Features of formation and distribution of pearlite disperstion degree in C82D steel (EN ISO 16120-2:2017) billet and after its hot plastic deformation at different stages of rolling, were established. The structure is a lamellar pearlite with different degrees of dispersity, regardless of the technological area of samples selection. The interplate distance of pearlite increases and reaches the largest values in the center with the approach to the axial rolling zone. The hot plastic deformation should be completed at a controlled temperature of the metal output at the last stage of rolling. Here stage of water cooling was excluded what reduces the temperature gradient between the surface and the center rolling.

The main way to improve the quality characteristics of the steel is a properly selected heat treatment, the purpose of which is to create a structure that would meet the requirements of the manufacturer. The size of the austenite grain in the steel depends significantly on the heating temperature during heat treatment, which in turn affects the final structure of the steel and mechanical properties after treatment. The experimental chromium molybdenum vanadium steel 31CrMoV9 (EN 10085:2001) was heated to temperatures in the range of 850...1050°C and cooled in water and air. The structure formation of the Cr-Mo-V steel after heating to different temperatures, quenching and tempering, consists of tempered martensite (including residual austenite) and alloyed carbides, and after normalization - bainite and martensite with different ratios. As the normalization temperature increases, the total area occupied by martensite increases without changing its morphology. The increase in the heating temperature leads to an increase in the amount of martensite from 10% at 850°С to 50% at 1050°С, correspondingly, the microhardness of the steel increases, shown by research. With an increase in the tempering temperature, the average microhardness of steel decreases. The change in microhardness is probably associated with an increase in the amount of the residual austenite and the dissolution of alloyed carbides. Detected that with an increase in the heating temperature, the increase in the initial austenite grain led to the coarsening of martensite needles. The effect of the increasing the austenization temperature on the change in the structure of the Cr-Mo-V steel was shown during investigations. Detected that the temperature of the austenization affects the cooling rate. To obtain more detailed results of this effect, it is necessary to conduct additional investigations of the kinetics of the transformations in the Cr-Mo-V steel upon cooling from different temperatures. The obtained results make it possible to tentatively predict the final structure of the Cr-Mo-V steel after various modes of the heat treatment. Heating should be carried out at a temperature of 850ºС, which will additionally lead to resource saving to get a uniform structure in the Cr-Mo-V steel.

The purpose of the work was to research ways to reduce the duration of the intermediate heat treatment mode of alloy steels while ensuring the required level of properties and reducing energy costs. For further mechanical processing of alloy rolled metal, it is necessary to obtain the original rolled product with a structure that has high plasticity and low hardness. In order to achieve the required mechanical properties, a special softening heat treatment is carried out - annealing at subcritical temperatures (A1 - 25...40 °С) with a long exposure. Thanks to annealing, structural transformations take place and the resulting structure has a granular morphology. Research has established that in steels with a mixed structure during annealing, structural transformations begin with thermodynamically less stable structures. For medium-carbon steels alloyed with such elements as chromium, molybdenum, vanadium, increasing the cooling rate after hot rolling by ~1.5 times makes it possible to obtain a structure that is more susceptible to annealing. Thus, for chrome-molybdenum steel, the structure consists of at least 80% bainite, ~15% ferrite and 5% pearlite. For chromium molybdenum vanadium steel, the basis of the structure is bainite (~90%), martensite (~5%) and ferrite (up to 5%). In the resulting structures, upon subsequent annealing, structural transformations occur at reduced temperatures and require a shorter exposure time. The values of hardness and strength of the studied steels processed under the reduced regime decrease by 15-30%, and the plasticity indicators increase by ~50%. Obtaining such a structure after rolling makes it possible to speed up the intermediate thermal softening treatment, thereby reducing energy costs, and ensuring high plasticity of steel before mechanical processing (stamping, forging, etc.)

Invistigations are directed to detection of the metallurgical reasons of the breakage of alloy steels during production of the small diameter wire. Several batches of the coiled bar(s) of low carbon silicon-manganese steels of the Sv-08G2S and G3Si1 were taken as materials for invistigation. The main influence on the metal breakage during drawing is exerted by its structure and mechanical characteristics, it is known. The initial structure of the coiled bar(s) before drawing to small diameters from the differents smelting within one sort was not differ and represented globular pearlite for Sv-08G2S steel, ferrite and pearlite for G3Si1 steel. The influence of equipment and drawing technology on the wire breakage is minimal, as the processing is performed with the same conditnions. Special attention in the invistigations was paid to the contamination of steel with nonmetallic inclusions during the drawing. Analysis is showed in the wire of low carbon silicon-manganese steels the presence of the point oxides, non-deformable silicates and sulfides, and fragile inclusions, which are located along the rolling direction. The feature of the wire, which has observed the breakage during the production, is the presence of the large quantity of the line inclusions (3...5 points), which are located in some cases over the entire surface of the polished section of the Sv-08G2S steel. The structure of all investigated steels does not differ, the mechanical characteristics of the wire with breakage corresponded to the required standard values. It is shown that contamination of the original wire rod with (more than 2 points) of nonmetallic inclusions are significantly increases the chance of difficulties arising during production of the wire with a diameter of 1,2...0,8 mm and can lead to breaks. The high purity of steel in terms of nonmetallic inclusions guarantees the absence of breakage due to this defect during the production of wire, which contributes to increase of the performance and decrease in the cost of finished products.

