Static pressure forming process for stainless steel marine pipeline

Abstract

The invention discloses a static pressure forming process for a stainless steel marine pipeline. The performance of the pipeline is improved through material optimization and process innovation. The method comprises the following steps: selecting materials according to a service environment and carrying out cold rolling-annealing pretreatment: respectively carrying out 30%-40% cold rolling on 316L and 2205 dual-phase steel or N08367 corresponding to different working conditions, then carrying out protective atmosphere recrystallization annealing and water quenching at the temperature of more than or equal to 200 DEG C / s, and cooperating with pulse laser cleaning to retain a CrOfilm; in the forming stage, a silicon-based shear thickening fluid medium is adopted, segmented pressurization is matched with axial tension regulation, and the wall thickness deviation is smaller than or equal to 0.1 mm. And optimizing the 2205 steel phase proportion through subsequent pulsed magnetic field annealing. Electrochemical polishing Ra < = 0.15 mu m and a graphene coating are adopted for surface finishing, so that the fluid resistance is reduced, and the salt mist service life is 3 times that of a traditional process. According to the technology, through the synergistic effect of fluid dynamic response and magnetic field regulation and control, the problems that a traditional formed wall is uneven in thickness, insufficient in corrosion resistance and the like are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of plastic processing of metal pipes, and in particular relates to a static pressure forming process for stainless steel marine pipes. Background Art

[0002] Marine stainless steel pipes must meet high strength, seawater corrosion resistance, and high dimensional accuracy requirements. The manufacture of stainless steel pipes in marine engineering has long faced three major technical bottlenecks. Conventional hydroforming processes use Newtonian fluids, whose viscosity cannot adapt to shear rate. This results in excessive fluidity during high-pressure stages, making it impossible to effectively control localized thinning of the material. Experimental data shows that using conventional hydroforming can result in wall thickness deviations of up to ±0.3mm for a 200mm diameter pipe, increasing stress corrosion cracking susceptibility by over 30%.

[0003] Duplex stainless steel is prone to forming a brittle σ phase during heat treatment. When annealing temperatures are below 1050°C, the σ phase content of 2205 duplex steel can reach 5-8%, causing the impact toughness to drop below 40J. Conventional continuous annealing processes, however, result in 15%-20% Cr / Mo segregation, which reduces the pitting potential (Epit) by approximately 200mV.

[0004] Pipeline surface treatment technologies struggle to balance corrosion resistance and drag reduction. Traditional sandblasting (SA2.5 grade) results in an inner surface Ra value of approximately 3.2μm, increasing fluid pressure loss. Conventional epoxy coatings exhibit over 5% surface area delamination after just 500 hours in salt spray testing, failing to meet the 30-year service life requirements for ship pipelines.

[0005] Therefore, there is an urgent need to develop a pipe forming process that can achieve precise wall thickness control to meet the stringent requirements of modern ship engineering for high-performance pipes. Summary of the Invention

[0006] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a pipe forming process with precise wall thickness control and excellent corrosion resistance of the produced pipe to meet the needs of ship engineering.

[0007] To achieve the above objectives, the present invention provides the following technical solutions: A static pressure forming process for stainless steel marine pipes comprises the following steps: S1. Select the type of stainless steel and pre-treat it according to the service environment: Conventional seawater pipelines: 316L (Cr, 16.5-18.5%; Mo, 2.0-2.5%; PREN ≥ 28); Highly corrosive environment: 2205 duplex steel (Cr, 22%; Mo, 3%; PREN ≥ 35); Low temperature conditions: N08367 super austenitic steel (Cr, 20%; Ni, 24%; Mo, 6.3%).

[0008] The raw material plate is cold rolled to reduce by 30%-40%, and then recrystallized annealed in a protective atmosphere (Ar, 95% + H2, 5%): annealing temperature: 1050℃ (316L) or 1100℃ (2205 dual-phase steel); Furthermore, the annealed material is kept warm, and the holding time is calculated based on the thickness of the plate at 1.2 min / mm.

