Training Report on Bokaro Steel Plant

SAIL AT A GLANCE

Steel Authority of India Limited is India’s largest steel producing company and ranks 24th in the World. It was founded in 19th January 1954 with headquarter at New Delhi. It was incorporated to central public sector on 24 January 1973.  With an annual turnover of Rs. 49350 crore, the company is among the seven Maharatnas of the country’s Central Public Sector Enterprises.

SAIL manufactures and sells a broad range of steel products, including hot and cold rolled sheets and coils, galvanised sheets, electrical sheets, structurals, railway products, plates, bars and rods, stainless steel and other alloy steels. It has been possible through 5 integrated plants and 3 special steel plants which employs over a lakh of employees and produces 13.5 million metric ton of steel annually. Along with this there are various subsidiary and joint venture plant set up with SAIL.
SAIL has its own Research and Development centre at Ranchi to examine and improve the quality of steel production and update the technologies to be in par with rest of the world. Besides, SAIL has its own in-house Centre for Engineering and Technology (CET), Management Training Institute (MTI) and Safety Organisation at Ranchi. For promoting and selling various long and flat products produced in domestic market and abroad, SAIL has its own Central Marketing Organisation with branches spread all across the India and world. SAIL's International Trade Division ( ITD), in New Delhi- an ISO 9001:2000 accredited unit of CMO, undertakes exports of Mild Steel products and Pig Iron from SAIL’s five integrated steel plants. 

Sail Integrated Plants:

1. Raurkela Steel Plant
2. Bhilai Steel Plant    
3. Durgapur Steel Plant    
4. Bokaro Steel Plant    IISCO Steel Plant

Special Steel Plant    

1. Alloy Special Steel Plant, West Bengal    
2. Salem Steel Plant, Tamil Nadu   
3. Visvesvaraya Iron and Steel Plant, Karnataka

Subsidiary

•     Maharashtra Elektrosmelt Limited (MEL) in Maharashtra

Joint Venture

•     NTPC SAIL Power Company Pvt. Ltd (NSPCL)

A 50:50 joint venture between Steel Authority of India Ltd. (SAIL) and National Thermal Power Corporation Ltd. (NTPC Ltd.); manages the captive power plants at Rourkela, Durgapur and Bhilai with a combined capacity of 314 megawatts (MW). It has installed additional capacity by implementation of 500 MW (2 x 250 MW Units) power plant at Bhilai. The commercial generation of 1st Unit has commenced in April’2009 and 2nd Unit is likely to start commercial generation by July 2009.

•    Bokaro Power Supply Company Pvt. Limited (BPSCL)

This 50:50 joint venture between SAIL and the Damodar Valley Corporation formed in January 2002 is managing the 302-MW power generating station and 660 tons per hour steam generation facilities at Bokaro Steel Plant. BPSCL has proposed to expand its capacity by installing 2x250 MW coal based thermal unit at Bokaro. In addition, construction activities are underway for installation of 9th Boiler (300T/Hr) & 36 MW Back Pressure Turbo Generator (BPTG) project at Bokaro.

 

BOKARO STEEL PLANT

Bokaro Steel Plant is the fourth integrated steel plant of SAIL. Established in the year 1964 with Soviet collaboration, it is the first Swadeshi Steel Plant of India with maximum indigenous components and equipment. It was lately merged with SAIL first as a subsidiary and then as a unit under Public Sector Iron & Steel Companies (Restructuring & Miscellaneous Provisions) Act 1978.
Its 1st blast furnace was started in 2nd October 1972 and on 26th February 1978 it completed the 1st phase of 1.7 MT of ingot steel production. Presently it has five blast furnace and a total capacity to produce 4.5 MT of liquid steel. With the continuous up gradation in technology incorporated, it is expected to cross 10MT target by 2016. The first shop of Bokaro Steel plant got the ISO 9001 certificate in 1994 and its SAIL JYOTI products enjoys a loyal market.

Bokaro Steel Plant is designed to produce a wide range of flat products:

• Hot rolled coils
• Hot rolled plates
• Hot rolled sheets
• Cold rolled coils(CRO)
• Cold rolled sheets
• Tin mill black plates (tmbp)
• Galvanised plain and corrugated (gp/gc) sheets
• Oxygen Gas Produced in Oxygen Plant.
• Hydrogen Gas

The new features added in modernization of SMS-II include two twin-strand slab casters along with a Steel Refining Unit. The Steel Refining Unit was inaugurated on 19th September, 1997 and the Continuous Casting Machine on 25th April, 1998. The modernization of the Hot Strip Mill saw addition of new features like high pressure de-scalars, work roll bending, hydraulic automatic gauge control, quick work roll change, laminar cooling etc. New walking beam reheating furnaces are replacing the less efficient pusher type furnace.
A new hydraulic coiler has been added and two of the existing ones revamped. With the completion of Hot Strip Mill modernization, Bokaro is producing top quality hot rolled products that are well accepted in the global market.
Bokaro is designed to produce flat products like Hot Rolled Coils, Hot Rolled Plates, Hot Rolled Sheets, Cold Rolled Coils, Cold Rolled Sheets, Tin Mill Black Plates (TMBP) and Galvanised Plain and Corrugated (GP/GC) Sheets. Bokaro has provided a strong raw material base for a variety of modern engineering industries including automobile, pipe and tube, LPG cylinder, barrel and drum producing industries.
 

