Steels can be classified by a variety of
different systems depending on:
The
composition, such as carbon, low-alloy or stainless steel.
The
manufacturing methods, such as open hearth, basic oxygen
process, or electric furnace methods.
The
finishing method, such as hot rolling or cold rolling
The
product form, such as bar plate, sheet, strip, tubing or
structural shape
The
deoxidation practice, such as killed, semi-killed, capped or
rimmed steel
The
microstructure, such as ferritic, pearlitic and martensitic
The
required strength level, as specified in ASTM standards
The heat
treatment, such as annealing, quenching and tempering, and
thermomechanical processing
Quality
descriptors, such as forging quality and commercial quality.
Carbon Steels
The American Iron and Steel Institute (AISI) defines carbon
steel as follows:
Steel is considered to be carbon steel when no minimum content
is specified or required for chromium, cobalt, columbium
[niobium], molybdenum, nickel, titanium, tungsten, vanadium or
zirconium, or any other element to be added to obtain a desired
alloying effect; when the specified minimum for copper does not
exceed 0.40 per cent; or when the maximum content specified for
any of the following elements does not exceed the percentages
noted: manganese 1.65, silicon 0.60, copper 0.60.
Carbon steel can be classified, according to various deoxidation
practices, as rimmed, capped, semi-killed, or killed steel.
Deoxidation practice and the steelmaking process will have an
effect on the properties of the steel. However, variations in
carbon have the greatest effect on mechanical properties, with
increasing carbon content leading to increased hardness and
strength. As such, carbon steels are generally categorized
according to their carbon content. Generally speaking, carbon
steels contain up to 2% total alloying elements and can be
subdivided into low-carbon steels, medium-carbon steels,
high-carbon steels, and ultrahigh-carbon steels; each of these
designations is discussed below.
As a group, carbon steels are by far the most frequently used
steels. More than 85% of the steel produced and shipped in the
United States is carbon steel.
Low-carbon steels contain up to 0.30% C. The largest
category of this class of steel is flat-rolled products (sheet
or strip), usually in the cold-rolled and annealed condition.
The carbon content for these high-formability steels is very
low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in
automobile body panels, tin plate, and wire products.
For rolled steel structural plates and sections, the carbon
content may be increased to approximately 0.30%, with higher
manganese content up to 1.5%. These materials may be used for
stampings, forgings, seamless tubes, and boiler plate.
Medium-carbon steels are similar to low-carbon steels
except that the carbon ranges from 0.30 to 0.60% and the
manganese from 0.60 to 1.65%. Increasing the carbon content to
approximately 0.5% with an accompanying increase in manganese
allows medium carbon steels to be used in the quenched and
tempered condition. The uses of medium carbon-manganese steels
include shafts, axles, gears, crankshafts, couplings and
forgings. Steels in the 0.40 to 0.60% C range are also used for
rails, railway wheels and rail axles.
High-carbon steels contain from 0.60 to 1.00% C with
manganese contents ranging from 0.30 to 0.90%. High-carbon
steels are used for spring materials and high-strength wires.
Ultrahigh-carbon steels are experimental alloys containing 1.25
to 2.0% C. These steels are thermomechanically processed to
produce microstructures that consist of ultrafine, equiaxed
grains of spherical, discontinuous proeutectoid carbide
particles.
High-Strength Low-Alloy Steels
High-strength low-alloy (HSLA) steels, or microalloyed steels,
are designed to provide better mechanical properties and/or
greater resistance to atmospheric corrosion than conventional
carbon steels in the normal sense because they are designed to
meet specific mechanical properties rather than a chemical
composition.
The HSLA steels have low carbon contents
(0.05-0.25% C) in order to produce adequate formability and
weldability, and they have manganese contents up to 2.0%. Small
quantities of chromium, nickel, molybdenum, copper, nitrogen,
vanadium, niobium, titanium and zirconium are used in various
combinations.
HSLA Classification:
Weathering steels, designated to exhibit superior atmospheric
corrosion resistance
Control-rolled steels, hot rolled according to a predetermined
rolling schedule, designed to develop a highly deformed austenite structure that will transform to a very fine equiaxed
ferrite structure on cooling
Pearlite-reduced
steels, strengthened by very fine-grain ferrite and
precipitation hardening but with low carbon content and
therefore little or no pearlite in the microstructure
Microalloyed steels, with very small additions of such elements
as niobium, vanadium, and/or titanium for refinement of grain
size and/or precipitation hardening
Acicular
ferrite steel, very low carbon steels with sufficient
hardenability to transform on cooling to a very fine
high-strength acicular ferrite structure rather than the usual
polygonal ferrite structure
Dual-phase steels, processed to a micro-structure of ferrite
containing small uniformly distributed regions of high-carbon
martensite, resulting in a product with low yield strength and a
high rate of work hardening, thus providing a high-strength
steel of superior formability.
The various types of HSLA steels may also have small additions
of calcium, rare earth elements, or zirconium for sulfide
inclusion shape control.
Low-alloy Steels
Low-alloy steels constitute a category of ferrous materials that
exhibit mechanical properties superior to plain carbon steels as
the result of additions of alloying elements such as nickel,
chromium, and molybdenum. Total alloy content can range from
2.07% up to levels just below that of stainless steels, which
contain a minimum of 10% Cr.
For many low-alloy steels, the primary function of the alloying
elements is to increase hardenability in order to optimize
mechanical properties and toughness after heat treatment. In
some cases, however, alloy additions are used to reduce
environmental degradation under certain specified service
conditions.
As with steels in general, low-alloy steels can be classified
according to:
Chemical
composition, such as nickel steels, nickel-chromium steels,
molybdenum steels, chromium-molybdenum steels
Heat
treatment, such as quenched and tempered, normalized and
tempered, annealed.
Because of the wide variety of chemical compositions possible
and the fact that some steels are used in more than one
heat-treated, condition, some overlap exists among the alloy
steel classifications. In this article, four major groups of
alloy steels are addressed: (1) low-carbon quenched and tempered
(QT) steels, (2) medium-carbon ultrahigh-strength steels, (3)
bearing steels, and (4) heat-resistant chromium-molybdenum
steels.
Low-carbon quenched and tempered steels combine high
yield strength (from 350 to 1035 MPa) and high tensile strength
with good notch toughness, ductility, corrosion resistance, or
weldability. The various steels have different combinations of
these characteristics based on their intended applications.
However, a few steels, such as HY-80 and HY-100, are covered by
military specifications. The steels listed are used primarily as
plate. Some of these steels, as well as other, similar steels,
are produced as forgings or castings.
Medium-carbon ultrahigh-strength steels are structural
steels with yield strengths that can exceed 1380 MPa. Many of
these steels are covered by SAE/AISI designations or are
proprietary compositions. Product forms include billet, bar,
rod, forgings, sheet, tubing, and welding wire.
Bearing steels used for ball and roller bearing
applications are comprised of low carbon (0.10 to 0.20% C)
case-hardened steels and high carbon (-1.0% C) through-hardened
steels. Many of these steels are covered by SAE/AISI
designations.
Chromium-molybdenum heat-resistant steels contain 0.5 to
9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below
0.2%. The chromium provides improved oxidation and corrosion
resistance, and the molybdenum increases strength at elevated
temperatures. They are generally supplied in the normalized and
tempered, quenched and tempered or annealed condition.
Chromium-molybdenum steels are widely used in the oil and gas
industries and in fossil fuel and nuclear power plants.
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