Advanced high strength steels for automotive application...

       Advanced high strength steels for automotive application Advanced high strength steels (AHSS) are being developed for automotive applications. These automotive grades are different when compared with the conventional low and high strength steels. AHSS have superior mechanical properties which are developed in it due to the steel’s structure and due to its distinct processing. AHSS are manufactured by adopting control cooling from austenite or austenite plus ferrite phases on the run out roller table in a hot rolling mill or in the cooling section of a continuous annealing furnace in cold rolled product. A comparison of AHSS with other types of high strength steel is given in Fig 1. Some types of these steels are described below. Fig 1 Comparison of high strength steel with AHSS Dual Phase (DP) steel This steel has two phases namely ferrite and martensite. The hard second phase of martensite is present in the form of islands in a matrix of ferrite. Higher is the volume fraction of second phase higher is the strength. During the production of this steel, a portion of the austenite phase is first converted into ferrite before rapid cooling to transform the remaining austenite to martensite. Some hot rolled steels can have a microstructure which contains considerable amount of bainite. The property of good ductility to this steel is imparted by soft ferrite phase which is generally continuous. During the working of this steel, lower strength ferrite phase gets strained giving the steel a distinctive high work hardening rate. The work hardening rate along with good elongation provides DP steel better ultimate tensile strength (UTS) values then conventional steel of similar yield strength (YS). Accordingly DP steel has low YS/TS ratios. DP steel also has a bake hardening effect which is...