1.Where does the porosity defect in cold-rolled coils originate?
Porosity defects in cold-rolled coils are almost entirely inherited from the upstream continuously cast slab, originating from voids formed by gases within the solidified steel during steelmaking and continuous casting. They can be mainly classified into two categories:
Subcutaneous bubbles: Tiny pores hidden beneath the surface of the slab. This is the primary source of surface defects in cold-rolled sheets.
Pinholes/pores: Even smaller pores or pores located within the slab.
Based on their origin, these pores can be further classified into argon bubbles and reaction pores (such as CO bubbles), etc.
2.How do argon bubbles form during continuous casting and ultimately lead to defects in cold rolling?
Bubble Generation: During continuous casting, argon gas is typically blown in through stoppers or the top nozzle to prevent clogging of the submerged entry nozzle. Under the turbulent flow of the molten steel, this argon gas is broken into tiny bubbles.
Bubble Entrainment: Most argon bubbles rise to the surface and are absorbed by the protective slag. However, if the argon flow rate is unstable or the flow field is not properly controlled, some argon bubbles will penetrate deep into the crystallizer with the molten steel stream and be captured by the solidifying billet shell, forming subcutaneous pores.
Rolling Evolution: These subcutaneous pores are stretched and flattened during hot and cold rolling. If the pore walls fail to weld properly during rolling, they will crack or expand during subsequent processing, eventually forming peeling, flaking, or dot-like defects on the surface of the cold-rolled sheet.
3.Besides argon bubbles, what other metallurgical factors can cause porosity?
Poor deoxidation: When the oxygen content in molten steel is too high, or when insufficient deoxidizers (such as aluminum or silicon) are added, a carbon-oxygen reaction occurs during solidification, generating carbon monoxide (CO) gas. If the molten steel has high viscosity or solidifies too quickly, the CO gas cannot rise and escape, becoming trapped inside the billet and forming pores.
Gas supersaturation: If the content of dissolved hydrogen, nitrogen, or other gases in the molten steel is too high, they will precipitate during solidification due to a sudden drop in solubility, potentially accumulating and forming pores.
Poor drying of nozzles or protective slag: If the protective slag or nozzle material used in continuous casting contains moisture, it will generate water vapor upon contact with the molten steel at high temperatures, instantly forming bubbles that are then trapped inside the steel.
Studies indicate that oxide particles sometimes accompany these bubbles, and may even be removed during subsequent pickling, leaving only simple pores.
4.How do porosity defects evolve during the rolling process? What form do they ultimately take on the cold-rolled coil?
Initial State: In continuously cast slabs, pores are spherical or ellipsoidal, ranging in size from microscopic to visible to the naked eye, distributed subsurface or internally.
Hot Rolling Stage: Under high temperature and immense rolling pressure, pores are flattened and elongated, extending along the rolling direction. If the pore surfaces can make good contact and weld together at high temperature, the defect may disappear; if welding is poor, micro-delamination or discontinuity will form.
Cold Rolling and Annealing Stage:
Scale Formation: Unwelded subsurface pores will be exposed on or near the surface during cold rolling thinning, forming tongue-like bulges (i.e., heavy scale).
Void Formation: Severe pores or bubble clusters, when rolled to very thin dimensions, cannot be completely covered by metal, eventually rupturing to form perforations.
Blister Formation: Gas trapped inside the strip expands during annealing heating, causing localized bulging on the surface.
Final form: In cold-rolled products, porosity defects usually manifest as dot-like or strip-like peeling, single or multiple clustered small holes, and intermittent peeling.
5.How can we identify defects caused by pores at both the macroscopic and microscopic levels?
Macroscopic Identification Characteristics:
Distribution Location: Defects caused by argon bubbles are often distributed at the edges of the strip (20-50mm from the edge), as the edges are where bubbles are easily trapped. Defects caused by internal reaction pores can appear at any location across the strip width.
Defect Morphology:
Pit-like Peeling: Appears as small, raised dots with no obvious inclusions around them; common in high-carbon steel or deep-drawing steel.
Single Small Hole: An isolated small hole appears on the thin sheet; the area around the hole is smooth, with no signs of crack propagation.
Microscopic Confirmation:
Metallographic Analysis: A cross-section is cut from the defect location, ground, and observed. Peeling caused by bubbles often reveals voids or delamination extending along the rolling direction beneath, with no obvious non-metallic inclusions in the surrounding matrix.
Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis:
By opening the peeling skin or observing the inner wall of the pores, the detection of argon (Ar) directly proves that the defect originates from argon bubbles (although the argon may have already dissipated, making detection difficult).
More commonly, trace amounts of oxide particles (such as FeO, MnO, SiO₂, etc.) are detected, indicating that the pores underwent internal wall oxidation at high temperatures.
If a large amount of protective slag components such as Ca, Na, and K are detected, it indicates that the defect is more likely caused by slag entrapment than simple bubbles.
Key criterion: If no large amount of macroscopic inclusions are found at the defect site, but the morphology conforms to the characteristics of "layering, bubbling, and single pores," and the location is at the edge or there is a history of argon entrapment in the process, it can generally be determined to be a porosity defect.