Effect of different cooling rates on thermomechanically processed high-strength rebar steel

The present article is dealing with 0.2% C, 0.1% V and 0.02% Nb steel. Billets with 130 mm x 130 mm cross-section were austenitized and hold at 1080 C. The billets were hot rolled to 22 mm bar diameter. Hot rolling was finished at 980-1000 C. The final bars were air-cooled. On a parallel way, an experimental hot deformation investigation, on the same steel, was carried out at deformation temperature range 1200-800 C with the same amount of deformation (97% reduction in area). However, cooling regimes after deformation were air cooling, water quenching to 600 C followed by air cooling, and water quenching to room temperature. Microstructure investigation was done using both optical and scanning electron microscopes. Further evaluation was done using mechanical testing. The industrial trial has unsatisfied results with poorer yield strength with higher ultimate strength. Bainitic aggregates are detected in the hard phases islands. Air cooling after pilot hot deformation creates banded ferrite-pearlite microstructure with 9.11 mu m ferrite grains. However, quick water quenching to 600 C followed by air cooling develops tempered and softened coarse bainite phase. On the other hand, water quenching to room temperature develops fine bainite texture. Water quenching to 600 C followed by air cooling is the best regime creating accepted mechanical properties. (C) 2008 Elsevier B.V. All rights reserved.