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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 specific mechanical properties. They have a carbon content between 0.05 and 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. [1] [2] Copper, titanium, vanadium, and niobium are added for strengthening purposes. [2] 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. [2]
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. [2]
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. [2] HSLA steel cross-sections and structures are usually 20 to 30% lighter than a carbon steel with the same strength. [3] [4]
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. [5] HSLA steels usually have densities of around 7800 kg/m3. [6]
Military armour plate is mostly made from alloy steels, although some civilian armour against small arms is now made from HSLA steels with extreme low temperature quenching. [7]
A common type of micro-alloyed steel is improved-formability HSLA. It has a yield strength up to 80,000 psi (550 MPa) but costs only 24% more than A36 steel (36,000 psi (250 MPa)). One of the disadvantages of this steel is that it is 30 to 40% less ductile. In the U.S., these steels are dictated by the ASTM 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. [2] [8]
The Society of Automotive Engineers (SAE) maintains standards for HSLA steel grades because they are often used in automotive applications.
Grade | % Carbon (max) | % Manganese (max) | % Phosphorus (max) | % Sulfur (max) | % Silicon (max) | Notes |
---|---|---|---|---|---|---|
942X | 0.21 | 1.35 | 0.04 | 0.05 | 0.90 | Niobium or vanadium treated |
945A | 0.15 | 1.00 | 0.04 | 0.05 | 0.90 | |
945C | 0.23 | 1.40 | 0.04 | 0.05 | 0.90 | |
945X | 0.22 | 1.35 | 0.04 | 0.05 | 0.90 | Niobium or vanadium treated |
950A | 0.15 | 1.30 | 0.04 | 0.05 | 0.90 | |
950B | 0.22 | 1.30 | 0.04 | 0.05 | 0.90 | |
950C | 0.25 | 1.60 | 0.04 | 0.05 | 0.90 | |
950D | 0.15 | 1.00 | 0.15 | 0.05 | 0.90 | |
950X | 0.23 | 1.35 | 0.04 | 0.05 | 0.90 | Niobium or vanadium treated |
955X | 0.25 | 1.35 | 0.04 | 0.05 | 0.90 | Niobium, vanadium, or nitrogen treated |
960X | 0.26 | 1.45 | 0.04 | 0.05 | 0.90 | Niobium, vanadium, or nitrogen treated |
965X | 0.26 | 1.45 | 0.04 | 0.05 | 0.90 | Niobium, vanadium, or nitrogen treated |
970X | 0.26 | 1.65 | 0.04 | 0.05 | 0.90 | Niobium, vanadium, or nitrogen treated |
980X | 0.26 | 1.65 | 0.04 | 0.05 | 0.90 | Niobium, vanadium, or nitrogen treated |
Grade | Form | Yield strength (min) [psi (MPa)] | Ultimate tensile strength (min) [psi (MPa)] |
---|---|---|---|
942X | Plates, shapes & bars up to 4 in. | 42,000 (290) | 60,000 (414) |
945A, C | Sheet & strip | 45,000 (310) | 60,000 (414) |
Plates, shapes & bars: | |||
0–0.5 in. | 45,000 (310) | 65,000 (448) | |
0.5–1.5 in. | 42,000 (290) | 62,000 (427) | |
1.5–3 in. | 40,000 (276) | 62,000 (427) | |
945X | Sheet, strip, plates, shapes & bars up to 1.5 in. | 45,000 (310) | 60,000 (414) |
950A, B, C, D | Sheet & strip | 50,000 (345) | 70,000 (483) |
Plates, shapes & bars: | |||
0–0.5 in. | 50,000 (345) | 70,000 (483) | |
0.5–1.5 in. | 45,000 (310) | 67,000 (462) | |
1.5–3 in. | 42,000 (290) | 63,000 (434) | |
950X | Sheet, strip, plates, shapes & bars up to 1.5 in. | 50,000 (345) | 65,000 (448) |
955X | Sheet, strip, plates, shapes & bars up to 1.5 in. | 55,000 (379) | 70,000 (483) |
960X | Sheet, strip, plates, shapes & bars up to 1.5 in. | 60,000 (414) | 75,000 (517) |
965X | Sheet, strip, plates, shapes & bars up to 0.75 in. | 65,000 (448) | 80,000 (552) |
970X | Sheet, strip, plates, shapes & bars up to 0.75 in. | 70,000 (483) | 85,000 (586) |
980X | Sheet, strip & plates up to 0.375 in. | 80,000 (552) | 95,000 (655) |
Rank | Weldability | Formability | Toughness |
---|---|---|---|
Worst | 980X | 980X | 980X |
970X | 970X | 970X | |
965X | 965X | 965X | |
960X | 960X | 960X | |
955X, 950C, 942X | 955X | 955X | |
945C | 950C | 945C, 950C, 942X | |
950B, 950X | 950D | 945X, 950X | |
945X | 950B, 950X, 942X | 950D | |
950D | 945C, 945X | 950B | |
950A | 950A | 950A | |
Best | 945A | 945A | 945A |
Controlled rolling
Controlled rolling is a method of refining the grain of steel by introducing a large amount of nucleation sites for ferrite in the austenite matrix by rolling it at precisely controlled temperature, thereby increasing the strength of the steel. There are three main stages in controlled rolling: [12]
1) Deformation in recrystallization regions. In this stage, austenite is being recrystallized and refined, enabling refinement of ferrite grains in a later stage.
