The details of the cold rolling and drawing of the tube

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


Compared with cold rolling, the advantages of cold drawing are large deformation per pass, fewer processing passes, short production cycles, and low metal consumption. The disadvantages are that tool manufacturing is difficult and it is inconvenient to change specifications. The production flexibility is poor, the equipment investment is high and the maintenance is complicated; because it is a 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, great flexibility, and simple equipment and tool manufacturing; the disadvantages are small deformation per pass, many processing passes, long production cycles, and high metal consumption.


First, cold drawing of steel pipes
(1) Drawing steel pipes without mandrels, which is used to reduce only the outer diameter of steel pipes.
(2) Drawing steel pipes with fixed short mandrels, which is used to reduce both the outer diameter and wall thickness of steel pipes; this method has large friction resistance and large drawing force due to the fixed mandrel and the use of fixed external molds, and small deformation per pass; the advantage is that the drawing method is simple, so it is widely used.
(3) Drawing steel pipes with floating mandrels, which is often used for reel drawing steel pipes and can produce very long steel pipes (more than 100m). The drawing force is small during floating mandrel drawing, which can increase the deformation per pass; since there is no restriction on the pull rod, steel pipes with very small diameters can be drawn with mandrels.
(4) Long mandrel drawing: When drawing steel pipes, the mandrel moves with the steel pipe, which eliminates the friction resistance on the core, thereby reducing the drawing force and increasing the deformation per pass; the movement of the mandrel can also reduce the roughness of the inner surface of the steel pipe; the disadvantage of long mandrel drawing is that it has strict requirements on the mandrel, and the rod must be removed after drawing. There are two methods to remove the rod: one is to roll the steel pipe and the mandrel together on the inclined rolling mill to expand the diameter of the steel pipe a little, and then pull the mandrel out on the rod extraction machine; the other is to use double-die drawing to expand the diameter before pulling the mandrel, the latter die is an additional die, the steel pipe wall has only a small amount of deformation when passing through the additional die, the steel pipe diameter is slightly expanded, and the rod removal force can be reduced when removing the rod; two four-roller rolling dies can also be used for rolling and removing the rod.
(5) Diameter expansion drawing: the wall thickness of the steel pipe is reduced, the diameter is increased, and the length of the steel pipe is slightly shortened; when expanding the diameter of the steel pipe, the steel pipe is fixed and the pull rod drives the mandrel through the steel pipe.


To overcome the shortcomings of conventional cold drawing methods, the following steel pipe drawing methods have been continuously developed:
(1) Conical mandrel steel pipe drawing, which has a smaller pull rod force Pc than the general short mandrel.
(2) Semi-continuous and continuous steel pipe drawing. Various linear steel pipe drawing machines work intermittently. After pulling a steel pipe, they need to wait for the trolley to return before pulling the next steel pipe. Therefore, semi-continuous and continuous steel pipe drawings have been developed. There are two forms of semi-continuous cold-drawn steel pipe. One is to use a double-moving steel pipe drawing trolley, which is fixed on the motion chain and runs along a guide rail with a certain shape. After the upper steel pipe drawing trolley pulls out the previous steel pipe, the lower steel pipe drawing trolley arrives in front of the center frame and clamps the next steel pipe for drawing. The other is a semi-continuous steel pipe drawing machine that moves the double steel pipe drawing mold during drawing. Continuous steel pipe drawing machines are crawler-type and double-trolley reciprocating type. The crawler-type steel pipe drawing machine consists of several frames with steel pipe drawing molds installed at the front end. There are chain links on both sides of the frame, and track links are installed on the chain shaft to compress the steel pipe and force it into the steel pipe drawing die. This continuous steel pipe drawing machine can be used for drawing without a core rod and a long core rod, which can increase the output by 3 times compared with ordinary steel pipe drawing machines; the problem is that it is easy to produce uneven longitudinal wall thickness when drawing without a core rod, and the resistance of the fixed die is large, the energy consumption is high, the product surface quality is poor, and it is difficult to remove the rod. In addition, there is a continuous steel pipe drawing with double trolleys for reciprocating motion, which is used for drawing without a core rod, see continuous welding and continuous drawing of precision steel pipes.
(3) Long core rod rolling die drawing process of round steel pipe. The process of multi-roller rolling die long core rod drawing round steel pipe is: before drawing, the steel pipe head is first shrunk to match the conical transition area of the core rod to clamp the steel pipe. Then the core rod is inserted, and the thin end of the long core rod passes through the rolling die. During drawing, the cold drawing machine jaws hold the thin end of the mandrel to make linear motion and pull the steel pipe and the mandrel out of the rolling die together, to achieve wall reduction and elongation deformation of the steel pipe, and change the simple cold drawing method into a combined drawing and rolling method. This method can obtain a large deformation amount per pass, reduce energy consumption, have a simple structure, and have good surface quality of the steel pipe; the disadvantage is that it is difficult to remove the rod.
(4) Ultrasonic vibration steel pipe drawing.
(5) Drum steel pipe drawing.
(6) Warm drawing of steel pipe.
(7) Cold drawing of steel pipe with rotary die.
(8) Reverse tensile force steel pipe drawing.
(9) Torsion steel pipe drawing, that is, the drawing process of twisting the steel pipe while drawing. The straightness of the steel pipe after the drawing is good, and the straightening process can be omitted. When the steel pipe material 1 is drawn, it passes through the elliptical die 3 and the working die 4 is placed on the center frame 2. With the help of the rotation of chuck 5 on the steel pipe drawing trolley 6, a torque is applied to the steel pipe after leaving the deformation zone, causing it to twist, and the steel pipe is prevented from bending due to the twisting.
(10) Fluid dynamic lubrication steel pipe drawing. When drawing steel pipes, a pressure steel pipe is installed in front of the steel pipe drawing die. The pressure steel pipe has a certain length of lH, and there is only a small gap between its inner wall and the outer surface of the steel pipe material. During the drawing process, the lubricant is carried by the steel pipe moving forward through the pressure steel pipe to the front of the deformation zone. Since the gap between the inner wall of the pressure steel pipe and the surface of the steel pipe is small, and the lubricant that leaves the deformation zone with the steel pipe after drawing is relatively small, the lubricant is strongly squeezed when passing through the pressure steel pipe, and the pressure P of the lubricating film gradually increases. This is the effect of fluid dynamic pressure, which is the physical basis for establishing fluid dynamic lubrication drawing. If the pressure of the lubricating film in front of the deformation zone reaches enough to separate the surface of the steel pipe and the die wall, so that the lubricant is filled between the two, forming liquid friction, this can greatly improve the lubrication conditions, achieve the purpose of reducing the drawing force, increasing the deformation amount per pass, and reducing mold consumption.


