Main Wear Mechanisms of Steel Flow Bricks and Factors Affecting Refractoriness
The chemical composition of mullite steel flow bricks mainly consists of the mullite phase, with a small amount of low-melting iron oxide. The various oxide components in steel flow bricks have different melting points, so different mass fractions of these components result in different refractoriness of the bricks.
The procedure for calculating refractoriness from the chemical composition is as follows: first, calculate the product of the melting point and the corresponding mass fraction for each oxide, and then sum these products to obtain the liquidus temperature of the steel flow brick. The refractoriness of the steel flow brick can be estimated by correcting the liquidus temperature. Meanwhile, the types and amounts of various complex oxides formed in the reaction layer between the steel flow brick and molten steel can also be inferred.
The brick types used in the mould casting system include funnel bricks, nozzle bricks, center bricks, straight flow bricks, end flow bricks, and mould bottom bricks.
The raw mineral composition of steel flow bricks mainly consists of kaolinite (Al₂O₃·2SiO₂·2H₂O) and 6%–7% impurities (oxides of potassium, sodium, calcium, titanium, and iron). The firing process mainly involves the continuous dehydration and decomposition of kaolinite to form mullite (3Al₂O₃·2SiO₂) crystals. Mullite crystals are the only stable form of aluminosilicate at high temperatures, with a theoretical conversion rate of 87.64%. During firing, SiO₂ and Al₂O₃ in the steel flow bricks react with impurities to form eutectic low-melting silicates, which surround the mullite crystals.
01 Composition of Steel Flow Bricks from Different Batches
According to the inspection data, the main component of steel flow bricks is alumina, followed by silica, with a small amount of iron oxide and other impurities. It can be inferred that the physical and chemical properties of steel flow bricks are closely related to the contents of these main components, and changes in these components will have a significant impact on the physical and chemical indices of steel flow bricks.
02 Main Wear Mechanisms of Steel Flow Bricks in Steel–Brick Reactions
During the contact between steel flow bricks and molten steel, the interactions between molten steel and refractory materials include the following aspects:
Firstly, the erosion of molten steel on the refractory causes spalling, in which the refractory falls into the molten steel as a whole. This is a physical process, which usually occurs easily when unstable points such as low-melting pores exist in local areas of the refractory.
Secondly, components in the refractory, such as oxides, carbon, nitrides, etc., chemically react with elements in the molten steel, including Al, Si, Mn, Fe, etc. The reaction interface is large, but the thickness of the chemical reaction layer is small, and this reaction occurs throughout the entire contact process between the refractory and the molten steel.
Since physical melting usually occurs before chemical reactions, and the rate of physical melting loss is much faster than that of chemical reactions, it can be concluded that the main wear mechanism of steel flow bricks in contact with molten steel is physical melting.
The reaction between steel flow bricks and molten steel follows the sequence of physical melting first, followed by chemical decomposition. The decomposition reaction layer is very thin and the reaction rate is slow.
It is reasonable to characterize the refractoriness by the melting point of steel flow bricks, and the calculation method is relatively simple.
03 Calculation Process of the Chemical Composition Method
3.1 Chemical Composition of a Batch of Steel Flow Bricks
After a batch of steel flow bricks arrives at the factory, samples are taken to test their chemical composition.
According to the data in Table 1, the chemical composition is normal, and there is an obvious difference in the mass fractions between the main components and the minor components.
3.2 Calculation of Liquidus Temperature for a Batch of Steel Flow Bricks
The formula for calculating the number of molecules of oxide MxOy is:
a₍MxOy₎ = X₍MxOy₎ / A₍MxOy₎ (1)
Where:
a₍MxOy₎ = number of molecules of oxide MxOy;
X₍MxOy₎ = mass fraction of oxide MxOy, %;
A₍MxOy₎ = molecular weight of oxide MxOy.
During the contact between the reaction layer of the steel flow brick and high-temperature molten steel, the oxides in the reaction layer tend to undergo combination reactions to form low-melting phases.
When only components such as Al₂O₃, SiO₂, CaO, MgO, and Fe₂O₃ are considered, the liquidus temperature of the steel flow brick with an Al₂O₃ content of 65.29% is 1854 °C.
In addition to the oxide components forming mullite, such as Al₂O₃ and SiO₂, the raw material high-alumina bauxite used for steel flow bricks also contains a certain amount of impurities. Among these impurities, Fe₂O₃, K₂O, Na₂O, and others have relatively low melting points. During the contact between molten steel and the refractory surface, the high-temperature molten steel easily dissolves these impurities on the refractory surface, forming liquid channels that penetrate into the interior of the refractory. Therefore, low-melting oxides will reduce the refractoriness of steel flow bricks.
The actual liquidus temperature of the steel flow brick with an Al₂O₃ content of 65.29% should be below 1854 °C.
04 Mineral Phase Composition Formed in Steel Flow Bricks During High-Temperature Reactions
From a thermodynamic perspective, during the contact between molten steel and steel flow bricks, various oxides in the steel flow bricks tend to undergo combination reactions to form low-melting complex oxides.
During the contact reaction between steel flow bricks and high-temperature molten steel, complex compounds are continuously formed in the reaction layer of the bricks. When the reaction temperature of the molten steel is higher than the melting points of these complex oxides, a liquid phase of complex oxides forms in the reaction layer. These liquid-phase complex oxides are generated at the reaction interface and are then entrained into the molten steel by the flowing steel.
Based on the principle that complex oxides with lower melting points form preferentially, combined with the molecular numbers of various oxides, the molecular formulas and quantities of complex compounds in the reaction layer of steel flow bricks can be inferred.
There are 46 molecules of SiO₂, 64 molecules of Al₂O₃, and only 1 molecule each of CaO and Fe₂O₃. Therefore, SiO₂ and Al₂O₃ will combine preferentially. As shown in Table 6, the most preferentially formed complex compound is mullite, followed by calcium aluminate, both of which have low melting points. Fe₂O₃ has the lowest melting point, and the single Fe₂O₃ molecule still exists as an independent oxide.
The molar ratio of Al₂O₃ to SiO₂ in 3Al₂O₃·2SiO₂ is 3:2. Accordingly, approximately 64 parts of Al₂O₃ and 43 parts of SiO₂ form mullite, leaving 3 molecules of SiO₂ remaining. There is only 1 molecule of CaO and 1 molecule of Fe₂O₃. One molecule of CaO, one molecule of Al₂O₃, and one molecule of SiO₂ form one molecule of CaO·FeO·SiO₂. Finally, 2 molecules of SiO₂ remain.
05 Conclusion
(1)The main erosion occurring during the contact between steel flow bricks and molten steel is the physical melting process. The oxide decomposition reaction layer at the interface is thin, and the reaction rate is slow. The liquidus temperature of the reaction between steel flow bricks and molten steel is determined by both the melting points and the corresponding mass fractions of the various oxides in the bricks.
(2)During the interfacial reaction between steel flow bricks and molten steel, the reaction mechanism involves the formation of single oxide complexes from multiple individual oxides. The lower the melting point of a complex oxide, the faster its formation; the higher the mass fraction of an individual oxide, the more readily it undergoes combination reactions.
(3)The method for deriving refractoriness using chemical composition calculations is to obtain an accurate liquidus temperature based on the mass fractions and melting points of each component.