New technologies in rolling production. Modern technologies for the production of rolled products and the formation of structure and properties

The essence of metallurgical ideas from a technological point of view lies in the formation of an optimal structure for a particular product and in the impact on the process of structure formation itself. Since the structure of the metal is determined by the composition and technology, they cannot be considered separately, since the composition of the steel must correspond to the technological scheme.
A number of influences on the structure of steel are known:
- doping - changing the structure;
- microalloying - impact on the processes of grain growth, recrystallization; dispersion hardening, etc.;
- introduction into the metal of particles that change the processes of structure formation (for example, titanium oxides);
- impact on the crystallization process (cooling, soft reduction, etc.);
- thermal and deformation effects on the 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 of the applied technological schemes 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 get out of this situation, heat treatment was applied (normalization or hardening followed by high tempering).
Normalization does not provide a high range of properties of pipe steels (mainly combinations of strength, cold resistance and weldability). As a result of metallurgical research, a number of ideas on the composition of steels were formulated: steels with carbonitride hardening (for example, 16G2AF) and steels hardened in air to martensite (for example, 12Kh2G2NM), etc.
Quenching and tempering is already a double heat treatment, which is associated with high costs and low productivity. In addition, to increase the hardenability, additional alloying is necessary (hence, an increase in the cost of steel).
Hardening of large-sized rolled products is a very complex process, since it is associated with solving the problems of inhomogeneous cooling and metal warping. By the way, Chelyabinsk Profit http://cheliab-profit.ru/ sells similar products.
Experiments with hot rolling regimes led to the creation of controlled rolling, the most important result of which is grain refinement. The idea of ​​KP has been developed for several decades, which led to the creation of various technological schemes and the 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 rolled products made it possible to remove diffusion-mobile hydrogen from rolled products, relieve stresses 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 to almost ambient temperature, are regulated from the point of view of optimizing the formation of the structure.
Specialists of JFE Steel Corporation (Japan) proposed one more of the possible technological actions (between the completion of accelerated cooling and the beginning of slow cooling), heating of rolled products in a stream (HOP technology - heat-treatment on-line process).
Consequently, not all possibilities have been exhausted, and new ideas may appear.

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The starting material for the production of rolled products are ingots cast into molds - for billet mills, and for finished rolled mills - blooms, slabs and billets, rolled and continuously cast.

When using ingots, the technological scheme of rolling provides for the following operations: heating of ingots, rolling on blooming or slabbing, trimming the ends of the rolled product and cutting it into cut lengths. Further, slabs and large blooms are sent to finished rolled mills, and part of the blooms goes to continuous billet mills (CWM), where they are used to produce smaller billets for small-section and wire mills.

When using continuously cast billets (blooms, slabs), after heating or preheating, they are fed directly to the finished rolled mills, bypassing the blanking operations.

Ingots are cast from steels, which are subdivided according to a number of characteristics: by chemical composition, by production method, by structure, by purpose, by degree of deoxidation. Among them, carbon steels of ordinary quality (GOST 380), high-quality carbon steels (GOST 1050) and low-alloy structural steels (GOST 5058) occupy the largest share by weight.

Preparation of raw materials for rolling consists in removing surface defects and heating. Removal of surface defects - captivity, cracks, non-metallic inclusions, etc., is a very time-consuming operation. In old workshops, up to 70% of workers are employed on it. It is performed with a blade tool, cleaning with abrasive wheels, fire cleaning, machine tool peeling, etc.

Heating of the metal before rolling is carried out in heating wells, methodical furnaces and furnaces with a bogie hearth. The main purpose of heating a metal is to increase its ductility and reduce its resistance to deformation. However, heating can also have undesirable consequences - scale formation, decarburization of surface layers, overheating and burnout of the metal. And if the last three can be avoided by observing certain modes, then under normal conditions, scale formation is inevitable and leads to a loss of 1-2% of the metal or more, as well as a deterioration in surface quality.

The heating temperature of the metal is determined by the temperature regime of rolling - the temperature of the beginning (t n) and the end of rolling (t k). Usually, the temperature t n is taken 150-200 0 C below the solidus line of the state diagram of iron-carbon alloys so that the temperature t k lies in the region of single-phase gamma iron, i.e. in the temperature range above the transformation line. Usually for low and medium carbon steels t n = 1250 ... 1280 0 C, for high carbon t n = 1050 ... 1150 0 C, and t to 950 ... 1050 0 C.

In recent years, in order to save energy and material resources, improve the quality of rolled products, they switch to low-temperature heating and rolling.

9.1 Technology of semi-product production.

Semi-finished products include blooms with a section side of 240…350 mm, billets 50…240 mm, slabs up to 350 mm thick and up to 2500 mm wide. Semi-finished products are produced on blooming, slabbing and billet mills. Single-cell bloomings are the most common. According to the diameter of the rolls, they are divided into small (Æ 850 ... 1000 mm), medium (Æ 1050 ... 1170 mm) and large (Æ 1200 ... 1500 mm).

