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: step S1, the raw material of seamless steel tube is sequentially dosed, smelted, first heated, pierced, rolled, first cooled, second heated, sizing or reducing, to obtain first steel pipe;Step S2, the inner surface of first steel pipe is water jet cooling, to obtain second steel pipe;Step S3, the outer surface of second steel pipe is atomized cooling, to obtain third steel pipe;And step S4, the second cooling of third steel pipe is carried out, to obtain seamless steel tube.The present application can solve the problem that the super-thick wall hot-rolled seamless steel pipe cannot be quickly and uniformly cooled in the cooling process due to its large heat storage, and the inner surface temperature of the pipe is raised when using the through cooling method, which reduces the cooling speed.

Description

Technical Field

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

[0002] In recent years, the requirements for the strength, toughness, and dimensional accuracy of seamless steel pipe products have become increasingly stringent. Currently, seamless steel pipe grades above 345 are mainly carbon-manganese steel with a small amount of microalloying elements. For example, the performance requirements for E470 grade steel pipes are a tensile strength ≥650MPa, a yield strength ≥470MPa, and a low-temperature impact resistance of ≥34J at -20℃. As the wall thickness of steel pipes increases, even exceeding 30mm, online normalizing or tempering processes cannot reliably meet the high requirements for the strength and toughness of seamless steel pipes. Currently, one way to improve the strength and toughness of seamless steel pipes is to add expensive alloying elements, and another is to use a quenching and tempering heat treatment process. However, both methods significantly increase production costs, making it difficult to balance strength and toughness with production costs, and are also detrimental to subsequent welding of the steel pipes and the safety of the components.

[0003] To improve the strength and toughness of steel pipes, an online normalizing and rapid cooling process after sizing has been proposed. Firstly, online normalizing technology refines austenite grains through recrystallization. Accelerated cooling after sizing prevents grain growth during cooling. The rapidly cooled steel pipe after sizing further refines the grains, ultimately improving both strength and toughness by controlling rolling and cooling. However, this process still has shortcomings: 1) It cannot precisely control the bainite content; improper microstructure control can drastically deteriorate both the strength and toughness of the steel pipe. 2) The hollow cross-section of the steel pipe results in larger cross-sectional dimensions and a wider range of size variations, making uniform cooling impossible. Uneven cooling during accelerated cooling can cause bending and deformation. 3) Thick-walled steel pipes have high heat storage capacity. When using a through-cooling method, the reheating of the inner surface reduces the cooling rate, and inconsistent cooling between the inner and outer surfaces leads to significant differences in microstructure and properties.

[0004] Therefore, there is an urgent need to develop a method for preparing ultra-thick-walled seamless steel pipes that can precisely control the cooling path of the steel pipes and achieve the production of ultra-thick-walled hot-rolled seamless steel pipes with excellent strength and toughness. Summary of the Invention

[0005] The main objective of this invention is to provide a seamless steel pipe and its preparation method, in order to solve the problem that the ultra-thick-walled seamless steel pipes in the prior art cannot be cooled uniformly and rapidly, resulting in poor strength and toughness. In order to solve the above problems, this application provides a seamless steel pipe and its preparation method.

[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: step S1, sequentially batching, smelting, first heating, piercing, rolling, first cooling, second heating, and sizing or reducing the diameter of the raw materials for the seamless steel pipe to obtain a first steel pipe; step S2, spraying water to cool the inner surface of the first steel pipe to obtain a second steel pipe; step S3, atomizing the outer surface of the second steel pipe to obtain a third steel pipe; and step S4, subjecting the third steel pipe to a second cooling to obtain a seamless steel pipe.

[0007] Further, in step S2 above, the temperature of the inner surface of the second steel pipe is 150~300℃ lower than the temperature of the inner surface of the first steel pipe; and / or, the temperature of the inner surface of the second steel pipe is 550~650℃; and / or, the cooling rate of the water spray cooling is 30~60℃ / s; and / or, the water flow rate of the water spray cooling is 500~1000m³. 3 / h.

[0008] Further, in step S3 above, the temperature of the outer surface of the third steel pipe is 100~260℃ lower than the temperature of the outer surface of the second steel pipe; and / or, the temperature of the outer surface of the third steel pipe is 550~650℃; and / or, the cooling rate of atomization cooling is 5~20℃ / s; the second steel pipe is atomized and cooled by passing it through an atomization bed, and the speed at which the second steel pipe passes through the atomization bed is 30~70s / step; and / or, in step S4, the second cooling is air cooling.

[0009] Furthermore, step S1 above also includes: step S11, batching and smelting the raw materials sequentially to obtain a billet; step S12, subjecting the billet to a first heating and piercing sequentially to obtain a rough tube; step S13, rolling the rough tube to obtain a blank tube; step S14, subjecting the blank tube to a first cooling to obtain a first cooled blank tube; step S15, subjecting the first cooled blank tube to a second heating to obtain a second heated blank tube; and step S16, subjecting the second heated blank tube to sizing or reducing its diameter to obtain a first steel pipe.

[0010] Furthermore, by mass percentage, the elemental composition of the raw material includes: 0.10~0.22% C, 0.15~0.65% Si, 1.05~1.70% Mn, 0.03~0.12% V, 0.020~0.06% Nb, 0.020~0.06% Al, Ti ≤0.010%, Cr ≤0.20%, Ni ≤0.10%, Mo ≤0.10%, P ≤0.020%, S ≤0.010%, N ≤0.006%, with the balance being Fe; wherein the mass ratio of the total mass of V, Al, and Nb to the mass of C is 1~2.2:1.

