Effect of critical temperatures on microstructures and mechanical properties of Nb–Ti stabilized IF steel processed by multiaxial forging

Abstract

In the present study, evolution of microstructure during multiaxial forging (MAF) of a Nb–Ti stabilized IF steel and its mechanical properties have been investigated. The forging schedule was designed on the basis of critical temperatures Ar3, Ar1 (evaluated from dilatometric curve through thermomechanical simulator) and recrystallization stop temperature, Tnr (determined from Boratto equation). MAF was performed for 5cycles in 3 different phase regimes; in pure γ-region (1050°C), γ→α transformation zone (800°C) and pure α-region (650°C). The deformed samples were cooled by normal air cooling. EBSD and optical microscopy investigation confirmed the formation of fine ferrite grains (~5μm) due to strain induced transformation of unstable γ at 800°C and ultrafine ferrites (~1μm) through subgrains formation at pure α-ferritic region at 650°C. The specimen forged in pure α-region showed a 4-fold improvement of yield strength (YS) compared to that of the starting material (141MPa) without much interfering its ductility (25%). This is ascertained to the development of bimodal grain structures and formation of ultrafine carbide precipitates which were confirmed by EBSD and TEM analysis. The theoretical YS was estimated through analysis of different strengthening mechanisms and found to be highly corroborated with the experimentally obtained result.