Теоретичні основи формування субструктури переохолодженого аустеніту та механічних властивостей мікролегованих будівельних сталей

Abstract

UK: Дисертація присвячена вирішенню науково-прикладної проблеми отримання в металопрокаті для зварних будівельних конструкцій підвищеного рівня міцністних характеристик при збереженні пластичних та в’язкісних властивостей, шляхом розробки удосконаленої технологічної схеми виробництва високоміцного прокату з низьковуглецевих, низько-, мікро- та нелегованих сталей. Проблему вирішено шляхом виявлення загальних закономірностей формування в області дифузійного механізму розпаду переохолодженого аустеніту нанорозмірних структурних елементів та особливостей їх впливу на підвищення механічних властивостей будівельного металопрокату відповідального призначення. Використаний в роботі комплексний підхід щодо з’ясування принципів формування тонкої структури сталей в області дифузійного розпаду аустеніту (фериту та перліту), дозволив розробити технологічну схему безперервної контрольованої прокатки низьковуглецевих низько-, мікро та нелегованих сталей, яка дозволяє розкрити резервні можливості вітчизняних технологій виробництва та вдосконалити структуру і властивості високоміцного товстолистового прокату, що задовольняє вимогам сучасної вітчизняної будівельної індустрії.

EN: The work deals with solving scientific-applied problem of getting in metal-roll for welding building constructions of increased level of operating capacity by means of elaboration of the improved technological production scheme of high-strength rolled metal from low-carbon low-, micro- and non-alloyed steels. The problem is solved by means of the detection of general regularities of nanoscale structural elements formation in the area of diffusive mechanism of supercooled austenite decay and the peculiarities of their influence on the increase in mechanic properties of building metal-roll from low-carbon steels. It is stated that ductile deformation starts from the places with the enlarged level of free energy and spreads along crystallographic areas that make the smallest angle with the direction of maximal tangential stress from outer loading. It is shown that ductile deformation creates periodically inhomogeneous hardened environment, which will define the direction of crack growth. Under the conditions of loading, which happen in standard tensile tests, the resistance of material detachment is overcome in structural components with a lower resistance of ductile deformation, i.e. in ferrite because during the process of ductile deformation, residual strengths appear which gradually grow to reach the boundary state. Such concentration of strengths is observed close to grain boundaries. At the even strength spreading in the critical cross section of the steel part, ductile deformation can develop almost simultaneously in a number of ferrite grains. At the monotonous increase in loading a great number of micro cracks appear, which eventually unite, making the fracture surface with a large number of imperfections and the change of the direction from grain to grain. Grain boundaries create a net that evens the separation of the deformations among the grains of poly-crystal structure. The models of initiation and spreading the destruction in steels with ferrite-perlite structure are perfected. It is found out that the initiation of viscous destruction can be described by the activity of three mechanisms: the initial stage – the formation of dislocation loops near the part (Broek`s model); the next stage of pore growth happens owing to the accumulation of dislocations. Herewith, this stage occurs under the activity of both strength components normal and sliding. The urgency of the work is conditioned by the search for new additional opportunities to improve the structure and the properties of metal-roll from low-carbon low-, micro- and non-alloyed steels. The elaboration of new and improvement of the existing technologies of production of high-strength plate metal-roll that satisfies the modern demands of building industry was also of primary importance. The peculiarities of structure-creation processes are analyzed, which happen in metal-roll with low-carbon low-, micro- and non-alloyed steels during the use of the most wide-spread technological production schemes: hot and controlled rolling. It is found out that the effective and stable deformation influence on the steel structure is determined by the presence and the size of three temperature intervals limited by critical points of transformation and the temperatures of recrystallization of the deformed austenite. With the application of mathematical apparatus of the theory of experimental data array processing, the interconnection between the technological parameters of production schemes of low-carbon low-, micro- and non-alloyed steels (hot rolling, controlled rolling) 41 and the main mechanic metal-roll properties is quantitatively analyzed. Hereby, it is shown that the change of the temperature interval of finish rolling leads to the change of strength characteristics (for the technological scheme of controlled rolling). Ductile peculiarities remain constant. With the application of mathematic apparatus of one-parameter (for hot rolling) and multiparmetry regression analysis (for controlled rolling), mathematic models of interconnection between mechanic characteristics and temperatures of the beginning and the end of finish rolling are created. The analysis of the models showed that the increase in the temperature of the beginning of finish rolling (for controlled rolling) would lead to the increase in the level of mechanic characteristics, particularly strength. For the technological scheme of hot rolling, widening of temperature range of hot deformation will have a positive influence on the properties complex. On the basis of the obtained results, temperature-deformation regime of hot rolling in intercritical temperature interval is theoretically grounded, which leads to the conservation of the dislocation austenite substructure to the lowest boundary of intercritical temperature interval. As a result, the deformation in intercritical temperature interval will enable to obtain a fine structure of the deformed ferrite and perlite, the peculiarity of which is the absence of ferrite-perlite striation. This conception is realized for low-, micro- and almost non-alloyed steels. Thus, the analysis of the research results proved the possibility to correct the processes of the formation of structure and substructure components by means of constant deformation in intercritical temperature interval. Herewith, the decrease in the temperature of the end of hot rolling to the lowest boundary of intercritical interval will make it possible to improve further and stabilize the complex of mechanic characteristics of rolling for building constructions. The conservation of a stable polygonal structure during the deformation in the intercritical temperature interval leads to the formation of a developed ferrite-perlite structure with a certain type of cementite carcass of the perlite component. The complex approach, applied in the work, as for the clarification of principles of thin structure steel formation in the area of diffusive decay of austenite (ferrite and perlite) made it possible to work out the technological scheme of rolling production with lowcarbon low-, micro- and non-alloyed steels, which enables to reveal the backup capabilities of domestic production technologies and to improve the structure and properties of high-strength roll from low-carbon steels the way to meet the demands of modern domestic building industry. The obtained results can be classified as the solution to the important scientific-applied problem, which has a significant importance in terms of rolling improvement with low-carbon low-, micro- and non-alloyed steels produced by domestic enterprises.