This article first analyzes the damage mechanism of magnesia-chromium refractories, and explores the application of chromium-free refractories such as magnesia-zirconium, magnesia-aluminum spinel series, magnesia-calcium, MgO-C refractories, and magnesium-aron bonding. Advantages and disadvantages, and finally some promotional plans for chromium-free refractory materials for RH refining furnaces.
RH refining furnace; damage mechanism; chromium-free refractories; application development
Magnesia-chrome bricks, which are widely used in current RH refining furnaces, often chemically react with molten steel during actual use, and produce highly toxic carcinogenic substances-hexavalent chromium salt compounds. Pollution to groundwater resources, but also cause serious damage to the ecological environment. In addition, if the residual bricks containing magnesia-chrome bricks are dumped at will, it will also lead to oxidation reaction of Cr3+ into Cr6+ under certain temperature and acid-base environment, which will eventually lead to serious environmental pollution.
In recent years, many developed countries have successively promulgated laws and regulations prohibiting the production and use of chromium-containing products. However, if my country’s iron and steel enterprises want to seek long-term development under the background of fully implementing the sustainable development strategy, they must actively promote the chromium-free refractory material of the RH refining furnace. However, although a series of attempts have been made to apply materials such as magnesia-zirconium, magnesia spinel, magnesia spinel, magnesia-spinel zirconium bricks, and unfired low-carbon magnesia-carbon bricks, none of them has Fundamentally solve the problems of low service life and high cost. After comparative analysis of various materials, the performance of unburned magnesia spinel bricks in various parts of the RH refining furnace is not far behind that of chromium-containing materials, and can significantly reduce material costs, making a big step towards the goal of chromium-free .
Damage mechanism of magnesia-chromium refractories used in RH refining furnace
At present, the refractories commonly used in RH refining furnaces are mainly magnesia-chrome materials. The main damage of magnesia-chrome bricks is the structural spalling and thermal shock damage caused by the infiltration of slag. Among them, the damage mechanism is as follows.
The slag continuously infiltrates along the matrix of the magnesia-chrome brick. During this process, the silicate phase CaSiO3 and Ca MgSi2O6 will continuously fill the periclase grain boundaries in the infiltration layer. The low-melting point phase filling in this process will not only lead to a significant decrease in the degree of bonding of the main crystal phase, but also intensify the dissolution of components inside the magnesia-chrome brick and the continuous separation between the various structures. A large number of silicate phases with a melting point less than 1600°C are produced . Based on this, when the dipping tube is in contact with molten steel, it will directly affect the thermal strength of the magnesia-chrome brick due to the low melting point phase, which will eventually lead to peeling off of the infiltrated layer. In the process of slag infiltration, magnesia-chrome bricks are often affected by many factors such as density, low-melting silicate phase, molten steel erosion and vibration, and gradually crack and peel off. In short, the focus is on two aspects: First, the silicate phase generated in the permeable layer will destroy the crystal phase skeleton structure of refractory materials such as magnesia and reduce the high temperature strength of magnesia-chrome bricks. Second, the formation of this substance will also lead to liquid phase migration, which will directly lead to shrinkage of refractory materials, which will lead to continuous cracks in the interior of magnesia-chrome bricks, and the speed of slag corrosion and peeling will also increase.
On the reaction surface of molten steel and lined magnesia-chrome bricks, the components inside the magnesia-chrome bricks will not only have a corresponding chemical reaction with the components in the molten steel, but the components inside the magnesia-chrome bricks will also appear to the molten steel phenomenon of gradual dissolution. Among the dissolved substances of magnesia-chrome bricks, the degree of dissolution of the material Cr2O3 is relatively large, and Cr will basically be replaced by Al and Fe during the dissolution reaction.
Thermal shock damage
When the working layer of the magnesia-chrome brick undergoes oxidation and reduction changes, the magnesia-chrome spinel inside it will be continuously converted to the magnesia-faust body, and in the process, the volume of this substance will also change. Obvious changes have occurred, which has led to the cracking problem of the magnesia-chrome brick body.
When the dipping tube is put into use for a period of time, cracks will appear in the working layer and the permeable layer. In this state, the molten steel in the smelting process of the refining furnace will gradually penetrate into the cracks. The liquid steel will be solidified under the influence of temperature drop, and the molten steel infiltrated into the crack will melt again after the working temperature gradually rises. In the process of continuous solidification and melting, molten steel also presents obvious volume effect.
In the process of gradual solidification, the cracking of magnesia-chrome bricks will also lead to discontinuous stress, and further lead to the occurrence of obvious regular cracks between the interior of magnesia-chrome bricks, the permeable layer, and the original bricks. , whose direction tends to be parallel to the surface angle of the working layer. It is precisely because the magnesia-chrome brick is washed by molten steel that its actual force direction is parallel to the surface of the working layer, which leads to the problem of structural spalling.
