Property Evaluation and Application of Ultrafine Grained Steel

为了评价亚微米级细晶粒钢的性能，探索其应用的可能性，本研究已进行了三年。我们研究亚微米级细晶粒钢有8个主题，但分为三大类：(1)亚微米级细晶粒钢的制备与加工技术，(2)组织分析，(3)力学性能评价，(4)强化机理分析。此外，还开发了热漂移法成形直径数厘米的棒材，用于汽车零部件。在显微组织分析方面，用X射线衍射仪对所获得的微区进行了局域结构分析和位错密度测量，亚微米晶组织和位错亚结构为Caref…进一步用透射电子显微镜观察，用FE-SEM-EBSP方法分析了取向分布和织构。通过拉伸试验研究了从液氮低温到高温的不同温度下的力学性能。结果表明，亚微米级细晶粒钢即使在低温下也不会导致脆性断裂，而是延性断裂，在高温下表现出超塑性，具有良好的变形性能。此外，我们还认为，细化强化与其他强化机制的结合是实现进一步强化的重要途径。在此基础上，研究了晶粒细化和纳米铜沉淀的联合作用，发现钢的屈服强度有时可以用这两种强化机制的共同作用来解释。然而，也发现了超细晶粒钢没有时效硬化和位错强化的新事实，因此我们认为下一阶段应将亚微米级细晶粒钢转向纳米晶材料。较少

For the purpose of evaluating the properties of submicron fine grained steels and exploring their possibilities of application, this study had been carried out for three years. We had 8 themes for investigating the submicron fine grained steels, but they are classified into three categories : (1)Fabrication and processing technique of submicron fine grained steels, (2)Microstructure analysis, (3)Evaluation of mechanical properties, and (4)Analysis of strengthening mechanism.We succeeded to fabricate submicron fine grained steels by adopting powder metallurgy technique (mechanical milling process). In addition, hot excursion method to form rod steel of several cm in diameter was also developed for applying to automobile parts. As for the analysis of the microstructure obtained, the local structure analysis and measurement of the dislocation density were performed by X ray diffractometry for fine selected areas ; the submicron grained structure and the dislocation substructure were caref … More ully observed with TEM ; the orientation distribution and the texture were examined with FE-SEM-EBSP method. The mechanical properties were investigated by means of tensile testing at various temperatures ranging from low temperature of liquid nitrogen to elevated temperature. As a result, it was clarified that the submicron fine grained steels do not cause brittle fracture but ductile fracture even at low temperature, and that they exhibit superplasticity at elevated temperature, leading to the excellent deformability. Besides, we insisted that the combination of the grain refinement strengthening and other strengthening mechanism should be important to realize further strengthening. On the basis of this idea, the effect of combination of grain refinement and nano-Cu precipitation was investigated, and then we found that the yield strength of the steel is sometimes explained by the addition of both strengthening mechanism. However, it was also found the new facts that the ultrafine grained steel never exhibits age hardening and dislocation strengthening; thus we feel that the submicron fine grained steels should be moved to nano-grained material in the next stage. Less