Carbon and Low Alloy Steels

Carbon and Low Alloy Steels

Steel is the most widely used category of the metallic materials. It is mainly because it can be produced relatively in an economic manner in large quantities to very precise specifications. Steels also provide a wide range of mechanical properties, from moderate yield strength levels (200 MPa to 300 MPa) with excellent ductility to yield strengths exceeding 1400 MPa with fracture toughness levels as high as 110 MPa under-root meter. Steels form one of the most complex group of alloys in common use. The synergistic effect of alloying elements and heat treatment in steels helps in producing a tremendous variety of microstructures and properties (characteristics).

The term low alloy steel rather than the more general term alloy steel is being used to differentiate the steels from the high alloy steels. High alloy steels include steels with a high degree of fracture toughness.  High alloy steels are ultra high strength steels and consist of corrosion resistant steels, heat resistant steels, and wear resistant steels. High alloy steels also include maraging steels, austenitic manganese steels, tool steels, and stainless steels.

Steels can be classified (Fig 1) in several ways as given below, but the classification based upon the chemical composition is the most widely used classification internationally.

  • The composition such as C, micro alloy, low alloy, high alloy, or stainless steel.
  • The production processes such as basic oxygen steelmaking, energy optimizing furnace steelmaking, or electric furnace (electric arc furnace or induction furnace) steelmaking.
  • The finishing processes such as hot rolling, cold rolling, casting or forging etc.
  • The product type such as flat products (plate, sheet, and strip), long products (wire rods, reinforcement bars, rounds and structural shapes), pipes and tubes, cast or forged products.
  • The de oxidation practice method such as killed, semi-killed, rimmed or capped steel.
  • The steel microstructure such as ferritic, austenitic, pearlitic, bainitic, or martensitic etc.
  • The strength levels as per the specifications of various standards.
  • The process of heat treatment such as annealing, normalizing, thermo mechanical treatment, quenching and tempering etc.
  • Quality defining descriptions such as forging quality, commercial quality, drawing quality or welding quality etc.

Fig 1 Classification of steels

Chemical composition is normally used as the basis for classifying steels or assigning standard designations to steels. A heat analysis is generally considered to be an accurate representation of the composition of the entire heat of the steel. However, since the segregation of some alloying elements is intrinsic in the solidification process, some variation in the composition of individual rolled product can be expected. The composition of the individual rolled product may not conform to the applicable specification, even though the heat analysis does. The chemical composition of an individual product taken from a large heat of the steel is called the product analysis or check analysis. Ranges and limits for product analyses are generally broader and less restrictive than the corresponding ranges and limits for heat analyses.

Residual elements normally enter steel products from raw materials used to produce hot metal or from scrap steel used in steelmaking. Through careful steelmaking practices, the amounts of these residual elements are generally held to acceptable levels. Sulphur (S) and phosphorus (P) are usually considered deleterious to the mechanical properties of steels and hence, restrictions are placed on the allowable amounts of these elements for most grades. The amounts of S and P are invariably reported in the analyses of both C steels and alloy steels. Other residual alloying elements normally exert a lesser influence than S and P on the properties of steel. For many grades of steel, limitations on the amounts of these residual elements are either optional or omitted entirely. Amounts of residual alloying elements are generally not reported in either heat or product analyses, except for special reasons.

The composition requirements for several steels, particularly plain C steels, contain no specific restriction on silicon (Si) content. The lack of a Si requirement is not an omission, but instead indicates recognition that the amount of Si in a steel can frequently be traced directly to the deoxidation practice employed in making it.

Carbon steels

The American Iron and Steel Institute (AISI) defines carbon steel as ‘Steel is considered to be carbon steel when no minimum content is specified or required or chromium Cr), cobalt (Co), niobium (Nb), molybdenum (Mo), nickel (Ni), titanium (Ti), tungsten (W), vanadium (V), or zirconium (Zr), or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper (Cu) does not exceed 0.40 %; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese (Mn) 1.65 %, silicon (Si) 0.60 %, copper (Cu) 0.60 %’.

Carbon (C) steel can be classified, according to various deoxidation practices, as rimmed, capped, semikilled, or killed steel. Deoxidation practices and the steelmaking processes have an effect on the characteristics and properties of the steel. However, variations in C have the maximum effect on the mechanical properties, with increasing C content leading to an increase in the hardness and the strength. As such, C steels are normally categorized according to their C content. Generally speaking, C steels contain upto 2 % of total alloying elements and can be subdivided into ultra low C steels, low C steels, medium C steels, high C steels, and ultra high C steels. As a group, C steels are by far the most frequently used steels.

Ultra low carbon steels – These steels contain very low C normally less than 0.1 %. These steels also contain very low Mn and very low Si. These steels are having extra deep drawing properties and normally produced in the form of flat and rod products.

Low carbon steels – These steels contain upto 0.3 % C. This category includes mild steel with C content normally in the range of 0.15 % -0.3 %. The largest category of this class of steel is flat rolled products in the form of sheet or strip usually in the cold rolled and annealed condition. The C content for these deep drawing quality steels is around 0.1 % C, with upto 0.5 %  Mn. For structural use produced in the form of plates and sections, the C content can be increased upto 0.3 %, with higher Mn content upto 1.6 %.

Medium carbon steels – These steels are similar to low C steels except that the C content in these steels are higher and normally in the range of 0.31% to 0.6 % and the Mn content in the range of 0.6 % to 1.65 %. Due to Increased C content, the medium C steels can be used in the quenched and tempered condition. The medium C steels are used as shafts, axles, gears, crankshafts, couplings and forgings etc. Steels having C in the range of 0.4 % to 0.6 % C are also used for rails, railway wheels and rail axles.

