A method of drawing stainless steel tube

Abstract

The application provides a stainless steel pipe drawing method and relates to the technical field of metal plastic working. The stainless steel pipe drawing method comprises the following steps: after cleaning, lubricating and preheating a solid solution annealed austenitic stainless steel pipe blank, the pipe blank is sent to a deformation zone in front of a drawing die; the pipe blank is drawn by using a gradient die, and a pulse magnetic field, a multi-frequency ultrasonic field and a gradient temperature field are synchronously applied to the pipe blank; and the pipe blank is subjected to online aging treatment by using the residual heat generated by the drawing deformation. While the pipe blank is subjected to mechanical deformation of the die, the pulse electromagnetic field and the multi-frequency composite ultrasonic field which are accurately designed are synchronously applied to the pipe blank, and the precise temperature field control is assisted, so that the physical fields of force, electricity, sound and heat are deeply cooperated, the dislocation movement in the material is actively intervened, the expected high-performance product is directly obtained, the traditional intermediate annealing process is completely cancelled, and the process is greatly shortened and the energy consumption is reduced.

Description

Technical Field

[0001] This invention relates to the field of metal plastic processing technology, and in particular to a method for drawing stainless steel tubes. Background Technology

[0002] Stainless steel tubes, especially austenitic stainless steel tubes, are widely used in high-end fields such as aerospace, energy and chemical industry, medical devices, and precision instruments due to their excellent corrosion resistance, good comprehensive mechanical properties, and biocompatibility. Cold drawing is a key process for manufacturing high-precision, high-performance stainless steel tubes. However, traditional cold drawing processes suffer from severe work hardening and lengthy procedures.

[0003] During cold deformation, metastable austenitic stainless steel experiences rapid dislocation proliferation and entanglement, accompanied by deformation-induced martensitic transformation, leading to rapid material hardening and a sharp decline in plasticity. To ensure continued processing, energy-intensive and time-consuming intermediate annealing and recrystallization annealing processes must be interspersed between multiple drawing passes to restore the material's plasticity. This results in a long production process, low efficiency, high energy consumption, and the repeated heating can easily cause grain growth and surface oxidation.

[0004] To address these issues, existing technologies have explored various approaches, such as using ultrasonic-assisted drawing to reduce drawing force and improve surface quality, or studying the strengthening mechanism of deformation-induced martensitic phase transformation. However, these improvements are mostly localized optimizations: the single external field assistance, such as ultrasound, has limited effect and cannot fundamentally avoid intermediate annealing; the utilization of deformation-induced phase transformation is often passive and uncontrollable, resulting in unstable performance. Current technologies have not yet achieved a fundamental breakthrough in obtaining the desired performance in a single process by actively, precisely, and programmably controlling the evolution of the material's microstructure during deformation. Summary of the Invention

[0005] In view of this, in order to overcome the above-mentioned defects of existing stainless steel tube drawing technology, the purpose of this invention is to propose a stainless steel tube drawing method based on pulsed magnetic field and gradient phase transformation control. By actively and precisely controlling the deformation-induced martensitic phase transformation during a single drawing process, while the tube is subjected to mechanical deformation by the die, a precisely designed pulsed electromagnetic field and a multi-frequency composite ultrasonic field are simultaneously applied to it, supplemented by precise temperature field control, forming a deep synergy of force, electricity, sound and heat physical fields. This actively intervenes in the dislocation movement inside the material, directly obtaining the expected high-performance product, completely eliminating the traditional intermediate annealing process, significantly shortening the process and reducing energy consumption.

[0006] To achieve the above objectives, the present invention provides the following technical solution: To achieve the above objectives, the present invention provides a method for drawing stainless steel tubes, comprising: S1. Perform surface cleaning and lubrication treatment on the solution-annealed austenitic stainless steel tube blank; S2. Before the tube blank enters the deformation zone of the drawing die, it is preheated, and the preheating temperature is controlled at 200-400℃. S3. During the drawing deformation process, a pulsed magnetic field, a multi-frequency ultrasonic field, and a gradient temperature field are simultaneously applied to the tube blank, wherein: (a) The pulsed magnetic field is applied as follows: a high-frequency pulsed magnetic field with a frequency of 1-10 MHz and an intensity of 0.5-2 T is applied to the surface region of the tube blank, and a low-frequency pulsed magnetic field with a frequency of 0.1-1 MHz and an intensity of 2-5 T is applied to the core region. The pulse width is 1-100 μs and the duty cycle is 10-50%. (b) The multi-frequency ultrasonic field includes three frequency components: 20 kHz, 100 kHz and 1 MHz, with corresponding sound pressure amplitudes of 10-30 MPa, 5-20 MPa and 1-10 MPa, respectively, and the phase difference between the three frequency components is controlled within 0-90°. (c) The gradient temperature field is controlled by heating the surface of the tube blank using an infrared laser array, with a heating power density of 100-1000 W / cm². 2 Meanwhile, the core is cooled by micro-liquid nitrogen injection at a rate of 10-100℃ / s, so that the temperature gradient in the deformation zone is 20-100℃ higher on the surface than on the core. S4. Simultaneously with step S3, a gradient die is used to draw and deform the tube blank. The main deformation section of the gradient die is made of Al2O3 or ZrO2 ceramic composite material reinforced with Fe3O4 magnetic particles, and the volume fraction of Fe3O4 increases linearly from 5% at the inlet to 30% at the outlet. S5, the single-pass drawing diameter reduction rate reaches 35-50%, and no intermediate annealing treatment is required for the tube blank throughout the process; S6. Utilize the residual heat generated by the drawing deformation to perform online aging treatment on the formed tube at 300-400℃ for 10-60 seconds.

