Řízení vlastností tekuté oceli před jejím plynulým odléváním

Abstract

Based on literature analyze as well as analyze of experimental heats results, submitted dissertation analyze basic knowledge from the area of non metallic inclusions modification in liquid steel before its continuous casting. First part of the dissertation analyzes micro cleanness of the steel from the non metallic inclusions which are present in the steel point of view. Main focus is given to oxide and sulfide inclusions. Morphology, types and shapes are especially discussed. Second part of the dissertation is focused on the concrete questions regarding modification of Al2O3 inclusions by the calcium to liquid globular calcium aluminates xCaO.yAl2O3 and issues with nozzle clogging of the tundish submerged nozzles during continuous casting of the steel. Especially three basic elements were discussed which influence the nozzle clogging – calcium, aluminum and sulfur and their effect on the technological steel properties. Further dissertation analyzes possible types of Al2O3 inclusions modified by calcium with respect to sulfur content in the steel. Third part of the dissertation is focused on methods of mathematical statistics which were used for statistic evaluation of experimental heats results. Graphical statistic methods are described here (histogram and box plot) as well as numeric numerical statistic methods (binate and partial correlation analyze and linear and multiple linear regression). Theoretical knowledge is verified in the experimental part of dissertation in two chosen sequences of SBQ steel grades (Special Bar Quality). Steel grade 30MnVS6 was chosen as a typical SBQ steel grade where Al2O3 inclusions are modified by CaCN2, and steel grade C35RKL was chosen as a typical SBQ steel grade where Al2O3 inclusions are modified by CaSi. The influence of the chosen parameters of the steel on the nozzle clogging stage during continuous casting is evaluated at the end of experimental part of dissertation by using methods of mathematical statistic. Results of the dissertation were among others a part of successfully resolved grant project GA ČR reg. No. 106/04/0029 entitled “Control of properties of liquid steel before its continuous casting aimed at minimization of defects of continuously cast steel” under financial support of the GAČR.