High carbon steels – These steels have C content ranging from 0.61 % to 1 % with Mn content ranging from 0.3 % to 0.9 %. The use of high C steels includes in springs, pre stressed concrete wires and high strength wires etc.

Ultra high carbon steels – These steels contains C in the range of 1.25 % to 2 %.  These steels are normally thermo mechanically processed to produce microstructures consisting of ultra-fine, equi-axed grains of spherical, discontinuous pro-eutectoid carbide particles.

High strength low alloy steels

High-strength low-alloy (HSLA) steels, or micro-alloyed steels, are designed to provide better mechanical properties and /or greater resistance to atmospheric corrosion than normal C steels. They are not considered to be alloy steels in the usual sense since they are designed to meet specific mechanical properties rather than a chemical composition. HSLA steels have yield strengths of more than 275 MPa. The chemical composition of specific HSLA steel can vary for different product thickness to meet mechanical property requirements.

The C content of HSLA steels can range from 0.05 % to 0.25 % and Mn content upto 2 % in order to provide adequate formability and welding properties. In these steels, small quantities of Cr, Ni, Mo, Cu, V, Nb, Ti, Zr and N2 (nitrogen) are also used in different combinations. The HSLA steels can also have small additions of Zr, Ca (calcium), or rare earth elements for sulphide inclusion shape control.

The HSLA steels are normally produced in the as-rolled condition. They can also be produced in a controlled-rolled, normalized, or precipitation-hardened condition to meet specific property requirements. Primary applications for HSLA steels include oil and gas line pipe, ships, offshore structures, automobiles, off-highway equipment, and pressure vessels. The types of HSLA steels normally used include the following.

Weathering steels or atmospheric corrosion resistant steels – These steels show better atmospheric corrosion resistance due to an adherent oxide layer formed on it. These steels normally have Cu content of around 0.35 %.

Control rolled steels – These steels are hot rolled as per predetermined rolling schedule designed to develop a highly deformed austenite structure which then transforms into a very fine equi-axed ferrite structure on cooling.

Pearlite reduced steels – The strength of these steels is obtained by very fine grain ferrite and by precipitation hardening but with low C content and hence these steels have little or no pearlite in the microstructure.

Micro-alloyed steels – These steels contain small content of Nb, V, and /or Ti (normally less than 0.1 % each) for refinement of grain size as well as for precipitation hardening.

Acicular ferrite steels – These are very low C steels with sufficient level of hardenability. The structure of these steels transform on cooling to a very fine high strength acicular ferrite (low C bainite) structure instead of the usual polygonal ferrite structure.

Dual-phase steels – These steels are processed to a micro-structure of ferrite containing small uniformly distributed regions of high C martensite. These steels have low yield strength and a high rate of work hardening. These steels are of high strength with superior formability.

Low alloy steels

Low alloy steels are those steels which show mechanical properties superior to the properties of plain C steels due to the additions of alloying elements such as Ni, Cr, and Mo. Total content of alloying elements in these steels may from 2.07 % upto the levels just below that in the stainless steels which contain a minimum of 10 % of Cr. In many of the low alloy steels, the primary function of the alloying elements is to increase the hardenability of the steel so as to optimize the mechanical properties and toughness after heat treatment. However in some cases the addition of alloying elements is done to reduce environmental degradation under specified service conditions.

As with steels in general, low alloy steels can be classified as per chemical composition, and heat treatment. In case of chemical composition, the classification is based on the alloying element such as tungsten steels, nickel steels, nickel chromium steels, molybdenum steels, and chromium molybdenum steels etc. In case of heat treatment, low alloy steels can be classified such as quenched and tempered steels, normalized and tempered steels or annealed steels and so on.

Since there are wide variety of chemical compositions are possible in low alloy steels and also due to the fact that some steels can be used in more than one heat treated condition, there exist certain overlap in the classification of low alloy steels. Hence, it is rather difficult to classify low alloy steels. There are four major groups of low alloy steels which are normally used. These groups are (i) low C quenched and tempered steels, (ii) medium C ultra high strength steels, (iii) bearing steels, and (iv) heat resistant Cr-Mo steels.

Low-carbon quenched and tempered steels – These steels combine high yield strength (from 350 MPa to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The different types of low C quenched and tempered steels have various combinations of these characteristics based on their intended uses. Some of these steels are used in military for armour purpose mostly as plates. Some of these steel are also produced as forgings or castings.

Medium carbon ultra high strength steels – These are structural steels which are having yield strengths which can exceed 1380 MPa. Some of these steels are covered by designations given in various standards while some other steels are having proprietary compositions. Product forms for these steels include billets, bars, rods, forgings, sheets, pipes and welding wires.

Bearing steels – These steels are used for ball and roller bearing applications. These steels consist of low carbon (0.1 % C to 0.2 % C) case hardened steels and high carbon (around 1 % C) through hardened steels. Some of these steels are covered by designations given in different standards.

Chromium-molybdenum heat-resistant steels – These steels contain 0.5 % Cr to 9 % Cr and 0.5 % Mo to 1 % Mo. The C content is normally below 0.2 %. The Cr provides improved oxidation and corrosion resistance while the Mo increases strength at elevated temperatures. These steels are normally produced in the normalized and tempered, quenched and tempered or annealed conditions. Cr-Mo steels are extensively used in the oil and gas industries and in fossil fuel and nuclear power plants.

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