Effect of thermomechanical history on the hot ductility of a Nb-Ti microalloyed steel

It has been observed that the thermal history of the surface of the continuously cast strands affects the microstructure and the hot ductility of steel, and may therefore influence the probability of transverse cracking. During the continuous casting operation, mechanical strains also occur, due to the changes in temperature, and may in turn affect the hot ductility of steel, via microstructural changes. In this work, a Nb-Ti microalloyed steel was subjected to a physical simulation of a thermal history typical of billet casting. At various times during this thermal history, the specimens were subjected to plastic deformation, and the effect on hot ductility of this thermomechanical history was measured. It was found that applying strain immediately after solidification causes significant improvement in hot ductility, while imposing strain at temperatures close to the gamma to alpha transformation temperature severely embrittles the structure.