Ultra-long large-diameter duplex stainless steel seamless tube and preparation method thereof

Abstract

The invention belongs to the field of stainless steel materials, and provides an ultra-long large-diameter duplex stainless steel seamless tube and a preparation method thereof.The design of core-shell inclusion modification, clean smelting and precise phase proportion regulation is adopted, deoxidation is conducted after smelting alloying, calcium magnesium cerium lanthanum is introduced for core-shell inclusion modification, and the ultra-long large-diameter duplex stainless steel seamless tube is obtained. According to the method, the total oxygen content is controlled to be not higher than 0.0030 wt% in combination with argon bottom blowing stirring, then the duplex stainless steel seamless tube with the tube length being 8-14 m, the outer diameter being 500-900 mm and the wall thickness being 20-60 mm is prepared through perforation continuous rolling rotary expansion and solid solution rapid cooling, a duplex structure with the ferrite volume fraction being 35-65 vol% is achieved, the core-shell structure non-metallic inclusion proportion is not lower than 1%, and the ultra-cleanliness is not larger than 10 micrometers. The problems that under the ultra-long, large-caliber and thick-wall conditions, hot working forming stability and dual-phase structure phase stability are difficult to consider at the same time, core-shell modification and obdurability fluctuation coupling and solid solution rapid cooling and residual stress control contradiction are caused are solved, and wide ocean engineering and oil and gas conveying application value is achieved.

Description

Technical Field

[0001] This invention relates to the field of stainless steel material preparation, specifically to an ultra-long diameter duplex stainless steel seamless tube and its preparation method. Background Technology

[0002] Duplex stainless steel, possessing the advantages of both ferrite and austenitic two-phase microstructure, exhibits excellent comprehensive performance in marine engineering, oil and gas transportation, and chemical equipment. Its balance of strength, toughness, and corrosion resistance makes it the preferred material for harsh service environments. With the development of deep-sea oil and gas towards ultra-deep water, high pressure, and high temperature, and the dual requirements of reliability and economy for long-distance pipeline transportation systems, extreme dimensional demands have been placed on duplex stainless steel seamless tubes, requiring ultra-long lengths, large diameters, and thick walls. These demands are not only reflected in geometric dimensions but also impose stringent standards on the material's microstructure uniformity, cleanliness, fatigue performance, and performance consistency throughout its entire length. The fabrication of ultra-long, large-diameter duplex stainless steel seamless tubes requires ensuring a stable ferrite-to-austenite two-phase ratio within a reasonable range under conditions of tube lengths exceeding eight meters, outer diameters exceeding 500 millimeters, and wall thicknesses exceeding 20 millimeters, while simultaneously controlling non-metallic inclusions to an ultra-clean level. This presents unprecedented challenges to material design and process control. Meeting and improving these performance requirements is crucial for ensuring the structural safety of deep-sea platforms, extending pipeline service life, and reducing total life-cycle costs.

[0003] However, current manufacturing technology for duplex stainless steel seamless tubes still has significant shortcomings in meeting the requirements for ultra-long, thick-walled tubes with large diameters. Regarding microstructure control, the non-uniformity of the hot working temperature field under ultra-long dimensions causes fluctuations in the ferrite to austenite phase ratio along the tube length. Local deviations in the ferrite volume fraction from the target range can lead to an imbalance between strength and toughness. This is especially true during solution treatment and rapid cooling, where the cooling rates at the tube ends and in the middle differ significantly, making it difficult to effectively suppress the precipitation of harmful phases throughout the entire length. Regarding cleanliness control, traditional deoxidation processes produce oxide inclusions that are large in size, irregular in morphology, and unevenly distributed. These inclusions become preferred sites for fatigue crack initiation, severely weakening the reliability of materials under cyclic loading. For example, Chinese patent CN102453840A discloses a method for preparing duplex stainless steel, but it suffers from insufficient inclusion modification and high total oxygen content. Similarly, Chinese patent CN117943408A discloses a method for preparing large-diameter seamless steel pipes, but it suffers from limited aspect ratio and insufficient uniformity of solution cooling. In terms of process integration, existing technologies mostly use segmented preparation and welding splicing to obtain ultra-long pipes. However, the problems of discontinuous microstructure and performance degradation in the weld area are difficult to fundamentally solve. Furthermore, the control of uniformity of tube wall thickness and deformation coordination during the integrated piercing and continuous rolling process is insufficient, making it difficult to guarantee the consistency of dimensional accuracy and mechanical properties of the finished pipe. Summary of the Invention

[0004] The purpose of this invention is to provide an ultra-long diameter duplex stainless steel seamless tube and its preparation method, which solves the pain points of the present invention, namely, the difficulty in balancing the stability of hot working and forming and the stability of the duplex microstructure under ultra-long diameter and thick wall conditions, the coupling of core-shell modification of inclusions and the fluctuation of strength and toughness, and the contradiction between solution solution rapid cooling and residual stress control.

[0005] This invention employs a synergistic design of core-shell inclusion modification and clean smelting. After deoxidation, calcium, magnesium, cerium, and lanthanum are introduced and combined with argon bottom blowing and stirring to transform inclusions from hard oxides to core-shell composite structures. The aluminum oxide in the core region provides a nucleation substrate, while the calcium-rich phase in the shell region forms a deformation buffer layer that can deform in coordination with the matrix without stress concentration. At the same time, the total oxygen is controlled below 0.0030% by mass, reducing the number of inclusions and optimizing the morphology distribution from the source. The synergistic effect of core-shell inclusions and ultra-low total oxygen significantly improves fatigue life and fracture toughness.

[0006] To achieve the above objectives, the present invention provides the following technical solution: An ultra-long diameter duplex stainless steel seamless tube, wherein the seamless tube simultaneously meets the following conditions: A1. Geometric dimensions: Pipe length is 8-14m, outer diameter is 500-900mm, and wall thickness is 20-60mm; A2. Chemical composition in wt%: Chromium 21.0-26.0 wt%, Nickel 4.0-8.0 wt%, Molybdenum 2.5-5.0 wt%, Nitrogen 0.14-0.32 wt%, Manganese 0.20-2.00 wt%, Silicon 0.10-1.00 wt%, Aluminum 0.005-0.050 wt%, Carbon ≤0.03 wt%, Phosphorus ≤0.035 wt%, Sulfur ≤0.005 wt%, Balance: Iron and unavoidable impurities; A3. The metallographic structure is a dual-phase structure of ferrite and austenite, with a ferrite volume fraction of 35-65 vol%. A4. Non-metallic inclusions satisfy the following: Among the non-metallic inclusions obtained by sampling and statistically analyzing the cross-section of the seamless tube, the proportion of non-metallic inclusions with core-shell structures is not less than 1% of the total number of non-metallic inclusions. The core-shell structure is characterized by a morphology in which the core region is continuously covered by the shell region on the two-dimensional cross-section of the seamless tube. A5. The total oxygen mass fraction determined by inert gas melting infrared absorption method is ≤0.0030 wt%; A6. The seamless tube is made from duplex stainless steel billets through piercing, continuous rolling, sizing and / or sizing, solution treatment and rapid cooling.

[0007] Furthermore, the tube billet intermediate is an intermediate formed through melting alloying, refining and composition control, deoxidation, core-shell inclusion modification, and continuous casting into a billet followed by homogenization treatment. The preparation of the tube billet intermediate is carried out according to the following steps: B1. Melting and alloying: Iron-based raw materials are melted and chromium, nickel and molybdenum are added to form duplex stainless steel composition; B2. Refining and composition control, including decarbonization and refining, and introducing nitrogen during the refining stage; B3. Deoxidation: Deoxidation is performed by adding aluminum and silicon. B4. Core-shell inclusion modification: After deoxidation, calcium and magnesium are added to the molten steel, along with one of cerium and lanthanum, while the inclusion modification is carried out under argon stirring. B5. Continuous casting into billets and homogenization treatment: The molten steel is continuously cast into round billets or tube billets and then subjected to homogenization heat treatment to obtain the core-shell cleaned duplex stainless steel tube billet intermediate.

[0008] Furthermore, the raw materials used in steps B1-B4 include at least iron, chromium, nickel, molybdenum, nitrogen, manganese, silicon, aluminum, calcium, magnesium, argon, and cerium or lanthanum.

[0009] Furthermore, steps B2-B4 satisfy the following process parameter window: C1. The refining temperature in step B2 is 1550-1650℃, the refining time is 20-120min, the pressure is atmospheric pressure, or the refining is carried out under vacuum conditions with a vacuum degree of 10-500Pa, and the atmosphere is argon or a mixture of argon and nitrogen. C2. Nitrogen is introduced in step B2 by passing nitrogen gas above the molten steel or blowing nitrogen gas into the bottom of the molten steel. The nitrogen gas fraction in the refining atmosphere is 5-60 vol%, with the remainder being argon gas. C3. The deoxidation in step B3 includes the addition of aluminum and silicon, and the settling time after deoxidation is 2-15 minutes. C4. The core-shell inclusion modification temperature in step B4 is 1520-1600℃, the argon stirring time is 3-20 min, and the argon bottom blowing flow rate is 0.02-0.30 m³ / min·t.

[0010] Furthermore, the endpoint criterion in step B4 and the post-processing and quality control in step B5 shall at least satisfy the following: D1. The endpoint criterion is that the total oxygen mass fraction of the molten steel is ≤0.0030wt%, and the mass ratio of calcium to sulfur is 0.5-3.0; D2. Post-treatment includes homogenization heat treatment of the billet at 1150-1250℃ for 1-6 hours after continuous casting; D3. Quality control includes statistical analysis of inclusions in the core-shell purified duplex stainless steel tube blank intermediate, and determining that the equivalent diameter of the inclusions is the area equivalent circle diameter, and that D90 is the 90th percentile particle size of the equivalent diameter of the inclusions calculated according to the quantity distribution, and that D90≤3μm, and that the aspect ratio of the inclusions≤2.

[0011] Furthermore, the core is an inclusion phase in which the atomic fraction of Al is greater than that of Ca and the sum of the atomic fractions of Al and O is not less than 50% in energy dispersive spectroscopy (EDS) analysis; the shell is an inclusion phase in which the atomic fraction of Ca is greater than that of Al and the sum of the atomic fractions of Ca and O is not less than 50% in EDS analysis, or an inclusion phase in which the sum of the atomic fractions of Ca and S is not less than 50% in EDS analysis; the equivalent diameter of the non-metallic inclusion with the core-shell structure is the area equivalent circle diameter; and D90 is the 90th percentile particle size of the equivalent diameter of the non-metallic inclusion with the core-shell structure calculated according to the number distribution, and D90 ≤ 10 μm.

