Blast Furnace Steelmaking
Integrated steel mills use basic chemical elements to create steel. They create steel "from scratch," so to speak. To combine iron ore and coke (carbon, a product of coal), the ingredients in pig iron, integrated mills use massive furnaces called blast furnaces. Inside the blast furnace, the temperatures can soar to well above 3000 degrees Fahrenheit. Iron ore and coke are poured in through the top, and chemical reactions combine the elements into pig iron, which empties out of the bottom. The pig iron then gets reduced into steel using a basic oxygen furnace.
Blast furnaces alone require billions of dollars in capital to build and maintain, so to make a profit, integrated steel mills need large order books. These mills must operate continuously - because the operational temperatures of the blast furnaces are so high, the refractory brick contracts and crumbles if they are turned off. And if the blast furnaces are producing pig iron, the rest of the operations must be running to consume the hot metal. One advantage of these mills is the total control they have over the steelmaking process; they able capable of producing steel with very low residuals, which, when present in higher quantities, may constrain the quality of the steel.
1. Creating pig iron in the blast furnace
Blast furnaces use heated blast gases to drive chemical reactions that produce pig iron (iron with 4-5% carbon) from iron ore. The ore and coke are poured in layers through the top of the furnace, and as they liquefy into pig iron they are emptied into vessels called torpedo cars at the bottom of the furnace.
2. Reducing the carbon content in the basic oxygen furnace to make steel
The torpedo car carrying the hot pig iron is then brought to the steelmaking shop. Here the pig iron is transferred to the basic oxygen furnace, which uses oxygen to reduce the carbon content and impurity levels to make steel. The oxygen is blown in at supersonic velocities, and the results exothermic reactions raise the temperature of the steel to about 2900 degrees Fahrenheit.
3. Refining at the ladle metallurgy station and vacuum degassing
The composition of the steel is refined at the ladle metallurgy station. Argon is used to stir the steel and improve homogenization, final alloys are added, and desulfurization occurs. Final chemistry checks take place here, and this one heat of steel is ready for casting.
Some integrated mills also have a vacuum degassing station to further improve the chemistry of the steel by removing dissolved gases.
4. Continuous Casting
The steel from the ladle metallurgy station is poured into a tundish, which looks like a giant bathtub that feeds the continuous caster. The caster is made from a water-cooled copper mold, which is essentially a vertical box-shaped funnel that oscillates up and down. Steel is poured into the top of the mold, and steel in contact with the cooled copper begins to solidify and form a shell that holds the liquid steel. As the steel is extruded from the caster, it is cut into lengths.
After casting, the slab is transferred to a storage area to cool. Once it is time for the slab to be rolled, it will be brought to the hot strip mill where the slab will be heated in a furnace before rolling.
Integrated steel mills produce much thicker slabs during casting - usually around 8 to 10" thick slabs as opposed to the 2 to 3" slabs produced by mini mills. This gives the integrated mills an advantage, because there's a higher probability that surface-level defects produced during casting will "roll out," or get reduced to the point of disappearance.
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