Seamless steel pipe and method for producing the same

Abstract

The present application provides a kind of seamless steel tube and its preparation method.The preparation method includes the following steps: step S1, the metallurgical raw material is smelted, and blank is obtained;Step S2, blank is subjected to piercing treatment, and the pipe is obtained;Step S3, the pipe is rolled, and the obtained pipe is subjected to first rapid cooling, first rapid cooling includes first rapid cooling and second rapid cooling, and the temperature of the pipe decreases by 30-150 DEG C during first rapid cooling;The temperature of the pipe decreases by 30-150 DEG C during second rapid cooling;Step S4, the pipe after first rapid cooling is subjected to sizing or reducing treatment, and the obtained steel pipe is subjected to second rapid cooling, and the temperature of the steel pipe after second rapid cooling is 550-650 DEG C;Step S5, the steel pipe obtained by second rapid cooling is subjected to air cooling.The present application can obtain hot-rolled seamless steel pipe with small straightness and ovality and excellent strength and toughness by reasonably setting cooling path.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and more specifically, to a seamless steel pipe and its preparation method. Background Technology

[0002] In recent years, with the increasing application scenarios of various extreme working conditions such as low temperature, high temperature and high pressure, the requirements for the strength and toughness of seamless steel pipes have become increasingly stringent. For example, the S460NH grade requires a yield strength ≥460MPa, a tensile strength ≥540MPa, and a low-temperature impact strength of ≥40J at -20℃. However, hot-rolled seamless steel pipes with a yield strength of 345 and above suffer from insufficient strength and toughness, and poor low-temperature impact toughness. Traditional online normalizing or tempering processes are difficult to meet the application requirements, and existing technologies often use quenching and tempering. However, the quenching and tempering process not only significantly increases production costs, but also results in poor microstructural stability and higher internal stress in the steel pipe. It also significantly increases the yield strength ratio of the steel pipe, which is detrimental to subsequent welding of the steel pipe and the safety of the components.

[0003] Manufacturing companies aim to steadily and rapidly improve the strength and toughness of steel pipes by controlling the deformation, cooling rate, and cooling path. However, although TMCP technology has been maturely applied in the production of hot-rolled strip steel, medium and heavy plates, and profiles, the complex deformation during steel pipe rolling and the small adjustable range, coupled with the fact that steel pipes have a hollow cross-section with larger cross-sectional dimensions and a wider range of size variations compared to other long products, make it very difficult to control the rolling and implement online heat treatment.

[0004] To address the above issues, Chinese patent application CN109868352A discloses a water-cooled self-tempering process and apparatus for steel pipes. This process enables ordinary low-carbon manganese steel to be rapidly cooled to between 550 and 650°C via water spraying devices on the outer and inner walls of the steel pipe, resulting in a fine pearlitic structure. This improves the strength of the steel pipe and stabilizes the material's impact toughness. However, this apparatus is complex in design, cannot utilize existing production equipment, requires significant modifications to existing equipment, and the entire production process is time-consuming, slowing down the production pace and hindering production organization. Summary of the Invention

[0005] The main objective of this invention is to provide a seamless steel pipe and its preparation method, so as to solve the problems of poor stability or complex preparation process of high-strength and tough seamless steel pipes in the prior art.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for preparing a seamless steel pipe is provided, the method comprising the following steps: Step S1, smelting metallurgical raw materials to obtain a billet; Step S2, piercing the billet to obtain a rough tube; Step S3, rolling the rough tube, and subjecting the obtained rough tube to a first rapid cooling, the first rapid cooling including a first rapid cooling and a second rapid cooling, wherein the temperature of the rough tube decreases by 30-150°C during the first rapid cooling and the temperature of the rough tube decreases by 30-150°C during the second rapid cooling; Step S4, sizing or reducing the diameter of the rough tube after the first rapid cooling, and subjecting the obtained steel pipe to a second rapid cooling, wherein the temperature of the steel pipe after the second rapid cooling is 550-650°C; Step S5, air-cooling the steel pipe obtained after the second rapid cooling.

[0007] Furthermore, the chemical composition of the metallurgical raw materials, by weight percentage, is as follows: C 0.10%–0.20%, Si 0.15%–0.45%, Mn 1.05%–1.45%, V 0.02%–0.09%, Nb 0.020%–0.06%, Al 0.015%–0.06%, Ti ≤ 0.020%, Cr ≤ 0.30%, Ni ≤ 0.25%, Mo ≤ 0.10%, P ≤ 0.020%, S ≤ 0.010%, N ≤ 0.012%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, in step S2, the temperature of the billet is 1150–1250°C during the piercing process;

[0009] Preferably, the perforation process is a tapered enlarged diameter perforation.

[0010] Further, step S2 includes: heating the billet in a ring furnace at a temperature of 1200-1290°C for 3.5-4.5 hours;

[0011] Preferably, the billet is sequentially passed through a heat recovery section, preheating section I, heating section I, heating section II, heating section III, soaking section I, and soaking section II; preferably, the temperature of preheating section I is 400℃~650℃, the temperature of heating section I is 650~850℃, the temperature of heating section II is 850~1120℃, the temperature of heating section III is 1120~1290℃, the temperature of soaking section I is 1200~1290℃, and the temperature of soaking section II is 1200~1290℃; preferably, the billet exit temperature is 1190~1280℃, and the total heating time is ≥3.8 hours.

