A method for manufacturing a large-diameter seamless steel pipe

Abstract

The present application is suitable for the field of seamless steel pipe manufacturing, and provides a preparation method of large-diameter seamless steel pipe, comprising the following steps: step 1, preparing a blank; step 2, heating by an annular furnace; step 3, piercing rolling; step 4, continuous rolling; step 5, determining an outer diameter; and step 6, air cooling. The present application uses a small-section blank instead of a large-section blank, and redesigns a rack and a top head, which has great significance for achieving the company's production capacity target, reducing cost and improving competitiveness.

Description

Technical Field

[0001] This invention belongs to the field of seamless steel pipe manufacturing, and particularly relates to a method for preparing large-diameter seamless steel pipes. Background Technology

[0002] In the existing technology, the only difference between producing 508mm diameter seamless steel pipes and 457mm diameter seamless steel pipes is the use of a sizing stand. When producing 457mm diameter seamless steel pipes, a billet with an end face of 500mm is used, the heating cycle is as fast as 130 seconds, the hourly production capacity is low, and the head and tail cutting rate of 457mm diameter seamless steel pipes is 0.5% to 1% higher than that of 508mm diameter seamless steel pipes.

[0003] Therefore, in order to increase hourly production capacity, reduce the cost of grinding the unfinished pipes, reduce the head and tail cutting rate, and increase the yield, it is urgent to develop a method for producing large-diameter seamless steel pipes with a 457mm specification using a 481mm die, in order to overcome the shortcomings in current practical applications. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing large-diameter seamless steel pipes, aiming to solve the problems mentioned in the background art.

[0005] The present invention is implemented as follows: a method for preparing a large-diameter seamless steel pipe includes the following steps:

[0006] Step 1: Prepare the billet

[0007] The double-length and triple-length tube blanks provided by the steelmaking continuous casting workshop are cut into single-length tube blanks with a cross-sectional diameter of 450mm.

[0008] Step 2: Heating the ring furnace

[0009] The single-length tube blank is placed in the ring furnace for heating, and the heated tube blank is transferred to the platform.

[0010] Step 3, piercing rolling

[0011] The tube blank is processed into a hollow tube by the piercing roller, guide plate and mandrel in the piercing mill. The outer diameter of the tube is 538mm and the wall thickness is 25mm. A three-section mandrel is used for processing. The mandrel diameter is 438mm and the length is 1182mm. The exit angle of the guide plate is increased by 1° and the working length and width of the guide plate are increased.

[0012] Step 4: Continuous rolling

[0013] The perforated tube is fitted onto the mandrel and rolled into a rough tube on a continuous rolling mill with five stands arranged in sequence and the roll gaps of adjacent stands intersecting by 120°. The rough tube has an outer diameter of 481 mm and a wall thickness of 9 mm. Then, a three-stand tube stripping machine is used to separate the rough tube from the mandrel after continuous rolling and send the rough tube into the sizing area via a roller conveyor.

[0014] Step 5: Determine the outer diameter

[0015] The rough pipe is continuously rolled and sized on 1 to 12 sizing mills to obtain finished steel pipes with an outer diameter of 471 mm and a wall thickness of 9 mm.

[0016] Step 6: Air cooling

[0017] After sizing, the finished steel pipes are transported by roller conveyor to a large cooling bed for air cooling.

[0018] In a further technical solution, in step 2, a ring furnace is used to heat the tube blank to 1240-1290°C. The heated tube blank is clamped out by the discharge machine and placed on the discharge platform. It is then sent to the chain conveyor via a lever device. The chain conveyor sends the tube blank to a 5m high platform.

[0019] In a further technical solution, in step 4, each stand of the continuous rolling mill is composed of 3 rolls forming the roll profile, which consists of the bottom arc, the release arc, the connecting arc, and the roll gap, with each arc segment being tangent to the others.

