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

为了稳定经机械铣削处理后形成的铁的超细晶组织，在铁粉成分中加入了Y2O3氧化物粉末。采用热挤压法对MMed粉末进行固结。结果表明，与常规的MM法相比，我们可以得到更细的铁素体结构。此外，该组织具有完整的等轴铁素体晶粒。结果表明，在热挤压过程中，Y2O3抑制了铁素体的晶界移动，抑制了铁素体的晶粒长大。这使得在变形过程中发生超塑性变形成为可能，形成了细小的轴状组织。制备的MMed材料经晶粒细化强化，服从Hall-Petch关系，抗应力高达1.5 Gpa。虽然该材料延性较差，在屈服后产生缩颈，但将材料的韧脆转变温度降低到液氮温度。另一方面，在高温（约900 K）下，由于上述超塑性，该材料具有优异的成形性。但由于在高温下，Y2O3颗粒仍能保持超细晶粒结构，因此冷却至室温后可恢复高强度。这种材料的可逆特性对于利用工业技术形成高强度钢是非常重要的。

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.