It is necessary to obtain initial rolled product with a structure which has high ductility and low hardness for further mechanical processing of alloyed rolled metal products into the ready product. In this case the enterprises carry out a special softening heat treatment to increase the ductility of the metal. The treatment represents the annealing at subcritical temperatures (А1 — 25…40 °С) with long holding. During annealing the structural transformations are running and the formed structure has a granular morphology. To obtain spheroidizing in the steels with a mixed structure (contains at the same time pearlite, bainite or martensite) usually requires a lot of time and energy, because the structural transformations during heat treatment are running with different intensities in different phases. Established by study, that in the steels with a mixed structure during annealing the structural transformations begin from the thermodynamically less stable structures. So, to reduce the mode of the softening heat treatment (annealing) of the investigated steels it is rationally to change the cooling conditions after rolling and to provide the increased quantity of bainite and martensite. For medium-carbon steels alloyed by chromium, molybdenum and vanadium and also for low-carbon steel alloyed by manganese and silicon, an increase of the cooling rate after hot rolling by ~1.5 times allows to obtain more sensitive structure to annealing. So, the structure consists at least 80 % of the bainite, ~15 % of the ferrite and 5 % of the pearlite for chromium-molybdenum steel. The base of the structure contains bainite (~90 %), martensite (~5 %) and ferrite (up to 5 %) for chromium-molybdenum-vanadium steel. For low-carbon silicon-manganese steel an increase of the cooling rate leads to the formation of the structure which contains at least 35 % of the bainite-martensite, ~60 % of the ferrite and 5 % of the pearlite. The structural transformations proceed at the low temperatures and require a shorter duration of the holding in the obtained structures during further annealing. Receiving such structure after rolling, allows to speed up intermediate softening heat treatment, thereby reducing energy costs, and provides high ductility of steel before mechanical processing.

Rebars and coiled bar have a tendency to softening over time. After holding at the room temperature the ductility of the rolled products can be partially restored due to the processes of return and relaxation and also deposition of the diffusion-movable hydrogen from the steel. The reverse hydrogen brittleness and ageing are often observed, during production of the thermally strengthening rolled products. The variability of the mechanical properties of rebars and coiled bar of 25G2S, С70D and С82D steels with different diameters has been studied. It has been shown that tendency to softening is decreases during increasing of the strength class of the rebars (steel 25G2S) and it is related to deposition of the hydrogen in different quantities. Dynamics of increase of specific elongation for rebars turns out to be maximum in the first 30 days and growing rapidly (till 4,0…4,5 % abs.) over 90...120 days, then stabilizes and practically does not change over time. After use of the accelerated cooling at the final stage of the deformation heat treatment of the coiled bar of С70D high carbon steel the variability of specific elongation and reduction of area are characterized by a significant increase of ductility indexes within 3 days after primary testing. By research of variability of mechanical properties of coiled bar of С82D steel was established that during initial testing of samples (immediately after rolling) were recorded a low values of specific elongation and reduction of area (7,6…8,4 % and 15…24 % respectively) and while the progress of embrittlement have a tends to growth with increasing diameter. After holding of the coiled bar within 48...72 hours the parameters of the ductility for all diameters of rolled products are grow to 30...34% and stabilize over time. It has been shown that variability of the mechanical properties have a direct relations with the parameters of the structure, the initial hydrogen content in the steel and with degree of deformation processing of blank continuous casting.

Based on mathematical and physicochemical modeling a studied the effect of alloying elements (chromium, molybdenum, vanadium) in the formation of the mechanical properties of steel. The intervals of the content of alloying elements that ensure the fulfillment of the required norms are determined. Dependences of the change in the mechanical properties of doped rolled steel from the index of the structural state d are constructed.

