Ultra-large diameter thin-walled seamless steel pipe and its preparation method and application

Abstract

The present application discloses an ultra-large diameter thin-wall seamless steel pipe and its preparation method and application, which belongs to the technical field of seamless steel pipe. Step S1: According to the proportion of seamless steel pipe elements, the material is mixed and smelted to obtain a blank; Step S2: The blank is heated I and perforated in sequence to obtain a rough pipe; Step S3: The rough pipe is rolled to obtain a rough pipe; Step S4: The rough pipe is subjected to a fixed diameter reduction I to obtain a hot-rolled steel pipe; Step S5: The steel pipe is thermally expanded to obtain a hot-expanded steel pipe; The thermal expansion outer diameter D 热扩 and target outer diameter D 设定 The ratio is 1.05~1.10, 450<D 设定 ≤630mm; Step S6: The steel pipe is heated II, reduced in diameter II, water quenched, and tempered in sequence to obtain a seamless steel pipe; the outer diameter of the seamless steel pipe obtained by reducing in diameter II is D 设定 The present application can mass-produce ultra-large-caliber thin-walled high-strength seamless steel pipes with small ovality, no concave surface, excellent dimensional accuracy and toughness, thereby reducing production costs and improving production efficiency.

Description

Technical Field

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

[0002] In recent years, with the increase in the number of medium-sized, large-sized, and extra-large construction projects, construction machinery has gradually developed towards large-tonnage, extra-large-tonnage, and extremely harsh usage environments, and steel pipes used in construction machinery and other industries have gradually developed towards seamless, ultra-large diameter thin-walled, and high strength and toughness. At present, high-strength ultra-large diameter thin-walled steel pipes are mainly produced by welding high-strength steel plates into welded pipes or by hot rolling + (hot expansion) + quenching and tempering, etc. However, the reduction in strength and toughness at the weld of welded pipes is often the source of failure of engineering machinery and equipment. Therefore, when the safety factor requirements are high or the usage environment is extremely harsh, such as the outer drill pipe of a large-tonnage rotary drilling rig, an ultra-large diameter oil cylinder pipe used in an environment containing weak acid, etc., seamless steel pipes with excellent strength and toughness must be used to ensure the safety of the equipment. High-strength ultra-large diameter thin-walled seamless steel pipes (for example, the yield strength is not less than 420 MPa, the outer diameter ≥ 356 mm, and the outer diameter / wall thickness exceeds 35) usually cannot be produced by hot rolling or normalizing, and products with a set outer diameter need to be obtained by hot rolling + hot expansion, and then quenching and tempering heat treatment is carried out to make the steel pipe have an excellent strength and toughness ratio. However, during the quenching and tempering heat treatment process of ultra-large diameter thin-walled steel pipes, after the steel pipe is heated to a certain temperature above the critical temperature, due to the thin wall thickness of the steel pipe, and a large temperature drop during high-pressure water descaling and transportation, and a low water entry temperature during the quenching process, the strength of the steel pipe after heat treatment is low and the strength and toughness are poor. Moreover, the steel pipe is often affected by its own gravity, resulting in deformation or ovalization problems, and it is easy to be knocked by rigid components such as walking beams during heating in the walking beam furnace, causing surface concave defects.

[0003] In order to solve the above problems, at present, some manufacturers use the method of setting an internal support frame at the pipe end to reduce the deformation of the steel pipe. However, this method can only relieve the deformation or ovalization problems caused by the self-weight of the steel pipe at the pipe end, and cannot eliminate the deformation in the middle of the steel pipe or the surface concave defects caused by being knocked by rigid components. Subsequently, additional cold deformation or hot expansion repair processes are required. Therefore, the above production method greatly reduces the production efficiency, and it is difficult to ensure excellent strength and toughness matching and high dimensional accuracy of the produced steel pipe. Summary of the Invention

[0004] The main object of the present application is to provide an ultra-large diameter thin-walled seamless steel pipe, a preparation method thereof, and an application thereof, so as to solve the problems of deformation, ovalization, surface concave, low precision, and poor strength and toughness of ultra-large diameter thin-walled seamless steel pipes in the prior art.

[0005] In order to achieve the above object, according to one aspect of the present application, a preparation method of an ultra-large diameter thin-walled seamless steel pipe is provided, including the following steps:

[0006] Step S1: Ingredients are proportioned and melted according to the proportion of each element in the seamless steel pipe to obtain a billet.

[0007] By weight percentage, C is 0.20 - 0.32%, Si is 0.15 - 0.45%, Mn is 1.15 - 1.60%, V is 0.035 - 0.08%, Nb ≤ 0.025%, Al is 0.02 - 0.05%, Cr is 0.15 - 0.30%, Ni ≤ 0.15%, Mo ≤ 0.10%, P ≤ 0.020%, S ≤ 0.010%, B is 0.00070 - 0.0020%, N ≤ 0.012%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the relationship between the contents of V, Al, and N satisfies: 5.5 ≤ (V + Al): N ≤ 21.5.

[0008] Step S2: The billet is successively heated I and pierced to obtain a rough tube.

[0009] Step S3: The rough tube is rolled to obtain a semifinished tube.

[0010] Step S4: The semifinished tube is subjected to sizing I to obtain a hot-rolled steel pipe.

[0011] Step S5: The hot-rolled steel pipe is hot-expanded to obtain a hot-expanded steel pipe.

[0012] Among them, the working temperature of the hot expansion is 720 - 780 °C; the outer diameter of the hot-expanded steel pipe after hot expansion is D 热扩 , the target outer diameter is D 设定 , D 热扩 and D 设定 The ratio is 1.05 - 1.10, 450 mm < D 设定 ≤ 630 mm.

[0013] Step S6: The hot-expanded steel pipe is successively heated II, sized II, quenched in water, and tempered to obtain an extra-large diameter thin-walled seamless steel pipe; the outer diameter of the seamless steel pipe obtained after sizing II is D 设定 , and the wall thickness is 6 - 20 mm.

[0014] Furthermore, in step S5, the ratio of D 热扩 and D 设定 is 1.06 - 1.09.

[0015] Furthermore, in step S5, the advancing speed of the conical mandrel used for hot expansion is 150 - 350 mm / min.

[0016] Furthermore, in step S4, the process of sizing I includes: the semifinished tube first passes through the oval pass sizing stand I for sizing, and then passes through the round pass sizing stand I for sizing to obtain a hot-rolled steel pipe.

[0017] Furthermore, the number of sizing stands I with oval pass is 1 to 10.

[0018] Furthermore, the number of sizing stands I with round pass is 1 to 2.

[0019] Furthermore, the reduction ratios of the sizing stands I with oval pass and the sizing stands I with round pass are respectively less than 5%, and the total reduction ratio is less than 25%.

[0020] Furthermore, in step S6, the process of sizing II includes: the hot-expanded steel pipe after heating II first undergoes sizing through the sizing stands II with oval pass, and then undergoes sizing through the sizing stands II with round pass. The diameter of the seamless steel pipe after sizing is D 设定 。

[0021] Furthermore, the number of sizing stands II with oval pass is 1 to 4.

