Application of magnesia chrome brick in RH furnace

Magnesium-chromium refractory brick for RH furnace

High-temperature fired magnesia-chrome bricks are refractory bricks (such as direct-bonded, rebonded, semi-rebonded magnesia-chrome bricks), which have been widely used in refining furnace linings due to their strong resistance to low alkalinity slag corrosion. There are many different varieties of magnesia-chromium refractory bricks, and the production process, structure and performance of the bricks are also very different. According to the content of Cr2O3, magnesia-chromium refractory bricks can be divided into magnesia-chromium bricks (Cr2O3 content is 5~20%), chrome magnesia bricks (Cr2O3 content is 20~35%) and chrome bricks (Cr2O3 content is more than 35%). The production process is divided into sintered bricks and fused cast bricks.

Varieties of magnesia chrome brick：

(1) Silicate-bonded magnesia-chromium refractory bricks (ordinary fired magnesia-chromium bricks). This kind of brick is made of common chrome ore with high content of impurities (SiO2 and CaO) and brick-making magnesia, and the firing temperature is about 1550 ℃. The structural characteristics are: the refractory grains are combined by silicate, the apparent porosity is high, the resistance to slag erosion is poor, and the high temperature volume stability is poor.

(2) Pre-reacted magnesia-chromium refractory bricks. The magnesia (light-burned magnesia powder) and the chrome ore are co-milled and fired in a kiln, and the synthetic magnesia-chrome sand is used as the raw material to make bricks to form "pre-reacted magnesia-chrome bricks". Pre-reacted magnesia-chrome bricks are an improved type of silicate-bonded magnesia-chrome bricks.

(3) Directly combined with magnesia-chromium refractory bricks. The direct-bonded magnesia-chromium refractory is made of chromium concentrate with low impurity content and relatively pure magnesia, and the firing temperature is above 1700 ℃. Its structural characteristics are: the refractory grains are mostly in direct contact, and the direct bonding degree of periclase (solid solution)-periclase (solid solution) and periclase (solid solution) spinel (solid solution) in the brick is high, so Its high temperature performance, good slag corrosion resistance, and high temperature volume stability are better than ordinary magnesia-chrome bricks.

(4) The molten grains are combined with magnesia-chromium refractory bricks (electro-dissolved and then combined with magnesia-chromium). The magnesia and chrome ore (light burnt magnesia powder or magnesite and chrome ore) are fully and uniformly reacted by the electrofusion method to synthesize periclase solid solution and spinel solid solution magnesia-chromium raw materials with more ideal structure, and then combine magnesia-chromium The brick is made of this raw material, which is called molten grain recombined magnesia-chrome brick. Since the raw materials for making refractory bricks are relatively pure, they all need to be fired at a high temperature or ultra-high temperature above 1750 °C. Its microstructure is characterized by uniform distribution of spinel and other components, low porosity, direct contact between refractory grains, high compressive strength, good corrosion resistance, and high temperature strength, but the disadvantage is thermal shock stability. poor.

(5) Semi-recombined magnesia-chromium refractory bricks. The magnesia-chrome bricks made of synthetic raw materials and the fine powder of chrome concentrate and magnesia should be called semi-recombined magnesia-chrome bricks. Domestically, fused magnesia-chromium materials are used as particles, and co-sintered materials are used as fine powder or chrome concentrate and magnesia powder are mixed as fine powder. The firing temperature is above 1700 ℃, and the refractory grains in the brick are mainly combined directly. The advantages are good thermal shock resistance, corrosion resistance and erosion resistance.

(6) Co-sintered magnesia-chromium refractory bricks (also known as fully synthetic magnesia-chromium). Using 100% sintered synthetic magnesia-chromium sand as the raw material for making bricks, the magnesia-chromium bricks produced by high temperature firing are co-sintered magnesia-chromium bricks. It is characterized by good corrosion resistance and high temperature volume stability.

(7) Unburned magnesia-chromium refractory bricks (or chemically combined magnesia-chromium). Chemically bonded unburned magnesia-chrome bricks generally use magnesia and chrome ore as brick raw materials, and use sodium polyphosphate or sodium hexametaphosphate or water glass as binders to press magnesia-chrome bricks. No need for high temperature firing, only about 200 ℃ temperature baking. Since it is not fired at high temperature, magnesia will hydrate and cannot be stored for a long time.

