Super steel bulks with nano-sized grains

本研究的目的是展示利用多结构制备超细晶材料，并研究这种超细晶材料的变形机制。首先，研究了铁素体-马氏体双相钢的晶粒长大行为，发现与单相合金相比，晶粒长大速度极慢。通过拉拔和挤压等重塑性成形，获得了超细的纳米级微观组织。一个典型的例子是高碳珠光体钢，其拉丝被称为钢琴丝。在此基础上，对铸铁的显微组织进行了热处理研究。在整个研究过程中提出，在不含高合金化的情况下，应获得良好的组织。其次，利用原位中子衍射研究了超细晶材料的变形机理。采用逐级拉弦的方法，得到了每一步的中子衍射曲线。轮廓分析结果表明，相对于外部应力，铁素体基体中的晶格应变变大。这意味着铁素体基体本身得到强化，这与退火钢的情况不同，退火钢中渗碳体板或颗粒比基体承受更大的应力，即通常的弥散硬化机制。

The objectives of the present study are to show the fabrication of ultra-fine grained materials by using multi-structure and to examine the deformation mechanism of such a fine grained material.First, the grain growth behavior in ferrite-martensite dual phase steels was studied and the growth rate was found extremely slow compared with that for single phase alloys. The ultra-fine, i. e., nano-sized, microstructures have been obtained by heavy plastic forming like drawing and swaging. One typical example is high carbon pearlite steels whose drawn wire is so called piano wire. Based on the above findings, thermo-mechanical process was studied to obtain fine microstructure for cast irons. It has been proposed throughout this study that the fine microstructure should be achieved without high alloying.Second, the deformation mechanism for ultra-fine grained materials was studied by means of in situ neutron diffraction. A piano wire was pulled in a step by step manner and neutron diffraction profiles were obtained at each step. The results of profile analysis have revealed that the lattice strain in the ferrite matrix becomes comparably larger with respect to the external stress. It means that the ferrite matrix itself is strengthened, which is different for the cases of annealed steels, in which cementite plates or particles bears larger stress than the matrix, i. e., usual dispersion hardening mechanism.