Preparation method of steel pipe

Abstract

The invention discloses a preparation method of a steel pipe, which comprises the following steps: carrying out powdering treatment and surface modification on high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder according to a mass ratio, and uniformly mixing to obtain alloy powder; the alloy powder is subjected to compression molding and directional local strengthening, and a blank is obtained; carrying out sintering and multi-stage heat treatment on the green body to obtain a sinter; the sinter is subjected to multi-pass drawing and upsetting treatment, and a steel pipe is formed; straightening and rounding the steel pipe and polishing the inner and outer surfaces; the microstructure and performance of the alloy are optimized, defects and impurities in the alloy are reduced, and the uniformity and compactness of the alloy are improved, so that heat is transferred more effectively; stress concentration points and microdefects are reduced by changing the lattice structure and the chemical bond type in the alloy; compared with the prior art, excessive stress concentration of part of the prepared steel pipe is reduced, the uniformity of the alloy is improved, and the stress distribution of the steel pipe is more uniform.

Description

Technical Field

[0001] The present invention relates to a method for preparing steel pipes, and more particularly to a method for preparing steel pipes. Background Art

[0002] In the field of steel pipe manufacturing, existing technologies cover a variety of preparation methods and processes. Among them, common steel pipe preparation methods include hot-rolled seamless steel pipes, cold-drawn seamless steel pipes, high-frequency welded steel pipes, and submerged arc welded steel pipes, etc. These methods have their own characteristics and are suitable for the production of steel pipes with different specifications and uses; Hot-rolled seamless steel pipes: By heating the steel billet to a certain temperature, piercing to form a hollow billet, and then performing hot rolling on the pipe mill, gradually reducing the wall thickness and increasing the length, finally forming seamless steel pipes of the required size. This method is suitable for the production of large-diameter seamless steel pipes; Cold-drawn seamless steel pipes: First, heat one end of the steel pipe to soften it, and then draw it through a pipe drawing machine, gradually reducing the diameter and wall thickness to reach the required size and accuracy. The cold-drawing process is mainly used for the production of small-diameter, high-precision seamless steel pipes; High-frequency welded steel pipes This method has high production efficiency and good weld quality, and is widely used in the production of welded steel pipes for low-pressure fluid transportation; Submerged arc welded steel pipes The steel pipes produced by the submerged arc welding method have stable and reliable weld quality and are suitable for the production of medium and thick-walled steel pipes.

[0003] In the process of steel pipe preparation, powder metallurgy technology is also involved. Powder metallurgy technology is a method of preparing metal products by mixing, compacting, and sintering metal powders. In steel pipe preparation, powder metallurgy technology can be used to produce steel pipes with special properties, such as high strength, high corrosion resistance, etc.

[0004] However, in the existing steel pipe preparation process, the prepared steel pipes will have the problem of uneven alloy, and using them for too long will lead to a short service life of the steel pipes, because the uneven alloy causes stress concentration and affects the reduction of the service life of the steel pipes. Summary of the Invention

[0005] The present invention overcomes the deficiencies of the prior art and provides a method for preparing steel pipes.

[0006] To achieve the above object, the technical solution adopted by the present invention is: A method for preparing steel pipes, including: S1. Powderize and surface-modify high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder according to a mass ratio, and after uniformly mixing, obtain alloy powder;

[0007] S2. Compact and directionally locally strengthen the alloy powder to obtain a blank;

[0008] S3. Sinter and perform multi-stage heat treatment on the blank to obtain a sintered product;

[0009] S4. Perform multi-pass drawing and upsetting on the sintered material to form a steel pipe;

[0010] S5. Straighten and round the steel pipe and polish its inner and outer surfaces.

[0011] In a preferred embodiment of the present invention, in step S1, the mass ratio is specifically: 21 - 23:7 - 12:2 - 6:1 - 3:0.5 - 0.9:0.1 - 0.6:0.05 - 0.2:0.3:0.07:62.1.

[0012] In a preferred embodiment of the present invention, in step S1, the powdering treatment is specifically to convert high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder into fine and uniform metal powders with an average particle size of about 40 - 60 μm through gas atomization.

[0013] In a preferred embodiment of the present invention, the specific steps for preparing by gas atomization are as follows:

[0014] S11. Melt high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder respectively in a vacuum induction melting furnace to form a uniform molten metal;

[0015] S12. Introduce the molten metal into the gas atomization equipment, spray and break it into tiny droplets with high-pressure argon gas, and then quickly cool and solidify it into solid powder in the cooling chamber;

[0016] S13. Collect the generated powder and select powder particles with an average particle size of about 40 - 60 μm through a screening device.

[0017] In a preferred embodiment of the present invention, S14. Wash the powder with ethanol to remove surface impurities, and place the washed powder in a drying oven to remove moisture;

[0018] S15. Combine and use three modification methods: surface chemical coating modification method, precipitation reaction modification method, and mechanochemical modification method;

[0019] S16. After the modification treatment, wash the powder to remove unreacted modifiers and impurities;

[0020] After washing, place the powder in a drying oven for drying to remove moisture.

