Open-Hearth Production

the production of ingot steel of a given chemical composition in open-hearth furnaces of metallurgical or machine-building plants. The steel is produced by oxidizing melting of iron-containing materials charged into the furnace (pig iron, steel scrap, iron ore, and flux) as a result of complex physical and chemical interactions among the metal, the slag, and the gaseous medium of the furnace. Together with other forms of steel production, open-hearth production is the second stage in the overall production cycle of ferrous metallurgy. The other two major stages are the smelting of pig iron in blast furnaces and the rolling of steel ingots and bars.

Because of the advantages of open-hearth production relative to other processes of mass production of steel, it became the basic steel founding process in the late 19th and first half of the 20th century (from 1940 to 1955, about 80 percent of world steel production was prepared using the open-hearth process). The advantages of open-hearth production are great flexibility, the possibility of using the process on any production scale, less stringent requirements for the initial materials, the relative simplicity of monitoring and control of the smelting process, the high quality and wide variety of smelted steels, and the relatively low cost of conversion.

However, with the rapid development of oxygen-converter steel production, the construction of open-hearth units has virtually stopped; the proportion of open-hearth steel is decreasing continuously. In 1970, world open-hearth production was about 240 million tons (about 40 percent), of which about 84 million tons (about 72 percent of the country’s total) was produced in the USSR. Open-hearth production is the major consumer of steel scrap (about 50 percent).

Historical survey. Ideas for the conversion of scrap and pig iron into steel on the hearth of an open-flame furnace were put forward repeatedly. The greatest contribution to the development of the open-hearth process was made by F. Siemens of Germany, who in 1856 proposed use of the principle of regenerating the heat of flue gas for raising the temperature of the melting chamber of a smelting furnace, and by P. Martin of France, who in 1864 succeeded in constructing and operating the first reverberatory regenerative furnace for smelting ingot steel. In Russia the first open-hearth furnace, with a capacity of 2.5 tons, was put into operation by A. A. Iznoskov at the Sormovo Plant (now the Krasnoe Sormovo Plant in Gorky) in 1870.

Initially, open-hearth furnaces had an acid hearth. Furnaces with basic hearths were widely used after their introduction in 1879-80 in France at the Creusot and Terrenoire plants and in 1881 in Russia at the Aleksandrov Plant in St. Petersburg. In 1894 two Russian metallurgists, the brothers A. M. and lu. M. Goriainov, developed a method for open-hearth smelting of molten pig iron and successfully used the method at the Aleksandrovsk Plant in Ekaterinoslav (now the Petrovskii Works in Dnepropetrovsk). In France, Russia, and other countries, the process was called the Martin process; in Germany it was called the Siemens-Martin process.

The development of open-hearth production was characterized by three periods. In the first period (up to the early 20th century), smelting was done in furnaces of small capacity (up to 70 tons), heated by producer gas and with natural draft provided by a chimney flue. The second period (first half of the 20th century) was characterized by the conversion to coke blast-furnace gas, forced draft (provided by blowers), automation of the furnace heating mode, the use of waste-heat boilers, and the construction of furnaces with a capacity of 185-250 tons (later, 370-500 tons). The third period began in the 1950’s and was characterized by intensification of the process using oxygen, conversion to fuel with a high heat of combustion (mainly natural gas), construction of new plants with units of 600-900 tons’ capacity, and the design of a new type of furnace. Open-hearth production achieved its greatest scale in the USSR and the USA. The largest furnaces in the world, with a capacity of 900 tons, were in operation in the USSR in 1974. Significant contributions to the theory and practice of open-hearth production were made by the Soviet metallurgists V. E. Grum-Grzhimailo, A. A. Baikov, M. A. Pavlov, M. M. Karnaukhov, N. N. Dobrokhotov, V. I. Tyzhnov, and K. G. Trubin.

