Large-caliber thin-walled seamless steel pipe and its preparation method and application

Abstract

The present application discloses a large-diameter thin-walled seamless steel pipe, its preparation method and application, belonging to the technical field of seamless steel pipes. Step S1: Mix and smelt various metallurgical raw materials to obtain a tube blank; Step S2: Perform first heating, piercing, rolling and first sizing and reducing on the tube blank to obtain a hot-rolled steel pipe; hot-expand the hot-rolled steel pipe to obtain a hot-expanded steel pipe, and the hot-expanded outer diameter is 350-450 mm; Step S3: Perform second heating and second sizing and reducing on the hot-expanded steel pipe: the steel pipe first enters the second oval pass sizing and reducing stand for sizing and reducing, and then enters the second round pass sizing and reducing stand for sizing and reducing; the outer diameter of the seamless steel pipe is the same as the hot-expanded outer diameter, which is 350-450 mm, and the wall thickness is 5-20 mm; perform water quenching and tempering on the seamless steel pipe in sequence to obtain a large-diameter thin-walled seamless steel pipe. The large-diameter thin-walled high-strength seamless steel pipe has a uniform microstructure, a smooth surface, no concave surface collapse, and excellent strength and toughness.

Description

Technical Field

[0001] The present application relates to the technical field of seamless steel pipes, and more particularly, to a large-diameter thin-wall seamless steel pipe, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, the manufacture of high-strength large-diameter thin-wall steel pipes mainly relies on welded pipes synthesized from high-strength steel plates through welding technology, or is produced by a process combining hot rolling, hot expansion, and subsequent quenching and tempering treatments. Although welded pipes are common, the decrease in the strength and toughness of the weld area often directly causes mechanical equipment failures. Especially in occasions where safety standards are strict or harsh conditions need to be adapted (such as the application of oil cylinders containing weak acid media, the outer drill pipes of large-tonnage rotary drilling rigs, etc.), only seamless steel pipes with excellent strength and toughness can ensure the safe operation of the equipment. For high-strength large-diameter thin-wall seamless steel pipes, with the characteristics of a yield strength of at least 420 MPa, an outer diameter of not less than 356 mm, and an outer diameter-to-wall thickness ratio exceeding 30, it is usually difficult to directly obtain them only through hot rolling or normalizing processes. The manufacture of such steel pipes often requires prior hot rolling and then hot expansion to achieve the set outer diameter size, and subsequent quenching and tempering heat treatments are aimed at endowing them with excellent strength and toughness matching. However, during the quenching and tempering heat treatment process, when the steel pipe reaches above the critical temperature, due to its thin wall thickness, combined with the significant temperature drop during high-pressure water jet descaling and transportation, the water entry temperature during quenching is relatively low. The common result is that the comprehensive strength of the steel pipe after heat treatment is insufficient, and the strength and toughness performance is only mediocre. In addition, due to the self-weight of the steel pipe, deformation or ovalization problems frequently occur; during step furnace heating, the steel pipe is easily knocked by rigid components in the furnace (such as step beams), resulting in dents and damages on the outer surface.

[0003] To overcome the above challenges, the industry has tried to configure internal support structures at both ends of the steel pipe to reduce the deformation or ovalization caused by self-weight at the ends. Unfortunately, this method can only partially alleviate the end problems and is ineffective for the deformation or outer surface dents in the middle part of the steel pipe, thus requiring cold working or hot expansion trimming in subsequent processes, which undoubtedly increases the manufacturing cost and reduces the production efficiency. More critically, for steel pipes produced by traditional processes, it is difficult to achieve an ideal state in the balance of strength and toughness performance and dimensional accuracy. In view of this, developing a more efficient, economical production method that can significantly improve the strength, toughness, and dimensional accuracy of steel pipes has become an urgent need for the development of the industry. Summary of the Invention

[0004] The main purpose of the present application is to provide a large-diameter thin-wall seamless steel pipe, a preparation method thereof, and an application thereof, so as to solve the problems of easy deformation, concave surface, and uneven structure on the outer surface of large-diameter thin-wall seamless steel pipes in the prior art.

[0005] To achieve the above purpose, according to one aspect of the present application, a preparation method of a large-diameter thin-wall seamless steel pipe is provided, including:

[0006] Step S1: Mix and smelt various metallurgical raw materials to obtain a tube blank.

[0007] Step S2: First heat, pierce, roll, and first sizing and reducing the tube blank in sequence to obtain a hot-rolled seamless steel tube; perform hot expansion on the hot-rolled seamless steel tube to obtain a hot-expanded steel tube, and the outer diameter after hot expansion is 350 - 450 mm.

[0008] Step S3: Second heat and second sizing and reducing the hot-expanded steel tube in sequence; the process of the second sizing and reducing includes: the hot-expanded steel tube first enters the second oval pass sizing stand for the first sizing and reducing, and then enters the second round pass sizing stand for the second sizing and reducing; the number of the second oval pass sizing stands is 1 - 4, and the number of the second round pass sizing stands is 1 - 2; the second oval pass sizing stand includes a first roll, a second roll, and a third roll; the three rolls are the same; the axis of the first roll is arranged horizontally, and the axes of the second roll and the third roll are respectively arranged on both sides of the vertical line of the axis of the first roll; the first roll, the second roll, and the third roll are spliced to form an oval pass; the radial direction from the center of the oval pass of the first roll to the bottom of its roll is defined as the first short semi-axis direction; along the steel tube transportation direction, the included angle of the first short semi-axis directions of two adjacent second oval pass sizing stands is 180°; the outer diameter of the seamless steel tube obtained by the second sizing and reducing is the same as the outer diameter after hot expansion; the outer diameter of the seamless steel tube is 350 - 450 mm, and the wall thickness is 6 - 15 mm; finally, perform water quenching and tempering on the seamless steel tube in sequence to obtain a large-diameter thin-wall seamless steel tube.

[0009] Further, in step S1, by weight percentage, the element ratios are as follows: C 0.18 - 0.30%, Si 0.15 - 0.45%, Mn 1.25 - 1.65%, V 0.02 - 0.06%, Nb 0.015 - 0.035%, Ti ≤ 0.015%, Al 0.015 - 0.045%, Cr ≤ 0.15%, Ni ≤ 0.15%, Mo ≤ 0.10%, P ≤ 0.020%, S ≤ 0.010%, B 0.00080 - 0.0025%, N ≤ 0.010%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the relationship between the contents of V, Nb, Al, and N satisfies the formula: 5.0 ≤ (V + Nb + Al): N ≤ 19.5.

[0010] Further, in step S1, the element ratios are as follows by weight percentage: C 0.22~0.30%, Si 0.25~0.40%, Mn 1.25~1.50%, V 0.025~0.06%, Nb 0.020~0.035%, Ti ≤0.015%, Al 0.02~0.045%, Cr ≤0.15%, Ni ≤0.10%, Mo ≤0.05%, P ≤0.015%, S ≤0.008%, B 0.0012~0.0022%, N ≤0.080%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the weight percentage contents of V, Nb, Al, and N satisfy the relationship: 8.5 ≤ (V + Nb + Al): N ≤ 17.5.

[0011] Further, the yield strength of the large-diameter thin-walled seamless steel pipe is ≥550 MPa, the tensile strength is ≥650 MPa, the elongation is ≥18%, and the impact energy AKV at -40°C is ≥55 J.

[0012] Further, the outer diameter of the seamless steel pipe is 368~450 mm, and the wall thickness is 6~15 mm.

[0013] Further, the value range of the ovality α of the oval pass of the second oval-pass sizing stand: 1 < α ≤ 1.05.

[0014] Further, the temperature of the second heating is 860~960°C, the holding time = holding coefficient × wall thickness of the steel pipe, the holding coefficient is 2~3 min / mm, and the unit of the wall thickness of the steel pipe is mm; the second heating is carried out in a walking beam furnace and adopts a stepping mode.

[0015] Further, in step S2, the temperature of the intermediate frequency holding section of the hot expansion is 720~780°C, and the temperature deviation is not more than 10°C.

