Large-diameter thin-wall seamless steel pipe and preparation method and application thereof

Abstract

The invention discloses a large-diameter thin-wall seamless steel pipe and a preparation method and application thereof, and belongs to the technical field of seamless steel pipes. The method comprises the steps that S1, all metallurgical raw materials are mixed and smelted, and a pipe blank is obtained; s2, the pipe blank is sequentially subjected to first heating, perforating, rolling and first certain diameter reducing, and a hot-rolled steel pipe is obtained; the hot-rolled steel pipe is subjected to hot expansion to obtain a hot-expanded steel pipe, and the hot-expanded outer diameter is 350-450 mm; s3, sequentially carrying out second heating and second sizing and reducing on the hot-expanded steel pipe: firstly feeding the steel pipe into a second elliptical hole type sizing rack for sizing and reducing, and then feeding the steel pipe into a second circular hole type sizing rack for sizing and reducing; the outer diameter of the seamless steel pipe is the same as the outer diameter of the thermal expansion and is 350-450 mm, and the wall thickness is 5-20 mm; and sequentially carrying out water quenching and tempering on the seamless steel tube to obtain the large-caliber thin-wall seamless steel tube. The large-diameter thin-wall high-strength seamless steel pipe is uniform in microstructure, smooth in surface, free of concave collapse and excellent in obdurability.

Description

Technical Field

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

[0002] At present, the manufacture of high-strength large-diameter thin-walled steel pipes mainly relies on welded pipes synthesized from high-strength steel plates through welding technology, or through hot rolling combined with hot expansion and subsequent quenching and tempering. Although welded pipes are common, the decrease in strength and toughness in the weld area often becomes the direct cause of mechanical equipment failure, especially in situations where safety standards are strict or need to adapt to harsh conditions (such as the application of oil cylinders containing weak acid media, the outer drill pipe of large-tonnage rotary drilling rigs, etc.), only seamless steel pipes with excellent strength and toughness can ensure the safe operation of the equipment. High-strength large-diameter thin-walled seamless steel pipes, with a yield strength of at least 420MPa, an outer diameter of not less than 356mm, and an outer diameter to wall thickness ratio of more than 30, are usually difficult to obtain directly only through hot rolling or normalizing processes. The manufacture of such steel pipes often requires hot rolling and then a hot expansion process to achieve the set outer diameter size, and the subsequent quenching and tempering heat treatment is intended to give it 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, high-pressure water jet descaling and significant temperature drop during transportation, the water temperature during quenching is low. The common result is that the comprehensive strength of the steel pipe after heat treatment is insufficient, and the toughness performance is unsatisfactory. In addition, due to the deadweight of the steel pipe, deformation or ovalization problems occur frequently; during the heating of the walking furnace, the steel pipe is easily hit by the rigid components in the furnace (such as the walking beam), causing concave damage 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 of the ends due to their own weight. Unfortunately, this method can only partially alleviate the end problem, and it is difficult to work for the deformation of the middle part of the steel pipe or the depression of the outer surface, which requires the addition of cold processing or hot expansion and finishing in the subsequent process, which undoubtedly increases the manufacturing cost and reduces production efficiency. More importantly, the balance of strength and toughness and dimensional accuracy of steel pipes produced by traditional processes is difficult to achieve the ideal state. In view of this, the development of 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-walled seamless steel pipe and its preparation method and application, so as to solve the problems of deformation, concave surface, uneven structure and the like of the large-diameter thin-walled seamless steel pipe in the prior art.

[0005] In order to achieve the above object, according to one aspect of the present application, a method for preparing a large-diameter thin-walled seamless steel pipe is provided, comprising:

[0006] Step S1: mixing and smelting various metallurgical raw materials to obtain a tube blank;

[0007] Step S2: performing first heating, perforation, 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;

[0008] 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 the second elliptical hole type sizing rack for the first sizing reduction, and then enters the second circular hole type sizing rack for the second sizing reduction; the number of the second elliptical hole type sizing racks is 1 to 4, and 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 arranged in the first roller, the second roller and the third roller, respectively. On both sides of the vertical line of the axis of a roller; the first roller, the second roller and the third roller are spliced ​​to form an elliptical hole type; the radial direction from the center of the elliptical hole type 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 racks 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 water quenched and tempered in turn to obtain a large-diameter thin-walled seamless steel pipe.

[0009] Further, in step S1, the proportions of each element are as follows, in terms of weight percentage: 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 remainder is Fe and unavoidable impurities, totaling 100%; wherein the contents of V, Nb, Al and N satisfy the relationship: 5.0≤(V+Nb+Al): N≤19.5.

[0010] Further, in 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 1.25-1.50%, 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 remainder is Fe and unavoidable impurities, totaling 100%; wherein the weight percentages of V, Nb, Al and N satisfy the relationship: 8.5≤(V+Nb+Al): N≤17.5.

