temperature is below 1050℃, some alloying elements, such as carbon and nitrogen, may not completely dissolve, resulting in uneven grain structure and affecting the performance of the pipe. If the solution temperature is above 1050℃, it may cause grain coarsening, affecting the mechanical properties of the pipe. In particular, it may cause processing difficulties in subsequent processing, such as cracks, surface defects and other adverse conditions.

[0121] In this application, when implementing the annealing and softening process, solution annealing can improve the uniformity of the microstructure and reduce the hardness of the material; subsequently, water quenching and cooling at a rate >100℃ / s can suppress undesirable precipitation and maintain the stability of the microstructure after solution treatment, thus providing a good process basis for subsequent rotary rolling finishing.

[0122] Preferably, after annealing and softening, the hardness of the pipe is ≤180HB.

[0123] In this application, after annealing and softening, the hardness of the pipe is ≤180HB, which ensures that the pipe maintains sufficient strength and wear resistance, and will not cause processing difficulties or material damage due to excessive hardness in subsequent processing.

[0124] In this application, by implementing the annealing and softening process, we can avoid the processing difficulties caused by the excessive hardness of the pipe material and ensure the smooth progress of subsequent processing.

[0125] It is important to note that during the annealing and softening stage, there is a synergistic control relationship between solution temperature, holding time, and cooling rate. Solution temperature promotes the full dissolution of alloying elements and improves microstructure uniformity; holding time ensures the solution process is fully completed across the entire cross-section; and cooling rate suppresses undesirable precipitation and stabilizes the microstructure after solution treatment. When these three factors are properly matched, the material can be effectively softened, its hardness reduced, and its subsequent spin rolling finishing improved. This avoids microstructure inhomogeneity and excessive hardness caused by insufficient solution treatment, as well as grain coarsening, increased residual stress, and microstructure instability caused by overheating or improper cooling control. Consequently, the tube achieves more suitable plasticity and a more uniform initial microstructure before entering the spin rolling finishing process.

[0126] S7. Spin Roll Finishing: A three-roll spin mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 60-120℃, the feed rate is 0.2-0.5mm / r, the rolling force is 50-80kN, the roll speed is 60-90rpm, the spin rolling finishing is 1-3 passes, the instantaneous cross-sectional compression rate per pass is ≥10%, and the cooling rate of the water mist cooling system is 5-10℃ / s.

[0127] Excessively high spinning temperatures may cause the tube to soften excessively, affecting its strength and subsequent processing; while excessively low spinning temperatures may cause the material to become too hard during spinning, making it impossible to form smoothly. Therefore, this application uses spinning at a low temperature of 60–120°C to avoid material softening and microstructure fluctuations caused by heat accumulation during processing.

[0128] Feed rate refers to the axial distance the tube travels per revolution of the rolls during the spinning process. In this application, the feed rate is controlled within the range of 0.2 to 0.5 mm / r, which effectively controls the reduction amount in each spinning operation, ensuring the accuracy and surface quality of the tube.

[0129] Rolling force is the pressure applied to the tube during the spinning process. In this application, the rolling force is controlled within the range of 50 to 80 kN, which ensures that the tube is fully compressed during spinning, while avoiding excessive plastic deformation or local stress concentration, thus preventing material cracking or deformation.

[0130] The rotational speed of the rolls determines the speed at which the tube rotates, affecting the efficiency and effectiveness of the spun rolling process. In this application, controlling the roll rotational speed within the range of 60–90 rpm helps to achieve a stable spun rolling process and prevents processing problems caused by excessively fast or slow rotational speeds.

[0131] In the spinning finishing stage, if the number of spinning finishing passes is less than one (i.e., no spinning finishing is performed), it is difficult to fully utilize the effects of spinning on weakening fiber texture, redistributing residual stress, and dimensional finishing. If the number of spinning finishing passes exceeds three, it is easy to cause excessive accumulation of cold working effects, which in turn leads to the re-accumulation of residual stress, increased risk of surface damage, and decreased dimensional control stability, which is not conducive to subsequent solution treatment and overall performance stability. Therefore, in this application, the number of spinning finishing passes is controlled to 1 to 3, which can ensure the finishing effect while taking into account surface quality, dimensional accuracy, and process economy.

[0132] During the rotary rolling finishing stage, a water mist cooling system is used to cool the tube to prevent overheating of the tube surface, maintain surface smoothness, and prevent scratches or deformation. Simultaneously, a cooling rate of 5–10°C / s is set to avoid cracks or thermal stress caused by excessively rapid cooling.

[0133] It is important to emphasize that during the rotary rolling process, the tube surface experiences triaxial composite stress—radial compression, circumferential shear, and axial tension / compression—through circumferential shear and radial compression. This triaxial composite stress helps to alter the axial fiber texture formed by traditional cold rolling, achieving texture randomization and significantly reducing the difference in longitudinal and transverse properties. Especially during the rotary rolling finishing stage, achieving a single-pass instantaneous cross-sectional compression rate ≥10% can effectively improve the tube's overall mechanical properties and dimensional accuracy.

[0134] It should be noted that during the rotary rolling finishing stage, when the instantaneous cross-sectional compression rate of a single pass is ≥10%, a more sufficient radial compression and circumferential shear coupling effect can be formed within the tube wall, enabling the material to generate sufficient shear strain and plastic flow. This significantly promotes texture randomization, reduces residual stress difference, reduces longitudinal and transverse performance differences, and simultaneously improves dimensional finishing effect and comprehensive mechanical properties. When the instantaneous cross-sectional compression rate of a single pass is <10%, the above effects are significantly weakened, making it difficult to fully achieve the low anisotropy, high dimensional accuracy, and high strength targets required by this application.

[0135] In this application, after rotary rolling finishing, the longitudinal and transverse yield strength, tensile strength, and elongation differences of the pipe are all ≤5%, the residual stress difference is controlled within 30MPa, and the texture strength is controlled below 3 times the randomness. Therefore, this method fundamentally improves the anisotropy of the material.

[0136] In the rotary rolling finishing stage, the rotary rolling temperature, feed rate, rolling force, roll speed, instantaneous cross-sectional compression ratio per pass, and cooling conditions jointly determine the stress state, shear deformation degree, and surface forming quality of the tube in the rotary rolling zone. Among them, the rotary rolling temperature determines the deformation resistance of the material under cold working conditions; the feed rate and roll speed jointly affect the deformation rhythm per unit time; the rolling force and instantaneous cross-sectional compression ratio per pass determine the strength of radial compression and circumferential shearing; and the cooling conditions are used to control the accumulation of processing heat and surface condition. After synergistic optimization of these parameters, a stable triaxial composite stress state can be formed in the tube during the rotary rolling process, promoting the weakening of fiber texture, reduction of residual stress, and further dimensional refinement. This avoids uneven deformation, surface damage, and insufficient randomization of the microstructure caused by excessively low temperature, excessive feed, excessive loading, or insufficient compression ratio, and also avoids surface overheating, dimensional fluctuations, and performance instability caused by excessively high temperature, uncontrolled rolling force, or insufficient cooling. Thus, a synergistic improvement in low anisotropy, high dimensional accuracy, and low magnetic permeability stability is achieved.

[0137] S8. Tempering and heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1050~1100℃, the solution annealing time is 30min, the cooling rate during water quenching is >100℃ / s, the tempering temperature is 450~500℃, and the tempering time is 120min.

