Cold-drawn stainless steel seamless steel pipe for fluid transportation and preparation method thereof

Abstract

The invention discloses a cold-drawn stainless steel seamless steel tube for fluid transportation and a preparation method thereof, and relates to the technical field of seamless steel tube manufacturing. The steel pipe comprises the following components in percentage by mass: less than or equal to 0.02% of C, 0.4 to 0.6% of Si, 7.5 to 8.0% of Mn, 16.5 to 17.5% of Cr, 3.5 to 4.5% of Ni, 0.01 to 0.05% of Al, 0.25 to 0.30% of N, 0.015 to 0.002% of S, 0.003 to 0.004% of Ca and the balance of Fe, and the mass ratio of Ca to S is controlled to be 1 to 2. Optionally, the alloy also comprises microalloy elements such as Nb, Ti, Mo and the like, and key proportion parameters such as Mn / N, Mn / Ni, Ti / N and the like are limited. According to the steel pipe, through cooperation of a Ca / S inclusion metamorphism mechanism and austenite stable component proportion design, spheroidization and plasticization of inclusion morphology are achieved, and cold-drawing crack sensitivity is remarkably reduced; and meanwhile, a synergistic strengthening system of obdurability and corrosion resistance is constructed, and the service reliability of the material is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of seamless steel pipe manufacturing, in particular to a cold-drawn stainless steel seamless steel pipe for fluid transportation and a preparation method thereof. Background Art

[0002] Due to their excellent mechanical properties and corrosion resistance, seamless stainless steel pipes are widely used in high-demand fluid transport scenarios in industries such as petrochemicals, chemicals, electricity, medicine, and food. Cold-drawn seamless stainless steel pipes, with their high dimensional accuracy, excellent surface quality, and uniform and dense structure, are key pipe types for achieving high-precision and high-stability fluid transmission. However, as application environments expand to more extreme conditions such as high pressure, high flow rate, strong corrosion, and low temperature, traditional cold-drawn seamless stainless steel pipes are facing a series of performance bottlenecks during the forming, processing, and service life.

[0003] During the cold drawing process, steel pipes undergo multiple passes of plastic deformation, leading to localized stress concentration and uneven microstructural strain that can easily lead to crack initiation and propagation. This is a significant factor affecting pipe yield and reliability. Inclusions with irregular morphology, uneven distribution, and poor plasticity are particularly susceptible to microcrack initiation. Furthermore, the induced martensitic transformation during cold working can also cause localized microstructural embrittlement in the austenite matrix, reducing the fracture toughness and corrosion stability 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 micro-alloyed 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, 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, structural stability and corrosion resistance and its preparation method. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the object of the present invention is to provide a cold-drawn stainless steel seamless pipe for fluid transportation and a preparation method thereof.

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

[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 the Ca content to the S content is controlled to be 1-2.

[0019] The above-mentioned components of the seamless steel pipe of the present invention mainly lie in the inclusion modification control mechanism based on the Ca / S ratio on the one hand, and the stable austenite structure design regulated by high Mn and nitrogen content on the other hand. The synergistic effect of the two significantly improves the structural stability, mechanical reliability and service durability of the steel pipe during the cold drawing process, and is particularly suitable for the fluid transportation field with extremely high requirements on forming processing performance and usage performance.

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

[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, helping them to deform in coordination with the matrix during hot and cold working, avoiding becoming a source of cracks and thereby improving the overall deformation consistency and continuity of the steel pipe. At the same time, the inclusion size is reduced and evenly distributed, effectively avoiding the occurrence of localized structural stress concentration, which is a key factor in achieving both high strength and toughness and high processing performance.

[0022] In addition, the stable control of the austenite phase is achieved through the multi-element collaborative design of high manganese (Mn) content (7.1% to 8.0%), medium nickel (Ni, 3.5% to 4.5%) and relatively high nitrogen (N, 0.25% to 0.30%). Both manganese and nitrogen are strong austenite stabilizing elements, of which manganese can significantly enhance the solid solution strengthening ability and delay the induction of the martensite phase, while nitrogen further increases the thermodynamic stability of the austenite phase. During the cold drawing process, even after multiple accumulations of plastic strain, the steel can still maintain a stable austenite structure without induced martensite transformation, thus avoiding embrittlement or uneven performance caused by phase transformation. This stable organizational foundation is of great significance for improving the service reliability of finished steel pipes in low temperature, high pressure or corrosive media environments. The composition ratio of 16.5% to 17.5% Cr and an appropriate amount of Ni, combined with a high nitrogen and high manganese design, makes this stainless steel pipe have 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] By designing the ratios between the aforementioned components, the inclusion optimization effect brought about by the Ca / S modification mechanism significantly improves the macroscopic and microscopic uniformity of the base metal, providing a structural foundation for the stability of the austenite phase, reducing stress concentration points and microcrack initiation sites, and thus avoiding the induction of martensitic phase transformation due to strain concentration. Furthermore, the optimization of inclusion morphology by Ca also helps reduce localized corrosion sensitivity. Combined with the corrosion-resistant system containing Cr and Ni, the steel pipe has a more durable passivation capability and pitting corrosion resistance in corrosive media.

