How to improve the efficiency of induction furnace production of cast steel?（二）
- Leave it alone after power outage
After the molten steel with qualified composition reaches the maximum melting temperature, the power is turned off immediately and the electromagnetic stirring in the molten steel stops, which facilitates the oxides suspended in the molten steel generated after pre-deoxidation to float into the slag layer. For induction furnaces with a capacity of 100-150kg, the standing time during a power outage is generally controlled at 2-3 minutes. As the power is turned off, the temperature of the molten steel begins to drop. When the temperature is too low, the viscosity of the molten steel increases and inclusions are less likely to float. out, so too long standing time has no effect. If there are too many inclusions in the steel, the temperature can be raised to the highest melting temperature again, and then left to stand for a second time without power. During the power outage, the molten steel should be strictly covered, and the furnace mouth surface can also be covered with aluminum silicate fiber wool.
★Tips: The purpose of standing still during a power outage is to allow the oxidized inclusions produced in the early stage of the molten steel to float upward into the slag layer without the interference of electromagnetic stirring force to achieve the purification of the molten steel. The key to achieve this goal is to increase the temperature of the molten steel to reduce its viscosity and reduce the floating resistance of inclusions, so that the purpose of leaving the scum standing can be achieved. However, if the temperature of the molten steel is too high, there is a risk of oxidation and gas absorption. It is important to determine the appropriate maximum temperature of the molten steel when it is left to rest, that is, the temperature at which it starts to stand.
Generally speaking, the liquidus line of steel with high alloying elements is lower than that of carbon steel and low alloy steel, but the viscosity of molten steel is higher, so the selected superheat, that is, the range between liquidus temperature and resting temperature should be higher. For example, the liquidus temperature of 304 stainless steel is 1454°C. When the standing temperature is selected as 1700°C, the superheat temperature reaches 246°C. The liquidus temperature of 8620 low alloy steel is 1504°C. When the standing temperature is also selected as 1700°C, the overheating temperature is only 196°C.
- Final deoxidation and refining
Deoxidation time: After the molten steel is allowed to stand, use an inserted thermocouple to measure the temperature. If the temperature is close to the pouring temperature, final deoxidation can be carried out. If the temperature is too high or too low, it should be adjusted to the pouring temperature range before final deoxidation can be carried out.
Selection of final deoxidizer: For carbon steel, use 0.08~0.1% pure aluminum as the final deoxidizer, for stainless steel, use 0.15~0.3% silicon calcium manganese alloy and then add 0.04% pure aluminum as the final deoxidizer, or use a multi-component composite deoxidizer. For silicon-calcium-aluminum-barium alloy, the addition amount should be calculated based on the aluminum content of the alloy.
The order and method of adding the final deoxidizer: push away the slag surface and add silicon-calcium-manganese alloy first, then add aluminum immediately. Aluminum has a small specific gravity and calcium-containing alloys are easy to burn. The recovery rate must be considered in the adding method. It can be added by inserting or bell jar pressing.
Refining treatment after final deoxidation: For some steel types, according to process requirements, refining agents such as rare earth alloys or calcium-containing alloys can be added to the molten steel after final deoxidation, or grain refiners can be added to improve and enhance the properties of the steel. . The amount and method of addition are specified in the process documents.
★Tips 1: Aluminum has a strong affinity with oxygen and is the strongest deoxidizer for steel, so it is used as the final deoxidizer for steel. The disadvantage of aluminum deoxidation is that the deoxidation product Al2O3 formed is a cluster oxide. When aluminum is added to the surface of the molten steel, it will also form an aluminum oxide film with the oxygen in the air. When it is stirred into the molten steel, it will not easily float out and form Hard and brittle inclusions, so stainless steel polishing parts are taboo on using aluminum as a deoxidizer. The way to overcome this shortcoming is to add silicon, manganese, calcium and other elements while adding aluminum for deoxidation. The deoxidation product formed is not a single alumina, but a multi-component complex oxygen, which is easy to separate from the molten steel and float out.
★Tips 2: Rare earth and calcium-containing alloys have a refining effect on molten steel for the following three reasons:
① They have a strong affinity with oxygen, so they have a deep deoxidation effect on molten steel;
② The melting point of rare earths and their oxides is higher than that of steel. When steel solidifies, it increases the number of crystallization cores, thus refining the grains;
③ When there are trace amounts of calcium and rare earth elements in the molten steel, the small amount of oxidized inclusions left behind by the final deoxidation of the molten steel will be refined and spherical, so it will be very harmful to the steel.
