The deoxidation reaction of steel
Oxygen is to ensure the quality of steel ingot and an important operation. Oxidation of steel is a refining process, liquid steel inevitably a certain amount of dissolved oxygen. 1600, the oxygen solubility in liquid steel up to 0.23% (see Fe-O state diagram). Oxidation refining stage, the oxygen content of liquid steel by steel-making methods, steel specifications vary, generally about 0.02 to 0.08% range, while the oxygen solubility in solid iron is very low (for example, [kg2] – Fe [kg2] the solubility of up to 0.0082%). In the solidification of liquid steel, the oxygen precipitates to form FeO, distributed on the grain boundaries, reducing the steel’s ductility. Grain boundary on the formation of FeO and FeS also low melting point (910) material, the thermal processing of steel in the heat when crisp. Not fully deoxidized liquid steel in the ingot mold during solidification, the solid steel as low dissolved oxygen, oxygen concentration in the liquid steel in gradually, over the value of the excess carbon and oxygen balance of oxygen will react with carbon to produce CO gas bubbles within the ingot, serious will happen, “take up” phenomenon. Thus, in the final stages of steel must be oxygen. There are three main methods: precipitation deoxidation, degassing and vacuum oxygen diffusion.
Precipitated by adding oxygen to the liquid steel in the oxygen affinity of the larger elements than iron (oxygen scavenger), so that dissolved oxygen and steel together to form insoluble in liquid steel and precipitation of oxides or mixed oxides, the reaction is:
[160-01] (1) where [kg2] [M], [O] represent the dissolved oxygen in the steel elements and oxygen;, representing the steel [M] and [O] [kg2] of activity ; is the equilibrium constant of oxygen, its last known oxygen constant, the smaller the value, the stronger the ability of the element oxygen. Constant table of the elements of oxygen [reaction of the elements of oxygen and oxygen constant]. The element oxygen capacity from strong to weak order: cerium, zirconium, aluminum, titanium, boron, silicon, carbon, vanadium, manganese, chromium. In the use of cheaper production of manganese, silicon, aluminum for deoxidation agent. And its alloy (such as ferromanganese, ferrosilicon) in the form of added steel. Precipitation deoxidation products if not removed, will become a solid non-metallic inclusions in steel, affecting the quality of steel. The density of deoxidation products (typically 3 to 5 g / cm) than the liquid steel (7.1 g / cm) is small, can go up ruled out, the floating rate (cm / s) may Stokes (Stokes) obtained approximate formula : [160-02] (2) for the acceleration of gravity (981 cm / sec); for the viscosity of liquid steel (Park); for the deoxidation product of the radius (cm); [160-100], [160-101 ] are the product of deoxidation of molten steel and the density (g / cm). By (2) shows that the radius increases, the floating rate increased significantly. If the “composite deoxidizer” such as Si-Mn, Si-Ca, Si-Mn-Al, Mn-Si-Ca, etc. to deoxidation, deoxidation product is a mixture of oxides of these elements, and its melting point than a single element oxides The lower melting point, easy to aggregate into big particles can quickly go up excluded. Has been used in the production of these composite deoxidation agent. Since the 1970s, with the gun through the development of the carrier gas (argon or nitrogen) would powder deoxidizer (such as calcium, magnesium, rare earth metals, aluminum, ferrosilicon, etc.) directly into the liquid steel approach allows more oxygen to the removal low levels (ppm order of magnitude). Blown calcium (boiling point about 1484) into a bubble in the steel-making temperatures rise in the bubble – liquid interface of steel deoxidation reactions also occur. This oxygen is also known as dusting oxygen, and its essence is still a sediment oxygen (see injection metallurgy).
Reduction in the electric arc furnace steelmaking oxygen diffusion period, the slag containing [kg2] FeO [kg2] very little oxygen in liquid steel into the slag will react in the following:
[160-03] This is called the spread of oxygen. For oxygen diffusion constant, it is necessary to add oxygen into the slag batches agent (commonly ferrosilicon powder, toner, electricity powder, also used in some alloy aluminum powder, calcium powder powerful deoxidizing agent) to residue to maintain low [kg2] FeO [kg2] content. The product of diffusion deoxidation of liquid steel does not stain, which is in a better quality steel smelting deoxidation method. The disadvantage is that the reaction was slow, take a longer time, resulting in erosion of the lining of high temperature slag more serious.
These two methods have advantages and disadvantages of oxygen, to give full play their respective advantages, but also widely spread by precipitation with an integrated oxygen. Reduction in the electric furnace that period at the beginning, first settling in liquid steel deoxidation quickly reduce the oxygen content. When a thin slag formation, and then spread oxygen. In the oxygen diffusion during precipitation deoxidation products have sufficient time to go up, then plug the end of the last in the reduction of aluminum deoxidation. This will improve the quality of steel, but also shorten the refining time.
Vacuum deoxidation in the molten steel in low pressure oxygen to respond more fully and to the oxygen reaction:
[161-01], when a certain temperature in a vacuum, the lower the [% C] [% O] plot also reduce carbon deoxidation capacity enhancement. Vacuum is actually a low pressure oxygen liquid containing carbon steel deoxidation. Its advantages are: deoxidation products is a gas, not the formation of non-metallic inclusions, CO floating bubbles, stirring bath will help to nitrogen dehydrogenase. Can not be carried out at atmospheric pressure and oxygen reaction of molten steel in a vacuum may be continued under the oxygen reaction, the oxygen reduced to a lower value, while reducing the value of carbon, compared with a decline in the value of the oxygen of 3 / 4. Vacuum deoxidation of liquid steel in a vacuum melting or vacuum processing for.
Refining blown into the ladle with argon gas to liquid steel deoxidation, decarbonization, in addition to the inclusions of gas and process, is due to argon partial pressure of CO in the bubble is very low, prompting more complete oxygen reaction, which can also play a similar to the oxygen and the role of the vacuum (see vacuum metallurgy).