A gradient structure metal-based seamless tube and its preparation method

Abstract

This invention relates to the field of materials processing technology and discloses a gradient structure metal-based seamless tube and its preparation method. The preparation method includes: (1) three-stage temperature-controlled rolling; (2) interface reconstruction: applying a pulsed electromagnetic field with an asymmetric angle to the rolling direction to the tube rolled in step (1); the specific conditions of the pulsed electromagnetic field are: magnetic field strength of 0.5~5T; pulse frequency f satisfies: f=(v / d)×k; where v is the tube moving speed (m / s), d is the tube wall thickness (mm), and k is an adjustment coefficient of 103~105; microstructure control: laser selective impact and electromagnetic oscillation coupling treatment is performed on the tube obtained in step (2); the impact energy density is 5~15J / cm. 2 The oscillation frequency is 20~100kHz. This invention can synergistically improve the strength and toughness of pipes, significantly extend fatigue life, and achieve a breakthrough improvement in corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of materials processing technology, and in particular to a gradient structure metal-based seamless tube and its preparation method. Background Technology

[0002] Metal pipes are crucial "blood vessels" in industrial equipment, acting as the "blood vessels" for transporting gases and liquids. However, in some applications, the strength and toughness of metal pipes often fail to meet requirements. Furthermore, the pipe surface is subjected to impacts from solid and liquid media, leading to various defects such as wear, corrosion, and fatigue, ultimately causing pipe failure and reducing their service life. Improving the strength, toughness, and surface performance of metal pipes has always been a focus of research.

[0003] Gradient structure metals refer to metallic materials in which the microstructure, composition, or properties exhibit continuous or stepwise changes along a certain direction within the material. The design of a gradient structure allows for a smooth transition in properties such as hardness, strength, and toughness within the material, avoiding stress concentration and usage risks caused by abrupt property changes.

[0004] Traditional seamless metal tube manufacturing mainly employs processes such as hot rolling (e.g., skew rolling with piercing), cold drawing, extrusion, and spinning. Hot rolling, which uses high-temperature plastic deformation at 1100–1250℃ to form the tube blank, is highly efficient but leads to grain coarsening (20–50 μm) and severe surface oxidation. Cold drawing, through multiple cold deformation passes, can achieve high-precision tubes (tolerance ±0.05 mm), but residual stress reaches 200–400 MPa, exacerbating the risk of stress corrosion. Extrusion, while capable of processing difficult-to-deform metals, is limited by high die costs (US$50,000–200,000 per set) and microstructural anisotropy (performance differences ≥15%). These traditional processes, aiming for a uniform microstructure, generally suffer from a reversal of strength and toughness (e.g., elongation ≤10% when yield strength ≥600 MPa in cold-drawn tubes) and low fatigue life (<1.5 × 10⁻⁶). 6 It suffers from drawbacks such as high energy consumption (≥300 kWh / ton for hot-rolled pipes) and the inability to coordinate and control the radial performance gradient of the pipe. For example, the external pressure resistance of homogeneous hot-rolled pipes is only 500~700 MPa, while single cold drawing strengthening will double the sensitivity to grain boundary corrosion due to residual tensile stress (pitting potential decreases by 0.1~0.15V), which seriously restricts its reliability in extreme environments such as aerospace and deep-sea engineering. Summary of the Invention

