Steel
Steel is an alloy primarily made from iron and carbon, along with various other elements such as chromium, nickel, and manganese, which enhance its properties. This versatile material is known for its strength, malleability, durability, and resistance to corrosion, making it a crucial component in construction and manufacturing industries globally. The development of steel has a rich history dating back to around 2000 B.C.E, with early forms including wrought iron. Over time, various methods of production emerged, including cementation, crucible, and the revolutionary Bessemer process, which allowed for mass production of high-quality steel in the 19th century.
Modern steel-making techniques have evolved further, with processes like the basic oxygen process and electric arc furnaces optimizing efficiency and quality. Steel is foundational in constructing infrastructure, such as buildings and bridges, and is integral to manufacturing vehicles and machinery. Its many forms and alloys cater to a wide range of applications, reflecting its significance in both historical and contemporary contexts. Understanding steel's properties and production methods can provide valuable insights into its role in technological advancement and industrialization.
Steel
A variety of related alloys make up the products called “steel.” Since large-scale steel manufacturing processes were developed in the mid-nineteenth century, steel has been essential to the construction and transportation industries. Worldwide raw steel production exceeds 1 billion metric tons a year.
Background
Steel is produced when iron ore is combined with alloying elements—including carbon, chromium, nickel, manganese, molybdenum, silicon, and tungsten—and reduced by high heat to a molten state. Immediately after molten steel becomes solid, it is malleable, making it a highly versatile construction material. Steel has been produced with countless alloys, each of which lends unique characteristics to the finished product.
Because steel can be produced relatively cheaply and because it is durable, malleable, strong, corrosion-resistant, and ductile, it is among the most important products worldwide, especially in industrial nations. Depending on how it is made, it also can be heat-resistant, heat-conductive, and magnetic-permeable, a versatility that makes it indispensable in building automobiles, locomotives, airplanes, ships, and a host of other products that serve the transportation industry. Its value in the construction of buildings is incalculable. Before steel was mass-produced and developed in specialized ways, the skyscrapers that form the skylines of most major cities could not be built.
Steel’s Early History
Early iron production dates to about 2000 b.c.e., probably beginning in Anatolia (Turkey). The manufacture of iron for making weapons, instruments, and various utensils was well established one thousand years later. By 500 b.c.e., the Iron Age had touched all of Western Europe; there is evidence that a century later it had reached China.
Wrought iron has a longer history than steel, but some of the early wrought iron, whose carbon content ranged from 0.07 percent to 0.8 percent, qualified as steel. When iron has a carbon content above 0.3 percent, it possesses the qualities of steel. It becomes strong and, when quenched with cold water, very brittle. When it is reheated, however, in the process called tempering, the brittleness is reduced, increasing the metal’s malleability substantially.
The history of steel dates back three and a half millennia, probably to ancient inhabitants of what is now called Armenia. Sometime around 1500 b.c.e., it is speculated, a fire raged over a landmass that had iron ore deposits close to its surface. As the iron ore melted and mixed with the elements that surrounded it, certainly silicon and probably at least trace elements of several other minerals, an alloy was formed. When people discovered the properties this alloy had and realized its unique qualities, they devised ways of repeating the process under controlled conditions.
It is thought that the earliest manufactured steel was produced in small ovens or furnaces stoked with charcoal whose carbon mixed with iron ore to form the alloy. The draft that kept the fire blazing probably came from a bellows made from goat skin or from some other readily available material. As air was blown into the fire, oxygen, still a crucial component of steelmaking, became part of the mix.
