Rolling it. Cold rolling of metal

General provisions

Rolling of metals is carried out mainly at high temperatures, using the reduction of resistance to deformation. At the same time, cold rolling is also necessary, which is advisable with a small thickness of the rolled product, when, due to the large surface-to-volume ratio, rapid cooling of the metal does not make it possible to provide a high temperature in the deformation zone (rolling of thin sheets). Cold rolling gives products high dimensional accuracy and surface quality, which is impossible with hot rolling, as well as special physical, in particular magnetic properties.

AT last years production cold rolled sheet, tinplate and tape is increasingly increasing. This is because in many industries National economy constantly growing need for sheet steel with high mechanical properties, precise dimensions, good quality surfaces. Cold rolling combined with heat treatment makes it possible to produce steel sheets that meet these requirements. In 1977, the share of cold-rolled sheet

in the total volume of sheet-rolling production in the USSR, was approximately 19%; in the future, this share will grow steadily and by 1990 will presumably reach 37%.

in a modern way cold rolling of sheet steel is rolled, in which the metal in the form of long strips is wound into rolls of large mass. For rolling thin sheet steel in coils, mainly continuous mills are used, and with a small amount of production, single-stand reversing mills with a four-roll stand and multi-roll mills? Roll rolling on continuous and single-stand mills occurs with strip tension. Sheet cold rolling is much less frequently used on single-stand reversing mills (without tension).

Improving the technology of cold rolling is on the way to improving accuracy finished products due to: rigidity of working stands; application of means of elastic anti-bending of rolling rolls; improving the quality of rolls and equipping the mills with systems for automatic control of sheet thickness during the rolling process.

The rigidity of the working stands is determined mainly by the elastic deformation of the rolls and beds. Increasing the diameter of work and back-up rolls reduces their wear, improves rolling accuracy by reducing deflection, and increases heat transfer. The elastic deformation of the bed in the vertical direction on modern cold rolling mills is 0.3-0.5 mm; the reduction of this deformation is achieved by increasing the section of the racks and crossbars.

Metal rolling is a type of plastic processing when the original workpiece is compressed by rotating rolls of a rolling mill in order to reduce the cross section of the workpiece and give it a given shape. There are three main rolling methods:

longitudinal,

transverse,

transverse helical (or oblique).

Hydrogen nitrogen The closure of this cover with the base is airtight, as the annealing must be carried out in a controlled atmosphere. Bell furnaces, commonly used in cold sheet annealing, are furnaces that consist of a base on which the material is placed. Heating can be done with radiant tubes mounted on the inside wall of the hood or by direct firing of burners located on the base. Then the gas heating hood is placed for 17 hours. by introducing an inert gas, such as hydrogen, into the hood with a suitable device commonly used by these furnaces.

At longitudinal rolling the deformation of the workpiece is carried out between rolls rotating in different directions. The axes of the rolling rolls and the workpiece being processed are parallel (or intersect at a slight angle). Both rolls rotate in the same direction, while the round billet rotates in the opposite direction.

During cross rolling the workpiece to be processed is held in the rolls with the help of a special device. Compression of the workpiece in diameter and giving it the desired cross-sectional shape are provided by the appropriate profiling of the rolls and changing the distance between them. This method produces products that are bodies of revolution (balls, axles, gears, etc.).

Cross-helical or oblique rolling is carried out in rolls rotating in the same direction, installed in the rolling stand at some angle to each other. Oblique rolling mills are used in the production of pipes, mainly for piercing an ingot or billet into a sleeve. At the moment of contact of the metal with the rotating rolls, which are inclined to the axis of the workpiece, there are forces directed along the axis of the workpiece, and forces directed tangentially to its cross section. The combined action of these forces ensures rotation, retraction of the workpiece into the narrowing slot and deformation.

The main technological operations of any technological scheme for the production of rolled products include: preparation of raw materials; heating before rolling (except for cold rolling, when, however, another operation is often required - an appropriate heat treatment); hot and cold rolling; calibration and production of bent profiles; finishing with cutting, straightening, heat treatment, removal of surface defects, etching, etc.

metal drawing

metal drawing- this is the pulling of a product of a round or shaped profile through the hole of a drawing point (drawing point), the area of ​​\u200b\u200bthe output section of which is less than the cross-sectional area of ​​\u200b\u200bthe original product. Drawing is performed by traction applied to the front end of the workpiece. In this way, wire of all types, rods with high accuracy of transverse dimensions and pipes of various sections are obtained.

Metal processing by drawing is widely used in the metallurgical, cable and machine-building industries. By drawing, a wire with a minimum diameter of 0.002 mm, rods with a diameter of up to 100 mm, and not only of a circular cross section, are obtained, pipes are mainly of small diameter and with a thin wall. Drawing processes steels of various chemical compositions, precision alloys, as well as almost all non-ferrous metals (gold, silver, copper, aluminum, etc.) and their alloys. The products obtained by drawing have a high surface quality and a high dimensional accuracy of the cross section. If the product needs to be given mainly these characteristics, then this type of processing is called calibration.

