Steel producers are the largest-volume producers in the metal sector. Within the United States, steel is produced in two types of plants: integrated mills and EAF (electric arc furnace) mills, also known as minimills. Companies that produce steel in traditional, integrated mills perform every step of the steelmaking process themselves, using blast furnaces or oxygen furnaces. This older technology has a number of drawbacks because of its negative impact on the environment, its extensive use of energy, and its relatively low yield. Also, integrated mills must be located in industrial areas, near the sources of their raw materials.
Minimills typically process scrap iron or steel and use electric arc furnaces to continuously cast metal into blooms and billets. They produce fewer finished products than integrated mills but their costs are lower because they are typically nonunion and can operate near their customers, thus reducing transportation costs. They often locate in rural areas where costs are typically lower, and they use newer technology that reduces production costs and improves quality. These minimills, which began to appear after the depression of the early 1980s, have had a substantial impact on the structure of the U.S. steel industry and have taken a growing share of the market. There are about 110 minimills operated by around 58 companies in the U.S. today, which is a decline from past years, according to a Mineral Commodity Summary by the United States Geological Survey.
In traditional steel production, three main operations are involved: the blast furnace, where iron ore is heated and combined with other elements to eliminate impurities and leave only the iron itself; the steel furnace, where steel is made by adding alloying agents such as silicon and manganese; and rolling and finishing operations, where raw steel is shaped into finished and semifinished products.
To reduce their production costs and compete more effectively with minimills, many integrated mills have changed to continuous casting to make semifinished steel. This process produces steel ready for final processing directly from liquid steel, bypassing the processing steps associated with ingot casting. This makes the steel production process more efficient because it is faster and uses less energy.
Integrated steel manufacturers are also experimenting with a process called direct steelmaking, which eliminates the need for coke ovens. This process uses a coal-based, continuous in-bath melting process with a single vessel. The three principal methods of shaping metal in plants are rolling, casting, and forging.
The rolling process simply reduces or changes the cross-sectional area of a metal piece by compressive forces exerted by rotating rolls. Metal processed in this way is most likely to be used in construction or manufacturing rather than in precision applications.
Casting, a process used in foundries, is a method of metal forming in which molten metal is poured into a prepared mold, often having a core within the mold, to form a hollow cavity within the finished metal casting. A casting, then, is a metal object formed to a predetermined shape by pouring or injecting liquid metal into a mold. The largest tonnage of castings is produced in sand molds. Other molds are made of metal, plaster, ceramics, resin-bonded sands, and chemically bonded sands.
The forging process shapes metal by hammering or by tremendous pressure that forces the metal into cavities in a set of matching dies. This produces a metal part that is unusually strong and resistant to strain. Forging workers are employed by independent forging plants and in the forging departments of automobile, steel, and farm machinery manufacturers. The metals most commonly forged are iron, steel, nickel, titanium, aluminum, and bronze.
Throughout the United States, foundries are generally either jobbing foundries or captive foundries. Jobbing foundries are operated independently, and even though they may have machining, welding, and other facilities, the final processing of finished products is not their business. They produce castings for customers. Captive foundries are owned and operated by manufacturing organizations that use them to produce castings for their own use. An automotive foundry, for example, produces thousands of brakes, engine blocks, housings, engine heads, manifolds, crankshafts, valves, and smaller castings.
Since the early 1990s, the steel investment casting business has been growing. It is well suited to producing certain types of metal products that require intricate molds. In fact, it is one of the areas in which product shipments have been matched by growth in employment. Investment casting is also used to create bronze sculptures and jewelry that has intricate designs. Investment molds are not particularly harmful to the environment and therefore are not impacted by the environmental laws that are so influential on other types of metal processing.
Secondary, or scrap, manufacturing of metals is a sector of the industry that is being revitalized. Secondary manufacturers smelt, refine, and sometimes blend metal recovered from either the shaping and trimming of primary metals during production or fabrication, or from recycled metals. Metals that lend themselves to this process include aluminum, precious metals, copper, lead, and zinc. Products that are commonly recycled include beverage cans, coins, automobiles, household appliances, and ammunition.
Jobs within the metals industry include machine setters, operators, and tenders for various types of machines as well as tools; pourers and casters; model makers and patternmakers; foundry mold and coremakers; welding, soldering, and brazing machine operators; heat treating equipment workers; and plating and coating machine workers, among many others. According to the Department of Labor, there are approximately 1.1 million metal and plastic machine workers employed in the United States.