An acid service type heavy wall seamless pipe and a method of making the same

Abstract

The present application relates to alloy pipe preparation technical field, specifically a kind of acid service type thick-walled seamless pipe and its preparation method.The present application designs 14MnVE low alloy system with Fe as matrix.The system uses the economic alloy design without Mo, Nb, while ensuring the cost advantage of material, ensure the hot working adaptability of thick-walled seamless pipe.Through Ca-S synergistic control and the addition of rare earth elements, realize the dual effect of inclusion modification and grain boundary optimization, overcome the technical problem that economic X65QS pipeline pipe considers performance and cost under thick-walled specification.At the same time, using rolling and synergistic quenching combined process, obtain the uniform structure high-performance seamless pipe product.And corrosion-resistant layer is prepared on the surface of seamless pipe, further improve the performance of seamless pipe.In summary, the prepared seamless pipe has excellent comprehensive performance, so it has broad application prospect in alloy pipe preparation technical field.

Description

Technical Field

[0001] This invention relates to the field of alloy pipe manufacturing technology, specifically to an acid-resistant, service-ready thick-walled seamless pipe and its manufacturing method. Background Technology

[0002] In today's energy industry, especially in the development and transportation of highly corrosive oil and gas resources, acid-resistant pipeline steel pipes, as the core pressure-bearing components of the transportation system, are key materials driving technological progress in the industry. However, traditional X65QS thick-walled seamless pipeline pipes still face a series of mutually restrictive technical bottlenecks in meeting increasingly demanding service environments, large-diameter, and thick-walled market requirements.

[0003] First, to ensure resistance to hydrogen-induced cracking and basic mechanical strength in acidic environments, traditional formulations generally rely on adding expensive alloying elements such as Mo and Nb, directly leading to high production costs and making it difficult to achieve economical mass production of thick-walled specifications. Second, in terms of inclusion control, traditional processes are mostly limited to simply controlling sulfur content, failing to effectively regulate sulfide morphology. This causes strip-shaped inclusions to extend longitudinally during rolling, making them not only a preferential source of hydrogen-induced cracking but also resulting in significantly lower transverse impact toughness than longitudinal toughness, causing obvious anisotropy in mechanical properties. Furthermore, due to the significant heat conduction and deformation gradients during rolling and heat treatment of thick-walled tubes (wall thickness ≥ 20 mm), traditional processes struggle to avoid the problem of uneven microstructure between the surface and core (fine grains on the surface, coarse grains in the core), directly affecting the homogeneity of mechanical properties across the entire cross-section. Moreover, because thick-walled tubes operate under acidic conditions for extended periods, they are easily corroded by acidic chemicals, damaging the matrix structure and leading to a decline in material properties. These problems are intertwined, collectively restricting product specification expansion and performance improvement.

[0004] To overcome the shortcomings of the prior art, the present invention provides an acid-resistant service-grade thick-walled seamless tube and its preparation method. Summary of the Invention

[0005] The purpose of this invention is to provide an acid-resistant, service-ready, thick-walled, seamless tube and its preparation method, in order to solve the problems raised in the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A thick-walled seamless tube for acid-resistant service has the following chemical composition by mass percentage: C: 0.10-0.16wt%, Si: 0.20-0.40wt%, Mn: 1.15-1.40wt%, P≤0.020wt%, S≤0.002wt%, Ca: 0.0014-0.0020wt%, Ca / S: 2.5-6.0, V: 0.07-0.09wt%, Cr: 0.07-0.13wt%, Ce: 0.002-0.005wt%, Cu≤0.20wt%, Ni≤0.30wt%, Al≤0.045wt%, with the balance being Fe and unavoidable impurities; the carbon equivalent CE is 0.37-0.42wt%, and the weld crack susceptibility index Pcm is 0.20-0.22wt%.