[0011] Further, the final temperature of the first heating is 1210~1300℃; and / or, the first heating process includes: the billet is heated in the ring furnace by sequentially passing through a heat recovery section, a preheating section I, a heating section I, a heating section II, a heating section III, a soaking section I, and a soaking section II; wherein, the temperature of the heat recovery section is 200~400℃, 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 1120~1300℃, the temperature of the soaking section I is 1210~1300℃, the temperature of the soaking section II is 1210~1300℃, the billet exiting the furnace is 1200~1290℃, and the total heating time of the first heating is 3.5~4.5h.

[0012] Furthermore, piercing is performed using a tapered diameter expansion method at a temperature of 1170~1270℃; and / or, rolling is performed at a temperature of 960~1160℃.

[0013] Furthermore, the temperature of the rough tube after the first cooling is 450~600℃; A c3 The temperature is 845~860℃; and / or, the temperature of the rough tube after the second heating is higher than that of A. c3 The temperature is 50-60°C higher than A; and / or, the rough tube after the second heating is sized or reduced within the temperature range of the non-recrystallization zone, where the temperature of the non-recrystallization zone is higher than A. c3 The temperature is 20-30℃ higher.

[0014] According to another aspect of the present invention, a seamless steel pipe is provided, which is prepared by the above-described preparation method.

[0015] Furthermore, the thickness of the seamless steel pipe is 20~60mm; and / or, the average grain size of the seamless steel pipe is 9~19μm; and / or, the tensile strength of the seamless steel pipe is 560~650MPa, the yield strength of the seamless steel pipe is 501~550MPa, and the elongation of the seamless steel pipe is 24~30%; at -20℃, the average impact energy AkV of the seamless steel pipe is 162~230J.

[0016] By applying the technical solution of this invention, this application utilizes a two-stage rapid cooling process—water spraying cooling on the inner surface and atomizing cooling on the outer surface—to solve the problem that ultra-thick-walled hot-rolled seamless steel pipes, due to their large heat storage capacity, experience a decrease in cooling rate caused by the reheating of the inner surface during the cooling process using a through-type cooling method, thus preventing the seamless steel pipe from cooling quickly and uniformly. Specifically, reheating after the first cooling (second heating) can refine the grain size through a secondary phase transformation. Water spraying cooling on the inner surface of the first steel pipe not only enables rapid cooling of the inner surface, allowing the entire steel pipe to pass uniformly through the high-temperature phase transformation region, preventing the newly formed phase from growing and coarsening rapidly, but also preserves the hardened austenite to the phase transformation point. The new phase and carbonitrides surround the stress deformation nucleus, thereby greatly refining the grain structure. Atomizing cooling on the outer surface of the second steel pipe enables rapid cooling of the outer surface or local high-temperature areas of the second steel pipe, thereby achieving simultaneous, uniform, and rapid cooling of the inner and outer surfaces of the steel pipe to the set temperature. The second cooling process on the third steel pipe allows for slow cooling within the bainite transformation region, avoiding the upper bainite transformation region and thus preventing the formation of brittle upper bainite. This significantly refines the ferrite and pearlite microstructure, resulting in an ultra-thick-walled seamless steel pipe with excellent strength, toughness, and low-temperature toughness. Furthermore, the cooling process described in this application significantly reduces the production cost of seamless steel pipes and improves production efficiency. Detailed Implementation

[0017] 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.

[0018] As analyzed in the background section of this application, the existing ultra-thick-walled seamless steel pipes suffer from poor strength and toughness due to the inability to cool evenly and rapidly. In order to solve the above problems, this application provides a seamless steel pipe and its preparation method.

[0019] In a typical embodiment of this application, a method for preparing a seamless steel pipe is provided. The method includes: step S1, sequentially batching, smelting, first heating, piercing, rolling, first cooling, second heating, and sizing or reducing the diameter of the raw materials for the seamless steel pipe to obtain a first steel pipe; step S2, spraying water to cool the inner surface of the first steel pipe to obtain a second steel pipe; step S3, atomizing the outer surface of the second steel pipe to obtain a third steel pipe; and step S4, subjecting the third steel pipe to a second cooling to obtain a seamless steel pipe.

[0020] This application utilizes a two-stage rapid cooling process—internal surface water spray cooling and external surface atomization cooling—to address the problem of slow and uniform cooling of ultra-thick-walled hot-rolled seamless steel pipes during cooling due to the large heat storage capacity and subsequent surface reheating during through-flow cooling. Specifically, the second heating (reheating after the first cooling) refines the grain size through a secondary phase transformation. Water spray cooling of the inner surface of the first steel pipe not only rapidly cools the inner surface, causing the entire pipe to pass uniformly through the high-temperature phase transformation region, preventing the newly formed phases from growing and coarsening rapidly, but also preserves the hardened austenite at the transformation point. New phases and carbonitrides surround the stress nuclei, significantly refining the grain structure. Atomization cooling of the outer surface of the second steel pipe rapidly cools the outer surface or localized high-temperature areas, achieving simultaneous, uniform, and rapid cooling of both the inner and outer surfaces to the set temperature. The second cooling process on the third steel pipe allows for slow cooling within the bainite transformation region, avoiding the upper bainite transformation region and thus preventing the formation of brittle upper bainite. This significantly refines the ferrite and pearlite microstructure, resulting in an ultra-thick-walled seamless steel pipe with excellent strength, toughness, and low-temperature toughness. Furthermore, the cooling process described in this application significantly reduces the production cost of seamless steel pipes and improves production efficiency.