[0012] Furthermore, in step S3, the rolling temperature is 950–1150°C.

[0013] Furthermore, the first rapid cooling and / or the second rapid cooling adopt a through-cooling method;

[0014] Preferably, the first rapid cooling is carried out after the raw pipe exits the pipe-removing machine, and preferably, the cooling rate of the first rapid cooling is 1.5 to 50°C / s;

[0015] Preferably, the second rapid cooling is carried out before the rough pipe enters the sizing or reducing equipment, and preferably, the cooling rate of the second rapid cooling is 1.5 to 50°C / s.

[0016] Furthermore, the sizing or reducing temperature in step S4 is in the non-recrystallization zone above Ar3+20℃.

[0017] Furthermore, the second rapid cooling is carried out in one or more cooling correction groups, which include a rapid cooling equipment frame and a sizing mill finished product frame arranged sequentially along the pipe transport direction; the rapid cooling equipment frame includes a water spray device, and the center line of the water ring of the water spray device is parallel to and at the same horizontal position as the center line of the sizing mill finished product frame;

[0018] Preferably, the frame of the rapid cooling equipment is an air-pass cooling equipment;

[0019] Preferably, the water spraying device is fixed in the frame of the rapid cooling equipment. The water spraying device has a cavity that can accommodate the pipe through which it passes. The water spraying device is equipped with nozzles that are evenly distributed around the inner wall of the cavity of the water spraying device, one or more times. Preferably, the direction of water spraying from the nozzles is along the direction of pipe transportation, and the angle between the nozzles and the pipe is 30° to 60°. Preferably, the water flow rate of a single water spraying device is 10 to 300 m³ / h. 3 / h; Preferably, the cooling rate of the steel pipe in the frame of the rapid cooling equipment is 1.5~50℃ / s;

[0020] Preferably, the finishing frame of the sizing mill is a round hole type finishing mill frame; preferably, the round hole type finishing mill frame has alternating arrangement of the hole type roller gap and roller bottom;

[0021] Preferably, the number of cooling correction groups is 2 to 12.

[0022] Furthermore, the water spraying device is a truncated cone with its axis parallel to the ground. Along the transport direction of the pipe, there are small and large openings of the water spraying device. The small opening is the opening with a smaller cavity diameter of the water spraying device, and the large opening is the opening with a larger cavity diameter of the water spraying device. The water spraying device has a jacket for containing the cooling medium, and the nozzle is a hole set on the inner wall of the water spraying device.

[0023] Preferably, the nozzle orifice diameter is 0.5 to 5 mm.

[0024] According to another aspect of the present invention, a seamless steel pipe is provided, which is prepared by any of the above-described preparation methods.

[0025] By applying the technical solution of this invention, the rolled rough tube undergoes a second rapid cooling process before sizing or reducing. This not only lowers the final rolling temperature of the steel tube through two-stage rapid cooling, keeping the tube in the non-recrystallization region during the sizing or reducing process, but also preserves the hardened austenite to the phase transformation point. New phases and carbonitrides surround the strain nucleus, significantly refining the grain structure. Rapid cooling after sizing or reducing allows the steel tube structure to quickly pass through the phase transformation region, preventing the newly formed phases from growing and coarsening. Simultaneously, slow cooling in the low-temperature upper bainite region avoids the formation of brittle upper bainite, ultimately resulting in a significantly refined ferrite + pearlite structure. This application, through a rationally designed cooling path, can obtain hot-rolled seamless steel tubes with low straightness and ellipticity and excellent strength and toughness. Moreover, the preparation method of this application can fully utilize existing seamless steel tube equipment, requiring only minor modifications to complete the preparation of high-strength and high-toughness seamless steel tubes without the need for any complex rapid cooling equipment. This facilitates online mass production, significantly reducing production costs and improving production efficiency. Attached Figure Description

[0026] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0027] Figure 1 A schematic diagram of the cooling correction assembly according to an embodiment of the present invention is shown;

[0028] Figure 2 A schematic diagram of the structure of a rapid cooling equipment frame according to an embodiment of the present invention is shown;

[0029] Figure 3 A schematic diagram showing the relative positions of the water spray device and the steel pipe according to an embodiment of the present invention is shown.

[0030] The above-mentioned attached drawings include the following reference numerals: 1. Cooling correction group; 01. Rapid cooling equipment frame; 02. Sizing machine finished product frame; 010. Water spraying device; 011. Nozzle; 012. Cooling water supply device interface; 013. Finished product frame; 014. Large opening; 015. Small opening. Detailed Implementation

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0032] As analyzed in the background section of this application, the requirements for the strength and toughness of seamless steel pipes are becoming increasingly stringent. However, existing high-strength and high-toughness seamless steel pipes suffer from poor stability or complex manufacturing processes. To address this issue, this application provides a seamless steel pipe and its manufacturing method.

