Summary of basic principles of steelmaking

　　Make iron before making steel. Steel comes from pig iron. Pig iron smelted from iron ore contains high carbon content and many impurities (such as silicon, manganese, phosphorus, sulfur, etc.). Therefore, pig iron lacks plasticity and toughness, and its mechanical properties are poor. Besides melting and casting, it cannot be processed by pressure, which limits its use.

　　In order to overcome these shortcomings of pig iron and make it play a greater role in industry, it is also necessary to use oxygen from various sources at high temperature to oxidize and remove impurities in pig iron to a certain extent, so as to obtain iron carbon alloy steel with certain composition and properties. This method of oxidizing and removing impurities from pig iron at high temperature is called steelmaking.

　　Basic principles of steelmaking

　　Various impurities in pig iron have great affinity with oxygen to varying degrees in high temperature environment. Therefore, oxidation method can be used to make them become liquid, solid or gas oxides. Liquid and solid oxides react with furnace lining and flux added into the furnace at high temperature to form slag, which is discharged outside the furnace during slag removal, and the gas is also brought outside the furnace by CO during molten steel boiling.

　　In the steelmaking furnace, the oxidation of impurities mainly depends on the existence of FeO.

　　2Fe+O2→2FeO
 

　　1. oxidation of silicon

　　Silicon has a large affinity with oxygen, so the oxidation of silicon is very rapid. It has been completely oxidized at the initial stage of smelting to produce SiO2:

　　Si+2FeO→SiO2+2Fe

　　At the same time, SiO2 reacts with FeO to form silicate:

　　2FeO+SiO2→2FeO·SiO2

　　This salt is a very important part of the slag. It reacts with Cao to form stable compounds 2CaO · SiO2 and FeO. The former is firmly present in the slag, and the latter becomes a free component in the slag, increasing the content of FeO in the slag, which is more beneficial to promoting the oxidation of impurities. The reaction is as follows:

　　2FeO·SiO2+2CaO→2CaO·SiO2+2FeO
 

　　2. oxidation of manganese

　　Manganese is also an element easy to oxidize. The MnO produced by it has a high melting point. MnO does not dissolve in liquid metal, but it forms a compound with SiO2 to float on the surface of liquid metal and become a part of slag.

　　Mn+FeO→MnO+Fe

　　2MnO+SiO2→2MnO·SiO2

　　The oxidation reaction of silicon and manganese releases a large amount of heat, which can rapidly increase the furnace temperature (which is particularly important for converter steelmaking), and greatly accelerate the oxidation process of carbon.
 

　　3. oxidation of carbon element

　　The oxidation of carbon needs to absorb a lot of heat energy, so it must be carried out at a higher temperature. Carbon oxidation is also a very important reaction in the steelmaking process:

　　C+FeO→CO+Fe

　　As CO gas is generated during carbon oxidation, it plays a strong stirring role when escaping from liquid metal, which is called "boiling". The result of boiling can make the composition and temperature of molten pool uniform, accelerate the reaction between metal and slag interface, and is also conducive to the removal of gas and inclusions in steel.
 

　　4. oxidation of phosphorus

　　The oxidation of phosphorus can take place at a low temperature. The dephosphorization process consists of several reactions. The reactions are as follows:

　　2P+5FeO→P2O5+5Fe

　　P2O5+3FeO→3FeO·P2O5

　　When there is enough Cao in the alkaline slag, the following reactions will occur:

　　3FeO·P2O5+4CaO→4CaO·P2O5+3FeO

　　The generated 4cao · P2O5 is a stable compound, which is firmly maintained in the slag, thus achieving the goal of dephosphorization.

　　It must be noted that in the deoxidation process of molten steel, deoxidizers such as ferrosilicon and ferromanganese are added. Therefore, after deoxidation, the slag is often acidic, which destroys 3feo · P2O5 and reduces P2O5. P2O5 is an unstable oxide. It is easy to be reduced by carbon at high temperature, resulting in phosphorus recovery. This also shows that it is very difficult to remove phosphorus in acid furnace. In order to prevent this phenomenon, it is necessary to properly increase the basicity and amount of slag and improve the oxidizability of slag.
 