[0020] This invention provides a method for manufacturing large-diameter seamless steel pipes. By using smaller cross-section billets instead of larger ones and redesigning the frame and mandrel, the method can improve the output rhythm of the annular furnace, increase hourly steel production capacity, and reduce the cost per ton of steel consumed by natural gas. The reduced outer diameter of the billet also positively impacts the uniformity of heating the billet cross-section. Furthermore, the smaller billet cross-section means an increase in pipe length; cutting the same length of billet into fixed-length pipes requires 1-2 fewer cuts, which also reduces saw blade costs and saves metal loss from the saw blade. Overall, this method is of great significance for achieving the company's production capacity targets, reducing costs, and improving competitiveness. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of piercing mill rolling provided in an embodiment of the present invention.

[0022] Figure 2 A schematic diagram of the structure of the newly designed top head provided for an embodiment of the present invention.

[0023] Figure 3 This is a schematic diagram of the diameter reduction rate and diameter-to-wall ratio provided in an embodiment of the present invention.

[0024] Figure 4This is a schematic diagram of CH and wall thickness provided for an embodiment of the present invention.

[0025] Figure 5 This is a trend chart of capillary wall thickness measurement provided in an embodiment of the present invention.

[0026] Figure 6 This is a trend chart of capillary outer diameter measurement provided in an embodiment of the present invention.

[0027] Figure 7 This is a schematic diagram of the parameters of each arc segment in continuous rolling provided in an embodiment of the present invention.

[0028] Figure 8 A diagram showing the elongation coefficient of a five-stand continuous rolling mill provided for an embodiment of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0030] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0031] The basic design concept of the Φ481mm die is as follows: The 5-stand three-roll limited-motion mandrel continuous rolling mill of the Φ508mm PQF tube mill was introduced from the German company SMS Meer. The maximum die size is designed to be Φ530mm, and the maximum outer diameter of the product that can be produced is 508mm. The 530mm die produces a continuous rolling outer diameter of 530mm. By reducing the wall thickness and extending the tube through the tube stripper and sizing mill, a 457mm specification can be produced. However, the production of 457mm tubes still uses 500mm end face billets, which limits the production pace. The tube head and tail bends exceed the standard, the head and tail cutting rate is high, and the yield is low. Therefore, the Φ481mm die was designed based on the original mandrel.

[0032] The PQF mill pass design is generally based on the minimum nominal wall thickness rolled by that pass. A Ф481mm×9mm specification is selected for design. Considering the finished product outer diameter requirements and existing mandrel specifications, a tube outer diameter of approximately 540mm is chosen. Based on the billet supply, a Φ450mm continuously cast billet is selected as the raw material. The product specification change process of the mill is as follows: Ф450mm (cast billet) → Ф538mm×26.1mm (tube) → Ф481mm×9mm (continuously rolled tube) → Ф471mm×9mm (finished tube). The total expansion rate of the pierced tube is 20%, and the total reduction in diameter during continuous rolling is 57mm.

[0033] like Figure 1-8 As shown, a method for preparing a large-diameter seamless steel pipe according to an embodiment of the present invention includes the following steps:

[0034] Step 1: Prepare the billet

[0035] The double-length and triple-length tube blanks provided by the steelmaking continuous casting workshop are cut into single-length tube blanks with a cross-sectional diameter of 450mm.

[0036] In this step, continuously cast billets are stored and promptly supplied to the hot rolling line to ensure normal production turnover. The billets are inspected to supply high-quality billets for the skew rolling piercing deformation process; this is a prerequisite for producing high-quality steel pipes and a crucial condition for ensuring the normal operation of the production process, especially the piercing process. The double-length and triple-length billets provided by the steelmaking continuous casting workshop are cut into single-length billets to provide precise length billets for hot rolling. Double-length and triple-length are woodworking units of measurement; a double-length billet is twice the length of a single billet, and a triple-length billet is three times the length of a single billet.

[0037] Step 2: Heating the ring furnace

[0038] The single-length tube blank is placed in the ring furnace for heating, and the heated tube blank is transferred to the platform.

[0039] In this step, the tube blank is heated to 1240-1290℃ using a ring furnace. The heated tube blank is then clamped out by the discharge machine and placed on the discharge platform. It is then conveyed to the chain conveyor via a lever device. The chain conveyor then delivers the tube blank to a 5m high platform.

