Ti 미량합금강의 오스테나이트 결정립 성장 거동에 미치는 Nb 복합첨가의 영향

Abstract

The influence of Nb addition on the precipitation behavior of carbonitride on the austenite grain growth of Timicroalloyed steels during reheating to the austenite region has been studied using optical microscope and transmission electron microscope. An addition of small amount of Nb in a Ti-microalloyed steel is not effective in raising the austenite grain coarsening temperature. This is because the coarsening rate of carbonitride is distinctively high in a Ti-Nb containing steel as compared to that in a Ti containing steel due to the high diffusion rate and equilibrium concentration of Nb in austenite. Addition of large amount of Nb however tends to slightly raise the austenite grain coarsening temperature, despite a similar coarsening rate of carbonitride, because of an increase in the volume fraction of carbonitrides with increasing Nb content. The precipitation of Ti as a carbonitride is so slow that a significant amount of Ti remains supersaturated in as-cast state, whereas Nb reaches near-equilibrium concentration even in the as-cast state because of its high diffusion rate.