The Effect of Thermal History on the Hot Ductility of Microalloyed Steels
Transverse cracks in continuous cast steels can form if the hot ductility of 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 fractured. In this work, high temperature tensile testing was used to determine the hot ductility of a Nb-Ti and a Ti-B microalloyed steel. However, instead of cooling directly to the test temperature after melting, the specimens were subjected to thermal histories typical of a continuously cast billet surface, and then, at the unbending temperature, subjected to a tensile test to fracture. In other words, physical simulations of the continuous casting procedure were performed. The results were compared with those generated by conventional isothermal tensile testing.<br> For the isothermal tests, both steels exhibited a temperature range of low ductility. However, the physical simulations did not reveal such hot ductility behaviour. For both steels, almost all the physical simulation variants led to hot ductility values lower than predicted by the isothermal tests at the corresponding tensile test temperature. For the Nb-Ti steel, it was revealed that there is a critical minimum temperature, attained by the specimen during the thermal history, below which the hot ductility, measured at the tensile test temperature, is much reduced. It is assumed that this critical minimum temperature leads to the formation of grain boundary ferrite, which probably enhances the rate of formation of Nb precipitates, decreasing 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.
- ISIJ international
ISIJ international 39(3), 253-262, 1999-03-15
The Iron and Steel Institute of Japan