Effect of Interpass Time on the Dynamic Transformation of a Plain C-Mn and a Nb Microalloyed Steel

Strip rolling simulations were carried out on a 0.06%C-0.3%Mn-0.01%Si and a 0.11%C-1.0%Mn-0.11%Si-0.03%Al-0.034%Nb steel over the temperature range 1000 degrees C to 883 degrees C. Pass strains of 0.4 were applied at a strain rate of 1 s(-1) with interpass times of 0.5 s, 1 s, 1.5 s, 3 s and 5 s. Two different temperature schedules were employed, namely i) continuous cooling at 6 degrees C/s and ii) isothermal holding. The mean flow stresses (MFS's) applicable to strip rolling were determined by integration. The flow stress levels and MFS's decrease when the interpass times are short. When they are long, the flow stress increases with decreasing temperature. These observations indicate that the austenite is transforming dynamically into ferrite and statically into austenite. The nucleation and growth of the ferrite reduce the rolling load and modify the microstructure. The addition of Nb in solid solution delays the occurrence of dynamic transformation (and the retransformation of ferrite back into austenite). The forward nucleation of ferrite occurs displacively while the retransformation back into austenite takes place by a diffusional mechanism.