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In an effort to optimize outer body panel steel utilization with respect to dent resistance performance and weight reduction, the automotive industry continues to investigate the application of higher strength steels. Most recently, dual phase steel has been recognized as a very promising material substrate for outer body panel application, due to its inherent formability and final part performance attributes. This paper presents a comprehensive study of Ispat Inland's new electrogalvanized dual phase “DI-FORM 500” product, which was specifically designed to meet automotive exposed quality standards. It reviews the mechanical properties, aging characteristics, formability, dent resistance, weldability and fatigue strength of this product, along with a representation of its application advantages to the automotive industry, in terms of part performance, weight savings and cost avoidance.

To further the application of Advanced High Strength Steels (AHSS) in automotive body and structural parts, a good knowledge and experience base must be developed regarding the press formability of these materials. As a first step towards accomplishing this goal, the American Iron and Steel Institute, in collaboration with the United States Department of Energy, jointly funded under the Technology Roadmap Program, a study by Ispat Inland Research Laboratories to characterize the formability of AHSS using simulative laboratory tests. Splitting limits under different conditions and springback behavior of several grades of conventional high strength steels (HSS) such as bake-hardenable and HSLA steels, advanced high strength steels (AHSS) such as dual-phase and TRIP steels, and ultra-high strength steels (UHSS) such as recovery-annealed and tempered martensitic steels were characterized.

Forming Limit Curves (FLCs) have been used in press shops for decades during die development and more recently as failure criteria when used in conjunction with FEA for part feasibility analysis. Around the world there are different techniques used to determine the FLC. The differences between the techniques lie in tooling, specimen geometry and in the method used to determine the critical strains. A comprehensive study on FLCs of selected AHSS was carried out at ArcelorMittal Global R&D, where the different commonly used techniques and a new technique employing Digital Image Correlation (DIC) were employed to determine the FLCs. This paper presents results of these comparisons.

Drop tower axial crush testing was performed on hat section samples of various steel grades ranging in minimum tensile strength from 410 MPa to 1300 MPa. It was demonstrated that the energy absorption capability increases with the tensile strength of the steel. However, steels of very high strength, greater than 980 MPa tensile strength, exhibited a greater tendency for weld button pullout or material fracture, and thus limited energy the absorption capability. The effect of the closeout plate and the yield strength of the steel on energy absorption were also investigated. FEA simulations were performed and correlated to the experimental results. A flow stress based material criterion is introduced based on the analytical approach to compare the crush performance of steels.

The component being formed experiences some type of prestrain that may have an effect on its fatigue strength. This study investigated the forming effects on material fatigue strength of dual phase sheet steel (DP600) subjected to various uniaxial prestrains. In the as-received condition, DP600 specimens were tested for tensile properties to determine the prestraining level based on the uniform elongation corresponding to the maximum strength of DP600 on the stress-strain curve. Three different levels of prestrain at 90%, 70% and 50% of the uniform elongation were applied to uniaxial prestrain specimens for tensile tests and fatigue tests. Fatigue tests were conducted with strain controlled to obtain fatigue properties and compare them with the as-received DP600. The fatigue test results were presented with strain amplitude and Neuber's factor.

Tensile properties of sheet steels at dynamic conditions are becoming more important for automotives in recent years due to the positive strain rate effect of steels which significantly improves energy absorption capability during crash events. However, several testing techniques are used by different testing laboratories, no testing standards are available, and the quality of data generated by different laboratories is often not comparable. In order to improve the data quality at high strain rate testing conditions and thus to improve the accuracy of crash simulation results, The International Iron and Steel Institute (IISI) initiated a project to develop the “Recommendations for Dynamic Tensile Testing of Sheet Steels”. The document provides guidelines for key elements of high strain rate testing, testing techniques, input methods, specimen geometry and stress/strain measurement instrumentations.

Spot weld fatigue performance of dual phase steels is of great interest due to much higher fatigue strength of its base steel. In this study, 0.8mm DP500-EG and 1.4mm DP600-GI were tested for both tensile shear and cross tension conditions. For comparison, tensile shear test was also conducted for 1.6mm HSLA350-GI and 0.8mm DQSK-GI. Although fatigue strength was different due to their different gages, by using the stress index, Ki, a parameter to describe the local stress condition, fatigue strength of all four steels merged to a narrow scatter band, indicating very little dependence of spot weld fatigue on the strength of the base steel. In addition, the effect of weld surface cracking on fatigue strength of dual phase steels is of concern due to their high strength, despite the fact that it can occur to any steels under conditions of high current or electrode misalignment.

Bending under tension is an important deformation mode during stamping and has been observed to limit achievable ductility for high strength steels. This paper presents experimental results from Angular Stretch Bend (ASB) testing, which has been used to characterize bending under tension behavior for several conventional, advanced high strength steels and ultra-high strength steels. Steels that were studied include Bake Hardenable steels, High Strength Low Alloy (HSLA) steels, Dual Phase (DP) steels, Transformation Induced Plasticity (TRIP) steels, and tempered martensitic steels. Failure heights were determined under sample lockout conditions for different punch radii. By comparing absolute formability measured by the failure height, the results can be used to provide material formability ranking for different R/t ratios. In addition, strain distributions were analyzed to provide bending under tension forming limits for the different steel grades.