Research on the Strengthening Mechanism of Nb-Ti Microalloyed Ultra Low Carbon If Steel

Abstract

The microstructure evolution and carbides precipitation behavior during the hot rolling, cold rolling and annealing processes have been studied. The results indicates that the addition of Nb into Ti bearing IF steel promotes the recrystallization at 900 ℃ and obtains fine recrystallized grains. Based on the fine and uniform original structure, Nb-Ti microalloyed steel obtains fine structure after cold rolling and annealing. The study results of the carbides precipitation behavior suggests that the TiN precipitates at high temperature above 1100 ℃, and a large amount of  (Nb,Ti)C and TiC precipitate at about 900 ℃ in Nb-Ti and Ti bearing steels, respectively. There is no significantly difference between the amount of carbides in hot deformed steels at 900 ℃ and the hot rolled plates, which indicates that the carbides mainly precipitate during the hot rolling process due to the ultra low carbon and nitrogen content in the steel. However, there is few carbides precipitate during the coiling process at 600 ℃. The thermodynamic and kinetic calculation results are consistent with the TEM observation results. In Nb-Ti containing steel, the solid solution of Nb and Ti elements are significantly reduced due to the large amount precipitation of (Nb,Ti)C during the final rolling process, which effectively promotes the recrystallization of austenite. Therefore, the grains of hot-rolled, cold-rolled, and annealed strips are refined, and the fine-grained strengthening effect is improved. Finally, the strength of Nb-Ti containing steel is higher than that of Ti containing steel.

Keywords: ultra low carbon steel, Nb-Ti microalloying, structure, Precipitation, recrystallization