Structural steels

Structural steels

Structural steel is a standard construction material made from specific steel grades and is available in industry standard cross sectional shapes. This steel exhibits desirable physical properties such as strength, uniformity of properties, light weight and ease of use etc. This makes it one of the most versatile structural materials in use. Major applications for these steels are in high rise and tall multi-storey buildings, industrial buildings, towers, tunnels, bridges, road barriers and industrial structures etc.

The common shapes in which structural steels are available consist of sections (beams, channels, Tees section and angles), squares and rounds, hexagons, plates, pipes, hollow square sections, steel cable, Z sections and cold formed sections etc. (Fig 1).

Structural steel shapes

Fig 1  Some shapes of structural steels

Structural grade steels have specific chemical compositions and mechanical properties required as per their application. These steels are produced as per the specifications included in different standards which are issued for structural steels. Structural steels for use at ambient or moderately elevated temperatures are of the following types.

  1. Carbon and carbon-manganese steels – In these steels the maximum content for alloying elements does not exceed the following: (i) manganese – 1.65 %, silicon – 0.40 % and (iii) copper – 0.6 %. The specified minimum of copper does not exceed 0.4 % and also there is no minimum content is specified for other elements to obtain a desired alloying effect.
  2. High strength low alloy (HSLA) steels – These steels have specified minimum yield strengths greater than 280 Newtons /Sq cm and achieve that strength in hot rolled condition rather than by heat treatment.
  3. Heat treated high tensile steels – Both Carbon and HSLA steels can be heat treated to provide yield strengths in the range of 350 to 520 Newtons/Sq cm.
  4. Constructional alloy steels – These steels contain alloying elements in excess of the limits for carbon steels and are heat treated to obtain a combination of high strength and toughness. These are the strongest steels in general structural use with yield strength of 700 Newtons / Sq cm.
  5. High performance steels – These steels are with enhanced notch toughness and generally used in construction of bridges. These steels are higher in strength, lighter in weight and have greater atmospheric corrosion resistance than conventional steels.

Elements used in structural steels

The following are the important chemical elements which are used in the structural steels.

  • Carbon – Carbon is the most important chemical element in the structural steel. Increase in the carbon percentage improves strength but reduces ductility. Hence structural steels have carbon content in the range of 0.15 % to 0.30 %.
  • Manganese – Manganese content in structural steels in up to 1.65 % maximum. It has effects similar to those of carbon. Manganese and carbon in structural steels are to be in such combination so as to have the desired properties in the steel.
  • Silicon – It is one of the principal deoxidizer for the structural steel. It is the element that is most commonly used for the production of semi killed or fully killed structural steels. Silicon content in structural steels is less than 0.40 %.
  • Aluminum – Aluminum is used as deoxidizer in the production of semi killed or fully killed structural steels. It forms a more fine grained crystalline micro structure in the structural steel.
  • Phosphorus and sulphur – Both phosphorus and sulphur are usually undesirable elements in the structural steels. Sulphur promotes internal segregation in the steel matrix. Both these elements act to reduce the ductility of the structural steel. There detrimental effect on the steel weldability is also significant. Hence the contents of these elements in the structural steel are limited to less than 0.04 % to 0.05 %.
  • Copper – It is a corrosion resistance element and is a primary anti corrosion component in the weathering structural steels. In such steels the copper content should not be less than 0.2 % but ideally it should be in the range of o.25 % to 0.55 %.
  • Niobium – Niobium is the usual strength enhancing element in HSLA steels. It also has some corrosion resistance properties.
  • Chromium – Chromium is present in some structural steels in small amounts. It is usually used to enhance the corrosion resistance of the structural steel and hence often it is used in combination with nickel and copper.
  • Molybdenum – It is used in some grades of structural steels to increase the strength of the steel at the higher temperature. It also improves corrosion resistance. It is often used in structural steels in combination with either manganese or vanadium.
  • Nickel – Nickel improves the low temperature behaviour of the structural steel by improving the fracture toughness. It also has favorable effect on the corrosion resistance of the structural steel.
  • Vanadium – The effect of vanadium is similar to those of manganese, niobium and molybdenum. It helps structural steels to develop a finer crystalline micro structure and have increased fracture toughness.
  • Other chemical elements – Some structure steels grades have small amounts of some other alloying elements such as boron, nitrogen and titanium. These minor alloying elements along with the major alloying elements enhance certain capabilities of the structural steels.

Weldability of sructural steels

The weldability of structural steels depends on its carbon euivalent (CE) which is defined below.

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

Structural steels with carbon equivalent below 0.45 are readily weldable with appropriate procedures. CE of strutural steels greater than 0.45 indicates caution is to be observed during welding of such steels.

Properties of structural steels

The following are the important properties of structural steels

  • It has high strength which means that the weight of the structure made of steel is less.
  • It has uniform properties which do not change as oppose to concrete
  • Elasticity of structural steel follows Hooke’s law accurately
  • It has good ductility due to which structural steels can withstand extensive deformation without failure under high tensile stresses.
  • It has good toughness which means that structural steel has got both strength and ductility.
  • It has got flexibility which allows extension of existing structures made of structural steels can be done easily.
  • Structural steels are non combustible materials but they lose strength when heated sufficiently. When heated to temperatures normally associated with fires, the strength and stiffness of the structural steel are significantly reduced. Structural steels have a critical temperature. This critical temperature is the temperature at which the structural steel cannot safely support its load. Critical temperature is usually the temperature at which its yield stress has been reduced to 60 % of the room temperature yield stress.
  • Structural steels can corrode when in contact with water.
  • Structural steels can be coated or painted to provide it corrosion and fire resistance.