Ferritic Bainitic Steels...

Ferritic Bainitic Steels  Ferritic bainitic steels are also known as FB steels. These steels are one of the types of advanced high strength steels which have been developed for automotive application. Since these steels have two phases, hence these steels are also a type of dual phase (DP) steel. FB steels are mostly available as hot rolled products. These steels are normally cold-drawn. Ferritic bainitic range of hot rolled high strength steels has been developed to meet weight reduction requirements of the automobiles. . They are fully killed steels and are usually available in four strength levels namely FB 450, FB 540, FB 560 and FB 590. FB family of steels extends the HSLA range of micro alloyed steels to include products combining high ultimate tensile strength (UTS) with excellent formability. Typical additions for grain refinement in these steels are Al (aluminum), B (boron), Nb (niobium), and Ti (titanium). These elements are added individually or in combination. Nitrogen (N) binding is also used sometimes. FB steels are with soft ferrite and hard bainite. They have a microstructure of fine ferrite and bainite. Their micro structure is finer than the typical DP steel. Strengthening is obtained by both grain refinement and second phase hardening with bainite. The micro structure of FB steels gives these steels a marked improved ductility. Fig 1 shows a typical microstructure for the FB steel. Fig 1 Typical micro structure of FB steel  FB steels are utilized to meet specific customer application requirements that require stretch flangeable (SF) or high hole expansion (HHE) capabilities for improved edge stretch capability. SF capabilities of FB steels are based on their ferrite bainite micro structure. The micro structure is usually even more finely tuned to be SF. This characteristic can be measured by the...

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...