Cold machining method of super duplex stainless steel seamless tube

Abstract

The invention discloses a cold machining method of a super duplex stainless steel seamless tube, relates to the technical field of stainless steel tube machining, and aims to solve the problems of long period, high cost and structure coarsening caused by multiple times of intermediate annealing in the prior art. The method adopts a three-stage cold-drawing process which does not need intermediate annealing and is sequentially and continuously carried out, and comprises the following steps: carrying out low-temperature first-stage pre-deformation, introducing alpha'martensite and inducing a strong gamma-phase texture; medium-temperature secondary main deformation is carried out, and full reversal of alpha'martensite and in-situ precipitation of nanoscale Cr2N in an alpha phase are synchronously realized by controlling the temperature, the strain rate and the deformation in a specific synergistic window; according to the seamless pipe and the preparation method thereof, the final deformation close to the reverse transformation temperature is adopted to stabilize the structure, triple cooperative regulation and control of the phase proportion, the nanometer precipitation and the crystal texture are achieved, the prepared seamless pipe has high yield strength, high toughness and excellent stress corrosion resistance, meanwhile, the production period is greatly shortened, and the cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of stainless steel pipe processing technology, specifically to a cold working method for seamless pipes of super duplex stainless steel (SDSS, typical grades such as 2507, S32760, S32750). Background Technology

[0002] Super duplex stainless steel (SDSS) seamless tubes are widely used in harsh environments due to their excellent strength and corrosion resistance. Their core performance depends on the balanced ratio of 40-60% austenite (γ phase) and 40-60% ferrite (α phase). At the same time, the morphology of Cr2N precipitates in the α phase and the crystal texture of the γ phase play a key role in regulating the performance.

[0003] Existing cold working technologies mostly use "room temperature cold drawing + multiple intermediate annealing" to control the phase ratio, but this method has shortcomings: First, the cold drawing process itself induces the transformation of the γ phase into α' martensite and causes the α phase to grow, disrupting the phase equilibrium. Second, the annealing process, which is carried out to restore the phase ratio, will cause the α phase grains to coarsen and the coarse precipitation of phases such as Cr2N (particle size ≥50nm), which will impair toughness.

[0004] Although other technologies have attempted to control the precipitates through aging treatment, they have been unable to solve the phase imbalance problem caused by cold processing and have extended the production cycle. Summary of the Invention

[0005] This invention provides a cold working method for super duplex stainless steel seamless tubes, the core of which is a three-stage cold drawing process that does not require intermediate annealing and is carried out sequentially to solve the above-mentioned problems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A cold working method for super duplex stainless steel seamless tubes includes the following three consecutive cold drawing stages, with no intermediate annealing treatment required between the three cold drawing stages: (1) First-stage pre-deformation cold drawing: carried out in a low-temperature environment below the initiation temperature As of the super duplex stainless steel reversal transformation, the engineering deformation is controlled at 12-15%. This stage aims to introduce 5-8% strain-induced α' martensite to provide driving force for subsequent reversal transformation and to initially induce favorable crystallographic orientation. (2) Secondary main deformation cold drawing: carried out in a medium temperature environment higher than As, and the engineering deformation is controlled at 22-25%; this stage is the key to triple synergistic regulation, and two core objectives need to be achieved at the same time: to fully reverse the α' martensite generated in the first stage into the γ phase (reverse transformation rate ≥95%), and to form nanoscale Cr2N precipitates in situ in the ferrite (α) phase (average particle size ≤20nm), thereby stabilizing the phase ratio and improving the strength simultaneously; (3) Third-stage final deformation cold drawing: carried out at 15-20℃ (close to the temperature environment of As), and the engineering deformation is controlled at 5-8%. The main purpose of this stage is to stabilize the microstructure after the second-stage main deformation adjustment and to accurately correct the final dimensional accuracy of the pipe.

[0007] The inversion temperature As of the super duplex stainless steel was determined by differential scanning calorimetry (DSC) according to GB / T 19466.2 standard, with a heating rate of 10℃ / min. As is defined as the inversion temperature of the α'→γ inversion peak. The α' martensite content was determined by magnetic method according to ASTM A801 ​​standard, or by X-ray diffraction (XRD) quantitative phase analysis. The γ / α phase ratio was determined by the Rietveld full-spectrum fitting method of X-ray diffraction (XRD).

[0008] Preferably, in order to achieve the dual objectives of "reverse deformation" and "nanoprecipitation" in the second-order principal deformation, the inventors have discovered that the key lies in matching the thermodynamic driving force (mainly determined by the temperature difference between temperature T and the inversion initiation temperature As) and the kinetic conditions (mainly determined by the strain rate ε̇ and the amount of deformation) within a precise synergistic window. Through a large number of systematic experiments, this invention abandons the traditional mode of relying on a single parameter or complex empirical formulas and creatively proposes a set of multi-parameter coupled process windows that can be directly monitored and executed.

