Process Overview
The Basic Oxygen Furnace — also called the LD converter, from Linz and Donawitz in Austria where VOEST developed the process in 1952 — is the dominant steelmaking vessel worldwide, accounting for approximately 71% of global crude steel production. The furnace is a barrel-shaped, refractory-lined steel vessel of 150–400 tonnes capacity, mounted on a trunnion ring that allows it to tilt for charging and tapping. A water-cooled oxygen lance is lowered through the open top to within 1.5–3 m of the melt surface.
The charge consists of approximately 70–80% liquid hot metal from the blast furnace, arriving at 1,280–1,380 °C, and 20–30% cold steel scrap. Once charged, the vessel is brought upright and the oxygen blow begins. High-purity oxygen (≥99.5%) is injected at Mach 2 velocity at flow rates of 500–900 Nm³/min — the entire blow lasting only 12–18 minutes.
During the blow, dissolved silicon, manganese, carbon, and phosphorus are oxidised in a sequence governed by thermodynamic stability. Silicon and manganese oxidise first, within the initial 2–3 minutes. Carbon oxidation — the "carbon boil" — dominates the middle period as CO bubbles violently from the bath. Phosphorus is removed toward the end of the blow once sufficient slag basicity is established. The net result of all these exothermic oxidation reactions is that the bath temperature rises from approximately 1,300 °C at charge to 1,620–1,660 °C at tapping, with no external fuel required.
Pause and think
The BOF blow raises 300 tonnes of metal by roughly 300 °C in under 20 minutes — yet no fuel is burned. Where does this energy come from?
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. Each element contributes to the heat balance through its exothermic oxidation. Silicon is the dominant heat source: every 0.1% Si in the hot metal liberates approximately 16 MJ/t of charge, equivalent to allowing an additional 6–8 kg of cold scrap per tonne of hot metal.
The heat balance calculation — performed before each heat using a process control model — sets the scrap charge weight based on available hot metal temperature and silicon content, target tap temperature, and any cooling agents (iron ore, sinter, or limestone). A typical 300 t heat uses approximately 230–250 t of hot metal and 50–70 t of scrap. Higher-silicon hot metal (>0.5% Si) allows up to 30% scrap; lower-silicon hot metal (<0.3% Si) may restrict scrap to 15–18%.
Lime additions of 40–60 kg/t steel are made at the start of the blow and during the early stages. Lime dissolves progressively into the slag as the temperature rises. Achieving a fully dissolved, fluid slag with basicity (CaO/SiO₂) of 3.0–4.0 is the key challenge of the first half of the blow: undissolved lime particles retard reaction rates and can cause slopping. Fluorspar (CaF₂) or pre-melted lime flux are used as dissolution aids when lime uptake is slow.
Vessel Design and Refractory
The BOF converter is a barrel or pear-shaped steel shell mounted on a trunnion ring that allows the vessel to rotate through 360° for charging, tapping, and deskulling. A 300 t converter has an internal diameter of approximately 8–9 m at the widest point and an internal height of 9–10 m. The trunnion mounting transmits the full vessel weight — typically 500–700 t for a lined, steel-charged converter — through a robust support structure to the floor bearings.
The refractory lining is the critical wear component. Dolomite or magnesia-carbon (MgO-C) bricks, 0.8–1.2 m thick in the wear zone, line the vessel. MgO-C bricks resist both the high temperature (>1,700 °C peak at the lance impact zone) and the corrosive action of the highly basic, high-FeO slag. Lining thickness is monitored by laser profiling after every heat — a robotic laser scanner measures the profile of the lining from the vessel mouth and detects worn zones before they become critical.
Slag splashing is the key technique that transformed converter campaign life from ~1,000 heats (early LD era) to 3,000–5,000 heats today. After tapping, nitrogen is blown through the lance at high flow rate, throwing the residual slag against the vessel walls and mouth. The slag solidifies on the lining surface, building up a protective ceramic coating over the worn brick. The process takes 2–4 minutes and adds no cost beyond nitrogen consumption. Slag splashing was developed at Nippon Steel in the 1980s and is now universal practice worldwide. Without it, the economics of BOF steelmaking — where reline cost and downtime are major production cost components — would be substantially worse.