[0022] Furthermore, the number of sizing stands II with round pass is 1 to 2.

[0023] Furthermore, the reduction ratios of the sizing stands II with oval pass and the sizing stands II with round pass are respectively less than 5%, and the total reduction ratio is less than 10%.

[0024] Furthermore, in step S6, the temperature of heating II 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.

[0025] Furthermore, heating II is carried out in a walking beam furnace.

[0026] Furthermore, the temperature of tempering is 650 - 670 °C, and 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.

[0027] Furthermore, high-pressure water descaling is also included between heating II and sizing II, and the pressure ≥ 20 MPa.

[0028] Furthermore, in step S2, the process of heating I includes: the billet sequentially passes through the preheating section, heating zone I, heating zone II, heating zone III, soaking zone I, and soaking zone II.

[0029] Furthermore, the temperature of the preheating section is the furnace temperature.

[0030] Furthermore, the temperature of heating zone I is 830 - 990 °C, and the heating time is ≥ 42 min.

[0031] Furthermore, the temperature of heating zone II is 990 - 1130 °C, and the heating time is ≥ 42 min.

[0032] Further, the temperature in the third heating zone is 1130 - 1290 °C, and the heating time is ≥ 42 min.

[0033] Further, the temperature in the first soaking zone is 1200 - 1290 °C, and the heating time is ≥ 42 min.

[0034] Further, the temperature in the second soaking zone is 1200 - 1290 °C, and the heating time is ≥ 42 min.

[0035] Further, the discharging temperature of the billet is 1200 - 1280 °C.

[0036] Further, the total heating time of the first heating is ≥ 3.5 hours.

[0037] Further, the first heating is carried out in a rotary hearth furnace.

[0038] Further, in step S2, the piercing temperature is 1130 - 1230 °C; a conical piercing mill is used for piercing.

[0039] Further, in step S3, the rolling temperature is 950 - 1150 °C; a tandem mill is used for rolling.

[0040] Further, in step S1, by weight percentage, the element ratios of the seamless steel pipe are as follows: C 0.25 - 0.30%, Si 0.25 - 0.35%, Mn 1.30 - 1.50%, V 0.05 - 0.07%, Nb ≤ 0.020%, Al 0.025 - 0.04%, Cr 0.20 - 0.25%, Ni ≤ 0.10%, Mo ≤ 0.05%, P ≤ 0.012%, S ≤ 0.008%, B 0.001 - 0.002%, N ≤ 0.010%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the weight percentage contents of V, Al, and N satisfy the relationship: 6.5 ≤ (V + Al): N ≤ 20.5.

[0041] Further, the diameter of the ultra - large - diameter thin - wall seamless steel pipe is 480 - 630 mm.

[0042] Further, the wall thickness of the ultra - large - diameter thin - wall seamless steel pipe is 6 - 15 mm.

[0043] Further, the yield strength of the ultra - large - diameter thin - wall 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.

[0044] According to the second aspect of the present application, a super-large diameter thin-walled seamless steel pipe is provided, and the super-large diameter thin-walled seamless steel pipe is prepared by using the preparation method of the above-mentioned super-large diameter thin-walled seamless steel pipe.

[0045] According to the third aspect of the present application, there is provided the application of the super-large diameter thin-walled seamless steel pipe prepared by the preparation method of the above-mentioned super-large diameter thin-walled seamless steel pipe or the above-mentioned super-large diameter thin-walled seamless steel pipe in fluid transportation, alumina ore transportation, natural gas transportation pipelines and engineering machinery equipment.

[0046] Applying the technical solution of the present application, a super-large diameter thin-walled seamless steel pipe, its preparation method and application are provided. By utilizing the large deformation in the process of round tube sizing and reducing, and the rapid quenching and cooling after sizing, the steel pipe has the characteristics of small ovality, no concave surface on the outer surface, excellent dimensional accuracy, etc. during the quenching process. During the quenching process, each part of the steel pipe can be cooled evenly and quickly to room temperature, so as to obtain a super-large diameter thin-walled high-strength seamless steel pipe with excellent quenched structure, uniform structure and performance; this production process does not require adding any original processes, and does not require multiple hot expansion or cold drawing process repairs to eliminate the concave surface after hot expansion. It can batch-produce super-large diameter thin-walled high-strength seamless steel pipes with small ovality, no concave surface on the outer surface and excellent dimensional accuracy, greatly reducing the production cost and improving the production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] 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:

[0048] Figure 1 It shows a physical photograph of the outer surface of the super-large diameter thin-walled seamless steel pipe prepared in Example 2 of the present application;

[0049] Figure 2 It shows a physical photograph of the outer surface of the super-large diameter thin-walled seamless steel pipe prepared in Comparative Example 3 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] 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.

[0051] As described in the background technology, during the tempering heating process, the ultra-large diameter thin-walled steel pipe is affected by its own gravity, which causes deformation or elliptical changes. When heated in a walking furnace, it is easy to be bumped by rigid components such as walking beams, resulting in defects such as concave surfaces. The existing technology usually adopts 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. In order to better solve a series of problems such as deformation, concave surfaces, and reduced toughness of steel pipes, a new production process suitable for ultra-large diameter thin-walled high-strength seamless steel pipes is designed.

[0052] According to one aspect of the present application, a method for preparing an ultra-large diameter thin-walled seamless steel pipe is provided, comprising the following steps:

[0053] Step S1: preparing ingredients and smelting according to the proportion of each element of the seamless steel pipe to obtain a blank;

[0054] In terms of weight percentage, the proportion of each element in the seamless steel pipe is as follows: C 0.20~0.32%, Si 0.15~0.45%, Mn 1.15~1.60%, V 0.035~0.08%, Nb≤0.025%, Al 0.02~0.05%, Cr 0.15~0.30%, Ni≤0.15%, Mo≤0.10%, P≤0.020%, S≤0.010%, B 0.00070~0.0020%, N≤0.012%, the balance is Fe and unavoidable impurities, totaling 100%; Among them, the content of V, Al and N satisfies the relationship between: 5.5≤(V+Al): N≤21.5;

[0055] Step S2: the blank is heated and perforated in sequence to obtain a rough tube;

[0056] Step S3: The rough pipe is rolled to obtain a rough pipe;

[0057] Step S4: the rough pipe is subjected to a certain diameter reduction I to obtain a hot-rolled steel pipe;

[0058] Step S5: the hot-rolled steel pipe is subjected to thermal expansion to obtain a thermally expanded steel pipe;

[0059] The working temperature of thermal expansion is 720~780℃; the outer diameter of thermal expansion steel pipe is D 热扩 , the target outer diameter is D 设定 ;D 热扩 and D 设定 The ratio is 1.05~1.10; 450 mm<D 设定 ≤630mm;

[0060] Step S6: The hot-expanded steel pipe is successively subjected to heating II, sizing and reducing II, water quenching, and tempering to obtain an extra-large diameter thin-walled seamless steel pipe; the outer diameter of the seamless steel pipe obtained by sizing and reducing II is D 设定 , and the wall thickness is 6 - 20 mm.