[0138] In this application, when implementing the quenching and tempering heat treatment process, water quenching at a cooling rate >100℃ / s rapidly cools the tube to room temperature, ensuring grain refinement and maintaining a uniform microstructure, which is beneficial for subsequent material processing. It also avoids martensite formation and excessive hardness due to overcooling. Furthermore, a cooling rate >100℃ / s can suppress undesirable precipitation and maintain the stability of the microstructure after solution treatment. Simultaneously, combined with annealing softening treatment, it helps to reduce the deformation microstructure introduced by rotary rolling finishing and lower residual stress.

[0139] During tempering, if the tempering temperature is too low, residual stress cannot be fully released, and the overall stability of the material's properties is insufficient; if the tempering temperature is too high, it may affect the material's strength, microstructure stability, and corrosion resistance. Therefore, in this application, the tempering temperature is controlled at 450–500°C to further reduce residual stress, adjust the strength-toughness ratio, and stabilize the material's overall properties.

[0140] Correspondingly, during tempering, if the tempering time is too short, the internal stress may not be fully released, affecting the stability of the material properties; if the tempering time is too long, it may lead to the risk of microstructural changes and adversely affect the strength and corrosion resistance. Therefore, in this application, the tempering time is controlled at 120 minutes to ensure the tempering effect while taking into account the stability of the overall performance.

[0141] During the quenching and tempering heat treatment stage, solution temperature, holding time, cooling rate, tempering temperature, and tempering duration work together to restore the microstructure, eliminate residual stress, maintain austenite stability, and regulate the overall performance of the rolled tube. Solution temperature and holding time primarily improve the microstructure and reduce residual stress after rolling. Water quenching suppresses undesirable precipitation and stabilizes the low magnetic structure. Tempering further reduces residual stress, adjusts the strength-toughness ratio, and stabilizes the overall performance of the material. By controlling these parameters synergistically, yield strength can be improved and overall performance stabilized while maintaining low magnetic permeability and corrosion resistance. This avoids insufficient microstructure recovery, magnetic permeability fluctuations, and residual stress caused by insufficient solution treatment or cooling, as well as the risk of precipitation, decreased corrosion resistance, and strength-toughness imbalance caused by uncontrolled tempering temperature or time. This ensures that the finished drill collar possesses stable high strength, low magnetic permeability, and reliable service performance.

[0142] It should be noted that in this application, annealing and softening is the first solution treatment, and tempering heat treatment is the second solution treatment. The first solution treatment is to soften the material, reduce hardness, and give the pipe more suitable plasticity and a more uniform initial microstructure before entering the spinning and rolling finishing process. The second solution treatment is to improve the microstructure of the material after spinning and rolling finishing, reduce residual stress, and achieve stable control of low magnetic permeability. If the annealing and softening treatment is skipped in this application, and only tempering heat treatment is performed, the pipe will have higher hardness during the spinning and rolling finishing process, increasing the forming difficulty and significantly raising the risk of surface quality issues. If only annealing and softening treatment is performed on the pipe without tempering heat treatment, the spinning and rolling finishing will generate residual stress inside the pipe, and the microstructure will be unstable. This is very likely to cause fluctuations in the magnetic permeability and corrosion resistance of the pipe, affecting the overall performance of the pipe. Therefore, this application uses a two-stage solution treatment process to ensure that the pipe has excellent performance and stable quality during subsequent processing and use.

[0143] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0144] In this application, the machining process specifically includes: blanking to length, rough turning of the outer diameter, finishing of the inner diameter, machining of the ends and transition parts, machining of the connecting parts, finishing, and cleaning and final inspection, wherein:

[0145] The blanking process involves cutting the non-magnetic stainless steel drill collar blank to near-net-shape length according to the target drill collar length, and leaving a total machining allowance of 2-8mm at both ends.

[0146] Rough turning of the outer diameter is to rough turn the outer diameter of the near-net-shape blank of the non-magnetic stainless steel drill collar to remove oxide scale, heat treatment deformation layer and local surface defects, leaving a finishing allowance of 0.5 to 2.0 mm on each side;

[0147] Internal hole finishing involves using one or more of the following methods: boring, reaming, rolling, and honing to finish the internal hole in order to improve its roundness, coaxiality, and surface quality.

[0148] End and transition machining involves machining the end faces, shoulders, chamfers, and transition fillets of both ends of the drill collar;

[0149] The machining of the connection parts involves machining the threads, mating surfaces, or sealing surfaces of the drill collar joint connection parts;

[0150] Precision machining involves one or more of the following processes: precision turning, grinding, and polishing, on the outer diameter, inner hole, ends, and connecting parts, so that the outer diameter accuracy of the finished product reaches ±0.05mm and the surface roughness Ra≤0.8μm.

[0151] Cleaning and final inspection involve degreasing and cleaning the finished product, and inspecting its outer diameter, inner diameter, wall thickness, straightness, coaxiality, surface roughness, thread accuracy, magnetic permeability, and mechanical properties. Once qualified, the finished product is a non-magnetic stainless steel drill collar.

[0152] During the machining stage, there is a synergistic relationship between blanking allowance, roughing allowance, internal hole trimming method, and finishing parameters. Blanking and the reserved allowance determine the subsequent machining correction space; roughing removes oxide scale, deformed layers, and surface defects; internal hole trimming ensures roundness, coaxiality, and surface quality; and finishing directly determines the outer diameter accuracy, surface roughness, and forming quality of the connecting parts. By properly matching these parameters and steps, while fully utilizing the near-net-shape results from the preceding steps, machining efficiency, dimensional accuracy, and surface integrity can be balanced. This avoids incomplete defect removal and insufficient dimensional correction space due to insufficient allowance, as well as increased machining deformation, decreased efficiency, and dimensional consistency fluctuations caused by excessive allowance or improper process connections. This ensures a stable production of non-magnetic stainless steel drill collars that meet the requirements for outer diameter accuracy, surface roughness, and assembly.

[0153] In this application, the non-magnetic stainless steel drill collars obtained have a yield strength ≥980MPa, magnetic permeability μr≤1.005, outer diameter accuracy up to ±0.05mm, surface roughness Ra≤0.8μm, residual stress difference of 18~23MPa, texture strength of 1.9~2.3 times random, average grain size of 16.9~20.1μm, longitudinal and transverse yield, tensile and elongation differences of 3~5%, neutral salt spray test duration of 800~900h, pitting corrosion observation results of no obvious pitting corrosion, G150 critical pitting corrosion temperature of 48~58℃; the worst result in the G48 crevice corrosion test is slight crevice corrosion, and the maximum crevice corrosion depth is 5~18μm.

[0154] The second aspect of the present invention provides a non-magnetic stainless steel drill collar, which is manufactured using a method for preparing a non-magnetic stainless steel drill collar.

[0155] The third aspect of this invention provides an application of a non-magnetic stainless steel drill collar in the field of oil drilling and production.

[0156] Based on the aforementioned method for preparing a non-magnetic stainless steel drill collar, the present invention conducted the following sets of experiments to verify the performance of the non-magnetic stainless steel drill collar prepared by this method.

[0157] Example 1

[0158] A method for preparing a non-magnetic stainless steel drill collar includes the following steps:

[0159] S1. Billet Production: After the billet is melted into molten steel in an electric arc furnace, it is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel. The chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C 0.04wt.%, N 0.65wt.%, Mo 0.60wt.%, Mn 21.0wt.%, Cr 18.2wt.%, Ni 10.5wt.%, S 0.008wt.%, P 0.020wt.%, with the balance being Fe and unavoidable impurity elements.