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

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

[0026] Furthermore, 0.5-1.0 wt% of Mo is included.

[0027] The present invention further introduces the above-mentioned Nb (niobium), Ti (titanium) and Mo (molybdenum) microalloying elements, and controls them within an appropriate range, thereby achieving a comprehensive improvement in the strength, corrosion resistance, creep resistance and thermal stability of the steel pipe without destroying the stability of the austenite structure and the controllability of inclusions.

[0028] First, the Nb element has a significant grain-refining strengthening effect. Nb can effectively form stable carbide particles such as NbC during hot and cold working. These fine dispersed phases play a pinning role at the grain boundaries, significantly refining the grain structure and improving the yield strength and plastic toughness of the steel. At the same time, Nb can also inhibit grain boundary migration and improve the structural stability of the steel during heat treatment, especially during the cold drawing process, which helps to control the texture orientation and improve the dimensional consistency and processing stability of the steel pipe. In addition, Nb can also partially inhibit the precipitation of intergranular carbides, helping to improve the resistance to intergranular corrosion.

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

[0030] Again, the addition of Mo significantly enhances the corrosion resistance of the steel pipe. Mo is recognized as a key element to improve the resistance of steel to chloride ion pitting and crevice corrosion. It forms a stable passivation film in corrosive media (such as seawater, chloride ion-containing fluids, etc.), thereby effectively preventing local corrosion failure. At the same time, Mo can also synergize with Cr to enhance the reconstruction ability of the passivation film, so that the steel can maintain good corrosion resistance even in a dynamic corrosion environment. In addition, Mo can also enhance high-temperature creep resistance and improve the service stability of steel pipes in thermal stress environments.

[0031] More importantly, the three microalloying elements, Nb, Ti, and Mo, form a highly coupled synergistic system in terms of strengthening, stabilization, and inclusion control. Specifically, the carbonitride dispersed phase formed by Nb and Ti not only finely regulates the grain structure but also stabilizes the dislocation structure during deformation, effectively improving fatigue resistance and stress dispersion. Mo's corrosion resistance complements the stable microstructure created by the first two, effectively delaying microstructure degradation and local crack initiation during corrosion. Furthermore, the presence of Nb and Ti helps optimize the inclusion formation path, allowing Ca / S inclusions to tend toward stable spheroidization, further enhancing deformation coordination and overall plasticity.

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

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

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

[0035] By further limiting the ratios between manganese (Mn), nitrogen (N), nickel (Ni) and titanium (Ti), the structural 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 can improve the solid solubility of nitrogen in steel, preventing nitrogen precipitation from forming brittle nitrides, thereby effectively improving the utilization rate of nitrogen in steel. The presence of nitrogen not only strengthens the austenite structure, but also improves the level of solid solution strengthening, enhancing hardness and wear resistance. By limiting the ratio of manganese (Mn) and nitrogen (N), it is difficult to induce martensitic phase transformation during cold drawing, ensuring the thermodynamic stability of the austenite structure, and thus improving the deformation consistency and toughness of the material during cold working.

[0037] Secondly, nickel, as a classic austenite stabilizing element, helps to 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 high manganese components and limiting the ratio of manganese (Mn) and nickel (Ni), the dual goals of cost control and performance balance can be achieved while achieving austenite stability. At the same time, it can also prevent uneven segregation or inclusion enrichment caused by excessive manganese, ensure the uniformity of steel composition, and facilitate the consistency of structure and deformation continuity during subsequent heat treatment and cold working.

[0038] Furthermore, Ti and N have extremely strong chemical affinity, preferentially forming a TiN dispersed phase, which can effectively lock free nitrogen and prevent it from participating in adverse secondary precipitation reactions at high temperatures or during strain. By precisely controlling the ratio of Ti to N, not only is the coarsening and uneven precipitation of TiN particles avoided, but the dispersion and thermal stability of the particle distribution are also ensured, thereby improving the grain pinning ability and strengthening the ability to resist 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-plasticity brittle second phase or inclusion segregation due to excessive Ti, maintaining the material's good ductility and fracture toughness.

[0039] The present invention achieves stable maintenance of the austenite structure during multiple cold drawing processes by precisely balancing the proportional relationship between alloying elements, significantly inhibiting the induced martensitic phase transformation and structural embrittlement; at the same time, the solid solution strengthening and dispersion precipitation strengthening mechanisms synergistically improve the strength, toughness and processing forming ability of the material during cold deformation, significantly reducing the risk of fracture and microcrack propagation; in addition, the rationality of the ratio of each element ensures that no brittle inclusions, segregation bands or coarse precipitates will be generated due to interaction, further improving the structural uniformity and corrosion stability of the material under service.

