Kotobu Nagai

日本コンクリート工学会では，2003年より英文論文集JACT（Journal of Advanced Concrete Technology）の発行を開始し，コンクリート工学分野で国際的にも高く評価される論文集となった。一方，昨今の国際的な研究競争は激化し，インパクトファクターによる数値的な格付けによって，JACTの相対的な位置づけが低下しつつあることも事実である。本稿では，JACTなどの英文論文集の現状に関するデータを示すとともに，編集委員会で実施したアンケート結果の概要を示し，JACTの現状と課題，そして今後の方向性について，編集委員会での議論を踏まえた内容を紹介する。

A recycled steel with 0.2 % offset stress higher than 295 MPa was verified as a candidate for fire-resistant use, based on an academic standard in Japan. The chemical compositions were 0.15 mass% carbon-0.02 mass% silicon-0.5% mass% manganese and unavoidable alloy elements. The amount of elements inevitably contained in recycling was kept within the regulation, and then the dispersion of tensile properties among hot coils was controlled to be sufficiently small. The recycled steel satisfied the requirements by the academic standard in terms of tensile properties at elevated temperatures. No significant evolution was observed in the ferrite-based microstructure within the temperature and time range tested. The plastic deformation is apparently a strain-controlled one up to 400°C and a strain rate-controlled one at 600°C and 700°C. At 500°C, it is in the middle of the two. In the adopted test method, a constant strain rate was realized over the entire strain range except for the beginning stage of deformation. A peak strain rate occurs at about 0.2 % strain, which brings about the uncertainty in determination of 0.2 % offset stress at elevated temperatures. However, the constant strain rate requirement is satisfied in the determination of 1 % offset stress that the academic standard specifies.

Knowing the essence of metallurgical structure is vital for quality assurance and development of steels. Etching has been widely applied as micro-order evaluation of the structure, however not always reliable especially for fine and complex microstructures. The purpose of this study, therefore, is to clarify the application of Kelvin Probe Force Microscopy (KFM) for the evaluation of the fine and complex microstructures.Potential distribution images obtained by KFM were compared with the various images obtained by SEM, EBSD and EPMA for a duplex stainless steel (ASTM A 182 UNS S32750) and a Cu contained low alloy steel (ASTM A707 steel).The KFM was able to clearly identify α, γ, σ and a fresh γ phase in the duplex stainless steel. Moreover, it is concluded that the σ phase itself does not lower corrosion resistance, but on the other hand, the fresh γ phase co-generated with the σ phase can be attributed to the poor corrosion resistance and low toughness of the duplex stainless steel. From the KFM results for the Cu contained low alloy steel, the surface potential distribution obviously depends on surface crystal orientation and chemical compositions, which generally correspond to a work function. The KFM is able to directly show the work function of the surface as the potential distribution. Using the KFM, microstructural features and differences were obviously detected after the two-phase heat treatment for the Cu contained low alloy steel, which corresponds to a better balance of strength and toughness.The present study clarified that the KFM is an effective method to evaluate the metallurgical essence of microstructure especially for the steel having various phases.

Mechanical test at cryogenic temperature shows the advantages on characterization of deformation and cracking behavior in alloys from the viewpoint of microstructure. Microstructural crack initiation in high-cycle fatigue is clearly detected at the specimen interior. The high-strength alloys also show higher strain rate dependence on tensile deformation and low temperature creep deformation under thermal activation process. Not only microstructural analyses using noble techniques but also modeling works on the subsurface crack generation and tensile deformation are focused.

Multilayered metal composites consisting of alternating metals or alloys have been well developed to obtain the superior mechanical properties different from those in any of the constituent materials. The layer integrated steel and metals research project sponsored by Japan government started from 2006 and aimed to fabricate steel sheets with high strength (>1200 MPa) and superior elongation (>20), through the composite design of high strength steels and ductile steel layers. In the present work, two kinds of hot rolling bonded multilayered composites consisting of austenitic stainless steel/bainite (martensite) steel are focused. The effects of hot rolled microstructure and the annealed microstructure on the tensile behaviour of a 25-layer laminated composite steel are discussed. On the other hand, the tensile deformation microstructure of the hot rolling bonded and annealed 15-layer composite steel is systematically investigated in order to find the reason for the enhanced plasticity of the brittle layer in the composite.

The strain distribution in a multi-layered steel composed of brittle martensitic steel (WT-780C) and ductile austenitic steel (SUS304) during a compressive test was measured by the electron moiré method. A very fine model grid with 5 μm spacing was fabricated using electron lithography techniques on the surface of the specimen and an electron beam scan in which spaces are almost the same as those of the model grid can be used for the master grid. The difference between the amounts of secondary electrons per primary electron causes moiré fringes that consist of bright and dark parts. The spacing of the electron moiré fringe was measured, and the strain on the specimen was calculated by using the spacing of the electron moiré fringe and the spacing of the electron beam scan. The strain distribution was not uniform for each steel sheet and the boundaries. The strain near the boundary between the different steel sheets is larger than that in the SUS304 sheet and that in the WT-780C sheet.