A pass structure and rolling method for continuous rolling seamless steel pipes

Abstract

The application provides a hole type structure and a rolling method for continuously rolling seamless steel pipes, wherein the hole type structure comprises, in sequence along a rolling direction, a rough rolling hole type group, a transition hole type group and a finish rolling hole type group; the rough rolling hole type group is an elliptical hole type, and the curvature radius of the side wall of the hole type gradually increases from the hole type entrance to the hole type exit; and the finish rolling hole type group is a circular hole type, and the circular hole type is composed of at least three tangent circular arcs. The rolling hole type structure and the rolling method can effectively guide metal blank extension, reduce stress concentration, reduce pipe defects, improve the size precision and roundness of finished pipes, and can produce large-size finished steel pipes (φ225-235 mm) from small-diameter blanks (as low as 250 mm).

Description

Technical Field

[0001] This invention belongs to the field of steel pipe rolling technology, specifically relating to a die structure and rolling method for continuously rolled seamless steel pipes. Background Technology

[0002] The die design in continuous rolling mill production of seamless steel pipes directly determines the dimensional accuracy, surface quality, production stability, and economic benefits of the final product. When using continuous rolling mills to produce high-precision, large-diameter, thin-walled seamless steel pipes, traditional die designs often face a series of challenges. Especially when producing certain outer diameter specifications at the edge of the mill's design capacity, using conventional die and billet matching schemes often leads to quality defects such as uneven wall thickness, internal surface scratches, or dents due to difficulty in controlling metal flow and unreasonable deformation distribution. Simultaneously, unreasonable deformation can drastically increase the rolling load, not only impacting the rolling mill equipment and easily causing operational failures, but also accelerating the wear and tear of key tools such as rolls and mandrels, significantly increasing production costs.

[0003] Currently, the industry commonly uses a die structure designed for larger sizes to produce high-precision thin-walled gas cylinder tubes with specific outer diameters, and rolls them with corresponding large-diameter round billets. While this method operates within the allowable spacing of the equipment, it reveals significant limitations when actually producing target specifications, especially when the product wall thickness requirement is relatively thin. Because the target product wall thickness approaches or exceeds the ideal production limit of this die structure, coupled with the material's inherent high deformation resistance, the metal deformation process becomes excessively violent. This not only makes it difficult to stably control the wall thickness tolerance and roundness of the finished tube within strict ranges, increasing the risk of internal surface quality issues, but also easily causes the rolling load to exceed the equipment's safety threshold, resulting in instability in the rolling process and even production accidents. Furthermore, this method relies on large-sized billets and matching large die-cutting tools, which also means higher raw material costs and tool consumption.

[0004] Therefore, there is an urgent need for an innovative die design that can enable the stable production of high-quality large-diameter thin-walled seamless steel pipes using smaller blank sizes. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a die structure and rolling method for continuously rolled seamless steel pipes, enabling the stable production of high-quality large-diameter thin-walled seamless steel pipes.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a pass structure for continuously rolled seamless steel pipe, wherein the pass structure comprises, in sequence along the rolling direction, a roughing pass group, a transition pass group, and a finishing pass group;

[0008] All roughing mill pass groups are elliptical passes, and the radius of curvature of the sidewalls of the roughing mill pass groups gradually increases from the pass inlet to the outlet.

[0009] The finishing mill pass group is a near-circular pass, which is composed of at least three tangent circular arcs.

[0010] The die structure designed in this invention employs an elliptical die for roughing, which facilitates workpiece bite and large deformation reduction, minimizing rolling force fluctuations. A gradual curvature design is used, optimizing the sidewall curvature radius to guide the metal to extend laterally uniformly and smoothly from the initial circular billet, reducing local stress concentration at the corners of the workpiece. During the transition stage, the die gradually transitions from elliptical to circular, progressively correcting the ellipticity of the workpiece cross-section, reducing finishing rolling pressure, preventing abrupt changes in metal deformation, and preventing folding and excessive internal stress. In the finishing rolling stage, a multi-segment circular arc fitting is used to construct the die, resulting in higher contour fit and better constraint and calibration of the steel pipe's shape, significantly reducing the roundness deviation of the finished pipe and ensuring good outer diameter accuracy and inner surface finish. Through specific die design and combination, the wall thickness deviation and roundness error of the finished pipe are effectively reduced, improving the accuracy and quality of the finished pipe.

