10-Low Alloy Steel.pdf

Low Alloy Steel

Introduction

Introduction

High AlloyPlain Low Alloy

Steel

►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 total content of alloying elements is < 5 %, and Al>0.3%

Cr >0.03%

Co>0.03%

Mo>0.08%

Ni>0.6 %

►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.

ClassificationAISI/SAE

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.

General Application

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 SteelSupplements: special low alloy 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


►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


►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


►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


►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.


►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.


►Application: Structural, automotive

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


►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.


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)

Documents

10-Low Alloy Steel.pdf