[0007] The seamless tube adopts a 14MnVE low-alloy system with Fe as the matrix, without adding Mo or Nb alloying elements. Mn, V, and Cr are used as the main alloying elements, and the carbon equivalent CE is controlled at 0.37-0.42wt% and the weld crack sensitivity index Pcm is controlled at 0.20-0.22wt%, taking into account the mechanical performance requirements and weldability of thick-walled seamless tubes. In addition, rare earth element Ce is added to the composition, Ce: 0.002-0.005wt%, based on the following design principles: First, grain boundary purification effect, Ce preferentially agglomerates at grain boundaries, adsorbs and fixes harmful impurities such as S and P, effectively reducing the sensitivity of grain boundary hydrogen-induced cracking; Second, grain refinement effect, Ce can inhibit the coarsening of austenite grains during heating and deformation, laying the foundation for obtaining a fine-grained structure in subsequent heat treatment; Third, inclusion modification effect, the first three points can significantly refine the grains of the material, purify the matrix structure, improve the purity of the structure, and ensure the strength and impact toughness of the material.

[0008] In a more optimized manner, the metallographic structure of the seamless tube matrix is ​​tempered sorbite, or a small amount of ferrite may also be present; according to the GB / T 10561 inclusion rating standard, the morphology of sulfide inclusions in the matrix is ​​D-class spherical inclusions, the size of the sulfide inclusions is ≤15μm, and they are uniformly dispersed in the matrix; the production wall thickness of the seamless tube is 20mm-35mm, and the production outer diameter is 210mm-325mm; the specifications of the seamless tube after finishing are: outer diameter 219.1-323.9mm, wall thickness 22.23-33.32mm, specifically any one of 273.1mm×25.4mm, 273.1mm×28.58mm, 219.1mm×22.23mm, and 323.9mm×33.32mm, where all specifications adopt uniform composition and process parameters.

[0009] The seamless tube exhibits a relatively optimized yield strength ≥495MPa and a yield-to-tensile ratio controlled at ≤0.84; a full-size longitudinal impact energy at 0℃ of 300-325J, with a transverse and longitudinal impact toughness deviation ≤6%; and no HIC cracks were observed during hydrogen-induced cracking tests conducted according to NACE TM0284 standards. Its resistance to sulfide hydrogen-induced cracking meets the requirements of the X65QS steel grade in the API 5L standard, and no hydrogen-induced cracking was observed after immersion in an acidic environment with an H2S volume fraction of 0.01%-1.0% for 72 hours.

[0010] A method for manufacturing acid-resistant service-grade thick-walled seamless tubes involves using a solid round billet as raw material, the chemical composition of which is consistent with that of the tube. The billet undergoes heating, piercing, controlled rolling, and tempering heat treatment to obtain a seamless tube. The billet is heated in a stepped manner, with the final temperature reaching 1260±10℃. After piercing at 1170-1200℃, the temperature is lowered to 980-1100℃ for rolling. During the rolling process, dynamic recrystallization is used to refine the austenite grains, resulting in a shaped seamless tube. The shaped tube is then subjected to tempering heat treatment via quenching and tempering to obtain the seamless tube.

[0011] In a more optimized manner, a stepped heating method is used: the round billet is heated in 7 temperature zones, with zone 1 at 1120-1140℃, zone 2 at 1200-1220℃, zone 3 at 1240-1250℃, zone 4 at 1250-1260℃, zone 5 at 1260-1270℃, zone 6 at 1265-1275℃, and zone 7 at 1270-1280℃.

[0012] By using a stepped heating process, uneven temperature distribution of the raw material for thick-walled seamless tubes can be effectively avoided during the heating process. At the same time, by precisely controlling the initial rolling temperature and combining it with the precipitation strengthening effect of V element, the uniform refinement of austenite grains can be further promoted, effectively improving the overall microstructure uniformity of the alloy billet.

