BASIC OXYGEN STEELMAKING The Processing Route

INTRODUCTION

Nowadays, about 67% of the total worldwide crude steel production is made by a Basic Oxygen Steelmaking processing route which involves the following stages:

• BLAST FURNACE (BF)
• BASIC OXYGEN FURNACE (BOF)
• SECONDARY METALLURGY
• CONTINUOUS CASTING (CC)

BLAST FURNACE (BF)

A typical blast furnace consists of a cylindrical steel shell lined with a refractory. The shell is tapered at the top and at the bottom. The lower portion of the furnace, called the bosh, is equipped with several tubular openings or tuyeres through which the air blast is forced. Near the bottom of the bosh there is a hole through which the molten pig iron flows when the furnace is tapped, and above this hole, but below the tuyeres, there is another hole for draining the slag. The top of the furnace, which is about 25-30 m in height, contains vents for the escaping gases, and a pair of round hoppers closed with bell-shaped valves through which the charge is introduced into the furnace. The materials are brought up to the hoppers in small dump cars or skips that are hauled up an inclined external skip hoist.

The basic materials used for the manufacture of pig iron (also called hot metal) are iron ore, coke, and limestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke gives off carbon monoxide, which combines with the iron oxides in the ore, reducing them to metallic iron. The limestone in the furnace charge is used as an additional source of carbon monoxide and as a “flux” to combine with the infusible silica present in the ore to form fusible calcium silicate. Calcium silicate plus other impurities form a slag that floats on top of the molten metal at the bottom of the furnace. Ordinary pig iron as produced by blast furnaces contains about 92 % iron, 3 to 4 % carbon, 0.2 to 0.8 % silicon, 0.25 to 2.5 % manganese, 0.08 to 0.18 % phosphorous and a trace of sulfur.

Here below the chemical reactions that are involved into the blast furnace process:

BASIC OXYGEN FURNACE (BOF)

From the blast furnace, molten pig iron (hot metal) is poured into a torpedo car and subsequently into a ladle. The metal in the ladle is sent directly to the basic oxygen furnace (BOF) or to a pretreatment stage.

A water cooled lance is lowered down into it. This lance blows 99% pure oxygen into the converter, igniting the carbon dissolved in the steel and burning it to form a carbon monoxide and carbon dioxide, causing the temperature to rise about 1700 °C. This melts the scrap, lower the carbon content of the molten iron and helps remove unwanted chemical elements. Moreover, fluxes (like burnt lime) are fed into the vessel to form slag, which absorbs impurities of the steelmaking process.

Typical BOF has a vertical vessel lined with refractory bricks. Only 8-12 % of the volume is filled with the treated molten steel. The converter is capable to rotate about its horizontal axis. This rotation is necessary for charging raw materials, sampling the melt and pouring the steel out of the converter.

At the end of the entire process the BOF is tilted again and steel is poured into a giant ladle.

SECONDARY METALLURGY

The ladle is transferred to a device called Ladle Furnace (LF), where it's places under a cover equipped with three graphite electrodes columns which are connected to an AC transformer.

The primary functions of the LF is the reheating of liquid steel through electric power conducted by the graphite electrodes and homogenization of steel temperature and chemistry through inert gas stirring. In fact the ladle bottom has a porous plug which allows the gas inert argon supply. Moreover the LF stand is also equipped with an addition hopper mounted on the cover (ferroalloys addition) and a lance for injection of desulfurizing agent.

During the treatment, molten steel is covered by a layer of slag. The fumes formed during the operation are extracted through a suction system installed at the cover.

Additional devices, such as RH degasser, VD (vacuum degasser) could be adopted in the secondary metallurgy in order to produce special kind of steel.

CONTINUOUS CASTING (CC)

The molten metal is now cast through a mold, the casting takes the two dimensional profile of the mold but its length is indeterminate. The casting will keep traveling downward and its length increases with time. New molten steel is constantly supplied to the mold with a specific rate to keep up with the solidifying casting.

Molten metal, from the ladle, is poured into a tundish, which is a container that is located above the mold, it holds the liquid metal for the casting. This particular casting operation uses the force of gravity to fill the mold and to help move along the continuous metal casting. The tundish is constantly supplied with molten steel to keep the process going. The whole process is controlled to ensure there is smooth flow of molten steel through tundish.

Further, the impurities and slag are filtered in tundish before they move into the mold. The entrance of the mold is filled with inert gases to prevent reaction of molten steel with the gases in the environment like oxygen. The molten metal moves swiftly through the mold and it does not completely solidify in it. The entire mold is cooled with water that flows along the outer surface.

The metal casting moves outside the mold with the help of different sets of rollers. While one set of rollers bend the metal cast, another set will straighten it. This helps to change the direction of flow of the steel slab from vertical to horizontal.

Author: Eng. Matteo Sporchia

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