Mechanisms for the non-equilibrium segregation of solutes to static grain boundary during cooling (quenching-induced segregation) and to moving grain boundary during recrystallization (moving-induced segregation) are proposed. For quenching-induced segregation, in consideration of the local equilibrium among vacancies, solute atoms and vacancy-solute atom complexes, as well as the influence of equilibrium grain-boundary segregation, the theoretical dynamic formulae for this non-equilibrium segregation have been derived on the basis of the vacancy-dragging mechanism. Theoretical calculations have been carried out for the non-equilibrium segregation of boron to austenitic grain boundaries during isothermal holding and continuous cooling after heating at high temperature; the results agree well with those obtained from experiments. The model has also successfully explained the different behaviours of boron segregation during cooling in alpha-Fe and in gamma-Fe. For moving-induced segregation, based on the interaction between dislocations and the moving boundaries during recrystallization, a dislocation relaxation and widening grain-boundary mechanism of solute segregation on moving boundaries is proposed. Applying this model, we have calculated the boron segregation on moving boundaries during recrystallization in Fe-3% Si alloy; the results of these calculations agree with experimental results.