Summary of seamless steel pipe production methods

(1) Automatic tube rolling production

One of the ways to produce seamless steel pipes. The production equipment consists of a piercing machine, an automatic tube rolling machine, a leveling machine, a sizing machine, and a reducing machine. The production process is shown in the figure.

(2) Continuous tube rolling production

The production equipment consists of a piercing machine, a continuous tube rolling machine, and a tension-reducing machine. After the round billet is punched into a rough tube, a mandrel is inserted and continuously rolled by a two-roller mill with 7 to 9 roller axes arranged at 90 degrees to each other. After rolling, the mandrel is pulled out and tension-reduced after reheating. It can be rolled into a steel pipe up to 165m long. The 140mm continuous tube rolling unit has an annual output of 400,000 to 600,000 tons, which is 2 to 4 times that of the automatic tube rolling unit. The characteristics of this unit are that it is suitable for producing steel pipes with an outer diameter of less than 168mm, with large equipment investment, large installed capacity, a mandrel of up to 30m, and complex processing and manufacturing. The limited mandrel continuous tube mill (MPM) that appeared in the late 1970s forced the mandrel to move at a speed lower than the speed of the steel pipe during rolling, which improved the metal flow conditions and allowed the use of short mandrels to roll long tubes and large-diameter steel pipes.

(3) Cyclic tube rolling production

Polygonal and round steel ingots or continuous-casting billets are used as raw materials. After heating, they are perforated by water pressure into cup-shaped blanks, and then rolled into rough tubes by a two-roller inclined rolling and stretching machine. Then, on a cyclic tube mill with variable diameter holes and grooves, the rollers rotate one circle to roll out a section of steel pipe. Cyclic tube mills are also called Pilger tube mills. Cyclic tube rolling production uses steel ingots as raw materials and is suitable for rolling large-diameter thick-walled steel pipes and variable-section pipes.

(4) Three-roller tube rolling production

It is mainly used to produce thick-walled pipes with high dimensional accuracy. The wall thickness accuracy of the pipes produced by this method reaches ±5%, which is about twice as high as the accuracy of pipes produced by other methods. In the 1960s, this method was rapidly developed due to the invention of a new three-roller oblique rolling mill (called the Transval rolling mill). The new rolling mill is characterized by rapidly rotating the entrance rotary frame to change the rolling angle when rolling to the tail, thereby preventing the tail from forming a triangle, and expanding the ratio of the outer diameter to the wall thickness of the product from 12 to 35. It can not only produce thin-walled tubes but also improve production capacity.

(5) Pipe top production

The traditional method is to extend the square billet into a cup-shaped rough tube through hydraulic perforation and oblique rolling, insert the long mandrel into the bottom of the rough tube cup by a push rod, and sequentially pass through a series of roller molds with gradually decreasing holes and slots to roll into a pipe. This production method requires less equipment investment and can use continuous casting billets. It can produce extra-large and extra-thick tubes with a diameter of up to 1070mm and a wall thickness of up to 200mm, but the production efficiency is low, the wall thickness is relatively thick, and the tube length is relatively short. After the emergence of the new process of CPE, the tube blank is perforated by oblique rolling to form a rough tube, and then the tube is extended by top rolling after closing. This overcomes some shortcomings of the traditional method and has become a method with better economic benefits in the production of seamless tubes.

(6) Extruded tube production

First, the peeled round billet is perforated or expanded, then heated by induction heating or salt bath heating, and lubricant is applied to the inner surface and fed into the extruder. The metal is extruded into a tube through the annular gap between the die hole and the core rod. It is mainly used to produce low-plasticity high-temperature alloy tubes, special-shaped tubes and composite tubes, non-ferrous metal tubes, etc. This method has a wide production range but low output. In recent years, due to improvements in mold materials, lubricants, extrusion speeds, etc., extruded tube production has also developed.

