Steel Making Processes PDF

STEEL MAKING (C09 MET-404) 1. Introduction to steel making 1.1 Explain the WROUGHT IRON MAKING process? 1.1 Ans: - WROUGHT IRON MAKING: Wrought iron was then the purest form of iron produced commercially on a large scale It is quiet malleable and...

STEEL MAKING (C09 MET-404) 1. Introduction to steel making 1.1 Explain the WROUGHT IRON MAKING process? 1.1 Ans: - WROUGHT IRON MAKING: Wrought iron was then the purest form of iron produced commercially on a large scale It is quiet malleable and was used for making hooks, chains, etc. Pure iron melts at 15360 C, the presence of impurities decrease the melting point of iron so that pig iron is tapped from the B/F at around 1250 – 13500 C. Prior to 1850’s the max f/c temperature was 14500 C and the pig iron is readily re-melted. During refining of this pig iron, as the impurity content decreases the melting point of iron increases With the progress of refining, the metal become more and more viscous and the separation of metal from the slag become impossible. The majority of the slag was squeezed out from the pasty mass by mechanical working and finally producing a nearby impurity free ferrous material containing a small amount of mechanically entrapped slag particles. This fibrous structure material is called as wrought iron A reverberatory type of furnace was used for making wrought iron. The process was known as pudding process. CONSTRUCTION: Charge receives the heat energy from the dome shaped roof and Hearth of puddling furnace is lined with iron oxide and other parts are lined with fireclay rich in alumina. Supply of coal and hot air is provided near to hearth OPERATION: Wrought iron is produced by oxidation of impurities present in pig iron. While introducing pig iron from charging door iron oxide also added during melting for oxidation of carbon and other elements. 1  Refining process takes place in three stages. Removal of carbon takes place at the end of the process. By removal of impurities the temperature of bath increases creating pasty mass. At this stage the bath is stirred for easy removal of impurities. FIRST STAGE OF REFINING Si + 2 Fe3O4 = SiO2 + 6 FeO Mn + Fe3O4 = MnO + 3 FeO MnO + SiO2 = MnO.SiO2 FeO + SiO2 = FeO.SiO2 SECOND STAGE OF REFINING 2 P + 5 Fe3O4 = P2O5 + 15 FeO P2O5 + 3 FeO = Fe3(PO4)2 THIRD STAGE OF REFINING 2 C + O2 = 2CO 2 CO + O2 = 2 CO2 PROPERTIES OF WROUGHT IRON –Ductility, Low strength, Corrosion resistance, Easily forged and welded –purest form of iron with 99 % of iron ,softest , toughest and most ductile USES OF WROUGHT IRON –Crane hooks, Chains, Railway couplings, Pipe fittings, Boiler tubes, Plates, Sheets and bars. 1.2 Write the principle of steel making? Ans: - It is produced by refining or purification of pig iron or hot metal which is produced by reduction smelting of iron ore in a blast furnace. The hot metal composition consists of 92-95 % Fe & 5-8% impurities like C, Si, Mn, P, S, etc., Steel making is a process of selective oxidation of impurities where as iron making is a process of reduction smelting of iron ore. During steel making the impurities are oxidized to their respective oxides (except Sulphur) and the oxides are eliminated as gas or liquid slag. The composition of steel consists of 97 - 98% Ferro and 2 – 3% impurities like C, Si, Mn, S & P. 1.3 List out the steel making processes? 1.3 Ans: - Historical Steel Making Processes are a) Wrought iron making b) Cementation steel making c) Crucible steel making. 1.4 List the raw materials for steel making? 1.4 Ans: - i) Source of metallic iron ii) Oxidizing agents iii) Fluxes iv) Source of heat v) De-oxidizers vi) alloying agents and vii) Furnace Refractories 2 1.5 Write the principle of Bessemer process? 1.5 Ans: - General Principal of Bessemer process:- When a current of air is blown through molten pig iron, which contains C, Si, Mn, these elements are oxidized and form a slag. In Acid Bessemer process, the oxidation of ‘Si’ produces a large amount of heat to maintain the fluidity and temp. of metal. In Basic process, the oxidation of ‘P’ produces necessary heat. The blown in prolonged, during after blow, the ‘P’ is eliminated. Refining of molten pig iron takes place in both Bessemer processes. Selection of Bessemer process depends upon the composition of pig iron. If silicon content is more in pig iron acid Bessemer process is opted. If phosphorus content is more in pig iron then basic Bessemer process is followed. 1.6 Explain Acid Bessemer process? 1.6 Ans: -Depending upon the nature of slag steel making classified as acid steel making and basic steel making. The slag produced from this process is acidic nature Hot metal charge is supplied to the converter around 1200 to 1300°c Being an autogeneous process it runs on hot metal charge supplied at around 1200 -13000 C The composition of pig iron should meet 2 requirements: 1. Enough heat should be generated to heat up and maintain the bath at around 1550 – 16000 C 2. It should produced an oxide slag which should separate out very quickly from the melt Silicon: It is the main impurity and generates the heat required by its oxidation. It is kept around 2.5% if it falls below 1% the blow is bound to go cold and if it exceeds 3% the blow will be hot. Hot blow effects both lining life and slag metal separation. Carbon is in the range of 3.5 – 4.0%. High carbon leads to higher fluidity, but it prolongs the heat. Manganese is important to obtain proper quality slag. The acid Bessemer must produce a dry slag to separate itself from the metal. A high Mn content thins the slag and attack lining. Si should be in the range of 0.75 – 1.0%. COMPOSITION OF PIG IRON Carbon------3.5 to 4.0 %, Silicon------2.0 to 2.5 %, Manganese------0.75 to 1.0 % Sulphur----0.05 % max, Phosphorus------0.05 % max. CONSTRUCTION OF CONVERTER Bessemer process is carried out in a pear shaped vessel with a detachable bottom called converter. It is a concentric vessel where tapping of molten metal takes place at nose Body of converter is made up of steel plate and lined with soft silica bricks 3  Converter is capable to rotate 360°, normally it rotates more than 220° where electrical or hydraulic rotate mechanism is used. One of the trunions is hollow to allow air to blow through bottom of the cell contains tuyeres and wind box to equalize the air pressure at all the tuyeres.Wind box is connected to hollow trunion. Jack car is provided under the bottom which is useful for resting of bottom part WORKING PROCESS: contains the steps a) Charging b) Blowing c) Slagging and d) Tapping. CHARGING: The converter is rotated to the horizontal position on its charging side. BLOWING: The Converter is rotated to upright position when the blast of air is forced through. Slagging: Converter rotated to enable the molten slag to be poured out. Tapping: Converter rotated to enable the molten steel to be poured into ladles. PHYSICAL CHEMISTRY OF THE PROCESS 2 Fe + O2 = 2 FeO , Si + 2 FeO = 2 Fe + SiO2 Mn + FeO = Fe + MnO , C + FeO = Fe + CO SLAG COMPOSITION SiO2 ------ 55 to 65 % MnO ------ 12 to 18 % FeO ------ 18 to 12 % 4  Silicon and manganese which gives stable oxides are removed at beginning stage of blow Si and Mn oxidation increase the temp. of bath which is favorable for removal of carbon. Carbon gets oxidized and produces CO and CO2 at the nose of converter using atmospheric oxygen and aflame is seen The length and brightness of the flame increases with increasing rate of decarburization The flame reaches to its maximum length 6 to 8 m The length and luminosity of the flame are indications of carbon level in the melt The temperature of steel vessel at the end of the bow is 1550°c to 1580°c Scrap can be used as coolant to reutilize it ACID BESSEMER PRODUCTS Rails, Axles, Nuts, Bolts, Castings. 1.7 Explain the Basic Bessemer process – fore blow & after blow 1.7 Ans: - Basic Bessemer process Depends upon the nature of slag steel making classified like acid steel making and basic steel making The slag produced from this process is basic nature Hot metal charge is supplied to the converter around 1200 to 1300°c Basic Bessemer steel making is similar to acid Bessemer process in vessel design and operation This process is also called as Thomas process where pig iron contains 1.5 % of phosphorus Pig iron leads formation of highly basic slag with a minimum of external flux consumption 5  This process should maintain a highly basic un-oxidizing slag to stabilize Phosphorus & Sulphur in the slag The composition of pig iron should be such that, it should form a highly basic slag with Min. external flux It should produce enough heat to maintain the temperature of steel till the end of blow Carbon: As usual 3.5 – 4.0% Silicon: It is readily remove and needs lime to neutralize it. It should be 0.5 -1.0%. It is not the chief heat producer in a basic process Manganese: It forms a basic oxide and prevents excessive oxidation of iron. It is in the range of a 1-3%. It is useful for effective de-sulphurization of metal. Phosphorous: It is the heat producer in basic process. A min. of 1.5% is required to blow the metal. High percentage of P is useful to produce a fertilizer grade slag(18-22% P2O5). Sulphur: Basic slag can remove some sulphur, but the slag metal contact is very low. It should be less than 0.05%. COMPOSITION OF PIG IRON Carbon ------ 3.4 to 4.0 % Silicon ------ 0.5 to 1.0 % Manganese ------ 1.0 to 3.0 % Sulphur ------ 1.5 % min Phosphorus ------ 0.05 % max CONSTRUCTION OF CONVERTER Is same as acid Bessemer converter Physical chemistry process: Is same as acid Bessemer converter Sulphur is removed during blow period and passes in to the slag in the form of calcium sulphide. FeS + CaO = FeO + CaS Phosphorus is oxidized and slagged according to the reaction 2P + 5 FeO + 4 CaO = (3 CaO P2O5) + 5Fe Phosphorous is oxidized by iron oxide and slagged by calcium oxide (lime) SLAG COMPOSITION SiO2------ 5 to 10 %, MnO------2 to 6 %, FeO ------15 % CaO------50 %, P2O5------18 to 22 % Silicon and manganese which gives stable oxides are removed at beginning stage of blow Si and Mn oxidation increase the tem. of bath which is favorable for removal of carbon Phosphorus is removed before removal of carbon Carbon gets oxidized and produces CO and CO2 and driven out at the nose BASIC BESSEMER PRODUCTS: Tube making, Rails, Girders, Nuts, Bolts, Castings. 6  1.8 State the limitations of Bessemer process? 1.8 Ans: -i) N2 content appears in the finished steel. ii) C and P are not completely oxidized. iii) Removal of phosphorus is possible only in basic Bessemer process. 1.9 Define the mixer? 