Steelmaking using Induction Furnace

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1 15 Steelmaking using Topics to discuss... Introduction and brief history Furnace design Steelmaking process Furnace operations

2 Introduction and Brief History The development of s starts as far back as Michael Faraday, who discovered the principle of electromagnetic induction. In 1870 De Ferranti started experiments in Europe on induction furnaces. The first induction furnace for melting metals was patented by Edward Allen Colby in The first steel made in an induction furnace in United States was in 1907 in a Colby furnace near Philadelphia. First three-phase induction furnace was built in Germany in 1906 by Rochling- Rodenhauser. 3/16 In India the use of induction furnaces started in mid-60s. Imported medium frequency induction furnaces were used from mid-70s. Initially induction furnaces were used for melting stainless steel scrap but use of these furnaces for mild steel production was started from mid-80s. Early 80s to mid-90s saw a sudden growth of using induction furnace. During this period indigenous manufacture of the induction furnaces was also started. In late 90, BSRM of Bangladesh installed the first steelmaking unit based on induction melting technology. Now, all industries of Bangladesh, bar one, produce steel using induction furnace. 4/16

3 Salient Features of IF Steelmaking Coreless induction furnace is usually regarded as dead melting unit, where effectively only minimal changes occur during the process. Hence the raw materials play an important role during steelmaking. Besides the quality of steel produced, raw materials also affect (i) volume of slag produced (ii) refractory lining life, and (iii) safety of both the plant and the working personnel. Raw materials and their charging practice have a considerable influence on the specific consumption of electric energy and furnace productivity. The important parameters to be controlled in raw materials are: (i) size (ii) bulk density (iii) chemical composition (iv) cleanliness, amount of contamination, and freedom from rust, scale, sand, dirt, oils/grease, and (v) non-metallic coatings. 5/16 Amongst the various raw materials used for making a heat, metallics take the lion s share both in terms of technology and economics. The main raw materials for steelmaking in induction furnace are: (i) (ii) steel scrap, iron scrap or/and pig iron, (iii) sponge iron, (iv) carburizer (petroleum coke, anthracite coal), (v) additives (ferro alloys) 6/16

4 Dirty or contaminated scrap tends to deposit a slag layer on the furnace refractory. restricts the quantity of power which is drawn by the furnace. effectively reduce internal diameter of the furnace and makes charging difficult, which again affects the energy efficiency of the furnace. Rusty scrap not only takes more time to melt but also contains less metal per charging. Scrap is to be checked to ensure that pre-coated steels such as tinned plate and zinc coated are not included, since these materials produce excessive amounts of metallurgical fume and slag. For every 1 % slag formed at 1500 C energy loss is 10 kwh per ton. 7/16 Energy consumption is significantly increased by incorrect charging practices. Irrespective of charging mode, sponge iron is always charged after initial formation of molten pool by melting of steel scrap. FeO of sponge iron reacts vigorously with C in the liquid bath, produce carbon boil, and help producing clean steel by improving [1] heat transfer, [2] slag metal contact, [3] homogeneity of the bath, and [4] removing nitrogen and hydrogen. Slags developed in induction furnaces are not fluid and is quite heavy and sticky and often dry and in the form of a dross. periodic removal of slag using de-slagging spoons is necessary. if slag coagulants used, their use is to be strictly controlled to prevent chemical attack on furnace lining material. Slag volumes can be reduced by [1] selecting clean and proper charge materials, and [2] with sponge iron having higher percentage of total iron. 8/16

5 Metal losses for metallic charge materials depend upon the physical size of the component and their quality, but are normally less than 5 %, with a fair proportion of this loss being due to spillage and splash during the de-slagging and pouring operations. Recovery of carbon depends on the size and quality of the carburizer, method of addition, and time of addition. It can be expected to be within a range of 85 % to 95 %. 9/16 Electric Arc Furnace vs. Steel Chemistry Ability to use different quality charge material Control of composition Thanks to the use of proper slag mix and oxygen supply, charge of any quality can be used. Steel containing any level of P and C can be made. S removal to any level is possible (using a double slag process) As refining is limited, the required grade can only be achieved with a proper scrap selection, which results in a limited product mix and/or higher production cost due to use of more expensive and cleaner scrap. P level = % C level = % S cannot be removed due to low slag basicity and low slag volume

6 Electrical Energy Consumption Electrical energy consumption kwh/ton (continuous scrap charging) kwh/ton (bucket charging) Use of oxygen and fuel burning, electrical energy consumption is less High and narrow melting vessel (low d/h ratio) Low crucible wall thickness Low slag temperature Powerful bath motion kwh/ton Reduction in electrical energy is limited by operational practice and type of scrap used (e.g., shredded scrap needs low energy for its high density and lower melting loss) 11/16 Electrical energy consumption (contd...) operation can be designed for different scrap charging method, different operational practice, and advanced process control, which have strong impact on electrical energy consumption Some practices that can reduce electrical energy consumption are (a) initial scrap charging by bucket, (b) power on control during deslagging, (c) ensuring full crucible before tapping, (d) minimum holding time, (e) proper scheduling of furnace, (f) scrap quality 12/16

7 Operational Parameters Electrode, kg/ton Nil Refractory, kg/ton Oxygen, Nm 3 /h Nil Flux, kg/ton Nil Slag generation, kg/ton Noise level, db (A) /16 Others Manpower low high Environmental effect Low is equipped with quenching tower, cyclone and filter (fines de-dusting) together with possibility to install heat exchangers High induction furnace needs housing for dust and fume collection 14/16

8 The induction furnace has the following technical advantages over electric arc furnace. i) Low requirement on the electric grid ii) Higher yields iiii) Lower consumption of ferro-alloys iv) No cost on electrodes v) Lesser capital expenditure vi) Lower space requirement vii) Induction furnace is suitable for charging addition agents any time due to the characteristics of the bath agitation. viii) Has low load and no flicker disturbance ix) Automated application in a simple way 15/16 The disadvantages are: i) The requirement of minimal wall thickness of the refractory lining is having risk of crack formation resulting in stoppage of operations. ii) Induction furnaces puts more stringent requirement on the quality of scrap iii) Decarburizing, desulphurizing and dephosphorizing is restricted due to refractory wear. iv) The nonmetallic component of the charge materials is to be kept under control so that volume of the slag remains under limit and does not have adverse effect on the lining. v) Compared to s, Induction furnaces of very high capacities are not presently available. 16/16