[0160] S2. Piercing: The tube blank is heated to 1200℃ for a heating time of 3 min / mm (thickness of tube blank). Then, the uniformly heated tube blank is pierced using a skew rolling piercing mill to obtain the tube. After piercing, the tube is cooled to 850℃ at a cooling rate of 5℃ / s. The piercing process is as follows: mandrel temperature is 1150℃, piercing speed is 40mm / s, and mandrel compression ratio is 1.8.

[0161] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature at a cooling rate of 10℃ / s. The rolling temperature is 1000℃, the rolling speed is 12m / min, the roll pressure is 150 tons, the number of hot rolling passes is 6, and the reduction per pass is 20%.

[0162] S4. Reduction Finishing: The tube is further reduced and finished by a multi-stand reduction mill. At the same time, a water mist cooling system is used to cool the tube during the reduction finishing process. The single-stand reduction ratio is 1.05, the total reduction ratio is 1.22, the rolling speed is 18m / min, and the cooling rate of the water mist cooling system is 14℃ / s.

[0163] S5. Segmented Straightening: The reduced-diameter and finished pipe is air-cooled to 540℃ at a cooling rate of 1.5℃ / s. Then, a hot straightener is used to hot-straighten the air-cooled pipe. After hot straightening, the pipe is naturally cooled to room temperature. Finally, a cold multi-roller straightener is used to perform final fine straightening on the pipe cooled to room temperature. The hot straightening process parameters are: straightening temperature 480℃, straightening pressure 120 tons, hot straightening times 2 times, single reduction 3mm, straightening speed 0.8m / s. The final fine straightening process parameters are: fine straightening pressure 40 tons, fine straightening times 1 time, fine straightening single reduction 1.5mm, fine straightening speed 0.4m / s.

[0164] S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is 120℃ / s.

[0165] S7. Spin Roll Finishing: A three-roll spin mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 90℃, the feed rate is 0.30mm / r, the rolling force is 60kN, the roll speed is 80rpm, the spin rolling finishing is done in 2 passes, the instantaneous cross-sectional compression rate per pass is 10%, and the cooling rate of the water mist cooling system is 8℃ / s.

[0166] S8. Tempering and heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1050℃, the solution annealing time is 30min, the water quenching cooling rate is 120℃ / s, the tempering temperature is 500℃, and the tempering time is 120min.

[0167] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0168] In this application, metallographic analysis was used to observe the microstructure of the non-magnetic stainless steel drill collar prepared by the method described in Example 1. The resulting metallographic structure diagram is shown below. Figure 2 As shown. From Figure 2 It can be seen that the obtained non-magnetic stainless steel drill collar has a relatively uniform structure and fine grains.

[0169] Example 2

[0170] A method for preparing a non-magnetic stainless steel drill collar includes the following steps:

[0171] S1. Billet Production: After the billet is melted into molten steel in an electric arc furnace, it is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel. The chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C 0.06wt.%, N 0.50wt.%, Mo 0.50wt.%, Mn 18.0wt.%, Cr 16.0wt.%, Ni 8.0wt.%, S0.02wt.%, P 0.03wt.%, with the balance being Fe and unavoidable impurity elements.

[0172] S2. Piercing: The tube blank is heated to 1200℃ for a heating time of 3 min / mm (thickness of tube blank). Then, the uniformly heated tube blank is pierced using a skew rolling piercing mill to obtain the tube. After piercing, the tube is cooled to 840℃ at a cooling rate of 4℃ / s. The piercing process is as follows: mandrel temperature is 1150℃, piercing speed is 40mm / s, and mandrel compression ratio is 1.8.

[0173] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature at a cooling rate of 9℃ / s. The rolling temperature is 1000℃, the rolling speed is 12m / min, the roll pressure is 150 tons, the number of hot rolling passes is 6, and the reduction per pass is 20%.

[0174] S4. Reduction Finishing: The tube is further reduced and finished by a multi-stand reduction mill. At the same time, a water mist cooling system is used to cool the tube during the reduction finishing process. The single-stand reduction ratio is 1.05, the total reduction ratio is 1.22, the rolling speed is 18m / min, and the cooling rate of the water mist cooling system is 12℃ / s.

[0175] S5. Segmented Straightening: The reduced-diameter and finished pipe is air-cooled to 545℃ at a cooling rate of 1℃ / s. Then, a hot straightener is used to hot-straighten the air-cooled pipe. After hot straightening, the pipe is naturally cooled to room temperature. Finally, a cold multi-roller straightener is used to perform final fine straightening on the pipe cooled to room temperature. The hot straightening process parameters are: straightening temperature 470℃, straightening pressure 100 tons, hot straightening times 2 times, single reduction 2mm, straightening speed 0.6m / s. The final fine straightening process parameters are: fine straightening pressure 30 tons, fine straightening times 1 time, fine straightening single reduction 1mm, fine straightening speed 0.3m / s.

[0176] S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is 120℃ / s.

[0177] S7. Spin Roll Finishing: A three-roll spin mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 90℃, the feed rate is 0.30mm / r, the rolling force is 60kN, the roll speed is 80rpm, the spin rolling finishing is done in 2 passes, the instantaneous cross-sectional compression rate per pass is 10%, and the cooling rate of the water mist cooling system is 7℃ / s.

[0178] S8. Tempering and heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1050℃, the solution annealing time is 30min, the water quenching cooling rate is 120℃ / s, the tempering temperature is 500℃, and the tempering time is 120min.

[0179] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0180] Example 3

[0181] A method for preparing a non-magnetic stainless steel drill collar includes the following steps:

[0182] S1. Billet Production: After the billet is melted into molten steel in an electric arc furnace, it is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel. The chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C 0.02wt.%, N 0.80wt.%, Mo 0.80wt.%, Mn 25.0wt.%, Cr 20.0wt.%, Ni 12.0wt.%, S0.003wt.%, P 0.01wt.%, with the balance being Fe and unavoidable impurity elements.

[0183] S2. Piercing: The tube blank is heated to 1200℃ for a heating time of 3 min / mm (thickness of tube blank). Then, the uniformly heated tube blank is pierced using a skew rolling piercing mill to obtain the tube. After piercing, the tube is cooled to 860℃ at a cooling rate of 6℃ / s. The piercing process is as follows: mandrel temperature is 1150℃, piercing speed is 40mm / s, and mandrel compression ratio is 1.8.

[0184] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature at a cooling rate of 11℃ / s. The rolling temperature is 1050℃, the rolling speed is 12m / min, the roll pressure is 150 tons, the number of hot rolling passes is 6, and the reduction per pass is 21%.

[0185] S4. Reduction Finishing: The tube is further reduced and finished by a multi-stand reduction mill. At the same time, a water mist cooling system is used to cool the tube during the reduction finishing process. The single-stand reduction ratio is 1.05, the total reduction ratio is 1.22, the rolling speed is 18m / min, and the cooling rate of the water mist cooling system is 16℃ / s.

[0186] S5. Segmented Straightening: The reduced-diameter and finished pipe is air-cooled to 535℃ at a cooling rate of 2℃ / s. Then, a hot straightening machine is used to hot-straighten the air-cooled pipe. After hot straightening, the pipe is naturally cooled to room temperature. Finally, a cold multi-roller straightening machine is used to perform final fine straightening on the pipe cooled to room temperature. The hot straightening process parameters are: straightening temperature 490℃, straightening pressure 140 tons, hot straightening times 2 times, single reduction 4mm, and straightening speed 1m / s. The final fine straightening process parameters are: fine straightening pressure 50 tons, fine straightening times 1 time, fine straightening single reduction 2mm, and fine straightening speed 0.5m / s.

[0187] S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is 120℃ / s.