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

[0041] S1: Smelt the raw materials in proportion; during the smelting process, add the Ca element precisely through the core wire;

[0042] S2: removing surface oxide scale and defects from the steel ingot obtained after smelting in step S1, heating it to 1180-1195°C, performing piercing and hot rolling, maintaining the temperature at 1150-1180°C during the piercing process, and controlling the deformation at 35-50%; controlling the rolling temperature at 850-900°C; and finally cooling it to room temperature by air cooling;

[0043] S3: The tube is subjected to a multi-pass cold drawing deformation treatment of 4 to 6 times, 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., kept at this temperature for 1.5 minutes, and then rapidly cooled;

[0044] S4: Heat the cold-drawn pipe to 1050-1080°C, keep it warm for 1-3 minutes, and then quickly cool it with water or convection air;

[0045] S5: The heat-treated pipes are mechanically straightened, pickled to remove scale and residual inclusions, and subjected to surface passivation treatment.

[0046] Through the above preparation method, a uniform structure control path for the entire process from smelting to product formation is constructed, which ensures the stability of the material's austenitic structure. The strength and toughness, processing adaptability and service reliability of the pipe are also improved through the dual control of stress and structure, which is significantly better than the traditional stainless steel seamless pipe preparation process.

[0047] Step S1 uses the cored wire method to introduce Ca, providing technical support for precise control of the Ca / S ratio. This method allows Ca to be quickly and evenly distributed in the molten steel, reacting with residual S to form fine spherical Ca-Al-Si-O inclusions, thereby achieving spheroidization, plasticization, and high-temperature softening properties of the inclusion morphology.

[0048] During the S2 piercing and hot rolling stages, the ingot is heated to an optimized piercing temperature range of 1180-1195°C to fully stimulate the thermoplasticity and austenite phase stability of the steel, while avoiding the risk of grain coarsening and center tearing. Controlling the deformation to 35-50% during the piercing process not only facilitates the establishment of a complete continuous deformation band but also lays the strain-inducing foundation for subsequent microstructure refinement. The piercing temperature range is further limited to 1150-1180°C, which can effectively alleviate the uneven thermal stress caused by friction concentration and inclusion accumulation. At the same time, the hot rolling exit temperature is controlled at 850-900°C, and combined with an air cooling strategy, a certain metastable austenite structure and cooling-induced dislocation structure can be retained, providing structural support for stress coordination and strain homogenization during the S3 cold drawing process. This step, through the fine-tuning of thermal deformation conditions and cooling paths, forms an intermediate microstructure that is conducive to cold drawing.

[0049] The S3 step utilizes a multi-pass cold drawing process, distributing the total drawing deformation across four to six small passes. This reduces the peak stress of each deformation. Combined with an intermediate annealing operation between passes, this effectively releases some of the processing strain energy, slows the rate of dislocation accumulation, and improves the work hardening limit in the later stages of drawing. The intermediate annealing, with its short heating time at 1050°C and rapid cooling, achieves substructure reconstruction without causing grain growth, providing a good stress-coordination buffer zone for subsequent drawing.

[0050] During the S4 final heat treatment stage, a recrystallization treatment at 1050-1080°C is used, combined with a short-term rapid heating of 1-3 minutes, and then water cooling or high-pressure mist cooling to complete the equiaxed fine-grain reconstruction process of the structure. This temperature range is highly compatible with the austenite phase stability zone of the steel grade, which helps to quickly eliminate the dislocation density and structural distortion caused by processing strain, while avoiding excessive grain boundary migration or structural coarsening caused by heat treatment. Rapid cooling "locks" the fine-grained structure formed by recrystallization at a submicron to fine-grained scale, avoiding strengthening phase coarsening and stress back-seepage, effectively improving the service stability of the pipe, especially in terms of stress corrosion resistance, hydrogen embrittlement resistance, and fatigue resistance.

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

[0052] S1-1: After the matrix elements and main stabilizing alloying elements are mixed in proportion, vacuum induction melting is carried out; the temperature is raised to 1550-1580℃ in 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 within 0.015-0.035%; maintain electromagnetic stirring or bottom argon stirring for 1-3 minutes during the deoxidation process; take a jaw sample and test 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 content to 3.5-4.5%, and add ferrochrome in appropriate amounts; then, based on the results of the central control test, add iron nitride or manganese nitride to adjust the nitrogen content to 0.25-0.30%. The nitrogen content is confirmed by testing with an oxygen and nitrogen analyzer;

[0055] S1-4: The molten steel is transferred to an argon-oxygen refining furnace for refining, the slag basicity is adjusted, and top and bottom blowing is continued; the molten steel temperature is controlled at 1600-1650°C. In this stage, CaSi alloy wire is added twice using the cored wire injection method, with an interval of 1-2 minutes in between, and argon bottom blowing at 15-30 L / min is maintained for continuous stirring;

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

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

[0058] The S1 step, through vacuum induction melting combined with argon-oxygen refining, establishes a highly clean and stable metallurgical environment, laying the foundation for fine microstructure control and controlled inclusion modification. The vacuum environment of vacuum induction melting facilitates the precise solid solution of highly volatile elements such as Mn and N, while also providing high degassing efficiency, effectively reducing inclusion sources. The argon-oxygen refining process provides a flexible atmosphere adjustment window and the ability to fine-tune alloy composition.

