|New Designations for Hot-Dip Coated Steel Sheet
L ast month, ASTM Committee A05 on Metallic-Coated Iron and Steel Products released requirements for two new designations from ASTM in Specification A 653 for Hot-Dip Zinc- or Zinc-Iron Alloy-Coated Sheet. These new designationssolution hardened steel (SHS) and bake hardenable steel (BHS)are now available in five different grades with minimum yield strengths between 180 and 300 MPa (26 and 44 ksi).
These steels employ additions of elements such as manganese, phosphorus and silicon, which provide strengthening through a mechanism referred to by metallurgists as solid-solution hardening, says ASTM standards developer Keith Taylor, supervisor, Light Flat Rolled Products, ISG Bethlehem Research, Bethlehem, Pa. In addition, he says, the chemical composition and processing of BHS grades are controlled so that additional hardening occurs when the steel is subject to a paint-baking or similar thermal treatment, through a mechanism known as strain aging. A 653 prescribes a minimum bake hardening increment for BHS products, and a new annex in A 653 describes the method for measuring the degree of bake hardening.
A task group of Subcommittee A05.11 on Sheet Specifications, which developed the standard, included metallurgists and quality engineers from the producer and general interest communities.
Automakers have used solution hardened and bake hardenable sheet steels for many years, principally for outer body panels requiring high degrees of formability and strength, Taylor explains. Compared with conventional drawing and deep drawing steels, the SHS and BHS grades develop higher strength levels after forming and baking. These steels have provided automakers opportunities for reducing vehicle weight and improving fuel economy through down-gauging of parts.
Additionally, he concludes, the improved dent-resistance afforded by these steels can reduce warranty claims. Many automakers have developed their own specifications for these steels. The new revisions to A 653 represent a compromise between the various automotive specifications, and may help to stimulate use of solution hardened and bake hardenable products in other applications.
For further technical information, contact Keith Taylor, Light Flat Rolled Products, ISG Bethlehem Research, Bethlehem, Pa. (phone: 610/694-6747). Committee A05 meets Nov. 16-18 in Tampa, Fla. For meeting or membership details, contact Maxine Topping, manager, Technical Committee Operations, ASTM International (phone: 610/832-9737). //
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