11 Modern Steelmaking Processes 1. Charge Materials Topics to discuss 1. Metallics 2. Auxiliary charges 3. Oxidants 4. Fuels 5. Refractories
Raw Materials Principal raw materials used for steel manufacture metallics (metallic charge, metallic additions), auxiliary charge (fluxes, deoxidisers), oxidants, fuels, and refractories 3/28 1. Metallics 1.1 Metallic Charge The primary source contains (a) (b) pig iron (liquid or solid), and solid products of direct conversion of iron from iron ores (a.k.a. sponge iron or DRI) The secondary source (a) Ferro-alloys (a) steel scrap (also include pig iron scrap in some cases). ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, etc. (used primarily for deoxidation and alloy additions) 4/28
On average, 1130-1140 kg metallic charge used per ton of steel produced The proportion pig iron and scrap used for making steel depends on the type of steelmaking process In electric steelmaking, the use of pig iron is very low (less than 5%) and the metallic charge consists mainly of scrap or a semi-product premelted in a separate plant. In open-hearth processes, the charge is roughly 55% pig iron (the balance is scrap) In converter steelmaking, 70-85% pig iron (the balance is scrap). 5/28 Pig iron In an integrated steelmaking plant, liquid iron, rather pig iron, is used The composition of liquid/solid iron produced is always controlled to suit the steelmaking process. Merchant pig iron (in solid form, 15-45 kg/pc) merchant pig iron Acid, hematite or Swedish iron (0.05% max. P) used in acid steelmaking processes Basic iron (0.2-0.4% P) general purpose pig iron used in basic steelmaking processes Thomas iron (1.5% min. P) used in special basic steelmaking processes 6/28
Steel Scrap Steel scrap consists of discarded steel or steel products Generally segregated by composition and size or grade suitable for melting. A scrap is off grade if it fails to meet (i) (ii) applicable size limitations, applicable requirements for the type of scrap, and (iii) applicable requirement with respect to the scrap quality. Three main types of scrap : (i) internal scrap / home scrap 30-45% of total scrap generated spillage, sheared ends, rejected materials this scrap of known composition is mainly used at the same plant as metallic charge (ii) prompt scrap / process scrap 18-20% of total scrap chips, trimming, forging and stamping wastes (iii) dormant scrap / obsolete scrap 30% of the total scrap used or worn-off machines, rails, domestic appliances 7/28 Hydraulically compressed #1 bundle scrap #1 Heavy melting scrap Shredded steel scrap Hydraulically compressed #2 bundle scrap #2 Heavy melting scrap Different types of steel scrap Busheling 8/28
Cast iron scrap Steel turnings Internal re-melt scrap (home scrap) Rail scrap Ship scrap Different types of steel scrap Internal re-melt scrap (ladle scrap) 9/28 Collection of scraps is carried out by specialised organisations. returned to steel works either assorted or in classified forms. often composition is not known exactly. often contaminated with (1) sulphur-containing lubricating oils (2) non-ferrous metals (lead, aluminium, tin, copper, etc.), and (3) soil, rust, plastics, etc. some of these impurities are harmful for the working personnel, steelmaking plants and some influence the melting practice and steel quality steel quality. An appreciable percentage of scrap, is rather inconvenient for charging into steelmaking furnaces (low density, low loose mass, inconvenient size, contamination with soil, etc.). Proper preparation of scrap (sorting, removing dirt and plastics, bundling loose scraps, etc.) can (1) raise productivity of steelmaking shop (by reducing the time of charging), (2) increase yield of steel (by lowering oxidation losses), and (3) improve quality of finished product (by reducing inclusions). 10/28
Proportion of scrap consumption in various steelmaking processes Conventional processes Oxygen steelmaking processes Bessemer Up to 8% Bessemer (with modification) Up to 12% Open-hearth* Up to 75% Electric* Up to 100% LD Up to 25% LD (with modification) Up to 45% Kaldo and Rotor Up to 45% *The open-hearth and the electric processes were developed chiefly to remelt the available scrap. Until the advent of these two processes, there was no way to melt steel scrap because of the limitations of the furnace not being capable of attaining steelmaking temperatures i.e.150-1600 C. 11/28 Sponge iron, or DRI The products of direct reduction of iron ores containing 90-98% iron is usually used in steelmaking plants, mainly to dilute the impurities, as it contains the lowest residual impurities. Specifications of metallised materials, also known as direct reduced iron (DRI) or hot briquetted iron (HBI), or sponge iron to be used in steelmaking should satisfy the following requirements: Metallisation degree, % > 90 Iron content, % > 90 Oxygen content (as FeO), % 2 Sulphur content, % < 0.01 Phosphorus content, % < 0.045 Gangue, % 4 Some special features of DRI practically free from impurity, which are common in steel scrap contains 5-8% gangue (SiO 2 ) more slag, more CaO required porous and low density needs briquetting contain 1-2.5% C contains ~2% FeO needs reducing porous, oxidises easily and self-ignites to be careful in storing
