Deoxidation of molten steel is the last key process operation before casting, which is directly related to the metallurgical quality of cast steel. Deoxidation is also the basis of nitride enhancement and nodular modification of cast steel. Deoxidizer selection should follow the following principles:
- Compared with Fe, deoxidizer elements have a greater affinity with oxygen;
- The melting point of the deoxidizer is lower than the temperature of molten steel, which can ensure the alloy melting, uniform distribution and uniform deoxidation;
- Deoxidation products are insoluble in molten steel and easy to float and discharge;
- Deoxidation elements remaining in steel are harmless to the performance of steel;
- Wide source of deoxidizer, low price. According to the above principles, the commonly used deoxidizer in the production practice is aluminum, silicon, manganese and their combination of silicon manganese, silicon aluminum alloy, etc., and the order of element deoxidation capacity is Al>Si>Mn.
Manganese is an element with weak deoxidation capacity. Mn deoxidation can produce liquid deoxidation product MnO·FeO, which is easy to exclude liquid steel. The deoxidation capacity of manganese increases with the decrease of temperature. However, even at lower temperatures, the deoxidation capacity of manganese is weak, but Mn is still an indispensable deoxidation element in production.
Silicon is a strong deoxidizing element. The deoxidation product of silicon is solid SiO2 insoluble in liquid steel, which is moistened by liquid steel and is not easy to polymerize into larger particles, and it is difficult to float, so it takes a longer time to eliminate. The deoxidation capacity of silicon increases with decreasing temperature. When the induction furnace is deoxidized with Si, the oxygen reduction rate is faster, and the Si is added for 5 ~ 10min, the oxygen can be reduced to the lowest value.
Aluminum is a commonly used deoxidation element in steelmaking technology, aluminum and oxygen have a strong affinity, adding too much aluminum will aggravate the secondary oxidation of liquid steel, and cast steel is easy to produce brittle fracture defects. The surface of the brittle fracture of the cast steel is smooth, bright and slightly curved, similar to the rock sugar fracture. The change of the solubility of N and Al in steel depends on the chemical composition of steel, AlN in the austenite grain formation, then precipitates in the grain boundary in the shape of a thick film, and produces macro structure defects, reduces the plasticity and toughness of the cast steel, and increases the tendency of the cast steel to crack.
Titanium is a strong deoxidation element, and its deoxidation capacity is stronger than silicon and weaker than aluminum. Depending on the Ti content, different oxides can be generated when titanium is deoxidized. When Ti < 0.2%, the deoxidation product is TiO2 or Ti3O5. The deoxidation products of titanium are 16 ~ 18μm in size, approximately spherical, and uniformly distributed in the grain. In addition, titanium can also form TiN with high melting point and high dispersion in the liquid steel, which can be used as the core of non-uniform nucleation, so that the steel can obtain fine grain structure, and at the same time, TiN can also prevent the brittle fracture of the cast steel. Adding an appropriate amount of Ca to the aluminum deoxidized liquid steel can further deoxidize, and the deoxidized product is easy to float and discharge when combined with the aluminum deoxidized product. After Ca treatment, the shape of aluminum oxide inclusion can be changed, and the properties of liquid steel and the purity of steel can be improved. Ca in silicon aluminum calcium barium is a strong deoxidizer, the boiling point of Ca is 1484℃, it enters the liquid steel quickly becomes steam, reacts with the oxide during the floating process, and changes and refines the shape of the inclusions, but the yield of Ca is particularly important in the deoxidizer operation, which is mainly controlled by the type of deoxidizer, adding method and adding amount.
The application of rare earth deoxidizer in steel is mainly based on two elements, La and Ce, and mainly added to steel in the form of rare earth iron alloy to play a deoxidizing effect. Rare earth has a strong deoxidation capacity, but the shape of the deoxidation products formed is irregular, and the melting point is low, it is difficult to float to the surface of the liquid steel, and can not play the role of purifying the liquid steel. Rare earth can denaturate the inclusions in steel, especially MnS inclusions, which are conducive to their floating and improving mechanical properties.