[0040] Step 3, piercing rolling

[0041] like Figure 1 As shown, the tube blank is processed by the piercing roller, guide plate and mandrel in the piercing mill to form a hollow tube with an outer diameter of 538mm and a wall thickness of 25mm.

[0042] In this step, the mandrel is redesigned with a diameter of 438mm and a length of 1182mm; a newly designed guide plate is used, with the guide plate exit angle increased by 1°, and the working length and width of the guide plate are lengthened, and a three-section mandrel is selected for processing.

[0043] The piercing mill of the Ф508mm PQF continuous rolling mill uses a closed rolling pass consisting of a guide plate, a conical piercing roll, and a mandrel. After analysis, it was decided not to modify the piercing roll, and the existing Ф450mm guide plate (with a 1° increase in exit angle), along with its extended working length and width, could meet the diameter expansion requirements. Based on experience, after adjusting the piercing ellipticity parameters, no additional guide plate was added. Theoretically, a two-stage mandrel would be chosen based on the selected tube size; however, during production on the 508mm mill, a three-stage mandrel proved more advantageous. Comparing the two-stage and three-stage mandrels, the three-stage mandrel reduces piercing load, minimizes iron filings at the tube tail, and produces a better tube tail bevel. Therefore, a three-stage mandrel design was chosen for production. By calculating the length of the perforation deformation zone and the top reduction rate, and taking into account the gradual change in the gap between the cap and the mandrel as the cap tube specifications range from thin-walled to thick-walled, with the longest fixed length of 450mm end face, the size of the mandrel in the perforation area and the corresponding cap tube size are considered.

[0044] The 481mm bore was designed and developed to address the yield problem of 457mm diameter tubes, especially for ultra-thin-walled tubes. For example, a Ф457mm*9mm tube requires a Ф538mm×26.1mm diameter tube. However, existing mandrels cannot produce Ф538mm×26.1mm tubes. Therefore, the mandrel needed to be redesigned. Based on the mandrel design principles in CARTA2006 provided by Meer, combined with practical production experience, and using AutoCAD software to simulate the deformation zone of the perforation, the lengths, angles, and radius of curvature of each section of the mandrel were determined. A Ф438mm mandrel was successfully designed. Figure 2 As shown.

[0045] like Figure 3 and 4 As shown, the detailed calculation process for the Ф438mm mandrel is as follows:

[0046] Given: billet diameter DB = 450 mm, outer diameter of capillary tube DH = 538 mm, capillary tube wall thickness SH = 25 mm, inlet cone angle α1 = 2°, inlet cone angle α2 = 4°, outlet cone angle β = 4°, bite angle γ = 9.5°.

[0047] Based on the production experience of 454-hole thin-walled tubes, the mandrel thermal expansion (internal expansion) CH is taken. CTP =50mm; Roller gap E = 388.8mm, take E = 388mm (select diameter reduction rate of 86.4% of DB, see appendix) Figure 3 ).

[0048] DD = DH - 2 × SH - CH CTP DD = 438mm;

[0049] The rolling cone coefficient SF of the mandrel is taken as 1.7;

[0050] LGT2=SF×π×DH×tan(γ) / 2;

[0051] LGT2 = 240.29mm, rounded to 240mm;

[0052] The mandrel rolling cone angle betaGT2 = roll exit cone angle β + compensation angle (0.3°~0.6°). Based on the production experience of other roll types in the 508mm unit, betaGT2 = 4.45° is taken.

[0053] DA=DD-2×LGT2×tan(betaGT2);

[0054] DA = 400.64 mm;

[0055] Based on relevant experience with 454mm and 530mm hole-type mandrels.

[0056] LA = π × DH × tan(γ);

[0057] LA = 282.69 mm, rounded to 280 mm; betaA is taken as 6.5°;

[0058] DR = DA - 2 × LA × tanbetaA;

[0059] DR = 336.84 mm;

[0060] The number of rotations of the billet between the first and second bites is taken as GF, which is 1.15.

