Causes and Solutions of Hot-Rolled Sheet Surface Defects: 5 Tips to Improve Product Qualification Rate

Surface defects in hot-rolled sheets—such as scale, cracks, and indentations—directly reduce product qualification rates and increase production costs. For manufacturers, understanding the root causes of these defects and implementing targeted solutions is key to optimizing hot-rolling processes and enhancing sheet quality control.​

Common Hot-Rolled Sheet Defects: Causes & Targeted Solutions​

1. Scale (Oxide Layer)​
Causes: Formed when hot steel (1,100–1,250°C) reacts with oxygen in the air during heating or rolling. Thick scale can peel off during processing, leaving uneven marks on the sheet surface.​
Solutions: Use a high-temperature descaling system (e.g., high-pressure water jets at 15–20 MPa) before and during rolling to remove oxide layers. Control furnace atmosphere by adding nitrogen to reduce oxygen exposure.​

2. Surface Cracks​
Causes: Often stem from internal defects in steel billets (e.g., segregation of alloy elements) or excessive rolling pressure. Rapid cooling after rolling can also create thermal stress, leading to crack formation.​
Solutions: Inspect steel billets for internal defects using ultrasonic testing before rolling. Adjust rolling pressure to match the steel’s ductility (e.g., reduce pressure for high-carbon steel grades) and adopt gradient cooling (slow cooling in the first stage) to minimize thermal stress.​

3. Indentations & Scratches​
Causes: Caused by foreign particles (e.g., iron filings) on rolling mill rolls, or worn/damaged roll surfaces. Loose parts in the conveyor system can also scratch the sheet during post-rolling transport.​
Solutions: Clean rolls regularly with wire brushes and replace worn rolls (when surface roughness exceeds Ra 1.6 μm). Install particle filters in the rolling mill and add protective rubber pads to conveyor belts.​

4. Wrinkles​
Causes: Uneven rolling speed (e.g., front rolls moving faster than rear rolls) or uneven sheet thickness, leading to material accumulation and wrinkling.​
Solutions: Calibrate rolling mill speed to ensure consistent rotation across all rolls. Use laser thickness gauges to monitor sheet thickness in real time and adjust roll gaps promptly.​

5. Stains (Oil or Grease Marks)​
Causes: Leakage of lubricating oil from rolling mill bearings or improper application of cooling oil during rolling. Stains affect subsequent steel surface treatment like painting or galvanizing.​
Solutions: Inspect bearing seals regularly and replace damaged ones. Use metered oiling systems to control cooling oil volume, ensuring only necessary areas are lubricated.​

5 Tips to Improve Hot-Rolled Sheet Qualification Rate​

1. Pre-Rolling Billet Inspection: Adopt 100% ultrasonic testing of steel billets to eliminate defective raw materials—this reduces defect rates by 30–40% at the source.​
2. Optimize Descaling Processes: Upgrade to high-pressure descaling jets and adjust water temperature (50–60°C) to enhance scale removal efficiency, minimizing oxide-related defects.​
3. Real-Time Quality Monitoring: Install online surface inspection systems (e.g., machine vision cameras) to detect defects in real time, allowing immediate process adjustments before more sheets are affected.​
4. Regular Equipment Maintenance: Schedule weekly roll inspections and monthly lubrication system checks to prevent indentations, scratches, and oil stains caused by equipment issues.​
5. Train Operators on Process Control: Provide training on adjusting rolling parameters (speed, pressure, cooling rate) based on steel grades—skilled operators can reduce human-error-related defects by 25%.​
By addressing defect causes at each stage of the hot-rolling process and implementing these 5 tips, manufacturers can significantly improve product qualification rates, reduce waste, and enhance the competitiveness of hot-rolled sheet products.