[0033] According to a typical embodiment of this application, a method for preparing a seamless steel pipe is provided, the method comprising the following steps: Step S1, smelting metallurgical raw materials to obtain a billet; Step S2, piercing the billet to obtain a rough tube; Step S3, rolling the rough tube, and subjecting the obtained rough tube to a first rapid cooling, the first rapid cooling including a first rapid cooling and a second rapid cooling, wherein the temperature of the rough tube decreases by 30-150°C during the first rapid cooling and the temperature of the rough tube decreases by 30-150°C during the second rapid cooling; Step S4, sizing or reducing the diameter of the rough tube after the first rapid cooling, and subjecting the obtained steel pipe to a second rapid cooling, wherein the temperature of the steel pipe after the second rapid cooling is 550-650°C; Step S5, air cooling the steel pipe obtained by the second rapid cooling.

[0034] In the seamless steel pipe manufacturing method of this application, the rolled rough pipe undergoes a second rapid cooling process before sizing or reducing. This two-stage rapid cooling not only lowers the final rolling temperature of the steel pipe, keeping it in the non-recrystallization region during the sizing or reducing process, but also preserves the hardened austenite to the phase transformation point. New phases and carbonitrides surround the stress nucleus, significantly refining the grain structure. The rapid cooling after sizing or reducing allows the steel pipe structure to quickly pass through the phase transformation region, preventing the newly formed phases from growing and coarsening. Simultaneously, slow cooling in the low-temperature upper bainite region avoids the formation of brittle upper bainite, ultimately resulting in a highly refined ferrite + pearlite structure. By rationally setting the cooling path, this application can obtain hot-rolled seamless steel pipes with low straightness and ellipticity, and excellent strength and toughness. Furthermore, the manufacturing method of this application can fully utilize existing seamless steel pipe equipment; with slight modifications, high-strength and high-toughness seamless steel pipes can be manufactured without adding any complex rapid cooling equipment. This facilitates online mass production, significantly reducing production costs and improving production efficiency.

[0035] In some embodiments of this application, the chemical composition of the metallurgical raw materials, by weight percentage, is: C 0.10%–0.20%, Si 0.15%–0.45%, Mn 1.05%–1.45%, V 0.02%–0.09%, Nb 0.020%–0.06%, Al 0.015%–0.06%, Ti ≤ 0.020%, Cr ≤ 0.30%, Ni ≤ 0.25%, Mo ≤ 0.10%, P ≤ 0.020%, S ≤ 0.010%, N ≤ 0.012%, with the balance being Fe and unavoidable impurities. By using metallurgical raw materials with the above composition to prepare seamless steel pipes, the non-recrystallization region of hot-rolled seamless steel pipes can be expanded, allowing the hardened austenite to be maintained until the final rolling stage. This avoids the nucleation and growth of deformed microstructures in the steel pipe, which would ultimately lead to the deterioration of the steel pipe's strength, toughness, and low-temperature toughness. At the same time, through the synergistic effect between the components, dispersed carbonitrides are precipitated within the grains during the production process, preventing the formation of brittle upper bainite. This results in hot-rolled seamless steel pipes with certain strength, toughness, and excellent low-temperature toughness.

[0036] In some embodiments of this application, during step S2, the billet temperature is 1150–1250°C during piercing. Specifically, it can be 1150°C, 1170°C, 1190°C, 1210°C, 1230°C, 1250°C, or other values ​​within this range, which are not limited here. Piercing within this temperature range can effectively reduce internal defects and improve the mechanical properties of the steel pipe. Preferably, the piercing process is tapered expansion piercing. Using tapered expansion piercing can achieve a larger total deformation, allowing for flexible adjustment of the deformation in each process, such as piercing and rolling.

[0037] Understandably, the billet can be heat-treated before the piercing process. In some embodiments of this application, step S2 includes heating the billet in a ring furnace at a temperature of 1200–1290°C for 3.5–4.5 hours.

[0038] In some embodiments of this application, the billet sequentially passes through a heat recovery section, preheating section I, heating section I, heating section II, heating section III, homogenization section I, and homogenization section II. The heat recovery section primarily utilizes the heat from the exhaust gas discharged from the furnace to preheat the billet, reducing energy consumption. In this area, the billet temperature varies according to the exhaust gas temperature, serving as the preheating stage. In preheating section I, the billet begins the actual heating process. The heating rate at this stage needs to be controlled to avoid excessive temperature gradients between the billet surface and interior, leading to stress concentration; preferably, the temperature in preheating section I is 400°C to 650°C. In heating section I, the billet begins to experience a more significant temperature rise, preferably 650°C to 850°C, which promotes uniformity within the billet. The preferred temperature range for heating section II is 850°C to 1120°C, where the billet is heated to near its deformation temperature, aiming to facilitate better plastic deformation during subsequent rolling. Heating Zone III: The preferred heating temperature is 1120–1290℃. This is the stage where the billet reaches its final deformation temperature, allowing it to deform smoothly during subsequent piercing and continuous rolling processes. Soaking Zone I: The preferred heating temperature is 1200–1290℃. This heating zone ensures uniform overall billet temperature, reducing differences in mechanical properties caused by temperature inconsistencies. Soaking Zone II: The preferred temperature is 1200–1290℃. This further extends the billet's residence time at high temperatures, ensuring the entire billet reaches ideal temperature consistency and microstructure before exiting the furnace. Preferably, the billet's exit temperature is 1190–1280℃, with a total heating time ≥3.8 hours. By precisely controlling the process parameters during heating, especially temperature, uniform heating of the billet can be ensured, avoiding thermal stress and cracking, and improving the overall performance of the finished seamless steel pipe.

