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Circle grid analysis is often used throughout the different stages of stamping tool development to locate and correct critical strains in stampings based on the forming limit diagram of the sheet steel. Understanding the interaction among the grid, tooling, and sheet steel to be formed is important in making a reliable part strain analysis. It has been observed that a given sheet steel may produce acceptable parts in soft tool, yet split or neck in the hard tool/production stage even with similar tooling geometry. Four different automotive sheet steels and five different fixed bead material types were selected for draw bead simulator testing to determine the effect of electro-etching of the grid on frictional characteristics. Of the sheet steels examined - uncoated cold roll, hot dip galvanized, hot dip galvanneal, and electrogalvanized, circle gridding tended to increase the coefficient of friction for all but hot dip galvanneal, where the friction decreased with gridding.

A Draw Bead Simulator (DBS), with modified draw beads, was employed in this study to understand the springback behavior of sheet metal subjected to multiple bending-unbending cycles. The investigations were carried out in both the rolling and the transverse rolling directions on four types of materials: Electro-Galvanized DQ steel, light and heavy gauge Hot-Dip Galvanealed High Strength Steels, and Aluminum alloy AL6111. The sheet geometries, thickness strains, pulling forces and clamping forces were measured and analyzed for the purpose of establishing a benchmark database for numerical predictions of springback. The results indicate that the springback curvature changes dramatically with the die holding force. The conditions at which the springback is minimized was observed and found to depend on the material properties and the sheet thickness. Analysis with an implicit FEM showed that the predicted and the experimental results are in very good agreement.

The high strain rate behavior of three types of low carbon steel has been investigated in the strain rate regime of .006-100 sec-1. These types included hot and/or cold rolled (i) rimmed steels, (ii) aluminum killed steels and (iii) high strength low alloy steels with various combinations of cold rolling and aging treatments. The tensile properties reported at four strain rates in the above range are yield strength, tensile strength, uniform and total elongation, strain hardening exponent and strength coefficient. Strain rate hardening exponent at several strain levels is also reported. New experimental techniques are discussed. The results are presented in tabular form and are discussed with representative graphs. In general, strength increases and ductility decreases with increasing strain rate.

The use of tailor welded blank sheets for body-in-white applications has brought many new forming challenges. Mechanical properties of tailor welded blanks measured from the tension test showed significantly higher strengths and lower elongations than in the base metal. The forming limit of tailor welded blanks was significantly reduced when compared to that in the base metal. For the weld made from materials with dissimilar thicknesses/strengths, additional formability issues were also raised in addition to those for the weld made from materials with similar thickness/strength. These issues increase the potential for failure in the lesser thickness/lower strength material in the area immediately adjacent to the weld when the difference in thicknesses or strengths of the two materials increases.

In order to increase the strength and formability of batch annealed sheet steels, design of experiment method was used to set up an experimental matrix with five factors, including C, Mn, P in steels, coiling, and batch annealing temperatures, at two levels. Effects of these factors were analyzed using analysis of variance and linear regression methods for cold spot and hot spot, respectively. Linear regression results showed that higher alloying element contents and coiling temperature will increase strength and deteriorate elongation, which is opposite to the effect of annealing temperature. Analysis of variance showed similar results to those of linear regression, except the effect of C on elongation and effect of coiling temperature on tensile strength and elongation are negligible for cold spot. For hot spot, effect of coiling temperature on tensile strength is small.

The effects of key alloy and process parameters on the mechanical properties of high strength, highly formable low carbon sheet steels have been determined by a combination of laboratory processing, Taguchi Design of Experiments method, and multiple linear regression analysis. Factors studied include C, Mn, P, hot mill coiling temperature, in-line annealing temperature, and annealing line speed. Tensile strength levels were varied from 40 to 50 kg/mm2 with elongation of 30 to 40%. Annealing conditions were varied from partial to full recrystallization in both the single and α + γ two phase region. Effects of the chemistry and process parameters on the mechanical properties will be discussed, with particular emphasis on conditions required to produce a hot dipped galvannealed product meeting 45 kg/mm2 tensile strength and 35% elongation minimum requirements.

Interface friction between sheet metal and tooling in sheet metal forming is examined in different forming modes using laboratory simulative tests. Stretchability is studied by the limiting dome height test; drawability is investigated by a four inch Swift cup draw test and the coefficient of friction is measured by the draw bead simulator under bending and unbending deformation. The responses of the interface friction in six different coated and uncoated steel sheets are studied using seven different lubricants. It is found that the interface friction between sheet metal and tooling is very sensitive to the forming mode and the type of coating. For the same lubricant and coated material, two different forming modes may produce very different results in interface friction. However, overall good and bad lubricants for all forming modes can be determined for a given coated material using these three tests.

The ambient temperature aging behavior of hot dip coated cold-rolled bake hardenable steel sheets manufactured by an in-line continuous annealing technique has been investigated. These materials exhibit good and uniform mechanical and bake hardening properties as-produced and after aging. For the same chemistry, carbon and nitrogen solute levels can be higher in galvanized (GI) products than in galvannealed (GA) products, with the result, of course, of higher bake hardening and aging indexes. Based on the classical static strain aging theory, the interactions between interstitial solutes and dislocations during the ambient aging of these steel sheets can be classified into four stages. Optimal press forming or stamping operations are best performed in the first or second stages of aging but no significant forming difficulties should be expected even if these operations are conducted during the later aging stages.

Previous applications of 980 XK steels in the automotive industry have been limited. However, to meet increased structural requirements of MVSS-301, AMC has incorporated 980 XK steel in the 1977 Gremlin and Hornet underbody rear sill subassemblies. This paper emphasizes how formability and spot weldability characteristics were optimized in order to meet the vehicular crashworthiness required in this structural application. Traditional mild steel design, forming, and spot welding procedures were successfully modified to utilize 980 XK. These modifications are practical and have been successfully incorporated in production operations.