[0061] GL = π × DB × tan(γ) × GF

[0062] GL = 271.92mm, rounded to 272mm;

[0063] LD1 = Le - GL;

[0064]

[0065] Le = 487.25 mm, rounded to 487 mm;

[0066] LD1 = 487 - 272 = 215 mm;

[0067]

[0068] LD2 = 715.03mm, rounded to 715mm;

[0069] LD=LD1+LD2=215+715=930mm;

[0070] The top nose dimension ФF = (0.18-0.2) × DD, but according to the company's production experience ФF = 0.15 × DD = 65.7mm, so ФF is rounded to 65mm;

[0071] LR=LD-LGT2-LALR=309mm;

[0072]

[0073] RD = 947.34 mm.

[0074] Trial rolling was conducted using a newly designed Ф438mm mandrel. The outer diameter and wall thickness of the tube were measured and sampled every 200mm, and the dimensions fully met the requirements. Figure 5 and Figure 6 This is a trend graph of the measured values ​​for wall thickness and outer diameter.

[0075] The calculation parameters for 481mm piercing rolling are shown in Table 1.

[0076]

[0077] Step 4: Continuous rolling

[0078] After being perforated, the tube is fitted onto the mandrel and rolled into a rough tube on a continuous rolling mill with five stands arranged in sequence and adjacent stands having roll gaps staggered by 120°. The rough tube has an outer diameter of 481 mm and a wall thickness of 9 mm. Then, a three-stand tube stripping machine is used to separate the rough tube from the mandrel after continuous rolling and the rough tube is sent to the sizing area via a roller conveyor.

[0079] In this step, the rolled tube has a thinner wall, a longer length, and improved wall thickness accuracy and surface finish.

[0080] In the PQF continuous rolling mill, each stand consists of three rolls forming the roll pass. The roll pass profile is composed of a groove bottom arc, a release arc, a connecting arc, and a roll gap, with each arc segment being tangent to ensure a smooth transition. Figure 7 As shown, the structural parameters of each arc segment are: the radius, cross-sectional angle, and horizontal and vertical coordinates of the center of each arc segment, as well as the height, width, eccentricity, and roll gap of the pass. In the figure: R1 - radius of the bottom arc; R2 - radius of the departure arc; R5 - radius of the connecting arc; C1, C2, C5 - centers of R1, R2, and R5; b - connection point between the bottom arc and the departure arc; t - connection point between the departure arc and the connecting arc; s - connection point between the connecting arc and the edge of the roll. The core process technology of the PQF unit and the FQM high-quality tube rolling mill unit—the composition and design of the pass of the continuous tube rolling mill—is completely consistent. The pass structure of the four three-roll continuous tube rolling mill units, namely PQF, FQM, TCM (three-roll limited-movement mandrel continuous tube rolling mill) and CCTM (China Metallurgical Saidi continuous tube rolling mill), is completely consistent, and the same method can be used to design the pass.

[0081] The PQF pass of the 508mm mill is generally calculated based on the thinnest wall thickness rolled using that pass. Typically, the first two stands use a larger deformation amount, while the last two stands use a smaller deformation amount, allowing the rough tube to be rounded at the exit and smoothly removed from the bar. Calculations show that the continuous rolling mill is designed with a minimum dimension of Φ481mm × 9mm.

[0082] like Figure 8 As shown, the total elongation coefficient μz is calculated, μz = AO / A5 (AO is the cross-sectional area of ​​the capillary tube; A5 is the cross-sectional area of ​​the rough tube exiting the continuous rolling mill).

[0083] Based on the deformation characteristics of the unit, the PQF continuous rolling mill's three-roll pass design reduces the difference in circumferential speed between the bottom of the pass and the sidewall of the rolling groove. The continuous rolling mill uses five stands; the basic parameters of each stand were calculated and are shown in Table 2.

[0084] 1 1.396 35 3 1.008 -7.2 2 1.384 35 3 1.03 -4.8 3 1.269 35 3 1.018 0 4 1.148 24 2.5 1.014 0 5 1.036 24 2.5 1.018 0

[0085] The key parameters for continuous rolling pass design are calculated, and the data in the table are entered into the digitization table. After verification by the digitization program, the rolling table is printed out, and the mandrel specifications are determined according to different wall thicknesses.

