Effect of Yield Strength, Pre-strain, and Curvature on Stiffness and Static Dent Resistance of Formed Panel

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 (eo), 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 bake-hardened 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.