Diffusion-controlled growth and

The growth and coarsening kinetics of MnS precipitation were investigated during the hot deformation of electrical steels. The size distributions of the MnS particles in the three tested steels were followed at 800 °C, 900 °C, and 1000 °C, and the relevant precipitation start(P _s) and finish(P _f) times were also determined. The results show that the growth of these precip- itates obeys a parabolic law and is controlled by the diffusion of Mn atoms. Since the Mn concentration at the particle/matrix interface X¹ _Mn is far below the equilibrium solubility X^e _Mn, the growth rate increases with increasing overall Mn concentration. Analysis of the experimental data demonstrates that the effective diffusivity of Mn during particle growth is increasingly modified by pipe diffusion as the temperature is decreased. The experimental results regarding coarsening indicate that the kinetics during this stage are limited mainly by bulk diffusion at the lowest temperature. However, both bulk and grain boundary diffusion processes become rate controlling at the two higher temperatures. The latter process appears to become more and more important as the temperature is raised, a trend which can be attributed to the location of an increasing fraction of the MnS precipitates at the grain boundaries.