The ladle degassing refining method is the main method of refining outside the furnace (also known as secondary steelmaking). Out-of-furnace refining is the process of transferring the molten steel smelted in a roughing furnace (electric furnace and converter) to another high-temperature vessel (mainly a ladle) for refining: Refining is a process of using different metallurgical treatment methods and devices outside the furnace according to the type of steel to be smelted.
One method is classified as:
(1) Ladle processing type, such as ladle argon blowing, ladle powder spraying and vacuum cycle degassing (RH);
(2) Ladle refining type, such as vacuum oxygen decarburization (VOD), argon oxygen decarburization (AOD), ladle refining (LF, ASEA-CKF, VAD, etc.). Only by understanding the basic structure and technology of these devices can we understand the construction of refractory materials and the lining materials used in these devices. There are 6 common ladle refining devices as follows.
The RH method is a suction-type vacuum degassing refining method, also known as a circulating method vacuum degassing device, as shown in Figure 1. This method was developed in 1985 by Rheinstahl and Heraeus companies in Germany. The method and device are suitable for vacuum processing of large containers, and are widely used in the production of special steel, ordinary steel and stainless steel.
The main functions of this method are:
(1) Degassing functions such as dehydrogenation, deoxygenation and denitrification;
(2) Vacuum decarburization function;
(3) The composition and temperature are homogenized by stirring, and the function of floating and separating non-metallic inclusions is realized. The device is composed of a vacuum degassing chamber, Ar gas blowing device, immersion pipe, exhaust system and alloy silo. Two immersion pipes are arranged under the vacuum degassing chamber, and high-speed Ar gas is blown into one immersion pipe (rising pipe). According to the principle of the air pump, the molten steel in the ladle is brought into the lower part of the vacuum degassing chamber, and then from the other immersion pipe The (downcomer) returns to the ladle to realize the circulation degassing. The circulation speed is controlled by the amount of Ar gas blown in. The high-speed argon blowing and the flow of molten steel in the ladle will definitely aggravate the damage of the tank lining refractories.
The RH furnace works under vacuum and high temperature. It is divided into three replaceable parts: upper, middle and lower parts. The service life of the refractory material in the lower part is shorter, and the service life of the riser pipe is the shortest. The lining material of RH furnace is usually made of high-quality magnesia chrome bricks or alkaline rammed overall lining, and the masonry is divided into different parts.
AOD is the English abbreviation for Argon Oxygen Decarburization. This device is a stainless steel refining device developed by Union Carbid Company in 1967. In October of the same year, the world’s first AOD furnace was built in Jocelyn Stainless Steel Company in the United States. Due to the low equipment cost and few operational problems of this device, it quickly gained popularity in the world. 75% of the world’s stainless steel is refined by AOD. The schematic diagram of A0D device is shown in Figure 2.
The appearance is similar to the converter, but in fact it is still a refining steel ladle. The main function is to blow the molten steel in the primary smelting furnace (such as electric arc furnace), blow argon and oxygen in the A0D to further decarburize the molten steel, and then refine it through reduction and desulfurization to complete the purpose of adjusting the composition. There are 2~4 double casing tuyere on the side wall of the molten pool near the bottom of the furnace. The (O₂+Ar) mixed gas is blown into the molten steel from the inner pipe, and Ar gas is blown into the protective tuyere from the gap between the inner pipe and the outer pipe. . The generated CO gas is diluted with Ar gas and its partial pressure is reduced, and decarburization is performed while suppressing the oxidation loss of Cr. Before AOD decarburization, the carbon content of molten steel is 1%~2.5%, and the temperature is about 1550°C. Change the Ar/O₂ ratio in order to efficiently gradually decarburize from the high-carbon zone to the low-carbon zone while suppressing the oxidation of Cr. When the carbon content is reduced to the set carbon content, stop blowing oxygen and use Ar gas for stirring.
During the beginning of oxygen blowing and oxidative decarburization, the oxidizability of the slag increased, and the temperature rose above 1700°C. In the reduction period, add ferrosilicon or aluminum to reduce the chromium in the slag and increase the alloy yield. Although the alkalinity of the slag is very low during this period, high alkalinity slag is required during the final lime desulfurization period. During the entire refining process, the slag changes from acidic to alkaline, and the atmosphere changes from oxidizing atmosphere to reducing atmosphere. In addition to intermittent operation, the furnace lining temperature is high and fluctuates greatly. AOD furnace lining generally uses magnesia-chrome bricks, which are gradually replaced by magnesia-calcium bricks.
