Page 33 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

STRUCTURAL STEELS, STEELMAKING, AND FABRICATION 1.31

gases produced. Also, various fluxes are added to reduce the sulfur and phosphorus contents to
desired levels. In this batch process, large heats of steel may be produced in less than an hour.
An electric-arc furnace does not require a hot metal charge but relies mainly on steel scrap. The
metal is heated by an electric arc between large carbon electrodes that project through the furnace
roof into the charge. Oxygen is injected to speed the process. This is a versatile batch process that
can be adapted to producing small heats where various steel grades are required, but it also can be
used to produce large heats.
Ladle treatment is an integral part of most steelmaking processes. The ladle receives the prod-
uct of the steelmaking furnace so that it can be moved and poured into either ingot molds or a con-
tinuous casting machine. While in the ladle, the chemical composition of the steel is checked, and
alloying elements are added as required. Also, deoxidizers are added to remove dissolved oxygen.
Processing can be done at this stage to reduce further sulfur content, remove undesirable non-
metallics, and change the shape of remaining inclusions. Thus significant improvements can be made
in the toughness, transverse properties, and through-thickness ductility of the finished product.
Vacuum degassing, argon bubbling, induction stirring, and the injection of rare earth metals are some
of the many procedures that may be employed.
Killed steels usually are deoxidized by additions to both furnace and ladle. Generally, silicon
compounds are added to the furnace to lower the oxygen content of the liquid metal and stop oxidation
of carbon (block the heat). This also permits addition of alloying elements that are susceptible to
oxidation. Silicon or other deoxidizers, such as aluminum, vanadium, and titanium, may be added to
the ladle to complete deoxidation. Aluminum, vanadium, and titanium have the additional beneficial
effect of inhibiting grain growth when the steel is normalized. (In the hot-rolled conditions, such
steels have about the same ferrite grain size as semikilled steels.) Killed steels deoxidized with aluminum
and silicon (made to fine-grain practice) often are used for structural applications because of better
notch toughness and lower transition temperatures than semikilled steels of the same composition.
(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron and
Steel Engineers, Pittsburgh, Pa.)

1.24 CASTING AND HOT ROLLING

Today, the continuous casting process is used to produce semifinished products directly from liq-
uid steel, thus eliminating the ingot molds and primary mills used previously. With continuous cast-
ing, the steel is poured from sequenced ladles to maintain a desired level in a tundish above an
oscillating water-cooled copper mold (Fig. 1.14). The outer skin of the steel strand solidifies as it
passes through the mold, and this action is further aided by water sprayed on the skin just after the
strand exits the mold. The strand passes through sets of supporting rolls, curving rolls, and straight-
ening rolls and is then rolled into slabs. The slabs are cut to length from the moving strand and held
for subsequent rolling into finished product. Not only is the continuous casting process a more effi-
cient method, but it also results in improved quality through more consistent chemical composition
and better surfaces on the finished product.
Plates, produced from slabs or directly from ingots, are distinguished from sheet, strip, and flat bars
by size limitations in ASTM A6. Generally, plates are heavier, per linear foot, than these other prod-
ucts. Plates are formed with straight horizontal rolls and later trimmed (sheared or gas cut) on all edges.
Slabs usually are reheated in a furnace and descaled with high-pressure water sprays before they
are rolled into plates. The plastic slabs are gradually brought to desired dimensions by passage
through a series of rollers. In the last rolling step, the plates pass through leveling, or flattening,
rollers. Generally, the thinner the plate, the more flattening required. After passing through the lev-
eler, plates are cooled uniformly, then sheared or gas cut to desired length, while still hot.
Some of the plates may be heat treated, depending on grade of steel and intended use. For car-
bon steel, the treatment may be annealing, normalizing, or stress relieving. Plates of HSLA or con-
structional alloy steels may be quenched and tempered. Some mills provide facilities for on-line heat
treating or for thermomechanical processing (controlled rolling). Other mills heat treat off-line.

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