Characterization of scale and sub-scale development in ultrahigh-strength steels during processing

Ductile Ultra High Strength Steels (UHSS), the so-called third generation of Advanced High Strength Steels (AHSS), are increasingly used for automotive applications. However, the strengthening alloying additions in these steels may have consequences for the production of the final product. External and internal oxides form during coiling of the hot rolled product and can affect the formability of the steel. The work presented here focuses on the simulation of surface and subsurface reactions that lead to the formation of the internal oxides. Further it aims to establish a link between the near surface conditions and the formability of the final product. The internal oxidation depth largely depends on the temperature, atmosphere, top surface composition and alloying additions. Rolling, hot coil cooling, pickling, cold rolling and annealing were simulated under lab conditions to create a better understanding of how internal oxides develop during the whole strip steel production process. Microstructural characterization, including EDS, EBSD and STEM, were used to determine the nature of the external and internal oxides that formed throughout the steel processing.