1.9 Ans: -The hot metal from the B/F is stored in hot condition in a vessel is called mixer A hot metal mixers serves the purposes : It acts as a reservoir of molten metal, being a buffer between the blast furnace and Bessemer plant It mixes various grades of pig iron and homogenizes the composition of metal It maintains the temp. and keep the metal in liquid state 1.10 Classify the mixers? 1.10 Ans: - There are 2 types of mixers: 1. Inactive mixer 2. Active mixer 1.11 Differentiate the active mixer and in active mixer? 1.00 CLASSIFICATION OF STEELS 7  2. Open hearth process 2.1 Write the general principles for open-hearth furnace? 2.1 Ans: - Regeneration Principle The exhaust gases from the furnace are drawn through one of a series of chambers containing a mass of brickwork and give up most of their heat to the bricks. Then the flow through the furnace is reversed and the fuel and air pass through the heated chambers and are warmed by the bricks. Through this method, an open-hearth furnace can reach temperatures high enough (16500) to melt steel. During the 1st cycle the checkers on the right side are being heated by the outgoing gases. Then those on left side are giving up their heat to incoming air or fuel. During the 2nd cycle the checkers on the right side having been heated, give out their heat to incoming air or fuel gas. Then on the left side, the checkers are now being heated by the outgoing gases. 2.2 Classify the Open-hearth Processes of Steel Making? 2.2 Ans: -Classification of Open-hearth Processes of Steel Making Acid Open-hearth ,Basic Open-hearth Flush Slag, Duplexing Tilting Open-hearth, Twin Hearth 8 2.3 Explain the construction & lining of basic open-hearth f/c? 2.3 Ans: - Main Parts of Open-hearth Furnace: are Reaction Chamber, Hearth, and Furnace walls, Roof, Ports, Gas and Air Uptakes / Down takes ,Slag Pockets, Regenerators and Stack, Reversing (Switching) Valves, Launder, furnace Instrumentation. Reaction chamber: Reaction chamber consists of Hearth, Walls, Roof and Ports. 2.3 i) Hearth: It is a steel pan lined from inside with a layer of fire bricks and then at least three layers of Basic bricks. The working layer of is given slag wash with appropriate slag to decrease the attack of similar slag on the lining. The finished hearth contour must be sloping towards the tap hole from all the sides. For furnaces meant for cold metal charges shallower baths are designed than those for hot metal practice. WALLS The walls are heavily braced with vertical Steel sections and are tied across the roof by tie beams. The front wall of the furnace is vertical and the back wall, at least in the lower part is inclined. The front walls are made up of doors with water cooled steel frames. A peep hole called wicket is provided in each door for inspection Longitudinal Vertical section 9 Transverse Section 2.3 ii) ROOF The roof is the most vulnerable part of an open hearth furnace. The roof of basic or acid furnace is made from silica. The roof is sprung arched between front and back walls from skewback bricks. The skewback bricks for each ring are held in longitudinal skewback channels. Basic Roof design 2.3 iii) Ports These are the openings through which the air and the gaseous fuel enter the furnace chamber. The port should be as big as possible to deliver maximum fuel and air to liberate maximum heat in the chamber. These are a silica brick construction but for fast driven furnaces a basic brick construction with a backing layer of fire bricks is preferred. 10 2.3 iv) Slag Pockets The lower part of an uptake is called a slag pocket. These are used to collect slag and other particles carried away from the furnace by the high velocity gases. This is a silica brick construction but for faster driven basic furnaces basic bricks are also being used. The slag pocket is connected to the regenerator by what is known as fantail flue. Uptake, Slag pocket and Checkers 2.4 Explain the chemistry of the Basic Open Hearth process? 2.4 Ans: - Basic Open Hearth Practice of Steel Making  The following are the major changes responsible for increase in production rate with Basic Open-hearth Process. Rapid materials handling and proper scheduling of facilities in the entire shop. Fast Melting of Cold Charges Improved furnace design and refractories to allow faster working at higher temperatures. Rapid Refining Using Oxygen lances, Efficient control and charge balancing Steps Involved in Each Heat 1. Charging and Heating of solid charge & Hot Metal. 2. Melting 3.Refining. 4. Blocking, 5.De-oxidation and Finishing, 6.Tapping. 11 Reactions that Lead to Slag formation 2fe (scrap) +o2 (gases) =2(FeO) (FeO)+ [Si] = sio2 (FeO)+ [Mn] =MnO (FeO)+[c] = {co} + (sio2) + (MnO) + (FeO) =cao.sio2.mno.feo The flushing should be over before the carbon monoxide formation is vigorous or else high basicity slag also would be flushed out. After Flushing is over fresh lime is required to make high basicity slag to retain Phosphorous oxidized from bath by Oxygen lancing. Blocking At the end of the refining the Oxygen pressure is reduced to decrease the vigor of carbon oxidation and then it is completely stopped to block the heat. For blocking the heat, lumpy Fe-Mn and/ or Fe-Si are added to the bath. Addition of Fe-Mn and/or Fe-Si raises the manganese content of the bath by about 0.2% and silicon content b about 0.050- 0.20%. De-oxidation and Finishing De-oxidation inside the furnace has to be carried out without the danger of Phosphorus reversion. The bath is usually de-oxidized and alloyed further in the ladle during tapping. Tapping After Finishing of the heat in the furnace, temperature of the bath is measured to ensure correct tapping temperature. The Launder is very thoroughly dried before opening the tap hole. The tap hole is cleaned and opened to a below the level of the hearth. 2.5 write the types of steels produced by the open hearth Process B.O.H Hot metal Practice enhanced 30 -50% of Production rate over the Cold Metal Practice. Adopted For Integrated Steel Plants. Wide Variety of Plain Carbon Steels and Low Alloy steels can be Produced LOG Sheet illustrates the heat details of the Furnace. 2.6 State the advantages of open hearth process as compared to Bessemer process? 12 2.7 Explain recent trends in open hearth process: i) Ajax ii) Tandem iii) Tilting O.H. iv) Twin hearth 2.7(iii) Tilting Open hearth Furnace This Process makes use of Tilting Open hearth furnace. Tilting open hearth furnaces differ from the fixed furnaces in the design of reaction chamber and Ports. Constructional Features of The Tilting Open hearth Furnace The massive concrete and steel structure of the foundation is replaced by rollers or rockers on which the furnace is able to rotate. The Tilting furnace rotates concentrically with the furnace ports where as a rocking furnace rotates eccentrically to the ports. The hearth is much deeper than that of fixed furnace. The walls are reinforced with steel plates to with stand extra strain in tilted position. The back wall slope is reduced, as it can be altered by tilting for fettling. The roof is similar in design to that of the fixed furnace. The tap hole is always open and projects upwards. The furnace is tilted with the help of gears operated either electrically or hydraulically. The f/c can generally be tilted by 10-15O on the charging side and 20-35O on the tapping side. A small gap is left between the ports and the chamber to allow smooth tilting of hearth. The furnace gases escape through this gap. Advantages of Tilting hearth Practice A tilting hearth practice is suitable when a large load of metal and slag is to be handled within a much shorter period. This practice facilitates removal of slag from the furnace readily to make for new slag. Desired fraction of refined steel and slag an be removed from the furnace readily. Disadvantages of Tilting hearth Practice Tilting hearth furnaces have Lower thermal efficiency. The capital as well as the maintenance costs is higher. 13 2.7(iv) Steel Making by Twin Hearth Process The Stationary Open hearth is sub divided into two equal hearths by putting refractory wall on the line of the tap hole. The arrangement resembles as two halves of hearths acting as an independent chamber with independent oxygen lances. The carbon monoxide evolved in one chamber during lancing is burnt in the other chamber to preheat the solid charge. The chemical heat of carbon monoxide can be fully recovered for scrap heating in the second chamber. No Traditional Regenerators are necessary. The roof height is raised to accommodate intensive lancing without endangering the roof life. Advantages of Twin hearth process All Varieties of steel Produced by standard open hearth can be produced by twin hearth process. Treble the production rate of the same unit. Design is simple as there are no generators. What is meant by Twin hearth practice of steel making? Twin hearth practice of steel making In this practice, The Stationary Open hearth is sub divided into two equal hearths by putting refractory wall on the line of the tap hole. What are the advantages of twin hearth practice of steel making? Advantages of Twin hearth process All Varieties of steel Produced by standard open hearth can be produced by twin hearth process. Treble the production rate of the same unit. Design is simple as there are no generators. 14  3. Oxygen Steel Making 3.1 State the principles of oxygen steel making process? 3.1 Ans: - principles of oxygen steel making process: Refining of pig iron is done by pure Oxygen. Jet of Oxygen strikes the bath surface. Creating large quantities of hot iron oxides. These lime fluxes form basic oxidizing slag. Formation of metal –slag-gas emulsion increases the interfacial areas between Metal-Slag, Metal-Gas and Gas-Slag systems. Creating conditions for the removal of P and C to eliminate P before C Removal. 3.2 List the oxygen steel making processes? 3.2 Ans: O2 SM processes are LD PROCESS, KALDO PROCESS, ROTOR PROCESS, and LD-AC PROCESS. 3.3 Write advantages of using pure oxygen in steel making? 3.3 Ans: - Advantages of using pure oxygen over air are The problem of nitrogen pick up is minimized. Faster Rate of Refining. The volume of waste gases is reduced by 60 to 70%. The loss of sensible heat is reduced. The Net endothermic reactions are replaced by net exothermic reactions. The sluggishness of carbon boil is eliminated. 3.4 State the raw materials for L.D. 3.4 Ans: - Molten Metal, Cold Pig Iron, Steel Scrap, Iron Ore/Mill Scale, Fluxes, Gaseous Oxygen. 3.5 Explain the constructional details of converter & design of the lance? 