[0021] In a preferred embodiment of the present invention, in step S1, the specific steps for uniform mixing are: place it in a V-type mixer, set the rotation speed to 20 r / min, and the mixing time to 24 hours to ensure that the components are fully and evenly distributed.

[0022] In a preferred embodiment of the present invention, in step S2, the specific steps for press forming are:

[0023] S21. Put the mixed alloy powder into a special cylindrical mold, and use hot isostatic pressing (HIP) technology to hold for 10 minutes at a temperature of 600 °C and a pressure of 150 MPa to form a green body with a preliminary shape;

[0024] Here, the high-pressure condition with a pressure of 150 MPa can significantly improve the bonding force between powders, increase the density of the green body, reduce internal pores, and lower the risk of pores and microcracks that may appear in the finished product.

[0025] In a preferred embodiment of the present invention, in step S2, the steps of directional local strengthening are as follows:

[0026] S22. Place the green body on the workbench of the laser melting equipment and ensure its position is stable and easy to be irradiated by the laser beam;

[0027] S23. Use a selective laser melting (SLM) equipment, set the laser power to 400 W, the scanning speed to 10 mm / s, and the spot diameter to 0.4 mm, and locally heat and melt the key parts of the green body to refine the grain structure and promote the more uniform diffusion of alloying elements;

[0028] S24. Start scanning according to the scanning path, and the energy of the laser beam will be concentrated on a specific area of the green body, causing the metal powders in this area to quickly melt and bond with each other;

[0029] S25. After completing the laser melting, let the green body cool naturally to room temperature in the equipment.

[0030] In a preferred embodiment of the present invention, in step S3, the specific steps for sintering are: S31. Put the pressed green body into a vacuum furnace, gradually heat it to 1200 °C under argon protection, keep it warm for 2 hours to ensure sufficient bonding between powder particles without generating excessive thermal stress, and then cool it to room temperature with the furnace.

[0031] In a preferred embodiment of the present invention, in step S3, the specific steps for multi-stage heat treatment are: S32. Put the sintered green body into the vacuum furnace again, heat it to 1050 °C, keep it warm for 1 hour and then quickly cool it to dissolve the alloy phase and homogenize the structure;

[0032] S33. Then keep the green body at 480 °C for 8 hours to precipitate alloying elements to form strengthening phases and improve the strength and toughness of the material; rapid cooling inhibits the precipitation of secondary phases, maintains a single austenite or martensite structure, and improves the plasticity and welding performance of the material.

[0033] The present invention solves the defects existing in the background technology, and the present invention has the following beneficial effects:

[0034] (1) The alloy powder is obtained by powdering and surface modifying high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder according to the mass ratio, and then uniformly mixing them; the alloy powder is pressed and shaped and directionally locally strengthened to obtain a blank; the blank is sintered and multi-stage heat treated to obtain a sintered product; the sintered product is subjected to multi-pass drawing and upsetting treatments to form a steel pipe; the steel pipe is straightened and roundness corrected and the inner and outer surfaces are polished; the microstructure and properties of the alloy are optimized, the defects and impurities inside the alloy are reduced, and the uniformity and density of the alloy are improved, so as to transfer heat more effectively; by changing the lattice structure and chemical bond type inside the alloy, the stress concentration points and micro-defects are reduced; compared with the prior art, the stress concentration at some positions of the prepared steel pipe is reduced, the uniformity of the alloy is improved, and the stress distribution of the steel pipe is more uniform.

[0035] (2) When performing the modification treatment, a combination of three modification methods is adopted, which can give full play to the advantages of each modification method and form a complementary effect; surface chemical coating can improve the quality and properties of the alloy surface; precipitation reaction can form uniformly distributed strengthening phases inside the alloy, improving the strength and hardness of the alloy; mechanochemical modification can make the arrangement of molecules / atoms inside the alloy more compact through mechanical action, significantly enhancing the properties of the alloy.

[0036] (3) The green compact is directionally locally melted. By making the alloy rapidly melt and solidify in a local area, the grain size is refined, defects are reduced, the uniformity and density of the alloy are improved; the arrangement of molecules / atoms inside the alloy becomes more orderly, the interaction force between atoms is enhanced, the non-uniformity inside the alloy is reduced, and the properties of the alloy are further improved. Description of the Drawings

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings;

[0038] Figure 1 It is a flowchart of a preferred embodiment of the present invention; Detailed Embodiments

[0039] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0040] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways than those specifically described herein, and thus, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0041] In the description of the present application, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the scope of protection of the present application. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0042] In the description of the present application, it should be noted that, unless otherwise clearly specified and limited, the terms "mounted", "connected" and "coupled" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific circumstances.