Blooming can roll both blooms and slabs, while slabs can only roll slabs.

Small blooming mills are mainly used as swaging stands for billets and rail and beam mills.

On fig. 9.1. a scheme of blooming 1300 is presented. It is located in four spans - furnace (I), camp or main (II), machine (III), scrap (IV) and adjusting (V). Ingots from the stripper section of the steelmaking shop are delivered on railway platforms to the furnace bay, ingots of boiling steel in a stripped state, and ingots of calm steel in molds in a state undermined from the sprues and without profitable extensions.

The ingots are put into heating wells (1) with a bridge tong crane - regenerative or recuperative types. Due to a number of disadvantages inherent in regenerative wells (direct contact of the flame with the ingot, uneven heating, lack of a representative point for temperature control in the cell, etc.), wells of the regenerative type are more often used.

Up to 90% of the ingots are put into wells in a hot state, which reduces the heating time of the ingots by about half and, accordingly, the fuel consumption and loss of metal in the scale.

Depending on the temperature, ingots of hot fitting, warm fitting and cold fitting are distinguished with a temperature above 800 0 С, from 400 to 800 0 С and below 400 0 С, respectively.

From the wells, the heated ingots are placed by a tong crane onto an ingot carrier of a shuttle or ring type (3). Shuttle ones have a limited throughput and are a bottleneck in the technological chain, especially when supplying ingots from further cells. Therefore, ring ingot carriers are more preferable. Up to 3…4 trolleys are placed on the ring, moving at a speed of up to 6 m/s.

With a side pusher (2) from the ingot carrier, the ingots are pushed onto the turntable, then onto the receiving roller table and transferred along it to the back span to the blooming (5), where they are rolled into blooms or slabs.

The main feature of blooming is the possibility of lifting the upper roll between passes to a height of up to 1500 mm and reversing the rolls, which ensures the rolling of ingots in the forward and reverse directions until the rolls of the specified dimensions are obtained. To calibrate blooming rolls, a system of box gauges with a consistent or symmetrical arrangement of gauges is used (Fig. 9.2-a, b).

The rolling force on the blooming machine reaches 18 MN, the rolling moment is up to 5 MNm. The rolls are driven by a single motor through a gear cage or individually for each roll. The total power of the engines is up to 12 thousand kW.

The transfer of the roll from the caliber to the caliber along the axis of the rolls is carried out by manipulators. In the line of the front manipulator, a hook tilter is mounted on the drive side. Behind the blooming there is a fire cleaning machine (7) and further - scissors (8). On the fire cleaning machine (MOZ), surface defects are removed. Depending on the area and depth of stripping, the loss of metal is up to 3%.

On scissors, the front and rear ends of the roll are removed and cut to length. Here, on the front end of each bloom and slab, the passport data of the ingot is stamped with a stamp. The head and bottom trimmings from under the scissors are transferred by an inclined conveyor (9) to the scrap span onto the railway platforms.

Crank shears provide cutting force up to 16 MN and number of cuts up to 12 per minute.

From the shears, part of the blooms is sent along a roller table (10) to a continuous billet mill (CWM), and the other part and slabs along a conveyor (11) are sent to an adjustment for cooling and repair.

The capacity of blooming 1150 is 3...4 million tons/year, and that of blooming 1300 is up to 6 million tons/year (by planting).

Slabings are in many ways similar to bloomings in terms of composition and arrangement of equipment. The main difference of slabing is the presence, in addition to horizontal rolls, of a pair of vertical ones located in front of or behind the stand. In addition, the slab rolls are not calibrated, but smooth.

It is not economically feasible to roll billets of small cross section on blooming. Therefore, usually behind the blooming there is a non-ferrous surfacing station, on which billets are rolled from the blooms without heating. On fig. 9.3 shows a diagram of the NZS 900/700/500. The mill consists of three groups and ensures the production of square blanks with a section side of 240, 190 and 150 mm from the second group and 120, 100 and 80 mm from the third.

Through the supply roller table (1), the blooms enter the rotary device for directing the roll with the healthy end forward, and from it to the first group of two stands (3) with rolls 900 mm in diameter. The second group of six stands - two with rolls with a diameter of 900 mm (5) and four - 700 mm each (6.7). In order to avoid tilting of the roll between the stands, the rolls of the two stands 700 are arranged vertically (6). A tilter (4) is installed in front of the group.

From the second group, the rolls with a cross section of 150 mm and above are transferred by schleppers to a bypass roller table (8) and then to shears with a lower cut with a force of 10 MN.

To obtain blanks with a smaller cross section, the rolls enter the third group of six stands with a roll diameter of 500 mm, three of which are with vertical (11) and three with horizontal rolls (12). Pendulum scissors (9) are installed in front of the group to remove the front end and a tilter (10).

