An ultrafine grained microstructure was produced in a Nb microalloyed steel by cold rolling of martensite followed by an annealing heat treatment. Different hold times were used to determine the effect of annealing at 550°C on the microstructure and mechanical behaviour of the ultrafine grained steel. The results have shown that with increasing annealing time, both the ductility and strength decrease. This behaviour has been attributed to the relaxation of long range stress field within ultrafine grains/subgrains and carbide particle growth.
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