Iron and Steel: Blast Furnace, Alternative Ironmaking, Slags, Primary and Secondary Steelmaking, and Continuous Casting

A material balance tracks each element in and out: iron (almost all of it reports to the hot metal), carbon (from coke and injectant, leaving in the hot metal and the top gas), oxygen (from the ore and the blast), and the gangue oxides, which with the flux form the slag. Slag volume is fixed by the gangue: SiO₂ and Al₂O₃ from ore and coke ash must be fluxed with CaO and MgO to a target basicity. A heat balance sets the heat released (combustion of carbon at the tuyeres to CO, sensible heat of the hot blast, the exothermic indirect reduction) against the heat consumed (direct reduction and the Boudouard reaction, calcination of any raw limestone, melting and superheating metal and slag, the sensible heat of the top gas, and losses through the shell and cooling staves). The Rist operating diagram combines the two into one straight line on oxygen-carbon coordinates, and its slope is the carbon rate. Two quick balances recur in GATE numericals: the ore rate from the iron balance, and the flux rate from the basicity.

Ore fines cannot be charged directly because they choke the gas flow, so they are agglomerated. Sintering mixes fines with coke breeze, flux and return fines, lays the moist mix on a travelling grate, ignites the top and draws air down through the bed; the combustion front passes through, partially melting the mix and bonding it into a porous cake that is crushed and screened. Adding the flux here gives self-fluxing (and super-fluxed) sinter, which moves the calcination out of the furnace and improves reducibility. Pelletising is for very fine concentrates: they are rolled into green balls of about 9-16 mm with a binder (bentonite), dried and indurated at about 1250-1350 °C on a travelling grate or in a grate-kiln, giving strong, uniform, porous pellets that travel well and reduce evenly. Burden quality is tested for cold strength (tumbler index, shatter index), reducibility, reduction degradation at low temperature, and softening-melting behaviour.

Metallurgical coke is made by carbonising a blend of coking coals at about 1000-1100 °C for 16-20 hours in slot ovens, usually by-product ovens that recover tar, benzol and ammonia, then quenched by water or dry quenching. In the furnace it is the fuel, the reductant, the source of the carbon that dissolves in the iron, and — below the cohesive zone, where it is the only solid left — the permeable grid through which gas rises and metal and slag drain. So its tests measure both cold strength (the micum M40 and M10 indices and shatter and tumbler tests) and hot behaviour: the Coke Reactivity Index (CRI), the weight loss on reacting with CO₂ at 1100 °C for two hours, and the Coke Strength after Reaction (CSR), the tumbler strength of what remains. A good blast-furnace coke has low CRI and high CSR, low ash, low sulphur and a narrow size range.

Direct reduction reduces iron ore in the solid state, below its melting point, to direct reduced iron (DRI, or sponge iron) that retains the gangue and is melted later, usually in an electric arc furnace. Its quality is expressed by the degree of metallisation, metallic Fe/total Fe × 100. Coal-based DRI is made in rotary kilns: lump ore or pellets and non-coking coal with a desulphurising agent tumble through an inclined kiln against a counter-current of hot gas, reduction occurring by CO generated in the bed, with careful control to stay below the temperature at which the charge sticks and forms accretions. Gas-based DRI dominates world production; in the MIDREX process natural gas is reformed with top gas (CO₂ and H₂O) over a catalyst to a hot CO + H₂ gas that is blown into a vertical shaft, where it reduces a descending bed of pellets and lump ore at about 800-900 °C; the product is discharged cold or as hot briquetted iron, typically above 90% metallised.

Smelting reduction produces liquid hot metal without coke. COREX, the first commercial process, has two reactors: an upper reduction shaft in which lump ore and pellets are reduced to DRI of high metallisation by gas from below, and a lower melter-gasifier in which non-coking coal is gasified with pure oxygen, generating that reducing gas and the heat to melt the DRI into hot metal and slag of blast-furnace quality. It avoids coke ovens and sinter plants, which is its main advantage, at the cost of a large oxygen plant and a large export gas stream. Its fluidised-bed successor, FINEX, takes ore fines directly.