[0061] During the hot expansion process in the above step S5, the designed D 热扩 and D 设定 The ratio is any value among 1.05, 1.06, 1.07, 1.08, 1.09, 1.10 or the range value between any two of them; the above extra-large diameter is greater than 450 mm and less than or equal to 630 mm, such as any value among 455 mm, 460 mm, 470 mm, 480 mm, 490 mm, 500 mm, 510 mm, 520 mm, 530 mm, 540 mm, 550 mm, 560 mm, 570 mm, 580 mm, 590 mm, 600 mm, 610 mm, 620 mm, 630 mm or the range value between any two of them; the wall thickness is, for example, any value among 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.

[0062] In step S1 of this application, the value of (V + Al): N is any value among 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, 20, 20.5, 21, 21.5 or the range value between any two of them; the purpose of 5.5 ≤ (V + Al): N ≤ 21.5 is not only to form specific carbonitrides of V and Al well, refine the grains, 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, and reduce the AC3 temperature of the steel grade to avoid excessive temperature drop of the extra-large diameter thin-walled steel pipe during high-pressure water descaling and transportation, resulting in the steel pipe entering the two-phase region before entering the water, and ultimately leading to poor strength and toughness of the steel pipe.

[0063] In the hot expansion temperature of this application, the working temperature is controlled at any value among 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C or the range value between any two of them; the deviation ≤ 10°C; the upper and lower deviations of the working temperature are controlled at ≤ 9°C, ≤ 8°C, ≤ 7°C, ≤ 6°C, ≤ 5°C, ≤ 4°C, ≤ 3°C, ≤ 2°C, ≤ 1°C, etc., which is more precise. The hot expansion process includes an intermediate frequency heating section and an intermediate frequency holding section. By precisely controlling the hot expansion temperature, the hot expansion deformation of the steel pipe is ensured to be uniform and stable, and the high strength and excellent toughness of the extra-large diameter thin-walled seamless steel pipe are ensured.

[0064] The present application mainly considers and studies the process defects such as deformation and concave surface which are most likely to occur in seamless steel pipes during the heating process after thermal expansion, and finds that for ultra-large diameter thin-walled high-strength and toughness seamless steel pipes, the actual outer diameter is first expanded to a specific proportional range slightly larger than the preset target outer diameter during thermal expansion. When the heat-expanded steel pipe exhibits defects such as collapse, concave surface, and deformation in the pipe body during the quenching and heating processes, the heat-expanded steel pipe with a slightly larger diameter in advance is reduced to an ultra-large diameter thin-walled high-strength seamless steel pipe with a target outer diameter by a fixed diameter reduction process of first elliptical hole type and then circular hole type with a specific diameter reduction rate; wherein the ratio of the actual heat-expanded outer diameter to the set heat-expanded outer diameter is controlled within a range of 1.05 to 1.10, that is, the actual heat-expanded outer diameter is within a specific proportional range slightly larger than the preset target outer diameter. The amount of outer diameter expansion slightly larger than the set outer diameter after thermal expansion is 5%~10% of the set outer diameter after thermal expansion; after numerous experiments and theoretical research verifications, there is a certain correlation between the actual amount of excess thermal expansion outer diameter and the degree of concavity and deformation on the surface of the steel pipe during the quenching and heating process after thermal expansion in actual production; when the excess thermal expansion amount is controlled within the range of 5%~10%, the concavity and deformation on the surface of the steel pipe during the quenching and heating process can be trimmed and eliminated by coordinating with the subsequent specific fixed diameter reduction process, and the outer diameter of the seamless steel pipe after fixed diameter reduction can reach the set outer diameter, and the obtained seamless steel pipe has no concave surface, small ovality and excellent dimensional accuracy, which ensures the high dimensional accuracy and excellent strength and toughness of the seamless steel pipe.

[0065] The preparation of ultra-large diameter thin-walled seamless steel pipes in this application mainly adopts hot rolling + quenching and tempering, and the specific process includes:

[0066] Unloading of billets → heating in annular furnace → piercing → rolling → high-pressure water descaling → fixed diameter reduction I → thermal expansion → cutting head and tail → entering intermediate warehouse → quenching and heating in walking furnace → high-pressure water descaling → fixed diameter reduction II → water quenching → tempering → physical and chemical inspection → straightening → non-destructive testing → cutting to size → manual inspection → inkjet printing → packaging and storage.

[0067] The present application improves and eliminates the concave surface of the steel pipe by combining the heat expansion and the fixed diameter reduction process, reduces the deformation and ovality of the steel pipe, and the prepared ultra-large diameter thin-walled high-strength seamless steel pipe still has good strength and toughness; the process is suitable for ultra-large diameter thin-walled high-strength seamless steel pipes; the components in its element composition formula are more matched with the optimized process after synergy, which is not only conducive to eliminating the concave surface, reducing the ovality and reducing the deformation, but also can obtain ultra-large diameter thin-walled seamless steel pipes with excellent strength and toughness.

[0068] In some embodiments, in step S5, D 热扩 and D 设定The ratio is further controlled within 1.06 - 1.09. According to the caliber size and the defects such as concave surfaces and deformations generated on the surface of the steel pipe during the subsequent heating process, the amount of additional expansion of the actual hot-expanded outer diameter is further precisely controlled to better match the subsequent sizing and reducing process, thereby improving the dimensional accuracy of the seamless steel pipe after sizing and reducing, reducing the ovality, ensuring no concave surface on the outer surface, and further guaranteeing the high strength and excellent toughness of the seamless steel pipe.

[0069] In some embodiments, the advancing speed of the conical mandrel used in the hot expansion in step S5 is 150 - 350 mm / min, such as any value among 150, 180, 200, 220, 250, 280, 300, 320, 350 mm / min or any range value between any two of them. By precisely controlling the hot expansion speed, the hot expansion deformation of the steel pipe is ensured to be uniform and stable, and the high strength and excellent toughness of the super-large-caliber thin-walled seamless steel pipe are guaranteed.

[0070] In some embodiments, in step S4, the process of sizing and reducing I includes: the raw pipe first undergoes sizing and reducing through the oval pass sizing stand I, and then undergoes sizing and reducing through the round pass sizing stand I to obtain the hot-rolled steel pipe. Further, the number of oval pass sizing stands I is 1 - 10, such as any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or any range value between any two of them; the number of round pass sizing stands I is 1 - 2, such as 1 or 2; and the reduction rate of each oval pass sizing stand I and each round pass sizing stand I is less than 5%, and the total reduction rate of the sizing stands of the two pass types is less than 25%. By adopting the sequence of first oval pass + then round pass and specific reduction rates during the sizing and reducing I process, the raw pipe is ensured to gradually deform during the hot rolling process to obtain a hot-rolled steel pipe with stable, uniform dimensions, structure, and performance; in this process, the pass structures of each type can adopt the sizing stands commonly used in hot rolling.

