Deoxidation treatment of steel

The steelmaking process consists of refining hot metal into steel， which takes place in an oxidizing atmosphere. During the refining process， oxygen is dissolved in the steel. The following are the main sources of oxygen in steel.

Oxygen blowing

Use of oxidizing slag and iron ore in the steelmaking process

Oxygen drawn from the atmosphere during stirring of the steel

Oxidizing refractory materials in the lining

rusty and wet scrap steel.

Deoxygenation is the final stage of steelmaking. During the steelmaking process， the molten steel at the time of discharge contains 400 to 800 ppm of reactive oxygen. Deoxidation is carried out by adding the appropriate amount of ferroalloys or other special deoxidizers to the ladle during the steel removal process. If the carbon content of the steel is below specification at the end of the strike， the molten steel is also recarburized in the ladle. However， large additions in the ladle have a detrimental effect on the temperature of the molten steel.

The solubility of oxygen in the steel is negligibly small. During the solidification of molten steel， excess oxygen is rejected by the solidified steel. At 1700 degrees Celsius， the solubility of oxygen in liquid steel is 0.23%. The solubility of oxygen decreases during cooling and then falls sharply during solidification of the liquid steel， reaching 0.003% in the solid steel.

The excess oxygen released from the solid solution oxidizes components of the steel， such as C， Fe and alloying elements， resulting in pores and non-metallic inclusions that become entrapped in the cast steel structure. Pores and inclusions have a considerable impact on the mechanical properties and adversely affect the quality of the steel.

In order to prevent the steel composition from being oxidized during solidification， the oxygen content of the molten steel needs to be reduced. This is achieved by deoxidation of the steel， a technical steelmaking operation in which the concentration (activity) of oxygen dissolved in the molten steel is reduced to the desired level. In addition to producing good steel by eliminating porosity and minimizing non-metallic inclusions， deoxidation is also used to control grain size to improve the toughness of the steel.

Several strategies for deoxidizing steel have been developed. This can be achieved by adding a metal deoxidizer to the molten steel before or after steel removal， or by vacuum treatment in which the carbon dissolved in the steel is the deoxidizer. In addition to deoxidation with metal deoxidizers and vacuum deoxidation， another method of deoxidation is sometimes used， namely diffusion deoxidation.

Depending on the degree of deoxidation， there are four types of deoxidation， ranging from complete deoxidation to slight deoxidation. None of the types is better than the others， but each type is useful in its own way. Depending on the degree of deoxidation， carbon steels can be subdivided into four groups.

Rim steels - These are partially deoxidized or non-deoxidized low carbon steels that evolve a sufficient amount of carbon monoxide during solidification. Edge steel ingots are characterized by good surface quality and a fair amount of porosity. Fringed steels are usually produced without the addition of a deoxidizer to the steel in the furnace or with only a small amount of deoxidizer to the steel in the ladle in order to have enough oxygen present to obtain the desired gas evolution by reacting with carbon in the mold. The exact procedure followed depends on whether the carbon content of the steel is in the higher range (C=0.12 % to 0.15 %) or in the lower range (C=0.10 % or more). As the liquid steel in the ingot mold begins to solidify， carbon monoxide (CO) gas evolves rapidly， resulting in a relatively clean ingot skin with a low carbon and other solute content. This type of ingot is best suited for the manufacture of electrode rods and steel plates.

Capped Steel - The capped steel practice is a variation of the trimmed steel practice. It allows trimming to begin normally， but after a minute or more， the trimming action is terminated by sealing the mold with a cast iron cap. This practice is usually applied to steels with carbon content greater than 0.15%. The practice of capping ingots is commonly used in the production of plate， strip， wire and bar.

Semi-Dead Steels - These are incompletely deoxidized steels that contain a certain amount of excess oxygen that reacts with carbon to form a sufficient amount of carbon monoxide during the solidification of the steel to offset the solidification shrinkage. These steels typically have a carbon content between 0.15% and 0.30% and have most applications in structural shapes.

Killed Steels - These steels are highly deoxidized and have no carbon monoxide formation and evolution during solidification. Ingots and castings of killed steel have a homogeneous structure without gas pores (porosity). Aluminum is used for deoxidation， as well as iron alloys of manganese and silicon. In some cases， calcium silicide or other special strong deoxidizing agents are used. To minimize piping， almost all killed steel is cast in a hot-topped， big-end mold. For continuous casting， the steel is completely killed to achieve defect-free casting. Killed steel is typically used when the finished steel needs to have a uniform structure. Alloy steels， forged steels and steels used for carburizing all fall into this category， when the basic quality is sound. In the production of certain ultra-deep stamped steels， low carbon steels (C=0.12 % or more) are killed， usually with large amounts of aluminum added to the ladle， die or both. Although the deoxidizing effect of aluminum on steel inhibits the formation of carbon monoxide during solidification， thus inhibiting blow holes， the killing of steel by aluminum is undesirable in many steel processes.