[0005] Objective: To address the problems existing in the prior art, this invention provides a gradient structure metal-based seamless tube and its preparation method. Through three-stage gradient temperature-controlled rolling (T1→T2→T3 temperature gradient synergistic with ε1→ε2→ε3 deformation), a fine outer (≤5μm) and coarse inner (15-25μm) grain structure is constructed radially in the tube, overcoming the strength-toughness inversion limitation (external pressure resistance 880MPa / elongation 15%). Combined with an asymmetric pulsed electromagnetic field (30°-60° angle, 0.5-5T) to break grain boundary oxides, a 10-30nm amorphous interface layer is formed, increasing the interface strength by 50% to 300MPa. Laser-electromagnetic coupling is used to reconstruct the three-dimensional dislocation network, eliminating anisotropy (difference ≤5%), and the residual compressive stress layer (300-500MPa) increases the fatigue life to 3.5×10⁻⁶. 6 (Traditional process 1.2×10) 6 (Times); integrating intelligent lubrication and digital twin model, the rolling temperature is reduced to 800℃ (energy saving 40%), the surface roughness Ra≤0.4μm, and the pitting potential reaches 0.38V (improvement of 52%), meeting the requirements of extreme working conditions such as deep sea (pressure resistance 120MPa) and aerospace, and realizing the design and manufacture of high-performance pipes.

[0006] Technical solution: In a first aspect, the present invention provides a method for preparing a gradient structure metal-based seamless tube, comprising the following steps:

[0007] (1) Rolling and forming:

[0008] (a) Heat the metal billet to a first temperature T1 and hold it for 20 to 60 minutes;

[0009] (b) The metal billet obtained in step (a) is subjected to a first rolling and then water-cooled to a second temperature T2; the specific conditions for the first rolling are: deformation ε1 = 40%~60%, rolling speed v1 = 0.5~1.2m / s;

[0010] (c) The metal billet obtained in step (b) is rolled a second time and then water-cooled to a third temperature T3; the specific conditions for the second rolling are: deformation ε2 = 30%~50%, rolling speed v2 = 0.8~1.5m / s;

[0011] (d) The metal billet obtained in step (c) is subjected to a third rolling process; the specific conditions for the third rolling process are: deformation ε3 = 20%~40%, rolling speed v3 = 1.0~2.0m / s;

[0012] Wherein, the first temperature T1 = 0.8Tm~0.9Tm, where Tm is the melting point of the metal; the second temperature T2 = T1 - 150℃~200℃; and the third temperature T3 = T2 - 100℃~150℃.

[0013] The gradient temperature-controlled rolling process can significantly improve the overall performance of seamless metal tubes: through the synergistic effect of temperature gradient control (T1→T2→T3) and deformation gradient (ε1→ε2→ε3), a dynamic recrystallization gradient is formed during the rolling process. The outer layer, due to the strong thermo-mechanical coupling effect of low temperature (400-600℃) and large deformation (55%), experiences suppressed grain boundary migration, resulting in dynamic recrystallization and the formation of ultrafine grains (≤5μm). Meanwhile, the inner layer, under relatively high temperature (600-800℃) and small deformation (35%), gradually coarsens the grains (15-25μm) through dislocation recrystallization. This gradient structure, with fine outer grains and coarse inner grains, increases the material's resistance to external pressure to ≥880MPa (fine-grained Hall-Petch strengthening), while the coarse inner grains absorb crack propagation energy, resulting in a fatigue life of 3.5×10⁻⁶. 6 This second cycle (3 times better than traditional homogeneous structures) combines a surface layer with a 300-500 MPa gradient compressive stress (due to thermal expansion differences caused by temperature gradients) and a fine-grained, dense oxide film (pitting potential 0.38V), achieving a comprehensive performance breakthrough overcoming the inherent strength-toughness contradiction of traditional processes. If homogeneous rolling (single temperature ≥1000℃) is used, the uniform coarse-grained structure (approximately 15 μm) leads to stress concentration sensitivity (fatigue life of only 1.2 × 10⁻⁶). 6 The excessive grain growth (>20μm) caused by high temperature further weakens the strength (≤620MPa) and increases energy consumption by more than 30%. The thickened oxide scale deteriorates corrosion resistance (pitting potential 0.25V). The gradient temperature control process fundamentally solves the industry problem of the inability to synergistically optimize microstructure and properties under a single process parameter through thermodynamic regulation of spatial partitioning.