The open-hearth process. The charge for open-hearth furnaces is divided into a metal fraction, including pig iron, steel scrap, deoxidizers, and alloying additives, and a nonmetallic fraction, consisting of iron ore, open-hearth sinter, limestone, lime, bauxite, and fluorite. Pig iron, which is used in the molten state or in the form of ingots, is the major carbon source and provides normal running of the process. The amount of pig iron and steel scrap in the charge may vary in any proportion, depending on the type of process, economic considerations, and the grades of the steel produced. Ferroalloys and some pure metals, such as aluminum and nickel, are used as deoxidizers and alloying addiitives in open-hearth production. Iron ore and open-hearth sinter are used as oxidizing agents, and also as a flux, providing accelerated formation of the active slag. Scale may also be used as the oxidizing agent. Limestone, lime, bauxite, and fluorite are used to form slag with the required composition and consistency, which supports the oxidizing reactions, removal of harmful impurities, and heating of the metal.

In the open-hearth process, as distinct from the converter processes, the heat evolved as a result of oxidation of impurities in the metal bath is insufficient for smelting. Thus, additional heat from the combustion of fuel in the melting chamber is supplied to the furnace. Natural gas, fuel oil, coke, and blastfurnace gases are used as such fuels. To provide complete combustion of the fuel, the quantity of air supplied for combustion is slightly in excess of the theoretically required quantity. This produces an excess of oxygen in the products of combustion, in which the gaseous oxides CO₂ and H₂O, which partially dissociate at high temperatures, are also present. As a result, oxidation of iron and other elements in the charge takes place. To improve combustion, some of the air introduced into the furnace may be replaced by oxygen; gaseous oxygen is also supplied to the bath to intensify the processes of oxidation. The slag coating the metal in all subsequent stages of smelting is composed of FeO, Fe₂O₃, CaO, SiO₂, MnO, P₂O₅, and other oxides, along with the gradually decomposing refractory materials of the lining, the fluxes, and impurities carried by the charge. The slag plays an important role: it binds all the impurities that must be removed from the charge, transfers oxygen from the furnace atmosphere to the molten metal, transfers heat from the cone of flame to the metal, and protects the metal from saturation by gases in the furnace atmosphere and from overoxidation of the iron. In the various stages of smelting, the slag must have the chemical composition required for fluidity and must be present in definite amounts in the furnace.

In open-hearth smelting, the following stages are usually distinguished: dressing the furnace, filling and heating the charge, pouring the molten iron or charging solid pig iron, smelting, rimming, deoxidation and alloying, and tapping. Dressing the furnace serves to maintain all the elements of the melting chamber lining in working order. This is accomplished by introduction of refractory materials, such as crushed calcined dolomite or magnesite powder, by a fettling machine at the moment of tapping the cast discharge onto the hearth and walls as slag is removed from them. After tapping of the metal and slag from the furnace, the hearth is examined carefully, and uneven areas, such as bumps or pits, are repaired. The charge is filled by means of charging apparatus. All solid charge materials are introduced into the furnace in special boxes called charging boxes, with a volume of up to 3.3 cu m. The duration of the filling is 1 to 3 hr. Initial heating of the charge for up to 1.5 hr before pouring the pig iron into the furnace is used to achieve additional heating of all the steel scrap. The pouring of the pig iron takes 20-60 min.

The melting stage begins immediately after completion of pouring of the pig iron and requires 1-5 hr. In this stage the maximum amount of fuel is supplied to the furnace, and the bath is blown with oxygen. During the pouring of the pig iron and at the beginning of melting, rapid formation of slag occurs as the silicon and part of the manganese contained in the pig iron are oxidized (iron oxides also partially pass into the slag). The thick layer of slag that forms hinders heat transfer from the cone of flame to the metal. Therefore, some of the slag is removed from the furnace in the first half of melting by draining into slag ladles. Removal of most of the phosphorus is also accomplished during the melting stage. The chemical composition of the metal bath upon complete melting differs significantly from the required composition of the steel at the time of tapping of the melt, and the temperature of the metal is relatively low. Therefore, the purpose of subsequent stages of the melting, called finishing, is to provide the necessary heating of the metal and to bring it to a given chemical composition. Thus, the rimming stage is the most critical in open-hearth smelting.