[0011] In this invention, the curvature of the sidewall of the roughing die group is the curvature of the endpoint of the major axis of the ellipse corresponding to the elliptical die.

[0012] Preferably, the ratio of the length of the major axis to the minor axis of the ellipse of the roughing die group is 1.3-1.6, for example, it can be 1.3, 1.4, 1.5 or 1.6, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0013] Preferably, the transition pass group includes at least two sub-segments along the rolling direction, and the first sub-segment to the last sub-segment changes from an elliptical pass to a circular pass.

[0014] Preferably, in the transition hole group, the cross-sectional compression ratio of any segment is 1.2-1.4, for example, it can be 1.2, 1.25, 1.3, 1.35 or 1.4, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0015] The section compression ratio refers to the ratio of the area of ​​the die section of the previous sub-segment to the area of ​​the die section of the current sub-segment along the rolling direction.

[0016] Preferably, the near-circular die of the precision rolling die group includes a first circular arc, a second circular arc, and a third circular arc that are tangent to each other in sequence.

[0017] Preferably, the radius of the first arc is 5-8mm, for example, it can be 5mm, 6mm, 7mm or 8mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] Preferably, the radius of the second arc is 10-15mm, for example, it can be 10mm, 11mm, 12mm, 13mm, 14mm or 15mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0019] Preferably, the radius of the third arc is 0.85-0.95 times the radius of the target finished tube, for example, it can be 0.85, 0.88, 0.9, 0.92 or 0.95, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] Preferably, in the hole structure, the center distance between adjacent holes is 1.5-2.0 times the diameter of the target finished pipe, for example, it can be 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] Preferably, the working band width of the hole structure is 3-8mm, for example, it can be 3mm, 4mm, 5mm, 6mm, 7mm or 8mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0022] The working zone refers to the functional section on the profile of each roll groove in a continuous rolling pass that is in direct and continuous contact with the outer surface of the workpiece and transmits rolling force. This section is a straight or slightly curved line segment with a constant curvature on the upper surface of the cross-sectional profile of the roll pass. By controlling the width of the working zone, the cross-sectional shape and dimensional accuracy of the workpiece can be controlled, and uniform metal flow can be guided.

[0023] Preferably, the sidewall opening angle of the hole structure is 3°-6°, for example, it can be 3°, 4°, 5° or 6°, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] The opening angle of the hole sidewall refers to the angle formed between the line connecting the geometric center of the hole section and the midpoint of the bottom of the hole (i.e., the longitudinal baseline of the hole) and the tangent direction of the generatrix of the working zone of the hole sidewall.

[0025] Preferably, the sidewall opening angle of the pass structure increases sequentially from the roughing pass group to the finishing pass group.

[0026] By designing and optimizing the sidewall opening angle, the cross-section of the roll pass is nearly circular during the rolling process, which can adapt to the radial constraint requirements of the metal and solve the problem of rod removal and improve the inner surface quality.

[0027] Preferably, the radius of the bottom fillet of the hole structure is 2-5mm, for example, it can be 2mm, 3mm, 4mm or 5mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0028] The bottom fillet refers to the concave arc-shaped transition structure between the sidewall of the die (or the effective contact surface of the working zone) and the bottom area of ​​the die's inner side in the die cross-section. The bottom fillet connects the stress-bearing surface of the die with the bottom bearing area. By controlling the bottom fillet, local stress concentration at the corners of the die during rolling is avoided, and the surface of the rolled piece is prevented from being scratched by sharp corners, ensuring the smoothness of metal flow.

[0029] In a second aspect, the present invention provides a rolling method for continuously rolled seamless steel pipes, wherein the rolling method uses the die structure described in the first aspect.

[0030] Preferably, the rolling method includes the following steps:

[0031] After the tube blank is heated, it is sequentially rolled through the roughing pass group, the transition pass group and the finishing pass group to obtain a seamless steel tube.

