Steels_ Metallurgy and Applications, Third Edition

116 Steels: Metallurgy and Applications

Kilometres travelled per litre

26 [--

24=~

20

18

16"

14

12 r.

lo~._

800

(^1992) I
B
982 9 "~
9 9 ~ .= II
II
41,
1000 1200 1400 1600 1800 2000 2200
Weight (kgs)
11~992
4k 1982
Figure 1.115 Plots illustrating the variation in kilometres travelled per litre of fuel with
car weight for 1982 and 1992 (After Is~
car with a low weight and hence low fuel consumption, will be attractive from
an economic point of view but it will also lead to a reduction in the formation
of the 'so-called' greenhouse gas, CO2, that can lead to global warming. 17s
Weight reduction and the use of high-strength steels
During the early 1970s, higher-strength steels were introduced in the United
States for safety-related or structural members such as bumper reinforcements,
side door intrusion beams and seat belt anchors. These components were manu-
factured mainly from hot-rolled, niobium-treated, micro-alloyed steels which
provided a favourable cost/strength/weight ratio compared with lower-strength
unalloyed steel or any other strip steel available at that time. In addition, their
use necessitated only minor changes in manufacturing methods and facilities. It
was unfortunate that since these were new components, their use inevitably led
to an increase in vehicle weight. More recently, higher-strength thinner gauge
steels have been used for some of these components to take out some of the
additional weight that had been introduced. These have included the ultra-high-
strength steels, with a tensile strength above 1000 MPa, strengthened by a high
proportion of transformation product, and steels which develop their ultra-high
strength by heat treatment and quenching after forming.
The oil crises of 1973 and 1979 provided the initial stimuli for weight reduction
itself. The car body is assembled from large body panels and constitutes about
25-30% of the total weight of a medium-size car. It is, therefore, the heaviest
vehicle component. It was clear, therefore, that reducing the weight of the body
could have a significant impact on the total weight of the complete car. It was
realized that this could be done by the substitution of steel by aluminium or plas-
tics, but the penalty would have been increased cost as illustrated in Figure 1.116.
It was generally accepted that the use of high-strength steel, which would enable
component performance to be maintained at a reduced thickness, was the only
way to achieve both weight and cost savings.