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High-strength low-alloy steel (HSLA) is a type of
alloy steel Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The differe ...
that provides better mechanical properties or greater resistance to corrosion than
carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...
. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather specific mechanical properties. They have a carbon content between 0.05 and 0.25% to retain
formability Formability is the ability of a given metal workpiece to undergo plastic deformation without being damaged. The plastic deformation capacity of metallic materials, however, is limited to a certain extent, at which point, the material could experienc ...
and
weldability The weldability, also known as joinability,. of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others (see Rheological weldability). A material's weldability is used to ...
. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel,
niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has si ...
, nitrogen,
vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pass ...
,
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...
, molybdenum, titanium,
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
,
rare-earth elements The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
, or
zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
. 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
carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of t ...
s 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 . 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 Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...
than most carbon steels because of their lack of pearlite – the fine layers of ferrite (almost pure iron) and cementite in pearlite. HSLA steels usually have densities of around 7800 kg/m3. Military
armour plate Military vehicles are commonly armoured (or armored; see spelling differences) to withstand the impact of shrapnel, bullets, shells, rockets, and missiles, protecting the personnel inside from enemy fire. Such vehicles include armoured fight ...
is mostly made from alloy steels, although some civilian armour against small arms is now made from HSLA steels with extreme low temperature quenching.


Classifications

*
Weathering steel Weathering steel, often referred to by the genericised trademark COR-TEN steel and sometimes written without the hyphen as corten steel, is a group of steel alloys which were developed to eliminate the need for painting, and form a stable ru ...
s: Steels which have better corrosion resistance. A common example is COR-TEN. * Control-rolled steels: Hot rolled steels which have a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure upon cooling. * Pearlite-reduced steels: Low carbon content steels which lead to little or no pearlite, but rather a very fine grain ferrite matrix. It is strengthened by precipitation hardening. *
Acicular ferrite Acicular ferrite is a microstructure of ferrite in steel that is characterised by needle-shaped crystallites or grains when viewed in two dimensions. The grains, actually three-dimensional in shape, have a thin lenticular shape. This microstructu ...
steels: These steels are characterized by a very fine high strength acicular ferrite structure, a very low carbon content, and good
hardenability The hardenability of a metal alloy is the depth to which a material is hardened after putting it through a heat treatment process. It should not be confused with hardness, which is a measure of a sample's resistance to indentation or scratching. I ...
. *
Dual-phase steel Dual-phase steel (DP steel) is a high-strength steel that has a ferritic–martensitic microstructure. DP steels are produced from low or medium carbon steels that are quenched from a temperature above A1 but below A3 determined from continuous c ...
s: These steels have a ferrite microstructure that contain small, uniformly distributed sections of martensite. This microstructure gives the steels a low yield strength, high rate of work hardening, and good formability. *
Microalloyed steel Microalloyed steel is a type of alloy steel that contains small amounts of alloying elements (0.05 to 0.15%), including niobium, vanadium, titanium, molybdenum, zirconium, boron, and rare-earth metals. They are used to refine the grain microstructu ...
s: Steels which contain very small additions of niobium, vanadium, and/or titanium to obtain a refined grain size and/or precipitation hardening. A common type of micro-alloyed steel is improved-formability HSLA. It has a yield strength up to but costs only 24% more than
A36 steel A36 steel is a common structural steel alloy utilized in the United States.''Steel Construction Manual'', 8th Edition, second revised edition, American Institute of Steel Construction, 1986, Ch. 1 pp. 1–5. The A36 (UNS K02600) standard was establ ...
(). One of the disadvantages of this steel is that it is 30 to 40% less
ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
. In the U.S., these steels are dictated by the
ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
standards A1008/A1008M and A1011/A1011M for sheet metal and A656/A656M for plates. These steels were developed for the automotive industry to reduce weight without losing strength. Examples of uses include door-intrusion beams, chassis members, reinforcing and mounting brackets, steering and suspension parts, bumpers, and wheels.


SAE grades

The Society of Automotive Engineers (SAE) maintains standards for HSLA steel grades because they are often used in automotive applications.


Controlled-rolling of HSLA steels


Mechanism

Controlled rolling Controlled rolling is a method of refining grains of steel by introducing large amount of nucleation sites for ferrite in austenite matrix by rolling with temperature control, therefore increasing the strength of steel. There are three main stages during controlled rolling: 1) Deformation in recrystallization region. In this stage, austenite is being recrystallized and refined and can thereby refine the ferrite grains in the later stage. 2) Deformation in non-recrystallization region. Austenite grains are elongated by rolling. Deformation bands might present within the band as well. Elongated grain boundaries and deformation bands are all nucleation sites for ferrite. 3) Deformation in austenite-ferrite two phase region. Ferrite nucleates and austenite are further work-hardened. Strengthening Mechanism Control-rolled HSLA steels contain a combination of different strengthening mechanisms. The main strengthening effect come from grain refinement (
Grain boundary strengthening In materials science, grain-boundary strengthening (or Hall–Petch strengthening) is a method of strengthening materials by changing their average crystallite (grain) size. It is based on the observation that grain boundaries are insurmounta ...
), where strength increase as the grain size decrease. The other mechanisms include
solid solution strengthening In metallurgy, solid solution strengthening is a type of alloying that can be used to improve the strength of a pure metal. The technique works by adding atoms of one element (the alloying element) to the crystalline lattice of another element ( ...
and precipitate hardening from micro-alloyed elements. After the steel passes the temperature of austenite-ferrite region, it is then further strengthened by
work hardening In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. This strengt ...
.


Mechanical properties

Control-rolled HSLA steels usually have higher strength and toughness, as well as lower
ductile-brittle transition temperature Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
and ductile fracture properties. Below are some common micro-alloyed elements used to improve the mechanical properties. Effect of micro-alloyed elements: Niobium: Nb can increase the recrystallization temperature by around 100 °C, thereby extending the non-recrystallization region and slow down the grain growth. Nb can both increase the strength and toughness by precipitate strengthening and grain refinement. Moreover, Nb is a strong carbide/nitride former, the Nb(C, N) formed can hinder grain growth during austenite-to-ferrite transition.   Vanadium: V can significantly increase the strength and transition temperature by precipitate strengthening. Titanium: Ti have a slight increase in strengthen via both grain refinement and precipitate strengthening. Nb, V, and Ti are three common alloying elements in HSLA steels. They are all good carbide and nitride former, where the precipitates formed can prevent grain growth by pinning grain boundary. They are also all ferrite former, which increase the transition temperature of austenite-ferrite two phase region and reduce the non-recrystallization region. The reduction in non-recrystallization region induces the formation of deformation bands and activated grain boundaries, which are alternative ferrite nucleation site other than grain boundaries. Other alloying elements are mainly for solid solution strengthening including Silicon, Manganese, Chromium, Copper, and Nickel.


References


Sources

* . * {{Authority control Steels Armour