In the first stands, a system of box gauges is usually used, in subsequent stands, a rhombus is a square.

Flying shears (13) with a force of 1.5 MN are installed behind the third group. After cutting, the workpieces are fed to the stacking roller table (19) and then to the refrigerator (21).

The performance of the CW usually corresponds to the performance of the blooming plant behind which it is installed.

In addition to the NZS for the production of blanks, there are also used blanking mills of a linear type and with a sequential arrangement of stands.

9.2 Technology for the production of rolled products on rail and beam mills

The range of rail and beam mills includes railway rails weighing from 38 to 75 kg/r.m., tram and crane rails, I-beams and channels over No. 24, equilateral and unequal angles, zetoid, round and square profiles of large sizes, etc.

As an example, let's consider the production technology of the most critical and complex profile - railway rails on the 800 mill.

The mill is of a linear type, the stands are arranged in two lines (Fig. 7.12). In the first one there is a 900 double-reversing crimping stand (small blooming), in the second there are three 800 stands - a roughing and pre-finishing trio and a finishing duo with a separate drive. Billets with a cross section of 300´340 mm are heated in methodical furnaces to a temperature of 1180-1200 0 C. In the swaging stand, rolling is carried out in box and three-four T-type calibers, and in the rest - in seam calibers (Fig. 9.4).

A rail about 75 m long leaves the finishing stand with a temperature of 900 0 .

With circular saws, the roll is cut to a standard length of 12.5 or 25 m, taking into account thermal shrinkage and allowance for machining the ends.

To compensate for thermal bending when the rail is cooled to the head, it is pre-bent onto the sole and cooled in this form in a refrigerator to a temperature of approximately 600 0 C. Then slow cooling (anti-flake treatment) in the pits follows, to a temperature of 150 ... 200 0 C for 7 …8 hours.

The cooled rails are straightened in roller straightening machines (RPM) and, additionally, the ends of the rails are straightened on stamp presses. After that, the ends of the rails are milled to a standard size and bolt holes are drilled. The presence of defects in the rails is monitored by ultrasonic testing.

This is followed by heat treatment of the rails - normalization in continuous furnaces or hardening of the rail head (heating of the HDTV to 1000 0 C and cooling with an air-water mixture). The final straightening of the rails is carried out on the RPM in the standing position and under the pressure of the ends of the rails in the position on the side.

The acceptance of rails is carried out by the Quality Control Department and inspectors of the Ministry of Railways. They control the chemical composition and structure of rail steel, its strength and plastic properties, impact strength, fracture of samples, full-profile rails under a headframe, etc.

Rolling of beams, channels and other profiles is carried out according to the same technological scheme with some simplifications: a wider temperature range for heating the billet (1200 ... 1280 0 C), there is no preliminary bending of the roll before the cooler and slow cooling, less finishing and quality control of profiles.

9.3 Rolling of large, medium, small sections and wire rod.

A large grade is rolled on modern mills with a sequential arrangement of stands (Fig. 7.15), less often on linear-type mills, similar to rail and beam mills.

The starting material is blooms and blanks, rolled and continuously cast, of square section with a side of up to 310 mm. Heated in methodical furnaces with an end task and the issuance of workpieces along a roller table, they enter a continuous group (one or two) of several alternating stands with horizontal and vertical arrangement of rolls. Then, the rolls are transferred by schleppers to the second line, where the rolling is carried out in the opposite direction in a group of several successively arranged stands. The distance between adjacent stands exceeds the length of the rolls, and this eliminates the need to comply with the condition of constancy of the second volumes of metal. Therefore, profiles of complex shape can be rolled on such mills.

After the second line, the peals are transferred by schleppers to the third line, from where from the finishing stand to the hot cutting saws and then to the refrigerator. Finished rolled products are cut on cold saws to cut lengths, straightened in RPM, surface defects are removed and packaged for shipment to the finished product warehouse.

All stands of the mill have an individual drive. Each group and stand-alone stands are equipped with tilters.

The productivity of such mills reaches 2 million tons/year.

Medium and small grades are rolled on continuous and semi-continuous types of mills with a sequential arrangement of stands. The technological scheme is similar to that of large grade rolling.

Wire rod is produced on modern wire continuous mills. Heated blanks in front of the mill are welded end-to-end into an endless whip. In a continuous draft group (one or two), rolling is carried out in four threads. Then the flow is split into two intermediate continuous groups of stands (two threads for each), and after them it is again split into four threads, which are rolled in blocks of finishing stands - two or three rolls.

To ensure uniform cooling of the wire rod, it is intensively cooled at the exit from the finishing blocks and placed in coils on a moving conveyor with controlled cooling, after which it is placed in coils weighing up to 2 tons. Then the coils are compacted, tied and sent to the finished product warehouse.