[0071] In some embodiments, in step S6, the process of sizing and reducing II includes: the hot-expanded steel pipe after heating II first undergoes sizing and reducing through the oval pass sizing stand II, and then undergoes sizing and reducing through the round pass sizing stand II, and the diameter of the seamless steel pipe after sizing and reducing is the target outer diameter D 设定The number of sizing stands II with elliptical pass is 1 to 4, such as 1, 2, 3, or 4; the number of sizing stands II with round pass is 1 to 2, such as 1 or 2; and the reduction ratio of each sizing stand II with elliptical pass and each sizing stand II with round pass is less than 5% respectively, and the total reduction ratio of the sizing stands of the two passes is less than 10%. The combined sizing stand equipment structure can be selected from the prior art. By adopting the special sizing pass of first elliptical pass + then round pass, reasonably setting the outer diameter and wall thickness of the hot-expanded steel pipe before sizing, the elliptical pass and round pass of sizing, as well as the reduction amount of a single stand and the total reduction amount, problems with poor dimensions such as collapse caused by excessive deformation of a single stand, insufficient total deformation rate, large ovality of the outer diameter caused by non-full pass, and non-eliminated concave surface on the outer surface can be avoided, thereby obtaining an extra-large diameter thin-walled seamless steel pipe with small ovality, no concave surface on the outer surface, and excellent dimensional accuracy, further ensuring the high strength and excellent toughness of this type of steel pipe.

[0072] In some embodiments, in step S6, the temperature of heating II is 860 to 960 °C, such as any value among 860 °C, 880 °C, 900 °C, 920 °C, 940 °C, 960 °C or the range value between any two of them; the holding time = holding coefficient × wall thickness of the steel pipe, the holding coefficient is 3 to 4 min / mm, and the unit of the wall thickness of the steel pipe is mm; that is, the holding time is controlled according to 2 to 3 min / mm × nominal wall thickness of the steel pipe in mm; for example, when the wall thickness of the hot-expanded steel pipe is 10 mm, the holding time of heating II is 20 to 30 min. This process of heating II is carried out in a walking beam furnace. By controlling the heating temperature of the hot-expanded steel pipe and the accurate holding time, it helps to gradually 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 crack generation during quenching and cooling, and is conducive to forming a more uniform microstructure and obtaining better strength and toughness.

[0073] In some embodiments, in step S6, the sized steel pipe is transported to a quenching tank for quenching and cooling (water quenching) to room temperature, such as 20 to 30 °C; the tempering temperature is 650 to 670 °C, the holding time = holding coefficient × wall thickness of the steel pipe, the holding coefficient is 3 to 4 min / mm, and the unit of the wall thickness of the steel pipe is mm; that is, the holding time is controlled according to 3 to 4 min / mm × nominal wall thickness of the steel pipe in mm; for example, when the wall thickness of the hot-expanded steel pipe is 10 mm, the holding time of tempering is 30 to 40 min. By adopting rapid water quenching and tempering after the large deformation process of round tube sizing, the obtained steel pipe has the characteristics of small ovality, no concave surface on the outer surface, and excellent dimensional accuracy. During the quenching process, each part of the steel pipe can be cooled evenly and quickly to room temperature, thereby obtaining an extra-large diameter thin-walled high-strength seamless steel pipe with excellent metallographic structure, uniform organization and performance. Through the tempering process, it is ensured that the extra-large diameter thin-walled seamless steel pipe has stable strength and toughness.

[0074] In some embodiments, a high-pressure water descaling step is further included between heating II and sizing and reducing II; the pressure of the high-pressure water descaling is ≥ 20 MPa. By performing high-pressure water descaling after heating II, the scale generated on the outer surface of the hot-expanded steel pipe during the heating process can be removed.

[0075] In some embodiments, in step S6, straightening, non-destructive flaw detection, inspection, etc. of the ultra-large diameter thin-walled seamless steel pipe after tempering are all conventional operations and can be selected from the prior art.

[0076] In some embodiments, in step S2, the process of heating I includes: the billet sequentially passes through a preheating section, a heating I zone, a heating II zone, a heating III zone, a soaking I zone, and a soaking II zone; wherein, the temperature of the preheating section is the furnace temperature; the temperature of the heating I zone is 830 - 990 °C, and the heating time is ≥ 42 min; the temperature of the heating II zone is 990 - 1130 °C, and the heating time is ≥ 42 min; the temperature of the heating III zone is 1130 - 1290 °C, and the heating time is ≥ 42 min; the temperature of the soaking I zone is 1200 - 1290 °C, and the heating time is ≥ 42 min; the temperature of the soaking II zone is 1200 - 1290 °C, and the heating time is ≥ 42 min; the billet's discharging temperature is 1200 - 1280 °C; the total heating time of heating I is ≥ 3.5 hours; heating I is performed in a rotary hearth furnace. By adopting the above heating process, the billet can be uniformly and fully heated, which is beneficial to subsequent piercing and rolling deformation.

[0077] In some embodiments, in step S2, a conical piercing mill is used for piercing; the piercing temperature is 1130 - 1230 °C; the billet 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 pipe body can be smoothly pierced, and it is helpful for the subsequent hot rolling process.

[0078] In some embodiments, in step S3, a continuous rolling mill is used to roll the mandrel pipe to obtain a rough pipe, and the rolling temperature is 950 - 1150 °C; the rolling process of this process can be selected from the prior art. After rolling, a high-pressure water descaling operation is performed on the rough pipe to remove the scale on the pipe body surface, which is beneficial for subsequent sizing operation.

[0079] In some embodiments, in step S1, based on weight percentage, the element ratios of the seamless steel pipe are as follows: C 0.25 - 0.30%, Si 0.25 - 0.35%, Mn 1.30 - 1.50%, V 0.05 - 0.07%, Nb ≤ 0.020%, Al 0.025 - 0.04%, Cr 0.20 - 0.25%, Ni ≤ 0.10%, Mo ≤ 0.05%, P ≤ 0.012%, S ≤ 0.008%, B 0.001 - 0.002%, N ≤ 0.010%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the weight percentage contents of V, Al, and N satisfy the relationship: 6.5 ≤ (V + Al):N ≤ 20.5. The further adjusted and optimized element ratios are more conducive to the preparation of seamless steel pipes with super-large diameter, thin walls, high strength and toughness, and the formula and preparation process are more matched.

[0080] In some embodiments, based on weight percentage, the element ratios of the seamless steel pipe are as follows: C 0.28 - 0.30%, Si 0.30 - 0.40%, Mn 1.30 - 1.40%, V 0.05 - 0.06%, Nb ≤ 0.015%, Al 0.03 - 0.035%, Cr 0.22 - 0.25%, Ni ≤ 0.08%, Mo ≤ 0.04%, P ≤ 0.012%, S ≤ 0.005%, B 0.0012 - 0.0015%, N ≤ 0.008%, and the balance is Fe and inevitable impurities, totaling 100%; among them, the weight percentage contents of V, Al, and N satisfy the relationship: 7.5 ≤ (V + Al):N ≤ 19.5. With the further optimized element ratios combined with the improved process, the obtained steel pipes have better appearance and performance. For example, seamless steel pipes with super-large diameter, thin walls, high strength and toughness, without concave surfaces on the outside, small ovality, small deformation.

[0081] In some embodiments, the diameter of the seamless steel pipe is 480 - 630 mm; the above hot expansion + sizing and reducing process is more suitable for seamless steel pipes with super-large diameter within this range. Within this range, the larger the diameter, compared with conventional technologies, the more obvious the process effect of first hot expanding the allowance and then sizing and reducing in this application, the more obvious the improvement of the surface quality of the seamless steel pipe, and the obvious improvement of the mechanical properties.