There are two main types of refractories used in AOD furnaces, one is magnesia-chromium refractories, and the other is dolomite refractories. At present, there is a trend towards dolomite refractories. Dolomite bricks or magnesia-aluminum-carbon bricks are mostly used for tapping; the furnace body and bottom are made of fired dolomite bricks or resin combined dolomite bricks; the upper part is made of fired dolomite bricks or resin combined dolomite bricks; the slag line is used for firing Mainly magnesia-forming dolomite bricks; the tuyere adopts zirconium-containing sintered magnesian dolomite bricks; bulk materials are mainly dolomite-based ramming materials or cement-combined magnesia-based ramming materials.
VOD is the abbreviation of vacuum oxygen decarburization, also known as vacuum oxygen decarburization. In this type of external refining equipment (see Figure 3), the ladle containing the molten steel in the converter or electric furnace is placed in a vacuum chamber, and argon is blown from the bottom of the ventilating brick to stir, and at the same time, vacuum is degassed, and then oxygen is blown to the molten steel from the top to decarbonize And join the ferroalloy. This method of producing low-carbon stainless steel with less hydrogen, oxygen and non-metallic inclusions is a technology developed by Witten in West Germany in collaboration with Standard-Messo in 1965, so it is also called the Witten method.
The high temperature of oxygen blowing (above 1700°C), slag intrusion and vacuum conditions have caused VOD ladle lining refractories to withstand harsh service conditions. The slag line and the molten pool wall adopt recombined magnesia-chrome bricks and direct-combined magnesia-chrome bricks respectively. In order to eliminate the harm of hexavalent chromium to human health, magnesia chrome bricks are being gradually replaced by ultra-high temperature fired directly bonded magnesia dolomite bricks. Recently, low-carbon magnesia-carbon bricks, low-carbon magnesia dolomite bricks or aluminum-magnesium preforms have been developed and applied.
VOD furnace refractory configuration of several steel mills:
|Bottom||Slag line||The rest|
|Slag line||Slag line|
|High aluminum||Direct magnesia chrome brick and recombined magnesia chrome brick|
High aluminum bricks
|Fired oil-impregnated dolomite brick||Resin bonded dolomite brick||Burning oil immersion directly combined with dolomite bricks|
High aluminum bricks
|Aluminum magnesium castable||Semi-recombined magnesia chrome brick|
Directly combined with magnesia chrome brick
High temperature fired magnesia dolomite brick
Ordinary fired dolomite brick
Magnesia chrome brick and dolomite brick
|Ordinary Dolomite Brick|
Ordinary Dolomite Brick
LF device is also called ladle refining furnace. This method was developed by Japan in 1971. The reduction refining process of the electric arc furnace is transferred to the ladle refining device with heating refining function, and the influence of the melting oxidation refining process of the electric arc furnace on the reduction refining process is minimized to ensure the reduction refining effect of the device.
The LF device (see Figure 4) has a simple structure, basically adding a top cover that can insert 3 graphite electrodes in the molten steel casting ladle (the electrode is used for three-phase AC heating). Through the breathable brick installed at the bottom of the ladle, inert gas (such as Ar gas) can be blown into the molten steel, and the molten steel can be heated and stirred under the reducing slag, which is excellent in deoxidation, desulfurization and reduction of non-metal effect. Some LF furnaces are used in conjunction with VD (vacuum degas s ing).
The refractory materials commonly used in LF furnace are divided into slag line part, furnace wall, furnace cover and bag bottom. The slag line is more severely damaged under the erosion of high-alkalinity slag, and magnesia-carbon bricks with good thermal shock properties are often used; LF furnace fireplace slag penetration is more serious, and it is easy to cause structural peeling. The traditional selection of high alumina bricks can no longer meet the needs The use of carbon and MgO-added materials is used to overcome, and the refined package lining has also been developed from the finalized to the indeterminate direction; the furnace cover is basically castable; the furnace bottom is made of cast large bricks or dry ramming. Beaten up.
A ladle refining equipment jointly developed by ASEA and SKF is shown in Figure 5.
