Interaction Between Oxides And Solute Atoms In Steel And The Effects On The Formation Of Ferrite

Non metallic inclusions has been considered to be harmful impurities in steel, but three solute atoms(Manganese, Carbon and Boron) in steels can react with some inclusions(Ti2O3, ZrO2 and Al2O3) which have appropriate morphology, size and distribution. The defect oxides would be formed by substitution, interval, and chemical reaction during the process. The contents of solute atoms would be reduced and so that of the stability of austenite, then it is benefit to the formation of ferrite. In this paper, the formation energies of various defect oxides can be got using first principles calculation,and the mechanism of the interaction between solute atoms and oxides is revealed. The formation mechanism of ferrite and its effect on the strength and toughness are discussed deeply.The samples were prepared by double heat pressure bonding experiment and the interactions of three solute atoms(Mn, C and B) in steels and three oxides(Ti2O3, ZrO2 and Al2O3) were studied, respectively. The formation of ferrite was observed by optical microscopy(OM). The change of elements’ content across the oxide layer was analyzed by electron probe micro-analyzer(EPMA). The physical properties, such as energy,magnetic moment and density of states, were calculated by first principle. Experimental results and theoretical calculations were verified mutually. Combined with the results of theoretical research, the microstructure structure and mechanical properties of industrial Ti-Zr killed steel and Al killed steel were compared, discussed and analysied.Industrial production of steel validated the results in the laboratory furtherly.The main conclusions of this paper are as follows:(1) The first principle calculation results show that Ti2O3 is a cation vacancy oxide;ZrO2 is anion vacancy oxide, whereas Al2O3 is hard to form vacancy in oxide. Gibbs2 has been utilized to extend the first principle calculation for reaction temperature.Results show that the difference between the calculated results in the condition of reaction temperature(high temperature) and the one in the condition of 0 K and 0 Pa is approximately 5%.(2) Experimental characterization and calculation data show that: Mn can enter into the vacancy of Ti2O3 and Zr O2 to form Mn-doped oxides, and Mn depleted zones(MDZ)are formed obviously in the vicinity of there two oxides. It was found that Mn ion has similar radius with Zr ion by the analysis of their electronic structure, so ZrO2 is easy to be doped by Mn ions and crystal structure is more stable. Manganese is an austenite stabilizer; the presence of Mn depleted zone(MDZ) can therefore promote the formation of ferrite.(3) The first principle calculation results show that the formation energies of B-doped oxides are positive. Boron segregated at the austenite grain boundaries can not enter into oxides(Ti2O3, ZrO2 and Al2O3) during the interaction with steel matrix. But the boronwith large activity can react with excess oxygen of Ti2O3 which has cation vacancies to produce boron oxide. Oxides with cation vacancy can be transformed to perfect oxides during this process. The presence of boron will inhibit the formation of ferrite, and the formation of boron oxide can not inhibit the formation of ferrite any longer.(4) There are more carbon in austenite than in ferrite. The results of first principle calculation show that for the steel containing only carbon, carbon can not enter into oxides(Ti2O3, Zr O2 and Al2O3) during the interaction with steel matrix. Carbon can react with excess oxygen of Ti2O3 has cation vacancies to produce carbon monoxide.Oxides with cation vacancies can be transformed to to perfect oxides during this process.The decrease of carbon content is helpful to the transformation from austenite to ferrite.(5) The stability, electrical and magnetic properties of oxides of Ti, Zr and Al(MxOy)and Mn-doped oxides(MnMx-1Oy) were analysied based on the first principle calculation. The results show that Ti2O3 has more cation vacancies, and the formation energy of Ti3MnO6 is the lowest. The density of states shows that Ti2O3 has a little conductivity, whereas ZrO2 and Al2O3 are insulators. The chemical environment of Mn in the Mn-doped oxide has been changed, which leads to the asymmetry of the density of states of the doped oxide, therefore it has magnetic moments.(6) Based on the above theoretical research, the high-strength welding structure steel killed by Zr-Ti was developed. Fine dispersed complex Zr-Ti oxides have been formed.These oxides provide nucleation sites for the precipitation of MnS particles, resulting in the spheroidization and evenly dispersed MnS inclusions in the steel matrix. Because of?ne oxides dispersed in the matrix uniformly, the microstructure and mechanical properties are uniform. Compared with the Al-killed steel, the strength, ductility and toughness of the Zr-Ti-killed steel have been considerably improved. The results validated the above theoretical results, that the first principle calculation has a good theoretical guidance for the development of high performance steel materials, especially the development of component system.Two interaction mechanisms for the solute atoms and oxides in the steel are put forward from the point of view of the quantum mechanics based on the calculating formation energies and optimizing the structure of oxides. The promotions for the formation of ferrite by oxides interaction of solute atoms were explained theoretically.Research ideas, means and methods were provided on the research of the interaction of other solute atoms and oxides and their influence on the phase transformation.