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Traditional and modern technologies for obtaining rolled products. Modern technologies for the production of rolled products and the formation of structure and properties. Profile and rail straightening machines

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Details Category: Long products

Long products

Widely used in mechanical engineering, construction, and transport rolled metal: sheets, strips, tapes, rails, beams etc. It is obtained by compressing a metal ingot in a hot or cold state between the rotating rolls of a rolling mill. Steel, non-ferrous metals and their alloys are processed in this way.

Rental profile (its cross-sectional shape) depends on the shape of the rolls. The figures show the main profiles of rolling production products, called long products.

The following profiles are distinguished: long products: simple (circle, square, hexagon, stripe, sheet); shaped (rail, beam, channel, brand and etc.); special (wheels, reinforcing steel and etc.).

Most often, rolled products are used as blanks for various parts. For example, from hexagonal rod make bolts and nuts. From round steel cylindrical parts are turned on lathes. Angle rolled products used in the production of frames, frames, shelving, etc.

By rolling, you can give the workpiece the shape of the finished part, thereby avoiding additional processing and, therefore, reducing metal waste and saving time.

Below are several examples of common types of rolled products: pipe, reinforcement, beam, channel, sheet, angle, strip, etc.

Long products - one of the types of semi-finished products. This is the name given to a labor product intended for further processing and production of finished products.
You are already familiar with some types of semi-finished products - lumber, plywood, wire.
Sheet metal divided into thin sheet (up to 4 mm) and thick sheet (over 4 mm

Types and properties of steel

Steel- This iron-carbon alloy(up to 2%) and other chemical elements. It is widely used in mechanical engineering, transport, construction, and everyday life.
Depending on the composition there are different carbonaceous And alloyed steel. Carbon steel contains 0.4...2% carbon. Carbon gives steel hardness, but increases brittleness and reduces ductility. When adding other elements to steel during melting: chromium, nickel, vanadium etc. - its properties change. Some elements increase hardness and strength, others increase elasticity, others impart anti-corrosion, heat resistance, etc. Steels containing these elements are called alloyed. In alloy steel grades, additives are designated by letters: N - nickel , IN - tungsten ,G - manganese , D - copper , TO - cobalt , T - titanium .

By purpose they distinguish structural, instrumental and special become.
Structural carbon steel is of ordinary quality and high-quality. First- plastic, but has low strength. Used for making rivets, washers, bolts, nuts, soft wire, nails. Second is characterized by increased strength. Shafts, pulleys, lead screws, and gears are made from it.
Tool steel has greater hardness and strength than structural steel, and is used for the manufacture of chisels, hammers, thread-cutting tools, drills, and cutters.
Special steels - these are steels with special properties: heat-resistant, wear-resistant, stainless, etc.
All types of steel are marked in a certain way. So, structural steel ordinary quality is indicated by letters St. and serial number from 0 before 7 (Art. ABOUT, Art. 1 etc. - the higher the steel number, the higher the carbon content and tensile strength), high quality - two digits 05 , 08 , 10 etc., showing the carbon content in hundredths of a percent. Using the reference book, you can determine the chemical composition of steel and its properties.
The properties of steel can be changed using heat - heat treatment (heat treatment). It consists of heating to a certain temperature, holding at this temperature and subsequent rapid or slow cooling. The temperature range can be wide depending on the type of heat treatment and the carbon content of the steel.
Main types of heat treatment - hardening, tempering, annealing, normalization .
To increase the hardness of steel it is used hardening - heating a metal to a certain temperature (for example, up to 800 ° C) and rapid cooling in water, oil or other liquids.
When exposed to significant heat and rapid cooling, steel becomes hard and brittle. Brittleness after hardening can be reduced by vacations - the cooled, hardened steel part is again heated to a certain temperature (for example, 200...300°C), and then cooled in air.
For some instruments, only their working part is hardened. This increases the durability of the entire tool.
At annealing the workpiece is heated to a certain temperature, maintained at this temperature and slowly(this is the main difference from hardening) cool down. Annealed steel becomes softer and therefore easier to process.
Normalization - a type of annealing, only cooling occurs in air. This type of heat treatment helps to increase the strength of steel.

Heat treatment of steel at industrial enterprises is carried out thermal workers. Thermist must have a good knowledge of the internal structure of metals, their physical and technological properties, heat treatment modes, skillfully use thermal furnaces, and strictly observe labor safety rules.

The most important mechanical properties of steel - hardness and strength . On hardness steel is tested using special hardness testers. The measurement method is based on pressing a harder material into the sample: a hard steel ball, a diamond cone or a diamond pyramid.

Hardness value NV determined by dividing the load by the surface area of ​​the imprint left in the metal ( Brinell method ) (Fig. on the right, A),

or by the depth of immersion into the metal of a diamond tip, a steel ball ( Rockwell method ) (rice. 6 ).

Strength steel is determined using tensile testing machines by testing samples of a special shape, stretching them in the longitudinal direction until they break (Fig. on the left). When determining strength, divide the greatest load that preceded the rupture of the sample by the area of ​​its original cross-section.

In combination with free rolling (to free dimensions), this made it possible to increase the flexibility of the production process. The introduction of continuous casting of beam blanks with dimensions close to those of the finished profile has made significant changes in the process of producing large sections. The number of rolling passes has decreased, rolling mills have reduced their dimensions, the rolling process has been simplified, its economic performance has improved, and energy consumption has been reduced. In addition, when rolling rails and beams, measures such as temperature control and cooling of profiles, and when rolling rails also the possibility of strengthening them in the mill line, led to improved product quality.

Combined small-section wire rolling mills

Over the past 25 years, the maximum exit speed of wire rod mills has increased from 80 m/s to 120 m/s as a result of technological improvements driven by productivity demands. The most important step on this path, accompanied by an increase in production flexibility and dimensional accuracy of rolled products, was the introduction of the thermomechanical rolling process.

In addition, the weight of wire rod coils increased to 2 tons or more. Another direction for improving the wire rod rolling process was the expansion of the use of continuously cast billets. Since, based on metallurgical considerations, it is desirable to use workpieces with a maximum cross-section, even with a minimum speed at the entrance of the rolling mill, in this case it is necessary to increase the exit speed.