[0016] Further, the advancing speed of the conical mandrel used in the hot expansion is 150~350 mm / min.

[0017] Further, the rolling temperature is 950~1150°C; continuous rolling mills are used for rolling.

[0018] Further, in step S2, the process of the first sizing and reducing includes: the rolled steel pipe first enters the first oval-pass sizing stand for sizing and reducing, and then enters the first round-pass sizing stand for sizing and reducing. The number of the first oval-pass sizing stands is 1~10, and the number of the round-pass sizing stands is 1~2; the reduction rate of a single stand is controlled to be less than 5%, and the total reduction rate is less than 25%.

[0019] Further, in step S2, the process of the first heating includes: the tube blank sequentially passes through a preheating section, a heating zone I, a heating zone II, a heating zone III, a soaking zone I, and a soaking zone II; wherein, the temperature of the preheating section is the furnace temperature; the temperature of the heating zone I is 830 - 990 °C, and the heating time is ≥ 42 min; the temperature of the heating zone II is 990 - 1130 °C, and the heating time is ≥ 42 min; the temperature of the heating zone III is 1130 - 1290 °C, and the heating time is ≥ 42 min; the temperature of the soaking zone I is 1200 - 1290 °C, and the heating time is ≥ 42 min; the temperature of the soaking zone II is 1200 - 1290 °C, and the heating time is ≥ 42 min; the discharging temperature of the tube blank is 1200 - 1280 °C; the total time of the first heating is ≥ 3.5 hours; the first heating is carried out in a rotary hearth furnace.

[0020] Further, the temperature of piercing is 1130 - 1230 °C; a conical piercing mill is selected for piercing.

[0021] Further, in step S3, the temperature of tempering is 640 - 660 °C; the holding time = holding coefficient × wall thickness of the steel pipe, the holding coefficient is 3 - 4 min / mm, and the unit of the wall thickness of the steel pipe is mm.

[0022] Further, high-pressure water descaling is also included between the second heating and the second sizing and reducing, and the pressure is ≥ 20 MPa.

[0023] According to the second aspect of the present application, a large-diameter thin-walled seamless steel pipe is provided, which is prepared by using the above preparation method.

[0024] According to the third aspect of the present application, the application of the above large-diameter thin-walled seamless steel pipe in fluid transportation, alumina ore transportation, natural gas transportation pipelines, and engineering machinery equipment is provided.

[0025] Compared with the prior art, the present application has the following beneficial effects:

[0026] The present application provides a large-diameter thin-walled seamless steel pipe, a preparation method thereof, and an application thereof. The large-diameter thin-walled high-strength seamless steel pipe prepared by the process of heating, piercing, rolling, first sizing and reducing, hot expansion, reheating, second sizing and reducing, water quenching, and tempering in sequence has a uniform microstructure, a smooth surface, no concave surface collapse, good roundness, and excellent strength and toughness; especially in the process of the second sizing and reducing, an innovative combination mode of alternately arranging elliptical pass grooves is designed, which can realize the alternation of the maximum deformation direction, and the reduction rate is very small or almost zero; the hot-expanded steel pipe that is prone to elliptical deformation during the quenching heating process can be well sized into a circular steel pipe; it can also avoid problems with poor dimensions such as the large ellipticity of the steel pipe caused by too large ellipticity of a single stand and the longitudinal rolling marks formed by the metal squeezing into the roll gap, or the large outer diameter ellipticity and the uneliminated concave surface on the outer surface caused by too small ellipticity of a single stand and the unfilled pass groove, so as to obtain a large-diameter thin-walled seamless steel pipe with small ellipticity, no concave surface on the outer surface, and excellent dimensional accuracy, further ensuring the strength and toughness of this type of steel pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The specification drawings forming a part of the present application are used to provide a further understanding of the present application. The schematic embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation to the present application. In the drawings:

[0028] Figure 1 It is a physical photograph of the outer surface of the large-diameter thin-walled seamless steel pipe prepared in Example 2 of the present application;

[0029] Figure 2 It is a physical photograph of the outer surface of the large-diameter thin-walled seamless steel pipe prepared in Comparative Example 3 of the present application;

[0030] Figure 3 It is a physical photograph of the inner surface of the large-diameter thin-walled seamless steel pipe prepared in Comparative Example 3 of the present application;

[0031] Figure 4 It is a schematic diagram of the alternate arrangement of each pass groove in the second elliptical pass sizing stand in Example 1 of the present application;

[0032] Figure 5 It is a schematic diagram of the alternate arrangement of each pass groove in the second elliptical pass sizing stand in Example 2 of the present application;

[0033] Figure 6 It is a schematic diagram of the alternate arrangement of each pass groove in the second elliptical pass sizing stand in Example 3 of the present application.

[0034] Reference numerals:

[0035] 1. Second oval pass sizing stand 1; 2. Second oval pass sizing stand 2; 21. Second oval pass sizing stand 3; 22. Second oval pass sizing stand 4; 3. Second round pass sizing stand; 4. Steel pipe transportation direction; 5. Long semi-axis direction; 6. Short semi-axis direction; 61. First short semi-axis direction; 7. Roll gap; 8. Roll bottom; 9. Oval pass center; 10. First roll; 11. Second roll; 12. Third roll. Detailed implementation manners

[0036] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application will be described in detail below with reference to the drawings and embodiments.

[0037] As described in the background art, for large-diameter thin-walled steel pipes, problems such as deformation or becoming oval due to the influence of their own gravity during quenching and tempering heating, and defects such as concave surfaces on the outer surface are easily caused by being bumped by rigid components such as walking beams during walking beam furnace heating. The prior art usually adopts methods such as multiple hot expansion or setting support frames at the pipe ends of steel pipes to reduce defects such as steel pipe deformation and concave surfaces, but this method cannot guarantee the strength, toughness and high-precision dimensions of steel pipes.

[0038] According to one aspect of the present application, a production process for large-diameter thin-walled high-strength seamless steel pipes is provided, including:

[0039] Step S1: Mix and smelt various metallurgical raw materials according to the proportion of each element to obtain a tube blank.

[0040] Step S2: Perform first heating, piercing, rolling and first sizing and reducing on the tube blank in sequence to obtain a hot-rolled seamless steel pipe; perform hot expansion on the hot-rolled seamless steel pipe on a hot expansion line to obtain a hot-expanded steel pipe, and the hot-expanded outer diameter is 350 - 450 mm.

[0041] Step S3: Second heating and second sizing and reducing are successively performed on the hot-expanded steel pipe; the process of the second sizing and reducing includes: the hot-expanded steel pipe first enters the second oval pass sizing stand for the first sizing and reducing, and then enters the second round pass sizing stand for the second sizing and reducing; the number of the second oval pass sizing stands is 1 to 4, and the number of the second round pass sizing stands is 1 to 2; the second oval pass sizing stand includes a first roll, a second roll, and a third roll; the three rolls are the same; the axis of the first roll is arranged horizontally, and the axes of the second roll and the third roll are respectively arranged on both sides of the perpendicular line of the axis of the first roll; the first roll, the second roll, and the third roll are spliced to form an oval pass; an adjacent two rolls are spliced to form a roll gap; the bottom of the roll groove of each roll is a roll bottom; the radial direction from the center of the oval pass of the first roll to its roll bottom is defined as the first minor semi-axis direction; along the steel pipe transportation direction, the included angle between the first minor semi-axis directions of two adjacent second oval pass sizing stands is 180°; the outer diameter of the seamless steel pipe obtained by the second sizing and reducing is the same as the hot-expanded outer diameter; the outer diameter of the seamless steel pipe is 350 to 450 mm, and the wall thickness is 6 to 15 mm; finally, the seamless steel pipe is successively quenched and tempered with water to obtain a large-diameter thin-wall seamless steel pipe.

[0042] The outer diameter of the large-diameter thin-wall seamless steel pipe of the present application is any value among 350 mm, 360 mm, 370 mm, 380 mm, 390 mm, 400 mm, 410 mm, 420 mm, 430 mm, 440 mm, 450 mm or the range value between any two of them; the wall thickness is any value among 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 mm or the range value between any two of them.