[0188] S7. Spin Roll Finishing: A three-roll spin mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 90℃, the feed rate is 0.30mm / r, the rolling force is 60kN, the roll speed is 80rpm, the spin rolling finishing is done in 2 passes, the instantaneous cross-sectional compression rate per pass is 10%, and the cooling rate of the water mist cooling system is 9℃ / s.

[0189] S8. Tempering and heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1050℃, the solution annealing time is 30min, the water quenching cooling rate is 120℃ / s, the tempering temperature is 500℃, and the tempering time is 120min.

[0190] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0191] Example 4

[0192] A method for preparing a non-magnetic stainless steel drill collar includes the following steps:

[0193] S1. Billet Production: After the billet is melted into molten steel in an electric arc furnace, it is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel. The chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C 0.04wt.%, N 0.65wt.%, Mo 0.60wt.%, Mn 21.0wt.%, Cr 18.2wt.%, Ni 10.5wt.%, S 0.008wt.%, P 0.020wt.%, with the balance being Fe and unavoidable impurity elements.

[0194] S2. Piercing: The tube blank is heated to 1150℃ for a heating time of 3 min / mm (thickness of tube blank). Then, the uniformly heated tube blank is pierced using a skew rolling piercing mill to obtain the tube. After piercing, the tube is cooled to 820℃ at a cooling rate of 2℃ / s. The piercing process is as follows: mandrel temperature is 1050℃, piercing speed is 30 mm / s, and mandrel compression ratio is 1.5.

[0195] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature at a cooling rate of 5℃ / s. The rolling temperature is 950℃, the rolling speed is 10m / min, the roll pressure is 100 tons, the number of hot rolling passes is 6, and the reduction per pass is 18%.

[0196] S4. Reduction Finishing: The tube is further reduced and finished by a multi-stand reduction mill. At the same time, a water mist cooling system is used to cool the tube during the reduction finishing process. The single-stand reduction ratio is 1.02, the total reduction ratio is 1.10, the rolling speed is 15m / min, and the cooling rate of the water mist cooling system is 8℃ / s.

[0197] S5. Segmented Straightening: The reduced-diameter and finished pipe is air-cooled to 545℃ at a cooling rate of 0.5℃ / s. Then, a hot straightener is used to hot-straighten the air-cooled pipe. After hot straightening, the pipe is naturally cooled to room temperature. Finally, a cold multi-roller straightener is used to perform final fine straightening on the pipe cooled to room temperature. The hot straightening process parameters are: straightening temperature 450℃, straightening pressure 50 tons, hot straightening times 1, single reduction 1mm, straightening speed 0.2m / s. The final fine straightening process parameters are: fine straightening pressure 10 tons, fine straightening times 1, fine straightening single reduction 0.5mm, fine straightening speed 0.1m / s.

[0198] S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is 120℃ / s.

[0199] S7. Spin Roll Finishing: A three-roll spin mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 90℃, the feed rate is 0.30mm / r, the rolling force is 60kN, the roll speed is 80rpm, the spin rolling finishing is done in 1 pass, the instantaneous cross-sectional compression rate per pass is 10%, and the cooling rate of the water mist cooling system is 6℃ / s.

[0200] S8. Heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1050℃, the solution annealing time is 30min, the water quenching cooling rate is 120℃ / s, the tempering temperature is 450℃, and the tempering time is 120min.

[0201] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0202] Example 5

[0203] A method for preparing a non-magnetic stainless steel drill collar includes the following steps:

[0204] S1. Billet Production: After the billet is melted into molten steel in an electric arc furnace, it is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel. The chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C 0.04wt.%, N 0.65wt.%, Mo 0.60wt.%, Mn 21.0wt.%, Cr 18.2wt.%, Ni 10.5wt.%, S 0.008wt.%, P 0.020wt.%, with the balance being Fe and unavoidable impurity elements.

[0205] S2. Piercing: The tube blank is heated to 1250℃ for a heating time of 3 min / mm (thickness of tube blank). Then, the uniformly heated tube blank is pierced using a skew rolling piercing mill to obtain the tube. After piercing, the tube is cooled to 880℃ at a cooling rate of 8℃ / s. The piercing process is as follows: mandrel temperature is 1200℃, piercing speed is 50mm / s, and mandrel compression ratio is 2.

[0206] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature at a cooling rate of 15℃ / s. The rolling temperature is 1100℃, the rolling speed is 15m / min, the roll pressure is 200 tons, the number of hot rolling passes is 6, and the reduction per pass is 24%.

[0207] S4. Reduction Finishing: The tube is further reduced and finished by a multi-stand reduction mill. At the same time, a water mist cooling system is used to cool the tube during the reduction finishing process. The single-stand reduction ratio is 1.05, the total reduction ratio is 1.28, the rolling speed is 20m / min, and the cooling rate of the water mist cooling system is 20℃ / s.

[0208] S5. Segmented Straightening: The reduced-diameter and finished pipe is air-cooled to 530℃ at a cooling rate of 3℃ / s. Then, a hot straightening machine is used to hot-straighten the air-cooled pipe. After hot straightening, the pipe is naturally cooled to room temperature. Finally, a cold multi-roller straightening machine is used to perform final fine straightening on the pipe cooled to room temperature. The hot straightening process parameters are: straightening temperature 500℃, straightening pressure 200 tons, hot straightening times 3 times, single reduction 6mm, straightening speed 1.5m / s. The final fine straightening process parameters are: fine straightening pressure 80 tons, fine straightening times 2 times, fine straightening single reduction 3mm, fine straightening speed 1m / s.

[0209] S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is 120℃ / s.

[0210] S7. Spin Roll Finishing: A three-roll spin mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 120℃, the feed rate is 0.50mm / r, the rolling force is 80kN, the roll speed is 90rpm, the spin rolling finishing is done in 3 passes, the instantaneous cross-sectional compression rate per pass is 12%, and the cooling rate of the water mist cooling system is 10℃ / s.

[0211] S8. Tempering and heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1100℃, the solution annealing time is 30min, the water quenching cooling rate is 120℃ / s, the tempering temperature is 500℃, and the tempering time is 120min.

[0212] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0213] Example 6

[0214] A method for preparing a non-magnetic stainless steel drill collar includes the following steps:

[0215] S1. Billet Production: After the billet is melted into molten steel in an electric arc furnace, it is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel. The chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C 0.04wt.%, N 0.65wt.%, Mo 0.60wt.%, Mn 21.0wt.%, Cr 18.2wt.%, Ni 10.5wt.%, S 0.008wt.%, P 0.020wt.%, with the balance being Fe and unavoidable impurity elements.

[0216] S2. Piercing: The tube blank is heated to 1200℃ for a heating time of 3 min / mm (thickness of tube blank). Then, the uniformly heated tube blank is pierced using a skew rolling piercing mill to obtain the tube. After piercing, the tube is cooled to 850℃ at a cooling rate of 5℃ / s. The piercing process is as follows: mandrel temperature is 1150℃, piercing speed is 40mm / s, and mandrel compression ratio is 1.8.

[0217] S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature at a cooling rate of 10℃ / s. The rolling temperature is 1000℃, the rolling speed is 12m / min, the roll pressure is 150 tons, the number of hot rolling passes is 6, and the reduction per pass is 20%.

[0218] S4. Reduction Finishing: The tube is further reduced and finished by a multi-stand reduction mill. At the same time, a water mist cooling system is used to cool the tube during the reduction finishing process. The single-stand reduction ratio is 1.05, the total reduction ratio is 1.22, the rolling speed is 18m / min, and the cooling rate of the water mist cooling system is 14℃ / s.