[0009] Specifically, the secondary principal deformation must simultaneously satisfy the following three directly controllable process conditions: 1. Temperature conditions: The deformation temperature T must satisfy As+25℃≤T≤As+35℃. This temperature range provides a sufficiently high thermodynamic driving force to ensure that α' martensite can undergo rapid and sufficient reverse transformation (reverse transformation rate ≥95%), while avoiding excessive temperature leading to grain coarsening or unfavorable phase precipitation.

[0010] 2. Strain rate condition: The strain rate ε̇ needs to be controlled at 0.004 s. -1 up to 0.008 s -1 Within this range, the dislocation density and distribution are suitable: they can promote the nucleation of reverse transformation, provide effective channels and nucleation sites for the short-range diffusion of Cr and N atoms, and suppress the excessive growth of precipitates.

[0011] 3. Deformation conditions: The engineering deformation amount needs to be controlled at 22-25%. This deformation amount is the key to accumulating sufficient stored energy to drive phase transition and forming a large number of uniform dislocation networks to guide nanoprecipitation.

[0012] When the above three conditions are met simultaneously, a high-ratio reverse transformation of α' martensite (restoring and stabilizing the γ / α phase ratio) and the in-situ generation of fine nanoscale Cr2N precipitates (average particle size ≤20nm) can be achieved in one process.

[0013] In summary, the three-stage cold drawing process of this invention constitutes a tightly controlled regulatory system: the first-stage pre-deformation precisely implants α' martensite "seeds" at low temperatures and initiates texture orientation; the second-stage main deformation, within a medium-temperature synergistic window, simultaneously completes reverse transformation to restore phase equilibrium and utilizes deformation defects to drive nano-precipitation for strengthening; the third-stage final deformation, at near-As temperatures, "locks" and shapes the optimized microstructure; this continuous "drive-synergistic-lock" process replaces the traditional "deformation-annealing" cycle, achieving integrated and precise control of the microstructure and properties of super duplex stainless steel seamless tubes.

[0014] Furthermore, the inventors discovered that the temperature (T), strain rate (ε̇), and engineering deformation (ε) of the secondary principal deformation are not isolated parameters, and their optimal values ​​are all strongly correlated with the inversion temperature (As) of the material. Preferably, the temperature T should be set around (As + 30℃). Correspondingly, the strain rate ε̇ needs to be coordinated with the difference between (T-As) to ensure the dynamic balance between dislocation multiplication and atomic diffusion. The engineering deformation ε can also be finely adjusted with As to adapt to the differences in energy storage requirements of different grades of materials. This multi-parameter coupled control with As as the intrinsic benchmark is the key to the ability of this scheme to be widely adaptable to different grades of super duplex stainless steel and achieve stable results.

[0015] Simultaneously, the strong γ phase {110} induced by this invention <111> Texture (extreme density G) is the core of obtaining ultra-high stress corrosion resistance. The mechanism is that this strong texture can significantly promote the selective formation and stabilization of the protective Cr2O3 passivation film on the material surface in the corrosive environment. As a result, the product’s chloride ion stress corrosion threshold (σth) and texture extreme density (G) show a significant positive correlation. Within the preferred texture extreme density range (3.5-4.5) obtained by this invention, the product can achieve excellent performance of σth≥350MPa.

[0016] Preferably, in order to achieve and stabilize a favorable crystal texture, the present invention designs a mold for primary pre-deformation cold drawing; A variable curvature micro-spiral groove composite mold is adopted, with a cone angle of 10-12°. The radius of curvature of the first half (near the pipe inlet side) of the inner surface of the working zone is 50 mm, and the radius of curvature of the second half (near the outlet side) is 80 mm. The inner wall is provided with micro-spiral grooves (groove depth 0.1-0.3 μm, pitch 5-10 μm, which can be formed on the inner wall by precision machining methods such as micro-electrical discharge machining or laser etching). The micro-spiral grooves have a constant direction of rotation (e.g., right-handed), and the helix angle is preferably 3-5°. Controlled torsional strain can be introduced in the pre-deformation stage to effectively guide the γ phase along the curve. <110> Directional slippage ultimately leads to the presence of the γ phase {110} in the finished pipe. <111> The texture density remains consistently high at 3.5-4.5, and this high texture density is essential for achieving excellent resistance to chloride ion stress corrosion (σ). th ≥350MPa, σ th The key is to test according to ASTM G30 standard (the test environment is 3.5% NaCl solution, 25℃, constant load). Secondary main deformation mold: cone angle 8-10°, total working belt length 0.08-0.12m. To optimize deformation uniformity, the inner surface of the working belt can preferably adopt a gradient design with a first half curvature radius of 60mm and a second half curvature radius of 90mm. The third-stage final deformation mold has a cone angle of 6-8°, a total working belt length preferably of 0.06-0.09m, and an inner wall roughness Ra≤0.2μm to correct dimensional accuracy.

