Optimization of Global and Local Formability Properties Through Nb Microalloying

Automotive high strength steels have specific microstructure-dependent forming characteristics. More than two decades of research has demonstrated that Nb-induced microstructural optimization is applicable to a comprehensive variety of high-strength automotive steels. Commercial production of niobium microalloyed high strength low alloy (HSLA), dual phase (DP), multiphase/complex phase (MP/CP), transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP), and press hardening (PHS) steels has since been realized. Two primary effects of niobium microalloying are microstructural refinement and precipitation as carbide or carbonitride. These mechanisms are utilized primarily to provide additional strength. In addition, solute niobium affects the progress of phase transformations and recrystallization as well as the diffusivity of carbon during processing. Dedicated use of all these metallurgical functionalities in combination with appropriate processing conditions allows optimization of the properties and distribution of individual phases within the microstructure. This is not only beneficial to strength but also to global and local forming properties. Improvements are reflected in bendability, hole expansion, and work hardening behavior. Global formability is generally associated with drawability, stretch formability and necking resistance (re FLC)—driven by pronounced work hardening and efficient strain distribution. On the other hand, local formability is associated with fracture-dominant forming modes such as bending and sheared-edge stretching, and various fracture strain concepts have been developed to quantify this aspect. Niobium (Nb) microalloying is a powerful means for improving both aspects in general and achieving property balance in particular. 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 [1–3]. Such maps can be used to visualize the beneficial effects of niobium microalloying and to define feasible optimization vectors, as demonstrated for various steel types in the following.