The basic oxygen converter resembles a Bessemer converter. These, along with any additional gases from the burned fuel, are used to heat incoming air and this is why the open-hearth process is sometimes called the regenerative open-hearth. Oxygen released from the ore and additional injected oxygen combine with carbon in the molten pig iron to form carbon gases. ![]() A heat is refined into steel during an 8-12 hour time period. An open-hearth furnace is tapped through a hole in the furnace's bottom. When the contents of the heat are acceptable and the temperature is right, the furnace is tapped and the molten metal is poured into a ladle. The quantity of deoxidizers used must be closely controlled to allow a limited evolution of gas from the carbon-oxygen reaction. Semikilled steels are a compromise between rimmed and killed steels and are mainly limited to steels to be rolled into sheet bar, plate, and structural sections. Rolling processes are used in later operations to remove these defects. "Rimmed" steel is steel that has not been deoxidized and gas pockets and holes from free oxygen form in the center of the ingot while the rim near the surface of the ingot is free of defects. Aluminum ferrosilicon is added if the steel is to be "killed." A killed steel is one that has been deoxidized to prevent gas evolution in the ingot mold, making a more uniform steel. Mill scale, a form of iron oxide (Fe 3O 4), is used to reduce carbon content. Fluxing agents (primarily lime, added in the form of either limestone or burnt lime and supplemented by magnesia, MgO, and lime from the furnace bottom and sides) melt and combine with the impurities to form slag at the top of the melt which is poured off into a separate slag pot. These elements can be added either in the furnace charge, the melt or "bath," ladle, or the ingot molds to meet the desired chemical composition of the finished steel or to eliminate or counteract the effect of oxides or other impurities. ![]() Other elements, such as fluxing agents, carbon (usually in the form of anthracite coal pellets), and alloying materials, are then added to improve the steel. Scrap is placed in the furnace with a charging machine which usually serves a series of open hearth furnaces in a single building. Then the furnace is charged with many tons of molten pig iron. These materials are heated for about two hours at temperatures 2,700–3,000☏ (1,482–1,649☌) until they begin to fuse. To make a "heat," or one batch of steel, pig iron, limestone, and scrap steel, are initially "charged," or loaded, into the hearth. ![]() Open-hearth furnaces got their name from a shallow area called a hearth that is exposed to a blast of flames that alternately sweeps across the hearth from one side for a period of time and then to the side of the furnace. Because of stricter air pollution regulations and more efficient processes, the Bessemer converter is no longer used.įrom 1909 until the 1960s, the open-hearth process was the most popular method of steel production. This was solved by adding the necessary elements back into the converter after the blow. Unfortunately, the blowing air contained contaminants (such as nitrogen) and also removed some desirable elements such as carbon and manganese. The whole process took less than 15 minutes. Then 30-ft (9-m) flames replace the smoke as the oxygen combines with the carbon fuel and burns. Sparks and thick, brown smoke pour from the converter's mouth as the oxygen in the blow combines with the iron, silicon, and manganese to form slag. Air is then blown through holes in its bottom at a typical rate of 30,000 cubic feet per minute. After the converter is charged with hot metal, it is swung to the upright position. It is balanced on axles so that its open top can be tilted one way to take a charge and the other way to pour out steel. A Bessemer converter looks like a huge pear-shaped pot and can hold anywhere from 5-25 tons. ![]() Until 1909, most steel made in the United States came from Bessemer converters.
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