Crystallization conditions of liquid steel
The production of liquid steel is the process of placing a certain kind of material - usually steel - in a highly liquid state, usually from the ore smelting process. It has a variety of applications including sheet, strip, slug and billet production. Specific conditions for crystallization are required for each type of liquid steel depending on its intended application.
Crystallization of liquid steel is the process of allowing the liquid to solidify, or “freeze”, into individual crystals. These crystals are typically larger than the grain boundaries of the steel due to a process known as nucleation. As the grain boundary is crossed, the crystalline structure expands, assisted by the thermal energy provided by the freezing process. Formation of these crystals is influenced by temperature, solid content, and the presence of certain elements.
Crystallization occurs in two stages: nucleation and growth. Nucleation occurs when the solution cooling to the chromium steel solidifies, leading to solidification of liquid steel into larger grain structures. These crystals, or particles, begin to aggregate as they grow. Nucleation is typically initiated at around 930C (1700F) and finishes at around 780C (1400F). Growth occurs during the next stage of crystallization, during which the grain structures formed by nucleation continue to grow by the addition of solidifying liquid. Growth is typically finished at around 680C (1260F).
The cooling rate of the steel plays an important role in the crystallization process, as it influences the size and spheroidization of the crystals. Generally, slow cooling leads to larger crystal size and better spheroidization. The temperature of the coolant used during cooling is also important; it should be equal or slightly lower than the temperature of the steel, otherwise the steel could suffer from excessive grain growth.
Crystallization for strip production is usually done at a temperature of 700C (1290F), while for billet production a temperature of 900C (1650F) is used. For sheet production, the steel is typically cooled at a rate of 10°C per minute (18°F per minute).
The chemical composition of the steel also plays an important role in its crystallization behaviour. If the chemical composition of the steel contains elements such as nickel, molybdenum, chromium, and vanadium, these have an important influence on the nucleation and growth of the crystals. The presence of these elements alter the solidification time, which in turn effects the size and spheroidization of the crystals.
Through the combination of alloying elements, cooling rate, temperature and other factors, liquid steel can be crystallized in a variety of ways. The effects of these variables can be adjusted to ensure that the steel is crystallized in such a way as to best suit its intended application. By understanding and manipulating the crystallization conditions of liquid steel, it is possible to optimize the final product.
