The principal methods of manufacturing different grades of steels are: 1. Cementation Process 2. Crucible Process 3. Bessemer Process 4. Open Hearth (Acid or Basic) Process 5. Electric Process 6. L.D. Process 7. Duplex Process 8. Kaldo Process.
1. Cementation Process:
The pure wrought iron bars are arranged in the furnace between the layers of powdered charcoal and maintained at higher temperature (about 1500°C) for 5 to 15 days depending upon the quality of steel requited. The wrought iron gets covered with blister due to combination of carbon and thus the steel so produced is termed as ‘blister steel’. Blister steel has fissures and cavities and also its structure is not homogeneous. It may be employed for machined parts and facing hammer but less suitable for edge tools.
2. Crucible Process:
This process of manufacturing steel consists in heating in fire clay crucible either fragments of ‘blister steel’ or short lengths of wrought iron bars with charcoal and running the metal into iron moulds. The steel thus produced is known as cast steel. It is extremely hard and uniform in quality. It finds its use in making finest cutlery and hardest cutting tools.
3. Bessemer Process:
This process makes use of pear shaped iron converter which is supplied with pig iron melted in a cupola. The converter is lined with fire bricks and can revolve about the horizontal turn ions. The bottom is pierced by a number of tuyeres connecting with wind box underneath and made detachable since the wear on it is particular is very severe. It is usually changed after every 15-20 blows.
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To receive the charge, the converter is turned down in the horizontal position and again brought to vertical position i.e., working position. A strong blast of air is supplied to the charge for about 20 minutes. The metal becomes agitated by the passage of air blast and a shower of sparks is continuously emitted from the mouth of the converter. Iron oxide is formed which in turn reacts with the impurities present in the iron. Silicon is the first to be removed as it is most easily oxidised of the elements present (reaction given below)-
Si + 2FeO → SiO2 + 2Fe
Due to this reaction an enormous amount of heat is generated, therefore, it is important that iron should contain adequate quantity of silicon for the purpose. Iron having less than 1.5 per cent of silicon will blow cold, while more than 2.5 per cent will generate too much heat.
As the blow proceeds, a flame gradually appears at the mouth of the vessel and manganese starts oxidising. Some of the oxides of silicon, manganese and iron are ejected in the form of brown fumes but those remaining in the converter combine to form a slag which floats on the metal.
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The last element to be eliminated is the carbon which follows the reaction:
C + FeO → CO + Fe
This stage of process is known as ‘the boil’ owing to the violent agitation caused by the evolution of carbon monoxide gas from the metal. As the carbon is eliminated the boil gradually subsides and large white flame finally drops after sometime. The metal in the converter is now virtually pure iron in the liquid form.
The blast is shut-off and a quantity of ferro-manganese (an alloy of iron containing manganese upto 80% and carbon upto 5%, used in making additions of Mn to steel or cast iron), spiegeleisen (an alloy of iron containing 1.5-30% Mn and 4-5% carbon; added to steel as deoxidising agent and to raise the Mn-content of the steel) or some other suitable alloy steel as required is added and the blast resumed for a few minutes more to ensure the alloy thoroughly mixing with iron.
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The converter is then swung into discharging position and discharged into ladles from which the metal is poured into moulds where it solidifies t6 form ingots. To remove the blow holes formed due to bubbles of nitrogen or oxygen, a small quantity of aluminium or ferro-silicon is also added.
A Bessemer converter is furnished with an acidic lining (ganister) if the pig iron is free from sulphur and phosphorus, otherwise a basic lining of dolomite is employed. For removal of sulphur, calcined lime is added to the charge and then heated to red heat. A portion of it burns out while a portion combines with lime to form slag. The basic lining is least affected by this slag containing phosphorus.
Advantages and Disadvantages of Bessemer Converter:
I. The cost of building a Bessemer converter plant is comparatively less than that of an open hearth plant of equivalent capacity.
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II. A Bessemer converter, with its small size and ease of handling, makes it possible to produce small tonnages of many different grades of steel, actually three grades of steel in less than an hour. As compared to open hearth furnace, major repairs can be made in a fraction of time.
III. The flexibility is all the more attractive in times of slack demand when quick deliveries of small tonnages are desirable from standpoint of the manufacture.
