During which mission phase did the Space Shuttle Columbia disintegrate?

Atmospheric re-entry.

The Space Shuttle Columbia was lost during the most dynamic and stressful phase of its return profile: atmospheric re-entry. While the root cause—damage to the Thermal Protection System (TPS) from a foam strike—occurred shortly after launch, the physical disintegration did not manifest until the shuttle encountered the extreme speeds and resulting superheated plasma of re-entry. This allowed the hot gases to penetrate the compromised wing structure, leading to aerodynamic heating, internal structure damage, and subsequent breakup over Texas.