　　5. oxidation of sulfur

　　Sulfur exists in the form of FES. When there is enough Cao in the slag, the sulfur can also be removed. The reaction is as follows:

　　FeS+CaO→CaS+FeO

　　The generated CAS does not dissolve in the molten steel, but forms the slag floating on the surface of the molten steel.

　　The above reaction is reversible and is carried out in the slag containing FeO. When FeO reacts with CAS, the sulfur will return to the molten steel. Therefore, the desulfurization efficiency increases with the decrease of FeO content in the slag.

　　When the slag contains enough carbon, the reaction is different:

　　CaO+FeS+C→CaS+Fe+CO

　　Because carbon takes away the oxygen in FeO, the possibility of the interaction between CAS and FeO is lost, so the reaction cannot be reversed. This is why the desulfurization of electric furnace steelmaking is more complete than the other two methods.

　　In the process of sulfur removal, manganese also plays a role in promoting sulfur removal. The process is as follows:

　　FeS+MnO→MnS+FeO

　　The generated MNS is almost insoluble in molten steel and enters the slag. Therefore, the effect of sulfur removal increases with the oxidation of manganese.
 

　　6. Deoxidation of FeO

　　After the above series of oxidation reactions, although the impurities are oxidized to achieve the purpose of removal, but also because of the oxidation results, the steel water contains more FeO, that is, there is a large amount of oxygen in the steel water, which brings great harm to the steel. On the one hand, there are a large number of bubbles in the steel strip; On the other hand, the hot and cold embrittlement of steel also occur, and the harm increases with the increase of carbon content.

　　Therefore, at the end of the steelmaking process, we must try to remove a large amount of oxygen in the molten steel. The usual method is to add some deoxidizers to molten steel, such as ferromanganese, ferrosilicon, aluminum, etc., which strongly capture oxygen from FeO to achieve the purpose of deoxidation. The reaction is as follows:

　　FeO+Mn→MnO+Fe

　　2FeO+Si→SiO2+2Fe

　　3FeO+2Al→Al2O3+3Fe
 

　　7. role of slag

　　The whole steelmaking process consists of two processes: oxidation and reduction. Generally, the oxidation of carbon, silicon, manganese and phosphorus is called the reaction in the oxidation period, and desulfurization and deoxidation are called the reaction in the reduction period. From the above reaction equations, it can be seen that in order to remove impurities in metal, various factors must be considered, but the most important factor is slagging and slag removal.

　　Slag plays the following important roles in the steelmaking process:

　　① The slag shall ensure that the steelmaking process is carried out in a certain reaction direction (oxidation or reduction).

　　② The slag shall ensure the maximum removal of harmful impurities (phosphorus and sulfur) in the metal and prevent the gas (nitrogen and hydrogen) in the furnace gas from entering the metal.

　　③ The slag shall ensure the minimum loss of iron and other valuable elements during operation.

　　Basic methods of steelmaking

　　① Converter steelmaking

　　Converter steelmaking method is a steelmaking method that uses air or oxygen to oxidize the elements in molten iron to the specified limit by bottom blowing, side blowing and top blowing, so as to obtain qualified steel.

　　② Electric furnace steelmaking

　　Electric furnace is used to convert electric energy into heat energy to make steel. There are two kinds of electric furnaces commonly used: electric arc furnace and induction furnace. Electric arc furnace is the most widely used, suitable for smelting high-quality steel and alloy steel; Induction furnace is used for smelting high-grade alloy steel and non-ferrous alloy.

　　③ Open hearth steelmaking

　　With the development of industry, a large amount of scrap steel has been accumulated in the metal processing industry. At that time, it was impossible to convert it into steel again with a converter, so steelmakers looked for a steel-making method using scrap steel as raw material. In 1864, Martin, a Frenchman, invented the open hearth steelmaking process.

　　With the rapid development of oxygen top blowing converter steelmaking process, it has gradually replaced open hearth steelmaking process. With the progress of science and technology, some new steel-making methods are emerging, such as vacuum treatment of molten steel, electroslag furnace smelting, vacuum induction furnace smelting and other methods, which have already been used more and more.