Ultra Grain Refinement During the Simulated Thermomechanical-processing of Low Carbon Steel

Grain refinement is a useful method to improve the strength and toughness of steels without changing their chemical composition. In this study, critical temperatures for the thermomechanical treatment of niobium microalloyed steel were determined experimentally and through thermodynamic data. Simulations of conventional and controlled thermomechanical processing and a thermomechanical treatment to obtain ultrafine-grained microstructures were conducted using torsion tests. The final microstructures displayed significant grain size refinement. Conventional processing produced grains with an average size of 12 μm, while controlled processing led to an average grain size of 4.9 μm and severe plastic deformation at warm temperatures resulted in a grain size of 1.3 μm. The ultra refinement of ferrite grains was associated with strain-induced dynamic phase transformation and dynamic recrystallization of as-transformed ferrite.