Evolution of non-uniform grain structure during hot defornnation of a Nb-Ti microalloyed steel

Abstract

Recrystallisation and the evolution of the abnormally grown austenite grains were investigated for a continuously cast slab of a 0.13 C-1.41 Mn-0.027 Nb-0.012 Ti steel during reheating and after the subsequent deformation. The stability of the recrystallised structure and the uniformity of the final microstructure were also studied. The abnormally grown grains appear in a few minutes at reheating temperatures around 1200 °C. All grains in the bimodal grain structure recrystallised at 1100 °C for strains > 0.2 within about 40 s. The coarse grains are refined, while the fine grains become slightly larger. Some abnormal grain growth can occur again in the recrystallised structure within 10 min. After cooling at 1° C/s the coarse austenite grains transform into large areas of upper bainite, while the finer grains transform to fine ferrite and pearlite. The transformed microstructure in specimens cooled at l°C/s consists of large upper bainitic areas corresponding to the prior coarse austenite grains, surrounded by fine ferrite-pearlite grains.