Fluid conveying cold-drawn stainless steel seamless pipe and method for manufacturing the same

Abstract

The application discloses a cold-drawing stainless steel seamless pipe for fluid delivery and a preparation method thereof, and relates to the technical field of seamless pipe manufacturing. The steel pipe comprises the following components in percentage by mass: C: <=0.02%, Si: 0.4-0.6%, Mn: 7.5-8.0%, Cr: 16.5-17.5%, Ni: 3.5-4.5%, Al: 0.01-0.05%, N: 0.25-0.30%, S: 0.015-0.002%, Ca: 0.003-0.004%, Fe as the balance, and the mass ratio of Ca to S is controlled to be 1-2. Optionally, the steel pipe further comprises micro-alloy elements such as Nb, Ti and Mo, and key proportion parameters such as Mn / N, Mn / Ni and Ti / N are limited. The steel pipe is designed through a Ca / S inclusion modification mechanism and an austenite stable component proportioning design, balling and plasticizing of inclusion morphology are realized, cold-drawing crack sensitivity is significantly reduced, a synergistic strengthening system of strength and toughness and corrosion resistance is constructed, and material service reliability is improved.

Description

Technical Field

[0001] This invention relates to the technical field of seamless steel pipe manufacturing, and in particular to cold-drawn stainless steel seamless pipes for fluid transportation and their preparation methods. Background Technology

[0002] Stainless steel seamless pipes are widely used in demanding fluid transport applications in industries such as petrochemicals, chemicals, power, pharmaceuticals, and food due to their excellent mechanical properties and corrosion resistance. Among them, stainless steel seamless pipes manufactured using the cold-drawing process offer advantages such as high dimensional accuracy, excellent surface quality, and uniform, dense microstructure, making them a key pipe type for achieving high-precision, high-stability fluid transport. However, as application environments expand towards extreme conditions such as high pressure, high flow rate, strong corrosion, and low temperatures, traditional cold-drawn stainless steel seamless pipes have revealed a series of performance bottlenecks during forming, processing, and service.

[0003] During cold drawing, steel pipes undergo multiple plastic deformations, leading to localized stress concentrations and uneven strain, which can easily cause crack initiation and propagation. This becomes a significant factor affecting the yield and reliability of the finished pipe. In particular, irregularly shaped, unevenly distributed inclusions with poor plasticity are more likely to become microcrack initiation sites. Furthermore, the induced martensitic transformation during cold working can also cause localized embrittlement in the austenitic matrix, thereby reducing the fracture toughness and corrosion resistance of the finished pipe.

[0004] On the other hand, to meet the corrosion resistance requirements during the transportation of complex fluid media, it is often necessary to increase the content of alloying elements such as Cr and Ni in the material or introduce strengthening elements such as Mo and N. However, such adjustments are often accompanied by side effects such as increased costs, increased difficulty in microstructure control, and inclusion variation. Especially in high-nitrogen and microalloyed steels, the coordinated design of inclusion control, alloying element ratio balance, and microstructure stability faces technical challenges. Traditional processes have limited ability to control inclusion morphology, making it difficult to balance inclusion plasticity and microstructure consistency, thus affecting the stability of the cold drawing process and the overall performance consistency of the steel pipe.

[0005] Therefore, there is an urgent need to develop a high-performance cold-drawn stainless steel seamless pipe with good cold working adaptability, microstructure stability and corrosion resistance, and its preparation method. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a cold-drawn stainless steel seamless pipe for fluid transportation and its preparation method.

[0007] The first aspect of the present invention discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by weight percentage:

[0008] C: ≤0.02%;

[0009] Si: 0.4–0.6%;

[0010] Mn: 7.1–8.0%;

[0011] Cr: 16.5–17.5%;

[0012] Ni: 3.5–4.5%;

[0013] Al: 0.01–0.05%;

[0014] N: 0.25–0.30%;

[0015] S: 0.015~0.002%;

[0016] Ca: 0.003–0.004%;

[0017] Fe: Balance;

[0018] The ratio of Ca content to S content is controlled to be 1 to 2.

[0019] The seamless steel pipe components of the present invention are mainly based on the inclusion modification control mechanism of Ca / S ratio and the stable austenitic structure design regulated by high Mn synergistic nitrogen content. The synergistic effect of the two results in a significant improvement in the structural stability, mechanical reliability and service durability of the steel pipe during cold drawing process, making it particularly suitable for fluid transportation fields with extremely high requirements for forming and processing performance and service performance.

[0020] By controlling the sulfur (S) content, calcium (Ca) content, and Ca / S ratio, it is possible to effectively promote the transformation of sulfur-formed sulfide inclusions in molten steel from hard, brittle, and irregular MnS morphology into well-plastic, spherical or ellipsoidal Ca-Al-Si-O composite inclusions. These modified inclusions are less likely to become stress concentration points during cold drawing, effectively inhibiting the initiation and propagation of microcracks, significantly reducing cold working crack sensitivity, and significantly improving dimensional stability and yield during cold drawing.

[0021] Secondly, this inclusion modification mechanism also achieves systematic optimization of the controllability and plasticity of inclusion morphology. The introduction of Ca increases the softening temperature range of non-metallic inclusions, which helps them deform in coordination with the matrix during hot and cold working, avoiding them from becoming crack initiation sites, and thus improving the overall deformation consistency and continuity of the steel pipe. At the same time, the reduction in inclusion size and uniform distribution effectively avoids the generation of local stress concentration, which is a key factor in achieving a balance between high strength and toughness and high machinability.

[0022] Furthermore, through a multi-element synergistic design with high manganese (Mn) content (7.1%–8.0%), moderate nickel (Ni, 3.5%–4.5%), and relatively high nitrogen (N, 0.25%–0.30%) content, stable control of the austenitic phase was achieved. Both manganese and nitrogen are strong austenite stabilizing elements; manganese significantly enhances solid solution strengthening and delays the induction of martensite, while nitrogen further increases the thermodynamic stability of the austenitic phase. During cold drawing, even after multiple accumulations of plastic strain, the steel maintains a stable austenitic microstructure, preventing induced martensite transformation and avoiding embrittlement or performance inhomogeneity caused by phase transformation. This stable microstructure is of great significance for improving the service reliability of finished steel pipes in low-temperature, high-pressure, or corrosive environments. With a composition of 16.5%–17.5% Cr and an appropriate amount of Ni, combined with a high-nitrogen and high-manganese design, this stainless steel pipe possesses both excellent pitting corrosion resistance and high-temperature oxidation resistance, meeting the dual requirements of high corrosion resistance and toughness in fluid transportation scenarios.