The stands of roughing groups can have a common or individual drive, as well as blocks of finishing stands. Rolling speed on such mills reaches 120 m/sec, productivity - up to 1 million tons/year.

Emergency flying shears are installed in the draft groups, and after the finishing blocks - for cutting to a given mass of rebellion.

9.4 Sheet production technology

9.4.1 Production of hot-rolled sheets and strips. Thick sheets are rolled on specialized thick plate mills (TLS) and broadband hot rolling mills (SHSGP). Sheets with a thickness of 5 to 160 mm or more are rolled sheet by sheet at TLS, strips up to 20 mm thick are rolled at ShSGP, followed by cutting into sheets.

Two- and three-stand TLS with a sequential arrangement of stands are mainly used, for example, mill 3600 MK Azovstal. Continuously cast and rolled slabs up to 350 mm thick and weighing up to 16 tons are used as blanks, and ingots weighing up to 30 tons or more are used for especially thick sheets and slabs. Slabs are heated in method furnaces, and ingots in heating wells or bogie hearth furnaces.

The first stand with vertical or horizontal rolls is used as a scale breaker. The second stand is a draft duo or quarto, more often of a universal type, in which the width is broken down and the slab is reduced in thickness.

After the second stand, especially thick sheets and plates are sent by transfer trolley to the heat treatment and finishing department. To obtain sheets of smaller thickness, the rolls are rolled in the finishing quarto stand, which accounts for approximately 25% of the total reduction.

The removal of scale from the surface of the sheets on all stands is carried out with the help of hydraulic knockers with a water pressure of up to 17 MPa. The stands are equipped with manipulators on the front and rear sides, and roller tables with tapered rollers for turning slabs.

From the finishing stand, the rolls enter the roller hardening machine and then for cooling and finishing. They are cut into sheets of specified dimensions, which are corrected in the RPM, subjected to ultrasonic, visual and other types of control. To improve the service properties, the sheets are subjected to heat treatment (normalization, hardening, etc.).

The capacity of TLS is more than 1 million tons/year.

Hot-rolled strips, including thick ones, are rolled on continuous or semi-continuous SHGP. They produce up to 90% of sheet steel, due to their higher productivity and high technical and economic indicators compared to TLS.

At ShSGP, slabs are used as blanks, which are heated in continuous furnaces (1, Fig. 9.5). The heated slabs are fed through a roller table (2) into a rough scale breaker (3) with horizontal or vertical arrangement of rolls and then into an expansion stand (4), after which a press (5) is sometimes installed to reduce the width of the slab.

After that, the slabs enter the roughing group of sequentially located stands (6, 7, 8), as a rule, of a universal type quarto, and then to the finishing continuous group of stands - quarto (11…16). Flying shears for trimming the front end (9) and a finishing scale breaker (10) are installed in front of it. Removal of scale from the surface of the rolls is carried out with the help of hydraulic threshers.

After the finishing group of stands, the strips are intensively cooled in showering devices and wound on coilers into a roll.

Strip cutting into sheets of specified dimensions is carried out on longitudinal and transverse cutting units. Part of the strips in coils goes to the cold rolling shops (CHP).

Semi-continuous SHSHP are a combination of TLS as a roughing group and a continuous finishing group of stands. Thick sheets are issued from the draft group, and thick and thin strips wound into a roll are issued from the finishing group.

9.4.2 Production of cold-rolled sheet steel. On ShSGP produce strips with a thickness of 0.8 mm or more. Meanwhile, many products require sheets of smaller thicknesses. In addition, hot-rolled sheets have a surface unsuitable for the manufacture of front parts of products. Therefore, coils of hot-rolled strips are sent to the CHP for further rolling.

The technology provides for the following operations: pickling, rolling, surface cleaning, annealing, temper rolling, finishing.

Etching of the strips is carried out in order to remove mill scale from their surface. To do this, use continuous pickling units (NTA) with sulfuric or hydrochloric acid (Fig. 9.6). On guillotine shears (4) cut off the rear end of the previous strip and the front end of the next and weld them into a continuous tape on the butt welding machine (5). The joint is cleaned on the deburring tool (6). These operations are performed on a fixed belt. To ensure the continuity of the pickling process, a loop accumulator (8) is provided, from which the strip continuously enters the pickling baths (10).

In the washing bath (11), the remains of acid solutions are washed off the surface of the strips and dried in the chamber (13). The side edges of the strips are cut on the disk shears (14), then on the shears of the transverse cutting (15) the places of their butt welding are removed and again wound into rolls on the winder (16).

Cold rolling of strips is carried out on single-stand (four- or multi-roll) mills in the reverse rolling mode in several passes or on multi-stand mills from coil to coil. During the rolling process, a cutting fluid (coolant) is intensively supplied to the rolls - a mixture of emulsol with water.

