Metal deoxidation. Big encyclopedia of oil and gas

With all methods of steel production - open-hearth, converter, electric steelmaking - the oxygen content continuously increases as impurities (Si, Mn, C) burn out. At the end of melting, the content of dissolved oxygen in the liquid metal is determined mainly by the carbon concentration; The oxygen concentration reaches its maximum values ​​at a low carbon content. Upon reaching the specified carbon content in order to avoid the harmful effects of oxygen (in the form of inclusions of ferrous oxide and in dissolved form) on the properties solid metal and in order to obtain a high-quality ingot at the end of the melting, the steel is subjected to deoxidation. During deoxidation, the steel is cleared of oxygen (decrease in the concentration of dissolved oxygen) and, possibly, the deoxidation products are completely removed from the metal. The oxygen remaining in the metal in an inactive form affects the properties of the finished steel to a much lesser extent.
In metallurgical practice, the following ways steel deoxidation, a) precipitating deoxidation; b) diffusion deoxidation; c) deoxidation with synthetic slags; d) deoxidation in vacuum.
With precipitating deoxidation, the most common method, a decrease in the concentration of oxygen dissolved in a liquid metal is achieved by binding it with deoxidizing elements (Mn, Si, Ti, Zr, Al, Ca), which have a greater affinity for oxygen than iron.
When a deoxidizer (E) is added, the following interaction occurs in the metal:

x[O] + y[E] = EyOxg.l.t

with the formation of an oxide of the deoxidizing element in a gaseous, liquid or solid state, insoluble in steel. The degree of decrease in the concentration of dissolved oxygen is due to the deoxidizing ability of the deoxidizing element, which depends on the concentration of oxygen dissolved in liquid iron, which is in equilibrium with a certain concentration of the deoxidizing element,
With an increase in the affinity of the deoxidizing element for oxygen, its deoxidizing ability increases (Fig. 57).
Thermodynamic data of the deoxidation reaction are given in table. 14.

The resulting deoxidation products, due to their lower specific gravity, are removed from the metal to one degree or another. The completeness of purification of liquid steel from deoxidation products depends on the size, composition and properties of the particles, their ability to enlarge, metal wettability, viscosity and temperature of the metal. The most favorable conditions for the coarsening of particles and their floating from liquid steel are created during the formation of liquid low-melting deoxidation products, which is usually characteristic of oxides of elements (Mn, Si) with low deoxidizing ability. With an increase in the deoxidizing ability of the elements (Al, Ti, Zr), the melting point of the particles increases, which makes it difficult to obtain steel with a low oxygen content. It is advisable to use complex deoxidizers (Si-Mn, Si-Ca, Al-Mn-Si, Si-Al-V-Zr, Al-Si-Ca, etc.), under the action of which relatively fusible, capable of coarsening and rapid floating up are formed. deoxidation products.
The most widely used deoxidizers are manganese, silicon (in the form of ferroalloys), and aluminum.
Manganese is a relatively weak deoxidizer, but it is used in the deoxidation of all steels and is indispensable in the production of boiling steel. When deoxidized by manganese, depending on its content in liquid steel, xMnO * yFeO solutions are formed in a solid or liquid state. As the residual manganese in the metal increases, the content of MnO in the deoxidation products increases up to the formation of free MnO.
Silicon is a stronger deoxidizer. The products of silicon deoxidation with an increase in its content in steel are liquid iron silicates up to solid silica.
The joint deoxidation with manganese and silicon gives rise to manganese and iron silicates, the composition of which depends on the ratio of the concentrations of Mn, Si, and oxygen. In the presence of manganese, the deoxidizing ability of silicon increases.
Aluminum is a very active deoxidizer. With the introduction of aluminum in excess, which is usually done in practice, solid fine particles of alumina are formed. With an insufficient concentration of aluminum in the metal, particles of FeO*Al2O3 are formed.
Diffusion deoxidation, based on the law of iron oxide distribution between metal and slag, is reduced to slag deoxidation. A decrease in the concentration of FeO in the slag due to its deoxidation causes the diffusion of oxygen from the metal into the slag until an equilibrium distribution between both phases at a given temperature:

To deoxidize the slag, powdered deoxidizing mixtures are applied to its surface - coke, charcoal, rich in ferrosilicon, aluminum. During diffusion deoxidation, the metal is not contaminated by deoxidation products, but its implementation requires a reducing atmosphere and a long time, which is associated with a decrease in furnace productivity. This deoxidation method is widely used in the smelting of high-quality steel in electric arc furnaces, where a reducing atmosphere can be created without much difficulty.
The deoxidation of steel with synthetic slags (acidic or basic with a low content of FeO) is also based on the extraction of FeO from the metal, according to the distribution law. According to this method of deoxidation, steel is poured into a ladle with liquid synthetic slag. Due to the emulsification of the slag, the deoxidation proceeds at a high rate. When processing steel with synthetic basic slags, in addition to deoxidation, desulfurization of the metal is possible.
Deacidification practice. Depending on the degree of deoxidation of steel, boiling, semi-calm and calm steel are distinguished (Fig. 58).
Boiling steel - partially deoxidized (by manganese and carbon during boiling) steel, solidifying in molds with abundant release of gases, which are mainly (up to 90% CO) products of the interaction of carbon and oxygen dissolved in the liquid metal. The structure and quality of the boiling steel ingot depends on the intensity of gas evolution (Fig. 58, a). Boiling steel is smelted in open-hearth furnaces and converters; it contains from 0.02 to 0.27% carbon (rarely up to 0.35%) and up to 0.6% manganese. The main deoxidizer of boiling steel is carbon 75% ferromanganese, which is introduced into the furnace or into the ladle. Deoxidation in a ladle is more economically feasible, while the consumption of ferromanganese is reduced (up to 25%) and the duration of melting is reduced (by 5-15 minutes). Manganese waste during deoxidation in a ladle is 20-40%, during deoxidation in a furnace - 35-70%.
Semi-calm steel in terms of degree of deoxidation occupies an intermediate position between boiling and calm. The amount of deoxidizers added to the metal is not enough to completely prevent the release of gases, therefore, gas bubbles and an underdeveloped shrinkage cavity are observed in the semi-quiet steel ingot (Fig. 58, b). Semi-quiet steel ingots have greater chemical homogeneity than boiling steel ingots.

