ELECTRIC STEELMAKING
-:ABSTRACT:-
Now a days steels are of great demand because of its great applicability. Steels are iron-carbon alloys that may contain appreciable concentrations of other alloying elements. There are thousands of alloys that have a different compositions and/or heat treatment. Also various methods are employed for the preparation of steels. The mechanical properties are sensitive to content of carbon which is normally less than 1.0wt%.Here I have explained more details on the electric steel making process, with the introductory idea on other types of steel making processes & the classification of common steels.
INTROCTION:-
The composition of steels broadly divides it into
1)Plain carbon steels
-low carbon/mild steel ( up to 0.25%c)
-medium carbon steel (0.25%-0.65%c)
-high carbon steels (0 .65%-1.7%c)
2)Alloy steels
- Low alloy steel (alloy content <5%)>10%)
This classification is also important from the point of view of steel making. There are a number of steel making processes developed as given below.
Historical processes-
- wrought iron making
-cementation steel making
-crucible steel making
Modern steel making processes-
-Bessemer process
-open hearth process
-Electric furnace process
-Oxygen steelmaking process
Electric furnace process
Electric furnace are of three types-
1) Resistance furnace
2) Induction furnace
3) Electric arc furnace
Electric Resistance heating is not useful for steelmaking for a variety of reasons.
ELECTRIC INDUCTION FURNACE
In this process heat is applied by Induction Heating of a conductive medium in a crucible around which water cooled magnetic coils are wound.
Advantages:-
This method is clean, energy-efficient& well controllable melting process compared to other means of metal melting. Its capacity is 1kg-100 tones for which this procedure is used in research laboratories.
Drawbacks of induction furnace:-
It is lack of refining capacity. Charge material used here must clean of oxidation products & of a known composition.
ELECTRIC ARC FURNACE (EAF) METHOD:-
An electric arc furnace (EAF) is a furnace that heats charged material by means of an electric arc.Arc furnaces range in size from small units of approximately one ton capacity (used in foundries for producing cast iron products) up to about 400 ton units used for secondary steelmaking. Arc furnaces used in research laboratories and by dentists may have a capacity of only a few dozen grams
The temperature of an electric arc using carbon electrodes exceeds 1,800 degrees celcius. Hence steel making temperature can be readily maintained in an arc furnace. The first electric arc furnaces were developed by Paul Héroult, of France, with a commercial plant established in the United States in 1907.
Arc furnace are of two types-
a) Indirect arc furnaces
b) Direct arc furnaces
a) Indirect arc furnaces:
Here arc is struck between two carbon electrodes. Heat is transferred to the charge by radiations. It has small capacity. It does not develop steelmaking temperature readily. This is used generally for nonferrous materials.
b)Direct arc furnaces:
Here current flows from the electrode to the charge.Heat is transferred from the arc to the charge primarily by radiation but a part of the heat is also generated in the charge itself.
Design of arc furnace:-
Arc furnaces are of two different design,
a) The roof along with the electrodes swings clearly off the body to facilitate charging from it.
b) The roof is lifted a little & the furnace body moves to one side clearly off the roof to facilitate charging.
For smaller furnaces both of these alternatives are equally well suited but for bigger sizes the body becomes too heavy to move & hence the 1st one is favoured.
The furnace unit consists of the following parts –
a) furnace body(shell, hearth, walls, sprout, door)
b) gears for furnace body movements
c) roof & roof lift arrangements
d) electrodes, their holders & supports
e) electrical equipments( transformer, cables, electrode control mechanism etc)
Basic equipments & operation of furnace unit:-
The basic set-up of an electric arc furnace consists of a furnace shell with a roof on top. The furnace shell is lined with ceramic bricks (usually carbon bonded magnesia bricks) insulating the furnace from the liquid steel. On the upper side walls and on the inside of the roof, water cooled elements are used instead of ceramic insulation. These water cooled panels are positioned so there will be no direct contact with the liquid steel.
Before the melting and heating operations start, the furnace is charged with recycled steel scrap using a scrap basket that has been carefully loaded at the scrap yard. After scrap charging, the roof is closed and three graphite electrodes are lowered towards the scrap. The electrical power is switched on and on contact electrical power is transformed into heat as arcing takes place between the electrodes and the solid feedstock. As the scrap melts, a liquid steel pool starts to form at the bottom of the furnace.
The electrodes are moved downwards as the scrap is melted and caves into the scrap. The vertical ceramic bricks (usually carbon bonded magnesia bricks) insulating the furnace from the liquid steel. On the upper side walls and on the inside of the roof, water cooled elements are used instead of ceramic insulation. These water cooled panels are positioned so there will be no direct contact with the liquid steel.
