Study On Recrystallization Behavior Of Ferritic Rolled IF Steels

As a new generation steel sheet for deep drawing applications, Interstitial-Free (IF) steels have been experiencing a rapid development in recent 20 years. Rolling in ferrite temperature region for IF steels is a new process which was developed about 10 years ago. After so many years' study, it is recognized that annealing is the last critical process which determines the properties of IF steels. Thus, in order to make proper annealing process parameters, microstructure evolution and its influence factors are needed to know clearly.Recrystallization behavior of a Ti-IF steel, a Ti+P-IF steel and a ultra low carbon (ULC) steel was studied in this paper, influence of chemical composition on recrystallization behavior was also analyzed and the mechanical properties of three annealed steels were compared at the same time.Based on the experiment results and their analyses, following conclusions are drawn:(1)The recrystallization of the Ti-IF steel is influenced greatly by annealing temperature. Even if steels annealed at 650℃for three hours, the microstructure of steels are still non-uniform, and some deformed microstructure are remained. On the contrary, recrystallization was completed when the temperature is raised to 700℃or 750℃. And the higher the temperature, the more uniform the microstructure is.(2)The recrystallization of the Ti+P-IF steel is influenced greatly by both the annealing temperature and holding time. The recrystallization is guaranteed as steels were annealed at 700℃for three hours. (3)The nucleation rate of the ULC steel was low, which makes it difficult to find the classic recrystallization program, i.e. new grain generation and growth. The recrystallization can be completed as steels were annealed at 600℃for more than three hours.(4) As a result of the influence of dynamic strain aging (DSA) behavior, the microstructure of the ULC steel rolled in ferrite range is different from that of cold rolling ones, no in-grain shear band is found except in the surface. In both Ti-IF steels and the Ti +P-IF steels, the carbon and nitrogen, which would normally otherwise be in solution, are scavenged as precipitates by the addition of Ti elements. There are many in-grain shear bands after ferritic rolling. The r-values of the Ti-IF steel, the Ti-P-IF steel and the ULC steel are 1.66, 1.54 and 0.75 respectively. The recrystallization temperature of IF steels is higher than that of the ULC steel as a result of the existence of numerous precipitates.