Microstructural stability of ultrafine grained low-carbon steel containing vanadium fabricated by intense plastic strain
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I. INTRODUCTION
OF the strengthening mechanisms of polycrystalline solids, grain refinement is the most preferred method for increasing the strength without degradation of ductility and toughness. This well-known fact in metallurgy was recently reappraised through progress in the fabrication of bulk, porosity-free, metallic materials with submicrometer-order grain size by imposing severe plastic straining. The representatives are equal channel angular pressing (ECAP),[1] accumulative roll bonding,[2] severe torsional straining,[3] etc. However, the microstructure of ultrafine-grained (UFG), metallic materials manufactured by severe plastic straining was reported to be in a nonequilibrium state due to the accumulation of a large amount of strain.[4] That is, such an UFG microstructure is thermally unstable and grain growth can occur readily, causing a loss of physical and mechanical superiority associated with the ultrafine grain size. Accordingly, a thermally stable equilibrium UFG structure should be ensured during service conditions and/or thermal exposure for the practical application of such materials. The most effective way to ensure the microstructural stability is the uniform introduction of fine, incoherent, second-phase particles into the matrix so that grain boundary migration causing grain growth is suppressed by the grain boundary pinning effect of the second-phase particles.
Recently, Shin and co-workers[5–8] reported the grain refinement of a commercial low-carbon steel (hereafter, CS steel) by utilizing ECAP. In their investigations, the asECAPed steel subjected to a total effective strain of ⬃4 exhibited ferrite grain refinement from 30 to ⬃0.2 m and an increase of the ultimate tensile strength from 480 MPa to over 900 MPa. In addition, by static annealing for 1 hour in the temperature range of 693 to 873 K, it was found that ultrafine ferrite grains of as-ECAPed CS steel were relatively stable up to 783 K. In the present study, ultrafine ferrite grains were produced in a low-carbon steel containing a small amount of vanadium (hereafter, CSV steel) by using ECAP, and the microstructural changes during static annealing were observed. The thermal stability of UFG CSV steel was examined by comparing its microstructural evolution behavior with that of UFG CS steel under identical annealing conditions. The addition of vanadium, which either exists as solute atoms or forms particles, such as carbides, nitrides, or carbonitrides, is known to be effective in retarding recrystallization and suppressing grain growth in severely worked steels.[9] Accordingly, the enhancement of the thermal stability is anticipated in the UFG CSV steel compared to the UFG CS steel.
II. EXPERIMENTAL PROCEDURE A. Materials and Heat Treatment
KYUNG-TAE PARK, Assistant Professor, is with the Division of Advanced Materials Science and Engineering, Hanbat National University, Taejon, 305719 Korea. YONG-SEOG KIM, Professor, is with the Department of Metallurgy and Materials Science, Hongik University, Seoul, 121-791 Korea. DONG HYU
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