Cementation
Cementation, one of the most effective ways of producing steel, began in Europe in the early seventeenth century. In this process, wrought iron is mixed with carbon in an environment from which most of the air has been removed. Through this process, the outer edges of wrought iron are hardened to the point that they can be sharpened for use as weapons. Sometimes a piece of strong, tempered steel was welded to a low-carbon wrought iron base. The result was a serviceable weapon that had a sharp cutting edge. Although much steel at this time was produced by trial and error, some formidable weapons were produced throughout the Middle Ages and into the seventeenth and eighteenth centuries. Another means of achieving cementation was to stack sheets of soft iron that were carbon-free in alternating layers with sheets of iron with a high carbon content and to heat the mass, then work it into a combined product that was stronger than wrought iron.
When the cementation process was applied to the mass production of steel in the mid-eighteenth century, bars of wrought iron were put into clay boxes. Each bar was surrounded by charcoal and put into a furnace, which was closed tightly to limit the amount of air that entered it. The boxes were then heated at high temperatures so that the carbon from the charcoal mixed with the molten iron to yield a product with sufficient carbon content to qualify as steel.
Crucible Method
Around 1740, Benjamin Huntsman, a British clock and instrument manufacturer, found that clock springs made through the cementation process were unsuited to the intricate work he was doing. He experimented with melting steel produced through cementation in a large retort to assure a more even distribution of carbon and other elements that would result in a more standard, homogeneous form of steel.
Huntsman cut cemented steel into small pieces that he then melted in a closed clay crucible. After he skimmed off the slag, he poured the remaining molten steel into a mold, where it became solid. Then he could work the resulting ingot into any form he required by reheating and hammering it. His product was much purer than the steel produced solely through cementation, making it a much better material than was previously available for making precision instruments.
Bessemer Process
The cementation and crucible processes were replaced in the last half of the nineteenth century by the Bessemer process, invented almost simultaneously in England by Henry Bessemer and in the United States by William Kelly. Bessemer’s English patent was granted in 1856, Kelly’s U.S. patent in the following year. The Bessemer process can be either an acid or a base process, depending on whether the lining of the refractory is acid or base. When an acid such as sandstone is used, molten steel is made beneath an acid slag that covers the molten surface. In this process, compressed air is blown through holes in the bottom of a pear-shaped converter. oxidation of most of the carbon, manganese, and silicon occurs. This oxidation produces the heat required for the process.
The base process invented by Sidney Thomas was not patented until 1879. It works on a principle similar to that of the acid process except that it uses a base such as dolomite or magnesite rather than an acid in the refractory. The resulting steel, made under a basic slag, is absent the phosphorus and most of the sulfur in the original pig iron.
The Bessemer process made possible for the first time the mass production of low-cost, high-quality steel, an essential component of most technological progress in the late nineteenth century and throughout most of the twentieth century. The first Bessemer steel factory was established in Michigan in 1864 and within a year was turning out the steel rails that made possible the building and rapid expansion of a transcontinental railway system, an essential link in the chain of westward expansion in the United States.
By 1886 the United States, which provided steel rails for the world, was producing about 2.7 million metric tons of steel a year, making it the world’s leading steel manufacturer. The Bessemer process made this possible. It was the chief method of making steel until 1907, when the open-hearth process, which had been gaining in popularity since the earliest days of the Bessemer process, essentially replaced it.
Open-Hearth Process
The steel industry quickly became so central to the industrial and economic development of the United States and other steel-producing countries that experimentation was constantly afoot to find new and better methods of producing steel and of refining and improving the product. As early as 1856, the year in which the Bessemer process was first patented in Britain, Carl Friedrich von Siemens invented the regenerative process of heating, later improved upon by his brother Karl Wilhelm. In regeneration, the direction of the gases used to heat furnaces is reversed in an alternating pattern so that the heat left by the trapped gases is held and used to preheat gases as they enter the vessel. The result is a considerable increase in temperature with a minimal expenditure of fuel.
