Page 343 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

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"‘ *f~ " Section 13.3 Flat rollnng Pract|ce 2

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ing New grains forming
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FIGURE |3.6 Changes in the grain structure of cast or of large-grain Wrought metals during
hot rolling. Hot rolling is an effective way to reduce grain size in metals for improved strength
and ductility. Cast structures of ingots or continuous castings are converted to a wrought
structure by hot working.

includes coarse and nonuniform grains; this structure usually is brittle and may be
porous. Hot rolling converts the cast structure to a wrought structure (Fig. 13.6) with
finer grains and enhanced ductility, both of which result from the breaking up of brit-
tle grain boundaries and the closing up of internal defects (especially porosity). Typical
temperature ranges for hot rolling are about 450°C for aluminum alloys, up to
125 O°C for alloy steels, and up to 1650°C for refractory alloys (see also Table 14.3).
The product of the first hot-rolling operation is called a bloom, a slab, or a billet
(see Fig. 13.1). A bloom usually has a square cross section, at least 150 mm on the side;
a slab usually is rectangular in cross section. Blooms are processed further by shape
rolling into structural shapes such as I-beams and railroad rails (Section 13.5). Slabs
are rolled into plates and sheets. Billets usually are square (With a cross-sectional
area smaller than blooms) and later are rolled into various shapes, such as round
rods and bars, using shaped rolls. Hot-rolled round rods (wire rods) are used as the
starting material for rod- and vvire-drawing operations (Chapter 15).
In the hot rolling of blooms, billets, and slabs, the surface of the material
usually is conditioned (prepared for a subsequent operation) prior to rolling them.
Conditioning is often done by means of a torch (scarfing) to remove heavy scale or by
rough grinding to smoothen surfaces. Prior to cold rolling, the scale developed during
hot rolling may be removed by pickling with acids (acid etching), by such mechanical
means as blasting with Water, or by grinding to remove other defects as vvell.
Cold rolling is carried out at room temperature and, compared with hot
rolling, produces sheets and strips With a much better surface finish (because of
lack of scale), better dimensional tolerances, and enhanced mechanical properties
(because of strain hardening).
Pack rolling is a flat-rolling operation in which two or more layers of metal are
rolled together, thus improving productivity. Aluminum foil, for example, is pack
rolled in two layers, so only the top and bottom outer layers have been in contact
with the rolls. Note that one side of aluminum foil is matte, While the other side is
shiny. The foil-to~foil side has a matte and satiny finish, but the foil-to-roll side is
shiny and bright because it has been in contact under high contact stresses with the
polished rolls during rolling.
Rolled mild steel, when subsequently stretched during sheet-forming operations,
undergoes yield-point elongation (Section 163)-a phenomenon that causes surface
irregularities called stretcher strains or L1}ia'er’s bands. To correct this situation, the