[0017] Specifically, the γ phase {110} <111> The polar density of the texture was measured using the X-ray diffraction (XRD) pole figure method. Specifically, a sample was cut from the finished seamless tube, and the {110} crystal plane pole figure of the γ phase was measured using an X-ray diffractometer. The inverse pole figure was then calculated using supporting material texture analysis software (such as MTEX), and the data was read from the sample. <111> The maximum value of the polar density corresponding to the orientation is the texture polar density.

[0018] Preferably, to ensure the stable implementation of the above-mentioned precise control, a specially formulated temperature-sensitive chelated nano-lubricant is required throughout the three-stage cold drawing process.

[0019] Preferably, the temperature-sensitive chelated nano-lubricant comprises the following raw materials in parts by weight: 60-70 parts of polyα-olefin, 5-8 parts of polymethacrylate, 0.5-1.0 parts of TiN nanoparticles with a particle size of 50 nm, 0.3-0.8 parts of fatty amine polyoxyethylene ether, and 0.2-0.5 parts of aminotrimethylenephosphonic acid. Its preparation method includes the following steps: (1) Heat the poly-α-olefin to 50-60℃, add polymethacrylate and stir for 25-35 min to obtain the base liquid; (2) Add TiN nanoparticles, fatty amine polyoxyethylene ether and aminotrimethylene phosphonic acid to the base liquid, shear at high speed of 3000-4000r / min for 20-30min, and cool to room temperature to obtain the product.

[0020] This lubricant has stable viscosity (33-35 mm) at 40-60℃. 2 The viscosity at -10 to 5°C is 45-50 mm³ / s, with a friction coefficient of 0.09-0.11. 2 / s, with a friction coefficient of 0.12-0.14; viscosity at 15-20℃ is 38-40 mm. 2 / s, with a friction coefficient of 0.10-0.12; to improve the dispersion stability of TiN nanoparticles across the entire temperature range, 0.1-0.3 parts by weight of a dispersant (such as polyisobutylene succinate) can be added, which contains aminotrimethylene phosphonic acid that can chelate free Cr during processing. 3+ This reduces mold adhesion; TiN nanoparticles can fill the tiny gaps between the mold's micro-spiral grooves and the tube surface, forming a micro-rolling lubricating layer, which together ensures a stable friction coefficient, providing a guarantee for stable texture induction and precise execution of process parameters.

[0021] Preferably, the complete process flow is as follows: Billet pretreatment: SDSS hot-rolled tube billets undergo solution treatment and water quenching. To obtain a stable and suitable initial phase ratio (γ / α=45-55%), the following solution treatment parameters can be selected for different grades: Grade 2507: 1080-1100℃, holding for 40-60 min; Grade S32760: 1090-1110℃, holding for 45-55 min; Grade S32750: 1060-1080℃, holding for 35-50 min; After solution treatment, the surface is polished to Ra≤0.8μm. First-stage pre-deformation: Temperature control in a low-temperature cold drawing machine (refer to the table according to the grade), lubricant applied, cold drawing in a mold, deformation rate 3-4 m / min, engineering deformation 12-15%, generating 5-8% strain-induced α' martensite, inducing γ phase {110} <111> Texture; Secondary main deformation: temperature control of medium-temperature cold drawing machine (refer to table), continuous application of lubricant, cold drawing, speed 1.5-2.2m / min, engineering deformation amount 22-25%, to achieve full reverse transformation of α' martensite and in-situ generation of nano-scale Cr2N precipitates in the α phase; Third-level final deformation: Precision cold drawing machine temperature control 15-20℃, cold drawing, speed 2-3m / min, engineering deformation amount 5-8%, maintaining phase ratio and texture; Post-treatment: perform pickling, washing and drying, then rinse with 60℃ deionized water for 5 minutes, and dry at 80-100℃ for 30-60 minutes.

[0022] To accommodate the differences in oxide scale characteristics on the surface of different grades of pipes, the pickling parameters can be optimized as follows: For grade 2507, the pickling solution concentration (vol%) is 10% nitric acid + 2% hydrofluoric acid, the temperature is 40-45℃, and the time is 10-12 minutes; for grade S32760, the pickling solution concentration (vol%) is 11% nitric acid + 2.5% hydrofluoric acid, the temperature is 45-50℃, and the time is 12-15 minutes; for grade S32750, the pickling solution concentration (vol%) is 9% nitric acid + 2% hydrofluoric acid, the temperature is 40-45℃, and the time is 11-13 minutes.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: It achieves a synergistic effect of phase ratio, nano-precipitation and strong texture, giving the product high strength, high toughness and ultra-high resistance to stress corrosion. Eliminating all intermediate annealing processes simplifies the traditional multi-pass "cold drawing-annealing" cycle into a single continuous three-stage cold drawing, shortening the processing cycle and reducing energy consumption and production costs. Attached Figure Description

[0024] Figure 1 A flowchart of a cold working method for a super duplex stainless steel seamless tube provided by the present invention. Detailed Implementation

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings of this application are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings of this application are used to distinguish different objects, not to describe a particular order.

