The function of the basic open-hearth furnace is to convert various types of ferrous material into finished steel of given composition and quality. In comparison with other steel-making methods, the basic open-hearth process is one of the most versatile, both from the standpoint of the raw materials that can be used and the types of steel that can be produced. The process can be considered as a sequence involving melting, refining, and deoxidation. In the case of some types of liquid-metal charges the melting stage is not present, but refining and deoxidation in varying degrees are fundamental features of the process.

General Discussion of the Basic Open-hearth Process

The first consideration in open-hearth practice is the type of charge on which the operation is based. This consists of three components — fluxing, oxidizing, and metallic materials.

Fluxes. — In the basic process the fluxing material is primarily lime, added in the form of either limestone (CaCO₃) or burnt lime (CaO). This is supplemented by magnesia (MgO) and lime from the furnace bottom and banks. The function of these fluxes is to form a slag with the silica (Si0₂), manganese oxide (MnO), iron oxides (FeO and Fe₂O₃), and phosphorus oxide (P₂O₅) that are produced by the oxidation of varying amounts of the silicon, manganese, iron, and phosphorus in the charge during melting and refining. The slag composed of the basic fluxes and the neutral and acid oxides, floats on top of the steel bath when the charge is melted. The composition of this slag depends on the amount of each slag-forming element present in the charge and on the extent of oxidation of these elements during the course of the refining operation. This slag is basic (i.e., it has an excess of basic over acid components) during the refining period and contains an excess of free iron oxide, so that it has an oxidizing effect on the liquid metal underneath.

Sources of Oxygen. — The important feature to be kept in mind when studying open-hearth practice is that it is primarily an oxidizing process. Refining depends on the oxidation of impurities under basic conditions and, with the exception of sulphur, only impurities that will be oxidized under these conditions can be removed. The sources of oxygen are rust and scale on the scrap, iron ore in the charge and added during the refining period, carbon dioxide (CO₂) obtained from the calcination of limestone, and air and other oxygen-containing gases in the furnace atmosphere. The rust and scale, and the oxides formed by the oxidation of the scrap by the furnace gases, supply oxygen during the melting period. When iron ore and limestone are added with the charge these are also important sources of oxygen during this period. After the charge is melted and the liquid metal bath is covered by slag, oxygen is transmitted to the bath by the slag. The oxygen in the slag is obtained from oxides carried over from the melting period and from oxidation of the slag by the furnace gases. Iron ore is also usually added during refining, and this supplies oxygen both to the slag and directly to the bath.

The type and amount of fluxing and oxidizing materials used depend on the composition of the metallic part of the charge. The metallic charge consists of various combinations of blast-furnace hot metal and cold pig-iron, cupola iron, cast-iron scrap, steel scrap, and Bessemer-blown metal. With the exception of the scrap steel and usually *the Bessemer metal, the metallic components of the charge contain relatively high percentages of carbon, silicon, manganese, and phosphorus. The fluxing and oxidizing additions must therefore be adjusted to the type and amount of these materials in the metallic charge.

Types of Metallic Charges. — The five fundamental types of metal­lic charge that are used in the basic open-hearth process — and the typical materials charged — are:

Type 1. All liquid iron. (Blast-furnace hot metal.)

Type 2. Liquid iron and liquid steel. (Blast-furnace hot metal and steel scrap.)

Type 3. Liquid iron and solid steel. (Blast-furnace hot metal and steel scrap.)

Type 4. Solid iron and solid steel. (Blast-furnace pig-iron and steel scrap.)

Type 5. All solid steel. (Steel scrap.)

The type of metallic charge used depends on the equipment of the plant and on the cost, availability, and composition of the metallic components of the charge.

The word iron, unqualified by a descriptive adjective as used in this article, does not mean the chemical element. In the parlance of the iron and steel industry it means an impure alloy of iron and 3 to 4.5 per cent carbon. Pig-iron and cast iron are specific varieties of "iron". Blast-furnace hot metal or blast-furnace metal has the same composition as pig-iron, but it is in the molten condition. Cupola metal or cupola iron has the same composition as cast iron but it, too, is molten. Bessemer blown metal is partly refined blast-furnace hot metal and is also molten.

Charging the Furnace. — The following description is based on the practice used for a 150-ton furnace charged with 55 per cent scrap and 45 per cent liquid iron (hot metal), with limestone as a flux. The solid material to be charged is brought onto* the floor of the open-hearth building in boxes which are carried on small railroad cars or buggies, each of which usually holds four boxes. Each box is picked up by a charging car which has a ram that fits a groove on the end of the box. The box is lifted from the car, inserted through the furnace door, and the contents are dumped inside the furnace.

The usual sequence in charging is to spread several boxes of light scrap on the bottom and place the limestone on top of this scrap. The reason for charging some scrap first is to prevent the lime from adhering to the bottom. The remainder of the scrap is then charged on top of the limestone. The flame is turned on partly while this material is being charged, and a full flame is carried after the charging is completed. The scrap is heated during the next period in the charging cycle, which lasts from 2 to 3 hr. This time varies with the type of scrap (light or heavy) and with the condition of the furnace. At the end of this heating period the scrap is partly melted and considerable iron oxide has been formed. An important feature of the furnace oper­ation is to obtain the correct amount of oxidation, as this iron oxide is the principal agent for oxidizing the impurities in the hot metal (liquid iron) during the melting period.

