| The susceptibility of steels to transverse cracking during continuous casting depends, in part, on the hot ductility of the steel. In the past, hot ductility has been measured in the laboratory by tensile testing specimens reheated to a high temperature (preferably melted), and then cooled down to the desired test temperature before isothermally testing to fracture. More recently, isothermal tensile testing has been performed after imposing different thermal histories prior to reaching the test temperature. The most relevant thermal history is that of the solidifying strand surface. Such thermal histories have been found to be quite complex, and invariably involve rapid cooling and heating cycles. This will therefore lead to high thermal gradients, which, in turn, can generate strains in the surface of the solidifying strand. Thus, the purpose of this thesis was to consider the effect of the thermomechanical history on the hot ductility of steel.; A Nb-Ti microalloyed steel was chosen to be studied, since microalloying often increases susceptibility to transverse cracking. After in-situ melting and solidification, tensile test specimens were subjected to the thermal history typical of a continuously cast billet. Different amounts of prior deformations in various schedules were imposed on the specimens at selected stages during the thermal history, before tensile testing to fracture at the time and temperature corresponding to the unbending stage of the billet casting. It was found that the hot ductility varied from 1% to 98%, depending on the stage in the thermal history at which prior deformation was executed. Microstructural studies showed remarkable changes in the precipitation pattern occurred due to the employed prior deformation. Similar hot ductility measurements executed on a low carbon steel, that was free from microalloy elements, exhibited an effect of prior hot deformation which was quite different to the Nb-Ti microalloyed steel. In particular, the effect on the hot ductility was not as dramatic. The effects were also postulated to be due to precipitation. |
