1.Medium frequency furnace melting principle
The induction furnace does not have the function of smelting steel, but is only a fast remelting equipment for qualified metals. Therefore, the metal charge must be pure and have qualified or similar composition.
(1)Hump Effect
During the smelting process, the upper part of the molten steel bulges due to the stirring of electromagnetic force. The hump phenomenon is a characteristic of induction furnace smelting. Since the electromagnetic stirring force is limited, the hump height is also limited. Therefore, when we build the furnace, the furnace collar cannot be too high. It is best to be slightly higher than the furnace table plane. Otherwise, the following problems will occur:
- The furnace collar is too high and the hump cannot reach the liquid surface, so the slag in the molten steel cannot be captured by the slag remover and the slag in the molten steel cannot be effectively removed;
- When it is necessary to add components, the hump cannot reach the liquid surface, resulting in the upper components remaining unchanged and inaccurate component measurement;
- The temperature measurement is inaccurate, the upper liquid surface temperature is low, and it cannot truly reflect the temperature of the molten steel.
(2)The role of electromagnetic stirring
Uniform molten steel temperature
Uniform molten steel temperature.Zone 1 is a low temperature zone. Because the molten steel supplies heat to the slag and maintains a certain temperature of the slag, and the slag dissipates heat to the atmosphere. The heat lost through the slag accounts for about 30% of the total heat loss. Therefore, the temperature of the molten steel in this zone is lower than that in other zones.
Zones 2 and 4 are medium temperature zones. Due to the skin effect of the induced current, the current density in the molten steel on the inner surface of the crucible is large, so more heat is generated. However, the heat lost to the outside through the crucible wall by the molten steel in this zone accounts for about 50% of the total heat loss, so it is not the highest temperature zone.
Zone 3 is also a low temperature zone. The molten steel in this zone transfers heat to the outside through the bottom of the crucible, and there is no heat source itself, so the temperature is relatively low.
Zone 5 is a high temperature zone. This zone is surrounded by high-temperature molten steel, and the heat is not easy to dissipate, becoming the highest temperature zone. This zone is generally located in the lower center of the crucible.
Electromagnetic stirring can accelerate the uniformity of the temperature of the molten steel in the furnace, which is beneficial to improving the quality of steel.
- Promote uniform composition
- Favorable for diffusion deoxidation and inclusion floating.
2.Classification of furnace lining materials
(1) Acidic
Crucibles made of refractory materials of acidic oxides are mainly represented by SiO2, i.e. quartz sand. The melting point of pure quartz is 1713℃, the refractoriness is about 1650℃, and the allowable working temperature of the crucible is 1550℃. It is mainly used for smelting cast iron and carbon steel with high carbon content.
(2) Alkaline
Crucibles made of refractory materials of alkaline oxides are mainly made of magnesia sand. Its main component is MgO, and it contains impurities such as CaO, SiO2, and Fe2O3.
(3) Neutral
Crucibles made of refractory materials of neutral oxides are mainly represented by Al2O3, i.e. aluminum oxide, of which α-Al2O3 is the main crystal form, also known as corundum. Corundum has a melting point of 2030℃, high density, and low expansion coefficient, so it is a high-quality crucible material.
Requirements for furnace lining materials
(1) It should have high refractoriness to avoid the premature appearance of liquid phase inside the crucible and reduce the maximum operating temperature of the crucible.
(2) It should have high stability and be able to resist chemical erosion by slag and molten steel. It generally depends on the purity of the material. The higher the purity, the higher the stability.
(3) Resistance to rapid cooling and heating. The crucible is subjected to periodic heating and cooling during use, and expansion and contraction occur due to temperature changes, which makes cracks very likely to occur. Resistance to rapid cooling and heating is related to the particle ratio of the refractory material, the density of the knot and the expansion coefficient of the material itself.
Other performance requirements also require high high temperature strength, insulation and other performance requirements.
3.Deoxidation and standing
From the lecture that night, the deoxidation and standing link was the link with the most questions. The two teachers talked about the following details:
(1) The time and temperature of final deoxidation. Final deoxidation is carried out after high-temperature standing, when the temperature is close to the casting temperature. There is no fixed temperature. It is determined by your casting temperature. The sooner the casting is done after final deoxidation, the better.
(2) The way to add the final deoxidizer. First, cover the surface of the molten steel with a thin layer of deslagging agent, then add the final deoxidizer, and then cover it with the deslagging agent. Press down with a pressure spoon to promote the reaction between the deoxidizer and the molten steel; or after sprinkling the deslagging agent, make a hole in the middle, put in the deoxidizer, and quickly cover it. The most undesirable way: sprinkle the deoxidizer on the surface of the molten steel, sparks fly.
(3) Final deoxidation belongs to diffusion deoxidation, which is an interface reaction. Final deoxidation is carried out only after more than 90% of the inclusions have been removed by high-temperature standing in the early stage. There is no need to worry about the problem of slag generation, and the final deoxidizer generally has the ability to make the slag float easily.
(4) Do not remove slag during high-temperature standing to avoid oxidation of molten steel at high temperatures. Teacher Wang said that a timer was installed on the smelting furnace, and slag removal can only be carried out after the time is reached.
