What is a non-metallic inclusions in seamless steel pipe

What is a non-metallic inclusions in seamless steel pipe and where is it come from?
Non-metallic compounds present in the steel, such as an oxide, sulfide, nitride or complex compounds, collectively referred to as non-metallic inclusions.
Non-metallic inclusions in steel comes from two aspects: one is produced with the smelting process, the other is brought in due to various reasons from the outside. The former we call it endogenous inclusion, and the latter is called foreign inclusion.
(1) endogenous inclusions. Can be produced in the following situations:
① in the smelting process, the product of deoxygenation is not fully ruled out, while remaining in the liquid steel. Some of them are small particles present in the basic structure of the steel, the others are gathered into large particles, and some take solid solution state in the steel;
② deoxygenation reactions are mostly exothermic reaction, the temperature drops in the casting process, then deoxygenation reaction continues to proceed, deoxidation products generated too late to float and remain in the steel;
③ In the tapping and casting process, molten steel is oxidized in contact with air, oxygen in combination with steel elements, to form a secondary oxide remaining in the steel;
④ during solidification of the ingot, as a result of "selected sub-crystalline" of the liquid steel, the low melting point of FeS, FeO precipitated between the grain boundaries and the dendrite ingot, becoming non-metallic inclusions.

(2) foreign inclusions can be produced in the following situations:
① soil, rust brought by scrap materials and impurities brought by refractories;
② in a steel casting process, molten steel tank washed out, ladle, tundish, the refractory material washed down to the molten steel to go, if too late float, then left in the molten steel;
③ When the punch mixed with slag, the steel slag to be wrapped into liquid forming mixed;
④ ladle as residue or a steel cover agent when falling ash and steel ladle and tundish stream sediment, tundish and mold will be mixed with the liquid steel to form inclusions;
In general, the endogenous particles are relatively small, and are relatively uniform distributed in the seamless steel pipe, but foreign inclusions are mostly irregular in shape, size and relatively large, the distribution is not uniform, sometimes appear here, sometimes there appear.