1.What is the mechanism by which inclusions cause cracking in cold-rolled coils?
The core mechanism by which inclusions cause cracking is the disruption of the matrix continuity, becoming a source of stress concentration.
Stress Concentration: The plasticity of non-metallic inclusions (such as oxides, sulfides, silicates, etc.) is generally much lower than that of the steel matrix. During cold rolling or subsequent stamping, extremely high stress concentrations occur at the interface between the inclusion and the matrix.
Microcrack Initiation: When the stress exceeds the interfacial bonding strength, the inclusion itself will break or peel off from the matrix, forming microcracks.
Crack Propagation: Under continuous external force, these microcracks will propagate and connect. Especially when inclusions are distributed in a chain-like or banded pattern, cracks will propagate rapidly along the inclusion aggregation area, eventually leading to macroscopic cracking.
2.What are some real-world examples that demonstrate that inclusions can cause cold-rolled coils to crack or break?
Cold Rolling Strip Breakage: Statistics from Xinjiang Baosteel regarding strip breakage accidents in its cold rolling mills show that raw material issues (mainly inclusions) accounted for 88.6% of all breakages. Electron microscopy analysis revealed that the composition of inclusions at the fracture surface was similar to that of the mold flux in the crystallizer, confirming that the breakage was caused by slag entrapment during the continuous casting process.
Edge Damage: A steel mill's SPHC cold-rolled base material exhibited edge damage. Research revealed that the cause was a high content of type B (Al₂O₃) inclusions in the steel, leading to micro-cracks at the slab edge before rolling. These cracks then tore and expanded after rough rolling.
Stamping Delamination Cracking: An automobile factory reported delamination cracking in cold-rolled galvanized sheets after stamping. The main causes included slag entrapment and subcutaneous inclusions. These defects became crack initiation points under stamping stress.
Bending and Coiling Brittle Fracture: A cold-rolled sheet experienced brittle fracture during bending and coiling, with a clean fracture surface and no plastic deformation. Analysis shows that the cause is the presence of a large number of Mg-Si inclusions and banded segregation inside, which disrupts the grain continuity and becomes the fracture initiation point.
3.Which types of inclusions have the greatest impact on the risk of cracking?
Brittle inclusions (e.g., Al₂O₃, TiN): These inclusions are hard and have poor plasticity, making them difficult to deform during rolling. They easily break and form chain-like distributions, severely damaging the matrix. Studies have confirmed that high Al₂O₃ inclusion content is a direct cause of edge cracks in slabs.
Large composite inclusions (e.g., slag entrapment): These are protective slags entrapped during continuous casting. They have complex compositions (containing Ca, Na, K, etc.), are usually large in size and irregular in shape, and have extremely poor bonding with the matrix, posing a significant hazard.
Deformable inclusions (e.g., MnS): Although MnS deforms and extends during hot working, if its size is too large or it forms a network, it will significantly reduce the transverse plasticity of the steel, leading to edge cracking during rolling.
Combined defects of bubbles and inclusions: When bubbles on the surface of the steel billet rupture during rolling, if they simultaneously attach to inclusions, they form a peeling defect, which is extremely prone to delamination and cracking during subsequent processing.
4.What is the relationship between cracking risk and the size and distribution of inclusions?
Size Effect: Generally, the larger the inclusion size, the higher the risk of cracking. Large inclusions are macroscopic defects that directly damage the matrix. Studies have shown that even micron-sized (1~10μm) spherical inclusions, if located in stress concentration areas (such as the subsurface), can become fatigue crack initiations.
Distribution Morphology: The distribution morphology of inclusions is more important than their absolute number. A single, isolated, small inclusion is relatively less harmful. However, if inclusions are distributed in chains, bands, or clusters, they form a weak surface along which cracks easily propagate, leading to delamination or brittle fracture.
5.How to detect and prevent cracks caused by inclusions?
Detection Methods:
Online Non-Destructive Testing: Utilizing an automated surface defect detection system (such as Parsytec), inclusions, scratches, and other defects on the strip surface can be detected in real-time on continuous annealing and pickling lines, providing timely warnings.
Offline Microscopic Analysis: For suspected inclusions, scanning electron microscopy and energy dispersive spectroscopy are used for microscopic morphology observation and compositional analysis. This is the most reliable method for determining the nature and source of inclusions.
Preventive Measures (Through the Entire Steelmaking-Rolling Process):
Steelmaking and Continuous Casting Control:
Improve the accuracy of liquid level control in the crystallizer to avoid excessive liquid level fluctuations that could lead to the entrapment of protective slag.
Optimize the parameters of the submerged entry nozzle to improve the flow field in the crystallizer and reduce slag entrapment.
Strengthen ladle refining to promote the flotation and removal of inclusions.
Bill Quality Control:
Conduct surface and internal quality inspections on continuously cast billets. Billets with defects are finished or discarded.