HSLA steel

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Iron alloy phases
Austenite (γ-iron; hard) Bainite Martensite Cementite (iron carbide; Fe₃C) Ledeburite (ferrite - cementite eutectic, 4.3% carbon) Ferrite (α-iron; soft) Pearlite (88% ferrite, 12% cementite) Spheroidite
Types of Steel
Plain-carbon steel (up to 2.1% carbon) Stainless steel (alloy with chromium) HSLA steel (high strength low alloy) Tool steel (very hard; heat-treated)
Other Iron-based materials
Cast iron (>2.1% carbon) Wrought iron (almost no carbon) Ductile iron

HSLA steel (high strength low alloy steel) is a type of steel alloy that provides many benefits over regular steel alloys. In general, HSLA alloys are much stronger and tougher than ordinary plain-carbon steels. They are used in cars, trucks, cranes, bridges, and other structures that are designed to handle large amounts of stress, often at very low temperatures.

HSLA steels are so called because they only contain a very small percentage of carbon. A typical HSLA steel may contain 0.15% carbon, 1.65% manganese and low levels (under 0.035%) of phosphorus and sulphur^[1]. It may also contain small amounts of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, silicon, or zirconium. HSLA steels are therefore also referred to as "microalloyed", as they are indeed alloyed in extremely small amounts by comparison to other main commercial alloy steels. As little as 0.10% niobium and vanadium can have profound effects on the mechanical properties of a 0.1% C, 1.3% Mn steel. These added elements are intended to alter the microstructure of plain-carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite. This eliminates the toughness-reducing effect of a pearlitic volume fraction, yet maintains and even increases the material's strength by precipitation strengthening and by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Their yield strengths can be anywhere between 250–590 MPa (35,000–85,000 psi).

HSLA steels are also more resistant to rust then most carbon steels, due to their lack of pearlite - the fine layers of ferrite (almost pure iron) and cementite in pearlite. The Angel of the North at Gateshead, England is a well known example of an unpainted HSLA structure (the actual alloy used is called COR-TEN and includes a small amount of copper). HSLA steels usually have densities of around 7800 kg/m³. [1]