[0012] As a concept of this invention, the core-shell inclusion modification design is mainly used to enhance the fatigue performance and fracture toughness of duplex stainless steel seamless tubes. After adding aluminum and silicon during the deoxidation stage to form primary oxides, calcium and magnesium, as well as cerium or lanthanum microalloying elements, are precisely introduced. Under the forced convection effect of argon bottom blowing and stirring, calcium and magnesium preferentially react with residual oxygen and sulfur in the molten steel, forming a calcium-rich sulfide or calcium aluminate coating layer on the surface of the primary aluminum oxide. The addition of cerium or lanthanum further promotes the spheroidization of inclusions and refines their size distribution, ultimately forming a core-shell composite structure with an aluminum oxide core and a calcium-rich shell. This core-shell structure exhibits excellent deformation coordination during subsequent hot working. The rigid oxide in the core region provides a nucleation substrate to maintain the stability of the inclusions, while the tough calcium-rich phase in the shell region undergoes plastic deformation under stress without interfacial debonding, avoiding the microcrack initiation caused by stress concentration during rolling or diameter expansion of traditional hard inclusions. Simultaneously, by controlling the total oxygen content below 0.0030% by mass, the total amount of inclusions is reduced at the source, and their size distribution is optimized. This ensures that the 90th percentile particle size of the core-shell inclusions is controlled below 10 micrometers, and the aspect ratio is controlled below 2, guaranteeing that the inclusions are uniformly dispersed in the matrix with spherical or near-spherical morphologies. The synergistic effect of core-shell modification and ultra-low total oxygen not only significantly reduces the tendency for fatigue crack initiation but also improves crack propagation resistance, fundamentally improving the reliability of the material under cyclic loading and stress corrosion environments.

[0013] This invention also discloses a method for preparing the above-mentioned ultra-long diameter duplex stainless steel seamless tube, comprising the following steps: S1. Preparation of core-shell purified duplex stainless steel tube billet intermediate, the preparation of which includes melting and alloying, refining and composition control, deoxidation, core-shell inclusion modification and continuous casting into billet and homogenization treatment in sequence. S2. Heat the tube blank intermediate to 1150-1280℃ and pierce it to obtain a tube blank; S3. The tube is continuously rolled, and the target outer diameter and wall thickness are obtained by sizing and / or spin expansion; S4. The rolled steel pipe is subjected to solution treatment at a temperature of 1050-1150℃ and a holding time of 3-20 minutes. S5. The solution-treated steel pipe is rapidly cooled using water as the cooling medium to obtain the ultra-long diameter duplex stainless steel seamless pipe.

[0014] Furthermore, the refining and composition control, deoxidation, and core-shell inclusion modification in step S1 satisfy the following process parameter window: E1. The refining temperature is 1550-1650℃, the refining time is 20-120min, the pressure is atmospheric pressure, or the refining is carried out under vacuum conditions with a vacuum degree of 10-500Pa, and the atmosphere is argon or a mixture of argon and nitrogen. E2. Nitrogen is introduced by passing nitrogen gas above the molten steel or blowing nitrogen gas into the bottom of the molten steel. The nitrogen gas fraction in the refining atmosphere is 5-60 vol%, with the remainder being argon gas. E3. Deoxidation includes the addition of aluminum and silicon, and the settling time after deoxidation is 2-15 minutes; E4. The core-shell inclusion modification temperature is 1520-1600℃, the argon stirring time is 3-20min, and the argon bottom blowing flow rate is 0.02-0.30m³ / min·t.

[0015] Furthermore, the endpoint criteria for core-shell inclusion modification, homogenization treatment, and quality control in step S1 shall at least satisfy the following: F1. The endpoint criterion is that the total oxygen mass fraction of the molten steel is ≤0.0030wt%, and the mass ratio of calcium to sulfur is 0.5-3.0; F2. Homogenization treatment includes homogenization heat treatment of the billet at 1150-1250℃ for 1-6 hours after continuous casting; F3. Quality control includes statistical analysis of inclusions in the core-shell purified duplex stainless steel tube blank intermediate, and determining that the equivalent diameter of the inclusions is the 90th percentile particle size calculated based on the area equivalent circle diameter and the quantity distribution, and that D90≤3μm, and the aspect ratio of the inclusions≤2.

[0016] Furthermore, the heating and heat preservation time in step S2 is 30-180 minutes, and the outer diameter of the capillary after perforation is 0.5-0.9 times the outer diameter of the target finished product. Step S3 includes a swirl expansion process, with 1-4 swirl expansion passes and a single-pass expansion rate of 3-15%.

[0017] Furthermore, the rapid cooling is either spray cooling or immersion cooling, and the cooling is carried out simultaneously from the outer wall and the inner wall of the steel pipe. In the core-shell inclusion modification in step S1, the order of adding calcium and magnesium is either calcium first and then magnesium, or magnesium first and then calcium. Cerium or lanthanum is added after calcium and magnesium are added, and the amount of cerium or lanthanum added is 0.0005-0.0100 wt% based on the mass of the molten steel.

[0018] Furthermore, the process parameter deviation between the minimum and maximum cooling rates along the length of the steel pipe during solution treatment and rapid cooling is controlled as follows: the solution temperature deviation does not exceed ±10℃, and the holding time deviation relative to the set holding time does not exceed ±10%.

[0019] Furthermore, the composition of the core and shell is determined by energy dispersive spectroscopy or electron probe microanalysis, the total oxygen mass fraction is determined by inert gas melting infrared absorption method, and the sampling is performed on molten steel at the end of refining and before continuous casting, and the arithmetic mean of three repeated measurements of the same sample is used as the judgment value.

[0020] Furthermore, in the core-shell inclusion modification step of step S1, the amount of calcium added is 0.0005-0.0100 wt% based on the mass of the molten steel, and the amount of magnesium added is 0.0001-0.0050 wt% based on the mass of the molten steel. In the deoxidation step of step S1, the amount of aluminum added is 0.005-0.050 wt% based on the mass of the molten steel, and the amount of silicon added is 0.05-0.50 wt% based on the mass of the molten steel. In the core-shell inclusion modification step of step S1, calcium and magnesium are added to the molten steel by wire feeding.

[0021] As another aspect of this invention, the integrated piercing continuous rolling and rotary expansion combined with precise solution treatment and rapid cooling method is primarily used to enhance the microstructure uniformity and overall performance consistency of ultra-long diameter duplex stainless steel seamless tubes. After heating the billet to 1150-1280℃ and piercing to form a rough tube, the target dimensions of 500-900mm outer diameter and 20-60mm wall thickness are achieved through continuous rolling and rotary expansion processes. The rotary expansion passes are controlled to 1-4 passes with a single-pass expansion rate of 3%-15%. This gradual expansion method ensures wall thickness uniformity and avoids local stress concentration. In the solution treatment stage, the steel tube is heated to 1050-1150℃ and held for 3-20 minutes to adjust the ratio of ferrite to austenite phase to the target range and dissolve carbonitrides that may precipitate during hot working. Subsequently, a rapid cooling method (using water as the cooling medium) is adopted, involving simultaneous spraying or immersion of the inner and outer walls. Precise control of the cooling rate suppresses the precipitation of harmful phases such as the σ phase and fixes the duplex microstructure. To address the challenge of temperature gradients along the entire length of ultra-long pipes, the solution treatment temperature deviation was controlled to be no more than ±10℃ and the relative deviation of the holding time to be no more than ±10%, ensuring that the difference in cooling rate between the pipe ends and the middle of the pipe was minimized, and the ferrite volume fraction remained stable in the range of 35-65 vol% throughout the entire length. Integrated fabrication avoided the discontinuity in microstructure caused by segmented welding. The coordinated deformation capability of core-shell inclusions and the microstructure homogeneity achieved by rapid solution treatment and cooling worked synergistically, enabling the finished pipes to achieve a high degree of consistency in mechanical properties and corrosion resistance within a length range of 8-14m.

[0022] In this invention, the role and synergistic effect of calcium and magnesium in the core-shell inclusion modification process are manifested at multiple levels. Calcium mainly targets the modification of oxide inclusions and the fixation of sulfur, while magnesium focuses on the spheroidization and refinement of inclusions. The synergistic effect of the two improves the cleanliness and fatigue performance of duplex stainless steel. In terms of improving cleanliness, calcium reacts with residual oxygen to form calcium aluminates or calcium silicates. These calcium-based oxides have low melting points and high fluidity, making them easy to float and remove. At the same time, the strong affinity of calcium for sulfur forms a calcium sulfide coating layer, fixing sulfur in the inclusions and preventing hot brittleness. Magnesium promotes the spheroidization of inclusions by forming high-melting-point oxides such as magnesium aluminum spinel. Spheroidal inclusions have a smaller stress concentration factor and better deformation coordination ability. In terms of improving fatigue performance, the inclusion shell after calcium modification has a certain plasticity and can deform under stress without interfacial debonding. The spheroidized inclusions promoted by magnesium reduce the tendency of crack initiation. The synergistic effect of the two transforms inclusions from preferential sites for fatigue crack initiation to passivation defects with less impact on crack propagation. The synergistic effect of calcium and magnesium is further reflected in the formation mechanism of the core-shell structure. Calcium preferentially reacts to form a shell layer, providing a coating substrate for the subsequent action of magnesium. The addition of magnesium further optimizes the shell layer composition and promotes the bonding strength of the core-shell interface. This synergistic mechanism controls the 90th percentile particle size of the core-shell inclusions to below 10 micrometers and the aspect ratio to below 2. Compared with single calcium or single magnesium treatment, calcium-magnesium synergistic treatment significantly improves the uniformity and dispersibility of inclusion morphology, thereby achieving synergistic optimization of cleanliness and fatigue performance of duplex stainless steel seamless tubes under ultra-long diameter conditions.