1.2 Metallic Additions or, Ferro Alloys Added principally to improve properties like tensile strength, ductility, fatigue strength and corrosion resistance. Additionally, there can be several other tasks for ferroalloys refining deoxidation control of non-metallic inclusions and precipitates added as iron alloys to the bath to lower cost, and increase melting and dissolution rate (by reducing melting point of the alloy) 13/28 analysis of some common ferro alloys of standard grades Chemical Composition, wt% Ferro alloy Grade C Si Mn Cr Other Fe Ferro silicon 50-90% Si 1-2 50-90 -- -- -- Balance Ferro manganese Standard HC 7 -- 78-82 -- -- Balance Low carbon <0.50 6.5 85-90 -- -- Balance Silico manganese EM <0.08 28-32 56-61 -- -- Balance Standard 1 22 66 -- -- Balance High carbon 7 max. 3 max. -- 58-65 -- Balance Ferro chrome Low carbon <0.75 2 max. -- 67-73 -- Balance LC 65/5 <0.5 5 max. -- 64-68 -- Balance Simplex LC <0.02 2 max. -- 68-71 -- Balance Ferro moly 0.5 1.5 -- -- Mo 60-67 Balance Ferro vanadium 0.1 1.25 -- -- V 50 Balance Ferro niobium 0.25 4 max. 2 max. -- Nb 63.5 Balance Ferro titanium 0.1 2 max. -- -- Ti 30-40 Balance Besides chemical composition and residual elements, size of ferroalloys is also important 14/28
2. Auxiliary Materials Materials include: limestone lime bauxite, fluorspar, crushed fireclay, and mixtures and briquettes of one or more of those previously mentioned (e.g., mixture of lime and fluorspar or lime and bauxite). Used as fluxes to bring down the softening point of gangue materials and reduce viscosity of slag, and increase slag basicity, and decrease activity of some components to make it stable in the slag phase. 15/28 Typical characteristics of auxiliary materials Material Typical Composition, wt% Principal Activity Limestone 50-54 CaO, 0.5-3.0 MgO, 0.6-1.0 SiO 2, 0.01 S Increase slag basicity Calcines dolomite 55 CaO, 34-38 MgO, 3-3 SiO 2, 0.10 S Increase slag basicity Lime 90-95 CaO, 2-3 MgO, 1.5 SiO 2, 0.1-0.2 S Increase slag basicity Ganister 0.5 CaO, 0.1 MgO, 94 SiO 2 Increase slag acidity Bauxite 54-56 Al 2 O 3, 11-14 Fe 2 O 3, 1-2 SiO 2, 1-2 TiO 2 Decrease slag viscosity Fluorite 75-95 CaF 2, 10 max. SiO 2, 0.8 max. S Decrease slag viscosity Crushed fireclay 35 Al 2 O 3, 60 SiO 2 Quickly decrease slag viscosity of high-basic slag Mixtures and briquettes Mixture of lime-dolomite, lime-bauxite, etc. Accelerate slag formation SiO 2 in dolomite or lime should be kept to a minimum since it decreases the available base (%CaO %SiO x Slag basicity). Presence of MgO in limestone is also detrimental to its quality since MgO is not as effective as CaO in retaining phosphorus and sulphur in slag. 16/28
3. Oxidants Added to the bath to accelerate the oxidation of carbon and other impurities. Used either in the solid state (iron ore, sinter, ore pellets, rolling scale, fluxed sinter, briquetted ore fines) or, in the gaseous state (compressed air, oxygen, various mixtures including oxygen, steam, carbon dioxide, etc.). Solid oxidant should have a high concentration of iron oxide and the least content of silica a high density where possible. Gaseous oxidants should be clean and have as low nitrogen (<0.5%) as possible to ensures proper conditions for making steels free from nitrogen. 17/28 4. Refractories Inorganic non-metallic material, capable to withstand high temperature (~1600 C) without undergoing physico chemical changes while remaining in contact with chemically reactive molten slag, metal and gases. Have a crucial impact on the cost and quality of steel products. The diversification on steel products and their cleanliness requirement in recent years caused an increased demand for high quality refractory. It becomes necessary to produce range of refractory materials with different properties to meet range of processing conditions. 18/28
