Iron and Steel: a trip inside a steel mill

Pure iron, prepared by the electrolysis of ferrous sulfate solution, has limited use.  Commercial iron invariably contains small amounts of carbon and other impurities that alter its physical properties, which are considerably improved by the further addition of carbon and other alloying elements. By far the greatest amount of iron is used in processed forms, such as wrought iron, cast iron, and steel.  The image below shows how iron is "cast" into forms that, when cool, will be referred to as "cast iron".  The iron cast is made of sand, possibly mined in Michigan.

The differences between the various types of iron and steel are sometimes confusing because of the nomenclature used. Steel in general is an alloy of iron and carbon, often with an admixture of other elements. Some alloys that are commercially called irons contain more carbon than commercial steels.
    Modern steelmaking employs blast furnaces, like the one seen below, that are merely refinements of the furnaces used by the old ironworkers. The process of refining molten iron with blasts of air was accomplished by the British inventor Sir Henry Bessemer who developed the Bessemer furnace, or converter, in 1855. Since the 1960s, several so-called minimills have been producing steel from scrap metal in electric furnaces. Although such mills are an important component of total US steel production, the giant steel mills that create steel directly from iron ore remain essential for the initial production of steel.

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Steel is a remanufactured product that uses pig iron as its main raw material.  

Pig-Iron Production
The basic materials used for the manufacture of pig iron are iron ore, coke, and limestone.  Coke is the hard, porous residue left after the destructive distillation of coal. Used as a reducing agent in the smelting of pig iron and as a fuel, coke is blackish-gray and has a metallic luster. It is composed largely of carbon, usually about 92%. When used as a fuel, it has a high heating value of 13,800 Btu/lb.
    Coke was first produced as a by-product in the manufacture of illuminating gas. The growth of the steel industry, however, produced a rising demand for metallurgical coke, making it inevitable that coke should be manufactured as a chief product rather than as a by-product.  The earliest method of coking coal was simply to pile it in large heaps out-of-doors, leaving a number of horizontal and vertical flues through the piles. These flues were filled with wood, which was lighted and which, in turn, ignited the coal. When most of the volatile elements in the coal were driven off, the flames would die down; the fire would then be partly smothered with coal dust, and the heap sprinkled with water.
    Coke is burned as a fuel to heat the blast furnace; as it burns, the coke gives off carbon monoxide, which combines with the iron oxides in the ore, reducing them to metallic iron. This is the basic chemical reaction in the blast furnace; it has the equation:
Fe2O3 + 3CO = 3CO2 + 2Fe.

The limestone in the furnace is used as an additional source of carbon monoxide and as a "flux" to combine with the infusible silica present in the ore to form fusible calcium silicate. Without the limestone, iron silicate would be formed, with a resulting loss of metallic iron. Calcium silicate plus other impurities form a slag that floats on top of the molten metal at the bottom of the furnace. Ordinary pig iron as produced by blast furnaces contains about 92% iron 3-4% carbon, 0.5-3% silicon, and trace amounts of manganese, phosphorus, and sulfur.