The charge is completed by the addition of blast-furnace, metal, which is brought to the furnaces in ladles of 40- to 60-ton capacity. A stand with a spout projecting into the furnace is placed at one of the doors, and the hot metal is poured into the furnace. Two ladles of hot metal are required, and the addition of this takes from 30 to 60 min. When the hot metal is being added, a vigorous boiling action often occurs as the metal mixes with the scrap in the furnace, owing to the reaction between the iron oxide on the scrap and the carbon in the hot metal. This action may cause the slag to foam and run out over the sills of the doors and is corrected by slowing down the rate of adding the hot metal until the action subsides. The addition of the last ladle of hot metal completes the charging period.

Refining Period. The beginning of the refining period is usually taken as the time when the metallic charge has melted and the limestone has been partly calcined and has floated up from the bottom of the furnace. The time from the finish of charging to the point when the heat is melted varies with the size and condition of furnace, the fuel, and the type of charge.

The following discussion covers the practice for 150-ton furnaces using oil. In type 1, a charge consisting of lime, ore, and liquid iron, the melting time will be from 3 to 7 hr. In type 2, which is an entirely liquid metallic charge, the first liquid steel (Bessemer-blown metal) poured into the furnace freezes over the lime on the bottom, but the later additions remain molten. The chilled metal melts rapidly and releases the lime from the bottom so that this type of heat reaches the refining stage within 30 to 45 min. after the last ladle of liquid metal is added. In type 3, cold steel scrap and liquid iron, the nor­mal melting time is from 3 to 5 hr., but when ore is added with charge and a larger proportion of liquid iron is used, the time is usually decreased. In the case of types 4 and 5, which are wholly cold charges, the melting time varies from 4 to 8 hr.

The Working Period. — The stage in a heat following the charging and melting periods is the "working" or final refining period. This extends from the time the heat is melted to the deoxidation or tapping time. It has been pointed out that during melting a certain proportion of the carbon, manganese, phosphorus, and silicon was eliminated by oxidation. The oxygen during this period was obtained from the iron oxide present and formed on the scrap during charging and melting, from the furnace gases after the heat was "under cover", i. e., when the charge was covered by slag, and from the carbon dioxide in the limestone. An additional source of oxygen during the melting period is iron ore in the types of practice where this is added with the charge. During the working period the refining is also accomplished by oxidation, but the sources of oxygen during this period are the furnace gases and iron ore added to the bath. The oxygen that comes from furnace gases is a relatively small proportion of the amount required to oxidize the elements in the bath within a reasonable length of time. Consequently, iron ore is used to control and accelerate the rate of oxidation. The ore also accelerates the "shaping up" or solution of the undissolved lime in the slag, and creates a boiling action in the bath which is advantageous in eliminating gases from the steel and controlling the temperature.

Deoxidation and Final Additions. The purpose of the basic open-hearth process is to produce steel with a desired composition and with other characteristics that determine the manner in which the molten steel solidifies after it is tapped from the furnace and poured into molds or castings.

Obtaining the Required Final Compositions. — The composition of the finished steel will depend on the chemical specification for which it is made. The specification will usually require that the analysis of the steel be within certain ranges of carbon, manganese, phosphorus, and sulphur contents. Other components, such as silicon and the alloying elements, may also be specified. In the case of elements which are considered undesirable in a certain grade, only the maximum percentages are given.

It is usually necessary to add manganese, and the addition of other elements may be required for various grades of steel. Some elements, such as nickel, molybdenum, and copper, may be added with the charge or during refining, since they are not oxidized; the elements that are oxidized such as manganese and silicon, may be added to the bath just before tapping or to the ladle.

An important element in steel which is not usually shown in the analysis is oxygen. The amount of oxygen in the steel varies with the carbon content, temperature, and slag composition. Although oxygen is not included in chemical specifications for finished steel, the amount and form in which it is present influence the type of solidification which will result when the steel is cast into molds, and also the properties of the product.

In the case of type which is termed killed steel it is necessary to remove some of the oxygen present in the bath at the end of the refining period and to fix the remainder in an inactive form. This is accomplished by making deoxidizing additions to the steel before tapping or to the ladle. These convert most of the oxygen in the bath to insoluble oxides such as MnO and SiO₂, which tend to float upward and pass into the slag. The furnace deoxidizing additions are selected, and the additions are spaced so that maximum elimination of the oxides formed will take place before the heat is tapped from the furnace. When the heat is tapped, other deoxidizing additions are usually added to the steel as it runs into the ladle. The object of these is to complete the deoxidation and to obtain the type of oxides desired in the finished steel.

Pouring the Heat. — The final operation in the production of a heat of steel is teeming or pouring of the steel into molds to make ingots or castings. The tapping ladle is carried from the furnace to the pouring stand by the pit crane and suspended over the molds. The steel is poured into each mold in sequence through a nozzle in the bottom of the ladle. The stream of steel passing through the nozzle is controlled by a stopper which fits into a well at the top of the nozzle. The stopper is attached to a stopper rod which is suspended from the top of the ladle by a linkage through which the steel pourer can con­trol the movement of the stopper. A spoon is used to take steel sample — the spoon being filled from the stream flowing out of the nozzle. There are usually at least two ladle tests taken from each heat, one from the first part and another from the last part of the heat, to obtain a representative average analysis of the steel in the ladle. These tests are sent to the laboratory where they are drilled and a determination is made of the significant elements. This is called the ladle analysis, and the application of the heat is governed by this analysis, supplemented on some grades by additional check analyses obtained from blooms, billets, or the finished product after the ingots are rolled. After the heat is poured into the molds, the cars are held at the pouring stand for varying lengths of time, depending on the grade of steel. The cars are then shunted to the stripper crane.