3.5 LD SHOP Multi floor shop is required. Tall shop is also required to raise or lower the lance. Elaborate gas cleaning facilities. Two or Three Vessels operate at a time. Computerized operated shops. Automatic spectrum – Chemical analysis Methods. MAJOR SECTIONS IN THE LD PLANT The vessel including foundations, rotating gears etc. The Lance Including its Auxiliary gears. The hood and the waste gas treatment plant. The material handling & storage facilities. Instrumentation & Control Pulpit. The vessel lining & wrecking accessories. Ld plant. LD Vessel It is referred to as LD converter or basic. Oxygen furnace (BOF) Vessel is divided into Three segments: 1. Spherical Bottom, 2.Cylindrical Body (or shell), 3.Conical Top. These segments are welded to form single piece. The top may be concentric or eccentric for the easy removal of slag, easy insertion of lance & construction hood. The Vessel top is a truncated frustum of cone staggered by 8 – 100 to the main axis of the converter and is obliquely attached to the cylindrical body 15  Tap hole is at the junction of cylindrical body and conical top Two trunions are attached to the ring to support the vessel freely in split bearings. Tilting gear is attached to one of trunions. Capable of rotation through 3600, but rarely it exceeds 2200. The bath depth is in the range of 110 – 180 cm. depending upon the capacity It should be maximum to prevent damage to the bottom during lancing Height to diameter ratio is about 1.5. VESSEL DESIGN Earlier vessels are 30-50 ton capacity. Modern vessels are up to 400 ton capacity The height-to-diameter ratio is 1.2 - 1.5.The specific volume is 0.75/m3. The thickness of the lining is 600-1000mm depends on vessel capacity. As the capacity increases if the diameter increased. Bath depth is nearly same. Example 200 tonnes 300 tonnes Height of the 9 9 Shell (m) Dia of bath (m) 5.0 6.5 Depth of bath (m) 1.5 1.8 LD LANCE Oxygen is fed through a water cooled lance. Made of three concentric steel tubes. O2 is fed through inner most tube. Water is circulated around central tube. Copper tip nozzle is welded to steel tubes. Oxygen is blown at 8 – 10 atm. pressure through a laval shaped nozzle.The jet issuing at the nozzle exit is supersonic and has a velocity of 1.5 – 2.5 times the velocity of sound In the blowing position the lance height from the still bath level should be 40 – 50 times the dia of the nozzle Lance is 8-10m long. Diameter is 20-25cms.Suspended by wire rope. It can be inserted in or withdrawn. Operated by electrically operated lance gear. 16 Ld lances: i) OXYGEN LANCE MULTI NOZZLE LANCE A multi nozzle lance has the advantages :- 1. Sloping and sparking lesson are minimized 2.FeO content of the slag is reduced by few percent 3. Oxygen consumption per ton of steel is less. 4. Increased production rate SUITABLE REFRACTORY MATERIALS The Lining of LD converter consists of Tarred dolomite bricks. Dolomite enriched with magnesite bricks. Magnesite bricks 3.6 Give the lining details of LD Converter? LINING DETAILS OF LD VESSEL: The life of LD lining is very important factor. 17  Increased lining life directly reduces the Refractory consumption per ton of steel made and the production cost. In a good LD plant the lining life of LD converter is over 1000 heats. 3-5 Kg Refractory material is consumed per ton of steel made. Lining Life Depends On Composition of refractory bricks, Lining design, Charge material composition Vessel operation & blowing conditions, Slag chemistry ,Temperature inside the Vessel Quality of end product, the blow period, heat. The sequence of operation is as follows. Scrap is charged from charging chute. The hot metal is then charged. Vessel is rotated to vertical position. 3.7 Explain the operation of L.D. process with chemical reactions. THE EFFECT OF SUPER SONIC JET ON BATH CIRCULATION The sequence of elimination of impurities during a LD blow is as follows Removal of silicon is quite fast completed with in 6 min. after commencement of blow Mn comes down concurrently with Si up to a certain point and it remains there for the rest of the blow Carbon and Phosphorous oxidise together and it starts after a few minutes of blowing BLOW CONDITIONS 18  REACTIONS IN THE LD CONVERTER CONTD Oxygen jet strikes the metal bath. It preferably oxidise Iron 2[Fe] +O2 2(FeO) Iron oxide reacts with impurities 2(FeO) + [Si] (Sio2) + [2Fe] (FeO)+ [Mn] (MnO) + [Fe] (FeO)+[C] Co+ [Fe] REACTIONS IN THE LD CONVERTER CONTD A certain quantity of impurities can also be oxidised by direct interaction with blow oxygen. [Si]+O2 (SiO2) 2[Mn] +O2 2(MnO) 2[C] +O2 2CO Reactions with blow oxygen & FeO depends on blowing conditions REACTIONS IN THE LD CONVERTER CONTD De-Phosphorisation occurs as the basic slag is being formed. “P” is oxidized by FeO & CaO 2[P] +5(FeO) +4(CaO) (4CaO.P2O5) +5[Fe] The oxidation of Carbon takes place during the whole period of blowing. The fast refining is due to tremendous increase in interfacial areas. 19 REACTIONS IN THE LD CONVERTER CONTD Two zones of refining in LD Vessel are Emulsion Refining & Bulk Phase Refining. Bulk phase refining is dominant in the beginning & in the end. Removal of “Si” completed within 5-6 minutes. “Mn” comes down along with “Si”. Early Oxidation of “Si” & “Mn” raise the temperature then dissolution of lime and formation of metal-slag-gas emulsion takes place. Previous slag in the converter also helps to form early slag. De-Phosphorisation is very rapid in emulsion. “P” should be fully eliminated before emulsion collapses. Collapse of emulsion begins at around 0.3%. De-”P” should be over by the time “C” comes down to 0.7-1%. The rate of “C” Oxidation depends on the intensity of O2 supply, shape of O2 jet, and Depth of penetration into the bath. A Jet penetrating more deep forms many gas cavities & bubbles. Many interfaces which facilitate separation “CO” De-”P” & De-”C” rates are controlled by adjusting the Lance height, the flow rate of O2. Raising the Lance or decreasing the O2 pressure increase slag-metal reactions Increase De-”P” lowering the Lance or increasing the pressure of O2 Increase gas metal reactions Increase De-”C”. SLAG CONTROL Slag is under very careful watch in Open Hearth Process but the situation is different in LD process which is very fast refining process. Final analysis of the slag is carried out just for checking. Excess Lime in the slag Prolongs lining life.Ensures complete De-”P” & De-”S” without reversion. Slag emulsion all around the “Hot Spot” protects the lining from radiations. Improves thermal efficiency. 20 3.8 Appreciate the advantages& uses of L.D. process. ADVANTAGES OF LD PROCESS Maximum production rate. Minimum capital & running cost. Autogenous process (No need of fuel supply). Better quality of the product (Least N2 & H2 content). High thermal efficiency. Maximum amount of coolant (Scrap) consumed. Maximum maneuverability to produce wide range of products. Maximum cleanliness of product. USES OF LD CONVERTER Production of soft steels. Production of high Carbon & alloy steels. Phosphorous can be reduced from 0.15-0.020 to 0.012-0.017%. Nitrogen content in the order of 0.002 is readily attained. The carbon can be readily reduced to 0.04 %.( not possible in O-H practice). 3.9 Give the constructional details of operation of KALDO PROCESS? KALDO PROCESS Oxygen steel making by rotating the vessel in inclined position. Developed at Domnarfvet works of Sweden by Prof.B.Kalling. Major portion of CO is burned to CO2 Rotation is essential to safeguard the lining. CONSTRUCTIONAL DETAILS Vessel is similar to concentric LD vessel. Placed in a cradle. Rotated around its long axis. Rotational speed is up to a maximum of 30 rpm. Cradle is mounted on Trunions. Vessel can be tilted to various positions. Vessel is held at an angle of 16-200 to the horizontal. Water cooled hood is placed to the mouth of vessel. Oxygen Lance is inserted through the hood. Feeder Lance is provided for charging without interruption of the blow. Oxygen Lance is capable of oscillation in vertical plane (20-370 ) 21  Oscillation is about 15-20 times per minute. The Lance can be held in any fixed inclined position during blowing. Feeder Lance can feed 2 T / min solid materials during the blow. 3.10 Explain the operation of Kaldo? OPERATION OF KALDO CONVERTER Charging of Lime, Ore & Scrap. Then charging of hot metal. Vessel is tilted to blowing angle of 16-200.The hood is held firmly & lance is inserted. Rate of oxygen supply & speed of rotation varied during blowing. PROCESS CONTROL Controlling parameters are speed of rotation of vessel rotation ,Flow rate of oxygen, And Lance angle. Si, Mn are oxidized in the early part of the blow. High speed of rotation is a major tool to control refining. As the vessel rotates at high speeds a shower of metal droplets fall. The droplets oxidised directly by the gas phase. The De-Carburization is initially slow. As the metal cascades De-Carburization builts up very rapidly. De-”P” & De-”C” are adjusted to obtain the correct analysis. The rate of De-Carburization is increased by; Increasing the speed of rotation. Increasing the velocity of oxygen flow. Lowering the Lance further down. Making the Lance steeper. The rate of DePhosphorisation is increased by; Decreasing the speed of rotation of the vessel. Decreasing the velocity of oxygen flow. Raising the Lance further up. Making the Lance angle shallower. ADVANTAGES OF KALDO PROCESS Most part of Co can be burned to Co2. Scrap can be used up to 40-50% Process can be used to refine pig iron of any composition. The yield of metal is high. CONCLUSIONS: The tap-to-tap time is almost double to LD process. A low life of the lining i.e. only 50-100 Heats. Mechanical equipment is complicated & bulky. Therefore the KALDO process has found only a limited application. 3.11 Differentiate the between the L.D. and KALDO process? 3.12 Explain the construction and operation of Rotor Oxygen process? ROTOR OXYGEN PROCESS: Originally developed for pre treatment of hot metal. Modified to refine high “P” iron directly. Rotating on its long axis horizontally. Using two Lances.One is immersed in the bath for refining 22  Another is for burning of CO to Co2. Rotor furnace is a cylindrical retort. Capable of rotation on longitudinal axis at a speed of 0.2 - 4.0 rpm. Furnace is mounted on a turning table. Capable of rotation through 3600 in horizontal plane It can be tilted 900 in vertical plane ROTOR FURNACE OPERATION OF ROTOR PROCESS Rotor containing previous slag is turned to charging position. Burnt lime, ore & Scrap are charged. Hot metal is charged. Lances are arranged from one end. Hood is placed at opposite end. Refining is controlled by varying the distribution of Oxygen between the Lances. When bath reaches 2.0%C, the first phospjhoric slag is removed. Second slag is made by fresh additions. Second blowing is made to carryout De-”C”. Refining is completed, metal & slag samples are taken out. Tapping is carried out by tilting the furnace. Slag is retained in the vessel for next heat 23  CONCLUSIONS High cost & complexity of the equipment. Low life of the refractory lining Poor control of the heat. Lower productivity than common converters. Hence Rotor process has not found wide application. 