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

Abstract

The invention discloses an ultra-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 following steps: S1, burdening according to the seamless steel tube element proportion, and smelting to obtain a blank; s2, the blank is sequentially subjected to heating I and perforating, and a tubular billet is obtained; s3, rolling the tubular billet to obtain a pierced billet; s4, the pierced billet is subjected to sizing and reducing I, and a hot-rolled steel pipe is obtained; s5, the steel pipe is subjected to heat expansion, and a heat-expanded steel pipe is obtained; the set ratio of the thermal expansion outer diameter D to the target outer diameter D is 1.05-1.10, and the D is larger than 450 and smaller than or equal to 630 mm; s6, the steel pipe is sequentially subjected to heating II, sizing and reducing II, water quenching and tempering, and the seamless steel pipe is obtained; and setting the outer diameter of the seamless steel tube obtained by reducing the diameter II as D setting. The ultra-large-diameter thin-wall high-strength seamless steel pipe which is small in ovality, free of a concave surface in appearance and excellent in size precision and obdurability can be produced in batches, the production cost is reduced, and the production efficiency is improved.

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-wall seamless steel pipe and a preparation method and application thereof. Background Art

[0002] In recent years, with the increase in the number of medium-sized, large and super-large construction projects, construction machinery has gradually developed towards large tonnage, super-large tonnage and extremely harsh use environments, and steel pipes for construction machinery and other industries have gradually developed towards seamless, super-large diameter thin-walled and high strength and toughness. At present, high-strength super-large diameter thin-walled steel pipes are mainly produced by welding high-strength steel plates into welded pipes or hot rolling + (hot expansion) + quenching and tempering, but the reduction in strength and toughness at the weld of the welded pipe is often the source of failure of construction machinery and equipment. Therefore, when the safety factor is required to be high or the use environment is extremely harsh, such as the outer drill pipe of a large-tonnage rotary excavator, super-large diameter cylinder pipes used in weak acid environments, etc., it is necessary to use seamless steel pipes with excellent strength and toughness to ensure the safety of the equipment. High-strength ultra-large diameter thin-walled seamless steel pipes (e.g., yield strength not less than 420MPa, outer diameter ≥356mm, outer diameter / wall thickness exceeding 35) are usually not produced by hot rolling or normalizing. Products with a set outer diameter need to be obtained by hot rolling + hot expansion, and then quenched and tempered heat treatment is performed to give the steel pipe an excellent strength-toughness ratio. However, during the quenched and tempered heat treatment of ultra-large diameter thin-walled steel pipes, after the steel pipes are heated to a temperature above the critical temperature, due to the thin wall thickness of the steel pipes, the large temperature drop during high-pressure water descaling and transportation, and the low water inlet temperature during quenching, the steel pipes have low strength and poor toughness after heat treatment. In addition, the steel pipes are often affected by their own gravity, resulting in deformation or elliptical problems. When heated in a stepping furnace, they are easily bumped by rigid components such as stepping beams, resulting in defects such as concave surfaces.

[0003] In order to solve the above problems, some manufacturers currently 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 alleviate the deformation or ovalization of the steel pipe end due to the deadweight of the steel pipe, and cannot eliminate the deformation of the middle part of the steel pipe or the concave surface defects caused by the collision of rigid components. Cold deformation or hot expansion repair processes need to be added later. Therefore, the above production method greatly reduces the production efficiency and the produced steel pipes are difficult to ensure excellent strength and toughness matching and high dimensional accuracy. Summary of the invention

[0004] The main purpose of the present application is to provide an ultra-large diameter thin-walled seamless steel pipe and its preparation method and application, so as to solve the problems of deformation, elliptical change, concave surface, 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 method for preparing an ultra-large diameter thin-walled seamless steel pipe is provided, comprising the following steps:

[0006] Step S1: preparing ingredients and smelting according to the proportion of each element of the seamless steel pipe to obtain a blank;

[0007] 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 remainder is Fe and unavoidable impurities, totaling 100%; wherein, the contents of V, Al and N satisfy the relationship: 5.5≤(V+Al): N≤21.5;

[0008] Step S2: the blank is heated and perforated in sequence to obtain a rough tube;

[0009] Step S3: The rough pipe is rolled to obtain a rough pipe;

[0010] Step S4: the rough pipe is subjected to a certain diameter reduction I to obtain a hot-rolled steel pipe;

[0011] Step S5: the hot-rolled steel pipe is subjected to thermal expansion to obtain a thermally expanded steel pipe;

[0012] 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;

[0013] Step S6: The hot-expanded steel pipe is subjected to heating II, diameter reduction II, water quenching, and tempering in sequence to obtain an ultra-large diameter thin-walled seamless steel pipe; the outer diameter of the seamless steel pipe obtained by diameter reduction II is D 设定 , wall thickness is 6~20 mm.

