State-of-Art High Strength Steel Sheets for the Automotive Application

New type of IF cold-rolled high strength steels (HSSs) with the strength level of 390 and 440MPa have been developed under the chemistry of the extra-low carbon steel containing around 60ppm C with an intentional addition of niobium by hybridizing the precipitation hardening with niobium carbides and the supplemental solid-solution hardening. In this steel, Precipitation Free Zone (PFZ) nearby recrystallized grain boundaries forms during continuous annealing. This structure leads to unique mechanical properties such as lower yielding and superior anti-secondary-work embrittlement under fine grain structure strictly required for the exposed panels in Body-in-White. Principles of the unique mechanical properties of the steel are introduced related with the formation of PFZ during annealing, and the results of further approach to improve them as the state-of-the-art product, which is widely used for the exposed panels in Body in White, are introduced in the paper.