[0007] As a further embodiment of the present invention, in step S3, the application of the pulsed magnetic field begins 10-50 mm before the drawing die inlet and ends 10-50 mm after the die outlet, and the magnetic field frequency is periodically switched between 1-10 MHz and 0.1-1 MHz with a period of 0.1-1 s to generate periodic phase transition waves in the tube blank.

[0008] As a further aspect of the present invention, in step S3, the three frequency components of the multi-frequency ultrasonic field are applied using a time-series modulation method, with a modulation period of 0.01-0.1 s. Within each period, 20 kHz ultrasonic waves are applied sequentially for 1-10 ms, 100 kHz ultrasonic waves for 1-10 ms, and 1 MHz ultrasonic waves for 1-10 ms, and a transition time of 1-5 ms is provided between adjacent frequency components.

[0009] As a further embodiment of the present invention, in step S4, the gradient mold further includes a guiding lubrication section and a sizing stabilizing section, wherein the surface of the guiding lubrication section is a diamond-like carbon coating with a thickness of 10-100 nm and a friction coefficient of less than 0.05; the sizing stabilizing section is made of nanocrystalline cubic boron nitride with a hardness of not less than 3000 HV.

[0010] As a further aspect of the present invention, step S3 further includes applying an auxiliary pulse current to the deformed region, wherein the density of the pulse current is 10. 2 -10 4 A / cm 2 The pulse frequency is 10-1000 Hz and the pulse width is 10-100 μs, which is used to promote dislocation motion and phase transition processes.

[0011] As a further aspect of the present invention, step S7 is also included: during the drawing process, the contact stress, temperature and friction state are monitored in real time by a sensor array integrated in the mold, with a sampling frequency of not less than 10 kHz. When the contact stress exceeds ±10% of the preset value or the temperature exceeds ±5% of the set range, the intensity of the pulse magnetic field, the sound pressure amplitude of the ultrasonic field or the drawing speed are automatically adjusted, with an adjustment range of 5-20%.

[0012] As a further aspect of the present invention, step S8 is also included: applying an electromagnetic pulse to the formed tube, wherein the intensity of the electromagnetic pulse is 1-3 T, the frequency is 10-100 Hz, and the duration is 1-10 s, for adjusting the residual stress distribution so that a residual compressive stress of 50-200 MPa is formed on the surface.

[0013] As a further aspect of the present invention, the stainless steel tube prepared by the stainless steel tube drawing method has a radial gradient structure, wherein: Within a surface depth of 10-50 μm, the austenite volume fraction is greater than 85%, and the average grain size is 1-3 μm. The transition layer depth is in the range of 50-200 μm, the austenite volume fraction is 50-85%, and the average grain size is 0.5-1 μm; In the core region, the volume fraction of deformation-induced martensite is 40-80%, and the average grain size is 0.1-0.5 μm; Furthermore, the pipe has a tensile strength of 1000-1500 MPa, an elongation of 20-40%, and a strength-ductility product of 20-50 GPa·s.

[0014] As a further aspect of the present invention, step S9 is also included: during the drawing process, when the martensite content of the tube is detected to exceed ±5% of the target value, the frequency and intensity of the pulse magnetic field are automatically adjusted. The adjustment strategy is as follows: if the martensite content is too high, the surface magnetic field frequency is increased by 1-5 MHz and the core magnetic field intensity is decreased by 0.5-2 T; if the martensite content is too low, the surface magnetic field frequency is decreased by 1-5 MHz and the core magnetic field intensity is increased by 0.5-2 T.

[0015] As a further aspect of the present invention, step S11 is also included: during the drawing process, an electromagnetic acoustic sensor is used to monitor the phase composition change of the pipe in real time, with a sampling frequency of 100 kHz-1 MHz, and the monitoring data is fed back to the digital twin model in real time to correct the prediction parameters of the tissue evolution sub-model, with a correction period of no more than 1 s.