Why required? To minimize heat losses from the reaction chamber To allow thermal energy dependent conversion of chemically reactive reactants into products because metallic vessels are not suitable. Refractory materials are required to withstand: 1. A wide range of temperature, up to 2200 C. 2. Sudden changes in temperature to cause thermal shock, resulting crack/fracturing. 3. Compressive stresses at both high and low temperatures. 4. Abrasive forces at both high and low temperatures. 5. The corrosive action of slags, ranging from acidic to basic in character. 6. great pressures and buoyant forces of molten metal 7. The corrosive action of gasses/volatile oxides/salts of metals. 19/28 Steelmaking furnaces are lines with suitable refractory materials, which erode during steel making and hence the material of lining is also required as a recurring consumable raw material. The lining of furnace is made either by laying bricks (or blocks) or by shaping the required contour in situ using a refractory mix. Often, the lining is repaired after certain number of heat to maintain it in a proper shape and state. 20/28
Common manufacturing/installation methods Brick Castable precast/vibrated/self-flow Shortcrete (wet gunning) Gunning Plastic Ramming material Mortar Insulation board/blanket 21/28 Refractory Classification Refractory materials and products are classified by several features chemical composition (acid, basic or normal) physical form (brick or monolithic/ ramming mass/castable/mortar/plastic mass) refractoriness (high (>2000 C)/medium (1770-2000 C)/low (1580-1770 C)) porosity and slag permeability strength density spalling resistance thermal expansion and thermal conductivity 22/28
A. Based on chemical composition 1. Acid refractory raw materials: SiO 2, ZrO 2 and alumino-silicate Typical refractories are fireclay, quartz and silica. 3. Neutral refractory chemically stable to both acids and bases manufactured from alumina, chromia, and carbon 2. Basic refractory raw materials: CaO, MgO, dolomite, chrome-magnesite. produced from dead burnt limestone, dolomite, magnesite, chrome ore. a) Magnesite cannot resist thermal stock, loose strength at high temperature and are not resistant to abrasion. b) Chrome-magnesite good resistance to chemical action of basic slag and mechanical strength and volume stability at high temperatures. c) Magnesite-carbon excellent resistance to chemical attack by steelmaking slags 23/28 B. Physical Form 1. Shaped refractories (bricks) have standard dimensions. machine pressed and have uniform properties. Special shapes are hand molded different types are: (i) Ramming refractory material (dry/wet) (ii) Castables (iii) Mortars (iv) Plastic mass 3. Insulating materials common insulating materials: ceramic fibres (produced from molten SiO 2, TiO 2, Zr O2, etc) in the form of wool, short fibres and long fibres 2. Monolithic refractories (using ramming mass) loose materials used to form joint free lining can be installed by casting, spraying etc. used mostly in cold condition so that desired shapes can be obtained with accuracy. main advantages grater volume stability better spalling tendency eliminating joint can be installed in hot standby mode easier transportation 24/28
Property Requirements Refractoriness Porosity and slag permeability Strength Specific gravity Resistance to spalling Permanent linear change on heating Thermal conductivity Bulk density 25/28 Properties of some selected refractories Refractory Major Composition, wt% Refractoriness, % Porosity, % Thermal Resistance at 1600 C Abrasion Resistance Resistance Against Acidic Slag and Fluxes Resistance Against Basic Slag and Fluxes Silica 93-96 SiO 2 1700 16-17 Good Good Fair Poor Low Alumina 40-45 SiO 2 35-55 Al 2 O 3 1700 20-23 Good Good Good Medium High Alumina 1750 20-29 Good Good Good Poor Magnesite 80-95 MgO <2000 20-23 Poor Medium Poor Good Dolomite 58 CaO 40 MgO 1650 20 Poor Medium Poor Good Carbon 85-90 C <2000 22-32 Good Poor Good Fair Chromite 15-23 Al 2 O 3 15-30 Cr 2 O 3 10-17 Fe 2 O 3 14-20 MgO 1800-1950 18-25 Fair Medium Fair Good Carborundum 89-91 SiC <2000 17-20 Good Good Good Poor Zirconia 67 ZrO 2 2500 - Good Good Good Poor 26/28
Refractory Consumption The melting and casting of steel require an enormous quantity (nearly 30 kg per ton of steel) of refractory materials. Life of the working lining usually ranges from 20-100 heats. Stability of refractories determines not only their consumption, but also the productivity of steelmaking plant (the frequency and extent of repairs), and the quality of steel produced. Modern techniques of the off-furnace treatment of metal (inert-gas blowing, vacuum treatment, etc.), which are associated with intense metal stirring, require refractories of especially high quality. 27/28 Measures to decrease refractories consumption: 1. using better quality refractory; 2. collecting and reusing refractory waste left from furnace repair; 3. running the heats properly according to the operating possibilities of the refractories; 4. replacing, fully or partially, the refractory lining by cooled (say, watercooled) elements; and 5. repairing the working layer of refractory lining periodically 28/28