[0014] (2) Interface reconstruction: A pulsed electromagnetic field with an asymmetric angle to the rolling direction is applied to the tube rolled in step (1); the specific conditions of the pulsed electromagnetic field are: magnetic field strength of 0.5~5T; pulse frequency f satisfies:

[0015] f = (v / d)×k

[0016] In the formula, v is the pipe moving speed (m / s), d is the pipe wall thickness (mm), and k is the adjustment coefficient 10. 3 ~10 5 ;

[0017] Dynamic interface reconstruction processing achieves cross-scale optimization of the pipe interface structure through the directional interaction between an asymmetric pulsed electromagnetic field and the microscopic defects of the material. When a pulsed magnetic field (0.5~5T) is applied at an angle of 30°~60° to the rolling direction, the dislocation motion driven by the Lorentz force and the atomic diffusion induced by the electron wind effect work synergistically. The rapid rising edge pulse (≤50μs) breaks the original grain boundary oxide through high-energy electron impact, the flat-top duration (accounting for 1 / 3~1 / 2 of the cycle) promotes dislocation recombination to form a low-energy interface, and the slow falling edge (≥100μs) guides solute atoms (such as Cr, Mo) to segregate at the interface (concentration gradient of 5~8 at.%). This spatiotemporal asymmetric energy input increases the interfacial bonding strength by more than 50% (up to 300MPa) and forms a dense amorphous transition layer with a thickness of 10~30nm, increasing the energy required for crack initiation to 2.3 times that of traditional interfaces. Compared to non-gradient processes, this treatment reduces the intergranular corrosion rate to 1 / 4 (0.02 mm / year) and creates a stress-structure dual-gradient match with the fine-grained surface layer produced by gradient temperature-controlled rolling (outer layer compressive stress of 300 MPa + synergistic suppression of crack propagation by nanocrystalline and amorphous interfaces). Without this treatment, coarse precipitates (size > 500 nm) and oxygen segregation (> 3 at.%) at conventional homogeneous interfaces (bonding strength ≤ 200 MPa) will lead to early fatigue failure (life < 1 × 10⁻⁶). 6 The method addresses the sensitivity to both intergranular corrosion (pitting potential decreases by 0.15V) and intergranular corrosion. Dynamic interface reconstruction, through electromagnetic-mechanical-chemical multi-field coupling, overcomes the technical bottleneck of simultaneously achieving strong and tough interfaces and resistance to environmental damage in metal pipes.

[0018] (3) Tissue control: The surface of the pipe obtained in step (2) is subjected to laser shock coupled with electromagnetic oscillation; the shock energy density is 5~15J / cm. 2 The oscillation frequency is 20~100kHz.

[0019] Further, in step (1), the temperature between the first temperature T1, the second temperature T2, and the third temperature T3 is achieved by a high-frequency coil and an intermediate-frequency coil; wherein, the high-frequency coil controls the surface temperature of the pipe, and the intermediate-frequency coil controls the core temperature of the pipe.

[0020] Furthermore, the high-frequency coil operates at a frequency of 100~300kHz; the intermediate-frequency coil operates at a frequency of 10~50kHz.

[0021] Furthermore, in step (2), the pulsed electromagnetic field adopts asymmetric waveform modulation technology, which includes the following in a single cycle: rise time tr≤50μs; flat-top duration tp = (1 / 3~1 / 2)T, where T is the pulse period; and fall time tf≥100μs.

[0022] Furthermore, the magnetic poles of the pulsed electromagnetic field are arranged at an angle of θ = 30°~60° with the rolling center line.

[0023] Furthermore, in step (3), the spatiotemporal coupling relationship between the laser selective impact and the electromagnetic oscillation satisfies the following: the phase difference between the laser pulse and the electromagnetic oscillation is Δφ = 90°±10°, and the spatial overlap between the laser action area and the peak electromagnetic field intensity area is ≥85%.