[0013] In a more optimized manner, during the quenching and tempering heat treatment stage, the quenching heating temperature is 920-960℃, water quenching is performed, the water quenching cooling rate is 30-50℃ / s, the tempering heating temperature is 620-680℃, the tempering holding time is 2.5-3.0h, and after the holding time is completed, the furnace is cooled to room temperature to obtain a seamless tube.

[0014] The quenching temperature of 920-960℃ can effectively match the austenitizing temperature of the 14MnVE alloy system, ensuring that the carbides and vanadium carbonitrides in the matrix are fully dissolved; after tempering, the microstructure is transformed into tempered sorbite, or a fine-grained microstructure with a small amount of ferrite, achieving microstructure uniformity from the surface to the core of the seamless tube.

[0015] In a more optimized manner, the surface of the seamless tube is polished and washed sequentially to obtain a pretreated seamless tube; the pretreated seamless tube is first subjected to laser cladding using Fe-Cr-Si metal powder; then electrophoretic deposition is performed using silicon carbide dispersion to obtain the finished seamless tube.

[0016] In a more optimized manner, iron powder, chromium powder, silicon powder, carbon powder, nickel powder, and cobalt powder are mixed and ball-milled at a speed of 110-120 r / min for 1.5-2.0 h to obtain Fe-Cr-Si metal powder; silicon carbide ceramic powder and polyvinyl butyral are added to acetone, stirred evenly, and then n-butylamine is added and stirred continuously to obtain silicon carbide dispersion.

[0017] In a more optimized manner, the content of each component of the Fe-Cr-Si metal powder is as follows: by mass fraction, 18-23% chromium powder, 4-7% silicon powder, 2-3% carbon powder, 3.5-4.5% nickel powder, 3.5-4.5% cobalt powder, with the balance being iron powder; the concentration of silicon carbide ceramic powder in the silicon carbide dispersion is 35-40 g / L, and the concentration of polyvinyl butyral is 6-8 g / L.

[0018] The optimized laser cladding process parameters are as follows: laser power 1.6-2.0kW, laser scanning speed 550-600mm / min, overlap rate 50%, nitrogen is used for powder feeding, the powder feeding rate is controlled at 11.0-11.5g / min, and argon is used as the protective gas. The seamless tube treated by the laser cladding process is polished, alkali washed to remove oil, acid washed to activate, and alcohol washed, and then used as the anode and graphite plate as the cathode. The two electrodes are placed in a silicon carbide dispersion for electrophoretic deposition to obtain the finished seamless tube. The power supply voltage is 25-35V, the deposition time is 10-12min, and the temperature is 25-30℃.

[0019] The beneficial effects of this invention are: This invention focuses on three core requirements for acid service (resistance to H2S hydrogen-induced cracking and high toughness), economy (Mo and Nb-free alloys), and adaptability to thick-walled forming (wall thickness ≤35mm), and designs a 14MnVE low alloy system based on Fe.

[0020] This alloy system employs an economical design free of Mo and Nb, effectively reducing material costs and ensuring good compatibility between the composition and the hot forming of thick-walled seamless pipes. Furthermore, by precisely controlling the carbon equivalent (CE) and weld crack sensitivity index (Pcm), the steel pipe can be guaranteed to have excellent field weldability, meeting the construction requirements of pipeline projects.

[0021] This alloy system, through the design of a calcium-sulfur synergistic regulation system and the joint optimization of rare earth elements, achieves dual optimization of inclusion modification and grain boundaries. Specifically, calcium treatment combined with sulfur content control can transform harmful linear inclusions into dispersed spherical deformed inclusions with a size ≤15μm, uniformly dispersed in the matrix. This fundamentally reduces the initiation source of hydrogen-induced cracking and significantly improves the steel pipe's resistance to sulfide stress corrosion cracking. Simultaneously, this optimization effect results in a breakthrough improvement in the material's transverse impact toughness. Furthermore, the added rare earth element Ce plays a crucial grain boundary purification role, inhibiting the segregation of harmful elements such as sulfur and phosphorus at grain boundaries, thereby further enhancing the steel pipe's resistance to hydrogen-induced cracking and its service reliability in acidic environments. This optimized design systematically achieves a balance between performance, cost, and thick-wall formability from two key aspects: inclusion morphology control and grain boundary purification.

