| Under traditional technical conditions, continuous casting steel billet with coarse columnar grains and centerline segregation is almost inevitable. Further more, during subsequent processing and heat treatment, the quality of as-cast steel billet is subject to high-temperature cracking and ridging. The key to solve this problem is to refine the solidification microstructure and improve the proportion of equiaxed grain zone. In the presence of effective heterogeneous nucleation sites ahead of the solidifying front, fine equiaxed grains form directly in the melt, so that the equiaxed grain structure may override the inherent columnar grain structure, which, in turn, give rise to hot ductility and hot cracking resistance. In recent year, the beneficial effect of the second phase particles on improving the solidification microstructure has been highlighted and recognized. At present, there is several heterogeneous nucleation mechanism ofδ-Fe, however, they are controversial and disagree with the experimental phenomena in the grain refinement. Although, no grain refiners are commercially available for steels, as opposed to cast iron, magnesium alloy and aluminum alloy where such remedies widely used to refine the solidification microstructure. Therefore, the exploration of heterogeneous nucleation mechanism ofδ-Fe and the preparation of grain refiner for steel have important practical and theoretical significance.In this thesis, the surface properties of TiN and the adsorption and stacking sequence of Fe atoms on the TiN surface were revealed through computer simulation using first-principles calculations and ab-initio molecular dynamics simulations, which will contribute to the understanding of heterogeneous nucleation mechanism ofδ-Fe. The nucleant potencies of TiN, TiC and ZrN particles were also analyzed, which is beneficial to provide theoretical basis for choosing the best heterogeneous nucleation sites forδ-Fe. Meanwhile, a preparation method of grain refiner for 409L ferritic stainless steel has been put forward. The Fe-Ti-N master alloy has been synthesized by reaction of Fe-Ti melts with nitrogen gas. The processing parameters of preparation of the Fe-Ti-N master alloy have been optimized and the grain refinement performance of the Fe-Ti-N master alloy has been tested. The main results are listed as follows:Four kinds of low-index surfaces of TiN have been studied by means of the first-principles calculations, it appears that 5, 11, 13, and 11 layers are thick enough to make interlayer distance converge and thus assure the bulk-like interior for (001), (110), Ti-and N-terminated(111)surfaces, respectively. The surface energy of the (001) surface is the lowest among all surfaces over most range of the nitrogen chemical potential, so that the (001) surface is thermodynamically more favorable than other surfaces.The three kinds of adsorption mold of atomic Fe-Ti(001) surface was investigated by dynamics simulation, it was found that the N site is more preferable than Ti and bridge sites for Fe atom. After the configurations of the threeδ-Fe/TiN initial interfaces have been fully relaxed, it was found that both of the configurations of the Fe-Ti type and the Fe-bridge type ofδ-Fe/TiN interface has been transformed into Fe-N typeδ-Fe/TiN interface after configuration relaxation, which means that the Fe-N typeδ-Fe/TiN interface is more stable than other interfaces.The degrees of difficulties for the Fe atomic adsorption and for theδ-Fe heterogeneous nucleation on the surface of TiN, TiC and ZrN have been investigated using ab-initio molecular dynamics simulations in a energy sense. The calculation results shows that the adsorption energies of Fe atom on TiN(001), TiC(001) and ZrN(001) surface are -3.15eV/atom ,-3.12eV/atom and -1.68eV/atom, respectively. The interface energies of forδ-Fe/TiN,δ-Fe/TiC andδ-Fe/ZrN interface are 1.08J/m~2,1.09J/m~2 and 1.14J/m~2, respectively. By comparing the adsorption energy and the interfacial energy, it can be concluded that the nucleant potency of TiN onδ-Fe is the strongest, followed by TiC, ZrN is the weakest.By optimizing the Fe-Ti-N master alloy preparation parameters including initial Ti content, nitrogen partial pressure, reaction time, cooling method and so on, a set of optimally preparation parameters has been obtained: reaction temperature is 1600℃, initial Ti content is 5wt.%, the nitrogen partial pressure in the furnace is 0.06Mpa, the gas flow rate of nitrogen through corundum tube is 400ml/min, the total reaction time is 10min, the cooling method is cooling within the furnace.The grain refinement experiments of the Fe-Ti-N master alloy on solidification structure of industry pure iron has been carried out. 2wt.% Fe-Ti-N master alloy has been added into the industry pure iron and the gain refinement performance is clear, the proportion of equiaxed grain zone increases from 30% to 50%, and the average equiaxed grain size decreases from 700μm to 400μm. In order to test the grain refinement performance of the Fe-Ti-N master alloy on solidification structure of 409L ferritic stainless steel, three processing parameters including the addition level, the melts temperature, have been tested. The mechanisms of these experimental phenomena have been analyzed in terms of thermodynamics and kinetics. It was found that the shows that decrease the addition level, lower the melts temperature, decrease the thermal holding time, have a significant impact on refining the grain size and improving the proportion of equiaxed grain zone of the as-cast solidification structure of 409L ferritic stainless steel.In order to investigate the feasibility of the grain refinement effectiveness of the Fe-Ti-N master alloy, some reference heats were done with addition of 409L ferritic stainless steel and with addition of elemental Ti. It was found that the addition of Fe-Ti-N master alloy caused a substantially greater refinement effect than that obtained through addition of 409L ferritic stainless steel or elemental Ti. Meanwhile, a group of pilot experiments have been carried out on 50KG castings at Baoshan Iron and Steel Corporation, the validity of refinement effectiveness of the Fe-Ti-N master alloy on the 409L ferritic stainless steel solidification structure has been confirmed. Furthermore, it was found that there was no segregation of the second-phase particles and the harmful elements on the grain boundary as a result of the Fe-Ti-N master alloy addition. |
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