Although the content of non-metallic inclusions in the square tube is small, it has a great influence on the performance of the tube, so the detection of non-metallic inclusions has become one of the important items in the production process. A correct understanding of the source, composition, form, hazard principle, distribution, and microscopic characteristics of inclusions is of great significance for controlling or reducing the content of inclusions in square tubes and improving quality.

The following characteristics can be used to determine the category of non-metallic inclusions in rectangular tubes:

(1) Alumina: most of them are undeformed, angular, and have small black or bluish particles, arranged in a row along the rolling direction, and have at least three particles.

(2) Spherical oxides: non-deformed, angular or round, black or bluish in shape, irregularly distributed particles.

(3) Single-particle spherical type: round or nearly round, single-particle inclusions with a diameter ≥ 13 μm.

(4) Sulfides: single gray inclusions with high ductility and a wide range of shape ratios, and generally end rounded corners in square rectangular tube materials.

(5) Silicate: with high ductility, a single black or dark gray inclusion with a wide range of aspect ratios, generally with an acute angle at the end.

The evaluation of the traditional type of inclusions can also be done by comparing its shape with the above five types of inclusions and indicating its chemical characteristics. The evaluation of deposits such as borides, carbides, carbonitrides or nitrides in square rectangular tubes can also be compared with the above five types of inclusions according to their morphology, and their chemical characteristics are expressed according to the above methods.