The equipment mainly includes arc heating device, vacuum degassing device, electromagnetic induction stirring device, special ladle with non-magnetic steel plate, alloy and other auxiliary raw material input device and ladle mobile trolley capable of tilting and removing slag. The outstanding feature of this equipment: remove the ladle vacuum degassing cover and replace it with the electrode heating cover, it will perform arc heating and refining like an electric furnace. The device is a device with multiple functions, which can ensure the safety of molten steel degassing, the deoxidizer and alloy added into the tank are evenly distributed, the temperature of molten steel can be adjusted, and the refining of molten steel in the steelmaking furnace can be partially transferred to this device. To shorten the refining time of the steelmaking furnace. This method is also called arc heating electromagnetic stirring refining method or electromagnetic stirring vacuum degassing method.
The service conditions of ASEA-SKF ladle refractory lining are much harsher than ordinary ladle. The temperature of molten steel in the tank is increased by 50~100℃, and the residence time of molten steel in the tank is almost doubled. The slag resistance of refractory materials in an oxidizing atmosphere is reduced by half when the temperature is increased by 30-50°C. Under vacuum conditions, molten slag penetrates into the brick along the pores of the binder and destroys its structure. When the temperature of molten steel increases from 1650°C to 1700°C, the corrosion of all refractory materials (except fused-cast bricks) increases by 1.5 to 2 times.
ASEA-SKF ladle is in the use condition of severe temperature fluctuation, Radex-DB605 containing complex spinel combined with magnesia chrome brick (MgO57.3%, Al ₂0₃7.7%, Cr ₂O₃ 20.2%, Fe₂0₃ 12.7%, SiO₂0 .6%, bulk density of 3.3g/cm³, apparent porosity of 13.5%, compressive strength of 35~45MPa, load softening temperature higher than 1750℃, 4h expansion at 1550℃ after heating by 0.1%, water cooling at 1300℃, weight loss of 20% The thermal shock resistance of 10 times) has been recommended by most countries for ASEA-SKF ladle working linings with severe temperature fluctuations. (Some countries have tried this brick in the ASEA-SKF 150t ladle slag line, and the service life is only 8 times). The slag line closest to the electrode is best to be cast magnesia chrome brick. In recent years, these magnesia chrome bricks have been gradually replaced by directly bonded magnesia dolomite bricks or magnesia carbon bricks.
Typical properties of recombined (semi-recombined) magnesia-chrome bricks at home and abroad
|performance||MgO/%||Cr2O3/%||CaO/%||SiO2/%||Al2O3/%||Fe2O3/%||Apparent porosity /%||Bulk density /g.cm-3||Compressive strength /MPa||Load softening temperature /℃||Linear expansion rate /%|
VAD is the abbreviation for vacuum arc degassing. The vacuum arc degassing device is a device of the ladle degassing refining method (see Figure 6).
VAD is a technology developed in 1968 by Finkl of the United States in collaboration with Standard-Messo of West Germany. It is a method of producing low-carbon stainless steel with less hydrogen, oxygen and non-metallic inclusions. The device performs special refining in a ladle that receives molten steel from a converter or electric furnace.
This method is very similar to ASEA-SKF, with the following two differences:
(1) Its stirring is carried out with Ar gas like VOD;
(2) It can be heated under vacuum (150~300Torr). Pack the steel that accepts molten steel in the converter or electric furnace into a vacuum chamber, and conduct electric arc heating while exhausting. In order to avoid the risk of discharge below 100 Torr, the arc heating is stopped when the pressure of the vacuum chamber is reduced to 200 Torr. When the exhaust is below 1 Torr, blow Ar gas and stir for 6 to 10 minutes, then increase the pressure to 100 to 200 Torr for arc heating, and blow Ar gas for 30 to 45 minutes for deoxygenation treatment. (1Torr=133.3224Pa) The above are the six common refining devices outside the furnace. In order to protect the metal structure of the cladding from the high temperature of molten steel and molten slag, and to ensure the residence time of molten steel in the ladle, the inner lining The choice of refractory materials is also particularly important. At present, because the inner lining of the ladle is damaged by molten steel and molten slag, erosion, high temperature, etc., its service life is not very satisfactory. Both the refractory industry and the steel industry are troubled by it, and the inner lining of the ladle is damaged. Afterwards, major repairs or replacements are required. These all take time. The unit consumption of domestic refractory materials is still very high. The increase in refractory loss not only increases the cost of steelmaking, but also affects the quality of steel. Coupled with this year’s national carbon neutrality and carbon peaking plan for the steel industry, all have a great test for the refractory industry, requiring technicians from various units to develop more energy-saving, consumption-reducing and long-lived refractories.