[0016] Compared with existing technologies, the stainless steel tube drawing method proposed in this invention has the following advantages: 1. The stainless steel tube drawing method of the present invention, through the synergistic effect of pulsed magnetic field, multi-frequency ultrasonic wave, and gradient temperature field, actively controls dislocation movement and phase transformation behavior during deformation, fundamentally solving the industry pain point that austenitic stainless steel must rely on intermediate annealing due to severe work hardening. It eliminates the energy-intensive intermediate heat treatment process and utilizes ultrasonic waves to reduce deformation resistance and magnetic fields to promote phase transformation, thereby reducing mechanical energy consumption. This results in lower overall energy consumption, reduced equipment footprint, increased production efficiency, and a significant decrease in the manufacturing cost per unit product.

[0017] 2. The stainless steel tube drawing method of the present invention produces tubes with ultra-high strength while maintaining excellent plasticity through gradient phase transformation and precise microstructure configuration, thereby improving the strength-ductility product. Utilizing the synergy of a high-frequency magnetic field and a gradient temperature field, a gradient structure with high plasticity and toughness on the surface and high strength in the core can be precisely constructed on the tube cross-section, greatly improving material flowability and surface friction. The strong coupling effect of multiple physical fields, especially the vibration of ultrasound, effectively breaks up coarse grains and suppresses deformation bands. Combined with online intelligent control, a uniform and fine microstructure can be obtained across the entire cross-section.

[0018] These or other aspects of this application will become more apparent from the following description of embodiments. It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the application. Detailed Implementation

[0019] The present application will be further described below with reference to specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the embodiments of this invention are further described in detail below with reference to specific examples. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit this application.

[0021] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two different entities or different parameters with the same name. Therefore, "first" and "second" are merely for convenience of expression and should not be construed as limiting the embodiments of the present invention. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, such as other steps or units inherent in a process, method, system, product, or device that includes a series of steps or units.

[0022] The technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] The following describes some embodiments of this application in detail. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0024] Example 1 An embodiment of the present invention provides a method for drawing stainless steel tubes, the specific steps of which are as follows: S1. Tube blank pretreatment: First, the surface of the solution-annealed 316L austenitic stainless steel tube blank is cleaned to remove oil and oxide layers. Then, a lubricating coating is applied to the cleaned tube blank surface to reduce frictional resistance during the subsequent drawing process.

[0025] S2. Preheating treatment: Before the tube blank enters the main drawing deformation zone, the entire tube blank is preheated. The preheating temperature is controlled at 300℃ to appropriately soften the tube blank material and reduce its deformation resistance.

[0026] S3, Gradient deformation assisted by composite physical fields: An intelligent gradient functional die is used to draw the tube blank, and pulsed magnetic field, multi-frequency ultrasonic field, and gradient temperature field are simultaneously applied to the tube blank inside the die to achieve active control of the deformation process. Specifically: The intelligent gradient functional die consists of a guiding lubrication section, a main deformation section, and a sizing and stabilizing section, arranged sequentially along the drawing direction. The guiding lubrication section has a 50 nm thick diamond-like carbon coating with a friction coefficient below 0.05, ensuring smooth tube blank introduction. The main deformation section is made of Fe3O4 magnetic particle-reinforced ZrO2 ceramic matrix composite material, with the Fe3O4 particle volume fraction linearly increasing from 5% at the inlet to 30% at the outlet, forming a gradient functional structure to achieve an optimized match between deformation resistance and wear resistance. The sizing and stabilizing section is made of nanocrystalline cubic boron nitride with a hardness of 3500 HV, ensuring the final dimensional accuracy of the tube.

[0027] Gradient temperature field application: The surface of the tube blank entering the mold deformation zone is selectively heated using an infrared laser array, with the laser power density set to 500 W / cm². 2 Simultaneously, the core region of the tube blank is cooled by a precisely directional micro-jet of liquid nitrogen, with the cooling rate controlled at 50℃ / s. Through regulation, a stable and controllable temperature gradient is formed in the deformation zone, making the surface temperature of the tube blank 60℃ higher than the core temperature. This gradient temperature field can induce differentiated rheological behavior in the material.

[0028] Pulsed magnetic field application: During the drawing deformation of the tube blank within the gradient die, a spatially and frequency-coupled pulsed magnetic field is simultaneously applied. Specifically, a high-frequency pulsed magnetic field with a frequency of 5 MHz and an intensity of 1 T is applied to the surface region of the tube blank; a low-frequency pulsed magnetic field with a frequency of 0.5 MHz and an intensity of 3 T is applied to the core region of the tube blank. The pulse width is set to 50 μs, and the duty cycle is 30%. The application area of ​​the pulsed magnetic field begins 30 mm before the die inlet and ends 30 mm after the die outlet, completely covering the deformation and adjacent areas. The magnetic field frequency switches periodically between 5 MHz (surface frequency) and 0.5 MHz (core frequency) with a period of 0.5 s, thereby inducing periodic phase transition waves inside the tube blank, promoting dislocation reorganization and phase transformation to refine the grains.