PROCESS FLOW OF STEEL PRODUCTION

Diagram

Raw material required for the production of steel are iron ore, limestone, manganese, dolomite, coke, and sinter. These are supplied by RMHP, coke oven and sinter plant. Blast furnace is charged with iron ore lump along with limestone, coke and sinter. Intense heat from in BF converts iron oxide to hot metal. This hot metal is fed to Steel Melting Shop (SMS). Making of steel takes place here by blowing 99.95% pure oxygen through the hot metal to remove impurities. Suitable alloys are added to produce different grade of steel. Raw material such as limestone, ferrous alloys, dolomite, and aluminium are also required. The steel ingot produced in SMS is goes to Slabbing mill where it is converted to rectangular slabs of appropriate thickness. From the slabbing mill the slab goes to Hot strip mill. It also receives the slab form SMS 2 which is based on continuous casting. It directly produced slab without producing ingot. These slabs are heated and made into sheets of different thickness in HSF. These steel sheets are rolled in the form of coil in HRCF. These coils are cut in appropriate dimension as specified by the customer and packed for sale.
Also the coil from the HSF is sent to Cold Rolling Mill. It produces high quality Tin Mill Black plate and galvanised product with better strength and finish. Cold Rolling is done at temperature below re- crystallisation temperature to produce thin gauge plate of high density. There is no heating of the coil here. Hot Dip Galvanisation complex integrated with CRM produces high quality zinc coated cold roll steel.

Raw Material Handling Plant

Raw Material Handling Plant (RMHP) is one of the most important units of an integral steel plant. The major input for steel making comes from mines to this section.
Different raw materials handling by this department are as follows:

• Iron-Ore of different grades
• Limestone of different grades
• Dolomite of different grades
• Manganese
• Coals of different grades

Basic functions of RMHP are as follows:

• Release of different raw materials wagons from different mines by wagon tipplers.
• To do the blending by making homogeneous pile of raw material with the help of stacker and reclaimer.
• Supply of raw materials to different customer departments as per their requirement.
• Keep a buffer stock of raw materials in order to take care of irregularities of supply.
• 100% screening of high-grade limestone and dolomite.

Basic customer departments of RMHP are as follow:

1. Refractory Material plant
2. Sinter plant
3. Blast furnace

Iron ore and fluxes are sourced from the captive mines of SAIL situated at Kiriburu, Meghahataburu, Bhawanathpur, Tulsidamar and Kuteshwar. Washed coal is supplied from different washeries at Dugda, Kathara, Kargali and Giddi, while raw coal is obtained from Jharia coalfields.

Coke-Oven & By Product Plant

In an integrated steel plant, Coke-Oven & By Product Plant plays an important role; it supplies coke, one of the two principal raw materials to the Blast Furnace. In addition to coke-oven gas produced during coke manufacturing is used as major energy source within the integrated steel plant along with B.F gas produced during combustion of coke in Blast Furnace.
The entire resources of coal can be classified into two groups:

1. Coking Coal
2. Non-Coking Coal

If the coal when heated to bright red in absence of air softens swells and eventually solidifies into a more or less coke, it is classified as coking or caking coal. All coking coals are caking in nature but all caking coal are not coking. Coking coal on carbonization gives metallurgical or blast furnace coke. Non-coking coal fails to produce blast furnace coke on carbonization. The moisture, ash, sulphur and phosphorous content in the coal play a vital role in deciding the quality of coke that will be produced in the coke-oven area. They all should be on the lower side.

There are four sections in coke-ovens and by product plant, they are :

1. Coal Handling Plant
2. Coke-Oven Batteries
3. Coke Sorting Pit
4. By Product Plant

The main purpose of this plant is to produce quality coke in large quantities along with useful coke-oven gas, while conserving limited reserve of quality coking coal and practicing economy in environment control measures.
The process of making coke consists of destructive distillation of coal in the absence of air. The coal is heated in a closed chamber thereby driving out of volatile components present in the coal and leave behind the residue called coke. These closed chambers are known as Coke-Oven Batteries, there are 8 batteries in BSL. There are 69 ovens in each battery. The carbonization temperature is usually kept in the range 1473k-1573k.
The composition achieved after carbonization is

• Fixed carbon: 77%
• Ash content: 22%
• Volatile matter: 1%
• A typical composition of coke-oven gas is:
• Methane: 22-25%
• Hydrogen: 55-60%
• Nitrogen: 2-3%
• Carbon-monoxide: 8-10%
• Carbon dioxide: 4-6%
• Calorific value: 4200-4400Kcal/cubic meter

 

Schematic diagram of Coke-Oven Batteries

After coke is produced it is stored in different sizes according to the place of consumption, acid treatment and absorption process in coal chemicals and By Product Plant. It is a technological necessity in order to make Coke-Oven gas suitable for use as a major source of fuel in the steel plant.
The different by products produced are as follows:

• Crude tar
• Naphthalene
• Naphthalene oil
• Wash oil
• Press oil
• Pitch
• Ammonia
• Benzene
• Toluene
• Xylene
• Naphtha
• Benzol

SINTERING PLANT

Sinter is porous agglomerate of mineral fines formed after incept fusion. Sintering is a process of preparing sinter in which a high temperature is maintained   for two basic purposes:

1. Waste management
2. Advantageous properties of sinter

Sintering as a process uses all wastes generated in the mines site, handling and the mill zone, so it is a good source of waste management. Apart from this, sinter has lots of advantages than lumps to use as a burden material in Blast Furnace. Now days, 70% of Blast Furnace burden is sinter.

• Ore Fines
• Flux (limestone+Dolomite)
• Coke
• Additives

1. Flux Dust (from blast furnace)
2. Mix scale (from Slabbing Mill & Hot Strip Mill)
3. L.D Slag (from Steel Melting Shops)
4. Calcined Dolomite (from Refractory Materials Plant)
5. Lime Dust (from Refractory Material Plant)

• Sinter Return (own generation)

Different sections of Sintering Plant are:

• Raw materials section: this sections deals with the raw materials coming from the various sources and processes it to suit the needs of the Sintering Plant.
• Proportioning Section & Stock Bins: the function of this section is to receive materials, prepare the raw materials and then store it in the bins. There are 23 bins /machine. The capacity of bins for various raw materials are:

1. Iron-Ore fines:3600T
2. Limestone fines:3000T
3. Coke Breeze: 1000T
4. Mix Scale & Flux Dust: 6000T
5. Hot & Cold Sinter: 1800T

• Waste Bin Section: This section receives wastes materials from different departments like flux dust from Blast Furnace, mill scale from Slabbing Mill, limestone from Refractory Materials Plant. Then these are proportioned and then send to the stock bins.
• Machine Building: This section has three machines producing 6.2 MT sinter per year. Nearly 52KWH of power is consumed/tone of gross section.