[0082] In some embodiments, the wall thickness of the seamless steel pipe is 6 - 15 mm; the above process is more applicable to seamless steel pipes with a diameter of 480 - 630 mm and a wall thickness of 6 - 15 mm; seamless steel pipes of this size have a large diameter and thin walls, and are more likely to have serious surface depressions, collapses, deformations, etc. during the hot expansion and quenching heating processes. The improvement effect is more obvious by adopting the above hot expansion allowance + sizing and reducing process.

[0083] In some embodiments, the seamless steel pipe has a yield strength ≥ 550 MPa, a tensile strength ≥ 650 MPa, an elongation ≥ 18%, and an impact energy AKV at -40°C ≥ 55 J; further, the seamless steel pipe has a yield strength ≥ 600 MPa, an elongation ≥ 20%, and an impact energy AKV at -40°C ≥ 70 J; still further, the seamless steel pipe has a yield strength ≥ 620 MPa, an elongation ≥ 21%, and an impact energy AKV at -40°C ≥ 120 J.

[0084] The outer surface of the super-large diameter thin-walled seamless steel pipe prepared in this application has no concave surface or collapse. The super-large diameter thin-walled seamless steel pipe prepared by the above process and the matching formula not only solves the surface defects, but also improves the mechanical strength and the performance uniformity of each part.

[0085] The super-large diameter thin-walled high-strength seamless steel pipe of this application uses a specific element ratio and is more suitable for super-large diameter thin-walled seamless steel pipes; it can avoid problems such as low strength of the steel pipe, poor impact toughness, and poor performance uniformity at the head and tail caused by excessive temperature drop during high-pressure water descaling and transportation during the production of traditional super-large diameter thin-walled high-strength seamless steel pipe grades, and has very excellent strength and toughness.

[0086] This application utilizes the large deformation during the round tube sizing and reducing II process and the rapid quenching and cooling after sizing, so that the steel pipe has the characteristics of small ovality, no concave surface on the outer surface, and excellent dimensional accuracy during the quenching process. During the quenching process, each part of the steel pipe can be cooled evenly and quickly to room temperature, thereby obtaining a super-large diameter thin-walled high-strength seamless steel pipe with excellent metallographic structure and uniform organization and performance.

[0087] This application adopts a specially set sizing and reducing mill pass, reasonably sets the outer diameter (i.e., the actual hot-expanded outer diameter) and wall thickness of the steel pipe before sizing and reducing, as well as the oval pass, round pass of sizing and reducing, and the single-stand reduction amount and total reduction amount, to avoid problems such as collapse caused by excessive deformation of a single stand, insufficient total deformation rate, and large outer diameter ovality and uneliminated concave surface on the outer surface caused by the pass not being filled, thereby obtaining a super-large diameter thin-walled seamless steel pipe with small ovality, no concave surface on the outer surface, and excellent dimensional accuracy.

[0088] The production process of this application does not require adding any original processes, and even less requires multiple hot expansion or cold drawing processes for repairing to eliminate concave surfaces after hot expansion. It can mass-produce super-large diameter thin-walled high-strength seamless steel pipes with small ovality, no concave surface on the outer surface, and excellent dimensional accuracy, greatly reducing the production cost and improving the production efficiency.

[0089] According to the second aspect of this application, a super-large diameter thin-walled seamless steel pipe is provided, and the super-large diameter thin-walled seamless steel pipe is prepared by using the preparation method of the above super-large diameter thin-walled seamless steel pipe.

[0090] According to the third aspect of the present application, there is provided an ultra-large diameter thin-walled seamless steel pipe prepared by the above-mentioned method for preparing an ultra-large diameter thin-walled seamless steel pipe, or the use of the above-mentioned ultra-large diameter thin-walled seamless steel pipe in fluid transportation, transportation of ores such as alumina, natural gas transportation pipelines and engineering machinery and equipment; in particular, the ultra-large diameter thin-walled seamless steel pipe has better wear resistance when used for transportation of ores such as alumina.

[0091] 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.

[0092] The smelting raw materials used in the examples of the present application are prior art and can be purchased commercially.

[0093] Example 1

[0094] The overall process of ultra-large diameter thin-walled seamless steel pipes includes: blank cutting → ring furnace heating → perforation → rolling → high-pressure water descaling → fixed diameter reduction I → thermal expansion → cutting head and tail → storage in intermediate warehouse → walking furnace quenching and heating → high-pressure water descaling → fixed diameter reduction II → water quenching → tempering → physical and chemical inspection → straightening → non-destructive testing → cutting to size → manual inspection → inkjet printing → packaging and storage.

[0095] Step S1: preparing ingredients and smelting according to the following proportions of each element to obtain a blank;

[0096] In terms of weight percentage, C 0.20%, Si 0.15%, Mn 1.15%, V 0.035%, Nb 0.005%, Al0.02%, Cr 0.15%, Ni 0.015%, Mo 0.017%, P 0.020%, S 0.010%, B 0.00070%, N 0.010%, and the remainder is Fe and unavoidable impurities, totaling 100%; wherein, the contents of V, Al and N satisfy the relationship: (V+Al): N=5.5.

[0097] Step S2: Heat the blank in an annular furnace I. The blank 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 blank out of the furnace is 1200°C; the total heating time is 3.5 hours. The blank 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;

[0098] Step S3: Roll the pilger tube using a continuous rolling mill at a rolling temperature of 950°C to obtain a rough tube; perform high-pressure water descaling on the rough tube with a water pressure ≥ 20 MPa;

[0099] Step S4: Send the rough tube to the sizing and reducing I stand for sizing and reducing to obtain a hot-rolled seamless steel tube; among them, the sizing and reducing I stand includes 3 oval pass sizing stands and 1 round pass sizing stand; along the steel tube transportation direction, the first oval pass stand, the second oval pass stand, the third oval pass stand, and the round pass sizing stand are arranged in sequence; the reduction rate of a single stand for each pass type is 1.52%, 3.1%, 1.55%, and 0.1% respectively, and the total reduction rate is 6.15%.