Cast Iron
Cast iron is a study in contradictions. It is a symbol of strength, sturdy enough to support a massive industrial building, and of weakness, brittle enough to shatter when dropped. Tamed in a foundry furnace to a molten mass, this mineral pulled from the earth flows obediently into the intricate details of a sand mold.
    As far back as 3500 BC, Egyptians mined iron from meteorites, the only form in which it exists as a pure element. But it took another 1,500 years to figure out how to smelt it--extract it from ore where it lives as an oxidized compound. Iron was probably cast for the first time soon afterward. Europeans began casting iron in the15th century, but the black metal remained a rare and precious substance for nearly 300 years because melting iron required enormous amounts of wood for fuel. In the 1600s, England went so far as to ban cast iron production to protect its forests. Yet ironically, it was an Englishman who made possible iron’s modern ascendance. In 1709, he discovered that coke, a baked coal that burns hotter than wood or coal, could be used to efficiently smelt iron, then heat it to the 2,800F that renders it castable.
    With its 2-4.5% carbon content, cast iron is more brittle and rust-prone than its low-carbon (less than 0.03%) hammer-forged cousin, wrought iron. But the casting process is better suited to mass production than black-smithing, so the molded metal’s star rose meteorically during the Industrial Revolution. From frying pans to steam engines, from bathtubs to drain pipes, cast iron had an effect on every aspect of people’s lives. Here was a versatile, durable, easily formable material.
    Cast iron’s architectural heyday ended with the development of steel, a stronger and more durable material that is itself an iron-based alloy with a very low carbon content (0.015 to 0.5%). Today’s foundries make their cast iron mostly from recycled scrap steel, or scrap mixed with pig iron--smelted, carbon-infused iron. By throwing in scrap, they create a mix or iron, carbon, and minerals like silicon and manganese that are in modern-day cast-iron alloys.
    Melted in an electric furnace, the iron is poured into a sand mold made from wooden patterns. Pattern making is a highly skilled trade. Foundries employ teams of designers to create, on computers, exact replicas of original pieces, then hand-cut the designs in wood. The pattern is carved slightly larger than the intended product to account for cooling shrinkage.
    The wooden pattern is set into a wood form that is filled with clay-infused sand to make half of one mold; to make the other half, another sand-filled form is packed around the protruding pattern, which is then removed. The liquid iron comes to the mold via a crucible, a large bucket that travels on a track or crane from the furnace. A foundry worker ladles the white-hot liquid into the mold or, for a large cast, pours it straight from the crucible. After the iron cools and hardens, usually within hours, the sand cast is broken off and foundry workers blast away the last sand grains with metal shot. Because of its brittle nature, cast iron shouldn’t be shaped after cooling.


The recipe for steel
    Here's the recipe for a typical "batch" of molten pig iron. For each ton of molten pig iron, you need:
    2600 lbs iron ore or iron ore pellets,
    1000 lbs coke,
    and a few hundred lbs of flux (slag, calcite, dolomite, limestone, etc).  Calcite or dolomite is used to make steel. In some instances, burnt lime(manufactured by heating calcite or dolomite) is substituted. The lime in the stone or burnt lime (when melted in blast furnaces, basic oxygen furnaces, or electric furnaces) combines with the impurities in the ore or hot metal to form slag, which, because it is lighter, floats on top of the molten metal.  Take a few minutes and "walk through" the process of steel-making as nicely illustrated in the 12-step diagrams below.

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Note the steel mill below, with its blast furnaces, and the raw materials for them: piles of iron ore pellets being offloaded from the ore freighter, AND piles of crushed limestone to be used for flux.
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Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University


The image below shows a Great Lakes' freighter unloading iron ore pellets at the Algoma Steel plant, in Sault Ste. Marie, Ontario.  Note the large piles of pellets onshore, and the two large blast furnaces in the background.
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Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

A typical blast furnace consists of a cylindrical steel shell lined with a a nonmetallic substance such as firebrick. The shell is tapered at the top and at the bottom and is widest at a point about one-quarter of the distance from the bottom. The lower portion of the furnace, called the bosh, is equipped with several tubular openings or tuyeres through which the air blast is forced. Near the bottom of the bosh is a hole through which the molten pig iron flows when the furnace is tapped, and above this hole is another one for draining the slag. The top of the furnace, which is about 27 m (about 90 ft) in height, contains vents for the escaping gases, and a pair of round hoppers closed with bell-shaped valves through which the charge is introduced into the furnace. The materials are brought up to the hoppers in small dump cars that are hauled up an inclined external skip hoist.
    Blast furnaces operate continuously and are never shut down. The raw material to be fed into the furnace (see the recipe above) is divided into a number of small charges that are introduced into the furnace at 10- to 15-min intervals. Slag is drawn off from the top of the melt about once every 2 hr, and the molten iron itself is drawn off or tapped about five times a day. The air used to supply the blast in a blast furnace is preheated to temperatures between approximately 540 and 870 C (approximately 1000 and 1600 F).  An important development in blast furnace technology, the pressurizing of furnaces, was introduced after World War II. By "throttling" the flow of gas from the furnace vents, the pressure within the furnace may be built up to 1.7 atm or more. The pressurizing technique makes possible better combustion of the coke and higher output of pig iron. The output of many blast furnaces can be increased 25% by pressurizing.   Experimental installations have also shown that the output of blast furnaces can be increased by enriching the air blast with oxygen. 