[0021] Water spray cooling is a cooling method in which a pressure pump sprays liquid water through a nozzle at a high pressure, a certain flow rate, and a certain angle onto the inner surface of a steel pipe. It mainly relies on convection heat transfer to remove the heat from the steel pipe.

[0022] Atomized cooling is a cooling method that uses nozzles to mix water vapor and break liquid water into extremely fine droplets to form a water mist, which is then sprayed onto the outer surface of the steel pipe at different angles. It mainly relies on the rapid evaporation of the water mist due to its extremely high surface area to absorb heat.

[0023] In one embodiment of this application, in step S2 above, the temperature of the inner surface of the second steel pipe is 150-300°C lower than the temperature of the inner surface of the first steel pipe; and / or, the temperature of the inner surface of the second steel pipe is 550-650°C; and / or, the cooling rate of the water spray cooling is 30-60°C / s; and / or, the water flow rate of the water spray cooling is 500-1000 m³ / s. 3 / h.

[0024] Preferably, the first steel pipe is cooled by spraying water in an online water quenching tank. It is also preferable to control the temperature of the inner surface of the second steel pipe to be 150-300°C lower than that of the first steel pipe. Controlling the cooling rate and water flow rate within this range helps to increase the cooling speed of the first steel pipe and regulate the temperature of the inner surface of the second steel pipe to achieve the aforementioned range. This allows the entire steel pipe to pass uniformly through the high-temperature phase transformation region, mitigating the rapid growth and coarsening of newly formed phases. Furthermore, it helps to retain the hardened austenite to the phase transformation point, with new phases and carbonitrides surrounding the stress nuclei, thereby further refining the grain structure.

[0025] In one embodiment of this application, in step S3 above, the temperature of the outer surface of the third steel pipe is 100~260℃ lower than the temperature of the outer surface of the second steel pipe; and / or, the temperature of the outer surface of the third steel pipe is 550~650℃; and / or, the cooling rate of atomization cooling is 5~20℃ / s; the second steel pipe is atomized and cooled by passing it through an atomization bed, and the speed at which the second steel pipe passes through the atomization bed is 30~70s / step; and / or, in step S4 above, the second cooling is air cooling.

[0026] Preferably, the temperature of the outer surface of the third steel pipe is controlled to be 100-260°C lower than that of the outer surface of the second steel pipe. Controlling the cooling rate of the atomization cooling and the speed at which the second steel pipe passes through the atomization bed within this range helps to rapidly cool the outer surface or localized high-temperature areas of the second steel pipe. This allows the inner and outer surfaces of the steel pipe to be cooled simultaneously, uniformly, and rapidly to the set temperature, thereby promoting the synchronicity and uniformity of the phase transformation process inside and outside the steel pipe. The atomization bed is a stepping-type device, where s / step represents the number of seconds taken to advance one step (one step distance). For example, 30s / step means advancing one step distance every 30 seconds.

[0027] By rapidly cooling through the above two stages, the inner and outer surface temperatures of the thick-walled steel pipe are precisely controlled within a narrow range, that is, the temperature difference between the outer surface and the inner surface of the third steel pipe is 0~50℃. This helps to reduce the temperature difference between the inside and outside of the steel pipe, thereby improving the uniformity of the steel pipe's performance in the thickness direction.

[0028] The preferred cooling rate of water spray cooling is 15~50℃ / s higher than that of atomization cooling. This helps to reduce the reduction in the cooling rate of the second stage of atomization cooling caused by heat accumulation on the inner surface of the steel pipe due to the short-term rapid cooling of the inner surface of the steel pipe through the first stage of water spray cooling. At the same time, the second stage of atomization cooling rapidly cools the outer surface and local areas of the steel pipe, which helps to quickly reduce the overall temperature of the steel pipe to the set temperature, thereby accurately and rapidly reducing the overall temperature of the steel pipe to a specific temperature at a certain cooling rate.

[0029] In one embodiment of this application, step S1 further includes: step S11, batching and smelting the raw materials sequentially to obtain a billet; step S12, subjecting the billet to a first heating and piercing sequentially to obtain a rough tube; step S13, rolling the rough tube to obtain a blank tube; step S14, subjecting the blank tube to a first cooling to obtain a first cooled blank tube; step S15, subjecting the first cooled blank tube to a second heating to obtain a second heated blank tube; and step S16, subjecting the second heated blank tube to sizing or reducing its diameter to obtain a first steel pipe.

[0030] According to the chemical composition requirements of the target seamless steel pipe, the prepared raw materials are smelted to obtain a billet. Through initial heating and piercing, a rough tube is obtained. The rough tube is then rolled to adjust its size and shape, forming a blank tube. The blank tube undergoes initial cooling on a small cooling bed, which helps control the cooling rate and temperature, reducing cracks and deformation, and preparing it for subsequent reheating. The blank tube after initial cooling is then reheated (secondary heating), which helps promote secondary phase transformation, thereby further refining the grains. Size adjustment is performed by sizing or reducing to achieve the precise diameter specifications of the first steel pipe.

