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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 × 10 1 s after the beginning of strain holding increased with the increase of plastic strain. Also, the values of dislocation velocity stress exponent and dislocation velocity coef cient on various plastic strain were obtained to discuss the plastic strain dependence of dislocation velocity. Although it was reported that the dislocation velocity v in elastic and considerably small plastic region increased with ow stress σ or effective stress σ e and also followed the empirical equation v = B σ m * or v = Bσ e m * , with constant value of dislocation velocity stress exponent m* and dislocation velocity coef cient B or B , 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.

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