[0032] The rolling method of the present invention rapidly reduces the surface area and removes surface defects of the billet during the rough rolling process. The intermediate rolling process connects the rough rolling and finish rolling, which alleviates stress concentration after rough rolling, adjusts the cross-sectional shape of the rolled piece, and finally uses finish rolling to precisely control the dimensional accuracy and roundness of the finished product, ensuring that the stringent requirements of gas cylinder pipes are met.

[0033] Preferably, the heating temperature is 1200-1300℃, for example, it can be 1200℃, 1225℃, 1250℃, 1275℃ or 1300℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, the initial rolling temperature of the roughing roll is 1140-1180℃, for example, it can be 1140℃, 1150℃, 1160℃, 1170℃ or 1180℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, the initial rolling temperature of the finishing mill is 950-1200℃, for example, it can be 950℃, 1000℃, 1050℃, 1100℃, 1150℃ or 1200℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, the deformation amounts of the roughing, intermediate, and finishing rolling processes decrease sequentially.

[0037] Preferably, the deformation per pass of the rough rolling is 26%-30%, for example, it can be 26%, 27%, 28%, 29% or 30%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] Preferably, the deformation per pass of the intermediate rolling mill is 12%-23%, for example, it can be 12%, 14%, 16%, 18%, 20% or 23%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0039] Preferably, the deformation amount per pass of the finishing rolling is 1%-8%, for example, it can be 1%, 2%, 4%, 6% or 8%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] The deformation amount refers to the shrinkage rate of the rolled section.

[0041] Preferably, the elongation of the roughing, intermediate rolling and finishing rolling decreases sequentially.

[0042] Preferably, the single-pass elongation of the roughing roll is 1.30-1.50, for example, it can be 1.30, 1.35, 1.40, 1.45 or 1.50, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the single-pass elongation of the intermediate rolling mill is 1.10-1.35, for example, it can be 1.10, 1.15, 1.20, 1.25, 1.30 or 1.35, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0044] Preferably, the single-pass elongation of the finishing roll is 1.00-1.10, for example, it can be 1.00, 1.02, 1.05, 1.08 or 1.10, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] The elongation rate refers to the ratio of the length of the workpiece after rolling to its length before rolling.

[0046] Preferably, the rolling method uses a mandrel with limited movement.

[0047] Preferably, the difference between the diameter of the limiting mandrel and the inner diameter of the tube blank is 0.5-1.0 mm, for example, it can be 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm or 1.0 mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0048] Preferably, the taper of the limiting mandrel is 0.5%-1.5%, for example, it can be 0.5%, 0.8%, 1.0%, 1.2% or 1.5%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0049] Preferably, the chemical composition of the seamless steel pipe, in mass percentage, includes: C 0.30%-0.37%, Cr 0.90%-1.20%, Mo 0.15%-0.30%, Si 0.10%-0.40%, Mn 0.60%-0.90%, P≤0.035%, and S≤0.025%.

[0050] Preferably, the diameter of the steel billet is ≥250mm.

[0051] Preferably, the diameter of the seamless steel pipe is 225-235mm, for example, it can be 225mm, 229mm, 232mm or 235mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0052] Preferably, the wall thickness of the seamless steel pipe is 5.4-6.0 mm, for example, it can be 5.4 mm, 5.5 mm, 5.6 mm, 5.7 mm, 5.8 mm, 5.9 mm or 6.0 mm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0053] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0054] The rolling die structure and rolling method provided by this invention can effectively guide the elongation of metal billets, reduce stress concentration, reduce pipe defects, improve the dimensional accuracy and roundness of finished pipes, control the wall thickness tolerance within ±0.15mm (or not exceeding ±3% of the target wall thickness), and control the pipe roundness below 0.8mm. It can produce large-size finished steel pipes (φ225-235mm) from small diameter billets (as low as 250mm). Detailed Implementation

[0055] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0056] Example 1

[0057] This embodiment provides a pass structure for continuously rolled seamless steel pipes, wherein the pass structure includes, in sequence along the rolling direction, a roughing pass group, a transition pass group, and a finishing pass group.