[0009] Furthermore, the material after the insulation is completed is quenched, and the material is water quenched to room temperature with a cooling rate of ≥200°C / s to inhibit carbide precipitation.

[0010] Furthermore, the surface of the quenched material is cleaned using a pulsed laser to reduce the average thickness of the oxide scale to ≤50 nm while retaining a completeness of Cr2O3 passivation film ≥95%.

[0011] S2. Placing the sheet in a mold and injecting a shear-thickening non-Newtonian fluid medium; Furthermore, the mold cavity surface meets the following requirements: roughness Ra≤0.4μm; demoulding slope 0.5°-1°; the mold operating temperature range is -20°C to 300°C, adapting to the temperature rise of the medium.

[0012] Furthermore, the non-Newtonian fluid medium adopts a silicon-based non-Newtonian fluid, and its rheological properties meet the following requirements: dynamic viscosity η = 0.5-50 Pa·s (at 25°C); shear thickening index n ≥ 1.2 (Herschel-Bulkley model fitting); thixotropy recovery time < 10s.

[0013] S3. Segmented pressurization and axial tension applied inside the mold cavity to control the elongation of the material, including: The first stage: pressurize at a rate of 5-8 MPa / s to 50-80 MPa, maintain the pressure for 5-8 seconds, and make the plate initially fit the mold; The second stage: the pressure is increased to 150-200 MPa at a rate of 8-12 MPa / s and maintained for 15-20 seconds, during which axial tension is applied simultaneously; Axial tensile force = 5%-8% of material yield strength; After the above pressure maintenance, the material is slowly depressurized at a rate of 2 MPa / s to make the rebound of the material ≤0.1 mm.

[0014] S4. Magnetic field annealing and surface finishing: Pulse magnetic field annealing is performed under a nitrogen protective atmosphere: the heating rate of the pulse magnetic field annealing process is controlled at 10-20°C / min, the temperature is raised to 850°C-900°C and kept warm.

[0015] Furthermore, the magnetic field parameters of the pulsed magnetic field annealing include: a sinusoidal alternating magnetic field with an intensity of 0.3-0.8 T and a frequency of 5-10 Hz.

[0016] Furthermore, the holding time is calculated based on the material thickness at 10 min / mm, and the furnace is cooled to 400-500° C. and then air-cooled.

[0017] Furthermore, the surface finishing includes: electrochemical polishing of the inner wall and coating treatment of the outer wall.

[0018] As a preferred technical solution, an acoustic emission sensor (frequency band 50-200kHz) is embedded in the mold cavity to collect signal characteristics in real time: Normal forming: signal amplitude <200mV, energy value <50pJ; Microcrack warning: When the amplitude is greater than 500mV and the energy is greater than 200pJ, the pressure regulation system is triggered. The pressure regulation system is preferably a clamping device of the clamping mechanism.

[0019] The advantages and beneficial effects of the present invention are: 1. Through the shear thickening characteristics of non-Newtonian fluids (η increases with increasing shear rate γ), the flow resistance is reduced in the initial molding stage (low γ), while the local pressure transmission efficiency is improved in the high-pressure stage (high γ), which greatly improves the uniformity of wall thickness.

[0020] 2. The alternating magnetic field induces eddy currents within the austenite grains, promoting uniform diffusion of the Cr / Mo elements and inhibiting the precipitation of σ phase. In 2205 duplex steel annealed in the magnetic field, the ferrite / austenite ratio is optimized from 50 / 50 to 45 / 55, and the PREN value is increased from 35 to 40.

[0021] 3. The ultra-smooth surface of the inner wall (Ra≤0.2μm) reduces fluid resistance by 10%-15%, and the graphene coating on the outer wall forms a dense barrier (oxygen permeability <0.05cm³ / m²·day), extending the salt spray test life to three times that of traditional processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a process flow chart shown in the present invention. DETAILED DESCRIPTION

[0023] The static pressure forming process of this invention solves the problems of uneven wall thickness, material embrittlement, and insufficient surface corrosion resistance in traditional processes through the coordinated optimization of multiple links. The following systematically explains the technical principles and the relationship between various technical features.