Drawing is most often performed at room temperature, when hardening accompanies plastic deformation of most metals. This property, in combination with heat treatment, is used to improve some of the mechanical characteristics of the metal. So, for example, reinforcing wire with a diameter of 3 ... 12 mm from carbon structural steel (0.70 ... 0.90% C), when produced by drawing, provides a tensile strength of 1400 ... 1900 MPa and a yield strength of 1200 .. .1500 MPa.

As a starting material for drawing, rolled and pressed blanks are used. In the production of aluminum, copper and other wire, wire rod obtained directly from melting furnace through the mold and continuous rolling mill. Regardless of the method of obtaining the original workpiece, before drawing, it undergoes thorough preliminary preparation, which consists in carrying out one or another type of heat treatment, dross removal and surface preparation for fixing and holding lubricant on it during the drawing process. These preliminary operations ensure the normal performance of plastic deformation in the drawing hole, contribute to obtaining a high quality surface of the product, reduce the force and energy for drawing, and reduce the wear of the drawing tool.

Heat treatment of the metal before drawing removes hardening, gives the metal the necessary plastic properties, and ensures the most optimal structure. Therefore, heat treatment is chosen such that, in combination with plastic deformation, it provides maximum mechanical and other characteristics of the workpiece. Depending on the chemical composition of the metal and the purpose of the drawing product, annealing, normalization, hardening, and patenting are used. Patenting is applied to carbon steels. The patenting process consists in heating the metal above the critical point and cooling it in an environment with a temperature of 450...500°C. Molten lead or salts are used as such a quenching medium.

In the production of wire and rods by drawing, great attention is paid to the preparation of the surface of the processed product before drawing. Scale removal in calibration and drawing shops is carried out by mechanical, chemical and electrochemical methods, as well as combinations of these methods. During mechanical cleaning of the surface from scale, the wire or rod is subjected to periodic kinks in different planes between the rollers, after which the metal enters the final cleaning with steel brushes. This method is economically feasible, suitable for surface cleaning mainly from carbon steel, the scale of which, when bent, is relatively easily destroyed and falls off. Of the mechanical methods that provide a fairly successful cleaning of the metal surface, shot blasting is used. Under the influence of shots made of chilled cast iron. cast steel or high-strength finely cut steel wire, the scale on the surface of the workpiece is loosened and removed. This method of cleaning the metal surface from scale in many cases does not require additional pickling and is most often used in calibration shops.

Chemical descaling methods are widely used due to their reliability, although they are less economical than mechanical methods. Etching of carbon and a number of alloy steels is carried out in sulfuric or hydrochloric acids. High-alloy steels (acid-resistant, stainless, etc.) are pickled in mixtures of acids (sulfuric and hydrochloric, sulfuric and nitric, etc.). Copper and its alloys are pickled in 5...10% sulfuric acid at a temperature of 30...60°C. Pickling of metal in acids for descaling is usually carried out with the addition of additives (etching inhibitors) to the bath, which significantly reduce the rate of dissolution of the base metal, but do not affect the rate of dissolution of scale, which prevents overpickling. In addition, additives reduce the diffusion of hydrogen (H 2) into the metal, reduce the gas contamination of pickling departments, and improve working conditions.

Immediately after pickling, the metal is thoroughly washed to remove residual acid solution, iron salts, sludge, pickling additive, and dirt. Washing is carried out immediately after pickling, since the delay leads to drying of the pickling liquid and the release of sparingly soluble iron salts. Typically, washing is carried out first in hot water, which provides intensive dissolution of salts, and then for better removal of sludge - in a stream of cold water from a hose at a pressure of about 0.7 MPa.

After the scale is removed, a sub-lubricating layer is applied, which should retain the lubricant well during drawing and help prevent metal sticking to the working surface of the die.

After pickling, washing, applying a sublubricating layer, the metal is dried in special chambers with air circulation at a temperature of 300...350°C. Drying removes moisture, and also eliminates possible pickling (hydrogen) brittleness, which can arise from the fact that part of the hydrogen formed during pickling diffuses into the metal and causes deterioration of its plastic properties.

All operations to prepare the metal surface for drawing are performed in a special isolated room. For pickling and surface treatment of wires and bars, there are batch and continuous pickling machines. Processing in continuous machines provides fast and uniform pickling of products of any cross section. This method is the most progressive, since in a continuous process it is possible to combine heat treatment, descaling and application of a sub-lubricating layer. Such in-line processing provides full automation of the process, improves the quality of the metal, and reduces the complexity of operations.

After the drawing process, in addition to heat treatment, in many cases, the bars are straightened, ground, polished, and, depending on the purpose, protective coatings are applied to them, for example, by zinc plating, tinning, chromium plating, cadmium plating, aluminizing, varnishing, etc. Editing is usually performed on roller straightening machines, which installed either in the production stream or separately. Grinding the surface of calibrated bars to a depth of 0.15...0.30 mm is used to remove surface defects, remove the decarburized layer, give precise dimensions to the cross section of the bar, etc.