Effect of Hot Deformation on Bainite Structure in Low Carbon Steels

Abstract

Effects of deformation in unrecrystallized austentie region on bainite microstructure were investigated in low carbon Nb-B bearing steels. Particular emphases were placed on the variation of the morphology of bainitic ferrite in isothermally transformed specimen.
SEM observation shows that, in the case of non-deformation, a number of straightly elongated bainitic ferrite laths form parallel from austenite grain boundaries, showing an aspect of typical upper bainite structure. In the case of 30%-deformation, although bainitic ferrite laths are shaped like curves of bows, the deformation has a llittle influence on their length. In the case of 50%-deformation, on the contrary, the length is significantly decreased in the growth direction of the lath. Furthermore, the number of bainitic ferrite lath in a "bainite packet" (an aggregate of bainitic ferrite laths with same crystallographic orientation) is decreased by the enhancement of the nucleation within austenite grains. The decrease of both the length and the number of bainitic ferrite lath leads to complicate the appearance of a microstructure.
TEM observation, however, confirmed that those bainitic ferrite laths were also surrounded by two sets of parallel planes close to {451}_α in the case of 50%-deformation, as previously reported for typical bainitic ferrite laths in the case of non-deformation. Therefore it suggests that bainitic ferrite laths transformed from heavily deformed austenite has also the same crystallographic characteristics with typical one.