3.13 State the principle of oxygen lime process (LDAC process) PRINCIPLE: LDAC process is meant to refine Iron containing more than 0.4%P. Increasing the rate of De Phosphorisation. The De Phosphorisation is over well before De carburization. This was possible by introducing lime powder through Lance at Hot Spot. At least one intermediate Phosphoric Slag removed. COMPOSITION OF THOMAS IRON Carbon 3.2-3.5% Phosphorous 1.5-2.0% Manganese 0.7-1.0% Silicon 0.4-0.6% Sulphur 0.05% or so 4. Electric furnaces 4.1 Classify the Electric Furnaces? Classification: Two principal commercial types for steel making are DIRECT ARC FURNACE: Suitable for melting & refining INDUCTION FURNACE: Suitable for Melting only. Hardly any refining takes place. 4.2 State the principle of arc furnace PRINCIPLE OF ARC FURNACE: When the current flows from the electrode to the charge, Electric Arc is struck between ELECTRODE & CHARGE then the temp of electric arc exceeds 40000C. Heat is transferred primarily by radiation. Part of heat is generated in the charge itself. 4.3 List out the raw materials for EAF? The chief raw materials are Sources of metallic iron(is Hot Metal, or Steel Scrap ), Oxidizing agents (Iron Ore&/ Mill scale),Modern trend is to use pure O2 , Fluxes: Lime, fluorspar, Dolomite etc…, De Oxidizers & Alloying additions. 4.4 State the sequence of the charging? SEQUENCE OF CHARGING: The furnace is cleared with the roof & electrodes. Small furnaces are charged through doors, big furnaces are charged from top. Top charging is more common, Charging is done by charging baskets. A little of light scrap is placed on hearth bottom. It protects damage from heavy scrap. The heavy scrap is charged above this, it is charged mainly in the centre. The remaining light scrap is loaded around heavy and at the top. Burnt lime & fluorspar are added in the basket. If refining is to be carried out, Lumpy iron ore and/mill scale is also included in the charge. 24  Broken pieces of electrodes, coke etc are added, this is for required carbon at the melt out. The raw materials are loaded in charging basket, the charging basket is brought to the top of the furnace by over head crane, and the bottom of the basket is opened. The charge drops out in the furnace; the charge should be put in the furnace in one lot. 4.5 Write about electrode material? The electrodes are either of Carbon or Graphite. Capable of carrying current at high density. Graphite is generally preferred because better electrical conductivity than Carbon. Electrode size depends on furnace capacity. Size varies from few cms to 100-110cms. Circular in cross-section, each piece is about 1-3m long, Electrodes are consumed. Electrode pieces are joined to one another, with the help of a nipple joint, Act as an endless electrode, Electrode material is costly, and its consumption should be minimum. It varies with practice, generally 3-6 kg/ ton of steel made. 4.6 Explain the construction and lining details of furnace? The furnace proper looks like sausepan, Covered from top with inverted sauser, Electrodes are inserted from top. ARC FURNACE PLANT The roof along with the electrodes swings, the body to facilitate charging from top. It is suitable for small & big furnaces. The roof is lifted a little & the body moves to one side only to facilitate charging. It is suitable for small furnaces only. Various parts of the furnace are furnace body (the shell, the hearth, the walls, the spout, the door); Gears for furnace body movements, Roof & Roof lift arrangements. Electrodes, their holders & supporters (Electrical equipment, The transformer, The cables, Electrode control mechanism). ELECTRIC ARC FURNACE SHOP ELECTRIC ARC FURNACE REFRACTORY LINING THE HEARTH: A layer of fire bricks at the bottom. Followed by Magnesite brick to form sub-hearth. SIDE WALL: The cylindrical part of the shell.Magnesite, Dolomite, or Chrome/Magnesite bricks used TAP HOLE: Gap left at the proper place. For this a round former is inserted. The shape left around is rammed. SPOUT: Teeming ladle firebricks are used next to steel plate. The rest is rammed along with the hearth. It is a part of hearth LINING DETAILS OF ARC FURNACE 25  DOOR: Lined with basic monolithic. Water cooled. It should be at proper place for slag removal. Still it should be well protected to stand slagging operations. ROOF: The roof is a domed construction. It has three holes. Located symmetrically for insertion of electrodes. Holes are made from ring type bricks. Superior in performance to Si. Roof fails due to splash of iron oxide rich slags. More than one roof is provided for each f/c. 4.7 Explain the melting practice in EAF? The essential steps are 1. f/c Preparation, 2. Charging, 3.Melt-Down, 4.Refining, Finishing & tapping of single slag heat, 5. Slag-off & making reducing, 6. Reducing period, 7. Finishing & tapping of the heat. 1. FURNACE PREPARATION: Lining is inspected; the eroded portion is repaired in hot condition by using granular dolomite or magnesite. Fettling may be done manually or by machines. The tap hole is repaired & plugged. 2. CHARGING: The raw materials are loaded in charging baskets. The basket is held just above the furnace by an over head crane. The bottom of the basket is opened. The charge drops out in the furnace. All the charge should be put in one lot. One or more baskets can be used. 3. MELT DOWN: After charging is over, the roof is held in position, the electrodes are lowered. Arc is struck, Electrodes are put on automatic control, and the metal below the arc melts, Molten metal drips down, Electrodes travel automatically further down. In some furnaces, furnace is rotated in horizontal plane; Melting can be made more even. By striking the arc at different points, if such provision is not there, the electrodes are lowered, and then allowed to bore the charge to some extent, and then raised the electrodes, the voltage is raised. This is repeated & a pool of molten metal is formed at the bottom. Melting is continued till all the charge is fully molten. Super heated to the desired level. Lime & Spar forms slag during melting. Most of “P” & “Si” will be oxidized (by ore) during melting. Charge ore is essential for oxidizing refining. 4. REFINING Actual amount of impurities to be oxidized is very small. The melt-out stage must be quiescent. Obtain a metal sample for analysis. The “C” content indicates the extent of refining. The opening carbon should be 0.15-0.30% Add iron ore (feed ore) 26  Refining is commenced. During refining samples are periodically taken out. To asses the progress of refining. If the analysis & temp are at desired level. The heat is ready for De-Oxidation. Preliminary de-oxidation by Fe-Si, Fe-Mn Block the heat by dipping the electrodes in the bath. 5. Finishing & Tapping of a single heat: Bath temperature should be high at the time of blocking. Alloying additions (Mn, Cr etc) can be made to blocked heat. Lowered O2 prevents oxidation of alloy. Recovery is high. Final de-oxidation may be done in the ladle. The heat is tapped by opening the tap hole. The furnace is tilted by a jerk. It forms carbide as (CaO) += (CaC2) {CO} It is quite effective for De-Sulphurisation. The lime to coke ratio may be 6:1 to12:1 Slag without coke is white in colour which is known as lime slag Slag with high coke is grey in colour which is known as carbide slag For low Carbon heats less than 0.15%C lime slag is preferred. This slag is made by the addition of lime, Fe-Si & Al. No coke is added. It re carburize the bath. 4.8 Explain the single slag and double slag practice? 4.8 SINGLE SLAG PRACTICE: Gained popularity in recent years. Shorten the tap-to-tap time. A single oxidizing slag is converted to reducing slag. Good enough for charge, containing valuable alloying elements. Not suitable for poor quality charge. The early oxidizing slag is converted to reducing, towards the end of melting. Alloying additions of W, Cr, Mo, Ni, Co etc are made with the charge itself. During oxidizing refining P, Si, C & Mn are oxidized. After oxidizing, the slag is converted to reducing. The slag is de-oxidized with powdered Fe-Si& CO; the metal is brought to specification level. The aims of reducing period are:  De-oxidation of the metal, Removal of Sulphur  Adjustment of the composition to specification  The most common De-Oxidizers used in arc furnace are:  Calcium carbide, Fe-Mn ,Fe-Si, Aluminum etc., DOUBLE SLAG PRACTICE If the charge is of poor quality, good quality scrap is always beneficial. Dirty & rusty scrap may be used. “C” is added in the charge to obtain required Carbon at the melt out. It is for boiling of the bath. Scrap proportion may vary from plant to plant: Ex.35%heavy scrap, 40% medium scrap, 25% light scraps. Charging should be completed in the beginning. The power & electrode consumption is max during melting, melting period should be min. Lime & ore may be added after 2/3 of the charge is molten. Some operators prefer to add partly with the charge & the remainder later. The lime addition should not be delayed. Acidic impurities (Si & P) attack the basic lining. Low temperature favors oxidation of P & Cr to that of Carbon. During oxidizing refining P, Si, C, & Mn are oxidized The oxidizing slag of 60% CaO & 15% FeO is obtained in 15 minutes after clear melting. This gives a good boil& oxidizes all the P. Initially the temperature low Sufficient lime added to retain “P”. As the C falls below the specification level, the oxidizing period should be over. De-Oxidation prior to slag-off by Fe-Mn. The first slag is removed. The bath is further de-oxidized by Fe-Si & Al. Add Lime, Spar, Coke, and Fe-Si. Reducing slag is made. The duration of reducing period depends on “S” content. 27 The FeO content of slag is maintained below 0.5% Fe-Si addition helps to De-Sulphurise the bath. Final test analysis is obtained. Required alloying additions are made. Bath temp is checked. Heat is tapped. APPLICATIONS  Production of low Carbon steels.  Production of low alloy steels. 