How did “iron” transform into “steel”?

Steelmaking begins with ironmaking. Steel comes from pig iron. Pig iron smelted from iron ore has a high carbon content and contains many impurities (such as silicon, manganese, phosphorus, sulfur, etc.). As a result, pig iron lacks plasticity and resistance, and has poor mechanical properties. It cannot be subjected to pressure processing except for melting and casting, thus restricting its usefulness.
In order to overcome these shortcomings of pig iron and make it more effective in industry, it is necessary to use oxygen from various sources at high temperature to eliminate the impurities in pig iron to a certain degree to obtain a certain composition and composition. Certain properties of iron-carbon alloy-steel.
This method of removing impurities in pig iron by oxidation at high temperatures is called steelmaking.
The fundamental principle of steelmaking.
Various impurities in pig iron have a greater affinity with oxygen to varying degrees under high temperature environments. Therefore, oxidation can be used to make them into liquid, solid or gas oxides. The liquid and solid oxides react with the furnace lining and the flux in the furnace at high temperatures to form slag, and are removed from the furnace during slagging. The gas is also taken out of the furnace by CO when the molten steel is jubilant.
In the steelmaking furnace, the oxidation of impurities is mainly accomplished by the presence of FeO.
2Fe+O2→2FeO
01 The silicon oxide of
silicon has a greater affinity with oxygen, so the oxidation of silicon is very rapid. It has been completely oxidized to form SiO2 in the early stage of smelting:
Si+2FeO→SiO2+2Fe
together with SiO2 It reacts with FeO to form silicate:
2FeO+SiO2→2FeO·SiO2
This salt is a very important part of the slag. It interacts with CaO to form stable compounds 2CaO·SiO2 and FeO. The former is solidly present in the slag, and the latter becomes The free components in the slag are removed, and the FeO content in the slag is added to promote the oxidation of impurities. The response is as follows:
2FeO·SiO2+2CaO→2CaO·SiO2+2FeO
02 Oxidation of manganese element
Manganese is also an element that is easy to oxidize. The MnO produced by it has a higher melting point. MnO does not dissolve in the molten metal, but its compound with SiO2 floats on the surface of the liquid metal and becomes a part of the slag. The oxidation reaction of
Mn+FeO→MnO+Fe
2MnO+SiO2→2MnO·SiO2
silicon and manganese releases a lot of heat, which can make the furnace temperature advance rapidly (this is particularly important for converter steelmaking) and greatly accelerate the carbon oxidation process.
03 oxidation of carbon
oxide needs a lot of energy absorption of carbon, it is necessary ability at higher temperatures. The oxidation of carbon is a very important reaction in the steelmaking process:
C+FeO→CO+Fe
Because CO gas is generated when carbon is oxidized, it will have a violent mixing effect when it escapes from the liquid metal. This effect is called ” jubilation”. The result of jubilation can promote the uniformity of the composition and temperature of the molten pool, speed up the reaction of the metal and slag interface, and also help to remove the gas and inclusions in the steel.
04 The oxidation of
phosphorus oxide phosphorus can occur at a not too high temperature. The
dephosphorization process is composed of several reactions, and the reactions are as follows: 2P+5FeO→P2O5+5Fe
P2O5+3FeO→3FeO·P2O5
When in When there is sufficient CaO in the alkaline slag, the following reaction will occur:
3FeO·P2O5+4CaO→4CaO·P2O5+3FeO The 4CaO·P2O5
produced is a stable compound, which is firmly held in the slag, thus reaching the dephosphorization intention.
It is necessary to pay attention to the fact that in the deoxidation process of molten steel, it is necessary to participate in deoxidizers such as ferrosilicon and ferromanganese. Therefore, after deoxidation, the slag becomes acidic, and 3FeO·P2O5 is damaged, and P2O5 is recovered from it, and P2O5 is unstable. It is easy to be restored by carbon at high temperature, and the phenomenon of phosphorus recovery occurs. This also shows that it is difficult to remove phosphorus in an acid furnace. In order to avoid the occurrence of this phenomenon, it is necessary to appropriately add slag basicity and slag amount to advance the slag oxidation property.
Oxidation of elemental sulfur 05
