First Principles Study On Thermodynamic Of Aluminum-deoxidation In Liquid Steel

The large size of alumina inclusions forming in aluminum-deoxidation process seriouslyaffect the quality of steel and nozzle blocking in the process of casting. One of the problemmetallurgical workers pay close attention to is how to reduce the alumina inclusions harm tothe steelmaking and steel quality for a long time. Several main technologies that possiblyreduce the harm of alumina inclusion is strengthen removal, modified calcium treatment,refining the size. The study considers special thermodynamic properties of the metastablenanometer scale in the early deoxidization, and the deoxidation reaction nucleation problemis studied by using first principles method, expect to provide theory basis for refining controlalumina inclusions in the process of deoxidation.Firstly, the thermodynamic properties of metastable phase, such as aluminum clusters,Al2O3nano-particles which the deoxidization reaction may form is calculated by using thefirst principles method. Structure and thermodynamic properties of clusters (Al2O3)n, n=1~10,15,30are calculated by DMol3, and thus the formation Gibbs change of (Al2O3)nis got insteelmaking temperature.The results show that:(1) CP,H and S of (Al2O3)nincreases withincreasing T; GVdecreases with increasing T and is two two phase in the range of400-500K,indicating that the stability of clusters is changing;(2)Δ G θrmwhich [Al],[O] generate clustersand clusters form macro α-Al2O3shows a trend of decrease step by step;(3)The stability of(Al2O3)2is higher than (Al2O3)1. The stability of (Al2O3)n, n=3~10,15,30stability higher than(Al2O3)2is close and the law of chang is not a simple linear relationship, indicating that(Al2O3)n, n=3-10,15,30may reciprocally transform in certain conditions.The thermodynamicproperties of Al2O3nano-particles were calculated by CASTEP, and gets [Al],[O] reaction ofAl2O3nano-particles. The results show that:(1)CV, H decreases and FV, S size increases withincreasing particle size;(2) There is the obvious size effect in CV, H, FV, S.andΔ G θrmin the2-15nm of the particle size;(3) CV, H, FV, S andΔ G θrmof Al2O3nano-particles and macroAl2O3particles are very close in50-100nm, which illustrates the influence of the size effect onthermodynamics has been very limited.Nucleation mechanism of aluminum-deoxidation is investigated using the laws ofthermodynamics, shows the potential process clusters may experience is that [Al],[O]reactionfirst form (Al2O3)1, and then (Al2O3)1transform into(Al2O3)2,(Al2O3)2transform into(Al2O3)n,n=3~10,15,30by further, finally (Al2O3)naggregate into the α-Al2O3nuclei. We get the log[mass%Al]~log[mass%O] using theΔ G θrmof clusters and Al2O3nano particles,and analyzethe problem of “excess oxygen” in aluminum-deoxidation. The results shows that the mainfactors of the "excess oxygen" is the metastable phase in liquid steel.Using iron powder and iron oxide as raw materials, we carry out smelting deoxidationexperiment in tube furnace through proportioning control[O] content, mixing, briquetting.Thecomposition, size and morphology of inclusions is observed by scanning electron microscopein solid iron,It shows: most of alumina inclusions is small in size, about1-5μm; part ofalumina is wrapped around the iron oxide, because of shortage aluminum, outer layer of ironoxide may be formed by “excess oxygen” and iron reaction during the cooling process. Itinitially illuminates “excess oxygen” has been gathered in alumina inclusions around in thehigh temperature melt or cooling process,...