[0011] Furthermore, 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.

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

[0013] Furthermore, the value range of the elliptical hole of the second elliptical hole sizing rack is: 1<α≤1.05.

[0014] Furthermore, 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 steel pipe wall thickness is mm; the second heating is carried out in a stepping furnace and adopts a stepping mode.

[0015] Furthermore, in step S2, the temperature of the medium frequency insulation section of the thermal expansion is 720-780°C, and the temperature deviation is not greater than 10°C.

[0016] Furthermore, the advancement speed of the tapered mandrel used in the thermal expansion is 150-350 mm / min.

[0017] Furthermore, the rolling temperature is 950-1150°C; and the rolling is performed using a continuous rolling mill.

[0018] Further, in step S2, the first sizing process includes: the rolled steel pipe first enters the first elliptical hole sizing rack for sizing, and then enters the 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 less than 25%.

[0019] Further, in step S2, the first heating process includes: the tube billet 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 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 ≥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 temperature of the tube billet out of the furnace is 1200-1280°C; the total time of the first heating is ≥3.5 hours; and the first heating is performed in a ring furnace.

[0020] Furthermore, the perforation temperature is 1130-1230°C; and a conical perforator is used for perforation.

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

[0022] Furthermore, high-pressure water descaling is also included between the second heating and the second fixed diameter reduction, with a pressure ≥ 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 the above-mentioned preparation method.

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

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

[0026] The present application provides a large-diameter thin-walled seamless steel pipe and a preparation method and application thereof. The large-diameter thin-walled high-strength seamless steel pipe prepared by the process of sequential heating, perforation, rolling, first sizing reduction, heat expansion, reheating, second sizing reduction, water quenching and tempering has uniform microstructure, smooth surface, no concave collapse, good roundness and excellent strength and toughness. In particular, in the process of secondary sizing reduction, an innovative combination of elliptical hole types is designed to achieve alternating maximum deformation directions, and the sizing rate is very small or almost 0. The hot-expanded steel pipe that is prone to elliptical deformation during quenching and heating can be better sized into a circular steel pipe. It can also avoid the large ellipticity of the steel pipe caused by the large ellipticity of the single frame, the metal squeezed into the roller gap to form a longitudinal rolling mark, or the small ellipticity of the single frame, the large outer diameter ellipticity caused by the hole type not being filled, and the outer surface concave surface not being eliminated. The problem of poor size, thereby obtaining a large-diameter thin-walled seamless steel pipe with small ellipticity, no concave surface and excellent dimensional accuracy, further ensuring the strength and toughness of this type of steel pipe. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

[0031] Figure 4 This is a schematic diagram of the alternating arrangement of various hole types in the second elliptical hole type sizing machine frame in Example 1 of the present application;

[0032] Figure 5 This is a schematic diagram of the alternating arrangement of various hole types in the second elliptical hole type sizing machine frame in Example 2 of the present application;

[0033] Figure 6 This is a schematic diagram of the alternating arrangement of various hole types in the second elliptical hole sizing rack in Example 3 of the present application.

[0034] Reference numerals:

[0035] 1. Second elliptical hole type sizing rack one; 2. Second elliptical hole type sizing rack two; 21. Second elliptical hole type sizing rack three; 22. Second elliptical hole type sizing rack four; 3. Second circular hole type sizing rack; 4. Steel pipe transportation direction; 5. Long semi-axis direction; 6. Short semi-axis direction; 61. First short semi-axis direction; 7. Roller gap; 8. Roller bottom; 9. Elliptical hole center; 10. First roller; 11. Second roller; 12. Third roller. DETAILED DESCRIPTION

[0036] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below in conjunction with the drawings and embodiments.

[0037] As mentioned in the background technology, during the quenching and tempering heating process, large-diameter thin-walled steel pipes are affected by their own gravity, which causes deformation or elliptical problems. When heated in a walking furnace, they are easily knocked by rigid components such as walking beams, resulting in defects such as concave surfaces. The existing technology usually uses multiple thermal expansions or sets support frames at the ends of steel pipes to reduce defects such as deformation and concave surfaces of steel pipes, but this method cannot guarantee the toughness and high-precision dimensions of the steel pipe.