[0014] Further, in step S5, D 热扩 and D 设定 The ratio is 1.06~1.09.

[0015] Furthermore, in step S5, the advancement speed of the tapered mandrel used for thermal expansion is 150-350 mm / min.

[0016] Furthermore, in step S4, the process of sizing 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 a hot rolled steel pipe.

[0017] Further, the number of the elliptical hole sizing racks 1 is 1 to 10.

[0018] Further, the number of the round hole type sizing racks 1 is 1 to 2.

[0019] Furthermore, 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%.

[0020] Further, in step S6, the process of reducing the diameter II includes: the hot expanded steel pipe after heating II is first reduced in diameter by an elliptical hole type sizing rack II, and then reduced in diameter by a round hole type sizing rack II. The diameter of the seamless steel pipe after the reduction is D 设定 .

[0021] Furthermore, the number of the elliptical hole sizing racks II is 1 to 4.

[0022] Furthermore, the number of the round hole type sizing racks II is 1 to 2.

[0023] Furthermore, the diameter reduction rates of the elliptical hole type sizing rack II and the round hole type sizing rack II are respectively less than 5%, and the total diameter reduction rate is less than 10%.

[0024] Further, in step S6, the temperature of heating II is 860-960°C, the insulation time = insulation coefficient x steel pipe wall thickness, the insulation coefficient is 2-3 min / mm, and the unit of steel pipe wall thickness is mm.

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

[0026] Furthermore, the tempering temperature is 650~670℃, the holding time = insulation coefficient x steel pipe wall thickness, the insulation coefficient is 3~4min / mm, and the unit of steel pipe wall thickness is mm.

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

[0028] Furthermore, in step S2, the heating process I includes: the blank 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.

[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] Furthermore, the temperature of heating zone III is 1130~1290℃, and the heating time is ≥42min.

[0033] Furthermore, the temperature of the soaking zone I is 1200-1290° C., and the heating time is ≥42 min.

[0034] Furthermore, the temperature of the soaking zone II is 1200-1290° C., and the heating time is ≥42 min.

[0035] Furthermore, the furnace temperature of the billet is 1200-1280°C.

[0036] Furthermore, the total heating time of heating I is ≥3.5 hours.

[0037] Furthermore, the heating I is carried out in a ring heating furnace.

[0038] Furthermore, in step S2, the perforation temperature is 1130-1230°C; the perforation is performed using a conical perforator.

[0039] Furthermore, in step S3, the rolling temperature is 950-1150°C; and the rolling is performed using a continuous rolling mill.

[0040] Further, in step S1, the proportions of the elements of the seamless steel pipe are as follows, in terms of weight percentage: 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%, Cr0.20-0.25%, Ni≤0.10%, Mo≤0.05%, P≤0.012%, S≤0.008%, B 0.001-0.002%, N≤0.010%, and the remainder is Fe and unavoidable impurities, totaling 100%; wherein the weight percentages of V, Al and N satisfy the relationship: 6.5≤(V+Al): N≤20.5.

[0041] Furthermore, the diameter of the ultra-large diameter thin-walled seamless steel pipe is 480~630mm.

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

[0043] Furthermore, the yield strength of the ultra-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.

[0044] According to the second aspect of the present application, an ultra-large diameter thin-walled seamless steel pipe is provided, and the ultra-large diameter thin-walled seamless steel pipe is prepared by the above-mentioned method for preparing the ultra-large diameter thin-walled seamless steel pipe.

[0045] 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 the ultra-large diameter thin-walled seamless steel pipe or the application of the above-mentioned ultra-large diameter thin-walled seamless steel pipe in fluid transportation, alumina ore transportation, natural gas transportation pipelines and engineering machinery and equipment.