Improvement of the process over the past 25 years has made it possible to cool individual rolled strands in the mill line and implement thermomechanical rolling of wire rod, and as a result, obtain products that are more focused on customer requirements, i.e., achieve and control the required mechanical properties of products already at the hot rolling stage.

Trends in the modern market, especially the market for high-quality steels, are manifested in a decrease in the range of sizes of finished products in the mill range and in a greater variety of steel grades. To meet these trends, different rolling strategies must be applied. The productivity of a rolling mill largely depends on the duration of the changeover process, due to the transition to rolling a different finished size or when changing the grade of rolled steel.

Multiline rolling technology. This technology, used to increase the productivity and production flexibility of high-quality wire rod mills, allows for standardized roll calibration, right down to the finishing blocks (Fig. 1). This eliminates the downtime of crimping stands, intermediate group stands and finishing blocks of the small-section wire mill, which is observed in traditional shops during mill readjustment associated with the transition to rolling of a different size.

Rice. 1. Multiline rolling technology using a loop device: rolling options on a small-section wire mill from Acominas, Brazil

The basis of the concept is a combination of a loop device, an eight-stand block group and an FRS (FlexibleReducing and Sizing) block with four stands and a device for fast handling (Fig. 2).

Rice. 2. FRS block

The device for quick transfer of the FRS block allows you to switch to another rolling size in 5 minutes. Since minimal time is required for setup after handling, it is possible to create a flexible program for rolling products of different sizes from different steel grades.

The new rolling mill concept also makes it possible to switch from traditional to thermomechanical rolling by simply pressing a button on the control panel. The choice of a rolling route and the direction of the rolled metal along a route equipped with retractable devices for cooling and temperature equalization (see Fig. 1) allows you to switch to a different size of rolled product or another grade of steel in accordance with the adopted rolling strategy without operator intervention and without any manual equipment settings. This concept also implies a significant reduction in equipment downtime.

The general concept is complemented by the CCT (Controlled Cooling Technology) technological system, which makes it possible to simulate the temperature conditions of rolling, the formation of a microstructure and the required mechanical properties. Only after the simulation is completed, the real rolling process begins with the regulation of its parameters in the mill line and automatic regulation of the cooling mode in the refrigerator sections.

To meet the requirements associated with tighter dimensional tolerances for hot-rolled sections and wire rods, three- and four-strand rolling has been abandoned and a return to rolling mills with a maximum of two strings, which are separated into single-strand finishing lines as early as possible in the process.

The past few years have also seen increased use of precision rolling systems to achieve even tighter dimensional tolerances for bars and wire rods.

Hydraulic control systems cross-sectional dimensions of rolled products. Section mills use hydraulic sizing control systems, such as the ASC (Automatic Size Control) system, designed to complement mechanical precision sizing control systems. These systems (Fig. 3) use only two stands in mills with alternating vertical and horizontal stands and allow the entire range of products to be rolled (round, flat, square, hexagonal and corner sections) to tolerances corresponding to 1/4 of the DIN 1013 standard.

Rice. 3. Precision ASC system for regulating the dimensions of long products

Both stands are equipped with hydraulic pressing devices and provide fully automated control using monitors. The regulation applies to the entire length of the rolled product. A special measuring device placed between the stands ensures tension-free rolling. To switch to another size, it is enough to pull out only the cassettes with rolls and wires from the mill line and replace them within 5 minutes with others using a quick transfer device. Adjustment of the gap between the rolls is fully automated. In the roll preparation area, only roll barrels and wires are replaced.

Rolling technology in three-roll stands

This technology began to be used on an industrial scale when rolling long profiles in the late 1970s and was then constantly improved.

A special feature of this technology is the combination of crimping and calibration passes in one block of stands (in the finishing block when rolling rods and in the roughing block when producing wire rod). This block is called RSB (Reducing and Sizing Block). In accordance with the technology, rolling with free dimensions was introduced, which made it possible to obtain a wide range of finished product sizes with fairly tight tolerances, using a single calibration of the rolls, only by adjusting the position of the rolls. With one finishing gauge system, the RSB block makes it possible to produce products with dimensional accuracy within 1/4 tolerances of the DIN 1013 standard (Fig. 4).

Rice. 4. Five-stand block RSB (370 mm)

Endless rolling

The ECR (Endless Casting Rolling) process (Fig. 5) combines continuous casting and rolling processes in one production line using a tunnel furnace. As a result of the integration of thermal equipment into a single production complex, the duration of the technological process from liquid steel to the finished product does not exceed 4 hours. The ECR process can be used on mills for rolling billets and shaped profiles, as well as on mills for rolling grades and wire rods. The ECR line includes a continuous casting machine, a roller hearth furnace, a rolling mill with roughing, intermediate and finishing groups of stands, a refrigerator, a heat treatment section, equipment for cutting, surface quality control, packaging (forming and tying bags).

Rice. 5. Endless long section casting and rolling (ECR) process

In a roller hearth furnace, the temperature of the metal is equalized and heated to the rolling temperature. In addition, the furnace acts as a buffer equipment in case of disruption of the rolling mill.

The rolling line is equipped with frameless stands and a hydraulic device for rapid transfer, allowing this operation to be fully automated. Changing the shape or size of the rolled product can be done in a few minutes. A top-level computerized control system pre-calculates and sets the nominal parameters of the rolling process. Triangulation laser sensors are installed on the output sides of the intermediate and finishing groups, which measure the shape and dimensions of the rolled product. The measurement results are sent to the monitor of the mill operation control system to calculate corrective effects on the process parameters. A top-level computerized control system accumulates an archive of production information in order to obtain products of guaranteed quality.

At the output of the production line there is equipment for heat treatment in the mill flow, for hot and cold leveling, as well as for coil winding. The entire line (from the casting unit to heat treatment and finishing) is controlled by an automated system.

The first ECR unit for endless rolling of long products made from special steels was put into operation in 2000.

The know-how and equipment used on the endless rolling unit served as the basis for the creation of section mills with high productivity and increased yield. On the EBROS unit (Endless Bar Rolling System - endless rolling of sectional profiles), heated workpieces are connected by butt welding. After deburring the weld, the “endless” billet enters the rolling mill stands. Since the operating cycle eliminates idle time and the appearance of trim, the productivity of the unit increases by 10-15%, and the yield increases by 2-3%.

