PHYSICAL SIMULATION OF CONTROLLED ROLLING AND COOLING OF NB-TI MICROALLOYED STEEL STRIP

Reversible two-stand two-high laboratory mill TANDEM was used to study the effect of simulation parameters on the final properties of HSLA steel strip with 0.08 % C, 1.26 % Mn, 0.05 % Ti and 0.03 % Nb. The flat samples were heated at 1230 degrees C for 20 or 60 minutes and rolled from thickness of 24.0 mm to 3.2 mm by 6 passes. After finishing at approx. 868 degrees C, three different regimes of cooling were applied: free air cooling only, accelerated water cooling down to 560 degrees C followed by 1-hour stay in furnace at this temperature (simulating the conditions in industrial strip coil), or water spray cooling followed by free air cooling to room temperature. Ductility of the laboratory rolled products was almost constant. Yield stress and strength was only little influenced by simulation complexity after accelerated cooling, but these values were much lower after simple free air cooling (minus 80 MPa in average). After free air cooling, final equiaxed ferrite grain size was rather greater in the case of longer heating (5.2 vs 4.5 mu m), but the microstructure was quite different after accelerated cooling when acicular ferrite occurs and pearlite disappears. The obtained results are important from the point of view of the physical simulation methodology.