[0043] As Figure 1 shown, a method for preparing a steel pipe includes:

[0044] S1. Powderizing and surface-modifying high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder according to a mass ratio, and uniformly mixing them to obtain alloy powder;

[0045] S2. Compacting and directionally locally strengthening the alloy powder to obtain a blank;

[0046] S3. Sintering and multi-stage heat-treating the blank to obtain a sintered product;

[0047] S4. Performing multi-pass drawing and upsetting on the sintered product to form a steel pipe;

[0048] S5. Straightening and rounding the steel pipe and polishing its inner and outer surfaces.

[0049] In step S1, the mass ratio is specifically: 21 - 23:7 - 12:2 - 6:1 - 3:0.5 - 0.9:0.1 - 0.6:0.05 - 0.2:0.3:0.07:62.1.

[0050] In step S1, the pulverization treatment is specifically to convert high - purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder into fine and uniform metal powders with an average particle size of about 40 - 60 μm through gas atomization.

[0051] The specific steps for preparing by gas atomization are as follows:

[0052] S11. Respectively melt high - purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder in a vacuum induction melting furnace to form a uniform molten metal;

[0053] S12. Introduce the molten metal into a gas atomization device, use high - pressure argon gas to spray and break it into tiny droplets, and then quickly cool and solidify into solid powders in a cooling chamber.

[0054] S13. Collect the generated powders, and select powder particles with an average particle size of about 40 - 60 μm through a screening device.

[0055] Specifically, since the molten metal is broken into a large number of tiny droplets instantaneously, each droplet almost contains all the components of the original alloy, which ensures the high uniformity of the final powder; at the same time, the generated powder particles are mostly spherical or near - spherical, and this shape helps to improve the powder fluidity and facilitates subsequent mixing and pressing operations;

[0056] Using an inert gas as the atomization medium can effectively prevent metal oxidation, which maintains the purity of the powder, and this is especially important for superalloys.

[0057] In addition, by adjusting parameters such as gas pressure and spraying angle, the average particle size and its distribution range of the powder can be controlled to meet the requirements of different application scenarios.

[0058] In step S1, the specific steps of the modification treatment are as follows:

[0059] S14. Wash the powder with ethanol to remove surface impurities, and place the washed powder in a drying oven to remove moisture;

[0060] S15. Combine and use three modification methods: surface chemical coating modification method, precipitation reaction modification method, and mechanochemical modification method;

[0061] S16. After the modification treatment, wash the powder to remove unreacted modifiers and impurities;

[0062] After cleaning, place the powder in a drying oven for drying to remove moisture;

[0063] More specifically, in step S15, the specific modification method is as follows:

[0064] S151. Place high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder in different containers respectively; add an appropriate amount of silane coupling agent solution (KH-550) to each container to ensure that the solution can completely cover the powder; at 60 °C, make the silane coupling agent uniformly adsorbed on the powder surface by stirring or ultrasonic means; after 2 - 4 h, take out the powder, wash it with a solvent such as ethanol to remove the unreacted silane coupling agent; place the washed powder in an oven for drying to obtain the modified powder;

[0065] S152. Mix the high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder modified in step S151 evenly; add sodium hydroxide solution to the mixed powder to ensure that the concentration and pH value of the solution are appropriate; at 60 °C, make sodium hydroxide react with the metal ions on the powder surface by stirring to form hydroxide precipitates; after the reaction is completed, filter and wash the precipitate to remove the unreacted sodium hydroxide and impurities; place the washed precipitate in an oven for drying to obtain the modified powder;

[0066] S153. Mix the high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder modified in step S152 evenly; place the mixed powder in an ultrafine comminution device and activate the particle surface through strong mechanical force; during the comminution process, add stearic acid, and make the modifier react or adsorb with the powder surface through the action of mechanical force; after comminution and modification are completed, take out the powder, and perform screening and drying treatments to obtain the final modified powder.

[0067] Specifically, through the uniform adsorption of modifiers such as silane coupling agent (KH-550) on the powder surface, the wettability between the powder and the subsequent treatment liquid (such as the melt during sintering) can be improved. This helps the melt to wet and penetrate more evenly between the powder particles during the sintering process, thereby improving the density and uniformity of the material. One end of the silane coupling agent can form a chemical bond with the metal atoms or functional groups on the powder surface, and the other end may carry functional groups that can react with the components in the subsequent treatment liquid. This chemical bonding not only enhances the activity of the powder surface but also promotes the close combination between the powder and the subsequent treatment liquid.

[0068] Depositing an inorganic substance (hydroxide) on the powder surface through a precipitation reaction can change the polarity and charge properties of the powder surface, thereby regulating its interaction with the subsequent treatment liquid. This regulation improves the wettability of the melt and reduces the porosity.

[0069] More specifically, the modification treatment can also improve the fluidity of the powder, making it easier to form a uniform green body and a dense sintered body during pressing and sintering. The improvement in fluidity helps to reduce the formation of pores and defects, thereby enhancing the overall performance and viscosity of the material.