Magnesium-zirconium brick is a typical basic refractory material. The element ZrO2 not only has outstanding high temperature resistance and excellent chemical inertness, but also can achieve extremely low saturated vapor pressure even in high temperature environments. Strong wear resistance, so ZrO2 material has received widespread attention as a substitute material for magnesia-chrome bricks in RH refining furnaces. This material has obvious weak acidity, so it has high resistance to the erosion of acidic and neutral slag, while MgO material can effectively resist the erosion of alkaline slag. Therefore, adding a certain amount of ZrO2 material to the MgO material can make this material still applicable under the condition of large changes in the basicity of slag, and it has excellent corrosion resistance while showing a high degree of thermal stability. Vibration stability, and most importantly, the application of this material will pollute the natural environment and groundwater sources. The application of ZrO2 in magnesia-zirconium refractories has very similar characteristics to the application of Cr2O3 in magnesia-chrome bricks.
Magnesium-aluminum spinel series refractories
In the context of the continuous development of the material MgO-Al2O3, based on the application of fused magnesia, high-purity magnesia, spinel and other raw materials, further introduction of ZrO2, Fe2O3, TiO2, FeO, Al2O3 and other materials can promote the production of products Significant improvement in quality. Compared with magnesia-chrome bricks, this material can not only avoid the harm of hexavalent chromium to the environment, but also achieve a significant improvement in the performance of refractory materials in terms of corrosion resistance, reduction and oxidation resistance. , thermal shock resistance and volume stability have also been significantly improved simultaneously.
Magnesium aluminum spinel refractory material
Because the refractory performance of magnesia-alumina spinel is not much different from that of magnesia-chrome bricks, it can be used as a substitute for RH refining furnace refractories. However, the Al2O3 element in the magnesia-alumina brick made of high-purity synthetic spinel as raw material dissolves faster than Cr2O3. Therefore, the magnesia-alumina spinel brick is used instead of the magnesia-chrome brick in the lining of the RH refining furnace. It will lead to a significant reduction in the practical performance of the lining. Magnesium-aluminum spinel, a refractory synthetic material, mainly includes industrial alumina, light burnt magnesia, bauxite clinker, etc., and is synthesized by electrofusion or sintering. In the process of MgO+α-Al2O3→MgO·Al2O3 (MgAlO4) reaction, there is a volume expansion rate of 6.9%. In addition, the ability of spinel to aggregate and recrystallize is very weak, so the refractory material Brick firing is very difficult. The main components of aluminum-magnesium unfired bricks are fused magnesia, corundum powder, magnesium-aluminum spinel and binder. The main purpose is to produce secondary spinel, so as to ensure that the performance of the brick matrix can reach According to the corresponding standards, the values of MgO and Al2O3 will have a greater impact on the physical and chemical properties of the synthetic spinel sample. The phase diagram of the binary system is shown in Figure 1.
Fig.1 Phase diagram of MgO-Al2O3 binary system
Magnesium aluminum spinel titanium refractory material
MgO-Al2O3-TiO2 brick refers to a new type of material made by adding TiO2 on the basis of MgO-Al2O3 brick, a refractory material. Among them, although MgO has good resistance to slag corrosion, it has poor resistance in terms of permeability, thermal shock, and hydration. The material TiO2 has good permeability resistance, so adding it to MgO and using it together can significantly improve the thermal shock resistance of refractory bricks. However, there are few studies on the practical application of this refractory material.
Magnesium aluminum spinel zirconium refractory material
MgO-Al2O3-ZrO2 brick is added ZrO2 on the basis of MgO-Al2O3-TiO2 brick. By adding an appropriate amount of zirconia material or zircon micropowder to the MgO-Al2O3 casting material, the further development of the structural bonding phase of the refractory material is promoted, thereby improving the corrosion resistance of the refractory material. According to relevant practical research, the actual use effect of this refractory material is very small compared with the performance of magnesia-chrome bricks, but the cost is relatively high, so this material has not been widely used in industrial development.
The currently applied magnesia-calcium refractories mainly cover magnesia-calcium bricks, many magnesia-calcium bricks, magnesia-calcium tundish coatings, dry ramming materials and so on. Because the content of iron oxide in refining slag is low, it will not react with CaO in magnesia-calcium bricks to generate too much low-melting calcium ferrite, so magnesia-calcium bricks can be applied to the outer lining of refining furnaces without producing Toxic substances will affect the quality of steel, so it is very suitable for use in the refining of clean steel. In addition, my country is rich in dolomite, magnesia dolomite and magnesia resources, which have great advantages in the development of magnesia-calcium refractories. Therefore, magnesia-calcium refractories will gradually replace Magnesia chrome brick.
