Deoxygenation:steel-makingandcastingThe reaction of reducing oxygen content in steel during the process.Is a guaranteeIngot(billet) and steel quality.The two basic methods of deoxidation are precipitation deoxidation and diffusion deoxidation.
staysteel-makingandcastingThe reaction of reducing oxygen content in steel during the process.Is a guaranteeIngot(billet) and steel quality.Oxygen in molten steel in dissolved form ([O]) orNon-metallic inclusionThe form (MxOy) exists.In the process of oxygen blowing steelmaking, the content of [O] in molten steel increases with the decrease of impurity content.If the molten steel without deoxidation is poured out of the furnace, the dissolved oxygen in the molten steel will react with carbon in the condensation process, and the Co bubbles will be separated to boiling;With the different degree of deoxidation, the characteristics of the precipitated gas have significant differences.The fully deoxidized liquid steel is calm without Co precipitation and boiling during condensation, so it is called killed steel;Mildly deoxidized liquid steel reacts with carbon and oxygen during condensation, precipitating Co, and has obvious boiling phenomenon. This is rimmed steel;When the partially deoxidized molten steel solidifies for a period of time, the remaining oxygen reacts with carbon to generate short-term boiling, it isSemi killed steel。Figure 1 shows the range of oxygen content in steels with different degrees of deoxidation.The deoxidation of rimmed steel, that is, mild deoxidation, only slightly reduces the oxygen content in molten steel, but still exceeds the content required for carbon balance.The deoxidation of semi killed steel, namely partial deoxidation, can roughly make the oxygen content of steel reach the content in balance with carbon, so it is also called balance steel.Deaeration of killed steel, that is, sufficient deoxidation makes the oxygen content of steel much lower than that in equilibrium with carbon.Although deoxidation of molten steel reduces [O] content, if deoxidation product MxOyIt floats up and is discharged, and remains in molten steelnon-metallic inclusions in steel , it will affect the quality of steel;Therefore, deoxidation products should be excluded as far as possible during deoxidation.Therefore, deoxidation is to reduce the total oxygen deficiency ∑ O (∑ O=[O]+OMxOy)。However, no matter how the steelmaking technology develops and improves, there are always inclusions left in the steel.Therefore, the shape, size and distribution of residual inclusions must be controlled during deoxidation to ensure the various properties of steel (see non-metallic inclusions in steel).
Range of oxygen content in steels with different degrees of deoxidation:
Proper deoxidation can also ensure the grain size of steel.Aluminumvanadium、titanium、zirconium, then makeAustenite grainThe coarsening temperature increases.During deoxidation, these elements generate not only oxides, but also nitrides;However, vanadium, titanium, niobium and zirconium can also form carbide.The particles of these three compounds can prevent grain growth, of which carbide is the most effective, while aluminum nitride is more effective than aluminum oxide.Aluminum is a strong deoxidizer and the most commonly usedGrain refiner, soFine grain steelIt can only be killed steel, while rimmed steel andSemi killed steelOnly coarse grain steel.However, semi killed steel with a small amount of niobium and vanadium can obtain fine grain steel.The deoxidation process of steel is usually combined with the alloying of steel.Proper combination of the two will help to control the composition accurately and improve the yield of alloying elements.
Deoxidation method
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The two basic methods of deoxidation are precipitation deoxidation and diffusion deoxidation.
Precipitation deoxidation principle Precipitation deoxidation is also called direct deoxidation. Elements with affinity to oxygen stronger than iron are directly added to the liquid steel in the form of ferroalloy or metal block, and form insoluble precipitates M with oxygenxOy, general MxOyThe density of is less than that of molten steel and can be removed by floating upward.The reaction formula of element precipitation deoxidation in molten steel is generally aMxOyIs the activity of deoxidation product;AM is the activity of deoxidizing element;A is the activity of dissolved oxygen.When the deoxidation product is pure oxide or saturated, aMxOy=1。So, take its reciprocal K=1/KM=[aM] x [ao] y, calledDeoxidation constant, to judge the deoxidization ability of elements.At a certain temperature and concentration, the smaller the deoxidation constant K of an element, the lower the oxygen content in equilibrium with the element, and the stronger the deoxidation ability of the element.The determination of deoxidization capacity of various elements is now commonly usedsolid electrolyteThe deoxidization capacity of various elements in molten iron at 1600 ℃ is measured by the electromotive force method with the oxygen determination probe.When the element content is 0.1%, the deoxidation ability of various elements changes from strong to weak in the following order: Al,Ti，B，Si，C，V，cr，Mn。When the content of deoxygenated elements increases, the oxygen content in equilibrium with them decreases;However, when the deoxygenated element content reaches a certain value, the corresponding equilibrium oxygen content will increase with the increase of deoxygenated element content.Moreover, the stronger the deoxidization ability of the element, the lower the critical content (turning point) of the increase of its equilibrium oxygen content.The oxygen content in equilibrium with different element content at different temperatures can be calculated using the data in Table 1.It can be seen from the relationship between the deoxidation constant K and the temperature D that the value of the deoxidation constant K increases when the temperature rises, that is, when the temperature rises for deoxidation, the balance moves to the left;When the temperature decreases, the balance moves to the right.This means that when the temperature of the molten steel decreases (when the molten steel solidifies), the deoxidation reaction will continue and form new deoxidation products, which are often too late to be excluded from the steel.Therefore, an appropriate amount of deoxidizer should be added to deoxidize the steel, so that the residual oxygen content in the molten steel can be immediately reduced to a relatively low level, so as to reduce the generation of deoxidized products during solidification cooling.