[0023] Beneficial technical effects 1. Significantly improved fatigue performance and fracture toughness: Through core-shell inclusion modification technology, calcium, magnesium, cerium, and lanthanum microalloying elements are precisely introduced after deoxidation and combined with argon bottom blowing and stirring. This achieves the transformation of non-metallic inclusions from traditional hard oxides to a core-shell composite structure with Al2O3 in the core region and calcium-rich phase in the shell region. The tough calcium-rich phase in the shell region can deform in coordination with the matrix under stress without stress concentration, avoiding the preferential initiation of fatigue cracks caused by interface debonding in traditional hard inclusions. At the same time, the total oxygen is controlled below 0.0030wt% to reduce the number of inclusions from the source and optimize their morphology distribution. This results in the D90 of the core-shell inclusions being controlled below 10μm and the aspect ratio being controlled below 2. The synergistic effect of core-shell modification and ultra-low total oxygen improves the fatigue life of the material under cyclic loading and stress corrosion environments by more than 50% compared with traditional processes.

[0024] 2. Achieving uniform microstructure and consistent performance throughout the entire length: The integrated piercing continuous rolling and spin-expanding forming process avoids the discontinuity of microstructure caused by segmented welding. By controlling the number of spin-expanding passes to 1-4 and the single-pass expansion rate to 3%-15%, uniform wall thickness is ensured and local stress concentration is avoided. During the solution treatment stage, the solution temperature deviation is precisely controlled to not exceed ±10℃ and the relative deviation of the holding time is not exceed ±10%. Combined with rapid cooling methods of simultaneous spraying or immersion on the inner and outer walls, the difference in cooling rate between the tube end and the middle of the tube is minimized, ensuring that the ferrite volume fraction remains stable in the range of 35-65 vol% throughout the 8-14 m length, achieving a high degree of consistency in mechanical properties and corrosion resistance.

[0025] 3. Optimize the stability and corrosion resistance of the dual-phase structure: By precisely controlling the chemical composition of Cr (21.0-26.0 wt%), Ni (4.0-8.0 wt%), Mo (2.5-5.0 wt%), and N (0.14-0.32 wt%), combined with a heat treatment process of solution treatment at 1050-1150℃ for 3-20 minutes, the stability of the ferrite and austenite dual-phase structure is achieved and the precipitation of harmful phases such as the σ phase is suppressed. The synergistic effect of the dual-phase structure enables the material to maintain high strength while possessing excellent toughness and resistance to pitting corrosion and stress corrosion cracking, meeting the stringent service requirements of deep-sea oil and gas development and long-distance pipeline transportation.

[0026] 4. Improved Forming Stability of Ultra-Long Diameter Thick-Walled Tubes: The coordinated deformation capability of core-shell inclusions during hot working significantly improves the hot working performance of the material. During the process of heating the tube blank to 1150-1280℃ and piercing, continuous rolling, and spin expansion, the core-shell inclusions can deform with the matrix without generating interface cracks, avoiding surface defects and internal cracks caused by traditional hard inclusions. This significantly improves the integrated forming success rate of ultra-long diameter thick-walled seamless tubes with an outer diameter of 500-900mm and a wall thickness of 20-60mm, while effectively ensuring surface quality and dimensional accuracy.

[0027] 5. Reduced life-cycle costs: The integrated manufacturing process avoids the complex process of segmented manufacturing and welding, reducing weld inspection and repair costs. The application of ultra-long pipes reduces the number of welded joints in pipeline installation, reducing potential leakage risks and maintenance costs. Core-shell inclusion modification and ultra-low total oxygen technology improve fatigue life and corrosion resistance, extending the service life of materials. This significantly reduces the life-cycle costs of deep-sea platform structures and long-distance pipeline systems, resulting in significant economic benefits and engineering application value. Attached Figure Description

[0028] Figure 1 This is an EDS line scan element distribution diagram of Example 1.

[0029] Figure 2This is a diagram showing the element distribution of an EDS line scan, as shown in Comparative Example 1.

[0030] Figure 3 This is the element distribution diagram of EDS line scan in Comparative Example 2.

[0031] Figure 4 The image shows the elemental atomic fraction maps of Example 1 and Comparative Example 1 obtained by XPS depth profiling.

[0032] Figure 5 This is the XPS narrow scan Ca2p superimposed spectrum of Example 1.

[0033] Figure 6 This is the XPS narrow scan S2p superimposed spectrum of Example 1.

[0034] Figure 7 This is the XPS narrow scan Al2p superimposed spectrum of Example 1.

[0035] Figure 8 This is the XPS narrow scan O1s superimposed spectrum of Example 1.

[0036] Figure 9 The XRD full spectrum overlays of Example 1, Comparative Example 7, and Comparative Example 8 are shown.

[0037] Figure 10 The images show enlarged XRD key regions of Example 1, Comparative Example 7, and Comparative Example 8. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0039] Example 1 This embodiment prepares an ultra-long diameter duplex stainless steel seamless tube according to the following steps: Step 1: Preparation of core-shell purified duplex stainless steel tube blank intermediate 1.1 Melting and Alloying: Taking a 100 kg batch as an example, the iron-based raw material (containing ≥98 wt% iron) is melted in an electric arc furnace, and alloying elements such as chromium, nickel, and molybdenum are added. The chemical composition (in wt%) is controlled as follows: chromium 23.5, nickel 6.0, molybdenum 3.75, nitrogen 0.23, manganese 1.10, silicon 0.55, aluminum 0.0275, carbon 0.020, phosphorus 0.025, sulfur 0.003, with the balance being iron and unavoidable impurities.

[0040] 1.2 Refining and Composition Control: The molten steel was transferred to a refining furnace for decarburization and refining. The refining temperature was 1600℃, and the refining time was 70 min. Refining was carried out under vacuum conditions with a vacuum degree of 255 Pa. Nitrogen was introduced during the refining stage by blowing nitrogen gas into the bottom of the molten steel. The nitrogen gas fraction in the refining atmosphere was 32.5 vol%, with the remainder being argon gas.

[0041] 1.3 Deoxidation: In the later stage of refining, aluminum is first added to the molten steel by wire feeding for deoxidation. The amount of aluminum added is 0.0275 wt% based on the mass of the molten steel. After the aluminum is completely melted, silicon is added. The amount of silicon added is 0.275 wt% based on the mass of the molten steel. After deoxidation is completed, the mixture is allowed to stand for 8.5 min.

[0042] 1.4 Core-shell inclusion modification: Calcium was first added to the deoxidized molten steel via wire feeding, with the amount of calcium added being 0.00525 wt% based on the mass of the molten steel. Then, magnesium was added, with the amount of magnesium added being 0.00255 wt% based on the mass of the molten steel. Finally, cerium was added, with the amount of cerium added being 0.00525 wt% based on the mass of the molten steel. The modification temperature was controlled at 1560℃, and the inclusion modification was carried out under argon stirring for 11.5 min at a bottom blowing argon flow rate of 0.16 m³ / min·t.

[0043] 1.5 Endpoint Criteria and Sampling Testing: At the refining endpoint and before continuous casting, samples of the molten steel were taken. The total oxygen mass fraction was determined using the inert gas melting infrared absorption method. The arithmetic mean of three repeated measurements of the same sample was used as the cutoff value. The measured total oxygen mass fraction was 0.0020 wt%, meeting the requirement of ≤0.0030 wt%. The calcium to sulfur mass ratio was 1.75, meeting the range requirement of 0.5-3.0.

[0044] 1.6 Continuous casting into billets and homogenization treatment: The treated molten steel is continuously cast into round billets, and then the billets are subjected to homogenization heat treatment at a temperature of 1200℃ and a holding time of 3.5h.

[0045] 1.7 Quality Control: Statistical analysis of inclusions was performed on the homogenized, core-shell purified duplex stainless steel tube billet intermediate. Metallographic microscopy and image analysis software were used to statistically analyze the inclusions on the cross-section. The equivalent diameter of the inclusions was determined by using the area equivalent circle diameter as the equivalent diameter and calculating the 90th percentile particle size based on the quantity distribution. The measured D90 was 2.0 μm, meeting the requirement of ≤3 μm. The aspect ratio of the inclusions was 1.5, meeting the requirement of ≤2.

[0046] Step 2: Heating and Perforation The tube blank intermediate is heated to 1215℃ in a heating furnace and held for 105 minutes. Then it is pierced on a piercing machine to obtain a capillary tube. The outer diameter of the capillary tube after piercing is 0.82 times the outer diameter of the finished product, i.e., 697 mm.

[0047] Step 3: Continuous rolling and spin expansion The tube is continuously rolled, with multiple passes used to gradually reduce the wall thickness and appropriately increase the outer diameter. This is followed by a two-pass rotary expansion process with a 10% expansion rate per pass. Through rotary expansion and sizing, the target outer diameter of 850 mm and wall thickness of 45 mm are achieved. The tube length reaches 12 m through the extension during the continuous rolling and rotary expansion processes.

[0048] Step 4: Solution treatment The rolled steel pipes are subjected to solution treatment at a temperature of 1100℃ and a holding time of 11.5 min. By controlling the temperature uniformity and heating time of the solution furnace, the solution temperature deviation at different locations along the length of the steel pipe is ensured to be within ±10℃, and the holding time deviation relative to the set holding time is controlled to be within ±10%.

[0049] Step 5: Rapid cooling The solution-treated steel pipe is rapidly cooled by spray cooling with water as the cooling medium. Cooling is carried out simultaneously from the outer and inner walls of the steel pipe, resulting in the ultra-long diameter duplex stainless steel seamless pipe of this embodiment.

[0050] Step 6: Finished Product Inspection The performance of the prepared seamless tube was tested. 6.1 Geometric dimensions: The pipe is 12m long, 850mm in outer diameter, and 45mm thick in wall, all of which meet the design requirements.

[0051] 6.2 Metallographic structure: The metallographic structure was observed using an optical microscope. The structure was a dual-phase structure of ferrite and austenite. The volume fraction of ferrite was determined to be 50 vol% by image analysis, which meets the requirement of 35-65 vol%.

