Process Overview
The Basic Oxygen Furnace — also called the LD converter after its Austrian birthplace of Linz and Donawitz — accepts a charge of approximately 70–80% liquid hot metal and 20–30% cold steel scrap. A water-cooled oxygen lance is lowered to within 1.5–3 m of the melt surface and high-purity oxygen (≥99.5%) is blown at supersonic velocity (Mach 2) at flow rates of 500–900 Nm³/min. The blow oxidises carbon to CO and CO₂, silicon to SiO₂, manganese to MnO, and phosphorus to P₂O₅, all of which report to the slag. The exothermic heat from these oxidation reactions — totalling around 1,000 MJ per heat — provides all the energy needed to heat the bath to tapping temperature; no external fuel is required.
Charge Composition and Heat Balance
Hot metal from the blast furnace typically contains 4.0–4.5% C, 0.3–0.8% Si, 0.2–0.5% Mn, and 0.08–0.12% P. The silicon content is the principal driver of heat generation: 1% Si in the hot metal liberates roughly 160 MJ/t, equivalent to allowing an additional 60–80 kg of scrap per tonne of hot metal. Steelmakers therefore balance hot metal Si content, scrap ratio, and target tap temperature in a pre-blow heat balance calculation. Lime (CaO) additions of 40–60 kg/t are made at the start of the blow to form a fluid, foamy slag with basicity (CaO/SiO₂) of 3.0–4.0, which is essential for efficient phosphorus removal.
Blowing Practice and Lance Height
Lance height profiling is a critical control variable. A high lance position at blow start promotes slag foaming and prevents metallic splash; the lance is progressively lowered through the blow to increase the carbon removal rate as the bath cools and the reaction front migrates deeper. Modern plants use dynamic blowing models that adjust lance height, oxygen flow rate, and lime addition rate in real time using off-gas analysis (CO/CO₂ ratio and flow rate) to predict end-point carbon and temperature. Sublance measurements — a disposable probe dropped through the lance — provide a bath temperature and carbon sample at approximately 90% blow completion, enabling final corrections before oxygen cut.
Key BOF Operating Parameters
| Parameter | Typical value |
|---|---|
| Converter capacity | 150–400 t |
| Hot metal / scrap ratio | 70–80% HM / 20–30% scrap |
| Oxygen blow time | 12–18 min |
| Oxygen flow rate | 500–900 Nm³/min |
| Lance height (blow) | 1.5–3.0 m above bath |
| Lime addition | 40–60 kg/t steel |
| Slag basicity (CaO/SiO₂) | 3.0–4.0 |
| Tap temperature | 1,620–1,660 °C |
| Tap carbon | 0.03–0.15% C |
| Tap-to-tap time | 35–45 min |
Typical ranges for a 300 t converter at a modern integrated steel plant.
End-Point Chemistry and Tapping
At oxygen cut, the bath typically contains 0.03–0.06% C for low-carbon grades or up to 0.15% C for medium-carbon grades, with temperatures of 1,620–1,660 °C. The converter is tilted and steel tapped into a ladle through a taphole in the vessel trunnion. Deoxidation alloys (aluminium, silicon, ferromanganese) and slag-forming fluxes are added to the ladle during tapping to kill the heat and begin secondary metallurgy chemistry adjustment. Slag splashing — blowing nitrogen through the oxygen lance after tap to coat the vessel lining — extends refractory life to 3,000–5,000 heats.
Productivity and Tap-to-Tap Time
BOF steelmaking is characterised by extremely high throughput: a modern 300 t converter completes a heat — charge, blow, sample, tap, deskulling — in 35–45 minutes, corresponding to 30–40 heats per day per vessel. At 300 t per heat this yields 9,000–12,000 t/day per converter, which is why integrated BOF-route plants account for the majority of the world's crude steel production. Converter availability is managed by rotating vessels through maintenance campaigns for refractory relining.
Important Note on Secondary Metallurgy
The BOF delivers a crude steel that is high in oxygen (~500–800 ppm dissolved O). All quality steels require secondary metallurgy treatment in a ladle furnace (LF), vacuum degasser (VD/RH), or combined unit (CAS-OB) to achieve final composition, temperature, and cleanliness targets before casting.