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Technical Paper

New High Strength Steels Help Automakers Reach Future Goals for Safety, Affordability, Fuel Efficiency and Environmental Responsibility

Vehicle weight reduction, reduced costs and improved safety performance are the main driving forces behind material selection for automotive applications. High strength steels (HSS) have demonstrated their ability to meet these demands and consequently have been the fastest growing light-weighting material in vehicle structures for the past decade. The evolution in steel technology in recent years has produced new grades of highly formable, advanced high strength steel (AHSS) grades that will continue to meet these automotive demands into the next decade. This paper describes how the remarkable combination of formability, strength, ductility, durability, strain-rate sensitivity and strain hardening characteristics of the AHSS grades enable affordable weight reduction while improving crash safety.
Technical Paper

Springback of Sheet Metal Subjected to Multiple Bending-Unbending Cycles

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.
Technical Paper

Ambient Aging Behavior of Hot Dip Coated Bake Hardenable Steel Sheets Relevant to Automotive Industry

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.
Technical Paper

Static and Dynamic Dent Resistance Performance of Automotive Steel Body Panels

In recent years, strict weight reduction targets have pushed auto manufacturers to use lighter gauge sheet steels in all areas of the vehicle including exterior body panels. As sheet metal thicknesses are reduced, dentability of body panels becomes of increasing concern. Thus, the goal becomes one of reducing sheet metal thickness while maintaining acceptable dent resistance. Most prior work in this area has focused on quasi-static loading conditions. In this study, both quasi-static and dynamic dent tests are evaluated. Fully assembled doors made from mild, medium strength bake hardenable and non-bake hardenable steels are examined. The quasi-static dent test is run at a test speed of 0.1 m/minute while the dynamic dent test is run at a test speed of 26.8 m/minute. Dynamic dent testing is of interest because it more closely approximates real life denting conditions such as in-plant handling and transit damage, and parking lot damage from car door and shopping cart impact.
Technical Paper

Development of High Strength Batch Annealed Low Carbon Steel for Automotive Application

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.
Technical Paper

Development of 40 to 50 kg/mm2 Tensile Strength, Highly Formable Cold-Rolled Low Carbon Steel by α + γ Phase Annealing

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.
Technical Paper

An Evaluation of Interface Friction in Different Forming Models for Coated Steel Sheets

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.
Technical Paper

The Effects of Die Materials and Electro-Etching on Frictional Characteristics of Automotive Sheet Steels

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.
Technical Paper

Strain Rate Sensitivity of Automotive Steels

Strain rate sensitivity is an important material property in the formability of sheet metal. In this study, strain rate sensitivity is evaluated for several different grades of steel. Strain rate sensitivity varies from 0.01 to 0.022 for the steels tested. It was found that formable steels such as IF and AKDQ steels have both high n-value (strain hardening) and m-value (strain rate sensitivity). Positive strain rate sensitivity results in a significant increase in the yield strength and tensile strength at higher strain rates. The n-value decreases with strain rate for all of the steels. The total elongation decreases slightly with strain rate for the lower strength steels but is constant or even increases slightly with strain rate for high strength steels. For a typical AKDQ steel, the increase in yield strength can be as high as 43% for an increase in strain rate from 0.002 /s to 2.0 /s.
Technical Paper

980 XK: A Critical Automotive Application for HSLA Steel

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.
Technical Paper

High Strain Rate Behavior of Some Hot and Cold Rolled Low Carbon Steels

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.
Technical Paper

Strain Rate Effects on the Properties of High Strength, Low Alloy Steels

New design techniques will require that more be known about the response of metals to various service conditions. This paper discusses experimental work which was done to define the response of several hot worked metals to several deformation rates. The results show that ferrous materials with low alloy contents are very strain rate sensitive under the conditions of testing, regardless of the static strength level. These materials exhibit strength and absorbed-energy increases and uniform elongation losses with strain rate increases. A 6061-T6 aluminum alloy tested for comparison showed no strain rate sensitivity over the range of testing conditions. In a practical sense, ferrous alloys will be stronger at high loading rates than expected from ordinary mechanical property measurements. This can be an important advantage when considering dent and crash resistance.
Technical Paper

Fatigue Properties of Galvanized Steel and Hot Rolled Steel Before and After Exposure to Salt Spray

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.