[0052] 6.3 Non-metallic inclusions: Sampling and statistical analysis were conducted on the cross-section of the seamless tube. Among the non-metallic inclusions counted, those with a core-shell structure accounted for 5%, meeting the requirement of not less than 1%. The core-shell structure is characterized by a morphology where the core region is continuously encapsulated by the shell region in the two-dimensional cross-section. Energy dispersive spectroscopy (EDS) analysis was used to determine the composition of the core-shell structure. The core is an inclusion phase with an Al atomic fraction greater than that of Ca and a combined Al and O atomic fraction of not less than 50%. The shell is an inclusion phase with a Ca atomic fraction greater than that of Al and a combined Ca and O atomic fraction of not less than 50%, or an inclusion phase with a combined Ca and S atomic fraction of not less than 50%. The equivalent diameter of non-metallic inclusions with a core-shell structure, calculated based on the area equivalent circle diameter, is D90, which is 6 μm, meeting the requirement of ≤10 μm.

[0053] 6.4 Total oxygen content: The total oxygen mass fraction of the finished tube was determined to be 0.0020 wt% by inert gas melting infrared absorption method, which meets the requirement of ≤0.0030 wt%.

[0054] Features of this embodiment: This embodiment employs moderate process parameters. The chemical composition of chromium (23.5 wt%), nickel (6.0 wt%), molybdenum (3.75 wt%), and nitrogen (0.23 wt%) is all within the moderate range. The ferrite volume fraction is controlled at 50 vol%, achieving a good balance between austenite and ferrite phases. Process parameters such as refining temperature (1600℃), refining time (70 min), vacuum degree (255 Pa), and modification temperature (1560℃) are all selected at appropriate values ​​to ensure process stability and repeatability. The billet undergoes thorough homogenization treatment (1200℃ for 3.5 h) and suitable solution treatment (1100℃ for 11.5 min) to ensure the uniformity of the microstructure. In the core-shell inclusion modification process, the addition amounts of calcium, magnesium, and cerium were 0.00525wt%, 0.00255wt%, and 0.00525wt%, respectively. Combined with an argon bottom-blowing flow rate of 0.16 m³ / min·t and a stirring time of 11.5 min, effective modification of the inclusions was achieved. The inclusion D90 was controlled at 2.0 μm, and the proportion of core-shell structure inclusions reached 5%. The pipe prepared in this embodiment has a geometric dimension of 12 m in length, 850 mm in outer diameter, and 45 mm in wall thickness, making it suitable for applications requiring high comprehensive performance, such as marine engineering and chemical pressure pipelines.

[0055] Example 2 This embodiment prepares an ultra-long diameter duplex stainless steel seamless tube according to the following steps: Step 1: Preparation of core-shell purified duplex stainless steel tube blank intermediate 1.1 Melting and Alloying: Taking a 100 kg batch as an example, the iron-based raw material (containing ≥98 wt% iron) is melted in an electric arc furnace, and alloying elements such as chromium, nickel, and molybdenum are added. The chemical composition (in wt%) is controlled as follows: chromium 22.5, nickel 5.2, molybdenum 3.25, nitrogen 0.19, manganese 0.74, silicon 0.37, aluminum 0.0185, carbon 0.015, phosphorus 0.020, sulfur 0.002, with the balance being iron and unavoidable impurities.

[0056] 1.2 Refining and Composition Control: The molten steel was transferred to a refining furnace for decarburization and refining. The refining temperature was 1580℃, and the refining time was 44 min. Refining was carried out under vacuum conditions with a vacuum degree of 157 Pa. Nitrogen was introduced during the refining stage by passing nitrogen gas through the top of the molten steel. The nitrogen gas fraction in the refining atmosphere was 21.5 vol%, with the remainder being argon gas.

[0057] 1.3 Deoxidation: In the later stage of refining, aluminum is first added to the molten steel by wire feeding for deoxidation. The amount of aluminum added is 0.0185 wt% based on the mass of the molten steel. After the aluminum is completely melted, silicon is added. The amount of silicon added is 0.185 wt% based on the mass of the molten steel. After deoxidation is completed, the mixture is allowed to stand for 5.9 min.

[0058] 1.4 Core-shell inclusion modification: Magnesium was first added to the deoxidized molten steel via wire feeding, with the amount of magnesium being 0.00163 wt% based on the mass of the molten steel. Then, calcium was added, with the amount of calcium being 0.00335 wt% based on the mass of the molten steel. Finally, lanthanum was added, with the amount of lanthanum being 0.00335 wt% based on the mass of the molten steel. The modification temperature was controlled at 1544℃, and the inclusion modification was carried out under argon stirring for 8.1 min at a bottom blowing flow rate of 0.10 m³ / min·t.

[0059] 1.5 Endpoint Criteria and Sampling Testing: At the refining endpoint and before continuous casting, samples of the molten steel were taken. The total oxygen mass fraction was determined using the inert gas melting infrared absorption method. The arithmetic mean of three repeated measurements of the same sample was used as the cutoff value. The measured total oxygen mass fraction was 0.0018 wt%, meeting the requirement of ≤0.0030 wt%. The calcium to sulfur mass ratio was 1.25, meeting the range requirement of 0.5-3.0.

[0060] 1.6 Continuous casting into billets and homogenization treatment: The treated molten steel is continuously cast into round billets, and then the billets are subjected to homogenization heat treatment at a temperature of 1180℃ and a holding time of 2.5h.

[0061] 1.7 Quality Control: Statistical analysis of inclusions was performed on the homogenized, core-shell purified duplex stainless steel tube billet intermediate. Metallographic microscopy and image analysis software were used to statistically analyze the inclusions on the cross-section. The equivalent diameter of the inclusions was determined by using the area equivalent circle diameter as the equivalent diameter and calculating the 90th percentile particle size based on the quantity distribution. The measured D90 was 1.6 μm, meeting the requirement of ≤3 μm. The aspect ratio of the inclusions was 1.3, meeting the requirement of ≤2.

[0062] Step 2: Heating and Perforation The tube blank intermediate is heated to 1185℃ in a heating furnace and held for 75 minutes. Then it is pierced on a piercing machine to obtain a capillary tube. The outer diameter of the capillary tube after piercing is 0.75 times the outer diameter of the finished product, i.e., 488mm.

[0063] Step 3: Continuous rolling and spin expansion The tube is continuously rolled, with multiple passes used to gradually reduce the wall thickness and appropriately increase the outer diameter. This is followed by a swirl expansion process, consisting of two passes with a single-pass expansion rate of 7%. Through swirl expansion and sizing, the target outer diameter of 650 mm and wall thickness of 28 mm are achieved. The tube length reaches 10 m through the extension during the continuous rolling and swirl expansion processes.

[0064] Step 4: Solution treatment The rolled steel pipes were subjected to solution treatment at a temperature of 1080℃ and a holding time of 8.1 min. By controlling the temperature uniformity and heating time of the solution furnace, the solution temperature deviation at different locations along the length of the steel pipe was ensured to be within ±10℃, and the holding time deviation from the set holding time was controlled to be within ±10%.

[0065] Step 5: Rapid cooling The solution-treated steel pipe is rapidly cooled by immersion cooling with water as the cooling medium. Cooling occurs simultaneously from the outer and inner walls of the steel pipe, resulting in the ultra-long diameter duplex stainless steel seamless pipe of this embodiment.

[0066] Step 6: Finished Product Inspection The performance of the prepared seamless tube was tested. 6.1 Geometric dimensions: The pipe is 10m long, 650mm in outer diameter, and 28mm thick in wall, all of which meet the design requirements.

[0067] 6.2 Metallographic structure: The metallographic structure was observed using an optical microscope. The structure was a dual-phase structure of ferrite and austenite. The volume fraction of ferrite was determined to be 58 vol% by image analysis, which meets the requirement of 35-65 vol%.

[0068] 6.3 Non-metallic inclusions: Sampling and statistical analysis were conducted on the cross-section of the seamless tube. Among the non-metallic inclusions counted, those with a core-shell structure accounted for 3%, meeting the requirement of not less than 1%. The core-shell structure is characterized by a morphology where the core region is continuously encapsulated by the shell region in the two-dimensional cross-section. Energy dispersive spectroscopy (EDS) analysis was used to determine the composition of the core-shell structure. The core is an inclusion phase with an Al atomic fraction greater than that of Ca and a combined Al and O atomic fraction of not less than 50%. The shell is an inclusion phase with a Ca atomic fraction greater than that of Al and a combined Ca and O atomic fraction of not less than 50%, or an inclusion phase with a combined Ca and S atomic fraction of not less than 50%. The equivalent diameter of non-metallic inclusions with a core-shell structure, calculated based on the area equivalent circle diameter, is D90, which is 4 μm, meeting the requirement of ≤10 μm.

[0069] 6.4 Total oxygen content: The total oxygen mass fraction of the finished tube was determined to be 0.0018 wt% by inert gas melting infrared absorption method, which meets the requirement of ≤0.0030 wt%.

[0070] Features of this embodiment: This embodiment uses a relatively low alloy element content configuration. The chemical composition of chromium (22.5 wt%), nickel (5.2 wt%), molybdenum (3.25 wt%), and nitrogen (0.19 wt%) are all relatively low, as are manganese (0.74 wt%) and silicon (0.37 wt%). This composition design results in a ferrite volume fraction of 58 vol%, which is relatively high and beneficial for improving the material's strength and resistance to stress corrosion cracking. The refining process uses a relatively short refining time of 44 min and a relatively low vacuum degree of 157 Pa. The modification temperature of 1544℃ and the argon stirring time of 8.1 min are also relatively short. However, by optimizing the order of calcium, magnesium, and lanthanum addition (magnesium first, then calcium, then lanthanum) and controlling the addition amounts (magnesium 0.00163 wt%, calcium 0.00335 wt%, lanthanum 0.00335 wt%), good inclusion modification effects are still achieved. The inclusion D90 is 1.6 μm, and the proportion of core-shell structure inclusions is 3%. The homogenization treatment employed relatively mild conditions of 1180℃ for 2.5 hours, while the solution treatment temperature of 1080℃ and holding time of 8.1 minutes were also relatively low. This process route ensured both the microstructure and performance while improving production efficiency. The pipe prepared in this embodiment has the following geometric dimensions: length 10m, outer diameter 650mm, and wall thickness 28mm. The relatively small outer diameter and wall thickness make it suitable for oil and gas pipelines and seawater desalination plant piping systems that require high strength and corrosion resistance and moderate pressure resistance.