Molten silicate slags are ionic liquids built on the SiO₄⁴⁻ tetrahedron. Pure silica is a fully connected three-dimensional network, every oxygen bridging two silicon atoms. Network formers (SiO₂, P₂O₅, B₂O₃) build it; network modifiers (CaO, MgO, FeO, MnO, Na₂O) supply O²⁻ ions that break bridging oxygens into non-bridging ones, depolymerising the network into chains, rings and finally isolated tetrahedra; amphoteric oxides (Al₂O₃, and Fe₂O₃) act as either. Basicity is expressed as the ratio CaO/SiO₂ (the V-ratio), or with more terms such as (CaO + MgO)/(SiO₂ + Al₂O₃), or as optical basicity. As basicity rises the network breaks down, so viscosity falls (it also falls with temperature) until the solid-oxide saturation limit is reached, and the activity of free O²⁻ — the slag’s capacity to absorb sulphur and phosphorus — rises.

Desulphurisation, (O²⁻) + [S] = (S²⁻) + [O], needs a basic slag (many O²⁻), low oxygen potential in the metal (deoxidised steel, or the carbon-saturated hot metal of the blast furnace) and high temperature; the slag’s sulphide capacity C_S and the sulphur partition ratio L_S = (%S)/[%S] measure it. Dephosphorisation, 2[P] + 5[O] + 3(O²⁻) = 2(PO₄³⁻), is an OXIDATION: it needs a basic slag, a HIGH oxygen potential (high FeO in the slag) and LOW temperature, since the reaction is strongly exothermic. So sulphur and phosphorus want opposite oxygen potentials, and a converter, which is oxidising, is good at removing phosphorus and poor at removing sulphur — which is why hot metal is desulphurised before the converter and steel after it, in the ladle.

Steelmaking is controlled oxidation of hot metal. In the basic oxygen furnace (BOF, or LD converter) high-purity oxygen is blown at supersonic speed through a water-cooled lance onto a bath of hot metal and scrap under a lime-rich slag. The Ellingham order explains the sequence: silicon oxidises first, then manganese, then carbon, with phosphorus going to the slag as the basic, FeO-rich slag forms. All these oxidations are exothermic, so the process needs no fuel and melts 15-25% scrap as coolant. Decarburisation proceeds in three periods: an initial one while silicon is removed, a middle one at a maximum rate set by the oxygen supply, and a final one, below roughly 0.3% C, in which the rate is controlled by carbon mass transfer to the reaction sites and falls with the carbon content. The carbon-oxygen equilibrium, [C] + [O] = CO(g), makes the product [%C][%O] nearly constant at a given temperature and p_CO — about 0.002-0.0025 at 1600 °C and 1 atm — so the lower the final carbon, the higher the dissolved oxygen that must later be removed. Combined (top and bottom) blowing stirs the bath with inert gas through bottom tuyeres, bringing the melt closer to equilibrium and lowering the FeO in the slag and the oxygen in the steel.

Kinetically, CO cannot form homogeneously in the melt: a bubble must overcome the pressure of the metal above it plus the surface-tension term 2σ/r, which for a nascent bubble is enormous. CO therefore nucleates heterogeneously — on refractory pores, on the gas-metal interface of injected oxygen or argon, on metal droplets ejected into the slag, where the emulsion of metal, slag and gas gives a huge reaction area. The electric arc furnace melts a cold charge of scrap, DRI or hot metal with arcs from graphite electrodes, supplemented by oxygen lancing and carbon injection. Practice today uses ultra-high-power transformers, water-cooled panels, eccentric bottom tapping (EBT) for slag-free tapping, and a foamy slag — made by injecting carbon so that CO bubbles swell the slag — that buries the arc, shields the walls from radiation and raises thermal efficiency. The EAF has become a melting unit; refining is completed in the ladle.

