Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 121

mid-size BIW and representing a saving of 14%. It has been said, therefore, that
there would be a case for manufacturing a ULSAB body even if no savings in
weight had been predicted.
It is useful to note that steels with a yield stress above 210 MPa have been
specified for two-thirds of the parts in the ULSAB design and a high proportion of
the steel used is also coated with one or other of the corrosion protective coatings
available. It is clear, therefore, that corrosion protection, as well as mechanical
performance, has been an important consideration of the design. Several steel
bodies are being built to confirm and demonstrate the phase 1 design and perfor-
mance. These are to be exhibited around the world during 1998.

Improved corrosion resistance

In recent years, the use of zinc-coated steels in car body construction has
increased dramatically as manufacturers have attempted to improve their
warranties against corrosion. Two types of corrosion have to be considered,
namely:

1. Perforation corrosion which takes place from the inside of the vehicle to the
   outside, causing a hole to appear.


2. Cosmetic corrosion which takes place on the external surfaces of the vehicle.

Most manufacturers have incorporated about 70% coated steel in the body-in-
white and it is reported that Audi has adopted a 100% coated steel construction.
Both hot dip and electro-galvanized steels are being used in place of uncoated,
cold-reduced steel and their application to different parts of the car body has
been described by Dasarathy and Goodwin. 45 This is illustrated in Figure 1.120
which refers to the materials listed below. It is useful to note, however, that there
is a general trend away from the use of electro-coated material towards the use
of galvanneal and that hot dip aluminium coated steels are used for car silencers.

9 CRIFF: uncoated steel
, Galvatite: hot dip zinc
e Galvanneal: hot dip iron-zinc alloy

CR 1 FF

Durasteel

Figure 1.120 Typical automotive body panels fabricated from metallic-coated steels
(After Dasarathy and Goodwin 45)