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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.

Springback prediction is one of the roadblocks for using advanced high strength steel in the automotive industry. Accurate characterization and modeling of the mechanical behavior of AHSS is recognized as one of the critical factors for successful prediction of springback. Conventional tensile test based material characterization and constitutive modeling may lead to poor springback simulation accuracy. Aiming to accurately predict springback, a series of advanced material characterizations including bi-axial material testing, large-strain loading path reversal testing, unloading tests at large strain, stress-strain behavior beyond uniform elongation, were performed for selected AHSS and associated constitutive models were developed to incorporate these characterizations. Validations through lab samples and industrial parts show that the AHSS springback prediction accuracy is significantly improved with these improved material models.

With interest in improving vehicle quality and customer satisfaction, Ford Motor Company initiated an effort aimed at improving dent resistance of closure panels. An investigation of various means of product improvement led to the recognition of dual phase steels, due to their inherent formability and strain hardening attributes, as the most appropriate steel panel for outer panel applications. Ispat Inland's new Electro-galvanized dual phase steel DI-FORM 500 (henceforth referred to by the generic designation, DP500), which meets 500 MPa minimum tensile strength, was specifically designed to meet automotive exposed quality standards. This paper compares the dent resistance performance of automotive door assemblies manufactured with both Bake Hardenable 210 (BH210) and DP500 door outer panels. Results indicate the achievement of significantly improved outer panel dent resistance through the use of the DP500 product.

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.

To aid in understanding die wear when stamping AHSS, a study to characterize the contact pressure distribution in drawbeads during stamping had been undertaken. As direct measurement of contact pressure for a drawbead is not feasible during metal flow, a combination of experimental and Finite Element (FE) simulation techniques were used to determine the contact pressure distributions and the maximum contact pressure for a number of different conditions. Testing was conducted using the Drawbead Simulator (DBS) for two different bead configurations. The materials in this investigation were 0.7mm and 0.8mm EG BH210 and EG DP500. Static Implicit FE analyses were conducted with ABAQUS Standard using 2D plane strain continuum elements. A combined hardening model in conjunction with strain rate effects was used to describe material behavior as it flows through the drawbeads.

The risk of skid lines for Class A panels has to be assessed before releasing the die development for hard tooling. Criteria are needed to predict skid lines in the formability evaluation stage to avoid expensive changes to tooling and process for resolving skid line issue in production. In this study, criteria using three different measured parameters were developed and validated. A draw-stretch-draw (DSD) test procedure was developed to generate skid lines on lab samples for the physical evaluation. This was done using tooling with various die entry radii and different draw beads. The skid line severity of lab samples was rated by specialists in the inspection of automotive outer panel surface quality. The skid line rating was correlated with geometric measurements of the lab samples after the DSD test. The sensitivity of the appearance of skid lines to tooling and process parameter variations was identified.

Forming limit curve (FLC) and fracture forming limit curve (FFLC) are valuable tools for failure prediction in forming simulation and die try-out in press shops. In this paper, methods are presented to determine FLC and FFLC for sheets of advanced high strength steels (AHSS) using digital image correlation (DIC). Dome tests were conducted on AHSS specimens using DIC system for strain measurement. For generating FLCs, two approaches are introduced to determine the onset of localized necking by analyzing the strain history at critical locations, one of which has been implemented into the commercial DIC software Vic-3D (Correlated Solution inc.). For determination of FFLC, a method for measuring fracture strains based on the strain path evolution is presented. The measured FLCs for several AHSS were compared to the FLCs using ISO 12004-2, and conventional North American experimental measurements and empirical equations.

To improve the formability of advanced high strength steels, the interaction between steel sheet, tool material and tool coating was investigated. Square cup drawing experiments were conducted to determine the range of binder forces for forming good cups without wrinkling or splitting. Binder Span of Control (BSC) tests were conducted for DP590, TRIP590, DP780, DP780 EG and DP980 using three uncoated tool steels and two coatings on a standard tool steel substrate. The experimental results indicate that the binder span for forming good cups is sensitive to the choice of tool material and tool coating and the effect of lubricant on formability also varies with tooling material and coating. The obtained binder spans were compared and the best coating plus tool steel combinations for steels of different grades were identified. In addition, roughness of the tooling surface was measured before and after stamping.

Accurate description of the plastic yield locus is important for accurate prediction of sheet metal formability and springback using FEM. This paper presents experimental results obtained for the initial plastic yield locus and its evolution for some selected Advanced High Strength Steels (AHSS). A review of available experimental methods was conducted to select appropriate techniques for testing. For loading in tension-shear, the Arcan test was selected, however because of lack of uniformity of the stress distribution, the test was not included in the final series of tests. Shear testing, uniaxial tensile testing, plane strain testing and stacked compression testing were used to determine the yield locus. From the test results and analysis for the selected AHSS, it seems that the onset of initial yielding and its isotropic evolution to 4% plastic strain is best described by the von Mises yield function.

Application of Advanced High Strength Steels (AHSS) to closure and structural parts in the automotive industry is increasing in future models. In addition to weight reduction, the other primary motivation to consider these products is the improvement of structural performance that is needed to meet future stringent safety standards. AHSS products have a combination of unique microstructures and mechanical behavior. It is important to develop basic knowledge and understanding of all the manufacturing aspects of forming these products, so that robust forming processes can be engineered to successfully form parts in a production environment. The edge condition obtained after post-draw operations such as trimming has a significant influence on processes such as stretch flanging. A study to investigate the influence of punching clearance on the edge characteristics of various AHSS products has been initiated at Mittal Steel R&D.

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.

This study introduces a new practical calibration approach of ductile fracture models by performing square punch tests on metal sheets. During square punch tests, ductile fracture occurs at either the corner of die or punch radius when applying different clamping loads and lubrication conditions. At the corner of die radius, in-plane pure shear is induced at the intersection between the side-walls and the flange by combined tension and compression. On the other hand, the material at the corner of the punch radius is under combined bending and biaxial tension. The material studied in this paper is advanced high strength steel (AHSS) DP780 from ArcelorMittal. Isotropic J2 plasticity model with mixed Swift-Voce hardening rule is calibrated from uniaxial tensile tests.

Prediction of dent resistance of automotive closure panels using Finite Element Analysis (FEA) has increased in popularity as an effective screening tool to determine the optimum combination of material properties and thickness for a given exposed panel application. The analysis process typically includes formability analysis, the results of which are mapped onto a structural model for analysis of dent resistance. Although this method is general, allowing for consideration of the entire panel assembly and boundary conditions, the approach can be very time consuming especially when evaluating several combinations of material grade and thicknesses. This paper presents an alternative approach for the prediction of dent resistance. Using a number of screening simulations the most influential variables affecting dent resistance were determined. A rectangular bi-curved plate was used to approximate the panel assembly.