On multi-stand mills, tinplate and thin strips with a thickness of 0.14 mm are rolled, and on single-stand multi-roll mills - the thinnest strip with a thickness of up to 0.002 mm.

To remove hardening, the metal is annealed in bell-type furnaces (rolls) or in continuous annealing units (strip) at a temperature of about 900 0 C. Preliminary, emulsion residues and various contaminants are removed from the surface of the strips in electrolytic cleaning units.

To increase the stampability, the sheets are subjected to training by rolling with a slight reduction - 1 ... 2%.

During the finishing process, the strips are cut into sheets of specified dimensions on slitting and cross-cutting units, straightened, protective and/or decorative coatings are applied, etc.

In addition to the roll method, in recent years, the CCP began to introduce the principles of endless rolling and finishing in continuous units for pickling, rolling, surface cleaning, annealing and skin pass.

There is a transition to a new qualitative stage of development. This is due to many factors: from the creation, implementation and development of advanced technologies, including in steelmaking, to a change in the very concept in relation to rolling production. One of the most important factors in this development in the rolling industry is the ability to exercise absolute control over the temperature-deformation process during rolling on the latest generation of mills. This trend is most pronounced in rolling mills designed for the production of wire rod and small grades. Let's try to assess the reasons for this, taking into account the opportunities provided by the use of new approaches in the technology of wire rod rolling. In the process of hot rolling, high-temperature thermomechanical metal treatment (TMT) takes place. However, TMT, as a rule, is understood not only as the physical essence of the process, but also as a purposeful 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 varieties of thermomechanical processing of steel. They can be divided into the following groups:

• Modes of thermomechanical processing, in which the deformation is carried out 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).
• Thermomechanical processing with deformation during the transformation of supercooled austenite.

Modes of thermomechanical processing associated with the deformation carried out after the transformation of austenite into martensite or bainite. An example of such treatment is the hardening method associated with strain aging of martensite. To harden steel, various combinations of thermomechanical treatment modes can be used, for example, HTMT with LTMT, HTMT with strain aging of martensite, etc. Thermomechanical 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 the final heat treatment, including martensite quenching and tempering. Traditionally, when considering the problem of achieving the required properties in the finished product 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 these processes that influence the formation of structure in the finished product. In practice, technologists used to obtain the necessary mechanical properties; in finished rolled products, only such mechanisms were used in the manufacture of steels as alloying and heat treatment. As an example, let us cite the disadvantages of using traditional methods for manufacturing finished rolled products from ordinary steel grades. In this class of steels, the structure consists of ferrite with a known small fraction of pearlite. If you want to obtain less metal-intensive structures and steel products with increased reliability at low manufacturing costs, the problem arises of increasing the strength of rolled products obtained in a hot-rolled state. If only an increase in the proportion of perlite by increasing the carbon content is used to increase the strength, then this possibility is limited, since with an increase in strength due to an increase in the carbon content, the ductility, toughness and weldability of steel sharply decrease, which leads to the rejection of this rolled product, since along with strength in rolling, it is also necessary to ensure the above 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 the deterioration of the processability of processing (additional cleaning, etc.). Additional heat treatment after rolling, such as quenching + tempering, allows you to get an increase in 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 exploiting the special state of hot-rolled steel obtained during the deformation process was the use of accelerated cooling facilities after rolling, especially the use of water cooling. The use of this technology directly in the rolling lines made it possible to reduce the effect of the full flow of recrystallization processes that previously formed the structure and mechanical properties in the finished rolled 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 to carry out rolling and heating, first of all, the effectiveness of the influence of the chemical composition and heat treatment on the final properties of rolled metal depends. The chemical composition has a great influence on changes in the structure and during TMT, and its effect on 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 on 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 by this structure during the rolling process. Heat treatment outside the rolling mill with the rise in the cost of energy is becoming more and more impractical. A number of modes of thermomechanical processing can provide, along with high strength properties, increased plasticity and toughness. Often, the use of TMT makes it possible to obtain a set of mechanical properties that cannot be achieved by conventional heat treatment and conventional alloying. By changing the deformation conditions during TMT, it is possible to control the density and nature of the 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 a rapid development and interest of manufacturers of metal products in the TMT process. It should be noted that the development of the TMT process in the production of wire rod is promising. This is due to the peculiarities of production and geometric dimensions (high strain rates and a particularly small cross section, unlike other types of metal products obtained by hot rolling). The fact is that only when rolling wire rod for a large grade range is it possible to implement and control the processes of hot work hardening 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 grade, usually austenitic grade steel or low rolling temperatures). This allows you to control the strength properties of hot rolled products, and a high degree of deformation in combination with chemical composition and heat treatment is plastic. Another very important factor from the point of view of thermomechanical processing can be attributed to the features of wire rod rolling - the time between deformations can reach very small values, especially in the last stands, up to 0.0005 s. In order to preserve the structure obtained in the TMT process, the method of performing cooling after rolling is of great importance. In this case, two problems arise: transporting the rolled product to the cooling device and cooling the metal over the entire cross section to ensure the uniformity of the structure, and, consequently, the properties over the cross section of the finished rolled product. A small cross section of wire rod (diameter up to 8 mm) will allow us to consider it as a thermally thin body.