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

[0039] Step S1: mixing and smelting various metallurgical raw materials according to the proportion of each element to obtain a tube blank;

[0040] Step S2: performing first heating, perforation, rolling and first sizing reduction on the tube blank in sequence to obtain a hot-rolled seamless steel tube; hot-expanding the hot-rolled seamless steel tube on a hot expansion line to obtain a hot-expanded steel tube, wherein the hot-expanded outer diameter is 350-450 mm;

[0041] Step S3: sequentially performing a second heating and a second sizing on the hot expanded steel pipe; the second sizing process comprises: the hot expanded steel pipe first enters the second elliptical hole sizing rack for the first sizing, and then enters the second round hole sizing rack for the second sizing; the number of the second elliptical hole sizing racks is 1 to 4, and the number of the second round hole sizing racks is 1 to 2; the second elliptical hole 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 arranged on the vertical line of the axis of the first roller respectively On both sides; the first roller, the second roller and the third roller are spliced ​​to form an elliptical hole type; two adjacent rollers are spliced ​​to form a roller gap; the bottom of the roller groove of each roller is the roller bottom; the radial direction from the center of the elliptical hole type 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 type sizing racks 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 water quenched and tempered in turn to obtain a large-diameter thin-walled seamless steel pipe.

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

[0043] The number of the second elliptical hole type sizing racks designed in the present application is 1 to 4, such as 1, 2, 3, 4, for example 2 to 4; the number of the second circular hole type sizing racks is 1 to 2, such as 1 or 2; the diameter reduction rate of a single rack is controlled to be 0, and the total diameter reduction rate is 0; the second elliptical hole type sizing rack includes a first roller 10, a second roller 11 and a third roller 12; the three rollers are the same; the axis of the first roller 10 is arranged in the horizontal direction, and the axis of the second roller 11 and the axis of the third roller 12 are respectively arranged on both sides of the vertical line of the axis of the first roller 10; the first roller 10, the second roller 11 and the third roller 12 are spliced ​​to form an elliptical hole; Two adjacent 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 elliptical hole center 9 to the elliptical hole roll gap 7 is defined as the major semi-axis direction 5, and the radial direction from the elliptical hole center 9 to the elliptical hole roll bottom 8 is defined as the minor semi-axis direction 6. There are three major semi-axis directions and three minor semi-axis directions in an elliptical hole frame; among them, the radial direction from the elliptical hole center 9 of the first roll 10 to the elliptical hole roll bottom 8 of the first roll is defined as the first minor semi-axis direction 61; when the number of second elliptical hole sizing frames is more than 2, along the steel pipe transportation direction, the angle between the first minor semi-axis directions of two adjacent second elliptical hole sizing frames is 180°. For example, the first minor semi-axis direction 61 of the previous elliptical hole is at the 12 o'clock direction (or the 6 o'clock direction), and the first minor semi-axis direction 61 of the next elliptical hole is at the 6 o'clock direction (or the 12 o'clock direction), and they are arranged alternately in this way. For another example, when the second sizing process includes 4 second elliptical hole sizing racks and 1 second circular hole sizing rack, in the steel pipe transportation direction, different hole racks are sequentially arranged as: 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, 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, 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 second elliptical hole sizing rack 4 22 with the first minor semi-axis direction 61 of the elliptical hole at the 6 o'clock direction of the hole. The above-mentioned alternating arrangement can make the maximum deformation direction alternate, and the roundness effect of the sized steel pipe gradually becomes better. The alternating arrangement of the above-mentioned multiple elliptical hole types in the present application is very suitable for the case where the diameter reduction rate is very small or almost zero. There is no need to reduce the diameter of the heat-expanded steel pipe. As long as the above-mentioned alternating arrangement method is adopted, the steel pipe that is easy to be elliptical in the quenching and heating process after heat expansion can be better sized into a round steel pipe.Therefore, the above-mentioned combined sizing machine hole type is adopted to reasonably set the outer diameter and wall thickness of the hot expanded steel pipe before sizing, and the elliptical hole type and circular hole type of sizing, so as to avoid the problems of poor size such as the large ovality of the steel pipe that cannot be eliminated due to the large ovality of a single frame, the longitudinal rolling mark formed by the metal squeezing into the roller gap, the large outer diameter ovality caused by the small ovality of a single frame and the incomplete hole type, and the failure to eliminate the concave surface on the surface. As a result, large-diameter thin-walled seamless steel pipe with small ovality, no concave surface and excellent dimensional accuracy is obtained, which further ensures the strength and toughness of this type of steel pipe.

[0044] As a preferred embodiment, in terms of weight percentage, 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 remainder is Fe and unavoidable impurities, totaling 100%; wherein the contents of V, Nb, Al and N satisfy the relationship: 5.0≤(V+Nb+Al): N≤19.5.

[0045] The relationship between the four elements in the steel pipe formula of the present application is as follows (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 any range between two values; in the formula of the large-diameter seamless steel pipe designed in the present application, the proportion of each element is optimized to make the formula and process more compatible; in particular, 5.0≤(V+Nb+Al): The purpose of N≤19.5 is not only to form specific V, Nb, Al carbonitrides, refine the grains, and improve the strength and toughness of the steel pipe, but also to fix the N element, maximize the effect of the B element in improving the hardenability, reduce the content of elements such as C and Cr, improve the weldability, and reduce production costs. The synergistic addition of V, Nb, and Al microalloys can reduce the overall alloy dosage and reduce the AC3 temperature of the steel grade, avoiding excessive temperature drop in large-diameter thin-walled steel pipes during high-pressure water descaling and transportation, which causes the steel pipe to enter the two-phase zone before entering the water, ultimately resulting in the deterioration of the strength and toughness of the steel pipe.