[0024] "PREN" stands for Pitting Equivalent Value, a parameter used to evaluate the pitting corrosion resistance of stainless steel and nickel-based alloys in chloride-containing environments. Its calculation formula is PREN = %Cr + 3.3×%Mo + 16×%N. Cr (chromium) forms a passive film, providing basic corrosion resistance, Mo (molybdenum) enhances resistance to chloride ions, and N (nitrogen) comes from alloying elements artificially added to the plate raw materials during processing and manufacturing.

[0025] Material selection involves selecting different stainless steel types (316L, 2205 duplex steel, and N08367) based on the service environment. Corrosion resistance (PREN value) and low-temperature performance are enhanced by adjusting the content of elements such as Cr, Mo, and Ni. For example, the high Cr / Mo content (PREN ≥ 35) of 2205 duplex steel inhibits pitting corrosion, while the ultra-high Ni content of N08367 enhances low-temperature toughness.

[0026] In the method provided by this invention, cold rolling reduces the sheet material by 30%-40% relative to the original material, refining the grains through dislocation multiplication and improving strength. Annealing then eliminates internal stresses and promotes uniform recrystallization. A holding time of 1.2 min / mm ensures uniform temperature across the thickness of the sheet, avoiding grain size gradients caused by differential heat conduction.

[0027] Water quenching (with a cooling rate ≥200°C / s) inhibits carbide precipitation and reduces the risk of intergranular corrosion. Pulsed laser cleaning removes oxide scale (≤50 nm) while preserving the Cr2O3 passivation film (≥95% integrity), maintaining the material's intrinsic corrosion resistance. Pretreatment involves cold rolling to refine the grains, annealing to optimize the phase composition, and rapid cooling to suppress harmful phases, providing a highly uniform substrate for subsequent forming. Preserving the passivation film also lays the foundation for subsequent surface treatment of the formed tube.

[0028] Shear-thickening fluids and staged pressurization are optional. Non-Newtonian fluids can be selected from high-temperature resistant types such as high-temperature greases, ceramic suspensions, and molten salt-based nanofluids. As a preferred implementation, a chemically inert silicone-based medium can be used. Its shear-thickening properties (n ≥ 1.2) maintain low viscosity during the initial molding phase (low shear rates), facilitating mold filling. During the high-pressure phase (high shear rates), viscosity increases, enhancing local pressure transfer efficiency and preventing excessive material thinning. Stage 1 (50-80 MPa): Low pressure is applied to the mold to prevent stress concentration caused by rapid initial deformation. Stage 2 (150-200 MPa): High pressure combined with axial tension controls elongation, compensates for material deformation variations, and reduces wall thickness deviations. Slow pressure relief reduces elastic rebound and ensures dimensional accuracy. Dynamic matching of the fluid medium and pressurization strategy achieves uniform plastic deformation through adaptive viscosity changes and tension-assisted flow, addressing the problem of material thinning in high-pressure zones encountered in traditional processes.

[0029] A shear-thickening non-Newtonian fluid (n≥1.2) is used as the pressure transmission medium, and its viscosity changes with the shear rate in a power law: η=Kγ n−1 ; Where K is the consistency coefficient, γ is the shear rate, n is the flow behavior index of non-Newtonian fluid (also known as the non-Newtonian index), and η is the viscosity.

[0030] During the molding process: Low-pressure fitting stage (γ<50 s⁻¹): The fluid maintains low viscosity (η=25-50 Pa·s), achieving rapid filling and reducing fluid resistance work (W=∫P·dV reduced by 15-20%, P is the fluid pressure, V is the fluid volume.) High-pressure molding stage (γ>200 s⁻¹): The viscosity surges to 3-5 times the initial value, forming a solid-state pressure transmission network, which improves the uniformity of the mold cavity pressure distribution.

