Quality problems in the production process of seamless steel pipes - quality defects of rolling pipes and their prevention

Quality defects of rolling pipes and their prevention
The rolling process is the main process to determine the wall thickness of seamless steel pipes. Its function is to reduce the wall of perforated rough pipes. The main defects are uneven wall thickness of rough pipes, surface cracks, pits, abrasions, rolling folds, punctures (breaks), etc.

1. Quality defects of automatic rolling pipes
When the automatic steel pipe rolling machine rolls rough pipes, the rough pipe quality defects produced include uneven wall thickness of rough pipes, rough pipe "ears" and rolling folds, rough pipe inner surface straight lines and pits, rough pipe surface cracks and abrasions, etc.
The uneven wall thickness of rough pipes: The main quality defects produced when the automatic steel pipe rolling machine rolls steel pipes are related to the uneven wall thickness of the rough pipes and the uneven temperature of the rough pipes, as well as the structural type of the steel pipe rolling machine itself, the shape of the roller hole, the deformation, the shape of the head, and the operating technology.
"Ears" and roll-breaks on rough pipes: are mainly caused by the unreasonable distribution of deformation, incorrect hole shape, tool wear, and other reasons. Any conditions that are not conducive to the axial extension of the rough pipe and promote the lateral expansion of the metal are likely to cause "ears" and roll-breaks on the rough pipe.
Straight lines and pits on the inner surface of rough pipes: Internal straight lines are a quality defect on the inner surface of rough pipes that often occurs in automatic steel pipe rolling machines, and are also a difficult problem to solve. In most cases, the inner straight lines of rough pipes are slight and will not turn the steel pipes into scrap. The inner straight lines are linear scratches along the axial direction on the inner surface of the rough pipe, mainly produced in the wall reduction area of the rough pipe. The surface state of the plug, the iron oxide scale on the inner wall of the rough pipe, the distribution of deformation, and the lubrication conditions are all related to the inner straight lines of the rough pipe. Spraying lubricant into the inner hole of the rough pipe to improve the lubrication effect of the plug can reduce the rolling pressure, reduce the wear of the plug, and avoid the plug sticking to the steel, thereby reducing the occurrence of inner straight lines on the rough pipe. Internal pitting is manifested as uneven pitting in a certain area of the inner surface of the rough pipe. It is mainly because the numerous iron oxide scales on the inner surface of the rough pipe are not blown clean, or it may be caused by the accumulation of anti-oxidants or lubricants in a certain area of the inner surface of the rough pipe after being damp and agglomerated. To reduce the internal pitting of the rough pipe, the angle of the air nozzle used in the iron oxide removal device should be properly adjusted and sufficient wind pressure should be ensured to completely remove the iron oxide scale.
Surface cracks and scratches on rough pipes: There are two types of cracks on the surface of rough pipes, one is caused by the rough pipe itself, and the other is caused by the automatic steel pipe rolling process. When the deformation is too large, the deformation distribution is unreasonable, or the hole type is improperly adjusted, and the surface wear of the rolling tool is relatively serious, surface cracks will occur. Any measures to reduce the uneven wall thickness of the rough pipe and reduce the uneven deformation of the metal are conducive to reducing the surface cracks of the rough pipe. The surface scratches of the rough pipe are caused by the rough surface of the deformation tool, or by the relative movement between the rough pipe and the workpiece in contact with the rough pipe surface. To reduce the surface scratch defects of rough pipes, the inspection and adjustment of steel pipe rolling tools and workpieces should be strengthened.

2. Quality defects of periodic steel pipes
The rough pipe defects of periodic steel pipe rolling mills are caused by the rough pipes themselves, and the other is caused by the self-pipe tying process. The defects include: uneven wall thickness (uneven transverse wall thickness, uneven longitudinal wall thickness, and wall thickness run, etc.), surface defects (surface scratches, internal and external scars, internal straight lines, cracks, internal pits, etc.), rolling (rolling, puncture, "baby mouth", etc.)
The uneven wall thickness of rough pipes: It is the main quality defect caused by the steel pipe rolling mill, including uneven transverse wall thickness, uneven longitudinal wall thickness, convex wall thickness, and "bamboo" wall thickness. Factors affecting uneven wall thickness of rough pipe: A. Roller hole shape; B. Process parameters; C. Steel pipe rolling tools; D. Operation technology;
Surface defects of rough pipe: mainly surface scratches, inner straights, inner pits, inner and outer scars, rough pipe rolling folds (transverse rolling folds, longitudinal rolling folds), etc. of rough pipe; surface scratches of rough pipe are mainly caused by surface defects of mandrel and roller, and scratches on the surface of rough pipe by the rear guide groove of steel pipe rolling machine. The inner straights are caused by the steel sticking, severe wear, "flesh falling" of the mandrel surface, or the hard iron oxide scale and iron filings remaining on the inner surface of the rough pipe, which scratch the inner surface of the rough pipe during the rolling process and form straight grooves of rough pipe. To reduce surface scratches and inner straights of rough pipe, the surface of the mandrel and roller should be smooth, and the cooling of the roller and lubrication of the mandrel should be strengthened. The rolling and folding of the rough pipe are mainly divided into transverse rolling and folding caused by the deformation of the metal being accumulated on the surface of the rough pipe due to the obstruction of axial flow, and longitudinal rolling and folding caused by the wide metal forming "ears" at the roll gap of the roll. For the quality defects caused by periodic pipe tying, the quality of the rough pipe should be guaranteed first, and the control of the tool quality should be strengthened, the hole design of the roll and the adjustment of the process parameters should be emphasized, and the adjustment and operation level of the pipe tying machine should be improved.

3. Quality defects of continuous rolling steel pipes
The remarkable characteristics of the continuous rolling steel pipe machine are its large production capacity and high production efficiency. The rough pipes rolled are long in length, good in product quality, and have a wide range of specifications. However, the investment is large, the relevant technical requirements are many, and the degree of automation is high. The quality defects of the continuous rolling rough pipe mainly include uneven wall thickness of the rough pipe, surface defects rolling and folding (including concave), and so on. The cause of concave: The uneven heating temperature of the rough pipe will increase the uneven deformation of the rough pipe, which is easy to cause concave and rolling of the rough pipe. The larger the rough pipe diameter-to-wall ratio D/S value, the smaller the tensile force required for the rough pipe wall to be stretched and deformed, and the greater the possibility of denting and rolling. The prevention of denting mainly involves strengthening the adjustment of rolling process parameters and hole type of the continuous rolling steel pipe mill.

4. Quality defects of oblique rolled steel pipes
The defects of rough pipe oblique rolling are caused by the rough pipe itself, and the other is caused by the process of self-rolling steel pipes. The defects are related to the hole shape and adjustment of the oblique rolling mill, deformation process parameters, and adjustment, tool quality, mandrel lubrication conditions, etc. The defects are surface defects, uneven wall thickness, torsion, and irregular head and tail shapes. To prevent the quality defects of the rough pipe caused by the oblique rolling process, it is necessary not only to improve the steel quality of the tube blank, improve the heating quality and perforation quality of the tube blank, ensure the quality of the perforated rough tube, and eliminate the defects such as rolling folds, cracks, pits, and scars on the surface of the rough tube but also to reasonably design the hole type, optimize the deformation process parameters, carefully adjust the hole type according to the process parameters, ensure the quality of the steel pipe rolling tools and improve the lubrication conditions of the tools, etc.