MANUFACTURE OF STEEL :

1. The Bessemer Process

2. Open Hearth Process

3. Electrical Furnace Process

The above process can be subdivided into (a) acid process and (b) basic process, according to the type of lining used in the furnace and chemical nature of the slags.

In acid process,) the Bessemer Converter is lined with silica bricks because of acid slag formation. The acid process does not eliminate phosphorus or sulphur from the metal.

In the basic process, the converter is lined with burnt dolomite (CaO Mgo) or burnt magnesite (Mgo). It removes phosphorus and to some extent sulphur.

The Bessemer Process :

The Bessemer Converter consists of a steel casing lined with appropriate refractory depending upon the steelmaking process, acid or basic. The converter shaped like a huge concrete mixer can be rotated from a vertical to a horizontal position in order to fascilitate charging and pouring. The converter is charged with pig iron (molten)and blown with air and slowly rotated in vertical position so that air is discharged through the molten contents.

There are three distinctive stages : (1) the slag formation or blowing period, (2), the brilliant flame blowing period and (3) the reddish smoke period.

The first stage commences as soon as the blast is put on under a pressure of about 200-300 KN/m gauge. The oxygen of the blast air oxidises silicon and manganese and converts them to slag. Then oxygen of the blast oxidises iron to Ferrous Oxide. These reactions are accompanied by the evolution of heat which raises the temperature from 125O°C to 1525°C. This stage lasts for 3 6r 4 minutes.

The second stage commences after the oxidation of iron,) silicon and manganese, when the metal has reached a sufficiently high temperature. This helps dissolved carbon to oxidise by the ferrous oxide of the slag. The gas evolved during this stage is rich in carbon monoxide which burns at the nose of the converter with a dazzling white flame. It takes about eight to twelve minutes to remove the carbon. In the acid process, the flame dies down approximately 20 minutes after the commencement of blow and this indicates that the bulk of the impurities have been removed. In the basic process,, the blow is extended for 5 minutes, known as "after blow", which is necessary for removal of phosphorus.

The third stage begins when the flame "drops" a sign that the carbon has been practically removed from the charge. This stage lasts for 1 or 2 minutes after which the converter is turned to the horizontal position. To remove excess of iron oxide which is dissolved in the molten metal,, deoxidisers such as ferromanganese (75-80% Mn, 6-7% Carbon and iron), ferrosilicon or aluminium are added to the metal bath.

FeO + Mn = Fe + MnO (insoluble in steel, joins slag)

Any excess manganese, together with the carbon, dissolves in the steel, so that, in addition to deoxidising, the ferromanganeze acts as an agent to adjust the carbon content •

OPEN-HEARTH PROCESS :

This process also can be either acid or basic in nature, depending upon the type of charge used, and consequently the nature of the slag produced. Whereas the Bessemer process uses no external source of heat, the open-hearth furnace is fired by procuder gas and coke-oven gas (a by-product of blast furnace coke manufacture) or by fuel oil. This independent source of heat allows a somewhat greater variation in the composition of the charge, since the impurities are no longer needed to act as fuel.

In order to attain the necessary high temperature and to economise in fuel, a regenerative system is used to pre-heat the in-going air and gas. The chequered brickwork stoves on the right are being reheated by the hot waste gases as they leave the furnace. When they have attained a temperature between 900° and 1200°C, the change-over valves will be operated in order to reverse the flow of gases so that those stoves on the left, which up to now have been heating the in-going (and being cooled themselves in the process) will now receive the out-going gases and begin to heat up. In the mean time the pair of stoves on the right will come into action to pre-heat the in-going gas and air. The chage-over will operate approx. every half-hour.

The furnace itself consists essentially of a hearth in the shape of a large, elongated basin able to hold between 60 and 300 tons of steel according to size. Along one side is a row of charging doors, whilst on the other side, which contains the tap hole, is the casting pit. At each end is a pair of ports (or inlets) for gas and air respectively.

The main requirement of the raw material for the acid process are that it shall be low in sulpher and phosphorus, since none of these elements will be removed during the process. The basic process,) however, is most adaptable of all steel-making processes, since apart from sulphur, the impurity content is not critical. Even a small amount of sulphur is allowable, but to remove sulphur as manganese sulphide (MnS), manganese content must be fairly high. The basic open-hearth process, consists charge of steel scrap, phosphoric pig iron and burnt limestone.

In acid process, a lining of silica sand is used on the hearth and on acid slag high in silica is produced on the metal during the refining process. Acid slag cannot remove phosphorus for which basic lining is used. In basic process, a lining of magnesite is used and limestone is charged with the raw material. A basic slag high in lime is produced on the bath after the metal melts.

When the charge, either acid or basic is melted and the impurities (silicon, manganese) are brought down to required level, oxide ore or mill-scale is added as an oxygen carrier whilst in basic process some lime may be added at this stage. Since the total time is between six to fourteen hours, the test samples are taken from time to time.

The acid open hearth process is used mainly for the production of high grade steel for use as axles, wire ropes, springs, castings, piston-rods and alloy steels, while the basic process used for the production of steel of wide variety at low cost.

THE ELECTRICAL FURNACE PROCESS :

The electric furnace is used for the manufacture of high-grade tool steels and alloy steels. Though electricity is an expensive fuel, the utilisation of medium-grade scrap, which can be bought cheaply, is possible in this process, since conditions favour the removal of phosphorus and sulphur. The high cost of electricity used is thus largely offset.

The furnace used is of the arc type, and employs carbon rods striking an arc on to the charge. The lining can be either acid or basic, though the latter is more generally employed, particularly when the purpose of the process is to refine the scrap charged and basic slags must in consequence be used.

In the basic process the charge is melted under a basic oxidising slag containing lime and mill-scale, Consequently the silicon, manganese and phosphorus present are absorbed* into the slag, and when this process is found to be completed, the slag is removed completely, so that only impurity present is sulphur. Another slag is formed on the surface of the melt, but a basic reducing slag composed of lime and anthracite. The main function of the slag is the effective complete removal of sulphur, and this process is the only steel- making process where this occurs.

FeS + C + CaO = Fe + CO + CaS (insoluble in steely joins the slag)

Main advantages of the electric process are -

a) There is a definite and reliable removal of sulphur.

b) Conditions are chemically clean,) and there can be no
contamination of the charge,, as is possible even with
gaseous fuels.

c) The furnace atmosphere and the slag can fee made
either oxidising or reducing at will.

d) The temperature can be controlled easily.-

e) The carbon content of steel can be held constant^
thus making its adjustments within fine limits
possible.

f) The addition of alloying elements can be made with
precision.

CLASSIFICATION OF STEEL :

On the basis of carbon content^ steel is classified as :

a) Low carbon or mild steel - 0.05-0.30% carbon.

b) Medium Carbon Steel - ^l0.30-0.60% carbon

c) High Carbon Steel - 0.60-1.50% carbon

d) Tool Steel - 1.00-1.50% carbon

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