[0059] During the process, aluminum is added at the initial stage of deoxidation, with the residual content controlled at 0.015-0.035%. This range satisfies deoxidation requirements while preventing the formation of large amounts of coarse Al2O3 inclusions. The resulting fine alumina particles serve as precursors for calcium-modified inclusions, subsequently reacting with calcium to form CaO·Al2O3 inclusions with low melting points and good plasticity. These inclusions exhibit excellent wettability and deformation coordination, facilitating stress relief during cold drawing.

[0060] Manganese and nitrogen are added in a controlled ratio to ensure the Mn / N atomic ratio remains stable within the austenite phase stability range, 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, making it the key to ensuring a high strength-to-toughness ratio.

[0061] During the argon-oxygen refining process, the stepwise addition of calcium, combined with argon agitation, ensures that calcium is fully diffused and evenly distributed throughout the molten steel, preventing localized calcium accumulation or oxidative burnout. Maintaining the Ca / S atomic ratio within the range of 2.2 to 3.8 is key to achieving the transformation of Al₂O₃ to CaO·Al₂O₃, inhibiting CaS coarsening and inclusion agglomeration. This transformation mechanism significantly improves inclusion plasticity, enhances strain coordination between the matrix and inclusions during drawing, and reduces the risk of crack initiation.

[0062] At the same time, the argon stirring process also promotes the floating and uniform distribution of inclusions, helping to obtain a clean initial structure free of concentrated inclusions in the ingot, thereby avoiding microcracks induced by inclusion accumulation during subsequent hot piercing. Ultimately, rapid steel casting and controlled cooling rhythm effectively maintain compositional uniformity and structural thermodynamic stability, providing a foundation for good fluidity and plastic response during S2 hot piercing, and providing a source of guarantee for stress conduction, structural refinement, and cold working crack resistance in the S3-S5 stages.

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

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

[0065] Electromagnetic induction excitation assisted drawing arranged on the periphery of 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 significantly alters the stress-strain field distribution in the die-tube contact area. Ultrasonic excitation promotes interface vibration, lubricant film reconstruction, and localized thermal softening of the material at a microscopic scale, reducing frictional resistance and critical drawing stress. Electromagnetic induction induces low-frequency vibration stress waves in the material, forming periodic dislocation migration windows that facilitate the diffusion of the plastic expansion zone to the surrounding area and suppress local strain concentration.

[0067] When a composite excitation system is employed, the two external field effects are coupled and synergistic, not only improving the drawing force distribution and metal fluidity, but also dynamically adjusting the stress state in various regions during the drawing process. In the inclusion region in particular, the external field vibrations promote the diffusion of strain energy in multiple directions, mitigating microcrack initiation points caused by inconsistent inclusion deformation.

[0068] Furthermore, mold temperature control measures, maintaining the mold temperature between 35 and 45°C, help maintain a constant friction coefficient, prevent thermal expansion of the mold from affecting deformation stability, and avoid uneven strain on the metal surface caused by localized mold temperature rise. This process design provides a uniform cold-worked microstructure for the subsequent S4 rapid recrystallization heat treatment and significantly reduces crack susceptibility during the drawing process.

[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, inclusion spheroidization and plasticization are achieved, effectively inhibiting the initiation and expansion of microcracks during cold drawing, significantly improving the processing stability and service safety of steel pipes. By controlling the high manganese and high nitrogen ratios and the ratio between austenite stabilizing elements, it is ensured that the austenite structure does not undergo martensitic transformation during cold working, comprehensively improving the strength, toughness and structural stability of the material.

[0071] (2) The three microalloying elements Nb, Ti, and Mo are rationally introduced to form a synergistic strengthening system, which can achieve significant improvements in grain refinement, corrosion resistance, and creep resistance while maintaining the stability of the austenite phase.

[0072] (3) Construct a closed loop for microstructure and stress control throughout the entire S1-S4 process to ensure uniform composition, controlled inclusions, coordinated stress, and microstructure reconstruction at each stage, achieving excellent cold drawing adaptability and consistency of the final product. Ultrasonic vibration and electromagnetic induction are used in combination during the cold drawing stage to optimize stress-strain field distribution, alleviate strain concentration, and significantly improve forming stability and crack resistance under complex deformation conditions. DETAILED DESCRIPTION

[0073] The present invention is described in detail below 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 in percentage by mass:

[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] Among them, the ratio of Ca content to S content is controlled to be 1.4.

[0078] The ratio of Mn content to N content is controlled to be 28.89.