[0046] The present application conducts a large number of experimental studies on the main processes in which seamless steel pipes are most likely to suffer from deformation, concave surfaces, etc. during the quenching and heating process after hot expansion. It is found that for large-diameter, thin-walled, high-strength and toughness seamless steel pipes, the proportion of steel pipe elements can be optimized and combined with production processes such as precisely controlled heating temperature, holding time, hot expansion, fixed diameter reduction rate, and rapid water quenching cooling after fixed diameter reduction. Through the optimization and coordination of formulation and process, the defects such as concave surfaces and deformation on the surface of seamless steel pipes that are prone to occur during the second heating process can be solved; there is no need to use traditional technology of multiple hot expansions or cold drawing repairs to solve the surface defects that occur during the second heating process.

[0047] As a preferred embodiment, the proportion of each element is as follows, in terms of weight percentage: C 0.22~0.30%, Si0.25~0.40%, 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 remainder is Fe and unavoidable impurities, totaling 100%; wherein the weight percentages 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 the present application adopts a special composition range and a certain component ratio to avoid the problems of low steel pipe strength, poor impact toughness, poor head-to-tail performance uniformity caused by high-pressure water descaling and excessive temperature drop during transportation during the production of traditional large-diameter, thin-walled, high-strength seamless steel pipe brands, and has very excellent strength and toughness.

[0048] As a preferred embodiment, the proportion of each element is as follows, in terms of weight percentage: 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 remainder is Fe and unavoidable impurities, totaling 100%; wherein the weight percentages of V, Nb, Al and N satisfy the relationship: 12.5≤(V+Nb+Al): N≤17.5. By further optimizing the ratio of each element in the steel pipe raw material, it is more suitable for coordination with the process of this application, improving the synergistic effect, and is more conducive to eliminating defects such as concave surfaces on the surface of seamless steel pipes.

[0049] As a preferred embodiment, 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%, and 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.

[0050] As a preferred embodiment, in terms of weight percentage, C 0.25%, Si 0.15%, Mn 1.25%, V0.020%, Nb 0.015%, Ti 0.0017%, Al 0.04%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.015%, S0.008%, B 0.0012%, N 0.010%, and 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=7.5.

[0051] As a preferred embodiment, C 0.27%, Si 0.35%, Mn 1.35%, V 0.04%, Nb 0.030%, Ti0.0017%, Al 0.030%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.015%, S 0.008%, B 0.002%, N0.008%, and 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.

[0052] As a preferred embodiment, the outer diameter of the large-diameter thin-walled seamless steel pipe is 368~450mm, and the wall thickness is 6~15mm. The formula and process of the steel pipe in this application are more suitable for the steel pipes with the preferred caliber and wall thickness mentioned above; 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℃ is ≥55J; for example, the yield strength is ≥600MPa, and the yield strength is further ≥620MPa, further reaching more than 650MPa; the tensile strength is ≥700MPa, and the tensile strength is further ≥720MPa, further reaching more than 750MPa; the elongation is ≥19%, and the elongation is further ≥20%, further reaching 21%; the impact energy AKV is ≥70J, and the impact energy AKV is further ≥100J, further reaching more than 150J; the seamless steel pipe has no concave surface and collapse, and the ovality is small.

[0053] As a preferred embodiment, in 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, soaking zone II in sequence; wherein, the temperature of the preheating section is the furnace temperature; the temperature of heating zone I is 830~990℃, and the heating time is ≥42min; the temperature of heating zone II is 990~1130℃, and the heating time is ≥42min; the temperature of heating zone III is 1130~1290℃, and the heating time is ≥42min; the temperature of soaking zone I is 1200~1290℃, and the heating time is ≥42min; the temperature of soaking zone II is 1200~1290℃, and the heating time is ≥42min; the furnace temperature of the tube billet is 1200~1280℃; the total time of the first heating is ≥3.5 hours; the first heating is heated in a ring furnace. The above heating process conditions can fully heat the tube billet, which is more conducive to the perforation and rolling process.

[0054] As a preferred embodiment, a conical piercing machine is used 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 of the heating furnace and sent to the piercing machine for piercing through a conveying device. By controlling the piercing temperature within the above range, the tube body can be pierced smoothly, which is helpful for the subsequent hot rolling process.

