We put a glass of water (such as 20°C) in a cold storage at -20°C. When the temperature of the water drops to 0°C, crystals appear in the cup. At this time, water and water crystals coexist, and the temperature is still 0°C. , only when the water is completely frozen, the entire temperature of the cup drops to the same temperature as the cold storage. Therefore, the temperature at which water begins to freeze is called the freezing temperature. The solidification and crystallization process of molten steel is the same as that of water. When the temperature drops to the solidification temperature (1535°C), crystals appear. It can be seen that, to realize the process of converting liquid into solid, two conditions must be satisfied, namely, a certain degree of subcooling and a crystallization core. The so-called subcooling degree is the degree that the actual temperature is lower than the solidification temperature. For pure iron, only when the supercooling degree reaches 295 °C, many atomic groups with small volumes and short-range orderly arrangement in the liquid metal can form embryonic nuclei as crystalline cores and gradually grow up. However, in actual production, when molten steel is poured into the mold, the subcooling degree required for crystallization is only a few degrees.
This is because:
1) The mold temperature is low, the molten steel temperature is high, and the mold wall provides cooling power.
2) The unevenness of the surface of the model provides a “support” and is conducive to the formation of crystal nuclei.
3) The particles suspended in molten steel can also be used as crystallization cores.