[0100] Step S5: Send the hot-rolled seamless steel tube to the hot expansion line for hot expansion. The hot expansion process includes an intermediate frequency heating section and an intermediate frequency holding section, with the working temperature controlled at 720°C, and the advancing speed of the conical mandrel is 150 mm / min; design the actual hot-expanded outer diameter D 热扩 to be slightly larger than the preset target hot-expanded outer diameter, D 热扩 is 477.75 mm, D 设定 is 455 mm, D 热扩 : D 设定 = is 1.05; the wall thickness is 11 mm;

[0101] Step S6: Heat the hot-expanded steel tube in a walking beam furnace for heating II before quenching. The temperature of heating II is 860°C, and the holding time is 2 times the wall thickness of the hot-expanded steel tube, about 22 min; perform high-pressure water descaling on the heated tube body with a water pressure ≥ 20 MPa; perform sizing and reducing II on the steel tube after high-pressure water descaling. The sizing and reducing II stand includes 4 oval pass sizing stands and 1 round pass sizing stand, and the first oval pass sizing stand, the second oval pass sizing stand, the third oval pass sizing stand, the fourth oval pass sizing stand, and the round pass sizing stand are arranged in sequence along the transportation direction; the reduction rate of a single stand is 0.65%, 1.70%, 1.70%, 0.70%, and 0.1% respectively, and the total reduction rate is 4.76%. The outer diameter of the seamless steel tube obtained after sizing and reducing II is D 设定 = 455 mm; send the steel tube after sizing and reducing II to a quenching tank for water quenching to room temperature; perform tempering treatment on the quenched hot-expanded seamless steel tube. The tempering temperature is 650°C, and the holding time is 3 times the wall thickness of the hot-expanded steel tube, about 33 min; after straightening, flaw detection, and inspection processes, an extra-large diameter thin-walled high-strength seamless steel tube is finally obtained.

[0102] Example 2

[0103] The overall process of ultra-large diameter thin-walled seamless steel pipes includes: blank cutting → ring furnace heating → perforation → rolling → high-pressure water descaling → fixed diameter reduction I → thermal expansion → cutting head and tail → storage in intermediate warehouse → walking furnace quenching and heating → high-pressure water descaling → fixed diameter reduction II → water quenching → tempering → physical and chemical inspection → straightening → non-destructive testing → cutting to size → manual inspection → inkjet printing → packaging and storage.

[0104] Step S1: preparing ingredients and smelting according to the following proportions of each element to obtain a blank;

[0105] In terms of weight percentage, C 0.29%, Si 0.35%, Mn 1.35%, V 0.055%, Nb 0.005%, Al0.035%, Cr 0.22%, Ni 0.015%, Mo 0.017%, P 0.015%, S 0.005%, B 0.001%, N 0.008%, and the remainder is Fe and unavoidable impurities, totaling 100%; wherein, the contents of V, Al and N satisfy the relationship: (V+Al): N=11.25.

[0106] Step S2: Heat the blank in an annular furnace I. The blank 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 42 minutes, the temperature of heating zone II is 1110°C, the heating time is 42 minutes, the temperature of heating zone III is 1210°C, the heating time is 42 minutes, the temperature of soaking zone I is 1250°C, the heating time is 42 minutes, the temperature of soaking zone II is 1250°C, the heating time is 42 minutes; the temperature of the blank out of the furnace is 1245°C; the total heating time is 3.5 hours. The blank 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;

[0107] 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;

[0108] Step S4: Send the rough pipe to the sizing and reducing rack I for sizing and reducing to obtain a hot-rolled seamless steel pipe; wherein the sizing and reducing rack I includes 4 elliptical hole sizing racks and 1 round hole sizing rack; along the steel pipe transportation direction, the first elliptical hole rack, the second elliptical hole rack, the third elliptical hole rack, the fourth elliptical hole rack, and the round hole sizing rack are arranged in sequence; the sizing reduction rates of the single racks are 1.09%, 2.0%, 2.1%, 1.0%, and 0.1% in sequence, and the total sizing reduction rate is 6.15%.

[0109] 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. 热扩 626.4mm, D 设定 580mm, D 热扩 :D 设定 = is 1.08; wall thickness is 11mm;

[0110] Step S6: Heat the hot-expanded steel pipe in a step furnace before quenching II, the temperature of heating II is 910°C, and the holding time is 2.5 times the wall thickness of the hot-expanded steel pipe, which is about 27.5 minutes; descale the heated pipe body with high-pressure water, and the water pressure is ≥20MPa; reduce the steel pipe after high-pressure water descaling II, and the reduction II rack includes 4 elliptical hole sizing racks and 1 circular hole sizing rack. Along the transportation direction, the first elliptical hole sizing rack, the second elliptical hole sizing rack, the third elliptical hole sizing rack, the fourth elliptical hole sizing rack, and the circular hole sizing rack are arranged in sequence; the reduction rates of a single rack are 1.22%, 2.5%, 2.5%, 1.3%, and 0.1%, respectively, and the total reduction rate is 7.41%. The outer diameter of the seamless steel pipe obtained after reduction II is D 设定 =580mm; the steel pipe after the fixed reduction II 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 660°C and the holding time is 3 times the wall thickness of the hot-expanded steel pipe, which is about 33 minutes; after straightening, flaw detection and inspection processes, an ultra-large diameter thin-walled high-strength seamless steel pipe is finally obtained; the surface of the steel pipe is as follows Figure 1 As shown, there is no deformation or collapse, the outer surface is smooth without concavity, and the ovality is small.

[0111] Example 3

[0112] The overall process of ultra-large diameter thin-walled seamless steel pipes includes: blank cutting → ring furnace heating → perforation → rolling → high-pressure water descaling → fixed diameter reduction I → thermal expansion → cutting head and tail → storage in intermediate warehouse → walking furnace quenching and heating → high-pressure water descaling → fixed diameter reduction II → water quenching → tempering → physical and chemical inspection → straightening → non-destructive testing → cutting to size → manual inspection → inkjet printing → packaging and storage.

[0113] Step S1: preparing ingredients and smelting according to the following proportions of each element to obtain a blank;

[0114] By weight percentage, C is 0.32%, Si is 0.45%, Mn is 1.60%, V is 0.08%, Nb is 0.005%, Al is 0.05%, Cr is 0.30%, Ni is 0.015%, Mo is 0.017%, P is 0.020%, S is 0.010%, B is 0.0020%, N is 0.0061%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the relationship between the contents of V, Al, and N satisfies the formula: (V + Al):N = 21.3.

[0115] Step S2: Heat the billet in a rotary hearth furnace for Heating I. 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 of the six zones are as follows: The preheating zone follows the furnace temperature. The temperature in Heating Zone I is 990°C, and the heating time is 42 minutes. The temperature in Heating Zone II is 1130°C, and the heating time is 52 minutes. The temperature in Heating Zone III is 1290°C, and the heating time is 42 minutes. The temperature in soaking zone I is 1290°C, and the heating time is 42 minutes. The temperature in soaking zone II is 1290°C, and the heating time is 42 minutes. The billet's discharging temperature is 1280°C; the total heating time is 2 hours and 40 minutes. The billet 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.

[0116] Step S3: Roll the rough tube using a continuous rolling mill. The rolling temperature is 1150°C to obtain a semifinished tube; perform high-pressure water descaling on the semifinished tube, with the water pressure ≥ 20 MPa.

[0117] Step S4: Send the semifinished tube to the first sizing and reducing stand for sizing and reducing to obtain a hot-rolled seamless steel tube; among them, the first sizing and reducing stand includes 4 oval pass sizing stands and 1 round pass sizing stand; along the steel tube transportation direction, they are the first oval pass stand, the second oval pass stand, the third oval pass stand, the fourth oval pass stand, and the round pass sizing stand in sequence; the reduction ratio of a single stand is 1.09%, 2.0%, 2.1%, 1.0%, 0.1% respectively, and the total reduction ratio is 6.15%.