[0029] Multi-frequency ultrasonic field application: A time-series modulation method is used to apply an ultrasonic field containing three frequency components: 20 kHz, 100 kHz, and 1 MHz, to the tube blank in the deformation zone. The modulation period is 0.05 s. Within a single modulation period, a 20 kHz ultrasonic wave with a sound pressure level of 20 MPa is applied sequentially for 5 ms, followed by a 2 ms transition period, then a 100 kHz ultrasonic wave with a sound pressure level of 10 MPa for 5 ms, and finally, after another 2 ms interval, a 1 MHz ultrasonic wave with a sound pressure level of 5 MPa for 5 ms. The phase difference between the three frequency components is controlled at 45°. This multi-frequency, time-series ultrasonic field effectively reduces material flow stress, improves plasticity, and assists in the breakup of coarse grains.

[0030] S4, Large Deformation Drawing: Under the synergistic effect of the composite physical field in step S3, the tube blank completes a single-pass drawing deformation in the intelligent gradient functional mold. The diameter reduction rate of this drawing reaches 40%, and no intermediate annealing treatment is required for the tube blank during the entire drawing process, achieving efficient and continuous processing.

[0031] S5. Online Timeliness Processing: Utilizing the residual heat from the deformation after the tube is drawn from the mold, it is immediately subjected to online aging treatment. The formed stainless steel tube is held at 350°C for 30 seconds to promote the precipitation of fine second-phase particles in the supersaturated solid solution, partially eliminate residual stress, stabilize the microstructure, and thus further improve the comprehensive mechanical properties of the material.

[0032] In this embodiment, during the drawing process, a multi-physics sensor array integrated within the mold monitors the stress and temperature of the tube / mold contact surface in real time, with a sampling frequency of 20 kHz. The control system performs dynamic feedback adjustments based on a preset process model and real-time monitoring data. When the system detects that the instantaneous value of the contact stress exceeds 10% of the model's preset value, it automatically adjusts the intensity of the pulsed magnetic field, increasing the surface magnetic field intensity from 1 T to 1.1 T to optimize the deformation process.

[0033] In this embodiment, after the online aging treatment in step S5, an electromagnetic pulse post-treatment can be applied to the stainless steel pipe. Specific parameters are: pulse magnetic field strength 2 T, frequency 50 Hz, and duration 5 seconds. This treatment can further adjust the residual stress distribution inside the pipe, forming a residual compressive stress layer of approximately 100 MPa on its surface, which is beneficial for improving fatigue performance.

[0034] The stainless steel tube prepared by the method in this embodiment has excellent mechanical properties and a unique radial gradient microstructure, wherein, Gradient microstructure: In the surface layer of the pipe within a depth of 30 μm, the volume fraction of austenite is greater than 90%, with an average grain size of about 2 μm; in the transition layer with a depth of 50-150 μm, the volume fraction of austenite is about 70%, with an average grain size of about 0.8 μm; in the core region, the volume fraction of deformation-induced martensite is about 60%, with an average grain size of about 0.3 μm.

[0035] The stainless steel pipe prepared by the method in this embodiment has a tensile strength of 1200 MPa, an elongation of 30%, and a strength-ductility product of up to 36 GPa·%, achieving a synergistic improvement in high strength and good ductility.

[0036] Example 2 An embodiment of the present invention provides a method for drawing stainless steel tubes, the specific steps of which are as follows: S1. Tube blank pretreatment: The surface treatment of 316L austenitic stainless steel tube blanks in the solution annealed state involves first thoroughly removing surface contaminants by pickling and ultrasonic cleaning, and then coating with a polymer-based lubricant to form a uniform lubricating film.

[0037] S2. Preheating treatment: Before the tube blank enters the main deformation zone, it is preheated as a whole using a medium-frequency induction coil. The preheating temperature is precisely controlled at 250℃ to initially activate the material's plastic deformation capacity.

[0038] S3, Intelligent drawing deformation assisted by multi-field coupling: The tube blank is drawn in an intelligent gradient functional die and simultaneously subjected to the synergistic effects of multiple physical fields. The intelligent gradient functional die consists of three sections. The guiding and lubricating section has an 80 nm thick diamond-like carbon (DLC) coating with a friction coefficient of approximately 0.03, ensuring low-friction introduction. The main deformation section is made of Fe3O4 particle-reinforced Al2O3 ceramic matrix composite material, where the Fe3O4 volume fraction increases linearly from 5% at the inlet to 30% at the outlet, achieving continuous variation in the die's mechanical properties to adapt to changes in stress state during large deformation. The sizing and stabilizing section is made of nanocrystalline cubic boron nitride (cBN) with a Vickers hardness of up to 3200 HV, ensuring high dimensional accuracy and high surface quality of the final tube.

[0039] When the gradient temperature field is applied, a high-power infrared laser array is used to rapidly scan and heat the surface of the tube blank, with the power density set to 800 W / cm². 2 Simultaneously, a small amount of gaseous / liquid mixed nitrogen is injected into the core of the tube blank through a multi-nozzle array for forced cooling, with the cooling rate controlled at 80 ℃ / s. Through precise control, a stable temperature gradient is established and maintained in the deformation zone, ensuring that the surface temperature of the tube blank is 80 ℃ higher than that of the core.