[0043] The number of the second oval-hole sizing stands designed in this application is 1 to 4, such as 1, 2, 3, 4, for example 2 to 4; the number of the second round-hole sizing stands is 1 to 2, for example 1 or 2; the reduction ratio of a single stand is 0, and the total reduction ratio is 0; the second oval-hole sizing stand includes a first roll 10, a second roll 11 and a third roll 12; the three rolls are the same; the axis of the first roll 10 is arranged in the horizontal direction, and the axes of the second roll 11 and the third roll 12 are respectively arranged on both sides of the vertical line of the axis of the first roll 10; the first roll 10, the second roll 11 and the third roll 12 are spliced to form an oval hole; the roll gap 7 is spliced between two adjacent rolls; the bottom of the roll groove of each roll is the roll bottom 8; the radial direction from the center 9 of the oval hole to the roll gap 7 of the oval hole is defined as the long semi-axis direction 5, and the radial direction from the center 9 of the oval hole to the roll bottom 8 of the oval hole is defined as the short semi-axis direction 6, and there are three long semi-axis directions and three short semi-axis directions in an oval-hole stand; among them, the radial direction from the center 9 of the oval hole of the first roll 10 to the roll bottom 8 of the oval hole of the first roll is defined as the first short semi-axis direction 61; when the number of the second oval-hole sizing stands is more than 2, along the steel pipe transportation direction, the included angle between the first short semi-axis directions of two adjacent second oval-hole sizing stands is 180°. For example, the first short semi-axis direction 61 of the previous oval hole is at the 12 o'clock direction (or 6 o'clock direction), and the first short semi-axis direction 61 of the next oval hole is at the 6 o'clock direction (or 12 o'clock direction), and they are arranged alternately in this way. Another example, when the second sizing process includes 4 second oval-hole sizing stands and 1 second round-hole sizing stand, in the steel pipe transportation direction, the different hole-type stands are arranged in sequence as follows: the second oval-hole sizing stand one 1 with the first short semi-axis direction 61 of the oval hole at the 12 o'clock direction of the hole, the second oval-hole sizing stand two 2 with the first short semi-axis direction 61 of the oval hole at the 6 o'clock direction of the hole, the second oval-hole sizing stand three 21 with the first short semi-axis direction 61 of the oval hole at the 12 o'clock direction of the hole, and the second oval-hole sizing stand four 22 with the first short semi-axis direction 61 of the oval hole at the 6 o'clock direction of the hole. Adopting the above alternating arrangement method can make the maximum deformation directions alternate, and the roundness effect of the sized steel pipe is gradually better. The above method of alternating multiple oval holes in this application is very suitable for the situation where the reduction ratio is very small or almost 0. It is not necessary to reduce the diameter of the hot-expanded steel pipe. As long as the above alternating arrangement method is adopted, the steel pipe that is easy to be oval-shaped during the quenching heating process after hot expansion can be well sized into a round steel pipe.Therefore, by adopting the above-mentioned sizing machine pass, reasonably setting the outer diameter and wall thickness of the hot-expanded steel pipe before sizing, and the oval pass and round pass of sizing, problems with poor dimensions such as the large ovality of the steel pipe caused by excessive ovality of a single stand that cannot be eliminated, metal squeezing into the roll gap to form longitudinal rolling marks, too small ovality of a single stand, large ovality of the outer diameter caused by the unfilled pass, and the concave surface on the outer surface not being eliminated are avoided. As a result, large-diameter thin-walled seamless steel pipes with small ovality, no concave surface on the outer surface, and excellent dimensional accuracy are obtained, further ensuring the strength and toughness of this type of steel pipe.

[0044] As a preferred embodiment, by weight percentage, C is 0.18 - 0.30%, Si is 0.15 - 0.45%, Mn is 1.25 - 1.65%, V is 0.02 - 0.06%, Nb is 0.015 - 0.035%, Ti ≤ 0.015%, Al is 0.015 - 0.045%, Cr ≤ 0.15%, Ni ≤ 0.15%, Mo ≤ 0.10%, P ≤ 0.020%, S ≤ 0.010%, B is 0.00080 - 0.0025%, N ≤ 0.010%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the contents of V, Nb, Al, and N satisfy the relationship: 5.0 ≤ (V + Nb + Al): N ≤ 19.5.

[0045] The relationship among the four elements in the steel pipe formula of this application, such as (V + Nb + Al): N, is any value among 5.0, 5.5, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or the range value between any two of them; in the formula of the large-diameter seamless steel pipe designed in this application, the proportion of each element is optimized to make the formula and process better adapted; in particular, the purpose of 5.0 ≤ (V + Nb + Al): N ≤ 19.5 is not only to form specific carbonitrides of V, Nb, and Al, refine the grains, and improve the strength and toughness of the steel pipe, but also to fix the N element, give full play to the effect of element B in improving hardenability as much as possible, reduce the contents of elements such as C and Cr, improve the weldability, reduce the production cost, and the synergistic addition of V, Nb, and Al microalloys reduces the overall alloy dosage. Moreover, it can reduce the AC3 temperature of the steel grade, avoid excessive temperature drop of the large-diameter thin-walled steel pipe during high-pressure water descaling and transportation, which may cause the steel pipe to enter the two-phase region before entering the water, ultimately resulting in poor strength and toughness of the steel pipe.

[0046] This application conducts a large number of experimental studies on the main processes such as deformation and concave surfaces that are most likely to occur during the quenching heating process of seamless steel pipes after hot expansion. It is found that for large-diameter thin-walled and high-strength and tough seamless steel pipes, by optimizing the proportion of steel pipe elements and combining precise control of heating temperature, holding time, hot expansion, sizing reduction rate, and rapid water quenching cooling after sizing, through the optimization and coordination of the formula processing technology, the defects such as external concave surfaces and deformation that are likely to occur during the second heating process of seamless steel pipes can be solved; there is no need to solve the external defects that occur during the second heating process through multiple hot expansions or cold drawing repairs in traditional technologies.

[0047] As a preferred embodiment, by weight percentage, the element ratios are as follows: C 0.22~0.30%, Si 0.25~0.40%, Mn 1.25~1.50%, V 0.025~0.06%, Nb 0.020~0.035%, Ti ≤0.015%, Al 0.02~0.045%, Cr ≤0.15%, Ni ≤0.10%, Mo ≤0.05%, P ≤0.015%, S ≤0.008%, B 0.0012~0.0022%, N ≤0.080%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the weight percentage contents of V, Nb, Al, and N satisfy the relationship: 8.5 ≤ (V + Nb + Al): N ≤ 17.5. The large-diameter thin-walled high-strength seamless steel pipe of this application adopts a special composition range and a certain component ratio to avoid problems such as low steel pipe strength, poor impact toughness, and poor head and tail performance uniformity caused by excessive temperature drop during high-pressure water descaling and transportation when producing traditional large-diameter thin-walled high-strength seamless steel pipe grades, and has very excellent strength and toughness.

[0048] As a preferred embodiment, by weight percentage, the element ratios are as follows: C 0.25~0.30%, Si 0.30~0.40%, Mn 1.30~1.40%, V 0.03~0.05%, Nb 0.025~0.035%, Ti ≤0.015%, Al 0.02~0.035%, Cr ≤0.15%, Ni ≤0.08%, Mo ≤0.04%, P ≤0.010%, S ≤0.005%, B 0.0015~0.0022%, N ≤0.006%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the weight percentage contents of V, Nb, Al, and N satisfy the relationship: 12.5 ≤ (V + Nb + Al): N ≤ 17.5. By further optimizing the element ratios in the steel pipe raw materials, it is more suitable for coordinating with the process of this application, improving the coordination effect, and is more conducive to eliminating defects such as external concave surfaces of seamless steel pipes.

