Effects of cooling rate and isothermal holding on precipitation behavior during solidification of Nb-Ti bearing HSLA steels

Abstract

The effects of the cooling rate and isothermal holding on precipitation behavior during solidification have been investigated in 0.063C-0.017Ti-0.056Nb HSLA steels. The precipitates identified in an as-cast slab were semidendritic, dendritic or rodlike Nb-rich (Nb,Ti)(C,N). The morphology and chemistry were quite different when these precipitates formed after reheating and/or hot-rolling processes. No precipitation has been observed at the end of the solidification and the continuous cooling down to 800degreesC. ICP and TEM analyses indicated that most of the Nb and Ti was mainly precipitated into carbonitrides by the isothermal holding at a temperature range of 900degrees to 1000degreesC. In the case of a continuous casting process, the isothermal holding region corresponds to a certain flat cooling region, probably due to the latent heat of solidification. The Nb-rich carbonitrides formed during solidification are associated with the microsegregation of Ti and Nb in the interdendritic region.