Inclusions in steel are mostly regarded as harmful. However, if the size and chemistry of the inclusions are controlled, they can act as functional particles in improving the mechanical properties of the steel. Titanium as oxides or carbides can be used to control the microstructure and mechanical properties of steel. The composition, shape and size of the inclusions can vary depending on the conditions. Reactions and wetting between titanium inclusions and steel with different alloying elements are key phenomena in developing steel production processes. In this thesis the interfacial phenomena between liquid steel and titanium-containing inclusions were studied. The aim was to determine the inclusion types after additions of titanium and TiO2, the effect and role of aluminium and manganese on the inclusions, the wetting between titanium carbide and steel and the effect of chromium, nickel and molybdenum on wetting and interfacial reactions. The experiments in a vacuum furnace consisted of adding Ti and TiO2 into liquid steel before and during casting. The inclusions formed after the additions of Ti/TiO2 were comprehensively studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS) and electron backscatter diffraction (EBSD), focusing on titanium oxide inclusions. The study examined inclusion types, and the composition and size of the inclusions existing in the cast ingots. Another experimental method used was wetting experiments between TiO2/TiC and steel or iron alloys. The wetting experiments were performed using two different methods: conventional sessile drop technique and separate heating of the TiC substrate and metal droplet. During the wetting experiments the contact angles were recorded and evaluated. The samples from these experiments were investigated by SEM-EDS. The interface, particles and new phases were studied. Thermodynamic calculations (FactSage) of the formation of inclusions in low-alloyed steel in the melt and during solidification as well as carbide formation in stainless steel were performed to support the experimental part. The results of the inclusion study after the addition of TiO2 into liquid steel were novel at the time of publication of the articles. The extensive study on the effect of alloying elements on the wetting and interfacial reactions between stainless steel and TiC were unique and remains pioneering work. The results showed the complexity of the titanium-containing inclusions in steel. Mastering of the inclusions demands profound knowledge of the dominating phenomena. This thesis provides valuable information for developing the production processes of steels that have improved mechanical properties by using titanium-containing functional particles.
|Translated title of the contribution||Titaanioksidi- ja karbidi-mikropartikkelien hyödyntäminen teräksen ominaisuuksien parantamisessa – tutkimus sulkeumien hallinnasta ja rajapintailmiöistä|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- functional particles
- titanium oxide
- titanium carbide
- mechanical properties