BLAST FURNACE

Blast Furnace can be termed as the core metallurgical reactor for the production of hot metal. It is a counter-current heat & mass exchange rector in which there is continuous transfer of mass (oxygen, carbon, hydrogen) & thermal energy between descending burden materials and ascending gases generated at tuyer level.

The inputs required for the Blast Furnace are as follows:

• Iron-Ore Lamp
• Sinter
• Metallurgical Coke (25mm to 80mm)
• Lime-Stone
• Manganese
• Nut Coke

The outputs from the Blast Furnace are as follows:

• Hot Metal
• Blast Furnace (BF) gas
• Gangue Material in the form of slag

Different sections of Blast Furnace are as follows:

1. Raw Material Charging Section: This section receives raw materials from RMHP, Sintering Plant, Cock-Ovens and then supplies them to the furnace top by means of a skip car.
2. Furnace Power: Raw materials are changed into it and then hot blast of air at 1373 K is supplied. Due to the occurrence of exothermic reaction, reduction of iron oxide to hot metal takes place. It has five main sections, they are as follows:
• Hearth
• Bosh
• Belly
• Stack
• Throat
3. Stoves: Hot air blast is supplied through stoves, which are maintained at 1523 K with the help of combustion if mixed gas. The stoves have refractory lining of chrome-magnetite from inside & fire clay lining from outside.

There are total 5 Blast Furnaces in the BSL, each having a volume of 2000cu.m and working volume of 1756cu.m.
A Blast of Hot air at 1373 k is temperature is given to the Blast Furnace. This blast is given at a pressure of 3.2 Kg/sq.cm.
The hot metal from the furnace has the following specifications:

•         Carbon=4.0-45%
•         Silicon=0.6-1.2%
•         Manganese=0.6-1.2%
•         Phosphorous=0.25%
•         Sulphur=0.05%

Cock Rate: Cock Rate is the amount of cock required per ton of hot metal. Cock Rate at present is 560 Kg/THM. Earlier it was 630 Kg/THM
THM = Tone of Hot Metal

  STEEL MELTING SHOPS

Steel making involves the process of removal of impurities life carbon, Manganese, Sulphur, Phosphorous and other impurities from the hot metal by oxidation and formation of basic slag. For this the hot metal from blast furnace is taken to converter. Pure oxygen is blown through water-cooled lance to remove impurities by oxidation and then steel is made. Purity of oxygen used is 99.5%
There are two Steel Melting Shop (SMS), SMS-I & SMS-II. Three types of products are produced from these shops. They are:

1. Killed Steel
2. Semi-Killed Steel
3. Reamed Steel

Apart from the hot metal from the Blast Furnace, the different raw materials required for the production of steel are:

• Limestone
• Dolomite
• Aluminum
• Ferro-Alloys

 

S.M.S.-I

There are five numbers of converters, each of 100-130T capacities. The total capacity of this shop is 2.5 M.T/year. For oxygen blowing there are two water-cooled lances. The oxygen pressure is 15-20 Kg/sq.cm.
Various sections of the SMS-I are:

1. Mixers: The mixer is of 1300 T capacity and the temperature is about 1573-1593 K. the mixer acts as a buffer stock. Hot metal from mixture is supplied to converter by locos.
2. Converters: This converts hot metal into steel by oxidation following an exothermic reaction. Lime, scrap & following hot metal are changed in a definite sequence and then oxygen lancing is done. The lime and other additions are done during blowing through slide gate system. Lime is added as a flux to maintain the basicity of the bath. Basicity is defined as the ratio of calcium oxide & silicon oxide and is kept between 3-3.5.
3. Pit Slid: Steel tapped from this side into the teeming ladles (140 T) and is transferred to teeming bay after proper de-oxidation.
4. Teeming Bay: Steel is poured from teeming ladle into moulds (placed in mould train) by operating slide gate and sent to stripper yard.
5. Stripper Yard: The ready ingot is stripped here from the mould and sent back to the mould-cleaning yard, where it is cleaned by water spray and coated with tar or graphite.
6. Waste Heat Boiler: It cools the converter gas and generates steam, which is cooled and collected in the boiler for further use.

Data:

•     Capacity pf SMS-I: 2.5MT/year
•     No. Converters: 0.5 (100-130 T each)
•     No. Of Lances/Converter: 02(280-320 N Cu. m/min) Oxygen
•     Mode of Combustion: Free Combustion

S.M.S.-II & C.C.S.

SMS-II differs from SMS-I because of blowing process and converter gas recovery system. The mode of combustion in SMS-II is suppressed combustion where atmosphere air is not allowed in hood area and combustion of converter gas is suppressed which is further used as flux.
Inputs: Hot metal from the blast furnace
Outputs: Semi-Killed or Killed steel to continuous casting machine exclusively.
Sections: Major sections of SMS-II are as follows:

• Mixer: The mixer is of 2500 T capacity and the temperature is about 1573-1593 K. The mixer acts as a reservoir of hot metal, homogenizes it and maintains the temperature. It acts as a buffer stock. Hot metal from mixer is supplied to converter by locos. There are two mixers in SMS-II
• Converters: This converts hot metal into steel by oxidation following an exothermic reaction. Lime, Scrap & Following hot metal are charged in a definite sequence and then oxygen lancing is done. The lime and other addition are done during blowing through slide gate system. Lime is added as a flux to maintain the basicity of bath. Basicity is defined as the ratio of Calcium Oxide & Silicon Oxide and is kept between 3-3.5. There are two converters of 300 T capacities each.
• Steel Refining Unit: This unit homogenizes the temperature and comical composition of liquid steel received from different converters. Desulphurization of steel is done up to a sulphur level of 0.01%. Adding pre-requisite quantities of Ferro-alloys also dose chemical correction of steel. End point temperature control of liquid steel is done by heating or cooling the melt.
• Continuous Caster: It is the heart of SMS-II and has following main parts:
• Tundish: It is refractory liquid buffer vessel designed to continuously feed liquid metal from ladle to continuous casting mould at regulated flow rate.
• Mould: Solidification of metal starts in the mould. Copper using primary cooling mode generally makes it. At the beginning of casting, a dummy bar is inserted inside the mould to avoid free fall of steel. Continuous lubrication between and steel is provided by mould powder. It also acts as coolant.
• Secondary Cooling Zone: The metal, after leaving the mould enters in its secondary cooling comprising arrays of rollers and water spray nozzles. Rollers support and guide its downward movement of strand and water spray cools the strand resulting in complete casting.
• Straightening Rolls: These are located at the end of strand 2 where fully solidified strand is straightened by pairs of internally water cooled rolls and made to move over horizontally placed roller tables. After the stand has been straightened, it is cut to required length using acetylene-cutting machine.

Advantages of Continuous Casting:

1. It gives higher yield.
2. Energy consumption is low.
3. Operating cast is low.
4. Role of primary mills gets eliminated.

 SLABBING MILL

Slabbing mill acts as the primary means of producing slabs of different specifications by rolling the steel ingots SMS-I, but with the advent of continuous casting route, the slabbing mill has rendered useless.
Inputs: Ingots from stripper yard of SMS-I.
Outputs: It provides slab to the internal customer i.e. Hot Strip Mill (H.S.M) as well as to the external customers.
Functions: The main function of Slabbing Mill is heating, rolling and conditioning.
Sections: There are main sections in the Slabbing Mill, they are:
Ingot Storage Yard: It has a capacity to store 70,000 T of ingots and served with two 75 T overhead tong cranes.

• Soaking Pit: With the help of soaker cranes ingots are charging into the soaking pits where they are heated by mixed gas (C-O gas &B.F gas) to a temperature of 1553K-1593K for about 3-4Hrs for hot ingots and up to 10Hrs for the cold ingots. Soaking pit is lined with refractory material.
• Mill Power: After soaking is completed, the ingots are taken out and placed on the roller table in a tilter.  For the above purpose ingot buggy and pusher cars are used. The receiving temperature is about 1523 K at which slabs are rolled. The mill proper is has been divided into parts which are as follows:

1. Post I: Pushing the ingots from the tilter to the roller table
2. .Post II: Hot working or milling of ingots is done here. The mill is of universal type, since rolling og ingots can be done from all sides. The mill produces a slab of thickness 150-300mm &width750-1350mm and length up to 10,300mm. the mill has two horizontal rolls of diameter 1250mm and two vertical rolls of diameter 1000mm. Rollers are balanced hydraulically and thickness and thickness is controlled by moving gap between the rolls.
3. Post III: Hot or cold scarfing of rolled slab is done here. The mill uses oxygen flame to melt 1mm sheet of slab so as to remove surface defects viz. scarfing losses are about 2% of total ingot weight.
4. Post IV: Here, the fish tail of slab is cut from the front end & the tail end with the help of shearing line.
5. Post V: Here, online painting/making & slab weighing is done.
6. Post VI: Here, piling up of various slabs for sending to slab yard with the help of two pillars is done.

•  Slab Yard: It receives & stocks slabs, which are further, supplied to the HSM. It is also required for the purpose of supplying to the external customers. The total slab yard area is divided into stacks and further piling is done in a particular manner with 15-16 slabs in single pile.

 HOT STRIP MILL

The plastic deformation of metal above its re-crystallization temperature by squeezing it between a pair of rolls is called hot rolling. The Hot Strip Mill, here is continuous 2000mm mill. It is referred to, as 2000mm mill because the barrel length of rolls used in this mill is 2000mm.
Inputs: The input in this section comes from Slabbing Mill & C.C.S.
Outputs: It produces slabs with the flowing dimensions:

•         Thickness: 170-230mm
•         Width: 950-1650mm
•         Maximum Weight: 38 T
•         Length: 7,700-10,500mm

    It produces coils of the following dimension:

•         Strips Thickness: 1.9-12mm
•         Strip Width: 950-1650mm
•         Maximum Weight: 27 T
•         Coil Inside Diameter: 850 mm
•         Coil Outside Diameter: 2,300 mm

There are five main units in the Hot Strip Mill, they are as follows:

1. Reheating Furnace: Its principal function is to heat the cold or hot slabs to the rolling temperature of 1523 K. It has three main sections, they are:
1. Slab Charging System
2. Furnace Proper
3. Slab Discharging System
2. Evaporation Cooling System: Its purpose is to cool the skid pipes, using deaerated & dematerialized water at it boiling point at 7-8 Kg/sq cm pressure. Each furnace has 3-4 skid pipes.
3. Roughing Group: The roughing group train consists of a vertical scale breaker, two high roughing stand and five high universal roughing stands, followed by seven finishing stands.
4. Finishing Stands: It consists of a flying shear, finishing scale breaker & seven (four-high) finishing stands and run-out table follows it.
5. Coilers: Mill is provided with four coilers out of which two are continuously used and other two are spare coilers. Coilers are designed for coiling up to 12mm thick strip.

