Basic Oxygen Furnace (BOF) Steelmaking

Before charging to the BOF, hot metal frequently undergoes pre-treatment to reduce sulphur and, at some plants, silicon and phosphorus content. Hot metal desulphurisation (HMD) is the most universal step: magnesium powder or calcium carbide is injected into the torpedo ladle or hot metal ladle, reducing sulphur from 0.020–0.040% as tapped from the blast furnace to below 0.005% — or below 0.001% for demanding grades such as linepipe and automotive. Pre-treating sulphur in the torpedo ladle is far more efficient than desulphurising in the BOF or ladle furnace, because the reducing conditions of unoxidised hot metal strongly favour sulphur transfer to the desulphurising reagent.

Silicon pre-treatment — blowing oxygen into the torpedo ladle to reduce Si from 0.5–0.8% to below 0.25% — is practiced at some Japanese integrated plants where tight heat balance control and high scrap ratios are priorities. Phosphorus pre-treatment using a basic flux charge in the torpedo ladle is practiced where high-phosphorus domestic ores are the feedstock, achieving an initial phosphorus reduction before the BOF double-slag process handles the remainder.

Lance height profiling is the principal control variable in BOF blowing practice. A high lance position at blow start (2.0–3.0 m above the bath) generates a soft, spreading oxygen impact that promotes slag foaming and prevents metallic splash onto the lance and vessel mouth. As the blow progresses and bath carbon falls, the lance is lowered progressively to 1.5–2.0 m, increasing penetration depth and maintaining reaction intensity.

The dynamic blowing model — standard at all modern BOF plants — adjusts lance height, oxygen flow rate, and lime addition rate in real time using off-gas analysis. The mass flow and CO/CO₂ ratio of the converter off-gas, measured continuously at the baghouse inlet, allow the model to compute a running carbon-balance estimate of bath carbon and temperature without physical access to the vessel during the blow. The objective is to arrive at oxygen cut precisely at target carbon and temperature — avoiding a re-blow (costly in time and yield) or an over-blown condition (high slag FeO, poor metallic yield, erosive slag chemistry).

A sublance — a disposable thermocouple-and-sampler probe dropped through the lance guidance tube — is used at approximately 90% of blow completion to take a direct bath temperature reading and a carbon sample, confirming the model's predicted end-point or enabling a final correction to the remaining oxygen quantity. Modern sublance systems return a result within 45 seconds, providing data-driven confirmation before oxygen cut.

Modern BOF vessels are fitted with tuyeres in the vessel base through which nitrogen or argon is injected throughout the blow at low flow rates (0.05–0.15 Nm³/t·min). This "combined blowing" practice — bottom gas stirring plus top oxygen blowing — achieves significantly better bath mixing than top-blowing alone, with two measurable benefits: a lower residual FeO content in the slag at end of blow (improving metallic yield by 0.5–1.0%), and faster homogenisation of temperature and composition, which reduces variance in tap carbon and temperature.

Nitrogen is used as the stirring gas during the bulk of the blow, when dissolved nitrogen in the steel is acceptable. For grades with tight nitrogen specifications (<50 ppm N), argon is switched in at approximately 85% blow completion, as bath carbon drops and gas solubility rises — preventing nitrogen pickup from the stirring gas in the final minutes of the blow.