End-Point Chemistry and Tapping
At oxygen cut, the bath contains 0.03–0.06% C for low-carbon grades or 0.08–0.15% C for medium-carbon structural grades, with temperatures of 1,620–1,660 °C. The converter tilts to the tapping position and steel flows through the taphole — a drilled refractory opening in the trunnion-side nose — into the ladle below. A refractory dart plugs the taphole initially to prevent BOF slag from entering the ladle; the dart is withdrawn once 5–10% of the heat is in the ladle and the steel stream is established.
Deoxidation and alloying additions — aluminium, ferrosilicon, ferromanganese, and any required microalloying elements — are added to the ladle via conveyor or pre-weighed hopper during tapping. Adding alloys into the falling steel stream maximises mixing energy, ensuring rapid dissolution and avoiding undissolved lumps. Lime and synthetic slag-forming fluxes are also added during tapping to establish the ladle slag chemistry for the downstream LF treatment. Temperature loss during tapping is typically 20–40 °C and is included in the tap temperature target.
Total dissolved oxygen at tap is 400–800 ppm, directly linked to tap carbon through the C–O equilibrium at steelmaking temperature. Residual dissolved oxygen above ~100 ppm after aluminium deoxidation forms alumina inclusions (Al₂O₃) that require calcium treatment in the LF to modify them to liquid calcium aluminate — preventing nozzle clogging at the continuous caster.
Productivity and Vessel Campaign Life
BOF steelmaking is characterised by exceptional throughput. A modern 300 t converter completes a full cycle — charge, blow, sublance, tap, deskulling, turnaround — in 35–45 minutes, corresponding to 30–40 heats per day. At 300 t per heat, a single vessel produces 9,000–12,000 t/day. Integrated BOF steelplants typically operate two or three vessels in rotation: while one vessel is tapping, another is being charged, and a third may be under maintenance or resting between heats.
Vessel campaign life — the number of heats between major refractory relinings — is a primary measure of operational efficiency. A modern converter, fitted with a magnesia-carbon brick lining, achieves 3,000–5,000 heats per campaign when slag splashing is practiced. Without slag splashing, campaign life typically falls to 1,500–2,500 heats. A reline requires the vessel to be taken offline for 3–7 days; campaign length therefore directly determines plant availability and annual production capacity. Refractory lining cost — brick cost plus downtime cost — is the primary motivation for the industry-wide adoption of slag splashing since its development at Nippon Steel in the 1980s.
Metallic Yield and Loss Mechanisms
Metallic yield — the fraction of charged metallic iron recovered as steel in the ladle — is typically 89–92% for a BOF heat. The remaining 8–11% of charged iron is lost through several mechanisms:
Iron oxidised to FeO in slag (~4–6% of charge): FeO is thermodynamically necessary in the BOF slag to drive the oxidation reactions, but each kilogram of FeO in the final slag represents iron lost from the heat. End-of-blow FeO in the slag typically runs 15–25% for a top-blown converter; combined blowing reduces this to 12–18% by improving bath mixing and allowing the equilibrium to approach more closely at the slag-metal interface. Reducing slag FeO from 20% to 15% recovers approximately 4 kg Fe/t steel — significant at steel volumes of hundreds of thousands of tonnes per year.
Dust and fume in off-gas (~1.5–2% of charge): The intense CO boil carries iron-rich droplets and FeO particles into the off-gas system. These are collected in the gas cleaning plant (baghouse or wet scrubber) as a zinc-bearing dust. Recovery of these dusts is complex because zinc contamination from galvanised scrap restricts recycling options — the zinc must be separated before the iron can be returned to the furnace.
Iron in slag at tap (~0.5–1%): Metallic iron droplets are entrained in the slag at tapping — particularly if the tapping angle or dart closure is imprecise, allowing slag to enter the ladle early. Each 1% FeO in the final BOF slag represents roughly 4–5 kg Fe/t steel lost. Iron loss in slag is a major economic driver for combined blowing investment and for careful end-point control.
Key BOF terms
Tap each card to reveal the definition.
Key BOF operating parameters
| param | 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 (during blow) | 1.5–3.0 m above bath |
| Lime addition | 40–60 kg/t steel |
| Slag basicity (CaO/SiO₂) | 3.0–4.0 |
| Bottom stirring gas flow | 0.05–0.15 Nm³/t·min |
| Tap temperature | 1,620–1,660 °C |
| Tap carbon | 0.03–0.15% C (grade dependent) |
| Tap-to-tap time | 35–45 min |
| Dissolved O at tap | 400–800 ppm |
| Refractory campaign life | 3,000–5,000 heats |
Typical ranges for a 300 t converter at a modern integrated steel plant.
Secondary metallurgy follows every BOF heat
The BOF delivers crude steel with dissolved oxygen of 400–800 ppm and composition held deliberately short of final specification. All quality steels — flat-rolled, structural, tubular, and engineering grades — require subsequent treatment in a ladle furnace (LF), vacuum degasser (VD/RH), or combined unit to achieve final temperature, composition, and inclusion cleanliness targets before casting. The BOF is not the last metallurgical step; it is the first.