Studium příčin výskytu vnitřních necelistvostí ve vývalcích z Cr-Mo ocelí

Abstract

The scope of presented thesis is dedicated to study of internal defects initiation in hot rolled billets from low-alloyed Cr-Mo steels. The problem with the occurence of internal defects was already studied by many authors in the past; however, the achieved results were not satisfactory enough. All data and materials discussed in the thesis are originally from Třinecké železárny a.s. Since internal defects are indicated during an ultrasonic inspection in already cooled billets, it can be said that the studied problem is very complicated and complex. The thesis includes a theoretical analysis of possible reasons for the occurence of internal defects in hot rolled billets. A statistical analysis has been performed based on longterm monitoring of industrial process data. The analysis showed dominant factors influencing the probability of internal defects initiation. The most influential are vanadium microalloying, casting speed and steel superheat in the tundish. On the basis of a theoretical analysis, mathematical-physical simulations and performance of a wide scale of experiments (microchemical, fractography, metallography, extraction replicas analysis, microhardness and impact work determination), other factors have been investigated that can be directly connected to the occurence of internal defects. The most crucial is the influence of the heating strategy in the soaking pit before hot rolling and a proper rolling strategy at the blooming. Counter measures have been adopted based on the knowledge gained, which led to a decrease of defects during the production process. Nowdays it is possible to say that the initiation and propagation of internal defects in Cr-Mo based low-alloyed steels is not a problem anymore.