Before the melting and heating operations start, the furnace is charged with recycled steel scrap using a scrap basket that has been carefully loaded at the scrap yard. After scrap charging, the roof is closed and three graphite electrodes are lowered towards the scrap. The electrical power is switched on and on contact electrical power is transformed into heat as arcing takes place between the electrodes and the solid feedstock. As the scrap melts, a liquid steel pool starts to form at the bottom of the furnace.
The electrodes are moved downwards as the scrap is melted and caves into the scrap. The movement of the electrodes is obtained by adjusting the electrode arms positions, which are controlled by the feedback from the electrical system, constantly supervising the electrical performance and aiming for an optimum power input at a predefined set-point.
As the scrap is melted, more volume is made available inside the furnace and at a certain point power is switched off, the furnace roof is opened, and another scrap basket will be loaded into the furnace. The power is again switched on and melting of the second basket starts.
When all scrap baskets (usually 2 or 3) have been melted, the heating continues for some time in order to superheat the steel to the target temperature at tapping. In more modern furnaces, oxygen is also lanced into the scrap, combusting or cutting the steel and burning out carbon , and sometimes chemical heat is provided by wall-mounted oxy-fuel burners. Both processes accelerate scrap meltdown.
During this period - usually referred to as the refining period - some metallurgical operations such as desulfurization, dephosphorization and decarburization, may be performed. When the steel has obtained the correct composition and temperature, the furnace power is switched off and the furnace is tapped. In the ladle furnace, alloys are added to obtain the desired properties in the steel required to meet product specification. The molten steel then passes through a continuous casting machine. The billet which emerges is cut to convenient lengths, that are further processed in a rolling mill.
ELECTRICAL EQUIPMENTS:-
The primary side, with a voltage of 25-50 kV, enters a furnace transformer where the voltage is decreased to a level suitable for the furnace operating conditions (secondary side), normally between 400-1000 V. In the case of alternating current, three phases are used and three electrodes will be needed. Each of the three phases is connected to one of three graphite electrodes.
The graphite electrodes play an important role since they carry the electrical power into the furnace. Graphite material is used because it withstands high temperatures and is a good electrical conductor. When the electrode is near the scrap an arc is created and an electric circuit is formed. These arcs provide the heat energy needed to melt the scrap – the higher the voltage, the longer the arc. The aim is to always have an even uptake of the provided power. Thus the electrodes have to be raised or lowered depending on the voltage reading.
Operating an EAF requires careful monitoring of the furnace electrical parameters at all times. By adjusting the tap settings, i.e. the pre-defined combinations of voltage and current, the electrical characteristics may be altered in order to fit the present operating conditions.
The electrodes have an upper limit in maximum current allowed, which in practice leaves the secondary voltage as the main regulating parameter when changing tap setting. The secondary voltage is directly proportional to the arc length inside the furnace.
RAW MATERIALS SELECTION:-
EAF is a based on recycling steel scrap with a little addition of ferro alloy in order to reach a target composition. A wide range of raw materials are used in the EAF. Where possible, the lowest cost alternative is selected – usually a coarse heavy scrap – so long as the material stays within the chemical composition requirements.
In some locations of the furnace a coarse scrap is not advisable and a more expensive fine scrap should be used. The finer scrap is used in order to avoid operating problems in the furnace, or practical problems when handling the scrap.
SLAG FOAMING:-
Slag foamability is as critical a parameter as CO gas generation in order to obtain foam, which floats on the surface of the molten steel. Foamability is controlled by the slag phase physical properties viscosity, surface tension and density. These properties vary with slag composition.
Foaming slag is used to also increase the thermal efficiency of the furnace during the refining period, when the side walls are fully exposed to the arc radiation. A foaming slag will rise and cover the electric arcs, thus permitting the use of a high tap setting without increasing the thermal load on the furnace walls. In addition, an electrical arc covered by a foaming slag will have a higher efficiency in transferring the energy into the steel phase.
For a furnace with basic refractories, which includes most carbon steel producing furnaces, the usual slag formers are calcium oxide (CaO, in the form of burnt lime) and magnesium oxide (MgO, in the form of dolomite and magnetite ). These slag formers are either charged with the scrap, or blown into the furnace during meltdown. Later in the heat, carbon (in the form of coke) is lanced into this slag layer, partially combusting to form carbon monoxide gas, which then causes the slag to foam, allowing greater thermal efficiency, and better arc stability and electrical efficiency. The slag blanket also covers the arcs, prevents damage to the furnace roof and sidewalls from radiant heat.
Slag foaming is obtained by injecting oxygen into the liquid steel, where mainly iron is oxidized according to the reaction:
O2 + 2 Fe = 2 (FeO)
Carbon powder is then injected simultaneously into the slag phase where iron oxide is reduced.
