Page 345 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

ii Section 13 4 Rolling Mills 2

Sheet
thickness
l

Tension Compression Tension Compression

(8) (D)
FIGURE |3.9 (a) Residual stresses developed in rolling with small-diameter rolls or at small
reductions in thickness per pass. (b) Residual stresses developed in rolling With large-diameter
rolls or at high reductions per pass. Note the reversal of the residual stress patterns.

Dimensional Tolerances. Thickness tolerances for cold-rolled sheets usually range
from ;i:0.1 to 0.35 mm, depending on the thickness. Tolerances are much greater for
hot-rolled plates, because of thermal effects. Flcztness tolerances are usually within
i15 mm/m for cold rolling and ;t55 mm/m for hot rolling.

Surface Roughness. The ranges of surface roughness in cold and hot rolling are
given in Fig. 23.13, which, for comparison, includes other manufacturing processes.
Note that cold rolling can produce a very fine surface finish; hence, products made
of cold-rolled sheets may not require additional finishing operations, depending on
the application. Note also that hot rolling and sand casting produce the same range
of surface roughness.

Gage Numbers. The thickness of a sheet usually is identified by a gage number:
the smaller the number, the thicker the sheet. Several numbering systems are used in
industry, depending on the type of sheet metal being classified. Rolled sheets of cop-
per and of brass also are identified by thickness changes during rolling, such as %
hard, % hard, and so on.

l3.4 Rolling Mills

Several types of rolling mills and equipment are available with diverse roll arrange-
ments. Although the equipment for hot and cold rolling is essentially the same, there
are important differences in the roll materials, process parameters, lubricants, and
cooling systems. The design, construction, and operation of rolling mills (Fig. 13.1O)
require major investments. Highly automated mills produce close-tolerance, high-
quality plates and sheets at high production rates and low cost per unit Weight, par-
ticularly vvhen integrated with continuous casting. Rolling speeds may range up to
40 m/s. The width of rolled products may range up to 5 m.
Two-high rolling mills (Fig. 13.3b) are used for hot rolling in initial breakdown
passes (primary roughing or cogging mills) on cast ingots or in continuous casting,