A method for manufacturing a highly homogeneous ultra-large diameter thin-walled seamless steel pipe

Abstract

A method for manufacturing a highly homogeneous ultra-large-caliber thin-walled seamless steel pipe relates to the technical field of seamless steel pipe manufacturing. It includes the following steps: S1, sequentially performing oblique rolling perforation, rolling expansion with a top head once, and rolling expansion with a top head twice to obtain a rolled rough pipe; S2, heating and heat-insulating the rolled rough pipe and then forging expansion, the forging ratio is ≥2, and the hole expansion and wall reduction are achieved by forging to obtain a forged rough pipe; S3, the forged rough pipe is heated in an online medium-frequency induction furnace, and then one or more hot expansions are performed to obtain a hot-expanded rough pipe; S4, heat-treating the hot-expanded rough pipe, then grinding it inside and outside to remove the surface oxide scale and defects, and then cold-rolling it into a finished product to obtain a finished seamless steel pipe with high finish on the inner and outer surfaces. The present invention combines a highly efficient tube-piercing rolling process and a forging expansion process, has a high degree of automation, reduces the cost yield, and effectively improves production efficiency, is conducive to large-scale and efficient production, and also has the performance of a forged steel pipe.

Description

Technical Field

[0001] The invention relates to the technical field of seamless steel pipe manufacturing, in particular to a method for manufacturing a highly homogeneous ultra-large-caliber thin-wall seamless steel pipe. Background Art

[0002] The traditional large-diameter thin-walled seamless steel pipe production process can be divided into forging and rolling. The process of the forging method is: the steel ingot is forged and blanked, and the water riser is removed before upsetting the cover plate, and a set of punches are used to punch the upset blank, and the relatively small holes punched out are pre-expanded using a horse lever and a mandrel, and the mandrel is passed into the rough pipe with the expanded hole to be drawn to the required length, and the rough pipe is expanded multiple times using a horse lever and a horse frame until it reaches the finished blank specification, and the forged outer skin is removed by machining. Due to the unevenness of the inside and outside of the forging and the severe small steps, the machining allowance accounts for a relatively large proportion. For example, for a finished pipe of φ2000*30mm, the specifications of the forged finished blank are generally: φ2040*70mm, and the machining loss material rate is: 57.1%. It can be seen that the forging process of large-size thin-walled tubes has many fires (generally at least 7 fires), low degree of automation, extremely low yield rate, extremely low production efficiency, and extremely high cost, which is not conducive to large-scale and efficient production.

[0003] Conventional piercing and rolling production efficiency is high, but the seamless pipe produced by the rolling method is greatly restricted by the outer diameter. This is mainly because the hole expansion process is mainly rolling hole expansion. The length of the billet is greatly extended during the rolling deformation process, resulting in a smaller outer diameter extension (during forging hole expansion, the length of the pipe remains basically unchanged, but the outer diameter expansion is large). In addition, the pipe produced by the rolling method is in a rolled state. Compared with the forged steel pipe, the overall performance is poor and cannot meet the requirements of many fields. Summary of the invention

[0004] The purpose of the present invention is to provide a method for manufacturing a highly homogeneous ultra-large diameter thin-wall seamless steel pipe, which can effectively solve the problems in the background technology.

[0005] The technical solution to achieve the above purpose is:

[0006] A method for manufacturing a highly homogeneous ultra-large diameter thin-wall seamless steel pipe, characterized in that it comprises the following steps:

[0007] S1, heating and keeping a cylindrical continuous casting billet or a cylindrical continuous forging tube billet, and then performing oblique rolling piercing, primary rolling with a mandrel for hole expansion, and secondary rolling with a mandrel for hole expansion to obtain a rolled rough tube;

[0008] S2, heating and keeping the rolled rough pipe, and then forging it, with a forging ratio of ≥2, and achieving hole expansion and wall reduction through forging to obtain a forged rough pipe;

[0009] S3, heating the forged rough pipe in an online medium frequency induction furnace, and then performing one or more heat expansion steps to obtain a heat expanded rough pipe;

[0010] S4. Heat-treat the hot-expanded pipe, grind it inside and outside to remove the surface oxide scale and defects, and then cold-roll it into a finished product to obtain a finished seamless steel pipe with high internal and external surface finish.