[0031] Through the thermo-rheological coupling characteristics of silicon-based media (activation energy E a =45 kJ / mol), at mold preheating temperature (60-100°C): reduce zero shear viscosity (η0 decreases by 30-40%), improve permeability in thin-walled areas; enhance shear thickening response speed (τ increases to 1.5×10 4 Pa·s⁻¹), achieving millisecond viscosity switching. In the high-pressure stage, 6-8%σ axial tension is applied to induce the material to flow preferentially along the main strain direction, compensating the elongation at both ends of the pipe, establishing a plane strain state at the notch, and controlling the wall thickness deviation within ±0.12mm. At this time, through the stress superposition effect (σ 总 =σ 介质 +σ 拉力 ) Breaking through the material yield platform can further reduce the pressure required for molding by 20-25%.

[0032] Establish the fluid pressure distribution equation inside the mold cavity: P(x,y,z)=P0+ (k1x 2 +k2y 2 +k3z 2 ) Where: P0 is the mold clamping force (MPa), h is the average thickness of the fluid layer, and k is the dimensionless mold geometry factor, which is determined by the regularity of the medium flow path within the mold. The closer the mold is to the formed tube and the smaller the gap, the higher the value. Ideally, the cavity and tube structures should be similar, with a volume ratio between 1:(1.1-1.2). Within this range, the k value is approximately 0.85-0.95. x, y, and z are the distances along the length of the coordinate axes of the three-dimensional rectangular coordinate system established within the mold cavity. h, x, y, and z are all expressed in the same dimension, and their units are negated. By adjusting the viscosity η in real time, the pressure field uniformity coefficient is increased from 0.65 in traditional processes to above 0.85, thereby improving the uniformity of tube wall thickness.

[0033] The shear force applied to the non-Newtonian fluid of the present invention is derived from the mold pressure, and the process effects in each stage are as follows. The pulsed magnetic field induces eddy currents within the material, promoting the diffusion of Cr / Mo elements, reducing segregation from 15%-20% to below 5%, and inhibiting the precipitation of the brittle σ phase. Furthermore, the magnetic field optimizes the austenite grain orientation, improving toughness. A suitable heating rate reduces thermal stresses, while a holding time ensures sufficient element diffusion. Furnace cooling to 400-500°C followed by air cooling ensures a controlled cooling path to avoid the formation of secondary brittle phases. Magnetic field annealing optimizes the ferrite / austenite phase ratio in the duplex steel, while eliminating residual stresses and enhancing the material's overall performance.

[0034] During the surface finishing process, the inner surface of the pipe is electrochemically polished with a mixed acid solution. At a current density of 20-30A / dm², surface protrusions are selectively dissolved, reducing the inner surface roughness (Ra), thereby reducing fluid pressure loss by 10%-15%. A graphene-modified epoxy coating (60%-70% solids content) is sprayed on the outer surface of the pipe, forming a dense network structure that reduces oxygen permeability and extends its salt spray life. This ultra-smooth inner surface reduces energy loss while, in combination with the outer coating, providing a dual physical and chemical barrier, extending the pipe's service life.

[0035] As a preferred embodiment, an acoustic emission sensor is installed in the mold cavity to capture microcrack signals in real time, triggering the pressure regulation system to prevent defect expansion. Wall thickness deviations are dynamically monitored, and feedback is provided to adjust the holding time to ensure thickness uniformity. The monitoring system is linked to molding parameters in real time, forming a closed-loop control loop, improving process stability and yield. Through this multi-scale, multi-step collaborative innovation, this invention overcomes the technical bottlenecks of traditional processes and achieves high-performance manufacturing of ship piping.

[0036] The following embodiments are further described in conjunction with the accompanying drawings and examples. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0037] Example 1 In this embodiment, a 90° elbow with a nominal diameter of DN200 is processed, and a 316L austenitic stainless steel plate with a thickness of 8.0 mm (conforming to ASTM A240 standard, composition: C≤0.03%, Cr 16.8%, Ni 10.2%, Mo 2.1%) is selected.

