Classification of Non-Metals Impurities by Chemical Composition

Impurities in steel can be classified according to their composition, performance, source and size. According to the composition of inclusions, the impurities in steel can be divided into three categories: oxide impurities, sulfide impurities and nitride impurities.



Oxide impurities can be divided into simple oxides, including FeO, Fe2O3, MnO, Al2O3, SiO2, TiO2, etc. Complex oxides include spinel-type impurities and calcium calcium aluminates. The spinel-type impurities have an octahedral crystal structure of MgO · Al2O3. Although calcium is also a divalent metal element, because its ionic radius is too large, its oxide is not a spinel structure, but calcium aluminate CaO · Al2O3. It is aluminum in steel and suspended in molten steel The reaction product of alkaline slag, or the product of deoxidation of calcium alloy and aluminum. Silicates are complex compounds composed of metal oxides and silicon dioxide, so they also belong to the oxide-type impurities. This type of impurity is related to the secondary oxidation of eroded refractory materials, slag and steel slag. In addition, oxides can also form solid solutions before, the most common is FeO-MnO, commonly known as manganese-containing oxides.

Impurity of sulfide substance: When the sulfur content is high, it is precipitated in the form of FeS with a melting point of only 1190 ° C in the as-cast steel, resulting in hot brittleness of the steel. In order to reduce or eliminate this hazard, the general method is to add a certain amount of manganese to the steel to form a MnS substance with a higher melting point. Therefore, under normal circumstances, the sulfide impurity in steel is mainly FeS, MnS and their solid solutions. In the smelting process, Al2S3 impurities will be formed in the steel when the amount of aluminum added is large. When the rare earth elements lanthanum and cerium are added to the steel, for example rare earth ferro silicon magnesium, the corresponding rare earth sulfides La2S3, Ce2S3, etc. will be formed. In most steels, sulfides are more important impurities than oxides. Depending on the degree of deoxidation of the steel, the content of the remaining deoxidizing elements, sulfides will form various forms such as spherical, chain, and irregular shapes in the steel, and its positive effect on the hot brittleness of the steel is also large to small.

Impurity of nitrides: In general, the nitrogen content of the molten steel is not high, so the nitride impurities in the steel are also less. However, if the molten steel contains elements such as aluminum, titanium, niobium, vanadium, etc., which have a greater affinity for nitrogen, the molten steel will absorb nitrogen in the air during tapping and pouring and increase the amount of nitrogen inclusions in the steel.

In summary, under normal circumstances, the inclusions in steel are mainly oxides and sulfides, and the nitride inclusions are not many and often exist in the form of composites.