Role of niobium in advanced sheet steels for automotive applications

Two major drivers for the use of newer steels in the automotive industry is fuel efficiency and increased safety performance. Fuel efficiency is mainly a function of weight of steel parts, which in turn, is controlled by gauge and design. Safety is determined by the energy absorbing capacity of the steel used to make the part. All of these factors are incentives for the U.S. automakers to use both Highly Formable and Advanced High Strength Steels (AHSS) to replace the conventional steels used to manufacture automotive parts in the past. Highly Formable Steels are generally ultra-low carbon steels fully or partially stabilized by alloying elements such as Ti or Nb. The role of Nb in these steels will be described in this paper. AHSS is a general term used to describe various families of steels. The most common AHSS is the dual-phase steel that consists of a ferrite-martensite microstructure. These steels are characterized by high strength, good ductility, low tensile to yield strength ratio and high bake -hardenability. Another class of AHSS is the multi-phase steel which have a complex microstructure consisting of various phase constituents and a high yield to tensile strength ratio.Transformation Induced Plasticity (TRIP) steels is the latest class of AHSS steels finding interest among the U.S. automakers. These steels consist of a ferrite-bainite microstructure with significant amount of retained austenite phase and show the highest combination of strength and elongation, so far, among the AHSS in use. The role of Nb in all of the above families of AHSS will be discussed in the paper. (AU)