[0024] Furthermore, during the rolling process in step (1), an axial vibration load is also applied simultaneously; the specific conditions of the axial vibration load are: vibration frequency fv = 50~200Hz; amplitude A = (0.005~0.015)D, where D is the outer diameter of the pipe.

[0025] Applying axial vibration loads (frequency 50-500Hz, amplitude 0.01-0.5mm) during seamless steel pipe rolling induces a dynamic softening effect in the material through high-frequency micro-amplitude vibration, reducing rolling force by 10-20%. Simultaneously, it promotes dislocation movement and dynamic recrystallization, refines grain size by 1-2 levels, eliminates internal wall folding defects, and improves dimensional accuracy. This technology combines the synergistic control of vibration energy and plastic deformation, overcoming the limitations of traditional static rolling for forming high-strength materials. It offers the dual advantages of energy saving (energy consumption reduced by 12-18%) and optimized microstructure and properties, making it suitable for the efficient processing of difficult-to-deform metals.

[0026] Secondly, the present invention provides a gradient structure metal-based seamless tube prepared by any of the methods described above, the tube having a three-layer gradient structure: outer layer: nanocrystalline layer with a grain size ≤500nm and a thickness accounting for 10%~15% of the tube wall thickness; transition layer: dislocation cell structure with a cell wall spacing of 50~200nm and a thickness accounting for 20%~30% of the tube wall thickness; inner layer: submicron grains with a grain size of 1~3μm and a thickness accounting for 55%~70% of the tube wall thickness.

[0027] Furthermore, the pipe material simultaneously satisfies the following parameters in the pipe wall thickness direction: microhardness gradient HV outer layer / HV inner layer ≥ 1.5, residual stress σ of gradient compressive stress layer ≥ 300MPa, gradient change rate ≥ 50MPa / mm; texture strength ratio, {110} outer layer / {111} inner layer = 2.5~4.0.

[0028] This invention overcomes the strength-toughness contradiction by using gradient temperature-controlled rolling (outer layer 400-600℃ fine grain strengthening / inner layer 600-800℃ coarse grain toughening) combined with asymmetric electromagnetic pulse treatment (0.5-5T, 30°-60° angle), achieving an external compressive strength of 800-950MPa (40% increase) and an elongation ≥15%, while suppressing the stress corrosion rate to ≤10 through a surface 300-500MPa compressive stress layer.-8 mm / s. Dynamic interface reconstruction technology removes grain boundary oxides and constructs a 10-30 nm amorphous transition layer, increasing the pitting potential to 0.38 V and extending the fatigue life to 3.5 × 10⁻⁶ mm / s. 6 This process achieves three times the performance of traditional methods. Multi-scale microstructure control (laser shock + electromagnetic oscillation) induces a three-dimensional dislocation network, eliminating anisotropy (difference ≤5%). Combined with gradient structural design, this increases buckling strength to 120 MPa, enabling it to withstand thermal shock cycles from -196 to 300℃. Process energy consumption is reduced by 40% (≤180 kWh / ton), solving the challenges of high-performance pipes in aerospace hydraulic systems and deep-sea oil and gas transportation, achieving a leap from homogeneous manufacturing to performance-designable upgrades.

[0029] Beneficial effects: Compared with the prior art, the specific beneficial effects of this invention are as follows:

[0030] 1. Enhanced resilience through synergistic improvement

[0031] By coupling gradient temperature-controlled rolling with a 55% large deformation in the outer layer, a gradient structure of fine outer grains (≤5μm) and coarse inner grains (15-25μm) is formed, breaking through the contradiction between strength and toughness in traditional homogeneous materials. The external pressure strength reaches 880MPa (40% higher than the traditional process) while the elongation remains ≥15%, solving the industry problem of "high strength inevitably leads to brittleness" in aerospace hydraulic pipes.

[0032] 2. Significantly extended fatigue life.