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The process of tapping the blast furnace (pouring out the molten pig iron, and casting them into small rectangular blocks known as "pigs") consists of knocking out a clay plug from the iron hole near the bottom and allowing the molten metal to flow into a clay-lined runner and then into a large, brick-lined metal container, which may be either a ladle or a rail car capable of holding as much as 100 tons of metal. Any slag that may flow from the furnace with the metal is skimmed off before it reaches the container. The container of molten pig iron is then transported to the steelmaking shop.  The machine below is a pig caster, which accepts molten iron into rectangular receptacles, and allows it to harden into "pigs".  The pig casting machine below makes pigs that are 10 lbs, approximately 5" x 6" x 3" in size.  The 165 ft. (50 m.) long machine can cast 150 tons of molten iron in about ninety minutes, for a daily capacity in excess of 1,500 tons.


Here's another pig casting operation (below), in which the iron is tapped into a torpedo ladle car, which then transports it to a pig casting machine. You can see the ladle cars, which are essentially an assembly line of molds that move along as molten iron, here being poured from the blast furnace, is discharged into them.
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    The oldest process for making steel in large quantities, the Bessemer process, made use of a tall, pear-shaped furnace, called a Bessemer converter, that could be tilted sideways for charging and pouring. Great quantities of air were blown through the molten metal; its oxygen united chemically with the impurities and carried them off. In the basic oxygen process, steel is also refined in a pear-shaped furnace that tilts sideways for charging and pouring. Air, however, has been replaced by a high-pressure stream of nearly pure oxygen.  The molten iron is next cast into a variety of shapes.  In our example we will be discussing how steel that is cast into rectangular ingots or slabs (see below) is reworked into sheet steel.

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Steel
Steel is marketed in a wide variety of sizes and shapes, such as rods, pipes, railroad rails, tees, channels, and I-beams. These shapes are produced at steel mills by rolling and otherwise forming heated ingots to the required shape. The working of steel also improves the quality of the steel by refining its crystalline structure and making the metal tougher. The basic process of working steel is known as hot rolling.
    Hot rolling is especially applicable to ingots like the one shown above. In hot rolling the cast ingot is first heated to bright-red heat (2300 F) in a furnace called a soaking pit and is then passed between a series of pairs of metal rollers that squeeze it to the desired size and shape.  The distance between the rollers diminishes for each successive pair as the steel is elongated and reduced in thickness. Continuous mills roll steel strips and sheets in widths of up to 2.4 m (8 ft). Such mills process thin sheet steel rapidly, before it cools and becomes unworkable. A slab of hot steel over 11 cm (about 4.5 in) thick is fed through a series of rollers which reduce it progressively in thickness to 0.127 cm (0.05 in) and increase its length from 4 m (13 ft) to 370 m (1210 ft).  The rollers can generate up to 3500 tons of pressure per square inch; 5000-10,000 hp motors are used to turn the rollers.  The steel ingot is eventually passed to many roller mills that reduce it to the correct thickness. The rollers of mills used to produce railroad rails and such structural shapes as I-beams, H-beams, and angles are grooved to give the required shape.
    Modern manufacturing requires a large amount of thin sheet steel.  Think of all the products you use every day that have sheet steel in them: cars, appliances, buildings, desks, trucks, tables, cans, boxes, etc!  they all started out as rolls of steel like the one shown below.

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Many of Michigan's steel mills produce primarily rolled steel.  The steel is rolled "cold"--that is, it is rolled at room temperature and not at high temperatures.  Eventually, the optimal thickness of the steel is achieved, as at the exit reel area. 

And finally, the steel is "on the road", on its way to an automobile body or assembly plant.
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Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

Parts of the text on this page have been taken from "This Old House" magazine.

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.