First, according to the chemical composition of cast steel can be divided into carbon cast steel and alloy cast steel two categories. Among them, carbon cast steel is the most widely used, accounting for more than 80% of the total cast steel output.
1, carbon cast steel
In general, low carbon steel ZG15 has a high melting point and poor casting properties. It is only used for the manufacture of motor parts or carburizing parts. Medium carbon steels ZG25-ZG45 have a higher comprehensive performance than various types of cast iron, ie high strength and good quality. The plasticity and toughness are therefore suitable for manufacturing parts with complex shapes, high strength and toughness requirements, such as train wheels, forging hammer frames and anvils, rolls and high pressure valves, etc., which are the most widely used in carbon cast steel; High-carbon steel ZG55 has a low melting point, and its casting performance is better than that of medium carbon steel, but its plasticity and toughness are poor, and it is only used to make a few wearable parts.
2, alloy cast steel
According to the total amount of alloy elements, the alloy cast steel can be divided into two categories of low alloy steel and high alloy steel. For low-alloy cast steel, manganese, manganese-silicon, and chromium are mainly used in China. Such as ZG40Mn, ZG30MnSi1, ZG30Cr1MnSi1 and so on. It is used to make gears, hydraulic cylinders and turbine rotors, while the ZG40Cr1 is often used to make important parts such as high-strength gears and high-strength shafts. High alloy cast steel with special properties such as wear resistance, heat resistance or corrosion resistance. Such as high manganese steel ZGMn13, is a wear-resistant steel, mainly used in the manufacture of parts used in dry friction working conditions, such as excavator grab front and grab teeth, tractor and tank tracks, etc.; chromium nickel stainless steel ZG1Cr18Ni9 and chromium stainless steel ZG1Cr13 and ZGCr28, etc., have high corrosion resistance to nitric acid, and are mainly used for manufacturing parts on equipment such as chemical, petroleum, chemical fiber and food.
Second, the cast steel casting process characteristics
Cast steel has higher mechanical properties than cast iron, but its casting performance is inferior to cast iron. Because the melting point of the cast steel is high, the molten steel is easily oxidized, the flowability of the molten steel is poor, and the shrinkage is large, the body shrinkage rate is 10 to 14%, and the line shrinkage is 1.8 to 2.5%. In order to prevent the cast steel parts from being insufficiently cast, cold-separated, shrinkage-reduced, shrink-splayed, cracked, and sticky sand, complex technological measures must be taken in comparison with cast iron: Due to the poor fluidity of the steel castings, cold-separation is prevented in order to prevent the steel castings. Insufficient pouring, the wall thickness of steel castings shall not be less than 8mm; the structure of casting system shall be simple and the cross-sectional dimension shall be larger than that of cast iron; dry cast or hot cast shall be adopted; the casting temperature shall be appropriately increased, generally 1520° to 1600°C, Because the pouring temperature is high, the degree of superheat of the molten steel is large, and the time for maintaining the liquid is long, and the fluidity can be improved. However, if the temperature is too high, coarse grains, hot cracks, pores and sticky sand will be caused. Therefore, generally small, thin-walled and complex shape of the casting, the pouring temperature is about the melting temperature of the steel +150 °C; large, thick-walled castings casting temperature higher than its melting point of about 100 °C.
2. Because the shrinkage of cast steel greatly exceeds that of cast iron, in order to prevent the shrinkage and shrinkage defects of castings, most of the casting process adopts measures such as riser, cold iron and subsidies to achieve solidification in order. In addition, in order to prevent shrinkage, shrinkage, porosity and crack defects in cast steel parts, uniform wall thicknesses, sharp corners and right angle structures, sawdust in the mold sand, and coke addition in the core should be avoided. Hollow cores and oil sand cores are used to improve the concession and air permeability of sand or cores. The melting point of cast steel is high, and the corresponding casting temperature is also high. The molten steel interacts with the mold material at high temperatures and is susceptible to sticky sand defects. Therefore, artificial quartz sand with a higher refractoriness should be used as a mold, and a paint made of quartz powder or zircon sand powder should be brushed on the surface of the mold. In order to reduce the source of gas, improve the fluidity of molten steel and the strength of molds, most steel castings are cast with a dry or quick-drying type, such as CO2 hardened water glass sand type.
Third, the heat treatment of steel castings
Steel castings should be used after heat treatment. Because the as-cast steel castings have internal porosity, cracks, shrinkage and shrinkage, coarse grains, unequal structure and residual internal stress and other casting defects, the strength, especially the plasticity and toughness of the steel castings are greatly reduced. In order to refine grains, uniform structure, and eliminate internal stresses, steel castings must be normalized or annealed. The normalized steel has higher mechanical properties than the annealed ones and has a lower cost, so it is used more often. However, since the normalizing treatment will cause greater internal stress than annealing, it is only applicable to steel castings with a carbon content of less than 0.35%. Because of the good plasticity of low-carbon steel castings, it is not easy to crack when cooled. In order to reduce the internal stress, the steel castings should be tempered at high temperatures after normalizing. For steel castings with a carbon content of ≥ 0.35%, which are complex in structure and prone to cracking, they can only be annealed. Cast steel should not be hardened, otherwise it is easy to crack.
Fourth, the melting of cast steel
The smelting of cast steel generally uses open hearth, electric arc furnace and induction furnace. Open hearth furnace is characterized by large capacity, can use scrap as raw material, can accurately control the composition of steel and can smelt high-quality steel and low-alloy steel, and is mostly used for smelting molten steel used for large-scale steel castings with high quality requirements. The three-phase electric arc furnace has convenient operation for opening and shutting down the furnace, can guarantee the composition and quality of the molten steel, and is not strict with the requirements of the charge material, and is easy to raise the temperature. Therefore, the high-quality steel, the high-grade alloy steel and the special steel can be produced, and the production molding is performed. Common equipment for steel castings. In addition, using high-frequency or medium-frequency induction furnaces can smelt all kinds of high-grade alloy steels and steels with extremely low carbon content. Induction furnace melting speed, low alloying element loss, energy consumption, and high quality of molten steel, that is, less impurities, less inclusions, suitable for small steel casting workshop!