[0031] In one embodiment of this application, the elemental composition of the raw material, by mass percentage, includes: 0.10~0.22% C, 0.15~0.65% Si, 1.05~1.70% Mn, 0.03~0.12% V, 0.020~0.06% Nb, 0.020~0.06% Al, Ti ≤0.010%, preferably 0.005~0.010%, and Cr ≤0.20%, preferably 0.1~0.20%. The content of Ni is ≤0.10%, preferably 0.08~0.10%; Mo is ≤0.10%, preferably 0.06~0.10%; P is ≤0.020%, preferably 0.006~0.020%; S is ≤0.010%, preferably 0.002~0.010%; N is ≤0.006%, preferably 0.004~0.006%; and the balance is Fe. The ratio of the total mass of V, Al and Nb to the mass of C is 1~2.2:1.

[0032] Using raw materials with the above-mentioned elemental composition and content helps to expand the non-recrystallization region of hot-rolled seamless steel pipes, allowing the hardened austenite to be maintained until the final rolling stage. This alleviates the nucleation and growth of deformed microstructures in the steel pipe, thus improving the strength, toughness, and low-temperature toughness of the seamless steel pipe. At the same time, through the synergistic effect of the above components, dispersed carbides are precipitated within the grains during the preparation process, reducing the formation of brittle upper bainite, thereby obtaining ultra-thick-walled seamless steel pipes with excellent strength, toughness, and low-temperature toughness.

[0033] In one embodiment of this application, the final temperature of the first heating is 1210~1300℃; and / or, the first heating process includes: the billet is heated in a ring furnace by sequentially passing through a heat recovery section, a preheating section I, a heating section I, a heating section II, a heating section III, a soaking section I, and a soaking section II; wherein, the temperature of the heat recovery section is 200~400℃, 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 1120~1300℃, the temperature of the soaking section I is 1210~1300℃, the temperature of the soaking section II is 1210~1300℃, the billet exiting the furnace is 1200~1290℃, and the total heating time of the first heating is 3.5~4.5h.

[0034] Heating the billet to the aforementioned temperature range in the ring furnace helps it achieve ideal uniform temperature and microstructure, facilitating subsequent piercing and rolling steps. Simultaneously, this process allows the austenite phase in the billet to become more abundant, improving the plasticity of the steel pipe. Preferably, the temperature of the heat recovery section is within the aforementioned range (i.e., furnace temperature), which helps preheat the billet and recover waste heat from the furnace exhaust gases, improving energy efficiency and reducing production costs. Preferably, the temperature of preheating zone I is within the aforementioned range, which helps further preheat the billet, reducing stress concentration and cracking caused by sudden temperature changes. Preferably, the temperature of heating zone I is within the aforementioned range, which helps the billet continue to heat up, initiating the formation of the austenite phase. Preferably, the temperature of heating zone II is within the aforementioned range, which helps the billet reach the higher temperature level required for piercing, improving the material's plasticity and reducing its deformation resistance. Preferably, the temperature of heating zone III is within the aforementioned range, which helps prepare for the next process. Preferably, the temperatures of homogenizing zones I and II are within the aforementioned range, which helps to achieve overall temperature uniformity in the billet, reducing potential temperature gradients and thus facilitating subsequent piercing and rolling processes. The temperature of the heat spreader II is the final temperature of the first heating.

[0035] In one embodiment of this application, piercing is performed by tapering and the piercing temperature is 1170~1270℃; and / or, the rolling temperature is 960~1160℃.

[0036] Using a tapered diameter expansion method for piercing, and controlling the piercing temperature within the aforementioned range, helps to obtain a larger total deformation, thereby flexibly adjusting the deformation of each process, such as piercing and rolling. Using a continuous rolling mill or an Assel mill for rolling, preferably controlling the rolling temperature within the aforementioned range, helps to refine the grains, thereby enhancing the toughness and strength of the steel pipe material.

[0037] In one embodiment of this application, the temperature of the first cooled rough tube is 450~600℃; A c3The temperature is 845~860℃; and / or, the temperature of the rough tube after the second heating is higher than that of A. c3 The temperature is 50-60°C higher than A; and / or, the rough tube after the second heating is sized or reduced within the temperature range of the non-recrystallization zone, where the temperature of the non-recrystallization zone is higher than A. c3 The temperature is 20-30℃ higher.

[0038] The rough tube is first cooled in a small cooling bed, and then cooled to the above-mentioned temperature range. It is then heated to the above-mentioned temperature range in a walking beam furnace, which helps to promote secondary phase transformation and further refine the grains.

[0039] A c3 It is the critical temperature at which ferrite completely dissolves into austenite. Based on the composition of the steel pipe, A... c3 The temperature is within the above range. Preferably, the temperature of the rough tube after the second heating is controlled to be higher than A. c3 A temperature increase of 50-60℃ helps promote secondary phase transformation, thereby further refining the austenite grains.

[0040] The non-recrystallization region is the temperature range in which grains do not recrystallize and grow again after the material is deformed at high temperatures. Preferably, the second heated rough tube is sized or reduced within the aforementioned temperature range of the non-recrystallization region, which helps to maintain the plasticity of the material, while reducing grain coarsening, ultimately resulting in a first steel tube with finer grains.

