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

Abstract

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.

Investigating the incremental forming capabilities of extra deep drawn steel

“…Furthermore, low yield strength also makes EDD steel susceptible to denting. 13 Due to these properties, which are unique for a steel alloy, EDD steel is extensively used for non-structural components 14 and preferred by the automobile industry for making bodyin-white components, door inners, dash panels, bodyside inners, and floor pans with spare tire tub. 15,16 To extend the benefits of ISF to the automotive industry, it is important to gauge the scope of ISF and its limitations while forming components out of EDD steel sheets.…”

Formability study of bake hardening steel and its correlation with microstructure

“…A comparison of FLC of BH steel estimated experimentally in the present study with (a) experimental data reported in literature 1,3 as well as (b) numerically estimated using Keeler and Brazier 27 and Paul 28 models.…”

“…In recent years, one of the major emphases of automobile industries is to use thinner gauge steel sheets to improve environmental performance by reducing fuel consumption and greenhouse gas emissions without compromising passenger safety. 1–3 The use of thinner gauge steel sheets necessitates a favorable combination of desired properties, including high tensile strength and ductility, good surface finish, dent resistance, and good formability. 4 However, these requirements are often contradictory as it is difficult to achieve high strength without compromising formability.…”

“…Further, experimentally determined FLC is compared with reported literature which exhibits a similar experimental study of BH steel grades. 1,3 In addition to that, FLC of BH steel is predicted using Keeler and Brazier 27 and Paul 28 models with a purpose to gain better insight into the forming characteristic. An attempt is also made to correlate the specimens’ forming behavior with microstructure and mechanical properties by characterizing the specimens of varying width.…”

“…21,22 Hariharan et al 1 proposed a methodology to estimate the forming limit diagram of BH steel based on geometric interpolation of the orientation-specific limit strains between the uniaxial, plane strain, and biaxial strain paths. Manikandan et al 3 experimentally investigated the formability of different high-strength automotive steel sheets and reported the better formability of BH 180 steel than BH 220 steel. The authors reported that an increase in the strength of steel generally reduced its formability.…”