[0011] Furthermore, in step S1, the continuous casting billet or forged tube billet is heated to a temperature of 1100° C. to 1300° C., and the holding time is 0.4 to 1.2 min / mm according to the outer diameter of the continuous casting billet or forged tube billet.

[0012] Furthermore, in step S2, the rolled rough pipe is heated to 1100° C. to 1300° C., and the holding time is 1 to 3 min / mm according to the wall thickness of the rolled rough pipe.

[0013] Furthermore, in step S3, the forged rough pipe is heated to a temperature of Ac3+ (50°C-300°C).

[0014] The beneficial effects of the present invention are as follows: compared with traditional forging, the present invention has a simple production process and high production efficiency, and the three processes of oblique rolling piercing, rolling and leveling can be completed by heating once to obtain a rolled rough tube; while traditional forging requires processes such as blanking, upsetting, punching, mandrel drawing, and horse bar expansion, and requires the use and replacement of a variety of tooling (forging anvils, upsetting cover plates, a group of punches, mandrels, horse bars, horse frames, etc.) and multiple firing times to obtain the above-mentioned rough tubes of the same specifications.

[0015] The present invention preheats the horse bar during forging and expanding the hole, and rotates it at a small angle to ensure that all surfaces can be forged. The hole is expanded by using an expanding anvil, and the forging ratio is ≥2 during expansion. During forging, the three-dimensional compressive stress can penetrate the center of the wall thickness, so that the internal structure of the entire cross section is fully broken, so that the outer diameter is further expanded and the wall thickness is further reduced. The performance of the forged rough seamless steel pipe is better than that of the rolled steel pipe.

[0016] The present invention adopts medium frequency induction heating, and then utilizes lubricant and the smooth working surface of the inner mold to thermally expand the steel pipe, so as to eliminate the uneven surfaces inside and outside the forged rough pipe, and obtain a rough pipe with a further enlarged outer diameter, a further reduced wall thickness and a relatively smooth appearance.

[0017] The present invention performs heat treatment and internal and external grinding on the hot expanded steel pipe to remove surface oxide scale and defects, and then sends it to a cold rolling mill for cold rolling to produce a finished product. The cold rolling single-side wall thickness reduction is ≥2mm, ensuring that the inner and outer surfaces reach the smoothness of the cold rolled state. Compared with the traditional forged seamless steel pipe, which finally uses a machining process of internal boring and external grinding, the yield rate of large-diameter thin-walled seamless steel pipe is increased by more than 50%.

[0018] Compared with traditional forged pipes, the seamless steel pipe produced by the present invention has fewer processing times, a narrow overall deformation temperature range during each processing time, and relatively uniform deformation, that is, a high degree of product homogenization, so the overall uniformity of the steel pipe's organizational properties is better.

[0019] The present invention combines a highly efficient tube-piercing rolling process with a forging hole-expanding process, has a high degree of automation, reduces the cost, effectively improves the yield rate, and is conducive to large-scale and efficient production, while also having the performance of a forged steel pipe. DETAILED DESCRIPTION

[0020] The following is a further description of a method for manufacturing a highly homogeneous ultra-large diameter thin-walled seamless steel pipe of the present invention in conjunction with an embodiment.