[0055] As a preferred embodiment, rolling adopts a continuous rolling mill to roll the rough pipe to obtain a rough pipe; the rolling temperature is 950-1150°C; the rolling process of this process can be selected from the existing technology. After rolling, the rough pipe is descaled with high-pressure water to remove the iron oxide on the surface of the pipe body, which is beneficial to the subsequent sizing operation.

[0056] As a preferred embodiment, in step S2, the first sizing process includes: the rolled steel pipe first enters the first elliptical hole sizing rack for sizing, and then enters the 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 of each hole type is controlled to be less than 5%, and the total sizing rate is less than 25%; in the first sizing process, the elliptical hole type and the circular hole type are used for sizing to ensure that the steel pipe is gradually deformed during the hot rolling process, so as to obtain a hot-rolled steel pipe with excellent size, uniform structure and stable performance.

[0057] As a preferred embodiment, the temperature of the medium frequency insulation section of thermal expansion is 720~780℃, and the overall deviation is not more than 10℃. The temperature of the medium frequency insulation section of thermal expansion can be selected from 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, or any range between them. The advancement speed of the conical mandrel used in thermal expansion is 150~350 mm / min. The advancement speed of the conical mandrel used in thermal expansion is 150~350mm / min, for example, any value among 150, 180, 200, 220, 250, 280, 300, 320, 350 mm / min, or any range between them. By accurately controlling the temperature and speed of thermal expansion, the thermal expansion deformation of the steel pipe can be gradually uniform and the organizational structure can be stable, which is beneficial to improving the strength and toughness of large-diameter thin-walled seamless steel pipes.

[0058] As a preferred embodiment, in step S3, the temperature of the second heating is 860-960°C, the holding time = holding coefficient x steel pipe wall thickness, the holding coefficient is 2-3 min / mm, and the unit of steel pipe wall thickness is mm; for example, the wall thickness of the thermally expanded steel pipe is 8 mm, and the holding time of the second heating is 16-24 min. By controlling the heating temperature and holding time of the thermally expanded steel pipe, it is helpful to release the residual stress generated in the steel pipe due to the thermal expansion process, improve the strength and toughness of the steel pipe, reduce the possibility of cracks 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 furnace and a stepping mode is adopted. The stepping mode can prevent the steel pipe from stopping rotating when the straightening machine is calibrated or a fault occurs, thereby alleviating the deterioration of the ovality or collapse of the steel pipe when it is stationary in the furnace.

[0060] As a preferred embodiment, the ellipticity α (major semi-axis / minor semi-axis) of the elliptic hole of the second elliptic hole sizing frame is in the range of 1<α≤1.05. By limiting the ellipticity and the combination of each frame, the ellipticity of the prepared large-diameter seamless steel pipe is further improved.

[0061] As a preferred embodiment, the steel pipe after the fixed diameter reduction is transported to the quenching tank for quenching cooling (water quenching) to room temperature, such as 20~30℃; the tempering temperature is 640~660℃, the holding time = insulation coefficient x steel pipe wall thickness, the insulation coefficient is 2~3min / mm, and the unit of steel pipe wall thickness is mm; for example, the wall thickness of the hot expanded steel pipe is 8mm, and the tempering holding time is 24~32min. By using rapid water quenching and tempering after the round pipe fixed diameter reduction process, the obtained steel pipe has the characteristics of small ovality, no concave surface, excellent dimensional accuracy, etc. During the quenching process, all parts of the steel pipe can be evenly and quickly cooled to room temperature, thereby obtaining a large-diameter thin-walled high-strength seamless steel pipe with excellent quenching structure, uniform structure and performance, and ensuring that the large-diameter thin-walled seamless steel pipe has stable strength and toughness through the tempering and heat preservation process.

[0062] As a preferred embodiment, high-pressure water descaling is also included between the second heating and the second fixed diameter reduction, and the pressure is ≥20MPa. The oxidized iron scale on the outer surface of the hot-expanded steel pipe can be removed by high-pressure water descaling after quenching and heating; straightening, non-destructive testing, inspection, etc. of the large-diameter thin-walled seamless steel pipe after tempering are all conventional operations and can be selected from the existing technology.

[0063] The present application mainly adopts hot rolling + tempering method to prepare large-diameter thin-walled seamless steel pipes. The specific process includes: blank feeding → ring furnace heating → perforation → rolling → high-pressure water descaling → first fixed diameter reduction → hot expansion → cutting head and tail → entering intermediate warehouse → walking 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 warehousing.

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

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

[0066] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0067] The smelting raw materials used in the examples 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 a medium frequency heating section and a medium frequency insulation section. The working temperature is controlled at 720°C, and the advancement speed of the conical mandrel is 150mm / min. The outer diameter of the hot expanded steel pipe is 450mm and the wall thickness is 13mm.