[0070] More specifically, during the mechanochemical modification process, the strong mechanical force causes microcracks, deformation, or exfoliation on the surface of the powder particles, increasing the surface roughness. This rough surface structure is conducive to the mechanical interlocking between the powder particles, thereby enhancing the overall viscosity of the material.

[0071] More specifically, modifiers such as stearic acid can adhere to the powder surface through physical adsorption or chemical adsorption. The intermolecular forces (such as van der Waals forces, hydrogen bonds, etc.) between these modifier molecules help to increase the viscosity between the powder particles.

[0072] In step S1, the specific steps for uniform mixing are as follows: Place it in a V-type mixer, set the rotation speed to 20 r / min, and the mixing time to 24 hours to ensure that the components are fully and evenly distributed; among them, the amount of the loaded material should be controlled at about 3 / 5 of the volume of the V-type mixer to ensure the mixing effect. The V-type mixer, through its unique geometric shape, enables powders with different densities and particle sizes to continuously flip and flow in the container under low-speed rotation (20 r / min), thereby achieving sufficient mixing; at the same time, setting a long mixing time of 24 hours allows each powder to have enough time to contact and be evenly distributed with other components, reducing the possibility of local component segregation;

[0073] This makes each element in the final mixture evenly distributed, providing an ideal raw material basis for subsequent processing, and enabling more uniform diffusion and reaction during the subsequent heat treatment process to form a consistent microstructure.

[0074] It should be noted that during the mixing process, silicate needs to be added as a dispersant, and the dispersant usually has high stability and heat resistance; because the powder particles are prone to agglomeration during the mixing process, and the dispersant can form a protective film on the particle surface to prevent direct contact and agglomeration between the particles.

[0075] In step S2, the specific steps for pressing and forming are as follows:

[0076] S21. Put the mixed alloy powder into a special cylindrical mold. Using hot isostatic pressing (HIP) technology, maintain it at 600 °C and 150 MPa for 10 min to form a green body with a preliminary shape.

[0077] Here, the high-pressure condition of 150 MPa can significantly improve the bonding force between powders, increase the density of the green body, reduce internal pores, and lower the risk of pores and microcracks that may appear in the finished product.

[0078] In step S2, the steps of directional local strengthening are as follows:

[0079] S22. Place the green body on the working table of the laser melting equipment and ensure its position is stable and easy to be irradiated by the laser beam.

[0080] S23. Use a selective laser melting (SLM) equipment. Set the laser power to 400 W, the scanning speed to 10 mm / s, and the spot diameter to 0.4 mm. Locally heat and melt the key parts of the green body (such as the place where the wall thickness changes) to refine the grain structure and promote the more uniform diffusion of alloying elements.

[0081] S24. Start scanning according to the scanning path. The energy of the laser beam will be concentrated on a specific area of the green body, causing the metal powders in this area to quickly melt and bond with each other.

[0082] S25. After completing the laser melting, let the green body cool naturally to room temperature in the equipment.

[0083] More specifically, using the SLM equipment to locally heat and melt the key parts, the energy of the laser beam is concentrated on a specific area, causing the metal powders in this area to quickly melt and bond with each other, forming a fine and uniform grain structure. During the melting process, the alloying elements are more uniformly diffused, improving the chemical homogeneity of the material. And by treating the stress concentration areas such as the place where the wall thickness changes, the mechanical properties of these positions are enhanced, improving the overall durability, especially being obvious under the working conditions of high stress.

[0084] More specifically, after directional local strengthening, the microstructure of the key parts is optimized, enhancing the overall strength and toughness of the material. Especially the tensile strength and yield strength are significantly improved. The local melting and slow cooling process helps to reduce the thermal stress generated by rapid cooling, avoiding the generation of cracks and other defects.

[0085] In step S3, the specific steps of sintering are as follows: S31. Put the pressed green body into a vacuum furnace. Gradually heat it to 1200 °C under argon protection and keep it warm for 2 hours to ensure sufficient bonding between powder particles without generating excessive thermal stress, and then cool it to room temperature with the furnace.

[0086] Specifically, at high temperatures, diffusion and recrystallization occur between metal atoms, forming a continuous metallurgical bonding interface, enhancing the overall strength and toughness of the material; the process of furnace cooling to room temperature reduces the thermal stress generated by rapid cooling, avoiding cracks and other defects, and this also promotes the homogenization of the microstructure, improving the overall performance of the material.

[0087] In step S3, the specific steps of multi-stage heat treatment are as follows: S32. Put the sintered blank into a vacuum furnace again, heat it up to 1050 °C, hold for 1 hour and then rapidly cool it to dissolve the alloy phase and homogenize the structure;

[0088] S33. Then hold the blank at 480 °C for 8 hours to precipitate alloying elements to form strengthening phases, enhancing the strength and toughness of the material; rapid cooling inhibits the precipitation of secondary phases, maintaining a single austenite or martensite structure and improving the plasticity and weldability of the material.