[0040] When applying the pulsed magnetic field, spatially differentiated pulsed magnetic fields are applied to the deformation zone of the mold and its preceding and following regions, specifically from 20 mm before the inlet to 20 mm after the outlet. A pulsed magnetic field with a frequency of 2 MHz and an intensity of 1.5 T is applied to the surface region; a pulsed magnetic field with a frequency of 0.2 MHz and an intensity of 4 T is applied to the core region. The pulse width is 20 μs, and the duty cycle is 40%. Furthermore, the magnetic field frequency is controlled to periodically switch between 2 MHz and 0.2 MHz with a period of 0.3 s to induce dynamic stress waves within the material, promoting dislocation motion and phase transition.

[0041] When applying a time-series modulated multi-frequency ultrasonic field, a time-series modulation technique is used. The modulation period is set to 0.03 s. Within each period, the following ultrasonic waves are applied sequentially and individually: a 20 kHz ultrasonic wave with a sound pressure level of 25 MPa for 3 ms, followed by a 1 ms pause as a transition time; then a 100 kHz ultrasonic wave with a sound pressure level of 15 MPa for 3 ms, followed by another 1 ms pause; finally, a 1 MHz ultrasonic wave with a sound pressure level of 8 MPa for 3 ms. The phase difference between the three frequency components is set to 30°. This time-series application method can more effectively utilize the activation effect of different frequency ultrasonic waves on defects of different scales in the material.

[0042] When applying the auxiliary pulse current, a high-density pulse current is simultaneously applied to the tube blank in the deformation zone while the aforementioned physical field is being applied. The pulse current density is 5 × 10⁻⁶. 3 A / cm 2 The frequency is 500 Hz and the pulse width is 50 μs. The electroplastic and Joule heating effects generated by this pulsed current can further promote dislocation motion and phase transition dynamics.

[0043] S4. Large Deformation Pulling and Online Control: Under the synergistic effect of the composite field described in step S3, the drawing process achieves a single-pass diameter reduction of 45%. The entire drawing process is monitored and adjusted online intelligently: a miniature sensor array integrating piezoelectric stress sensors and infrared temperature sensors, integrated into the die guiding and sizing sections, monitors the contact stress and temperature distribution at the billet / die interface in real time at a sampling frequency of 50 kHz. The monitoring data is transmitted to the central controller in real time. When the system detects that the temperature in a certain area exceeds the set upper limit by 5%, the controller automatically reduces the sound pressure amplitude of the ultrasonic field in the corresponding area by 10% according to a preset algorithm, lowering the 20 kHz component from 25 MPa to 22.5 MPa, and simultaneously reducing the drawing speed by 8%.

[0044] S5. Online timeliness and post-processing: After the pipe is demolded, it is immediately introduced into an insulated channel for online aging treatment, utilizing the residual heat from deformation. The aging temperature is 380℃, and the time is 15 seconds. Then, an electromagnetic pulse post-treatment is performed. An additional electromagnetic pulse treatment is applied to the pipe that has completed online aging, with the following parameters: pulse magnetic field strength 2.5 T, frequency 80 Hz, and duration 3 seconds. This creates a residual compressive stress of approximately 150 MPa on the pipe surface, improving its fatigue and stress corrosion resistance.

[0045] The stainless steel pipe prepared by the method in this embodiment also exhibits excellent gradient microstructure and comprehensive properties. In the gradient microstructure, the surface layer within a depth of approximately 20 μm has a high austenite content with an average grain size of 1.5 μm; in the transition layer from 50 to 180 μm, the austenite content is approximately 65%, with a grain size refined to 0.6 μm; and in the core region, the deformation-induced martensite content reaches 70%, with an average grain size of only 0.2 μm. The prepared pipe exhibits a good combination of high strength and high plasticity, with a tensile strength as high as 1350 MPa, an elongation maintained at 25%, and a strength-ductility product of 33.8 GPa·%. Furthermore, due to the residual compressive stress in the surface layer, its fatigue limit is significantly improved.

[0046] Example 3 An embodiment of the present invention provides a method for drawing stainless steel tubes, the specific steps of which are as follows: S1. Tube blank pretreatment: 304 austenitic stainless steel tube blanks in a solution-annealed state are selected. First, chemical degreasing and pickling activation are performed, followed by surface cleaning using high-pressure water jet. After cleaning, a high-performance lubricating coating containing molybdenum disulfide nano-solid lubricant is uniformly coated onto the outer surface of the tube blank.

[0047] S2. Preheating treatment: Before the tube blank enters the main deformation zone, it is preheated uniformly using a resistance radiant heater. The preheating temperature is set at 280℃ to ensure that the tube blank reaches a suitable temperature for plastic deformation as a whole, while avoiding local overheating.

