Evolution of non-uniform grain structure during hot defornnation of a Nb-Ti microalloyed steel
T Katajarinne*, M. Somani'*, R Karjalainen** and D. Porter***
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 …ver más…

Medina et aÜ^ have investigated the abnormal grain growth in a medium-carbon microalloyed steel and calculated the pinning force by applying different models. Non-uniform austenite grain size may result in a heterogeneous final microstructure, which may cause the harmful scatter in impact toughness values occasionally encountered in hot-rolled steel plates and sheets. However, the evolution of abnormally grown austenite grain structure during the course of rough rolling has not been reported in detail. Karjalainen and Perttula^^ have earlier investigated the recrystallisation of partially recrystallised austenite, i.e. in a dual-grain structure. The present paper reports stress relaxation and optical microscopy investigations of the recrystallisation and the grain size evolution of non-homogeneous reheated grain structures. 2. MATERIALS AND EXPERIMENTAL PROCEDURE A piece of a continuous cast slab of a Nb-Ti microalloyed steel with the chemical composition (wt %) 0.13 C-0.20 SÍ4.41 Mn-0.037 Al-0.019 V-0.027 Nb-0.012 Ti-0.0054 N was used in the investigation. Specimens were taken from both close to the surface and the centre-line. Reheating and hot deformation were performed in a Gleeble 1500 thermo-mechanical simulator using cylindrical specimens with dimensions 0lO X 12 mm. After the reheating treatment, the