[0086] Step 5: Determine the outer diameter

[0087] The rough pipe is continuously rolled and sized on 1 to 12 sizing mills to obtain finished steel pipes with an outer diameter of 471 mm and a wall thickness of 9 mm.

[0088] In this step, the sizing process involves continuously rolling the steel pipe (with or without reheating) after it has been removed from the pipe stripper on 1 to 12 sizing mills. The purpose of sizing is to achieve the desired sizing effect under certain total reduction rate and relatively small reduction rate per stand. Furthermore, the sizing process can also enable the production of multiple specifications of finished pipes from a single specification of pipe.

[0089] Step 6: Air cooling

[0090] After sizing, the finished steel pipes are transported by roller conveyor to a large cooling bed for air cooling.

[0091] The above embodiments of the present invention provide a method for preparing large-diameter seamless steel pipes. This method involves research on the production die type for seamless steel pipes and improvements to product structure and quality based on production characteristics. This approach uses smaller cross-section billets instead of larger cross-section billets (450mm instead of 500mm), and redesigns the frame and mandrel. This is of great significance for achieving the company's production capacity targets, reducing costs, and improving competitiveness. Specifically:

[0092] 1) It can improve the discharge rhythm of the ring furnace and increase the hourly steel production capacity per ton;

[0093] 2) As the discharge rate increases, the cost per ton of steel consumed by natural gas will also decrease;

[0094] 3) Reducing the outer diameter of the billet will also have a positive effect on the uniformity of heating of the billet cross section;

[0095] 4) The reduction in the cross-section of the tube blank means the increase in the length of the tube. When the same length of tube blank is cut into a fixed length blank, the number of cuts is reduced by 1-2, which also has a positive effect on reducing the cost of saw blades and saving metal loss on the cutting edge.

[0096] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0097] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a large-diameter seamless steel pipe, characterized in that, Based on the Ф508mm PQF continuous rolling mill, smaller cross-section billets were used instead of larger cross-section billets. The mill stand and mandrel were redesigned, and a Ф481mm roll pass was designed and developed, including the following steps: Step 1: Prepare the billet The double-length and triple-length tube blanks provided by the steelmaking continuous casting workshop are cut into single-length tube blanks with a cross-sectional diameter of 450mm. Step 2: Heating the ring furnace The single-length tube blank is placed in the ring furnace for heating, and the heated tube blank is transferred to the platform. Step 3, piercing rolling The tube blank is processed into a hollow tube by the piercing roller, guide plate and mandrel in the piercing mill. The outer diameter of the tube is 538mm and the wall thickness is 25mm. A three-section mandrel is used for processing. The mandrel diameter is 438mm and the length is 1182mm. The inlet angle of the guide plate remains unchanged at 3°, the outlet angle is increased to 7°, and the working surface length of the guide plate is increased from 515mm to 640mm to ensure the stability of the tube tail. Step 4: Continuous rolling The perforated tube is fitted onto the mandrel and rolled into a rough tube on a continuous rolling mill with five stands arranged in sequence and the roll gaps of adjacent stands intersecting by 120°. The rough tube has an outer diameter of 481 mm and a wall thickness of 9 mm. Then, a three-stand tube stripping machine is used to separate the rough tube from the mandrel after continuous rolling and send the rough tube into the sizing area via a roller conveyor. Step 5: Determine the outer diameter The rough pipe is continuously rolled and sized on 1 to 12 sizing mills to obtain finished steel pipes with an outer diameter of 471 mm and a wall thickness of 9 mm. Step 6: Air cooling After sizing, the finished steel pipes are transported by roller conveyor to a large cooling bed for air cooling. In step 2, the tube blank is heated to 1240-1290°C using a ring furnace; The heated tube blank is clamped out by the discharge machine and placed on the discharge platform. It is then sent to the chain conveyor via a lever device. The chain conveyor sends the tube blank to a 5m high platform. In step 4, each stand of the continuous rolling mill consists of three rolls forming the roll pass. The roll pass profile is composed of the bottom arc, the release arc, the connecting arc, and the roll gap, with each arc segment being tangent to the others.

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