To clarify the effects of vanadium additions on the strengthening mechanisms of tempered martensitic steel, the microstructures, precipitates, dislocation densities, and tensile properties of water-quenched and tempered Fe0.2C0.5Si2.5MnxV (mass%; x = 00.82) steels were analyzed. The vanadium carbide precipitates were plate-shaped and had a BakerNutting orientation relationship with the ferrite matrix. The size and shape of the vanadium carbide precipitates on the slip plane were considered when evaluating the contribution of precipitation strengthening. The increase in yield strength upon adding 0.82 mass% vanadium to the tempered steel was mainly caused by precipitation strengthening owing to the vanadium carbide precipitates and dislocation strengthening owing to the high dislocation density. This study demonstrates that the contribution of precipitation strengthening might be overestimated if it is assumed that the precipitates all hinder dislocation motion by the Orowan mechanism.
A computer model is constructed to simulate the dissolution of V carbide and carbonitride with size distribution in steels. Assuming local equilibrium of carbon, nitrogen, and V at the particle/matrix interface, the dissolution rate is calculated using the mean-field and invariant field approximations. The fraction of particles and size distribution (PSD) of V carbide are in good agreement with those in an Fe-C-V alloy reported in the literature. The V mass fraction and PSD of carbonitride, measured by extraction replica in this study, were also reproduced well by simulation in an Fe-C-V-N alloy (N~20 ppm). Moreover, simulation with an equilibrium tie-line passing through the bulk alloy composition, as often done in the calculation of precipitate dissolution rate, yielded a large error.
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