Cold rolling and drawing of tube

The cold processing of pipes by rolling and drawing at room temperature on the cold-rolled and cold-drawn pipe unit belongs to the deep processing of hot-rolled pipes (including hot-extruded pipes) and welded pipes.It can obtain pipes with smaller diameter, thinner wall thickness, higher dimensional accuracy, lower surface roughness and better performance, as well as various cross-sectional shapes.Various pipes with a diameter of 0.2-765mm and a wall thickness of 0.015-50mm can be produced by cold drawing.Various pipes with a diameter of 4-450mm and a wall thickness of 0.03-35mm can be produced by cold rolling.

Compared with cold rolling, cold rolling has the advantages of large amount of deformation per pass, less processing passes, short production cycle and low metal consumption.The disadvantage is that it is difficult to manufacture tools and it is inconvenient to change specifications.Poor production flexibility, high equipment investment and complicated maintenance; due to the periodic rolling, the rolling time is long, the productivity is low, and the energy consumption is high.The advantages of cold drawing are high productivity, convenient specification change during production, high flexibility, and simple manufacturing of equipment and tools; the disadvantages are small deformation per pass, many processing passes, long production cycle, and large metal consumption.

Various conventional methods for cold drawing pipes are shown in Figure 1.

(1) Extubation without mandrel, used to reduce only the outer diameter of the tube.

(2) Immobilizing the short mandrel to pull out the pipe, which is used to reduce the outer diameter and wall thickness of the pipe at the same time; this method is due to the fixed mandrel and the use of a fixed outer mold, which has large frictional resistance, large drawing force, and pass deformation Small; the advantage is that the drawing method is simple, so it is widely used.

(3) Floating mandrel extubation, commonly used in drum extubation, can produce very long pipes (above 100m).The drawing force of the floating mandrel is small, which can increase the deformation of each pass; because there is no restriction of the tie rod, the pipe with a small diameter can be drawn with the mandrel.

(4) Long mandrel extubation, since the mandrel moves together with the tube during extubation, the friction resistance on the mandrel is basically eliminated, so the pulling force can be reduced and the deformation of the pass can be increased; the movement of the mandrel can also reduce The roughness of the inner surface of the pipe; the disadvantage of long mandrel extubation is that it has strict requirements on the mandrel, and the rod must be taken off after drawing.There are two ways to take off the rod: one is to roll the pipe and the mandrel together on the skew rolling mill to expand the diameter of the pipe a small amount, and then pull out the mandrel on the rod drawing machine; the other is to use double-die drawing to Expand the diameter before pulling the core rod (see Figure 2). The latter mold is an additional mold. When passing through the additional mold, the pipe wall has only a small amount of deformation, and the diameter of the pipe is slightly enlarged, which can reduce the rod removal force when removing the rod; Two four-roll rolling dies can also be used for rolling and stripping.

(5) Expanding and pulling out the tube, the wall thickness of the tube decreases, the diameter increases, and the tube length is somewhat shortened; when the tube is expanded and pulled out, the tube is fixed and the pull rod drives the mandrel to pass through the tube.

In order to overcome the shortcomings of the conventional cold drawing method, the following extubation methods have been continuously studied:

(1) The conical mandrel extubation (Figure 3), compared with the general short mandrel, the pull rod force Pc is smaller.

(2) Semi-continuous and continuous extubation.All kinds of linear extubation machines work intermittently. After pulling out a tube, it is necessary to wait for the trolley to return before pulling out a tube.Therefore, semi-continuous and continuous extubation were developed.There are two types of semi-continuous cold drawn tubes.One is to use a double-moving extubation trolley (Fig. 4). The extubation trolley is fixed on the kinematic chain and runs along a guide rail with a certain shape.After the tube pulling trolley on the upper level pulls out the previous tube, the tube pulling trolley on the lower level reaches the front of the center frame and clamps the next tube for drawing.The other is a semi-continuous tube pulling machine that moves double tube pulling dies during drawing, as shown in Figure 5.

The continuous extubation machine has a crawler type (Figure 6) and a double-car reciprocating type.The crawler extubation machine consists of several frames with extubation dies installed at the front end.Ring chains are installed on the upper and lower sides of the frame, and crawler joints are installed on the ring chain shafts, which are used to compress the pipes and force them to be sent into the extubation die.This continuous extubation machine can be used for drawing without mandrel and long mandrel, which can increase the output by 3 times compared with ordinary extubation machine; the existing problem is that the longitudinal wall thickness is easy to be uneven when drawing without mandrel , the use of fixed die has large resistance, high energy consumption, poor product surface quality and difficulty in removing the rod.In addition, there is continuous tube drawing with double trolley reciprocating motion, which is used for drawing without mandrel, see continuous welding and continuous drawing of precision tubes.

(3) Long mandrel rolling die drawing round tube process (Figure 7).The process of drawing a round tube with a multi-roller rolling die long mandrel successfully researched by the University of Science and Technology Beijing in China is: before drawing, the tube head is shrunk so that it matches the mandrel tapered transition zone to block the tube.The mandrel is then inserted and the thin end of the long mandrel passed through the roller die.When drawing, the jaws of the cold drawing machine bite the thin end of the mandrel to make a linear motion, and pull the pipe and the mandrel together from the rolling die, so as to realize the extension and deformation of the pipe wall reduction, and change the simple cold drawing method into a drawing and rolling combination method.This method can obtain large pass deformation, reduce energy consumption, simple structure, and good surface quality of the pipe; the disadvantage is that it is difficult to remove the rod.

(4) Ultrasonic vibration extubation.

