In the previous article, we discussed how to optimize mold flux for high-speed continuous casting. Today, we focus on a common defect that directly affects the surface quality of cold-rolled thin sheets: blisters. Blisters are flaky or scale-like defects on the surface of cold-rolled sheets, manifested as separation between the surface layer and the base metal, forming "raised skin." This defect cannot be concealed during subsequent painting or galvanizing processes, directly leading to product rejection. For high-end products such as automotive outer panels, home appliance panels, and packaging steel, blisters are a "zero tolerance" fatal defect. The root cause of blisters is often not the rolling process, but inclusions, bubbles, or cracks on or just beneath the surface of the continuous casting slab that are "flattened" and "torn" during rolling. How can you eliminate blister defects on cold-rolled sheet surfaces at the source to produce high-end thin sheets with smooth surfaces? Wuxi WeiDa Cored Wire Co.,Ltd provides a comprehensive solution based on continuous casting slab surface layer quality control, mold flux optimization, and molten steel cleanliness improvement.
The Root Cause of Blisters: The "Time Bomb" Hidden in the Slab Surface Layer
Blister defects on cold-rolled sheets can almost always be traced back to defects on or just beneath the surface of the continuous casting slab. These defects are "flattened" during hot rolling, further extended and torn during cold rolling, and ultimately exposed on the surface as blisters. The main root causes of blisters include the following four categories.
First, subsurface inclusions. This is the most common cause of blisters. During continuous casting, mold flux droplets, deoxidation products (Al₂O₃), secondary oxidation products, etc., are captured by the solidifying shell, forming large inclusions beneath the slab surface. These inclusions have mismatched deformation capacity with the matrix during rolling, leading to interface separation and blister formation. Among these, mold flux entrapment is the most important source of subsurface inclusions.
Second, subsurface bubbles. Hydrogen, nitrogen, or argon in the molten steel are captured during solidification, forming tiny pores beneath the slab surface. These pores are flattened but cannot be welded shut during hot rolling, forming elongated "seams" that further expand into blisters during cold rolling. Subsurface bubbles are usually related to insufficient steel degassing or excessively high moisture content in the mold flux.
Third, subsurface cracks. Surface microcracks generated during continuous casting due to meniscus fluctuations, poor mold flux lubrication, improper mold taper, etc., are "torn open" during rolling, forming blisters. These types of blisters are usually distributed in straight lines or grids.
Fourth, abnormal slab surface oscillation marks. Excessively deep oscillation marks cannot be completely eliminated during rolling, forming strip-like defects that appear as blisters after cold rolling. This situation is usually related to improper mold oscillation parameters.
Limitations of Traditional Inspection Methods
The "terrible" aspect of blisters is that their root cause – subsurface defects in the continuous casting slab – is often invisible to the naked eye in the slab state. Conventional slab surface inspection can only find macroscopic cracks or slag entrapment, and cannot detect defects 2-15mm beneath the surface. By the time blisters are found after cold rolling, multiple processes have already been performed – hot rolling, cold rolling, annealing, tempering – and the value added is high. The loss from rejection at this stage is enormous.
Our Solution: Eliminate Subsurface Defects at the Source
Wuxi WeiDa Cored Wire Co.,Ltd's strategy is "don't leave problems for the rolling mill" – eliminate the source of subsurface defects at the continuous casting stage.
First, prevent mold flux entrapment to reduce subsurface inclusions at the source. Mold flux entrapment is the most important source of subsurface inclusions. We help you reduce entrapment through the following methods:
•Optimize submerged entry nozzle (SEN) design: Adjust the port angle (15-25° downward) and immersion depth (120-160mm) to ensure a stable steel flow field that does not scour the meniscus.
•Use high-stability mold flux: Select flux with appropriate viscosity (0.8-1.2 poise) and appropriate density that is not easily entrained by the steel flow.
•Control mold level fluctuation: Keep level fluctuations within ±3mm – this is the foundation for preventing entrapment.
•Optimize argon flow rate: Control SEN argon flow rate within 3-8 liters/minute to avoid excessively large argon bubbles carrying mold flux into the steel.
Second, improve molten steel cleanliness to reduce deoxidation products and secondary oxidation. Al₂O₃ inclusions are another important source of subsurface defects.
•Precision calcium treatment: Using our pure calcium wire or calcium iron wire, modify Al₂O₃ into low-melting-point globular calcium aluminates, promoting their flotation and removal. Control the Ca/Al ratio within the optimal window of 0.08-0.15.
•Full protective casting: Use long nozzle argon sealing, tundish covering flux, and submerged entry nozzle to prevent secondary oxidation of the molten steel.
Third, prevent subsurface bubble formation to eliminate porosity defects at the source.
•Reduce hydrogen content in the steel: Use vacuum degassing (VD/RH) to control hydrogen content below 2ppm. Our high-purity alloy cored wires have extremely low moisture content and do not introduce additional hydrogen.
•Use low-moisture mold flux: Our mold fluxes undergo rigorous drying during production, with moisture content controlled below 0.2% .
•Optimize argon stirring: Avoid excessively strong argon stirring that would cause argon bubbles to be entrained deep into the molten steel.
Fourth, optimize mold metallurgy conditions to reduce subsurface cracks.
•Optimize mold oscillation: Use a high-frequency, small-amplitude mode (frequency >180 cpm, amplitude 3-6mm) to reduce oscillation mark depth.
•Control mold taper: Use parabolic taper or multi-stage taper to match the shell shrinkage curve.
•Stabilize mold level: Use an electromagnetic level control system to keep level fluctuations within ±3mm.
Fifth, enhance slab surface inspection to identify defective slabs early. Although subsurface defects are invisible to the naked eye, they can be detected through the following methods:
•Eddy current testing: Online eddy current inspection can identify surface cracks and inclusions.
•Spot acid etching: Periodically acid etch slabs to check macro-etch structure and evaluate the extent of subsurface defects.
•Ultrasonic testing: For high-end products, implement 100% ultrasonic testing of slabs.
Integrated Optimization of Process Parameters
Eliminating blister defects requires coordination across multiple processes. Taking low-carbon aluminum-killed steel for automotive outer panels as an example, Wuxi WeiDa's technical team assisted a customer in integrating and optimizing the following parameters: SEN port angle 18° downward, immersion depth 140mm, mold level fluctuation ±2mm, mold flux viscosity 1.0 poise, calcium treatment Ca/Al ratio 0.10, hydrogen content after vacuum degassing 1.5ppm. After implementation, the blister defect rate on cold-rolled sheets was reduced from 1.2% to 0.15% .
From "Passive Sorting" to "Active Prevention"
Many steel enterprises' approach to blister defects is "sort after rolling" – manually or automatically identifying defective sheets after cold rolling and downgrading or scrapping them. This approach not only wastes significant processing costs but also fails to solve the problem fundamentally. Wuxi WeiDa advocates "active prevention" – eliminating the source of subsurface defects at the continuous casting stage. Through an integrated four-pronged approach of mold flux optimization, molten steel cleanliness improvement, mold metallurgy control, and slab quality inspection, you can significantly reduce the blister defect rate on your cold-rolled sheets.
If you are troubled by blisters on cold-rolled sheet surfaces and wish to eliminate subsurface defects at the source, please visit our website https://www.weidamaterials.com/ to obtain the complete solution for continuous casting slab surface layer quality control and cold-rolled sheet surface defect prevention.