Property Evaluation and Application of Ultrafine Grained Steel

为了评价亚微米细晶钢的性能和探索其应用的可能性，本研究进行了三年。我们对亚微米细晶钢的研究有8个主题，但它们分为三类：(1)亚微米细晶钢的制备与加工技术，(2)显微组织分析，(3)力学性能评价，(4)强化机理分析。我们成功地采用粉末冶金技术（机械铣削工艺）制备了亚微米细晶钢。此外，还开发了适用于汽车零部件的几厘米直径棒钢的热偏移成形方法。对于获得的微观组织分析，采用X射线衍射法对精挑细选区域进行了局部结构分析和位错密度测量；透射电镜观察到的亚微米晶粒组织和位错亚结构较为清晰；用FE-SEM-EBSP方法分析了其取向分布和织构。通过低温和高温的拉伸试验，研究了复合材料的力学性能。结果表明，亚微米细晶钢在低温下不发生脆性断裂，而发生韧性断裂，在高温下表现出超塑性，具有优异的变形性能。此外，我们认为晶粒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cu析出相结合对钢屈服强度的影响，发现两种强化机制的结合有时可以解释钢的屈服强度。同时也发现了超细晶钢不表现时效硬化和位错强化的新现象；因此，我们认为下一阶段应将亚微米细晶钢转移到纳米材料。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