[0046] By applying the technical solution of the present application, an ultra-large diameter thin-walled seamless steel pipe and a preparation method and application thereof are provided. By utilizing the large deformation of the round pipe during the sizing process and the rapid quenching and cooling after sizing, the steel pipe has the characteristics of small ovality, no concave surface, and excellent dimensional accuracy during the quenching process. During the quenching process, all parts of the steel pipe can be cooled to room temperature evenly and quickly, thereby obtaining an ultra-large diameter thin-walled high-strength seamless steel pipe with excellent quenching structure, uniform structure and performance. The production process does not need to add any original processes, and does not need to perform multiple hot expansion or cold drawing process repairs to eliminate the concave surface after hot expansion. It can mass-produce ultra-large diameter thin-walled high-strength seamless steel pipes with small ovality, no concave surface and excellent dimensional accuracy, which greatly reduces production costs and improves production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] 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:

[0048] Figure 1 A real photograph of the outer surface of the ultra-large diameter thin-walled seamless steel pipe prepared in Example 2 of the present application is shown;

[0049] Figure 2 A photograph of the outer surface of the ultra-large diameter thin-walled seamless steel pipe prepared in Comparative Example 3 of the present application is shown. DETAILED DESCRIPTION

[0050] 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.

[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 subjected to heating II, diameter reduction II, water quenching, and tempering in sequence to obtain an ultra-large diameter thin-walled seamless steel pipe; the outer diameter of the seamless steel pipe obtained by diameter reduction II is D 设定 , wall thickness is 6~20 mm.

[0061] The D designed in the thermal expansion process of step S5 above 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 a range value between any two of them; the above-mentioned super-large caliber is greater than 450mm and less than or equal to 630mm, for example, 455mm, 460mm, 470mm, 480mm, 490mm, 500mm, 510mm, 520mm, 530mm, 540mm, 550mm, 560mm, 570mm, 580mm, 590mm, 600mm, 610mm, 620mm, 630mm, or any value between 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 a range value between them.

[0062] In step S1 of the present application, the value of (V+Al): N is 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, or any range between two thereof; 5.5≤(V+Al): The purpose of N≤21.5 is not only to form specific V and Al carbonitrides well, refine the grains, improve the strength and toughness of the steel pipe, fix the N element at the same time, maximize the effect of the B element on improving the hardenability, but also to reduce the AC3 temperature of the steel grade, and avoid excessive temperature drop during high-pressure water descaling and transportation of ultra-large diameter thin-walled steel pipes, 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.

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

[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 at 1.06~1.09. According to the caliber size and the concave surface, deformation and other defects generated on the steel pipe surface during the subsequent heating process, the actual thermal expansion amount of the outer diameter is further accurately controlled to better match the subsequent fixed diameter reduction process, thereby improving the dimensional accuracy of the seamless steel pipe after fixed diameter reduction, reducing ovality, ensuring that there is no concave surface on the outer surface, and further ensuring the high strength and excellent toughness of the seamless steel pipe.

[0069] In some embodiments, the advancement speed of the conical mandrel used for thermal expansion in step S5 is 150-350 mm / min, for example, any value of 150, 180, 200, 220, 250, 280, 300, 320, 350 mm / min or any range of two values. By precisely controlling the thermal expansion speed, the thermal expansion deformation of the steel pipe is ensured to be uniform and stable, and the high strength and excellent toughness of the ultra-large diameter thin-walled seamless steel pipe are ensured.

[0070] In some embodiments, in step S4, the process of reducing the diameter I includes: the rough pipe is first reduced by an elliptical hole sizing rack I, and then reduced by a circular hole sizing rack I to obtain a hot-rolled steel pipe. Further, the number of elliptical hole sizing racks I is 1 to 10, for example, any value of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or a range value between any two; the number of circular hole sizing racks I is 1 to 2, for example, 1 or 2; and the diameter reduction rate of each elliptical hole sizing rack I and each circular hole sizing rack I is less than 5%, respectively, and the total diameter reduction rate of the two hole sizing racks is less than 25%. In the process of reducing the diameter I, an elliptical hole type first + a circular hole type and a specific diameter reduction rate are used to ensure that the rough pipe is gradually deformed during the hot rolling process to obtain a hot-rolled steel pipe with stable size, structure, and performance and uniformity; in this process, each hole structure can use a conventional hot rolling sizing rack.

