Sources of Inclusions in X70 Pipeline Steel

According to the sources, the inclusions in X70 pipeline steel can be divided into two categories: endogenous and exogenous. Among them, the endogenous inclusions are mainly oxide inclusions, which come from the following sources:Deoxidation products are precipitated during alloying and final deoxidation of aluminum at steelmaking temperatures, known as primary deoxidation products; the deoxidation product precipitated during the cooling of the X70 molten steel from the refining temperature to the liquidus temperature is called the secondary deoxidation product; the deoxidation product precipitated when X70 molten steel is cooled from the liquidus temperature to the solidus temperature is called the third deoxidation product. The distribution of endogenous inclusions in X70 pipeline steel is relatively uniform, and their particles are generally fine.

The foreign inclusions are mainly in the process of smelting, tapping and casting of X70 steel. X70 molten steel is in contact with refractory materials and slag, and slag is involved in X70 molten steel (especially when tapping). Refractories are eroded at high temperatures, and its particles are mixed into the X70 molten steel, these situations are unavoidable. The refractory materials and slag particles mixed into the X70 molten steel undergo chemical reactions with the X70 molten steel and constantly change in composition and structure. These particles, whether they have just been brought into the X70 molten steel mechanically, or have undergone considerable changes, and once they are retained in the casting, they become inclusions in the X70 steel, which are called foreign inclusions. Foreign inclusions are generally large in size and irregular in shape, with structures that appear here and there only by accident.