Page 157 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications

the dummy bar. The bar is withdrawn at the same rate at which the metal is poured.
The cooling rate is such that the metal develops a solidified skin (shell), so as to sup-
port itself during its travel downward, typically at speeds of about 25 mm/s. The
shell thickness at the exit end of the mold is about 12 to 18 mm. Additional cooling
is provided by water sprays along the travel path of the solidifying metal. The molds
generally are coated with graphite or similar solid lubricants in order to reduce both
friction and adhesion at the mold-metal interfaces. Also, the molds are vibrated in
order to reduce friction and sticking.
The continuously cast metal may be cut into desired lengths by shearing or
computer-controlled torch cutting, or it may be fed directly into a rolling mill for
further reduction in thickness and for the shaping of products such as channels
and I-beams. In addition to costing less, continuously cast metals have more uni-
form compositions and properties than those obtained by ingot casting. Modern
facilities use computer-controlled hot-rolling operations on continuously cast
strands, with final sheet thicknesses on the order of 2 to 6 mm for carbon, stain-
less, and electrical steels and with capabilities for a rapid switchover from one
type of steel to another. Afterwards, steel plates or other shapes undergo one or
more further processes, such as (a) cleaning and pickling by chemicals to remove
surface oxides, (b) cold rolling to improve strength and surface finish, (c) anneal-
ing, and (d) coating (galvanizing or aluminizing) to improve resistance to corro-
sion.
In strip casting, thin slabs, or strips, are produced from molten metal. The
metal solidifies in similar fashion to strand casting, but the hot solid then is rolled to
form the final shape (Fig. 5.4b). The compressive stresses in rolling (see Section 13.2)
serve to reduce porosity and provide better material properties. In effect, strip casting
eliminates a hot-rolling operation in the production of metal strips or slabs. In mod-
ern facilities, final thicknesses on the order of 2 to 6 mm can be obtained for carbon,
stainless, and electrical steels and other metals.

5.5 Carbon and Alloy Steels

Carbon and alloy steels are among the most commonly used metals and have a wide
variety of compositions, processing options, and applications (Table 5.1). These
steels are available in various basic product shapes: plate, sheet, strip, bar, wire,
tube, castings, and forgings.

5.5.l Effects of Various Elements in Steels
Various elements are added to steels in order to impart properties such as harden-
ability, strength, hardness, toughness, wear resistance, workability, weldability,
and machinability. These elements are listed below (in alphabetical order) with
summaries of their beneficial and detrimental effects. Generally, the higher the
percentages of these elements in steels, the greater are the particular properties
that they impart. For example, the higher the carbon content, the greater the
hardenability of the steel and the greater its strength, hardness, and wear resist-
ance. Cn the other hand, ductility, weldability, and toughness are reduced with
increasing carbon content.
Boron improves hardenability without the loss of (or even with some improve-
ment in) machinability and formability.
Calcium deoxidizes steels, improves toughness, and may improve formability
and machinability.