[0039] In step 3, the tube is rolled to obtain a rough tube, which is then subjected to a first rapid cooling process after exiting the tube stripping machine. In some embodiments of this application, the rolling temperature is 950–1150°C, specifically 950°C, 970°C, 990°C, 1000°C, 1020°C, 1050°C, 1080°C, 1100°C, 1120°C, 1140°C, 1150°C, etc., or other values ​​within this range, which are not limited here. Controlling the rolling temperature within the above range is beneficial for obtaining rough tubes with better dimensional accuracy.

[0040] In some embodiments of this application, the rolling unit includes a continuous rolling mill, which is beneficial to improving the dimensional accuracy of the rolled rough tubes.

[0041] In some embodiments of this application, the aforementioned first and / or second rapid cooling employs a through-cooling method. This through-cooling method allows for more uniform cooling of the rough tube and provides a controllable cooling rate, adaptable to steel tubes of different sizes and shapes. In seamless steel tube production, the use of through-cooling, particularly in the first rapid cooling process of this application which includes two rapid cooling cycles, can significantly improve the strength and toughness of the steel tube. By controlling the cooling process, the internal microstructure of the material is optimized, thereby improving material performance.

[0042] During the first rapid cooling process, the temperature of the rough tube drops by 30–150°C. This means the first rapid cooling cools the rough tube to a temperature 30–150°C below the temperature before entering the rapid cooling zone. Specifically, this can be 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, or other values ​​within the aforementioned range. Preferably, the cooling rate of the first rapid cooling is 1.5–50°C / s, specifically 1.5°C / s, 10°C / s, 20°C / s, 30°C / s, 40°C / s, 50°C / s, or other values ​​within the aforementioned range. In some embodiments of this application, the first rapid cooling is performed after the rough tube exits the tube-removing machine. This allows the first rapid cooling to occur during the transportation of the rough tube, without affecting the normal production rhythm of the steel pipe, thus improving the production efficiency of seamless steel pipes and effectively reducing costs.

[0043] During the second rapid cooling process, the temperature of the raw pipe drops by 30–150°C. In other words, the second rapid cooling cools the raw pipe to a temperature 30–150°C below the temperature before entering the second rapid cooling zone. Specifically, this can be 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, or other values ​​within the aforementioned range. Preferably, the cooling rate of the second rapid cooling is 1.5–50°C / s, specifically 1.5°C / s, 10°C / s, 20°C / s, 30°C / s, 40°C / s, 50°C / s, or other values ​​within the aforementioned range. In some embodiments of this application, the second rapid cooling is performed before the raw pipe enters the sizing or reducing equipment. The two rapid cooling processes in the first rapid cooling occur at different locations during the transportation of the steel pipe, without affecting the normal production rhythm of the steel pipe. This allows for better utilization of existing equipment through modification and is relatively simple and easy to implement.

[0044] It should be noted that the first and second rapid cooling processes can proceed at the existing pace; for example, the location of the raw tube can be changed in between, during which the raw tube can undergo a certain degree of natural cooling. Preferably, the temperature drop of the raw tube between the first and second rapid cooling processes is 0–200°C.

[0045] In some embodiments of this application, the rough tube that has undergone the first rapid cooling process is subjected to high-pressure water descaling to remove the iron oxide scale generated during heating or rolling, thereby improving the surface quality of the steel tube.

[0046] The rough tube after the first rapid cooling process is then subjected to sizing or reduction treatment. The sizing or reduction treatment process can refer to existing technologies. In some embodiments of this application, the sizing or reduction treatment temperature in step S4 is in the non-recrystallization region above Ar3 + 20°C. It is understood that Ar3 is one of the thermodynamic transformation temperatures, specifically referring to the temperature at which austenite begins to transform into ferrite. The Ar3 temperature is related to the chemical composition in metallurgical principles. The temperature of the non-recrystallization region is lower than the recrystallization temperature of the steel tube structure. Sizing or reducing the diameter in the non-recrystallization region prevents recrystallization of the metal grains, allowing them to remain in a hardened state after processing. This helps to form a finer grain structure, thereby improving the strength and toughness of the material. The temperature of the non-recrystallization region is related to the composition of the steel tube and the processing technology, and can be obtained by calculation and experimentation using existing methods.

[0047] In some typical embodiments of this application, the second rapid cooling is performed in one or more cooling correction groups 1, such as... Figure 1 As shown, the cooling and straightening assembly 1 includes a rapid cooling equipment frame 01 and a sizing mill finished product frame 02 arranged sequentially along the pipe transport direction; the rapid cooling equipment frame 01 includes a water spray device 010 (e.g., ...). Figure 2 As shown, the center line of the water ring of the water spray device 010 is parallel to the center line of the finished product frame 02 of the sizing machine and is at the same horizontal position.

