Carbon and low alloy steels

Carbon and low alloy steels

 The definition of the carbon steels by American Iron and Steel Institute (AISI) is 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 any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 %; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65 %, silicon 0.60 %, copper 0.60 %.”

Steels can be classified based on different systems depending upon:

  • The composition: Carbon, micro alloy, low alloy, high alloy or stainless steel.
  • The manufacturing processes: Open hearth furnace, basic oxygen process, energy optimizing furnace or electric arc furnace.
  • The finishing methods: Hot rolling, cold rolling or forging etc.
  • The type of product: Flat such as plate, sheet, strip, long such as wire rods, reinforcement bars, rounds and shapes, pipes and tubes or forged products.
  • The de oxidation method: Killed, semi-killed, rimmed or capped steel
  • The microstructure: Ferritic, austenitic, pearlitic, bainitic or martensitic
  • The strength levels: HSS, HSLA or normal strength to meet standard requirement
  • The heat treatment process: Annealing, normalizing, thermo mechanical treatment, quenching and tempering etc.
  • Quality defining designations: Forging quality, commercial quality, drawing quality or welding quality etc.

Carbon steels

As a group carbon steels are the most frequently produced and used steels. More than 85 % of the steels produced presently are carbon steels. Variations in the carbon content of the steels have the greatest impact on the mechanical properties of steels. Increase in the carbon content also results into increase in the hardness of the steels as well as their strengths. Hence carbon steels are generally defined according to their carbon content. Normally carbon steels contain up to 2 % total alloying elements and can be sub divided into ultra low carbon, low carbon, medium carbon, high carbon and ultra high carbon steels. These steels are described below.

Ultra low carbon steels – These steels contain very low carbon usually less than 0.10 %. These steels also contain very low manganese and very low silicon. These steels are having extra deep drawing properties and usually produced in the form of flat and wire rod products.

Low carbon steels – These steels contain up to 0.30 % C. This category includes mild steel with carbon content usually in the range of 0.15-0.30%. 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 carbon content for these deep drawing quality steels is around 0.10 % C, with up to 0.5 % manganese. For structural use produced in the form of plates and sections, the carbon content may be increased up to 0.30 %, with higher manganese content up to 1.6 %.

Medium-carbon steels – These steels are similar to low carbon steels except that the carbon content in these steels are higher and normally in the range of 0.31% to 0.60 % and the manganese from 0.60 % to 1.65 %. Due to Increased carbon content, the medium carbon steels can be used in the quenched and tempered condition. The medium carbon steels are used as 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 rail axles.

High carbon steels – These steels have carbon content ranging from 0.61 % to 1.00% with manganese content ranging from 0.30 % to 0.90 %. The use of high carbon steels includes in springs, pre stressed concrete wires and high strength wires etc.

Ultrahigh carbon steels – These steels contains carbon in the range of 1.25 % to 2.0 %.  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 meet the specific requirement of mechanical properties rather than a chemical composition. These steels are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels.

The carbon content in HSLA steels may range from 0.05 % to 0.25 % and manganese content up to 2.0 % in order to provide adequate deep drawing and welding properties. In these steels small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are also used in various combinations. The HSLA steels may also have small additions of calcium, rare earth elements, or zirconium for sulfide inclusion shape control.

HSLA are classified as below:

  1. Weathering steels or atmospheric corrosion resistant steels – These steels exhibit better atmospheric corrosion resistance due to an adherent oxide layer formed on it. These steels normally contain copper around 0.35 %.
  2. Control rolled steels – These steels are hot rolled as per predetermined rolling schedule designed to develop a highly deformed austenite structure that will transform to a very fine equi-axed ferrite structure on cooling.
  3. Pearlite reduced steels – The strength of these steels is obtained by very fine grain ferrite and by precipitation hardening. These steels are having very low carbon content and hence little or no pearlite in the microstructure.
  4. Micro alloyed steels – These steels contain small content of niobium, vanadium, and/or titanium for refinement of grain size as well as for precipitation hardening.
  5. Acicular ferrite steels – These are very low carbon steels with sufficient level of hardenability. The structure of these steels transform on cooling to a very fine high strength acicular ferrite structure instead of the usual polygonal ferrite structure.
  6. Dual phase steels – These steels are processed to a micro-structure of ferrite containing small uniformly distributed regions of high carbon 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 steels that exhibit mechanical properties superior to the properties of plain carbon steels due to the additions of alloying elements like nickel, chromium, and molybdenum. Total alloy content in these steels may from 2.01 % up to the levels just below those of stainless steels which contain a minimum of 10 % of Chromium.

In most of the low alloy steels, the primary function of the alloying elements is to increase hardenability 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 for specified service conditions.

Low alloy steels are generally classified as per:

  1. Chemical  composition – This classification is based on the alloying element such as tungsten steels, nickel steels, nickel chromium      steels, molybdenum steels, chromium molybdenum steels etc.
  2. Heat treatment – low alloy steels can be  classified based on heat treatment such as quenched and tempered steels, normalized and tempered steels or annealed steels.

Since there is a wide variety of chemical compositions possible in low alloy steels and also due to the fact that some steels can be used in more than one heat treated condition It is rather difficult to classify low alloy steels and there exists some overlap. However below there are given four major groups of alloy steels which are in common use.

i. Low-carbon quenched and tempered steels – These steels combine high yield strength (from 350 to 1035 N/Sq mm) and high tensile strength with good notch toughness, ductility, corrosion resistance and weldability. These steels have various combinations of these characteristics based on their intended applications. Some of these steels are produced as forgings or castings.

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

iii. Bearing steels – These steels are used for ball and roller bearing applications. These steels consist of low carbon (0.10 % to 0.20 % C) case hardened steels and high carbon (less than 1.0 % C) through hardened steels. Some of these steels are covered by designations given in different standards.

iv. Chromium-molybdenum heat-resistant steels – These 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 while the molybdenum increases strength at elevated temperatures. These steels are normally supplied in the normalized and tempered, quenched and tempered or annealed conditions.