Effect of chemical composition on the micro structural evolution in micro alloyed steels during deformation with in (Α + Γ) region

Laboratory hot torsion testing technique was carried out on one plain low carbon steel and two plain low carbon Nb and Nb-Ti micro alloyed steels to study the effects of chemical composition, micro alloying addition, on the dynamic softening and grain refinement of ferrite during warm deformation. Deformation schedule was conducted within two phase region (i.e. between Ar3-Ar1). The physical processes that occur during deformation were studied by analyzing the true stress-true strain warm flow curves of ferrite. Examination of the micro structural evolution was carried out by optical microscopy and electron back scattering diffraction. Results show that warm flow ferrite stress-strain curves are similar to those affected only by dynamic softening and an extended warm flow softening seen during deformation. The dynamic softening mechanism observed during warm deformation of ferrite was explained by continuous dynamic recrystallization. The addition of Nb and Ti has significant effects on ferrite grain refining in micro alloyed steels. It was observed that the Nb-Ti micro alloyed steel has the finest equiaxed ferrite grains, 1.5 μm, in compared with Nb and C-Mn steels. Moreover, plain low carbon steel has the largest ferrite grain size, 2.8 μm. This demonstrated that the effect of Nb-Ti on ferrite grain refinement is stronger than that of Nb alone. It was concluded that the addition of Nb and Ti enhances grain refinement through continuous dynamic recrystallization of ferrite.