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 (γ-recrystallization region at Tnr+50 °C, γ-nonrecrystallization region at Tnr−50 °C and (α+γ) region) was designed on the basis of critical temperatures, Ar3 and Ar1 (obtained from dilatometric study, Gleeble-3800), and Tnr (determined from Boratto equation). The combination of high yield strength (YS) and ductility of the forced air-cooled (FAC) and quenched specimens rolled in (α+γ) region is attributed to the high misorientation angles of matrix, formation of subgrain ferrite (~2 µm)+larger ferrite (~35 µm) 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 °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.