By 1870 an acid furnace that employed this principle was developed in Boston. The heat it produced was sufficient to melt pig iron and scrap steel, removing carbon and undesirable impurities in the molten mass through oxidation. Because the steel was melted on a hearth beneath the factory roof and could be seen, inspected, and sampled through furnace doors, the method came to be called the “open-hearth” process. In this process, the carbon in the pig iron is oxidized by the oxygen in the iron ore, producing carbon monoxide. Ferromanganese is added for deoxidization when the carbon content of the molten steel reaches an appropriate level. It is then poured into molds that form it into ingots.
The acid open-hearth furnace was soon replaced in the United States by the basic open-hearth furnace, whose basic lining was magnesite brick that had magnesite or burned dolomite spread over it. This process was an improvement over the Bessemer process because of its ability to remove phosphorus from the molten steel, a great advantage in the United States, many of whose largest deposits of iron ore were phosphorus-laden.
The first commercial application of the basic open-hearth process occurred in Pennsylvania in 1888. Within two years, sixteen such furnaces were operating in the United States; by 1950, nine hundred basic open-hearth furnaces were in operation. By the mid-twentieth century, such furnaces accounted for 90 percent of the steel production in the United States. This process had fallen out of favor, however, by 1970, when the basic oxygen process began replacing it in many venues. The open-hearth process was an improvement over the Bessemer process because it used larger quantities of scrap steel than could be used in Bessemer furnaces. Also, the basic open-hearth process eradicated more impurities from the iron ore it used than earlier processes had.
Basic Oxygen Process
The basic oxygen process consists of inducing pure oxygen under high pressure to enter the furnace through a water-cooled tube at the top of the basic refractory. This process removes impurities from the molten iron, which is then poured as molten steel into a ladle. This method offers the quickest, most economical means of converting molten iron to steel on a large scale.
Electric Furnace Method
The electric furnace process uses electricity to heat scrap metal in a furnace, thereby reducing the scrap metal to a molten state. This method is advantageous because it uses considerably more scrap metal than any of the previous processes used in mass-producing steel.
The electric arc furnace, invented in 1899 by Paul Héroult, was first used essentially for making precision instruments. Through the years, however, this furnace was able to produce large quantities of plain carbon steel. The use of the electric-arc furnace accelerated during World War II, when there was an urgent need to use as much scrap steel as possible.
Currently this type of furnace, which has a basic bottom of dolomite or magnesite with basic or silica brick sidewalls and roof, can hold a charge of more than 400 metric tons. Advances in electric delivery and technology have made possible the expanded use of Héroult-type furnaces, whose popularity has spread throughout the steelmaking industry.
A variation on the electric-arc furnace is the induction furnace, whose use is limited mostly to producing small quantities of the specialized steel used for making precision instruments. In this process, a round chamber is surrounded by a copper coil through which electrical current is passed after the furnace has been loaded. As the current vibrates within the chamber, extremely high temperatures, sufficient to reduce the charge quickly to a molten state, are produced.
Rolling
Perhaps the most important modern steel-manufacturing process is rolling. In this process, steel ingots are removed from their molds in a stripper, after which they are immersed in deep, refractory-lined furnaces and heated to temperatures of nearly 1,100° Celsius. The ingots, after reaching the requisite temperature, are then removed from the furnace and taken on an ingot buggy to huge tables in the rolling mill, where they are placed horizontally on the tables for primary rolling. Here the ingots are flattened by passing through two rollers revolving in opposite directions at the same speed. The primary mill produces blooms, billets, and slabs that are then taken to other mills to be transformed into the specific steel products that industry requires.
Other means of preparing steel for its eventual specialized uses include drawing, forging, extrusion, and casting. Cold drawing is used to produce sizes that cannot be achieved with the same precision by the various hot-working methods available. Forging, probably the oldest method of steel processing, involves the hammering and shaping of hot metal into the forms in which it can best be used. The extrusion method involves a hydraulic process by which hot molten steel is forced under high pressure through a die into a cylinder at one end that has been shaped into the desired configuration. Casting occurs when molten steel is poured into a mold of a given shape and size that will result, on cooling, in a product of the shape and size desired.
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