[0023] Through the design of the above-mentioned component ratios, the inclusion optimization effect brought about by the Ca / S modification mechanism significantly improves the macroscopic and microscopic uniformity of the matrix metal, providing a microstructural basis for the stability of the austenitic phase, reducing stress concentration points and microcrack initiation sites, thereby avoiding martensitic phase transformation induced by strain concentration. On the other hand, the optimization of inclusion morphology by Ca also helps to reduce local corrosion sensitivity. Combined with the corrosion-resistant system containing Cr and Ni, this gives the steel pipe a more durable passivation capability and pitting corrosion resistance in corrosive media.

[0024] Furthermore, it also includes 0.03 to 0.05 wt% Nb.

[0025] Furthermore, it also includes 0.02 to 0.04 wt% Ti.

[0026] Furthermore, it also includes 0.5 to 1.0 wt% Mo.

[0027] This invention further introduces the aforementioned Nb (niobium), Ti (titanium), and Mo (molybdenum) microalloying elements, and controls them within appropriate ranges, thereby achieving a comprehensive improvement in the strength, corrosion resistance, creep resistance, and thermal stability of the steel pipe without compromising the stability of the austenitic structure and the controllability of inclusions.

[0028] First, nitrogen (Nb) has a significant grain-refining strengthening effect. During hot and cold working, Nb can effectively form stable carbide particles such as NbC. These finely dispersed phases act as pinning agents at grain boundaries, significantly refining the grain structure and improving the yield strength and toughness of steel. Simultaneously, Nb can inhibit grain boundary migration, improving the microstructure stability of steel during heat treatment, especially during cold drawing, helping to control texture orientation and improve the dimensional consistency and processing stability of steel pipes. Furthermore, Nb can partially inhibit the precipitation of intergranular carbides, further enhancing resistance to intergranular corrosion.

[0029] Secondly, the introduction of Ti is mainly used to further improve the stability and crack resistance of inclusions. Ti has a strong affinity for carbon and nitrogen, preferentially forming dispersed precipitates such as TiC and TiN in steel. This further locks in free carbon and nitrogen in the steel, reducing their potential impact on grain boundary corrosion and improving the steel's resistance to intergranular corrosion and high-temperature stability. More importantly, the fine carbonitrides formed by Ti are uniformly distributed within the austenite grains, effectively improving the stress dispersion ability of the material during multiple cold drawing processes, inhibiting microcrack initiation, and improving fatigue resistance.

[0030] Furthermore, the addition of Mo significantly enhances the corrosion resistance of steel pipes. Mo is a recognized key element for improving the resistance of steel to chloride pitting and crevice corrosion. It forms a stable passivation film in corrosive media (such as seawater and chloride-containing fluids), effectively preventing localized corrosion failure. Simultaneously, Mo can synergistically enhance the passivation film's rebuilding ability with Cr, enabling the steel to maintain good corrosion resistance even in dynamic corrosive environments. In addition, Mo can improve high-temperature creep resistance and enhance the service stability of steel pipes in thermal stress environments.

[0031] More importantly, Nb, Ti, and Mo form a highly coupled synergistic system in terms of strengthening, stabilizing, and inclusion control mechanisms. Specifically, the carbonitride dispersion phases formed by Nb and Ti not only finely regulate the grain structure but also stabilize the dislocation structure during deformation, effectively improving fatigue resistance and stress dispersion. The corrosion resistance of Mo complements the stable microstructure formed by the two elements, effectively delaying microstructure degradation and the initiation of local cracks during corrosion. Furthermore, the presence of Nb and Ti helps optimize the inclusion formation path, making Ca / S inclusions tend to be stable and spherical, further improving deformation coordination and overall plasticity.

[0032] Furthermore, the ratio of Mn content to N content is controlled to be 25-35.

[0033] Furthermore, the ratio of Mn content to Ni content is controlled to be 1.6 to 2.1.

[0034] Furthermore, the ratio of Ti content to N content is 0.12 to 0.15.

[0035] By further limiting the proportions of manganese (Mn), nitrogen (N), nickel (Ni), and titanium (Ti), the microstructure stability and toughness of the steel pipe during cold working are significantly improved, while ensuring a high degree of synergy with the inclusion control system.

[0036] First, a high-manganese environment enhances the solid solution capacity of nitrogen in steel, preventing nitrogen precipitation and the formation of brittle nitrides, thus effectively improving the utilization rate of nitrogen in the steel. Furthermore, the presence of nitrogen not only strengthens the austenitic structure but also enhances solid solution strengthening, hardness, and wear resistance. By limiting the ratio of manganese (Mn) to nitrogen (N), martensitic phase transformation is less likely to be induced during cold drawing deformation, ensuring the thermodynamic stability of the austenitic microstructure and thereby improving the deformation consistency and toughness of the material during cold working.

[0037] Secondly, nickel, as a classic austenite stabilizing element, helps suppress the γ→α' martensite transformation that occurs under strain or low-temperature conditions. However, Ni resources are expensive and difficult to obtain. Therefore, by introducing a high manganese content and limiting the ratio of manganese (Mn) to nickel (Ni), austenite stabilization can be achieved while simultaneously balancing cost control and performance. Furthermore, it can prevent uneven segregation or inclusion enrichment caused by excessive manganese, ensuring the uniformity of steel composition and promoting microstructure consistency and deformation continuity during subsequent heat treatment and cold working.

[0038] Furthermore, Ti and N exhibit a strong chemical affinity, preferentially forming a TiN dispersed phase. This effectively locks in free nitrogen, preventing it from participating in unfavorable secondary precipitation reactions at high temperatures or during strain. By precisely controlling the Ti to N ratio, not only is coarsening and uneven precipitation of TiN particles avoided, but also the dispersion and thermal stability of the particle distribution are ensured, thereby improving grain pinning ability and enhancing resistance to recrystallization and slip band penetration. More importantly, this ratio range can stabilize the formation of the strengthening phase without causing the precipitation of a low-ductility, brittle second phase or inclusion segregation due to excessive Ti, thus maintaining the material's good ductility and fracture toughness.

[0039] This invention achieves stable maintenance of the austenitic structure during multi-pass cold drawing by precisely balancing the proportional relationship between alloying elements, significantly suppressing induced martensitic phase transformation and microstructure embrittlement. At the same time, the solid solution strengthening and dispersed precipitation strengthening mechanisms synergistically enhance the strength, toughness, and processing formability of the material during cold deformation, significantly reducing the risk of fracture and microcrack propagation. In addition, the rational proportion of each element ensures that brittle inclusions, segregation bands, or coarse precipitates will not be generated due to interactions, further improving the microstructure uniformity and corrosion stability of the material under service conditions.

