58 Section 1 Introduction to Collision Repair
the occupants in all types of collisions—front, rear, and
side. The vehicle must also protect its passengers in
the event of a rollover. To achieve these goals, today’s
vehicles are made from a variety of materials, including
steel, plastic, glass, magnesium, and aluminum.
Steel
Steel is one of the most commonly used materials l
in the manufacture of today’s vehicles. Steel is an alloy.
An alloy is a metallic substance that consists of one y
metal and at least one other alloying element. Alloying
elements are materials added to metals to give them
desired properties. Iron is the primary metal in steel.
Carbon is an alloying element. The amount of carbon
and other alloying elements in steel determines its
strength and other characteristics.
There are three basic types of steel used in auto-
mobile construction: mild steel, high-strength steel, and
advanced high-strength steel. See Figure 3-7. The type
of steel is determined by the types and percentages
of alloying elements added to the steel, as well as the
annealing procedure. Annealing is a controlled heating
and cooling procedure that affects the hardness and
strength of steel.
Mild steel has a carbon content of approximately l
0.25%. Small amounts of other elements, such as
manganese, silicon, or copper, may also be found in
mild steel. Full frames may be made from mild steel.
Body panels are also made from mild steel.
Yield strength is the amount of force required to
permanently change the shape of metal. Mild steel has
a yield strength of about 30,000 pounds per square
inch (psi). Because mild steel has a relatively low yield
strength, it is easily formed. Additional strength can be
obtained by increasing thickness of the steel. Unfor-r r
tunately, this also increases the overall weight of the
vehicle. Crowns, body lines, and convolutions give mild
steel body panels greater strength. Mild steel can be
heated and welded with no loss of strength.
There are two types of mild steel used to manu-
facture vehicles, drawing steel (DS) and deep drawing
steel (DDS). Drawing steel is the most common type of
steel used on a vehicle. Deep drawing steel has lower
yield strength than drawing steel, allowing it to be bent
into complex shapes.
High-strength steel has a different chemical l
composition than mild steel. Additional alloying elements
used in high-strength steel include manganese, phos-
phorus, sulfur, silicon, copper, nickel, chromium, and
molybdenum. The carbon content of high-strength steel
may be as high as 0.75%. There are two basic catego-
ries of high-strength steel: conventional high-strength
steel and advanced high-strength steel.
With a yield strength of 50,000 psi, conventional
high-strength steel is stronger than mild steel. Conven-
tional high-strength steel allows panels to be made from
thinner material, so vehicles weigh less. Less weight
means better fuel economy. However, conventional
high-strength steel is harder and more brittle than mild
steel. Conventional high-strength steel can be used to
make both structural and non-structural parts.
Great care is required when repairing damaged
conventional high-strength steel components. Only
moderately damaged conventional high-strength
steel panels can be repaired and retain strength. The
strength will be lost if a badly damaged conventional
high-strength steel panel is straightened.
One type of conventional high-strength steel is
high-strength, low-alloy steel (HSLA). High-strength,
low-alloy steel contains very small amounts of carbon.
Figure 3-6. This is a full-size van. Since it is designed
to transport passengers, it has side windows. (Ford)
Figure 3-7. Several different types of steel are used in
the construction of this unibody vehicle. (Saab)
Mild Steel
High-Strength Steel
Advanced High-Strength Steel