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Technical Paper

Die Materials and Treatments with Electrogalvaniied Steels

Frictional characteristics and galling tendencies were examined for three sheet metal forming die materials - cast steel, gray cast iron, and nodular cast iron - with four die treatments - unhardened, flame hardened, ion nitrided, and chrome plated. Repetitive testing was performed using a draw bead simulator and an electrogalvanized, free zinc coated steel with a nominal coating weight of 70 g/m2 per side. No die material/treatment condition significantly decreased friction. Higher friction was exhibited with chrome plated cast iron draw beads. Pickup of zinc on the draw beads varied with both die material and treatment. Surface irregularities arising from substrate characteristics interacting with surface treatments caused extreme die pickup. Depending on the natureand hardness of the draw bead surface treatment, the adherence of this zinc to the draw beads could be quite weak, minimizing continued local buildup.
Technical Paper

Effect of Steel Strengthening Mechanisms on Dent Resistance of Automotive Body Panels

The dent resistance of a panel is an important consideration when selecting a type of steel for manufacturing. Yield strength, thickness of the formed part, and stiffness (related to curvature) of the panel each contribute to the overall denting behavior. This study examines the dent resistance of several grades of steel; the objective is to determine if a fundamental difference in denting behavior exists between bake hardenable and non-bake hardenable grades after normalizing with respect to yield strength and thickness in the panel. Several key parameters that may contribute to the denting process are examined, including effects of yield strength, strain aging, work hardening, and stiffness. In addition, an apparent strain-path dependence of yield point return has been observed in bake hardenable steels; the effect of this on dent resistance has yet to be determined.
Technical Paper

The Effect of Zinc Coatings on the Formability of Automotive Sheet Steels

The growth of coated sheet steels in automotive applications continues to require new application technology in joining, finishing, painting and forming. The formability of zinc-coated steels depends on both the character of the substrate and nature of the coating. By eliminating the substrate as a variable in this study, the effect of various coatings (one-side electrogalvanized, hot-dip zinc and iron-zinc alloy) on formability was determined using simulative laboratory tests. Under conditions of plane strain and stretch, all coated and uncoated steels performed comparably and can be considered interchangeable with each other. However, for drawing conditions, the drawability parameter, rm, of the hot-dip iron-zinc alloy coated steels was inferior to that of both free zinc coatings and uncoated steels.
Technical Paper

The Interactions of Coated Steels, Die Materials, and Forming Lubricants

Sheet metal automotive components are developed in several stages, including “soft tool” (zinc alloy) prototype development and “hard” final tool tryout. Increased use of zinc and zinc-alloy coated steels for these stampings has raised concern about the reaction of metallic coatings on steels with the “soft” prototype die materials. Choice of forming lubricant adds further complications. To investigate this, a variety of hot-dip and electro-galvanized zinc and zinc-alloy coated mild steels were tested in several forming modes using both “hard” steel and “soft” prototype tools and several different lubricants. The zinc coating interaction with the prototype tooling depends on the nature of the coating on the steel, the deformation mode, and the lubricant. Therefore, proper selection of lubricant for soft tool tryout may be difficult. Hot-dip, alloy-coated steels pose special problems in this respect.
Technical Paper

The Limiting Dome Height Test for Assessing the Formability of Sheet Steel

The Limiting Dome Height (LDH) test has attracted much interest as a simulative tool for predicting press performance of materials. However, problems of test reproducibility have been observed. An experimental evaluation has been performed to determine the possible source or sources of these problems. The evaluation was performed using both uncoated and coated mild steels. A small tooling temperature rise and minimal punch - die misalignment had little effect on the LDHO value and on reproducibility. Increased clamping forces above a minimum threshold value improved reproducibility but did not significantly change the LDHO value. A low viscosity oil for cleaning and lubricating test samples was found to give the most satisfactory results. A correlation was found between LDHO and both the strain-hardening exponent and the total elongation obtained from uniaxial tensile tests. The lack of LDH test reproducibility remains unexplained.