[0058] The roughing mill pass group is an elliptical pass. From the pass inlet to the exit, the radius of curvature of the pass sidewall (i.e., at the endpoint of the major axis of the ellipse) gradually increases. At the pass inlet, the ratio of the length of the major axis to the minor axis of the ellipse is 1.5, and at the pass exit, the ratio is 1.3. It can be calculated that at the pass inlet, the radius of curvature of the pass sidewall is 0.33 times the length of the minor axis of the ellipse, and at the pass exit, the radius of curvature of the pass sidewall is 0.38 times the length of the minor axis of the ellipse. In the roughing mill pass group, the opening angle of the pass sidewall is 3°, the width of the pass working zone is 3mm, the radius of the bottom fillet of the pass at the pass inlet is 2mm, and the radius of the bottom fillet of the pass at the pass exit is 3mm. The bottom fillet radius increases synchronously with the change in radius of curvature.

[0059] The transition pass group includes a first segment and a second segment along the rolling direction. The first segment is an elliptical pass, and the second segment is a circular pass. The cross-sectional compression ratio of the first segment is 1.4, and that of the second segment is 1.2. The opening angle of the sidewalls of the pass in the transition pass group is 4°, the working zone width is 4mm, the bottom fillet radius of the first segment is 3mm, and that of the second segment is 4mm. The bottom fillet radius increases synchronously with the reduction in cross-sectional area.

[0060] The finishing mill pass group is a near-circular pass, which is composed of a first arc, a second arc, and a third arc that are tangent to each other in sequence. The radius of the first arc is 6 mm, and it is located close to the working zone. The radius of the second arc is 12 mm, and it is used to transition between the first arc and the third arc. The radius of the third arc is 0.9 times the radius of the target finished tube. The opening angle of the sidewall of the finishing mill pass group is 6°, the radius of the bottom fillet of the pass is 5 mm, and the bottom fillet is tangent to the third arc. The width of the working zone of the pass is 5 mm.

[0061] In the hole structure, the center distance between adjacent holes is 1.8 times the diameter of the target finished pipe.

[0062] Comparative Example 1

[0063] This comparative example provides a die structure for continuously rolled seamless steel pipes. Compared with Example 1, the roughing die group is set as an elliptical die. From the die inlet to the outlet, the radius of curvature of the die sidewall remains unchanged. The ratio of the length of the major axis to the minor axis of the elliptical die is kept at 1.5. All other aspects are the same as in Example 1.

[0064] Application Example 1

[0065] This application example provides a rolling method for continuously rolled seamless steel pipes, using the roll pass structure provided in Example 1. The chemical composition of the steel used, in mass percentage, includes: C 0.32%, Cr 0.95%, Mo 0.25%, Si 0.18%, Mn 0.65%, P 0.032%, and S 0.020%. The target finished pipe has an outer diameter of 232 mm.

[0066] The rolling method includes the following methods:

[0067] (1) The tube blank with a diameter of 250 mm is heated to 1200 °C in an annular furnace. A mandrel that moves synchronously with the rolled product is used. The diameter of the mandrel is 0.5 mm smaller than the inner diameter of the tube blank, and the taper is controlled at 0.8%.

[0068] (2) The tube blank is rough rolled in two passes using a rough rolling pass. The initial rolling temperature is 1160℃. The deformation of the rolled piece in each pass is 28.5%, and the elongation of the rolled piece is 1.430. During the rolling process, the rolled piece cools down naturally as the rolling progresses.

[0069] (3) After rough rolling, the billet is rolled in two passes using a transition pass. The deformation of the first pass is 21% and the elongation is 1.302. The deformation of the second pass is 14.4% and the elongation is 1.198.

[0070] (4) Water cooling is set after intermediate rolling and before finishing rolling to control the temperature of the rolled piece within the range of 950-1200℃. Two-pass finishing rolling is carried out using a finishing rolling pass. The deformation of the rolled piece in the first pass is 6% and the elongation is 1.078. The deformation of the rolled piece in the second pass is 1.6% and the elongation is 1.020. After rolling, the finished seamless steel pipe is obtained.