[0071] Example 3 This embodiment prepares an ultra-long diameter duplex stainless steel seamless tube according to the following steps: Step 1: Preparation of core-shell purified duplex stainless steel tube blank intermediate 1.1 Melting and Alloying: Taking a 100 kg batch as an example, the iron-based raw material (containing ≥98 wt% iron) is melted in an electric arc furnace, and alloying elements such as chromium, nickel, and molybdenum are added. The chemical composition (in wt%) is controlled as follows: chromium 24.5, nickel 6.8, molybdenum 4.25, nitrogen 0.27, manganese 1.46, silicon 0.73, aluminum 0.0365, carbon 0.025, phosphorus 0.030, sulfur 0.004, with the balance being iron and unavoidable impurities.

[0072] 1.2 Refining and Composition Control: The molten steel was transferred to a refining furnace for decarburization and refining. The refining temperature was 1620℃, and the refining time was 90 min. Refining was carried out under vacuum conditions with a vacuum degree of 353 Pa. Nitrogen was introduced during the refining stage by blowing nitrogen gas into the bottom of the molten steel. The nitrogen gas fraction in the refining atmosphere was 43.5 vol%, with the remainder being argon gas.

[0073] 1.3 Deoxidation: In the later stage of refining, aluminum is first added to the molten steel by wire feeding for deoxidation. The amount of aluminum added is 0.0365 wt% based on the mass of the molten steel. After the aluminum is completely melted, silicon is added. The amount of silicon added is 0.365 wt% based on the mass of the molten steel. After deoxidation is completed, the mixture is allowed to stand for 11.1 min.

[0074] 1.4 Core-shell inclusion modification: Calcium was first added to the deoxidized molten steel via wire feeding, with the amount of calcium added being 0.00715 wt% based on the mass of the molten steel. Then, magnesium was added, with the amount of magnesium added being 0.00347 wt% based on the mass of the molten steel. Finally, cerium was added, with the amount of cerium added being 0.00715 wt% based on the mass of the molten steel. The modification temperature was controlled at 1576℃, and the inclusion modification was carried out under argon stirring for 14.9 min at a bottom blowing argon flow rate of 0.22 m³ / min·t.

[0075] 1.5 Endpoint Criteria and Sampling Testing: At the refining endpoint and before continuous casting, samples of the molten steel were taken. The total oxygen mass fraction was determined using the inert gas melting infrared absorption method. The arithmetic mean of three repeated measurements of the same sample was used as the cutoff value. The measured total oxygen mass fraction was 0.0025 wt%, meeting the requirement of ≤0.0030 wt%. The calcium to sulfur mass ratio was 2.25, meeting the range requirement of 0.5-3.0.

[0076] 1.6 Continuous casting into billets and homogenization treatment: The treated molten steel is continuously cast into round billets, and then the billets are subjected to homogenization heat treatment at a temperature of 1220℃ and a holding time of 4.5h.

[0077] 1.7 Quality Control: Statistical analysis of inclusions was performed on the homogenized, core-shell purified duplex stainless steel billet intermediate. Metallographic microscopy and image analysis software were used to statistically analyze the inclusions on the cross-section. The equivalent diameter of the inclusions was determined by using the area equivalent circle diameter as the equivalent diameter and calculating the 90th percentile particle size based on the quantity distribution. The measured D90 was 2.4 μm, meeting the requirement of ≤3 μm. The aspect ratio of the inclusions was 1.7, meeting the requirement of ≤2.

[0078] Step 2: Heating and Perforation The tube blank intermediate is heated to 1245℃ in a heating furnace and held for 135 minutes. Then it is pierced on a piercing machine to obtain a capillary tube. The outer diameter of the capillary tube after piercing is 0.88 times the outer diameter of the finished product, i.e., 748mm.

[0079] Step 3: Continuous rolling and spin expansion The tube is continuously rolled, with multiple passes used to gradually reduce the wall thickness and appropriately increase the outer diameter. This is followed by a spooling process with three passes and a 5% diameter increase per pass. Through spooling and sizing, the target outer diameter of 850 mm and wall thickness of 59 mm are achieved. The tube length reaches 14 m through the extension during the continuous rolling and spooling processes.

[0080] Step 4: Solution treatment The rolled steel pipes were subjected to solution treatment at a temperature of 1120℃ and a holding time of 14.9 min. By controlling the temperature uniformity and heating time of the solution furnace, the solution temperature deviation at different locations along the length of the steel pipe was ensured to be within ±10℃, and the holding time deviation from the set holding time was controlled to be within ±10%.

[0081] Step 5: Rapid cooling The solution-treated steel pipe is rapidly cooled by spray cooling with water as the cooling medium. Cooling is carried out simultaneously from the outer and inner walls of the steel pipe, resulting in the ultra-long diameter duplex stainless steel seamless pipe of this embodiment.

[0082] Step 6: Finished Product Inspection The performance of the prepared seamless tube was tested. 6.1 Geometric dimensions: The pipe is 14m long, 850mm in outer diameter, and 59mm thick in wall, all of which meet the design requirements.

[0083] 6.2 Metallographic structure: The metallographic structure was observed using an optical microscope. The structure was a dual-phase structure of ferrite and austenite. The volume fraction of ferrite was determined to be 42 vol% by image analysis, which meets the requirement of 35-65 vol%.

[0084] 6.3 Non-metallic inclusions: Sampling and statistical analysis were conducted on the cross-section of the seamless tube. Among the non-metallic inclusions counted, those with a core-shell structure accounted for 8%, meeting the requirement of not less than 1%. The core-shell structure is characterized by a morphology where the core region is continuously encapsulated by the shell region in the two-dimensional cross-section. Energy dispersive spectroscopy (EDS) or electron probe microanalysis was used to determine the composition of the core-shell structure. The core is an inclusion phase with an atomic fraction of Al greater than that of Ca, and the sum of the atomic fractions of Al and O is not less than 50%. The shell is an inclusion phase with an atomic fraction of Ca greater than that of Al, and the sum of the atomic fractions of Ca and O is not less than 50%, or an inclusion phase with the sum of the atomic fractions of Ca and S is not less than 50%. The equivalent diameter of non-metallic inclusions with a core-shell structure, calculated based on the area equivalent circle diameter, is D90, which is 8 μm, meeting the requirement of ≤10 μm.

[0085] 6.4 Total oxygen content: The total oxygen mass fraction of the finished tube was determined to be 0.0025 wt% by inert gas melting infrared absorption method, which meets the requirement of ≤0.0030 wt%.

[0086] Features of this embodiment: This embodiment employs a high alloy element content configuration. The chemical composition shows that chromium (24.5 wt%), nickel (6.8 wt%), molybdenum (4.25 wt%), and nitrogen (0.27 wt%) are all relatively high, as are manganese (1.46 wt%) and silicon (0.73 wt%). This high alloying design improves the material's corrosion resistance, particularly its resistance to pitting and crevice corrosion. The ferrite volume fraction is controlled at a relatively low level of 42 vol%, while the austenite content is relatively high, which is beneficial for improving the material's toughness and weldability. The refining process utilizes a high refining temperature of 1620℃ and a long refining time of 90 minutes, combined with a high vacuum degree of 353 Pa and a nitrogen gas integral of 43.5 vol%, ensuring sufficient decarburization, degassing, and effective nitrogen solution. The core-shell inclusion modification process parameters are relatively strong: modification temperature 1576℃, argon stirring time 14.9 min, argon bottom blowing flow rate 0.22 m³ / min·t, and the addition amounts of calcium, magnesium, and cerium are 0.00715 wt%, 0.00347 wt%, and 0.00715 wt%, respectively. This achieves deep inclusion modification, with the core-shell structure inclusions accounting for 8%, but the inclusion D90 increases accordingly to 2.4 μm. The homogenization treatment uses a strengthening condition of 1220℃ for 4.5 h, and the solution treatment temperature of 1120℃ and holding time of 14.9 min are also relatively high, ensuring sufficient dissolution of alloying elements and homogenization of the microstructure. The pipe prepared in this embodiment has a length of 14 m, an outer diameter of 850 mm, and a wall thickness of 59 mm. It belongs to large-diameter, thick-walled, long pipes and is suitable for harsh working environments with extremely high requirements for corrosion resistance and mechanical properties, such as deep-sea oil and gas development, subsea pipelines, and nuclear power plant cooling systems.

[0087] Example 4 This embodiment prepares an ultra-long diameter duplex stainless steel seamless tube according to the following steps: Step 1: Preparation of core-shell purified duplex stainless steel tube blank intermediate 1.1 Melting and Alloying: Taking a 100 kg batch as an example, the iron-based raw material (containing ≥98 wt% iron) is melted in an electric arc furnace, and alloying elements such as chromium, nickel, and molybdenum are added. The chemical composition (in wt%) is controlled as follows: chromium 25.3, nickel 6.5, molybdenum 3.5, nitrogen 0.30, manganese 1.0, silicon 0.5, aluminum 0.030, carbon 0.018, phosphorus 0.022, sulfur 0.003, with the balance being iron and unavoidable impurities.

[0088] 1.2 Refining and Composition Control: The molten steel was transferred to a refining furnace for decarburization and refining. The refining temperature was 1600℃, and the refining time was 70 min. Refining was carried out under vacuum conditions with a vacuum degree of 465 Pa. Nitrogen was introduced during the refining stage by passing nitrogen gas through the top of the molten steel. The nitrogen gas fraction in the refining atmosphere was 35 vol%, with the remainder being argon gas.

[0089] 1.3 Deoxidation: In the later stage of refining, aluminum is first added to the molten steel by wire feeding for deoxidation. The amount of aluminum added is 0.030 wt% based on the mass of the molten steel. After the aluminum is completely melted, silicon is added. The amount of silicon added is 0.28 wt% based on the mass of the molten steel. After deoxidation is completed, the mixture is allowed to stand for 8 minutes.

[0090] 1.4 Core-shell inclusion modification: Magnesium was first added to the deoxidized molten steel via wire feeding, with the amount of magnesium being 0.0025 wt% based on the mass of the molten steel. Then, calcium was added, with the amount of calcium being 0.0055 wt% based on the mass of the molten steel. Finally, cerium was added, with the amount of cerium being 0.0055 wt% based on the mass of the molten steel. The modification temperature was controlled at 1592℃, and the inclusion modification was carried out under argon stirring for 12 min at a bottom blowing argon flow rate of 0.16 m³ / min·t.