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

[0080] This embodiment also discloses a method for preparing the cold-drawn seamless stainless steel pipe for fluid transportation, comprising 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 mixed according to the mass percentage of each element specifically contained in the above-mentioned seamless steel pipe, the raw materials are loaded into an induction furnace for vacuum induction melting, and the temperature is raised to 1550°C under an inert atmosphere to complete decarburization, degassing and preliminary alloying.

[0083] S1-2: Deoxidation is then initiated within this temperature range. Aluminum wire is added, controlling the amount required to maintain the target residual aluminum content after deoxidation. Electromagnetic stirring or bottom argon stirring is maintained for one minute during the deoxidation process to ensure sufficient diffusion of the aluminum into the molten steel. A jaw sample is then taken and tested for aluminum content using an optical emission spectrometer (OES) to confirm that the content meets the desired range.

[0084] S1-3: After deoxidation, continue adjusting the content of key alloying elements: add ferromanganese to control the Mn content to the target value, add ferronickel to adjust the Ni content to the target value, and add ferrochromium as needed. Then, based on the results of the central control test, add iron nitride to adjust the nitrogen content to the target value. Nitrogen content is confirmed using an oxygen and nitrogen analyzer (such as the LECO ONH836).

[0085] S1-4: After adjusting the main elements, the molten steel is transferred to an argon-oxygen refining furnace for refining. The slag basicity is adjusted, and top and bottom blowing is continued to promote inclusion floating and uniform composition. The molten steel temperature is controlled at 1600°C. During this stage, CaSi alloy wire is added in two steps using the cored wire injection method, with a 2-minute interval between each step. Continuous stirring is maintained while bottom blowing argon at 150 L / min to promote uniform calcium diffusion.

[0086] S1-5: After Ca is added, sample and analyze the S content. Calculate the Ca / S mass ratio based on the amount of Ca added to ensure it remains within the target value. If the S content is too high, add a small amount of CaC2 for slag-enhanced desulfurization. Finally, confirm that all elemental contents are stable. Once inclusion treatment is complete, immediately tap the steel for mold casting or continuous casting. Allow the ingot to cool 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°C before piercing. Use induction heating or a furnace to maintain a uniform temperature throughout the core of the ingot. Once the temperature stabilizes, hot pierce the heated ingot using a roller piercer. Adjust the piercing speed and roll angle to ensure uniform deformation. Maintain a temperature of 1150°C and control deformation to 35% during piercing.

[0088] After piercing, the initial rolling stage begins, where the hollow billet undergoes three consecutive hot rolling passes, with the exit temperature controlled at 850°C. After rolling, it is air-cooled to room temperature without slow cooling or holding treatment. The intermediate billet is then inspected for outer diameter, wall thickness, and surface condition. Once the surface quality meets the required standards, it undergoes mechanical shot blasting or pickling to remove residual oxide scale and inclusions attached by thermal deformation, providing a good surface foundation for the cold drawing stage.

[0089] S3: The intermediate tube billet, after hot piercing and hot rolling, is cold drawn at room temperature in four passes, with a deformation rate of 15% per pass. Mechanical lubrication and surface cleaning are performed between passes, and an intermediate annealing treatment is performed after the second intermediate pass. This involves heating the intermediate tube to 1050°C, holding for 1.5 minutes, and then rapidly cooling to release the 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 to simultaneously apply the following two external field excitation methods:

[0091] Ultrasonic excitation module: set 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 outside the mold, the applied frequency is 50Hz, and the magnetic density is controlled at a low-frequency alternating magnetic field of 0.08T.

[0093] The two operate in coordination using a synchronous control system, with the startup rhythm dynamically adjusted as the drawing force changes. To ensure external field effects and thermal stability, the drawing mold is equipped with an external water cooling system to stabilize the mold temperature at 35°C, and a temperature control feedback device is equipped to adjust the cooling flow in real time.

[0094] During the drawing process, the drawing force, surface temperature rise, and outer diameter change of the tube are monitored in real time. A non-contact infrared thermometer and force sensing system are used to record the data to assist in adjusting the drawing rhythm. After drawing, the tube is visually inspected and dimensionalally measured to confirm the absence of defects such as cracks, surface scratches, and wrinkles before proceeding to the next stage of recrystallization heat treatment.

[0095] S4: The stainless steel seamless steel pipe after cold drawing is immediately sent to the heat treatment device for recrystallization treatment, and heated using a continuous roller bottom heating furnace or a vertical resistance furnace. The pipe is heated to 1050°C, and the holding time is controlled at 3 minutes to ensure that the overall temperature is uniform and then immediately taken out of the furnace. After the heat treatment is completed, the steel pipe is rapidly cooled through the high-pressure mist cooling section and the water cooling section in sequence. The overall cooling rate is not less than 100°C / s, and the temperature of the pipe is finally quickly reduced to below 100°C. After cooling, the pipe size inspection and surface visual inspection are carried out. If qualified, it enters the straightening and surface treatment stage.

