How to improve the efficiency of induction furnace production of cast steel? （一）
- Preparation before smelting
Ingredients: Use high-quality profile scraps with the same or similar composition as the required steel grade or refined continuous casting bars as the main material. Generally, the proportion of recycled materials should be controlled within 30%. If the quality requirements of castings are high, recycled materials are not used. , the amount of ferroalloy used to adjust the composition shall not exceed 5%; accurately weigh the charge according to the ingredient list and record it.
Requirements for furnace materials: All metal furnace materials must be clean, free of rust, oil, dry and moisture-free. There is no remaining shell material after shot blasting of the recycled materials. Electrolytic metals such as electrolytic nickel need to be dehydrogenated by baking at 800°C. Other iron alloys added during smelting need to be weighed and placed in an oven at ≧100°C for later use; the slagging agents lime and fluorite should be placed in the oven for later use. , to prevent moisture absorption; the slag removal agent should be screened to remove the ash and dried at low temperature before being placed in front of the furnace for later use.
Charging: First, use 80-90% dry lime and 10-20% fluorite as slag-forming agent, add it to the bottom of the furnace to create bottom slag. When the steel material starts to melt, it can contact the slag-forming agent and cover the molten steel. The slag-forming agent The amount is about 1-2% of the weight of the furnace charge, and the principle is to completely cover the molten steel; place the fine steel materials on the bottom of the furnace, and then add refractory and difficult-to-oxidize ferroalloys and metals such as ferromolybdenum, ferrotungsten and electrolytic nickel When added to the bottom of the furnace, other ferroalloys and metals should be added after pre-deoxidation. The remaining main materials should be installed vertically and tightly in the furnace to facilitate pushing the materials downward when melting from bottom to top.
★Tips 1: Since the induction electric furnace has a large ratio of furnace depth to diameter, the metal charge is heated and melted by electromagnetic induction, and the furnace wall is thin to improve electrical efficiency, resulting in a small contact area between the molten steel and the slag layer, and the slag layer is formed by The temperature of metal is low due to heat transfer, and the furnace wall does not allow excessive impact. Therefore, the induction electric furnace does not have the smelting function of steel, but is only a rapid remelting equipment for qualified metals. Therefore, the charge must be accurate and clean.
★Tips 1: Alkaline slag should be made in the alkaline furnace lining. The oxides in the charge will react with the slag first, so as not to pollute and adhere to the furnace lining. Unless the charge is very clean, there is no need to make slag.
- Rapid melting
When the furnace material is loaded, the power supply can be started to deliver electricity at full power to speed up the melting speed, shorten the contact time of the steel material in the atmosphere, and reduce oxidation and air inhalation. However, when melting in a cold furnace with an alkaline furnace lining, because the magnesia furnace lining has cracks, it is necessary to use low-power power first. After the heated steel material bakes the furnace lining until it turns red, and the cracks are closed, the furnace lining can be melted quickly at full power; the melting process It is necessary to push and replenish the materials in time. The recycled materials of different shapes are easy to get stuck and can be added at the end. It is necessary to prevent "bridging" accidents caused by improper feeding or not pushing the materials in time.
During operation, attention should be paid to ensuring that the slag layer covers the molten steel. If the amount of slag is too small, a slag-forming agent can be added to prevent the molten steel from being exposed. If the slag layer is too thick, part of the slag liquid can be skimmed off because a single slag is used. Method, do not replace the slag liquid midway.
★Tips: During non-vacuum induction electric furnace melting, during the heating and melting process of the steel, the solid steel and the molten and exposed molten steel are constantly oxidized by oxygen in the air. Even if part of the steel has been melted, due to electromagnetic Due to the effect of stirring, the molten slag cannot cover the molten steel. Only when most of the steel material has melted can the molten slag cover the molten steel and isolate it from the air.
In order to reduce the oxidation of steel during the melting process, there are two commonly used measures:
① Increase the power supply and shorten the melting time, which shortens the contact time between the steel material and oxygen. Generally, the melting time is shortened to more than ten minutes;
② Install an argon (nitrogen) gas protection device at the furnace mouth, and turn on the inert gas protection device during the melting period of the steel material to drive away the air above the furnace mouth to form a protective layer of inert gas. When the molten steel is completely covered by the slag layer, then Turn it off.
- Pre-deoxidation
Pre-deoxidation time: When the steel material has been completely melted, but the temperature is not high, generally about ≦1550℃, it is the best time for pre-deoxidation.
Selection of pre-deoxidizer: Choose the one with better deoxidation effect. High residual manganese and silicon are allowed to be used as pre-deoxidizer in steel. Electrolytic manganese and crystalline silicon are used for steel with low carbon content. Low-carbon ferromanganese and high-quality silicon are used for carbon steel. iron.
Adding amount of pre-deoxidizer: Add as much as possible under the principle that the residual content of manganese and silicon in the final steel does not exceed the standard to improve the deoxidation effect. In order to form a manganese silicate composite compound that is easy to float, the manganese/silicon ratio needs to be greater than 1 or more. High, generally the minimum addition of manganese is 0.5%, and silicon is 0.25%.
The order of adding the pre-deoxidizer: add manganese first, then add silicon. After preheating, the deoxidizer is pushed away from the slag surface and added to the molten steel, immediately covering the molten steel.
★Tips 1: Manganese and silicon are the best deoxidizers for molten steel. Although their affinity with oxygen is not as good as aluminum, they have two major advantages: First, when they jointly deoxidize molten steel, they form a composite oxidation with a melting point lower than that of steel. The material MnSiO3 is easy to aggregate and emerge from the molten steel; secondly, the allowable residual amounts of manganese and silicon in the steel are relatively high, so adding an appropriate excess is beneficial to the progress of the deoxidation reaction.
There are two points to be aware of when using manganese and silicon deoxidizers:
First, add manganese and silicon for deoxidation when the molten steel is at low temperature, because this is the period when manganese and silicon deoxidation are best;
The second is to add manganese first, and then add silicon. This is because SiO2, the product of silicon deoxidation, is a floc that floats in the molten steel and is not easily discharged. Therefore, there must be a large amount of MnO in the molten steel before adding silicon. When SiO2 appears, it is surrounded by MnO to form a composite oxide of MnSiO3.
Experiments have proven that the inclusion content in steel is significantly different with different adding orders. At the same time, this is also the reason why we emphasize that the amount of manganese added should be greater than the amount of silicon added, and the manganese-silicon ratio should be at least greater than 1. There is also a saying that the manganese in steel When the content of silicon is high, the deoxidation effect of silicon will be better.
★Tips 2: Use manganese and silicon to remove most of the oxygen in the molten steel and form deoxidation products that are easy to eliminate. This is the purpose of pre-deoxidation. However, because manganese and silicon have slightly poor affinity with oxygen, the molten steel will Oxygen still exists, so manganese silicon deoxidation can only be called pre-deoxidation.
- Adjust ingredients and heat up
Immediately after pre-deoxidation, the molten steel is sampled for spectral analysis. At this time, the power supply is turned down to the insulation power, and the preheated alloy is added based on the analysis results. When the composition of the molten steel is qualified, the temperature is immediately raised at full power to reach the maximum melting temperature and measured with an inserted thermocouple. The maximum melting temperature should be determined according to different steel types. For example, 304 stainless steel is generally 1680~1700°C.
★Tips: After pre-deoxidation, the oxygen content in the molten steel decreases. At this time, adding various elemental alloys that are easier to oxidize into the furnace will result in less oxidation loss and less oxidized inclusions. This period is a critical period to ensure that the chemical composition of the steel is qualified through alloy addition and rapid pre-furnace analysis.
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