CARBONITRIDE PRECIPITATION BEHAVIORS IN Ti AND Ti-Nb MICROALLOYED STEELS

摘要

Carbonitride precipitation behaviors of 0.15C-0.3Si -1.0Mn-Ti-Nb microalloyed steels during casting and slab reheating processes were investigated. Effects of microalloying compositions and solidification cooling rate on the carbonitride precipitation were discussed. In both Ti and Ti-Nb steels solidified with fast cooling rate of 35℃/min, the carbonitrides in cast and reheated slab were very fine regardless of the microalloying composition. However, in continuous cast Ti steel with normal cooling rate of 15℃/min, the size of carbonitrides in cast slab was fairly large and a large portion of added Ti remained in the matrix as solute. The free Ti in as-cast slab induced the coarse carbonitride precipitates during reheating process and resulted in coarse reheated austenite grain. The size of carbonitride in reheated Ti steel increased very sensitively with the increase of Ti/N ratio, because the larger Ti/N ratio caused larger free Ti in as-cast state. In case of continuous cast Ti-Nb steel with cooling rate of 15℃/min, most of the microalloying elements finely precipitated as carbonitrides during continuous casting and these precipitates grew very slowly. Thus, it was shown that the combined addition of Nb and Ti to this steel was the most effective to the refinement of carbonitride precipitates in normal continuous-cast and reheated slab.