[0089] Specifically, heating up to 1050 °C and rapidly cooling it dissolve the alloy phase and homogenize the structure, eliminating the internal stress caused by work hardening and restoring the ductility and toughness of the material; after multi-stage heat treatment, the material not only maintains good ductility and toughness but also has higher strength and hardness, being particularly suitable for applications under high-temperature and high-pressure environments.

[0090] In step S4, the specific steps of precision drawing are as follows:

[0091] S41. Primary drawing: Calculate the target size of the first drawing according to the outer diameter and wall thickness of the initial steel pipe to ensure that the deformation amount does not exceed 20%; load the steel pipe into the drawing machine and perform primary drawing through a die;

[0092] S42. Carefully put the steel pipe after the first drawing into an annealing furnace, set the annealing temperature at 650 °C, and keep it at a constant temperature for 1 hour after reaching the target temperature, then cool it to room temperature with the furnace. Slow cooling helps reduce internal stress concentration and prevent deformation or crack generation.

[0093] S43. According to the requirements of the final product, plan 3 - 4 drawing operations, with the deformation amount controlled within 20% each time.

[0094] Specifically, during the drawing process, the material will undergo plastic deformation, and the magnitude of the deformation amount directly affects the mechanical properties and microstructure of the material; controlling the total deformation amount within 20% can ensure that the material will not generate cracks, fractures or severe work hardening phenomena due to excessive stretching during the deformation process, thus maintaining the good performance of the material.

[0095] In step S4, the total upsetting deformation amount is also controlled within 20%;

[0096] Specifically, by appropriate deformation, the cross-sectional shape of the steel pipe is changed to better meet the requirements of the final product. The total deformation is controlled within 20% to ensure that the mechanical properties of the material are not affected. Appropriate upsetting can enhance the strength of specific areas, especially at connection parts or where large loads are applied, improving the overall structural stability.

[0097] If the upsetting deformation is too large, it may cause severe plastic deformation and residual stress inside the material, even leading to cracking and failure of the material. If the upsetting deformation is too small, the internal strengthening of the material will be insufficient. Controlling the total deformation within 20% can avoid these adverse factors and ensure the stability and reliability of the material.

[0098] In step S5, the specific steps of straightening and rounding are as follows: S51. Correctly install the steel pipe processed in S4 on the straightening machine for straightening.

[0099] S52. When rounding, feed the preliminarily straightened steel pipe into the rounding device, and use multiple rollers to slightly squeeze the steel pipe to restore its circular cross-section.

[0100] In step S5, the specific steps for polishing the inner and outer surfaces are as follows: S53. Clean the inner and outer surfaces of the steel pipe to remove oil, rust, and other contaminants; use abrasives with different grit sizes in sequence from coarse to fine to polish the inner and outer surfaces of the steel pipe; after polishing, galvanize or apply anti-rust paint to the steel pipe.

[0101] Specifically, after fine polishing treatment, the inner and outer surfaces of the steel pipe reach a very high surface finish standard. The surface treatment not only improves the appearance quality but also increases the corrosion resistance and extends the service life of the steel pipe, especially suitable for applications in harsh environments.

[0102] Example 1

[0103] S1. Powderize and surface-modify high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder in a mass ratio of 22:9:4:1.5:0.7:0.3:0.1:0.3:0.07:62.1. After uniform mixing, obtain alloy powder.

[0104] S11. Melt high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder respectively in a vacuum induction melting furnace to form uniform molten metal.

[0105] S12. Introduce the molten metal into the gas atomization equipment, use high-pressure argon gas to spray and break it into tiny droplets, and then quickly cool and solidify into solid powder in the cooling chamber.

[0106] S13. Collect the generated powder and select powder particles with an average particle size of about 50 μm through a screening device.

[0107] S14. Wash the powder with ethanol to remove surface impurities, and place the washed powder in a drying oven to remove moisture.

[0108] S15. Combine and use three modification methods: surface chemical coating modification method, precipitation reaction modification method, and mechanochemical modification method.

[0109] S151. Place high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder in different containers respectively; add an appropriate amount of silane coupling agent solution (KH-550) to each container to ensure that the solution can completely cover the powder; at 60 °C, ultrasonically adsorb the silane coupling agent evenly on the powder surface; after 2 - 4 h, take out the powder, wash it with solvents such as ethanol to remove the unreacted silane coupling agent; place the washed powder in an oven to dry and obtain the modified powder.

[0110] S152. Mix the high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder modified in step S151 evenly; add sodium hydroxide solution to the mixed powder to ensure that the concentration and pH value of the solution are appropriate; at 60 °C, stir to make sodium hydroxide react with the metal ions on the powder surface to form hydroxide precipitates; after the reaction is completed, filter and wash the precipitate to remove the unreacted sodium hydroxide and impurities; place the washed precipitate in an oven to dry and obtain the modified powder.

