Studium fázových změn v oceli během jejího tuhnutí

Abstract

The vast majority of metals and alloys used in technical practice are produced from a liquid state - a liquid metal. A most important phase transformation at steelmaking process is crystallization of steel (phase transformation of liquid steel from liquid to solid state) and process of own solidification of steel. This Ph.D. thesis deals with study of phase transformations in steel during its solidification process. Specifically, is focused on possibility of experimental refinement of liquidus (TL) and solidus (TS) temperatures of real industrially produced steel grades by the direct thermal analysis method. Knowledge of these temperatures is important not only for correct setting of the steel casting process and solidification of steel, but also for precise setting of the boundary conditions of numerical modelling of steel solidification process, which are often used to optimize modern metallurgical processes. TL is connected with superheat setting of steel before casting. TS is related with own solidification process of steel and segregation phenomena formation. TL and TS defines a width of the two-phase region (steel solidification temperature interval), which is a significant source of defects in the cast steel, which significantly reduce utility properties of cast steel and operational reliability of final steel product. Experimental study of steel solidification process is one of the ways to eliminate their possible impacts on quality of cast steel during solidification and to achieve required properties of steel without any additional treatment. Dozens of steel grades (continuously cast and cast into mould) by direct thermal analysis method on Netzsch STA 449 F3 Jupiter experimental system were analysed. A series of methodological experiments on pure standard in order to generate a methodology of direct thermal analysis methods to study real steel grades and identification of the influence of selected experimental conditions were performed. Practical implementation of the method of DOE to system of evaluation of correctness of measured data was also realized. Comparison of the results of the parallel methods used for TL and TS identification was also performed. A result of the whole process of refinement of TL and TS temperatures represent a final recommendation of one proper TL and TS temperature of each studied steel grade.