[0022] This system successfully solved the technical bottleneck of balancing thick-walled specifications and low cost in economical X65QS pipeline pipes. By developing a large-diameter, thick-walled seamless pipe specification system covering outer diameters of 219.1-323.9 mm and wall thicknesses of 22.23-33.32 mm, and adopting unified low-cost component design and process parameters, stable mass production of thick-walled seamless pipes that balance economy and acid service performance (meeting standards such as API 5L) has been achieved. These pipes can be directly applied to high-pressure acidic gathering and transmission pipelines, effectively meeting engineering requirements and reducing overall costs.

[0023] Furthermore, this invention utilizes a composite process of rolling and synergistic tempering to produce seamless pipes with a fine-grained tempered sorbite microstructure across the entire cross-section. This uniform and dense microstructure, free from delamination, ensures excellent adaptability to thick walls. Moreover, the pipe exhibits a yield strength Rt0.5 ≥ 495 MPa and a tensile strength ≥ 590 MPa, consistently meeting the mechanical property standards of X65QS steel grade. With a yield-to-tensile ratio controlled at approximately 0.84, it combines high strength with good plasticity, effectively avoiding the risk of brittle fracture due to stress concentration in acidic service environments. Simultaneously, the seamless pipe demonstrates excellent toughness at low temperatures, with a full-size longitudinal impact energy of ≥ 300 J at 0°C, and balanced transverse and longitudinal impact toughness, effectively adapting to low-temperature impact conditions during pipeline transportation and construction.

[0024] Furthermore, this invention prepares a silicon carbide ceramic layer on the surface of a seamless tube via electrophoretic deposition. SiC is a high-hardness, high-density ceramic material with extremely low permeability to acidic corrosive media. When a complete, defect-free SiC ceramic layer is formed on the steel surface, it can effectively isolate the steel substrate from the corrosive environment. However, seamless tubes typically have a high coefficient of thermal expansion, while silicon carbide ceramic layers with good corrosion resistance typically have a very low coefficient of thermal expansion. Therefore, when the two are directly bonded, enormous thermal stress is generated during temperature changes (especially cooling or thermal cycling), leading to coating cracking and peeling. Therefore, a transition layer, Fe-Cr-Si alloy, is incorporated, with a coefficient of thermal expansion between the seamless tube substrate and the silicon carbide ceramic layer, thus establishing a gentle gradient transition zone of thermal expansion between them. By adding silicon powder to the Fe-Cr-Si alloy transition layer, the silicon powder, which has a very low coefficient of thermal expansion, is dissolved into the Fe-Cr matrix, effectively lowering the alloy's coefficient of thermal expansion and achieving a relatively stable change in the coefficient of thermal expansion, thereby mitigating thermal stress. Meanwhile, the main component of the Fe-Cr-Si layer, Fe, is in the same system as the steel substrate, enabling effective metallurgical bonding with extremely strong adhesion. Therefore, the outer SiC ceramic layer, with its high hardness and high chemical inertness, serves as the first active barrier against corrosion. The inner Fe-Cr-Si alloy layer not only buffers thermal stress but also possesses good corrosion resistance due to its high chromium content, thus acting as a second line of defense against corrosion.