[0026] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0027] The process flow of the method described in this invention can be found in [reference needed]. Figure 1Its core lies in achieving the triple goal of microstructure control simultaneously within a single process by sequentially performing three-stage cold drawing and precisely controlling the direct process parameters at each stage (especially the secondary main deformation). To facilitate implementation, this invention provides a fully validated quick reference table of process parameters for different grades (see Table 1 below), allowing users to directly refer to the table to set key parameters based on the pipe grade.

[0028] Table 1 Quick Reference Table of Secondary Main Deformation Process Parameters for Typical Grades The strain rate ε̇ in the table is estimated based on typical parameters of 0.08-0.12m working zone length and 8-10° cone angle. It is used to guide equipment settings. In actual production, the core objectives are to achieve the "temperature", "deformation amount" and "deformation rate" in the table.

[0029] Example 1: Preparation of seamless tube of grade 2507, Φ100mm×3mm Pre-treatment of billet: Take a Φ120mm×8mm hot-rolled tube billet of 2507 super duplex stainless steel, and perform solution treatment at 1080℃ for 50 minutes, followed by water quenching; after testing, the initial microstructure has a γ / α phase ratio of about 52%, and the surface roughness Ra after grinding is 0.6μm; First-stage pre-deformation cold drawing: This is performed in a low-temperature cold drawing machine, with the deformation zone temperature controlled at -5℃ (refer to Table 1, within the recommended range of -10 to 0℃ for grade 2507). A variable curvature micro-spiral groove composite mold is used, and a temperature-sensitive chelated nano-lubricant is applied (specific ratio: 65 parts polyα-olefin, 6 parts polymethacrylate, 0.8 parts TiN nanoparticles, 0.5 parts fatty amine polyoxyethylene ether, and 0.3 parts aminotrimethylenephosphonic acid). The tube moving speed (deformation rate) is controlled at 3.5 m / min, achieving a 13% engineering deformation. After this stage, approximately 6% strain-induced α' martensite is generated in the microstructure, and the γ phase {110} is induced. <111> Texture, the measured texture density is approximately 4.0; Secondary main deformation cold drawing: This is carried out in a medium-temperature cold drawing machine, with the temperature of the deformation zone controlled at 48℃ (refer to Table 1, satisfying the synergistic window of T between As+25℃ and As+35℃, and within the recommended range of 45-55℃ for grade 2507). The above-mentioned lubricant is continued to be used, and the tube moving speed is controlled at 1.8 m / min (refer to Table 1, within the recommended range of 1.5-2.0 m / min), achieving 23% engineering deformation (refer to Table 1, within the range of 22-25%). The core of this stage is to simultaneously satisfy the coupling conditions of temperature, strain rate, and deformation. After this process, the α' martensite generated by the primary pre-deformation undergoes full reverse transformation, with a residual amount ≤0.3% (reverse transformation rate ≥95%). At the same time, nanoscale Cr2N precipitates with an average particle size of about 16nm are precipitated in situ in the α phase, and finally the γ / α phase ratio is stabilized at 50 / 50. The third-stage final deformation cold drawing is carried out in a precision cold drawing machine, with the temperature controlled at 18℃ (i.e., within the range of 15-20℃) and the tube moving speed controlled at 2.5 m / min, to achieve 6% engineering deformation. The main purpose of this stage is to release some internal stress, further homogenize the structure, and accurately correct the tube dimensions. Post-treatment: Pickle in pickling solution (10 vol% nitric acid + 2 vol% hydrofluoric acid) at 45°C for 12 minutes, then rinse with deionized water at 60°C for 5 minutes, and dry at 90°C for 40 minutes.

[0030] Test results: The finished pipe has a uniform microstructure, with γ phase accounting for 50% and α phase accounting for 50%. It exhibits excellent mechanical and corrosion properties: yield strength (Rp0.2) is 560MPa, tensile strength is 780MPa, Charpy V-notch impact energy (Akv, tested according to GB / T 229 standard) is 115J, and resistance to chloride ion stress corrosion threshold (σ) is [not specified]. th (Tested according to ASTM G30 standard) up to 360MPa, with high dimensional accuracy, roundness deviation within ±0.15mm, and a product qualification rate of 100%.

