Abstract
With the aim of reducing surface cracks of continuously cast low carbon Nb containing steel, effects of morphology and composition of carbonitrides on high temperature ductility has been investigated under the condition which simulates thermal cycle of steel slab surface passing through spray cooling and bending zones of a vertical-bending continuous caster. Fine Nb carbonitrides precipitation at prior γ grain boundaries has been shown to be enhanced by the amount of temperature variation at slab surface caused by repetitive cooling by spray water and subsequent reheating just after contact with delivering rolls.
Microalloying of titanium to the steel has been shown to have marked effects on the degree of supercooling during solidification and δ/γ transformation by high temperature differential themal analysis. In addition to titanium, the similar effect of other nitride forming elements such as Zr, Y microalloyed to the steel has been cofirmed by change in high temperature ductility test. From thermochemical calculation on precipitation of carbonitrides of these elements, it has been suggested that these elements precipitates during solidification and act as nucleation site for precipitation of niobium carbonitrides under nitrogen content reduced by preceding nitride formation of these elements. The improvement in high temperature ductility due to the microalloying of nitride forming element is considered to be caused by the change in chemistry and morphology of carbonitrides.