[0075] Step S6: The hot expanded steel pipe is subjected to a second heating in a walking furnace before quenching, the heating temperature is 860°C, and the holding time is 2 times the wall thickness of the hot expanded steel pipe, which is about 26 minutes; the heated pipe body is descaled with high-pressure water, and the water pressure is ≥20MPa; the steel pipe after high-pressure water descaling is subjected to a second diameter reduction; the sizing machine structure used for the second diameter reduction is: 4 second elliptical hole sizing machine frames and 1 second circular hole sizing machine frame are sequentially arranged along the transportation direction; the diameter reduction rate of a single sizing machine frame is 0%, and the total diameter reduction rate is 0%; the ellipticity α (major semi-axis / minor semi-axis) of the second elliptical hole is 1.035, 1.025, 1.020, and 1.015, respectively;

[0076] The second elliptical hole type sizing stand comprises a first roller 10, a second roller 11 and a third roller 12; the three rollers are identical; the axis of the first roller 10 is arranged in the horizontal direction, and the axis of the second roller 11 and the axis of the third roller 12 are respectively arranged on both sides of the vertical line of the axis of the first roller 10; the first roller 10, the second roller 11 and the third roller 12 are spliced ​​to form an elliptical hole type; two adjacent rollers are spliced ​​to form a roller gap 7; the bottom of the roller groove of each roller is a roller bottom 8; the radial direction from the center 9 of the elliptical hole type to the roller gap 7 is defined as the major semi-axis direction 5, and the radial direction from the center 9 of the elliptical hole type to the roller bottom 8 is defined as the minor semi-axis direction 6; wherein, the radial direction from the center 9 of the elliptical hole type of the first roller 10 to the roller bottom 8 of the elliptical hole type of the first roller 10 is defined as the first minor semi-axis direction 61; the angle between the first minor semi-axis directions 61 of two adjacent second elliptical hole type sizing stands is 180°;

[0077] The sizing racks sequentially include: along the steel pipe transportation direction 4, the second elliptical hole type sizing rack 1 with the first minor semi-axis direction 61 of the elliptical hole type at the 12 o'clock direction of the hole type, the second elliptical hole type sizing rack 2 with the first minor semi-axis direction 61 of the elliptical hole type at the 6 o'clock direction of the hole type, the second elliptical hole type sizing rack 3 21 with the first minor semi-axis direction 61 of the elliptical hole type at the 12 o'clock direction of the hole type, the second elliptical hole type sizing rack 4 22 with the first minor semi-axis direction 61 of the elliptical hole type at the 6 o'clock direction of the hole type, and the second circular hole type sizing rack 3; the overall structure of the sizing rack is as follows 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: 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 round 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 round 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%.

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

[0085] Step S6: The hot expanded steel pipe is subjected to a second heating in a walking furnace before quenching, the heating temperature is 910°C, and the holding time is 2.5 times the wall thickness of the hot expanded steel pipe, which is about 32.5 minutes; the heated pipe body is descaled with high-pressure water, and the water pressure is ≥20MPa; the steel pipe after high-pressure water descaling is subjected to a second diameter reduction; the sizing machine structure used for the second diameter reduction is: 3 second elliptical hole sizing racks and 1 second circular hole sizing rack are sequentially arranged along the transportation direction; the ellipticity α (major semi-axis / minor semi-axis) of the second elliptical hole is 1.035, 1.025, and 1.015 respectively; the diameter reduction rate of a single sizing rack is 0%, and the total diameter reduction rate is 0%;

[0086] The second elliptical hole type sizing stand comprises a first roller 10, a second roller 11 and a third roller 12; the three rollers are identical; the axis of the first roller 10 is arranged in the horizontal direction, and the axis of the second roller 11 and the axis of the third roller 12 are respectively arranged on both sides of the vertical line of the axis of the first roller 10; the first roller 10, the second roller 11 and the third roller 12 are spliced ​​to form an elliptical hole type; two adjacent rollers are spliced ​​to form a roller gap 7; the bottom of the roller groove of each roller is a roller bottom 8; the radial direction from the center 9 of the elliptical hole type to the roller gap 7 is defined as the major semi-axis direction 5, and the radial direction from the center 9 of the elliptical hole type to the roller bottom 8 is defined as the minor semi-axis direction 6; wherein, the radial direction from the center 9 of the elliptical hole type of the first roller 10 to the roller bottom 8 of the elliptical hole type of the first roller 10 is defined as the first minor semi-axis direction 61; the angle between the first minor semi-axis directions 61 of two adjacent second elliptical hole type 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 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 990°C, the heating time is 42 minutes, the temperature of heating zone II is 1130°C, the heating time is 42 minutes, the temperature of heating zone III is 1290°C, the heating time is 42 minutes, the temperature of soaking zone I is 1290°C, the heating time is 42 minutes, the temperature of soaking zone II is 1290°C, the heating time is 42 minutes; the temperature of the billet out of the furnace is 1280°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 1230°C, and a rough tube is obtained after piercing;

[0092] Step S3: rolling the rough pipe with a continuous rolling mill at a rolling temperature of 1150° C. to obtain a rough pipe; dephosphorizing the rough pipe with high-pressure water at a water pressure of ≥20 MPa;

[0093] 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 round 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 round 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%.