[0033] Asymmetric electromagnetic pulse treatment (0.5-5T, 30°-60° angle) generates a 300-500MPa gradient compressive stress layer on the surface. Combined with the outer fine grain barrier effect, this results in a fatigue life of 3.5×10⁻⁶. 6 Second cycle (traditional process 1.2×10) 6 (times), crack propagation rate reduced to ≤10 -8 mm / cycle, meeting the 20-year maintenance-free requirement for deep-sea oil and gas pipelines.

[0034] 3. Breakthrough improvement in corrosion resistance

[0035] Dynamic interface reconstruction technology removes grain boundary oxides (oxygen content ≤0.05wt%), forming a dense amorphous transition layer of 10-30nm. The pitting potential is increased to 0.38V (a 52% increase compared to the traditional 0.25V), and the intergranular corrosion rate is reduced to 0.02mm / year (compared to 0.08mm / year in the traditional process). This technology is suitable for high-Cl... - Marine environment.

[0036] 4. Both energy consumption and costs have decreased.

[0037] The gradient temperature control process reduces the maximum rolling temperature to 800℃ (300℃ lower than traditional hot rolling). Combined with an intelligent lubrication system (reducing the amount of nano-graphene lubricant by 50%) and digital twin optimization, the overall energy consumption is ≤180kWh / ton (a 40% reduction), the product qualification rate increases from 85% to 98%, and the production cost is reduced by 35%.

[0038] 5. Increased adaptability to extreme environments

[0039] Cross-scale tissue regulation (laser shock + electromagnetic oscillation) constructs a three-dimensional dislocation network, making the anisotropy index ≤5% (traditional ≥15%), the buckling strength up to 120MPa (traditional 80MPa), and able to withstand thermal shock cycles of -196~300℃ ≥50 times (traditional ≤5 times), adapting to the alternating working conditions of ultra-low temperature deep space exploration and high temperature geothermal pipelines. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the embodiments.

[0041] In this invention, gradient structure metal-based seamless tubes are prepared by using a multi-band composite induction rolling mill, an asymmetric electromagnetic pulse generator, and a laser-electromagnetic coupling processing device.

[0042] Example 1:

[0043] This embodiment provides a method for preparing a gradient structure metal-based seamless tube, the specific steps of which are as follows:

[0044] (1) Rolling forming: (a) Heat 304 stainless steel to 1100℃ (austenitizing temperature) and hold for t1=30min; (b) Perform the first stage deformation in the rolling equipment, with deformation ε1=50% (rolling force controlled ≤800kN), rolling speed v1=0.8m / s, and immediately water-cool to 950℃ after rolling; (c) Perform the second stage deformation at 950℃, with deformation ε2=40% (roll inclination angle 10°), rolling speed v2=1.0m / s, and air-cool to 800℃ after rolling; (d) Perform the third stage deformation at 800℃, with deformation ε3=30% (roll liquid nitrogen cooling), rolling speed v3=1.5m / s;

[0045] (2) Interface reconstruction: A pulsed electromagnetic field is applied to the rolled tube. The magnetic poles of the pulsed electromagnetic field are arranged at an angle of θ=45° with the rolling center line. The magnetic field strength is 2T, and the pulse frequency f satisfies: f= (v / d) × k=(1.0 / 5)×10 4 =2000Hz (k=10) 4 (Unit matching correction), where v is the pipe moving speed (m / s), d is the pipe wall thickness (mm), and k is the adjustment coefficient 10. 3 ~105 ;

[0046] The pulsed electromagnetic field employs asymmetric waveform modulation technology, and within a single cycle, it includes: rise time tr ≤ 40μs; flat-top duration tp = (1 / 3~1 / 2)T = 0.3ms, where T is the pulse period, and when T = 1ms; fall time tf ≥ 120μs.

[0047] Tissue control: In the final heat treatment stage, laser shock coupled with electromagnetic oscillation is used, with the laser shock energy density being 10 J / cm³. 2 (Pulse width 20ns), electromagnetic oscillation frequency 50kHz; phase difference Δφ = 90° between laser pulse and electromagnetic oscillation, and spatial overlap of laser action area and electromagnetic field intensity peak area 90%.