Thus, having obtained the required structure on the rolling mill, we can fix it in 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 intensity of cooling after rolling, it is also possible to achieve a different structure in the cross-sectional layers and obtain certain properties. Since the rate of heat removal in a larger section from the inner layers is limited, it is problematic, and sometimes even impossible, to maintain the advantages of the induced structure during rolling. When conducting an experiment on a rolling mill, the most important point is to take into account the factors most influencing 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 the subsequent assessment of their influence on the structure, such already known data as:
- the effect of temperature and exposure in the furnace on the growth of grain in the workpiece;
- influence of grain size and metal temperature on transformations from austenite;
- change in the structure of hot-worked austenite during post-deformation exposure;
- structure formation at hot
rolling.

To determine the effect of rolling parameters on the structure of hot-worked 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 the intermediate temperatures in the mill line, the following values ​​are determined: strain rate; deformation temperature; time between deformations. In a controlled rolling process, temperature is one of the most important factors in targeting 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 stock. Most often, the first two levers of influence are used to influence the temperature of the roll during rolling. In order to apply inter-stand cooling and heating, an installation is required
additional equipment. In addition, a preliminary assessment of the cooling possibilities is required (at rolling speeds above 30 m/s and an interstand distance of no more than 1 m, the time to provide the necessary heat removal is limited). It is also a big task to know the influence of the temperature fields of rolled products during rolling for a certain grade assortment on the structure of the metal, in particular
for 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 energy and power parameters of the rolling mill, the forces acting on the rolls (washers) and other details of the working stands, the accuracy of the profile dimensions, the shape and quality of the surface of the finished rolled product, the durability of the rolling rolls, and the stability of the entire technological process depend on the thermal regime. At the same time, it is directly related to the modes of compression, speeds and tensions. Most rolling mills do not directly measure the temperature of the intermediate roll over the entire length of the mill. This is due to both the high cost of the installation and the operating conditions of the instruments, which often does not allow accurate determination of the temperature of the metal, and can lead to breakage of the measuring equipment in case of an emergency deviation of the metal from the rolling line. Also, when using interdeformation cooling, even the determination of the surface temperature of the roll does not give an accurate picture of the mass-average temperature of the metal, which, in turn, is the most significant for evaluating 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 heat balance, which depends on all types of heat transfer that take place during hot rolling: heat loss by thermal conduction in contact with washers and water cooling, convection and radiation. The biggest problem in determining the heat transfer during rolling is to establish the patterns of temperature changes at any point of the roll 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 addition, each of the types of heat transfer makes its own contribution, which is not always possible to accurately determine. The 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 peculiar only to this site. The resulting effect of complex heat transfer depends not only on the intensity of specific types of transfer, but also on the features of their interaction (serial or parallel, stationary or non-stationary). In contrast to the stationary regime, in which the temperature field does not change with time, the thermal rolling process is characterized as non-stationary. In this case, the temperature field of the roll is a function of time. A non-stationary process is associated with a change in enthalpy with time. In this case, the intensity of heat removal is not constant in time. Solving the problem of non-stationary heat conduction means finding the dependences of temperature changes and the amount of heat transferred over time for
any point on the body. Each of the processes of unsteady heat transfer is described by a system of differential equations. However, these equations describe an innumerable set of 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 a metal during rolling, it is necessary to consider the flowing heat at each stage and give a complete mathematical description of all the particular features inherent in 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 roll.
- Boundary conditions characterizing the features of the process
at the edges of the body.
- Temporal conditions characterizing the features of the process
in time.