[0219] S5. Segmented Straightening: The reduced-diameter and finished pipe is air-cooled to 540℃ at a cooling rate of 1.5℃ / s. Then, a hot straightener is used to hot-straighten the air-cooled pipe. After hot straightening, the pipe is naturally cooled to room temperature. Finally, a cold multi-roller straightener is used to perform final fine straightening on the pipe cooled to room temperature. The hot straightening process parameters are: straightening temperature 480℃, straightening pressure 120 tons, hot straightening times 2 times, single reduction 3mm, straightening speed 0.8m / s. The final fine straightening process parameters are: fine straightening pressure 40 tons, fine straightening times 1 time, fine straightening single reduction 1.5mm, fine straightening speed 0.4m / s.

[0220] S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is 120℃ / s.

[0221] S7. Spin Roll Finishing: A three-roll spin roll mill is used to spin roll the annealed and softened tubes. At the same time, a water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 60℃, the feed rate is 0.20mm / r, the rolling force is 50kN, the roll speed is 60rpm, the spin rolling finishing is done in 2 passes, the instantaneous cross-sectional compression rate per pass is 10%, and the cooling rate of the water mist cooling system is 5℃ / s.

[0222] S8. Tempering and heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1050℃, the solution annealing time is 30min, the water quenching cooling rate is 120℃ / s, the tempering temperature is 500℃, and the tempering time is 120min.

[0223] S9. Machining: Machining is performed on the near-net-shape blank of the non-magnetic stainless steel drill collar to obtain the finished non-magnetic stainless steel drill collar.

[0224] Comparative Example 1

[0225] Unlike Example 1, Comparative Example 1 did not perform "S7, Spinning Finishing".

[0226] Comparative Example 2

[0227] Unlike Example 1, in Comparative Example 2, "S7, rotary rolling finishing" was replaced with traditional cold drawing finishing, the drawing speed was 0.8 to 1.5 m / min, the drawing force was 300 to 450 kN, the number of drawing times was 1, and no intermediate annealing was performed during the cold drawing process.

[0228] Comparative Example 3

[0229] Unlike Example 1, Comparative Example 3 did not perform "S6, Annealing and Softening".

[0230] Comparative Example 4

[0231] Unlike Example 1, Comparative Example 4 did not perform "S8, tempering heat treatment".

[0232] Comparative Example 5

[0233] Unlike Example 1, the instantaneous cross-sectional compression rate in step "S7, rotary rolling finishing" of Comparative Example 5 is 8%.

[0234] Comparative Example 6

[0235] Unlike Example 1, the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel in Comparative Example 6 has an N content of 0.45 wt.% in its chemical element composition.

[0236] Performance testing: room temperature tensile properties testing, magnetic permeability testing, hardness testing, surface roughness testing, dimensional accuracy testing, XRD residual stress testing, texture strength analysis, grain size testing, and corrosion resistance testing.

[0237] In this experiment, room temperature tensile properties were tested according to the requirements of GB / T 228.1 standard. The yield strength, tensile strength, and elongation after fracture of the finished non-magnetic stainless steel drill collars were determined. For the room temperature tensile properties test, samples were taken along both the axial and transverse directions, with at least three parallel samples in each group, and the average value was taken.

[0238] In this experiment, the magnetic permeability of the samples was measured at room temperature using a magnetic permeability tester. At least five test points were taken on both the outer and inner surfaces, and the average and maximum values ​​were recorded. The Vickers hardness HV10 test method was used to measure the hardness of the samples, with tests conducted at the outer surface layer, 1 / 4 wall thickness, and 1 / 2 wall thickness, with at least three points at each location. The surface roughness Ra value of the samples was measured using a surface roughness meter, with at least three tests conducted along the axial direction for each sample, and the average value was recorded. The outer diameter, inner diameter, and wall thickness of the samples were measured using an outside micrometer, inside diameter gauge, and coordinate measuring machine, and dimensional deviations were calculated. The residual stress on the sample surface was measured using X-ray diffraction, with axial and circumferential residual stresses tested at at least three test points in each direction, and the average value was recorded. The texture analysis of the samples was performed using the X-ray diffraction pole figure method to evaluate the orientation intensity of the main crystal planes and the degree of texture concentration. The grain size of the samples was measured using metallographic methods, and the average grain size was statistically analyzed according to standard methods, with at least five fields of view for each sample.

[0239] In this experiment, the local corrosion resistance of the prepared non-magnetic stainless steel drill collar in a chlorine-containing environment was comprehensively analyzed through neutral salt spray test, pitting corrosion observation, crevice corrosion test, and critical pitting temperature test. The crevice corrosion test was conducted in accordance with ASTM G48 standard, using a ferric chloride solution system to evaluate the crevice corrosion sensitivity of the sample, recording whether corrosion occurred on the sample surface and in the crevice area, and measuring the maximum depth of crevice corrosion. The critical pitting temperature test was conducted in accordance with ASTM G150 standard, using the critical pitting temperature as a characterization index of the material's resistance to pitting corrosion.

[0240] In this experiment, the performance of the finished non-magnetic stainless steel drill collars prepared in Examples 1-6 and Comparative Examples 1-6 was tested. The test results are shown in Tables 1-5. Table 1 shows the test results of room temperature tensile properties, magnetic permeability and hardness. Table 2 shows the test results of surface roughness and dimensional accuracy. Table 3 shows the test results of XRD residual stress, texture strength and grain size. Table 4 shows the test results of longitudinal and transverse performance differences. Table 5 shows the test results of engineering applicability and corrosion resistance.

[0241] It should be noted that the longitudinal and transverse performance difference test results in this experiment are based on the axial and transverse tensile test results from the room temperature tensile performance test. ; ; In this experiment, the test results for longitudinal and transverse performance differences were retained to one decimal place.

[0242]

[0243]

[0244]

[0245]

[0246]

[0247] As can be seen from Tables 1-2, the non-magnetic stainless steel drill collars prepared in Examples 1-6 have a yield strength of 982-1002 MPa, a tensile strength of 1096-1145 MPa, an average permeability of 1.003-1.004, a maximum permeability of 1.004-1.005, an outer diameter deviation of -0.04-0.04 mm, and a surface roughness Ra of 0.58-0.73 μm. The non-magnetic stainless steel drill collars prepared in Comparative Examples 1-6 have a yield strength of 795-970 MPa, a tensile strength of 1020-1092 MPa, an average permeability of 1.006-1.01, a maximum permeability of 1.008-1.012, an outer diameter deviation of 0.03-0.07 mm, and a surface roughness Ra of 0.79-1.12 μm. Therefore, the non-magnetic stainless steel drill collar prepared by this invention exhibits excellent performance in terms of mechanical properties, magnetic properties, dimensional accuracy, and surface quality. It can achieve the comprehensive performance requirements of yield strength ≥980MPa, magnetic permeability μr≤1.005, outer diameter accuracy controlled within ±0.05mm, and surface roughness Ra≤0.8μm, which is significantly better than the non-magnetic stainless steel drill collars prepared by comparative examples 1-6.

