The effect of thermal history on the hot ductility of microalloyed steels

Transverse cracks in continuous cast steels can form if the hot ductility o f the cast steel at the unbending stage is poor. To measure hot ductility, tensile specimens are usually reheated to a high temperature (preferably to the melting point), cooled to the test temperature and then isothermally f ractured. In this work, high temperature tensile testing was used to determ ine the hot ductility of a Nb-Ti and a Ti-B microalloyed steel. However, in stead of cooling directly to the test temperature after melting, the specim ens were subjected to thermal histories typical of a continuously cast bill et surface, and then, at the unbending temperature, subjected to a tensile test to fracture. In other words, physical simulations of the continuous ca sting procedure were performed. The results were compared with those genera ted by conventional isothermal tensile testing. For the isothermal tests, both steels exhibited a temperature range of low ductility. However, the physical simulations did not reveal such hot ductil ity behaviour. For both steels, almost all the physical simulation variants led to hot ductility values lower than predicted by the isothermal tests a t the corresponding tensile test temperature. For the Nb-Ti steel, it was r evealed that there is a critical minimum temperature, attained by the speci men during the thermal history, below which the hot ductility, measured at the tensile test temperature, is much reduced. It is assumed that this crit ical minimum temperature leads to the formation of grain boundary ferrite, which probably enhances the rate of formation of Nb precipitates, decreasin g the hot ductility in this way. However, for the TI-B steel, the effect of thermal history could not be explained in such a straightforward manner.