In soft-magnetic, nanocrystalline materials like Vitroperm®-type iron-based  ones, long-time heat-treatment is the standard way for achieving a nanocrystalline phase with a fine-grained structure. In search of those nanocrystalline materials with an enhanced saturation flux density, materials with higher Fe-content are investigated. However, in spite of increasing the content of Fe and/or that of Nb as a grain growth inhibitor, the structure would not be as fine grained as the standard Vitroperm®800 at least after the standard heat-treatment. Therefore for the new Fe-rich materials, a new type of flash annealing on ribbon-wound cores was tested. After applying the heat-treatment, the structures of different layers in the cores were investigated. The results show good soft magnetic properties together with a fine-grained structure of crystallites of 10-30 nm in diameter being comparable to that of nanocrystals in the standard Vitroperm®800.