Controlling Scab Defects in Hot-Rolled Tubes

Hot-rolled tubes are a widely used product in the steel pipe production process, widely used in the petroleum, chemical, construction, and machinery manufacturing industries. However, scab is a typical and significant surface quality issue in hot-rolled tubes. Scab not only degrades the surface quality of the steel pipe but can also become a stress concentration point during subsequent use. In severe cases, it can even affect the mechanical properties and service life of the steel pipe. Therefore, effectively controlling and reducing scab defects in hot-rolled tubes is a critical step in the steel pipe manufacturing process that must be addressed.

1. Formation Mechanism of Scab Defects

Scab defects in hot-rolled tubes primarily manifest as metal accumulation or adhesion on the surface or in localized areas, often accompanied by surface roughness and unevenness. Its formation mechanisms include:

Scaling and secondary indentation of metal surface oxides: During the heating or rolling process, oxide scale on the steel billet surface is not promptly removed and is carried into the rolling process and indented into the metal surface, forming scabs.

Roller surface defects cause metal accumulation: Cracks, pits, or metal adhesion on the roll surface can cause localized compression and tearing of the steel surface during rolling, resulting in scarring.

Slab surface quality issues: If the billet has primary defects such as inclusions, folds, or subcutaneous bubbles, these defects can easily expand during the hot rolling process and form scarring.

Mechanical damage during rolling: Scratches and tears caused by equipment abnormalities during rolling, conveying, or cooling can also cause secondary scarring.

II. Common Causes of Scarring Defects

Based on actual production experience, scarring in hot-rolled pipes is primarily related to the following factors:

Improper heating process: Excessively high or uneven heating temperatures lead to excessively thick oxide scale with poor adhesion, making it easy to flake off and press into the surface.

Incomplete descaling: Insufficient pressure in the high-pressure water descaling system or improper nozzle placement can result in incomplete removal of oxide scale.

Poor roll management: Severe wear on the roll surface or failure to promptly repair it can easily lead to metal adhesion and surface defects.

Unqualified billet quality: Cracks, shrinkage cavities, or slag inclusions on the billet surface expand into scabs during the hot rolling process.

Improper lubrication and cooling: Inadequate lubrication or uneven cooling can cause adhesion or irregular friction marks on the rolled piece surface.

III. Detection and Identification of Scab Defects

To promptly detect and control scab defects, steel pipe manufacturers generally use the following inspection methods:

Visual inspection: Visible surface scabs are detected through visual inspection or manual inspections.

Surface eddy current testing: Sensitive to minute surface defects on steel pipes, it can quickly detect scab locations.

Ultrasonic testing: Used to detect potential internal defects beneath scabs.

Automated online inspection system: Leveraging high-resolution cameras and AI recognition technology, it enables full process monitoring.

IV. Control Measures for Scab Defects

Effective control of scabs in hot-rolled pipes requires multiple approaches, including raw materials, equipment, processes, and management:

Bill Quality Control

Strictly control the smelting and continuous casting quality of billets to reduce initial defects such as slag inclusions and cracks.

Conduct surface inspection before entering the furnace and perform grinding if necessary.

Heating and Descaling Optimization

Control heating temperature and holding time to avoid excessive temperatures that lead to thickening of oxide scale.

Optimize the high-pressure water descaling system to ensure that nozzle pressure, angle, and coverage meet process requirements.

Roller Management and Maintenance

Regularly inspect the roll surface and promptly regrind or replace it.

Use roll materials with better wear resistance and anti-adhesion properties.

Process Parameter Control

Rationally control rolling speed and reduction to avoid local overload.

Improve lubrication and cooling conditions to reduce adhesion between metal and equipment surfaces.

Process Monitoring and Inspection

Use automated inspection equipment to detect signs of scaling in real time and take corrective measures.

Establish a defect data analysis system to compile statistics and track defect occurrence patterns.

V. Process Optimization and Development Trends

With the steel pipe industry's increasing demands for product quality, scab control is evolving towards more refined and intelligent technologies:

Intelligent Heating Control: Optimal control of the billet heating process is achieved through the integration of big data and temperature control systems.

High-Efficiency Descaling Technology: Ultra-High-Pressure Water Descaling or combined descaling technologies are employed to improve descaling efficiency.

Intelligent Detection and Feedback Systems: Utilizing artificial intelligence and machine vision, these systems automatically identify and provide early warnings for scab defects.

Green Processes and Environmentally Friendly Materials: Developing new environmentally friendly lubricants and antioxidant coatings reduces the incidence of surface defects.

VI. Conclusion

Scab defects in hot-rolled pipes not only affect product appearance but can also compromise performance and operational safety. By strengthening billet quality management, optimizing heating and descaling processes, improving roll maintenance, and introducing intelligent detection and process control technologies, the incidence of scab defects can be significantly reduced. In the future, with the in-depth application of intelligent manufacturing and process optimization, surface quality control of hot-rolled pipes will become even more refined and efficient, providing a solid foundation for the high-quality development of the steel pipe industry.