State of art Nb bearing cold rolled steel sheet for automotive application

A new type of IF cold-rolled high strength steel (HSS) with strength levels of 390 and 440 MPa has been developed using the chemistry of extra-low carbon steel containing around 60 ppm C with an intentional addition of niobium by hybridizing the precipitation hardening with niobium carbides and the supplemental solid-solution hardening. In this steel, the formation of PFZ (Precipitation Free Zones) in the neighborhood of grain boundaries is the most typical microstructural feature responsible for the unique mechanical properties described as follows: 1) The yield strength hardly increases by the existence of PFZ even when grain refinement is taking place. 2) An outstanding high Lankford value is obtained under the fine grain structure. 3) The secondary-work embrittlement resistance is improved by not only the grain refinement but also the existence of PFZ. 4) A sufficiently good surface quality is achieved for galvannealed sheet. Principles of the unique mechanical properties of the steel have been evealed based on the substructure formation during continuous annealing. The results of a further approach to improve these as a state-of-the-art product being widely used for the exposed panels in the body-in-white are introduced in the paper. (AU) © TMS (The Minerals, Metals & Materials Society), 2006