Abstract
The grain growth characteristics of dual-phase steels have been investigated using 3%Si-0.3%Cr-0.020.13%C steels which are composed of α- and γ-phases at 300°C.
The grain growth in single-phase region proceeds by grain boundary migration, and the relation between mean radius r and annealing time t is described as follows,
(r)2-(r0)2=k2·t……(1)
In the case of dual-phase steels, the grain growth is in need of diffusion of alloying elements, because the chemical composition of α- and γ-phases differs each other. The grains of the minor phase grow slowly in a mode of Ostwald ripening, while the grain boundaries in the major phase migrate under a restriction of pinning by the minor phase grains. It follows as a consequence that the grains of both the phases grow slowly, obeying the following equations,
(r)3-(r0)3=k3·t……(2)
(r)4-(r0)4=k4·t……(3)
In the α-rich dual-phase steels, the growth rate is controlled by volume-diffusion in the α-phase, and the growth law is expressed by eq. (2). In the γ-rich dual-phase steels, however, boundary-diffusion is predominant than volume-diffusion in the γ-phase, and the growth law is expressed by eq. (3).