[0049] As a preferred embodiment, by weight percentage, C is 0.18%, Si is 0.15%, Mn is 1.25%, V is 0.02%, Nb is 0.015%, Ti is 0.0017%, Al is 0.015%, Cr is 0.055%, Ni is 0.021%, Mo is 0.013%, P is 0.020%, S is 0.010%, B is 0.00080%, N is 0.010%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the contents of V, Nb, Al, and N satisfy the relationship: (V + Nb + Al): N = 5.0.

[0050] As a preferred embodiment, by weight percentage, C is 0.25%, Si is 0.15%, Mn is 1.25%, V is 0.020%, Nb is 0.015%, Ti is 0.0017%, Al is 0.04%, Cr is 0.055%, Ni is 0.021%, Mo is 0.013%, P is 0.015%, S is 0.008%, B is 0.0012%, N is 0.010%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the contents of V, Nb, Al, and N satisfy the relationship: (V + Nb + Al)): N = 7.5.

[0051] As a preferred embodiment, C is 0.27%, Si is 0.35%, Mn is 1.35%, V is 0.04%, Nb is 0.030%, Ti is 0.0017%, Al is 0.030%, Cr is 0.055%, Ni is 0.021%, Mo is 0.013%, P is 0.015%, S is 0.008%, B is 0.002%, N is 0.008%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the contents of V, Nb, Al, and N satisfy the relationship: (V + Nb + Al)): N = 12.5.

[0052] As a preferred embodiment, the outer diameter of the large-diameter thin-walled seamless steel pipe is 368 - 450 mm, and the wall thickness is 6 - 15 mm. The formula and process of the above steel pipe in this application are more applicable to the steel pipe with the above preferred diameter and wall thickness; the yield strength of the large-diameter thin-walled seamless steel pipe is ≥550 MPa, the tensile strength is ≥650 MPa, the elongation is ≥18%, and the impact energy AKV at -40 °C is ≥55 J; for example, the yield strength is ≥600 MPa, further the yield strength is ≥620 MPa, and further it reaches above 650 MPa; the tensile strength is ≥700 MPa, further the tensile strength is ≥720 MPa, and further it reaches above 750 MPa; the elongation is ≥19%, further the elongation is ≥20%, and further it reaches 21%; the impact energy AKV is ≥70 J, further the impact energy AKV is ≥100 J, and further it reaches above 150 J; the outer surface of the seamless steel pipe has no concave surface and collapse, and the roundness is small.

[0053] As a preferred embodiment, in step S2, the process of the first heating includes: the tube blank sequentially passes through a preheating section, a heating zone I, a heating zone II, a heating zone III, a soaking zone I, and a soaking zone II; wherein, the temperature of the preheating section is the furnace temperature; the temperature of the heating zone I is 830 - 990 °C, and the heating time is ≥42 min; the temperature of the heating zone II is 990 - 1130 °C, and the heating time is ≥42 min; the temperature of the heating zone III is 1130 - 1290 °C, and the heating time is ≥42 min; the temperature of the soaking zone I is 1200 - 1290 °C, and the heating time is ≥42 min; the temperature of the soaking zone II is 1200 - 1290 °C, and the heating time is ≥42 min; the discharging temperature of the tube blank is 1200 - 1280 °C; the total time of the first heating is ≥3.5 hours; the first heating is carried out in a rotary hearth furnace. Using the above heating process conditions, the tube blank can be fully heated, which is more conducive to the piercing and rolling processes.

[0054] As a preferred implementation manner, a conical piercing mill is selected for piercing, and the piercing temperature is 1130 - 1230 °C; for example, 1130 °C, 1180 °C, 1230 °C. The heated tube blank is taken out from the heating furnace and sent to the piercing mill through a conveying device for piercing. By controlling the piercing temperature within the above range, the tube body can be smoothly pierced, and it is helpful for the subsequent hot rolling process.

[0055] As a preferred embodiment, continuous rolling mills are used for rolling the mandrel tube to obtain a rough tube; the rolling temperature is 950 - 1150 °C; the rolling process of this process can be selected from the prior art. After rolling, high-pressure water descaling operation is carried out on the rough tube to remove the scale on the surface of the tube body, which is beneficial to the subsequent sizing operation.

[0056] As a preferred embodiment, in step S2, the process of the first sizing and reducing includes: the rolled steel pipe first enters the first sizing and reducing stand with an oval pass for sizing and reducing, and then enters the first sizing and reducing stand with a round pass for sizing and reducing. The number of the first sizing and reducing stands with an oval pass is 1 to 10, and the number of the sizing and reducing stands with a round pass is 1 to 2; it is controlled that the reducing rate of a single stand of each pass is less than 5%, and the total reducing rate is less than 25%; in this first sizing and reducing process, sizing and reducing are carried out by using the oval pass first + the round pass later, which can ensure that the steel pipe is gradually deformed during the hot rolling process, and a hot-rolled steel pipe with excellent dimensions, uniform structure and stable performance is obtained.

[0057] As a preferred embodiment, the temperature of the intermediate frequency heat preservation section of hot expansion is 720 - 780 °C, and the overall deviation is not greater than 10 °C. The temperature of the intermediate frequency heat preservation section of hot expansion can be selected as any value or the range value between any two of 720 °C, 730 °C, 740 °C, 750 °C, 760 °C, 770 °C, 780 °C. The advancing speed of the conical mandrel used in hot expansion is 150 - 350 mm / min. The advancing speed of the conical mandrel used in hot expansion is 150 - 350 mm / min, such as any value or the range value between any two of 150, 180, 200, 220, 250, 280, 300, 320, 350 mm / min. By accurately controlling the temperature and speed of hot expansion, the hot expansion deformation of the steel pipe can be gradually made uniform and the organizational structure can be stabilized, which is beneficial to improving the strength and toughness of the large-diameter thin-walled seamless steel pipe.

[0058] As a preferred embodiment, in step S3, the temperature of the second heating is 860 - 960 °C, and the holding time = holding coefficient × wall thickness of the steel pipe. The holding coefficient is 2 - 3 min / mm, and the unit of the wall thickness of the steel pipe is mm; for example, when the wall thickness of the hot-expanded steel pipe is 8 mm, the holding time of the second heating is 16 - 24 min. By controlling the heating temperature and holding time of the hot-expanded steel pipe, it is helpful to release the residual stress generated inside the steel pipe during the hot expansion process, improve the strength and toughness of the steel pipe, reduce the possibility of cracks generated during quenching and cooling, form a more uniform microstructure, and obtain better strength and toughness.

[0059] As a preferred embodiment, in step S3, the second heating is carried out in a walking beam furnace and adopts a stepping mode. Adopting the stepping mode can prevent the steel pipe from stopping rotating when the straightening machine is being corrected or fails, and can reduce the deterioration of the ovality or collapse of the steel pipe in the static state in the furnace.

[0060] As a preferred embodiment, the range of the ovality α (major semi-axis / minor semi-axis) of the oval pass of the second oval pass sizing and reducing stand is: 1 < α ≤ 1.05. By limiting the ovality and the combination of each stand, the ovality of the prepared large-diameter seamless steel pipe is further improved.

[0061] As a preferred embodiment, the pipe after sizing is transported to a quenching tank for quenching and cooling (water quenching) to room temperature, such as 20 - 30 °C; the tempering temperature is 640 - 660 °C, and the holding time = holding coefficient × pipe wall thickness, the holding coefficient is 2 - 3 min / mm, and the unit of the pipe wall thickness is mm; for example, when the wall thickness of the hot-expanded pipe is 8 mm, the holding time for tempering is 24 - 32 min. By adopting rapid water quenching and tempering after the sizing and circularizing process of the round pipe, the obtained pipe has the characteristics of small ovality, no concave surface on the outer surface, excellent dimensional accuracy, etc. During the quenching process, each part of the pipe can be cooled evenly and rapidly to room temperature, so as to obtain a large-diameter thin-walled high-strength seamless pipe with excellent quenched structure, uniform structure and properties. Through the tempering and holding process, it is ensured that the large-diameter thin-walled seamless pipe has stable strength and toughness.

