  By direct quenching or post-annealing followed by quenching, we have
successfully obtained a series of KxFe2-ySe2 samples with different properties.
It is found that the samples directly quenched in the cooling process of growth
show superconductivity and the one cooled with furnace is insulating even
though their stoichiometries are similar. The sample cooled with furnace can be
tuned from insulating to superconducting by post-annealing and then quenching.
Based on the two points mentioned above, we conclude that the superconducting
state in KxFe2-ySe2 is metastable, and quenching is the key point to achieve
the superconducting state. The similar stoichiometries of all the
non-superconducting and superconducting samples indicate that the iron valence
doesn't play a decisive role in determining whether a KxFe2-ySe2 sample is
superconducting. Combining with the result got in the KxFe2-ySe2 thin films
prepared by molecular beam epitaxy (MBE), we argue that our superconducting
sample partly corresponds to the phase without iron vacancies as evidenced by
scanning tunnelling microscopy (STM) and the insulating sample mainly
corresponds to the phase with the $\sqrt{5}\times\sqrt{5}$ vacancy order.
Quenching may play a role of freezing the phase without iron vacancies.
