Tkachenko I. Development of scientific and methodological bases of forecasting and optimization for compositions and heat treatment technologies of multi-component alloy steels

Research area- structural states and performance properties of thermally strengthened multi-component structural steels for demanding applications. Research object –influence of chemical compositions and heat treatment on structures, the combination of properties and its stability for low-carbon multi-alloyed structural steels. Research methods – the combined methodological approach is used, which includes theoretical, experimental and computer experiments methods. The main theoretical methods are: thermodynamic models of regular and quasi-regular solutions; kinetic theory; linear elastic theory; models for dislocation substructure and its components interactions with solute atoms. The main experimental methods are: optical metallography in polarized and nonpolarized light; standard mechanical tests; serial impact tests; transmission electron microscopy; scanning electron fractography; electron microdifraction; X-ray phase quantitative analysis; dilatometric analysis. Computer experiments were conducted using complex computer technology which includes: primary statistical treatment of experimental data; "Data Mining" techniques; multiple regression models development; Monte-Carlo simulations. Computer investigations were carried out within the frame of softwares See-5-demo, WizWhy-3.01-demo, SciLab 4.1, MuPad. The dissertation gives a solution of the scientific and technical problem of reaching stably high combination of operating properties for multi-component thermally strengthened alloy steels based on a proposed systematic approach, which includes: a priori forecasting their structure and properties; prevention of embrittlement; multi-purpose optimization of production technologies parameters; the use of complexes of preliminary and final heat treatment technologies. A combined computer technology is developed, which provides realization of the proposed approach. By means of thermodynamics and kinetic theory: possibility of carbon stratification of supercooled austenite is shown; a combined diagram of Bainitic and Martensitic transformations is developed; causes of the austenite stabilization to Bainitic and Martensitic transformations due to alloying are revealed; chemical elements able to form clusters and segregations are determined; a new model for computer forecasting the influence of carbon and alloying elements on kinetics of supercooled austenite decomposition is developed and executed. A new quantitative model for the hydrogen embrittlement and flakes formation in steels is developed and used for optimization of the industrial technologies. Using the offered computer technology, optimum methods are developed for reaching an assured quality level of thick sheets made of different strength level low carbon steels due to the multy-purpose optimization of chemical compositions, forming optimum initial microstructures before the final thermal strengthening. Developed and applied in «Azovstal» Iron and Steel Works new heat treatment technologies provide upgrading the quality and production efficiencyfor: high weight sheet electroflux ingots; thick sheets made of steels of different strength levels. The annual general real economic profit due to industrial application of the dissertation results is more than 4 mln.hrn.