[0025] Furthermore, by setting the mass fraction of silicon powder in the Fe-Cr-Si alloy transition layer to 4-7%, this invention ensures a smooth transition in the coefficient of thermal expansion of the transition layer while avoiding the sacrifice of material toughness and process reliability due to excessive silicon powder addition. This achieves a balance between thermal compatibility and mechanical integrity, guaranteeing the long-term structural stability of the composite coating under thermal cycling conditions. Otherwise, when the silicon content is below 4%, the decrease in the alloy's coefficient of thermal expansion is insufficient, and a significant difference in the coefficient of thermal expansion remains between the transition layer and the upper silicon carbide ceramic layer. This large step change means that high residual thermal stress will still be generated at the interface during temperature fluctuations, leading to a decrease in coating bonding strength and even premature cracking or peeling of the ceramic layer, severely affecting the protective effect. Conversely, when the silicon content exceeds 7%, although the coefficient of thermal expansion of the alloy further decreases and the compatibility with the ceramic layer is better, excessive silicon content will lead to the formation of excessive brittle metal silicide phases in the alloy. These hard and brittle phases significantly deteriorate the plasticity and toughness of the transition layer, making it prone to microcracks when subjected to thermal or mechanical stress. This not only weakens the load-bearing capacity of the transition layer itself, but the cracks may also extend to the ceramic layer, leading to the failure of the protective system. Attached Figure Description

[0026] Figure 1 This is a typical metallographic structure of the seamless tube of the present invention; Figure 2 This is a distribution diagram of inclusions in the seamless tube matrix of the present invention. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The technical solution of the present invention will be described below with reference to Example 1 and Comparative Examples 1-3. The alloy design systems of Example 1 and Comparative Examples 1-3 are shown in Table 1 (wt%).

[0029] Table 1

[0030] Example 1: Steel was smelted according to the elemental composition system of Example 1 in Table 1. The chemical composition by mass percentage was as follows: C: 0.13wt%, Mn: 1.28wt%, V: 0.08wt%, Ca: 0.0014wt%, P: 0.018wt%, S: 0.00056wt%, Ca / S: 2.5, Ce: 0.0035wt%, Si: 0.3wt%, Cu: 0.1wt%, Ni: 0.3wt%, Cr: 0.1wt%, Al: 0.03wt%; the balance was Fe and unavoidable impurities; carbon equivalent CE = 0.39, and welding crack sensitivity index Pcm = 0.21.

[0031] The molten steel is smelted according to the above system, and then cast into round billets. The alloy billets are heated in a furnace using a stepped heating method. After reaching 1080℃, they are rolled with an initial rolling temperature of 1070℃. During the rolling process, dynamic recrystallization is used to refine the austenite grains, resulting in a shaped steel pipe. The shaped steel pipe is then subjected to a synergistic tempering process using quenching and tempering to obtain a seamless pipe. The stepped heating temperature is as follows: Zone 1: 1120℃, Zone 2: 1200℃, Zone 3: 1250℃, Zone 4: 1255℃, Zone 5: 1266℃, Zone 6: 1270℃, and Zone 7: 1275℃.

[0032] Quenching and tempering heat treatment: The formed steel pipe is quenched at 940℃, and after heating, it is water quenched at a cooling rate of 40℃ / s, and then tempered at 640℃ for 2.5h. After the tempering is completed, it is cooled to 25℃ in the furnace to obtain a seamless pipe. The specifications of the seamless pipe are: outer diameter 273.1mm, wall thickness 28.58mm; the sulfide inclusions are spherical, with a size ≤8μm, and are uniformly dispersed.

[0033] Comparative Examples 1, 2, and 3 were smelted according to the elemental composition system of the corresponding examples in Table 1. The molten steel was cast into round billets and then processed to obtain seamless tubes. The subsequent process parameters and product specifications were consistent with those of Example 1.

[0034] Table 2

[0035] Analysis: The mechanical property test results of Example 1 are shown in Table 2. According to the NACE TM0284 standard, hydrogen-induced cracking test was conducted. No HIC cracks were found. The SSC resistance meets the requirements of API 5L standard X65QS steel grade. The batch-to-batch mechanical property deviation is 3.2%, which fully meets the requirements of economy and performance.

