►Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.
►The primary function of the alloying elements in low alloy steels is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.
►Wide range of manufacturing methods i.e. welding, casting, machining, forming.
ClassificationBased on application
►Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications.
►Medium-carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels are covered by SAE/AISI designations or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and welding wire.
ClassificationBased on application►Bearing steels used for ball and roller bearing applications
are comprised of low carbon (0.10 to 0.20% C) case-hardened steels and high carbon (-1.0% C) through-hardened steels. Many of these steels are covered by SAE/AISI designations.
►Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.2%. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalized and tempered, quenched and tempered or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants.
Low Alloy SteelMn steels (13xx)
►Chemical composition : 0,3 - 0,45% C, 0,25 - 1% Mn. To gain higher strength & weldability than mild steel 1,6 – 1,9 % Mn
►High Strength, good hardenability
►Application : axle, shaft, gear, tie rod for automobiles, farm equiptment
►Heat treatment : quenching, tempering, annealing, normalizing
Low Alloy SteelMn steels (13xx)
►Effect of Mn in strengthening plain carbon steel : Solid-solution hardening
Grain size refinement
Increasing proportion of pearlite
Low Alloy SteelCr steels (5xxx)
►Divided into 3 groups based on their Cr contents 50xx : 0,40% Cr
51xx : 0,8 – 1% Cr
52xxx : 1,03% Cr
►Carbon contents: 0,2 – 1,04% C
►High strength, hard, high wear resistance, low ductility
Low Alloy SteelCr steels (5xxx)
►Application : ball & roller bearing, spring steel
►Susceptible to temper embrittlement
►Heat treatment :quenching, tempering, normalizing, annealing
Low Alloy SteelMo steels (4xxx)
►Restricted to about 0,25% Mo optimum value based on experiment
►Good hardenability, high strength, good toughness
►Application : rear-axle, automatic transmission components, pinion gear
►Heat treatment : quenching, tempering
Others (XXXX)
Please read :
►Avner’s Introduction to Physical Metallurgy
►William F. Smith’s Structure & Properties of Engineering Alloy
Low Alloy SteelSupplements: special low alloy steels
►Special and modern low alloy steels
►Relatively, it is “better” than common low alloy steel for the same class or application
►More advance production methods
►HSLA steels
►TRIP steels
►Maraging steels
Low Alloy SteelHSLA steels
►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/plain carbon steels.
►Good formability, fatigue strength and weldability
Low Alloy SteelHSLA steels
►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 (HSLA steels have yield strengths greater than 275 MPa, or 40 ksi). The chemical composition of a specific HSLA steel may vary for different product thicknesses to meet mechanical property requirements
Low Alloy SteelHSLA steels
►The HSLA steels in sheet or plate form have low carbon content (0.05 to 0.25% C) in order to produce adequate formability and weldability, and they have manganese content up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium are used in various combinations
Low Alloy SteelHSLA steels – Application
►Oil and gas pipelines, heavy-duty highway and off-road vehicles, construction and farm machinery, industrial equipment, storage tanks, mine and railroad cars, barges and dredges, snowmobiles, lawn mowers, and passenger car components. Bridges, offshore structures, power transmission towers, light poles, and building beams and panels, structural parts of the vehicle
►Material savings by using HSLA for automotive application = approx. $23/vehicle. Total = $11,101,714/yr. (AISI 2004)
Low Alloy SteelHSLA steels – Categories
►Weathering steels, which contain small amounts of alloying elements such as copper and phosphorus for improved atmospheric corrosion resistance and solid-solution strengthening
►Microalloyed ferrite-pearlite steels, which contain very small (generally, less than 0.10%) additions of strong carbide or carbonitrideforming elements such as niobium, vanadium, and/or titanium for precipitation strengthening, grain refinement, and possibly transformation temperature control
Low Alloy SteelHSLA steels – Categories
►As-rolled pearlitic steels/pearlite-reduced 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
►Acicular ferrite (low-carbon bainite) steels, which are low-carbon (less than 0.05% C) steels with an excellent combination of high yield strengths, (as high as 690 MPa, or 100 ksi) weldability, formability,and good toughness
Low Alloy SteelHSLA steels – Categories
►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
►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 is changed from elongated stringers to small, dispersed, almost spherical globules
Low Alloy SteelTRIP steels
►TRIP (TRansformation Induced Plasticity) steels are one of the newest family of advanced high strength steels (AHSS) developed for the automotive industry.
►The steels have a microstructure of soft ferrite grains with bainite and retained austenite. The retained austenite transforms into martensite (a hard phase) during plastic deformation (like stamping or crash event) stage.
Low Alloy SteelTRIP steels
►The hard martensite delays the onset of necking resulting in a product with high total elongation, excellent formability and high crash energy absorption. In addition, TRIP steels also exhibit extremely high fatigue endurance limit, thereby providing excellent durability performance
►TRIP steels can therefore be engineered or tailored to provide excellent formability for manufacturing complex parts. In addition, these steels can be designed into the automotive body structure to provide excellent crash energy absorption.
Low Alloy SteelTRIP steels – Characteristics
►Work hardening – As compared with other high strength steels, TRIP steel displays higher work hardening rate in entire range of plastic deformation.
►Yield point elongation (YPE) - Tested as delivered TRIP steels usually show YPE; however, some grades may have no YPE.
►Formability – Due to high work hardening rate TRIP steel behaves in a stable way in stamping processes (resistance to onset necking) and displays remarkably high formability (high potential to form parts of complex geometry).
►Bendability – TRIP steel demonstrates good bendabilty. As a result, product and process design solutions leading to springback control are easier to implement
Low Alloy SteelTRIP steels – Characteristics
►Bake hardening – TRIP steels have an excellent bake-hardening capacity. The increase in the yield strength in typical paint baking cycle is approximately 10 ksi (70 MPa).
►Product mass reduction capacity – TRIP steels have high potential for part downgauging and weight reduction.
►Fatigue performance – TRIP steels have higher fatigue strength than equivalent conventional HSLA steels.
Low Alloy SteelTRIP steels – Characteristics
Ferrite-bainite-austenite
Yes
Very Good
Excellent
Excellent
Good
More difficult than DUAL-TEN®
Microstructure
Yield Point Elongation
Formability for strength level
Bake Hardening
Strain Rate Sensitivity
Fatigue
Weldability
Ferrite-martensite
None
Good
Excellent
Good
Good
OK
TRIP SteelTopic AreaDUAL-TEN® Steel
(Dual Phase Steel)
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