Thesis for a Doctor's degree in technical sciences in speciality 05.02.01 «Material science» (13 – Mechanical engineering). – State Higher Educational Institution of «Prydniprovska State Academia of Civil Engineering and Architecture» Ministry of education and science of Ukraine, Dnipro, 2020.
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 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.
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 low-carbon 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.
Key words: nanoscale elements, quasi-eutectoid, cementite carcass, polygonal and special boundaries, ductile deformation, mathematical modelling, continuous controlled rolling.
