Silicon carbide (SiC) is produced by high-temperature smelting of electric furnaces such as quartz sand, petroleum coke (or coal char), and wood chips (which require the addition of salt when producing green silicon carbide). Silicon carbide is also a rare mineral in nature, Moissanite. Silicon carbide is also known as carbon silica.
Among the non-oxide high-tech refractory raw materials such as C, N and B, silicon carbide is the most widely used and economical one, and it can be called gold steel sand or refractory sand. At present, China's industrial production of silicon carbide is divided into black silicon carbide and green silicon carbide, both hexagonal crystals, specific gravity of 3.20 ~ 3.25, microhardness of 2840 ~ 3320kg / mm2.
Silicon Carbide is a kind of carbide accidentally discovered by the American Acheson in the 1891 fused diamond experiment. It was mistaken for a mixture of diamonds at that time, so it was named Emery. In 1893, Acheson came out. The method of industrial smelting of silicon carbide, which is commonly known as the Acheson furnace, has been used until now, with a carbonaceous material as the core of the resistance furnace, energized to heat the mixture of quartz SIO2 and carbon to form silicon carbide.
Due to the low natural content, silicon carbide is mostly artificial. The common method is to mix quartz sand with coke, use the silica and petroleum coke, add salt and wood chips, put it into an electric furnace, heat it to a high temperature of about 2000 °C, and obtain silicon carbide powder after various chemical processes. .
What are the specific uses of silicon carbide and recarburizer in cast iron smelting? In recent years, as the country's requirements for the atmospheric environment have increased, it has become a consensus to strictly limit the pollution emissions of industrial enterprises. The melting of cast iron has changed from cupola to induction furnace melting. With the widespread promotion of electric furnace smelting, there are two kinds of non-metallic materials - silicon carbide and recarburizer, which are also increasing in the application of cast iron smelting.
The superheating temperature of the electric furnace smelting is obviously lower than that of the cupola, and the metallurgical condition is also worse than that of the cupola, especially the white mouth tendency and the shrinking tendency are higher than that of the cupola smelting. In order to improve the metallurgical quality of electric furnace smelting, reduce the heritability of pig iron and improve the mechanical properties of castings, the application of non-metallic materials such as silicon carbide and recarburizer has become an inevitable trend, especially when electric furnace smelting produces high quality cast iron parts.
At present, the silicon carbide products in the domestic market are black and green, their purity is different, and the price is different. Different companies can choose different silicon carbide products according to their own conditions and conditions. The microstructure and properties of cast iron depend to a large extent on the microstructure and quality of the raw materials. Low grade silicon carbide (about 85% SiC) is an excellent deoxidizer, which speeds up steelmaking and facilitates the control of chemical composition. Improve the quality of steel.
Silicon carbide can be used as a deoxidizer for steel making and a modifier for cast iron structures. It can be used as a raw material for the manufacture of silicon tetrachloride, and is the main raw material for the silicone resin industry. Silicon carbide deoxidizer is a new type of strong composite deoxidizer, which replaces the traditional silicon powder carbon powder for deoxidation. Compared with the original process, the physical and chemical properties are more stable, the deoxidation effect is better, the deoxidation time is shortened, energy is saved, and the energy is improved. Steelmaking efficiency, improving the quality of steel, reducing the consumption of raw and auxiliary materials, reducing environmental pollution, improving working conditions, and improving the comprehensive economic benefits of electric furnaces are of great value.
Due to the genetic influence of graphite in pig iron, a large amount of scrap steel and recarburizer have to be used for electric furnace melting. The use of recarburizers is very important, especially for cylinders and cylinder head castings with high requirements for compactness. It is necessary to use high temperature graphitized recarburizer. If there is no carbonization agent that has been graphitized by high temperature, it has a large amount of impurities, a large amount of ash, and it takes a long time to diffuse into the molten iron.
If the smelting time is short, the composition of the upper and lower irons in the electric furnace is not uniform, and the casting is likely to cause shrinkage defects after casting. In addition, the high-temperature-treated recarburizer has a high nitrogen content, and the nitrogen-containing scrap is increased to increase the nitrogen content in the molten iron. When a certain content is reached, crack-like pore defects are formed in the casting.
The method of adding the recarburizer: in the early stage, it is added simultaneously with the charge, and the middle stage charge has been partially melted, and the molten iron can be added in the late stage of melting. However, do not add more than 0.2% later to avoid coarse graphite. The use of recarburizers must use graphite recarburizers of good quality and low nitrogen content.
If the iron contains low Ti, it is impossible to neutralize a large amount of nitrogen, and at this time, a large amount of pores are generated due to the N2 removal. A good recarburizer containing graphite carbon should be between 95% and 98%, and the S content should be below 0.5%. N content of 500~4000ppm is a poor quality recarburizer.
In short, the use of two non-metallic materials, such as silicon carbide and recarburizer, on cast iron has its advantages and disadvantages. It must be carefully selected and used reasonably to achieve the desired results.