Mills for the production of long products

As in the production of wire rod, only continuously cast billets are currently used in section rolling mills. Based on considerations of the dimensional accuracy of rolled products, when rolling long profiles, the trend is to abandon multi-thread mills. The vast majority of modern section mills are designed and operate as single-thread mills, with alternating horizontal and vertical stands.

To ensure high productivity when rolling reinforcing profiles and compliance with the required tight tolerances on the dimensions of long products made from high-quality and corrosion-resistant steels, rolling of these types of metal products is currently carried out separately. As in the production of wire rods, technological rolling with controlled temperatures and thermomechanical rolling have been introduced into the production of long products over the past 25 years. Currently, Garrett winders can wind finished profiles with a diameter of up to 70 mm into coils.

To avoid bottlenecks in the production process, when producing profiles both in cut lengths and in coils, finishing operations are performed on continuous lines. To control quality and ensure its high level, laser sensors and eddy current flaw detectors are used to control the dimensions and identify surface defects of hot-rolled steel.

Large section and rail and beam mills

The main objective of large-section mills is the cost-effective production of high-quality products. When producing large sections, you can adhere to one of two concepts: the first is continuous mills, the second is reversing mills with a sequential arrangement of stands and a finishing sizing stand. On continuous mills the ECR process can be applied.

Rolling technology on reversing tandem mills

This technology is suitable for the production of medium and large sections, beams up to 1000 mm high (with a flange width up to 400 mm), angles, special profiles and rails.

Tandem reversing rolling mills include a twin-roll crimping stand, a group of three identical universal/twin-roll stands in series, a finishing universal/twin-roll stand and a finishing line with a cooler, leveler, shears, stackers and packing machines.

Compared to a concept without a free-standing finishing stand, this mill configuration has the following advantages:

  • compact arrangement of rolling equipment - a crimping stand, an intermediate group of tandem stands and a separate finishing stand;
  • a sizing stand operating in a continuous mode at the exit of the mill makes it possible to achieve fairly tight tolerances on the dimensions of rolled products and significantly reduce roll wear;
  • the number of rolling stands is reduced and the use of rolls and wires is improved;
  • the flexibility of the applied roll calibration is increased due to the use of identical, interchangeable universal/double-roll stands;
  • the range of spare parts and parts is reduced due to the identical design of the stands;
  • frameless stands with hydraulic pressing devices that can operate under load (SCC – Stand Core Concept); in addition to the standard system for automatic control of profile dimensions, it is possible to use higher-level control systems with output to a monitor connected to a triangulometric laser sensor installed in the mill line to measure the rolled profile;
  • short time for readjusting the mill when switching to rolling of a different size (20 min).

When rolling medium-grade profiles (HE 100-260, IPE 100-550, angles 100-200), the following advantages of rolling on reversible tandem mills can be noted compared to traditional rolling on a mill without a separate calibration stand:

  • planned downtime associated with the transfer of rolls is reduced to 40%;
  • the labor intensity of work and costs associated with transferring rolls and replacing input and output wiring are reduced to 20%;
  • Roll costs are reduced by 40-60% depending on the finished rolled profile.

Rolling technology on universal mills and HH mills

In accordance with the main trends in the global market for large sections, section rolling shops with a shortened technological cycle and low production costs are in increasing demand. Mastering the casting of beam blanks and the combination of casting blanks close in size to the finished profile, followed by their rolling, prepared the prerequisites for combining the casting and rolling processes into an integrated line for the production of a wide range of large-section profiles, including the highly sought-after tongue and groove profiles.

When rolling large-section profiles, the use of modern universal stands as part of a reversible tandem mill (CN rolling technology) has become the dominant solution (Fig. 6). When rolling, all three stands are used in each pass, with the first universal stand having a calibration according to the X scheme, and the second universal stand, acting as a finishing stand, having a calibration according to the H scheme, corresponding to the finished profile.

Rice. 6. Reversible group of the mill with a sequential arrangement of stands (tandem) for rolling according to the XN scheme

On large-section and rail-and-beam mills, rolling is used in a reversible group of universal tandem stands not only to produce beams and other large-section profiles (channels, angles, profiles for shipbuilding, special profiles and tongues), but also as a compact group of stands for the economical production of rails intended for work in conditions of heavily loaded and high-speed railways (Fig. 7). This technology made it possible to produce rails with increased dimensional accuracy, improved surface quality and less wear on the rolling rolls.

Rice. 7. Large section and rail and beam mill with heat treatment and finishing lines

Features of rail production

Rails– These are rolled products that are subject to extremely high demands. Specifications for physical properties and geometric parameters such as curvature, dimensional tolerances, surface condition, microstructure and residual stress levels are of paramount importance. To meet these requirements, rolled rails are processed using horizontal and vertical straightening machines during finishing. The horizontal leveling machine is also used in the production of large-grade profiles. Currently, it is possible to produce and ship rails up to 135 m long. Rails intended for severe operating conditions are subjected to special heat treatment to give their heads special wear resistance along the entire length of the rail.

On medium-grade mills (Fig. 8), both universal and two-roll stands are used for rolling steel construction profiles - beams, channels, angles, strip steel and special profiles.

Rice. 8. Layout of a medium grade mill

Rolling of beams and profiles from beam blanks

Once continuous casting of thin-walled beam blanks became possible, reductions and rolling forces were reduced.

The example shown in Fig. 9 shows that a beam blank with a wall height of approximately 810 mm and a thickness of 90 mm can be compressed to the dimensions acceptable at the entrance to the universal finishing stand. The number of rib gauges depends on the degree of deformation of the beam blank required for rolling in a universal stand. A possible scheme for compressing a beam blank is shown in Fig. 9 .

Rice. 9. Maximum and minimum change in the shape of flanges and walls when rolling beams from beam blanks

The maximum and minimum compression limits for the profile flange and wall are also shown. In all four cases, the drawing ratios at which the largest beam profile (with the greatest wall height) is obtained, and compression ratios in vertical (edger) rolls to obtain a profile of the minimum size (with a minimum cross-sectional area) are illustrated.

After mastering the rolling of beam blanks and introducing the compact beam production technology CBP (CompactBeamProduction), the question arose about whether (and how exactly) beam blanks can be used in the production of sheet piling profiles.