[0048] S3, Intelligent drawing deformation driven by digital twin: The tube blank is drawn in an intelligent gradient functional die and simultaneously subjected to precise action of a composite physical field. The entire process is monitored and optimized in real time by a digital twin system. The intelligent gradient functional die includes a guiding lubrication section, a main deformation section, and a sizing and stabilizing section. The guiding lubrication section has a 20 nm thick diamond-like carbon (DLC) coating with a friction coefficient of less than 0.04. The main deformation section is made of Fe3O4 particle-reinforced ZrO2-based ceramic composite material, with the Fe3O4 volume fraction linearly increasing from 5% at the inlet to 30% at the outlet to provide gradient mechanical properties. The sizing and stabilizing section is made of nanocrystalline cubic boron nitride (cBN) with a hardness ≥3100 HV to ensure dimensional accuracy.

[0049] When the gradient temperature field is applied, a ring-shaped infrared laser is used to uniformly heat the surface of the tube blank in the deformation zone, with a heating power density of 300 W / cm². 2 Simultaneously, atomized liquid nitrogen is sprayed from a micro-nozzle array surrounding the inner wall of the tube blank to rapidly dissipate heat from the core, with the cooling rate controlled at 30℃ / s. Ultimately, a stable gradient is formed in the deformation zone, maintaining a surface temperature 40℃ higher than that of the core.

[0050] When the pulsed magnetic field is applied, a differentiated pulsed magnetic field is applied to the mold deformation zone, ranging from 40 mm before the inlet to 40 mm after the outlet. The surface magnetic field parameters are: frequency 8 MHz, intensity 0.8 T; the core magnetic field parameters are: frequency 0.8 MHz, intensity 2.2 T. The pulse width is 80 μs, and the duty cycle is 20%. The magnetic field frequency is periodically switched between 8 MHz and 0.8 MHz with a period of 1.0 s, inducing periodic stress disturbances within the material.

[0051] When applying the time-series modulated multi-frequency ultrasonic field, time-series modulation technology is used to cyclically apply multi-frequency ultrasonic waves with a period of 0.08 s. Within each cycle, a 20 kHz ultrasonic wave with a sound pressure amplitude of 15 MPa is applied sequentially for 5 ms, with a 2 ms transition time; then a 100 kHz ultrasonic wave with a sound pressure amplitude of 8 MPa is applied for 5 ms, with another 2 ms interval; finally, a 1 MHz ultrasonic wave with a sound pressure amplitude of 3 MPa is applied for 5 ms. The phase difference between the three frequency components is set to 60°.

[0052] During the real-time control driven by digital twin, a miniature piezoelectric sensor array integrated in the mold with a sampling frequency of 50 kHz is used to monitor the contact stress and friction state in real time during the drawing process. At the same time, a non-contact electromagnetic acoustic sensor with a sampling frequency of 500 kHz is used to monitor the phase composition changes of the tube online and non-destructively in real time, and the monitoring data is transmitted to the digital twin system in real time.

[0053] S4, Large Deformation Drawing: Drawing is performed under the intelligent control environment described in step S3, achieving a single-pass diameter reduction rate of 38%, and requiring no intermediate annealing throughout the process, resulting in high process continuity.

[0054] S5, Online Timeliness Processing After demolding, the pipe is immediately subjected to online aging treatment at 320°C for 40 seconds, utilizing residual heat, to stabilize the microstructure and properties. Based on the prediction results of the residual stress distribution of the pipe using a digital twin model, a customized electromagnetic pulse treatment is applied to the aged pipe. The treatment parameters are dynamically determined by the model according to the predicted stress state. In this embodiment, an electromagnetic pulse with an intensity of 1.8 T and a frequency of 30 Hz is used, with an application time of 8 seconds. After treatment, a residual compressive stress of approximately 100 MPa is formed on the surface of the pipe, effectively improving fatigue performance.

[0055] The stainless steel pipe prepared by the method in this embodiment exhibits highly uniform and excellent microstructure and properties under intelligent control. In the gradient microstructure, the surface austenite volume fraction of 10-30 μm is as high as 92%, with an average grain size of 2.2 μm; the transition layer austenite volume fraction of 50-150 μm is 75%, with an average grain size of 0.7 μm; and the deformation-induced martensite volume fraction in the core region is 55%, with an average grain size of 0.4 μm. The microstructure transitions uniformly along the radial direction. The pipe exhibits a balance between high strength and good plasticity, with a tensile strength of 1100 MPa, an elongation of 35%, and a strength-ductility product as high as 38.5 GPa·%. Thanks to intelligent control, the performance dispersion of the same batch of pipes is significantly reduced.