[0041] Seamless steel pipes undergo subsequent processes such as end trimming, straightening, physical and chemical testing, non-destructive testing, manual inspection, lettering, packaging, and warehousing to obtain the finished product.

[0042] In another typical embodiment of this application, a seamless steel pipe is provided, which is prepared by the above-described preparation method.

[0043] The seamless steel pipes prepared by the above method have excellent strength, toughness and low-temperature toughness, and can meet high performance requirements even with thicker walls.

[0044] In one embodiment of this application, the thickness of the seamless steel pipe is 20~60mm; and / or, the average grain size of the seamless steel pipe is 9~19μm; and / or, the tensile strength of the seamless steel pipe is 560~610MPa, the yield strength of the seamless steel pipe is 501~550MPa, and the elongation of the seamless steel pipe is 24~30%; at -20℃, the average impact energy AkV of the seamless steel pipe is 162~230J.

[0045] The preferred thickness of the seamless steel pipe is within the above-mentioned range, which helps to meet different application requirements. The preferred average grain size of the seamless steel pipe is within the above-mentioned range, which helps to improve the strength and toughness of the seamless steel pipe.

[0046] It should be noted that, due to the special nature of the alloy field and the limitations of existing testing and characterization methods, it is difficult to fully and quantitatively characterize the complex microstructure of the seamless steel pipe obtained above. However, experiments show that the seamless steel pipe obtained in this application has higher strength and toughness.

[0047] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0048] Example 1

[0049] The raw materials for seamless steel pipes are smelted by mass percentage according to the following composition: 0.15% C, 0.40% Si, 1.60% Mn, 0.07% V, 0.04% Nb, 0.04% Al, 0.005% Ti, 0.15% Cr, 0.05% Ni, 0.05% Mo, 0.001% P, 0.005% S, 0.003% N, with unavoidable impurities totaling ≤0.15% and the balance being Fe. The smelting process involves electric furnace smelting, LF refining, VD vacuum degassing, and continuous casting to obtain billets. The starting temperature for continuous casting is 1560℃.

[0050] The billet is sequentially subjected to a first heating and then to conical expansion and piercing at 1200℃ to obtain a rough tube. The first heating process includes: sequentially passing through a heat recovery section, preheating section I, heating section I, heating section II, heating section III, soaking section I, and soaking section II in a ring furnace; the temperature in the heat recovery section is 300℃ (following the furnace temperature), the temperature in preheating section I is 500℃, the temperature in heating section I is 750℃, the temperature in heating section II is 1000℃, the temperature in heating section III is 1200℃, the temperature in soaking section I is 1250℃, the temperature in soaking section II (the final temperature of the first heating) is 1280℃, the billet exiting the furnace is 1280℃, and the total heating time for the first heating is 4.5 hours.

[0051] The rough tube is rolled at 1000℃ using a continuous rolling mill to obtain a blank tube. The blank tube undergoes a first cooling process on a small cooling bed until its temperature reaches 550℃. After the first cooling, the blank tube undergoes a second heating process in a walking beam furnace until it reaches a temperature higher than that of tube A. c3 The temperature is 55°C higher, resulting in a second heated rough tube at 910°C. This second heated rough tube is then placed in a sizing mill and sized at a temperature higher than A. c3 The first steel pipe is obtained by sizing at a temperature above 20°C in the non-recrystallization zone. Among them, A... c3 The temperature is 855℃.

[0052] The inner surface of the first steel pipe was cooled by spraying water into an online water quenching tank until the temperature of the inner surface of the second steel pipe reached 600℃. The cooling rate was 50℃ / s, and the water flow rate was 800m³ / s. 3 / h, the temperature of the inner surface of the second steel pipe is 200℃ lower than the temperature of the inner surface of the first steel pipe.

[0053] The outer surface of the second steel pipe is cooled by atomization in an atomizing bed, and the temperature of the outer surface of the third steel pipe is set to 600℃. The cooling rate of atomization is 10℃ / s, the speed of the second steel pipe passing through the atomizing bed is 50s / step, the temperature of the outer surface of the third steel pipe is 150℃ lower than that of the outer surface of the second steel pipe, and the temperature difference between the outer surface and the inner surface of the third steel pipe is 20℃.

[0054] The third steel pipe is transported to a large cooling bed for a second cooling air cooling process, resulting in a seamless steel pipe with a thickness of 50mm.

[0055] Example 2

[0056] The raw materials for seamless steel pipes are smelted by mass percentage according to the following composition: 0.20% C, 0.65% Si, 1.05% Mn, 0.12% V, 0.02% Nb, 0.06% Al, 0.005% Ti, 0.20% Cr, 0.08% Ni, 0.1% Mo, 0.006% P, 0.01% S, 0.004% N, with unavoidable impurities totaling ≤0.15% and the balance being Fe. The smelting process involves electric furnace smelting, LF refining, VD vacuum degassing, and continuous casting to obtain billets. The starting temperature for continuous casting is 1560℃.

