Physical Chemistry of AI-Ti Deoxidation Reaction of Molten Iron and Formed Inclusions

摘要

Many grades of Ti-alloyed steels have been widely utilized in many applications because Ti addition improves various physical and chemical properties of steel.Titanium forms several types of non-metallic inclusions such as oxide,nitride,carbide or sulfide and it is well known that titanium oxide and nitride become nucleation sites during austenite to ferrite transformation,or pinning particles against austenite grain growth.Therefore,the evolution of fine inclusions containing Ti would be beneficial to the production of fine grained low-carbon and low-alloy steels with high strength.Titanium is normally added after deoxidation reaction by A1 addition,because Ti has strong affinity with dissolved oxygen.Therefore,A1-Ti complex deoxidation is commonly operated during secondary refining process of Ti-alloyed steel.However,thermodynamics of Al-Ti complex deoxidation has been open to question because reported thermodynamic data have been inconsistent each other.This discrepancy is due to various valences of Ti and insufficient information about phase diagrams between Al2O3,Ti2O3,Ti3O5 and FeO.In the present study,firstly the thermodynamics of the FeA1-Ti-O system was focused on and equilibrium between Al2TiO5 oxide,which stability is unknown,and molten Fe-Al-Ti alloy was studied.Although A12TiO5 oxide contacted with the melt,the interface between oxide and melt showed the nonstoichiometric Al-Ti-Fe-O oxide.Furthermore,observations and analyses of inclusions in solidified steels after equilibrium revealed the similar oxide compositions.Secondly,comparison of the evolution and changing behaviors of inclusions in steels deoxidized by A1-Ti addition by observing inclusions in as-cast and heated alloys were investigated.Heating of the alloy at1473K affects on the compositional change of A1-Ti-O oxide inclusions,as well as the evolution and growth of TiN and TiS inclusions.Increase in the nitrogen content of the Fe-A1-Ti alloy enhanced the formation of TiN and TiS in both as-cast and heated alloys.