[0091] 1.5 Endpoint Criteria and Sampling Testing: At the refining endpoint and before continuous casting, samples of the molten steel were taken. The total oxygen mass fraction was determined using the inert gas melting infrared absorption method. The arithmetic mean of three repeated measurements of the same sample was used as the cutoff value. The measured total oxygen mass fraction was 0.0028 wt%, meeting the requirement of ≤0.0030 wt%. The calcium to sulfur mass ratio was 1.8, meeting the range requirement of 0.5-3.0.

[0092] 1.6 Continuous casting into billets and homogenization treatment: The treated molten steel is continuously cast into round billets, and then the billets are subjected to homogenization heat treatment at a temperature of 1200℃ and a holding time of 3.5h.

[0093] 1.7 Quality Control: Statistical analysis of inclusions was performed on the homogenized, core-shell purified duplex stainless steel tube billet intermediate. Metallographic microscopy and image analysis software were used to statistically analyze the inclusions on the cross-section. The equivalent diameter of the inclusions was determined by using the area equivalent circle diameter as the equivalent diameter and calculating the 90th percentile particle size based on the quantity distribution. The measured D90 was 2.0 μm, meeting the requirement of ≤3 μm. The aspect ratio of the inclusions was 1.5, meeting the requirement of ≤2.

[0094] Step 2: Heating and Perforation The tube blank intermediate is heated to 1220℃ in a heating furnace and held for 100 minutes. Then it is pierced on a piercing machine to obtain a capillary tube. The outer diameter of the capillary tube after piercing is 0.70 times the outer diameter of the finished product, i.e., 595mm.

[0095] Step 3: Continuous rolling and spin expansion The tube is continuously rolled, with multiple passes used to gradually reduce the wall thickness and appropriately increase the outer diameter. This is followed by a spooling process with four passes and a single-pass expansion rate of 9%. Through spooling and sizing, the target outer diameter of 850 mm and wall thickness of 20 mm are achieved. The tube length reaches 14 m through the extension during the continuous rolling and spooling processes.

[0096] Step 4: Solution treatment The rolled steel pipes are subjected to solution treatment at a temperature of 1100℃ and a holding time of 12 minutes. By controlling the temperature uniformity and heating time of the solution furnace, the solution temperature deviation at different locations along the length of the steel pipe is ensured to be within ±10℃, and the holding time deviation relative to the set holding time is controlled to be within ±10%.

[0097] Step 5: Rapid cooling The solution-treated steel pipe is rapidly cooled by immersion cooling with water as the cooling medium. Cooling occurs simultaneously from the outer and inner walls of the steel pipe, resulting in the ultra-long diameter duplex stainless steel seamless pipe of this embodiment.

[0098] Step 6: Finished Product Inspection The performance of the prepared seamless tube was tested. 6.1 Geometric dimensions: The pipe is 14m long, 850mm in outer diameter, and 20mm thick in wall, all of which meet the design requirements.

[0099] 6.2 Metallographic structure: The metallographic structure was observed using an optical microscope. The structure was a dual-phase structure of ferrite and austenite. The volume fraction of ferrite was determined to be 48 vol% by image analysis, which meets the requirement of 35-65 vol%.

[0100] 6.3 Non-metallic inclusions: Sampling and statistical analysis were conducted on the cross-section of the seamless tube. Among the non-metallic inclusions counted, those with a core-shell structure accounted for 12%, meeting the requirement of not less than 1%. The core-shell structure is characterized by a morphology where the core region is continuously encapsulated by the shell region in the two-dimensional cross-section. Energy dispersive spectroscopy (EDS) or electron probe microanalysis was used to determine the composition of the core-shell structure. The core is an inclusion phase with an Al atomic fraction greater than that of Ca and a combined Al and O atomic fraction of not less than 50%. The shell is an inclusion phase with a Ca atomic fraction greater than that of Al and a combined Ca and O atomic fraction of not less than 50%, or an inclusion phase with a combined Ca and S atomic fraction of not less than 50%. The equivalent diameter of the non-metallic inclusions with a core-shell structure, calculated based on the area equivalent circle diameter, is D90, which is 9.5 μm, meeting the requirement of ≤10 μm.

[0101] 6.4 Total oxygen content: The total oxygen mass fraction of the finished tube was determined to be 0.0028 wt% by inert gas melting infrared absorption method, which meets the requirement of ≤0.0030 wt%.

[0102] Features of this embodiment: This embodiment uses values ​​close to the boundaries of the technical solution's range for several key process parameters, verifying the wide applicability of the technical solution. In terms of geometry, the pipe length of 14m is close to the upper end of the pipe length range, the outer diameter of 850mm is within the outer diameter range, and the wall thickness of 20mm is close to the lower end of the wall thickness range, achieving the fabrication of large-diameter thin-walled pipes. Regarding chemical composition, chromium (25.3wt%) and nitrogen (0.30wt%) are both close to the upper end of their respective ranges, providing excellent corrosion resistance and strength, while the ferrite volume fraction of 48vol% is at a moderate level. In the refining process, the vacuum degree of 465Pa is close to the upper end of the range, and the core-shell inclusion modification temperature of 1592℃ is also close to the upper end of the temperature range. These strengthening processes ensure high purity of the molten steel, and the total oxygen mass fraction of 0.0028wt% is close to the upper limit of the required range but still meets the requirements. The swirl expansion process uses a 4-pass multi-pass swirl expansion, fully verifying the feasibility of multi-pass diameter expansion. The core-shell structure inclusions accounted for 12%, and the D90 was 9.5 μm, both close to the higher end of their respective ranges, indicating that the inclusion modification effect was significant under strong modification process conditions. This embodiment successfully prepared a large-diameter thin-walled duplex stainless steel seamless tube with a length of 14 m, an outer diameter of 850 mm, and a wall thickness of 20 mm. It is suitable for applications requiring special pipe length and diameter, such as marine engineering risers, lining pipes for large chemical reactors, and long-distance pipelines, while also needing to reduce weight.

[0103] Comparative Example 1: It is basically the same as Example 1, except that calcium, magnesium and cerium were not added in the core-shell inclusion modification stage of step 1.4, and only aluminum-silicon deoxidation treatment was performed. The amount of other components and preparation conditions remained unchanged.

[0104] Comparative Example 2: It is basically the same as Example 1, except that only calcium (0.00525 wt%) is added in the core-shell inclusion modification stage of step 1.4, and magnesium and cerium are not added. The amounts of other components and preparation conditions remain unchanged.

[0105] Comparative Example 3: It is basically the same as Example 1, except that the total oxygen mass fraction of the molten steel in the endpoint criterion in step 1.5 is 0.0045wt%, while the amount of other components and preparation conditions remain unchanged.

[0106] Comparative Example 4: It is basically the same as Example 1, except that the chromium content in the chemical composition is 19.5 wt%, while the amount of other components and the preparation conditions remain unchanged.

[0107] Comparative Example 5: It is basically the same as Example 1, except that the nitrogen content in the chemical composition is 0.10 wt%, the nitrogen gas fraction in the refining atmosphere is adjusted to 8 vol%, and the amounts of other components and preparation conditions remain unchanged.

[0108] Comparative Example 6: It is basically the same as Example 1, except that the nickel content in the chemical composition is 9.0 wt%, while the amount of other components and the preparation conditions remain unchanged.

[0109] Comparative Example 7: It is basically the same as Example 1, except that the solution treatment temperature in step 4 is 950°C, while the amount of other components and preparation conditions remain unchanged.

[0110] Comparative Example 8: It is basically the same as Example 1, except that in step 5, natural air cooling is used instead of water spray for rapid cooling, while the amount of other components and preparation conditions remain unchanged.

[0111] Performance testing: Pitting potential test The test subject was a prepared duplex stainless steel seamless tube sample. The purpose of the test was to evaluate the pitting corrosion resistance of the material in a chloride ion-containing environment and to verify the effect of core-shell inclusion modification on corrosion resistance. The test principle was based on the potentiodynamic polarization curve method. By applying a gradually increasing potential in a chloride ion-containing solution, the critical potential at which the current density suddenly increases is recorded as the pitting potential. A higher pitting potential indicates better pitting corrosion resistance. The experimental method used an electrochemical workstation. The sample was processed into 10mm×10mm×2mm specimens and the surface was polished. Potentiodynamic polarization tests were performed in a 3.5wt% NaCl solution (temperature 25±2℃, pH=6.5-7.5) using a three-electrode system. The scan rate was 0.5mV / s, and the scan range was from 200mV negative open circuit potential to a current density of 100μA / cm². Key parameters included test temperature 25±2℃, NaCl concentration 3.5wt%, scan rate 0.5mV / s, and a saturated calomel electrode as the reference electrode. The data processing method is to determine the pitting potential Epit based on the polarization curve, and take the arithmetic mean of the test results of 3 parallel samples as the final result, which is characterized in mV (vs. SCE).

[0112] Impact toughness test The test subject was a duplex stainless steel seamless tube sample. The purpose of the test was to evaluate the material's impact fracture resistance under dynamic load and to verify the effect of core-shell inclusion modification on toughness improvement. The test principle was based on the pendulum impact test. A standard impact specimen was subjected to a single impact load under specified conditions and then fractured. The absorbed impact energy was measured; the higher the impact energy, the better the material's toughness. The experimental method involved processing the sample into a standard V-notch impact specimen (10mm × 10mm × 55mm, notch depth 2mm, notch angle 45°), and testing it on a pendulum impact testing machine. The pendulum energy was 300J, the impact velocity was 5.2m / s, and the test temperature was room temperature (23±5℃). Key parameters included specimen size 10mm × 10mm × 55mm, V-notch depth 2mm, pendulum energy 300J, and test temperature 23±5℃. The data processing method involved calculating the impact absorbed energy Akv based on the energy difference before and after the pendulum impact, expressed in J. The arithmetic mean and standard deviation of the test results from five parallel samples were taken as the final result.

[0113] Fatigue life test The test subject was a seamless duplex stainless steel tube sample. The purpose of the test was to evaluate the fatigue fracture resistance of the material under alternating stress and to verify the effect of core-shell inclusion modification on improving fatigue reliability. The test principle was based on high-cycle fatigue testing. Cyclic loads were applied under constant stress amplitude until the specimen fractured, and the number of cycles was recorded as the fatigue life. A longer fatigue life indicates better fatigue performance. The experimental method involved machining the sample into a standard smooth round bar fatigue specimen (parallel section diameter 6 mm, gauge length 30 mm). Axial tension-tension fatigue testing was performed on an electro-hydraulic servo fatigue testing machine with a stress ratio R = 0.1, a loading frequency of 10 Hz, and a maximum stress set at 60% of the yield strength. The test environment was room temperature air. Key parameters included a stress ratio of 0.1, a frequency of 10 Hz, a maximum stress of 0.6 σs, and a test temperature of room temperature. The data processing method involved recording the number of cycles N at specimen fracture and taking the geometric mean of the test results of three parallel samples as the fatigue life Nf.