[0062] As a preferred embodiment, high-pressure water descaling is also included between the second heating and the second sizing, and the pressure ≥ 20 MPa. By performing high-pressure water descaling after quenching heating, the scale on the outer surface of the hot-expanded pipe can be removed; straightening, non-destructive flaw detection, inspection, etc. of the large-diameter thin-walled seamless pipe after tempering are all conventional operations and can be selected from the prior art.

[0063] The present application mainly uses the hot rolling + quenching and tempering method to prepare the large-diameter thin-walled seamless pipe. The specific process includes: blank cutting → heating in a ring furnace → piercing → rolling → high-pressure water descaling → first sizing → hot expansion → cutting the head and tail → entering the intermediate storage → quenching heating in a walking beam furnace → high-pressure water descaling → second sizing → water quenching → tempering → physical and chemical inspection → straightening → non-destructive flaw detection → cutting to fixed length → manual inspection → spraying characters → packaging and warehousing.

[0064] According to the second aspect of the present application, a large-diameter thin-walled seamless pipe is provided, which is prepared by the above method.

[0065] According to the third aspect of the present application, an application of the large-diameter thin-walled seamless pipe prepared by the method for preparing a large-diameter thin-walled seamless pipe or the above large-diameter thin-walled seamless pipe in fluid transportation, transportation of ores such as alumina, natural gas transportation pipelines and engineering machinery equipment is provided.

[0066] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0067] The smelting raw materials used in the embodiments of the present application are prior art and can be purchased commercially.

[0068] Example 1

[0069] The overall process of large-diameter thin-walled seamless steel pipes includes: blank cutting → ring furnace heating → perforation → rolling → high-pressure water descaling → first fixed diameter reduction → thermal expansion → cutting head and tail → storage in intermediate warehouse → stepping furnace quenching and heating → high-pressure water descaling → second fixed diameter reduction → water quenching → tempering → physical and chemical inspection → straightening → non-destructive testing → cutting to size → manual inspection → inkjet printing → packaging and storage.

[0070] Step S1: preparing ingredients according to the proportion of each element, in terms of weight percentage, C 0.18%, Si 0.15%, Mn 1.25%, V0.02%, Nb 0.015%, Ti 0.0017%, Al 0.015%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.020%, S0.010%, B 0.00080%, N 0.010%, the remainder is Fe and unavoidable impurities, totaling 100%; wherein the contents of V, Nb, Al and N satisfy the relationship: (V+Nb+Al): N=5.0; mixing and smelting the metallurgical raw materials to obtain a tube blank;

[0071] Step S2: The tube billet is subjected to the first heating in the annular furnace, and the billet passes through six heating zones in sequence: preheating section, heating zone I, heating zone II, heating zone III, soaking zone I, soaking zone II; the heating process of the six zones is as follows: the preheating section follows the furnace temperature, the temperature of heating zone I is 830°C, the heating time is 42 minutes, the temperature of heating zone II is 990°C, the heating time is 42 minutes, the temperature of heating zone III is 1130°C, the heating time is 42 minutes, the temperature of soaking zone I is 1200°C, the heating time is 42 minutes, the temperature of soaking zone II is 1200°C, the heating time is 42 minutes; the temperature of the billet out of the furnace is 1200°C; the total heating time is 3.5 hours. The tube billet is taken out of the annular heating furnace and sent to a conical piercing machine for piercing, the piercing temperature is 1130°C, and a rough tube is obtained after piercing;

[0072] Step S3: rolling the rough pipe with a continuous rolling mill at a rolling temperature of 950° C. to obtain a rough pipe; dephosphorizing the rough pipe with high-pressure water at a water pressure of ≥20 MPa;

[0073] Step S4: sending the rough pipe to the first sizing reducing rack for sizing to obtain a hot-rolled seamless steel pipe; wherein the first sizing reducing rack includes 3 first elliptical hole sizing racks and 1 first circular hole sizing rack; along the steel pipe transportation direction, the first elliptical hole rack 1, the second elliptical hole rack 2, the third elliptical hole rack 3, and the first circular hole sizing rack are arranged in sequence; the sizing reduction rates of the single racks are 1.8%, 3.4%, 1.75%, and 0.1%, respectively, and the total sizing reduction rate is 6.9%.

[0074] Step S5: Send the hot-rolled seamless steel pipe to the hot expansion line for hot expansion. The hot expansion process includes an intermediate frequency heating section and an intermediate frequency heat preservation section. The working temperature is controlled at 720°C, and the advancing speed of the conical mandrel is 150 mm / min. The outer diameter of the hot-expanded steel pipe is 450 mm, and the wall thickness is 13 mm.

[0075] Step S6: Perform the second heating before quenching on the hot-expanded steel pipe in the walking beam furnace. The heating temperature is 860°C, and the heat preservation time is twice the wall thickness of the hot-expanded steel pipe, about 26 min. Remove the scale from the heated pipe body with high-pressure water, and the water pressure ≥ 20 MPa. Perform the second sizing and reducing on the steel pipe after high-pressure water descaling. The sizing machine structure used for the second sizing and reducing: Four second elliptical pass sizing stands and one second round pass sizing stand are arranged in sequence along the transportation direction. The reduction ratio of a single sizing stand of the sizing machine is 0%, and the total reduction ratio is 0%. The ovality α (major semi-axis / minor semi-axis) of the second elliptical pass is 1.035, 1.025, 1.020, and 1.015 in sequence.

[0076] The second elliptical pass sizing stand includes a first roll 10, a second roll 11, and a third roll 12. The three rolls are the same. The axis of the first roll 10 is arranged horizontally, and the axes of the second roll 11 and the third roll 12 are respectively arranged on both sides of the vertical line of the axis of the first roll 10. The first roll 10, the second roll 11, and the third roll 12 are spliced to form an elliptical pass. The roll gap 7 is spliced between adjacent two rolls. The bottom of the roll groove of each roll is the roll bottom 8. The radial direction from the center 9 of the elliptical pass to the roll gap 7 is defined as the major semi-axis direction 5, and the radial direction from the center 9 of the elliptical pass to the roll bottom 8 is defined as the minor semi-axis direction 6. Among them, the radial direction from the center 9 of the elliptical pass of the first roll 10 to the roll bottom 8 of the elliptical pass of the first roll 10 is defined as the first minor semi-axis direction 61. The included angle between the first minor semi-axis directions 61 of adjacent two second elliptical pass sizing stands is 180°.

[0077] The sizing stands include in sequence: The second elliptical pass sizing stand one 1 in which the first minor semi-axis direction 61 of the elliptical pass is at the 12 o'clock direction of the pass along the steel pipe transportation direction 4, the second elliptical pass sizing stand two 2 in which the first minor semi-axis direction 61 of the elliptical pass is at the 6 o'clock direction of the pass, the second elliptical pass sizing stand three 21 in which the first minor semi-axis direction 61 of the elliptical pass is at the 12 o'clock direction of the pass, the second elliptical pass sizing stand four 22 in which the first minor semi-axis direction 61 of the elliptical pass is at the 6 o'clock direction of the pass, and the second round pass sizing stand 3. The overall structure of the sizing stands is as Figure 4As shown; the outer diameter of the seamless steel pipe after the second sizing is 450mm. The steel pipe after sizing is transported to the quenching tank for water quenching to room temperature; the quenched hot-expanded seamless steel pipe is tempered at a temperature of 640°C and a holding time of 3 times the wall thickness of the hot-expanded steel pipe, which is about 39 minutes; after straightening, flaw detection and inspection processes, a large-diameter thin-walled high-strength seamless steel pipe is finally obtained, with an outer diameter of 450mm and a wall thickness of 13mm.

[0078] Example 2

[0079] The overall process of large-diameter thin-walled seamless steel pipes includes: blank cutting → ring furnace heating → perforation → rolling → high-pressure water descaling → first fixed diameter reduction → thermal expansion → cutting head and tail → storage in intermediate warehouse → stepping furnace quenching and heating → high-pressure water descaling → second fixed diameter reduction → water quenching → tempering → physical and chemical inspection → straightening → non-destructive testing → cutting to size → manual inspection → inkjet printing → packaging and storage.

