Microstructure evolution during recrystallization in dual-phase steels

The microstructure and texture of rolled and annealed dual-phase steels with 0.147 wt. % C, 1.868 wt. % Mn, and 0.403 wt. % Si were analyzed using SEM, EDX, and EBSD. Hot rolled sheets showed a ferritic-pearlitic microstructure with a pearlite volume fraction of about 40 % and ferrite grain size of about 6 mu m. Ferrite and pearlite were heterogeneously distributed at the surface and distributed in bands at the center of the sheets. The hot rolled sheets revealed a through-thickness texture inhomogeneity with a plane-strain texture with strong alpha-fiber and gamma-fiber at the center and a shear texture at the surface. After cold rolling, the ferrite grains showed elongated morphology and larger orientation gradients, the period of the ferrite-pearlite band structure at the center of the sheets was decreased, and the plane-strain texture components were strengthened in the entire sheet. Recrystallization, phase transformation, and the competition of both processes were of particular interest with respect to the annealing experiments. For this purpose, various annealing techniques were applied, i.e., annealing in salt bath, conductive annealing, and industrial hot-dip coating. The sheets were annealed in the ferritic, intercritical, and austenitic temperature regimes in a wide annealing time range including variation of heating and cooling rates.