Seamless steel tube and preparation method thereof

Abstract

The invention provides a seamless steel tube and a preparation method thereof. The preparation method comprises the steps that S1, raw materials of the seamless steel pipe are sequentially subjected to burdening, smelting, first heating, perforating, rolling, first cooling, second heating and sizing or reducing, and a first steel pipe is obtained; s2, the inner surface of the first steel pipe is subjected to water spraying cooling, and a second steel pipe is obtained; s3, the outer surface of the second steel pipe is atomized and cooled, and a third steel pipe is obtained; and S4, the third steel pipe is subjected to second cooling, and the seamless steel pipe is obtained. By means of the two-stage rapid cooling process of inner surface water spraying cooling and outer surface atomization cooling, the problem that in the cooling process of the ultra-thick-wall hot-rolled seamless steel pipe, the heat storage amount is large; and when a pass-type cooling mode is adopted, the cooling speed is reduced due to temperature return of the inner surface of the seamless steel pipe, so that the seamless steel pipe cannot be quickly and uniformly cooled.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel pipes, in particular to a seamless steel pipe and a preparation method thereof. BACKGROUND

[0002] In recent years, the requirements for the strength and toughness and dimensional accuracy of seamless steel pipes are increasingly high, and the current seamless steel pipe grades above 345 mainly use carbon manganese steel with a small amount of micro-alloying elements. For example, the performance requirements of E470 grade steel pipes are that the tensile strength is required to be greater than or equal to 650 MPa, the yield strength is required to be greater than or equal to 470 MPa, and the low-temperature impact at-20 DEG C is required to be greater than or equal to 34 J. With the increasing thickness of the steel pipe, even more than 30 mm, the on-line normalizing or normalizing process cannot stably meet the higher requirements for the strength and toughness of the seamless steel pipe. At present, the ways to improve the strength and toughness of the seamless steel pipe are to add expensive alloying elements or to use a quenching and tempering heat treatment process, but both of the above methods will greatly increase the production cost, it is difficult to balance the strength and toughness and the production cost, and it is not conducive to the subsequent welding of the steel pipe and the safety of the component.

[0003] In order to improve the strength and toughness of the steel pipe, an on-line normalizing and sizing rapid cooling process is proposed. First, the on-line normalizing technology refines the austenite grains by recrystallization, and the accelerated cooling after sizing avoids the grain growth during the cooling process, the steel pipe grains are further refined after sizing, and finally the strength and toughness of the steel pipe are improved by controlling the rolling and cooling. However, this process still has some shortcomings: 1) the content of bainite cannot be accurately controlled, and once the microstructure is not properly controlled, the strength and toughness of the steel pipe will be sharply deteriorated; 2) the shape of the steel pipe is a hollow section with a larger cross-sectional size and a larger size specification range, so the steel pipe cannot be uniformly cooled at each part, and the non-uniform cooling in the accelerated cooling process will cause the steel pipe to bend and deform; 3) the thick-walled steel pipe has a large heat storage capacity, and when the through cooling method is used for cooling, the inner surface temperature will be reduced, and the cooling speed will be reduced, and the cooling of the inner and outer surfaces is not uniform, which will cause a large difference in the microstructure and performance between the inner and outer surfaces.

[0004] Therefore, it is urgent to develop a preparation method of an ultra-thick-walled seamless steel pipe, which can accurately control the cooling path of the steel pipe and realize the production of an ultra-thick-walled hot-rolled seamless steel pipe with excellent strength and toughness. SUMMARY

[0005] The main purpose of the present application is to provide a seamless steel pipe and a preparation method thereof, to solve the problem that the existing ultra-thick-walled seamless steel pipe cannot be uniformly and rapidly cooled, resulting in poor strength and toughness, in order to solve the above problems, the present application provides a seamless steel pipe and a preparation method thereof.

[0006] In order to achieve the above object, according to one aspect of the present application, a seamless steel pipe manufacturing method is provided, which comprises: step S1, sequentially performing batching, smelting, first heating, piercing, rolling, first cooling, second heating, sizing or reducing on a raw material of the seamless steel pipe to obtain a first steel pipe; step S2, performing water spray cooling on an inner surface of the first steel pipe to obtain a second steel pipe; step S3, performing atomization cooling on an outer surface of the second steel pipe to obtain a third steel pipe; and step S4, performing second cooling on the third steel pipe to obtain the seamless steel pipe.

[0007] Further, in the step S2, the temperature of the inner surface of the second steel pipe is 150-300℃ lower than that 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 of the water spray cooling is 500-1000m 3 / h.