[0071] In some embodiments, in step S6, the process of reducing the diameter II includes: the heat-expanded steel pipe after heating II is first reduced in diameter by an elliptical hole sizing rack II, and then reduced in diameter by a round hole sizing rack II. The diameter of the seamless steel pipe after the reduction is the target outer diameter D 设定. The number of elliptical hole sizing racks II is 1 to 4, such as 1, 2, 3, and 4; the number of circular hole sizing racks II is 1 to 2, such as 1 or 2; and the diameter reduction rate of each elliptical hole sizing rack II and each circular hole sizing rack II is less than 5%, respectively, and the total diameter reduction rate of the two hole sizing racks is less than 10%. The combined sizing rack equipment structure can be selected from the prior art. By adopting a special sizing machine hole type of first elliptical hole type + then circular hole type, the outer diameter and wall thickness of the hot expanded steel pipe before sizing reduction, the elliptical hole type and circular hole type of sizing reduction, and the diameter reduction amount and total diameter reduction amount of a single rack are reasonably set to avoid collapse caused by excessive deformation of a single rack, insufficient total deformation rate, large outer diameter ellipticity caused by incomplete hole type, and poor dimensional problems such as uneliminated outer surface concave surface, thereby obtaining an ultra-large diameter thin-walled seamless steel pipe with small ellipticity, no concave surface, and excellent dimensional accuracy, further ensuring the high strength and excellent toughness of such steel pipes.

[0072] In some embodiments, in step S6, the temperature of heating II is 860-960°C, for example, any value of 860°C, 880°C, 900°C, 920°C, 940°C, 960°C or any range between the two; insulation time = insulation coefficient x steel pipe wall thickness, insulation coefficient is 3-4min / mm, and the unit of steel pipe wall thickness is mm; that is, the insulation time is controlled according to 2-3min / mm x nominal steel pipe wall thickness mm; for example, the wall thickness of the hot-expanded steel pipe is 10mm, and the insulation time of heating II is 20-30min. The heating II process is carried out in a stepping furnace. By controlling the heating temperature and precise insulation time of the hot-expanded steel pipe, it is helpful to gradually release the residual stress generated in the steel pipe due to the hot expansion process, improve the strength and toughness of the steel pipe, reduce the possibility of cracks during quenching and cooling, and help to form a more uniform microstructure and obtain better strength and toughness.

[0073] In some embodiments, in step S6, 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°C; the tempering temperature is 650-670°C, the holding time = insulation coefficient x steel pipe wall thickness, the insulation coefficient is 3-4min / mm, and the unit of steel pipe wall thickness is mm; that is, the holding time is controlled according to 3-4min / mm x nominal steel pipe wall thickness mm; for example, the wall thickness of the hot expanded steel pipe is 10mm, and the holding time of tempering is 30-40min. By adopting rapid water quenching and tempering after the large deformation process of the round pipe fixed diameter reduction, 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 an ultra-large diameter thin-walled high-strength seamless steel pipe with excellent metallographic structure, uniform structure and performance, and ensuring that the ultra-large diameter thin-walled seamless steel pipe has stable strength and toughness through the tempering process.

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

[0075] In some embodiments, in step S6, straightening, nondestructive testing, and inspection of the ultra-large diameter thin-walled seamless steel pipe after tempering are conventional operations that can be selected from existing technologies.

[0076] In some embodiments, in step S2, the process of heating I includes: the 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 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 temperature of the billet out of the furnace is 1200~1280℃; the total heating time of heating I is ≥3.5 hours; heating I is heated in a ring heating furnace. The above heating process can be used to uniformly heat the billet, which is conducive to subsequent perforation and rolling deformation.

[0077] In some embodiments, in step S2, a conical puncher is used for punching; the punching temperature is 1130-1230°C; the billet is taken out of the heating furnace and sent to the puncher for punching through a conveying device. By controlling the punching temperature within the above range, the tube body can be punched smoothly, which is helpful for the subsequent hot rolling process.

[0078] In some embodiments, in step S3, a continuous rolling mill is used to roll the rough 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, 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.

[0079] In some embodiments, in step S1, the ratio of each element of the seamless steel pipe is as follows in terms of weight percentage: 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 remainder is Fe and unavoidable impurities, totaling 100%; wherein the weight percentage of V, Al and N satisfies the relationship: 6.5≤(V+Al): N≤20.5. Further adjustment and optimization of the element ratio is more conducive to the preparation of ultra-large diameter thin-walled, high-strength and toughness seamless steel pipes, and the formula is more matched with the preparation process.