  HOT ROLLED COIL FINISHING

Through Hot Rolled Coil Finishing (HRCF)is situated at a distance from HSM, still it is considered as part of HSM. In HRCF mill, the coils are rolled in the 2000mm wide strip mill and are further subjected as shearing and slitting process to obtain sheet & slit coils of desired and pre-determined sizes as per the customer needs.
Major sections of HRCF are as follows:

• Coil Yard: It is meant for storing the coils. It is connected to HSM by inter-shop conveyors. Coil yard has a storage capacity of 1200 sq. m, it is serviced with 6 cranes of 50/10 T capacity fitted with lifting tongs.
• Shearing & Slitting Lines (1& 2): This is a finishing line and all the finishing lines have their feeding points in the coil yard and outlet in the finishing shipping area. Shearing Line 1 is used for shearing trick gauge material (above 5mm thick) and line 2 for thin gauge material (below 5mm thick). Plates are sheared in line 1 and sheets in line 2.
• Off Gauge Shearing Lines: These shearing lines are used to provide finishing to the materials already sheared in the Shearing lines.
• Additional Coil Yard: This Coil Yard provides with extra storage space.
• Shipping Area: This area is used to load the finishing products (coil or sheets) in the wagons and then these are sent to the customer or to the different stockyards in different parts of the country and also in abroad.

  COLD ROLLING MILL

Cold rolling can be defined as a compressive deformation process in which there is either a continuous or a stepwise deformation with more than one rolls. Hot rolled steel is generally cold rolled at ambient temperature to produce material of following characteristics:

• Thinner gauge material
• Material with improved strength
• Bright, Smooth & Dense surface finish

Inputs: Coils from HSM through HRCF
Products: Sheets & Coils of different dimensions
Major sections of CRM are as follows:

1. Picking Lines: The strip of coil coming from HSM has a layer of scale (mixture of different iron oxides) with a thickness of 5-20 micrometer. The strop is decaled with a combination of mechanical & chemical dissolution in picking line. Removal of scale is performed by chemically treating the surface of hot rolled strip with acid. Here, there are two continuous picking lines consisting (PL-I is Sulphuric Acid line & PL-II is Hydrochloric Acid line) of uncoiled, welding machine, lopper, acid tank and coiler.
2. Tandem Mill: In Tandem Mill pickled strips is fed between vary hard rolls in a continuous fashion where the strips is engaged in several rolls stands simultaneously enabling high tension force is applied. The roll arrangement is 4-high with five strands. Application of coolant, water with mineral oil emulsion is provided continuously during the cold rolling thus reducing the friction and heat generation at the roll site. There are two tandem mills i.e. TM-I & TM-II. TM-II being automated.
3. Annealing Zone: During the cold rolling the material is work hardened which causes loss in ductility & build up of internal stress and to restore the lost ductility and gain structure. Annealing involves heat treating the strip to a temperature of 873-923 K. There are three annealing lines in the CRM.
4. Hot Dip Galvanizing Section: In this section the coils are uncoiled, heated and then galvanized in the liquid zinc. Galvanization is a process in which of coating of zinc is done on the surface of steel so that it can be protected from rust and other atmospherics hazards. This section consists of the following different section:

• Hot Dip Galvanizing line
• Continuous Coil Corrugation Line
• Galvanized Sheet Shearing Line

After the required processes are completed in the Hot Dip Galvanizing Section are done, then the coils or the corrugated sheets are packed and then these are sent to the customer or to the different stockyards in different parts of the country and also in abroad.

CAPITAL REPAIR (ELECTRICAL)

Capital Repair (Electrical) is one of the most important auxiliary department in Bokaro Steel Plant. It undertakes all capital repair jobs throughout the plant
It carries out major electrical repair in planned (Shutdown) and unplanned (Breakdown) situation in Bokaro Steel Plant.
Tasks undertaken by CR (E) are:

• Installation of board distribution board for powering different equipment during capital repair.
• Installation of telpher panel, winch panel etc.
• Changing of blast furnace carbon plates in blast furnace.
• Depending upon the time required for capital repair, it could be categorised into 3 categories:
• Category 1 -> 2 to 3 days
• Category 2 -> 10 to 15 days
• Category 3 -> 10 to 12 months
• Repairs of cranes in various departments.

Basic circuit components used by CR (E) department:

1. Moulded Case Circuit Breaker
2. Contactors
3. Overload Relay
4. Push Button

MCCB:

 

Moulded case circuit breaker is a protective device for protecting the load as well as the conducting wire from over current. When a current greater than the set value flows through MCCB, it trips the power off and protects the device form damage. In case of motors it protects the winding from burning due to high current. Unlike fuses, they can be repeatedly used by manually restoring it to initial condition.
MCCB has a electromagnetic coil through which the current is made to flow. When over current flows through it, the magnetic field created by it is strong enough to pull the plunger and break the contact between fixed and movable contact. To make it operational again, it is manually restored to its original position.

Contactors:
 

Contactors are devices for making and breaking of contact. For small loads like bulbs and fans, we directly control the on and off operation through switch. But doing the same for high rated machine especially motors could be dangerous as this would ionise the air and produce spark and burn the contact leads of the switch. So we employ contactors to avoid such risks.
It has an electromagnetic coil which when energised attracts the contact metal from NO position to NC position. This completes the power circuit and current flows through the circuit via metallic contact. As the coil is de-energized the metallic contact returns to its initial position under the effect of spring force. Auxiliary NO and NC contacts are also available for developing different logic in control circuit like HOLD option or interlocking etc.

Overload Relay:

 
Overload relay are provided to protect the motors against overheating. This improves there operation life and also prevents damage to motor. Unlike circuit breakers which trips as soon as the current value exceeds the rating specified, overload relay allow heavy inrush of current for a specific period of time. This is necessary to prevent tripping at the time of starting of motor since they draw larger amount of current during start. Their operation is based on a bimetallic strip. When overcurrent flows through the bimetallic strip, it bend after some time due to heat generated. This breaks its contact with the fixed contact and the circuit breaks.

Push Button:
 

Push button are switch with just one stable state. These are employed for controlling the direction of movement of the motors.