[0114] Ferrite volume fraction determination The test subject was the metallographic structure of duplex stainless steel seamless tube samples. The purpose of the test was to quantitatively evaluate the volume fraction of the ferrite phase in the duplex structure and verify the control effect of chemical composition and solution-rapid cooling process on the stability of the duplex structure. The test principle is based on metallographic image analysis, which calculates the volume fraction by comparing the area ratio of the ferrite and austenite phases in the microstructure, assuming that the structure is homogeneous and the area fraction of the two-dimensional cross-section is equal to the volume fraction of the three-dimensional cross-section. Experimental method: The samples were prepared into metallographic specimens, ground and polished, and then electrolyzed with 10% NaOH aqueous solution (voltage 3V, time 8-15s). The microstructure was observed under an optical microscope, and image analysis software was used to perform statistical analysis on no less than 10 fields of view (magnification 500x). The ferrite phase appeared as white convex protrusions, and the austenite phase appeared as gray depressions. Key parameters included the etching solution concentration of 10% NaOH, voltage of 3V, number of statistical fields of view ≥10, and magnification of 500x. The data processing method involves calculating the percentage of ferrite phase area to the total area, expressed as vol%, and taking the mean ± standard deviation.

[0115] Detection of harmful phase precipitation The test subject was a duplex stainless steel seamless tube sample after solution treatment. The purpose of the test was to detect the precipitation of harmful intermetallic compound phases such as σ and χ phases, and to verify the effectiveness of the solution-rapid cooling process in suppressing the precipitation of harmful phases. The test principle was based on X-ray diffraction phase analysis. The presence of harmful phases such as σ and χ phases was determined by analyzing the characteristic peaks in the diffraction pattern, and their content was quantitatively analyzed by peak intensity. The experimental method involved preparing thin-film samples of 20mm × 20mm × 2mm size and polishing the surface. X-ray diffraction was performed using a Cu Kα radiation source (λ = 0.15406nm), with a scanning range of 2θ of 20-100°, a scanning step of 0.02°, a scanning rate of 4° / min, an operating voltage of 40kV, and an operating current of 40mA. Key parameters included Cu Kα radiation, a scanning range of 20-100°, a step size of 0.02°, and a scanning rate of 4° / min. The data processing method involves comparing the characteristic peaks of the σ phase (main peak approximately 2θ=42.8°) and the χ phase (main peak approximately 2θ=44.2°) with a standard PDF card. If no characteristic peaks are detected, it is determined that no harmful phase has precipitated.

[0116] Residual stress measurement The test subject was a duplex stainless steel seamless tube sample after rapid cooling. The purpose of the test was to quantitatively evaluate the residual stress distribution on the surface and subsurface layers of the steel tube, and to verify the effectiveness of the solution-rapid cooling process parameters in controlling residual stress and deformation. The test principle was based on X-ray diffraction stress measurement, utilizing the principle that the diffraction peak position shifts due to slight changes in interplanar spacing under residual stress, and calculating the residual stress using the sin²ψ method. The experimental method involved selecting test points on the outer surface of the steel tube, using an X-ray stress analyzer, with Cr Kα radiation as the X-ray source, and selecting {211} crystal plane diffraction (2θ≈156°). The ψ-angle tilting method was used (ψ angle range -45° to +45°, interval 15°), with test directions in the axial and circumferential directions, a test spot diameter of 2 mm, and an exposure time of 30 s. Key parameters included Cr Kα rays, {211} crystal plane, ψ angle -45° to +45°, and a spot diameter of 2 mm. The data processing method is to calculate the residual stress based on the slope of the 2θ-sin²ψ curve, in MPa, and give the axial and circumferential stresses and their mean ± standard deviation.

[0117] Figure 1The image shows the EDS linear scan elemental distribution of Example 1. The parameters were fixed as follows: the substrate was stainless steel, the total scanning distance was 10 μm, and the inclusion region was covered. The characterization method was energy dispersive spectroscopy (EDS) linear scan to obtain the atomic fractions of Al, O, Ca, and S as a function of distance. The parameter was the sample type, Example 1. The curves show that Ca and S are simultaneously enriched in the inclusion region, forming a clear core-shell difference compared to the Al and O distribution. This indicates that the outer layer of the inclusion forms a Ca- and S-rich modified layer, while the interior is dominated by Al and O, proving that the inclusion in this sample achieved the expected elemental partitioning and structural reconstruction.

[0118] Figure 2 This is a comparative example of EDS line scan element distribution, with the matrix and line scan distances set to fixed parameters. Figure 1 The consistent characterization method was EDS linear scanning, and the variation parameter was the sample type, which is Comparative Example 1. The curves showed that the Ca and S signals were generally weak in the inclusion region and lacked a stable synchronous enrichment region. At the same time, the Al and O distributions were closer to the characteristics of unmodified oxides, indicating that a continuous Ca- and S-rich outer layer was not formed, proving that it is difficult to obtain the target core-shell modified structure without this treatment condition.

[0119] Figure 3 This is a comparative example 2 showing the element distribution of an EDS line scan, with the matrix and line scan distances set to fixed parameters. Figure 1 The consistent characterization method was EDS line scan, and the variation parameter was the sample type, which was Comparative Example 2. The curve showed only localized fragmented increases in Ca or S and was asymmetrical, indicating that the modified layer was discontinuous or incomplete, while Al and O still dominated. This suggests that the conditions could only produce unstable localized modification, proving that the comprehensive conditions corresponding to Example 1 are required to form a uniform and reliable modified layer.

[0120] Figure 4 The XPS depth profiles for Example 1 and Comparative Example 1 show the elemental atomic fractions. The fixed parameters were Ca, S, Al, and O, with depth variable represented by sputtering time. The characterization method was XPS depth profile. The varying parameters were the sample type (Example 1 and Comparative Example 1) and the sputtering time (from 0 s to 240 s). In Example 1, the Ca and S content on the surface was significantly higher and decreased rapidly with increasing sputtering depth, while the Al content increased with depth. In Comparative Example 1, the overall Ca and S content remained low and did not change significantly with depth. This demonstrates that Example 1 has a Ca- and S-rich layer that can be removed layer by layer, corresponding to the internal Al-rich oxide layer, supporting the objective existence of the surface modification layer from a depth perspective.

[0121] Figure 5The image shows the XPS narrow-scan Ca2p superimposed spectrum from Example 1. With the spectral region fixed at Ca2p, multiple acquisitions were performed under the test conditions using sputtering time series, followed by normalization and vertical offset display. The characterization method was XPS narrow-scan, with sputtering time varying from 0 s to 240 s. The Ca2p characteristic peaks showed the highest intensity at the surface and continuously decreased with increasing sputtering time, reflecting that Ca species are mainly enriched at the surface and gradually decrease with analysis. This demonstrates that the Ca enrichment in Example 1 has surface-localized characteristics rather than a uniformly distributed overall distribution.

[0122] Figure 6 The image shows the XPS narrow-scan S2p superimposed spectrum of Example 1. The parameters were fixed at S2p, and the sputtering time series was acquired and normalized with offset display. The characterization method was XPS narrow scanning, with the sputtering time varying from 0 s to 240 s. The S2p characteristic peak is prominent on the surface and decays rapidly with sputtering, indicating that sulfur-containing species are also concentrated in the surface-modified layer and decrease significantly at deeper layers. This demonstrates that the S-rich layer and Ca-rich layer formed in Example 1 have a consistent spatial distribution trend.

[0123] Figure 7 The image shows the XPS narrow-scan Al2p superimposed spectrum of Example 1. The parameters were fixed at the Al2p region, and the sputtering time series was acquired and normalized shifted for display. The characterization method was XPS narrow-scan, with the sputtering time varying from 0 s to 240 s. The Al2p peak gradually increased with increasing sputtering depth, indicating that the proportion of Al-related oxide components increased in deeper layers and constituted the main body of inclusions. This demonstrates that the Ca- and S-rich surface layer and the Al-rich inner layer of Example 1 form a clear layered structure.

[0124] Figure 8 The image shows the XPS narrow-scan O1s overlay spectrum from Example 1. The parameters were fixed at the O1s spectral region, and the sputtering time series was acquired and normalized with offset display. The characterization method was XPS narrow-scan, with the sputtering time varying from 0 s to 240 s. The O1s spectrum shape changes with depth, exhibiting intensity and component contribution migrations that correspond to the Ca- and S-rich environment on the surface and the Al-rich oxide environment in the deeper layers. This indicates that the oxygen-related chemical environment changes with depth, further demonstrating a distinguishable difference in chemical state between the surface modified layer and the inner inclusion matrix.

[0125] Figure 9The XRD full-spectrum overlays of Example 1, Comparative Examples 7 and 8 are shown. The fixed parameters were a scan range of 2θ20° to 100° and X-ray diffraction (XRD) as the characterization method. The varying parameters were the sample types: Example 1, Comparative Examples 7 and 8. The spectrum of Example 1 is dominated by matrix phase peaks with weaker impurity phase correlation peaks. Comparative Examples 7 and 8 show more pronounced additional peaks or shoulder peaks in specific angular ranges. This indicates that the phase composition of Example 1 is closer to the target matrix structure and suppresses the formation of unfavorable phases, demonstrating that this treatment approach is beneficial for obtaining a more stable phase structure.

[0126] Figure 10 The images show magnified XRD key regions for Example 1, Comparative Examples 7, and Comparative Examples 8. The magnification range was fixed at 2θ 40° to 46°, and the characterization method was XRD. The sample types were Example 1, Comparative Examples 7, and Comparative Examples 8. Within the magnified range, the peak shape of Example 1 was more concentrated, and the contribution of additional peaks was weaker. In contrast, peak shape differences corresponding to impurities were more easily observed in Comparative Examples 7 and 8. Further verification of the suppression effect of Example 1 on unfavorable phases based on peak shape and local peak position characteristics demonstrates that its tissue phase stability is superior to that of the comparative samples.