Roll calibration shown in Fig. 10, represents the process of rolling Larsen sheet piles (trough-shaped) on a mill with a universal stand, providing two passes in horizontal rolls to obtain a universal beam profile and two passes in vertical (edger) rolls of a group of reversible tandem stands to form a profile with the shape and dimensions required by the entering the finishing cage.

Rice. 10. Rolling of sheet piling profiles (Larsen profile) from beam blanks

Currently, as noted above, beam profiles are rolled from blanks using the CN technological scheme. In addition, beam blanks are used for the production of Larsen sheet piles and rails. The entire range of standard beam profiles can be rolled from just four sizes of continuously cast beam blanks. Further optimization of the beam rolling process followed the path of adapting the well-known compact strip production (CSP) technology to the production of beams. This process, called CBP, significantly reduced the number of rolling passes.

In addition, it is possible to roll Vignelle rails (with a flat base) from beam blanks, as shown in Fig. 11. In this case, the number of passes is significantly reduced compared to the classical scheme of rolling rails in two-roll stands.

Rice. 11. Calibration of rolls for rolling Vignelle rails from beam blanks

In the production of rails, head hardening and heat treatment in the mill line have become traditional operations to obtain products of the required quality.

Hydraulic push systems

Modern billet and long-section mills, which include universal/twin-roll stands, are equipped with automated hydraulic pressing systems that allow finished products to be rolled to very tight tolerances. The bed on the operator’s side is movable and has the ability to extend along with the rolls (which can have different barrel lengths) and wires (Fig. 12). Setting up the mill when switching to rolling of a different size takes only 20 minutes, which makes the production of small batches of products economically justified.

Rice. 12. Compact universal/twin-roll stand

Using a digital process control system (TSC – TechnologicalControlSystem) (Fig. 13), the installation of the rolls by means of hydraulic devices can be maintained constant along the entire length of the rolled profile. Each hydraulic cylinder is positioned so that the gaps between the horizontal and vertical rolls correspond to the pre-calculated nominal values. The hydraulic system for regulating the roll gap (HGC - Hydraulic Gap Control) also helps prevent the destruction of the rolls and bed when overloads occur. In addition, during the rolling process, the lower roller is positioned relative to the upper roller. The deformation of the stands, which occurs under the influence of various rolling forces, is compensated during rolling using a system for automatically controlling the dimensions of rolled products (AGC - Automatic Gage Control). All this allows the use of reproducible and relatively simple calibration schemes.

Rice. 13. Process control system

Aerosol Cooling Refrigerator, Selective Cooling Line and Laser Profile Measuring System

Using water mist as a cooling medium in a specific area of ​​the refrigerator speeds up the cooling process and provides the following benefits:

  • specific influence on the cooling curve (Fig. 14);
  • smaller refrigerator area;
  • reduction of capital costs;
  • low operating costs;
  • the possibility of using a modular cooling system with selective on/off sections;
  • increasing the productivity of refrigerators in existing workshops.

Rice. 14. Comparison of different cooling methods and aerosol cooling refrigerator

To ensure uniform temperature distribution in the steel profile when rolling I-beams and rails, a selective cooling device is installed between the output side of the mill and the cooler, the geometry of which corresponds to the shape and dimensions of the profile. In combination with a process control system, this solution makes it possible to cool specific sections of the cross-section of the rolled profile (Fig. 15).

Rice. 15. Selective cooling of rails and beams

This not only improves the straightness of the rolled profiles on the refrigerator, but also reduces residual stresses in the metal due to a more uniform occurrence of structural transformations.

In addition, the mechanical properties of rolled products can be improved. Selective cooling sections can also be mounted on refrigerators in existing workshops.

Finished rails, beams and other profiles after rolling are measured in a hot state using the beam splitting method. A laser beam directed at the surface of the profile being measured is reflected and captured by a high-speed, high-resolution sensor. The distance to the profile surface is calculated depending on the position at which the reflected beam is captured by the sensor. Based on the measurement results, the contour of the measured profile can be drawn.

Profile and rail straightening machines

Modern roller-type CRS machines with a compact layout for straightening profiles (Fig. 16, a) are equipped with nine two-support prefabricated leveling rollers with a fixed location. All nine rollers have individual drives. Hydraulic cylinders can adjust the position of the rollers under load or the gap between them. Compared to traditional leveling equipment, such machines have the following advantages:

  • uniform and symmetrical application of load, as well as more favorable distribution of residual stresses in profiles;
  • compensation of elastic springing of rollers by adjusting their position using hydraulic cylinders;
  • hydraulic mechanism for axial installation of each roller;
  • assembly of the correct rollers with minimal gaps and maximum accuracy of their installation during the straightening process;
  • automated replacement of rollers, taking no more than 20 minutes.

Rice. 16. Leveling machine for steel profiles (a) and rails (b), arranged according to the H-V scheme

Rail straightening machines (Fig. 16, b) consist of horizontal and vertical blocks and are characterized by increased structural rigidity and individual drive of straightening rollers. In combination with off-line rail straightening machines and special tension control systems between straightening rollers, these machines make it possible to achieve a minimum level of residual stress in the rails, which significantly increases their service life.

Distinctive features of rail straightening machines are:

  • backlash-free assembly of straight rollers, bushings and supports on adjustable shafts;
  • installation of correct bushings on shafts using bayonet rings and high-pressure hydraulic systems;
  • automated machine adjustment when changing product sizes;
  • Replacing the correct rollers within 30 minutes.

Prospects

The increasing demands made by consumers of long rolled products regarding properties and dimensional accuracy, as well as the need to introduce resource-saving technologies, forced technologists to master the production of finished products directly from rolling heating and without additional heat treatment. In some cases, this provides material properties that cannot be obtained using traditional heat treatment processes.

Advances in modern instrumentation and automation, as well as improvements in the design of rolling mills, have made it possible to achieve a high level of automation in the production process. This has resulted in a number of important achievements, including increased yield, improved product quality and more consistent properties, the ability to respond instantly to process deviations, fine-tuning of rolling equipment, reduced scrap, and reliable documentation of the entire process to ensure guaranteed quality. products.

