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, the fluidity of the molten steel is poor, and the shrinkage is large. The volume shrinkage rate is 10 to 14%, and the linear shrinkage is 1.8 to 2.5%. In order to prevent insufficient pouring and cold isolation of steel castings, , shrinkage cavities, shrinkage porosity, cracks, sand sticking and other defects, more complex process measures must be taken than cast iron:

1. Due to the poor fluidity of molten steel, in order to prevent cold shut and insufficient pouring of steel castings, the wall thickness of steel castings must not 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 or hot casting should be used; the pouring temperature should be increased appropriately, generally 1520° to 1600°C. Because the pouring temperature is high, the molten steel will be superheated and remain liquid. over a long period of time, liquidity 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 temperature 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, in order 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, in order 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. In order 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.
2. 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. In order 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. In order to reduce internal stress, steel castings should be tempered at high temperature 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.
3. 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 stop, can ensure the composition and quality of the molten steel, has not 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 an ideal tool for producing 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 extremely low carbon content. The induction furnace has fast melting speed, small burning loss of alloy elements, low energy consumption, and high quality of molten steel, that is, low impurity content and low inclusions. Learn more about related casting techniques.
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