[0040] Another aspect of the present invention discloses a method for preparing cold-drawn stainless steel seamless pipes for fluid transportation, comprising the following steps:

[0041] S1: Smelt the raw materials according to the specified proportions; during the smelting process, add Ca elements precisely through core-spun wire;

[0042] S2: After removing the surface oxide scale and defects from the steel ingot obtained after smelting in step S1, heat it to 1180-1195℃ for piercing and hot rolling. During piercing, maintain the temperature at 1150-1180℃ and control the deformation at 35-50%. The exit rolling temperature is controlled at 850-900℃. Finally, air cooling is used to cool it to room temperature.

[0043] S3: The tube blank is subjected to 4 to 6 passes of cold drawing deformation treatment, with the deformation rate of each pass controlled at 10 to 18%; after the second or third pass, an intermediate annealing treatment is performed, that is, the drawn intermediate tube is heated to 1050°C, held for 1.5 minutes and then rapidly cooled.

[0044] S4: Heat the cold-drawn pipe to 1050-1080℃, hold for 1-3 minutes, and then quickly cool with water or by convection air cooling;

[0045] S5: After heat treatment, the pipes are mechanically straightened, pickled to remove oxide scale and residual inclusions, and then passivated.

[0046] The above preparation method establishes a uniform microstructure control path throughout the entire process from smelting to finished product, ensuring the stability of the austenitic microstructure of the material. Furthermore, the stress-microstructure dual control method enhances the strength, toughness, processing adaptability, and service reliability of the pipe, which is significantly superior to the traditional stainless steel seamless pipe preparation process.

[0047] Step S1 introduces Ca using a core-wrapped wire method, providing technical support for precise control of the Ca / S ratio. This method allows Ca to distribute rapidly and uniformly in the molten steel, reacting with residual S to form fine spherical Ca-Al-Si-O inclusions, thereby achieving spheroidization, plasticity, and high-temperature softening characteristics of the inclusion morphology.

[0048] In the S2 piercing and hot rolling stage, heating the ingot to an optimized piercing temperature range of 1180–1195℃ fully stimulates the thermoplasticity and austenitic phase stability of the steel, while avoiding the risks of grain coarsening and central tearing. Controlling the deformation during piercing to 35–50% not only facilitates the establishment of a complete continuous deformation band but also lays the strain-induced foundation for subsequent microstructure refinement. Further limiting the piercing temperature range to 1150–1180℃ effectively alleviates the uneven thermal stress caused by friction concentration and inclusion aggregation. Simultaneously, controlling the hot rolling exit temperature at 850–900℃, combined with an air-cooling strategy, preserves a certain degree of metastable austenitic structure and cooling-induced dislocation structure, providing microstructural support for stress coordination and strain homogenization during the S3 cold drawing process. This step, through precise control of hot deformation conditions and cooling paths, forms an intermediate microstructure state conducive to cold drawing.

[0049] The S3 step employs a multi-pass cold drawing method, distributing the total drawing deformation over 4 to 6 small passes. This reduces the peak stress in each deformation pass, and combined with inter-pass annealing, effectively releases some of the processing strain energy, slows down the dislocation accumulation rate, and improves the work hardening limit in the later stages of drawing. Intermediate annealing, with its short-time heating and rapid cooling at 1050℃, can complete substructure reconstruction without causing grain growth, providing a good stress-coordination buffer zone for subsequent drawing operations.

[0050] In the final S4 heat treatment stage, recrystallization at 1050–1080℃ is combined with short-duration rapid heating for 1–3 minutes, followed by water cooling or high-pressure mist cooling to complete the equiaxed fine-grain reconstruction process of the microstructure. This temperature range closely matches the austenitic phase stability region of the steel grade, helping to quickly eliminate dislocation density and microstructure distortion caused by processing strain, while avoiding excessive grain boundary migration or microstructure coarsening caused by heat treatment. Rapid cooling "locks" the fine-grained structure formed by recrystallization at the submicron to fine-grained scale, preventing coarsening of the strengthening phase and stress back penetration, effectively improving the service stability of the pipe, especially in terms of resistance to stress corrosion, hydrogen embrittlement, and fatigue performance.

[0051] Furthermore, step S1 specifically includes the following steps:

[0052] S1-1: After the base elements and main stabilizing alloying elements are proportioned, vacuum induction melting is carried out; the temperature is raised to 1550-1580℃ in a vacuum or inert gas to complete decarburization, degassing and preliminary alloying.

[0053] S1-2: Add pure aluminum ingots or aluminum wires, controlling the amount added so that the residual aluminum content after deoxidation is controlled at 0.015-0.035%; maintain electromagnetic stirring or bottom blowing argon stirring for 1-3 minutes during the deoxidation process; take a clamp sample and detect the Al content using a photoelectric direct-reading spectrometer to confirm that it meets the range.

[0054] S1-3: Add ferromanganese to control the Mn content to 7.0-8.0%, add ferronickel to adjust the Ni to 3.5-4.5%, and add an appropriate amount of ferrochromium; then add ferric nitride or manganese nitride according to the central control test results to adjust the nitrogen content to 0.25-0.30%; the nitrogen content is confirmed by testing with an oxygen-nitrogen analyzer;

[0055] S1-4: Transfer the molten steel to the argon-oxygen refining furnace for refining, adjust the slag basicity, and continue top and bottom blowing; control the temperature of the molten steel at 1600-1650℃, and add CaSi alloy wire in two batches using the cored wire injection method during this stage, with an interval of 1-2 minutes between them, and maintain continuous stirring with argon bottom blowing at 15-30L / min.

[0056] S1-5: After adding Ca, sample and analyze the S content to ensure it is within the set range of 0.0015% to 0.0020%. Calculate the Ca / S atomic ratio based on the amount of Ca added, ensuring it is controlled between 2.2 and 3.8. If the S content is too high, a small amount of CaF2 or CaC2 desulfurizing agent can be added to enhance slag desulfurization.

[0057] By controlling the specific process and parameters of step S1.

[0058] Step S1, through vacuum induction melting combined with argon-oxygen refining, establishes a high-cleanliness and high-stability metallurgical environment, laying the foundation for fine microstructure control and controllable inclusion modification. The vacuum environment of vacuum induction melting facilitates the precise solid solution of highly volatile elements such as Mn and N, while its high degassing efficiency effectively reduces inclusion sources; the argon-oxygen refining process provides a flexible atmosphere adjustment window and the ability to finely modify alloy composition.

[0059] In the process, aluminum is added at the initial stage of deoxidation, and the residual amount is controlled within 0.015% to 0.035%. This range satisfies the deoxidation requirements without generating a large number of coarse Al2O3 inclusions. The resulting fine alumina particles act as precursors for Ca-modified inclusions, reacting with subsequent calcium elements to generate CaO·Al2O3-type low-melting-point, highly ductile inclusions with good wettability and deformation compatibility, which is beneficial for stress relief during cold drawing.

