Stavrovska V. Increase of the mechanical properties of microalloyed steel by improving the chemical composition and the cooling mode from the rolling heating

The work’s aim is increasing of complex of mechanical properties or plates from the steel S355 by using of heat treatment and thermo-mechanical rolling, optimization f it`s chemical composition, determination of optimum parameters of thermo-mechanical rolling. The research’s object is processes of phase transformation in low-alloyed structural steels at heat treatment and thermo-mechanical rolling. The research’s subject is the formation of the microstructure and mechanical properties of plates from structural V-Nb-Al micro-alloyed steel at treating with using of the heat from rolling heating. The research methods: optical microscopy, scanning electron and transmission electron microscopy, dilatometric, heating analysis methods; test on mechanical properties, methods of the math statistics and regression analisis. The thesis is devoted of the problem of increasing the mechanical properties of plates of S355 by optimization of its chemical composition and parameters of heat treatment. Regression models of the influence of chemical elements, plate thickness, test temperature on the mechanical properties of normalized steel with a composition close to S355 steel have been developed. The efficiency of using energy-saving technologies of thermal and thermo-mechanical treatment of S355 steel from rolling heating in exchange of normalization from furnace heating is shown. The optimal temperature of the end of deformation at normalizing rolling (830-850 оС) is found. The contribution of structural strengthening mechanisms to the total strength after the implementation of various schemes of thermal deformation treatment with rolling heating has been determined; the increasing contribution of the dislocation mechanism after controlled rolling with accelerated cooling to 460-530 оC is shown. The possibility of replacement of furnace heating of steel with the accelerated cooling by water from rolling heating is proved, which ensures full compliance with the strength class S690QL.