[0080] In some embodiments, the proportions of the elements in the seamless steel pipe are as follows, in terms of weight percentage: 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%, Cr0.22-0.25%, Ni≤0.08%, Mo≤0.04%, P≤0.012%, S≤0.005%, B 0.0012-0.0015%, N≤0.008%, and the remainder is Fe and unavoidable impurities, totaling 100%; wherein the weight percentages of V, Al and N satisfy the relationship: 7.5≤(V+Al): N≤19.5. The further optimized element ratio combined with the improved process results in steel pipes with better appearance and performance, such as ultra-large diameter thin-walled seamless steel pipes with no concave surfaces, small ovality, small deformation, and high strength and toughness.

[0081] In some embodiments, the diameter of the seamless steel pipe is 480~630mm; the above-mentioned hot expansion + fixed diameter reduction process is more suitable for ultra-large diameter seamless steel pipes within this range. Within this range, the larger the diameter, the more obvious the process effect of first hot expansion of the excess and then fixed diameter reduction is compared with conventional technology, the more obvious the improvement of the surface quality of the seamless steel pipe is, and the mechanical properties are also significantly improved.

[0082] In some embodiments, the wall thickness of the seamless steel pipe is 6~15mm; the above process is more suitable for seamless steel pipes with a diameter of 480~630mm and a wall thickness of 6~15mm; seamless steel pipes of this size have a large diameter and a thin wall, and are more likely to have serious surface depressions, collapses, deformations and other problems during thermal expansion and quenching heating. The improvement effect of the above-mentioned thermal expansion allowance + fixed diameter reduction process is more obvious.

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

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

[0085] The ultra-large diameter thin-walled high-strength seamless steel pipe of the present application adopts a specific element ratio that is more suitable for ultra-large diameter thin-walled seamless steel pipes; it can 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 ultra-large diameter thin-walled high-strength seamless steel pipe brands, and has very excellent strength and toughness.

[0086] The present application utilizes the large deformation of the round tube in the process of sizing reduction II and the rapid quenching and cooling after sizing, so that the steel tube has the characteristics of small ovality, no concave surface, and excellent dimensional accuracy during the quenching process. During the quenching process, all parts of the steel tube can be cooled to room temperature evenly and quickly, thereby obtaining an ultra-large diameter thin-walled high-strength seamless steel tube with excellent metallographic structure, uniform structure and performance.

[0087] The present application adopts a specially set hole type of the sizing machine, reasonably sets the outer diameter (i.e., the actual thermal expansion outer diameter) and wall thickness of the steel pipe before sizing, as well as the elliptical hole type, circular hole type, single-frame sizing amount and total sizing amount of the sizing machine, so as to avoid the collapse caused by excessive deformation of a single frame, and the problems of large outer diameter ovality and failure to eliminate the concave surface caused by insufficient total deformation rate and incomplete hole type, thereby obtaining an ultra-large diameter thin-walled seamless steel pipe with small ovality, no concave surface and excellent dimensional accuracy.

[0088] The production process of the present application does not require any addition of any existing process steps, nor does it require multiple hot expansion or cold drawing process repairs to eliminate the concave surface after hot expansion. It can mass-produce ultra-large diameter thin-walled high-strength seamless steel pipes with small ovality, no concave surface and excellent dimensional accuracy, greatly reducing production costs and improving production efficiency.

[0089] According to the second aspect of the present application, an ultra-large diameter thin-walled seamless steel pipe is provided, and the ultra-large diameter thin-walled seamless steel pipe is prepared by the above-mentioned method for preparing the ultra-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: 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;

[0099] 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 3 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, and the round hole sizing rack are arranged in sequence; the sizing reduction rates of a single rack for each hole type are 1.52%, 3.1%, 1.55%, and 0.1%, respectively, and the total sizing reduction rate is 6.15%.

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

[0101] Step S6: Heat the hot-expanded steel pipe in a step furnace before quenching II, the temperature of heating II is 860°C, and the holding time is twice the wall thickness of the hot-expanded steel pipe, which is about 22 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 sizing II, and the sizing II rack includes 4 elliptical hole sizing racks and 1 circular hole sizing rack. 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 along the transportation direction; the sizing rates of a single rack are 0.65%, 1.70%, 1.70%, 0.70%, and 0.1% respectively, and the total sizing rate is 4.76%. The outer diameter of the seamless steel pipe obtained after sizing II is D 设定 =455mm; 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 650°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.