IV. Probably because of the higher ferrous oxide content of acid grade, it has peculiar properties of machinability, stiffness, weldability and sensitivity to cold work. High- sulphur Bessemer steel possesses a tremendous inherent advantage by reason of ease of machinability. Bessemer steel, both hot rolled and cold drawn, is generally stiffer than open hearth steel of equivalent tensile properties.
4. Open Hearth Process:
The furnace used in this process resembles a reverberatory furnace. Here pig iron is melted in the furnace and large quantity of scrap iron or steel is dissolved in the liquid bath. The source of heat is heated air and coal gas. In open hearth furnace a great economy in fuel is obtained by the utilisation of heat of the hot gases of combustion. These gases are made to pass through brick gratings of two regenerators before leaving the chimney. The air and gas on the other hand traverse through two other chambers.
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After an hour direction of flow is reversed. In this way, when the entering air and gas are being adequately heated in their passage through the heated gratings in the two regenerators, the other two regenerators get heated by the hot gases escaping into the chimney.
The intense heat produced by burning of air and coal gas keeps the charge in the molten condition. Silicon gets converted into silicate and is removed as a slag and carbon burns to carbon dioxide leaving practically pure iron. A measured quantity of ferro-manganese or suitable alloy steel is added to supply the necessary carbon, manganese, chromium or nickel. The liquid steel, finally, is poured into the moulds through ladles.
The furnace lining may be basic or acidic depending upon the fact whether phosphorus is present or absent in the pig iron. Acid lining comprises either silica or ganister (83% quartz, 13% clay, 4% moisture and other impurities). Basic lining is usually magnesite (MgCO3) or dolomite (CaCO3.MgCO3).
The basic open hearth process is steadily replacing the acid open hearth due to the following reasons:
I. The process is not limited to relatively low phosphorus and sulphur materials but, instead makes possible the removal of relatively large amounts of these elements and, by so doing, open up vast tonnages of ores and steel scrap which ordinarily could not be available for acid steel.
II. It permits the use of all types of steel scraps, light and bulky, heavy and dense, dirty and clean to produce high grade steel of any chemical analysis.
III. It is much more flexible process permitting operator to vary the proportion of steel scrap, solid pig iron and molten metal to meet local and economic conditions.
IV. There is relatively, wide permissible range in the chemical composition and physical characteristics of lining materials, the slag and molten metal.
V. Steels of wide range of carbon content, from as low as 0.03 to as high as 1.1 percent can be made. As compared to Bessemer process, the basic open hearth is a leisurely method giving the operator an ample time in which to make chemical analysis and slag and metal adjustment.
Table 2.4. Comparison between Bessemer Process and Open Hearth Process:
Bessemer Process:
1. It can use comparatively higher phosphatic pigs of wider ranges of composition.
2. Scrap iron cannot be used.
3. Refining and finishing require 10 to 20 minutes.
4. Operations depend entirely on eye judgement, so it is very difficult to produce a uniform product.
5. Bessemer process produces an inferior quality steel associated with blow holes and inclusions.
6. Carbon is completely eliminated first and so requires large amount of recarburizer and the product becomes less homogeneous.
7. Low capital investment.
8. Output low (80% of pig used).
9. Because of rapidity of the process, the control of different operations is rather difficult and careful working is necessitated.
Open Heart Process:
1. It can use lower phosphatic pigs of narrower ranges of composition.
2. Scarp iron can be used.
3. Refining and finishing, are completed within 8 to 10 hours.
4. Operation is guided by laboratory analysis, so it is comparatively possible to produce a uniform product.
5. Open hearth steel is of much superior quality containing lesser blowholes and inclusions.
6. Carbon is partially removed and requires smaller amount of recarburizer and the product is more homogeneous.
7. High capital investment.
8. Output high comparatively.
9. Operation is comparatively easy and finishing upto the desired analysis is feasible.
5. Electric Process:
This process makes use of two types of electric furnaces:
(a) Arc type
(b) High frequency type.
(a) Arc Type Furnace:
A three phase direct arc type furnace. It consists of a circular steel casting lined inside with refractory material. The roof is removable and a spare is usually kept for rapid replacement. The roof is provided with three holes through which pass the electrodes.