The aim of the work is to determine the content intervals of alloying elements in structural alloyed steels, which ensure the obtaining of mechanical properties and the conformity of rolled products to the requirements of European standards. The studies were conducted using a predictive model developed by the Iron and Steel Institute of the National Academy of Sciences of Ukraine, taking into account the full chemical composition of the steel. The regularities of changes in the interatomic interaction parameter on the number of alloying elements in the steel composition and its relationship with mechanical properties are revealed. The dependences of mechanical properties (tensile strength, relative elongation) on the chemical composition of steel are constructed through the physicochemical criterion – the average statistical distance between interacting atoms (structural parameter d). The interrelation between the chemical composition and mechanical properties of chrome-molybdenum structural steels has been established. It is shown that increasing the chromium content increases the tensile strength, and doping with molybdenum and vanadium increases the ductility of rolled products. It was determined that in order to guarantee compliance with the requirements of the ultimate strength (900-1100 MPa) and relative elongation (> 11%) for steel 31CrMoV9, the content of alloying elements should correspond to the following intervals: 2.42-2.62%Cr, 0.2-0, 23%Mo and 0.17-0.20%V. The results obtained make it possible to predict the mechanical properties of doped steel, depending on the actual chemical composition of the steel.

Influence of austenitization temperature of chrome-molybdenum-vanadium steel on structure formation at the softening heat treatment is studied. It is shown that the decline of the austenitization temperature promotes to reduce the micro-hardness values due to the intensification of spheroidizing of pearlite after the overcooling and high tempering. Increasing the austenitization temperature leads to formation of an uneven structure after tempering.

The impact of the austenite grain growth of chromo-molybdenum steel on the kinetics of structural decomposition under continuous cooling was studied. It is shown that when the grain is enlarged with an increase in the austenitization temperature from 850 till 1050 °C, the stability of the supercooled austenite in the structure of the chromo-molybdenum steel during the decay process by the intermediate and shear mechanism increases, which contributes to a decrease in the critical quenching rate.

Influence of austenite grain size of chrome-molybdenum-vanadium steel on formation of structure at continuous cooling has been studied. It is shown, that at the coarsen of grain with increasing temperature of austenitization from 850 till 1050°С stability of the supercooled austenite in structure of chrome-molybdenum-vanadium steel is increased, that is promoted decrease in critical quenching rate.

Influence of austenite grain size of chrome-molybdenum-vanadium steel on formation of structure at continuous cooling has been studied. It is shown, that at the coarsen of grain with increasing temperature of austenitization from 850 till 1050 ° С stability of the supercooled austenite in structure of chrome-molybdenum-vanadium steel is increased, that is promoted decrease in critical quenching rate.

Austenitizing studied the effect of temperature on austenite grain size of the chrome-molybdenum-vanadium steel. It is shown that at austenitizing 850–1050ºS in the structure of chrome-molybdenum-vanadium steel nominal diameter of austenite grain varies from 0,063 to 0,084 mm.

It is shown that after thermo-mechanical processing of chrome-molybdenic rolled metal including cooling with a speed of 0,6-0,8 ° C / sec, the quantity of bainite increases to 75% and the amount of lamellar pearlite decreases to 5-10% in structure that promotes reduction of the mode of subsequent softening processing. It is established that at cooling with a specified speed the transformations in chrome-molybdenic steel come to an end at temperatures of 350-370 ° C. The offered technology of the reduced mode of thermomechanical processing provides necessary quality of rolled metal and promotes decrease of energy resources consumption.

On the basis of the drawn card of the hardness values distribution according to the temperature and time parameters of heat treatment of chrome-molybdenic steel it is revealed that required values of hardness are reached at 680–650 °C within 4 hours. It is shown that decrease of hardness occurs due to large quantity of carbides which have changed their morphology.

It is shown that after thermo-mechanical processing of chrome-molybdenic rolled metal including cooling with a speed of 0,6-0,8 ° C / sec, the quantity of bainite increases to 75% and the amount of lamellar pearlite decreases to 5-10% in structure that promotes reduction of the mode of subsequent softening processing. It is established that at cooling with a specified speed the transformations in chrome-molybdenic steel come to an end at temperatures of 350-370 ° C. The offered technology of the reduced mode of thermomechanical processing provides necessary quality of rolled metal and promotes decrease of energy resources consumption.

Peculiarities of formation and distribution of pearlitic structure of steel 80K in a blank and after deformation in various areas of wire mill (in semi-finished rolled products and rolled wire) are established. It is shown that irregularity of temperature field by the section of semi-finished rolled products influences on processes of structure formation. It is shown by researches that the increase of deformation degree and the subsequent controlled accelerated cooling in the line of wire mill 150 leads to increase in uniformity of pearlitic structure and its dispersion.

Peculiarities of formation and distribution of pearlitic structure of steel 80K in a blank and after deformation in various areas of wire mill (in semi-finished rolled products and rolled wire) are established. It is shown that irregularity of temperature field by the section of semi-finished rolled products influences on processes of structure formation. It is shown by researches that the increase of deformation degree and the subsequent controlled accelerated cooling in the line of wire mill 150 leads to increase in uniformity of pearlitic structure and its dispersion.