[0248] As shown in Table 3, the non-magnetic stainless steel drill collars prepared in Examples 1-6 have an axial residual stress of 110–128 MPa, a circumferential residual stress of 92–105 MPa, a residual stress difference of 18–23 MPa, a texture strength of 1.9–2.3 times random, and an average grain size of 16.9–20.1 μm. In contrast, the non-magnetic stainless steel drill collars prepared in Comparative Examples 1-6 have an axial residual stress of 182–308 MPa, a circumferential residual stress of 122–182 MPa, a residual stress difference of 60–126 MPa, a texture strength of 3–5.8 times random, and an average grain size of 19.4–25.4 μm. Therefore, the non-magnetic stainless steel drill collars prepared in this invention exhibit significantly reduced axial and circumferential residual stress differences, significantly weakened texture strength, and finer average grain size. This further confirms that the synergistic combination of rotary rolling finishing and two solution treatments in this invention can effectively weaken the axial fiber texture formed during traditional processing, reduce residual stress concentration, and promote microstructure homogenization, thereby reducing material anisotropy.

[0249] Furthermore, the results of dimensional accuracy and surface quality show that the present invention, through the combination of "spin rolling finishing + subsequent machining", can not only obtain a near-net-shape finished non-magnetic stainless steel drill collar with high dimensional accuracy, but also stably achieve an outer diameter accuracy of ±0.05mm and a surface roughness Ra≤0.8μm after final machining, meeting the high-precision manufacturing requirements of drill collars.

[0250] As shown in Table 4, the longitudinal and transverse yield differences of the non-magnetic stainless steel drill collars prepared in Examples 1-6 were 3.4-3.8%, the longitudinal and transverse tensile differences were 3.1-3.4%, and the longitudinal and transverse elongation differences were 3.8-5.0%. In contrast, the longitudinal and transverse yield differences of the non-magnetic stainless steel drill collars prepared in Comparative Examples 1-6 were 5.8-8.9%, the longitudinal and transverse tensile differences were 5.9-8%, and the longitudinal and transverse elongation differences were 6.6-11.0%. Therefore, the longitudinal and transverse yield, tensile, and elongation differences of the non-magnetic stainless steel drill collars prepared by this invention are all controlled within 3-5%, while the longitudinal and transverse yield, tensile, and elongation differences of the non-magnetic stainless steel drill collars prepared in Comparative Examples 1-6 generally reach 6-11%. This further demonstrates that the method provided by this invention can significantly improve the anisotropy problem of materials, that is, reduce the longitudinal and transverse yield, tensile, and elongation differences, making the drill collar have a more stable and uniform mechanical response during actual service. Furthermore, this invention further confirms that the rotary rolling finishing plus two solution treatments provided by the present invention can effectively improve the anisotropy of materials.

[0251] As can be seen from Table 5, the non-magnetic stainless steel drill collars prepared in Examples 1-6 all have a magnetic permeability ≤1.005, an outer diameter deviation ≤±0.05mm, a surface roughness Ra ≤0.8μm, a yield strength ≥980MPa, a neutral salt spray test duration of 800-900h, and no obvious pitting corrosion observed. The worst result in the G48 crevice corrosion test was slight crevice corrosion, with a maximum crevice corrosion depth of 5-18μm. The critical pitting temperature of G150 was 48-58℃. The non-magnetic stainless steel drill collars prepared in Comparative Examples 1-6 all exhibited a magnetic permeability >1.005, an outer diameter deviation not completely ≤±0.05mm, a surface roughness Ra not completely ≤0.8μm, and a yield strength <980MPa. The neutral salt spray test duration was 600–720h. The best result observed in pitting corrosion was an increased tendency to pit. The best result in the G48 crevice corrosion test was moderate crevice corrosion, with a maximum crevice corrosion depth of 31–62μm. The critical pitting temperature for G150 was 34–43℃. Furthermore, even the least desirable G48 crevice corrosion test results in Examples 1-6 were superior to the best-performing G48 crevice corrosion test results in Comparative Examples 1-6. This fully demonstrates that the preparation method provided by this invention can still achieve corrosion resistance and engineering applicability far exceeding the optimal level of the comparative examples, even under relatively unfavorable combinations of material composition and process parameters. Therefore, it can be seen that the non-magnetic stainless steel drill collar prepared by the present invention exhibits good corrosion resistance in neutral salt spray test, pitting corrosion observation, G48 crevice corrosion test and G150 critical pitting temperature test, and also shows excellent performance in mechanical properties, magnetic properties, dimensional accuracy and surface quality. It has good comprehensive performance and strong engineering applicability.

[0252] It should be noted that the actual service environment of drill collars typically includes environments containing Cl. -In environments characterized by mud, H2S / CO2 corrosion, and high temperature and pressure, a single neutral salt spray test is insufficient to comprehensively characterize the service performance of a drill collar. Therefore, this invention combines multiple corrosion evaluation results to comprehensively analyze the applicability of the prepared non-magnetic stainless steel drill collar in the complex environment of oil drilling and production. The results show that the non-magnetic stainless steel drill collar prepared by this invention has good application potential.

[0253] As shown in Tables 1-5, Example 3, due to its high-end parameter combination in the design of key components (N, Mn, Cr, Ni, and Mo content), achieved a yield strength of 1002 MPa, a tensile strength of 1145 MPa, an average and maximum magnetic permeability of 1.003 and 1.004 respectively, and a surface roughness Ra of 0.58 μm. Furthermore, its residual stress difference, texture strength, and average grain size were the lowest among all examples. In addition, its longitudinal and transverse yield difference and longitudinal and transverse elongation difference were also the lowest, while its longitudinal and transverse tensile difference, although not the lowest, was the second lowest. Moreover, it exhibited the best corrosion resistance in neutral salt spray tests, pitting corrosion observation, G48 crevice corrosion tests, and G150 critical pitting temperature tests. Based on the above performance indicators, higher N, Mn, Cr, Ni, and Mo contents are beneficial for further improving austenite stability, enhancing solid solution strengthening effect, and improving resistance to localized corrosion. Furthermore, with the synergistic effect of rotary rolling finishing and quenching and tempering heat treatment, the material can achieve a more uniform and stable microstructure, thereby significantly improving its overall performance.

[0254] In this experiment, Examples 1-6 employed median baseline design, low-value material composition design, high-value material composition design, low-value process parameter design, high-value process parameter design, and low-value rotary rolling design, respectively, to systematically investigate the influence of variations in material composition parameters and process parameters within the scope of this invention on mechanical properties, magnetic properties, dimensional accuracy, anisotropy, and corrosion resistance. The test results in Tables 1-5 show that, within the scope defined by this invention, median or medium-to-high value matching designs are more conducive to obtaining non-magnetic stainless steel drill collars with balanced comprehensive performance. While excessive bias towards low values ​​can still meet basic performance requirements, it is relatively weaker in terms of strength, dimensional accuracy, surface quality, residual stress control, and anisotropy improvement. Example 1, employing a median matching design of material composition and process parameters, serves as the baseline implementation of this invention. It exhibits balanced performance in yield strength, permeability, dimensional accuracy, surface roughness, residual stress difference, texture strength, and corrosion resistance, indicating that the median parameter combination can effectively balance structural stability, forming stability, and overall service performance. Example 2 employs a low-value design for key material components, with N, Mo, Mn, Cr, and Ni all taking relatively low values. Compared to Example 1, Example 2 shows a slight decrease in yield strength, tensile strength, surface roughness, residual stress difference, texture strength, grain refinement effect, anisotropy, and corrosion resistance. However, it still meets the requirements of this invention for low magnetic permeability, high dimensional accuracy, and high strength, indicating that using low values ​​within the material composition range of this invention is still feasible, although the overall performance is slightly inferior to the combination of median or high-value material compositions. Example 3 employs a high-value design for key material components, with N, Mo, Mn, Cr, and Ni all taking values ​​close to the upper limit. Simultaneously, the process parameters are maintained at a relatively optimal matching level. Its yield strength, tensile strength, magnetic property stability, surface roughness, residual stress control, texture weakening degree, grain refinement effect, anisotropy, and corrosion resistance all reach optimal or near-optimal levels, indicating that higher contents of austenite stabilizing elements and corrosion-strengthening elements are more conducive to further improving the overall performance of the material. Example 4 employed a low-value design for the piercing, hot rolling sizing, reduction finishing, segmented straightening, and quenching and tempering heat treatment processes, and a slightly lower-value design for the rotary rolling finishing process. Although it met the basic performance requirements of this invention, it was relatively weak in terms of yield strength, surface roughness, dimensional accuracy, residual stress control, texture strength, grain refinement effect, and anisotropy. This indicates that when the overall process parameters are in the low-value window, the effects of material plastic flow, dimensional finishing, and texture control are relatively weakened. Example 5 employed a high-value design for the overall process parameters, which performed better in terms of strength, corrosion resistance, surface quality, and microstructure control, and its overall performance was superior to the combination of medium and low-value processes. However, because some process parameters were at a high level, its overall results were not superior to Example 3 in all indicators. This shows that although high-value process parameters are beneficial for strengthening forming and microstructure control, they still need to be matched with the material composition and subsequent heat treatment.Example 6 mainly examines the effects of low values ​​in the rotary finishing process parameters, where the rotary temperature, feed rate, rolling force, roll speed, and cooling rate are all set to low values. The test results show that it can still achieve good overall performance, but it is slightly inferior to Example 1 and Example 5 in terms of surface quality, residual stress control, and anisotropy improvement. This indicates that when the rotary finishing process parameters are at a low level, the synergistic effect of circumferential shear and radial compression is relatively weakened, which has an adverse effect on the randomization of the microstructure and the finishing effect.

