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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.

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.

This paper outlines the advantage of using galvanized steel to protect critical structural members from corrosion. For example, the mean 2 X 106 cycle fatigue strength for 0.070 and 0.085 in thick galvanized steel was approximately 26,000 psi. It was unaffected by exposure to 96 h of salt spray before testing. Comparable fatigue strength for 0.095 in thick hot rolled steel of nearly equivalent chemistry was about 28,000 psi. Exposed to 96 h of salt spray, it dropped to about 25,400 psi.

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.

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.