[0111] S153. Mix the high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder modified in step S152 evenly; place the mixed powder in an ultrafine grinding device and activate the particle surface through strong mechanical force; during the grinding process, add stearic acid and make the modifier react or adsorb on the powder surface through the action of mechanical force; after grinding and modification are completed, take out the powder and perform screening and drying treatments to obtain the final modified powder.

[0112] S16. After the modification treatment, wash the powder to remove the unreacted modifier and impurities.

[0113] S21. Put the mixed alloy powder into a special cylindrical mold, and use hot isostatic pressing (HIP) technology to hold for 10 min at a temperature of 600 °C and a pressure of 150 MPa to form a green body with a preliminary shape.

[0114] S22. Place the green body on the working table of a laser melting device and ensure its position is stable and easy to be irradiated by the laser beam.

[0115] S23. Use a selective laser melting (SLM) device, set the laser power to 400 W, the scanning speed to 10 mm / s, and the spot diameter to 0.4 mm. Locally heat and melt the area where the wall thickness of the blank changes to refine the grain structure and promote more uniform diffusion of alloying elements.

[0116] S24. Start scanning according to the scanning path. The energy of the laser beam will be concentrated on a specific area of the blank, causing the metal powder in this area to quickly melt and combine with each other.

[0117] S25. After completing the laser melting, let the blank cool naturally to room temperature in the device.

[0118] S31. Place the pressed blank into a vacuum furnace, gradually heat it to 1200 °C under argon protection, and hold it for 2 hours to ensure sufficient bonding between powder particles without generating excessive thermal stress. Then cool it to room temperature with the furnace.

[0119] S32. Place the sintered blank into the vacuum furnace again, heat it to 1050 °C, hold it for 1 hour and then quickly cool it to dissolve the alloy phase and homogenize the structure.

[0120] S33. Then hold the blank at 480 °C for 8 hours to precipitate alloying elements to form strengthening phases and improve the strength and toughness of the material. Quick cooling inhibits the precipitation of secondary phases, maintains a single austenite or martensite structure, and improves the plasticity and welding performance of the material.

[0121] S41. For the first drawing, calculate the target size of the first drawing according to the outer diameter and wall thickness of the initial steel pipe to ensure that the deformation amount does not exceed 20%. Load the steel pipe into the drawing machine and perform the first drawing through the die.

[0122] S42. Carefully place the steel pipe after the first drawing into the annealing furnace, set the annealing temperature to 650 °C, and keep it at a constant temperature for 1 hour after reaching the target temperature. Then cool it to room temperature with the furnace. Slow cooling helps to reduce internal stress concentration and prevent deformation or crack generation.

[0123] S43. According to the requirements of the final product, plan 3 - 4 drawing operations, with the deformation amount controlled within 20% each time.

[0124] S51. Correctly install the steel pipe after S4 treatment on the straightening machine and straighten it.

[0125] S52. When rounding, feed the preliminarily straightened steel pipe into the rounding device and use multiple rollers to slightly squeeze the steel pipe to restore its circular cross-section.

[0126] S53. Clean the inner and outer surfaces of the steel pipe to remove oil stains, rust and other contaminants; use abrasives with different particle sizes in sequence from coarse to fine to polish the inner and outer surfaces of the steel pipe; after polishing, galvanize or paint the steel pipe with antirust paint.

[0127] Comparative Example 1

[0128] Based on Example 1, the difference is that in S13, collect the generated powder and select powder particles with an average particle size of about 40 μm through a screening device.

[0129] Comparative Example 2

[0130] Based on Example 1, the difference is that in S13, collect the generated powder and select powder particles with an average particle size of about 60 μm through a screening device.

[0131] Comparative Example 3

[0132] Based on Example 1, the difference is that no surface modification treatment is carried out.

[0133] Comparative Example 4

[0134] Based on Example 1, the difference is that in step S15, only the combination of three modification methods, namely surface chemical coating modification method, precipitation reaction modification method and mechanochemical modification method, is used;

[0135] Place high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder in different containers respectively; add an appropriate amount of silane coupling agent solution (KH-550) to each container to ensure that the solution can completely cover the powder; at 60 °C, make the silane coupling agent evenly adsorbed on the powder surface by ultrasonic treatment; after 2 - 4 h, take out the powder, wash it with solvents such as ethanol to remove the unreacted silane coupling agent; place the washed powder in an oven to dry to obtain the modified powder;

[0136] Comparative Example 5

[0137] Based on Example 1, the difference is that in step S15, only the precipitation reaction modification method is used;

[0138] Mix the high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder modified through step S151 evenly; add sodium hydroxide solution to the mixed powder to ensure that the concentration and pH value of the solution are appropriate; at 60 °C, make sodium hydroxide react with the metal ions on the powder surface by stirring to generate hydroxide precipitation; after the reaction is completed, filter and wash the precipitate to remove the unreacted sodium hydroxide and impurities; place the washed precipitate in an oven to dry to obtain the modified powder; Comparative Example 6

[0139] Based on Example 1, the difference is that in step S15, only the mechanochemical modification method is used.

