We analyze residual frozen flux in a long narrow superconducting strip cooled through its transition temperature T_c in a small perpendicular magnetic field. This problem is relevant for the issue of trapped magnetic flux in superconducting electronic devices. During cooling, the low-temperature vortex configuration is formed at temperatures very close to T_c, where the flux density is determined by dynamic balance between the thermally-activated exits and entries of vortices over the geometrical energy barrier formed by the interaction with the strip edges and the Meissner screening current. In the field range between the minimum flux-expulsion field and the penetration field, the equilibrium flux density is finite due to thermal activation and rapidly decreases with decreasing temperature. During cooling, however, the escape rate decreases exponentially, and the vortex density falls out of equilibrium at a field-dependent freezing temperature T_fr. We derive and solve the dynamic-balance equation for this process, which yields definite quantitative results for T_fr and the frozen vortex density. The relative freezing temperature 1-T_fr/T_c exceeds the fluctuation width of the transition by a large logarithmic factor, rapidly increases when the magnetic field approaches the minimum flux-expulsion field, and logarithmically increases with decreasing cooling rate. The resulting frozen flux density has a very strong magnetic-field dependence which can be used to define the effective flux-expulsion magnetic field.