[0021] This embodiment takes the production of high-homogeneity ultra-large-diameter thin-walled seamless steel pipes with a grade of P91 and a finished product specification of φ3845*25mm (outer diameter*wall thickness) as an example, and specifically includes the following steps:

[0022] S1, oblique rolling and piercing: the raw material is φ1000mm P91 continuous casting billet, which is heated to 1200℃ and kept warm for 10h. After being taken out of the furnace, it is successively obliquely rolled and pierced to φ1000*130mm (outer diameter*wall thickness), rolled to φ1150*105mm (outer diameter*wall thickness) with a top head, and rolled to φ1350*80mm (outer diameter*wall thickness) with a top head for a second time to obtain a rolled rough pipe;

[0023] S2. Forging and expanding the hole: The rolled rough pipe is returned to the furnace for reheating to 1200℃ and kept warm for 3h. At the same time, the horse frame and the upper flat anvil are preheated to 250℃, and the hole is forged and expanded on the horse frame. Specifically, the horse bar is inserted into the inner hole of the pipe fitting and placed on the horse frame. The length direction of the upper flat anvil is consistent with the longitudinal direction of the steel pipe. The horse bar is rotated while forging to drive the steel pipe to rotate. The two adjacent forging presses have overlapping areas to ensure that the entire surface is fully forged. The forging specifications are: φ3580*30mm (outer diameter*wall thickness), and the forged rough pipe is obtained;

[0024] S3, thermal expansion: The forged rough pipe is heated to 1050℃ by online medium frequency induction, and the forged rough pipe is thermally expanded and reduced in diameter by using the inner die of the hole expander, and the hole is expanded to φ3850*28mm (length*wall thickness) to obtain a thermally expanded rough pipe;

[0025] S4. Cold rolling: The hot expanded pipe is subjected to normalizing heat treatment, and then internal and external grinding is performed to remove surface oxide scale and defects, and then the finished product is cold rolled to φ3845*25mm (outer diameter*wall thickness) to obtain a finished seamless steel pipe with high internal and external surface finish.

[0026] The manufacturing method of the high-homogeneity ultra-large-diameter thin-wall seamless steel pipe of the present invention realizes the industrialized production of the high-homogeneity ultra-large-diameter thin-wall seamless steel pipe. On the basis of ensuring that the steel pipe has forging performance, it greatly reduces and improves production efficiency, reduces production costs, saves material utilization, and has certain economic benefits.

Claims

1. A method for manufacturing a highly homogeneous ultra-large diameter thin-walled seamless steel pipe, characterized in that: The steps include: S1, heating and keeping a cylindrical continuous casting billet or a cylindrical continuous forging tube billet, and then performing oblique rolling piercing, primary rolling with a mandrel for hole expansion, and secondary rolling with a mandrel for hole expansion to obtain a rolled rough tube; S2. The rolled rough tube is heated and kept warm before forging, and the forging ratio is ≥2. The hole is expanded and the wall is reduced by forging to obtain a forged rough tube. The horse bar is preheated during forging and expanded, and is rotated at a small angle to ensure that the surface can be forged. The hole is expanded using an anvil, and the forging ratio is ≥2 during expansion. During forging, the three-dimensional compressive stress can penetrate the center of the wall thickness, so that the internal structure of the entire cross section is fully broken; S3, heating the forged rough pipe in an online medium frequency induction furnace, and then performing one or more heat expansion steps to obtain a heat expanded rough pipe; S4. Heat-treat the hot-expanded pipe, grind it inside and outside to remove the surface oxide scale and defects, and then cold-roll it into a finished product to obtain a finished seamless steel pipe with high internal and external surface finish. The cold-rolled single-side wall thickness reduction is ≥2mm; In step S1, the continuous casting billet or forged tube billet is heated to a temperature of 1100° C. to 1300° C., and the holding time is 0.4 to 1.2 min / mm according to the outer diameter of the continuous casting billet or forged tube billet; In step S2, the rolled rough pipe is heated to 1100° C. to 1300° C., and the holding time is 1 to 3 min / mm according to the wall thickness of the rolled rough pipe; In step S3, the forged rough pipe is heated to a temperature of Ac3+50°C to Ac3+300°C.

2. The method for manufacturing a highly homogeneous ultra-large diameter thin-walled seamless steel pipe according to claim 1, characterized in that: The super-large diameter thin-wall seamless steel pipe is a super-large diameter thin-wall seamless steel pipe with an outer diameter ≥φ1400mm.