The electrodes may be of graphite or amorphous carbon, the former material has double the conductivity and will carry 2.5 times the current: hence, the graphite electrodes are usually about 2/ 3rd diameter of amorphous carbon electrodes. Though graphite electrodes are costly yet the above advantages are sufficient to account for their choice for most arc furnaces.
To maintain a desired length of arc, the electrodes are raised and lowered individually by electric motors operated by automatic regulators. The voltage between steel and electrodes may be 40-145 volts; the longer the arc, higher the voltage required and the less the input of heat to furnace. Electric power is supplied in bulk in the form of three phase alternating current at 6.6 or 10 kV.
A transformer set up close to the furnace reduces the voltage down to that required for the arcs and its primary winding having tappings to allow for adjustments to the arc voltage. As the power supply is a three phase circuit, three electrodes are arranged in an equilateral triangle over the metal. Owing to low voltage required by the arc, the current must be very high to obtain the desired output.
The hearth of an electric furnace may have acid and basic linings depending upon the process adopted. Basic process is used for making steel ingots and some castings while the acid process mostly for making steel castings.
The usual size of this furnace is between 5 to 10 tonnes, though 50 and 100 tonnes furnaces have been produced. This type of furnace is used for making alloy steels such as stainless, high speed steel etc.
The advantage of this furnace is that purer product is obtained and composition con be exactly controlled during refining process. This is the reason that direct arc furnace even being costlier in initial as well as operating cost is preferred. Though this furnace is employed for melting and refining but due to higher cost its use is restricted to refining than melting. It operates at a power factor of about 0.8 lagging.
(b) High Frequency Electric Furnace:
This type of furnace works on the principle that when a piece of metal is held in a coil of wire carrying alternating current, eddy currents flow in steel clue to the alternating magnetic field being produced. Due to these currents steel is heated up.
When the charge gets melted/ the eddy currents flowing through molten metal agitate or stir it and mix the constituents thoroughly. Fig. 2.33 shows a high frequency induction furnace which works upto 10000 cycles/second and is most suitable when the charge is of the same composition as the casting.
In this furnace tubular shaped conductors are used to:
(i) Produce- inductive effect and
(ii) Withstand high temperature at high frequency.
The wire solenoid is not used as it cannot stand as high a temperature as 1800°C which may be produced in this furnace.
It is usual practice to operate the high frequency furnace purely as a melting furnace and only rarely arc refining operations attempted. For this reason it is possible to avoid oxidising conditions during melting and steels containing high percentages of easily oxidised elements like chromium, manganese and vanadium can be melted with practically no loss of these elements.
The perfection cannot be attained in the arc furnace, so that the high-frequency furnace reigns supreme for melting such materials as stainless steel, high speed steel, magnet steel and other high alloy steels.
Even when operated intermittently, the high-frequency furnace can prove economical owing to very small weight refractory material which has to be heated up with steel. A high frequency furnace costs about three times as much to install as does an arc furnace of the same capacity.
Advantages of the Electric Furnaces:
I. The generation of heat within an electric furnace is independent of the furnace atmosphere, and this may be maintained oxidising, neutral or reducing, at will.
II. The temperature is more under control than in an open hearth furnace and .very high temperatures can be attained by means of the arc.
III. The close control over steel-making conditions possible in the electric furnace permits high-grade steel to be made from low grade scrap, often without using pig iron. Electric steels are mostly of superior qualities not normally made by an open hearth process.
IV. The gas, fumes and impurities which exist in fuel bed furnaces are absent.
6. L.D. Process:
This process originated in 1953 at Linz steel works in Austria. In India it is being used in the Hindustan steel plant at Rourkela (Orissa). It combines the high productivity of the acid Bessemer process and the superior quality of the basic open hearth steel.
The furnace is a vessel similar to that of the Bessemer converter except that the capacity is between 30 to 40 tonnes (double that of Bessemer) and there is no detachable bottom with its row of tuyere bricks. The vessel has a basic lining.
In this process a jet of pure oxygen (99%) is blown at a pressure of 7.0 to 10.5 bar and at a speed even greater than the sound to the molten bath of hot metal. The temperature thus produced is about 2550°C due to which the impurities like carbon, nitrogen, phosphorus and sulphur etc. are burnt. For a converter of capacity 50 tonnes, the blow lasts for about half an hour while the complete operation consumes about one hour.
