The effect of isothermal heat treatments (1 hour at 200, 400, 600 and 800 degrees C) on mechanical properties of thermo-mechanically rolled S700MC steel has been investigated by extensive mechanical characterizations. Treatments at 600 degrees C increase yield and tensile strength and decrease impact energy. Below 600 degrees C the steel retains its bainitic structure. Simulations of precipitation kinetics suggest that this hardening effect arises from the nucleation of fine (Ti,Nb)C particles, indicating that the bainitic structure is unstable above 600 degrees C due to its high supersaturation with respect to C, Nb and Ti. These results can help to optimize the operating practices for post-weld heat treatments.