Fabrication of Ultra Fine-grained Steel from Mechanically Milled Iron Powder and Evaluation of Its Properties

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In order to stabilize ultra fine-grained structure of iron formed through the mechanical milling treatment, Y2O3 oxide powder was added to the ingredient iron powder. Hot-extrusion method was applied to consolidate the MMed powder. As a result, we could obtain a much finer ferritic structure than that obtained by the usual MM method. Besides, the structure has complete equiaxed ferrite grains. It was found that the Y2O3 pins the movement of grain boundary and suppresses grain growth of ferrite during the hot-extrusion. This made superplastic deformation possible on the deformation and resulted in the formation of the fine and eqiaxed structure.The developed MMed material was strengthened by grain refining strengthening yielding to the Hall-Petch relationship and the proof stress was as much as 1.5 Gpa. Although this material has rather poor ductility and caused necking just after yielding, the ductile-brittle transition temperature of the material was lowered to the liquid nitrogen temperature.On the other hand, at high temperature (around 900 K), this material has an excellent formability due to the superplasticity as mentioned above. But the high strength can recover again when it is cooled to room temperature because the ultra fine-grained structure is kept by the Y2O3 particles even at high temperature. Such a reversible property of the material is very important in terms of forming high strength steels using industrial technology.