[0255] Furthermore, as can be clearly seen from Tables 1-5, Comparative Example 1, due to the lack of rotary rolling finishing, exhibited a decrease in yield strength to 795 MPa, an increase in average and maximum magnetic permeability to 1.01 and 1.012 respectively, an increase in outer diameter deviation to 0.07 mm, and a deterioration in surface roughness Ra to 1.12 μm. Simultaneously, the residual stress difference reached as high as 126 MPa, the texture strength increased to 5.8 times that of random, the average grain size increased to 25.4 μm, the longitudinal and transverse yield difference reached 8.9%, the longitudinal and transverse tensile difference reached 8%, the longitudinal and transverse elongation difference reached 11.0%, the neutral salt spray test duration was only 600 h with significant local pitting corrosion, the G48 crevice corrosion test results showed significant crevice corrosion with a crevice corrosion depth as high as 62 μm, and the G150 critical pitting temperature was as low as 34 °C. Overall, its performance was the worst among Examples 1-6 and Comparative Examples 1-6. It can be seen that without spin rolling finishing, the strong fiber texture inside the material cannot be effectively weakened, the residual stress is difficult to redistribute, and the uniformity of the structure and the ability to finish the dimensions are significantly reduced, resulting in the simultaneous deterioration of strength, magnetic properties, dimensional accuracy, surface quality and anisotropy. Therefore, spin rolling finishing is the key step for the present invention to achieve comprehensive performance improvement.

[0256] Comparative Example 2, after replacing rotary rolling finishing with traditional cold drawing finishing, although the outer diameter deviation was controlled to 0.05 mm and the dimensional accuracy was improved compared to Comparative Example 1, its residual stress difference was as high as 117 MPa, the texture strength was 5.1 times that of random, the longitudinal and transverse yield difference reached 8%, the longitudinal and transverse tensile difference reached 7.9%, the longitudinal and transverse elongation difference reached 9.8%, the maximum magnetic permeability was 1.009, the surface roughness Ra was 0.91 μm, the salt spray test was 680 h and local pitting corrosion was obvious, the G48 crevice corrosion test results showed obvious crevice corrosion with a crevice corrosion depth of 48 μm, and the G150 critical pitting temperature was 38℃. This shows that although traditional cold drawing finishing can improve the dimensions to a certain extent, it cannot achieve texture randomization, residual stress reduction and magnetic property stability at the same time as the rotary rolling finishing provided by this invention. Therefore, its comprehensive performance is still significantly inferior to the non-magnetic stainless steel drill collars obtained in Examples 1-6 of this invention.

[0257] After the first solution treatment was removed in Comparative Example 3, the yield strength was 948 MPa, the hardness increased to 238 HV, the surface roughness Ra was 0.95 μm, the residual stress difference was 94 MPa, the texture strength was 4.3 times that of random, the longitudinal and transverse yield difference reached 7%, the longitudinal and transverse tensile difference reached 7.2%, and the longitudinal and transverse elongation difference reached 9.2%. This indicates that if the first solution softening is not performed, the material before spinning will have high hardness and insufficient plasticity, which will weaken the forming stability and surface quality of the subsequent spinning finishing, thereby affecting the material's microstructure control effect and comprehensive performance, resulting in the inability to meet the precision requirements of the finished non-magnetic stainless steel drill collar.

[0258] After the second solution treatment was removed in Comparative Example 4, the outer diameter deviation was controlled at 0.04 mm, and the surface roughness Ra was 0.83, which was significantly improved compared with Comparative Examples 1-3. However, the average and maximum magnetic permeability increased to 1.008 and 1.01, respectively, the residual stress difference was 110 MPa, the texture strength was 4.6 times that of random, the salt spray test was 690 h, and the pitting tendency increased. The critical pitting temperature of G150 was 39℃. This indicates that if the second solution treatment is not performed after spinning, the deformed structure and residual stress cannot be fully eliminated, the austenite stability is insufficient, and the magnetic properties and corrosion resistance will fluctuate. Therefore, the second solution treatment plays an important role in stabilizing low magnetic permeability and improving overall service performance.

[0259] Comparative Example 5, after reducing the instantaneous cross-sectional compression ratio of a single pass in the rotary rolling process to 8%, showed a yield strength of 952 MPa, a residual stress difference of 78 MPa, a texture strength of 3.7 times that of random, a longitudinal and transverse yield difference of 6%, a longitudinal and transverse tensile difference of 6.4%, a longitudinal and transverse elongation difference of 8.3%, an outer diameter deviation of 0.06 mm, and a surface roughness Ra of 0.88 μm. This indicates that when the instantaneous cross-sectional compression ratio of a single pass is lower than the limit (10%) of this invention, the coupling effect of radial compression and circumferential shear is weakened, and the effects of texture weakening, residual stress control, and dimensional finishing are not as good as those of the embodiments of this invention. Therefore, it is difficult to achieve the low anisotropy and high dimensional accuracy required by this invention. Therefore, the requirement of "instantaneous cross-sectional compression ratio of a single pass ≥ 10%" in this application has obvious criticality and necessity.

[0260] Comparative Example 6, by adjusting the key component parameters outside the scope of this invention, showed a decrease in yield strength to 918 MPa, an increase in average and maximum magnetic permeability to 1.009 and 1.011 respectively, an increase in average grain size to 24.8 μm, a salt spray test duration of 640 h with significant pitting corrosion, and a G48 crevice corrosion test with significant crevice corrosion and a crevice corrosion depth of 45 μm. The critical pitting temperature of G150 was 37 °C. This indicates that when the key components deviate from the scope defined by this invention, the stability of austenite, the solid solution strengthening effect, and the basis of corrosion resistance are all weakened. Even if other process conditions remain unchanged, it is difficult to obtain the low magnetic permeability, high strength, and excellent corrosion resistance achieved by the non-magnetic stainless steel drill collars prepared in Examples 1-6 of this invention. Therefore, the limitation of the Cr-Ni-Mo-N system composition range in this invention also plays a crucial role.