[0057] The billet is sequentially subjected to a first heating and then to conical expansion and piercing at 1170℃ to obtain a rough tube. The first heating process includes: sequentially passing through a heat recovery section, preheating section I, heating section I, heating section II, heating section III, soaking section I, and soaking section II in a ring furnace; the temperature in the heat recovery section is 200℃ (following the furnace temperature), the temperature in preheating section I is 400℃, the temperature in heating section I is 650℃, the temperature in heating section II is 850℃, the temperature in heating section III is 1120℃, the temperature in soaking section I is 1210℃, the temperature in soaking section II (the final temperature of the first heating) is 1210℃, the billet exit temperature is 1200℃, and the total heating time for the first heating is 4.5 hours.

[0058] The rough tube is rolled at 960℃ using a continuous rolling mill to obtain a blank tube. The blank tube undergoes a first cooling process on a small cooling bed until its temperature reaches 450℃. After the first cooling, the blank tube undergoes a second heating process in a walking beam furnace until it reaches a temperature higher than that of tube A. c3 The temperature is increased by 50°C, resulting in a second heated rough tube at 900°C. This second heated rough tube is then placed in a sizing mill, and the rough tube is sized at a temperature higher than A. c3 The first steel pipe is obtained by sizing at a temperature above 20°C in the non-recrystallization zone. Among them, A... c3 The temperature is 850℃.

[0059] The inner surface of the first steel pipe was cooled by spraying water into an online water quenching tank until the temperature of the inner surface of the second steel pipe reached 550℃. The cooling rate was 30℃ / s, and the water flow rate was 500m³ / s. 3 / h, the temperature of the inner surface of the second steel pipe is 150℃ lower than the temperature of the inner surface of the first steel pipe.

[0060] The outer surface of the second steel pipe is cooled by atomization in an atomizing bed, and the temperature of the outer surface of the third steel pipe is set to 550℃. The cooling rate of atomization is 5℃ / s, the speed of the second steel pipe passing through the atomizing bed is 60s / step, the temperature of the outer surface of the third steel pipe is 100℃ lower than that of the outer surface of the second steel pipe, and the temperature difference between the outer surface and the inner surface of the third steel pipe is 5℃.

[0061] The third steel pipe is transported to a large cooling bed for a second cooling air cooling process, resulting in a seamless steel pipe with a thickness of 50mm.

[0062] Example 3

[0063] The raw materials for seamless steel pipes are smelted by mass percentage according to the following composition: 0.10% C, 0.15% Si, 1.70% Mn, 0.03% V, 0.06% Nb, 0.02% Al, 0.01% Ti, 0.1% Cr, 0.10% Ni, 0.06% Mo, 0.02% P, 0.002% S, and 0.006% N, with the total unavoidable impurities content ≤0.15% and the balance being Fe. The smelting process involves electric furnace smelting, LF refining, VD vacuum degassing, and continuous casting to obtain billets. The starting temperature for continuous casting is 1560℃.

[0064] The billet is sequentially heated and then pierced in a conical shape at 1270℃ to obtain a rough tube. The first heating process includes: heating in a ring furnace sequentially through a heat recovery section, preheating section I, heating section I, heating section II, heating section III, soaking section I, and soaking section II; the temperature in the heat recovery section is 400℃ (following the furnace temperature), the temperature in preheating section I is 650℃, the temperature in heating section I is 850℃, the temperature in heating section II is 1120℃, the temperature in heating section III is 1300℃, the temperature in soaking section I is 1300℃, the temperature in soaking section II (the final temperature of the first heating) is 1300℃, the billet exit temperature is 1290℃, and the total heating time for the first heating is 3.5 hours.

[0065] The rough tube is rolled at 1160℃ using a continuous rolling mill to obtain a blank tube. The blank tube undergoes a first cooling process on a small cooling bed until its temperature reaches 600℃. After the first cooling, the blank tube undergoes a second heating process in a walking beam furnace until it reaches a temperature higher than that of tube A. c3 The temperature is increased by 60°C, resulting in a second heated rough tube at 920°C. This second heated rough tube is then sized in a sizing mill at a temperature in the non-recrystallization zone, which is at least 20°C higher than the temperature of Ac3, to obtain the first steel tube. Wherein, A... c3 The temperature is 860℃.

[0066] The inner surface of the first steel pipe was cooled by spraying water into an online water quenching tank until the temperature of the inner surface of the second steel pipe reached 550℃. The cooling rate was 60℃ / s, and the water flow rate was 1000 m³ / s. 3 / h, the temperature of the inner surface of the second steel pipe is 150℃ lower than the temperature of the inner surface of the first steel pipe.

[0067] The outer surface of the second steel pipe is cooled by atomization in an atomizing bed, and the temperature of the outer surface of the third steel pipe is set to 550℃. The cooling rate of atomization is 20℃ / s, the speed of the second steel pipe passing through the atomizing bed is 35s / step, the temperature of the outer surface of the third steel pipe is 260℃ lower than that of the outer surface of the second steel pipe, and the temperature difference between the outer and inner surfaces of the third steel pipe is 50℃.

[0068] The third steel pipe is transported to a large cooling bed for a second cooling air cooling process, resulting in a seamless steel pipe with a thickness of 50mm.

[0069] Example 4

[0070] The difference from Example 1 is that an online water-cooling method is used to spray water onto the inner surface of the first steel pipe, cooling it to a temperature of 550°C on the inner surface of the second steel pipe. The cooling rate of the water spray is 60°C / s, and the water flow rate is 1000 m³ / s. 3 / h, the temperature of the inner surface of the second steel pipe is 150℃ lower than that of the inner surface of the first steel pipe, and finally a seamless steel pipe is obtained.