[0008] Further, in the step S3, the temperature of the outer surface of the third steel pipe is 100-260℃ lower than that 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 the atomization cooling is 5-20℃ / s; the second steel pipe is atomized and cooled by a spray bed, the speed of the second steel pipe passing through the spray bed is 30-70s / step; and / or, in the step S4, the second cooling is air cooling.

[0009] Further, the step S1 further comprises: step S11, sequentially performing batching and smelting on the raw material to obtain a billet; step S12, sequentially performing first heating and piercing on the billet to obtain a rough pipe; step S13, performing rolling on the rough pipe to obtain a roughed pipe; step S14, performing first cooling on the roughed pipe to obtain a first cooled roughed pipe; step S15, performing second heating on the first cooled roughed pipe to obtain a second heated roughed pipe; and step S16, performing sizing or reducing on the second heated roughed pipe to obtain the first steel pipe.

[0010] Further, the element composition of the raw material comprises, in mass percentage: 0.10-0.22% of C element, 0.15-0.65% of Si element, 1.05-1.70 of Mn element, 0.03-0.12% of V element, 0.020-0.06% of Nb element, 0.020-0.06% of Al element, Ti element ≤0.010%, Cr element ≤0.20%, Ni element ≤0.10%, Mo element ≤0.10%, P element ≤0.020%, S element ≤0.010%, N element ≤0.006%, and the balance is Fe element; wherein, the total mass of the V element, Al element and Nb element to the mass of the C element is 1-2.2:1.

[0011] Further, the final temperature of the first heating is 1210-1300℃; and / or, the first heating process comprises that the billet is heated in the annular furnace in turn through a heat recovery section, a preheating section I zone, a heating I zone, a heating II zone, a heating III zone, a soaking I zone and a soaking II zone; wherein the temperature of the heat recovery section is 200-400℃, the temperature of the preheating section I zone is 400-650℃, the temperature of the heating I zone is 650-850℃, the temperature of the heating II zone is 850-1120℃, the temperature of the heating III zone is 1120-1300℃, the temperature of the soaking I zone is 1210-1300℃, the temperature of the soaking II zone is 1210-1300℃, the billet discharge temperature is 1200-1290℃, and the total heating time of the first heating is 3.5-4.5h.

[0012] Further, the piercing is performed in a conical expanding manner, and the piercing temperature is 1170-1270℃; and / or, the rolling temperature is 960-1160℃.

[0013] Further, the temperature of the first cooled hollow pipe is 450-600℃; A c3 ; and / or, the temperature of the second heated hollow pipe is 50-60℃ higher than the temperature of A c3 ; and / or, the second heated hollow pipe is sized or reduced in diameter in a non-recrystallization zone, and the temperature of the non-recrystallization zone is 20-30℃ higher than the temperature of A c3 .

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

[0015] Further, 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, the elongation of the seamless steel pipe is 24-30%, and the average impact energy AkV of the seamless steel pipe at-20℃ is 162-230J.

[0016] By the two-stage rapid cooling process of the inner surface water spray cooling and the outer surface atomization cooling, the problem that the ultra-thick wall hot-rolled seamless steel pipe cannot be rapidly and uniformly cooled due to the reduced cooling speed caused by the inner surface temperature rise when the through type cooling mode is used because of the large heat storage of the ultra-thick wall hot-rolled seamless steel pipe during the cooling process can be solved. Specifically, the first cooling and then reheating (second heating) can refine the grains through the second phase transformation. The inner surface of the first steel pipe is subjected to the water spray cooling, which can not only drive the whole steel pipe to pass through the high temperature phase transformation region uniformly through the rapid cooling of the inner surface, so that the newly formed phase cannot grow rapidly and coarsen, but also can keep the hardened austenite to the phase transformation point, so that the new phase and the carbonitride will surround the strain nucleation, thereby greatly refining the grain structure. The outer surface of the second steel pipe is subjected to the atomization cooling, which can rapidly cool the outer surface or the local high temperature region of the second steel pipe, so that the inner and outer surfaces of the steel pipe are simultaneously and uniformly cooled to the set temperature. The third steel pipe is subjected to the second cooling, which can slowly cool the steel pipe in the bainite transformation region, avoids the upper bainite transformation region, thereby avoiding the formation of the brittle upper bainite, greatly refines the ferrite and pearlite structure, and further obtains the ultra-thick wall seamless steel pipe with excellent strength and toughness and low temperature toughness. In addition, the above cooling process of the present application greatly reduces the production cost of the seamless steel pipe and improves the production efficiency. DETAILED DESCRIPTION

[0017] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the embodiments.