[0096] S5: After recrystallization heat treatment, the cold-drawn stainless steel seamless pipe is sent to a straightening machine for mechanical straightening. A six- or eight-roll straightener adjusts straightness and ovality, controlling straightening deformation to no more than 0.5%. After straightening, the pipe is sent to a pickling unit for immersion pickling using a mixed acid system of HF and HNO3. After pickling, the pipe surface is rinsed with high-pressure pure water to remove residual acid and reaction residues.

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

[0098] Example 2

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

[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] Among them, the ratio of Ca content to S content is controlled to be 1.6.

[0102] The ratio of Mn content to N content is controlled to be 29.42.

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

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

[0105] S1:

[0106] Step S1-1 is the same as that of 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 that in Example 1, except that electromagnetic stirring or bottom blowing argon stirring is maintained for 3 minutes during the deoxidation process.

[0108] Step S1-3 is the same as that in Example 1, except that the ratio of Mn, Ni, and N elements is controlled according to this embodiment.

[0109] Step S1-4 is the same as that in Example 1, except that the temperature of the molten steel is controlled at 1650° C., the cored wire injection method is used for two additions with a 1-minute interval between them, and argon is blown at 30 L / min for continuous stirring.

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

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

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

[0113] S4 is the same as Example 1, except that the cold-drawn tube 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 in percentage by mass:

[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] Among them, the ratio of Ca content to S content is controlled to be 1.7.

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

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

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

[0122] S1:

[0123] Step S1-1 is the same as that of 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 that in Example 1, except that electromagnetic stirring or bottom blowing argon stirring is maintained for 2 minutes during the deoxidation process.

[0125] Step S1-3 is the same as that in Example 1, except that the ratio of Mn, Ni, and N elements is controlled according to this embodiment.

[0126] Step S1-4 is the same as that in Example 1, except that the temperature of the molten steel is controlled at 1630° C., the cored wire injection method is used for two additions with an interval of 1.5 minutes between them, and argon is blown at 20 L / min for continuous stirring.

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

[0128] S2 is the same as Example 1, except that the blank is heated to 1185°C before piercing, maintained at 1165°C during piercing, and the deformation is controlled at 37%. Furthermore, during the initial rolling stage, the blank is hot-rolled in five consecutive passes, with the outlet temperature controlled at 880°C. After rolling, the blank is air-cooled to room temperature.

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

[0130] S4 is the same as Example 1, except that the cold-drawn tube 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 in percentage by mass:

[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] Among them, the ratio of Ca content to S content is controlled to be 1.8.

[0136] The ratio of Mn content to N content is controlled to be 2:4.

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

[0138] This embodiment also discloses a method for preparing a cold-drawn stainless steel seamless pipe for fluid transportation, which is the same as that of Example 1.

[0139] Example 5

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

[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] Among them, the ratio of Ca content to S content is controlled to be 1.6.

[0143] The ratio of Mn content to N content is controlled to be 28.85.

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

[0145] This embodiment also discloses a method for preparing a cold-drawn stainless steel seamless pipe for fluid transportation, which is the same as that of Example 1.

[0146] Example 6

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

[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] Among them, the ratio of Ca content to S content is controlled to be 1.4.

[0150] The ratio of Mn content to N content is controlled to be 28.89.

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

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

[0153] The method for preparing a cold-drawn stainless steel seamless pipe for fluid transportation disclosed in this embodiment is different from that in Example 1 in that steps S1-3 are adjusted to:

[0154] After deoxidation, continue adjusting the content of the main alloying elements: add ferromanganese to control the Mn content to the target value, add ferronickel to adjust the Ni content to the target value, and add an appropriate amount of ferrochromium to adjust the Cr content to the target value. Add ferromolybdenum to adjust the Mo content to the target value to improve pitting resistance and high-temperature strength. Add ferrotitanium to adjust the Ti content to the target value, and control the melt stirring intensity during addition to avoid Ti burnout. Add ferroniobium to control the Nb content to the target value, adding it after the titanium, and control the overheating time to prevent carbide precipitation and affect the stability of the austenite. Then, according to the results of the central control test, add iron nitride to adjust the nitrogen content to the target value. The nitrogen content is confirmed using an oxygen and nitrogen analyzer (such as the LECO ONH836).

[0155] Example 7

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

[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] Among them, the ratio of Ca content to S content is controlled to be 1.4.

[0159] The ratio of Mn content to N content is controlled to be 27.78.

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

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

[0162] This embodiment also discloses a method for preparing a cold-drawn stainless steel seamless pipe for fluid transportation, which is the same as that of Example 6.

[0163] Example 8

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

[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] Among them, the ratio of Ca content to S content is controlled to be 1.5.

[0167] The ratio of Mn content to N content is controlled to be 27.45.

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

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

[0170] This embodiment also discloses a method for preparing a cold-drawn stainless steel seamless pipe for fluid transportation, which is the same as that of Example 6.

