Non-metallic inclusions in steel

The quality of a metal alloy, i.e. its ability to satisfy a certain number of processing and usage properties, depends on several factors, including its physico-chemical structure. This includes the macrostructure (primary structure, wrought or not), inherited from solidification and transformation phenomena, and the microstructure (secondary structure, modifiable by thermal, mechanical or chemical treatments, etc.), which gives the metal its final hardness or toughness.

Lamacrostructure is a chemical heterogeneity of elements in solution (segregations) or elements out of solution. The latter form separate phases, insoluble in the metal matrix even before solidification. These phases are chemical compounds of the component metals and certain metalloids, generally oxygen, sulfur and nitrogen in the case of ferrous alloys. They remain insoluble in solid steel and are inherited throughout the life of the mechanical part.

The microstructure can also include these same separate phases (precipitates), but finer and more dispersed, soluble by certain heat treatments in the same way as conventional steel grains and phases.

Oxides and sulfides have long been regarded as foreign and harmful substances; this sentiment is certainly well-founded for large inclusions visible to the naked eye (up to 0.1 mm in diameter), but as all steels contain inclusions of all sizes, down to a few nanometers, and some of them are useful, we must acknowledge their presence, study their nature and behavior, and control their physico-chemical, geometric and topographical parameters as best we can.

Since knowledge of the oxygen, sulfur and other element content is insufficient to predict the properties of steels, we will successively examine the nature of inclusions, their origin, distribution, nature and morphology, their distribution, their behavior during steel processing and during their use.

As is customary in the industry, element contents are expressed in terms of mass.