[0031] Example 2: Preparation of Φ80mm×2mm thin-walled seamless tube of grade 2507 Pre-treatment of billet: Φ100mm×6mm 2507 tube billet, solution treated at 1080℃ for 40 minutes, water quenched, initial γ / α≈50%, Ra=0.7μm; First-stage pre-deformation: temperature controlled at -8℃, using the same lubricant as in Example 1, speed 3.2 m / min, engineering deformation amount 14%, generating approximately 7% α' martensite, texture density 3.8; Secondary principal deformation: temperature control 46℃ (meeting the synergistic temperature window), rate 1.6 m / min (within the recommended range in Table 1), engineering deformation amount 22% (within the recommended range in Table 1), to achieve full reverse transformation of α' martensite (residual ≤0.3%) and generate Cr2N precipitates of about 14 nm; Level III final deformation: Temperature controlled at 16℃, speed at 2.2 m / min, engineering deformation of 7%; Post-treatment: Follow the pickling, washing and drying process of Example 1.

[0032] Test results: γ / α≈49 / 51, yield strength 550MPa, Akv=112J, σ th =355MPa, wall thickness deviation ±1.8%, no cracks.

[0033] Example 3: Preparation of S32760 grade Φ120mm×4mm seamless tube Pre-treatment of billet: Φ140mm×10mm S32760 tube billet, solution treated at 1090℃ for 50 minutes, water quenched, initial γ / α≈51%, Ra=0.6μm; First-stage pre-deformation: temperature control -3℃ (refer to Table 1, within the recommended range of -5 to 0℃ for grade S32760), rate 3.8 m / min, engineering deformation 15%, generating approximately 8% α' martensite, texture density 4.2; Secondary principal deformation: temperature controlled at 55℃ (refer to Table 1, meeting the recommended range of 50-60℃ and conforming to the synergistic temperature window), rate 2.0 m / min (within the recommended range of 1.8-2.2 m / min in Table 1), engineering deformation amount 25% (within the recommended range in Table 1), to achieve full reverse transformation of α' martensite (residual ≤0.4%) and generate Cr2N precipitates of approximately 18 nm; Level III final deformation: Temperature controlled at 19℃, speed at 2.8 m / min, engineering deformation amount 5%; Post-treatment: Pickle in pickling solution (11% nitric acid + 2.5% hydrofluoric acid) at 48°C for 14 minutes, then wash with water and dry.

[0034] Test results: γ=51%, α=49%, yield strength 570MPa, Akv=110J, σ th =370MPa, long-term service phase ratio deviation 0.8%.

[0035] Example 4: Preparation of S32750 grade Φ90mm×2.5mm seamless tube Pre-treatment of billet: Φ110mm×7mm S32750 tube billet, solution treated at 1060℃ for 40 minutes, water quenched, initial γ / α≈49%, Ra=0.7μm; First-stage pre-deformation: temperature control at 0℃ (refer to Table 1, within the recommended range of -5 to 5℃ for grade S32750), rate at 3.0 m / min, engineering deformation at 12%, generating approximately 5% α' martensite, with a texture density of 3.6; Secondary principal deformation: temperature controlled at 50℃ (refer to Table 1, meeting the recommended range of 45-50℃ and conforming to the synergistic temperature window), rate 1.6 m / min (within the recommended range of 1.6-1.9 m / min in Table 1), engineering deformation amount 24% (within the recommended range in Table 1), achieving full reverse transformation of α' martensite (residual ≤0.3%) and generating Cr2N precipitates of approximately 15 nm; Level 3 final deformation: temperature controlled at 20℃, speed at 2.3 m / min, engineering deformation amount 6%; Post-treatment: Pickling at 44℃ (9% nitric acid + 2% hydrofluoric acid) for 13 minutes, followed by washing with water and drying.

[0036] Test results: γ / α=50 / 50, yield strength 565MPa, Akv=113J, σ th =358MPa.

[0037] Example 5: Preparation of large-diameter seamless tubes of grade 2507, Φ150mm×5mm Pre-treatment of billet: Φ180mm×12mm 2507 hot-rolled tube billet is used. After solution treatment at 1100℃ for 60 minutes, it is water quenched. The initial microstructure has a γ / α phase ratio of about 53%. The surface is polished to Ra=0.6μm. First-stage pre-deformation cold drawing: temperature controlled at -10℃ (refer to Table 1, within the recommended range for grade 2507). Using the mold described in claim 5 (8μm pitch), a temperature-sensitive chelated nano-lubricant is coated. The deformation rate is controlled at 4.0 m / min, achieving 15% engineering deformation. After this stage, approximately 8% strain-induced α' martensite is generated, and the γ phase texture density reaches 4.5.