[0094] Step S5: Send the hot-rolled seamless steel pipe to the hot expansion line for hot expansion. The hot expansion process includes a medium frequency heating section and a medium frequency insulation section. The working temperature is 780°C and the conical mandrel advancement speed is 350mm / min. The outer diameter of the hot expanded steel pipe is 425mm and the wall thickness is 13mm.

[0095] Step S6: The hot expanded steel pipe is subjected to a second heating in a walking furnace before quenching, the heating temperature is 960°C, and the holding time is 3 times the wall thickness of the hot expanded steel pipe, which is about 39 minutes; the heated pipe body is descaled with high-pressure water, and the water pressure is ≥20MPa; the steel pipe after high-pressure water descaling is subjected to a second sizing reduction; the sizing machine structure used for the second sizing reduction is as follows: 2 second elliptical hole sizing racks and 1 second circular hole sizing rack are sequentially arranged along the transportation direction; the ellipticity α (major semi-axis / minor semi-axis) of the second elliptical hole is 1.035 and 1.015 respectively; the sizing rate of a single sizing rack is 0%, and the total sizing rate is 0%;

[0096] The second elliptical hole type sizing stand comprises a first roller 10, a second roller 11 and a third roller 12; the three rollers are identical; the axis of the first roller 10 is arranged in the horizontal direction, and the axis of the second roller 11 and the axis of the third roller 12 are respectively arranged on both sides of the vertical line of the axis of the first roller 10; the first roller 10, the second roller 11 and the third roller 12 are spliced ​​to form an elliptical hole type; two adjacent rollers are spliced ​​to form a roller gap 7; the bottom of the roller groove of each roller is a roller bottom 8; the radial direction from the center 9 of the elliptical hole type to the roller gap 7 is defined as the major semi-axis direction 5, and the radial direction from the center 9 of the elliptical hole type to the roller bottom 8 is defined as the minor semi-axis direction 6; wherein, the radial direction from the center 9 of the elliptical hole type of the first roller 10 to the roller bottom 8 of the elliptical hole type of the first roller 10 is defined as the first minor semi-axis direction 61; the angle between the first minor semi-axis directions 61 of two adjacent second elliptical hole type sizing stands is 180°;

[0097] 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, and a second circular hole sizing rack 3; the overall structure of the sizing rack is as follows Figure 6 As shown; the outer diameter of the seamless steel pipe after the second sizing reduction is 425mm. 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 660°C and the holding time is 4 times the wall thickness of the heat-expanded steel pipe, which is about 52 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 425mm and a wall thickness of 13mm; the outer surface of the steel pipe is smooth without defects such as concave surfaces or collapse.

[0098] Example 4

[0099] The difference between Example 4 and Example 2 is that the ratio of each element of the seamless steel pipe is;

[0100] In terms of weight percentage, C 0.25%, Si 0.15%, Mn 1.25%, V 0.020%, Nb 0.015%, Ti0.0017%, Al 0.04%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.015%, S 0.008%, B 0.0012%, N0.010%, and the balance is Fe and unavoidable impurities, totaling 100%; wherein the contents of V, Nb, Al and N satisfy the relationship: (V+Nb+Al)): N=7.5.

[0101] Example 5

[0102] The difference between Example 5 and Example 2 is that the ratio of each element of the seamless steel pipe is;

[0103] In terms of weight percentage, C 0.30%, Si 0.45%, Mn 1.50%, V 0.06%, Nb 0.025%, Ti0.0017%, Al 0.045%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.015%, S 0.005%, B 0.0015%, N0.008%, and the balance is Fe and unavoidable impurities, totaling 100%; wherein, the contents of V, Nb, Al and N satisfy the relationship: (V+Nb+Al): N=16.25.

[0104] Example 6

[0105] The difference between Example 6 and Example 2 is that the ratio of each element of the seamless steel pipe is:

[0106] In terms of weight percentage, C 0.15%, Si 0.10%, Mn 1.1%, V 0.015%, Nb 0.040%, Ti0.0017%, Al 0.01%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.025%, S 0.015%, B 0.0005%, N0.003%, and the balance is Fe and unavoidable impurities, totaling 100%; wherein, the contents of V, Nb, Al and N satisfy the relationship: (V+Nb+Al): N=21.67.