The major reaction of the rimming stage is the oxidation of carbon dissolved in the molten metal. The carbon oxide bubbles that form as a result of the reaction rise to the surface of the metal and break through the layer of slag, creating the impression that the bath is boiling. The rate of oxidation of carbon in this stage may be controlled by the addition of iron ore and other fluxes or by blowing the bath with oxygen and compressed air. The composition of the slag to provide optimum heating of the metal and removal of undesirable impurities (particularly sulfur) is controlled by the addition of lime, ores, and other flux materials. The emerging carbon oxide bubbles play an important role in the open-hearth process as they mix the lower, less heated layers of the metal with the upper, more heated leayers and thus accelerate heating of the entire volume of the metal. In addition, as they ascend the bubbles capture other gases and nonmetallic particles, whose presence in finished steel reduces its quality.

The rimming stage is sometimes arbitrarily divided into the ore rimming stage, in which ore (oxygen), lime, and fluxes are introduced into the furnace, and the pure rimming stage, in which oxidation of the carbon dissolved in the metal continues without any additives, using the oxygen dissolved in the slag and metal. The final finishing of the metal to provide the required temperature and chemical composition takes place during the pure rimming stage. The duration of pure rimming is strictly controlled and depends on the grade of steel being smelted. From the moment of complete melting of the bath to the end of the rimming stage, the composition of the metal and slag and the metal temperature are monitored. The overall duration of the rimming stage is 1-2.5 hr.

Deoxidation and alloying form the final stage of smelting, whose main purpose is to reduce the oxygen content in the metal and to bring the metal to a particular content of all elements, including the alloying elements. The finishing additives are introduced into the furnace or the steel-smelting ladle during tapping of the metal, depending on the grade of the steel.

To tap the metal from the furnace, a tap hole at the back wall is pierced or burned through with a jet of gaseous oxygen. The metal flows out through a trough into a steel-pouring ladle installed below it (in large furnaces, the melt is tapped into two or three ladles). The total duration of the tapping is up to 20 min. After tapping and the necessary examination, the hole is resealed with refractory materials. The metal is poured from the ladle to casting molds or into devices for continuous teeming. A method developed in the USSR, in which the metal is treated in the ladle (during tapping from the furnace) with synthetic slags prepared in a special smelting unit designed to improve the quality of open-hearth steel, has become widespread.

Types of processes. Open-hearth processes are divided into acid and basic processes, depending on the composition of the refractory materials used in preparing the furnace hearth (in the basic process, mainly the basic oxides CaO and MgO; in the acid process, SiO₂). The slag in the basic process consists primarily of basic oxides; that in the acid process consists of acid oxides. Open-hearth processes are divided into several technological variations, depending on the composition of the charge (more precisely, on the ratio of pig iron to scrap in the charge). In the carbonizer, or scrap-carbon, process, the metal fraction of the charge consists almost exclusively of steel scrap, and the required amount of carbon is introduced into the charge by carbon-containing materials called carbonizers, such as anthracite, coke, graphite, and pit coal. The carbonizer process has become very widespread.

The scrap process is characterized by a charge consisting mainly of scrap. Pig iron consumption depends on the amount of carbon in the melted metal required for carrying out the rimming stage and varies from 20 to 45 percent. The scrap process is usually used in plants without blast furnaces, as well as in open-hearth units in machine-building plants.

The scrap-ore process is the most widespread process; it is so named because the solid fraction of the charge consists mainly of scrap and ore. The process is characterized by use of large quantities of molten pig iron (50-80 percent of the weight of the metal fraction of the charge), which are added to the furnace. The scrap-ore process is used in open-hearth units in plants with blast furnaces. Because of the large quantity of pig iron in the charge, many impurities (carbon, manganese, silicon, phosphorus, and sulfur) are introduced into the bath. The oxidation of the impurities requires increased amounts of oxygen (in gaseous form or as oxides of the ore).