The basic conditions for realizing fluid dynamic lubrication steel pipe drawing are: there is a pressure steel pipe with a certain length, a high drawing speed, and a lubricant with a high viscosity. Fluid dynamic lubrication is mainly used for drawing steel pipes without mandrels.


Second, cold rolling of steel pipes
There are two types of conventional cold rolling mills, namely two-roller cold rolling mills and multi-roller (generally 3-4 roller) cold rolling mills.


Due to the small diameter of the rolls, the small rolling force, and the small rolling unit pressure between the metal and the tool, the elastic deformation of the rolls is small. In addition, the use of support rolls has high rigidity and is suitable for rolling thin-walled and ultra-thin-walled precision steel pipes with a minimum wall thickness of 0.03mm (see ultra-thin-wall steel pipe production); the disadvantage is that the deformation per pass is small and the productivity is low. Double-line rolling can increase productivity by 50% to 70%. During rolling, the frame and the rolls reciprocate. When the working frame reaches the rear limit position, the steel pipe material is fed into the rolling direction by a feed amount of m with the help of a special feeding mechanism. Then, due to the forward movement of the frame, the rollers also rotate, and the roller neck rolls along the support plate. The special shape of the support plate gradually reduces the hole radius, and the feeding part of the steel pipe material is reduced in diameter and thinned in wall thickness. During the rolling process, the cylindrical mandrel of the steel pipe material does not move in the axial direction (locked by a special device). When the working frame reaches the front limit position, the rolling is completed, and the steel pipe material is rotated by a certain angle at the same time so that all parts of the cross-section of the steel pipe are processed. After that, the working frame moves in the opposite direction, the rolled section of the steel pipe material is further refined, and the metal corresponding to the opening part of the hole is rolled on the mandrel. As a result of the lateral flow of the metal, the inner diameter of the steel pipe increases, so that the inner surface of the steel pipe material in the working cone part is separated from the mandrel, creating conditions for the next feeding of the pipe material, and this is repeated until the full length of the steel pipe material is rolled.


The two-roller cold rolling mill is a widely used cold rolling mill. The advantages are large deformation per pass and elongation coefficient of 14 to 18; the disadvantages are that the roll pass is difficult to manufacture, the equipment investment is high, and the dimensional accuracy of the rolled steel pipe is not as good as that of the multi-roller cold rolling mill. The working principle of the two-roller cold rolling mill is the same as that of the multi-roller cold rolling mill. The difference is that the two-roller cold rolling mill uses two rolls with variable cross-sectional shapes to replace 3 to 4 rolls with constant cross-sectional dimensions and a conical mandrel.


To overcome the inherent disadvantages of conventional cold rolling mills, the following processes and equipment have been developed:
(1) Multi-line and high-speed rolling are adopted on traditional cold rolling mills, annular pass shapes are applied, the roll diameter is reduced, and the reciprocating motion of the frame is changed to the reciprocating motion of the roll box to reduce the mass of the moving parts, reduce energy consumption and increase the rolling speed. Various new two-roller cold rolling mills with support rolls have been developed. Double-row rolls and multi-row rolls are used on multi-roller cold rolling mills.
(2) Planetary cold rolling mills have been used for cold rolling of non-ferrous metal steel pipes.
(3) Continuous cold rolling mill: This type of mill has the advantages of high output, large deformation per pass, and short rolling rhythm time, but it also has the disadvantages of uneven size along the length of the steel pipe, long mandrel, high requirements and difficult manufacturing, and high equipment investment. A continuous cold rolling mill has been used for cold reduction of steel pipes.