Improvement of strength-ductility balance for low carbon ultrafine-grained steels through strain hardening design

Grain refinement is the only one method to increase both strength and toughness for low carbon steel. However, ultrafine-grained steel has some problems. One of these is the shortness of uniform elongation. Ashby et al. show that an increase in volume fraction of second phase and a decrease in its diameter increase strain hardening rate and therefore increase uniform elongation. We have tried to apply this idea to the low carbon ultrafine-grained steel by using carbides as the second phase. Superior uniform elongation is promised by increasing the carbon content to lead an increase in the volume fraction of carbide (f) or by decreasing the rolling temperature to make the diameter of carbide (d) finer. It is evident that an increase in fid increases the strain hardening rate or the uniform elongation of low carbon ultrafine-grained steel.