Steels_ Metallurgy and Applications, Third Edition

118 Steels: Metallurgy and Applications

the equivalent grade of non-bake-hardening steel but with the performance in
service of one grade higher.
Many applications of bake-hardening steels aim to provide improved plastic
dent resistance mainly in lightly deformed parts of panels with low curvature such
as bonnets and boot lids. Drewes and Engl 1s3 presented a relationship between
yield stress, sheet thickness and load required to give the same residual dent
depth of 0.2 mm as illustrated in Figure 1.117. This may be used to calculate
what reduction in gauge could be obtained for the same dent resistance by using
a higher yield stress. The curves for rephosphorized, micro-alloyed and bake-
hardening steel produce similar effects for the same sheet thickness but the
dual-phase steel gives a different effect due to the high rate of work hardening.
Figure 1.118 Is4 gives an example of the various types of high-strength steel
that may typically be used for the various parts of an existing motor vehicle
with an indication of change for the future. Figure 1.119 gives a summary of the
historical development of the use of high-strength steel in cars from a European
point of view.
Hitherto, weight reduction has mainly been achieved by substitution of thinner
steels for thicker ones without any major change in design concept. Recently,
a major project Is5 has been initiated, with phase 1 sponsored by 32 large steel
companies across the world, to maximize weight reduction using any existing
manufacturing processes and steel grades that are available. The project known
as the ULSAB (ultra light steel auto body) project, aims to design a body-in-
white (BIW) with the same or higher-performance features as existing designs
but with a reduced weight. Phase 1 of this project has been completed 1s5 and
progress has been reviewed. 186,187
A bench mark of existing designs was obtained by taking mean values over a
number of suitable, mid-size, existing designs and normalizing the figures to take

6OO

500

z~ 400

LL o

'3oo

200

100

,. ,.

Rephosphorized and /

micro-alloyed steels/0.88 mm

..... Dual-phase / /~

steels /" // 0.75 mm

//

I I I

200 3so ,oo

YS (N/ram 2)

Figure 1.117 Effect of yield strength and sheet thickness on dent resistance under
quasistatic localized loading (Fo.2 = load producing 0.2 mm remaining dent depth) (After
Drewes and Engl. ~s3)