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Chapter 3 Vehicle Construction
Instead, various combinations of alloys are used to
produce various strengths of HSLA steels. The greater
strength of a conventional high strength steel part
compared to a mild steel part is caused by the small
and uniform grain structure of the conventional high
strength steel. HSLA steel parts are sensitive to heat.
Heat in excess of vehicle manufacturers limits will
change the small grains in HSLA to large grains and
weaken the part.
Advanced high-strength steel contains different
amounts and types of alloying elements than conven-
tional high-strength steel. One distinctive alloying
element used in advanced high-strength steel is
boron. Advanced high-strength steel has yield point
of up to approximately 200,000 psi. The grain struc-
ture of advanced high strength steel is different from
the grain structure of conventional high strength steel.
Advanced high strength steel has small islands, called
microstructures, dispersed among the small grains. As
with conventional high strength steel, advanced high
strength steel is sensitive to heat. Body parts, such
as door bumper brackets, crash beams and internal
reinforcements inside pillars and roofs are made from
advanced high-strength steel. Most damaged advanced
high-strength steel components cannot be repaired;
they must be replaced. Some types of advanced high
strength steels are martensitic, dual phase (DP), and
transformation induced plasticity (TRIP).
In order to protect the vehicle occupants with
a stronger vehicle but to make the vehicle lighter
for better gas mileage, the use of conventional and
advanced high strength steel will increase in the
future. In 2010 one vehicle manufacturer will produce
vehicles composed of 50% conventional or advanced
high strength steel and 50% mild steel compared to
vehicles made of 5% conventional high strength steel
and 95% mild steel in 1992.
Another type of steel used in vehicles is laminated
steel. Laminated steel is made of two l layers of steel
bonded together with a solid layer of adhesive. The
advantage of laminated steel over conventional single
layer steel is less noise. Driving a vehicle and even
just running the engine will cause the vehicle panels to
vibrate. The vibration can be felt and heard by the vehicle
occupants. Laminated steel reduces noise, vibration, and
harshness (NVH) by damping the vibrations. The layer of
adhesive adsorbs and does not transmit the vibrations,
keeping the steel layers quiet. Laminated steel is used in
the cowl and floor areas on some vehicles.
Depending on the application, both hot-rolled steel
and cold-rolled steel are used in vehicle construction.
Hot-rolled steel is formed by heating a relatively thick
slab of steel and processing it through a series of rollers
until the desired thickness is achieved. Hot-rolled steel is
frequently used for frame rails, hinges, cross members,
engine mounts, and other heavy applications where
appearance is not an issue. Cold-rolled steel is manu-
factured by further processing hot-rolled steel. After the
hot-rolled steel is cooled, it is again processed through
another series of rollers. This time, however, the steel is
not heated. Cold rolling achieves a more uniform thick-
ness than hot rolling, with a much better surface finish.
Galvanized steel has a zinc coating. The zinc
coating helps prevent the steel from rusting. Galvanized
steel is used on many lower body parts.
Aluminum
The use of aluminum is increasing in vehicle
construction. Some vehicles have aluminum body parts,
such as the fenders, hood, deck lid, radiator support,
engine cradle, and bumper reinforcement. Aluminum
is only about one third the weight of steel, but it can
be just as strong. Aluminum is easier to recycle than
steel, but aluminum parts are more expensive than
steel parts. Like steel, aluminum is an alloy. Alloying
elements, including manganese, silicon, and magne-
sium, are added to produce desired characteristics such
as strength or corrosion resistance.
Aluminum alloys are divided into two types: wrought
and cast. Either type can be heat treatable or non-heat
treatable. The shape of a wrought aluminum alloy part
is formed using mechanical force to alter the arrange-
ment of the molecules. Heat-treatable aluminum alloy is
heated after forming for increased strength. The strength
of non-heat treatable aluminum alloy is not affected by
heat. Wrought-type aluminum alloy can be in the form of
sheets or extrusions. Aluminum alloy sheets are pressed
between rollers to achieve the desired thickness. The
sheets are then shaped in a press. Component parts
can be joined together to make an assembled part such
as a hood. Extruded parts are formed by forcing soft-
ened aluminum alloy through a die. The die shapes the
softened aluminum alloy to the design shape. A rocker
panel is an example of an extruded part.
Cast aluminum alloy parts are made by adding liquid
aluminum alloy to a mold. As the hot liquid aluminum
alloy cools to a solid, it takes on the shape of the mold.
A shock tower is an example of a cast aluminum part.
The strength of an aluminum alloy is determined
by the type of aluminum (wrought or cast), the alloying
elements, and the thickness of the part. Compared to
steel, aluminum melts at a lower temperature (approxi-
mately 1,200°F [650°C] for aluminum alloy compared to
2,700°F [1480°C] for steel), transfers heat more readily,
and does not change color when heated.
Slightly damaged aluminum panels can be repaired
by heating, shaping, or welding. One type of vehicle has
an aluminum full frame and another type of vehicle has
an aluminum unibody. See Figure 3-8.