[0038] In S1, the plate was pre-cold rolled using a four-roll reversing mill at a rolling force of 1200 kN to a thickness of 6.0 mm (25% deformation). Recrystallization annealing was then performed in a nitrogen atmosphere using a high-temperature heating process at 1080°C ± 5°C for 7.2 minutes (calculated at 1.2 minutes per mm thickness), followed by water quenching to achieve a uniform austenitic structure. The annealed material had a hardness of HV220 ± 10. During the surface treatment phase, an IPG YLS-6000 fiber laser was used for cleaning, with settings of 4.2 J / cm², a pulse frequency of 20 Hz, and a scan speed of 200 mm / s to ensure complete removal of the oxide layer while retaining a 2.3 nm thick passivating Cr2O3 film (verified by XPS).

[0039] In step S2, the sheet is placed in a mold for compression molding. The mold cavity surface is precision-ground to a roughness of Ra = 0.35 μm (measured by white light interferometry), with a draft angle of 0.8°. The mold is preheated to 60°C to optimize fluidity. The injected silicone-based non-Newtonian fluid exhibits shear-thickening properties, with a base viscosity of 25 Pa·s (shear rate 10 s⁻¹) at 25°C and a shear-thickening index n = 1.25.

[0040] S3: Adjust the pressure inside the mold. The pressurization process is controlled in two stages: in the first stage, increase the pressure to 80 MPa at a rate of 7 MPa / s and hold for 8 seconds to ensure that the sheet initially fits the mold surface. In the second stage, increase the pressure to 180 MPa at a rate of 10 MPa / s. Simultaneously apply an axial tensile force of 13.2 MPa (calculated as 6% of the material's yield strength of 220 MPa). Hold for 18 seconds, then release the pressure at a rate of 2 MPa / s. The total molding time is 45 seconds. The joints of the pipe are welded, polished, and the weld slag is removed.

[0041] The formed elbow was annealed at 860°C in a pulsed magnetic field, kept warm for 45 minutes in a 0.5T alternating magnetic field (frequency 8Hz), then furnace cooled to 500°C and air cooled to eliminate residual stress and optimize the Cr element distribution.

[0042] In the surface treatment stage S4, the inner wall was electrochemically polished using an electrolyte with a volume ratio of H3PO4 to H2SO4 of 3:1. The treatment was carried out at 50°C and 20A / dm² for 12 minutes, reducing the inner wall roughness from 0.75μm to 0.18μm. The outer wall was sprayed with an epoxy resin coating containing 1.2wt% graphene with a film thickness of 80±5μm. After step curing at 120°C×2h+150°C×1h, the coating adhesion reached 17.3MPa (ISO 2409 cross-cut test).

[0043] Example 2 In this embodiment, for the DN150 tee component, a 2205 duplex steel plate with a thickness of 10.0 mm (ASTM A928 standard, composition: C≤0.02%, Cr 22.5%, Ni 5.5%, Mo 3.2%) is selected.

[0044] S1 was pre-cold rolled at 1800 kN to 6.0 mm (40% deformation). Annealing was then performed at 1120°C ± 5°C in a nitrogen atmosphere for 12 minutes, followed by a water quench at a cooling rate of 250°C / s. This resulted in a uniform duplex structure with an austenite / ferrite ratio of 52 / 48 (EBSD analysis). Laser cleaning was performed using a power density of 4.8 J / cm² and a scanning speed of 180 mm / s. After treatment, the surface residual stress was reduced to -125 MPa (measured by X-ray diffraction), effectively improving stress corrosion resistance.