4.9 Define Carbide slag? 4.9 Ans: - Slag with high percentage of coke is in grey colour is known as carbide slag. COMPOSITION OF CARBIDE SLAG  CaO - 65-70% ; SiO2 - 20-25%  FeO - 0.5%max; MnO - 5-10% 4.10 IMPORTANCE OF CARBIDE SLAG 4.10 Quite effective for de-sulphurisation of metal Fes+2CaO+CaC2 3Fe+3CaS+2Co. Possible to attain very low sulphur levels in the steel (0.005%) Used for making tool, ball bearing steels etc. Recovery of alloying elements is high under carbide slag. 4.11 Appreciate the advantages and disadvantages of Arc furnaces? ADVANTAGES OF ARC FURNACE: All types of carbon steels and alloy steels can be produced i) Wide variety of charge materials is acceptable even for high alloy steel making processes. ii) Metallized pellets can also be used iv) the recovery of alloying elements of scrap is high v) Yield is nearly 91%. vi) Favorable cost pattern of electrical energy. Vii) Wide range of furnace capacities. Viii) Flexibility in terms of intermittent operation, for irregular production is good. DISADVANTAGES OF ARC FURNACE i) The electric power, relative to chemical fuels, is costly which is not economical for bulk steel production. High sound levels. ii) Heavy truck traffic for scrap, materials handling and products 4.12Write the uses of gaseous oxygen in EAF 4.12 Oxygen lancing in BAF is now a routine practice. The use of ore for Carbon oxidation involves large losses of chromium and time consuming Which causes quick deterioration of furnace lining. Ore oxidation is impossible for stainless steels containing less than 0.1% Carbon To produce stainless steels very low in carbon and high in chromium 4.13 Explain stainless steel making by electric arc furnace (Rust less process) 4.14Write the principle of induction furnace? There are two concentric conductors. The inductor being the external conductor. The metal charge is internal conductor. Induction furnace consists of a crucible. A magnesite monolithic construction, with a spout for pouring. The metal charge acts as secondary winding. The crucible is surrounded by several turns of water cooled copper tubing. This carries high frequency primary current. Since current flow in opposite direction through them. They repel each other. 4.15Write about the melting practice in induction furnace? 4.15 Selection of raw materials is very important. What goes in must comes out. 28  No change in chemical composition. Sampling is not carried out during melting. It is carried out just before tapping. Light scrap is charged at the bottom Heavy scrap at the top to prevent atmospheric oxidation. Scrap should be of known composition. After melting necessary alloying additions are made Temperature is measured. Tapped in to a ladle by tilting the furnace Little De-C can be carried out. Iron-ore is incorporated in the charge for this purpose Oxygen spraying may be adopted. Excellent stirring action of the metal. Homogenization of the metal. Low oxidation of metals & alloys Alloy steels can be made with out much of virgin alloy additions No de-oxidation is carried out. Hence better cleanliness of the product A thin layer of slag is maintained to prevent atmospheric oxidation Slag is an insulator & not heated by induction. Very dry & does not take part in refining. Process can be carried out under vacuum. This is known as vacuum induction melting. Power consumption is 900KWH/ton for 0.5t furnace. 650KWH/t for 5.0ton furnace. Costlier than Arc furnace. But flexibility to produce different steels in small lots. 4.16 Appreciate the advantages of induction furnace? Ability to make small orders economically. Since there are no electrodes, it is possible to melt steels very low in Carbon. The absence of Arc ensures the metal made low in gases. No oxidation & hence alloy recovery is 100% Productivity is high. Temperature is controlled accurately. Excellent stirring action. (Homogenization). Max control over the charge. The only commercial process for making steels on a small scale. -------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------- 7. Ingot Production and defects INGOT MOULDS The moulds are massive castings Cross-sections like square, rectangular, round, polygonal etc. Used according to subsequent mechanical working Square cross-section moulds are used for rolling into Billets Rails Structural sections Girders Merchant products. Rectangular moulds are suitable for rolling into flat products like sheets, strips and plates. Round moulds are used for tube making Polygonal moulds are used for tyres, wheels, forgings etc. To help strip the mould off the ingot, the mould walls are tapered 7.1 Classify ingot moulds? Types of moulds: Narrow end up (NEU) or big end down. Wide end up (WEU) or narrow end down FIG2. TYPES OF INGOT MOULDS NEU moulds are used to produce 29  Rimming and semi-skilled steel ingots Bottle top moulds used for making a capped rimming steel ingot WEU moulds are used to produce Forging ingots of killed plain carbon or alloy steels. The inner walls of the mould may be Plane Cambered Corrugated Fluted. The specific surface area of the mould walls increases from plane to fluted. Faster cooling increases ingot skin thickness. FIG3.CROSS-SECTIONAL SHAPES OF MOULDS 7.2 MOULD MATERIAL Cast iron has good thermal shock resistance Earlier moulds are made of cupola metal. Now moulds are made of blast furnace metal These are cheaper Low in S content than cupola metal THE HOT TOP Killed steels solidify with the formation of pipe. 30  It leads to loss of yield Hot top acts as a reservoir It is also called feeder head. To feed metal to the main part of the ingot. To avoid the formation of pipe in the main part of the ingot hot top is used The pipe is confined to hot top. Hot tops are made in two ways. In one fireclay lining is given to the mould top from inside. Needs to be made a new for every casting broken during stripping of the ingot The other uses a cast iron box lined from inside with fire bricks placed on the top of WEU moulds Exothermic or insulating materials are added in the hot top. FIG4.HOT TOPS OF VARIOUS DESIGNS 7.3. TEEMING METHODS Teeming means pouring of liquid steel in mould. The quality of ingot depends on method of teeming Methods of teeming are Direct pouring, Tundish teeming, Bottom teeming or Indirect Teeming 7.4 Explain the Direct pouring? 7.4 DIRECT POURING: Metal is teemed from the ladle directly into the mould. The rate of pouring can be controlled by, the use of different sizes and designs of nozzles. Rate of pouring increases as the diameter of nozzle increases. FIG1.DIRECT TEEMING 1. TEEMING LADLE 2. STOPPER ROD 3. MOULD Enlargement of diameter of nozzle is due to erosion of fireclay nozzles Magnesite and graphite nozzles are better than fireclay nozzles. The size of the nozzle depends on the type of steel to be teemed For soft steels ( more viscous ) ,Slightly larger diameter (35mm) nozzle is preferred. 31  For medium and high carbon steels 20mm dia nozzles are preferred. Since the metal stream directly hits the bottom plate, the wear of bottom plate is quite severe. Direct pouring is used for rolling ingots 7.5 Explain Tundish teeming 7.5 TUNDISH TEEMING A tundish is inserted between the ladle and mould. To ensure uniform pipe like metal stream The tundish has its own nozzle and stopper The metal level is maintained constant through out the teeming period. The size of tundish nozzle is slightly bigger than ladle nozzle. Tundish may have no. of nozzles, to teem many moulds at a time. Tundish teeming is used for teeming Forging ingots Special alloy steel ingots. FIG. 2 TUNDISH TEEMING 1. TEEMING LADLE 2. STOPPER ROD 3. TUNDISH 4. MOULD 7.6BOTTOM TEEMING 8 Also known as uphill or indirect teeming 9 Steel is teemed into a vertical runner. 10 The top of the vertical runner is shaped like a trumpet. Or bell. 11 The height of the runner is more than mould height. 12 One vertical runner may feed 2-12 moulds at a time 13 Vertical & horizontal runners are cast iron pipes 14 The through runners are set in the bottom plates in slots 15 Compressed air is used to blow off any loose particles in the runners. Advantages: The quality of bottom teemed ingot is much superior. Wear of the bottom plate is much less. Disadvantages: More refractory wear elaborate preparation of mould. Steel solidified in the runners should be scrapped. It is economical only for superior quality ingots. FIG3.BOTTOM TEEMING 32  1. TEEMING LADLE 2. STOPPER ROD 3. MOULD 7.7CLASSIFICATION OF STEELS BASING ON GAS EVOLUTION Steels can be classified on the basis of gas evolution (De-oxidation) : Killed Steel Semi killed steel Rimmed steel Capped steel 7.8 KILLED STEEL No gas evolution takes place during solidification. All steels containing more than 0.3%C are killed The heat is worked in such a way that by the time the C level drops close to the specification level, The refining should be over. The heat is then blocked by adding Fe-Si, high Si Pig Iron, Silico – Manganese. Blocking stops the Carbon-Oxygen reaction by lowering the oxygen content of the bath. Ferro alloys are added in the ladle while tapping to fully deoxidise the steel The de-oxidation products should be given adequate time to rise to the surface. Or else it will form non-metallic inclusions Killed steels are always cast in WEU moulds with hot tops Killed steel is accompanied by segregation. Killed steel structure is quite sound & dense. This is suitable for forgings Alloy steels are fully killed steels because soundness of the ingot is what is essentially required 7.9 DEMI KILLED STEELS Partially de-oxidized steels Small amount of gas is evolved during solidification. The carbon content has to be in the range or 0.15-0.30% The required de-oxidation may be carried out in the furnace 33  Fe-Mn, Fe-Si & Al are used as de oxidizers Gas is not evolved immediately after pouring. Top level freezes before gas evolution commences. Gas is evolved towards end of ingot solidification. The pipe is compensated by gas evolution and its entrapment. By de-oxidizing the top of the ingot by adding aluminum just before completion of pouring , the top of the ingot may freeze quickly Therefore the blow holes are formed mainly in the middle part of the ingot with very little in the lower part. The blow holes are absent in the top portion Semi killed steels find a very wide use in the manufacture of Structural shapes Plates Merchant products 7.10RIMMING STEELS 8 Lot of gas evolution during solidification 9 It gives appearance of boiling to liquid steel 10 The boiling action is termed as rimming. 11 These steels must contain enough dissolved oxygen 12 This is possible only in low carbon steel. 