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High Strength Steels

A Key Material in Critical Safety Components

Auto manufacturers are challenged by increasing oil prices, stricter emission standards, and greater demands on safety. Lighter and stronger steels in structural components are now required from OEMs and suppliers. How does a die designer handle forming and piercing with these new High Strength Steels (HSS)?

Typical High Strength Steel components in Automotive bodies

Typical High Strength Steel Components in Automotive Bodies
(Courtesy: VW-EU)

Types of high strength steels

HSS grades are varied. Some grades have elongation similar to that of conventional cold or hot rolled steels with higher yield and tensile strengths such as High Strength Low Alloy (HSLA) 420. Other grades have very little elongation and exhibit very high yield tensile strengths. Some grades achieve high tensile strengths through a hardening process. These materials behave differently enough to warrant special treatment in the die design and presswork.

Dent resistant grades

One of the earlier forms of high strength steel used for automotive components came in the form of Dent Resistance (DR) grades. These DR grades were desirable in auto components that required increased load carrying capability and improved crash energy management. But their most important factor for mass reduction was a reduction in sheet metal thickness. However, it is well known that an increase in strength typically comes with a loss in ductility or formability. Steel manufacturers made these grades available in two flavors — a Bake Hardenable (BH) grade and a Non-Bake Hardenable (NBH) grade.

The difference between BH and NBH

BH grades achieve an increase in strength through the process of post draw baking cycle. This is typically done in a paint dry oven after forming. NBH grades gain their increase during the forming operation through work hardening. Die design should account for larger forming forces on tools used in NBH forming, and should favor a relatively more generous drawing geometry (corner radii). You could expect higher contact pressures during forming with NBH grades. Severe galling is a possibility and coatings may be recommended on certain applications.