Steelmaking / BOS Plant

What is steel?

Steel consists in the majority of iron but with high levels of carbon e.g. 4% removed and a controlled amount remaining. The amount of carbon in the steel is very important to the way the material behaves after heat treatment. Table 1 gives some indication of these properties.

What else is added to steel?

Various additives are used to improve the basic carbon steel properties; for example molybdenum is used to improve the hot strength and hardness, nickel improves toughness and ductility. The different properties required of the steel for different uses (e.g. hammer head, car body) will dictate the type of steel used and the way it is treated by controlled heating and hot and cold rolling. Table 2 gives further details on steel additives.

BOS is a sequential process centering around the converter positioning and contents processing. Automation of the total process is possible from centrally stored process recipe information to the front end instrumentation, while still allowing manual intervention and re-processing steps.

The converter is a vessel that tilts 180° from the upright vertical on the charging side, and over 90° on the tapping side. The converter is tilted to one of several pre-determined angles during the sequence as shown in Figure 2.

The converter is rotated to its charging position to be loaded with scrap metal (50 tonnes), molten iron (300 tonnes), lime (15 tonnes) and the ‘basic’ fluxes magnesium and limestone (6 tonnes).

The converter returns to the vertical and the main lance lowers, injecting oxygen at high velocity onto the molten metal surface for 15 to 20 minutes.

After injecting a pre-determined quantity of oxygen, the ‘blow’ is complete and the main lance is withdrawn. At 80% and 100% oxygen flows as calculated by a thermochemical model, a sub lance is lowered to measure temperature which is used to calculate carbon arrest.

Assuming that temperature and sample analysis are acceptable, the converter is rotated from vertical to tapping position.

After tapping, the converter is rotated back to its slag position for emptying occasionally, returned to the vertical for addition of thickeners, then gently rocked for slag-wash prior to emptying which is conducted by taking the converter to the opposite (180°) vertical position.

Practically, there are other intermediate steps depending on other variables. For example:

• re-blows may be necessary according to temperature or analysis
• an anti-slop interruption may be necessary to reduce slop by increasing the inert gas flows for a period
• increased inert gas flow stirring may be needed to reduce temperature

Throughout the cycle, inert gases such as nitrogen and argon (during blow and re-blow) continuously flow to agitate or stir the converter contents, and to keep the converter stirring elements clear for the duration of the converter campaign.

The result is steel of very low carbon content. The percentage needed for the type of steel being manufactured will be higher than this, so a controlled amount of carbon is added to meet the specification.

After the metal has been poured, the cycle starts again and if no problems delay the steelmakers, typically 8 ‘blows’ will be achieved in one shift of 8 hours producing nearly 3,000 tonnes of steel.

Converter angle control

The converter is positioned to the desired angle by comparing the corresponding absolute desired angle with the absolute actual angle and driving the positioning motors in the correct direction.

The positioning motors are driven progressively at increasing and subsequently decreasing speeds according to the error size and time intervals until the position is within the tolerance band set.

Gas flows

The BOS process uses expensive gases (Argon, Oxygen and Nitrogen) and the accurate measurement and totalization of these gases assists economic operation and tight quality control by using these values in the generation of spreadsheet reports and logs.