[0171] Example 9

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

[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] Among them, the ratio of Ca content to S content is controlled to be 1.4.

[0175] The ratio of Mn content to N content is controlled to be 27.69.

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

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

[0178] This embodiment also discloses a method for preparing a cold-drawn stainless steel seamless pipe for fluid transportation, which is the same as that of Example 6.

[0179] Examples 10-11

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

[0181]

[0182]

[0183] The preparation method of the cold-drawn stainless steel seamless pipe for fluid transportation in Examples 10-11 is the same as that 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] Scrap stainless steel containing iron, chromium, nickel, manganese, and silicon is pre-blended with alloys such as ferrochrome, ferronickel, ferromanganese, and ferrosilicon according to a predetermined composition. Smelting is performed in a DC arc furnace, heated to approximately 1600°C, completing the metal melting, preliminary alloying, and decarburization.

[0188] S1-2: Oxygen-argon double-blowing refining

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

[0190] S1-3: Calcium treatment (Ca adjustment)

[0191] During the final refining stage of oxygen-argon dual-blowing refining, calcium is added using a CaSi alloy cored wire feed method. The total Ca addition is controlled to maintain a stable Ca content in the molten steel at 0.0035%. To improve the uniformity of calcium distribution in the molten steel, argon is blown 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 the tundish-crystallizer system for continuous casting. Electromagnetic stirring technology is used to improve compositional uniformity in the mold, and the resulting slab (or ingot) is air-cooled. Composition analysis confirms that all elements meet 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 obtained in step S2 is directly subjected to four cold drawing passes at room temperature, with the deformation rate of each pass controlled at 15%. Ordinary carbide dies are used during the drawing process, and graphite powder or grease is coated on the inner wall of the die 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 ratio of each element in the seamless steel pipe is as follows:

[0199]

[0200]

[0201] Comparative Example 2

[0202] The difference between Comparative Example 2 and Example 1 is that the ratio of each element in the seamless steel pipe is 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 method

[0205] 1. Cold drawing crack sensitivity test

[0206] method:

[0207] Simulated drawing experiment: The cold drawing process is simulated with 6 passes and 15% deformation rate in each pass to evaluate the crack sensitivity of steel pipes during cold working.

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

[0209] condition:

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

[0211] Surface crack detection: Refer to the C5 level standard in GB / T 5777-1996 "Ultrasonic Flaw Detection Method for Seamless Steel Tubes" for surface crack detection.

[0212] index:

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

[0214] Fracture rate (%)

[0215] 2. Mechanical properties test (room temperature strength and toughness)

[0216] method:

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

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

[0219] standard:

[0220] Tensile test: According to GB / T 228.1-2021 "Tensile test of metallic materials - Part 1: Room temperature test method".

[0221] Charpy impact test: Based on GB / T 229-2020 "Charpy pendulum impact test method for metallic materials"

[0222] 3. Corrosion resistance test (pitting performance)

[0223] method:

[0224] Constant potential polarization test: The pitting potential and corrosion current density of stainless steel materials are measured in 3.5% NaCl solution.

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

[0226] Standard: Based on the detection 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 detection.

[0227] The above three test results are shown in Table 1 below.

[0228] Table 1

[0229]

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

[0231] In terms of cold drawing crack sensitivity, the microcrack density of Examples 1 to 3 and 6 to 8 is less than 0.35 / cm 2 , and there is no breakage, showing good cold drawing adaptability, which is attributed to the fact that the Ca / S ratio is controlled in the range of 1.4 to 1.6 to form a spheroidized, fine, and plastic Ca-Al-O inclusion structure, combined with a reasonable Mn / N (about 28 to 30) and Mn / Ni ratio (1.84 to 1.95) to stabilize the austenite structure and not easily induce cracks 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 and 10% and 12% fractures were found, which verified that insufficient Ca / S or unqualified Mn / N will directly lead to inclusion degradation and austenite instability, resulting in a significant increase in crack initiation during the cold drawing process.

[0232] In terms of mechanical properties, Examples 6-8 achieved tensile strengths of 710-721 MPa and elongations exceeding 42%, demonstrating the combined effects of Nb / Ti dispersion strengthening, Mo solid solution strengthening, and improved deformation coordination due to inclusions. Comparative Example 2, on the other hand, achieved a tensile strength of only 601 MPa and an elongation of 19.9%, slightly better than Comparative Example 1 but significantly lower than the Examples. This suggests that failure of Mn / N, Ni, and other elements to maintain a stable austenitic structure, or the presence of coarse MnS inclusions, can easily lead to work hardening and embrittlement failure.

[0233] In terms of corrosion resistance, the pitting potential of Examples 6 to 8 reached 355 to 359 mV, and the corrosion current density was as low as 1.02 to 1.13 μA / cm 2 , indicating that the passivation film is stable and the corrosion microbattery is well suppressed. In comparison, the pitting potential of Comparative Example 2 is 248.7mV and the corrosion current is as high as 3.58μA / cm 2 , reflecting the phenomenon of rupture of the passivation film and drastic expansion of the corrosion micro-area caused by the main inclusion of MnS. Comparative Example 1 also has a similar but slightly lighter trend.

