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

Abstract

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 (~2µm)+larger ferrite (~35µm) and precipitation of NbC (<10nm). 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.