sulfur FeS based approach exist. When there is sufficient CaO in the slag, the sulfur can also be removed, and the response is as follows:
CaS
generated by FeS+CaO→CaS+FeO is not soluble in molten steel, and constitutes the slag floating on the surface of molten steel.
The above reaction is a reversible reaction, and it is carried out in the slag containing FeO. When FeO and CaS have an effect, the sulfur will be returned to the molten steel from the beginning, so the desulfurization power will increase as the FeO content in the slag is reduced.
When the slag contains sufficient carbon, the response is different:
CaO+FeS+C→CaS+Fe+CO
Because carbon deprives FeO of oxygen, the possibility of the effect of CaS and FeO is lost, so the response cannot be reversed. This is why the sulfur removal in electric furnace steelmaking is more thorough than the other two methods.
In the process of desulfurization, manganese also plays a role in promoting desulfurization. The process is as follows:
MnS
produced by FeS+MnO→MnS+FeO is almost insoluble in molten steel and enters the slag. Therefore, the effect of desulfurization increases with the oxidation of manganese.
After the deoxidation of 06FeO
after the above-mentioned series of oxidation reactions, although the impurities are oxidized, the intention of removal is reached, but because of the results of oxidation, the molten steel contains more FeO, which means that there is a lot of oxygen in the molten steel. Bringing great damage to steel, on the one hand, it makes the steel with many bubbles; on the other hand, it also makes the steel show hot and cold brittleness, and the damage is increased with the addition of carbon content.
Therefore, at the end of the steelmaking process, it is necessary to try to remove a lot of oxygen present in the molten steel. The generally selected method is to add some deoxidizers in the molten steel, such as ferromanganese, ferrosilicon, aluminum, etc., which violently grab oxygen from FeO to achieve the intention of deoxidation. The response is as follows:
FeO+Mn→MnO+Fe
2FeO+ The effect of Si→SiO2+2Fe
3FeO+2Al→Al2O3+3Fe
07 slag
The entire steelmaking process consists of two processes: oxidation and recovery. Generally, the oxidation of carbon, silicon, manganese, and phosphorus is called the response during the oxidation period, and the desulfurization and deoxidation are called the response during the recovery period. It can be seen from the above responses that in order to eliminate impurities in metals, it is necessary to consider various factors, but the most important factors are slagging and slag removal.
The slag has the following important effects in the steelmaking process:
①The slag should ensure that the steelmaking process is carried out according to a certain response time (oxidation or restoration).
②The slag should ensure the maximum removal of harmful impurities (phosphorus and sulfur) in the metal, and prevent the gas (nitrogen and sulfur) in the furnace gas from entering the metal.
③The slag should ensure the minimum loss of iron and other valuable elements during operation.
The basic method of
steelmaking ① Converter steelmaking The
converter steelmaking method uses air or oxygen, adopts bottom blowing, side blowing and top blowing methods to oxidize the elements in the molten iron to the regulatory limit, and then obtain a kind of steel with qualified composition. Steelmaking methods.

② Electric furnace steelmaking
Electric furnace uses electric energy to transform heat energy to make steel. There are two commonly used electric furnaces: electric arc furnace and induction electric 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 alloys.

③
Following the development of the open-hearth steelmaking industry, a lot of scrap steel has been accumulated in the metal processing industry. At that time, it was not possible to convert it into steel from scratch with a converter, so the steel workers looked for a steelmaking method using scrap steel as a raw material. In 1864, the Frenchman Martin invented the open-hearth steelmaking method.

The rapid development of the oxygen top-blown converter steelmaking method has gradually replaced the open-hearth steelmaking method. With the advancement of science and technology, some new steelmaking methods continue to emerge, such as the vacuum treatment of molten steel, electroslag furnace smelting, and vacuum induction electric furnace smelting, which have long been used more and more.