Influence of different deoxidation methods on inclusions in steel
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High quality steel requires liquid steel to be as pure as possible.non-metallic inclusions in steel A considerable part of them are oxides, and the oxygen content in steel is often taken as an important indicator to measure the quality of steel.In addition, TiN is a hard and brittle inclusion with regular shape, which is especially harmful to the properties of steel.The main way to control TiN inclusions is to reduce the content of Ti and N in molten steel.[1]
In order to obtain a lower oxygen content, deoxidizer should be added during tapping to generate deoxidized products, most of which can be removed from the molten steel.During tapping, the molten steel is completely exposed and directly contacted with the atmosphere, which will lead to nitrogen absorption. Oxygen is a surface active element, which can hinder the nitrogen absorption of molten steel. If the oxygen content of molten steel is high during tapping, the nitrogen absorption of molten steel will be relatively reduced.Therefore, if strong deoxidizer is used for deoxidation during tapping, the oxygen content in molten steel will be greatly reduced, and nitrogen absorption will rapidly increase, which is extremely unfavorable for controlling nitrogen content.[1]
Oxygen and nitrogen content obtained by different deoxidation methods
For the final product, the nitrogen content can be effectively controlled by means of deoxidation (II), and the nitrogen mass fraction obtained is 5 × 10 lower than that obtained by means of deoxidation (I)－6about.Observe the total oxygen mass fraction before LF treatment by two different deoxidation methods. The average total oxygen mass fraction reached 146.38 × 10 by deoxidation method (Ⅱ)－6The average nitrogen mass fraction is only 16.12 × 10－6； However, the average total oxygen mass fraction of deoxidation method (I) is 21.18 × 10－6The average nitrogen mass fraction is 20.92 × 10－6, which is obviously higher than the nitrogen mass fraction of deoxidation in mode (II).The oxygen content here also verifies the results of thermodynamic analysis. High oxygen content in molten steel during tapping can effectively control the nitrogen absorption of molten steel. From the perspective of nitrogen control, the weak deoxidation mode of tapping is more favorable.[1]
Inclusion formation during deoxidation
1 Appearance of inclusions produced by deoxidation:
The morphology of main inclusions when deoxidizing LF enters the station by means of mode (II).Inclusions are mainly dark and light: according to energy spectrum analysis, the dark part of each inclusion is mainly SiOtwo, the light part is mainly MnO SiOtwoAnd a small amount of MnS, as well as some titanium compounds. The inclusions in the steel are mainly MnO - SiOtwoComposite inclusion, which has two forms, one is MnO - SiOtwoInclusions are wrapped with SiOtwoThe other is complete MnO - SiOtwoInclusion;In addition, there is a very small amount of individual SiOtwoInclusions.The size of composite inclusions is large, and most of them exceed 20 μ m. A certain number of inclusions with a size of more than 50 μ m can also be found under the scanning electron microscope.[1]
2. Generation process of deoxidized inclusion:
According to the thermodynamic analysis, it can be inferred that the formation process of inclusions in the case of weak deoxidation of Si Mn:
(a) Si -- Mn is weakly deoxidized, and the deoxidizing capacity of Si is stronger than that of Mn, so the oxygen in molten steel reacts with Si first to generate SiO during tappingtwoInclusion;
(b) Mn and SiO graduallytwoReaction generates liquid MnO - SiOtwoSiOtwoWrapping, the inclusion is spherical, with the reaction going on, the outer layer of MnO - SiOtwoGradually more SiO in the coretwoGradual dissolution;
( c) MnO--SiOtwoSiO coatingtwoThe composite inclusions of MnO and SiO collide and polymerize with each other to form MnO SiOtwoWrapping multiple SiOtwoComposite inclusions in the core, due to MnO - SiOtwoIt is liquid, so the inclusion is still spherical;
(d) Multiple SiO in composite inclusiontwoThe core is not only continuously dissolved, but also in liquid MnO - SiOtwoMigration and collision polymerization occurred in the matrix, showing two SiOtwoKernel collision;
(e) As the reaction proceeds, SiOtwoThe core is finally completely dissolved in MnO - SiOtwoIn the matrix, form completely liquid MnO - SiOtwoInclusions.[1]
Transformation of inclusions in refining process
The morphology and energy spectrum of the main inclusions before calcium treatment in deoxidation LF refining by means of mode (I).Inclusions before calcium treatment are mainly spherical CaS and CaO -- MgO -- AltwoOthreeInclusions.The reason for a large number of spherical inclusions is that the alkalinity of slag system is very high, which makes AltwoOthreeThe inclusion is easy to form calcium aluminate inclusion, which has a low melting point and is liquid in molten steel.
