Steel

Steel is a very ductile alloy consisting of iron and carbon. Based on the percentage of carbon content, steel is classified as follows:

Low-carbon or mild steel: carbon content up to 0.25%
Medium-carbon or medium hard steel: carbon content from 0.25 to 0.70%
High-carbon or hard steel: carbon content from 0.70 to 1.5%

1. Manufacturing of Steel

There are five processes by which steel is manufactured:

(i) Bessemer Process

A Bessemer convertor (a furnace) is used in this process, which is wide at the bottom and narrow at the top and rotates about the horizontal axis. Pig iron is filled in the convertor and hot blast air is forced into it. The impurities get oxidised by the blast air and a reddish-yellow flame is seen at the mouth of the convertor, and the flame diminishes gradually. Now the supply of air is stopped and adequate quantities of materials like ferromanganese is added to make steel of desired quality. The blast is again started for a few minutes and the molten metal is poured into moulds to form blocks called ingots.

(ii) Cementation Process

Here a dome-shaped furnace called a cementation furnace is used. In this process the pig iron is first converted into pure wrought iron. After adjusting the carbon content, steel is obtained.

(iii) Open-hearth Process

Here a mixture of pre-heated air and coal gas is used as a fuel. The steel produced by this process is homogeneous and is of high reliability.

(iv) Electric Process

Here the raw material is melted using electricity. The other procedure is the same as that of the Bessemer process and more control of temperature is feasible here. It is a quick process and special-grade steels are produced by this process.

(v) Duplex Process

It is a combination of the Bessemer and the basic open-hearth process.

Figure 11.1 shows a flow chart for the production of steel.

Figure 11.1 Flow diagram for the production of steel.

2. Uses of Steel

Mild steel is used to manufacture tools, machine parts, tubes, sheet metal, tin plate, structural steel, etc.
Medium hard steel is used to manufacture machine/engine components, boiler plates, rails, hammers, pressing dies, structural steel, agricultural implements, aero-engine, cylinders, springs, etc.
Hard steel is used in the manufacture of plates, cutlery, springs, miner’s drills, heavy tools, sledge hammers, axes, planning and slotting machines, lathes, etc.

3. Properties of Mild Steel

The properties of mild steel are as follows:

It has a high tensile strength.
It has a high elastic property reflected by a high modulus of rigidity.
It develops an effective bond with concrete.
Because of a good bond, it is capable of transferring stresses.
As it has an almost equal coefficient of thermal expansion and contraction, there is no room for the development of thermal stresses.
It is readily available and is comparatively cheap.

4. Properties of Medium Hard Steel

The properties of medium hard steel are as follows:

It has higher strength than mild steel.
It can be hardened to a certain extent.
It is tougher and harder than mild steel.
It can not easily be forged or welded.
It has a high resistance to shocks and vibrations.

5. Properties of Hard Steel

The properties of hard steel are as follows:

It has a granular structure.
It is very hard and is tougher than other steels.
It is possible to magnetise permanently.
It has very high compressive and tensile strengths.
It can be tempered and hardened.
It has a very high resistance to shocks and vibrations.
It is difficult to forge.

6. Suitability of Steel

The suitability of steel for different works as per Indian Standards is given in Table 11.2.

Table 11.2 Suitability of Indian Standard Steel as construction material