[0080] Step S1: preparing ingredients according to the proportion of each element, in terms of weight percentage, C 0.27%, Si 0.35%, Mn 1.35%, V0.04%, Nb 0.030%, Ti 0.0017%, Al 0.030%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.015%, S0.008%, B 0.002%, N 0.008%, the remainder is Fe and unavoidable impurities, totaling 100%; wherein the contents of V, Nb, Al and N satisfy the relationship: (V+Nb+Al)): N=12.5; mixing and smelting the metallurgical raw materials to obtain a tube blank;

[0081] Step S2: The tube billet is subjected to the first heating in the annular furnace, and the billet passes through six heating zones in sequence: preheating section, heating zone I, heating zone II, heating zone III, soaking zone I, soaking zone II; the heating process of the six zones is as follows: the preheating section follows the furnace temperature, the temperature of heating zone I is 900°C, the heating time is 42min, the temperature of heating zone II is 1110°C, the heating time is 42min, the temperature of heating zone III is 1210°C, the heating time is 42min, the temperature of soaking zone I is 1250°C, the heating time is 42min, the temperature of soaking zone II is 1250°C, the heating time is 42min; the temperature of the billet out of the furnace is 1245°C; the total heating time is 3.5 hours. The tube billet is taken out of the annular heating furnace and sent to a conical piercing machine for piercing, the piercing temperature is 1180°C, and a rough tube is obtained after piercing;

[0082] Step S3: rolling the rough pipe with a continuous rolling mill at a rolling temperature of 1050° C. to obtain a rough pipe; dephosphorizing the rough pipe with high-pressure water at a water pressure of ≥20 MPa;

[0083] Step S4: Send the raw pipe to the first sizing-reducing mill for sizing and reducing to obtain a hot-rolled seamless steel pipe; among them, the first sizing-reducing mill includes 3 first oval-pass sizing stands and 1 first round-pass sizing stand; along the steel pipe transportation direction, they are the first oval-pass stand one, the second oval-pass stand two, the third oval-pass stand three, and the first round-pass sizing stand in sequence; the reduction rate of a single stand is 1.8%, 3.4%, 1.75%, 0.1% respectively, and the total reduction rate is 6.9%.

[0084] Step S5: Send the hot-rolled seamless steel pipe to the hot expansion line for hot expansion. The hot expansion process includes an intermediate frequency heating section and an intermediate frequency heat preservation section. The working temperature is controlled at 750°C, and the advancing speed of the conical mandrel is 250 mm / min; the outer diameter of the hot-expanded steel pipe is 430 mm, and the wall thickness is 13 mm.

[0085] Step S6: Conduct the second heating before quenching on the hot-expanded steel pipe in a walking beam furnace. The heating temperature is 910°C, and the heat preservation time is 2.5 times the wall thickness of the hot-expanded steel pipe, about 32.5 min; conduct high-pressure water descaling on the heated pipe body, and the water pressure ≥ 20 MPa; conduct the second sizing and reducing on the steel pipe after high-pressure water descaling; the sizing machine structure used for the second sizing and reducing: 3 second oval-pass sizing stands and 1 second round-pass sizing stand are arranged in sequence along the transportation direction; the ovality α (major semi-axis / minor semi-axis) of the second oval-pass is 1.035, 1.025, 1.015 in sequence; the reduction rate of a single sizing stand is 0%, and the total reduction rate is 0%;

[0086] The second oval-pass sizing stand includes a first roll 10, a second roll 11, and a third roll 12; the three rolls are the same; the axis of the first roll 10 is arranged horizontally, and the axes of the second roll 11 and the third roll 12 are respectively arranged on both sides of the vertical line of the axis of the first roll 10; the first roll 10, the second roll 11, and the third roll 12 are spliced to form an oval pass; the roll gap 7 is spliced between adjacent two rolls; the bottom of the roll groove of each roll is the roll bottom 8; the radial direction from the oval-pass center 9 to the roll gap 7 is defined as the major semi-axis direction 5, and the radial direction from the oval-pass center 9 to the roll bottom 8 is defined as the minor semi-axis direction 6; among them, the radial direction from the oval-pass center 9 of the first roll 10 to the roll bottom 8 of the oval pass of the first roll 10 is defined as the first minor semi-axis direction 61; the included angle between the first minor semi-axis directions 61 of adjacent two second oval-pass sizing stands is 180°;

[0087] The sizing racks sequentially include: a second elliptical hole sizing rack 1 with the first minor semi-axis direction 61 of the elliptical hole at the 12 o'clock direction of the hole along the steel pipe transportation direction 4, a second elliptical hole sizing rack 2 with the first minor semi-axis direction 61 of the elliptical hole at the 6 o'clock direction of the hole, a second elliptical hole sizing rack 3 21 with the first minor semi-axis direction 61 of the elliptical hole at the 12 o'clock direction of the hole, and a second circular hole sizing rack 3; the overall structure of the sizing rack is as follows Figure 5 As shown; the outer diameter of the seamless steel pipe after the second sizing reduction is 430mm. The steel pipe after sizing reduction is transported to the quenching tank for water quenching to room temperature; the quenched heat-expanded seamless steel pipe is tempered at a temperature of 650℃ and a holding time of 3.5 times the wall thickness of the heat-expanded steel pipe, which is about 45.5min; after straightening, flaw detection and inspection processes, a large-diameter thin-walled high-strength seamless steel pipe is finally obtained, with an outer diameter of 430mm and a wall thickness of 13mm; the outer surface of the steel pipe is smooth, without defects such as concave or collapse, such as Figure 1 This is a physical picture of the seamless steel pipe after heat expansion + tempering treatment.

[0088] Example 3

[0089] The overall process of large-diameter thin-walled seamless steel pipes includes: blank cutting → ring furnace heating → perforation → rolling → high-pressure water descaling → first fixed diameter reduction → thermal expansion → cutting head and tail → storage in intermediate warehouse → stepping furnace quenching and heating → high-pressure water descaling → second fixed diameter reduction → water quenching → tempering → physical and chemical inspection → straightening → non-destructive testing → cutting to size → manual inspection → inkjet printing → packaging and storage.

[0090] Step S1: preparing ingredients according to the proportion of each element, in terms of weight percentage, C 0.30%, Si 0.45%, Mn 1.65%, V0.06%, Nb 0.035%, Ti 0.0017%, Al 0.045%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.020%, S0.010%, B 0.0025%, N 0.0072%, the remainder is Fe and unavoidable impurities, totaling 100%; wherein the contents of V, Nb, Al and N satisfy the relationship: (V+Nb+Al)): N=19.5; mixing and smelting the metallurgical raw materials to obtain a tube blank;

[0091] Step S2: The tube blank is first heated in a rotary hearth furnace. The billet passes through six heating zones in sequence: the preheating zone, heating zone I, heating zone II, heating zone III, soaking zone I, and soaking zone II. The heating processes in the six zones are as follows: In the preheating zone, it follows the furnace temperature. The temperature in heating zone I is 990 °C, and the heating time is 42 min. The temperature in heating zone II is 1130 °C, and the heating time is 42 min. The temperature in heating zone III is 1290 °C, and the heating time is 42 min. The temperature in soaking zone I is 1290 °C, and the heating time is 42 min. The temperature in soaking zone II is 1290 °C, and the heating time is 42 min. The billet's outgoing furnace temperature is 1280 °C. The total heating time is 3.5 hours. The tube blank is taken out from the rotary hearth furnace and sent to a conical piercing mill for piercing. The piercing temperature is 1230 °C, and a rough tube is obtained after piercing.

[0092] Step S3: The rough tube is rolled by a continuous rolling mill at a rolling temperature of 1150 °C to obtain a semifinished tube. High-pressure water descaling is performed on the semifinished tube, and the water pressure is ≥ 20 MPa.

