For machine parts requiring higher strength, plasticity, and toughness, steel castings are required. The output of steel castings is second only to cast iron, accounting for about 15% of the total castings output.
1.Classification of chemical ingredients
According to the chemical composition, cast steel can be divided into two categories: carbon cast steel and alloy cast steel. Among them, carbon cast steel is the most widely used, accounting for more than 80% of the total cast steel production.
1). Carbon cast steel
Generally, low carbon steel ZG15 has a higher melting point and poor casting performance, and is only used to manufacture motor parts or carburized parts; medium carbon steel ZG25~ZG45 has higher comprehensive properties than all types of cast iron, that is, high strength and excellent properties. It has excellent plasticity and toughness, so it is suitable for manufacturing parts with complex shapes and high strength and toughness requirements, such as train wheels, forging hammer frames and anvils, rolls, high-pressure valves, etc. It is the most widely used type of carbon cast steel; High carbon steel ZG55 has a low melting point and its casting performance is better than that of medium carbon steel. However, its plasticity and toughness are poor and it is only used to manufacture a few wear-resistant parts.
2). Alloy cast steel
According to the total amount of alloying elements, alloy cast steel can be divided into two categories: low alloy steel and high alloy steel.
① Low alloy cast steel, mainly used in my country is the manganese series, manganese-silicon series, and chromium series. Such as ZG40Mn, ZG30MnSi1, ZG30Cr1MnSi1, etc. It is used to make parts such as gears, hydraulic press working cylinders, and turbine rotors, while ZG40Cr1 is often used to make important stress-bearing parts such as high-strength gears and high-strength shafts.
② High alloy cast steel with special properties such as wear resistance, heat resistance, or corrosion resistance. For example, high manganese steel ZGMn13 is a kind of anti-wear steel, mainly used to manufacture parts used under dry friction working conditions, such as the grab front wall and grab teeth of excavators, tracks of tractors and tanks, 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 to manufacture parts for chemical, petroleum, chemical fiber, and food equipment.
2. Casting process characteristics of cast steel
The mechanical properties of cast steel are higher than those of cast iron, but their casting properties are worse than those of cast iron. Because the melting point of cast steel is high, the molten steel is easy to oxidize, has poor fluidity, and shrinks greatly. Its volume shrinkage is 10 to 14%, and its linear shrinkage is 1.8 to 2.5%. To prevent steel castings from defects such as insufficient pouring, cold shut, shrinkage cavities and porosity, cracks, and sand adhesion, more complex process measures than cast iron must be taken:
1). Due to the poor fluidity of molten steel, to prevent cold isolation and insufficient pouring of steel castings, the wall thickness of steel castings cannot be less than 8mm; the structure of the pouring system should be simple and the cross-sectional size should be larger than that of cast iron; dry casting should be used. In hot casting mold; increase the pouring temperature appropriately, generally 1520° to 1600°C, because the high pouring temperature will superheat the molten steel and maintain the liquid state for a long time, and the fluidity can be improved. However, if the pouring temperature is too high, it will cause defects such as coarse grains, thermal cracks, pores, and sand sticking. Therefore, for general small, thin-walled, and complex-shaped castings, the pouring temperature is about the melting point of steel + 150°C; for large, thick-walled castings, the pouring temperature is about 100°C higher than its melting point.
2). Since the shrinkage of cast steel is much greater than that of cast iron, to prevent shrinkage cavities and shrinkage porosity defects in castings, measures such as risers, cold iron, and subsidies are mostly used in the casting process to achieve sequential solidification.
In addition, to prevent shrinkage cavities, shrinkage porosity, pores and crack defects in steel castings, the wall thickness should be uniform, sharp corners and right-angle structures should be avoided, sawdust should be added to the casting sand, coke should be added to the core, and Hollow cores and oil sand cores are used to improve the concession and air permeability of the sand mold or core.
The melting point of cast steel is high, and the corresponding pouring temperature is also high. The interaction between molten steel and casting materials at high temperatures can easily produce sand-sticking defects. Therefore, artificial quartz sand with higher refractoriness should be used as the casting mold, and paint made of quartz powder or zirconium sand powder should be brushed on the surface of the casting mold. To reduce the source of gas and improve the fluidity of molten steel and the strength of the mold, most steel castings are cast with dry or quick-drying molds, such as CO2-hardened water glass sand molds.
3. Heat treatment of steel castings
Steel castings should be used after heat treatment. Because there are casting defects such as pores, cracks, shrinkage cavities, shrinkage porosity, coarse grains, uneven structure, and residual internal stress inside the cast steel castings, the strength, especially the plasticity and toughness of the steel castings are greatly reduced.
To refine the grains, uniformize the structure, and eliminate internal stress, steel castings must be normalized or annealed. The mechanical properties of normalized steel are higher than those after annealing, and the cost is lower, so it is widely used. However, since normalizing treatment will cause greater internal stress than annealing, it is only suitable for steel castings with a carbon content of less than 0.35%. Because low-carbon steel castings have good plasticity, they are not easy to crack when cooled. To reduce internal stress, steel castings should be tempered at high temperatures after normalizing. For steel castings with carbon content ≥0.35%, complex structures, and prone to cracks, only annealing can be performed. Steel castings should not be quenched, otherwise they will easily crack.
4. Smelting of cast steel
Cast steel is generally smelted using open hearth furnaces, electric arc furnaces, and induction furnaces. The characteristics of the open hearth are that it has a large capacity, can use scrap steel as raw material, can accurately control the composition of steel, and can melt high-quality steel and low-alloy steel. It is mostly used to smelt liquid steel for large steel castings with high-quality requirements.
The three-phase electric arc furnace is easy to start and shut down, can ensure the composition and quality of the molten steel, has no strict requirements on the charge, and is easy to heat up. Therefore, it can make high-quality steel, high-grade alloy steel, special steel, etc., and is ideal for the production of molded steel. Commonly used equipment for steel castings.
In addition, the use of power frequency or medium frequency induction furnaces can melt various high-grade alloy steels and steels with shallow carbon content. The induction furnace has fast melting speed, low alloy element burning loss, low energy consumption, and high quality of molten steel, that is, low impurity content and inclusions, making it suitable for use in small steel casting workshops.
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