The various steps involved are:
I. The converter still hot from previous blow is charged with 15% steel scrap and 85% molten pig iron from the mixer.
II. Oxygen is blown through water cooled nozzle at a pressure of 7 to 10.5 bar and lime added.
III. The blow lasts for about half an hour, the dropping of flame indicates that the molten mass has been refined.
IV. The separation of low carbon steel is effected by pouring out steel into a ladle and the liquid slag thickened with burnt lime is left mostly in the converter.
LD process is the only process where sulphur can be effectively reduced.
The reasons for this are as follows:
A. Large volume of slag which is highly rich in lime is formed and is kept fluid due to intense heat. Sulphur is eliminated as gas.
B. The residual manganese in bath is higher and MnS gets oxidised giving out SO2 gas.
7. Duplex Process:
This process is designed to combine the advantages of the Bessemer and open hearth process while avoiding their disadvantages.
For duplexing the following two methods are used:
1. In one method, hot metal from the blast furnace or mixture is partially blown in a basic converter and then transferred to a basic open-hearth furnace, where refining is completed in 5-6 hours.
2. In another method, five acid Bessemer converters are used to eliminate silicon, manganese and carbon and the soft steel so made is passed on to a 200 tonnes basic tilting furnace for removal of sulphur and phosphorus. A sixth converter is used to supply partially blown metal for recarburizing the steel in the tilting furnace, which is tapped every 4.30 hours, leaving 50 tonnes of steel in the furnace.
Duplex process has the following characteristics:
I. The life of open hearth is increased because of the absence of silica oxide which has detrimental effect on the basic lining.
II. Pig iron associated with silica and phosphorus contents is converted into a good quality of steel. Silica undergoes oxidation in acid Bessemer converter while phosphorus in the basic open hearth process.
III. Since the time of open hearth heat is reduced, labour, maintenance and overhead costs are less. This system is used at Tata Iron and Steel Works, Jamshedpur (Bihar).
8. Kaldo Process:
Kaldo process was introduced by Professor Killing. The fundamental departure from the LD process is the rotation of the bath during oxygen injection whereby slag metal contact and reactions are affected. In the Kaldo rotary oxygen process conversion of pig iron into steel is carried out into a rotating converter into which oxygen is introduced through a water cooled lance which enters the converter through a central opening at one end, which simultaneously acts as outlet for the exhaust gases.
The speed of the rotation can be varied upto 30 r.p.m., which is most suitable speed for main stage of the process. The furnace or converter is lined with ordinary tarred dolomite and has a life of 50 heats.
The usual procedure is as follows:
I. To inject oxygen until most of the phosphorus has been removed at which stage the carbon will approximate 1.5%, when the first slag is removed. The first slag contains about 22% P2O5 with only 3% Fe. Owing to heat evolved during oxygen injection it is necessary to add either iron ore or scrap as a cooler. The usual practice is to employ iron ore for cooling purposes, since this increases the metallic yield and reduces the oxygen.
II. After the first slag has been removed blowing is continued until the carbon has been reduced to about 1.0% by which time the phosphorus will be less than 0.1%. At this stage second slag is removed. The second slag contains about 17 to 20% P2O5 with about 6% iron. These two phosphoric slags, which average 16 to 32% P2O5, can therefore be sold for fertilizing purposes.
III. After removing the second slag, a final slag is collected with lime additions and blowing is continued until the metal has been refined to the desired specification. The final slag remains in the vessel as a base for subsequent heat. The metallic yield is about 92% and the average blowing time is 35 to 40 minutes for 30 tonnes heat.
Kaldo process entails the following advantages:
(i) Gives high rates of productivity at a low capital cost.
(ii) Particularly suitable for manufacturing high grade steel from high phosphorus pig iron, and the surplus heat produced in the process permits the employment of an appreciable quantity of iron ore as a cooling agent; thereby increasing the molten yield.
(iii) The satisfactory by-product value of high phosphoric acid slag produced is another economic advantage.
The main disadvantage of this process is the number of moving parts and their weight which probably exercise the adverse effect on the maintenance cost.