Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 4: Steel

Steel is one of several products derived from iron ore, traditionally made in a blast furnace by combining the iron ore with carbon (originally in the form of charcoal, then refined coal or coke), which acts as a "reducing agent" by drawing off oxygen from the iron oxides of the ore; and limestone, which removes impurities by forming a slag of lighter density which can be scraped off the top as the ingredients are heated to a molten mix.

The product of such a process is pig iron which contains quite a large percentage of carbon, and can be reheated and formed directly into cast iron. But cast iron, while strong in compression, is hard and brittle, especially in tension, and therefore not suitable for use as a modern structural material. Wrought iron is made by driving virtually all the carbon from the iron mix, which results in a material that is more ductile than cast iron, and can be safely stressed in tension. However, it is softer, more malleable, and less strong, making it also less suitable for structural use, especially when an alternative (mild carbon steel) became available at or about the beginning of the twentieth century which combined some of the hardness and compressive strength of cast iron with the ductility and tensile strength of wrought iron. While all three of these materials — cast iron, wrought iron, and steel — were still in use in 1900, mild carbon steel soon became the de facto standard and is now the only iron-based (ferrous) structural material in use (the other two remain in use primarily as ornamental materials and railings, or as pipes in the case of cast iron). A summary of the three iron-based (ferrous) metals follows:

  1. Wrought iron contains 0.05–0.1% carbon. It is soft, malleable, and resists corrosion.

  2. Plain carbon steel contains less than 2% carbon (while mild carbon steel contains less than 0.3% carbon); it is strong, stiff, and ductile in both compression and tension.

  3. Cast iron contains more than 1.71% carbon. It is hard, brittle, strong in compression, but weak in tension. It is used in pipes and for ornamental metal applications.

Note that carbon is the most important ingredient in terms of the structural properties that result. Alloys of steel can be made by adding other ingredients into the mix; two examples are weathering steel (Cor-ten is one proprietary brand name) and stainless steel (which contains chromium and/or nickel).

In the U.S., wide-flange (W) shapes are no longer commonly manufactured from iron ore in a "basic oxygen" process, but are made almost entirely from recycled cars in an "electric arc" furnace, a continuous casting process that takes approximately three hours from car to W-shape.