13 Hence these steels are produced only if carbon is less than 0.15% 14 No de-oxidation is carried out in the furnace. 15 Small amount of de-oxidation is carried out using Fe-Mn or Al as de-oxidizers 16 De oxidation is carried out either in the ladle or in the mould 17 The exact procedure depends on Carbon content. 18 The correct thickness of ingot skin , without blow holes can be obtained by controlling the oxygen content & temperature of liquid steel while teeming 19 Primary blow holes are formed next to the ingot skin of adequate thickness. 20 Secondary blow holes are formed still inside 21 The zone between these two is known as RIM. 22 This is a characteristic of rimming ingot. 23 Rimming ingot has a smooth surface. 24 Suitable for rolling into flat products. 25 These are cast in NEU moulds 26 Rimming ingot has a smooth surface. 27 Suitable for rolling into flat products. 34 28 These are cast in NEU moulds 29 Fig1.TYPICAL INGOT STRUCTURES RESULTING FROM GAS EVOLUTION DURING SOLIDIFICATION The amount of gas evolved increases progressively from No.1 to 8 1.KILLED STEEL 2.SEMI KILLED STEEL 5.CAPPED STEEL 7.RIMMING STEELS 7.11CAPPED STEELS Another variety of rimming steels steels containing around 0.15% carbon Gas evolution is much less brisk than rimming steels. Steel is cast in bottle shaped NEU moulds Constricted top facilities mechanical capping of the ingot. Early gas evolutions are prohibited by adding al to the mould during teeming. The gas evolution in the later stages raises the metal level This is stopped by mechanical capping Segregation is more in rimming steels. This is less in capped steel due to short rimming time These are used for producing flats, wires and bars. 7.12INGOT DEFECTS The aim of the operator is to produce ingot Both physically and chemically homogeneous. Which would have an equiaxed structure? Would be free of segregation Without non metallic inclusions, cavities Would have a smooth surface finish. The Important ingot defects are : Pipe Blowholes Chemical segregation Non metallic inclusions Columnar structure or Ingotism Internal fissures and hairline cracking Ingot cracks. Surface defects. PIPE The volumetric contraction resulting on solidification. Appears in the form of a cavity known as pipe This is 2.5-3.0% of the total apparent volume of the ingot Fig1.KILLED STEEL INGOTS SHOWING PIPE FORMATION 35  a) NEU mould showing long primary & secondary pipe b) WEU mould showing short pipe c) WEU with hot top d) NEU with hot top showing pipe confined to hot top alone Rimming and semi killed steels show slight tendency for piping. It can be eliminated by careful practice. Capped steel is practically free of pipe Pipe formation is serious in killed steels. The shape and location of pipe depends on mould type. In WEU mould the pipe is short and wide In NEU mould it is narrow and long Yield is reduced due to pipe formation. Two types of pipe Primary pipe and secondary pipe. Primary or open pipes get oxidized. Does not weld during rolling This portion has to be discarded Secondary pipe is deeply seated does not get oxidized. Welded up during rolling. Remedies: The detrimental effect of pipe formation on the yield is reduced by Adopting a hot top feeder Pipe is confirmed to feeder box The volume of feeder box is about 15% of the ingot volume. Use of insulating or exothermic materials keeps the hot top molten for long. It prevents extending pipe to main part of the ingot. BLOW HOLES The entrapment of gas evolved during solidification produces cavities known as blow holes Types of blow holes : Primary blow holes. Secondary blow holes. Primary blow holes are elongated (honeycomb) Locate next to the ingot skin Secondary blow holes are more spherical & these are located further in BLOW HOLES Formation of blow holes eliminate Partially or fully the pipe Increase the yield during rolling. 36  These must be located at proper depth from surface. Deeply seated blow holes are welded during rolling Blow holes closer to the surface often get oxidized during soaking These do not heal up during rolling Produce surface defects (seams) on the product. Gas evolution should be controlled during solidification. Blow holes should be formed only after adequate thick ingot skin is formed COLUMNAR STRUCTURE OR INGOTISM Steel is a crystalline solid During solidifications, a chill layer is formed. Further dendrites are formed. These grow along their principal axis perpendicular to the mould walls. Their lateral growth is restricted. This is due to the growth of adjoining dendrites. This gives rise to elongated crystals. If the length of these is appreciable, it is known is columnar structure or ingotism. These ingots tend to crack during rolling. Fig3.STRUCTURE OF A KILLED INGOT SHOWING THE THREE ZONES OF SOLIDIFICATION In general, columnar structure does not extend to the centre of ingot. Middle portion of the ingot solidifies as equiaxed grains These two are adjusted to keep the ingotism minimum. Adjustment is carried out with respect to Composition of steel. Pouring temperature. Mould temperature. Gas evolutions during solidification. 7.13SEGREGATION Segregation means departure from average composition. If the concentration is greater than the average. It is called positive segregation If the concentration is less than the average. It is called negative segregation. Segregation is the result of differential solidification. It is a characteristic of all liquid solutions. 37  Steel is a liquid solution of S, Si, C, P, Mn etc., in iron. Hence steel is prone to segregation during solidification Fig4. KILLED STEEL INGOT SHOWING SEGREGATION Segregation depends on composition of steel Segregation tendency increases in the order of Mn, Si, C, P & S. Segregation increases with increasing time of solidification. Large ingots tend to segregate more than small ingots Segregation is increased due to gas evolution during solidification Segregation increases in the order of killed, semi-killed, capped and rimming steels. Segregation can be minimized by prolonged soaking of ingots before working. NON METALLIC INCLUSIONS Cleanliness of the ingot means free from the entrapped non-particles. Most important in judging the quality of ingot. Types of inclusions: Indigenous- those arising in the course of steel making Exogenous- those arising from erosion of lining. Indigenous inclusion are due to faulty de-oxidation practice. Enough time should be allowed for de-oxidation products to rise to the surface. Proper care during de-oxidation can minimize these inclusions Exogenous inclusion arise from Mechanical erosion of ladle lining. Refractories used in the assembly of mould Use of strong refractory can eliminate these inclusions. INTERNAL FISSURES & HAIR LINE CRACKING The term CLINKED ingot is used to denote internally cracked ingot These cracks or fissures arise due to two causes 1. Too rapid heating of an ingot. the outer layer expands more rapidly than the core giving rise to internal rupture. Alloy steels are prone to such cracking because of their coarse & their weak crystal structure 2.Too rapid cooling of an ingot After stripping the mould cause uneven contraction at the surface & in the core, finally resulting in the internal fissures REMIDIES 38  Prevent too rapid cooling and reheating of an ingot HAIR LINE CRACKING These are formed due to disorption of dissolved H2 The solubility of H2 is decreased during solidifications. Hydrogen is desorbed very slowly. Even after cooling for days or weeks. Hydrogen desorption depends on Type of steel Cross section Residual stresses Hydrogen content of steel. Alloy steels are more prone to hair line cracking. The safe limit of H2 is 2.0-2.5 CC /100gm. To reduce H2 content below this level ingot is held at 600-650 0C Holding time depends on cross section. Many times the ingot is stripped off very early. Transferred to soaking pit Prolonged soaking is carried out to diffuse out hydrogen. Alternatively by vacuum treatment Hydrogen content can be decreased 7.17INGOT CRACKS Immediately after pouring, a thin solid layer is formed on the side faces and bottom of the ingot It is due to chilling effect of the mould This is known as ingot skin. The contraction of the ingot and expansion of the mould on heating. Tends to separate the two An air gap is formed between the two The ingot skin has to withstand to the ferro-static pressure of liquid core If the skin is not thick, it ruptures. Giving rise to cracks in the surface. CAUSES : to decrease skin thickness are Too high teeming temp. Rapid rate of pouring. Too high mould temperature. Formation of hot spot. Friction between mould and ingot. Pouring more on one side. Types of ingot cracks: Longitudinal cracks Transverse cracks Restriction cracks Sub-cutaneous cracks Longitudinal cracks: More or less parallel to vertical axis of the ingot. The tendency to form these cracks increases If is the ratio of cross section to height increases. Alloy steels are more prone to these cracks than mild steels. 