[0018] As analyzed in the background art of the present application, the existing ultra-thick wall seamless steel pipe has the problem of poor strength and toughness due to the non-uniform and rapid cooling. In order to solve the above problem, the present application provides a seamless steel pipe and a preparation method thereof.

[0019] In a typical embodiment of the present application, a preparation method of a seamless steel pipe is provided, which comprises: step S1, sequentially performing batching, smelting, first heating, piercing, rolling, first cooling, second heating, sizing or reducing sizing on a raw material of the seamless steel pipe to obtain a first steel pipe; step S2, performing water spray cooling on the inner surface of the first steel pipe to obtain a second steel pipe; step S3, performing atomization cooling on the outer surface of the second steel pipe to obtain a third steel pipe; and step S4, performing second cooling on the third steel pipe to obtain the seamless steel pipe.

[0020] The application can solve the problem that the ultra-thick-wall hot-rolled seamless steel pipe cannot be cooled quickly and uniformly due to the decrease of cooling speed caused by the temperature rise of the inner surface of the pipe when the through cooling mode is used because of the large heat storage of the pipe during the cooling process by using a two-stage rapid cooling process of water spray cooling on the inner surface and atomization cooling on the outer surface. Specifically, the first cooling and then reheating (second heating) can refine the grains through secondary phase transformation. The water spray cooling on the inner surface of the first steel pipe can not only drive the whole steel pipe to pass through the high-temperature phase transformation region uniformly through the rapid cooling of the inner surface, so that the newly formed phase does not grow and coarsen quickly, but also can keep the hardened austenite to the phase transformation point, and the new phase and carbonitride will surround the strain nucleation, thereby greatly refining the grain structure. The atomization cooling on the outer surface of the second steel pipe can quickly cool the outer surface or local high-temperature region of the second steel pipe, thereby realizing the simultaneous and uniform rapid cooling of the inner and outer surfaces of the steel pipe to the set temperature. The second cooling of the third steel pipe can make the steel pipe slowly cool in the bainite transformation region, avoiding the upper bainite transformation region, thereby avoiding the formation of brittle upper bainite, thereby greatly refining the ferrite structure and pearlite structure, and further obtaining an ultra-thick-wall seamless steel pipe with excellent strength and toughness and low-temperature toughness. In addition, the above cooling process of the application greatly reduces the production cost of the seamless steel pipe and improves the production efficiency.

[0021] The water spray cooling is a cooling mode in which a pressure pump sprays liquid water to the inner surface of the steel pipe at a high pressure, a certain flow rate and a certain angle through a nozzle, mainly relying on convection heat transfer to take away the heat of the steel pipe.

[0022] The atomization cooling is a cooling mode in which water vapor is mixed by using a nozzle or the like, liquid water is broken into extremely small water droplets to form water mist, and the water mist is sprayed at different angles to the outer surface of the steel pipe, mainly relying on the extremely high surface area of the water mist to quickly evaporate to absorb heat.

[0023] In an embodiment of the application, in the step S2, the temperature of the inner surface of the second steel pipe is 150-300℃ lower than that 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 500-1000m 3 / h.

[0024] Preferably, the first steel pipe is subjected to water spray cooling in an online water quenching tank. Preferably, the temperature of the inner surface of the second steel pipe is controlled to be 150-300℃ lower than that of the first steel pipe, and the cooling rate of the water spray cooling and the water flow rate of the water spray cooling are controlled to be in the above range, which helps to increase the cooling speed of the first steel pipe and to control the temperature of the inner surface of the second steel pipe to be in the above range, so as to drive the whole steel pipe to uniformly pass through the high-temperature phase transition region, to relieve the rapid growth and coarsening of the newly formed phase, and to help to keep the hardened austenite to the phase transition point, with the new phase and carbonitride surrounding the strain nucleus, so as to further refine the grain structure.

[0025] In an embodiment of the present application, in the step S3, the temperature of the outer surface of the third steel pipe is 100-260℃ lower than that 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 the atomization cooling is 5-20℃ / s; the second steel pipe is subjected to atomization cooling in an atomization bed, and the speed of the second steel pipe passing through the atomization bed is 30-70s / step; and / or, in the step S4, 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℃ lower than that of the outer surface of the second steel pipe, and the cooling rate of the atomization cooling and the speed of the second steel pipe passing through the atomization bed are controlled to be in the above range, which helps to rapidly cool the outer surface or the local high-temperature region of the second steel pipe, so as to uniformly and rapidly cool the inner and outer surfaces of the steel pipe to the set temperature, and to further promote the synchronization and uniformity of the phase transition process of the inner and outer portions of the steel pipe. The atomization bed is a step-by-step device, and s / step represents the number of seconds spent for each step (one step distance), for example, 30s / step means advancing one step distance every 30s.