Starter:
Induction motor drives very high current which is 4 6 times the rated value at the time of starting. It is higher in larger motors. Such high current need to be limited as it causes nuisance tripping of the circuit breakers. Here comes the role of starter. It reduces the heavy inrush of current at the time of starting of motor.
Two types of starter that I came across in CR (E) are:

1. DOL starter
2. Two Way Starter

An important thing about both these starters is that neither of them limits the current flow. Sometimes I wonder why we call them a starter! However they do one important task that is to place the make and break point of the circuit (load) far away from the operator, thus protecting him from sparks that might be produced at the push button itself.

Direct On Line Starter

DOL starter are used for driving induction motors only in one direction. They are used with motors up to 5HP generally.
Components of DOL starter:

• MCCB
• Contactor
• 2 Push Buttons – OFF, ON

Power Circuit
The incoming three phase wires are first connected to 3 main NO contact terminals of the contactor. From the contactor, the wires pass through the thermal overload relay and then finally to the load (motor).


Control Circuit
Any one phase wire first of all goes to the NC terminal of the Overload relay and in series to the NC of the OFF push button. From the other NC terminal wire is drawn to the NO terminal of the ON push button. The other NO terminal is connected to the contactor coil terminal in series. It goes to A1 terminal and A2 terminal goes to the neutral point.  The NO auxiliary contact of the contactor is connected in parallel to the ON push button. Since the load of control circuit is small, thin wires are used.
 

Operation
To start the motor ON push button is pressed. This energises the contactor’s coil and the NO contact get closed. This completes the power circuit and the motor starts. Now even when the push button is released, the motor still stays ON. The reason being the parallel auxiliary contacts which provides alternate path to keep the contactor coil energised. To power off the motor, the off push button in series to the contactor coil is pressed. This de-energises the coil and the contacts get opened.

Two Way Starter

Unlike DOL starter, two way starter allows reversal of the direction of rotation of the motor.
Components required:

1. MCCB
2. 2 Contactors
3. Overload Relay
4. 3 Push Buttons – OFF, Forward, Reverse

Power Circuit
The incoming 3 phase wire is fed to the NO points of the contactors parallel such that the incoming phase wire to the second contactor has any two phase interchanged. This phase interchange allows for the reversal of direction of rotation of the motor. The output of both the contactors are connected with matching terminals and then given to the motor after passing through the overload relay.

Control Circuit
Power to the control circuit is provided by the any one of the phase wires. Any one of the  phase wire first of all goes to the NC terminal of the Overload relay and in series to the NC of the OFF push button. From the other terminal two parallel path arises.
One containing the auxiliary NO of contactor 1, auxiliary NC of contactor 2 and coil of contactor 1 in series. NO terminals of the forward push button are connected in parallel to the auxiliary NO of contactor 1.
Other containing the auxiliary NO of contactor 2, auxiliary NC of contactor 1 and coil of contactor 2 in series. NO terminals of the reverse push button are connected in parallel to the auxiliary NO of contactor 2.
Finally the wire goes to the neutral point, thus finishing the control circuit.
 

Operation
When say forward push button is pressed, the coil of contactor 1 get energised. This closed the contacts and the power circuit gets completed in the phase sequence RYB. Releasing the push button brings it in hold state as explained earlier. Similarly when reverse push button is pressed, the coil of contactor 2 picks up and the power circuit gets completed but with changed phase sequence. On releasing it comes to hold state. On pressing the OFF push button, power to the control circuit is cut thus de-energising the coil whichever is active at that instance.
There are possibilities that both the forward and reverse button are pressed simultaneously. In that case two different phase will get short circuited. To avoid this situation interlocking is provided prevent both the contactors to pick up during the same period. Here this is done by connecting the auxiliary NO of one contactor in series with the auxiliary NC of the other.


 

Telpher

Telpher is an overhead trolley which lifts object from one place and drops it at other. For this it uses two motors:
Travel Motor: It allows it to move over the beam from one position to another.
Hoist Motor: It allows it to pick the object and place it down using a ropes or chain
Along with moving objects, we may require to hold them at some position for some time. For this we require brakes.  For making all these possible we require a control panel through which we can safely operate them.
Main Parts of Telpher are:

• Travel Motor
• Lifting Motor
• Brakes
• Rope/ chain
• Rope Drum
• Rope Guide
• Load Block Assembly

Telpher Control Panel


Components:

• MCCB
• 5 contactor – 2 each for both the motors  and one master
• 2 - Overload Relay
• Push Buttons -6

Operations available at the pendent:

1. Stop button
2. Forward
3. Reverse
4. Up
5. Down
6. Main contactor control – On/Off

Power Circuit
 
The 3 phase wires are fed into the MCCB and from there to the master contactor C1. From here the connections may be seen at two Two way starter connection for each motor. C2 and C3 for controlling the travel motor and C4 and C5 for hoist motor. Each of the connection goes through the overload relay before going to the individual motors finally.

Control Circuit
 
The control circuit is similar to two way starter except they don’t have hold facility for both the motors. Which means the parallel connection of auxiliary NC terminals across the push button has been removed. Only the master contactor has hold on circuit connection. Other connection remains the same.

Operation
 
When ON push button (Master contactor) is pressed and released, power supply reaches the i/p terminals of the C2, C3, C4 and C5. Depending upon which button is pressed thereafter, the appropriate action takes place. However as holding facility is absent, the action ceases as soon as the button is released. Both the motors work independently. Interlocking is provided between the two modes of operation of individual motor to prevent short circuiting of phases.

 

GENERAL MAINTENANCE

Steel plant is a flow based company. Being a flow based company damage to any small instrument needs to be instantaneously replaced or repaired. To be more economical repairing is more preferred than replacing. Here comes the role of general maintenance. It ensures smooth functioning of various electrical equipment of plant. Since the line coming to GM is 440V only, it can’t undertake the repair of heavy instruments.

General Maintenance undertakes the task of maintenance and repair of

• Low power motors ( up to 30kW)
• Voltage stabilizer
• Welding Machines
• Small transformers (less than 10kVA) 

Transformers

 

The type of transformer that come here are basically step down transformer used for control and lighting arrangements. For lighting they use 24V supply. All of these transformer are shell type transformer. Unlike core transformers, shell type transformers have a wider central limb compared to side limbs. High voltage winding are wound on low voltage winding.
Most of the transformer coming here have damaged insulation or winding. These are re-winded and if needed extra insulation packing is given. Testing is done before sending them back.