[0060] The addition of manganese and nitrogen is controlled according to a set ratio to ensure that the Mn / N atomic ratio remains stable within the austenitic phase stable region, preventing strain-induced martensitic transformation during subsequent cold working and ensuring microstructure plasticity at the phase structure level. Nitrogen control not only provides solid solution strengthening but also enhances the thermal stability of austenite, which is crucial for ensuring a high strength-to-toughness ratio.

[0061] In the argon-oxygen refining process, the stepwise addition of calcium combined with argon stirring ensures sufficient and uniform diffusion of Ca in the molten steel, preventing localized calcium enrichment or oxidation loss. Controlling the Ca / S atomic ratio within the window of 2.2–3.8 is crucial for achieving the transformation of Al₂O₃ into CaO·Al₂O₃ and suppressing CaS coarsening and inclusion agglomeration. This transformation mechanism significantly improves inclusion plasticity, enhances strain compatibility between the matrix and inclusions during drawing, and reduces the risk of crack initiation.

[0062] Meanwhile, the argon agitation process promotes the flotation and uniform distribution of inclusions, helping to obtain a clean initial microstructure without concentrated inclusion aggregation in the ingot, thus avoiding microcracks induced by inclusion aggregation during subsequent hot piercing. Finally, rapid tapping and casting, along with controlled cooling, effectively maintain compositional homogeneity and thermodynamic stability of the microstructure, providing a good foundation for fluidity and plastic response in S2 hot piercing, and also providing a source of guarantee for stress transmission, microstructure refinement, and cold working crack resistance in the S3 to S5 stages.

[0063] Furthermore, in step S3, the cold drawing process simultaneously employs the following two auxiliary stress control methods:

[0064] Ultrasonic vibration-assisted drawing method is set near the inner wall of the mold: frequency 20-25kHz, amplitude 10-20μm;

[0065] Electromagnetic induction excitation assisted drawing, arranged around the mold: frequency 50-100Hz, magnetic density 0.05-0.08T.

[0066] In step S3, the introduction of two external field excitation mechanisms—ultrasonic vibration and electromagnetic induction—during the cold drawing process can significantly alter the stress-strain field distribution in the mold-tube contact area. Ultrasonic excitation can induce interface vibration, lubricant film reconstruction, and local thermal softening of the material at the microscale, reducing frictional resistance and the critical drawing stress. Electromagnetic induction excitation can generate low-frequency vibrational stress waves in the material, forming periodic dislocation migration windows, which facilitates the diffusion of the plastic expansion zone to the surrounding area and suppresses local strain concentration.

[0067] When a composite excitation system is used, the two external field effects are coupled and synergistic, which not only improves the drawing force distribution and metal fluidity, but also dynamically adjusts the stress state in each region during the drawing process. Especially in the inclusion region, the external field vibration promotes the diffusion of strain energy in multiple directions, alleviating the microcrack initiation points caused by inconsistent deformation of inclusions.

[0068] Furthermore, by controlling the mold temperature within the range of 35–45°C through mold temperature control measures, it helps maintain a constant coefficient of friction, prevents thermal expansion of the mold from affecting deformation stability, and avoids uneven strain on the metal surface caused by localized temperature rise in the mold. This process design provides a uniform cold working microstructure for the subsequent S4 rapid recrystallization heat treatment, while also significantly reducing crack sensitivity during drawing.

[0069] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in the following aspects:

[0070] (1) By precisely controlling the Ca / S mass ratio, inclusions are spheroidized and plasticized, effectively suppressing the initiation and propagation of microcracks during cold drawing, and significantly improving the processing stability and service safety of steel pipes. By controlling the proportion of high manganese and high nitrogen and the ratio of austenite stabilizing elements, it is ensured that the austenite structure does not undergo martensitic transformation during cold working, thus comprehensively improving the strength, toughness and structural stability of the material.

[0071] (2) By rationally introducing three microalloying elements, Nb, Ti and Mo, and forming a synergistic strengthening system, the grain refinement, corrosion resistance and creep resistance are significantly improved while maintaining the stability of the austenite phase.

[0072] (3) Construct a closed loop for the entire process of S1 to S4 to control the microstructure and stress, ensuring uniform composition, controllable inclusions, stress coordination and microstructure reconstruction at each stage, so as to achieve excellent cold drawing adaptability and consistency of the final product. The cold drawing stage combines ultrasonic vibration and electromagnetic induction to optimize the stress-strain field distribution, alleviate strain concentration, and significantly improve the forming stability and crack resistance under complex deformation conditions. Detailed Implementation

[0073] The present invention will now be described in detail with reference to the embodiments.

[0074] Example 1

[0075] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0076] element C Si Mn Cr Ni Al N S Ca Fe content(%) 0.015 0.5 7.8 17 4 0.03 0.27 0.0025 0.0035 70.379

[0077] The ratio of Ca content to S content is controlled at 1.4.

[0078] The ratio of Mn content to N content was controlled at 28.89.

[0079] The ratio of Mn content to Ni content is controlled at 1.95.

[0080] This embodiment also discloses a method for preparing the above-mentioned cold-drawn stainless steel seamless pipe for fluid transportation, including the following steps:

[0081] S1:

[0082] S1-1: The required matrix elements and main stabilizing alloying elements (Cr, Ni, Mn and Fe-based raw materials) are proportioned according to the mass percentage of each element specifically contained in the seamless steel pipe. The raw materials are loaded into an induction furnace for vacuum induction melting. The temperature is raised to 1550℃ under an inert atmosphere to complete decarburization, degassing and preliminary alloying.

[0083] S1-2: Subsequently, deoxidation is initiated within this temperature range. Aluminum wire is added, and the amount added is controlled to ensure that the residual aluminum content after deoxidation is within the target value. During the deoxidation process, electromagnetic stirring or bottom-blowing argon stirring is maintained for 1 minute to ensure sufficient diffusion of aluminum in the molten steel. A sample is taken from the jaws and the Al content is detected using an optical emission spectrometer (OES) to confirm that it meets the range.

[0084] S1-3: After deoxidation, continue to adjust the content of major alloying elements: add ferromanganese to control the Mn content to the target value, add ferronickel to adjust the Ni to the target value, and add an appropriate amount of ferrochromium. Subsequently, add ferric nitride according to the central control test results to adjust the nitrogen content to the target value. The nitrogen content is confirmed by testing with an oxygen-nitrogen analyzer (such as LECO ONH836).

[0085] S1-4: After the main element adjustment is completed, the molten steel is transferred to an argon-oxygen refining furnace for refining. The slag basicity is adjusted, and top and bottom blowing continues to promote inclusion flotation and composition uniformity. The molten steel temperature is controlled at 1600℃. During this stage, CaSi alloy wire is added in two batches using the cored wire injection method, with a 2-minute interval between them. Argon bottom blowing is maintained at 150L / min with continuous stirring to promote uniform calcium diffusion.

