読者の皆様 今月も先月に引き続き、超微細粒鋼の力学的性質についての話題です。 筆者はポストドクトラルフェロー趙 明純さん（IMR: Institute of Metal Research, Chinaから）です。 超微細粒鋼のシャルピー靭性について、花村年裕さんのもとで研究をされておられます。 今回は微差粒鋼の吸収エネルギーと破面の特長についての話題提供です。 ----------------------------------------------------------------------- C-Mn steels with the ultrafine grained ferrite/cementite (UGF/C) microstructure have attracted a great commercial interest as advanced structural materials in the future application due to their potentially high strength and low DBTT with a relatively low cost, characterized by the ultrafine grained ferrite matrix plus finely globular cementite particles [1]. Structural steels must meet the toughness requirements. In order to comply with the demands for applying C-Mn steels with the UGF/C microstructure to commercial production, clear understanding of their impact toughness is inevitable. Charpy impact energy transition curves of C-Mn steels with the UGF/C microstructure exhibit a clear transition from upper shelf energy (USE) to lower shelf energy (LSE) with decreasing test temperatures. In comparison with the coarse grained ferrite/pearlite (CGF/P) microstructure, the UGF/C microstructure has lower USE but higher LSE. Also, the energy transition is slower and occurs at lower temperature. The separations occur in the UGF/C microstructure, but the occurrence or the frequency of separation itself has no relation with the change of Charpy absorbed energy [2]. The large-sized dimples appear in the upper shelf region can well account for higher USE and also the typical cleavage fracture surface shows the nil LSE for the CGF/P microstructure. And it is quite evident that the transition is distinct due to the abrupt change in fracture surface from a ductile mode to a brittle one in a narrow temperature range. Large-sized dimples mixed by numerous small-sized dimples appear in the upper shelf region while small-sized dimples mixed by cleavages in the lower shelf region for the UGF/C microstructure. Large-sized dimples are much less in size and area/volume fraction than those in the CGF/P microstructure, which appears to successfully explain their difference in USE. Also, the change from large-sized dimples mixed by numerous small-sized dimples in the upper shelf region to small-sized dimples mixed by cleavages in the lower shelf region leads to a distinct change from USE to LSE. Large sized dimples revealed a surprisingly stronger contribution on improving impact energies. Interestingly some small sized dimples were found in the lower shelf region even down to liquid helium test temperature, with a ductile fracture in a certain extent under this circumstance, showing higher LSE rather than nil LSE of the CGF/P microstructure. Reference:
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