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

[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 embodiment. All technical solutions based on the concept of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart 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 pipe for fluid transportation, characterized in that: The following components are included in mass percentage: 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 the Ca content to the S content is controlled to be 1-2.

2. The cold-drawn seamless stainless steel pipe for fluid transportation according to claim 1, characterized in that: It also contains 0.03-0.05 wt% of Nb.

3. The cold-drawn seamless stainless steel pipe for fluid transportation according to claim 1, characterized in that: It also contains 0.02-0.04 wt% of Ti.

4. The cold-drawn seamless stainless steel pipe for fluid transportation according to claim 1, characterized in that: It also contains 0.5-1.0 wt% of Mo.

5. The cold-drawn seamless stainless steel pipe for fluid transportation according to claim 1, characterized in that: The ratio of the Mn content to the N content is controlled to be 25-35.

6. The cold-drawn seamless stainless steel pipe for fluid transportation according to claim 5, characterized in that: The ratio of the Mn content to the Ni content is controlled to be 1.6 to 2.

1.

7. The cold-drawn seamless stainless steel pipe for fluid transportation according to claim 6, characterized in that: The ratio of the Ti content to the N content is 0.12 to 0.

15.

8. A method for preparing a cold-drawn seamless stainless steel pipe for fluid transportation according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1: Smelt the raw materials in proportion; during the smelting process, add the Ca element precisely through the core wire; S2: removing surface oxide scale and defects from the steel ingot obtained after smelting in step S1, heating it to 1180-1195°C, performing piercing and hot rolling, maintaining the temperature at 1150-1180°C during the piercing process, and controlling the deformation at 35-40%; controlling the rolling temperature at 850-900°C; and finally cooling it to room temperature by air cooling; S3: The tube is subjected to a multi-pass cold drawing deformation treatment of 4 to 6 times, with the deformation rate of each pass controlled at 8 to 15%; after the second or third pass, an intermediate annealing treatment is performed, that is, the drawn intermediate tube is heated to 1050° C., kept at this temperature for 1.5 minutes, and then rapidly cooled; S4: Heat the cold-drawn pipe to 1050-1080°C, keep it warm for 1-3 minutes, and then quickly cool it with water or convection air; S5: The heat-treated pipes are mechanically straightened, pickled to remove scale and residual inclusions, and subjected to surface passivation treatment.

9. The method for preparing a cold-drawn seamless stainless steel pipe for fluid transportation according to claim 8, characterized in that: The S1 step specifically includes the following steps: S1-1: After the matrix elements and main stabilizing alloying elements are mixed in proportion, vacuum induction melting is carried out; the temperature is raised to 1550-1580℃ in vacuum or inert gas to complete decarburization, degassing and preliminary alloying; S1-2: Add pure aluminum ingots or aluminum wires, controlling the amount added so that the residual aluminum content after deoxidation is controlled within 0.015-0.035%; maintain electromagnetic stirring or bottom argon stirring for 1-3 minutes during the deoxidation process; take a jaw sample and test the Al content using a photoelectric direct reading spectrometer to confirm that it meets the range; S1-3: Add ferromanganese to control the Mn content to 7.1-8.0%, add ferronickel to adjust the Ni content to 3.5-4.5%, and add ferrochrome in appropriate amounts; then, based on the results of the central control test, add iron nitride or manganese nitride to adjust the nitrogen content to 0.25-0.30%. The nitrogen content is confirmed by testing with an oxygen and nitrogen analyzer; S1-4: The molten steel is transferred to an argon-oxygen refining furnace for refining, the slag basicity is adjusted, and top and bottom blowing is continued; the molten steel temperature is controlled at 1600-1650°C. In this stage, CaSi alloy wire is added twice using the cored wire injection method, with an interval of 1-2 minutes in between, and argon bottom blowing at 15-30 L / min is maintained for continuous stirring; S1-5: After Ca is added, sample and analyze the S content to ensure it is within the set range of 0.0015-0.0020%. Calculate the Ca / S atomic ratio based on the amount of Ca added and ensure it is controlled between 2.2 and 3.

8. If the S content is too high, add a small amount of CaF or CaC desulfurizer to enhance slag desulfurization.

10. The method for preparing a cold-drawn seamless stainless steel pipe for fluid transportation according to claim 8, characterized in that: In the cold drawing process of step S3, the following two auxiliary stress control methods are simultaneously adopted: Ultrasonic vibration assisted drawing near the inner wall of the mold: frequency 20-25kHz, amplitude 10-20μm; Electromagnetic induction excitation assisted drawing arranged on the periphery of the mold: frequency 50~100Hz, magnetic density 0.05~0.08T.

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