To investigate the plastic strain dependence of dislocation velocity, stress relaxation tests with various plastic strains were performed using 590-MPa grade ferrite-bainite
steel at room temperature. The amount of the stress drop at both 1.00 s and 6.00 × 101 s after the begin-ning of strain holding increased with the increase of plastic strain. Also,
the values of dislocation velocity-stress exponent and dislocation ve-locity coefcient on various plastic strain were obtained to discuss the plastic strain dependence of dislocation
velocity. Although it was re-ported that the dislocation velocity v in elastic and considerably small plastic region increased with ow stress ÿÿor effective stress ÿeÿ and also followed
the empirical equation v = B ÿmÿ ÿor v = Bÿe mÿ ÿ, with constant value of dislocation velocity-stress exponent m* and dislocation velocity coefcient Bÿor Bÿ, we found that the m*
decreased and B increased with the increase of ow stress and plastic strain. Taking the plastic strain dependence of m* and B into account, the dislocation velocity was independent
of plastic strain. Furthermore, mobile dislocation density, which was estimated from the value of athermal stress obtained from the stress relaxation test, increased as plastic strain
increased.