[0027] The above two stages of rapid cooling precisely control the inner and outer surface temperatures of the thick-walled steel pipe in a relatively narrow range, i.e., the temperature difference between the outer surface of the third steel pipe and the inner surface of the third steel pipe is 0-50℃, which helps to reduce the temperature difference between the inner and outer surfaces of the steel pipe, thereby improving the uniformity of the performance of the steel pipe in the thickness direction.

[0028] Preferably, the cooling rate of the water spray cooling is 15-50℃ / s greater than that of the atomization cooling, which helps to reduce the decrease in the cooling speed of the second stage of atomization cooling due to the heat accumulation of the inner surface of the steel pipe through the short-time rapid cooling of the inner surface of the steel pipe in the first stage of water spray cooling, while the second stage of atomization cooling rapidly cools the outer surface and the local portion of the steel pipe, which helps to rapidly reduce the whole steel pipe to the set temperature, so as to precisely and rapidly reduce the whole steel pipe to the specific temperature at a certain cooling rate.

[0029] In an embodiment of the present application, the step S1 further comprises: a step S11 of sequentially preparing and smelting the raw material to obtain a billet; a step S12 of sequentially first heating and piercing the billet to obtain a rough pipe; a step S13 of rolling the rough pipe to obtain a hollow pipe; a step S14 of first cooling the hollow pipe to obtain a first cooled hollow pipe; a step S15 of second heating the first cooled hollow pipe to obtain a second heated hollow pipe; and a step S16 of sizing or reducing the second heated hollow pipe to obtain the first steel pipe.

[0030] According to the chemical composition requirements of the target seamless steel pipe, the prepared raw material is smelted to obtain a billet. Through first heating and piercing, a rough pipe is obtained. The rough pipe is adjusted in size and shape through rolling to form a hollow pipe. The hollow pipe is first cooled on a small cooling bed, which helps to control the cooling rate and temperature, reduce cracks and deformation, and prepare for subsequent reheating. The first cooled hollow pipe is heated again (second heating), which helps to promote secondary phase transformation, thereby further refining the grains. Through sizing or reducing, the size of the first steel pipe is adjusted to reach the precise diameter specification.

[0031] In an embodiment of the present application, the elemental composition of the raw material, in terms of mass percentage, comprises: 0.10-0.22% of C element, 0.15-0.65% of Si element, 1.05-1.70 of Mn element, 0.03-0.12% of V element, 0.020-0.06% of Nb element, 0.020-0.06% of Al element, Ti element ≤0.010%, preferably 0.005-0.010%, Cr element ≤0.20%, preferably 0.1-0.20%, Ni element ≤0.10%, preferably 0.08-0.10%, Mo element ≤0.10%, preferably 0.06-0.10%, P element ≤0.020%, preferably 0.006-0.020%, S element ≤0.010%, preferably 0.002-0.010%, N element ≤0.006%, preferably 0.004-0.006%, and the balance being Fe element; wherein the total mass of V element, Al element and Nb element to the mass of C element is 1-2.2:1.

[0032] Preferably, the raw material with the above elemental composition and content is used, which helps to expand the unrecrystallized region of the hot-rolled seamless steel pipe, keeps the hardened austenite to the finish rolling stage, relieves the formation of coarse structure due to the nucleation and growth of deformed structure in the steel pipe, thereby improving the strength and toughness of the seamless steel pipe and low-temperature toughness, and through the synergistic cooperation between the above components, dispersoids of carbides are precipitated in the grains during the preparation process, the generation of brittle upper bainite is reduced, thereby obtaining an ultra-thick wall seamless steel pipe with excellent strength, toughness and low-temperature toughness.

[0033] In an embodiment of the present application, the final temperature of the first heating is 1210-1300℃; and / or, the first heating process comprises: the billet is heated in the annular furnace in turn through a heat recovery section, a preheating section I zone, a heating I zone, a heating II zone, a heating III zone, a soaking I zone and a soaking II zone; wherein the temperature of the heat recovery section is 200-400℃, the temperature of the preheating section I zone is 400-650℃, the temperature of the heating I zone is 650-850℃, the temperature of the heating II zone is 850-1120℃, the temperature of the heating III zone is 1120-1300℃, the temperature of the soaking I zone is 1210-1300℃, the temperature of the soaking II zone is 1210-1300℃, the billet discharge temperature is 1200-1290℃, and the total heating time of the first heating is 3.5-4.5h.