  • P.-Y. Mok
  • K. Overhagen
  • W. Stelmacher

Over the past years, when improving the technology of long rolling, the main attention has been paid to obtaining the required properties of long products and wire rods directly from rolling heating and the possibility of further processing of rolled products without preliminary heat treatment. In combination with free rolling (to free dimensions), this made it possible to increase the flexibility of the production process. The introduction of continuous casting of beam blanks with dimensions close to those of the finished profile has made significant changes in the process of producing large sections. The number of rolling passes has decreased, rolling mills have reduced their dimensions, the rolling process has been simplified, its economic performance has improved, and energy consumption has been reduced. In addition, when rolling rails and beams, measures such as temperature control and cooling of profiles, and when rolling rails also the possibility of strengthening them in the mill line, led to improved product quality.

  • long products,
  • small section wire mill,
  • large section mill,
  • rail and beam mill,
  • rolling process,
  • finishing,
  • heat treatment.
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The essence of metallurgical ideas from a technological point of view lies in the formation of an optimal structure for a specific product and in influencing the process of structure formation itself. Since the structure of the metal is determined by composition and technology, they cannot be considered separately, since the composition of the steel must correspond to the technological scheme.
A number of effects on the structure of steel are known:
- doping - changing the structure;
- microalloying - influence on the processes of grain growth and recrystallization; dispersion hardening, etc.;
- introduction into the metal of particles that change the processes of structure formation (for example, titanium oxides);
- influence on the crystallization process (cooling, soft compression, etc.);
- thermal and deformation effects on metal in the solid state.
This material deals mainly with thermal deformation effects on steel in the solid state, taking into account the necessary changes in its chemical composition.
The first technological scheme used for the production of rolled metal for electric-welded pipes was hot rolling, after which the steel has a rough structure and a low level of properties. To overcome this situation, heat treatment was used (normalization or hardening followed by high tempering).
Normalization does not provide a high range of properties of pipe steels (mainly a combination of strength, cold resistance and weldability). As a result of metallurgical research, a number of ideas were formulated on the composition of steels: steels with carbonitride hardening (for example, 16G2AF) and steels that are hardened in air to martensite (for example, 12Kh2G2NM), etc.
Hardening and tempering is already a double heat treatment, which is associated with high costs and low productivity. In addition, to increase hardenability, additional alloying is necessary (hence, increasing the cost of steel).
Hardening of large-sized rolled products is a very complex process, since it is associated with solving the problems of non-uniform cooling and warping of the metal. By the way, Chelyabinsk Profit http://cheliab-profit.ru/ sells similar products.
Experiments with hot rolling conditions led to the creation of controlled rolling, the most important result of which is grain refinement. The idea of ​​CP has been developing for several decades, which has led to the creation of various technological schemes and corresponding steel compositions.
The development of technology for accelerated cooling of rolled products by controlling phase transformations has dramatically increased the possibilities of thermomechanical rolling in terms of strength, toughness, meeting special requirements, assortment and purpose of rolled products.
Slow cooling of the rolled product made it possible to remove diffusion-mobile hydrogen from the rolled product, relieve stress and improve its continuity and ductility. It seems that this is the last stage of the technology and all technological operations, from heating for rolling to cooling almost to ambient temperature, are regulated from the point of view of optimizing the formation of the structure.
Specialists from JFE Steel Corporation (Japan) proposed another possible technological effect (between the end of accelerated cooling and the beginning of slow cooling), heating of rolled products in a flow (HOP technology - heat-treatment on-line process).
Consequently, not all possibilities have been exhausted; new ideas may also appear.

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There is a transition to a newer qualitative round of development. This is due to many factors: from the creation, implementation and development of advanced technologies, including in steelmaking, to changes in the very concept in relation to rolling production. One of the most important factors of this development in rolling production is the emerging opportunity to exercise absolute control over the temperature-strain process during rolling on the latest generation mills. This trend is most clearly manifested in rolling mills designed for the production of wire rod and small grades. Let's try to assess what causes this, taking into account the opportunities provided by the use of new approaches in wire rod rolling technology. During the hot rolling process, high-temperature thermomechanical processing of the metal (TMT) occurs. However, TMT, as a rule, is understood not only as the physical essence of the process, but also as a targeted complex effect on the structure of a metal alloy by a set of operations of deformation, heating and cooling, as a result of which the final structure of the metal alloy is formed, and, consequently, its properties . There are a large number of types of thermomechanical processing of steel. They can be divided into the following groups:

  • Thermomechanical processing modes in which deformation occurs in the austenitic state. This group includes the most well-known and studied hardening methods: high-temperature thermomechanical treatment (HTMT) and low-temperature thermomechanical treatment (LTMT).
  • Thermo-mechanical treatment with deformation during the transformation of supercooled austenite.

Regimes of thermomechanical processing associated with deformation carried out after the transformation of austenite into martensite or bainite. An example of such a treatment is the hardening method associated with strain aging of martensite. To strengthen steel, various combinations of thermomechanical treatment modes can be used, for example, HTMT with LTMT, HTMT with strain aging of martensite, etc. Thermo-mechanical treatment is most often the final operation in the manufacture of parts. But it can also be used as a preliminary operation, which ensures the formation of a favorable structure during final heat treatment, including hardening for martensite and tempering. Traditionally, when considering the problem of achieving the required properties in finished products from a metal alloy, the influence of chemical elements on the properties of the metal and heat treatment is used. At the same time, the formation of a structure during heating, and especially during rolling, remained a “black box” for a long time. But it is precisely these processes that influence the formation of the structure in the finished product. In practice, technologists used to obtain the necessary mechanical properties; in finished rolled products they used only such mechanisms in the manufacture of steel as alloying and heat treatment. As an example, we will give the disadvantages of using traditional methods for producing finished rolled products from ordinary steel grades. This class of steels has a structure consisting of ferrite with a known small proportion of pearlite. If there is a desire to obtain less metal-intensive structures and steel products that have increased reliability at low manufacturing costs, the problem arises of increasing the strength of rolled products obtained in the hot-rolled state. If, to increase strength, only increasing the proportion of pearlite by increasing the carbon content is used, then this possibility is limited, since with an increase in strength due to an increase in carbon content, the ductility, toughness and weldability of steel sharply decrease, which leads to the abandonment of this rolled product, since along with strength in rolled products it is also necessary to ensure the above-mentioned properties of the metal. The production of rolled products from high-alloy steels leads to a sharp increase in the cost of finished products due to the high price of alloying elements and deterioration in processing technology (additional stripping, etc.). Additional heat treatment after rolling, such as quenching + tempering, makes it possible to increase the strength and plastic properties of steel, but this effect can only be obtained for low-alloy steel grades. At the same time, there is also an increase in the cost of finished steel products. The first step in using the special state of hot-rolled steel obtained during the deformation process was the use of accelerated cooling units after rolling, in particular the use of water cooling. The use of this technology directly in rolling lines made it possible to reduce the influence of the full course of recrystallization processes, which previously formed the structure and mechanical properties of the finished product.