[0038] Secondary main deformation cold drawing: temperature controlled at 52℃ (refer to Table 1, meeting its recommended range of 45-55℃, and conforming to the synergistic temperature window of As+25℃ to As+35℃), deformation rate controlled at 1.9 m / min (within the recommended range of 1.5-2.0 m / min in Table 1), achieving 25% engineering deformation (within the recommended range in Table 1); by simultaneously meeting the synergistic process window, full reversal of α' martensite was achieved (residual amount ≤0.5%), and nanoscale Cr2N precipitates with an average particle size of about 17nm were formed in situ in the α phase.

[0039] Three-stage final deformation cold drawing: temperature controlled at 17℃, deformation rate at 3.0 m / min, achieving 8% engineering deformation.

[0040] Post-treatment: Pickling at 45℃ (10 vol% nitric acid + 2 vol% hydrofluoric acid) for 12 minutes.

[0041] Test results: The final microstructure has an γ / α ratio of approximately 51 / 49, a yield strength of 580 MPa, an impact energy of 111 J (Akv), and a chloride ion stress corrosion resistance threshold σ. th It has a strength of 365 MPa (tested according to ASTM G30 standard), high dimensional accuracy, roundness deviation of ±0.2 mm, and a pass rate of 100%.

[0042] Example 6: S32750 grade, Φ25mm×1.0mm ultra-thin wall precision seamless tube Pre-treatment of billet: S32750 hot-rolled tube billet with Φ40mm×3mm is used. After solution treatment at 1060℃ for 40 minutes, it is water quenched. The initial γ / α phase ratio is about 49%. The surface is polished to Ra≤0.7μm.

[0043] First-stage pre-deformation cold drawing: temperature control at 0℃ (refer to Table 1, within the recommended range of grade S32750). To adapt to ultra-thin wall deformation, a variable curvature micro-spiral groove mold with a smaller pitch (6μm) is used to reduce the unit deformation force. The deformation rate is 3.0 m / min, the engineering deformation is 14%, and an appropriate amount of α' martensite is generated and texture is induced.

[0044] Secondary primary deformation cold drawing: temperature control at 48℃ (refer to Table 1, within the recommended range of 45-50℃, and conforming to the synergistic temperature window), deformation rate of 1.7 m / min (within the recommended range of 1.6-1.9 m / min in Table 1), and engineering deformation of 23% (within the recommended range in Table 1). This parameter combination ensures synergistic control, achieving complete α' martensite reversal (residual ≤0.3%) and generating Cr2N precipitates with an average particle size of approximately 16 nm. To ensure stable deformation of the ultrathin wall, a higher frequency of lubricant spraying is used (once every 10 seconds, 5 μL / cm per spray). 2 ).

[0045] Three-stage final deformation cold drawing: temperature control 20℃, deformation rate 2.0 m / min, engineering deformation amount 7%.

[0046] Post-treatment: Pickle according to standard procedure (9% nitric acid + 2% hydrofluoric acid), and dry at 80-100℃ for 30-60 minutes.

[0047] Test results: The pipe dimensions are precise, with a wall thickness tolerance of ±0.05mm, microstructure γ / α=50 / 50, Cr2N particle size 16nm, texture density 3.7, yield strength 555MPa, impact energy Akv=112J, and chloride ion stress corrosion threshold σ. th ≥355MPa.

[0048] Example 7: Validation of process optimization under normal fluctuations in raw material composition In industrial continuous production, the composition of tube billets of the same grade (2507) from different heats will fluctuate normally within the standard range. In this example, a heat of 2507 tube billet with qualified composition but at the upper limit of the standard range (measured composition: Cr: 25.8%, Ni: 7.0%, N: 0.29%) was selected. Its billet specifications, solution treatment and surface pretreatment are the same as those in Example 1. The actual inversion temperature As was determined to be 24℃, slightly higher than the typical value (20℃) used for grade 2507 in Table 1. To verify the adaptive adjustment effect, the following two paths were followed, and all process steps and parameters (including but not limited to: primary / tertiary deformation temperature and deformation amount, deformation rate of each stage, mold used, lubricant formulation and coating method, pickling post-treatment, etc.) were strictly kept consistent with those in Example 1, except for the secondary main deformation temperature. Path A (using the old parameters for comparison): Ignoring the difference in As, directly adopt the process parameters in Table 1 for As=20℃, that is, the secondary main deformation temperature is 48℃.

[0049] Path B (adaptive adjustment using the present invention): Based on the core principles of the present invention, for the measured As=24℃, the secondary main deformation temperature is adjusted to the range of (As+25℃) to (As+35℃), i.e. 49℃ to 59℃. To obtain the best effect, 52℃ is selected as the optimization temperature.

[0050] Using the same tube blank, seamless tubes of Φ100mm×3mm were prepared according to path A and path B respectively.

