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Technical Paper

Corrosion Performance and Fuel Compatibility of Prepainted Zinc-Nickel Coated Steel for Fuel Tank Applications

Automotive demands for increased service life and use of flexible fuel blends of alcohol and gasoline have propelled the development of new materials for automotive fuel systems. Traditional fuel system materials, i.e., bare or prepainted terne coated steel sheet, which do not meet the new requirements are being replaced with prepainted zinc-nickel coated steel sheet. Automotive fuel tanks and fuel system components made from the new prepainted zinc-nickel steel sheet offer increased service life and compatibility with the entire range of flexible fuel blends. This paper describes the results of several laboratory corrosion studies which examined the environmental corrosion performance and the fuel compatibility of prepainted zinc-nickel coated steel as a function of several system properties. Performance is compared to prepainted terne, prepainted hot dip tin, and prepainted galvanneal.
Technical Paper

Effects of Coating Weight and Pretreatment on the Painted Corrosion Performance of Coated Steel Sheet

A program was initiated to determine corrosion performance of coated sheet products. The program examined the performance of galvannealed, hot-dip galvanized and electroplated zinc coated steel sheet in a variety of cyclic laboratory corrosion tests. Both perforation and cosmetic corrosion susceptibility were evaluated. Materials were pretreated using immersion and spray phosphate systems and were also prepared without a phosphate pretreatment. Results of these studies will be presented in this paper.
Technical Paper

Effects of Steel Surface Texture on Appearance After Painting

An extensive investigation was conducted to assess the influence of steel sheet surface finish (i.e. topography or “texture”) on painted surface appearance. Ten sets of steel panels representing a variety of shot blast, EDT, laser, and bright surface textures were painted simultaneously using advanced solvent-based luxury vehicle paint systems. Paint appearance was measured using a relatively new Autospect instrument and also in terms of conventional Distinctness of Reflected Image (DOI). The results are discussed in detail with respect to 1) the influence of different steel surface textures on paint appearance, 2) the evolution of surface topography during painting, and 3) the implications of this work.
Technical Paper

Effects of Substrate and Phosphate Type on the Painted Corrosion Performance of Galvanneal Coated Sheet

Galvannealed sheet steel is gaining acceptance by automotive manufacturers due to its combination of properties and cost. Galvanneal can be produced by several processes including the most common method termed the “hot” line hot dip coating process, but also using “flux” line hot dip coating application. Similarly, several types of phosphate coatings: Zn, Zn-Ni, and Zn-Ni-Mn phosphates can be applied to these materials prior to painting to enhance paint adhesion and painted corrosion resistance. This manuscript reports the affects of commercial phosphate type and galvanneal type (based on production method and properties) on the corrosion performance of galvanneal. In addition, painted corrosion performance of galvanneal is compared to painted corrosion performance of electroplated zinc. The manuscript also describes the use of electrochemical methods as one alternative to conventional test methods resulting in substantial time savings.
Technical Paper

Influence of Strain State on Powdering of Galvannealed Sheet Steel

Galvanneal powdering was examined on a stabilized ultra low carbon steel substrate as a function of strain state using cup drawing and in-plane stretching experiments to simulate deformations encountered in production stampings. Significant powdering was encountered in drawing while minimal powdering occurred in in-plane stretching. Powdering was measured at specific locations and correlated with strains in those locations. A powdering map was generated in strain space using the experimental data. A few measurements of powdering on selected regions of an automotive stamped part are reported.
Technical Paper

Performance of Coatings for Underbody Structural Components

The Auto/Steel Partnership established the Light Truck Frame Project Group in 1996 with two objectives: (a) to develop materials, design and fabrication knowledge that would enable the frames on North American OEM (original equipment manufacturer) light trucks to be reduced in weight, and (b) to improve corrosion resistance of frames on these vehicles, thereby allowing a reduction in the thickness of the components and a reduction in frame weight. To address the issues relating to corrosion, a subgroup of the Light Truck Frame Project Group was formed. The group comprised representatives from the North American automotive companies, test laboratories, frame manufacturers, and steel producers. As part of a comprehensive test program, the Corrosion Subgroup has completed tests on frame coatings. Using coated panels of a low carbon hot rolled and pickled steel sheet and two types of accelerated cyclic corrosion tests, seven frame coatings were tested for corrosion performance.
Technical Paper

Recent Progress in the Development of Forming Limit Curves for Automotive Sheet Steels

Forming Limit Diagrams (FLD) are extensively used in North American press shops during tooling trials and in production for problem identification/resolution. The Keeler-Goodwin FLC shape and the correlation developed by Keeler and Brazier (based on n-value and thickness) have been widely accepted as the Standard FLC Method to predict forming limit curves for commercial steels. In this paper, the Standard FLC Method is reviewed, and an alternative approach used at the authors' laboratory (Bethlehem FLC Method) is described. The two methods are discussed in the context of more recent experimental determinations of FLC's for a variety of “modern” sheet steels including DQSK, Interstitial Free and Bake-Hardening steels, as well as coated sheet products. Some specific press-shop examples are also presented, which further highlight the value to industry of re-examining the Standard FLC Methodology used in circle-grid analysis.
Technical Paper

Sheet Steel Surface Treatments for Enhanced Formability

Surface friction is an important characteristic which influences the formability of sheet steel products. Numerous friction tests have been developed, and many previous investigations have reported effects of surface characteristics, coatings, lubrication, etc., on formability. Recently, increased attention has been focussed on reducing friction via the application of solid film lubricants or special surface post-treatments such as phosphates, metallics/intermetallics, etc. This paper presents the results of selected laboratory evaluations conducted using a variety of steels and surface treatments. Formability was measured using Limiting Dome Height and Drawbead Simulator friction testing, along with Limiting Draw Ratio testing in one instance. The examples highlight some potential opportunities which may be considered for improving formability in industrial stamping operations.
Technical Paper

The Effects of Coating Composition, Substrate, and Welding Machine on the Resistance Spot Welding Behavior of Hot-Dip Galvanized and Galvannealed Sheet Steels

This joint research study by Bethlehem Steel Corporation and the U. S. Steel Group of the USX Corporation reveals that several significant factors affect the welding performance (electrode life) of hot-dip galvanized and galvannealed sheet steels. The statistical analyses of the electrode life data reveal that coating aluminum content affects electrode wear behavior of both the hot-dip galvanized and the galvannealed coatings. Higher levels of aluminum content cause more rapid electrode wear and shorten electrode life. Iron content in the galvannealed coatings seems to have some positive effect on increasing electrode life, but its effect is not nearly as significant as the negative effect of aluminum content. As the Fe/Al coating content ratio of galvannealed coatings increases, electrode life increases. Substrate type also affects electrode life, with the IF grade consistently producing shorter electrode lives.