Page 32 - 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

1.30 CHAPTER ONE

In practice, carbon content is limited so as not to impair ductility, notch toughness, and weld-
ability. To obtain high strength, therefore, resort is had to other strengthening agents that improve
these desirable properties or at least do not impair them as much as carbon. Often, the better these
properties are required to be at high strengths, the more costly the steels are likely to be.
Attempts have been made to relate chemical composition to weldability by expressing the rela-
tive influence of chemical content in terms of carbon equivalent. One widely used formula, which
is a supplementary requirement in ASTM A6 for structural steels, is
+
+
+
C eq = C + Mn + Cr Mo V + Ni Cu (1.5)
6 5 15
where C = carbon content, %
Mn = manganese content, %
Cr = chromium content, %
Mo = molybdenum, %
V = vanadium, %
Ni = nickel content, %
Cu = copper, %
Carbon equivalent is related to the maximum rate at which a weld and adjacent plate may be
cooled after welding, without underbead cracking occurring. The higher the carbon equivalent, the
lower will be the allowable cooling rate. Also, use of low-hydrogen welding electrodes and preheat-
ing becomes more important with increasing carbon equivalent. (Structural Welding Code—Steel,
American Welding Society, Miami, Fla.)
Though carbon provides high strength in steels economically, it is not a necessary ingredient. Very-
high-strength steels are available that contain so little carbon that they are considered carbon-free.
Maraging steels, carbon-free iron-nickel martensites, develop yield strengths from 150 to 300 ksi,
depending on alloying composition. As pointed out in Art. 1.19, iron-carbon martensite is hard and
brittle after quenching and becomes softer and more ductile when tempered. In contrast, maraging
steels are relatively soft and ductile initially but become hard, strong, and tough when aged. They
are fabricated while ductile and later strengthened by an aging treatment. These steels have high
resistance to corrosion, including stress-corrosion cracking.
(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron and
Steel Engineers, Pittsburgh, Pa.)

1.23 STEELMAKING METHODS

Structural steel is usually produced today by one of two production processes. In the traditional
process, iron or “hot metal” is produced in a blast furnace and then further processed in a basic oxygen
furnace to make the steel for the desired products. Alternatively, steel can be made in an electric arc
furnace that is charged mainly with steel scrap instead of hot metal. In either case, the steel must be
produced so that undesirable elements are reduced to levels allowed by pertinent specifications to
minimize adverse effects on properties.
In a blast furnace, iron ore, coke, and flux (limestone and dolomite) are charged into the top of a
large refractory-lined furnace. Heated air is blown in at the bottom and passed up through the bed of
raw materials. A supplemental fuel such as gas, oil, or powdered coal is also usually charged. The iron
is reduced to metallic iron and melted; then it is drawn off periodically through tap holes into transfer
ladles. At this point, the molten iron includes several other elements (manganese, sulfur, phosphorus,
and silicon) in amounts greater than permitted for steel, and thus further processing is required.
In a basic oxygen furnace, the charge consists of hot metal from the blast furnace and steel scrap.
Oxygen, introduced by a jet blown into the molten metal, reacts with the impurities present to facil-
itate the removal or reduction in level of unwanted elements, which are trapped in the slag or in the

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