Steel tapped from the BOF or EAF carries several hundred ppm of dissolved oxygen and must be deoxidised in the ladle. Deoxidisers are chosen from the Ellingham diagram: manganese is weak, silicon stronger, aluminium strongest (with calcium and titanium also used). Each gives an equilibrium [%M]ˣ[%O]ʸ product, and the oxide it forms — Al₂O₃ clusters, silicates — remains as non-metallic inclusions that must be floated out. Steels are classified by deoxidation: killed (fully deoxidised with Al and Si, no gas evolution on solidification), semi-killed, and rimmed (little deoxidation, CO evolved in the mould). Argon stirring through a porous plug or lance homogenises temperature and composition, promotes inclusion flotation and speeds slag-metal reactions; the ladle furnace adds arc reheating so that the ladle can be held and trimmed. Desulphurisation to below 0.005% uses a synthetic CaO-CaF₂ or CaO-Al₂O₃ slag with vigorous argon stirring in well-deoxidised steel, or calcium injection.

Vacuum degassing exploits Sieverts’ law for hydrogen and nitrogen and the C-O reaction for carbon: lowering the pressure lowers the equilibrium [H] and [N] as √p and drives [C] + [O] = CO forward, which is vacuum decarburisation for ultra-low-carbon steel. The RH (Ruhrstahl-Heraeus) degasser circulates steel through two snorkels into a vacuum vessel, argon lift gas in one leg driving the flow; VD and tank degassers put the whole ladle under vacuum with argon stirring. Inclusion shape control modifies the inclusions that remain: calcium treatment converts solid alumina clusters, which clog nozzles, into liquid calcium aluminates that are globular and float out, and replaces elongated MnS stringers, which make rolled plate weak in the through-thickness direction, with hard, globular CaS; rare-earth additions do the same.

Stainless steel needs at least about 10.5% Cr and, for most grades, very low carbon. The difficulty is that chromium oxidises along with carbon when oxygen is blown: at a given temperature the equilibrium between [Cr], [C] and p_CO means that removing carbon without losing chromium to the slag requires either a very high temperature or a low partial pressure of CO. Argon-oxygen decarburisation (AOD) blows oxygen diluted with argon (progressively richer in argon as the carbon falls) through side tuyeres into a converter: the argon lowers p_CO, so carbon is removed preferentially, and any chromium oxidised is recovered by a final reduction with ferrosilicon. Vacuum-oxygen decarburisation (VOD) reaches the same end by lowering the total pressure. The electric arc furnace melts the charge, and the AOD or VOD refines it.

In continuous casting, steel flows from the ladle into a tundish, which distributes it to one or more strands and gives inclusions a last chance to float, and through a submerged entry nozzle into a water-cooled, oscillating copper mould. A thin solid shell forms against the mould wall; the strand is withdrawn continuously, supported by rolls and cooled by water sprays in the secondary cooling zone until it is fully solid, then straightened and cut to length. The mould oscillates with a period of negative strip, when it moves down faster than the strand, to stop the shell from sticking and tearing; mould powder melts on the meniscus to lubricate the shell, insulate the steel surface and absorb inclusions. Shell growth follows the parabolic law d = K√t (with K of order 20-30 mm/min^½), so the metallurgical length — the distance from the meniscus to the point of final solidification — is L = v t_f = v (D/2K)² for a strand of thickness D cast at speed v. Electromagnetic stirring in the mould or strand breaks up the columnar front and increases the equiaxed zone.

Thin slab casting casts slabs about 50-90 mm thick, against about 200-250 mm for conventional slab casters, using a funnel-shaped mould that leaves room for the submerged nozzle at the top. The thin slab solidifies faster, casts at higher speed and goes hot, through a tunnel furnace for equalisation, directly into a short hot-strip mill — compact strip production — eliminating the roughing mill and the reheating furnace, with large savings in energy and capital. Its challenges are mould-level turbulence and heat-transfer control at high speed.