Mechanical Treatment of SteelCast steel, in the form of slabs, billets, or bars (these latter two differ
somewhat arbitrarily in size) is treated further by various combinations
of hot and cold deformation to produce a finished product for sale from
the mill. Further treatments by fabricators usually occur before delivery
to the final customer. These treatments have three purposes: (1) to
change the shape by deformation or metal removal to desired tolerances;
(2) to break up—at least partially—the segregation and large
grain sizes inevitably formed during the solidification process and to
redistribute the nonmetallic inclusions which are present; and (3) to
change the properties. For example, these may be functional—strength
or toughness—or largely aesthetic, such as reflectivity.
These purposes may be separable or in many cases may be acting
simultaneously. An example is hot-rolled sheet or plate in which often
the rolling schedule (reductions and temperature of each pass, and the
cooling rate after the last reduction) is a critical path to obtain the
properties and sizes desired and is often known as ‘‘heat treatment on
the mill.’’
Most steels are reduced after appropriate heating (to above 1,000°C)
in various multistand hot rolling mills to produce sheet, strip, plate, tubes,
shaped sections, or bars. More specialized deformation, e.g., by hammer
forging, can result in working in more than one direction, with a distribution
of inclusions which is not extended in one direction. Rolling,
e.g., more readily imparts anisotropic properties. Press forging at slow
strain rates changes the worked structure to greater depths and is preferred
for high-quality products. The degree of reduction required to
eliminate the cast structure varies from 4:1 to 10:1; clearly smaller
reductions would be desirable but are currently not usual.
The slabs, blooms, and billets from the caster must be reheated in an
atmosphere-controlled furnace to the working temperature, often from
room temperature, but if practices permit, they may be charged hot to
save energy. Coupling the hot deformation process directly to slabs at
the continuous caster exit is potentially more efficient, but practical
difficulties currently limit this to a small fraction of total production.
The steel is oxidized during heating to some degree, and this oxidation
is removed by a combination of light deformation and high-pressure
water sprays before the principal deformation is applied. There are
differences in detail between processes, but as a representative example,
the conventional production of wide ‘‘hot-rolled sheet’’ [.1.5 m
(60 in)] will be discussed.
The slab, about 0.3 m (12 in) thick at about 1200°C is passed through
a scale breaker and high-pressure water sprays to remove the oxide
film. It then passes through a set of roughing passes (possibly with
some modest width reduction) to reduce the thickness to just over
25 mm (1 in), the ends are sheared perpendicular to the length to remove
irregularities, and finally they are fed into a series of up to seven roll
stands each of which creates a reduction of 50 to 10 percent passing
along the train. Process controls allow each mill stand to run sufficiently
faster than the previous one to maintain tension and avoid pileups between
stands. The temperature of the sheet is a balance between heat
added by deformation and that lost by heat transfer, sometimes with
interstand water sprays. Ideally the temperature should not vary between
head and tail of the sheet, but this is hard to accomplish.
The deformation encourages recrystallization and even some grain
growth between stands; even though the time is short, temperatures are
high. Emerging from the last stand between 815 and 950°C, the austenite
may or may not recrystallize, depending on the temperature. At
higher temperatures, when austenite does recrystallize, the grain size is
usually small (often in the 10- to 20-mm range). At lower exit temperatures
austenite grains are rolled into ‘‘pancakes’’ with the short dimension
often less than 10 mm. Since several ferrite grains nucleate from
each austenite grain during subsequent cooling, the ferrite grain size can
be as low as 3 to 6 mm (ASTM 14 to 12). We shall see later that small
ferrite grain sizes are a major contributor to the superior properties of
today’s carbon steels, which provide good strength and superior toughness
simultaneously and economically.
Some of these steels also incorporate strong carbide and nitride
formers in small amounts to provide extra strength from precipitation
hardening; the degree to which these are undissolved in austenite during
hot rolling affects recrystallization significantly. The subject is too
complex to treat briefly here; the interested reader is referred to the
ASM ‘‘Metals Handbook,’’ 10th ed., vol. 1, pp. 389–423.
After the last pass, the strip may be cooled by programmed water
sprays to between 510 and 730°C so that during coiling, any desired
precipitation processes may take place in the coiler. The finished coil,
usually 2 to 3 mm (0.080 to 0.120 in) thick and sometimes 1.3 to 1.5 mm
(0.052 to 0.060 in) thick, which by now has a light oxide coating, is
taken off line and either shippped directly or retained for further processing
to make higher value-added products. Depending on composition,
typical values of yield strength are from 210 up to 380 MPa (30 to
55 ksi), UTS in the range of 400 to 550 MPa (58 to 80 ksi), with an
elongation in 200 mm (8 in) of about 20 percent. The higher strengths
correspond to low-alloy steels.
About half the sheet produced is sold directly as hot-rolled sheet. The
remainder is further cold-worked after scale removal by pickling and
either is sold as cold-worked to various tempers or is recrystallized to
form a very formable product known as cold-rolled and annealed, or more
usually as cold-rolled, sheet. Strengthening by cold work is common in
sheet, strip, wire, or bars. It provides an inexpensive addition to strength
but at the cost of a serious loss of ductility, often a better surface finish,
and finished product held to tighter tolerances. It improves springiness
by increasing the yield strength, but does not change the elastic moduli.
Examples of the effect of cold working on carbon-steel drawn wires are
shown in Figs. 6.2.2 and 6.2.3.
To make the highest class of formable sheet is a very sophisticated
operation. After pickling, the sheet is again reduced in a multistand
(three, four, or five) mill with great attention paid to tolerances and
surface finish. Reductions per pass range from 25 to 45 percent in early
passes to 10 to 30 percent in the last pass. The considerable heat generated
necessitates an oil-water mixture to cool and to provide the necessary
lubrication. The finished coil is degreased prior to annealing.
The purpose of annealing is to provide, for the most demanding applications,
pancake-shaped grains after recrystallization of the coldworked
ferrite, in a matrix with a very sharp crystal texture containing
little or no carbon or nitrogen in solution. The exact metallurgy is complex
but well understood. Two types of annealing are possible: slow
heating, holding, and cooling of coils in a hydrogen atmosphere (box
annealing) lasting several days, or continuous feeding through a furnace
with a computer-controlled time-temperature cycle. The latter is much
quicker but very capital-intensive and requires careful and complex
process control.
As requirements for formability are reduced, production controls can
be relaxed. In order of increasing cost, the series is commercial quality
(CQ), drawing quality (DQ), deep drawing quality (DDQ), and extra
deep drawing quality (EDDQ). Even more formable steels are possible,
but they are not often commercially interesting
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