[0051] Path A (48℃) results: Product performance is qualified, but not optimal. Testing revealed an α' martensite reversal rate of approximately 96%, an average Cr2N particle size of 18 nm, a yield strength of 555 MPa, and a σ... th =350MPa, the tissue is at the edge of the control window.

[0052] Pathway B (52℃) Results: Product performance reached its optimal level, with complete α' martensite reversal (less residue, >97%), more uniform and finer Cr2N precipitation (average particle size 16nm), yield strength increased to 565MPa, and stress corrosion resistance threshold significantly optimized to σ. th =365MPa.

[0053] As can be seen from the above, by adjusting the main deformation temperature to a better position within the synergistic window based on the measured As temperature, the material potential can be further stimulated, resulting in products with better and more stable performance.

[0054] Comparative Example 1: Existing technology (traditional "cold drawing-annealing" process) Using the same 2507 billet as in Example 1, the billet was first cold-drawn at room temperature (approximately 25°C) with an engineering deformation of 20%. Then, it was annealed at 850°C for 30 minutes, followed by cold rolling at room temperature with an engineering deformation of 30%. It was then annealed again at 850°C for 30 minutes, and finally fine-drawn at room temperature with an engineering deformation of 8%. Conventional lubricant and ordinary conical molds were used throughout the process.

[0055] Test results: The final microstructure showed only 38% γ phase and 62% α phase, indicating a phase imbalance. The Cr₂N precipitates were coarse, with an average particle size of approximately 60 nm. The yield strength was low (470 MPa), toughness was poor (Akv=75 J), and resistance to stress corrosion was weak (σ). th =240MPa), with a product qualification rate of only 83%.

[0056] Comparative Example 2: Phase ratio + nano-precipitate regulation (no texture regulation) The same blank, the same two-stage and three-stage cold drawing process parameters (temperature, rate, deformation amount) and temperature-sensitive chelated nano lubricant were used as in Example 1. The difference was that the first-stage pre-deformation used a regular conical mold without micro-spiral grooves (the cone angle was the same, but the inner wall was smooth).

[0057] Test results: The phase ratio (γ / α=50 / 50) and Cr2N particle size (16nm) were well controlled. The yield strength (560MPa) and toughness (Akv=114J) were comparable to those of Example 1. However, due to the lack of texture induction, the γ phase {110} <111> The texture density is only 2.2, which leads to a significant decrease in resistance to chloride ion stress corrosion (σ). th =270MPa), and the phase ratio stability deteriorates after deep processing (deviation reaches 1.6%).

[0058] Comparative Example 3: Process parameters deviate from the synergistic window (leading to dual failure of phase transition and precipitation). Using the same billet, die (including micro-spiral grooves) and lubricant as in Example 1, but simultaneously changing two key parameters of the secondary principal deformation to remove them from the cooperative window: The temperature was lowered to 35°C (well below the lower limit of 45°C for As+25°C).

[0059] The deformation rate was significantly increased to 3.0 m / min (far exceeding the 1.5-2.0 m / min range recommended in Table 1, resulting in a strain rate ε̇ as high as about 0.012, exceeding the upper limit of the synergistic window of this invention).

[0060] The engineering deformation remains unchanged at 23%.

[0061] Test results: Incomplete α' martensite reversal (approximately 3% remaining), coarse Cr2N precipitates (average particle size 45nm) and extremely uneven distribution, leading to a comprehensive deterioration in the product's overall performance: yield strength 490MPa, impact energy Akv=85J, stress corrosion threshold σ th =310MPa, and the texture is also weakened due to the disorder of deformation parameters in the early stage (extreme density 3.0).

[0062] The key process parameters and final performance indicators of each embodiment and comparative example are summarized in Table 2 below: Table 2 Summary of Key Parameters and Performance of Examples and Comparative Examples In summary: First, eliminating intermediate annealing and adopting continuous three-stage cold drawing is the basis for improving efficiency in this solution.

[0063] Secondly, the temperature, strain rate, and deformation amount of the secondary principal deformation must all fall within a specific synergistic window based on the actual reverse transformation temperature As of the material. This is a necessary and sufficient condition for simultaneously achieving full reverse transformation of α' martensite (restoring phase equilibrium) and in-situ precipitation of nanoscale Cr2N in the α phase (achieving strengthening). Deviating from this window (as in Comparative Example 3) will lead to the failure of both phase transformation and precipitation.

[0064] Finally, a strong γ phase {110} is induced to form through a primary pre-deformation mold. <111> Texture (extreme density G≥3.5) is what enables the product to obtain the chloride ion stress corrosion threshold σ. th The key independent factor for the excellent performance of ≥350MPa is the lack of texture control (such as in Comparative Example 2). Even with good control of the phase ratio and precipitates, the corrosion resistance will be greatly reduced.