[0086] S1-5: After adding Ca, sample and analyze the S content, and calculate the Ca / S mass ratio based on the amount of Ca added to ensure it is controlled within the target value. If the S content is too high, a small amount of CaC2 can be added to enhance slag desulfurization. After confirming that the content of all elements is stable, tap the steel immediately after the inclusion treatment is completed, and perform ingot casting or continuous casting. Cool the ingot to room temperature for later use.

[0087] S2: After removing surface oxide scale and defects from the ingot obtained in S1, heat it to 1180℃ before piercing. Induction heating or furnace holding is used to ensure uniform core temperature. Once the temperature stabilizes, hot piercing is performed on the heated ingot using a roller piercing mill. The piercing speed and roller angle are adjusted to ensure uniform deformation. During piercing, the temperature is maintained at 1150℃, and the deformation is controlled within 35%.

[0088] After piercing, the blank enters the initial rolling stage, where it undergoes three consecutive hot rolling passes, with the exit temperature controlled at 850℃. After rolling, it is air-cooled to room temperature without slow cooling or heat preservation treatment, yielding an intermediate tube blank whose outer diameter, wall thickness, and surface condition are inspected. Once the surface quality meets the standards, mechanical shot blasting or pickling is performed to remove residual oxide scale and heat-deformed inclusions, providing a good surface foundation for the cold drawing stage.

[0089] S3: The intermediate tube blank, after hot piercing and hot rolling, is cold-drawn at room temperature in four passes, with the deformation rate controlled at 15% in each pass. Mechanical lubrication and surface cleaning are performed between each pass, and an intermediate annealing treatment is performed after the second intermediate pass. This involves heating the drawn intermediate tube to 1050℃, holding it at that temperature for 1.5 minutes, and then rapidly cooling it to release drawing hardening stress, adjust the hardness gradient, and prevent crack accumulation.

[0090] During the drawing process, a combined external field excitation system is installed on the drawing die frame, and the following two external field excitation methods are applied simultaneously:

[0091] Ultrasonic excitation module: located near the inner wall of the mold, with a frequency controlled at 20kHz and an amplitude of 18μm;

[0092] Electromagnetic excitation module: Arranged in the coil system around the mold, it applies a low-frequency alternating magnetic field with a frequency of 50Hz and a magnetic density controlled at 0.08T.

[0093] Both operate in coordination using a synchronous control system, with the start-up rhythm dynamically adjusted according to changes in drawing force. To ensure optimal performance and thermal stability in the external field, the drawing die is equipped with an external water cooling system to stabilize the die temperature at 35°C, and a temperature control feedback device is provided to adjust the cooling flow rate in real time.

[0094] During the drawing process, the drawing force, surface temperature rise of the tube, and outer diameter changes are monitored in real time. A non-contact infrared thermometer and force sensing system are used to record the data to help adjust the drawing rhythm. After drawing, the tube is visually inspected and its dimensions are measured to confirm that there are no defects such as drawing cracks, surface scratches, or wrinkles before proceeding to the next stage of recrystallization heat treatment.

[0095] S4: Immediately after cold drawing, the seamless stainless steel pipe is sent to a heat treatment unit for recrystallization treatment, using a continuous roller hearth furnace or a vertical resistance furnace. The pipe is heated to 1050℃, held for 3 minutes, and removed from the furnace immediately after ensuring uniform temperature. After heat treatment, the pipe is rapidly cooled sequentially through a high-pressure mist cooling section and a water cooling section, with an overall cooling rate of not less than 100℃ / s, ultimately reducing the pipe temperature rapidly to below 100℃. After cooling, the pipe dimensions and surface are inspected visually. Once qualified, it proceeds to the straightening and surface treatment stage.

[0096] S5: The cold-drawn stainless steel seamless tube, after recrystallization heat treatment, is fed into a straightening device for mechanical straightening. A six-roll or eight-roll straightener is used to adjust the straightness and ovality, controlling the straightening deformation to not exceed 0.5%. After straightening, the tube is sent to an acid pickling device for immersion pickling using an HF+HNO3 mixed acid system. After pickling, the tube surface is rinsed with high-pressure pure water to remove residual acid and reaction residue.

[0097] Next, the pickled pipes are placed in a passivation treatment tank, using a nitric acid or nitric acid + chromate composite passivation system. After passivation, they are thoroughly rinsed with pure water and then air-dried or dried at low temperature until completely dry.

[0098] Example 2

[0099] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0100] element C Si Mn Cr Ni Al N S Ca Fe content(%) 0.02 0.55 7.65 16.80 4.25 0.03 0.26 0.0022 0.00352 70.2

[0101] The ratio of Ca content to S content is controlled at 1.6.

[0102] The ratio of Mn content to N content was controlled at 29.42.

[0103] The ratio of Mn content to Ni content is controlled at 1.8.

[0104] This embodiment also discloses a method for preparing the above-mentioned cold-drawn stainless steel seamless pipe for fluid transportation, including the following steps:

[0105] S1:

[0106] Step S1-1 is the same as in Example 1, except that the temperature is raised to 1580°C to complete decarburization, degassing and preliminary alloying.

[0107] Step S1-2 is the same as in Example 1, except that electromagnetic stirring or bottom-blowing argon stirring is maintained for 3 minutes during the deoxygenation process.

[0108] Steps S1-3 are the same as in Example 1, except that the proportions of Mn, Ni, and N elements are controlled according to the proportions in this example.

[0109] Steps S1-4 are the same as in Example 1, except that the temperature of the molten steel is controlled at 1650℃, the cored wire injection method is used to add the steel twice with a 1-minute interval between the two additions, and argon gas is continuously stirred at a bottom blowing rate of 30L / min.

[0110] Steps S1-5 are the same as in Example 1, except that the ratio of Ca and S elements is controlled according to the ratio in this example.

[0111] S2 is the same as Example 1, except that it is heated to 1195°C before piercing, and the temperature is maintained at 1180°C during piercing, with the deformation controlled at 40%; secondly, when entering the initial rolling stage, the hollow billet is subjected to 5 consecutive hot rolling passes, and the rolling exit temperature is controlled at 900°C. After rolling, it is air-cooled to room temperature.

[0112] S3 is the same as Example 1, except that it is completed in 6 passes, with the deformation rate controlled at 8% for each pass. An intermediate annealing process is performed after the third pass. In addition, the frequency of the ultrasonic excitation module is controlled at 25kHz and the amplitude is 10μm. The electromagnetic excitation module is applied at a frequency of 100Hz and the magnetic density is controlled at a low-frequency alternating magnetic field of 0.05T.

[0113] S4 is the same as Example 1, except that the cold-drawn pipe is heated to 1080°C and the holding time is controlled to 1 minute.

