Balancing global and local formability properties through Nb microalloying

David Martin, CBMM Asia Bernardo Barile, CBMM Europe BV Caio Pisano, CBMM Europe BV Automotive high strength steels have specific microstructure-dependent forming characteristics. Global formability is generally associated with high uniform strain values which imply good drawability and stretch forming properties driven by pronounced work hardening. Local formability on the other hand is often measured by various fracture strain values—generally higher in single phase steels. In this respect, the so-called ‘local/global formability map’ concept has been established not only to provide a comprehensive methodology to characterize existing automotive steels but also to enable improvement strategies toward more balanced forming characteristics. Niobium (Nb) microalloying is a powerful tool to achieve both property improvement in general and property balance in particular. More than two decades of research has demonstrated that Nb-induced microstructural optimization is applicable to HSLA steels, AHSS (DP, CP, TRIP, TWIP) and PHS, and it has been realized in commercial production of such steels. This contribution details the underlying metallurgical and processing effects of Nb microalloying in automotive high-strength steels and highlights achieved global and local formability improvements. Respective optimization vectors are demonstrated through intrinsic formability mapping, where the possibilities and limitations are indicated.