[0071] In this application example, the obtained seamless steel pipe was inspected. Its outer diameter was 232.0 ± 0.09 mm, the measured maximum outer diameter was 232.07 mm, and the minimum outer diameter was 231.98 mm; the pipe wall thickness was 5.7 mm, and the measured average wall thickness was 5.71 mm; the wall thickness tolerance was ± 0.11 mm, the maximum positive deviation was 0.10 mm, and the maximum negative deviation was 0.09 mm; the roundness of the steel pipe (the difference between the maximum and minimum outer diameters of the same cross section) was 0.62 mm; there were no obvious defects such as scratches, dents, folds, and cracks on the inner and outer surfaces, and it reached Grade I (no harmful defects) according to GB / T6402-2018 "Ultrasonic Testing and Classification of Steel Forgings" flaw detection, with a surface roughness Ra = 0.72 μm.

[0072] Application Example 2

[0073] This application example provides a rolling method for continuously rolled seamless steel pipes, using the die structure provided in Example 1. The chemical composition of the steel used, in mass percentage, includes: C 0.35%, Cr 1.15%, Mo 0.17%, Si 0.30%, Mn 0.84%, P 0.032%, and S 0.020%. The target finished pipe has an outer diameter of 229 mm.

[0074] The rolling method includes the following methods:

[0075] (1) The tube blank with a diameter of 250 mm is heated to 1300 °C in an annular furnace. A mandrel that moves synchronously with the rolled product is used. The diameter of the mandrel is 1.0 mm smaller than the inner diameter of the tube blank, and the taper is controlled at 1.4%.

[0076] (2) The tube blank is rough rolled in two passes using a rough rolling pass. The initial rolling temperature is 1160℃. The deformation of the rolled piece in each pass is 28.5%, and the elongation of the rolled piece is 1.430. During the rolling process, the rolled piece cools down naturally as the rolling progresses.

[0077] (3) After rough rolling, the billet is rolled in two passes using a transition pass. The deformation of the first pass is 21% and the elongation is 1.302. The deformation of the second pass is 14.4% and the elongation is 1.198.

[0078] (4) Water cooling is set after intermediate rolling and before finishing rolling to control the temperature of the rolled piece within the range of 950-1200℃. Two-pass finishing rolling is carried out using a finishing rolling pass. The deformation of the rolled piece in the first pass is 6% and the elongation is 1.078. The deformation of the rolled piece in the second pass is 1.6% and the elongation is 1.020. After rolling, the finished seamless steel pipe is obtained.

[0079] In this application example, the obtained seamless steel pipe was inspected. Its outer diameter was 229.0±0.10mm, the measured maximum outer diameter was 229.06mm, and the minimum outer diameter was 228.96mm; the pipe wall thickness was 5.5mm, and the measured average wall thickness was 5.52mm; the wall thickness tolerance was ±0.12mm, the maximum positive deviation was 0.11mm, and the maximum negative deviation was 0.10mm; the roundness of the steel pipe was 0.68mm; there were no oxide scale residues, local thinning, or sharp protrusions on the inner and outer surfaces. It met the Class I standard according to GB / T 9443-2007 "Penetration Testing and Classification of Cast Steel Parts", and the surface roughness Ra=0.78μm.

[0080] Comparative Application Example 1

[0081] This comparative application example provides a rolling method for continuously rolled seamless steel pipes. Compared with Application Example 1, it adopts the die structure provided in Comparative Example 1, and all other aspects are the same as Application Example 1.

[0082] In this comparative application example, the obtained seamless steel pipe was inspected. Its outer diameter was 232.0±0.23mm, the measured maximum outer diameter was 232.25mm, and the minimum outer diameter was 231.78mm; the pipe wall thickness was 5.7mm, and the measured average wall thickness was 5.73mm; the wall thickness tolerance was ±0.29mm, the maximum positive deviation was 0.27mm, and the maximum negative deviation was 0.25mm; the roundness of the steel pipe was 1.35mm; there were 3 minor scratches with a length of 2-3mm on the outer surface, and local dents (depth ≤0.15mm) on the inner surface. The flaw detection according to GB / T6402-2018 "Ultrasonic Testing and Classification of Steel Forgings" only reached Grade II (with 3 small point defects), and the surface roughness Ra=1.45μm.

