What is Steel? Structural Shapes and Alloys


Steel is any of a range of alloys of iron that contain less than 2 percent carbon. Ordinary structural steel, called mild steel, contains less than three tenths of 1 percent carbon, plus traces of beneficial elements such as manganese and silicon, and of detrimental impurities such as phosphorus, sulfur, oxygen, and nitrogen. In contrast, Ordinary cast iron contains 3 to 4 percent carbon and greater quantities of impurities than steel, while wrought iron contains even less carbon than most . Carbon content is a crucial determinant of the properties of any ferrous (iron-based) metal: Too much carbon makes a hard but brittle metal (like cast iron), while too little produces a malleable, relatively weak material (like wrought iron). Thus, mild steel is iron whose properties have been optimized for structural purposes by controlling the amounts of carbon and other elements in the metal.

The process of converting iron ore to steel begins with the smelting of ore into cast iron. Cast iron is produced in a blast furnace charged with alternating layers of iron ore (oxides of iron), coke (coal whose volatile constituents have been distilled out, leaving only carbon), and crushed limestone . The coke is burned by large quantities of air forced into the bottom of the furnace to produce carbon monoxide, which reacts with the ore to reduce it to elemental iron. The limestone forms a slag with various impurities, but large amounts of carbon and other elements are inevitably incorporated into the iron. The molten iron is drawn off at the bottom of the furnace and held in a liquid state for processing into steel.

Most steel that is converted from iron is manufactured by the basic oxygen process , in which a hollow, water-cooled lance is lowered into a container of molten iron and recycled steel scrap. A stream of pure oxygen at ver y high pressure is blown from the lance into the metal to burn off the excess carbon and impurities. A ß ux of lime and ß uorspar is added to the metal to react with other impurities, particularly phosphorus, and forms a slag that is discarded. New metallic elements may be added to the container at the end of the process to adjust the composition of the steel as desired: Manganese gives resistance to abrasion and impact, molybdenum gives strength, vanadium imparts strength and toughness, and nickel and chromium give corrosion resistance, toughness, and stiffness. The entire process takes place with the aid of careful sampling and laboratory analysis techniques to ensure the finished quality of the steel and takes less than an hour from start to finish.

Today, most structural steel for frames of buildings is produced from scrap steel in so-called  mini-mills, utilizing electric arc furnaces. These mills are miniature only in comparison to the conventional mills that they have replaced; they are housed in enormous buildings and roll structural shapes up to 40 inches (1 m) deep. The scrap from which structural steel is made comes mostly from defunct automobiles, one mini-mill alone consuming 300,000 junk cars in an average year. Through careful metallurgical testing and control, these are recycled into .

Where steel without any protective finish will remain exposed to exterior conditions in the completed , weathering steel (ASTM A588) may be specified. This steel alloy develops a tenacious oxide coating when exposed to the atmosphere that, once formed, protects against further corrosion and eliminates the need for paint or another protective coating. While mostly used for highway and bridge construction where it reduces maintenance costs, weathering steel also finds occasional use in buildings, where the deep, warm hue of the oxide coating can be exploited as an aesthetic feature. With the addition of nickel and chromium to steel, various grades of stainless steel (ASTM A240 and A276) with even greater corrosion resistance and costing significantly more than conventional structural steel, can be produced. Steel can also be protected from corrosion by galvanizing, the application of a zinc coating.

In the structural mill or breakdown mill, the beam blank is reheated as necessary and then passed through a succession of rollers that squeeze the metal into progressively more refined approximations of the desired shape and size. The finished shape exits from the last set of rollers as a continuous length that is cut into shorter segments by a hot saw. These segments are cooled on a cooling bed. Then a roller straightener corrects any residual crookedness. Finally, each piece is cut to length and labeled with its shape designation and the number of the batch of steel from which it was rolled. Later, when the Regardless of the particular steelmaking process, finished steel is cast continuously into beam blanks or blooms, ver y thick approximations of the desired final shape, which are then rolled into final form, as described later in this chapter.

Steel Alloys

By adjusting the mix of metallic elements used in the production of steel, its strength and other properties can be manipulated. Mild structural steel, known by its ASTM designation of A36, was for decades the predominant steel type used in building frames. But today is mini-mills, using scrap as their primary raw material, routinely produce stronger, less expensive high-strength, low-alloy steels, such as those designated ASTM A992 or ASTM A572. ASTM A992 steel is the preferred steel type for standard wide-ß angel structural shapes, while ASTM A36 steel, or, where higher strength is needed, ASTM A572 steel, are specified for angles, channels, plates, and bars. Piece is shipped to a fabricator, it will be accompanied by a certificate that gives the chemical analysis of that particular batch, as evidence that the steel meets standard structural specifications.

The roller spacings in the structural mill are adjustable; by varying the spacings between rollers, a number of different shapes with the same nominal dimensions can be produced (Figure 11.10). This provides the architect and the structural engineer with a finely graduated selection of shapes from which to select each structural member in a building, thereby avoiding the waste of steel through the specification of shapes that are larger than required.

Wide-flange shapes are used for most beams and columns, superseding the older American Standard (I-beam) shapes American Standard shapes are less efficient structurally than wide ß angles because the roller arrangement that produces them is incapable of increasing the amount of steel

Examples of the standard shapes of structural steel

Figure 11.10

Examples of the standard shapes of structural steel. Where two shapes are superimposed, they illustrate different weights of the same section, produced by varying the spacing of the rollers in the structural mill. Structural steel shapes and their general requirements are defined in ASTM A6. Bars are round, rectangular, and hexagonal solid shapes generally not greater than 8 inches (203 mm) in any cross-sectional dimension. Wider solid shapes are called or sheet, depending on their thickness in relation to their width. Plate is thicker than sheet.

in the ß angles without also adding steel to the web, where it does little to increase the load-carrying capacity of the member. Wide ß angles are available in a vast range of sizes and weights. The smallest available in the United States is nominally 4 inches (100 mm), and the largest is 44 inches (1117 mm). Weights per linear foot of member range from 9 to 730 pounds (13Ð 1080 kg/m), the latter for a nominal 14-inch (360mm) shape with ß angles nearly 5 inches (130 mm) thick. Some producers construct heavier wide-ß angle sections by welding together ß angle and web plates rather than rolling, a procedure that is also used for producing ver y deep, long-span plate girders.

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