Key words: hot rolled SMLS pipe
The inner surface of hot rolled SMLS pipes is defined as irregular distribution of helical, semi-helical or serrated folds, some of which are straight Alice skin. The head of steel tube has some internal folds, some exist in the whole steel tube, or periodically or irregularly. The defective steel pipe can be used for grinding, heavy-duty use or scrap after being broken. Internal defects have always been one of the main factors affecting steel pipe production and yield index. There are two main reasons for internal defects: continuous casting slab (internal) and piercing process system (external factors).


Analysis of the causes of internal folds
When the steel tube is pierced by inclined rolling, it is easy to crack the metal center and form holes through continuous casting of the core by tensile stress. The metal surface of the cavity is rough and will be rapidly oxidized by the head of cooling water and air, forming a fold attached to the inner surface of the pipe during subsequent rolling. Therefore, to eliminate internal defects, we must avoid forming holes.


Relationship between quality of continuous casting billet and defects of steel tube
In the case of continuous casting billet rolling pipe, if it can be obtained without folding, internal and external surface cracks and delamination defects in capillary, generally speaking, qualified product tubes can be produced. Therefore, the smelting process is the key to the production of qualified steel tubes. In recent years, through experimental analysis, experts and scholars have produced defects and loose centers, core shrinkage and the depth of columnar crystals in continuous casting tubes.


The relationship between piercing process system and internal defects
Deformation of inclined rolling perforation includes basic deformation and additional deformation. Foundation deformation means that solid continuous casting billet enters capillary when it enters rolling geometry and size change. It is intuitive deformation, and the deformation of geometry size, shape and material has nothing to do with rolling. Additional deformation is due to the non-uniform deformation of the metal in the main body, which mainly includes torsional deformation, longitudinal shear deformation and tangential shear deformation. This kind of deformation will reduce the quality of products and increase energy consumption, so it should be minimized in production. Any factor that reduces the plasticity and brittle fracture strength of metal or increases the transverse tensile force of continuous casting pipe during perforation will lead to the formation of internal defects, and the internal defects mainly occur in the process of perforation, but the specific reasons are various.