[0071] Example 5

[0072] The difference from Example 1 is that an online water-cooling method is used to spray water onto the inner surface of the first steel pipe, cooling it to a temperature of 700°C on the inner surface of the second steel pipe. The cooling rate of the water spray is 20°C / s, and the water flow rate is 300 m³ / s. 3 / h, the temperature of the inner surface of the second steel pipe is 100℃ lower than that of the inner surface of the first steel pipe, and finally a seamless steel pipe is obtained.

[0073] Example 6

[0074] The difference from Example 1 is that the outer surface of the second steel pipe is subjected to atomized cooling, and the temperature of the outer surface of the third steel pipe is set to 550°C. The atomized cooling rate is 20°C / s, the speed of the second steel pipe passing through the atomized bed is 35s / step, the temperature of the outer surface of the third steel pipe is 260°C lower than that of the second steel pipe, and the temperature difference between the outer and inner surfaces of the third steel pipe is 50°C, ultimately resulting in a seamless steel pipe.

[0075] Example 7

[0076] The difference from Example 1 is that the outer surface of the second steel pipe is subjected to atomized cooling, and the temperature of the outer surface of the third steel pipe is set to 500°C. The atomized cooling rate is 25°C / s, the speed of the second steel pipe passing through the atomized bed is 75s / step, the temperature of the outer surface of the third steel pipe is 290°C lower than that of the second steel pipe, and the temperature difference between the outer and inner surfaces of the third steel pipe is 80°C, ultimately resulting in a seamless steel pipe.

[0077] Example 8

[0078] The difference from Example 1 is that the cooling rate of water spray cooling is 60°C / s, while the cooling rate of atomization cooling is 20°C / s. The cooling rate of water spray cooling is 40°C / s higher than that of atomization cooling, and a seamless steel pipe is finally obtained.

[0079] Example 9

[0080] The difference from Example 1 is that the cooling rate of water spray cooling is 60°C / s, while the cooling rate of atomization cooling is 5°C / s. The cooling rate of water spray cooling is 55°C / s higher than that of atomization cooling, and a seamless steel pipe is finally obtained.

[0081] Example 10

[0082] The difference from Example 1 is that the rough tube undergoes a first cooling process on a small cooling bed, cooling it to a temperature of 600°C after the first cooling. The first-cooled rough tube is then subjected to a second heating process in a walking beam furnace, heating it to a temperature higher than A. c3 The temperature is 60°C higher, resulting in a rough tube with a second heating temperature of 920°C. A c3 The temperature is 860℃, and seamless steel pipes are finally obtained.

[0083] Example 11

[0084] The difference from Example 1 is that the rough tube undergoes a first cooling process on a small cooling bed, cooling it to a temperature of 650°C after the first cooling. The first-cooled rough tube is then subjected to a second heating process in a walking beam furnace, heating it to a temperature higher than A. c3 The temperature is 65°C higher, resulting in a second heated rough tube at 925°C. A c3 The temperature is 860℃, and seamless steel pipes are finally obtained.

[0085] Example 12

[0086] The difference from Example 1 is that the total mass content of V, Al and Nb is 0.22%, the mass content of C is 0.10%, and the ratio of the total mass of V, Al and Nb to the mass of C is 2.2:1, resulting in a seamless steel pipe.

[0087] Example 13

[0088] The difference from Example 1 is that the total mass content of V, Al and Nb is 0.24%, the mass content of C is 0.10%, and the ratio of the total mass of V, Al and Nb to the mass of C is 2.4:1, resulting in a seamless steel pipe.

[0089] Comparative Example 1

[0090] The difference from Example 1 is that the rough tube is not cooled for the first time on a small cooling bed, but is directly transported to a walking beam furnace for the second heating and diameter reduction, and finally a seamless steel tube is obtained.

[0091] Comparative Example 2

[0092] The difference from Example 1 is that only atomization cooling is performed on the outer surface of the first steel pipe, while the inner surface is not cooled separately, resulting in a seamless steel pipe.

[0093] Comparative Example 3

[0094] The difference from Example 1 is that the second steel pipe is directly transported to the large cooling bed for a second cooling to obtain a seamless steel pipe.

[0095] Test method:

[0096] Average grain size test: Tested according to GB / T 6394 standard.

[0097] Mechanical properties (tensile strength, yield strength, elongation) and low-temperature impact performance tests: tested according to GB / T228.1 and GB / T 229 standards respectively.

[0098] The test results are shown in Table 1.

[0099] Table 1

[0100]

[0101] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0102] This application utilizes a two-stage rapid cooling process—internal surface water spray cooling and external surface atomization cooling—to address the problem of slow and uniform cooling of ultra-thick-walled hot-rolled seamless steel pipes during cooling due to the large heat storage capacity and subsequent surface reheating during through-flow cooling. Specifically, the second heating (reheating after the first cooling) refines the grain size through a secondary phase transformation. Water spray cooling of the inner surface of the first steel pipe not only rapidly cools the inner surface, causing the entire pipe to pass uniformly through the high-temperature phase transformation region, preventing the newly formed phases from growing and coarsening rapidly, but also preserves the hardened austenite at the transformation point. New phases and carbonitrides surround the stress nuclei, significantly refining the grain structure. Atomization cooling of the outer surface of the second steel pipe rapidly cools the outer surface or localized high-temperature areas, achieving simultaneous, uniform, and rapid cooling of both the inner and outer surfaces to the set temperature. The second cooling process on the third steel pipe allows for slow cooling within the bainite transformation region, avoiding the upper bainite transformation region and preventing the formation of brittle upper bainite. This significantly refines the ferrite and pearlite microstructure, resulting in an ultra-thick-walled seamless steel pipe with excellent strength, toughness, and low-temperature toughness. Furthermore, the cooling process described in this application significantly reduces the production cost of seamless steel pipes and improves production efficiency.

