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Application Considerations

Commercial Steel (CS) This designation is for the basic product, useful for many applications requiring the strength of steel combined with the workability needed for bending and moderate forming. It provides the strength and formability levels that most users need for general purpose applications. CS sheet is subject to a phenomenon called aging; therefore, if the sheet must be free from strain lines (stretcher strains, flutes, etc.) after fabrication, it should be roller leveled just before the forming operation.

Forming Steel (FS) The primary difference between FS and CS sheet is that FS sheet is processed to be more formable, i.e., easier to bend, etc. FS sheet is slightly more formable than CS and is typically used for applications where breakage (splitting) might be encountered when CS sheet is not quite formable enough to make a part. Often, FS is selected to allow easier bending with less springback. FS sheet is produced from specially selected steels, processed for improved and more uniform formability properties. FS sheet is subject to the same aging phenomenon as CS sheet; therefore, to avoid strain lines, FS sheet should be roller leveled just before the forming operation.

Deep Drawing Steel (DDS) DDS sheet is produced from aluminum-killed steel employing special steelmaking practices. It can be produced using restricted low-carbon steels or interstitial-free steels depending on the application requirement and the producing facility. It has forming characteristics superior to CS and FS sheet. These characteristics make it excellent for applications involving deep drawing or combinations of drawing and stretching. DDS sheet is non-aging. DDS sheet is recommended for use when improved drawability is required and the effects of aging (slight hardening, loss of ductility, and strain lines) are undesirable or roller-leveling equipment is not available.

Extra Deep Drawing Steel (EDDS) Coated sheet of this quality is characterized by excellent uniformity and exceptional formability. EDDS Sheet is produced from vacuum degassed steel to achieve a very low carbon content. Also, stabilizing elements such as titanium and niobium (columbium) are added during steel production to  combine with the residual amounts of carbon and nitrogen remaining in the degassed steel to make it "interstitial-free". The final product is excellent for deep drawn parts in that the sheet exhibits a high resistance to thinning during drawing. EDDS Sheet is non-aging. Thus, coil breaks, strain lines and fluting during fabrication are not encountered.

ASTM References for Hot-Dip Galvannealed

The mechanical properties of the base steel of galvannealed sheet are similar to the properties of regular galvanized sheet.

The following table contains typical values for U. S. Steel product when purchased to the above-listed designators. These numbers represent typical data. A broad range can be expected because the final sheet thickness has a significant influence on the mechanical properties and formability. Normally, the yield strength value tends to increase and the formability values tend to decrease as the steel sheet thickness decreases.

*The typical mechanical property values presented in this table are nonmandatory. They are intended solely to provide the purchaser with as much information as possible to make an informed decision on the steel to be specified. Since the mechanical properties often vary as a function of steel thickness, the yield strength for thicker steels may be lower than the number listed; similarly, the yield strength for thinner sheets may be higher than this number.

Coating Thickness
By far, the most common coating thickness used for galvannealed product (other than for automotive applications) is the A40 category. Additional coating weights are available; refer to ASTM Specification A653 and the following table for all the coating categories that are available. The table includes metric categories that can be purchased to g/sq. meter coating weights. Since U. S. Steel Galvannealed Sheet is designed to be painted, these coating weights provide the necessary corrosion resistance for most painted-product applications.

A40 is often preferred over A60 because the amount of powdering (actual loss of coating) that occurs during forming is significantly less and, after painting, the corrosion resistance is quite good.  

(1) The coating category number is the term by which this product is ordered.

(2) The weight of coating in oz. per sq. ft. refers to the total coating on both surfaces.  Typically, about half this coating is on each side.  The triple spot average encompasses an edge-center-edge sampling to determine adequate coverage across the sheet width.

(3). Ordered coating weight is specified on a per-side basis, e.g., 50/50 requires each surface to have a minimum weight of 50 g/m2.

(4). To convert from oz/sq ft to g/sq m, apply the following conversion factor: g/sq m = oz/sq ft X 305.15

Surface Finish

Galvannealed sheet has a uniform matte finish instead of the bright spangled finish associated with galvanized sheet. It is also available with an Extra Smooth finish, achieved by  temper rolling after coating. The Extra Smooth finish resembles the matte finish of cold rolled sheet.

Surface Treatment
Galvannealed product is susceptible to storage staining similar to galvanized when the coils get wet during transit or storage.  Instead of the "white rust" associated with galvanized, the surface of galvannealed product often turns black when storage staining occurs. This stain does not usually affect the product performance as long as the stain is tightly adhering to the surface. Brushing to remove the stain is not necessary as long as the paint adhesion is acceptable and the paint hides the dark surface effectively.

Galvannealed sheet can be shipped safely by ordering the product "oiled". The oil provides protection from humid-storage staining during shipment and storage. However, oils are often difficult to remove prior to painting due to the surface morphology of the  galvannealed coating. Good cleaning practices are needed to achieve good performance after painting.

Galvannealed product can be shipped "dry". When shipped "dry", care must be used to prevent conditions (condensation and/or moisture penetration) that lead to storage staining. Careful handling and transit practices must be rigidly applied. Proper packaging with vapor-inhibitive paper is recommended.

A rust inhibitive "chemical treatment" can also be applied to galvannealed product to prevent storage staining. To allow the use of a mill-applied chemical treatment, tests must be conducted to assure that the pretreating and painting practices are not adversely affected by the chemical treatment.

Galvannealed sheet can also be supplied with a phosphate coating. The phosphate coating is applied over the galvannealed coating by either a spray-application process or a rollcoating process. The spray-application process is used to provide a product that can be readily painted. The phosphate provides enhanced corrosion resistance by improving the resistance to undercutting corrosion of the paint. The tendency for paint delamination is reduced.

The roll-applied phosphate is intended to serve as a lubricant for deep-drawing operations. Actually, both types of phosphate coatings provide enhanced lubricating characteristics to the product. In practice, the phosphate coating decreases the need to apply special drawing compounds during deep drawing.