Motors

Similar is the case with motors. Most of the motors are 3 phase induction motors whose winding and insulation have been damaged. These are small motors of low rating. Type of winding is identified and re-winding is done. Mini wax papers are used for insulation purpose. Joining of the wire ends is done using welding gas and electrode.

Voltage Stabilizer

Voltage stabilizer are used to provide constant voltage to the ac appliances like computers, ACs etc. Thus protecting the appliance from damage due to voltage fluctuation. They work on the same principle as that of transformer. Input is given to the primary side. Secondary side appliance is connected. When the input voltage falls, the electromagnetic relays are activated which increase the number of turns on the secondary to prevent the voltage change in the secondary side. Reverse happens when the voltage on primary increases thus preventing any fluctuation on the primary side.

In general maintenance, readymade set of turns are kept. In case the turns of the voltage stabilizers are damaged, they replace it with new one by referring to the data sheet they maintain

HEAVY MAINTENANCE

MAJOR JOBS OF THE DEPARTMENT

This department undertakes the following major jobs in the various zones.

• Routine maintenance and inspection of all electrical machines above 200 KW
• Medium Repairs of electrical machine above 200 KW.
• Breakdown Repairs of electrical machine above 200 KW
• Alignment with mechanism of the electrical machines after Medium Repairs and Capital Repairs.
• Balancing of armature and rotor at site
• Maints. of large spare armature, rotor & large coupling mounting
• Condition monitoring of large electrical drives and connected equipment's by vibration and shock Pulse measurement, analysis.

UNPLANNED JOBS

Some unplanned jobs are also carried out to facilitate the work of other departments like removal of coupling, alignment and commissioning etc. with approval from GM(Elect).

SPECIAL JOBS

• HME specializes in the following jobs which require skill and precision.
• Alignment of Vertical Pump motors of 1000 KW to 5000 KW rating which requires considerable skill, time and man-power.
• Alignment of machines with rigid couplings.
• Scrapping and matching of sliding friction bearing segments.
• Scrapping and matching of Mill Main Drive bearings having a dia. as high as 900 mm.
• Repair of synchronous motor poles, their removal and fixing back after repair and balancing of rotor if needed.
• Balancing of rotor at site.
• Condition monitoring of large Electrical Drives and connected equipment's by vibrations and shock pulse measurement and analysis of defects and liquidation.

 Motors used in plants are basically 4 types:

1. DC Compound Motors
2. AC slip ring Induction Motors
3. AC squirrel cage Induction Motors
4. Synchronous Motors

DC Compound Motors

How does it rotate?

DC Compound Motors are used where we need speed regulation. It is preferred over shunt motor because of its high torque capacity.
However DC Motors are less preferred over AC motors because of high maintenance cost incurred. The brush needs to be replace quite frequently. These brushed are costly.
DC Motors are used in rolling mills.

How can you differentiate it?

Only DC Motors have commutator. This could be one of the criteria to differentiate it from other if the Motor is open. In case it is closed, the bolts can be seen on the outer covering which hold the poles. They are sure conformation that the motor is a DC Motor.

AC squirrel cage Induction and Slip ring

How does it rotate?

3 phase supplied to the stator produced rotating magnetic field. The rotor contains bars which are short circuited at the ends. The flux cuts the conductors of the rotor and induces current in it. According to LENZ law, the rotors will try to oppose the cause that created it. Here it’s the rotating magnetic field. So it starts rotating in the same direction to catch the speed of rotation of magnetic field. Same for slip ring induction motor. However in slip ring, 3 phase rotor winding is provided which can be connected to external resistance to limit the current through winding.
They have rugged construction and easy to maintain, hence most of the motors are squirrel cage motors in the plant. The only limitation to it is speed regulation which can’t be achieved in squirrel cage motors. Also the speed keeps on varying close to speed of rotation of magnetic field. It is not constant, hence it can’t be used in places like rolling mills where speed requires to be constant.

The problem of speed regulation is overcome in AC slip ring Induction motor where we can connect external resistance to the armature circuit. This limits the current through the armature and hence the torque experienced by it.

Synchronous Motors

Synchronous Motors start as an induction motor. Rotating magnetic field is produced. Then the magnetic poles below the rotor bars are excited by giving a dc supply through slip ring. The magnetic field of the dc poles and the rotating magnetic field of stator get interlocked and the rotor rotates at a constant speed of the magnetic field.
There is no scope for speed regulation in synchronous motor. Also synchronous motor are costly in manufacturing as well as maintaining.
However one of the significant advantage it has on other motors is that unlike other motors it is capacitive. There is lot of inductive load in the plant. Increase in inductive load reduced the active power generated. So to counter it we need capacitive load.

How to differentiate?

The stator of AC squirrel cage, slip ring and synchronous look almost the same. We can differentiate it looking at the rotor or the armature.

In squirrel cage, you can see the rotor bars on the periphery. If you see slip ring of the rotor then it is Slip ring. In case you see the rotor bars or conductors and below them poles and slip ring on the shaft, then it is synchronous motor.


Motors can be differentiated on the basis of salient and non-salient poles.
Salient Poles means that there is only one set of poles. These poles are not even differentiable. These types of induction motors are used in generating power plant.
Non salient poles Motor have more than one pair of Poles which can be easily distinguished.

Motors can also be differentiated as:

1. LT Motors 
2. HT Motors

Motors below 1kW rating are LT Motors while motors above that are HT motors. Motors available blow 1kW rating are using 415W motors. HT motors are usually 6.6kW or 11kW motors.