However, it should be emphasized that the rare earth alloy must be added after the final deoxidation is completed, otherwise too many rare earth oxides will be produced and contaminate the molten steel. In addition, nano-titanium nitride is a very pure and fine grain refiner. The addition amount is only a few ten thousandths to have a good effect, so it will not have the side effect of contaminating the molten steel, but it must be added inside the molten steel to have the effect. Effect.
- Removal of slag
After the final deoxidation is completed, immediately carry out preliminary slag removal, that is, use a steel drill to remove large pieces of covering slag, then use dry granular slag remover to spread on the slag surface, dip the covering slag and pick it out with a steel drill, and repeat the removal. After removing the slag twice, sprinkle the slag remover evenly on the molten steel surface to cover the molten steel, and you are ready to tap the steel ladle. If a furnace is used instead of a ladle for casting the fork shell, turn off the power after the initial slag removal, evenly spread the slag remover to form a slag layer, and use a steel drill to move the slag layer to remove the thin slag on the steel liquid surface and the furnace wall. After three to four quick slag removal operations, the molten steel surface is clean and free of slag, and the shell can be poured. During the slag removal process, the molten steel is exposed to the air, and the slag removal operation must be fast and effective to reduce re-oxidation pollution of the molten steel. At the same time, before starting to remove slag, notify the pouring or fork shell personnel to prepare for tapping and pouring.
★Tips 1: For direct casting with the furnace fork shell, slag removal is a very important step, because the steel slag is thin at high temperatures and is difficult to separate from the molten steel. If it is poured into the mold shell, it will inevitably leave fine particles on the surface of the casting. The defects of slag holes often do not attract the attention of operators and managers. To do this well, on the one hand, you must choose a good quality slag removal agent, and on the other hand, you must have practiced slag removal operation skills. If a teapot pouring ladle is used to cast the shell, the slag removal requirements in the furnace do not need to be too strict, because the slag skimming function of the teapot pouring ladle ensures this.
★Tips 2: The slag remover commonly used in industry at present is perlite, which is an acidic silicate vitreous volcanic lava mineral. After crushing, screening and low-temperature baking to remove free water particles, it is added When it reaches the high-temperature molten steel and slag layer, it immediately expands and foams, so it has a strong function of adsorbing thin slag on the surface of the molten steel. When using a slag remover to remove slag, the particles should be ash-free, evenly spread, and light. Do it according to the methods and principles of "kneading the slag, picking it out quickly, focusing on the effect, and reducing the frequency", because the slag removing agent is different from the slag forming agent. It has many impurities and contains crystal water. Excessive use will also pollute the molten steel. At the same time, its melting point is low, and it will turn into thin slag if left on the surface of the molten steel for too long, but it will not achieve the effect of adsorbing the thin slag.
- Steel tapping and pouring
Clean the furnace surface and use an air nozzle to blow off the dust, sand and impurities on the furnace surface and the tapping trough. The tapping trough should not be too long. If it has been repaired, it must be bottom-dried and dried in advance to minimize the flow distance of the molten steel. And prevent the liquid steel from absorbing moisture. At this time, a whole piece of aluminum silicate fiber wool can be used to cover the furnace mouth to achieve the effect of heat preservation; when the fork shell is poured, the fork shell and the pouring should be closely coordinated to achieve accurate, continuous and fast pouring operations. Generally, the pouring time of each furnace should be controlled within 3 to 4 minutes. If the pouring time is too long and the liquid steel is seriously oxidized, another aluminum block needs to be put into the furnace to supplement deoxidation; if it is subcontracted pouring, half of the final deoxidizer needs to be left. , added to the bottom of the ladle in batches, and the molten steel is poured into the ladle for deoxidation. The ladle needs to be baked until dark red before it can be filled with molten steel, and a teapot-nozzle skimming ladle should be used; during the tapping and pouring process, as the furnace As the molten steel gradually decreases, the power supply should be gradually reduced to keep the pouring temperature of the molten steel relatively stable.
★Tips: During the tapping and pouring process, the molten steel is completely exposed to the atmosphere. At this time, the qualified molten steel that has been deoxidized will be oxidized again, and gradually become more serious as time goes by. This is the so-called secondary oxidation, which will affect the castings. quality. In order to reduce the secondary oxidation of molten steel, two measures can be chosen:
The first is to select the appropriate capacity of the induction furnace. The purpose is to shorten the pouring time of molten steel in each furnace. The average molten steel weight of each group of mold shells should be calculated according to the product objects of the respective companies. Under the principle of ensuring that each furnace of molten steel is poured in 3-4 minutes, Calculate and select the optimal capacity of the induction furnace;
The second is to set up an inert gas protection device on the furnace mouth surface, and open the inert gas protection device during the slag removal and tapping section to form an inert gas layer on the molten steel surface to protect the exposed molten steel from secondary oxidation.
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