[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] In terms of weight percentage, C 0.32%, Si 0.45%, Mn 1.60%, V 0.08%, Nb 0.005%, Al0.05%, Cr 0.30%, Ni 0.015%, Mo 0.017%, P 0.020%, S 0.010%, B 0.0020%, N 0.0061%, and the remainder is Fe and unavoidable impurities, totaling 100%; among which, the contents of V, Al and N satisfy the relationship: (V+Al): N=21.3.

[0115] Step S2: Heat the blank in an annular furnace I, and 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 990°C, the heating time is 42 minutes, the temperature of heating zone II is 1130°C, the heating time is 52 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 blank out of the furnace is 1280°C; the total heating time is 2 hours and 40 minutes. The blank 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;

[0116] 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;

[0117] Step S4: sending 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, there are 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; the sizing rates of the single racks are 1.09%, 2.0%, 2.1%, 1.0%, 0.1%, respectively, and the total sizing rate is 6.15%.

[0118] 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 780°C and the advancement speed of the conical mandrel is 350mm / min. 热扩 693mm, D 设定 630mm, D 热扩 :D 设定 = is 1.10; wall thickness is 11mm;

[0119] Step S6: Heat the hot-expanded steel pipe in a step furnace before quenching II, the temperature of heating II is 960°C, and the holding time is 3 times the wall thickness of the hot-expanded steel pipe, which is about 33 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 1.51%, 3.15%, 3.15%, 1.50%, and 0.1% respectively, and the total reduction rate is 9.09%. The outer diameter of the seamless steel pipe obtained after reduction II is D 设定 =630mm; 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 670°C and the holding time is 4 times the wall thickness of the hot-expanded steel pipe, which is about 44 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 shown.

[0120] Example 4

[0121] The difference between Example 4 and Example 2 is that the ratio of each element of the seamless steel pipe is;

[0122] In terms of weight percentage, C 0.28%, Si 0.25%, Mn 1.20%, V 0.045%, Nb 0.005%, Al0.03%, Cr 0.20%, Ni 0.015%, Mo 0.017%, P 0.015%, S 0.008%, B 0.0012%, 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=7.5.

[0123] Example 5

[0124] The difference between Example 5 and Example 2 is that the ratio of each element of the seamless steel pipe is;

[0125] In terms of weight percentage, C 0.30%, Si 0.40%, Mn 1.40%, V 0.06%, Nb 0.005%, Al 0.035%, Cr 0.25%, Ni 0.015%, Mo 0.017%, P 0.015%, S 0.005%, B 0.0015%, 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=11.875.

[0126] Example 6

[0127] The difference between Example 6 and Example 2 is that the ratio of each element of the seamless steel pipe is:

[0128] In terms of weight percentage, C 0.18%, Si 0.15%, Mn 1.10%, V 0.02%, Nb 0.005%, Al0.015%, Cr 0.15%, Ni 0.015%, Mo 0.017%, P 0.025%, S 0.01%, B 0.008%, N 0.007%, and the balance is Fe and unavoidable impurities, totaling 100%; wherein, 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 is that the ratio of each element of the seamless steel pipe is:

[0131] In terms of weight percentage, C 0.35%, Si 0.50%, Mn 1.70%, V 0.10%, Nb 0.005%, Al0.05%, Cr 0.35%, Ni 0.015%, Mo 0.017%, P 0.02%, S 0.01%, B 0.004%, N 0.012%, and the remainder is Fe and unavoidable impurities, totaling 100%; wherein, 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 thermal expansion outer diameter D 热扩 Designed to be slightly larger than the preset target thermal expansion outer diameter, D 热扩 508.8mm, D 设定 480mm, D 热扩 :D 设定 = is 1.06; wall thickness is 11mm.

[0134] Example 9

[0135] The difference between Example 9 and Example 2 is that in step S5, the actual thermal expansion outer diameter D 热扩 Designed to be slightly larger than the preset target thermal expansion outer diameter, D 热扩 567.1mm, D 设定 530mm, D 热扩 :D 设定 = is 1.07; wall thickness is 11mm.

[0136] Example 10

[0137] The difference between Example 10 and Example 2 is that in step S5, the actual thermal expansion outer diameter D 热扩 Designed to be slightly larger than the preset target thermal expansion outer diameter, D 热扩 664.9mm, D 设定 610mm, D 热扩 :D 设定 = is 1.09; wall thickness is 11mm.

[0138] Embodiment 11

[0139] The difference between Example 11 and Example 2 is that the thermal expansion temperature in step S5 is 800°C.

