High-strength low-alloy steel (HSLA) is a type of alloy
steel that provides better mechanical properties or greater resistance to
corrosion than carbon steel. HSLA steels vary from other steels in that they
are not made to meet a specific chemical composition but rather to specific
mechanical properties. They have a carbon content between 0.05–0.25% to retain
formability and weldability. Other alloying elements include up to 2.0%
manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium,
chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium.
Copper, titanium, vanadium, and niobium are added for strengthening purposes. These
elements are intended to alter the microstructure of carbon steels, which is
usually a ferrite-pearlite aggregate, to produce a very fine dispersion of
alloy carbides in an almost pure ferrite matrix. This eliminates the
toughness-reducing effect of a pearlitic volume fraction yet maintains and
increases the material's strength by refining the grain size, which in the case
of ferrite increases yield strength by 50% for every halving of the mean grain
diameter. Precipitation strengthening plays a minor role, too. Their yield
strengths can be anywhere between 250–590 megapascals (36,000–86,000 psi).
Because of their higher strength and toughness HSLA steels usually require 25
to 30% more power to form, as compared to carbon steels.
Copper, silicon, nickel, chromium, and phosphorus are added
to increase corrosion resistance. Zirconium, calcium, and rare earth elements
are added for sulfide-inclusion shape control which increases formability.
These are needed because most HSLA steels have directionally sensitive
properties. Formability and impact strength can vary significantly when tested
longitudinally and transversely to the grain. Bends that are parallel to the
longitudinal grain are more likely to crack around the outer edge because it
experiences tensile loads. This directional characteristic is substantially
reduced in HSLA steels that have been treated for sulfide shape control.
They are used in cars, trucks, cranes, bridges, roller
coasters and other structures that are designed to handle large amounts of
stress or need a good strength-to-weight ratio. HSLA steel cross-sections
and structures are usually 20 to 30% lighter than a carbon steel with the same
strength.
HSLA steels are also more resistant to rust than most carbon
steels because of their lack of pearlite – the fine layers of ferrite (almost
pure iron) and cementite in pearlite.[citation needed] HSLA steels usually have
densities of around 7800 kg/m³.
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