Effects of Cooling Rate and Isothermal Holding on the Precipitation Behavior during Solidification of Nb-Ti Bearing HSLA Steels

摘要

Effects of cooling rate and isothermal holding on thernprecipitation behavior during solidification have beenrninvestigated in 0.063C-0.017Ti-0.056Nb HSLA steels. Thernprecipitates identified in as-cast slab were semi-dendritic,rndendritic or rod-like Nb-rich (Nb,Ti)(C,N). The morphology andrnchemistry were quite different from these precipitates formedrnafter reheating and/or hot rolling processes. No precipitation hasrnbeen observed at the end of the solidification and the continuousrncooling down to 800 °C. ICP and TEM analyses indicated thatrnmost Nb and Ti were mainly precipitated into carbonitrides byrnthe isothermal holding at temperature range from 900 to 1000rn°C. In case of continuous casting process;;the isothermalrnholding region corresponds to a certain flat cooling regionrnprobably due to the latent heat of solidification. The Nb-richrncarbonitrides formed during solidification are associated withrnthe micro-segregation of Ti and Nb in interdendritic region.