[0048] The aforementioned cooling and correction group 1, which consists of the rapid cooling equipment frame 01 and the sizing mill finished product frame 02, is arranged sequentially along the pipe transport direction. This cooling method allows the steel pipe to be rapidly cooled and immediately enter the sizing mill finished product frame 02 for immediate correction. This avoids bending caused by uneven cooling during the rapid cooling process. In particular, when the aforementioned rapid cooling device contains multiple cooling and correction groups 1, multiple rapid cooling equipment frames 01 and sizing mill finished product frames 02 are arranged alternately along the pipe transport direction. The alternating cooling and correction of multiple frames can greatly improve the cooling speed of the steel pipe, thereby obtaining hot-rolled seamless steel pipes with low straightness and ovality and excellent strength and toughness.

[0049] In some embodiments of this application, the aforementioned rapid cooling equipment frame 01 is an air-pass cooling device, capable of online cooling of the pipe. This facilitates precise adjustment of the cooling rate and accurate control of the cooling path based on the pipe's outer diameter, thickness, and length, thereby effectively and precisely controlling the microstructure of the cooled pipe. Preferably, the cooling rate of the steel pipe in the rapid cooling equipment frame is 1.5–50°C / s, specifically 1.5°C / s, 10°C / s, 20°C / s, 30°C / s, 40°C / s, 50°C / s, etc., or other values ​​within the aforementioned range.

[0050] In some embodiments of this application, such as Figure 2 As shown, the aforementioned rapid cooling equipment frame 01 includes a water spraying device 010, which is fixed within the frame. The water spraying device 010 has a cavity capable of accommodating pipes. The water spraying device 010 is equipped with nozzles 011, which are evenly distributed around the inner wall of the cavity of the water spraying device 010, ensuring a more uniform and rapid temperature drop across the pipes, thereby improving pipe performance. Preferably, the direction of water spraying from the nozzles 011 is along the direction of pipe transport, with an angle of 30° to 60° with the pipe. That is, the water flow from the water spraying device 010 is directed towards the pipe transport direction, and the angle between the direction of the water flow and the pipe transport direction is 30° to 60°. Preferably, the water flow rate of a single water spraying device 010 is 10 to 300 m³ / h. 3 / h. For example... Figure 1 As shown, the frame of the aforementioned rapid cooling equipment also includes a cooling water supply device interface 012, which is connected to the water spray device 010 and can continuously supply cooling water to the water spray device.

[0051] In some embodiments, such as Figure 2 As shown, the water spray device 010 is installed on the finished product frame 013. In some embodiments, the above-mentioned rapid cooling equipment frame can be modified from the finished product frame of the sizing mill. Specifically, the rolls on the finished product frame of the sizing mill are removed to obtain the finished product frame 013, and the water spray device 010 and the cooling water supply device interface 012 are installed on the finished product frame. That is to say, the above-mentioned rapid cooling equipment frame can be simply modified based on the original equipment. Through the cooperation with the finished product frame of the sizing mill, the rapidly cooled steel pipe is immediately corrected, which can not only greatly increase the cooling speed, but also help to obtain hot-rolled seamless steel pipes with small straightness and ovality and excellent strength and toughness.

[0052] In some embodiments of this application, such as Figure 3 As shown, the water spray device 010 is a frustum with its axis parallel to the ground, along the pipe (the pipe is as shown in the image). Figure 3In the transport direction of the cylinder shown, a water spray device 010 is provided with a small opening 015 and a large opening 014. The small opening 015 is the opening with a smaller diameter cavity of the water spray device 010, and the large opening 014 is the opening with a larger diameter cavity (the cavity shape is also a frustum) of the water spray device 010. The water spray device 010 has a jacket for containing the cooling medium. The nozzle 011 is a hole provided on the inner wall of the water spray device 010. The water spray device 010 uses the opening in the inner wall of the frustum-shaped water spray device as the nozzle 011, and the angle of water spraying is controlled by controlling the taper of the frustum. This water spray device 010 not only has a good cooling effect, but also has a simple structure, is easy to process and manufacture, and has a low failure rate. Those skilled in the art will understand that, for pipes of different thicknesses or specifications, the cooling intensity of the water spray device 010 can be adjusted by adjusting the water spray volume of the water spray device 010 and the density of the nozzle 011 to adapt to pipes of different thicknesses or specifications and achieve the purpose of rapid cooling.

[0053] In some embodiments of this application, the orifice diameter of the nozzle 011 is 0.5 to 5 mm, specifically 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, etc., or other values ​​within the above range, which helps to achieve rapid cooling of the pipe and further improve the strength and toughness of the seamless steel pipe.

[0054] In some embodiments of this application, the above-mentioned sizing mill finished product frame 02 is a round hole type sizing mill finished product frame, which is particularly suitable for the correction of seamless steel pipes after cooling.