The next step in improving the mechanical properties was the use of the so-called controlled rolling process using the principles of thermomechanical processing. Let us consider in more detail the use of these principles in the TMT process. Depending on how rolling and heating are carried out, the effectiveness of the influence of the chemical composition and heat treatment on the final properties of rolled metal primarily depends. The chemical composition has a great influence on changes in the structure during the TMT process, and its influence on the mechanical properties should be considered from the standpoint of all stages of metal processing: from heating to cooling. Heat treatment from rolling heating only fixes the state of the structure obtained at the rolling mill, and although there are many options for its implementation with obtaining various sets of properties, the increase in their values ​​is limited to a given structure during the rolling process. Heat treatment outside the rolling mill is becoming increasingly impractical as energy costs rise. A number of thermomechanical processing modes can provide, along with high strength properties, increased plasticity and toughness. Often, the use of TMT makes it possible to obtain a complex of mechanical properties that cannot be achieved by conventional heat treatment and traditional alloying. By changing the conditions of deformation during TMT, it is possible to regulate the density and distribution of defects in the crystalline structure, which makes it possible to control the structure and properties of steel over a wide range. It is these reasons that were the basis for such rapid development and interest among metal product manufacturers in the TMT process. It is necessary to note the prospects for the development of the TMT process in the production of wire rod. This is due to the peculiarities of production and geometric dimensions (high strain rates and particularly small cross-sections, unlike other types of metal products produced by hot rolling). The fact is that only when rolling wire rod for a large range of grades is it possible to carry out and control the processes of hot peening and recrystallization, which, due to the lack of high strain rates in the production of other types of rolled products, is not feasible in a rolling line, or is possible when certain restrictions are imposed (limited grade range, as a rule, steels of the austenitic class or low rolling temperatures). This allows you to control the strength properties of hot rolled products, and the high degree of deformation in combination with the chemical composition and heat treatment makes it plastic. The peculiarities of rolling wire rod include another very important factor from the standpoint of thermomechanical processing - the time between deformations can reach very small values, especially in the last stands, down to 0.0005 s. To preserve the structure obtained during the TMT process, the method of cooling after rolling is of great importance. In this case, two problems arise: transporting the rolled product to a cooling device and cooling the metal over the entire cross-section to ensure uniformity of the structure, and, consequently, the properties across the cross-section of the finished rolled product. The small cross-section of the wire rod (diameter up to 8 mm) will allow us to consider it as a thermally thin body.

Thus, having obtained the necessary structure at the rolling mill, we can fix it throughout the entire cross-section and along the entire length, which improves the uniformity of properties and the quality of hot rolled products. If necessary, by changing the cooling intensity after rolling, it is also possible to achieve a different structure across the cross-section layers and obtain certain properties. Since the rate of heat removal in a larger section from the inner layers is limited, maintaining the advantages of the induced structure during the rolling process is problematic, and sometimes even impossible. When conducting an experiment on a rolling mill, the most important point is to take into account the factors that most influence the structure. To do this, it is necessary to carry out mathematical modeling of the rolling process, which makes it possible to determine the values ​​of the parameters affecting the structure. For subsequent assessment of their influence on the structure, the following already known data can be used:
- the influence of temperature and exposure in the oven on grain growth in the workpiece;
- influence of grain size and metal temperature on transformations from austenite;
- change in the structure of hot-deformed austenite during post-deformation holding;
- structure formation during hot
rolling.


To determine the influence of rolling parameters on the structure of hot-deformed metal, it is necessary to create a thermokinetic model of the wire mill on which the experiment is carried out. On the basis of which, based on the speed of the end of rolling and intermediate temperatures in the mill line, the following values ​​are determined: strain rate; deformation temperatures; time between deformations. When implementing a controlled rolling process, temperature is one of the most important factors in the targeted influence on the structure and final properties in the production of wire rod. There are several ways to directly control the temperature of the rolled product during the rolling process: changing the heating temperature, regulating the rolling speed, inter-stand cooling and heating the rolled product. Most often, the first two levers of influence are used to influence the temperature of the rolled material during rolling. To use inter-stand cooling and heating, an installation is required
additional equipment. In addition, a preliminary assessment of cooling capabilities is required (at rolling speeds above 30 m/s and an inter-stand distance of no more than 1 m, the time to ensure the necessary heat removal is limited). Also, a big task is to know the influence of the temperature fields of the rolled product during the rolling process for a certain grade range on the structure of the metal, in particular
by grain size. When using control over the rolling temperature, it must be taken into account that the range of possible control has certain limitations. The thermal conditions determine the energy-power parameters of the rolling mill, the forces acting on the rolls (washers) and other parts of the working stands, the accuracy of the profile dimensions, the shape and surface quality of the finished product, the durability of the rolling rolls, and the stability of the entire technological process. Moreover, it is directly related to the modes of compression, speed and tension. Most rolling mills do not directly measure the temperature of the intermediate strip along the entire length of the mill. This is due to both the high cost of the installation and the operating conditions of the devices, which often does not allow one to accurately determine the temperature of the metal and can lead to breakdown of the measuring equipment in the event of an emergency deviation of the metal from the rolling line. Also, when using interstrain cooling, even determining the surface temperature of the rolled product does not give an accurate picture of the average mass temperature of the metal, which, in turn, is the most significant for assessing the above parameters. The temperature during metal rolling is not uniformly distributed over the cross section, and since it is not possible to determine this distribution by direct measurement, it is advisable to resort to calculating thermal characteristics. The thermal regime is calculated taking into account the thermal balance, which depends on all types of heat exchange that take place during hot rolling: heat loss by thermal conductivity in contact with the washers and water cooling, convection and radiation. The greatest problem in determining heat transfer during rolling is to establish the patterns of temperature changes at any point in the rolling during the time from heating to obtaining the finished wire rod. The change in the temperature of the rolled product during rolling is associated with the occurrence of all types of thermal processes: thermal conductivity, convection and radiation. In this case, each type of heat transfer makes its own contribution, which is not always possible to accurately determine. Deformation of metal by rolling from the position of heat transfer consists of a large number of different stages (cycles). At each such stage, certain processes operate with conditions unique to this area. The resulting effect of complex heat transfer depends not only on the intensity of specific types of transfer, but also on the characteristics of their interaction (sequential or parallel, stationary or non-stationary). In contrast to the stationary mode, in which the temperature field does not change over time, the thermal rolling process is characterized as non-stationary. In this case, the temperature field of the roll is a function of time. An unsteady process is associated with a change in enthalpy over time. In this case, the intensity of heat removal is not constant over time. Solving the problem of non-stationary thermal conductivity means finding the dependence of the change in temperature and the amount of transferred heat over time for
any point of the body. Each of the processes of unsteady heat transfer is described by a system of differential equations. However, these equations describe countless heat transfer processes derived from consideration of an elementary section in a physical body. In order to solve a specific problem associated with a change in the temperature of the metal during rolling, it is necessary to consider the thermal processes occurring at each stage and give a complete mathematical description of all the particular features characteristic of this case. To do this, it is necessary to solve a system of differential equations when determining the following boundary conditions:
- Geometric conditions characterizing the shape and dimensions of the roll.
- Physical conditions characterizing the physical properties of the medium and the roll.
- Boundary conditions characterizing the features of the process
at the boundaries of the body.
- Temporary conditions characterizing the peculiarities of the process
in time.

