Alloy steels are defined as those steels that:
The low-alloy steels are those steels containing alloy
elements, including carbon, up to a total alloy content of about 8.0%.
- contain manganese, silicon, or copper in quantities
greater than the maximum limits (1.65% Mn, 0.60%
Si, and 0.60% Cu) of carbon steel; or
- that have specified ranges or minimums for one or more other alloying additions.
Except for plain carbon steels that are micro alloyed with just vanadium,
niobium, and/or titanium, most low-alloy steels are suitable as engineering quenched and tempered steels and are generally heat treated for engineering use.
Low-alloy steels with suitable alloy compositions have
greater hardenability than structural carbon steel and,
thus, can provide high strength and good toughness in
thicker sections by heat treatment. Their alloy contents
may also provide improved heat and corrosion resistance.
Effect of manganese on precipitation strengthening is
greater than its effect in niobium steels. However, the
absolute strength of niobium steel with 1.2% Mn is only
about 50 MPa less than that of vanadium steel but at a
much lower alloy level (that is, 0.06% Nb versus
Another factor affecting the strength of vanadium steels
is the ferrite grain size produced after cooling from-the
austenitizing temperature. Finer ferrite grain sizes can
be produced by either lower austenite-to-ferrite
transformation temperatures or by the formation of
finer austenite grain sizes prior to transformation.
The austenite grain size of hot-rolled steels is
determined by the recrystallization and grain growth of
austenite during rolling. Vanadium hot-rolled steels
usually undergo conventional rolling but are also
produced by recrystallization controlled roiling.
With conventional rolling, vanadium steels provide
moderate precipitation strengthening and relatively
little strengthening from grain refinement. The maximum
yield strength of conventionally hot-rolled vanadium
steels with 0.25% C and 0.08% V is about 450 MPa. The
practical limit of yield strengths for hot-rolled
vanadium-microalloyed steel is about 415 MPa.
Niobium Microalloyed Steels. Like vanadium, niobium
increases yield strength by precipitation hardening; the
magnitude of the increase depends on the size and amount
of precipitated niobium carbides. However, niobium is
also a more effective grain refiner than vanadium. Thus,
the combined effect of precipitation strengthening and
ferrite grain refinement makes niobium a more effective
strengthening agent than vanadium. The usual niobium
addition is 0.02 to 0.04%, which is about one-third the
optimum vanadium addition. Strengthening by niobium is
35 to 40 MPa per 0.01% addition.
Niobium steels are produced by controlled rolling,
recrystalization controlled rolling, accelerating cooling,
and direct quenching. The recrystallization controlled
rolling of niobium steel can be effective without titanium,
while recrystallization rolling of vanadium steels requires
titanium for grain refinement.
Vanadium-Niobium Microalloyed Steels. Steels
microalloyed with both niobium and vanadium provide
higher yield strength in the conventionally hot-rolled
condition than that achievable with either element alone.
As conventionally hot rolled, the niobium-vanadium steels
derive almost all of their increased strength from
precipitation strengthening and therefore have high
ductile-brittle transition temperatures. If the steel
is controlled rolled, the addition of both niobium and
vanadium together is especially advantageous for increasing
the yield strength and lowering ductile-brittle transition
temperatures by grain refinement.
Usually niobium-vanadium steels are made with relatively
low carbon contents. This reduces the amount of pearlite
and improves toughness, ductility, and weldability. These
steels are frequently referred to as pearlite-reduced
Niobium-Molybdenum Microalloyed Steels. Steels
microalloyed with niobium and molybdenum may have either
a ferrite-pearlite microstructure or an acicular ferrite
microstructure. In ferrite-pearlite niobium steels, the
addition of molybdenum increases the yield strength and
tensile strength by about 20 MPa and 30 MPa, respectively,
per 0.1% Mo, over a range of 0% to 0.27% Mo.
The principal effect of molybdenum on the microstructure
is to alter the morphology of the pearlite and to
introduce upper bainite as a partial replacement for
pearlite. However, because the individual strength values
of pearlite and bainite are somewhat similar, it has been
proposed that the strength increase is due to solid-solution
strengthening and enhanced precipitation strengthening
caused by a molybdenum-niobium synergism.
Vanadium-Nitrogen Microalloyed Steels. Vanadium
combines more strongly with nitrogen than niobium does,
and forms VN precipitates in vanadium-nitrogen steel.
Nitrogen additions to high-strength steels containing
vanadium have become commercially important because the
additions enhance precipitation hardening.
Some producers use nitrogen additions to assist in the
precipitation strengthening of controlled-cooled sheet
and plate with thicknesses above 9.5 mm. In one case,
hot-rolled plates with vanadium and 0.018 to 0.022% N
have been produced by controlled cooling in thicknesses
up to 16 mm with yield strengths of 550 MPa. However,
delayed cracking is a major problem in these steels.
The use of nitrogen is not recommended for steels that
will be welded because of its detrimental effect on notch
toughness in the heat-affected zone.
Titanium-Microalloyed Steels. Titanium in low-carbon
steels forms into a number of compounds that provide
grain refinement, precipitation strengthening, and
sulfide shape control. However, because titanium is
also a strong deoxidizer, titanium can be used only
in fully killed (aluminum deoxidized) steels so that
titanium is available for forming into compounds other
than titanium oxide. Commercially, steels precipitation
strengthened with titanium are produced in thicknesses
up to 9.5 mm in the minimum yield strength range from
345 to 550 MPa with controlled rolling required to
maximize strengthening and improve toughness.
Like niobium and/or vanadium steels, titanium microalloyed
steels are strengthened by mechanisms that involve a
combination of grain refinement and precipitation
strengthening; the combination depends on the amount
of alloy additions and processing methods. In reheated
or continuously cast steels, small amounts of titanium
(<0.025% Ti) are effective grain refiners because austenite
grain growth is retarded by titanium nitride.
Titanium-Niobium Microalloyed Steels. Although
precipitation-strengthened titanium steels have
limitations in terms of toughness and variability
of mechanical properties, research has shown that
an addition of titanium to low-carbon niobium steels
results in an overall improvement in properties.
Titanium increases the efficiency of niobium because
it combines with the nitrogen-forming titanium nitrides,
thus preventing the formation of niobium nitrides.
Acicular Ferrite (Low-Carbon Bainite) Steels.
Another approach to the development of HSLA steels is
to obtain a very fine, high-strength acicular ferrite
microstructure, instead of the usual polygonal ferrite
microstructure during the cooling transformation of
ultra-low carbon (<0.08% C) steels with sufficient
hardenability (by additions of manganese, molybdenum,
and/ or boron). Niobium can also be used for precipitation
strengthening and grain refinement. The principal
difference between the structure of acicular ferrite
(which is also referred to as low-carbon bainite) and
that of polygonal ferrite is that the former is
characterized by a high dislocation density and fine,
highly elongated grains that are not exhibited in
Acicular ferrite steels can be obtained by quenching or,
preferably, by air-cooling with suitable alloys for
hardenability. The principal advantage of this type of
HSLA steel is the unusual combination of high yield
strengths (415 to 690 MPa), high toughness, and good
weldability. A major application of these steels is
line pipe in arctic conditions.