(5) Reel extubation.

(6) Pipe warm drawing.

(7) Cold drawing of pipe rotating die.

(8) Anti-pull extubation.

(9) Twist extubation, that is, the drawing process in which the pipe is twisted while drawing.The straightness of the pipe after drawing is good, and the straightening process can be omitted.The drawing process is shown in Figure 8.When the tube material 1 is drawn, it passes through the elliptical die 3 (used to prevent the pipe from rotating) and the working die 4 placed on the center frame 2 .By means of the rotation of the collet 5 on the extubation trolley 6, torque is applied to the pipe after leaving the deformation zone, so that it is twisted, and the pipe prevents bending due to the twist.

(10) Extubation with hydrodynamic lubrication.Figure 9 is a schematic diagram of extubation with hydrodynamic lubrication.When pulling out the tube, install a pressure tube before the tube pulling die. The pressure tube has a certain length lH, and there is only a small gap between its inner wall and the outer surface of the tube material.During the drawing process, the lubricant is carried by the forwardly moving tube through the pressure tube before reaching the deformation zone.Since the gap between the inner wall of the pressure tube and the surface of the tube is small, and the lubricant that leaves the deformation zone with the tube after drawing is relatively small, the lubricant is strongly squeezed when passing through the pressure tube, and the pressure of the lubricating film P gradually increases, which is the effect of hydrodynamic pressure, which is the physical basis for establishing hydrodynamic lubrication.If the pressure of the lubricating film before the deformation zone reaches enough to separate the surface of the pipe from the die wall, so that the two are completely filled with lubricant to form liquid friction, this can greatly improve the lubrication conditions, reduce the drawing force and increase the deformation of the pass. And the purpose of reducing mold consumption.

The basic conditions for realizing hydrodynamic lubricating extubation are: there is a pressure tube with a certain length, the drawing speed is higher, and the lubricant with higher viscosity is used.Hydrodynamic lubrication is primarily used for extubation without mandrels.

There are two types of cold rolling mills commonly used in pipe cold rolling, namely two-roll cold rolling mills and multi-roll (generally 3 to 4 roll) cold rolling mills.

The multi-roll cold-rolled pipe mill was invented by the former Soviet Union Institute of Metallurgical Machinery Science.Because the diameter of the roll is small, the rolling force is small, the rolling unit pressure between the metal and the tool is small, so the elastic deformation of the roll is small, and the back-up roll is used, the rigidity of the rolling mill is high, and it is suitable for rolling thin-walled and ultra-thin-walled precision tubes , the minimum wall thickness is 0.03mm (see ultra-thin-wall tube production); the disadvantage is that the deformation of the pass is small and the productivity is low.Adopting double-wire rolling can increase productivity by 50% to 70%.The working principle of the multi-roll cold rolling mill is shown in Figure 10.During rolling, the frame and the rolls reciprocate.When the working frame reaches the rear limit position (I in the figure), the pipe material is sent to the rolling direction by a feeding amount m by means of a special feeding mechanism, and then due to the forward movement of the frame, the roll also rotates, and the roll neck Rolling along the support plate, the special shape of the support plate makes the radius of the hole gradually decrease, and the feeding part of the tube is reduced in diameter and wall thickness.During the rolling process, the cylindrical mandrel of the tube does not move in the axial direction (locked by a special device).When the working frame reaches the front limit position (Ⅱ in the figure), the rolling ends, and at the same time, the tube material is rotated at a certain angle, so that all parts of the tube cross section can be processed.After that, the working frame reversely moves, and the rolled section of pipe is further finished, and the metal corresponding to the opening part of the hole pattern is spread on the mandrel.As a result of the metal lateral flow, the inner diameter of the pipe increases, which makes the inner surface of the pipe at the working cone part break away from the mandrel, creating conditions for the next feeding of the pipe, and so on until the full length of the pipe is rolled.

The two-roll cold rolling mill is a widely used cold rolling mill.The advantage is that the pass deformation is large, and the elongation coefficient can reach 14-18; the disadvantage is that it is difficult to manufacture the roll pass, the equipment investment is high, and the dimensional accuracy of the rolled pipe is not as good as that of the multi-roll cold rolling mill.The working principle of the two-roll cold-rolled pipe mill (see Figure 11) is the same as that of the multi-roll cold-rolled pipe mill. The difference is that the two-roll cold-rolled pipe mill uses two rolls with variable cross-section passes instead of 3 to 4 passes. Rolls with constant cross-sectional dimensions and tapered mandrels are used.

In order to overcome the inherent shortcomings of conventional cold rolling mills, the following processes and equipment have been developed:

(1) Adopt multi-line, high-speed rolling on the traditional cold-rolled pipe mill, apply ring pass (see cold-rolled pipe tools), reduce the roll diameter, change the reciprocating motion of the rack to the reciprocating motion of the roll box (the rack is fixed), To reduce the quality of moving parts, reduce energy consumption and increase rolling speed.Various new two-roll cold-rolling mills with back-up rolls have been developed.Double-row rolls and multi-row rolls are used on multi-roll cold-rolled pipe mills.

(2) The planetary cold-rolling pipe mill has been applied to cold-rolling non-ferrous metal pipes.

(3) Continuous cold-rolled pipe mill, which has the advantages of high output, large pass deformation, and short rolling rhythm time, but also has uneven dimensions along the length of the pipe, long mandrel, high requirements and manufacturing Difficulties and high investment in equipment and other shortcomings.The continuous cold rolling mill has been used for pipe cold reduction (see pipe cold reduction).