Solving this system of equations will make it possible to obtain a description of the rolling temperature field in any section of the rolling mill at any time. This problem of determining the temperature fields along the cross section of the rolled product at any moment of rolling was solved for the small-section wire mill 300 No3 of OJSC MMK. As an example
The diagram in Figure 1 shows the temperature distribution across the cross section
intermediate roll. Using the results of this model made it possible to evaluate the existing temperature-strain regime
rolling, and by changing the main factors of rolling - to predict and obtain the required mode from the position of forming the necessary structure. In order to obtain a new level of properties on wire rod intended for reinforcement, studies were carried out at OJSC MMK on the 250#2 mill using a temperature-strain model and a newly installed water cooling unit. The installation in 2004 of a new water cooling line at mill 250#2 (manufactured by NPP Inzhmet) made it possible to conduct experimental studies in order to obtain thermomechanically strengthened reinforcement of small diameters. The production of thermomechanically strengthened reinforcement on the 250No2 mill consisted of carrying out the process of hardening the surface layer of the wire rod in a water cooling line located after the finishing stand No. 16 in the flow of the rolling mill. Next, the rolled products are laid by a winder in the form of coils on a mesh conveyor, after which they are collected on a coil collector into coils weighing up to 300 kg. Cooling is carried out using a high-pressure nozzle and in successively arranged tubes, at the inlet and outlet of which the cooling of the wire rod is interrupted by cut-off devices. The length of the active cooling zone depends on the diameter of the rolled wire rod and can be ≈ 7.2 m and ≈ 9.7 m.
Thermo-mechanical hardening of wire rod can be divided into three stages. At the first stage, the wire rod leaving the finishing stand No. 16 enters the heat strengthening line, where it is subjected to intensive cooling with water. This process must provide cooling of the surface of the wire rod at a rate exceeding the critical cooling rate necessary to obtain a martensite structure in the surface layer of the wire rod. However, the technology of the thermal hardening process must ensure a temperature in the central layers of the wire rod at which the austenitic structure is preserved during cooling. This process can be divided into a second stage, which will make it possible, with further cooling at a rate lower than the critical speed, to obtain a ferrite-pearlite structure in the core of the wire rod, which will ensure high plasticity of the resulting reinforcement (Fig. 2). At the third stage, the high temperature of the central layers of the wire rod after the end of the intensive cooling operation will facilitate the self-tempering process of the hardened surface layer. This process, in turn, also makes it possible to increase the plasticity of the surface layer while maintaining its high strength.
The metal located between the surface and central layers has an intermediate cooling rate, which leads to a layer with a bainite structure. As a result of such cooling, it turns out that the wire rod in cross section represents two zones in the form of a ring: with a martensitic and bainite structure and a ferrite-pearlite structure in the central
parts. As a result of experimental rolling on the 250#2 mill, a wire rod with the indicated structure was obtained (Fig. 3).
Study of the structure of sections of thermomechanically strengthened wire rod
showed that the resulting rolled product, as a rule, has one or several hardened crescent-shaped layers. This is apparently due to the fact that cooling is carried out by only one nozzle per cooling cycle. In such conditions, if a situation arises of “accidental” washing of any one area of ​​the rolled product in a single cooling chamber, there is no further possibility of carrying out several more cooling cycles, which would allow for more uniform cooling of the wire rod across the cross section. Further cooling of the wire rod on a mesh conveyor without directional air blowing also leads to an uneven temperature field both across the cross-section and along the length of the wire rod coil. Also from the experience of conducted
rolling, a change in the temperature of the wire rod after water cooling along the length of the coil was revealed (temperature change along one coil
∆Т=30-50 °С). Since the cooling time and conditions are the same along the entire length of the coil, it was concluded that the reason for this temperature difference is the uneven heating along the length of the billets in the heating furnace of the rolling mill.