[0034] Heating the billet in the annular furnace to the above temperature range helps the billet to obtain the ideal uniform temperature and organizational state for the subsequent piercing and rolling steps, and meanwhile, the austenite phase in the billet becomes more sufficient, which is beneficial to improve the plasticity of the steel pipe. Preferably, the temperature of the heat recovery section is in the above range (i.e. with the furnace temperature), which helps to preheat the billet while recovering the waste heat of the exhaust gas in the furnace, so as to improve the energy utilization efficiency and reduce the production cost. Preferably, the temperature of the preheating section I zone is in the above range, which helps to further preheat the billet and reduce the stress concentration and cracks caused by sudden temperature change. Preferably, the temperature of the heating I zone is in the above range, which helps the billet to continue to heat up and begin to form the austenite phase. Preferably, the temperature of the heating II zone is in the above range, which helps the billet to reach a higher temperature level required for piercing and improve the plasticity of the material and reduce its deformation resistance. Preferably, the temperature of the heating III zone is in the above range, which helps to prepare for the next process. Preferably, the temperatures of the soaking I zone and the soaking II zone are in the above range, which helps to make the overall temperature of the billet uniform and reduce the temperature gradient that may occur in the billet, thereby facilitating the subsequent piercing and rolling processes. The temperature of the soaking II zone is the final temperature of the first heating.

[0035] In an embodiment of the present application, the piercing is performed in a tapered expanding manner, and the piercing temperature is 1170-1270℃; and / or, the rolling temperature is 960-1160℃.

[0036] The piercing is performed in a tapered expanding manner, and the piercing temperature is controlled in the above range, which helps to obtain a larger total deformation, thereby flexibly adjusting the deformation of each process such as piercing and rolling. The rolling is performed by using a pilger mill or an asel mill, and the rolling temperature is preferably controlled in the above range, which helps to refine the grains, thereby enhancing the toughness and strength of the steel pipe material.

[0037] In an embodiment of the present application, the temperature of the first cooling after the piercing is 450-600℃;A c3temperature of the second heated hollow shell is 50-60°C higher than A c3 temperature of the second heated hollow shell is 50-60°C higher than A c3 temperature of the second heated hollow shell is 20-30°C higher than A

[0038] The first cooling of the hollow shell in the small cooling bed, the cooling of the first cooled hollow shell to the above-mentioned temperature range, and the second heating of the second heated hollow shell in the step furnace to the above-mentioned temperature range are helpful to promote the secondary phase transformation, thereby further refining the grains.

[0039] A c3 is the critical temperature at which ferrite is completely dissolved in austenite. According to the composition of the steel pipe, the temperature of A c3 is in the above-mentioned range. Preferably, the temperature of the second heated hollow shell is 50-60°C higher than A c3 , which is helpful to promote the secondary phase transformation, thereby further refining the austenite grains.

[0040] The unrecrystallization zone is a temperature zone in which the grains of the material will not be re-nucleated and grown by heating after the material is deformed at high temperature. Preferably, the sizing or reducing of the second heated hollow shell is performed in the above-mentioned temperature range of the unrecrystallization zone, which is helpful to maintain the plasticity of the material while reducing the grain coarsening, and finally obtain the first steel pipe with finer grains.

[0041] The seamless steel pipe is subjected to subsequent processes of cutting head and tail, straightening, physical and chemical inspection, nondestructive testing, manual inspection, letter spraying, and packaging into the warehouse, to obtain the finished product.

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

[0043] The seamless steel pipe prepared by the above-mentioned preparation method has excellent strength and toughness and low-temperature toughness, and can meet the higher performance requirements even if the wall thickness is thick.

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

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

[0046] It should be particularly pointed out that due to the particularity of the alloy field and the limitation of the existing test and characterization means, it is difficult to comprehensively quantitatively characterize the complex microstructure of the obtained seamless steel pipe, but experiments show that the obtained seamless steel pipe has higher strength and toughness.

[0047] The beneficial effects of the present application will be further illustrated below in combination with examples.