There are three main elements used in the deoxidation of steel. They are manganese (Mn)， silicon (Si) and aluminum (Al). Manganese and silicon are added in the form of high or low carbon iron alloys or silicon-manganese alloys. The purity of aluminum added for deoxidation is about 98%. Sometimes calcium (Ca) is also used for deoxidation.

Calcium is the most effective deoxidizer and silicon is not as efficient compared to calcium. Al is also a strong deoxidizing element compared to Si. Although Ca and Al are very efficient deoxidizers， they oxidize very quickly and， in addition， they have a much lower density than steel. In addition， Ca has a boiling point of 1485 degrees Celsius， which means that Ca is gaseous at steelmaking temperatures. An appropriate injection method or addition method is necessary for deoxidation with Ca.

Deoxidation can be carried out by single elements， such as Si， Al， Mn， etc.， or by mixed elements， such as Si+Mn， Ca-Si-Al， etc. Deoxygenation by single elements is called simple deoxygenation.

And the deoxidation of mixed elements is called complex deoxidation. In both simple and complex deoxidation， oxides are formed; therefore it is also known as precipitation deoxidation. Deoxidation is also carried out by carbon under vacuum; this is known as vacuum deoxidation. Elements are added to ferroalloys in the form of FeSi， FeMn or FeSi + FeMn. In complex deoxidation using Si + Mn， Ca + Si， Ca + Si + Al mixtures， there are the following advantages compared to simple deoxidation

Lower dissolved oxygen.

Due to the formation of liquid deoxygenation products， large size product agglomerates can be easily obtained and can be easily floated.

Deoxidation with Fe-Mn

When steel is partially deoxygenated with Mn， iron is also involved in the reaction， forming liquid or solid Mn (Fe) O as a deoxygenation product.

[Mn] + [O] = MnO

[Fe] + [O] = FeO

The equilibrium state of steel with the deoxygenation product Mn (Fe) O is shown in Figure 1.

Fig. 1 Manganese and oxygen content of Fe in equilibrium with FeO-MnO liquid or solid solutions

 Deoxidation with Si and Mn

The deoxidation of Si is more complete than that of Mn， and using both elements together will result in less residual oxygen in solution because of the reduced activity of Si. Depending on the concentration of Si and Mn added to the ladle， the deoxidation product will be either molten manganese silicate (MnO.SiO2) or solid silicon dioxide (SiO2).

[Si] + 2[O] = SiO2 (1)

[Mn] + [O] = MnO (2)

One of the early studies of the equilibrium of the slag-metal reaction is attributed to Korber and Oelsen， who measured the equilibrium distribution of manganese and silicon between liquid iron and MnO-FeO-SiO2 slag saturated with SiO2. The results of their experiments at 1600 ± 10 degrees C are shown in Figure 2.

Fig. 2 Concentrations of Mn， Si and O in liquid iron in equilibrium with SiO2. Saturated manganese silicate melted at 1600±10 degrees C

Deoxygenation with Si， Mn and Al

Half-dead steels with residual dissolved oxygen in the range of 40-23 ppm are deoxidized by adding small amounts of Al and Si-Mn or a combination of FeSi and FeMn to the ladle. In this case， the deoxidation product is a liquid Mn-Al-SiO2 with a composition similar to 3MnO.Al2O3.SiO2. By adding a small amount of Al， e.g. about 15 kg of Al and Si/Mn in 100 tons of heat， almost all of the Al is consumed in this combined deoxidation with Si and Mn. The residual dissolved aluminum in the steel will be less than 10 ppm. figure 3 shows the deoxidation balance with Si and Mn， compared to the deoxidation product with Al， Si and Mn， saturated with Al2O3.

Figure 3 Deoxygenation equilibrium with Si and Mn， compared with deoxygenation equilibrium with Al， Si and Mn saturated with deoxygenation products

 Deoxidation with aluminum

Aluminum is a very effective deoxidizer and is used in most steelmaking operations. Usually， the deoxidation of aluminum takes place in the ladle. In some cases， the addition of aluminum also takes place in the mold during ingot or continuous casting. Figure 4 shows the apparent equilibrium relationship of the deoxidation products: pure Al2O3 and molten calcium aluminate with a CaO/Al2O3 ratio of 1.

Fig. 4 Deoxidation of Al with Al2O3 or liquid calcium aluminate in equilibrium， with CaO/Al2O3 of 1

 When the killed Al steel is treated with Ca-Si， the alumina inclusions are converted to molten calcium aluminate. When the ratio of CaO/Al2O3 is 1， the activity of Al2O3 is 0.064 in the temperature range of 1500-1700 degrees Celsius， compared to pure Al2O3.