The solution of this system of equations will make it possible to obtain a description of the temperature field of the rolled product in any section of the rolling mill at any time. This problem of determining the temperature fields along the cross section of the roll at any moment of rolling was solved for the fine-section wire mill 300 No3 of OJSC MMK. As an example
shows a diagram in Figure 1 of the temperature distribution over the cross section
intermediate roll. Using the results of this model made it possible to evaluate the existing temperature-deformation regime
rolling, and by changing the main factors of rolling - to predict and obtain the required mode from the standpoint of the formation of 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 at mill 250#2 using a temperature-deformation model and a newly installed water cooling unit. Installation in 2004 of a new water cooling line at mill 250#2 (manufactured by NPP Inzhmet) made it possible to carry out experimental studies in order to obtain thermomechanically hardened reinforcement of small diameters. Obtaining thermomechanically hardened reinforcement on the 250No2 mill consisted in carrying out the process of hardening the surface layer of the wire rod in the water cooling line located after the finishing stand No16 in the flow of the rolling mill. Further, the rolled stock is placed by a coiler in the form of coils on a mesh conveyor, after which it is collected on a coil collector into riots weighing up to 300 kg. Cooling is carried out with the help of a high pressure nozzle and in successive 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.
Thermomechanical hardening of wire rod can be divided into three stages. At the first stage, wire rod leaving the finishing stand No. 16 enters the heat-strengthening line, where it is subjected to intensive cooling with water. This process should ensure that the surface of the wire rod is cooled at a rate exceeding the critical cooling rate necessary to obtain a martensite structure in the surface layer of the wire rod. However, in this case, the technology of the heat hardening process should provide such 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 the second stage, which will allow, upon its further cooling at a rate of a lower critical rate, 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 contribute to the self-tempering 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 the central layer has an intermediate cooling rate, which leads to a layer with a bainitic structure. As a result of such cooling, it turns out that the wire rod in cross section consists of two zones in the form of a ring: with a martensitic and bainitic structure and a ferrite-pearlite in the central
parts. As a result of experimental rolling on the 250#2 mill, wire rod with the indicated structure was obtained (Fig. 3).
Investigation of the structure of thin sections of thermomechanically hardened wire rod
showed in the resulting rolled products, as a rule, the presence of one or more hardened crescent-shaped layers. This, apparently, is due to the fact that cooling is carried out by only one nozzle in one cooling cycle. Under such conditions, if a situation arises of “accidental” washing of any one area of ​​rolled metal in a single cooling chamber, there is no possibility of further cooling cycles that would allow more uniform cooling of the wire rod over the section. Further cooling of the wire rod on a mesh conveyor without directional air blowing also leads to an uneven temperature field both over the cross section and along the length of the rod coil. Also from the experience of
rolling revealed a change in the temperature of the wire rod after water cooling along the length of the coil (temperature change for 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.

The measurement of the billet temperature at the exit from the furnace and after the roughing group (the temperature change was ∆T=50–80 °C) subsequently confirmed this assumption. The factors listed above ultimately lead to a large non-uniformity of the structural components along the length of the rolled product, which directly causes a significant spread (up to 50–80 N/mm2) of mechanical properties within the batch. Such a structure in wire rod 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 even on wire rod from some low-alloy steel grades with standard rolling and cooling in air (Fig. 4). Obtaining the above wire rod requires precise adherence to the heat strengthening technology. The setting of the water cooling line depends on many factors: the steel grade, the required mechanical properties, the diameter of the wire rod, the composition of the equipment of the cooling line, the setting of the high pressure nozzle, the rolling speed, the flow rate and pressure of water (Fig. 5).
To determine the technological parameters, depending on the listed factors, experimental studies were carried out with the measurement of the self-tempering temperature. Samples were taken from wire rod coils obtained during experimental rolling 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. At the same time, 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 the optimal technological regime that satisfies the needs of consumers. One of the most promising applications for thermomechanical
hardened reinforcement of small diameters is to use it for
ligaments of reinforcing cage in high-strength reinforced concrete slabs. The scope of this reinforcement may in the future be other various 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 research allowed to determine the main parameters of the process of thermomechanical hardening of wire rod of small diameters. Subsequently, during the start-up of mill 170 at OJSC MMK, after adapting the results obtained to the rolling conditions at the new mill, this range will be mastered in mass production.
FINDINGS
- Considered the processes occurring during the deformation of the metal in a hot state. The factors most influencing 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: a particularly small cross section and high strain rates, unlike other types of metal products obtained by hot rolling.
- The results of using such a tool as temperature modeling in order to obtain the necessary mechanical properties of wire rod during hot rolling are shown, taking into account the existing technological capabilities of the mill, as well as from the point of view of the effect of hot plastic deformation and chemical composition on the structure.
- The results of the application of the use of thermomechanical treatment during rolling on the structure of the finished wire rod are given.

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Details Category: Long products

Long products

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

Rental Profile (its cross-sectional shape) depends on the shape of the rolls. The figures show the main profiles of rolling products, called grade rental.

There are the following profiles long products: simple (circle, square, hexagon, stripe, leaf); shaped (rail, beam, channel, taurus and etc.); special (wheels, reinforcing steel and etc.).

Most often, long products are used as blanks for various parts. For example, from hex bar make bolts and nuts. From round bars turning cylindrical parts on lathes. Angle bars used in the production of frames, frames, racks, etc.

Rolling can give the workpiece the shape of a finished part, thereby avoiding additional processing and, consequently, reducing metal waste and saving time.

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

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

Types and properties of steel

Steel- This alloy of iron and carbon(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 carbonaceous and alloyed steel. Carbon steel contains 0.4...2% carbon. Carbon gives steel hardness, but increases brittleness, reduces ductility. When added to steel during melting of other elements: chromium, nickel, vanadium and others - its properties change. Some elements increase hardness, strength, others - elasticity, others give anti-corrosion, heat resistance, etc. Steels that contain these elements are called alloyed. In alloy steel grades, additives are denoted by letters: H - nickel , AT - tungsten ,G - manganese , D - copper , To - cobalt , T - titanium .

