Thermomechanical processing of low carbon Nb-Ti stabilized microalloyed steel: Microstructure and mechanical properties

Aim of the present study is to investigate the effect of deformation temperature and cooling rate on microstructural features and mechanical properties of Nb-Ti stabilized microalloyed steel. Rolling schedule in 3 different phase regimes (gamma-recrystallization region at T-nr+50 degrees C, gamma-nonrecrystallization region at T-nr-50 degrees C and (alpha+gamma) region) was designed on the basis of critical temperatures, Ar-3 and Ar-1 (obtained from dilatometric study, Gleeble-3800), and T-nr (determined from Boratto equation). The combination of high yield strength (YS) and ductility of the forced air-cooled (FAC) and quenched specimens rolled in (alpha+gamma) region is attributed to the high misorientation angles of matrix, formation of subgrain ferrite (similar to 2 mu m)+ larger ferrite (similar to 35 mu m) and precipitation of NbC ( < 10 nm). Whereas, the good combination of YS, ductility and high impact energy of the forced air cooled samples deformed at 1050 degrees C is endorsed to the high fraction of acicular ferrite (76%), formation of degenerate pearlite and precipitation of nanosize TiC. TEM investigation substantiated the formation of shear bands and nanosize carbide precipitates; whereas, EBSD analysis confirmed subgrain formation and misorientation angles of matrix grains. (C) 2015 Elsevier B.V. All rights reserved.