Development of an ultrafine grained ferrite in a low C-Mn and Nb-Ti microalloyed steels after warm torsion and intercritical annealing

In recent years, several studies on grain refinement of ferrite have been conducted by different methods (equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB)) in order to optimize the relationship between mechanical properties and microstructure of steels. The present work deals with the strain-induced dynamic transformation of ferrite. Samples of low C-Mn and Nb-Ti microalloyed steels were intensively deformed in warm torsion. This resulted in the formation of ultrafine ferrite grains (∼1 μm) thereby enhancing the mechanical properties compared to the ordinary hot rolled products without the necessity of implementing an extremely high strain accumulation. The following annealing has resulted in recrystallization and growth of ferrite grains accompanied by a homogenization of the final microstructure and a decrease in hardness. With increase in annealing time the volume fraction of martensite/austenite (MA) constituent increased. The results have shown that the effect of steel chemistry was insignificant in comparison with the effect of thermomechanical processing parameters.