[0056] To verify the effectiveness of the stainless steel tube drawing method described in this invention and the excellent performance of its products, performance tests were conducted on the austenitic stainless steel tubes (material 304 or 316L) drawn in Examples 1 to 3 of this invention. The tests are as follows: I. Based on relevant national standards (GB / T) and industry-standard methods, tensile property tests, hardness gradient tests, fatigue performance tests, and process stability assessments were conducted on the finished pipes. For tensile property testing, standard tensile specimens were cut axially from the pipes prepared in each embodiment. The gauge length diameter was 0.8 times the inner diameter of the pipe, and the gauge length was 5.65 times √So. Room temperature tensile tests were conducted on a universal testing machine according to GB / T 228.1. The tensile strength (Rm), specified plastic extension strength (Rp0.2), elongation after fracture (A), and strength-ductility product (Rm×A) were recorded and calculated.

[0057] During the hardness gradient test, along the cross-section of the pipe, from the outer surface to the inner surface, a micro Vickers hardness tester (according to GB / T 4340.1) is used to make test dots at 20μm intervals, with a load of 300g.

[0058] During fatigue performance testing, the pipes of Examples 2 and 3, which underwent electromagnetic pulse post-treatment, and Example 1, which served as a comparison and did not undergo electromagnetic pulse post-treatment, were processed into rotary bending fatigue specimens. Fatigue tests were conducted on a rotary bending fatigue testing machine according to GB / T 4337 to determine their respective stresses and lifespans, and their fatigue limits were compared.

[0059] During the process stability assessment, comprehensive mechanical properties and phase composition tests were conducted on five tubes from the same batch produced in Example 3. The standard deviation (SD) and coefficient of variation (CV) of tensile strength, elongation, and core martensite content were calculated to evaluate the stability and product consistency of the intelligent drawing process driven by digital twins.

[0060] II. The test results are shown in Table 1 below: Table 1 Test data results of pipes from Examples 1 to 3 Note: "-" in the table indicates that no test was conducted or no comparison was made. The numerical format is "mean ± standard deviation".

[0061] The results above show that the pipes prepared in all three embodiments achieved high strength exceeding 1000 MPa and high elongation exceeding 20%, with a strength-ductility product exceeding 30 GPa·%, achieving a good synergy between strength and ductility. Embodiment 2, due to the use of a higher diameter reduction ratio and auxiliary pulse current, achieved the highest strength (1350 MPa), but with a slight sacrifice in ductility. Embodiment 3, under the optimized control of the digital twin system, achieved the highest elongation (34.8%) and strength-ductility product (38.5 GPa·%), exhibiting the best combination of toughness and strength.

[0062] Example 1, without post-treatment, had surface residual stress close to zero or only a small compressive stress. Examples 2 and 3, through electromagnetic pulse post-treatment, successfully introduced significant residual compressive stress (-140 to -155 MPa and -95 to -105 MPa) on the outer surface. Fatigue test results (410 MPa and 380 MPa vs 320 MPa) directly demonstrate that this residual compressive stress layer effectively inhibited the initiation of fatigue cracks and significantly improved the fatigue life of the pipe. The residual stress in Example 2 was higher, consistent with the stronger electromagnetic pulse parameters used.

[0063] The drawing method provided by this invention, which utilizes a composite physical field synergy and gradient functional mold assistance, can successfully produce high-performance stainless steel tubes with radial gradient microstructure, high strength, high plasticity, and good fatigue life. Specifically, the integrated auxiliary pulse current further enhances the material to achieve ultra-high strength; while the introduction of intelligent real-time control driven by digital twins ensures excellent performance while achieving precise process control and high product consistency, representing an important path for advanced manufacturing towards intelligence and high precision.

[0064] This invention provides a stainless steel tube drawing method that, through the synergistic effect of pulsed magnetic field, multi-frequency ultrasound, and gradient temperature field, actively controls dislocation movement and phase transformation behavior during deformation, fundamentally solving the industry pain point that austenitic stainless steel must rely on intermediate annealing due to severe work hardening. It eliminates the energy-intensive intermediate heat treatment process and utilizes ultrasound to reduce deformation resistance and magnetic field to promote phase transformation, thereby reducing mechanical energy consumption. This results in lower overall energy consumption, reduced equipment footprint, and increased production efficiency, significantly reducing the manufacturing cost per unit product. The tubes prepared through gradient phase transformation and precise microstructure configuration achieve ultra-high strength while maintaining excellent plasticity, enhancing the strength-ductility product. Utilizing the synergy of high-frequency magnetic field and gradient temperature field, a gradient structure with high plasticity and toughness on the surface and high strength in the core can be precisely constructed on the tube cross-section, greatly improving material flowability and surface friction. The strong coupling effect of multiple physical fields, especially the vibration energy of ultrasound, effectively breaks up coarse grains and suppresses deformation bands. Combined with online intelligent control, a uniform and fine microstructure can be obtained across the entire cross-section.

[0065] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.