(8) Cast magnesia-chromium refractory bricks. Using magnesia and chrome ore as the main raw materials, adding a small amount of additives, mixing, compacting and biscuit burning, crushing into blocks, melting in an electric arc furnace, then injecting into the mold, annealing, and producing mother bricks, which are cut and ground. It is cold processed into products of various specific shapes. The structural characteristics of fused-cast magnesia-chrome bricks are uniform distribution of components, direct contact between refractory grains, and island-like silicates. The bricks are particularly resistant to melt erosion, penetration and erosion, but have poor thermal shock stability.

The magnesia-chromium refractory brick is a magnesia refractory material containing chromium trioxide. The chromium oxide can increase the wetting angle of the liquid relative to the refractory phase by solid-melting in the periclase and forming a low-expansion magnesia-chromium spinel. The degree of direct bonding produces an ideal periclase-magnesium-chromium spinel-microcrack composite structure, so that the magnesia-chromium refractory brick has good load softening temperature, high temperature strength, corrosion resistance, thermal shock stability, low Thermal conductivity and better hanging kiln skin. Magnesia-chromium refractory bricks have high performance/price ratio among existing fired zone refractories, and thus become a widely used refractory material.

Types and Performance Analysis of Corundum Refractory Bricks for RH Furnace

The RH vacuum degassing furnace was only used as a degassing device at the beginning, and the refractory lining at that time mainly used clay bricks and high alumina bricks. Now, the function of RH furnace has been extended to oxygen blowing and powder spraying, and the applicable conditions of lining refractory materials have become more severe, so refractory materials are selected. Especially with the increase in the production of special steel, the method of increasing the circulation flow and blowing a large amount of gas is being vigorously promoted to carry out stable production and high-speed processing of ultra-low carbon steel. Increasing the circulating flow increases the wear of the refractory lining; increasing the amount of cold air blowing causes high temperature spalling; increasing the intake of ladle slag increases structural spalling and erosion, all of which will lead to increased damage to the lining material. Therefore, the current RH/RH-OB lining is dominated by directly bonded corundum bricks, while semi-bonded or re-bonded corundum bricks are used around the oxygen blowing ports of the RH/RH-OB lining, and some use magnesia-carbon bricks.

The refractories used in the top and upper tanks of the RH unit are generally less damaged than the lower ones because they do not come into direct contact with molten steel and slag. The middle part is damaged due to contact with molten steel and slag erosion or due to high temperature spalling, so that the refractory lining is damaged. The refractory lining of the lower tank including the dip tube is the high corrosion area of ​​the RH device, which often determines the service life of the RH furnace. Therefore, the lining of the lower tank should be fired at high temperature and directly combined with corundum bricks. The seriously damaged part of the lower part of the furnace body is the circulation pipe, because the structure of the lining limits its thickness, and the complex-shaped refractory products need to be heated twice, so no refractory material has sufficient service life. In addition, in the RH-OB furnace, OB also has an important influence on the use of refractory materials. When the upper lance method is used, the refractory materials are eroded by the oxides and high-temperature reactive gases generated by the blown oxygen and iron elements in molten steel, especially The formation of oxides will rapidly erode the working surface of refractory materials, so it is necessary to choose MgO-Cr2O3 bricks with high Cr2O3 content to have a longer service life, while those exposed to high temperature gas to choose MgO-Cr2O3 bricks with low Cr2O3 content will have a longer life. Good comprehensive performance.

