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INTRODUCTIONContinuous Casting Shop is a part of SMS-2 (steel melting shop-2). Continuous Casting Shop consumes most of the production of SMS-2. SMS2 comprises of Converter Shop & Continuous Casting Shop. The Converter Shop has 3 converter of 130T capacity each. Tap time of converter is 60 min with an average heat of nearly 120 Tons. Tapping is a process of pouring of hot metal into the ladle.
The operation cycle of the converter is as follows: - Ψ Charging 5min Ψ Oxygen blowing 20min Ψ Sampling & temperature measurement 8min Ψ Correction before tapping 5min Ψ Tapping 5min Ψ Nitrogen splashing, coating 7min Ψ Total 50min
The converter shop has many sections as described below: MIXTURE SECTION: The main function of mixture is to supply hot metal (pig iron) to the converter at a fairly uniform temperature & composition, in required quantity whenever demanded. The mixture acts as buffer between Blast Furnace and Converters. There are two mixture of 1300 T capacity each. The pig iron reaches mixture in ladle of 100 T capacities. Burners are provided in the mixture to maintain the temperature of hot metal. Hot metal from mixture is supplied to the converter as per demand. SCRAP YARD: Scrap yard receives the scraps generated from all over the plant in wagons and truck. Two magnet cranes do the unloading / loading of scraps. Scraps are supplied to the converters, which is melt with hot metal. LADLE PREPARATION SECTION: The steel-casting ladle, required for tapping of steel from converter is prepared here. There are 26 ladles of 130 T capacities each. These entire ladles are of bottom purging through porous plugs and fitted with slide gate system for pouring of steel for casting. The ladles are lined with magnesia carbon bricks to with stand the heat and erosion during treatment in the VAD, LF, & RH units.
SLAG POT HANDLING SECTION: The slag produced during steel making is poured out of the converter into 16m3 slag pots. The tilting slag car are taken by locomotives to slag yard where the slag cups are tilted and emptied. BLUK MATERIAL HANDLING SECTION: The following bulk material are handled in this section: ό Lime ό Calcined dolomite ό Lime stone ό Coke ό Raw material ό Iron ore Lime and calcined dolomite are used as fluxes, iron ore and limestone are used as coolants. Coke is used for preheating the converter and coating. These materials are and charged into the converter. Very sophisticated microprocessor based system, of weighing and then charging of bulk material into converter, is installed in the main control pulpit. GENERAL DESCIRPTION OF CONTINUOUS CASTING PROCESS: - Continuous Casting is a process of casting without interruption. Liquid steel form the converter is taken to the Argon Rising Unit. Argon is blown into the ladle. This homogenizes the temperature and composition of the molten steel. Aluminum may be added to it to reduce oxygen content. Ferro alloy are added are for trimming addition. The molten steel in the ladle is then delivered to a reservoir above the continuous casting machine called a tundish. The flow of steel from the tundish into one or more open-ended, water-cooled copper mould is controlled by a stopper rod or a slide gate. To initiate a cast, a dummy bar is inserted into the mould and sealed so that the initial flow of steel is contained in the mould and a solid skin is formed. After the mould has been filled to desired height, the dummy bar is gradually withdrawn at the same rate at which casting is done. The initial liquid steel solidifies on a suitable attachment of the dummy bar so that the cast stand can be withdrawn through the machine. Solidification of a shell begins immediately at the surface of the copper mould and the casting speed is such that the shell thickness is capable of withstanding the pressure of the molten core upon exiting from the copper mould. To prevent sticking of the frozen shell to the copper mould, the mould is oscillated during the casting operation and a lubricant is added to the mould, the strand is mechanically supported by rolls below the mould where secondary cooling is achieved by spraying cooling water or air mist onto the strand surface to complete the solidification process. After the stand has fully solidified, it is cut into desired length by cut off torch (normally an oxy-acetylene torch), which moves along with the stand. This final portion for disengagement and storage of the dummy bar. MODIFIED SLAB CASTING MACHINE
Different Sections:- Continuous Casting consist of following sections: · Argon rinsing Bay · Tundish preparation Bay · Casting Bay · VAD (Vacuum Arc Degassing Unit) · Ladle Furnace · RH Degasser · Discharge Bay · Slab & Bloom Storage Yard (SBS) · Mould Repair Shop · Casting Powder Plant
