Formation of ultrafine grained ferrite during thermomechanical treatment in microalloyed steel

Abstract

In the present work, the formation of ultrafine grained ferrite has been studied by applying suitable thermomechanical treatment. A high amount of deformation (～80%) at varying strain rates (0·01–10 s^?1) was applied in the temperature range of Ar₃ to Ac₃ followed by water quenching. This treatment resulted in a two-phase ferrite–martensite microstructure as compared to fully martensite structure after quenching without deformation. The formation of ultrafine ferrite (?3 μm) during deformation was favourable at a lower temperature and a slower strain rate. A maximum ～50% ferrite formed during deformation at 780°C with a strain rate of 0·01 s^?1. Experimental rolling with a high strain (～1·3) with finish rolling temperature just above Ar₃ (～750°C) resulted in fine ferrite–pearlite of ?3 μm, and the properties showed a high value of strength as compared to steels rolled in a conventional way. Dual phase microstructure (ferrite and martensite) was produced after partial austenisation to 780°C followed by quenching in water, and this resulted in an excellent combination of properties (high ultimate tensile strength, low yield strength/ultimate tensile strength, high elongation and high n values).