[0114] S5 is the same as in Example 1.

[0115] Example 3

[0116] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0117] element C Si Mn Cr Ni Al N S Ca Fe content(%) 0.02 0.45 8.00 17.30 4.21 0.02 0.25 0.00230 0.00391 69.80

[0118] The ratio of Ca content to S content is controlled at 1.7.

[0119] The ratio of Mn content to N content is controlled at 32.

[0120] The ratio of Mn content to Ni content is controlled at 1.9.

[0121] This embodiment also discloses a method for preparing the above-mentioned cold-drawn stainless steel seamless pipe for fluid transportation, including the following steps:

[0122] S1:

[0123] Step S1-1 is the same as in Example 1, except that the temperature is raised to 1570°C to complete decarburization, degassing and preliminary alloying.

[0124] Step S1-2 is the same as in Example 1, except that electromagnetic stirring or bottom-blowing argon stirring is maintained for 2 minutes during the deoxygenation process.

[0125] Steps S1-3 are the same as in Example 1, except that the proportions of Mn, Ni, and N elements are controlled according to the proportions in this example.

[0126] Steps S1-4 are the same as in Example 1, except that the temperature of the molten steel is controlled at 1630℃, the cored wire injection method is used to add the steel twice with a 1.5-minute interval between the two additions, and argon gas is continuously stirred at a bottom blowing rate of 20L / min.

[0127] Steps S1-5 are the same as in Example 1, except that the ratio of Ca and S elements is controlled according to the ratio in this example.

[0128] S2 is the same as Example 1, except that it is heated to 1185°C before piercing, and the temperature is maintained at 1165°C during piercing, with the deformation controlled at 37%; secondly, when entering the initial rolling stage, the hollow billet is subjected to 5 consecutive hot rolling passes, and the rolling exit temperature is controlled at 880°C. After rolling, it is air-cooled to room temperature.

[0129] S3 is the same as Example 1, except that it is completed in 5 passes, with the deformation rate controlled at 9% for each pass. An intermediate annealing process is performed after the third pass. In addition, the frequency of the ultrasonic excitation module is controlled at 23kHz and the amplitude is 15μm. The applied frequency of the electromagnetic excitation module is 75Hz and the magnetic density is controlled at a low-frequency alternating magnetic field of 0.06T.

[0130] S4 is the same as Example 1, except that the cold-drawn pipe is heated to 1080°C and the holding time is controlled to 1 minute.

[0131] S5 is the same as in Example 1.

[0132] Example 4

[0133] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0134] element C Si Mn Cr Ni Al N S Ca Fe content(%) 0.02 0.44 7.20 17.00 5.10 0.04 0.3 0.00250 0.00450 69.893

[0135] The ratio of Ca content to S content is controlled at 1.8.

[0136] The ratio of Mn content to N content is controlled at 24.

[0137] The ratio of Mn content to Ni content is controlled at 2.

[0138] The preparation method of the cold-drawn stainless steel seamless pipe for fluid transportation disclosed in this embodiment is the same as that in Embodiment 1.

[0139] Example 5

[0140] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0141] element C Si Mn Cr Ni Al N S Ca Fe content(%) 0.02 0.58 7.50 16.70 3.00 0.03 0.26 0.00230 0.00368 71.90

[0142] The ratio of Ca content to S content is controlled at 1.6.

[0143] The ratio of Mn content to N content was controlled at 28.85.

[0144] The ratio of Mn content to Ni content is controlled at 2.5.

[0145] The preparation method of the cold-drawn stainless steel seamless pipe for fluid transportation disclosed in this embodiment is the same as that in Embodiment 1.

[0146] Example 6

[0147] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0148] element C Si Mn Cr Ni Al N S Ca Nb Ti Mo Fe content 0.02 0.5 7.8 17 4 0.03 0.27 0.0025 0.0035 0.04 0.035 0.8 69.504

[0149] The ratio of Ca content to S content is controlled at 1.4.

[0150] The ratio of Mn content to N content was controlled at 28.89.

[0151] The ratio of Mn content to Ni content is controlled at 1.95.

[0152] The ratio of Ti content to N content is 0.129.

[0153] This embodiment discloses a method for preparing a cold-drawn seamless stainless steel pipe for fluid transportation. The difference between this method and Embodiment 1 is that steps S1-3 are adjusted as follows:

[0154] After deoxidation, the content of major alloying elements was further adjusted: Ferromanganese was added to control the Mn content to the target value; ferronickel was added to adjust the Ni content to the target value; and ferrochromium was added as needed to adjust the Cr content to the target value. Ferromolybdenum was added to adjust the Mo content to the target value to improve pitting corrosion resistance and high-temperature strength; ferrotitanium was added to adjust the Ti content to the target value, with the molten metal stirring intensity controlled during addition to prevent Ti burn-out; ferroniobium was added to control the Nb content to the target value, added after titanium, and the overheating time was controlled to prevent carbide precipitation from affecting austenite stability. Subsequently, ferric nitride was added according to the central control test results to adjust the nitrogen content to the target value. The nitrogen content was confirmed by an oxygen-nitrogen analyzer (such as a LECO ONH836).

[0155] Example 7

[0156] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0157] element C Si Mn Cr Ni Al N S Ca Nb Ti Mo Fe content 0.015 0.5 7.5 17 4. 0.025 0.27 0.0025 0.0035 0.045 0.035 0.7 69.877

[0158] The ratio of Ca content to S content is controlled at 1.4.

[0159] The ratio of Mn content to N content was controlled at 27.78.

[0160] The ratio of Mn content to Ni content is controlled at 1.88.

[0161] The ratio of Ti content to N content is 0.13.

[0162] The preparation method of the cold-drawn stainless steel seamless pipe for fluid transportation disclosed in this embodiment is the same as that in Embodiment 6.

[0163] Example 8

[0164] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0165] element C Si Mn Cr Ni Al N S Ca Nb Ti Mo Fe content 0.017 0.45 7.1 17.2 3.8 0.035 0.255 0.002 0.003 0.03 0.038 0.9 70.025

[0166] The ratio of Ca content to S content is controlled at 1.5.

[0167] The ratio of Mn content to N content was controlled at 27.45.

[0168] The ratio of Mn content to Ni content is controlled at 1.84.

[0169] The ratio of Ti content to N content is 0.149.

[0170] The preparation method of the cold-drawn stainless steel seamless pipe for fluid transportation disclosed in this embodiment is the same as that in Embodiment 6.

[0171] Example 9

[0172] This embodiment discloses a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0173] element C Si Mn Cr Ni Al N S Ca Nb Ti Mo Fe content 0.02 0.5 7.2 17.1 3.9 0.03 0.26 0.0025 0.0035 0.035 0.028 0.85 69.957

[0174] The ratio of Ca content to S content is controlled at 1.4.