[0036] As shown in Tables 1-2, the significant difference between Comparative Example 1 and Example 1 in terms of elemental composition design is whether rare earth element Ce is added, and the insufficient Ca content leads to a lower Ca / S ratio in Comparative Example 1 compared to Example 1. Although the final strength is comparable, the longitudinal and transverse impact energy of Comparative Example 1 is lower than that of Example 1, and the longitudinal and transverse impact energy deviation reaches 11%, which is significantly higher than that of Example 1.

[0037] As shown in Tables 1-2, the significant difference in elemental composition design between Comparative Example 2 and Example 1 is whether rare earth element Ce is added and whether the S content is too high. The high S content is the main reason for the excessive size and elongated shape of the sulfide inclusions, which is manifested as a decrease in longitudinal and transverse impact energy and a significant increase in the longitudinal and transverse impact energy deviation, reaching 13%.

[0038] As shown in Tables 1-2, compared with Example 1, Comparative Example 3 has a lower amount of Ce and Ca added in its elemental composition design, while the S content is too high. Ce and Ca have limited effect on improving inclusions. At the same time, the high S content results in unsatisfactory morphology, quantity, and distribution of inclusions in the matrix. This is manifested in a reduction in longitudinal and transverse impact energy, and a large deviation in longitudinal and transverse impact energy, reaching 8%, which is inferior to Example 1.

[0039] In addition, a Mo and Nb-containing X65QS thick-walled seamless pipeline was designed as Comparative Example 4. The chemical composition by mass percentage is as follows: C: 0.14wt%, Mn: 1.30wt%, V: 0.08wt%, Si: 0.03wt%, S: 0.0018wt%, Mo: 0.25wt%, Nb: 0.05wt%, P: 0.019wt%, with the remainder being Fe and unavoidable impurities; carbon equivalent CE = 0.45, and welding crack sensitivity index Pcm = 0.24. The core difference from Example 1 is the addition of Mo and Nb noble alloys, while Ca and Ce elements are not added.

[0040] Preparation process: The same controlled rolling, quenching and tempering process parameters as in Example 1 were used (step heating, initial rolling temperature 1070℃, quenching at 940℃, water quenching, and tempering at 650℃ for 2.5h). Product specifications: consistent with Example 1.

[0041] Performance test results: Yield strength Rt0.5 = 515 MPa, tensile strength = 615 MPa; 0℃ full-size transverse impact energy = 302 J, longitudinal impact energy = 345 J, transverse and longitudinal impact toughness deviation = 12.5% ​​(significantly higher than the longitudinal and transverse impact toughness deviation of 5% in this invention); sulfide inclusions are distributed in strips (different from the spherical distribution in this invention); hydrogen-induced cracking test was conducted according to NACE TM0284 standard, no HIC cracks were found, and the SSC resistance meets the requirements of API 5L standard X65QS steel grade. However, due to the addition of Mo and Nb precious alloys, the production cost is 18% higher than that of Example 1, and the microstructure segregation on the inner wall of the thick-walled tube is more severe, which is inferior to Example 1.

[0042] As can be seen from the test results of Example 1 and Comparative Example 4 above, Example 1 of the present invention strictly follows the core design of "alloy without adding Mo and Nb". Compared with Comparative Example 4 which adds Mo and Nb, the production cost is significantly reduced, achieving the economic design goal. At the same time, through Ca-S synergistic regulation, Ce microalloying and precise control of carbon equivalent, the technical problems of large deviation in transverse and longitudinal impact toughness, poor inclusion morphology and microstructure segregation in the traditional Comparative Example 4 are solved, taking into account both performance stability and practicality. The test methods for mechanical properties in Table 2 are as follows:

[0043] Mechanical property testing: Yield strength, tensile strength, and yield ratio are tested according to GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test method at room temperature".

[0044] Impact toughness test: 0℃ full-size transverse impact energy, longitudinal impact energy, and transverse and longitudinal impact toughness deviation refer to GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials".