39  Transverse cracks: Parallel to the base of ingot The tendency to form these cracks increases. If the ratio of height to cross section increases This is the most common type of ingot cracks. Restriction cracks This may be longitudinal or transverse in direction Located at the corners of the ingot Longitudinal restriction cracks are due to large corner radius of the ingot The transverse restriction cracks are due to friction between the mould & the ingot of a small corner radius Sub cretaceous cracks These are internal fissures close to the surface These are formed due to thermal shocks These open up during soaking and /or rolling. Surface cracks can be eliminated by Designing the mould properly Using thick mould walls for effective chilling Use of corrugated or fluted mould walls. Smooth corner of the mould Dressing the mould before pouring. SURFACE DEFECTS These are formed mainly due to faulty teeming practices and the use of faulty moulds. Scab: A projection on the side surface of an ingot caused by freezing of steel in a cavity in the mould. Lappiness: Lap is a fold in the ingot skin caused by freezing of a slowly rising top surface of the metal in the mould before the pouring is over Causes: Slow pouring , Low teeming temperature, Double pouring Splash: Metal drops are thrown off due to the impact of the metal stream on the mould bottom. These drops stick to the mould wall These are oxidized and form seams in the rolled products. Crazing: If a large no. of cracks are present in the mould wall Steel may freeze in these cracks Giving rise to a network of fins on the ingot face. Also known as crocodile skin. These may form seams during mechanical working Spongy Top The viscous top tends to rise due to the late gas evolution & there makes the top spongy. Flash Solidified steel formed due to steel entering crevices in the mould assembly At the joining of mould with bottom plate or hot top Boot Leg Sinking of an ingot top below the original level in the mould. This is due to decreased evolution of gas. 40  Calls for extra front crop during rolling Skin Holes These are formed due to entrapment of gas evolved from mould dressing Leads to seams in rolled products 8. Continuous casting of steel CONTINUOUS CASTING INTRODUCTION Earlier, steel has been cast into ingots Desired finished shapes was obtained by mechanical working Ingots are rolled in primary mills to produce blooms or slabs. Subsequently processed to make structurals, rails, merchant products etc.. This method of producing blooms, slabs, etc by primary mills has the following disadvantages : 1) Large amount of capital has to be invested 2) In spite of all care defects do occur in ingots i.e., segregation 3) The ends of the ingots has to be discarded and it decreases the yield 4) If billets are needed additional rolling mills are required The development of continuous casting has made a significant impact on the steel production throughout the world It has greatly improved the efficiency of material utilization In continuous casting the process yield is more than 95% and there is a marked improvement in the quality of products Continuous casting was successfully applied in 1943 only In India Mukund iron & Steel Company Established first machine in 1965 In Continuous casting the liquid steel is poured into a mould through tundish 41  During solidification this steel attains the shape of the mould in which it is allowed to solidify Continuous casting may be defined as teeming of liquid metal, in a short mould with a false bottom, through which partially solidified ingot is continuously with drawn, at the same rate at which metal is poured in the mould. The feed rate of liquid metal into the mould is synchronized with a rate of solidification and hence with drawl of solidified steel from the mould There by a continuous link of a cast material is build up which is known as Continuous casting of steel 8.2 Advantages of continuous casting: Much of the problems in conventional ingot production can be eliminated Operation is more economical Good quality of the product High rate of production Direct production of blooms, slabs or even billets 8.3 PRINCIPLE OF CONTINUOUS CASTING MACHINE: Pouring of liquid metal in a short mould with a false bottom. Partially solidified ingot is continuously with drawn from the mould. The rate of withdrawl should be at the same rate at which metal is poured in the mould. Fig.1 PRINCIPLE OF CONTINUOUS CASTING MACHINE 42  fig3PRINCIPLE OFCONTINUOUS CASTING The equipment for continuous casting of steel consists of 1) The ladle to hold steel for teeming 2) The tundish to closely regulate the flow of steel in the mould 3) The mould to allow adequate solidification of the product 4) The withdrawl rolls to pull out the ingot continuously from the mould 5) The cooling sprays to solidify the ingot completely 6) The bending and/or cutting devices to obtain handlable lengths of the products 7) The auxiliary electrical and/or mechanical gears to help run the machine smoothly.  The mould is open at both ends and is water cooled  The operation is started by fixing a dummy plug bar to temporarily close the bottom of the mould  Steel is slowly poured in the mould via a tundish  As soon as the mould is full to a certain level withdrawl of the plug begins  The rate of withdrawl must exactly match with the rate of pouring  Uninterrupted pouring and simultaneous withdrawl gives rise to the whole cast in the form of one piece.  This may be cut into smaller pieces as per thr the requirement.  Ingot does not completely solidify in the mould.  As soon as a sufficiently thick skin is formed, withdrawing from the mould commences 43   It is then cooled by secondary cooling  If the bar is withdrawn rapidly, this seal may fracture  This may produce cracks in the ingot or in the leak outs  This can be eliminated by adopting a moving mould rather than a stationary mould. 8.4 JUNGHAN’S PRINCIPLE The principle of moving the mould is known as junghan’s principle The mould is moved up and down through stroke of 12-40mm The ratio of speeds of downward to upward stroke is nearly 1:3 The downward speed being equal to that of the rate of withdrawl If the down speed is evenly slightly less than that of the rate of withdrawl major transverse cracks are formed Therefore the speed of down strokes has been increased to little more than the speed of with drawl This results in negetive stripping of the ingot ADVANTAGES The initially crystalysed skin of the ingot is further compacted & Formation of tensile stresses is prevented Eliminates the possibility of transverse cracking of the skin Transverse cracks that may be formed earlier are liable to be welded again It allows maximum rate of withdrawl i.e., maximum production from a given machine 8.5CLASSIFICATION OF CONTINUOUS CASTING MACHINES 9 Continuous casting machines can be classified : 9.4 According to their products 9.5 According to the path which castings describes 10 According to their products, Continuous casting machines can be classified as : 11 Billet Casters 12 Bloom Casters 13 Slab Casters 14 Round Casters According to the path which castings describes, Continuous casting machines can be classified as: 1) Vertical type CCM 44 2) Vertical – mould Horizontal discharge type CCM 3) The curved mould (S-TYPE)CCM 4) Horizontal type  Equipment for “C-C” Machines  Mould : It is the most important part of the machine. It controls the extraction of heat and solidification of steel. The shape, size and design of the mould are responsible for the quality of the product. Moulds are primarily made of copper.  Mould Guide : It is responsible for controlling the movement of the mould.  With drawl Rolls : They are responsible for the exit of semi- solidified steel strand from the mould bottom  Cooling Spray : It is responsible for mould life and system solidification. Mould cooling also determines thickness of solidified wall of the strand.  Strand Bending : They are responsible for shaping the & Straightners C-C Strand. Different types of C-C machines differ in their bending or straightening Mechanism. Cut Off Torch: It is used to cut the strand in to appropriate lengths. Shrouding : It protects the liquid steel while it is system poured in to the mould. Different types of shrouds are used depending on the requirement and cost benefits. Dummy bar : It is a device used to prevent the liquid steel from pouring out of mould bottom at the start of casting process. Ladle : It is used to transfer the liquid steel from the steel melting shop to the Tundish of C-C machine. The ladle lining, pre-heat cycle and nozzle are the important features. Tundish : It acts as reservoir vessel for the C-C machine and supplies liquid steel to the mould. Tundish design, capacity and slide gating system are important features. 8.6 VERTICAL TYPE CONTINUOUS CASTING MACHINE 45  It is the first continuous casting system The mould and the discharge are both vertical Liquid steel is brought to the machine in a stopper controlled ladle. Steel is teemed in a stopper controlled tundish This regulates the flow of steel to the mould Fig1.VERTICAL TYPE CONTINUOUS CASTING MACHINE Below the mould is the secondary cooling zone Rollers are set in to make close contact with the ingot The water spray nozzles are interspersed in between these rolls. This is known as roller apron These hold the product fairly tightly to support it The main withdrawl rolls are situated just below the roller apron The cut - off torch travels at the same speed as that of the withdrawl by clamping the product After cutting, the torch goes back to its position quickly The product is then laid horizontal Hoisted to the normal flour level Normally CCM has more than one strand As many as 8- strand machines are in use This type of plant is very tall Needs either a tall shop or a large pit to accommodate the equipment This plant is used for large and medium sections It is good for the production of slabs In the event of brake down it is easy to repair and restart the machine Most simple in construction and most reliable to operate. All steel qualities can be cast at high speeds No fear of damage to the strand by bending and straightening 8.7 VERTICAL-MOULD HORIZONTAL DISCHARGE TYPE C-C M’s 9 This is a modification over the earlier vertical design to reduce the over all height of the machine 10 The mould, the roller apron design and the pinch rolls are similar to vertical machine. 11 After the product emerges from the pinch rolls it is bent to obtain the discharge horizontal Fig.1VERTICAL-MOULD HORIZONTAL DISCHARGE TYPE CCM 46  The cutting torch moves horizontally A horizontal set of straightening rolls becomes necessary A saving of 30% in height is possible by this design However flour space requirement is more Heavy sections being difficult to bend Cannot be cast by this machine In the event of break down it is more difficult to repair and restart. Popular for small and medium size cross-sections Essential details of continuous casting machine The basic requirements of any casting machine are as follows: i) Hot metal handling system as a source of molten finished steel ii) Tundish for supply and distribution of liquid steel to the mould iii) iii) Mould to freeze the skin of the casting iv) iv) Water sprays to complete solidification and required cooling v) v) Drive system to withdraw the strand continuously at a predetermined rate vi) vi) Cut-off machine to cut the continuously solidified piece into required lengths 8.8 PROBLEMS IN THE PROUCTION OF INGOTS BY CCM  The common causes of defects in continuous cast products are as follows :  1. Improper tapping temp. of steel  2. Improper casting speed  3. Improper de-oxidation of steel  4. Improper cooling rate  5. Entrapment of slag in steel  6. Entrapment of refractory in steel 47  7. Entrapment of casting powder Cracks  The most common problem in continuous casting of steel is the formation of cracks  The cracks may appear any where, at the surface or In the interior  Due to contact with air, the surface cracks are oxidized and don’t get welded during rolling  The most common process to remove the surface cracks is either scarfing or grinding Different types of cracks are :  1. longitudinal corner cracks  2. Transverse Mid-face or Corner cracks  3. Star cracks  4. Triple Point Cracks  5. Center Line Cracks  6. Diagonal Cracks  7. Pinch Roll Cracks   Pin Holes/ Blow Holes  These holes are cause by the difference of solubility of various gases like CO, N2, H2, etc., In the liquid and solid metals  They are distinguished by their size i.e,. Pin Holes are less than 3mm in dia. and Blow Holes are more than 3 mm in dia.  