[0045] S2, the above-mentioned sheet is placed in a mold for pressing. The surface roughness of the mold cavity is Ra = 0.28μm, the demoulding slope is 0.5°, and the preheating temperature is 80℃ to adapt to high viscosity media. A silicone-based fluid with stronger shear thickening properties (η = 40Pa·s, n = 1.35) is selected. S3, adjust the pressure inside the mold. In the first stage of the pressurization process, increase the pressure to 100 MPa at a rate of 8 MPa / s and hold the pressure for 6 seconds. In the second stage, increase the pressure to 200 MPa at a rate of 12 MPa / s. Simultaneously apply 38.5 MPa axial tension (calculated as 7% of the yield strength of 550 MPa). After holding the pressure for 20 seconds, release the pressure at 1.5 MPa / s. The total molding time is 50 seconds.

[0046] The formed parts were annealed at 900°C in a pulsed magnetic field, held in a 0.7T high-intensity magnetic field (10Hz frequency) for 60 minutes, and water-quenched, increasing the austenite content to 54% and the σ phase content to less than 0.5% (verified by EDS surface scanning). The joints of the pipes were welded, polished, and the weld slag removed.

[0047] S4, the inner wall of the pipe is electrochemically polished with a high current density of 25A / dm² for 15 minutes to obtain an ultra-smooth surface with Ra=0.12μm; the outer wall of the 100±8μm thick graphene epoxy coating has an adhesion of 19.2MPa after step curing.

[0048] Example 3 In this embodiment, in response to the special requirements of cryogenic transmission pipelines for liquefied natural gas (LNG) transport ships, a straight pipe with a nominal diameter of DN300 is selected as the processing object, and the material is 12.0 mm thick N08367 super austenitic stainless steel plate (compliant with ASTM B688 standard, composition: C ≤ 0.03%, Cr 20.2%, Ni 24.5%, Mo 6.3%).

[0049] S1. During the plate pretreatment phase, the steel was first rolled on a six-roll cold rolling mill at a rolling force of 2500kN to a thickness of 8.0mm (33.3% deformation). The steel was then recrystallized in a vacuum furnace by heating to 1150°C ± 10°C and holding for 14.4 minutes (calculated at 1.2 minutes / mm thickness). The steel was then quenched at a rate of 300°C / s with argon gas injection to achieve an ASTM grade 7 uniform grain structure (average grain size 25μm). The surface was treated with high-power-density laser cleaning (5.0J / cm², pulse frequency 25Hz, scanning speed 150mm / s) to ensure a residual oxygen content of ≤0.5at% (XPS analysis) to prevent oxidation inclusions during subsequent forming.

[0050] S2. The sheet material was placed in a mold for compression molding. High-precision molds were used. Atomic force microscopy revealed a surface roughness of Ra = 0.20 μm, and a draft angle of 0.3°. The mold temperature was controlled at 100 ± 5°C to accommodate high-viscosity media. The injected silicone-based non-Newtonian fluid exhibited shear-thickening properties, with a base viscosity of η = 50 Pa·s at 25°C and a shear-thickening index n = 1.45.

[0051] S3. Adjust the pressure inside the mold. The pressurization process is precisely controlled in two stages: in the first stage, increase the pressure to 120 MPa at a rate of 6 MPa / s and maintain the pressure for 10 seconds to allow the material to initially stretch and fit the mold; in the second stage, increase the pressure to 220 MPa at a rate of 15 MPa / s, and simultaneously apply an axial tensile force of 28 MPa (calculated as 8% of the material's yield strength of 350 MPa). After maintaining the pressure for 25 seconds, slowly release the pressure at a rate of 1 MPa / s. The total molding time is 60 seconds.

[0052] The formed straight pipes undergo pulsed magnetic field annealing at 920°C, then hold in a 0.8T high-intensity alternating magnetic field (12Hz frequency) for 72 minutes. They are then cooled to 400°C and air-cooled, significantly improving their structural stability at low temperatures. The joints of the pipes are then welded and polished, and the weld slag is removed.

[0053] In the S4 surface finishing stage, the inner wall is electrochemically polished using an electrolyte with a volume ratio of H3PO4 to HNO3 of 4:1. The treatment is carried out at a high current density of 30A / dm² for 18 minutes. The inner wall roughness is reduced to Ra=0.10μm, effectively reducing the fluid transportation resistance. The outer wall adopts a double-layer composite coating: the bottom layer is a 50μm nickel-based alloy coating (microhardness HV0.3=320), and the surface layer is an 80μm polytetrafluoroethylene-graphene composite coating (friction coefficient μ=0.08).