[0048] Optionally, during the rolling process, an axial vibration load is also applied simultaneously, with a vibration frequency of fv = 100Hz and an amplitude of A = (0.005~0.015)D = 0.5mm (when D = 89mm, A = 0.005×89≈0.45mm), where D is the outer diameter of the pipe.

[0049] The gradient-structured metal-based seamless tubing prepared in this embodiment has a three-layer gradient structure: outer layer: nanocrystalline layer (grain size 400nm, thickness 0.6mm, accounting for 12%); transition layer: dislocation cell structure (cell wall spacing 120nm, thickness 1.5mm, accounting for 30%); inner layer: submicron crystals (grain size 2μm, thickness 2.9mm, accounting for 58%). Mechanical properties: tensile strength ≥850MPa (680MPa with traditional process), elongation 35%, grain size gradient difference reaches level 3 (ASTM level 9 to 6).

[0050] Example 2:

[0051] This embodiment provides a method for preparing a gradient structure metal-based seamless tube, the specific steps of which are as follows:

[0052] (1) Rolling forming: (a) Heat the 42CrMo alloy to 1150℃ (austenitization + carbide dissolution), and hold for t1=45min; (b) First stage deformation: deformation amount ε1=55% (rolling force ≤1200kN), rolling speed v1=0.6m / s, and water cooling to 1000℃ after rolling; (c) Second stage deformation: carried out at 1000℃, deformation amount ε2=45% (roll inclination angle 8°), rolling speed v2=0.9m / s, and air cooling to 850℃; (d) Third stage deformation: carried out at 850℃, deformation amount ε3=35% (roll liquid nitrogen cooling), rolling speed v3=1.2m / s.

[0053] (2) Interface reconstruction: A pulsed electromagnetic field is applied to the rolled tube. The magnetic poles of the pulsed electromagnetic field are arranged at an angle of θ=50° with the rolling center line. The magnetic field strength is 3T, and the pulse frequency f satisfies: f=v / d×k=0.90 / 008×8×10 3 =900 Hz, (v=0.9m / s, d=8mm=0.008m, k=8×10 3 )

[0054] In the formula, v is the pipe moving speed (m / s), d is the pipe wall thickness (mm), and k is the adjustment coefficient 10. 3 ~10 5 ;

[0055] The pulsed electromagnetic field employs asymmetric waveform modulation technology, and within a single cycle, it includes: rise time tr ≤ 30μs; flat-top duration tp = (1 / 3~1 / 2)T = 0.25ms (T = 0.8ms), where T is the pulse period; and fall time tf ≥ 150μs.

[0056] Tissue control: In the final heat treatment stage, laser shock coupled with electromagnetic oscillation is used, with the laser shock energy density being 12 J / cm². 2 The laser pulse induces a surface phase transition, and the electromagnetic oscillation frequency is 80kHz. The phase difference between the laser pulse and the electromagnetic oscillation is Δφ = 85°, and the spatial overlap between the laser action area and the peak electromagnetic field intensity area is 88%.

[0057] Optionally, during the rolling process, an axial vibration load is also applied simultaneously, with a vibration frequency of fv = 150Hz and an amplitude of A = (0.005~0.015)D = 0.8mm (when D = 159mm, A = 0.005D), where D is the outer diameter of the pipe.

[0058] The gradient structure metal-based seamless tube prepared in this embodiment has a three-layer gradient structure: outer layer: nanoscale bainite layer (grain size 300nm, thickness 1.2mm, accounting for 15%); transition layer: lath martensite with dislocation cell structure + retained austenite (lath width 200nm, thickness 2.4mm, accounting for 30%); inner layer: submicron tempered sorbite (grain size 1.5μm, thickness 4.4mm, accounting for 55%).

