Classification of Carbon and Low-Alloy Steels

Steels can be classified according to a number of different systems:

• Composition, such as carbon, low alloy or stainless steel.
• The production method, such as open hearth, basic oxygen or electric arc furnace.
• The finishing process, such as hot rolling or cold rolling
• The product form, such as plate, sheet, strip, tube or structural shapes
• The deoxidation practice, such as killed, semi-killed, capped or rimmed steel
• The microstructure, such as ferritic, pearlitic and martensitic
• The required level of strength, as specified in ASTM standards
• Heat treatment, such as annealing, quenching and tempering and thermo-mechanical processing
• Quality descriptors, such as forging quality and commercial quality.

Carbon steels

The American Iron and Steel Institute (AISI) defines carbon steel as follows

Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium, or zirconium, or for any other element added to obtain a desired alloying effect; when the minimum content specified for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages indicated: manganese 1.65, silicon 0.60, copper 0.60.

Carbon steel can be classified according to different deoxidation practices as rimmed, capped, semi-killed or killed steel. The deoxidation practice and the steel making process will affect the properties of the steel. However, variations in carbon content have the greatest effect on mechanical properties, with increasing carbon content leading to increased hardness and strength. Carbon steels are therefore generally categorised according to their carbon content. Typically, carbon steels contain up to 2% total alloying elements and can be divided into low carbon, medium carbon, high carbon and ultra high carbon steels, each of which is discussed below.

As a group, carbon steels are by far the most widely used steels. More than 85% of the steel produced and shipped in the United States is carbon steel.

Low carbon steels contain up to 0.30% C. The largest category of this class of steel is flat rolled products (sheet or strip), usually in the cold rolled and annealed condition. The carbon content of these high formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical applications include automotive body panels, tinplate and wire products.

For rolled structural steel plates and sections, the carbon content can be increased to about 0.30%, with higher manganese contents up to 1.5%. These materials can be used for stampings, forgings, seamless tubes and boiler plate.

Medium carbon steels are similar to low carbon steels except that the carbon content ranges from 0.30 to 0.60% and the manganese content ranges from 0.60 to 1.65%. Increasing the carbon content to about 0.5% with a concomitant increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. Applications for medium carbon manganese steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and railway axles.

High carbon steels have a carbon content of 0.60 to 1.00% and a manganese content of 0.30 to 0.90%. High carbon steels are used for spring materials and high strength wires.

Ultrahigh carbon steels are experimental alloys containing 1.25 to 2.0% C. These steels are thermomechanically processed to produce microstructures consisting of ultrafine, equiaxed grains of spherical, discontinuous proeutectoid carbide particles.

High strength low alloy steels

High Strength Low Alloy (HSLA) or micro-alloyed steels are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense, because they are designed to meet specific mechanical properties rather than a chemical composition.

HSLA steels have low carbon contents (0.05-0.25% C) to provide adequate formability and weldability, and they have manganese contents up to 2.0%. Small amounts of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

HSLA Classification:

Weathering steels, designated for superior atmospheric corrosion resistance.

Controlled rolled steels, hot-rolled according to a predetermined rolling schedule, designed to develop a highly deformed austenitic microstructure which is converted to a very fine equiaxed ferrite microstructure on cooling.

Pearlite-reduced steels, strengthened by very fine grain ferrite and precipitation hardening, but with low carbon content and therefore little or no pearlite in the microstructure.

Micro-alloyed steels, with very small additions of elements such as niobium, vanadium and/or titanium for grain refinement and/or precipitation hardening.

Acicular ferrite steels, very low carbon steels with sufficient hardenability to transform on cooling into a very fine high strength acicular ferrite structure rather than the usual polygonal ferrite structure.

Dual phase steels, processed to a microstructure of ferrite containing small uniformly distributed regions of high carbon martensite, resulting in a product with low yield strength and high work hardening rate, thus providing a high strength steel with superior formability.

The various types of HSLA steels may also have small additions of calcium, rare earth elements or zirconium to control the shape of the sulphide inclusions.

Low alloy steels

Low alloy steels are a category of ferrous materials that have superior mechanical properties to plain carbon steels as a result of the addition of alloying elements such as nickel, chromium and molybdenum. The total alloy content can range from 2.07% to levels just below that of stainless steels, which contain a minimum of 10% Cr.

In many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimise mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.

As with steels in general, low alloy steels can be classified according to their

Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels.

Heat treatment, such as quenched and tempered, normalised, annealed.

Due to the wide variety of possible chemical compositions and the fact that some steels are used in more than one heat treatment condition, there is some overlap between alloy steel classifications. In this article, four major groups of alloy steels are considered:

(1) low carbon quenched and tempered (QT) steels,

(2) medium carbon high strength steels,

(3) bearing steels, and

(4) heat resistant chromium-molybdenum steels.

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. Different steels have different combinations of these properties depending on their intended applications. However, some steels, such as HY-80 and HY-100, are covered by military specifications. The steels listed are mainly used in plate form. Some of these and other similar steels are produced as forgings or castings.

Medium carbon high strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels have SAE/AISI designations or are proprietary compositions. Product forms include billets, bars, rods, forgings, plates, tubes and welding wire.

Bearing steels used in ball and roller bearing applications include 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 less than 0.2%. The chromium provides improved oxidation and corrosion resistance and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalised, quenched and tempered or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas, fossil fuel and nuclear power industries.

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