Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.71
Ideally, an ingot should be homogeneous, with a fine, equiaxial crystal structure.
It should not contain nonmetallic inclusions or cavities and should be free of chemical
segregation. In practice, however, because of uneven cooling and release of
gases in the mold, an ingot may develop any of a number of internal and external
defects. Some of these may be eliminated or minimized during the rolling operation.
Prevention or elimination of the others often adds to the cost of steels.
Steel cools unevenly in a mold, because the liquid at the mold walls solidifies
first and cools more rapidly than metal in the interior of the ingot. Gases, chiefly
oxygen, dissolved in the liquid, are released as the liquid cools. Four types of ingot
may result—killed, semikilled, capped, and rimmed—depending on the amount of
gases dissolved in the liquid, the carbon content of the steel, and the amount of
deoxidizers added to the steel.
A fully killed ingot develops no gas; the molten steel lies dead in the mold. The
top surface solidifies relatively fast. Pipe, an intermittently bridged shrinkage cavity,
forms below the top. Fully killed steels usually are poured in big-end-up molds
with ‘‘hot tops’’ to confine the pipe to the hot top, which is later discarded. A
semikilled ingot develops a slight amount of gas. The gas, trapped when the metal
solidifies, forms blowholes in the upper portion of the ingot. A capped ingot develops
rimming action, a boiling caused by evolution of gas, forcing the steel to
rise. The action is stopped by a metal cap secured to the mold. Strong upward
currents along the sides of the mold sweep away bubbles that otherwise would form
blowholes in the upper portion of the ingot. Blowholes do form, however, in the
lower portion, separated by a thick solid skin from the mold walls. A rimmed ingot
develops a violent rimming action, confining blowholes to only the bottom quarter
of the ingot.
Rimmed or capped steels cannot be produced if too much carbon is present
(0.30% or more), because insufficient oxygen will be dissolved in the steels to
cause the rimming action. Killed and semikilled steels require additional costs for
deoxidizers if carbon content is low, and the deoxidation products form nonmetallic
inclusions in the ingot. Hence, it often is advantageous for steel producers to make
low-carbon steels by rimmed or capped practice, and high-carbon steels by killed
or semikilled practice.
Pipe, or shrinkage cavities, generally is small enough in most steels to be eliminated
by rolling. Blowholes in the interior of an ingot, small voids formed by
entrapped gases, also usually are eliminated during rolling. If they extend to the
surface, they may be oxidized and form seams when the ingot is rolled, because
the oxidized metal cannot be welded together. Properly made ingots have a thick
enough skin over blowholes to prevent oxidation.
Segregation in ingots depends on the chemical composition and on turbulence
from gas evolution and convection currents in the molten metal. Killed steels have
less segregation than semikilled steels, and these types of steels have less segregation
than capped or rimmed steels. In rimmed steels, the effects of segregation
are so marked that interior and outer regions differ enough in chemical composition
to appear to be different steels. The boundary between these regions is sharp.
Rimmed steels are made without additions of deoxidizers to the furnace and
with only small additions to the ladle, to ensure sufficient evolution of gas. When
properly made, rimmed ingots have little pipe and a good surface. Such steels are
preferred where surface finish is important and the effects of segregation will not
be harmful.
Capped steels are made much like rimmed steels but with less rimming action.
Capped steels have less segregation. They are used to make sheet, strip, skelp,
tinplate, wire, and bars.