[0059] Mechanical properties: tensile strength 1100MPa (30% improvement over traditional process), elongation 18%, impact energy AKU=65J (40% improvement), hardness gradient HRC 50-42-35.

[0060] Example 3:

[0061] This embodiment provides a method for preparing a gradient structure metal-based seamless tube, the specific steps of which are as follows:

[0062] (1) Rolling: (a) Heat pure titanium TA2 to 920℃ and hold for t1=50min; (b) Perform the first stage deformation in the rolling equipment, with a deformation amount ε1=60% and a rolling speed v1=0.5m / s, and immediately water cool to 750℃ after rolling; (c) Perform the second stage deformation at 750℃, with a deformation amount ε2=40% and a rolling speed v2=0.8m / s, and air cool to 600℃ after rolling; (d) Perform the third stage deformation at 600℃, with a deformation amount ε3=25% and a rolling speed v3=1.0m / s;

[0063] (2) Interface reconstruction: A pulsed electromagnetic field is applied to the rolled tube. The magnetic poles of the pulsed electromagnetic field are arranged at an angle of θ=60° with the rolling centerline. The magnetic field strength is 1.5T, and the pulse frequency f satisfies:

[0064] f = (v / d) × k=0.7 / 0.006×5×10 3 ≈583 kHz;

[0065] In the formula, v is the pipe moving speed (m / s), d is the pipe wall thickness (mm), and k is the adjustment coefficient 10. 3 ~10 5 ;

[0066] The pulsed electromagnetic field employs asymmetric waveform modulation technology, and within a single cycle, it includes: rise time tr = 20 μs; flat-top duration tp = (1 / 3~1 / 2)T = 0.2 ms (T = 0.6 ms), where T is the pulse period; and fall time tf = 200 μs.

[0067] Tissue control: In the final heat treatment stage, laser shock coupled with electromagnetic oscillation is used, with a laser shock energy density of 8 J / cm³. 2 The oscillation frequency of the electromagnetic oscillation is 30kHz; the phase difference between the laser pulse and the electromagnetic oscillation is Δφ = 95°; and the spatial overlap between the laser action area and the peak area of ​​the electromagnetic field intensity is ≥92%.

[0068] Optionally, during the rolling process, an axial vibration load is also applied simultaneously, with a vibration frequency of fv = 80Hz; the amplitude A = (0.005~0.015)D = 0.57mm (when D = 114mm, A = 0.005D), where D is the outer diameter of the pipe.

[0069] The gradient structure metal-based seamless tube prepared in this embodiment has a three-layer gradient structure: outer layer: ultrafine equiaxed α-crystal nanocrystal layer with a grain size of 200 nm and a thickness accounting for 15% of the tube wall thickness, approximately 0.9 mm; transition layer: dislocation entanglement + nanotwin structure with a twin thickness of 50 nm, a cell wall spacing of 50~200 nm, and a thickness accounting for 25% of the tube wall thickness, approximately 1.5 mm; inner layer: submicron grains, coarse α-lamellae (crystal width 1.2 μm) with a grain size of 1~3 μm and a thickness accounting for 60% of the tube wall thickness, approximately 3.6 mm.

[0070] The properties of the pipes prepared in Examples 1-3 and the pipes prepared by conventional manufacturing methods were tested, and the specific data are as follows:

[0071]

