Since steel smelting takes place under substantially oxidizing conditions, oxygen inevitably dissolves in large quantities in the iron melt. Oxygen solubility in pure iron at a temperature 1600 ºС is 0,23 % and reaches 0,48 % at 1800 ºS. The oxygen content in iron is inversely proportional to the content of impurities in it, as well as carbon. Therefore, in the process of refining steel from impurities, the oxygen content increases. By the end of refining, liquid steel contains a significant amount of oxygen - about 0,05-0,10 %. If such steel is immediately poured, then since the solubility of oxygen in solid iron is only 0,03 %, excess oxygen is released as gas, which leads to the rejection of steel castings and ingots. therefore, even very clean steel can be irreversibly damaged, unless this excess oxygen is removed from the casting operation prior to casting.
Steel deoxidation = killing of steel
The process of removing residual oxygen from refined steel is known as steel deoxidation.. Interesting, that deoxidation in English uses two terms: deoxidation и killing. The first can be translated as deoxidation, and the second literally means "mortification", which is somewhere close to the term "calming", which is sometimes applied in relation to the deoxidation of steel.
There are two main approaches to deoxidation.:
– diffusion deoxidation of steel;
– deoxidation of steel by precipitation.
Diffusive deoxidation of steel
Dissolved oxygen is reduced by forcing it to diffuse into the slag within the steel furnace itself or by out-of-furnace vacuum treatment in a ladle. This method has limitations in its oxygen removal efficiency..
Deoxidation of steel by precipitation
There are chemical elements, which have a higher affinity for oxygen, that is, they react more easily with him, than iron. Such elements are called deoxidizers.. When a deoxidizer is added to an iron melt, it reacts with oxygen in the melt to form oxides. This method is widely used and is very effective in reducing the oxygen content of steel.. Several factors are taken into account when choosing a deoxidizer..
Thermodynamics of steel deoxidation
The thermodynamically best oxidizer is that, which provides a minimum oxygen content with its minimum residual content in steel.
In this sense, the best deoxidizer is aluminum and it is widely used in practice. Silicon is also an effective deoxidizer. Aluminum and silicon, together with manganese and carbon, are most often used for steel deoxidation due to their relative cheapness..
Elements such as zirconium can be used as deoxidizers., titanium, bor, vanadium, niobium and others, but they are much more expensive, than conventional deoxidizers.
The residual content of the deoxidizer in the steel should not lead to any harmful effect on the properties of the steel.. The content of these elements must be within acceptable limits according to the technical specifications for the final product..
Kinetics of deoxidation of steel
Kinetically, the oxidant must act quickly, to achieve a high proportion of its assimilation in the deoxidation reaction. However, such data are usually absent and when choosing a deoxidizer they are guided mainly by thermodynamic considerations..
Besides, the physics of the process is also very important, so the deoxidation reaction product should not remain in the form of particles mechanically captured by the steel. These particles are known as non-metallic inclusions., which negatively affect the mechanical properties of steel. Mechanical properties of steel vary considerably depending on the quantity, sizes, shape, distribution and composition of these inclusions. Therefore, all sorts of measures are being taken, to purify steel from these deoxidation products and obtain pure steel. The term "steel cleanliness" refers to the relative freedom of steel from these inclusions.
Generally, the kinetics of the deoxidation reaction is not so much the kinetics of the formation of its product, how much is the kinetics of the subsequent purification of steel from this product.
Deoxidation of steel with carbon
Therefore, the ideal deoxidation product is a gaseous product - carbon monoxide. Only carbon produces the gaseous product of the deoxidation operation. At atmospheric pressure, carbon is not an effective deoxidizer. However, its effectiveness is greatly enhanced under reduced pressure.. To obtain a cleaner steel, vacuum treatment of steel is used.
Source: B.P. Bhardwaj, Steel and Iron Handbook