Abstract
Two major drivers for the use of newer steels in the automotive industry are 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 US 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. AHSS is a general term used to describe various families of steels. The most common AHSS is dual-phase steel which consists of a ferrite–martensite microstructure. These steels are characterized by high strength, good ductility, low yield to tensile strength ratio and high bake-hardenability. Another class of AHSS is the multi-phase steel which has 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 US automakers. These steels consist of a ferrite–bainite microstructure with significant amount of retained austenite phase and exhibit the highest combination of strength and elongation, so far, among the AHSS in use. High level of energy absorbing capacity combined with a sustained level of high n value up to the limit of uniform elongation as well as high bake-hardenability make these steels particularly attractive for safety critical parts and parts requiring complex forming. Finally, martensitic steels with very high strengths are also in use for certain parts. All of the above kinds of steels will be discussed in this paper.
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