[0140] Mix the high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder, and iron powder after being modified in step S152 evenly; place the mixed powder in an ultrafine grinding device, and activate the particle surface through the action of strong mechanical force; during the grinding process, add stearic acid, and make the modifier react or adsorb with the powder surface through the action of mechanical force; after grinding and modification are completed, take out the powder, and perform screening and drying treatments to obtain the final modified powder.

[0141] Comparative Example 6

[0142] Based on Example 1, the difference is that the green compact is not subjected to directional local melting;

[0143] Comparative Example 7

[0144] Based on Example 1, the difference is that multi-stage heat treatment is not carried out.

[0145] Experimental Example 1

[0146] It is verified through 2 experimental categories; namely alloy composition analysis, mechanical property testing, thermal property testing, and microstructure observation. Specifically:

[0147] Alloy composition analysis: Take steel pipes from Example 1 and Comparative Examples 1-7 respectively, and use a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS) to analyze the microstructure and element distribution of the samples; Data collection: Record the distribution of alloy elements in the SEM images, as well as the content and distribution uniformity index of each element in the EDS data (defined as the ratio of the standard deviation to the average value of the element content in the sample, and the smaller the ratio, the more uniform the element distribution); As shown in Table 1

[0148]

[0149]

[0150] Experimental Example 2

[0151] Use an X-ray diffractometer to analyze the crystal phase structure of the samples. The X-ray diffractometer is set with a scanning speed of 5° / min, a step size of 0.03°, a tube voltage of 30 kV, and a tube current of 50 mA. Record the XRD pattern, and observe and analyze the characteristic peak intensity and position of the crystal phase structure to evaluate the crystal phase composition and possible stress state of the samples.

[0152] At the same time, a scanning electron microscope (SEM) equipped with an EBSD probe was used to measure the residual stress distribution of the steel pipes in Example 1 and Comparative Examples 1-7; the distance between the sample and the probe was adjusted to 15 mm in the SEM to ensure that the test area was located at a representative part of the sample. The acceleration voltage was 20 kV, the aperture size was 60 μm, and the step size was 1.2 μm for EBSD scanning to collect the Kikuchi pattern data of the sample, and the corresponding algorithm was applied to calculate the residual stress distribution; the average value and the maximum value of the residual stress, as well as other relevant statistical data were recorded

[0153]

[0154]

[0155] It can be seen from the data in Table 1 and Table 2;

[0156] Comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the uniformity index of the alloy element distribution in Example 1 is the lowest among all samples, indicating that the steel pipe alloy formed by powder particles with an average particle size of about 50 μm in Example 1 has the most uniform distribution. The reason is that in Example 1, the particle size is appropriate, the surface area is appropriate, and the contact area between the powder and the modifier is also reasonable. The adsorption and reaction of the modifier on the powder surface are more sufficient, thereby improving the uniformity of the alloy;

[0157] Comparing Example 1 with Comparative Examples 3, 4 and 5, it can be seen that the steel pipe alloy formed by combining the three modification methods of surface chemical coating modification method, precipitation reaction modification method and mechanochemical modification method in Example 1 has the most uniform distribution. The reason is that: in Example 1, by combining the three methods of surface chemical coating, precipitation reaction and mechanochemical modification, the microstructure and properties of the alloy can be more comprehensively optimized. This combined modification method can reduce the defects and impurities inside the alloy, improve the uniformity and density of the alloy, and thus transfer heat more effectively; by changing the lattice structure and chemical bond type inside the alloy, the thermal conductivity is further improved; in terms of the coefficient of thermal expansion, in Example 1, the combined modification method can more comprehensively optimize the microstructure and properties of the alloy, reduce the stress concentration points and micro-defects; this enables the alloy to expand and contract more uniformly when the temperature changes.

[0158] Comparing Example 1 with Comparative Example 6 shows that the alloy distribution in the steel pipe formed by directional local melting in Example 1 is the most uniform. The reason is that in Example 1, through directional local melting, the microstructure of the alloy is significantly refined, the grain size is reduced, and the phase region distribution is more uniform. This change in microstructure enables heat to conduct more smoothly through the alloy during the conduction process, improving the thermal conductivity. In addition, directional local melting may further improve the thermal conductivity by changing the lattice orientation and chemical bond type inside the alloy; at the same time, directional local melting makes the microstructure of the alloy more uniform and delicate, reducing stress concentration and microdefects;

[0159] Comparing Example 1 with Comparative Example 7 shows that the alloy distribution in the steel pipe formed by multi-stage heat treatment in Example 1 is the most uniform. The reason is that in Example 1, during the multi-stage heat treatment process, molecules inside the powder and the substrate undergo phase transformation, diffusion, recrystallization and other phenomena at different temperatures, thereby improving their microstructure and properties. However, in Comparative Example 7 without multi-stage heat treatment, the molecules inside the powder and the substrate cannot undergo the expected phase transformation and diffusion phenomena, resulting in a more uniform distribution of the steel pipe.