[0048] Example 1

[0049] The raw materials of the seamless steel pipe are compounded according to 0.15% of C element, 0.40% of Si element, 1.60% of Mn element, 0.07% of V element, 0.04% of Nb element, 0.04% of Al element, 0.005% of Ti element, 0.15% of Cr element, 0.05% of Ni element, 0.05% of Mo element, 0.001% of P element, 0.005% of S element, 0.003% of N element, and the total content of inevitable impurities is ≤0.15%, and the balance is Fe element, and then smelting, i.e. electric furnace smelting, LF refining, VD vacuum degassing and continuous casting, to obtain a billet, and the starting temperature of continuous casting is 1560℃.

[0050] The billet is sequentially subjected to first heating and conical expanding piercing at 1200℃ to obtain a rough pipe. The process of the first heating includes sequentially passing through a heat recovery section, a preheating section I zone, a heating I zone, a heating II zone, a heating III zone, a soaking I zone and a soaking II zone in an annular furnace for heating; the temperature of the heat recovery section is 300℃ with the furnace temperature, the temperature of the preheating section I zone is 500℃, the temperature of the heating I zone is 750℃, the temperature of the heating II zone is 1000℃, the temperature of the heating III zone is 1200℃, the temperature of the soaking I zone is 1250℃, the temperature of the soaking II zone (the final temperature of the first heating) is 1280℃, the billet out of the furnace temperature is 1280℃, and the total heating time of the first heating is 4.5h.

[0051] The rough pipe is rolled at 1000℃ by using a continuous rolling pipe mill set to obtain a crude pipe. The crude pipe is subjected to first cooling in a small cooling bed to a temperature of 550℃ after the first cooling. The crude pipe after the first cooling is subjected to second heating in a step furnace to a temperature which is 55℃ higher than the temperature of A c3 , to obtain the crude pipe after the second heating with a temperature of 910℃. The crude pipe after the second heating is subjected to sizing in a sizing machine at a temperature which is more than 20℃ higher than the temperature of the non-recrystallization zone of A c3 . The first steel pipe is obtained. The temperature of A c3 is 855℃.

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

[0053] The outer surface of the second steel pipe is cooled by atomization in an atomization bed, and the temperature of the outer surface of the third steel pipe is set to be 600℃. The cooling rate of the atomization cooling is 10℃ / s, the speed of the second steel pipe passing through the atomization 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 of the third steel pipe and the inner surface of the third steel pipe is 20℃.

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

[0055] Example 2

[0056] The raw materials of the seamless steel pipe are compounded according to 0.20% of C element, 0.65% of Si element, 1.05% of Mn element, 0.12% of V element, 0.02% of Nb element, 0.06% of Al element, 0.005% of Ti element, 0.20% of Cr element, 0.08% of Ni element, 0.1% of Mo element, 0.006% of P element, 0.01% of S element, 0.004% of N element, and the total content of inevitable impurities is ≤0.15%, and the balance is Fe element, and then smelting, i.e. electric furnace smelting, LF refining, VD vacuum degassing and continuous casting are carried out to obtain a billet, and the starting temperature of the continuous casting is 1560℃.

[0057] The billet is sequentially subjected to first heating and conical expanding piercing at 1170℃ to obtain a rough pipe. The first heating process includes sequentially passing through a heat recovery section, a preheating section I zone, a heating I zone, a heating II zone, a heating III zone, a soaking I zone and a soaking II zone in an annular furnace for heating; the temperature of the heat recovery section is 200℃ higher than the furnace temperature, the temperature of the preheating section I zone is 400℃, the temperature of the heating I zone is 650℃, the temperature of the heating II zone is 850℃, the temperature of the heating III zone is 1120℃, the temperature of the soaking I zone is 1210℃, the temperature of the soaking II zone (the final temperature of the first heating) is 1210℃, the billet out-of-furnace temperature is 1200℃, and the total heating time of the first heating is 4.5h.

[0058] The hollow shell is obtained by rolling the blank at 960℃ using a continuous rolling pipe mill. The hollow shell is first cooled on a small cooling bed until the temperature of the first cooled hollow shell is 450℃. The first cooled hollow shell is second heated in a walking beam furnace until the temperature of the second heated hollow shell is 50℃ higher than the temperature A c3 , and the temperature of the second heated hollow shell is 900℃. The second heated hollow shell is sized in a sizing mill at a temperature higher than the temperature A c3 by 20℃ or more in the unrecrystallized region, and a first steel pipe is obtained. The temperature A c3 is 850℃.