[0045] Example 2: 61.5% iron powder, 23% chromium powder, 6% silicon powder, 2.5% carbon powder, 3.5% nickel powder, and 3.5% cobalt powder were mixed by mass fraction and ball-milled at 115 r / min for 1.7 h to obtain Fe-Cr-Si metal powder. Silicon carbide ceramic powder and polyvinyl butyral were added to acetone, stirred evenly, and then n-butylamine was added and stirred continuously to obtain a silicon carbide dispersion. The concentration of silicon carbide ceramic powder in the silicon carbide dispersion was 35 g / L, and the concentration of polyvinyl butyral was 8 g / L.

[0046] The surface of the seamless tube prepared in Example 1 was polished and washed in sequence to obtain a pretreated seamless tube. The pretreated seamless tube was then subjected to laser cladding using Fe-Cr-Si metal powder. The laser cladding process parameters were: laser power 1.8kW, laser scanning speed 570mm / min, overlap rate 50%, nitrogen was used for powder feeding, the powder feeding rate was controlled at 11.3g / min, and argon was used as the protective gas.

[0047] The seamless tube, after being processed by laser cladding, is polished, degreased by alkali washing, activated by acid washing, and washed with alcohol. It is then used as the anode and a graphite plate as the cathode. The two electrodes are placed in a silicon carbide dispersion for electrophoretic deposition to obtain the finished seamless tube. The power supply voltage is 30V, the deposition time is 11min, and the temperature is 27℃.

[0048] Corrosion resistance test: The seamless tubes prepared in Example 1 and Example 2 were placed in a 30wt% hydrochloric acid etching solution and immersed at 100℃ for 120h. After the etching process, the degree of corrosion of the seamless tubes was tested. The test results are shown in Table 3.

[0049] Table 3

[0050] Example 2 is a seamless tube product obtained by first laser cladding and then electrophoretic deposition of the seamless tube prepared in Example 1. The laser cladding bottom layer provides a corrosion-resistant transition layer, and then the electrophoretically deposited SiC layer tightly covers the surface of the cladding layer, forming a continuous, dense ceramic layer that is resistant to strong acids. Therefore, the corrosion rate of Example 2 after hydrochloric acid corrosion is 2.55 mm / year, which is significantly lower than that of Example 1.

Claims

1. A thick-walled seamless tube for acid-resistant service, characterized in that: The chemical composition by mass percentage is as follows: C: 0.10-0.16wt%, Si: 0.20-0.40wt%, Mn: 1.15-1.40wt%, P≤0.020wt%, S≤0.002wt%, Ca: 0.0014-0.0020wt%, Ca / S: 2.5-6.0, V: 0.07-0.09wt%, Cr: 0.07-0.13wt%, Ce: 0.002-0.005wt%, Cu≤0.20wt%, Ni≤0.30wt%, Al≤0.045wt%, with the balance being Fe and unavoidable impurities; the carbon equivalent CE is 0.37-0.42wt%, and the weld crack susceptibility index Pcm is 0.20-0.22wt%.

2. The acid-resistant, service-ready, thick-walled seamless tube according to claim 1, characterized in that: The metallographic structure of the seamless tube matrix is ​​tempered sorbite, or ferrite may also be present; the production thickness of the seamless tube is 20mm-35mm, and the production outer diameter is 210mm-325mm; according to the GB / T 10561 inclusion rating standard, the morphology of the sulfidation inclusions in the matrix is ​​Class D spherical inclusions, the size of the sulfidation inclusions is ≤15μm, and they are uniformly dispersed in the matrix.

3. The acid-resistant, service-ready, thick-walled seamless tube according to claim 2, characterized in that: Yield strength ≥495MPa, yield strength ratio controlled ≤0.84; full-size longitudinal impact energy at 0℃: 300-325J, transverse and longitudinal impact toughness deviation ≤6%; hydrogen-induced cracking test according to NACE TM0284 standard, no HIC cracks; resistance to sulfide hydrogen-induced cracking meets the requirements of X65QS steel grade in API 5L standard, no hydrogen-induced cracking phenomenon after immersion in an acidic environment with H2S volume fraction of 0.01%-1.0% for 72h.