[0160] Based on the ideal embodiments of the present invention as inspiration, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A method for preparing a steel pipe, characterized in that: include: S1. Powdering and surface-modifying high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder according to mass proportion, and uniformly mixing them to obtain alloy powder; S2, pressing and locally strengthening the alloy powder to obtain a green body; S3, sintering and multi-stage heat treatment the green body to obtain a sintered product; S4, performing multiple drawing and roughening processes on the sintered product to form a steel pipe; S5. Straighten and round the steel pipe and polish the inner and outer surfaces.

2. The method for preparing a steel pipe according to claim 1, characterized in that: In step S1, the mass ratio is specifically: 21-23: 7-12: 2-6: 1-3: 0.5-0.9:0.1-0.6:0.05-0.2:0.3:0.07：62.1。 3. The method for preparing a steel pipe according to claim 1, characterized in that: In step S1, the powdering process is specifically to convert high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder into fine and uniform metal powder with an average particle size of 40-60 μm by gas atomization.

4. The method for preparing a steel pipe according to claim 3, characterized in that: The specific steps of the aerosolization method are as follows: S11, respectively melting high-purity chromium powder, nickel powder, molybdenum powder, manganese powder, silicon powder, titanium powder, niobium powder, copper powder and iron powder in a vacuum induction melting furnace to form a uniform molten metal; S12, introducing the molten metal into a gas atomization device, breaking it into tiny droplets by spraying high-pressure argon gas, and then rapidly cooling and solidifying it into solid powder in a cooling chamber; S13, collecting the generated powder, and selecting powder particles with an average particle size of 40-60 μm through a screening device.

5. The method for preparing a steel pipe according to claim 1, characterized in that: S14, washing the powder with ethanol to remove surface impurities, and placing the washed powder in a drying oven to remove moisture; S15, using a combination of three modification methods: surface chemical coating modification method, precipitation reaction modification method and mechanochemical modification method; S16, after the modification treatment, washing the powder to remove unreacted modifier and impurities; After washing, the powder is dried in a drying oven to remove moisture.

6. The method for preparing a steel pipe according to claim 1, characterized in that: In step S1, the specific steps of uniform mixing are: placing it in a V-type mixer, setting the speed to 20 r / min, and mixing time to 24 hours to ensure that the ingredients are fully and evenly distributed.

7. The method for preparing a steel pipe according to claim 1, characterized in that: In step S2, the specific steps of compression molding are: S21, putting the mixed alloy powder into a special cylindrical mold, using hot isostatic pressing technology, keeping it at a temperature of 600°C and a pressure of 150MPa for 10 minutes to form a preliminary shape of the green body; The high pressure condition of 150MPa is used here to significantly improve the bonding force between powders, increase the density of the green body, reduce internal porosity, and reduce the risk of pores and microcracks in the finished product.

8. The method for preparing a steel pipe according to claim 1, characterized in that: The steps of directional local strengthening in step S2 are: S22, placing the blank on a workbench of the laser melting equipment, and ensuring that the blank is in a stable position and is easily irradiated by the laser beam; S23, using laser selective melting equipment, setting the laser power to 400W, the scanning speed to 10mm / s, and the spot diameter to 0.4mm, locally heating and melting the key parts of the blank, refining the grain structure, and promoting more uniform diffusion of alloy elements; S24, scanning starts according to the scanning path, and the energy of the laser beam is concentrated on a specific area of ​​the blank, causing the metal powder in the area to melt quickly and combine with each other; S25. After laser melting is completed, the blank is allowed to cool naturally to room temperature in the equipment.

9. The method for preparing a steel pipe according to claim 1, characterized in that: In step S3, the specific steps of sintering are as follows: S31, placing the pressed green body into a vacuum furnace, gradually heating it to 1200°C under argon protection, and keeping it warm for 2 hours to ensure that the powder particles are fully bonded without generating excessive thermal stress, and then cooling it to room temperature with the furnace.

10. The method for preparing a steel pipe according to claim 1, characterized in that: In step S3, the specific steps of the multi-stage heat treatment are as follows: S32, placing the sintered green body into the vacuum furnace again, heating it to 1050°C, keeping it at this temperature for 1 hour, and then rapidly cooling it to dissolve the alloy phase and homogenize the structure; S33. Then, the blank is kept at 480°C for 8 hours to allow the alloy elements to precipitate to form a strengthening phase, thereby improving the strength and toughness of the material; rapid cooling inhibits the precipitation of the secondary phase, maintains a single austenite or martensite structure, and improves the plasticity and welding properties of the material.

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