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Home  >  Journal list  >  MATERIALS TRANSACTIONS  >  Vol.45  No.7 (2004)  >  pp.2224-2231

MATERIALS TRANSACTIONS
<<Previous article Vol.45  No.7 (2004)   pp.2224 - 2231 Next article>>

Effect of Deformation Temperature and Strain Rate on Evolution of Ultrafine Grained Structure through Single-Pass Large-Strain Warm Deformation in a Low Carbon Steel

Akio Ohmori1), Shiro Torizuka1), Kotobu Nagai1), Naoshi Koseki2) and Yasuo Kogo2)
1) National Institute for Materials Science
2) Department of Material Science and Technology, Faculty of Industrial Science and Technology, Tokyo University of Science


Ultrafine grained structure formed dynamically through a severe warm deformation in the temperature range from 773 K to 923 K has been investigated in a 0.16%C-0.4%Si-1.4%Mn steel. The effects of the deformation conditions such as deformation temperature and strain rate on microstructural evolution were examined using a single-pass compression technique with a pair of anvils. A large plastic strain up to 4 was imposed on the specimen interior at a strain rate of 1 or 0.01 s−1. Ultrafine ferrite grains surrounded by high angle boundaries, whose nominal grain size ranged from 0.26 to 1.1 μm, evolved when the equivalent plastic strain exceeded the critical value about 0.5 to 1, and increased with an increase in strain without any large-scale migration of high angle boundaries. The effects of deformation conditions on microstructural evolution of ultrafine grained structures can be summarized into the Zener-Hollomon(Z-H) parameter dependences. The average size and the volume fraction of newly evolved ultrafine grains depend on the Z-H parameter. Decreasing Z-H parameter enhances the formation of equiaxed ultrafine grains. These indicate that the mechanism forming ultrafine grained structures through the warm severe deformation in the present study is similar to “continuous recrystallization” or “in-situ recrystallization” and that some activation process during or after the deformation plays an important role in the microstructural evolution.


Keyword:
low carbon steel, severe deformation, warm deformation, ultrafine grain, ferrite, microstructure, large strain, Zener-Hollomon parameter, grain size, misorientation

Received: March 17, 2004
Accepted: May 11, 2004 , Published online: June 24, 2005
Copyright (c) 2005 The Japan Institute of Metals

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