MgO-C refractory material
The lining refractories used in the early stage of RH refining furnaces were basically MgO-Cr2O3 bricks. However, under the environment of increasing awareness of environmental protection, many experts and scholars have carried out corresponding research on the application of MgO-C. In order to avoid the oxidation of MgO-C material during use, metal Al or Al alloy materials can be added appropriately to improve the strength of this refractory brick.
Carbon-containing refractories have the characteristics of poor wettability, so the application in RH refining furnace lining has the advantages of strong permeability resistance, thermal spalling resistance, and structural spalling resistance. Therefore, magnesia carbon bricks are very suitable for refining furnace lining of high-quality carbon alloy steel. For some low-carbon steels with relatively high requirements, in order to reduce the entry of titanium-magnesium and magnesium-calcium-carbon bricks into the molten steel as much as possible, experts, scholars and relevant staff also need low-carbon magnesium refractory based on carbon content below 5%. Research and development of materials. It should be specially mentioned that at present, Kyushu Refractories Co., Ltd. of Japan has used agglomerate nano-carbon black to combine with a binder made of a small amount of B4C resin material to develop a low-carbon magnesium carbon with only 3% carbon content. Brick, the performance of this material is not much different from that of magnesia carbon bricks containing 18% graphite, but it has lower thermal conductivity. From this, we can know that the application of nanotechnology in the research and development of refractory materials for RH refining furnace lining is very worthy of in-depth research, and only by reducing the cost of nanometer raw materials can this technology be widely promoted.
Magnesium Arlon bonded magnesia refractories
The basic refractory material combined with MgAlON mainly refers to the combination of MgAlON and fused magnesia, sintered magnesia, magnesium aluminum spinel and other materials. The reason why MgAlON bonding material can adapt to the application of RH refining furnace refractory bricks lies in the following five aspects: (1) This material will not produce harmful substances and will not pollute the environment. (2) In the refining process of ultra-low carbon steel and steel with high nitrogen content, the application of this material will not pollute the molten steel. (3) MgAlON is a combined solid solution of AlON and MgO-Al2O3. However, AlON has the disadvantages of being unstable below 1650°C, decomposing, and not being oxidized in a high oxygen pressure environment.
RH refractory chrome-free promotion plan
During the use of the RH refining furnace, the performance of the refractory material is required to withstand high temperature and vacuum for a long time. It must not only be able to withstand the erosion of slag and the erosion of molten steel, but also be able to withstand temperature changes without being affected. Magnesia-chrome bricks, a refractory material used in RH refining furnaces, have the advantages of high refractoriness, good thermal shock resistance, good slag erosion resistance and strong adaptability to slag alkalinity, so they have been widely used. From a technical point of view, it is very difficult to realize the chromium-free application of RH refractories in a true sense. Therefore, the author has formulated the following systematic plan for the promotion of chromium-free refractories commonly used in RH refining furnaces.
(1) In order to truly realize the chromium-free of RH refractories, the design of its technical route must fully consider the quality of steel grades, service life and cost of use.
(2) Based on the actual needs of the tank body, circulation pipe, dipping pipe lining and other aspects, the slag resistance strength, hot state strength, thermal shock resistance and thermal expansion rate of the chromium-free refractory materials of the RH refining furnace are correspondingly carried out The balance match.
(3) In the selection of materials, it is strictly forbidden to use elements such as carbon, zirconium and titanium that have adverse effects on steel quality and cost.
(4) In the process of researching and developing products, carbon-free magnesia should be used as the main product, and corundum-based integral pouring materials should be combined in local locations, and this material should be used in combination with the corresponding means of casing repair to provide technical support. It provides guarantee for full play of its economical and sound effects.
(5) According to the characteristics of the RH refining furnace, according to the different parts of the device, the research and development of chromium-free refractory materials and corresponding practical application experiments are carried out in stages, so as to realize the RH refining furnace in the process of step-by-step trials Chromium-free exploration and practice of refractory materials.
All in all, from the analysis of the development trend of my country’s current steelmaking industry, my country’s demand for high-quality stainless steel and special steel will inevitably expand significantly in the future. From another perspective, this also means waiting for RH refining Furnace refractory materials will also usher in rapid update iterations. Under the sustainable development strategy, chromium-free refractory materials with the characteristics of long service life, functionalization, energy saving and environmental protection, and recycling will inevitably become the target of key application and development. The use of spinel series refractories to replace magnesia-chrome bricks in RH refining furnaces will also become an inevitable trend in the development of chromium-free refractory materials for RH refining furnaces in my country.