Grain refinement mechanisms in BCC ferritic steel and FCC austenitic steel highly deformed under different temperatures and strain rates

Abstract

An interstitial free (IF) steel having single-phase bcc-structure (<~900 °C) and a SUS-304 stainless steel having single-phase fcc-structure were deformed to high strains by torsion at different temperatures and strain rates. Various grain sizes ranging from 0.6 μm to 46 μm in the IF steel and those from 0.4 μm to 44 μm in the SUS-304 steel were produced by torsion. The dependence of the ultrafine and fine grain sizes (D) on the torsion flow stresses (σ) was analyzed in both materials. Accordingly, the σ - D relationship could be expressed in a power-law form known as the Derby relation, with multiple power-law exponents. This result indicated that different grain refinement mechanisms, i.e., grain subdivision and dynamic recrystallization, were dominant under different deformation conditions. Differences between the IF steel and the SUS-304 steel with respect to the types of microstructure refinement and the corresponding mechanisms in these steels were also discussed.