The ore process acquired its name because the solid fraction of the charge consists mainly of iron ore. The metal fraction of the charge consists only of molten pig iron. The ore process is not widely used.

More than 95 percent of open-hearth steel is smelted by basic scrap and scrap-ore processes. The acid open-hearth process is used much less than the basic process because of the difficulty of removing sulfur and phosphorus from the metal in the acid process; therefore, the acid process requires charge materials of higher purity (which are more expensive). Smelting in the acid process is of longer duration than in the basic process. However, the nature of the interaction of the metal with an acid hearth lining and acid slag, which has reduced gas permeability relative to the basic process, and also the use of high-purity charge materials, makes possible the production of high-quality steel in the acid process, free of harmful impurities and with low anisotropy of properties along and across the direction of subsequent pressure working. Thus, acid open-hearth steel is widely used in the production of turbine rotors, large crankshafts, and artillery barrels, which require high mechanical strength along and across the grain.

Open-hearth shop. In open-hearth shops, the charge may be supplied by rail or by crane. Most open-hearth steel is produced in shops in which the charge is supplied by rail. Modern open-hearth shops include a bedding plant, mixer section, main building, ingot stripping section, and mold preparation section. The bedding plant is used to receive and store shipments of charge and fettling materials. Magnetic overhead traveling and bucket cranes are used for loading and unloading in the bedding plant. The charge is transferred to the furnaces in charging boxes mounted on rail cars.

The mixer section is usually immediately adjacent to the main building and contains one or two mixers used for holding the molten pig iron entering from the blast furnace. The pig iron is transferred by rail from the mixer to the open-hearth furnaces in ladle cars. In plants without a mixer section, pig iron is transferred from the blast furnace to the open-hearth furnaces in mixer-type ladles.

The main building of an open-hearth shop consists of a charge wing and furnace and casting bays. The charge wing is situated at the height of the floor of the furnace melting chamber and adjacent to the furnace bay; it is used to supply charge materials to the furnaces. The open-hearth furnaces and control panels are located in the furnace bay. The furnaces are arranged in a single line along the central columns of the main building. The control panels are located on the side of the charge wing. The working area of the furnace bay is at a level of 6-7 m above the plant floor. Usually, three rail lines are laid on the working area to supply charging boxes to the furnaces, to move the underslung charging apparatus, and to supply ladle cars with molten pig iron from the mixer section to the furnaces. Traveling overhead casting cranes are used to pour the pig iron into the furnaces. The casting bay is immediately adjacent to the furnace bay. The major function of the casting bay is to receive the steel from the furnaces, to pour it into casting molds or continuous casters, and to remove process slag. The open-hearth furnaces are located on one side of the casting bay, and the pouring floors are placed along the walls on the other side (in the case of pouring steel into casting molds). A number of rail lines are installed in the casting bay for the casting unit and for servicing the slag and waste removal operation. Stands for pouring ladles and slag ladles, as well as stopper drying ovens and ladle repair pits, are also located in the casting bay. Traveling overhead steel pouring cranes and bracket-jib cranes (for servicing the running castings and steel discharge spout) are installed in the casting bay.

The ingot stripping section is usually located in a separate building near the soaking pits for the blooming and slabbing mills. Here the ingots are removed from the molds.

The mold preparation section (casting bed) is designed for the assembly of casting units for steel pouring and is usually near the casting bay. A number of rail lines are laid in the mold preparation section, and there are units for preparing new feeder heads, drying ovens for feeder heads, a burner for heating ingots, a runner rack, and runner drying ovens. There are usually several bridge cranes.

The annual output of modern open-hearth plants in metallurgical factories is 250,000-3,000,000 tons of ingots.