Iron and Steel Manufacture

V

Open-Hearth Process

Essentially the production of steel from pig iron by any process consists of burning out the excess carbon and other impurities present in the iron. One difficulty in the manufacture of steel is its high melting point, about 1,370° C (about 2,500° F), which prevents the use of ordinary fuels and furnaces. To overcome this difficulty the open-hearth furnace was developed; this furnace can be operated at a high temperature by regenerative preheating of the fuel gas and air used for combustion in the furnace. In regenerative preheating, the exhaust gases from the furnace are drawn through one of a series of chambers containing a mass of brickwork and give up most of their heat to the bricks. Then the flow through the furnace is reversed and the fuel and air pass through the heated chambers and are warmed by the bricks. Through this method open-hearth furnaces can reach temperatures as high as 1,650° C (approximately 3,000° F).

The furnace itself consists typically of a flat, rectangular brick hearth about 6 m by 10 m (about 20 ft by 33 ft), which is roofed over at a height of about 2.5 m (about 8 ft). In front of the hearth a series of doors opens out onto a working floor in front of the hearth. The entire hearth and working floor are one story above ground level, and the space under the hearth is taken up by the heat-regenerating chambers of the furnace. A furnace of this size produces about 100 metric tons of steel every 11 hr.

The furnace is charged with a mixture of pig iron (either molten or cold), scrap steel, and iron ore that provides additional oxygen. Limestone is added for flux and fluorspar to make the slag more fluid. The proportions of the charge vary within wide limits, but a typical charge might consist of 56,750 kg (125,000 lb) of scrap steel, 11,350 kg (25,000 lb) of cold pig iron, 45,400 kg (100,000 lb) of molten pig iron, 11,800 kg (26,000 lb) of limestone, 900 kg (2,000 lb) of iron ore, and 230 kg (500 lb) of fluorspar. After the furnace has been charged, the furnace is lighted and the flames play back and forth over the hearth as their direction is reversed by the operator to provide heat regeneration.

Chemically the action of the open-hearth furnace consists of lowering the carbon content of the charge by oxidization and of removing such impurities as silicon, phosphorus, manganese, and sulfur, which combine with the limestone to form slag. These reactions take place while the metal in the furnace is at melting heat, and the furnace is held between 1,540° and 1,650° C (2,800° and 3,000° F) for many hours until the molten metal has the desired carbon content. Experienced open-hearth operators can often judge the carbon content of the metal by its appearance, but the melt is usually tested by withdrawing a small amount of metal from the furnace, cooling it, and subjecting it to physical examination or chemical analysis. When the carbon content of the melt reaches the desired level, the furnace is tapped through a hole at the rear. The molten steel then flows through a short trough to a large ladle set below the furnace at ground level. From the ladle the steel is poured into cast-iron molds that form ingots usually about 1.5 m (about 5 ft) long and 48 cm (19 in) square. These ingots, the raw material for all forms of fabricated steel, weigh approximately 2.25 metric tons in this size. Recently, methods have been put into practice for the continuous processing of steel without first having to go through the process of casting ingots.

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VI

Basic Oxygen Process

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. After the furnace has been charged and turned upright, an oxygen lance is lowered into it. The water-cooled tip of the lance is usually about 2 m (about 6 ft) above the charge although this distance can be varied according to requirements. Thousands of cubic meters of oxygen are blown into the furnace at supersonic speed. The oxygen combines with carbon and other unwanted elements and starts a high-temperature churning reaction that rapidly burns out impurities from the pig iron and converts it into steel. The refining process takes 50 min or less; approximately 275 metric tons of steel can be made in an hour.

VII

Electric-Furnace Steel

In some furnaces, electricity instead of fire supplies the heat for the melting and refining of steel. Because refining conditions in such a furnace can be regulated more strictly than in open-hearth or basic oxygen furnaces, electric furnaces are particularly valuable for producing stainless steels and other highly alloyed steels that must be made to exacting specifications. Refining takes place in a tightly closed chamber, where temperatures and other conditions are kept under rigid control by automatic devices. During the early stages of this refining process, high-purity oxygen is injected through a lance, raising the temperature of the furnace and decreasing the time needed to produce the finished steel. The quantity of oxygen entering the furnace can always be closely controlled, thus keeping down undesirable oxidizing reactions.

Most often the charge consists almost entirely of scrap. Before it is ready to be used, the scrap must first be analyzed and sorted, because its alloy content will affect the composition of the refined metal. Other materials, such as small quantities of iron ore and dry lime, are added in order to help remove carbon and other impurities that are present. The additional alloying elements go either into the charge or, later, into the refined steel as it is poured into the ladle.

After the furnace is charged, electrodes are lowered close to the surface of the metal. The current enters through one of the electrodes, arcs to the metallic charge, flows through the metal, and then arcs back to the next electrode. Heat is generated by the overcoming of resistance to the flow of current through the charge. This heat, together with that coming from the intensely hot arc itself, quickly melts the metal. In another type of electric furnace, heat is generated in a coil. See Electric Furnace.

VIII

Finishing Processes

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. In hot rolling the cast ingot is first heated to bright-red heat 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.

The first pair of rollers through which the ingot passes is commonly called the blooming mill, and the square billets of steel that the ingot produces are known as blooms. From the blooming mill, the steel is passed on to roughing mills and finally to finishing mills that reduce it to the correct cross section. 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. 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 (1,210 ft). Continuous mills are equipped with a number of accessory devices including edging rollers, descaling devices, and devices for coiling the sheet automatically when it reaches the end of the mill. The edging rollers are sets of vertical rolls set opposite each other at either side of the sheet to ensure that the width of the sheet is maintained. Descaling apparatus removes the scale that forms on the surface of the sheet by knocking it off mechanically, loosening it by means of an air blast, or bending the sheet sharply at some point in its travel. The completed coils of sheet are dropped on a conveyor and carried away to be annealed and cut into individual sheets. A more efficient way to produce thin sheet steel is to feed thinner slabs through the rollers. Using conventional casting methods, ingots must still be passed through a blooming mill in order to produce slabs thin enough to enter a continuous mill.

By devising a continuous casting system that produces an endless steel slab less than 5 cm (2 in) thick, German engineers have eliminated any need for blooming and roughing mills. In 1989, a steel mill in Indiana became the first outside Europe to adopt this new system.

IX

Pipe

Cheaper grades of pipe are shaped by bending a flat strip, or skelp, of hot steel into cylindrical form and welding the edges to complete the pipe. For the smaller sizes of pipe, the edges of the skelp are usually overlapped and passed between a pair of rollers curved to correspond with the outside diameter of the pipe. The pressure on the rollers is great enough to weld the edges together. Seamless pipe or tubing is made from solid rods by passing them between a pair of inclined rollers that have a pointed metal bar, or mandrel, set between them in such a way that it pierces the rods and forms the inside diameter of the pipe at the same time that the rollers are forming the outside diameter.

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