Measurements of the temperature of the workpiece at the outlet of the furnace and after the roughing group (temperature change was ∆T=50–80 °C) subsequently confirmed this assumption. The factors listed above ultimately lead to a large unevenness of the structural components along the length of the rolled product, which directly causes a significant scatter (up to 50-80 N/mm2) of the mechanical properties within the batch. Such a structure in wire rod made from ordinary low-carbon steel grades makes it possible to obtain a unique set of mechanical properties: high yield strength with good ductility, which is not always possible to obtain even on wire rod made from some low-alloy steel grades with standard rolling and air cooling (Fig. 4). Obtaining the above wire rod requires strict adherence to heat strengthening technology. The setting of the water cooling line depends on many factors: steel grade, required mechanical properties, wire rod diameter, composition of the cooling line equipment, setting of the high pressure nozzle, rolling speed, water flow and pressure (Fig. 5).
To determine the technological parameters depending on the listed factors, experimental studies were carried out by measuring the self-tempering temperature. From the wire rod coils obtained during experimental rolling, samples were taken for mechanical tests and metallographic analysis of the resulting microstructure. The results obtained show that there is a fairly large range of changes in mechanical properties. In this case, the same trend is observed as with an increase in the carbon content in carbon steel grades: with an increase in strength properties, plastic properties decrease (Fig. 5).
Based on the brand assortment, the level of mechanical properties and the nominal diameter, it is possible to obtain an optimal technological regime that satisfies the needs of consumers. One of the most promising areas of application is thermomechanical
reinforced reinforcement of small diameters is to use it for
reinforcement cage bundles in high-strength reinforced concrete slabs. The scope of application of this reinforcement may in the future include various other reinforced concrete structures, foundations, etc. Today, this can ensure the improvement of regulatory and technical documentation (GOST, TU, etc.) and the study of the possibilities of using this new type of product. The conducted studies made it possible to determine the main parameters of the process of thermomechanical hardening of small diameter wire rods. Subsequently, when the 170 mill is launched at OJSC MMK, after adapting the results obtained to the rolling conditions at the new mill, it will be possible to master this assortment for mass production.
CONCLUSIONS
- The processes occurring during the deformation of metal in a hot state are considered. The factors that most influence the formation of the metal structure after deformation are determined.
- The prospects for the development of the TMT process in the production of wire rod are shown, taking into account its geometric dimensions and production features: particularly small cross-section and high deformation rates, in contrast to other types of metal products obtained by hot rolling.
- The results of using such a tool as temperature modeling are shown in order to obtain the necessary mechanical properties of wire rod during hot rolling, taking into account the existing technological capabilities of the mill, as well as from the point of view of the influence of hot plastic deformation and chemical composition on the structure.
- The results of using thermomechanical processing during rolling on the structure of the finished wire rod are presented.

The start and end temperatures of hot deformation are determined depending on the melting and recrystallization temperatures. Rolling of most grades begins at a temperature of 1200...1150 0 C, and ends at a temperature of 950...900 0 C.

The cooling mode is essential. Rapid and uneven cooling leads to cracking and warping.

During rolling, the temperature at the beginning and end of the process, the compression mode, and the adjustment of the rolls are controlled as a result of monitoring the size and shape of the rolled product. To monitor the condition of the surface of rolled products, samples are taken regularly.

Finishing of rolled products includes cutting to cut lengths, straightening, removal of surface defects, etc. The finished rolled products are subjected to final control.

The rolling process is carried out on special rolling mills.

Rolling mill – a set of machines for deforming metal in rotating rolls and performing auxiliary operations (transportation, control, etc.).

The equipment for deforming metal is called the main equipment and is located on the main line of the rolling mill (line of working stands).

Figure 1 — Rolling mill diagram

1 – rolling rolls; 2 – plate; 3 – club spindle; 4 – universal spindle; 5 – working cage; 6 – gear cage; 7 – coupling; 8 – gearbox; 9 – engine

The main line of a rolling mill consists of a working stand and a drive line, including a motor, gearbox, gear stand, couplings, and spindles.

Rolling stand

Rolling rolls 1 are installed in the working stand 5, which receives the rolling pressure. The defining characteristic of the working stand is the dimensions of the rolling rolls: the diameter (for long products) or the length (for sheet products) of the barrel. Depending on the number and location of the rolls in the working stand, rolling mills are distinguished: two-roll (duo-mill), three-roll (trio-mill), four-roll (quatro-mill) and universal (Figure 2).

In two-roll stands (Figure 2, position a), only one pass of metal is carried out in one direction. Metal in three-roll stands (Figure 2, position b) moves in one direction between the lower and upper rolls, and in the opposite direction - between the middle and upper rolls.

In four-roll stands (Figure 2, position c) support rolls are installed, which allow the use of small-diameter work rolls, thereby increasing the drawing strength and reducing the deforming forces.

Universal stands (Figure 2, position d) have non-driven vertical rolls, which are located between the bearing supports of the horizontal rolls and in the same plane with them.

Gear cage 6 is designed to distribute engine torque between the rolls. This is a single-stage gearbox, the gear ratio of which is equal to one, and the role of gears is performed by gear rollers.

The spindles are designed to transmit torque from the gear stand to the rolling rolls with a deviation from coaxiality of up to 10...12 0. For minor movement in the vertical plane, club type 3 spindles are used complete with a club clutch. The internal contours of club couplings correspond to the cross-sectional shape of the roll shank or spindle. The coupling provides a gap of 5...8 mm, which allows the possibility of working with a misalignment of 1...2 0. With significant movements of the rolls in the vertical plane, the spindle axis can make a significant angle with the horizontal plane; in this case, articulated or universal spindles 4 are used, which can transmit torque to the rolling rolls when the spindle is misaligned to 10...12 0.

Figure 2 — Rolling stands

DC and current motors are used as the motor of the rolling mill 9; the type and power depend on the productivity of the mill.

Gearbox 8 is used to change the speed when transmitting motion from the engine to the rolls. Gears are usually herringbone with a helix inclination of 30 0 .

According to their purpose, rolling mills are divided into mills for the production of semi-products and mills for the production of finished products.

Heating of the metal is carried out in flame and electric furnaces. According to the furnace temperature distribution, there can be and. In periodically heated chamber furnaces, the temperature is the same throughout the entire working space. In methodical furnaces, the temperature of the working space constantly increases from the place where the workpieces are loaded to the place where they are unloaded. The metal is heated gradually, methodically. The furnaces are characterized by high productivity. They are used in rolling and forging shops for heating non-ferrous metal ingots. Large ingots are heated before rolling in a type of chamber, flame furnace.

The following are used as transport devices in rolling production:

  • ingot carriers and various types of carts for feeding ingots and billets from heating devices to the mill;
  • roller tables are the main means of transport in rolling shops (with rotating rollers installed in series, they ensure longitudinal movement of metal; with the oblique arrangement of the rollers, the possibility of transverse movement of the strip arises);
  • manipulators designed for the correct task of the strip into the caliber;
  • tilters designed to rotate the workpiece around a horizontal axis.