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Formability is an important property requirement in sheet metal forming and mostly useful to automobile and aerospace industries. Every product design starts with finite element simulation before the die design and material selection stage. The forming limit curve along with finite element results acts as tool to investigate whether the design is feasible. Hence, it is of utmost importance that the forming limit curve is accurate over a wide range of strain path. Generally, a forming limit curve is derived from discrete failure strain points corresponding to different strain ratios by fitting a smooth curve. It has been observed from many studies that the generated forming limit curves are devoid of any failure points corresponding to equibiaxial strain path. This is found to be true even when the test conditions are supposed to produce biaxial state of strain. To understand the reason behind this observation, a set of forming limit diagram experiments and finite element analyses were carried out for high-strength interstitial free steel and interstitial free galvannealed steel with three different friction conditions. It was observed that friction plays an important role in right-hand side of the forming limit curve. Finite element results and experimental validation suggest that failure strain points for biaxial strain paths can be obtained only if the tests are conducted with proper lubrication system.

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The purpose of this study is to develop a phenomenological model for prediction of the entire forming limit diagram from simple tensile material properties. The phenomenological model is based on the necking and ductile damage theories. In the proposed model, void nucleation is described as a function of the equivalent plastic strain, and void growth is a function of the stress triaxiality. The forming limit curves calculated from the proposed phenomenological model matched reasonably well in the region of uniaxial tension to balance biaxial tension with the experimental forming limit curves generated on C-Mn 440 steel, interstitial-free 340 steel, and interstitial-free steel sheets.

Increased importance on weight reduction is driving automotive industries to reduce thickness of the steel panels without compromising the vehicle safety and performance. High strength steels are looked at as a candidate for automotive applications. To overcome the limitation of less formability in high strength steel, steel makers introduced the bake hardening steel (BH) grades. This study compares the formability of extra deep draw (EDD) steel grades which are mainly used for body panels with that of bake hardening steel. The influence of material yield strength, pre-strain (ε o ), and curvature (R) of product on static dent resistance (SDR) and its stiffness is studied experimentally. It was found that higher yield strength provides higher dent resistance, whereas high panel thickness and smaller curvature resulted in higher stiffness. Higher dent resistance observed in bake-hardened steel is due to the increase in strength by bake hardening process. The use of bakehardened steel in automotive applications represents a good opportunity for weight reduction, increased stiffness, and dent resistance. The experimental result was used to validate the SDR regression formulae developed by Tata Steel and then same formulae used to predict and compare the dent resistance value of back hardening and high strength interstitial free steel (HIF) grades.