4. A method for preparing the acid-resistant, service-ready thick-walled seamless tube according to any one of claims 1-3, characterized in that: Using solid round billets as raw materials, the chemical composition of the billets is consistent with that of the seamless tubes. The round billets are heated, pierced, rolled under controlled conditions, and tempered to obtain seamless tubes. The billets are heated in a stepped manner, and the billets are finally heated to 1260±10℃. After piercing at 1170-1200℃, the temperature is reduced to 980-1100℃ for rolling. During the rolling process, dynamic recrystallization is used to refine the austenite grains to obtain the shaped seamless tubes. The shaped tubes are then tempered and quenched to obtain seamless tubes.

5. The method for preparing an acid-resistant, service-ready thick-walled seamless tube according to claim 4, characterized in that: Stepped heating: The round billet is heated in 7 temperature zones: Zone 1 is 1120-1140℃, Zone 2 is 1200-1220℃, Zone 3 is 1240-1250℃, Zone 4 is 1250-1260℃, Zone 5 is 1260-1270℃, Zone 6 is 1265-1275℃, and Zone 7 is 1270-1280℃.

6. The method for preparing an acid-resistant, service-ready thick-walled seamless tube according to claim 4, characterized in that: In the quenching and tempering heat treatment stage, the quenching heating temperature is 920-960℃, water quenching is performed, and the water quenching cooling rate is 30-50℃ / s. The tempering heating temperature is 620-680℃, and the tempering holding time is 2.5-3.0h. After the holding time is completed, the tube is cooled to room temperature in the furnace to obtain a seamless tube. The specifications of the seamless tube are: outer diameter 219.1-323.9mm and wall thickness 22.23-33.32mm.

7. The method for preparing an acid-resistant, service-ready thick-walled seamless tube according to claim 4, characterized in that: The surface of the seamless tube is polished and washed in sequence to obtain a pretreated seamless tube. The pretreated seamless tube is then subjected to laser cladding using Fe-Cr-Si metal powder. Electrophoretic deposition is then performed using silicon carbide dispersion to obtain the finished seamless tube.

8. The method for preparing an acid-resistant, service-ready thick-walled seamless tube according to claim 7, characterized in that: Iron powder, chromium powder, silicon powder, carbon powder, nickel powder, and cobalt powder are mixed and ball-milled at 110-120 r / min for 1.5-2.0 h to obtain Fe-Cr-Si metal powder. Silicon carbide ceramic powder and polyvinyl butyral are added to acetone, stirred evenly, and then n-butylamine is added and stirred continuously to obtain silicon carbide dispersion.

9. The method for preparing an acid-resistant, service-ready thick-walled seamless tube according to claim 8, characterized in that: The Fe-Cr-Si metal powder contains the following components by mass fraction: 18-23% chromium powder, 4-7% silicon powder, 2-3% carbon powder, 3.5-4.5% nickel powder, 3.5-4.5% cobalt powder, with the balance being iron powder; the concentration of silicon carbide ceramic powder in the silicon carbide dispersion is 35-40 g / L, and the concentration of polyvinyl butyral is 6-8 g / L.

10. The method for preparing an acid-resistant, service-ready thick-walled seamless tube according to claim 7, characterized in that: Laser cladding process parameters: laser power 1.6-2.0kW, laser scanning speed 550-600mm / min, overlap rate 50%, nitrogen is used for powder feeding, powder feeding rate is controlled at 11.0-11.5g / min, and argon is used as the protective gas; the seamless tube after laser cladding is polished, alkali washed to remove oil, acid washed to activate, and alcohol washed, and then used as the anode and graphite plate as the cathode. The two electrodes are placed in silicon carbide dispersion for electrophoretic deposition to obtain the finished seamless tube; the power supply voltage is 25-35V, the deposition time is 10-12min, and the temperature is 25-30℃.

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