2) Deformation in non-recrystallization regions. 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 comes from grain refinement (Grain boundary strengthening), in which strength increases as the grain size decreases. The other mechanisms include solid solution strengthening and precipitate hardening from micro-alloyed elements. [13] [14] After the steel passes the temperature of austenite-ferrite region, it is then further strengthened by work hardening. [13] [12]
Control-rolled HSLA steels usually have higher strength and toughness, as well as lower ductile-brittle transition temperature [14] and ductile fracture properties. [13] 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, [12] thereby extending the non-recrystallization region and slowing down the grain growth. Nb can increase both the strength and toughness by precipitate strengthening and grain refinement. [14] Moreover, Nb is a strong carbide/nitride former, the Nb(C, N) formed can hinder grain growth during austenite-to-ferrite transition. [14]
Vanadium: V can significantly increase the strength and transition temperature by precipitate strengthening. [14]
Titanium: Ti produces 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 formers, [12] where the precipitates formed can prevent grain growth by pinning grain boundaries. They are also all ferrite formers, which increase the transition temperature of austenite-ferrite two phase region and reduce the non-recrystallization region. [12] The reduction in the non-recrystallization region induces the formation of deformation bands and activated grain boundaries, which are alternative ferrite nucleation sites other than grain boundaries. [12]
Other alloying elements are mainly for solid solution strengthening including silicon, manganese, chromium, copper, and nickel. [14]
An alloy is a mixture of chemical elements of which in most cases at least one is a metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Most alloys are metallic and show good electrical conductivity, ductility, opacity, and luster, and may have properties that differ from those of the pure elements such as increased strength or hardness. In some cases, an alloy may reduce the overall cost of the material while preserving important properties. In other cases, the mixture imparts synergistic properties such as corrosion resistance or mechanical strength.
Steel is an alloy of iron and carbon with improved strength and fracture resistance compared to other forms of iron. Because of its high tensile strength and low cost, steel is one of the most commonly manufactured materials in the world. Steel is used in buildings, as concrete reinforcing rods, in bridges, infrastructure, tools, ships, trains, cars, bicycles, machines, electrical appliances, furniture, and weapons.
Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.
Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.
Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, and is a frequently found and important constituent in ferrous metallurgy. While cementite is present in most steels and cast irons, it is produced as a raw material in the iron carbide process, which belongs to the family of alternative ironmaking technologies. The name cementite originated from the theory of Floris Osmond and J. Werth, in which the structure of solidified steel consists of a kind of cellular tissue, with ferrite as the nucleus and Fe3C the envelope of the cells. The carbide therefore cemented the iron.
Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902). It exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.
Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C. First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite is cooled past a temperature where it is no longer thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as similar in appearance to tempered martensite.
Pearlite is a two-phased, lamellar structure composed of alternating layers of ferrite and cementite that occurs in some steels and cast irons. During slow cooling of an iron-carbon alloy, pearlite forms by a eutectoid reaction as austenite cools below 723 °C (1,333 °F). Pearlite is a microstructure occurring in many common grades of steels.
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:
Maraging steels are steels that possess superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of very-low-carbon ultra-high-strength steels that derive their strength from precipitation of intermetallic compounds rather than from carbon. The principal alloying metal is 15 to 25 wt% nickel. Secondary alloying metals, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates.
Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.
Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.
In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.
Alloy steel is steel that is alloyed with a variety of elements in amounts between 1.0% and 50% by weight, typically to improve its mechanical properties.
Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.
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 cooling transformation diagram. This results in a microstructure consisting of a soft ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength). Therefore, the overall behaviour of DP steels is governed by the volume fraction, morphology (size, aspect ratio, interconnectivity, etc.), the grain size and the carbon content. For achieving these microstructures, DP steels typically contain 0.06–0.15 wt.% C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or bainite formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and microstructure refinement). The desire to produce high strength steels with formability greater than microalloyed steel led to development of DP steels in 2007 by Tata Steel.
In metallurgy, hot working refers to processes where metals are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is important because recrystallization keeps the materials from strain hardening, which ultimately keeps the yield strength and hardness low and ductility high. This contrasts with cold working.
Microalloyed steel is a type of alloy steel that contains small amounts of alloying elements, including niobium, vanadium, titanium, molybdenum, zirconium, boron, and rare-earth metals. They are used to refine the grain microstructure or facilitate precipitation hardening.
TRIP steel are a class of high-strength steel alloys typically used in naval and marine applications and in the automotive industry. TRIP stands for "Transformation induced plasticity," which implies a phase transformation in the material, typically when a stress is applied. These alloys are known to possess an outstanding combination of strength and ductility.
Boron steel refers to steel alloyed with a small amount of boron, usually less than 1%. The addition of boron to steel greatly increases the hardenability of the resulting alloy.
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