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

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 Ar-3, Ar-1 (evaluated fromdilatometric curve through thermomechanical simulator) and recrystallization stop temperature, Tnr (determined from Boratto equation). MAF was performed for 5 cycles in 3 different phase regimes; in pure.-region (1050 degrees C), gamma ->alpha transformation zone (800 degrees C) and pure a-region (650 degrees C). The deformed samples were cooled by normal air cooling. EBSD and optical microscopy investigation confirmed the formation of fine ferrite grains (similar to 5 mu m) due to strain induced transformation of unstable. at 800 degrees C and ultrafine ferrites (similar to 1 mu m) through subgrains formation at pure alpha-ferritic region at 650 degrees C. The specimen forged in pure alpha-region showed a 4-fold improvement of yield strength (YS) compared to that of the starting material (141 MPa) 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. (C) 2016 Elsevier Ltd. All rights reserved.