Dr. Dmitri Kopeliovich
Non-metallic inclusions are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, silicates, oxysulfides, carbonitrides).
Despite of small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as:
Depending on the source, from which non-metallic inclusion are derived, they are subdivided into two groups: indigenous and exogenous inclusions.
Indigenous inclusions are formed in liquid, solidified or solid steel as a result of chemical reactions (deoxidation, desulfurization) between the elments dissolved in steel.
Exogenous inclusions are derived from external sources such as furnace refractories, ladle lining, mold materials etc. Amount of exogenous inclusions and their influence on the steel properties are insufficient.
FeO, Al2O3, SiO2, MnO, Cr2O3 etc.
Al2O3*SiO2, Al2O3*FeO, Cr2O3*FeO, MgO*Al2O3, MnO*SiO2 etc.
FeS, MnS, CaS, MgS, Ce2S3 etc.
MnS*MnO, Al2O3*CaS, FeS*FeO etc.
Fe3C, WC, Cr3C2, Mn3C, Fe3W3C etc.
TiN, AlN, VN, BN etc.
Titanium carbonitrides, vanadium carbonitrides, niobium carbonitrides etc.
Fe3P, Fe2P, Mn5P2
There are three stages of inclusions formation:
At this stage nuclei of new phase are formed as a result of supersaturation of the solution (liquid or solid steel) with the solutes (eg. Al and O) due to dissolution of the additives (deoxidation or desulfurization reagents) or cooling down of the metal.
The nucleation process is determined by surface tension on the boundary inclusion-liquid steel. The less the surface tension, the lower supersaturation is required for formation of the new phase nuclei.
The nucleation process is much easier in the presence of other phase (other inclusions) in the melt. In this case the new phase formation is determined by the wetting angle between a nucleus and the substrate inclusion. Wetting condition (low wetting angle) are favorable for the new phase nucleation.
Growth of a separate inclusion continues until the chemical equilibrium is achieved (no supersaturation).
Growth of inclusions in solid steel is very slow process therefore a certain level of non-equilibrium supersaturation may be retained (eg. martensite).
3. Coalescence and agglomeration
Motion of the molten steel due to thermal convection or forced stirring causes collisions of the inclusions, which may result in their coalescence (merging of liquid inclusions) or agglomeration (merging of solid inclusions). The coalescence/agglomeration process is driven by the energetic benefit obtained from decrease of the boundary surface between the inclusion and the liquid steel. Inclusions with higher surface energy have higher chance to merge when collide.
Large inclusions float up faster than the smaller ones. The floating inclusions are absorbed by the slag. The floating process may be intensified by moderate stirring. Vigorous stirring will result in breaking the large inclusions into small droplets/fragments. Gas bubbles moving up through the molten steel also promote the inclusions floating and absorption by the slag. Blowing inert gas in Ladle Furnace (LF) or Vacuum ladle degassing of steel in result in obtaining cleaner steel.
Besides of the shape of non-metallic inclusions their distribution throughout the steel grain structure is very important factor determining mechanical properties of the steel.
Distribution of non-metallic inclusions may change as a result of metal forming (eg. Rolling). Ductile inclusions are deformed and elongated in the rolling direction. The less ductile inclusions are breaking forming chains of fragments. Elongated inclusions and chains result anisotropy of mechanical and other properties. Mechanical properties in transverse direction are lower than those parallel to the rolling direction.