DIFFRENT PARTS IN CONTINEOUS CASTING MACHINE: - · Ladle with slide gate opening · Lift and turn mechanism · Hydraulic cylinder for opening slide gate · Tundish car · Stroud holding and adjusting mechanism · Stroud · Tundish nozzle with tundish block integral · Stopper · Stopper mechanism · Nozzle (submersed) · Mould · Mould supporting & oscillating frame · Foot roll integral with mould · Five roll set · Pinch roll set · Dummy bar · Roll table · Oxyacetylene gas cutting system · Roll cooling system · Hydraulic system · Roll bearing lubrication system · Roll drive system and reducer · Transfer car and cranes
BREAKDOWN MAINTENACE: -Break own maintenance is a process of maintaining the machine component and equipment as soon as it is detected. It is done so because due to failure of that machine component or equipment caster operation will be directly affected and even it may bring it to rest immediately. In it as soon as fault or failure is detected operation is brought to rest and that component is change by spare one if available. Before going for maintenance following steps are taken: · Machine is switched off and operation is stopped · Machine is allowed to cool down. It takes about 3 hours for cooling. It is cooled through secondary equipment and cooling water
· It may be the case that cooling of machine is not require for maintaining that part bur for starting the process if it is essential · Then the water supply is stopped and takes full shut down of the machine. For this shut down form is filled up. · Next step in integral inspection of machine. All pinch roll are kept in lifted position. Inspector goes inside the machine and check for bearings and rollers and breakdown & minor defect list is prepared · Defective rollers and other broken or failed components are changed by spare one. · Local alignment is checked. · Full machine alignment is checked and correction is made for that. · Top sets are fixed and roll gap is fixed. Secondary cooling system is checked. A SAMPLELIST OF BREAKDOWN MAINTENANCE: -1. Tundish car (Hoist drive) : - Limit switch was by passed thereby damaging the locking nut of bolts reducer. 2. Pinch rolls (Pinch roll stand) : - Pinch roll no 03 roller broken. 3. Dummy bar unit (dummy bar unit) : - Dummy bar link (first) broken. 4. Receiving roll table (Drive for roller rotation): - Bearing broken thereby damaging gear teeth.
CONDITION BASED MAINTENANCE (CBM) (PROTECTIVE MAINTENANCE) Condition based maintenance is a type of preventive maintenance. During break down maintenance and after certain no of heats machine in brought to rest and all the components and equipments are checked and checklist is prepared about the defect and its extent. It is also mentioned that how long a defective component may work properly without breakage. According to that schedule of preventive maintenance is prepared. A sample checklist of component is as follows: (For slab caster at BSP Bhilai)
EQUIPMENT JOB
PARENT EQUIPMENT: Lift & turn stand
GENERAL: · Check for smoothness of operations during rotation DRIVE FOR TURNINIG: · Check brakes and regulation · Check main gear auxiliary reducer bull gear reducer · Check lubrication of bull gear
FRAME STRUCTURE OF BODY: · Check condition of plate form and rolling
PARENT EQUIPMENT: Tundish car GENERAL: · check protection area on motors & hoist mechanism HOIST DRIVE: · Check limit switch & striker for hoist · Check hoist reducer oil leakage & oil level &bottom cover bolts · Check hoist brake drum coupling brakes & intermediate shaft coupling DIRVE FOR CAR MOVEMENT: · Check floating shaft coupling · Check wheel pedestal, bearing, coupling & brake condition TUNDISH CAR BODY: · Check limit switch & striker · Check protection cover on motor front, platform beam condition · Check the freens of centering bolts &condition of the guide roller · Check tundish adjustment bolts
PAENT EQUIPMENT: Moved Oscillating Mechanism GENERAL: · Check for any jam in the clearance · Check for the smooth running of mechanism for any abnormal sound
MOULD OSCILLATION SYSTEM: · Check for coupling bolts shaft bolts shaft pedestal · Check the tightness of all lock
PARENT EQUIPMENT: Secondary cooling zone SECONDARY COOLING BUNKER: · Check bunker plate and structure
PARENT EQUIPMENT: Pinch Roll GENERAL: · Check condition of plate form & rolling DRIVE FOR PINCH ROLL: · Check the condition of PR brakes · Check lubrication spindle pusher PINCH ROLL STAND: · Check top frame fixing firm locking, condition of spindle locking & slide blocks · Check the condition of buffer spring side pins · Check the condition of rollers bearing & uprooting rollers SECONDARY COOLING SYSETM: Check rollers cooling outlet flow of water from rolls
TIME BASED MAINTENANCE(CAPITAL MAINTENANCE)From experience and experiments it is decided that a machine will operate satisfactorily for certain no of heats after that machine is maintained completely over all machine is dismantled and goes for repair in different shops. There in hops they see what are minor and major problem in the component or equipment & then they rectify all. The problem and equipment is prepared completely to work in machine. In this maintenance process check list of defects and maintenance is prepared.