[0140] Example 12

[0141] The difference between Example 12 and Example 2 is that the advancement speed of the conical core rod during the thermal expansion process in step S5 is 100 mm / min.

[0142] Embodiment 13

[0143] The difference between Example 13 and Example 2 is that the advancement speed of the conical core rod during the thermal expansion process in step S5 is 400 mm / min.

[0144] Embodiment 14

[0145] The difference between Example 14 and Example 2 is that the heating temperature II in step S6 is 800°C.

[0146] Embodiment 15

[0147] The difference between Example 15 and Example 2 is that the heating temperature II in step S6 is 1000°C.

[0148] Comparative Example 1

[0149] The difference between Comparative Example 1 and Example 2 is that D 热扩 518.16mm, D 设定 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 636mm, D 设定 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 530mm, D 设定 530mm, D 热扩： D 设定 = is 1.0, that is, when hot expansion is performed, the pipe is directly hot expanded to the set diameter, and then heated, descaled with high-pressure water, water quenched, and tempered. If the steel pipe surface is concave, it is repaired by multiple hot expansion or cold drawing processes. Figure 2 As shown, there is obvious concavity and collapse.

[0154] The outer surface quality, dimensional accuracy and performance of the ultra-large diameter thin-walled seamless steel pipes prepared in Examples 1 to 15 and Comparative Examples 1 to 3 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.

[0155] Table 1

[0156]

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

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

[0159] Although comparative examples 1 and 2 adopt the element composition and process of the present application, D 热扩 and D 设定 The ratio of is not well controlled, and the prepared seamless steel pipe still has obvious concave surface, deformation, and large ovality, indicating that within a certain range of super-large diameter, D 热扩 and D 设定 It must be controlled within a specific range to significantly eliminate the concave surface of the steel pipe, reduce deformation and ovality, and ensure excellent strength and toughness.

[0160] Comparative Example 3 did not adopt the specific hot expansion process of the present application. When defects such as concave surface, collapse, and large ovality appeared in the hot expanded steel pipe during quenching and heating, the concave surface was 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.

[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 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.

[0162] 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 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 certain 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; The working temperature of the thermal expansion is 720-780°C; the thermal expansion outer diameter of the thermal expansion steel pipe is D 热扩 , the target outer diameter is D 设定 , the D 热扩 and the D 设定 The ratio is 1.05~1.10, 450 mm<D 设定 ≤630mm; Step S6: The hot expanded steel pipe is subjected to heating II, diameter reduction II, water quenching, and tempering in sequence to obtain the ultra-large diameter thin-walled seamless steel pipe; the outer diameter of the seamless steel pipe obtained by the diameter reduction II is D 设定 , wall thickness is 6~20 mm.

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

3. The method for preparing an ultra-large diameter 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 claim 1, characterized in that: In step S6, the process of reducing the diameter II includes: the hot expanded steel pipe after heating II is first reduced in diameter by an elliptical hole sizing rack II, and then reduced in diameter by a round hole sizing rack II. The diameter of the seamless steel pipe after the reduction is D 设定 ; The number of the oval hole type sizing racks II is 1 to 4; the number of the round hole type sizing racks II is 1 to 2; The diameter reduction rates of the elliptical hole type sizing rack II and the circular hole type sizing rack II are respectively less than 5%, and the total diameter reduction rate is less than 10%.

5. 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 temperature of the heating II is 860-960°C, the insulation time = 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 performed in a stepping furnace; And / or, the tempering temperature is 650-670°C, the holding time = holding coefficient x 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 also included between the heating II and the sizing reduction II, with a pressure ≥ 20 MPa.

6. 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 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 is ≥3.5 hours; the heating I is carried out in a ring heating furnace; And / or, in step S2, the perforation temperature is 1130-1230°C; the perforation is performed using a conical perforator; And / or, in step S3, the rolling temperature is 950-1150° C.; and the rolling is performed using a continuous rolling 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, the composition of each element of the seamless steel pipe is as follows, in terms of weight percentage: 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%; among which, the weight percentages of V, Al and N satisfy the relationship: 6.5≤(V+Al): N≤20.

5.

8. 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: The diameter of the super-large diameter thin-walled seamless steel pipe is 480-630 mm; And / or, the wall thickness of the ultra-large diameter thin-walled seamless steel pipe is 6 to 15 mm; And / or, the yield strength of the ultra-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.

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

10. Use of the ultra-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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