[0059] The inner surface of the first steel pipe is water cooled by spraying water in an on-line water quenching tank until the temperature of the inner surface of the second steel pipe is 550℃. The cooling rate of the water cooling is 30℃ / s, the water flow of the water cooling is 500m 3 / h, and 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 atomized cooled in an atomized bed until the temperature of the outer surface of the third steel pipe is 550℃. The cooling rate of the atomized cooling is 5℃ / s, the speed of the second steel pipe passing through the atomized bed is 60s / step, the temperature of the outer surface of the third steel pipe is 100℃ lower than the temperature of the outer surface of the second steel pipe, and the temperature difference between the outer surface of the third steel pipe and the inner surface of the third steel pipe is 5℃.

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

[0062] Example 3

[0063] The raw materials of the seamless steel pipe are smelted by ingredients of 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, 0.006% N, and ≤0.15% inevitable impurities, and the balance of Fe, in mass percent, i.e. electric furnace smelting, LF refining, VD vacuum degassing and continuous casting to obtain a billet, and the starting temperature of the continuous casting is 1560℃.

[0064] The blank is sequentially subjected to first heating and conical expanding piercing at 1270℃ to obtain a rough pipe. The first heating process comprises sequentially passing through a heat recovery section, a preheating section I zone, a heating I zone, a heating II zone, a heating III zone, a soaking I zone and a soaking II zone in an annular furnace. The temperature of the heat recovery section is 400℃, the temperature of the preheating section I zone is 650℃, the temperature of the heating I zone is 850℃, the temperature of the heating II zone is 1120℃, the temperature of the heating III zone is 1300℃, the temperature of the soaking I zone is 1300℃, the temperature of the soaking II zone (the final temperature of the first heating) is 1300℃, the discharge temperature of the blank is 1290℃, and the total heating time of the first heating is 3.5h.

[0065] The rough pipe is rolled at 1160℃ by using a continuous rolling mill to obtain a crude pipe. The crude pipe is subjected to first cooling in a small cooling bed to a temperature of 600℃. The crude pipe after the first cooling is subjected to second heating in a walking beam furnace to a temperature 60℃ higher than the temperature of A c3 , to obtain a crude pipe after the second heating with a temperature of 920℃. The crude pipe after the second heating is subjected to sizing in a sizing mill at a temperature 20℃ higher than the temperature of Ac3 in the unrecrystallized zone to obtain a first steel pipe. The temperature of A c3 is 860℃.

[0066] The inner surface of the first steel pipe is subjected to water spraying cooling in an online water quenching tank to a temperature of 550℃ of the inner surface of a second steel pipe. The cooling rate of the water spraying cooling is 60℃ / s, the water flow of the water spraying cooling is 1000m 3 / h, and 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 subjected to atomization cooling in an atomization bed to a temperature of 550℃ of the outer surface of a third steel pipe. The cooling rate of the atomization cooling is 20℃ / s, the speed of the second steel pipe passing through the atomization bed is 35s / step, the temperature of the outer surface of the third steel pipe is 260℃ lower than the temperature of the outer surface of the second steel pipe, and the temperature difference between the outer surface of the third steel pipe and the inner surface of the third steel pipe is 50℃.

[0068] The third steel pipe is transported to a large cooling bed to be subjected to second cooling air cooling to obtain a seamless steel pipe with a thickness of 50mm.

[0069] Example 4

[0070] The difference from Example 1 is that the inner surface of the first steel pipe is subjected to water spraying cooling in an online water quenching tank to a temperature of 550℃ of the inner surface of a second steel pipe. The cooling rate of the water spraying cooling is 60℃ / s, the water flow of the water spraying cooling is 1000m 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, and finally the seamless steel pipe is obtained.

[0071] Example 5

[0072] The difference from Example 1 is that the inner surface of the first steel pipe is sprayed and cooled in the online water quenching tank, and the temperature of the inner surface of the second steel pipe is 700℃. Among them, the cooling rate of the spray cooling is 20℃ / s, the water flow of the spray cooling is 300m 3 / h, the temperature of the inner surface of the second steel pipe is 100℃ lower than the temperature of the inner surface of the first steel pipe, and finally the 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 atomized and cooled, and the temperature of the outer surface of the third steel pipe is set to 550℃. Among them, the cooling rate of the atomization cooling is 20℃ / s, the speed of the second steel pipe passing through the atomization bed is 35s / step, the temperature of the outer surface of the third steel pipe is 260℃ lower than the temperature of the outer surface of the second steel pipe, the temperature difference between the outer surface and the inner surface of the third steel pipe is 50℃, and finally the seamless steel pipe is obtained.

