Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 33

resistance may be improved by reducing the carbon content and by controlling

the size and morphology of carbides. 54

Stretcher strain markings

Temper rolling serves several functions. As mentioned previously it imprints a

surface roughness onto the steel surface and may improve the steel flatness, but

it also removes the yield point elongation. It is found, however, that a number of

steels exhibit a phenomenon called strain ageing, whereby the yield point returns

on storage at room temperature due to the diffusion of carbon or nitrogen in

solution onto free dislocations. Normally, it is considered that a visible marking

would occur on the surface of a pressed component if the yield point elongation

increases above about 0.3%. These markings are called stretcher strain markings

and would lead to the rejection of the pressing if the markings are on the exposed

surface of, for example, a motor vehicle body component. The markings are

avoided if the strain ageing rate is sufficiently low or if the steel is not stored

for very long periods before pressing. It will be seen below that most low-

strength strip steel grades are guaranteed not to exhibit stretcher strain markings

on forming for at least six months after delivery.

Surface roughness

The final surface roughness is imprinted onto the steel surface by the temper
mill rolls as indicated in the previous section. The roughness is usually defined
by the average peak height and the peak count per inch across the surface and
typical values to use would be 1-2 Ixm and 200 peaks per inch, depending on the
application. The surface of the temper mill rolls may be roughened using one of
four different methods, 55 namely shot blasting, spark erosion, using a pulsed laser
(the lasertex process) or electron beam roughening, but each method produces a
different type of surface.
The roughness is important because it influences the way in which forming
lubricant is held on the surface and has, therefore, a direct influence on formability
as a result of frictional effects. The roughness also has an influence on the
appearance of a steel surface after painting.

Use of laser.welded tailored blanks

Traditionally, each body panel of a motor vehicle is press formed from a single
sheet of steel cut to a suitable blank size and shape prior to .forming. The impli-
cation was that each part of a pressing was made from the same grade of steel
with the same strength and thickness. The result has been that certain parts of a
pressing may have been stronger than is strictly necessary or additional strength-
ening members may have been needed to provide strength in parts of a pressing
that may otherwise have been too weak.
This problem may now be overcome by making blanks from two or more pieces
which have been laser welded together before forming to make what have become
known as tailored blanks. The separate pieces may have different strength,
formability, thickness or coating, depending on the different requirements for