[0103] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. 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, The preparation method includes: Step S1 involves sequentially feeding, smelting, first heating, piercing, rolling, first cooling, second heating, and sizing or reducing the diameter of the seamless steel pipe raw material to obtain the first steel pipe. Step S2: Spray water to cool the inner surface of the first steel pipe to obtain the second steel pipe; Step S3: Atomize and cool the outer surface of the second steel pipe to obtain the third steel pipe; and Step S4: The third steel pipe is subjected to a second cooling to obtain a seamless steel pipe; The temperature of the first cooled rough tube is 450~600℃; A c3 The temperature is 845~860℃; the temperature of the second heated rough tube is higher than that of A. c3 The temperature is 50-60℃ higher; The temperature of the inner surface of the second steel pipe is 150~300℃ lower than the temperature of the inner surface of the first steel pipe; The temperature of the outer surface of the third steel pipe is 100~260℃ lower than the temperature of the outer surface of the second steel pipe; The cooling rate of the water spray cooling is 15~50℃ / s greater than that of the atomization cooling. The temperature difference between the outer surface and the inner surface of the third steel pipe is 0~50℃.

2. The preparation method according to claim 1, characterized in that, In step S2, the temperature of the inner surface of the second steel pipe is 550~650℃; and / or, the cooling rate of the water spray cooling is 30~60℃ / s; and / or, the water flow rate of the water spray cooling is 500~1000m³. 3 / h.

3. The preparation method according to claim 1 or 2, characterized in that, In step S3, the temperature of the outer surface of the third steel pipe is 550~650℃; and / or, the cooling rate of the atomization cooling is 5~20℃ / s; the second steel pipe is subjected to the atomization cooling through the atomization bed, and the speed at which the second steel pipe passes through the atomization bed is 30~70s / step. And / or, in step S4, the second cooling is air cooling.

4. The preparation method according to claim 1 or 2, characterized in that, Step S1 further includes: Step S11: The raw materials are sequentially processed through batching and smelting to obtain a billet; Step S12: The blank is subjected to the first heating and the piercing in sequence to obtain a tube; Step S13: Roll the tube to obtain a rough tube; Step S14: Perform the first cooling on the raw tube to obtain the first cooled raw tube; Step S15: The first cooled rough tube is subjected to the second heating to obtain the second heated rough tube; Step S16: The second heated rough tube is sized or reduced in diameter to obtain the first steel tube.

5. The preparation method according to claim 1 or 2, characterized in that, The raw material, by mass percentage, comprises: 0.10-0.22% C, 0.15-0.65% Si, 1.05-1.70% Mn, 0.03-0.12% V, 0.020-0.06% Nb, 0.020-0.06% Al, Ti ≤0.010%, Cr ≤0.20%, Ni ≤0.10%, Mo ≤0.10%, P ≤0.020%, S ≤0.010%, N ≤0.006%, with the balance being Fe; wherein the ratio of the total mass of V, Al, and Nb to the mass of C is 1-2.2:

1.

6. The preparation method according to claim 4, characterized in that, The final temperature of the first heating is 1210~1300℃; And / or, the first heating process includes: the billet is heated in a ring furnace by sequentially passing through a heat recovery section, a preheating section I, a heating section I, a heating section II, a heating section III, a soaking section I, and a soaking section II; wherein, the temperature of the heat recovery section is 200~400℃, 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 1120~1300℃, the temperature of the soaking section I is 1210~1300℃, the temperature of the soaking section II is 1210~1300℃, the furnace exit temperature of the billet is 1200~1290℃, and the total heating time of the first heating is 3.5~4.5h.

7. The preparation method according to claim 4, characterized in that, The perforation is performed using a tapered diameter expansion method, and the perforation temperature is 1170~1270℃; And / or, the rolling temperature is 960~1160℃.

8. The preparation method according to claim 4, characterized in that, The second heated rough tube is subjected to sizing or reduction within the temperature range of the non-recrystallization zone, wherein the temperature of the non-recrystallization zone is higher than that of A. c3 The temperature is 20-30℃ higher.

9. A seamless steel pipe, characterized in that, The seamless steel pipe is prepared by the preparation method according to any one of claims 1 to 8.

10. The seamless steel pipe according to claim 9, characterized in that, The thickness of the seamless steel pipe is 20~60mm; and / or, the average grain size of the seamless steel pipe is 9~19μm; and / or, the tensile strength of the seamless steel pipe is 560~650MPa, the yield strength of the seamless steel pipe is 501~550MPa, and the elongation of the seamless steel pipe is 24~30%; at -20℃, the average impact energy AkV of the seamless steel pipe is 162~230J.

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