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

为了稳定通过机械球磨处理形成的铁的超细晶结构，将Y2 O3氧化物粉末添加到成分铁粉中。采用热挤压法对MMed粉末进行了固结。因此，我们可以得到一个更精细的铁素体组织比通常的MM方法得到的。此外，该组织具有完整的等轴铁素体晶粒。结果表明，Y2 O3在热挤压过程中对晶界的移动起到了钉扎作用，抑制了铁素体晶粒的长大。结果表明，所研制的MMed材料通过细化晶粒强化，屈服强度满足Hall-Petch关系，屈服强度可达1.5GPa。虽然该材料的延展性较差，在刚屈服后就发生颈缩，但该材料的韧脆转变温度低于液氮温度，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。但是，当冷却到室温时，高强度可以再次恢复，这是因为即使在高温下，Y2 O3颗粒也保持了超细晶结构。材料的这种可逆性质在使用工业技术形成高强度钢方面非常重要。

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.