Page 155 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

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

aluminum, silicon, manganese, and vanadium that have been added to the
melt. These elements have an affinity for oxygen and form metallic oxides. If
aluminum is used, the product is called aluminum-killed steel (see Table 16.4.)
The term killed comes from the fact that the steel lies quietly after being
poured into the mold.
If they are sufficiently large, the oxide inclusions in the molten bath float
out and adhere to (or are dissolved in) the slag. A fully killed steel thus is free
of any porosity caused by gases; it also is free of any blowholes (large spheri-
cal holes near the surfaces of the ingot). Consequently, the chemical and me-
chanical properties of a killed-steel ingot are relatively uniform throughout.
Because of shrinkage during the solidification, however, an ingot of this type
develops a pipe at the top (also called a shrinkage cavity). It has the appear-
ance of a funnel-like shape. This pipe can take up a substantial volume of the
ingot, as it has to be cut off and scrapped.
2. Semikilled Steel. Semi-killed steel is a partially deoxiclized steel. It contains
some porosity (generally in the upper central section of the ingot), but it has
little or no pipe. As a result, scrap is reduced. Although the piping in semi-
killed steels is less, this advantage is offset by the presence of porosity in that
region. Semi-killed steels are economical to produce.
3. Rimmed Steel. In a rimmed steel, which generally has a low carbon content
(less than 0.15%), the evolved gases are only partially killed (or controlled) by
the addition of other elements, such as aluminum. The gases produce blow-
holes along the outer rim of the ingot--hence the term rimmed. Rimmed steels
have little or no piping, and they have a ductile skin with good surface finish.
However, if they are not controlled properly, blowholes may break through
the skin. Furthermore, impurities and inclusions tend to segregate toward the
center of the ingot. Thus, products made from this steel may be defective, and
thorough inspection is essential.

Refining. The properties and manufacturing characteristics of ferrous alloys are
affected adversely by the amount of impurities, inclusions, and other elements pres-
ent. (See Section 2.10.) The removal of impurities is known as rejqning. Most refin-
ing is done in melting furnaces or in ladles, by the addition of various elements.
There is an increasing demand for cleaner steels with improved and more uniform
properties and a greater consistency of composition.
Refining is important particularly in producing high-grade steels and alloys for
high-performance and critical applications, such as aircraft components. Moreover,
warranty periods on shafts, camshafts, crankshafts, and similar parts can be in-
creased significantly by using higher quality steels. Such steels are subjected to
secondary refining in ladles (ladle metallurgy) and ladle refining (injection refining),
which generally consists of melting and processing the steel in a vacuum. Several
processes using controlled atmospheres (such as electron-beam melting, vacuum-arc
remelting, argon-oxygen decarburization, and vacuum-arc double-electrode remelt-
ing) have been developed.

5.4 Continuous Casting

The inefficiencies and the problems involved in making steels in the traditional form
of ingots are alleviated by the continuous-casting process, which produces higher
quality steels at reduced costs (see also Section 13.5.1 on minimills). Conceived in
the 1860s, continuous or strand casting was first developed for casting nonferrous