[0175] The ratio of Mn content to N content was controlled at 27.69.

[0176] The ratio of Mn content to Ni content is controlled at 1.846.

[0177] The ratio of Ti content to N content is 0.1.

[0178] The preparation method of the cold-drawn stainless steel seamless pipe for fluid transportation disclosed in this embodiment is the same as that in Embodiment 6.

[0179] Examples 10-11

[0180] Examples 10-11 disclose a cold-drawn stainless steel seamless pipe for fluid transportation, comprising the following components by mass percentage:

[0181]

[0182]

[0183] The preparation methods of the cold-drawn stainless steel seamless pipes for fluid transportation in Examples 10-11 are the same as those in Example 6.

[0184] Example 12

[0185] The difference between this embodiment and embodiment 6 is that step S1 is as follows:

[0186] S1-1: Raw material preparation and electric arc furnace melting

[0187] Waste stainless steel containing iron, chromium, nickel, manganese, silicon, etc., is pre-mixed with alloys such as ferrochrome, ferronickel, ferromanganese, and ferrosilicon according to a set composition. The mixture is then smelted using a DC electric arc furnace, heated to approximately 1600℃, to complete the metal melting, preliminary alloying, and decarburization processes.

[0188] S1-2: Oxygen-Argon Dual-Blowing Refining

[0189] The molten steel from the electric arc furnace is transferred to an oxygen-argon dual-blowing refining furnace, where oxygen and argon (O2+Ar) are simultaneously blown in for further decarburization and refining. The carbon removal rate is controlled by adjusting the oxygen / argon flow ratio, while chromium burn-off is avoided. In the later refining stage, the oxygen proportion is gradually reduced and the argon flow rate is increased to promote desulfurization and composition homogenization. During this process, aluminum granules or aluminum-iron alloys are added for deoxidation, adjusting the Al content to 0.03%; appropriate amounts of iron nitride are added or nitrogen is blown in the high-temperature section to increase the nitrogen content to 0.27%; a slag-forming agent (lime + fluorite) system is used to adjust the slag basicity to above 2.0 for desulfurization, controlling the S content to ≤0.0025%.

[0190] S1-3: Calcium Treatment (Ca Adjustment)

[0191] In the tail end of the oxygen-argon dual-blowing refining process, calcium is added using a CaSi alloy cored wire feeding method to control the total amount of Ca added, thereby stabilizing the Ca content in the molten steel at 0.0035%. To improve the uniformity of calcium distribution in the molten steel, argon is purged from the bottom of the ladle for 10 minutes.

[0192] S1-4: Continuous casting

[0193] After being tapped from the ladle, the molten steel is fed into a tundish-crystallizer system for continuous casting. Electromagnetic stirring technology is used to improve the compositional uniformity in the crystallizer, and the resulting billet (or ingot) is then air-cooled. Compositional analysis results confirm that all elements meet the target requirements.

[0194] Example 13

[0195] The difference between this embodiment and embodiment 6 is that step S3 is as follows:

[0196] The intermediate tube blank obtained in step S2 is directly cold-drawn in 4 passes at room temperature, with the deformation rate controlled at 15% per pass. Ordinary cemented carbide molds are used during the drawing process, and graphite powder or grease is coated on the inner wall of the molds to reduce friction. No external excitation field or intermediate annealing treatment is used.

[0197] Comparative Example 1

[0198] The difference between Comparative Example 1 and Example 1 is that the proportions of each element in the seamless steel pipe are as follows:

[0199]

[0200]

[0201] Comparative Example 2

[0202] The difference between Comparative Example 2 and Example 1 is that the proportions of each element in the seamless steel pipe are as follows:

[0203] element C Si Mn Cr Ni Al N S Ca Fe content(%) 0.018 0.57 6.6 17.2 4.125 0.024 0.22 0.0023 0.00345 70

[0204] Detection methods

[0205] 1. Cold drawing crack sensitivity testing

[0206] method:

[0207] Simulated drawing experiment: The cold drawing process of steel pipe was simulated by passing 6 times with a deformation rate of 15% per pass to evaluate the crack sensitivity of steel pipe during cold working.

[0208] Surface crack statistics: During the cold drawing process, surface cracks are statistically analyzed, and their density and distribution are recorded.

[0209] condition:

[0210] Standard: For simulated drawing experiments, refer to the deformation control method for flattening tests in GB / T 246-2017 "Metallic Materials Tube Flattening Test Method", and adjust the parameters according to the actual cold drawing process.

[0211] Surface crack detection: Refer to the C5 standard in GB / T 5777-1996 "Ultrasonic Testing Method for Seamless Steel Pipes" for the detection of surface cracks.

[0212] index:

[0213] Surface microcrack density (cracks / cm) 2 )

[0214] Breakage rate (%)

[0215] 2. Mechanical property testing (room temperature strength and toughness)

[0216] method:

[0217] Tensile test: used to determine the yield strength, tensile strength and elongation of a material.

[0218] Charpy impact test (-20℃): used to evaluate the impact toughness of materials under low temperature conditions and to test their resistance to brittle fracture.

[0219] standard:

[0220] Tensile test: in accordance with GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature".

[0221] Charpy impact test: According to GB / T 229-2020 "Metallic Materials - Charpy Pendulum Impact Test Method"

[0222] 3. Corrosion resistance testing (pitting corrosion resistance)

[0223] method:

[0224] Potentially constant polarization test: In a 3.5% NaCl solution, the pitting potential and corrosion current density of stainless steel materials were measured.

[0225] Electrolyte: 3.5% NaCl solution, temperature 25℃.

[0226] Standard: Based on the test method for intergranular corrosion in GB / T 4334-2008 "Corrosion of metals and alloys - Test method for intergranular corrosion of stainless steel", and adjusted to pitting corrosion test.

[0227] The results of the three tests are shown in Table 1 below.

[0228] Table 1

[0229]

[0230] Examples 1-13 are generally superior to Comparative Examples 1 and 2 in terms of cold drawing crack sensitivity, mechanical strength and corrosion resistance, demonstrating the significant technical effects of specific component control and inclusion modification mechanism in the technical solution of the present invention.

[0231] Regarding sensitivity to cold drawing cracks, the microcrack density in Examples 1-3 and 6-8 was all below 0.35 cracks / cm². 2 Furthermore, no breakage was observed, demonstrating excellent cold-drawing adaptability. This is attributed to the formation of spheroidized, fine, and ductile Ca-Al-O inclusions within the Ca / S ratio range of 1.4–1.6, coupled with a suitable Mn / N ratio (approximately 28–30) and Mn / Ni ratio (1.84–1.95), which stabilized the austenitic structure and reduced the likelihood of crack induction during cold working. In contrast, Comparative Example 1 and Comparative Example 2 showed 1.48 and 2.00 cracks / cm, respectively.2 The microcrack density was measured, and fracture rates of 10% and 12% were observed, confirming that insufficient Ca / S or unqualified Mn / N ratios directly lead to inclusion deterioration and austenite instability, resulting in a significant increase in crack initiation during the cold drawing process.