[0055] In some embodiments of this application, the die-cutting roll gaps and roll bottoms of the finished product frame of the circular die-cutting sizing mill are arranged alternately. The die-cutting roll gap refers to the gap between two adjacent die-cutting rolls used to form the steel pipe in the circular die-cutting sizing mill, and the roll bottom refers to the bottom of the die-cutting roll. The alternating arrangement of the die-cutting roll gaps and roll bottoms means that in the arrangement of the finished product frame of the circular die-cutting mill, the die-cutting roll gaps and roll bottoms are not continuous, but alternate. That is, a certain position of the steel pipe is at the roll bottom (or roll gap) of one frame's die-cutting roll, while the same position is at the roll gap (or roll bottom) of the next frame's die-cutting roll. This design has the following effects: the alternating arrangement of the die-cutting roll gaps and roll bottoms ensures that when the steel pipe passes through the sizing mill, the difference in deformation at the same position of the steel pipe over a long period at the roll bottom (or roll gap) can be eliminated, resulting in a more uniform deformation of the entire circular cross-section. This helps to eliminate ellipticity or bending that may occur during rapid cooling or deformation, thereby improving the straightness and roundness of the product.

[0056] Understandably, each cooling and straightening unit 1 can be equipped with a conveying mechanism to transfer the pipe from the cooling and straightening unit 01 to the sizing mill finished product frame 02. Similarly, when the rapid cooling device contains multiple cooling and straightening units 1, a conveying mechanism can also be provided between adjacent cooling and straightening units 1 to transport the pipe from the sizing mill finished product frame 02 to the next cooling and straightening unit 01. It should be noted that sometimes, to accommodate the production rhythm of seamless steel pipes, such as when the number of cooling and straightening units is less than the number of sizing mill frame positions, additional transport frames are added to transport the steel pipe from the cooling and straightening units to the transport roller conveyor.

[0057] The specific number of cooling correction groups 1 in the aforementioned rapid cooling device is not limited. In some embodiments of this application, as examples, the number of cooling correction groups 1 is 1 to 12, specifically 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12. Preferably, the number of cooling correction groups is 2 to 12, which has a more significant effect on improving the strength, toughness, straightness, and ovality of hot-rolled seamless steel pipes.

[0058] Step S5 involves air cooling the steel pipe that has undergone a second rapid cooling process at a temperature of 550–650°C. The air cooling method can be selected from existing technologies, such as air cooling on a large cooling bed.

[0059] After air cooling, the steel pipe can be post-processed using existing technologies to obtain seamless steel pipe products. The post-processing methods can refer to existing technologies, and specifically, include, but are not limited to, cutting off the ends, straightening, physical and chemical testing, non-destructive testing, manual inspection, lettering, and packaging for warehousing.

[0060] According to another embodiment of this application, a seamless steel pipe is provided, which is prepared by any of the above-described preparation methods. The seamless steel pipe prepared by the above methods not only has low straightness and ellipticity and excellent strength and toughness, but also can operate stably in harsh environments such as high pressure, high temperature, deep sea, and extreme climates. Furthermore, the preparation cost is low, and it has good market application prospects.

[0061] The beneficial effects that this application can achieve will be further illustrated below with reference to embodiments and comparative examples.

[0062] Example 1

[0063] 1) The metallurgical raw materials are smelted to obtain billets. According to the percentage of each component in the total weight of the metallurgical raw materials, the metallurgical raw materials include C: 0.15%, Si: 0.30%, Mn: 1.25%, V: 0.05%, Nb: 0.035%, Al: 0.025%, Ti: 0.0070%, Cr: 0.050%, Ni: 0.080%, Mo: 0.053%, P: 0.008%, S: 0.0025%, N: 0.0058%, with the balance being Fe and unavoidable impurities.

[0064] 2) The obtained billet is heated in a ring furnace at a temperature of 1240℃, sequentially passing through seven heating zones: heat recovery zone, preheating zone I, heating zone I, heating zone II, heating zone III, soaking zone I, and soaking zone II. The heating temperatures in the seven zones are: furnace temperature, 550℃, 750℃, 950℃, 1200℃, 1240℃, and 1240℃, respectively. The billet exits the furnace at a temperature of 1230℃, and the total heating time is controlled at 3.8 hours. After heating, the billet undergoes conical expansion and piercing at 1200℃ to form a tube.

[0065] 3) A continuous rolling mill is used to continuously roll the tube at 1100℃ to obtain a rough tube with initially determined dimensions. The rough tube is immediately subjected to rapid cooling after exiting the mill, using a through-cooling method at a rate of 10℃ / s, cooling it to 100℃ below the temperature before entering the rapid cooling zone. The rough tube undergoes rapid cooling again before entering the sizing mill at a rate of 10℃ / s, cooling it to 100℃ below the temperature before entering this rapid cooling zone.

[0066] 4) The Ar3 temperature of the rough pipe was tested to be 720℃, and the temperature of the non-recrystallized zone was 720~850℃. The rough pipe was first descaled with high-pressure water, and then the rough pipe was subjected to a sizing process at a temperature of 800℃.

[0067] 5) The steel pipes undergo a third rapid cooling process immediately after exiting the sizing mill. This third rapid cooling equipment includes five round-hole sizing mill finished product frames and five... Figure 2 and 3 The rapid cooling equipment frame shown depicts steel pipes that immediately enter another circular hole sizing mill frame after passing through one rapid cooling unit. This process is repeated until the final circular hole sizing mill frame, where the steel pipes are rapidly cooled to 560°C. The nozzle 011 of the rapid cooling equipment has an angle of 45°, meaning the angle between the water spray direction and the pipe transport direction is 45°, and the water flow is along the direction of the steel pipe's movement. The water flow rate of a single rapid cooling equipment frame's spray device is 50 m³ / h. 3 / h or so.