Motor testing

Meggering of the insulation: Generally it is in mega ohm. The ends of the megger are on the 1. Body and the other on the insulation. If the insulation resistance is less than expected then more coating of insulation plates are put. If the resistance of insulation is poor due to moisture than it is put in furnace for a while.
For HT motors, for meggering we apply 2500 or 5000 V and for LT motors 500V is applied.
Resistance and inductance test of the coil.
Resistance and inductance test of the coil of each phase is takes and matched if they are same in each phase. If it is not so, it indicates that some of the winding may be short circuited.

Bearing

 The type of bearing used in motor are:

• Ball bearing
• Cylindrical Bearing

Ball Bearing

Ball bearing have metal balls placed in the form of ring. They are separated from each other by cage like construction. The contact surface with the shaft is very small, just a point. So the friction is very less. However it does not allow the movement of the shaft along its axial direction.

Cylindrical Bearing

This form of bearing have rolling cylinders arranged in the circular form around the shaft. They allow axial movement. However the point of contact of with the shaft is more and hence grater friction.
Usually in cylinders we have 2 bearing- one ball and the other cylinder. This ensures that there is room for expansion and contraction of the shaft when it heats and cools down.

ZONES AND SECTIONS

Heavy Maint.(Elect.) is divided into the following five zones & one central workshop.

• Thermal Power Plant
• It covers the repairs and maintenance of electrical machines in Pump House Nos. 1 and 3 including canal pump house of water management department.
• Sintering Plant Zone
• It covers the repair and maintenance of big electrical machines in CO, BPP, Sintering Plant. Blast Furnace, Refractory, RM & MHP and D.N.W.
• Hot Strip Mill Zone
• It covers the maintenance of electrical machines of capacity 200 KW in Hot Strip Mill, Slabbing Mill, Air Blower  Station. And the Pump Houses 2,8,9 and 10 in the mill area.
• Cold Rolling Mill Zone
• It consist of the maintenance of electrical machines sof capacity 200 KW in HRCF, SSD, CRM, Gas Booster Station and Central Compressor Plant No.2.
• SMS Zone:
• It covers SMS-I, SMS-2, Oxygen Plant & CCP-I, Traffic and PH-4.

Central Workshop:
It is for centralized medium repair/ breakdown repair jobs required to be done for the various heavy maintenance zones of the  department.

MAINTENANCE PROCEDURE

Routine Maintenance & Inspection

The following jobs are carried out during Routine Maintenance Inspection

1. Checking, operation of motor with special regard to working of bearings, brush gear ventilation, motor heating and vibrations.
2. Insulation Resistance of Windings.
3. Checking of bandages during winding maintenance.
4. Change of bearing lubricant if required. Greasing of bearings
5. Checking of brush gear, replacement of worn out parts  i.e. brushes, holders etc.
6. Checking and cleaning of commutators
7. Tightening of all nuts bolts and fixtures.
8. Terminal leads to be checked
9. Foundation bolts to be checked
10. Checking of coolers (Heat exchangers which are mounted on motors)

Medium Repair

1. Important jobs carried out during Medium Repairs are listed below
2. Same as for routine checking and maintenance
3. Removal of Motor and thorough checking of stator and rotor winding
4. Maintenance of windings, drying and varnishing, rewinding and rewinding if required
5. Maintenance of slip rings/commutators, matching of slip rings and commutators if required
6. Re-insulation of jumpers/terminal leads if required.
7. Checking resistance of winding, quality of joints and resoldering.
8. Measurement of bearing clearance, air gap and adjustment
9. Repair of shaft necks, coupling seating and end shields if necessary
10. Replacement of bearing if necessary
11. Checking alignment with mechanism and rectification.
12. Measurement of vibration and rectification/balancing
13. Checking magnetic axis and adjustment.

CAPITAL REPAIR

It consists of the following jobs :

1.  Same as for medium repair
2. Complete dismantling of units and their thorough checking
3. Cleaning of windings and determining condition of body insulation and inter-turn insulation by testing procedure. Partial repair or complete repair rewinding if necessary.
4. Rewinding changing and insulation of jumpers, terminals insulating block for jumpers and charging terminal boards.
5. Drying, impregnating and varnishing windings
6. Replacement of bearings.
7. Full replacement of brushes, brush holders and insulation parts of brush gear
8. Renewal of surfaces of slip rings/commutator
9. Restoration of shaft surfaces under bearing and couplings to size.
10. Repair of key ways
11. Repair of end shield, fan land covers.
12. Repair of motor body, foundation and fixtures
13. Paintings
14. Core repairing/staggering of core
15. Re-caging of rotor bars
16. Replacement of rotor shaft.

FREQUENCY OF MAINTENANCE

It is decided depending on the criticality of job, environmental conditions, type of machine, its location and maintenance. For CGT jobs the frequency is once in 3 months , whereas in DC machines it is once in 2 months. For critical drives this frequency is reduced as per application. .For MR  plans AC motors are taken once in 3 yrs and DC motors are taken once in 4 yrs.

TBS (TURBO BLOWER STATION) AND ODPL (OUTDOOR PIPELINE)

Function of this department is to supply compressed air to boiler and to oxygen plant.
There are 8 Turbo blower unit and 4 turbo compressor unit.
Turbine: Turbine is a device to convert potential energy stored in the steam to kinetic energy. The kinetic energy appears as rotation of turbine.


Super-heated steam enters the nozzle of the turbine at 540c and 100kg/m3 pressure. It strikes the pulse blades which produces high torque to drive the turbine. 80% pressure drop takes place here only. Then the steam keeps on oscillating between the guide blades and the reaction blades and finally loses its temperature as well as pressure. It is pumped into the condenser. It is cooled and pumped back to the boiler for heating. Makeup water is added to makeup the loss.
 
The turbine is coupled with a blower. Blower does the task opposite to that of the turbine ie. the kinetic energy of the rotor is used to compress the air. Air is sucked through a nozzle and then compressed to the required amount. These compressed air are sent to boiler and oxygen plant.
** The above report is a contribution of not just one but few more.