[0232] In terms of mechanical properties, the tensile strength of Examples 6-8 reached 710-721 MPa, and the elongation exceeded 42%, demonstrating the combined effect of Nb / Ti dispersed phase strengthening synergistic with Mo solid solution reinforcement and improved inclusion deformation coordination. In contrast, the tensile strength of Comparative Example 2 was only 601 MPa, and the elongation dropped to 19.9%. Comparative Example 1 was slightly better but still significantly lower than the Examples, indicating that if Mn / N, Ni, etc., fail to stably maintain the austenitic structure or if coarse MnS inclusions are present, work hardening and embrittlement failure are likely to occur.

[0233] Regarding corrosion resistance, the pitting potentials of Examples 6-8 reached 355-359 mV, and the corrosion current densities were as low as 1.02-1.13 μA / cm². 2 This indicates that the passivation film is stable and effectively suppresses corrosion microcells. In contrast, Comparative Example 2 exhibits a pitting potential of 248.7 mV and a corrosion current as high as 3.58 μA / cm. 2 This reflects the phenomenon of passivation film rupture and severe expansion of corrosion micro-regions caused by MnS main inclusions. Comparative Example 1 also shows a similar but slightly milder trend.

[0234] In summary, this invention effectively improves the crack resistance, toughness, and corrosion resistance of stainless steel seamless pipes during cold drawing by introducing specific ranges of Mn, Ni, N, and Ti content ratio control, precisely regulating the Ca / S inclusion modification mechanism, and combining it with intermediate annealing, external field excitation, and other process measures.

[0235] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A cold-drawn stainless steel seamless steel pipe for fluid delivery, characterized by, Comprise the following components in percentage by mass: C ：≤ 0.02%； Si: 0.4~0.6%; Mn: 7.1~8.0%; Cr ：16.5～17.5%； Ni: 3.5~4.5%; Al ：0.01～0.05%； N ：0.25～0.30%； S ：0.015～0.002%； Ca: 0.003~0.004%; Fe: balance; The ratio of Ca content to S content is controlled to be 1~2; The method for preparing a cold-drawn stainless steel seamless pipe for fluid delivery comprises the following steps: S1: proportionally smelting raw materials; in the smelting process, Ca element is accurately added through the way of cored wire; S2: after removing the surface oxide skin and defects of the steel ingot obtained after the step S1, heating to 1180~1195℃, performing piercing and hot rolling treatment, maintaining the temperature at 1150~1180℃ during the piercing process, and controlling the deformation amount at 35~40%; the out-rolling temperature is controlled at 850~900℃; finally, adopting air cooling to cool to room temperature; S3: performing 4~6 times of multi-pass cold-drawing deformation treatment on the pipe blank, and controlling the deformation rate at each pass to be 8~15%; after the completion of the second or third pass, performing once of intermediate annealing treatment, i.e. heating the intermediate pipe to 1050℃, maintaining for 1.5 minutes, and then rapidly cooling; In the step S3, the following two auxiliary stress regulation methods are synchronously adopted during the cold-drawing process: Ultrasonic vibration auxiliary drawing set near the inner wall of the die: frequency 20~25 kHz, amplitude 10~20 μm; Electromagnetic induction excitation auxiliary drawing arranged at the periphery of the die: frequency 50~100 Hz, magnetic density 0.05~0.08 T; S4: heating the cold-drawn pipe to 1050~1080℃, maintaining for 1~3 minutes, and then rapidly water cooling or convection air cooling; S5: performing mechanical straightening, pickling to remove the oxide skin and residual inclusions, and surface passivation treatment on the pipe after the heat treatment.

2. The cold-drawn stainless steel seamless pipe for fluid delivery according to claim 1, characterized by Further comprising 0.03~0.05wt% of Nb.

3. The cold-drawn stainless steel seamless tube for fluid delivery according to claim 1, characterized by Further comprising 0.02~0.04wt% of Ti.

4. The cold-drawn stainless steel seamless tube for fluid delivery according to claim 1, characterized by Further comprising 0.5~1.0wt% of Mo.

5. The cold-drawn stainless steel seamless tube for fluid delivery according to claim 1, characterized by The ratio of Mn content to N content is controlled to be 25~35.

6. The cold-drawn stainless steel seamless tube for fluid delivery according to claim 5, characterized by The ratio of Mn content to Ni content is controlled to be 1.6~2.

1.

7. The cold-drawn stainless steel seamless tube for fluid delivery according to claim 6, characterized by The ratio of Ti content to N content is 0.12~0.

15.

8. The cold-drawn stainless steel seamless tube for fluid delivery according to claim 1, characterized by The step S1 specifically comprises the following steps: S1-1: after proportionally batching the base elements and main stable alloying elements, performing vacuum induction smelting; in vacuum or inert gas, heating to 1550~1580℃, completing decarburization, degassing and preliminary alloying; S1-2: adding pure aluminum ingot or aluminum wire, and controlling the adding amount to control the residual aluminum content after deoxidization to be 0.015~0.035%; maintaining electromagnetic stirring or bottom argon stirring for 1~3 minutes during the deoxidization process; taking the sample at the tongs to detect the Al content through the photoelectric direct-reading spectrometer, and confirming that the Al content meets the range; S1-3: adding manganese iron to control the Mn content to be 7.1~8.0%, adding nickel iron to adjust the Ni to be 3.5~4.5%, and appropriately adding chromium iron; then, according to the detection results of the intermediate control, adding nitride iron or nitride manganese to adjust the nitrogen content to be 0.25~0.30%; the nitrogen content is confirmed through the oxygen-nitrogen analyzer; S1-4: The liquid steel is transferred into the argon-oxygen refining furnace for refining, the slag basicity is adjusted, and the combined blowing is continued; the temperature of the liquid steel is controlled at 1600-1650 ℃, the CaSi alloy wire is added twice by the cored wire injection method, the interval is 1-2 minutes, and the argon bottom blowing is continuously stirred at 15-30 L / min; S1-5: After the Ca is added, the S content is analyzed by sampling, it is ensured that the S content is in the set interval of 0.0015-0.0020%, the Ca / S atomic ratio is calculated combined with the Ca addition amount, and it is ensured that the Ca / S atomic ratio is controlled in the range of 2.2-3.8; if the S is high, a small amount of CaF2 or CaC2 desulfurizer is supplemented to strengthen the desulfurization of the slag.

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