[0261] In summary, this invention, by limiting the content of each element in Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel and combining it with the process route of "piercing-hot rolling sizing-reduction finishing-segment straightening-annealing softening-spin-roll finishing-quenching and tempering heat treatment," especially the spin-roll finishing process, effectively improves the axial fiber texture during the tube forming process, significantly reduces the residual stress distribution and longitudinal and transverse performance differences, and obtains non-magnetic stainless steel drill collars with low magnetic permeability, high strength, high dimensional accuracy, excellent corrosion resistance, and low anisotropy. In addition, compared with non-magnetic stainless steel drill collars produced by traditional process routes, the process route provided by this invention effectively improves material utilization, reduces manufacturing costs, and has broad application prospects.

[0262] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a non-magnetic stainless steel drill collar, characterized in that, Includes the following steps: S1. Billet Production: After the billet is melted into molten steel in an electric arc furnace, it is refined through AOD refining and VOD vacuum degassing processes. Then, the refined molten steel is continuously cast into billets using a continuous casting process. The billet is a Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel. The chemical element content of the Cr-Ni-Mo-N series low-carbon austenitic non-magnetic stainless steel is as follows: C≤0.06wt.%, N 0.5~0.8wt.%, Mo 0.5~0.8wt.%, Mn 18~25wt.%, Cr 16~20wt.%, Ni 8~12wt.%, S≤0.02wt.%, P≤0.03wt.%, with the balance being Fe and unavoidable impurity elements. S2. Piercing: The tube blank is uniformly heated and then pierced to obtain the tube. After piercing, the tube is cooled to 820-880℃. S3. Hot rolling sizing: The pipe is reheated to reach the rolling temperature, and then the pipe is hot rolled in multiple passes. After hot rolling, the pipe is cooled to room temperature. S4. Reduction and Finishing: The pipe is further reduced and finished by a multi-stand reduction machine. At the same time, a water mist cooling system is used to cool the pipe during the reduction and finishing process. S5. Segmented Straightening: The reduced-diameter finished pipe is air-cooled to below 550℃ at a cooling rate of 0.5–3℃ / s. Then, a hot straightener is used to hot-straighten the air-cooled pipe. After hot straightening, the pipe is naturally cooled to room temperature. Finally, a cold multi-roller straightener is used to perform final straightening on the cooled pipe. The hot straightening process parameters are: straightening temperature 450–500℃, straightening pressure 50–200 tons, number of hot straightening cycles 1–3, single reduction 1–6 mm. The straightening speed is 0.2–1.5 m / s; the final fine straightening process parameters are: fine straightening pressure of 10–80 tons, fine straightening times of 1–2 times, single fine straightening reduction of 0.5–3.0 mm, and fine straightening speed of 0.1–1.0 m / s; after hot straightening of the air-cooled pipe, the straightness of the pipe is ≤0.3 mm / m; after final fine straightening of the pipe cooled to room temperature, the straightness of the pipe is ≤0.3 mm / m, and the outer diameter deviation and wall thickness deviation are both controlled within ±0.2 mm; S6. Annealing and softening: The segmented straightened pipe is subjected to solution annealing, followed by water quenching to room temperature. The solution annealing temperature is 1050℃, the solution annealing time is 30min, and the cooling rate is >100℃ / s. After annealing and softening, the hardness of the pipe is ≤180HB. S7. Spin Roll Finishing: A three-roll spin mill is used to spin roll the annealed and softened tubes. A water mist cooling system is used to cool the tubes during the spin rolling finishing process. The spin rolling temperature is 60–120℃, the feed rate is 0.2–0.5 mm / r, the rolling force is 50–80 kN, the roll speed is 60–90 rpm, and the number of spin rolling finishing passes is 1–3. The instantaneous cross-sectional compression rate per pass is ≥10%, and the cooling rate of the water mist cooling system is 5–10℃ / s. After spin rolling finishing, the longitudinal and transverse yield strength, tensile strength, and elongation differences of the tubes are all ≤5%, the residual stress difference is controlled within 30 MPa, and the texture strength is controlled below 3 times the randomness. S8. Heat treatment: The tube after rotary rolling and finishing is subjected to solution annealing, followed by water quenching to room temperature, and then tempering. After tempering, the tube is naturally air-cooled to room temperature to obtain a near-net-shape blank for non-magnetic stainless steel drill collars. The solution annealing temperature is 1050~1100℃, the solution annealing time is 30min, the cooling rate during water quenching is >100℃ / s, the tempering temperature is 450~500℃, and the tempering time is 120min. S9. Machining: The near-net-shape blank of the non-magnetic stainless steel drill collar is machined to obtain the finished non-magnetic stainless steel drill collar. The yield strength of the obtained non-magnetic stainless steel drill collar is ≥980MPa, the magnetic permeability μr≤1.005, the outer diameter accuracy can reach ±0.05mm, the surface roughness Ra≤0.8μm, and the difference in yield strength, tensile strength and elongation in the longitudinal and transverse directions is 3~5%.

2. The method for preparing a non-magnetic stainless steel drill collar according to claim 1, characterized in that, The uniform heat treatment process specifically involves heating the tube blank to 1150–1250℃. The heating time is determined based on the tube blank thickness, and the calculation formula is shown below: ， In the formula, Heating time, in minutes; The thickness of the tube blank is in mm; Heating time per millimeter of tube blank thickness. =3min / mm; In step S2, a skew rolling piercing mill is used to pierce the uniformly heated tube blank. The piercing process is as follows: the mandrel temperature is 1050-1200℃, the piercing speed is 30-50mm / s, and the mandrel compression ratio is 1.5-2.

0. After piercing, the tube is cooled to 820-880℃ by a constant-speed air cooling system at a cooling rate of 2-8℃ / s.

3. The method for preparing a non-magnetic stainless steel drill collar according to claim 1, characterized in that, During hot rolling sizing, the rolling temperature is 950–1100℃, the rolling speed is 10–15 m / min, the roll pressure is 100–200 tons, the hot rolling is 6 passes, and the reduction per pass is 18–24%. After hot rolling, a water mist cooling system is used to cool the tube to room temperature at a cooling rate of 5–15℃ / s. During the diameter reduction finishing process, the single-stand diameter reduction ratio is 1.02 to 1.05, the total diameter reduction ratio is 1.1 to 1.3, the rolling speed is 15 to 20 m / min, and the cooling rate of the water mist cooling system is 8 to 20 ℃ / s.

4. The method for preparing a non-magnetic stainless steel drill collar according to claim 1, characterized in that, After hot rolling and sizing, the cumulative cross-sectional reduction rate of the pipe is 70% to 80%.

5. The method for preparing a non-magnetic stainless steel drill collar according to claim 1, characterized in that, The obtained non-magnetic stainless steel drill collars had a residual stress difference of 18–23 MPa, a texture strength of 1.9–2.3 times random, an average grain size of 16.9–20.1 μm, a neutral salt spray test duration of 800–900 h, and no obvious pitting corrosion was observed. The critical pitting temperature of G150 was 48–58 °C. The worst result in the G48 crevice corrosion test was slight crevice corrosion, and the maximum crevice corrosion depth was 5–18 μm.

6. A non-magnetic stainless steel drill collar, characterized in that, It is manufactured using the preparation method of a non-magnetic stainless steel drill collar according to any one of claims 1-5.

7. A non-magnetic stainless steel drill collar according to claim 6, characterized in that, The non-magnetic stainless steel drill collar is used in the field of oil drilling and production.

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