[0068] 6) The cooled steel pipe is transported to a large cooling bed for air cooling, and finally a seamless steel pipe with a specification of 168×14mm is obtained.

[0069] Example 2

[0070] The difference from Example 1 is that in step 5), the third rapid cooling device includes a finished product frame of a round hole sizing machine and a rapid cooling device frame. After passing through one rapid cooling device, the steel pipe enters another finished product frame of a round hole sizing machine to cool the steel pipe to the same temperature.

[0071] Example 3

[0072] The difference from Example 1 is that, in step 1), the metallurgical raw materials include C: 0.21%, Si: 0.46%, Mn: 1.46%, V: 0.10%, Nb: 0.065%, Al: 0.065%, Ti: 0.0065%, Cr: 0.055%, Ni: 0.080%, Mo: 0.053%, P: 0.010%, S: 0.003%, N: 0.006%, with the balance being Fe and unavoidable impurities.

[0073] Example 4

[0074] The difference from Example 1 is that in step 4), the sizing temperature is 740°C.

[0075] Example 5

[0076] The difference from Example 1 is that in step 4), the sizing temperature is 850°C.

[0077] Example 6

[0078] The difference from Example 1 is that in step 4), the sizing temperature is 880°C.

[0079] Example 7

[0080] The difference from Example 1 is that in step 3), the rough tube is rapidly cooled immediately after exiting the continuous rolling mill using a through-cooling method at a rate of 10°C / s, cooling to 150°C below the temperature of the steel tube before entering the rapid cooling zone. The rough tube is then rapidly cooled again before entering the sizing mill at a rate of 10°C / s, cooling to 150°C below the temperature of the steel tube before entering the rapid cooling zone.

[0081] Example 8

[0082] The difference from Example 1 is that in step 3), the rough tube is rapidly cooled immediately after exiting the continuous rolling mill using a through-cooling method at a rate of 10°C / s, cooling to 50°C below the temperature of the steel tube before entering the rapid cooling zone. The rough tube is then rapidly cooled again before entering the sizing mill at a rate of 10°C / s, cooling to 50°C below the temperature of the steel tube before entering the rapid cooling zone.

[0083] Example 9

[0084] The difference from Example 1 is that in step 4), the diameter reduction process of the rough tube is carried out at a temperature of 800°C.

[0085] Comparative Example 1

[0086] The difference from Example 1 is that in step 3), after the rough tube leaves the continuous rolling mill, it is conveyed to the high-pressure water descaling via the conveyor rollers, and then undergoes a diameter reduction process. After the diameter reduction process, step 5) is omitted, and the tube is directly conveyed to the large cooling bed via the conveyor rollers for air cooling, finally obtaining a hot-rolled seamless steel tube.

[0087] Comparative Example 2

[0088] The difference from Example 1 is that in step 3), the rough tube is only subjected to rapid cooling once after leaving the continuous rolling mill. The rough tube is rapidly cooled before entering the sizing mill at a cooling rate of 15°C / s, until the temperature of the steel tube is 180°C below the temperature before entering the rapid cooling zone.

[0089] Comparative Example 3

[0090] The difference from Example 1 is that in step 3), the continuous rolling mill performs continuous rolling at 1150°C to obtain a rough tube with initially determined dimensions. The rough tube is immediately subjected to rapid cooling after exiting the mill, using a through-cooling method at a rate of 15°C / s, cooling to a temperature 180°C below the temperature before entering the rapid cooling zone. The rough tube undergoes rapid cooling again before entering the sizing mill at a rate of 15°C / s, cooling to a temperature 180°C below the temperature before entering the rapid cooling zone.

[0091] Comparative Example 4

[0092] The difference from Example 1 is that step 5 is omitted, and the steel pipe is directly air-cooled on the large cooling bed after leaving the sizing machine.

[0093] The seamless steel pipes prepared in the above embodiments and comparative examples were subjected to performance tests according to the following methods, and the test results are listed in Table 1 below.

[0094] The dimensional accuracy, tensile properties, and impact properties of the seamless steel pipes prepared in Examples 1-9 and Comparative Examples 1-4 were tested using vernier calipers, spirit levels, and in accordance with GB / T228.1 and GB / T229, respectively. The results are shown in Table 1.

[0095] Table 1

[0096]

[0097]