MAINTANANCE OF DIFFERENT PARTS OF MACHINE: These are mainly set of rollers and there failure criterion is different thus maintenance procedure is different. A) MAINTENANCE OF FIVE ROLL & EIGHT ROLL SET: FAILURE CRITERION: Following are the main failure criterion for five & eight roll set: 1. Roller bending 2. Bearing failure 3. Misalignment
POSSIBLE CAUSES:
1. Bearing failure may be due to improper greasing and improper secondary cooling may also lead to bearing failure. 2. Low temperature of metal from ladle and stoppage of strand in the rollers and low bending strength of the rollers may lead to the permanent bending of rollers. 3. Misalignment is cause by both bearing failure & roll bending.
REPARE PROCESS:
1. Roller set is brought from the shop is dismantled completely. 2. All the rollers are removed form the strand and are fully opened. 3. There are two bearing in each side of roller, which is removed by removing outer covering from the shaft. 4. Bearings are checked if any problem is found bearing is removed & spar bearing is placed. 5. Spar rollers are placed on the frame & tighten through bolts. 6. In case of five roll set bottom set is bigger radius this set is first maintained. 7. First and last row of rollers are maintained at a height of 310mm from reference plane using plate below the shaft. 8. Above the reference plane plate of different thickness based on cross section required 9. Then upper or smaller radius at the op is placed & is locked with bottom with by studs and bolts. 10. Now rollers gap is maintained using tiller gauge & taking correction factor for shrinkage of slab.
ROLL GAP FROM TOP TO BOTTOM FOR FIVE ROLL SET GAP (mm) 1 201.70 2 201.58 3 201.45 4 201.33 5 201.20
FOR EIGHT ROLL SET GAP (mm) 1 201.20 2 201.13 3 201.06 4 200.99 5 200.92 6 200.85 7 200.77 8 200.70 11. Only different fire roll & eight roll repair is that in eight roll set is small radius bottom set is larger set. MAINTENANCE OF PINCH ROLL: CRITERION OF FAILURE: Following are the main criterion for rejecting the roll. · Roller bending Max allowable 2mm · Bearing failure · Fire cracks Max allowable depth is 30 to 40 mm. · Roller breakage · Bearing surface failure of roller.
POSSIBLE CAUSEC: 1. Bearing failure may be due to improper lubrication bending of roll bulging of strand etc. 2. Roller bending may be due to improper cooling of rolls bulged strand and stoppage of standing the roll due to stoppage of process. 3. Fire cracks due to composition of roller material & was not detect earlier.
REPAIR PROCESS: Ψ To make the blended rollers concentric rollers are turned on lathe to make concentric. Ψ Bearing failure is maintained by replacing break bearing by spare one & damaged one is sent for repair. Ψ Roller rejected. Ψ From pinch rolls is used in roll table set where cutting by oxyacetylene is done if it is not totally breaked. Ψ Breaked rolls con not be repaired thus they become scrap. Ψ If bearing surface is wearied due to breaked bearing or due to lack of lubrication it is sent to reclamation and is machined there to bring it to the proper size. . MAINTENANCE OF ROLL TABLE: Failure criterion: Following are the failure criterion of roll table a) Breakage of rolls. b) Excessive wear of rolls due to molten metal facing and cutting flame during cutting process of slab.
Maintenance of mould: maintenance of mould is done in mould repair shop. Criterion of mould failure: v Leakage of cooling water from the copper wall gap. There should be no leakage normal working pressure is 7kgf/cm2 during the operation. v Improper gap between two walls at corner. This cause bursting of skin formed during primary cooling and can lead to breakout also. Max allowable gap for satisfactory operation is 0.2mm but normal practice is 0.1mm(safer side). v Sticking of metal on walls due to break out of skin and all. v In case of capital it is preferred to repair because it is to bring the initial position, although it is capable of working satisfactorily for some more heats to secure no interruption in process for long time. v Primary cooling water may not be soft thus it may case rusting in the passage and may create problem in proper distribution of water through out the mould walls. Repair process: Repair process of mould consist of following steps be followed: § Dismantling of mould § Machining copper walls § Assembly of mould § Check the comer gap rectify if required § Put abestor mills board fix up sheets § Check alignment of mould. Blast Furnace Blast furnace department consists of 7 blast furnace three of them are of 1033m3 useful volume and production is of about 2000T/day, three of them are 1719m3 useful volume and production of about 2500T/day, and blast furnace no.7 is of 2000cum useful volume and produce 3500T of hot metal/day. The annual revised rated capacity for hot metal is 4.5MT with only five-blast operation in operation.
There are 3 main section of the department: 1. Raw material section 2. Furnace section 3. Auxiliary section
Raw material section: The raw materials used for the production of pig iron are: Coke (fuel), Iron Ore, Sinter (Fe-bearing material), Limestone (Flux) & Mn-Ore and Quartzite (additives).
