The classical Johnson-Mehl-Avrami-Kolmogorov approach for nucleation and growth
models of diffusive phase transitions is revisited and applied to model the growth of ferrite in mul-
tiphase steels. For the prediction of mechanical properties of such steels, a deeper knowledge of the
grain structure is essential. To this end, a Fokker-Planck evolution law for the volume distribution
of ferrite grains is developed and shown to exhibit a log-normally distributed solution. Numerical
parameter studies are given and confirm expected properties qualitatively. As a preparation for
future work on parameter identification, a strategy is presented for the comparison of volume dis-
tributions with area distributions experimentally gained from polished micrograph sections.