Page 344 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

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24 Chapter 13 Metal-Rolling Processes and Equipment
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ia) (b) Rollers

FIGURE l3.7 (a) A method of roller leveling to flatten rolled sheets. (b) Roller leveling to
straighten drawn bars.

sheet metal is subjected to a final, light pass of 0.5 to 1.5% reduction known as
temper rolling or skin pass shortly before stretching.
A rolled sheet may not be sufficiently flat as it leaves the roll gap, due to fac-
tors such as variations in the incoming material or in the processing parameters dur-
ing rolling. To improve flatness, the rolled strip typically goes through a series of
leveling rolls. Several roller arrangements are used, as shown in Fig. 13.7. The work-
piece is flexed in opposite directions as it passes through the sets of rollers. Each roll
usually is driven separately by an individual electric motor.

I3.3.I Defects in Rolled Plates and Sheets

Defects may be present on the surfaces of rolled plates and sheets, or there may be in-
ternal structural defects. Defects are undesirable not only because they compromise
surface appearance, but also because they may adversely affect strength, formability,
and other manufacturing characteristics. Several surface defects (such as scale, rust,
scratches, gouges, pits, and cracks) have been identified in sheet metals. These defects
may be caused by inclusions and impurities in the original cast material or by various
other conditions related to material preparation and to the rolling operation.
Wavy edges on sheets (Fig. 13.8a) are the result of roll bending. The strip is
thinner along its edges than at its center (see Fig. l3.4a); thus, the edges elongate
more than the center. Consequently, the edges buckle because they are constrained
by the central region from expanding freely in the longitudinal (rolling) direction.
The cracks shown in Figs. 13.8b and c are usually the result of poor material ductil-
ity at the rolling temperature. Because the quality of the edges of the sheet may affect
sheet-metal-forming operations, edge defects in rolled sheets often are removed by
shearing and slitting operations (Section 162). Alligatoring (Fig. 13.8d) is a com-
plex phenomenon and typically is caused by nonuniform bulk defor-
Fiolling direction mation of the billet during rolling or by the presence of defects in the
original cast material.

I3.3.2 Other Characteristics of Rolled Metals
(H) (D)
Residual Stresses. Because of nonuniform deformation of the mate-
rial in the roll gap, residual stresses can develop in rolled plates and
sheets, especially during cold rolling. Small-diameter rolls or small
thickness reductions per pass tend to plastically deform the metal more
at its surfaces than in the bulk (Fig. 13.9a). This situation results in
compressive residual stresses on the surfaces and tensile stresses in the
(C) (d)
bulk. Conversely, large-diameter rolls or high reductions per pass tend
FIGURE l3.8 Schematic illustration of to deform the bulk more than the surfaces (Fig. 13.9b). This is due to
typical defects in flat rolling: (a) wavy the higher frictional constraint at the surfaces along the arc of contact-
edges; (b) zipper cracks in the center of the a situation that produces residual stress distributions that are the oppo-
strip; (c) edge cracks; and (d) alligatoring. site of those with small-diameter rolls.