In the production of thick plates (medium- and thick-gauge plates), surface quality is the core indicator determining the grade of the finished product. Surface defects are typically classified into two major categories based on their origin: steel-related defects (originating from the slab) and process-related defects (originating from the rolling process). This article will provide an in-depth analysis of the causes of these defects and offer standardized preventive management solutions.
Steel-Related Defects: Source Control Is Key
Steel-related defects refer to inherent defects present in the billet itself. If not detected or addressed prior to rolling, these defects will propagate into the finished product as the material is deformed.
1. Cracks and Fissures
Characteristics: Fish-scale or rippled patterns of varying depths. Cracks may be randomly distributed, but dense clusters are commonly found along the plate edges.
Causes:
Existing longitudinal or transverse cracks in the continuous-cast billet were not thoroughly removed.
Subsurface bubbles or inclusions rupture and become exposed under rolling pressure.
Uneven heating causes localized differences in plasticity, leading to stress concentration.
Preventive Measures:
Thorough Quality Inspection: Strictly enforce surface cleaning of slabs to ensure no cracks enter the furnace.
Precise Heating: Optimize the heating curve, strictly control the heating rate, and ensure a uniform temperature distribution.
High-Pressure Rolling: Apply a large reduction ratio during the high-temperature stage to promote the sealing of subsurface gas bubbles.
2. Gas Bubbles
Characteristics: Round protrusions with smooth edges, distributed irregularly.
Causes: Accumulation of gas within the cast billet; during rolling, surface bulges form because the gas cannot be welded shut.
Preventive Measures:
Vacuum Degassing: Strengthen degassing treatment during the smelting process to reduce gas content in the steel.
Strict Incoming Inspection: Establish a billet quality sorting system and strictly prohibit billets with subsurface bubbles from entering the furnace.
3. Surface Inclusions
Characteristics: Non-metallic substances visible to the naked eye (in the form of spots, lumps, or streaks), typically reddish-brown, light yellow, or grayish-white in color.
Causes:
Non-metallic impurities inherent in the steel or entrained protective slag.
Flaking of the furnace lining or refractory materials that is pressed into the surface of the steel plate.
Preventive Measures:
Cleaning and Derusting: Intensify desulfurization of slabs and ensure sufficient pressure in the high-pressure water desulfurization system.
Refractory Management: Perform regular maintenance on the refractory materials of the heating furnace roof and walls to prevent spalling.
Process Defects: Operating Standards as a Safeguard
Process defects arise during production stages such as heating, rolling, and heat treatment; they are acquired defects caused by human error or mechanical factors.
1. Bulges and Indentations
Characteristics: Bulges are localized, regular protrusions, while indentations are depressions of various shapes.
Causes: Material loss on the surface of rolling or straightening rolls, or foreign matter (such as iron oxide scale or steel slag) adhering to the roll surface and being pressed into the steel plate.
Preventive Measures:
Roll System Monitoring: Regularly inspect the condition of rolling and straightening rolls; if surface damage is detected, grind or replace them promptly.
Foreign Object Prevention: Strictly implement cooling water spraying to prevent “imprinting.”
2. Pitting
Characteristics: Rough, dot-like pits distributed locally or in clusters.
Causes: Corrosion of fuel nozzles, severe oxidation during the heating process leading to scale detachment, and foreign matter adhering to the bottom rollers of the heat treatment furnace.
Preventive Measures:
Furnace Condition Adjustment: Precisely control the temperature and atmosphere of the heating furnace to avoid localized overheating or underheating.
Furnace Roller Maintenance: Regularly clean the heat treatment furnace rollers to reduce the accumulation of foreign matter.
3. Embedded Iron Oxide Scale
Characteristics: Grayish-brown or reddish-brown in color, distributed in bands or lumps.
Causes: Primary or secondary iron oxide scale that was not thoroughly rinsed away by high-pressure water and became embedded in the matrix during the rolling process.
Preventive Measures:
Precise Descaling: Optimize the descaling process to ensure proper descaling pressure and flow rate in the four-high finishing mill.
Atmosphere Control: Properly control the heating environment to reduce secondary oxidation.
4. Overburn
Characteristics: Diamond-shaped or chicken-claw-shaped cracks appear at the edges, and the fracture surface reveals a distinctly coarse, fibrous microstructure.
Causes: Excessively high heating temperatures and prolonged holding times lead to oxidation at grain boundaries.
Preventive Measures:
Process Compliance: Establish strict upper limits for heating temperatures and holding times based on the characteristics of the steel grade.
Flame Adjustment: Strictly prohibit direct flame contact with the slab surface and ensure proper combustion.
Expert Recommendations: Quality Management Logic
To reduce the aforementioned defects at the source, it is recommended to implement “End-to-End Quality Traceability Management”:
Slab End: Strengthen quality inspections for hot delivery and hot charging of continuous cast slabs, and establish a defect identification database.
Rolling End: Implement real-time monitoring and closed-loop control of descaling pressure.
Equipment End: Adopt an online surface inspection system (AI visual recognition) to detect bulges, indentations, and pitting in real time.
Through standardized operating procedures (SOPs) and preventive maintenance, the production line’s defect rate can be significantly reduced, enhancing the market competitiveness of medium- and heavy-gauge steel plates.
E-mail： cykc@cykcgroup.com.cn