1. Fine ore : 85,000 tones 2. Iron ore (lumps) : 1,80,000 tones 3. Limestone : 40,000 tones 4. Mn.Ore : 11,000 tones 5. Quartzite : 3,000 tones Furnace Section: Blast Furnace is a vertical shaft furnace, enclose in a welded shell, lined with fire-clay bricks of high alumina content. The hearth bottom, hearth, bosh and the cylinder part of the bosh are cooled by means of plate coolers. In the shaft, there are cantilever cooler. The walls of the furnace top are protected by steel refractory lined plates. In BF#7 for cooling the shaft, there are inverted L shapes cooler.
TOP CHARGING EQUIPMENT The bell type chaging device (exhisting in BF 1,2 & 3) has a big bell, small bell with hoppers, gas seal, and receiving hopper; and receives raw material from an oval shaped receiving hopper. In BF-3 RCU (Rotating Charging Unit) is provided, by means of which charging can be distributed in desired sector inside the furnace BFs. #4,5,6 & 7 are equipped with paul gate, upper sealing valve, lower material gate and lower sealing valve. This system also has a gearbox to operate a rotating chute. The latter distributes the material inside the furnace peripherally in different rings. This facilities better distribution of charge inside the furnace.
CHARGING PROGRAMME To facilitate smooth working of furnaces, the following charging programme is normally practiced. A. Programme COC / COOCC / CCOOC B. Programme CCOO Each charging cycle consist of 3 programmes of A & B exclusively or in combination depending on peripheral conditions. Generally in Bell-less Top Furnace the programme selected is B i.e. CCOO, C=COKE; O=NON-COKE i.e. ore, sinter, Mn ore, lime atone or quartzite.
STOVES Form bottom of the furnace the hot blast is blown into the furnace through tuyers with the help of turbo-blowers & hot blast stove. There are 3-4 stoves for each furnace. Each stove consists of a combustion chamber and a regenerative chamber. Stoves are kept on blast for 2 hrs. the following are the provision in a stove : cold blast valves (1) , Hot blast valve (2) , Chimney valve (2) , By-pass chimney valve (3) , gas butter fly valve (1) , Gas burner (1) , Air fan (1). Blast of Air reaches as follows: Cold blast CB Main Stoves HB Main Bustle pipe Gooseneck pipe Tuyer stock Furnace
The liquid product of smelting namely pig iron and accumulator in the hearth. Slag floats on the Pig Iron because of difference in density.
FLUSHING OF SLAG: TAPPING AND DISPOSAL OF HOT METAL The furnace is provided with a tap hole, which is enclosed in a steel cast frame and is cooled by two Tap hole coolers, which have their own supply of water. The hot metal is tapped out at an interval of 1-2 hrs depending upon the furnace condition. The usual way of opening the tap hole is to drill the tap hole until the skull is reached. Oxygen lancing is carried out melt the skull. The hot metal continues to flow down the bend runner form, which it is diverted into individual metal ladles. At the end of tapping the tap hole is closed with mudgun, which is electrically or hydraulically operated. The hot metal ladle is a refractory lined 100 T capacity containers. It transport hot metal form Blast Furnace to SMSs, PCM and Foundry. The distribution of hot metal is 90% to SMS, 3% to foundry and rest to Pig Casting Machines. Fleets of 78-iron ladles car are available, out of which normally 48 are in circulation in cases all seven furnace are in operation.
SPECIFICATION OF RAW MATERIALS Material Chemical Analysis Specification SizeIron Ore Fe 64.2 % min 10-40 mmSiO2 2.5+-0.5%P 0.1 % maxAl2O3 / SiO2 0.88 % max _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Limestone CaO 39.5 % 0.1 % 6-50 mmSiO2 6.5 +- 0.25 % MgO 8.5 +- 0.5 % _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Mn. Ore Mn 30 % min 25-80 mmSiO2 30 % min Al2O3 5 % min P 0.30 % max
Source of Raw Materials: 1. Iron Ore . Dalli, Rajahara Mines 2. Lime Stone . Nandini Mines 3. Manganese . Balaghat Mines 4. Sinter . Sintering Plants (SP-2, SP-3) 5. Coke . Coke Ovens (Coke sorting plants) 6. Converter Slag . SMS l l
Requirements for producing of one ton of Hot Metal(Specific Consumption)
Iron Ore . 459 Kg Lime Stone . 850 Kg(Depending on Sinter Usage) Manganese . 800 Kg(50% in burden) Sinter . 35 Kg Coke . 08 Kg Converter Slag
. 30 Kg Services / By-Product per ton of Hot MetalSteam . 120 KgCompressed Air . 25 M3Air Blast . 2600 M3Electricity . 33.7 Kwh Water . 40 M3 Slag . 435 Kg Flue Dust . 25 KgBF Gas . 1830 Kg(CV-800Kcal / M3)
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