[0093] Step S4: The semifinished tube is sent to the first sizing and reducing stand for sizing and reducing to obtain a hot-rolled seamless steel tube. The first sizing and reducing stand includes 3 first oval pass sizing stands and 1 first round pass sizing stand. Along the steel tube transportation direction, they are the first oval pass stand one, the second oval pass stand two, the third oval pass stand three, and the first round pass sizing stand in sequence. The reduction ratio of a single stand is 1.8%, 3.4%, 1.75%, and 0.1% respectively, and the total reduction ratio is 6.9%.

[0094] Step S5: The hot-rolled seamless steel tube is sent to a hot expanding line for hot expansion. The hot expansion process includes an intermediate frequency heating-up section and an intermediate frequency heat preservation section. The working temperature is 780 °C, and the advancing speed of the conical mandrel is 350 mm / min. The outer diameter of the hot-expanded steel tube is 425 mm, and the wall thickness is 13 mm.

[0095] Step S6: The hot-expanded steel tube is secondarily heated in a walking beam furnace before quenching. The heating temperature is 960 °C, and the holding time is about 39 min, which is 3 times the wall thickness of the hot-expanded steel tube. High-pressure water descaling is performed on the heated tube body, and the water pressure is ≥ 20 MPa. Second sizing and reducing is performed on the steel tube after high-pressure water descaling. The structure of the sizing and reducing mill used for the second sizing and reducing: Along the transportation direction, 2 second oval pass sizing stands and 1 second round pass sizing stand are arranged in sequence. The ovality α (major semi-axis / minor semi-axis) of the second oval pass is 1.035 and 1.015 respectively. The reduction ratio of a single sizing stand is 0%, and the total reduction ratio is 0%.

[0096] The second elliptical pass sizing stand includes a first roll 10, a second roll 11 and a third roll 12; the three rolls are identical; the axis of the first roll 10 is arranged horizontally, and the axes of the second roll 11 and the third roll 12 are respectively arranged on both sides of the perpendicular line of the axis of the first roll 10; the first roll 10, the second roll 11 and the third roll 12 are spliced to form an elliptical pass; the adjacent two rolls are spliced to form a roll gap 7; the bottom of the roll groove of each roll is a roll bottom 8; the radial direction from the center 9 of the elliptical pass to the roll gap 7 is defined as the long semi-axis direction 5, and the radial direction from the center 9 of the elliptical pass to the roll bottom 8 is defined as the short semi-axis direction 6; among them, the radial direction from the center 9 of the elliptical pass of the first roll 10 to the roll bottom 8 of the elliptical pass of the first roll 10 is defined as the first short semi-axis direction 61; the included angle between the first short semi-axis directions 61 of two adjacent second elliptical pass sizing stands is 180°;

[0097] The sizing stands successively include: along the steel pipe transportation direction 4, the second elliptical pass sizing stand one 1 with the first short semi-axis direction 61 of the elliptical pass at the 12 o'clock direction of the pass, the second elliptical pass sizing stand two 2 with the first short semi-axis direction 61 of the elliptical pass at the 6 o'clock direction of the pass, and the second round pass sizing stand 3; the overall structure of the sizing stands is as Figure 6 shown; the outer diameter of the seamless steel pipe after the second sizing and reduction is 425 mm. The sized and reduced steel pipe is transported to a quenching tank for water quenching to room temperature; the quenched hot-expanded seamless steel pipe is subjected to tempering treatment, the tempering temperature is 660 °C, and the holding time is 4 times the wall thickness of the hot-expanded steel pipe, about 52 min; after straightening, flaw detection, and inspection processes, a large-diameter thin-walled high-strength seamless steel pipe with an outer diameter of 425 mm and a wall thickness of 13 mm is finally obtained; the outer surface of the steel pipe is smooth, without defects such as concave surfaces or collapses.

[0098] Example 4

[0099] The difference between Example 4 and Example 2 lies in the element ratios of the seamless steel pipe;

[0100] By weight percentage, C 0.25%, Si 0.15%, Mn 1.25%, V 0.020%, Nb 0.015%, Ti 0.0017%, Al 0.04%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.015%, S 0.008%, B 0.0012%, N 0.010%, the balance is Fe and unavoidable impurities, totaling 100%; among them, the contents of V, Nb, Al and N satisfy the relationship: (V + Nb + Al)): N = 7.5.

[0101] Example 5

[0102] Example 5 is different from Example 2 in the element ratios of the seamless steel pipe;

[0103] By weight percentage, C is 0.30%, Si is 0.45%, Mn is 1.50%, V is 0.06%, Nb is 0.025%, Ti is 0.0017%, Al is 0.045%, Cr is 0.055%, Ni is 0.021%, Mo is 0.013%, P is 0.015%, S is 0.005%, B is 0.0015%, N is 0.008%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the contents of V, Nb, Al, and N satisfy the relationship: (V + Nb + Al): N = 16.25.

[0104] Example 6

[0105] Example 6 is different from Example 2 in the element ratios of the seamless steel pipe;

[0106] By weight percentage, C is 0.15%, Si is 0.10%, Mn is 1.1%, V is 0.015%, Nb is 0.040%, Ti is 0.0017%, Al is 0.01%, Cr is 0.055%, Ni is 0.021%, Mo is 0.013%, P is 0.025%, S is 0.015%, B is 0.0005%, N is 0.003%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the contents of V, Nb, Al, and N satisfy the relationship: (V + Nb + Al): N = 21.67.

[0107] Example 7

[0108] Example 7 is different from Example 2 in the element ratios of the seamless steel pipe;

[0109] By weight percentage, C is 0.35%, Si is 0.55%, Mn is 1.70%, V is 0.080%, Nb is 0.045%, Ti is 0.0017%, Al is 0.06%, Cr is 0.055%, Ni is 0.021%, Mo is 0.013%, P is 0.02%, S is 0.01%, B is 0.003%, N is 0.012%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the contents of V, Nb, Al, and N satisfy the relationship: (V + Nb + Al): N = 15.41.

[0110] Example 8

[0111] Example 8 is different from Example 2 in that the hot-expanded outer diameter is 420 mm; the wall thickness is 13 mm.

[0112] Example 9

[0113] Example 9 is different from Example 2 in that the hot-expanded outer diameter is 415 mm and the wall thickness is 13 mm.

[0114] Example 10

[0115] Example 10 is different from Example 2 in that the hot-expanded outer diameter is 406 mm and the wall thickness is 13 mm.

[0116] Example 11

[0117] Example 11 is different from Example 2 in that the hot-expanded outer diameter is 377 mm and the wall thickness is 13 mm.

[0118] Example 12

[0119] Example 12 is different from Example 2 in that the hot expansion temperature is 800 °C.

[0120] Example 13

[0121] Example 13 is different from Example 2 in that the advancing speed of the tapered mandrel during hot expansion is 100 mm / min.

[0122] Example 14

[0123] Example 14 is different from Example 2 in that the advancing speed of the tapered mandrel during hot expansion is 400 mm / min.

[0124] Example 15

[0125] Example 15 is different from Example 2 in that the second heating temperature is 800 °C.

[0126] Example 16

[0127] Example 16 is different from Example 2 in that the second heating temperature is 1000 °C.

[0128] Comparative Example 1

[0129] Comparative Example 1 is different from Example 2 in that in step S6, after high-pressure water descaling, the second sizing and reducing is not carried out, and direct water quenching and tempering are carried out. It is found that there are concave surfaces on the outer surface of the steel pipe, and then multiple hot expansion or cold drawing processes are carried out for repair; the inner and outer surfaces of the steel pipe are as Figure 2 and Figure 3 shown, with obvious concave surfaces and collapses.

[0130] The inner and outer surface quality, dimensional accuracy and performance of the large-diameter thin-walled seamless steel pipes prepared in Examples 1 to 15 and Comparative Example 1 were detected by visual inspection, vernier caliper, spirit level and in accordance with GB / T 228.1 and GB / T 229 respectively, and the results are shown in Table 1.