[0083] In summary, this invention, through its die structure design and rolling process, can effectively improve material ductility, produce larger-sized finished steel pipes (φ225-235mm), improve the diameter accuracy of the steel pipes, control the wall thickness tolerance within ±0.15mm, and improve the roundness of the steel pipes to below 0.8mm. This can effectively improve the dimensional accuracy of the finished steel pipes and reduce defects.

[0084] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A die structure for continuously rolled seamless steel pipes, characterized in that, The pass structure includes, in sequence along the rolling direction, a roughing pass group, a transition pass group, and a finishing pass group; All roughing mill pass groups are elliptical passes. From the inlet to the outlet of the pass, the radius of curvature of the sidewall of the pass gradually increases. The ratio of the length of the major axis to the minor axis of the ellipse of the roughing mill pass group is 1.3-1.

6. The transition pass group includes at least two sub-segments along the rolling direction, and the first sub-segment to the last sub-segment changes from an elliptical pass to a circular pass; in the transition pass group, the cross-sectional compression ratio of any sub-segment is 1.2-1.4; The finishing mill pass group is a near-circular pass shape, which is composed of three tangent circular arcs. The near-circular pass shape of the finishing mill pass group includes a first circular arc, a second circular arc, and a third circular arc that are tangent to each other in sequence. The radius of the first circular arc is 5-8 mm, the radius of the second circular arc is 10-15 mm, and the radius of the third circular arc is 0.85-0.95 times the radius of the target finished tube. In the aforementioned die structure, the center distance between adjacent dies is 1.5-2.0 times the diameter of the target finished tube; the working zone width of the die structure is 3-8 mm; the sidewall opening angle of the die structure is 3°-6°; the sidewall opening angle of the die structure increases sequentially from the roughing die group to the finishing die group; and the radius of the bottom fillet of the die structure is 2-5 mm.

2. A rolling method for continuously rolled seamless steel pipes, wherein the rolling method uses the roll pass structure as described in claim 1; the rolling method includes the following steps: After the tube blank is heated, it is sequentially rolled through the roughing pass group, the transition pass group and the finishing pass group to obtain a seamless steel tube.

3. The rolling method according to claim 2, characterized in that, The heating temperature is 1200-1300℃.

4. The rolling method according to claim 2, characterized in that, The initial rolling temperature of the roughing mill is 1140-1180℃.

5. The rolling method according to claim 2, characterized in that, The initial rolling temperature of the finishing mill is 950-1200℃.

6. The rolling method according to claim 2, characterized in that, The deformation of the rolled pieces decreases sequentially from rough rolling to intermediate rolling to finish rolling.

7. The rolling method according to claim 6, characterized in that, The deformation per pass of the roughing mill is 26%-30%; The deformation per pass of the intermediate rolling mill is 12%-23%; The deformation per pass of the finishing mill is 1%-8%.

8. The rolling method according to claim 2, characterized in that, The elongation of the rolled pieces decreases sequentially from roughing to intermediate rolling to finishing.

9. The rolling method according to claim 8, characterized in that, The single-pass elongation of the roughing mill is 1.30-1.50; The single-pass elongation of the intermediate rolling mill is 1.10-1.35; The single-pass elongation of the finishing mill is 1.00-1.

10.

10. The rolling method according to claim 2, characterized in that, The rolling method employs a limited-movement mandrel; The difference between the diameter of the limiting mandrel and the inner diameter of the tube blank is 0.5-1.0 mm; The taper of the limiting mandrel is 0.5%-1.5%.

11. The rolling method according to claim 2, characterized in that, The chemical composition of the seamless steel pipe, in mass percentage, includes: C 0.30%-0.37%, Cr 0.90%-1.20%, Mo 0.15%-0.30%, Si 0.10%-0.40%, Mn 0.60%-0.90%, P≤0.035%, S≤0.025%; The diameter of the tube blank is ≥250mm; The diameter of the seamless steel pipe is 225-235mm; The wall thickness of the seamless steel pipe is 5.4-6.0 mm.