[0072] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent transformations or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a gradient structure metal-based seamless tube, characterized in that, Includes the following steps: (1) Rolling and forming: (a) Heat the metal billet to a first temperature T1 and hold it for 20 to 60 minutes; (b) The metal billet obtained in step (a) is subjected to a first rolling and then water-cooled to a second temperature T2; the specific conditions for the first rolling are: deformation ε1 = 40%~60%, rolling speed v1 = 0.5~1.2m / s; (c) The metal billet obtained in step (b) is rolled a second time and then water-cooled to a third temperature T3; the specific conditions for the second rolling are: deformation ε2 = 30%~50%, rolling speed v2 = 0.8~1.5m / s; (d) The metal billet obtained in step (c) is subjected to a third rolling process; the specific conditions for the third rolling process are: deformation ε3 = 20%~40%, rolling speed v3 = 1.0~2.0m / s; Wherein, the first temperature T1 = 0.8Tm~0.9Tm, where Tm is the melting point of the metal; the second temperature T2 = T1 - 150℃~200℃; and the third temperature T3 = T2 - 100℃~150℃. (2) Interface reconstruction: A pulsed electromagnetic field with an asymmetric angle to the rolling direction is applied to the tube rolled in step (1); the specific conditions of the pulsed electromagnetic field are: magnetic field strength of 0.5~5T; pulse frequency f satisfies: f = (v / d)×k In the formula, v is the pipe moving speed (m / s), d is the pipe wall thickness (mm), and k is the adjustment coefficient 10. 3 ~10 5 ; (3) Tissue control: The surface of the pipe obtained in step (2) is subjected to laser shock coupled with electromagnetic oscillation; the laser shock energy density is 5~15J / cm. 2 The electromagnetic oscillation frequency is 20~100kHz.

2. The method for preparing gradient structure metal-based seamless tubing according to claim 1, characterized in that: In step (1), the temperatures between the first temperature T1, the second temperature T2, and the third temperature T3 are achieved by a high-frequency coil and an intermediate-frequency coil; wherein, the high-frequency coil controls the surface temperature of the pipe, and the intermediate-frequency coil controls the core temperature of the pipe.

3. The method for preparing gradient structure metal-based seamless tubing according to claim 2, characterized in that: The high-frequency coil operates at a frequency of 100~300kHz; the intermediate-frequency coil operates at a frequency of 10~50kHz.

4. The method for preparing the gradient structure metal-based seamless tubing according to claim 1, characterized in that: In step (2), the pulsed electromagnetic field adopts asymmetric waveform modulation technology, which includes the following in a single cycle: rise time tr≤50μs; flat-top duration tp = (1 / 3~1 / 2)T, where T is the pulse period; fall time tf≥100μs.

5. The method for preparing gradient structure metal-based seamless tubing according to claim 4, characterized in that: The magnetic poles of the pulsed electromagnetic field are arranged at an angle of θ = 30°~60° with the rolling center line.

6. The method for preparing gradient structure metal-based seamless tubing according to claim 1, characterized in that: In step (3), the relationship between the laser shock and electromagnetic oscillation coupling process satisfies the following: the phase difference between the laser pulse and the electromagnetic oscillation is Δφ = 90° ± 10°, and the spatial overlap between the laser action area and the peak electromagnetic field intensity area is ≥ 85%.

7. The method for preparing the gradient structure metal-based seamless tubing according to claim 1, characterized in that: During the rolling process in step (1), an axial vibration load is also applied simultaneously; the specific conditions of the axial vibration load are: vibration frequency fv = 50~200Hz; amplitude A = (0.005~0.015)D, where D is the outer diameter of the pipe.

8. A gradient-structured metal-based seamless tube prepared by the method according to any one of claims 1-7, characterized in that: The pipe has a three-layer gradient structure: outer layer: nanocrystalline layer with a grain size ≤500nm and a thickness accounting for 10%~15% of the pipe wall thickness; transition layer: dislocation cell structure with a cell wall spacing of 50~200nm and a thickness accounting for 20%~30% of the pipe wall thickness; inner layer: submicron grains with a grain size of 1~3μm and a thickness accounting for 55%~70% of the pipe wall thickness.

9. The gradient structure metal-based seamless tubing according to claim 8, characterized in that: The pipe material simultaneously meets the following parameters in the pipe wall thickness direction: microhardness gradient HV outer layer / HV inner layer ≥ 1.5, residual stress σ of gradient compressive stress layer ≥ 300MPa, gradient change rate ≥ 50MPa / mm; texture strength ratio, {110} outer layer / {111} inner layer = 2.5~4.0.

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