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Material World
Process

Basic oxygen steelmaking

Blowing pure oxygen through molten pig iron to burn out its carbon — the step that turns brittle iron into steel, in about twenty minutes.

Iron leaves a blast furnace saturated with carbon — four per cent or so — which makes it pourable and useless for anything needing tensile strength. Steelmaking is the removal of most of that carbon, and the basic oxygen process does it by blowing pure oxygen through the melt at supersonic velocity.

The reaction is violently exothermic and needs no external heat: the carbon burning out supplies it, which is why a converter can also swallow a quarter of its charge as cold scrap. A heat of three hundred tonnes takes roughly twenty minutes, against many hours for the open-hearth furnaces it replaced, and that speed is the reason steel became cheap enough to use casually.

Uses

The great majority of the world's primary steel, made from blast furnace iron. The alternative route — the electric arc furnace — melts scrap rather than refining pig iron, and takes a growing share as recycled steel accumulates and as decarbonisation pressure rises.

"Basic" in the name is chemistry rather than simplicity: the converter is lined with basic refractory, magnesia or dolomite, so that lime added to the melt can form a basic slag and strip out phosphorus. An acid lining would be eaten by it, and phosphorus left in the steel makes it brittle when cold.

History

Bessemer's converter of 1856 established the principle — air blown through molten iron burns the carbon out — and it worked poorly with phosphoric ores, which is most European ore. Thomas and Gilchrist's basic lining solved that in 1879.

Substituting pure oxygen for air, industrialised at Linz and Donawitz in the early 1950s, removed the nitrogen that had been carried through the melt embrittling the steel and absorbing heat. Within twenty years it had displaced both Bessemer and open-hearth steelmaking almost entirely.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

  • Cast iron — alloy · as pig iron tapped from the blast furnace, carrying the carbon the process removes
  • Oxygen — element · blown through the melt at supersonic velocity; the carbon burning out supplies all the heat needed
  • Quicklime — compound · forms a basic slag that strips phosphorus and sulfur out of the melt

produces

  • Steel — alloy · most of the world's primary steel

succeeded

  • Smelting — process · smelting produces the iron; steelmaking refines it, and the two are consecutive stages rather than alternatives

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Basic oxygen steelmaking back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Basic oxygen steelmaking → takes as input (forms a basic slag that strips phosphorus and sulfur out of the melt) → Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (heated until the carbonate decomposes to lime and carbon dioxide) → Limestone → is sourced from (precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Basic oxygen steelmaking → takes as input (as pig iron tapped from the blast furnace, carrying the carbon the process removes) → Cast iron → is composed of (the balance) → Iron → is produced by (as pig iron, high in carbon and brittle until refined) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Basic oxygen steelmaking → takes as input (blown through the melt at supersonic velocity; the carbon burning out supplies all the heat needed) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Basic oxygen steelmaking → takes as input (forms a basic slag that strips phosphorus and sulfur out of the melt) → Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (supplies the silica and alumina that combine with lime in the cement kiln) → Clay → is sourced from (clay is chiefly what feldspar becomes when it weathers) → Orthoclase
  • Basic oxygen steelmaking → takes as input (forms a basic slag that strips phosphorus and sulfur out of the melt) → Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (at around 150 °C, which is driving off water rather than decomposing a carbonate — so no carbon dioxide comes out of the rock) → Gypsum plaster → is sourced from (calcined to drive off three quarters of the water, and it takes it back when mixed) → Gypsum
  • Basic oxygen steelmaking → takes as input (forms a basic slag that strips phosphorus and sulfur out of the melt) → Quicklime → is produced by (the solid residue once carbon dioxide has been driven off) → Calcination → takes as input (gently calcined to plaster of Paris, a far lower temperature than lime burning) → Gypsum

These are the most distinct paths back. Basic oxygen steelmaking can be traced through others besides.

Downstream — what it becomes

  • Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is associated with (produced in small quantities long before it could be made reliably) → Iron Age complete chain
  • Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is an input to (and the same steel becomes a spring, a cutting edge or a machinable bar depending only on the schedule) → Heat treatment → produces (laid up as fibre and resin and cured under heat and pressure, usually in an autoclave — and the cure schedule is as much of the specification as the fibre is) → Carbon fibre composite → is used as (aircraft primary structure since the 1990s, where stiffness per unit mass is what is being bought) → Structural engineering
  • Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is used in → Construction
  • Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is used as (the default structural material of the industrial world — strong in tension and compression alike, and the only common one that is) → Structural engineering
  • Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is an input to (and the same steel becomes a spring, a cutting edge or a machinable bar depending only on the schedule) → Heat treatment → produces (the same, and mostly without the autoclave, which is why it costs a fraction as much) → Glass fibre composite → is used as (hulls, tanks and blades — the cheap composite that everything else is compared against) → Structural engineering
  • Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is an input to (and the same steel becomes a spring, a cutting edge or a machinable bar depending only on the schedule) → Heat treatment → is used in (gears, shafts and springs, and case hardening to give a gear a hard face and a tough core) → Automotive manufacture

These are the most distinct paths onward. Basic oxygen steelmaking ends up in others besides.