(FeO) + C = Fe + CO (g)
The resulting CO gas is a critical component in order to obtain a foaming slag.
More slag formers are introduced and more oxygen is lanced into the bath, burning out impurities such as silicon, sulfur, phosphorus, aluminiums, manganese and calcium and removing their oxides to the slag. Metals that have a poorer affinity for oxygen than iron, such as nickel and copper, cannot be removed through oxidation and must be controlled through scrap chemistry alone, such as introducing the direct reduced iron and pig iron mentioned earlier. A foaming slag is maintained throughout, and often overflows the furnace to pour out of the slag door into the slag pit.
Once the temperature and chemistry are correct, the steel is tapped out into a preheated ladle through tilting the furnace. As soon as slag is detected during tapping the furnace is rapidly tilted back towards the deslagging side, minimising slag entering the ladle. During tapping some alloy additions are introduced into the metal stream. Often, a few tonnes of liquid steel and slag is left in the furnace in order to form a 'hot heel', which helps preheat the next charge of scrap and accelerate its meltdown. During and after tapping, the furnace is 'turned around': the slag door is cleaned of solidified slag, repairs may take place, and electrodes are inspected for damage or lengthened through the addition of new segments; the taphole is filled with sand at the completion of tapping. For a 90-tonne, medium-power furnace, the whole process will usually take about 60-70 minutes from the tapping of one heat to the tapping of the next (the tap-to-tap time).
Electrode Breakage:-
Electrode breakage occurs occasionally in the EAF, predominantly during the melt-down operation. This should be avoided due to the high costs associated with a breakage. Besides the cost for the high density graphite electrode – which is a rather expensive material – the down-time equals loss of production which represents high values. Normally a broken electrode is changed within 10 minutes, but in difficult cases considerably longer down-times can be expected.
Electrode breakage is usually a consequence of mechanical overload on the electrode from the surrounding scrap.
-Scrap is caving in from the side as the electrode penetrates into the scrap pile inside the furnace. If heavy pieces hit directly on the electrode side breakage may occur.
-The electrodes are moving downward without detecting that a non-conducting material is present at the tip of the electrode. As the electrode continues to push downwards, breakage may occur.
The combination of arc length and scrap material is also of importance when considering the probability of an electrode breakage. The furnace operator then needs to balance the tap settings (voltage and current) with the need of highest possible power input and suitable arc length
TAPPING:-
Tapping of the furnace is initiated by the operator when the processing in the furnace is finalized and the target temperature has been reached. Tapping should be performed as fast as possible in order to save time.
There are two common furnace designs that have different tapping configurations.
1)Eccentrically-Bottom Tapping (EBT) furnaces have a taphole positioned off-center in the base of the furnace. Such a configuration enables slag-free tapping. In these cases a "hot heel" (small amount of remaining metal and slag) is retained in the furnace between the heats.
2)Spout furnaces are used for some steel grades. Tapping via a spout causes the slag to be carried over to the ladle, where it is thoroughly mixed with the steel. In these cases all the metal is poured out, without any hot heel remaining in the furnace.
ENVIRONMENTAL ISSUES:-
EAF causes high sound effect. It produces dust & off gas which pollutes the environment. Slag is produced in this process. EAF demands cooling water which creates water problem. It requires heavy truck for scrap & material handling & products which creates traffic problem. since electricity is the main source for EAF , electricity generation causes environmental effect .
ADVANTAGES:-
EAF primarily uses scrap steels as feed unlike the more traditional Basic oxysen method, which uses iron ore and coking coal as feed. Also 80 metric tonnes of liquid steel takes approximately 60 mins, where as in other process like basic oxigen furnace can have a capacity of 150-300 tonnes per batch in 30-40 min. but in induction furnace 1 tonne can be melted in 1 hr. This method is economical where there is a plentiful of supply of electric power with a well developed electrical grid.
CONCLUSION:-
The share of electric arc furnace steel making technology in world crude production has risen from 15% in 1970 to 34% in 1997.In japan, the EAF share of steel production is forecast to rise from 32.85 in 1997 to 37% in 2010 & from 43.8% to 50.1% in United States over the same period. Continuation of this trend will have significant for Iron ore & coking coal consumption in the longer term.
REFERENCES :-
1) Introduction to modern steel making.
-R. H. Tupkary
2)physical metallurgy principles
-R.E. Reed Hill
3)Material & manufacturing processing
-De Garmo
4)material science & engg. an introduction
- W. D. callister
5)Paul crompton-The diffusion of new steel making technology,Resources policy, vol-27,issue-2,june 2001,page 87-95.
ELECTRIC STEELMAKING
