HSLA Steels
General Introduction
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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.
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These are a
group of low-carbon steels that utilize small amounts of alloying elements to
attain yield strengths greater than 275 MPa (40 ksi) in the as-rolled or
normalized condition.
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These are
developed by controlled chemical composition and mechanical treatment to obtain
desired better mechanical properties.
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They are not
considered to be alloy steels in the normal sense because they are designed to
meet specific mechanical properties rather than a chemical composition
Composition
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Primarily
low-carbon (C≤ 0.20%) steels with about 1-2% Mn and small quantities (<
0.50%) of other elements.
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Small quantities
of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium,
and zirconium are used in various combinations.
Properties
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These steels
have better mechanical properties and sometimes better corrosion resistance
than as-rolled carbon steels.
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Yield Strength=
290-480 MPa, Tensile Strength= 415-620 Mpa.
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Give good
mechanical properties with reduced weight.
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Weldability of
many HSLA steels is comparable to or better than that of mild steel.
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Not strengthened
by heat treatment.
Processing
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These steels are
usually used in hot finished condition
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Processing may
also involve special hot-mill processing that further improves the mechanical
properties.
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These processing
methods include:
Ø The controlled rolling of precipitation-strengthened HSLA steels
Ø The accelerated cooling of, preferably, controlled-rolled HSLA steels
Ø The quenching or accelerated air or water cooling of low-carbon steels (≤0.08% C)
Ø The normalizing of
vanadium-containing HSLA steels
Ø The intercritical annealing of HSLA steels
•
HSLA steels are
also furnished as cold-rolled sheet and forgings.
Classification
HSLA steels can be divided into seven categories:
Ø Weathering steels, which contain small amounts of alloying elements
such as copper and phosphorus for improved atmospheric corrosion resistance and
solid-solution strengthening
Ø Microalloyed ferrite-pearlite steels, which contain very small (generally, less than
0.10%) additions of strong carbide or carbonitride-forming elements such as
niobium, vanadium, and/or titanium for precipitation strengthening, grain
refinement, and possibly transformation temperature control
Ø As-rolled pearlitic steels, which may include carbon-manganese steels but
which may also have small additions of other alloying elements to enhance
strength, toughness, formability, and weldability
Ø Acicular ferrite (low-carbon bainite) steels, which are low-carbon (<0.08% C) steels with an
excellent combination of high yield strengths, weldability, formability, and
good toughness
Ø Dual-phase steels, which have a microstructure of martensite
dispersed in a ferritic matrix and provide a good combination of ductility and
high tensile strength
Ø Inclusion shape controlled steels, which provide improved ductility and
through-thickness toughness by the small additions of calcium, zirconium, or
titanium, or perhaps rare-earth elements so that the shape of the sulfide
inclusions are changed from elongated stringers to small, dispersed, almost
spherical globules
Ø Hydrogen-induced cracking resistant steels with
low carbon, low sulfur, inclusion shape control, and limited manganese
segregation, plus copper contents greater than 0.26%
Applications
Primary applications for HSLA steels include
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Oil and gas line
pipe
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Ships
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offshore
structures
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Automotive crank
shaft
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Crane and
vehicles
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off-highway
equipment, and
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pressure
vessels.