The inhomogeneity of elements in inclusions and the composition of compounds are the main forms of inclusions in steel, which are seriously harmful to steel structures. The hazards of inclusions of different elements and their compounds are simply classified as follows:
Sulfur and sulfides in steel are inclusions in steel and are seriously harmful to steel structures.
Since the affinity of S with elements such as Ni, Mn, Ti, and Zr is much greater than that of Fe, sulfides such as MnS, TiS (titanium-containing steel), and NiS (high-nickel steel) are commonly found in steel.
Since MnS is easily soluble in water containing Cl ions, its main hazard is to reduce the corrosion resistance of steel, especially the pitting and crevice corrosion resistance.
Another hazard of sulfides in steel is that they reduce the plasticity, toughness, and fatigue resistance of steel. If the sulfide content in steel exceeds a certain standard, it will cause cracks in the ingot during smelting production and rolling.
O in steel reacts with elements such as Ai, Si, Cr, Mn, and Fe to form complex oxide inclusions such as Al2O3, SiO2, Cr2O3, MnO, Fe2O3, FeO oxides, and FeO·Cr2O3.
The presence of oxides and silicates in steel destroys the continuity of the steel matrix and leads to stress concentration, thereby reducing the plasticity, toughness, and fatigue resistance of the steel. On the one hand, the directionality of the mechanical properties of the steel is enhanced, which deteriorates the lateral properties; on the other hand, the cutting performance of the steel is reduced, and it is difficult to polish.
The surface defects of some steels, such as skinning, scarring, unevenness, and cracks, are all related to non-metallic inclusions in the steel, which are mainly SiO2·FeO·Al2O3 and titanium oxides and nitrides.
Common nitrides in steels containing Ti and Nb include TiN, NbN, and AlN inclusions. Since nitride inclusions have high hardness, are not easy to deform, and are mostly distributed in groups in steel, their adverse effects on steel are similar to those of oxide inclusions such as SiO2 and Al2O3.
The harm of nitrides to the mechanical properties of steel. With the increase of nitrides in steel, the brittleness of steel increases significantly. Studies have shown that geometric nitrides are the source of cracks that lead to brittle transgranular fracture of some steels.
