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How Steel Is Made : Carbon Steel making via the Integrated Route

In summary, carbon integrated steelmaking involves four main steps:

  • purifying coal into a high-carbon fuel called coke
  • burning the coke in a blast furnace to melt the iron ore and using limestone to help drive off impurities in the ore, thus producing a high-iron-content product called pig iron
  • combining molten pig iron with a little steel scrap in a basic oxygen furnace for a second cooking phase to remove most of the remaining carbon from the pig iron, thus producing steel
  • casting the steel into a semifinished shape that can be rolled into a variety of finished shapes.

Much of the modernization efforts of the integrated steel producers have focused on cutting the cost of each part of the process, eliminating unnecessary steps, reducing the labor involved in each step, and decreasing the amount of waste generated by the process.



The recipe for making 1 ton of pig iron in a blast furnace calls for 1¾ tons of iron ore, ¾ ton of coke, ¼ ton of limestone, and 4 tons of air. Integrated has traditionally meant owning the raw materials as well as the steelmaking plant much like an “integrated” oil producer would own both oil in the ground as well as a refinery.Interestingly, in an effort to reduce costs and balance-sheet leverage, many of the integrated producers have closed or sold mines and coke ovens, and have instead taken on long-term supply agreements for raw materials. Concurrently, more minimills are arranging for longer-term supplies of raw materials, frequently through ownership interests. Today, integrated has more to do with how the steel is made—from virgin raw materials in a blast furnace—than with who owns theraw materials.

Iron makes up about 5% of the earth’s crust. Commercial reserves of iron ore average 30-35% iron content (in a range of 25-65%). Ores with a 35% iron con-tent, which is typical for U.S. ores, must be concentrated to 60-70% iron content through a process of either crushing and roasting, magnetic separation, or chemical/gravitational flotation. Concentrated iron ore will usually be agglomerated into pellets, briquettes, or pebble-like nodules before being used in steel-making.

The use of agglomerated ore allows for proper air circulation around the ore during the reduction to pig iron in the blast furnace. Coke is made by baking coal in ovens at 1,650-2,000 degrees Fahrenheit, a process that drives off the water and impurities in the coal, leaving behind an almost pure form of carbon. Without the water and other impurities, coke is a cellular and porous material that weighs very little for the space it occupies. If the coke is not structurally strong enough, the weight of the ore burden piled onto it in the blast furnace will crush the coke and not allow for proper airflow in the cooking section of the furnace. A modern coke oven is relatively narrow (12-22 inches) but tall (6-22 feet) and deep (30-52 feet). Several coke ovens together form a battery. Because coke battery emissions pose potential environmental problems and are expensive to rebuild, coke ovens are sometimes closed when they reach the end of their useful lives. Domestic coke batteries were required to elect one of two environmental compliance standards, or tracks, several years ago. Those that elected the MACT track (maximum achievable control technology) are compliant until 2003 but must meet a standard to be set in October 2001. Most of these batteries will likely be closed in 2004. Batteries electing the LAER track (lowest achievable emission rate) will get a new, more difficult standard assigned in 2010,but most will probably be okay until 2020. New, “recovery” batteries, which convert recovered waste heat into electrical power, are available; therefore, a coke shortage is not anticipated


The Blast Furnace

The ingredients of coke, iron ore, and limestone are continuously conveyed and poured, or charged, into the top of a giant, 10-story blast furnace. The inside of a blast furnace will include a hearth that might be 30 feet in diameter and 20 feet high with an 85-foot working space, or stack, above it. The temperature inside the blast furnace is approximately 3,500 degrees Fahrenheit. Preheated air is forced, or blasted, into the furnace (hence the name blast furnace), which causes the coke to burn furiously and produce carbon monoxide.

As the ingredients sift and sink to the bottom of the furnace and the heat breaks down the virgin iron ore, the carbon monoxide attracts the oxygen from the iron oxide ore to form carbon di-oxide, which exhausts. The residue is a purer iron material known as pig iron,which gathers in a liquid state at the bottom of the furnace. In the lower portion of the blast furnace, molten limestone attracts other impurities in the cooking ore and floats them on the bath of molten pig iron forming in the bottom of the furnace. The limestone layer is called slag, which attracts certain elements and repels others as those elements precipitate out of the molten solution. After enough molten pig iron has accumulated in the blast furnace, a tap hole is opened on the bottom of the furnace and the pig iron is tapped, or poured,into vessels for transport to the next step in the steelmaking process. The slag is processed for other markets, principally as construction aggregates.


The Basic Oxygen Furnace (BOF)

Blast furnace pig iron, containing 3-4% carbon, can be used to make cast iron or refined further to make steel. To make steel, the molten pig iron from the blast furnace is poured into a basic oxygen furnace (BOF), where most of the remaining carbon will be removed. Both limestone, to remove impurities, and some scrap steel, to act as a coolant, are added to the oxygen furnace at this point to improve the refining process. A BOF might be 30-35 feet high and have an inside diameter of over 20 feet. In a BOF, pure oxygen is blown (at speeds up to Mach 2.3) through a long tube,or lance, inserted into the furnace. As the carbon and silicon in the mixture oxidize,a tremendous amount of heat is released, and the scrap melts into the molten mass. The oxygen acts to remove the carbon, which otherwise would make the steel brittle. The batch of refined metal in the BOF is referred to as a heat,and sometimes steel company management will refer to heats per shift or a 200-ton heat, etc. A Q-BOP is a type of basic oxygen furnace in which oxygen is blown into the bottom of the furnace rather than through a lance at the top. Other variations of a BOF exist, including some with both top and bottom blowing. After about 20 minutes of oxygen blowing in the BOF, slag is poured off the top of the molten bath in one direction, and the steel is poured in the other direction onto a huge bucket, or ladle, up to 20 feet tall. Increasingly the common practice is to place the ladle into a ladle metallurgy ladle met station, which is much like a miniature electric arc furnace using electrical energy to further refine the heat of steel.

By using the ladle met station, the producer gains both greater control over the chemistry of the steel and better throughputs through the basic oxygen furnace. Once satisfied with the chemistry of the molten steel, it is teemed, or poured, from the ladle into either a mold-casting operation to produce an ingot, or more frequently today, into a continuous caster to produce a slab, billet, or bloom. 

After the steel is cast into one of these shapes, it is referred to as semifinished.Operations in the blast furnace can be changed a bit from the basic approach outlined here. For example, pulverized coal can be injected (PCI) into the blast furnace to replace more expensive coke. Oxygen can be injected as well to stimulate the burning of the coke and enhance furnace output. How material is charged into the furnace can be an area of efficiency gains. How much scrap is put into the basic oxygen furnace can change its output potential; however, the amount will have implications for other furnace variables, such as required refining time and energy. Some methods of altering furnace operation are more expensive but are worthwhile in a tight market when selling prices are also higher.

Similarly, when the market weakens, output can be reduced slightly while leaving operations technically full. These kinds of operating practices have helped the variable cost performance of the industry on the margin.

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