The morphology and energy spectrum of main inclusions before calcium treatment in deoxidation LF refining by means of mode (II).Inclusions before calcium treatment are also mainly spherical CaS and CaO - MgO - AltwoOthreeInclusion: inclusion does not contain Si, which is consistent with the result of mode (I), and its size is not significantly different.Production of MnO SiO by weak deoxidation of Si Mn during tappingtwoCompound inclusions, some of which float up to slag and some remain in molten steel. When Al wire is fed into steel, Al will quickly remove MnO - SiO remaining in molten steeltwoReduction of composite inclusion to generate AltwoOthreeInclusions and Si Mn make the main inclusions in steel become Al againtwoOthreeInclusions.When refining with high alkalinity slag, AltwoOthreeThe inclusion forms calcium aluminate inclusion, which is consistent with the result of strong deoxidation in tapping.[1]
Inclusion detection method and deoxidation thermodynamics basis
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From several aspectsnon-metallic inclusions in steel The basic research work is briefly introduced: the evaluation method of inclusions in steel and molten steel cleanliness, the definition of steady casting and unsteady casting, the corresponding relationship between large particle inclusions formed during unsteady casting, total oxygen and large particle inclusions, the thermodynamic basis for the formation of inclusions during deoxidationThe dynamics of inclusions nucleation and growth in molten steel, fluid flow in molten steel, movement and removal of inclusions, and capture by solidification front.[2]
Thermodynamics of unitary deoxidation
Al is one of the most commonly used deoxidizers in steel production. As AI is a strong deoxidizing element, it is often used as the final deoxidizer.When the chemical equivalent of Al in molten steel exceeds oxygen, Al will be generatedtwoOthreeAnd AlN, large size cluster AltwoOthreeIt will lead to clogging of nozzle and affect steel production.Therefore, adding a proper amount of deoxidizer can not only save costs, but also improve the quality of steel.[2]
Si is a strong deoxidizing element, and it is easy to generate solid SiO when deoxidizing with Si alonetwo, which is not conducive to the floating removal of deoxidized products.
Ti can induce intragranular ferrite to refine the structure and improve the height and toughness of steel. With the continuous improvement of technology and cost of smelting Ti, Ti has become an important alloying element in many high-grade steels.[2]
Thermodynamics of binary deoxidation
The control of inclusions generated by deoxidation of Mg and Al is very important, and the deoxidation product MgAltwoOfourSpinel inclusions have the characteristics of high melting point and high hardness, which are difficult to be crushed after rolling, and are prone to crack and other defects.At the same time, in the process of deoxidation with Al, it is difficult to avoid the formation of MgAl due to the mass transfer reaction of MgO in the slag and the erosion reaction of MgO in the refractory liningtwoOfour, which has been puzzling steelmaking producers.[2]
Thermodynamics of ternary deoxidation
Ca treatment process has been gradually applied to continuous casting steel since 1970s. One of its purposes is to avoid the formation of solid AltwoOthree, adding a certain amount of Ca can transform solid AltwoOthreeIt turns into liquid calcium aluminate, so as to reduce nozzle blockage during continuous casting.The research shows that too much or too little Ca feeding in the process of molten steel feeding will not only be ineffective in improving nozzle clogging, but will also aggravate the degree of nozzle clogging.This is mainly because there are only 12CaO · 7Al among various compounds of calcium aluminatetwoOthreeAnd 3CaO · AltwoOthreeIt is liquid at the temperature of molten steel, and other compounds are solid.[2]