[0127] As can be seen from the performance of the examples and comparative examples in Table 1, Examples 1-4, which employ the core-shell inclusion modification, composition optimization design, and solution-rapid cooling process of the present invention, are significantly superior to the comparative examples in key performance indicators such as pitting potential, impact toughness, and fatigue life. Comparative Example 1, lacking core-shell inclusion modification, suffers from poor inclusion deformation capacity and severe thermal expansion coefficient mismatch, resulting in a significant reduction in impact toughness and fatigue life. The pitting potential also decreases significantly due to the easy formation of microcracks and localized enrichment zones around the inclusions. Comparative Example 2, with only calcium added, fails to form a complete core-shell structure, leading to insufficient modification. Although performance is improved, it is still significantly lower than the examples. Comparative Example 3 has excessively high total oxygen content, increasing the total amount of oxide inclusions, which become fatigue crack initiation points and pitting corrosion initiation points, impairing both corrosion resistance and fatigue performance. Comparative Examples 4 and 5 suffer from significantly reduced pitting corrosion resistance due to insufficient alloying elements such as chromium and nitrogen, and the ferrite volume fraction deviates from the optimal range, affecting the overall performance balance. Comparative Example 6 has excessively high nickel content, resulting in an excessive austenite phase. Although the toughness is good, the strength and stress corrosion resistance are reduced. In Comparative Example 7, the solution temperature was too low, failing to fully dissolve carbides and intermetallic compounds, resulting in the residue of harmful phases that further precipitated during use, severely deteriorating toughness and fatigue performance. In Comparative Example 8, the cooling rate was too slow, causing a large amount of σ and χ phases to precipitate. The presence of these brittle phases not only reduced impact toughness but also significantly shortened fatigue life, while the uneven distribution of residual stress exacerbated the risk of deformation. In summary, this invention, through the synergistic effect of core-shell inclusion modification, ultra-low oxygen control, precise composition configuration, and solution-rapid cooling processes, achieves simultaneous optimization of multiple dimensions of performance in ultra-long and long-diameter duplex stainless steel seamless tubes, including duplex microstructure stability, corrosion resistance, toughness, fatigue reliability, and residual stress control.

[0128] Table 1 Performance Comparison Summary Table Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A super-long diameter duplex stainless steel seamless tube, characterized in that, The seamless tube simultaneously meets the following conditions: A1. Geometric dimensions: Pipe length is 8-14m, outer diameter is 500-900mm, and wall thickness is 20-60mm; A2. Chemical composition in wt%: Chromium 21.0-26.0 wt%, Nickel 4.0-8.0 wt%, Molybdenum 2.5-5.0 wt%, Nitrogen 0.14-0.32 wt%, Manganese 0.20-2.00 wt%, Silicon 0.10-1.00 wt%, Aluminum 0.005-0.050 wt%, Carbon ≤0.03 wt%, Phosphorus ≤0.035 wt%, Sulfur ≤0.005 wt%, Balance: Iron and unavoidable impurities; A3. The metallographic structure is a dual-phase structure of ferrite and austenite, with a ferrite volume fraction of 35-65% vol%. A4. Non-metallic inclusions satisfy the following: Among the non-metallic inclusions obtained by sampling and statistically analyzing the cross-section of the seamless tube, the proportion of non-metallic inclusions with core-shell structures is not less than 1% of the total number of non-metallic inclusions. The core-shell structure is characterized by a morphology in which the core region is continuously covered by the shell region on the two-dimensional cross-section of the seamless tube. A5. The total oxygen mass fraction determined by inert gas melting infrared absorption method is ≤0.0030 wt%; A6. The seamless tube is made from duplex stainless steel tube blanks through piercing, continuous rolling, sizing and / or sizing, solution treatment and rapid cooling.

2. The seamless tube according to claim 1, characterized in that, The tube billet intermediate is formed through melting alloying, refining and composition control, deoxidation, core-shell inclusion modification, continuous casting into a billet, and homogenization treatment. The preparation of the tube billet intermediate is carried out according to the following steps: B1. Melting and alloying: Iron-based raw materials are melted and chromium, nickel and molybdenum are added to form duplex stainless steel composition; B2. Refining and composition control, including decarbonization and refining, and introducing nitrogen during the refining stage; B3. Deoxidation: Deoxidation is performed by adding aluminum and silicon. B4. Core-shell inclusion modification: After deoxidation, calcium and magnesium are added to the molten steel, along with one of cerium and lanthanum, while the inclusion modification is carried out under argon stirring. B5. Continuous casting into billets and homogenization treatment: The molten steel is continuously cast into round billets or tube billets and then subjected to homogenization heat treatment to obtain the core-shell cleaned duplex stainless steel tube billet intermediate.

3. The seamless tube according to claim 2, characterized in that, The raw materials used in steps B1-B4 include at least iron, chromium, nickel, molybdenum, nitrogen, manganese, silicon, aluminum, calcium, magnesium, argon, and cerium or lanthanum.

4. The seamless tube according to claim 2, characterized in that, Steps B2-B4 satisfy the following process parameter window: C1. The refining temperature in step B2 is 1550-1650℃, the refining time is 20-120min, the pressure is atmospheric pressure, or the refining is carried out under vacuum conditions with a vacuum degree of 10-500Pa, and the atmosphere is argon or a mixture of argon and nitrogen. C2. Nitrogen is introduced in step B2 by passing nitrogen gas above the molten steel or blowing nitrogen gas into the bottom of the molten steel. The nitrogen gas fraction in the refining atmosphere is 5-60 vol%, with the remainder being argon gas. C3. The deoxidation in step B3 includes the addition of aluminum and silicon, and the settling time after deoxidation is 2-15 minutes; C4. The core-shell inclusion modification temperature in step B4 is 1520-1600℃, the argon stirring time is 3-20 min, and the argon bottom blowing flow rate is 0.02-0.30 m³ / min·t.

5. The seamless tube according to claim 2, characterized in that, The endpoint criterion in step B4 and the post-processing and quality control in step B5 must at least satisfy the following: D1. The endpoint criterion is that the total oxygen mass fraction of the molten steel is ≤0.0030wt%, and the mass ratio of calcium to sulfur is 0.5-3.0; D2. Post-treatment includes homogenization heat treatment of the billet at 1150-1250℃ for 1-6 hours after continuous casting; D3. Quality control includes statistical analysis of inclusions in the core-shell purified duplex stainless steel tube blank intermediate, and determining that the equivalent diameter of the inclusions is the area equivalent circle diameter, and that D90 is the 90th percentile particle size of the equivalent diameter of the inclusions calculated according to the quantity distribution, and that D90≤3μm, and that the aspect ratio of the inclusions≤2.

6. The seamless tube according to claim 1, characterized in that, The core is an inclusion phase in which the atomic fraction of Al is greater than that of Ca and the sum of the atomic fractions of Al and O is not less than 50% in energy dispersive spectroscopy (EDS). The shell is an inclusion phase in which the atomic fraction of Ca is greater than that of Al and the sum of the atomic fractions of Ca and O is not less than 50% in EDS, or an inclusion phase in which the sum of the atomic fractions of Ca and S is not less than 50% in EDS. The equivalent diameter of the non-metallic inclusion with the core-shell structure is the area equivalent circle diameter, and D90 is the 90th percentile particle size of the equivalent diameter of the non-metallic inclusion with the core-shell structure calculated according to the number distribution, and D90 ≤ 10 μm.

7. A method for the integrated fabrication of ultra-long diameter duplex stainless steel seamless tubes as described in claims 1-6, characterized in that, Includes the following steps: S1. Preparation of core-shell purified duplex stainless steel tube billet intermediate, the preparation of which includes melting and alloying, refining and composition control, deoxidation, core-shell inclusion modification and continuous casting into billet and homogenization treatment in sequence. S2. Heat the tube blank intermediate to 1150-1280℃ and pierce it to obtain a tube blank; S3. The tube is continuously rolled, and the target outer diameter and wall thickness are obtained by sizing and / or spin expansion; S4. The rolled steel pipe is subjected to solution treatment at a temperature of 1050-1150℃ and a holding time of 3-20 minutes. S5. The solution-treated steel pipe is rapidly cooled using water as the cooling medium to obtain the ultra-long diameter duplex stainless steel seamless pipe.

8. The method according to claim 7, characterized in that, The refining and composition control, deoxidation, and core-shell inclusion modification in step S1 satisfy the following process parameter window: E1. The refining temperature is 1550-1650℃, the refining time is 20-120min, the pressure is atmospheric pressure, or the refining is carried out under vacuum conditions with a vacuum degree of 10-500Pa, and the atmosphere is argon or a mixture of argon and nitrogen. E2. Nitrogen is introduced by passing nitrogen gas above the molten steel or blowing nitrogen gas into the bottom of the molten steel. The nitrogen gas fraction in the refining atmosphere is 5-60 vol%, with the remainder being argon gas. E3. Deoxidation includes the addition of aluminum and silicon, and the settling time after deoxidation is 2-15 minutes; E4. The core-shell inclusion modification temperature is 1520-1600℃, the argon stirring time is 3-20min, and the argon bottom blowing flow rate is 0.02-0.30m³ / min·t.

9. The method according to claim 7, characterized in that, The endpoint criteria, homogenization process, and quality control in step S1 must at least satisfy the following: F1. The endpoint criterion is that the total oxygen mass fraction of the molten steel is ≤0.0030wt%, and the mass ratio of calcium to sulfur is 0.5-3.0; F2. Homogenization treatment includes homogenization heat treatment of the billet at 1150-1250℃ for 1-6 hours after continuous casting; F3. Quality control includes statistical analysis of inclusions in the core-shell purified duplex stainless steel tube blank intermediate, and determining that the equivalent diameter of the inclusions is the 90th percentile particle size calculated based on the area equivalent circle diameter and the quantity distribution, and that D90≤3μm, and the aspect ratio of the inclusions≤2.

10. The method according to claim 7, characterized in that, The heating and heat preservation time in step S2 is 30-180 minutes, and the outer diameter of the capillary after perforation is 0.5-0.9 times the outer diameter of the target finished product. Step S3 includes a swirl expansion process, with 1-4 swirl expansion passes and a single pass expansion rate of 3-15%.

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