They can be solved by :  1. Efficient de-oxidation using a Mn/ Si ratio of 3:1  2. Maintaining the temp. gradient as low as possible  3. Argon rinsing the melt  4. Electro magnetic stirring in the mould  Oxygen dissolved in the steel reacts with carbon to generate bubbles of carbon monoxide 48  This reaction extremely fast and violent generating large amounts of hot gas  Concast product is more susceptible to hydrogen pick up because of use of lubricating oil in the mould  This can cause formation of pin holes  Rhomboidity  The most common mould related problem  The Percentage Rhomboidity is defined as a ratio of the difference of diagonals to the average diagonal length  The Percentage Rhomboidity = 2 (D2 – D1)/ (D1 + D2)  Reasons :  1. Non-symmetrical cooling  2. Poor Mould alignment  3. Non Uniform spray cooling  4. Wobbly mould oscillation  A major problem that may occur in continuous casting is breakout  This is when the thin shell of the strand breaks, allowing the still-molten metal inside the strand to spill out  And foul the machine, requiring an expensive shutdown  Often, breakout is due to too high a withdrawal rate  As the shell has not had the time to solidify to the required thickness  Breakout can also occur if solidifying steel sticks to the mould surface,  Another problem that may occur is a carbon boil 49  5. Secondary Steel Making Processes 5.1 State the objectives of secondary steel making. 1. improve the quality of steel wrt cleanliness, tighter grain size control, narrow hardenablity range etc, for air crafts, submarines, shipbuilding, arctic pipe lines, electrical steels etc, 2. Improvement in production rate 3. Decrease in energy consumption 4. Use of relatively cheaper grade materials 5. Use of alternate sources of energy 6. Higher recovery of alloying elements 5.2 Classification of Secondary steel making processes. 1. Stirring treatments, 2. Synthetic slag refining with stirring, 3. Vaccum treatments, 4. Decarburization techniques, 5. Injection metallurgy, 6. Plunging techniques 7. Post solidification treatments. 5.3 Explain Furnace based processes-A.O.D process. AOD (Argon – Oxygen – Decarburization process): Here oxygen gas is passed in to a charge of stainless steel scrap and high carbon Ferro – Chrome melt to remove C to get low carbon stain less steels. The reaction Cr2O3 + 3C = 2Cr + 3CO shift to right at high temp and low pressures (pCO) It means, at that high temperature (1700OC), low CO gas pressures (pCO = 0.1 atm), dissolved C in the melt is decarburized as CO and Cr oxidation is avoided. Low Partial pressure of CO (pCO) achieved by using a mixture of O2 and inert 1 Ar gas Stages of AOD 1. Decarburization Stage 1 : Oxygen: Argon = 3:1, C = 0.3% after reaction, Stage 2 : Oxygen: Argon = 2:1, C = 0.09% - 0.12% after reaction Stage 3 :Oxygen: Argon = 1:2, C = 0.02% after reaction 2. Deoxidation: this is done for Cr recovery by using reducing slag. (by Ferro – Silicon and lime mixture) 3. Desulphurization: this is done for desulphurization. Desulphurization mixture consists of lime, Ferro – Silicon and spar in the ratio of 7:15:15 5.4 Explain Ladle based processes-Ladle furnace.. Ladle furnace: It is a simple ladle provided with arrangements, like 1. Bottom plug inert gas purging, 2. Lid with electrodes for arc heating and also for vacuum treatment, 3. Chutes for alloying additions, 4. Openings for injection etc, 5.5 Explain Remelting processes-E.S.R. and V.A.R. The Electro Slag Refining In this process the steel to be refined forms the consumable electrode (cast ingot). Heat is not produced by arcing but by the passage of current across a conducting liquid basic slag (CaF2 + Al2O3 + CaO) due to its resistance. The slag is held between the electrode and the melt surface. The electrode bottom tip is always kept immersed in the slag. The electrode melts progressively and after passing through the slag, it settles to the bottom and solidifies uni directionally as casting. voltage is 40 – 50 Volts A.C., current = 5- 80 A. Power = 1500kWh/ ton. 2  This process is exceptionally suited for the production of different shapes (Rectangular, Round, Square and Hollow shaped), particularly large sections (20 – 60 inches) Multiple Electrodes can be melted into a single mold High alloy tool steels, bearing steels. Refining occurs. Improved cleanliness (reduce larger sized non metallic inclusions) and decreased segregation but cannot eliminate dissolved gases. Spacing between mold wall & Electrodes is not critical. 5.6 Explain Remelting processes-( Vacuum Arc Re-melting (V.A.R) process ) 3  V.A.R. :-The process consists of remelting a solid Electrode by striking an Arc between crucible and the ingot surface, the system being held under vacuum. Pressure = 0.1 torr ( vacuum) The liquid metal falling from the Electrode progressively builds an ingot inside the water cooled Copper crucible. The heat required is supplied by Arc. ; 23 – 28 Volts D.C Due to vacuum, degassing occurs during remelting hence, O2 and H2 levels in the casting is after remelting is very low. Advantages. The process produce very sound ingots of dense crystal structure, low Hydrogen and Oxygen contents with minimal chemical and non metallic segregations 4  6. Vacuum treatment of liquid steel. 6.1Know the gasses dissolved in liquid steel. 6.1 Dissolved gases in steel are Oxygen, Nitrogen and Hydrogen. Solubility of these dissolved gases depends upon composition of steel and it increases with temperature. The amount of dissolved gases depends upon the quality of raw materials used and steel making process used. On solidification excess dissolved gases are liberated which may form skin or pin holes, blow holes, etc., These cavities are in general detrimental to the mechanical properties 6.2State the objectives of degassing 1. The Vacuum melting processes are meant to produce steel having low gas content and inclusions in a very small amount 2. To remove Hydrogen from steel, so that the prolonged annealing treatment is not required 3. To improve cleanliness by removing Oxygen in the form of CO gas 4. To produce steel of very low ‘C’ content (below 0.03%)by transferring part of the refining from furnace to the degassing unit 5. To bring about De – Sulphurization of steel by reagents carried along with an inert gas like argon bubbled through the bath 6. To ensure the better control of chemical composition of steel by adding the requisite amount of additions under vacuum 7. The processes are called Vacuum De-gassing Processes 6.3Classify the degassing processes. 1 6.4 Explain the Ladle degassing, Steam degassing, and Circulation degassing (R-H &D – H process). De-gassing processes: 1.Ladle, 2. Stream 3. Circulation De – gassing Ladle De–gassing: In this liquid steel is held in a ladle which is put inside a vacuum chamber.Steel may be stirred by bubbling an inert gas or by an electro magnetic stirrer while being exposed to vacuum Stream De – gassing: Liquid steel flows down in the form of stream Steel is exposed to vacuum while the liquid steel pour like stream from the furnace to ladle, ladle to another ladle or mould. Circulation De – gassing :In this process liquid steel is either continuously or intermittently circulated during its exposed to vacuum. Ex. D – H and R – H continuous De – gassing circulation systems 2  R – H DEGASSING PROCESS PRINCIPLE: Molten metal is exposed to vacuum in the form of either a circulating stream or a small portion of a metal pool. This process was developed by Rheinstahl Heinrich shutte in Germany in 1957. 3  CONSTRUCTION OF EXHAUST CHAMBER The chamber as a cylinder shell with two legs, often called snorkels(high alumina refractory) Openings at the top side are provided for exhaust and alloying additions, observation and control The cylinder is lined inside with fire bricks (upper portion) and high alumina bricks (lower portion) here, the metal comes in contact with bricks. During degassing metal enters the cylindrical vacuum chamber through 4  one chamber and flows back under gravity through the other The inlet snorkel is in laval shape using refractory while the out let snorkel is tubular. A refractory tube is attached to each snorkel. This tube is to be dipped in liquid steel for degassing These are made of pure Alumina Lifter gas like argon is introduced in the inlet snorkel at the point where alumina tubular tip is attached to it. OPERATION Before the actual used the chamber is variously heated to 900 – 15000C at different points by forging fuel up in snorkels The chamber is lifted and lowered to an appropriate level in the ladle containing mouth steel. The chamber is evacuated and the liquid steel just rises in the chamber Lifter gas is then introduced the inlet snorkel The lifter gas expands and rises up their by using the velocity of the steel in the inlet snorkel. The lifter gas bursts in the chamber and likewise, the steel also explodes on melting vacuum as in stream degassing. The net result is that degassing takes place very efficiently Gravity causes the steel to flow back in the ladle through the other snorkel. Degassed steel is slightly cooler and denser. CONDITIONS OF R-H DEGASSING The rate of circulation of metal is controlled by adjusting the vacuum and the rate of flow of lifter gas The average arte of circulation is nearly 12t/ min and 20 min are required to that 100 t of steel The degassing is quite efficient and more than 90% gas is removed. At end of degassing alloy additions can be made depending upon the available super heat The metals cools down by about 20 – 500C depending upon the ladle size Amount of argon required is 0.015 – 0.075 m3 /t ADVANTAGES Pumping capacity is required to very small Heat losses are relatively low It can produce steels with less than 0.02% C Alloy additions can be made to adjust the specification more closely D –H DEGASSING PROCESS It was developed by Dortmund – Horder in Germany It was also known as lifter gas process In this process a small portion about 10 – 15% of the total steel in the ladle in treated at a time under vacuum The process is repeated until required degassing is achieved FIG: D-H PROCESS OF DEGASSING