Refractory for Vacuum Circulation Degassing (RH) Furnace

The molten steel vacuum cycle degassing method (RH method) was jointly developed by Ruhrstal and Heraeus in 1957, and the prefix of the names of the two companies is called RH method. The working principle of the RH device is that the molten steel is degassed in a vacuum chamber lined with refractory materials. There are two insertion pipes in the lower part of the vacuum degassing chamber, namely the molten steel rising pipe and the descending pipe. During vacuum treatment, the two pipes are inserted into the molten steel. Since the vacuum chamber has been evacuated, the molten steel will rise from the two pipes to the height of the pressure difference. At the same time, the driving gas (argon or nitrogen) is blown from the lower third of the riser. Due to the turbulent flow, a large number of bubble nuclei will be formed in the riser, and the gas in the molten steel will diffuse into the argon (or nitrogen) bubbles. , the volume will increase hundreds of times, the molten steel is sprayed into the vacuum chamber in a fountain shape at a speed of about 5m/s, and the degassing process is accelerated. The degassed molten steel converges to the bottom of the vacuum chamber and returns to the ladle through the descending pipe at a speed of 1-2m/s. The molten steel that is not degassed will continuously enter the vacuum chamber from the riser to be degassed. In this way, after the molten steel is continuously circulated several times, the degassing process ends. Figure 12-54 shows the working principle of the RH device. The main equipments of the RH method are vacuum degassing chamber, lifting device, heating device and air extraction device, as shown in Figure 12-55. The vacuum degassing chamber is slender, and the two insertion tubes are generally parallel. The shape depends on the degassing efficiency, the stirring of molten steel, the accuracy of composition adjustment, the rising status of the molten steel in the room and the height of the droplet splashing. etc.

Vacuum degassing chambers are supported by:
(1) The degassing chamber rotates up and down.
(2) The degassing chamber lifts up and down.
(3) Degassing chamber fixed type, etc. At present, the degassing chamber fixing method is widely used. The lifting device generally uses a hydraulic device to lift the ladle, so that the ladle is vertically lifted under the degassing chamber, and the ladle is generally transported under the degassing chamber by a trolley. The heating device is used to preheat the vacuum degassing chamber to reduce the temperature drop of molten steel during vacuum treatment and to prevent splash droplets from adhering to the refractory material in the chamber. Various gases (coke oven gas, blast furnace gas, natural gas, etc.) or liquid fuels (heavy oil, kerosene, etc.) can be used for heating by spraying and burning through the preheating holes, and it can also be used to insert electric heating in the vacuum degassing chamber. radiant heating.

The air extraction device adopts the ordinary steam jet pump method. The ferroalloy addition device is generally installed in the upper part of the vacuum degassing chamber, and the alloy material is added with a sealed hopper and a vibrating feeder. Compared with other refining treatment methods, RH degassing method has the following advantages:
(1) The degassing effect is better. Due to the input of driving gas, a large number of bubble nuclei are formed, and the molten steel entering the vacuum chamber will be sprayed into very fine droplets, which greatly increases the degassing area of ​​the molten steel, which is conducive to the degassing.
(2) The temperature drop of molten steel is small. Generally, the temperature drop of molten steel after one treatment is only about 30-50 °C, and it can also be heated during the degassing process.
(3) The degassing chamber has a wide range of applications. Different volumes of molten steel can be processed in the same equipment, as well as in electric furnaces and induction furnaces.
(4) There are many types of steel that can be processed. There are forging steel, high-strength steel, structural steel, bearing steel, tool steel, stainless steel, electrical steel, deep-drawing steel, etc.
(5) The equipment is flexible in operation and stable and reliable in operation. The RH device works under high temperature and vacuum conditions. The driving gas is blown into the rising pipe, and the molten steel is continuously sprayed upward. The refractory materials in the middle and lower parts of the vacuum degassing chamber are subjected to the scouring effect of molten steel splash and molten steel circulation, and the damage is more serious. For the refractory material inserted into the pipe, the inner wall will be scoured by high-speed air flow (flow rate up to several meters per second) and molten steel, the outer wall and bottom will be severely impacted by molten steel, chemical erosion of slag, and rapid cooling and rapid heating, as well as mechanical loss. Serious, prone to thermal spalling. Compared with other degassing methods, the erosion of refractory materials in the RH method is more serious, and the working environment of the refractory materials inserted into the pipe is particularly harsh, which is the weak link of the RH device. The refractory materials used in the device are: high alumina refractory bricks, magnesia dolomite bricks, periclase spinel bricks, magnesia bricks, magnesia chrome bricks, refractory ramming materials and refractory castables, etc. At present, countries tend to use directly combined magnesia-chrome bricks as the lining of the vacuum chamber, and the inserting tube lining is generally made of high-alumina refractory ramming material or refractory castable, and also alkaline bulk materials.