[0054] The pipe fittings prepared in the above embodiments were subjected to performance testing, and the testing methods and test results are shown in the following table. Test items Test standards Example 1 (316L) Example 2 (2205) Example 3 Tensile strength (MPa) GB / T 228.1 655 785 720 Yield strength (MPa) GB / T 228.1 285 550 360 Elongation (%) GB / T 228.1 42 28 38 Critical pitting temperature (℃) ASTM G150 42 65 82 Wall thickness uniformity (%) EN 10216-5 98.3 97.8 96.5 Time for 5% of the salt spray test area to peel off (h) ISO 9227 5200 6000+ 5400 The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A static pressure forming process for stainless steel marine pipes, characterized in that: The following steps are involved: S1, cold rolling and annealing pretreatment of stainless steel plates, and surface laser cleaning; S2, placing the sheet in a mold and injecting a shear-thickening non-Newtonian fluid medium; S3, pressurizing the tube formed by the plate in the mold in a two-stage gradient manner, while applying axial tension to control the elongation of the tube; S4, performing pulse magnetic field annealing and surface finishing treatment on the formed pipe.

2. The static pressure forming process according to claim 1, characterized in that: In S1, the annealed material is kept warm, and the holding time is calculated based on the thickness of the plate at 1.2 min / mm.

3. The static pressure forming process according to claim 2, characterized in that: In S1, the material after heat preservation is quenched, and the material is water quenched to room temperature at a cooling rate of ≥200°C / s to inhibit carbide precipitation.

4. The static pressure forming process according to claim 3, characterized in that: In S1: the surface of the quenched material is cleaned by using a pulsed laser to make the average thickness of the oxide scale ≤ 50 nm, while retaining the integrity of the Cr2O3 passivation film ≥ 95%.

5. The static pressure forming process according to claim 1, characterized in that: In S2, the mold cavity surface satisfies the following requirements: roughness Ra≤0.4 μm; demoulding slope 0.5°-1°; the mold operating temperature range is -20°C to 300°C, adapting to the temperature rise of the medium.

6. The static pressure forming process according to claim 5, characterized in that: In S2, the non-Newtonian fluid medium is a silicon-based non-Newtonian fluid, and its rheological properties meet the following requirements: dynamic viscosity η = 0.5-50 Pa·s (at 25°C); shear thickening index n ≥ 1.2 (Herschel-Bulkley model fitting); and thixotropy recovery time < 10s.

7. The static pressure forming process according to claim 1, characterized in that: In S3, the pressure in the first stage of the gradient pressurization is 50-80 MPa, the pressurization rate is 5-8 MPa / s, and the holding time is 5-8 s; The pressurization rate in the second stage is 8-12MPa / s, the final pressure is 150-200MPa, and the holding time is 15-20s.

8. The static pressure forming process according to claim 7, characterized in that: In S3, the axial tension value is 5%-8% of the yield strength of the stainless steel plate, the tension direction is consistent with the axial direction of the pipeline, and the material after pressure maintenance is completed is slowly depressurized at a rate of 2 MPa / s to make the material rebound ≤0.1 mm.

9. The static pressure forming process according to claim 1, characterized in that: In S4, the pulse magnetic field annealing conditions are: in a protective atmosphere, the annealing temperature is 850-900° C., the magnetic field intensity is 0.3-0.8 T, the alternating frequency is 5-10 Hz, and the pulse magnetic field annealing is performed and kept warm.

10. The static pressure forming process according to claim 9, characterized in that: In the above-mentioned S4, the surface finishing includes electrochemical polishing of the inner wall to Ra≤0.2 μm, and spraying of a graphene-modified epoxy coating on the outer wall with a coating thickness of 50-100 μm.

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