[0131] Table 1

[0132]

[0133] Note: ΔD represents the nominal outside diameter tolerance.

[0134] The test data in Table 1 show that for the large-diameter thin-walled seamless steel pipes prepared by using the element composition and innovative process of the present application in Examples 1 to 16, there are no concave surfaces and collapses on the outer surface, the outer surface is smooth without concave surfaces, the ovality is small, the dimensional accuracy is high, and they have relatively excellent strength and toughness.

[0135] In Comparative Example 1, in step S6, after the hot-expanded steel pipe was descaled by high-pressure water, the second sizing and reducing was not carried out, and water quenching and tempering were directly carried out. When concave surfaces, collapses, and excessive ovality and other defects occurred during the quenching heating of the hot-expanded steel pipe, the concave surfaces were eliminated by repeating hot expansion or cold drawing. However, obviously, the elimination effect of this method was not good, and there were still obvious concave surfaces and collapses on the outer surface of the steel pipe and the ovality was large, etc., and the strength and toughness of the seamless steel pipe produced by this method were poor.

[0136] It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects and do not necessarily have to be used to describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances so that the embodiments of the present application described herein can be implemented in an order other than those described herein, for example.

[0137] The above are only the preferred embodiments of the present application and are not used to limit the present application. For those skilled in the art, the present application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A method for preparing a large-diameter thin-walled seamless steel pipe, characterized in that: include: Step S1: mixing and smelting various metallurgical raw materials to obtain a tube blank; In terms of weight percentage, the proportion of each element is as follows: C 0.18~0.30%, Si 0.15~0.45%, Mn 1.35~1.65%, V 0.02~0.06%, Nb 0.015~0.035%, Ti≤0.015%, Al 0.015~0.045%, Cr 0.055~0.15%, Ni0.021~0.10%, Mo 0.013~0.04%, P≤0.020%, S≤0.010%, B 0.00080~0.0025%, N≤0.010%, the remainder is Fe and unavoidable impurities, a total of 100%; Among them, the contents of V, Nb, Al and N satisfy the relationship: 5.0≤(V+Nb+Al): N≤19.5; Step S2: performing first heating, punching, rolling and first sizing reduction on the tube blank in sequence to obtain a hot-rolled seamless steel tube; performing heat expansion on the hot-rolled seamless steel tube to obtain a heat-expanded steel tube, wherein the heat-expanded outer diameter is 350-450 mm; Step S3: sequentially performing a second heating and a second sizing reduction on the hot expanded steel pipe; the second sizing reduction process comprises: the hot expanded steel pipe first enters a second elliptical hole type sizing rack for a first sizing reduction, and then enters a second circular hole type sizing rack for a second sizing reduction; the number of the second elliptical hole type sizing racks is 1 to 4, and the value range of the elliptical hole type of the second elliptical hole type sizing rack is: 1<α≤1.05; the number of the second circular hole type sizing racks is 1 to 2; the second elliptical hole type sizing rack comprises a first roller, a second roller and a third roller; the three rollers are the same; the axis of the first roller is arranged in the horizontal direction, and the axes of the second roller and the third roller are respectively arranged on both sides of the vertical line of the axis of the first roller; The first roller, the second roller and the third roller are spliced ​​to form an elliptical hole profile; the radial direction from the center of the elliptical hole profile of the first roller to its roller bottom is defined as the first short semi-axis direction; along the steel pipe transportation direction, the angle between the first short semi-axis directions of two adjacent second elliptical hole sizing stands is 180°; the outer diameter of the seamless steel pipe obtained by the second sizing reduction is the same as the thermal expansion outer diameter; the outer diameter of the seamless steel pipe is 350~450mm, and the wall thickness is 6~15mm; finally, the seamless steel pipe is sequentially water quenched and tempered, and the tempering temperature is 640~660℃; the insulation time = insulation coefficient x steel pipe wall thickness, the insulation coefficient is 3~4min / mm, and the unit of the steel pipe wall thickness is mm; the large-diameter thin-walled seamless steel pipe is obtained.

2. The method for preparing a large-diameter thin-walled seamless steel pipe according to claim 1, characterized in that: In the step S1, the proportion of each element is as follows, in terms of weight percentage: C 0.22-0.30%, Si 0.25-0.40%, Mn 1.35-1.50%, V 0.025-0.06%, Nb 0.020-0.035%, Ti≤0.015%, Al 0.02-0.045%, Cr 0.055-0.15%, Ni0.021-0.08%, Mo 0.013-0.04%, P≤0.015%, S≤0.008%, B 0.0012-0.0022%, N≤0.080%, and the remainder is Fe and unavoidable impurities, totaling 100%; The weight percentages of V, Nb, Al and N satisfy the relationship: 8.5≤(V+Nb+Al): N≤17.

5.

3. The method for preparing a large-diameter thin-walled seamless steel pipe according to claim 1 or 2, characterized in that: The yield strength of the large-diameter thin-walled seamless steel pipe is ≥550MPa, the tensile strength is ≥650MPa, the elongation is ≥18%, and the impact energy AKV at -40°C is ≥55J; and / or the outer diameter of the seamless steel pipe is 368~450mm.

4. The method for preparing a large-diameter thin-walled seamless steel pipe according to claim 1 or 2, characterized in that: The temperature of the second heating is 860~960℃, the insulation time = insulation coefficient x steel pipe wall thickness, the insulation coefficient is 2~3min / mm, and the unit of the steel pipe wall thickness is mm; the second heating is carried out in a stepping furnace and adopts a stepping mode.

5. The method for preparing a large-diameter thin-walled seamless steel pipe according to claim 1 or 2, characterized in that: In step S2, the temperature of the intermediate frequency insulation section of the thermal expansion is 720-780°C, and the temperature deviation is not greater than 10°C; and / or, the advancement speed of the tapered mandrel used in the thermal expansion is 150-350 mm / min; And / or, the rolling temperature is 950-1150° C.; the rolling is performed using a continuous rolling mill.

6. The method for preparing a large-diameter thin-walled seamless steel pipe according to claim 1 or 2, characterized in that: In the step S2, the first sizing process includes: the rolled steel pipe first enters a first elliptical hole sizing rack for sizing, and then enters a first circular hole sizing rack for sizing, the number of the first elliptical hole sizing racks is 1 to 10, and the number of the circular hole sizing racks is 1 to 2; the sizing rate of a single rack is controlled to be less than 5%, and the total sizing rate is controlled to be less than 25%.

7. The method for preparing a large-diameter thin-walled seamless steel pipe according to claim 1 or 2, characterized in that: In the step S2, the first heating process includes: the tube billet passes through the preheating section, heating zone I, heating zone II, heating zone III, soaking zone I, and soaking zone II in sequence; wherein the temperature of the preheating section is the furnace temperature; the temperature of the heating zone I is 830-990°C, and the heating time is ≥42min; the temperature of the heating zone II is 990-1130°C, and the heating time is ≥42min; the temperature of the heating zone III is 1130-1290°C, and the heating time is ≥42min; the temperature of the soaking zone I is 1200-1290°C, and the heating time is ≥42min; the temperature of the soaking zone II is 1200-1290°C, and the heating time is ≥42min; the furnace exit temperature of the tube billet is 1200-1280°C; the total time of the first heating is ≥3.5 hours; the first heating is performed in a ring furnace; And / or, the perforation temperature is 1130-1230° C.; the perforation is performed using a conical perforator.

8. The method for preparing a large-diameter thin-walled seamless steel pipe according to claim 1 or 2, characterized in that: In the step S3, high-pressure water descaling is further performed between the second heating and the second fixed diameter reduction, and the pressure is ≥20 MPa.

9. A large-diameter thin-walled seamless steel pipe, characterized in that: The large-diameter thin-walled seamless steel pipe is prepared by the method for preparing a large-diameter thin-walled seamless steel pipe according to any one of claims 1 to 8.

10. Use of the large-diameter thin-wall seamless steel pipe according to claim 9 in fluid transportation, alumina ore transportation, natural gas transportation pipelines and engineering machinery equipment.

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