Distinguish according to purpose structural, instrumental and special become.
Structural carbon steel is of ordinary quality and high-quality. First- plastic, but has low strength. It is used for the manufacture of rivets, washers, bolts, nuts, soft wire, nails. Second differs in the increased durability. Shafts, pulleys, lead screws, gears are made from it.
Tool steel has greater hardness, strength than structural, and is used for the manufacture of chisels, hammers, thread-cutting tools, drills, 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. O, Art. one etc. - the higher the steel number, the higher the carbon content and tensile strength), quality - two digits 05 , 08 , 10 etc., showing the carbon content in hundredths of a percent. According to the reference book, you can determine the chemical composition of steel and its properties.
The properties of steel can be changed by thermal action - heat treatment (heat treatment). It consists in 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.
The main types of heat treatment - hardening, tempering, annealing, normalizing .
Used to increase the hardness of steel hardening - heating the metal to a certain temperature (for example, up to 800 ° C) and rapid cooling in water, oil or other liquids.
With significant heating and rapid cooling, steel becomes hard and brittle. Brittleness after hardening can be reduced by holidays - the cooled hardened steel part is again heated to a certain temperature (for example, 200 ... 300 ° C), and then cooled in air.
For some tools, 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 better machined.
Normalization - kind of annealing, only cooling occurs in air. This type of heat treatment improves the strength of the steel.

Heat treatment of steel in industrial plants is carried out thermal workers. The 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 the hardness steel is tested using special hardness testers. The measurement method is based on the indentation of a harder material into the sample: a hard steel ball, a diamond cone or a diamond pyramid.

Hardness value HB is determined by dividing the load by the surface area of ​​the imprint left in the metal ( Brinell method ) (Fig. right, a),

or according to the depth of immersion in the metal of the diamond point, steel ball ( Rockwell method ) (rice. 6 ).

Strength steels are determined on tensile testing machines by testing samples of a special shape, stretching them in the longitudinal direction until they break (fig. on the left). To determine the strength, divide the maximum load that preceded the rupture of the specimen by the area of ​​its original cross section.

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

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

During rolling, the temperature of the beginning and end of the process, the reduction mode, and the adjustment of the rolls are controlled as a result of monitoring the dimensions and shape of the rolled products. To control the condition of the surface of the rolled products, samples are taken regularly.

Finishing of rolled products includes cutting to length, straightening, removal of surface defects, etc. 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 metal deformation in rotating rolls and for performing auxiliary operations (transportation, control, etc.).

Equipment for metal deformation is called the main one and is located on the main line of the rolling mill (lines of working stands).

Figure 1 - Scheme of a rolling mill

1 - rolling rolls; 2 - plate; 3 - club spindle; 4 - universal spindle; 5 - working stand; 6 - gear cage; 7 - clutch; 8 - reducer; 9 - engine

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

rolling stand

Rolls 1 are installed in the working stand 5, which takes 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 flat products) of the barrel. Depending on the number and location of 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.

Back-up rolls are installed in the four-roll stands (Figure 2, position c), which allow the use of work rolls of small diameter, thereby increasing the draft and reducing 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 stand 6 is designed to distribute the 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 rolls.

Spindles are designed to transmit torque from the gear stand to rolling rolls with misalignment up to 10…12 0 . With a slight movement in the vertical plane, club type 3 spindles are used complete with a club clutch. The inner outlines of the 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 skewed up to 10 ... 12 0 .

Figure 2 - Rolling stands

As the engine of the rolling mill 9, DC and current motors are used, the type and power depend on the performance of the mill.

Reducer 8 is used to change the number of revolutions when transferring motion from the engine to the rolls. Gears are usually chevron with a helix inclination of 30 0 .

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

Metal heating is carried out in flame and electric furnaces. According to the temperature distribution, furnaces can be and . In chamber furnaces of periodic heating, the temperature is the same throughout the working space. In methodical furnaces, the temperature of the working space is constantly increasing from the place of loading of blanks to the place of their unloading. The metal is heated gradually, methodically. Furnaces are characterized by high productivity. They are used in rolling and forging and stamping shops for heating non-ferrous ingots. Large ingots are heated before rolling in - a variety of chamber, flame furnaces.

As transport devices in rolling production use:

• ingot carriers and various types of carts for supplying ingots and blanks from heating devices to the mill;
• roller tables - the main vehicle of rolling shops (with rotating rollers installed in series, they provide longitudinal movement of the metal; with an oblique arrangement of the rollers, there is a possibility of transverse movement of the strip);
• manipulators designed for the correct task of the strip into the caliber;
• tilters designed to rotate the workpiece around a horizontal axis.