Continuous Cooling Transformation Behavior and Microstructure of Low Carbon Nb-Bearing Microalloyed Steel

The transformation temperatures at different cooling rates of low-carbon Nb-bearing microalloyed steels with different Nb contents were obtained by thermal dilation method using Gleeble-1500D thermo-mechanical simulator. The transformation behavior of austenite during continuous cooling and the effects of Nb content and cooling rate on transformation microstructure and hardness were studied. The results show that the γ→α transformation temperature lowered, the microstructure became more complex and the microhardness increased with the increase of Nb content and cooling rate. The microstructure of the steel with low Nb content (<0.024wt%) was composed of coarser ferrite and minor pearlite at slow cooling rate of 3 ℃·s-1, the sharp-angle shaped and polygon shaped ferrite as well as banite were got at higher cooling rates. The acicular ferrite was got at higher cooling rate (50 ℃·s-1 ) in the higher Nb content (0.06wt%) steel.