[0107] Example 7

[0108] The difference between Example 7 and Example 2 is that the ratio of each element of the seamless steel pipe is:

[0109] In terms of weight percentage, C 0.35%, Si 0.55%, Mn 1.70%, V 0.080%, Nb 0.045%, Ti0.0017%, Al 0.06%, Cr 0.055%, Ni 0.021%, Mo 0.013%, P 0.02%, S 0.01%, B 0.003%, N0.012%, and the balance is Fe and unavoidable impurities, totaling 100%; wherein, the contents of V, Nb, Al and N satisfy the relationship: (V+Nb+Al): N=15.41.

[0110] Example 8

[0111] The difference between Example 8 and Example 2 is that the thermal expansion outer diameter is 420 mm and the wall thickness is 13 mm.

[0112] Example 9

[0113] The difference between Example 9 and Example 2 is that the thermal expansion outer diameter is 415 mm and the wall thickness is 13 mm.

[0114] Example 10

[0115] The difference between Example 10 and Example 2 is that the thermal expansion outer diameter is 406 mm and the wall thickness is 13 mm.

[0116] Embodiment 11

[0117] The difference between Example 11 and Example 2 is that the thermal expansion outer diameter is 377 mm and the wall thickness is 13 mm.

[0118] Example 12

[0119] The difference between Example 12 and Example 2 is that the thermal expansion temperature is 800°C.

[0120] Embodiment 13

[0121] The difference between Example 13 and Example 2 is that the advancement speed of the conical core rod during the thermal expansion process is 100 mm / min.

[0122] Embodiment 14

[0123] The difference between Example 14 and Example 2 is that the advancement speed of the conical core rod during the thermal expansion process is 400 mm / min.

[0124] Embodiment 15

[0125] The difference between Example 15 and Example 2 is that the second heating temperature is 800°C.

[0126] Example 16

[0127] The difference between Example 16 and Example 2 is that the second heating temperature is 1000°C.

[0128] Comparative Example 1

[0129] The difference between Comparative Example 1 and Example 2 is that in step S6, after high-pressure water descaling, the second fixed diameter reduction is not performed, and water quenching and tempering are directly performed. It is found that the surface of the steel pipe has a concave surface, and then multiple hot expansion or cold drawing processes are performed for repair; the inner and outer surfaces of the steel pipe are as follows Figure 2 and Figure 3 As shown, there is obvious concavity and collapse.

[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 tested visually, with a vernier caliper and a level ruler, and in accordance with GB / T228.1 and GB / T229. 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 the large-diameter thin-walled seamless steel pipes prepared in Examples 1 to 16 using the element composition and innovative process of the present application have no concavity or collapse on the outer surface, are smooth and have no concavity, have small ovality, high dimensional accuracy, and have excellent strength and toughness.

[0135] In comparative example 1, in step S6, the hot-expanded steel pipe is directly water quenched and tempered without the second fixed diameter reduction after being descaled by high-pressure water. When the hot-expanded steel pipe has defects such as concave surface, collapse, and large ovality during quenching heating, the concave surface is eliminated by repeated hot expansion or cold drawing. However, it is obvious that the elimination effect of this method is not good. The surface of the steel pipe still has obvious concave surface and collapse, and the ovality is large, etc., and the strength and toughness of the seamless steel pipe produced by this method are poor.

[0136] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, 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 description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in 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; 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 sizing rack for a first sizing reduction, and then enters a second round hole sizing rack for a second sizing reduction; The number of the second elliptical hole type sizing racks is 1 to 4, and the number of the second circular hole type sizing racks is 1 to 2; the second elliptical hole type sizing rack includes 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; the radial direction from the elliptical hole center of the first roller to the bottom of the roller 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 type sizing racks 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 to 450 mm, and the wall thickness is 6 to 15 mm; finally, the seamless steel pipe is sequentially water quenched and tempered to obtain the large-diameter thin-walled seamless steel pipe.

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 0.18-0.30%, Si 0.15-0.45%, Mn 1.25-1.65%, V0.02-0.06%, 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 remainder is Fe and unavoidable impurities, totaling 100%; Among them, the contents of V, Nb, Al and N satisfy the relationship: 5.0≤(V+Nb+Al): N≤19.

5.

3. 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 1.25-1.50%, V0.025-0.06%, 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 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.

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4. The method for preparing a large-diameter thin-walled seamless steel pipe according to any one of claims 1 to 3, 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 368-450 mm; And / or, the value range of the elliptical hole α of the second elliptical hole sizing rack is: 1<α≤1.05; And / or, 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 any one of claims 1 to 3, 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 any one of claims 1 to 3, 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 any one of claims 1 to 3, 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 any one of claims 1 to 3, characterized in that: In the step S3, the tempering temperature is 640-660°C; the holding time = the holding coefficient x the steel pipe wall thickness, the holding coefficient is 3-4 min / mm, and the unit of the steel pipe wall thickness is mm; And / or, high-pressure water descaling is further performed between the second heating and the second sizing reduction, with a pressure ≥ 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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