[0066] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of different aspects of the invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A method for drawing stainless steel tubes, characterized in that, include: After cleaning, lubricating and preheating the solution-annealed austenitic stainless steel tube blank, the tube blank is sent into the deformation zone of the drawing die. A gradient die is used to draw and deform the tube blank, and a pulsed magnetic field, a multi-frequency ultrasonic field and a gradient temperature field are applied to the tube blank simultaneously. The residual heat generated by the drawing deformation is used to perform online aging treatment on the formed tube. The pulsed magnetic field is applied as follows: a high-frequency pulsed magnetic field with a frequency of 1-10 MHz and an intensity of 0.5-2 T is applied to the surface region of the tube blank, and a low-frequency pulsed magnetic field with a frequency of 0.1-1 MHz and an intensity of 2-5 T is applied to the core region. The pulse width is 1-100 μs and the duty cycle is 10-50%. The multi-frequency ultrasonic field includes three frequency components: 20 kHz, 100 kHz and 1 MHz, with corresponding sound pressure amplitudes of 10-30 MPa, 5-20 MPa and 1-10 MPa, respectively, and the phase difference between the three frequency components is controlled within 0-90°. The gradient temperature field is controlled by heating the surface of the tube blank using an infrared laser array, with a heating power density of 100-1000 W / cm². 2 Meanwhile, the core is cooled by micro-liquid nitrogen injection at a rate of 10-100 ℃ / s, so that the temperature gradient in the deformation zone is 20-100 ℃ higher on the surface than on the core.

2. The stainless steel tube drawing method as described in claim 1, characterized in that, Before the tube blank enters the deformation zone of the drawing die, the temperature of the tube blank is controlled at 200-400℃ for preheating treatment.

3. The stainless steel tube drawing method as described in claim 2, characterized in that, When using a gradient die to draw and deform the tube blank, the main deformation section of the gradient die is made of Al2O3 or ZrO2 ceramic composite material reinforced with Fe3O4 magnetic particles, and the volume fraction of Fe3O4 increases linearly from 5% at the inlet to 30% at the outlet, with a single-pass drawing diameter reduction rate of 35-50%.

4. The stainless steel tube drawing method as described in claim 3, characterized in that, The formed pipe is subjected to online aging treatment at 300-400℃ for 10-60 seconds.

5. The stainless steel tube drawing method as described in claim 1, characterized in that, The pulsed magnetic field is applied starting 10-50 mm before the drawing die inlet and ending 10-50 mm after the die outlet. The magnetic field frequency is periodically switched between 1-10 MHz and 0.1-1 MHz with a period of 0.1-1 s to generate periodic phase transition waves in the tube blank.

6. The stainless steel tube drawing method as described in claim 3, characterized in that, The gradient mold further includes a guiding lubrication section and a sizing stabilizing section. The surface of the guiding lubrication section is a diamond-like carbon coating with a thickness of 10-100 nm and a friction coefficient of less than 0.

05. The sizing stabilizing section is made of nanocrystalline cubic boron nitride with a hardness of not less than 3000 HV.

7. The stainless steel tube drawing method as described in claim 1, characterized in that, The online aging treatment of the formed tube using the residual heat generated by the drawing deformation also includes: applying an electromagnetic pulse to the formed tube, wherein the intensity of the electromagnetic pulse is 1-3 T, the frequency is 10-100 Hz, and the duration is 1-10 s, to adjust the residual stress distribution and form a residual compressive stress of 50-200 MPa on the surface.

8. The stainless steel tube drawing method as described in claim 1, characterized in that, The stainless steel tube prepared by the stainless steel tube drawing method has a radial gradient structure, wherein: Within a surface depth of 10-50 μm, the austenite volume fraction is greater than 85%, and the average grain size is 1-3 μm. The transition layer has a depth of 50-200 μm, an austenite volume fraction of 50-85%, and an average grain size of 0.5-1 μm. In the core region, the volume fraction of deformation-induced martensite is 40-80%, and the average grain size is 0.1-0.5 μm; Furthermore, the pipe has a tensile strength of 1000-1500 MPa, an elongation of 20-40%, and a strength-ductility product of 20-50 GPa·s.

9. The stainless steel tube drawing method as described in claim 1, characterized in that, During the drawing process, when the martensite content of the tube is detected to exceed ±5% of the target value, the frequency and intensity of the pulsed magnetic field are automatically adjusted. The adjustment strategy is as follows: if the martensite content is too high, the surface magnetic field frequency is increased by 1-5 MHz and the core magnetic field intensity is decreased by 0.5-2 T; if the martensite content is too low, the surface magnetic field frequency is decreased by 1-5 MHz and the core magnetic field intensity is increased by 0.5-2 T.

10. The stainless steel tube drawing method as described in claim 7, characterized in that, During the drawing process, electromagnetic acoustic sensors are used to monitor the phase composition changes of the tube in real time, with a sampling frequency of 100 kHz-1 MHz. The monitoring data is fed back to the digital twin model in real time to correct the prediction parameters of the tissue evolution sub-model, with a correction period of no more than 1 s.

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