Reheating furnace is also one of the heat treatment furnaces in the rolling mill. It consists of furnace cover, furnace wall, furnace bottom and heat exchanger. Its function is to heat the steel ingot uniformly to the rolling temperature, so that the rolling mill can roll the steel ingot into large, medium and small billets, and then roll into various steel materials. If it is a continuous casting billet, it can be directly rolled into various steel materials without reheating furnace heating.
Selection of working environment and refractory materials for reheating furnace
In addition to high temperature flames and furnace slag erosion, the furnace mouth, furnace wall and furnace bottom of the reheating furnace also withstand the impact of steel ingots and fixtures. In addition to roasting the furnace mouth and movable furnace cover, the furnace mouth and movable furnace cover are also affected by temperature changes and mechanical vibration. Therefore, reheating furnaces should be built with refractory materials with high refractoriness, high mechanical strength, good slag resistance and good thermal shock resistance. The furnace cover and furnace wall are usually made of clay bricks; the lower part of the furnace wall uses silica bricks with good mechanical properties; magnesium bricks are generally used for the furnace bottom to resist the corrosion of alkaline melts. When the furnace slag does not melt, the furnace bottom can also be built with high alumina bricks. There should be a transition of 2 layers of high aluminum bricks between the magnesium bricks of the furnace bottom and the silica bricks of the furnace wall.
In recent years, due to the development and wide application of unshaped refractory materials, refractory casting materials or refractory plastics have been used in many parts of reheating furnaces instead of refractory brick structures with good results. Clay refractory casting material NL can be used for hand furnace covers and upper furnace walls. The lower part of the furnace wall can adopt phosphate refractory casting material, and the furnace bottom can adopt magnesium refractory casting material, or select from five brands according to GB10695-89 “Alkali-resistant and Heat-resistant Casting Material”.
The working environment of the reheating furnace liner is: (1) long-term high temperature; (2) steel ingot wear and loader clamp collision; the furnace cover is often opened and closed, and the furnace liner is affected by cold and hot.
In the past, reheating furnaces were mainly made of clay bricks, high alumina bricks and silica bricks. Due to slag erosion, the working layer of the furnace bottom and the foot of the furnace wall generally adopts magnesium chromium bricks and magnesium bricks. At present, unshaped refractory materials or prefabricated blocks are mainly used for reheating furnaces. Unshaped refractory materials mainly use high alumina mullite casting materials containing stainless steel fibers. The inner lining of the furnace cover adopts high-strength lightweight casting material.
On the reheating furnace, lightweight insulation casting materials and refractory fiber products are its development trend. In terms of furnace wall casting materials, low-cement and ultra-low cement Al2O3 accounting for 50%-75% of aluminum-silicon system casting materials are increasingly used.
It is worth noting that the application of fiber spray coating in heat treatment furnaces is worth paying attention to. It has played a very good role in convenient construction, insulation and maintenance, and is a development direction.
The function of the reheating furnace is to heat the steel ingot uniformly to the rolling temperature in the high temperature furnace. There are many types of reheating furnaces. The reheating furnace consists of furnace hearth, furnace wall, furnace bottom, heat exchanger, etc. The following introduces the refractory materials for reheating furnaces.
1. Furnace cover.
The average temperature of reheating furnaces is generally 1200-1300°C, and the working surface of furnace covers reaches 135-1450°C. When the furnace is opened, the temperature drops below 300°C, the temperature changes suddenly and frequently, and the mechanical vibration is large. Therefore, the inner lining of the furnace cover refractory material often cracks and peels off, and sometimes it can be eroded.
The furnace cover is usually made of clay bricks and high alumina bricks with good heat resistance, small changes in heavy burning lines and high compressive strength. Or the whole phosphate refractory casting material is poured.
2. Furnace wall.
The weight load of the upper part of the side wall of the reheating furnace is relatively large. It is often subjected to vibration from steel ingots, clamps and wear, as well as the movement of furnace wall.
3. Furnace bottom.
The furnace bottom temperature is basically the same as the steel ingot reheating temperature, about 1200-1300°C. The furnace bottom works in an oxidizing atmosphere. The iron oxide skin peeled off from the surface of the steel ingot accumulates on the furnace bottom, sometimes with a thickness of more than 100-150mm, causing severe chemical corrosion to the furnace bottom. When dry slagging, the structure inside the furnace bottom from bottom to top is: thermal insulation felt (or red brick), lightweight clay brick (or high-strength diatomite brick), clay brick, and magnesium brick. The coke layer is laid on the magnesium brick. When liquid slagging is used, the upper layer of the furnace bottom is magnesium refractory casting material and the lower layer is clay refractory casting material.
4. Furnace mouth.
The furnace mouth temperature changes frequently due to frequent opening. It is also affected by loading and unloading timbers and vibration. A high alumina brick or high-strength clay brick structure, or phosphate refractory casting material or aluminate cement refractory casting material constitute an integral structure.
5. Exhaust port.
The exhaust port is located under the furnace. Aluminum cement refractory casting material with low calcium or phosphate refractory casting material should withstand the purchase load and temperature changes of the furnace wall.
6. Heat exchanger or heat accumulator.
In the past, refractory clay tubes were mainly used for heat exchangers. Now, in addition to some still using clay tubes, each uses cast or sintered magnesium chromium bricks. Stabilized dolomite bricks can also be used. Silicon carbide tubes are corrosion resistant and have good heat exchange efficiency.
The thermodynamic operation method of reheating furnace
In the initial rolling process, the consumption of reheating furnaces accounts for a large proportion. The great potential for energy saving in reheating furnaces is to make full use of the physical heat of hot ingots, reduce the settling time and downtime after casting of steel ingots. The old process waits for the steel ingot to solidify completely before demolding, and then sends it to the average heating furnace, and then a large part of the physical heat is lost. By the time the furnace is loaded, the temperature of the steel ingot has dropped to 500-600 degrees. Therefore, the development trend of modern reheating furnace thermal technology is to adopt liquid core steel ingot reheating and micro energy heating.
Practice has proved that boiling steel and half-quenched steel can be demolded into the furnace when the steel ingot is completely solidified except that it needs to be completely solidified before demolding. That is, when the steel ingot core still has about 30% liquid core rate, it can ensure the quality and safe transportation of the steel ingot. For example, 10 minutes after boiling steel casting, the boiling steel and half quenching solidification rate can be demolded between 60% and 70%, and the cold condensation rate can be installed between 70% and 80%. At this time, the surface temperature of the steel ingot can reach about 1000 degrees, bringing a lot of condensation latent heat to the steel ingot, greatly reducing the heat consumption of the furnace and significantly improving productivity. Moreover, the heating period of liquid core steel ingots is short, the burning loss rate is hot, which can greatly reduce the heat consumption of the furnace and significantly increase productivity. In addition, since the core steel ingot does not need to be burned at high temperature, the service life of the furnace can be extended. Modern reheating furnaces strive to increase the liquid core rate of hot ingots.