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Application Considerations

Properties  |  Coating Thickness  |  Surface Treatments

Properties of the Base Steel
U. S. Steel Terne Sheet and U. S. Steel NI-TERNE Sheet are available in three basic designations related to differing types of steel strength and formability.

• Commercial Steel (CS)
• Deep Drawing Steel (DDS)
• Extra Deep Drawing Steel (EDDS)

The mechanical properties of terne coated sheet are similar to those found with cold rolled sheet of similar strength and formability levels. The steel chemistries and mechanical properties for these designations are given in tables below. These are consistent with the requirements found in ASTM Specification A308.

When an ellipses (...) appears in this table, there is no requirement, but the analysis shall be reported.

^A  For CS Designation, specify Type B to avoid carbon levels below 0.02%.

^B  When a deoxidized steel is required for the application, CS may be ordered to a minimum of  0.02% total aluminum.

^C  CS Type A describes the typical commercial quality product previously included in
ASTM A366.

^D  May be furnished as a stabilized steel at producer's options.

^E  Furnish EDDS as a stabilized steel. Analyze and report the elements used for chemical stabilization as part of the heat analysis.

^B  These typical mechanical properties apply to a full range of steel sheet thicknesses. The yield strength tends to increase and some of the formability values tend to decrease as the steel sheet thickness decreases.

^C r_m Value - Average plastic strain rates as determined by method in Test Method E 517.

^D  n Value - Strain hardening exponent as determined by method in Test Method E 646.

^E  No typical mechanical properties have been established.

^F The DDS designation encompasses the previous Drawing Quality, Special Killed product.

^G Furnish EDDS as a stabilized steel. It will be free from changes in mechanical properties over time, that is, nonaging.

The hot-dip coating process is used to apply the terne coating. However, it is different than the standard hot-dip process in that a flux-coating process is used to achieve good bonding of the coating. This process is discussed in the section on hot-dip coating. In this process, the steel is annealed prior to coating so that the final mechanical properties are already imparted to the steel before processing on the coating line. Thus, the term "cold process" is used. It involves the application of a fluxing agent (a water-based solution) onto the steel strip surfaces prior to immersion in the molten terne metal bath. The chemical reaction between the flux and the steel surface essentially provide a clean steel surface so that the tin in the terne coating metal can diffuse into the steel forming a metallurgical bond. This bonding reaction is required to achieve a pore-free coating as well as to achieve a smooth, continuous, tightly adhering coating. The fluxing action is comparable to that involved with conventional soldering. 

U. S. Steel Terne Sheet and U. S. Steel NI-TERNE Sheet are produced to the general requirements contained in ASTM Specification A924, the general requirement specification for hot-dip coated products. As stated previously, the properties related to the steel chemistry for the different designations are contained in ASTM Specification A308.

Coating Thickness
Coating thickness (measured as coating weight in ounces per square foot or grams per square meter) is an important factor in the effective application of terne coated sheet.  Heavier coating weights generally provide greater corrosion resistance. ASTM Specification A308 defines the range of coating weights commonly produced. The terms, LT25, LT40, etc. are typically used to specify the coating weight requirements to the producing plant. The specific meaning of these designations is presented in the following table:

• Chemical treatment.
• Oil.
• Chemical treatment and oil.

Chemical Treatment
"Chemical treatment" consists of the application of a thin, water-based film that contains inorganic corrosion inhibitors. After drying, the treatment is almost colorless. The treatment is applied to both surfaces of the sheet.

As with most metallic coatings on steel, the terne-metal coating is susceptible to staining if the steel in coil form or stacked cut sheets becomes wet. The stain generally appears as a dark stain. It is composed of corrosion products from the reaction between the coating and water in the presence of limited oxygen. The chemical treatment makes the surface more resistant to "humid-storage staining" than an untreated surface.

Oil
Terne coated sheet can also be ordered with "oil" to inhibit the onset of storage staining if the coil or sheets become wet during transit or storage. The oil is applied at the exit end of the terne coating facility. Its primary purpose is to prevent staining during storage, although it can also serve as a lubricant for mild forming operations. Its lubricating characteristics are not optimized since the oil's main requirement is to serve as a corrosion inhibitor.

Oiling is generally less effective than a chemical treatment for protection against humid-storage staining, but it clearly offers good protection as long as the product is handled with reasonable care. If the terne coated sheet is intended to be painted, oil should be used as the corrosion inhibitor, unless the effect of the chemical treatment on paint adherence has been evaluated.  Oil is often used when the terne coated sheet subsequently will be painted.

Chemical Treatment and Oil
A combination of "chemical treatment and oil" can be specified when the effectiveness of chemical treatment is desired for humid-storage stain resistance along with rust preventive oil for enhanced formability. This combination offers very good protection to the coiled steel.

Dry
Terne coated sheet is shipped "dry" if neither chemical treatment nor oil is compatible with the customer's requirements.  Because the untreated terne coated surface is susceptible to humid-storage stain, terne coated sheet produced dry must be transported and stored with special precautions to preserve low humidity conditions within the body of the coil or lift of stacked sheets