[0065] This invention, through the aforementioned three-stage "drive-coordination-locking" process, achieves for the first time, the triple synergistic regulation of phase ratio, nanoprecipitation, and crystal texture in super duplex stainless steel during continuous cold drawing without intermediate annealing, thereby combining high strength, high toughness, and ultra-high corrosion resistance. It should be noted that, for the sake of simplicity, the foregoing embodiments are all described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0066] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still combine, add, delete, or otherwise adjust the features of the various embodiments of the present invention according to the circumstances without conflict or creative effort, thereby obtaining different technical solutions that do not fundamentally depart from the concept of the present invention. These technical solutions also fall within the scope of protection of the present invention.

Claims

1. A cold working method for a super duplex stainless steel seamless tube, characterized in that, This includes a three-stage cold drawing process performed sequentially without intermediate annealing: (1) First-stage pre-deformation cold drawing: carried out in a low-temperature environment below the inversion and transformation initiation temperature As of the super duplex stainless steel, and the engineering deformation is controlled at 12-15%; (2) Secondary main deformation cold drawing: carried out in a medium temperature environment higher than that of As, and the engineering deformation is controlled at 22-25%; (3) Third-stage final deformation cold drawing: carried out at 15-20℃, and the engineering deformation is controlled at 5-8%.

2. The cold working method for super duplex stainless steel seamless tubes according to claim 1, characterized in that, The temperatures for the first-stage pre-deformation are: -10~0℃ for grade 2507, -5~0℃ for grade S32760, and -5~5℃ for grade S32750; the temperatures for the second-stage main deformation are: 45-55℃ for grade 2507, 50-60℃ for grade S32760, and 45-50℃ for grade S32750.

3. The cold working method for super duplex stainless steel seamless tubes according to claim 1, characterized in that, The secondary principal deformation must simultaneously meet the following conditions: Temperature conditions: The deformation temperature T satisfies As+25℃≤T≤As+35℃, where As is the inversion temperature of the super duplex stainless steel. Strain rate condition: The strain rate ε̇ is controlled at 0.004 s. -1 up to 0.008 s -1 between; Deformation condition: The engineering deformation is 22-25%.

4. The cold working method for super duplex stainless steel seamless tubes according to claim 1, characterized in that, The cold drawing die used for the first-stage pre-deformation is a variable curvature micro-spiral groove composite structure: the cone angle is 10-12°, the radius of curvature of the first half of the inner surface of the working strip is 50mm and the second half is 80mm; the inner wall is provided with micro-spiral grooves with a groove depth of 0.1-0.3μm and a pitch of 5-10μm; the die used for the second-stage main deformation cold drawing has a cone angle of 8-10° and a total length of 0.08-0.12m for the working strip.

5. The cold working method for super duplex stainless steel seamless tubes according to claim 1, characterized in that, The temperature-sensitive chelated nano-lubricant is used in the three-stage cold drawing process. The viscosity of the temperature-sensitive chelated nano-lubricant is 33-35 mm at 40-60℃. 2 / s, with a friction coefficient of 0.09-0.

11.

6. The cold working method for the super duplex stainless steel seamless tube according to claim 1, characterized in that, The deformation rate of the first-stage pre-deformation is 3-4 m / min, the deformation rate of the second-stage main deformation is 1.5-2.2 m / min, and the deformation rate of the third-stage final deformation is 2-3 m / min.

7. A super duplex stainless steel seamless tube, characterized in that, The seamless tube is prepared by the method according to any one of claims 1-6, wherein the γ / α phase ratio is 48-52%, the average particle size of the Cr2N precipitates in the α phase is 15±3 nm, and the γ phase {110} <111> The texture density is 3.5-4.5; the seamless tube has a yield strength of 550-580 MPa, an impact energy Akv ≥ 110 J, and a chloride ion stress corrosion resistance threshold σ. th ≥350MPa.

8. A temperature-sensitive chelated nano-lubricant for use in the cold working method of the super duplex stainless steel seamless tube according to any one of claims 1-6, characterized in that, By weight, it includes 60-70 parts of polyα-olefin, 5-8 parts of polymethacrylate, 0.5-1.0 parts of TiN nanoparticles with a particle size of 50 nm, 0.3-0.8 parts of fatty amine polyoxyethylene ether, and 0.2-0.5 parts of aminotrimethylenephosphonic acid.

9. The method for preparing the temperature-sensitive chelated nano-lubricant according to claim 8, characterized in that, Includes the following steps: (1) Heat the poly-α-olefin to 50-60℃, add polymethacrylate and stir for 25-35 min to obtain the base liquid; (2) Add TiN nanoparticles, fatty amine polyoxyethylene ether and aminotrimethylene phosphonic acid to the base liquid, shear at high speed of 3000-4000r / min for 20-30min, and cool to room temperature to obtain the product.

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