Fabrication of Lotus-Type Porous Carbon Steel Slabs by Continuous Casting Technique in Nitrogen Atmosphere
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INTRODUCTION
RECENTLY, porous metals have attracted much attention because of their various unique properties such as lightweight materials, catalysts, electrodes, vibration and acoustic energy-damping materials, impact energy absorption materials, and so on.[1–3] Among porous metals, lotus-type porous metals (lotus metals)[2] and Gasar metals,[3] which have a lot of long cylindrical pores aligned in one direction, are investigated actively. Lotus metals not only possess the properties of conventional porous metals but also have the unique properties originating from their anisotropic directional pores. In particular, lotus metals have mechanical properties superior to those of the conventional porous metals.[4] The specific strength of lotus metals in the pore growth direction does not decrease because of a lack of stress concentration even if the pores exist, whereas the specific strength of porous metals with spherical pores decreases with increasing porosity because of the stress concentration. The lotus metals can be fabricated by unidirectional solidification in pressurized hydrogen or nitrogen gas.[2,3] The solubility gap between liquid and solid is MAKOTO KASHIHARA, Engineer, and HIROSHI YONETANI, General Manager, are with Material Laboratory, Mori Seiki Co. Ltd., Iga, Mie 519-1414, Japan. Contact e-mail: m-kashihara@moriseiki. co.jp SHINSUKE SUZUKI, formerly Associate Professor, The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan, is now, Associate Professor, Waseda Institute for Advanced Study, Waseda University, Shinjuku, Tokyo 169-8555, Japan. YUKI KAWAMURA, formerly Master Course Student, The Institute of Scientific and Industrial Research, Osaka University, is now Engineer, Automobile R&D Center, Honda R&D Co. Ltd., Haga, Tochigi 321-3393, Japan. SANG-YOUL KIM, formerly Post Doctoral Fellow, The Institute of Scientific and Industrial Research, Osaka University, is now, Assistant Manager, Changwon Branch Office, Korea Industrial Complex Corp., Changwon, Gyeongnam 641-020, Korea. HIDEO NAKAJIMA, Professor, is with The Institute of Scientific and Industrial Research, Osaka University. Manuscript submitted January 8, 2009. Article published online June 3, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
used to produce lotus metals; the pores are evolved by insoluble gas during solidification from the melt in which hydrogen or nitrogen is dissolved. Because carbon steel has been used as structural materials, lotus carbon steel is expected to be used for lightweight structural metals. Kujime et al.[5] and Kashihara et al.[6,7] produced lotus carbon steel by unidirectional solidification using a continuous zone melting technique.[8] It was reported that the yield strength of the lotus carbon steels fabricated in a nitrogen atmosphere is similar to that of the nonporous carbon steel in the porosity range up to 30 pct, which is attributed to the solid-solution hardening as a result of nitrogen in the steel.[7] Thus, the nitrogen atmosphere can improve the mechanical prop
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