Property Evaluation and Application of Ultrafine Grained Steel

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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