[0098] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects: In the seamless steel pipe preparation method of this application, the rolled rough pipe is subjected to secondary rapid cooling before sizing or reducing treatment. This not only reduces the final rolling temperature of the steel pipe through two-stage rapid cooling, keeping the steel pipe in the non-recrystallization region during the sizing or reducing process, but also maintains the hardened austenite to the phase transformation point. New phases and carbonitrides surround the stress deformation nucleus, which can greatly refine the grain structure and fully disperse the precipitation of carbonitrides. The rapid cooling of the steel pipe after sizing or reducing treatment allows the steel pipe structure to quickly pass through the phase transformation region, preventing the newly formed phase from growing and coarsening. At the same time, slow cooling in the low-temperature upper bainite region avoids the formation of brittle upper bainite, ultimately obtaining a greatly refined ferrite + pearlite structure. By reasonably setting the cooling path, this application can obtain hot-rolled seamless steel pipes with small straightness and ellipticity and excellent strength and toughness. Moreover, the preparation method of this application can make full use of existing seamless steel pipe equipment. With slight modifications, the preparation of high-strength and high-toughness seamless steel pipe production equipment can be completed without adding any complex rapid cooling equipment. This facilitates online mass production, which can greatly reduce production costs and improve production efficiency.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a seamless steel pipe, characterized in that, Includes the following steps: Step S1: Smelt the metallurgical raw materials to obtain billets; Step S2: The blank is perforated to obtain a tube. Step S3: Roll the tube into a rough tube and then subject the resulting rough tube to a first rapid cooling process. The first rapid cooling process includes a first rapid cooling and a second rapid cooling. During the first rapid cooling process, the temperature of the rough tube decreases by 30-150°C. During the second rapid cooling process, the temperature of the rough tube decreases by 30-150°C. Step S4: The rough tube after the first rapid cooling is sized or reduced in diameter, and the resulting steel pipe is subjected to a second rapid cooling. The temperature of the steel pipe after the second rapid cooling is 550~650℃. Step S5: Air-cool the steel pipe obtained by the second rapid cooling. Step S2 includes: heating the billet in a ring furnace at a temperature of 1200~1290℃ for 3.5~4.5 hours; passing the billet sequentially through a heat recovery section, a preheating section I, a heating section I, a heating section II, a heating section III, a homogenization section I, and a homogenization section II; the temperature of the preheating section I is 400℃~650℃, the temperature of the heating section I is 650~850℃, the temperature of the heating section II is 850~1120℃, the temperature of the heating section III is 1120~1290℃, the temperature of the homogenization section I is 1200~1290℃, and the temperature of the homogenization section II is 1200~1290℃; the billet exits the furnace at a temperature of 1190~1280℃, and the total heating time is ≥3.8 hours; The first rapid cooling and / or the second rapid cooling adopt a through-cooling method; the first rapid cooling is carried out after the rough pipe exits the pipe-removing machine, and the cooling rate of the first rapid cooling is 1.5~50℃ / s; the second rapid cooling is carried out before the rough pipe enters the sizing equipment or the reducing equipment, and the cooling rate of the second rapid cooling is 1.5~50℃ / s. The second rapid cooling is carried out in one or more cooling correction groups (1), the cooling correction group (1) includes a rapid cooling equipment frame (01) and a sizing machine finished product frame (02) arranged sequentially along the pipe transport direction; the rapid cooling equipment frame (01) includes a water spray device (010), the center line of the water ring of the water spray device (010) and the center line of the sizing machine finished product frame (02) are parallel and at the same horizontal position; The rapid cooling equipment frame (01) is an air-pass cooling device; the water spray device (010) is fixed in the rapid cooling equipment frame (01), the water spray device (010) has a cavity that can accommodate the pipe material, the water spray device (010) is provided with nozzles (011), the nozzles (011) are evenly distributed on the water spray device (010) around one or more times the inner wall of the cavity of the water spray device (010); the direction of water spraying by the nozzles (011) is along the direction of pipe material transportation, and the angle with the pipe material is 30°~60°; the water flow rate of a single water spray device (010) is 10~300m³ / h. 3 / h; The cooling rate of the steel pipe in the frame (01) of the rapid cooling equipment is 1.5~50℃ / s; The sizing mill finished product frame (02) is a round hole type sizing mill finished product frame; the round hole type sizing mill finished product frame has alternating arrangement of the hole type roller gap and roller bottom; the number of cooling correction groups (1) is 2~12; The water spray device (010) is a frustum with its axis parallel to the ground. Along the transport direction of the pipe, the water spray device (010) is provided with a small opening (015) and a large opening (014). The small opening (015) is the opening with a smaller cavity diameter of the water spray device (010), and the large opening (014) is the opening with a larger cavity diameter of the water spray device (010). The water spray device (010) has a jacket for containing cooling medium. The nozzle (011) is a hole provided on the inner wall of the water spray device (010). The diameter of the nozzle (011) is 0.5~5mm.

2. The method for preparing seamless steel pipe according to claim 1, characterized in that, The chemical composition of the metallurgical raw material, by weight percentage, is: C 0.10%~0.20%, Si 0.15%~0.45%, Mn 1.05%~1.45%, V 0.02%~0.09%, Nb 0.020%~0.06%, Al 0.015%~0.06%, Ti≤0.020%, Cr≤0.30%, Ni≤0.25%, Mo≤0.10%, P≤0.020%, S≤0.010%, N≤0.012%, with the balance being Fe and unavoidable impurities.

3. The method for preparing seamless steel pipe according to claim 1, characterized in that, In step S2, the temperature of the billet is 1150~1250℃ when the piercing process is performed. The perforation process is a tapered, enlarged-diameter perforation.

4. The method for preparing seamless steel pipe according to claim 1, characterized in that, In step S3, the rolling temperature is 950~1150℃.

5. The method for preparing seamless steel pipe according to claim 1, characterized in that, The sizing or reducing temperature in step S4 is in the non-recrystallization zone above Ar3+20℃.

6. A seamless steel pipe, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 5.

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