Semi-quiet steel is smelted in open-hearth furnaces and converters; it contains 0.1-0.3% C, 0.35-0.85% Mn and up to 0 15% Si. Semi-quiet steel is deoxidized either in a furnace (ferromanganese, blast-furnace ferrosilicon), and then in a ladle (ferrosilicon, silicon carbide, aluminum, ferrotitanium), or only in a ladle. Sometimes a small amount of aluminum (0.018-0.05 kg/t) is added to the mold. Semi-quiet steel is also obtained in bottle molds, in which metal boiling can be controlled.
Quiet steel is deoxidized by an excess of strong deoxidizers, which exclude the possibility of the interaction of dissolved oxygen with carbon during cooling and solidification of the metal in the mold. A calm steel ingot is characterized by a relatively small segregation, a dense structure, and the presence of a shrinkage cavity, concentrated in the profitable part (Figure 58, c). In terms of chemical composition, calm steel is very diverse; carbon (up to 2.0% C) and alloy steels are smelted. Quiet steel is smelted in open-hearth, electric arc furnaces and in converters.
The practice of deoxidizing calm steel is very diverse. With all methods, they strive to obtain a well-deoxidized steel with the lowest possible content of oxide inclusions, in the presence of which the quality of the metal deteriorates greatly. Contamination of steel with oxide inclusions depends on the method and sequence of introduction of deoxidizers.
Carbon and low-carbon ferromanganese, mirror cast iron, blast-furnace and 45% ferrosilicon, oily manganese, aluminum, ferroaluminum, silicocalcium, silicoaluminum, alsical silicon carbide, silicozirconium, etc. are used as deoxidizers. ladle. Sometimes steel is deoxidized in a ladle without first being deoxidized with silicon in a furnace.
To reduce the contamination of steel with oxide inclusions and for their more uniform distribution, aluminum, silicocalcium or alsikal has recently been introduced into the ladle through special tubes. A method has also been proposed for deoxidizing steel in a ladle with liquid aluminum.

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The deoxidation of steels during welding is carried out by alloying the weld pool with elements with a high affinity for oxygen: manganese, silicon, titanium, aluminum. These elements are introduced either from the electrode wire, or from the electrode coating, or from welding fluxes as a result of exchange reactions.

Steel deoxidation is carried out in order to reduce the number of non-metallic inclusions and refine grains. The fewer non-metallic inclusions and the more uniformly they are distributed in the steel, the lower its brittleness threshold and the higher the strength and toughness. The impact strength also depends on the grain size of the steel: the finer the natural grain, the higher the impact strength.

Steel deoxidation is carried out with the help of such metals, in which the chemical affinity for oxygen is greater than that of iron. Typically, manganese, silicon and aluminum are used for deoxidation.

The deoxidation of steel in the main scrap process begins even at pure boiling, as FeO is consumed. The final deoxidation is carried out similarly to the deoxidation of converter steel. Ferromanganese and blast-furnace ferrosilicon are loaded into the furnace, rich in ferrosilicon (45% Si) and aluminum, when tapping the melt, into a chute or ladle. The main scrap process, as a rule, is smelted calm steel.

The deoxidation of steel in the main scrap process begins even at pure boiling, as FeO is consumed. The final deoxidation is carried out similarly to the deoxidation of converter steel. Ferromanganese and blast-furnace ferrosilicon are loaded into the furnace, rich in ferrosilicon (45% Si) and aluminum, when tapping the melt, into a chute or ladle. The main scrap process, as a rule, is smelting calm steel.

The deoxidation of steel is carried out to remove oxides from it and mainly iron, which causes red brittleness of steel and a decrease in mechanical properties. According to the conditions of deoxidation, calm and boiling steel are distinguished. Quiet steel is deoxidized with ferromanganese, ferrosilicon and aluminum; the fracture of the ingot is dense. Boiling steel is deoxidized only by ferromanganese; it contains a significant amount of gas bubbles that are welded during rolling. Deoxidation with blast-furnace ferromanganese and ferrosilicon is carried out directly in the converter, and deoxidation with 45% ferrosilicon and aluminum is carried out in the ladle when they are introduced into the metal stream when it is drained from the converter into the ladle. Putting them into the converter will not achieve the goal - because of their lightness, they will not sink into the metal.

Steel deoxidation in the acid process is similar to the open-hearth process, but there is also a difference in diffusion deoxidation, which requires the removal of oxidizing slag and the production of a new one from ferromanganese, fireclay and ground ferrosilicon, coke or charcoal. After exposure under it for 20 - 40 minutes, the slag brightens due to the reduction of oxides and the metal deoxidizes.

Steel deoxidation should be carried out at the following consumption of aluminum: not more than 0 5 kg / t for molybdenum steel grades and not more than 0 7 kg / t for chromium-molybdenum steel grades.