[0118] Step S5: Send the hot-rolled seamless steel tube to the hot expansion 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 controlled at 780°C, and the advancing speed of the conical mandrel is 350 mm / min; D 热扩 is 693 mm, D 设定 is 630 mm, D 热扩 :D 设定 = is 1.10; the wall thickness is 11 mm.

[0119] Step S6: Heat the hot-expanded steel pipe II before quenching in a walking beam furnace. The temperature of the heat treatment II is 960 °C, and the holding time is three times the wall thickness of the hot-expanded steel pipe, about 33 min. Descaling the heated pipe body with high-pressure water, with the water pressure ≥ 20 MPa. Perform sizing and reducing II on the steel pipe after high-pressure water descaling. The sizing and reducing II stand includes 4 oval pass sizing stands and 1 round pass sizing stand. Along the transportation direction, the first oval pass sizing stand, the second oval pass sizing stand, the third oval pass sizing stand, the fourth oval pass sizing stand, and the round pass sizing stand are arranged in sequence. The reduction ratio of a single stand is 1.51%, 3.15%, 3.15%, 1.50%, and 0.1% respectively, and the total reduction ratio is 9.09%. The outer diameter of the seamless steel pipe obtained after sizing and reducing II is D 设定 = 630 mm. Transport the steel pipe after sizing and reducing II to a quenching tank for water quenching to room temperature. Temper the quenched hot-expanded seamless steel pipe. The tempering temperature is 670 °C, and the holding time is four times the wall thickness of the hot-expanded steel pipe, about 44 min. After straightening, flaw detection, and inspection processes, an ultra-large diameter thin-walled high-strength seamless steel pipe is finally obtained. The surface of the steel pipe is as Figure 1 shown.

[0120] Example 4

[0121] The difference between Example 4 and Example 2 lies in the element ratio of the seamless steel pipe;

[0122] By weight percentage, C is 0.28%, Si is 0.25%, Mn is 1.20%, V is 0.045%, Nb is 0.005%, Al is 0.03%, Cr is 0.20%, Ni is 0.015%, Mo is 0.017%, P is 0.015%, S is 0.008%, B is 0.0012%, N is 0.010%, and the balance is Fe and unavoidable impurities, totaling 100%. Among them, the relationship between the contents of V, Al, and N satisfies the formula: (V + Al): N = 7.5.

[0123] Example 5

[0124] The difference between Example 5 and Example 2 lies in the element ratio of the seamless steel pipe;

[0125] By weight percentage, C is 0.30%, Si is 0.40%, Mn is 1.40%, V is 0.06%, Nb is 0.005%, Al is 0.035%, Cr is 0.25%, Ni is 0.015%, Mo is 0.017%, 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, Al, and N satisfy the relationship: (V + Al):N = 11.875.

[0126] Example 6

[0127] The difference between Example 6 and Example 2 lies in the element ratios of the seamless steel pipe;

[0128] By weight percentage, C is 0.18%, Si is 0.15%, Mn is 1.10%, V is 0.02%, Nb is 0.005%, Al is 0.015%, Cr is 0.15%, Ni is 0.015%, Mo is 0.017%, P is 0.025%, S is 0.01%, B is 0.008%, N is 0.007%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the contents of V, Al, and N satisfy the relationship: (V + Al):N = 5.0.

[0129] Example 7

[0130] The difference between Example 7 and Example 2 lies in the element ratios of the seamless steel pipe;

[0131] By weight percentage, C is 0.35%, Si is 0.50%, Mn is 1.70%, V is 0.10%, Nb is 0.005%, Al is 0.05%, Cr is 0.35%, Ni is 0.015%, Mo is 0.017%, P is 0.02%, S is 0.01%, B is 0.004%, N is 0.012%, and the balance is Fe and unavoidable impurities, totaling 100%; among them, the contents of V, Al, and N satisfy the relationship: (V + Al):N = 12.5.

[0132] Example 8

[0133] The difference between Example 8 and Example 2 is that in step S5, the actual hot-expanded outer diameter D 热扩 is designed to be slightly larger than the preset target hot-expanded outer diameter, D 热扩 is 508.8 mm, D 设定 is 480 mm, D 热扩 : D 设定 = is 1.06; the wall thickness is 11 mm.

[0134] Example 9

[0135] Example 9 is different from Example 2 in that in step S5, the actual hot expansion outer diameter D 热扩 is designed to be slightly larger than the preset target hot expansion outer diameter, D 热扩 is 567.1 mm, D 设定 is 530 mm, D 热扩 : D 设定 = is 1.07; the wall thickness is 11 mm.

[0136] Example 10

[0137] Example 10 is different from Example 2 in that in step S5, the actual hot expansion outer diameter D 热扩 is designed to be slightly larger than the preset target hot expansion outer diameter, D 热扩 is 664.9 mm, D 设定 is 610 mm, D 热扩 : D 设定 = is 1.09; the wall thickness is 11 mm.

[0138] Example 11

[0139] Example 11 is different from Example 2 in that the hot expansion temperature in step S5 is 800 °C.

[0140] Example 12

[0141] Example 12 is different from Example 2 in that the advancing speed of the tapered mandrel during hot expansion in step S5 is 100 mm / min.

[0142] Example 13

[0143] Example 13 is different from Example 2 in that the advancing speed of the tapered mandrel during hot expansion in step S5 is 400 mm / min.

[0144] Example 14

[0145] Example 14 is different from Example 2 in that the heating II temperature in step S6 is 800 °C.

[0146] Example 15

[0147] Example 15 is different from Example 2 in that the heating II temperature in step S6 is 1000 °C.

[0148] Comparative Example 1

[0149] Comparative Example 1 is different from Example 2 in that D 热扩 is 518.16 mm, D 设定 is 508, D 热扩： D 设定= is 1.02.

[0150] Comparative Example 2

[0151] The difference between Comparative Example 2 and Example 2 is that D 热扩 is 636 mm, D 设定 is 530, D 热扩： D 设定 = is 1.20.

[0152] Comparative Example 3

[0153] The difference between Comparative Example 3 and Example 2 is that D 热扩 is 530 mm, D 设定 is 530 mm, D 热扩： D 设定 = is 1.0, that is, directly hot expand to the set diameter during hot expansion, then perform heating, high-pressure water descaling, water quenching, and tempering. It is found that there are concave surfaces on the outer surface of the steel pipe, and then perform multiple hot expansion or cold drawing processes for repair; as shown in Figure 2 the figure, obvious concave surfaces and collapses appear.

[0154] Visually, using vernier calipers, spirit levels, and in accordance with GB / T 228.1 and GB / T 229 respectively, the outer surface quality, dimensional accuracy, and performance of the extra-large diameter thin-walled seamless steel pipes prepared in Examples 1-15 and Comparative Examples 1-3 were detected, and the results are shown in Table 1.

[0155] Table 1

[0156]

[0157] Note: ΔD represents the nominal outer diameter tolerance.

[0158] The detection data in Table 1 show that the extra-large diameter thin-walled seamless steel pipes prepared by the collaborative innovation process of element ratio in Examples 1-15 of the present application have no deformation and collapse, smooth outer surface without concave surfaces, small ovality, high dimensional accuracy, and excellent strength and toughness.