(7) Guide plate rolling tube production

It is called the Diesel method. After perforation, the rough tube with a long core rod is rolled into a thin-walled tube on a guide plate tube rolling mill. The rolling mill is similar to a two-roll oblique rolling piercing machine, except that the fixed guide plate is replaced by an active guide plate. Since it is produced with a long mandrel, the inner wall of the pipe is smooth and scratch-free; however, the tool cost is high and the adjustment is complicated. It is mainly used to produce carbon steel pipes for general use with an outer diameter of less than 150 mm. It is currently less used and has no great development prospects.

(8) Spinning pipe production

The flat plate or hollow blank is spun on a spinning machine once or multiple times to form a thin-walled pipe. The pipe has high precision, good mechanical properties, and a wide range of sizes, but the production efficiency is low. It is mainly used to produce non-ferrous metal pipes but is also increasingly used to produce steel pipes. In addition to being used to produce living utensils, chemical containers, and machine parts, spun pipes are mostly used in the military industry.

(9) Cold rolling and cold drawing pipe production

It is used to produce small-diameter thin-walled, precision, and special-shaped pipes. The production feature is a multi-process cycle process. The elongation can reach 6 to 8 when cold rolled using a cyclic cold rolling mill. In the 1960s, it began to develop in the direction of high-speed, multi-line, long-stroke, and long pipe billets. In addition, small roller cold rolling mills have also been developed. It is mainly used to produce ultra-thin precision tubes with a wall thickness of less than 1mm. The cold rolling equipment is complex, the tool processing is difficult, and the variety and specification changes are not flexible. Usually, a combined process of cold rolling and cold drawing is used, that is, cold rolling is used to reduce the wall first to obtain a large deformation, and then cold drawing is used to obtain a variety of specifications.

Production equipment for seamless steel pipes: two-roller oblique rolling piercing process commonly used by piercing machines. The round tube billet is pierced and rolled into a hollow thick-walled tube (rough tube), and the axes of the two rollers form an inclination angle with the rolling line. In recent years, the inclination angle has increased from 6° to 12° to 13° to 17°, which speeds up the piercing speed. For the production of steel pipes with a diameter of more than 250mm, secondary piercing is used to reduce the wall thickness of the rough tube. New processes such as piercing with active rotating guide discs, piercing with back thrust, axial discharge, and circulating top welding have also made certain developments, thereby strengthening the piercing process and improving the quality of rough tubes.

Automatic tube rolling machine: Roll thick-walled rough tubes into thin-walled rough tubes. Generally, after 2 to 3 passes, the finished wall thickness is rolled to a total elongation of about 1.8 to 2.2. Since the 1970s, the use of single-hole groove rollers, double-frame tandem rolling mills, double-groove tracking rolling, and spherical head technologies have improved production efficiency and realized the mechanization of tube rolling.

Leveling machine: The structure is similar to that of the piercing machine. The purpose of leveling is to eliminate internal and external surface defects and the ovality of the rough pipe and reduce the unevenness of the transverse wall thickness. In recent years, the three-roller leveling machine has been adopted to improve the deformation and leveling efficiency of the leveling machine.

Sizing machine: It consists of 3 to 12 frames, and the reducing mill consists of 12 to 24 frames, with a reduction rate of about 3 to 28%. The tension-reducing mill that appeared in the 1950s controls the wall thickness with appropriate tension while adjusting the roller speed and reducing the diameter. The new tension-reducing mill generally uses a three-roller type with 18 to 28 frames, with a maximum reduction rate of 80%, a wall reduction rate of 44%, and an outlet speed of 18 mm per second. The tension reducer has the disadvantage of thickening at both ends, which can be eliminated by "sudden electrical control at the head and tail ends" or micro-tension reduction.

Automatic pipe rolling unit: Commonly used series have four types of outer diameters of 100mm, 140mm, 250mm, and 400mm, producing steel pipes with outer diameters of 17 to 426mm. The unit is characterized by achieving the main deformation on the perforator, flexible specification changes, and a wide range of production varieties. Due to the development of continuous pipe rolling technology, units below 140mm are no longer built.