[0075] Example 7

[0076] The difference from Example 1 is that the outer surface of the second steel pipe is atomized and cooled, and the temperature of the outer surface of the third steel pipe is set to 500℃. Among them, the cooling rate of the atomization cooling is 25℃ / s, the speed of the second steel pipe passing through the atomization bed is 75s / step, the temperature of the outer surface of the third steel pipe is 290℃ lower than the temperature of the outer surface of the second steel pipe, the temperature difference between the outer surface and the inner surface of the third steel pipe is 80℃, and finally the seamless steel pipe is obtained.

[0077] Example 8

[0078] The difference from Example 1 is that the cooling rate of the spray cooling is 60℃ / s, the cooling rate of the atomization cooling is 20℃ / s, and the cooling rate of the spray cooling is 40℃ / s greater than the cooling rate of the atomization cooling. / s, and finally the seamless steel pipe is obtained.

[0079] Example 9

[0080] The difference from Example 1 is that the cooling rate of the spray cooling is 60℃ / s, the cooling rate of the atomization cooling is 5℃ / s, and the cooling rate of the spray cooling is 55℃ / s greater than the cooling rate of the atomization cooling. / s, and finally the seamless steel pipe is obtained.

[0081] Example 10

[0082] The difference from Example 1 is that the hollow pipe is first cooled in a small cooling bed to a temperature of 600°C after the first cooling. The first cooled hollow pipe is second heated in a step furnace to a temperature 60°C higher than A c3 , to obtain a second heated hollow pipe with a temperature of 920°C, A c3 has a temperature of 860°C, and finally a seamless steel pipe is obtained.

[0083] Example 11

[0084] The difference from Example 1 is that the hollow pipe is first cooled in a small cooling bed to a temperature of 650°C after the first cooling. The first cooled hollow pipe is second heated in a step furnace to a temperature 65°C higher than A c3 , to obtain a second heated hollow pipe with a temperature of 925°C, A c3 has a temperature of 860°C, and finally a seamless steel pipe is obtained.

[0085] Example 12

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

[0087] Example 13

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

[0089] Comparative Example 1

[0090] The difference from Example 1 is that the hollow pipe is not first cooled in a small cooling bed, but is directly transported to a step furnace for second heating and sizing, and finally a seamless steel pipe is obtained.

[0091] Comparative Example 2

[0092] The difference from Example 1 is that only the outer surface of the first steel pipe is atomized cooled, and the inner surface is not separately cooled, and finally a seamless steel pipe is obtained.

[0093] Comparative Example 3

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

[0095] Test method:

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

[0097] Mechanical properties (tensile strength, yield strength, elongation) and low temperature impact property test: test according to GB / T 228.1, GB / T 229 standard respectively.

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

[0099] Table 1

[0100]

[0101] From the above description, it can be seen that the above-mentioned embodiments of the present application achieve the following technical effects:

[0102] The two-stage rapid cooling process of the inner surface water spray cooling and the outer surface atomization cooling can solve the problem that the super-thick wall hot-rolled seamless steel pipe cannot be rapidly and uniformly cooled due to the decrease of the cooling speed caused by the inner surface temperature rise when the through type cooling method is used because of the large heat storage of the super-thick wall hot-rolled seamless steel pipe. Specifically, the first cooling and then reheating (second heating) can refine the grains through the second phase change. The inner surface of the first steel pipe is subjected to water spray cooling, which can not only drive the whole steel pipe to pass through the high temperature phase change region uniformly through the rapid cooling of the inner surface, so that the newly formed phase does not grow rapidly and coarsen, but also can keep the hardened austenite to the phase change point, and the new phase and carbonitride will surround the strain nucleation, thereby greatly refining the grain structure. The outer surface of the second steel pipe is subjected to atomization cooling, which can rapidly cool the outer surface or local high temperature region of the second steel pipe, so as to realize the simultaneous and uniform rapid cooling of the inner and outer surfaces of the steel pipe to the set temperature. The second cooling of the third steel pipe can slowly cool in the bainite transformation region, avoid the upper bainite transformation region, and avoid the formation of brittle upper bainite, thereby greatly refining the ferrite structure and pearlite structure, and further obtaining a super-thick wall seamless steel pipe with excellent strength and toughness and low temperature toughness. In addition, the above cooling process of the present application greatly reduces the production cost of the seamless steel pipe and improves the production efficiency.

[0103] The above is only an embodiment of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

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.

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 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.

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 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 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 temperature of the first cooled rough tube is 450~600℃; A c3 The temperature is 845~860℃; And / or, the temperature of the second heated rough tube is higher than that of A. c3 The temperature is 50-60℃ higher; And / or, the second heated rough tube is subjected to the 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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