[0159] Although Comparative Examples 1-2 adopt the element composition and process of the present application, the ratio control of D 热扩 and D 设定 is not good, and the prepared seamless steel pipes still have obvious concave surfaces, deformation, and large ovality, indicating that within a certain extra-large diameter range, D 热扩 and D 设定 must be controlled within a specific range to significantly eliminate the concave surfaces on the outer surface of the steel pipe, reduce deformation and ovality, and ensure excellent strength and toughness.

[0160] In Comparative Example 3, the specific hot expansion process of the present application was not adopted. When defects such as concave surfaces, collapses, and excessive ovality 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 the outer surface of the steel pipe still had obvious concave surfaces and collapses and excessive ovality, etc., and the strength and toughness of the seamless steel pipe produced by this method were poor.

[0161] 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 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.

[0162] 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 an ultra-large diameter thin-walled seamless steel pipe, characterized in that, include: Step S1: preparing ingredients and smelting according to the proportion of each element of the seamless steel pipe to obtain a blank; Among them, in terms of weight percentage: C 0.20~0.32%, Si 0.15~0.45%, Mn 1.15~1.60%, V 0.035~0.08%, Nb≤0.025%, Al 0.02~0.05%, Cr 0.15~0.30%, Ni≤0.15%, Mo≤0.10%, P≤0.020%, S≤0.010%, B 0.00070~0.0020%, N≤0.012%, the balance is Fe and unavoidable impurities, a total of 100%; the content of V, Al and N satisfies the relationship: 5.5≤(V+Al): N≤21.5; Step S2: the blank is heated and perforated in sequence to obtain a rough tube; Step S3: the rough pipe is rolled to obtain a rough pipe; Step S4: the rough pipe is subjected to a fixed diameter reduction I to obtain a hot-rolled steel pipe; Step S5: the hot-rolled steel pipe is subjected to thermal expansion to obtain a thermally expanded steel pipe; Among them, the working temperature of the hot expansion is 720~780°C; the outer diameter of the hot-expanded steel pipe after hot expansion is D 热扩 , the target outer diameter is D 设定 , the ratio of the D 热扩 to the D 设定 is 1.05~1.10, and 450 mm < D 设定 ≤ 630 mm; Step S6: The hot-expanded steel pipe is successively subjected to heating II, sizing and reducing II, water quenching, and tempering to obtain the ultra-large diameter thin-walled seamless steel pipe; the outer diameter of the seamless steel pipe obtained by sizing and reducing II is D 设定 , and the wall thickness is 6 to 20 mm; Wherein, the temperature of heating II is 860~960℃; The sizing and reducing II: The hot-expanded steel pipe after heating II first undergoes sizing and reducing through the oval pass sizing and reducing stand II, and then undergoes sizing and reducing through the round pass sizing and reducing stand II. The diameter of the seamless steel pipe after sizing and reducing is the D 设定 ; The number of the oval pass sizing and reducing stands II is 1 to 4; The number of the round pass sizing and reducing stands II is 1 to 2; The reducing ratios of the oval pass sizing and reducing stand II and the round pass sizing and reducing stand II are respectively less than 5%, and the total reducing ratio is less than 10%.

2. The manufacturing method of the super-large-caliber thin-walled seamless steel pipe according to claim 1, characterized in that, In the step S5, the D 热扩 and the D 设定 The ratio is 1.06 to 1.09; And / or, in step S5, the advancement speed of the conical mandrel used for the thermal expansion is 150-350 mm / min.

3. The preparation method of the super-large-caliber thin-walled seamless steel pipe according to claim 1, characterized in that, In the step S4, the process of reducing the diameter I includes: the rough pipe is firstly reduced in diameter by an elliptical hole type sizing rack I, and then reduced in diameter by a round hole type sizing rack I to obtain the hot rolled steel pipe; The number of the oval hole type sizing racks 1 is 1 to 10; the number of the round hole type sizing racks 1 is 1 to 2; The diameter reduction rates of the elliptical hole type sizing rack 1 and the circular hole type sizing rack 1 are respectively less than 5%, and the total diameter reduction rate is less than 25%.

4. The method for preparing an ultra-large diameter thin-walled seamless steel pipe according to any one of claims 1 to 3, characterized in that, In the step S6, the insulation time in the heating II = insulation coefficient x steel pipe wall thickness, the insulation coefficient is 2-3 min / mm, and the unit of the steel pipe wall thickness is mm; the heating II is carried out in a stepping furnace.

5. The manufacturing method of the super-large diameter thin-walled seamless steel pipe according to any one of claims 1 to 3, characterized in that, The tempering temperature is 650-670°C, the holding time = holding coefficient x steel pipe wall thickness, the holding coefficient is 3-4min / mm, and the unit of the steel pipe wall thickness is mm; And / or, high-pressure water descaling is also included between the heating II and the sizing reduction II, with a pressure ≥ 20 MPa.

6. The method for preparing an extra-large diameter thin-walled seamless steel pipe according to any one of claims 1 to 3, characterized in that, In the step S2, the heating process I includes: the blank is sequentially passed 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; 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 temperature of the blank is 1200-1280°C; The total heating time of the heating I ≥ 3.5 hours; the heating I is performed in a rotary hearth furnace; And / or, in the step S2, the temperature of the piercing is 1130 - 1230 °C; the piercing is performed using a conical piercer; And / or, in the step S3, the temperature of the rolling is 950 - 1150 °C; the rolling is performed using a tandem mill.

7. The method for preparing an ultra-large diameter thin-walled seamless steel pipe according to any one of claims 1 to 3, characterized in that, In the step S1, by weight percentage, the element ratios of the seamless steel pipe are as follows: C 0.25 - 0.30%, Si 0.25 - 0.35%, Mn 1.30 - 1.50%, V 0.05 - 0.07%, Nb ≤ 0.020%, Al 0.025 - 0.04%, Cr 0.20 - 0.25%, Ni ≤ 0.10%, Mo ≤ 0.05%, P ≤ 0.012%, S ≤ 0.008%, B 0.001 - 0.002%, N ≤ 0.010%, the balance is Fe and unavoidable impurities, totaling 100%; wherein, the weight percentage contents of V, Al and N satisfy the relationship: 6.5 ≤ (V + Al): N ≤ 20.

5.

8. The manufacturing method of the super-large diameter thin-walled seamless steel pipe according to any one of claims 1 to 3, characterized in that, The diameter of the extra-large diameter thin-walled seamless steel pipe is 480 - 630 mm; And / or, the wall thickness of the extra-large diameter thin-walled seamless steel pipe is 6 - 15 mm; And / or, the yield strength of the extra-large diameter thin-walled seamless steel pipe ≥ 550 MPa, the tensile strength ≥ 650 MPa, the elongation ≥ 18%, the impact energy AKV at -40 °C ≥ 55 J.

9. A super-large-caliber thin-walled seamless steel pipe, characterized in that, The extra-large diameter thin-walled seamless steel pipe is prepared by using the preparation method of the extra-large diameter thin-walled seamless steel pipe according to any one of claims 1 to 8.

10. Application of the extra-large diameter thin-walled 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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