Steels for Automotive Applications...

Steels for Automotive Applications Steel has been a leader in automobile applications since 1920s. Currently, steel is the primary material in body and chassis structures. It is the backbone of the entire vehicle. In cars, these days, steel makes up about 65 % weight. It plays many roles in present day vehicles. It protects occupants, provides positive driving experience, reacts to road loads, provides comforts, and provides attachment points to other components of the vehicle. As there is a high emphasis on greenhouse gas reductions and improving fuel efficiency in the transportation sector, the automobile industry is investing significantly in lightweight materials. The industry is moving towards the objective of increasing the use of lightweight materials. It is giving priority to the activities connected with the development of new materials, forming technologies, and manufacturing processes. The weight reduction is still the most cost-effective means to reduce fuel consumption and greenhouse gases. It has been estimated that for every 10 % of weight eliminated from a vehicle’s total weight, fuel economy improves by 7 %. This also means that for every kilogram of weight reduced in a vehicle, there is around 20 kg of carbon dioxide reduction. Over the last decade, a strong competition between steel and low density metals has been observed in the automobile industry due to the increasing requirements of passenger safety, vehicle performance and fuel economy. The materials used in automotive industry need to fulfill several criteria before being approved. Some of the criteria are the results of regulation and legislation with the environmental and safety concerns and some are the requirements of the automobile users. In many occasions, different factors are conflicting and therefore a successful automobile design is only be possible through an optimized and balanced solution. Around 65...

Twinning Induced Plasticity Steels...

Twinning Induced Plasticity Steels  Twinning induced plasticity steels, also known as TWIP steel, are a class of austenitic steels which can deform by both glide of individual dislocations and mechanical twinning. The steels are fully austenitic and nonmagnetic, with no phase transformation. These steels represent one of the types of new families of high strength steels which have been developed in recent years to meet the increased demands of weight reduction in automobiles. The formation of deformation twins causes a large degree of deformation. This deformation mode is the reason behind the naming of this group of steels. First steel based on plasticity induced by mechanical twinning was found in 1998 which had strength of 800 MPa with a total elongation of above 85 %. These values varied with deformation temperature, strain rate and chemical composition. Composition and micro structure  TWIP steels contain a high level of manganese (Mn) content. These steels are normally composed of iron (Fe), 15 % to 30 % manganese (Mn), 1 % to 3 % silicon (Si) and 1 % to 3 % aluminum (Al). The steels usually contain large concentrations of Mn because it is crucial to preserve the austenitic structure based on the ternary system of Fe-Mn-Al. The main influence of Mn in TWIP steels is to control the stacking fault energy (SFE) and therefore the deformation mode. The addition of aluminum to Fe-high Mn TWIP steels is because it increases SFE significantly and therefore stabilizes the austenite against phase transformations which can occurs in the Fe-Mn steels during deformation. Furthermore, it strengthens the austenitic micro structure by solid solution hardening. Al also improves corrosion resistance. Silicon sustains the austenite transformation into martensite, since its lowers the SFE and increases the number of stacking faults that are nucleation sites for martensite. It also...

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...